selection?_ . let there be a species which has varied into _two forms_ each adapted to certain existing conditions better than the parent form, which they soon supplant. . if these _two forms_, which are supposed to coexist in the same district, do not intercross, natural selection will accumulate all favourable variations till they become well suited to their conditions of life, and form two slightly differing species. . but if these _two forms_ freely intercross with each other, and produce hybrids, which are also quite fertile _inter se_, then the formation of the two distinct races or species will be retarded, or perhaps entirely prevented; for the offspring of the crossed unions will be _more vigorous_ owing to the cross, although _less adapted_ to their conditions of life than either of the pure breeds. . now, let a partial sterility of the hybrids of some considerable proportion of these two forms arise; and, as this would probably be due to some special conditions of life, we may fairly suppose it to arise in some definite portion of the area occupied by the two forms. . the result will be that, in that area, the hybrids (although continually produced by first crosses almost as freely as before) will not themselves increase so rapidly as the two pure forms; and as the two pure forms are, by the terms of the problem, better suited to their several conditions of life than the hybrids, they will inevitably increase more rapidly, and will continually tend to supplant the hybrids altogether at every recurrent severe struggle for existence. . we may fairly suppose, also, that as soon as any sterility appears some disinclination to _cross unions_ will appear, and this will further tend to the diminution of the production of hybrids. . in the other part of the area, however, where hybridism occurs with perfect freedom, hybrids of various degrees may increase till they equal or even exceed in number the pure species--that is, the incipient species will be liable to be swamped by intercrossing. . the first result, then, of a partial sterility of crosses appearing in one part of the area occupied by the two forms, will be--that the great majority of the individuals will there consist of the two pure forms only, while in the remaining part these will be in a minority,--which is the same as saying that the new _physiological variety_ of the two forms will be better suited to the conditions of existence than the remaining portion which has not varied physiologically. . but when the struggle for existence becomes severe, that variety which is best adapted to the conditions of existence always supplants that which is imperfectly adapted; therefore, _by natural selection_ the _varieties_ which are _sterile_ when crossed will become established as the only ones. . now let variations in the _amount of sterility_ and in the _disinclination to crossed unions_ continue to occur--also in certain parts of the area: exactly the same result must recur, and the progeny of this new physiological variety will in time occupy the whole area. . there is yet another consideration that would facilitate the process. it seems probable that the _sterility variations_ would, to some extent, concur with, and perhaps depend upon, the _specific variations_; so that, just in proportion as the _two forms_ diverged and became better adapted to the conditions of existence, they would become more sterile when intercrossed. if this were the case, then natural selection would act with double strength; and those which were better adapted to survive both structurally and physiologically would certainly do so.] [footnote : cases of this kind are referred to at p. . it must, however, be noted, that such sterility in first crosses appears to be equally rare between different species of the same genus as between individuals of the same species. mules and other hybrids are freely produced between very distinct species, but are themselves infertile or quite sterile; and it is this infertility or sterility of the hybrids that is the characteristic--and was once thought to be the criterion--of species, not the sterility of their first crosses. hence we should not expect to find any constant infertility in the first crosses between the distinct strains or varieties that formed the starting-point of new species, but only a slight amount of infertility in their mongrel offspring. it follows, that mr. romanes' theory of _physiological selection_--which assumes sterility or infertility between first crosses as the fundamental fact in the origin of species--does not accord with the general phenomena of hybridism in nature.] [footnote : the exact number is . , but the fractions are omitted for clearness.] chapter viii the origin and uses of colour in animals the darwinian theory threw new light on organic colour--the problem to be solved--the constancy of animal colour indicates utility--colour and environment--arctic animals white--exceptions prove the rule--desert, forest, nocturnal, and oceanic animals--general theories of animal colour--variable protective colouring--mr. poulton's experiments--special or local colour adaptations--imitation of particular objects--how they have been produced--special protective colouring of butterflies--protective resemblance among marine animals--protection by terrifying enemies--alluring coloration--the coloration of birds' eggs--colour as a means of recognition--summary of the preceding exposition--influence of locality or of climate on colour--concluding remarks. among the numerous applications of the darwinian theory in the interpretation of the complex phenomena presented by the organic world, none have been more successful, or are more interesting, than those which deal with the colours of animals and plants. to the older school of naturalists colour was a trivial character, eminently unstable and untrustworthy in the determination of species; and it appeared to have, in most cases, no use or meaning to the objects which displayed it. the bright and often gorgeous coloration of insect, bird, or flower, was either looked upon as having been created for the enjoyment of mankind, or as due to unknown and perhaps undiscoverable laws of nature. but the researches of mr. darwin totally changed our point of view in this matter. he showed, clearly, that some of the colours of animals are useful, some hurtful to them; and he believed that many of the most brilliant colours were developed by sexual choice; while his great general principle, that all the fixed characters of organic beings have been developed under the action of the law of utility, led to the inevitable conclusion that so remarkable and conspicuous a character as colour, which so often constitutes the most obvious distinction of species from species, or group from group, must also have arisen from survival of the fittest, and must, therefore, in most cases have some relation to the wellbeing of its possessors. continuous observation and research, carried on by multitudes of observers during the last thirty years, have shown this to be the case; but the problem is found to be far more complex than was at first supposed. the modes in which colour is of use to different classes of organisms is very varied, and have probably not yet been all discovered; while the infinite variety and marvellous beauty of some of its developments are such as to render it hopeless to arrive at a complete and satisfactory explanation of every individual case. so much, however, has been achieved, so many curious facts have been explained, and so much light has been thrown on some of the most obscure phenomena of nature, that the subject deserves a prominent place in any account of the darwinian theory. _the problem to be solved._ before dealing with the various modifications of colour in the animal world it is necessary to say a few words on colour in general, on its prevalence in nature, and how it is that the colours of animals and plants require any special explanation. what we term colour is a subjective phenomenon, due to the constitution of our mind and nervous system; while, objectively, it consists of light-vibrations of different wave-lengths emitted by, or reflected from, various objects. every visible object must be coloured, because to be visible it must send rays of light to our eye. the kind of light it sends is modified by the molecular constitution or the surface texture of the object. pigments absorb certain rays and reflect the remainder, and this reflected portion has to our eyes a definite colour, according to the portion of the rays constituting white light which are absorbed. interference colours are produced either by thin films or by very fine striae on the surfaces of bodies, which cause rays of certain wave-lengths to neutralise each other, leaving the remainder to produce the effects of colour. such are the colours of soap-bubbles, or of steel or glass on which extremely fine lines have been ruled; and these colours often produce the effect of metallic lustre, and are the cause of most of the metallic hues of birds and insects. as colour thus depends on molecular or chemical constitution or on the minute surface texture of bodies, and, as the matter of which organic beings are composed consists of chemical compounds of great complexity and extreme instability, and is also subject to innumerable changes during growth and development, we might naturally expect the phenomena of colour to be more varied here than in less complex and more stable compounds. yet even in the inorganic world we find abundant and varied colours; in the earth and in the water; in metals, gems, and minerals; in the sky and in the ocean; in sunset clouds and in the many-tinted rainbow. here we can have no question of _use_ to the coloured object, and almost as little perhaps in the vivid red of blood, in the brilliant colours of red snow and other low algae and fungi, or even in the universal mantle of green which clothes so large a portion of the earth's surface. the presence of some colour, or even of many brilliant colours, in animals and plants would require no other explanation than does that of the sky or the ocean, of the ruby or the emerald--that is, it would require a purely physical explanation only. it is the wonderful individuality of the colours of animals and plants that attracts our attention--the fact that the colours are localised in definite patterns, sometimes in accordance with structural characters, sometimes altogether independent of them; while often differing in the most striking and fantastic manner in allied species. we are thus compelled to look upon colour not merely as a physical but also as a biological characteristic, which has been differentiated and specialised by natural selection, and must, therefore, find its explanation in the principle of adaptation or utility. _the constancy of animal colour indicates utility._ that the colours and markings of animals have been acquired under the fundamental law of utility is indicated by a general fact which has received very little attention. as a rule, colour and marking are constant in each species of wild animal, while, in almost every domesticated animal, there arises great variability. we see this in our horses and cattle, our dogs and cats, our pigeons and poultry. now, the essential difference between the conditions of life of domesticated and wild animals is, that the former are protected by man, while the latter have to protect themselves. the extreme variations in colour that immediately arise under domestication indicate a tendency to vary in this way, and the occasional occurrence of white or piebald or other exceptionally coloured individuals of many species in a state of nature, shows that this tendency exists there also; and, as these exceptionally coloured individuals rarely or never increase, there must be some constant power at work to keep it in check. this power can only be natural selection or the survival of the fittest, which again implies that some colours are useful, some injurious, in each particular case. with this principle as our guide, let us see how far we can account both for the general and special colours of the animal world. _colour and environment._ the fact that first strikes us in our examination of the colours of animals as a whole, is the close relation that exists between these colours and the general environment. thus, white prevails among arctic animals; yellow or brown in desert species; while green is only a common colour in tropical evergreen forests. if we consider these cases somewhat carefully we shall find, that they afford us excellent materials for forming a judgment on the various theories that have been suggested to account for the colours of the animal world. in the arctic regions there are a number of animals which are wholly white all the year round, or which only turn white in winter. among the former are the polar bear and the american polar hare, the snowy owl and the greenland falcon; among the latter the arctic fox, the arctic hare, the ermine, and the ptarmigan. those which are permanently white remain among the snow nearly all the year round, while those which change their colour inhabit regions which are free from snow in summer. the obvious explanation of this style of coloration is, that it is protective, serving to conceal the herbivorous species from their enemies, and enabling carnivorous animals to approach their prey unperceived. two other explanations have, however, been suggested. one is, that the prevalent white of the arctic regions has a direct effect in producing the white colour in animals, either by some photographic or chemical action on the skin or by a reflex action through vision. the other is, that the white colour is chiefly beneficial as a means of checking radiation and so preserving animal heat during the severity of an arctic winter. the first is part of the general theory that colour is the effect of coloured light on the objects--a pure hypothesis which has, i believe, no facts whatever to support it. the second suggestion is also an hypothesis merely, since it has not been proved by experiment that a white colour, _per se_, independently of the fur or feathers which is so coloured, has any effect whatever in checking the radiation of low-grade heat like that of the animal body. but both alike are sufficiently disproved by the interesting exceptions to the rule of white coloration in the arctic regions, which exceptions are, nevertheless, quite in harmony with the theory of protection. whenever we find arctic animals which, from whatever cause, do not require protection by the white colour, then neither the cold nor the snow-glare has any effect upon their coloration. the sable retains its rich brown fur throughout the siberian winter; but it frequents trees at that season and not only feeds partially on fruits or seeds, but is able to catch birds among the branches of the fir-trees, with the bark of which its colour assimilates. then we have that thoroughly arctic animal, the musk-sheep, which is brown and conspicuous; but this animal is gregarious, and its safety depends on its association in small herds. it is, therefore, of more importance for it to be able to recognise its kind at a distance than to be concealed from its enemies, against which it can well protect itself so long as it keeps together in a compact body. but the most striking example is that of the common raven, which is a true arctic bird, and is found even in mid-winter as far north as any known bird or mammal. yet it always retains its black coat, and the reason, from our point of view, is obvious. the raven is a powerful bird and fears no enemy, while, being a carrion-feeder, it has no need for concealment in order to approach its prey. the colour of the raven and of the musk-sheep are, therefore, both inconsistent with any other theory than that the white colour of arctic animals has been acquired for concealment, and to that theory both afford a strong support. here we have a striking example of the exception proving the rule. in the desert regions of the earth we find an even more general accordance of colour with surroundings. the lion, the camel, and all the desert antelopes have more or less the colour of the sand or rock among which they live. the egyptian cat and the pampas cat are sandy or earth coloured. the australian kangaroos are of similar tints, and the original colour of the wild horse is supposed to have been sandy or clay coloured. birds are equally well protected by assimilative hues; the larks, quails, goatsuckers, and grouse which abound in the north african and asiatic deserts are all tinted or mottled so as closely to resemble the average colour of the soil in the districts they inhabit. canon tristram, who knows these regions and their natural history so well, says, in an often quoted passage: "in the desert, where neither trees, brushwood, nor even undulations of the surface afford the slightest protection to its foes, a modification of colour which shall be assimilated to that of the surrounding country is absolutely necessary. hence, without exception, the upper plumage of every bird, whether lark, chat, sylvain, or sand-grouse, and also the fur of all the smaller mammals, and the skin of all the snakes and lizards, is of one uniform isabelline or sand colour." passing on to the tropical regions, it is among their evergreen forests alone that we find whole groups of birds whose ground colour is green. parrots are very generally green, and in the east we have an extensive group of green fruit-eating pigeons; while the barbets, bee-eaters, turacos, leaf-thrushes (phyllornis), white-eyes (zosterops), and many other groups, have so much green in their plumage as to tend greatly to their concealment among the dense foliage. there can be no doubt that these colours have been acquired as a protection, when we see that in all the temperate regions, where the leaves are deciduous, the ground colour of the great majority of birds, especially on the upper surface, is a rusty brown of various shades, well corresponding with the bark, withered leaves, ferns, and bare thickets among which they live in autumn and winter, and especially in early spring when so many of them build their nests. nocturnal animals supply another illustration of the same rule, in the dusky colours of mice, rats, bats, and moles, and in the soft mottled plumage of owls and goatsuckers which, while almost equally inconspicuous in the twilight, are such as to favour their concealment in the daytime. an additional illustration of general assimilation of colour to the surroundings of animals, is furnished by the inhabitants of the deep oceans. professor moseley of the challenger expedition, in his british association lecture on this subject, says: "most characteristic of pelagic animals is the almost crystalline transparency of their bodies. so perfect is this transparency that very many of them are rendered almost entirely invisible when floating in the water, while some, even when caught and held up in a glass globe, are hardly to be seen. the skin, nerves, muscles, and other organs are absolutely hyaline and transparent, but the liver and digestive tract often remain opaque and of a yellow or brown colour, and exactly resemble when seen in the water small pieces of floating seaweed." such marine organisms, however, as are of larger size, and either occasionally or habitually float on the surface, are beautifully tinged with blue above, thus harmonising with the colour of the sea as seen by hovering birds; while they are white below, and are thus invisible against the wave-foam and clouds as seen by enemies beneath the surface. such are the tints of the beautiful nudibranchiate mollusc, glaucus atlanticus, and many others. _general theories of animal colour._ we are now in a position to test the general theories, or, to speak more correctly, the popular notions, as to the origin of animal coloration, before proceeding to apply the principle of utility to the explanation of some among the many extraordinary manifestations of colour in the animal world. the most generally received theory undoubtedly is, that brilliancy and variety of colour are due to the direct action of light and heat; a theory no doubt derived from the abundance of bright-coloured birds, insects, and flowers which are brought from tropical regions. there are, however, two strong arguments against this theory. we have already seen how generally bright coloration is wanting in desert animals, yet here heat and light are both at a maximum, and if these alone were the agents in the production of colour, desert animals should be the most brilliant. again, all naturalists who have lived in tropical regions know that the proportion of bright to dull coloured species is little if any greater there than in the temperate zone, while there are many tropical groups in which bright colours are almost entirely unknown. no part of the world presents so many brilliant birds as south america, yet there are extensive families, containing many hundreds of species, which are as plainly coloured as our average temperate birds. such are the families of the bush-shrikes and ant-thrushes (formicariidae), the tyrant-shrikes (tyrannidae), the american creepers (dendrocolaptidae), together with a large proportion of the wood-warblers (mniotiltidae), the finches, the wrens, and some other groups. in the eastern hemisphere, also, we have the babbling-thrushes (timaliidae), the cuckoo-shrikes (campephagidae), the honey-suckers (meliphagidae), and several other smaller groups which are certainly not coloured above the average standard of temperate birds. again, there are many families of birds which spread over the whole world, temperate and tropical, and among these the tropical species rarely present any exceptional brilliancy of colour. such are the thrushes, goatsuckers, hawks, plovers, and ducks; and in the last-named group it is the temperate and arctic zones that afford the most brilliant coloration. the same general facts are found to prevail among insects. although tropical insects present some of the most gorgeous coloration in the whole realm of nature, yet there are thousands and tens of thousands of species which are as dull coloured as any in our cloudy land. the extensive family of the carnivorous ground-beetles (carabidae) attains its greatest brilliancy in the temperate zone; while by far the larger proportion of the great families of the longicorns and the weevils, are of obscure colours even in the tropics. in butterflies, there is undoubtedly a larger proportion of brilliant colour in the tropics; but if we compare families which are almost equally developed over the globe--as the pieridae or whites and yellows, and the satyridae or ringlets--we shall find no great disproportion in colour between those of temperate and tropical regions. the various facts which have now briefly been noticed are sufficient to indicate that the light and heat of the sun are not the direct causes of the colours of animals, although they may favour the production of colour when, as in tropical regions, the persistent high temperature favours the development of the maximum of life. we will now consider the next suggestion, that light reflected from surrounding coloured objects tends to produce corresponding colours in the animal world. this theory is founded on a number of very curious facts which prove, that such a change does sometimes occur and is directly dependent on the colours of surrounding objects; but these facts are comparatively rare and exceptional in their nature, and the same theory will certainly not apply to the infinitely varied colours of the higher animals, many of which are exposed to a constantly varying amount of light and colour during their active existence. a brief sketch of these dependent changes of colour may, however, be advantageously given here. _variable protective colouring._ there are two distinct kinds of change of colour in animals due to the colouring of the environment. in one case the change is caused by reflex action set up by the animal _seeing_ the colour to be imitated, and the change produced can be altered or repeated as the animal changes its position. in the other case the change occurs but once, and is probably not due to any conscious or sense action, but to some direct influence on the surface tissues while the creature is undergoing a moult or change to the pupa form. the most striking example of the first class is that of the chameleon, which changes to white, brown, yellowish, or green, according to the colour of the object on which it rests. this change is brought about by means of two layers of pigment cells, deeply seated in the skin, and of bluish and yellowish colours. by suitable muscles these cells can be forced upwards so as to modify the colour of the skin, which, when they are not brought into action, is a dirty white. these animals are excessively sluggish and defenceless, and the power of changing their colour to that of their immediate surroundings is no doubt of great service to them. many of the flatfish are also capable of changing their colour according to the colour of the bottom they rest on; and frogs have a similar power to a limited extent. some crustacea also change colour, and the power is much developed in the chameleon shrimp (mysis chamaeleon) which is gray when on sand, but brown or green when among brown or green seaweed. it has been proved by experiment that when this animal is blinded the change does not occur. in all these cases, therefore, we have some form of reflex or sense action by which the change is produced, probably by means of pigment cells beneath the skin as in the chameleon. the second class consists of certain larvae, and pupae, which undergo changes of colour when exposed to differently coloured surroundings. this subject has been carefully investigated by mr. e.b. poulton, who has communicated the results of his experiments to the royal society.[ ] it had been noticed that some species of larvae which fed on several different plants had colours more or less corresponding to the particular plant the individual fed on. numerous cases are given in professor meldola's article on "variable protective colouring" (_proc. zool. soc._, , p. ), and while the general green coloration was attributed to the presence of chlorophyll beneath the skin, the particular change in correspondence to each food-plant was attributed to a special function which had been developed by natural selection. later on, in a note to his translation of weissmann's _theory of descent_, professor meldola seemed disposed to think that the variations of colour of some of the species might be phytophagic--that is, due to the direct action of the differently coloured leaves on which the insect fed. mr. poulton's experiments have thrown much light on this question, since he has conclusively proved that, in the case of the sphinx caterpillar of smerinthus ocellatus, the change of colour is not due to the food but to the coloured light reflected from the leaves. this was shown by feeding two sets of larvae on the same plant but exposed to differently coloured surroundings, obtained by sewing the leaves together, so that in one case only the dark upper surface, in the other the whitish under surface was exposed to view. the result in each case was a corresponding change of colour in the larvae, confirming the experiments on different individuals of the same batch of larvae which had been supplied with different food-plants or exposed to a different coloured light. an even more interesting series of experiments was made on the colours of pupae, which in many cases were known to be affected by the material on which they underwent their transformations. the late mr. t.w. wood proved, in , that the pupae of the common cabbage butterflies (pieris brassicae and p. rapae) were either light, or dark, or green, according to the coloured boxes they were kept in, or the colours of the fences, walls, etc., against which they were suspended. mrs. barber in south africa found that the pupae of papilio nireus underwent a similar change, being deep green when attached to orange leaves of the same tint, pale yellowish-green when on a branch of the bottle-brush tree whose half-dried leaves were of this colour, and yellowish when attached to the wooden frame of a box. a few other observers noted similar phenomena, but nothing more was done till mr. poulton's elaborate series of experiments with the larvae of several of our common butterflies were the means of clearing up several important points. he showed that the action of the coloured light did not affect the pupa itself but the larva, and that only for a limited period of time. after a caterpillar has done feeding it wanders about seeking a suitable place to undergo its transformation. when this is found it rests quietly for a day or two, spinning the web from which it is to suspend itself; and it is during this period of quiescence, and perhaps also the first hour or two after its suspension, that the action of the surrounding coloured surfaces determines, to a considerable extent, the colour of the pupa. by the application of various surrounding colours during this period, mr. poulton was able to modify the colour of the pupa of the common tortoise-shell butterfly from nearly black to pale, or to a brilliant golden; and that of pieris rapae from dusky through pinkish to pale green. it is interesting to note, that the colours produced were in all cases such only as assimilated with the surroundings usually occupied by the species, and also, that colours which did not occur in such surroundings, as dark red or blue, only produced the same effects as dusky or black. careful experiments were made to ascertain whether the effect was produced through the sight of the caterpillar. the ocelli were covered with black varnish, but neither this, nor cutting off the spines of the tortoise-shell larva to ascertain whether they might be sense-organs, produced any effect on the resulting colour. mr. poulton concludes, therefore, that the colour-action probably occurs over the whole surface of the body, setting up physiological processes which result in the corresponding colour-change of the pupa. such changes are, however, by no means universal, or even common, in protectively coloured pupae, since in papilio machaon and some others which have been experimented on, both in this country and abroad, no change can be produced on the pupa by any amount of exposure to differently coloured surroundings. it is a curious point that, with the small tortoise-shell larva, exposure to light from gilded surfaces produced pupae with a brilliant golden lustre; and the explanation is supposed to be that mica abounded in the original habitat of the species, and that the pupae thus obtained protection when suspended against micaceous rock. looking, however, at the wide range of the species and the comparatively limited area in which micaceous rocks occur, this seems a rather improbable explanation, and the occurrence of this metallic appearance is still a difficulty. it does not, however, commonly occur in this country in a natural state. the two classes of variable colouring here discussed are evidently exceptional, and can have little if any relation to the colours of those more active creatures which are continually changing their position with regard to surrounding objects, and whose colours and markings are nearly constant throughout the life of the individual, and (with the exception of sexual differences) in all the individuals of the species. we will now briefly pass in review the various characteristics and uses of the colours which more generally prevail in nature; and having already discussed those protective colours which serve to harmonise animals with their general environment, we have to consider only those cases in which the colour resemblance is more local or special in its character. _special or local colour adaptations._ this form of colour adaptation is generally manifested by markings rather than by colour alone, and is extremely prevalent both among insects and vertebrates, so that we shall be able to notice only a few illustrative cases. among our native birds we have the snipe and woodcock, whose markings and tints strikingly accord with the dead marsh vegetation among which they live; the ptarmigan in its summer dress is mottled and tinted exactly like the lichens which cover the stones of the higher mountains; while young unfledged plovers are spotted so as exactly to resemble the beach pebbles among which they crouch for protection, as beautifully exhibited in one of the cases of british birds in the natural history museum at south kensington. in mammalia, we notice the frequency of rounded spots on forest or tree haunting animals of large size, as the forest deer and the forest cats; while those that frequent reedy or grassy places are striped vertically, as the marsh antelopes and the tiger. i had long been of opinion that the brilliant yellow and black stripes of the tiger were adaptive, but have only recently obtained proof that it is so. an experienced tiger-hunter, major walford, states in a letter, that the haunts of the tiger are invariably full of the long grass, dry and pale yellow for at least nine months of the year, which covers the ground wherever there is water in the rainy season, and he adds: "i once, while following up a wounded tiger, failed for at least a minute to see him under a tree in grass at a distance of about twenty yards--jungle open--but the natives saw him, and i eventually made him out well enough to shoot him, but even then i could not see at what part of him i was aiming. there can be no doubt whatever that the colour of both the tiger and the panther renders them almost invisible, especially in a strong blaze of light, when among grass, and one does not seem to notice stripes or spots till they are dead." it is the black shadows of the vegetation that assimilate with the black stripes of the tiger; and, in like manner, the spotty shadows of leaves in the forest so harmonise with the spots of ocelots, jaguars, tiger-cats, and spotted deer as to afford them a very perfect concealment. in some cases the concealment is effected by colours and markings which are so striking and peculiar that no one who had not seen the creature in its native haunts would imagine them to be protective. an example of this is afforded by the banded fruit pigeon of timor, whose pure white head and neck, black wings and back, yellow belly, and deeply-curved black band across the breast, render it a very handsome and conspicuous bird. yet this is what mr. h.o. forbes says of it: "on the trees the white-headed fruit pigeon (ptilopus cinctus) sate motionless during the heat of the day in numbers, on well-exposed branches; but it was with the utmost difficulty that i or my sharp-eyed native servant could ever detect them, even in trees where we knew they were sitting."[ ] the trees referred to are species of eucalyptus which abound in timor. they have whitish or yellowish bark and very open foliage, and it is the intense sunlight casting black curved shadows of one branch upon another, with the white and yellow bark and deep blue sky seen through openings of the foliage, that produces the peculiar combination of colours and shadows to which the colours and markings of this bird have become so closely assimilated. even such brilliant and gorgeously coloured birds as the sun-birds of africa are, according to an excellent observer, often protectively coloured. mrs. m.e. barber remarks that "a casual observer would scarcely imagine that the highly varnished and magnificently coloured plumage of the various species of noctarinea could be of service to them, yet this is undoubtedly the case. the most unguarded moments of the lives of these birds are those that are spent amongst the flowers, and it is then that they are less wary than at any other time. the different species of aloes, which blossom in succession, form the principal sources of their winter supplies of food; and a legion of other gay flowering plants in spring and summer, the aloe blossoms especially, are all brilliantly coloured, and they harmonise admirably with the gay plumage of the different species of sun-birds. even the keen eye of a hawk will fail to detect them, so closely do they resemble the flowers they frequent. the sun-birds are fully aware of this fact, for no sooner have they relinquished the flowers than they become exceedingly wary and rapid in flight, darting arrow-like through the air and seldom remaining in exposed situations. the black sun-bird (nectarinea amethystina) is never absent from that magnificent forest-tree, the 'kaffir boom' (erythrina caffra); all day long the cheerful notes of these birds may be heard amongst its spreading branches, yet the general aspect of the tree, which consists of a huge mass of scarlet and purple-black blossoms without a single green leaf, blends and harmonises with the colours of the black sun-bird to such an extent that a dozen of them may be feeding amongst its blossoms without being conspicuous, or even visible."[ ] some other cases will still further illustrate how the colours of even very conspicuous animals may be adapted to their peculiar haunts. the late mr. swinhoe says of the kerivoula picta, which he observed in formosa: "the body of this bat was of an orange colour, but the wings were painted with orange-yellow and black. it was caught suspended, head downwards, on a cluster of the fruit of the longan tree (nephelium longanum). now this tree is an evergreen, and all the year round some portion of its foliage is undergoing decay, the particular leaves being, in such a stage, partially orange and black. this bat can, therefore, at all seasons suspend from its branches and elude its enemies by its resemblance to the leaves of the tree."[ ] even more curious is the case of the sloths--defenceless animals which feed upon leaves, and hang from the branches of trees with their back downwards. most of the species have a curious buff-coloured spot on the back, rounded or oval in shape and often with a darker border, which seems placed there on purpose to make them conspicuous; and this was a great puzzle to naturalists, because the long coarse gray or greenish hair was evidently like tree-moss and therefore protective. but an old writer, baron von slack, in his _voyage_ _to surinam_ ( ), had already explained the matter. he says: "the colour and even the shape of the hair are much like withered moss, and serve to hide the animal in the trees, but particularly when it has that orange-coloured spot between the shoulders and lies close to the tree; it looks then exactly like a piece of branch where the rest has been broken off, by which the hunters are often deceived." even such a huge animal as the giraffe is said to be perfectly concealed by its colour and form when standing among the dead and broken trees that so often occur on the outskirts of the thickets where it feeds. the large blotch-like spots on the skin and the strange shape of the head and horns, like broken branches, so tend to its concealment that even the keen-eyed natives have been known to mistake trees for giraffes or giraffes for trees. innumerable examples of this kind of protective colouring occur among insects; beetles mottled like the bark of trees or resembling the sand or rock or moss on which they live, with green caterpillars of the exact general tints of the foliage they feed on; but there are also many cases of detailed imitation of particular objects by insects that must be briefly described.[ ] _protective imitation of particular objects._ the insects which present this kind of imitation most perfectly are the phasmidae, or stick and leaf insects. the well-known leaf-insects of ceylon and of java, species of phyllium, are so wonderfully coloured and veined, with leafy expansions on the legs and thorax, that not one person in ten can see them when resting on the food-plant close beneath their eyes. others resemble pieces of stick with all the minutiae of knots and branches, formed by the insects' legs, which are stuck out rigidly and unsymmetrically. i have often been unable to distinguish between one of these insects and a real piece of stick, till i satisfied myself by touching it and found it to be alive. one species, which was brought me in borneo, was covered with delicate semitransparent green foliations, exactly resembling the hepaticae which cover pieces of rotten stick in the damp forests. others resemble dead leaves in all their varieties of colour and form; and to show how perfect is the protection obtained and how important it is to the possessors of it, the following incident, observed by mr. belt in nicaragua, is most instructive. describing the armies of foraging ants in the forest which devour every insect they can catch, he says: "i was much surprised with the behaviour of a green leaf-like locust. this insect stood immovably among a host of ants, many of which ran over its legs without ever discovering there was food within their reach. so fixed was its instinctive knowledge that its safety depended on its immovability, that it allowed me to pick it up and replace it among the ants without making a single effort to escape. this species closely resembles a green leaf."[ ] caterpillars also exhibit a considerable amount of detailed resemblance to the plants on which they live. grass-feeders are striped longitudinally, while those on ordinary leaves are always striped obliquely. some very beautiful protective resemblances are shown among the caterpillars figured in smith and abbott's _lepidopterous insects of georgia_, a work published in the early part of the century, before any theories of protection were started. the plates in this work are most beautifully executed from drawings made by mr. abbott, representing the insects, in every case, on the plants which they frequented, and no reference is made in the descriptions to the remarkable protective details which appear upon the plates. we have, first, the larva of sphinx fuciformis feeding on a plant with linear grass-like leaves and small blue flowers; and we find the insect of the same green as the leaves, striped longitudinally in accordance with the linear leaves, and with the head blue corresponding both in size and colour with the flowers. another species (sphinx tersa) is represented feeding on a plant with small red flowers situated in the axils of the leaves; and the larva has a row of seven red spots, unequal in size, and corresponding very closely with the colour and size of the flowers. two other figures of sphinx larvae are very curious. that of sphinx pampinatrix feeds on a wild vine (vitis indivisa), having green tendrils, and in this species the curved horn on the tail is green, and closely imitates in its curve the tip of the tendril. but in another species (sphinx cranta), which feeds on the fox-grape (vitis vulpina), the horn is very long and red, corresponding with the long red-tipped tendrils of the plant. both these larvae are green with oblique stripes, to harmonise with the veined leaves of the vines; but a figure is also given of the last-named species after it has done feeding, when it is of a decided brown colour and has entirely lost its horn. this is because it then descends to the ground to bury itself, and the green colour and red horn would be conspicuous and dangerous; it therefore loses both at the last moult. such a change of colour occurs in many species of caterpillars. sometimes the change is seasonal; and, in those which hibernate with us, the colour of some species, which is brownish in autumn in adaptation to the fading foliage, becomes green in spring to harmonise with the newly-opened leaves at that season.[ ] some of the most curious examples of minute imitation are afforded by the caterpillars of the geometer moths, which are always brown or reddish, and resemble in form little twigs of the plant on which they feed. they have the habit, when at rest, of standing out obliquely from the branch, to which they hold on by their hind pair of prolegs or claspers, and remain motionless for hours. speaking of these protective resemblances mr. jenner weir says: "after being thirty years an entomologist i was deceived myself, and took out my pruning scissors to cut from a plum tree a spur which i thought i had overlooked. this turned out to be the larva of a geometer two inches long. i showed it to several members of my family, and defined a space of four inches in which it was to be seen, but none of them could perceive that it was a caterpillar."[ ] one more example of a protected caterpillar must be given. mr. a. everett, writing from sarawak, borneo, says: "i had a caterpillar brought me, which, being mixed by my boy with some other things, i took to be a bit of moss with two exquisite pinky-white seed-capsules; but i soon saw that it moved, and examining it more closely found out its real character: it is covered with hair, with two little pink spots on the upper surface, the general hue being more green. its motions are very slow, and when eating the head is withdrawn beneath a fleshy mobile hood, so that the action of feeding does not produce any movement externally. it was found in the limestone hills at busan, the situation of all others where mosses are most plentiful and delicate, and where they partially clothe most of the protruding masses of rock." _how these imitations have been produced._ to many persons it will seem impossible that such beautiful and detailed resemblances as those now described--and these are only samples of thousands that occur in all parts of the world--can have been brought about by the preservation of accidental useful variations. but this will not seem so surprising if we keep in mind the facts set forth in our earlier chapters--the rapid multiplication, the severe struggle for existence, and the constant variability of these and all other organisms. and, further, we must remember that these delicate adjustments are the result of a process which has been going on for millions of years, and that we now see the small percentage of successes among the myriads of failures. from the very first appearance of insects and their various kinds of enemies the need of protection arose, and was usually most easily met by modifications of colour. hence, we may be sure that the earliest leaf-eating insects acquired a green colour as one of the necessities of their existence; and, as the species became modified and specialised, those feeding on particular species of plants would rapidly acquire the peculiar tints and markings best adapted to conceal them upon those plants. then, every little variation that, once in a hundred years perhaps, led to the preservation of some larva which was thereby rather better concealed than its fellows, would form the starting-point of a further development, leading ultimately to that perfection of imitation in details which now astonishes us. the researches of dr. weismann illustrate this progressive adaptation. the very young larvae of several species are green or yellowish without any markings; they then, in subsequent moults, obtain certain markings, some of which are often lost again before the larva is fully grown. the early stages of those species which, like elephant hawk-moths (chaerocampa), have the anterior segments elongated and retractile, with large eye-like spots to imitate the head of a vertebrate, are at first like those of non-retractile species, the anterior segments being as large as the rest. after the first moult they become smaller, comparatively; but it is only after the second moult that the ocelli begin to appear, and these are not fully defined till after the third moult. this progressive development of the individual--the ontogeny--gives us a clue to the ancestral development of the whole race--the phylogeny; and we are enabled to picture to ourselves the very slow and gradual steps by which the existing perfect adaptation has been brought about. in many larvae great variability still exists, and in some there are two or more distinctly-coloured forms--usually a dark and a light or a brown and a green form. the larva of the humming-bird hawk-moth (macroglossa stellatarum) varies in this manner, and dr. weismann raised five varieties from a batch of eggs from one moth. it feeds on species of bedstraw (galium verum and g. mollugo), and as the green forms are less abundant than the brown, it has probably undergone some recent change of food-plant or of habits which renders brown the more protective colour. _special protective colouring of butterflies._ we will now consider a few cases of special protective colouring in the perfect butterfly or moth. mr. mansel weale states that in south africa there is a great prevalence of white and silvery foliage or bark, sometimes of dazzling brilliancy, and that many insects and their larvae have brilliant silvery tints which are protective, among them being three species of butterflies whose undersides are silvery, and which are thus effectually protected when at rest.[ ] a common african butterfly (aterica meleagris) always settles on the ground with closed wings, which so closely resemble the soil of the district that it can with difficulty be seen, and the colour varies with the soil in different localities. thus specimens from senegambia were dull brown, the soil being reddish sand and iron-clay; those from calabar and cameroons were light brown with numerous small white spots, the soil of those countries being light brown clay with small quartz pebbles; while in other localities where the colours of the soil were more varied the colours of the butterfly varied also. here we have variation in a single species which has become specialised in certain areas to harmonise with the colour of the soil.[ ] many butterflies, in all parts of the world, resemble dead leaves on their under side, but those in which this form of protection is carried to the greatest perfection are the species of the eastern genus kallima. in india k. inachis, and in the larger malay islands k. paralekta, are very common. they are rather large and showy butterflies, orange and bluish on the upper side, with a very rapid flight, and frequenting dry forests. their habit is to settle always where there is some dead or decaying foliage, and the shape and colour of the wings (on the under surface), together with the attitude of the insect, is such as to produce an absolutely perfect imitation of a dead leaf. this is effected by the butterfly always settling on a twig, with the short tail of the hind wings just touching it and forming the leaf-stalk. from this a dark curved line runs across to the elongated tip of the upper wings, imitating the midrib, on both sides of which are oblique lines, formed partly by the nervures and partly by markings, which give the effect of the usual veining of a leaf. the head and antennae fit exactly between the closed upper wings so as not to interfere with the outline, which has just that amount of irregular curvature that is seen in dry and withered leaves. the colour is very remarkable for its extreme amount of variability, from deep reddish-brown to olive or pale yellow, hardly two specimens being exactly alike, but all coming within the range of colour of leaves in various stages of decay. still more curious is the fact that the paler wings, which imitate leaves most decayed, are usually covered with small black dots, often gathered into circular groups, and so exactly resembling the minute fungi on decaying leaves that it is hard at first to believe that the insects themselves are not attacked by some such fungus. the concealment produced by this wonderful imitation is most complete, and in sumatra i have often seen one enter a bush and then disappear like magic. once i was so fortunate as to see the exact spot on which the insect settled; but even then i lost sight of it for some time, and only after a persistent search discovered that it was close before my eyes.[ ] here we have a kind of imitation, which is very common in a less developed form, carried to extreme perfection, with the result that the species is very abundant over a considerable area of country. _protective resemblance among marine animals._ among marine animals this form of protection is very common. professor moseley tells us that all the inhabitants of the gulf-weed are most remarkably coloured, for purposes of protection and concealment, exactly like the weed itself. "the shrimps and crabs which swarm in the weed are of exactly the same shade of yellow as the weed, and have white markings upon their bodies to represent the patches of membranipora. the small fish, antennarius, is in the same way weed-colour with white spots. even a planarian worm, which lives in the weed, is similarly yellow-coloured, and also a mollusc, scyllaea pelagica." the same writer tells us that "a number of little crabs found clinging to the floats of the blue-shelled mollusc, ianthina, were all coloured of a corresponding blue for concealment."[ ] professor e.s. morse of salem, mass., found that most of the new england marine mollusca were protectively coloured; instancing among others a little red chiton on rocks clothed with red calcareous algae, and crepidula plana, living within the apertures of the shells of larger species of gasteropods and of a pure white colour corresponding to its habitat, while allied species living on seaweed or on the outside of dark shells were dark brown.[ ] a still more interesting case has been recorded by mr. george brady. he says: "amongst the nullipore which matted together the laminaria roots in the firth of clyde were living numerous small starfishes (ophiocoma bellis) which, except when their writhing movements betrayed them, were quite undistinguishable from the calcareous branches of the alga; their rigid angularly twisted rays had all the appearance of the coralline, and exactly assimilated to its dark purple colour, so that though i held in my hand a root in which were half a dozen of the starfishes, i was really unable to detect them until revealed by their movements."[ ] these few examples are sufficient to show that the principle of protective coloration extends to the ocean as well as over the earth; and if we consider how completely ignorant we are of the habits and surroundings of most marine animals, it may well happen that many of the colours of tropical fishes, which seem to us so strange and so conspicuous, are really protective, owing to the number of equally strange and brilliant forms of corals, sea-anemones, sponges, and seaweeds among which they live. _protection by terrifying enemies._ a considerable number of quite defenceless insects obtain protection from some of their enemies by having acquired a resemblance to dangerous animals, or by some threatening or unusual appearance. this is obtained either by a modification of shape, of habits, of colour, or of all combined. the simplest form of this protection is the aggressive attitude of the caterpillars of the sphingidae, the forepart of the body being erected so as to produce a rude resemblance to the figure of a sphinx, hence the name of the family. the protection is carried further by those species which retract the first three segments and have large ocelli on each side of the fourth segment, thus giving to the caterpillar, when the forepart of its body is elevated, the appearance of a snake in a threatening attitude. the blood-red forked tentacle, thrown out of the neck of the larvae of the genus papilio when alarmed, is, no doubt, a protection against the attacks of ichneumons, and may, perhaps, also frighten small birds; and the habit of turning up the tail possessed by the harmless rove-beetles (staphylinidae), giving the idea that they can sting, has, probably, a similar use. even an unusual angular form, like a crooked twig or inorganic substance, may be protective; as mr. poulton thinks is the case with the curious caterpillar of notodonta ziczac, which, by means of a few slight protuberances on its body, is able to assume an angular and very unorganic-looking appearance. but perhaps the most perfect example of this kind of protection is exhibited by the large caterpillar of the royal persimmon moth (bombyx regia), a native of the southern states of north america, and known there as the "hickory-horned devil." it is a large green caterpillar, often six inches long, ornamented with an immense crown of orange-red tubercles, which, if disturbed, it erects and shakes from side to side in a very alarming manner. in its native country the negroes believe it to be as deadly as a rattlesnake, whereas it is perfectly innocuous. the green colour of the body suggests that its ancestors were once protectively coloured; but, growing too large to be effectually concealed, it acquired the habit of shaking its head about in order to frighten away its enemies, and ultimately developed the crown of tentacles as an addition to its terrifying powers. this species is beautifully figured in abbott and smith's _lepidopterous insects of georgia_. _alluring coloration._ besides those numerous insects which obtain protection through their resemblance to the natural objects among which they live, there are some whose disguise is not used for concealment, but as a direct means of securing their prey by attracting them within the enemy's reach. only a few cases of this kind of coloration have yet been observed, chiefly among spiders and mantidae; but, no doubt, if attention were given to the subject in tropical countries, many more would be discovered. mr. h.o. forbes has described a most interesting example of this kind of simulation in java. while pursuing a large butterfly through the jungle, he was stopped by a dense bush, on a leaf of which he observed one of the skipper butterflies sitting on a bird's dropping. "i had often," he says, "observed small blues at rest on similar spots on the ground, and have wondered what such a refined and beautiful family as the lycaenidae could find to enjoy, in food apparently so incongruous for a butterfly. i approached with gentle steps, but ready net, to see if possible how the present species was engaged. it permitted me to get quite close, and even to seize it between my fingers; to my surprise, however, part of the body remained behind, adhering as i thought to the excreta. i looked closely, and finally touched with my finger the excreta to find if it were glutinous. to my delighted astonishment i found that my eyes had been most perfectly deceived, and that what seemed to be the excreta was a most artfully coloured spider, lying on its back with its feet crossed over and closely adpressed to the body." mr. forbes then goes on to describe the exact appearance of such excreta, and how the various parts of the spider are coloured to produce the imitation, even to the liquid portion which usually runs a little down the leaf. this is exactly imitated by a portion of the thin web which the spider first spins to secure himself firmly to the leaf; thus producing, as mr. forbes remarks, a living bait for butterflies and other insects so artfully contrived as to deceive a pair of human eyes, even when intently examining it.[ ] a native species of spider (thomisus citreus) exhibits a somewhat similar alluring protection by its close resemblance to buds of the wayfaring tree, viburnum lantana. it is pure creamy-white, the abdomen exactly resembling in shape and colour the unopened buds of the flowers among which it takes its station; and it has been seen to capture flies which came to the flowers. but the most curious and beautiful case of alluring protection is that of a wingless mantis in india, which is so formed and coloured as to resemble a pink orchis or some other fantastic flower. the whole insect is of a bright pink colour, the large and oval abdomen looking like the labellum of an orchid. on each side, the two posterior legs have immensely dilated and flattened thighs which represent the petals of a flower, while the neck and forelegs imitate the upper sepal and column of an orchid. the insect rests motionless, in this symmetrical attitude, among bright green foliage, being of course very conspicuous, but so exactly resembling a flower that butterflies and other insects settle upon it and are instantly captured. it is a living trap, baited in the most alluring manner to catch the unwary flower-haunting insects.[ ] _the coloration of birds' eggs._ the colours of birds' eggs have long been a difficulty on the theory of adaptive coloration, because, in so many cases it has not been easy to see what can be the use of the particular colours, which are often so bright and conspicuous that they seem intended to attract attention rather than to be concealed. a more careful consideration of the subject in all its bearings shows, however, that here too, in a great number of cases, we have examples of protective coloration. when, therefore, we cannot see the meaning of the colour, we may suppose that it has been protective in some ancestral form, and, not being hurtful, has persisted under changed conditions which rendered the protection needless. we may divide all eggs, for our present purpose, into two great divisions; those which are white or nearly so, and those which are distinctly coloured or spotted. egg-shells being composed mainly of carbonate of lime, we may assume that the primitive colour of birds' eggs was white, a colour that prevails now among the other egg-bearing vertebrates--lizards, crocodiles, turtles, and snakes; and we might, therefore, expect that this colour would continue where its presence had no disadvantages. now, as a matter of fact, we find that in all the groups of birds which lay their eggs in concealed places, whether in holes of trees or in the ground, or in domed or covered nests, the eggs are either pure white or of very pale uniform coloration. such is the case with kingfishers, bee-eaters, penguins, and puffins, which nest in holes in the ground; with the great parrot family, the woodpeckers, the rollers, hoopoes, trogons, owls, and some others, which build in holes in trees or other concealed places; while martins, wrens, willow-warblers, and australian finches, build domed or covered nests, and usually have white eggs. there are, however, many other birds which lay their white eggs in open nests; and these afford some very interesting examples of the varied modes by which concealment may be obtained. all the duck tribe, the grebes, and the pheasants belong to this class; but these birds all have the habit of covering their eggs with dead leaves or other material whenever they leave the nest, so as effectually to conceal them. other birds, as the short-eared owl, the goatsucker, the partridge, and some of the australian ground pigeons, lay their white or pale eggs on the bare soil; but in these cases the birds themselves are protectively coloured, so that, when sitting, they are almost invisible; and they have the habit of sitting close and almost continuously, thus effectually concealing their eggs. pigeons and doves offer a very curious case of the protection of exposed eggs. they usually build very slight and loose nests of sticks and twigs, so open that light can be seen through them from below, while they are generally well concealed by foliage above. their eggs are white and shining; yet it is a difficult matter to discover, from beneath, whether there are eggs in the nest or not, while they are well hidden by the thick foliage above. the australian podargihuge goatsuckers--build very similar nests, and their white eggs are protected in the same manner. some large and powerful birds, as the swans, herons, pelicans, cormorants, and storks, lay white eggs in open nests; but they keep careful watch over them, and are able to drive away intruders. on the whole, then, we see that, while white eggs are conspicuous, and therefore especially liable to attack by egg-eating animals, they are concealed from observation in many and various ways. we may, therefore, assume that, in cases where there seems to be no such concealment, we are too ignorant of the whole of the conditions to form a correct judgment. we now come to the large class of coloured or richly spotted eggs, and here we have a more difficult task, though many of them decidedly exhibit protective tints or markings. there are two birds which nest on sandy shores--the lesser tern and the ringed plover,--and both lay sand-coloured eggs, the former spotted so as to harmonise with coarse shingle, the latter minutely speckled like fine sand, which are the kinds of ground the two birds choose respectively for their nests. "the common sandpipers' eggs assimilate so closely with the tints around them as to make their discovery a matter of no small difficulty, as every oologist can testify who has searched for them. the pewits' eggs, dark in ground colour and boldly marked, are in strict harmony with the sober tints of moor and fallow, and on this circumstance alone their concealment and safety depend. the divers' eggs furnish another example of protective colour; they are generally laid close to the water's edge, amongst drift and shingle, where their dark tints and black spots conceal them by harmonising closely with surrounding objects. the snipes and the great army of sandpipers furnish innumerable instances of protectively coloured eggs. in all the instances given the sitting-bird invariably leaves the eggs uncovered when it quits them, and consequently their safety depends solely on the colours which adorn them."[ ] the wonderful range of colour and marking in the eggs of the guillemot may be imputed to the inaccessible rocks on which it breeds, giving it complete protection from enemies. thus the pale or bluish ground colour of the eggs of its allies, the auks and puffins, has become intensified and blotched and spotted in the most marvellous variety of patterns, owing to there being no selective agency to prevent individual variation having full sway. the common black coot (fulica atra) has eggs which are coloured in a specially protective manner. dr. william marshall writes, that it only breeds in certain localities where a large water reed (phragmites arundinacea) abounds. the eggs of the coot are stained and spotted with black on a yellowish-gray ground, and the dead leaves of the reed are of the same colour, and are stained black by small parasitic fungi of the uredo family; and these leaves form the bed on which the eggs are laid. the eggs and the leaves agree so closely in colour and markings that it is a difficult thing to distinguish the eggs at any distance. it is to be noted that the coot never covers up its eggs, as its ally the moor-hen usually does. the beautiful blue or greenish eggs of the hedge-sparrow, the song-thrush, and sometimes those of the blackbird, seem at first sight especially calculated to attract attention, but it is very doubtful whether they are really so conspicuous when seen at a little distance among their usual surroundings. for the nests of these birds are either in evergreens, as holly or ivy, or surrounded by the delicate green tints of our early spring vegetation, and may thus harmonise very well with the colours around them. the great majority of the eggs of our smaller birds are so spotted or streaked with brown or black on variously tinted grounds that, when lying in the shadow of the nest and surrounded by the many colours and tints of bark and moss, of purple buds and tender green or yellow foliage, with all the complex glittering lights and mottled shades produced among these by the spring sunshine and by sparkling raindrops, they must have a quite different aspect from that which they possess when we observe them torn from their natural surroundings. we have here, probably, a similar case of general protective harmony to that of the green caterpillars with beautiful white or purple bands and spots, which, though gaudily conspicuous when seen alone, become practically invisible among the complex lights and shadows of the foliage they feed upon. in the case of the cuckoo, which lays its eggs in the nests of a variety of other birds, the eggs themselves are subject to considerable variations of colour, the most common type, however, resembling those of the pipits, wagtails, or warblers, in whose nests they are most frequently laid. it also often lays in the nest of the hedge-sparrow, whose bright blue eggs are usually not at all nearly matched, although they are sometimes said to be so on the continent. it is the opinion of many ornithologists that each female cuckoo lays the same coloured eggs, and that it usually chooses a nest the owners of which lay somewhat similar eggs, though this is by no means universally the case. although birds which have cuckoos' eggs imposed upon them do not seem to neglect them on account of any difference of colour, yet they probably do so occasionally; and if, as seems probable, each bird's eggs are to some extent protected by their harmony of colour with their surroundings, the presence of a larger and very differently coloured egg in the nest might be dangerous, and lead to the destruction of the whole set. those cuckoos, therefore, which most frequently placed their eggs among the kinds which they resembled, would in the long run leave most progeny, and thus the very frequent accord in colour might have been brought about. some writers have suggested that the varied colours of birds' eggs are primarily due to the effect of surrounding coloured objects on the female bird during the period preceding incubation; and have expended much ingenuity in suggesting the objects that may have caused the eggs of one bird to be blue, another brown, and another pink.[ ] but no evidence has been presented to prove that any effects whatever are produced by this cause, while there seems no difficulty in accounting for the facts by individual variability and the action of natural selection. the changes that occur in the conditions of existence of birds must sometimes render the concealment less perfect than it may once have been; and when any danger arises from this cause, it may be met either by some change in the colour of the eggs, or in the structure or position of the nest, or by the increased care which the parents bestow upon the eggs. in this way the various divergences which now so often puzzle us may have arisen. _colour as a means of recognition._ if we consider the habits and life-histories of those animals which are more or less gregarious, comprising a large proportion of the herbivora, some carnivora, and a considerable number of all orders of birds, we shall see that a means of ready recognition of its own kind, at a distance or during rapid motion, in the dusk of twilight or in partial cover, must be of the greatest advantage and often lead to the preservation of life. animals of this kind will not usually receive a stranger into their midst. while they keep together they are generally safe from attack, but a solitary straggler becomes an easy prey to the enemy; it is, therefore, of the highest importance that, in such a case, the wanderer should have every facility for discovering its companions with certainty at any distance within the range of vision. some means of easy recognition must be of vital importance to the young and inexperienced of each flock, and it also enables the sexes to recognise their kind and thus avoid the evils of infertile crosses; and i am inclined to believe that its necessity has had a more widespread influence in determining the diversities of animal coloration than any other cause whatever. to it may probably be imputed the singular fact that, whereas bilateral symmetry of coloration is very frequently lost among domesticated animals, it almost universally prevails in a state of nature; for if the two sides of an animal were unlike, and the diversity of coloration among domestic animals occurred in a wild state, easy recognition would be impossible among numerous closely allied forms.[ ] the wonderful diversity of colour and of marking that prevails, especially in birds and insects, may be due to the fact that one of the first needs of a new species would be, to keep separate from its nearest allies, and this could be most readily done by some easily seen external mark of difference. a few illustrations will serve to show how this principle acts in nature. my attention was first called to the subject by a remark of mr. darwin's that, though, "the hare on her form is a familiar instance of concealment through colour, yet the principle partly fails in a closely allied species, the rabbit; for when running to its burrow it is made conspicuous to the sportsman, and no doubt to all beasts of prey, by its upturned white tail."[ ] but a little consideration of the habits of the animal will show that the white upturned tail is of the greatest value, and is really, as it has been termed by a writer in _the field_, a "signal flag of danger." for the rabbit is usually a crepuscular animal, feeding soon after sunset or on moonlight nights. when disturbed or alarmed it makes for its burrow, and the white upturned tails of those in front serve as guides and signals to those more remote from home, to the young and the feeble; and thus each following the one or two before it, all are able with the least possible delay to regain a place of comparative safety. the apparent danger, therefore, becomes a most important means of security. the same general principle enables us to understand the singular, and often conspicuous, markings on so many gregarious herbivora which are yet, on the whole, protectively coloured. thus, the american prong-buck has a white patch behind and a black muzzle. the tartarian antelope, the ovis poli of high asia, the java wild ox, several species of deer, and a large number of antelopes have a similar conspicuous white patch behind, which, in contrast to the dusky body, must enable them to be seen and followed from a distance by their fellows. where there are many species of nearly the same general size and form inhabiting the same region--as with the antelopes of africa--we find many distinctive markings of a similar kind. the gazelles have variously striped and banded faces, besides white patches behind and on the flanks, as shown in the woodcut. the spring-bok has a white patch on the face and one on the sides, with a curiously distinctive white stripe above the tail, which is nearly concealed when the animal is at rest by a fold of skin but comes into full view when it is in motion, being thus quite analogous to the upturned white tail of the rabbit. in the pallah the white rump-mark is bordered with black, and the peculiar shape of the horns distinguishes it when seen from the front. the sable-antelope, the gems-bok, the oryx, the hart-beest, the bonte-bok, and the addax have each peculiar white markings; and they are besides characterised by horns so remarkably different in each species and so conspicuous, that it seems probable that the peculiarities in length, twist, and curvature have been differentiated for the purpose of recognition, rather than for any speciality of defence in species whose general habits are so similar. [illustration: fig. .--gazella soemmerringi.] it is interesting to note that these markings for recognition are very slightly developed in the antelopes of the woods and marshes. thus, the grys-bok is nearly uniform in colour, except the long black-tipped ears; and it frequents the wooded mountains. the duyker-bok and the rhoode-bok are wary bush-haunters, and have no marks but the small white patch behind. the wood-haunting bosch-bok goes in pairs, and has hardly any distinctive marks on its dusky chestnut coat, but the male alone is horned. the large and handsome koodoo frequents brushwood, and its vertical white stripes are no doubt protective, while its magnificent spiral horns afford easy recognition. the eland, which is an inhabitant of the open country, is uniformly coloured, being sufficiently recognisable by its large size and distinctive form; but the derbyan eland is a forest animal, and has a protectively striped coat. in like manner, the fine speke's antelope, which lives entirely in the swamps and among reeds, has pale vertical stripes on the sides (protective), with white markings on face and breast for recognition. an inspection of the figures of antelopes and other animals in wood's _natural history_, or in other illustrated works, will give a better idea of the peculiarities of recognition markings than any amount of description. other examples of such coloration are to be seen in the dusky tints of the musk-sheep and the reindeer, to whom recognition at a distance on the snowy plains is of more importance than concealment from their few enemies. the conspicuous stripes and bands of the zebra and the quagga are probably due to the same cause, as may be the singular crests and face-marks of several of the monkeys and lemurs.[ ] [illustration: fig. --recognition marks of three african plovers.] among birds, these recognition marks are especially numerous and suggestive. species which inhabit open districts are usually protectively coloured; but they generally possess some distinctive markings for the purpose of being easily recognised by their kind, both when at rest and during flight. such are, the white bands or patches on the breast or belly of many birds, but more especially the head and neck markings in the form of white or black caps, collars, eye-marks or frontal patches, examples of which are seen in the three species of african plovers figured on page . recognition marks during flight are very important for all birds which congregate in flocks or which migrate together; and it is essential that, while being as conspicuous as possible, the marks shall not interfere with the general protective tints of the species when at rest. hence they usually consist of well-contrasted markings on the wings and tail, which are concealed during repose but become fully visible when the bird takes flight. such markings are well seen in our four british species of shrikes, each having quite different white marks on the expanded wings and on the tail feathers; and the same is the case with our three species of saxicola--the stone-chat, whin-chat, and wheat-ear--which are thus easily recognisable on the wing, especially when seen from above, as they would be by stragglers looking out for their companions. the figures opposite, of the wings of two african species of stone-curlew which are sometimes found in the same districts, well illustrates these specific recognition marks. though not very greatly different to our eyes, they are no doubt amply so to the sharp vision of the birds themselves. besides the white patches on the primaries here shown, the secondary feathers are, in some cases, so coloured as to afford very distinctive markings during flight, as seen in the central secondary quills of two african coursers (fig. ). [illustration: fig. .--oedicnemus vermiculatus (above). oe. senegalensis (below).] most characteristic of all, however, are the varied markings of the outer tail-feathers, whose purpose is so well shown by their being almost always covered during repose by the two middle feathers, which are themselves quite unmarked and protectively tinted like the rest of the upper surface of the body. the figures of the expanded tails of two species of east asiatic snipe, whose geographical ranges overlap each other, will serve to illustrate this difference; which is frequently much greater and modified in an endless variety of ways (fig. ). numbers of species of pigeons, hawks, finches, warblers, ducks, and innumerable other birds possess this class of markings; and they correspond so exactly in general character with those of the mammalia, already described, that we cannot doubt they serve a similar purpose.[ ] [illustration: fig. .--secondary quills.] [illustration: fig. .--scolopax megala (upper). s. stenura (lower).] those birds which are inhabitants of tropical forests, and which need recognition marks that shall be at all times visible among the dense foliage, and not solely or chiefly during flight, have usually small but brilliant patches of colour on the head or neck, often not interfering with the generally protective character of their plumage. such are the bright patches of blue, red, or yellow, by which the usually green eastern barbets are distinguished; and similar bright patches of colour characterise the separate species of small green fruit-doves. to this necessity for specialisation in colour, by which each bird may easily recognise its kind, is probably due that marvellous variety in the peculiar beauties of some groups of birds. the duke of argyll, speaking of the humming birds, made the objection that "a crest of topaz is no better in the struggle for existence than a crest of sapphire. a frill ending in spangles of the emerald is no better in the battle of life than a frill ending in spangles of the ruby. a tail is not affected for the purposes of flight, whether its marginal or its central feathers are decorated with white;" and he goes on to urge that mere beauty and variety for their own sake are the only causes of these differences. but, on the principles here suggested, the divergence itself is useful, and must have been produced _pari passu_ with the structural differences on which the differentiation of species depends; and thus we have explained the curious fact that prominent differences of colour often distinguish species otherwise very closely allied to each other. among insects, the principle of distinctive coloration for recognition has probably been at work in the production of the wonderful diversity of colour and marking we find everywhere, more especially among the butterflies and moths; and here its chief function may have been to secure the pairing together of individuals of the same species. in some of the moths this has been secured by a peculiar odour, which attracts the males to the females from a distance; but there is no evidence that this is universal or even general, and among butterflies, especially, the characteristic colour and marking, aided by size and form, afford the most probable means of recognition. that this is so is shown by the fact that "the common white butterfly often flies down to a bit of paper on the ground, no doubt mistaking it for one of its own species;" while, according to mr. collingwood, in the malay archipelago, "a dead butterfly pinned upon a conspicuous twig will often arrest an insect of the same species in its headlong flight, and bring it down within easy reach of the net, especially if it be of the opposite sex."[ ] in a great number of insects, no doubt, form, motions, stridulating sounds, or peculiar odours, serve to distinguish allied species from each other, and this must be especially the case with nocturnal insects, or with those whose colours are nearly uniform and are determined by the need of protection; but by far the larger number of day-flying and active insects exhibit varieties of colour and marking, forming the most obvious distinction between allied species, and which have, therefore, in all probability been acquired in the process of differentiation for the purpose of checking the intercrossing of closely allied forms.[ ] whether this principle extends to any of the less highly organised animals is doubtful, though it may perhaps have affected the higher mollusca. but in marine animals it seems probable that the colours, however beautiful, varied, and brilliant they may often be, are in most cases protective, assimilating them to the various bright-coloured seaweeds, or to some other animals which it is advantageous for them to imitate.[ ] _summary of the preceding exposition._ before proceeding to discuss some of the more recondite phenomena of animal coloration, it will be well to consider for a moment the extent of the ground we have already covered. protective coloration, in some of its varied forms, has not improbably modified the appearance of one-half of the animals living on the globe. the white of arctic animals, the yellowish tints of the desert forms, the dusky hues of crepuscular and nocturnal species, the transparent or bluish tints of oceanic creatures, represent a vast host in themselves; but we have an equally numerous body whose tints are adapted to tropical foliage, to the bark of trees, or to the soil or dead leaves on or among which they habitually live. then we have the innumerable special adaptations to the tints and forms of leaves, or twigs, or flowers; to bark or moss; to rock or pebble; by which such vast numbers of the insect tribes obtain protection; and we have seen that these various forms of coloration are equally prevalent in the waters of the seas and oceans, and are thus coextensive with the domain of life upon the earth. the comparatively small numbers which possess "terrifying" or "alluring" coloration may be classed under the general head of the protectively coloured. but under the next head--colour for recognition--we have a totally distinct category, to some extent antagonistic or complementary to the last, since its essential principle is visibility rather than concealment. yet it has been shown, i think, that this mode of coloration is almost equally important, since it not only aids in the preservation of existing species and in the perpetuation of pure races, but was, perhaps, in its earlier stages, a not unimportant factor in their development. to it we owe most of the variety and much of the beauty in the colours of animals; it has caused at once bilateral symmetry and general permanence of type; and its range of action has been perhaps equally extensive with that of coloration for concealment. _influence of locality or of climate on colour._ certain relations between locality and coloration have long been noticed. mr. gould observed that birds from inland or continental localities were more brightly coloured than those living near the sea-coast or on islands, and he supposed that the more brilliant atmosphere of the inland stations was the explanation of the phenomenon.[ ] many american naturalists have observed similar facts, and they assert that the intensity of the colours of birds and mammals increases from north to south, and also with the increase of humidity. this change is imputed by mr. j.a. allen to the direct action of the environment. he says: "in respect to the correlation of intensity of colour in animals with the degree of humidity, it would perhaps be more in accordance with cause and effect to express the law of correlation as a _decrease_ of intensity of colour with a _decrease_ of humidity, the paleness evidently resulting from exposure and the blanching effect of intense sunlight, and a dry, often intensely heated atmosphere. with the decrease of the aqueous precipitation the forest growth and the protection afforded by arborescent vegetation gradually also decreases, as of course does also the protection afforded by clouds, the excessively humid regions being also regions of extreme cloudiness, while the dry regions are comparatively cloudless districts."[ ] almost identical changes occur in birds, and are imputed by mr. allen to similar causes. it will be seen that mr. gould and mr. allen impute opposite effects to the same cause, brilliancy or intensity of colour being due to a brilliant atmosphere according to the former, while paleness of colour is imputed by the latter to a too brilliant sun. according to the principles which have been established by the consideration of arctic, desert, and forest animals respectively, we shall be led to conclude that there has been no direct action in this case, but that the effects observed are due to the greater or less need of protection. the pale colour that is prevalent in arid districts is in harmony with the general tints of the surface; while the brighter tints or more intense coloration, both southward and in humid districts, are sufficiently explained by the greater shelter due to a more luxuriant vegetation and a shorter winter. the advocates of the theory that intensity of light directly affects the colours of organisms, are led into perpetual inconsistencies. at one time the brilliant colours of tropical birds and insects are imputed to the intensity of a tropical sun, while the same intensity of sunlight is now said to have a "bleaching" effect. the comparatively dull and sober hues of our northern fauna were once supposed to be the result of our cloudy skies; but now we are told that cloudy skies and a humid atmosphere intensify colour. in my _tropical nature_ (pp. - ) i have called attention to what is perhaps the most curious and decided relation of colour to locality which has yet been observed--the prevalence of white markings in the butterflies and birds of islands. so many cases are adduced from so many different islands, both in the eastern and western hemisphere, that it is impossible to doubt the existence of some common cause; and it seems probable to me now, after a fuller consideration of the whole subject of colour, that here too we have one of the almost innumerable results of the principle of protective coloration. white is, as a rule, an uncommon colour in animals, but probably only because it is so conspicuous. whenever it becomes protective, as in the case of arctic animals and aquatic birds, it appears freely enough; while we know that white varieties of many species occur occasionally in the wild state, and that, under domestication, white or parti-coloured breeds are freely produced. now in all the islands in which exceptionally white-marked birds and butterflies have been observed, we find two features which would tend to render the conspicuous white markings less injurious--a luxuriant tropical vegetation, and a decided scarcity of rapacious mammals and birds. white colours, therefore, would not be eliminated by natural selection; but variations in this direction would bear their part in producing the recognition marks which are everywhere essential, and which, in these islands, need not be so small or so inconspicuous as elsewhere. _concluding remarks._ on a review of the whole subject, then, we must conclude that there is no evidence of the individual or prevalent colours of organisms being directly determined by the amount of light, or heat, or moisture, to which they are exposed; while, on the other hand, the two great principles of the need of concealment from enemies or from their prey, and of recognition by their own kind, are so wide-reaching in their application that they appear at first sight to cover almost the whole ground of animal coloration. but, although they are indeed wonderfully general and have as yet been very imperfectly studied, we are acquainted with other modes of coloration which have a different origin. these chiefly appertain to the very singular class of warning colours, from which arise the yet more extraordinary phenomena of mimicry; and they open up so curious a field of inquiry and present so many interesting problems, that a chapter must be devoted to them. yet another chapter will be required by the subject of sexual differentiation of colour and ornament, as to the origin and meaning of which i have arrived at different conclusions from mr. darwin. these various forms of coloration having been discussed and illustrated, we shall be in a position to attempt a brief sketch of the fundamental laws which have determined the general coloration of the animal world. footnotes: [footnote : _proceedings of the royal society_, no. , ; _transactions of the royal society_, vol. clxxviii. b. pp. - .] [footnote : _a naturalist's wanderings in the eastern archipelago_, p. .] [footnote : _trans. phil. soc._ (? _of s. africa_), , part iv, p. .] [footnote : _proc. zool. soc._, p. .] [footnote : with reference to this general resemblance of insects to their environment the following remarks by mr. poulton are very instructive. he says: "holding the larva of sphinx ligustri in one hand and a twig of its food-plant in the other, the wonder we feel is, not at the resemblance but at the difference; we are surprised at the difficulty experienced in detecting so conspicuous an object. and yet the protection is very real, for the larvae will be passed over by those who are not accustomed to their appearance, although the searcher may be told of the presence of a large caterpillar. an experienced entomologist may also fail to find the larvae till after a considerable search. this is general protective resemblance, and it depends upon a general harmony between the appearance of the organism and its whole environment. it is impossible to understand the force of this protection for any larva, without seeing it on its food-plant and in an entirely normal condition. the artistic effect of green foliage is more complex than we often imagine; numberless modifications are wrought by varied lights and shadows upon colours which are in themselves far from uniform. in the larva of papilio machaon the protection is very real when the larva is on the food-plant, and can hardly be appreciated at all when the two are apart." numerous other examples are given in the chapter on "mimicry and other protective resemblances among animals," in my _contributions to the theory of natural selection_.] [footnote : _the naturalist in nicaragua_, p. .] [footnote : r. meldola, in _proc. zool. soc._, , p. .] [footnote : _nature_, vol. iii. p. .] [footnote : _trans. ent. soc. lond._, , p. .] [footnote : _ibid._ (_proceedings_, p. xlii.)] [footnote : wallace's _malay archipelago_, vol. i. p. (fifth edition, p. ), with figure.] [footnote : moseley's _notes by a naturalist on the challenger_.] [footnote : _proceedings of the boston soc. of nat. hist._, vol. xiv. .] [footnote : _nature_, , p. .] [footnote : _a naturalist's wanderings in the eastern archipelago_, p. .] [footnote : a beautiful drawing of this rare insect, hymenopus bicornis (in the nymph or active pupa state), was kindly sent me by mr. wood-mason, curator of the indian museum at calcutta. a species, very similar to it, inhabits java, where it is said to resemble a pink orchid. other mantidae, of the genus gongylus, have the anterior part of the thorax dilated and coloured either white, pink, or purple; and they so closely resemble flowers that, according to mr. wood-mason, one of them, having a bright violet-blue prothoracic shield, was found in pegu by a botanist, and was for a moment mistaken by him for a flower. see _proc. ent. soc. lond._, , p. liii.] [footnote : c. dixon, in seebohm's _history of british birds_, vol. ii. introduction, p. xxvi. many of the other examples here cited are taken from the same valuable work.] [footnote : see a.h.s. lucas, in _proceedings of royal society of victoria_, , p. .] [footnote : professor wm.h. brewer of yale college has shown that the white marks or the spots of domesticated animals are rarely symmetrical, but have a tendency to appear more frequently on the left side. this is the case with horses, cattle, dogs, and swine. among wild animals the skunk varies considerably in the amount of white on the body, and this too was found to be usually greatest on the left side. a close examination of numerous striped or spotted species, as tigers, leopards, jaguars, zebras, etc., showed that the bilateral symmetry was not exact, although the general effect of the two sides was the same. this is precisely what we should expect if the symmetry is not the result of a general law of the organisation, but has been, in part at least, produced and preserved for the useful purpose of recognition by the animal's fellows of the same species, and especially by the sexes and the young. see _proc. of the am. ass. for advancement of science_, vol. xxx. p. .] [footnote : _descent of man_, p. .] [footnote : it may be thought that such extremely conspicuous markings as those of the zebra would be a great danger in a country abounding with lions, leopards, and other beasts of prey; but it is not so. zebras usually go in bands, and are so swift and wary that they are in little danger during the day. it is in the evening, or on moonlight nights, when they go to drink, that they are chiefly exposed to attack; and mr. francis galton, who has studied these animals in their native haunts, assures me, that in twilight they are not at all conspicuous, the stripes of white and black so merging together into a gray tint that it is very difficult to see them at a little distance. we have here an admirable illustration of how a glaringly conspicuous style of marking for recognition may be so arranged as to become also protective at the time when protection is most needed; and we may also learn how impossible it is for us to decide on the inutility of any kind of coloration without a careful study of the habits of the species in its native country.] [footnote : the principle of colouring for recognition was, i believe, first stated in my article on "the colours of animals and plants" in macmillan's _magazine_, and more fully in my volume on _tropical nature_. subsequently mrs. barber gave a few examples under the head of "indicative or banner colours," but she applied it to the distinctive colours of the males of birds, which i explain on another principle, though this may assist.] [footnote : quoted by darwin in _descent of man_, p. .] [footnote : in the _american naturalist_ of march , mr. j.e. todd has an article on "directive coloration in animals," in which he recognises many of the cases here referred to, and suggests a few others, though i think he includes many forms of coloration--as "paleness of belly and inner side of legs"--which do not belong to this class.] [footnote : for numerous examples of this protective colouring of marine animals see moseley's _voyage of the challenger_, and dr. e.s. morse in _proc. of bost. soc. of nat. hist._, vol. xiv. .] [footnote : see _origin of species_, p. .] [footnote : the "geographical variation of north american squirrels," _proc. bost. soc. of nat. hist._, , p. ; and _mammals and winter birds of florida_, pp. - .] chapter ix warning coloration and mimicry the skunk as an example of warning coloration--warning colours among insects--butterflies--caterpillars--mimicry--how mimicry has been produced--heliconidae--perfection of the imitation--other cases of mimicry among lepidoptera--mimicry among protected groups--its explanation--extension of the principle--mimicry in other orders of insects--mimicry among the vertebrata--snakes--the rattlesnake and the cobra--mimicry among birds--objections to the theory of mimicry--concluding remarks on warning colours and mimicry. we have now to deal with a class of colours which are the very opposite of those we have hitherto considered, since, instead of serving to conceal the animals that possess them or as recognition marks to their associates, they are developed for the express purpose of rendering the species conspicuous. the reason of this is that the animals in question are either the possessors of some deadly weapons, as stings or poison fangs, or they are uneatable, and are thus so disagreeable to the usual enemies of their kind that they are never attacked when their peculiar powers or properties are known. it is, therefore, important that they should not be mistaken for defenceless or eatable species of the same class or order, since in that case they might suffer injury, or even death, before their enemies discovered the danger or the uselessness of the attack. they require some signal or danger-flag which shall serve as a warning to would-be enemies not to attack them, and they have usually obtained this in the form of conspicuous or brilliant coloration, very distinct from the protective tints of the defenceless animals allied to them. _the skunk as illustrating warning coloration._ while staying a few days, in july , at the summit hotel on the central pacific railway, i strolled out one evening after dinner, and on the road, not fifty yards from the house, i saw a pretty little white and black animal with a bushy tail coming towards me. as it came on at a slow pace and without any fear, although it evidently saw me, i thought at first that it must be some tame creature, when it suddenly occurred to me that it was a skunk. it came on till within five or six yards of me, then quietly climbed over a dwarf wall and disappeared under a small outhouse, in search of chickens, as the landlord afterwards told me. this animal possesses, as is well known, a most offensive secretion, which it has the power of ejecting over its enemies, and which effectually protects it from attack. the odour of this substance is so penetrating that it taints, and renders useless, everything it touches, or in its vicinity. provisions near it become uneatable, and clothes saturated with it will retain the smell for several weeks, even though they are repeatedly washed and dried. a drop of the liquid in the eyes will cause blindness, and indians are said not unfrequently to lose their sight from this cause. owing to this remarkable power of offence the skunk is rarely attacked by other animals, and its black and white fur, and the bushy white tail carried erect when disturbed, form the danger-signals by which it is easily distinguished in the twilight or moonlight from unprotected animals. its consciousness that it needs only to be seen to be avoided gives it that slowness of motion and fearlessness of aspect which are, as we shall see, characteristic of most creatures so protected. _warning colours among insects._ it is among insects that warning colours are best developed, and most abundant. we all know how well marked and conspicuous are the colours and forms of the stinging wasps and bees, no one of which in any part of the world is known to be protectively coloured like the majority of defenceless insects. most of the great tribe of malacoderms among beetles are distasteful to insect-eating animals. our red and black telephoridae, commonly called "soldiers and sailors," were found, by mr. jenner weir, to be refused by small birds. these and the allied lampyridae (the fireflies and glow-worms) in nicaragua, were rejected by mr. belt's tame monkey and by his fowls, though most other insects were greedily eaten by them. the coccinellidae or lady-birds are another uneatable group, and their conspicuous and singularly spotted bodies serve to distinguish them at a glance from all other beetles. these uneatable insects are probably more numerous than is supposed, although we already know immense numbers that are so protected. the most remarkable are the three families of butterflies--heliconidae, danaidae, and acraeidae--comprising more than a thousand species, and characteristic respectively of the three great tropical regions--south america, southern asia, and africa. all these butterflies have peculiarities which serve to distinguish them from every other group in their respective regions. they all have ample but rather weak wings, and fly slowly; they are always very abundant; and they all have conspicuous colours or markings, so distinct from those of other families that, in conjunction with their peculiar outline and mode of flight, they can usually be recognised at a glance. other distinctive features are, that their colours are always nearly the same on the under surface of their wings as on the upper; they never try to conceal themselves, but rest on the upper surfaces of leaves or flowers; and, lastly, they all have juices which exhale a powerful scent, so that when one kills them by pinching the body, the liquid that exudes stains the fingers yellow, and leaves an odour that can only be removed by repeated washings. now, there is much direct evidence to show that this odour, though not very offensive to us, is so to most insect-eating creatures. mr. bates observed that, when set out to dry, specimens of heliconidae were less subject to the attacks of vermin; while both he and i noticed that they were not attacked by insect-eating birds or dragonflies, and that their wings were not found in the forest paths among the numerous wings of other butterflies whose bodies had been devoured. mr. belt once observed a pair of birds capturing insects for their young; and although the heliconidae swarmed in the vicinity, and from their slow flight could have been easily caught, not one was ever pursued, although other butterflies did not escape. his tame monkey also, which would greedily munch up other butterflies, would never eat the heliconidae. it would sometimes smell them, but always rolled them up in its hand and then dropped them. we have also some corresponding evidence as to the distastefulness of the eastern danaidae. the hon. mr. justice newton, who assiduously collected and took notes upon the lepidoptera of bombay, informed mr. butler of the british museum that the large and swift-flying butterfly charaxes psaphon, was continually persecuted by the bulbul, so that he rarely caught a specimen of this species which had not a piece snipped out of the hind wings. he offered one to a bulbul which he had in a cage, and it was greedily devoured, whilst it was only by repeated persecution that he succeeded in inducing the bird to touch a danais.[ ] besides these three families of butterflies, there are certain groups of the great genus papilio--the true swallow-tailed butterflies--which have all the characteristics of uneatable insects. they have a special coloration, usually red and black (at least in the females), they fly slowly, they are very abundant, and they possess a peculiar odour somewhat like that of the heliconidae. one of these groups is common in tropical america, another in tropical asia, and it is curious that, although not very closely allied, they have each the same red and black colours, and are very distinct from all the other butterflies of their respective countries. there is reason to believe also that many of the brilliantly coloured and weak-flying diurnal moths, like the fine tropical agaristidae and burnet-moths, are similarly protected, and that their conspicuous colours serve as a warning of inedibility. the common burnet-moth (anthrocera filipendula) and the equally conspicuous ragwort-moth (euchelia jacobeae) have been proved to be distasteful to insect-eating creatures. the most interesting and most conclusive example of warning coloration is, however, furnished by caterpillars, because in this case the facts have been carefully ascertained experimentally by competent observers. in the year , when mr. darwin was collecting evidence as to the supposed effect of sexual selection in bringing about the brilliant coloration of the higher animals, he was struck by the fact that many caterpillars have brilliant and conspicuous colours, in the production of which sexual selection could have no place. we have numbers of such caterpillars in this country, and they are characterised not only by their gay colours but by not concealing themselves. such are the mullein and the gooseberry caterpillars, the larvae of the spurge hawk-moth, of the buff-tip, and many others. some of these caterpillars are wonderfully conspicuous, as in the case of that noticed by mr. bates in south america, which was four inches long, banded across with black and yellow, and with bright red head, legs, and tail. hence it caught the eye of any one who passed by, even at the distance of many yards. mr. darwin asked me to try and suggest some explanation of this coloration; and, having been recently interested in the question of the warning coloration of butterflies, i suggested that this was probably a similar case,--that these conspicuous caterpillars were distasteful to birds and other insect-eating creatures, and that their bright non-protective colours and habit of exposing themselves to view, enabled their enemies to distinguish them at a glance from the edible kinds and thus learn not to touch them; for it must be remembered that the bodies of caterpillars while growing are so delicate, that a wound from a bird's beak would be perhaps as fatal as if they were devoured.[ ] at this time not a single experiment or observation had been made on the subject, but after i had brought the matter before the entomological society, two gentlemen, who kept birds and other tame animals, undertook to make experiments with a variety of caterpillars. mr. jenner weir was the first to experiment with ten species of small birds in his aviary, and he found that none of them would eat the following smooth-skinned conspicuous caterpillars--abraxas grossulariata, diloba caeruleocephala, anthrocera filipendula, and cucullia verbasci. he also found that they would not touch any hairy or spiny larvae, and he was satisfied that it was not the hairs or the spines, but the unpleasant taste that caused them to be rejected, because in one case a young smooth larva of a hairy species, and in another case the pupa of a spiny larva, were equally rejected. on the other hand, all green or brown caterpillars as well as those that resemble twigs were greedily devoured.[ ] mr. a.g. butler also made experiments with some green lizards (lacerta viridis), which greedily ate all kinds of food, including flies of many kinds, spiders, bees, butterflies, and green caterpillars; but they would not touch the caterpillar of the gooseberry-moth (abraxas grossulariata), or the imago of the burnet-moth (anthrocera filipendula). the same thing happened with frogs. when the gooseberry caterpillars were first given to them, "they sprang forward and licked them eagerly into their mouths; no sooner, however, had they done so, than they seemed to become aware of the mistake that they had made, and sat with gaping mouths, rolling their tongues about, until they had got quit of the nauseous morsels, which seemed perfectly uninjured, and walked off as briskly as ever." spiders seemed equally to dislike them. this and another conspicuous caterpillar (halia wavaria) were rejected by two species--the geometrical garden spider (epeira diadema) and a hunting spider.[ ] some further experiments with lizards were made by professor weismann, quite confirming the previous observations; and in mr. e.b. poulton of oxford undertook a considerable series of experiments, with many other species of larvae and fresh kinds of lizards and frogs. mr. poulton then reviewed the whole subject, incorporating all recorded facts, as well as some additional observations made by mr. jenner weir in . more than a hundred species of larvae or of perfect insects of various orders have now been made the subject of experiment, and the results completely confirm my original suggestion. in almost every case the protectively coloured larvae have been greedily eaten by all kinds of insectivorous animals, while, in the immense majority of cases, the conspicuous, hairy, or brightly coloured larvae have been rejected by some or all of them. in some instances the inedibility of the larvae extends to the perfect insect, but not in others. in the former cases the perfect insect is usually adorned with conspicuous colours, as the burnet and ragwort moths; but in the case of the buff-tip, the moth resembles a broken piece of rotten stick, yet it is partly inedible, being refused by lizards. it is, however, very doubtful whether these are its chief enemies, and its protective form and colour may be needed against insectivorous birds or mammals. mr. samuel h. scudder, who has largely bred north american butterflies, has found so many of the eggs and larvae destroyed by hymenopterous and dipterous parasites that he thinks at least nine-tenths, perhaps a greater proportion, never reach maturity. yet he has never found any evidence that such parasites attack either the egg or the larva of the inedible danais archippus, so that in this case the insect is distasteful to its most dangerous foes in all the stages of its existence, a fact which serves to explain its great abundance and its extension over almost the whole world.[ ] one case has been found of a protectively coloured larva,--one, moreover, which in all its habits shows that it trusts to concealment to escape its enemies--which was yet always rejected by lizards after they had seized it, evidently under the impression that from its colour it would be eatable. this is the caterpillar of the very common moth mania typica; and mr. poulton thinks that, in this case, the unpleasant taste is an incidental result of some physiological processes in the organism, and is itself a merely useless character. it is evident that the insect would not conceal itself so carefully as it does if it had not some enemies, and these are probably birds or small mammals, as its food-plants are said to be dock and willow-herb, not suggestive of places frequented by lizards; and it has been found by experiment that lizards and birds have not always the same likes and dislikes. the case is interesting, because it shows that nauseous fluids sometimes occur sporadically, and may thus be intensified by natural selection when required for the purpose of protection. another exceptional case is that of the very conspicuous caterpillar of the spurge hawk-moth (deilephila euphorbiae), which was at once eaten by a lizard, although, as it exposes itself on its food-plant in the daytime and is very abundant in some localities, it must almost certainly be disliked by birds or by some animals who would otherwise devour it. if disturbed while feeding it is said to turn round with fury and eject a quantity of green liquid, of an acid and disagreeable smell similar to that of the spurge milk, only worse.[ ] these facts, and mr. poulton's evidence that some larvae rejected by lizards at first will be eaten if the lizards are very hungry, show that there are differences in the amount of the distastefulness, and render it probable that if other food were wanting many of these conspicuous insects would be eaten. it is the abundance of the eatable kinds that gives value to the inedibility of the smaller number; and this is probably the reason why so many insects rely on protective colouring rather than on the acquisition of any kind of defensive weapons. in the long run the powers of attack and defence must balance each other. hence we see that even the powerful stings of bees and wasps only protect them against some enemies, since a tribe of birds, the bee-eaters, have been developed which feed upon them, and some frogs and lizards do so occasionally. the preceding outline will sufficiently explain the characteristics of "warning coloration" and the end it serves in nature. there are many other curious modifications of it, but these will be best appreciated after we have discussed the remarkable phenomenon of "mimicry," which is bound up with and altogether depends upon "warning colour," and is in some cases the chief indication we have of the possession of some offensive weapon to secure the safety of the species imitated. _mimicry._ this term has been given to a form of protective resemblance, in which one species so closely resembles another in external form and colouring as to be mistaken for it, although the two may not be really allied and often belong to distinct families or orders. one creature seems disguised in order to be made like another; hence the terms "mimic" and mimicry, which imply no voluntary action on the part of the imitator. it has long been known that such resemblances do occur, as, for example, the clear-winged moths of the families sesiidae and aegeriidae, many of which resemble bees, wasps, ichneumons, or saw-flies, and have received names expressive of the resemblance; and the parasitic flies (volucella) which closely resemble bees, on whose larvae the larvae of the flies feed. the great bulk of such cases remained, however, unnoticed, and the subject was looked upon as one of the inexplicable curiosities of nature, till mr. bates studied the phenomenon among the butterflies of the amazon, and, on his return home, gave the first rational explanation of it.[ ] the facts are, briefly, these. everywhere in that fertile region for the entomologist the brilliantly coloured heliconidae abound, with all the characteristics which i have already referred to when describing them as illustrative of "warning coloration." but along with them other butterflies were occasionally captured, which, though often mistaken for them, on account of their close resemblance in form, colour, and mode of flight, were found on examination to belong to a very distinct family, the pieridae. mr. bates notices fifteen distinct species of pieridae, belonging to the genera leptalis and euterpe, each of which closely imitates some one species of heliconidae, inhabiting the same region and frequenting the same localities. it must be remembered that the two families are altogether distinct in structure. the larvae of the heliconidae are tubercled or spined, the pupae suspended head downwards, and the imago has imperfect forelegs in the male; while the larvae of the pieridae are smooth, the pupae are suspended with a brace to keep the head erect, and the forefeet are fully developed in both sexes. these differences are as large and as important as those between pigs and sheep, or between swallows and sparrows; while english entomologists will best understand the case by supposing that a species of pieris in this country was coloured and shaped like a small tortoise-shell, while another species on the continent was equally like a camberwell beauty--so like in both cases as to be mistaken when on the wing, and the difference only to be detected by close examination. as an example of the resemblance, woodcuts are given of one pair in which the colours are simple, being olive, yellow, and black, while the very distinct neuration of the wings and form of the head and body can be easily seen. [illustration: fig. .--methona psidii (heliconidae). leptalis orise (pieridae).] besides these pieridae, mr. bates found four true papilios, seven erycinidae, three castnias (a genus of day-flying moths), and fourteen species of diurnal bombycidae, all imitating some species of heliconidae which inhabited the same district; and it is to be especially noted that none of these insects were so abundant as the heliconidae they resembled, generally they were far less common, so that mr. bates estimated the proportion in some cases as not one to a thousand. before giving an account of the numerous remarkable cases of mimicry in other parts of the world, and between various groups of insects and of higher animals, it will be well to explain briefly the use and purport of the phenomenon, and also the mode by which it has been brought about. _how mimicry has been produced._ the fact has been now established that the heliconidae possess an offensive odour and taste, which lead to their being almost entirely free from attack by insectivorous creatures; they possess a peculiar form and mode of flight, and do not seek concealment; while their colours--although very varied, ranging from deep blue-black, with white, yellow, or vivid red bands and spots, to the most delicate semitransparent wings adorned with pale brown or yellow markings--are yet always very distinctive, and unlike those of all the other families of butterflies in the same country. it is, therefore, clear that if any other butterflies in the same region, which are eatable and suffer great persecution from insectivorous animals, should come to resemble any of these uneatable species so closely as to be mistaken for them by their enemies, they will obtain thereby immunity from persecution. this is the obvious and sufficient reason why the imitation is useful, and therefore why it occurs in nature. we have now to explain how it has probably been brought about, and also why a still larger number of persecuted groups have not availed themselves of this simple means of protection. from the great abundance of the heliconidae[ ] all over tropical america, the vast number of their genera and species, and their marked distinctions from all other butterflies, it follows that they constitute a group of high antiquity, which in the course of ages has become more and more specialised, and owing to its peculiar advantages has now become a dominant and aggressive race. but when they first arose from some ancestral species or group which, owing to the food of the larvae or some other cause, possessed disagreeable juices that caused them to be disliked by the usual enemies of their kind, they were in all probability not very different either in form or coloration from many other butterflies. they would at that time be subject to repeated attacks by insect-eaters, and, even if finally rejected, would often receive a fatal injury. hence arose the necessity for some distinguishing mark, by which the devourers of butterflies in general might learn that these particular butterflies were uneatable; and every variation leading to such distinction, whether by form, colour, or mode of flight, was preserved and accumulated by natural selection, till the ancestral heliconoids became well distinguished from eatable butterflies, and thenceforth comparatively free from persecution. then they had a good time of it. they acquired lazy habits, and flew about slowly. they increased abundantly and spread all over the country, their larvae feeding on many plants and acquiring different habits; while the butterflies themselves varied greatly, and colour being useful rather than injurious to them, gradually diverged into the many coloured and beautifully varied forms we now behold. but, during the early stages of this process, some of the pieridae, inhabiting the same district, happened to be sufficiently like some of the heliconidae to be occasionally mistaken for them. these, of course, survived while their companions were devoured. those among their descendants that were still more like heliconidae again survived, and at length the imitation would become tolerably perfect. thereafter, as the protected group diverged into distinct species of many different colours, the imitative group would occasionally be able to follow it with similar variations,--a process that is going on now, for mr. bates informs us that in each fresh district he visited he found closely allied representative species or varieties of heliconidae, and along with them species of leptalis (pieridae), which had varied in the same way so as still to be exact imitations. but this process of imitation would be subject to check by the increasing acuteness of birds and other animals which, whenever the eatable leptalis became numerous, would surely find them out, and would then probably attack both these and their friends the heliconidae in order to devour the former and reject the latter. the pieridae would, however, usually be less numerous, because their larvae are often protectively coloured and therefore edible, while the larvae of the heliconidae are adorned with warning colours, spines, or tubercles, and are uneatable. it seems probable that the larvae and pupae of the heliconidae were the first to acquire the protective distastefulness, both because in this stage they are more defenceless and more liable to fatal injury, and also because we now find many instances in which the larvae are distasteful while the perfect insects are eatable, but i believe none in which the reverse is the case. the larvae of the pieridae are now beginning to acquire offensive juices, but have not yet obtained the corresponding conspicuous colours; while the perfect insects remain eatable, except perhaps in some eastern groups, the under sides of whose wings are brilliantly coloured although this is the part which is exposed when at rest. it is clear that if a large majority of the larvae of lepidoptera, as well as the perfect insects, acquired these distasteful properties, so as seriously to diminish the food supply of insectivorous and nestling birds, these latter would be forced by necessity to acquire corresponding tastes, and to eat with pleasure what some of them now eat only under pressure of hunger; and variation and natural selection would soon bring about this change. many writers have denied the possibility of such wonderful resemblances being produced by the accumulation of fortuitous variations, but if the reader will call to mind the large amount of variability that has been shown to exist in all organisms, the exceptional power of rapid increase possessed by insects, and the tremendous struggle for existence always going on, the difficulty will vanish, especially when we remember that nature has the same fundamental groundwork to act upon in the two groups, general similarity of forms, wings of similar texture and outline, and probably some original similarity of colour and marking. yet there is evidently considerable difficulty in the process, or with these great resources at her command nature would have produced more of these mimicking forms than she has done. one reason of this deficiency probably is, that the imitators, being always fewer in number, have not been able to keep pace with the variations of the much more numerous imitated form; another reason may be the ever-increasing acuteness of the enemies, which have again and again detected the imposture and exterminated the feeble race before it has had time to become further modified. the result of this growing acuteness of enemies has been, that those mimics that now survive exhibit, as mr. bates well remarks, "a palpably intentional likeness that is perfectly staggering," and also "that those features of the portrait are most attended to by nature which produce the most effective deception when the insects are seen in nature." no one, in fact, can understand the perfection of the imitation who has not seen these species in their native wilds. so complete is it in general effect that in almost every box of butterflies, brought from tropical america by amateurs, are to be found some species of the mimicking pieridae, erycinidae, or moths, and the mimicked heliconidae, placed together under the impression that they are the same species. yet more extraordinary, it sometimes deceives the very insects themselves. mr. trimen states that the male danais chrysippus is sometimes deceived by the female diadema bolina which mimics that species. dr. fritz müller, writing from brazil to professor meldola, says, "one of the most interesting of our mimicking butterflies is leptalis melite. the female alone of this species imitates one of our common white pieridae, which she copies so well that even her own male is often deceived; for i have repeatedly seen the male pursuing the mimicked species, till, after closely approaching and becoming aware of his error, he suddenly returned."[ ] this is evidently not a case of true mimicry, since the species imitated is not protected; but it may be that the less abundant leptalis is able to mingle with the female pieridae and thus obtain partial immunity from attack. mr. kirby of the insect department of the british museum informs me that there are several species of south american pieridae which the female leptalis melite very nearly resembles. the case, however, is interesting as showing that the butterflies are themselves deceived by a resemblance which is not so great as that of some mimicking species. _other examples of mimicry among lepidoptera._ in tropical asia, and eastward to the pacific islands, the danaidae take the place of the heliconidae of america, in their abundance, their conspicuousness, their slow flight, and their being the subjects of mimicry. they exist under three principal forms or genera. the genus euploea is the most abundant both in species and individuals, and consists of fine broad-winged butterflies of a glossy or metallic blue-black colour, adorned with pure white, or rich blue, or dusky markings situated round the margins of the wings. danais has generally more lengthened wings, of a semitransparent greenish or a rich brown colour, with radial or marginal pale spots; while the fine hestias are of enormous size, of a papery or semitransparent white colour, with dusky or black spots and markings. each of these groups is mimicked by various species of the genus papilio, usually with such accuracy that it is impossible to distinguish them on the wing.[ ] several species of diadema, a genus of butterflies allied to our vanessas, also mimic species of danais, but in this case the females only are affected, a subject which will be discussed in another chapter. another protected group in the eastern tropics is that of the beautiful day-flying moths forming the family agaristidae. these are usually adorned with the most brilliant colours or conspicuous markings, they fly slowly in forests among the butterflies and other diurnal insects, and their great abundance sufficiently indicates their possession of some distastefulness which saves them from attack. under these conditions we may expect to find other moths which are not so protected imitating them, and this is the case. one of the common and wide-ranging species (opthalmis lincea), found in the islands from amboyna to new ireland, is mimicked in a wonderful manner by one of the liparidae (the family to which our common "tussock" and "vapourer" moths belong). this is a new species collected at amboyna during the voyage of the _challenger_, and has been named artaxa simulans. both insects are black, with the apex of the fore wings ochre coloured, and the outer half of the hind wings bright orange. the accompanying woodcuts (for the use of which i am indebted to mr. john murray of the _challenger_ office) well exhibit their striking resemblance to each other. [illustration: fig. .--opthalmis lincea (agaristidae). artaxa simulans (liparidae).] in africa exactly similar phenomena recur, species of papilio and of diadema mimicking danaidae or acraeidae with the most curious accuracy. mr. trimen, who studied this subject in south africa, has recorded eight species or varieties of diadema, and eight of papilio, which each mimic some species of danais; while eight species or varieties of panopaea (another genus of nymphalidae), three of melanitis (eurytelidae), and two of papilio, resemble with equal accuracy some species of acraea.[ ] he has also independently observed the main facts on which the explanation of the phenomenon rests,--the unpleasant odour of the danais and acraea, extending to their larvae and pupae; their great abundance, slow flight, and disregard of concealment; and he states that while lizards, mantidae, and dragonflies all hunt butterflies, and the rejected wings are to be found abundantly at some of their feeding-places, those of the two genera danais and acraea were never among them. the two groups of the great genus papilio (the true swallow-tailed butterflies) which have been already referred to as having the special characteristics of uneatable insects, have also their imitators in other groups; and thus, the belief in their inedibility--derived mainly from their style of warning coloration and their peculiar habits--is confirmed. in south america, several species of the "aeneas" group of these butterflies are mimicked by pieridae and by day-flying moths of the genera castnia and pericopis. in the east, papilio hector, p. diphilus, and p. liris, all belonging to the inedible group, are mimicked by the females of other species of papilio belonging to very distinct groups; while in northern india and china, many fine day-flying moths (epicopeia) have acquired the strange forms and peculiar colours of some of the large inedible papilios of the same regions. in north america, the large and handsome danais archippus, with rich reddish-brown wings, is very common; and it is closely imitated by limenitis misippus, a butterfly allied to our "white admiral," but which has acquired a colour quite distinct from that of the great bulk of its allies. in the same country there is a still more interesting case. the beautiful dark bronzy green butterfly, papilio philenor, is inedible both in larva and perfect insect, and it is mimicked by the equally dark limenitis ursula. there is also in the southern and western states a dark female form of the yellow papilio turnus, which in all probability obtains protection from its general resemblance to p. philenor. mr. w.h. edwards has found, by extensive experiment, that both the dark and yellow females produce their own kinds, with very few exceptions; and he thinks that the dark form has the advantage in the more open regions and in the prairies, where insectivorous birds abound. but in open country the dark form would be quite as conspicuous as the yellow form, if not more so, so that the resemblance to an inedible species would be there more needed.[ ] the only probable case of mimicry in this country is that of the moth, diaphora mendica, whose female only is white, while the larva is of protective colours, and therefore almost certainly edible. a much more abundant moth, of about the same size and appearing about the same time, is spilosoma menthrasti, also white, but in this case both it and its larva have been proved to be inedible. the white colour of the female diaphora, although it must be very conspicuous at night, may, therefore, have been acquired in order to resemble the uneatable spilosoma, and thus gain some protection.[ ] _mimicry among protected (uneatable) genera._ before giving some account of the numerous other cases of warning colours and of mimicry that occur in the animal kingdom, it will be well to notice a curious phenomenon which long puzzled entomologists, but which has at length received a satisfactory explanation. we have hitherto considered, that mimicry could only occur when a comparatively scarce and much persecuted species obtained protection by its close external resemblance to a much more abundant uneatable species inhabiting its own district; and this rule undoubtedly prevails among the great majority of mimicking species all over the world. but mr. bates also found a number of pairs of species of different genera of heliconidae, which resembled each other quite as closely as did the other mimicking species he has described; and since all these insects appear to be equally protected by their inedibility, and to be equally free from persecution, it was not easy to see why this curious resemblance existed, or how it had been brought about. that it is not due to close affinity is shown by the fact that the resemblance occurs most frequently between the two distinct sub-families into which (as mr. bates first pointed out) the heliconidae are naturally divided on account of very important structural differences. one of these sub-families (the true heliconinae) consists of two genera only, heliconius and eueides, the other (the danaoid heliconinae) of no less than sixteen genera; and, in the instances of mimicry we are now discussing, one of the pairs or triplets that resemble each other is usually a species of the large and handsome genus heliconius, the others being species of the genera mechanitis, melinaea, or tithorea, though several species of other danaoid genera also imitate each other. the following lists will give some idea of the number of these curious imitative forms, and of their presence in every part of the neotropical area. the bracketed species are those that resemble each other so closely that the difference is not perceptible when they are on the wing. in the lower amazon region are found-- { heliconius sylvana. { melinaea egina. { heliconius numata. { melinaea mneme. { tithorea harmonia. { methona psidii. { thyridia ino. { ceratina ninonia. { melinaea mnasias. in central america are found-- { heliconius zuleika. nicaragua { melinaea hezia. { mechanitis sp. { heliconius formosus. { tithorea penthias. guatemala { heliconius telchina. { melinaea imitata. in the upper amazon region-- { heliconius pardalinus. { melinaea pardalis. { heliconius aurora. { melinaea lucifer. in new grenada-- { heliconius ismenius. { melinaea messatis. { heliconius messene. { melinaea mesenina. { (?) mechanitis sp. { heliconius hecalesia. { tithorea hecalesina. { heliconius hecuba. { tithorea bonplandi. in eastern peru and bolivia-- { heliconius aristona. { melinaea cydippe. { (?) mechanitis mothone. in pernambuco-- { heliconius ethra. { mechanitis nesaea. in rio janeiro-- { helieonius eucrate. { mechanitis lysimnia. in south brazil-- { thyridia megisto. { ituna ilione. { acraea thalia. { eueides pavana. besides these, a number of species of ithomia and napeogenes, and of napeogenes and mechanitis, resemble each other with equal accuracy, so that they are liable to be mistaken for each other when on the wing; and no doubt many other equally remarkable cases are yet unnoticed. [illustration: fig. .--wings of ituna ilione, female. wings of thyridia megisto, female.] the figures above of the fore and hind wings of two of these mimicking species, from dr. fritz müller's original paper in _kosmos_, will serve to show the considerable amount of difference, in the important character of the neuration of the wings, between these butterflies, which really belong to very distinct and not at all closely allied genera. other important characters are--( ) the existence of a small basal cell in the hind wings of ituna which is wanting in thyridia; ( ) the division of the cell between the veins _b_ and of the hind wings in the former genus, while it is undivided in the latter; and ( ) the existence in thyridia of scent-producing tufts of hair on the upper edge of the hind wing, while in ituna these are wanting; but in place of them are extensible processes at the end of the abdomen, also emitting a powerful scent. these differences characterise two marked subdivisions of the danaoid heliconinae, each containing several distinct genera; and these subdivisions are further distinguished by very different forms of larvae, that to which ituna belongs having from two to four long threadlike tentacles on the back, while in that containing thyridia these are always absent. the former usually feed on asclepiadeae, the latter on solanaceae or scrophulariaceae. the two species figured, though belonging to such distinct and even remote genera, have acquired almost identical tints and markings so as to be deceptively alike. the surface of the wings is, in both, transparent yellowish, with black transverse bands and white marginal spots, while both have similar black-and white-marked bodies and long yellow antennae. dr. müller states that they both show a preference for the same flowers growing on the edges of the forest paths.[ ] we will now proceed to give the explanation of these curious similarities, which have remained a complete puzzle for twenty years. mr. bates, when first describing them, suggested that they might be due to some form of parallel variation dependent on climatic influences; and i myself adduced other cases of coincident local modifications of colour, which did not appear to be explicable by any form of mimicry.[ ] but we neither of us hit upon the simple explanation given by dr. fritz müller in . his theory is founded on the assumed, but probable, fact, that insect-eating birds only learn by experience to distinguish the edible from the inedible butterflies, and in doing so necessarily sacrifice a certain number of the latter. the quantity of insectivorous birds in tropical america is enormous; and the number of young birds which every year have to learn wisdom by experience, as regards the species of butterflies to be caught or to be avoided, is so great that the sacrifice of life of the inedible species must be considerable, and, to a comparatively weak or scarce species, of vital importance. the number thus sacrificed will be fixed by the quantity of young birds, and by the number of experiences requisite to cause them to avoid the inedible species for the future, and not at all by the numbers of individuals of which each species consists. hence, if two species are so much alike as to be mistaken for one another, the fixed number annually sacrificed by inexperienced birds will be divided between them, and both will benefit. but if the two species are very unequal in numbers, the benefit will be comparatively slight for the more abundant species, but very great for the rare one. to the latter it may make all the difference between safety and destruction. to give a rough numerical example. let us suppose that in a given limited district there are two species of heliconidae, one consisting of only , the other of , individuals, and that the quota required annually in the same district for the instruction of young insectivorous birds is . by the larger species this loss will be hardly felt; to the smaller it will mean the most dreadful persecution resulting in a loss of half the total population. but, let the two species become superficially alike, so that the birds see no difference between them. the quota of will now be taken from a combined population of , butterflies, and if proportionate numbers of each suffer, then the weak species will only lose five individuals instead of as it did before. now we know that the different species of heliconidae are not equally abundant, some being quite rare; so that the benefit to be derived in these latter cases would be very important. a slight inferiority in rapidity of flight or in powers of eluding attack might also be a cause of danger to an inedible species of scanty numbers, and in this case too the being merged in another much more abundant species, by similarity of external appearance, would be an advantage. the question of fact remains. do young birds pursue and capture these distasteful butterflies till they have learned by bitter experience what species to avoid? on this point dr. müller has fortunately been able to obtain some direct evidence, by capturing several acraeas and heliconidae which had evidently been seized by birds but had afterwards escaped, as they had pieces torn out of the wing, sometimes symmetrically out of both wings, showing that the insect had been seized when at rest and with the two pairs of wings in contact. there is, however, a general impression that this knowledge is hereditary, and does not need to be acquired by young birds; in support of which view mr. jenner weir states that his birds always disregarded inedible caterpillars. when, day by day, he threw into his aviary various larvae, those which were edible were eaten immediately, those which were inedible were no more noticed than if a pebble had been thrown before the birds. the cases, however, are not strictly comparable. the birds were not young birds of the first year; and, what is more important, edible larvae have a comparatively simple coloration, being always brown or green and smooth. uneatable larvae, on the other hand, comprise all that are of conspicuous colours and are hairy or spiny. but with butterflies there is no such simplicity of contrast. the eatable butterflies comprise not only brown or white species, but hundreds of nymphalidae, papilionidae, lycaenidae, etc., which are gaily coloured and of an immense variety of patterns. the colours and patterns of the inedible kinds are also greatly varied, while they are often equally gay; and it is quite impossible to suppose that any amount of instinct or inherited habit (if such a thing exists) could enable young insectivorous birds to distinguish all the species of one kind from all those of the other. there is also some evidence to show that animals do learn by experience what to eat and what to avoid. mr. poulton was assured by rev. g.j. bursch that very young chickens peck at insects which they afterwards avoid. lizards, too, often seized larvae which they were unable to eat and ultimately rejected. although the heliconidae present, on the whole, many varieties of coloration and pattern, yet, in proportion to the number of distinct species in each district, the types of coloration are few and very well marked, and thus it becomes easier for a bird or other animal to learn that all belonging to such types are uneatable. this must be a decided advantage to the family in question, because, not only do fewer individuals of each species need to be sacrificed in order that their enemies may learn the lesson of their inedibility, but they are more easily recognised at a distance, and thus escape even pursuit. there is thus a kind of mimicry between closely allied species as well as between species of distinct genera, all tending to the same beneficial end. this may be seen in the four or five distinct species of the genus heliconius which all have the same peculiar type of coloration--a yellow band across the upper wings and radiating red stripes on the lower,--and are all found in the same forests of the lower amazon; in the numerous very similar species of ithomia with transparent wings, found in every locality of the same region; and in the very numerous species of papilio of the "aeneas" group, all having a similar style of marking, the resemblance being especially close in the females. the very uniform type of colouring of the blue-black euplaeas and of the fulvous acraeas is of the same character.[ ] in all these cases the similarity of the allied species is so great, that, when they are on the wing at some distance off, it is difficult to distinguish one species from another. but this close external resemblance is not always a sign of very near affinity; for minute examination detects differences in the form and scalloping of the wings, in the markings on the body, and in those on the under surface of the wings, which do not usually characterise the closest allies. it is to be further noted, that the presence of groups of very similar species of the same genus, in one locality, is not at all a common phenomenon among unprotected groups. usually the species of a genus found in one locality are each well marked and belong to somewhat distinct types, while the closely allied forms--those that require minute examination to discriminate them as distinct species--are most generally found in separate areas, and are what are termed representative forms. the extension we have now given to the theory of mimicry is important, since it enables us to explain a much wider range of colour phenomena than those which were first imputed to mimicry. it is in the richest butterfly region in the world--the amazon valley--that we find the most abundant evidence of the three distinct sets of facts, all depending on the same general principle. the form of mimicry first elucidated by mr. bates is characterised by the presence in each locality of certain butterflies, or other insects, themselves edible and belonging to edible groups, which derived protection from having acquired a deceptive resemblance to some of the inedible butterflies in the same localities, which latter were believed to be wholly free from the attacks of insectivorous birds. then came the extension of the principle, by dr. f. müller, to the case of species of distinct genera of the inedible butterflies resembling each other quite as closely as in the former cases, and like them always found in the same localities. they derive mutual benefit from becoming, in appearance, one species, from which a certain toll is taken annually to teach the young insectivorous birds that they are uneatable. even when the two or more species are approximately equal in numbers, they each derive a considerable benefit from thus combining their forces; but when one of the species is scarce or verging on extinction, the benefit becomes exceedingly great, being, in fact, exactly apportioned to the need of the species. the third extension of the same principle explains the grouping of allied species of the same genera of inedible butterflies into sets, each having a distinct type of coloration, and each consisting of a number of species which can hardly be distinguished on the wing. this must be useful exactly in the same way as in the last case, since it divides the inevitable toll to insectivorous birds and other animals among a number of species. it also explains the fact of the great similarity of many species of inedible insects in the same locality--a similarity which does not obtain to anything like the same extent among the edible species. the explanation of the various phenomena of resemblance and mimicry, presented by the distasteful butterflies, may now be considered tolerably complete. _mimicry in other orders of insects._ a very brief sketch of these phenomena will be given, chiefly to show that the same principle prevails throughout nature, and that, wherever a rather extensive group is protected, either by distastefulness or offensive weapons, there are usually some species of edible and inoffensive groups that gain protection by imitating them. it has been already stated that the telephoridae, lampyridae, and other families of soft-winged beetles, are distasteful; and as they abound in all parts of the world, and especially in the tropics, it is not surprising that insects of many other groups should imitate them. this is especially the case with the longicorn beetles, which are much persecuted by insectivorous birds; and everywhere in tropical regions some of these are to be found so completely disguised as to be mistaken for species of the protected groups. numbers of these imitations have been already recorded by mr. bates and myself, but i will here refer to a few others. in the recently published volumes on the longicorn and malacoderm beetles of central america[ ] there are numbers of beautifully coloured figures of the new species; and on looking over them we are struck by the curious resemblance of some of the longicorns to species of the malacoderm group. in some cases we discover perfect mimics, and on turning to the descriptions we always find these pairs to come from the same locality. thus the otheostethus melanurus, one of the prionidae, imitates the malacoderm, lucidota discolor, in form, peculiar coloration, and size, and both are found at chontales in nicaragua, the species mimicked having, however, as is usual, a wider range. the curious and very rare little longicorn, tethlimmena aliena, quite unlike its nearest allies in the same country, is an exact copy on a somewhat smaller scale of a malacoderm, lygistopterus amabilis, both found at chontales. the pretty longicorn, callia albicornis, closely resembles two species of malacoderms (silis chalybeipennis and colyphus signaticollis), all being small beetles with red head and thorax and bright blue elytra, and all three have been found at panama. many other species of callia also resemble other malacoderms; and the longicorn genus lycidola has been named from its resemblance to various species of the lycidae, one of the species here figured (lycidola belti) being a good mimic of calopteron corrugatum and of several other allied species, all being of about the same size and found at chontales. in these cases, and in most others, the longicorn beetles have lost the general form and aspect of their allies to take on the appearance of a distinct tribe. some other groups of beetles, as the elateridae and eucnemidae, also deceptively mimic malacoderms. wasps and bees are often closely imitated by insects of other orders. many longicorn beetles in the tropics exactly mimic wasps, bees, or ants. in borneo a large black wasp, whose wings have a broad white patch near the apex (mygnimia aviculus), is closely imitated by a heteromerous beetle (coloborhombus fasciatipennis), which, contrary to the general habit of beetles, keeps its wings expanded in order to show the white patch on their apex, the wing-coverts being reduced to small oval scales, as shown in the figure. this is a most remarkable instance of mimicry, because the beetle has had to acquire so many characters which are unknown among its allies (except in another species from java)--the expanded wings, the white band on them, and the oval scale-like elytra.[ ] another remarkable case has been noted by mr. neville goodman, in egypt, where a common hornet (vespa orientalis) is exactly imitated in colour, size, shape, attitude when at rest, and mode of flight, by a beetle of the genus laphria.[ ] the tiger-beetles (cicindelidae) are also the subjects of mimicry by more harmless insects. in the malay islands i found a heteromerous beetle which exactly resembled a therates, both being found running on the trunks of trees. a longicorn (collyrodes lacordairei) mimics collyris, another genus of the same family; while in the philippine islands there is a cricket (condylodeira tricondyloides), which so closely resembles a tiger-beetle of the genus tricondyla that the experienced entomologist, professor westwood, at first placed it in his cabinet among those beetles. [illustration: fig. .--mygnimia aviculus (wasp). coloborhombus fasciatipennis (beetle).] [illustration: fig. . a. doliops sp. (longicorn) mimics pachyrhynchus orbifae, (b) (a hard curculio). c. doliops curculionoides mimics (d) pachyrhynchus sp. e. scepastus pachyrhynchoides (a grasshopper), mimics (f) apocyrtus sp. (a hard curculio). g. doliops sp. mimics (h) pachyrhynchus sp. i. phoraspis (grasshopper) mimics (k) a coccinella. all the above are from the philippines. the exact correspondence of the colours of the insects themselves renders the mimicry much more complete in nature than it appears in the above figures.] one of the characters by which some beetles are protected is excessive hardness of the elytra and integuments. several genera of weevils (curculionidae) are thus saved from attack, and these are often mimicked by species of softer and more eatable groups. in south america, the genus heilipus is one of these hard groups, and both mr. bates and m. roelofs, a belgian entomologist, have noticed that species of other genera exactly mimic them. so, in the philippines, there is a group of curculionidae, forming the genus pachyrhynchus, in which all the species are adorned with the most brilliant metallic colours, banded and spotted in a curious manner, and are very smooth and hard. other genera of curculionidae (desmidophorus, alcides), which are usually very differently coloured, have species in the philippines which mimic the pachyrhynchi; and there are also several longicorn beetles (aprophata, doliops, acronia, and agnia), which also mimic them. besides these, there are some longicorns and cetonias which reproduce the same colours and markings; and there is even a cricket (scepastus pachyrhynchoides), which has taken on the form and peculiar coloration of these beetles in order to escape from enemies, which then avoid them as uneatable.[ ] the figures on the opposite page exhibit several other examples of these mimicking insects. innumerable other cases of mimicry occur among tropical insects; but we must now pass on to consider a few of the very remarkable, but much rarer instances, that are found among the higher animals. _mimicry among the vertebrata._ perhaps the most remarkable cases yet known are those of certain harmless snakes which mimic poisonous species. the genus elaps, in tropical america, consists of poisonous snakes which do not belong to the viper family (in which are included the rattlesnakes and most of those which are poisonous), and which do not possess the broad triangular head which characterises the latter. they have a peculiar style of coloration, consisting of alternate rings of red and black, or red, black, and yellow, of different widths and grouped in various ways in the different species; and it is a style of coloration which does not occur in any other group of snakes in the world. but in the same regions are found three genera of harmless snakes, belonging to other families, some few species of which mimic the poisonous elaps, often so exactly that it is with difficulty one can be distinguished from the other. thus elaps fulvius in guatemala is imitated by the harmless pliocerus equalis; elaps corallinus in mexico is mimicked by the harmless homalocranium semicinctum; and elaps lemniscatus in brazil is copied by oxyrhopus trigeminus; while in other parts of south america similar cases of mimicry occur, sometimes two harmless species imitating the same poisonous snake. a few other instances of mimicry in this group have been recorded. there is in south africa an egg-eating snake (dasypeltis scaber), which has neither fangs nor teeth, yet it is very like the berg adder (clothos atropos), and when alarmed renders itself still more like by flattening out its head and darting forward with a hiss as if to strike a foe.[ ] dr. a.b. meyer has also discovered that, while some species of the genus callophis (belonging to the same family as the american elaps) have large poison fangs, other species of the same genus have none; and that one of the latter (c. gracilis) resembles a poisonous species (c. intestinalis) so closely, that only an exact comparison will discover the difference of colour and marking. a similar kind of resemblance is said to exist between another harmless snake, megaerophis flaviceps, and the poisonous callophis bivirgatus; and in both these cases the harmless snake is less abundant than the poisonous one, as occurs in all examples of true mimicry.[ ] in the genus elaps, above referred to, the very peculiar style of colour and marking is evidently a "warning colour" for the purpose of indicating to snake-eating birds and mammals that these species are poisonous; and this throws light on the long-disputed question of the use of the rattle of the rattlesnake. this reptile is really both sluggish and timid, and is very easily captured by those who know its habits. if gently tapped on the head with a stick, it will coil itself up and lie still, only raising its tail and rattling. it may then be easily caught. this shows that the rattle is a warning to its enemies that it is dangerous to proceed to extremities; and the creature has probably acquired this structure and habit because it frequents open or rocky districts where protective colour is needful to save it from being pounced upon by buzzards or other snake-eaters. quite parallel in function is the expanded hood of the indian cobra, a poisonous snake which belongs also to the elapidae. this is, no doubt, a warning to its foes, not an attempt to terrify its prey; and the hood has been acquired, as in the case of the rattlesnake, because, protective coloration being on the whole useful, some mark was required to distinguish it from other protectively coloured, but harmless, snakes. both these species feed on active creatures capable of escaping if their enemy were visible at a moderate distance. _mimicry among birds._ the varied forms and habits of birds do not favour the production among them of the phenomena of warning colours or of mimicry; and the extreme development of their instincts and reasoning powers, as well as their activity and their power of flight, usually afford them other means of evading their enemies. yet there are a few imperfect, and one or two very perfect cases of true mimicry to be found among them. the less perfect examples are those presented by several species of cuckoos, an exceedingly weak and defenceless group of birds. our own cuckoo is, in colour and markings, very like a sparrow-hawk. in the east, several of the small black cuckoos closely resemble the aggressive drongo-shrikes of the same country, and the small metallic cuckoos are like glossy starlings; while a large ground-cuckoo of borneo (carpococcyx radiatus) resembles one of the fine pheasants (euplocamus) of the same country, both in form and in its rich metallic colours. more perfect cases of mimicry occur between some of the dull-coloured orioles in the malay archipelago and a genus of large honey-suckers--the tropidorhynchi or "friar-birds." these latter are powerful and noisy birds which go in small flocks. they have long, curved, and sharp beaks, and powerful grasping claws; and they are quite able to defend themselves, often driving away crows and hawks which venture to approach them too nearly. the orioles, on the other hand, are weak and timid birds, and trust chiefly to concealment and to their retiring habits to escape persecution. in each of the great islands of the austro-malayan region there is a distinct species of tropidorhynchus, and there is always along with it an oriole that exactly mimics it. all the tropidorhynchi have a patch of bare black skin round the eyes, and a ruff of curious pale recurved feathers on the nape, whence their name of friar-birds, the ruff being supposed to resemble the cowl of a friar. these peculiarities are imitated in the orioles by patches of feathers of corresponding colours; while the different tints of the two species in each island are exactly the same. thus in bouru both are earthy brown; in ceram they are both washed with yellow ochre; in timor the under surface is pale and the throat nearly white, and mr. h.o. forbes has recently discovered another pair in the island of timor laut. the close resemblance of these several pairs of birds, of widely different families, is quite comparable with that of many of the insects already described. it is so close that the preserved specimens have even deceived naturalists; for, in the great french work, _voyage de l'astrolabe_, the oriole of bouru is actually described and figured as a honey-sucker; and mr. forbes tells us that, when his birds were submitted to dr. sclater for description, the oriole and the honey-sucker were, previous to close examination, considered to be the same species. _objections to the theory of mimicry._ to set forth adequately the varied and surprising facts of mimicry would need a large and copiously illustrated volume; and no more interesting subject could be taken up by a naturalist who has access to our great collections and can devote the necessary time to search out the many examples of mimicry that lie hidden in our museums. the brief sketch of the subject that has been here given will, however, serve to indicate its nature, and to show the weakness of the objections that were at first made to it. it was urged that the action of "like conditions," with "accidental resemblances" and "reversion to ancestral types," would account for the facts. if, however, we consider the actual phenomena as here set forth, and the very constant conditions under which they occur, we shall see how utterly inadequate are these causes, either singly or combined. these constant conditions are-- . that the imitative species occur in the same area and occupy the very same station as the imitated. . that the imitators are always the more defenceless. . that the imitators are always less numerous in individuals. . that the imitators differ from the bulk of their allies. . that the imitation, however minute, is _external_ and _visible_ only, never extending to internal characters or to such as do not affect the external appearance. these five characteristic features of mimicry show us that it is really an exceptional form of protective resemblance. different species in the same group of organisms may obtain protection in different ways: some by a general resemblance to their environment; some by more exactly imitating the objects that surround them--bark, or leaf, or flower; while others again gain an equal protection by resembling some species which, from whatever cause, is almost as free from attack as if it were a leaf or a flower. this immunity may depend on its being uneatable, or dangerous, or merely strong; and it is the resemblance to such creatures for the purpose of sharing in their safety that constitutes mimicry. _concluding remarks on warning colours and mimicry._ colours which have been acquired for the purpose of serving as a warning of inedibility, or of the possession of dangerous offensive weapons, are probably more numerous than have been hitherto supposed; and, if so, we shall be able to explain a considerable amount of colour in nature for which no use has hitherto been conjectured. the brilliant and varied colours of sea-anemones and of many coral animals will probably come under this head, since we know that many of them possess the power of ejecting stinging threads from various parts of their bodies which render them quite uneatable to most animals. mr. gosse describes how, on putting an anthea into a tank containing a half-grown bullhead (cottus bubalis) which had not been fed for some time, the fish opened his mouth and sucked in the morsel, but instantly shot it out again. he then seized it a second time, and after rolling it about in his mouth for a moment shot it out again, and then darted away to hide himself in a hole. some tropical fishes, however, of the genera tetrodon, pseudoscarus, astracion, and a few others, seem to have acquired the power of feeding on corals and medusae; and the beautiful bands and spots and bright colours with which they are frequently adorned, may be either protective when feeding in the submarine coral groves, or may, in some cases, be warning colours to show that they themselves are poisonous and uneatable. a remarkable illustration of the wide extension of warning colours, and their very definite purpose in nature, is afforded by what may now be termed "mr. belt's frog." frogs in all parts of the world are, usually, protectively coloured with greens or browns; and the little tree-frogs are either green like the leaves they rest upon, or curiously mottled to imitate bark or dead leaves. but there are a certain number of very gaily coloured frogs, and these do not conceal themselves as frogs usually do. such was the small toad found by darwin at bahia blanca, which was intense black and bright vermilion, and crawled about in the sunshine over dry sand-hills and arid plains. and in nicaragua, mr. belt found a little frog gorgeously dressed in a livery of red and blue, which did not attempt concealment and was very abundant, a combination of characters which convinced him that it was uneatable. he, therefore, took a few specimens home with him and gave them to his fowls and ducks, but none would touch them. at last, by throwing down pieces of meat, for which there was a great competition among the poultry, he managed to entice a young duck into snatching up one of the little frogs. instead of swallowing it, however, the duck instantly threw it out of its mouth, and went about jerking its head as if trying to get rid of some unpleasant taste.[ ] the power of predicting what will happen in a given case is always considered to be a crucial test of a true theory, and if so, the theory of warning colours, and with it that of mimicry, must be held to be well established. among the creatures which probably have warning colours as a sign of inedibility are, the brilliantly coloured nudibranchiate molluscs, those curious annelids the nereis and the aphrodite or sea-mouse, and many other marine animals. the brilliant colours of the scallops (pecten) and some other bivalve shells are perhaps an indication of their hardness and consequent inedibility, as in the case of the hard beetles; and it is not improbable that some of the phosphorescent fishes and other marine organisms may, like the glow-worm, hold out their lamp as a warning to enemies.[ ] in queensland there is an exceedingly poisonous spider, whose bite will kill a dog, and cause severe illness with excruciating pain in man. it is black, with a bright vermilion patch on the middle of the body; and it is so well recognised by this conspicuous coloration that even the spider-hunting wasps avoid it.[ ] locusts and grasshoppers are generally of green protective tints, but there are many tropical species most gaudily decorated with red, blue, and black colours. on the same general grounds as those by which mr. belt predicted the inedibility of his conspicuous frog, we might safely predict the same for these insects; but we have fortunately a proof that they are so protected, since mr. charles home states that one of the bright coloured indian locusts was invariably rejected when offered to birds and lizards.[ ] * * * * * the examples now given lead us to the conclusion that colours acquired for the purpose of serving as a danger-signal to enemies are very widespread in nature, and, with the corresponding colours of the species which mimic them, furnish us with a rational explanation of a considerable portion of the coloration of animals which is outside the limits of those colours that have been acquired for either protection or recognition. there remains, however, another set of colours, chiefly among the higher animals, which, being connected with some of the most interesting and most disputed questions in natural history, must be discussed in a separate chapter. footnotes: [footnote : _nature_, vol. iii. p. . professor meldola observed that specimens of danais and euplaea in collections were less subject to the attacks of mites _(proc. ent. soc._, , p. xii.); and this was corroborated by mr. jenner weir. _entomologist_, , vol. xv. p. .] [footnote : see darwin's _descent of man_, p. .] [footnote : _transactions of the entomological society of london_, , p. .] [footnote : _ibid._, p. .] [footnote : _nature_, vol. iii. p. .] [footnote : stainton's _manual of butterflies and moths_, vol. i. p. ; e.b. poulton, _proceedings of the zool. soc. of london_, , pp. - .] [footnote : see _transactions of the linnean society_, vol. xxiii. pp. - , coloured plates.] [footnote : these butterflies are now divided into two sub-families, one of which is placed with the danaidae; but to avoid confusion i shall always speak of the american genera under the old term heliconidae.] [footnote : r. meldola in _ann. and mag. of nat. hist._, feb. , p. .] [footnote : see _trans. linn. soc._, vol. xxv. wallace, on variation of malayan papilionidae; and, wallace's _contributions to natural selection_ chaps. iii. and iv., where full details are given.] [footnote : see _trans. linn. soc._, vol. xxvi., with two coloured plates illustrating cases of mimicry.] [footnote : edwards's _butterflies of north america_, second series, part vi.] [footnote : professor meldola informs me that he has recorded another case of mimicry among british moths, in which acidalia subsericata imitates asthena candidata. see _ent. mo. mag._, vol. iv. p. .] [footnote : from professor meldola's translation of dr. f. müller's paper, in _proc. ent. soc. lond._, , p. xx.] [footnote : _island life_, p. .] [footnote : this extension of the theory of mimicry was pointed out by professor meldola in the paper already referred to; and he has answered the objections to dr. f. müller's theory with great force in the _annals and mag. of nat. hist._, , p. .] [footnote : godman and salvin's _biologia centrali-americana, insecta, coleoptera_, vol. iii. part ii., and vol. v.] [footnote : _trans. ent. soc._, , p. .] [footnote : _proc. cambridge phil. soc._, vol. iii. part ii., .] [footnote : _compte-rendu de la société entomologique de belgaue_, series ii., no. , .] [footnote : _nature_, vol. xxxiv. p. .] [footnote : _proceedings of the zool. soc. of london_, , p. .] [footnote : _the naturalist in nicaragua_, p. .] [footnote : mr. belt first suggested this use of the light of the lampyridae (fireflies and glow-worms)--_naturalist in nicaragua_, p. . mr. verrill and professor meldola made the same suggestion in the case of medusae and other phosphorescent marine organisms (_nature_, vol. xxx. pp. , ).] [footnote : w.e. armit, in _nature_, vol. xviii. p. .] [footnote : _proc. ent. soc._, , p. xiii.] chapter x colours and ornaments characteristic of sex sex colours in the mollusca and crustacea--in insects--in butterflies and moths--probable causes of these colours--sexual selection as a supposed cause--sexual coloration of birds--cause of dull colours of female birds--relation of sex colour to nesting habits--sexual colours of other vertebrates--sexual selection by the struggles of males--sexual characters due to natural selection--decorative plumage of males and its effect on the females--display of decorative plumage by the males--a theory of animal coloration--the origin of accessory plumes--development of accessory plumes and their display--the effect of female preference will be neutralised by natural selection--general laws of animal coloration--concluding remarks. in the preceding chapters we have dealt chiefly with the coloration of animals as distinctive of the several species; and we have seen that, in an enormous number of cases, the colours can be shown to have a definite purpose, and to be useful either as a means of protection or concealment, of warning to enemies, or of recognition by their own kind. we have now to consider a subordinate but very widespread phenomenon---the differences of colour or of ornamental appendages in the two sexes. these differences are found to have special relations with the three classes of coloration above referred to, in many cases confirming the explanation already given of their purport and use, and furnishing us with important aid in formulating a general theory of animal coloration. in comparing the colours of the two sexes we find a perfect gradation, from absolute identity of colour up to such extreme difference that it is difficult to believe that the two forms can belong to the same species; and this diversity in the colours of the sexes does not bear any constant relation to affinity or systematic position. in both insects and birds we find examples of complete identity and extreme diversity of the sexes; and these differences occur sometimes in the same tribe or family, and sometimes even in the same genus. it is only among the higher and more active animals that sexual differences of colour acquire any prominence. in the mollusca the two sexes, when separated, are always alike in colour, and only very rarely present slight differences in the form of the shell. in the extensive group of crustacea the two sexes as a rule are identical in colour, though there are often differences in the form of the prehensile organs; but in a very few cases there are differences of colour also. thus, in a brazilian species of shore-crab (gelasimus) the female is grayish-brown, while in the male the posterior part of the cephalo-thorax is pure white, with the anterior part of a rich green. this colour is only acquired by the males when they become mature, and is liable to rapid change in a few minutes to dusky tints.[ ] in some of the freshwater fleas (daphnoidae) the males are ornamented with red and blue spots, while in others similar colours occur in both sexes. in spiders also, though as a rule the two sexes are alike in colour, there are a few exceptions, the males being ornamented with brilliant colours on the abdomen, while the female is dull coloured. _sexual coloration in insects._ it is only when we come to the winged insects that we find any large amount of peculiarity in sexual coloration, and even here it is only developed in certain orders. flies (diptera), field-bugs (hemiptera), cicadas (homoptera), and the grasshoppers, locusts, and crickets (orthoptera) present very few and unimportant sexual differences of colour; but the last two groups have special musical organs very fully developed in the males of some of the species, and these no doubt enable the sexes to discover and recognise each other. in some cases, however, when the female is protectively coloured, as in the well-known leaf-insects already referred to (p. ), the male is smaller and much less protectively formed and coloured. in the bees and wasps (hymenoptera) it is also the rule that the sexes are alike in colour, though there are several cases among solitary bees where they differ; the female being black, and the male brown in anthophora retusa, while in andraena fulva the female is more brightly coloured than the male. of the great order of beetles (coleoptera) the same thing may be said. though often so rich and varied in their colours the sexes are usually alike, and mr. darwin was only able to find about a dozen cases in which there was any conspicuous difference between them.[ ] they exhibit, however, numerous sexual characters, in the length of the antennae, and in horns, legs, or jaws remarkably enlarged or curiously modified in the male sex. it is in the family of dragonflies (order neuroptera) that we first meet with numerous cases of distinctive sexual coloration. in some of the agrionidae the males have the bodies rich blue and the wings black, while the females have the bodies green and the wings transparent. in the north american genus hetaerina the males alone have a carmine spot at the base of each wing; but in some other genera the sexes hardly differ at all. the great order of lepidoptera, including the butterflies and moths, affords us the most numerous and striking examples of diversity of sexual colouring. among the moths the difference is usually but slight, being manifested in a greater intensity of the colour of the smaller winged male; but in a few cases there is a decided difference, as in the ghost-moth (hepialus humuli), in which the male is pure white, while the female is yellow with darker markings. this may be a recognition colour, enabling the female more readily to discover her mate; and this view receives some support from the fact that in the shetland islands the male is almost as yellow as the female, since it has been suggested that at midsummer, when this moth appears, there is in that high latitude sufficient twilight all night to render any special coloration unnecessary.[ ] butterflies present us with a wonderful amount of sexual difference of colour, in many cases so remarkable that the two sexes of the same species remained for many years under different names and were thought to be quite distinct species. we find, however, every gradation from perfect identity to complete diversity, and in some cases we are able to see a reason for this difference. beginning with the most extraordinary cases of diversity--as in diadema misippus, where the male is black, ornamented with a large white spot on each wing margined with rich changeable blue, while the female is orange-brown with black spots and stripes--we find the explanation in the fact that the female mimics an uneatable danais, and thus gains protection while laying its eggs on low plants in company with that insect. in the allied species, diadema bolina, the females are also very different from the males, but are of dusky brown tints, evidently protective and very variable, some specimens having a general resemblance to the uneatable euplaeas; so that we see here some of the earlier stages of both forms of protection. the remarkable differences in some south american pieridae are similarly explained. the males of pieris pyrrha, p. lorena, and several others, are white with a few black bands and marginal spots like so many of their allies, while the females are gaily coloured with yellow and brown, and exactly resemble some species of the uneatable heliconidae of the same district. similarly, in the malay archipelago, the female of diadema anomala is glossy metallic blue, while the male is brown; the reason for this reversal of the usual rule being, that the female exactly mimics the brilliant colouring of the common and uneatable euplaea midamus, and thus secures protection. in the fine adolias dirtea, the male is black with a few specks of ochre-yellow and a broad marginal band of rich metallic greenish-blue, while the female is brownish-black entirely covered with rows of ochre-yellow spots. this latter coloration does not appear to be protective when the insect is seen in the cabinet, but it really is so. i have observed the female of this butterfly in sumatra, where it settles on the ground in the forest, and its yellow spots so harmonise with the flickering gleams of sunlight on the dead leaves that it can only be detected with the greatest difficulty. a hundred other cases might be quoted in which the female is either more obscurely coloured than the male, or gains protection by imitating some inedible species; and any one who has watched these female insects flying slowly along in search of the plants on which to deposit their eggs, will understand how important it must be to them not to attract the attention of insect-eating birds by too conspicuous colours. the number of birds which capture insects on the wing is much greater in tropical regions than in europe; and this is perhaps the reason why many of our showy species are alike, or almost alike, in both sexes, while they are protectively coloured on the under side which is exposed to view when they are at rest. such are our peacock, tortoise-shell, and red admiral butterflies; while in the tropics we more commonly find that the females are less conspicuous on the upper surface even when protectively coloured beneath. we may here remark, that the cases already quoted prove clearly that either male or female may be modified in colour apart from the opposite sex. in pieris pyrrha and its allies the male retains the usual type of coloration of the whole genus, while the female has acquired a distinct and peculiar style of colouring. in adolias dirtea, on the other hand, the female appears to retain something like the primitive colour and markings of the two sexes, modified perhaps for more perfect protection; while the male has acquired more and more intense and brilliant colours, only showing his original markings by the few small yellow spots that remain near the base of the wings. in the more gaily coloured pieridae, of which our orange-tip butterfly may be taken as a type, we see in the female the plain ancestral colours of the group, while the male has acquired the brilliant orange tip to its wings, probably as a recognition mark. in those species in which the under surface is protectively coloured, we often find the upper surface alike in both sexes, the tint of colour being usually more intense in the male. but in some cases this leads to the female being more conspicuous, as in some of the lycaenidae, where the female is bright blue and the male of a blue so much deeper and soberer in tint as to appear the less brilliantly coloured of the two. _probable causes of these colours._ in the production of these varied results there have probably been several causes at work. there seems to be a constant tendency in the male of most animals--but especially of birds and insects--to develop more and more intensity of colour, often culminating in brilliant metallic blues or greens or the most splendid iridescent hues; while, at the same time, natural selection is constantly at work, preventing the female from acquiring these same tints, or modifying her colours in various directions to secure protection by assimilating her to her surroundings, or by producing mimicry of some protected form. at the same time, the need for recognition must be satisfied; and this seems to have led to diversities of colour in allied species, sometimes the female, sometimes the male undergoing the greatest change according as one or other could be modified with the greatest ease, and so as to interfere least with the welfare of the race. hence it is that sometimes the males of allied species vary most, as in the different species of epicalia; sometimes the females, as in the magnificent green species of ornithoptera and the "aeneas" group of papilio. the importance of the two principles--the need of protection and recognition--in modifying the comparative coloration of the sexes among butterflies, is beautifully illustrated in the case of the groups which are protected by their distastefulness, and whose females do not, therefore, need the protection afforded by sober colours. in the great families, heliconidae and acraeidae, we find that the two sexes are almost always alike; and, in the very few exceptions, that the female, though differently, is not less gaily or less conspicuously coloured. in the danaidae the same general rule prevails, but the cases in which the male exhibits greater intensity of colour than the female are perhaps more numerous than in the other two families. there is, however, a curious difference in this respect between the oriental and the american groups of distasteful papilios with warning colours, both of which are the subjects of mimicry. in the eastern groups--of which p. hector and p. coon may be taken as types--the two sexes are nearly alike, the male being sometimes more intensely coloured and with fewer pale markings; but in the american groups--represented by p. aeneas, p. sesostris, and allies--there is a wonderful diversity, the males having a rich green or bluish patch on the fore wings, while the females have a band or spots of pure white, not always corresponding in position to the green spot of the males. there are, however, transitional forms, by which a complete series can be traced, from close similarity to great diversity of colouring between the sexes; and this may perhaps be only an extreme example of the intenser colour and more concentrated markings which are a very prevalent characteristic of male butterflies. there are, in fact, many indications of a regular succession of tints in which colour development has occurred in the various groups of butterflies, from an original grayish or brownish neutral tint. thus in the "aeneas" group of papilios we have the patch on the upper wings yellowish in p. triopas, olivaceous in p. bolivar, bronzy-gray with a white spot in p. erlaces, more greenish and buff in p. iphidamas, gradually changing to the fine blue of p. brissonius, and the magnificent green of p. sesostris. in like manner, the intense crimson spots of the lower wings can be traced step by step from a yellow or buff tint, which is one of the most widespread colours in the whole order. the greater purity and intensity of colour seem to be usually associated with more pointed wings, indicating greater vigour and more rapid flight. _sexual selection as a supposed cause of colour development._ mr. darwin, as is well known, imputed most of the brilliant colours and varied patterns of butterflies' wings to sexual selection--that is, to a constant preference, by female butterflies, for the more brilliant males; the colours thus produced being sometimes transmitted to the males alone, sometimes to both sexes. this view has always seemed to me to be unsupported by evidence, while it is also quite inadequate to account for the facts. the only direct evidence, as set forth with his usual fairness by mr. darwin himself, is opposed to his views. several entomologists assured him that, in moths, the females evince not the least choice of their partners; and dr. wallace of colchester, who has largely bred the fine bombyx cynthia, confirmed this statement. among butterflies, several males often pursue one female, and mr. darwin says, that, unless the female exerts a choice the pairing must be left to chance. but, surely, it may be the most vigorous or most persevering male that is chosen, not necessarily one more brightly or differently coloured, and this will be true "natural selection." butterflies have been noticed to prefer some coloured flowers to others; but that does not prove, or even render probable, any preference for the colour itself, but only for flowers of certain colours, on account of the more agreeable or more abundant nectar obtained from them. dr. schulte called mr. darwin's attention to the fact, that in the diadema bolina the brilliant blue colour surrounding the white spots is only visible when we look towards the insect's head, and this is true of many of the iridescent colours of butterflies, and probably depends upon the direction of the striae on the scales. it is suggested, however, that this display of colour will be seen by the female as the male is approaching her, and that it has been developed by sexual selection.[ ] but in the majority of cases the males _follow_ the female, hovering over her in a position which would render it almost impossible for her to see the particular colours or patterns on his upper surface; to do so the female should mount higher than the male, and fly towards him--being the seeker instead of the sought, and this is quite opposed to the actual facts. i cannot, therefore, think that this suggestion adds anything whatever to the evidence for sexual selection of colour by female butterflies. this question will, however, be again touched upon after we have considered the phenomena of sexual colour among the vertebrata. _sexual coloration of birds._ the general rule among vertebrates, as regards colour, is, for the two sexes to be alike. this prevails, with only a few exceptions, in fishes, reptiles, and mammalia; but in birds diversity of sexual colouring is exceedingly frequent, and is, not improbably, present in a greater or less degree in more than half of the known species. it is this class, therefore, that will afford us the best materials for a discussion of the problem, and that may perhaps lead us to a satisfactory explanation of the causes to which sexual colour is due. the most fundamental characteristic of birds, from our present point of view, is a greater intensity of colour in the male. this is the case in hawks and falcons; in many thrushes, warblers, and finches; in pigeons, partridges, rails, plovers, and many others. when the plumage is highly protective or of dull uniform tints, as in many of the thrushes and warblers, the sexes are almost or quite identical in colour; but when any rich markings or bright tints are acquired, they are almost always wanting or much fainter in the female, as we see in the black-cap among warblers, and the chaffinch among finches. it is in tropical regions, where from a variety of causes colour has been, developed to its fullest extent, that we find the most remarkable examples of sexual divergence of colour. the most gorgeously coloured birds known are the birds of paradise, the chatterers, the tanagers, the humming-birds, and the pheasant-tribe, including the peacocks. in all these the females are much less brilliant, and, in the great majority of cases, exceptionally plain and dull coloured birds. not only are the remarkable plumes, crests, and gorgets of the birds of paradise entirely wanting in the females, but these latter are usually without any bright colour at all, and rank no higher than our thrushes in ornamental plumage. of the humming-birds the same may be said, except that the females are often green, and sometimes slightly metallic, but from their small size and uniform tints are never conspicuous. the glorious blues and purples, the pure whites and intense crimsons of the male chatterers are represented in the females by olive-greens or dull browns, as are the infinitely varied tints of the male tanagers. and in pheasants, the splendour of plumage which characterises the males is entirely absent in the females, which, though often ornamental, have always comparatively sober and protective tints. the same thing occurs with many other groups. in the eastern tropics are many brilliant birds belonging to the families of the warblers, flycatchers, shrikes, etc., but the female is always much less brilliant than the male and often quite dull coloured. _cause of dull colours of female birds._ the reason of this phenomenon is not difficult to find, if we consider the essential conditions of a bird's existence, and the most important function it has to fulfil. in order that the species may be continued, young birds must be produced, and the female birds have to sit assiduously on their eggs. while doing this they are exposed to observation and attack by the numerous devourers of eggs and birds, and it is of vital importance that they should be protectively coloured in all those parts of the body which are exposed during incubation. to secure this end all the bright colours and showy ornaments which decorate the male have not been acquired by the female, who often remains clothed in the sober hues which were probably once common to the whole order to which she belongs. the different amounts of colour acquired by the females have no doubt depended on peculiarities of habits and of environment, and on the powers of defence or of concealment possessed by the species. mr. darwin has taught us that natural selection cannot produce absolute, but only relative perfection; and as a protective colour is only one out of many means by which the female birds are able to provide for the safety of their young, those which are best endowed in other respects will have been allowed to acquire more colour than those with whom the struggle for existence is more severe. _relation of sex colour to nesting habits._ this principle is strikingly illustrated by the existence of considerable numbers of birds in which both sexes are similarly and brilliantly coloured,--in some cases as brilliantly as the males of many of the groups above referred to. such are the extensive families of the kingfishers, the woodpeckers, the toucans, the parrots, the turacos, the hangnests, the starlings, and many other smaller groups, all the species of which are conspicuously or brilliantly coloured, while in all of them the females are either coloured exactly like the males, or, when differently coloured, are equally conspicuous. when searching for some cause for this singular apparent exception to the rule of female protective colouring, i came upon a fact which beautifully explains it; for in all these cases, without exception, the species either nests in holes in the ground or in trees, or builds a domed or covered nest, so as completely to conceal the sitting-bird. we have here a case exactly parallel to that of the butterflies protected by distastefulness, whose females are either exactly like the males, or, if different, are equally conspicuous. we can hardly believe that so exact a parallel should exist between such remote classes of animals, except under the influence of a general law; and, in the need of protection by all defenceless animals, and especially by most female insects and birds, we have such a law, which has been proved to have influenced the colours of a considerable proportion of the animal kingdom.[ ] the general relation which exists between the mode of nesting and the coloration of the sexes in those groups of birds which need protection from enemies, may be thus expressed: when both sexes are brilliant or conspicuous, the nest is such as to conceal the sitting-bird; but when the male is brightly coloured and the female sits exposed on the nest, she is always less brilliant and generally of quite sober and protective hues. it must be understood that the mode of nesting has influenced the colour, not that the colour has determined the mode of nesting; and this, i believe, has been generally, though not perhaps universally, the case. for we know that colour varies more rapidly, and can be more easily modified and fixed by selection, than any other character; whereas habits, especially when connected with structure, and when they pervade a whole group, are much more persistent and more difficult to change, as shown by the habit of the dog turning round two or three times before lying down, believed to be that of the wild ancestral form which thus smoothed down the herbage so as to form a comfortable bed. we see, too, that the general mode of nesting is characteristic of whole families differing widely in size, form, and colours. thus, all the kingfishers and their allies in every part of the world nest in holes, usually in banks, but sometimes in trees. the motmots and the puff-birds (bucconidae) build in similar places; while the toucans, barbets, trogons, woodpeckers, and parrots all make their nests in hollow trees. this habit, pervading all the members of extensive families, must therefore be extremely ancient, more especially as it evidently depends in some degree on the structure of the birds, the bills, and especially the feet, of all these groups being unfitted for the construction of woven arboreal nests.[ ] but in all these families the colour varies greatly from species to species, being constant only in the one character of the similarity of the sexes, or, at all events, in their being equally conspicuous even though differently coloured. when i first put forward this view of the connection between the mode of nesting and the coloration of female birds, i expressed the law in somewhat different terms, which gave rise to some misunderstanding, and led to numerous criticisms and objections. several cases were brought forward in which the females were far less brilliant than the males, although the nest was covered. this is the case with the maluridae, or superb warblers of australia, in which the males are very brilliant during the pairing season and the females quite plain, yet they build domed nests. here, there can be little doubt, the covered nest is a protection from rain or from some special enemies to the eggs; while the birds themselves are protectively coloured in both sexes, except for a short time during the breeding season when the male acquires brilliant colours; and this is probably connected with the fact of their inhabiting the open plains and thin scrub of australia, where protective colours are as generally advantageous as they are in our north-temperate zones. as i have now stated the law, i do not think there are any exceptions to it, while there are an overwhelming number of cases which give it a strong support. it has been objected that the domed nests of many birds are as conspicuous as the birds themselves would be, and would, therefore, be of no use as a protection to the birds and young. but, as a matter of fact, they do protect from attack, for hawks or crows do not pluck such nests to pieces, as in doing so they would be exposed to the attack of the whole colony; whereas a hawk or falcon could carry off a sitting-bird or the young at a swoop, and entirely avoid attack. moreover, each kind of covered nest is doubtless directed against the attacks of the most dangerous enemies of the species, the purse-like nests, often a yard long, suspended from the extremity of thin twigs, being useful against the attacks of snakes, which, if they attempted to enter them, would be easily made to lose their hold and fall to the ground. such birds as jays, crows, magpies, hawks, and other birds of prey, have also been urged as an exception; but these are all aggressive birds, able to protect themselves, and thus do not need any special protection for their females during nidification. some birds which build in covered nests are comparatively dull coloured, like many of the weaver birds, but in others the colours are more showy, and in all the sexes are alike; so that none of these are in any way opposed to the rule. the golden orioles have, however, been adduced as a decided exception, since the females are showy and build in an open nest. but even here the females are less brilliant than the males, and are sometimes greenish or olivaceous on the upper surface; while they very carefully conceal their nests among dense foliage, and the male is sufficiently watchful and pugnacious to drive off most intruders. on the other hand, how remarkable it is that the only small and brightly coloured birds of our own country in which the male and female are alike--the tits and starlings--either build in holes or construct covered nests; while the beautiful hangnests (icteridae) of south america, which always build covered or purse-shaped nests, are equally showy in both sexes, in striking contrast with the chatterers and tanagers of the same country, whose females are invariably less conspicuous than the males. on a rough estimate, there are about species of birds in the class of showy males and females, with concealed nidification; while there are probably, from an equally rough estimate, about the same number in the contrasted class of showy males and dull females, with open nests. this will leave the great bulk of known birds in the classes of those which are more or less protectively coloured in both sexes; or which, from their organisation and habits, do not require special protective coloration, such as many of the birds of prey, the larger waders, and the oceanic birds. there are a few very curious cases in which the female bird is actually more brilliant than the male, and which yet have open nests. such are the dotterel (eudromias morinellus), several species of phalarope, an australian creeper (climacteris erythropus), and a few others; but in every one of these cases the relation of the sexes in regard to nidification is reversed, the male performing the duties of incubation, while the female is the stronger and more pugnacious. this curious case, therefore, quite accords with the general law of coloration.[ ] _sexual colours of other vertebrates._ we may consider a few of the cases of sexual colouring of other classes of vertebrates, as given by mr. darwin. in fishes, though the sexes are usually alike, there are several species in which the males are more brightly coloured, and have more elongated fins, spines, or other appendages, and in some few cases the colours are decidedly different. the males often fight together, and are altogether more vivacious and excitable than the females during the breeding season; and with this we may connect a greater intensity of coloration. in frogs and toads the colours are usually alike, or a little more intense in the males, and the same may be said of most snakes. it is in lizards that we first meet with considerable sexual differences, many of the species having gular pouches, frills, dorsal crests, or horns, either confined to the males, or more developed in them than in the females, and these ornaments are often brightly coloured. in most cases, however, the tints of lizards are protective, the male being usually a little more intense in coloration; and the difference in extreme cases may be partly due to the need of protection for the female, which, when laden with eggs, must be less active and less able to escape from enemies than the male, and may, therefore, have retained more protective colours, as so many insects and birds have certainly done.[ ] in mammalia there is often a somewhat greater intensity of colour in the male, but rarely a decided difference. the female of the great red kangaroo, however, is a delicate gray; while in the lemur macaco of madagascar the male is jet-black and the female brown. in many monkeys also there are some differences of colour, especially on the face. the sexual weapons and ornaments of male mammalia, as horns, crests, manes, and dewlaps, are well known, and are very numerous and remarkable. having thus briefly reviewed the facts, we will now consider the theories to which they have given rise. _sexual selection by the struggles of males._ among the higher animals it is a very general fact that the males fight together for the possession of the females. this leads, in polygamous animals especially, to the stronger or better armed males becoming the parents of the next generation, which inherits the peculiarities of the parents; and thus vigour and offensive weapons are continually increased in the males, resulting in the strength and horns of the bull, the tusks of the boar, the antlers of the stag, and the spurs and fighting instinct of the gamecock. but almost all male animals fight together, though not specially armed; even hares, moles, squirrels, and beavers fight to the death, and are often found to be scarred and wounded. the same rule applies to almost all male birds; and these battles have been observed in such different groups as humming-birds, finches, goatsuckers, woodpeckers, ducks, and waders. among reptiles, battles of the males are known to occur in the cases of crocodiles, lizards, and tortoises; among fishes, in those of salmon and sticklebats. even among insects the same law prevails; and male spiders, beetles of many groups, crickets, and butterflies often fight together. from this very general phenomenon there necessarily results a form of natural selection which increases the vigour and fighting power of the male animal, since, in every case, the weaker are either killed, wounded, or driven away. this selection would be more powerful if males were always in excess of females, but after much research mr. darwin could not obtain any satisfactory evidence that this was the case. the same effect, however, is produced in some cases by constitution or habits; thus male insects usually emerge first from the pupa, and among migrating birds the males arrive first both in this country and in north america. the struggle is thus intensified, and the most vigorous males are the first to have offspring. this in all probability is a great advantage, as the early breeders have the start in securing food, and the young are strong enough to protect themselves while the later broods are being produced. it is to this form of male rivalry that mr. darwin first applied the term "sexual selection." it is evidently a real power in nature; and to it we must impute the development of the exceptional strength, size, and activity of the male, together with the possession of special offensive and defensive weapons, and of all other characters which arise from the development of these or are correlated with them. but he has extended the principle into a totally different field of action, which has none of that character of constancy and of inevitable result that attaches to natural selection, including male rivalry; for by far the larger portion of the phenomena, which he endeavours to explain by the direct action of sexual selection, can only be so explained on the hypothesis that the immediate agency is female choice or preference. it is to this that he imputes the origin of all secondary sexual characters other than weapons of offence and defence, of all the ornamental crests and accessory plumes of birds, the stridulating sounds of insects, the crests and beards of monkeys and other mammals, and the brilliant colours and patterns of male birds and butterflies. he even goes further, and imputes to it a large portion of the brilliant colour that occurs in both sexes, on the principle that variations occurring in one sex are sometimes transmitted to the same sex only, sometimes to both, owing to peculiarities in the laws of inheritance. in this extension of sexual selection to include the action of female choice or preference, and in the attempt to give to that choice such wide-reaching effects, i am unable to follow him more than a very little way; and i will now state some of the reasons why i think his views are unsound. _sexual characters due to natural selection._ besides the acquisition of weapons by the male for the purpose of fighting with other males, there are some other sexual characters which may have been produced by natural selection. such are the various sounds and odours which are peculiar to the male, and which serve as a call to the female or as an indication of his presence. these are evidently a valuable addition to the means of recognition of the two sexes, and are a further indication that the pairing season has arrived; and the production, intensification, and differentiation of these sounds and odours are clearly within the power of natural selection. the same remark will apply to the peculiar calls of birds, and even to the singing of the males. these may well have originated merely as a means of recognition between the two sexes of a species, and as an invitation from the male to the female bird. when the individuals of a species are widely scattered, such a call must be of great importance in enabling pairing to take place as early as possible, and thus the clearness, loudness, and individuality of the song becomes a useful character, and therefore the subject of natural selection. such is especially the case with the cuckoo, and with all solitary birds, and it may have been equally important at some period of the development of all birds. the act of singing is evidently a pleasurable one; and it probably serves as an outlet for superabundant nervous energy and excitement, just as dancing, singing, and field sports do with us. it is suggestive of this view that the exercise of the vocal power seems to be complementary to the development of accessory plumes and ornaments, all our finest singing birds being plainly coloured, and with no crests, neck or tail plumes to display; while the gorgeously ornamented birds of the tropics have no song, and those which expend much energy in display of plumage, as the turkey, peacocks, birds of paradise, and humming-birds, have comparatively an insignificant development of voice. some birds have, in the wings or tail, peculiarly developed feathers which produce special sounds. in some of the little manakins of brazil, two or three of the wing-feathers are curiously shaped and stiffened in the male, so that the bird is able to produce with them a peculiar snapping or cracking sound; and the tail-feathers of several species of snipe are so narrowed as to produce distinct drumming, whistling, or switching sounds when the birds descend rapidly from a great height. all these are probably recognition and call notes, useful to each species in relation to the most important function of their lives, and thus capable of being developed by the agency of natural selection. _decorative plumage of birds and its display._ mr. darwin has devoted four chapters of his _descent of man_ to the colours of birds, their decorative plumage, and its display at the pairing season; and it is on this latter circumstance that he founds his theory, that both the plumage and the colours have been developed by the preference of the females, the more ornamented males becoming the parents of each successive generation. any one who reads these most interesting chapters will admit, that the fact of the display is demonstrated; and it may also be admitted, as highly probable, that the female is pleased or excited by the display. but it by no means follows that slight differences in the shape, pattern, or colours of the ornamental plumes are what lead a female to give the preference to one male over another; still less that all the females of a species, or the great majority of them, over a wide area of country, and for many successive generations, prefer exactly the same modification of the colour or ornament. the evidence on this matter is very scanty, and in most cases not at all to the point. some peahens preferred an old pied peacock; albino birds in a state of nature have never been seen paired with other birds; a canada goose paired with a bernicle gander; a male widgeon preferred a pintail duck to its own species; a hen canary preferred a male greenfinch to either linnet, goldfinch, siskin, or chaffinch. these cases are evidently exceptional, and are not such as generally occur in nature; and they only prove that the female does exert some choice between very different males, and some observations on birds in a state of nature prove the same thing; but there is no evidence that slight variations in the colour or plumes, in the way of increased intensity or complexity, are what determines the choice. on the other hand, mr. darwin gives much evidence that it is _not_ so determined. he tells us that messrs. hewitt, tegetmeier, and brent, three of the highest authorities and best observers, "do not believe that the females prefer certain males on account of the beauty of their plumage." mr. hewitt was convinced "that the female almost invariably prefers the most vigorous, defiant, and mettlesome male;" and mr. tegetmeier, "that a gamecock, though disfigured by being dubbed, and with his hackles trimmed, would be accepted as readily as a male retaining all his natural ornaments."[ ] evidence is adduced that a female pigeon will sometimes turn antipathy to a particular male without any assignable cause; or, in other cases, will take a strong fancy to some one bird, and will desert her own mate for him; but it is not stated that superiority or inferiority of plumage has anything to do with these fancies. two instances are indeed given, of male birds being rejected, which had lost their ornamental plumage; but in both cases (a widow-finch and a silver pheasant) the long tail-plumes are the indication of sexual maturity. such cases do not support the idea that males with the tail-feathers a trifle longer, or the colours a trifle brighter, are generally preferred, and that those which are only a little inferior are as generally rejected,--and this is what is absolutely needed to establish the theory of the development of these plumes by means of the choice of the female. it will be seen, that female birds have unaccountable likes and dislikes in the matter of their partners, just as we have ourselves, and this may afford us an illustration. a young man, when courting, brushes or curls his hair, and has his moustache, beard, or whiskers in perfect order, and no doubt his sweetheart admires them; but this does not prove that she marries him on account of these ornaments, still less that hair, beard, whiskers, and moustache were developed by the continued preferences of the female sex. so, a girl likes to see her lover well and fashionably dressed, and he always dresses as well as he can when he visits her; but we cannot conclude from this that the whole series of male costumes, from the brilliantly coloured, puffed, and slashed doublet and hose of the elizabethan period, through the gorgeous coats, long waistcoats, and pigtails of the early georgian era, down to the funereal dress-suit of the present day, are the direct result of female preference. in like manner, female birds may be charmed or excited by the fine display of plumage by the males; but there is no proof whatever that slight differences in that display have any effect in determining their choice of a partner. _display of decorative plumage._ the extraordinary manner in which most birds display their plumage at the time of courtship, apparently with the full knowledge that it is beautiful, constitutes one of mr. darwin's strongest arguments. it is, no doubt, a very curious and interesting phenomenon, and indicates a connection between the exertion of particular muscles and the development of colour and ornament; but, for the reasons just given, it does not prove that the ornament has been developed by female choice. during excitement, and when the organism develops superabundant energy, many animals find it pleasurable to exercise their various muscles, often in fantastic ways, as seen in the gambols of kittens, lambs, and other young animals. but at the time of pairing, male birds are in a state of the most perfect development, and possess an enormous store of vitality; and under the excitement of the sexual passion they perform strange antics or rapid flights, as much probably from an internal impulse to motion and exertion as with any desire to please their mates. such are the rapid descent of the snipe, the soaring and singing of the lark, and the dances of the cock-of-the-rock and of many other birds. it is very suggestive that similar strange movements are performed by many birds which have no ornamental plumage to display. goatsuckers, geese, carrion vultures, and many other birds of plain plumage have been observed to dance, spread their wings or tails, and perform strange love-antics. the courtship of the great albatross, a most unwieldy and dull coloured bird, has been thus described by professor moseley: "the male, standing by the female on the nest, raises his wings, spreads his tail and elevates it, throws up his head with the bill in the air, or stretches it straight out, or forwards, as far as he can, and then utters a curious cry."[ ] mr. jenner weir informs me that "the male blackbird is full of action, spreads out his glossy wing and tail, turns his rich golden beak towards the female, and chuckles with delight," while he has never seen the more plain coloured thrush demonstrative to the female. the linnet distends his rosy breast, and slightly expands his brown wings and tail; while the various gay coloured australian finches adopt such attitudes and postures as, in every case, to show off their variously coloured plumage to the best advantage.[ ] _a theory of animal coloration._ having rejected mr. darwin's theory of female choice as incompetent to account for the brilliant colours and markings of the higher animals, the preponderance of these colours and markings in the male sex, and their display during periods of activity or excitement, i may be asked what explanation i have to offer as a preferable substitute. in my _tropical nature_ i have already indicated such a theory, which i will now briefly explain, supporting it by some additional facts and arguments, which appear to me to have great weight, and for which i am mainly indebted to a most interesting and suggestive posthumous work by mr. alfred tylor.[ ] the fundamental or ground colours of animals ar has been shown in preceding chapters, very largely protective, and it is not improbable that the primitive colours of all animals were so. during the long course of animal development other modes of protection than concealment by harmony of colour arose, and thenceforth the normal development of colour due to the complex chemical and structural changes ever going on in the organism, had full play; and the colours thus produced were again and again modified by natural selection for purposes of warning, recognition, mimicry, or special protection, as has been already fully explained in the preceding chapters. mr. taylor has, however, called attention to an important principle which underlies the various patterns or ornamental markings of animals--namely, that diversified coloration follows the chief lines of structure, and changes at points, such as the joints, where function changes. he says, "if we take highly decorated species--that is, animals marked by alternate dark or light bands or spots, such as the zebra, some deer, or the carnivora, we find, first, that the region of the spinal column is marked by a dark stripe; secondly, that the regions of the appendages, or limbs, are differently marked; thirdly, that the flanks are striped or spotted, along or between the regions of the lines of the ribs; fourthly, that the shoulder and hip regions are marked by curved lines; fifthly, that the pattern changes, and the direction of the lines, or spots, at the head, neck, and every joint of the limbs; and lastly, that the tips of the ears, nose, tail, and feet, and the eye are emphasised in colour. in spotted animals the greatest length of the spot is generally in the direction of the largest development of the skeleton." this structural decoration is well seen in many insects. in caterpillars, similar spots and markings are repeated in each segment, except where modified for some form of protection. in butterflies, the spots and bands usually have reference to the form of the wing and the arrangement of the nervures; and there is much evidence to show that the primitive markings are always spots in the cells, or between the nervures, or at the junctions of nervures, the extension and coalescence of these spots forming borders, bands, or blotches, which have become modified in infinitely varied ways for protection, warning, or recognition. even in birds, the distribution of colours and markings follows generally the same law. the crown of the head, the throat, the ear-coverts, and the eyes have usually distinct tints in all highly coloured birds; the region of the furcula has often a distinct patch of colour, as have the pectoral muscles, the uropygium or root of the tail, and the under tail-coverts.[ ] mr. tylor was of opinion the primitive form of ornamentation consisted of spots, the confluence of these in certain directions forming lines or bands; and, these again, sometimes coalescing into blotches, or into more or less uniform tints covering a large portion of the surface of the body. the young lion and tiger are both spotted; and in the java hog (sus vittatus) very young animals are banded, but have spots over the shoulders and thighs. these spots run into stripes as the animal grows older; then the stripes expand, and at last, meeting together, the adult animal becomes of a uniform dark brown colour. so many of the species of deer are spotted when young, that darwin concludes the ancestral form, from which all deer are derived, must have been spotted. pigs and tapirs are banded or spotted when young; an imported young specimen of tapirus bairdi was covered with white spots in longitudinal rows, here and there forming short stripes.[ ] even the horse, which darwin supposes to be descended from a striped animal, is often spotted, as in dappled horses; and great numbers show a tendency to spottiness, especially on the haunches. ocelli may also be developed from spots, or from bars, as pointed out by mr. darwin. spots are an ordinary form of marking in disease, and these spots sometimes run together, forming blotches. there is evidence that colour markings are in some way dependent on nerve distribution. in the disease known as frontal herpes, an eruption occurs which corresponds exactly to the distribution of the ophthalmic division of the fifth cranial nerve, mapping out all its little branches even to the one which goes to the tip of the nose. in a hindoo suffering from herpes the pigment was destroyed in the arm along the course of the ulnar nerve, with its branches along both sides of one finger and the half of another. in the leg the sciatic and scaphenous nerves were partly mapped out, giving to the patient the appearance of an anatomical diagram.[ ] these facts are very interesting, because they help to explain the general dependence of marking on structure which has been already pointed out. for, as the nerves everywhere follow the muscles, and these are attached to the various bones, we see how it happens, that the tracts in which distinct developments of colour appear, should so often be marked out by the chief divisions of the bony structure in vertebrates, and by the segments in the annulosa. there is, however, another correspondence of even greater interest and importance. brilliant colours usually appear just in proportion to the development of tegumentary appendages. among birds the most brilliant colours are possessed by those which have developed frills, crests, and elongated tails like the humming-birds; immense tail-coverts like the peacock; enormously expanded wing-feathers, as in the argus-pheasant; or magnificent plumes from the region of the coracoids in many of the birds of paradise. it is to be noted, also, that all these accessory plumes spring from parts of the body which, in other species, are distinguished by patches of colour; so that we may probably impute the development of colour and of accessory plumage to the same fundamental cause. among insects, the most brilliant and varied coloration occurs in the butterflies and moths, groups in which the wing-membranes have received their greatest expansion, and whose specialisation has been carried furthest in the marvellous scaly covering which is the seat of the colour. it is suggestive, that the only other group in which functional wings are much coloured is that of the dragonflies, where the membrane is exceedingly expanded. in like manner, the colours of beetles, though greatly inferior to those of the lepidoptera, occur in a group in which the anterior pair of wings has been thickened and modified in order to protect the vital parts, and in which these wing-covers (elytra), in the course of development in the different groups, must have undergone great changes, and have been the seat of very active growth. _the origin of accessory plumes._ mr. darwin supposes, that these have in almost every case been developed by the preference of female birds for such males as possessed them in a higher degree than others; but this theory does not account for the fact that these plumes usually appear in a few definite parts of the body. we require some cause to initiate the development in one part rather than in another. now, the view that colour has arisen over surfaces where muscular and nervous development is considerable, and the fact that it appears especially upon the accessory or highly developed plumes, leads us to inquire whether the same cause has not primarily determined the development of these plumes. the immense tuft of golden plumage in the best known birds of paradise (paradisea apoda and p. minor) springs from a very small area on the side of the breast. mr. frank e. beddard, who has kindly examined a specimen for me, says that "this area lies upon the pectoral muscles, and near to the point where the fibres of the muscle converge towards their attachment to the humerus. the plumes arise, therefore, close to the most powerful muscle of the body, and near to where the activities of that muscle would be at a maximum. furthermore, the area of attachment of the plumes is just above the point where the arteries and nerves for the supply of the pectoral muscles, and neighbouring regions, leave the interior of the body. the area of attachment of the plume is, also, as you say in your letter, just above the junction of the coracoid and sternum." ornamental plumes of considerable size rise from the same part in many other species of paradise birds, sometimes extending laterally in front, so as to form breast shields. they also occur in many humming-birds, and in some sun-birds and honey-suckers; and in all these cases there is a wonderful amount of activity and rapid movement, indicating a surplus of vitality, which is able to manifest itself in the development of these accessory plumes.[ ] in a quite distinct set of birds, the gallinaceae, we find the ornamental plumage usually arising from very different parts, in the form of elongated tail-feathers or tail-coverts, and of ruffs or hackles from the neck. here the wings are comparatively little used, the most constant activities depending on the legs, since the gallinaceae are pre-eminently walking, running, and scratching birds. now the magnificent train of the peacock--the grandest development of accessory plumes in this order--springs from an oval or circular area, about three inches in diameter, just above the base of the tail, and, therefore, situated over the lower part of the spinal column near the insertion of the powerful muscles which move the hind limbs and elevate the tail. the very frequent presence of neck-ruffs or breast-shields in the males of birds with accessory plumes may be partly due to selection, because they must serve as a protection in their mutual combats, just as does the lion's or the horse's mane. the enormously lengthened plumes of the bird of paradise and of the peacock can, however, have no such use, but must be rather injurious than beneficial in the bird's ordinary life. the fact that they have been developed to so great an extent in a few species is an indication of such perfect adaptation to the conditions of existence, such complete success in the battle for life, that there is, in the adult male at all events, a surplus of strength, vitality, and growth-power which is able to expend itself in this way without injury. that such is the case is shown by the great abundance of most of the species which possess these wonderful superfluities of plumage. birds of paradise are among the commonest birds in new guinea, and their loud voices can be often heard when the birds themselves are invisible in the depths of the forest; while indian sportsmen have described the peafowl as being so abundant, that from twelve to fifteen hundred have been seen within an hour at one spot; and they range over the whole country from the himalayas to ceylon. why, in allied species, the development of accessory plumes has taken different forms, we are unable to say, except that it may be due to that individual variability which has served as the starting-point for so much of what seems to us strange in form, or fantastic in colour, both in the animal and vegetable world. _development of accessory plumes and their display._ if we have found a _vera causa_ for the origin of ornamental appendages of birds and other animals in a surplus of vital energy, leading to abnormal growths in those parts of the integument where muscular and nervous action are greatest, the continuous development of these appendages will result from the ordinary action of natural selection in preserving the most healthy and vigorous individuals, and the still further selective agency of sexual struggle in giving to the very strongest and most energetic the parentage of the next generation. and, as all the evidence goes to show that, so far as female birds exercise any choice, it is of "the most vigorous, defiant, and mettlesome male," this form of sexual selection will act in the same direction, and help to carry on the process of plume development to its culmination. that culmination will be reached when the excessive length or abundance of the plumes begins to be injurious to the bearer of them; and it may be this check to the further lengthening of the peacock's train that has led to the broadening of the feathers at the ends, and the consequent production of the magnificent eye-spots which now form its crowning ornament. the display of these plumes will result from the same causes which led to their production. just in proportion as the feathers themselves increased in length and abundance, the skin-muscles which serve to elevate them would increase also; and the nervous development as well as the supply of blood to these parts being at a maximum, the erection of the plumes would become a habit at all periods of nervous or sexual excitement. the display of the plumes, like the existence of the plumes themselves, would be the chief external indication of the maturity and vigour of the male, and would, therefore, be necessarily attractive to the female. we have, thus, no reason for imputing to her any of those aesthetic emotions which are excited in us, by the beauty of form, colour, and pattern of these plumes; or the still more improbable aesthetic tastes, which would cause her to choose her mate on account of minute differences in their forms, colours, or patterns. as co-operating causes in the production of accessory ornamental plumes, i have elsewhere suggested[ ] that crests and other erectile feathers may have been useful in making the bird more formidable in appearance, and thus serving to frighten away enemies; while long tail or wing feathers might serve to distract the aim of a bird of prey. but though this might be of some use in the earlier stages of their development, it is probably of little importance compared with the vigour and pugnacity of which the plumes are the indication, and which enable most of their possessors to defend themselves against the enemies which are dangerous to weaker and more timid birds. even the tiny humming-birds are said to attack birds of prey that approach too near to their nests. _the effect of female preference will be neutralised by natural selection._ the various facts and arguments now briefly set forth, afford an explanation of the phenomena of male ornament, as being due to the general laws of growth and development, and make it unnecessary to call to our aid so hypothetical a cause as the cumulative action of female preference. there remains, however, a general argument, arising from the action of natural selection itself, which renders it almost inconceivable that female preference could have been effective in the way suggested; while the same argument strongly supports the view here set forth. natural selection, as we have seen in our earlier chapters, acts perpetually and on an enormous scale in weeding out the "unfit" at every stage of existence, and preserving only those which are in all respects the very best. each year, only a small percentage of young birds survive to take the place of the old birds which die; and the survivors will be those which are best able to maintain existence from the egg onwards, an important factor being that their parents should be well able to feed and protect them, while they themselves must in turn be equally able to feed and protect their own offspring. now this extremely rigid action of natural selection must render any attempt to select mere ornament utterly nugatory, unless the most ornamented always coincide with "the fittest" in every other respect; while, if they do so coincide, then any selection of ornament is altogether superfluous. if the most brightly coloured and fullest plumaged males are _not_ the most healthy and vigorous, have _not_ the best instincts for the proper construction and concealment of the nest, and for the care and protection of the young, they are certainly not the fittest, and will not survive, or be the parents of survivors. if, on the other hand, there _is_ generally this correlation--if, as has been here argued, ornament is the natural product and direct outcome of superabundant health and vigour, then no other mode of selection is needed to account for the presence of such ornament. the action of natural selection does not indeed disprove the existence of female selection of ornament as ornament, but it renders it entirely ineffective; and as the direct evidence for any such female selection is almost _nil_, while the objections to it are certainly weighty, there can be no longer any reason for upholding a theory which was provisionally useful in calling attention to a most curious and suggestive body of facts, but which is now no longer tenable. the term "sexual selection" must, therefore, be restricted to the direct results of male struggle and combat. this is really a form of natural selection, and is a matter of direct observation; while its results are as clearly deducible as those of any of the other modes in which selection acts. and if this restriction of the term is needful in the case of the higher animals it is much more so with the lower. in butterflies the weeding out by natural selection takes place to an enormous extent in the egg, larva, and pupa states; and perhaps not more than one in a hundred of the eggs laid produces a perfect insect which lives to breed. here, then, the impotence of female selection, if it exist, must be complete; for, unless the most brilliantly coloured males are those which produce the best protected eggs, larvae, and pupae, and unless the particular eggs, larvae, and pupae, which are able to survive, are those which produce the most brilliantly coloured butterflies, any choice the female might make must be completely swamped. if, on the other hand, there _is_ this correlation between colour development and perfect adaptation to conditions in all stages, then this development will necessarily proceed by the agency of natural selection and the general laws which determine the production of colour and of ornamental appendages.[ ] _general laws of animal coloration._ the condensed account which has now been given of the phenomena of colour in the animal world will sufficiently show the wonderful complexity and extreme interest of the subject; while it affords an admirable illustration of the importance of the great principle of utility, and of the effect of the theories of natural selection and development in giving a new interest to the most familiar facts of nature. much yet remains to be done, both in the observation of new facts as to the relations between the colours of animals and their habits or economy, and, more especially, in the elucidation of the laws of growth which determine changes of colour in the various groups; but so much is already known that we are able, with some confidence, to formulate the general principles which have brought about all the beauty and variety of colour which everywhere delight us in our contemplation of animated nature. a brief statement of these principles will fitly conclude our exposition of the subject. . colour may be looked upon as a necessary result of the highly complex chemical constitution of animal tissues and fluids. the blood, the bile, the bones, the fat, and other tissues have characteristic, and often brilliant colours, which we cannot suppose to have been determined for any special purpose, as colours, since they are usually concealed. the external organs, with their various appendages and integuments, would, by the same general laws, naturally give rise to a greater variety of colour. . we find it to be the fact that colour increases in variety and intensity as external structures and dermal appendages become more differentiated and developed. it is on scales, hair, and especially on the more highly specialised feathers, that colour is most varied and beautiful; while among insects colour is most fully developed in those whose wing membranes are most expanded, and, as in the lepidoptera, are clothed with highly specialised scales. here, too, we find an additional mode of colour production in transparent lamellae or in fine surface striae which, by the laws of interference, produce the wonderful metallic hues of so many birds and insects. . there are indications of a progressive change of colour, perhaps in some definite order, accompanying the development of tissues or appendages. thus spots spread and fuse into bands, and when a lateral or centrifugal expansion has occurred--as in the termination of the peacocks' train feathers, the outer web of the secondary quills of the argus pheasant, or the broad and rounded wings of many butterflies--into variously shaded or coloured ocelli. the fact that we find gradations of colour in many of the more extensive groups, from comparatively dull or simple to brilliant and varied hues, is an indication of some such law of development, due probably to progressive local segregation in the tissues of identical chemical or organic molecules, and dependent on laws of growth yet to be investigated. . the colours thus produced, and subject to much individual variation, have been modified in innumerable ways for the benefit of each species. the most general modification has been in such directions as to favour concealment when at rest in the usual surroundings of the species, sometimes carried on by successive steps till it has resulted in the most minute imitation of some inanimate object or exact mimicry of some other animal. in other cases bright colours or striking contrasts have been preserved, to serve as a warning of inedibility or of dangerous powers of attack. most frequent of all has been the specialisation of each distinct form by some tint or marking for purposes of easy recognition, especially in the case of gregarious animals whose safety largely depends upon association and mutual defence. . as a general rule the colours of the two sexes are alike; but in the higher animals there appears a tendency to deeper or more intense colouring in the male, due probably to his greater vigour and excitability. in many groups in which this superabundant vitality is at a maximum, the development of dermal appendages and brilliant colours has gone on increasing till it has resulted in a great diversity between the sexes; and in most of these cases there is evidence to show that natural selection has caused the female to retain the primitive and more sober colours of the group for purposes of protection. _concluding remarks._ the general principles of colour development now sketched out enable us to give some rational explanation of the wonderful amount of brilliant colour which occurs among tropical animals. looking on colour as a normal product of organisation, which has either been allowed free play, or has been checked and modified for the benefit of the species, we can see at once that the luxuriant and perennial vegetation of the tropics, by affording much more constant means of concealment, has rendered brilliant colour less hurtful there than in the temperate and colder regions. again, this perennial vegetation supplies abundance of both vegetable and insect food throughout the year, and thus a greater abundance and greater variety of the forms of life are rendered possible, than where recurrent seasons of cold and scarcity reduce the possibilities of life to a minimum. geology furnishes us with another reason, in the fact, that throughout the tertiary period tropical conditions prevailed far into the temperate regions, so that the possibilities of colour development were still greater than they are at the present time. the tropics, therefore, present to us the results of animal development in a much larger area and under more favourable conditions than prevail to-day. we see in them samples of the productions of an earlier and a better world, from an animal point of view; and this probably gives a greater variety and a finer display of colour than would have been produced, had conditions always been what they are now. the temperate zones, on the other hand, have recently suffered the effects of a glacial period of extreme severity, with the result that almost the only gay coloured birds they now possess are summer visitors from tropical or sub-tropical lands. it is to the unbroken and almost unchecked course of development from remote geological times that has prevailed in the tropics, favoured by abundant food and perennial shelter, that we owe such superb developments as the frills and crests and jewelled shields of the humming-birds, the golden plumes of the birds of paradise, and the resplendent train of the peacock. this last exhibits to us the culmination of that marvel and mystery of animal colour which is so well expressed by a poet-artist in the following lines. the marvel will ever remain to the sympathetic student of nature, but i venture to hope that in the preceding chapters i have succeeded in lifting--if only by one of its corners--the veil of mystery which has for long shrouded this department of nature. _on a peacock's feather._ in nature's workshop but a shaving, of her poem but a word, but a tint brushed from her palette, this feather of a bird! yet set it in the sun glance, display it in the shine, take graver's lens, explore it, note filament and line, mark amethyst to sapphire, and sapphire to gold, and gold to emerald changing the archetype unfold! tone, tint, thread, tissue, texture, through every atom scan, conforming still, developing, obedient to plan. this but to form a pattern on the garment of a bird! what then must be the poem, this but its lightest word! sit before it; ponder o'er it, 'twill thy mind advantage more, than a treatise, than a sermon, than a library of lore. footnotes: [footnote : darwin's _descent of man_, p. .] [footnote : darwin's _descent of man_, p. , and footnote.] [footnote : _nature_, , p. .] [footnote : darwin in _nature_, , p. .] [footnote : see the author's _contributions to natural selection_, chap. vii. in which these facts were first brought forward.] [footnote : on this point see the author's _contributions to natural selection_, chap. v. i.] [footnote : seebohm's _history of british birds_, vol. ii., introduction, p. xiii.] [footnote : for details see darwin's _descent of man_, chap. xii.] [footnote : _descent of man_, pp. , , .] [footnote : _notes of a naturalist on the challenger._] [footnote : _descent of man_, pp. , .] [footnote : _coloration in animals and plants_, london, .] [footnote : _coloration of animals_, pl. x, p. ; and pls. ii, iii, and iv, pp. , , .] [footnote : see coloured fig. in _proc. zool. soc._, , p. .] [footnote : a. tylor's _coloration_, p. ; and photograph in hutchinson's _illustrations of clinical surgery_, quoted by tylor.] [footnote : for activity and pugnacity of humming-birds, see _tropical nature_, pp. , .] [footnote : _tropical nature_, p. . in chapter v of this work the views here advocated were first set forth, and the reader is referred there for further details.] [footnote : the rev. o. pickard-cambridge, who has devoted himself to the study of spiders, has kindly sent me the following extract from a letter, written in , in which he states his views on this question:-- "i myself doubt that particular application of the darwinian theory which attributes male peculiarities of form, structure, colour, and ornament to female appetency or predilection. there is, it seems to me, undoubtedly something in the male organisation of a special, and sexual nature, which, of its own vital force, develops the remarkable male peculiarities so commonly seen, and of no imaginable use to that sex. in as far as these peculiarities show a great vital power, they point out to us the finest and strongest individuals of the sex, and show us which of them would most certainly appropriate to themselves the best and greatest number of females, and leave behind them the strongest and greatest number of progeny. and here would come in, as it appears to me, the proper application of darwin's theory of natural selection; for the possessors of greatest vital power being those most frequently produced and reproduced, the external signs of it would go on developing in an ever-increasing exaggeration, only to be checked where it became really detrimental in some respect or other to the individual." this passage, giving the independent views of a close observer--one, moreover, who has studied the species of an extensive group of animals both in the field and in the laboratory--very nearly accords with my own conclusions above given; and, so far as the matured opinions of a competent naturalist have any weight, afford them an important support.] chapter xi the special colours of plants: their origin and purpose the general colour relations of plants--colours of fruits--the meaning of nuts--edible or attractive fruits--the colours of flowers--modes of securing cross-fertilisation--the interpretation of the facts--summary of additional facts bearing on insect fertilisation--fertilisation of flowers by birds--self-fertilisation of flowers--difficulties and contradictions--intercrossing not necessarily advantageous--supposed evil results of close interbreeding--how the struggle for existence acts among flowers--flowers the product of insect agency--concluding remarks on colour in nature. the colours of plants are both less definite and less complex than are those of animals, and their interpretation on the principle of utility is, on the whole, more direct and more easy. yet here, too, we find that in our investigation of the uses of the various colours of fruits and flowers, we are introduced to some of the most obscure recesses of nature's workshop, and are confronted with problems of the deepest interest and of the utmost complexity. so much has been written on this interesting subject since mr. darwin first called attention to it, and its main facts have become so generally known by means of lectures, articles, and popular books, that i shall give here a mere outline sketch, for the purpose of leading up to a discussion of some of the more fundamental problems which arise out of the facts, and which have hitherto received less attention than they deserve. _the general colour relations of plants._ the green colour of the foliage of leafy plants is due to the existence of a substance called chlorophyll, which is almost universally developed in the leaves under the action of light. it is subject to definite chemical changes during the processes of growth and of decay, and it is owing to these changes that we have the delicate tints of spring foliage, and the more varied, intense, and gorgeous hues of autumn. but these all belong to the class of intrinsic or normal colours, due to the chemical constitution of the organism; as colours they are unadaptive, and appear to have no more relation to the wellbeing of the plants themselves than have the colours of gems and minerals. we may also include in the same category those algae and fungi which have bright colours--the "red snow" of the arctic regions, the red, green, or purple seaweeds, the brilliant scarlet, yellow, white, or black agarics, and other fungi. all these colours are probably the direct results of chemical composition or molecular structure, and, being thus normal products of the vegetable organism, need no special explanation from our present point of view; and the same remark will apply to the varied tints of the bark of trunks, branches, and twigs, which are often of various shades of brown and green, or even vivid reds or yellows. there are, however, a few cases in which the need of protection, which we have found to be so important an agency in modifying the colours of animals, has also determined those of some of the smaller members of the vegetable kingdom. dr. burchell found a mesembryanthomum in south africa like a curiously shaped pebble, closely resembling the stones among which it grew;[ ] and mr. j.p. mansel weale states that in the same country one of the asclepiadeae has tubers growing above ground among stones which they exactly resemble, and that, when not in leaf, they are for this reason quite invisible.[ ] it is clear that such resemblances must be highly useful to these plants, inhabiting an arid country abounding in herbivorous mammalia, which, in times of drought or scarcity, will devour everything in the shape of a fleshy stem or tuber. true mimicry is very rare in plants, though adaptation to like conditions often produces in foliage and habit a similarity that is deceiving. euphorbias growing in deserts often closely resemble cacti. seaside plants and high alpine plants of different orders are often much alike; and innumerable resemblances of this kind are recorded in the names of plants, as veronica epacridea (the veronica like an epacris), limnanthemum nymphaeoides (the limnanthemum like a nymphaea), the resembling species in each case belonging to totally distinct families. but in these cases, and in most others that have been observed, the essential features of true mimicry are absent, inasmuch as the one plant cannot be supposed to derive any benefit from its close resemblance to the other, and this is still more certain from the fact that the two species usually inhabit different localities. a few cases exist, however, in which there does seem to be the necessary accordance and utility. mr. mansel weale mentions a labiate plant (ajuga ophrydis), the only species of the genus ajuga in south africa, which is strikingly like an orchid of the same country; while a balsam (impatiens capensis), also a solitary species of the genus in that country, is equally like an orchid, growing in the same locality and visited by the same insects. as both these genera of plants are specialised for insect fertilisation, and both of the plants in question are isolated species of their respective genera, we may suppose that, when they first reached south africa they were neglected by the insects of the country; but, being both remotely like orchids in form of flower, those varieties that approached nearest to the familiar species of the country were visited by insects and cross-fertilised, and thus a closer resemblance would at length be brought about. another case of close general resemblance, is that of our common white dead-nettle (lamium album) to the stinging-nettle (urtica dioica); and sir john lubbock thinks that this is a case of true mimicry, the dead-nettle being benefited by being mistaken by grazing animals for the stinging-nettle.[ ] _colours of fruits._ it is when we come to the essential parts of plants on which their perpetuation and distribution depends, that we find colour largely utilised for a distinct purpose in flowers and fruits. in the former we find attractive colours and guiding marks to secure cross-fertilisation by insects; in the latter attractive or protective coloration, the first to attract birds or other animals when the fruits are intended to be eaten, the second to enable them to escape being eaten when it would be injurious to the species. the colour phenomena of fruits being much the most simple will be considered first. the perpetuation and therefore the very existence of each species of flowering plant depend upon its seeds being preserved from destruction and more or less effectually dispersed over a considerable area. the dispersal is effected either mechanically or by the agency of animals. mechanical dispersal is chiefly by means of air-currents, and large numbers of seeds are specially adapted to be so carried, either by being clothed with down or pappus, as in the well-known thistle and dandelion seeds; by having wings or other appendages, as in the sycamore, birch, and many other trees; by being thrown to a considerable distance by the splitting of the seed-vessel, and by many other curious devices.[ ] very large numbers of seeds, however, are so small and light that they can be carried enormous distances by gales of wind, more especially as most of this kind are flattened or curved, so as to expose a large surface in proportion to their weight. those which are carried by animals have their surfaces, or that of the seed-vessel, armed with minute hooks, or some prickly covering which attaches itself to the hair of mammalia or the feathers of birds, as in the burdock, cleavers, and many other species. others again are sticky, as in plumbago europaea, mistletoe, and many foreign plants. all the seeds or seed-vessels which are adapted to be dispersed in any of these ways are of dull protective tints, so that when they fall on the ground they are almost indistinguishable; besides which, they are usually small, hard, and altogether unattractive, never having any soft, juicy pulp; while the edible seeds often bear such a small proportion to the hard, dry envelopes or appendages, that few animals would care to eat them. _the meaning of nuts._ there is, however, another class of fruits or seeds, usually termed nuts, in which there is a large amount of edible matter, often very agreeable to the taste, and especially attractive and nourishing to a large number of animals. but when eaten, the seed is destroyed and the existence of the species endangered. it is evident, therefore, that it is by a kind of accident that these nuts are eatable; and that they are not intended to be eaten is shown by the special care nature seems to have taken to conceal or to protect them. we see that all our common nuts are green when on the tree, so as not easily to be distinguished from the leaves; but when ripe they turn brown, so that when they fall on to the ground they are equally indistinguishable among the dead leaves and twigs, or on the brown earth. then they are almost always protected by hard coverings, as in hazel-nuts, which are concealed by the enlarged leafy involucre, and in the large tropical brazil-nuts and cocoa-nuts by such a hard and tough case as to be safe from almost every animal. others have an external bitter rind, as in the walnut; while in the chestnuts and beech-nuts two or three fruits are enclosed in a prickly involucre. notwithstanding all these precautions, nuts are largely devoured by mammalia and birds; but as they are chiefly the product of trees or shrubs of considerable longevity, and are generally produced in great profusion, the perpetuation of the species is not endangered. in some cases the devourers of nuts may aid in their dispersal, as they probably now and then swallow the seed whole, or not sufficiently crushed to prevent germination; while squirrels have been observed to bury nuts, many of which are forgotten and afterwards grow in places they could not have otherwise reached.[ ] nuts, especially the larger kinds which are so well protected by their hard, nearly globular cases, have their dispersal facilitated by rolling down hill, and more especially by floating in rivers and lakes, and thus reaching other localities. during the elevation of land areas this method would be very effective, as the new land would always be at a lower level than that already covered with vegetation, and therefore in the best position for being stocked with plants from it. the other modes of dispersal of seeds are so clearly adapted to their special wants, that we feel sure they must have been acquired by the process of variation and natural selection. the hooked and sticky seeds are always those of such herbaceous plants as are likely, from their size, to come in contact with the wool of sheep or the hair of cattle; while seeds of this kind never occur on forest trees, on aquatic plants, or even on very dwarf creepers or trailers. the winged seed-vessels or seeds, on the other hand, mostly belong to trees and to tall shrubs or climbers. we have, therefore, a very exact adaptation to conditions in these different modes of dispersal; while, when we come to consider individual cases, we find innumerable other adaptations, some of which the reader will find described in the little work by sir john lubbock already referred to. _edible or attractive fruits._ it is, however, when we come to true fruits (in a popular sense) that we find varied colours evidently intended to attract animals, in order that the fruits may be eaten, while the seeds pass through the body undigested and are then in the fittest state for germination. this end has been gained in a great variety of ways, and with so many corresponding adaptations as to leave no doubt as to the value of the result. fruits are pulpy or juicy, and usually sweet, and form the favourite food of innumerable birds and some mammals. they are always coloured so as to contrast with the foliage or surroundings, red being the most common as it is certainly the most conspicuous colour, but yellow, purple, black, or white being not uncommon. the edible portion of fruits is developed from different parts of the floral envelopes, or of the ovary, in the various orders and genera. sometimes the calyx becomes enlarged and fleshy, as in the apple and pear tribe; more often the integuments of the ovary itself are enlarged, as in the plum, peach, grape, etc.; the receptacle is enlarged and forms the fruit of the strawberry; while the mulberry, pineapple, and fig are examples of compound fruits formed in various ways from a dense mass of flowers. in all cases the seeds themselves are protected from injury by various devices. they are small and hard in the strawberry, raspberry, currant, etc., and are readily swallowed among the copious pulp. in the grape they are hard and bitter; in the rose (hip) disagreeably hairy; in the orange tribe very bitter; and all these have a smooth, glutinous exterior which facilitates their being swallowed. when the seeds are larger and are eatable, they are enclosed in an excessively hard and thick covering, as in the various kinds of "stone" fruit (plums, peaches, etc.), or in a very tough core, as in the apple. in the nutmeg of the eastern archipelago we have a curious adaptation to a single group of birds. the fruit is yellow, somewhat like an oval peach, but firm and hardly eatable. this splits open and shows the glossy black covering of the seed or nutmeg, over which spreads the bright scarlet arillus or "mace," an adventitious growth of no use to the plant except to attract attention. large fruit pigeons pluck out this seed and swallow it entire for the sake of the mace, while the large nutmeg passes through their bodies and germinates; and this has led to the wide distribution of wild nutmegs over new guinea and the surrounding islands. in the restriction of bright colour to those edible fruits the eating of which is beneficial to the plant, we see the undoubted result of natural selection; and this is the more evident when we find that the colour never appears till the fruit is ripe--that is, till the seeds within it are fully matured and in the best state for germination. some brilliantly coloured fruits are poisonous, as in our bitter-sweet (solanum dulcamara), cuckoo-pint (arum) and the west indian manchineel. many of these are, no doubt, eaten by animals to whom they are harmless; and it has been suggested that even if some animals are poisoned by them the plant is benefited, since it not only gets dispersed, but finds, in the decaying body of its victim, a rich manure heap.[ ] the particular colours of fruits are not, so far as we know, of any use to them other than as regards conspicuousness, hence a tendency to _any_ decided colour has been preserved and accumulated as serving to render the fruit easily visible among its surroundings of leaves or herbage. out of fruit-bearing plants in mongredien's _trees and shrubs_, and hooker's _british flora_, the fruits of no less than sixty-eight, or rather more than half, are red, forty-five are black, fourteen yellow, and seven white. the great prevalence of red fruits is almost certainly due to their greater conspicuousness having favoured their dispersal, though it may also have arisen in part from the chemical changes of chlorophyll during ripening and decay producing red tints as in many fading leaves. yet the comparative scarcity of yellow in fruits, while it is the most common tint of fading leaves, is against this supposition. there are, however, a few instances of coloured fruits which do not seem to be intended to be eaten; such are the colocynth plant (cucumis colocynthus), which has a beautiful fruit the size and colour of an orange, but nauseous beyond description to the taste. it has a hard rind, and may perhaps be dispersed by being blown along the ground, the colour being an adventitious product; but it is quite possible, notwithstanding its repulsiveness to us, that it may be eaten by some animals. with regard to the fruit of another plant, calotropis procera, there is less doubt, as it is dry and full of thin, flat-winged seeds, with fine silky filaments, eminently adapted for wind-dispersal; yet it is of a bright yellow colour, as large as an apple, and therefore very conspicuous. here, therefore, we seem to have colour which is a mere byproduct of the organism and of no use to it; but such cases are exceedingly rare, and this rarity, when compared with the great abundance of cases in which there is an obvious purpose in the colour, adds weight to the evidence in favour of the theory of the attractive coloration of edible fruits in order that birds and other animals may assist in their dispersal. both the above-named plants are natives of palestine and the adjacent arid countries.[ ] _the colours of flowers._ flowers are much more varied in their colours than fruits, as they are more complex and more varied in form and structure; yet there is some parallelism between them in both respects. flowers are frequently adapted to attract insects as fruits are to attract birds, the object being in the former to secure cross-fertilisation, in the latter dispersal; while just as colour is an index of the edibility of fruits which supply pulp or juice to birds, so are the colours of flowers an indication of the presence of nectar or of pollen which are devoured by insects. the main facts and many of the details, as to the relation of insects to flowers, were discovered by sprengel in . he noticed the curious adaptation of the structure of many flowers to the particular insects which visit them; he proved that insects do cross-fertilise flowers, and he believed that this was the object of the adaptations, while the presence of nectar and pollen ensured the continuance of their visits; yet he missed discovering the _use_ of this cross-fertilisation. several writers at a later period obtained evidence that cross-fertilisation of plants was a benefit to them; but the wide generality of this fact and its intimate connection with the numerous and curious adaptations discovered by sprengel, was first shown by mr. darwin, and has since been demonstrated by a vast mass of observations, foremost among which are his own researches on orchids, primulas, and other plants.[ ] by an elaborate series of experiments carried on for many years mr. darwin demonstrated the great value of cross-fertilisation in increasing the rapidity of growth, the strength and vigour of the plant, and in adding to its fertility. this effect is produced immediately, not as he expected would be the case, after several generations of crosses. he planted seeds from cross-fertilised and self-fertilised plants on two sides of the same pot exposed to exactly similar conditions, and in most cases the difference in size and vigour was amazing, while the plants from cross-fertilised parents also produced more and finer seeds. these experiments entirely confirmed the experience of breeders of animals already referred to (p. ), and led him to enunciate his famous aphorism, "nature abhors perpetual self-fertilisation".[ ] in this principle we appear to have a sufficient reason for the various contrivances by which so many flowers secure cross-fertilisation, either constantly or occasionally. these contrivances are so numerous, so varied, and often so highly complex and extraordinary, that they have formed the subject of many elaborate treatises, and have also been amply popularised in lectures and handbooks. it will be unnecessary, therefore, to give details here, but the main facts will be summarised in order to call attention to some difficulties of the theory which seem to require further elucidation. _modes of securing cross-fertilisation._ when we examine the various modes in which the cross-fertilisation of flowers is brought about, we find that some are comparatively simple in their operation and needful adjustments, others highly complex. the simple methods belong to four principal classes:--( ) by dichogamy--that is, by the anthers and the stigma becoming mature or in a fit state for fertilisation at slightly different times on the same plant. the result of this is that, as plants in different stations, on different soils, or exposed to different aspects flower earlier or later, the mature pollen of one plant can only fertilise some plant exposed to somewhat different conditions or of different constitution, whose stigma will be mature at the same time; and this difference has been shown by darwin to be that which is adapted to secure the fullest benefit of cross-fertilisation. this occurs in geranium pratense, thymus serpyllum, arum maculatum, and many others. ( ) by the flower being self-sterile with its own pollen, as in the crimson flax. this absolutely prevents self-fertilisation. ( ) by the stamens and anthers being so placed that the pollen cannot fall upon the stigma, while it does fall upon a visiting insect which carries it to the stigma of another flower. this effect is produced in a variety of very simple ways, and is often aided by the motion of the stamens which bend down out of the way of the stigmas before the pollen is ripe, as in malva sylvestris (see fig. ). ( ) by the male and female flowers being on different plants, forming the class dioecia of linnaeus. in these cases the pollen may be carried to the stigmas either by the wind or by the agency of insects. [illustration: fig. . malva sylvestris, adapted for insect-fertilisation. malva rotundifolia, adapted for self-fertilisation.] now these four methods are all apparently very simple, and easily produced by variation and selection. they are applicable to flowers of any shape, requiring only such size and colour as to attract insects, and some secretion of nectar to ensure their repeated visits, characters common to the great majority of flowers. all these methods are common, except perhaps the second; but there are many flowers in which the pollen from another plant is prepotent over the pollen from fertilisation, the same flower, and this has nearly the same effect as self-sterility if the flowers are frequently crossed by insects. we cannot help asking, therefore, why have other and much more elaborate methods been needed? and how have the more complex arrangements of so many flowers been brought about? before attempting to answer these questions, and in order that the reader may appreciate the difficulty of the problem and the nature of the facts to be explained, it will be necessary to give a summary of the more elaborate modes of securing cross-fertilisation. ( ) we first have dimorphism and heteromorphism, the phenomena of which have been already sketched in our seventh chapter. here we have both a mechanical and a physiological modification, the stamens and pistil being variously modified in length and position, while the different stamens in the same flower have widely different degrees of fertility when applied to the same stigma,--a phenomenon which, if it were not so well established, would have appeared in the highest degree improbable. the most remarkable case is that of the three different forms of the loosestrife (lythrum salicaria) here figured (fig. on next page). ( ) some flowers have irritable stamens which, when their bases are touched by an insect, spring up and dust it with pollen. this occurs in our common berberry. [illustration: fig. .--lythrum salicaria (purple loosestrife).] ( ) in others there are levers or processes by which the anthers are mechanically brought down on to the head or back of an insect entering the flower, in such a position as to be carried to the stigma of the next flower it visits. this may be well seen in many species of salvia and erica. ( ) in some there is a sticky secretion which, getting on to the proboscis of an insect, carries away the pollen, and applies it to the stigma of another flower. this occurs in our common milkwort (polygala vulgaris). ( ) in papilionaceous plants there are many complex adjustments, such as the squeezing out of pollen from a receptacle on to an insect, as in lotus corniculatus, or the sudden springing out and exploding of the anthers so as thoroughly to dust the insect, as in medicago falcata, this occurring after the stigma has touched the insect and taken off some pollen from the last flower. ( ) some flowers or spathes form closed boxes in which insects find themselves entrapped, and when they have fertilised the flower, the fringe of hairs opens and allows them to escape. this occurs in many species of arum and aristolochia. ( ) still more remarkable are the traps in the flower of asclepias which catch flies, butterflies, and wasps by the legs, and the wonderfully complex arrangements of the orchids. one of these, our common orchis pyramidalis, may be briefly described to show how varied and beautiful are the arrangements to secure cross-fertilisation. the broad trifid lip of the flower offers a support to the moth which is attracted by its sweet odour, and two ridges at the base guide the proboscis with certainty to the narrow entrance of the nectary. when the proboscis has reached the end of the spur, its basal portion depresses the little hinged rostellum that covers the saddle-shaped sticky glands to which the pollen masses (pollinia) are attached. on the proboscis being withdrawn, the two pollinia stand erect and parallel, firmly attached to the proboscis. in this position, however, they would be useless, as they would miss the stigmatic surface of the next flower visited by the moth. but as soon as the proboscis is withdrawn, the two pollen masses begin to diverge till they are exactly as far apart as are the stigmas of the flower; and then commences a second movement which brings them down till they project straight forward nearly at right angles to their first position, so as exactly to hit against the stigmatic surfaces of the next flower visited on which they leave a portion of their pollen. the whole of these motions take about half a minute, and in that time the moth will usually have flown to another plant, and thus effect the most beneficial kind of cross-fertilisation.[ ] this description will be better understood by referring to the illustration opposite, from darwin's _fertilisation of orchids_(fig. ). [illustration: fig. .--orchis pyramidalis.] _the interpretation of these facts._ having thus briefly indicated the general character of the more complex adaptations for cross-fertilisation, the details of which are to be found in any of the numerous works on the subject,[ ] we find ourselves confronted with the very puzzling question--why were these innumerable highly complex adaptations produced, when the very same result may be effected--and often is effected--by extremely simple means? supposing, as we must do, that all flowers were once of simple and regular forms, like a buttercup or a rose, how did such irregular and often complicated flowers as the papilionaceous or pea family, the labiates or sage family, and the infinitely varied and fantastic orchids ever come into existence? no cause has yet been suggested but the need of attracting insects to cross-fertilise them; yet the attractiveness of regular flowers with bright colours and an ample supply of nectar is equally great, and cross-fertilisation can be quite as effectively secured in these by any of the four simple methods already described. before attempting to suggest a possible solution of this difficult problem, we have yet to pass in review a large body of curious adaptations connected with insect fertilisation, and will first call attention to that portion of the phenomena which throw some light upon the special colours of flowers in their relation to the various kinds of insects which visit them. for these facts we are largely indebted to the exact and long-continued researches of professor hermann müller. _summary of additional facts bearing on insect fertilisation._ . that the size and colour of a flower are important factors in determining the visits of insects, is shown by the general fact of more insects visiting conspicuous than inconspicuous flowers. as a single instance, the handsome geranium palustre was observed by professor müller to be visited by sixteen different species of insects, the equally showy g. pratense by thirteen species, while the smaller and much less conspicuous g. molle was visited by eight species, and g. pusillum by only one. in many cases, however, a flower may be very attractive to only a few species of insects; and professor müller states, as the result of many years' assiduous observation, that "a species of flower is the more visited by insects the more conspicuous it is." . sweet odour is usually supplementary to the attraction of colour. thus it is rarely present in the largest and most gaudily coloured flowers which inhabit open places, such as poppies, paeonies, sunflowers, and many others; while it is often the accompaniment of inconspicuous flowers, as the mignonette; of such as grow in shady places, as the violet and primrose; and especially of white or yellowish flowers, as the white jasmine, clematis, stephanotis, etc. . white flowers are often fertilised by moths, and very frequently give out their scent only by night, as in our butterfly-orchis (habenaria chlorantha); and they sometimes open only at night, as do many of the evening primroses and other flowers. these flowers are often long tubed in accordance with the length of the moths' probosces, as in the genus pancratium, our butterfly orchis, white jasmine, and a host of others. . bright red flowers are very attractive to butterflies, and are sometimes specially adapted to be fertilised by them, as in many pinks (dianthus deltoides, d. superbus, d. atrorubens), the corn-cockle (lychnis githago), and many others. blue flowers are especially attractive to bees and other hymenoptera (though they frequent flowers of all colours), no less than sixty-seven species of this order having been observed to visit the common "sheep's-bit" (jasione montana). dull yellow or brownish flowers, some of which smell like carrion, are attractive to flies, as the arum and aristolochia; while the dull purplish flowers of the scrophularia are specially attractive to wasps. . some flowers have neither scent nor nectar, and yet attract insects by sham nectaries! in the herb-paris (paris quadrifolia) the ovary glistens as if moist, and flies alight on it and carry away pollen to another flower; while in grass of parnassus (parnassia palustris) there are a number of small stalked yellow balls near the base of the flower, which look like drops of honey but are really dry. in this case there is a little nectar lower down, but the special attraction is a sham; and as there are fresh broods of insects every year, it takes time for them to learn by experience, and thus enough are always deceived to effect cross-fertilisation.[ ] this is analogous to the case of the young birds, which have to learn by experience the insects that are inedible, as explained at page . . many flowers change their colour as soon as fertilised; and this is beneficial, as it enables bees to avoid wasting time in visiting those blossoms which have been already fertilised and their nectar exhausted. the common lungwort (pulmonaria officinalis), is at first red, but later turns blue; and h. müller observed bees visiting many red flowers in succession, but neglecting the blue. in south brazil there is a species of lantana, whose flowers are yellow the first day, orange the second, and purple the third; and dr. fritz müller observed that many butterflies visited the yellow flowers only, some both the yellow and the orange flowers, but none the purple. . many flowers have markings which serve as guides to insects; in some cases a bright central eye, as in the borage and forget-me-not; or lines or spots converging to the centre, as in geraniums, pinks, and many others. this enables insects to go quickly and directly to the opening of the flower, and is equally important in aiding them to obtain a better supply of food, and to fertilise a larger number of flowers. . flowers have been specially adapted to the kinds of insects that most abound where they grow. thus the gentians of the lowlands are adapted to bees, those of the high alps to butterflies only; and while most species of rhinanthus (a genus to which our common "yellow rattle" belongs) are bee-flowers, one high alpine species (r. alpinus) has been also adapted for fertilisation by butterflies only. the reason of this is, that in the high alps butterflies are immensely more plentiful than bees, and flowers adapted to be fertilised by bees can often have their nectar extracted by butterflies without effecting cross-fertilisation. it is, therefore, important to have a modification of structure which shall make butterflies the fertilisers, and this in many cases has been done.[ ] . economy of time is very important both to the insects and the flowers, because the fine working days are comparatively few, and if no time is wasted the bees will get more honey, and in doing so will fertilise more flowers. now, it has been ascertained by several observers that many insects, bees especially, keep to one kind of flower at a time, visiting hundreds of blossoms in succession, and passing over other species that may be mixed with them. they thus acquire quickness in going at once to the nectar, and the change of colour in the flower, or incipient withering when fertilised, enables them to avoid those flowers that have already had their honey exhausted. it is probably to assist the insects in keeping to one flower at a time, which is of vital importance to the perpetuation of the species, that the flowers which bloom intermingled at the same season are usually very distinct both in form and colour. in the sandy districts of surrey, in the early spring, the copses are gay with three flowers--the primrose, the wood-anemone, and the lesser celandine, forming a beautiful contrast, while at the same time the purple and the white dead-nettles abound on hedge banks. a little later, in the same copses, we have the blue wild hyacinth (scilla nutans), the red campion (lychnis dioica), the pure white great starwort (stellaria holosteum), and the yellow dead-nettle (lamium galeobdolon), all distinct and well-contrasted flowers. in damp meadows in summer we have the ragged robin (lychnis floscuculi), the spotted orchis (o. maculata), and the yellow rattle (rhinanthus crista-galli); while in drier meadows we have cowslips, ox-eye daisies, and buttercups, all very distinct both in form and colour. so in cornfields we have the scarlet poppies, the purple corn-cockle, the yellow corn-marygold, and the blue cornflower; while on our moors the purple heath and the dwarf gorse make a gorgeous contrast. thus the difference of colour which enables the insect to visit with rapidity and unerring aim a number of flowers of the same kind in succession, serves to adorn our meadows, banks, woods, and heaths with a charming variety of floral colour and form at each season of the year.[ ] _fertilisation of flowers by birds._ in the temperate regions of the northern hemisphere, insects are the chief agents in cross-fertilisation when this is not effected by the wind; but in warmer regions, and in the southern hemisphere, birds are found to take a considerable part in the operation, and have in many cases led to modifications in the form and colour of flowers. each part of the globe has special groups of birds which are flower-haunters. america has the humming-birds (trochilidae), and the smaller group of the sugar-birds (caerebidae). in the eastern tropics the sun-birds (nectarineidae) take the place of the humming-birds, and another small group, the flower-peckers (dicaeidae), assist them. in the australian region there are also two flower-feeding groups, the meliphagidae, or honey-suckers, and the brush-tongued lories (trichoglossidae). recent researches by american naturalists have shown that many flowers are fertilised by humming-birds, such as passion-flowers, trumpet-flowers, fuchsias, and lobelias; while some, as the salvia splendens of mexico, are specially adapted to their visits. we may thus perhaps explain the number of very large tubular flowers in the tropics, such as the huge brugmansias and bignonias; while in the andes and in chile, where humming-birds are especially plentiful, we find great numbers of red tubular flowers, often of large size and apparently adapted to these little creatures. such are the beautiful lapageria and philesia, the grand pitcairneas, and the genera fuchsia, mitraria, embothrium, escallonia, desfontainea, eccremocarpus, and many gesneraceae. among the most extraordinary modifications of flower structure adapted to bird fertilisation are the species of marcgravia, in which the pedicels and bracts of the terminal portion of a pendent bunch of flowers have been modified into pitchers which secrete nectar and attract insects, while birds feeding on the nectar, or insects, have the pollen of the overhanging flowers dusted on their backs, and, carrying it to other flowers, thus cross-fertilise them (see illustration). [illustration: fig. .--humming-bird fertilising marcgravia nepenthoides.] in australia and new zealand the fine "glory peas" (clianthus), the sophora, loranthus, many epacrideae and myrtaceae, and the large flowers of the new zealand flax (phormium tenax), are cross-fertilised by birds; while in natal the fine trumpet-creeper (tecoma capensis) is fertilised by nectarineas. the great extent to which insect and bird agency is necessary to flowers is well shown by the case of new zealand. the entire country is comparatively poor in species of insects, especially in bees and butterflies which are the chief flower fertilisers; yet according to the researches of local botanists no less than one-fourth of all the flowering plants are incapable of self-fertilisation, and, therefore, wholly dependent on insect or bird agency for the continuance of the species. the facts as to the cross-fertilisation of flowers which have now been very briefly summarised, taken in connection with darwin's experiments proving the increased vigour and fertility given by cross-fertilisation, seem amply to justify his aphorism that "nature abhors self-fertilisation," and his more precise statement, that, "no plant is perpetually self-fertilised;" and this view has been upheld by hildebrand, delpino, and other botanists.[ ] _self-fertilisation of flowers._ but all this time we have been only looking at one side of the question, for there exists an abundance of facts which seem to imply, just as surely, the utter uselessness of cross-fertilisation. let us, then, see what these facts are before proceeding further. . an immense variety of plants are habitually self-fertilised, and their numbers probably far exceed those which are habitually cross-fertilised by insects. almost all the very small or obscure flowered plants with hermaphrodite flowers are of this kind. most of these, however, may be insect fertilised occasionally, and may, therefore, come under the rule that no species are perpetually self-fertilised. . there are many plants, however, in which special arrangements exist to secure self-fertilisation. sometimes the corolla closes and brings the anthers and stigma into contact; in others the anthers cluster round the stigmas, both maturing together, as in many buttercups, stitchwort (stellaria media), sandwort (spergula), and some willow-herbs (epilobium); or they arch over the pistil, as in galium aparine and alisma plantago. the style is also modified to bring it into contact with the anthers, as in the dandelion, groundsel, and many other plants.[ ] all these, however, may be occasionally cross-fertilised. . in other cases precautions are taken to prevent cross-fertilisation, as in the numerous cleistogamous or closed flowers. these occur in no less than fifty-five different genera, belonging to twenty-four natural orders, and in thirty-two of these genera the normal flowers are irregular, and have therefore been specially modified for insect fertilisation.[ ] these flowers appear to be degradations of the normal flowers, and are closed up by various modifications of the petals or other parts, so that it is impossible for insects to reach the interior, yet they produce seed in abundance, and are often the chief means by which the species is continued. thus, in our common dog-violet the perfect flowers rarely produce seed, while the rudimentary cleistogamic flowers do so in abundance. the sweet violet also produces abundance of seed from its cleistogamic flowers, and few from its perfect flowers; but in liguria it produces only perfect flowers which seed abundantly. no case appears to be known of a plant which has cleistogamic flowers only, but a small rush (juncus bufonius) is in this condition in some parts of russia, while in other parts perfect flowers are also produced.[ ] our common henbit dead-nettle (lamium amplexicaule) produces cleistogamic flowers, as do also some orchids. the advantage gained by the plant is great economy of specialised material, since with very small flowers and very little expenditure of pollen an abundance of seed is produced. . a considerable number of plants which have evidently been specially modified for insect fertilisation have, by further modification, become quite self-fertile. this is the case with the garden-pea, and also with our beautiful bee-orchis, in which the pollen-masses constantly fall on to the stigmas, and the flower, being thus self-fertilised, produces abundance of capsules and of seed. yet in many of its close allies insect agency is absolutely required; but in one of these, the fly-orchis, comparatively very little seed is produced, and self-fertilisation would therefore be advantageous to it. when garden-peas were artificially cross-fertilised by mr. darwin, it seemed to do them no good, as the seeds from these crosses produced less vigorous plants than seed from those which were self-fertilised; a fact directly opposed to what usually occurs in cross-fertilised plants. . as opposed to the theory that there is any absolute need for cross-fertilisation, it has been urged by mr. henslow and others that many self-fertilised plants are exceptionally vigorous, such as groundsel, chickweed, sow-thistle, buttercups, and other common weeds; while most plants of world-wide distribution are self-fertilised, and these have proved themselves to be best fitted to survive in the battle of life. more than fifty species of common british plants are very widely distributed, and all are habitually self-fertilised.[ ] that self-fertilisation has some great advantage is shown by the fact that it is usually the species which have the smallest and least conspicuous flowers which have spread widely, while the large and showy flowered species of the same genera or families, which require insects to cross-fertilise them, have a much more limited distribution. . it is now believed by some botanists that many inconspicuous and imperfect flowers, including those that are wind-fertilised, such as plantains, nettles, sedges, and grasses, do not represent primitive or undeveloped forms, but are degradations from more perfect flowers which were once adapted to insect fertilisation. in almost every order we find some plants which have become thus reduced or degraded for wind or self-fertilisation, as poterium and sanguisorba among the rosaceae; while this has certainly been the case in the cleistogamic flowers. in most of the above-mentioned plants there are distinct rudiments of petals or other floral organs, and as the chief use of these is to attract insects, they could hardly have existed in primitive flowers.[ ] we know, moreover, that when the petals cease to be required for the attraction of insects, they rapidly diminish in size, lose their bright colour or almost wholly disappear.[ ] _difficulties and contradictions._ the very bare summary that has now been given of the main facts relating to the fertilisation of flowers, will have served to show the vast extent and complexity of the inquiry, and the extraordinary contradictions and difficulties which it presents. we have direct proof of the beneficial results of intercrossing in a great number of cases; we have an overwhelming mass of facts as to the varied and complex structure of flowers evidently adapted to secure this intercrossing by insect agency; yet we see many of the most vigorous plants which spread widely over the globe, with none of these adaptations, and evidently depending on self-fertilisation for their continued existence and success in the battle of life. yet more extraordinary is it to find numerous cases in which the special arrangements for cross-fertilisation appear to have been a failure, since they have either been supplemented by special means for self-fertilisation, or have reverted back in various degrees to simpler forms in which self-fertilisation becomes the rule. there is also a further difficulty in the highly complex modes by which cross-fertilisation is often brought about; for we have seen that there are several very effective yet very simple modes of securing intercrossing, involving a minimum of change in the form and structure of the flower; and when we consider that the result attained with so much cost of structural modification is by no means an unmixed good, and is far less certain in securing the perpetuation of the species than is self-fertilisation, it is most puzzling to find such complex methods resorted to, sometimes to the extent of special precautions against the possibility of self-fertilisation ever taking place. let us now see whether any light can be thrown on these various anomalies and contradictions. _intercrossing not necessarily advantageous._ no one was more fully impressed than mr. darwin with the beneficial effects of intercrossing on the vigour and fertility of the species or race, yet he clearly saw that it was not always and necessarily advantageous. he says: "the most important conclusion at which i have arrived is, that the mere act of intercrossing by itself does no good. the good depends on the individuals which are crossed differing slightly in constitution, owing to their progenitors having been subjected during several generations to slightly different conditions. this conclusion, as we shall hereafter see, is closely connected with various important physiological problems, such as the benefit derived from slight changes in the conditions of life."[ ] mr. darwin has also adduced much direct evidence proving that slight changes in the conditions of life are beneficial to both animals and plants, maintaining or restoring their vigour and fertility in the same way as a favourable cross seems to restore it.[ ] it is, i believe, by a careful consideration of these two classes of facts that we shall find the clue to the labyrinth in which this subject has appeared to involve us. _supposed evil results of close interbreeding._ just as we have seen that intercrossing is not necessarily good, we shall be forced to admit that close interbreeding is not necessarily bad. our finest breeds of domestic animals have been thus produced, and by a careful statistical inquiry mr. george darwin has shown that the most constant and long-continued intermarriages among the british aristocracy have produced no prejudicial results. the rabbits on porto santo are all the produce of a single female; they have lived on the same small island for years, and they still abound there and appear to be vigorous and healthy (see p. ). we have, however, on the other hand, overwhelming evidence that in many cases, among our domestic animals and cultivated plants, close interbreeding does produce bad results, and the apparent contradiction may perhaps be explained on the same general principles, and under similar limitations, as were found to be necessary in defining the value of intercrossing. it appears probable, then, that it is not interbreeding in itself that is hurtful, but interbreeding without rigid selection or some change of conditions. under nature, as in the case of the porto santo rabbits, the rapid increase of these animals would in a very few years stock the island with a full population, and thereafter natural selection would act powerfully in the preservation only of the healthiest and the most fertile, and under these conditions no deterioration would occur. among the aristocracy there has been a constant selection of beauty, which is generally synonymous with health, while any constitutional infertility has led to the extinction of the family. with domestic animals the selection practised is usually neither severe enough nor of the right kind. there is no natural struggle for existence, but certain points of form and colour characteristic of the breed are considered essential, and thus the most vigorous or the most fertile are not always those which are selected to continue the stock. in nature, too, the species always extends over a larger area and consists of much greater numbers, and thus a difference of constitution soon arises in different parts of the area, which is wanting in the limited numbers of pure bred domestic animals. from a consideration of these varied facts we conclude that an occasional disturbance of the organic equilibrium is what is essential to keep up the vigour and fertility of any organism, and that this disturbance may be equally well produced either by a cross between individuals of somewhat different constitutions, or by occasional slight changes in the conditions of life. now plants which have great powers of dispersal enjoy a constant change of conditions, and can, therefore, exist permanently, or at all events, for very long periods, without intercrossing; while those which have limited powers of dispersal, and are restricted to a comparatively small and uniform area, need an occasional cross to keep up their fertility and general vigour. we should, therefore, expect that those groups of plants which are adapted both for cross-and self-fertilisation, which have showy flowers and possess great powers of seed-dispersal, would be the most abundant and most widely distributed; and this we find to be the case, the compositae possessing all these characteristics in the highest degree, and being the most generally abundant group of plants with conspicuous flowers in all parts of the world. _how the struggle for existence acts among flowers._ let us now consider what will be the action of the struggle for existence under the conditions we have seen to exist. everywhere and at all times some species of plants will be dominant and aggressive; while others will be diminishing in numbers, reduced to occupy a smaller area, and generally having a hard struggle to maintain themselves. whenever a self-fertilising plant is thus reduced in numbers it will be in danger of extinction, because, being limited to a small area, it will suffer from the effects of too uniform conditions which will produce weakness and infertility. but while this change is in progress, any crosses between individuals of slightly different constitution will be beneficial, and all variations favouring either insect agency on the one hand, or wind-dispersal of pollen on the other, will lead to the production of a somewhat stronger and more fertile stock. increased size or greater brilliancy of the flower, more abundant nectar, sweeter odour, or adaptations for more effectual cross-fertilisation would all be preserved, and thus would be initiated some form of specialisation for insect agency in cross-fertilisation; and in every different species so circumstanced the result would be different, depending as it would on many and complex combinations of variation of parts of the flower, and of the insect species which most abounded in the district. species thus favourably modified might begin a new era of development, and, while spreading over a somewhat wider area, give rise to new varieties or species, all adapted in various degrees and modes to secure cross-fertilisation by insect agency. but in course of ages some change of conditions might prove adverse. either the insects required might diminish in numbers or be attracted by other competing flowers, or a change of climate might give the advantage to other more vigorous plants. then self-fertilisation with greater means of dispersal might be more advantageous; the flowers might become smaller and more numerous; the seeds smaller and lighter so as to be more easily dispersed by the wind, while some of the special adaptations for insect fertilisation being useless would, by the absence of selection and by the law of economy of growth, be reduced to a rudimentary form. with these modifications the species might extend its range into new districts, thereby obtaining increased vigour by the change of conditions, as appears to have been the case with so many of the small flowered self-fertilised plants. thus it might continue to exist for a long series of ages, till under other changes--geographical or biological--it might again suffer from competition or from other adverse circumstances, and be at length again confined to a limited area, or reduced to very scanty numbers. but when this cycle of change had taken place, the species would be very different from the original form. the flower would have been at one time modified to favour the visits of insects and to secure cross-fertilisation by their aid, and when the need for this passed away, some portions of these structures would remain, though in a reduced or rudimentary condition. but when insect agency became of importance a second time, the new modifications would start from a different or more advanced basis, and thus a more complex result might be produced. owing to the unequal rates at which the reduction of the various parts might occur, some amount of irregularity in the flower might arise, and on a second development towards insect cross-fertilisation this irregularity, if useful, might be increased by variation and selection. the rapidity and comparative certainty with which such changes as are here supposed do really take place, are well shown by the great differences in floral structure, as regards the mode of fertilisation, in allied genera and species, and even in some cases in varieties of the same species. thus in the ranunculaceae we find the conspicuous part of the flower to be the petals in ranunculus, the sepals in helleborus, anemone, etc., and the stamens in most species of thalictrum. in all these we have a simple regular flower, but in aquilegia it is made complex by the spurred petals, and in delphinium and aconitum it becomes quite irregular. in the more simple class self-fertilisation occurs freely, but it is prevented in the more complex flowers by the stamens maturing before the pistil. in the caprifoliaceae we have small and regular greenish flowers, as in the moschatel (adoxa); more conspicuous regular open flowers without honey, as in the elder (sambucus); and tubular flowers increasing in length and irregularity, till in some, like our common honeysuckle, they are adapted for fertilisation by moths only, with abundant honey and delicious perfume to attract them. in the scrophulariaceae we find open, almost regular flowers, as veronica and verbascum, fertilised by flies and bees, but also self-fertilised; scrophularia adapted in form and colour to be fertilised by wasps; and the more complex and irregular flowers of linaria, rhinanthus, melampyrum, pedicularis, etc., mostly adapted to be fertilised by bees. in the genera geranium, polygonum, veronica, and several others there is a gradation of forms from large and bright to small and obscure coloured flowers, and in every case the former are adapted for insect fertilisation, often exclusively, while in the latter self-fertilisation constantly occurs. in the yellow rattle (rhinanthus crista-galli) there are two forms (which have been named _major_ and _minor_), the larger and more conspicuous adapted to insect fertilisation only, the smaller capable of self-fertilisation; and two similar forms exist in the eyebright (euphrasia officinalis). in both these cases there are special modifications in the length and curvature of the style as well as in the size and shape of the corolla; and the two forms are evidently becoming each adapted to special conditions, since in some districts the one, in other districts the other is most abundant.[ ] these examples show us that the kind of change suggested above is actually going on, and has presumably always been going on in nature throughout the long geological epochs during which the development of flowers has been progressing. the two great modes of gaining increased vigour and fertility--intercrossing and dispersal over wider areas--have been resorted to again and again, under the pressure of a constant struggle for existence and the need for adaptation to ever-changing conditions. during all the modifications that ensued, useless parts were reduced or suppressed, owing to the absence of selection and the principle of economy of growth; and thus at each fresh adaptation some rudiments of old structures were re-developed, but not unfrequently in a different form and for a distinct purpose. the chief types of flowering plants have existed during the millions of ages of the whole tertiary period, and during this enormous lapse of time many of them may have been modified in the direction of insect fertilisation, and again into that of self-fertilisation, not once or twice only, but perhaps scores or even hundreds of times; and at each such modification a difference in the environment may have led to a distinct line of development. at one epoch the highest specialisation of structure in adaptation to a single species or group of insects may have saved a plant from extinction; while, at other times, the simplest mode of self-fertilisation, combined with greater powers of dispersal and a constitution capable of supporting diverse physical conditions, may have led to a similar result. with some groups the tendency seems to have been almost continuously to greater and greater specialisation, while with others a tendency to simplification and degradation has resulted in such plants as the grasses and sedges. we are now enabled dimly to perceive how the curious anomaly of very simple and very complex methods of securing cross-fertilisation--both equally effective--may have been brought about. the simple modes may be the result of a comparatively direct modification from the more primitive types of flowers, which were occasionally, and, as it were, accidentally visited and fertilised by insects; while the more complex modes, existing for the most part in the highly irregular flowers, may result from those cases in which adaptation to insect-fertilisation, and partial or complete degradation to self-fertilisation or to wind-fertilisation, have again and again recurred, each time producing some additional complexity, arising from the working up of old rudiments for new purposes, till there have been reached the marvellous flower structures of the papilionaceous tribes, of the asclepiads, or of the orchids. we thus see that the existing diversity of colour and of structure in flowers is probably the ultimate result of the ever-recurring struggle for existence, combined with the ever-changing relations between the vegetable and animal kingdoms during countless ages. the constant variability of every part and organ, with the enormous powers of increase possessed by plants, have enabled them to become again and again readjusted to each change of condition as it occurred, resulting in that endless variety, that marvellous complexity, and that exquisite colouring which excite our admiration in the realm of flowers, and constitute them the perennial charm and crowning glory of nature. _flowers the product of insect agency._ in his _origin of species_, mr. darwin first stated that flowers had been rendered conspicuous and beautiful in order to attract insects, adding: "hence we may conclude that, if insects had not been developed on the earth, our plants would not have been decked with beautiful flowers, but would have produced only such poor flowers as we see on our fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are all fertilised through the agency of the wind." the argument in favour of this view is now much stronger than when he wrote; for not only have we reason to believe that most of these wind-fertilised flowers are degraded forms of flowers which have once been insect fertilised, but we have abundant evidence that whenever insect agency becomes comparatively ineffective, the colours of the flowers become less bright, their size and beauty diminish, till they are reduced to such small, greenish, inconspicuous flowers as those of the rupture-wort (herniaria glabra), the knotgrass (polygonum aviculare), or the cleistogamic flowers of the violet. there is good reason to believe, therefore, not only that flowers have been developed in order to attract insects to aid in their fertilisation, but that, having been once produced, in however great profusion, if the insect races were all to become extinct, flowers (in the temperate zones at all events) would soon dwindle away, and that ultimately all floral beauty would vanish from the earth. we cannot, therefore, deny the vast change which insects have produced upon the earth's surface, and which has been thus forcibly and beautifully delineated by mr. grant allen: "while man has only tilled a few level plains, a few great river valleys, a few peninsular mountain slopes, leaving the vast mass of earth untouched by his hand, the insect has spread himself over every land in a thousand shapes, and has made the whole flowering creation subservient to his daily wants. his buttercup, his dandelion, and his meadow-sweet grow thick in every english field. his thyme clothes the hillside; his heather purples the bleak gray moorland. high up among the alpine heights his gentian spreads its lakes of blue; amid the snows of the himalayas his rhododendrons gleam with crimson light. even the wayside pond yields him the white crowfoot and the arrowhead, while the broad expanses of brazilian streams are beautified by his gorgeous water-lilies. the insect has thus turned the whole surface of the earth into a boundless flower-garden, which supplies him from year to year with pollen or honey, and itself in turn gains perpetuation by the baits that it offers for his allurement."[ ] _concluding remarks on colour in nature._ in the last four chapters i have endeavoured to give a general and systematic, though necessarily condensed view of the part which is played by colour in the organic world. we have seen in what infinitely varied ways the need of concealment has led to the modification of animal colours, whether among polar snows or sandy deserts, in tropical forests or in the abysses of the ocean. we next find these general adaptations giving way to more specialised types of coloration, by which each species has become more and more harmonised with its immediate surroundings, till we reach the most curiously minute resemblances to natural objects in the leaf and stick insects, and those which are so like flowers or moss or birds' droppings that they deceive the acutest eye. we have learnt, further, that these varied forms of protective colouring are far more numerous than has been usually suspected, because, what appear to be very conspicuous colours or markings when the species is observed in a museum or in a menagerie, are often highly protective when the creature is seen under the natural conditions of its existence. from these varied classes of facts it seems not improbable that fully one-half of the species in the animal kingdom possess colours which have been more or less adapted to secure for them concealment or protection. passing onward we find the explanation of a distinct type of colour or marking, often superimposed upon protective tints, in the importance of easy recognition by many animals of their fellows, their parents, or their mates. by this need we have been able to account for markings that seem calculated to make the animal conspicuous, when the general tints and well-known habits of the whole group demonstrate the need of concealment. thus also we are able to explain the constant symmetry in the markings of wild animals, as well as the numerous cases in which the conspicuous colours are concealed when at rest and only become visible during rapid motion. in striking contrast to ordinary protective coloration we have "warning colours," usually very conspicuous and often brilliant or gaudy, which serve to indicate that their possessors are either dangerous or uneatable to the usual enemies of their tribe. this kind of coloration is probably more prevalent than has been hitherto supposed, because in the case of many tropical animals we are quite unacquainted with their special and most dangerous enemies, and are also unable to determine whether they are or are not distasteful to those enemies. as a kind of corollary to the "warning colours," we find the extraordinary phenomena of "mimicry," in which defenceless species obtain protection by being mistaken for those which, from any cause, possess immunity from attack. although a large number of instances of warning colour and of mimicry are now recorded, it is probably still an almost unworked field of research, more especially in tropical regions and among the inhabitants of the ocean. the phenomena of sexual diversities of coloration next engaged our attention, and the reasons why mr. darwin's theory of "sexual selection," as regards colour and ornament, could not be accepted were stated at some length, together with the theory of animal coloration and ornament we propose to substitute for it. this theory is held to be in harmony with the general facts of animal coloration, while it entirely dispenses with the very hypothetical and inadequate agency of female choice in producing the detailed colours, patterns, and ornaments, which in so many cases distinguish the male sex. if my arguments on this point are sound, they will dispose also of mr. grant allen's view of the direct action of the colour sense on the animal integuments.[ ] he argues that the colours of insects and birds reproduce generally the colours of the flowers they frequent or the fruits they eat, and he adduces numerous cases in which flower-haunting insects and fruit-eating birds are gaily coloured. this he supposes to be due to the colour-taste, developed by the constant presence of bright flowers and fruits, being applied to the selection of each variation towards brilliancy in their mates; thus in time producing the gorgeous and varied hues they now possess. mr. allen maintains that "insects are bright where bright flowers exist in numbers, and dull where flowers are rare or inconspicuous;" and he urges that "we can hardly explain this wide coincidence otherwise than by supposing that a taste for colour is produced through the constant search for food among entomophilous blossoms, and that this taste has reacted upon its possessors through the action of unconscious sexual selection." the examples mr. allen quotes of bright insects being associated with bright flowers seem very forcible, but are really deceptive or erroneous; and quite as many cases could be quoted which prove the very opposite. for example, in the dense equatorial forests flowers are exceedingly scarce, and there is no comparison with the amount of floral colour to be met with in our temperate meadows, woods, and hillsides. the forests about para in the lower amazon are typical in this respect, yet they abound with the most gorgeously coloured butterflies, almost all of which frequent the forest depths, keeping near the ground, where there is the greatest deficiency of brilliant flowers. in contrast with this let us take the cape of good hope--the most flowery region probably that exists upon the globe,--where the country is a complete flower-garden of heaths, pelargoniums, mesembryanthemus, exquisite iridaceous and other bulbs, and numerous flowering shrubs and trees; yet the cape butterflies are hardly equal, either in number or variety, to those of any country in south europe, and are utterly insignificant when compared with those of the comparatively flowerless forest-depths of the amazon or of new guinea. neither is there any relation between the colours of other insects and their haunts. few are more gorgeous than some of the tiger-beetles and the carabi, yet these are all carnivorous; while many of the most brilliant metallic buprestidae and longicorns are always found on the bark of fallen trees. so with the humming-birds; their brilliant metallic tints can only be compared with metals or gems, and are totally unlike the delicate pinks and purples, yellows and reds of the majority of flowers. again, the australian honey-suckers (meliphagidae) are genuine flower-haunters, and the australian flora is more brilliant in colour display than that of most tropical regions, yet these birds are, as a rule, of dull colours, not superior on the average to our grain-eating finches. then, again, we have the grand pheasant family, including the gold and the silver pheasants, the gorgeous fire-backed and ocellated pheasants, and the resplendent peacock, all feeding on the ground on grain or seeds or insects, yet adorned with the most gorgeous colours. there is, therefore, no adequate basis of facts for this theory to rest upon, even if there were the slightest reason to believe that not only birds, but butterflies and beetles, take any delight in colour for its own sake, apart from the food-supply of which it indicates the presence. all that has been proved or that appears to be probable is, that they are able to perceive differences of colour, and to associate each colour with the particular flowers or fruits which best satisfy their wants. colour being in its nature diverse, it has been beneficial for them to be able to distinguish all its chief varieties, as manifested more particularly in the vegetable kingdom, and among the different species of their own group; and the fact that certain species of insects show some preference for a particular colour may be explained by their having found flowers of that colour to yield them a more abundant supply of nectar or of pollen. in those cases in which butterflies frequent flowers of their own colour, the habit may well have been acquired from the protection it affords them. it appears to me that, in imputing to insects and birds the same love of colour for its own sake and the same aesthetic tastes as we ourselves possess, we may be as far from the truth as were those writers who held that the bee was a good mathematician, and that the honeycomb was constructed throughout to satisfy its refined mathematical instincts; whereas it is now generally admitted to be the result of the simple principle of economy of material applied to a primitive cylindrical cell.[ ] in studying the phenomena of colour in the organic world we have been led to realise the wonderful complexity of the adaptations which bring each species into harmonious relation with all those which surround it, and which thus link together the whole of nature in a network of relations of marvellous intricacy. yet all this is but, as it were, the outward show and garment of nature, behind which lies the inner structure--the framework, the vessels, the cells, the circulating fluids, and the digestive and reproductive processes,--and behind these again those mysterious chemical, electrical, and vital forces which constitute what we term life. these forces appear to be fundamentally the same for all organisms, as is the material of which all are constructed; and we thus find behind the outer diversities an inner relationship which binds together the myriad forms of life. each species of animal or plant thus forms part of one harmonious whole, carrying in all the details of its complex structure the record of the long story of organic development; and it was with a truly inspired insight that our great philosophical poet apostrophised the humble weed-- flower in the crannied wall, i pluck you out of the crannies, i hold you here, root and all, in my hand, little flower--but _if_ i could understand what you are, root and all, and all in all, i should know what god and man is. footnotes: [footnote : burchell's _travels_, vol. i. p. .] [footnote : _nature_, vol. iii. p. .] [footnote : _flowers, fruits, and leaves_, p. (fig. ).] [footnote : for a popular sketch of these, see sir j. lubbock's _flowers, fruits, and leaves_, or any general botanical work.] [footnote : _nature_, vol. xv. p, .] [footnote : grant allen's _colour sense_, p. .] [footnote : canon tristram's _natural history of the bible_, pp. , .] [footnote : for a complete historical account of this subject with full references to all the works upon it, see the introduction to hermann müller's _fertilisation of flowers_, translated by d'arcy w. thompson.] [footnote : for the full detail of his experiments, see _cross-and self-fertilisation of plants_, .] [footnote : see darwin's _fertilisation of orchids_ for the many extraordinary and complex arrangements in these plants.] [footnote : the english reader may consult sir john lubbock's _british wild flowers in relation to insects_, and h. müller's great and original work, _the fertilisation of flowers_.] [footnote : müller's _fertilisation of flowers_, p. .] [footnote : "alpenblumen," by d.h. müller. see _nature_, vol. xxiii. p. .] [footnote : this peculiarity of local distribution of colour in flowers may be compared, as regards its purpose, with the recognition colours of animals. just as these latter colours enable the sexes to recognise each other, and thus avoid sterile unions of distinct species, so the distinctive form and colour of each species of flower, as compared with those that usually grow around it, enables the fertilising insects to avoid carrying the pollen of one flower to the stigma of a distinct species.] [footnote : see h. müller's _fertilisation of flowers_, p. .] [footnote : the above examples are taken from rev. g. henslow's paper on "self-fertilisation of plants," in _trans. linn. soc._ second series, _botany_, vol. i. pp. - , with plate. mr. h.o. forbes has shown that the same thing occurs among tropical orchids, in his paper "on the contrivances for insuring self-fertilisation in some tropical orchids," _journ. linn. soc._, xxi. p. .] [footnote : these are the numbers given by darwin, but i am informed by mr. hemsley that many additions have been since made to the list, and that cleistogamic flowers probably occur in nearly all the natural orders.] [footnote : for a full account of cleistogamic flowers, see darwin's _forms of flowers_, chap. viii.] [footnote : henslow's "self-fertilisation," _trans. linn. soc._ second series, _botany_, vol. i. p. .] [footnote : the rev. george henslow, in his _origin of floral structures_, says: "there is little doubt but that all wind-fertilised angiosperms are degradations from insect-fertilised flowers.... _poterium sanguisorba_ is anemophilous; and _sanguisorba officinalis_ presumably was so formerly, but has reacquired an entomophilous habit; the whole tribe poterieae being, in fact, a degraded group which has descended from potentilleae. plantains retain their corolla but in a degraded form. junceae are degraded lilies; while cyperaceae and gramineae among monocotyledons may be ranked with amentiferae among dicotyledons, as representing orders which have retrograded very far from the entomophilous forms from which they were possibly and probably descended" (p. ). "the genus plantago, like _thalictrum minus_, poterium, and others, well illustrate the change from an entomophilous to the anemophilous state. _p. lanceolata_ has polymorphic flowers, and is visited by pollen-seeking insects, so that it can be fertilised either by insects or the wind. _p. media_ illustrates transitions in point of structure, as the filaments are pink, the anthers motionless, and the pollen grains aggregated, and it is regularly visited by _bombus terrestris_. on the other hand, the slender filaments, versatile anthers, powdery pollen, and elongated protogynous style are features of other species indicating anemophily; while the presence of a degraded corolla shows its ancestors to have been entomophilous. _p. media_, therefore, illustrates, not a primitive entomophilous condition, but a return to it; just as is the case with _sanguisorba officinalis_ and _salix caprea_; but these show no capacity of restoring the corolla, the attractive features having to be borne by the calyx, which is purplish in sanguisorba, by the pink filaments of plantago, and by the yellow anthers in the sallow willow" (p. ). "the interpretation, then, i would offer of inconspicuousness and all kinds of degradations is the exact opposite to that of conspicuousness and great differentiations; namely, that species with minute flowers, rarely or never visited by insects, and habitually self-fertilised, have primarily arisen through the neglect of insects, and have in consequence assumed their present floral structures" (p. ). in a letter just received from mr. henslow, he gives a few additional illustrations of his views, of which the following are the most important: "passing to incompletae, the orders known collectively as 'cyclospermeae' are related to caryophylleae; and to my mind are degradations from it, of which orache is anemophilous. cupuliferae have an inferior ovary and rudimentary calyx-limb on the top. these, as far as i know, cannot be interpreted except as degradations. the whole of monocotyledons appear to me (from anatomical reasons especially) to be degradations from dicotyledons, and primarily through the agency of growth in water. many subsequently became terrestrial, but retained the effects of their primitive habitat through heredity. the -merous [sic] perianth of grasses, the parts of the flower being in whorls, point to a degradation from a sub-liliaceous condition." mr. henslow informs me that he has long held these views, but, as far as he knows, alone. mr. grant allen, however, set forth a similar theory in his _vignettes from nature_ (p. ) and more fully in _the colours of flowers_ (chap. v.), where he develops it fully and uses similar arguments to those of mr. henslow.] [footnote : h. müller gives ample proof of this in his _fertilisation of flowers_.] [footnote : _cross-and self-fertilisation_, p. .] [footnote : _animals and plants_, vol. ii. p. .] [footnote : müller's _fertilisation of flowers_, pp. , . other cases of recent degradation and readaptation to insect-fertilisation are given by professor henslow (see footnote, p. ).] [footnote : _the colour sense_, by grant allen, p. .] [footnote : _the colour sense_, chap. ix.] [footnote : see _origin of species_, sixth edition, p. .] chapter xii the geographical distribution of organisms the facts to be explained--the conditions which have determined distribution--the permanence of oceans--oceanic and continental areas--madagascar and new zealand--the teachings of the thousand-fathom line--the distribution of marsupials--the distribution of tapirs--powers of dispersal as illustrated by insular organisms--birds and insects at sea--insects at great altitudes--the dispersal of plants--dispersal of seeds by the wind--mineral matter carried by the wind--objections to the theory of wind-dispersal answered--explanation of north temperate plants in the southern hemisphere--no proof of glaciation in the tropics--lower temperature not needed to explain the facts--concluding remarks. the theory which we may now take as established--that all the existing forms of life have been derived from other forms by a natural process of descent with modification, and that this same process has been in action during past geological time--should enable us to give a rational account not only of the peculiarities of form and structure presented by animals and plants, but also of their grouping together in certain areas, and their general distribution over the earth's surface. in the absence of any exact knowledge of the facts of distribution, a student of the theory of evolution might naturally anticipate that all groups of allied organisms would be found in the same region, and that, as he travelled farther and farther from any given centre, the forms of life would differ more and more from those which prevailed at the starting-point, till, in the remotest regions to which he could penetrate, he would find an entirely new assemblage of animals and plants, altogether unlike those with which he was familiar. he would also anticipate that diversities of climate would always be associated with a corresponding diversity in the forms of life. now these anticipations are to a considerable extent justified. remoteness on the earth's surface is usually an indication of diversity in the fauna and flora, while strongly contrasted climates are always accompanied by a considerable contrast in the forms of life. but this correspondence is by no means exact or proportionate, and the converse propositions are often quite untrue. countries which are near to each other often differ radically in their animal and vegetable productions; while similarity of climate, together with moderate geographical proximity, are often accompanied by marked diversities in the prevailing forms of life. again, while many groups of animals--genera, families, and sometimes even orders--are confined to limited regions, most of the families, many genera, and even some species are found in every part of the earth. an enumeration of a few of these anomalies will better illustrate the nature of the problem we have to solve. as examples of extreme diversity, notwithstanding geographical proximity, we may adduce madagascar and africa, whose animal and vegetable productions are far less alike than are those of great britain and japan at the remotest extremities of the great northern continent; while an equal, or perhaps even a still greater, diversity exists between australia and new zealand. on the other hand, northern africa and south europe, though separated by the mediterranean sea, have faunas and floras which do not differ from each other more than do the various countries of europe. as a proof that similarity of climate and general adaptability have had but a small part in determining the forms of life in each country, we have the fact of the enormous increase of rabbits and pigs in australia and new zealand, of horses and cattle in south america, and of the common sparrow in north america, though in none of these cases are the animals natives of the countries in which they thrive so well. and lastly, in illustration of the fact that allied forms are not always found in adjacent regions, we have the tapirs, which are found only on opposite sides of the globe, in tropical america and the malayan islands; the camels of the asiatic deserts, whose nearest allies are the llamas and alpacas of the andes; and the marsupials, only found in australia and on the opposite side of the globe, in america. yet, again, although mammalia may be said to be universally distributed over the globe, being found abundantly on all the continents and on a great many of the larger islands, yet they are entirely wanting in new zealand, and in a considerable number of other islands which are, nevertheless, perfectly able to support them when introduced. now most of these difficulties can be solved by means of well-known geographical and geological facts. when the productions of remote countries resemble each other, there is almost always continuity of land with similarity of climate between them. when adjacent countries differ greatly in their productions, we find them separated by a sea or strait whose great depth is an indication of its antiquity or permanence. when a group of animals inhabits two countries or regions separated by wide oceans, it is found that in past geological times the same group was much more widely distributed, and may have reached the countries it inhabits from an intermediate region in which it is now extinct. we know, also, that countries now united by land were divided by arms of the sea at a not very remote epoch; while there is good reason to believe that others now entirely isolated by a broad expanse of sea were formerly united and formed a single land area. there is also another important factor to be taken account of in considering how animals and plants have acquired their present peculiarities of distribution,--changes of climate. we know that quite recently a glacial epoch extended over much of what are now the temperate regions of the northern hemisphere, and that consequently the organisms which inhabit those parts must be, comparatively speaking, recent immigrants from more southern lands. but it is a yet more important fact that, down to middle tertiary times at all events, an equable temperate climate, with a luxuriant vegetation, extended to far within the arctic circle, over what are now barren wastes, covered for ten months of the year with snow and ice. the arctic zone has, therefore, been in past times capable of supporting almost all the forms of life of our temperate regions; and we must take account of this condition of things whenever we have to speculate on the possible migrations of organisms between the old and new continents. _the conditions which have determined distribution._ when we endeavour to explain in detail the facts of the existing distribution of organic beings, we are confronted by several preliminary questions, upon the solution of which will depend our treatment of the phenomena presented to us. upon the theory of descent which we have adopted, all the different species of a genus, as well as all the genera which compose a family or higher group, have descended from some common ancestor, and must therefore, at some remote epoch, have occupied the same area, from which their descendants have spread to the regions they now inhabit. in the numerous cases in which the same group now occupies countries separated by oceans or seas, by lofty mountain-chains, by wide deserts, or by inhospitable climates, we have to consider how the migration which must certainly have taken place has been effected. it is possible that during some portion of the time which has elapsed since the origin of the group the interposing barriers have not been in existence; or, on the other hand, the particular organisms we are dealing with may have the power of overpassing the barriers, and thus reaching their present remote dwelling-places. as this is really the fundamental question of distribution on which the solution of all its more difficult problems depends, we have to inquire, in the first place, what is the nature of, and what are the limits to, the changes of the earth's surface, especially during the tertiary and latter part of the secondary periods, as it was during those periods that most of the existing types of the higher animals and plants came into existence; and, in the next place, what are the extreme limits of the powers of dispersal possessed by the chief groups of animals and plants. we will first consider the question of barriers, more especially those formed by seas and oceans. _the permanence of oceans._ it was formerly a very general belief, even amongst geologists, that the great features of the earth's surface, no less than the smaller ones, were subject to continual mutations, and that during the course of known geological time the continents and great oceans had again and again changed places with each other. sir charles lyell, in the last edition of his _principles of geology_ ( ), said: "continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages;" and this may be said to have been the orthodox opinion down to the very recent period when, by means of deep-sea soundings, the nature of the ocean bottom was made known. the first person to throw doubt on this view appears to have been the veteran american geologist, professor dana. in , in the report of wilke's exploring expedition, he adduced the argument against a former continent in the pacific during the tertiary period, from the absence of all native quadrupeds. in , in articles in the _american journal_, he discussed the development of the american continent, and argued for its general permanence; and in his _manual of geology_ in and later editions, the same views were more fully enforced and were latterly applied to all continents. darwin, in his _journal of researches_, published in , called attention to the fact that all the small islands far from land in the pacific, indian, and atlantic oceans are either of coralline or volcanic formation. he excepted, however, the seychelles and st. paul's rocks; but the former have since been shown to be no exception, as they consist entirely of coral rock; and although darwin himself spent a few hours on st. paul's rocks on his outward voyage in the _beagle_, and believed he had found some portions of them to be of a "cherty," and others of a "felspathic" nature, this also has been shown to be erroneous, and the careful examination of the rocks by the abbé renard clearly proves them to be wholly of volcanic origin.[ ] we have, therefore, at the present time, absolutely no exception whatever to the remarkable fact that all the oceanic islands of the globe are either of volcanic or coral formation; and there is, further, good reason to believe that those of the latter class in every case rest upon a volcanic foundation. in his _origin of species_, darwin further showed that no true oceanic island had any native mammals or batrachia when first discovered, this fact constituting the test of the class to which an island belongs; whence he argued that none of them had ever been connected with continents, but all had originated in mid-ocean. these considerations alone render it almost certain that the areas now occupied by the great oceans have never, during known geological time, been occupied by continents, since it is in the highest degree improbable that every fragment of those continents should have completely disappeared, and have been replaced by volcanic islands rising out of profound oceanic abysses; but recent research into the depth of the oceans and the nature of the deposits now forming on their floors, adds greatly to the evidence in this direction, and renders it almost a certainty that they represent very ancient if not primaeval features of the earth's surface. a very brief outline of the nature of this evidence will be now given. the researches of the _challenger_ expedition into the nature of the sea-bottom show, that the whole of the land debris brought down by rivers to the ocean (with the exception of pumice and other floating matter), is deposited comparatively near to the shores, and that the fineness of the material is an indication of the distance to which it has been carried. everything in the nature of gravel and sand is laid down within a very few miles of land, only the finer muddy sediments being carried out for or miles, and the very finest of all, under the most favourable conditions, rarely extending beyond , or at the utmost, miles from land into the deep ocean.[ ] beyond these distances, and covering the entire ocean floor, are various oozes formed wholly from the debris of marine organisms; while intermingled with these are found various volcanic products which have been either carried through the air or floated on the surface, and a small but perfectly recognisable quantity of meteoric matter. ice-borne rocks are also found abundantly scattered over the ocean bottom within a definite distance of the arctic and antarctic circles, clearly marking out the limit of floating icebergs in recent geological times. now the whole series of marine stratified rocks, from the earliest palaeozoic to the most recent tertiary beds, consist of materials closely corresponding to the land debris now being deposited within a narrow belt round the shores of all continents; while no rocks have been found which can be identified with the various oozes now forming in the deep abysses of the ocean. it follows, therefore, that all the geological formations have been formed in comparatively shallow water, and always adjacent to the continental land of the period. the great thickness of some of the formations is no indication of a deep sea, but only of slow subsidence during the time that the deposition was in progress. this view is now adopted by many of the most experienced geologists, especially by dr. archibald geikie, director of the geological survey of great britain, who, in his lecture on "geographical evolution," says: "from all this evidence we may legitimately conclude that the present land of the globe, though consisting in great measure of marine formations, has never lain under the deep sea; but that its site must always have been near land. even its thick marine limestones are the deposits of comparatively shallow water."[ ] but besides these geological and physical considerations, there is a mechanical difficulty in the way of repeated change of position of oceans and continents which has not yet received the attention it deserves. according to the recent careful estimate by mr. john murray, the land area of the globe is to the water area as · to · . the mean height of the land above sea-level is feet, while the mean depth of the ocean is , feet. hence the bulk of dry land is , , cubic miles, and that of the waters of the ocean , , cubic miles; and it follows that if the whole of the solid matter of the earth's surface were reduced to one level, it would be everywhere covered by an ocean about two miles deep. the accompanying diagram will serve to render these figures more intelligible. the length of the sections of land and ocean are in the proportion of their respective areas, while the mean height of the land and the mean depth of the ocean are exhibited on a greatly increased vertical scale. if we considered the continents and their adjacent oceans separately they would differ a little, but not very materially, from this diagram; in some cases the proportion of land to ocean would be a little greater, in others a little less. [illustration: fig. .] now, if we try to imagine a process of elevation and depression by which the sea and land shall completely change places, we shall be met by insuperable difficulties. we must, in the first place, assume a general equality between elevation and subsidence during any given period, because if the elevation over any extensive continental area were not balanced by some subsidence of approximately equal amount, an unsupported hollow would be left under the earth's crust. let us now suppose a continental area to sink, and an adjacent oceanic area to rise, it will be seen that the greater part of the land will disappear long before the new land has approached the surface of the ocean. this difficulty will not be removed by supposing a portion of a continent to subside, and the immediately adjacent portion of the ocean on the other side of the continent to rise, because in almost every case we find that within a comparatively short distance from the shores of all existing continents, the ocean floor sinks rapidly to a depth of from to fathoms, and maintains a similar depth, generally speaking, over a large portion of the oceanic areas. in order, therefore, that any area of continental extent be upraised from the great oceans, there must be a subsidence of a land area five or six times as great, unless it can be shown that an extensive elevation of the ocean floor up to and far above the surface could occur without an equivalent depression elsewhere. the fact that the waters of the ocean are sufficient to cover the whole globe to a depth of two miles, is alone sufficient to indicate that the great ocean basins are permanent features of the earth's surface, since any process of alternation of these with the land areas would have been almost certain to result again and again in the total disappearance of large portions, if not of all, of the dry land of the globe. but the continuity of terrestrial life since the devonian and carboniferous periods, and the existence of very similar forms in the corresponding deposits of every continent--as well as the occurrence of sedimentary rocks, indicating the proximity of land at the time of their deposit, over a large portion of the surface of all the continents, and in every geological period--assure us that no such disappearance has ever occurred. _oceanic and continental areas._ when we speak of the permanence of oceanic and continental areas as one of the established facts of modern research, we do not mean that existing continents and oceans have always maintained the exact areas and outlines that they now present, but merely, that while all of them have been undergoing changes in outline and extent from age to age, they have yet maintained substantially the same positions, and have never actually changed places with each other. there are, moreover, certain physical and biological facts which enable us to mark out these areas with some confidence. we have seen that there are a large number of islands which may be classed as oceanic, because they have never formed parts of continents, but have originated in mid-ocean, and have derived their forms of life by migration across the sea. their peculiarities are seen to be very marked in comparison with those islands which there is good reason to believe are really fragments of more extensive land areas, and are hence termed "continental." these continental islands consist in every case of a variety of stratified rocks of various ages, thus corresponding closely with the usual structure of continents; although many of the islands are small like jersey or the shetland islands, or far from continental land like the falkland islands or new zealand. they all contain indigenous mammalia or batrachia, and generally a much greater variety of birds, reptiles, insects, and plants, than do the oceanic islands. from these various characteristics we conclude that they have all once formed parts of continents, or at all events of much larger land areas, and have become isolated, either by subsidence of the intervening land or by the effects of long-continued marine denudation. now, if we trace the thousand-fathom line around all our existing continents we find that, with only two exceptions, every island which can be classed as "continental" falls within this line, while all that lie beyond it have the undoubted characteristics of "oceanic" islands. we, therefore, conclude that the thousand-fathom line marks out, approximately, the "continental area,"--that is, the limits within which continental development and change throughout known geological time have gone on. there may, of course, have been some extensions of land beyond this limit, while some areas within it may always have been ocean; but so far as we have any direct evidence, this line may be taken to mark out, approximately, the most probable boundary between the "continental area," which has always consisted of land and shallow sea in varying proportions, and the great oceanic basins, within the limits of which volcanic activity has been building up numerous islands, but whose profound depths have apparently undergone little change. _madagascar and new zealand._ the two exceptions just referred to are madagascar and new zealand, and all the evidence goes to show that in these cases the land connection with the nearest continental area was very remote in time. the extraordinary isolation of the productions of madagascar--almost all the most characteristic forms of mammalia, birds, and reptiles of africa being absent from it--renders it certain that it must have been separated from that continent very early in the tertiary, if not as far back as the latter part of the secondary period; and this extreme antiquity is indicated by a depth of considerably more than a thousand fathoms in the mozambique channel, though this deep portion is less than a hundred miles wide between the comoro islands and the mainland.[ ] madagascar is the only island on the globe with a fairly rich mammalian fauna which is separated from a continent by a depth greater than a thousand fathoms; and no other island presents so many peculiarities in these animals, or has preserved so many lowly organised and archaic forms. the exceptional character of its productions agrees exactly with its exceptional isolation by means of a very deep arm of the sea. new zealand possesses no known mammals and only a single species of batrachian; but its geological structure is perfectly continental. there is also much evidence that it does possess one mammal, although no specimens have been yet obtained.[ ] its reptiles and birds are highly peculiar and more numerous than in any truly oceanic island. now the sea which directly separates new zealand from australia is more than fathoms deep, but in a north-west direction there is an extensive bank under fathoms, extending to and including lord howe's island, while north of this are other banks of the same depth, approaching towards a submarine extension of queensland on the one hand, and new caledonia on the other, and altogether suggestive of a land union with australia at some very remote period. now the peculiar relations of the new zealand fauna and flora with those of australia and of the tropical pacific islands to the northward indicate such a connection, probably during the cretaceous period; and here, again, we have the exceptional depth of the dividing sea and the form of the ocean bottom according well with the altogether exceptional isolation of new zealand, an isolation which has been held by some naturalists to be great enough to justify its claim to be one of the primary zoological regions. _the teachings of the thousand-fathom line._ if now we accept the annexed map as showing us approximately how far beyond their present limits our continents may have extended during any portion of the tertiary and secondary periods, we shall obtain a foundation of inestimable value for our inquiries into those migrations of animals and plants during past ages which have resulted in their present peculiarities of distribution. we see, for instance, that the south american and african continents have always been separated by nearly as wide an ocean as at present, and that whatever similarities there may be in their productions must be due to the similar forms having been derived from a common origin in one of the great northern continents. the radical difference between the higher forms of life of the two continents accords perfectly with their permanent separation. if there had been any direct connection between them during tertiary times, we should hardly have found the deep-seated differences between the quadrumana of the two regions--no family even being common to both; nor the peculiar insectivora of the one continent, and the equally peculiar edentata of the other. the very numerous families of birds quite peculiar to one or other of these continents, many of which, by their structural isolation and varied development of generic and specific forms, indicate a high antiquity, equally suggest that there has been no near approach to a land connection during the same epoch. looking to the two great northern continents, we see indications of a possible connection between them both in the north atlantic and the north pacific oceans; and when we remember that from middle tertiary times backward--so far as we know continuously to the earliest palaeozoic epoch--a temperate and equable climate, with abundant woody vegetation, prevailed up to and within the arctic circle, we see what facilities may have been afforded for migration from one continent to the other, sometimes between america and europe, sometimes between america and asia. admitting these highly probable connections, no bridging of the atlantic in more southern latitudes (of which there is not a particle of evidence) will have been necessary to account for all the intermigration that has occurred between the two continents. if, on the other hand, we remember how long must have been the route, and how diverse must always have been the conditions between the more northern and the more southern portions of the american and euro-asiatic continents, we shall not be surprised that many widespread forms in either continent have not crossed into the other; and that while the skunks (mephitis), the pouched rats (saccomyidae), and the turkeys (meleagris) are confined to america, the pigs and the hedgehogs, the true flycatchers and the pheasants are found only in the euro-asiatic continent. but, just as there have been periods which facilitated intermigration between america and the old world, there have almost certainly been periods, perhaps of long duration even geologically, when these continents have been separated by seas as wide as, or even wider than, those of the present day; and thus may be explained such curious anomalies as the origination of the camel-tribe in america, and its entrance into asia in comparatively recent tertiary times, while the introduction of oxen and bears into america from the euro-asiatic continent appears to have been equally recent.[ ] we shall find on examination that this view of the general permanence of the oceanic and continental areas, with constant minor fluctuations of land and sea over the whole extent of the latter, enables us to understand, and offer a rational explanation of, most of the difficult problems of geographical distribution; and further, that our power of doing this is in direct proportion to our acquaintance with the distribution of fossil forms of life during the tertiary period. we must, also, take due note of many other facts of almost equal importance for a due appreciation of the problems presented for solution, the most essential being, the various powers of dispersal possessed by the different groups of animals and plants, the geological antiquity of the species and genera, and the width and depth of the seas which separate the countries they, inhabit. a few illustrations will now be given of the way in which these branches of knowledge enable us to deal with the difficulties and anomalies that present themselves. _the distribution of marsupials._ this singular and lowly organised type of mammals constitutes almost the sole representative of the class in australia and new guinea, while it is entirely unknown in asia, africa, or europe. it reappears in america, where several species of opossums are found; and it was long thought necessary to postulate a direct southern connection of these distant countries, in order to account for this curious fact of distribution. when, however, we look to what is known of the geological history of the marsupials the difficulty vanishes. in the upper eocene deposits of western europe the remains of several animals closely allied to the american opossums have been found; and as, at this period, a very mild climate prevailed far up into the arctic regions, there is no difficulty in supposing that the ancestors of the group entered america from europe or northern asia during early tertiary times. but we must go much further back for the origin of the australian marsupials. all the chief types of the higher mammalia were in existence in the eocene, if not in the preceding cretaceous period, and as we find none of these in australia, that country must have been finally separated from the asiatic continent during the secondary or mesozoic period. now during that period, in the upper and the lower oolite and in the still older trias, the jaw-bones of numerous small mammalia have been found, forming eight distinct genera, which are believed to have been either marsupials or some allied lowly forms. in north america also, in beds of the jurassic and triassic formations, the remains of an equally great variety of these small mammalia have been discovered; and from the examination of more than sixty specimens, belonging to at least six distinct genera, professor marsh is of opinion that they represent a generalised type, from which the more specialised marsupials and insectivora were developed. from the fact that very similar mammals occur both in europe and america at corresponding periods, and in beds which represent a long succession of geological time, and that during the whole of this time no fragments of any higher forms have been discovered, it seems probable that both the northern continents (or the larger portion of their area) were then inhabited by no other mammalia than these, with perhaps other equally low types. it was, probably, not later than the jurassic age when some of these primitive marsupials were able to enter australia, where they have since remained almost completely isolated; and, being free from the competition of higher forms, they have developed into the great variety of types we now behold there. these occupy the place, and have to some extent acquired the form and structure of distinct orders of the higher mammals--the rodents, the insectivora, and the carnivora,--while still preserving the essential characteristics and lowly organisation of the marsupials. at a much later period--probably in late tertiary times--the ancestors of the various species of rats and mice which now abound in australia, and which, with the aerial bats, constitute its only forms of placental mammals, entered the country from some of the adjacent islands. for this purpose a land connection was not necessary, as these small creatures might easily be conveyed among the branches or in the crevices of trees uprooted by floods and carried down to the sea, and then floated to a shore many miles distant. that no actual land connection with, or very close approximation to, an asiatic island has occurred in recent times, is sufficiently proved by the fact that no squirrel, pig, civet, or other widespread mammal of the eastern hemisphere has been able to reach the australian continent. _the distribution of tapirs._ these curious animals form one of the puzzles of geographical distribution, being now confined to two very remote regions of the globe--the malay peninsula and adjacent islands of sumatra and borneo, inhabited by one species, and tropical america, where there are three or four species, ranging from brazil to ecuador and guatemala. if we considered these living forms only, we should be obliged to speculate on enormous changes of land and sea in order that these tropical animals might have passed from one country to the other. but geological discoveries have rendered all such hypothetical changes unnecessary. during miocene and pliocene times tapirs abounded over the whole of europe and asia, their remains having been found in the tertiary deposits of france, india, burmah, and china. in both north and south america fossil remains of tapirs occur only in caves and deposits of post-pliocene age, showing that they are comparatively recent immigrants into that continent. they perhaps entered by the route of kamchatka and alaska, where the climate, even now so much milder and more equable than on the north-east of america, might have been warm enough in late pliocene times to have allowed the migration of these animals. in asia they were driven southwards by the competition of numerous higher and more powerful forms, but have found a last resting-place in the swampy forests of the malay region. _what these facts prove._ now these two cases, of the marsupials and the tapirs, are in the highest degree instructive, because they show us that, without any hypothetical bridging of deep oceans, and with only such changes of sea and land as are indicated by the extent of the comparatively shallow seas surrounding and connecting the existing continents, we are able to account for the anomaly of allied forms occurring only in remote and widely separated areas. these examples really constitute crucial tests, because, of all classes of animals, mammalia are least able to surmount physical barriers. they are obviously unable to pass over wide arms of the sea, while the necessity for constant supplies of food and water renders sandy deserts or snow-clad plains equally impassable. then, again, the peculiar kinds of food on which alone many of them can subsist, and their liability to the attacks of other animals, put a further check upon their migrations. in these respects almost all other organisms have great advantages over mammals. birds can often fly long distances, and can thus cross arms of the sea, deserts, or mountain ranges; insects not only fly, but are frequently carried great distances by gales of wind, as shown by the numerous cases of their visits to ships hundreds of miles from land. reptiles, though slow of movement, have advantages in their greater capacity for enduring hunger or thirst, their power of resisting cold or drought in a state of torpidity, and they have also some facilities for migration across the sea by means of their eggs, which may be conveyed in crevices of timber or among masses of floating vegetable matter. and when we come to the vegetable kingdom, the means of transport are at their maximum, numbers of seeds having special adaptations for being carried by mammalia or birds, and for floating in the water, or through the air, while many are so small and so light that there is practically no limit to the distances they may be carried by gales and hurricanes. we may, therefore, feel quite certain that the means of distribution that have enabled the larger mammalia to reach the most remote regions from a common starting-point, will be at least as efficacious, and usually far more efficacious, with all other land animals and plants; and if in every case the existing distribution of this class can be explained on the theory of oceanic and continental permanence, with the limited changes of sea and land already referred to, no valid objections can be taken against this theory founded on anomalies of distribution in other orders. yet nothing is more common than for students of this or that group to assort that the theory of oceanic permanence is quite inconsistent with the distribution of its various species and genera. because a few indian genera and closely allied species of birds are found in madagascar, a land termed "lemuria" has been supposed to have united the two countries during a comparatively recent geological epoch; while the similarity of fossil plants and reptiles, from the permian and miocene formations of india and south africa, has been adduced as further evidence of this connection. but there are also genera of snakes, of insects, and of plants, common to madagascar and south america only, which have been held to necessitate a direct land connection between these countries. these views evidently refute themselves, because any such land connections must have led to a far greater similarity in the productions of the several countries than actually exists, and would besides render altogether inexplicable the absence of all the chief types of african and indian mammalia from madagascar, and its marvellous individuality in every department of the organic world.[ ] _powers of dispersal as illustrated by insular organisms._ having arrived at the conclusion that our existing oceans have remained practically unaltered throughout the tertiary and secondary periods of geology, and that the distribution of the mammalia is such as might have been brought about by their known powers of dispersal, and by such changes of land and sea as have probably or certainly occurred, we are, of course, restricted to similar causes to explain the much wider and sometimes more eccentric distribution of other classes of animals and of plants. in doing so, we have to rely partly on direct evidence of dispersal, afforded by the land organisms that have been observed far out at sea, or which have taken refuge on ships, as well as by the periodical visitants to remote islands; but very largely on indirect evidence, afforded by the frequent presence of certain groups on remote oceanic islands, which some ancestral forms must, therefore, have reached by transmission across the ocean from distant lands. _birds._ these vary much in their powers of flight, and their capability of traversing wide seas and oceans. many swimming and wading birds can continue long on the wing, fly swiftly, and have, besides, the power of resting safely on the surface of the water. these would hardly be limited by any width of ocean, except for the need of food; and many of them, as the gulls, petrels, and divers, find abundance of food on the surface of the sea itself. these groups have a wide distribution _across_ the oceans; while waders--especially plovers, sandpipers, snipes, and herons--are equally cosmopolitan, travelling _along_ the coasts of all the continents, and across the narrow seas which separate them. many of these birds seem unaffected by climate, and as the organisms on which they feed are equally abundant on arctic, temperate, and tropical shores, there is hardly any limit to the range even of some of the species. land-birds are much more restricted in their range, owing to their usually limited powers of flight, their inability to rest on the surface of the sea or to obtain food from it, and their greater specialisation, which renders them less able to maintain themselves in the new countries they may occasionally reach. many of them are adapted to live only in woods, or in marshes, or in deserts; they need particular kinds of food or a limited range of temperature; and they are adapted to cope only with the special enemies or the particular group of competitors among which they have been developed. such birds as these may pass again and again to a new country, but are never able to establish themselves in it; and it is this organic barrier, as it is termed, rather than any physical barrier, which, in many cases, determines the presence of a species in one area and its absence from another. we must always remember, therefore, that, although the presence of a species in a remote oceanic island clearly proves that its ancestors must at one time have found their way there, the absence of a species does not prove the contrary, since it also may have reached the island, but have been unable to maintain itself, owing to the inorganic or organic conditions not being suitable to it. this general principle applies to all classes of organisms, and there are many striking illustrations of it. in the azores there are eighteen species of land-birds which are permanent residents, but there are also several others which reach the islands almost every year after great storms, but have never been able to establish themselves. in bermuda the facts are still more striking, since there are only ten species of resident birds, while no less than twenty other species of land-birds and more than a hundred species of waders and aquatics are frequent visitors, often in great numbers, but are never able to establish themselves. on the same principle we account for the fact that, of the many continental insects and birds that have been let loose, or have escaped from confinement, in this country, hardly one has been able to maintain itself, and the same phenomenon is still more striking in the case of plants. of the thousands of hardy plants which grow easily in our gardens, very few have ever run wild, and when the experiment is purposely tried it invariably fails. thus a. de candolle informs us that several botanists of paris, geneva, and especially of montpellier, have sown the seeds of many hundreds of species of exotic hardy plants, in what appeared to be the most favourable situations, but that in hardly a single case has any one of them become naturalised.[ ] still more, then, in plants than in animals the absence of a species does not prove that it has never reached the locality, but merely that it has not been able to maintain itself in competition with the native productions. in other cases, as we have seen, facts of an exactly opposite nature occur. the rat, the pig, and the rabbit, the water-cress, the clover, and many other plants, when introduced into new zealand, nourish exceedingly, and even exterminate their native competitors; so that in these cases we may feel sure that the species in question did not exist in new zealand simply because they had been unable to reach that country by their natural means of dispersal. i will now give a few cases, in addition to those recorded in my previous works, of birds and insects which have been observed far from any land. _birds and insects at sea._ captain d. fullarton of the ship _timaru_ recorded in his log the occurrence of a great number of small land-birds about the ship on th march , when in lat. ° ' n., long. ° ' w. he says: "a great many small land-birds about us; put about sixty into a coop, evidently tired out." and two days later, th march, "over fifty of the birds cooped on th died, though fed. sparrows, finches, water-wagtails, two small birds, name unknown, one kind like a linnet, and a large bird like a starling. in all there have been on board over seventy birds, besides some that hovered about us for some time and then fell into the sea exhausted." easterly winds and severe weather were experienced at the time.[ ] the spot where this remarkable flight of birds was met with is about miles due west of brest, and this is the least distance the birds must have been carried. it is interesting to note that the position of the ship is nearly in the line from the english and french coasts to the azores, where, after great storms, so many bird stragglers arrive annually. these birds were probably blown out to sea during their spring migration along the south coast of england to wales and ireland. during the autumnal migration, however, great flocks of birds--especially starlings, thrushes, and fieldfares--have been observed every year flying out to sea from the west coast of ireland, almost the whole of which must perish. at the nash lighthouse, in the bristol channel on the coast of glamorganshire, an enormous number of small birds were observed on d september, including nightjars, buntings, white-throats, willow-wrens, cuckoos, house-sparrows, robins, wheatears, and blackbirds. these had probably crossed from somersetshire, and had they been caught by a storm the larger portion of them must have been blown out to sea.[ ] these facts enable us to account sufficiently well for the birds of oceanic islands, the number and variety of which are seen to be proportionate to their facilities for reaching the island and maintaining themselves in it. thus, though more birds yearly reach bermuda than the azores, the number of residents in the latter islands is much larger, due to the greater extent of the islands, their number, and their more varied surface. in the galapagos the land-birds are still more numerous, due in part to their larger area and greater proximity to the continent, but chiefly to the absence of storms, so that the birds which originally reached the islands have remained long isolated and have developed into many closely allied species adapted to the special conditions. all the species of the galapagos but one are peculiar to the islands, while the azores possess only one peculiar species, and bermuda none--a fact which is clearly due to the continual immigration of fresh individuals keeping up the purity of the breed by intercrossing. in the sandwich islands, which are extremely isolated, being more than miles from any continent or large island, we have a condition of things similar to what prevails in the galapagos, the land-birds, eighteen in number, being all peculiar, and belonging, except one, to peculiar genera. these birds have probably all descended from three or four original types which reached the islands at some remote period, probably by means of intervening islets that have since disappeared. in st. helena we have a degree of permanent isolation which has prevented any land-birds from reaching the island; for although its distance from the continent, miles, is not so great as in the case of the sandwich islands, it is situated in an ocean almost entirely destitute of small islands, while its position within the tropics renders it free from violent storms. neither is there, on the nearest part of the coast of africa, a perpetual stream of migrating birds like that which supplies the innumerable stragglers which every year reach bermuda and the azores. _insects._ winged insects have been mainly dispersed in the same way as birds, by their power of flight, aided by violent or long-continued winds. being so small, and of such low specific gravity, they are occasionally carried to still greater distances; and thus no islands, however remote, are altogether without them. the eggs of insects, being often deposited in borings or in crevices of timber, may have been conveyed long distances by floating trees, as may the larvae of those species which feed on wood. several cases have been published of insects coming on board ships at great distances from land; and darwin records having caught a large grasshopper when the ship was miles from the coast of africa, whence the insect had probably come. in the _entomologists' monthly magazine_ for june , mr. maclachlan has recorded the occurrence of a swarm of moths in the atlantic ocean, from the log of the ship _pleione_. the vessel was homeward bound from new zealand, and in lat. ° ' n., long. ° ' w., hundreds of moths appeared about the ship, settling in numbers on the spars and rigging. the wind for four days previously had been very light from north, north-west, or north-east, and sometimes calm. the north-east trade wind occasionally extends to the ship's position at that time of year. the captain adds that "frequently, in that part of the ocean, he has had moths and butterflies come on board." the position is miles south-west of the cape verde islands, and about north-east of the south american coast. the specimen preserved is deiopeia pulchella, a very common species in dry localities in the eastern tropics, and rarely found in britain, but, mr. maclachlan thinks, not found in south america. they must have come, therefore, from the cape verde islands, or from some parts of the african coast, and must have traversed about a thousand miles of ocean with the assistance, no doubt, of a strong north-east trade wind for a great part of the distance. in the british museum collection there is a specimen of the same moth caught at sea during the voyage of the _rattlesnake_, in lat. ° n., long. - / ° w., being between the former position and sierra leone, thus rendering it probable that the moths came from that part of the african coast, in which case the swarm encountered by the _pleione_ must have travelled more than miles. a similar case was recorded by mr. f.a. lucas in the american periodical _science_ of th april . he states that in he met with numerous moths of many species while at sea in the south atlantic (lat. ° s., long. ° w.), about miles from the coast of brazil. as this position is just beyond the south-east trades, the insects may have been brought from the land by a westerly gale. in the _zoologist_ ( , p. ) is the record of a small longicorn beetle which flew on board a ship miles off the west coast of africa. numerous other cases are recorded of insects at less distances from land, and, taken in connection with those already given, they are sufficient to show that great numbers must be continually carried out to sea, and that occasionally they are able to reach enormous distances. but the reproductive powers of insects are so great that all we require, in order to stock a remote island, is that some few specimens shall reach it even once in a century, or once in a thousand years. _insects at great altitudes._ equally important is the proof we possess that insects are often carried to great altitudes by upward currents of air. humboldt noticed them up to heights of , and , feet in south america, and mr. albert müller has collected many interesting cases of the same character in europe.[ ] a moth (plusia gamma) has been found on the summit of mont blanc; small hymenoptera and moths have been seen on the pyrenees at a height of , feet, while numerous flies and beetles, some of considerable size, have been caught on the glaciers and snow-fields of various parts of the alps. upward currents of air, whirlwinds and tornadoes, occur in all parts of the world, and large numbers of insects are thus carried up into the higher regions of the atmosphere, where they are liable to be caught by strong winds, and thus conveyed enormous distances over seas or continents. with such powerful means of dispersal the distribution of insects over the entire globe, and their presence in the most remote oceanic islands, offer no difficulties. _the dispersal of plants._ the dispersal of seeds is effected in a greater variety of ways than are available in the case of any animals. some fruits or seed-vessels, and some seeds, will float for many weeks, and after immersion in salt water for that period the seeds will often germinate. extreme cases are the double cocoa-nut of the seychelles, which has been found on the coast of sumatra, about miles distant; the fruits of the sapindus saponaria (soap-berry), which has been brought to bermuda by the gulf stream from the west indies, and has grown after a journey in the sea of about miles; and the west indian bean, entada scandens, which reached the azores from the west indies, a distance of full miles, and afterwards germinated at kew. by these means we can account for the similarity in the shore flora of the malay archipelago and most of the islands of the pacific; and from an examination of the fruits and seeds, collected among drift during the voyage of the _challenger_, mr. hemsley has compiled a list of species which are probably widely dispersed by this means. a still larger number of species owe their dispersal to birds in several distinct ways. an immense number of fruits in all parts of the world are devoured by birds, and have been attractively coloured (as we have seen), in order to be so devoured, because the seeds pass through the birds' bodies and germinate where they fall. we have seen how frequently birds are forced by gales of wind across a wide expanse of ocean, and thus seeds must be occasionally carried. it is a very suggestive fact, that all the trees and shrubs in the azores bear berries or small fruits which are eaten by birds; while all those which bear larger fruits, or are eaten chiefly by mammals--such as oaks, beeches, hazels, crabs, etc.--are entirely wanting. game-birds and waders often have portions of mud attached to their feet, and mr. darwin has proved by experiment that such mud frequently contains seeds. one partridge had such a quantity of mud attached to its foot as to contain seeds from which eighty-two plants germinated; this proves that a very small portion of mud may serve to convey seeds, and such an occurrence repeated even at long intervals may greatly aid in stocking remote islands with vegetation. many seeds also adhere to the feathers of birds, and thus, again, may be conveyed as far as birds are ever carried. dr. guppy found a small hard seed in the gizzard of a cape petrel, taken about miles east of tristan da cunha. _dispersal of seeds by the wind._ in the preceding cases we have been able to obtain direct evidence of transportal; but although we know that many seeds are specially adapted to be dispersed by the wind, we cannot obtain direct proof that they are so carried for hundreds or thousands of miles across the sea, owing to the difficulty of detecting single objects which are so small and inconspicuous. it is probable, however, that the wind as an agent of dispersal is really more effective than any of those we have hitherto considered, because a very large number of plants have seeds which are very small and light, and are often of such a form as to facilitate aerial carriage for enormous distances. it is evident that such seeds are especially liable to be transported by violent winds, because they become ripe in autumn at the time when storms are most prevalent, while they either lie upon the surface of the ground, or are disposed in dry capsules on the plant ready to be blown away. if inorganic particles comparable in weight, size, or form with such seeds are carried for great distances, we may be sure that seeds will also be occasionally carried in the same way. it will, therefore, be necessary to give a few examples of wind-carriage of small objects. on th july a remarkable shower of small pieces of hay occurred at monkstown, near dublin. they appeared floating slowly down from a great height, as if falling from a dark cloud which hung overhead. the pieces picked up were wet, and varied from single blades of grass to tufts weighing one or two ounces. a similar shower occurred a few days earlier in denbighshire, and was observed to travel in a direction contrary to that of the wind in the lower atmosphere.[ ] there is no evidence of the distance from which the hay was brought, but as it had been carried to a great height, it was in a position to be conveyed to almost any distance by a violent wind, had such occurred at the time. _mineral matter carried by the wind._ the numerous cases of sand and volcanic dust being carried enormous distances through the atmosphere sufficiently prove the importance of wind as a carrier of solid matter, but unfortunately the matter collected has not been hitherto examined with a view to determine the maximum size and weight of the particles. a few facts, however, have been kindly furnished me by professor judd, f.r.s. some dust which fell at genoa on th october , and was believed to have been brought from the african desert, consisted of quartz, hornblende, and other minerals, and contained particles having a diameter of / inch, each weighing / , grain. this dust had probably travelled over miles. in the dust from krakatoa, which fell at batavia, about miles distant, during the great eruption, there are many solid particles even larger than those mentioned above. some of this dust was given me by professor judd, and i found in it several ovoid particles of a much larger size, being / inch long, and / wide and deep. the dust from the same eruption, which fell on board the ship _arabella_, miles from the volcano, also contained solid particles / inch diameter. mr. john murray of the _challenger_ expedition writes to me that he finds in the deep sea deposits and even miles west of the coast of africa, rounded particles of quartz, having a diameter of / inch, and similar particles are found at equally great distances from the south-west coasts of australia; and he considers these to be atmospheric dust carried to that distance by the wind. taking the sp. gr. of quartz at . , these particles would weigh about / , grain each. these interesting facts can, however, by no means be taken as indicating the extreme limits of the power of wind in carrying solid particles. during the krakatoa eruption no gale of special violence occurred, and the region is one of comparative calms. the grains of quartz found by mr. murray more nearly indicate the limit, but the very small portions of matter brought up by the dredge, as compared with the enormous areas of sea-bottom, over which the atmospheric dust must have been scattered, render it in the highest degree improbable that the maximum limit either of size of particles, or of distance from land has been reached. let us, however, assume that the quartz grains, found by mr. murray in the deep-sea ooze miles from land, give us the extreme limit of the power of the atmosphere as a carrier of solid particles, and let us compare with these the weights of some seeds. from a small collection of the seeds of thirty species of herbaceous plants sent me from kew, those in the above table were selected, and small portions of eight of them carefully weighed in a chemical balance.[ ] by counting these portions i was able to estimate the number of seeds weighing one grain. the three very minute species, whose numbers are marked with an asterisk (*), were estimated by the comparison of their sizes with those of the smaller weighed seeds. no| species. |approximate | approximate | remarks. | |no. of seeds| dimensions. | | |in one grain| | | | | in. in. in. | |draba verna | , | / x / x / |oval, flat. |hypericum perforatum | | / x / |cylindrical. |astilbe rivularis | , | / x / |elongate, flat, tailed, | | | | wavy. |saxifraga coriophylla| | / x / |surface rough, adhere | | | | to the dry capsules. |oenothera rosea | | / x / |ovate. |hypericum hirsutum | | / x / |cylindrical, rough. |mimulus luteus | , | / x / |oval, minute. |penthorum sedoides | , * | / x / |flattened, very minute. |sagina procumbens | , * | / |sub-triangular, flat. |orchis maculata | , * | --- |margined, flat, | | | | very minute. |gentiana purpurea | | / |wavy, rough, with this | | | | coriaceous margins. |silene alpina | --- | / |flat, with fringed | | | | margins. |adenophora communis | --- | / x / |very thin, wavy, light. |quartz grains | , | / |deep sea ... miles. |do. | , | / |genoa ... miles. if now we compare the seeds with the quartz grains, we find that several are from twice to three times the weight of the grains found by mr. murray, and others five times, eight times, and fifteen times as heavy; but they are proportionately very much larger, and, being usually irregular in shape or compressed, they expose a very much larger surface to the air. the surface is often rough, and several have dilated margins or tailed appendages, increasing friction and rendering the uniform rate of falling through still air immensely less than in the case of the smooth, rounded, solid quartz grains. with these advantages it is a moderate estimate that seeds ten times the weight of the quartz grains could be carried quite as far through the air by a violent gale and under the most favourable conditions. these limits will include five of the seeds here given, as well as hundreds of others which do not exceed them in weight; and to these we may add some larger seeds which have other favourable characteristics, as is the case with numbers - , which, though very much larger than the rest, are so formed as in all probability to be still more easily carried great distances by a gale of wind. it appears, therefore, to be absolutely certain that every autumnal gale capable of conveying solid mineral particles to great distances, must also carry numbers of small seeds at least as far; and if this is so, the wind alone will form one of the most effective agents in the dispersal of plants. hitherto this mode of conveyance, as applying to the transmission of seeds for great distances across the ocean, has been rejected by botanists, for two reasons. in the first place, there is said to be no direct evidence of such conveyance; and, secondly, the peculiar plants of remote oceanic islands do not appear to have seeds specially adapted for aerial transmission. i will consider briefly each of these objections. _objection to the theory of wind-dispersal._ to obtain direct evidence of the transmission of such minute and perishable objects, which do not exist in great quantities, and are probably carried to the greatest distances but rarely and as single specimens, is extremely difficult. a bird or insect can be seen if it comes on board ship, but who would ever detect the seeds of mimulus or orchis even if a score of them fell on a ship's deck? yet if but one such seed per century were carried to an oceanic island, that island might become rapidly overrun by the plant, if the conditions were favourable to its growth and reproduction. it is further objected that search has been made for such seeds, and they have not been found. professor kerner of innsbruck examined the snow on the surface of glaciers, and assiduously collected all the seeds he could find, and these were all of plants which grew in the adjacent mountains or in the same district. in like manner, the plants growing on moraines were found to be those of the adjacent mountains, plateaux, or lowlands. hence he concluded that the prevalent opinion that seeds may be carried through the air for very great distances "is not supported by fact."[ ] the opinion is certainly not supported by kerner's facts, but neither is it opposed by them. it is obvious that the seeds that would be carried by the wind to moraines or to the surface of glaciers would be, first and in the greatest abundance, those of the immediately surrounding district; then, very much more rarely, those from more remote mountains; and lastly, in extreme rarity, those from distant countries or altogether distinct mountain ranges. let us suppose the first to be so abundant that a single seed could be found by industrious search on each square yard of the surface of the glacier; the second so scarce that only one could possibly be found in a hundred yards square; while to find one of the third class it would be necessary exhaustively to examine a square mile of surface. should we expect that _one_ ever to be found, and should the fact that it could not be found be taken as a proof that it was not there? besides, a glacier is altogether in a bad position to receive such remote wanderers, since it is generally surrounded by lofty mountains, often range behind range, which would intercept the few air-borne seeds that might have been carried from a distant land. the conditions in an oceanic island, on the other hand, are the most favourable, since the land, especially if high, will intercept objects carried by the wind, and will thus cause more of the solid matter to fall on it than on an equal area of ocean. we know that winds at sea often blow violently for days together, and the rate of motion is indicated by the fact that miles an hour was the average velocity of the wind observed during twelve hours at the ben nevis observatory, while the velocity sometimes rises to miles an hour. a twelve hours' gale might, therefore, carry light seeds a thousand miles as easily and certainly as it could carry quartz-grains of much greater specific gravity, rotundity, and smoothness, or even miles; and it is difficult even to imagine a sufficient reason why they should not be so carried--perhaps very rarely and under exceptionally favourable conditions,--but this is all that is required. as regards the second objection, it has been observed that orchideae, which have often exceedingly small and light seeds, are remarkably absent from oceanic islands. this, however, may be very largely due to their extreme specialisation and dependence on insect agency for their fertilisation; while the fact that they do occur in such very remote islands as the azores, tahiti, and the sandwich islands, proves that they must have once reached these localities either by the agency of birds or by transmission through the air; and the facts i have given above render the latter mode at least as probable as the former. sir joseph hooker remarks on the composite plant of kerguelen island (cotula plumosa) being found also on lord auckland and macquarrie islands, and yet having no pappus, while other species of the genus possess it. this is certainly remarkable, and proves that the plant must have, or once have had, some other means of dispersal across wide oceans.[ ] one of the most widely dispersed species in the whole world (sonchus oleraceus) possesses pappus, as do four out of five of the species which are common to europe and new zealand, all of which have a very wide distribution. the same author remarks on the limited area occupied by most species of compositae, notwithstanding their facilities for dispersal by means of their feathered seeds; but it has been already shown that limitations of area are almost always due to the competition of allied forms, facilities for dispersal being only one of many factors in determining the wide range of species. it is, however, a specially important factor in the case of the inhabitants of remote oceanic islands, since, whether they are peculiar species or not, they or their remote ancestors must at some time or other have reached their present position by natural means. i have already shown elsewhere, that the flora of the azores strikingly supports the view of the species having been introduced by aerial transmission only, that is, by the agency of birds and the wind, because all plants that could not possibly have been carried by these means are absent.[ ] in the same way we may account for the extreme rarity of leguminosae in all oceanic islands. mr. hemsley, in his report on insular floras, says that they "are wanting in a large number of oceanic islands where there is no true littoral flora," as st. helena, juan fernandez, and all the islands of the south atlantic and south indian oceans. even in the tropical islands, such as mauritius and bourbon, there are no endemic species, and very few in the galapagos and the remoter pacific islands. all these facts are quite in accordance with the absence of facilities for transmission through the air, either by birds or the wind, owing to the comparatively large size and weight of the seeds; and an additional proof is thus afforded of the extreme rarity of the successful floating of seeds for great distances across the ocean.[ ] _explanation of north temperate plants in the southern hemisphere._ if we now admit that many seeds which are either minute in size, of thin texture or wavy form, or so fringed or margined as to afford a good hold to the air, are capable of being carried for many hundreds of miles by exceptionally violent and long-continued gales of wind, we shall not only be better able to account for the floras of some of the remotest oceanic islands, but shall also find in the fact a sufficient explanation of the wide diffusion of many genera, and even species, of arctic and north temperate plants in the southern hemisphere or on the summits of tropical mountains. nearly fifty of the flowering plants of tierra-del-fuego are found also in north america or europe, but in no intermediate country; while fifty-eight species are common to new zealand and northern europe; thirty-eight to australia, northern europe, and asia; and no less than seventy-seven common to new zealand, australia, and south america.[ ] on lofty mountains far removed from each other, identical or closely allied plants often occur. thus the fine primula imperialis of a single mountain peak in java has been found (or a closely allied species) in the himalayas; and many other plants of the high mountains of java, ceylon, and north india are either identical or closely allied forms. so, in africa, some species, found on the summits of the cameroons and fernando po in west africa, are closely allied to species in the abyssinian highlands and in temperate europe; while other abyssinian and cameroons species have recently been found on the mountains of madagascar. some peculiar australian forms have been found represented on the summit of kini balu in borneo. again, on the summit of the organ mountains in brazil there are species allied to those of the andes, but not found in the intervening lowlands. _no proof of recent lower temperature in the tropics._ now all these facts, and numerous others of like character, were supposed by mr. darwin to be due to a lowering of temperature during glacial epochs, which allowed these temperate forms to migrate across the intervening tropical lowlands. but any such change within the epoch of existing species is almost inconceivable. in the first place, it would necessitate the extinction of much of the tropical flora (and with it of the insect life), because without such extinction alpine herbaceous plants could certainly never spread over tropical forest lowlands; and, in the next place, there is not a particle of direct evidence that any such lowering of temperature in inter-tropical lowlands ever took place. the only alleged evidence of the kind is that adduced by the late professor agassiz and mr. hartt; but i am informed by my friend, mr. j.c. branner (now state geologist of arkansas, u.s.), who succeeded mr. hartt, and spent several years completing the geological survey of brazil, that the supposed moraines and glaciated granite rocks near rio janeiro and elsewhere, as well as the so-called boulder-clay of the same region, are entirely explicable as the results of sub-aerial denudation and weathering, and that there is no proof whatever of glaciation in any part of brazil. _lower temperature not needed to explain the facts._ but any such vast physical change as that suggested by darwin, involving as it does such tremendous issues as regards its effects on the tropical fauna and flora of the whole world, is really quite uncalled for, because the facts to be explained are of the same essential nature as those presented by remote oceanic islands, between which and the nearest continents no temperate land connection is postulated. in proportion to their limited area and extreme isolation, the azores, st. helena, the galapagos, and the sandwich islands, each possess a fairly rich--the last a very rich--indigenous flora; and the means which sufficed to stock them with a great variety of plants would probably suffice to transmit others from mountain-top to mountain-top in various parts of the globe. in the case of the azores, we have large numbers of species identical with those of europe, and others closely allied, forming an exactly parallel case to the species found on the various mountain summits which have been referred to. the distances from madagascar to the south african mountains and to kilimandjaro, and from the latter to abyssinia, are no greater than from spain to the azores, while there are other equatorial mountains forming stepping-stones at about an equal distance to the cameroons. between java and the himalayas we have the lofty mountains of sumatra and of north-western burma, forming steps at about the same distance apart; while between kini balu and the australian alps we have the unexplored snow mountains of new guinea, the bellenden ker mountains in queensland, and the new england and blue mountains of new south wales. between brazil and bolivia the distances are no greater; while the unbroken range of mountains from arctic america to tierra-del-fuego offers the greatest facilities for transmission, the partial gap between the lofty peak of chiriqui and the high andes of new grenada being far less than from spain to the azores. thus, whatever means have sufficed for stocking oceanic islands must have been to some extent effective in transmitting northern forms from mountain to mountain, across the equator, to the southern hemisphere; while for this latter form of dispersal there are special facilities, in the abundance of fresh and unoccupied surfaces always occurring in mountain regions, owing to avalanches, torrents, mountain-slides, and rock-falls, thus affording stations on which air-borne seeds may germinate and find a temporary home till driven out by the inroads of the indigenous vegetation. these temporary stations may be at much lower altitudes than the original habitat of the species, if other conditions are favourable. alpine plants often descend into the valleys on glacial moraines, while some arctic species grow equally well on mountain summits and on the seashore. the distances above referred to between the loftier mountains may thus be greatly reduced by the occurrence of suitable conditions at lower altitudes, and the facilities for transmission by means of aerial currents proportionally increased.[ ] _facts explained by the wind-carriage of seeds._ but if we altogether reject aerial transmission of seeds for great distances, except by the agency of birds, it will be difficult, if not impossible, to account for the presence of so many identical species of plants on remote mountain summits, or for that "continuous current of vegetation" described by sir joseph hooker as having apparently long existed from the northern to the southern hemisphere. it may be admitted that we can, possibly, account for the greater portion of the floras of remote oceanic islands by the agency of birds alone; because, when blown out to sea land-birds must reach some island or perish, and all which come within sight of an island will struggle to reach it as their only refuge. but, with mountain summits the case is altogether different, because, being surrounded by land instead of by sea, no bird would need to fly, or to be carried by the wind, for several hundred miles at a stretch to another mountain summit, but would find a refuge in the surrounding uplands, ridges, valleys, or plains. as a rule the birds that frequent lofty mountain tops are peculiar species, allied to those of the surrounding district; and there is no indication whatever of the passage of birds from one remote mountain to another in any way comparable with the flights of birds which are known to reach the azores annually, or even with the few regular migrants from australia to new zealand. it is almost impossible to conceive that the seeds of the himalayan primula should have been thus carried to java; but, by means of gales of wind, and intermediate stations from fifty to a few hundred miles apart, where the seeds might vegetate for a year or two and produce fresh seed to be again carried on in the same manner, the transmission might, after many failures, be at last effected. a very important consideration is the vastly larger scale on which wind-carriage of seeds must act, as compared with bird-carriage. it can only be a few birds which carry seeds attached to their feathers or feet. a very small proportion of these would carry the seeds of alpine plants; while an almost infinitesimal fraction of these latter would convey the few seeds attached to them safely to an oceanic island or remote mountain. but winds, in the form of whirlwinds or tornadoes, gales or hurricanes, are perpetually at work over large areas of land and sea. insects and light particles of matter are often carried up to the tops of high mountains; and, from the very nature and origin of winds, they usually consist of ascending or descending currents, the former capable of suspending such small and light objects as are many seeds long enough for them to be carried enormous distances. for each single seed carried away by external attachment to the feet or feathers of a bird, countless millions are probably carried away by violent winds; and the chance of conveyance to a great distance and in a definite direction must be many times greater by the latter mode than by the former.[ ] we have seen that inorganic particles of much greater specific gravity than seeds, and nearly as heavy as the smallest kinds, are carried to great distances through the air, and we can therefore hardly doubt that some seeds are carried as far. the direct agency of the wind, as a supplement to bird-transport, will help to explain the presence in oceanic islands of plants growing in dry or rocky places whose small seeds are not likely to become attached to birds; while it seems to be the only effective agency possible in the dispersal of those species of alpine or sub-alpine plants found on the summits of distant mountains, or still more widely separated in the temperate zones of the northern and southern hemispheres. _concluding remarks._ on the general principles that have been now laid down, it will be found that all the chief facts of the geographical distribution of animals and plants can be sufficiently understood. there will, of course, be many cases of difficulty and some seeming anomalies, but these can usually be seen to depend on our ignorance of some of the essential factors of the problem. either we do not know the distribution of the group in recent geological times, or we are still ignorant of the special methods by which the organisms are able to cross the sea. the latter difficulty applies especially to the lizard tribe, which are found in almost all the tropical oceanic islands; but the particular mode in which they are able to traverse a wide expanse of ocean, which is a perfect barrier to batrachia and almost so to snakes, has not yet been discovered. lizards are found in all the larger pacific islands as far as tahiti, while snakes do not extend beyond the fiji islands; and the latter are also absent from mauritius and bourbon, where lizards of seven or eight species abound. naturalists resident in the pacific islands would make a valuable contribution to our science by studying the life-history of the native lizards, and endeavouring to ascertain the special facilities they possess for crossing over wide spaces of ocean. footnotes: [footnote : see a. agassiz, _three cruises of the blake_ (cambridge, mass., ), vol. i. p. , footnote.] [footnote : even the extremely fine mississippi mud is nowhere found beyond a hundred miles from the mouths of the river in the gulf of mexico (a. agassiz, _three cruises of the blake_, vol. i. p. ).] [footnote : i have given a full summary of the evidence for the permanence of oceanic and continental areas in my _island life_, chap. vi.] [footnote : for a full account of the peculiarities of the madagascar fauna, see my _island life_, chap. xix.] [footnote : see _island life_, p. , and the whole of chaps. xxi. xxii. more recent soundings have shown that the map at p. , as well as that of the madagascar group at p. , are erroneous, the ocean around norfolk island and in the straits of mozambique being more than fathoms deep. the general argument is, however, unaffected.] [footnote : for some details of these migrations, see the author's _geographical distribution of animals_, vol. i. p. ; also heilprin's _geographical and geological distribution of animals_.] [footnote : for a full discussion of this question, see _island life_, pp. - .] [footnote : _géographie botanique_, p. .] [footnote : _nature_, st april .] [footnote : report of the brit. assoc. committee on migration of birds during .] [footnote : _trans. ent. soc._, , p. .] [footnote : _nature_ ( ), vol. xii. pp. , .] [footnote : i am indebted to professor r. meldola of the finsbury technical institute, and rev. t.d. titmas of charterhouse for furnishing me with the weights required.] [footnote : see _nature_, vol. vi. p. , for a summary of kerner's paper.] [footnote : it seems quite possible that the absence of pappus in this case is a recent adaptation, and that it has been brought about by causes similar to those which have reduced or aborted the wings of insects in oceanic islands. for when a plant has once reached one of the storm-swept islands of the southern ocean, the pappus will be injurious for the same reason that the wings of insects are injurious, since it will lead to the seeds being blown out to sea and destroyed. the seeds which are heaviest and have least pappus will have the best chance of falling on the ground and remaining there to germinate, and this process of selection might rapidly lead to the entire disappearance of the pappus.] [footnote : see _island life_, p. .] [footnote : mr. hemsley suggests that it is not so much the difficulty of transmission by floating, as the bad conditions the seeds are usually exposed to when they reach land. many, even if they germinate, are destroyed by the waves, as burchell noticed at st. helena; while even a flat and sheltered shore would be an unsuitable position for many inland plants. air-borne seeds, on the other hand, may be carried far inland, and so scattered that some of them are likely to reach suitable stations.] [footnote : for fuller particulars, see sir j. hooker's _introduction to floras of new zealand and australia_, and a summary in my _island life_, chaps. xxii. xxiii.] [footnote : for a fuller discussion of this subject, see my _island life_, chap. xxiii.] [footnote : a very remarkable case of wind conveyance of seeds on a large scale is described in a letter from mr. thomas hanbury to his brother, the late daniel hanbury, which has been kindly communicated to me by mr. hemsley of kew. the letter is dated "shanghai, st may ," and the passage referred to is as follows:-- "for the past three days we have had very warm weather for this time of year, in fact almost as warm as the middle of summer. last evening the wind suddenly changed round to the north and blew all night with considerable violence, making a great change in the atmosphere. "this morning, myriads of small white particles are floating about in the air; there is not a single cloud and no mist, yet the sun is quite obscured by this substance, and it looks like a white fog in england. i enclose thee a sample, thinking it may interest. it is evidently a vegetable production; i think, apparently, some kind of seed." mr. hemsley adds, that this substance proves to be the plumose seeds of a poplar or willow. in order to produce the effects described--_quite obscuring the sun like a white fog_,--the seeds must have filled the air to a very great height; and they must have been brought from some district where there were extensive tracts covered with the tree which produced them.] chapter xiii the geological evidences of evolution what we may expect--the number of known species of extinct animals--causes of the imperfection of the geological record--geological evidences of evolution--shells--crocodiles--the rhinoceros tribe--the pedigree of the horse tribe--development of deer's horns--brain development--local relations of fossil and living animals--cause of extinction of large animals--indications of general progress in plants and animals--the progressive development of plants--possible cause of sudden late appearance of exogens--geological distribution of insects--geological succession of vertebrata--concluding remarks. the theory of evolution in the organic world necessarily implies that the forms of animals and plants have, broadly speaking, progressed from a more generalised to a more specialised structure, and from simpler to more complex forms. we know, however, that this progression has been by no means regular, but has been accompanied by repeated degradation and degeneration; while extinction on an enormous scale has again and again stopped all progress in certain directions, and has often compelled a fresh start in development from some comparatively low and imperfect type. the enormous extension of geological research in recent times has made us acquainted with a vast number of extinct organisms, so vast that in some important groups--such as the mollusca--the fossil are more numerous than the living species; while in the mammalia they are not much less numerous, the preponderance of living species being chiefly in the smaller and in the arboreal forms which have not been so well preserved as the members of the larger groups. with such a wealth of material to illustrate the successive stages through which animals have passed, it will naturally be expected that we should find important evidence of evolution. we should hope to learn the steps by which some isolated forms have been connected with their nearest allies, and in many cases to have the gaps filled up which now separate genus from genus, or species from species. in some cases these expectations are fulfilled, but in many other cases we seek in vain for evidence of the kind we desire; and this absence of evidence with such an apparent wealth of material is held by many persons to throw doubt on the theory of evolution itself. they urge, with much appearance of reason, that all the arguments we have hitherto adduced fall short of demonstration, and that the crucial test consists in being able to show, in a great number of cases, those connecting links which we say must have existed. many of the gaps that still remain are so vast that it seems incredible to these writers that they could ever have been filled up by a close succession of species, since these must have spread over so many ages, and have existed in such numbers, that it seems impossible to account for their total absence from deposits in which great numbers of species belonging to other groups are preserved and have been discovered. in order to appreciate the force, or weakness, of these objections, we must inquire into the character and completeness of that record of the past life of the earth which geology has unfolded, and ascertain the nature and amount of the evidence which, under actual conditions, we may expect to find. _the number of known species of extinct animals._ when we state that the known fossil mollusca are considerably more numerous than those which now live on the earth, it appears at first sight that our knowledge is very complete, but this is far from being the case. the species have been continually changing throughout geological time, and at each period have probably been as numerous as they are now. if we divide the fossiliferous strata into twelve great divisions--the pliocene, miocene, eocene, cretaceous, oolite, lias, trias, permian, carboniferous, devonian, silurian, and cambrian,--we find not only that each has a very distinct and characteristic molluscan fauna, but that the different subdivisions often present a widely different series of species; so that although a certain number of species are common to two or more of the great divisions, the totality of the species that have lived upon the earth must be very much more than twelve times--perhaps even thirty or forty times--the number now living. in like manner, although the species of fossil mammals now recognised by more or less fragmentary fossil remains may not be much less numerous than the living species, yet the duration of existence of these was comparatively so short that they were almost completely changed, perhaps six or seven times, during the tertiary period; and this is certainly only a fragment of the geological time during which mammalia existed on the globe. there is also reason to believe that the higher animals were much more abundant in species during past geological epochs than now, owing to the greater equability of the climate which rendered even the arctic regions as habitable as the temperate zones are in our time. the same equable climate would probably cause a more uniform distribution of moisture, and render what are now desert regions capable of supporting abundance of animal life. this is indicated by the number and variety of the species of large animals that have been found fossil in very limited areas which they evidently inhabited at one period. m. albert gaudry found, in the deposits of a mountain stream at pikermi in greece, an abundance of large mammalia such as are nowhere to be found living together at the present time. among them were two species of mastodon, two different rhinoceroses, a gigantic wild boar, a camel and a giraffe larger than those now living, several monkeys, carnivora ranging from martens and civets to lions and hyaenas of the largest size, numerous antelopes of at least five distinct genera, and besides these many forms altogether extinct. such were the great herds of hipparion, an ancestral form of horse; the helladotherium, a huge animal bigger than the giraffe; the ancylotherium, one of the edentata; the huge dinotherium; the aceratherium, allied to the rhinoceros; and the monstrous chalicotherium, allied to the swine and ruminants, but as large as a rhinoceros; and to prey upon these, the great machairodus or sabre-toothed tiger. and all these remains were found in a space paces long by paces broad, many of the species existing in enormous quantities. the pikermi fossils belong to the upper miocene formation, but an equally rich deposit of upper eocene age has been discovered in south-western france at quercy, where m. filhol has determined the presence of no less than forty-two species of beasts of prey alone. equally remarkable are the various discoveries of mammalian fossils in north america, especially in the old lake bottoms now forming what are called the "bad lands" of dakota and nebraska, belonging to the miocene period. here are found an enormous assemblage of remains, often perfect skeletons, of herbivora and carnivora, as varied and interesting as those from the localities already referred to in europe; but altogether distinct, and far exceeding, in number and variety of species of the larger animals, the whole existing fauna of north america. very similar phenomena occur in south america and in australia, leading us to the conclusion that the earth at the present time is impoverished as regards the larger animals, and that at each successive period of tertiary time, at all events, it contained a far greater number of species than now inhabit it. the very richness and abundance of the remains which we find in limited areas, serve to convince us how imperfect and fragmentary must be our knowledge of the earth's fauna at any one past epoch; since we cannot believe that all, or nearly all, of the animals which inhabited any district were entombed in a single lake, or overwhelmed by the floods of a single river. but the spots where such rich deposits occur are exceedingly few and far between when compared with the vast areas of continental land, and we have every reason to believe that in past ages, as now, numbers of curious species were rare or local, the commoner and more abundant species giving a very imperfect idea of the existing series of animal forms. yet more important, as showing the imperfection of our knowledge, is the enormous lapse of time between the several formations in which we find organic remains in any abundance, so vast that in many cases we find ourselves almost in a new world, all the species and most of the genera of the higher animals having undergone a complete change. _causes of the imperfection of the geological record._ these facts are quite in accordance with the conclusions of geologists as to the necessary imperfection of the geological record, since it requires the concurrence of a number of favourable conditions to preserve any adequate representation of the life of a given epoch. in the first place, the animals to be preserved must not die a natural death by disease, or old age, or by being the prey of other animals, but must be destroyed by some accident which shall lead to their being embedded in the soil. they must be either carried away by floods, sink into bogs or quicksands, or be enveloped in the mud or ashes of a volcanic eruption; and when thus embedded they must remain undisturbed amid all the future changes of the earth's surface. but the chances against this are enormous, because denudation is always going on, and the rocks we now find at the earth's surface are only a small fragment of those which were originally laid down. the alternations of marine and freshwater deposits, and the frequent unconformability of strata with those which overlie them, tell us plainly of repeated elevations and depressions of the surface, and of denudation on an enormous scale. almost every mountain range, with its peaks, ridges, and valleys, is but the remnant of some vast plateau eaten away by sub-aerial agencies; every range of sea-cliffs tell us of long slopes of land destroyed by the waves; while almost all the older rocks which now form the surface of the earth have been once covered with newer deposits which have long since disappeared. nowhere are the evidences of this denudation more apparent than in north and south america, where granitic or metamorphic rocks cover an area hardly less than that of all europe. the same rocks are largely developed in central africa and eastern asia; while, besides those portions that appear exposed on the surface, areas of unknown extent are buried under strata which rest on them uncomformably, and could not, therefore, constitute the original capping under which the whole of these rocks must once have been deeply buried; because granite can only be formed, and metamorphism can only go on, deep down in the crust of the earth. what an overwhelming idea does this give us of the destruction of whole piles of rock, miles in thickness and covering areas comparable with those of continents; and how great must have been the loss of the innumerable fossil forms which those rocks contained! in view of such destruction we are forced to conclude that our palaeontological collections, rich though they may appear, are really but small and random samples, giving no adequate idea of the mighty series of organism which have lived upon the earth.[ ] admitting, however, the extreme imperfection of the geological record as a whole, it may be urged that certain limited portions of it are fairly complete--as, for example, the various miocene deposits of india, europe, and north america,--and that in these we ought to find many examples of species and genera linked together by intermediate forms. it may be replied that in several cases this really occurs; and the reason why it does not occur more often is, that the theory of evolution requires that distinct genera should be linked together, not by a direct passage, but by the descent of both from a common ancestor, which may have lived in some much earlier age the record of which is either wanting or very incomplete. an illustration given by mr. darwin will make this more clear to those who have not studied the subject. the fantail and pouter pigeons are two very distinct and unlike breeds, which we yet know to have been both derived from the common wild rock-pigeon. now, if we had every variety of living pigeon before us, or even all those which have lived during the present century, we should find no intermediate types between these two--none combining in any degree the characters of the pouter with that of the fantail. neither should we ever find such an intermediate form, even had there been preserved a specimen of every breed of pigeon since the ancestral rock-pigeon was first tamed by man--a period of probably several thousand years. we thus see that a complete passage from one very distinct species to another could not be expected even had we a complete record of the life of any one period. what we require is a complete record of all the species that have existed since the two forms began to diverge from their common ancestor, and this the known imperfection of the record renders it almost impossible that we should ever attain. all that we have a right to expect is, that, as we multiply the fossil forms in any group, the gaps that at first existed in that group shall become less wide and less numerous; and also that, in some cases, a tolerably direct series shall be found, by which the more specialised forms of the present day shall be connected with more generalised ancestral types. we might also expect that when a country is now characterised by special groups of animals, the fossil forms that immediately preceded them shall, for the most part, belong to the same groups; and further, that, comparing the more ancient with the more modern types, we should find indications of progression, the earlier forms being, on the whole, lower in organisation, and less specialised in structure than the later. now evidence of evolution of these varied kinds is what we do find, and almost every fresh discovery adds to their number and cogency. in order, therefore, to show that the testimony given by geology is entirely in favour of the theory of descent with modification, some of the more striking of the facts will now be given. _geological evidences of evolution._ in an article in _nature_ (vol. xiv. p. ), professor judd calls attention to some recent discoveries in the hungarian plains, of fossil lacustrine shells, and their careful study by dr. neumayr and m. paul of the austrian geological survey. the beds in which they occur have accumulated to the thickness of feet, containing throughout abundance of fossils, and divisible into eight zones, each of which exhibits a well-marked and characteristic fauna. professor judd then describes the bearing of these discoveries as follows-- "the group of shells which affords the most interesting evidence of the origin of new forms through descent with modification is that of the genus vivipara or paludina, which occurs in prodigious abundance throughout the whole series of freshwater strata. we shall not, of course, attempt in this place to enter into any details concerning the forty distinct _forms_ of this genus (dr. neumayr very properly hesitates to call them all _species_), which are named and described in this monograph, and between which, as the authors show, so many connecting links, clearly illustrating the derivation of the newer from the older types, have been detected. on the minds of those who carefully examine the admirably engraved figures given in the plates accompanying this valuable memoir, or still better, the very large series of specimens from among which the subjects of these figures are selected, and which are now in the museum of the reichsanstalt of vienna, but little doubt will, we suspect, remain that the authors have fully made out their case, and have demonstrated that, beyond all controversy, the series with highly complicated ornamentation were variously derived by descent--the lines of which are in most cases perfectly clear and obvious--from the simple and unornamented vivipara achatinoides of the congerien-schichten (the lower division of the series of strata). it is interesting to notice that a large portion of these unquestionably derived forms depart so widely from the type of the genus vivipara, that they have been separated on so high an authority as that of sandberger, as a new genus, under the name of tulotoma. and hence we are led to the conclusion that a vast number of forms, certainly exhibiting specific distinctions, and according to some naturalists, differences even entitled to be regarded of generic value, have all a common ancestry." it is, as professor judd remarks, owing to the exceptionally favourable circumstances of a long-continued and unbroken series of deposits being formed under physical conditions either identical or very slowly changing, that we owe so complete a record of the process of organic change. usually, some disturbing elements, such as a sudden change of physical conditions, or the immigration of new sets of forms from other areas and the consequent retreat or partial extinction of the older fauna, interferes with the continuity of organic development, and produces those puzzling discordances so generally met with in geological formations of marine origin. while a case of the kind now described affords evidence of the origin of species complete and conclusive, though on a necessarily very limited scale, the very rarity of the conditions which are essential to such completeness serves to explain why it is that in most cases the direct evidence of evolution is not to be obtained. another illustration of the filling up of gaps between existing groups is afforded by professor huxley's researches on fossil crocodiles. the gap between the existing crocodiles and the lizards is very wide, but as we go back in geological time we meet with fossil forms which are to some extent intermediate and form a connected series. the three living genera--crocodilus, alligator, and gavialis--are found in the eocene formation, and allied forms of another genus, holops, in the chalk. from the chalk backward to the lias another group of genera occurs, having anatomical characteristics intermediate between the living crocodiles and the most ancient forms. these, forming two genera belodon and stagonolepis, are found in a still older formation, the trias. they have characters resembling some lizards, especially the remarkable hatteria of new zealand, and have also some resemblances to the dinosaurians--reptiles which in some respects approach birds. considering how comparatively few are the remains of this group of animals, the evidence which it affords of progressive development is remarkably clear.[ ] among the higher animals the rhinoceros, the horse, and the deer afford good evidence of advance in organisation and of the filling up of the gaps which separate the living forms from their nearest allies. the earliest ancestral forms of the rhinoceroses occur in the middle eocene of the united states, and were to some extent intermediate between the rhinoceros and tapir families, having like the latter four toes to the front feet, and three to those behind. these are followed in the upper eocene by the genus amynodon, in which the skull assumes more distinctly the rhinocerotic type. following this in the lower miocene we have the aceratherium, like the last in its feet, but still more decidedly a rhinoceros in its general structure. from this there are two diverging lines--one in the old world, the other in the new. in the former, to which the aceratherium is supposed to have migrated in early miocene times, when a mild climate and luxuriant vegetation prevailed far within the arctic circle, it gave rise to the ceratorhinus and the various horned rhinoceroses of late tertiary times and of those now living. in america a number of large hornless rhinoceroses were developed--they are found in the upper miocene, pliocene, and post-pliocene formations--and then became extinct. the true rhinoceroses have three toes on all the feet.[ ] _the pedigree of the horse tribe._ yet more remarkable is the evidence afforded by the ancestral forms of the horse tribe which have been discovered in the american tertiaries. the family equidae, comprising the living horse, asses, and zebras, differ widely from all other mammals in the peculiar structure of the feet, all of which terminate in a single large toe forming the hoof. they have forty teeth, the molars being formed of hard and soft material in crescentic folds, so as to be a powerful agent in grinding up hard grasses and other vegetable food. the former peculiarities depend upon modifications of the skeleton, which have been thus described by professor huxley:-- "let us turn in the first place to the fore-limb. in most quadrupeds, as in ourselves, the fore-arm contains distinct bones, called the radius and the ulna. the corresponding region in the horse seems at first to possess but one bone. careful observation, however, enables us to distinguish in this bone a part which clearly answers to the upper end of the ulna. this is closely united with the chief mass of the bone which represents the radius, and runs out into a slender shaft, which may be traced for some distance downwards upon the back of the radius, and then in most cases thins out and vanishes. it takes still more trouble to make sure of what is nevertheless the fact, that a small part of the lower end of the bone of a horse's fore-arm, which is only distinct in a very young foal, is really the lower extremity of the ulna. "what is commonly called the knee of a horse is its wrist. the 'cannon bone' answers to the middle bone of the five metacarpal bones which support the palm of the hand in ourselves. the pastern, coronary, and coffin bones of veterinarians answer to the joints of our middle fingers, while the hoof is simply a greatly enlarged and thickened nail. but if what lies below the horse's 'knee' thus corresponds to the middle finger in ourselves, what has become of the four other fingers or digits? we find in the places of the second and fourth digits only two slender splintlike bones, about two-thirds as long as the cannon bone, which gradually taper to their lower ends and bear no finger joints, or, as they are termed, phalanges. sometimes, small bony or gristly nodules are to be found at the bases of these two metacarpal splints, and it is probable that these represent rudiments of the first and fifth toes. thus, the part of the horse's skeleton which corresponds with that of the human hand, contains one overgrown middle digit, and at least two imperfect lateral digits; and these answer, respectively, to the third, the second, and the fourth fingers in man. "corresponding modifications are found in the hind limb. in ourselves, and in most quadrupeds, the leg contains two distinct bones, a large bone, the tibia, and a smaller and more slender bone, the fibula. but, in the horse, the fibula seems, at first, to be reduced to its upper end; a short slender bone united with the tibia, and ending in a point below, occupying its place. examination of the lower end of a young foal's shin-bone, however, shows a distinct portion of osseous matter which is the lower end of the fibula; so that the, apparently single, lower end of the shin-bone is really made up of the coalesced ends of the tibia and fibula, just as the, apparently single, lower end of the fore-arm bone is composed of the coalesced radius and ulna. "the heel of the horse is the part commonly known as the hock. the hinder cannon bone answers to the middle metatarsal bone of the human foot, the pastern, coronary, and coffin bones, to the middle toe bones; the hind hoof to the nail; as in the forefoot. and, as in the forefoot, there are merely two splints to represent the second and the fourth toes. sometimes a rudiment of a fifth toe appears to be traceable. "the teeth of a horse are not less peculiar than its limbs. the living engine, like all others, must be well stoked if it is to do its work; and the horse, if it is to make good its wear and tear, and to exert the enormous amount of force required for its propulsion, must be well and rapidly fed. to this end, good cutting instruments and powerful and lasting crushers are needful. accordingly, the twelve cutting teeth of a horse are close-set and concentrated in the forepart of its mouth, like so many adzes or chisels. the grinders or molars are large, and have an extremely complicated structure, being composed of a number of different substances of unequal hardness. the consequence of this is that they wear away at different rates; and, hence, the surface of each grinder is always as uneven as that of a good millstone."[ ] we thus see that the equidae differ very widely in structure from most other mammals. assuming the truth of the theory of evolution, we should expect to find traces among extinct animals of the steps by which this great modification has been effected; and we do really find traces of these steps, imperfectly among european fossils, but far more completely among those of america. it is a singular fact that, although no horse inhabited america when discovered by europeans, yet abundance of remains of extinct horses have been found both in north and south america in post-tertiary and upper pliocene deposits; and from these an almost continuous series of modified forms can be traced in the tertiary formation, till we reach, at the very base of the series, a primitive form so unlike our perfected animal, that, had we not the intermediate links, few persons would believe that the one was the ancestor of the other. the tracing out of this marvellous history we owe chiefly to professor marsh of yale college, who has himself discovered no less than thirty species of fossil equidae; and we will allow him to tell the story of the development of the horse from a humble progenitor in his own words. "the oldest representative of the horse at present known is the diminutive eohippus from the lower eocene. several species have been found, all about the size of a fox. like most of the early mammals, these ungulates had forty-four teeth, the molars with short crowns and quite distinct in form from the premolars. the ulna and fibula were entire and distinct, and there were four well-developed toes and a rudiment of another on the forefeet, and three toes behind. in the structure of the feet and teeth, the eohippus unmistakably indicates that the direct ancestral line to the modern horse has already separated from the other perissodactyles, or odd-toed ungulates. "in the next higher division of the eocene another genus, orohippus, makes its appearance, replacing eohippus, and showing a greater, though still distant, resemblance to the equine type. the rudimentary first digit of the forefoot has disappeared, and the last premolar has gone over to the molar series. orohippus was but little larger than eohippus, and in most other respects very similar. several species have been found, but none occur later than the upper eocene. "near the base of the miocene, we find a third closely allied genus, mesohippus, which is about as large as a sheep, and one stage nearer the horse. there are only three toes and a rudimentary splint on the forefeet, and three toes behind. two of the premolar teeth are quite like the molars. the ulna is no longer distinct or the fibula entire, and other characters show clearly that the transition is advancing. "in the upper miocene mesohippus is not found, but in its place a fourth form, miohippus, continues the line. this genus is near the anchitherium of europe, but presents several important differences. the three toes in each foot are more nearly of a size, and a rudiment of the fifth metacarpal bone is retained. all the known species of this genus are larger than those of mesohippus, and none of them pass above the miocene formation. "the genus protohippus of the lower pliocene is yet more equine, and some of its species equalled the ass in size. there are still three toes on each foot, but only the middle one, corresponding to the single toe of the horse, comes to the ground. this genus resembles most nearly the hipparion of europe. "in the pliocene we have the last stage of the series before reaching the horse, in the genus pliohippus, which has lost the small hooflets, and in other respects is very equine. only in the upper pliocene does the true equus appear and complete the genealogy of the horse, which in the post-tertiary roamed over the whole of north and south america, and soon after became extinct. this occurred long before the discovery of the continent by europeans, and no satisfactory reason for the extinction has yet been given. besides the characters i have mentioned, there are many others in the skeleton, skull, teeth, and brain of the forty or more intermediate species, which show that the transition from the eocene eohippus to the modern equus has taken place in the order indicated"[ ] (see fig. ). [illustration: fig. .--geological development of the horse tribe (eohippus since discovered).] well may professor huxley say that this is demonstrative evidence of evolution; the doctrine resting upon exactly as secure a foundation as did the copernican theory of the motions of the heavenly bodies at the time of its promulgation. both have the same basis--the coincidence of the observed facts with the theoretical requirements. _development of deer's horns._ another clear and unmistakable proof of evolution is afforded by one of the highest and latest developed tribes of mammals--the true deer. these differ from all other ruminants in possessing solid deciduous horns which are always more or less branched. they first appear in the middle miocene formation, and continue down to our time; and their development has been carefully traced by professor boyd dawkins, who thus summarises his results:-- "in the middle stage of the miocene the cervine antler consists merely of a simple forked crown (as in cervus dicroceros), which increases in size in the upper miocene, although it still remains small and erect, like that of the roe. in cervus matheroni it measures · inches, and throws off not more than four tines, all small. the deer living in auvergne in the succeeding or pliocene age, present us with another stage in the history of antler development. there, for the first time, we see antlers of the axis and rusa type, larger and longer, and more branching than any antlers were before, and possessing three or more well-developed tines. deer of this type abounded in pliocene europe. they belong to the oriental division of the cervidae, and their presence in europe confirms the evidence of the flora, brought forward by the comte de saporta, that the pliocene climate was warm. they have probably disappeared from europe in consequence of the lowering of the temperature in the pleistocene age, while their descendants have found a congenial home in the warmer regions of eastern asia. "in the latest stage of the pliocene--the upper pliocene of the val d'arno--the cervus dicranios of nesti presents us with antlers much smaller than those of the irish elk, but very complicated in their branching. this animal survived into the succeeding age, and is found in the pre-glacial forest bed of norfolk, being described by dr. falconer under the name of sedgwick's deer. the irish elk, moose, stag, reindeer, and fallow deer appear in europe in the pleistocene age, all with highly complicated antlers in the adult, and the first possessing the largest antlers yet known. of these the irish elk disappeared in the prehistoric age, after having lived in countless herds in ireland, while the rest have lived on into our own times in euro-asia, and, with the exception of the last, also in north america. "from this survey it is obvious that the cervine antlers have increased in size and complexity from the mid-miocene to the pleistocene age, and that their successive changes are analogous to those which are observed in the development of antlers in the living deer, which begin with a simple point, and increase in number of tines till their limit of growth be reached. in other words, the development of antlers indicated at successive and widely-separated pages of the geological record is the same as that observed in the history of a single living species. it is also obvious that the progressive diminution of size and complexity in the antlers, from the present time back into the early tertiary age, shows that we are approaching the zero of antler development in the mid-miocene. no trace of any antler-bearing ruminant has been met with in the lower miocenes, either of europe or the united states."[ ] _progressive brain-development._ the three illustrations now given sufficiently prove that, whenever the geological record approaches to completeness, we have evidence of the progressive change of species in definite directions, and from less developed to more developed types--exactly such a change as we may expect to find if the evolution theory be the true one. many other illustrations of a similar change could be given, but the animal groups in which they occur being less familiar, the details would be less interesting, and perhaps hardly intelligible. there is, however, one very remarkable proof of development that must be briefly noticed--that afforded by the steady increase in the size of the brain. this may be best stated in the words of professor marsh:-- "the real progress of mammalian life in america, from the beginning of the tertiary to the present, is well illustrated by the brain-growth, in which we have the key to many other changes. the earliest known tertiary mammals all had very small brains, and in some forms this organ was proportionally less than in certain reptiles. there was a gradual increase in the size of the brain during this period, and it is interesting to find that this growth was mainly confined to the cerebral hemispheres, or higher portion of the brain. in most groups of mammals the brain has gradually become more convoluted, and thus increased in quality as well as quantity. in some also the cerebellum and olfactory lobes, the lower parts of the brain, have even diminished in size. in the long struggle for existence during tertiary time the big brains won, then as now; and the increasing power thus gained rendered useless many structures inherited from primitive ancestors, but no longer adapted to new conditions." this remarkable proof of development in the organ of the mental faculties, forms a fitting climax to the evidence already adduced of the progressive evolution of the general structure of the body, as illustrated by the bony skeleton. we now pass on to another class of facts equally suggestive of evolution. _the local relations of fossil and living animals._ if all existing animals have been produced from ancestral forms--mostly extinct--under the law of variation and natural selection, we may expect to find in most cases a close relation between the living forms of each country and those which inhabited it in the immediately preceding epoch. but if species have originated in some quite different way, either by any kind of special creation, or by sudden advances of organisation in the offspring of preceding types, such close relationship would not be found; and facts of this kind become, therefore, to some extent a test of evolution under natural selection or some other law of gradual change. of course the relationship will not appear when extensive migration has occurred, by which the inhabitants of one region have been able to take possession of another region, and destroy or drive out its original inhabitants, as has sometimes happened. but such cases are comparatively rare, except where great changes of climate are known to have occurred; and we usually do find a remarkable continuity between the existing fauna and flora of a country and those of the immediately preceding age. a few of the more remarkable of these cases will now be briefly noticed. the mammalian fauna of australia consists, as is well known, wholly of the lowest forms--the marsupials and monotremata--except only a few species of mice. this is accounted for by the complete isolation of the country from the asiatic continent during the whole period of the development of the higher animals. at some earlier epoch the ancestral marsupials, which abounded both in europe and north america in the middle of the secondary period, entered the country, and have since remained there, free from the competition of higher forms, and have undergone a special development in accordance with the peculiar conditions of a limited area. while in the large continents higher forms of mammalia have been developed, which have almost or wholly exterminated the less perfect marsupials, in australia these latter have become modified into such varied forms as the leaping kangaroos, the burrowing wombats, the arboreal phalangers, the insectivorous bandicoots, and the carnivorous dasyuridae or native cats, culminating in the thylacinus or "tiger-wolf" of tasmania--animals as unlike each other as our sheep, rabbits, squirrels, and dogs, but all retaining the characteristic features of the marsupial type. now in the caves and late tertiary or post-tertiary deposits of australia the remains of many extinct mammalia have been found, but all are marsupials. there are many kangaroos, some larger than any living species, and others more allied to the tree-kangaroos of new guinea; a large wombat as large as a tapir; the diprotodon, a thick-limbed kangaroo the size of a rhinoceros or small elephant; and a quite different animal, the nototherium, nearly as large. the carnivorous thylacinus of tasmania is also found fossil; and a huge phalanger, thylacoleo, the size of a lion, believed by professor owen and by professor oscar schmidt to have been equally carnivorous and destructive.[ ] besides these, there are many other species more resembling the living forms both in size and structure, of which they may be, in some cases, the direct ancestors. two species of extinct echidna, belonging to the very low monotremata, have also been found in new south wales. next to australia, south america possesses the most remarkable assemblage of peculiar mammals, in its numerous edentata--the sloths, ant-eaters, and armadillos; its rodents, such as the cavies and chinchillas; its marsupial opossums, and its quadrumana of the family cebidae. remains of extinct species of all these have been found in the caves of brazil, of post-pliocene age; while in the earlier pliocene deposits of the pampas many distinct genera of these groups have been found, some of gigantic size and extraordinary form. there are armadillos of many types, some being as large as elephants; gigantic sloths of the genera megatherium, megalonyx, mylodon, lestodon, and many others; rodents belonging to the american families cavidae and chinchillidae; and ungulates allied to the llama; besides many other extinct forms of intermediate types or of uncertain affinities.[ ] the extinct moas of new zealand--huge wingless birds allied to the living apteryx--illustrate the same general law. the examples now quoted, besides illustrating and enforcing the general fact of evolution, throw some light on the usual character of the modification and progression of animal forms. in the cases where the geological record is tolerably complete, we find a continuous development of some kind--either in complexity of ornamentation, as in the fossil paludinas of the hungarian lake-basins; in size and in the specialisation of the feet and teeth, as in the american fossil horses; or in the increased development of the branching horns, as in the true deer. in each of these cases specialisation and adaptation to the conditions of the environment appear to have reached their limits, and any change of these conditions, especially if it be at all rapid or accompanied by the competition of less developed but more adaptable forms, is liable to cause the extinction of the most highly developed groups. such we know was the case with the horse tribe in america, which totally disappeared in that continent at an epoch so recent that we cannot be sure that the disappearance was not witnessed, perhaps caused, by man; while even in the eastern hemisphere it is the smaller species--the asses and the zebras--that have persisted, while the larger and more highly developed true horses have almost, if not quite, disappeared in a state of nature. so we find, both in australia and south america, that in a quite recent period many of the largest and most specialised forms have become extinct, while only the smaller types have survived to our day; and a similar fact is to be observed in many of the earlier geological epochs, a group progressing and reaching a maximum of size or complexity and then dying out, or leaving at most but few and pigmy representatives. _cause of extinction of large animals._ now there are several reasons for the repeated extinction of large rather than of small animals. in the first place, animals of great bulk require a proportionate supply of food, and any adverse change of conditions would affect them more seriously than it would smaller animals. in the next place, the extreme specialisation of many of these large animals would render it less easy for them to be modified in any new direction suited to changed conditions. still more important, perhaps, is the fact that very large animals always increase slowly as compared with small ones--the elephant producing a single young one every three years, while a rabbit may have a litter of seven or eight young two or three times a year. now the probability of favourable variations will be in direct proportion to the population of the species, and as the smaller animals are not only many hundred times more numerous than the largest, but also increase perhaps a hundred times as rapidly, they are able to become quickly modified by variation and natural selection in harmony with changed conditions, while the large and bulky species, being unable to vary quickly enough, are obliged to succumb in the struggle for existence. as professor marsh well observes: "in every vigorous primitive type which was destined to survive many geological changes, there seems to have been a tendency to throw off lateral branches, which became highly specialised and soon died out, because they were unable to adapt themselves to new conditions." and he goes on to show how the whole narrow path of the persistent suilline type, throughout the entire series of the american tertiaries, is strewed with the remains of such ambitious offshoots, many of them attaining the size of a rhinoceros; "while the typical pig, with an obstinacy never lost, has held on in spite of catastrophes and evolution, and still lives in america to-day." _indications of general progression in plants and animals._ one of the most powerful arguments formerly adduced against evolution was, that geology afforded no evidence of the gradual development of organic forms, but that whole tribes and classes appeared suddenly at definite epochs, and often in great variety and exhibiting a very perfect organisation. the mammalia, for example, were long thought to have first appeared in tertiary times, where they are represented in some of the earlier deposits by all the great divisions of the class fully developed--carnivora, rodents, insectivora, marsupials, and even the perissodactyle and artiodactyle divisions of the ungulata--as clearly defined as at the present day. the discovery in of a single lower jaw in the stonesfield slate of oxfordshire hardly threw doubt on the generalisation, since either its mammalian character was denied, or the geological position of the strata, in which it was found, was held to have been erroneously determined. but since then, at intervals of many years, other remains of mammalia have been discovered in the secondary strata, ranging from the upper oolite to the upper trias both in europe and the united states, and one even (tritylodon) in the trias of south africa. all these are either marsupials, or of some still lower type of mammalia; but they consist of many distinct forms classed in about twenty genera. nevertheless, a great gap still exists between these mammals and those of the tertiary strata, since no mammal of any kind has been found in any part of the cretaceous formation, although in several of its subdivisions abundance of land plants, freshwater shells, and air-breathing reptiles have been discovered. so with fishes. in the last century none had been obtained lower than the carboniferous formation; thirty years later they were found to be very abundant in the devonian rocks, and later still they were discovered in the upper ludlow and lower ludlow beds of the silurian formation. we thus see that such sudden appearances are deceptive, and are, in fact, only what we ought to expect from the known imperfection of the geological record. the conditions favourable to the fossilisation of any group of animals occur comparatively rarely, and only in very limited areas; while the conditions essential for their permanent preservation in the rocks, amid all the destruction caused by denudation or metamorphism, are still more exceptional. and when they are thus preserved to our day, the particular part of the rocks in which they lie hidden may not be on the surface but buried down deep under other strata, and may thus, except in the case of mineral-bearing deposits, be altogether out of our reach. then, again, how large a proportion of the earth consists of wild and uncivilised regions in which no exploration of the rocks has been yet made, so that whether we shall find the fossilised remains of any particular group of animals which lived during a limited period of the earth's history, and in a limited area, depends upon at least a fivefold combination of chances. now, if we take each of these chances separately as only ten to one against us (and some are certainly more than this), then the actual chance against our finding the fossil remains, say of any one order of mammalia, or of land plants, at any particular geological horizon, will be about a hundred thousand to one. it may be said, if the chances are so great, how is it that we find such immense numbers of fossil species exceeding in number, in some groups, all those that are now living? but this is exactly what we should expect, because the number of species of organisms that have ever lived upon the earth, since the earliest geological times, will probably be many hundred times greater than those now existing of which we have any knowledge; and hence the enormous gaps and chasms in the geological record of extinct forms is not to be wondered at. yet, notwithstanding these chasms in our knowledge, if evolution is true, there ought to have been, on the whole, progression in all the chief types of life. the higher and more specialised forms should have come into existence later than the lower and more generalised forms; and however fragmentary the portions we possess of the whole tree of life upon the earth, they ought to show us broadly that such a progressive evolution has taken place. we have seen that in some special groups, already referred to, such a progression is clearly visible, and we will now cast a hasty glance over the entire series of fossil forms, in order to see if a similar progression is manifested by them as a whole. _the progressive development of plants._ ever since fossil plants have been collected and studied, the broad fact has been apparent that the early plants--those of the coal formation--were mainly cryptogamous, while in the tertiary deposits the higher flowering plants prevailed. in the intermediate secondary epoch the gymnosperms--cycads and coniferae--formed a prominent part of the vegetation, and as these have usually been held to be a kind of transition form between the flowerless and flowering plants, the geological succession has always, broadly speaking, been in accordance with the theory of evolution. beyond this, however, the facts were very puzzling. the highest cryptogams--ferns, lycopods, and equisetaceae--appeared suddenly, and in immense profusion in the coal formation, at which period they attained a development they have never since surpassed or even equalled; while the highest plants--the dicotyledonous and monocotyledonous angiosperms--which now form the bulk of the vegetation of the world, and exhibit the most wonderful modifications of form and structure, were almost unknown till the tertiary period, when they suddenly appeared in full development, and, for the most part, under the same generic forms as now exist. during the latter half of the present century, however, great additions have been made to our knowledge of fossil plants; and although there are still indications of vast gaps in our knowledge, due, no doubt, to the very exceptional conditions required for the preservation of plant remains, we now possess evidence of a more continuous development of the various types of vegetation. according to mr. lester f. ward, between and species of fossil plants have been described or indicated; and, owing to the careful study of the nervation of leaves, a large number of these are referable to their proper orders or genera, and therefore give us some notion--which, though very imperfect, is probably accurate in its main outlines--of the progressive development of vegetation on the earth.[ ] the following is a summary of the facts as given by mr. ward:-- the lowest forms of vegetable life--the cellular plants--have been found in lower silurian deposits in the form of three species of marine algae; and in the whole silurian formation fifty species have been recognised. we cannot for a moment suppose, however, that this indicates the first appearance of vegetable life upon the earth, for in these same lower silurian beds the more highly organised vascular cryptogams appear in the form of rhizocarps--plants allied to marsilea and azolla,--and a very little higher, ferns, lycopods, and even conifers appear. we have indications, however, of a still more ancient vegetation, in the carbonaceous shales and thick beds of graphite far down in the middle laurentian, since there is no other known agency than the vegetable cell by means of which carbon can be extracted from the atmosphere and fixed in the solid state. these great beds of graphite, therefore, imply the existence of abundance of vegetable life at the very commencement of the era of which we have any geological record.[ ] ferns, as already stated, begin in the middle silurian formation with the eopteris morrieri. in the devonian, we have species, in the carboniferous , and in the permian species; after which fossil ferns diminish greatly, though they are found in every formation; and the fact that fully living species are known, while the richest portion of the tertiary in fossil plants--the miocene--- has only produced species, will serve to indicate the extreme imperfection of the geological record. the equisetaceae (horsetails) which also first appear in the silurian and reach their maximum development in the coal formation, are, in all succeeding formations, far less numerous than ferns, and only thirty living species are known. lycopodiaceae, though still more abundant in the coal formation, are very rarely found in any succeeding deposit, though the living species are tolerably numerous, about having been described. as we cannot suppose them to have really diminished and then increased again in this extraordinary manner, we have another indication of the exceptional nature of plant preservation and the extreme and erratic character of the imperfection of the record. passing now to the next higher division of plants--the gymnosperms--we find coniferae appearing in the upper silurian, becoming tolerably abundant in the devonian, and reaching a maximum in the carboniferous, from which formation more than species are known, equal to the number recorded as now living. they occur in all succeeding formations, being abundant in the oolite, and excessively so in the miocene, from which species have been described. the allied family of gymnosperms, the cycadaceae, first appear in the carboniferous era, but very scantily; are most abundant in the oolite, from which formation species are known, and then steadily diminish to the tertiary, although there are seventy-five living species. we now come to the true flowering plants, and we first meet with monocotyledons in the carboniferous and permian formations. the character of these fossils was long disputed, but is now believed to be well established; and the sub-class continues to be present in small numbers in all succeeding deposits, becoming rather plentiful in the upper cretaceous, and very abundant in the eocene and miocene. in the latter formation species have been discovered; but the species in the eocene form a larger proportion of the total vegetation of the period. true dicotyledons appear very much later, in the cretaceous period, and only in its upper division, if we except a single species from the urgonian beds of greenland. the remarkable thing is that we here find the sub-class fully developed and in great luxuriance of types, all the three divisions--apetalae, polypetalae, and gamopetalae--being represented, with a total of no less than species. among them are such familiar forms as the poplar, the birch, the beech, the sycamore, and the oak; as well as the fig, the true laurel, the sassafras, the persimmon, the maple, the walnut, the magnolia, and even the apple and the plum tribes. passing on to the tertiary period the numbers increase, till they reach their maximum in the miocene, where more than species of dicotyledons have been discovered. among these the proportionate number of the higher gamopetalae has slightly increased, but is considerably less than at the present day. _possible cause of sudden late appearance of exogens._ the sudden appearance of fully developed exogenous flowering plants in the cretaceous period is very analogous to the equally sudden appearance of all the chief types of placental mammalia in the eocene; and in both cases we must feel sure that this suddenness is only apparent, due to unknown conditions which have prevented their preservation (or their discovery) in earlier formations. the case of the dicotyledonous plants is in some respects the most extraordinary, because in the earlier mesozoic formations we appear to have a fair representation of the flora of the period, including such varied forms as ferns, equisetums, cycads, conifers, and monocotyledons. the only hint at an explanation of this anomaly has been given by mr. ball, who supposes that all these groups inhabited the lowlands, where there was not only excessive heat and moisture, but also a superabundance of carbonic acid in the atmosphere--conditions under which these groups had been developed, but which were prejudicial to the dicotyledons. these latter are supposed to have originated on the high table-lands and mountain ranges, in a rarer and drier atmosphere in which the quantity of carbonic acid gas was much less; and any deposits formed in lake beds at high altitudes and at such a remote epoch have been destroyed by denudation, and hence we have no record of their existence.[ ] during a few weeks spent recently in the rocky mountains, i was struck by the great scarcity of monocotyledons and ferns in comparison with dicotyledons--a scarcity due apparently to the dryness and rarity of the atmosphere favouring the higher groups. if we compare coulter's _rocky mountain botany_ with gray's _botany of the northern (east) united states_, we have two areas which differ chiefly in the points of altitude and atmospheric moisture. unfortunately, in neither of these works are the species consecutively numbered; but by taking the pages occupied by the two divisions of dicotyledons on the one hand, monocotyledons and ferns on the other, we can obtain a good approximation. in this way we find that in the flora of the north-eastern states the monocotyledons and ferns are to the dicotyledons in the proportion of to ; in the rocky mountains they are in the proportion of only to ; while if we take an exclusively alpine flora, as given by mr. ball, there are not one-fifth as many monocotyledons as dicotyledons. these facts show that even at the present day elevated plateaux and mountains are more favourable to dicotyledons than to monocotyledons, and we may, therefore, well suppose that the former originated within such elevated areas, and were for long ages confined to them. it is interesting to note that their richest early remains have been found in the central regions of the north american continent, where they now, proportionally, most abound, and where the conditions of altitude and a dry atmosphere were probably present at a very early period. [illustration: fig. .--diagram illustrating the geological distribution of plants.] the diagram (fig. ), slightly modified from one given by mr. ward, will illustrate our present knowledge of the development of the vegetable kingdom in geological time. the shaded vertical bands exhibit the proportions of the fossil forms actually discovered, while the outline extensions are intended to show what we may fairly presume to have been the approximate periods of origin, and progressive increase of the number of species, of the chief divisions of the vegetable kingdom. these seem to accord fairly well with their respective grades of development, and thus offer no obstacle to the acceptance of the belief in their progressive evolution. _geological distribution of insects._ the marvellous development of insects into such an endless variety of forms, their extreme specialisation, and their adaptation to almost every possible condition of life, would almost necessarily imply an extreme antiquity. owing, however, to their small size, their lightness, and their usually aerial habits, no class of animals has been so scantily preserved in the rocks; and it is only recently that the whole of the scattered material relating to fossil insects and their allies have been brought together by mr. samuel h. scudder of boston, and we have thus learned their bearing on the theory of evolution.[ ] the most striking fact which presents itself on a glance at the distribution of fossil insects, is the completeness of the representation of all the chief types far back in the secondary period, at which time many of the existing families appear to have been perfectly differentiated. thus in the lias we find dragonflies "apparently as highly specialised as to-day, no less than four tribes being present." of beetles we have undoubted curculionidae from the lias and trias; chrysomelidae in the same deposits; cerambycidae in the oolites; scarabaeidae in the lias; buprestidae in the trias; elateridae, trogositidae, and nitidulidae in the lias; staphylinidae in the english purbecks; while hydrophilidae, gyrinidae, and carabidae occur in the lias. all these forms are well represented, but there are many other families doubtfully identified in equally ancient rocks. diptera of the families empidae, asilidae, and tipulidae have been found as far back as the lias. of lepidoptera, sphingidae and tineidae have been found in the oolite; while ants, representing the highly specialised hymenoptera, have occurred in the purbeck and lias. this remarkable identity of the families of very ancient with those of existing insects is quite comparable with the apparently sudden appearance of existing genera of trees in the cretaceous epoch. in both cases we feel certain that we must go very much farther back in order to find the ancestral forms from which they were developed, and that at any moment some fresh discovery may revolutionise our ideas as to the antiquity of certain groups. such a discovery was made while mr. scudder's work was passing through the press. up to that date all the existing orders of true insects appeared to have originated in the trias, the alleged moth and beetle of the coal formation having been incorrectly determined. but now, undoubted remains of beetles have been found in the coal measures of silesia, thus supporting the interpretation of the borings in carboniferous trees as having been made by insects of this order, and carrying back this highly specialised form of insect life well into palaeozoic times. such a discovery renders all speculation as to the origin of true insects premature, because we may feel sure that all the other orders of insects, except perhaps hymenoptera and lepidoptera, were contemporaneous with the highly specialised beetles. the less highly organised terrestrial arthropoda--the arachnida and myriapoda--are, as might be expected, much more ancient. a fossil spider has been found in the carboniferous, and scorpions in the upper silurian rocks of scotland, sweden, and the united states. myriapoda have been found abundantly in the carboniferous and devonian formations; but all are of extinct orders, exhibiting a more generalised structure than living forms. much more extraordinary, however, is the presence in the palaeozoic formations of ancestral forms of true insects, termed by mr. scudder palaeodictyoptera. they consist of generalised cockroaches and walking-stick insects (orthopteroidea); ancient mayflies and allied forms, of which there are six families and more than thirty genera (neuropteroidea); three genera of hemipteroidea resembling various homoptera and hemiptera, mostly from the carboniferous formation, a few from the devonian, and one ancestral cockroach (palaeoblattina) from the middle silurian sandstone of france. if this occurrence of a true hexapod insect from the middle silurian be really established, taken in connection with the well-defined coleoptera from the carboniferous, the origin of the entire group of terrestrial arthropoda is necessarily thrown back into the cambrian epoch, if not earlier. and this cannot be considered improbable in view of the highly differentiated land plants--ferns, equisetums, and lycopods--in the middle or lower silurian, and even a conifer (cordaites robbii) in the upper silurian; while the beds of graphite in the laurentian were probably formed from terrestrial vegetation. on the whole, then, we may affirm that, although the geological record of the insect life of the earth is exceptionally imperfect, it yet decidedly supports the evolution hypothesis. the most specialised order, lepidoptera, is the most recent, only dating back to the oolite; the hymenoptera, diptera, and homoptera go as far as the lias; while the orthoptera and neuroptera extend to the trias. the recent discovery of coleoptera in the carboniferous shows, however, that the preceding limits are not absolute, and will probably soon be overpassed. only the more generalised ancestral forms of winged insects have been traced back to silurian time, and along with them the less highly organised scorpions; facts which serve to show us the extreme imperfection of our knowledge, and indicate possibilities of a world of terrestrial life in the remotest palaeozoic times. _geological succession of vertebrata._ the lowest forms of vertebrates are the fishes, and these appear first in the geological record in the upper silurian formation. the most ancient known fish is a pteraspis, one of the bucklered ganoids or plated fishes--by no means a very low type--allied to the sturgeon (accipenser) and alligator-gar (lepidosteus), but, as a group, now nearly extinct. almost equally ancient are the sharks, which under various forms still abound in our seas. we cannot suppose these to be nearly the earliest fishes, especially as the two lowest orders, now represented by the amphioxus or lancelet and the lampreys, have not yet been found fossil. the ganoids were greatly developed in the devonian era, and continued till the cretaceous, when they gave way to the true osseous fishes, which had first appeared in the jurassic period, and have continued to increase till the present day. this much later appearance of the higher osseous fishes is quite in accordance with evolution, although some of the very lowest forms, the lancelet and the lampreys, together with the archaic ceratodus, have survived to our time. the amphibia, represented by the extinct labyrinthodons, appear first in the carboniferous rocks, and these peculiar forms became extinct early in the secondary period. the labyrinthodons were, however, highly specialised, and do not at all indicate the origin of the class, which may be as ancient as the lower forms of fishes. hardly any recognisable remains of our existing groups--the frogs, toads, and salamanders--are found before the tertiary period, a fact which indicates the extreme imperfection of the record as regards this class of animals. true reptiles have not been found till we reach the permian where prohatteria and proterosaurus occur, the former closely allied to the lizard-like sphenodon of new zealand, the latter having its nearest allies in the same group of reptiles--rhyncocephala, other forms of which occur in the trias. in this last-named formation the earliest crocodiles--phytosaurus (belodon) and stagonolepis occur, as well as the earliest tortoises--chelytherium, proganochelys, and psephoderma.[ ] fossil serpents have been first found in the cretaceous formation, but the conditions for the preservation of these forms have evidently been unfavourable, and the record is correspondingly incomplete. the marine plesiosauri and ichthyosauri, the flying pterodactyles, the terrestrial iguanodon of europe, and the huge atlantosaurus of colorado--the largest land animal that has ever lived upon the earth[ ]--all belong to special developments of the reptilian type which flourished during the secondary epoch, and then became extinct. birds are among the rarest of fossils, due, no doubt, to their aerial habits removing them from the ordinary dangers of flood, bog, or ice which overwhelm mammals and reptiles, and also to their small specific gravity which keeps them floating on the surface of water till devoured. their remains were long confined to tertiary deposits, where many living genera and a few extinct forms have been found. the only birds yet known from the older rocks are the toothed birds (odontornithes) of the cretaceous beds of the united states, belonging to two distinct families and many genera; a penguin-like form (enaliornis) from the upper greensand of cambridge; and the well-known long-tailed archaeopteryx from the upper oolite of bavaria. the record is thus imperfect and fragmentary in the extreme; but it yet shows us, in the few birds discovered in the older rocks, more primitive and generalised types, while the tertiary birds had already become specialised like those living, and had lost both the teeth and the long vertebral tail, which indicate reptilian affinities in the earlier mammalia have been found, as already stated, as far back as the trias formation, in europe in the united states and in south africa, all being very small, and belonging either to the marsupial order, or to some still lower and more generalised type, out of which both marsupials and insectivora were developed. other allied forms have been found in the lower and upper oolite both of europe and the united states. but there is then a great gap in the whole cretaceous formation, from which no mammal has been obtained, although both in the wealden and the upper chalk in europe, and in the upper cretaceous deposits of the united states an abundant and well-preserved terrestrial flora has been discovered. why no mammals have left their remains here it is impossible to say. we can only suppose that the limited areas in which land plants have been so abundantly preserved, did not present the conditions which are needed for the fossilisation and preservation of mammalian remains. when we come to the tertiary formation, we find mammals in abundance; but a wonderful change has taken place. the obscure early types have disappeared, and we discover in their place a whole series of forms belonging to existing orders, and even sometimes to existing families. thus, in the eocene we have remains of the opossum family; bats apparently belonging to living genera; rodents allied to the south american cavies and to dormice and squirrels; hoofed animals belonging to the odd-toed and even-toed groups; and ancestral forms of cats, civets, dogs, with a number of more generalised forms of carnivora. besides these there are whales, lemurs, and many strange ancestral forms of proboscidea.[ ] the great diversity of forms and structures at so remote an epoch would require for their development an amount of time, which, judging by the changes that have occurred in other groups, would carry us back far into the mesozoic period. in order to understand why we have no record of these changes in any part of the world, we must fall back upon some such supposition as we made in the case of the dicotyledonous plants. perhaps, indeed, the two cases are really connected, and the upland regions of the primeval world, which saw the development of our higher vegetation, may have also afforded the theatre for the gradual development of the varied mammalian types which surprise us by their sudden appearance in tertiary times. [illustration: geological distribution of mammalia.] notwithstanding these irregularities and gaps in the record, the accompanying table, summarising our actual knowledge of the geological distribution of the five classes of vertebrata, exhibits a steady progression from lower to higher types, excepting only the deficiency in the bird record which is easily explained. the comparative perfection of type in which each of these classes first appears, renders it certain that the origin of each and all of them must be sought much farther back than any records which have yet been discovered. the researches of palaeontologists and embryologists indicate a reptilian origin for birds and mammals, while reptiles and amphibia arose, perhaps independently, from fishes. _concluding remarks._ the brief review we have now taken of the more suggestive facts presented by the geological succession of organic forms, is sufficient to show that most, if not all, of the supposed difficulties which it presents in the way of evolution, are due either to imperfections in the geological record itself, or to our still very incomplete knowledge of what is really recorded in the earth's crust. we learn, however, that just as discovery progresses, gaps are filled up and difficulties disappear; while, in the case of many individual groups, we have already obtained all the evidence of progressive development that can reasonably be expected. we conclude, therefore, that the geological difficulty has now disappeared; and that this noble science, when properly understood, affords clear and weighty evidence of evolution. footnotes: [footnote : the reader who desires to understand this subject more fully, should study chap. x. of the _origin of species_, and chap. xiv. of sir charles lyell's _principles of geology_.] [footnote : on "stagonolepis robertsoni and on the evolution of the crocodilia," in _q.j. of geological society_, ; and abstract in _nature_, vol. xii. p. .] [footnote : from a paper by messrs. scott and osborne, "on the origin and development of the rhinoceros group," read before the british association in .] [footnote : american addresses, pp. - .] [footnote : lecture on the introduction and succession of vertebrate life in america, _nature_, vol. xvi. p. .] [footnote : _nature_, vol. xxv. p. .] [footnote : see _the mammalia in their relation to primeval times_, p. .] [footnote : for a brief enumeration and description of these fossils, see the author's _geographical distribution of animals_, vol. i. p. .] [footnote : sketch of palaeobotany in fifth annual report of u.s. geological survey, - , pp. - , with diagrams. sir j. william dawson, speaking of the value of leaves for the determination of fossil plants, says: "in my own experience i have often found determinations of the leaves of trees confirmed by the discovery of their fruits or of the structure of their stems. thus, in the rich cretaceous plant-beds of the dunvegan series, we have beech-nuts associated in the same bed with leaves referred to _fagus_. in the laramie beds i determined many years ago nuts of the _trapa_ or water-chestnut, and subsequently lesquereux found in beds in the united states leaves which he referred to the same genus. later, i found in collections made on the red deer river of canada my fruits and lesquereux's leaves on the same slab. the presence of trees of the genera _carya_ and _juglans_ in the same formation was inferred from their leaves, and specimens have since been obtained of silicified wood with the microscopic structure of the modern butternut. still we are willing to admit that determinations from leaves alone are liable to doubt."--_the geological history of plants_, p. .] [footnote : sir j. william dawson's _geological history of plants_, p. .] [footnote : "on the origin of the flora of the european alps," _proc. of roy. geog. society_, vol. i. ( ), pp. - .] [footnote : systematic review of our present knowledge of fossil insects, including myriapods and arachnids (_bull. of u.s. geol. survey_, no. , washington, ).] [footnote : for the facts as to the early appearance of the above named groups of reptiles i am indebted to mr. e. lydekker of the geological department of the natural history museum.] [footnote : according to professor marsh this creature was or feet long, and when erect, at least feet in height. it fed upon the foliage of the mountain forests of the cretaceous epoch, the remains of which are preserved with it.] [footnote : for fuller details, see the author's _geographical distribution of animals_, and heilprin's _geographical and geological distribution of animals_.] chapter xiv fundamental problems in relation to variation and heredity fundamental difficulties and objections--mr. herbert spencer's factors of organic evolution--disuse and effects of withdrawal of natural selection--supposed effects of disuse among wild animals--difficulty as to co-adaptation of parts by variation and selection--direct action of the environment--the american school of evolutionists--origin of the feet of the ungulates--supposed action of animal intelligence--semper on the direct influence of the environment--professor geddes's theory of variation in plants--objections to the theory--on the origin of spines--variation and selection overpower the effects of use and disuse--supposed action of the environment in imitating variations--weismann's theory of heredity--the cause of variation--the non-heredity of acquired characters--the theory of instinct--concluding remarks. having now set forth and illustrated at some length the most important of the applications of the development hypothesis in the explanation of the broader and more generally interesting phenomena presented by the organic world, we propose to discuss some of the more fundamental problems and difficulties which have recently been adduced by eminent naturalists. it is the more necessary to do this, because there is now a tendency to minimise the action of natural selection in the production of organic forms, and to set up in its place certain fundamental principles of variation or laws of growth, which it is urged are the real originators of the several lines of development, and of most of the variety of form and structure in the vegetable and animal kingdoms. these views have, moreover, been seized upon by popular writers to throw doubt and discredit on the whole theory of evolution, and especially on darwin's presentation of that theory, to the bewilderment of the general public, who are quite unable to decide how far the new views, even if well established, tend to subvert the darwinian theory, or whether they are really more than subsidiary parts of it, and quite powerless without it to produce any effect whatever. the writers whose special views we now propose to consider are: ( ) mr. herbert spencer, on modification of structures arising from modification of functions, as set forth in his _factors of organic evolution_. ( ) dr. e.d. cope, who advocates similar views in detail, in his work entitled _the origin of the fittest_, and may be considered the head of a school of american naturalists who minimise the agency of natural selection. ( ) dr. karl semper, who has especially studied the direct influence of the environment in the whole animal kingdom, and has set forth his views in a volume on _the natural conditions of existence as they affect animal life_. ( ) mr. patrick geddes, who urges that fundamental laws of growth, and the antagonism of vegetative and reproductive forces, account for much that has been imputed to natural selection. we will now endeavour to ascertain what are the more important facts and arguments adduced by each of the above writers, and how far they offer a substitute for the action of natural selection; having done which, a brief account will be given of the views of dr. aug. weismann, whose theory of heredity will, if established, strike at the very root of the arguments of the first three of the writers above referred to. _mr. herbert spencer's factors of organic evolution._ mr. spencer, while fully recognising the importance and wide range of the principle of natural selection, thinks that sufficient weight has not been given to the effects of use and disuse as a factor in evolution, or to the direct action of the environment in determining or modifying organic structures. as examples of the former class of actions, he adduces the decreased size of the jaws in the civilised races of mankind, the inheritance of nervous disease produced by overwork, the great and inherited development of the udders in cows and goats, and the shortened legs, jaws, and snout in improved races of pigs--the two latter examples being quoted from mr. darwin,--and other cases of like nature. as examples of the latter, mr. darwin is again quoted as admitting that there are many cases in which the action of similar conditions appears to have produced corresponding changes in different species; and we have a very elaborate discussion of the direct action of the medium in modifying the protoplasm of simple organisms, so as to bring about the difference between the outer surface and the inner part that characterises the cells or other units of which they are formed. now, although this essay did little more than bring together facts which had been already adduced by mr. darwin or by mr. spencer himself, and lay stress upon their importance, its publication in a popular review was immediately seized upon as "an avowed and definite declaration against some of the leading ideas on which the mechanical philosophy depends," and as being "fatal to the adequacy of the mechanical philosophy as any explanation of organic evolution,"[ ]--an expression of opinion which would be repudiated by every darwinian. for, even admitting the interpretation which mr. spencer puts on the facts he adduces, they are all included in the causes which darwin himself recognised as having acted in bringing about the infinitude of forms in the organic world. in the concluding chapter of the _origin of species_ he says: "i have now recapitulated the facts and considerations which have thoroughly convinced me that species have been modified during a long course of descent. this has been effected chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner--that is, in relation to adaptive structures whether past or present, by the direct action of external conditions, and by variations which seem to us, in our ignorance, to arise spontaneously." this passage, summarising darwin's whole inquiry, and explaining his final point of view, shows how very inaccurate may be the popular notion, as expressed by the duke of argyll, of any supposed additions to the causes of change of species as recognised by darwin. but, as we shall see presently, there is now much reason to believe that the supposed inheritance of acquired modifications--that is, of the effects of use and disuse, or of the direct influence of the environment--is not a fact; and if so, the very foundation is taken away from the whole class of objections on which so much stress is now laid. it therefore becomes important to inquire whether the facts adduced by darwin, spencer, and others, do really necessitate such inheritance, or whether any other interpretation of them is possible. i believe there is such an interpretation; and we will first consider the cases of disuse on which mr. spencer lays most stress. the cases mr. spencer adduces as demonstrating the effects of disuse in diminishing the size and strength of organs are, the diminished size of the jaws in the races of civilised men, and the diminution of the muscles used in closing the jaws in the case of pet-dogs fed for generations on soft food. he argues that the minute reduction in any one generation could not possibly have been useful, and, therefore, not the subject of natural selection; and against the theory of correlation of the diminished jaw with increased brain in man, he urges that there are cases of large brain development, accompanied by jaws above the average size. against the theory of economy of nutrition in the case of the pet-dogs, he places the abundant food of these animals which would render such economy needless. but neither he nor mr. darwin has considered the effects of the withdrawal of the action of natural selection in keeping up the parts in question to their full dimensions, which, of itself, seems to me quite adequate to produce the results observed. recurring to the evidence, adduced in chapter iii, of the constant variation occurring in all parts of the organism, while selection is constantly acting on these variations in eliminating all that fall below the best working standard, and preserving only those that are fully up to it; and, remembering further, that, of the whole number of the increase produced annually, only a small percentage of the best adapted can be preserved, we shall see that every useful organ will be kept up nearly to its higher limit of size and efficiency. now mr. galton has proved experimentally that, when any part has thus been increased (or diminished) by selection, there is in the offspring a strong tendency to revert to a mean or average size, which tends to check further increase. and this mean appears to be, not the mean of the actual existing individuals but a lower mean, or that from which they had been recently raised by selection.[ ] he calls this the law of "regression towards mediocrity," and it has been proved by experiments with vegetables and by observations on mankind. this regression, in every generation, takes place even when both parents have been selected for their high development of the organ in question; but when there is no such selection, and crosses are allowed among individuals of every grade of development, the deterioration will be very rapid; and after a time not only will the average size of the part be greatly reduced, but the instances of full development will become very rare. thus what weismann terms "panmixia," or free intercrossing, will co-operate with galton's law of "regression towards mediocrity," and the result will be that, whenever selection ceases to act on any part or organ which has heretofore been kept up to a maximum of size and efficiency, the organ in question will rapidly decrease till it reaches a mean value considerably below the mean of the progeny that has usually been produced each year, and very greatly below the mean of that portion which has survived annually; and this will take place by the general law of heredity, and quite irrespective of any _use_ or _disuse_ of the part in question. now, no observations have been adduced by mr. spencer or others, showing that the average amount of change supposed to be due to _disuse_ is greater than that due to the law of regression towards mediocrity; while even if it were somewhat greater, we can see many possible contributory causes to its production. in the case of civilised man's diminished jaw, there may well be some correlation between the jaw and the brain, seeing that increased mental activity would lead to the withdrawal of blood and of nervous energy from adjacent parts, and might thus lead to diminished growth of those parts in the individual. and in the case of pet-dogs, the selection of small or short-headed individuals would imply the unconscious selection of those with less massive temporal muscles, and thus lead to the concomitant reduction of those muscles. the amount of reduction observed by darwin in the wing-bones of domestic ducks and poultry, and in the hind legs of tame rabbits, is very small, and is certainly no greater than the above causes will well account for; while so many of the external characters of all our domestic animals have been subject to long-continued artificial selection, and we are so ignorant of the possible correlations of different parts, that the phenomena presented by them seem sufficiently explained without recurrence to the assumption that any changes in the individual, due to disuse, are inherited by the offspring. _supposed effects of disuse among wild animals._ it may be urged, however, that among wild animals we have many undoubted results of disuse much more pronounced than those among domestic kinds, results which cannot be explained by the causes already adduced. such are the reduced size of the wings of many birds on oceanic islands; the abortion of the eyes in many cave animals, and in some which live underground; and the loss of the hind limbs in whales and in some lizards. these cases differ greatly in the amount of the reduction of parts which has taken place, and may be due to different causes. it is remarkable that in some of the birds of oceanic islands the reduction is little if at all greater than in domestic birds, as in the water-hen of tristan d'acunha. now if the reduction of wing were due to the hereditary effects of disuse, we should expect a very much greater effect in a bird inhabiting an oceanic island than in a domestic bird, where the disuse has been in action for an indefinitely shorter period. in the case of many other birds, however--as some of the new zealand rails and the extinct dodo of mauritius--the wings have been reduced to a much more rudimentary condition, though it is still obvious that they were once organs of flight; and in these cases we certainly require some other causes than those which have reduced the wings of our domestic fowls. one such cause may have been of the same nature as that which has been so efficient in reducing the wings of the insects of oceanic islands--the destruction of those which, during the occasional use of their wings, were carried out to sea. this form of natural selection may well have acted in the case of birds whose powers of flight were already somewhat reduced, and to whom, there being no enemies to escape from, their use was only a source of danger. we may thus, perhaps, account for the fact that many of these birds retain small but useless wings with which they never fly; for, the wings having been reduced to this functionless condition, no power could reduce them further except correlation of growth or economy of nutrition, causes which only rarely come into play. the complete loss of eyes in some cave animals may, perhaps, be explained in a somewhat similar way. whenever, owing to the total darkness, they became useless, they might also become injurious, on account of their delicacy of organisation and liability to accidents and disease; in which case natural selection would begin to act to reduce, and finally abort them; and this explains why, in some cases, the rudimentary eye remains, although completely covered by a protective outer skin. whales, like moas and cassowaries, carry us back to a remote past, of whose conditions we know too little for safe speculation. we are quite ignorant of the ancestral forms of either of these groups, and are therefore without the materials needful for determining the steps by which the change took place, or the causes which brought it about.[ ] on a review of the various examples that have been given by mr. darwin and others of organs that have been reduced or aborted, there seems too much diversity in the results for all to be due to so direct and uniform a cause as the individual effects of disuse accumulated by heredity. for if that were the only or chief efficient cause, and a cause capable of producing a decided effect during the comparatively short period of the existence of animals in a state of domestication, we should expect to find that, in wild species, all unused parts or organs had been reduced to the smallest rudiments, or had wholly disappeared. instead of this we find various grades of reduction, indicating the probable result of several distinct causes, sometimes acting separately, sometimes in combination, such as those we have already pointed out. and if we find no positive evidence of _disuse_, acting by its direct effect on the individual, being transmitted to the offspring, still less can we find such evidence in the case of the _use_ of organs. for here the very fact of _use_, in a wild state, implies _utility_, and utility is the constant subject for the action of natural selection; while among domestic animals those parts which are exceptionally used are so used in the service of man, and have thus become the subjects of artificial selection. thus "the great and inherited development of the udders in cows and goats," quoted by spencer from darwin, really affords no proof of inheritance of the increase due to use, because, from the earliest period of the domestication of these animals, abundant milk-production has been highly esteemed, and has thus been the subject of selection; while there are no cases among wild animals that may not be better explained by variation and natural selection. _difficulty as to co-adaptation of parts by variation and selection._ mr. spencer again brings forward this difficulty, as he did in his _principles of biology_ twenty-five years ago, and urges that all the adjustments of bones, muscles, blood-vessels, and nerves which would be required during, for example, the development of the neck and fore-limbs of the giraffe, could not have been effected by "simultaneous fortunate spontaneous variations." but this difficulty is fully disposed of by the facts of simultaneous variation adduced in our third chapter, and has also been specially considered in chapter vi, p. . the best answer to this objection may, perhaps, be found in the fact that the very thing said to be impossible by variation and natural selection has been again and again effected by variation and artificial selection. during the process of formation of such breeds as the greyhound or the bulldog, of the race-horse and carthorse, of the fantail pigeon or the otter-sheep, many co-ordinate adjustments have been produced; and no difficulty has occurred, whether the change has been effected by a single variation--as in the last case named--or by slow steps, as in all the others. it seems to be forgotten that most animals have such a surplus of vitality and strength for all the ordinary occasions of life that any slight superiority in one part can be at once utilised; while the moment any want of balance occurs, variations in the insufficiently developed parts will be selected to bring back the harmony of the whole organisation. the fact that, in all domestic animals, variations do occur, rendering them swifter or stronger, larger or smaller, stouter or slenderer, and that such variations can be separately selected and accumulated for man's purposes, is sufficient to render it certain that similar or even greater changes may be effected by natural selection, which, as darwin well remarks, "acts on every internal organ, on every shade of constitututional difference, on the whole machinery of life." the difficulty as to co-adaptation of parts by variation and natural selection appears to me, therefore, to be a wholly imaginary difficulty which has no place whatever in the operations of nature. _direct action of the environment._ mr. spencer's last objection to the wide scope given by darwinians to the agency of natural selection is, that organisms are acted upon by the environment, which produces in them definite changes, and that these changes in the individual are transmitted by inheritance, and thus become increased in successive generations. that such changes are produced in the individual there is ample evidence, but that they are inherited independently of any form of selection or of reversion is exceedingly doubtful, and darwin nowhere expresses himself as satisfied with the evidence. the two very strongest cases he mentions are the twenty-nine species of american trees which all differed in a corresponding way from their nearest european allies; and the american maize which became changed after three generations in europe. but in the case of the trees the differences alleged may be partly due to correlation with constitutional peculiarities dependent on climate, especially as regards the deeper tint of the fading leaves and the smaller size of the buds and seeds in america than in europe; while the less deeply toothed or serrated leaves in the american species are, in our present complete ignorance of the causes and uses of serration, quite as likely to be due to some form of adaptation as to any direct action of the climate. again, we are not told how many of the allied species do not vary in this particular manner, and this is certainly an important factor in any conclusion we may form on the question. in the case of the maize it appears that one of the more remarkable and highly selected american varieties was cultivated in germany, and in three years nearly all resemblance to the original parent was lost; and in the sixth year it closely resembled a common european variety, but was of somewhat more vigorous growth. in this case no selection appears to have been practised, and the effects may have been due to that "reversion to mediocrity" which invariably occurs, and is more especially marked in the case of varieties which have been rapidly produced by artificial selection. it may be considered as a partial reversion to the wild or unimproved stock; and the same thing would probably have occurred, though perhaps less rapidly, in america itself. as this is stated by darwin to be the most remarkable case known to him "of the direct and prompt action of climate on a plant," we must conclude that such direct effects have not been proved to be accumulated by inheritance, independently of reversion or selection. the remaining part of mr. spencer's essay is devoted to a consideration of the hypothetical action of the environment on the lower organisms which consist of simple cells or formless masses of protoplasm; and he shows with great elaboration that the outer and inner parts of these are necessarily subject to different conditions; and that the outer actions of air or water lead to the formation of integuments, and sometimes to other definite modifications of the surface, whence arise permanent differences of structure. although in these cases also it is very difficult to determine how much is due to direct modification by external agencies transmitted and accumulated by inheritance, and how much to spontaneous variations accumulated by natural selection, the probabilities in favour of the former mode of action are here greater, because there is no differentiation of nutritive and reproductive cells in these simple organisms; and it can be readily seen that any change produced in the latter will almost certainly affect the next generation.[ ] we are thus carried back almost to the origin of life, and can only vaguely speculate on what took place under conditions of which we know so little. _the american school of evolutionists._ the tentative views of mr. spencer which we have just discussed, are carried much further, and attempts have been made to work them out in great detail, by many american naturalists, whose best representative is dr. e.d. cope of philadelphia.[ ] this school endeavours to explain all the chief modifications of form in the animal kingdom by fundamental laws of growth and the inherited effects of use and effort, returning, in fact, to the teachings of lamarck as being at least equally important with those of darwin. the following extract will serve to show the high position claimed by this school as original discoverers, and as having made important additions to the theory of evolution: "wallace and darwin have propounded as the cause of modification in descent their law of natural selection. this law has been epitomised by spencer as the 'survival of the fittest.' this neat expression no doubt covers the case, but it leaves the origin of the fittest entirely untouched. darwin assumes a 'tendency to variation' in nature, and it is plainly necessary to do this, in order that materials for the exercise of a selection should exist. darwin and wallace's law is then only restrictive, directive, conservative, or destructive of something already created. i propose, then, to seek for the originative laws by which these subjects are furnished; in other words, for the causes of the origin of the fittest."[ ] mr. cope lays great stress on the existence of a special developmental force termed "bathmism" or growth-force, which acts by means of retardation and acceleration "without any reference to fitness at all;" that "instead of being controlled by fitness it is the controller of fitness." he argues that "all the characteristics of generalised groups from genera up (excepting, perhaps, families) have been evolved under the law of acceleration and retardation," combined with some intervention of natural selection; and that specific characters, or species, have been evolved by natural selection with some assistance from the higher law. he, therefore, makes species and genera two absolutely distinct things, the latter not developed out of the former; generic characters and specific characters are, in his opinion, fundamentally different, and have had different origins, and whole groups of species have been simultaneously modified, so as to belong to another genus; whence he thinks it "highly probable that the same specific form has existed through a succession of genera, and perhaps in different epochs of geologic time." useful characters, he concludes, have been produced by the special location of growth-force by use; useless ones have been produced by location of growth-force without the influence of use. another element which determines the direction of growth-force, and which precedes use, is effort; and "it is thought that effort becomes incorporated into the metaphysical acquisitions of the parent, and is inherited with other metaphysical qualities by the young, which, during the period of growth, is much more susceptible to modifying influences, and is likely to exhibit structural change in consequence."[ ] from these few examples of their teachings, it is clear that these american evolutionists have departed very widely from the views of mr. darwin, and in place of the well-established causes and admitted laws to which he appeals have introduced theoretical conceptions which have not yet been tested by experiments or facts, as well as metaphysical conceptions which are incapable of proof. and when they come to illustrate these views by an appeal to palaeontology or morphology, we find that a far simpler and more complete explanation of the facts is afforded by the established principles of variation and natural selection. the confidence with which these new ideas are enunciated, and the repeated assertion that without them darwinism is powerless to explain the origin of organic forms, renders it necessary to bestow a little more time on the explanations they give us of well-known phenomena with which, they assert, other theories are incompetent to grapple. as examples of use producing structural change, mr. cope adduces the hooked and toothed beaks of the falcons and the butcher-birds, and he argues that the fact of these birds belonging to widely different groups proves that similarity of use has produced a similar structural result. but no attempt is made to show any direct causal connection between the use of a bill to cut or tear flesh and the development of a tooth on the mandible. such use might conceivably strengthen the bill or increase its size, but not cause a special tooth-like outgrowth which was not present in the ancestral thrush-like forms of the butcher-bird. on the other hand, it is clear that any variations of the bill tending towards a hook or tooth would give the possessor some advantage in seizing and tearing its prey, and would thus be preserved and increased by natural selection. again, mr. cope urges the effects of a supposed "law of polar or centrifugal growth" to counteract a tendency to unsymmetrical growth, where one side of the body is used more than the other. but the undoubted hurtfulness of want of symmetry in many important actions or functions would rapidly eliminate any such tendency. when, however, it has become useful, as in the case of the single enlarged claw of many crustacea, it has been preserved by natural selection. _origin of the feet of the ungulates._ perhaps the most original and suggestive of mr. cope's applications of the theory of use and effort in modifying structure are, his chapters "on the origin of the foot-structure of the ungulates;" and that "on the effect of impacts and strains on the feet of mammalia;" and they will serve also to show the comparative merits of this theory and that of natural selection in explaining a difficult case of modification, especially as it is an explanation claimed as new and original when first enunciated in . let us, then, see how he deals with the problem. the remarkable progressive change of a four or five-toed ancestor into the one-toed horse, and the equally remarkable division of the whole group of ungulate animals into the odd-toed and even-toed divisions, mr. cope attempts to explain by the effects of impact and use among animals which frequented hard or swampy ground respectively. on hard ground, it is urged, the long middle toe would be most used and subjected to the greatest strains, and would therefore acquire both strength and development. it would then be still more exclusively used, and the extra nourishment required by it would be drawn from the adjacent less-used toes, which would accordingly diminish in size, till, after a long series of changes, the records of which are so well preserved in the american tertiary rocks, the true one-toed horse was developed. in soft or swampy ground, on the other hand, the tendency would be to spread out the foot so that there were two toes on each side. the two middle toes would thus be most used and most subject to strains, and would, therefore, increase at the expense of the lateral toes. there would be, no doubt, an advantage in these two functional toes being of equal size, so as to prevent twisting of the foot while walking; and variations tending to bring this about would be advantageous, and would therefore be preserved. thus, by a parallel series of changes in another direction, adapted to a distinct set of conditions, we should arrive at the symmetrical divided hoofs of our deer and cattle. the fact that sheep and goats are specially mountain and rock-loving animals may be explained by their being a later modification, since the divided hoof once formed is evidently well adapted to secure a firm footing on rugged and precipitous ground, although it could hardly have been first developed in such localities. mr. cope thus concludes: "certain it is that the length of the bones in the feet of the ungulate orders has a direct relation to the dryness of the ground they inhabit, and the possibility of speed which their habit permits them or necessarily imposes on them."[ ] if there is any truth in the explanation here briefly summarised, it must entirely depend on the fact of individual modifications thus produced being hereditary, and we yet await the proof of this. in the meantime it is clear that the very same results could have been brought about by variation and natural selection. for the toes, like all other organs, vary in size and proportions, and in their degree of union or separation; and if in one group of animals it was beneficial to have the middle toe larger and longer, and in another set to have the two middle toes of the same size, nothing can be more certain than that these particular modifications would be continuously preserved, and the very results we see ultimately produced. the oft-repeated objections that the cause of variations is unknown, that there must be something to determine variations in the right direction; that "natural selection includes no actively progressive principle, but must wait for the development of variation, and then, after securing the survival of the best, wait again for the best to project its own variations for selection," we have already sufficiently answered by showing that variation--in abundant or typical species--is always present in ample amount; that it exists in all parts and organs; that these vary, for the most part, independently, so that any required combination of variations can be secured; and finally, that all variation is necessarily either in excess or defect of the mean condition, and that, consequently, the right or favourable variations are so frequently present that the unerring power of natural selection never wants materials to work upon. _supposed action of animal intelligence._ the following passage briefly summarises mr. cope's position: "intelligence is a conservative principle, and will always direct effort and use into lines which will be beneficial to its possessor. here we have the source of the fittest, _i.e._ addition of parts by increase and location of growth-force, directed by the influence of various kinds of compulsion in the lower, and intelligent option among higher animals. thus intelligent choice, taking advantage of the successive evolution of physical conditions, may be regarded as the _originator of the fittest_, while natural selection is the tribunal to which all results of accelerated growth are submitted. this preserves or destroys them, and determines the new points of departure on which accelerated growth shall build."[ ] this notion of "intelligence"--the intelligence of the animal itself--determining its own variation, is so evidently a very partial theory, inapplicable to the whole vegetable kingdom, and almost so to all the lower forms of animals, amongst which, nevertheless, there is the very same adaptation and co-ordination of parts and functions as among the highest, that it is strange to see it put forward with such confidence as necessary for the completion of darwin's theory. if "the various kinds of compulsion"--by which are apparently meant the laws of variation, growth, and reproduction, the struggle for existence, and the actions necessary to preserve life under the conditions of the animal's environment--are sufficient to have developed the varied forms of the lower animals and of plants, we can see no reason why the same "compulsion" should not have carried on the development of the higher animals also. the action of this "intelligent option" is altogether unproved; while the acknowledgment that natural selection is the tribunal which either preserves or destroys the variations submitted to it, seems quite inconsistent with the statement that intelligent choice is the "orginator of the fittest," since whatever is really "the fittest" can never be destroyed by natural selection, which is but another name for the survival of the fittest. if "the fittest" is always definitely produced by some other power, then natural selection is not wanted. if, on the other hand, both fit and unfit are produced, and natural selection decides between them, that is pure darwinism, and mr. cope's theories have added nothing to it. [illustration: fig. .--transformation of artemia salina to a. milhausenii; , tail-lobe of a. salina, and its transition through , , , , to , into that of a. milhausenii; , post-abdomen of a. salina; , post-abdomen of a form bred in brackish water; , gill of a. milhausenii; , gill of a. salina. (from schmankewitsch.)] _semper on the direct influence of the environment._ another eminent naturalist, professor karl semper of würzburg, also adopts the view of the direct transforming power of the environment, and has brought together an immense body of interesting facts showing the influence of food, of light, of temperature, of still water and moving water, of the atmosphere and its currents, of gravitation, and of other organisms, in modifying the forms and other characteristics of animals.[ ] he believes that these various influences produce a direct and important effect, and that this effect is accumulated by inheritance; yet he acknowledges that we have no direct evidence of this, and there is hardly a single case adduced in the book which is not equally well explained by adaptation, brought about by the survival of beneficial variations. perhaps the most remarkable case he has brought forward is that of the transformation of species of crustaceans by a change in the saltness of the water (see fig. ). artemia salina lives in brackish water, while a. milhausenii inhabits water which is much salter. they differ greatly in the form of the tail-lobes, and in the presence or absence of spines upon the tail, and had always been considered perfectly distinct species. yet either was transformed into the other in a few generations, during which the saltness of the water was gradually altered. yet more, a. salina was gradually accustomed to fresher water, and in the course of a few generations, when the water had become perfectly fresh, the species was changed into branchipus stagnalis, which had always been considered to belong to a different genus on account of differences in the form of the antennae and of the posterior segments of the body (see fig. ). this certainly appears to be a proof of change of conditions producing a change of form independently of selection, and of that change of form, while remaining under the same conditions, being inherited. yet there is this peculiarity in the case, that there is a chemical change in the water, and that this water permeates the whole body, and must be absorbed by the tissues, and thus affect the ova and even the reproductive elements, and in this way may profoundly modify the whole organisation. why and how the external effects are limited to special details of the structure we do not know; but it does not seem as if any far-reaching conclusions as to the cumulative effect of external conditions on the higher terrestrial animals and plants, can be drawn from such an exceptional phenomenon. it seems rather analogous to those effects of external influences on the very lowest organisms in which the vegetative and reproductive organs are hardly differentiated, in which case such effects are doubtless inherited.[ ] [illustration: fig. . _a._ branchipus stagnalis. _b._ artemia salina.] _professor geddes's theory of variation in plants._ in a paper read before the edinburgh botanical society in mr. patrick geddes laid down the outlines of a fundamental theory of plant variation, which he has further extended in the article "variation and selection" in the _encydopaedia britannica_, and in a paper read before the linnaean society but not yet published. a theory of variation should deal alike with the origin of specific distinctions and with those vaster differences which characterise the larger groups, and he thinks it should answer such questions as--how an axis comes to be arrested to form a flower? how the various forms of inflorescence were evolved? how did perigynous or epigynous flowers arise from hypogynous flowers? and many others equally fundamental. natural selection acting upon numerous accidental variations will not, he urges, account for such general facts as these, which must depend on some constant law of variation. this law he believes to be the well-known antagonism of vegetative and reproductive growth acting throughout the whole course of plant development; and he uses it to explain many of the most characteristic features of the structure of flowers and fruits. commencing with the origin of the flower, which all botanists agree in regarding as a shortened branch, he explains this shortening as an inevitable physiological fact, since the cost of the development of the reproductive elements is so great as necessarily to check vegetative growth. in the same manner the shortening of the inflorescence from raceme to spike or umbel, and thence to the capitulum or dense flower-head of the composite plants is brought about. this shortening, carried still further, produces the flattened leaf-like receptacle of dorstenia, and further still the deeply hollowed fruity receptacle of the fig. the flower itself undergoes a parallel modification due to a similar cause. it is formed by a series of modified leaves arranged round a shortened axis. in its earlier stages the number of these modified leaves is indefinite, as in many ranunculaceae; and the axis itself is not greatly shortened, as in myosurus. the first advance is to a definite number of parts and a permanently shortened axis, in the arrangement termed hypogynous, in which all the whorls are quite distinct from each other. in the next stage there is a further shortening of the central axis, leaving the outer portion as a ring on which the petals are inserted, producing the arrangement termed perigynous. a still further advance is made by the contraction of the axis, so as to leave the central part forming the ovary quite below the flower, which is then termed epigynous. these several modifications are said to be parallel and definite, and to be determined by the continuous checking of vegetation by reproduction along what is an absolute groove of progressive change. this being the case, the importance of natural selection is greatly diminished. instead of selecting and accumulating spontaneous indefinite variations, its function is to retard them after the stage of maximum utility has been independently reached. the same simple conception is said to unlock innumerable problems of vegetable morphology, large and small alike. it explains the inevitable development of gymnosperm into angiosperm by the checked vegetative growth of the ovule-bearing leaf or carpel; while such minor adaptations as the splitting fruit of the geranium or the cupped stigma of the pansy, can be no longer looked upon as achievements of natural selection, but must be regarded as naturally traceable to the vegetative checking of their respective types of leaf organ. again, a detailed examination of spiny plants practically excludes the hypothesis of mammalian selection altogether, and shows spines to arise as an expression of the diminishing vegetativeness--in fact, the ebbing vitality of a species.[ ] _objections to the theory._ the theory here sketched out is enticing, and at first sight seems calculated to throw much light on the history of plant development; but on further consideration, it seems wanting in definiteness, while it is beset with difficulties at every step. take first the shortening of the raceme into the umbel and the capitulum, said to be caused by arrest of vegetative growth, due to the antagonism of reproduction. if this were the whole explanation of the phenomenon, we should expect the quantity of seed to increase as this vegetative growth diminished, since the seed is the product of the reproductive energy of the plant, and its quantity the best measure of that energy. but is this the case? the ranunculus has comparatively few seeds, and the flowers are not numerous; while in the same order the larkspur and the columbine have far more seeds as well as more flowers, but there is no shortening of the raceme or diminution of the foliage, although the flowers are large and complex. so, the extremely shortened and compressed flower-heads of the compositae produce comparatively few seeds--one only to each flower; while the foxglove, with its long spike of showy flowers, produces an enormous number. again, if the shortening of the central axis in the successive stages of hypogynous, perigynous, and epigynous flowers were an indication of preponderant reproduction and diminished vegetation, we should find everywhere some clear indications of this fact. the plants with hypogynous flowers should, as a rule, have less seed and more vigorous and abundant foliage than those at the other extreme with epigynous flowers. but the hypogynous poppies, pinks, and st. john's worts have abundance of seed and rather scanty foliage; while the epigynous dogwoods and honeysuckles have few seeds and abundant foliage. if, instead of the number of the seeds, we take the size of the fruit as an indication of reproductive energy, we find this at a maximum in the gourd family, yet their rapid and luxuriant growth shows no diminution of vegetative power. so that the statement that plant modifications proceed "along an absolute groove of progressive change" is contradicted by innumerable facts indicating advance and regression, improvement or degradation, according as the ever-changing environment renders one form more advantageous than the other. as one instance i may mention the anonaceae or custard-apple tribe, which are certainly an advance from the ranunculaceae; yet in the genus polyalthea the fruit consists of a number of separate carpels, each borne on a long stalk, as if reverting to the primitive stalked carpellary leaves. _on the origin of spines._ but perhaps the most extraordinary application of the theory is that which considers spines to be an indication of the "ebbing vitality of a species," and which excludes "mammalian selection altogether." if this were true, spines should occur mainly in feeble, rare, and dying-out species, instead of which we have the hawthorn, one of our most vigorous shrubs or trees, with abundant vitality and an extensive range over the whole palaearctic region, showing that it is really a dominant species. in north america the numerous thorny species of crataegus are equally vigorous, as are the false acacia (robinia) and the honey-locust (gleditschia). neither have the numerous species of very spiny acacias been noticed to be rarer or less vigorous than the unarmed kinds. on the other point--that spines are not due to mammalian selection--we are able to adduce what must be considered direct and conclusive evidence. for if spines, admittedly produced by aborted branches, petioles, or peduncles, are due solely or mainly to diminished vegetativeness or ebbing vitality, they ought to occur in all countries alike, or at all events in all whose similar conditions tend to check vegetation; whereas, if they are, solely or mainly, developed as a protection against the attacks of herbivorous mammals, they ought to be most abundant where these are plentiful, and rare or absent where indigenous mammalia are wanting. oceanic islands, as compared with continents, would thus furnish a crucial test of the two theories; and mr. hemsley of kew, who has specially studied insular floras, has given me some valuable information on this point. he says: "there are no spiny or prickly plants in the indigenous element of the st. helena flora. the relatively rich flora of the sandwich isles is not absolutely without a prickly plant, but almost so. all the endemic genera are unarmed, and the endemic species of almost every other genus. even such genera as zanthoxylon, acacia, xylosoma, lycium, and solanum, of which there are many armed species in other countries, are only represented by unarmed species. the two endemic rubi have the prickles reduced to the setaceous condition, and the two palms are unarmed. "the flora of the galapagos includes a number of prickly plants, among them several cacti (these have not been investigated and may be american species), but i do not think one of the known endemic species of any family is prickly or spiny. "spiny and prickly plants are also rare in new zealand, but there are the formidably armed species of wild spaniard (aciphylla), one species of rubus, the pungent-leaved epacrideae and a few others." mr. j.g. baker of kew, who has specially studied the flora of mauritius and the adjacent islands, also writes me on this point. he says: "taking mauritius alone, i do not call to mind a single species that is a spinose endemic tree or shrub. if you take the whole group of islands (mauritius, bourbon, seychelles, and rodriguez), there will be about a dozen species, but then nine of these are palms. leaving out palms, the trees and shrubs of that part of the world are exceptionally non-spinose." these are certainly remarkable facts, and quite inexplicable on the theory of spines being caused solely by checked vegetative growth, due to weakness of constitution or to an arid soil and climate. for the galapagos and many parts of the sandwich islands are very arid, as is a considerable part of the north island of new zealand. yet in our own moist climate and with our very limited number of trees and shrubs we have about eighteen spiny or prickly species, more, apparently, than in the whole endemic floras of the mauritius, sandwich islands, and galapagos, though these are all especially rich in shrubby and arboreal species. in new zealand the prickly rubus is a leafless trailing plant, and its prickles are probably a protection against the large snails of the country, several of which have shells from two to three and a half inches long.[ ] the "wild spaniards" are very spiny herbaceous umbelliferae, and may have gained their spines to preserve them from being trodden down or eaten by the moas, which, for countless ages, took the place of mammals in new zealand. the exact use or meaning of the spines in palms is more doubtful, though they are, no doubt, protective against some animals; but it is certainly an extraordinary fact that in the entire flora of the mauritius, so largely consisting of trees and shrubs, not a single endemic species should be thorny or spiny. if now we consider that every continental flora produces a considerable proportion of spiny and thorny species, and that these rise to a maximum in south africa, where herbivorous mammalia were (before the settlement of the country), perhaps, more abundant and varied than in any other part of the world; while another district, remarkable for well-armed vegetation, is chile, where the camel-like vicugnas, llamas, and alpacas, and an abundance of large rodents wage perpetual war against shrubby vegetation, we shall see the full significance of the almost total absence of thorny and spiny plants in the chief oceanic islands; and so far from "excluding the hypothesis of mammalian selection altogether," we shall find in this hypothesis the only satisfactory explanation of the facts. from the brief consideration of professor geddes's theory now given, we conclude that, although the antagonism between vegetative and reproductive growth is a real agency, and must be taken account of in our endeavour to explain many of the fundamental facts in the structure and form of plants, yet it is so overpowered and directed at every step by the natural selection of favourable variations, that the results of its exclusive and unmodified action are nowhere to be found in nature. it may be allowed to rank as one of those "laws of growth," of which so many have now been indicated, and which were always recognised by darwin as underlying all variation; but unless we bear in mind that its action must always be subordinated to natural selection, and that it is continually checked, or diverted, or even reversed by the necessity of adaptation to the environment, we shall be liable to fall into such glaring errors as the imputing to "ebbing vitality" alone such a widespread phenomenon as the occurrence of spines and thorns, while ignoring altogether the influence of the organic environment in their production.[ ] the sketch now given of the chief attempts that have been made to prove that either the direct action of the environment or certain fundamental laws of variation are independent causes of modification of species, shows us that their authors have, in every case, failed to establish their contention. any direct action of the environment, or any characters acquired by use or disuse, can have no effect whatever upon the race unless they are inherited; and that they are inherited in any case, except when they directly affect the reproductive cells, has not been proved. on the other hand, as we shall presently show, there is much reason for believing that such acquired characters are in their nature non-heritable. _variation and selection overpower the effects of use and disuse._ but there is another objection to this theory arising from the very nature of the effects produced. in each generation the effects of use or disuse, or of effort, will certainly be very small, while of this small effect it is not maintained that the whole will be always inherited by the next generation. how small the effect is we have no means of determining, except in the case of disuse, which mr. darwin investigated carefully. he found that in twelve fancy breeds of pigeons, which are often kept in aviaries, or if free fly but little, the sternum had been reduced by about one-seventh or one-eighth of its entire length, and that of the scapula about one-ninth. in domestic ducks the weight of the wing-bones in proportion to that of the whole skeleton had decreased about one-tenth. in domestic rabbits the bones of the legs were found to have increased in weight in due proportion to the increased weight of the body, but those of the hind legs were rather less in proportion to those of the fore legs than in the wild animal, a difference which may be imputed to their being less used in rapid motion. the pigeons, therefore, afford the greatest amount of reduction by disuse--one-seventh of the length of the sternum. but the pigeon has certainly been domesticated four or five thousand years; and if the reduction of the wings by disuse has only been going on for the last thousand years, the amount of reduction in each generation would be absolutely imperceptible, and quite within the limits of the reduction due to the absence of selection, as already explained. but, as we have seen in chapter iii, the fortuitous variation of every part or organ usually amounts to one-tenth, and often to one-sixth of the average dimensions--that is, the fortuitous variation in one generation among a limited number of the individuals of a species is as great as the cumulative effects of disuse in a thousand generations! if we assume that the effects of use or of effort in the individual are equal to the effects of disuse, or even ten or a hundred times greater, they will even then not equal, in each generation, the amount of the fortuitous variations of the same part. if it be urged that the effects of use would modify all the individuals of a species, while the fortuitous variations to the amount named only apply to a portion of them, it may be replied, that that portion is sufficiently large to afford ample materials for selection, since it often equals the numbers that can annually survive; while the recurrence in each successive generation of a like amount of variation would render possible such a rapid adjustment to new conditions that the effects of use or disuse would be as nothing in comparison. it follows, that even admitting the modifying effects of the environment, and that such modifications are inherited, they would yet be entirely swamped by the greater effects of fortuitous variation, and the far more rapid cumulative results of the selection of such variations. _supposed action of the environment in initiating variations._ it is, however, urged that the reaction of the environment initiates variations, which without it would never arise; such, for instance, as the origin of horns through the pressures and irritations caused by butting, or otherwise using the head as a weapon or for defence. admitting, for the sake of argument, that this is so, all the evidence we possess shows that, from the very first appearance of the rudiment of such an organ, it would vary to a greater extent than the amount of growth directly produced by use; and these variations would be subject to selection, and would thus modify the organ in ways which use alone would never bring about. we have seen that this has been the case with the branching antlers of the stag, which have been modified by selection, so as to become useful in other ways than as a mere weapon; and the same has almost certainly been the case with the variously curved and twisted horns of antelopes. in like manner, every conceivable rudiment would, from its first appearance, be subject to the law of variation and selection, to which, thenceforth, the direct effect of the environment would be altogether subordinate. a very similar mode of reasoning will apply to the other branch of the subject--the initiation of structures and organs by the action of the fundamental laws of growth. admitting that such laws have determined some of the main divisions of the animal and vegetable kingdom, have originated certain important organs, and have been the fundamental cause of certain lines of development, yet at every step of the process these laws must have acted in entire subordination to the law of natural selection. no modification thus initiated could have advanced a single step, unless it were, on the whole, a useful modification; while its entire future course would be necessarily subject to the laws of variation and selection, by which it would be sometimes checked, sometimes hastened on, sometimes diverted to one purpose, sometimes to another, according as the needs of the organism, under the special conditions of its existence, required such modification. we need not deny that such laws and influences may have acted in the manner suggested, but what we do deny is that they could possibly escape from the ever-present and all-powerful modifying effects of variation and natural selection.[ ] _weismann's theory of heredity._ professor august weismann has put forth a new theory of heredity founded upon the "continuity of the germ-plasm," one of the logical consequences of which is, that acquired characters of whatever kind are not transmitted from parent to offspring. as this is a matter of vital importance to the theory of natural selection, and as, if well founded, it strikes away the foundations of most of the theories discussed in the present chapter, a brief outline of weismann's views must be attempted, although it is very difficult to make them intelligible to persons unfamiliar with the main facts of modern embryology.[ ] the problem is thus stated by weismann: "how is it that in the case of all higher animals and plants a single cell is able to separate itself from amongst the millions of most various kinds of which an organism is composed, and by division and complicated differentiation to reconstruct a new individual with marvellous likeness, unchanged in many cases even throughout whole geological periods?" darwin attempted to solve the problem by his theory of "pangenesis," which supposed that every individual cell in the body gave off gemmules or germs capable of reproducing themselves, and that portions of these germs of each of the almost infinite number of cells permeate the whole body and become collected in the generative cells, and are thus able to reproduce the whole organism. this theory is felt to be so ponderously complex and difficult that it has met with no general acceptance among physiologists. the fact that the germ-cells _do_ reproduce with wonderful accuracy not only the general characters of the species, but many of the individual characteristics of the parents or more remote ancestors, and that this process is continued from generation to generation, can be accounted for, weismann thinks, only on two suppositions which are physiologically possible. either the substance of the parent germ-cell, after passing through a cycle of changes required for the construction of a new individual, possesses the capability of producing anew germ-cells identical with those from which that individual was developed, or _the new germ-cells arise, as far as their essential and characteristic substance is concerned, not at all out of the body of the individual, but direct from the parent germ-cell_. this latter view weismann holds to be the correct one, and, on this theory, heredity depends on the fact that a substance of special molecular composition passes over from one generation to another. this is the "germ-plasm," the power of which to develop itself into a perfect organism depends on the extraordinary complication of its minutest structure. at every new birth a portion of the specific germ-plasm, which the parent egg-cell contains, is not used up in producing the offspring, but is reserved unchanged to produce the germ-cells of the following generation. thus the germ-cells--so far as regards their essential part the germ-plasm--are not a product of the body itself, but are related to one another in the same way as are a series of generations of unicellular organisms derived from one another by a continuous course of simple division. thus the question of heredity is reduced to one of growth. a minute portion of the very same germ-plasm from which, first the germ-cell, and then the whole organism of the parent, were developed, becomes the starting-point of the growth of the child. _the cause of variation._ but if this were all, the offspring would reproduce the parent exactly, in every detail of form and structure; and here we see the importance of sex, for each new germ grows out of the united germ-plasms of two parents, whence arises a mingling of their characters in the offspring. this occurs in each generation; hence every individual is a complex result reproducing in ever-varying degrees the diverse characteristics of his two parents, four grandparents, eight great-grandparents, and other more remote ancestors; and that ever-present individual variation arises which furnishes the material for natural selection to act upon. diversity of sex becomes, therefore, of primary importance as _the cause of variation_. where asexual generation prevails, the characteristics of the individual alone are reproduced, and there are thus no means of effecting the change of form or structure required by changed conditions of existence. under such changed conditions a complex organism, if only asexually propagated, would become extinct. but when a complex organism is sexually propagated, there is an ever-present cause of change which, though slight in any one generation, is cumulative, and under the influence of selection is sufficient to keep up the harmony between the organism and its slowly changing environment.[ ] _the non-heredity of acquired characters._ certain observations on the embryology of the lower animals are held to afford direct proof of this theory of heredity, but they are too technical to be made clear to ordinary readers. a logical result of the theory is the impossibility of the transmission of acquired characters, since the molecular structure of the germ-plasm is already determined within the embryo; and weismann holds that there are no facts which really prove that acquired characters can be inherited, although their inheritance has, by most writers, been considered so probable as hardly to stand in need of direct proof. we have already shown, in the earlier part of this chapter, that many instances of change, imputed to the inheritance of acquired variations, are really cases of selection; while the very fact that _use_ implies _usefulness_ renders it almost impossible to eliminate the action of selection in a state of nature. as regards mutilations, it is generally admitted that they are not hereditary, and there is ample evidence on this point. when it was the fashion to dock horses' tails, it was not found that horses were born with short tails; nor are chinese women born with distorted feet; nor are any of the numerous forms of racial mutilation in man, which have in some cases been carried on for hundreds of generations, inherited. nevertheless, a few cases of apparent inheritance of mutilations have been recorded,[ ] and these, if trustworthy, are difficulties in the way of the theory. the undoubted inheritance of disease is hardly a difficulty, because the predisposition to disease is a congenital, not an acquired character, and as such would be the subject of inheritance. the often-quoted case of a disease induced by mutilation being inherited (brown-sequard's epileptic guinea-pigs) has been discussed by professor weismann, and shown to be not conclusive. the mutilation itself--a section of certain nerves--was never inherited, but the resulting epilepsy, or a general state of weakness, deformity, or sores, was sometimes inherited. it is, however, possible that the mere injury introduced and encouraged the growth of certain microbes, which, spreading through the organism, sometimes reached the germ-cells, and thus transmitted a diseased condition to the offspring. such a transference of microbes is believed to occur in syphilis and tuberculosis, and has been ascertained to occur in the case of the muscardine silkworm disease.[ ] _the theory of instinct._ the theory now briefly outlined cannot be said to be proved, but it commends itself to many physiologists as being inherently probable, and as furnishing a good working hypothesis till displaced by a better. we cannot, therefore, accept any arguments against the agency of natural selection which are based upon the opposite and equally unproved theory that acquired characters are inherited; and as this applies to the whole school of what may be termed neo-lamarckians, their speculations cease to have any weight. the same remark applies to the popular theory of instincts as being inherited habits; though darwin gave very little weight to this, but derived almost all instincts from spontaneous useful variations which, like other spontaneous variations, are of course inherited. at first sight it appears as if the acquired habits of our trained dogs--pointers, retrievers, etc.--are certainly inherited; but this need not be the case, because there must be some structural or psychical peculiarities, such as modifications in the attachments of muscles, increased delicacy of smell or sight, or peculiar likes and dislikes, which are inherited; and from these, peculiar habits follow as a natural consequence, or are easily acquired. now, as selection has been constantly at work in improving all our domestic animals, we have unconsciously modified the structure, while preserving only those animals which best served our purpose in their peculiar faculties, instincts, or habits. much of the mystery of instinct arises from the persistent refusal to recognise the agency of imitation, memory, observation, and reason as often forming part of it. yet there is ample evidence that such agency must be taken into account. both wilson and leroy state that young birds build inferior nests to old ones, and the latter author observes that the best nests are made by birds whose young remain longest in the nest. so, migration is now well ascertained to be effected by means of vision, long flights being made on bright moonlight nights when the birds fly very high, while on cloudy nights they fly low, and then often lose their way. thousands annually fly out to sea and perish, showing that the instinct to migrate is imperfect, and is not a good substitute for reason and observation. again, much of the perfection of instinct is due to the extreme severity of the selection during its development, any failure involving destruction. the chick which cannot break the eggshell, the caterpillar that fails to suspend itself properly or to spin a safe cocoon, the bees that lose their way or that fail to store honey, inevitably perish. so the birds that fail to feed and protect their young, or the butterflies that lay their eggs on the wrong food-plant, leave no offspring, and the race with imperfect instincts perishes. now, during the long and very slow course of development of each organism, this rigid selection at every step of progress has led to the preservation of every detail of structure, faculty, or habit that has been necessary for the preservation of the race, and has thus gradually built up the various instincts which seem so marvellous to us, but which can yet be shown to be in many cases still imperfect. here, as everywhere else in nature, we find comparative, not absolute perfection, with every gradation from what is clearly due to imitation or reason up to what seems to us perfect instinct--that in which a complex action is performed without any previous experience or instruction.[ ] _concluding remarks._ having now passed in review the more important of the recent objections to, or criticisms of, the theory of natural selection, we have arrived at the conclusion that in no one case have the writers in question been able materially to diminish its importance, or to show that any of the laws or forces to which they appeal can act otherwise than in strict subordination to it. the direct action of the environment as set forth by mr. herbert spencer, dr. cope, and dr. karl semper, even if we admit that its effects on the individual are transmitted by inheritance, are so small in comparison with the amount of spontaneous variation of every part of the organism that they must be quite overshadowed by the latter. and if such direct action may, in some cases, have initiated certain organs or outgrowths, these must from their very first beginnings have been subject to variation and natural selection, and their further development have been almost wholly due to these ever-present and powerful causes. the same remark applies to the views of professor geddes on the laws of growth which have determined certain essential features in the morphology of plants and animals. the attempt to substitute these laws for those of variation and natural selection has failed in cases where we can apply a definite test, as in that of the origin of spines on trees and shrubs; while the extreme diversity of vegetable structure and form among the plants of the same country and of the same natural order, of itself affords a proof of the preponderating influence of variation and natural selection in keeping the many diverse forms in harmony with the highly complex and ever-changing environment. lastly, we have seen that professor weismann's theory of the continuity of the germ-plasm and the consequent non-heredity of acquired characters, while in perfect harmony with all the well-ascertained facts of heredity and development, adds greatly to the importance of natural selection as the one invariable and ever-present factor in all organic change, and that which can alone have produced the temporary fixity combined with the secular modification of species. while admitting, as darwin always admitted, the co-operation of the fundamental laws of growth and variation, of correlation and heredity, in determining the direction of lines of variation or in the initiation of peculiar organs, we find that variation and natural selection are ever-present agencies, which take possession, as it were, of every minute change originated by these fundamental causes, check or favour their further development, or modify them in countless varied ways according to the varying needs of the organism. whatever other causes have been at work, natural selection is supreme, to an extent which even darwin himself hesitated to claim for it. the more we study it the more we are convinced of its overpowering importance, and the more confidently we claim, in darwin's own words, that it "has been the most important, but not the exclusive, means of modification." footnotes: [footnote : see the duke of argyll's letter in _nature_, vol. xxxiv. p. .] [footnote : _journal of the anthropological institute,_ vol. xv. pp. - .] [footnote : the idea of the non-heredity of acquired variations was suggested by the summary of professor weismann's views, in _nature_, referred to later on. but since this chapter was written i have, through the kindness of mr. e.b. poulton, seen some of the proofs of the forthcoming translation of weismann's essays on heredity, in which he sets forth an explanation very similar to that here given. on the difficult question of the almost entire disappearance of organs, as in the limbs of snakes and of some lizards, he adduces "a certain form of correlation, which roux calls 'the struggle of the parts in the organism,'" as playing an important part. atrophy following disuse is nearly always attended by the corresponding increase of other organs: blind animals possess more developed organs of touch, hearing, and smell; the loss of power in the wings is accompanied by increased strength of the legs, etc. now as these latter characters, being useful, will be selected, it is easy to understand that a congenital increase of these will be accompanied by a corresponding congenital diminution of the unused organ; and in cases where the means of nutrition are deficient, every diminution of these useless parts will be a gain to the whole organism, and thus their complete disappearance will, in some cases, be brought about directly by natural selection. this corresponds with what we know of these rudimentary organs. it must, however, be pointed out that the non-heredity of acquired characters was maintained by mr. francis galton more than twelve years ago, on theoretical considerations almost identical with those urged by professor weismann; while the insufficiency of the evidence for their hereditary transmission was shown, by similar arguments to those used above and in the work of professor weismann already referred to (see "a theory of heredity," in _journ. anthrop. instit._, vol. v. pp. - ).] [footnote : this explanation is derived from weismann's theory of the continuity of the germ-plasm as summarised in _nature_.] [footnote : see a collection of his essays under the title, _the origin of the fittest: essays on evolution_, d. appleton and co. new york. .] [footnote : _origin of the fittest_, p. .] [footnote : _ibid._ p. . it may be here noted that darwin found these theories unintelligible. in a letter to professor e.t. morse in , he writes: "there is one point which i regret you did not make clear in your address, namely, what is the meaning and importance of professors cope and hyatt's views on acceleration and retardation? i have endeavoured, and given up in despair, the attempt to grasp their meaning" (_life and letters_, vol. iii. p. ).] [footnote : _origin of the fittest_, p. .] [footnote : _origin of the fittest_, p. .] [footnote : _the natural conditions of existence as they affect animal life._ london, .] [footnote : in dr. weismann's essay on "heredity," already referred to, he considers it not improbable that changes in organisms produced by climatic influences may be inherited, because, as these changes do not affect the external parts of an organism only, but often, as in the case of warmth or moisture permeate the whole structure, they may possibly modify the germ-plasm itself, and thus induce variations in the next generation. in this way, he thinks, may possibly be explained the climatic varieties of certain butterflies, and some other changes which seem to be effected by change of climate in a few generations.] [footnote : this brief indication of professor geddes's views is taken from the article "variation and selection" in the _encyclopedia britannica_, and a paper "on the nature and causes of variation in plants" in _trans. and proc. of the edinburgh botanical society_, ; and is, for the most part, expressed in his own words.] [footnote : placostylis bovinus, ½ inches long; paryphanta busbyi, in. diam.; p. hochstetteri, ¾ in. diam.] [footnote : the general arguments and objections here set forth will apply with equal force to professor g. henslow's theory of the origin of the various forms and structures of flowers as due to "the responsive actions of the protoplasm in consequence of the irritations set up by the weights, pressures, thrusts, tensions, etc., of the insect visitors" (_the origin of floral structures through insect and other agencies_, p. ). on the assumption that acquired characters are inherited, such irritations may have had something to do with the initiation of variations and with the production of certain details of structure, but they are clearly incompetent to have brought about the more important structural and functional modifications of flowers. such are, the various adjustments of length and position of the stamens to bring the pollen to the insect and from the insect to the stigma; the various motions of stamens and styles at the right time and the right direction; the physiological adjustments bringing about fertility or sterility in heterostyled plants; the traps, springs, and complex movements of various parts of orchids; and innumerable other remarkable phenomena. for the explanation of these we have no resource but variation and selection, to the effects of which, acting alternately with regression or degradation as above explained (p. ) must be imputed the development of the countless floral structures we now behold. even the primitive flowers, whose initiation may, perhaps, have been caused, or rendered possible, by the irritation set up by insects' visits, must, from their very origin, have been modified, in accordance with the supreme law of utility, by means of variation and survival of the fittest.] [footnote : in an essay on "the duration of life," forming part of the translation of dr. weismann's papers already referred to, the author still further extends the sphere of natural selection by showing that the average duration of life in each species has been determined by it. a certain length of life is essential in order that the species may produce offspring sufficient to ensure its continuance under the most unfavourable conditions; and it is shown that the remarkable inequalities of longevity in different species and groups may be thus accounted for. yet more, the occurrence of death in the higher organisms, in place of the continued survival of the unicellular organisms however much they may increase by subdivision, may be traced to the same great law of utility for the race and survival of the fittest. the whole essay is of exceeding interest, and will repay a careful perusal. a similar idea occurred to the present writer about twenty years back, and was briefly noted down at the time, but subsequently forgotten.] [footnote : the outline here given is derived from two articles in _nature_, vol. xxxiii. p. , and vol. xxxiv. p. , in which weismann's papers are summarised and partly translated.] [footnote : there are many indications that this explanation of the cause of variation is the true one. mr. e.b. poulton suggests one, in the fact that parthenogenetic reproduction only occurs in isolated species, not in groups of related species; as this shows that parthenogenesis cannot lead to the evolution of new forms. again, in parthenogenetic females the complete apparatus for fertilisation remains unreduced; but if these varied as do sexually produced animals, the organs referred to, being unused, would become rudimentary. even more important is the significance of the "polar bodies," as explained by weismann in one of his _essays_; since, if his interpretation of them be correct, variability is a necessary consequence of sexual generation.] [footnote : darwin's _animals and plants_, vol. ii. pp. , .] [footnote : in his essay on "heredity," dr. weismann discusses many other cases of supposed inheritance of acquired characters, and shows that they can all be explained in other ways. shortsightedness among civilised nations, for example, is due partly to the absence of selection and consequent regression towards a mean, and partly to its individual production by constant reading.] [footnote : weismann explains instinct on similar lines, and gives many interesting illustrations (see _essays on heredity_). he holds "that all instinct is entirely due to the operation of natural selection, and has its foundation, not upon inherited experiences, but upon variations of the germ." many interesting and difficult cases of instinct are discussed by darwin in chapter viii of the _origin of species_, which should be read in connection with the above remarks. since this chapter was written my attention has been directed to mr. francis galton's _theory of heredity_ (already referred to at p. ) which was published thirteen years ago as an alternative for darwin's theory of pangenesis. mr. galton's theory, although it attracted little attention, appears to me to be substantially the same as that of professor weismann. galton's "stirp" is weismann's "germ-plasm." galton supposes the sexual elements in the offspring to be directly formed from the residue of the _stirp_ not used up in the development of the body of the parent--weismann's "continuity of the germ-plasm." galton also draws many of the same conclusions from his theory. he maintains that characters acquired by the individual as the result of external influences cannot be inherited, unless such influences act directly on the reproductive elements--instancing the possible heredity of alcoholism, because the alcohol permeates the tissues and may reach the sexual elements. he discusses the supposed heredity of effects produced by use or disuse, and explains them much in the same manner as does weismann. galton is an anthropologist, and applies the theory, mainly, to explain the peculiarities of hereditary transmission in man, many of which peculiarities he discusses and elucidates. weismann is a biologist, and is mostly concerned with the application of the theory to explain variation and instinct, and to the further development of the theory of evolution. he has worked it out more thoroughly, and has adduced embryological evidence in its support; but the views of both writers are substantially the same, and their theories were arrived at quite independently. the names of galton and weismann should therefore be associated as discoverers of what may be considered (if finally established) the most important contribution to the evolution theory since the appearance of the _origin of species_.] chapter xv darwinism applied to man general identity of human and animal structure--rudiments and variations showing relation of man to other mammals--the embryonic development of man and other mammalia--diseases common to man and the lower animals--the animals most nearly allied to man--the brains of man and apes--external differences of man and apes--summary of the animal characteristics of man--the geological antiquity of man--the probable birthplace of man--the origin of the moral and intellectual nature of man--the argument from continuity--the origin of the mathematical faculty--the origin of the musical and artistic faculties--independent proof that these faculties have not been developed by natural selection--the interpretation of the facts--concluding remarks. our review of modern darwinism might fitly have terminated with the preceding chapter; but the immense interest that attaches to the origin of the human race, and the amount of misconception which prevails regarding the essential teachings of darwin's theory on this question, as well as regarding my own special views upon it, induce me to devote a final chapter to its discussion. to any one who considers the structure of man's body, even in the most superficial manner, it must be evident that it is the body of an animal, differing greatly, it is true, from the bodies of all other animals, but agreeing with them in all essential features. the bony structure of man classes him as a vertebrate; the mode of suckling his young classes him as a mammal; his blood, his muscles, and his nerves, the structure of his heart with its veins and arteries, his lungs and his whole respiratory and circulatory systems, all closely correspond to those of other mammals, and are often almost identical with them. he possesses the same number of limbs terminating in the same number of digits as belong fundamentally to the mammalian class. his senses are identical with theirs, and his organs of sense are the same in number and occupy the same relative position. every detail of structure which is common to the mammalia as a class is found also in man, while he only differs from them in such ways and degrees as the various species or groups of mammals differ from each other. if, then, we have good reason to believe that every existing group of mammalia has descended from some common ancestral form--as we saw to be so completely demonstrated in the case of the horse tribe,--and that each family, each order, and even the whole class must similarly have descended from some much more ancient and more generalised type, it would be in the highest degree improbable--so improbable as to be almost inconceivable--that man, agreeing with them so closely in every detail of his structure, should have had some quite distinct mode of origin. let us, then, see what other evidence bears upon the question, and whether it is sufficient to convert the probability of his animal origin into a practical certainty. _rudiments and variations as indicating the relation of man to other mammals._ all the higher animals present rudiments of organs which, though useless to them, are useful in some allied group, and are believed to have descended from a common ancestor in which they were useful. thus there are in ruminants rudiments of incisor teeth which, in some species, never cut through the gums; many lizards have external rudimentary legs; while many birds, as the apteryx, have quite rudimentary wings. now man possesses similar rudiments, sometimes constantly, sometimes only occasionally present, which serve intimately to connect his bodily structure with that of the lower animals. many animals, for example, have a special muscle for moving or twitching the skin. in man there are remnants of this in certain parts of the body, especially in the forehead, enabling us to raise our eyebrows; but some persons have it in other parts. a few persons are able to move the whole scalp so as to throw off any object placed on the head, and this property has been proved, in one case, to be inherited. in the outer fold of the ear there is sometimes a projecting point, corresponding in position to the pointed ear of many animals, and believed to be a rudiment of it. in the alimentary canal there is a rudiment--the vermiform appendage of the caecum--which is not only useless, but is sometimes a cause of disease and death in man; yet in many vegetable feeding animals it is very long, and even in the orang-utan it is of considerable length and convoluted. so, man possesses rudimentary bones of a tail concealed beneath the skin, and, in some rare cases, this forms a minute external tail. the variability of every part of man's structure is very great, and many of these variations tend to approximate towards the structure of other animals. the courses of the arteries are eminently variable, so that for surgical purposes it has been necessary to determine the probable proportion of each variation. the muscles are so variable that in fifty cases the muscles of the foot were found to be not strictly alike in any two, and in some the deviations were considerable; while in thirty-six subjects mr. j. wood observed no fewer than muscular variations. the same author states that in a single male subject there were no fewer than seven muscular variations, all of which plainly represented muscles proper to various kinds of apes. the muscles of the hands and arms--parts which are so eminently characteristic of man--are extremely liable to vary, so as to resemble the corresponding muscles of the lower animals. that such variations are due to reversion to a former state of existence mr. darwin thinks highly probable, and he adds: "it is quite incredible that a man should, through mere accident, abnormally resemble certain apes in no less than seven of his muscles, if there had been no genetic connection between them. on the other hand, if man is descended from some ape-like creature, no valid reason can be assigned why certain muscles should not suddenly reappear after an interval of many thousand generations, in the same manner as, with horses, asses, and mules, dark coloured stripes suddenly reappear on the legs and shoulders, after an interval of hundreds, or more probably of thousands of generations."[ ] _the embryonic development of man and other mammalia._ the progressive development of any vertebrate from the ovum or minute embryonic egg affords one of the most marvellous chapters in natural history. we see the contents of the ovum undergoing numerous definite changes, its interior dividing and subdividing till it consists of a mass of cells, then a groove appears marking out the median line or vertebral column of the future animal, and thereafter are slowly developed the various essential organs of the body. after describing in some detail what takes place in the case of the ovum of the dog, professor huxley continues: "the history of the development of any other vertebrate animal, lizard, snake, frog, or fish tells the same story. there is always to begin with, an egg having the same essential structure as that of the dog; the yelk of that egg undergoes division or segmentation, as it is called, the ultimate products of that segmentation constitute the building materials for the body of the young animal; and this is built up round a primitive groove, in the floor of which a notochord is developed. furthermore, there is a period in which the young of all these animals resemble one another, not merely in outward form, but in all essentials of structure, so closely, that the differences between them are inconsiderable, while in their subsequent course they diverge more and more widely from one another. and it is a general law that the more closely any animals resemble one another in adult structure, the larger and the more intimately do their embryos resemble one another; so that, for example, the embryos of a snake and of a lizard remain like one another longer than do those of a snake and a bird; and the embryos of a dog and of a cat remain like one another for a far longer period than do those of a dog and a bird, or of a dog and an opossum, or even than those of a dog and a monkey."[ ] we thus see that the study of development affords a test of affinity in animals that are externally very much unlike each other; and we naturally ask how this applies to man. is he developed in a different way from other mammals, as we should certainly expect if he has had a distinct and altogether different origin? "the reply," says professor huxley, "is not doubtful for a moment. without question, the mode of origin and the early stages of the development of man are identical with those of the animals immediately below him in the scale." and again he tells us: "it is very long before the body of the young human being can be readily discriminated from that of the young puppy; but at a tolerably early period the two become distinguishable by the different forms of their adjuncts, the yelk-sac and the allantois;" and after describing these differences he continues: "but exactly in those respects in which the developing man differs from the dog, he resembles the ape.... so that it is only quite in the latter stages of development that the young human being presents marked differences from the young ape, while the latter departs as much from the dog in its development as the man does. startling as this last assertion may appear to be, it is demonstrably true, and it alone appears to me sufficient to place beyond all doubt the structural unity of man with the rest of the animal world, and more particularly and closely with the apes."[ ] a few of the curious details in which man passes through stages common to the lower animals may be mentioned. at one stage the os coccyx projects like a true tail, extending considerably beyond the rudimentary legs. in the seventh month the convolutions of the brain resemble those of an adult baboon. the great toe, so characteristic of man, forming the fulcrum which most assists him in standing erect, in an early stage of the embryo is much shorter than the other toes, and instead of being parallel with them, projects at an angle from the side of the foot, thus corresponding with its permanent condition in the quadrumana. numerous other examples might be quoted, all illustrating the same general law. _diseases common to man and the lower animals._ though the fact is so well known, it is certainly one of profound significance that many animal diseases can be communicated to man, since it shows similarity, if not identity, in the minute structure of the tissues, the nature of the blood, the nerves, and the brain. such diseases as hydrophobia, variola, the glanders, cholera, herpes, etc., can be transmitted from animals to man or the reverse; while monkeys are liable to many of the same non-contagious diseases as we are. rengger, who carefully observed the common monkey (cebus azarae) in paraguay, found it liable to catarrh, with the usual symptoms, terminating sometimes in consumption. these monkeys also suffered from apoplexy, inflammation of the bowels, and cataract in the eye. medicines produced the same effect upon them as upon us. many kinds of monkeys have a strong taste for tea, coffee, spirits, and even tobacco. these facts show the similarity of the nerves of taste in monkeys and in ourselves, and that their whole nervous system is affected in a similar way. even the parasites, both external and internal, that affect man are not altogether peculiar to him, but belong to the same families or genera as those which infest animals, and in one case, scabies, even the same species.[ ] these curious facts seem quite inconsistent with the idea that man's bodily structure and nature are altogether distinct from those of animals, and have had a different origin; while the facts are just what we should expect if he has been produced by descent with modification from some common ancestor. _the animals most nearly allied to man._ by universal consent we see in the monkey tribe a caricature of humanity. their faces, their hands, their actions and expressions present ludicrous resemblances to our own. but there is one group of this great tribe in which this resemblance is greatest, and they have hence been called the anthropoid or man-like apes. these are few in number, and inhabit only the equatorial regions of africa and asia, countries where the climate is most uniform, the forests densest, and the supply of fruit abundant throughout the year. these animals are now comparatively well known, consisting of the orang-utan of borneo and sumatra, the chimpanzee and the gorilla of west africa, and the group of gibbons or long-armed apes, consisting of many species and inhabiting south-eastern asia and the larger malay islands. these last are far less like man than the other three, one or other of which has at various times been claimed to be the most man-like of the apes and our nearest relations in the animal kingdom. the question of the degree of resemblance of these animals to ourselves is one of great interest, leading, as it does, to some important conclusions as to our origin and geological antiquity, and we will therefore briefly consider it. if we compare the skeletons of the orang or chimpanzee with that of man, we find them to be a kind of distorted copy, every bone corresponding (with very few exceptions), but altered somewhat in size, proportions, and position. so great is this resemblance that it led professor owen to remark: "i cannot shut my eyes to the significance of that all-pervading similitude of structure--every tooth, every bone, strictly homologous--which makes the determination of the difference between _homo_ and _pithecus_ the anatomist's difficulty." the actual differences in the skeletons of these apes and that of man--that is, differences dependent on the presence or absence of certain bones, and not on their form or position--have been enumerated by mr. mivart as follows:--( ) in the breast-bone consisting of but two bones, man agrees with the gibbons; the chimpanzee and gorilla having this part consisting of seven bones in a single series, while in the orang they are arranged in a double series of ten bones. ( ) the normal number of the ribs in the orang and some gibbons is twelve pairs, as in man, while in the chimpanzee and gorilla there are thirteen pairs. ( ) the orang and the gibbons also agree with man in having five lumbar vertebrae, while in the gorilla and the chimpanzee there are but four, and sometimes only three. ( ) the gorilla and chimpanzee agree with man in having eight small bones in the wrist, while the orang and the gibbons, as well as all other monkeys, have nine.[ ] the differences in the form, size, and attachments of the various bones, muscles, and other organs of these apes and man are very numerous and exceedingly complex, sometimes one species, sometimes another agreeing most nearly with ourselves, thus presenting a tangled web of affinities which it is very difficult to unravel. estimated by the skeleton alone, the chimpanzee and gorilla seem nearer to man than the orang, which last is also inferior as presenting certain aberrations in the muscles. in the form of the ear the gorilla is more human than any other ape, while in the tongue the orang is the more man-like. in the stomach and liver the gibbons approach nearest to man, then come the orang and chimpanzee, while the gorilla has a degraded liver more resembling that of the lower monkeys and baboons. _the brains of man and apes._ we come now to that part of his organisation in which man is so much higher than all the lower animals--the brain; and here, mr. mivart informs us, the orang stands highest in rank. the height of the orang's cerebrum in front is greater in proportion than in either the chimpanzee or the gorilla. "on comparing the brain of man with the brains of the orang, chimpanzee, and baboon, we find a successive decrease in the frontal lobe, and a successive and very great increase in the relative size of the occipital lobe. concomitantly with this increase and decrease, certain folds of brain substance, called 'bridging convolutions,' which in man are conspicuously interposed between the parietal and occipital lobes, seem as utterly to disappear in the chimpanzee, as they do in the baboon. in the orang, however, though much reduced, they are still to be distinguished.... the actual and absolute mass of the brain is, however, slightly greater in the chimpanzee than in the orang, as is the relative vertical extent of the middle part of the cerebrum, although, as already stated, the frontal portion is higher in the orang; while, according to m. gratiolet, the gorilla is not only inferior to the orang in cerebral development, but even to his smaller african congener, the chimpanzee."[ ] on the whole, then, we find that no one of the great apes can be positively asserted to be nearest to man in structure. each of them approaches him in certain characteristics, while in others it is widely removed, giving the idea, so consonant with the theory of evolution as developed by darwin, that all are derived from a common ancestor, from which the existing anthropoid apes as well as man have diverged. when, however, we turn from the details of anatomy to peculiarities of external form and motions, we find that, in a variety of characters, all these apes resemble each other and differ from man, so that we may fairly say that, while they have diverged somewhat from each other, they have diverged much more widely from ourselves. let us briefly enumerate some of these differences. _external differences of man and apes._ all apes have large canine teeth, while in man these are no longer than the adjacent incisors or premolars, the whole forming a perfectly even series. in apes the arms are proportionately much longer than in man, while the thighs are much shorter. no ape stands really erect, a posture which is natural in man. the thumb is proportionately larger in man, and more perfectly opposable than in that of any ape. the foot of man differs largely from that of all apes, in the horizontal sole, the projecting heel, the short toes, and the powerful great toe firmly attached parallel to the other toes; all perfectly adapted for maintaining the erect posture, and for free motion without any aid from the arms or hands. in apes the foot is formed almost exactly like our hand, with a large thumb-like great toe quite free from the other toes, and so articulated as to be opposable to them; forming with the long finger-like toes a perfect grasping hand. the sole cannot be placed horizontally on the ground; but when standing on a level surface the animal rests on the outer edge of the foot with the finger and thumb-like toes partly closed, while the hands are placed on the ground resting on the knuckles. the illustration on the next page (fig. ) shows, fairly well, the peculiarities of the hands and feet of the chimpanzee, and their marked differences, both in form and use, from those of man. the four limbs, with the peculiarly formed feet and hands, are those of arboreal animals which only occasionally and awkwardly move on level ground. the arms are used in progression equally with the feet, and the hands are only adapted for uses similar to those of our hands when the animal is at rest, and then but clumsily. lastly, the apes are all hairy animals, like the majority of other mammals, man alone having a smooth and almost naked skin. these numerous and striking differences, even more than those of the skeleton and internal anatomy, point to an enormously remote epoch when the race that was ultimately to develop into man diverged from that other stock which continued the animal type and ultimately produced the existing varieties of anthropoid apes. [illustration: fig. .--chimpanzee (troglodytes niger).] _summary of the animal characteristics of man._ the facts now very briefly summarised amount almost to a demonstration that man, in his bodily structure, has been derived from the lower animals, of which he is the culminating development. in his possession of rudimentary structures which are functional in some of the mammalia; in the numerous variations of his muscles and other organs agreeing with characters which are constant in some apes; in his embryonic development, absolutely identical in character with that of mammalia in general, and closely resembling in its details that of the higher quadrumana; in the diseases which he has in common with other mammalia; and in the wonderful approximation of his skeleton to those of one or other of the anthropoid apes, we have an amount of evidence in this direction which it seems impossible to explain away. and this evidence will appear more forcible if we consider for a moment what the rejection of it implies. for the only alternative supposition is, that man has been specially created--that is to say, has been produced in some quite different way from other animals and altogether independently of them. but in that case the rudimentary structures, the animal-like variations, the identical course of development, and all the other animal characteristics he possesses are deceptive, and inevitably lead us, as thinking beings making use of the reason which is our noblest and most distinctive feature, into gross error. we cannot believe, however, that a careful study of the facts of nature leads to conclusions directly opposed to the truth; and, as we seek in vain, in our physical structure and the course of its development, for any indication of an origin independent of the rest of the animal world, we are compelled to reject the idea of "special creation" for man, as being entirely unsupported by facts as well as in the highest degree improbable. _the geological antiquity of man._ the evidence we now possess of the exact nature of the resemblance of man to the various species of anthropoid apes, shows us that he has little special affinity for any one rather than another species, while he differs from them all in several important characters in which they agree with each other. the conclusion to be drawn from these facts is, that his points of affinity connect him with the whole group, while his special peculiarities equally separate him from the whole group, and that he must, therefore, have diverged from the common ancestral form before the existing types of anthropoid apes had diverged from each other. now, this divergence almost certainly took place as early as the miocene period, because in the upper miocene deposits of western europe remains of two species of ape have been found allied to the gibbons, one of them, dryopithecus, nearly as large as a man, and believed by m. lartet to have approached man in its dentition more than the existing apes. we seem hardly, therefore, to have reached, in the upper miocene, the epoch of the common ancestor of man and the anthropoids. the evidence of the antiquity of man himself is also scanty, and takes us but very little way back into the past. we have clear proof of his existence in europe in the latter stages of the glacial epoch, with many indications of his presence in interglacial or even pre-glacial times; while both the actual remains and the works of man found in the auriferous gravels of california deep under lava-flows of pliocene age, show that he existed in the new world at least as early as in the old.[ ] these earliest remains of man have been received with doubt, and even with ridicule, as if there were some extreme improbability in them. but, in point of fact, the wonder is that human remains have not been found more frequently in pre-glacial deposits. referring to the most ancient fossil remains found in europe--the engis and neanderthal crania,--professor huxley makes the following weighty remark: "in conclusion, i may say, that the fossil remains of man hitherto discovered do not seem to me to take us appreciably nearer to that lower pithecoid form, by the modification of which he has, probably, become what he is." the californian remains and works of art, above referred to, give no indication of a specially low form of man; and it remains an unsolved problem why no traces of the long line of man's ancestors, back to the remote period when he first branched off from the pithecoid type, have yet been discovered. it has been objected by some writers--notably by professor boyd dawkins--that man did not probably exist in pliocene times, because almost all the known mammalia of that epoch are distinct species from those now living on the earth, and that the same changes of the environment which led to the modification of other mammalian species would also have led to a change in man. but this argument overlooks the fact that man differs essentially from all other mammals in this respect, that whereas any important adaptation to new conditions can be effected in them only by a change in bodily structure, man is able to adapt himself to much greater changes of conditions by a mental development leading him to the use of fire, of tools, of clothing, of improved dwellings, of nets and snares, and of agriculture. by the help of these, without any change whatever in his bodily structure, he has been able to spread over and occupy the whole earth; to dwell securely in forest, plain, or mountain; to inhabit alike the burning desert or the arctic wastes; to cope with every kind of wild beast, and to provide himself with food in districts where, as an animal trusting to nature's unaided productions, he would have starved.[ ] it follows, therefore, that from the time when the ancestral man first walked erect, with hands freed from any active part in locomotion, and when his brain-power became sufficient to cause him to use his hands in making weapons and tools, houses and clothing, to use fire for cooking, and to plant seeds or roots to supply himself with stores of food, the power of natural selection would cease to act in producing modifications of his body, but would continuously advance his mind through the development of its organ, the brain. hence man may have become truly man--the species, homo sapiens--even in the miocene period; and while all other mammals were becoming modified from age to age under the influence of ever-changing physical and biological conditions, he would be advancing mainly in intelligence, but perhaps also in stature, and by that advance alone would be able to maintain himself as the master of all other animals and as the most widespread occupier of the earth. it is quite in accordance with this view that we find the most pronounced distinction between man and the anthropoid apes in the size and complexity of his brain. thus, professor huxley tells us that "it may be doubted whether a healthy human adult brain ever weighed less than or ounces, or that the heaviest gorilla brain has exceeded ounces," although "a full-grown gorilla is probably pretty nearly twice as heavy as a bosjes man, or as many an european woman."[ ] the average human brain, however, weighs or ounces, and if we take the average ape brain at only ounces less than the very largest gorilla's brain, or ounces, we shall see better the enormous increase which has taken place in the brain of man since the time when he branched off from the apes; and this increase will be still greater if we consider that the brains of apes, like those of all other mammals, have also increased from earlier to later geological times. if these various considerations are taken into account, we must conclude that the essential features of man's structure as compared with that of apes--his erect posture and free hands--were acquired at a comparatively early period, and were, in fact, the characteristics which gave him his superiority over other mammals, and started him on the line of development which has led to his conquest of the world. but during this long and steady development of brain and intellect, mankind must have continuously increased in numbers and in the area which they occupied--they must have formed what darwin terms a "dominant race." for had they been few in numbers and confined to a limited area, they could hardly have successfully struggled against the numerous fierce carnivora of that period, and against those adverse influences which led to the extinction of so many more powerful animals. a large population spread over an extensive area is also needed to supply an adequate number of brain variations for man's progressive improvement. but this large population and long-continued development in a single line of advance renders it the more difficult to account for the complete absence of human or pre-human remains in all those deposits which have furnished, in such rich abundance, the remains of other land animals. it is true that the remains of apes are also very rare, and we may well suppose that the superior intelligence of man led him to avoid that extensive destruction by flood or in morass which seems to have often overwhelmed other animals. yet, when we consider that, even in our own day, men are not unfrequently overwhelmed by volcanic eruptions, as in java and japan, or carried away in vast numbers by floods, as in bengal and china, it seems impossible but that ample remains of miocene and pliocene man do exist buried in the most recent layers of the earth's crust, and that more extended research or some fortunate discovery will some day bring them to light. _the probable birthplace of man._ it has usually been considered that the ancestral form of man originated in the tropics, where vegetation is most abundant and the climate most equable. but there are some important objections to this view. the anthropoid apes, as well as most of the monkey tribe, are essentially arboreal in their structure, whereas the great distinctive character of man is his special adaptation to terrestrial locomotion. we can hardly suppose, therefore, that he originated in a forest region, where fruits to be obtained by climbing are the chief vegetable food. it is more probable that he began his existence on the open plains or high plateaux of the temperate or sub-tropical zone, where the seeds of indigenous cereals and numerous herbivora, rodents, and game-birds, with fishes and molluscs in the lakes, rivers, and seas supplied him with an abundance of varied food. in such a region he would develop skill as a hunter, trapper, or fisherman, and later as a herdsman and cultivator,--a succession of which we find indications in the palaeolithic and neolithic races of europe. in seeking to determine the particular areas in which his earliest traces are likely to be found, we are restricted to some portion of the eastern hemisphere, where alone the anthropoid apes exist, or have apparently ever existed. there is good reason to believe, also, that africa must be excluded, because it is known to have been separated from the northern continent in early tertiary times, and to have acquired its existing fauna of the higher mammalia by a later union with that continent after the separation from it of madagascar, an island which has preserved for us a sample, as it were, of the early african mammalian fauna, from which not only the anthropoid apes, but all the higher quadrumana are absent.[ ] there remains only the great euro-asiatic continent; and its enormous plateaux, extending from persia right across tibet and siberia to manchuria, afford an area, some part or other of which probably offered suitable conditions, in late miocene or early pliocene times, for the development of ancestral man. it is in this area that we still find that type of mankind--the mongolian--which retains a colour of the skin midway between the black or brown-black of the negro, and the ruddy or olive-white of the caucasian types, a colour which still prevails over all northern asia, over the american continents, and over much of polynesia. from this primary tint arose, under the influence of varied conditions, and probably in correlation with constitutional changes adapted to peculiar climates, the varied tints which still exist among mankind. if the reasoning by which this conclusion is reached be sound, and all the earlier stages of man's development from an animal form occurred in the area now indicated, we can better understand how it is that we have as yet met with no traces of the missing links, or even of man's existence during late tertiary times, because no part of the world is so entirely unexplored by the geologist as this very region. the area in question is sufficiently extensive and varied to admit of primeval man having attained to a considerable population, and having developed his full human characteristics, both physical and mental, before there was any need for him to migrate beyond its limits. one of his earliest important migrations was probably into africa, where, spreading westward, he became modified in colour and hair in correlation with physiological changes adapting him to the climate of the equatorial lowlands. spreading north-westward into europe the moist and cool climate led to a modification of an opposite character, and thus may have arisen the three great human types which still exist. somewhat later, probably, he spread eastward into north-west america and soon scattered himself over the whole continent; and all this may well have occurred in early or middle pliocene times. thereafter, at very long intervals, successive waves of migration carried him into every part of the habitable world, and by conquest and intermixture led ultimately to that puzzling gradation of types which the ethnologist in vain seeks to unravel. _the origin of the moral and intellectual nature of man._ from the foregoing discussion it will be seen that i fully accept mr. darwin's conclusion as to the essential identity of man's bodily structure with that of the higher mammalia, and his descent from some ancestral form common to man and the anthropoid apes. the evidence of such descent appears to me to be overwhelming and conclusive. again, as to the cause and method of such descent and modification, we may admit, at all events provisionally, that the laws of variation and natural selection, acting through the struggle for existence and the continual need of more perfect adaptation to the physical and biological environments, may have brought about, first that perfection of bodily structure in which he is so far above all other animals, and in co-ordination with it the larger and more developed brain, by means of which he has been able to utilise that structure in the more and more complete subjection of the whole animal and vegetable kingdoms to his service. but this is only the beginning of mr. darwin's work, since he goes on to discuss the moral nature and mental faculties of man, and derives these too by gradual modification and development from the lower animals. although, perhaps, nowhere distinctly formulated, his whole argument tends to the conclusion that man's entire nature and all his faculties, whether moral, intellectual, or spiritual, have been derived from their rudiments in the lower animals, in the same manner and by the action of the same general laws as his physical structure has been derived. as this conclusion appears to me not to be supported by adequate evidence, and to be directly opposed to many well-ascertained facts, i propose to devote a brief space to its discussion. _the argument from continuity._ mr. darwin's mode of argument consists in showing that the rudiments of most, if not of all, the mental and moral faculties of man can be detected in some animals. the manifestations of intelligence, amounting in some cases to distinct acts of reasoning, in many animals, are adduced as exhibiting in a much less degree the intelligence and reason of man. instances of curiosity, imitation, attention, wonder, and memory are given; while examples are also adduced which may be interpreted as proving that animals exhibit kindness to their fellows, or manifest pride, contempt, and shame. some are said to have the rudiments of language, because they utter several different sounds, each of which has a definite meaning to their fellows or to their young; others the rudiments of arithmetic, because they seem to count and remember up to three, four, or even five. a sense of beauty is imputed to them on account of their own bright colours or the use of coloured objects in their nests; while dogs, cats, and horses are said to have imagination, because they appear to be disturbed by dreams. even some distant approach to the rudiments of religion is said to be found in the deep love and complete submission of a dog to his master.[ ] turning from animals to man, it is shown that in the lowest savages many of these faculties are very little advanced from the condition in which they appear in the higher animals; while others, although fairly well exhibited, are yet greatly inferior to the point of development they have reached in civilised races. in particular, the moral sense is said to have been developed from the social instincts of savages, and to depend mainly on the enduring discomfort produced by any action which excites the general disapproval of the tribe. thus, every act of an individual which is believed to be contrary to the interests of the tribe, excites its unvarying disapprobation and is held to be immoral; while every act, on the other hand, which is, as a rule, beneficial to the tribe, is warmly and constantly approved, and is thus considered to be right or moral. from the mental struggle, when an act that would benefit self is injurious to the tribe, there arises conscience; and thus the social instincts are the foundation of the moral sense and of the fundamental principles of morality.[ ] the question of the origin and nature of the moral sense and of conscience is far too vast and complex to be discussed here, and a reference to it has been introduced only to complete the sketch of mr. darwin's view of the continuity and gradual development of all human faculties from the lower animals up to savages, and from savage up to civilised man. the point to which i wish specially to call attention is, that to prove continuity and the progressive development of the intellectual and moral faculties from animals to man, is not the same as proving that these faculties have been developed by natural selection; and this last is what mr. darwin has hardly attempted, although to support his theory it was absolutely essential to prove it. because man's physical structure has been developed from an animal form by natural selection, it does not necessarily follow that his mental nature, even though developed _pari passu_ with it, has been developed by the same causes only. to illustrate by a physical analogy. upheaval and depression of land, combined with sub-aerial denudation by wind and frost, rain and rivers, and marine denudation on coastlines, were long thought to account for all the modelling of the earth's surface not directly due to volcanic action; and in the early editions of lyell's _principles of geology_ these are the sole causes appealed to. but when the action of glaciers was studied and the recent occurrence of a glacial epoch demonstrated as a fact, many phenomena--such as moraines and other gravel deposits, boulder clay, erratic boulders, grooved and rounded rocks, and alpine lake basins--were seen to be due to this altogether distinct cause. there was no breach of continuity, no sudden catastrophe; the cold period came on and passed away in the most gradual manner, and its effects often passed insensibly into those produced by denudation or upheaval; yet none the less a new agency appeared at a definite time, and new effects were produced which, though continuous with preceding effects, were not due to the same causes. it is not, therefore, to be assumed, without proof or against independent evidence, that the later stages of an apparently continuous development are necessarily due to the same causes only as the earlier stages. applying this argument to the case of man's intellectual and moral nature, i propose to show that certain definite portions of it could not have been developed by variation and natural selection alone, and that, therefore, some other influence, law, or agency is required to account for them. if this can be clearly shown for any one or more of the special faculties of intellectual man, we shall be justified in assuming that the same unknown cause or power may have had a much wider influence, and may have profoundly influenced the whole course of his development. _the origin of the mathematical faculty._ we have ample evidence that, in all the lower races of man, what may be termed the mathematical faculty is, either absent, or, if present, quite unexercised. the bushmen and the brazilian wood-indians are said not to count beyond two. many australian tribes only have words for one and two, which are combined to make three, four, five, or six, beyond which they do not count. the damaras of south africa only count to three; and mr. galton gives a curious description of how one of them was hopelessly puzzled when he had sold two sheep for two sticks of tobacco each, and received four sticks in payment. he could only find out that he was correctly paid by taking two sticks and then giving one sheep, then receiving two sticks more and giving the other sheep. even the comparatively intellectual zulus can only count up to ten by using the hands and fingers. the ahts of north-west america count in nearly the same manner, and most of the tribes of south america are no further advanced.[ ] the kaffirs have great herds of cattle, and if one is lost they miss it immediately, but this is not by counting, but by noticing the absence of one they know; just as in a large family or a school a boy is missed without going through the process of counting. somewhat higher races, as the esquimaux, can count up to twenty by using the hands and the feet; and other races get even further than this by saying "one man" for twenty, "two men" for forty, and so on, equivalent to our rural mode of reckoning by scores. from the fact that so many of the existing savage races can only count to four or five, sir john lubbock thinks it improbable that our earliest ancestors could have counted as high as ten.[ ] when we turn to the more civilised races, we find the use of numbers and the art of counting greatly extended. even the tongas of the south sea islands are said to have been able to count as high as , . but mere counting does not imply either the possession or the use of anything that can be really called the mathematical faculty, the exercise of which in any broad sense has only been possible since the introduction of the decimal notation. the greeks, the romans, the egyptians, the jews, and the chinese had all such cumbrous systems, that anything like a science of arithmetic, beyond very simple operations, was impossible; and the roman system, by which the year would be written mdccclxxxviii, was that in common use in europe down to the fourteenth or fifteenth centuries, and even much later in some places. algebra, which was invented by the hindoos, from whom also came the decimal notation, was not introduced into europe till the thirteenth century, although the greeks had some acquaintance with it; and it reached western europe from italy only in the sixteenth century.[ ] it was, no doubt, owing to the absence of a sound system of numeration that the mathematical talent of the greeks was directed chiefly to geometry, in which science euclid, archimedes, and others made such brilliant discoveries. it is, however, during the last three centuries only that the civilised world appears to have become conscious of the possession of a marvellous faculty which, when supplied with the necessary tools in the decimal notation, the elements of algebra and geometry, and the power of rapidly communicating discoveries and ideas by the art of printing, has developed to an extent, the full grandeur of which can be appreciated only by those who have devoted some time (even if unsuccessfully) to the study. the facts now set forth as to the almost total absence of mathematical faculty in savages and its wonderful development in quite recent times, are exceedingly suggestive, and in regard to them we are limited to two possible theories. either prehistoric and savage man did not possess this faculty at all (or only in its merest rudiments); or they did possess it, but had neither the means nor the incitements for its exercise. in the former case we have to ask by what means has this faculty been so rapidly developed in all civilised races, many of which a few centuries back were, in this respect, almost savages themselves; while in the latter case the difficulty is still greater, for we have to assume the existence of a faculty which had never been used either by the supposed possessors of it or by their ancestors. let us take, then, the least difficult supposition--that savages possessed only the mere rudiments of the faculty, such as their ability to count, sometimes up to ten, but with an utter inability to perform the very simplest processes of arithmetic or of geometry--and inquire how this rudimentary faculty became rapidly developed into that of a newton, a la place, a gauss, or a cayley. we will admit that there is every possible gradation between these extremes, and that there has been perfect continuity in the development of the faculty; but we ask, what motive power caused its development? it must be remembered we are here dealing solely with the capability of the darwinian theory to account for the origin of the _mind_, as well as it accounts for the origin of the _body_ of man, and we must, therefore, recall the essential features of that theory. these are, the preservation of useful variations in the struggle for life; that no creature can be improved beyond its necessities for the time being; that the law acts by life and death, and by the survival of the fittest. we have to ask, therefore, what relation the successive stages of improvement of the mathematical faculty had to the life or death of its possessors; to the struggles of tribe with tribe, or nation with nation; or to the ultimate survival of one race and the extinction of another. if it cannot possibly have had any such effects, then it cannot have been produced by natural selection. it is evident that in the struggles of savage man with the elements and with wild beasts, or of tribe with tribe, this faculty can have had no influence. it had nothing to do with the early migrations of man, or with the conquest and extermination of weaker by more powerful peoples. the greeks did not successfully resist the persian invaders by any aid from their few mathematicians, but by military training, patriotism, and self-sacrifice. the barbarous conquerors of the east, timurlane and gengkhis khan, did not owe their success to any superiority of intellect or of mathematical faculty in themselves or their followers. even if the great conquests of the romans were, in part, due to their systematic military organisation, and to their skill in making roads and encampments, which may, perhaps, be imputed to some exercise of the mathematical faculty, that did not prevent them from being conquered in turn by barbarians, in whom it was almost entirely absent. and if we take the most civilised peoples of the ancient world--the hindoos, the arabs, the greeks, and the romans, all of whom had some amount of mathematical talent--we find that it is not these, but the descendants of the barbarians of those days--the celts, the teutons, and the slavs--who have proved themselves the fittest to survive in the great struggle of races, although we cannot trace their steadily growing success during past centuries either to the possession of any exceptional mathematical faculty or to its exercise. they have indeed proved themselves, to-day, to be possessed of a marvellous endowment of the mathematical faculty; but their success at home and abroad, as colonists or as conquerors, as individuals or as nations, can in no way be traced to this faculty, since they were almost the last who devoted themselves to its exercise. we conclude, then, that the present gigantic development of the mathematical faculty is wholly unexplained by the theory of natural selection, and must be due to some altogether distinct cause. _the origin of the musical and artistic faculties._ these distinctively human faculties follow very closely the lines of the mathematical faculty in their progressive development, and serve to enforce the same argument. among the lower savages music, as we understand it, hardly exists, though they all delight in rude musical sounds, as of drums, tom-toms, or gongs; and they also sing in monotonous chants. almost exactly as they advance in general intellect, and in the arts of social life, their appreciation of music appears to rise in proportion; and we find among them rude stringed instruments and whistles, till, in java, we have regular bands of skilled performers probably the successors of hindoo musicians of the age before the mahometan conquest. the egyptians are believed to have been the earliest musicians, and from them the jews and the greeks, no doubt, derived their knowledge of the art; but it seems to be admitted that neither the latter nor the romans knew anything of harmony or of the essential features of modern music.[ ] till the fifteenth century little progress appears to have been made in the science or the practice of music; but since that era it has advanced with marvellous rapidity, its progress being curiously parallel with that of mathematics, inasmuch as great musical geniuses appeared suddenly among different nations, equal in their possession of this special faculty to any that have since arisen. as with the mathematical, so with the musical faculty, it is impossible to trace any connection between its possession and survival in the struggle for existence. it seems to have arisen as a _result_ of social and intellectual advancement, not as a _cause_; and there is some evidence that it is latent in the lower races, since under european training native military bands have been formed in many parts of the world, which have been able to perform creditably the best modern music. the artistic faculty has run a somewhat different course, though analogous to that of the faculties already discussed. most savages exhibit some rudiments of it, either in drawing or carving human or animal figures; but, almost without exception, these figures are rude and such as would be executed by the ordinary inartistic child. in fact, modern savages are, in this respect hardly equal to those prehistoric men who represented the mammoth and the reindeer on pieces of horn or bone. with any advance in the arts of social life, we have a corresponding advance in artistic skill and taste, rising very high in the art of japan and india, but culminating in the marvellous sculpture of the best period of grecian history. in the middle ages art was chiefly manifested in ecclesiastical architecture and the illumination of manuscripts, but from the thirteenth to the fifteenth centuries pictorial art revived in italy and attained to a degree of perfection which has never been surpassed. this revival was followed closely by the schools of germany, the netherlands, spain, france, and england, showing that the true artistic faculty belonged to no one nation, but was fairly distributed among the various european races. these several developments of the artistic faculty, whether manifested in sculpture, painting, or architecture, are evidently outgrowths of the human intellect which have no immediate influence on the survival of individuals or of tribes, or on the success of nations in their struggles for supremacy or for existence. the glorious art of greece did not prevent the nation from falling under the sway of the less advanced roman; while we ourselves, among whom art was the latest to arise, have taken the lead in the colonisation of the world, thus proving our mixed race to be the fittest to survive. _independent proof that the mathematical, musical, and artistic faculties have not been developed under the law of natural selection._ the law of natural selection or the survival of the fittest is, as its name implies, a rigid law, which acts by the life or death of the individuals submitted to its action. from its very nature it can act only on useful or hurtful characteristics, eliminating the latter and keeping up the former to a fairly general level of efficiency. hence it necessarily follows that the characters developed by its means will be present in all the individuals of a species, and, though varying, will not vary very widely from a common standard. the amount of variation we found, in our third chapter, to be about one-fifth or one-sixth of the mean value--that is, if the mean value were taken at , the variations would reach from to , or somewhat more, if very large numbers were compared. in accordance with this law we find, that all those characters in man which were certainly essential to him during his early stages of development, exist in all savages with some approach to equality. in the speed of running, in bodily strength, in skill with weapons, in acuteness of vision, or in power of following a trail, all are fairly proficient, and the differences of endowment do not probably exceed the limits of variation in animals above referred to. so, in animal instinct or intelligence, we find the same general level of development. every wren makes a fairly good nest like its fellows; every fox has an average amount of the sagacity of its race; while all the higher birds and mammals have the necessary affections and instincts needful for the protection and bringing-up of their offspring. but in those specially developed faculties of civilised man which we have been considering, the case is very different. they exist only in a small proportion of individuals, while the difference of capacity between these favoured individuals and the average of mankind is enormous. taking first the mathematical faculty, probably fewer than one in a hundred really possess it, the great bulk of the population having no natural ability for the study, or feeling the slightest interest in it.[ ] and if we attempt to measure the amount of variation in the faculty itself between a first-class mathematician and the ordinary run of people who find any kind of calculation confusing and altogether devoid of interest, it is probable that the former could not be estimated at less than a hundred times the latter, and perhaps a thousand times would more nearly measure the difference between them. the artistic faculty appears to agree pretty closely with the mathematical in its frequency. the boys and girls who, going beyond the mere conventional designs of children, draw what they _see_, not what they _know_ to be the shape of things; who naturally sketch in perspective, because it is thus they see objects; who see, and represent in their sketches, the light and shade as well as the mere outlines of objects; and who can draw recognisable sketches of every one they know, are certainly very few compared with those who are totally incapable of anything of the kind. from some inquiries i have made in schools, and from my own observation, i believe that those who are endowed with this natural artistic talent do not exceed, even if they come up to, one per cent of the whole population. the variations in the amount of artistic faculty are certainly very great, even if we do not take the extremes. the gradations of power between the ordinary man or woman "who does not draw," and whose attempts at representing any object, animate or inanimate, would be laughable, and the average good artist who, with a few bold strokes, can produce a recognisable and even effective sketch of a landscape, a street, or an animal, are very numerous; and we can hardly measure the difference between them at less than fifty or a hundred fold. the musical faculty is undoubtedly, in its lower forms, less uncommon than either of the preceding, but it still differs essentially from the necessary or useful faculties in that it is almost entirely wanting in one-half even of civilised men. for every person who draws, as it were instinctively, there are probably five or ten who sing or play without having been taught and from mere innate love and perception of melody and harmony.[ ] on the other hand, there are probably about as many who seem absolutely deficient in musical perception, who take little pleasure in it, who cannot perceive discords or remember tunes, and who could not learn to sing or play with any amount of study. the gradations, too, are here quite as great as in mathematics or pictorial art, and the special faculty of the great musical composer must be reckoned many hundreds or perhaps thousands of times greater than that of the ordinary "unmusical" person above referred to. it appears then, that, both on account of the limited number of persons gifted with the mathematical, the artistic, or the musical faculty, as well as from the enormous variations in its development, these mental powers differ widely from those which are essential to man, and are, for the most part, common to him and the lower animals; and that they could not, therefore, possibly have been developed in him by means of the law of natural selection. * * * * * we have thus shown, by two distinct lines of argument, that faculties are developed in civilised man which, both in their mode of origin, their function, and their variations, are altogether distinct from those other characters and faculties which are essential to him, and which have been brought to their actual state of efficiency by the necessities of his existence. and besides the three which have been specially referred to, there are others which evidently belong to the same class. such is the metaphysical faculty, which enables us to form abstract conceptions of a kind the most remote from all practical applications, to discuss the ultimate causes of things, the nature and qualities of matter, motion, and force, of space and time, of cause and effect, of will and conscience. speculations on these abstract and difficult questions are impossible to savages, who seem to have no mental faculty enabling them to grasp the essential ideas or conceptions; yet whenever any race attains to civilisation, and comprises a body of people who, whether as priests or philosophers, are relieved from the necessity of labour or of taking an active part in war or government, the metaphysical faculty appears to spring suddenly into existence, although, like the other faculties we have referred to, it is always confined to a very limited proportion of the population. in the same class we may place the peculiar faculty of wit and humour, an altogether natural gift whose development appears to be parallel with that of the other exceptional faculties. like them, it is almost unknown among savages, but appears more or less frequently as civilisation advances and the interests of life become more numerous and more complex. like them, too, it is altogether removed from utility in the struggle for life, and appears sporadically in a very small percentage of the population; the majority being, as is well known, totally unable to say a witty thing or make a pun even to save their lives.[ ] _the interpretation of the facts._ the facts now set forth prove the existence of a number of mental faculties which either do not exist at all or exist in a very rudimentary condition in savages, but appear almost suddenly and in perfect development in the higher civilised races. these same faculties are further distinguished by their sporadic character, being well developed only in a very small proportion of the community; and by the enormous amount of variation in their development, the higher manifestations of them being many times--perhaps a hundred or a thousand times--stronger than the lower. each of these characteristics is totally inconsistent with any action of the law of natural selection in the production of the faculties referred to; and the facts, taken in their entirety, compel us to recognise some origin for them wholly distinct from that which has served to account for the animal characteristics--whether bodily or mental--of man. the special faculties we have been discussing clearly point to the existence in man of something which he has not derived from his animal progenitors--something which we may best refer to as being of a spiritual essence or nature, capable of progressive development under favourable conditions. on the hypothesis of this spiritual nature, superadded to the animal nature of man, we are able to understand much that is otherwise mysterious or unintelligible in regard to him, especially the enormous influence of ideas, principles, and beliefs over his whole life and actions. thus alone we can understand the constancy of the martyr, the unselfishness of the philanthropist, the devotion of the patriot, the enthusiasm of the artist, and the resolute and persevering search of the scientific worker after nature's secrets. thus we may perceive that the love of truth, the delight in beauty, the passion for justice, and the thrill of exultation with which we hear of any act of courageous self-sacrifice, are the workings within us of a higher nature which has not been developed by means of the struggle for material existence. it will, no doubt, be urged that the admitted continuity of man's progress from the brute does not admit of the introduction of new causes, and that we have no evidence of the sudden change of nature which such introduction would bring about. the fallacy as to new causes involving any breach of continuity, or any sudden or abrupt change, in the effects, has already been shown; but we will further point out that there are at least three stages in the development of the organic world when some new cause or power must necessarily have come into action. the first stage is the change from inorganic to organic, when the earliest vegetable cell, or the living protoplasm out of which it arose, first appeared. this is often imputed to a mere increase of complexity of chemical compounds; but increase of complexity, with consequent instability, even if we admit that it may have produced protoplasm as a chemical compound, could certainly not have produced _living_ protoplasm--protoplasm which has the power of growth and of reproduction, and of that continuous process of development which has resulted in the marvellous variety and complex organisation of the whole vegetable kingdom. there is in all this something quite beyond and apart from chemical changes, however complex; and it has been well said that the first vegetable cell was a new thing in the world, possessing altogether new powers--that of extracting and fixing carbon from the carbon-dioxide of the atmosphere, that of indefinite reproduction, and, still more marvellous, the power of variation and of reproducing those variations till endless complications of structure and varieties of form have been the result. here, then, we have indications of a new power at work, which we may term _vitality_, since it gives to certain forms of matter all those characters and properties which constitute life. the next stage is still more marvellous, still more completely beyond all possibility of explanation by matter, its laws and forces. it is the introduction of sensation or consciousness, constituting the fundamental distinction between the animal and vegetable kingdoms. here all idea of mere complication of structure producing the result is out of the question. we feel it to be altogether preposterous to assume that at a certain stage of complexity of atomic constitution, and as a necessary result of that complexity alone, an _ego_ should start into existence, a thing that _feels_, that is _conscious_ of its own existence. here we have the certainty that something new has arisen, a being whose nascent consciousness has gone on increasing in power and definiteness till it has culminated in the higher animals. no verbal explanation or attempt at explanation--such as the statement that life is the result of the molecular forces of the protoplasm, or that the whole existing organic universe from the amaeba up to man was latent in the fire-mist from which the solar system was developed--can afford any mental satisfaction, or help us in any way to a solution of the mystery. the third stage is, as we have seen, the existence in man of a number of his most characteristic and noblest faculties, those which raise him furthest above the brutes and open up possibilities of almost indefinite advancement. these faculties could not possibly have been developed by means of the same laws which have determined the progressive development of the organic world in general, and also of man's physical organism.[ ] these three distinct stages of progress from the inorganic world of matter and motion up to man, point clearly to an unseen universe--to a world of spirit, to which the world of matter is altogether subordinate. to this spiritual world we may refer the marvellously complex forces which we know as gravitation, cohesion, chemical force, radiant force, and electricity, without which the material universe could not exist for a moment in its present form, and perhaps not at all, since without these forces, and perhaps others which may be termed atomic, it is doubtful whether matter itself could have any existence. and still more surely can we refer to it those progressive manifestations of life in the vegetable, the animal, and man--which we may classify as unconscious, conscious, and intellectual life,--and which probably depend upon different degrees of spiritual influx. i have already shown that this involves no necessary infraction of the law of continuity in physical or mental evolution; whence it follows that any difficulty we may find in discriminating the inorganic from the organic, the lower vegetable from the lower animal organisms, or the higher animals from the lowest types of man, has no bearing at all upon the question. this is to be decided by showing that a change in essential nature (due, probably, to causes of a higher order than those of the material universe) took place at the several stages of progress which i have indicated; a change which may be none the less real because absolutely imperceptible at its point of origin, as is the change that takes place in the curve in which a body is moving when the application of some new force causes the curve to be slightly altered. _concluding remarks._ those who admit my interpretation of the evidence now adduced--strictly scientific evidence in its appeal to facts which are clearly what ought _not_ to be on the materialistic theory--will be able to accept the spiritual nature of man, as not in any way inconsistent with the theory of evolution, but as dependent on those fundamental laws and causes which furnish the very materials for evolution to work with. they will also be relieved from the crushing mental burthen imposed upon those who--maintaining that we, in common with the rest of nature, are but products of the blind eternal forces of the universe, and believing also that the time must come when the sun will lose his heat and all life on the earth necessarily cease--have to contemplate a not very distant future in which all this glorious earth--which for untold millions of years has been slowly developing forms of life and beauty to culminate at last in man--shall be as if it had never existed; who are compelled to suppose that all the slow growths of our race struggling towards a higher life, all the agony of martyrs, all the groans of victims, all the evil and misery and undeserved suffering of the ages, all the struggles for freedom, all the efforts towards justice, all the aspirations for virtue and the wellbeing of humanity, shall absolutely vanish, and, "like the baseless fabric of a vision, leave not a wrack behind." as contrasted with this hopeless and soul-deadening belief, we, who accept the existence of a spiritual world, can look upon the universe as a grand consistent whole adapted in all its parts to the development of spiritual beings capable of indefinite life and perfectibility. to us, the whole purpose, the only _raison d'être_ of the world--with all its complexities of physical structure, with its grand geological progress, the slow evolution of the vegetable and animal kingdoms, and the ultimate appearance of man--was the development of the human spirit in association with the human body. from the fact that the spirit of man--the man himself--_is_ so developed, we may well believe that this is the only, or at least the best, way for its development; and we may even see in what is usually termed "evil" on the earth, one of the most efficient means of its growth. for we know that the noblest faculties of man are strengthened and perfected by struggle and effort; it is by unceasing warfare against physical evils and in the midst of difficulty and danger that energy, courage, self-reliance, and industry have become the common qualities of the northern races; it is by the battle with moral evil in all its hydra-headed forms, that the still nobler qualities of justice and mercy and humanity and self-sacrifice have been steadily increasing in the world. beings thus trained and strengthened by their surroundings, and possessing latent faculties capable of such noble development, are surely destined for a higher and more permanent existence; and we may confidently believe with our greatest living poet-- that life is not as idle ore, but iron dug from central gloom, and heated hot with burning fears, and dipt in baths of hissing tears, and batter'd with the shocks of doom to shape and use. we thus find that the darwinian theory, even when carried out to its extreme logical conclusion, not only does not oppose, but lends a decided support to, a belief in the spiritual nature of man. it shows us how man's body may have been developed from that of a lower animal form under the law of natural selection; but it also teaches us that we possess intellectual and moral faculties which could not have been so developed, but must have had another origin; and for this origin we can only find an adequate cause in the unseen universe of spirit. footnotes: [footnote : _descent of man_, pp. - ; also pp. - .] [footnote : _man's place in nature_, p. .] [footnote : _man's place in nature_, p. . see figs. of embryos of man and dog in darwin's _descent of man_, p. .] [footnote : _the descent of man_, pp. , .] [footnote : _man and apes._ by st. george mivart, f.r.s., . it is an interesting fact (for which i am indebted to mr. e.b. poulton) that the human embryo possesses the extra rib and wrist-bone referred to above in ( ) and ( ) as occurring in some of the apes.] [footnote : _man and apes_, pp. , .] [footnote : for a sketch of the evidence of man's antiquity in america, see _the nineteenth century_ for november .] [footnote : this subject was first discussed in an article in the _anthropological review_, may , and republished in my _contributions to natural selection_, chap, ix, in .] [footnote : _man's place in nature_, p. .] [footnote : for a full discussion of this question, see the author's _geographical distribution of animals_, vol. i. p. .] [footnote : for a full discussion of all these points, see _descent of man_, chap. iii.] [footnote : _descent of man_, chap. iv.] [footnote : lubbock's _origin of civilisation_, fourth edition, pp. - ; tylor's _primitive culture_, chap. vii.] [footnote : it has been recently stated that some of these facts are erroneous, and that some australians can keep accurate reckoning up to , or more, when required. but this does not alter the general fact that many low races, including the australians, have no words for high numbers and never require to use them. if they are now, with a little practice, able to count much higher, this indicates the possession of a faculty which could not have been developed under the law of utility only, since the absence of words for such high numbers shows that they were neither used nor required.] [footnote : article arithmetic in _eng. cyc. of arts and sciences_.] [footnote : see "history of music," in _eng. cyc._, science and arts division.] [footnote : this is the estimate furnished me by two mathematical masters in one of our great public schools of the proportion of boys who have any special taste or capacity for mathematical studies. many more, of course, can be drilled into a fair knowledge of elementary mathematics, but only this small proportion possess the natural faculty which renders it possible for them ever to rank high as mathematicians, to take any pleasure in it, or to do any original mathematical work.] [footnote : i am informed, however, by a music master in a large school that only about one per cent have real or decided musical talent, corresponding curiously with the estimate of the mathematicians.] [footnote : in the latter part of his essay on heredity (pp. - of the volume of _essays_), dr. weismann refers to this question of the origin of "talents" in man, and, like myself, comes to the conclusion that they could not be developed under the law of natural selection. he says: "it may be objected that, in man, in addition to the instincts inherent in every individual, special individual predispositions are also found, of such a nature that it is impossible they can have arisen by individual variations of the germ-plasm. on the other hand, these predispositions--which we call talents--cannot have arisen through natural selection, because life is in no way dependent on their presence, and there seems to be no way of explaining their origin except by an assumption of the summation of the skill attained by exercise in the course of each single life. in this case, therefore, we seem at first sight to be compelled to accept the transmission of acquired characters." weismann then goes on to show that the facts do not support this view; that the mathematical, musical, or artistic faculties often appear suddenly in a family whose other members and ancestors were in no way distinguished; and that even when hereditary in families, the talent often appears at its maximum at the commencement or in the middle of the series, not increasing to the end, as it should do if it depended in any way on the transmission of acquired skill. gauss was not the son of a mathematician, nor handel of a musician, nor titian of a painter, and there is no proof of any special talent in the ancestors of these men of genius, who at once developed the most marvellous pre-eminence in their respective talents. and after showing that such great men only appear at certain stages of human development, and that two or more of the special talents are not unfrequently combined in one individual, he concludes thus-- "upon this subject i only wish to add that, in my opinion, talents do not appear to depend upon the improvement of any special mental quality by continued practice, but they are the expression, and to a certain extent the bye-product, of the human mind, which is so highly developed in all directions." it will, i think, be admitted that this view hardly accounts for the existence of the highly peculiar human faculties in question.] [footnote : for an earlier discussion of this subject, with some wider applications, see the author's _contributions to the theory of natural selection_, chap. x.] index =a= abbott, dr. c.c., instability of habits of birds, on american water-thrushes (seiurus), mr., drawings of caterpillars and their food plants, accessory plumes, development and display of, acclimatisation, achatinellidae, gulick on variations in, acquired characters, non-heredity of, acraeidae, mimicry of, adaptation to conditions at various periods of life, adolias dirtea, sexual diversity of, aegeriidae, mimicry by, agaristidae, mimicry of, agassiz, on species, on north american weeds, . agelaeus phoeniceus, diagram showing variations of, ; proportionate numbers which vary, albatross, courtship of great, allen, mr. grant, on forms of leaves, on degradation of wind-fertilised from insect-fertilised flowers, (note) on insects and flowers, on production of colour through the agency of the colour sense, mr. j.a., on the variability of birds, allen, mr. j.a., on colour as influenced by climate, alluring coloration, american school of evolutionists, anemone nemorosa, variability of, animal coloration, a theory of, general laws of, intelligence, supposed action of, characteristics of man, animals, the struggle among, wild, their enjoyment of life, usually die painless deaths, constitutional variation of, uses of colours of, supposed effects of disuse in wild, most allied to man, antelopes, recognition marks of, anthrocera filipendula inedible, apples, variations of, arctic animals, supposed causes of white colour of, argyll, duke of, on goose reared by a golden eagle, artemia salina and a. milhausenii, asclepias curassavica, spread of, asses running wild in quito, attractive fruits, australia, spread of the cape-weed in, fossil and recent mammals of, azara, on cause of horses and cattle not running wild in paraguay, azores, flora of, supports aerial transmission of seeds, =b= baker, mr. j.g., on rarity of spiny plants in mauritius, ball, mr., on cause of late appearance of exogens, barber, mrs., on variable colouring of pupae of papilio nireus, on protective colours of african sun-birds, barbs, barriers, importance of, in questions of distribution, bates, mr. h.w., on varieties of butterflies, on inedibility of heliconidae, on a conspicuous caterpillar, on mimicry, , , bathmism or growth-force, cope on, beddard, mr. f.e., variations of earthworms, on plumes of bird of paradise, beech trees, aggressive in denmark, beetle and wasp (figs.), beetle, fossil in coal measures of silesia, beginnings of important organs, belt, mr., on leaf-like locust, on birds avoiding heliconidae, belt's frog, birds, rate of increase of, how destroyed, variation among, variation of markings of, variation of wings and tails of, diagram showing variation of tarsus and toes, use of structural peculiarities of, eggs, coloration of, recognition marks of, and butterflies, white in tropical islands, sometimes seize inedible butterflies, mimicry among, birds, sexual coloration of, cause of dull colour of female, choice of female not known to be determined by colour, etc., decorative plumage of, antics of unornamented, which fertilise flowers, colours of, not dependent on the colours of flowers, no proof of aesthetic tastes in, dispersal of, and insects at sea, of oceanic islands, carrying seeds on their feet, ancestral forms of, birthplace, probable, of man, bombyx regia, protective form of larva of, boyd dawkins, on development of deer's horns, on origin of man, brady, mr. george, on protective colouring of starfishes, brain development, progressive, brains of man and apes, branner, mr. j.c., on supposed proofs of glaciation in brazil, brazil, supposed proof of glaciation in, brewer, professor w.h., on want of symmetry in colours of animals, bromelia, animals inhabiting leaves of, bronn, professor, on supposed uselessness of variations of ears and tails, butler, mr. a.g., on inedibility of conspicuous caterpillars, butterflies, varieties of, small, of isle of man, special protective colouring of, recognition by, inedibility of some, mimicry among, , colour development of, sexual coloration of, =c= caddis-fly larvae inhabiting bromelia leaves, callophis, harmless mimicking poisonous species, candolle, alp. de, on variation in oaks, on variability of papaver bracteatum, cardinalis virginianus, diagram showing proportionate numbers which vary, ; variations of, carpenter, dr. w.b., on variation in the foraminifera, carriers, caterpillars, resemblance of, to their food plants, - inedible, cattle, how they prevent the growth of trees, increase of, in st. domingo, mexico, and the pampas, ceylon, spread of lantana mixta in, chaffinch, change of habit of, in new zealand, chambers, robert, on origin of species, chance rarely determines survival, change of conditions, utility of, characters, non-adaptive, transferred from useless to useful class, charaxes psaphon persecuted by a bird, chile, numerous red tubular flowers in, chimpanzee, figure of, clark, mr. edwin, on cause of absence of forests on the pampas, on the struggle for life in the south american valleys, cleistogamous flowers, close interbreeding, supposed evil results of, clover, white, spread of, in new zealand, co-adaptation of parts by variation, no real difficulty, cobra, use of hood of, coccinella mimicked by grasshopper, (figure), collingwood, mr., on butterflies recognising their kind, coloration, alluring, of birds' eggs, a theory of animal, colour correlated with sterility, correlated with constitutional peculiarities, in nature, the problem to be solved, constancy, in animals indicates utility, and environment, general theories of animal, animal, supposed causes of, obscure, of many tropical animals, produced by surrounding objects, adaptations, local, for recognition, of wild animals not quite symmetrical, (note) as influenced by locality or climate, development in butterflies, more variable than habits, and nerve distribution, and tegumentary appendages, of flowers, change of, in flowers when fertilised, in nature, concluding remarks on, , of fruits, of flowers growing together contrasted, complexity of flowers due to alternate adaptation to insect and self-fertilisation, composite, a, widely dispersed without pappus, confinement, affecting fertility, continental and oceanic areas, continents and oceans cannot have changed places, possible connections between, continuity does not prove identity of origin, cope, dr. e.d., on non-adaptive characters, on fundamental laws of growth, on bathmism or growth-force, on use producing structural change, on law of centrifugal growth, on origin of the feet of ungulates, on action of animal intelligence, correlations in pigeons, horses, etc., corvus frugilegus, corone, coursers, figures of secondary quills, cowslip, two forms of, crab, sexual diversity of colour of, cretaceous period, dicotyledons of, crisp, dr., on variations of gall bladder and alimentary canal, crosses, a cause of variation, reciprocal, cross-fertilisation, modes of securing, difference in, crossing and changed conditions, parallelism of, cruciferae, variations of structure in, cuckoo, eggs of, cuckoos mimick hawks, cultivated plants, origin of useful, curculionidae mimicked by various insects (figs.), curves of variation, =d= dana, professor, on the permanence of continents, danaidae little attacked by mites, mimicry of, darwin, change of opinion effected by, the newton of natural history, his view of his own work, on the enemies of plants, on fir-trees destroyed by cattle, on change of plants and animals caused by planting, on absence of wild cattle in paraguay, on cats and red clover, on variety of plants in old turf, on the beneficent action of the struggle for existence, on variability of wild geraniums, on variability of common species, his non-recognition of extreme variability of wild species, on races of domestic pigeon, on constitutional variation in plants, on unconscious selection, on a case of divergence, on advantage of diversification of structure in inhabitants of one region, on species of plants in turf, on isolation, on origin of mammary glands, on eyes of flatfish, on origin of the eye, on useless characters, on use of ears and tails, on disappearance of sports, on tendency to vary in one direction, on rare perpetuation of sports, on utility of specific characters, (note) on importance of biological environment, on variable fertility of plants, on fertile hybrids among plants, darwin, on correlation of sterility and colour, on selective association, on infertility and natural selection, on cause of infertility of hybrids, on white tail of rabbit, on conspicuous caterpillars, on sexual selection in insects, on decorative plumage of male birds, on development of ocelli, on value of cross-fertilisation, on limits to utility of intercrossing, on flowers due to insects, on oceanic islands, on effects of disuse in domestic animals, , on direct action of environment, on unintelligibility of theory of retardation and acceleration, (note) on origin of man's moral nature, mr. george, on intermarriages of british aristocracy, darwinian theory, statement of, not opposed to spiritual nature of man, dawkins, professor boyd, on development of deer's horns, on recent origin of man, dawson, sir w., on determination of fossil plants by leaves, (note) death of wild animals usually painless, de candolle, definition of species, on difficulty of naturalising plants, on war between plants, on origin of useful cultivated plants, deer's horns, development of, degeneration, delboeuf's law of variation, dendraeca coronata, variation of wing-feathers of, denmark, struggle between trees in, denudation, evidences of, desert animals, colour of, deserts, effect of goats and camels in destroying vegetation in, development and display of accessory plumes, diadema anomala, misippus, great diversity of sexes in, diaphora mendica mimics spilosoma menthrasti, difficulties in the facts of fertilisation of flowers, dimorphism and trimorphism, dippers, probable origin of, disease and markings, diseases common to man and animals, display of decorative plumage, distribution of organisms should be explained by theory of descent, conditions which have determined the, of marsupials, of tapirs, disuse, effects of, among wild animals, no proof that the effects of, are inherited, divergence of character, - leads to maximum of forms of life in each area, diversity of fauna and flora with geographical proximity, dixon, mr. c, changed habits of chaffinch in new zealand, dogs, origin of, varieties of, dolichonyx oryzivorus, diagram showing variations of, domestic animals, varieties of, draba verna, varieties of, dress of men not determined by female choice, dust from krakatoa, size of particles of, =e= eastern butterflies, variation of, eaton, rev. a.e., on kerguelen insects, edwards, mr. w.h., on dark forms of papilio turnus, eggs protectively coloured, , theory of varied colours of, elaps mimicked by harmless snakes, embryonic development of man and other mammalia, ennis, mr. john, on willows driving out watercresses from rivers of new zealand, entomostraca, in bromelia leaves, environment never identical for two species, direct action of, direct influence of, as initiator of variations, action of, overpowered by natural selection, ethical aspect of the struggle for existence, euchelia jacobeae inedible, everett, mr. a., on a caterpillar resembling moss, evidence of evolution that may be expected among fossil forms, evolutionists, american school of, exogens, possible cause of sudden late appearance of, external differences of man and apes, extinct animals, number of species of, extinction of large animals, cause of, eye, origin of, eyes, explanation of loss of in cave animals, =f= facts of natural selection, summary of, falcons illustrating divergence, and butcher birds, hooked and toothed beaks of, fantails, female birds, why often dull coloured, female birds, what their choice of mates is determined by, butterflies, why dull coloured, brighter than male bird, choice a doubtful agent in selection, preference neutralised by natural selection, fertility of domestic animals, flatfish, eyes of, flesh-fly, enormous increase of, floral structure, great differences of, in allied genera and species, flowers, variations of, colours of, with sham nectaries, changing colour when fertilised, adapted to bees or to butterflies, contrasted colours of, at same season and locality, fertilisation of, by birds, self-fertilisation of, once insect-fertilised now self-fertile, how the struggle for existence acts among, repeatedly modified during whole tertiary period, the product of insect agency, forbes, mr. h.o., on protective colour of a pigeon, on spider imitating birds' dropping, fossil shells, complete series of transitional forms of, crocodiles afford evidence of evolution, horses in america, and living animals, local relations of, fowl, early domestication of, frill-back, indian, frog inhabiting bromelia leaves, fruits, use of characters of, colours of, edible or attractive, poisonous, fulica atra, protectively coloured eggs of, fulmar petrel, abundance of, =g= gallinaceae, ornamental plumes of, galton, mr. f., diagrams of variability used by, on markings of zebra, (note) on regression towards mediocrity, theory of heredity by, (note) on imperfect counting of the damaras, gaudry on extinct animals at pikermi, gay, mons. t., on variations of structure in cruciferae, gazella soemmerringi (figure), gazelles, recognition marks of, geddes, professor, on variation in plants, objection to theory of, geikie, dr. archibald, on formation of marine stratified rocks, geoffroy st. hilaire, on species, geological evidences of evolution, , record, causes of imperfection of, distribution of insects, antiquity of man, ghost-moth, colours of, glaciation, no proofs of, in brazil, glow-worm, light a warning of inedibility, gomphia oleaefolia, variability of, goose eating flesh, gosse, mr. p.h., on variation in the sea-anemones, on sea-anemone and bullhead, gould, mr., on colours of coast and inland birds, grant allen, on forms of leaves, on insects and flowers, graphite in laurentian implies abundant plant life, gray, dr. asa, on naturalised plants in the united states, dr. j.e., on variation of skulls of mammalia, great fertility not essential to rapid increase, great powers of increase of animals, green colour of birds in tropical forests, grouse, red, recent divergence of, gulick, rev. j.t., on variation of land-shells, on isolation and variation, , on divergent evolution, =h= habits of animals, variability of, hairy caterpillars inedible, hanbury, mr. thomas, on a remarkable case of wind conveyance of seed, (note) hansten-blangsted, on succession of trees in denmark, harvest mice, prehensile tails of young, hawkweed, species and varieties of british, hector, sir james, use of horns of deer, heliconidae, warning colours of, mimicry of, helix nemoralis, varieties of, hortensis, varieties of, hemsley, mr., on rarity of spines in oceanic islands, henslow, professor g., on vigour of self-fertilised plants, on wind-fertilised as degradations from insect-fertilised flowers, on origin of forms and structures of flowers, (note) herbert, dean, on species, on plant hybrids, herbivora, recognition marks of, heredity, weismann's theory of, herschel, sir john, on species, hooker, sir joseph, on attempts at naturalising australian plants in new zealand, home, mr. c, on inedibility of an indian locust, horns of deer, uses of, horse tribe, pedigree of, ancestral forms of, humming-birds, recognition marks of, huth, mr., on close interbreeding, huxley, professor on the struggle for existence, on fossil crocodiles, on anatomical peculiarities of the horse tribe, on development of vertebrates, on early man, on brains of man and the gorilla, hybridity, remarks on facts of, summary on, hybrids, infertility of, supposed test of distinct species, fertility of, fertile among animals, between sheep and goat, fertile between distinct species of moths, fertile among plants, hymenopus bicornis, resembling flower, =i= icterus baltimore, diagram showing proportionate numbers which vary, imitative resemblances, how produced, increase of organisms in a geometrical ratio, inedible fruits rarely coloured, insect and self-fertilisation, alternation of, in flowers, insect-fertilisation, facts relating to, insects, coloured for recognition, warning colours of, sexual coloration of, importance of dull colours to female, visiting one kind of flower at a time, and flowers, the most brilliant not found together, insects, no proof of love of colour by, and birds at sea, in mid-ocean, at great altitudes, geological distribution of, ancestral in silurian, fossil support evolution, instability of useless characters, instinct, the theory of, insular organisms illustrate powers of dispersal, interbreeding, close, injurious effects of, supposed evil results of close, intercrossing, swamping effects of, not necessarily useful, intermediate forms, why not found, islands, all oceanic are volcanic or coralline, isle of man, small butterflies of, isolation, the importance of, to prevent intercrossing, by variations of habits, etc., rev. j.g. gulick on, when ineffective, ituna ilione and thyridia megisto, figures of wings of, =j= jacobin, jenyns, rev. l., on internal variations of mammalia, jordan, mons. a., on varieties of draba verna, judd, professor, on dust fallen at genoa, on hungarian fossil lacustrine shells, =k= kerguelen island, wingless insects of, kerivoula picta, protective colour of, kerner, professor, on use of external characters of plants, on seeds found on glaciers, kingfishers illustrating divergence of character, =l= lacerta muralis, diagram of variation of, lagopus scoticus, origin of, lamarck, on origin of species, land debris deposited near coasts, and ocean, diagram showing comparative height and depth of, large animals, cause of extinction of, larvae of moths, variability of, laughers, frill-backs, nuns, spots, and swallows, law of relation of colour and nest, , laws of animal coloration, lawson tait, on uses of tails, leaf-butterflies, leguminosae, rare in oceanic islands, lemuria, an unsound hypothesis, lepidoptera, variation of, leyden museum, diagram showing variability of birds in, life, weismann on duration of, (note) limenitis misippus mimics danais archippus, ursula mimics papilio philenor, linnaeus, on rapid increase of the flesh-fly, livingstone, his sensations when seized by a lion, lizards, variation among, diagram of variation of, sexual colours of, local colour adaptations, locusts with warning colours inedible, longicorns mimic malacoderms, low, mr., on effects of close interbreeding, low, mr., on fertile crosses between sheep and goat, on selective association, low forms of life, continued existence of, explained, forms, persistence of, temperature of tropics not needed to explain plant dispersal, lower types, extinction of, among the higher animals, lubbock, sir john, on forms of leaves, on imperfect counting of early man, lyell, sir charles, on variation of species, on the shifting of continents, =m= madagascar and new zealand, madeira, wingless beetles of, maize, origin of, male rivalry, a real cause of selection, males of many animals fights together, malm, on eyes of flatfish, mammalia, variation of, sexual colours of, , afford crucial tests of theories of distribution, early forms of, geological distribution of, mammary glands, supposed origin of, man, summary of animal characteristics of, geological antiquity of, early remains of, in california, probably as old as the miocene, probable birthplace of, origin of moral and intellectual nature of, possesses mental qualities not derived exclusively from his animal progenitors, man's body that of an animal, development similar to that of animals, structure compared with that of the anthropoid apes, mania typica refused by lizards, mantidae resembling flowers, marcgravia nepenthoides fertilised by birds (woodcut), marine animals, protective resemblance among, with warning colours, marsh, mr., on destructiveness to vegetation of goats and camels, professor o., on the development of the horse tribe, on brain development of tertiary mammals, on specialised forms dying out, marsupials, distribution of, mathematical faculty, the origin of the, how developed, not developed by law of natural selection, mathematics, late development of, meldola, professor r., on variable protective colouring, on mimicry among british moths, (note) on an extension of the theory of mimicry, (note) melons, variations of, methona psidii and leptalis orise (figs.), meyer, dr. a.b., on mimicry of snakes, milne edwards, on variation of lizards, mimicking birds deceive naturalists, butterfly, figure of, mimicry, how it has been produced, among protected genera, extension of, in various orders of insects, among vertebrata, among birds, objections to theory of, mineral particles carried by wind, miocene fossils of north america, missing links, character of, mivart, dr. st. george, on variation of ribs and dorsal vertebrae, on supposed useless characters, (note) on resemblance of man and apes, modifications for special purposes, mongrels, sterility of, monkeys affected by medicines as are men, monocotyledons degradations from dicotyledons, (note) scarcity of, in rocky mountains, scarcity of, in alpine flora, moral nature of man, origin of, morse, professor e.t., on protective colouring of marine mollusca, moseley, professor, on protective resemblance among marine animals, on courtship of great albatross, moths, protected groups of, mountains, remote, with identical plants, müller, dr. fritz, on inhabitants of bromelia leaves, on butterfly, deceived by its mimic, his explanation of mimicry among protected genera, dr. hermann, on variability of myosurus minimus, murray, mr. john, on bulk of land and ocean, on quartz particles on ocean floor, rev. r.p., variation in the neuration of butterflies' wings, musical and artistic faculties, origin of, myosurns minimus, variability of, =n= natural selection with changed and unchanged conditions, and sterility, overpowers effects of use and disuse, the most important agency in modifying species, naturalist deceived by a mimicking insect, by mimicking birds, naudin, m., on varieties of melons, nectarinea amethystina, protective colouring of, nestor notabilis, variation of habits of, nests of birds influence the colour of females, new species, conditions favourable to origin of, newton, professor a., on fertile hybrid ducks, new zealand, european plants in, spread of white clover in, effects of introduced plants in, native rat and fly exterminated by european species, many plants of, incapable of self-fertilisation, fauna of, few spiny plants in, nocturnal animals, colours of, non-adaptive characters, instability of, normandy pigs, fleshy appendages to jaws of, north america, miocene fossils of, northern plants in southern hemisphere, nostus borbonicus, variability of, number of individuals which vary, nutmeg, how dispersed, nuts, not meant to be eaten, =o= oaks, great variability of, objections to darwin's theory, ocean floor, deposits on, oceanic animals, colours of, and continental areas, islands have no mammals or batrachia, oceans, the permanence of, oedicnemus, figures of wings of, opthalmis lincea and artaxa simulans (figs.), orang-utans, variations of skull of, orchideae, why scarce on oceanic islands, orchis pyramidalis, mode of fertilisation of, figures illustrating fertilisation of, organic development, three stages of, involving new cause or power, world, the development of, implies a spiritual world, organisation, advance of, by natural selection, degradation in, origin of species, objections, of accessory plumes, orioles mimicking honey-suckers, ornamental plumes and vitality, =p= pachyrhynchi subjects of mimicry, pampas, effects of drought in, papaver bracteatum, variability of, papilio, use of forked tentacle of larva of, protected groups of, mimicry of, paraguay, absence of wild cattle and horses, parnassia palustris, sham nectaries of, parrot, change of habits of new zealand, parus, species of, illustrate divergence, of character, passenger-pigeon, account of its breeding-places and numbers, pelagic animals, colours of, phasmidae, resemblance of, to sticks and leaves, physiological selection, pickard-cambridge, rev. o., on sexual selection, (note) pieridae, sexual diversity among, pigeons, varieties of, domestic, derived from wild rock-pigeons, curious correlations in, white eggs of, protective, pigs, great increase of, in south america and new zealand, pikermi, extinct animals of, pipits as illustrating divergence, planorbidae, variations of, plants, the enemies of, variability of, constitutional variation of, colour relations of, true mimicry rare in, exotic rarely naturalised in europe, dispersal of, northern, in southern hemisphere, identical on summits of remote mountains, progressive development of, geological development of (diagram), plovers, recognition marks of (figure), plumes, origin of accessory, muscular relation of ornamental, poisonous fruits, porto santo, rabbits of, poulton, mr. e.b., on variable colours of larvae and pupae, , on concealments of insects by resemblance to environment, on protective form of notodonta ziczac, on inedibility of conspicuous larvae, pouters, primulaceae, variations of structure in, problem, the, before darwin, problems in variation and heredity, progression in plants and animals, protection by terrifying enemies, protective colouring, variable, of white-headed fruit-pigeon, of african sun-birds, of kerivoula picta, of sloths, of larva of sphinx ligustri, of stick and leaf insects, of caterpillars, , of butterflies, ptilopus cinctus, protective colour of, pugnacity of birds with accessory plumes, =r= rabbit, use of white tail of, rapid increase of plants, raspail, m., on variability in a grass, rat, black, spread of, rattlesnake, use of rattle of, raven, why black in arctic regions, reciprocal crosses, recognition marks of herbivora, of birds, of tropical forest birds, of insects, reproductive functions, susceptibility of, reptiles, geological distribution of, rhinoceroses, evidence of evolution afforded by fossil, rocks, all stratified formed in shallow water, rocky mountains, scarcity of monocotyledons in, rodents, prevent woody vegetation in the pampas, romanes, professor g.j., on useless characters, , on meaningless peculiarities of structure, on supposed absence of simultaneous variations, on physiological selection, rook and crow, roses, mr. baker on varieties of, rubus, bentham and babington on species and varieties of, rudiments and variations in man, runts, rutaceae, variation of structure in, =s= st. helena, destruction of forests by goats, st. hilaire, m. aug., variability of gomphia oleaefolia, saxicola, divergence of character in species of, recognition marks of, scientific opinion before darwin, scolopax, figures of tails of, scudder, mr. s.h., on inedibility of danais archippus, on fossil insects, seebohm, mr., on swamping effects of intercrossing, seeds, how dispersed, how protected, floating great distances, dispersal of, by wind, weight and dimensions of, importance of wind-carriage of, remarkable case of wind-carriage of, seiurus carolinensis, diagram of variation, sp., habits of, selection, artificial, by man, circumstances favourable to, unconscious, selective association, isolation by, self-fertilisation of flowers, semper, professor, on casting hairs of reptiles and crustacea, (note) on direct influence of environment, sesiidae, mimicry by, sex colour and nests of birds, sex, colours characteristic of, sexual colours of insects, probable causes of, of birds, characters due to natural selection, diversity the cause of variation, sexual selection and colour, by struggles of males, neutralised by natural selection, - restricted to male struggles, shetland islands, variety of ghost-moth in, shrews and field-mice, internal variations of, shrikes, recognition marks of, similarity of forms of life not due to similarity of conditions, singing of male birds, use of, skull of wolf, diagram of variations of, of ursus labiatus, diagram of variations of, of sus cristatus, diagram of variations of, skunk an illustration of warning colour, slack, baron von, on protective markings of sloths, sloth, protective colour and marking of, snakes, mimicry of poisonous, snipe, tails of two species (figs.), sounds and odours peculiar to male, how useful, produced by peculiar feathers, south america, fossil and recent mammals of, species, definition of, , origin of, , lyell on, agassiz on, transmutation of, geoffroy st. hilaire on, dean herbert on, professor grant on, von buch on, allied, found in distinct areas, species, which vary little, closely allied inhabit distinct areas, vigour and fertility of, how kept up, spencer, mr. herbert, on factors of organic evolution, on effects of disuse, on difficulty as to co-adaptation of parts, on direct action of environment, sphingidae, protective attitudes of larvae, sphinx ligustri, general resemblance of larva to food plant, spider, alluring coloration of, spines, on origin of, rarity of, in oceanic islands, spiny plants abundant in south africa and chile, spots a primitive ornamentation of animals, sprengel on flowers and insects, staphylinidae, protective habit of, sterility of mongrels, correlated with colour, etc., and natural selection, of hybrids produced by natural selection, struggle for existence, among plants, for life, illustrations of, for existence on the pampas, for life between closely allied forms most severe, for existence, ethics of, how it acts among flowers, summary of facts of colouring for protection and recognition, survival of the fittest, , , swainson, definition of species, swamping effects of intercrossing, sweden, destruction of grass by larvae of moths in, swinhoe, mr., on protective colouring of a bat, symmetry, bilateral in colours of animals needful for recognition, =t= tails used as respirators, tapirs, distribution of, tegetmeier, mr., on feeding habits of pigeons and fowls, on sparrows and crocuses, on curious correlations in pigeons, tegumentary appendages and colour, thousand-fathom line divides oceanic from continental islands, the teachings of, map showing, thwaites, mr., on spread of lantana mixta in ceylon, tiger, use of stripes of, titmice as illustrating divergence, transformation of species of crustacea, transmutationists, the early, travers, mr. w.l., on effects of introduced plants in new zealand, trees, great variety of, in many forests, trimen, mr., on butterfly deceived by its mimic, on mimicry, tropical animals, why brilliantly coloured, tropics, no proof of lower temperature of, tropidorhynchi mimicked by orioles, trumpeter, tumblers, turbits and owls, tylor, mr. a., on _coloration in animals and plants_, =u= ungulates, origin of feet of, use and disuse, effects of, overpowered by natural selection, useless characters, not specific, useless specific characters, no proof of existence of, utriculariae inhabiting bromelias, =v= vanessa callirhoe, small variety in porto santo, variability of the lower animals, of the foraminifera, of sea-anemones, of land mollusca, of insects, of lizards, of birds, of primary wing-feathers, of wings and tail, of dolichonyx oryzivorus, of agelaeus phoeniceus, of cardinalis virginianus, of tarsus and toes, of birds in leyden museum, of sciurus carolinensis, of skulls of wolf, of skulls of a bear, of skulls of sus cristatus, of plants, of oaks, variation, lyell on, in internal organs, the facts of, proofs of generality of, of vegetables and fruits, of apples and melons, under domestication accords with that under nature, coincident not necessary, and heredity, problems of, professor geddes's theory of, the cause of, variations of flowers, of domestic animals, of domestic pigeons, conditions favourable to production of, beneficial, varieties, importance of, of same species adapted to self or to insect-fertilisation, vegetables, variation of, vegetation and reproduction, antagonism of, vertebrata, mimicry among, geological succession of, _vestiges of creation_, viola odorata, canina, violets, as illustrating species, von buch on species, =w= wallace, dr. alexander, on absence of choice by female moths, ward, mr. lester f., on progressive development of plants, warning coloration, warning colours of marine animals, wasps and bees, mimicry of, poisonous with warning colours, water-cress, chokes rivers in new zealand, driven out by willows, water-ouzels, probable origin of, weale, mr. mansel, on protective colours of butterflies, weeds of united states, weir, mr. jenner, on deceptive resemblance of a caterpillar to a twig, on inedibility of conspicuous caterpillars, on birds disregarding inedible larvae, weismann on progressive adaptation of colours of larva, on non-heredity of acquired characters, and galton's theories of heredity almost identical, (note) on origin of the mathematical faculty, (note) weismann's theory of heredity, westwood, professor, on variation of insects, deceived by a mimicking cricket, white coloration of insular birds and butterflies, whymper, mr., his sensations when falling on the matterhorn, willows, species and varieties of british, wilson, alexander, his account of the passenger-pigeon in north america, wind-carriage of seeds explains many facts of plant distribution, wind-dispersal of seeds, objections to, wind-fertilised degraded from insect-fertilised flowers, wings of stone-curlews (figure), why small but useless are retained, wit and humour, origin of faculties of, wollaston, mr. t.w., on variation of beetles, on small butterfly in porto santo, wolves, varieties of in catskill mountains, wood, mr. j., on muscular variations, mr. t.w., on variable colouring of pupae of cabbage butterflies, woodward, dr. s.p., on variation of mollusca, =y= youatt, on breeds of sheep, young animals often spotted, =z= zebra, markings for recognition and protection, (note) the end _at the deathbed of darwinism_ a series of papers by e. dennert, ph.d. authorized translation by e. v. o'harra and john h. peschges german literary board burlington, iowa copyright by r. neumann _contents_ preface introduction chapter i.--the return to wigand--the botanist, julius von sachs--the vienna zoologist, dr. schneider chapter ii.--professor goethe on "the present status of darwinism"--explains the reluctance of certain men of science to discard darwinism chapter iii.--professor korchinsky rejects darwinism--his theory of heterogenesis--professor haberlandt of graz--demonstration of a "vital force"--its nature--the sudden origination of a new organ--importance of the experiment. chapter iv.--testimony of a palaeontologist, professor steinmann--on haeckel's family trees--the principle of multiple origin--extinction of the saurians--"darwinism not the alpha and omega of the doctrine of descent"--steinmann's conclusions chapter v.--eimer's theory of organic growth--definite lines of development--rejects darwin's theory of fluctuating variations--opposes weismann--repudiates darwinian "mimicry"--discards the "romantic" hypothesis of sexual selection--"transmutation is a physiological process, a phyletic growth" chapter vi.--admissions of a darwinian--professor von wagner's explanation of the decay of darwinism--darwinism rejects the inductive method, hence unscientific--wagner's contradictory assertions chapter vii.--haeckel's latest production--his extreme modesty--reception of the weltraetsel--schmidt's apologia--the romanes incident--men of science who convicted haeckel of deliberate fraud chapter viii.--grottewitz writes on "darwinian myths"--darwinism incapable of scientific proof--"the principle of gradual development certainly untenable"--"darwin's theory of "chance" a myth" chapter ix.--professor fleischmann of erlangen--doctrine of descent not substantiated--missing links--"collapse of haeckel's theory"--descent hypothesis "antiquated"--fleischmann formerly a darwinian--haeckel's disreputable methods of defense chapter x.--hertwig, the berlin anatomist, protests against the materialistic view of life"--no empiric proof of darwinism--"the impotence of natural selection"--rejects haeckel's "biogenetic law" conclusion.--darwinism abandoned by men of science--supplanted by a theory in harmony with theistic principles preface. the general tendency of recent scientific literature dealing with the problem of organic evolution may fairly be characterized as distinctly and prevailingly unfavorable to the darwinian theory of natural selection. in the series of chapters herewith offered for the first time to english readers, dr. dennert has brought together testimonies which leave no room for doubt about the decadence of the darwinian theory in the highest scientific circles in germany. and outside of germany the same sentiment is shared generally by the leaders of scientific thought. that the popularizers of evolutionary conceptions have any anti-darwinian tendencies cannot, of course, be for a moment maintained. for who would undertake to popularize what is not novel or striking? but a study of the best scientific literature reveals the fact that the attitude assumed by one of our foremost american zoologists, professor thomas hunt morgan, in his recent work on "evolution and adaptation," is far more general among the leading men of science than is popularly supposed. professor morgan's position may be stated thus: he adheres to the general theory of descent, i.e., he believes the simplest explanation which has yet been offered of the structural _similarities_ between species within the same group, is the hypothesis of a common descent from a parent species. but he emphatically rejects the notion--and this is the quintessence of darwinism--that the _dissimilarities_ between species have been brought about by the purely mechanical agency of natural selection. to find out what, precisely, darwin meant by the term "natural selection" let us turn for a moment, to his great work, _the origin of species by means of natural selection_. in the second chapter of that work, darwin observes that small "fortuitous" variations in individual organisms, though of small interest to the systematist, are of the "highest importance" for his theory, since these minute variations often confer on the possessor of them, some advantage over his fellows in the quest for the necessaries of life. thus these chance individual variations become the "first steps" towards slight varieties, which, in turn, lead to sub-species, and, finally, to species. varieties, in fact, are "incipient species." hence, small "fortuitous" fluctuating, individual variations--i.e., those which chance to occur without predetermined direction--are the "first-steps" in the origin of species. this is the first element in the darwinian theory. in the third chapter of the same work we read: "it has been seen in the last chapter that amongst organic beings in a state of nature there is some individual variability. * * * but the mere existence of individual variability and of some few well-marked varieties, though necessary as a _foundation_ of the work, helps us but little in understanding how species arise in nature. how have all those exquisite adaptations of one part of the organization to another part, and to the conditions of life, and of one organic being to another being, been perfected? * * *" again it may be asked, how is it that varieties, which i have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? how do those groups of species which constitute what are called distinct genera arise? all of these results follow from the _struggle for life_. owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings, and to their physical conditions of life, will tend to the preservation of such individuals and will generally be inherited by the offspring. the offspring also will thus have a better chance of surviving, for of the many individuals of any species which are periodically born, but a small number can survive. i have called this principle by which each slight variation, if useful, is preserved, by the term, "natural selection." mr. darwin adds that his meaning would be more accurately expressed by a phrase of mr. spencer's coinage, "survival of the fittest." it may be observed that neither "natural selection" nor "survival of the fittest" gives very accurate expression to the idea which darwin seems to wish to convey. natural selection is at best a metaphorical description of a process, and "survival of the fittest" describes the result of that process. nor shall we find the moving principle of evolution in individual variability unless we choose to regard chance as an efficient agency. consequently, the only efficient principle conceivably connected with the process is the "struggle for existence;" and even this has only a purely negative function in the origination of species or of adaptations. for, the "surviving fittest" owe nothing more to the struggle for existence than our pensioned veterans owe to the death-dealing bullets which did _not_ hit them. mr. darwin has, however, obviated all difficulty regarding precision of terms by the remark that he intended to use his most important term, "struggle for existence" in "a large and metaphorical sense." we have now seen the second element of darwinism, namely, the "struggle for life." the theory of natural selection, then, postulates the accumulation of minute "fortuitions" individual modifications, which are useful to the possessor of them, by means of a struggle for life of such a sanguinary nature and of such enormous proportions as to result in the destruction of the overwhelming majority of adult individuals. these are the correlative factors in the process of natural selection. in view of the popular identification of darwinism with the doctrine of evolution, on the one hand, and with the theory of struggle for life, on the other hand, it is necessary to insist on the darwinian conception of small, fluctuating, useful variations as the "first-steps" in the evolutionary process. for, this conception distinguishes darwinism from the more recent evolutionary theory, e.g., of de vries who rejects the notion that species have originated by the accumulation of fluctuating variations; and it is quite as essential to the darwinian theory of natural selection as is the "struggle for life." it is, in fact, an integral element in the selection theory. the attitude of science towards darwinism may, therefore, be conveniently summarized in its answer to the following questions: . is there any evidence that such a struggle for life among mature forms, as darwin postulates, actually occurs? . can the origin of adaptive structures be explained on the ground of their _utility_ in this struggle, i.e., is it certain or even probable that the organism would have perished, had it lacked the particular adaptation in its present degree of perfection? on the contrary, is there not convincing proof that many, and presumably most, adaptations cannot be thus accounted for? the above questions are concerned with "the struggle for life." those which follow have to do with the problem of variations. . is there any reason to believe that new species may originate by the accumulation of fluctuating individual variations? . does the evidence of the geological record--which, as huxley observed, is the only direct evidence that can be had in the question of evolution--does this evidence tell for or against the origin of existing species from earlier ones by means of minute gradual modifications? we must be content here with the briefest outline of the reply of science to these inquiries. . darwin invites his readers to "keep steadily in mind that each organic being is striving to increase in geometrical ratio." if this tendency were to continue unchecked, the progeny of living beings would soon be unable to find standing room. indeed, the very bacteria would quickly convert every vestige of organic matter on earth into their own substance. for has not cohn estimated that the offspring of a single bacterium, at its ordinary rate of increase under favorable conditions, would in three days amount to , billions of individuals with an aggregate weight of seven thousand five hundred tons? and the , , elephants which, according to darwin, should to-day perpetuate the lives of each pair that mated in the twelfth century--surely these would be a "magna pars" in the sanguinary contest. when the imagination views these and similar figures, and places in contrast to this multitude of living beings, the limited supply of nourishment, the comparison of nature with a huge slaughterhouse seems tame enough. but reason, not imagination, as darwin observes more than once, should be our guide in a scientific inquiry. it is observed on careful reflection that darwin's theory is endangered by an extremely large disturbing element, viz., accidental destruction. under this term we include all the destruction of life which occurs in utter indifference to the presence or absence of any individual variations from the parent form. indeed, the greatest destruction takes place among immature forms before any variation from the parent stock is discernible at all. in this connection we may instance the vast amount of eggs and seeds destroyed annually irrespective of any adaptive advantage that would be possessed by the matured form. and the countless forms in every stage of individual development which meet destruction through "accidental causes which would not be in the least degree mitigated by certain changes of structure or of constitution which would otherwise be beneficial to the species." this difficulty, darwin himself recognized. but he was of opinion that if even "one-hundredth or one-thousandth part" of organic beings escaped this fortuitous destruction, there would supervene among the survivors a struggle for life sufficiently destructive to satisfy his theory. this suggestion, however, fails to meet the difficulty. for, as professor morgan points out, darwin assumes "that a second competition takes place after the first destruction of individuals has occurred, and this presupposes that more individuals reach maturity than there is room for in the economy of nature." it presupposes that the vast majority of forms that survive accidental destruction, succumb in the second struggle for life in which the determining factor is some slight individual variation, e.g., a little longer neck in the case of the giraffe, or a wing shorter than usual in the case of an insect on an island. the whole theory of struggle, as formulated by darwin, is, therefore, a violent assumption. men of science now recognize that "egoism and struggle play a very subordinate part in organic development, in comparison with co-operation and social action." what, indeed, but a surrender of the paramountcy of struggle for life, is huxley's celebrated romanes lecture in which he supplants the cosmic process by the ethical? the french free-thinker, charles robin, gave expression to the verdict of exact science when he declared: "darwinism is a fiction, a poetical accumulation of probabilities without proof, and of attractive explanations without demonstration." . the hopeless inadequacy of the struggle for life to account for adaptive structures has been dealt with at considerable length by professor morgan in the concluding chapters of the work already mentioned. we cannot here follow him in his study of the various kinds of adaptations, e.g., form and symmetry, mutual adaptation of colonial forms, protective coloration, organs of extreme perfection, tropisms and instincts, etc., in regard to the origin of each of which he is forced to abandon the darwinian theory. it will suffice to call attention to his conclusions concerning the phenomena of regeneration of organs. by his research in this special field professor morgan has won international recognition among men of science. it was while prosecuting his studies in this field that he became impressed with the utter bankruptcy of the theory of natural selection which darwinians put forward to explain the acquisition by organisms of this most useful power of regeneration. it is not difficult to show that regeneration could not in many cases, and presumably in none, have been acquired through natural selection (p. ). if an earth worm (_allolobophora foctida_) be cut in two in the middle, the posterior piece regenerates at its anterior cut end, not a head but a tail. "not by the widest stretch of the imagination can such a result be accounted for on the selection theory." quite the reverse case presents itself in certain planarians. if the head of _planaria lugubris_ is cut off just behind the eyes, there develops at the cut surface of the head-piece another head turned in the opposite direction. "these and other reasons," concludes professor morgan (p. ), "indicate with certainty that regeneration cannot be explained by the theory of natural selection." the ingenuity of the darwinian imagination, however, will hardly fail to assign some reason why two heads are more useful than one in the above instance, and thus reconcile the phenomenon with darwinism. for, according to professor morgan "to imagine that a particular organ is useful to its possessor and to account for its origin because of the imagined benefit conferred, is the general procedure of the followers of the darwinian school." "personal conviction, mere possibility," writes quatrefages, "are offered as proofs, or at least as arguments in favor of the theory." "the realms of fancy are boundless," is blanchard's significant comment on darwin's explanation of the blindness of the mole. "on this class of speculation," says bateson in his "materials for the study of variation," referring to darwinian speculation as to the beneficial or detrimental nature of variations, "on this class of speculation the only limitations are those of the ingenuity of the author." the general form of darwin's argument, declared the writer of a celebrated article in the north british review, is as follows: "all these things may have been, therefore my theory is possible; and since my theory is a possible one, all those hypotheses which it requires are rendered probable." . we pass now to the question of the possibility of building up a new species by the accumulation of chance individual variations. that species ever originate in this way is denied by the advocates of the evolutionary theory which is now superseding darwinism. typical of the new school is the botanist hugo de vries of amsterdam. the "first-steps" in the origin of new species according to de vries are not fluctuating individual variations, but mutations, i.e., definite and permanent modifications. according to the mutation theory a new species arises from the parent species, not gradually but suddenly. it appears suddenly "without visible preparation and without transitional steps." the wide acceptance with which this theory is meeting must be attributed to the fact that men of science no longer believe in the origin of species by the accumulation of slight fluctuating modifications. to quote the words of de vries, "fluctuating variation cannot overstep the limits of the species, even after the most prolonged selection--still less can it lead to the production of new, permanent characters." it has been the wont of darwinians to base their speculations on the assumption that "an inconceivably long time" could effect almost anything in the matter of specific transformations. but the evidence which has been amassed during the past forty years leaves no doubt that there is a limit to individual variability which neither time nor skill avail to remove. as m. blanchard asserts in his work, _la vie des etres animes_ (p. ), "all investigation and observation make it clear that, while the variability of creatures in a state of nature displays itself in very different degrees, yet, in its most astonishing manifestations, it remains confined within a circle beyond which it cannot pass." it is interesting to observe how writers of the darwinian school attempt to explain the origin of articulate language as a gradual development of animal sounds. "it does not," observes darwin, "appear altogether incredible that some unusually wise ape-like animal should have thought of imitating the growl of a beast of prey, so as to indicate to his fellow monkeys the nature of the expected danger. and this would have been a first step in the formation of a language." but what a tremendous step! an ape-like animal that "thought" of imitating a beast must certainly have been "unusually wise." in bridging the chasm which rational speech interposes between man and the brute creation, the darwinian is forced to assume that the whole essential modification is included in the first step. then he conceals the assumption by parcelling out the accidental modification in a supposed series of transitional stages. he endeavors to veil his inability to explain the first step, as chevalier bunsen remarked, by the easy but fruitless assumption of an infinite space of time, destined to explain the gradual development of animals into men; as if millions of years could supply the want of an agent necessary for the first movement, for the first step in the line of progress. "how can speech, the expression of thought, develop itself in a year or in millions of years, out of unarticulated sounds which express feelings of pleasure, pain, and appetite? the common-sense of mankind will always shrink from such theories." . the hopes and fears of darwinians have rightly been centered on the history of organic development as outlined in the geological record. it has been pointed out repeatedly by the foremost men of science that if the theory of genetic descent with the accumulation of small variations be the true account of the origin of species, a complete record of the ancestry of any existing species would reveal no distinction of species and genera. between any two well-defined species, if one be derived from the other, there must be countless transition forms. but palaeontology fails to support the theory of evolution by minute variations. darwinism has been shattered on the geologic rocks. "the complete absence of intermediate forms," says mr. carruthers, "and the sudden and contemporaneous appearance of highly organized and widely separated groups, deprive the hypothesis of genetic evolution of any countenance from the plant record of these ancient rocks. the whole evidence is against evolution (i.e., by minute modification) and there is none for it." (cf. _history of plant life and its bearing on theory of evolution_, ). similar testimony regarding the animal kingdom is borne by mr. mivart in the following carefully worded statement: "the mass of palaeontological evidence is indeed overwhelmingly against minute and gradual modification." "the darwinian theory," declared professor fleischmann of erlangen, recently, "has not a single fact to confirm it in the realm of nature. it is not the result of scientific research, but purely the product of the imagination." on one occasion huxley expressed his conviction that the pedigree of the horse as revealed in the geological record furnished demonstrative evidence for the theory of evolution. the question has been entered into in detail by professor fleischmann in his work, _die descendenstheorie_. in this book the erlangen professor makes great capital out of the "trot-horse" (paradepferd) of huxley and haeckel; and as regards the evolutionary theory, easily claims a verdict of "not proven." in this connection the moderate statement of professor morgan is noteworthy: "when he (fleischmann) says there is no absolute proof that the common plan of structure must be the result of blood relationship, he is not bringing a fatal argument against the theory of descent, for no one but an enthusiast sees anything more in the explanation than a very probable theory that appears to account for the facts. to demand an absolute proof is to ask for more than any reasonable advocate of the descent theory claims for it." (professor morgan, as we have already seen, rejects darwinism, and inclines to the mutation theory of de vries.) the vast majority of darwinians must, therefore, be classed as "enthusiasts" who are not "reasonable advocates of the descent theory." for has not professor marsh told his readers that "to doubt evolution is to doubt science?" and similar assertions have been so frequently made and reiterated by darwinians that the claim that darwinism has become a dogma contains, as professor morgan notes, more truth than the adherents of that school find pleasant to hear. more interesting, however, than huxley's geological pedigree of the horse is haeckel's geological pedigree of man. one who reads haeckel's _natural history of creation_ can hardly escape the impression that the author had actually seen specimens of each of the twenty-one ancestral forms of which his pedigree of man is composed. such, however, was not the case. quatrefages, speaking of this wonderful genealogical tree which haeckel has drawn up with such scientific accuracy of description, observes: "the first thing to remark is that _not one_ of the creatures exhibited in this pedigree has ever been seen, either living or in fossil. their existence is based entirely upon theory." (_les emules de darwin_, ii. _p._ ). "man's pedigree as drawn up by haeckel," says the distinguished savant, du bois-reymond, "is worth about as much as is that of homer's heroes for critical historians." in constructing his genealogies haeckel has frequent recourse to his celebrated "law of biogenesis." the "law of biogenesis" which is the dignified title haeckel has given to the discredited recapitulation theory, asserts that the embryological development of the individual (ontogeny), is a brief recapitulation, a summing up, of the stages through which the species passed in the course of its evolution in the geologic past, (phylogeny). ontogeny is a brief recapitulation of phylogeny. this, says haeckel, is what the "fundamental law of biogenesis" teaches us. (the reader of haeckel and other darwinians will frequently find laws put forward to establish facts: whereas other men of science prefer to have facts establish laws). when, therefore, as quatrefages remarks, the transition between the types which haeckel has incorporated into his genealogical tree, appears too abrupt, he often betakes himself to ontogeny and describes the embryo in the corresponding interval of development. this description he inserts in his genealogical mosaic, by virtue of the "law of biogenesis." many theories have been constructed to explain the phenomena of embryological development. of these the simplest and least mystical is that of his in the great classic work on embryology, "unsere koerperform." his tells us: "in the entire series of forms which a developing organism runs through, each form is the necessary antecedent step of the following. if the embryo is to reach the complicated end-form, it must pass, step by step, through the simpler ones. each step of the series is the physiological consequence of the preceding stage, and the necessary condition for the following." but whatever theory be accepted by men of science, it is certainly not that proposed by haeckel. carl vogt after giving haeckel's statement of the "law of biogenesis" wrote: "this law which i long held as well-founded, is absolutely and radically false." even oskar hertwig, perhaps the best known of haeckel's former pupils, finds it necessary to change haeckel's expression of the biogenetic law so that "a contradiction contained in it may be removed." professor morgan, finally, rejects haeckel's boasted "law of biogenesis" as "_in principle, false_." and he furthermore seems to imply that fleischmann merits the reproach of men of science, for wasting his time in confuting "the antiquated and generally exaggerated views of writers like haeckel." "antiquated and generally exaggerated views." such is the comment of science on haeckel's boast that darwin's pre-eminent service to science consisted in pointing out how purposive adaptations may be produced by natural selection without the direction of mind just as easily as they may be produced by artificial selection and human design. and yet the latest and least worthy production from the pen of this darwinian philosopher, _the riddle of the universe_, is being scattered broad-cast by the anti-christian press, in the name and guise of _popular_ science. it is therein that the evil consists. for the discerning reader sees in the book itself, its own best refutation. the pretensions of haeckel's "consistent and monistic theory of the eternal cosmogenetic process" are best met by pointing to the fact that its most highly accredited and notorious representative has given to the world in exposition and defense of pure darwinian philosophy, a work, which, for boldness of assertion, meagerness of proof, inconsequence of argument, inconsistency in fundamental principles and disregard for facts which tell against the author's theory, has certainly no equal in contemporary literature. in the apt and expressive phrase of professor paulsen, the book "fairly drips with superficiality" (von seichtigkeit triefen). if the man of science is to be justified, as huxley suggested, not by faith but by verification, haeckel and his docile darwinian disciples have good reason to tremble for their scientific salvation. edwin v. o'hara. st. paul, minn. _introduction._ during the last few years i have published under this title short articles dealing with the present status of darwinism. in view of the kind reception which has been accorded to these articles by the reading public i have thought it well to bring them together in pamphlet form. indeed, the darwinian movement and its present status are eminently deserving of consideration, especially on the part of those before whom darwinism has hitherto always been held up triumphantly as a scientific disproof of the very foundations of the christian faith. by way of introduction and explanation some general preliminary remarks may not be amiss here. previous to twenty or thirty years ago, it was justifiable to identify darwinism with the doctrine of descent, for at that time darwinism was the only doctrine of descent which could claim any general recognition. consequently, one who was an adherent of the doctrine of descent was also a darwinian. those to whom this did not apply were so few as to be easily counted. the dispute then hinged primarily on darwinism; hence, for those who did not admit the truth of that theory, the doctrine of descent was for the most part also a myth. i say, for the most part; for there were already even at that time a few clear-sighted naturalists (wigand, naegeli, koelliker and others) who saw plainly the residue of truth that would result from the discussion. but to the overwhelming majority, the alternatives seemed to be: either darwinism or no evolution at all. today, however, the state of things is considerably altered. the doctrine of descent is clearly and definitely distinguished from darwinism at least by the majority of naturalists. it is therefore of the utmost importance that this luminous distinction should likewise become recognized in lay circles. my object in these pages is to show that darwinism will soon be a thing of the past, a matter of history; that we even now stand at its death-bed, while its friends are solicitous only to secure for it a decent burial. out of the chaos of controversy which has obtained during the last four decades there has emerged an element of truth--for there lurks a germ of truth in most errors--which has gained almost universal recognition among contemporary men of science, namely, the doctrine of descent. the fact that living organisms form an ascending series from the less perfect to the more perfect; the further fact that they also form a series according as they display more or less homology of structure and are formed according to similar types; and, lastly, that the fossil remains of organisms found in the various strata of the earth's surface likewise represent an ascending series from the simple to the more complex--these three facts suggested to naturalists the thought that living organisms were not always as we find them to-day, but that the more perfect had developed from simpler forms through a series of modifications. these thoughts were at first advanced with some hesitation, and were confined to narrow circles. they received, however, material support when, during the fourth decade of the th century the splendid discovery was made (by k. e. von baer) that every organism is slowly developed from a germ, and in the process of development passes through temporary lower stages to a permanent higher one. even at that time many naturalists believed in a corresponding development of the whole series of organisms, without of course being able to form a clear conception of the process. such was the state of affairs when darwin in the year published his principal work, _the origin of species by means of natural selection_. in this work for the first time an exhaustive attempt was made to sketch a clear and completely detailed picture of the process of development. darwin started with the fact that breeders of animals and growers of plants, having at their disposal a large number of varieties, always diverging somewhat from each other, choose individuals possessing characteristics which they desired to strengthen, and use only these for procreation. in this manner the desired characteristic is gradually made more prominent, and the breeder appears to have obtained a new species. similar conditions are supposed to prevail in nature, only that there is lacking the selecting hand of the breeder. here the so-called principle of natural selection holds automatic sway by means of the struggle for existence. all the various forms of life are warring for the means of subsistence, each striving to obtain for itself the best nourishment, etc. in this struggle those organisms will be victorious which possess the most favorable characteristics; all others must succumb. hence those only will survive which are best adapted to their environment. but between those which survive, the struggle begins anew, and when the favoring peculiarities become more pronounced in some, (by chance, of course) these in turn win out. thus nature gradually improves her various breeds through the continued action of a self-regulating mechanism. such are the main features of darwinism, its real kernel, about which of course,--and this is a proof of its insufficiency,--from the very beginning a number of auxiliary hypotheses attached themselves. darwin's theory sounds so clear and simple, and seems at first blush so luminous that it is no wonder if many careful naturalists regarded it as an incontrovertible truth. the warning voice of the more prudent men of science was silenced by the loud enthusiasm of the younger generation over the solution of the greatest of the world-problems: the genesis of living beings had been brought to light, and--a thing which admitted of no doubt--man as well as the brute creation was a product of purely natural evolution. the doctrine which materialism had already proclaimed with prophetic insight, had at length been irrefragably established on a scientific basis: god, soul and immortality were contemptuously relegated to the domain of nursery tales. what further use was there for a god when, in addition to the kant-laplacian theory of the origin of the planetary system, it had been discovered that living organisms had likewise evolved spontaneously? how could man who had sprung from the irrational brute possess a soul? and thus, finally, disappeared the third delusion, the hope of immortality. for with death the functions of the body simply cease, as also do those of the brain, which people had foolishly believed to be something more than an aggregation of atoms. the body dissolves into its constituent elements and serves in its turn to build up other organisms: but as a human body it all turns to dust nor 'leaves a wrack behind'. thus darwinism was made the basis first for a materialistic, and then for a monistic, view of the world, and hence came to be rigorously opposed to every form of theism. but since, at that time, darwinism was the only theory of evolution recognized by the world of science, the opposition of the christian world was directed not specifically against darwinism, but against the theory of evolution as such. the wheat was rooted up with the tares. i will not discuss here which of the two views concerning creation; the origin of the world in one moment of time, or a gradual evolution of the world and its potentialities, is the more worthy of the creative power of god. manifestly the greatness and magnificence of creation will in no way be compromised by the concept of evolution. this, of course, is simply my opinion. any further statement would be out of place here. but what is the darwinian position? it is merely a special form of the evolutionary theory, one of the various attempts to explain how the process of development actually took place. darwinism as understood in the following chapters possesses the following characteristic traits: ( ) evolution began and continues without the aid or intervention of a creator. ( ) in the production of variations there is no definite law; chance reigns supreme. ( ) there is no indication of purpose or finality to be detected anywhere in the evolutionary process. ( ) the working factor in evolution is egoism, a war of each against his fellows: this is the predominating principle which manifests itself in nature. ( ) in this struggle the strongest, fleetest and most cunning will always prevail, (the darwinian term "fittest" has been the innocent source of a great deal of error). ( ) man, whether you regard his body or his mind, is nothing but a highly developed animal. a careful examination of darwinism shows that these are the necessary presuppositions, or, if you will, the inevitable consequences of that theory. to accept that theory is to repudiate the christian view of the world. the truth of the above propositions is utterly incompatible, not only with any religious views, but with our civil and social principles as well. the most patent facts of man's moral life, however, cannot be explained on any such hypothesis, and the logic of events has already shown that darwinism could never have won general acceptance but for the incautious enthusiasm of youth which intoxicated the minds of the rising generation of naturalists and incapacitated them for the exercise of sober judgment. to show that there is among contemporary men of science a healthy reaction against darwinism is the object of this treatise. the reader may now ask, what, then, is your idea of evolution? it certainly is easier to criticise than to do constructive work. an honest study of nature, however, inevitably leads us to the conclusion that the final solution of the problem is still far distant. many a stone has already been quarried for the future edifice of evolution by unwearied research during the last four decades. but in opposition to darwinism it may, at the present time, be confidently asserted that any future doctrine of evolution will have to be constructed on the following basic principles: ( ) all evolution is characterized by finality; it proceeds according to a definite plan, and tends to a definite end. ( ) chance and disorder find no place in nature; every stage of the evolutionary process is the result of law-controlled factors. ( ) egoism and struggle among living organisms are of very subordinate importance in comparison with co-operation and social action. ( ) the soul of man is an independent substance, and entirely unintelligible as a mere higher stage of development of animal instinct. a theory of evolution, however, resting on these principles cannot dispense with a creator and conserver of the world and of life. chapter i. "it was a happy day that people threw off the straight-jacket of logic and the burdensome fetters of strict method, and mounting the light-caparisoned steed of philosophic science, soared into the empyrean, high above the laborious path of ordinary mortals. one may not take offense if even the most sedate citizen, for the sake of a change, occasionally kicks over the traces, provided only that he returns in due time to his wonted course. and now in the domain of biology, one is led to think that the time has at length arrived for putting an end to mad masquerade pranks and for returning without reserve to serious and sober work, to find satisfaction therein." with these words did the illustrious wigand, twenty-five years ago, conclude the preface to the third volume of his large classical work against darwinism. true, he did not at that time believe that the mad campaign of darwinism had already ended to its own detriment, but he always predicted with the greatest confidence that the struggle would soon terminate in victory for the anti-darwinian camp. when wigand closed his eyes in death in , he was able to bear with him the consciousness that the era of darwinism was approaching its end, and that he had been in the right. today, at the dawn of the new century, nothing is more certain than that darwinism has lost its prestige among men of science. it has seen its day and will soon be reckoned a thing of the past. a few decades hence when people will look back upon the history of the doctrine of descent, they will confess that the years between and were in many respects a time of carnival; and the enthusiasm which at that time took possession of the devotees of natural science will appear to them as the excitement attending some mad revel. a justification of our hope that wigand's warning prediction will finally be fulfilled is to be found in the fact that to-day the younger generation of naturalists is departing more and more from darwinism. it is a fact worthy of special mention that the opposition to darwinism to-day comes chiefly from the ranks of the zoologists, whereas thirty years ago large numbers of zoologists from jena associated themselves with the darwinian school, hoping to find there a full and satisfactory solution for the profoundest enigmas of natural science. the cause of this reaction is not far to seek. there was at the time a whole group of enthusiastic darwinians among the university professors, haeckel leading the van, who clung to that theory so tenaciously and were so zealous in propagating it, that for a while it seemed impossible for a young naturalist to be anything but a darwinian. then the inevitable reaction gradually set in. darwin himself died, the darwinians of the sixties and seventies lost their pristine ardor, and many even went beyond darwin. above all, calm reflection took the place of excited enthusiasm. as a result it has become more and more apparent that the past forty years have brought to light nothing new that is of any value to the cause of darwinism. this significant fact has aroused doubts as to whether after all darwinism can really give a satisfactory explanation of the genesis of organic forms. the rising generation is now discovering what discerning scholars had already recognized and stated a quarter of a century ago. they are also returning to a study of the older opponents of darwinism, especially of wigand. it is only now, many years after his death, that a tribute has been paid to this distinguished savant which unfortunately was grudgingly withheld during his life. one day recently there was laid before his monument in the botanical garden of marburg a laurel-wreath with the inscription: "to the great naturalist, philosopher and man." it came from a young zoologist at vienna who had thoroughly mastered wigand's great anti-darwinian work, an intelligent investigator who had set to work in the spirit of wigand. another talented zoologist, hans driesch, dedicates to the memory of wigand two books in rapid succession and reprehends the contemporaries of that master of science for ignoring him. o. hammann abandons darwinism for an internal principle of development. w. haacke openly disavows darwinism; and even at the convention of naturalists in , l. wilser was allowed to assert without contradiction that, "anyone who has committed himself to darwinism can no longer be ranked as a naturalist." these are all signs which clearly indicate a radical revolution, and they are all the more significant since it is the younger generation, which will soon take the lead, that thinks and speaks in this manner. but it is none the less noteworthy that the younger naturalists are not alone in this movement. many of the older men of science are swelling the current. we shall recall here only the greatest of those whom we might mention in this connection. julius von sachs, the most gifted and brilliant botanist of the last century, who unfortunately is no longer among us, was in the sixties an outspoken darwinian, as is evident especially from his history of botany and from the first edition of his handbook of botany. soon, however, sachs began to incline toward the position assumed by naegeli; and as early as , wigand, in the third volume of his great work, expressed the hope that sachs would withdraw still further from darwinism. as years went by, sachs drifted more and more from his earlier position, and wigand was of opinion that to himself should be ascribed the credit of bringing about the change. during his last years sachs had become bitterly opposed to darwinism, and in his masterly "physiological notes" he took a firm stand on the "internal factors of evolution." during recent years i had the pleasure of occasional correspondence with sachs. on the th of september, , he wrote me: for more than twenty years i have recognized that if we are to build up a strictly scientific theory of organic structural processes, we must separate the doctrine of descent from darwinism. it was with this intention that he worked during the last years of his life and it is to be hoped that his school will continue his researches with this aim in view. the tendency among naturalists to return to wigand is well exemplified in an article contributed to the "preussischen jahrbuecher" for january, , by dr. karl camillo schneider, assistant at the zoological institute of the university of vienna. this article which is entitled the origin of species, pursues wigand's train of thought throughout, and whole sentences and even paragraphs are taken verbatim from his main work. this, at all events, is a very instructive indication of the present tendency which deserves prominence: and its significance becomes more evident when we recall how the work of wigand was received by the non-christian press a quarter of a century ago. it was either ridiculed or ignored. the two methods of treatment were applied to his writings which are always readily employed when the critic has nothing pertinent to say. it is interesting to note that darwin himself employed this method. wigand once told me that he had sent darwin a copy of his work and had addressed a letter to him at the same time merely stating that he had sent the book, making no reference to the line of thought contained in it. darwin answered immediately in the kindest manner that he had not as yet received the book, but when it arrived he would at once make a careful study of its contents. darwin did not write to him again, and when a new edition of his works appeared, the work of wigand, the most comprehensive answer to darwin ever written, was passed over without even a passing mention. thus darwin completely ignored his keenest antagonist. as has been said, the majority of those who wrote about wigand ridiculed him: very few regarded him seriously, and even these indulged chiefly in personal recriminations. thus matters stood twenty-five years ago. wigand's prediction passed unheeded. that a periodical not having a specifically christian circle of readers should now publish a condemnation of darwinism entirely in accordance with the views of wigand, is a fact which indicates a notable change of sentiment during the intervening years. i should not be at all astonished if many who sneered at wigand twenty years ago, now read the article in the preussischen jahrbuecher with entire approval. ill-will towards wigand has not altogether disappeared even to-day. this is evident from the fact that as yet dr. schneider does not venture to defend wigand publicly, nor to acknowledge him as his principal authority. we must be content, however, if only, the truth will finally prevail. chapter ii. striking testimony relative to the present position of darwinism is borne by the strasburg zoologist, dr. goette, who has won fame by his invaluable labors as an historian of evolutionary theory. in the "umschau," no. , , he discusses the "present status of darwinism," and the conclusions he arrives at, are identical with mine. at the outset goette indicates the distinction between darwinism and the doctrine of descent, and then points out that the distinguishing features of the former consist not so much in the three facts of heredity, variation, and over-production, but rather in selection, survival of the fittest, and also in that mystical theory of heredity--the doctrine of pangenesis--which is peculiarly darwinian. since this theory of pangenesis has found no adherents, the question may henceforth be restricted to the doctrine of natural selection. this goette very well observes. he points, moreover, to the fact that the misgivings that were entertained concerning the doctrine of natural selection on its first appearance, were, on the whole, precisely the same as they are to-day; only with this difference, that formerly they were disregarded by naturalists whose clearness of vision was obscured by excessive enthusiasm; whereas, to-day men have again returned to their sober senses and lend their attention more readily to objections. goette recalls the fact that m. wagner tried to supplement natural selection with his "law of migration," and that later on, romanes and gulick endeavored to supply the evident deficiencies in darwin's theory, by invoking other principles; and that even at that time, askenasy, braun, and naegeli--and more recently, the lately deceased eimer--insisted on the fact of definitely ordered variations, in opposition to the theory of selection. many naturalists recognize the difficulties but do not abandon the theory of selection, thinking that some supplementary principle would suffice to make it acceptable: many others refuse to decide either for or against darwinism and maintain towards it an attitude of indifference. the younger investigators, however, are utterly opposed to it. "there can be no doubt that since its first appearance the influence of darwinism on men's minds has notably diminished, although the theory has not been entirely discarded."--but the very fact that the younger naturalists are hostile to it, makes it evident that darwinism has a still darker future in store for it: that sooner or later it will come to possess a merely historical interest. "the present position of darwinism," says goette, "is characterized especially by the uncertainty of criticism which is unable to declare definitely in favor of either side." goette finds the chief cause of this uncertainty in the fact "that men of science (even darwin himself) have widened the concept of selection as a means of originating new species through the interaction of individuals in the same species, so as to express the mutually antagonistic relations existing between several such species." the latter alone is subject to experimental verification, but it can only cause the isolation of existing forms and is not a species-originating selection--with which alone we are here concerned. this kind of selection can enfeeble the existing flora and fauna, but cannot produce a new species. selection productive of new species "is not actually demonstrable; it is a purely theoretical invention." goette next points out that the investigator is everywhere confronted by definitely-directed variation: a fact which does not harmonize with the theory of selection, nor, consequently with darwinism. if some scientists have not as yet accepted eimer's presentation of this doctrine, their action is most probably to be attributed to the fear lest "they should have to accept not merely, variation according to definite laws, but likewise a principle of finality and other causes lying beyond the range of scientific investigation." the rejection of the theory of selection often promotes, as goette rightly observes, a reactionary tendency towards _a priori_ explanations of phenomena with which we are but slightly acquainted. "there are naturalists who do not discard the theory of selection simply because it seems to furnish a much-desired mechanical explanation of purposive adaptions" (a momentous admission to which we shall have occasion to revert). others have broken entirely with selection and the principle of utility and extend the idea of finality to the general capacity of organisms to persist. thus adaptation becomes a principle which transcends the limits of natural science and pervades the whole domain of life. goette observes that darwin spoke of useful, less useful and indifferent organisms, by which he meant those adaptations destined for particular vital functions which tend to make the organs more and more specialized. since the ability to live is threatened by this specialization it cannot be purposive. this is not wholly true, because the more specialized the individual organ becomes, the more perfect is the whole organism which is composed of these specialized organs. the functions of the individual organ may be restricted, but the power of the entire organism is notably increased, according to the law of the division of labor. goette therefore has not sufficient grounds for rejecting this expression. he considers that a real and permanent purpose for the individual living forms is out of the question, but that this purpose may be sought for in the development and history of the collective life of nature. definitely ordered variation, he thinks, a scientific explanation of which is indeed yet forthcoming, will explain adaptation equally as well as does selection. after what has been said this statement of goette must come as a surprise, for one would think that according to his view definite variation explains adaptations better than selection. goette sums up his main conclusion in the following words: "the doctrine of heredity or of descent, which comes from lamarck though it was first made widely known by darwin, has since continually gained a broader and surer foundation. but darwin's own doctrine regarding the causes and process of descent which alone can be called darwinism, has on the other hand doubtlessly waned in influence and prestige." this is exactly what we also maintain: the establishment of the theory of descent in general, and the continual retrogression of darwinism in particular. wigand was entirely right when he said that darwinism would not live beyond the century. we may, however, derive from the discussions of goette something else that is of the highest importance, namely, an admission in which is to be found the real and fundamental explanation of the conduct of the majority of naturalists who still cling to darwinism. it does not consist in the fact that they are convinced of the truth of darwinism but in their "reluctance to give up the mechanical explanation of finality proposed by darwin," or rather in the fear of being driven to the recognition of theistic principles. with commendable candor goette attacks this method of keeping up a system notwithstanding its recognized deficiencies. goette furthermore points out especially that this recognition is more widespread than one might be able to gather from occasional discussions on the subject. from the account which goette gives of the present status of darwinism we may safely conclude that darwinism had entered upon a period of decay; it is in the third stage of a development through which many a scientific doctrine has already passed. the four stages of this development are the following: . the incipient stage: a new doctrine arises, the older representatives of the science oppose it partly because of keener insight and greater experience, partly also from indolence, not wishing to allow themselves to be drawn out of their accustomed equilibrium; among the younger generation there arises a growing sentiment in favor of the new doctrine. . the stage of growth: the new doctrine continually gains greater favor among the young generation, finding vent in bursts of enthusiasm; some of the cautious seniors have passed away, others are carried along by the stream of youthful enthusiasm in spite of better knowledge, and the voices of the thoughtful are no longer heard in the general uproar, exultingly proclaiming that to live is bliss. . the period of decay: the joyous enthusiasm has vanished; depression succeeds intoxication. now that the young men have themselves grown older and become more sober, many things appear in a different light. the doubts already expressed by the old and prudent during the stage of growth are now better appreciated and gradually increase in weight. many become indifferent, the present younger generation becomes perplexed and discards the theory entirely. . the final stage: the last adherents of the "new doctrine" are dead or at least old and have ceased to be influential, they sit upon the ruins of a grandeur that even now belongs to the "good old time." the influential and directing spirits have abandoned this doctrine, once so important and seemingly invincible, for the consideration of living issues and the younger generation regards it as an interesting episode in the history of science. with reference to darwinism we are in the third stage which is characterized especially by the indifference of the present middle-aged generation and by growing opposition on the part of the younger coming generation. this very characteristic feature is brought into prominence by the discussion of goette. if all signs, however, are not deceptive, this third stage, that of decay, is drawing to an end; soon we shall enter the final stage and with that the tragic-comedy of darwinism will be brought to a close. if some one were to ask me how according to the count of years, i should determine the extent of the individual stages of darwinism, this would be my answer: . the incipient stage extends from (the year during which darwin's principal work, _the origin of species_, appeared) to the end of the sixties. . the stage of growth: from that time, for about years, to the end of the eighties. . the stage of decay: from that time on to about the year . . the final stage: the first decade of the new century. i am not by choice a prophet, least of all regarding the weather. but i think it may not be doubted that the fine weather, at least, has passed for darwinism. so having carefully scanned the firmament of science for signs of the weather, i shall for once make a forecast for darwinism, namely: increasing cloudiness with heavy precipitations, indications of a violent storm, which threatens to cause the props of the structure to totter, and to sweep it from the scene. chapter iii. as further witnesses to the passing of darwinism, two botanists may be cited; the first is professor korschinsky who in no. , , of the _naturwissenschaftliche wochenschrift_ published an article on "heterogenesis and evolution," which was to be followed later by a large work on this subject. with precision and emphasis he points to the numerous instances in which there occurs on or in a plant, suddenly and without intervention, a variation which may become hereditary under certain circumstances; thus during the last century a number of varieties of garden plants have been evolved. on the basis of such experiments korschinsky developed the theory which had been proposed by koelliker in wuerzburg thirty years earlier, namely, the theory of "heterogeneous production" or "heterogenesis," as korschinsky calls it. when one understands that a plant gives rise suddenly and without any intervention to a grain of seed, which produces a different plant, it becomes evident that all darwinistic speculations about selection and struggle for existence are forthwith absolutely excluded. the effect can proceed only from the internal vital powers inherent in the specified organism acting in connection, perhaps, with the internal conditions of life, which suddenly exert an influence in a new direction. korschinsky distinguishes clearly and definitely between the principles of heterogenesis and transmutation (gradual transformation through natural selection in the struggle for existence), and in so doing comes to a complete denial of darwinism. the other naturalist who has dealt darwinism a telling blow is the botanist of graz, professor haberlandt. he published some very interesting observations and experiments in the "festschrift fuer schwendener" (berlin , borntraeger). they are concerned with a liane javas of the family of mulberry plants (conocephalus ovatus.) the free leaves possess under the outer layer, a tissue composed of large, thin-walled, water-storing cells; flat cavities on the upper side, having, furthermore, organs that secrete water, which the botanist calls hydathodes. these are delicate, small, glandular cells over which are the bundles of vascular fibres (leaf-veins) that convey the water to them; over these in the top layer are so-called water-crevices through which the water can force itself to the outside. it is unnecessary to enter upon a closer explanation of the anatomical structure of these peculiar organs. the water which is forced upward by the root-pressure of the plant is naturally conveyed through the vascular fibres into the leaves and at every hydathode the superfluous water oozes out in drops, a phenomenon which one can also very nicely observe e.g. on the "lady's cloak" (alchemilla vulgaris) of the german flora. a portion of the night-dew must be attributed to this secretion of water. on the liane, then, haberlandt observed a very considerable secretion of water: a full-grown leaf secreted during one night . g. of water (that is per cent. of its own weight.) through this peculiarity the water supply within the plant is regulated and the danger avoided that any water should penetrate the surrounding tissue in consequence of strong root-pressure,--which would naturally obstruct the vital function of the entire leaf. besides it is to be noticed that in this way an abundant flow of water is produced: the plant takes up large quantities of water from the earth, laden with nutritive salts, and the distilled water is almost pure (it contains only . g. salts), so that the nutritive salts are absorbed by the plant. from these considerations it necessarily appears that the hydathodes are of great biological importance to the plant. haberlandt then "poisoned" the plant, by sprinkling it with a . per cent sublimate solution of alcohol. the purpose of this experiment was to ascertain whether in the secretion of water there was question of a merely physical process or of a vital process. in the first case the action of the hydathode should continue even after the treatment with the sublimate solution, while in the latter case it should not. as the secretion ceased the obvious conclusion to be deduced from this experiment is that the hydathodes do not act as purely mechanical filtration-apparatuses, as one might have thought, but that there is here evidence of an active vital process in the plant; the unusual term "poisoning" is therefore really justified under present circumstances. let me dwell for a moment on this result, for, although it may be somewhat foreign to our present purpose and to the further observations of haberlandt, it is very significant in itself. the water moves in the plant in closed cells, as the cells of the aqueous gland are entirely closed, but the organic membrane, as every one knows, has the peculiar physical property of allowing water to pass through, the pressure, of course, being applied on the side of least resistance; when therefore the water is forced into the cells by root-pressure, it is easily intelligible that according to purely physical laws it should come to the surface of the leaf on the side of the least resistance, that is, by way of the water-crevices. even the defenders of "vital force" would not find any reason in this for not considering the phenomenon of distillation in this case a purely physical phenomenon. and still according to haberlandt's experiments it is not. the sublimate could at most only impede the process of filtration, but should under no circumstances have destroyed it. but it does destroy it, and the hydathode dies. the conclusion certainly follows from this that this process is connected with some vital function. even if the hydathode is treated with sublimate solution, all the conditions for mechanical filtration still remain: the earth has moisture which can be taken up by the roots so that root-pressure still exists. the water is in all cases conveyed to the hydathodes through the vascular fibres, the cell walls of the hydathodes are still adapted for filtration, and yet they do not filter. hence some other factor must join itself to the physico-mechanical process of filtration and affect or destroy it, and this factor can be found only in the protoplasm, the vital element of the cells; for we know that the sublimate acts with pernicious effect on it and in such a manner that it destroys its entire power of reaction; it kills it, as we say. the experiment under discussion has, therefore, great significance for our view of the vital processes in the plant; it proves beyond doubt that these processes are in no way of a purely mechanical nature, but that there is something underlying all this, a hitherto inexplicable something, which we call "life." in all vital activities, physical and chemical processes certainly do occur; they do not, however, take place spontaneously but are made use of by the vital element of the plant to produce an effect that is desirable or necessary for the vital activity of the plant. if the vital element is dead, no matter how favorable the conditions may be for chemical and physical processes, these do not take place and the effect necessary for life is not obtained. it is very remarkable after all that according to the experiment of haberlandt this peculiar relation should become apparent in a process that is so open to our investigation as the filtration of water through the cell-wall of a plant. after what has been said i consider this simple experiment of haberlandt of great significance; for it is a direct proof of the existence of a vital force. one may resist to his heart's content, but without avail; vital force is again finding its way into science. more and more cognizance is being taken of the fact that and years ago people jumped at conclusions very imprudently when they believed that the first artificial preparation of organic matter (urea, by woehler) had proven the non-existence of a vital force. since then there has been great rejoicing in the camp of materialists who scoffed at the "ignorant" who would not as yet forsake vital force. "behold," they said, "in the chemist's retort the same matter is produced chemically that is produced in the body of the animal, without the direction of a hidden vital force, which, if it is not necessary in the one case, neither is it necessary in the other." any one who had given the matter careful consideration could even at that time have known where the "ignorant" really were. that in both cases chemical processes take place is clear and undisputed, but the materialists forgot entirely that even in the laboratory it was not the mere contact of the elements that produced the urea; a chemist was needed and in this case not any one arbitrarily chosen, but a man of the genius and knowledge of a woehler to watch over the process, and utilize and partly direct the laws of chemistry in order to obtain the desired result. hence it was even then absurd to deny vital force as a consequence of that experiment. since, however, it was well-adapted for materialistic purposes, this denial was proclaimed with the sound of trumpet throughout the land, and repeated again and again with surprising tenacity, with the result that even thoughtful investigators rejected vital force almost universally in the seventies and eighties. it has always been a problem to me how this could have happened. it can, indeed, be explained only on the supposition that naturalists were adverse to the introduction of anything into nature, that appeared to them mystical and mysterious. nor is such a procedure at all necessary: vital force is by no means a mysterious, ghostly power that soars above nature, but a force of nature like its other forces, as mysterious and as definite as they are, only that it dominates a specified group of beings, namely, living organisms. it may readily be compared with any other natural phenomenon. for instance, the phenomenon of crystallization has its well determined sphere of activity, viz., the mineral world. it employs definite mathematico-physical laws to obtain a specified result, and even acts differently in different mineral substances in so far as it produces in the one case this, in the other case that form; but still it should be a similarly directed force which has the effect of producing these peculiar forms. precisely similar is it with vital force. it has its determined sphere of activity, the kingdom of living organisms; it acts according to definite physico-chemical laws in producing a specified result; it acts differently in different living organisms; it is therefore a force of nature as clear yet as mysterious as the force of crystallization or as any other force of nature. hence one has no cause to complain of its mysteriousness, for all other forces of nature are just as much, or if you will, just as little mysterious as vital force. the only thing to be maintained is this, that living organisms are dominated by a special force with special phenomena and special activities, even as in mineral substances there is a special dominant force which produces special phenomena and exercises special activities. it is possible to produce crystals in the laboratory, but no one will be so foolish as to maintain that in nature crystals are not formed in consequence of a very definite force inherent in the mineral-substances; nor will any one deny the existence of the force of crystallization because it does not appear in living organisms. nor have i ever despaired of a return of the theory of vital force. a change of opinion has really taken place during this decade; at present the voices for a vital force are constantly growing stronger and it will most probably not be very long before it will be again universally recognized, not as something preternatural, of course, but as a force of nature on an equal footing with the other forces of nature, with activities, just as mysterious and just as well-attested as the activities of the other forces of nature. haberlandt's experiment, however, had also an indirect consequence that is of far-reaching importance. he observed that within a few days new water-secreting organs of an entirely different structure and of different origin were formed on the leaves that had been sprinkled with sublimate. over the bundles of vascular fibres, little knots as large as a pin head arose in larger numbers out of a tissue underlying the top layer; out of these the water now oozed every morning. closer investigation disclosed the fact that these organs develop only on young immature leaves where groups of peculiar, perishable gland-hairs are found; beneath these dead mucous glands the substitute secretive organs originate in the inner tissue. it is of no importance to state in what particular cells they originate. suffice it to say that they are colorless capillary tubes originating in various cells; projecting like the hairs of a brush, containing living protoplasm and evanescent chlorophyll. it is also important to note that this new organ is immediately connected with the water-conducting system consisting of bundles of vascular fibres. haberlandt furthermore indicates especially that these organs when viewed in connection with the process of secretion give evidence of an active vital principle as well as of simple mechanical filtration. these substitute organs are all indeed well adapted to their purpose and adequately replace the old secretive organs, but they so easily dry out and are so little protected that after a week they become parched and die because wound-cork forms under them. the leaf no longer produces new hydathodes, but on its lower side it produces growths that function as vesicles, by means of which it continues to sustain itself. haberlandt furthermore records a phenomenon perhaps analogous to this on the grape-vine, but with this exception the case described by him is unique. in order to pass any further judgment regarding it, we should have to ascertain whether the whole phenomenon is not a case of so-called adaptation; if so, processes should be found in nature, analogous to the poisoning of the hydathodes in this experiment, which result in the destruction of the hydathodes so that in consequence the plant would have gained the power of making good the loss, by means of the substitute organs. such processes, however, (even through poisoning or through parasites) would be very highly improbable. equally incredible is the alternative possibility that the new organs would be produced by the plant not as a substitute but as a supplementary apparatus when the old ones would not suffice for secretion in case of very large absorption of water. this also must doubtlessly be rejected, as haberlandt has observed. powers of adaptation should, of course, according to darwinism, be gradually acquired in the struggle for existence, as in that case they should also have stability; but since this is not possessed by the new organs, the presumption is that they do not possess the character of adaptation. they are therefore new organs that originated after an entirely unnatural and unforeseen interference with the normal vital functions and in consequence of a self-regulating activity of the organism. what then is there in the whole phenomenon worthy of notice with regard to the theory of descent? . an immediately well adapted new organ has here originated very suddenly without any previous incipient formation, without gradual perfection and without stages of transition. . in its formation struggle for existence and natural selection are entirely excluded, neither can find any application whatever even according to the newer exposition of weismann. haberlandt himself draws this conclusion. . if this phenomenon of a suddenly appearing change can take place in the course of the development of the individual, there can be no obvious reason why it should not take place in the same manner (without natural selection or struggle for existence) in the course of the phylogenetic development. it is manifestly of the greatest importance that in this case a direct, experimental proof has been given that an organ has originated suddenly and without the aid of darwinian principles. haberlandt's article is nothing less than a complete renunciation of darwinism on the part of haberlandt, a renunciation which we greet with great satisfaction. in fact one such observation would really suffice to set aside darwinism and prove the utter insufficiency of its principles to give explanation of the origin of natural species. on the other hand, this observation plainly proves two things: first, that the above mentioned doctrine of koelliker, now held by korschinsky is a move in the right direction for the discovery of the causes of descent; and secondly, that the principal cause of the evolution is not to be sought in environment and blind forces but in the systematically working, internal vital principle in plants and animals. with that, however, an important part of the foundation of the mechanical-materialistic view of the world is demolished. chapter iv. since we have heard the verdict of zoologists and botanists concerning darwinism, it is but right that we should now listen to a palaeontologist, a representative of the science, which investigates the petrified records of the earth's surface, and strives to collect information regarding the world of life during remote, by-gone ages of the earth. it is evident to every-one that the verdict of this science must be of very special importance in passing on the question of the development of living organisms. darwin himself recognized this at the outset. he and his followers, however, soon perceived that, while the revelations of palaeontology were on the whole favorable to the doctrine of descent, in so far as they proved the gradual change of organization, in consecutive strata, from the simple to more complex forms, palaeontology revealed nothing that would sustain the darwinian theory as to the method of that development. as soon as the darwinians, and first of all darwin himself, perceived this, they at once brought forward a very cheap subterfuge. since darwinism postulates a very gradual, uninterrupted development of living organisms, there must have been an immense number of transition-forms between any two animal or plant species which to-day, although otherwise related, are separated by characteristic features. consequently, on the darwinian hypothesis, all of these transition-forms must have perished for the singular reason that other better organized forms overcame them in the struggle for existence. if therefore the millions of transition-forms were still missing, and the known petrified forms of older strata of the earth did not reveal them, the darwinians were able to console themselves until from to years ago, with the assertion that our knowledge was still too deficient, that a more thorough investigation of the earth's surface and especially of out-of-the-way parts would eventually bring to light the supposed transition forms. such assertion affords very poor consolation, and is anything but scientific. the method of natural science consists in establishing general principles on the basis of the materials actually furnished by experiments and observation and not in excogitating general laws and then consoling oneself with the thought that while our knowledge of nature is as yet extremely imperfect, time will furnish the actual material necessary to substantiate our guesses. but since then many a year has come and gone and darwinism has caused, and for that alone it deserves credit, a diligent research in every field of natural science, and has promoted among palaeontologists a search for the missing transition-forms. the materials of investigation from the field of palaeontology have also wonderfully increased during these decades. hence it is worth while now at the dawn of the new century to examine this material with a view to its availableness for the theory of descent and especially for darwinism. professor steinmann has recently done so in freiburg in breisgau, on the occasion of an address as rector of the university. what conclusions did he reach? steinmann declares it to be the primary task of post-darwinian palaeontology "to arrange the fossil animal and plant-remains in the order of descent and thus to build up a truly natural, because historically demonstrable, classification of the animal and plant-world." at the outset it is to be noted that for various reasons palaeontology is unable to execute this momentous task in its full extent. the evidence of palaeontology is deficient, if for no other reason than that many animal organisms could not be preserved at all on account of their soft bodies; many animal groups have, nevertheless, received an unusual increase (mollusks, radiata, fish, saurians, vertebrates, and dendroid plants). as regards the attempt made in the sixties to draw up lines of descent, steinmann repudiates, without, of course, mentioning names, the family tree constructed by haeckel and his associates as wholly hypothetical and hence unjustified; he rightly remarks that their method smacks of the closet. he finds fault with them chiefly because they predicated actuality of this imaginary family-tree and fancied that the historical research of the future would have but isolated facts to establish. in speaking of the palaeontological research of the last few decades, steinmann says: "in the light of recent research, fossil discoveries have frequently appeared less intelligible and more ambiguous than before, and in those cases in which an attempt has been made to bring the descent-system into agreement with the actual facts, the incongruity between the two has become obvious." thus, for instance, the well-known archaeopteryx is not, as was maintained, a connecting link between reptile and bird, but a member of a blindly ending side branch. in fact palaeontological research has proven incapable of finding the transitions between different species, clearly determined by the theory. but the overwhelming abundance of matter called for new endeavors to master it. it was then further discovered--steinmann finds an illustration of this fact in the echinodermata--that the well-known "fundamental law of biogenesis" of haeckel can be accepted only in a very restricted sense and may even lead to conclusions absolutely false. we desire to remark here that a "fundamental principle" should never mislead; if it does so, it is not a fundamental principle. it is of importance to know that according to palaeontological investigation, empiric systematizing and phylogenetic classification do not always coincide, as, for instance, in the case of the ammonites. acording to palaeontological investigation the great systematic categories are only grades of organization. hence present day systematizing is being more and more discarded, and the said categories--as indeed also the lesser groups of forms--must be of polyphyletic origin, that is, they must have descended from different primitive stocks. it may be asked: what bearing has this principle of multiple origins? for a long time reptiles were the predominating vertebrates; when mammals and birds appeared, numerous, varied and strange saurians inhabited land and sea; but "with the end of the chalk-period most saurians seem to have vanished suddenly from the scene, and soon we behold the mainlands and oceans inhabited by mammals of most diverse kinds." the saurians have become almost extinct and the mammal-tribe suddenly shows a most extraordinary variability and power of development. how is either phenomenon to be explained? "the disappearance of a group of organisms has been preferably explained since the time of darwin, by defeat in the struggle with superior competitors. if ever an explanation lacked pertinency, it does so in this case, in which the succumbing group is represented by gigantic and well preserved animal forms, widely distributed and accustomed to the most varied methods of nutrition, whereas the competitor appears in the form of small, harmless marsupials. it would be equivalent to a struggle between the elephant and the mouse." we acknowledge with pleasure this clear rejection of darwinism on the part of steinmann. steinmann also rejects the natural extinction of those forms, perhaps from the weakness of old age; whether he is wholly warranted in doing so, seems somewhat doubtful. he tries to explain the phenomenon on the basis of the multiple origin of the mammals; and in fact there is already speculation regarding triple origin, viz: tambreets, marsupials, and the other mammals. now if the latter also possessed a multiple origin, the problem of the extinction of the saurians would, according to steinmann solve itself. one would not need to consider the number of extinct forms as large as is now done. however, he does not enter upon any closer consideration of this question. but he points out, for instance, that to-day the shells of mollusks (snails and conchylia) are regarded as structures that were acquired only in the course of time for the sake of protection, the disappearance of which, therefore, implied a disadvantage for the respective organisms. this transition would be something extraordinary--"but if on the contrary, one regards the shells as the necessary products of a special kind of assimilation and of the immoveableness of certain parts of the body, the gradual disappearance might well be considered a process which may take place in various animal-groups with a certain regularity in the course of the phyletic development." the snails devoid of shells, for instance, may be derived with certainty from those possessed of shells; this process has very probably also taken place in different genetic lines. this view is well worth consideration; it stands in sharp opposition, in fundamental principles, to the darwinian explanation. this calls for special emphasis here. how should one explain the origin of uncrusted mollusks from crusted ones through the struggle for existence, since in such a contest the latter must have had far greater prospect of survival than the former? this view together with the principle of multiple origin opens up, according to steinmann, "the prospect of an altered conception of the process of formation of the organic world." according to the new conception, the many extinct forms of antiquity are not, as darwin supposed, "unsuccessful attempts and continued aberrations of nature"--how this reminds one of that old, naive, much-ridiculed idea that fossils were models that god had discarded as unserviceable--but would gain new life and assume hitherto unsuspected relationship to the present organic creation. "science, which seeks after operative causes, at the beginning of the century regarded creation as a multiplicity of phenomena without any causal connection as to their origin. darwin taught as a fundamental principle the unity and the causal inter-relation of creation, but was not entirely able to save this hypothesis from a violent and sudden death. in the future sketch creation will appear as wholly restricted in itself and lasting, the causes of its limitation lie, up to the time of the intervention of men, solely in the balanced motion of the planet which it peoples." at the close of his address steinmann points out that behind the problem of the manner of development, there stands "the unsolved question regarding its operative causes." "regarding this point," he continues, "opinions have perhaps never been so divergent as they are to-day. the times have passed when the darwinian explanations were regarded with naive confidence as the alpha and omega of the doctrine of descent. not only are the adherents of darwinian ideas divided among themselves, but the theory of lamarck, somewhat altered, favored by the results of historical investigation, appears more striking and now seems more in harmony with facts than formerly. what is considered by one as the ruling factor in the evolution of organisms is regarded by another as a "quantite negligeable" or even as the greatest mistake of the century. in this discord of opinions the principle of descent alone forms the stable pole." thus steinmann, and we can but applaud his conclusions with undisguised pleasure, for they tend throughout in the direction of our anti-darwinian view, and deal darwinism another fatal blow. it is also worthy of special note that this time the blow is dealt from the side of palaeontology; for, even if now and again we dissent from steinmann, in this we fully agree with him that the historical method of considering the evidences of bygone periods of creation is at the very least quite as important for passing correct judgment regarding descent, as is the investigation of contemporary living organisms. indeed, family-trees were constructed without regard for palaeontology, almost exclusively from an examination of present conditions, and sometimes the author did not even shrink from falsification. this procedure has been bitterly revenged and will take further revenge unless at length a definite end be put to the family-tree nuisance and the respective books instead of being published anew, be relegated to the lumber-room of science, there to turn yellow amid dust and cobwebs--the curious evidence of gross folly. but only have patience, even that time will come. the conclusions of steinmann, that are most important for us, may be summarized as follows: . the family and transition forms demanded from palaeontology by darwinism for its family-trees, constructed not empirically but _a priori_, are nowhere to be found among the abundant materials which palaeontological investigation has already produced. . the results of the investigation do not correspond with the family groups drawn up according to the so-called "biogenetic principle," which principle has in fact led men of science into false paths. . at best, the biogenetic principle has a limited validity, (we add that later it will undoubtedly follow darwinism and its family trees into the lumber-room). . the results of palaeontology, in so far, for instance, as they testify to the sudden disappearance of the saurians and the advent of mammals, everywhere contradict the darwinian principle of the survival of the fittest in the struggle for existence. . "the time has long passed when the darwinian explanations were regarded with naive confidence as the alpha and omega of the doctrine of descent." . only the principle of descent is universally recognized; the "how" of it, its causes, are to-day entirely a matter of dispute. chapter v. the strongest evidence of the decay of darwinism is to be found in the fact that, since darwin first enunciated his theory, many and diverse attempts have been made to explain the origin of species on other principles. names of men, like m. wagner, naegeli, wigand, koelliker, and kerner mark these attempts; but of these investigators naegeli alone proposed a well-developed hypothesis. finally, however, eimer, professor of zoology in tuebingen came forward with a detailed theory of descent. as early as he published a comprehensive work dealing with it, under the title: "the origin of species by means of the transmission of acquired characters according to the laws of organic growth." as the title itself indicates, a very marked divergence was even at that time manifesting itself between eimer and his former teacher and friend, the great defender of darwinism in germany, aug. weismann, professor of zoology in freiburg in breisgau. for, while the latter vigorously attacks the transmission of acquired characters, eimer's whole theory is founded on this very transmission. observations regarding the coloring of animals, in fact, form the basis of eimer's theory. eimer attributes the origin of species to "organic growth" by which he means not merely increase in size, but also change of form, etc. this growth does not proceed blindly or aimlessly, but proceeds on rigidly determined lines, which depend upon the structure and constitution of the particular organism. external influences, however, also affect it. eimer specially emphasizes four points in this connection: . this rigidly determined development of a character exhibits well defined, regular stages, and the evolution of each individual repeats the whole series of transformations (the mueller-haeckel "biogenetic-law.") . new characters are first acquired by strong adult males (the law of male dominance). . new characters appear on definite parts of the body, spreading especially from the rear to the front, (the law of undulation). . varieties are stages in the process of development, through which all the individuals of the respective species must pass. these points indicate how important for eimer is the transmission of those characters which the parents themselves have acquired in the course of their own development. he conceives that this transmission takes place when the causative influences exert themselves permanently on many succeeding generations. eimer thinks that in this way the constitution of the respective species is gradually transformed. besides the effect of external influences (which may vary according to the climate, etc.: geoffroy st. hilaire), eimer mentions as important and active factors in this development, ( ). the use and disuse of organs (lamarck); ( ). the struggle for existence (darwin); ( ). the correlation of organs, that is, the inner relation of organs in consequence of which a change in one organ may occasion a sudden change in another organ; ( ). cross fertilization and hybridism. it is clear that with reference to the factors of evolution eimer is, and perhaps not unreasonably, an eclectic, whose aim is to do justice to the predecessors of darwin as well as to darwin himself. his antagonism to darwin and weismann in this work is still quite moderate, although even here it appears with sufficient clearness that selection and the struggle for existence, the two principles peculiarly characteristic of darwinism, do not give rise to new species, but can at best only separate and differentiate species already existing. the second part of eimer's work dealing with the origin of species, which appeared after an interval of ten years, bears the title: "orthogenesis of butterflies." the origin of species, ii. part ( tables and illustrations in the text). leipzig, . in this book substantially the same thoughts occupy the mind of the author as in the former volume, but in many respects they are more mature, and conspicuously more definite and precise. the most salient features are the following: . eimer establishes his theory by means of very minute observations on a definite species of animals, viz., butterflies. . he attributes evolution almost exclusively to development along definitely determined lines. . he proves the utter untenableness of darwinian principles and repudiates them unqualifiedly. . in a very distinct and severe manner he gives expression to his opposition to his former friend weismann. . he attacks with telling effect the fantastic darwinian "mimicry." in his "general introduction" eimer first treats of orthogenesis in opposition to the darwinian theory of selection. the very first sentence gives evidence of this antagonism: "according to my investigation, organic growth (organophysis), which is rendered dependent on the plasm by permanent external influences, climate and nourishment, and the expression of which is found in development along definitely determined lines, (orthogenesis), is the principal cause of transformation, its occasional interruption and its temporary cessation and is likewise the principal cause of the division of the series of organisms into species." lamarck's theory of the use and disuse of organs and darwin's hypothesis of natural selection are consequently pushed into the background. here also eimer at once places himself at variance with naegeli who had enunciated a similar theory. naegeli took as a starting point an inherent tendency in every being to perfect itself, thus presupposing an "inner principle of development," and making light of external influences as transforming causes. eimer flatly contradicts this view. we shall revert to this point in our criticism of his theory. in opposition to the theory of selection, eimer lays special stress on the fact that its underlying assumption, viz., fortuitous, indefinite variation in many different directions, is entirely devoid of foundation in fact, and that selection, in order to be effective, postulates the previous existence of the required useful characters, whereas the very point at issue is to explain how these characters have originated. since, therefore, according to eimer's investigations, there are everywhere to be found only a few, definitely determined lines of variation, selection is incapable of exercising any choice. the development, furthermore, proceeds without regard for utility, since, for instance, the features that characterize a species of plants are out of all reference to utility. "even if nothing exists that is essentially detrimental, nevertheless very much does exist that bears no reference whatever to immediate good, and was therefore never affected by selection." further on, eimer expresses still more clearly the opposition of his theory to that of darwin, and in so doing he attacks vigorously the omnipotence of selection, so unreasonably proclaimed by the followers of darwin. eimer's theory, consequently, asserts that: "the essential cause of transmutation is organic growth, a definite variation, which, during long periods of time proceeds unswervingly and without reference to utility, in but few directions and is conditioned by the action of external influences, of climate and nourishment." in consequence of an interruption of orthogenesis a stoppage ensues in certain stages of the development, and this stoppage is the great cause of the arrangement of forms in different species. of vital importance also "is development through different stages (hetero-epistase), which results in the arrested development of certain characters in an organism, while others progress and still others become retrogressive. as a rule use and disuse are of great efficacy in this regard, and conjointly with these compensation and correlation." occasionally also irregular development sets in, which proceeds by leaps. of course, eimer could not but in his turn burn incense before darwin by declaring that he would not dare to cross swords with such a man, while in reality he repudiates all of darwin's fundamental tenets. it may be well to state here in addition a few important supplementary considerations: "development can everywhere proceed in only a limited number of directions because the constitution, the material composition of the body, conditions these directions and prevents variation in all directions." this is an important statement because eimer clearly expresses therein the difference between his own theory and that of naegeli. he makes the direction of development dependent on the material composition of the body, whereas naegeli considers it dependent upon an internal tendency of every being to perfect itself, hence upon a power inherent in the body. eimer's view therefore tends towards a mechanical explanation, while naegeli postulates a vital energy. the "internal causes" according to eimer find their explanation in the material composition of the body. since the growth of the individual organism depends on this composition and on the external influences, eimer compares family-development with it and designates the latter as "organic growth." in opposition to naegeli he maintains that this "organic growth" does not always aim at perfection but often tends to simplification and retrogression. the following, then, according to eimer, are the directive principles of variation: ( ). the general law of coloration (stripes running lengthwise change into spots, stripes running crosswise change to a uniform color). ( ). the law of definitely directed local change (new colors spread from the rear to the front and from above downward or vice versa, old colors disappear in the same directions.) ( ). the law of male predominance (males are as a rule one step in advance of the females in development). female predominance is an exception. ( ). the law of age-predominance (new characters appear at a well-advanced age, and at the time of greatest strength). ( ). the law of wave-like development (during the course of the formation of the individual organism a series of changes proceed in a definite direction over the body of the animals). ( ). the law of independent uniformity of development (the same course of development is pursued in non-related forms and results in similar forms). ( ). the law of development through different stages (different characteristics of the same being may develop to a different degree and in different directions). ( ). the law of unilateral development (the progeny does not present a complete combination of the characters of the parents but manifests a preponderance of the characteristics of either parent). ( ). the law of the reversal of development (the direction of development may reverse and tend towards the starting point). ( ). the law of the cessation of development (a protracted cessation of development frequently ensues in one or the other stage). the origin (perhaps rather the distinction) of species is accounted for principally by the last named law, by means of which eimer also explains the so-called atavism or reversion. to this law are joined other factors, e.g., development proceeding in leaps, as demonstrated by koelliker and heer; local separation (through migration; prevention of fertilization, e.g., the impossibility of cross-fertilization between certain individual organisms) which romanes had already opposed to natural selection, and crossing. the second main division of the book is taken up with a very searching and detailed criticism of weismann. this criticism seems to me entirely warranted; because not only the latter's unintelligible position with regard to natural selection (the repudiation of which he seems to regard as synonymous "with cessation of all investigation into the causal nexus of phenomena in the domain of life") but likewise his fanciful theory of heredity, utterly devoid as it is of any support from actual observation, bespeak an utter lack of qualities essential to a naturalist; and the manner in which he ignores his former pupil and his labors, because they proved embarrassing to him, is entirely unworthy of a man of science. eimer devotes special attention to "mimicry"; and indeed he was forced to be very solicitous to dispel this fanciful conception of darwinism which radically contradicted his own views. moreover, the untenableness of the mimicry hypothesis must have revealed itself very clearly to him in the course of his investigations regarding the coloring of butterflies. mimicry, as our readers are well aware, consists in this, that living beings imitate other organisms or even inanimate objects; darwinism maintains that this is done for the sake of protection against enemies. this phenomenon is said to have been produced by selection. those animals that possessed, for instance, some similarity to a leaf, in consequence escaped their enemies more easily than others and survived, while those that had no leaf-like appearance succumbed; when this process had been repeated a few times, many animals (butterflies) gradually developed that marvelous leaf-like appearance, which frequently deceives the most practiced eye. it appears so simple and natural that one need not wonder that this peculiar phenomenon gained many an adherent for darwinism. but, of course, it is directly opposed to the views of eimer; and it is for this reason that he endeavors so assiduously to disprove the error of darwinism in this regard. as the underlying color design of the butterfly eimer designates eleven longitudinal designs; and the examination of the leaf-like forms leads him to the conclusion, that their appearance always depends on "the unaltered condition or the greater prominence of certain parts of this fundamental design." there is to be observed a shifting of the third band, so that in conjunction with the fourth, which is curved, it forms the mid-rib of the leaf. eimer finds the cause of this phenomenon in the alteration of the form. the leaf-like form results from an acumination and elongation of the wings, which in turn results from a marked elongation of the rim of the fore-wing. and this again is produced by the proportionately greater growth of one part of the wing-section than of the others. with reference to the reason of this growth it is of importance to note that experiments, consisting in the application of artificial heat to the chrysales of the swallow-tail and sailor-butterfly, demonstrated that by this means "the fore-wing is drawn out more toward the outer wing-vein, and the rim of the fore-wing becomes more elongated and curved." it is observed, however, that the natural heat-forms of the same genera and species, namely, the summer-forms and those which live in the warm southern climate, exhibit, for instance, in the case of butterflies akin to the sailor, the same features, the elongation and more marked curvature of the fore-rim of the fore-wings and the consequent more extended form, that are produced by the action of artificial heat. manifestly this is a matter of vital importance for the solution of the question: heat, whether artificial or natural, produces a difference in growth, which results in a change of form and coloring. there is consequently no room for natural selection or the struggle for existence. the leaf-like form is generally associated with the dark, faded colors of dry leaves, and when this similarity disappears even bright colors appear on the fore-wings. in many cases the resemblance to leaves is very imperfect; different forms of the same species live side by side and among them are to be found those, the resemblance of which to leaves is extremely slight. all these facts, and especially the frequently recurring retrogression of the leaf-like appearance, justify serious doubt regarding the darwinian assumption, that adaptation was a necessity for the forest-butterflies on account of the protection which it provided. an eye witness furthermore declares that the butterflies that resemble leaves most closely do not always alight on withered leaves, on which they would be almost invisible, but frequently rest on a green background, against which they show off very clearly, and therefore could not long escape the keen eye of birds. besides, these butterflies are but seldom pursued by the birds, of which there is question here, and hence are in no need of protection. the longer eimer devoted his attention to the origin of this resemblance the more "the poetic picture of the imitated leaf" vanished out of sight, and he became convinced that it involved the necessary expression of the lines of development, which the respective beings were bound to follow, and that there was no question of imitation. apart from the resemblance to leaves, by reason of regular changes of color, design, and wing-structure, numerous non-related butterflies often develop such wonderful similarities--which are not, as hitherto supposed, imitations or disguises produced by selection, but are either the outcome of an entirely independent uniformity of development or, at least, of its consequence--that it must be admitted that external similarity may arise by different means and in various ways. these relations of similarity are of such frequent recurrence because of the limited number of directions of development in which changes or color and design in butterflies may tend. eimer finds the reason of this small number of directions, in which development may proceed, in the fact "that the elementary external influences of climate and nourishment on the constitution of the organism are everywhere the cause of the transformations." another important point is the difference of sex. if the butterflies are of different sex, the males as a rule exhibit a more developed stage of design and color than the females. these frequently present on the upper side the stage of coloration, which the males present on the lower side, while the upper side of the males is one stage in advance. it is of special significance that the characters of the more advanced sex frequently correspond to those of a related, superior species, and occasionally to those of widely separated species. eimer endeavors to explain male predominance "by a more delicate and more developed, i.e., more complex, chemico-physical organization of the male organism." even this development tends toward simplification, the origin of dull-black colors. this most interesting question brings eimer into conflict with another darwinian principle, the so-called principle of "sexual election," according to which the more striking characteristics of the male sex become strengthened for the reason that females invariably give the preference to the males endowed with them, over those that are less "attractive." these exceedingly romantic ideas have been often and deservedly repudiated, e.g., even by wallace only a short time after their first appearance. eimer really does them too much honor when he again undertakes, even with a certain amount of respect, a thorough refutation of them, "as in every regard unfounded." it is of primary importance to note here, that in the case of dimorphism of the sexes abrupt modifications occur in connection with unilateral heredity. "it is impossible for sexual selection to produce a change of design and color, which results in the sudden kaleidoscopic formation of wholly different designs, as we find actually taking place through the action of artificial heat and cold and other factors in nature." this brings us to a brief consideration of the answer, which eimer proposes to give to the question of the real causes of the formation of species among butterflies. a precise and clear statement of this important part of eimer's theory of descent, is contained in the following extracts: "the transformation of organisms is primarily conditioned by the action of immediate external influences on the organisms. the same causes, which produce individual growth, especially climate and nourishment, also produce the organic growth of organisms, that is, transmutation, which is but a continuation in the progeny of individual growth, through the transmission of the characteristics acquired during the lifetime of the individual." hence, transmutation is simply a physiological process, a phyletic growth. "the changes, which the individual organism experiences during its life in its material, physiological and morphological organization, are in part transmitted to its progeny. the changes thus acquired become more marked from generation to generation, until finally they result in a perceptible new structure." "in this process, new or changing external influences undoubtedly exercise great activity, but the same influences, constantly repeated, must in the course of time also produce a change in the organisms through the physiological activity, which is conditioned by them, so that after a long time elapses, a species will have changed even in an unvarying environment and will react on new influences in a manner quite different from their progenitors; their "constitution" has undergone a change." "this organic growth of living beings takes place regardless of the active use of the organs and in many cases remains independent of this (lamarckian) factor of transformation. but use may exercise considerable influence on the formation resulting from the primitive organic growth, by modifying the growth, by restricting it to those parts most frequently called into use, or even by depriving other parts of the necessary matter (compensation)." "the lamarckian principle, therefore, offers but a possible and to transformation, the principal cause is to be found in organic growth." "* * * the organic growth of butterflies is primarily conditioned by climatic influences. * * * the proof is to be found in the facts revealed by the geographical distribution of butterflies, by the variations corresponding to the seasons, and by experiments regarding the influence of artificial heat and cold on development." experimental proof is naturally of vital importance for eimer's theory. he cites in this regard especially the experiments of merrifield, handfuss, fischer, fickert, and countess maria von linden. in eimer's own laboratory the latter performed experiments on papilionides, "which prove in the most striking manner the recapitulation of the family-history in the individual." "the fact that it is possible by raising or lowering the temperature during the time of development to breed butterflies, possessed of the characteristics of related varieties and species living in southern and northern regions respectively, characteristics not merely of color and design, but also of structure, is complete irrefragable proof of my views." eimer therefore belongs to the class of naturalists, like wigand, askenasy, naegeli, and many others, who reject the purely mechanical trend of darwinism and recognize an "immanent principle of development." he seeks the essential cause of evolution in the constitution of the plasm of organisms. this very analogy between the development of the family and that of the individual should, in fact, convince any one of this. if eimer chooses to refer the analogy to "growth" and to designate the evolution of the whole animated kingdom as also a process of growth, there is, strictly speaking, no room for objection. however, there is here a danger, which he does not seem to have guarded against. to designate the whole process as a growth, as eimer does, really explains nothing, but merely defines more clearly the status of the problem. for, what do we know of the so-called process of growth? in truth, nothing, so that very little is gained by referring evolution to organic growth; the problem remains unsolved. the most important and correct part of eimer's conclusion seems to be the establishment of definite lines of development. he has, in fact, permanently disposed of the darwinian assumption of universal chaos in evolution, upon which good mother nature could at will exercise her choice. fortuitously initiated development is a condition sine qua non of darwinism and weismannism. for any one, who has studied the work of eimer and still adheres to this fundamental error of darwinism, there is no possible escape from the labyrinth into which he has allowed the hand of darwinism to lead him. if, on the one hand, eimer recognizes the immanent principles of development, he, nevertheless, on the other hand, also accords due consideration and ascribes great efficacy to external influences; in fact, he represents them as perhaps the more essential factor. climate, nourishment, etc., affect the inner structure, the plasm, transform it and thus produce variation which is transmitted to the progeny. but, however great may be the influence of environment, eimer seems to overestimate it. indeed, the analogy of "growth" should have led eimer to a conception of the true relation between "internal" and "external" causes. warmth, air, light, moisture and nourishment, are undoubtedly necessary factors in the process of growth, but they are only the conditions which render it possible, and not the causes which produce it. the latter are to be found in the individual organism itself. the conditions may be ever so favorable and well-adapted for growth, still the organism will not develop unless it bear within itself the power to do so. on the other hand, although it is hampered and may become abnormal, it will readily grow even in an unfavorable environment, as long as it retains its inherent vital force. the same is very likely true of the genealogical growth. evolution took place in virtue of the power inherent in the developing organisms. but only when the environment was favorable and normal, did the evolution proceed favorably and normally, that is, toward the perfection of the animate kingdom. it appears as if the internal principle of development were losing influence and significance with eimer; but the ulterior reason for this is not far to seek. whoever recognizes the validity of the internal principle of development, eliminates chance, that stop-gap of materialism, from evolution, and is lead at once to a supreme intelligence which directs evolution. as soon as it comes in sight, however, certain persons take fright and turn aside or even turn back in order to avoid it. this was the case with eimer, although perhaps in a lesser degree. this is sincerely to be deplored, since his theory would have gained in depth if he had but done full justice to the internal principle of development. for the same reason he seems to have attacked naegeli's principle of perfection, another fact which is very much to be regretted. true, it is as anti-mechanical as it can be and hence has gained but few adherents; but it is based on truth nevertheless, and will some day prevail in the doctrine of descent. it is perfectly intelligible that the thought of "perfection" should not have occurred to eimer or should have slipped his memory during his observations on butterflies. the fact however, reveals a one-sidedness which he could have avoided. when the notion of utility is rejected--and eimer rejects it very emphatically in his discussions on mimicry--it is undoubtedly difficult to arrive at the concept of a perfecting tendency. this, however, can in no way mean that this concept should be entirely banished from nature, even as the notion of utility cannot be banished. even if the coloration and design of the wings of the butterfly do not reveal utility, other characteristics certainly do reveal it. it is one of the fatal mistakes of darwinism, that it fails to recognize the possibility of dividing the characters and qualities of organisms into two large groups, as i attempted to do with more detail, for instance, in my "catechism of botany." there i called them (p. ) "autochthon-morphological" and "adaptive-morphological characters." the former reveal no relation to utility, they are innate and distinguish the organism from other organisms; the latter can be explained by means of certain vital functions, hence they possess a certain utility and adapt themselves more or less to environment. the former are permanent, the latter changeable. darwinians regard all the characters of organisms as useful, physiological, and adaptive. if they have been hitherto unable to make good this assumption, they appeal to our lack of knowledge and console themselves with the thought that the future may yet reveal the missing relations. the presence on plants and animals of any autochthon-morphological characters means death to darwinism, because these can never be explained by means of selection and struggle for existence. eimer is too much inclined towards the other extreme; he does not admit the existence of adaptive-morphological characteristics. viewed in this aspect, his repudiation of mimicry may perhaps also seem somewhat harsh and one-sided. in this narrowness of view must also be sought the reason for his complete repudiation of naegeli's principle of perfection. it is an incontrovertible fact that in the organic world there exists an ascending scale from the imperfect to the perfect. every organism is indeed perfect in its own sphere and from its own point of view. but perfection with reference to things of earth is a very relative concept; many an organism which is perfect in itself, appears very imperfect when compared with others. if, then, there is a gradation of animals and plants from the lower to the higher, it is the task of the theory of descent to explain this gradual perfection. the crude and aimless activity of darwinian selection, which necessarily operates through "chance," can never explain this perfection, which remains, as far as selection is concerned, one of the greatest enigmas of nature. far from solving the enigma, selection but makes it obscurer. if, then, one refuses to recognize a directing creative intelligence, whose direction produces this perfection, nothing remains but naegeli's principle of perfection. the outer world with its influences can certainly not produce perfection, hence this power must lie within the organism itself. but when one has once brought himself to accept an immanent principle of development, it surely cannot be difficult to take the next step and ascribe to it the tendency towards perfection. that eimer does not take this step, is, to my mind, a mistake, which must be attributed to his one-sidedness, which, in turn, results from the fact that he generalizes too arbitrarily his observations on butterflies and the conclusions which he draws from them. animals and plants certainly possess many characteristics which cannot be explained by means of his theory alone. the conclusion will probably be finally arrived at, that nature is inexhaustible and many-sided, even in the lines on which it proceeds to attain this or that end. one thing, however, of primary importance is evident from the investigations of eimer, namely the proof that the same lines of development may be entered upon from entirely different starting-points, and that the number of these lines is limited. this fact is of importance because it enjoins more caution in arguing from uniformity of development to family-relation, than has been usually employed since the days of darwin. the method commonly employed is undoubtedly very convenient, but is somewhat liable to be misleading. hence, if one wishes to establish the genealogical relationship of forms, nothing remains but to set out on the laborious path of studying the development of both; and even then it remains questionable whether the truth will be arrived at. however, he who concludes to relationship from a comparison of developed forms, is much less likely to arrive at the truth. in one point eimer concedes too much to darwinism, in the matter of the famous fundamental principle of biogenesis, according to which an organism is said to repeat in its individual development the whole series of its progenitors. although he does not enter upon a discussion of the principle, it is evident from one passage that he accepts it. one is inclined to think that his careful observations and experiments should have convinced him of the contrary. it appears to me, at least, that the abundant materials of his observations bear evidence radically opposed to the principle. during late years, the antagonism to it has been on the increase, and the day is not very distant when it shall have passed into history. it would certainly be a laudable undertaking to enter upon a thorough investigation of the actual basis of the principle. chapter vi. in every disease, especially in a lingering one, there are times when life's flickering embers glow with an unnatural brightness. hence, it would not be a all surprising if a similar phenomenon were to be observed in the case of dying darwinism; for it cannot be doubted that its disease is chronic. it has, in fact, been dying this long time. certain indications render it very probable that we are at present witnessing such a phenomenon, for to-day we behold once more a few naturalists stepping before the public in defense of darwinism. we are desirous of presenting the present status of the darwinian theory as objectively as possible, hence, since we have hitherto heard exclusively anti-darwinian testimonies--as the nature of the case demanded--we shall now lend our attention to a darwinian. the reader may then decide for himself whether this treatise should not still bear the title, "at the death-bed of darwinism." the naturalist in question is the zoologist, professor f. von wagner. in the "umschau" (no. , ) he published an article, "regarding the present status of darwinism," which is highly instructive and important in more respects than one. we wish, in the first place, to call special attention to the following statements embodied in the article: "it is not to be denied that in serious professional circles the former enthusiasm has considerably decreased and a scepticism is gaining ground more and more, which betrays a widespread tendency towards revolutionizing current theories. the _fin de siecle_ therefore, finds darwinism not with the proud mien of a conqueror, but on the defensive against new antagonists." and again: "it seems, in fact, as if darwinism were about to enter a crisis, the outcome of which can scarcely be any longer a matter of doubt." to what outcome reference is made, appears from two sentences in the introduction: "thus it happens that a theory which was once accorded enthusiastic approval, is treated with cold disdain or vice versa. examples of this are to be found in the history of all sciences and circumstances seem to indicate that darwinism is to add another to the number of these theories." is not this exactly what we have repeatedly asserted? it is most significant that these words are not written by an opponent of darwinism, but by one who seems to be thoroughly convinced of the truth of darwinism. i am of opinion that it can be no longer a matter of doubt to any one, that the position of darwinism is hopeless. if this were not true, a darwinian would be very careful about making such an open and unreserved statement. we therefore accept professor von wagner's words as a very welcome endorsement of what we have constantly maintained. professor von wagner, however, proposes to himself the further question: whence comes the unfavorable attitude of present-day natural science towards darwinism? a discussion of this question by a darwinian cannot but be of interest to us, and indeed is an important contribution to the problem. with goette, professor von wagner admits that the objections, which are raised against darwinism to-day, are the very same which were raised from thirty to forty years ago. but when he then proceeds to assert that this is not to be explained on the assumption that the pristine enthusiasm for selection was due to a serious over-estimation of that theory, he fails to furnish even a shred of evidence in support of his assertion. anyone can readily point out that darwinism explains the totality of the world of organisms by interlinking them, but has generally failed to account for the individual case, wagner admits this as far as the "actual" is concerned, for it is quite impossible to trace with any certainty the action, in any particular case, of natural selection in the process which results in the production of a new species. at the outset it was reasonable to hope, that with the progress of science this difficulty would be solved or at least lessened; but this expectation has not been realized. * * * it is wholly unintelligible how a naturalist can make this statement five hundred years after bacon of verulam, without drawing therefrom the proper conclusion. this lack of logic reminds me strongly of the assertion recently made by an eminent authority, that the principal cause of the difficulties of many naturalists in matters of religion is their deficient philosophical training. wagner's statement implies that, in the case of darwinism one may in defiance of all established law, actually reverse the methods of natural science. how justifiable and how necessary was it not, then, that even three decades ago wigand should have written his comprehensive work: "darwinism and the scientific researches of newton and cuvier." ordinarily the scientific (inductive) method proceeds from the "actual" and attempts to deduce from the "individual case" an explanation, which applies to the whole. here, however, we are face to face with a theory, which, according to the candid confession of an advocate, fails in the individual case, but furnishes a unifying explanation of the whole. this means nothing less than a complete subversion of all scientific methods. usually a theory is deduced from separate observations regarding the "actual" but here--and this is what wigand constantly asserted--the theory was enunciated first, and then followed the attempt to establish it in fact. one could then rest content and trust to the future to establish the theory by producing evidences of the "actual" in the individual case. but forty years have elapsed since the darwinian hypothesis first became known, naturalists by the thousands have spent themselves in the endeavor to corroborate it by proofs based on actual facts, and to-day one of its own advocates has to confess that the endeavor has been a total failure. instead of drawing the conclusion, however, that the theory is unwarranted and that the decrease of enthusiasm for it is therefore a natural consequence, he gratuitously enters a flat denial of this inference. every intelligent observer must conclude with absolute certainty from this confession of a darwinian, that darwinism is, in fact, not a scientific but a philosophic theory of nature. but let us proceed to a consideration of the other reasons which wagner suggests as an explanation of the retrogression of darwinism. he states as a first reason, that scientific research since darwin "has amassed such an abundance of empiric materials for the truth of the principle of descent, that this doctrine has been able, even for some time past, to maintain an independent position and to draw proofs of its truth immediately from nature itself, without the intervention of darwinism." * * * "from which it follows as a matter of course, that the question, whether the manner indicated by darwin for the origin of species is the correct one, has decreased by no means inconsiderably in significance, inasmuch as darwin's theory could now, if it were necessary, be abandoned with less concern than formerly because it could be relinquished without detriment to the doctrine of descent." it is unintelligible how one can attempt to explain a fact of such importance so superficially. with naive unconcern there appears on the face of it the acknowledgement that darwinism has really not been based on actual observation but has been enunciated for the sake of the doctrine of descent. come what may, this must be vindicated. other means are now said to substantiate it, hence the darwinian crutches may safely be discarded. the principle of action twenty or thirty years ago was therefore: a poor explanation is better than no explanation. i cannot understand, how wagner dares to credit present-day naturalists with such motives. when he then proceeds to say "that with the advance of the principle of development, new lines were entered upon, which led primarily to the corroboration and empiric demonstration of the doctrine of descent, and not of darwinism"--that the theory of darwin was consequently neglected and, in fact, forced into the background--"that the labors specifically attributable to darwinism as compared with the theory of descent, put the former more and more into a false position to the detriment of its prestige"--when, i say, wagner has marshalled all these considerations to explain the present aversion to darwinism, he is guilty of a total subversion of facts. the true state of the case is the very contrary. the credit given by wagner to the darwinian theory for stimulating research, is the very same as i also accorded it. the purpose of this research undoubtedly was to substantiate not only the doctrine of evolution in general, but also the darwinian hypothesis in particular. to verify this, one need only glance over the various numbers of the "kosmos," the periodical, which haeckel and his associates established for that very purpose and which continued to publish good and bad indiscriminately until some time in the eighties when lack of interest compelled its discontinuance. wagner therefore misconstrues facts when he asserts that there have been no specifically darwinian researches. since the thoughts of darwin first found expression these researches have been most abundant and their results have been consigned to the printer's ink. no doubt--and this is the salient point, which wagner passes over in complete silence--they have been of service only to the doctrine of descent in general, and in spite of the energetic efforts of the darwinians, they have never led to the ardently desired proof from facts of the hypothesis of selection. this and no other is the state of the case. in view of these vain endeavors, however, intelligent investigators have gradually become perplexed and have turned away from darwinism, not because they have lost interest in it nor even because they no longer feel the need of it to assist the doctrine of descent, but for the one sole reason that its insufficiency has become more and more apparent and that all experiments undertaken on its behalf have made the fact clearer and clearer that the first criticism of the great naturalists of the sixties and seventies was perfectly justified. in forming a judgment concerning the whole question it cannot but be a matter of the utmost significance, that men have turned away from darwinism to entirely different theories of descent. it is a mistake to suppose, as wagner would have us suppose, that the last decades have produced nothing but generalities regarding the doctrine of descent. for they have also witnessed the publication of a number of significant works, which aimed at giving a better individual explanation than was found in darwinism. i need but recall naegeli, eimer, haacke and a host of others. the most noteworthy feature of these new views regarding theories of descent, is the constantly spreading conviction that the real determining causes of evolution are to be sought for in the constitution of the organisms themselves, hence in internal principles. this view, however, is not only absolutely and diametrically opposed to darwinism but completely destructive of it as well. the actual circumstances, therefore, are the very reverse of those pictured by wagner. darwinism has been rejected not on account of a lack of research but on account of an abundance of research, which provided its absolute insufficiency. besides these "general points of view," as he calls them, wagner finds two other "considerations of no less importance" for explaining the decay of darwinism. it is an incontrovertible fact, that the hereditary transmission of acquired characters has in no way been proved. on the contrary after it had at first received a general tacit recognition and was postulated by lamarck, darwin and haeckel, it was denied by weismann. wagner asserts "that the number of those who have allied themselves with weismann in this matter is obviously on the increase as is naturally the case, since, to the present day not a single incontestable case of hereditary transmission of acquired characters has been demonstrated, where as actual facts are at hand to prove the contrary." it is perfectly evident that the doctrine that acquired characters are not inherited is fatal to darwinism. hence wagner rightly considers its ascendancy a notable factor in bringing about the decay of darwinism. finally, wagner briefly indicates that certain new theories necessarily exercised an influence on darwinism. haeckel and the palaeontologists of north america supplemented it with a number of lamarckian elements without alteration of its essential principles (the neo-lamarckians); eimer regards the transmission of acquired characters as an established fact, but rejects natural selection as wholly worthless; weismann, on the contrary, denies the transmission of acquired characters, but nevertheless regards natural selection as the main factor in the formation of species (the theory of the neo-darwinians). eimer speaks of the impotence of natural selection, weismann of its omnipotence. all this has shaken men's confidence in the trustworthiness of the darwinian principles. this fact we are in no way inclined to doubt, but we must again differ from wagner with regard to its significance. we maintain that matters had to take this turn, since the reason why darwinism is now meeting with such serious opposition, is to be found in its very nature. this indeed should have been recognized forty years ago instead of just beginning to dawn on men of science at the present day. for if acquired characters are not transmitted by heredity, darwinism is an impossibility. forty years ago darwinism should have recognized that its first and supreme task was to prove the hereditary transmission of acquired characters, so as to establish itself, first of all, on a sound footing. one of the most peculiar incidents in this scientific tragi-comedy is the fact that weismann, the mainstay of contemporary decadent darwinism, attacks with might and main its fundamental assumption, the transmission of acquired characters, whereas eimer, who is thoroughly convinced that he has proved that doctrine, in his turn attacks darwinism and proves with telling effect the impotence of its principles. the amused observer can really demand nothing more. he can but rub his hands for joy and cheer on the heated combatants: well done! on with the struggle! and the last vestige of darwinism will soon have disappeared. if, then, we were to summarize our strictures on the reasons which wagner adduces to account for the decay of darwinism, we would say this: some of them are unwarranted, others are falsely interpreted. there is, however, a third point which is of special interest to us, in the article under consideration; we refer to the view, which there finds expression, regarding the nature and outcome of the present crisis--a crisis, which, as a candid naturalist, wagner is not in a position to deny. this view rests on the entirely gratuitous assertion, "that the decline, in the esteem enjoyed by darwinism, is not due to a better insight arising from widened experience, but is primarily the expression of a tendency--a tendency which resulted almost as a psychological necessity from the precarious position into which darwinism was forced under the sway of the theory of descent." this assertion rests, as stated above, on wholly erroneous assumptions. it is a serious mistake, to speak in this connection of tendencies and even to brand them as a "psychological necessity." the decline in esteem is essentially due to experience, and indeed to experience which has made it certain that darwinism has everywhere failed. the importance of the present crisis in darwinism is to be restricted even further, according to wagner, by the fact, "that the real objections, urged against the theory of darwin, are almost in every instance based on theoretic considerations, the validity of which can be put to the test only in fictitious cases. this manner of proceeding manifestly leads to the inevitable consequence, that the results thus obtained can claim no decisive weight against darwinism. a decisive critique can be constructed only on the basis of experience, and in this connection it cannot be emphasized sufficiently, that, as yet, the path to it has been scarcely indicated, to say nothing of its having been actually pursued." the reason for this fact according to wagner, is to be found "in the numerous and most extraordinary difficulties that arise in the way of the empiric investigation of the theory of selection." after we have read all this, we instinctively ask ourselves: do we actually live at the beginning of the th century? is it possible, that even at this late day the whole structure of scientific method is to be subverted in this fashion? just consider for a moment, what according to these words is the actual import of the whole article: darwinism is a unifying explanation of the origin of the totality of the world of organisms, but fails in the individual case; in any specified case it is "almost impossible" to trace with any certainty the action of natural selection in the process which results in the production of a new species; that is, darwinism was enunciated with a complete disregard for inductive method, as an hypothesis to explain the whole, and without actual proof in the concrete--a most unscientific procedure. immediately after, however, the adversaries of darwinism are asked in all seriousness to produce individual facts in disproof of the theory. in the same strain wagner goes on to say that "from no point of view is our vision so penetrating as to be able to grasp the coherence which according to darwin pervades the complex course of natural selection. when men of science take occasion to repudiate darwinism because of our inability to explain satisfactorily any particular case by means of the theory of selection, this inability arises not from the theory of darwin but from the inadequacy of our experience. for as yet the empiric prerequisites for an objective judgment regarding the validity or futility of the theory of selection are entirely lacking." every naturalist who believes in the inductive method must needs draw the conclusion from these naive admissions, that, as darwinism lacks the empiric prerequisites, it should be discarded. moreover, the demand is made in all seriousness, that, in order to refute darwinism which has not as yet been established empirically, empiric proofs should be forthcoming. to my mind, the scientific and logical bankruptcy of darwinism was never announced more bluntly and ingenuously. furthermore it must be remarked that wagner's statement, regarding "fictitious cases," is not even pertinent. he seems to have no idea of the observations and experiments of sachs, haberlandt, eimer, and a host of other investigators. the disproof of darwinism on the basis of scientific research is an accomplished fact. a word about the conclusion of wagner's article, which in view of what has been already said, cannot be a matter of surprise. he maintains that the considerations which he adduces, "clearly" prove that there is no "reasonable ground for despairing of the theory of darwin--; for a theory, which neither proceeds from questionable assumptions, nor loses itself in airy hypotheses, but rests throughout and exclusively on facts, need never fear the advance of science." but a moment ago it was asserted that the theory of selection is lacking "entirely as yet the empiric prerequisites" and now only twenty-three lines further on, it rests "throughout and exclusively on facts." it is difficult to know what conclusion to come to regarding a naturalist and university professor who can commit himself to such a contradiction. i shall abstain from any comment and let the reader form his own judgment. does this article betoken the death-bed of darwinism? for my own part i repeat what i said above, that i consider it the most valuable contribution to the characterization of decadent darwinism that has appeared up to the present time. the sooner a theory, which is thus treated and characterized by one of its own advocates, is stored away in the lumber-room of science, the better. in view of the sound judgment, which is to-day becoming more and more apparent in scientific circles, there is reason to hope that this article of professor von wagner will be additional incentive for many naturalists to break completely with darwinism. chapter vii. in the year haeckel published a new work, which he intended as a kind of testament; for with the close of the nineteenth century the author desired to put a finishing touch to his life-work. in the preface haeckel states with very remarkable modesty that his book cannot reasonably claim to present a complete solution of the riddles of existence; that his answer to the great questions can naturally be only subjective and only partly correct; that his attainments in the different branches is very unequal and imperfect; and that his book is really only a sketch book of studies of very unequal value. in this way the author naturally gains at once the confidence of his reader who is thus prepared to yield assent when the author makes pretense to sincerity of conviction and an honest search after truth. the reader's surprise at the contents of the book and at the manner of its presentation is, however, only increased by this ruse. all modesty has vanished, monistic doctrines are presented as absolute truth, every divergent opinion is contemptuously branded as heretical; in short, the book reveals a darwinian orthodoxy of the purest type, with all the signs of blind bigotry and odious intolerance which the author imagines he discovers in his christian adversaries. it is difficult to see where, in view of such a contradiction between the work and its preface, there is room for an honest striving after truth. personally i do not wish to deny haeckel all honesty of purpose, for it is my endeavor to understand the _whole_ man. the one prominent feature of the "weltraetsel" is the fact that, owing to a very marked deficiency in philosophical training, haeckel has become so completely absorbed in his system that he has lost all interest in everything else and takes cognizance only of what suits his purpose. what he lacks above all, is the ability to appreciate even the "honest" opinion of others; hence, from the very outset he brings into the discussion that bitterness of which he complains in others (in the weltraetsel he once makes this accusation against me). notwithstanding all this, honest conviction may be present, but if so, it is joined with total blindness. but what is to be thought of his search after truth since he completely ignores his adversaries? for instance, in spite of loofs' attacks, he continues to have his book reprinted without alteration, without submitting it to revision. the "reichsbote" is perfectly in the right when it says: haeckel, in fact, takes account only of what suits his purpose. as regards the contents of the "weltraetsel," it is not my intention to enter here upon a criticism of it but merely to discuss it as illustrating the general status of the theory of descent. it is to be noted, in the first place, that it is really not a scientific book at all; for of its pages, the first or "anthropological part," with which alone we are here concerned, occupies only (from pages to ), even less than one-sixth of the whole, whereas the "theological part" is almost twice as long. the book is, in fact, rather a theologico-natural-philosophical treatise than a work of natural science. the scientific part is, however, the foundation on which haeckel builds up his natural philosophy, and which he uses as the starting point of his criticism of theology. hence it is worth our while to discuss it. how then fares it with the anthropological basis of haeckel's whole system? as an attentive student of his age the naturalist-philosopher of jena must have perceived the true position of darwinism, namely, that the foremost naturalists of to-day have no more than an historical interest in it. since, in accordance with the well known tendency of old men to persevere in the position they have once assumed and not easily to accept innovations, haeckel is still an incorrigibly orthodox darwinian, we should naturally expect him to embody in this testament some new cogent evidence of the truth of darwinism. but nothing of that nature is to be found in the book. the first chapter of the "anthropological part" is taken up with a "general history of nineteenth century culture," in itself a sign of peculiar logical acumen, that he should include this and the "struggle regarding world-views" in the "anthropological part" instead of embodying it in a general introduction. the remaining chapters treat: "our bodily structure," "our life," "our embryonic-history," "our family-history." it is not to be supposed, however, that any arguments are here adduced, nothing but assertions; a large part of the chapter is taken up with historical sketches, in which haeckel again proves himself utterly devoid of all appreciation of history and all sense of justice. he attributes the decay of the natural sciences to the "flourishing condition of christianity" and dares to speak of the unfavorable influence of christianity on civilization. apart from the historical sketch, each chapter presents only the quintessence of darwinism, fairly bristling with assertions, which are boldly put forth as incontrovertible truths. in view of the author's demand to have at least his sincere love of truth recognized, we can but throw up our hands out of sheer astonishment. to illustrate haeckel's "love of truth" let it suffice to observe that in the second chapter he asserts that man is not only a true vertebrate, a true mammal, etc.--which indeed is passable--but even a true ape (having "all the anatomical characteristics of true apes"). with a wonderful elasticity he passes over the differences. what, indeed, is to be said, when he states as a "fact" that "physiologically compared (!), the sound-speech of apes is the preparatory stage to articulate human speech." it is so simply monstrous, that even garner's famous book of ape-speech, cannot surpass it. as a third illustration of haeckel's method of argumentation, if we are still justified in speaking of such a thing, we may mention his assertion (p. ) as a "certain historical fact," "that man is descended directly from the ape, and indirectly from a long line of lower vertebrates." if, in view of the results of research during the last forty years any one can assert this as a "certain historical fact" and can still wish to be credited with honest conviction and love of truth, there remains, to adopt haeckel's own expression, but one explanation for this psychological enigma, namely, intellectual _marasmus senilis_, which may very easily have set in with a man of sixty-six, who himself complains (p. ) of "divers warnings of approaching age." thus, the anthropological part of the "weltraetsel" contains nothing new; always the same old story, the same threadbare assertions without a shred of evidence to corroborate them. the remaining parts also contain various scientific assertions, which are proposed as facts without being such, but these parts do not immediately pertain to our theme. suffice it to say that, after reading haeckel's "weltraetsel," one would be led to think that there is no question of a "deathbed of darwinism," but that on the contrary darwinism, as remodeled by haeckel, is more in the ascendant to-day than ever. let us judge of its prestige by the reception accorded the "weltraetsel." one unaltered edition after the other, thousand after thousand, the book is given to the public. hence it must meet with approval. it does indeed meet with approval, but the question is, from whom? immature college and university students will doubtless receive it with reverential awe, just as they received the "natural history of creation" twenty-five years ago. bebel accepts the book as an infallible source of truth, and after him the social democrats and free-church members will add it to the list of their "body and stomach books," which alone will afford it a respectable clientele, at least in number. in no one of my "deathbed articles," however, have i as yet ever maintained that darwinism was decadent in _these_ circles. i know full well, that darwinism has filtered down into that sphere and there satisfies the anti-christian and anti-religious demands of thousands. nothing, however, really depends on these senseless blind adherents of haeckel's unproved assertions. we are now intent upon investigating how the world of eminent thinkers and natural science regards the latest product of haeckel's fancy. that alone is of importance in ascertaining the real status of darwinism. as regards, in the first place, the other parts of the book, it is well known that all of them were vigorously attacked. loofs in particular exposed haeckel's theology, according to its deserts, in the clear light of truth, and convicted haeckel of "ignorance" and "dishonesty;" while the philosopher paulsen made short work of the "weltraetsel" from his own standpoint, ("if a book could drip with superficiality, i should predicate that of the th chapter"). harnack also condemned the theological section in the "christliche welt," and troeltsch, hoenigswald, and hohlfeld took haeckel severely to task on philosophic grounds. the naturalists have thus far maintained silence. scientific journals, and, i believe, only the more popular ones, pass a varying judgment on the book according to the intellectual bent of their book reviewers; but no one of the eminent and leading naturalists has publicly expressed his opinion regarding it. they all maintain a very significant silence, which speaks for itself. now, however, just at the proper time a book, _die descendenz-theorie_ has appeared from the pen of the zoologist, professor fleischmann of erlangen, in which haeckel is severely condemned. (see chapter ix.) the press-notices of the weltraetsel, which are quoted in the book will be considered presently. it appears that with reference to natural science, only "laymen" discuss the book and approve of haeckel's views. this is a point of great importance since it proves satisfactorily that men of science will have nothing to do with the "weltraetsel." the large number of replies would, however, not allow haeckel's friends to remain silent. the most extensive defense forthcoming was a pamphlet published by a certain heinrich schmidt of jena. it cannot be gathered from his book (der kampf um die weltraetsel, bonn, e. strauss ) to what profession the author belongs, hence i am unable to judge whence he derives the right to treat haeckel's opponents in summary a manner. it is significant to note what class of men, according to schmidt, received the "weltraetsel" with enthusiasm and joy. they are august specht, the free-church editor of "menschentum" and of the "freien glocken," julius hart, professor keller-zuerich, the philosopher and "neokantian" professor spitzer of graz, the popular literateur w. boelsche, w. ule, and a few unknown great men, dr. zimmer, th. pappstein, r. steiner, a. haese; but stay, i came very near forgetting the great pillar, dodel of zuerich. but where is there mention of the professional colleagues of haeckel whose testimonies could be taken seriously? under the heading "literary humbug," which evidently has reference to the contents of his own work, schmidt then meets numerous objections. here vigorous epithets are bandied about, as, for instance, "absolute nonsense," "muddler," "foolish and senseless prattle," "idle talk," etc.; and from dodel he copies the words with which the latter once sought to annihilate me: job, verse , "thou hast spoken like one of the foolish women." and he ventures to express indignation at loofs' "invectives." as a compliment to lasson he declares that he could easily conceive of the possibility of an ape ascending the professor's chair and speaking as intelligently as he (lasson); which remark he probably intended as a witticism. he informs his readers that the criticism of haeckel by men like virchow, his, semper, haacke, baer, and wigand have been examined by professional specialists and proved practically worthless. this statement alone so clearly reveals schmidt's lack of critical faculty and judgment that by it he at once forfeits his right to be taken seriously. the whole book is nothing more than a collection of quotations from the reviews of the "weltraetsel," interspersed with characteristic expressions like "idle talk," "nonsense," etc., as exemplified above. a really pertinent reply and refutation of objections is entirely beyond schmidt's range; he waives the demand for a direct reply, for instance, in the following amusing way (p. ): "two reasons, however, prevent me from being more explicit: in the first place i do not like to dispute with people who adduce variant readings and church-fathers as proofs and can still remain serious. in the second place i would not like to fall into the hands of a loofs." in this manner it is indeed easy to evade an argument, which for good reasons one is not able to pursue. loofs' criticism is so serious and destructive that it should be of the utmost concern to haeckel's friends to refute it. since they are unable to do so, they content themselves with references to loofs' caustic style, which he should indeed have avoided. there are, nevertheless, cases in which one must employ trenchant phraseology, and haeckel himself has given an occasion for it; a dignified style is simply out of the question in his case. haeckel extricated himself with even greater ease, by declaring that he had "neither time nor inclination" for reply, and that a mutual understanding with loofs was impossible because their scientific views were entirely different. could anything be more suggestive of the words of mephistopheles: "but in each word must be a thought-- there is,--or we may so assume,-- not always found, nor always sought. while words--mere words supply its room. words answer well, when men enlist 'em, in building up a favorite system." there are two other points in schmidt's book that are of interest to us. the first of these is the manner in which the author treats the romanes incident. romanes ranks, as is well known, among the first of haeckel's authorities. hence it is a very painful fact that, but a short time before the publication of the first edition of the "weltraetsel," my translation into german of romanes' "thoughts on religion" should have appeared. from this book it was evident that haeckel and his associates could no longer count this man among their number since he--a life-long seeker after truth--had abandoned atheism for theism, and died a believing christian. troeltsch and the "reichsbote" asked whether haeckel had purposely concealed this fact, and schmidt now explains that haeckel first became acquainted with the "thoughts on religion" through him towards the end of january, . unfortunately he does not add that since then a number of new editions of the "weltraetsel" have appeared, in which haeckel could have explained himself in an honorable manner. schmidt has therefore not been successful in his attempt to clear up this matter. but how does he settle with romanes? he says: "_we are assured_ that the thoughts were written down by the english naturalist george john romanes"; and again: "the thoughts are published by a canon of westminster, charles gore, to whom _they are said_ to have been handed over after the death of romanes in the year ." then he has the audacity to place romanes in quotation marks. and finally he asserts that they would abide by romanes' former works as their authority, the more so, because these were not, like the "thoughts," "published and glossed by a canon only after his (romanes') death." by means of all this and of a comparison with the "letters of the obscurantists" he wishes to create the suspicion that there might be question here of forgery. such an insinuation, (i employ schmidt's own words) "cannot be characterized otherwise than as contemptible." "here it is even worse than contemptible." i must beg my reader's pardon for overstepping the bounds of reserve with these caustic words, although they originated with schmidt; but really the flush of anger rightfully mounts to one's cheeks when a man, from the mere fact that he is a disciple of the "great" haeckel assumes the right to charge canon gore and indirectly myself with forgery. it is really very significant that these men should have to resort to such base and despicable expedients to extricate themselves from their unpleasant predicament. apart from this, it was very amusing to me personally to think that for the sake of my unworthy self, schmidt should have borrowed from his lord and master the epithet "pious," which haeckel in his turn has drawn from his cherished friend dodel. in all probability they will continue to hawk it about in order to bring me into disrepute with the rest of their kind. the few remarks schmidt still finds it proper to make regarding the "thoughts," betray his inability to understand the book. but as i stated in the preface it was a difficult book to read and understand. it is obviously not reading matter for shallow minds. i refer schmidt to the biography of romanes, published by his wife, (the life and letters of g. j. romanes, london, longmans, green & co., ), where he will find romanes' religious development described by a well-informed hand. this development began as early as , hence during the time of his intimate friendship with darwin. in this book on pages and schmidt will also find the words in which, _before_ his death, romanes begged that, if he were personally unable to publish the "thoughts," they should be given to his friend canon gore after his own death. but why waste so many words on mr. schmidt, for since all these things must be doubly disagreeable and painful to him and haeckel, he will very probably resort without delay to personal insinuation and accuse mrs. romanes of forgery. to us, however, who thoroughly appreciate the situation, it is a matter of great moment that of one of the few really eminent naturalists, to whom haeckel thought to be able to lay full and exclusive claim, for the last twenty years of his life should have been moving towards the christian faith in his eager search for truth and should die not a monist, but a convinced christian. neither did he die an old man, to whom the adherents of monism would certainly have the effrontery to impute feeble-mindedness, but at the early age of forty-six years. nor was his a sudden deathbed conversion--an impression which schmidt attempts to create (p. ) in order to be able with h. heine to relegate the conversion to the domain of pathology--but followed after many years of diligent and honest study and research. the other point of which we must treat here, is the manner in which, after the example of dr. reh, schmidt attempts in the "umschau" to exonerate haeckel in the matter of the "history of the three cliches." to begin with, it is at the very least dishonest on the part of schmidt to say that, "in default of scientific arguments, theological adversaries have for the last thirty years been using it as the basis of their attacks." that is untrue, the "theological adversaries" have not had knowledge of it for that length of time. on the contrary haeckel's own scientific colleagues were the first to discover and publish the matter some time in the seventies, and in consequence excluded haeckel from their circle. why does schmidt not mention here the names of ruetimeyer, his, and semper? furthermore schmidt writes as if haeckel had satisfied his colleagues in the matter of his forgery by declaring soon after ( ) that he had been "guilty of a very ill-considered act of folly." why does schmidt not mention the fact that the weighty attacks of his (our bodily form and the physiological problem of its origin, leipzig, ) dates from the year , five years after haeckel's forced, palliative explanation? besides, this incident of the three cliches is only one instance; the other examples of haeckel's sense of truthfulness are for the most part entirely unknown to his "theological adversaries," who have nowhere to my knowledge made use of them; but _all_ of them have been brought to light and held up before haeckel by naturalists, namely, by bastian ( ), semper and kossmann ( and ), hensen and brandt ( ), and hamann ( ). does this in any way tend to establish schmidt's honesty? (dr. dennert has entered into a more searching criticism of haeckel in his book, _die wahrheit ueber haeckel_. aufl halle a. s., .) in a word, the manner in which the "weltraetsel" was received and in which haeckel has been defended by schmidt, are valuable indications of the decay of darwinism. i repeat that i am speaking of course of the leading scientific circles. those who hold back are never lacking, and one cannot be surprised that, in the case of darwinism, their number is considerable: for on the one hand, to understand it an extraordinarily slight demand is made on one's mental capacity; and on the other hand it is a very convenient and even a seemingly scientific means of obviating the necessity of belief in god. these facts appeal very strongly to the multitude. in concluding this section, we shall quote a positive testimony to the decay of darwinism. on page of his "outlines of the history of the development of man and of the mammals" (leipzig, w. engelmann, ) prof. o. schultze, anatomist in wuerzburg, says: "the idea entertained by darwin, that the development of species may be explained by a natural choice--selection--which operates through the struggle of individuals for existence, cannot permanently satisfy the spirit of inquiry. even the factors of variability, heredity, and adaptation, which are essential to the transformation of species, do not offer an exact explanation." chapter viii. i have already called attention several times to the fact that darwinism is indeed on the wane among men of science, but that it has gradually penetrated into lay circles where it is now posing as irrefragable truth. especially the circles dominated by the social democrats swear by nothing higher than darwin and haeckel. in fact, only a short time ago bebel publicly professed himself a convert to haeckel's wisdom. it is inevitable, however, that light should gradually dawn even in these circles, for it would be indeed strange, if no honest man could be found to tell them the truth regarding darwinism. this has occurred sooner than i dared to hope. this chapter can announce the glad tidings that even in "social-democratic science" darwinism is doomed to decay. much printer's ink will, of course, be yet wasted before it will be so entirely dead as to be no longer available as a weapon against christianity; but a beginning at least has been made. in the december number of the ninth year of the _sozialistische monatshefte_, a social-democratic writer, curt grottewitz, undertakes to bring out an article on "darwinian myths." it is stated there that darwin had a few eminent followers, but that the educated world took no notice of their work; that now, however, they seemed to be attracting more attention. "there is no doubt, that a number of darwinian views, which are still prevalent to-day, have sunk to the level of untenable myths. true, the main doctrine of darwin--the origin of new species from existing ones--is incontestably established, but apart from this even some very fundamental principles, which the master thought he discerned in the development of organisms, can scarcely be any longer maintained." it may be well to remark here, that this was not really darwin's main doctrine, for it already existed before his time (lamarck, geoffroy st. hilaire). darwin's main doctrine is the explanation of the origin of species by natural selection operating through the struggle for existence. it is therefore the old error repeated. darwinism is confounded with the doctrine of descent, of which it is merely one form. it is not our intention to derogate in the least from darwin's merit, which consists in the fact that he gained general recognition for the doctrine of descent; but that was not his main work. he wished above all to explain the _how_ of descent; this is his doctrine, and this doctrine we attack and declare to be on the point of expiring. grottewitz very frankly continues: "the difficulty with the darwinian doctrines consists in the fact that they are incapable of being strictly and irrefutably demonstrated. the origin of one species from another, the conservation of useful forms, the existence of countless intermediary links, are all assumptions, which could never be supported by concrete cases found in actual experience." some are said to be well established indirectly by proofs drawn from probabilities, while others are proved to be absolutely untenable. among the latter grottewitz includes "sexual selection," which is indeed a monstrous figment of the imagination. there was moreover really no reason for adhering to it so long. it is eminently untrue, that the biological research of the last few years proved for the _first_ time the untenableness of this doctrine, as grottewitz seems to think. clear thinkers recognized its untenableness long ago, and surely grottewitz and the whole band of darwinian devotees as well, could have known that as early as twenty-five years ago this doctrine had been subjected to a reductio ad absurdum with classic clearness in wigand's great work. it is certainly a very peculiar phenomenon; for decades we behold a doctrine reverently re-echoed; thoughtful investigators expose its folly, but still the worship continues, the zeitgeist must have its idol. it appears, however, as if the zeitgeist were gradually tiring of its golden calf and were on the point of casting it into the rubbish-heap. misgivings arise on all sides; here one class of objections are considered, there another. a closer examination reveals that these are by no means new reasons, based on new researches, but the very oldest, urged long ago and perhaps much more clearly and forcibly. at that time, however, the zeitgeist was under the spell of the suggestion of individual men: it heard and saw nothing but the captivating, obvious simplicity of the doctrine; but now when the subject begins to be tedious and the discussion lags, the interest consequently abates and the zeitgeist suddenly grasps the old objections, presented in a new garb, and what was hitherto truth, clear and irrefutable, now sinks into the dreary, gray mists of myth. sic transit gloria mundi! this has been the history of darwinism, and especially of darwin's theory of sexual selection. what grottewitz urges against it, was advanced decades ago by other and more eminent men; then people would not listen, to-day they are inclined to listen. of very special interest is the further admission, that "the principle of gradual development" has been "considerably shaken" and is "certainly untenable." grottewitz points out that it has been demonstrated that the progeny of the same parents are often entirely dissimilar, and that new organs very suddenly spring up in individuals even when they had had no previous existence. "a slight variation from the parent form is of no utility to the progeny; they must acquire at once a completely developed, new character, if it is to be of any use to them." quite right! but this one admission is destructive of the entire doctrine of natural selection. if one accepts saltatory evolution, as for instance, heer, koelliker, and wigand did long ago, then, as grottewitz now discovers, the difficulty arising for darwinism from the absence of the numerous intermediary forms which it postulates, naturally disappears. grottewitz attributes sudden variation to the influence of environment, just as geoffroy st. hilaire had already done before darwin. he likewise repudiates darwin's doctrine of adaptation and the theory of "chance," which is bound up with all his views. "darwin's theory of chance seems to me to be especially deserving of rejection." the article closed with these words: "there must evidently be a very definite principle, according to which the frequent and striking development from the homogeneous to the heterogeneous, from the no-longer adapted to the readapted, proceeds. we all of us are far from considering this principle a teleological, mystical or mythical one, but for that matter, darwin's theory of chance is nothing more than a myth." he is most certainly in the right. to place this whole wonderful, and so minutely regulated world of organisms at the mercy of chance is utterly monstrous, and for this very reason darwinism, which is throughout a doctrine of chance, must be rejected; it is indeed a myth. we are grateful to grottewitz for undertaking to tear the assumed mask of science from this myth and expose it before his associates. he should, however, have done so even more vigorously and unequivocally and should have stated plainly: darwinism is a complete failure; we believe indeed in a natural development of the organic world, but we are unable to prove it. in the conclusion of the article quoted there is, of course, again to be found the cloven-hoof: by all means no teleological principle! but why in the world should we not accept a teleological principle, since it is clearly evident that the whole world of life is permeated by teleology, that is, by design and finality? why not? forsooth, because then belief in god would again enter and create havoc in the ranks of the "brethren." but however much men may struggle against the teleologico-theistic principle and secure themselves against it, it is all of no avail, the principle stands at the gate and clamors loudly for admission; and if grottewitz could but bring himself to undertake a study of wigand's masterful work, perhaps his heresy would increase and we might perhaps then find another article in the "sozialistische monatshefte" tending still more strongly toward the truth. but what will brother bebel with his haeckelism say to the present article? all in all, instead of calling his article "darwinian myths" grottewitz might just as well have entitled it "at the deathbed of darwinism." may he bring out a series of "deathbed articles" to disclose the truth regarding darwinism to his associates. chapter ix. professor fleischmann, zoologist in erlangen, recently published a book bearing the title, "die descendenztheorie," in which he opposes every theory of descent. the book is made up of lectures delivered by the author before general audiences of professional students, hence is popular in form and of very special apologetic value. numerous excellent illustrations aid the reader in understanding the text. one statement in the introduction characterizes the decided position assumed by the author. he says: "after long and careful investigation i have come to the conclusion that the doctrine of descent has not been substantiated. i go even farther and maintain that the discussion of the question does not belong to the field of the exact sciences of zoology and botany." at the outset, fleischmann establishes the fact that in the animal kingdom there are rigidly separated types, which cannot be derived from each other, whereas the doctrine of descent postulates "one single common model of body-structure" from which all types have been developed. cuvier in his day, set up four such types of essentially different structure; when darwin's work appeared two more had been added; r. hertwig postulates even seven, boas nine (both ); j. kennel ( ) seventeen, and fleischmann himself sixteen. in consequence the doctrine of descent has become more complicated since it now embraces sixteen or seventeen different problems, each of which in turn gives rise to many subordinate problems. the discussion which the author inaugurates regarding the domain to which the question of descent belongs, is very well-timed. he forcibly and definitely discountenances the method which transfers it to the domain of religion. the question must be decided by the naturalists themselves according to the strict inductive method; that is, the solution must be based on well ascertained facts, without resorting to conclusions deduced from general principles. "exact research must show that living organisms actually have overstepped the bounds defining their species, and not merely that they conceivably may have done so." hence it is absolutely necessary to procure the intermediary forms. this is the foundation on which fleischmann builds and against which no opponent can prevail. fleischmann first discusses the differences between the classes of vertebrates; the mammals, birds, reptiles, amphibians and fish. for if the differences of their bodily structure could be shown to be one of degree and not radical, it could be supposed that the lines of demarcation which now delimitate the larger types might some day vanish. a single illustration suffices for fleischmann's purpose, viz., the plan of structure of the limbs of the different classes of vertebrates. the four higher classes are characterized by a common underlying plan of limb structure, whilst fish have one peculiar to themselves. on the other hand it is an inevitable postulate of the doctrine of descent that fish are the original progenitors of all other vertebrates. hence the five-joint limbs of the latter must have developed from the fins of fish. this derivation was actually attempted but without success, as fleischmann points out at considerable length. by means of citations taken from the writings of darwinian adherents, he illustrates the confusion which even now reigns among them on this matter. the evolution of the remaining vertebrates from the fish is therefore a wholly gratuitous assumption devoid of any foundation in fact. fleischmann further discusses the "parade-horse" of the theory of descent. it has been the common belief, especially fostered by haeckel, that the history of the descent of our present horse lies before us in its complete integrity as pictured in the drawings of marsh. here fleischmann again proves at great length the insufficiency of actually available materials. of special importance is his repeated demand that not only individual parts of the animals but the whole organism as well should be derived from the earlier forms. if, for instance, it be possible to arrange horses and their tertiary kindred in an unbroken line of descent according to the formation of their feet, whilst the other characteristics (teeth, skull-structure, etc.,) do not admit of arrangement in a corresponding series, the first line must be surrendered. very similar to this is the case of the "family history of birds," which as all know, has been traced back to reptiles. it is in this matter that the famous archaeopteryx plays an important part. unfortunately, however, grave difficulties are again encountered in this connection. this primitive form is a real bird according to zittel; and according to the same investigator as also according to marsh, dames, vetter, parker, tuerbringen, parlow and mehnert, it is inadmissible to connect birds with a definite class of reptiles. haeckel finds his way out of the difficulty by supplying hypothetical forms which no one has ever seen, but which his imagination has admirably depicted as transitional forms. in so doing, however, he abandons the inductive method of natural science. it is impossible for us to treat at such length all the remaining sections of this important book. we may mention in passing that fleischmann examines the "roots of the mammal stock," and enters upon a detailed discussion of "the origin of lung-breathing vertebrates," the "real phylo-genetic problem of the mollusks," and "the origin of the echinodermata." it is evident that he boldly takes up the most important problems connected with the theory of descent, and does not confine himself to a one-sided discussion of individual points. as he did not fear to examine thoroughly the famous, and as it hitherto appeared, invulnerable, "parade-horse," so neither does he hesitate to demolish the other reputed proof for the doctrine of descent, e.g., the fresh-water snail of steinheim, the remains of which hilzendorf and neumayr examined and were said to have arranged in lines of descent that "would actually stagger one." it is important to call especial attention to this because the adversaries of the book ignore it. he next shows up the so-called "fundamental principle of biogenesis" according to which organisms are supposed to repeat during their individual development the forms of their progenitors (enunciated by fritz mueller and haeckel). fleischmann points out the exceptions which haeckel attributes to "cenogenesis," (that is to falsification) and shows the disagreement among contemporary naturalists regarding this fundamental principle. even haeckel's friend and pupil, o. hertwig sounds the retreat. the th chapter deals with the "collapse of haeckel's doctrine," which is revealed in the fact that "the practical possibility of ascertaining anything regarding the primitive history of the animal kingdom is completely exhausted and the hope of so doing forever frustrated." "instead of scientists having been able from year to year to produce an increasing abundance of proof for the correctness of the doctrine of descent, the lack of proofs and the impossibility of procuring evidence is to-day notorious." in the last chapter fleischmann finally attempts to prove on logical principles the untenableness of the evolutionary idea. he starts from the fact that philosophers use the word development to designate a definite sequence of ideas, i.e., in a logical order. "metamorphosis, says hegel, belongs to the idea as such since its variation alone is development. rational speculation must get rid of such nebulous concepts as the evolution of the more highly developed animal organisms from the less developed, etc." naturalists use the word in a different sense. instead of a sequence of grades of being they posit a sequence of transformations; instead of a logical sequence of ideas they posit a transforming and progressive development. zoology constructs a system of specific and generic concepts, "an animal kingdom with logical relations." our concepts are derived from natural objects, but in reality do not perfectly correspond to them. the phylogenetic school commits the capital mistake of presenting a transformation which can be realized only in logical concepts, as an actually occurring process, and of confounding an abstract operation with concrete fact. "the logical transformation of the concept ape into the concept man is no genealogical process." the mathematician may logically 'develop' the concept of a circle from that of a polygon, but it by no means follows that the circle is phylo-genetically derived from the polygon. because the concept of species is variable, the species themselves, according to darwin, should be subject to a continual flux; whereas the real cause of the variability which he observed lies in the discrepancy between objective facts and their logical tabulation, in the narrowness of our concepts and in the lack of adequate means of expression. he thus makes natural objects responsible for our logical limitations. with regard to organisms the descent-school confounded the purely logical signification of the word "related" with that of blood or family affinity. but surely when they speak of the relation of forms in the crystal systems, they do not refer to genetic connection. to-day this interchange of concepts is so general that one needs to exercise great care if one would avoid it. the theory which postulates the blood-relationship of individuals of the same species may be correct, but it is utterly incapable of proof, and the same is true in a greater degree when there is question of individuals of the same class but of different species. since a direct proof is impossible, an attempt was made to construct an indirect proof by a comparison of bodily-organs. but in so doing the descent theorizers had to relinquish scientific analysis altogether. in conclusion fleischmann states that he does not mean to discard every hypothesis of descent. he simply gives warning against an over-estimation of the theory. in opposition to those who esteem it as the highest achievement of science, he looks upon it as a necessary evil. its proper sphere is the laboratory of the man of science, and not the thronging market-place. "the descent hypothesis will meet the same fate (be cast aside), since its incompatibility with facts of ordinary observation is manifesting itself. at the time of its appearance in a new form, forty years ago, it exercised a beneficial influence on scientific progress and induced a great number of capable minds to devote themselves to the study of anatomical, palaeontological and evolutionary problems. meanwhile, however, viewed in the light of a constantly increasing wealth of actual materials, the hypothesis has become antiquated and the labors of its industrious advocates makes it obvious to unbiased critics, that it is time to relegate it ad acta." * * * * * * * my own views agree with those of fleischmann as presented above, except in regard to his last chapter. i must, of course, admit that his criticism has discredited the doctrine of descent as a scientifically established theory. hence, as i have always asserted, it must be excluded from the realm of exact science. no doubt people will come gradually to see that the theory involves a creed and therefore belongs to the domain of cosmic philosophy. all this i readily admit. not so, however, as regards the concept of "development." it seems to me to be incorrect to regard this as a logical concept only, even with reference to organisms. true, the whole zoological system is in reality nothing more than a logical abstraction. and in view of this fact one must be on one's guard against confusing a logical transformation of concepts with a genealogical development. we must, however, not forget that we possess the wonderful analogy of ontogeny (individual development) and above all, the fact of mutation and of metagenesis. and even if we wish to avoid the error of haeckel and others who find a necessary connection between ontogeny and phylogeny, nevertheless the analogy will still entitle us to picture to ourselves the development of the whole range of living organisms. such a representation will, of course, have only a subjective value. no doubt, it is logically unjustifiable to argue from the variable concept to the variability of the species. still there is something real in plants and animals which corresponds to our specific concepts. in some cases the corresponding reality may be so well defined that it is not difficult to form the concept accurately; whereas in other cases where the task is more difficult, the difficulty must be due to the object. under these circumstances we may safely conclude from the lack of definiteness in our concepts to a certain lack of rigid delimitation in the organic forms. this blending of certain forms suggests the idea of transformation, but does not furnish definite proof of it. such proof can be had only by the direct observation of a transformation. and no doubt in certain cases a transformation may occur. as regards animals, i may call attention, for instance, to the experiments made with butterflies by standfuss, and as regards plants, to the experiments of haberlandt, of which i treated in chapter iii. the limits within which these transformations take place are indeed very narrow as are also the limits of those indisputable varieties which naturally arise within an otherwise rigidly defined species. i am aware that the transformation of one species into another has not yet been effected, but the above-mentioned attempts at transformation have nevertheless demonstrated that certain organic forms when subjected to changed conditions of life, display certain mutations which clearly show that variability is to be attributed, not, certainly, to the specific concepts, but to the corresponding reality. this observation and reflexion, joined with the fact that organisms form a progressive series from the simple to the more complex, and with the observed phenomena of individual development, lead me to regard the concept of descent as admissible, and in a certain sense, even probable. but i agree with fleischmann in saying that this is a mere belief, and that all attempts to give it a higher scientific value by inductive proof have signally failed. my standpoint, moreover, requires me to admit the validity of the hypothesis of descent as an heuristic maxim of natural science. i believe that we shall be justified in the future, as we were forty years ago, in directing our investigation in the direction of descent, and i do not consider such investigation so utterly hopeless as fleischmann represents it. however, i entirely concur with him in the opinion that we are here concerned (and shall be for a long time to come) with a mere hypothesis which belongs not in the market-place, nor among the world views of the multitude, but in the study of the man of science. above all it must not be mixed up with religious questions. whether the hypothesis will ever emerge from the study of the man of science as a well-attested law, is still an open question, incapable of immediate solution. * * * * * * * it is of interest for us to inquire what reception fleischmann's protest against the theory of descent has been accorded by his associates. fleischmann was formerly an advocate of the theory of descent. he was a pupil and assistant of selenka, who was then at erlangen (died in muenster ). he had previously written a number of scientific works from the standpoint of the descent theory. in the year , investigations regarding rodents led him to oppose that theory. during the winter term of - he gave evidence of this change in a public lecture. not until was there question of his appointment to the chair of zoology in erlangen. in he published a manual of zoology based on principles radically opposed to the doctrine of descent. this manual irritated haeckel so much that he issued one of his well-known articles, _ascending and descending zoology_, in which, after his usual manner, he casts suspicion on fleischmann of having received his appointment to the chair at erlangen by becoming an anti-darwinian in accordance with a desire expressed at the diet of bavaria. i am not aware that haeckel has paid any attention to the work of fleischmann which we have just reviewed. by its publication, however, the author disturbed a hornet's nest. dispassionate, but still entirely adverse is professor plate's review in the "biologisches zentralblatt," while the "umschau" publishes two criticisms, one by professor von wagner, the other by dr. reh, which for want of sense could not well be equalled. it was the former who furnished material for our sixth chapter and who there displayed such utter confusion of thought regarding the inductive method. the same confusion is apparent in his recent utterance in which he observes that fleischmann's whole aim is to accumulate observational data, meanwhile avoiding speculation as far as possible. his criticism is replete with bitter personal epithets, e.g., "reactionary," "mental incompetency," "dishonest mask of hypercritical exactness," which manifest the writer's inability to enter upon an objective discussion of the question. a still more reprehensible position is assumed by dr. reh, who censures fleischmann for introducing to the general public the question of descent which belongs properly to the forum of science. he claims that fleischmann, by so doing, forfeited his right to an unbiased hearing. dr. reh forgets that but a short time ago he had no word of censure for haeckel's _weltraetsel_ which was intended for a far wider circle of readers. he next appropriates haeckel's suspicion regarding fleischmann which we noticed above, and then adds the entirely untrue assertion that the first half of fleischmann's manual, written before he took possession of the chair in erlangen, is written in the spirit of darwin, whereas the second half which appeared at a later date is written in the contrary spirit. he then takes individual points of fleischmann's treatise out of their context in order to execute a cheap and nonsensical criticism of them. haeckel has evidently been giving instructions on the best manner of dealing with adversaries. and very docile disciples they are who imitate his method even to the extent of defaming and abusing their scientific opponents. but is not this another plain indication of the decay of darwinism? of course haeckel recognized at the very beginning of his career that it was necessary to support the theory by means of personal bitterness, forgeries and misrepresentations. but if the last surviving advocates of darwinism must needs have recourse to the same disreputable means, to what a low estate, indeed, has it fallen! let us hope that these last wild convulsions are really the signs of approaching dissolution. chapter x. in order to judge of the present status of darwinism it is of primary importance to note the position assumed by the few really eminent investigators, who as pupils of haeckel still seem to have remained true to him. among these i reckon oskar hertwig, the well known berlin anatomist. as early as in an address at the university on, _die lehre vom organismus und ihre beziehung zur sozialwissenschaft_, hertwig gave expression to views which are very little in harmony with the doctrines proceeding from jena, and which are also put forth in his manual, _the cell and the tissue_. in that address we read (p. ): "with the same right, with which, for the good of scientific progress, an energetic protest has been raised against a certain mysticism which attaches to the word vitality, i beg to give warning against an opposite extreme which is but too apt to lead to onesided and unreal, and hence also, ultimately to false notions of the vital process, against an extreme which would see in the vital process nothing but a chemico-physical and mechanical problem and thinks to arrive at true scientific knowledge only in so far as it succeeds in tracing back phenomena to the movements of repelling and attracting atoms and in subjecting them to mathematical calculation." with right does the physicist mach, with reference to such views and tendencies, speak of a 'mechanical mythology in opposition to the animistic mythology of the old religions' and considers both as "improper and fantastic exaggerations based on a one-sided judgment." "my position on the question just stated becomes apparent from the consideration that the living organism is not only a complex of chemical materials and a bearer of physical forces, but also possesses a special organization, a structure, by means of which it is very essentially differentiated from the inorganic world, and in virtue of which it alone is designated as living." here, then, the distinction between living and non-living nature is clearly and definitely expressed, and hertwig expresses himself just as definitely when he says (p. ): "whereas, but a few decades ago a scientific materialistic conception of the world issuing from a onesided, unhistorical point of view, misjudged the significance of the historic religious and ethical forces in the development of mankind, a change has become apparent in this regard." to this gratifying testimony against materialism the distinguished naturalist added an equally valuable testimony regarding darwinism on the occasion of the naturalists' convention in . he there sketched an excellent summary of the "development of biology in the nineteenth century," in which he decidedly opposes the materialistic-mechanical conception of life. in so doing he also touches upon haeckel's carbon-hypothesis, to which the latter still clings, and says: "that from the properties of carbon, combined with the properties of oxygen, hydrogen, nitrogen, etc., in certain proportions albumen should result, is a process which in its essence is as incomprehensible as that a living cell should arise from a certain organization of different albumina." then the speaker is inevitably led to speak of the doctrine of descent and darwinism. in the first place he declares definitely that ontogeny alone, i.e., the development of the individual being, is "capable of a direct scientific investigation." on the other hand we move in the domain of hypotheses in dealing with the further question: "how have the species of organisms living to-day originated in the course of the world's history?" this is a very valuable admission in view of haeckel's dogmatic assertion that the descent of man from the ape is a "certain historical fact." very moderate and pertinent are also the further words of the speaker: "of course, a philosophically trained investigator will regard it as axiomatic that the organisms which inhabit our earth to-day did not exist in their present form in earlier periods of the earth and that they had to pass through a process of development, beginning with the simplest forms." "but in the attempt to outline in detail the particular form in which a species of animals of our day existed in remote antiquity, we lose the safe ground of experience. for out of the countless millions of organisms, that lived in earlier periods of the earth, the duration of which is measured by millions of years, only scanty skeleton remains have by way of exception been preserved in a fossil state. from these naturally but a very imperfect and hypothetical representation can be formed of the soft bodies with which they were once clothed. and even then it remains forever doubtful whether the progeny of the prehistoric creature, the scant remains of which we study, has not become entirely extinct, so that it can in no way be regarded as the progenitor of any creature living at present." i should like to know wherein this differs radically from fleischmann's contention in his "descendenztheorie" (p. .) for we find stated here what fleischmann emphasizes so much, viz., that with the problem of descent we leave the domain of experience. it is worthy of special note in this connection that hertwig likewise evidently regards as the sole really empirically and inductively serviceable proof of descent, that which is drawn from palaeontology, from prehistoric animal and plant remains. he makes not the least mention of the indirect proofs taken from ontogenetic development or comparative anatomy, to which the darwinians and advocates of descent love so much to appeal, because they feel that the real inductive proof is lacking and totally fails to sustain their position. hertwig next points out that the problem of descent stirred scientific as well as lay circles twice during the past century. he then pays lamarck and darwin the necessary tribute, at which we cannot take offense since he was reared in the darwinian atmosphere of jena. i also willingly admit that darwinism served science as a "powerful ferment," even if i must emphasize just as decidedly how harmful it was that this "ferment" was introduced into lay circles at an unseasonable time by the apostles of materialism. for while it was very well adapted to bring about in educated circles a fermentation which produced beneficial results, in uncritical lay-circles this ferment produced nothing but a corruption of world-views. hertwig then designates "struggle for existence," survival of the fittest, and selection, as "very indefinite expressions." "with too general terms, one does not explain the individual case or produces only the appearance of an explanation whereas in every case the true causative relations remain in the dark. but it is the duty of scientific investigation to establish for each observed effect the prevenient cause, or more correctly, since nothing results from a single cause, to discover the various causes." "the origin of the world of organisms from natural causes, however, is certainly an unusually complicated and difficult problem. it is just as little capable of being solved by a single magic formula as every disease is of yielding to a panacea. by the very act of proclaiming the omnipotence of natural selection, weismann found he was forced to the admission that: "as a rule we cannot furnish the proof that a definite adaptation has originated through natural selection," in other words: we know nothing in reality of the complexity of causes which has produced the given phenomenon. so we may on the contrary, with spencer, speak of the "impotence of natural selection."" "in this scientific struggle with which the past century closed, it seems necessary to distinguish between the doctrine of evolution and the theory of selection. they are based on entirely different principles. for with huxley we can say: "even if the darwinian hypothesis were blown away, the doctrine of evolution would remain standing where it stood." in it we possess an acquisition of our century which rests on facts, and which undoubtedly ranks amongst its greatest." this last sentence affirms exactly what i have repeatedly asserted: the doctrine of descent remains, darwinism passes away. hertwig then is decidedly of opinion that darwinism entirely fails in the individual case because in its application the basis of experience vanishes. indeed, according to him, phylogeny is not at all capable of direct scientific investigation. these are all important admissions which one would certainly have considered impossible twenty years ago; they unequivocally indicate the decline of darwinian views, and in a certain way also harmonize with fleischmann's work. true, hertwig still clings to the thought of descent, but apparently no longer as to a conclusion of natural science. this appears from the assertion: "ontogeny alone is capable of a direct scientific (he evidently speaks of natural science) investigation," and from the other statement that a _philosophically_ trained investigator will accept it (descent) as axiomatic although it belongs to the domain of hypothesis. what else does this mean but that: we have no specific knowledge of descent but we believe in it. in short, this is not natural science but natural philosophy; it forms no constituent part of our certain knowledge of nature but it is one aspect of our world-view. all the above-quoted assertions of hertwig are calm and well-considered and show a decided deviation from the darwinian position. above all we are pleased to note that he appropriates spencer's phrase regarding the "impotence of natural selection" and that in the citation from huxley he at least admits the possibility that the darwinian doctrine will be "wafted away." it is also proper to mention here the fact that in another place hertwig no longer recognizes so fully the dogma set up by fritz mueller and haeckel which is so closely bound up with darwinism. i mean the so-called "biogenetic principle" according to which the individual organism is supposed to repeat in its development the development of the race during the course of ages. in his book: "the cell and the tissue" (die zelle und die gewebe, ii. jena , p. ) hertwig says: "we must drop the expression: 'repetition of forms of extinct ancestors' and employ instead: repetition of forms which accord with the laws of organic development and lead from the simple to the complex. we must lay special emphasis on the point that in the embryonic forms even as in the developed animal forms general laws of the development of the organized body-substance find expression." any one can subscribe to these statements; in truth they contain something totally different from the "biogenetic principle"; for haeckel has really no interest in so general a truth, but is intent only upon a proof of descent. hertwig continues: "in order to make our train of thought clear, let us take the egg-cell. since the development of every organism begins with it, the primitive condition is in no way recapitulated from the time when perhaps only single-celled amoebas existed on our planet. for according to our theory the egg-cell, for instance, of a now extant mammal is no simple and indifferent, purposeless structure, as it is often represented, (as according to haeckel's "biogenetic principle" it would necessarily be); we see in it, in fact, the extraordinarily complex end-product of a very long historic process of development, through which the organic substance has passed since that hypothetical epoch of single-celled organisms." "if the eggs of a mammal now differ very essentially from those of a reptile and of an amphibian because in their organization they represent the beginnings only of mammals, even as these represent only the beginnings of reptiles and amphibians, by how much more must they differ from those hypothetical single-celled amoebas which could as yet show no other characteristics than to reproduce amoebas of their own kind." this is a view which has frequently been clearly expressed by anti-darwinians: the egg-cells of the various animals are in themselves fundamentally different and can therefore have nothing in common but similarity of structure. in opposition to hertwig, haeckel in his superficial way deduces from it an internal similarity as well. after a few polite bows before his old teacher, haeckel, hertwig thus summarizes his view: "ontogenetic (that is, those stages in the individual development) stages therefore give us only a greatly changed picture of the phylogenetic (i.e., genealogical) stages as they may once have existed in primitive ages, but do not correspond to them in their actual content." this is a very resigned position, very far removed from haeckel's certainty and orthodoxy. to sum up: o. hertwig has become a serious heretic in matters darwinian. will haeckel, in his usual manner try to cast suspicion on hertwig also? for haeckel himself says (free science and free doctrine, stuttgart, , p. ): "since i am not bound by fear to the berlin tribunal of science or by anxieties regarding the loss of influential berlin connections, as are most of my like-minded colleagues, i do not hesitate here as elsewhere to express my honest conviction, frankly and freely, regardless of the anger which perhaps real or pretended privy councillors in berlin may feel upon hearing the unadorned truth." verily, it is a matter of suspense to know whether his school will now pour forth their wrath upon o. hertwig, or whether finally the discovery will not be made in jena that hertwig secretly possessed himself of his position in berlin, in the same manner as fleischmann obtained his at erlangen, viz., by a promise of desertion from darwinism. conclusion. we may conveniently summarize what we have said in the foregoing chapters in the following statement: the theory of descent is almost universally recognized to-day by naturalists as a working hypothesis. still, in spite of assertions to the contrary, no conclusive proof of it has as yet been forthcoming. nevertheless it cannot be denied that the theory provides us with an intelligible explanation of a series of problems and facts which cannot be so well explained on other grounds. on the other hand, darwinism, i.e., the theory of natural selection by means of the struggle for existence, is being pushed to the wall all along the line. the bulk of naturalists no longer recognizes its validity, and even those who have not yet entirely discarded it, are at least forced to admit that the darwinian explanation now possesses a very subordinate significance. in the place of darwinian principles, new ideas are gradually winning general acceptance, which, while they are in harmony with the principles of adaptation and use, (lamarck) enunciated before the time of darwin, nevertheless attribute a far-reaching importance to _internal forces of development_. these new conceptions necessarily involve the admission that _evolution has not been a purely mechanical process_. the book of the day _science and christianity_ _by f. bettex_ _translated from the german_ the author among other things says in the preface: i wish to make clear to my readers how little real science is hidden behind the fine phrases and sounding words or the infidel, and how little he himself understands of the material creation which he affirms to be the only one.... the christian and biblical conception of the universe is more logical, more harmonious, more in accordance with facts, therefore, more scientific than all philosophies, all systems, materialistic and atheistic. contents of the book: chapter i. progress chapter ii. evolution and modern science chapter iii. christians and science chapter iv. science chapter v. materialism one of the many favorable reviews: it is a view of much scope, and so far as it attempts reconciliation between science and christianity, is eminently successful. there can be no doubt that at present, when there is so pronounced a disposition to follow every fad in science, especially if it opposes the bible, such a book should have a wide reading and is adapted to accomplish much good. _price $ . _ german literary board, burlington, iowa transcriber's note: a few typographical errors have been corrected: they are listed at the end of the text. * * * * * on the origin of species. * * * * * "but with regard to the material world, we can at least go so far as this--we can perceive that events are brought about not by insulated interpositions of divine power, exerted in each particular case, but by the establishment of general laws." whewell: _bridgewater treatise_. "the only distinct meaning of the word 'natural' is _stated_, _fixed_, or _settled_; since what is natural as much requires and presupposes an intelligent agent to render it so, _i.e._ to effect it continually or at stated times, as what is supernatural or miraculous does to effect it for once." butler: _analogy of revealed religion_. "to conclude, therefore, let no man out of a weak conceit of sobriety, or an ill-applied moderation, think or maintain, that a man can search too far or be too well studied in the book of god's word, or in the book of god's works; divinity or philosophy; but rather let men endeavour an endless progress or proficience in both." bacon: _advancement of learning_. * * * * * _down, bromley, kent,_ _october st, ._ (_ st thousand_). * * * * * on the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. by charles darwin, m.a., fellow of the royal, geological, linnean, etc., societies; author of 'journal of researches during h. m. s. beagle's voyage round the world.' _fifth thousand._ london: john murray, albemarle street. . _the right of translation is reserved._ * * * * * london: printed by w. clowes and sons, stamford street, and charing cross. * * * * * {v} contents. * * * * * introduction page chapter i. variation under domestication. causes of variability--effects of habit--correlation of growth--inheritance--character of domestic varieties--difficulty of distinguishing between varieties and species--origin of domestic varieties from one or more species--domestic pigeons, their differences and origin--principle of selection anciently followed, its effects--methodical and unconscious selection--unknown origin of our domestic productions--circumstances favourable to man's power of selection - chapter ii. variation under nature. variability--individual differences--doubtful species--wide ranging, much diffused, and common species vary most--species of the larger genera in any country vary more than the species of the smaller genera--many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges - {vi} chapter iii. struggle for existence. its bearing on natural selection--the term used in a wide sense--geometrical powers of increase--rapid increase of naturalised animals and plants--nature of the checks to increase--competition universal--effects of climate--protection from the number of individuals--complex relations of all animals and plants throughout nature--struggle for life most severe between individuals and varieties of the same species; often severe between species of the same genus--the relation of organism to organism the most important of all relations - chapter iv. natural selection. natural selection--its power compared with man's selection--its power on characters of trifling importance--its power at all ages and on both sexes--sexual selection--on the generality of intercrosses between individuals of the same species--circumstances favourable and unfavourable to natural selection, namely, intercrossing, isolation, number of individuals--slow action--extinction caused by natural selection--divergence of character, related to the diversity of inhabitants of any small area, and to naturalisation--action of natural selection, through divergence of character and extinction, on the descendants from a common parent--explains the grouping of all organic beings - chapter v. laws of variation. effects of external conditions--use and disuse, combined with natural selection; organs of flight and of vision--acclimatisation--correlation of growth--compensation and economy of growth--false correlations--multiple, rudimentary, and lowly organised structures variable--parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable--species of the same genus vary in an analogous manner--reversions to long-lost characters--summary - {vii} chapter vi. difficulties on theory. difficulties on the theory of descent with modification--transitions--absence or rarity of transitional varieties--transitions in habits of life--diversified habits in the same species--species with habits widely different from those of their allies--organs of extreme perfection--means of transition--cases of difficulty--natura non facit saltum--organs of small importance--organs not in all cases absolutely perfect--the law of unity of type and of the conditions of existence embraced by the theory of natural selection - chapter vii. instinct. instincts comparable with habits, but different in their origin--instincts graduated--aphides and ants--instincts variable--domestic instincts, their origin--natural instincts of the cuckoo, ostrich, and parasitic bees--slave-making ants--hive-bee, its cell-making instinct--difficulties on the theory of the natural selection of instincts--neuter or sterile insects--summary - chapter viii. hybridism. distinction between the sterility of first crosses and of hybrids--sterility various in degree, not universal, affected by close interbreeding, removed by domestication--laws governing the sterility of hybrids--sterility not a special endowment, but incidental on other differences--causes of the sterility of first crosses and of hybrids--parallelism between the effects of changed conditions of life and crossing--fertility of varieties when crossed and of their mongrel offspring not universal--hybrids and mongrels compared independently of their fertility--summary - {viii} chapter ix. on the imperfection of the geological record. on the absence of intermediate varieties at the present day--on the nature of extinct intermediate varieties; on their number--on the vast lapse of time, as inferred from the rate of deposition and of denudation--on the poorness of our palæontological collections--on the intermittence of geological formations--on the absence of intermediate varieties in any one formation--on the sudden appearance of groups of species--on their sudden appearance in the lowest known fossiliferous strata - chapter x. on the geological succession of organic beings. on the slow and successive appearance of new species--on their different rates of change--species once lost do not reappear--groups of species follow the same general rules in their appearance and disappearance as do single species--on extinction--on simultaneous changes in the forms of life throughout the world--on the affinities of extinct species to each other and to living species--on the state of development of ancient forms--on the succession of the same types within the same areas--summary of preceding and present chapters - chapter xi. geographical distribution. present distribution cannot be accounted for by differences in physical conditions--importance of barriers--affinity of the productions of the same continent--centres of creation--means of dispersal, by changes of climate and of the level of the land, and by occasional means--dispersal during the glacial period co-extensive with the world - chapter xii. geographical distribution--_continued_. distribution of fresh-water productions--on the inhabitants of oceanic islands--absence of batrachians and of terrestrial mammals--on the relation of the inhabitants of islands to those of the nearest mainland--on colonisation from the nearest source with subsequent modification--summary of the last and present chapters - chapter xiii. mutual affinities of organic beings: morphology: embryology: rudimentary organs. classification, groups subordinate to groups--natural system--rules and difficulties in classification, explained on the theory of descent with modification--classification of varieties--descent always used in classification--analogical or adaptive characters--affinities, general, complex and radiating--extinction separates and defines groups--morphology, between members of the same class, between parts of the same individual--embryology, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age--rudimentary organs; their origin explained--summary - chapter xiv. recapitulation and conclusion. recapitulation of the difficulties on the theory of natural selection--recapitulation of the general and special circumstances in its favour--causes of the general belief in the immutability of species--how far the theory of natural selection may be extended--effects of its adoption on the study of natural history--concluding remarks - * * * * * { } on the origin of species. * * * * * introduction. when on board h.m.s. 'beagle,' as naturalist, i was much struck with certain facts in the distribution of the inhabitants of south america, and in the geological relations of the present to the past inhabitants of that continent. these facts seemed to me to throw some light on the origin of species--that mystery of mysteries, as it has been called by one of our greatest philosophers. on my return home, it occurred to me, in , that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. after five years' work i allowed myself to speculate on the subject, and drew up some short notes; these i enlarged in into a sketch of the conclusions, which then seemed to me probable: from that period to the present day i have steadily pursued the same object. i hope that i may be excused for entering on these personal details, as i give them to show that i have not been hasty in coming to a decision. my work is now nearly finished; but as it will take me two or three more years to complete it, and as my health is far from strong, i have been urged to publish this abstract. i have more especially been induced to do this, as mr. wallace, who is now studying the { } natural history of the malay archipelago, has arrived at almost exactly the same general conclusions that i have on the origin of species. last year he sent me a memoir on this subject, with a request that i would forward it to sir charles lyell, who sent it to the linnean society, and it is published in the third volume of the journal of that society. sir c. lyell and dr. hooker, who both knew of my work--the latter having read my sketch of --honoured me by thinking it advisable to publish, with mr. wallace's excellent memoir, some brief extracts from my manuscripts. this abstract, which i now publish, must necessarily be imperfect. i cannot here give references and authorities for my several statements; and i must trust to the reader reposing some confidence in my accuracy. no doubt errors will have crept in, though i hope i have always been cautious in trusting to good authorities alone. i can here give only the general conclusions at which i have arrived, with a few facts in illustration, but which, i hope, in most cases will suffice. no one can feel more sensible than i do of the necessity of hereafter publishing in detail all the facts, with references, on which my conclusions have been grounded; and i hope in a future work to do this. for i am well aware that scarcely a single point is discussed in this volume on which facts cannot be adduced, often apparently leading to conclusions directly opposite to those at which i have arrived. a fair result can be obtained only by fully stating and balancing the facts and arguments on both sides of each question; and this cannot possibly be here done. i much regret that want of space prevents my having the satisfaction of acknowledging the generous assistance which i have received from very many naturalists, some of them personally unknown to me. i cannot, however, { } let this opportunity pass without expressing my deep obligations to dr. hooker, who for the last fifteen years has aided me in every possible way by his large stores of knowledge and his excellent judgment. in considering the origin of species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties, from other species. nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified, so as to acquire that perfection of structure and coadaptation which most justly excites our admiration. naturalists continually refer to external conditions, such as climate, food, &c., as the only possible cause of variation. in one very limited sense, as we shall hereafter see, this may be true; but it is preposterous to attribute to mere external conditions, the structure, for instance, of the woodpecker, with its feet, tail, beak, and tongue, so admirably adapted to catch insects under the bark of trees. in the case of the misseltoe, which draws its nourishment from certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by the effects of external conditions, or of habit, or of the volition of the plant itself. the author of the 'vestiges of creation' would, i presume, say that, after a certain unknown number of { } generations, some bird had given birth to a woodpecker, and some plant to the missletoe, and that these had been produced perfect as we now see them; but this assumption seems to me to be no explanation, for it leaves the case of the coadaptations of organic beings to each other and to their physical conditions of life, untouched and unexplained. it is, therefore, of the highest importance to gain a clear insight into the means of modification and coadaptation. at the commencement of my observations it seemed to me probable that a careful study of domesticated animals and of cultivated plants would offer the best chance of making out this obscure problem. nor have i been disappointed; in this and in all other perplexing cases i have invariably found that our knowledge, imperfect though it be, of variation under domestication, afforded the best and safest clue. i may venture to express my conviction of the high value of such studies, although they have been very commonly neglected by naturalists. from these considerations, i shall devote the first chapter of this abstract to variation under domestication. we shall thus see that a large amount of hereditary modification is at least possible; and, what is equally or more important, we shall see how great is the power of man in accumulating by his selection successive slight variations. i will then pass on to the variability of species in a state of nature; but i shall, unfortunately, be compelled to treat this subject far too briefly, as it can be treated properly only by giving long catalogues of facts. we shall, however, be enabled to discuss what circumstances are most favourable to variation. in the next chapter the struggle for existence amongst all organic beings throughout the world, which inevitably follows from the high geometrical ratio of their { } increase, will be treated of. this is the doctrine of malthus, applied to the whole animal and vegetable kingdoms. as many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be _naturally selected_. from the strong principle of inheritance, any selected variety will tend to propagate its new and modified form. this fundamental subject of natural selection will be treated at some length in the fourth chapter; and we shall then see how natural selection almost inevitably causes much extinction of the less improved forms of life, and leads to what i have called divergence of character. in the next chapter i shall discuss the complex and little known laws of variation and of correlation of growth. in the four succeeding chapters, the most apparent and gravest difficulties on the theory will be given: namely, first, the difficulties of transitions, or in understanding how a simple being or a simple organ can be changed and perfected into a highly developed being or elaborately constructed organ; secondly, the subject of instinct, or the mental powers of animals; thirdly, hybridism, or the infertility of species and the fertility of varieties when intercrossed; and fourthly, the imperfection of the geological record. in the next chapter i shall consider the geological succession of organic beings throughout time; in the eleventh and twelfth, their geographical distribution throughout space; in the thirteenth, their classification or mutual affinities, both when mature and in an embryonic condition. in the last chapter i shall give a { } brief recapitulation of the whole work, and a few concluding remarks. no one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and varieties, if he makes due allowance for our profound ignorance in regard to the mutual relations of all the beings which live around us. who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare? yet these relations are of the highest importance, for they determine the present welfare, and, as i believe, the future success and modification of every inhabitant of this world. still less do we know of the mutual relations of the innumerable inhabitants of the world during the many past geological epochs in its history. although much remains obscure, and will long remain obscure, i can entertain no doubt, after the most deliberate study and dispassionate judgment of which i am capable, that the view which most naturalists entertain, and which i formerly entertained--namely, that each species has been independently created--is erroneous. i am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. furthermore, i am convinced that natural selection has been the main but not exclusive means of modification. * * * * * { } chapter i. variation under domestication. causes of variability--effects of habit--correlation of growth--inheritance--character of domestic varieties--difficulty of distinguishing between varieties and species--origin of domestic varieties from one or more species--domestic pigeons, their differences and origin--principle of selection anciently followed, its effects--methodical and unconscious selection--unknown origin of our domestic productions--circumstances favourable to man's power of selection. when we look to the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us, is, that they generally differ more from each other than do the individuals of any one species or variety in a state of nature. when we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treatment, i think we are driven to conclude that this great variability is simply due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent-species have been exposed under nature. there is also, i think, some probability in the view propounded by andrew knight, that this variability may be partly connected with excess of food. it seems pretty clear that organic beings must be exposed during several generations to the new conditions of life to cause any appreciable amount of variation; and that when the organisation has once begun to vary, it generally continues to vary for many generations. { } no case is on record of a variable being ceasing to be variable under cultivation. our oldest cultivated plants, such as wheat, still often yield new varieties: our oldest domesticated animals are still capable of rapid improvement or modification. it has been disputed at what period of life the causes of variability, whatever they may be, generally act; whether during the early or late period of development of the embryo, or at the instant of conception. geoffroy st. hilaire's experiments show that unnatural treatment of the embryo causes monstrosities; and monstrosities cannot be separated by any clear line of distinction from mere variations. but i am strongly inclined to suspect that the most frequent cause of variability may be attributed to the male and female reproductive elements having been affected prior to the act of conception. several reasons make me believe in this; but the chief one is the remarkable effect which confinement or cultivation has on the function of the reproductive system; this system appearing to be far more susceptible than any other part of the organisation, to the action of any change in the conditions of life. nothing is more easy than to tame an animal, and few things more difficult than to get it to breed freely under confinement, even in the many cases when the male and female unite. how many animals there are which will not breed, though living long under not very close confinement in their native country! this is generally attributed to vitiated instincts; but how many cultivated plants display the utmost vigour, and yet rarely or never seed! in some few such cases it has been discovered that very trifling changes, such as a little more or less water at some particular period of growth, will determine whether or not the plant sets a seed. i cannot here enter on the copious details which i have collected on { } this curious subject; but to show how singular the laws are which determine the reproduction of animals under confinement, i may just mention that carnivorous animals, even from the tropics, breed in this country pretty freely under confinement, with the exception of the plantigrades or bear family; whereas carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs. many exotic plants have pollen utterly worthless, in the same exact condition as in the most sterile hybrids. when, on the one hand, we see domesticated animals and plants, though often weak and sickly, yet breeding quite freely under confinement; and when, on the other hand, we see individuals, though taken young from a state of nature, perfectly tamed, long-lived, and healthy (of which i could give numerous instances), yet having their reproductive system so seriously affected by unperceived causes as to fail in acting, we need not be surprised at this system, when it does act under confinement, acting not quite regularly, and producing offspring not perfectly like their parents. sterility has been said to be the bane of horticulture; but on this view we owe variability to the same cause which produces sterility; and variability is the source of all the choicest productions of the garden. i may add, that as some organisms will breed freely under the most unnatural conditions (for instance, the rabbit and ferret kept in hutches), showing that their reproductive system has not been thus affected; so will some animals and plants withstand domestication or cultivation, and vary very slightly--perhaps hardly more than in a state of nature. a long list could easily be given of "sporting plants;" by this term gardeners mean a single bud or offset, which suddenly assumes a new and sometimes very different character from that of the rest of the plant. { } such buds can be propagated by grafting, &c., and sometimes by seed. these "sports" are extremely rare under nature, but far from rare under cultivation; and in this case we see that the treatment of the parent has affected a bud or offset, and not the ovules or pollen. but it is the opinion of most physiologists that there is no essential difference between a bud and an ovule in their earliest stages of formation; so that, in fact, "sports" support my view, that variability may be largely attributed to the ovules or pollen, or to both, having been affected by the treatment of the parent prior to the act of conception. these cases anyhow show that variation is not necessarily connected, as some authors have supposed, with the act of generation. seedlings from the same fruit, and the young of the same litter, sometimes differ considerably from each other, though both the young and the parents, as müller has remarked, have apparently been exposed to exactly the same conditions of life; and this shows how unimportant the direct effects of the conditions of life are in comparison with the laws of reproduction, of growth, and of inheritance; for had the action of the conditions been direct, if any of the young had varied, all would probably have varied in the same manner. to judge how much, in the case of any variation, we should attribute to the direct action of heat, moisture, light, food, &c., is most difficult: my impression is, that with animals such agencies have produced very little direct effect, though apparently more in the case of plants. under this point of view, mr. buckman's recent experiments on plants are extremely valuable. when all or nearly all the individuals exposed to certain conditions are affected in the same way, the change at first appears to be directly due to such conditions; but in some cases it can be shown that quite opposite conditions produce { } similar changes of structure. nevertheless some slight amount of change may, i think, be attributed to the direct action of the conditions of life--as, in some cases, increased size from amount of food, colour from particular kinds of food or from light, and perhaps the thickness of fur from climate. habit also has a decided influence, as in the period of flowering with plants when transported from one climate to another. in animals it has a more marked effect; for instance, i find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild-duck; and i presume that this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parent. the great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with the state of these organs in other countries, is another instance of the effect of use. not a single domestic animal can be named which has not in some country drooping ears; and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable. there are many laws regulating variation, some few of which can be dimly seen, and will be hereafter briefly mentioned. i will here only allude to what may be called correlation of growth. any change in the embryo or larva will almost certainly entail changes in the mature animal. in monstrosities, the correlations between quite distinct parts are very curious; and many instances are given in isidore geoffroy st. hilaire's great work on this subject. breeders believe that long limbs are almost always accompanied by an elongated head. some instances of correlation are quite whimsical: thus { } cats with blue eyes are invariably deaf; colour and constitutional peculiarities go together, of which many remarkable cases could be given amongst animals and plants. from the facts collected by heusinger, it appears that white sheep and pigs are differently affected from coloured individuals by certain vegetable poisons. hairless dogs have imperfect teeth: long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet. hence, if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly unconsciously modify other parts of the structure, owing to the mysterious laws of the correlation of growth. the result of the various, quite unknown, or dimly seen laws of variation is infinitely complex and diversified. it is well worth while carefully to study the several treatises published on some of our old cultivated plants, as on the hyacinth, potato, even the dahlia, &c.; and it is really surprising to note the endless points in structure and constitution in which the varieties and sub-varieties differ slightly from each other. the whole organisation seems to have become plastic, and tends to depart in some small degree from that of the parental type. any variation which is not inherited is unimportant for us. but the number and diversity of inheritable deviations of structure, both those of slight and those of considerable physiological importance, is endless. dr. prosper lucas's treatise, in two large volumes, is the fullest and the best on this subject. no breeder doubts how strong is the tendency to inheritance: like produces like is his fundamental belief: doubts have been thrown on this principle by theoretical writers alone. when any deviation of structure often appears, and we see it in the { } father and child, we cannot tell whether it may not be due to the same cause having acted on both; but when amongst individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in the parent--say, once amongst several million individuals--and it reappears in the child, the mere doctrine of chances almost compels us to attribute its reappearance to inheritance. every one must have heard of cases of albinism, prickly skin, hairy bodies, &c., appearing in several members of the same family. if strange and rare deviations of structure are truly inherited, less strange and commoner deviations may be freely admitted to be inheritable. perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly. the laws governing inheritance are quite unknown; no one can say why a peculiarity in different individuals of the same species, or in individuals of different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not exclusively to the like sex. it is a fact of some little importance to us, that peculiarities appearing in the males of our domestic breeds are often transmitted either exclusively, or in a much greater degree, to males alone. a much more important rule, which i think may be trusted, is that, at whatever period of life a peculiarity first appears, it tends to appear in the offspring at a corresponding age, though sometimes earlier. in many cases this could not be otherwise: thus the inherited peculiarities in the horns of cattle could appear only in { } the offspring when nearly mature; peculiarities in the silkworm are known to appear at the corresponding caterpillar or cocoon stage. but hereditary diseases and some other facts make me believe that the rule has a wider extension, and that when there is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent. i believe this rule to be of the highest importance in explaining the laws of embryology. these remarks are of course confined to the first _appearance_ of the peculiarity, and not to its primary cause, which may have acted on the ovules or male element; in nearly the same manner as in the crossed offspring from a short-horned cow by a long-horned bull, the greater length of horn, though appearing late in life, is clearly due to the male element. having alluded to the subject of reversion, i may here refer to a statement often made by naturalists--namely, that our domestic varieties, when run wild, gradually but certainly revert in character to their aboriginal stocks. hence it has been argued that no deductions can be drawn from domestic races to species in a state of nature. i have in vain endeavoured to discover on what decisive facts the above statement has so often and so boldly been made. there would be great difficulty in proving its truth: we may safely conclude that very many of the most strongly-marked domestic varieties could not possibly live in a wild state. in many cases we do not know what the aboriginal stock was, and so could not tell whether or not nearly perfect reversion had ensued. it would be quite necessary, in order to prevent the effects of intercrossing, that only a single variety should be turned loose in its new home. nevertheless, as our varieties certainly do occasionally { } revert in some of their characters to ancestral forms, it seems to me not improbable, that if we could succeed in naturalising, or were to cultivate, during many generations, the several races, for instance, of the cabbage, in very poor soil (in which case, however, some effect would have to be attributed to the direct action of the poor soil), that they would to a large extent, or even wholly, revert to the wild aboriginal stock. whether or not the experiment would succeed, is not of great importance for our line of argument; for by the experiment itself the conditions of life are changed. if it could be shown that our domestic varieties manifested a strong tendency to reversion,--that is, to lose their acquired characters, whilst kept under the same conditions, and whilst kept in a considerable body, so that free intercrossing might check, by blending together, any slight deviations in their structure, in such case, i grant that we could deduce nothing from domestic varieties in regard to species. but there is not a shadow of evidence in favour of this view: to assert that we could not breed our cart and race-horses, long and short-horned cattle, and poultry of various breeds, and esculent vegetables, for an almost infinite number of generations, would be opposed to all experience. i may add, that when under nature the conditions of life do change, variations and reversions of character probably do occur; but natural selection, as will hereafter be explained, will determine how far the new characters thus arising shall be preserved. when we look to the hereditary varieties or races of our domestic animals and plants, and compare them with closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species. domestic races of the same species, also, often have a somewhat monstrous character; by which i mean, that, although differing { } from each other, and from other species of the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when compared one with another, and more especially when compared with all the species in nature to which they are nearest allied. with these exceptions (and with that of the perfect fertility of varieties when crossed,--a subject hereafter to be discussed), domestic races of the same species differ from each other in the same manner as, only in most cases in a lesser degree than, do closely-allied species of the same genus in a state of nature. i think this must be admitted, when we find that there are hardly any domestic races, either amongst animals or plants, which have not been ranked by competent judges as mere varieties, and by other competent judges as the descendants of aboriginally distinct species. if any marked distinction existed between domestic races and species, this source of doubt could not so perpetually recur. it has often been stated that domestic races do not differ from each other in characters of generic value. i think it could be shown that this statement is hardly correct; but naturalists differ widely in determining what characters are of generic value; all such valuations being at present empirical. moreover, on the view of the origin of genera which i shall presently give, we have no right to expect often to meet with generic differences in our domesticated productions. when we attempt to estimate the amount of structural difference between the domestic races of the same species, we are soon involved in doubt, from not knowing whether they have descended from one or several parent-species. this point, if it could be cleared up, would be interesting; if, for instance, it could be shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, which we all know propagate their kind so truly, were the { } offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many very closely allied natural species--for instance, of the many foxes--inhabiting different quarters of the world. i do not believe, as we shall presently see, that the whole amount of difference between the several breeds of the dog has been produced under domestication; i believe that some small part of the difference is due to their being descended from distinct species. in the case of some other domesticated species, there is presumptive, or even strong evidence, that all the breeds have descended from a single wild stock. it has often been assumed that man has chosen for domestication animals and plants having an extraordinary inherent tendency to vary, and likewise to withstand diverse climates. i do not dispute that these capacities have added largely to the value of most of our domesticated productions; but how could a savage possibly know, when he first tamed an animal, whether it would vary in succeeding generations, and whether it would endure other climates? has the little variability of the ass or guinea-fowl, or the small power of endurance of warmth by the reindeer, or of cold by the common camel, prevented their domestication? i cannot doubt that if other animals and plants, equal in number to our domesticated productions, and belonging to equally diverse classes and countries, were taken from a state of nature, and could be made to breed for an equal number of generations under domestication, they would vary on an average as largely as the parent species of our existing domesticated productions have varied. in the case of most of our anciently domesticated animals and plants, i do not think it is possible to come to any definite conclusion, whether they have descended from one or several wild species. the argument mainly relied on by those who believe in the multiple origin { } of our domestic animals is, that we find in the most ancient records, more especially on the monuments of egypt, much diversity in the breeds; and that some of the breeds closely resemble, perhaps are identical with, those still existing. even if this latter fact were found more strictly and generally true than seems to me to be the case, what does it show, but that some of our breeds originated there, four or five thousand years ago? but mr. horner's researches have rendered it in some degree probable that man sufficiently civilized to have manufactured pottery existed in the valley of the nile thirteen or fourteen thousand years ago; and who will pretend to say how long before these ancient periods, savages, like those of tierra del fuego or australia, who possess a semi-domestic dog, may not have existed in egypt? the whole subject must, i think, remain vague; nevertheless, i may, without here entering on any details, state that, from geographical and other considerations, i think it highly probable that our domestic dogs have descended from several wild species. knowing, as we do, that savages are very fond of taming animals, it seems to me unlikely, in the case of the dog-genus, which is distributed in a wild state throughout the world, that since man first appeared one single species alone should have been domesticated. in regard to sheep and goats i can form no opinion. i should think, from facts communicated to me by mr. blyth, on the habits, voice, and constitution, &c., of the humped indian cattle, that these had descended from a different aboriginal stock from our european cattle; and several competent judges believe that these latter have had more than one wild parent. with respect to horses, from reasons which i cannot give here, i am doubtfully inclined to believe, in opposition to several authors, that all the races have descended from one { } wild stock. mr. blyth, whose opinion, from his large and varied stores of knowledge, i should value more than that of almost any one, thinks that all the breeds of poultry have proceeded from the common wild indian fowl (gallus bankiva). in regard to ducks and rabbits, the breeds of which differ considerably from each other in structure, i do not doubt that they have all descended from the common wild duck and rabbit. the doctrine of the origin of our several domestic races from several aboriginal stocks, has been carried to an absurd extreme by some authors. they believe that every race which breeds true, let the distinctive characters be ever so slight, has had its wild prototype. at this rate there must have existed at least a score of species of wild cattle, as many sheep, and several goats in europe alone, and several even within great britain. one author believes that there formerly existed in great britain eleven wild species of sheep peculiar to it! when we bear in mind that britain has now hardly one peculiar mammal, and france but few distinct from those of germany and conversely, and so with hungary, spain, &c., but that each of these kingdoms possesses several peculiar breeds of cattle, sheep, &c., we must admit that many domestic breeds have originated in europe; for whence could they have been derived, as these several countries do not possess a number of peculiar species as distinct parent-stocks? so it is in india. even in the case of the domestic dogs of the whole world, which i fully admit have probably descended from several wild species, i cannot doubt that there has been an immense amount of inherited variation. who can believe that animals closely resembling the italian greyhound, the bloodhound, the bull-dog, or blenheim spaniel, &c.--so unlike all wild canidæ--ever existed freely in a state of nature? it has often been loosely said that all our races of dogs have { } been produced by the crossing of a few aboriginal species; but by crossing we can only get forms in some degree intermediate between their parents; and if we account for our several domestic races by this process, we must admit the former existence of the most extreme forms, as the italian greyhound, bloodhound, bull-dog, &c., in the wild state. moreover, the possibility of making distinct races by crossing has been greatly exaggerated. there can be no doubt that a race may be modified by occasional crosses, if aided by the careful selection of those individual mongrels, which present any desired character; but that a race could be obtained nearly intermediate between two extremely different races or species, i can hardly believe. sir j. sebright expressly experimentised for this object, and failed. the offspring from the first cross between two pure breeds is tolerably and sometimes (as i have found with pigeons) extremely uniform, and everything seems simple enough; but when these mongrels are crossed one with another for several generations, hardly two of them will be alike, and then the extreme difficulty, or rather utter hopelessness, of the task becomes apparent. certainly, a breed intermediate between _two very distinct_ breeds could not be got without extreme care and long-continued selection; nor can i find a single case on record of a permanent race having been thus formed. _on the breeds of the domestic pigeon._--believing that it is always best to study some special group, i have, after deliberation, taken up domestic pigeons. i have kept every breed which i could purchase or obtain, and have been most kindly favoured with skins from several quarters of the world, more especially by the hon. w. elliot from india, and by the hon. c. murray from persia. many treatises in different languages have been published on pigeons, and some of them are very important, as being of { } considerable antiquity. i have associated with several eminent fanciers, and have been permitted to join two of the london pigeon clubs. the diversity of the breeds is something astonishing. compare the english carrier and the short-faced tumbler, and see the wonderful difference in their beaks, entailing corresponding differences in their skulls. the carrier, more especially the male bird, is also remarkable from the wonderful development of the carunculated skin about the head, and this is accompanied by greatly elongated eyelids, very large external orifices to the nostrils, and a wide gape of mouth. the short-faced tumbler has a beak in outline almost like that of a finch; and the common tumbler has the singular inherited habit of flying at a great height in a compact flock, and tumbling in the air head over heels. the runt is a bird of great size, with long, massive beak and large feet; some of the sub-breeds of runts have very long necks, others very long wings and tails, others singularly short tails. the barb is allied to the carrier, but, instead of a very long beak, has a very short and very broad one. the pouter has a much elongated body, wings, and legs; and its enormously developed crop, which it glories in inflating, may well excite astonishment and even laughter. the turbit has a very short and conical beak, with a line of reversed feathers down the breast; and it has the habit of continually expanding slightly the upper part of the oesophagus. the jacobin has the feathers so much reversed along the back of the neck that they form a hood, and it has, proportionally to its size, much elongated wing and tail feathers. the trumpeter and laugher, as their names express, utter a very different coo from the other breeds. the fantail has thirty or even forty tail feathers, instead of twelve or fourteen, the normal number in all members of the great pigeon family; and these feathers are kept expanded, and are { } carried so erect that in good birds the head and tail touch; the oil-gland is quite aborted. several other less distinct breeds might be specified. in the skeletons of the several breeds, the development of the bones of the face in length and breadth and curvature differs enormously. the shape, as well as the breadth and length of the ramus of the lower jaw, varies in a highly remarkable manner. the number of the caudal and sacral vertebræ vary; as does the number of the ribs, together with their relative breadth and the presence of processes. the size and shape of the apertures in the sternum are highly variable; so is the degree of divergence and relative size of the two arms of the furcula. the proportional width of the gape of mouth, the proportional length of the eyelids, of the orifice of the nostrils, of the tongue (not always in strict correlation with the length of beak), the size of the crop and of the upper part of the oesophagus; the development and abortion of the oil-gland; the number of the primary wing and caudal feathers; the relative length of wing and tail to each other and to the body; the relative length of leg and of the feet; the number of scutellæ on the toes, the development of skin between the toes, are all points of structure which are variable. the period at which the perfect plumage is acquired varies, as does the state of the down with which the nestling birds are clothed when hatched. the shape and size of the eggs vary. the manner of flight differs remarkably; as does in some breeds the voice and disposition. lastly, in certain breeds, the males and females have come to differ to a slight degree from each other. altogether at least a score of pigeons might be chosen, which if shown to an ornithologist, and he were told that they were wild birds, would certainly, i think, be ranked by him as well-defined species. moreover, i do not believe that any ornithologist would place the { } english carrier, the short-faced tumbler, the runt, the barb, pouter, and fantail in the same genus; more especially as in each of these breeds several truly-inherited sub-breeds, or species as he might have called them, could be shown him. great as the differences are between the breeds of pigeons, i am fully convinced that the common opinion of naturalists is correct, namely, that all have descended from the rock-pigeon (columba livia), including under this term several geographical races or sub-species, which differ from each other in the most trifling respects. as several of the reasons which have led me to this belief are in some degree applicable in other cases, i will here briefly give them. if the several breeds are not varieties, and have not proceeded from the rock-pigeon, they must have descended from at least seven or eight aboriginal stocks; for it is impossible to make the present domestic breeds by the crossing of any lesser number: how, for instance, could a pouter be produced by crossing two breeds unless one of the parent-stocks possessed the characteristic enormous crop? the supposed aboriginal stocks must all have been rock-pigeons, that is, not breeding or willingly perching on trees. but besides c. livia, with its geographical sub-species, only two or three other species of rock-pigeons are known; and these have not any of the characters of the domestic breeds. hence the supposed aboriginal stocks must either still exist in the countries where they were originally domesticated, and yet be unknown to ornithologists; and this, considering their size, habits, and remarkable characters, seems very improbable; or they must have become extinct in the wild state. but birds breeding on precipices, and good fliers, are unlikely to be exterminated; and the common rock-pigeon, which has the same habits with the domestic breeds, has not been exterminated { } even on several of the smaller british islets, or on the shores of the mediterranean. hence the supposed extermination of so many species having similar habits with the rock-pigeon seems to me a very rash assumption. moreover, the several above-named domesticated breeds have been transported to all parts of the world, and, therefore, some of them must have been carried back again into their native country; but not one has ever become wild or feral, though the dovecot-pigeon, which is the rock-pigeon in a very slightly altered state, has become feral in several places. again, all recent experience shows that it is most difficult to get any wild animal to breed freely under domestication; yet on the hypothesis of the multiple origin of our pigeons, it must be assumed that at least seven or eight species were so thoroughly domesticated in ancient times by half-civilized man, as to be quite prolific under confinement. an argument, as it seems to me, of great weight, and applicable in several other cases, is, that the above-specified breeds, though agreeing generally in constitution, habits, voice, colouring, and in most parts of their structure, with the wild rock-pigeon, yet are certainly highly abnormal in other parts of their structure; we may look in vain throughout the whole great family of columbidæ for a beak like that of the english carrier, or that of the short-faced tumbler, or barb; for reversed feathers like those of the jacobin; for a crop like that of the pouter; for tail-feathers like those of the fantail. hence it must be assumed not only that half-civilized man succeeded in thoroughly domesticating several species, but that he intentionally or by chance picked out extraordinarily abnormal species; and further, that these very species have since all become extinct or unknown. so many strange contingencies seem to me improbable in the highest degree. { } some facts in regard to the colouring of pigeons well deserve consideration. the rock-pigeon is of a slaty-blue, and has a white rump (the indian subspecies, c. intermedia of strickland, having it bluish); the tail has a terminal dark bar, with the bases of the outer feathers externally edged with white; the wings have two black bars; some semi-domestic breeds and some apparently truly wild breeds have, besides the two black bars, the wings chequered with black. these several marks do not occur together in any other species of the whole family. now, in every one of the domestic breeds, taking thoroughly well-bred birds, all the above marks, even to the white edging of the outer tail-feathers, sometimes concur perfectly developed. moreover, when two birds belonging to two distinct breeds are crossed, neither of which is blue or has any of the above-specified marks, the mongrel offspring are very apt suddenly to acquire these characters; for instance, i crossed some uniformly white fantails with some uniformly black barbs, and they produced mottled brown and black birds; these i again crossed together, and one grandchild of the pure white fantail and pure black barb was of as beautiful a blue colour, with the white rump, double black wing-bar, and barred and white-edged tail-feathers, as any wild rock-pigeon! we can understand these facts, on the well-known principle of reversion to ancestral characters, if all the domestic breeds have descended from the rock-pigeon. but if we deny this, we must make one of the two following highly improbable suppositions. either, firstly, that all the several imagined aboriginal stocks were coloured and marked like the rock-pigeon, although no other existing species is thus coloured and marked, so that in each separate breed there might be a tendency to revert to the very same colours and markings. or, secondly, { } that each breed, even the purest, has within a dozen or, at most, within a score of generations, been crossed by the rock-pigeon: i say within a dozen or twenty generations, for we know of no fact countenancing the belief that the child ever reverts to some one ancestor, removed by a greater number of generations. in a breed which has been crossed only once with some distinct breed, the tendency to reversion to any character derived from such cross will naturally become less and less, as in each succeeding generation there will be less of the foreign blood; but when there has been no cross with a distinct breed, and there is a tendency in both parents to revert to a character, which has been lost during some former generation, this tendency, for all that we can see to the contrary, may be transmitted undiminished for an indefinite number of generations. these two distinct cases are often confounded in treatises on inheritance. lastly, the hybrids or mongrels from between all the domestic breeds of pigeons are perfectly fertile. i can state this from my own observations, purposely made, on the most distinct breeds. now, it is difficult, perhaps impossible, to bring forward one case of the hybrid offspring of two animals _clearly distinct_ being themselves perfectly fertile. some authors believe that long-continued domestication eliminates this strong tendency to sterility: from the history of the dog i think there is some probability in this hypothesis, if applied to species closely related together, though it is unsupported by a single experiment. but to extend the hypothesis so far as to suppose that species, aboriginally as distinct as carriers, tumblers, pouters, and fantails now are, should yield offspring perfectly fertile, _inter se_, seems to me rash in the extreme. from these several reasons, namely, the improbability of man having formerly got seven or eight supposed { } species of pigeons to breed freely under domestication; these supposed species being quite unknown in a wild state, and their becoming nowhere feral; these species having very abnormal characters in certain respects, as compared with all other columbidæ, though so like in most other respects to the rock-pigeon; the blue colour and various marks occasionally appearing in all the breeds, both when kept pure and when crossed; the mongrel offspring being perfectly fertile;--from these several reasons, taken together, i can feel no doubt that all our domestic breeds have descended from the columba livia with its geographical sub-species. in favour of this view, i may add, firstly, that c. livia, or the rock-pigeon, has been found capable of domestication in europe and in india; and that it agrees in habits and in a great number of points of structure with all the domestic breeds. secondly, although an english carrier or short-faced tumbler differs immensely in certain characters from the rock-pigeon, yet by comparing the several sub-breeds of these varieties, more especially those brought from distant countries, we can make an almost perfect series between the extremes of structure. thirdly, those characters which are mainly distinctive of each breed, for instance the wattle and length of beak of the carrier, the shortness of that of the tumbler, and the number of tail-feathers in the fantail, are in each breed eminently variable; and the explanation of this fact will be obvious when we come to treat of selection. fourthly, pigeons have been watched, and tended with the utmost care, and loved by many people. they have been domesticated for thousands of years in several quarters of the world; the earliest known record of pigeons is in the fifth Ægyptian dynasty, about b.c., as was pointed out to me by professor lepsius; but mr. birch informs me that pigeons are given in a bill { } of fare in the previous dynasty. in the time of the romans, as we hear from pliny, immense prices were given for pigeons; "nay, they are come to this pass, that they can reckon up their pedigree and race." pigeons were much valued by akber khan in india, about the year ; never less than , pigeons were taken with the court. "the monarchs of iran and turan sent him some very rare birds;" and, continues the courtly historian, "his majesty by crossing the breeds, which method was never practised before, has improved them astonishingly." about this same period the dutch were as eager about pigeons as were the old romans. the paramount importance of these considerations in explaining the immense amount of variation which pigeons have undergone, will be obvious when we treat of selection. we shall then, also, see how it is that the breeds so often have a somewhat monstrous character. it is also a most favourable circumstance for the production of distinct breeds, that male and female pigeons can be easily mated for life; and thus different breeds can be kept together in the same aviary. i have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because when i first kept pigeons and watched the several kinds, knowing well how true they bred, i felt fully as much difficulty in believing that they could have descended from a common parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches, or other large groups of birds, in nature. one circumstance has struck me much; namely, that all the breeders of the various domestic animals and the cultivators of plants, with whom i have ever conversed, or whose treatises i have read, are firmly convinced that the several breeds to which each has attended, are descended from so many aboriginally distinct species. { } ask, as i have asked, a celebrated raiser of hereford cattle, whether his cattle might not have descended from long-horns, and he will laugh you to scorn. i have never met a pigeon, or poultry, or duck, or rabbit fancier, who was not fully convinced that each main breed was descended from a distinct species. van mons, in his treatise on pears and apples, shows how utterly he disbelieves that the several sorts, for instance a ribston-pippin or codlin-apple, could ever have proceeded from the seeds of the same tree. innumerable other examples could be given. the explanation, i think, is simple: from long-continued study they are strongly impressed with the differences between the several races; and though they well know that each race varies slightly, for they win their prizes by selecting such slight differences, yet they ignore all general arguments, and refuse to sum up in their minds slight differences accumulated during many successive generations. may not those naturalists who, knowing far less of the laws of inheritance than does the breeder, and knowing no more than he does of the intermediate links in the long lines of descent, yet admit that many of our domestic races have descended from the same parents--may they not learn a lesson of caution, when they deride the idea of species in a state of nature being lineal descendants of other species? _selection._--let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species. some little effect may, perhaps, be attributed to the direct action of the external conditions of life, and some little to habit; but he would be a bold man who would account by such agencies for the differences of a dray and race horse, a greyhound and bloodhound, a carrier and tumbler pigeon. one of the most remarkable features in our domesticated races { } is that we see in them adaptation, not indeed to the animal's or plant's own good, but to man's use or fancy. some variations useful to him have probably arisen suddenly, or by one step; many botanists, for instance, believe that the fuller's teazle, with its hooks, which cannot be rivalled by any mechanical contrivance, is only a variety of the wild dipsacus; and this amount of change may have suddenly arisen in a seedling. so it has probably been with the turnspit dog; and this is known to have been the case with the ancon sheep. but when we compare the dray-horse and race-horse, the dromedary and camel, the various breeds of sheep fitted either for cultivated land or mountain pasture, with the wool of one breed good for one purpose, and that of another breed for another purpose; when we compare the many breeds of dogs, each good for man in very different ways; when we compare the game-cock, so pertinacious in battle, with other breeds so little quarrelsome, with "everlasting layers" which never desire to sit, and with the bantam so small and elegant; when we compare the host of agricultural, culinary, orchard, and flower-garden races of plants, most useful to man at different seasons and for different purposes, or so beautiful in his eyes, we must, i think, look further than to mere variability. we cannot suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; indeed, in several cases, we know that this has not been their history. the key is man's power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. in this sense he may be said to make for himself useful breeds. the great power of this principle of selection is not hypothetical. it is certain that several of our eminent breeders have, even within a single lifetime, modified to { } a large extent some breeds of cattle and sheep. in order fully to realise what they have done, it is almost necessary to read several of the many treatises devoted to this subject, and to inspect the animals. breeders habitually speak of an animal's organisation as something quite plastic, which they can model almost as they please. if i had space i could quote numerous passages to this effect from highly competent authorities. youatt, who was probably better acquainted with the works of agriculturists than almost any other individual, and who was himself a very good judge of an animal, speaks of the principle of selection as "that which enables the agriculturist, not only to modify the character of his flock, but to change it altogether. it is the magician's wand, by means of which he may summon into life whatever form and mould he pleases." lord somerville, speaking of what breeders have done for sheep, says:--"it would seem as if they had chalked out upon a wall a form perfect in itself, and then had given it existence." that most skilful breeder, sir john sebright, used to say, with respect to pigeons, that "he would produce any given feather in three years, but it would take him six years to obtain head and beak." in saxony the importance of the principle of selection in regard to merino sheep is so fully recognised, that men follow it as a trade: the sheep are placed on a table and are studied, like a picture by a connoisseur; this is done three times at intervals of months, and the sheep are each time marked and classed, so that the very best may ultimately be selected for breeding. what english breeders have actually effected is proved by the enormous prices given for animals with a good pedigree; and these have now been exported to almost every quarter of the world. the improvement is by no means generally due to crossing different breeds; { } all the best breeders are strongly opposed to this practice, except sometimes amongst closely allied sub-breeds. and when a cross has been made, the closest selection is far more indispensable even than in ordinary cases. if selection consisted merely in separating some very distinct variety, and breeding from it, the principle would be so obvious as hardly to be worth notice; but its importance consists in the great effect produced by the accumulation in one direction, during successive generations, of differences absolutely inappreciable by an uneducated eye--differences which i for one have vainly attempted to appreciate. not one man in a thousand has accuracy of eye and judgment sufficient to become an eminent breeder. if gifted with these qualities, and he studies his subject for years, and devotes his lifetime to it with indomitable perseverance, he will succeed, and may make great improvements; if he wants any of these qualities, he will assuredly fail. few would readily believe in the natural capacity and years of practice requisite to become even a skilful pigeon-fancier. the same principles are followed by horticulturists; but the variations are here often more abrupt. no one supposes that our choicest productions have been produced by a single variation from the aboriginal stock. we have proofs that this is not so in some cases, in which exact records have been kept; thus, to give a very trifling instance, the steadily-increasing size of the common gooseberry may be quoted. we see an astonishing improvement in many florists' flowers, when the flowers of the present day are compared with drawings made only twenty or thirty years ago. when a race of plants is once pretty well established, the seed-raisers do not pick out the best plants, but merely go over their seed-beds, and pull up the "rogues," as they call the plants that deviate from the proper standard. with animals this { } kind of selection is, in fact, also followed; for hardly any one is so careless as to allow his worst animals to breed. in regard to plants, there is another means of observing the accumulated effects of selection--namely, by comparing the diversity of flowers in the different varieties of the same species in the flower-garden; the diversity of leaves, pods, or tubers, or whatever part is valued, in the kitchen-garden, in comparison with the flowers of the same varieties; and the diversity of fruit of the same species in the orchard, in comparison with the leaves and flowers of the same set of varieties. see how different the leaves of the cabbage are, and how extremely alike the flowers; how unlike the flowers of the heartsease are, and how alike the leaves; how much the fruit of the different kinds of gooseberries differ in size, colour, shape, and hairiness, and yet the flowers present very slight differences. it is not that the varieties which differ largely in some one point do not differ at all in other points; this is hardly ever, perhaps never, the case. the laws of correlation of growth, the importance of which should never be overlooked, will ensure some differences; but, as a general rule, i cannot doubt that the continued selection of slight variations, either in the leaves, the flowers, or the fruit, will produce races differing from each other chiefly in these characters. it may be objected that the principle of selection has been reduced to methodical practice for scarcely more than three-quarters of a century; it has certainly been more attended to of late years, and many treatises have been published on the subject; and the result has been, in a corresponding degree, rapid and important. but it is very far from true that the principle is a modern discovery. i could give several references to the full acknowledgment of the importance of the principle in works of high antiquity. in rude and barbarous periods { } of english history choice animals were often imported, and laws were passed to prevent their exportation: the destruction of horses under a certain size was ordered, and this may be compared to the "roguing" of plants by nurserymen. the principle of selection i find distinctly given in an ancient chinese encyclopædia. explicit rules are laid down by some of the roman classical writers. from passages in genesis, it is clear that the colour of domestic animals was at that early period attended to. savages now sometimes cross their dogs with wild canine animals, to improve the breed, and they formerly did so, as is attested by passages in pliny. the savages in south africa match their draught cattle by colour, as do some of the esquimaux their teams of dogs. livingstone shows how much good domestic breeds are valued by the negroes of the interior of africa who have not associated with europeans. some of these facts do not show actual selection, but they show that the breeding of domestic animals was carefully attended to in ancient times, and is now attended to by the lowest savages. it would, indeed, have been a strange fact, had attention not been paid to breeding, for the inheritance of good and bad qualities is so obvious. at the present time, eminent breeders try by methodical selection, with a distinct object in view, to make a new strain or sub-breed, superior to anything existing in the country. but, for our purpose, a kind of selection, which may be called unconscious, and which results from every one trying to possess and breed from the best individual animals, is more important. thus, a man who intends keeping pointers naturally tries to get as good dogs as he can, and afterwards breeds from his own best dogs, but he has no wish or expectation of permanently altering the breed. nevertheless i cannot doubt that this process, continued during centuries, { } would improve and modify any breed, in the same way as bakewell, collins, &c., by this very same process, only carried on more methodically, did greatly modify, even during their own lifetimes, the forms and qualities of their cattle. slow and insensible changes of this kind could never be recognised unless actual measurements or careful drawings of the breeds in question had been made long ago, which might serve for comparison. in some cases, however, unchanged, or but little changed individuals of the same breed may be found in less civilised districts, where the breed has been less improved. there is reason to believe that king charles's spaniel has been unconsciously modified to a large extent since the time of that monarch. some highly competent authorities are convinced that the setter is directly derived from the spaniel, and has probably been slowly altered from it. it is known that the english pointer has been greatly changed within the last century, and in this case the change has, it is believed, been chiefly effected by crosses with the fox-hound; but what concerns us is, that the change has been effected unconsciously and gradually, and yet so effectually, that, though the old spanish pointer certainly came from spain, mr. borrow has not seen, as i am informed by him, any native dog in spain like our pointer. by a similar process of selection, and by careful training, the whole body of english racehorses have come to surpass in fleetness and size the parent arab stock, so that the latter, by the regulations for the goodwood races, are favoured in the weights they carry. lord spencer and others have shown how the cattle of england have increased in weight and in early maturity, compared with the stock formerly kept in this country. by comparing the accounts given in old pigeon treatises of carriers and tumblers with these breeds as now existing in britain, { } india, and persia, we can, i think, clearly trace the stages through which they have insensibly passed, and come to differ so greatly from the rock-pigeon. youatt gives an excellent illustration of the effects of a course of selection, which may be considered as unconsciously followed, in so far that the breeders could never have expected or even have wished to have produced the result which ensued--namely, the production of two distinct strains. the two flocks of leicester sheep kept by mr. buckley and mr. burgess, as mr. youatt remarks, "have been purely bred from the original stock of mr. bakewell for upwards of fifty years. there is not a suspicion existing in the mind of any one at all acquainted with the subject that the owner of either of them has deviated in any one instance from the pure blood of mr. bakewell's flock, and yet the difference between the sheep possessed by these two gentlemen is so great that they have the appearance of being quite different varieties." if there exist savages so barbarous as never to think of the inherited character of the offspring of their domestic animals, yet any one animal particularly useful to them, for any special purpose, would be carefully preserved during famines and other accidents, to which savages are so liable, and such choice animals would thus generally leave more offspring than the inferior ones; so that in this case there would be a kind of unconscious selection going on. we see the value set on animals even by the barbarians of tierra del fuego, by their killing and devouring their old women, in times of dearth, as of less value than their dogs. in plants the same gradual process of improvement, through the occasional preservation of the best individuals, whether or not sufficiently distinct to be ranked at their first appearance as distinct varieties, and whether { } or not two or more species or races have become blended together by crossing, may plainly be recognised in the increased size and beauty which we now see in the varieties of the heartsease, rose, pelargonium, dahlia, and other plants, when compared with the older varieties or with their parent-stocks. no one would ever expect to get a first-rate heartsease or dahlia from the seed of a wild plant. no one would expect to raise a first-rate melting pear from the seed of the wild pear, though he might succeed from a poor seedling growing wild, if it had come from a garden-stock. the pear, though cultivated in classical times, appears, from pliny's description, to have been a fruit of very inferior quality. i have seen great surprise expressed in horticultural works at the wonderful skill of gardeners, in having produced such splendid results from such poor materials; but the art, i cannot doubt, has been simple, and, as far as the final result is concerned, has been followed almost unconsciously. it has consisted in always cultivating the best known variety, sowing its seeds, and, when a slightly better variety has chanced to appear, selecting it, and so onwards. but the gardeners of the classical period, who cultivated the best pear they could procure, never thought what splendid fruit we should eat; though we owe our excellent fruit, in some small degree, to their having naturally chosen and preserved the best varieties they could anywhere find. a large amount of change in our cultivated plants, thus slowly and unconsciously accumulated, explains, as i believe, the well-known fact, that in a vast number of cases we cannot recognise, and therefore do not know, the wild parent-stocks of the plants which have been longest cultivated in our flower and kitchen gardens. if it has taken centuries or thousands of years to improve or modify most of our plants up to their present { } standard of usefulness to man, we can understand how it is that neither australia, the cape of good hope, nor any other region inhabited by quite uncivilised man, has afforded us a single plant worth culture. it is not that these countries, so rich in species, do not by a strange chance possess the aboriginal stocks of any useful plants, but that the native plants have not been improved by continued selection up to a standard of perfection comparable with that given to the plants in countries anciently civilised. in regard to the domestic animals kept by uncivilised man, it should not be overlooked that they almost always have to struggle for their own food, at least during certain seasons. and in two countries very differently circumstanced, individuals of the same species, having slightly different constitutions or structure, would often succeed better in the one country than in the other; and thus by a process of "natural selection," as will hereafter be more fully explained, two sub-breeds might be formed. this, perhaps, partly explains what has been remarked by some authors, namely, that the varieties kept by savages have more of the character of species than the varieties kept in civilised countries. on the view here given of the all-important part which selection by man has played, it becomes at once obvious, how it is that our domestic races show adaptation in their structure or in their habits to man's wants or fancies. we can, i think, further understand the frequently abnormal character of our domestic races, and likewise their differences being so great in external characters and relatively so slight in internal parts or organs. man can hardly select, or only with much difficulty, any deviation of structure excepting such as is externally visible; and indeed he rarely cares for what is internal. he can never act by selection, excepting on variations { } which are first given to him in some slight degree by nature. no man would ever try to make a fantail, till he saw a pigeon with a tail developed in some slight degree in an unusual manner, or a pouter till he saw a pigeon with a crop of somewhat unusual size; and the more abnormal or unusual any character was when it first appeared, the more likely it would be to catch his attention. but to use such an expression as trying to make a fantail, is, i have no doubt, in most cases, utterly incorrect. the man who first selected a pigeon with a slightly larger tail, never dreamed what the descendants of that pigeon would become through long-continued, partly unconscious and partly methodical selection. perhaps the parent bird of all fantails had only fourteen tail-feathers somewhat expanded, like the present java fantail, or like individuals of other and distinct breeds, in which as many as seventeen tail-feathers have been counted. perhaps the first pouter-pigeon did not inflate its crop much more than the turbit now does the upper part of its oesophagus,--a habit which is disregarded by all fanciers, as it is not one of the points of the breed. nor let it be thought that some great deviation of structure would be necessary to catch the fancier's eye: he perceives extremely small differences, and it is in human nature to value any novelty, however slight, in one's own possession. nor must the value which would formerly be set on any slight differences in the individuals of the same species, be judged of by the value which would now be set on them, after several breeds have once fairly been established. many slight differences might, and indeed do now, arise amongst pigeons, which are rejected as faults or deviations from the standard of perfection of each breed. the common goose has not given rise to any marked varieties; hence the thoulouse and the common breed, which differ only in colour, that { } most fleeting of characters, have lately been exhibited as distinct at our poultry-shows. i think these views further explain what has sometimes been noticed--namely, that we know nothing about the origin or history of any of our domestic breeds. but, in fact, a breed, like a dialect of a language, can hardly be said to have had a definite origin. a man preserves and breeds from an individual with some slight deviation of structure, or takes more care than usual in matching his best animals and thus improves them, and the improved individuals slowly spread in the immediate neighbourhood. but as yet they will hardly have a distinct name, and from being only slightly valued, their history will be disregarded. when further improved by the same slow and gradual process, they will spread more widely, and will get recognised as something distinct and valuable, and will then probably first receive a provincial name. in semi-civilised countries, with little free communication, the spreading and knowledge of any new sub-breed will be a slow process. as soon as the points of value of the new sub-breed are once fully acknowledged, the principle, as i have called it, of unconscious selection will always tend,--perhaps more at one period than at another, as the breed rises or falls in fashion,--perhaps more in one district than in another, according to the state of civilization of the inhabitants,--slowly to add to the characteristic features of the breed, whatever they may be. but the chance will be infinitely small of any record having been preserved of such slow, varying, and insensible changes. i must now say a few words on the circumstances, favourable, or the reverse, to man's power of selection. a high degree of variability is obviously favourable, as freely giving the materials for selection to work on; not that mere individual differences are not amply { } sufficient, with extreme care, to allow of the accumulation of a large amount of modification in almost any desired direction. but as variations manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be much increased by a large number of individuals being kept; and hence this comes to be of the highest importance to success. on this principle marshall has remarked, with respect to the sheep of parts of yorkshire, that "as they generally belong to poor people, and are mostly _in small lots_, they never can be improved." on the other hand, nurserymen, from raising large stocks of the same plants, are generally far more successful than amateurs in getting new and valuable varieties. the keeping of a large number of individuals of a species in any country requires that the species should be placed under favourable conditions of life, so as to breed freely in that country. when the individuals of any species are scanty, all the individuals, whatever their quality may be, will generally be allowed to breed, and this will effectually prevent selection. but probably the most important point of all, is, that the animal or plant should be so highly useful to man, or so much valued by him, that the closest attention should be paid to even the slightest deviation in the qualities or structure of each individual. unless such attention be paid nothing can be effected. i have seen it gravely remarked, that it was most fortunate that the strawberry began to vary just when gardeners began to attend closely to this plant. no doubt the strawberry had always varied since it was cultivated, but the slight varieties had been neglected. as soon, however, as gardeners picked out individual plants with slightly larger, earlier, or better fruit, and raised seedlings from them, and again picked out the best seedlings and bred from them, then, there appeared (aided by some { } crossing with distinct species) those many admirable varieties of the strawberry which have been raised during the last thirty or forty years. in the case of animals with separate sexes, facility in preventing crosses is an important element of success in the formation of new races,--at least, in a country which is already stocked with other races. in this respect enclosure of the land plays a part. wandering savages or the inhabitants of open plains rarely possess more than one breed of the same species. pigeons can be mated for life, and this is a great convenience to the fancier, for thus many races may be kept true, though mingled in the same aviary; and this circumstance must have largely favoured the improvement and formation of new breeds. pigeons, i may add, can be propagated in great numbers and at a very quick rate, and inferior birds may be freely rejected, as when killed they serve for food. on the other hand, cats, from their nocturnal rambling habits, cannot be matched, and, although so much valued by women and children, we hardly ever see a distinct breed kept up; such breeds as we do sometimes see are almost always imported from some other country, often from islands. although i do not doubt that some domestic animals vary less than others, yet the rarity or absence of distinct breeds of the cat, the donkey, peacock, goose, &c., may be attributed in main part to selection not having been brought into play: in cats, from the difficulty in pairing them; in donkeys, from only a few being kept by poor people, and little attention paid to their breeding; in peacocks, from not being very easily reared and a large stock not kept; in geese, from being valuable only for two purposes, food and feathers, and more especially from no pleasure having been felt in the display of distinct breeds. to sum up on the origin of our domestic races of { } animals and plants. i believe that the conditions of life, from their action on the reproductive system, are so far of the highest importance as causing variability. i do not believe that variability is an inherent and necessary contingency, under all circumstances, with all organic beings, as some authors have thought. the effects of variability are modified by various degrees of inheritance and of reversion. variability is governed by many unknown laws, more especially by that of correlation of growth. something may be attributed to the direct action of the conditions of life. something must be attributed to use and disuse. the final result is thus rendered infinitely complex. in some cases, i do not doubt that the intercrossing of species, aboriginally distinct, has played an important part in the origin of our domestic productions. when in any country several domestic breeds have once been established, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of the crossing of varieties has, i believe, been greatly exaggerated, both in regard to animals and to those plants which are propagated by seed. in plants which are temporarily propagated by cuttings, buds, &c., the importance of the crossing both of distinct species and of varieties is immense; for the cultivator here quite disregards the extreme variability both of hybrids and mongrels, and the frequent sterility of hybrids; but the cases of plants not propagated by seed are of little importance to us, for their endurance is only temporary. over all these causes of change i am convinced that the accumulative action of selection, whether applied methodically and more quickly, or unconsciously and more slowly, but more efficiently, is by far the predominant power. * * * * * { } chapter ii. variation under nature. variability--individual differences--doubtful species--wide ranging, much diffused, and common species vary most--species of the larger genera in any country vary more than the species of the smaller genera--many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges. before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must briefly discuss whether these latter are subject to any variation. to treat this subject at all properly, a long catalogue of dry facts should be given; but these i shall reserve for my future work. nor shall i here discuss the various definitions which have been given of the term species. no one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. generally the term includes the unknown element of a distinct act of creation. the term "variety" is almost equally difficult to define; but here community of descent is almost universally implied, though it can rarely be proved. we have also what are called monstrosities; but they graduate into varieties. by a monstrosity i presume is meant some considerable deviation of structure in one part, either injurious to or not useful to the species, and not generally propagated. some authors use the term "variation" in a technical sense, as implying a modification directly due to the physical conditions of life; and "variations" in this sense are supposed not to be inherited: but who can say that the dwarfed condition of shells in the brackish waters of the baltic, or dwarfed { } plants on alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for at least some few generations? and in this case i presume that the form would be called a variety. again, we have many slight differences which may be called individual differences, such as are known frequently to appear in the offspring from the same parents, or which may be presumed to have thus arisen, from being frequently observed in the individuals of the same species inhabiting the same confined locality. no one supposes that all the individuals of the same species are cast in the very same mould. these individual differences are highly important for us, as they afford materials for natural selection to accumulate, in the same manner as man can accumulate in any given direction individual differences in his domesticated productions. these individual differences generally affect what naturalists consider unimportant parts; but i could show by a long catalogue of facts, that parts which must be called important, whether viewed under a physiological or classificatory point of view, sometimes vary in the individuals of the same species. i am convinced that the most experienced naturalist would be surprised at the number of the cases of variability, even in important parts of structure, which he could collect on good authority, as i have collected, during a course of years. it should be remembered that systematists are far from pleased at finding variability in important characters, and that there are not many men who will laboriously examine internal and important organs, and compare them in many specimens of the same species. i should never have expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species; i should have expected that changes of this nature could have been effected only { } by slow degrees: yet quite recently mr. lubbock has shown a degree of variability in these main nerves in coccus, which may almost be compared to the irregular branching of the stem of a tree. this philosophical naturalist, i may add, has also quite recently shown that the muscles in the larvæ of certain insects are very far from uniform. authors sometimes argue in a circle when they state that important organs never vary; for these same authors practically rank that character as important (as some few naturalists have honestly confessed) which does not vary; and, under this point of view, no instance of an important part varying will ever be found: but under any other point of view many instances assuredly can be given. there is one point connected with individual differences, which seems to me extremely perplexing: i refer to those genera which have sometimes been called "protean" or "polymorphic," in which the species present an inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which as varieties. we may instance rubus, rosa, and hieracium amongst plants, several genera of insects, and several genera of brachiopod shells. in most polymorphic genera some of the species have fixed and definite characters. genera which are polymorphic in one country seem to be, with some few exceptions, polymorphic in other countries, and likewise, judging from brachiopod shells, at former periods of time. these facts seem to be very perplexing, for they seem to show that this kind of variability is independent of the conditions of life. i am inclined to suspect that we see in these polymorphic genera variations in points of structure which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter will be explained. { } those forms which possess in some considerable degree the character of species, but which are so closely similar to some other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank them as distinct species, are in several respects the most important for us. we have every reason to believe that many of these doubtful and closely-allied forms have permanently retained their characters in their own country for a long time; for as long, as far as we know, as have good and true species. practically, when a naturalist can unite two forms together by others having intermediate characters, he treats the one as a variety of the other, ranking the most common, but sometimes the one first described, as the species, and the other as the variety. but cases of great difficulty, which i will not here enumerate, sometimes occur in deciding whether or not to rank one form as a variety of another, even when they are closely connected by intermediate links; nor will the commonly-assumed hybrid nature of the intermediate links always remove the difficulty. in very many cases, however, one form is ranked as a variety of another, not because the intermediate links have actually been found, but because analogy leads the observer to suppose either that they do now somewhere exist, or may formerly have existed; and here a wide door for the entry of doubt and conjecture is opened. hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgment and wide experience seems the only guide to follow. we must, however, in many cases, decide by a majority of naturalists, for few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges. { } that varieties of this doubtful nature are far from uncommon cannot be disputed. compare the several floras of great britain, of france or of the united states, drawn up by different botanists, and see what a surprising number of forms have been ranked by one botanist as good species, and by another as mere varieties. mr. h. c. watson, to whom i lie under deep obligation for assistance of all kinds, has marked for me british plants, which are generally considered as varieties, but which have all been ranked by botanists as species; and in making this list he has omitted many trifling varieties, but which nevertheless have been ranked by some botanists as species, and he has entirely omitted several highly polymorphic genera. under genera, including the most polymorphic forms, mr. babington gives species, whereas mr. bentham gives only ,--a difference of doubtful forms! amongst animals which unite for each birth, and which are highly locomotive, doubtful forms, ranked by one zoologist as a species and by another as a variety, can rarely be found within the same country, but are common in separated areas. how many of those birds and insects in north america and europe, which differ very slightly from each other, have been ranked by one eminent naturalist as undoubted species, and by another as varieties, or, as they are often called, as geographical races! many years ago, when comparing, and seeing others compare, the birds from the separate islands of the galapagos archipelago, both one with another, and with those from the american mainland, i was much struck how entirely vague and arbitrary is the distinction between species and varieties. on the islets of the little madeira group there are many insects which are characterized as varieties in mr. wollaston's admirable work, but which it cannot { } be doubted would be ranked as distinct species by many entomologists. even ireland has a few animals, now generally regarded as varieties, but which have been ranked as species by some zoologists. several most experienced ornithologists consider our british red grouse as only a strongly-marked race of a norwegian species, whereas the greater number rank it as an undoubted species peculiar to great britain. a wide distance between the homes of two doubtful forms leads many naturalists to rank both as distinct species; but what distance, it has been well asked, will suffice? if that between america and europe is ample, will that between the continent and the azores, or madeira, or the canaries, or ireland, be sufficient? it must be admitted that many forms, considered by highly-competent judges as varieties, have so perfectly the character of species that they are ranked by other highly-competent judges as good and true species. but to discuss whether they are rightly called species or varieties, before any definition of these terms has been generally accepted, is vainly to beat the air. many of the cases of strongly-marked varieties or doubtful species well deserve consideration; for several interesting lines of argument, from geographical distribution, analogical variation, hybridism, &c., have been brought to bear on the attempt to determine their rank. i will here give only a single instance,--the well-known one of the primrose and cowslip, or primula vulgaris and veris. these plants differ considerably in appearance; they have a different flavour, and emit a different odour; they flower at slightly different periods; they grow in somewhat different stations; they ascend mountains to different heights; they have different geographical ranges; and lastly, according to very numerous experiments made during several years by { } that most careful observer gärtner, they can be crossed only with much difficulty. we could hardly wish for better evidence of the two forms being specifically distinct. on the other hand, they are united by many intermediate links, and it is very doubtful whether these links are hybrids; and there is, as it seems to me, an overwhelming amount of experimental evidence, showing that they descend from common parents, and consequently must be ranked as varieties. close investigation, in most cases, will bring naturalists to an agreement how to rank doubtful forms. yet it must be confessed that it is in the best-known countries that we find the greatest number of forms of doubtful value. i have been struck with the fact, that if any animal or plant in a state of nature be highly useful to man, or from any cause closely attract his attention, varieties of it will almost universally be found recorded. these varieties, moreover, will be often ranked by some authors as species. look at the common oak, how closely it has been studied; yet a german author makes more than a dozen species out of forms, which are very generally considered as varieties; and in this country the highest botanical authorities and practical men can be quoted to show that the sessile and pedunculated oaks are either good and distinct species or mere varieties. when a young naturalist commences the study of a group of organisms quite unknown to him, he is at first much perplexed to determine what differences to consider as specific, and what as varieties; for he knows nothing of the amount and kind of variation to which the group is subject; and this shows, at least, how very generally there is some variation. but if he confine his attention to one class within one country, he will soon make up his mind how to rank most of the doubtful forms. his { } general tendency will be to make many species, for he will become impressed, just like the pigeon or poultry fancier before alluded to, with the amount of difference in the forms which he is continually studying; and he has little general knowledge of analogical variation in other groups and in other countries, by which to correct his first impressions. as he extends the range of his observations, he will meet with more cases of difficulty; for he will encounter a greater number of closely-allied forms. but if his observations be widely extended, he will in the end generally be enabled to make up his own mind which to call varieties and which species; but he will succeed in this at the expense of admitting much variation,--and the truth of this admission will often be disputed by other naturalists. when, moreover, he comes to study allied forms brought from countries not now continuous, in which case he can hardly hope to find the intermediate links between his doubtful forms, he will have to trust almost entirely to analogy, and his difficulties rise to a climax. certainly no clear line of demarcation has as yet been drawn between species and sub-species--that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at the rank of species; or, again, between sub-species and well-marked varieties, or between lesser varieties and individual differences. these differences blend into each other in an insensible series; and a series impresses the mind with the idea of an actual passage. hence i look at individual differences, though of small interest to the systematist, as of high importance for us, as being the first step towards such slight varieties as are barely thought worth recording in works on natural history. and i look at varieties which are in any degree more distinct and permanent, as steps leading to more { } strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to species. the passage from one stage of difference to another and higher stage may be, in some cases, due merely to the long-continued action of different physical conditions in two different regions; but i have not much faith in this view; and i attribute the passage of a variety, from a state in which it differs very slightly from its parent to one in which it differs more, to the action of natural selection in accumulating (as will hereafter be more fully explained) differences of structure in certain definite directions. hence i believe a well-marked variety may be called an incipient species; but whether this belief be justifiable must be judged of by the general weight of the several facts and views given throughout this work. it need not be supposed that all varieties or incipient species necessarily attain the rank of species. they may whilst in this incipient state become extinct, or they may endure as varieties for very long periods, as has been shown to be the case by mr. wollaston with the varieties of certain fossil land-shells in madeira. if a variety were to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the species as the variety; or it might come to supplant and exterminate the parent species; or both might co-exist, and both rank as independent species. but we shall hereafter have to return to this subject. from these remarks it will be seen that i look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. the term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience' sake. { } guided by theoretical considerations, i thought that some interesting results might be obtained in regard to the nature and relations of the species which vary most, by tabulating all the varieties in several well-worked floras. at first this seemed a simple task; but mr. h. c. watson, to whom i am much indebted for valuable advice and assistance on this subject, soon convinced me that there were many difficulties, as did subsequently dr. hooker, even in stronger terms. i shall reserve for my future work the discussion of these difficulties, and the tables themselves of the proportional numbers of the varying species. dr. hooker permits me to add, that after having carefully read my manuscript, and examined the tables, he thinks that the following statements are fairly well established. the whole subject, however, treated as it necessarily here is with much brevity, is rather perplexing, and allusions cannot be avoided to the "struggle for existence," "divergence of character," and other questions, hereafter to be discussed. alph. de candolle and others have shown that plants which have very wide ranges generally present varieties; and this might have been expected, as they become exposed to diverse physical conditions, and as they come into competition (which, as we shall hereafter see, is a far more important circumstance) with different sets of organic beings. but my tables further show that, in any limited country, the species which are most common, that is abound most in individuals, and the species which are most widely diffused within their own country (and this is a different consideration from wide range, and to a certain extent from commonness), often give rise to varieties sufficiently well-marked to have been recorded in botanical works. hence it is the most flourishing, or, as they may be called, the dominant species,--those { } which range widely over the world, are the most diffused in their own country, and are the most numerous in individuals,--which oftenest produce well-marked varieties, or, as i consider them, incipient species. and this, perhaps, might have been anticipated; for, as varieties, in order to become in any degree permanent, necessarily have to struggle with the other inhabitants of the country, the species which are already dominant will be the most likely to yield offspring, which, though in some slight degree modified, still inherit those advantages that enabled their parents to become dominant over their compatriots. if the plants inhabiting a country and described in any flora be divided into two equal masses, all those in the larger genera being placed on one side, and all those in the smaller genera on the other side, a somewhat larger number of the very common and much diffused or dominant species will be found on the side of the larger genera. this, again, might have been anticipated; for the mere fact of many species of the same genus inhabiting any country, shows that there is something in the organic or inorganic conditions of that country favourable to the genus; and, consequently, we might have expected to have found in the larger genera, or those including many species, a large proportional number of dominant species. but so many causes tend to obscure this result, that i am surprised that my tables show even a small majority on the side of the larger genera. i will here allude to only two causes of obscurity. fresh-water and salt-loving plants have generally very wide ranges and are much diffused, but this seems to be connected with the nature of the stations inhabited by them, and has little or no relation to the size of the genera to which the species belong. again, plants low in the scale of organisation are { } generally much more widely diffused than plants higher in the scale; and here again there is no close relation to the size of the genera. the cause of lowly-organised plants ranging widely will be discussed in our chapter on geographical distribution. from looking at species as only strongly-marked and well-defined varieties, i was led to anticipate that the species of the larger genera in each country would oftener present varieties, than the species of the smaller genera; for wherever many closely related species (_i.e._ species of the same genus) have been formed, many varieties or incipient species ought, as a general rule, to be now forming. where many large trees grow, we expect to find saplings. where many species of a genus have been formed through variation, circumstances have been favourable for variation; and hence we might expect that the circumstances would generally be still favourable to variation. on the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few. to test the truth of this anticipation i have arranged the plants of twelve countries, and the coleopterous insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the species on the side of the larger genera present varieties, than on the side of the smaller genera. moreover, the species of the large genera which present any varieties, invariably present a larger average number of varieties than do the species of the small genera. both these results follow when another division is made, and when all the smallest genera, with from only one to four species, are absolutely excluded from the tables. these { } facts are of plain signification on the view that species are only strongly marked and permanent varieties; for wherever many species of the same genus have been formed, or where, if we may use the expression, the manufactory of species has been active, we ought generally to find the manufactory still in action, more especially as we have every reason to believe the process of manufacturing new species to be a slow one. and this certainly is the case, if varieties be looked at as incipient species; for my tables clearly show as a general rule that, wherever many species of a genus have been formed, the species of that genus present a number of varieties, that is of incipient species beyond the average. it is not that all large genera are now varying much, and are thus increasing in the number of their species, or that no small genera are now varying and increasing; for if this had been so, it would have been fatal to my theory; inasmuch as geology plainly tells us that small genera have in the lapse of time often increased greatly in size; and that large genera have often come to their maxima, declined, and disappeared. all that we want to show is, that where many species of a genus have been formed, on an average many are still forming; and this holds good. there are other relations between the species of large genera and their recorded varieties which deserve notice. we have seen that there is no infallible criterion by which to distinguish species and well-marked varieties; and in those cases in which intermediate links have not been found between doubtful forms, naturalists are compelled to come to a determination by the amount of difference between them, judging by analogy whether or not the amount suffices to raise one or both to the rank of species. hence the amount of difference is one very important criterion in settling whether two forms { } should be ranked as species or varieties. now fries has remarked in regard to plants, and westwood in regard to insects, that in large genera the amount of difference between the species is often exceedingly small. i have endeavoured to test this numerically by averages, and, as far as my imperfect results go, they confirm the view. i have also consulted some sagacious and experienced observers, and, after deliberation, they concur in this view. in this respect, therefore, the species of the larger genera resemble varieties, more than do the species of the smaller genera. or the case may be put in another way, and it may be said, that in the larger genera, in which a number of varieties or incipient species greater than the average are now manufacturing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by a less than usual amount of difference. moreover, the species of the large genera are related to each other, in the same manner as the varieties of any one species are related to each other. no naturalist pretends that all the species of a genus are equally distinct from each other; they may generally be divided into sub-genera, or sections, or lesser groups. as fries has well remarked, little groups of species are generally clustered like satellites around certain other species. and what are varieties but groups of forms, unequally related to each other, and clustered round certain forms--that is, round their parent-species? undoubtedly there is one most important point of difference between varieties and species; namely, that the amount of difference between varieties, when compared with each other or with their parent-species, is much less than that between the species of the same genus. but when we come to discuss the principle, as i call it, of divergence of character, { } we shall see how this may be explained, and how the lesser differences between varieties will tend to increase into the greater differences between species. there is one other point which seems to me worth notice. varieties generally have much restricted ranges: this statement is indeed scarcely more than a truism, for if a variety were found to have a wider range than that of its supposed parent-species, their denominations ought to be reversed. but there is also reason to believe, that those species which are very closely allied to other species, and in so far resemble varieties, often have much restricted ranges. for instance, mr. h. c. watson has marked for me in the well-sifted london catalogue of plants ( th edition) plants which are therein ranked as species, but which he considers as so closely allied to other species as to be of doubtful value: these reputed species range on an average over . of the provinces into which mr. watson has divided great britain. now, in this same catalogue, acknowledged varieties are recorded, and these range over . provinces; whereas, the species to which these varieties belong range over . provinces. so that the acknowledged varieties have very nearly the same restricted average range, as have those very closely allied forms, marked for me by mr. watson as doubtful species, but which are almost universally ranked by british botanists as good and true species. finally, then, varieties have the same general characters as species, for they cannot be distinguished from species,--except, firstly, by the discovery of intermediate linking forms, and the occurrence of such links cannot affect the actual characters of the forms which they connect; and except, secondly by a certain amount of { } difference, for two forms, if differing very little, are generally ranked as varieties, notwithstanding that intermediate linking forms have not been discovered; but the amount of difference considered necessary to give to two forms the rank of species is quite indefinite. in genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties. in large genera the species are apt to be closely, but unequally allied together, forming little clusters round certain species. species very closely allied to other species apparently have restricted ranges. in all these several respects the species of large genera present a strong analogy with varieties. and we can clearly understand these analogies, if species have once existed as varieties, and have thus originated: whereas, these analogies are utterly inexplicable if each species has been independently created. we have, also, seen that it is the most flourishing or dominant species of the larger genera which on an average vary most; and varieties, as we shall hereafter see, tend to become converted into new and distinct species. the larger genera thus tend to become larger; and throughout nature the forms of life which are now dominant tend to become still more dominant by leaving many modified and dominant descendants. but by steps hereafter to be explained, the larger genera also tend to break up into smaller genera. and thus, the forms of life throughout the universe become divided into groups subordinate to groups. * * * * * { } chapter iii. struggle for existence. bears on natural selection--the term used in a wide sense--geometrical powers of increase--rapid increase of naturalised animals and plants--nature of the checks to increase--competition universal--effects of climate--protection from the number of individuals--complex relations of all animals and plants throughout nature--struggle for life most severe between individuals and varieties of the same species; often severe between species of the same genus--the relation of organism to organism the most important of all relations. before entering on the subject of this chapter, i must make a few preliminary remarks, to show how the struggle for existence bears on natural selection. it has been seen in the last chapter that amongst organic beings in a state of nature there is some individual variability: indeed i am not aware that this has ever been disputed. it is immaterial for us whether a multitude of doubtful forms be called species or sub-species or varieties; what rank, for instance, the two or three hundred doubtful forms of british plants are entitled to hold, if the existence of any well-marked varieties be admitted. but the mere existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in nature. how have all those exquisite adaptations of one part of the organisation to another part, and to the conditions of life, and of one distinct organic being to another being, been perfected? we see these beautiful co-adaptations most { } plainly in the woodpecker and missletoe; and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world. again, it may be asked, how is it that varieties, which i have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? how do those groups of species, which constitute what are called distinct genera, and which differ from each other more than do the species of the same genus, arise? all these results, as we shall more fully see in the next chapter, follow from the struggle for life. owing to this struggle for life, any variation, however slight, and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. the offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. i have called this principle, by which each slight variation, if useful, is preserved, by the term of natural selection, in order to mark its relation to man's power of selection. we have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of nature. but natural selection, as we shall hereafter see, is a power incessantly ready for action, and is as { } immeasurably superior to man's feeble efforts, as the works of nature are to those of art. we will now discuss in a little more detail the struggle for existence. in my future work this subject shall be treated, as it well deserves, at much greater length. the elder de candolle and lyell have largely and philosophically shown that all organic beings are exposed to severe competition. in regard to plants, no one has treated this subject with more spirit and ability than w. herbert, dean of manchester, evidently the result of his great horticultural knowledge. nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult--at least i have found it so--than constantly to bear this conclusion in mind. yet unless it be thoroughly engrained in the mind, i am convinced that the whole economy of nature, with every fact on distribution, rarity, abundance, extinction, and variation, will be dimly seen or quite misunderstood. we behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that though food may be now superabundant, it is not so at all seasons of each recurring year. i should premise that i use the term struggle for existence in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. two canine animals in a time of dearth, may be truly said to struggle with each other which shall get food and live. but a plant on the edge of a desert is said to struggle { } for life against the drought, though more properly it should be said to be dependent on the moisture. a plant which annually produces a thousand seeds, of which on an average only one comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. the missletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for if too many of these parasites grow on the same tree, it will languish and die. but several seedling missletoes, growing close together on the same branch, may more truly be said to struggle with each other. as the missletoe is disseminated by birds, its existence depends on birds; and it may metaphorically be said to struggle with other fruit-bearing plants, in order to tempt birds to devour and thus disseminate its seeds rather than those of other plants. in these several senses, which pass into each other, i use for convenience' sake the general term of struggle for existence. a struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. it is the doctrine of malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there { } can be no artificial increase of food, and no prudential restraint from marriage. although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them. there is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair. even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing room for his progeny. linnæus has calculated that if an annual plant produced only two seeds--and there is no plant so unproductive as this--and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. the elephant is reckoned the slowest breeder of all known animals, and i have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pair of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair. but we have better evidence on this subject than mere theoretical calculations, namely, the numerous recorded cases of the astonishingly rapid increase of various animals in a state of nature, when circumstances have been favourable to them during two or three following seasons. still more striking is the evidence from our domestic animals of many kinds which have run wild in several parts of the world: if the statements of the rate of increase of slow-breeding cattle and horses in south america, and latterly in australia, had not been well authenticated, they would have been incredible. so it is with plants: cases could be given of { } introduced plants which have become common throughout whole islands in a period of less than ten years. several of the plants, such as the cardoon and a tall thistle, now most numerous over the wide plains of la plata, clothing square leagues of surface almost to the exclusion of all other plants, have been introduced from europe; and there are plants which now range in india, as i hear from dr. falconer, from cape comorin to the himalaya, which have been imported from america since its discovery. in such cases, and endless instances could be given, no one supposes that the fertility of these animals or plants has been suddenly and temporarily increased in any sensible degree. the obvious explanation is that the conditions of life have been very favourable, and that there has consequently been less destruction of the old and young, and that nearly all the young have been enabled to breed. in such cases the geometrical ratio of increase, the result of which never fails to be surprising, simply explains the extraordinarily rapid increase and wide diffusion of naturalised productions in their new homes. in a state of nature almost every plant produces seed, and amongst animals there are very few which do not annually pair. hence we may confidently assert, that all plants and animals are tending to increase at a geometrical ratio, that all would most rapidly stock every station in which they could any how exist, and that the geometrical tendency to increase must be checked by destruction at some period of life. our familiarity with the larger domestic animals tends, i think, to mislead us: we see no great destruction falling on them, and we forget that thousands are annually slaughtered for food, and that in a state of nature an equal number would have somehow to be disposed of. the only difference between organisms which annually { } produce eggs or seeds by the thousand, and those which produce extremely few, is, that the slow-breeders would require a few more years to people, under favourable conditions, a whole district, let it be ever so large. the condor lays a couple of eggs and the ostrich a score, and yet in the same country the condor may be the more numerous of the two: the fulmar petrel lays but one egg, yet it is believed to be the most numerous bird in the world. one fly deposits hundreds of eggs, and another, like the hippobosca, a single one; but this difference does not determine how many individuals of the two species can be supported in a district. a large number of eggs is of some importance to those species which depend on a rapidly fluctuating amount of food, for it allows them rapidly to increase in number. but the real importance of a large number of eggs or seeds is to make up for much destruction at some period of life; and this period in the great majority of cases is an early one. if an animal can in any way protect its own eggs or young, a small number may be produced, and yet the average stock be fully kept up; but if many eggs or young are destroyed, many must be produced, or the species will become extinct. it would suffice to keep up the full number of a tree, which lived on an average for a thousand years, if a single seed were produced once in a thousand years, supposing that this seed were never destroyed, and could be ensured to germinate in a fitting place. so that in all cases, the average number of any animal or plant depends only indirectly on the number of its eggs or seeds. in looking at nature, it is most necessary to keep the foregoing considerations always in mind--never to forget that every single organic being around us may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of { } its life; that heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals. lighten any check, mitigate the destruction ever so little, and the number of the species will almost instantaneously increase to any amount. the causes which check the natural tendency of each species to increase in number are most obscure. look at the most vigorous species; by as much as it swarms in numbers, by so much will its tendency to increase be still further increased. we know not exactly what the checks are in even one single instance. nor will this surprise any one who reflects how ignorant we are on this head, even in regard to mankind, so incomparably better known than any other animal. this subject has been ably treated by several authors, and i shall, in my future work, discuss some of the checks at considerable length, more especially in regard to the feral animals of south america. here i will make only a few remarks, just to recall to the reader's mind some of the chief points. eggs or very young animals seem generally to suffer most, but this is not invariably the case. with plants there is a vast destruction of seeds, but, from some observations which i have made, i believe that it is the seedlings which suffer most from germinating in ground already thickly stocked with other plants. seedlings, also, are destroyed in vast numbers by various enemies; for instance, on a piece of ground three feet long and two wide, dug and cleared, and where there could be no choking from other plants, i marked all the seedlings of our native weeds as they came up, and out of the no less than were destroyed, chiefly by slugs and insects. if turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants { } gradually kill the less vigorous, though fully grown, plants: thus out of twenty species growing on a little plot of turf (three feet by four) nine species perished from the other species being allowed to grow up freely. the amount of food for each species of course gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species. thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin. if not one head of game were shot during the next twenty years in england, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually killed. on the other hand, in some cases, as with the elephant and rhinoceros, none are destroyed by beasts of prey: even the tiger in india most rarely dares to attack a young elephant protected by its dam. climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought, i believe to be the most effective of all checks. i estimated that the winter of - destroyed four-fifths of the birds in my own grounds; and this is a tremendous destruction, when we remember that ten per cent, is an extraordinarily severe mortality from epidemics with man. the action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food. even when climate, for instance extreme cold, { } acts directly, it will be the least vigorous, or those which have got least food through the advancing winter, which will suffer most. when we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing; and the change of climate being conspicuous, we are tempted to attribute the whole effect to its direct action. but this is a false view: we forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers, and, as each area is already fully stocked with inhabitants, the other species will decrease. when we travel southward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in other species being favoured, as in this one being hurt. so it is when we travel northward, but in a somewhat lesser degree, for the number of species of all kinds, and therefore of competitors, decreases northwards; hence in going northward, or in ascending a mountain, we far oftener meet with stunted forms, due to the _directly_ injurious action of climate, than we do in proceeding southwards or in descending a mountain. when we reach the arctic regions, or snow-capped summits, or absolute deserts, the struggle for life is almost exclusively with the elements. that climate acts in main part indirectly by favouring other species, we may clearly see in the prodigious number of plants in our gardens which can perfectly well endure our climate, but which never become naturalised, for they cannot compete with our native plants nor resist destruction by our native animals. { } when a species, owing to highly favourable circumstances, increases inordinately in numbers in a small tract, epidemics--at least, this seems generally to occur with our game animals--often ensue: and here we have a limiting check independent of the struggle for life. but even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through facility of diffusion amongst the crowded animals, been disproportionably favoured: and here comes in a sort of struggle between the parasite and its prey. on the other hand, in many cases, a large stock of individuals of the same species, relatively to the numbers of its enemies, is absolutely necessary for its preservation. thus we can easily raise plenty of corn and rape-seed, &c., in our fields, because the seeds are in great excess compared with the number of birds which feed on them; nor can the birds, though having a superabundance of food at this one season, increase in number proportionally to the supply of seed, as their numbers are checked during winter: but any one who has tried, knows how troublesome it is to get seed from a few wheat or other such plants in a garden: i have in this case lost every single seed. this view of the necessity of a large stock of the same species for its preservation, explains, i believe, some singular facts in nature, such as that of very rare plants being sometimes extremely abundant in the few spots where they do occur; and that of some social plants being social, that is, abounding in individuals, even on the extreme confines of their range. for in such cases, we may believe, that a plant could exist only where the conditions of its life were so favourable that many could exist together, and thus save the species from utter destruction. i should add that the good effects of frequent intercrossing, and { } the ill effects of close interbreeding, probably come into play in some of these cases; but on this intricate subject i will not here enlarge. many cases are on record showing how complex and unexpected are the checks and relations between organic beings, which have to struggle together in the same country. i will give only a single instance, which, though a simple one, has interested me. in staffordshire, on the estate of a relation, where i had ample means of investigation, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five years previously and planted with scotch fir. the change in the native vegetation of the planted part of the heath was most remarkable, more than is generally seen in passing from one quite different soil to another: not only the proportional numbers of the heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath. the effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the heath; and the heath was frequented by two or three distinct insectivorous birds. here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception that the land had been enclosed, so that cattle could not enter. but how important an element enclosure is, i plainly saw near farnham, in surrey. here there are extensive heaths, with a few clumps of old scotch firs on the distant hill-tops: within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. { } when i ascertained that these young trees had not been sown or planted, i was so much surprised at their numbers that i went to several points of view, whence i could examine hundreds of acres of the unenclosed heath, and literally i could not see a single scotch fir, except the old planted clumps. but on looking closely between the stems of the heath, i found a multitude of seedlings and little trees, which had been perpetually browsed down by the cattle. in one square yard, at a point some hundred yards distant from one of the old clumps, i counted thirty-two little trees; and one of them, with twenty-six rings of growth, had during many years tried to raise its head above the stems of the heath, and had failed. no wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs. yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually searched it for food. here we see that cattle absolutely determine the existence of the scotch fir; but in several parts of the world insects determine the existence of cattle. perhaps paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state; and azara and rengger have shown that this is caused by the greater number in paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. the increase of these flies, numerous as they are, must be habitually checked by some means, probably by birds. hence, if certain insectivorous birds (whose numbers are probably regulated by hawks or beasts of prey) were to increase in paraguay, the flies would decrease--then cattle and horses would became feral, and this would certainly greatly { } alter (as indeed i have observed in parts of south america) the vegetation: this again would largely affect the insects; and this, as we just have seen in staffordshire, the insectivorous birds, and so onwards in ever-increasing circles of complexity. we began this series by insectivorous birds, and we have ended with them, not that in nature the relations can ever be as simple as this. battle within battle must ever be recurring with varying success; and yet in the long-run the forces are so nicely balanced, that the face of nature remains uniform for long periods of time, though assuredly the merest trifle would often give the victory to one organic being over another. nevertheless so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life! i am tempted to give one more instance showing how plants and animals, most remote in the scale of nature, are bound together by a web of complex relations. i shall hereafter have occasion to show that the exotic lobelia fulgens, in this part of england, is never visited by insects, and consequently, from its peculiar structure, never can set a seed. many of our orchidaceous plants absolutely require the visits of moths to remove their pollen-masses and thus to fertilise them. i have, also, reason to believe that humble-bees are indispensable to the fertilisation of the heartsease (viola tricolor), for other bees do not visit this flower. from experiments which i have lately tried, i have found that the visits of bees are necessary for the fertilisation of some kinds of clover; but humble-bees alone visit the red clover (trifolium pratense), as other bees cannot reach the nectar. hence i have very little doubt, that if the { } whole genus of humble-bees became extinct or very rare in england, the heartsease and red clover would become very rare, or wholly disappear. the number of humble-bees in any district depends in a great degree on the number of field-mice, which destroy their combs and nests; and mr. h. newman, who has long attended to the habits of humble-bees, believes that "more than two-thirds of them are thus destroyed all over england." now the number of mice is largely dependent, as every one knows, on the number of cats; and mr. newman says, "near villages and small towns i have found the nests of humble-bees more numerous than elsewhere, which i attribute to the number of cats that destroy the mice." hence it is quite credible that the presence of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district! in the case of every species, many different checks, acting at different periods of life, and during different seasons or years, probably come into play; some one check or some few being generally the most potent, but all concur in determining the average number or even the existence of the species. in some cases it can be shown that widely-different checks act on the same species in different districts. when we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. but how false a view is this! every one has heard that when an american forest is cut down, a very different vegetation springs up; but it has been observed that ancient indian ruins in the southern united states, which must formerly have been cleared of trees, now display the same beautiful diversity and proportion of kinds as in the surrounding { } virgin forests. what a struggle between the several kinds of trees must here have gone on during long centuries, each annually scattering its seeds by the thousand; what war between insect and insect--between insects, snails, and other animals with birds and beasts of prey--all striving to increase, and all feeding on each other or on the trees or their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees! throw up a handful of feathers, and all must fall to the ground according to definite laws; but how simple is this problem compared to the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the proportional numbers and kinds of trees now growing on the old indian ruins! the dependency of one organic being on another, as of a parasite on its prey, lies generally between beings remote in the scale of nature. this is often the case with those which may strictly be said to struggle with each other for existence, as in the case of locusts and grass-feeding quadrupeds. but the struggle almost invariably will be most severe between the individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers. in the case of varieties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon decided; for instance, if several varieties of wheat be sown together, and the mixed seed be resown, some of the varieties which best suit the soil or climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years quite supplant the other varieties. to keep up a mixed stock of even such extremely close varieties as the variously { } coloured sweet-peas, they must be each year harvested separately, and the seed then mixed in due proportion, otherwise the weaker kinds will steadily decrease in numbers and disappear. so again with the varieties of sheep: it has been asserted that certain mountain-varieties will starve out other mountain-varieties, so that they cannot be kept together. the same result has followed from keeping together different varieties of the medicinal leech. it may even be doubted whether the varieties of any one of our domestic plants or animals have so exactly the same strength, habits, and constitution, that the original proportions of a mixed stock could be kept up for half-a-dozen generations, if they were allowed to struggle together, like beings in a state of nature, and if the seed or young were not annually sorted. as species of the same genus have usually, though by no means invariably, some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera. we see this in the recent extension over parts of the united states of one species of swallow having caused the decrease of another species. the recent increase of the missel-thrush in parts of scotland has caused the decrease of the song-thrush. how frequently we hear of one species of rat taking the place of another species under the most different climates! in russia the small asiatic cockroach has everywhere driven before it its great congener. one species of charlock will supplant another, and so in other cases. we can dimly see why the competition should be most severe between allied forms, which fill nearly the same place in the economy of nature; { } but probably in no one case could we precisely say why one species has been victorious over another in the great battle of life. a corollary of the highest importance may be deduced from the foregoing remarks, namely, that the structure of every organic being is related, in the most essential yet often hidden manner, to that of all other organic beings, with which it comes into competition for food or residence, or from which it has to escape, or on which it preys. this is obvious in the structure of the teeth and talons of the tiger; and in that of the legs and claws of the parasite which clings to the hair on the tiger's body. but in the beautifully plumed seed of the dandelion, and in the flattened and fringed legs of the water-beetle, the relation seems at first confined to the elements of air and water. yet the advantage of plumed seeds no doubt stands in the closest relation to the land being already thickly clothed by other plants; so that the seeds may be widely distributed and fall on unoccupied ground. in the water-beetle, the structure of its legs, so well adapted for diving, allows it to compete with other aquatic insects, to hunt for its own prey, and to escape serving as prey to other animals. the store of nutriment laid up within the seeds of many plants seems at first sight to have no sort of relation to other plants. but from the strong growth of young plants produced from such seeds (as peas and beans), when sown in the midst of long grass, i suspect that the chief use of the nutriment in the seed is to favour the growth of the young seedling, whilst struggling with other plants growing vigorously all around. look at a plant in the midst of its range, why does it not double or quadruple its numbers? we know { } that it can perfectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges into slightly hotter or colder, damper or drier districts. in this case we can clearly see that if we wished in imagination to give the plant the power of increasing in number, we should have to give it some advantage over its competitors, or over the animals which preyed on it. on the confines of its geographical range, a change of constitution with respect to climate would clearly be an advantage to our plant; but we have reason to believe that only a few plants or animals range so far, that they are destroyed by the rigour of the climate alone. not until we reach the extreme confines of life, in the arctic regions or on the borders of an utter desert, will competition cease. the land may be extremely cold or dry, yet there will be competition between some few species, or between the individuals of the same species, for the warmest or dampest spots. hence, also, we can see that when a plant or animal is placed in a new country amongst new competitors, though the climate may be exactly the same as in its former home, yet the conditions of its life will generally be changed in an essential manner. if we wished to increase its average numbers in its new home, we should have to modify it in a different way to what we should have done in its native country; for we should have to give it some advantage over a different set of competitors or enemies. it is good thus to try in our imagination to give any form some advantage over another. probably in no single instance should we know what to do, so as to succeed. it will convince us of our ignorance on the mutual relations of all organic beings; a conviction as necessary, as it seems to be difficult to acquire. all that we can do, is to keep steadily in mind that each { } organic being is striving to increase at a geometrical ratio; that each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life, and to suffer great destruction. when we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply. * * * * * { } chapter iv. natural selection. natural selection--its power compared with man's selection--its power on characters of trifling importance--its power at all ages and on both sexes--sexual selection--on the generality of intercrosses between individuals of the same species--circumstances favourable and unfavourable to natural selection, namely, intercrossing, isolation, number of individuals--slow action--extinction caused by natural selection--divergence of character, related to the diversity of inhabitants of any small area, and to naturalisation--action of natural selection, through divergence of character and extinction, on the descendants from a common parent--explains the grouping of all organic beings. how will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation? can the principle of selection, which we have seen is so potent in the hands of man, apply in nature? i think we shall see that it can act most effectually. let it be borne in mind in what an endless number of strange peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the hereditary tendency is. under domestication, it may be truly said that the whole organisation becomes in some degree plastic. let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life. can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? if such do occur, can we doubt { } (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? on the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. this preservation of favourable variations and the rejection of injurious variations, i call natural selection. variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic. we shall best understand the probable course of natural selection by taking the case of a country undergoing some physical change, for instance, of climate. the proportional numbers of its inhabitants would almost immediately undergo a change, and some species might become extinct. we may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of some of the inhabitants, independently of the change of climate itself, would seriously affect many of the others. if the country were open on its borders, new forms would certainly immigrate, and this also would seriously disturb the relations of some of the former inhabitants. let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. but in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the original inhabitants were in some manner modified; for, had the area been open to immigration, these same { } places would have been seized on by intruders. in such case, every slight modification, which in the course of ages chanced to arise, and which in any way favoured the individuals of any of the species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would thus have free scope for the work of improvement. we have reason to believe, as stated in the first chapter, that a change in the conditions of life, by specially acting on the reproductive system, causes or increases variability; and in the foregoing case the conditions of life are supposed to have undergone a change, and this would manifestly be favourable to natural selection, by giving a better chance of profitable variations occurring; and unless profitable variations do occur, natural selection can do nothing. not that, as i believe, any extreme amount of variability is necessary; as man can certainly produce great results by adding up in any given direction mere individual differences, so could nature, but far more easily, from having incomparably longer time at her disposal. nor do i believe that any great physical change, as of climate, or any unusual degree of isolation to check immigration, is actually necessary to produce new and unoccupied places for natural selection to fill up by modifying and improving some of the varying inhabitants. for as all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modifications in the structure or habits of one inhabitant would often give it an advantage over others; and still further modifications of the same kind would often still further increase the advantage. no country can be named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of { } them could anyhow be improved; for in all countries, the natives have been so far conquered by naturalised productions, that they have allowed foreigners to take firm possession of the land. and as foreigners have thus everywhere beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted such intruders. as man can produce and certainly has produced a great result by his methodical and unconscious means of selection, what may not nature effect? man can act only on external and visible characters: nature cares nothing for appearances, except in so far as they may be useful to any being. she can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. man selects only for his own good; nature only for that of the being which she tends. every selected character is fully exercised by her; and the being is placed under well-suited conditions of life. man keeps the natives of many climates in the same country; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a long and a short beaked pigeon on the same food; he does not exercise a long-backed or long-legged quadruped in any peculiar manner; he exposes sheep with long and short wool to the same climate. he does not allow the most vigorous males to struggle for the females. he does not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his productions. he often begins his selection by some half-monstrous form; or at least by some modification prominent enough to catch his eye, or to be plainly useful to him. under nature, the slightest difference of structure or constitution may well turn the nicely-balanced scale in the struggle for life, and so be { } preserved. how fleeting are the wishes and efforts of man! how short his time! and consequently how poor will his products be, compared with those accumulated by nature during whole geological periods. can we wonder, then, that nature's productions should be far "truer" in character than man's productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship? it may metaphorically be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. we see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were. although natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance, may thus be acted on. when we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather, and the black-grouse that of peaty earth, we must believe that these tints are of service to these birds and insects in preserving them from danger. grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey--so much so, that on { } parts of the continent persons are warned not to keep white pigeons, as being the most liable to destruction. hence i can see no reason to doubt that natural selection might be most effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant. nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect: we should remember how essential it is in a flock of white sheep to destroy every lamb with the faintest trace of black. in plants the down on the fruit and the colour of the flesh are considered by botanists as characters of the most trifling importance: yet we hear from an excellent horticulturist, downing, that in the united states smooth-skinned fruits suffer far more from a beetle, a curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums; whereas another disease attacks yellow-fleshed peaches far more than those with other coloured flesh. if, with all the aids of art, these slight differences make a great difference in cultivating the several varieties, assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of enemies, such differences would effectually settle which variety, whether a smooth or downy, a yellow or purple fleshed fruit, should succeed. in looking at many small points of difference between species, which, as far as our ignorance permits us to judge, seem quite unimportant, we must not forget that climate, food, &c., probably produce some slight and direct effect. it is, however, far more necessary to bear in mind that there are many unknown laws of correlation of growth, which, when one part of the organisation is modified through variation, and the modifications are accumulated by natural selection for { } the good of the being, will cause other modifications, often of the most unexpected nature. as we see that those variations which under domestication appear at any particular period of life, tend to reappear in the offspring at the same period;--for instance, in the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult;--so in a state of nature, natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations profitable at that age, and by their inheritance at a corresponding age. if it profit a plant to have its seeds more and more widely disseminated by the wind, i can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees. natural selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern the mature insect. these modifications will no doubt affect, through the laws of correlation, the structure of the adult; and probably in the case of those insects which live only for a few hours, and which never feed, a large part of their structure is merely the correlated result of successive changes in the structure of their larvæ. so, conversely, modifications in the adult will probably often affect the structure of the larva; but in all cases natural selection will ensure that modifications consequent on other modifications at a different period of life, shall not be in the least degree injurious: for if they became so, they would cause the extinction of the species. natural selection will modify the structure of the { } young in relation to the parent, and of the parent in relation to the young. in social animals it will adapt the structure of each individual for the benefit of the community; if each in consequence profits by the selected change. what natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another species; and though statements to this effect may be found in works of natural history, i cannot find one case which will bear investigation. a structure used only once in an animal's whole life, if of high importance to it, might be modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, used exclusively for opening the cocoon--or the hard tip to the beak of nestling birds, used for breaking the egg. it has been asserted, that of the best short-beaked tumbler-pigeons more perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching. now, if nature had to make the beak of a full-grown pigeon very short for the bird's own advantage, the process of modification would be very slow, and there would be simultaneously the most rigorous selection of the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish: or, more delicate and more easily broken shells might be selected, the thickness of the shell being known to vary like every other structure. _sexual selection._--inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, the same fact probably occurs under nature, and if so, natural selection will be able to modify one sex in its functional relations to the other sex, or in relation to wholly different habits of life in the two sexes, as is sometimes the case { } with insects. and this leads me to say a few words on what i call sexual selection. this depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. sexual selection is, therefore, less rigorous than natural selection. generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. but in many cases, victory depends not on general vigour, but on having special weapons, confined to the male sex. a hornless stag or spurless cock would have a poor chance of leaving offspring. sexual selection by always allowing the victor to breed might surely give indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, as well as the brutal cock-fighter, who knows well that he can improve his breed by careful selection of the best cocks. how low in the scale of nature the law of battle descends, i know not; male alligators have been described as fighting, bellowing, and whirling round, like indians in a war-dance, for the possession of the females; male salmons have been seen fighting all day long; male stag-beetles often bear wounds from the huge mandibles of other males. the war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons. the males of carnivorous animals are already well armed; though to them and to others, special means of defence may be given through means of sexual selection, as the mane to the lion, the shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield may be as important for victory, as the sword or spear. amongst birds, the contest is often of a more peaceful character. all those who have attended to the subject, { } believe that there is the severest rivalry between the males of many species to attract by singing the females. the rock-thrush of guiana, birds of paradise, and some others, congregate; and successive males display their gorgeous plumage and perform strange antics before the females, which, standing by as spectators, at last choose the most attractive partner. those who have closely attended to birds in confinement well know that they often take individual preferences and dislikes: thus sir r. heron has described how one pied peacock was eminently attractive to all his hen birds. it may appear childish to attribute any effect to such apparently weak means: i cannot here enter on the details necessary to support this view; but if man can in a short time give elegant carriage and beauty to his bantams, according to his standard of beauty, i can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect. i strongly suspect that some well-known laws, with respect to the plumage of male and female birds, in comparison with the plumage of the young, can be explained on the view of plumage having been chiefly modified by sexual selection, acting when the birds have come to the breeding age or during the breeding season; the modifications thus produced being inherited at corresponding ages or seasons, either by the males alone, or by the males and females; but i have not space here to enter on this subject. thus it is, as i believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is, individual males have had, in successive generations, some slight advantage over other { } males, in their weapons, means of defence, or charms; and have transmitted these advantages to their male offspring. yet, i would not wish to attribute all such sexual differences to this agency: for we see peculiarities arising and becoming attached to the male sex in our domestic animals (as the wattle in male carriers, horn-like protuberances in the cocks of certain fowls, &c.), which we cannot believe to be either useful to the males in battle, or attractive to the females. we see analogous cases under nature, for instance, the tuft of hair on the breast of the turkey-cock, which can hardly be either useful or ornamental to this bird;--indeed, had the tuft appeared under domestication, it would have been called a monstrosity. _illustrations of the action of natural selection._--in order to make it clear how, as i believe, natural selection acts, i must beg permission to give one or two imaginary illustrations. let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf is hardest pressed for food. i can under such circumstances see no reason to doubt that the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or selected,--provided always that they retained strength to master their prey at this or at some other period of the year, when they might be compelled to prey on other animals. i can see no more reason to doubt this, than that man can improve the fleetness of his greyhounds by careful and methodical selection, or by that unconscious selection which results from each man trying { } to keep the best dogs without any thought of modifying the breed. even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might be born with an innate tendency to pursue certain kinds of prey. nor can this be thought very improbable; for we often observe great differences in the natural tendencies of our domestic animals; one cat, for instance, taking to catch rats, another mice; one cat, according to mr. st. john, bringing home winged game, another hares or rabbits, and another hunting on marshy ground and almost nightly catching woodcocks or snipes. the tendency to catch rats rather than mice is known to be inherited. now, if any slight innate change of habit or of structure benefited an individual wolf, it would have the best chance of surviving and of leaving offspring. some of its young would probably inherit the same habits or structure, and by the repetition of this process, a new variety might be formed which would either supplant or coexist with the parent form of wolf. or, again, the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to hunt different prey; and from the continued preservation of the individuals best fitted for the two sites, two varieties might slowly be formed. these varieties would cross and blend where they met; but to this subject of intercrossing we shall soon have to return. i may add, that, according to mr. pierce, there are two varieties of the wolf inhabiting the catskill mountains in the united states, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks. let us now take a more complex case. certain plants excrete a sweet juice, apparently for the sake of eliminating something injurious from their sap: this is { } effected by glands at the base of the stipules in some leguminosæ, and at the back of the leaf of the common laurel. this juice, though small in quantity, is greedily sought by insects. let us now suppose a little sweet juice or nectar to be excreted by the inner bases of the petals of a flower. in this case insects in seeking the nectar would get dusted with pollen, and would certainly often transport the pollen from one flower to the stigma of another flower. the flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, we have good reason to believe (as will hereafter be more fully alluded to), would produce very vigorous seedlings, which consequently would have the best chance of flourishing and surviving. some of these seedlings would probably inherit the nectar-excreting power. those individual flowers which had the largest glands or nectaries, and which excreted most nectar, would be oftenest visited by insects, and would be oftenest crossed; and so in the long-run would gain the upper hand. those flowers, also, which had their stamens and pistils placed, in relation to the size and habits of the particular insects which visited them, so as to favour in any degree the transportal of their pollen from flower to flower, would likewise be favoured or selected. we might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nectar; and as pollen is formed for the sole object of fertilisation, its destruction appears a simple loss to the plant; yet if a little pollen were carried, at first occasionally and then habitually, by the pollen-devouring insects from flower to flower, and a cross thus effected, although nine-tenths of the pollen were destroyed, it might still be a great gain to the plant; and those individuals which produced more and more pollen, and had larger and larger anthers, would be selected. { } when our plant, by this process of the continued preservation or natural selection of more and more attractive flowers, had been rendered highly attractive to insects, they would, unintentionally on their part, regularly carry pollen from flower to flower; and that they can most effectually do this, i could easily show by many striking instances. i will give only one--not as a very striking case, but as likewise illustrating one step in the separation of the sexes of plants, presently to be alluded to. some holly-trees bear only male flowers, which have four stamens producing a rather small quantity of pollen, and a rudimentary pistil; other holly-trees bear only female flowers; these have a full-sized pistil, and four stamens with shrivelled anthers, in which not a grain of pollen can be detected. having found a female tree exactly sixty yards from a male tree, i put the stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without exception, there were pollen-grains, and on some a profusion of pollen. as the wind had set for several days from the female to the male tree, the pollen could not thus have been carried. the weather had been cold and boisterous, and therefore not favourable to bees, nevertheless every female flower which i examined had been effectually fertilised by the bees, accidentally dusted with pollen, having flown from tree to tree in search of nectar. but to return to our imaginary case: as soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence. no naturalist doubts the advantage of what has been called the "physiological division of labour;" hence we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in { } another flower or on another plant. in plants under culture and placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent; now if we suppose this to occur in ever so slight a degree under nature, then as pollen is already carried regularly from flower to flower, and as a more complete separation of the sexes of our plant would be advantageous on the principle of the division of labour, individuals with this tendency more and more increased, would be continually favoured or selected, until at last a complete separation of the sexes would be effected. let us now turn to the nectar-feeding insects in our imaginary case: we may suppose the plant of which we have been slowly increasing the nectar by continued selection, to be a common plant; and that certain insects depended in main part on its nectar for food. i could give many facts, showing how anxious bees are to save time; for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which they can, with a very little more trouble, enter by the mouth. bearing such facts in mind, i can see no reason to doubt that an accidental deviation in the size and form of the body, or in the curvature and length of the proboscis, &c., far too slight to be appreciated by us, might profit a bee or other insect, so that an individual so characterised would be able to obtain its food more quickly, and so have a better chance of living and leaving descendants. its descendants would probably inherit a tendency to a similar slight deviation of structure. the tubes of the corollas of the common red and incarnate clovers (trifolium pratense and incarnatum) do not on a hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of the common red { } clover, which is visited by humble-bees alone; so that whole fields of the red clover offer in vain an abundant supply of precious nectar to the hive-bee. thus it might be a great advantage to the hive-bee to have a slightly longer or differently constructed proboscis. on the other hand, i have found by experiment that the fertility of clover depends on bees visiting and moving parts of the corolla, so as to push the pollen on to the stigmatic surface. hence, again, if humble-bees were to become rare in any country, it might be a great advantage to the red clover to have a shorter or more deeply divided tube to its corolla, so that the hive-bee could visit its flowers. thus i can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by the continued preservation of individuals presenting mutual and slightly favourable deviations of structure. i am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open to the same objections which were at first urged against sir charles lyell's noble views on "the modern changes of the earth, as illustrative of geology;" but we now seldom hear the action, for instance, of the coast-waves, called a trifling and insignificant cause, when applied to the excavation of gigantic valleys or to the formation of the longest lines of inland cliffs. natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic { } beings, or of any great and sudden modification in their structure. _on the intercrossing of individuals._--i must here introduce a short digression. in the case of animals and plants with separated sexes, it is of course obvious that two individuals must always (with the exception of the curious and not well-understood cases of parthenogenesis) unite for each birth; but in the case of hermaphrodites this is far from obvious. nevertheless i am strongly inclined to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur for the reproduction of their kind. this view was first suggested by andrew knight. we shall presently see its importance; but i must here treat the subject with extreme brevity, though i have the materials prepared for an ample discussion. all vertebrate animals, all insects, and some other large groups of animals, pair for each birth. modern research has much diminished the number of supposed hermaphrodites, and of real hermaphrodites a large number pair; that is, two individuals regularly unite for reproduction, which is all that concerns us. but still there are many hermaphrodite animals which certainly do not habitually pair, and a vast majority of plants are hermaphrodites. what reason, it may be asked, is there for supposing in these cases that two individuals ever concur in reproduction? as it is impossible here to enter on details, i must trust to some general considerations alone. in the first place, i have collected so large a body of facts, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and { } fertility to the offspring; and on the other hand, that _close_ interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a general law of nature (utterly ignorant though we be of the meaning of the law) that no organic being self-fertilises itself for an eternity of generations; but that a cross with another individual is occasionally--perhaps at very long intervals--indispensable. on the belief that this is a law of nature, we can, i think, understand several large classes of facts, such as the following, which on any other view are inexplicable. every hybridizer knows how unfavourable exposure to wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully exposed to the weather! but if an occasional cross be indispensable, the fullest freedom for the entrance of pollen from another individual will explain this state of exposure, more especially as the plant's own anthers and pistil generally stand so close together that self-fertilisation seems almost inevitable. many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family; but in several, perhaps in all, such flowers, there is a very curious adaptation between the structure of the flower and the manner in which bees suck the nectar; for, in doing this, they either push the flower's own pollen on the stigma, or bring pollen from another flower. so necessary are the visits of bees to papilionaceous flowers, that i have found, by experiments published elsewhere, that their fertility is greatly diminished if these visits be prevented. now, it is scarcely possible that bees should fly from flower to flower, and not carry pollen from one to the other, to the great good, as i believe, of the plant. bees will act like a camel-hair pencil, and it is quite sufficient just to touch the anthers of { } one flower and then the stigma of another with the same brush to ensure fertilisation; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species; for if you bring on the same brush a plant's own pollen and pollen from another species, the former will have such a prepotent effect, that it will invariably and completely destroy, as has been shown by gärtner, any influence from the foreign pollen. when the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards it, the contrivance seems adapted solely to ensure self-fertilisation; and no doubt it is useful for this end: but, the agency of insects is often required to cause the stamens to spring forward, as kölreuter has shown to be the case with the barberry; and in this very genus, which seems to have a special contrivance for self-fertilisation, it is well known that if closely-allied forms or varieties are planted near each other, it is hardly possible to raise pure seedlings, so largely do they naturally cross. in many other cases, far from there being any aids for self-fertilisation, there are special contrivances, as i could show from the writings of c. c. sprengel and from my own observations, which effectually prevent the stigma receiving pollen from its own flower: for instance, in lobelia fulgens, there is a really beautiful and elaborate contrivance by which every one of the infinitely numerous pollen-granules are swept out of the conjoined anthers of each flower, before the stigma of that individual flower is ready to receive them; and as this flower is never visited, at least in my garden, by insects, it never sets a seed, though by placing pollen from one flower on the stigma of another, i raised plenty of seedlings; and whilst another species of lobelia growing close by, which is visited by bees, seeds freely. in very many other cases, though there { } be no special mechanical contrivance to prevent the stigma of a flower receiving its own pollen, yet, as c. c. sprengel has shown, and as i can confirm, either the anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that these plants have in fact separated sexes, and must habitually be crossed. how strange are these facts! how strange that the pollen and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of self-fertilisation, should in so many cases be mutually useless to each other! how simply are these facts explained on the view of an occasional cross with a distinct individual being advantageous or indispensable! if several varieties of the cabbage, radish, onion, and of some other plants, be allowed to seed near each other, a large majority, as i have found, of the seedlings thus raised will turn out mongrels: for instance, i raised seedling cabbages from some plants of different varieties growing near each other, and of these only were true to their kind, and some even of these were not perfectly true. yet the pistil of each cabbage-flower is surrounded not only by its own six stamens, but by those of the many other flowers on the same plant. how, then, comes it that such a vast number of the seedlings are mongrelized? i suspect that it must arise from the pollen of a distinct _variety_ having a prepotent effect over a flower's own pollen; and that this is part of the general law of good being derived from the intercrossing of distinct individuals of the same species. when distinct _species_ are crossed the case is directly the reverse, for a plant's own pollen is always prepotent over foreign pollen; but to this subject we shall return in a future chapter. in the case of a gigantic tree covered with { } innumerable flowers, it may be objected that pollen could seldom be carried from tree to tree, and at most only from flower to flower on the same tree, and that flowers on the same tree can be considered as distinct individuals only in a limited sense. i believe this objection to be valid, but that nature has largely provided against it by giving to trees a strong tendency to bear flowers with separated sexes. when the sexes are separated, although the male and female flowers may be produced on the same tree, we can see that pollen must be regularly carried from flower to flower; and this will give a better chance of pollen being occasionally carried from tree to tree. that trees belonging to all orders have their sexes more often separated than other plants, i find to be the case in this country; and at my request dr. hooker tabulated the trees of new zealand, and dr. asa gray those of the united states, and the result was as i anticipated. on the other hand, dr. hooker has recently informed me that he finds that the rule does not hold in australia; and i have made these few remarks on the sexes of trees simply to call attention to the subject. turning for a very brief space to animals: on the land there are some hermaphrodites, as land-mollusca and earth-worms; but these all pair. as yet i have not found a single case of a terrestrial animal which fertilises itself. we can understand this remarkable fact, which offers so strong a contrast with terrestrial plants, on the view of an occasional cross being indispensable, by considering the medium in which terrestrial animals live, and the nature of the fertilising element; for we know of no means, analogous to the action of insects and of the wind in the case of plants, by which an occasional cross could be effected with terrestrial animals without the concurrence of two individuals. of aquatic animals, there are many self-fertilising hermaphrodites; but here { } currents in the water offer an obvious means for an occasional cross. and, as in the case of flowers, i have as yet failed, after consultation with one of the highest authorities, namely, professor huxley, to discover a single case of an hermaphrodite animal with the organs of reproduction so perfectly enclosed within the body, that access from without and the occasional influence of a distinct individual can be shown to be physically impossible. cirripedes long appeared to me to present a case of very great difficulty under this point of view; but i have been enabled, by a fortunate chance, elsewhere to prove that two individuals, though both are self-fertilising hermaphrodites, do sometimes cross. it must have struck most naturalists as a strange anomaly that, in the case of both animals and plants, species of the same family and even of the same genus, though agreeing closely with each other in almost their whole organisation, yet are not rarely, some of them hermaphrodites, and some of them unisexual. but if, in fact, all hermaphrodites do occasionally intercross with other individuals, the difference between hermaphrodites and unisexual species, as far as function is concerned, becomes very small. from these several considerations and from the many special facts which i have collected, but which i am not here able to give, i am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct individual is a law of nature. i am well aware that there are, on this view, many cases of difficulty, some of which i am trying to investigate. finally then, we may conclude that in many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it occurs perhaps only at long intervals; but in none, as i suspect, can self-fertilisation go on for perpetuity. { } _circumstances favourable to natural selection._--this is an extremely intricate subject. a large amount of inheritable and diversified variability is favourable, but i believe mere individual differences suffice for the work. a large number of individuals, by giving a better chance for the appearance within any given period of profitable variations, will compensate for a lesser amount of variability in each individual, and is, i believe, an extremely important element of success. though nature grants vast periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving, it may be said, to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will soon be exterminated. in man's methodical selection, a breeder selects for some definite object, and free intercrossing will wholly stop his work. but when many men, without intending to alter the breed, have a nearly common standard of perfection, and all try to get and breed from the best animals, much improvement and modification surely but slowly follow from this unconscious process of selection, notwithstanding a large amount of crossing with inferior animals. thus it will be in nature; for within a confined area, with some place in its polity not so perfectly occupied as might be, natural selection will always tend to preserve all the individuals varying in the right direction, though in different degrees, so as better to fill up the unoccupied place. but if the area be large, its several districts will almost certainly present different conditions of life; and then if natural selection be modifying and improving a species in the several districts, there will be intercrossing with the other individuals of the same species on the confines of each. and in { } this case the effects of intercrossing can hardly be counterbalanced by natural selection always tending to modify all the individuals in each district in exactly the same manner to the conditions of each; for in a continuous area, the physical conditions at least will generally graduate away insensibly from one district to another. the intercrossing will most affect those animals which unite for each birth, which wander much, and which do not breed at a very quick rate. hence in animals of this nature, for instance in birds, varieties will generally be confined to separated countries; and this i believe to be the case. in hermaphrodite organisms which cross only occasionally, and likewise in animals which unite for each birth, but which wander little and which can increase at a very rapid rate, a new and improved variety might be quickly formed on any one spot, and might there maintain itself in a body, so that whatever intercrossing took place would be chiefly between the individuals of the same new variety. a local variety when once thus formed might subsequently slowly spread to other districts. on the above principle, nurserymen always prefer getting seed from a large body of plants of the same variety, as the chance of intercrossing with other varieties is thus lessened. even in the case of slow-breeding animals, which unite for each birth, we must not overrate the effects of intercrosses in retarding natural selection; for i can bring a considerable catalogue of facts, showing that within the same area, varieties of the same animal can long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from varieties of the same kind preferring to pair together. intercrossing plays a very important part in nature in keeping the individuals of the same species, or of the same variety, true and uniform in character. it will { } obviously thus act far more efficiently with those animals which unite for each birth; but i have already attempted to show that we have reason to believe that occasional intercrosses take place with all animals and with all plants. even if these take place only at long intervals, i am convinced that the young thus produced will gain so much in vigour and fertility over the offspring from long-continued self-fertilisation, that they will have a better chance of surviving and propagating their kind; and thus, in the long run, the influence of intercrosses, even at rare intervals, will be great. if there exist organic beings which never intercross, uniformity of character can be retained amongst them, as long as their conditions of life remain the same, only through the principle of inheritance, and through natural selection destroying any which depart from the proper type; but if their conditions of life change and they undergo modification, uniformity of character can be given to their modified offspring, solely by natural selection preserving the same favourable variations. isolation, also, is an important element in the process of natural selection. in a confined or isolated area, if not very large, the organic and inorganic conditions of life will generally be in a great degree uniform; so that natural selection will tend to modify all the individuals of a varying species throughout the area in the same manner in relation to the same conditions. intercrosses, also, with the individuals of the same species, which otherwise would have inhabited the surrounding and differently circumstanced districts, will be prevented. but isolation probably acts more efficiently in checking the immigration of better adapted organisms, after any physical change, such as of climate or elevation of the land, &c.; and thus new places in the natural economy of the country are left open for the old inhabitants to struggle for, and become adapted to, through { } modifications in their structure and constitution. lastly, isolation, by checking immigration and consequently competition, will give time for any new variety to be slowly improved; and this may sometimes be of importance in the production of new species. if, however, an isolated area be very small, either from being surrounded by barriers, or from having very peculiar physical conditions, the total number of the individuals supported on it will necessarily be very small; and fewness of individuals will greatly retard the production of new species through natural selection, by decreasing the chance of the appearance of favourable variations. if we turn to nature to test the truth of these remarks, and look at any small isolated area, such as an oceanic island, although the total number of the species inhabiting it, will be found to be small, as we shall see in our chapter on geographical distribution; yet of these species a very large proportion are endemic,--that is, have been produced there, and nowhere else. hence an oceanic island at first sight seems to have been highly favourable for the production of new species. but we may thus greatly deceive ourselves, for to ascertain whether a small isolated area, or a large open area like a continent, has been most favourable for the production of new organic forms, we ought to make the comparison within equal times; and this we are incapable of doing. although i do not doubt that isolation is of considerable importance in the production of new species, on the whole i am inclined to believe that largeness of area is of more importance, more especially in the production of species, which will prove capable of enduring for a long period, and of spreading widely. throughout a great and open area, not only will there be a better chance of favourable variations arising from the large number of individuals of the same species { } there supported, but the conditions of life are infinitely complex from the large number of already existing species; and if some of these many species become modified and improved, others will have to be improved in a corresponding degree or they will be exterminated. each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into competition with many others. hence more new places will be formed, and the competition to fill them will be more severe, on a large than on a small and isolated area. moreover, great areas, though now continuous, owing to oscillations of level, will often have recently existed in a broken condition, so that the good effects of isolation will generally, to a certain extent, have concurred. finally, i conclude that, although small isolated areas probably have been in some respects highly favourable for the production of new species, yet that the course of modification will generally have been more rapid on large areas; and what is more important, that the new forms produced on large areas, which already have been victorious over many competitors, will be those that will spread most widely, will give rise to most new varieties and species, and will thus play an important part in the changing history of the organic world. we can, perhaps, on these views, understand some facts which will be again alluded to in our chapter on geographical distribution; for instance, that the productions of the smaller continent of australia have formerly yielded, and apparently are now yielding, before those of the larger europæo-asiatic area. thus, also, it is that continental productions have everywhere become so largely naturalised on islands. on a small island, the race for life will have been less severe, and there will have been less modification and less { } extermination. hence, perhaps, it comes that the flora of madeira, according to oswald heer, resembles the extinct tertiary flora of europe. all fresh-water basins, taken together, make a small area compared with that of the sea or of the land; and, consequently, the competition between fresh-water productions will have been less severe than elsewhere; new forms will have been more slowly formed, and old forms more slowly exterminated. and it is in fresh water that we find seven genera of ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world, as the ornithorhynchus and lepidosiren, which, like fossils, connect to a certain extent orders now widely separated in the natural scale. these anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition. to sum up the circumstances favourable and unfavourable to natural selection, as far as the extreme intricacy of the subject permits. i conclude, looking to the future, that for terrestrial productions a large continental area, which will probably undergo many oscillations of level, and which consequently will exist for long periods in a broken condition, is the most favourable for the production of many new forms of life, likely to endure long and to spread widely. for the area first existed as a continent, and the inhabitants, at this period numerous in individuals and kinds, will have been subjected to very severe competition. when converted by subsidence into large separate islands, there will still exist many individuals of the same species on each island: intercrossing on the confines of the range of each species will thus be checked: after physical changes of any kind, immigration will be { } prevented, so that new places in the polity of each island will have to be filled up by modifications of the old inhabitants; and time will be allowed for the varieties in each to become well modified and perfected. when, by renewed elevation, the islands shall be re-converted into a continental area, there will again be severe competition: the most favoured or improved varieties will be enabled to spread: there will be much extinction of the less improved forms, and the relative proportional numbers of the various inhabitants of the renewed continent will again be changed; and again there will be a fair field for natural selection to improve still further the inhabitants, and thus produce new species. that natural selection will always act with extreme slowness, i fully admit. its action depends on there being places in the polity of nature, which can be better occupied by some of the inhabitants of the country undergoing modification of some kind. the existence of such places will often depend on physical changes, which are generally very slow, and on the immigration of better adapted forms having been checked. but the action of natural selection will probably still oftener depend on some of the inhabitants becoming slowly modified; the mutual relations of many of the other inhabitants being thus disturbed. nothing can be effected, unless favourable variations occur, and variation itself is apparently always a very slow process. the process will often be greatly retarded by free intercrossing. many will exclaim that these several causes are amply sufficient wholly to stop the action of natural selection. i do not believe so. on the other hand, i do believe that natural selection always acts very slowly, often only at long intervals of time, and generally on only a very few of the inhabitants of the same region at the same time. i further believe, that this very slow, { } intermittent action of natural selection accords perfectly well with what geology tells us of the rate and manner at which the inhabitants of this world have changed. slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, i can see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection. _extinction._--this subject will be more fully discussed in our chapter on geology; but it must be here alluded to from being intimately connected with natural selection. natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. but as from the high geometrical ratio of increase of all organic beings, each area is already fully stocked with inhabitants, it follows that as each selected and favoured form increases in number, so will the less favoured forms decrease and become rare. rarity, as geology tells us, is the precursor to extinction. we can, also, see that any form represented by few individuals will, during fluctuations in the seasons or in the number of its enemies, run a good chance of utter extinction. but we may go further than this; for as new forms are continually and slowly being produced, unless we believe that the number of specific forms goes on perpetually and almost indefinitely increasing, numbers inevitably must become extinct. that the number of specific forms has not indefinitely increased, geology shows us plainly; and indeed we can see reason why they should not have thus increased, for the number of places in the polity of nature is not indefinitely great,--not that we { } have any means of knowing that any one region has as yet got its maximum of species. probably no region is as yet fully stocked, for at the cape of good hope, where more species of plants are crowded together than in any other quarter of the world, some foreign plants have become naturalised, without causing, as far as we know, the extinction of any natives. furthermore, the species which are most numerous in individuals will have the best chance of producing within any given period favourable variations. we have evidence of this, in the facts given in the second chapter, showing that it is the common species which afford the greatest number of recorded varieties, or incipient species. hence, rare species will be less quickly modified or improved within any given period, and they will consequently be beaten in the race for life by the modified descendants of the commoner species. from these several considerations i think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct. the forms which stand in closest competition with those undergoing modification and improvement, will naturally suffer most. and we have seen in the chapter on the struggle for existence that it is the most closely-allied forms,--varieties of the same species, and species of the same genus or of related genera,--which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other. consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them. we see the same process of extermination amongst our domesticated productions, through the selection of improved forms by man. many curious { } instances could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds. in yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these "were swept away by the short-horns" (i quote the words of an agricultural writer) "as if by some murderous pestilence." _divergence of character._--the principle, which i have designated by this term, is of high importance on my theory, and explains, as i believe, several important facts. in the first place, varieties, even strongly-marked ones, though having somewhat of the character of species--as is shown by the hopeless doubts in many cases how to rank them--yet certainly differ from each other far less than do good and distinct species. nevertheless, according to my view, varieties are species in the process of formation, or are, as i have called them, incipient species. how, then, does the lesser difference between varieties become augmented into the greater difference between species? that this does habitually happen, we must infer from most of the innumerable species throughout nature presenting well-marked differences; whereas varieties, the supposed prototypes and parents of future well-marked species, present slight and ill-defined differences. mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a greater degree; but this alone would never account for so habitual and large an amount of difference as that between varieties of the same species and species of the same genus. as has always been my practice, let us seek light on { } this head from our domestic productions. we shall here find something analogous. a fancier is struck by a pigeon having a slightly shorter beak; another fancier is struck by a pigeon having a rather longer beak; and on the acknowledged principle that "fanciers do not and will not admire a medium standard, but like extremes," they both go on (as has actually occurred with tumbler-pigeons) choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks. again, we may suppose that at an early period one man preferred swifter horses; another stronger and more bulky horses. the early differences would be very slight; in the course of time, from the continued selection of swifter horses by some breeders, and of stronger ones by others, the differences would become greater, and would be noted as forming two sub-breeds; finally, after the lapse of centuries, the sub-breeds would become converted into two well-established and distinct breeds. as the differences slowly become greater, the inferior animals with intermediate characters, being neither very swift nor very strong, will have been neglected, and will have tended to disappear. here, then, we see in man's productions the action of what may be called the principle of divergence, causing differences, at first barely appreciable, steadily to increase, and the breeds to diverge in character both from each other and from their common parent. but how, it may be asked, can any analogous principle apply in nature? i believe it can and does apply most efficiently, from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers. { } we can clearly see this in the case of animals with simple habits. take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. if its natural powers of increase be allowed to act, it can succeed in increasing (the country not undergoing any change in its conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations, climbing trees, frequenting water, and some perhaps becoming less carnivorous. the more diversified in habits and structure the descendants of our carnivorous animal became, the more places they would be enabled to occupy. what applies to one animal will apply throughout all time to all animals--that is, if they vary--for otherwise natural selection can do nothing. so it will be with plants. it has been experimentally proved, that if a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbage can thus be raised. the same has been found to hold good when first one variety and then several mixed varieties of wheat have been sown on equal spaces of ground. hence, if any one species of grass were to go on varying, and those varieties were continually selected which differed from each other in at all the same manner as distinct species and genera of grasses differ from each other, a greater number of individual plants of this species of grass, including its modified descendants, would succeed in living on the same piece of ground. and we well know that each species and each variety of grass is annually sowing almost countless seeds; and thus, as it may be said, is striving its utmost to increase its numbers. { } consequently, i cannot doubt that in the course of many thousands of generations, the most distinct varieties of any one species of grass would always have the best chance of succeeding and of increasing in numbers, and thus of supplanting the less distinct varieties; and varieties, when rendered very distinct from each other, take the rank of species. the truth of the principle, that the greatest amount of life can be supported by great diversification of structure, is seen under many natural circumstances. in an extremely small area, especially if freely open to immigration, and where the contest between individual and individual must be severe, we always find great diversity in its inhabitants. for instance, i found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants, and these belonged to eighteen genera and to eight orders, which shows how much these plants differed from each other. so it is with the plants and insects on small and uniform islets; and so in small ponds of fresh water. farmers find that they can raise most food by a rotation of plants belonging to the most different orders: nature follows what may be called a simultaneous rotation. most of the animals and plants which live close round any small piece of ground, could live on it (supposing it not to be in any way peculiar in its nature), and may be said to be striving to the utmost to live there; but, it is seen, that where they come into the closest competition with each other, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders. the same principle is seen in the naturalisation of { } plants through man's agency in foreign lands. it might have been expected that the plants which have succeeded in becoming naturalised in any land would generally have been closely allied to the indigenes; for these are commonly looked at as specially created and adapted for their own country. it might, also, perhaps have been expected that naturalised plants would have belonged to a few groups more especially adapted to certain stations in their new homes. but the case is very different; and alph. de candolle has well remarked in his great and admirable work, that floras gain by naturalisation, proportionally with the number of the native genera and species, far more in new genera than in new species. to give a single instance: in the last edition of dr. asa gray's 'manual of the flora of the northern united states,' naturalised plants are enumerated, and these belong to genera. we thus see that these naturalised plants are of a highly diversified nature. they differ, moreover, to a large extent from the indigenes, for out of the genera, no less than genera are not there indigenous, and thus a large proportional addition is made to the genera of these states. by considering the nature of the plants or animals which have struggled successfully with the indigenes of any country, and have there become naturalised, we may gain some crude idea in what manner some of the natives would have to be modified, in order to gain an advantage over the other natives; and we may at least safely infer that diversification of structure, amounting to new generic differences, would be profitable to them. the advantage of diversification in the inhabitants of the same region is, in fact, the same as that of the physiological division of labour in the organs of the same individual body--a subject so well elucidated by milne { } edwards. no physiologist doubts that a stomach adapted to digest vegetable matter alone, or flesh alone, draws most nutriment from these substances. so in the general economy of any land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will a greater number of individuals be capable of there supporting themselves. a set of animals, with their organisation but little diversified, could hardly compete with a set more perfectly diversified in structure. it may be doubted, for instance, whether the australian marsupials, which are divided into groups differing but little from each other, and feebly representing, as mr. waterhouse and others have remarked, our carnivorous, ruminant, and rodent mammals, could successfully compete with these well-pronounced orders. in the australian mammals, we see the process of diversification in an early and incomplete stage of development. after the foregoing discussion, which ought to have been much amplified, we may, i think, assume that the modified descendants of any one species will succeed by so much the better as they become more diversified in structure, and are thus enabled to encroach on places occupied by other beings. now let us see how this principle of benefit being derived from divergence of character, combined with the principles of natural selection and of extinction, will tend to act. the accompanying diagram will aid us in understanding this rather perplexing subject. let a to l represent the species of a genus large in its own country; these species are supposed to resemble each other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the letters standing at unequal distances. i have said a large genus, because we have seen in the second chapter, { } that on an average more of the species of large genera vary than of small genera; and the varying species of the large genera present a greater number of varieties. we have, also, seen that the species, which are the commonest and the most widely-diffused, vary more than rare species with restricted ranges. let (a) be a common, widely-diffused, and varying species, belonging to a genus large in its own country. the little fan of diverging dotted lines of unequal lengths proceeding from (a), may represent its varying offspring. the variations are supposed to be extremely slight, but of the most diversified nature; they are not supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed to endure for equal periods. only those variations which are in some way profitable will be preserved or naturally selected. and here the importance of the principle of benefit being derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection. when a dotted line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to have formed a fairly well-marked variety, such as would be thought worthy of record in a systematic work. [illustration] the intervals between the horizontal lines in the diagram, may represent each a thousand generations; but it would have been better if each had represented ten thousand generations. after a thousand generations, species (a) is supposed to have produced two fairly well-marked varieties, namely a^ and m^ . these two varieties will generally continue to be exposed to the same conditions which made their parents variable, { } and the tendency to variability is in itself hereditary, consequently they will tend to vary, and generally to vary in nearly the same manner as their parents varied. moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages which made their parent (a) more numerous than most of the other inhabitants of the same country; they will likewise partake of those more general advantages which made the genus to which the parent-species belonged, a large genus in its own country. and these circumstances we know to be favourable to the production of new varieties. if, then, these two varieties be variable, the most divergent of their variations will generally be preserved during the next thousand generations. and after this interval, variety a^ is supposed in the diagram to have produced variety a^ , which will, owing to the principle of divergence, differ more from (a) than did variety a^ . variety m^ is supposed to have produced two varieties, namely m^ and s^ , differing from each other, and more considerably from their common parent (a). we may continue the process by similar steps for any length of time; some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modified condition, some producing two or three varieties, and some failing to produce any. thus the varieties or modified descendants, proceeding from the common parent (a), will generally go on increasing in number and diverging in character. in the diagram the process is represented up to the ten-thousandth generation, and under a condensed and simplified form up to the fourteen-thousandth generation. but i must here remark that i do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular. { } i am far from thinking that the most divergent varieties will invariably prevail and multiply: a medium form may often long endure, and may or may not produce more than one modified descendant; for natural selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely complex relations. but as a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will be increased. in our diagram the line of succession is broken at regular intervals by small numbered letters marking the successive forms which have become sufficiently distinct to be recorded as varieties. but these breaks are imaginary, and might have been inserted anywhere, after intervals long enough to have allowed the accumulation of a considerable amount of divergent variation. as all the modified descendants from a common and widely-diffused species, belonging to a large genus, will tend to partake of the same advantages which made their parent successful in life, they will generally go on multiplying in number as well as diverging in character: this is represented in the diagram by the several divergent branches proceeding from (a). the modified offspring from the later and more highly improved branches in the lines of descent, will, it is probable, often take the place of, and so destroy, the earlier and less improved branches: this is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines. in some cases i do not doubt that the process of modification will be confined to a single line of descent, and the number of the descendants will not be increased; although the amount { } of divergent modification may have been increased in the successive generations. this case would be represented in the diagram, if all the lines proceeding from (a) were removed, excepting that from a^ to a^{ }. in the same way, for instance, the english race-horse and english pointer have apparently both gone on slowly diverging in character from their original stocks, without either having given off any fresh branches or races. after ten thousand generations, species (a) is supposed to have produced three forms, a^{ }, f^{ }, and m^{ }, which, from having diverged in character during the successive generations, will have come to differ largely, but perhaps unequally, from each other and from their common parent. if we suppose the amount of change between each horizontal line in our diagram to be excessively small, these three forms may still be only well-marked varieties; or they may have arrived at the doubtful category of sub-species; but we have only to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into well-defined species: thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species. by continuing the same process for a greater number of generations (as shown in the diagram in a condensed and simplified manner), we get eight species, marked by the letters between a^{ } and m^{ }, all descended from (a). thus, as i believe, species are multiplied and genera are formed. in a large genus it is probable that more than one species would vary. in the diagram i have assumed that a second species (i) has produced, by analogous steps, after ten thousand generations, either two well-marked varieties (w^{ } and z^{ }) or two species, according to the amount of change supposed to be represented { } between the horizontal lines. after fourteen thousand generations, six new species, marked by the letters n^{ } to z^{ }, are supposed to have been produced. in each genus, the species, which are already extremely different in character, will generally tend to produce the greatest number of modified descendants; for these will have the best chance of filling new and widely different places in the polity of nature: hence in the diagram i have chosen the extreme species (a), and the nearly extreme species (i), as those which have largely varied, and have given rise to new varieties and species. the other nine species (marked by capital letters) of our original genus, may for a long period continue to transmit unaltered descendants; and this is shown in the diagram by the dotted lines not prolonged far upwards from want of space. but during the process of modification, represented in the diagram, another of our principles, namely that of extinction, will have played an important part. as in each fully stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms, there will be a constant tendency in the improved descendants of any one species to supplant and exterminate in each stage of descent their predecessors and their original parent. for it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution, and structure. hence all the intermediate forms between the earlier and later states, that is between the less and more improved state of a species, as well as the original parent-species itself, will generally tend to become extinct. so it probably will be with many whole collateral lines of descent, which will be conquered by later and improved lines of descent. if, however, the { } modified offspring of a species get into some distinct country, or become quickly adapted to some quite new station, in which child and parent do not come into competition, both may continue to exist. if then our diagram be assumed to represent a considerable amount of modification, species (a) and all the earlier varieties will have become extinct, having been replaced by eight new species (a^{ } to m^{ }); and (i) will have been replaced by six (n^{ } to z^{ }) new species. but we may go further than this. the original species of our genus were supposed to resemble each other in unequal degrees, as is so generally the case in nature; species (a) being more nearly related to b, c, and d, than to the other species; and species (i) more to g, h, k, l, than to the others. these two species (a) and (i), were also supposed to be very common and widely diffused species, so that they must originally have had some advantage over most of the other species of the genus. their modified descendants, fourteen in number at the fourteen-thousandth generation, will probably have inherited some of the same advantages: they have also been modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many related places in the natural economy of their country. it seems, therefore, to me extremely probable that they will have taken the places of, and thus exterminated, not only their parents (a) and (i), but likewise some of the original species which were most nearly related to their parents. hence very few of the original species will have transmitted offspring to the fourteen-thousandth generation. we may suppose that only one (f), of the two species which were least closely related to the other nine original species, has transmitted descendants to this late stage of descent. { } the new species in our diagram descended from the original eleven species, will now be fifteen in number. owing to the divergent tendency of natural selection, the extreme amount of difference in character between species a^{ } and z^{ } will be much greater than that between the most different of the original eleven species. the new species, moreover, will be allied to each other in a widely different manner. of the eight descendants from (a) the three marked a^{ }, q^{ }, p^{ }, will be nearly related from having recently branched off from a^{ }; b^{ } and f^{ }, from having diverged at an earlier period from a^ , will be in some degree distinct from the three first-named species; and lastly, o^{ }, e^{ } and m^{ }, will be nearly related one to the other, but from having diverged at the first commencement of the process of modification, will be widely different from the other five species, and may constitute a sub-genus or even a distinct genus. the six descendants from (i) will form two sub-genera or even genera. but as the original species (i) differed largely from (a), standing nearly at the extreme points of the original genus, the six descendants from (i) will, owing to inheritance alone, differ considerably from the eight descendants from (a); the two groups, moreover, are supposed to have gone on diverging in different directions. the intermediate species, also (and this is a very important consideration), which connected the original species (a) and (i), have all become, excepting (f), extinct, and have left no descendants. hence the six new species descended from (i), and the eight descended from (a), will have to be ranked as very distinct genera, or even as distinct sub-families. thus it is, as i believe, that two or more genera are produced by descent with modification, from two or more species of the same genus. and the two or { } more parent-species are supposed to have descended from some one species of an earlier genus. in our diagram, this is indicated by the broken lines, beneath the capital letters, converging in sub-branches downwards towards a single point; this point representing a single species, the supposed single parent of our several new sub-genera and genera. it is worth while to reflect for a moment on the character of the new species f^{ }, which is supposed not to have diverged much in character, but to have retained the form of (f), either unaltered or altered only in a slight degree. in this case, its affinities to the other fourteen new species will be of a curious and circuitous nature. having descended from a form which stood between the two parent-species (a) and (i), now supposed to be extinct and unknown, it will be in some degree intermediate in character between the two groups descended from these species. but as these two groups have gone on diverging in character from the type of their parents, the new species (f^{ }) will not be directly intermediate between them, but rather between types of the two groups; and every naturalist will be able to bring some such case before his mind. in the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but each may represent a million or hundred million generations, and likewise a section of the successive strata of the earth's crust including extinct remains. we shall, when we come to our chapter on geology, have to refer again to this subject, and i think we shall then see that the diagram throws light on the affinities of extinct beings, which, though generally belonging to the same orders, or families, or genera, with those now living, yet are often, in some degree, intermediate in character between existing groups; and we can understand this fact, for { } the extinct species lived at very ancient epochs when the branching lines of descent had diverged less. i see no reason to limit the process of modification, as now explained, to the formation of genera alone. if, in our diagram, we suppose the amount of change represented by each successive group of diverging dotted lines to be very great, the forms marked a^{ } to p^{ }, those marked b^{ } and f^{ }, and those marked o^{ } to m^{ }, will form three very distinct genera. we shall also have two very distinct genera descended from (i); and as these latter two genera, both from continued divergence of character and from inheritance from a different parent, will differ widely from the three genera descended from (a), the two little groups of genera will form two distinct families, or even orders, according to the amount of divergent modification supposed to be represented in the diagram. and the two new families, or orders, will have descended from two species of the original genus; and these two species are supposed to have descended from one species of a still more ancient and unknown genus. we have seen that in each country it is the species of the larger genera which oftenest present varieties or incipient species. this, indeed, might have been expected; for as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common. hence, the struggle for the production of new and modified descendants, will mainly lie between the larger groups, which are all trying to increase in number. one large group will slowly conquer another large group, reduce its numbers, and thus lessen its chance of further variation and improvement. within the same large { } group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. small and broken groups and sub-groups will finally disappear. looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which as yet have suffered least extinction, will for a long period continue to increase. but which groups will ultimately prevail, no man can predict; for we well know that many groups, formerly most extensively developed, have now become extinct. looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that of the species living at any one period, extremely few will transmit descendants to a remote futurity. i shall have to return to this subject in the chapter on classification, but i may add that on this view of extremely few of the more ancient species having transmitted descendants, and on the view of all the descendants of the same species making a class, we can understand how it is that there exist but very few classes in each main division of the animal and vegetable kingdoms. although extremely few of the most ancient species may now have living and modified descendants, yet at the most remote geological period, the earth may have been as well peopled with many species of many genera, families, orders, and classes, as at the present day. _summary of chapter._--if during the long course of ages and under varying conditions of life, organic beings { } vary at all in the several parts of their organisation, and i think this cannot be disputed; if there be, owing to the high geometrical ratio of increase of each species, a severe struggle for life at some age, season, or year, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, i think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same manner as so many variations have occurred useful to man. but if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. this principle of preservation, i have called, for the sake of brevity, natural selection; and it leads to the improvement of each creature in relation to its organic and inorganic conditions of life. natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed, or young, as easily as the adult. amongst many animals, sexual selection will give its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of offspring. sexual selection will also give characters useful to the males alone, in their struggles with other males. whether natural selection has really thus acted in nature, in modifying and adapting the various forms of life to their several conditions and stations, must be judged of by the general tenour and balance of evidence given in the following chapters. but we already see how it entails extinction; and how largely extinction { } has acted in the world's history, geology plainly declares. natural selection, also, leads to divergence of character; for more living beings can be supported on the same area the more they diverge in structure, habits, and constitution, of which we see proof by looking to the inhabitants of any small spot or to naturalised productions. therefore during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified these descendants become, the better will be their chance of succeeding in the battle for life. thus the small differences distinguishing varieties of the same species, steadily tend to increase till they come to equal the greater differences between species of the same genus, or even of distinct genera. we have seen that it is the common, the widely-diffused, and widely-ranging species, belonging to the larger genera, which vary most; and these tend to transmit to their modified offspring that superiority which now makes them dominant in their own countries. natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life. on these principles, i believe, the nature of the affinities of all organic beings may be explained. it is a truly wonderful fact--the wonder of which we are apt to overlook from familiarity--that all animals and all plants throughout all time and space should be related to each other in group subordinate to group, in the manner which we everywhere behold--namely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming { } sub-families, families, orders, sub-classes, and classes. the several subordinate groups in any class cannot be ranked in a single file, but seem rather to be clustered round points, and these round other points, and so on in almost endless cycles. on the view that each species has been independently created, i can see no explanation of this great fact in the classification of all organic beings; but, to the best of my judgment, it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram. the affinities of all the beings of the same class have sometimes been represented by a great tree. i believe this simile largely speaks the truth. the green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. at each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. the limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. from the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various { } sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only from having been found in a fossil state. as we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the ornithorhynchus or lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. as buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation i believe it has been with the great tree of life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications. * * * * * { } chapter v. laws of variation. effects of external conditions--use and disuse, combined with natural selection; organs of flight and of vision--acclimatisation--correlation of growth--compensation and economy of growth--false correlations--multiple, rudimentary, and lowly organised structures variable--parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable--species of the same genus vary in an analogous manner--reversions to long-lost characters--summary. i have hitherto sometimes spoken as if the variations--so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature--had been due to chance. this, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation. some authors believe it to be as much the function of the reproductive system to produce individual differences, or very slight deviations of structure, as to make the child like its parents. but the much greater variability, as well as the greater frequency of monstrosities, under domestication or cultivation, than under nature, leads me to believe that deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generations. i have remarked in the first chapter--but a long catalogue of facts which cannot be here given would be necessary to show the truth of the remark--that the reproductive system is eminently susceptible to changes in the conditions of life; and to { } this system being functionally disturbed in the parents, i chiefly attribute the varying or plastic condition of the offspring. the male and female sexual elements seem to be affected before that union takes place which is to form a new being. in the case of "sporting" plants, the bud, which in its earliest condition does not apparently differ essentially from an ovule, is alone affected. but why, because the reproductive system is disturbed, this or that part should vary more or less, we are profoundly ignorant. nevertheless, we can here and there dimly catch a faint ray of light, and we may feel sure that there must be some cause for each deviation of structure, however slight. how much direct effect difference of climate, food, &c., produces on any being is extremely doubtful. my impression is, that the effect is extremely small in the case of animals, but perhaps rather more in that of plants. we may, at least, safely conclude that such influences cannot have produced the many striking and complex co-adaptations of structure between one organic being and another, which we see everywhere throughout nature. some little influence may be attributed to climate, food, &c.: thus, e. forbes speaks confidently that shells at their southern limit, and when living in shallow water, are more brightly coloured than those of the same species further north or from greater depths. gould believes that birds of the same species are more brightly coloured under a clear atmosphere, than when living on islands or near the coast. so with insects, wollaston is convinced that residence near the sea affects their colours. moquin-tandon gives a list of plants which when growing near the sea-shore have their leaves in some degree fleshy, though not elsewhere fleshy. several other such cases could be given. the fact of varieties of one species, when they range { } into the zone of habitation of other species, often acquiring in a very slight degree some of the characters of such species, accords with our view that species of all kinds are only well-marked and permanent varieties. thus the species of shells which are confined to tropical and shallow seas are generally brighter-coloured than those confined to cold and deeper seas. the birds which are confined to continents are, according to mr. gould, brighter-coloured than those of islands. the insect-species confined to sea-coasts, as every collector knows, are often brassy or lurid. plants which live exclusively on the sea-side are very apt to have fleshy leaves. he who believes in the creation of each species, will have to say that this shell, for instance, was created with bright colours for a warm sea; but that this other shell became bright-coloured by variation when it ranged into warmer or shallower waters. when a variation is of the slightest use to a being, we cannot tell how much of it to attribute to the accumulative action of natural selection, and how much to the conditions of life. thus, it is well known to furriers that animals of the same species have thicker and better fur the more severe the climate is under which they have lived; but who can tell how much of this difference may be due to the warmest-clad individuals having been favoured and preserved during many generations, and how much to the direct action of the severe climate? for it would appear that climate has some direct action on the hair of our domestic quadrupeds. instances could be given of the same variety being produced under conditions of life as different as can well be conceived; and, on the other hand, of different varieties being produced from the same species under the same conditions. such facts show how indirectly { } the conditions of life act. again, innumerable instances are known to every naturalist of species keeping true, or not varying at all, although living under the most opposite climates. such considerations as these incline me to lay very little weight on the direct action of the conditions of life. indirectly, as already remarked, they seem to play an important part in affecting the reproductive system, and in thus inducing variability; and natural selection will then accumulate all profitable variations, however slight, until they become plainly developed and appreciable by us. _effects of use and disuse._--from the facts alluded to in the first chapter, i think there can be little doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited. under free nature, we can have no standard of comparison, by which to judge of the effects of long-continued use or disuse, for we know not the parent-forms; but many animals have structures which can be explained by the effects of disuse. as professor owen has remarked, there is no greater anomaly in nature than a bird that cannot fly; yet there are several in this state. the logger-headed duck of south america can only flap along the surface of the water, and has its wings in nearly the same condition as the domestic aylesbury duck. as the larger ground-feeding birds seldom take flight except to escape danger, i believe that the nearly wingless condition of several birds, which now inhabit or have lately inhabited several oceanic islands, tenanted by no beast of prey, has been caused by disuse. the ostrich indeed inhabits continents and is exposed to danger from which it cannot escape by flight, but by kicking it can defend itself from enemies, as well as any of the smaller { } quadrupeds. we may imagine that the early progenitor of the ostrich had habits like those of a bustard, and that as natural selection increased in successive generations the size and weight of its body, its legs were used more, and its wings less, until they became incapable of flight. kirby has remarked (and i have observed the same fact) that the anterior tarsi, or feet, of many male dung-feeding beetles are very often broken off; he examined seventeen specimens in his own collection, and not one had even a relic left. in the onites apelles the tarsi are so habitually lost, that the insect has been described as not having them. in some other genera they are present, but in a rudimentary condition. in the ateuchus or sacred beetle of the egyptians, they are totally deficient. there is not sufficient evidence to induce me to believe that mutilations are ever inherited; and i should prefer explaining the entire absence of the anterior tarsi in ateuchus, and their rudimentary condition in some other genera, by the long-continued effects of disuse in their progenitors; for as the tarsi are almost always lost in many dung-feeding beetles, they must be lost early in life, and therefore cannot be much used by these insects. in some cases we might easily put down to disuse modifications of structure which are wholly, or mainly, due to natural selection. mr. wollaston has discovered the remarkable fact that beetles, out of the species inhabiting madeira, are so far deficient in wings that they cannot fly; and that of the twenty-nine endemic genera, no less than twenty-three genera have all their species in this condition! several facts, namely, that beetles in many parts of the world are frequently blown to sea and perish; that the beetles in madeira, as observed by mr. wollaston, lie much concealed, { } until the wind lulls and the sun shines; that the proportion of wingless beetles is larger on the exposed desertas than in madeira itself; and especially the extraordinary fact, so strongly insisted on by mr. wollaston, of the almost entire absence of certain large groups of beetles, elsewhere excessively numerous, and which groups have habits of life almost necessitating frequent flight;--these several considerations have made me believe that the wingless condition of so many madeira beetles is mainly due to the action of natural selection, but combined probably with disuse. for during thousands of successive generations each individual beetle which flew least, either from its wings having been ever so little less perfectly developed or from indolent habit, will have had the best chance of surviving from not being blown out to sea; and, on the other hand, those beetles which most readily took to flight would oftenest have been blown to sea and thus have been destroyed. the insects in madeira which are not ground-feeders, and which, as the flower-feeding coleoptera and lepidoptera, must habitually use their wings to gain their subsistence, have, as mr. wollaston suspects, their wings not at all reduced, but even enlarged. this is quite compatible with the action of natural selection. for when a new insect first arrived on the island, the tendency of natural selection to enlarge or to reduce the wings, would depend on whether a greater number of individuals were saved by successfully battling with the winds, or by giving up the attempt and rarely or never flying. as with mariners shipwrecked near a coast, it would have been better for the good swimmers if they had been able to swim still further, whereas it would have been better for the bad swimmers if they had not been able to swim at all and had stuck to the wreck. { } the eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up by skin and fur. this state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. in south america, a burrowing rodent, the tuco-tuco, or ctenomys, is even more subterranean in its habits than the mole; and i was assured by a spaniard, who had often caught them, that they were frequently blind; one which i kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. as frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not indispensable to animals with subterranean habits, a reduction in their size with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would constantly aid the effects of disuse. it is well known that several animals, belonging to the most different classes, which inhabit the caves of styria and of kentucky, are blind. in some of the crabs the foot-stalk for the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. as it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, i attribute their loss wholly to disuse. in one of the blind animals, namely, the cave-rat, the eyes are of immense size; and professor silliman thought that it regained, after living some days in the light, some slight power of vision. in the same manner as in madeira the wings of some of the insects have been enlarged, and the wings of others have been reduced by natural selection aided by use and disuse, so in the case of the cave-rat natural selection seems to have struggled with the loss of light and { } to have increased the size of the eyes; whereas with all the other inhabitants of the caves, disuse by itself seems to have done its work. it is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar climate; so that on the common view of the blind animals having been separately created for the american and european caverns, close similarity in their organisation and affinities might have been expected; but, as schiödte and others have remarked, this is not the case, and the cave-insects of the two continents are not more closely allied than might have been anticipated from the general resemblance of the other inhabitants of north america and europe. on my view we must suppose that american animals, having ordinary powers of vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the kentucky caves, as did european animals into the caves of europe. we have some evidence of this gradation of habit; for, as schiödte remarks, "animals not far remote from ordinary forms, prepare the transition from light to darkness. next follow those that are constructed for twilight; and, last of all, those destined for total darkness." by the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have effected other changes, such as an increase in the length of the antennæ or palpi, as a compensation for blindness. notwithstanding such modifications, we might expect still to see in the cave-animals of america, affinities to the other inhabitants of that continent, and in those of europe, to the inhabitants of the european continent. and this is the case with some of the american cave-animals, as i hear from { } professor dana; and some of the european cave-insects are very closely allied to those of the surrounding country. it would be most difficult to give any rational explanation of the affinities of the blind cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation. that several of the inhabitants of the caves of the old and new worlds should be closely related, we might expect from the well-known relationship of most of their other productions. far from feeling any surprise that some of the cave-animals should be very anomalous, as agassiz has remarked in regard to the blind fish, the amblyopsis, and as is the case with the blind proteus with reference to the reptiles of europe, i am only surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the inhabitants of these dark abodes will probably have been exposed. _acclimatisation._--habit is hereditary with plants, as in the period of flowering, in the amount of rain requisite for seeds to germinate, in the time of sleep, &c., and this leads me to say a few words on acclimatisation. as it is extremely common for species of the same genus to inhabit very hot and very cold countries, and as i believe that all the species of the same genus have descended from a single parent, if this view be correct, acclimatisation must be readily effected during long-continued descent. it is notorious that each species is adapted to the climate of its own home: species from an arctic or even from a temperate region cannot endure a tropical climate, or conversely. so again, many succulent plants cannot endure a damp climate. but the degree of adaptation of species to the climates under which they live is often overrated. { } we may infer this from our frequent inability to predict whether or not an imported plant will endure our climate, and from the number of plants and animals brought from warmer countries which here enjoy good health. we have reason to believe that species in a state of nature are limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates. but whether or not the adaptation be generally very close, we have evidence, in the case of some few plants, of their becoming, to a certain extent, naturally habituated to different temperatures, or becoming acclimatised: thus the pines and rhododendrons, raised from seed collected by dr. hooker from trees growing at different heights on the himalaya, were found in this country to possess different constitutional powers of resisting cold. mr. thwaites informs me that he has observed similar facts in ceylon, and analogous observations have been made by mr. h. c. watson on european species of plants brought from the azores to england. in regard to animals, several authentic cases could be given of species within historical times having largely extended their range from warmer to cooler latitudes, and conversely; but we do not positively know that these animals were strictly adapted to their native climate, but in all ordinary cases we assume such to be the case; nor do we know that they have subsequently become acclimatised to their new homes. as i believe that our domestic animals were originally chosen by uncivilised man because they were useful and bred readily under confinement, and not because they were subsequently found capable of far-extended transportation, i think the common and extraordinary capacity in our domestic animals of not only withstanding the most different climates but of being perfectly { } fertile (a far severer test) under them, may be used as an argument that a large proportion of other animals, now in a state of nature, could easily be brought to bear widely different climates. we must not, however, push the foregoing argument too far, on account of the probable origin of some of our domestic animals from several wild stocks: the blood, for instance, of a tropical and arctic wolf or wild dog may perhaps be mingled in our domestic breeds. the rat and mouse cannot be considered as domestic animals, but they have been transported by man to many parts of the world, and now have a far wider range than any other rodent, living free under the cold climate of faroe in the north and of the falklands in the south, and on many islands in the torrid zones. hence i am inclined to look at adaptation to any special climate as a quality readily grafted on an innate wide flexibility of constitution, which is common to most animals. on this view, the capacity of enduring the most different climates by man himself and by his domestic animals, and such facts as that former species of the elephant and rhinoceros were capable of enduring a glacial climate, whereas the living species are now all tropical or sub-tropical in their habits, ought not to be looked at as anomalies, but merely as examples of a very common flexibility of constitution, brought, under peculiar circumstances, into play. how much of the acclimatisation of species to any peculiar climate is due to mere habit, and how much to the natural selection of varieties having different innate constitutions, and how much to both means combined, is a very obscure question. that habit or custom has some influence i must believe, both from analogy, and from the incessant advice given in agricultural works, even in the ancient encyclopædias of china, to be very { } cautious in transposing animals from one district to another; for it is not likely that man should have succeeded in selecting so many breeds and sub-breeds with constitutions specially fitted for their own districts: the result must, i think, be due to habit. on the other hand, i can see no reason to doubt that natural selection will continually tend to preserve those individuals which are born with constitutions best adapted to their native countries. in treatises on many kinds of cultivated plants, certain varieties are said to withstand certain climates better than others: this is very strikingly shown in works on fruit trees published in the united states, in which certain varieties are habitually recommended for the northern, and others for the southern states; and as most of these varieties are of recent origin, they cannot owe their constitutional differences to habit. the case of the jerusalem artichoke, which is never propagated by seed, and of which consequently new varieties have not been produced, has even been advanced--for it is now as tender as ever it was--as proving that acclimatisation cannot be effected! the case, also, of the kidney-bean has been often cited for a similar purpose, and with much greater weight; but until some one will sow, during a score of generations, his kidney-beans so early that a very large proportion are destroyed by frost, and then collect seed from the few survivors, with care to prevent accidental crosses, and then again get seed from these seedlings, with the same precautions, the experiment cannot be said to have been even tried. nor let it be supposed that no differences in the constitution of seedling kidney-beans ever appear, for an account has been published how much more hardy some seedlings appeared to be than others. on the whole, i think we may conclude that habit, { } use, and disuse, have, in some cases, played a considerable part in the modification of the constitution, and of the structure of various organs; but that the effects of use and disuse have often been largely combined with, and sometimes overmastered by the natural selection of innate variations. _correlation of growth._--i mean by this expression that the whole organisation is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified. this is a very important subject, most imperfectly understood. the most obvious case is, that modifications accumulated solely for the good of the young or larva, will, it may safely be concluded, affect the structure of the adult; in the same manner as any malconformation affecting the early embryo, seriously affects the whole organisation of the adult. the several parts of the body which are homologous, and which, at an early embryonic period, are alike, seem liable to vary in an allied manner: we see this in the right and left sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and limbs, varying together, for the lower jaw is believed to be homologous with the limbs. these tendencies, i do not doubt, may be mastered more or less completely by natural selection: thus a family of stags once existed with an antler only on one side; and if this had been of any great use to the breed it might probably have been rendered permanent by natural selection. homologous parts, as has been remarked by some authors, tend to cohere; this is often seen in monstrous plants; and nothing is more common than the union of homologous parts in normal structures, as the union of { } the petals of the corolla into a tube. hard parts seem to affect the form of adjoining soft parts; it is believed by some authors that the diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape of their kidneys. others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child. in snakes, according to schlegel, the shape of the body and the manner of swallowing determine the position of several of the most important viscera. the nature of the bond of correlation is very frequently quite obscure. m. is. geoffroy st. hilaire has forcibly remarked, that certain malconformations very frequently, and that others rarely coexist, without our being able to assign any reason. what can be more singular than the relation between blue eyes and deafness in cats, and the tortoise-shell colour with the female sex; the feathered feet and skin between the outer toes in pigeons, and the presence of more or less down on the young birds when first hatched, with the future colour of their plumage; or, again, the relation between the hair and teeth in the naked turkish dog, though here probably homology comes into play? with respect to this latter case of correlation, i think it can hardly be accidental, that if we pick out the two orders of mammalia which are most abnormal in their dermal covering, viz. cetacea (whales) and edentata (armadilloes, scaly anteaters, &c.), that these are likewise the most abnormal in their teeth. i know of no case better adapted to show the importance of the laws of correlation in modifying important structures, independently of utility and, therefore, of natural selection, than that of the difference between the outer and inner flowers in some compositous and umbelliferous plants. every one knows the { } difference in the ray and central florets of, for instance, the daisy, and this difference is often accompanied with the abortion of parts of the flower. but, in some compositous plants, the seeds also differ in shape and sculpture; and even the ovary itself, with its accessory parts, differs, as has been described by cassini. these differences have been attributed by some authors to pressure, and the shape of the seeds in the ray-florets in some compositæ countenances this idea; but, in the case of the corolla of the umbelliferæ, it is by no means, as dr. hooker informs me, in species with the densest heads that the inner and outer flowers most frequently differ. it might have been thought that the development of the ray-petals by drawing nourishment from certain other parts of the flower had caused their abortion; but in some compositæ there is a difference in the seeds of the outer and inner florets without any difference in the corolla. possibly, these several differences may be connected with some difference in the flow of nutriment towards the central and external flowers: we know, at least, that in irregular flowers, those nearest to the axis are oftenest subject to peloria, and become regular. i may add, as an instance of this, and of a striking case of correlation, that i have recently observed in some garden pelargoniums, that the central flower of the truss often loses the patches of darker colour in the two upper petals; and that when this occurs, the adherent nectary is quite aborted; when the colour is absent from only one of the two upper petals, the nectary is only much shortened. with respect to the difference in the corolla of the central and exterior flowers of a head or umbel, i do not feel at all sure that c. c. sprengel's idea that the ray-florets serve to attract insects, whose agency is highly advantageous in the fertilisation of plants of { } these two orders, is so far-fetched, as it may at first appear: and if it be advantageous, natural selection may have come into play. but in regard to the differences both in the internal and external structure of the seeds, which are not always correlated with any differences in the flowers, it seems impossible that they can be in any way advantageous to the plant: yet in the umbelliferæ these differences are of such apparent importance--the seeds being in some cases, according to tausch, orthospermous in the exterior flowers and coelospermous in the central flowers,--that the elder de candolle founded his main divisions of the order on analogous differences. hence we see that modifications of structure, viewed by systematists as of high value, may be wholly due to unknown laws of correlated growth, and without being, as far as we can see, of the slightest service to the species. we may often falsely attribute to correlation of growth, structures which are common to whole groups of species, and which in truth are simply due to inheritance; for an ancient progenitor may have acquired through natural selection some one modification in structure, and, after thousands of generations, some other and independent modification; and these two modifications, having been transmitted to a whole group of descendants with diverse habits, would naturally be thought to be correlated in some necessary manner. so, again, i do not doubt that some apparent correlations, occurring throughout whole orders, are entirely due to the manner alone in which natural selection can act. for instance, alph. de candolle has remarked that winged seeds are never found in fruits which do not open: i should explain the rule by the fact that seeds could not gradually become winged through natural selection, except in fruits which opened; so that the individual plants producing { } seeds which were a little better fitted to be wafted further, might get an advantage over those producing seed less fitted for dispersal; and this process could not possibly go on in fruit which did not open. the elder geoffroy and goethe propounded, at about the same period, their law of compensation or balancement of growth; or, as goethe expressed it, "in order to spend on one side, nature is forced to economise on the other side." i think this holds true to a certain extent with our domestic productions: if nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is difficult to get a cow to give much milk and to fatten readily. the same varieties of the cabbage do not yield abundant and nutritious foliage and a copious supply of oil-bearing seeds. when the seeds in our fruits become atrophied, the fruit itself gains largely in size and quality. in our poultry, a large tuft of feathers on the head is generally accompanied by a diminished comb, and a large beard by diminished wattles. with species in a state of nature it can hardly be maintained that the law is of universal application; but many good observers, more especially botanists, believe in its truth. i will not, however, here give any instances, for i see hardly any way of distinguishing between the effects, on the one hand, of a part being largely developed through natural selection and another and adjoining part being reduced by this same process or by disuse, and, on the other hand, the actual withdrawal of nutriment from one part owing to the excess of growth in another and adjoining part. i suspect, also, that some of the cases of compensation which have been advanced, and likewise some other facts, may be merged under a more general principle, namely, that natural selection is continually trying to economise in every part of the organisation. if under { } changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure. i can thus only understand a fact with which i was much struck when examining cirripedes, and of which many other instances could be given: namely, that when a cirripede is parasitic within another and is thus protected, it loses more or less completely its own shell or carapace. this is the case with the male ibla, and in a truly extraordinary manner with the proteolepas: for the carapace in all other cirripedes consists of the three highly-important anterior segments of the head enormously developed, and furnished with great nerves and muscles; but in the parasitic and protected proteolepas, the whole anterior part of the head is reduced to the merest rudiment attached to the bases of the prehensile antennæ. now the saving of a large and complex structure, when rendered superfluous by the parasitic habits of the proteolepas, though effected by slow steps, would be a decided advantage to each successive individual of the species; for in the struggle for life to which every animal is exposed, each individual proteolepas would have a better chance of supporting itself, by less nutriment being wasted in developing a structure now become useless. thus, as i believe, natural selection will always succeed in the long run in reducing and saving every part of the organisation, as soon as it is rendered superfluous, without by any means causing some other part to be largely developed in a corresponding degree. and, conversely, that natural selection may perfectly well succeed in largely developing any organ, without requiring as a necessary compensation the reduction of some adjoining part. { } it seems to be a rule, as remarked by is. geoffroy st. hilaire, both in varieties and in species, that when any part or organ is repeated many times in the structure of the same individual (as the vertebræ in snakes, and the stamens in polyandrous flowers) the number is variable; whereas the number of the same part or organ, when it occurs in lesser numbers, is constant. the same author and some botanists have further remarked that multiple parts are also very liable to variation in structure. inasmuch as this "vegetative repetition," to use prof. owen's expression, seems to be a sign of low organisation, the foregoing remark seems connected with the very general opinion of naturalists, that beings low in the scale of nature are more variable than those which are higher. i presume that lowness in this case means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why natural selection should have preserved or rejected each little deviation of form less carefully than when the part has to serve for one special purpose alone. in the same way that a knife which has to cut all sorts of things may be of almost any shape; whilst a tool for some particular object had better be of some particular shape. natural selection, it should never be forgotten, can act on each part of each being, solely through and for its advantage. rudimentary parts, it has been stated by some authors, and i believe with truth, are apt to be highly variable. we shall have to recur to the general subject of rudimentary and aborted organs; and i will here only add that their variability seems to be owing to their uselessness, and therefore to natural selection having no power to check deviations in their structure. thus { } rudimentary parts are left to the free play of the various laws of growth, to the effects of long-continued disuse, and to the tendency to reversion. _a part developed in any species in an extraordinary degree or manner, in comparison with the same part in allied species, tends to be highly variable._--several years ago i was much struck with a remark, nearly to the above effect, published by mr. waterhouse. i infer also from an observation made by professor owen, with respect to the length of the arms of the ourang-outang, that he has come to a nearly similar conclusion. it is hopeless to attempt to convince any one of the truth of this proposition without giving the long array of facts which i have collected, and which cannot possibly be here introduced. i can only state my conviction that it is a rule of high generality. i am aware of several causes of error, but i hope that i have made due allowance for them. it should be understood that the rule by no means applies to any part, however unusually developed, unless it be unusually developed in comparison with the same part in closely allied species. thus, the bat's wing is a most abnormal structure in the class mammalia; but the rule would not here apply, because there is a whole group of bats having wings; it would apply only if some one species of bat had its wings developed in some remarkable manner in comparison with the other species of the same genus. the rule applies very strongly in the case of secondary sexual characters, when displayed in any unusual manner. the term, secondary sexual characters, used by hunter, applies to characters which are attached to one sex, but are not directly connected with the act of reproduction. the rule applies to males and females; but as females more rarely offer remarkable secondary sexual characters, it applies { } more rarely to them. the rule being so plainly applicable in the case of secondary sexual characters, may be due to the great variability of these characters, whether or not displayed in any unusual manner--of which fact i think there can be little doubt. but that our rule is not confined to secondary sexual characters is clearly shown in the case of hermaphrodite cirripedes; and i may here add, that i particularly attended to mr. waterhouse's remark, whilst investigating this order, and i am fully convinced that the rule almost invariably holds good with cirripedes. i shall, in my future work, give a list of the more remarkable cases; i will here only briefly give one, as it illustrates the rule in its largest application. the opercular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures, and they differ extremely little even in different genera; but in the several species of one genus, pyrgoma, these valves present a marvellous amount of diversification: the homologous valves in the different species being sometimes wholly unlike in shape; and the amount of variation in the individuals of several of the species is so great, that it is no exaggeration to state that the varieties differ more from each other in the characters of these important valves than do other species of distinct genera. as birds within the same country vary in a remarkably small degree, i have particularly attended to them, and the rule seems to me certainly to hold good in this class. i cannot make out that it applies to plants, and this would seriously have shaken my belief in its truth, had not the great variability in plants made it particularly difficult to compare their relative degrees of variability. when we see any part or organ developed in a remarkable degree or manner in any species, the fair { } presumption is that it is of high importance to that species; nevertheless the part in this case is eminently liable to variation. why should this be so? on the view that each species has been independently created, with all its parts as we now see them, i can see no explanation. but on the view that groups of species have descended from other species, and have been modified through natural selection, i think we can obtain some light. in our domestic animals, if any part, or the whole animal, be neglected and no selection be applied, that part (for instance, the comb in the dorking fowl) or the whole breed will cease to have a nearly uniform character. the breed will then be said to have degenerated. in rudimentary organs, and in those which have been but little specialised for any particular purpose, and perhaps in polymorphic groups, we see a nearly parallel natural case; for in such cases natural selection either has not or cannot come into full play, and thus the organisation is left in a fluctuating condition. but what here more especially concerns us is, that in our domestic animals those points, which at the present time are undergoing rapid change by continued selection, are also eminently liable to variation. look at the breeds of the pigeon; see what a prodigious amount of difference there is in the beak of the different tumblers, in the beak and wattle of the different carriers, in the carriage and tail of our fantails, &c., these being the points now mainly attended to by english fanciers. even in the sub-breeds, as in the short-faced tumbler, it is notoriously difficult to breed them nearly to perfection, and frequently individuals are born which depart widely from the standard. there may be truly said to be a constant struggle going on between, on the one hand, the tendency to reversion to a less modified state, as well as an innate tendency to further { } variability of all kinds, and, on the other hand, the power of steady selection to keep the breed true. in the long run selection gains the day, and we do not expect to fail so far as to breed a bird as coarse as a common tumbler from a good short-faced strain. but as long as selection is rapidly going on, there may always be expected to be much variability in the structure undergoing modification. it further deserves notice that these variable characters, produced by man's selection, sometimes become attached, from causes quite unknown to us, more to one sex than to the other, generally to the male sex, as with the wattle of carriers and the enlarged crop of pouters. now let us turn to nature. when a part has been developed in an extraordinary manner in any one species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of modification since the period when the species branched off from the common progenitor of the genus. this period will seldom be remote in any extreme degree, as species very rarely endure for more than one geological period. an extraordinary amount of modification implies an unusually large and long-continued amount of variability, which has continually been accumulated by natural selection for the benefit of the species. but as the variability of the extraordinarily-developed part or organ has been so great and long-continued within a period not excessively remote, we might, as a general rule, expect still to find more variability in such parts than in other parts of the organisation which have remained for a much longer period nearly constant. and this, i am convinced, is the case. that the struggle between natural selection on the one hand, and the tendency to reversion and variability on the other hand, will in the { } course of time cease; and that the most abnormally developed organs may be made constant, i can see no reason to doubt. hence when an organ, however abnormal it may be, has been transmitted in approximately the same condition to many modified descendants, as in the case of the wing of the bat, it must have existed, according to my theory, for an immense period in nearly the same state; and thus it comes to be no more variable than any other structure. it is only in those cases in which the modification has been comparatively recent and extraordinarily great that we ought to find the _generative variability_, as it may be called, still present in a high degree. for in this case the variability will seldom as yet have been fixed by the continued selection of the individuals varying in the required manner and degree, and by the continued rejection of those tending to revert to a former and less modified condition. the principle included in these remarks may be extended. it is notorious that specific characters are more variable than generic. to explain by a simple example what is meant. if some species in a large genus of plants had blue flowers and some had red, the colour would be only a specific character, and no one would be surprised at one of the blue species varying into red, or conversely; but if all the species had blue flowers, the colour would become a generic character, and its variation would be a more unusual circumstance. i have chosen this example because an explanation is not in this case applicable, which most naturalists would advance, namely, that specific characters are more variable than generic, because they are taken from parts of less physiological importance than those commonly used for classing genera. i believe this explanation is partly, yet only indirectly, true; i shall, however, have to { } return to this subject in our chapter on classification. it would be almost superfluous to adduce evidence in support of the above statement, that specific characters are more variable than generic; but i have repeatedly noticed in works on natural history, that when an author has remarked with surprise that some _important_ organ or part, which is generally very constant throughout large groups of species, has _differed_ considerably in closely-allied species, that it has, also, been _variable_ in the individuals of some of the species. and this fact shows that a character, which is generally of generic value, when it sinks in value and becomes only of specific value, often becomes variable, though its physiological importance may remain the same. something of the same kind applies to monstrosities: at least is. geoffroy st. hilaire seems to entertain no doubt, that the more an organ normally differs in the different species of the same group, the more subject it is to individual anomalies. on the ordinary view of each species having been independently created, why should that part of the structure, which differs from the same part in other independently-created species of the same genus, be more variable than those parts which are closely alike in the several species? i do not see that any explanation can be given. but on the view of species being only strongly marked and fixed varieties, we might surely expect to find them still often continuing to vary in those parts of their structure which have varied within a moderately recent period, and which have thus come to differ. or to state the case in another manner:--the points in which all the species of a genus resemble each other, and in which they differ from the species of some other genus, are called generic characters; and these characters in common i attribute to { } inheritance from a common progenitor, for it can rarely have happened that natural selection will have modified several species, fitted to more or less widely-different habits, in exactly the same manner: and as these so-called generic characters have been inherited from a remote period, since that period when the species first branched off from their common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight degree, it is not probable that they should vary at the present day. on the other hand, the points in which species differ from other species of the same genus, are called specific characters; and as these specific characters have varied and come to differ within the period of the branching off of the species from a common progenitor, it is probable that they should still often be in some degree variable,--at least more variable than those parts of the organisation which have for a very long period remained constant. in connexion with the present subject, i will make only two other remarks. i think it will be admitted, without my entering on details, that secondary sexual characters are very variable; i think it also will be admitted that species of the same group differ from each other more widely in their secondary sexual characters, than in other parts of their organisation; compare, for instance, the amount of difference between the males of gallinaceous birds, in which secondary sexual characters are strongly displayed, with the amount of difference between their females; and the truth of this proposition will be granted. the cause of the original variability of secondary sexual characters is not manifest; but we can see why these characters should not have been rendered as constant and uniform as other parts of the organisation; for secondary sexual characters have been accumulated by sexual selection, which { } is less rigid in its action than ordinary selection, as it does not entail death, but only gives fewer offspring to the less favoured males. whatever the cause may be of the variability of secondary sexual characters, as they are highly variable, sexual selection will have had a wide scope for action, and may thus readily have succeeded in giving to the species of the same group a greater amount of difference in their sexual characters, than in other parts of their structure. it is a remarkable fact, that the secondary sexual differences between the two sexes of the same species are generally displayed in the very same parts of the organisation in which the different species of the same genus differ from each other. of this fact i will give in illustration two instances, the first which happen to stand on my list; and as the differences in these cases are of a very unusual nature, the relation can hardly be accidental. the same number of joints in the tarsi is a character generally common to very large groups of beetles, but in the engidæ, as westwood has remarked, the number varies greatly; and the number likewise differs in the two sexes of the same species: again in fossorial hymenoptera, the manner of neuration of the wings is a character of the highest importance, because common to large groups; but in certain genera the neuration differs in the different species, and likewise in the two sexes of the same species. this relation has a clear meaning on my view of the subject: i look at all the species of the same genus as having as certainly descended from the same progenitor, as have the two sexes of any one of the species. consequently, whatever part of the structure of the common progenitor, or of its early descendants, became variable; variations of this part would, it is highly probable, be taken advantage of by natural and sexual selection, in order to fit { } the several species to their several places in the economy of nature, and likewise to fit the two sexes of the same species to each other, or to fit the males and females to different habits of life, or the males to struggle with other males for the possession of the females. finally, then, i conclude that the greater variability of specific characters, or those which distinguish species from species, than of generic characters, or those which the species possess in common;--that the frequent extreme variability of any part which is developed in a species in an extraordinary manner in comparison with the same part in its congeners; and the slight degree of variability in a part, however extraordinarily it may be developed, if it be common to a whole group of species;--that the great variability of secondary sexual characters, and the great amount of difference in these same characters between closely allied species;--that secondary sexual and ordinary specific differences are generally displayed in the same parts of the organisation,--are all principles closely connected together. all being mainly due to the species of the same group having descended from a common progenitor, from whom they have inherited much in common,--to parts which have recently and largely varied being more likely still to go on varying than parts which have long been inherited and have not varied,--to natural selection having more or less completely, according to the lapse of time, overmastered the tendency to reversion and to further variability,--to sexual selection being less rigid than ordinary selection,--and to variations in the same parts having been accumulated by natural and sexual selection, and having been thus adapted for secondary sexual, and for ordinary specific purposes. { } _distinct species present analogous variations; and a variety of one species often assumes some of the characters of an allied species, or reverts to some of the characters of an early progenitor._--these propositions will be most readily understood by looking to our domestic races. the most distinct breeds of pigeons, in countries most widely apart, present sub-varieties with reversed feathers on the head and feathers on the feet,--characters not possessed by the aboriginal rock-pigeon; these then are analogous variations in two or more distinct races. the frequent presence of fourteen or even sixteen tail-feathers in the pouter, may be considered as a variation representing the normal structure of another race, the fantail. i presume that no one will doubt that all such analogous variations are due to the several races of the pigeon having inherited from a common parent the same constitution and tendency to variation, when acted on by similar unknown influences. in the vegetable kingdom we have a case of analogous variation, in the enlarged stems, or roots as commonly called, of the swedish turnip and ruta baga, plants which several botanists rank as varieties produced by cultivation from a common parent: if this be not so, the case will then be one of analogous variation in two so-called distinct species; and to these a third may be added, namely, the common turnip. according to the ordinary view of each species having been independently created, we should have to attribute this similarity in the enlarged stems of these three plants, not to the _vera causa_ of community of descent, and a consequent tendency to vary in a like manner, but to three separate yet closely related acts of creation. with pigeons, however, we have another case, namely, the occasional appearance in all the breeds, of slaty-blue birds with two black bars on the wings, a white { } rump, a bar at the end of the tail, with the outer feathers externally edged near their bases with white. as all these marks are characteristic of the parent rock-pigeon, i presume that no one will doubt that this is a case of reversion, and not of a new yet analogous variation appearing in the several breeds. we may i think confidently come to this conclusion, because, as we have seen, these coloured marks are eminently liable to appear in the crossed offspring of two distinct and differently coloured breeds; and in this case there is nothing in the external conditions of life to cause the reappearance of the slaty-blue, with the several marks, beyond the influence of the mere act of crossing on the laws of inheritance. no doubt it is a very surprising fact that characters should reappear after having been lost for many, perhaps for hundreds of generations. but when a breed has been crossed only once by some other breed, the offspring occasionally show a tendency to revert in character to the foreign breed for many generations--some say, for a dozen or even a score of generations. after twelve generations, the proportion of blood, to use a common expression, of any one ancestor, is only in ; and yet, as we see, it is generally believed that a tendency to reversion is retained by this very small proportion of foreign blood. in a breed which has not been crossed, but in which _both_ parents have lost some character which their progenitor possessed, the tendency, whether strong or weak, to reproduce the lost character might be, as was formerly remarked, for all that we can see to the contrary, transmitted for almost any number of generations. when a character which has been lost in a breed, reappears after a great number of generations, the most probable hypothesis is, not that the offspring suddenly takes after an ancestor some hundred generations { } distant, but that in each successive generation there has been a tendency to reproduce the character in question, which at last, under unknown favourable conditions, gains an ascendancy. for instance, it is probable that in each generation of the barb-pigeon, which produces most rarely a blue and black-barred bird, there has been a tendency in each generation in the plumage to assume this colour. this view is hypothetical, but could be supported by some facts; and i can see no more abstract improbability in a tendency to produce any character being inherited for an endless number of generations, than in quite useless or rudimentary organs being, as we all know them to be, thus inherited. indeed, we may sometimes observe a mere tendency to produce a rudiment inherited: for instance, in the common snapdragon (antirrhinum) a rudiment of a fifth stamen so often appears, that this plant must have an inherited tendency to produce it. as all the species of the same genus are supposed, on my theory, to have descended from a common parent, it might be expected that they would occasionally vary in an analogous manner; so that a variety of one species would resemble in some of its characters another species; this other species being on my view only a well-marked and permanent variety. but characters thus gained would probably be of an unimportant nature, for the presence of all important characters will be governed by natural selection, in accordance with the diverse habits of the species, and will not be left to the mutual action of the conditions of life and of a similar inherited constitution. it might further be expected that the species of the same genus would occasionally exhibit reversions to lost ancestral characters. as, however, we never know the exact character of the common ancestor of a group, we could not distinguish these two { } cases: if, for instance, we did not know that the rock-pigeon was not feather-footed or turn-crowned, we could not have told, whether these characters in our domestic breeds were reversions or only analogous variations; but we might have inferred that the blueness was a case of reversion, from the number of the markings, which are correlated with the blue tint, and which it does not appear probable would all appear together from simple variation. more especially we might have inferred this, from the blue colour and marks so often appearing when distinct breeds of diverse colours are crossed. hence, though under nature it must generally be left doubtful, what cases are reversions to an anciently existing character, and what are new but analogous variations, yet we ought, on my theory, sometimes to find the varying offspring of a species assuming characters (either from reversion or from analogous variation) which already occur in some other members of the same group. and this undoubtedly is the case in nature. a considerable part of the difficulty in recognising a variable species in our systematic works, is due to its varieties mocking, as it were, some of the other species of the same genus. a considerable catalogue, also, could be given of forms intermediate between two other forms, which themselves must be doubtfully ranked as either varieties or species; and this shows, unless all these forms be considered as independently created species, that the one in varying has assumed some of the characters of the other, so as to produce the intermediate form. but the best evidence is afforded by parts or organs of an important and uniform nature occasionally varying so as to acquire, in some degree, the character of the same part or organ in an allied species. i have collected a long list of such cases; but { } here, as before, i lie under a great disadvantage in not being able to give them. i can only repeat that such cases certainly do occur, and seem to me very remarkable. i will, however, give one curious and complex case, not indeed as affecting any important character, but from occurring in several species of the same genus, partly under domestication and partly under nature. it is a case apparently of reversion. the ass not rarely has very distinct transverse bars on its legs, like those on the legs of the zebra: it has been asserted that these are plainest in the foal, and from inquiries which i have made, i believe this to be true. it has also been asserted that the stripe on each shoulder is sometimes double. the shoulder-stripe is certainly very variable in length and outline. a white ass, but _not_ an albino, has been described without either spinal or shoulder stripe; and these stripes are sometimes very obscure, or actually quite lost, in dark-coloured asses. the koulan of pallas is said to have been seen with a double shoulder-stripe. the hemionus has no shoulder-stripe; but traces of it, as stated by mr. blyth and others, occasionally appear: and i have been informed by colonel poole that the foals of this species are generally striped on the legs, and faintly on the shoulder. the quagga, though so plainly barred like a zebra over the body, is without bars on the legs; but dr. gray has figured one specimen with very distinct zebra-like bars on the hocks. with respect to the horse, i have collected cases in england of the spinal stripe in horses of the most distinct breeds, and of _all_ colours; transverse bars on the legs are not rare in duns, mouse-duns, and in one instance in a chestnut: a faint shoulder-stripe may sometimes be seen in duns, and i have seen a trace in a { } bay horse. my son made a careful examination and sketch for me of a dun belgian cart-horse with a double stripe on each shoulder and with leg-stripes; and a man, whom i can implicitly trust, has examined for me a small dun welch pony with _three_ short parallel stripes on each shoulder. in the north-west part of india the kattywar breed of horses is so generally striped, that, as i hear from colonel poole, who examined the breed for the indian government, a horse without stripes is not considered as purely-bred. the spine is always striped; the legs are generally barred; and the shoulder-stripe, which is sometimes double and sometimes treble, is common; the side of the face, moreover, is sometimes striped. the stripes are plainest in the foal; and sometimes quite disappear in old horses. colonel poole has seen both gray and bay kattywar horses striped when first foaled. i have, also, reason to suspect, from information given me by mr. w. w. edwards, that with the english racehorse the spinal stripe is much commoner in the foal than in the full-grown animal. without here entering on further details, i may state that i have collected cases of leg and shoulder stripes in horses of very different breeds, in various countries from britain to eastern china; and from norway in the north to the malay archipelago in the south. in all parts of the world these stripes occur far oftenest in duns and mouse-duns; by the term dun a large range of colour is included, from one between brown and black to a close approach to cream-colour. i am aware that colonel hamilton smith, who has written on this subject, believes that the several breeds of the horse have descended from several aboriginal species--one of which, the dun, was striped; and that the above-described appearances are all due to ancient { } crosses with the dun stock. but i am not at all satisfied with this theory, and should be loth to apply it to breeds so distinct as the heavy belgian cart-horse, welch ponies, cobs, the lanky kattywar race, &c., inhabiting the most distant parts of the world. now let us turn to the effects of crossing the several species of the horse-genus. rollin asserts, that the common mule from the ass and horse is particularly apt to have bars on its legs: according to mr. gosse, in certain parts of the united states about nine out of ten mules have striped legs. i once saw a mule with its legs so much striped that any one would at first have thought that it must have been the product of a zebra; and mr. w. c. martin, in his excellent treatise on the horse, has given a figure of a similar mule. in four coloured drawings, which i have seen, of hybrids between the ass and zebra, the legs were much more plainly barred than the rest of the body; and in one of them there was a double shoulder-stripe. in lord morton's famous hybrid from a chestnut mare and male quagga, the hybrid, and even the pure offspring subsequently produced from the mare by a black arabian sire, were much more plainly barred across the legs than is even the pure quagga. lastly, and this is another most remarkable case, a hybrid has been figured by dr. gray (and he informs me that he knows of a second case) from the ass and the hemionus; and this hybrid, though the ass seldom has stripes on his legs and the hemionus has none and has not even a shoulder-stripe, nevertheless had all four legs barred, and had three short shoulder-stripes, like those on the dun welch pony, and even had some zebra-like stripes on the sides of its face. with respect to this last fact, i was so convinced that not even a stripe of colour appears from what would commonly be called an { } accident, that i was led solely from the occurrence of the face-stripes on this hybrid from the ass and hemionus to ask colonel poole whether such face-stripes ever occur in the eminently striped kattywar breed of horses, and was, as we have seen, answered in the affirmative. what now are we to say to these several facts? we see several very distinct species of the horse-genus becoming, by simple variation, striped on the legs like a zebra, or striped on the shoulders like an ass. in the horse we see this tendency strong whenever a dun tint appears--a tint which approaches to that of the general colouring of the other species of the genus. the appearance of the stripes is not accompanied by any change of form or by any other new character. we see this tendency to become striped most strongly displayed in hybrids from between several of the most distinct species. now observe the case of the several breeds of pigeons: they are descended from a pigeon (including two or three sub-species or geographical races) of a bluish colour, with certain bars and other marks; and when any breed assumes by simple variation a bluish tint, these bars and other marks invariably reappear; but without any other change of form or character. when the oldest and truest breeds of various colours are crossed, we see a strong tendency for the blue tint and bars and marks to reappear in the mongrels. i have stated that the most probable hypothesis to account for the reappearance of very ancient characters, is--that there is a _tendency_ in the young of each successive generation to produce the long-lost character, and that this tendency, from unknown causes, sometimes prevails. and we have just seen that in several species of the horse-genus the stripes are either plainer or appear more commonly in the young than in the old. call the breeds of pigeons, some of which have bred true for { } centuries, species; and how exactly parallel is the case with that of the species of the horse-genus! for myself, i venture confidently to look back thousands on thousands of generations, and i see an animal striped like a zebra, but perhaps otherwise very differently constructed, the common parent of our domestic horse, whether or not it be descended from one or more wild stocks, of the ass, the hemionus, quagga, and zebra. he who believes that each equine species was independently created, will, i presume, assert that each species has been created with a tendency to vary, both under nature and under domestication, in this particular manner, so as often to become striped like other species of the genus; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus. to admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. it makes the works of god a mere mockery and deception; i would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore. _summary._--our ignorance of the laws of variation is profound. not in one case out of a hundred can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents. but whenever we have the means of instituting a comparison, the same laws appear to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus. the external conditions of life, as { } climate and food, &c., seem to have induced some slight modifications. habit in producing constitutional differences, and use in strengthening and disuse in weakening and diminishing organs, seem to have been more potent in their effects. homologous parts tend to vary in the same way, and homologous parts tend to cohere. modifications in hard parts and in external parts sometimes affect softer and internal parts. when one part is largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of the structure which can be saved without detriment to the individual, will be saved. changes of structure at an early age will generally affect parts subsequently developed; and there are very many other correlations of growth, the nature of which we are utterly unable to understand. multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialised to any particular function, so that their modifications have not been closely checked by natural selection. it is probably from this same cause that organic beings low in the scale of nature are more variable than those which have their whole organisation more specialised, and are higher in the scale. rudimentary organs, from being useless, will be disregarded by natural selection, and hence probably are variable. specific characters--that is, the characters which have come to differ since the several species of the same genus branched off from a common parent--are more variable than generic characters, or those which have long been inherited, and have not differed within this same period. in these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ; but we have also seen in the second chapter that the same principle applies to the whole individual; { } for in a district where many species of any genus are found--that is, where there has been much former variation and differentiation, or where the manufactory of new specific forms has been actively at work--there, on an average, we now find most varieties or incipient species. secondary sexual characters are highly variable, and such characters differ much in the species of the same group. variability in the same parts of the organisation has generally been taken advantage of in giving secondary sexual differences to the sexes of the same species, and specific differences to the several species of the same genus. any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. but when a species with any extraordinarily-developed organ has become the parent of many modified descendants--which on my view must be a very slow process, requiring a long lapse of time--in this case, natural selection may readily have succeeded in giving a fixed character to the organ, in however extraordinary a manner it may be developed. species inheriting nearly the same constitution from a common parent and exposed to similar influences will naturally tend to present analogous variations, and these same species may occasionally revert to some of the characters of their ancient progenitors. although new and important modifications may not arise from reversion and analogous { } variation, such modifications will add to the beautiful and harmonious diversity of nature. whatever the cause may be of each slight difference in the offspring from their parents--and a cause for each must exist--it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive. * * * * * { } chapter vi. difficulties on theory. difficulties on the theory of descent with modification--transitions--absence or rarity of transitional varieties--transitions in habits of life--diversified habits in the same species--species with habits widely different from those of their allies--organs of extreme perfection--means of transition--cases of difficulty--natura non facit saltum--organs of small importance--organs not in all cases absolutely perfect--the law of unity of type and of the conditions of existence embraced by the theory of natural selection. long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader. some of them are so grave that to this day i can never reflect on them without being staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, i think, fatal to my theory. these difficulties and objections may be classed under the following heads:--firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? why is not all nature in confusion instead of the species being, as we see them, well defined? secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some animal with wholly different habits? can we believe that natural selection could produce, on the one hand, organs of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, organs of { } such wonderful structure, as the eye, of which we hardly as yet fully understand the inimitable perfection? thirdly, can instincts be acquired and modified through natural selection? what shall we say to so marvellous an instinct as that which leads the bee to make cells, which has practically anticipated the discoveries of profound mathematicians? fourthly, how can we account for species, when crossed, being sterile and producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired? the two first heads shall be here discussed--instinct and hybridism in separate chapters. _on the absence or rarity of transitional varieties._--as natural selection acts solely by the preservation of profitable modifications, each new form will tend in a fully-stocked country to take the place of, and finally to exterminate, its own less improved parent or other less-favoured forms with which it comes into competition. thus extinction and natural selection will, as we have seen, go hand in hand. hence, if we look at each species as descended from some other unknown form, both the parent and all the transitional varieties will generally have been exterminated by the very process of formation and perfection of the new form. but, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth? it will be much more convenient to discuss this question in the chapter on the imperfection of the geological record; and i will here only state that i believe the answer mainly lies in the record being incomparably less perfect than is generally supposed; the imperfection of the record being chiefly due to organic beings not inhabiting { } profound depths of the sea, and to their remains being embedded and preserved to a future age only in masses of sediment sufficiently thick and extensive to withstand an enormous amount of future degradation; and such fossiliferous masses can be accumulated only where much sediment is deposited on the shallow bed of the sea, whilst it slowly subsides. these contingencies will concur only rarely, and after enormously long intervals. whilst the bed of the sea is stationary or is rising, or when very little sediment is being deposited, there will be blanks in our geological history. the crust of the earth is a vast museum; but the natural collections have been made only at intervals of time immensely remote. but it may be urged that when several closely-allied species inhabit the same territory we surely ought to find at the present time many transitional forms. let us take a simple case: in travelling from north to south over a continent, we generally meet at successive intervals with closely allied or representative species, evidently filling nearly the same place in the natural economy of the land. these representative species often meet and interlock; and as the one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other. but if we compare these species where they intermingle, they are generally as absolutely distinct from each other in every detail of structure as are specimens taken from the metropolis inhabited by each. by my theory these allied species have descended from a common parent; and during the process of modification, each has become adapted to the conditions of life of its own region, and has supplanted and exterminated its original parent and all the transitional varieties between its past and present states. hence we ought not to expect at the { } present time to meet with numerous transitional varieties in each region, though they must have existed there, and may be embedded there in a fossil condition. but in the intermediate region, having intermediate conditions of life, why do we not now find closely-linking intermediate varieties? this difficulty for a long time quite confounded me. but i think it can be in large part explained. in the first place we should be extremely cautious in inferring, because an area is now continuous, that it has been continuous during a long period. geology would lead us to believe that almost every continent has been broken up into islands even during the later tertiary periods; and in such islands distinct species might have been separately formed without the possibility of intermediate varieties existing in the intermediate zones. by changes in the form of the land and of climate, marine areas now continuous must often have existed within recent times in a far less continuous and uniform condition than at present. but i will pass over this way of escaping from the difficulty; for i believe that many perfectly defined species have been formed on strictly continuous areas; though i do not doubt that the formerly broken condition of areas now continuous has played an important part in the formation of new species, more especially with freely-crossing and wandering animals. in looking at species as they are now distributed over a wide area, we generally find them tolerably numerous over a large territory, then becoming somewhat abruptly rarer and rarer on the confines, and finally disappearing. hence the neutral territory between two representative species is generally narrow in comparison with the territory proper to each. we see the same fact in ascending mountains, and sometimes { } it is quite remarkable how abruptly, as alph. de candolle has observed, a common alpine species disappears. the same fact has been noticed by e. forbes in sounding the depths of the sea with the dredge. to those who look at climate and the physical conditions of life as the all-important elements of distribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly. but when we bear in mind that almost every species, even in its metropolis, would increase immensely in numbers, were it not for other competing species; that nearly all either prey on or serve as prey for others; in short, that each organic being is either directly or indirectly related in the most important manner to other organic beings, we must see that the range of the inhabitants of any country by no means exclusively depends on insensibly changing physical conditions, but in large part on the presence of other species, on which it depends, or by which it is destroyed, or with which it comes into competition; and as these species are already defined objects (however they may have become so), not blending one into another by insensible gradations, the range of any one species, depending as it does on the range of others, will tend to be sharply defined. moreover, each species on the confines of its range, where it exists in lessened numbers, will, during fluctuations in the number of its enemies or of its prey, or in the seasons, be extremely liable to utter extermination; and thus its geographical range will come to be still more sharply defined. if i am right in believing that allied or representative species, when inhabiting a continuous area, are generally so distributed that each has a wide range, with a comparatively narrow neutral territory between them, in which they become rather suddenly rarer and rarer; then, as varieties do not essentially differ from species, { } the same rule will probably apply to both; and if we in imagination adapt a varying species to a very large area, we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone. the intermediate variety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser area; and practically, as far as i can make out, this rule holds good with varieties in a state of nature. i have met with striking instances of the rule in the case of varieties intermediate between well-marked varieties in the genus balanus. and it would appear from information given me by mr. watson, dr. asa gray, and mr. wollaston, that generally when varieties intermediate between two other forms occur, they are much rarer numerically than the forms which they connect. now, if we may trust these facts and inferences, and therefore conclude that varieties linking two other varieties together have generally existed in lesser numbers than the forms which they connect, then, i think, we can understand why intermediate varieties should not endure for very long periods;--why as a general rule they should be exterminated and disappear, sooner than the forms which they originally linked together. for any form existing in lesser numbers would, as already remarked, run a greater chance of being exterminated than one existing in large numbers; and in this particular case the intermediate form would be eminently liable to the inroads of closely allied forms existing on both sides of it. but a far more important consideration, as i believe, is that, during the process of further modification, by which two varieties are supposed on my theory to be converted and perfected into two distinct species, the two which exist in larger numbers from inhabiting larger areas, will have a great advantage over the intermediate variety, which exists { } in smaller numbers in a narrow and intermediate zone. for forms existing in larger numbers will always have a better chance, within any given period, of presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers. hence, the more common forms, in the race for life, will tend to beat and supplant the less common forms, for these will be more slowly modified and improved. it is the same principle which, as i believe, accounts for the common species in each country, as shown in the second chapter, presenting on an average a greater number of well-marked varieties than do the rarer species. i may illustrate what i mean by supposing three varieties of sheep to be kept, one adapted to an extensive mountainous region; a second to a comparatively narrow, hilly tract; and a third to wide plains at the base; and that the inhabitants are all trying with equal steadiness and skill to improve their stocks by selection; the chances in this case will be strongly in favour of the great holders on the mountains or on the plains improving their breeds more quickly than the small holders on the intermediate narrow, hilly tract; and consequently the improved mountain or plain breed will soon take the place of the less improved hill breed; and thus the two breeds, which originally existed in greater numbers, will come into close contact with each other, without the interposition of the supplanted, intermediate hill-variety. to sum up, i believe that species come to be tolerably well-defined objects, and do not at any one period present an inextricable chaos of varying and intermediate links: firstly, because new varieties are very slowly formed, for variation is a very slow process, and natural selection can do nothing until favourable { } variations chance to occur, and until a place in the natural polity of the country can be better filled by some modification of some one or more of its inhabitants. and such new places will depend on slow changes of climate, or on the occasional immigration of new inhabitants, and, probably, in a still more important degree, on some of the old inhabitants becoming slowly modified, with the new forms thus produced and the old ones acting and reacting on each other. so that, in any one region and at any one time, we ought only to see a few species presenting slight modifications of structure in some degree permanent; and this assuredly we do see. secondly, areas now continuous must often have existed within the recent period in isolated portions, in which many forms, more especially amongst the classes which unite for each birth and wander much, may have separately been rendered sufficiently distinct to rank as representative species. in this case, intermediate varieties between the several representative species and their common parent, must formerly have existed in each broken portion of the land, but these links will have been supplanted and exterminated during the process of natural selection, so that they will no longer exist in a living state. thirdly, when two or more varieties have been formed in different portions of a strictly continuous area, intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will generally have had a short duration. for these intermediate varieties will, from reasons already assigned (namely from what we know of the actual distribution of closely allied or representative species, and likewise of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they { } tend to connect. from this cause alone the intermediate varieties will be liable to accidental extermination; and during the process of further modification through natural selection, they will almost certainly be beaten and supplanted by the forms which they connect; for these from existing in greater numbers will, in the aggregate, present more variation, and thus be further improved through natural selection and gain further advantages. lastly, looking not to any one time, but to all time, if my theory be true, numberless intermediate varieties, linking most closely all the species of the same group together, must assuredly have existed; but the very process of natural selection constantly tends, as has been so often remarked, to exterminate the parent-forms and the intermediate links. consequently evidence of their former existence could be found only amongst fossil remains, which are preserved, as we shall in a future chapter attempt to show, in an extremely imperfect and intermittent record. _on the origin and transitions of organic beings with peculiar habits and structure._--it has been asked by the opponents of such views as i hold, how, for instance, a land carnivorous animal could have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted? it would be easy to show that within the same group carnivorous animals exist having every intermediate grade between truly aquatic and strictly terrestrial habits; and as each exists by a struggle for life, it is clear that each is well adapted in its habits to its place in nature. look at the mustela vison of north america, which has webbed feet and which resembles an otter in its fur, short legs, and form of tail; during summer this animal { } dives for and preys on fish, but during the long winter it leaves the frozen waters, and preys like other polecats on mice and land animals. if a different case had been taken, and it had been asked how an insectivorous quadruped could possibly have been converted into a flying bat, the question would have been far more difficult, and i could have given no answer. yet i think such difficulties have very little weight. here, as on other occasions, i lie under a heavy disadvantage, for out of the many striking cases which i have collected, i can give only one or two instances of transitional habits and structures in closely allied species of the same genus; and of diversified habits, either constant or occasional, in the same species. and it seems to me that nothing less than a long list of such cases is sufficient to lessen the difficulty in any particular case like that of the bat. look at the family of squirrels; here we have the finest gradation from animals with their tails only slightly flattened, and from others, as sir j. richardson has remarked, with the posterior part of their bodies rather wide and with the skin on their flanks rather full, to the so-called flying squirrels; and flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to glide through the air to an astonishing distance from tree to tree. we cannot doubt that each structure is of use to each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey, or to collect food more quickly, or, as there is reason to believe, by lessening the danger from occasional falls. but it does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive under all natural conditions. let the climate and vegetation change, let other competing { } rodents or new beasts of prey immigrate, or old ones become modified, and all analogy would lead us to believe that some at least of the squirrels would decrease in numbers or become exterminated, unless they also became modified and improved in structure in a corresponding manner. therefore, i can see no difficulty, more especially under changing conditions of life, in the continued preservation of individuals with fuller and fuller flank-membranes, each modification being useful, each being propagated, until by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was produced. now look at the galeopithecus or flying lemur, which formerly was falsely ranked amongst bats. it has an extremely wide flank-membrane, stretching from the corners of the jaw to the tail, and including the limbs and the elongated fingers: the flank-membrane is, also, furnished with an extensor muscle. although no graduated links of structure, fitted for gliding through the air, now connect the galeopithecus with the other lemuridæ, yet i see no difficulty in supposing that such links formerly existed, and that each had been formed by the same steps as in the case of the less perfectly gliding squirrels; and that each grade of structure was useful to its possessor. nor can i see any insuperable difficulty in further believing it possible that the membrane-connected fingers and forearm of the galeopithecus might be greatly lengthened by natural selection; and this, as far as the organs of flight are concerned, would convert it into a bat. in bats which have the wing-membrane extended from the top of the shoulder to the tail, including the hind-legs, we perhaps see traces of an apparatus originally constructed for gliding through the air rather than for flight. { } if about a dozen genera of birds had become extinct or were unknown, who would have ventured to have surmised that birds might have existed which used their wings solely as flappers, like the logger-headed duck (micropterus of eyton); as fins in the water and front legs on the land, like the penguin; as sails, like the ostrich; and functionally for no purpose, like the apteryx. yet the structure of each of these birds is good for it, under the conditions of life to which it is exposed, for each has to live by a struggle; but it is not necessarily the best possible under all possible conditions. it must not be inferred from these remarks that any of the grades of wing-structure here alluded to, which perhaps may all have resulted from disuse, indicate the natural steps by which birds have acquired their perfect power of flight; but they serve, at least, to show what diversified means of transition are possible. seeing that a few members of such water-breathing classes as the crustacea and mollusca are adapted to live on the land; and seeing that we have flying birds and mammals, flying insects of the most diversified types, and formerly had flying reptiles, it is conceivable that flying-fish, which now glide far through the air, slightly rising and turning by the aid of their fluttering fins, might have been modified into perfectly winged animals. if this had been effected, who would have ever imagined that in an early transitional state they had been inhabitants of the open ocean, and had used their incipient organs of flight exclusively, as far as we know, to escape being devoured by other fish? when we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we should bear in mind that animals displaying early { } transitional grades of the structure will seldom continue to exist to the present day, for they will have been supplanted by the very process of perfection through natural selection. furthermore, we may conclude that transitional grades between structures fitted for very different habits of life will rarely have been developed at an early period in great numbers and under many subordinate forms. thus, to return to our imaginary illustration of the flying-fish, it does not seem probable that fishes capable of true flight would have been developed under many subordinate forms, for taking prey of many kinds in many ways, on the land and in the water, until their organs of flight had come to a high stage of perfection, so as to have given them a decided advantage over other animals in the battle for life. hence the chance of discovering species with transitional grades of structure in a fossil condition will always be less, from their having existed in lesser numbers, than in the case of species with fully developed structures. i will now give two or three instances of diversified and of changed habits in the individuals of the same species. when either case occurs, it would be easy for natural selection to fit the animal, by some modification of its structure, for its changed habits, or exclusively for one of its several different habits. but it is difficult to tell, and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed habits; both probably often change almost simultaneously. of cases of changed habits it will suffice merely to allude to that of the many british insects which now feed on exotic plants, or exclusively on artificial substances. of diversified habits innumerable instances could be given: i have often watched a tyrant flycatcher (saurophagus sulphuratus) in south america, hovering over one spot { } and then proceeding to another, like a kestrel, and at other times standing stationary on the margin of water, and then dashing like a kingfisher at a fish. in our own country the larger titmouse (parus major) may be seen climbing branches, almost like a creeper; it often, like a shrike, kills small birds by blows on the head; and i have many times seen and heard it hammering the seeds of the yew on a branch, and thus breaking them like a nuthatch. in north america the black bear was seen by hearne swimming for hours with widely open mouth, thus catching, almost like a whale, insects in the water. as we sometimes see individuals of a species following habits widely different from those of their own species and of the other species of the same genus, we might expect, on my theory, that such individuals would occasionally have given rise to new species, having anomalous habits, and with their structure either slightly or considerably modified from that of their proper type. and such instances do occur in nature. can a more striking instance of adaptation be given than that of a woodpecker for climbing trees and for seizing insects in the chinks of the bark? yet in north america there are woodpeckers which feed largely on fruit, and others with elongated wings which chase insects on the wing; and on the plains of la plata, where not a tree grows, there is a woodpecker, which in every essential part of its organisation, even in its colouring, in the harsh tone of its voice, and undulatory flight, told me plainly of its close blood-relationship to our common species; yet it is a woodpecker which never climbs a tree! petrels are the most aërial and oceanic of birds, yet in the quiet sounds of tierra del fuego, the puffinuria berardi, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when { } unwillingly it takes flight, would be mistaken by any one for an auk or grebe; nevertheless, it is essentially a petrel, but with many parts of its organisation profoundly modified. on the other hand, the acutest observer by examining the dead body of the water-ouzel would never have suspected its sub-aquatic habits; yet this anomalous member of the strictly terrestrial thrush family wholly subsists by diving,--grasping the stones with its feet and using its wings under water. he who believes that each being has been created as we now see it, must occasionally have felt surprise when he has met with an animal having habits and structure not at all in agreement. what can be plainer than that the webbed feet of ducks and geese are formed for swimming? yet there are upland geese with webbed feet which rarely or never go near the water; and no one except audubon has seen the frigate-bird, which has all its four toes webbed, alight on the surface of the sea. on the other hand grebes and coots are eminently aquatic, although their toes are only bordered by membrane. what seems plainer than that the long toes of grallatores are formed for walking over swamps and floating plants, yet the water-hen is nearly as aquatic as the coot; and the landrail nearly as terrestrial as the quail or partridge. in such cases, and many others could be given, habits have changed without a corresponding change of structure. the webbed feet of the upland goose may be said to have become rudimentary in function, though not in structure. in the frigate-bird, the deeply-scooped membrane between the toes shows that structure has begun to change. he who believes in separate and innumerable acts of creation will say, that in these cases it has pleased the creator to cause a being of one type to take the place of one of another type; but this seems to me only { } restating the fact in dignified language. he who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being vary ever so little, either in habits or structure, and thus gain an advantage over some other inhabitant of the country, it will seize on the place of that inhabitant, however different it may be from its own place. hence it will cause him no surprise that there should be geese and frigate-birds with webbed feet, living on the dry land or most rarely alighting on the water; that there should be long-toed corncrakes living in meadows instead of in swamps; that there should be woodpeckers where not a tree grows; that there should be diving thrushes, and petrels with the habits of auks. _organs of extreme perfection and complication._--to suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, i freely confess, absurd in the highest possible degree. yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. how a nerve comes to be sensitive to { } light, hardly concerns us more than how life itself first originated; but i may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. in looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition. amongst existing vertebrata, we find but a small amount of gradation in the structure of the eye, and from fossil species we can learn nothing on this head. in this great class we should probably have to descend far beneath the lowest known fossiliferous stratum to discover the earlier stages, by which the eye has been perfected. in the articulata we can commence a series with an optic nerve merely coated with pigment, and without any other mechanism; and from this low stage, numerous gradations of structure, branching off in two fundamentally different lines, can be shown to exist, until we reach a moderately high stage of perfection. in certain crustaceans, for instance, there is a double cornea, the inner one divided into facets, within each of which there is a lens-shaped swelling. in other crustaceans the transparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends and must act by convergence; and at their lower ends there seems to be an imperfect vitreous substance. { } with these facts, here far too briefly and imperfectly given, which show that there is much graduated diversity in the eyes of living crustaceans, and bearing in mind how small the number of living animals is in proportion to those which have become extinct, i can see no very great difficulty (not more than in the case of many other structures) in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great articulate class. he who will go thus far, if he find on finishing this treatise that large bodies of facts, otherwise inexplicable, can be explained by the theory of descent, ought not to hesitate to go further, and to admit that a structure even as perfect as the eye of an eagle might be formed by natural selection, although in this case he does not know any of the transitional grades. his reason ought to conquer his imagination; though i have felt the difficulty far too keenly to be surprised at any degree of hesitation in extending the principle of natural selection to such startling lengths. it is scarcely possible to avoid comparing the eye to a telescope. we know that this instrument has been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. but may not this inference be presumptuous? have we any right to assume that the creator works by intellectual powers like those of man? if we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing { } slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. further we must suppose that there is a power always intently watching each slight accidental alteration in the transparent layers; and carefully selecting each alteration which, under varied circumstances, may in any way, or in any degree, tend to produce a distincter image. we must suppose each new state of the instrument to be multiplied by the million; and each to be preserved till a better be produced, and then the old ones to be destroyed. in living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. let this process go on for millions on millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the creator are to those of man? if it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. but i can find out no such case. no doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, round which, according to my theory, there has been much extinction. or again, if we look to an organ common to all the members of a large class, for in this latter case the organ must have been first formed at an extremely remote period, since which all the many members of the class have been developed; and in order to discover the early transitional grades through which the organ has { } passed, we should have to look to very ancient ancestral forms, long since become extinct. we should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind. numerous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions; thus the alimentary canal respires, digests, and excretes in the larva of the dragon-fly and in the fish cobites. in the hydra, the animal may be turned inside out, and the exterior surface will then digest and the stomach respire. in such cases natural selection might easily specialise, if any advantage were thus gained, a part or organ, which had performed two functions, for one function alone, and thus wholly change its nature by insensible steps. two distinct organs sometimes perform simultaneously the same function in the same individual; to give one instance, there are fish with gills or branchiæ that breathe the air dissolved in the water, at the same time that they breathe free air in their swimbladders, this latter organ having a ductus pneumaticus for its supply, and being divided by highly vascular partitions. in these cases one of the two organs might with ease be modified and perfected so as to perform all the work by itself, being aided during the process of modification by the other organ; and then this other organ might be modified for some other and quite distinct purpose, or be quite obliterated. the illustration of the swimbladder in fishes is a good one, because it shows us clearly the highly important fact that an organ originally constructed for one purpose, namely flotation, may be converted into one for a wholly different purpose, namely respiration. the swimbladder has, also, been worked in as an accessory to the auditory organs of certain fish, or, for i do not know { } which view is now generally held, a part of the auditory apparatus has been worked in as a complement to the swimbladder. all physiologists admit that the swimbladder is homologous, or "ideally similar" in position and structure with the lungs of the higher vertebrate animals: hence there seems to me to be no great difficulty in believing that natural selection has actually converted a swimbladder into a lung, or organ used exclusively for respiration. i can, indeed, hardly doubt that all vertebrate animals having true lungs have descended by ordinary generation from an ancient prototype, of which we know nothing, furnished with a floating apparatus or swimbladder. we can thus, as i infer from professor owen's interesting description of these parts, understand the strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachea, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. in the higher vertebrata the branchiæ have wholly disappeared--the slits on the sides of the neck and the loop-like course of the arteries still marking in the embryo their former position. but it is conceivable that the now utterly lost branchiæ might have been gradually worked in by natural selection for some quite distinct purpose: in the same manner as, on the view entertained by some naturalists that the branchiæ and dorsal scales of annelids are homologous with the wings and wing-covers of insects, it is probable that organs which at a very ancient period served for respiration have been actually converted into organs of flight. in considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another, that i will give one more instance. pedunculated cirripedes have two minute folds of skin, { } called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until they are hatched within the sack. these cirripedes have no branchiæ, the whole surface of the body and sack, including the small frena, serving for respiration. the balanidæ or sessile cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at the bottom of the sack, in the well-enclosed shell; but they have large folded branchiæ. now i think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiæ of the other family; indeed, they graduate into each other. therefore i do not doubt that little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly aided the act of respiration, have been gradually converted by natural selection into branchiæ, simply through an increase in their size and the obliteration of their adhesive glands. if all pedunculated cirripedes had become extinct, and they have already suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiæ in this latter family had originally existed as organs for preventing the ova from being washed out of the sack? although we must be extremely cautious in concluding that any organ could not possibly have been produced by successive transitional gradations, yet, undoubtedly, grave cases of difficulty occur, some of which will be discussed in my future work. one of the gravest is that of neuter insects, which are often very differently constructed from either the males or fertile females; but this case will be treated of in the next chapter. the electric organs of fishes offer another case of special difficulty; it is impossible to conceive by what steps these wondrous organs have been produced; but, as owen and others have remarked, { } their intimate structure closely resembles that of common muscle; and as it has lately been shown that rays have an organ closely analogous to the electric apparatus, and yet do not, as matteucci asserts, discharge any electricity, we must own that we are far too ignorant to argue that no transition of any kind is possible. the electric organs offer another and even more serious difficulty; for they occur in only about a dozen fishes, of which several are widely remote in their affinities. generally when the same organ appears in several members of the same class, especially if in members having very different habits of life, we may attribute its presence to inheritance from a common ancestor; and its absence in some of the members to its loss through disuse or natural selection. but if the electric organs had been inherited from one ancient progenitor thus provided, we might have expected that all electric fishes would have been specially related to each other. nor does geology at all lead to the belief that formerly most fishes had electric organs, which most of their modified descendants have lost. the presence of luminous organs in a few insects, belonging to different families and orders, offers a parallel case of difficulty. other cases could be given; for instance in plants, the very curious contrivance of a mass of pollen-grains, borne on a foot-stalk with a sticky gland at the end, is the same in orchis and asclepias,--genera almost as remote as possible amongst flowering plants. in all these cases of two very distinct species furnished with apparently the same anomalous organ, it should be observed that, although the general appearance and function of the organ may be the same, yet some fundamental difference can generally be detected. i am inclined to believe that in nearly the same way as two men have sometimes independently hit on { } the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings, which beings owe but little of their structure in common to inheritance from the same ancestor. although in many cases it is most difficult to conjecture by what transitions organs could have arrived at their present state; yet, considering that the proportion of living and known forms to the extinct and unknown is very small, i have been astonished how rarely an organ can be named, towards which no transitional grade is known to lead. the truth of this remark is indeed shown by that old but somewhat exaggerated canon in natural history of "natura non facit saltum." we meet with this admission in the writings of almost every experienced naturalist; or, as milne edwards has well expressed it, nature is prodigal in variety, but niggard in innovation. why, on the theory of creation, should this be so? why should all the parts and organs of many independent beings, each supposed to have been separately created for its proper place in nature, be so commonly linked together by graduated steps? why should not nature have taken a leap from structure to structure? on the theory of natural selection, we can clearly understand why she should not; for natural selection can act only by taking advantage of slight successive variations; she can never take a leap, but must advance by the shortest and slowest steps. _organs of little apparent importance._--as natural selection acts by life and death,--by the preservation of individuals with any favourable variation, and by the destruction of those with any unfavourable deviation of structure,--i have sometimes felt much difficulty in { } understanding the origin of simple parts, of which the importance does not seem sufficient to cause the preservation of successively varying individuals. i have sometimes felt as much difficulty, though of a very different kind, on this head, as in the case of an organ as perfect and complex as the eye. in the first place, we are much too ignorant in regard to the whole economy of any one organic being, to say what slight modifications would be of importance or not. in a former chapter i have given instances of most trifling characters, such as the down on fruit and the colour of its flesh, which, from determining the attacks of insects or from being correlated with constitutional differences, might assuredly be acted on by natural selection. the tail of the giraffe looks like an artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better, for so trifling an object as driving away flies; yet we should pause before being too positive even in this case, for we know that the distribution and existence of cattle and other animals in south america absolutely depends on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great advantage. it is not that the larger quadrupeds are actually destroyed (except in some rare cases) by flies, but they are incessantly harassed and their strength reduced, so that they are more subject to disease, or not so well enabled in a coming dearth to search for food, or to escape from beasts of prey. organs now of trifling importance have probably in some cases been of high importance to an early progenitor, and, after having been slowly perfected at a { } former period, have been transmitted in nearly the same state, although now become of very slight use; and any actually injurious deviations in their structure will always have been checked by natural selection. seeing how important an organ of locomotion the tail is in most aquatic animals, its general presence and use for many purposes in so many land animals, which in their lungs or modified swimbladders betray their aquatic origin, may perhaps be thus accounted for. a well-developed tail having been formed in an aquatic animal, it might subsequently come to be worked in for all sorts of purposes, as a fly-flapper, an organ of prehension, or as an aid in turning, as with the dog, though the aid must be slight, for the hare, with hardly any tail, can double quickly enough. in the second place, we may sometimes attribute importance to characters which are really of very little importance, and which have originated from quite secondary causes, independently of natural selection. we should remember that climate, food, &c., probably have some little direct influence on the organisation; that characters reappear from the law of reversion; that correlation of growth will have had a most important influence in modifying various structures; and finally, that sexual selection will often have largely modified the external characters of animals having a will, to give one male an advantage in fighting with another or in charming the females. moreover when a modification of structure has primarily arisen from the above or other unknown causes, it may at first have been of no advantage to the species, but may subsequently have been taken advantage of by the descendants of the species under new conditions of life and with newly acquired habits. to give a few instances to illustrate these latter { } remarks. if green woodpeckers alone had existed, and we did not know that there were many black and pied kinds, i dare say that we should have thought that the green colour was a beautiful adaptation to hide this tree-frequenting bird from its enemies; and consequently that it was a character of importance and might have been acquired through natural selection; as it is, i have no doubt that the colour is due to some quite distinct cause, probably to sexual selection. a trailing bamboo in the malay archipelago climbs the loftiest trees by the aid of exquisitely constructed hooks clustered around the ends of the branches, and this contrivance, no doubt, is of the highest service to the plant; but as we see nearly similar hooks on many trees which are not climbers, the hooks on the bamboo may have arisen from unknown laws of growth, and have been subsequently taken advantage of by the plant undergoing further modification and becoming a climber. the naked skin on the head of a vulture is generally looked at as a direct adaptation for wallowing in putridity; and so it may be, or it may possibly be due to the direct action of putrid matter; but we should be very cautious in drawing any such inference, when we see that the skin on the head of the clean-feeding male turkey is likewise naked. the sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt they facilitate, or may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals. we are profoundly ignorant of the causes producing slight and unimportant variations; and we are { } immediately made conscious of this by reflecting on the differences in the breeds of our domesticated animals in different countries,--more especially in the less civilised countries where there has been but little artificial selection. careful observers are convinced that a damp climate affects the growth of the hair, and that with the hair the horns are correlated. mountain breeds always differ from lowland breeds; and a mountainous country would probably affect the hind limbs from exercising them more, and possibly even the form of the pelvis; and then by the law of homologous variation, the front limbs and even the head would probably be affected. the shape, also, of the pelvis might affect by pressure the shape of the head of the young in the womb. the laborious breathing necessary in high regions would, we have some reason to believe, increase the size of the chest; and again correlation would come into play. animals kept by savages in different countries often have to struggle for their own subsistence, and would be exposed to a certain extent to natural selection, and individuals with slightly different constitutions would succeed best under different climates; and there is reason to believe that constitution and colour are correlated. a good observer, also, states that in cattle susceptibility to the attacks of flies is correlated with colour, as is the liability to be poisoned by certain plants; so that colour would be thus subjected to the action of natural selection. but we are far too ignorant to speculate on the relative importance of the several known and unknown laws of variation; and i have here alluded to them only to show that, if we are unable to account for the characteristic differences of our domestic breeds, which nevertheless we generally admit to have arisen through ordinary generation, we ought not to lay too much stress on our ignorance of the precise cause { } of the slight analogous differences between species. i might have adduced for this same purpose the differences between the races of man, which are so strongly marked; i may add that some little light can apparently be thrown on the origin of these differences, chiefly through sexual selection of a particular kind, but without here entering on copious details my reasoning would appear frivolous. the foregoing remarks lead me to say a few words on the protest lately made by some naturalists, against the utilitarian doctrine that every detail of structure has been produced for the good of its possessor. they believe that very many structures have been created for beauty in the eyes of man, or for mere variety. this doctrine, if true, would be absolutely fatal to my theory. yet i fully admit that many structures are of no direct use to their possessors. physical conditions probably have had some little effect on structure, quite independently of any good thus gained. correlation of growth has no doubt played a most important part, and a useful modification of one part will often have entailed on other parts diversified changes of no direct use. so again characters which formerly were useful, or which formerly had arisen from correlation of growth, or from other unknown cause, may reappear from the law of reversion, though now of no direct use. the effects of sexual selection, when displayed in beauty to charm the females, can be called useful only in rather a forced sense. but by far the most important consideration is that the chief part of the organisation of every being is simply due to inheritance; and consequently, though each being assuredly is well fitted for its place in nature, many structures now have no direct relation to the habits of life of each species. thus, we can hardly believe that the webbed feet of the upland { } goose or of the frigate-bird are of special use to these birds; we cannot believe that the same bones in the arm of the monkey, in the fore-leg of the horse, in the wing of the bat, and in the nipper of the seal, are of special use to these animals. we may safely attribute these structures to inheritance. but to the progenitor of the upland goose and of the frigate-bird, webbed feet no doubt were as useful as they now are to the most aquatic of existing birds. so we may believe that the progenitor of the seal had not a nipper, but a foot with five toes fitted for walking or grasping; and we may further venture to believe that the several bones in the limbs of the monkey, horse, and bat, which have been inherited from a common progenitor, were formerly of more special use to that progenitor, or its progenitors, than they now are to these animals having such widely diversified habits. therefore we may infer that these several bones might have been acquired through natural selection, subjected formerly, as now, to the several laws of inheritance, reversion, correlation of growth, &c. hence every detail of structure in every living creature (making some little allowance for the direct action of physical conditions) may be viewed, either as having been of special use to some ancestral form, or as being now of special use to the descendants of this form--either directly, or indirectly through the complex laws of growth. natural selection cannot possibly produce any modification in any one species exclusively for the good of another species; though throughout nature one species incessantly takes advantage of, and profits by, the structure of another. but natural selection can and does often produce structures for the direct injury of other species, as we see in the fang of the adder, and in the ovipositor of the ichneumon, by which its eggs are { } deposited in the living bodies of other insects. if it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection. although many statements may be found in works on natural history to this effect, i cannot find even one which seems to me of any weight. it is admitted that the rattlesnake has a poison-fang for its own defence and for the destruction of its prey; but some authors suppose that at the same time this snake is furnished with a rattle for its own injury, namely, to warn its prey to escape. i would almost as soon believe that the cat curls the end of its tail when preparing to spring, in order to warn the doomed mouse. but i have not space here to enter on this and other such cases. natural selection will never produce in a being anything injurious to itself, for natural selection acts solely by and for the good of each. no organ will be formed, as paley has remarked, for the purpose of causing pain or for doing an injury to its possessor. if a fair balance be struck between the good and evil caused by each part, each will be found on the whole advantageous. after the lapse of time, under changing conditions of life, if any part comes to be injurious, it will be modified; or if it be not so, the being will become extinct, as myriads have become extinct. natural selection tends only to make each organic being as perfect as, or slightly more perfect than, the other inhabitants of the same country with which it has to struggle for existence. and we see that this is the degree of perfection attained under nature. the endemic productions of new zealand, for instance, are perfect one compared with another; but they are now rapidly yielding before the advancing legions of plants { } and animals introduced from europe. natural selection will not produce absolute perfection, nor do we always meet, as far as we can judge, with this high standard under nature. the correction for the aberration of light is said, on high authority, not to be perfect even in that most perfect organ, the eye. if our reason leads us to admire with enthusiasm a multitude of inimitable contrivances in nature, this same reason tells us, though we may easily err on both sides, that some other contrivances are less perfect. can we consider the sting of the wasp or of the bee as perfect, which, when used against many attacking animals, cannot be withdrawn, owing to the backward serratures, and so inevitably causes the death of the insect by tearing out its viscera? if we look at the sting of the bee, as having originally existed in a remote progenitor as a boring and serrated instrument, like that in so many members of the same great order, and which has been modified but not perfected for its present purpose, with the poison originally adapted to cause galls subsequently intensified, we can perhaps understand how it is that the use of the sting should so often cause the insect's own death: for if on the whole the power of stinging be useful to the community, it will fulfil all the requirements of natural selection, though it may cause the death of some few members. if we admire the truly wonderful power of scent by which the males of many insects find their females, can we admire the production for this single purpose of thousands of drones, which are utterly useless to the community for any other end, and which are ultimately slaughtered by their industrious and sterile sisters? it may be difficult, but we ought to admire the savage instinctive hatred of the queen-bee, which urges her instantly to destroy the { } young queens her daughters as soon as born, or to perish herself in the combat; for undoubtedly this is for the good of the community; and maternal love or maternal hatred, though the latter fortunately is most rare, is all the same to the inexorable principle of natural selection. if we admire the several ingenious contrivances, by which the flowers of the orchis and of many other plants are fertilised through insect agency, can we consider as equally perfect the elaboration by our fir-trees of dense clouds of pollen, in order that a few granules may be wafted by a chance breeze on to the ovules? _summary of chapter._--we have in this chapter discussed some of the difficulties and objections which may be urged against my theory. many of them are very serious; but i think that in the discussion light has been thrown on several facts, which on the theory of independent acts of creation are utterly obscure. we have seen that species at any one period are not indefinitely variable, and are not linked together by a multitude of intermediate gradations, partly because the process of natural selection will always be very slow, and will act, at any one time, only on a very few forms; and partly because the very process of natural selection almost implies the continual supplanting and extinction of preceding and intermediate gradations. closely allied species, now living on a continuous area, must often have been formed when the area was not continuous, and when the conditions of life did not insensibly graduate away from one part to another. when two varieties are formed in two districts of a continuous area, an intermediate variety will often be formed, fitted for an intermediate zone; but from reasons assigned, the intermediate variety will usually exist in lesser numbers than { } the two forms which it connects; consequently the two latter, during the course of further modification, from existing in greater numbers, will have a great advantage over the less numerous intermediate variety, and will thus generally succeed in supplanting and exterminating it. we have seen in this chapter how cautious we should be in concluding that the most different habits of life could not graduate into each other; that a bat, for instance, could not have been formed by natural selection from an animal which at first could only glide through the air. we have seen that a species may under new conditions of life change its habits, or have diversified habits, with some habits very unlike those of its nearest congeners. hence we can understand, bearing in mind that each organic being is trying to live wherever it can live, how it has arisen that there are upland geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with the habits of auks. although the belief that an organ so perfect as the eye could have been formed by natural selection, is more than enough to stagger any one; yet in the case of any organ, if we know of a long series of gradations in complexity, each good for its possessor, then, under changing conditions of life there is no logical impossibility in the acquirement of any conceivable degree of perfection through natural selection. in the cases in which we know of no intermediate or transitional states, we should be very cautious in concluding that none could have existed, for the homologies of many organs and their intermediate states show that wonderful metamorphoses in function are at least possible. for instance, a swim-bladder has apparently been converted into an air-breathing lung. the same organ having performed { } simultaneously very different functions, and then having been specialised for one function; and two very distinct organs having performed at the same time the same function, the one having been perfected whilst aided by the other, must often have largely facilitated transitions. we are far too ignorant, in almost every case, to be enabled to assert that any part or organ is so unimportant for the welfare of a species, that modifications in its structure could not have been slowly accumulated by means of natural selection. but we may confidently believe that many modifications, wholly due to the laws of growth, and at first in no way advantageous to a species, have been subsequently taken advantage of by the still further modified descendants of this species. we may, also, believe that a part formerly of high importance has often been retained (as the tail of an aquatic animal by its terrestrial descendants), though it has become of such small importance that it could not, in its present state, have been acquired by natural selection,--a power which acts solely by the preservation of profitable variations in the struggle for life. natural selection will produce nothing in one species for the exclusive good or injury of another; though it may well produce parts, organs, and excretions highly useful or even indispensable, or highly injurious to another species, but in all cases at the same time useful to the owner. natural selection in each well-stocked country, must act chiefly through the competition of the inhabitants one with another, and consequently will produce perfection, or strength in the battle for life, only according to the standard of that country. hence the inhabitants of one country, generally the smaller one, will often yield, as we see they do yield, to the inhabitants of another and generally larger country. for in { } the larger country there will have existed more individuals, and more diversified forms, and the competition will have been severer, and thus the standard of perfection will have been rendered higher. natural selection will not necessarily produce absolute perfection; nor, as far as we can judge by our limited faculties, can absolute perfection be everywhere found. on the theory of natural selection we can clearly understand the full meaning of that old canon in natural history, "natura non facit saltum." this canon, if we look only to the present inhabitants of the world, is not strictly correct, but if we include all those of past times, it must by my theory be strictly true. it is generally acknowledged that all organic beings have been formed on two great laws--unity of type, and the conditions of existence. by unity of type is meant that fundamental agreement in structure, which we see in organic beings of the same class, and which is quite independent of their habits of life. on my theory, unity of type is explained by unity of descent. the expression of conditions of existence, so often insisted on by the illustrious cuvier, is fully embraced by the principle of natural selection. for natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them during long-past periods of time: the adaptations being aided in some cases by use and disuse, being slightly affected by the direct action of the external conditions of life, and being in all cases subjected to the several laws of growth. hence, in fact, the law of the conditions of existence is the higher law; as it includes, through the inheritance of former adaptations, that of unity of type. * * * * * { } chapter vii. instinct. instincts comparable with habits, but different in their origin--instincts graduated--aphides and ants--instincts variable--domestic instincts, their origin--natural instincts of the cuckoo, ostrich, and parasitic bees--slave-making-ants--hive-bee, its cell-making instinct--difficulties on the theory of the natural selection of instincts--neuter or sterile insects--summary. the subject of instinct might have been worked into the previous chapters; but i have thought that it would be more convenient to treat the subject separately, especially as so wonderful an instinct as that of the hive-bee making its cells will probably have occurred to many readers, as a difficulty sufficient to overthrow my whole theory. i must premise, that i have nothing to do with the origin of the primary mental powers, any more than i have with that of life itself. we are concerned only with the diversities of instinct and of the other mental qualities of animals within the same class. i will not attempt any definition of instinct. it would be easy to show that several distinct mental actions are commonly embraced by this term; but every one understands what is meant, when it is said that instinct impels the cuckoo to migrate and to lay her eggs in other birds' nests. an action, which we ourselves should require experience to enable us to perform, when performed by an animal, more especially by a very young one, without any experience, and when performed by many individuals in the same way, without their knowing for what purpose it is performed, is usually said to be instinctive. { } but i could show that none of these characters of instinct are universal. a little dose, as pierre huber expresses it, of judgment or reason, often comes into play, even in animals very low in the scale of nature. frederick cuvier and several of the older metaphysicians have compared instinct with habit. this comparison gives, i think, a remarkably accurate notion of the frame of mind under which an instinctive action is performed, but not of its origin. how unconsciously many habitual actions are performed, indeed not rarely in direct opposition to our conscious will! yet they may be modified by the will or reason. habits easily become associated with other habits, and with certain periods of time and states of the body. when once acquired, they often remain constant throughout life. several other points of resemblance between instincts and habits could be pointed out. as in repeating a well-known song, so in instincts, one action follows another by a sort of rhythm; if a person be interrupted in a song, or in repeating anything by rote, he is generally forced to go back to recover the habitual train of thought: so p. huber found it was with a caterpillar, which makes a very complicated hammock; for if he took a caterpillar which had completed its hammock up to, say, the sixth stage of construction, and put it into a hammock completed up only to the third stage, the caterpillar simply re-performed the fourth, fifth, and sixth stages of construction. if, however, a caterpillar were taken out of a hammock made up, for instance, to the third stage, and were put into one finished up to the sixth stage, so that much of its work, was already done for it, far from feeling the benefit of this, it was much embarrassed, and, in order to complete its hammock, seemed forced to start from the third stage, where it had left off, and thus tried to complete the already finished work. { } if we suppose any habitual action to become inherited--and i think it can be shown that this does sometimes happen--then the resemblance between what originally was a habit and an instinct becomes so close as not to be distinguished. if mozart, instead of playing the pianoforte at three years old with wonderfully little practice, had played a tune with no practice at all, he might truly be said to have done so instinctively. but it would be the most serious error to suppose that the greater number of instincts have been acquired by habit in one generation, and then transmitted by inheritance to succeeding generations. it can be clearly shown that the most wonderful instincts with which we are acquainted, namely, those of the hive-bee and of many ants, could not possibly have been thus acquired. it will be universally admitted that instincts are as important as corporeal structure for the welfare of each species, under its present conditions of life. under changed conditions of life, it is at least possible that slight modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so little, then i can see no difficulty in natural selection preserving and continually accumulating variations of instinct to any extent that may be profitable. it is thus, as i believe, that all the most complex and wonderful instincts have originated. as modifications of corporeal structure arise from, and are increased by, use or habit, and are diminished or lost by disuse, so i do not doubt it has been with instincts. but i believe that the effects of habit are of quite subordinate importance to the effects of the natural selection of what may be called accidental variations of instincts;--that is of variations produced by the same unknown causes which produce slight deviations of bodily structure. no complex instinct can possibly be produced through { } natural selection, except by the slow and gradual accumulation of numerous, slight, yet profitable, variations. hence, as in the case of corporeal structures, we ought to find in nature, not the actual transitional gradations by which each complex instinct has been acquired--for these could be found only in the lineal ancestors of each species--but we ought to find in the collateral lines of descent some evidence of such gradations; or we ought at least to be able to show that gradations of some kind are possible; and this we certainly can do. i have been surprised to find, making allowance for the instincts of animals having been but little observed except in europe and north america, and for no instinct being known amongst extinct species, how very generally gradations, leading to the most complex instincts, can be discovered. changes of instinct may sometimes be facilitated by the same species having different instincts at different periods of life, or at different seasons of the year, or when placed under different circumstances &c.; in which case either one or the other instinct might be preserved by natural selection. and such instances of diversity of instinct in the same species can be shown to occur in nature. again as in the case of corporeal structure, and conformably with my theory, the instinct of each species is good for itself, but has never, as far as we can judge, been produced for the exclusive good of others. one of the strongest instances of an animal apparently performing an action for the sole good of another, with which i am acquainted, is that of aphides voluntarily yielding their sweet excretion to ants: that they do so voluntarily, the following facts show. i removed all the ants from a group of about a dozen aphides on a dock-plant, and prevented their attendance during several hours. after this interval, i felt sure that the aphides { } would want to excrete. i watched them for some time through a lens, but not one excreted; i then tickled and stroked them with a hair in the same manner, as well as i could, as the ants do with their antennæ; but not one excreted. afterwards i allowed an ant to visit them, and it immediately seemed, by its eager way of running about, to be well aware what a rich flock it had discovered; it then began to play with its antennæ on the abdomen first of one aphis and then of another; and each aphis, as soon as it felt the antennæ, immediately lifted up its abdomen and excreted a limpid drop of sweet juice, which was eagerly devoured by the ant. even the quite young aphides behaved in this manner, showing that the action was instinctive, and not the result of experience. but as the excretion is extremely viscid, it is probably a convenience to the aphides to have it removed; and therefore probably the aphides do not instinctively excrete for the sole good of the ants. although i do not believe that any animal in the world performs an action for the exclusive good of another of a distinct species, yet each species tries to take advantage of the instincts of others, as each takes advantage of the weaker bodily structure of others. so again, in some few cases, certain instincts cannot be considered as absolutely perfect; but as details on this and other such points are not indispensable, they may be here passed over. as some degree of variation in instincts under a state of nature, and the inheritance of such variations, are indispensable for the action of natural selection, as many instances as possible ought to be here given; but want of space prevents me. i can only assert, that instincts certainly do vary--for instance, the migratory instinct, both in extent and direction, and in its total loss. so it is with the nests of birds, which vary partly { } in dependence on the situations chosen, and on the nature and temperature of the country inhabited, but often from causes wholly unknown to us: audubon has given several remarkable cases of differences in the nests of the same species in the northern and southern united states. fear of any particular enemy is certainly an instinctive quality, as may be seen in nestling birds, though it is strengthened by experience, and by the sight of fear of the same enemy in other animals. but fear of man is slowly acquired, as i have elsewhere shown, by various animals inhabiting desert islands; and we may see an instance of this, even in england, in the greater wildness of all our large birds than of our small birds; for the large birds have been most persecuted by man. we may safely attribute the greater wildness of our large birds to this cause; for in uninhabited islands large birds are not more fearful than small; and the magpie, so wary in england, is tame in norway, as is the hooded crow in egypt. that the general disposition of individuals of the same species, born in a state of nature, is extremely diversified, can be shown by a multitude of facts. several cases also, could be given, of occasional and strange habits in certain species, which might, if advantageous to the species, give rise, through natural selection, to quite new instincts. but i am well aware that these general statements, without facts given in detail, can produce but a feeble effect on the reader's mind. i can only repeat my assurance, that i do not speak without good evidence. the possibility, or even probability, of inherited variations of instinct in a state of nature will be strengthened by briefly considering a few cases under domestication. we shall thus also be enabled to see the respective parts which habit and the selection of { } so-called accidental variations have played in modifying the mental qualities of our domestic animals. a number of curious and authentic instances could be given of the inheritance of all shades of disposition and tastes, and likewise of the oddest tricks, associated with certain frames of mind or periods of time. but let us look to the familiar case of the several breeds of dogs: it cannot be doubted that young pointers (i have myself seen a striking instance) will sometimes point and even back other dogs the very first time that they are taken out; retrieving is certainly in some degree inherited by retrievers; and a tendency to run round, instead of at, a flock of sheep, by shepherd-dogs. i cannot see that these actions, performed without experience by the young, and in nearly the same manner by each individual, performed with eager delight by each breed, and without the end being known,--for the young pointer can no more know that he points to aid his master, than the white butterfly knows why she lays her eggs on the leaf of the cabbage,--i cannot see that these actions differ essentially from true instincts. if we were to see one kind of wolf, when young and without any training, as soon as it scented its prey, stand motionless like a statue, and then slowly crawl forward with a peculiar gait; and another kind of wolf rushing round, instead of at, a herd of deer, and driving them to a distant point, we should assuredly call these actions instinctive. domestic instincts, as they may be called, are certainly far less fixed or invariable than natural instincts; but they have been acted on by far less rigorous selection, and have been transmitted for an incomparably shorter period, under less fixed conditions of life. how strongly these domestic instincts, habits, and dispositions are inherited, and how curiously they become mingled, is well shown when different breeds of dogs are { } crossed. thus it is known that a cross with a bull-dog has affected for many generations the courage and obstinacy of greyhounds; and a cross with a greyhound has given to a whole family of shepherd-dogs a tendency to hunt hares. these domestic instincts, when thus tested by crossing, resemble natural instincts, which in a like manner become curiously blended together, and for a long period exhibit traces of the instincts of either parent: for example, le roy describes a dog, whose great-grandfather was a wolf, and this dog showed a trace of its wild parentage only in one way, by not coming in a straight line to his master when called. domestic instincts are sometimes spoken of as actions which have become inherited solely from long-continued and compulsory habit, but this, i think, is not true. no one would ever have thought of teaching, or probably could have taught, the tumbler-pigeon to tumble,--an action which, as i have witnessed, is performed by young birds, that have never seen a pigeon tumble. we may believe that some one pigeon showed a slight tendency to this strange habit, and that the long-continued selection of the best individuals in successive generations made tumblers what they now are; and near glasgow there are house-tumblers, as i hear from mr. brent, which cannot fly eighteen inches high without going head over heels. it may be doubted whether any one would have thought of training a dog to point, had not some one dog naturally shown a tendency in this line; and this is known occasionally to happen, as i once saw in a pure terrier: the act of pointing is probably, as many have thought, only the exaggerated pause of an animal preparing to spring on its prey. when the first tendency to point was once displayed, methodical selection and the inherited effects of compulsory training in each successive generation would soon complete the { } work; and unconscious selection is still at work, as each man tries to procure, without intending to improve the breed, dogs which will stand and hunt best. on the other hand, habit alone in some cases has sufficed; no animal is more difficult to tame than the young of the wild rabbit; scarcely any animal is tamer than the young of the tame rabbit; but i do not suppose that domestic rabbits have ever been selected for tameness; and i presume that we must attribute the whole of the inherited change from extreme wildness to extreme tameness, simply to habit and long-continued close confinement. natural instincts are lost under domestication: a remarkable instance of this is seen in those breeds of fowls which very rarely or never become "broody," that is, never wish to sit on their eggs. familiarity alone prevents our seeing how universally and largely the minds of our domestic animals have been modified by domestication. it is scarcely possible to doubt that the love of man has become instinctive in the dog. all wolves, foxes, jackals, and species of the cat genus, when kept tame, are most eager to attack poultry, sheep, and pigs; and this tendency has been found incurable in dogs which have been brought home as puppies from countries, such as tierra del fuego and australia, where the savages do not keep these domestic animals. how rarely, on the other hand, do our civilised dogs, even when quite young, require to be taught not to attack poultry, sheep, and pigs! no doubt they occasionally do make an attack, and are then beaten; and if not cured, they are destroyed; so that habit, with some degree of selection, has probably concurred in civilising by inheritance our dogs. on the other hand, young chickens have lost, wholly by habit, that fear of the dog and cat which no doubt was originally instinctive in them, in the same way as it is so plainly instinctive in { } young pheasants, though reared under a hen. it is not that chickens have lost all fear, but fear only of dogs and cats, for if the hen gives the danger-chuckle, they will run (more especially young turkeys) from under her, and conceal themselves in the surrounding grass or thickets; and this is evidently done for the instinctive purpose of allowing, as we see in wild ground-birds, their mother to fly away. but this instinct retained by our chickens has become useless under domestication, for the mother-hen has almost lost by disuse the power of flight. hence, we may conclude, that domestic instincts have been acquired and natural instincts have been lost partly by habit, and partly by man selecting and accumulating during successive generations, peculiar mental habits and actions, which at first appeared from what we must in our ignorance call an accident. in some cases compulsory habit alone has sufficed to produce such inherited mental changes; in other cases compulsory habit has done nothing, and all has been the result of selection, pursued both methodically and unconsciously; but in most cases, probably, habit and selection have acted together. we shall, perhaps, best understand how instincts in a state of nature have become modified by selection, by considering a few cases. i will select only three, out of the several which i shall have to discuss in my future work,--namely, the instinct which leads the cuckoo to lay her eggs in other birds' nests; the slave-making instinct of certain ants; and the comb-making power of the hive-bee; these two latter instincts have generally, and most justly, been ranked by naturalists as the most wonderful of all known instincts. it is now commonly admitted that the more immediate and final cause of the cuckoo's instinct is, that { } she lays her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on her own eggs, those first laid would have to be left for some time unincubated, or there would be eggs and young birds of different ages in the same nest. if this were the case, the process of laying and hatching might be inconveniently long, more especially as she has to migrate at a very early period; and the first hatched young would probably have to be fed by the male alone. but the american cuckoo is in this predicament; for she makes her own nest and has eggs and young successively hatched, all at the same time. it has been asserted that the american cuckoo occasionally lays her eggs in other birds' nests; but i hear on the high authority of dr. brewer, that this is a mistake. nevertheless, i could give several instances of various birds which have been known occasionally to lay their eggs in other birds' nests. now let us suppose that the ancient progenitor of our european cuckoo had the habits of the american cuckoo; but that occasionally she laid an egg in another bird's nest. if the old bird profited by this occasional habit, or if the young were made more vigorous by advantage having been taken of the mistaken maternal instinct of another bird, than by their own mother's care, encumbered as she can hardly fail to be by having eggs and young of different ages at the same time; then the old birds or the fostered young would gain an advantage. and analogy would lead me to believe, that the young thus reared would be apt to follow by inheritance the occasional and aberrant habit of their mother, and in their turn would be apt to lay their eggs in other birds' nests, and thus be successful in rearing their young. by a continued process of this nature, i believe that the strange instinct of our cuckoo could be, and has been, { } generated. i may add that, according to dr. gray and to some other observers, the european cuckoo has not utterly lost all maternal love and care for her own offspring. the occasional habit of birds laying their eggs in other birds' nests, either of the same or of a distinct species, is not very uncommon with the gallinaceæ; and this perhaps explains the origin of a singular instinct in the allied group of ostriches. for several hen ostriches, at least in the case of the american species, unite and lay first a few eggs in one nest and then in another; and these are hatched by the males. this instinct may probably be accounted for by the fact of the hens laying a large number of eggs; but, as in the case of the cuckoo, at intervals of two or three days. this instinct, however, of the american ostrich has not as yet been perfected; for a surprising number of eggs lie strewed over the plains, so that in one day's hunting i picked up no less than twenty lost and wasted eggs. many bees are parasitic, and always lay their eggs in the nests of bees of other kinds. this case is more remarkable than that of the cuckoo; for these bees have not only their instincts but their structure modified in accordance with their parasitic habits; for they do not possess the pollen-collecting apparatus which would be necessary if they had to store food for their own young. some species, likewise, of sphegidæ (wasp-like insects) are parasitic on other species; and m. fabre has lately shown good reason for believing that although the tachytes nigra generally makes its own burrow and stores it with paralysed prey for its own larvæ to feed on, yet that when this insect finds a burrow already made and stored by another sphex, it takes advantage of the prize, and becomes for the occasion parasitic. in this case, as with the supposed case of the cuckoo, i can { } see no difficulty in natural selection making an occasional habit permanent, if of advantage to the species, and if the insect whose nest and stored food are thus feloniously appropriated, be not thus exterminated. _slave-making instinct._--this remarkable instinct was first discovered in the formica (polyerges) rufescens by pierre huber, a better observer even than his celebrated father. this ant is absolutely dependent on its slaves; without their aid, the species would certainly become extinct in a single year. the males and fertile females do no work. the workers or sterile females, though most energetic and courageous in capturing slaves, do no other work. they are incapable of making their own nests, or of feeding their own larvæ. when the old nest is found inconvenient, and they have to migrate, it is the slaves which determine the migration, and actually carry their masters in their jaws. so utterly helpless are the masters, that when huber shut up thirty of them without a slave, but with plenty of the food which they like best, and with their larvae and pupæ to stimulate them to work, they did nothing; they could not even feed themselves, and many perished of hunger. huber then introduced a single slave (f. fusca), and she instantly set to work, fed and saved the survivors; made some cells and tended the larvæ, and put all to rights. what can be more extraordinary than these well-ascertained facts? if we had not known of any other slave-making ant, it would have been hopeless to have speculated how so wonderful an instinct could have been perfected. another species, formica sanguinea, was likewise first discovered by p. huber to be a slave-making ant. this species is found in the southern parts of england, and its habits have been attended to by mr. f. smith, of { } the british museum, to whom i am much indebted for information on this and other subjects. although fully trusting to the statements of huber and mr. smith, i tried to approach the subject in a sceptical frame of mind, as any one may well be excused for doubting the truth of so extraordinary and odious an instinct as that of making slaves. hence i will give the observations which i have myself made, in some little detail. i opened fourteen nests of f. sanguinea, and found a few slaves in all. males and fertile females of the slave-species (f. fusca) are found only in their own proper communities, and have never been observed in the nests of f. sanguinea. the slaves are black and not above half the size of their red masters, so that the contrast in their appearance is very great. when the nest is slightly disturbed, the slaves occasionally come out, and like their masters are much agitated and defend the nest: when the nest is much disturbed and the larvæ and pupæ are exposed, the slaves work energetically with their masters in carrying them away to a place of safety. hence, it is clear, that the slaves feel quite at home. during the months of june and july, on three successive years, i have watched for many hours several nests in surrey and sussex, and never saw a slave either leave or enter a nest. as, during these months, the slaves are very few in number, i thought that they might behave differently when more numerous; but mr. smith informs me that he has watched the nests at various hours during may, june and august, both in surrey and hampshire, and has never seen the slaves, through present in large numbers in august, either leave or enter the nest. hence he considers them as strictly household slaves. the masters, on the other hand, may be constantly seen bringing in materials for the nest, and food of all kinds. during the present year, however, in the month { } of july, i came across a community with an unusually large stock of slaves, and i observed a few slaves mingled with their masters leaving the nest, and marching along the same road to a tall scotch-fir-tree, twenty-five yards distant, which they ascended together, probably in search of aphides or cocci. according to huber, who had ample opportunities for observation, in switzerland the slaves habitually work with their masters in making the nest, and they alone open and close the doors in the morning and evening; and, as huber expressly states, their principal office is to search for aphides. this difference in the usual habits of the masters and slaves in the two countries, probably depends merely on the slaves being captured in greater numbers in switzerland than in england. one day i fortunately witnessed a migration of f. sanguinea from one nest to another, and it was a most interesting spectacle to behold the masters carefully carrying (instead of being carried by, as in the case of f. rufescens) their slaves in their jaws. another day my attention was struck by about a score of the slave-makers haunting the same spot, and evidently not in search of food; they approached and were vigorously repulsed by an independent community of the slave-species (f. fusca); sometimes as many as three of these ants clinging to the legs of the slave-making f. sanguinea. the latter ruthlessly killed their small opponents, and carried their dead bodies as food to their nest, twenty-nine yards distant; but they were prevented from getting any pupæ to rear as slaves. i then dug up a small parcel of the pupæ of f. fusca from another nest, and put them down on a bare spot near the place of combat; they were eagerly seized, and carried off by the tyrants, who perhaps fancied that, after all, they had been victorious in their late combat. { } at the same time i laid on the same place a small parcel of the pupæ of another species, f. flava, with a few of these little yellow ants still clinging to the fragments of the nest. this species is sometimes, though rarely, made into slaves, as has been described by mr. smith. although so small a species, it is very courageous, and i have seen it ferociously attack other ants. in one instance i found to my surprise an independent community of f. flava under a stone beneath a nest of the slave-making f. sanguinea; and when i had accidentally disturbed both nests, the little ants attacked their big neighbours with surprising courage. now i was curious to ascertain whether f. sanguinea could distinguish the pupæ of f. fusca, which they habitually make into slaves, from those of the little and furious f. flava, which they rarely capture, and it was evident that they did at once distinguish them: for we have seen that they eagerly and instantly seized the pupæ of f. fusca, whereas they were much terrified when they came across the pupæ, or even the earth from the nest of f. flava, and quickly ran away; but in about a quarter of an hour, shortly after all the little yellow ants had crawled away, they took heart and carried off the pupæ. one evening i visited another community of f. sanguinea, and found a number of these ants returning home and entering their nests, carrying the dead bodies of f. fusca (showing that it was not a migration) and numerous pupæ. i traced a long file of ants burthened with booty, for about forty yards, to a very thick clump of heath, whence i saw the last individual of f. sanguinea emerge, carrying a pupa; but i was not able to find the desolated nest in the thick heath. the nest, however, must have been close at hand, for two or three individuals of f. fusca were rushing about in the greatest { } agitation, and one was perched motionless with its own pupa in its mouth on the top of a spray of heath, an image of despair, over its ravaged home. such are the facts, though they did not need confirmation by me, in regard to the wonderful instinct of making slaves. let it be observed what a contrast the instinctive habits of f. sanguinea present with those of the continental f. rufescens. the latter does not build its own nest, does not determine its own migrations, does not collect food for itself or its young, and cannot even feed itself: it is absolutely dependent on its numerous slaves. formica sanguinea, on the other hand, possesses much fewer slaves, and in the early part of the summer extremely few: the masters determine when and where a new nest shall be formed, and when they migrate, the masters carry the slaves. both in switzerland and england the slaves seem to have the exclusive care of the larvæ, and the masters alone go on slave-making expeditions. in switzerland the slaves and masters work together, making and bringing materials for the nest: both, but chiefly the slaves, tend, and milk as it may be called, their aphides; and thus both collect food for the community. in england the masters alone usually leave the nest to collect building materials and food for themselves, their slaves and larvæ. so that the masters in this country receive much less service from their slaves than they do in switzerland. by what steps the instinct of f. sanguinea originated i will not pretend to conjecture. but as ants, which are not slave-makers, will, as i have seen, carry off pupæ of other species, if scattered near their nests, it is possible that such pupæ originally stored as food might become developed; and the foreign ants thus unintentionally reared would then follow their proper instincts, and do { } what work they could. if their presence proved useful to the species which had seized them--if it were more advantageous to this species to capture workers than to procreate them--the habit of collecting pupae originally for food might by natural selection be strengthened and rendered permanent for the very different purpose of raising slaves. when the instinct was once acquired, if carried out to a much less extent even than in our british f. sanguinea, which, as we have seen, is less aided by its slaves than the same species in switzerland, i can see no difficulty in natural selection increasing and modifying the instinct--always supposing each modification to be of use to the species--until an ant was formed as abjectly dependent on its slaves as is the formica rufescens. _cell-making instinct of the hive-bee._--i will not here enter on minute details on this subject, but will merely give an outline of the conclusions at which i have arrived. he must be a dull man who can examine the exquisite structure of a comb, so beautifully adapted to its end, without enthusiastic admiration. we hear from mathematicians that bees have practically solved a recondite problem, and have made their cells of the proper shape to hold the greatest possible amount of honey, with the least possible consumption of precious wax in their construction. it has been remarked that a skilful workman, with fitting tools and measures, would find it very difficult to make cells of wax of the true form, though this is perfectly effected by a crowd of bees working in a dark hive. grant whatever instincts you please, and it seems at first quite inconceivable how they can make all the necessary angles and planes, or even perceive when they are correctly made. but the difficulty is not { } nearly so great as it at first appears: all this beautiful work can be shown, i think, to follow from a few very simple instincts. i was led to investigate this subject by mr. waterhouse, who has shown that the form of the cell stands in close relation to the presence of adjoining cells; and the following view may, perhaps, be considered only as a modification of his theory. let us look to the great principle of gradation, and see whether nature does not reveal to us her method of work. at one end of a short series we have humble-bees, which use their old cocoons to hold honey, sometimes adding to them short tubes of wax, and likewise making separate and very irregular rounded cells of wax. at the other end of the series we have the cells of the hive-bee, placed in a double layer: each cell, as is well known, is an hexagonal prism, with the basal edges of its six sides bevelled so as to fit on to a pyramid, formed of three rhombs. these rhombs have certain angles, and the three which form the pyramidal base of a single cell on one side of the comb, enter into the composition of the bases of three adjoining cells on the opposite side. in the series between the extreme perfection of the cells of the hive-bee and the simplicity of those of the humble-bee, we have the cells of the mexican melipona domestica, carefully described and figured by pierre huber. the melipona itself is intermediate in structure between the hive and humble bee, but more nearly related to the latter: it forms a nearly regular waxen comb of cylindrical cells, in which the young are hatched, and, in addition, some large cells of wax for holding honey. these latter cells are nearly spherical and of nearly equal sizes, and are aggregated into an irregular mass. but the important point to notice, is that these cells are always made at that degree of nearness to each other, that they would have { } intersected or broken into each other, if the spheres had been completed; but this is never permitted, the bees building perfectly flat walls of wax between the spheres which thus tend to intersect. hence each cell consists of an outer spherical portion and of two, three, or more perfectly flat surfaces, according as the cell adjoins two, three, or more other cells. when one cell comes into contact with three other cells, which, from the spheres being nearly of the same size, is very frequently and necessarily the case, the three flat surfaces are united into a pyramid; and this pyramid, as huber has remarked, is manifestly a gross imitation of the three-sided pyramidal bases of the cell of the hive-bee. as in the cells of the hive-bee, so here, the three plane surfaces in any one cell necessarily enter into the construction of three adjoining cells. it is obvious that the melipona saves wax by this manner of building; for the flat walls between the adjoining cells are not double, but are of the same thickness as the outer spherical portions, and yet each flat portion forms a part of two cells. reflecting on this case, it occurred to me that if the melipona had made its spheres at some given distance from each other, and had made them of equal sizes and had arranged them symmetrically in a double layer, the resulting structure would probably have been as perfect as the comb of the hive-bee. accordingly i wrote to professor miller, of cambridge, and this geometer has kindly read over the following statement, drawn up from his information, and tells me that it is strictly correct:-- if a number of equal spheres be described with their centres placed in two parallel layers; with the centre of each sphere at the distance of radius × [root] , or radius × . (or at some lesser distance), from the centres of the six surrounding spheres in the same { } layer; and at the same distance from the centres of the adjoining spheres in the other and parallel layer; then, if planes of intersection between the several spheres in both layers be formed, there will result a double layer of hexagonal prisms united together by pyramidal bases formed of three rhombs; and the rhombs and the sides of the hexagonal prisms will have every angle identically the same with the best measurements which have been made of the cells of the hive-bee. hence we may safely conclude that if we could slightly modify the instincts already possessed by the melipona, and in themselves not very wonderful, this bee would make a structure as wonderfully perfect as that of the hive-bee. we must suppose the melipona to make her cells truly spherical, and of equal sizes; and this would not be very surprising, seeing that she already does so to a certain extent, and seeing what perfectly cylindrical burrows in wood many insects can make, apparently by turning round on a fixed point. we must suppose the melipona to arrange her cells in level layers, as she already does her cylindrical cells; and we must further suppose, and this is the greatest difficulty, that she can somehow judge accurately at what distance to stand from her fellow-labourers when several are making their spheres; but she is already so far enabled to judge of distance, that she always describes her spheres so as to intersect largely; and then she unites the points of intersection by perfectly flat surfaces. we have further to suppose, but this is no difficulty, that after hexagonal prisms have been formed by the intersection of adjoining spheres in the same layer, she can prolong the hexagon to any length requisite to hold the stock of honey; in the same way as the rude humble-bee adds cylinders of wax to the circular mouths of her old cocoons. by such { } modifications of instincts in themselves not very wonderful,--hardly more wonderful than those which guide a bird to make its nest,--i believe that the hive-bee has acquired, through natural selection, her inimitable architectural powers. but this theory can be tested by experiment. following the example of mr. tegetmeier, i separated two combs, and put between them a long, thick, square strip of wax: the bees instantly began to excavate minute circular pits in it; and as they deepened these little pits, they made them wider and wider until they were converted into shallow basins, appearing to the eye perfectly true or parts of a sphere, and of about the diameter of a cell. it was most interesting to me to observe that wherever several bees had begun to excavate these basins near together, they had begun their work at such a distance from each other, that by the time the basins had acquired the above stated width (_i.e._ about the width of an ordinary cell), and were in depth about one sixth of the diameter of the sphere of which they formed a part, the rims of the basins intersected or broke into each other. as soon as this occurred, the bees ceased to excavate, and began to build up flat walls of wax on the lines of intersection between the basins, so that each hexagonal prism was built upon the scalloped edge of a smooth basin, instead of on the straight edges of a three-sided pyramid as in the case of ordinary cells. i then put into the hive, instead of a thick, square piece of wax, a thin and narrow, knife-edged ridge, coloured with vermilion. the bees instantly began on both sides to excavate little basins near to each other, in the same way as before; but the ridge of wax was so thin, that the bottoms of the basins, if they had been excavated to the same depth as in the former { } experiment, would have broken into each other from the opposite sides. the bees, however, did not suffer this to happen, and they stopped their excavations in due time; so that the basins, as soon as they had been a little deepened, came to have flat bottoms; and these flat bottoms, formed by thin little plates of the vermilion wax having been left ungnawed, were situated, as far as the eye could judge, exactly along the planes of imaginary intersection between the basins on the opposite sides of the ridge of wax. in parts, only little bits, in other parts, large portions of a rhombic plate had been left between the opposed basins, but the work, from the unnatural state of things, had not been neatly performed. the bees must have worked at very nearly the same rate on the opposite sides of the ridge of vermilion wax, as they circularly gnawed away and deepened the basins on both sides, in order to have succeeded in thus leaving flat plates between the basins, by stopping work along the intermediate planes or planes of intersection. considering how flexible thin wax is, i do not see that there is any difficulty in the bees, whilst at work on the two sides of a strip of wax, perceiving when they have gnawed the wax away to the proper thinness, and then stopping their work. in ordinary combs it has appeared to me that the bees do not always succeed in working at exactly the same rate from the opposite sides; for i have noticed half-completed rhombs at the base of a just-commenced cell, which were slightly concave on one side, where i suppose that the bees had excavated too quickly, and convex on the opposed side, where the bees had worked less quickly. in one well-marked instance, i put the comb back into the hive, and allowed the bees to go on working for a short time, and again examined the cell, and i found that the rhombic { } plate had been completed, and had become _perfectly flat_: it was absolutely impossible, from the extreme thinness of the little rhombic plate, that they could have effected this by gnawing away the convex side; and i suspect that the bees in such cases stand in the opposed cells and push and bend the ductile and warm wax (which as i have tried is easily done) into its proper intermediate plane, and thus flatten it. from the experiment of the ridge of vermilion wax, we can clearly see that if the bees were to build for themselves a thin wall of wax, they could make their cells of the proper shape, by standing at the proper distance from each other, by excavating at the same rate, and by endeavouring to make equal spherical hollows, but never allowing the spheres to break into each other. now bees, as may be clearly seen by examining the edge of a growing comb, do make a rough, circumferential wall or rim all round the comb; and they gnaw into this from the opposite sides, always working circularly as they deepen each cell. they do not make the whole three-sided pyramidal base of any one cell at the same time, but only the one rhombic plate which stands on the extreme growing margin, or the two plates, as the case may be; and they never complete the upper edges of the rhombic plates, until the hexagonal walls are commenced. some of these statements differ from those made by the justly celebrated elder huber, but i am convinced of their accuracy; and if i had space, i could show that they are conformable with my theory. huber's statement that the very first cell is excavated out of a little parallel-sided wall of wax, is not, as far as i have seen, strictly correct; the first commencement having always been a little hood of wax; but i will not here enter on these details. we see how important { } a part excavation plays in the construction of the cells; but it would be a great error to suppose that the bees cannot build up a rough wall of wax in the proper position--that is, along the plane of intersection between two adjoining spheres. i have several specimens showing clearly that they can do this. even in the rude circumferential rim or wall of wax round a growing comb, flexures may sometimes be observed, corresponding in position to the planes of the rhombic basal plates of future cells. but the rough wall of wax has in every case to be finished off, by being largely gnawed away on both sides. the manner in which the bees build is curious; they always make the first rough wall from ten to twenty times thicker than the excessively thin finished wall of the cell, which will ultimately be left. we shall understand how they work, by supposing masons first to pile up a broad ridge of cement, and then to begin cutting it away equally on both sides near the ground, till a smooth, very thin wall is left in the middle; the masons always piling up the cut-away cement, and adding fresh cement, on the summit of the ridge. we shall thus have a thin wall steadily growing upward; but always crowned by a gigantic coping. from all the cells, both those just commenced and those completed, being thus crowned by a strong coping of wax, the bees can cluster and crawl over the comb without injuring the delicate hexagonal walls, which are only about one four-hundredth of an inch in thickness; the plates of the pyramidal basis being about twice as thick. by this singular manner of building, strength is continually given to the comb, with the utmost ultimate economy of wax. it seems at first to add to the difficulty of understanding how the cells are made, that a multitude of bees all work together; one bee after working a short time at one cell going to another, so that, as huber has stated, { } a score of individuals work even at the commencement of the first cell. i was able practically to show this fact, by covering the edges of the hexagonal walls of a single cell, or the extreme margin of the circumferential rim of a growing comb, with an extremely thin layer of melted vermilion wax; and i invariably found that the colour was most delicately diffused by the bees--as delicately as a painter could have done with his brush--by atoms of the coloured wax having been taken from the spot on which it had been placed, and worked into the growing edges of the cells all round. the work of construction seems to be a sort of balance struck between many bees, all instinctively standing at the same relative distance from each other, all trying to sweep equal spheres, and then building up, or leaving ungnawed, the planes of intersection between these spheres. it was really curious to note in cases of difficulty, as when two pieces of comb met at an angle, how often the bees would pull down and rebuild in different ways the same cell, sometimes recurring to a shape which they had at first rejected. when bees have a place on which they can stand in their proper positions for working,--for instance, on a slip of wood, placed directly under the middle of a comb growing downwards so that the comb has to be built over one face of the slip--in this case the bees can lay the foundations of one wall of a new hexagon, in its strictly proper place, projecting beyond the other completed cells. it suffices that the bees should be enabled to stand at their proper relative distances from each other and from the walls of the last completed cells, and then, by striking imaginary spheres, they can build up a wall intermediate between two adjoining spheres; but, as far as i have seen, they never gnaw away and finish off the angles of a cell till a large part both of that cell and of { } the adjoining cells has been built. this capacity in bees of laying down under certain circumstances a rough wall in its proper place between two just-commenced cells, is important, as it bears on a fact, which seems at first quite subversive of the foregoing theory; namely, that the cells on the extreme margin of wasp-combs are sometimes strictly hexagonal; but i have not space here to enter on this subject. nor does there seem to me any great difficulty in a single insect (as in the case of a queen-wasp) making hexagonal cells, if she work alternately on the inside and outside of two or three cells commenced at the same time, always standing at the proper relative distance from the parts of the cells just begun, sweeping spheres or cylinders, and building up intermediate planes. it is even conceivable that an insect might, by fixing on a point at which to commence a cell, and then moving outside, first to one point, and then to five other points, at the proper relative distances from the central point and from each other, strike the planes of intersection, and so make an isolated hexagon: but i am not aware that any such case has been observed; nor would any good be derived from a single hexagon being built, as in its construction more materials would be required than for a cylinder. as natural selection acts only by the accumulation of slight modifications of structure or instinct, each profitable to the individual under its conditions of life, it may reasonably be asked, how a long and graduated succession of modified architectural instincts, all tending towards the present perfect plan of construction, could have profited the progenitors of the hive-bee? i think the answer is not difficult: it is known that bees are often hard pressed to get sufficient nectar; and i am informed by mr. tegetmeier that it has been experimentally found that no less than from twelve to fifteen pounds of dry sugar { } are consumed by a hive of bees for the secretion of each pound of wax; to that a prodigious quantity of fluid nectar must be collected and consumed by the bees in a hive for the secretion of the wax necessary for the construction of their combs. moreover, many bees have to remain idle for many days during the process of secretion. a large store of honey is indispensable to support a large stock of bees during the winter; and the security of the hive is known mainly to depend on a large number of bees being supported. hence the saving of wax by largely saving honey must be a most important element of success in any family of bees. of course the success of any species of bee may be dependent on the number of its parasites or other enemies, or on quite distinct causes, and so be altogether independent of the quantity of honey which the bees could collect. but let us suppose that this latter circumstance determined, as it probably often does determine, the numbers of a humble-bee which could exist in a country; and let us further suppose that the community lived throughout the winter, and consequently required a store of honey: there can in this case be no doubt that it would be an advantage to our humble-bee, if a slight modification of her instinct led her to make her waxen cells near together, so as to intersect a little; for a wall in common even to two adjoining cells, would save some little wax. hence it would continually be more and more advantageous to our humble-bee, if she were to make her cells more and more regular, nearer together, and aggregated into a mass, like the cells of the melipona; for in this case a large part of the bounding surface of each cell would serve to bound other cells, and much wax would be saved. again, from the same cause, it would be advantageous to the melipona, if she were to make her cells closer together, and more regular in every way { } than at present; for then, as we have seen, the spherical surfaces would wholly disappear, and would all be replaced by plane surfaces; and the melipona would make a comb as perfect as that of the hive-bee. beyond this stage of perfection in architecture, natural selection could not lead; for the comb of the hive-bee, as far as we can see, is absolutely perfect in economising wax. thus, as i believe, the most wonderful of all known instincts, that of the hive-bee, can be explained by natural selection having taken advantage of numerous, successive, slight modifications of simpler instincts; natural selection having by slow degrees, more and more perfectly, led the bees to sweep equal spheres at a given distance from each other in a double layer, and to build up and excavate the wax along the planes of intersection. the bees, of course, no more knowing that they swept their spheres at one particular distance from each other, than they know what are the several angles of the hexagonal prisms and of the basal rhombic plates. the motive power of the process of natural selection having been economy of wax; that individual swarm which wasted least honey in the secretion of wax, having succeeded best, and having transmitted by inheritance its newly acquired economical instinct to new swarms, which in their turn will have had the best chance of succeeding in the struggle for existence. no doubt many instincts of very difficult explanation could be opposed to the theory of natural selection,--cases, in which we cannot see how an instinct could possibly have originated; cases, in which no intermediate gradations are known to exist; cases of instinct of apparently such trifling importance, that they could { } hardly have been acted on by natural selection; cases of instincts almost identically the same in animals so remote in the scale of nature, that we cannot account for their similarity by inheritance from a common parent, and must therefore believe that they have been acquired by independent acts of natural selection. i will not here enter on these several cases, but will confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. i allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind. the subject well deserves to be discussed at great length, but i will here take only a single case, that of working or sterile ants. how the workers have been rendered sterile is a difficulty; but not much greater than that of any other striking modification of structure; for it can be shown that some insects and other articulate animals in a state of nature occasionally become sterile; and if such insects had been social, and it had been profitable to the community that a number should have been annually born capable of work, but incapable of procreation, i can see no very great difficulty in this being effected by natural selection. but i must pass over this preliminary difficulty. the great difficulty lies in the working ants differing widely from both the males and the fertile females in structure, as in the shape of the thorax and in being destitute of wings and sometimes of eyes, and in instinct. as far as instinct alone is concerned, the prodigious difference in this respect between the workers and the perfect females, would have been far better exemplified by the hive-bee. if a working ant or other neuter insect had been an animal { } in the ordinary state, i should have unhesitatingly assumed that all its characters had been slowly acquired through natural selection; namely, by an individual having been born with some slight profitable modification of structure, this being inherited by its offspring, which again varied and were again selected, and so onwards. but with the working ant we have an insect differing greatly from its parents, yet absolutely sterile; so that it could never have transmitted successively acquired modifications of structure or instinct to its progeny. it may well be asked how is it possible to reconcile this case with the theory of natural selection? first, let it be remembered that we have innumerable instances, both in our domestic productions and in those in a state of nature, of all sorts of differences of structure which have become correlated to certain ages, and to either sex. we have differences correlated not only to one sex, but to that short period alone when the reproductive system is active, as in the nuptial plumage of many birds, and in the hooked jaws of the male salmon. we have even slight differences in the horns of different breeds of cattle in relation to an artificially imperfect state of the male sex; for oxen of certain breeds have longer horns than in other breeds, in comparison with the horns of the bulls or cows of these same breeds. hence i can see no real difficulty in any character having become correlated with the sterile condition of certain members of insect-communities: the difficulty lies in understanding how such correlated modifications of structure could have been slowly accumulated by natural selection. this difficulty, though appearing insuperable, is lessened, or, as i believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the { } desired end. thus, a well-flavoured vegetable is cooked, and the individual is destroyed; but the horticulturist sows seeds of the same stock, and confidently expects to get nearly the same variety: breeders of cattle wish the flesh and fat to be well marbled together; the animal has been slaughtered, but the breeder goes with confidence to the same family. i have such faith in the powers of selection, that i do not doubt that a breed of cattle, always yielding oxen with extraordinarily long horns, could be slowly formed by carefully watching which individual bulls and cows, when matched, produced oxen with the longest horns; and yet no one ox could ever have propagated its kind. thus i believe it has been with social insects: a slight modification of structure, or instinct, correlated with the sterile condition of certain members of the community, has been advantageous to the community: consequently the fertile males and females of the same community flourished, and transmitted to their fertile offspring a tendency to produce sterile members having the same modification. and i believe that this process has been repeated, until that prodigious amount of difference between the fertile and sterile females of the same species has been produced, which we see in many social insects. but we have not as yet touched on the climax of the difficulty; namely, the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an almost incredible degree, and are thus divided into two or even three castes. the castes, moreover, do not generally graduate into each other, but are perfectly well defined; being as distinct from each other, as are any two species of the same genus, or rather as any two genera of the same family. thus in eciton, there are working and soldier neuters, with jaws and instincts extraordinarily { } different: in cryptocerus, the workers of one caste alone carry a wonderful sort of shield on their heads, the use of which is quite unknown: in the mexican myrmecocystus, the workers of one caste never leave the nest; they are fed by the workers of another caste, and they have an enormously developed abdomen which secretes a sort of honey, supplying the place of that excreted by the aphides, or the domestic cattle as they may be called, which our european ants guard or imprison. it will indeed be thought that i have an overweening confidence in the principle of natural selection, when i do not admit that such wonderful and well-established facts at once annihilate my theory. in the simpler case of neuter insects all of one caste or of the same kind, which have been rendered by natural selection, as i believe to be quite possible, different from the fertile males and females,--in this case, we may safely conclude from the analogy of ordinary variations, that each successive, slight, profitable modification did not probably at first appear in all the individual neuters in the same nest, but in a few alone; and that by the long-continued selection of the fertile parents which produced most neuters with the profitable modification, all the neuters ultimately came to have the desired character. on this view we ought occasionally to find neuter-insects of the same species, in the same nest, presenting gradations of structure; and this we do find, even often, considering how few neuter-insects out of europe have been carefully examined. mr. f. smith has shown how surprisingly the neuters of several british ants differ from each other in size and sometimes in colour; and that the extreme forms can sometimes be perfectly linked together by individuals taken out of the same nest: i have myself compared perfect gradations of this kind. it often happens that the larger or the smaller sized workers { } are the most numerous; or that both large and small are numerous, with those of an intermediate size scanty in numbers. formica flava has larger and smaller workers, with some of intermediate size; and, in this species, as mr. f. smith has observed, the larger workers have simple eyes (ocelli), which though small can be plainly distinguished, whereas the smaller workers have their ocelli rudimentary. having carefully dissected several specimens of these workers, i can affirm that the eyes are far more rudimentary in the smaller workers than can be accounted for merely by their proportionally lesser size; and i fully believe, though i dare not assert so positively, that the workers of intermediate size have their ocelli in an exactly intermediate condition. so that we here have two bodies of sterile workers in the same nest, differing not only in size, but in their organs of vision, yet connected by some few members in an intermediate condition. i may digress by adding, that if the smaller workers had been the most useful to the community, and those males and females had been continually selected, which produced more and more of the smaller workers, until all the workers had come to be in this condition; we should then have had a species of ant with neuters very nearly in the same condition with those of myrmica. for the workers of myrmica have not even rudiments of ocelli, though the male and female ants of this genus have well-developed ocelli. i may give one other case: so confidently did i expect to find gradations in important points of structure between the different castes of neuters in the same species, that i gladly availed myself of mr. f. smith's offer of numerous specimens from the same nest of the driver ant (anomma) of west africa. the reader will perhaps best appreciate the amount of difference in these { } workers, by my giving not the actual measurements, but a strictly accurate illustration: the difference was the same as if we were to see a set of workmen building a house of whom many were five feet four inches high, and many sixteen feet high; but we must suppose that the larger workmen had heads four instead of three times as big as those of the smaller men, and jaws nearly five times as big. the jaws, moreover, of the working ants of the several sizes differed wonderfully in shape, and in the form and number of the teeth. but the important fact for us is, that though the workers can be grouped into castes of different sizes, yet they graduate insensibly into each other, as does the widely-different structure of their jaws. i speak confidently on this latter point, as mr. lubbock made drawings for me with the camera lucida of the jaws which i had dissected from the workers of the several sizes. with these facts before me, i believe that natural selection, by acting on the fertile parents, could form a species which should regularly produce neuters, either all of large size with one form of jaw, or all of small size with jaws having a widely different structure; or lastly, and this is our climax of difficulty, one set of workers of one size and structure, and simultaneously another set of workers of a different size and structure;--a graduated series having been first formed, as in the case of the driver ant, and then the extreme forms, from being the most useful to the community, having been produced in greater and greater numbers through the natural selection of the parents which generated them; until none with an intermediate structure were produced. thus, as i believe, the wonderful fact of two distinctly defined castes of sterile workers existing in the same nest, both widely different from each other and from { } their parents, has originated. we can see how useful their production may have been to a social community of insects, on the same principle that the division of labour is useful to civilised man. as ants work by inherited instincts and by inherited organs or tools, and not by acquired knowledge and manufactured instruments, a perfect division of labour could be effected with them only by the workers being sterile; for had they been fertile, they would have intercrossed, and their instincts and structure would have become blended. and nature has, as i believe, effected this admirable division of labour in the communities of ants, by the means of natural selection. but i am bound to confess, that, with all my faith in this principle, i should never have anticipated that natural selection could have been efficient in so high a degree, had not the case of these neuter insects convinced me of the fact. i have, therefore, discussed this case, at some little but wholly insufficient length, in order to show the power of natural selection, and likewise because this is by far the most serious special difficulty, which my theory has encountered. the case, also, is very interesting, as it proves that with animals, as with plants, any amount of modification in structure can be effected by the accumulation of numerous, slight, and as we must call them accidental, variations, which are in any manner profitable, without exercise or habit having come into play. for no amount of exercise, or habit, or volition, in the utterly sterile members of a community could possibly affect the structure or instincts of the fertile members, which alone leave descendants. i am surprised that no one has advanced this demonstrative case of neuter insects, against the well-known doctrine of lamarck. _summary._--i have endeavoured briefly in this chapter { } to show that the mental qualities of our domestic animals vary, and that the variations are inherited. still more briefly i have attempted to show that instincts vary slightly in a state of nature. no one will dispute that instincts are of the highest importance to each animal. therefore i can see no difficulty, under changing conditions of life, in natural selection accumulating slight modifications of instinct to any extent, in any useful direction. in some cases habit or use and disuse have probably come into play. i do not pretend that the facts given in this chapter strengthen in any great degree my theory; but none of the cases of difficulty, to the best of my judgment, annihilate it. on the other hand, the fact that instincts are not always absolutely perfect and are liable to mistakes;--that no instinct has been produced for the exclusive good of other animals, but that each animal takes advantage of the instincts of others;--that the canon in natural history, of "natura non facit saltum," is applicable to instincts as well as to corporeal structure, and is plainly explicable on the foregoing views, but is otherwise inexplicable,--all tend to corroborate the theory of natural selection. this theory is, also, strengthened by some few other facts in regard to instincts; as by that common case of closely allied, but certainly distinct, species, when inhabiting distant parts of the world and living under considerably different conditions of life, yet often retaining nearly the same instincts. for instance, we can understand on the principle of inheritance, how it is that the thrush of south america lines its nest with mud, in the same peculiar manner as does our british thrush: how it is that the male wrens (troglodytes) of north america, build "cock-nests," to roost in, like the males of our distinct kitty-wrens,--a habit wholly unlike that of { } any other known bird. finally, it may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers,--ants making slaves,--the larvae of ichneumonidæ feeding within the live bodies of caterpillars,--not as specially endowed or created instincts, but as small consequences of one general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die. * * * * * { } chapter viii. hybridism. distinction between the sterility of first crosses and of hybrids--sterility various in degree, not universal, affected by close interbreeding, removed by domestication--laws governing the sterility of hybrids--sterility not a special endowment, but incidental on other differences--causes of the sterility of first crosses and of hybrids--parallelism between the effects of changed conditions of life and crossing--fertility of varieties when crossed and of their mongrel offspring not universal--hybrids and mongrels compared independently of their fertility--summary. the view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms. this view certainly seems at first probable, for species within the same country could hardly have kept distinct had they been capable of crossing freely. the importance of the fact that hybrids are very generally sterile, has, i think, been much underrated by some late writers. on the theory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility. i hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences. in treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded together; namely, the sterility of two species { } when first crossed, and the sterility of the hybrids produced from them. pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the organs themselves are perfect in structure, as far as the microscope reveals. in the first case the two sexual elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed. this distinction is important, when the cause of the sterility, which is common to the two cases, has to be considered. the distinction has probably been slurred over, owing to the sterility in both cases being looked on as a special endowment, beyond the province of our reasoning powers. the fertility of varieties, that is of the forms known or believed to have descended from common parents, when intercrossed, and likewise the fertility of their mongrel offspring, is, on my theory, of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species. first, for the sterility of species when crossed and of their hybrid offspring. it is impossible to study the several memoirs and works of those two conscientious and admirable observers, kölreuter and gärtner, who almost devoted their lives to this subject, without being deeply impressed with the high generality of some degree of sterility. kölreuter makes the rule universal; but then he cuts the knot, for in ten cases in which he found two forms, considered by most authors as distinct species, quite fertile together, he unhesitatingly ranks { } them as varieties. gärtner, also, makes the rule equally universal; and he disputes the entire fertility of kölreuter's ten cases. but in these and in many other cases, gärtner is obliged carefully to count the seeds, in order to show that there is any degree of sterility. he always compares the maximum number of seeds produced by two species when crossed and by their hybrid offspring, with the average number produced by both pure parent-species in a state of nature. but a serious cause of error seems to me to be here introduced: a plant to be hybridised must be castrated, and, what is often more important, must be secluded in order to prevent pollen being brought to it by insects from other plants. nearly all the plants experimentised on by gärtner were potted, and apparently were kept in a chamber in his house. that these processes are often injurious to the fertility of a plant cannot be doubted; for gärtner gives in his table about a score of cases of plants which he castrated, and artificially fertilised with their own pollen, and (excluding all cases such as the leguminosæ, in which there is an acknowledged difficulty in the manipulation) half of these twenty plants had their fertility in some degree impaired. moreover, as gärtner during several years repeatedly crossed the primrose and cowslip, which we have such good reason to believe to be varieties, and only once or twice succeeded in getting fertile seed; as he found the common red and blue pimpernels (anagallis arvensis and coerulea), which the best botanists rank as varieties, absolutely sterile together; and as he came to the same conclusion in several other analogous cases; it seems to me that we may well be permitted to doubt whether many other species are really so sterile, when intercrossed, as gärtner believes. { } it is certain, on the one hand, that the sterility of various species when crossed is so different in degree and graduates away so insensibly, and, on the other hand, that the fertility of pure species is so easily affected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins. i think no better evidence of this can be required than that the two most experienced observers who have ever lived, namely, kölreuter and gärtner, should have arrived at diametrically opposite conclusions in regard to the very same species. it is also most instructive to compare--but i have not space here to enter on details--the evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties, with the evidence from fertility adduced by different hybridisers, or by the same author, from experiments made during different years. it can thus be shown that neither sterility nor fertility affords any clear distinction between species and varieties; but that the evidence from this source graduates away, and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences. in regard to the sterility of hybrids in successive generations; though gärtner was enabled to rear some hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten generations, yet he asserts positively that their fertility never increased, but generally greatly decreased. i do not doubt that this is usually the case, and that the fertility often suddenly decreases in the first few generations. nevertheless i believe that in all these experiments the fertility has been diminished by an independent cause, namely, from close interbreeding. i have collected so large a body of facts, showing { } that close interbreeding lessens fertility, and, on the other hand, that an occasional cross with a distinct individual or variety increases fertility, that i cannot doubt the correctness of this almost universal belief amongst breeders. hybrids are seldom raised by experimentalists in great numbers; and as the parent-species, or other allied hybrids, generally grow in the same garden, the visits of insects must be carefully prevented during the flowering season: hence hybrids will generally be fertilised during each generation by their own individual pollen; and i am convinced that this would be injurious to their fertility, already lessened by their hybrid origin. i am strengthened in this conviction by a remarkable statement repeatedly made by gärtner, namely, that if even the less fertile hybrids be artificially fertilised with hybrid pollen of the same kind, their fertility, notwithstanding the frequent ill effects of manipulation, sometimes decidedly increases, and goes on increasing. now, in artificial fertilisation pollen is as often taken by chance (as i know from my own experience) from the anthers of another flower, as from the anthers of the flower itself which is to be fertilised; so that a cross between two flowers, though probably on the same plant, would be thus effected. moreover, whenever complicated experiments are in progress, so careful an observer as gärtner would have castrated his hybrids, and this would have insured in each generation a cross with a pollen from a distinct flower, either from the same plant or from another plant of the same hybrid nature. and thus, the strange fact of the increase of fertility in the successive generations of _artificially fertilised_ hybrids may, i believe, be accounted for by close interbreeding having been avoided. now let us turn to the results arrived at by the third most experienced hybridiser, namely, the hon. and { } rev. w. herbert. he is as emphatic in his conclusion that some hybrids are perfectly fertile--as fertile as the pure parent-species--as are kölreuter and gärtner that some degree of sterility between distinct species is a universal law of nature. he experimentised on some of the very same species as did gärtner. the difference in their results may, i think, be in part accounted for by herbert's great horticultural skill, and by his having hothouses at his command. of his many important statements i will here give only a single one as an example, namely, that "every ovule in a pod of crinum capense fertilised by c. revolutum produced a plant, which (he says) i never saw to occur in a case of its natural fecundation." so that we here have perfect, or even more than commonly perfect, fertility in a first cross between two distinct species. this case of the crinum leads me to refer to a most singular fact, namely, that there are individual plants of certain species of lobelia and of some other genera, which can be far more easily fertilised by the pollen of another and distinct species, than by their own pollen; and all the individuals of nearly all the species of hippeastrum seem to be in this predicament. for these plants have been found to yield seed to the pollen of a distinct species, though quite sterile with their own pollen, notwithstanding that their own pollen was found to be perfectly good, for it fertilised distinct species. so that certain individual plants and all the individuals of certain species can actually be hybridised much more readily than they can be self-fertilised! for instance, a bulb of hippeastrum aulicum produced four flowers; three were fertilised by herbert with their own pollen, and the fourth was subsequently fertilised by the pollen of a compound hybrid descended from three other and distinct { } species: the result was that "the ovaries of the three first flowers soon ceased to grow, and after a few days perished entirely, whereas the pod impregnated by the pollen of the hybrid made vigorous growth and rapid progress to maturity, and bore good seed, which vegetated freely." in a letter to me, in , mr. herbert told me that he had then tried the experiment during five years, and he continued to try it during several subsequent years, and always with the same result. this result has, also, been confirmed by other observers in the case of hippeastrum with its sub-genera, and in the case of some other genera, as lobelia, passiflora and verbascum. although the plants in these experiments appeared perfectly healthy, and although both the ovules and pollen of the same flower were perfectly good with respect to other species, yet as they were functionally imperfect in their mutual self-action, we must infer that the plants were in an unnatural state. nevertheless these facts show on what slight and mysterious causes the lesser or greater fertility of species when crossed, in comparison with the same species when self-fertilised, sometimes depends. the practical experiments of horticulturists, though not made with scientific precision, deserve some notice. it is notorious in how complicated a manner the species of pelargonium, fuchsia, calceolaria, petunia, rhododendron, &c., have been crossed, yet many of these hybrids seed freely. for instance, herbert asserts that a hybrid from calceolaria integrifolia and plantaginea, species most widely dissimilar in general habit, "reproduced itself as perfectly as if it had been a natural species from the mountains of chile." i have taken some pains to ascertain the degree of fertility of some of the complex crosses of rhododendrons, and i am assured that many of them { } are perfectly fertile. mr. c. noble, for instance, informs me that he raises stocks for grafting from a hybrid between rhod. ponticum and catawbiense, and that this hybrid "seeds as freely as it is possible to imagine." had hybrids, when fairly treated, gone on decreasing in fertility in each successive generation, as gärtner believes to be the case, the fact would have been notorious to nurserymen. horticulturists raise large beds of the same hybrids, and such alone are fairly treated, for by insect agency the several individuals of the same hybrid variety are allowed to freely cross with each other, and the injurious influence of close interbreeding is thus prevented. any one may readily convince himself of the efficiency of insect-agency by examining the flowers of the more sterile kinds of hybrid rhododendrons, which produce no pollen, for he will find on their stigmas plenty of pollen brought from other flowers. in regard to animals, much fewer experiments have been carefully tried than with plants. if our systematic arrangements can be trusted, that is if the genera of animals are as distinct from each other, as are the genera of plants, then we may infer that animals more widely separated in the scale of nature can be more easily crossed than in the case of plants; but the hybrids themselves are, i think, more sterile. i doubt whether any case of a perfectly fertile hybrid animal can be considered as thoroughly well authenticated. it should, however, be borne in mind that, owing to few animals breeding freely under confinement, few experiments have been fairly tried: for instance, the canary-bird has been crossed with nine other finches, but as not one of these nine species breeds freely in confinement, we have no right to expect that the first crosses between them and the canary, or that their hybrids, { } should be perfectly fertile. again, with respect to the fertility in successive generations of the more fertile hybrid animals, i hardly know of an instance in which two families of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of close interbreeding. on the contrary, brothers and sisters have usually been crossed in each successive generation, in opposition to the constantly repeated admonition of every breeder. and in this case, it is not at all surprising that the inherent sterility in the hybrids should have gone on increasing. if we were to act thus, and pair brothers and sisters in the case of any pure animal, which from any cause had the least tendency to sterility, the breed would assuredly be lost in a very few generations. although i do not know of any thoroughly well-authenticated cases of perfectly fertile hybrid animals, i have some reason to believe that the hybrids from cervulus vaginalis and reevesii, and from phasianus colchicus with p. torquatus and with p. versicolor are perfectly fertile. there is no doubt that these three pheasants, namely, the common, the true ring-necked, and the japan, intercross, and are becoming blended together in the woods of several parts of england. the hybrids from the common and chinese geese (a. cygnoides), species which are so different that they are generally ranked in distinct genera, have often bred in this country with either pure parent, and in one single instance they have bred _inter se_. this was effected by mr. eyton, who raised two hybrids from the same parents but from different hatches; and from these two birds he raised no less than eight hybrids (grandchildren of the pure geese) from one nest. in india, however, these cross-bred geese must be far more fertile; for i am assured by two eminently capable judges, namely { } mr. blyth and capt. hutton, that whole flocks of these crossed geese are kept in various parts of the country; and as they are kept for profit, where neither pure parent-species exists, they must certainly be highly fertile. a doctrine which originated with pallas, has been largely accepted by modern naturalists; namely, that most of our domestic animals have descended from two or more wild species, since commingled by intercrossing. on this view, the aboriginal species must either at first have produced quite fertile hybrids, or the hybrids must have become in subsequent generations quite fertile under domestication. this latter alternative seems to me the most probable, and i am inclined to believe in its truth, although it rests on no direct evidence. i believe, for instance, that our dogs have descended from several wild stocks; yet, with perhaps the exception of certain indigenous domestic dogs of south america, all are quite fertile together; and analogy makes me greatly doubt, whether the several aboriginal species would at first have freely bred together and have produced quite fertile hybrids. so again there is reason to believe that our european and the humped indian cattle are quite fertile together; but from facts communicated to me by mr. blyth, i think they must be considered as distinct species. on this view of the origin of many of our domestic animals, we must either give up the belief of the almost universal sterility of distinct species of animals when crossed; or we must look at sterility, not as an indelible characteristic, but as one capable of being removed by domestication. finally, looking to all the ascertained facts on the intercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses { } and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal. _laws governing the sterility of first crosses and of hybrids._--we will now consider a little more in detail the circumstances and rules governing the sterility of first crosses and of hybrids. our chief object will be to see whether or not the rules indicate that species have specially been endowed with this quality, in order to prevent their crossing and blending together in utter confusion. the following rules and conclusions are chiefly drawn up from gärtner's admirable work on the hybridisation of plants. i have taken much pains to ascertain how far the rules apply to animals, and considering how scanty our knowledge is in regard to hybrid animals, i have been surprised to find how generally the same rules apply to both kingdoms. it has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from zero to perfect fertility. it is surprising in how many curious ways this gradation can be shown to exist; but only the barest outline of the facts can here be given. when pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust. from this absolute zero of fertility, the pollen of different species of the same genus applied to the stigma of some one species, yields a perfect gradation in the number of seeds produced, up to nearly complete or even quite complete fertility; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond that which the plant's own pollen will produce. so in hybrids themselves, there are some which never have produced, and probably never would produce, even { } with the pollen of either pure parent, a single fertile seed: but in some of these cases a first trace of fertility may be detected, by the pollen of one of the pure parent-species causing the flower of the hybrid to wither earlier than it otherwise would have done; and the early withering of the flower is well known to be a sign of incipient fertilisation. from this extreme degree of sterility we have self-fertilised hybrids producing a greater and greater number of seeds up to perfect fertility. hybrids from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus produced--two classes of facts which are generally confounded together--is by no means strict. there are many cases, in which two pure species can be united with unusual facility, and produce numerous hybrid-offspring, yet these hybrids are remarkably sterile. on the other hand, there are species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. even within the limits of the same genus, for instance in dianthus, these two opposite cases occur. the fertility, both of first crosses and of hybrids, is more easily affected by unfavourable conditions, than is the fertility of pure species. but the degree of fertility is likewise innately variable; for it is not always the same when the same two species are crossed under the same circumstances, but depends in part upon the constitution of the individuals which happen to have been chosen for the experiment. so it is with hybrids, for their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same capsule and exposed to exactly the same conditions. { } by the term systematic affinity is meant, the resemblance between species in structure and in constitution, more especially in the structure of parts which are of high physiological importance and which differ little in the allied species. now the fertility of first crosses between species, and of the hybrids produced from them, is largely governed by their systematic affinity. this is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility. but the correspondence between systematic affinity and the facility of crossing is by no means strict. a multitude of cases could be given of very closely allied species which will not unite, or only with extreme difficulty; and on the other hand of very distinct species which unite with the utmost facility. in the same family there may be a genus, as dianthus, in which very many species can most readily be crossed; and another genus, as silene, in which the most persevering efforts have failed to produce between extremely close species a single hybrid. even within the limits of the same genus, we meet with this same difference; for instance, the many species of nicotiana have been more largely crossed than the species of almost any other genus; but gärtner found that n. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised by, no less than eight other species of nicotiana. very many analogous facts could be given. no one has been able to point out what kind, or what amount, of difference in any recognisable character is sufficient to prevent two species crossing. it can be shown that plants most widely different in habit and general appearance, and having strongly marked { } differences in every part of the flower, even in the pollen, in the fruit, and in the cotyledons, can be crossed. annual and perennial plants, deciduous and evergreen trees, plants inhabiting different stations and fitted for extremely different climates, can often be crossed with ease. by a reciprocal cross between two species, i mean the case, for instance, of a stallion-horse being first crossed with a female-ass, and then a male-ass with a mare: these two species may then be said to have been reciprocally crossed. there is often the widest possible difference in the facility of making reciprocal crosses. such cases are highly important, for they prove that the capacity in any two species to cross is often completely independent of their systematic affinity, or of any recognisable difference in their whole organisation. on the other hand, these cases clearly show that the capacity for crossing is connected with constitutional differences imperceptible by us, and confined to the reproductive system. this difference in the result of reciprocal crosses between the same two species was long ago observed by kölreuter. to give an instance: mirabilis jalapa can easily be fertilised by the pollen of m. longiflora, and the hybrids thus produced are sufficiently fertile; but kölreuter tried more than two hundred times, during eight following years, to fertilise reciprocally m. longiflora with the pollen of m. jalapa, and utterly failed. several other equally striking cases could be given. thuret has observed the same fact with certain sea-weeds or fuci. gärtner, moreover, found that this difference of facility in making reciprocal crosses is extremely common in a lesser degree. he has observed it even between forms so closely related (as matthiola annua and glabra) that many botanists rank them only as varieties. it is also a remarkable fact, that hybrids raised from reciprocal crosses, though { } of course compounded of the very same two species, the one species having first been used as the father and then as the mother, generally differ in fertility in a small, and occasionally in a high degree. several other singular rules could be given from gärtner: for instance, some species have a remarkable power of crossing with other species; other species of the same genus have a remarkable power of impressing their likeness on their hybrid offspring; but these two powers do not at all necessarily go together. there are certain hybrids which instead of having, as is usual, an intermediate character between their two parents, always closely resemble one of them; and such hybrids, though externally so like one of their pure parent-species, are with rare exceptions extremely sterile. so again amongst hybrids which are usually intermediate in structure between their parents, exceptional and abnormal individuals sometimes are born, which closely resemble one of their pure parents; and these hybrids are almost always utterly sterile, even when the other hybrids raised from seed from the same capsule have a considerable degree of fertility. these facts show how completely fertility in the hybrid is independent of its external resemblance to either pure parent. considering the several rules now given, which govern the fertility of first crosses and of hybrids, we see that when forms, which must be considered as good and distinct species, are united, their fertility graduates from zero to perfect fertility, or even to fertility under certain conditions in excess. that their fertility, besides being eminently susceptible to favourable and unfavourable conditions, is innately variable. that it is by no means always the same in degree in the first cross and in the hybrids produced { } from this cross. that the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent. and lastly, that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other. this latter statement is clearly proved by reciprocal crosses between the same two species, for according as the one species or the other is used as the father or the mother, there is generally some difference, and occasionally the widest possible difference, in the facility of effecting an union. the hybrids, moreover, produced from reciprocal crosses often differ in fertility. now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature? i think not. for why should the sterility be so extremely different in degree, when various species are crossed, all of which we must suppose it would be equally important to keep from blending together? why should the degree of sterility be innately variable in the individuals of the same species? why should some species cross with facility, and yet produce very sterile hybrids; and other species cross with extreme difficulty, and yet produce fairly fertile hybrids? why should there often be so great a difference in the result of a reciprocal cross between the same two species? why, it may even be asked, has the production of hybrids been permitted? to grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related to the facility of the first union between their parents, seems to be a strange arrangement. the foregoing rules and facts, on the other hand, { } appear to me clearly to indicate that the sterility both of first crosses and of hybrids is simply incidental or dependent on unknown differences, chiefly in the reproductive systems, of the species which are crossed. the differences being of so peculiar and limited a nature, that, in reciprocal crosses between two species the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction. it will be advisable to explain a little more fully by an example what i mean by sterility being incidental on other differences, and not a specially endowed quality. as the capacity of one plant to be grafted or budded on another is so entirely unimportant for its welfare in a state of nature, i presume that no one will suppose that this capacity is a _specially_ endowed quality, but will admit that it is incidental on differences in the laws of growth of the two plants. we can sometimes see the reason why one tree will not take on another, from differences in their rate of growth, in the hardness of their wood, in the period of the flow or nature of their sap, &c.; but in a multitude of cases we can assign no reason whatever. great diversity in the size of two plants, one being woody and the other herbaceous, one being evergreen and the other deciduous, and adaptation to widely different climates, does not always prevent the two grafting together. as in hybridisation, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft trees together belonging to quite distinct families; and, on the other hand, closely allied species, and varieties of the same species, can usually, but not invariably, be grafted with ease. but this capacity, as in hybridisation, is by no means absolutely governed by systematic affinity. although many distinct genera within the same family have been grafted { } together, in other cases species of the same genus will not take on each other. the pear can be grafted far more readily on the quince, which is ranked as a distinct genus, than on the apple, which is a member of the same genus. even different varieties of the pear take with different degrees of facility on the quince; so do different varieties of the apricot and peach on certain varieties of the plum. as gärtner found that there was sometimes an innate difference in different _individuals_ of the same two species in crossing; so sagaret believes this to be the case with different individuals of the same two species in being grafted together. as in reciprocal crosses, the facility of effecting an union is often very far from equal, so it sometimes is in grafting; the common gooseberry, for instance, cannot be grafted on the currant, whereas the currant will take, though with difficulty, on the gooseberry. we have seen that the sterility of hybrids, which have their reproductive organs in an imperfect condition, is a very different case from the difficulty of uniting two pure species, which have their reproductive organs perfect; yet these two distinct cases run to a certain extent parallel. something analogous occurs in grafting; for thouin found that three species of robinia, which seeded freely on their own roots, and which could be grafted with no great difficulty on another species, when thus grafted were rendered barren. on the other hand, certain species of sorbus, when grafted on other species, yielded twice as much fruit as when on their own roots. we are reminded by this latter fact of the extraordinary case of hippeastrum, lobelia, &c., which seeded much more freely when fertilised with the pollen of distinct species, than when self-fertilised with their own pollen. { } we thus see, that although there is a clear and fundamental difference between the mere adhesion of grafted stocks, and the union of the male and female elements in the act of reproduction, yet that there is a rude degree of parallelism in the results of grafting and of crossing distinct species. and as we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems, so i believe that the still more complex laws governing the facility of first crosses, are incidental on unknown differences, chiefly in their reproductive systems. these differences, in both cases, follow to a certain extent, as might have been expected, systematic affinity, by which every kind of resemblance and dissimilarity between organic beings is attempted to be expressed. the facts by no means seem to me to indicate that the greater or lesser difficulty of either grafting or crossing together various species has been a special endowment; although in the case of crossing, the difficulty is as important for the endurance and stability of specific forms, as in the case of grafting it is unimportant for their welfare. _causes of the sterility of first crosses and of hybrids._--we may now look a little closer at the probable causes of the sterility of first crosses and of hybrids. these two cases are fundamentally different, for, as just remarked, in the union of two pure species the male and female sexual elements are perfect, whereas in hybrids they are imperfect. even in first crosses, the greater or lesser difficulty in effecting a union apparently depends on several distinct causes. there must sometimes be a physical impossibility in the male element reaching the ovule, as would be the case with a plant { } having a pistil too long for the pollen-tubes to reach the ovarium. it has also been observed that when pollen of one species is placed on the stigma of a distantly allied species, though the pollen-tubes protrude, they do not penetrate the stigmatic surface. again, the male element may reach the female element, but be incapable of causing an embryo to be developed, as seems to have been the case with some of thuret's experiments on fuci. no explanation can be given of these facts, any more than why certain trees cannot be grafted on others. lastly, an embryo may be developed, and then perish at an early period. this latter alternative has not been sufficiently attended to; but i believe, from observations communicated to me by mr. hewitt, who has had great experience in hybridising gallinaceous birds, that the early death of the embryo is a very frequent cause of sterility in first crosses. i was at first very unwilling to believe in this view; as hybrids, when once born, are generally healthy and long-lived, as we see in the case of the common mule. hybrids, however, are differently circumstanced before and after birth: when born and living in a country where their two parents can live, they are generally placed under suitable conditions of life. but a hybrid partakes of only half of the nature and constitution of its mother, and therefore before birth, as long as it is nourished within its mother's womb or within the egg or seed produced by the mother, it may be exposed to conditions in some degree unsuitable, and consequently be liable to perish at an early period; more especially as all very young beings seem eminently sensitive to injurious or unnatural conditions of life. in regard to the sterility of hybrids, in which the sexual elements are imperfectly developed, the case is { } very different. i have more than once alluded to a large body of facts, which i have collected, showing that when animals and plants are removed from their natural conditions, they are extremely liable to have their reproductive systems seriously affected. this, in fact, is the great bar to the domestication of animals. between the sterility thus superinduced and that of hybrids, there are many points of similarity. in both cases the sterility is independent of general health, and is often accompanied by excess of size or great luxuriance. in both cases, the sterility occurs in various degrees; in both, the male element is the most liable to be affected; but sometimes the female more than the male. in both, the tendency goes to a certain extent with systematic affinity, for whole groups of animals and plants are rendered impotent by the same unnatural conditions; and whole groups of species tend to produce sterile hybrids. on the other hand, one species in a group will sometimes resist great changes of conditions with unimpaired fertility; and certain species in a group will produce unusually fertile hybrids. no one can tell, till he tries, whether any particular animal will breed under confinement or any exotic plant seed freely under culture; nor can he tell, till he tries, whether any two species of a genus will produce more or less sterile hybrids. lastly, when organic beings are placed during several generations under conditions not natural to them, they are extremely liable to vary, which is due, as i believe, to their reproductive systems having been specially affected, though in a lesser degree than when sterility ensues. so it is with hybrids, for hybrids in successive generations are eminently liable to vary, as every experimentalist has observed. thus we see that when organic beings are placed under new and unnatural conditions, and when hybrids { } are produced by the unnatural crossing of two species, the reproductive system, independently of the general state of health, is affected by sterility in a very similar manner. in the one case, the conditions of life have been disturbed, though often in so slight a degree as to be inappreciable by us; in the other case, or that of hybrids, the external conditions have remained the same, but the organisation has been disturbed by two different structures and constitutions having been blended into one. for it is scarcely possible that two organisations should be compounded into one, without some disturbance occurring in the development, or periodical action, or mutual relation of the different parts and organs one to another, or to the conditions of life. when hybrids are able to breed _inter se_, they transmit to their offspring from generation to generation the same compounded organisation, and hence we need not be surprised that their sterility, though in some degree variable, rarely diminishes. it must, however, be confessed that we cannot understand, excepting on vague hypotheses, several facts with respect to the sterility of hybrids; for instance, the unequal fertility of hybrids produced from reciprocal crosses; or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent. nor do i pretend that the foregoing remarks go to the root of the matter: no explanation is offered why an organism, when placed under unnatural conditions, is rendered sterile. all that i have attempted to show, is that in two cases, in some respects allied, sterility is the common result,--in the one case from the conditions of life having been disturbed, in the other case from the organisation having been disturbed by two organisations having been compounded into one. it may seem fanciful, but i suspect that a similar { } parallelism extends to an allied yet very different class of facts. it is an old and almost universal belief, founded, i think, on a considerable body of evidence, that slight changes in the conditions of life are beneficial to all living things. we see this acted on by farmers and gardeners in their frequent exchanges of seed, tubers, &c., from one soil or climate to another, and back again. during the convalescence of animals, we plainly see that great benefit is derived from almost any change in the habits of life. again, both with plants and animals, there is abundant evidence, that a cross between very distinct individuals of the same species, that is between members of different strains or sub-breeds, gives vigour and fertility to the offspring. i believe, indeed, from the facts alluded to in our fourth chapter, that a certain amount of crossing is indispensable even with hermaphrodites; and that close interbreeding continued during several generations between the nearest relations, especially if these be kept under the same conditions of life, always induces weakness and sterility in the progeny. hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings, and on the other hand, that slight crosses, that is crosses between the males and females of the same species which have varied and become slightly different, give vigour and fertility to the offspring. but we have seen that greater changes, or changes of a particular nature, often render organic beings in some degree sterile; and that greater crosses, that is crosses between males and females which have become widely or specifically different, produce hybrids which are generally sterile in some degree. i cannot persuade myself that this parallelism is an accident or an illusion. both series of facts seem to be connected together by some { } common but unknown bond, which is essentially related to the principle of life. _fertility of varieties when crossed, and of their mongrel offspring._--it may be urged, as a most forcible argument, that there must be some essential distinction between species and varieties, and that there must be some error in all the foregoing remarks, inasmuch as varieties, however much they may differ from each other in external appearance, cross with perfect facility, and yield perfectly fertile offspring. i fully admit that this is almost invariably the case. but if we look to varieties produced under nature, we are immediately involved in hopeless difficulties; for if two hitherto reputed varieties be found in any degree sterile together, they are at once ranked by most naturalists as species. for instance, the blue and red pimpernel, the primrose and cowslip, which are considered by many of our best botanists as varieties, are said by gärtner not to be quite fertile when crossed, and he consequently ranks them as undoubted species. if we thus argue in a circle, the fertility of all varieties produced under nature will assuredly have to be granted. if we turn to varieties, produced, or supposed to have been produced, under domestication, we are still involved in doubt. for when it is stated, for instance, that the german spitz dog unites more easily than other dogs with foxes, or that certain south american indigenous domestic dogs do not readily cross with european dogs, the explanation which will occur to every one, and probably the true one, is that these dogs have descended from several aboriginally distinct species. nevertheless the perfect fertility of so many domestic varieties, differing widely from each other in appearance, for instance of the pigeon or of the cabbage, is { } a remarkable fact; more especially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed. several considerations, however, render the fertility of domestic varieties less remarkable than at first appears. it can, in the first place, be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed; and we may apply the same rule to domestic varieties. in the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids which were at first only slightly sterile; and if this be so, we surely ought not to expect to find sterility both appearing and disappearing under nearly the same conditions of life. lastly, and this seems to me by far the most important consideration, new races of animals and plants are produced under domestication by man's methodical and unconscious power of selection, for his own use and pleasure: he neither wishes to select, nor could select, slight differences in the reproductive system, or other constitutional differences correlated with the reproductive system. he supplies his several varieties with the same food; treats them in nearly the same manner, and does not wish to alter their general habits of life. nature acts uniformly and slowly during vast periods of time on the whole organisation, in any way which may be for each creature's own good; and thus she may, either directly, or more probably indirectly, through correlation, modify the reproductive system in the several descendants from any one species. seeing this difference in the process of selection, as carried on by man and nature, we need not be surprised at some difference in the result. i have as yet spoken as if the varieties of the same { } species were invariably fertile when intercrossed. but it seems to me impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases, which i will briefly abstract. the evidence is at least as good as that from which we believe in the sterility of a multitude of species. the evidence is, also, derived from hostile witnesses, who in all other cases consider fertility and sterility as safe criterions of specific distinction. gärtner kept during several years a dwarf kind of maize with yellow seeds, and a tall variety with red seeds, growing near each other in his garden; and although these plants have separated sexes, they never naturally crossed. he then fertilised thirteen flowers of the one with the pollen of the other; but only a single head produced any seed, and this one head produced only five grains. manipulation in this case could not have been injurious, as the plants have separated sexes. no one, i believe, has suspected that these varieties of maize are distinct species; and it is important to notice that the hybrid plants thus raised were themselves _perfectly_ fertile; so that even gärtner did not venture to consider the two varieties as specifically distinct. girou de buzareingues crossed three varieties of gourd, which like the maize has separated sexes, and he asserts that their mutual fertilisation is by so much the less easy as their differences are greater. how far these experiments may be trusted, i know not; but the forms experimentised on, are ranked by sagaret, who mainly founds his classification by the test of infertility, as varieties. the following case is far more remarkable, and seems at first quite incredible; but it is the result of an astonishing number of experiments made during many years on nine species of verbascum, by so good an observer { } and so hostile a witness, as gärtner: namely, that yellow and white varieties of the same species of verbascum when intercrossed produce less seed, than do either coloured varieties when fertilised with pollen from their own coloured flowers. moreover, he asserts that when yellow and white varieties of one species are crossed with yellow and white varieties of a _distinct_ species, more seed is produced by the crosses between the similarly coloured flowers, than between those which are differently coloured. yet these varieties of verbascum present no other difference besides the mere colour of the flower; and one variety can sometimes be raised from the seed of the other. from observations which i have made on certain varieties of hollyhock, i am inclined to suspect that they present analogous facts. kölreuter, whose accuracy has been confirmed by every subsequent observer, has proved the remarkable fact, that one variety of the common tobacco is more fertile, when crossed with a widely distinct species, than are the other varieties. he experimentised on five forms, which are commonly reputed to be varieties, and which he tested by the severest trial, namely, by reciprocal crosses, and he found their mongrel offspring perfectly fertile. but one of these five varieties, when used either as father or mother, and crossed with the nicotiana glutinosa, always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with n. glutinosa. hence the reproductive system of this one variety must have been in some manner and in some degree modified. from these facts; from the great difficulty of ascertaining the infertility of varieties in a state of nature, for a supposed variety if infertile in any degree would generally be ranked as species; from man selecting only { } external characters in the production of the most distinct domestic varieties, and from not wishing or being able to produce recondite and functional differences in the reproductive system; from these several considerations and facts, i do not think that the very general fertility of varieties can be proved to be of universal occurrence, or to form a fundamental distinction between varieties and species. the general fertility of varieties does not seem to me sufficient to overthrow the view which i have taken with respect to the very general, but not invariable, sterility of first crosses and of hybrids, namely, that it is not a special endowment, but is incidental on slowly acquired modifications, more especially in the reproductive systems of the forms which are crossed. _hybrids and mongrels compared, independently of their fertility._--independently of the question of fertility, the offspring of species when crossed and of varieties when crossed may be compared in several other respects. gärtner, whose strong wish was to draw a marked line of distinction between species and varieties, could find very few and, as it seems to me, quite unimportant differences between the so-called hybrid offspring of species, and the so-called mongrel offspring of varieties. and, on the other hand, they agree most closely in very many important respects. i shall here discuss this subject with extreme brevity. the most important distinction is, that in the first generation mongrels are more variable than hybrids; but gärtner admits that hybrids from species which have long been cultivated are often variable in the first generation; and i have myself seen striking instances of this fact. gärtner further admits that hybrids between very closely allied species are more variable { } than those from very distinct species; and this shows that the difference in the degree of variability graduates away. when mongrels and the more fertile hybrids are propagated for several generations an extreme amount of variability in their offspring is notorious; but some few cases both of hybrids and mongrels long retaining uniformity of character could be given. the variability, however, in the successive generations of mongrels is, perhaps, greater than in hybrids. this greater variability of mongrels than of hybrids does not seem to me at all surprising. for the parents of mongrels are varieties, and mostly domestic varieties (very few experiments having been tried on natural varieties), and this implies in most cases that there has been recent variability; and therefore we might expect that such variability would often continue and be superadded to that arising from the mere act of crossing. the slight degree of variability in hybrids from the first cross or in the first generation, in contrast with their extreme variability in the succeeding generations, is a curious fact and deserves attention. for it bears on and corroborates the view which i have taken on the cause of ordinary variability; namely, that it is due to the reproductive system being eminently sensitive to any change in the conditions of life, being thus often rendered either impotent or at least incapable of its proper function of producing offspring identical with the parent-form. now hybrids in the first generation are descended from species (excluding those long cultivated) which have not had their reproductive systems in any way affected, and they are not variable; but hybrids themselves have their reproductive systems seriously affected, and their descendants are highly variable. but to return to our comparison of mongrels and { } hybrids: gärtner states that mongrels are more liable than hybrids to revert to either parent-form; but this, if it be true, is certainly only a difference in degree. gärtner further insists that when any two species, although most closely allied to each other, are crossed with a third species, the hybrids are widely different from each other; whereas if two very distinct varieties of one species are crossed with another species, the hybrids do not differ much. but this conclusion, as far as i can make out, is founded on a single experiment; and seems directly opposed to the results of several experiments made by kölreuter. these alone are the unimportant differences, which gärtner is able to point out, between hybrid and mongrel plants. on the other hand, the resemblance in mongrels and in hybrids to their respective parents, more especially in hybrids produced from nearly related species, follows according to gärtner the same laws. when two species are crossed, one has sometimes a prepotent power of impressing its likeness on the hybrid; and so i believe it to be with varieties of plants. with animals one variety certainly often has this prepotent power over another variety. hybrid plants produced from a reciprocal cross, generally resemble each other closely; and so it is with mongrels from a reciprocal cross. both hybrids and mongrels can be reduced to either pure parent-form, by repeated crosses in successive generations with either parent. these several remarks are apparently applicable to animals; but the subject is here excessively complicated, partly owing to the existence of secondary sexual characters; but more especially owing to prepotency in transmitting likeness running more strongly in one sex than in the other, both when one species is crossed with another, and when, one variety is crossed with { } another variety. for instance, i think those authors are right, who maintain that the ass has a prepotent power over the horse, so that both the mule and the hinny more resemble the ass than the horse; but that the prepotency runs more strongly in the male-ass than in the female, so that the mule, which is the offspring of the male-ass and mare, is more like an ass, than is the hinny, which is the offspring of the female-ass and stallion. much stress has been laid by some authors on the supposed fact, that mongrel animals alone are born closely like one of their parents; but it can be shown that this does sometimes occur with hybrids; yet i grant much less frequently with hybrids than with mongrels. looking to the cases which i have collected of cross-bred animals closely resembling one parent, the resemblances seem chiefly confined to characters almost monstrous in their nature, and which have suddenly appeared--such as albinism, melanism, deficiency of tail or horns, or additional fingers and toes; and do not relate to characters which have been slowly acquired by selection. consequently, sudden reversions to the perfect character of either parent would be more likely to occur with mongrels, which are descended from varieties often suddenly produced and semi-monstrous in character, than with hybrids, which are descended from species slowly and naturally produced. on the whole i entirely agree with dr. prosper lucas, who, after arranging an enormous body of facts with respect to animals, comes to the conclusion, that the laws of resemblance of the child to its parents are the same, whether the two parents differ much or little from each other, namely in the union of individuals of the same variety, or of different varieties, or of distinct species. laying aside the question of fertility and sterility, { } in all other respects there seems to be a general and close similarity in the offspring of crossed species, and of crossed varieties. if we look at species as having been specially created, and at varieties as having been produced by secondary laws, this similarity would be an astonishing fact. but it harmonises perfectly with the view that there is no essential distinction between species and varieties. _summary of chapter._--first crosses between forms sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile. the sterility is of all degrees, and is often so slight that the two most careful experimentalists who have ever lived, have come to diametrically opposite conclusions in ranking forms by this test. the sterility is innately variable in individuals of the same species, and is eminently susceptible of favourable and unfavourable conditions. the degree of sterility does not strictly follow systematic affinity, but is governed by several curious and complex laws. it is generally different, and sometimes widely different, in reciprocal crosses between the same two species. it is not always equal in degree in a first cross and in the hybrid produced from this cross. in the same manner as in grafting trees, the capacity of one species or variety to take on another, is incidental on generally unknown differences in their vegetative systems, so in crossing, the greater or less facility of one species to unite with another, is incidental on unknown differences in their reproductive systems. there is no more reason to think that species have been specially endowed with various degrees of sterility to prevent them crossing and blending in nature, than to think that trees have been specially endowed with various and { } somewhat analogous degrees of difficulty in being grafted together in order to prevent them becoming inarched in our forests. the sterility of first crosses between pure species, which have their reproductive systems perfect, seems to depend on several circumstances; in some cases largely on the early death of the embryo. the sterility of hybrids, which have their reproductive systems imperfect, and which have had this system and their whole organisation disturbed by being compounded of two distinct species, seems closely allied to that sterility which so frequently affects pure species, when their natural conditions of life have been disturbed. this view is supported by a parallelism of another kind;--namely, that the crossing of forms only slightly different is favourable to the vigour and fertility of their offspring; and that slight changes in the conditions of life are apparently favourable to the vigour and fertility of all organic beings. it is not surprising that the degree of difficulty in uniting two species, and the degree of sterility of their hybrid-offspring should generally correspond, though due to distinct causes; for both depend on the amount of difference of some kind between the species which are crossed. nor is it surprising that the facility of effecting a first cross, the fertility of the hybrids produced from it, and the capacity of being grafted together--though this latter capacity evidently depends on widely different circumstances--should all run, to a certain extent, parallel with the systematic affinity of the forms which are subjected to experiment; for systematic affinity attempts to express all kinds of resemblance between all species. first crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not quite { } universally, fertile. nor is this nearly general and perfect fertility surprising, when we remember how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences, and not of differences in the reproductive system. in all other respects, excluding fertility, there is a close general resemblance between hybrids and mongrels. finally, then, the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties. * * * * * { } chapter ix. on the imperfection of the geological record. on the absence of intermediate varieties at the present day--on the nature of extinct intermediate varieties; on their number--on the vast lapse of time, as inferred from the rate of deposition and of denudation--on the poorness of our palæontological collections--on the intermittence of geological formations--on the absence of intermediate varieties in any one formation--on the sudden appearance of groups of species--on their sudden appearance in the lowest known fossiliferous strata. in the sixth chapter i enumerated the chief objections which might be justly urged against the views maintained in this volume. most of them have now been discussed. one, namely the distinctness of specific forms, and their not being blended together by innumerable transitional links, is a very obvious difficulty. i assigned reasons why such links do not commonly occur at the present day, under the circumstances apparently most favourable for their presence, namely on an extensive and continuous area with graduated physical conditions. i endeavoured to show, that the life of each species depends in a more important manner on the presence of other already defined organic forms, than on climate; and, therefore, that the really governing conditions of life do not graduate away quite insensibly like heat or moisture. i endeavoured, also, to show that intermediate varieties, from existing in lesser numbers than the forms which they connect, will generally be beaten out and exterminated during the course of further modification and improvement. the main cause, however, of innumerable intermediate links not now occurring everywhere throughout nature { } depends on the very process of natural selection, through which new varieties continually take the places of and exterminate their parent-forms. but just in proportion as this process of extermination has acted on an enormous scale, so must the number of intermediate varieties, which have formerly existed on the earth, be truly enormous. why then is not every geological formation and every stratum full of such intermediate links? geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory. the explanation lies, as i believe, in the extreme imperfection of the geological record. in the first place it should always be borne in mind what sort of intermediate forms must, on my theory, have formerly existed. i have found it difficult, when looking at any two species, to avoid picturing to myself, forms _directly_ intermediate between them. but this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all its modified descendants. to give a simple illustration: the fantail and pouter pigeons have both descended from the rock-pigeon; if we possessed all the intermediate varieties which have ever existed, we should have an extremely close series between both and the rock-pigeon; but we should have no varieties directly intermediate between the fantail and pouter; none, for instance, combining a tail somewhat expanded with a crop somewhat enlarged, the characteristic features of these two breeds. these two breeds, moreover, have become so much modified, that if we had no historical or indirect evidence regarding their origin, it would not have been possible to have { } determined from a mere comparison of their structure with that of the rock-pigeon, whether they had descended from this species or from some other allied species, such as c. oenas. so with natural species, if we look to forms very distinct, for instance to the horse and tapir, we have no reason to suppose that links ever existed directly intermediate between them, but between each and an unknown common parent. the common parent will have had in its whole organisation much general resemblance to the tapir and to the horse; but in some points of structure may have differed considerably from both, even perhaps more than they differ from each other. hence in all such cases, we should be unable to recognise the parent-form of any two or more species, even if we closely compared the structure of the parent with that of its modified descendants, unless at the same time we had a nearly perfect chain of the intermediate links. it is just possible by my theory, that one of two living forms might have descended from the other; for instance, a horse from a tapir; and in this case _direct_ intermediate links will have existed between them. but such a case would imply that one form had remained for a very long period unaltered, whilst its descendants had undergone a vast amount of change; and the principle of competition between organism and organism, between child and parent, will render this a very rare event; for in all cases the new and improved forms of life tend to supplant the old and unimproved forms. by the theory of natural selection all living species have been connected with the parent-species of each genus, by differences not greater than we see between the varieties of the same species at the present { } day; and these parent-species, now generally extinct, have in their turn been similarly connected with more ancient species; and so on backwards, always converging to the common ancestor of each great class. so that the number of intermediate and transitional links, between all living and extinct species, must have been inconceivably great. but assuredly, if this theory be true, such have lived upon this earth. _on the lapse of time._--independently of our not finding fossil remains of such infinitely numerous connecting links, it may be objected, that time will not have sufficed for so great an amount of organic change, all changes having been effected very slowly through natural selection. it is hardly possible for me even to recall to the reader, who may not be a practical geologist, the facts leading the mind feebly to comprehend the lapse of time. he who can read sir charles lyell's grand work on the principles of geology, which the future historian will recognise as having produced a revolution in natural science, yet does not admit how incomprehensively vast have been the past periods of time, may at once close this volume. not that it suffices to study the principles of geology, or to read special treatises by different observers on separate formations, and to mark how each author attempts to give an inadequate idea of the duration of each formation or even each stratum. a man must for years examine for himself great piles of superimposed strata, and watch the sea at work grinding down old rocks and making fresh sediment, before he can hope to comprehend anything of the lapse of time, the monuments of which we see around us. it is good to wander along lines of sea-coast, when formed of moderately hard rocks, and mark the { } process of degradation. the tides in most cases reach the cliffs only for a short time twice a day, and the waves eat into them only when they are charged with sand or pebbles; for there is good evidence that pure water can effect little or nothing in wearing away rock. at last the base of the cliff is undermined, huge fragments fall down, and these remaining fixed, have to be worn away, atom by atom, until reduced in size they can be rolled about by the waves, and then are more quickly ground into pebbles, sand, or mud. but how often do we see along the bases of retreating cliffs rounded boulders, all thickly clothed by marine productions, showing how little they are abraded and how seldom they are rolled about! moreover, if we follow for a few miles any line of rocky cliff, which is undergoing degradation, we find that it is only here and there, along a short length or round a promontory, that the cliffs are at the present time suffering. the appearance of the surface and the vegetation show that elsewhere years have elapsed since the waters washed their base. he who most closely studies the action of the sea on our shores, will, i believe, be most deeply impressed with the slowness with which rocky coasts are worn away. the observations on this head by hugh miller, and by that excellent observer mr. smith of jordan hill, are most impressive. with the mind thus impressed, let any one examine beds of conglomerate many thousand feet in thickness, which, though probably formed at a quicker rate than many other deposits, yet, from being formed of worn and rounded pebbles, each of which bears the stamp of time, are good to show how slowly the mass has been accumulated. in the cordillera i estimated one pile of conglomerate at ten thousand feet in thickness. let the { } observer remember lyell's profound remark that the thickness and extent of sedimentary formations are the result and measure of the degradation which the earth's crust has elsewhere suffered. and what an amount of degradation is implied by the sedimentary deposits of many countries! professor ramsay has given me the maximum thickness, in most cases from actual measurement, in a few cases from estimate, of each formation in different parts of great britain; and this is the result:-- feet. palæozoic strata (not including igneous beds) , secondary strata , tertiary strata , --making altogether , feet; that is, very nearly thirteen and three-quarters british miles. some of the formations, which are represented in england by thin beds, are thousands of feet in thickness on the continent. moreover, between each successive formation, we have, in the opinion of most geologists, enormously long blank periods. so that the lofty pile of sedimentary rocks in britain, gives but an inadequate idea of the time which has elapsed during their accumulation; yet what time this must have consumed! good observers have estimated that sediment is deposited by the great mississippi river at the rate of only feet in a hundred thousand years. this estimate has no pretension to strict exactness; yet, considering over what wide spaces very fine sediment is transported by the currents of the sea, the process of accumulation in any one area must be extremely slow. but the amount of denudation which the strata have in many places suffered, independently of the rate of accumulation of the degraded matter, probably offers the best evidence of the lapse of time. i remember { } having been much struck with the evidence of denudation, when viewing volcanic islands, which have been worn by the waves and pared all round into perpendicular cliffs of one or two thousand feet in height; for the gentle slope of the lava-streams, due to their formerly liquid state, showed at a glance how far the hard, rocky beds had once extended into the open ocean. the same story is still more plainly told by faults,--those great cracks along which the strata have been upheaved on one side, or thrown down on the other, to the height or depth of thousands of feet; for since the crust cracked, the surface of the land has been so completely planed down by the action of the sea, that no trace of these vast dislocations is externally visible. the craven fault, for instance, extends for upwards of miles, and along this line the vertical displacement of the strata has varied from to feet. prof. ramsay has published an account of a downthrow in anglesea of feet; and he informs me that he fully believes there is one in merionethshire of , feet; yet in these cases there is nothing on the surface to show such prodigious movements; the pile of rocks on the one or other side having been smoothly swept away. the consideration of these facts impresses my mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity. i am tempted to give one other case, the well-known one of the denudation of the weald. though it must be admitted that the denudation of the weald has been a mere trifle, in comparison with that which has removed masses of our palæozoic strata, in parts ten thousand feet in thickness, as shown in prof. ramsay's masterly memoir on this subject: yet it is an admirable lesson to stand on the intermediate hilly country and look on the one hand at the north downs, and { } on the other hand at the south downs; for, remembering that at no great distance to the west the northern and southern escarpments meet and close, one can safely picture to oneself the great dome of rocks which must have covered up the weald within so limited a period as since the latter part of the chalk formation. the distance from the northern to the southern downs is about miles, and the thickness of the several formations is on an average about feet, as i am informed by prof. ramsay. but if, as some geologists suppose, a range of older rocks underlies the weald, on the flanks of which the overlying sedimentary deposits might have accumulated in thinner masses than elsewhere, the above estimate would be erroneous; but this source of doubt probably would not greatly affect the estimate as applied to the western extremity of the district. if, then, we knew the rate at which the sea commonly wears away a line of cliff of any given height, we could measure the time requisite to have denuded the weald. this, of course cannot be done; but we may, in order to form some crude notion on the subject, assume that the sea would eat into cliffs feet in height at the rate of one inch in a century. this will at first appear much too small an allowance; but it is the same as if we were to assume a cliff one yard in height to be eaten back along a whole line of coast at the rate of one yard in nearly every twenty-two years. i doubt whether any rock, even as soft as chalk, would yield at this rate excepting on the most exposed coasts; though no doubt the degradation of a lofty cliff would be more rapid from the breakage of the fallen fragments. on the other hand, i do not believe that any line of coast, ten or twenty miles in length, ever suffers degradation at the same time along its whole indented length; and we { } must remember that almost all strata contain harder layers or nodules, which from long resisting attrition form a breakwater at the base. we may at least confidently believe that no rocky coast feet in height commonly yields at the rate of a foot per century; for this would be the same in amount as a cliff one yard in height retreating twelve yards in twenty-two years; and no one, i think, who has carefully observed the shape of old fallen fragments at the base of cliffs, will admit any near approach to such rapid wearing away. hence, under ordinary circumstances, i should infer that for a cliff feet in height, a denudation of one inch per century for the whole length would be a sufficient allowance. at this rate, on the above data, the denudation of the weald must have required , , years; or say three hundred million years. but perhaps it would be safer to allow two or three inches per century, and this would reduce the number of years to one hundred and fifty or one hundred million years. the action of fresh water on the gently inclined wealden district, when upraised, could hardly have been great, but it would somewhat reduce the above estimate. on the other hand, during oscillations of level, which we know this area has undergone, the surface may have existed for millions of years as land, and thus have escaped the action of the sea: when deeply submerged for perhaps equally long periods, it would, likewise, have escaped the action of the coast-waves. so that it is not improbable that a longer period than million years has elapsed since the latter part of the secondary period. i have made these few remarks because it is highly important for us to gain some notion, however imperfect, of the lapse of years. during each of these years, { } over the whole world, the land and the water has been peopled by hosts of living forms. what an infinite number of generations, which the mind cannot grasp, must have succeeded each other in the long roll of years! now turn to our richest geological museums, and what a paltry display we behold! _on the poorness of our palæontological collections._--that our palæontological collections are very imperfect, is admitted by every one. the remark of that admirable palæontologist, the late edward forbes, should not be forgotten, namely, that numbers of our fossil species are known and named from single and often broken specimens, or from a few specimens collected on some one spot. only a small portion of the surface of the earth has been geologically explored, and no part with sufficient care, as the important discoveries made every year in europe prove. no organism wholly soft can be preserved. shells and bones will decay and disappear when left on the bottom of the sea, where sediment is not accumulating. i believe we are continually taking a most erroneous view, when we tacitly admit to ourselves that sediment is being deposited over nearly the whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil remains. throughout an enormously large proportion of the ocean, the bright blue tint of the water bespeaks its purity. the many cases on record of a formation conformably covered, after an enormous interval of time, by another and later formation, without the underlying bed having suffered in the interval any wear and tear, seem explicable only on the view of the bottom of the sea not rarely lying for ages in an unaltered condition. the remains which do become embedded, if in sand or gravel, will when the beds are upraised generally be dissolved { } by the percolation of rain-water. i suspect that but few of the very many animals which live on the beach between high and low watermark are preserved. for instance, the several species of the chthamalinæ (a subfamily of sessile cirripedes) coat the rocks all over the world in infinite numbers: they are all strictly littoral, with the exception of a single mediterranean species, which inhabits deep water and has been found fossil in sicily, whereas not one other species has hitherto been found in any tertiary formation: yet it is now known that the genus chthamalus existed during the chalk period. the molluscan genus chiton offers a partially analogous case. with respect to the terrestrial productions which lived during the secondary and palæozoic periods, it is superfluous to state that our evidence from fossil remains is fragmentary in an extreme degree. for instance, not a land shell is known belonging to either of these vast periods, with the exception of one species discovered by sir c. lyell and dr. dawson in the carboniferous strata of north america, of which shell several specimens have now been collected. in regard to mammiferous remains, a single glance at the historical table published in the supplement to lyell's manual, will bring home the truth, how accidental and rare is their preservation, far better than pages of detail. nor is their rarity surprising, when we remember how large a proportion of the bones of tertiary mammals have been discovered either in caves or in lacustrine deposits; and that not a cave or true lacustrine bed is known belonging to the age of our secondary or palæozoic formations. but the imperfection in the geological record mainly results from another and more important cause than any of the foregoing; namely, from the several formations { } being separated from each other by wide intervals of time. when we see the formations tabulated in written works, or when we follow them in nature, it is difficult to avoid believing that they are closely consecutive. but we know, for instance, from sir r. murchison's great work on russia, what wide gaps there are in that country between the superimposed formations; so it is in north america, and in many other parts of the world. the most skilful geologist, if his attention had been exclusively confined to these large territories, would never have suspected that during the periods which were blank and barren in his own country, great piles of sediment, charged with new and peculiar forms of life, had elsewhere been accumulated. and if in each separate territory, hardly any idea can be formed of the length of time which has elapsed between the consecutive formations, we may infer that this could nowhere be ascertained. the frequent and great changes in the mineralogical composition of consecutive formations, generally implying great changes in the geography of the surrounding lands, whence the sediment has been derived, accords with the belief of vast intervals of time having elapsed between each formation. but we can, i think, see why the geological formations of each region are almost invariably intermittent; that is, have not followed each other in close sequence. scarcely any fact struck me more when examining many hundred miles of the south american coasts, which have been upraised several hundred feet within the recent period, than the absence of any recent deposits sufficiently extensive to last for even a short geological period. along the whole west coast, which is inhabited by a peculiar marine fauna, tertiary beds are so poorly developed, that no record of several { } successive and peculiar marine faunas will probably be preserved to a distant age. a little reflection will explain why along the rising coast of the western side of south america, no extensive formations with recent or tertiary remains can anywhere be found, though the supply of sediment must for ages have been great, from the enormous degradation of the coast-rocks and from muddy streams entering the sea. the explanation, no doubt, is, that the littoral and sub-littoral deposits are continually worn away, as soon as they are brought up by the slow and gradual rising of the land within the grinding action of the coast-waves. we may, i think, safely conclude that sediment must be accumulated in extremely thick, solid, or extensive masses, in order to withstand the incessant action of the waves, when first upraised and during subsequent oscillations of level. such thick and extensive accumulations of sediment may be formed in two ways; either, in profound depths of the sea, in which case, judging from the researches of e. forbes, we may conclude that the bottom will be inhabited by extremely few animals, and the mass when upraised will give a most imperfect record of the forms of life which then existed; or, sediment may be accumulated to any thickness and extent over a shallow bottom, if it continue slowly to subside. in this latter case, as long as the rate of subsidence and supply of sediment nearly balance each other, the sea will remain shallow and favourable for life, and thus a fossiliferous formation thick enough, when upraised, to resist any amount of degradation, may be formed. i am convinced that all our ancient formations, which are rich in fossils, have thus been formed during subsidence. since publishing my views on this subject in , i have watched the progress of { } geology, and have been surprised to note how author after author, in treating of this or that great formation, has come to the conclusion that it was accumulated during subsidence. i may add, that the only ancient tertiary formation on the west coast of south america, which has been bulky enough to resist such degradation as it has as yet suffered, but which will hardly last to a distant geological age, was certainly deposited during a downward oscillation of level, and thus gained considerable thickness. all geological facts tell us plainly that each area has undergone numerous slow oscillations of level, and apparently these oscillations have affected wide spaces. consequently formations rich in fossils and sufficiently thick and extensive to resist subsequent degradation, may have been formed over wide spaces during periods of subsidence, but only where the supply of sediment was sufficient to keep the sea shallow and to embed and preserve the remains before they had time to decay. on the other hand, as long as the bed of the sea remained stationary, _thick_ deposits could not have been accumulated in the shallow parts, which are the most favourable to life. still less could this have happened during the alternate periods of elevation; or, to speak more accurately, the beds which were then accumulated will have been destroyed by being upraised and brought within the limits of the coast-action. thus the geological record will almost necessarily be rendered intermittent. i feel much confidence in the truth of these views, for they are in strict accordance with the general principles inculcated by sir c. lyell; and e. forbes subsequently but independently arrived at a similar conclusion. one remark is here worth a passing notice. during periods of elevation the area of the land and of the { } adjoining shoal parts of the sea will be increased, and new stations will often be formed;--all circumstances most favourable, as previously explained, for the formation of new varieties and species; but during such periods there will generally be a blank in the geological record. on the other hand, during subsidence, the inhabited area and number of inhabitants will decrease (excepting the productions on the shores of a continent when first broken up into an archipelago), and consequently during subsidence, though there will be much extinction, fewer new varieties or species will be formed; and it is during these very periods of subsidence, that our great deposits rich in fossils have been accumulated. nature may almost be said to have guarded against the frequent discovery of her transitional or linking forms. from the foregoing considerations it cannot be doubted that the geological record, viewed as a whole, is extremely imperfect; but if we confine our attention to any one formation, it becomes more difficult to understand, why we do not therein find closely graduated varieties between the allied species which lived at its commencement and at its close. some cases are on record of the same species presenting distinct varieties in the upper and lower parts of the same formation, but, as they are rare, they may be here passed over. although each formation has indisputably required a vast number of years for its deposition, i can see several reasons why each should not include a graduated series of links between the species which then lived; but i can by no means pretend to assign due proportional weight to the following considerations. although each formation may mark a very long lapse of years, each perhaps is short compared with the period requisite to change one species into another. i am { } aware that two palæontologists, whose opinions are worthy of much deference, namely bronn and woodward, have concluded that the average duration of each formation is twice or thrice as long as the average duration of specific forms. but insuperable difficulties, as it seems to me, prevent us coming to any just conclusion on this head. when we see a species first appearing in the middle of any formation, it would be rash in the extreme to infer that it had not elsewhere previously existed. so again when we find a species disappearing before the uppermost layers have been deposited, it would be equally rash to suppose that it then became wholly extinct. we forget how small the area of europe is compared with the rest of the world; nor have the several stages of the same formation throughout europe been correlated with perfect accuracy. with marine animals of all kinds, we may safely infer a large amount of migration during climatal and other changes; and when we see a species first appearing in any formation, the probability is that it only then first immigrated into that area. it is well known, for instance, that several species appeared somewhat earlier in the palæozoic beds of north america than in those of europe; time having apparently been required for their migration from the american to the european seas. in examining the latest deposits of various quarters of the world, it has everywhere been noted, that some few still existing species are common in the deposit, but have become extinct in the immediately surrounding sea; or, conversely, that some are now abundant in the neighbouring sea, but are rare or absent in this particular deposit. it is an excellent lesson to reflect on the ascertained amount of migration of the inhabitants of europe during the glacial period, which forms only a part of one whole geological period; { } and likewise to reflect on the great changes of level, on the inordinately great change of climate, on the prodigious lapse of time, all included within this same glacial period. yet it may be doubted whether in any quarter of the world, sedimentary deposits, _including fossil remains_, have gone on accumulating within the same area during the whole of this period. it is not, for instance, probable that sediment was deposited during the whole of the glacial period near the mouth of the mississippi, within that limit of depth at which marine animals can flourish; for we know what vast geographical changes occurred in other parts of america during this space of time. when such beds as were deposited in shallow water near the mouth of the mississippi during some part of the glacial period shall have been upraised, organic remains will probably first appear and disappear at different levels, owing to the migration of species and to geographical changes. and in the distant future, a geologist examining these beds, might be tempted to conclude that the average duration of life of the embedded fossils had been less than that of the glacial period, instead of having been really far greater, that is extending from before the glacial epoch to the present day. in order to get a perfect gradation between two forms in the upper and lower parts of the same formation, the deposit must have gone on accumulating for a very long period, in order to have given sufficient time for the slow process of variation; hence the deposit will generally have to be a very thick one; and the species undergoing modification will have had to live on the same area throughout this whole time. but we have seen that a thick fossiliferous formation can only be accumulated during a period of subsidence; and to keep the depth approximately the same, which is necessary in { } order to enable the same species to live on the same space, the supply of sediment must nearly have counterbalanced the amount of subsidence. but this same movement of subsidence will often tend to sink the area whence the sediment is derived, and thus diminish the supply whilst the downward movement continues. in fact, this nearly exact balancing between the supply of sediment and the amount of subsidence is probably a rare contingency; for it has been observed by more than one palæontologist, that very thick deposits are usually barren of organic remains, except near their upper or lower limits. it would seem that each separate formation, like the whole pile of formations in any country, has generally been intermittent in its accumulation. when we see, as is so often the case, a formation composed of beds of different mineralogical composition, we may reasonably suspect that the process of deposition has been much interrupted, as a change in the currents of the sea and a supply of sediment of a different nature will generally have been due to geographical changes requiring much time. nor will the closest inspection of a formation give any idea of the time which its deposition has consumed. many instances could be given of beds only a few feet in thickness, representing formations, elsewhere thousands of feet in thickness, and which must have required an enormous period for their accumulation; yet no one ignorant of this fact would have suspected the vast lapse of time represented by the thinner formation. many cases could be given of the lower beds of a formation having been upraised, denuded, submerged, and then re-covered by the upper beds of the same formation,--facts, showing what wide, yet easily overlooked, intervals have occurred in its accumulation. in other cases we have the plainest evidence { } in great fossilised trees, still standing upright as they grew, of many long intervals of time and changes of level during the process of deposition, which would never even have been suspected, had not the trees chanced to have been preserved: thus messrs. lyell and dawson found carboniferous beds feet thick in nova scotia, with ancient root-bearing strata, one above the other, at no less than sixty-eight different levels. hence, when the same species occur at the bottom, middle, and top of a formation, the probability is that they have not lived on the same spot during the whole period of deposition, but have disappeared and reappeared, perhaps many times, during the same geological period. so that if such species were to undergo a considerable amount of modification during any one geological period, a section would not probably include all the fine intermediate gradations which must on my theory have existed between them, but abrupt, though perhaps very slight, changes of form. it is all-important to remember that naturalists have no golden rule by which to distinguish species and varieties; they grant some little variability to each species, but when they meet with a somewhat greater amount of difference between any two forms, they rank both as species, unless they are enabled to connect them together by close intermediate gradations. and this from the reasons just assigned we can seldom hope to effect in any one geological section. supposing b and c to be two species, and a third, a, to be found in an underlying bed; even if a were strictly intermediate between b and c, it would simply be ranked as a third and distinct species, unless at the same time it could be most closely connected with either one or both forms by intermediate varieties. nor should it be forgotten, as before explained, that a might be the actual progenitor { } of b and c, and yet might not at all necessarily be strictly intermediate between them in all points of structure. so that we might obtain the parent-species and its several modified descendants from the lower and upper beds of a formation, and unless we obtained numerous transitional gradations, we should not recognise their relationship, and should consequently be compelled to rank them all as distinct species. it is notorious on what excessively slight differences many palæontologists have founded their species; and they do this the more readily if the specimens come from different sub-stages of the same formation. some experienced conchologists are now sinking many of the very fine species of d'orbigny and others into the rank of varieties; and on this view we do find the kind of evidence of change which on my theory we ought to find. moreover, if we look to rather wider intervals, namely, to distinct but consecutive stages of the same great formation, we find that the embedded fossils, though almost universally ranked as specifically different, yet are far more closely allied to each other than are the species found in more widely separated formations; but to this subject i shall have to return in the following chapter. one other consideration is worth notice: with animals and plants that can propagate rapidly and are not highly locomotive, there is reason to suspect, as we have formerly seen, that their varieties are generally at first local; and that such local varieties do not spread widely and supplant their parent-forms until they have been modified and perfected in some considerable degree. according to this view, the chance of discovering in a formation in any one country all the early stages of transition between any two forms, is small, for the successive changes are supposed to have been local or { } confined to some one spot. most marine animals have a wide range; and we have seen that with plants it is those which have the widest range, that oftenest present varieties; so that with shells and other marine animals, it is probably those which have had the widest range, far exceeding the limits of the known geological formations of europe, which have oftenest given rise, first to local varieties and ultimately to new species; and this again would greatly lessen the chance of our being able to trace the stages of transition in any one geological formation. it should not be forgotten, that at the present day, with perfect specimens for examination, two forms can seldom be connected by intermediate varieties and thus proved to be the same species, until many specimens have been collected from many places; and in the case of fossil species this could rarely be effected by palæontologists. we shall, perhaps, best perceive the improbability of our being enabled to connect species by numerous, fine, intermediate, fossil links, by asking ourselves whether, for instance, geologists at some future period will be able to prove, that our different breeds of cattle, sheep, horses, and dogs have descended from a single stock or from several aboriginal stocks; or, again, whether certain sea-shells inhabiting the shores of north america, which are ranked by some conchologists as distinct species from their european representatives, and by other conchologists as only varieties, are really varieties or are, as it is called, specifically distinct. this could be effected only by the future geologist discovering in a fossil state numerous intermediate gradations; and such success seems to me improbable in the highest degree. geological research, though it has added numerous species to existing and extinct genera, and has made the { } intervals between some few groups less wide than they otherwise would have been, yet has done scarcely anything in breaking down the distinction between species, by connecting them together by numerous, fine, intermediate varieties; and this not having been effected, is probably the gravest and most obvious of all the many objections which may be urged against my views. hence it will be worth while to sum up the foregoing remarks, under an imaginary illustration. the malay archipelago is of about the size of europe from the north cape to the mediterranean, and from britain to russia; and therefore equals all the geological formations which have been examined with any accuracy, excepting those of the united states of america. i fully agree with mr. godwin-austen, that the present condition of the malay archipelago, with its numerous large islands separated by wide and shallow seas, probably represents the former state of europe, whilst most of our formations were accumulating. the malay archipelago is one of the richest regions of the whole world in organic beings; yet if all the species were to be collected which have ever lived there, how imperfectly would they represent the natural history of the world! but we have every reason to believe that the terrestrial productions of the archipelago would be preserved in an excessively imperfect manner in the formations which we suppose to be there accumulating. i suspect that not many of the strictly littoral animals, or of those which lived on naked submarine rocks, would be embedded; and those embedded in gravel or sand, would not endure to a distant epoch. wherever sediment did not accumulate on the bed of the sea, or where it did not accumulate at a sufficient rate to protect organic bodies from decay, no remains could be preserved. i believe that fossiliferous formations could be formed { } in the archipelago, of thickness sufficient to last to an age as distant in futurity as the secondary formations lie in the past, only during periods of subsidence. these periods of subsidence would be separated from each other by enormous intervals, during which the area would be either stationary or rising; whilst rising, each fossiliferous formation would be destroyed, almost as soon as accumulated, by the incessant coast-action, as we now see on the shores of south america. during the periods of subsidence there would probably be much extinction of life; during the periods of elevation, there would be much variation, but the geological record would then be least perfect. it may be doubted whether the duration of any one great period of subsidence over the whole or part of the archipelago, together with a contemporaneous accumulation of sediment, would _exceed_ the average duration of the same specific forms; and these contingencies are indispensable for the preservation of all the transitional gradations between any two or more species. if such gradations were not fully preserved, transitional varieties would merely appear as so many distinct species. it is, also, probable that each great period of subsidence would be interrupted by oscillations of level, and that slight climatal changes would intervene during such lengthy periods; and in these cases the inhabitants of the archipelago would have to migrate, and no closely consecutive record of their modifications could be preserved in any one formation. very many of the marine inhabitants of the archipelago now range thousands of miles beyond its confines; and analogy leads me to believe that it would be chiefly these far-ranging species which would oftenest produce new varieties; and the varieties would at first generally be local or confined to one place, but if possessed { } of any decided advantage, or when further modified and improved, they would slowly spread and supplant their parent-forms. when such varieties returned to their ancient homes, as they would differ from their former state, in a nearly uniform, though perhaps extremely slight degree, they would, according to the principles followed by many palæontologists, be ranked as new and distinct species. if then, there be some degree of truth in these remarks, we have no right to expect to find in our geological formations, an infinite number of those fine transitional forms, which on my theory assuredly have connected all the past and present species of the same group into one long and branching chain of life. we ought only to look for a few links, some more closely, some more distantly related to each other; and these links, let them be ever so close, if found in different stages of the same formation, would, by most palæontologists, be ranked as distinct species. but i do not pretend that i should ever have suspected how poor a record of the mutations of life, the best preserved geological section presented, had not the difficulty of our not discovering innumerable transitional links between the species which appeared at the commencement and close of each formation, pressed so hardly on my theory. _on the sudden appearance of whole groups of allied species._--the abrupt manner in which whole groups of species suddenly appear in certain formations, has been urged by several palæontologists--for instance, by agassiz, pictet, and by none more forcibly than by professor sedgwick--as a fatal objection to the belief in the transmutation of species. if numerous species, belonging to the same genera or families, have really { } started into life all at once, the fact would be fatal to the theory of descent with slow modification through natural selection. for the development of a group of forms, all of which have descended from some one progenitor, must have been an extremely slow process; and the progenitors must have lived long ages before their modified descendants. but we continually over-rate the perfection of the geological record, and falsely infer, because certain genera or families have not been found beneath a certain stage, that they did not exist before that stage. we continually forget how large the world is, compared with the area over which our geological formations have been carefully examined; we forget that groups of species may elsewhere have long existed and have slowly multiplied before they invaded the ancient archipelagoes of europe and of the united states. we do not make due allowance for the enormous intervals of time, which have probably elapsed between our consecutive formations,--longer perhaps in most cases than the time required for the accumulation of each formation. these intervals will have given time for the multiplication of species from some one or some few parent-forms; and in the succeeding formation such species will appear as if suddenly created. i may here recall a remark formerly made, namely that it might require a long succession of ages to adapt an organism to some new and peculiar line of life, for instance to fly through the air; but that when this had been effected, and a few species had thus acquired a great advantage over other organisms, a comparatively short time would be necessary to produce many divergent forms, which would be able to spread rapidly and widely throughout the world. i will now give a few examples to illustrate these { } remarks, and to show how liable we are to error in supposing that whole groups of species have suddenly been produced. i may recall the well-known fact that in geological treatises, published not many years ago, the great class of mammals was always spoken of as having abruptly come in at the commencement of the tertiary series. and now one of the richest known accumulations of fossil mammals, for its thickness, belongs to the middle of the secondary series; and one true mammal has been discovered in the new red sandstone at nearly the commencement of this great series. cuvier used to urge that no monkey occurred in any tertiary stratum; but now extinct species have been discovered in india, south america, and in europe even as far back as the eocene stage. had it not been for the rare accident of the preservation of footsteps in the new red sandstone of the united states, who would have ventured to suppose that, besides reptiles, no less than at least thirty kinds of birds, some of gigantic size, existed during that period? not a fragment of bone has been discovered in these beds. notwithstanding that the number of joints shown in the fossil impressions correspond with the number in the several toes of living birds' feet, some authors doubt whether the animals which left the impressions were really birds. until quite recently these authors might have maintained, and some have maintained, that the whole class of birds came suddenly into existence during an early tertiary period; but now we know, on the authority of professor owen (as may be seen in lyell's 'manual'), that a bird certainly lived during the deposition of the upper greensand. i may give another instance, which from having passed under my own eyes has much struck me. in a memoir on fossil sessile cirripedes, i have stated that, from the { } number of existing and extinct tertiary species; from the extraordinary abundance of the individuals of many species all over the world, from the arctic regions to the equator, inhabiting various zones of depths from the upper tidal limits to fathoms; from the perfect manner in which specimens are preserved in the oldest tertiary beds; from the ease with which even a fragment of a valve can be recognised; from all these circumstances, i inferred that had sessile cirripedes existed during the secondary periods, they would certainly have been preserved and discovered; and as not one species had then been discovered in beds of this age, i concluded that this great group had been suddenly developed at the commencement of the tertiary series. this was a sore trouble to me, adding as i thought one more instance of the abrupt appearance of a great group of species. but my work had hardly been published, when a skilful palæontologist, m. bosquet, sent me a drawing of a perfect specimen of an unmistakeable sessile cirripede, which he had himself extracted from the chalk of belgium. and, as if to make the case as striking as possible, this sessile cirripede was a chthamalus, a very common, large, and ubiquitous genus, of which not one specimen has as yet been found even in any tertiary stratum. hence we now positively know that sessile cirripedes existed during the secondary period; and these cirripedes might have been the progenitors of our many tertiary and existing species. the case most frequently insisted on by palæontologists of the apparently sudden appearance of a whole group of species, is that of the teleostean fishes, low down in the chalk period. this group includes the large majority of existing species. lately, professor pictet has carried their existence one sub-stage further back; and some palæontologists believe that certain { } much older fishes, of which the affinities are as yet imperfectly known, are really teleostean. assuming, however, that the whole of them did appear, as agassiz believes, at the commencement of the chalk formation, the fact would certainly be highly remarkable; but i cannot see that it would be an insuperable difficulty on my theory, unless it could likewise be shown that the species of this group appeared suddenly and simultaneously throughout the world at this same period. it is almost superfluous to remark that hardly any fossil-fish are known from south of the equator; and by running through pictet's palæontology it will be seen that very few species are known from several formations in europe. some few families of fish now have a confined range; the teleostean fish might formerly have had a similarly confined range, and after having been largely developed in some one sea, might have spread widely. nor have we any right to suppose that the seas of the world have always been so freely open from south to north as they are at present. even at this day, if the malay archipelago were converted into land, the tropical parts of the indian ocean would form a large and perfectly enclosed basin, in which any great group of marine animals might be multiplied; and here they would remain confined, until some of the species became adapted to a cooler climate, and were enabled to double the southern capes of africa or australia, and thus reach other and distant seas. from these and similar considerations, but chiefly from our ignorance of the geology of other countries beyond the confines of europe and the united states; and from the revolution in our palæontological ideas on many points, which the discoveries of even the last dozen years have effected, it seems to me to be about as rash in us to dogmatize on the succession of organic { } beings throughout the world, as it would be for a naturalist to land for five minutes on some one barren point in australia, and then to discuss the number and range of its productions. _on the sudden appearance of groups of allied species in the lowest known fossiliferous strata._--there is another and allied difficulty, which is much graver. i allude to the manner in which numbers of species of the same group, suddenly appear in the lowest known fossiliferous rocks. most of the arguments which have convinced me that all the existing species of the same group have descended from one progenitor, apply with nearly equal force to the earliest known species. for instance, i cannot doubt that all the silurian trilobites have descended from some one crustacean, which must have lived long before the silurian age, and which probably differed greatly from any known animal. some of the most ancient silurian animals, as the nautilus, lingula, &c., do not differ much from living species; and it cannot on my theory be supposed, that these old species were the progenitors of all the species of the orders to which they belong, for they do not present characters in any degree intermediate between them. if, moreover, they had been the progenitors of these orders, they would almost certainly have been long ago supplanted and exterminated by their numerous and improved descendants. consequently, if my theory be true, it is indisputable that before the lowest silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures. { } to the question why we do not find records of these vast primordial periods, i can give no satisfactory answer. several of the most eminent geologists, with sir e. murchison at their head, are convinced that we see in the organic remains of the lowest silurian stratum the dawn of life on this planet. other highly competent judges, as lyell and the late e. forbes, dispute this conclusion. we should not forget that only a small portion of the world is known with accuracy. m. barrande has lately added another and lower stage to the silurian system, abounding with new and peculiar species. traces of life have been detected in the longmynd beds, beneath barrande's so-called primordial zone. the presence of phosphatic nodules and bituminous matter in some of the lowest azoic rocks, probably indicates the former existence of life at these periods. but the difficulty of understanding the absence of vast piles of fossiliferous strata, which on my theory no doubt were somewhere accumulated before the silurian epoch, is very great. if these most ancient beds had been wholly worn away by denudation, or obliterated by metamorphic action, we ought to find only small remnants of the formations next succeeding them in age, and these ought to be very generally in a metamorphosed condition. but the descriptions which we now possess of the silurian deposits over immense territories in russia and in north america, do not support the view, that the older a formation is, the more it has always suffered the extremity of denudation and metamorphism. the case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained. to show that it may hereafter receive some explanation, i will give the following hypothesis. from the nature of the organic remains which { } do not appear to have inhabited profound depths, in the several formations of europe and of the united states; and from the amount of sediment, miles in thickness, of which the formations are composed, we may infer that from first to last large islands or tracts of land, whence the sediment was derived, occurred in the neighbourhood of the existing continents of europe and north america. but we do not know what was the state of things in the intervals between the successive formations; whether europe and the united states during these intervals existed as dry land, or as a submarine surface near land, on which sediment was not deposited, or as the bed of an open and unfathomable sea. looking to the existing oceans, which are thrice as extensive as the land, we see them studded with many islands; but not one oceanic island is as yet known to afford even a remnant of any palæozoic or secondary formation. hence we may perhaps infer, that during the palæozoic and secondary periods, neither continents nor continental islands existed where our oceans now extend; for had they existed there, palæozoic and secondary formations would in all probability have been accumulated from sediment derived from their wear and tear; and would have been at least partially upheaved by the oscillations of level, which we may fairly conclude must have intervened during these enormously long periods. if then we may infer anything from these facts, we may infer that where our oceans now extend, oceans have extended from the remotest period of which we have any record; and on the other hand, that where continents now exist, large tracts of land have existed, subjected no doubt to great oscillations of level, since the earliest silurian period. the coloured map appended to my volume on coral reefs, led me to conclude that the great oceans are still mainly areas of { } subsidence, the great archipelagoes still areas of oscillations of level, and the continents areas of elevation. but have we any right to assume that things have thus remained from the beginning of this world? our continents seem to have been formed by a preponderance, during many oscillations of level, of the force of elevation; but may not the areas of preponderant movement have changed in the lapse of ages? at a period immeasurably antecedent to the silurian epoch, continents may have existed where oceans are now spread out; and clear and open oceans may have existed where our continents now stand. nor should we be justified in assuming that if, for instance, the bed of the pacific ocean were now converted into a continent, we should there find formations older than the silurian strata, supposing such to have been formerly deposited; for it might well happen that strata which had subsided some miles nearer to the centre of the earth, and which had been pressed on by an enormous weight of superincumbent water, might have undergone far more metamorphic action than strata which have always remained nearer to the surface. the immense areas in some parts of the world, for instance in south america, of bare metamorphic rocks, which must have been heated under great pressure, have always seemed to me to require some special explanation; and we may perhaps believe that we see in these large areas, the many formations long anterior to the silurian epoch in a completely metamorphosed condition. the several difficulties here discussed, namely our not finding in the successive formations infinitely numerous transitional links between the many species which now exist or have existed; the sudden manner { } in which whole groups of species appear in our european formations; the almost entire absence, as at present known, of fossiliferous formations beneath the silurian strata, are all undoubtedly of the gravest nature. we see this in the plainest manner by the fact that all the most eminent palæontologists, namely cuvier, agassiz, barrande, falconer, e. forbes, &c., and all our greatest geologists, as lyell, murchison, sedgwick, &c., have unanimously, often vehemently, maintained the immutability of species. but i have reason to believe that one great authority, sir charles lyell, from further reflexion entertains grave doubts on this subject. i feel how rash it is to differ from these authorities, to whom, with others, we owe all our knowledge. those who think the natural geological record in any degree perfect, and who do not attach much weight to the facts and arguments of other kinds given in this volume, will undoubtedly at once reject my theory. for my part, following out lyell's metaphor, i look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. each word of the slowly-changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely separated, formations. on this view, the difficulties above discussed are greatly diminished, or even disappear. * * * * * { } chapter x. on the geological succession of organic beings. on the slow and successive appearance of new species--on their different rates of change--species once lost do not reappear--groups of species follow the same general rules in their appearance and disappearance as do single species--on extinction--on simultaneous changes in the forms of life throughout the world--on the affinities of extinct species to each other and to living species--on the state of development of ancient forms--on the succession of the same types within the same areas--summary of preceding and present chapters. let us now see whether the several facts and rules relating to the geological succession of organic beings, better accord with the common view of the immutability of species, or with that of their slow and gradual modification, through descent and natural selection. new species have appeared very slowly, one after another, both on the land and in the waters. lyell has shown that it is hardly possible to resist the evidence on this head in the case of the several tertiary stages; and every year tends to fill up the blanks between them, and to make the percentage system of lost and new forms more gradual. in some of the most recent beds, though undoubtedly of high antiquity if measured by years, only one or two species are lost forms, and only one or two are new forms, having here appeared for the first time, either locally, or, as far as we know, on the face of the earth. if we may trust the observations of philippi in sicily, the successive changes in the marine inhabitants of that island have been many and most gradual. the secondary formations are more broken; but, as bronn has remarked, neither the appearance { } nor disappearance of their many now extinct species has been simultaneous in each separate formation. species of different genera and classes have not changed at the same rate, or in the same degree. in the oldest tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms. falconer has given a striking instance of a similar fact, in an existing crocodile associated with many strange and lost mammals and reptiles in the sub-himalayan deposits. the silurian lingula differs but little from the living species of this genus; whereas most of the other silurian molluscs and all the crustaceans have changed greatly. the productions of the land seem to change at a quicker rate than those of the sea, of which a striking instance has lately been observed in switzerland. there is some reason to believe that organisms, considered high in the scale of nature, change more quickly than those that are low: though there are exceptions to this rule. the amount of organic change, as pictet has remarked, does not strictly correspond with the succession of our geological formations; so that between each two consecutive formations, the forms of life have seldom changed in exactly the same degree. yet if we compare any but the most closely related formations, all the species will be found to have undergone some change. when a species has once disappeared from the face of the earth, we have reason to believe that the same identical form never reappears. the strongest apparent exception to this latter rule, is that of the so-called "colonies" of m. barrande, which intrude for a period in the midst of an older formation, and then allow the pre-existing fauna to reappear; but lyell's explanation, namely, that it is a case of temporary migration from a distinct geographical province, seems to me satisfactory. { } these several facts accord well with my theory. i believe in no fixed law of development, causing all the inhabitants of a country to change abruptly, or simultaneously, or to an equal degree. the process of modification must be extremely slow. the variability of each species is quite independent of that of all others. whether such variability be taken advantage of by natural selection, and whether the variations be accumulated to a greater or lesser amount, thus causing a greater or lesser amount of modification in the varying species, depends on many complex contingencies,--on the variability being of a beneficial nature, on the power of intercrossing, on the rate of breeding, on the slowly changing physical conditions of the country, and more especially on the nature of the other inhabitants with which the varying species comes into competition. hence it is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, that it should change less. we see the same fact in geographical distribution; for instance, in the land-shells and coleopterous insects of madeira having come to differ considerably from their nearest allies on the continent of europe, whereas the marine shells and birds have remained unaltered. we can perhaps understand the apparently quicker rate of change in terrestrial and in more highly organised productions compared with marine and lower productions, by the more complex relations of the higher beings to their organic and inorganic conditions of life, as explained in a former chapter. when many of the inhabitants of a country have become modified and improved, we can understand, on the principle of competition, and on that of the many all-important relations of organism to organism, that any form which does not become in some degree modified and improved, { } will be liable to be exterminated. hence we can see why all the species in the same region do at last, if we look to wide enough intervals of time, become modified; for those which do not change will become extinct. in members of the same class the average amount of change, during long and equal periods of time, may, perhaps, be nearly the same; but as the accumulation of long-enduring fossiliferous formations depends on great masses of sediment having been deposited on areas whilst subsiding, our formations have been almost necessarily accumulated at wide and irregularly intermittent intervals; consequently the amount of organic change exhibited by the fossils embedded in consecutive formations is not equal. each formation, on this view, does not mark a new and complete act of creation, but only an occasional scene, taken almost at hazard, in a slowly changing drama. we can clearly understand why a species when once lost should never reappear, even if the very same conditions of life, organic and inorganic, should recur. for though the offspring of one species might be adapted (and no doubt this has occurred in innumerable instances) to fill the exact place of another species in the economy of nature, and thus supplant it; yet the two forms--the old and the new--would not be identically the same; for both would almost certainly inherit different characters from their distinct progenitors. for instance, it is just possible, if our fantail-pigeons were all destroyed, that fanciers, by striving during long ages for the same object, might make a new breed hardly distinguishable from our present fantail; but if the parent rock-pigeon were also destroyed, and in nature we have every reason to believe that the parent-form will generally be supplanted and exterminated by its improved offspring, it is quite { } incredible that a fantail, identical with the existing breed, could be raised from any other species of pigeon, or even from the other well-established races of the domestic pigeon, for the newly-formed fantail would be almost sure to inherit from its new progenitor some slight characteristic differences. groups of species, that is, genera and families, follow the same general rules in their appearance and disappearance as do single species, changing more or less quickly, and in a greater or lesser degree. a group does not reappear after it has once disappeared; or its existence, as long as it lasts, is continuous. i am aware that there are some apparent exceptions to this rule, but the exceptions are surprisingly few, so few that e. forbes, pictet, and woodward (though all strongly opposed to such views as i maintain) admit its truth; and the rule strictly accords with my theory. for as all the species of the same group have descended from some one species, it is clear that as long as any species of the group have appeared in the long succession of ages, so long must its members have continuously existed, in order to have generated either new and modified or the same old and unmodified forms. species of the genus lingula, for instance, must have continuously existed by an unbroken succession of generations, from the lowest silurian stratum to the present day. we have seen in the last chapter that the species of a group sometimes falsely appear to have come in abruptly; and i have attempted to give an explanation of this fact, which if true would have been fatal to my views. but such cases are certainly exceptional; the general rule being a gradual increase in number, till the group reaches its maximum, and then, sooner or later, it gradually decreases. if the number of the species of a genus, or the number of { } the genera of a family, be represented by a vertical line of varying thickness, crossing the successive geological formations in which the species are found, the line will sometimes falsely appear to begin at its lower end, not in a sharp point, but abruptly; it then gradually thickens upwards, sometimes keeping for a space of equal thickness, and ultimately thins out in the upper beds, marking the decrease and final extinction of the species. this gradual increase in number of the species of a group is strictly conformable with my theory; as the species of the same genus, and the genera of the same family, can increase only slowly and progressively; for the process of modification and the production of a number of allied forms must be slow and gradual,--one species giving rise first to two or three varieties, these being slowly converted into species, which in their turn produce by equally slow steps other species, and so on, like the branching of a great tree from a single stem, till the group becomes large. _on extinction._--we have as yet spoken only incidentally of the disappearance of species and of groups of species. on the theory of natural selection the extinction of old forms and the production of new and improved forms are intimately connected together. the old notion of all the inhabitants of the earth having been swept away at successive periods by catastrophes, is very generally given up, even by those geologists, as elie de beaumont, murchison, barrande, &c., whose general views would naturally lead them to this conclusion. on the contrary, we have every reason to believe, from the study of the tertiary formations, that species and groups of species gradually disappear, one after another, first from one spot, then from another, and finally from the world. both single species and whole { } groups of species last for very unequal periods; some groups, as we have seen, having endured from the earliest known dawn of life to the present day; some having disappeared before the close of the palæozoic period. no fixed law seems to determine the length of time during which any single species or any single genus endures. there is reason to believe that the complete extinction of the species of a group is generally a slower process than their production: if the appearance and disappearance of a group of species be represented, as before, by a vertical line of varying thickness, the line is found to taper more gradually at its upper end, which marks the progress of extermination, than at its lower end, which marks the first appearance and increase in numbers of the species. in some cases, however, the extermination of whole groups of beings, as of ammonites towards the close of the secondary period, has been wonderfully sudden. the whole subject of the extinction of species has been involved in the most gratuitous mystery. some authors have even supposed that as the individual has a definite length of life, so have species a definite duration. no one i think can have marvelled more at the extinction of species, than i have done. when i found in la plata the tooth of a horse embedded with the remains of mastodon, megatherium, toxodon, and other extinct monsters, which all co-existed with still living shells at a very late geological period, i was filled with astonishment; for seeing that the horse, since its introduction by the spaniards into south america, has run wild over the whole country and has increased in numbers at an unparalleled rate, i asked myself what could so recently have exterminated the former horse under conditions of life apparently so favourable. but how utterly groundless was my astonishment! { } professor owen soon perceived that the tooth, though so like that of the existing horse, belonged to an extinct species. had this horse been still living, but in some degree rare, no naturalist would have felt the least surprise at its rarity; for rarity is the attribute of a vast number of species of all classes, in all countries. if we ask ourselves why this or that species is rare, we answer that something is unfavourable in its conditions of life; but what that something is, we can hardly ever tell. on the supposition of the fossil horse still existing as a rare species, we might have felt certain from the analogy of all other mammals, even of the slow-breeding elephant, and from the history of the naturalisation of the domestic horse in south america, that under more favourable conditions it would in a very few years have stocked the whole continent. but we could not have told what the unfavourable conditions were which checked its increase, whether some one or several contingencies, and at what period of the horse's life, and in what degree, they severally acted. if the conditions had gone on, however slowly, becoming less and less favourable, we assuredly should not have perceived the fact, yet the fossil horse would certainly have become rarer and rarer, and finally extinct;--its place being seized on by some more successful competitor. it is most difficult always to remember that the increase of every living being is constantly being checked by unperceived injurious agencies; and that these same unperceived agencies are amply sufficient to cause rarity, and finally extinction. we see in many cases in the more recent tertiary formations, that rarity precedes extinction; and we know that this has been the progress of events with those animals which have been exterminated, either locally or wholly, through { } man's agency. i may repeat what i published in , namely, that to admit that species generally become rare before they become extinct--to feel no surprise at the rarity of a species, and yet to marvel greatly when it ceases to exist, is much the same as to admit that sickness in the individual is the forerunner of death--to feel no surprise at sickness, but when the sick man dies, to wonder and to suspect that he died by some unknown deed of violence. the theory of natural selection is grounded on the belief that each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of less-favoured forms almost inevitably follows. it is the same with our domestic productions: when a new and slightly improved variety has been raised, it at first supplants the less improved varieties in the same neighbourhood; when much improved it is transported far and near, like our short-horn cattle, and takes the place of other breeds in other countries. thus the appearance of new forms and the disappearance of old forms, both natural and artificial, are bound together. in certain flourishing groups, the number of new specific forms which have been produced within a given time is probably greater than that of the old specific forms which have been exterminated; but we know that the number of species has not gone on indefinitely increasing, at least during the later geological periods, so that looking to later times we may believe that the production of new forms has caused the extinction of about the same number of old forms. the competition will generally be most severe, as formerly explained and illustrated by examples, between the forms which are most like each other in all respects. { } hence the improved and modified descendants of a species will generally cause the extermination of the parent-species; and if many new forms have been developed from any one species, the nearest allies of that species, _i.e._ the species of the same genus, will be the most liable to extermination. thus, as i believe, a number of new species descended from one species, that is a new genus, comes to supplant an old genus, belonging to the same family. but it must often have happened that a new species belonging to some one group will have seized on the place occupied by a species belonging to a distinct group, and thus caused its extermination; and if many allied forms be developed from the successful intruder, many will have to yield their places; and it will generally be allied forms, which will suffer from some inherited inferiority in common. but whether it be species belonging to the same or to a distinct class, which yield their places to other species which have been modified and improved, a few of the sufferers may often long be preserved, from being fitted to some peculiar line of life, or from inhabiting some distant and isolated station, where they have escaped severe competition. for instance, a single species of trigonia, a great genus of shells in the secondary formations, survives in the australian seas; and a few members of the great and almost extinct group of ganoid fishes still inhabit our fresh waters. therefore the utter extinction of a group is generally, as we have seen, a slower process than its production. with respect to the apparently sudden extermination of whole families or orders, as of trilobites at the close of the palæozoic period and of ammonites at the close of the secondary period, we must remember what has been already said on the probable wide intervals of time { } between our consecutive formations; and in these intervals there may have been much slow extermination. moreover, when by sudden immigration or by unusually rapid development, many species of a new group have taken possession of a new area, they will have exterminated in a correspondingly rapid manner many of the old inhabitants; and the forms which thus yield their places will commonly be allied, for they will partake of some inferiority in common. thus, as it seems to me, the manner in which single species and whole groups of species become extinct, accords well with the theory of natural selection. we need not marvel at extinction; if we must marvel, let it be at our presumption in imagining for a moment that we understand the many complex contingencies, on which the existence of each species depends. if we forget for an instant, that each species tends to increase inordinately, and that some check is always in action, yet seldom perceived by us, the whole economy of nature will be utterly obscured. whenever we can precisely say why this species is more abundant in individuals than that; why this species and not another can be naturalised in a given country; then, and not till then, we may justly feel surprise why we cannot account for the extinction of this particular species or group of species. _on the forms of life changing almost simultaneously throughout the world._--scarcely any palæontological discovery is more striking than the fact, that the forms of life change almost simultaneously throughout the world. thus our european chalk formation can be recognised in many distant parts of the world, under the most different climates, where not a fragment of the mineral chalk itself can be found; namely, in north { } america, in equatorial south america, in tierra del fuego, at the cape of good hope, and in the peninsula of india. for at these distant points, the organic remains in certain beds present an unmistakeable degree of resemblance to those of the chalk. it is not that the same species are met with; for in some cases not one species is identically the same, but they belong to the same families, genera, and sections of genera, and sometimes are similarly characterised in such trifling points as mere superficial sculpture. moreover other forms, which are not found in the chalk of europe, but which occur in the formations either above or below, are similarly absent at these distant points of the world. in the several successive palæozoic formations of russia, western europe and north america, a similar parallelism in the forms of life has been observed by several authors: so it is, according to lyell, with the several european and north american tertiary deposits. even if the few fossil species which are common to the old and new worlds be kept wholly out of view, the general parallelism in the successive forms of life, in the stages of the widely separated palæozoic and tertiary periods, would still be manifest, and the several formations could be easily correlated. these observations, however, relate to the marine inhabitants of distant parts of the world: we have not sufficient data to judge whether the productions of the land and of fresh water change at distant points in the same parallel manner. we may doubt whether they have thus changed: if the megatherium, mylodon, macrauchenia, and toxodon had been brought to europe from la plata, without any information in regard to their geological position, no one would have suspected that they had co-existed with still living sea-shells; but as these anomalous monsters co-existed with the { } mastodon and horse, it might at least have been inferred that they had lived during one of the later tertiary stages. when the marine forms of life are spoken of as having changed simultaneously throughout the world, it must not be supposed that this expression relates to the same thousandth or hundred-thousandth year, or even that it has a very strict geological sense; for if all the marine animals which live at the present day in europe, and all those that lived in europe during the pleistocene period (an enormously remote period as measured by years, including the whole glacial epoch), were to be compared with those now living in south america or in australia, the most skilful naturalist would hardly be able to say whether the existing or the pleistocene inhabitants of europe resembled most closely those of the southern hemisphere. so, again, several highly competent observers believe that the existing productions of the united states are more closely related to those which lived in europe during certain later tertiary stages, than to those which now live here; and if this be so, it is evident that fossiliferous beds deposited at the present day on the shores of north america would hereafter be liable to be classed with somewhat older european beds. nevertheless, looking to a remotely future epoch, there can, i think, be little doubt that all the more modern _marine_ formations, namely, the upper pliocene, the pleistocene and strictly modern beds, of europe, north and south america, and australia, from containing fossil remains in some degree allied, and from not including those forms which are only found in the older underlying deposits, would be correctly ranked as simultaneous in a geological sense. the fact of the forms of life changing simultaneously, in the above large sense, at distant parts of the world, has greatly struck those admirable observers, mm. { } de verneuil and d'archiac. after referring to the parallelism of the palæozoic forms of life in various parts of europe, they add, "if struck by this strange sequence, we turn our attention to north america, and there discover a series of analogous phenomena, it will appear certain that all these modifications of species, their extinction, and the introduction of new ones, cannot be owing to mere changes in marine currents or other causes more or less local and temporary, but depend on general laws which govern the whole animal kingdom." m. barrande has made forcible remarks to precisely the same effect. it is, indeed, quite futile to look to changes of currents, climate, or other physical conditions, as the cause of these great mutations in the forms of life throughout the world, under the most different climates. we must, as barrande has remarked, look to some special law. we shall see this more clearly when we treat of the present distribution of organic beings, and find how slight is the relation between the physical conditions of various countries, and the nature of their inhabitants. this great fact of the parallel succession of the forms of life throughout the world, is explicable on the theory of natural selection. new species are formed by new varieties arising, which have some advantage over older forms; and those forms, which are already dominant, or have some advantage over the other forms in their own country, would naturally oftenest give rise to new varieties or incipient species; for these latter must be victorious in a still higher degree in order to be preserved and to survive. we have distinct evidence on this head, in the plants which are dominant, that is, which are commonest in their own homes, and are most widely diffused, having produced the greatest number of new varieties. it is also natural that the { } dominant, varying, and far-spreading species, which already have invaded to a certain extent the territories of other species, should be those which would have the best chance of spreading still further, and of giving rise in new countries to new varieties and species. the process of diffusion may often be very slow, being dependent on climatal and geographical changes, or on strange accidents, but in the long run the dominant forms will generally succeed in spreading. the diffusion would, it is probable, be slower with the terrestrial inhabitants of distinct continents than with the marine inhabitants of the continuous sea. we might therefore expect to find, as we apparently do find, a less strict degree of parallel succession in the productions of the land than of the sea. dominant species spreading from any region might encounter still more dominant species, and then their triumphant course, or even their existence, would cease. we know not at all precisely what are all the conditions most favourable for the multiplication of new and dominant species; but we can, i think, clearly see that a number of individuals, from giving a better chance of the appearance of favourable variations, and that severe competition with many already existing forms, would be highly favourable, as would be the power of spreading into new territories. a certain amount of isolation, recurring at long intervals of time, would probably be also favourable, as before explained. one quarter of the world may have been most favourable for the production of new and dominant species on the land, and another for those in the waters of the sea. if two great regions had been for a long period favourably circumstanced in an equal degree, whenever their inhabitants met, the battle would be prolonged and severe; and some from one birthplace and some from the other might be victorious. but in the course of time, the { } forms dominant in the highest degree, wherever produced, would tend everywhere to prevail. as they prevailed, they would cause the extinction of other and inferior forms; and as these inferior forms would be allied in groups by inheritance, whole groups would tend slowly to disappear; though here and there a single member might long be enabled to survive. thus, as it seems to me, the parallel, and, taken in a large sense, simultaneous, succession of the same forms of life throughout the world, accords well with the principle of new species having been formed by dominant species spreading widely and varying; the new species thus produced being themselves dominant owing to inheritance, and to having already had some advantage over their parents or over other species; these again spreading, varying, and producing new species. the forms which are beaten and which yield their places to the new and victorious forms, will generally be allied in groups, from inheriting some inferiority in common; and therefore as new and improved groups spread throughout the world, old groups will disappear from the world; and the succession of forms in both ways will everywhere tend to correspond. there is one other remark connected with this subject worth making. i have given my reasons for believing that all our greater fossiliferous formations were deposited during periods of subsidence; and that blank intervals of vast duration occurred during the periods when the bed of the sea was either stationary or rising, and likewise when sediment was not thrown down quickly enough to embed and preserve organic remains. during these long and blank intervals i suppose that the inhabitants of each region underwent a considerable amount of modification and extinction, and that there was much migration from { } other parts of the world. as we have reason to believe that large areas are affected by the same movement, it is probable that strictly contemporaneous formations have often been accumulated over very wide spaces in the same quarter of the world; but we are far from having any right to conclude that this has invariably been the case, and that large areas have invariably been affected by the same movements. when two formations have been deposited in two regions during nearly, but not exactly the same period, we should find in both, from the causes explained in the foregoing paragraphs, the same general succession in the forms of life; but the species would not exactly correspond; for there will have been a little more time in the one region than in the other for modification, extinction, and immigration. i suspect that cases of this nature occur in europe. mr. prestwich, in his admirable memoirs on the eocene deposits of england and france, is able to draw a close general parallelism between the successive stages in the two countries; but when he compares certain stages in england with those in france, although he finds in both a curious accordance in the numbers of the species belonging to the same genera, yet the species themselves differ in a manner very difficult to account for, considering the proximity of the two areas,--unless, indeed, it be assumed that an isthmus separated two seas inhabited by distinct, but contemporaneous, faunas. lyell has made similar observations on some of the later tertiary formations. barrande, also, shows that there is a striking general parallelism in the successive silurian deposits of bohemia and scandinavia; nevertheless he finds a surprising amount of difference in the species. if the several formations in these regions have not been deposited during the same exact { } periods,--a formation in one region often corresponding with a blank interval in the other,--and if in both regions the species have gone on slowly changing during the accumulation of the several formations and during the long intervals of time between them; in this case, the several formations in the two regions could be arranged in the same order, in accordance with the general succession of the form of life, and the order would falsely appear to be strictly parallel; nevertheless the species would not all be the same in the apparently corresponding stages in the two regions. _on the affinities of extinct species to each other, and to living forms._--let us now look to the mutual affinities of extinct and living species. they all fall into one grand natural system; and this fact is at once explained on the principle of descent. the more ancient any form is, the more, as a general rule, it differs from living forms. but, as buckland long ago remarked, all fossils can be classed either in still existing groups, or between them. that the extinct forms of life help to fill up the wide intervals between existing genera, families, and orders, cannot be disputed. for if we confine our attention either to the living or to the extinct alone, the series is far less perfect than if we combine both into one general system. with respect to the vertebrata, whole pages could be filled with striking illustrations from our great palaeontologist, owen, showing how extinct animals fall in between existing groups. cuvier ranked the ruminants and pachyderms, as the two most distinct orders of mammals; but owen has discovered so many fossil links, that he has had to alter the whole classification of these two orders; and has placed certain pachyderms in the same sub-order with ruminants: for example, he dissolves by fine gradations the apparently { } wide difference between the pig and the camel. in regard to the invertebrata, barrande, and a higher authority could not be named, asserts that he is every day taught that palaeozoic animals, though belonging to the same orders, families, or genera with those living at the present day, were not at this early epoch limited in such distinct groups as they now are. some writers have objected to any extinct species or group of species being considered as intermediate between living species or groups. if by this term it is meant that an extinct form is directly intermediate in all its characters between two living forms, the objection is probably valid. but i apprehend that in a perfectly natural classification many fossil species would have to stand between living species, and some extinct genera between living genera, even between genera belonging to distinct families. the most common case, especially with respect to very distinct groups, such as fish and reptiles, seems to be, that supposing them to be distinguished at the present day from each other by a dozen characters, the ancient members of the same two groups would be distinguished by a somewhat lesser number of characters, so that the two groups, though formerly quite distinct, at that period made some small approach to each other. it is a common belief that the more ancient a form is, by so much the more it tends to connect by some of its characters groups now widely separated from each other. this remark no doubt must be restricted to those groups which have undergone much change in the course of geological ages; and it would be difficult to prove the truth of the proposition, for every now and then even a living animal, as the lepidosiren, is discovered having affinities directed towards very distinct groups. yet if we compare the older reptiles and { } batrachians, the older fish, the older cephalopods, and the eocene mammals, with the more recent members of the same classes, we must admit that there is some truth in the remark. let us see how far these several facts and inferences accord with the theory of descent with modification. as the subject is somewhat complex, i must request the reader to turn to the diagram in the fourth chapter. we may suppose that the numbered letters represent genera, and the dotted lines diverging from them the species in each genus. the diagram is much too simple, too few genera and too few species being given, but this is unimportant for us. the horizontal lines may represent successive geological formations, and all the forms beneath the uppermost line may be considered as extinct. the three existing genera, a^{ }, q^{ }, p^{ }, will form a small family; b^{ } and f^{ } a closely allied family or sub-family; and o^{ }, e^{ }, m^{ }, a third family. these three families, together with the many extinct genera on the several lines of descent diverging from the parent-form (a), will form an order; for all will have inherited something in common from their ancient and common progenitor. on the principle of the continued tendency to divergence of character, which was formerly illustrated by this diagram, the more recent any form is, the more it will generally differ from its ancient progenitor. hence we can understand the rule that the most ancient fossils differ most from existing forms. we must not, however, assume that divergence of character is a necessary contingency; it depends solely on the descendants from a species being thus enabled to seize on many and different places in the economy of nature. therefore it is quite possible, as we have seen in the case of some silurian forms, that a species might go on being slightly modified in relation to its slightly altered conditions of { } life, and yet retain throughout a vast period the same general characteristics. this is represented in the diagram by the letter f^{ }. all the many forms, extinct and recent, descended from (a), make, as before remarked, one order; and this order, from the continued effects of extinction and divergence of character, has become divided into several sub-families and families, some of which are supposed to have perished at different periods, and some to have endured to the present day. by looking at the diagram we can see that if many of the extinct forms, supposed to be embedded in the successive formations, were discovered at several points low down in the series, the three existing families on the uppermost line would be rendered less distinct from each other. if, for instance, the genera a^ , a^ , a^{ }, f^ , m^ , m^ , m^ , were disinterred, these three families would be so closely linked together that they probably would have to be united into one great family, in nearly the same manner as has occurred with ruminants and pachyderms. yet he who objected to call the extinct genera, which thus linked the living genera of three families together, intermediate in character, would be justified, as they are intermediate, not directly, but only by a long and circuitous course through many widely different forms. if many extinct forms were to be discovered above one of the middle horizontal lines or geological formations --for instance, above no. vi.--but none from beneath this line, then only the two families on the left hand (namely, a^{ }, &c., and b^{ }, &c.) would have to be united into one family; and the two other families (namely, a^{ } to f^{ } now including five genera, and o^{ } to m^{ }) would yet remain distinct. these two families, however, would be less distinct from each other than they were before the discovery of the fossils. if, for instance, we suppose the existing genera of the two families to differ from each { } other by a dozen characters, in this case the genera, at the early period marked vi., would differ by a lesser number of characters; for at this early stage of descent they have not diverged in character from the common progenitor of the order, nearly so much as they subsequently diverged. thus it comes that ancient and extinct genera are often in some slight degree intermediate in character between their modified descendants, or between their collateral relations. in nature the case will be far more complicated than is represented in the diagram; for the groups will have been more numerous, they will have endured for extremely unequal lengths of time, and will have been modified in various degrees. as we possess only the last volume of the geological record, and that in a very broken condition, we have no right to expect, except in very rare cases, to fill up wide intervals in the natural system, and thus unite distinct families or orders. all that we have a right to expect, is that those groups, which have within known geological periods undergone much modification, should in the older formations make some slight approach to each other; so that the older members should differ less from each other in some of their characters than do the existing members of the same groups; and this by the concurrent evidence of our best palæontologists seems frequently to be the case. thus, on the theory of descent with modification, the main facts with respect to the mutual affinities of the extinct forms of life to each other and to living forms, seem to me explained in a satisfactory manner. and they are wholly inexplicable on any other view. on this same theory, it is evident that the fauna of any great period in the earth's history will be intermediate in general character between that which preceded and that which succeeded it. thus, the species which lived at the sixth great stage of descent in the { } diagram are the modified offspring of those which lived at the fifth stage, and are the parents of those which became still more modified at the seventh stage; hence they could hardly fail to be nearly intermediate in character between the forms of life above and below. we must, however, allow for the entire extinction of some preceding forms, and in any one region for the immigration of new forms from other regions, and for a large amount of modification, during the long and blank intervals between the successive formations. subject to these allowances, the fauna of each geological period undoubtedly is intermediate in character, between the preceding and succeeding faunas. i need give only one instance, namely, the manner in which the fossils of the devonian system, when this system was first discovered, were at once recognised by palæontologists as intermediate in character between those of the overlying carboniferous, and underlying silurian system. but each fauna is not necessarily exactly intermediate, as unequal intervals of time have elapsed between consecutive formations. it is no real objection to the truth of the statement, that the fauna of each period as a whole is nearly intermediate in character between the preceding and succeeding faunas, that certain genera offer exceptions to the rule. for instance, mastodons and elephants, when arranged by dr. falconer in two series, first according to their mutual affinities and then according to their periods of existence, do not accord in arrangement. the species extreme in character are not the oldest, or the most recent; nor are those which are intermediate in character, intermediate in age. but supposing for an instant, in this and other such cases, that the record of the first appearance and disappearance of the species was perfect, we have no reason to believe that forms successively produced necessarily endure for { } corresponding lengths of time: a very ancient form might occasionally last much longer than a form elsewhere subsequently produced, especially in the case of terrestrial productions inhabiting separated districts. to compare small things with great: if the principal living and extinct races of the domestic pigeon were arranged as well as they could be in serial affinity, this arrangement would not closely accord with the order in time of their production, and still less with the order of their disappearance; for the parent rock-pigeon now lives; and many varieties between the rock-pigeon and the carrier have become extinct; and carriers which are extreme in the important character of length of beak originated earlier than short-beaked tumblers, which are at the opposite end of the series in this same respect. closely connected with the statement, that the organic remains from an intermediate formation are in some degree intermediate in character, is the fact, insisted on by all palæontologists, that fossils from two consecutive formations are far more closely related to each other, than are the fossils from two remote formations. pictet gives as a well-known instance, the general resemblance of the organic remains from the several stages of the chalk formation, though the species are distinct in each stage. this fact alone, from its generality, seems to have shaken professor pictet in his firm belief in the immutability of species. he who is acquainted with the distribution of existing species over the globe, will not attempt to account for the close resemblance of the distinct species in closely consecutive formations, by the physical conditions of the ancient areas having remained nearly the same. let it be remembered that the forms of life, at least those inhabiting the sea, have changed almost simultaneously throughout the world, and therefore under the most different climates and conditions. consider the { } prodigious vicissitudes of climate during the pleistocene period, which includes the whole glacial period, and note how little the specific forms of the inhabitants of the sea have been affected. on the theory of descent, the full meaning of the fact of fossil remains from closely consecutive formations, though ranked as distinct species, being closely related, is obvious. as the accumulation of each formation has often been interrupted, and as long blank intervals have intervened between successive formations, we ought not to expect to find, as i attempted to show in the last chapter, in any one or two formations all the intermediate varieties between the species which appeared at the commencement and close of these periods; but we ought to find after intervals, very long as measured by years, but only moderately long as measured geologically, closely allied forms, or, as they have been called by some authors, representative species; and these we assuredly do find. we find, in short, such evidence of the slow and scarcely sensible mutation of specific forms, as we have a just right to expect to find. _on the state of development of ancient forms._--there has been much discussion whether recent forms are more highly developed than ancient. i will not here enter on this subject, for naturalists have not as yet defined to each other's satisfaction what is meant by high and low forms. the best definition probably is, that the higher forms have their organs more distinctly specialised for different functions; and as such division of physiological labour seems to be an advantage to each being, natural selection will constantly tend in so far to make the later and more modified forms higher than their early progenitors, or than the slightly modified descendants of such progenitors. in a more general sense the { } more recent forms must, on my theory, be higher than the more ancient; for each new species is formed by having had some advantage in the struggle for life over other and preceding forms. if under a nearly similar climate, the eocene inhabitants of one quarter of the world were put into competition with the existing inhabitants of the same or some other quarter, the eocene fauna or flora would certainly be beaten and exterminated; as would a secondary fauna by an eocene, and a palæozoic fauna by a secondary fauna. i do not doubt that this process of improvement has affected in a marked and sensible manner the organisation of the more recent and victorious forms of life, in comparison with the ancient and beaten forms; but i can see no way of testing this sort of progress. crustaceans, for instance, not the highest in their own class, may have beaten the highest molluscs. from the extraordinary manner in which european productions have recently spread over new zealand, and have seized on places which must have been previously occupied, we may believe, if all the animals and plants of great britain were set free in new zealand, that in the course of time a multitude of british forms would become thoroughly naturalized there, and would exterminate many of the natives. on the other hand, from what we see now occurring in new zealand, and from hardly a single inhabitant of the southern hemisphere having become wild in any part of europe, we may doubt, if all the productions of new zealand were set free in great britain, whether any considerable number would be enabled to seize on places now occupied by our native plants and animals. under this point of view, the productions of great britain may be said to be higher than those of new zealand. yet the most skilful naturalist from an examination of the { } species of the two countries could not have foreseen this result. agassiz insists that ancient animals resemble to a certain extent the embryos of recent animals of the same classes; or that the geological succession of extinct forms is in some degree parallel to the embryological development of recent forms. i must follow pictet and huxley in thinking that the truth of this doctrine is very far from proved. yet i fully expect to see it hereafter confirmed, at least in regard to subordinate groups, which have branched off from each other within comparatively recent times. for this doctrine of agassiz accords well with the theory of natural selection. in a future chapter i shall attempt to show that the adult differs from its embryo, owing to variations supervening at a not early age, and being inherited at a corresponding age. this process, whilst it leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more difference to the adult. thus the embryo comes to be left as a sort of picture, preserved by nature, of the ancient and less modified condition of each animal. this view may be true, and yet it may never be capable of full proof. seeing, for instance, that the oldest known mammals, reptiles, and fish strictly belong to their own proper classes, though some of these old forms are in a slight degree less distinct from each other than are the typical members of the same groups at the present day, it would be vain to look for animals having the common embryological character of the vertebrata, until beds far beneath the lowest silurian strata are discovered--a discovery of which the chance is very small. _on the succession of the same types within the same { } areas, during the later tertiary periods._--mr. clift many years ago showed that the fossil mammals from the australian caves were closely allied to the living marsupials of that continent. in south america, a similar relationship is manifest, even to an uneducated eye, in the gigantic pieces of armour like those of the armadillo, found in several parts of la plata; and professor owen has shown in the most striking manner that most of the fossil mammals, buried there in such numbers, are related to south american types. this relationship is even more clearly seen in the wonderful collection of fossil bones made by mm. lund and clausen in the caves of brazil. i was so much impressed with these facts that i strongly insisted, in and , on this "law of the succession of types,"--on "this wonderful relationship in the same continent between the dead and the living." professor owen has subsequently extended the same generalisation to the mammals of the old world. we see the same law in this author's restorations of the extinct and gigantic birds of new zealand. we see it also in the birds of the caves of brazil. mr. woodward has shown that the same law holds good with sea-shells, but from the wide distribution of most genera of molluscs, it is not well displayed by them. other cases could be added, as the relation between the extinct and living land-shells of madeira; and between the extinct and living brackish-water shells of the aralo-caspian sea. now what does this remarkable law of the succession of the same types within the same areas mean? he would be a bold man, who after comparing the present climate of australia and of parts of south america under the same latitude, would attempt to account, on the one hand, by dissimilar physical conditions for the dissimilarity of the inhabitants of these two continents, { } and, on the other hand, by similarity of conditions, for the uniformity of the same types in each during the later tertiary periods. nor can it be pretended that it is an immutable law that marsupials should have been chiefly or solely produced in australia; or that edentata and other american types should have been solely produced in south america. for we know that europe in ancient times was peopled by numerous marsupials; and i have shown in the publications above alluded to, that in america the law of distribution of terrestrial mammals was formerly different from what it now is. north america formerly partook strongly of the present character of the southern half of the continent; and the southern half was formerly more closely allied, than it is at present, to the northern half. in a similar manner we know from falconer and cautley's discoveries, that northern india was formerly more closely related in its mammals to africa than it is at the present time. analogous facts could be given in relation to the distribution of marine animals. on the theory of descent with modification, the great law of the long enduring, but not immutable, succession of the same types within the same areas, is at once explained; for the inhabitants of each quarter of the world will obviously tend to leave in that quarter, during the next succeeding period of time, closely allied though in some degree modified descendants. if the inhabitants of one continent formerly differed greatly from those of another continent, so will their modified descendants still differ in nearly the same manner and degree. but after very long intervals of time and after great geographical changes, permitting much inter-migration, the feebler will yield to the more dominant forms, and there will be nothing immutable in the laws of past and present distribution. { } it may be asked in ridicule, whether i suppose that the megatherium and other allied huge monsters have left behind them in south america, the sloth, armadillo, and anteater, as their degenerate descendants. this cannot for an instant be admitted. these huge animals have become wholly extinct, and have left no progeny. but in the caves of brazil, there are many extinct species which are closely allied in size and in other characters to the species still living in south america; and some of these fossils may be the actual progenitors of living species. it must not be forgotten that, on my theory, all the species of the same genus have descended from some one species; so that if six genera, each having eight species, be found in one geological formation, and in the next succeeding formation there be six other allied or representative genera with the same number of species, then we may conclude that only one species of each of the six older genera has left modified descendants, constituting the six new genera. the other seven species of the old genera have all died out and have left no progeny. or, which would probably be a far commoner case, two or three species of two or three alone of the six older genera will have been the parents of the six new genera; the other old species and the other whole old genera having become utterly extinct. in failing orders, with the genera and species decreasing in numbers, as apparently is the case of the edentata of south america, still fewer genera and species will have left modified blood-descendants. _summary of the preceding and present chapters._--i have attempted to show that the geological record is extremely imperfect; that only a small portion of the globe has been geologically explored with care; that { } only certain classes of organic beings have been largely preserved in a fossil state; that the number both of specimens and of species, preserved in our museums, is absolutely as nothing compared with the incalculable number of generations which must have passed away even during a single formation; that, owing to subsidence being necessary for the accumulation of fossiliferous deposits thick enough to resist future degradation, enormous intervals of time have elapsed between the successive formations; that there has probably been more extinction during the periods of subsidence, and more variation during the periods of elevation, and during the latter the record will have been least perfectly kept; that each single formation has not been continuously deposited; that the duration of each formation is, perhaps, short compared with the average duration of specific forms; that migration has played an important part in the first appearance of new forms in any one area and formation; that widely ranging species are those which have varied most, and have oftenest given rise to new species; and that varieties have at first often been local. all these causes taken conjointly, must have tended to make the geological record extremely imperfect, and will to a large extent explain why we do not find interminable varieties, connecting together all the extinct and existing forms of life by the finest graduated steps. he who rejects these views on the nature of the geological record, will rightly reject my whole theory. for he may ask in vain where are the numberless transitional links which must formerly have connected the closely allied or representative species, found in the several stages of the same great formation. he may disbelieve in the enormous intervals of time which have elapsed between our consecutive formations; he { } may overlook how important a part migration must have played, when the formations of any one great region alone, as that of europe, are considered; he may urge the apparent, but often falsely apparent, sudden coming in of whole groups of species. he may ask where are the remains of those infinitely numerous organisms which must have existed long before the first bed of the silurian system was deposited: i can answer this latter question only hypothetically, by saying that as far as we can see, where our oceans now extend they have for an enormous period extended, and where our oscillating continents now stand they have stood ever since the silurian epoch; but that long before that period, the world may have presented a wholly different aspect; and that the older continents, formed of formations older than any known to us, may now all be in a metamorphosed condition, or may lie buried under the ocean. passing from these difficulties, all the other great leading facts in palæontology seem to me simply to follow on the theory of descent with modification through natural selection. we can thus understand how it is that new species come in slowly and successively; how species of different classes do not necessarily change together, or at the same rate, or in the same degree; yet in the long run that all undergo modification to some extent. the extinction of old forms is the almost inevitable consequence of the production of new forms. we can understand why when a species has once disappeared it never reappears. groups of species increase in numbers slowly, and endure for unequal periods of time; for the process of modification is necessarily slow, and depends on many complex contingencies. the dominant species of the larger dominant groups tend to leave many modified { } descendants, and thus new sub-groups and groups are formed. as these are formed, the species of the less vigorous groups, from their inferiority inherited from a common progenitor, tend to become extinct together, and to leave no modified offspring on the face of the earth. but the utter extinction of a whole group of species may often be a very slow process, from the survival of a few descendants, lingering in protected and isolated situations. when a group has once wholly disappeared, it does not reappear; for the link of generation has been broken. we can understand how the spreading of the dominant forms of life, which are those that oftenest vary, will in the long run tend to people the world with allied, but modified, descendants; and these will generally succeed in taking the places of those groups of species which are their inferiors in the struggle for existence. hence, after long intervals of time, the productions of the world will appear to have changed simultaneously. we can understand how it is that all the forms of life, ancient and recent, make together one grand system; for all are connected by generation. we can understand, from the continued tendency to divergence of character, why the more ancient a form is, the more it generally differs from those now living. why ancient and extinct forms often tend to fill up gaps between existing forms, sometimes blending two groups previously classed as distinct into one; but more commonly only bringing them a little closer together. the more ancient a form is, the more often, apparently, it displays characters in some degree intermediate between groups now distinct; for the more ancient a form is, the more nearly it will be related to, and consequently resemble, the common progenitor of groups, since { } become widely divergent. extinct forms are seldom directly intermediate between existing forms; but are intermediate only by a long and circuitous course through many extinct and very different forms. we can clearly see why the organic remains of closely consecutive formations are more closely allied to each other, than are those of remote formations; for the forms are more closely linked together by generation: we can clearly see why the remains of an intermediate formation are intermediate in character. the inhabitants of each successive period in the world's history have beaten their predecessors in the race for life, and are, in so far, higher in the scale of nature; and this may account for that vague yet ill-defined sentiment, felt by many palæontologists, that organisation on the whole has progressed. if it should hereafter be proved that ancient animals resemble to a certain extent the embryos of more recent animals of the same class, the fact will be intelligible. the succession of the same types of structure within the same areas during the later geological periods ceases to be mysterious, and is simply explained by inheritance. if then the geological record be as imperfect as i believe it to be, and it may at least be asserted that the record cannot be proved to be much more perfect, the main objections to the theory of natural selection are greatly diminished or disappear. on the other hand, all the chief laws of palæontology plainly proclaim, as it seems to me, that species have been produced by ordinary generation: old forms having been supplanted by new and improved forms of life, produced by the laws of variation still acting round us, and preserved by natural selection. * * * * * { } chapter xi. geographical distribution. present distribution cannot be accounted for by differences in physical conditions--importance of barriers--affinity of the productions of the same continent--centres of creation--means of dispersal, by changes of climate and of the level of the land, and by occasional means--dispersal during the glacial period co-extensive with the world. in considering the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions. of late, almost every author who has studied the subject has come to this conclusion. the case of america alone would almost suffice to prove its truth: for if we exclude the northern parts where the circumpolar land is almost continuous, all authors agree that one of the most fundamental divisions in geographical distribution is that between the new and old worlds; yet if we travel over the vast american continent, from the central parts of the united states to its extreme southern point, we meet with the most diversified conditions; the most humid districts, arid deserts, lofty mountains, grassy plains, forests, marshes, lakes, and great rivers, under almost every temperature. there is hardly a climate or condition in the old world which cannot be paralleled in the new--at least as closely as the same species generally require; for it is a most rare case to find a group of organisms confined to any small spot, having conditions peculiar in only a slight { } degree; for instance, small areas in the old world could be pointed out hotter than any in the new world, yet these are not inhabited by a peculiar fauna or flora. notwithstanding this parallelism in the conditions of the old and new worlds, how widely different are their living productions! in the southern hemisphere, if we compare large tracts of land in australia, south africa, and western south america, between latitudes ° and °, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. or again we may compare the productions of south america south of lat. ° with those north of °, which consequently inhabit a considerably different climate, and they will be found incomparably more closely related to each other, than they are to the productions of australia or africa under nearly the same climate. analogous facts could be given with respect to the inhabitants of the sea. a second great fact which strikes us in our general review is, that barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions. we see this in the great difference of nearly all the terrestrial productions of the new and old worlds, excepting in the northern parts, where the land almost joins, and where, under a slightly different climate, there might have been free migration for the northern temperate forms, as there now is for the strictly arctic productions. we see the same fact in the great difference between the inhabitants of australia, africa, and south america under the same latitude: for these countries are almost as much isolated from each other as is possible. on each continent, also, we see the same fact; for on the opposite sides of { } lofty and continuous mountain-ranges, and of great deserts, and sometimes even of large rivers, we find different productions; though as mountain-chains, deserts, &c., are not as impassable, or likely to have endured so long as the oceans separating continents, the differences are very inferior in degree to those characteristic of distinct continents. turning to the sea, we find the same law. no two marine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of south and central america; yet these great faunas are separated only by the narrow, but impassable, isthmus of panama. westward of the shores of america, a wide space of open ocean extends, with not an island as a halting-place for emigrants; here we have a barrier of another kind, and as soon as this is passed we meet in the eastern islands of the pacific, with another and totally distinct fauna. so that here three marine faunas range far northward and southward, in parallel lines not far from each other, under corresponding climates; but from being separated from each other by impassable barriers, either of land or open sea, they are wholly distinct. on the other hand, proceeding still further westward from the eastern islands of the tropical parts of the pacific, we encounter no impassable barriers, and we have innumerable islands as halting-places, or continuous coasts, until after travelling over a hemisphere we come to the shores of africa; and over this vast space we meet with no well-defined and distinct marine faunas. although hardly one shell, crab or fish is common to the above-named three approximate faunas of eastern and western america and the eastern pacific islands, yet many fish range from the pacific into the indian ocean, and many shells are common to the eastern islands of the pacific { } and the eastern shores of africa, on almost exactly opposite meridians of longitude. a third great fact, partly included in the foregoing statements, is the affinity of the productions of the same continent or sea, though the species themselves are distinct at different points and stations. it is a law of the widest generality, and every continent offers innumerable instances. nevertheless the naturalist in travelling, for instance, from north to south never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other. he hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. the plains near the straits of magellan are inhabited by one species of rhea (american ostrich), and northward the plains of la plata by another species of the same genus; and not by a true ostrich or emu, like those found in africa and australia under the same latitude. on these same plains of la plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits and belonging to the same order of rodents, but they plainly display an american type of structure. we ascend the lofty peaks of the cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the american type. innumerable other instances could be given. if we look to the islands off the american shore, however much they may differ in geological structure, the inhabitants, though they may be all peculiar species, are essentially american. we may look back to past ages, as shown in the last chapter, and we find american types then prevalent on { } the american continent and in the american seas. we see in these facts some deep organic bond, prevailing throughout space and time, over the same areas of land and water, and independent of their physical conditions. the naturalist must feel little curiosity, who is not led to inquire what this bond is. this bond, on my theory, is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like, or, as we see in the case of varieties, nearly like each other. the dissimilarity of the inhabitants of different regions may be attributed to modification through natural selection, and in a quite subordinate degree to the direct influence of different physical conditions. the degree of dissimilarity will depend on the migration of the more dominant forms of life from one region into another having been effected with more or less ease, at periods more or less remote;--on the nature and number of the former immigrants;--and on their action and reaction, in their mutual struggles for life;--the relation of organism to organism being, as i have already often remarked, the most important of all relations. thus the high importance of barriers comes into play by checking migration; as does time for the slow process of modification through natural selection. widely-ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely-extended homes will have the best chance of seizing on new places, when they spread into new countries. in their new homes they will be exposed to new conditions, and will frequently undergo further modification and improvement; and thus they will become still further victorious, and will produce groups of modified descendants. on this principle of inheritance with modification, we can understand how it is that sections of genera, whole genera, { } and even families are confined to the same areas, as is so commonly and notoriously the case. i believe, as was remarked in the last chapter, in no law of necessary development. as the variability of each species is an independent property, and will be taken advantage of by natural selection, only so far as it profits the individual in its complex struggle for life, so the degree of modification in different species will be no uniform quantity. if, for instance, a number of species, which stand in direct competition with each other, migrate in a body into a new and afterwards isolated country, they will be little liable to modification; for neither migration nor isolation in themselves can do anything. these principles come into play only by bringing organisms into new relations with each other, and in a lesser degree with the surrounding physical conditions. as we have seen in the last chapter that some forms have retained nearly the same character from an enormously remote geological period, so certain species have migrated over vast spaces, and have not become greatly modified. on these views, it is obvious, that the several species of the same genus, though inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they have descended from the same progenitor. in the case of those species, which have undergone during whole geological periods but little modification, there is not much difficulty in believing that they may have migrated from the same region; for during the vast geographical and climatal changes which will have supervened since ancient times, almost any amount of migration is possible. but in many other cases, in which we have reason to believe that the species of a genus have been produced within comparatively recent times, there is great difficulty on this head. it { } is also obvious that the individuals of the same species, though now inhabiting distant and isolated regions, must have proceeded from one spot, where their parents were first produced: for, as explained in the last chapter, it is incredible that individuals identically the same should ever have been produced through natural selection from parents specifically distinct. we are thus brought to the question which has been largely discussed by naturalists, namely, whether species have been created at one or more points of the earth's surface. undoubtedly there are very many cases of extreme difficulty, in understanding how the same species could possibly have migrated from some one point to the several distant and isolated points, where now found. nevertheless the simplicity of the view that each species was first produced within a single region captivates the mind. he who rejects it, rejects the _vera causa_ of ordinary generation with subsequent migration, and calls in the agency of a miracle. it is universally admitted, that in most cases the area inhabited by a species is continuous; and when a plant or animal inhabits two points so distant from each other, or with an interval of such a nature, that the space could not be easily passed over by migration, the fact is given as something remarkable and exceptional. the capacity of migrating across the sea is more distinctly limited in terrestrial mammals, than perhaps in any other organic beings; and, accordingly, we find no inexplicable cases of the same mammal inhabiting distant points of the world. no geologist will feel any difficulty in such cases as great britain having been formerly united to europe, and consequently possessing the same quadrupeds. but if the same species can be produced at two separate points, why do we not find a single mammal common to europe and { } australia or south america? the conditions of life are nearly the same, so that a multitude of european animals and plants have become naturalised in america and australia; and some of the aboriginal plants are identically the same at these distant points of the northern and southern hemispheres? the answer, as i believe, is, that mammals have not been able to migrate, whereas some plants, from their varied means of dispersal, have migrated across the vast and broken interspace. the great and striking influence which barriers of every kind have had on distribution, is intelligible only on the view that the great majority of species have been produced on one side alone, and have not been able to migrate to the other side. some few families, many sub-families, very many genera, and a still greater number of sections of genera are confined to a single region; and it has been observed by several naturalists, that the most natural genera, or those genera in which the species are most closely related to each other, are generally local, or confined to one area. what a strange anomaly it would be, if, when coming one step lower in the series, to the individuals of the same species, a directly opposite rule prevailed; and species were not local, but had been produced in two or more distinct areas! hence it seems to me, as it has to many other naturalists, that the view of each species having been produced in one area alone, and having subsequently migrated from that area as far as its powers of migration and subsistence under past and present conditions permitted, is the most probable. undoubtedly many cases occur, in which we cannot explain how the same species could have passed from one point to the other. but the geographical and climatal changes, which have certainly occurred within recent geological times, must have interrupted or rendered discontinuous the { } formerly continuous range of many species. so that we are reduced to consider whether the exceptions to continuity of range are so numerous and of so grave a nature, that we ought to give up the belief, rendered probable by general considerations, that each species has been produced within one area, and has migrated thence as far as it could. it would be hopelessly tedious to discuss all the exceptional cases of the same species, now living at distant and separated points; nor do i for a moment pretend that any explanation could be offered of many such cases. but after some preliminary remarks, i will discuss a few of the most striking classes of facts; namely, the existence of the same species on the summits of distant mountain-ranges, and at distant points in the arctic and antarctic regions; and secondly (in the following chapter), the wide distribution of freshwater productions; and thirdly, the occurrence of the same terrestrial species on islands and on the mainland, though separated by hundreds of miles of open sea. if the existence of the same species at distant and isolated points of the earth's surface, can in many instances be explained on the view of each species having migrated from a single birthplace; then, considering our ignorance with respect to former climatal and geographical changes and various occasional means of transport, the belief that this has been the universal law, seems to me incomparably the safest. in discussing this subject, we shall be enabled at the same time to consider a point equally important for us, namely, whether the several distinct species of a genus, which on my theory have all descended from a common progenitor, can have migrated (undergoing modification during some part of their migration) from the area inhabited by their progenitor. if it can be shown to be almost invariably the case, that a region, of which { } most of its inhabitants are closely related to, or belong to the same genera with the species of a second region, has probably received at some former period immigrants from this other region, my theory will be strengthened; for we can clearly understand, on the principle of modification, why the inhabitants of a region should be related to those of another region, whence it has been stocked. a volcanic island, for instance, upheaved and formed at the distance of a few hundreds of miles from a continent, would probably receive from it in the course of time a few colonists, and their descendants, though modified, would still be plainly related by inheritance to the inhabitants of the continent. cases of this nature are common, and are, as we shall hereafter more fully see, inexplicable on the theory of independent creation. this view of the relation of species in one region to those in another, does not differ much (by substituting the word variety for species) from that lately advanced in an ingenious paper by mr. wallace, in which he concludes, that "every species has come into existence coincident both in space and time with a pre-existing closely allied species." and i now know from correspondence, that this coincidence he attributes to generation with modification. the previous remarks on "single and multiple centres of creation" do not directly bear on another allied question,--namely whether all the individuals of the same species have descended from a single pair, or single hermaphrodite, or whether, as some authors suppose, from many individuals simultaneously created. with those organic beings which never intercross (if such exist), the species, on my theory, must have descended from a succession of improved varieties, which will never have blended with other individuals or varieties, but will have supplanted each other; so that, at each { } successive stage of modification and improvement, all the individuals of each variety will have descended from a single parent. but in the majority of cases, namely, with all organisms which habitually unite for each birth, or which often intercross, i believe that during the slow process of modification the individuals of the species will have been kept nearly uniform by intercrossing; so that many individuals will have gone on simultaneously changing, and the whole amount of modification will not have been due, at each stage, to descent from a single parent. to illustrate what i mean: our english racehorses differ slightly from the horses of every other breed; but they do not owe their difference and superiority to descent from any single pair, but to continued care in selecting and training many individuals during many generations. before discussing the three classes of facts, which i have selected as presenting the greatest amount of difficulty on the theory of "single centres of creation," i must say a few words on the means of dispersal. _means of dispersal._--sir c. lyell and other authors have ably treated this subject. i can give here only the briefest abstract of the more important facts. change of climate must have had a powerful influence on migration: a region when its climate was different may have been a high road for migration, but now be impassable; i shall, however, presently have to discuss this branch of the subject in some detail. changes of level in the land must also have been highly influential: a narrow isthmus now separates two marine faunas; submerge it, or let it formerly have been submerged, and the two faunas will now blend or may formerly have blended: where the sea now extends, land may at a former period have connected islands or { } possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other. no geologist will dispute that great mutations of level have occurred within the period of existing organisms. edward forbes insisted that all the islands in the atlantic must recently have been connected with europe or africa, and europe likewise with america. other authors have thus hypothetically bridged over every ocean, and have united almost every island to some mainland. if indeed the arguments used by forbes are to be trusted, it must be admitted that scarcely a single island exists which has not recently been united to some continent. this view cuts the gordian knot of the dispersal of the same species to the most distant points, and removes many a difficulty: but to the best of my judgment we are not authorized in admitting such enormous geographical changes within the period of existing species. it seems to me that we have abundant evidence of great oscillations of level in our continents; but not of such vast changes in their position and extension, as to have united them within the recent period to each other and to the several intervening oceanic islands. i freely admit the former existence of many islands, now buried beneath the sea, which may have served as halting places for plants and for many animals during their migration. in the coral-producing oceans such sunken islands are now marked, as i believe, by rings of coral or atolls standing over them. whenever it is fully admitted, as i believe it will some day be, that each species has proceeded from a single birthplace, and when in the course of time we know something definite about the means of distribution, we shall be enabled to speculate with security on the former extension of the land. but i do not believe that it will ever be proved that within the { } recent period continents which are now quite separate, have been continuously, or almost continuously, united with each other, and with the many existing oceanic islands. several facts in distribution,--such as the great difference in the marine faunas on the opposite sides of almost every continent,--the close relation of the tertiary inhabitants of several lands and even seas to their present inhabitants,--a certain degree of relation (as we shall hereafter see) between the distribution of mammals and the depth of the sea,--these and other such facts seem to me opposed to the admission of such prodigious geographical revolutions within the recent period, as are necessitated on the view advanced by forbes and admitted by his many followers. the nature and relative proportions of the inhabitants of oceanic islands likewise seem to me opposed to the belief of their former continuity with continents. nor does their almost universally volcanic composition favour the admission that they are the wrecks of sunken continents;--if they had originally existed as mountain-ranges on the land, some at least of the islands would have been formed, like other mountain-summits, of granite, metamorphic schists, old fossiliferous or other such rocks, instead of consisting of mere piles of volcanic matter. i must now say a few words on what are called accidental means, but which more properly might be called occasional means of distribution. i shall here confine myself to plants. in botanical works, this or that plant is stated to be ill adapted for wide dissemination; but for transport across the sea, the greater or less facilities may be said to be almost wholly unknown. until i tried, with mr. berkeley's aid, a few experiments, it was not even known how far seeds could resist the injurious action of sea-water. to my surprise i found that { } out of kinds, germinated after an immersion of days, and a few survived an immersion of days. for convenience' sake i chiefly tried small seeds, without the capsule or fruit; and as all of these sank in a few days, they could not be floated across wide spaces of the sea, whether or not they were injured by the salt-water. afterwards i tried some larger fruits, capsules, &c., and some of these floated for a long time. it is well known what a difference there is in the buoyancy of green and seasoned timber; and it occurred to me that floods might wash down plants or branches, and that these might be dried on the banks, and then by a fresh rise in the stream be washed into the sea. hence i was led to dry stems and branches of plants with ripe fruit, and to place them on sea-water. the majority sank quickly, but some which whilst green floated for a very short time, when dried floated much longer; for instance, ripe hazel-nuts sank immediately, but when dried they floated for days, and afterwards when planted they germinated; an asparagus plant with ripe berries floated for days, when dried it floated for days, and the seeds afterwards germinated; the ripe seeds of helosciadium sank in two days, when dried they floated for above days, and afterwards germinated. altogether out of the dried plants, floated for above days, and some of the floated for a very much longer period. so that as / seeds germinated after an immersion of days; and as / plants with ripe fruit (but not all the same species as in the foregoing experiment) floated, after being dried, for above days, as far as we may infer anything from these scanty facts, we may conclude that the seeds of / plants of any country might be floated by sea-currents during days, and would retain their power of germination. in johnston's physical atlas, the average { } rate of the several atlantic currents is miles per diem (some currents running at the rate of miles per diem); on this average, the seeds of / plants belonging to one country might be floated across miles of sea to another country; and when stranded, if blown to a favourable spot by an inland gale, they would germinate. subsequently to my experiments, m. martens tried similar ones, but in a much better manner, for he placed the seeds in a box in the actual sea, so that they were alternately wet and exposed to the air like really floating plants. he tried seeds, mostly different from mine; but he chose many large fruits and likewise seeds from plants which live near the sea; and this would have favoured the average length of their flotation and of their resistance to the injurious action of the salt-water. on the other hand he did not previously dry the plants or branches with the fruit; and this, as we have seen, would have caused some of them to have floated much longer. the result was that / of his seeds floated for days, and were then capable of germination. but i do not doubt that plants exposed to the waves would float for a less time than those protected from violent movement as in our experiments. therefore it would perhaps be safer to assume that the seeds of about / plants of a flora, after having been dried, could be floated across a space of sea miles in width, and would then germinate. the fact of the larger fruits often floating longer than the small, is interesting; as plants with large seeds or fruit could hardly be transported by any other means; and alph. de candolle has shown that such plants generally have restricted ranges. but seeds may be occasionally transported in another manner. drift timber is thrown up on most islands, { } even on those in the midst of the widest oceans; and the natives of the coral-islands in the pacific, procure stones for their tools, solely from the roots of drifted trees, these stones being a valuable royal tax. i find on examination, that when irregularly shaped stones are embedded in the roots of trees, small parcels of earth are very frequently enclosed in their interstices and behind them,--so perfectly that not a particle could be washed away in the longest transport: out of one small portion of earth thus _completely_ enclosed by wood in an oak about years old, three dicotyledonous plants germinated: i am certain of the accuracy of this observation. again, i can show that the carcasses of birds, when floating on the sea, sometimes escape being immediately devoured; and seeds of many kinds in the crops of floating birds long retain their vitality: peas and vetches, for instance, are killed by even a few days' immersion in sea-water; but some taken out of the crop of a pigeon, which had floated on artificial salt-water for days, to my surprise nearly all germinated. living birds can hardly fail to be highly effective agents in the transportation of seeds. i could give many facts showing how frequently birds of many kinds are blown by gales to vast distances across the ocean. we may i think safely assume that under such circumstances their rate of flight would often be miles an hour; and some authors have given a far higher estimate. i have never seen an instance of nutritious seeds passing through the intestines of a bird; but hard seeds of fruit pass uninjured through even the digestive organs of a turkey. in the course of two months, i picked up in my garden kinds of seeds, out of the excrement of small birds, and these seemed perfect, and some of them, which i tried, germinated. { } but the following fact is more important: the crops of birds do not secrete gastric juice, and do not in the least injure, as i know by trial, the germination of seeds; now after a bird has found and devoured a large supply of food, it is positively asserted that all the grains do not pass into the gizzard for or even hours. a bird in this interval might easily be blown to the distance of miles, and hawks are known to look out for tired birds, and the contents of their torn crops might thus readily get scattered. mr. brent informs me that a friend of his had to give up flying carrier-pigeons from france to england, as the hawks on the english coast destroyed so many on their arrival. some hawks and owls bolt their prey whole, and after an interval of from twelve to twenty hours, disgorge pellets, which, as i know from experiments made in the zoological gardens, include seeds capable of germination. some seeds of the oat, wheat, millet, canary, hemp, clover, and beet germinated after having been from twelve to twenty-one hours in the stomachs of different birds of prey; and two seeds of beet grew after having been thus retained for two days and fourteen hours. freshwater fish, i find, eat seeds of many land and water plants: fish are frequently devoured by birds, and thus the seeds might be transported from place to place. i forced many kinds of seeds into the stomachs of dead fish, and then gave their bodies to fishing-eagles, storks, and pelicans; these birds after an interval of many hours, either rejected the seeds in pellets or passed them in their excrement; and several of these seeds retained their power of germination. certain seeds, however, were always killed by this process. although the beaks and feet of birds are generally quite clean, i can show that earth sometimes adheres to them: in one instance i removed twenty-two grains { } of dry argillaceous earth from one foot of a partridge, and in this earth there was a pebble quite as large as the seed of a vetch. thus seeds might occasionally be transported to great distances; for many facts could be given showing that soil almost everywhere is charged with seeds. reflect for a moment on the millions of quails which annually cross the mediterranean; and can we doubt that the earth adhering to their feet would sometimes include a few minute seeds? but i shall presently have to recur to this subject. as icebergs are known to be sometimes loaded with earth and stones, and have even carried brushwood, bones, and the nest of a land-bird, i can hardly doubt that they must occasionally have transported seeds from one part to another of the arctic and antarctic regions, as suggested by lyell; and during the glacial period from one part of the now temperate regions to another. in the azores, from the large number of the species of plants common to europe, in comparison with the plants of other oceanic islands nearer to the mainland, and (as remarked by mr. h. c. watson) from the somewhat northern character of the flora in comparison with the latitude, i suspected that these islands had been partly stocked by ice-borne seeds, during the glacial epoch. at my request sir c. lyell wrote to m. hartung to inquire whether he had observed erratic boulders on these islands, and he answered that he had found large fragments of granite and other rocks, which do not occur in the archipelago. hence we may safely infer that icebergs formerly landed their rocky burthens on the shores of these mid-ocean islands, and it is at least possible that they may have brought thither the seeds of northern plants. considering that the several above means of transport, and that several other means, which without { } doubt remain to be discovered, have been in action year after year, for centuries and tens of thousands of years, it would i think be a marvellous fact if many plants had not thus become widely transported. these means of transport are sometimes called accidental, but this is not strictly correct: the currents of the sea are not accidental, nor is the direction of prevalent gales of wind. it should be observed that scarcely any means of transport would carry seeds for very great distances; for seeds do not retain their vitality when exposed for a great length of time to the action of sea-water; nor could they be long carried in the crops or intestines of birds. these means, however, would suffice for occasional transport across tracts of sea some hundred miles in breadth, or from island to island, or from a continent to a neighbouring island, but not from one distant continent to another. the floras of distant continents would not by such means become mingled in any great degree; but would remain as distinct as we now see them to be. the currents, from their course, would never bring seeds from north america to britain, though they might and do bring seeds from the west indies to our western shores, where, if not killed by so long an immersion in salt-water, they could not endure our climate. almost every year, one or two land-birds are blown across the whole atlantic ocean, from north america to the western shores of ireland and england; but seeds could be transported by these wanderers only by one means, namely, in dirt sticking to their feet, which is in itself a rare accident. even in this case, how small would the chance be of a seed falling on favourable soil, and coming to maturity! but it would be a great error to argue that because a well-stocked island, like great britain, has not, as far as is known { } (and it would be very difficult to prove this), received within the last few centuries, through occasional means of transport, immigrants from europe or any other continent, that a poorly-stocked island, though standing more remote from the mainland, would not receive colonists by similar means. i do not doubt that out of twenty seeds or animals transported to an island, even if far less well-stocked than britain, scarcely more than one would be so well fitted to its new home, as to become naturalised. but this, as it seems to me, is no valid argument against what would be effected by occasional means of transport, during the long lapse of geological time, whilst an island was being upheaved and formed, and before it had become fully stocked with inhabitants. on almost bare land, with few or no destructive insects or birds living there, nearly every seed, which chanced to arrive, if fitted for the climate, would be sure to germinate and survive. _dispersal during the glacial period._--the identity of many plants and animals, on mountain-summits, separated from each other by hundreds of miles of lowlands, where the alpine species could not possibly exist, is one of the most striking cases known of the same species living at distant points, without the apparent possibility of their having migrated from one to the other. it is indeed a remarkable fact to see so many of the same plants living on the snowy regions of the alps or pyrenees, and in the extreme northern parts of europe; but it is far more remarkable, that the plants on the white mountains, in the united states of america, are all the same with those of labrador, and nearly all the same, as we hear from asa gray, with those on the loftiest mountains of europe. even as long ago as , such facts led gmelin to conclude that the { } same species must have been independently created at several distinct points; and we might have remained in this same belief, had not agassiz and others called vivid attention to the glacial period, which, as we shall immediately see, affords a simple explanation of these facts. we have evidence of almost every conceivable kind, organic and inorganic, that within a very recent geological period, central europe and north america suffered under an arctic climate. the ruins of a house burnt by fire do not tell their tale more plainly, than do the mountains of scotland and wales, with their scored flanks, polished surfaces, and perched boulders, of the icy streams with which their valleys were lately filled. so greatly has the climate of europe changed, that in northern italy, gigantic moraines, left by old glaciers, are now clothed by the vine and maize. throughout a large part of the united states, erratic boulders, and rocks scored by drifted icebergs and coast-ice, plainly reveal a former cold period. the former influence of the glacial climate on the distribution of the inhabitants of europe, as explained with remarkable clearness by edward forbes, is substantially as follows. but we shall follow the changes more readily, by supposing a new glacial period to come slowly on, and then pass away, as formerly occurred. as the cold came on, and as each more southern zone became fitted for arctic beings and ill-fitted for their former more temperate inhabitants, the latter would be supplanted and arctic productions would take their places. the inhabitants of the more temperate regions would at the same time travel southward, unless they were stopped by barriers, in which case they would perish. the mountains would become covered with snow and ice, and their former alpine inhabitants would descend to the plains. by the time that the cold had reached { } its maximum, we should have a uniform arctic fauna and flora, covering the central parts of europe, as far south as the alps and pyrenees, and even stretching into spain. the now temperate regions of the united states would likewise be covered by arctic plants and animals, and these would be nearly the same with those of europe; for the present circumpolar inhabitants, which we suppose to have everywhere travelled southward, are remarkably uniform round the world. we may suppose that the glacial period came on a little earlier or later in north america than in europe, so will the southern migration there have been a little earlier or later; but this will make no difference in the final result. as the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by the productions of the more temperate regions. and as the snow melted from the bases of the mountains, the arctic forms would seize on the cleared and thawed ground, always ascending higher and higher, as the warmth increased, whilst their brethren were pursuing their northern journey. hence, when the warmth had fully returned, the same arctic species, which had lately lived in a body together on the lowlands of the old and new worlds, would be left isolated on distant mountain-summits (having been exterminated on all lesser heights) and in the arctic regions of both hemispheres. thus we can understand the identity of many plants at points so immensely remote as on the mountains of the united states and of europe. we can thus also understand the fact that the alpine plants of each mountain-range are more especially related to the arctic forms living due north or nearly due north of them: for the migration as the cold came on, and the re-migration on the returning warmth, will generally { } have been due south and north. the alpine plants, for example, of scotland, as remarked by mr. h. c. watson, and those of the pyrenees, as remarked by ramond, are more especially allied to the plants of northern scandinavia; those of the united states to labrador; those of the mountains of siberia to the arctic regions of that country. these views, grounded as they are on the perfectly well-ascertained occurrence of a former glacial period, seem to me to explain in so satisfactory a manner the present distribution of the alpine and arctic productions of europe and america, that when in other regions we find the same species on distant mountain-summits, we may almost conclude without other evidence, that a colder climate permitted their former migration across the low intervening tracts, since become too warm for their existence. if the climate, since the glacial period, has ever been in any degree warmer than at present (as some geologists in the united states believe to have been the case, chiefly from the distribution of the fossil gnathodon), then the arctic and temperate productions will at a very late period have marched a little further north, and subsequently have retreated to their present homes; but i have met with no satisfactory evidence with respect to this intercalated slightly warmer period, since the glacial period. the arctic forms, during their long southern migration and re-migration northward, will have been exposed to nearly the same climate, and, as is especially to be noticed, they will have kept in a body together; consequently their mutual relations will not have been much disturbed, and, in accordance with the principles inculcated in this volume, they will not have been liable to much modification. but with our alpine productions, left isolated from the moment of the returning warmth, { } first at the bases and ultimately on the summits of the mountains, the case will have been somewhat different; for it is not likely that all the same arctic species will have been left on mountain ranges distant from each other, and have survived there ever since; they will, also, in all probability have become mingled with ancient alpine species, which must have existed on the mountains before the commencement of the glacial epoch, and which during its coldest period will have been temporarily driven down to the plains; they will, also, have been exposed to somewhat different climatal influences. their mutual relations will thus have been in some degree disturbed; consequently they will have been liable to modification; and this we find has been the case; for if we compare the present alpine plants and animals of the several great european mountain-ranges, though very many of the species are identically the same, some present varieties, some are ranked as doubtful forms, and some few are distinct yet closely allied or representative species. in illustrating what, as i believe, actually took place during the glacial period, i assumed that at its commencement the arctic productions were as uniform round the polar regions as they are at the present day. but the foregoing remarks on distribution apply not only to strictly arctic forms, but also to many sub-arctic and to some few northern temperate forms, for some of these are the same on the lower mountains and on the plains of north america and europe; and it may be reasonably asked how i account for the necessary degree of uniformity of the sub-arctic and northern temperate forms round the world, at the commencement of the glacial period. at the present day, the sub-arctic and northern temperate productions of the old and new worlds are separated from each other by the { } atlantic ocean and by the extreme northern part of the pacific. during the glacial period, when the inhabitants of the old and new worlds lived further southwards than at present, they must have been still more completely separated by wider spaces of ocean. i believe the above difficulty may be surmounted by looking to still earlier changes of climate of an opposite nature. we have good reason to believe that during the newer pliocene period, before the glacial epoch, and whilst the majority of the inhabitants of the world were specifically the same as now, the climate was warmer than at the present day. hence we may suppose that the organisms now living under the climate of latitude °, during the pliocene period lived further north under the polar circle, in latitude °- °; and that the strictly arctic productions then lived on the broken land still nearer to the pole. now if we look at a globe, we shall see that under the polar circle there is almost continuous land from western europe, through siberia, to eastern america. and to this continuity of the circumpolar land, and to the consequent freedom for intermigration under a more favourable climate, i attribute the necessary amount of uniformity in the sub-arctic and northern temperate productions of the old and new worlds, at a period anterior to the glacial epoch. believing, from reasons before alluded to, that our continents have long remained in nearly the same relative position, though subjected to large, but partial oscillations of level, i am strongly inclined to extend the above view, and to infer that during some earlier and still warmer period, such as the older pliocene period, a large number of the same plants and animals inhabited the almost continuous circumpolar land; and that these plants and animals, both in the old and { } new worlds, began slowly to migrate southwards as the climate became less warm, long before the commencement of the glacial period. we now see, as i believe, their descendants, mostly in a modified condition, in the central parts of europe and the united states. on this view we can understand the relationship, with very little identity, between the productions of north america and europe,--a relationship which is most remarkable, considering the distance of the two areas, and their separation by the atlantic ocean. we can further understand the singular fact remarked on by several observers, that the productions of europe and america during the later tertiary stages were more closely related to each other than they are at the present time; for during these warmer periods the northern parts of the old and new worlds will have been almost continuously united by land, serving as a bridge, since rendered impassable by cold, for the intermigration of their inhabitants. during the slowly decreasing warmth of the pliocene period, as soon as the species in common, which inhabited the new and old worlds, migrated south of the polar circle, they must have been completely cut off from each other. this separation, as far as the more temperate productions are concerned, took place long ages ago. and as the plants and animals migrated southward, they will have become mingled in the one great region with the native american productions, and have had to compete with them; and in the other great region, with those of the old world. consequently we have here everything favourable for much modification,--for far more modification than with the alpine productions, left isolated, within a much more recent period, on the several mountain-ranges and on the arctic lands of the two worlds. hence it has come, that when we compare { } the now living productions of the temperate regions of the new and old worlds, we find very few identical species (though asa gray has lately shown that more plants are identical than was formerly supposed), but we find in every great class many forms, which some naturalists rank as geographical races, and others as distinct species; and a host of closely allied or representative forms which are ranked by all naturalists as specifically distinct. as on the land, so in the waters of the sea, a slow southern migration of a marine fauna, which during the pliocene or even a somewhat earlier period, was nearly uniform along the continuous shores of the polar circle, will account, on the theory of modification, for many closely allied forms now living in areas completely sundered. thus, i think, we can understand the presence of many existing and tertiary representative forms on the eastern and western shores of temperate north america; and the still more striking case of many closely allied crustaceans (as described in dana's admirable work), of some fish and other marine animals, in the mediterranean and in the seas of japan,--areas now separated by a continent and by nearly a hemisphere of equatorial ocean. these cases of relationship, without identity, of the inhabitants of seas now disjoined, and likewise of the past and present inhabitants of the temperate lands of north america and europe, are inexplicable on the theory of creation. we cannot say that they have been created alike, in correspondence with the nearly similar physical conditions of the areas; for if we compare, for instance, certain parts of south america with the southern continents of the old world, we see countries closely corresponding in all their physical conditions, but with their inhabitants utterly dissimilar. { } but we must return to our more immediate subject, the glacial period. i am convinced that forbes's view may be largely extended. in europe we have the plainest evidence of the cold period, from the western shores of britain to the oural range, and southward to the pyrenees. we may infer from the frozen mammals and nature of the mountain vegetation, that siberia was similarly affected. along the himalaya, at points miles apart, glaciers have left the marks of their former low descent; and in sikkim, dr. hooker saw maize growing on gigantic ancient moraines. south of the equator, we have some direct evidence of former glacial action in new zealand; and the same plants, found on widely separated mountains in that island, tell the same story. if one account which has been published can be trusted, we have direct evidence of glacial action in the south-eastern corner of australia. looking to america; in the northern half, ice-borne fragments of rock have been observed on the eastern side as far south as lat. °- °, and on the shores of the pacific, where the climate is now so different, as far south as lat. °; erratic boulders have, also, been noticed on the rocky mountains. in the cordillera of equatorial south america, glaciers once extended far below their present level. in central chili i was astonished at the structure of a vast mound of detritus, about feet in height, crossing a valley of the andes; and this i now feel convinced was a gigantic moraine, left far below any existing glacier. further south on both sides of the continent, from lat. ° to the southernmost extremity, we have the clearest evidence of former glacial action, in huge boulders transported far from their parent source. we do not know that the glacial epoch was strictly simultaneous at these several far distant points on { } opposite sides of the world. but we have good evidence in almost every case, that the epoch was included within the latest geological period. we have, also, excellent evidence, that it endured for an enormous time, as measured by years, at each point. the cold may have come on, or have ceased, earlier at one point of the globe than at another, but seeing that it endured for long at each, and that it was contemporaneous in a geological sense, it seems to me probable that it was, during a part at least of the period, actually simultaneous throughout the world. without some distinct evidence to the contrary, we may at least admit as probable that the glacial action was simultaneous on the eastern and western sides of north america, in the cordillera under the equator and under the warmer temperate zones, and on both sides of the southern extremity of the continent. if this be admitted, it is difficult to avoid believing that the temperature of the whole world was at this period simultaneously cooler. but it would suffice for my purpose, if the temperature was at the same time lower along certain broad belts of longitude. on this view of the whole world, or at least of broad longitudinal belts, having been simultaneously colder from pole to pole, much light can be thrown on the present distribution of identical and allied species. in america, dr. hooker has shown that between forty and fifty of the flowering plants of tierra del fuego, forming no inconsiderable part of its scanty flora, are common to europe, enormously remote as these two points are; and there are many closely allied species. on the lofty mountains of equatorial america a host of peculiar species belonging to european genera occur. on the highest mountains of brazil, some few european genera were found by gardner, which do not exist in the wide { } intervening hot countries. so on the silla of caraccas the illustrious humboldt long ago found species belonging to genera characteristic of the cordillera. on the mountains of abyssinia, several european forms and some few representatives of the peculiar flora of the cape of good hope occur. at the cape of good hope a very few european species, believed not to have been introduced by man, and on the mountains, some few representative european forms are found, which have not been discovered in the intertropical parts of africa. on the himalaya, and on the isolated mountain-ranges of the peninsula of india, on the heights of ceylon, and on the volcanic cones of java, many plants occur, either identically the same or representing each other, and at the same time representing plants of europe, not found in the intervening hot lowlands. a list of the genera collected on the loftier peaks of java raises a picture of a collection made on a hill in europe! still more striking is the fact that southern australian forms are clearly represented by plants growing on the summits of the mountains of borneo. some of these australian forms, as i hear from dr. hooker, extend along the heights of the peninsula of malacca, and are thinly scattered, on the one hand over india and on the other as far north as japan. on the southern mountains of australia, dr. f. müller has discovered several european species; other species, not introduced by man, occur on the lowlands; and a long list can be given, as i am informed by dr. hooker, of european genera, found in australia, but not in the intermediate torrid regions. in the admirable 'introduction to the flora of new zealand,' by dr. hooker, analogous and striking facts are given in regard to the plants of that large island. hence we see that throughout the world, the plants growing on the { } more lofty mountains, and on the temperate lowlands of the northern and southern hemispheres, are sometimes identically the same; but they are much oftener specifically distinct, though related to each other in a most remarkable manner. this brief abstract applies to plants alone: some strictly analogous facts could be given on the distribution of terrestrial animals. in marine productions, similar cases occur; as an example, i may quote a remark by the highest authority, prof. dana, that "it is certainly a wonderful fact that new zealand should have a closer resemblance in its crustacea to great britain, its antipode, than to any other part of the world." sir j. richardson, also, speaks of the reappearance on the shores of new zealand, tasmania, &c., of northern forms of fish. dr. hooker informs me that twenty-five species of algæ are common to new zealand and to europe, but have not been found in the intermediate tropical seas. it should be observed that the northern species and forms found in the southern parts of the southern hemisphere, and on the mountain-ranges of the intertropical regions, are not arctic, but belong to the northern temperate zones. as mr. h. c. watson has recently remarked, "in receding from polar towards equatorial latitudes, the alpine or mountain floras really become less and less arctic." many of the forms living on the mountains of the warmer regions of the earth and in the southern hemisphere are of doubtful value, being ranked by some naturalists as specifically distinct, by others as varieties; but some are certainly identical, and many, though closely related to northern forms, must be ranked as distinct species. now let us see what light can be thrown on the foregoing facts, on the belief, supported as it is by a large { } body of geological evidence, that the whole world, or a large part of it, was during the glacial period simultaneously much colder than at present. the glacial period, as measured by years, must have been very long; and when we remember over what vast spaces some naturalised plants and animals have spread within a few centuries, this period will have been ample for any amount of migration. as the cold came slowly on, all the tropical plants and other productions will have retreated from both sides towards the equator, followed in the rear by the temperate productions, and these by the arctic; but with the latter we are not now concerned. the tropical plants probably suffered much extinction; how much no one can say; perhaps formerly the tropics supported as many species as we see at the present day crowded together at the cape of good hope, and in parts of temperate australia. as we know that many tropical plants and animals can withstand a considerable amount of cold, many might have escaped extermination during a moderate fall of temperature, more especially by escaping into the lowest, most protected, and warmest districts. but the great fact to bear in mind is, that all tropical productions will have suffered to a certain extent. on the other hand, the temperate productions, after migrating nearer to the equator, though they will have been placed under somewhat new conditions, will have suffered less. and it is certain that many temperate plants, if protected from the inroads of competitors, can withstand a much warmer climate than their own. hence, it seems to me possible, bearing in mind that the tropical productions were in a suffering state and could not have presented a firm front against intruders, that a certain number of the more vigorous and dominant temperate forms might have penetrated the native ranks and have reached or { } even crossed the equator. the invasion would, of course, have been greatly favoured by high land, and perhaps by a dry climate; for dr. falconer informs me that it is the damp with the heat of the tropics which is so destructive to perennial plants from a temperate climate. on the other hand, the most humid and hottest districts will have afforded an asylum to the tropical natives. the mountain-ranges north-west of the himalaya, and the long line of the cordillera, seem to have afforded two great lines of invasion: and it is a striking fact, lately communicated to me by dr. hooker, that all the flowering plants, about forty-six in number, common to tierra del fuego and to europe still exist in north america, which must have lain on the line of march. but i do not doubt that some temperate productions entered and crossed even the _lowlands_ of the tropics at the period when the cold was most intense,--when arctic forms had migrated some twenty-five degrees of latitude from their native country and covered the land at the foot of the pyrenees. at this period of extreme cold, i believe that the climate under the equator at the level of the sea was about the same with that now felt there at the height of six or seven thousand feet. during this the coldest period, i suppose that large spaces of the tropical lowlands were clothed with a mingled tropical and temperate vegetation, like that now growing with strange luxuriance at the base of the himalaya, as graphically described by hooker. thus, as i believe, a considerable number of plants, a few terrestrial animals, and some marine productions, migrated during the glacial period from the northern and southern temperate zones into the intertropical regions, and some even crossed the equator. as the warmth returned, these temperate forms would naturally ascend the higher mountains, being exterminated on the { } lowlands; those which had not reached the equator would re-migrate northward or southward towards their former homes; but the forms, chiefly northern, which had crossed the equator, would travel still further from their homes into the more temperate latitudes of the opposite hemisphere. although we have reason to believe from geological evidence that the whole body of arctic shells underwent scarcely any modification during their long southern migration and re-migration northward, the case may have been wholly different with those intruding forms which settled themselves on the intertropical mountains, and in the southern hemisphere. these being surrounded by strangers will have had to compete with many new forms of life; and it is probable that selected modifications in their structure, habits, and constitutions will have profited them. thus many of these wanderers, though still plainly related by inheritance to their brethren of the northern or southern hemispheres, now exist in their new homes as well-marked varieties or as distinct species. it is a remarkable fact, strongly insisted on by hooker in regard to america, and by alph. de candolle in regard to australia, that many more identical plants and allied forms have apparently migrated from the north to the south, than in a reversed direction. we see, however, a few southern vegetable forms on the mountains of borneo and abyssinia. i suspect that this preponderant migration from north to south is due to the greater extent of land in the north, and to the northern forms having existed in their own homes in greater numbers, and having consequently been advanced through natural selection and competition to a higher stage of perfection or dominating power, than the southern forms. and thus, when they became commingled during the glacial period, the northern forms { } were enabled to beat the less powerful southern forms. just in the same manner as we see at the present day, that very many european productions cover the ground in la plata, and in a lesser degree in australia, and have to a certain extent beaten the natives; whereas extremely few southern forms have become naturalised in any part of europe, though hides, wool, and other objects likely to carry seeds have been largely imported into europe during the last two or three centuries from la plata, and during the last thirty or forty years from australia. something of the same kind must have occurred on the intertropical mountains: no doubt before the glacial period they were stocked with endemic alpine forms; but these have almost everywhere largely yielded to the more dominant forms, generated in the larger areas and more efficient workshops of the north. in many islands the native productions are nearly equalled or even outnumbered by the naturalised; and if the natives have not been actually exterminated, their numbers have been greatly reduced, and this is the first stage towards extinction. a mountain is an island on the land; and the intertropical mountains before the glacial period must have been completely isolated; and i believe that the productions of these islands on the land yielded to those produced within the larger areas of the north, just in the same way as the productions of real islands have everywhere lately yielded to continental forms, naturalised by man's agency. i am far from supposing that all difficulties are removed on the view here given in regard to the range and affinities of the allied species which live in the northern and southern temperate zones and on the mountains of the intertropical regions. very many difficulties remain to be solved. i do not pretend to { } indicate the exact lines and means of migration, or the reason why certain species and not others have migrated; why certain species have been modified and have given rise to new groups of forms, and others have remained unaltered. we cannot hope to explain such facts, until we can say why one species and not another becomes naturalised by man's agency in a foreign land; why one ranges twice or thrice as far, and is twice or thrice as common, as another species within their own homes. i have said that many difficulties remain to be solved: some of the most remarkable are stated with admirable clearness by dr. hooker in his botanical works on the antarctic regions. these cannot be here discussed. i will only say that as far as regards the occurrence of identical species at points so enormously remote as kerguelen land, new zealand, and fuegia, i believe that towards the close of the glacial period, icebergs, as suggested by lyell, have been largely concerned in their dispersal. but the existence of several quite distinct species, belonging to genera exclusively confined to the south, at these and other distant points of the southern hemisphere, is, on my theory of descent with modification, a far more remarkable case of difficulty. for some of these species are so distinct, that we cannot suppose that there has been time since the commencement of the glacial period for their migration, and for their subsequent modification to the necessary degree. the facts seem to me to indicate that peculiar and very distinct species have migrated in radiating lines from some common centre; and i am inclined to look in the southern, as in the northern hemisphere, to a former and warmer period, before the commencement of the glacial period, when the antarctic lands, now covered with ice, supported a highly peculiar { } and isolated flora. i suspect that before this flora was exterminated by the glacial epoch, a few forms were widely dispersed to various points of the southern hemisphere by occasional means of transport, and by the aid, as halting-places, of existing and now sunken islands: by these means, as i believe, the southern shores of america, australia, new zealand, have become slightly tinted by the same peculiar forms of vegetable life. sir c. lyell in a striking passage has speculated, in language almost identical with mine, on the effects of great alternations of climate on geographical distribution. i believe that the world has recently felt one of his great cycles of change; and that on this view, combined with modification through natural selection, a multitude of facts in the present distribution both of the same and of allied forms of life can be explained. the living waters may be said to have flowed during one short period from the north and from the south, and to have crossed at the equator; but to have flowed with greater force from the north so as to have freely inundated the south. as the tide leaves its drift in horizontal lines, though rising higher on the shores where the tide rises highest, so have the living waters left their living drift on our mountain-summits, in a line gently rising from the arctic lowlands to a great height under the equator. the various beings thus left stranded may be compared with savage races of man, driven up and surviving in the mountain-fastnesses of almost every land, which serve as a record, full of interest to us, of the former inhabitants of the surrounding lowlands. * * * * * { } chapter xii. geographical distribution--_continued_. distribution of fresh-water productions--on the inhabitants of oceanic islands--absence of batrachians and of terrestrial mammals--on the relation of the inhabitants of islands to those of the nearest mainland--on colonisation from the nearest source with subsequent modification--summary of the last and present chapters. as lakes and river-systems are separated from each other by barriers of land, it might have been thought that fresh-water productions would not have ranged widely within the same country, and as the sea is apparently a still more impassable barrier, that they never would have extended to distant countries. but the case is exactly the reverse. not only have many fresh-water species, belonging to quite different classes, an enormous range, but allied species prevail in a remarkable manner throughout the world. i well remember, when first collecting in the fresh waters of brazil, feeling much surprise at the similarity of the fresh-water insects, shells, &c., and at the dissimilarity of the surrounding terrestrial beings, compared with those of britain. but this power in fresh-water productions of ranging widely, though so unexpected, can, i think, in most cases be explained by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream; and liability to wide dispersal would follow from this capacity as an almost necessary consequence. we can here consider only a few cases. in regard to { } fish, i believe that the same species never occur in the fresh waters of distant continents. but on the same continent the species often range widely and almost capriciously; for two river-systems will have some fish in common and some different. a few facts seem to favour the possibility of their occasional transport by accidental means; like that of the live fish not rarely dropped by whirlwinds in india, and the vitality of their ova when removed from the water. but i am inclined to attribute the dispersal of fresh-water fish mainly to slight changes within the recent period in the level of the land, having caused rivers to flow into each other. instances, also, could be given of this having occurred during floods, without any change of level. we have evidence in the loess of the rhine of considerable changes of level in the land within a very recent geological period, and when the surface was peopled by existing land and fresh-water shells. the wide difference of the fish on opposite sides of continuous mountain-ranges, which from an early period must have parted river-systems and completely prevented their inosculation, seems to lead to this same conclusion. with respect to allied fresh-water fish occurring at very distant points of the world, no doubt there are many cases which cannot at present be explained: but some fresh-water fish belong to very ancient forms, and in such cases there will have been ample time for great geographical changes, and consequently time and means for much migration. in the second place, salt-water fish can with care be slowly accustomed to live in fresh water; and, according to valenciennes, there is hardly a single group of fishes confined exclusively to fresh water, so that we may imagine that a marine member of a fresh-water group might travel far along the shores of the sea, and { } subsequently become modified and adapted to the fresh waters of a distant land. some species of fresh-water shells have a very wide range, and allied species, which, on my theory, are descended from a common parent and must have proceeded from a single source, prevail throughout the world. their distribution at first perplexed me much, as their ova are not likely to be transported by birds, and they are immediately killed by sea-water, as are the adults. i could not even understand how some naturalised species have rapidly spread throughout the same country. but two facts, which i have observed--and no doubt many others remain to be observed--throw some light on this subject. when a duck suddenly emerges from a pond covered with duck-weed, i have twice seen these little plants adhering to its back; and it has happened to me, in removing a little duckweed from one aquarium to another, that i have quite unintentionally stocked the one with fresh-water shells from the other. but another agency is perhaps more effectual: i suspended a duck's feet, which might represent those of a bird sleeping in a natural pond, in an aquarium, where many ova of fresh-water shells were hatching; and i found that numbers of the extremely minute and just-hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. these just hatched molluscs, though aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and would be sure to alight on a pool or rivulet, if blown across sea to an oceanic island or to any other distant point. sir charles lyell also { } informs me that a dyticus has been caught with an ancylus (a fresh-water shell like a limpet) firmly adhering to it; and a water-beetle of the same family, a colymbetes, once flew on board the 'beagle,' when forty-five miles distant from the nearest land: how much farther it might have flown with a favouring gale no one can tell. with respect to plants, it has long been known what enormous ranges many fresh-water and even marsh-species have, both over continents and to the most remote oceanic islands. this is strikingly shown, as remarked by alph. de candolle, in large groups of terrestrial plants, which have only a very few aquatic members; for these latter seem immediately to acquire, as if in consequence, a very wide range. i think favourable means of dispersal explain this fact. i have before mentioned that earth occasionally, though rarely, adheres in some quantity to the feet and beaks of birds. wading birds, which frequent the muddy edges of ponds, if suddenly flushed, would be the most likely to have muddy feet. birds of this order i can show are the greatest wanderers, and are occasionally found on the most remote and barren islands in the open ocean; they would not be likely to alight on the surface of the sea, so that the dirt would not be washed off their feet; when making land, they would be sure to fly to their natural fresh-water haunts. i do not believe that botanists are aware how charged the mud of ponds is with seeds: i have tried several little experiments, but will here give only the most striking case: i took in february three table-spoonfuls of mud from three different points, beneath water, on the edge of a little pond; this mud when dry weighed only ¾ ounces; i kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were { } of many kinds, and were altogether in number; and yet the viscid mud was all contained in a breakfast cup! considering these facts, i think it would be an inexplicable circumstance if water-birds did not transport the seeds of fresh-water plants to vast distances, and if consequently the range of these plants was not very great. the same agency may have come into play with the eggs of some of the smaller fresh-water animals. other and unknown agencies probably have also played a part. i have stated that fresh-water fish eat some kinds of seeds, though they reject many other kinds after having swallowed them; even small fish swallow seeds of moderate size, as of the yellow water-lily and potamogeton. herons and other birds, century after century, have gone on daily devouring fish; they then take flight and go to other waters, or are blown across the sea; and we have seen that seeds retain their power of germination, when rejected in pellets or in excrement, many hours afterwards. when i saw the great size of the seeds of that fine water-lily, the nelumbium, and remembered alph. de candolle's remarks on this plant, i thought that its distribution must remain quite inexplicable; but audubon states that he found the seeds of the great southern water-lily (probably, according to dr. hooker, the nelumbium luteum) in a heron's stomach; although i do not know the fact, yet analogy makes me believe that a heron flying to another pond and getting a hearty meal of fish, would probably reject from its stomach a pellet containing the seeds of the nelumbium undigested; or the seeds might be dropped by the bird whilst feeding its young, in the same way as fish are known sometimes to be dropped. in considering these several means of distribution, { } it should be remembered that when a pond or stream is first formed, for instance, on a rising islet, it will be unoccupied; and a single seed or egg will have a good chance of succeeding. although there will always be a struggle for life between the individuals of the species, however few, already occupying any pond, yet as the number of kinds is small, compared with those on the land, the competition will probably be less severe between aquatic than between terrestrial species; consequently an intruder from the waters of a foreign country, would have a better chance of seizing on a place, than in the case of terrestrial colonists. we should, also, remember that some, perhaps many, freshwater productions are low in the scale of nature, and that we have reason to believe that such low beings change or become modified less quickly than the high; and this will give longer time than the average for the migration of the same aquatic species. we should not forget the probability of many species having formerly ranged as continuously as fresh-water productions ever can range, over immense areas, and having subsequently become extinct in intermediate regions. but the wide distribution of fresh-water plants and of the lower animals, whether retaining the same identical form or in some degree modified, i believe mainly depends on the wide dispersal of their seeds and eggs by animals, more especially by fresh-water birds, which have large powers of flight, and naturally travel from one to another and often distant piece of water. nature, like a careful gardener, thus takes her seeds from a bed of a particular nature, and drops them in another equally well fitted for them. _on the inhabitants of oceanic islands._--we now come to the last of the three classes of facts, which i { } have selected as presenting the greatest amount of difficulty, on the view that all the individuals both of the same and of allied species have descended from a single parent; and therefore have all proceeded from a common birthplace, notwithstanding that in the course of time they have come to inhabit distant points of the globe. i have already stated that i cannot honestly admit forbes's view on continental extensions, which, if legitimately followed out, would lead to the belief that within the recent period all existing islands have been nearly or quite joined to some continent. this view would remove many difficulties, but it would not, i think, explain all the facts in regard to insular productions. in the following remarks i shall not confine myself to the mere question of dispersal; but shall consider some other facts, which bear on the truth of the two theories of independent creation and of descent with modification. the species of all kinds which inhabit oceanic islands are few in number compared with those on equal continental areas: alph. de candolle admits this for plants, and wollaston for insects. if we look to the large size and varied stations of new zealand, extending over miles of latitude, and compare its flowering plants, only in number, with those on an equal area at the cape of good hope or in australia, we must, i think, admit that something quite independently of any difference in physical conditions has caused so great a difference in number. even the uniform county of cambridge has plants, and the little island of anglesea , but a few ferns and a few introduced plants are included in these numbers, and the comparison in some other respects is not quite fair. we have evidence that the barren island of ascension aboriginally possessed under half-a-dozen flowering plants; { } yet many have become naturalised on it, as they have on new zealand and on every other oceanic island which can be named. in st. helena there is reason to believe that the naturalised plants and animals have nearly or quite exterminated many native productions. he who admits the doctrine of the creation of each separate species, will have to admit, that a sufficient number of the best adapted plants and animals have not been created on oceanic islands; for man has unintentionally stocked them from various sources far more fully and perfectly than has nature. although in oceanic islands the number of kinds of inhabitants is scanty, the proportion of endemic species (_i.e._ those found nowhere else in the world) is often extremely large. if we compare, for instance, the number of the endemic land-shells in madeira, or of the endemic birds in the galapagos archipelago, with the number found on any continent, and then compare the area of the islands with that of the continent, we shall see that this is true. this fact might have been expected on my theory, for, as already explained, species occasionally arriving after long intervals in a new and isolated district, and having to compete with new associates, will be eminently liable to modification, and will often produce groups of modified descendants. but it by no means follows, that, because in an island nearly all the species of one class are peculiar, those of another class, or of another section of the same class, are peculiar; and this difference seems to depend partly on the species which do not become modified having immigrated with facility and in a body, so that their mutual relations have not been much disturbed; and partly on the frequent arrival of unmodified immigrants from the mother-country, and the consequent intercrossing with them. with respect to the effects of this intercrossing, { } it should be remembered that the offspring of such crosses would almost certainly gain in vigour; so that even an occasional cross would produce more effect than might at first have been anticipated. to give a few examples: in the galapagos islands nearly every land-bird, but only two out of the eleven marine birds, are peculiar; and it is obvious that marine birds could arrive at these islands more easily than land-birds. bermuda, on the other hand, which lies at about the same distance from north america as the galapagos islands do from south america, and which has a very peculiar soil, does not possess one endemic land-bird; and we know from mr. j. m. jones's admirable account of bermuda, that very many north american birds, during their great annual migrations, visit either periodically or occasionally this island. madeira does not possess one peculiar bird, and many european and african birds are almost every year blown there, as i am informed by mr. e. v. harcourt. so that these two islands of bermuda and madeira have been stocked by birds, which for long ages have struggled together in their former homes, and have become mutually adapted to each other; and when settled in their new homes, each kind will have been kept by the others to their proper places and habits, and will consequently have been little liable to modification. any tendency to modification will, also, have been checked by intercrossing with the unmodified immigrants from the mother-country. madeira, again, is inhabited by a wonderful number of peculiar land-shells, whereas not one species of sea-shell is confined to its shores: now, though we do not know how sea-shells are dispersed, yet we can see that their eggs or larvae, perhaps attached to seaweed or floating timber, or to the feet of wading-birds, might be transported far more easily than { } land-shells, across three or four hundred miles of open sea. the different orders of insects in madeira apparently present analogous facts. oceanic islands are sometimes deficient in certain classes, and their places are apparently occupied by the other inhabitants; in the galapagos islands reptiles, and in new zealand gigantic wingless birds, take the place of mammals. in the plants of the galapagos islands, dr. hooker has shown that the proportional numbers of the different orders are very different from what they are elsewhere. such cases are generally accounted for by the physical conditions of the islands; but this explanation seems to me not a little doubtful. facility of immigration, i believe, has been at least as important as the nature of the conditions. many remarkable little facts could be given with respect to the inhabitants of remote islands. for instance, in certain islands not tenanted by mammals, some of the endemic plants have beautifully hooked seeds; yet few relations are more striking than the adaptation of hooked seeds for transportal by the wool and fur of quadrupeds. this case presents no difficulty on my view, for a hooked seed might be transported to an island by some other means; and the plant then becoming slightly modified, but still retaining its hooked seeds, would form an endemic species, having as useless an appendage as any rudimentary organ,--for instance, as the shrivelled wings under the soldered elytra of many insular beetles. again, islands often possess trees or bushes belonging to orders which elsewhere include only herbaceous species; now trees, as alph. de candolle has shown, generally have, whatever the cause may be, confined ranges. hence trees would be little likely to reach distant oceanic islands; and an herbaceous plant, though it would have no chance of { } successfully competing in stature with a fully developed tree, when established on an island and having to compete with herbaceous plants alone, might readily gain an advantage by growing taller and taller and overtopping the other plants. if so, natural selection would often tend to add to the stature of herbaceous plants when growing on an oceanic island, to whatever order they belonged, and thus convert them first into bushes and ultimately into trees. with respect to the absence of whole orders on oceanic islands, bory st. vincent long ago remarked that batrachians (frogs, toads, newts) have never been found on any of the many islands with which the great oceans are studded. i have taken pains to verify this assertion, and i have found it strictly true. i have, however, been assured that a frog exists on the mountains of the great island of new zealand; but i suspect that this exception (if the information be correct) may be explained through glacial agency. this general absence of frogs, toads, and newts on so many oceanic islands cannot be accounted for by their physical conditions; indeed it seems that islands are peculiarly well fitted for these animals; for frogs have been introduced into madeira, the azores, and mauritius, and have multiplied so as to become a nuisance. but as these animals and their spawn are known to be immediately killed by sea-water, on my view we can see that there would be great difficulty in their transportal across the sea, and therefore why they do not exist on any oceanic island. but why, on the theory of creation, they should not have been created there, it would be very difficult to explain. mammals offer another and similar case. i have carefully searched the oldest voyages, but have not finished my search; as yet i have not found a single { } instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by the natives) inhabiting an island situated above miles from a continent or great continental island; and many islands situated at a much less distance are equally barren. the falkland islands, which are inhabited by a wolf-like fox, come nearest to an exception; but this group cannot be considered as oceanic, as it lies on a bank connected with the mainland; moreover, icebergs formerly brought boulders to its western shores, and they may have formerly transported foxes, as so frequently now happens in the arctic regions. yet it cannot be said that small islands will not support small mammals, for they occur in many parts of the world on very small islands, if close to a continent; and hardly an island can be named on which our smaller quadrupeds have not become naturalised and greatly multiplied. it cannot be said, on the ordinary view of creation, that there has not been time for the creation of mammals; many volcanic islands are sufficiently ancient, as shown by the stupendous degradation which they have suffered and by their tertiary strata: there has also been time for the production of endemic species belonging to other classes; and on continents it is thought that mammals appear and disappear at a quicker rate than other and lower animals. though terrestrial mammals do not occur on oceanic islands, aërial mammals do occur on almost every island. new zealand possesses two bats found nowhere else in the world: norfolk island, the viti archipelago, the bonin islands, the caroline and marianne archipelagoes, and mauritius, all possess their peculiar bats. why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands? on my view this question can easily be answered; for no { } terrestrial mammal can be transported across a wide space of sea, but bats can fly across. bats have been seen wandering by day far over the atlantic ocean; and two north american species either regularly or occasionally visit bermuda, at the distance of miles from the mainland. i hear from mr. tomes, who has specially studied this family, that many of the same species have enormous ranges, and are found on continents and on far distant islands. hence we have only to suppose that such wandering species have been modified through natural selection in their new homes in relation to their new position, and we can understand the presence of endemic bats on islands, with the absence of all terrestrial mammals. besides the absence of terrestrial mammals in relation to the remoteness of islands from continents, there is also a relation, to a certain extent independent of distance, between the depth of the sea separating an island from the neighbouring mainland, and the presence in both of the same mammiferous species or of allied species in a more or less modified condition. mr. windsor earl has made some striking observations on this head in regard to the great malay archipelago, which is traversed near celebes by a space of deep ocean; and this space separates two widely distinct mammalian faunas. on either side the islands are situated on moderately deep submarine banks, and they are inhabited by closely allied or identical quadrupeds. no doubt some few anomalies occur in this great archipelago, and there is much difficulty in forming a judgment in some cases owing to the probable naturalisation of certain mammals through man's agency; but we shall soon have much light thrown on the natural history of this archipelago by the admirable zeal and researches of mr. wallace. i have not as yet had time to { } follow up this subject in all other quarters of the world; but as far as i have gone, the relation generally holds good. we see britain separated by a shallow channel from europe, and the mammals are the same on both sides; we meet with analogous facts on many islands separated by similar channels from australia. the west indian islands stand on a deeply submerged bank, nearly fathoms in depth, and here we find american forms, but the species and even the genera are distinct. as the amount of modification in all cases depends to a certain degree on the lapse of time, and as during changes of level it is obvious that islands separated by shallow channels are more likely to have been continuously united within a recent period to the mainland than islands separated by deeper channels, we can understand the frequent relation between the depth of the sea and the degree of affinity of the mammalian inhabitants of islands with those of a neighbouring continent,--an inexplicable relation on the view of independent acts of creation. all the foregoing remarks on the inhabitants of oceanic islands,--namely, the scarcity of kinds--the richness in endemic forms in particular classes or sections of classes,--the absence of whole groups, as of batrachians, and of terrestrial mammals notwithstanding the presence of aërial bats,--the singular proportions of certain orders of plants,--herbaceous forms having been developed into trees, &c.,--seem to me to accord better with the view of occasional means of transport having been largely efficient in the long course of time, than with the view of all our oceanic islands having been formerly connected by continuous land with the nearest continent; for on this latter view the migration would probably have been more complete; and if modification be admitted, all the forms of life would have been more { } equally modified, in accordance with the paramount importance of the relation of organism to organism. i do not deny that there are many and grave difficulties in understanding how several of the inhabitants of the more remote islands, whether still retaining the same specific form or modified since their arrival, could have reached their present homes. but the probability of many islands having existed as halting-places, of which not a wreck now remains, must not be overlooked. i will here give a single instance of one of the cases of difficulty. almost all oceanic islands, even the most isolated and smallest, are inhabited by land-shells, generally by endemic species, but sometimes by species found elsewhere. dr. aug. a. gould has given several interesting cases in regard to the land-shells of the islands of the pacific. now it is notorious that land-shells are very easily killed by salt; their eggs, at least such as i have tried, sink in sea-water and are killed by it. yet there must be, on my view, some unknown, but highly efficient means for their transportal. would the just-hatched young occasionally crawl on and adhere to the feet of birds roosting on the ground, and thus get transported? it occurred to me that land-shells, when hybernating and having a membranous diaphragm over the mouth of the shell, might be floated in chinks of drifted timber across moderately wide arms of the sea. and i found that several species did in this state withstand uninjured an immersion in sea-water during seven days: one of these shells was the helix pomatia, and after it had again hybernated i put it in sea-water for twenty days, and it perfectly recovered. as this species has a thick calcareous operculum, i removed it, and when it had formed a new membranous one, i immersed it for fourteen days in sea-water, and it recovered and crawled away: but more experiments are wanted on this head. { } the most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. numerous instances could be given of this fact. i will give only one, that of the galapagos archipelago, situated under the equator, between and miles from the shores of south america. here almost every product of the land and water bears the unmistakeable stamp of the american continent. there are twenty-six land-birds, and twenty-five of these are ranked by mr. gould as distinct species, supposed to have been created here; yet the close affinity of most of these birds to american species in every character, in their habits, gestures, and tones of voice, was manifest. so it is with the other animals, and with nearly all the plants, as shown by dr. hooker in his admirable memoir on the flora of this archipelago. the naturalist, looking at the inhabitants of these volcanic islands in the pacific, distant several hundred miles from the continent, yet feels that he is standing on american land. why should this be so? why should the species which are supposed to have been created in the galapagos archipelago, and nowhere else, bear so plain a stamp of affinity to those created in america? there is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which resembles closely the conditions of the south american coast: in fact there is a considerable dissimilarity in all these respects. on the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in climate, height, and size of the islands, between the galapagos and cape de verde archipelagos: but what an entire and absolute difference in their inhabitants! the inhabitants of the cape de verde islands are related to { } those of africa, like those of the galapagos to america. i believe this grand fact can receive no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the galapagos islands would be likely to receive colonists, whether by occasional means of transport or by formerly continuous land, from america; and the cape de verde islands from africa; and that such colonists would be liable to modification;--the principle of inheritance still betraying their original birthplace. many analogous facts could be given: indeed it is an almost universal rule that the endemic productions of islands are related to those of the nearest continent, or of other near islands. the exceptions are few, and most of them can be explained. thus the plants of kerguelen land, though standing nearer to africa than to america, are related, and that very closely, as we know from dr. hooker's account, to those of america: but on the view that this island has been mainly stocked by seeds brought with earth and stones on icebergs, drifted by the prevailing currents, this anomaly disappears. new zealand in its endemic plants is much more closely related to australia, the nearest mainland, than to any other region: and this is what might have been expected; but it is also plainly related to south america, which, although the next nearest continent, is so enormously remote, that the fact becomes an anomaly. but this difficulty almost disappears on the view that both new zealand, south america, and other southern lands were long ago partially stocked from a nearly intermediate though distant point, namely from the antarctic islands, when they were clothed with vegetation, before the commencement of the glacial period. the affinity, which, though feeble, i am assured by dr. hooker is real, between the flora of the south-western corner of australia and of the cape of good { } hope, is a far more remarkable case, and is at present inexplicable: but this affinity is confined to the plants, and will, i do not doubt, be some day explained. the law which causes the inhabitants of an archipelago, though specifically distinct, to be closely allied to those of the nearest continent, we sometimes see displayed on a small scale, yet in a most interesting manner, within the limits of the same archipelago. thus the several islands of the galapagos archipelago are tenanted, as i have elsewhere shown, in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world. and this is just what might have been expected on my view, for the islands are situated so near each other that they would almost certainly receive immigrants from the same original source, or from each other. but this dissimilarity between the endemic inhabitants of the islands may be used as an argument against my views; for it may be asked, how has it happened in the several islands situated within sight of each other, having the same geological nature, the same height, climate, &c., that many of the immigrants should have been differently modified, though only in a small degree. this long appeared to me a great difficulty: but it arises in chief part from the deeply-seated error of considering the physical conditions of a country as the most important for its inhabitants; whereas it cannot, i think, be disputed that the nature of the other inhabitants, with which each has to compete, is as least as important, and generally a far more important element of success. now if we look to those inhabitants of the galapagos archipelago which are found in other parts of the world (laying on one side for the moment the { } endemic species, which cannot be here fairly included, as we are considering how they have come to be modified since their arrival), we find a considerable amount of difference in the several islands. this difference might indeed have been expected on the view of the islands having been stocked by occasional means of transport--a seed, for instance, of one plant having been brought to one island, and that of another plant to another island. hence when in former times an immigrant settled on any one or more of the islands, or when it subsequently spread from one island to another, it would undoubtedly be exposed to different conditions of life in the different islands, for it would have to compete with different sets of organisms: a plant for instance, would find the best-fitted ground more perfectly occupied by distinct plants in one island than in another, and it would be exposed to the attacks of somewhat different enemies. if then it varied, natural selection would probably favour different varieties in the different islands. some species, however, might spread and yet retain the same character throughout the group, just as we see on continents some species spreading widely and remaining the same. the really surprising fact in this case of the galapagos archipelago, and in a lesser degree in some analogous instances, is that the new species formed in the separate islands have not quickly spread to the other islands. but the islands, though in sight of each other, are separated by deep arms of the sea, in most cases wider than the british channel, and there is no reason to suppose that they have at any former period been continuously united. the currents of the sea are rapid and sweep across the archipelago, and gales of wind are extraordinarily rare; so that the islands are far more effectually separated from each other than they appear to be on a map. nevertheless a good many { } species, both those found in other parts of the world and those confined to the archipelago, are common to the several islands, and we may infer from certain facts that these have probably spread from some one island to the others. but we often take, i think, an erroneous view of the probability of closely-allied species invading each other's territory, when put into free intercommunication. undoubtedly if one species has any advantage whatever over another, it will in a very brief time wholly or in part supplant it; but if both are equally well fitted for their own places in nature, both probably will hold their own places and keep separate for almost any length of time. being familiar with the fact that many species, naturalised through man's agency, have spread with astonishing rapidity over new countries, we are apt to infer that most species would thus spread; but we should remember that the forms which become naturalised in new countries are not generally closely allied to the aboriginal inhabitants, but are very distinct species, belonging in a large proportion of cases, as shown by alph. de candolle, to distinct genera. in the galapagos archipelago, many even of the birds, though so well adapted for flying from island to island, are distinct on each; thus there are three closely-allied species of mocking-thrush, each confined to its own island. now let us suppose the mocking-thrush of chatham island to be blown to charles island, which has its own mocking-thrush: why should it succeed in establishing itself there? we may safely infer that charles island is well stocked with its own species, for annually more eggs are laid there than can possibly be reared; and we may infer that the mocking-thrush peculiar to charles island is at least as well fitted for its home as is the species peculiar to chatham island. sir c. lyell and mr. wollaston have communicated to me a remarkable fact bearing on this { } subject; namely, that madeira and the adjoining islet of porto santo possess many distinct but representative land-shells, some of which live in crevices of stone; and although large quantities of stone are annually transported from porto santo to madeira, yet this latter island has not become colonised by the porto santo species: nevertheless both islands have been colonised by some european land-shells, which no doubt had some advantage over the indigenous species. from these considerations i think we need not greatly marvel at the endemic and representative species, which inhabit the several islands of the galapagos archipelago, not having universally spread from island to island. in many other instances, as in the several districts of the same continent, pre-occupation has probably played an important part in checking the commingling of species under the same conditions of life. thus, the south-east and south-west corners of australia have nearly the same physical conditions, and are united by continuous land, yet they are inhabited by a vast number of distinct mammals, birds, and plants. the principle which determines the general character of the fauna and flora of oceanic islands, namely, that the inhabitants, when not identically the same, yet are plainly related to the inhabitants of that region whence colonists could most readily have been derived,--the colonists having been subsequently modified and better fitted to their new homes,--is of the widest application throughout nature. we see this on every mountain, in every lake and marsh. for alpine species, excepting in so far as the same forms, chiefly of plants, have spread widely throughout the world during the recent glacial epoch, are related to those of the surrounding lowlands;--thus we have in south america, alpine humming-birds, alpine rodents, alpine plants, { } &c., all of strictly american forms, and it is obvious that a mountain, as it became slowly upheaved, would naturally be colonised from the surrounding lowlands. so it is with the inhabitants of lakes and marshes, excepting in so far as great facility of transport has given the same general forms to the whole world. we see this same principle in the blind animals inhabiting the caves of america and of europe. other analogous facts could be given. and it will, i believe, be universally found to be true, that wherever in two regions, let them be ever so distant, many closely-allied or representative species occur, there will likewise be found some identical species, showing, in accordance with the foregoing view, that at some former period there has been intercommunication or migration between the two regions. and wherever many closely-allied species occur, there will be found many forms which some naturalists rank as distinct species, and some as varieties; these doubtful forms showing us the steps in the process of modification. this relation between the power and extent of migration of a species, either at the present time or at some former period under different physical conditions, and the existence at remote points of the world of other species allied to it, is shown in another and more general way. mr. gould remarked to me long ago, that in those genera of birds which range over the world, many of the species have very wide ranges. i can hardly doubt that this rule is generally true, though it would be difficult to prove it. amongst mammals, we see it strikingly displayed in bats, and in a lesser degree in the felidæ and canidæ. we see it, if we compare the distribution of butterflies and beetles. so it is with most fresh-water productions, in which so many genera range over the world, and many individual species have { } enormous ranges. it is not meant that in world-ranging genera all the species have a wide range, or even that they have on an _average_ a wide range; but only that some of the species range very widely; for the facility with which widely-ranging species vary and give rise to new forms will largely determine their average range. for instance, two varieties of the same species inhabit america and europe, and the species thus has an immense range; but, if the variation had been a little greater, the two varieties would have been ranked as distinct species, and the common range would have been greatly reduced. still less is it meant, that a species which apparently has the capacity of crossing barriers and ranging widely, as in the case of certain powerfully-winged birds, will necessarily range widely; for we should never forget that to range widely implies not only the power of crossing barriers, but the more important power of being victorious in distant lands in the struggle for life with foreign associates. but on the view of all the species of a genus having descended from a single parent, though now distributed to the most remote points of the world, we ought to find, and i believe as a general rule we do find, that some at least of the species range very widely; for it is necessary that the unmodified parent should range widely, undergoing modification during its diffusion, and should place itself under diverse conditions favourable for the conversion of its offspring, firstly into new varieties and ultimately into new species. in considering the wide distribution of certain genera, we should bear in mind that some are extremely ancient, and must have branched off from a common parent at a remote epoch; so that in such cases there will have been ample time for great climatal and geographical changes and for accidents of transport; and consequently for the migration of some of the species into all { } quarters of the world, where they may have become slightly modified in relation to their new conditions. there is, also, some reason to believe from geological evidence that organisms low in the scale within each great class, generally change at a slower rate than the higher forms; and consequently the lower forms will have had a better chance of ranging widely and of still retaining the same specific character. this fact, together with the seeds and eggs of many low forms being very minute and better fitted for distant transportation, probably accounts for a law which has long been observed, and which has lately been admirably discussed by alph. de candolle in regard to plants, namely, that the lower any group of organisms is, the more widely it is apt to range. the relations just discussed,--namely, low and slowly-changing organisms ranging more widely than the high,--some of the species of widely-ranging genera themselves ranging widely,--such facts, as alpine, lacustrine, and marsh productions being related (with the exceptions before specified) to those on the surrounding low lands and dry lands, though these stations are so different,--the very close relation of the distinct species which inhabit the islets of the same archipelago,--and especially the striking relation of the inhabitants of each whole archipelago or island to those of the nearest mainland,--are, i think, utterly inexplicable on the ordinary view of the independent creation of each species, but are explicable on the view of colonisation from the nearest or readiest source, together with the subsequent modification and better adaptation of the colonists to their new homes. _summary of last and present chapters._--in these chapters i have endeavoured to show, that if we make due allowance for our ignorance of the full effects of all { } the changes of climate and of the level of the land, which have certainly occurred within the recent period, and of other similar changes which may have occurred within the same period; if we remember how profoundly ignorant we are with respect to the many and curious means of occasional transport,--a subject which has hardly ever been properly experimentised on; if we bear in mind how often a species may have ranged continuously over a wide area, and then have become extinct in the intermediate tracts, i think the difficulties in believing that all the individuals of the same species, wherever located, have descended from the same parents, are not insuperable. and we are led to this conclusion, which has been arrived at by many naturalists under the designation of single centres of creation, by some general considerations, more especially from the importance of barriers and from the analogical distribution of sub-genera, genera, and families. with respect to the distinct species of the same genus, which on my theory must have spread from one parent-source; if we make the same allowances as before for our ignorance, and remember that some forms of life change most slowly, enormous periods of time being thus granted for their migration, i do not think that the difficulties are insuperable; though they often are in this case, and in that of the individuals of the same species, extremely great. as exemplifying the effects of climatal changes on distribution, i have attempted to show how important has been the influence of the modern glacial period, which i am fully convinced simultaneously affected the whole world, or at least great meridional belts. as showing how diversified are the means of occasional transport, i have discussed at some little length the means of dispersal of fresh-water productions. { } if the difficulties be not insuperable in admitting that in the long course of time the individuals of the same species, and likewise of allied species, have proceeded from some one source; then i think all the grand leading facts of geographical distribution are explicable on the theory of migration (generally of the more dominant forms of life), together with subsequent modification and the multiplication of new forms. we can thus understand the high importance of barriers, whether of land or water, which separate our several zoological and botanical provinces. we can thus understand the localisation of sub-genera, genera, and families; and how it is that under different latitudes, for instance in south america, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are in so mysterious a manner linked together by affinity, and are likewise linked to the extinct beings which formerly inhabited the same continent. bearing in mind that the mutual relation of organism to organism is of the highest importance, we can see why two areas having nearly the same physical conditions should often be inhabited by very different forms of life; for according to the length of time which has elapsed since new inhabitants entered one region; according to the nature of the communication which allowed certain forms and not others to enter, either in greater or lesser numbers; according or not, as those which entered happened to come in more or less direct competition with each other and with the aborigines; and according as the immigrants were capable of varying more or less rapidly, there would ensue in different regions, independently of their physical conditions, infinitely diversified conditions of life,--there would be an almost endless amount of organic action and reaction,--and we should find, as we do find, some groups of beings greatly, and some only slightly modified,--some { } developed in great force, some existing in scanty numbers--in the different great geographical provinces of the world. on these same principles, we can understand, as i have endeavoured to show, why oceanic islands should have few inhabitants, but of these a great number should be endemic or peculiar; and why, in relation to the means of migration, one group of beings, even within the same class, should have all its species endemic, and another group should have all its species common to other quarters of the world. we can see why whole groups of organisms, as batrachians and terrestrial mammals, should be absent from oceanic islands, whilst the most isolated islands possess their own peculiar species of aërial mammals or bats. we can see why there should be some relation between the presence of mammals, in a more or less modified condition, and the depth of the sea between an island and the mainland. we can clearly see why all the inhabitants of an archipelago, though specifically distinct on the several islets, should be closely related to each other, and likewise be related, but less closely, to those of the nearest continent or other source whence immigrants were probably derived. we can see why in two areas, however distant from each other, there should be a correlation, in the presence of identical species, of varieties, of doubtful species, and of distinct but representative species. as the late edward forbes often insisted, there is a striking parallelism in the laws of life throughout time and space: the laws governing the succession of forms in past times being nearly the same with those governing at the present time the differences in different areas. we see this in many facts. the endurance of each species and group of species is continuous in time; for the exceptions to the rule are so few, that they may { } fairly be attributed to our not having as yet discovered in an intermediate deposit the forms which are therein absent, but which occur above and below: so in space, it certainly is the general rule that the area inhabited by a single species, or by a group of species, is continuous; and the exceptions, which are not rare, may, as i have attempted to show, be accounted for by migration at some former period under different conditions or by occasional means of transport, and by the species having become extinct in the intermediate tracts. both in time and space, species and groups of species have their points of maximum development. groups of species, belonging either to a certain period of time, or to a certain area, are often characterised by trifling characters in common, as of sculpture or colour. in looking to the long succession of ages, as in now looking to distant provinces throughout the world, we find that some organisms differ little, whilst others belonging to a different class, or to a different order, or even only to a different family of the same order, differ greatly. in both time and space the lower members of each class generally change less than the higher; but there are in both cases marked exceptions to the rule. on my theory these several relations throughout time and space are intelligible; for whether we look to the forms of life which have changed during successive ages within the same quarter of the world, or to those which have changed after having migrated into distant quarters, in both cases the forms within each class have been connected by the same bond of ordinary generation; and the more nearly any two forms are related in blood, the nearer they will generally stand to each other in time and space; in both cases the laws of variation have been the same, and modifications have been accumulated by the same power of natural selection. * * * * * { } chapter xiii. mutual affinities of organic beings: morphology: embryology: rudimentary organs. classification, groups subordinate to groups--natural system--rules and difficulties in classification, explained on the theory of descent with modification--classification of varieties--descent always used in classification--analogical or adaptive characters--affinities, general, complex and radiating--extinction separates and defines groups--morphology, between members of the same class, between parts of the same individual--embryology, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age--rudimentary organs; their origin explained--summary. from the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups. this classification is evidently not arbitrary like the grouping of the stars in constellations. the existence of groups would have been of simple signification, if one group had been exclusively fitted to inhabit the land, and another the water; one to feed on flesh, another on vegetable matter, and so on; but the case is widely different in nature; for it is notorious how commonly members of even the same sub-group have different habits. in our second and fourth chapters, on variation and on natural selection, i have attempted to show that it is the widely ranging, the much diffused and common, that is the dominant species belonging to the larger genera, which vary most. the varieties, or incipient species, thus produced ultimately become converted, as i believe, into new and distinct species; and these, on the principle of inheritance, tend to produce other new and dominant { } species. consequently the groups which are now large, and which generally include many dominant species, tend to go on increasing indefinitely in size. i further attempted to show that from the varying descendants of each species trying to occupy as many and as different places as possible in the economy of nature, there is a constant tendency in their characters to diverge. this conclusion was supported by looking at the great diversity of the forms of life which, in any small area, come into the closest competition, and by looking to certain facts in naturalisation. i attempted also to show that there is a constant tendency in the forms which are increasing in number and diverging in character, to supplant and exterminate the less divergent, the less improved, and preceding forms. i request the reader to turn to the diagram illustrating the action, as formerly explained, of these several principles; and he will see that the inevitable result is that the modified descendants proceeding from one progenitor become broken up into groups subordinate to groups. in the diagram each letter on the uppermost line may represent a genus including several species; and all the genera on this line form together one class, for all have descended from one ancient but unseen parent, and, consequently, have inherited something in common. but the three genera on the left hand have, on this same principle, much in common, and form a sub-family, distinct from that including the next two genera on the right hand, which diverged from a common parent at the fifth stage of descent. these five genera have also much, though less, in common; and they form a family distinct from that including the three genera still further to the right hand, which diverged at a still earlier period. and all these genera, descended from (a), form an order distinct from the { } genera descended from (i). so that we here have many species descended from a single progenitor grouped into genera; and the genera are included in, or subordinate to, sub-families, families, and orders, all united into one class. thus, the grand fact in natural history of the subordination of group under group, which, from its familiarity, does not always sufficiently strike us, is in my judgment explained. naturalists try to arrange the species, genera, and families in each class, on what is called the natural system. but what is meant by this system? some authors look at it merely as a scheme for arranging together those living objects which are most alike, and for separating those which are most unlike; or as an artificial means for enunciating, as briefly as possible, general propositions,--that is, by one sentence to give the characters common, for instance, to all mammals, by another those common to all carnivora, by another those common to the dog-genus, and then by adding a single sentence, a full description is given of each kind of dog. the ingenuity and utility of this system are indisputable. but many naturalists think that something more is meant by the natural system; they believe that it reveals the plan of the creator; but unless it be specified whether order in time or space, or what else is meant by the plan of the creator, it seems to me that nothing is thus added to our knowledge. such expressions as that famous one of linnæus, and which we often meet with in a more or less concealed form, that the characters do not make the genus, but that the genus gives the characters, seem to imply that something more is included in our classification, than mere resemblance. i believe that something more is included; and that propinquity of descent,--the only known cause of the similarity of organic beings,--is the bond, hidden as it is by various degrees of { } modification, which is partially revealed to us by our classifications. let us now consider the rules followed in classification, and the difficulties which are encountered on the view that classification either gives some unknown plan of creation, or is simply a scheme for enunciating general propositions and of placing together the forms most like each other. it might have been thought (and was in ancient times thought) that those parts of the structure which determined the habits of life, and the general place of each being in the economy of nature, would be of very high importance in classification. nothing can be more false. no one regards the external similarity of a mouse to a shrew, of a dugong to a whale, of a whale to a fish, as of any importance. these resemblances, though so intimately connected with the whole life of the being, are ranked as merely "adaptive or analogical characters;" but to the consideration of these resemblances we shall have to recur. it may even be given as a general rule, that the less any part of the organisation is concerned with special habits, the more important it becomes for classification. as an instance: owen, in speaking of the dugong, says, "the generative organs being those which are most remotely related to the habits and food of an animal, i have always regarded as affording very clear indications of its true affinities. we are least likely in the modifications of these organs to mistake a merely adaptive for an essential character." so with plants, how remarkable it is that the organs of vegetation, on which their whole life depends, are of little signification, excepting in the first main divisions; whereas the organs of reproduction, with their product the seed, are of paramount importance! we must not, therefore, in classifying, trust to resemblances in parts of the organisation, however important { } they may be for the welfare of the being in relation to the outer world. perhaps from this cause it has partly arisen, that almost all naturalists lay the greatest stress on resemblances in organs of high vital or physiological importance. no doubt this view of the classificatory importance of organs which are important is generally, but by no means always, true. but their importance for classification, i believe, depends on their greater constancy throughout large groups of species; and this constancy depends on such organs having generally been subjected to less change in the adaptation of the species to their conditions of life. that the mere physiological importance of an organ does not determine its classificatory value, is almost shown by the one fact, that in allied groups, in which the same organ, as we have every reason to suppose, has nearly the same physiological value, its classificatory value is widely different. no naturalist can have worked at any group without being struck with this fact; and it has been fully acknowledged in the writings of almost every author. it will suffice to quote the highest authority, robert brown, who in speaking of certain organs in the proteaceæ, says their generic importance, "like that of all their parts, not only in this but, as i apprehend, in every natural family, is very unequal, and in some cases seems to be entirely lost." again in another work he says, the genera of the connaraceæ "differ in having one or more ovaria, in the existence or absence of albumen, in the imbricate or valvular æstivation. any one of these characters singly is frequently of more than generic importance, though here even when all taken together they appear insufficient to separate cnestis from connarus." to give an example amongst insects, in one great division of the hymenoptera, the antennæ, as westwood has remarked, are most constant in structure; { } in another division they differ much, and the differences are of quite subordinate value in classification; yet no one probably will say that the antennae in these two divisions of the same order are of unequal physiological importance. any number of instances could be given of the varying importance for classification of the same important organ within the same group of beings. again, no one will say that rudimentary or atrophied organs are of high physiological or vital importance; yet, undoubtedly, organs in this condition are often of high value in classification. no one will dispute that the rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary bones of the leg, are highly serviceable in exhibiting the close affinity between ruminants and pachyderms. robert brown has strongly insisted on the fact that the rudimentary florets are of the highest importance in the classification of the grasses. numerous instances could be given of characters derived from parts which must be considered of very trifling physiological importance, but which are universally admitted as highly serviceable in the definition of whole groups. for instance, whether or not there is an open passage from the nostrils to the mouth, the only character, according to owen, which absolutely distinguishes fishes and reptiles--the inflection of the angle of the jaws in marsupials--the manner in which the wings of insects are folded--mere colour in certain algæ--mere pubescence on parts of the flower in grasses--the nature of the dermal covering, as hair or feathers, in the vertebrata. if the ornithorhynchus had been covered with feathers instead of hair, this external and trifling character would, i think, have been considered by naturalists as important an aid in determining the degree of affinity of this strange creature to { } birds and reptiles, as an approach in structure in any one internal and important organ. the importance, for classification, of trifling characters, mainly depends on their being correlated with several other characters of more or less importance. the value indeed of an aggregate of characters is very evident in natural history. hence, as has often been remarked, a species may depart from its allies in several characters, both of high physiological importance and of almost universal prevalence, and yet leave us in no doubt where it should be ranked. hence, also, it has been found, that a classification founded on any single character, however important that may be, has always failed; for no part of the organisation is universally constant. the importance of an aggregate of characters, even when none are important, alone explains, i think, that saying of linnæus, that the characters do not give the genus, but the genus gives the characters; for this saying seems founded on an appreciation of many trifling points of resemblance, too slight to be defined. certain plants, belonging to the malpighiaceæ, bear perfect and degraded flowers; in the latter, as a. de jussieu has remarked, "the greater number of the characters proper to the species, to the genus, to the family, to the class, disappear, and thus laugh at our classification." but when aspicarpa produced in france, during several years, only degraded flowers, departing so wonderfully in a number of the most important points of structure from the proper type of the order, yet m. richard sagaciously saw, as jussieu observes, that this genus should still be retained amongst the malpighiaceæ. this case seems to me well to illustrate the spirit with which our classifications are sometimes necessarily founded. practically when naturalists are at work, they do { } not trouble themselves about the physiological value of the characters which they use in defining a group, or in allocating any particular species. if they find a character nearly uniform, and common to a great number of forms, and not common to others, they use it as one of high value; if common to some lesser number, they use it as of subordinate value. this principle has been broadly confessed by some naturalists to be the true one; and by none more clearly than by that excellent botanist, aug. st. hilaire. if certain characters are always found correlated with others, though no apparent bond of connexion can be discovered between them, especial value is set on them. as in most groups of animals, important organs, such as those for propelling the blood, or for aërating it, or those for propagating the race, are found nearly uniform, they are considered as highly serviceable in classification; but in some groups of animals all these, the most important vital organs, are found to offer characters of quite subordinate value. we can see why characters derived from the embryo should be of equal importance with those derived from the adult, for our classifications of course include all ages of each species. but it is by no means obvious, on the ordinary view, why the structure of the embryo should be more important for this purpose than that of the adult, which alone plays its full part in the economy of nature. yet it has been strongly urged by those great naturalists, milne edwards and agassiz, that embryonic characters are the most important of any in the classification of animals; and this doctrine has very generally been admitted as true. the same fact holds good with flowering plants, of which the two main divisions have been founded on characters derived from the embryo,--on the number and position of the { } embryonic leaves or cotyledons, and on the mode of development of the plumule and radicle. in our discussion on embryology, we shall see why such characters are so valuable, on the view of classification tacitly including the idea of descent. our classifications are often plainly influenced by chains of affinities. nothing can be easier than to define a number of characters common to all birds; but in the case of crustaceans, such definition has hitherto been found impossible. there are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the articulata. geographical distribution has often been used, though perhaps not quite logically, in classification, more especially in very large groups of closely allied forms. temminck insists on the utility or even necessity of this practice in certain groups of birds; and it has been followed by several entomologists and botanists. finally, with respect to the comparative value of the various groups of species, such as orders, sub-orders, families, sub-families, and genera, they seem to be, at least at present, almost arbitrary. several of the best botanists, such as mr. bentham and others, have strongly insisted on their arbitrary value. instances could be given amongst plants and insects, of a group of forms, first ranked by practised naturalists as only a genus, and then raised to the rank of a sub-family or family; and this has been done, not because further research has detected important structural differences, at first overlooked, but because numerous allied species, with slightly different grades of difference, have been subsequently discovered. { } all the foregoing rules and aids and difficulties in classification are explained, if i do not greatly deceive myself, on the view that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike. but i must explain my meaning more fully. i believe that the _arrangement_ of the groups within each class, in due subordination and relation to the other groups, must be strictly genealogical in order to be natural; but that the _amount_ of difference in the several branches or groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification which they have undergone; and this is expressed by the forms being ranked under different genera, families, sections, or orders. the reader will best understand what is meant, if he will take the trouble of referring to the diagram in the fourth chapter. we will suppose the letters a to l to represent allied genera, which lived during the silurian epoch, and these have descended from a species which existed at an unknown anterior period. species of three of these genera (a, f, and i) have transmitted modified descendants to the present day, represented by the fifteen genera (a^{ } to z^{ }) on the uppermost horizontal line. now all these modified descendants from a single species, are represented as related in blood or descent to the same { } degree; they may metaphorically be called cousins to the same millionth degree; yet they differ widely and in different degrees from each other. the forms descended from a, now broken up into two or three families, constitute a distinct order from those descended from i, also broken up into two families. nor can the existing species, descended from a, be ranked in the same genus with the parent a; or those from i, with the parent i. but the existing genus f^{ } may be supposed to have been but slightly modified; and it will then rank with the parent-genus f; just as some few still living organic beings belong to silurian genera. so that the amount or value of the differences between organic beings all related to each other in the same degree in blood, has come to be widely different. nevertheless their genealogical _arrangement_ remains strictly true, not only at the present time, but at each successive period of descent. all the modified descendants from a will have inherited something in common from their common parent, as will all the descendants from i; so will it be with each subordinate branch of descendants, at each successive period. if, however, we choose to suppose that any of the descendants of a or of i have been so much modified as to have more or less completely lost traces of their parentage, in this case, their places in a natural classification will have been more or less completely lost,--as sometimes seems to have occurred with existing organisms. all the descendants of the genus f, along its whole line of descent, are supposed to have been but little modified, and they yet form a single genus. but this genus, though much isolated, will still occupy its proper intermediate position; for f originally was intermediate in character between a and i, and the several genera descended from these two genera will { } have inherited to a certain extent their characters. this natural arrangement is shown, as far as is possible on paper, in the diagram, but in much too simple a manner. if a branching diagram had not been used, and only the names of the groups had been written in a linear series, it would have been still less possible to have given a natural arrangement; and it is notoriously not possible to represent in a series, on a flat surface, the affinities which we discover in nature amongst the beings of the same group. thus, on the view which i hold, the natural system is genealogical in its arrangement, like a pedigree; but the degrees of modification which the different groups have undergone, have to be expressed by ranking them under different so-called genera, sub-families, families, sections, orders, and classes. it may be worth while to illustrate this view of classification, by taking the case of languages. if we possessed a perfect pedigree of mankind, a genealogical arrangement of the races of man would afford the best classification of the various languages now spoken throughout the world; and if all extinct languages, and all intermediate and slowly changing dialects, had to be included, such an arrangement would, i think, be the only possible one. yet it might be that some very ancient language had altered little, and had given rise to few new languages, whilst others (owing to the spreading and subsequent isolation and states of civilisation of the several races, descended from a common race) had altered much, and had given rise to many new languages and dialects. the various degrees of difference in the languages from the same stock, would have to be expressed by groups subordinate to groups; but the proper or even only possible arrangement would still be genealogical; and this would be strictly natural, as { } it would connect together all languages, extinct and modern, by the closest affinities, and would give the filiation and origin of each tongue. in confirmation of this view, let us glance at the classification of varieties, which are believed or known to have descended from one species. these are grouped under species, with sub-varieties under varieties; and with our domestic productions, several other grades of difference are requisite, as we have seen with pigeons. the origin of the existence of groups subordinate to groups, is the same with varieties as with species, namely, closeness of descent with various degrees of modification. nearly the same rules are followed in classifying varieties, as with species. authors have insisted on the necessity of classing varieties on a natural instead of an artificial system; we are cautioned, for instance, not to class two varieties of the pine-apple together, merely because their fruit, though the most important part, happens to be nearly identical; no one puts the swedish and common turnips together, though the esculent and thickened stems are so similar. whatever part is found to be most constant, is used in classing varieties: thus the great agriculturist marshall says the horns are very useful for this purpose with cattle, because they are less variable than the shape or colour of the body, &c.; whereas with sheep the horns are much less serviceable, because less constant. in classing varieties, i apprehend if we had a real pedigree, a genealogical classification would be universally preferred; and it has been attempted by some authors. for we might feel sure, whether there had been more or less modification, the principle of inheritance would keep the forms together which were allied in the greatest number of points. in tumbler pigeons, though some sub-varieties differ from the others { } in the important character of having a longer beak, yet all are kept together from having the common habit of tumbling; but the short-faced breed has nearly or quite lost this habit; nevertheless, without any reasoning or thinking on the subject, these tumblers are kept in the same group, because allied in blood and alike in some other respects. if it could be proved that the hottentot had descended from the negro, i think he would be classed under the negro group, however much he might differ in colour and other important characters from negroes. with species in a state of nature, every naturalist has in fact brought descent into his classification; for he includes in his lowest grade, or that of a species, the two sexes; and how enormously these sometimes differ in the most important characters, is known to every naturalist: scarcely a single fact can be predicated in common of the males and hermaphrodites of certain cirripedes, when adult, and yet no one dreams of separating them. the naturalist includes as one species the several larval stages of the same individual, however much they may differ from each other and from the adult; as he likewise includes the so-called alternate generations of steenstrup, which can only in a technical sense be considered as the same individual. he includes monsters; he includes varieties, not solely because they closely resemble the parent-form, but because they are descended from it. he who believes that the cowslip is descended from the primrose, or conversely, ranks them together as a single species, and gives a single definition. as soon as three orchidean forms (monochanthus, myanthus, and catasetum), which had previously been ranked as three distinct genera, were known to be sometimes produced on the same spike, they were immediately included as a single species. { } as descent has universally been used in classing together the individuals of the same species, though the males and females and larvæ are sometimes extremely different; and as it has been used in classing varieties which have undergone a certain, and sometimes a considerable amount of modification, may not this same element of descent have been unconsciously used in grouping species under genera, and genera under higher groups, though in these cases the modification has been greater in degree, and has taken a longer time to complete? i believe it has thus been unconsciously used; and only thus can i understand the several rules and guides which have been followed by our best systematists. we have no written pedigrees; we have to make out community of descent by resemblances of any kind. therefore we choose those characters which, as far as we can judge, are the least likely to have been modified in relation to the conditions of life to which each species has been recently exposed. rudimentary structures on this view are as good as, or even sometimes better than, other parts of the organisation. we care not how trifling a character may be--let it be the mere inflection of the angle of the jaw, the manner in which an insect's wing is folded, whether the skin be covered by hair or feathers--if it prevail throughout many and different species, especially those having very different habits of life, it assumes high value; for we can account for its presence in so many forms with such different habits, only by its inheritance from a common parent. we may err in this respect in regard to single points of structure, but when several characters, let them be ever so trifling, occur together throughout a large group of beings having different habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor. { } and we know that such correlated or aggregated characters have especial value in classification. we can understand why a species or a group of species may depart, in several of its most important characteristics, from its allies, and yet be safely classed with them. this may be safely done, and is often done, as long as a sufficient number of characters, let them be ever so unimportant, betrays the hidden bond of community of descent. let two forms have not a single character in common, yet if these extreme forms are connected together by a chain of intermediate groups, we may at once infer their community of descent, and we put them all into the same class. as we find organs of high physiological importance--those which serve to preserve life under the most diverse conditions of existence--are generally the most constant, we attach especial value to them; but if these same organs, in another group or section of a group, are found to differ much, we at once value them less in our classification. we shall hereafter, i think, clearly see why embryological characters are of such high classificatory importance. geographical distribution may sometimes be brought usefully into play in classing large and widely-distributed genera, because all the species of the same genus, inhabiting any distinct and isolated region, have in all probability descended from the same parents. we can understand, on these views, the very important distinction between real affinities and analogical or adaptive resemblances. lamarck first called attention to this distinction, and he has been ably followed by macleay and others. the resemblance, in the shape of the body and in the fin-like anterior limbs, between the dugong, which is a pachydermatous animal, and the whale, and between both these mammals and fishes, is analogical. amongst insects there are innumerable { } instances: thus linnæus, misled by external appearances, actually classed an homopterous insect as a moth. we see something of the same kind even in our domestic varieties, as in the thickened stems of the common and swedish turnip. the resemblance of the greyhound and racehorse is hardly more fanciful than the analogies which have been drawn by some authors between very distinct animals. on my view of characters being of real importance for classification, only in so far as they reveal descent, we can clearly understand why analogical or adaptive character, although of the utmost importance to the welfare of the being, are almost valueless to the systematist. for animals, belonging to two most distinct lines of descent, may readily become adapted to similar conditions, and thus assume a close external resemblance; but such resemblances will not reveal--will rather tend to conceal their blood-relationship to their proper lines of descent. we can also understand the apparent paradox, that the very same characters are analogical when one class or order is compared with another, but give true affinities when the members of the same class or order are compared one with another: thus the shape of the body and fin-like limbs are only analogical when whales are compared with fishes, being adaptations in both classes for swimming through the water; but the shape of the body and fin-like limbs serve as characters exhibiting true affinity between the several members of the whale family; for these cetaceans agree in so many characters, great and small, that we cannot doubt that they have inherited their general shape of body and structure of limbs from a common ancestor. so it is with fishes. as members of distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances,--to inhabit for instance { } the three elements of land, air, and water,--we can perhaps understand how it is that a numerical parallelism has sometimes been observed between the sub-groups in distinct classes. a naturalist, struck by a parallelism of this nature in any one class, by arbitrarily raising or sinking the value of the groups in other classes (and all our experience shows that this valuation has hitherto been arbitrary), could easily extend the parallelism over a wide range; and thus the septenary, quinary, quaternary, and ternary classifications have probably arisen. as the modified descendants of dominant species, belonging to the larger genera, tend to inherit the advantages, which made the groups to which they belong large and their parents dominant, they are almost sure to spread widely, and to seize on more and more places in the economy of nature. the larger and more dominant groups thus tend to go on increasing in size; and they consequently supplant many smaller and feebler groups. thus we can account for the fact that all organisms, recent and extinct, are included under a few great orders, under still fewer classes, and all in one great natural system. as showing how few the higher groups are in number, and how widely spread they are throughout the world, the fact is striking, that the discovery of australia has not added a single insect belonging to a new class; and that in the vegetable kingdom, as i learn from dr. hooker, it has added only two or three orders of small size. in the chapter on geological succession i attempted to show, on the principle of each group having generally diverged much in character during the long-continued process of modification, how it is that the more ancient forms of life often present characters in some slight degree intermediate between existing groups. a few { } old and intermediate parent-forms having occasionally transmitted to the present day descendants but little modified, will give to us our so-called osculant or aberrant groups. the more aberrant any form is, the greater must be the number of connecting forms which on my theory have been exterminated and utterly lost. and we have some evidence of aberrant forms having suffered severely from extinction, for they are generally represented by extremely few species; and such species as do occur are generally very distinct from each other, which again implies extinction. the genera ornithorhynchus and lepidosiren, for example, would not have been less aberrant had each been represented by a dozen species instead of by a single one; but such richness in species, as i find after some investigation, does not commonly fall to the lot of aberrant genera. we can, i think, account for this fact only by looking at aberrant forms as failing groups conquered by more successful competitors, with a few members preserved by some unusual coincidence of favourable circumstances. mr. waterhouse has remarked that, when a member belonging to one group of animals exhibits an affinity to a quite distinct group, this affinity in most cases is general and not special: thus, according to mr. waterhouse, of all rodents, the bizcacha is most nearly related to marsupials; but in the points in which it approaches this order, its relations are general, and not to any one marsupial species more than to another. as the points of affinity of the bizcacha to marsupials are believed to be real and not merely adaptive, they are due on my theory to inheritance in common. therefore we must suppose either that all rodents, including the bizcacha, branched off from some very ancient marsupial, which will have had a character in some degree intermediate with respect to all existing marsupials; or { } that both rodents and marsupials branched off from a common progenitor, and that both groups have since undergone much modification in divergent directions. on either view we may suppose that the bizcacha has retained, by inheritance, more of the character of its ancient progenitor than have other rodents; and therefore it will not be specially related to any one existing marsupial, but indirectly to all or nearly all marsupials, from having partially retained the character of their common progenitor, or of an early member of the group. on the other hand, of all marsupials, as mr. waterhouse has remarked, the phascolomys resembles most nearly, not any one species, but the general order of rodents. in this case, however, it may be strongly suspected that the resemblance is only analogical, owing to the phascolomys having become adapted to habits like those of a rodent. the elder de candolle has made nearly similar observations on the general nature of the affinities of distinct orders of plants. on the principle of the multiplication and gradual divergence in character of the species descended from a common parent, together with their retention by inheritance of some characters in common, we can understand the excessively complex and radiating affinities by which all the members of the same family or higher group are connected together. for the common parent of a whole family of species, now broken up by extinction into distinct groups and sub-groups, will have transmitted some of its characters, modified in various ways and degrees, to all; and the several species will consequently be related to each other by circuitous lines of affinity of various lengths (as may be seen in the diagram so often referred to), mounting up through many predecessors. as it is difficult to show the blood-relationship between the numerous kindred { } of any ancient and noble family, even by the aid of a genealogical tree, and almost impossible to do this without this aid, we can understand the extraordinary difficulty which naturalists have experienced in describing, without the aid of a diagram, the various affinities which they perceive between the many living and extinct members of the same great natural class. extinction, as we have seen in the fourth chapter, has played an important part in defining and widening the intervals between the several groups in each class. we may thus account even for the distinctness of whole classes from each other--for instance, of birds from all other vertebrate animals--by the belief that many ancient forms of life have been utterly lost, through which the early progenitors of birds were formerly connected with the early progenitors of the other vertebrate classes. there has been less entire extinction of the forms of life which once connected fishes with batrachians. there has been still less in some other classes, as in that of the crustacea, for here the most wonderfully diverse forms are still tied together by a long, but broken, chain of affinities. extinction has only separated groups: it has by no means made them; for if every form which has ever lived on this earth were suddenly to reappear, though it would be quite impossible to give definitions by which each group could be distinguished from other groups, as all would blend together by steps as fine as those between the finest existing varieties, nevertheless a natural classification, or at least a natural arrangement, would be possible. we shall see this by turning to the diagram: the letters, a to l, may represent eleven silurian genera, some of which have produced large groups of modified descendants. every intermediate link between these eleven genera and their primordial parent, and every { } intermediate link in each branch and sub-branch of their descendants, may be supposed to be still alive; and the links to be as fine as those between the finest varieties. in this case it would be quite impossible to give any definition by which the several members of the several groups could be distinguished from their more immediate parents; or these parents from their ancient and unknown progenitor. yet the natural arrangement in the diagram would still hold good; and, on the principle of inheritance, all the forms descended from a, or from i, would have something in common. in a tree we can specify this or that branch, though at the actual fork the two unite and blend together. we could not, as i have said, define the several groups; but we could pick out types, or forms, representing most of the characters of each group, whether large or small, and thus give a general idea of the value of the differences between them. this is what we should be driven to, if we were ever to succeed in collecting all the forms in any class which have lived throughout all time and space. we shall certainly never succeed in making so perfect a collection: nevertheless, in certain classes, we are tending in this direction; and milne edwards has lately insisted, in an able paper, on the high importance of looking to types, whether or not we can separate and define the groups to which such types belong. finally, we have seen that natural selection, which results from the struggle for existence, and which almost inevitably induces extinction and divergence of character in the many descendants from one dominant parent-species, explains that great and universal feature in the affinities of all organic beings, namely, their subordination in group under group. we use the element of descent in classing the individuals of both sexes and of all ages, although having few characters in common, { } under one species; we use descent in classing acknowledged varieties, however different they may be from their parent; and i believe this element of descent is the hidden bond of connexion which naturalists have sought under the term of the natural system. on this idea of the natural system being, in so far as it has been perfected, genealogical in its arrangement, with the grades of difference between the descendants from a common parent, expressed by the terms genera, families, orders, &c., we can understand the rules which we are compelled to follow in our classification. we can understand why we value certain resemblances far more than others; why we are permitted to use rudimentary and useless organs, or others of trifling physiological importance; why, in comparing one group with a distinct group, we summarily reject analogical or adaptive characters, and yet use these same characters within the limits of the same group. we can clearly see how it is that all living and extinct forms can be grouped together in one great system; and how the several members of each class are connected together by the most complex and radiating lines of affinities. we shall never, probably, disentangle the inextricable web of affinities between the members of any one class; but when we have a distinct object in view, and do not look to some unknown plan of creation, we may hope to make sure but slow progress. _morphology._--we have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organisation. this resemblance is often expressed by the term "unity of type;" or by saying that the several parts and organs in the different species of the class are homologous. the whole subject is included under { } the general name of morphology. this is the most interesting department of natural history, and may be said to be its very soul. what can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions? geoffroy st. hilaire has insisted strongly on the high importance of relative connexion in homologous organs: the parts may change to almost any extent in form and size, and yet they always remain connected together in the same order. we never find, for instance, the bones of the arm and forearm, or of the thigh and leg, transposed. hence the same names can be given to the homologous bones in widely different animals. we see the same great law in the construction of the mouths of insects: what can be more different than the immensely long spiral proboscis of a sphinx-moth, the curious folded one of a bee or bug, and the great jaws of a beetle?--yet all these organs, serving for such different purposes, are formed by infinitely numerous modifications of an upper lip, mandibles, and two pairs of maxillæ. analogous laws govern the construction of the mouths and limbs of crustaceans. so it is with the flowers of plants. nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same class, by utility or by the doctrine of final causes. the hopelessness of the attempt has been expressly admitted by owen in his most interesting work on the 'nature of limbs.' on the ordinary view of the independent creation of each being, we can only say that so it is;--that it has so pleased the creator to construct each animal and plant. the explanation is manifest on the theory of the { } natural selection of successive slight modifications,--each modification being profitable in some way to the modified form, but often affecting by correlation of growth other parts of the organisation. in changes of this nature, there will be little or no tendency to modify the original pattern, or to transpose parts. the bones of a limb might be shortened and widened to any extent, and become gradually enveloped in thick membrane, so as to serve as a fin; or a webbed foot might have all its bones, or certain bones, lengthened to any extent, and the membrane connecting them increased to any extent, so as to serve as a wing: yet in all this great amount of modification there will be no tendency to alter the framework of bones or the relative connexion of the several parts. if we suppose that the ancient progenitor, the archetype as it may be called, of all mammals, had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain signification of the homologous construction of the limbs throughout the whole class. so with the mouths of insects, we have only to suppose that their common progenitor had an upper lip, mandibles, and two pair of maxillæ, these parts being perhaps very simple in form; and then natural selection, acting on some originally created form, will account for the infinite diversity in structure and function of the mouths of insects. nevertheless, it is conceivable that the general pattern of an organ might become so much obscured as to be finally lost, by the atrophy and ultimately by the complete abortion of certain parts, by the soldering together of other parts, and by the doubling or multiplication of others,--variations which we know to be within the limits of possibility. in the paddles of the extinct gigantic sea-lizards, and in the mouths of certain suctorial crustaceans, the { } general pattern seems to have been thus to a certain extent obscured. there is another and equally curious branch of the present subject; namely, the comparison not of the same part in different members of a class, but of the different parts or organs in the same individual. most physiologists believe that the bones of the skull are homologous with--that is correspond in number and in relative connexion with--the elemental parts of a certain number of vertebræ. the anterior and posterior limbs in each member of the vertebrate and articulate classes are plainly homologous. we see the same law in comparing the wonderfully complex jaws and legs in crustaceans. it is familiar to almost every one, that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves, arranged in a spire. in monstrous plants, we often get direct evidence of the possibility of one organ being transformed into another; and we can actually see in embryonic crustaceans and in many other animals, and in flowers, that organs, which when mature become extremely different, are at an early stage of growth exactly alike. how inexplicable are these facts on the ordinary view of creation! why should the brain be enclosed in a box composed of such numerous and such extraordinary shaped pieces of bone? as owen has remarked, the benefit derived from the yielding of the separate pieces in the act of parturition of mammals, will by no means explain the same construction in the skulls of birds. why should similar bones have been created in the formation of the wing and leg of a bat, used as they are for such totally different purposes? why should one crustacean, which has an extremely complex { } mouth formed of many parts, consequently always have fewer legs; or conversely, those with many legs have simpler mouths? why should the sepals, petals, stamens, and pistils in any individual flower, though fitted for such widely different purposes, be all constructed on the same pattern? on the theory of natural selection, we can satisfactorily answer these questions. in the vertebrata, we see a series of internal vertebræ bearing certain processes and appendages; in the articulata, we see the body divided into a series of segments, bearing external appendages; and in flowering plants, we see a series of successive spiral whorls of leaves. an indefinite repetition of the same part or organ is the common characteristic (as owen has observed) of all low or little-modified forms; therefore we may readily believe that the unknown progenitor of the vertebrata possessed many vertebræ; the unknown progenitor of the articulata, many segments; and the unknown progenitor of flowering plants, many spiral whorls of leaves. we have formerly seen that parts many times repeated are eminently liable to vary in number and structure; consequently it is quite probable that natural selection, during a long-continued course of modification, should have seized on a certain number of the primordially similar elements, many times repeated, and have adapted them to the most diverse purposes. and as the whole amount of modification will have been effected by slight successive steps, we need not wonder at discovering in such parts or organs, a certain degree of fundamental resemblance, retained by the strong principle of inheritance. in the great class of molluscs, though we can homologise the parts of one species with those of other and distinct species, we can indicate but few serial homologies; that is, we are seldom enabled to say that one { } part or organ is homologous with another in the same individual. and we can understand this fact; for in molluscs, even in the lowest members of the class, we do not find nearly so much indefinite repetition of any one part, as we find in the other great classes of the animal and vegetable kingdoms. naturalists frequently speak of the skull as formed of metamorphosed vertebræ: the jaws of crabs as metamorphosed legs; the stamens and pistils of flowers as metamorphosed leaves; but it would in these cases probably be more correct, as professor huxley has remarked, to speak of both skull and vertebræ, both jaws and legs, &c.,--as having been metamorphosed, not one from the other, but from some common element. naturalists, however, use such language only in a metaphorical sense: they are far from meaning that during a long course of descent, primordial organs of any kind--vertebræ in the one case and legs in the other--have actually been modified into skulls or jaws. yet so strong is the appearance of a modification of this nature having occurred, that naturalists can hardly avoid employing language having this plain signification. on my view these terms may be used literally; and the wonderful fact of the jaws, for instance, of a crab retaining numerous characters, which they would probably have retained through inheritance, if they had really been metamorphosed during a long course of descent from true legs, or from some simple appendage, is explained. _embryology._--it has already been casually remarked that certain organs in the individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike. the embryos, also, of distinct animals within the same class are often strikingly similar: a better proof of this cannot be given, than a { } circumstance mentioned by agassiz, namely, that having forgotten to ticket the embryo of some vertebrate animal, he cannot now tell whether it be that of a mammal, bird, or reptile. the vermiform larvæ of moths, flies, beetles, &c., resemble each other much more closely than do the mature insects; but in the case of larvæ, the embryos are active, and have been adapted for special lines of life. a trace of the law of embryonic resemblance, sometimes lasts till a rather late age: thus birds of the same genus, and of closely allied genera, often resemble each other in their first and second plumage; as we see in the spotted feathers in the thrush group. in the cat tribe, most of the species are striped or spotted in lines; and stripes can be plainly distinguished in the whelp of the lion. we occasionally though rarely see something of this kind in plants: thus the embryonic leaves of the ulex or furze, and the first leaves of the phyllodineous acaceas, are pinnate or divided like the ordinary leaves of the leguminosæ. the points of structure, in which the embryos of widely different animals of the same class resemble each other, often have no direct relation to their conditions of existence. we cannot, for instance, suppose that in the embryos of the vertebrata the peculiar loop-like course of the arteries near the branchial slits are related to similar conditions,--in the young mammal which is nourished in the womb of its mother, in the egg of the bird which is hatched in a nest, and in the spawn of a frog under water. we have no more reason to believe in such a relation, than we have to believe that the same bones in the hand of a man, wing of a bat, and fin of a porpoise, are related to similar conditions of life. no one will suppose that the stripes on the whelp of a lion, or the spots on the young blackbird, { } are of any use to these animals, or are related to the conditions to which they are exposed. the case, however, is different when an animal during any part of its embryonic career is active, and has to provide for itself. the period of activity may come on earlier or later in life; but whenever it comes on, the adaptation of the larva to its conditions of life is just as perfect and as beautiful as in the adult animal. from such special adaptations, the similarity of the larvæ or active embryos of allied animals is sometimes much obscured; and cases could be given of the larvæ of two species, or of two groups of species, differing quite as much, or even more, from each other than do their adult parents. in most cases, however, the larvæ, though active, still obey, more or less closely, the law of common embryonic resemblance. cirripedes afford a good instance of this: even the illustrious cuvier did not perceive that a barnacle was, as it certainly is, a crustacean; but a glance at the larva shows this to be the case in an unmistakeable manner. so again the two main divisions of cirripedes, the pedunculated and sessile, which differ widely in external appearance, have larvæ in all their stages barely distinguishable. the embryo in the course of development generally rises in organisation: i use this expression, though i am aware that it is hardly possible to define clearly what is meant by the organisation being higher or lower. but no one probably will dispute that the butterfly is higher than the caterpillar. in some cases, however, the mature animal is generally considered as lower in the scale than the larva, as with certain parasitic crustaceans. to refer once again to cirripedes: the larvæ in the first stage have three pairs of legs, a very simple single eye, and a probosciformed mouth, with which they feed largely, for they increase much in { } size. in the second stage, answering to the chrysalis stage of butterflies, they have six pairs of beautifully constructed natatory legs, a pair of magnificent compound eyes, and extremely complex antennæ; but they have a closed and imperfect mouth, and cannot feed: their function at this stage is, to search by their well-developed organs of sense, and to reach by their active powers of swimming, a proper place on which to become attached and to undergo their final metamorphosis. when this is completed they are fixed for life: their legs are now converted into prehensile organs; they again obtain a well-constructed mouth; but they have no antennæ, and their two eyes are now reconverted into a minute, single, and very simple eye-spot. in this last and complete state, cirripedes may be considered as either more highly or more lowly organised than they were in the larval condition. but in some genera the larvæ become developed either into hermaphrodites having the ordinary structure, or into what i have called complemental males: and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction. we are so much accustomed to see differences in structure between the embryo and the adult, and likewise a close similarity in the embryos of widely different animals within the same class, that we might be led to look at these facts as necessarily contingent in some manner on growth. but there is no obvious reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all the parts in proper proportion, as soon as any structure became visible in the embryo. and in some whole groups of animals and in certain members of other groups, the embryo does not at any period differ widely from the { } adult: thus owen has remarked in regard to cuttle-fish, "there is no metamorphosis; the cephalopodic character is manifested long before the parts of the embryo are completed;" and again in spiders, "there is nothing worthy to be called a metamorphosis." the larvæ of insects, whether adapted to the most diverse and active habits, or quite inactive, being fed by their parents or placed in the midst of proper nutriment, yet nearly all pass through a similar worm-like stage of development; but in some few cases, as in that of aphis, if we look to the admirable drawings by professor huxley of the development of this insect, we see no trace of the vermiform stage. how, then, can we explain these several facts in embryology,--namely the very general, but not universal difference in structure between the embryo and the adult;--of parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike;--of embryos of different species within the same class, generally, but not universally, resembling each other;--of the structure of the embryo not being closely related to its conditions of existence, except when the embryo becomes at any period of life active and has to provide for itself;--of the embryo apparently having sometimes a higher organisation than the mature animal, into which it is developed? i believe that all these facts can be explained, as follows, on the view of descent with modification. it is commonly assumed, perhaps from monstrosities often affecting the embryos at a very early period, that slight variations necessarily appear at an equally early period. but we have little evidence on this head--indeed the evidence rather points the other way; for it is notorious that breeders of cattle, horses, and various { } fancy animals, cannot positively tell, until some time after the animal has been born, what its merits or form will ultimately turn out. we see this plainly in our own children; we cannot always tell whether the child will be tall or short, or what its precise features will be. the question is not, at what period of life any variation has been caused, but at what period it is fully displayed. the cause may have acted, and i believe generally has acted, even before the embryo is formed; and the variation may be due to the male and female sexual elements having been affected by the conditions to which either parent, or their ancestors, have been exposed. nevertheless an effect thus caused at a very early period, even before the formation of the embryo, may appear late in life; as when an hereditary disease, which appears in old age alone, has been communicated to the offspring from the reproductive element of one parent. or again, as when the horns of cross-bred cattle have been affected by the shape of the horns of either parent. for the welfare of a very young animal, as long as it remains in its mother's womb, or in the egg, or as long as it is nourished and protected by its parent, it must be quite unimportant whether most of its characters are fully acquired a little earlier or later in life. it would not signify, for instance, to a bird which obtained its food best by having a long beak, whether or not it assumed a beak of this particular length, as long as it was fed by its parents. hence, i conclude, that it is quite possible, that each of the many successive modifications, by which each species has acquired its present structure, may have supervened at a not very early period of life; and some direct evidence from our domestic animals supports this view. but in other cases it is quite possible that each successive modification, or { } most of them, may have appeared at an extremely early period. i have stated in the first chapter, that there is some evidence to render it probable, that at whatever age any variation first appears in the parent, it tends to reappear at a corresponding age in the offspring. certain variations can only appear at corresponding ages, for instance, peculiarities in the caterpillar, cocoon, or imago states of the silk-moth; or, again, in the horns of almost full-grown cattle. but further than this, variations which, for all that we can see, might have appeared earlier or later in life, tend to appear at a corresponding age in the offspring and parent. i am far from meaning that this is invariably the case; and i could give a good many cases of variations (taking the word in the largest sense) which have supervened at an earlier age in the child than in the parent. these two principles, if their truth be admitted, will, i believe, explain all the above specified leading facts in embryology. but first let us look at a few analogous cases in domestic varieties. some authors who have written on dogs, maintain that the greyhound and bulldog, though appearing so different, are really varieties most closely allied, and have probably descended from the same wild stock; hence i was curious to see how far their puppies differed from each other: i was told by breeders that they differed just as much as their parents, and this, judging by the eye, seemed almost to be the case; but on actually measuring the old dogs and their six-days old puppies, i found that the puppies had not nearly acquired their full amount of proportional difference. so, again, i was told that the foals of cart and race-horses differed as much as the full-grown animals; and this surprised me greatly, as i think it probable that the difference between these two breeds has been wholly { } caused by selection under domestication; but having had careful measurements made of the dam and of a three-days old colt of a race and heavy cart-horse, i find that the colts have by no means acquired their full amount of proportional difference. as the evidence appears to me conclusive, that the several domestic breeds of pigeon have descended from one wild species, i compared young pigeons of various breeds, within twelve hours after being hatched; i carefully measured the proportions (but will not here give details) of the beak, width of mouth, length of nostril and of eyelid, size of feet and length of leg, in the wild stock, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. now some of these birds, when mature, differ so extraordinarily in length and form of beak, that they would, i cannot doubt, be ranked in distinct genera, had they been natural productions. but when the nestling birds of these several breeds were placed in a row, though most of them could be distinguished from each other, yet their proportional differences in the above specified several points were incomparably less than in the full-grown birds. some characteristic points of difference--for instance, that of the width of mouth--could hardly be detected in the young. but there was one remarkable exception to this rule, for the young of the short-faced tumbler differed from the young of the wild rock-pigeon and of the other breeds, in all its proportions, almost exactly as much as in the adult state. the two principles above given seem to me to explain these facts in regard to the later embryonic stages of our domestic varieties. fanciers select their horses, dogs, and pigeons, for breeding, when they are nearly grown up: they are indifferent whether the desired qualities and structures have been acquired earlier or { } later in life, if the full-grown animal possesses them. and the cases just given, more especially that of pigeons, seem to show that the characteristic differences which give value to each breed, and which have been accumulated by man's selection, have not generally first appeared at an early period of life, and have been inherited by the offspring at a corresponding not early period. but the case of the short-faced tumbler, which when twelve hours old had acquired its proper proportions, proves that this is not the universal rule; for here the characteristic differences must either have appeared at an earlier period than usual, or, if not so, the differences must have been inherited, not at the corresponding, but at an earlier age. now let us apply these facts and the above two principles--which latter, though not proved true, can be shown to be in some degree probable--to species in a state of nature. let us take a genus of birds, descended on my theory from some one parent-species, and of which the several new species have become modified through natural selection in accordance with their diverse habits. then, from the many slight successive steps of variation having supervened at a rather late age, and having been inherited at a corresponding age, the young of the new species of our supposed genus will manifestly tend to resemble each other much more closely than do the adults, just as we have seen in the case of pigeons. we may extend this view to whole families or even classes. the fore-limbs, for instance, which served as legs in the parent-species, may have become, by a long course of modification, adapted in one descendant to act as hands, in another as paddles, in another as wings; and on the above two principles--namely of each successive modification supervening at a rather late age, and being inherited at a { } corresponding late age--the fore-limbs in the embryos of the several descendants of the parent-species will still resemble each other closely, for they will not have been modified. but in each of our new species, the embryonic fore-limbs will differ greatly from the fore-limbs in the mature animal; the limbs in the latter having undergone much modification at a rather late period of life, and having thus been converted into hands, or paddles, or wings. whatever influence long-continued exercise or use on the one hand, and disuse on the other, may have in modifying an organ, such influence will mainly affect the mature animal, which has come to its full powers of activity and has to gain its own living; and the effects thus produced will be inherited at a corresponding mature age. whereas the young will remain unmodified, or be modified in a lesser degree, by the effects of use and disuse. in certain cases the successive steps of variation might supervene, from causes of which we are wholly ignorant, at a very early period of life, or each step might be inherited at an earlier period than that at which it first appeared. in either case (as with the short-faced tumbler) the young or embryo would closely resemble the mature parent-form. we have seen that this is the rule of development in certain whole groups of animals, as with cuttle-fish and spiders, and with a few members of the great class of insects, as with aphis. with respect to the final cause of the young in these cases not undergoing any metamorphosis, or closely resembling their parents from their earliest age, we can see that this would result from the two following contingencies: firstly, from the young, during a course of modification carried on for many generations, having to provide for their own wants at a very early stage { } of development, and secondly, from their following exactly the same habits of life with their parents; for in this case, it would be indispensable for the existence of the species, that the child should be modified at a very early age in the same manner with its parents, in accordance with their similar habits. some further explanation, however, of the embryo not undergoing any metamorphosis is perhaps requisite. if, on the other hand, it profited the young to follow habits of life in any degree different from those of their parent, and consequently to be constructed in a slightly different manner, then, on the principle of inheritance at corresponding ages, the active young or larvæ might easily be rendered by natural selection different to any conceivable extent from their parents. such differences might, also, become correlated with successive stages of development; so that the larvæ, in the first stage, might differ greatly from the larvæ in the second stage, as we have seen to be the case with cirripedes. the adult might become fitted for sites or habits, in which organs of locomotion or of the senses, &c., would be useless; and in this case the final metamorphosis would be said to be retrograde. as all the organic beings, extinct and recent, which have ever lived on this earth have to be classed together, and as all have been connected by the finest gradations, the best, or indeed, if our collections were nearly perfect, the only possible arrangement, would be genealogical. descent being on my view the hidden bond of connexion which naturalists have been seeking under the term of the natural system. on this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. for the embryo is the animal in its less modified state; { } and in so far it reveals the structure of its progenitor. in two groups of animals, however much they may at present differ from each other in structure and habits, if they pass through the same or similar embryonic stages, we may feel assured that they have both descended from the same or nearly similar parents, and are therefore in that degree closely related. thus, community in embryonic structure reveals community of descent. it will reveal this community of descent, however much the structure of the adult may have been modified and obscured; we have seen, for instance, that cirripedes can at once be recognised by their larvæ as belonging to the great class of crustaceans. as the embryonic state of each species and group of species partially shows us the structure of their less modified ancient progenitors, we can clearly see why ancient and extinct forms of life should resemble the embryos of their descendants,--our existing species. agassiz believes this to be a law of nature; but i am bound to confess that i only hope to see the law hereafter proved true. it can be proved true in those cases alone in which the ancient state, now supposed to be represented in existing embryos, has not been obliterated, either by the successive variations in a long course of modification having supervened at a very early age, or by the variations having been inherited at an earlier period than that at which they first appeared. it should also be borne in mind, that the supposed law of resemblance of ancient forms of life to the embryonic stages of recent forms, may be true, but yet, owing to the geological record not extending far enough back in time, may remain for a long period, or for ever, incapable of demonstration. thus, as it seems to me, the leading facts in embryology, which are second in importance to none in natural history, are explained on the principle of slight { } modifications not appearing, in the many descendants from some one ancient progenitor, at a very early period in the life of each, though perhaps caused at the earliest, and being inherited at a corresponding not early period. embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals. _rudimentary, atrophied, or aborted organs._--organs or parts in this strange condition, bearing the stamp of inutility, are extremely common throughout nature. for instance, rudimentary mammæ are very general in the males of mammals: i presume that the "bastard-wing" in birds may be safely considered as a digit in a rudimentary state: in very many snakes one lobe of the lungs is rudimentary; in other snakes there are rudiments of the pelvis and hind limbs. some of the cases of rudimentary organs are extremely curious; for instance, the presence of teeth in foetal whales, which when grown up have not a tooth in their heads; and the presence of teeth, which never cut through the gums, in the upper jaws of our unborn calves. it has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds. nothing can be plainer than that wings are formed for flight, yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together! the meaning of rudimentary organs is often quite unmistakeable: for instance there are beetles of the same genus (and even of the same species) resembling each other most closely in all respects, one of which will have full-sized wings, and another mere rudiments of membrane; and here it is impossible to doubt, that the { } rudiments represent wings. rudimentary organs sometimes retain their potentiality, and are merely not developed: this seems to be the case with the mammæ of male mammals, for many instances are on record of these organs having become well developed in full-grown males, and having secreted milk. so again there are normally four developed and two rudimentary teats in the udders of the genus bos, but in our domestic cows the two sometimes become developed and give milk. in plants of the same species the petals sometimes occur as mere rudiments, and sometimes in a well-developed state. in plants with separated sexes, the male flowers often have a rudiment of a pistil; and kölreuter found that by crossing such male plants with an hermaphrodite species, the rudiment of the pistil in the hybrid offspring was much increased in size; and this shows that the rudiment and the perfect pistil are essentially alike in nature. an organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose; and remain perfectly efficient for the other. thus in plants, the office of the pistil is to allow the pollen-tubes to reach the ovules protected in the ovarium at its base. the pistil consists of a stigma supported on the style; but in some compositæ, the male florets, which of course cannot be fecundated, have a pistil, which is in a rudimentary state, for it is not crowned with a stigma; but the style remains well developed, and is clothed with hairs as in other compositæ, for the purpose of brushing the pollen out of the surrounding anthers. again, an organ may become rudimentary for its proper purpose, and be used for a distinct object: in certain fish the swim-bladder seems to be nearly rudimentary for its proper function of giving buoyancy, but has become converted into a { } nascent breathing organ or lung. other similar instances could be given. organs, however little developed, if of use, should not be called rudimentary; they cannot properly be said to be in an atrophied condition; they may be called nascent, and may hereafter be developed to any extent by natural selection. rudimentary organs, on the other hand, are essentially useless, as teeth which never cut through the gums; in a still less developed condition, they would be of still less use. they cannot, therefore, under their present condition, have been formed by natural selection, which acts solely by the preservation of useful modifications; they have been retained, as we shall see, by inheritance, and relate to a former condition of their possessor. it is difficult to know what are nascent organs; looking to the future, we cannot of course tell how any part will be developed, and whether it is now nascent; looking to the past, creatures with an organ in a nascent condition will generally have been supplanted and exterminated by their successors with the organ in a more perfect and developed condition. the wing of the penguin is of high service, and acts as a fin; it may, therefore, represent the nascent state of the wings of birds; not that i believe this to be the case, it is more probably a reduced organ, modified for a new function: the wing of the apteryx is useless, and is truly rudimentary. the mammary glands of the ornithorhynchus may, perhaps, be considered, in comparison with the udder of a cow, as in a nascent state. the ovigerous frena of certain cirripedes, which are only slightly developed and which have ceased to give attachment to the ova, are nascent branchiæ. rudimentary organs in the individuals of the same species are very liable to vary in degree of development { } and in other respects. moreover, in closely allied species, the degree to which the same organ has been rendered rudimentary occasionally differs much. this latter fact is well exemplified in the state of the wings of the female moths in certain groups. rudimentary organs may be utterly aborted; and this implies, that we find in an animal or plant no trace of an organ, which analogy would lead us to expect to find, and which is occasionally found in monstrous individuals of the species. thus in the snapdragon (antirrhinum) we generally do not find a rudiment of a fifth stamen; but this may sometimes be seen. in tracing the homologies of the same part in different members of a class, nothing is more common, or more necessary, than the use and discovery of rudiments. this is well shown in the drawings given by owen of the bones of the leg of the horse, ox, and rhinoceros. it is an important fact that rudimentary organs, such as teeth in the upper jaws of whales and ruminants, can often be detected in the embryo, but afterwards wholly disappear. it is also, i believe, a universal rule, that a rudimentary part or organ is of greater size relatively to the adjoining parts in the embryo, than in the adult; so that the organ at this early age is less rudimentary, or even cannot be said to be in any degree rudimentary. hence, also, a rudimentary organ in the adult is often said to have retained its embryonic condition. i have now given the leading facts with respect to rudimentary organs. in reflecting on them, every one must be struck with astonishment: for the same reasoning power which tells us plainly that most parts and organs are exquisitely adapted for certain purposes, tells us with equal plainness that these rudimentary or atrophied organs, are imperfect and useless. in works { } on natural history rudimentary organs are generally said to have been created "for the sake of symmetry," or in order "to complete the scheme of nature;" but this seems to me no explanation, merely a re-statement of the fact. would it be thought sufficient to say that because planets revolve in elliptic courses round the sun, satellites follow the same course round the planets, for the sake of symmetry, and to complete the scheme of nature? an eminent physiologist accounts for the presence of rudimentary organs, by supposing that they serve to excrete matter in excess, or injurious to the system; but can we suppose that the minute papilla, which often represents the pistil in male flowers, and which is formed merely of cellular tissue, can thus act? can we suppose that the formation of rudimentary teeth, which are subsequently absorbed, can be of any service to the rapidly growing embryonic calf by the excretion of precious phosphate of lime? when a man's fingers have been amputated, imperfect nails sometimes appear on the stumps: i could as soon believe that these vestiges of nails have appeared, not from unknown laws of growth, but in order to excrete horny matter, as that the rudimentary nails on the fin of the manatee were formed for this purpose. on my view of descent with modification, the origin of rudimentary organs is simple. we have plenty of cases of rudimentary organs in our domestic productions,--as the stump of a tail in tailless breeds,--the vestige of an ear in earless breeds,--the reappearance of minute dangling horns in hornless breeds of cattle, more especially, according to youatt, in young animals,--and the state of the whole flower in the cauliflower. we often see rudiments of various parts in monsters. but i doubt whether any of these cases throw light on the origin of rudimentary organs in a state of nature, { } further than by showing that rudiments can be produced; for i doubt whether species under nature ever undergo abrupt changes. i believe that disuse has been the main agency; that it has led in successive generations to the gradual reduction of various organs, until they have become rudimentary,--as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced to take flight, and have ultimately lost the power of flying. again, an organ useful under certain conditions, might become injurious under others, as with the wings of beetles living on small and exposed islands; and in this case natural selection would continue slowly to reduce the organ, until it was rendered harmless and rudimentary. any change in function, which can be effected by insensibly small steps, is within the power of natural selection; so that an organ rendered, during changed habits of life, useless or injurious for one purpose, might be modified and used for another purpose. or an organ might be retained for one alone of its former functions. an organ, when rendered useless, may well be variable, for its variations cannot be checked by natural selection. at whatever period of life disuse or selection reduces an organ, and this will generally be when the being has come to maturity and to its full powers of action, the principle of inheritance at corresponding ages will reproduce the organ in its reduced state at the same age, and consequently will seldom affect or reduce it in the embryo. thus we can understand the greater relative size of rudimentary organs in the embryo, and their lesser relative size in the adult. but if each step of the process of reduction were to be inherited, not at the corresponding age, but at an extremely early period of life (as we have good { } reason to believe to be possible), the rudimentary part would tend to be wholly lost, and we should have a case of complete abortion. the principle, also, of economy, explained in a former chapter, by which the materials forming any part or structure, if not useful to the possessor, will be saved as far as is possible, will probably often come into play; and this will tend to cause the entire obliteration of a rudimentary organ. as the presence of rudimentary organs is thus due to the tendency in every part of the organisation, which has long existed, to be inherited--we can understand, on the genealogical view of classification, how it is that systematists have found rudimentary parts as useful as, or even sometimes more useful than, parts of high physiological importance. rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue in seeking for its derivation. on the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the ordinary doctrine of creation, might even have been anticipated, and can be accounted for by the laws of inheritance. _summary._--in this chapter i have attempted to show, that the subordination of group to group in all organisms throughout all time; that the nature of the relationship, by which all living and extinct beings are united by complex, radiating, and circuitous lines of affinities into one grand system; the rules followed and the difficulties encountered by naturalists in their classifications; the value set upon characters, if constant and prevalent, whether of high vital importance, or of the most trifling { } importance, or, as in rudimentary organs, of no importance; the wide opposition in value between analogical or adaptive characters, and characters of true affinity; and other such rules;--all naturally follow on the view of the common parentage of those forms which are considered by naturalists as allied, together with their modification through natural selection, with its contingencies of extinction and divergence of character. in considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, and acknowledged varieties of the same species, however different they may be in structure. if we extend the use of this element of descent,--the only certainly known cause of similarity in organic beings,--we shall understand what is meant by the natural system: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms varieties, species, genera, families, orders, and classes. on this same view of descent with modification, all the great facts in morphology become intelligible,--whether we look to the same pattern displayed in the homologous organs, to whatever purpose applied, of the different species of a class; or to the homologous parts constructed on the same pattern in each individual animal and plant. on the principle of successive slight variations, not necessarily or generally supervening at a very early period of life, and being inherited at a corresponding period, we can understand the great leading facts in embryology; namely, the resemblance in an individual embryo of the homologous parts, which when matured will become widely different from each other in structure and function; and the resemblance in different species of a class of the homologous parts or { } organs, though fitted in the adult members for purposes as different as possible. larvæ are active embryos, which have become specially modified in relation to their habits of life, through the principle of modifications being inherited at corresponding ages. on this same principle--and bearing in mind, that when organs are reduced in size, either from disuse or selection, it will generally be at that period of life when the being has to provide for its own wants, and bearing in mind how strong is the principle of inheritance--the occurrence of rudimentary organs and their final abortion, present to us no inexplicable difficulties; on the contrary, their presence might have been even anticipated. the importance of embryological characters and of rudimentary organs in classification is intelligible, on the view that an arrangement is only so far natural as it is genealogical. finally, the several classes of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that i should without hesitation adopt this view, even if it were unsupported by other facts or arguments. * * * * * { } chapter xiv. recapitulation and conclusion. recapitulation of the difficulties on the theory of natural selection--recapitulation of the general and special circumstances in its favour--causes of the general belief in the immutability of species--how far the theory of natural selection may be extended--effects of its adoption on the study of natural history--concluding remarks. as this whole volume is one long argument, it may be convenient to the reader to have the leading facts and inferences briefly recapitulated. that many and serious objections may be advanced against the theory of descent with modification through natural selection, i do not deny. i have endeavoured to give to them their full force. nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected, not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor. nevertheless, this difficulty, though appearing to our imagination insuperably great, cannot be considered real if we admit the following propositions, namely,--that gradations in the perfection of any organ or instinct which we may consider, either do now exist or could have existed, each good of its kind,--that all organs and instincts are, in ever so slight a degree, variable,--and, lastly, that there is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct. the truth of these propositions cannot, i think, be disputed. { } it is, no doubt, extremely difficult even to conjecture by what gradations many structures have been perfected, more especially amongst broken and failing groups of organic beings; but we see so many strange gradations in nature, that we ought to be extremely cautious in saying that any organ or instinct, or any whole being, could not have arrived at its present state by many graduated steps. there are, it must be admitted, cases of special difficulty on the theory of natural selection; and one of the most curious of these is the existence of two or three defined castes of workers or sterile females in the same community of ants; but i have attempted to show how this difficulty can be mastered. with respect to the almost universal sterility of species when first crossed, which forms so remarkable a contrast with the almost universal fertility of varieties when crossed, i must refer the reader to the recapitulation of the facts given at the end of the eighth chapter, which seem to me conclusively to show that this sterility is no more a special endowment than is the incapacity of two trees to be grafted together; but that it is incidental on constitutional differences in the reproductive systems of the intercrossed species. we see the truth of this conclusion in the vast difference in the result, when the same two species are crossed reciprocally; that is, when one species is first used as the father and then as the mother. the fertility of varieties when intercrossed and of their mongrel offspring cannot be considered as universal; nor is their very general fertility surprising when we remember that it is not likely that either their constitutions or their reproductive systems should have been profoundly modified. moreover, most of the varieties which have been experimentised on have been { } produced under domestication; and as domestication (i do not mean mere confinement) apparently tends to eliminate sterility, we ought not to expect it also to produce sterility. the sterility of hybrids is a very different case from that of first crosses, for their reproductive organs are more or less functionally impotent; whereas in first crosses the organs on both sides are in a perfect condition. as we continually see that organisms of all kinds are rendered in some degree sterile from their constitutions having been disturbed by slightly different and new conditions of life, we need not feel surprise at hybrids being in some degree sterile, for their constitutions can hardly fail to have been disturbed from being compounded of two distinct organisations. this parallelism is supported by another parallel, but directly opposite, class of facts; namely, that the vigour and fertility of all organic beings are increased by slight changes in their conditions of life, and that the offspring of slightly modified forms or varieties acquire from being crossed increased vigour and fertility. so that, on the one hand, considerable changes in the conditions of life and crosses between greatly modified forms, lessen fertility; and on the other hand, lesser changes in the conditions of life and crosses between less modified forms, increase fertility. turning to geographical distribution, the difficulties encountered on the theory of descent with modification are grave enough. all the individuals of the same species, and all the species of the same genus, or even higher group, must have descended from common parents; and therefore, in however distant and isolated parts of the world they are now found, they must in the course of successive generations have passed from some one part to the others. we are often wholly unable { } even to conjecture how this could have been effected. yet, as we have reason to believe that some species have retained the same specific form for very long periods, enormously long as measured by years, too much stress ought not to be laid on the occasional wide diffusion of the same species; for during very long periods of time there will always have been a good chance for wide migration by many means. a broken or interrupted range may often be accounted for by the extinction of the species in the intermediate regions. it cannot be denied that we are as yet very ignorant of the full extent of the various climatal and geographical changes which have affected the earth during modern periods; and such changes will obviously have greatly facilitated migration. as an example, i have attempted to show how potent has been the influence of the glacial period on the distribution both of the same and of representative species throughout the world. we are as yet profoundly ignorant of the many occasional means of transport. with respect to distinct species of the same genus inhabiting very distant and isolated regions, as the process of modification has necessarily been slow, all the means of migration will have been possible during a very long period; and consequently the difficulty of the wide diffusion of species of the same genus is in some degree lessened. as on the theory of natural selection an interminable number of intermediate forms must have existed, linking together all the species in each group by gradations as fine as our present varieties, it may be asked, why do we not see these linking forms all around us? why are not all organic beings blended together in an inextricable chaos? with respect to existing forms, we should remember that we have no right to expect (excepting in rare cases) to discover _directly_ connecting { } links between them, but only between each and some extinct and supplanted form. even on a wide area, which has during a long period remained continuous, and of which the climate and other conditions of life change insensibly in going from a district occupied by one species into another district occupied by a closely allied species, we have no just right to expect often to find intermediate varieties in the intermediate zone. for we have reason to believe that only a few species are undergoing change at any one period; and all changes are slowly effected. i have also shown that the intermediate varieties which will at first probably exist in the intermediate zones, will be liable to be supplanted by the allied forms on either hand; and the latter, from existing in greater numbers, will generally be modified and improved at a quicker rate than the intermediate varieties, which exist in lesser numbers; so that the intermediate varieties will, in the long run, be supplanted and exterminated. on this doctrine of the extermination of an infinitude of connecting links, between the living and extinct inhabitants of the world, and at each successive period between the extinct and still older species, why is not every geological formation charged with such links? why does not every collection of fossil remains afford plain evidence of the gradation and mutation of the forms of life? we meet with no such evidence, and this is the most obvious and forcible of the many objections which may be urged against my theory. why, again, do whole groups of allied species appear, though certainly they often falsely appear, to have come in suddenly on the several geological stages? why do we not find great piles of strata beneath the silurian system, stored with the remains of the progenitors of the silurian groups of fossils? for certainly on my theory such { } strata must somewhere have been deposited at these ancient and utterly unknown epochs in the world's history. i can answer these questions and grave objections only on the supposition that the geological record is far more imperfect than most geologists believe. it cannot be objected that there has not been time sufficient for any amount of organic change; for the lapse of time has been so great as to be utterly inappreciable by the human intellect. the number of specimens in all our museums is absolutely as nothing compared with the countless generations of countless species which certainly have existed. we should not be able to recognise a species as the parent of any one or more species if we were to examine them ever so closely, unless we likewise possessed many of the intermediate links between their past or parent and present states; and these many links we could hardly ever expect to discover, owing to the imperfection of the geological record. numerous existing doubtful forms could be named which are probably varieties; but who will pretend that in future ages so many fossil links will be discovered, that naturalists will be able to decide, on the common view, whether or not these doubtful forms are varieties? as long as most of the links between any two species are unknown, if any one link or intermediate variety be discovered, it will simply be classed as another and distinct species. only a small portion of the world has been geologically explored. only organic beings of certain classes can be preserved in a fossil condition, at least in any great number. widely ranging species vary most, and varieties are often at first local,--both causes rendering the discovery of intermediate links less likely. local varieties will not spread into other and distant regions until they are considerably modified and { } improved; and when they do spread, if discovered in a geological formation, they will appear as if suddenly created there, and will be simply classed as new species. most formations have been intermittent in their accumulation; and their duration, i am inclined to believe, has been shorter than the average duration of specific forms. successive formations are separated from each other by enormous blank intervals of time; for fossiliferous formations, thick enough to resist future degradation, can be accumulated only where much sediment is deposited on the subsiding bed of the sea. during the alternate periods of elevation and of stationary level the record will be blank. during these latter periods there will probably be more variability in the forms of life; during periods of subsidence, more extinction. with respect to the absence of fossiliferous formations beneath the lowest silurian strata, i can only recur to the hypothesis given in the ninth chapter. that the geological record is imperfect all will admit; but that it is imperfect to the degree which i require, few will be inclined to admit. if we look to long enough intervals of time, geology plainly declares that all species have changed; and they have changed in the manner which my theory requires, for they have changed slowly and in a graduated manner. we clearly see this in the fossil remains from consecutive formations invariably being much more closely related to each other, than are the fossils from formations distant from each other in time. such is the sum of the several chief objections and difficulties which may justly be urged against my theory; and i have now briefly recapitulated the answers and explanations which can be given to them. i have felt these difficulties far too heavily during many years to { } doubt their weight. but it deserves especial notice that the more important objections relate to questions on which we are confessedly ignorant; nor do we know how ignorant we are. we do not know all the possible transitional gradations between the simplest and the most perfect organs; it cannot be pretended that we know all the varied means of distribution during the long lapse of years, or that we know how imperfect the geological record is. grave as these several difficulties are, in my judgment they do not overthrow the theory of descent from a few created forms with subsequent modification. now let us turn to the other side of the argument. under domestication we see much variability. this seems to be mainly due to the reproductive system being eminently susceptible to changes in the conditions of life; so that this system, when not rendered impotent, fails to reproduce offspring exactly like the parent-form. variability is governed by many complex laws,--by correlation of growth, by use and disuse, and by the direct action of the physical conditions of life. there is much difficulty in ascertaining how much modification our domestic productions have undergone; but we may safely infer that the amount has been large, and that modifications can be inherited for long periods. as long as the conditions of life remain the same, we have reason to believe that a modification, which has already been inherited for many generations, may continue to be inherited for an almost infinite number of generations. on the other hand we have evidence that variability, when it has once come into play, does not wholly cease; for new varieties are still occasionally produced by our most anciently domesticated productions. { } man does not actually produce variability; he only unintentionally exposes organic beings to new conditions of life, and then nature acts on the organisation, and causes variability. but man can and does select the variations given to him by nature, and thus accumulate them in any desired manner. he thus adapts animals and plants for his own benefit or pleasure. he may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed. it is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be quite inappreciable by an uneducated eye. this process of selection has been the great agency in the production of the most distinct and useful domestic breeds. that many of the breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether very many of them are varieties or aboriginal species. there is no obvious reason why the principles which have acted so efficiently under domestication should not have acted under nature. in the preservation of favoured individuals and races, during the constantly-recurrent struggle for existence, we see the most powerful and ever-acting means of selection. the struggle for existence inevitably follows from the high geometrical ratio of increase which is common to all organic beings. this high rate of increase is proved by calculation,--by the rapid increase of many animals and plants during a succession of peculiar seasons, or when naturalised in a new country. more individuals are born than can possibly survive. a grain in the balance will determine which individual shall live and which shall die,--which variety or species shall increase in number, and which { } shall decrease, or finally become extinct. as the individuals of the same species come in all respects into the closest competition with each other, the struggle will generally be most severe between them; it will be almost equally severe between the varieties of the same species, and next in severity between the species of the same genus. but the struggle will often be very severe between beings most remote in the scale of nature. the slightest advantage in one being, at any age or during any season, over those with which it comes into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will turn the balance. with animals having separated sexes there will in most cases be a struggle between the males for possession of the females. the most vigorous individuals, or those which have most successfully struggled with their conditions of life, will generally leave most progeny. but success will often depend on having special weapons or means of defence, or on the charms of the males; and the slightest advantage will lead to victory. as geology plainly proclaims that each land has undergone great physical changes, we might have expected that organic beings would have varied under nature, in the same way as they generally have varied under the changed conditions of domestication. and if there be any variability under nature, it would be an unaccountable fact if natural selection had not come into play. it has often been asserted, but the assertion is quite incapable of proof, that the amount of variation under nature is a strictly limited quantity. man, though acting on external characters alone and often capriciously, can produce within a short period a great result by adding up mere individual differences in his domestic productions; and every one admits that there are at least individual differences in species under { } nature. but, besides such differences, all naturalists have admitted the existence of varieties, which they think sufficiently distinct to be worthy of record in systematic works. no one can draw any clear distinction between individual differences and slight varieties; or between more plainly marked varieties and sub-species, and species. let it be observed how naturalists differ in the rank which they assign to the many representative forms in europe and north america. if then we have under nature variability and a powerful agent always ready to act and select, why should we doubt that variations in any way useful to beings, under their excessively complex relations of life, would be preserved, accumulated, and inherited? why, if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products? what limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure, and habits of each creature,--favouring the good and rejecting the bad? i can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life. the theory of natural selection, even if we looked no further than this, seems to me to be in itself probable. i have already recapitulated, as fairly as i could, the opposed difficulties and objections: now let us turn to the special facts and arguments in favour of the theory. on the view that species are only strongly marked and permanent varieties, and that each species first existed as a variety, we can see why it is that no line of demarcation can be drawn between species, commonly supposed to have been produced by special acts of creation, and varieties which are acknowledged to have been produced by secondary laws. on this same { } view we can understand how it is that in each region where many species of a genus have been produced, and where they now flourish, these same species should present many varieties; for where the manufactory of species has been active, we might expect, as a general rule, to find it still in action; and this is the case if varieties be incipient species. moreover, the species of the larger genera, which afford the greater number of varieties or incipient species, retain to a certain degree the character of varieties; for they differ from each other by a less amount of difference than do the species of smaller genera. the closely allied species also of the larger genera apparently have restricted ranges, and in their affinities they are clustered in little groups round other species--in which respects they resemble varieties. these are strange relations on the view of each species having been independently created, but are intelligible if all species first existed as varieties. as each species tends by its geometrical ratio of reproduction to increase inordinately in number; and as the modified descendants of each species will be enabled to increase by so much the more as they become diversified in habits and structure, so as to be enabled to seize on many and widely different places in the economy of nature, there will be a constant tendency in natural selection to preserve the most divergent offspring of any one species. hence during a long-continued course of modification, the slight differences, characteristic of varieties of the same species, tend to be augmented into the greater differences characteristic of species of the same genus. new and improved varieties will inevitably supplant and exterminate the older, less improved and intermediate varieties; and thus species are rendered to a large extent defined and distinct objects. dominant species belonging to the { } larger groups tend to give birth to new and dominant forms; so that each large group tends to become still larger, and at the same time more divergent in character. but as all groups cannot thus succeed in increasing in size, for the world would not hold them, the more dominant groups beat the less dominant. this tendency in the large groups to go on increasing in size and diverging in character, together with the almost inevitable contingency of much extinction, explains the arrangement of all the forms of life, in groups subordinate to groups, all within a few great classes, which we now see everywhere around us, and which has prevailed throughout all time. this grand fact of the grouping of all organic beings seems to me utterly inexplicable on the theory of creation. as natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modification; it can act only by very short and slow steps. hence the canon of "natura non facit saltum," which every fresh addition to our knowledge tends to make truer, is on this theory simply intelligible. we can plainly see why nature is prodigal in variety, though niggard in innovation. but why this should be a law of nature if each species has been independently created, no man can explain. many other facts are, as it seems to me, explicable on this theory. how strange it is that a bird, under the form of woodpecker, should have been created to prey on insects on the ground; that upland geese, which never or rarely swim, should have been created with webbed feet; that a thrush should have been created to dive and feed on sub-aquatic insects; and that a petrel should have been created with habits and structure fitting it for the life of an auk or grebe! and so on in endless other cases. but on the view of each { } species constantly trying to increase in number, with natural selection always ready to adapt the slowly varying descendants of each to any unoccupied or ill-occupied place in nature, these facts cease to be strange, or perhaps might even have been anticipated. as natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates; so that we need feel no surprise at the inhabitants of any one country, although on the ordinary view supposed to have been specially created and adapted for that country, being beaten and supplanted by the naturalised productions from another land. nor ought we to marvel if all the contrivances in nature be not, as far as we can judge, absolutely perfect; and if some of them be abhorrent to our ideas of fitness. we need not marvel at the sting of the bee causing the bee's own death; at drones being produced in such vast numbers for one single act, with the great majority slaughtered by their sterile sisters; at the astonishing waste of pollen by our fir-trees; at the instinctive hatred of the queen bee for her own fertile daughters; at ichneumonidæ feeding within the live bodies of caterpillars; and at other such cases. the wonder indeed is, on the theory of natural selection, that more cases of the want of absolute perfection have not been observed. the complex and little known laws governing variation are the same, as far as we can see, with the laws which have governed the production of so-called specific forms. in both cases physical conditions seem to have produced but little direct effect; yet when varieties enter any zone, they occasionally assume some of the characters of the species proper to that zone. in both varieties and species, use and disuse seem to have produced some effect; for it is difficult to resist this { } conclusion when we look, for instance, at the logger-headed duck, which has wings incapable of flight, in nearly the same condition as in the domestic duck; or when we look at the burrowing tucutucu, which is occasionally blind, and then at certain moles, which are habitually blind and have their eyes covered with skin; or when we look at the blind animals inhabiting the dark caves of america and europe. in both varieties and species correlation of growth seems to have played a most important part, so that when one part has been modified other parts are necessarily modified. in both varieties and species reversions to long-lost characters occur. how inexplicable on the theory of creation is the occasional appearance of stripes on the shoulder and legs of the several species of the horse-genus and in their hybrids! how simply is this fact explained if we believe that these species have descended from a striped progenitor, in the same manner as the several domestic breeds of pigeon have descended from the blue and barred rock-pigeon! on the ordinary view of each species having been independently created, why should the specific characters, or those by which the species of the same genus differ from each other, be more variable than the generic characters in which they all agree? why, for instance, should the colour of a flower be more likely to vary in any one species of a genus, if the other species, supposed to have been created independently, have differently coloured flowers, than if all the species of the genus have the same coloured flowers? if species are only well-marked varieties, of which the characters have become in a high degree permanent, we can understand this fact; for they have already varied since they branched off from a common progenitor in certain characters, by which they have come to be specifically distinct from each other; { } and therefore these same characters would be more likely still to be variable than the generic characters which have been inherited without change for an enormous period. it is inexplicable on the theory of creation why a part developed in a very unusual manner in any one species of a genus, and therefore, as we may naturally infer, of great importance to the species, should be eminently liable to variation; but, on my view, this part has undergone, since the several species branched off from a common progenitor, an unusual amount of variability and modification, and therefore we might expect this part generally to be still variable. but a part may be developed in the most unusual manner, like the wing of a bat, and yet not be more variable than any other structure, if the part be common to many subordinate forms, that is, if it has been inherited for a very long period; for in this case it will have been rendered constant by long-continued natural selection. glancing at instincts, marvellous as some are, they offer no greater difficulty than does corporeal structure on the theory of the natural selection of successive, slight, but profitable modifications. we can thus understand why nature moves by graduated steps in endowing different animals of the same class with their several instincts. i have attempted to show how much light the principle of gradation throws on the admirable architectural powers of the hive-bee. habit no doubt sometimes comes into play in modifying instincts; but it certainly is not indispensable, as we see, in the case of neuter insects, which leave no progeny to inherit the effects of long-continued habit. on the view of all the species of the same genus having descended from a common parent, and having inherited much in common, we can understand how it is that allied species, when placed under considerably different conditions of life, { } yet should follow nearly the same instincts; why the thrush of south america, for instance, lines her nest with mud like our british species. on the view of instincts having been slowly acquired through natural selection we need not marvel at some instincts being apparently not perfect and liable to mistakes, and at many instincts causing other animals to suffer. if species be only well-marked and permanent varieties, we can at once see why their crossed offspring should follow the same complex laws in their degrees and kinds of resemblance to their parents,--in being absorbed into each other by successive crosses, and in other such points,--as do the crossed offspring of acknowledged varieties. on the other hand, these would be strange facts if species have been independently created, and varieties have been produced by secondary laws. if we admit that the geological record is imperfect in an extreme degree, then such facts as the record gives, support the theory of descent with modification. new species have come on the stage slowly and at successive intervals; and the amount of change, after equal intervals of time, is widely different in different groups. the extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection; for old forms will be supplanted by new and improved forms. neither single species nor groups of species reappear when the chain of ordinary generation has once been broken. the gradual diffusion of dominant forms, with the slow modification of their descendants, causes the forms of life, after long intervals of time, to appear as if they had changed simultaneously throughout the world. the fact of the fossil remains of each formation being in some degree intermediate in character between the { } fossils in the formations above and below, is simply explained by their intermediate position in the chain of descent. the grand fact that all extinct organic beings belong to the same system with recent beings, falling either into the same or into intermediate groups, follows from the living and the extinct being the offspring of common parents. as the groups which have descended from an ancient progenitor have generally diverged in character, the progenitor with its early descendants will often be intermediate in character in comparison with its later descendants; and thus we can see why the more ancient a fossil is, the oftener it stands in some degree intermediate between existing and allied groups. recent forms are generally looked at as being, in some vague sense, higher than ancient and extinct forms; and they are in so far higher as the later and more improved forms have conquered the older and less improved organic beings in the struggle for life. lastly, the law of the long endurance of allied forms on the same continent,--of marsupials in australia, of edentata in america, and other such cases,--is intelligible, for within a confined country, the recent and the extinct will naturally be allied by descent. looking to geographical distribution, if we admit that there has been during the long course of ages much migration from one part of the world to another, owing to former climatal and geographical changes and to the many occasional and unknown means of dispersal, then we can understand, on the theory of descent with modification, most of the great leading facts in distribution. we can see why there should be so striking a parallelism in the distribution of organic beings throughout space, and in their geological succession throughout time; for in both cases the beings have been connected by the bond of ordinary generation, and the means of { } modification have been the same. we see the full meaning of the wonderful fact, which must have struck every traveller, namely, that on the same continent, under the most diverse conditions, under heat and cold, on mountain and lowland, on deserts and marshes, most of the inhabitants within each great class are plainly related; for they will generally be descendants of the same progenitors and early colonists. on this same principle of former migration, combined in most cases with modification, we can understand, by the aid of the glacial period, the identity of some few plants, and the close alliance of many others, on the most distant mountains, under the most different climates; and likewise the close alliance of some of the inhabitants of the sea in the northern and southern temperate zones, though separated by the whole intertropical ocean. although two areas may present the same physical conditions of life, we need feel no surprise at their inhabitants being widely different, if they have been for a long period completely separated from each other; for as the relation of organism to organism is the most important of all relations, and as the two areas will have received colonists from some third source or from each other, at various periods and in different proportions, the course of modification in the two areas will inevitably be different. on this view of migration, with subsequent modification, we can see why oceanic islands should be inhabited by few species, but of these, that many should be peculiar. we can clearly see why those animals which cannot cross wide spaces of ocean, as frogs and terrestrial mammals, should not inhabit oceanic islands; and why, on the other hand, new and peculiar species of bats, which can traverse the ocean, should so often be found on islands far distant from any continent. such facts { } as the presence of peculiar species of bats, and the absence of all other mammals, on oceanic islands, are utterly inexplicable on the theory of independent acts of creation. the existence of closely allied or representative species in any two areas, implies, on the theory of descent with modification, that the same parents formerly inhabited both areas; and we almost invariably find that wherever many closely allied species inhabit two areas, some identical species common to both still exist. wherever many closely allied yet distinct species occur, many doubtful forms and varieties of the same species likewise occur. it is a rule of high generality that the inhabitants of each area are related to the inhabitants of the nearest source whence immigrants might have been derived. we see this in nearly all the plants and animals of the galapagos archipelago, of juan fernandez, and of the other american islands being related in the most striking manner to the plants and animals of the neighbouring american mainland; and those of the cape de verde archipelago and other african islands to the african mainland. it must be admitted that these facts receive no explanation on the theory of creation. the fact, as we have seen, that all past and present organic beings constitute one grand natural system, with group subordinate to group, and with extinct groups often falling in between recent groups, is intelligible on the theory of natural selection with its contingencies of extinction and divergence of character. on these same principles we see how it is, that the mutual affinities of the species and genera within each class are so complex and circuitous. we see why certain characters are far more serviceable than others for classification;--why adaptive characters, though of paramount importance to the being, are of hardly any { } importance in classification; why characters derived from rudimentary parts, though of no service to the being, are often of high classificatory value; and why embryological characters are the most valuable of all. the real affinities of all organic beings are due to inheritance or community of descent. the natural system is a genealogical arrangement, in which we have to discover the lines of descent by the most permanent characters, however slight their vital importance may be. the framework of bones being the same in the hand of a man, wing of a bat, fin of the porpoise, and leg of the horse,--the same number of vertebræ forming the neck of the giraffe and of the elephant,--and innumerable other such facts, at once explain themselves on the theory of descent with slow and slight successive modifications. the similarity of pattern in the wing and leg of a bat, though used for such different purpose,--in the jaws and legs of a crab,--in the petals, stamens, and pistils of a flower, is likewise intelligible on the view of the gradual modification of parts or organs, which were alike in the early progenitor of each class. on the principle of successive variations not always supervening at an early age, and being inherited at a corresponding not early period of life, we can clearly see why the embryos of mammals, birds, reptiles, and fishes should be so closely alike, and should be so unlike the adult forms. we may cease marvelling at the embryo of an air-breathing mammal or bird having branchial slits and arteries running in loops, like those in a fish which has to breathe the air dissolved in water, by the aid of well-developed branchiæ. disuse, aided sometimes by natural selection, will often tend to reduce an organ, when it has become useless by changed habits or under changed conditions { } of life; and we can clearly understand on this view the meaning of rudimentary organs. but disuse and selection will generally act on each creature, when it has come to maturity and has to play its full part in the struggle for existence, and will thus have little power of acting on an organ during early life; hence the organ will not be much reduced or rendered rudimentary at this early age. the calf, for instance, has inherited teeth, which never cut through the gums of the upper jaw, from an early progenitor having well-developed teeth; and we may believe, that the teeth in the mature animal were reduced, during successive generations, by disuse or by the tongue and palate having been better fitted by natural selection to browse without their aid; whereas in the calf, the teeth have been left untouched by selection or disuse, and on the principle of inheritance at corresponding ages have been inherited from a remote period to the present day. on the view of each organic being and each separate organ having been specially created, how utterly inexplicable it is that parts, like the teeth in the embryonic calf or like the shrivelled wings under the soldered wing-covers of some beetles, should thus so frequently bear the plain stamp of inutility! nature may be said to have taken pains to reveal, by rudimentary organs and by homologous structures, her scheme of modification, which it seems that we wilfully will not understand. i have now recapitulated the chief facts and considerations which have thoroughly convinced me that species have been modified, during a long course of descent, by the preservation or the natural selection of many successive slight favourable variations. i cannot believe that a false theory would explain, as it seems to me that the theory of natural selection does explain, { } the several large classes of facts above specified. i see no good reason why the views given in this volume should shock the religious feelings of any one. a celebrated author and divine has written to me that "he has gradually learnt to see that it is just as noble a conception of the deity to believe that he created a few original forms capable of self-development into other and needful forms, as to believe that he required a fresh act of creation to supply the voids caused by the action of his laws." why, it may be asked, have all the most eminent living naturalists and geologists rejected this view of the mutability of species? it cannot be asserted that organic beings in a state of nature are subject to no variation; it cannot be proved that the amount of variation in the course of long ages is a limited quantity; no clear distinction has been, or can be, drawn between species and well-marked varieties. it cannot be maintained that species when intercrossed are invariably sterile, and varieties invariably fertile; or that sterility is a special endowment and sign of creation. the belief that species were immutable productions was almost unavoidable as long as the history of the world was thought to be of short duration; and now that we have acquired some idea of the lapse of time, we are too apt to assume, without proof, that the geological record is so perfect that it would have afforded us plain evidence of the mutation of species, if they had undergone mutation. but the chief cause of our natural unwillingness to admit that one species has given birth to other and distinct species, is that we are always slow in admitting any great change of which we do not see the intermediate steps. the difficulty is the same as that felt by so many geologists, when lyell first insisted that long { } lines of inland cliffs had been formed, and great valleys excavated, by the slow action of the coast-waves. the mind cannot possibly grasp the full meaning of the term of a hundred million years; it cannot add up and perceive the full effects of many slight variations, accumulated during an almost infinite number of generations. although i am fully convinced of the truth of the views given in this volume under the form of an abstract, i by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. it is so easy to hide our ignorance under such expressions as the "plan of creation," "unity of design," &c., and to think that we give an explanation when we only restate a fact. any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject my theory. a few naturalists, endowed with much flexibility of mind, and who have already begun to doubt on the immutability of species, may be influenced by this volume; but i look with confidence to the future, to young and rising naturalists, who will be able to view both sides of the question with impartiality. whoever is led to believe that species are mutable will do good service by conscientiously expressing his conviction; for only thus can the load of prejudice by which this subject is overwhelmed be removed. several eminent naturalists have of late published their belief that a multitude of reputed species in each genus are not real species; but that other species are real, that is, have been independently created. this seems to me a strange conclusion to arrive at. they admit that a multitude of forms, which till lately { } they themselves thought were special creations, and which are still thus looked at by the majority of naturalists, and which consequently have every external characteristic feature of true species,--they admit that these have been produced by variation, but they refuse to extend the same view to other and very slightly different forms. nevertheless they do not pretend that they can define, or even conjecture, which are the created forms of life, and which are those produced by secondary laws. they admit variation as a _vera causa_ in one case, they arbitrarily reject it in another, without assigning any distinction in the two cases. the day will come when this will be given as a curious illustration of the blindness of preconceived opinion. these authors seem no more startled at a miraculous act of creation than at an ordinary birth. but do they really believe that at innumerable periods in the earth's history certain elemental atoms have been commanded suddenly to flash into living tissues? do they believe that at each supposed act of creation one individual or many were produced? were all the infinitely numerous kinds of animals and plants created as eggs or seed, or as full grown? and in the case of mammals, were they created bearing the false marks of nourishment from the mother's womb? although naturalists very properly demand a full explanation of every difficulty from those who believe in the mutability of species, on their own side they ignore the whole subject of the first appearance of species in what they consider reverent silence. it may be asked how far i extend the doctrine of the modification of species. the question is difficult to answer, because the more distinct the forms are which we may consider, by so much the arguments fall away in force. but some arguments of the greatest weight { } extend very far. all the members of whole classes can be connected together by chains of affinities, and all can be classified on the same principle, in groups subordinate to groups. fossil remains sometimes tend to fill up very wide intervals between existing orders. organs in a rudimentary condition plainly show that an early progenitor had the organ in a fully developed state; and this in some instances necessarily implies an enormous amount of modification in the descendants. throughout whole classes various structures are formed on the same pattern, and at an embryonic age the species closely resemble each other. therefore i cannot doubt that the theory of descent with modification embraces all the members of the same class. i believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number. analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype. but analogy may be a deceitful guide. nevertheless all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction. we see this even in so trifling a circumstance as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree. therefore i should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed by the creator. when the views advanced by me in this volume, and by mr. wallace in the linnean journal, or when analogous views on the origin of species are generally { } admitted, we can dimly foresee that there will be a considerable revolution in natural history. systematists will be able to pursue their labours as at present; but they will not be incessantly haunted by the shadowy doubt whether this or that form be in essence a species. this i feel sure, and i speak after experience, will be no slight relief. the endless disputes whether or not some fifty species of british brambles are true species will cease. systematists will have only to decide (not that this will be easy) whether any form be sufficiently constant and distinct from other forms, to be capable of definition; and if definable, whether the differences be sufficiently important to deserve a specific name. this latter point will become a far more essential consideration than it is at present; for differences, however slight, between any two forms, if not blended by intermediate gradations, are looked at by most naturalists as sufficient to raise both forms to the rank of species. hereafter we shall be compelled to acknowledge that the only distinction between species and well-marked varieties is, that the latter are known, or believed, to be connected at the present day by intermediate gradations, whereas species were formerly thus connected. hence, without rejecting the consideration of the present existence of intermediate gradations between any two forms, we shall be led to weigh more carefully and to value higher the actual amount of difference between them. it is quite possible that forms now generally acknowledged to be merely varieties may hereafter be thought worthy of specific names, as with the primrose and cowslip; and in this case scientific and common language will come into accordance. in short, we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations { } made for convenience. this may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species. the other and more general departments of natural history will rise greatly in interest. the terms used by naturalists of affinity, relationship, community of type, paternity, morphology, adaptive characters, rudimentary and aborted organs, &c., will cease to be metaphorical, and will have a plain signification. when we no longer look at an organic being as a savage looks at a ship, as at something wholly beyond his comprehension; when we regard every production of nature as one which has had a history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, nearly in the same way as when we look at any great mechanical invention as the summing up of the labour, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting, i speak from experience, will the study of natural history become! a grand and almost untrodden field of inquiry will be opened, on the causes and laws of variation, on correlation of growth, on the effects of use and disuse, on the direct action of external conditions, and so forth. the study of domestic productions will rise immensely in value. a new variety raised by man will be a more important and interesting subject for study than one more species added to the infinitude of already recorded species. our classifications will come to be, as far as they can be so made, genealogies; and will then truly give what may be called the plan of creation. the rules for classifying will no doubt become simpler when we have a definite object in view. we possess no { } pedigrees or armorial bearings; and we have to discover and trace the many diverging lines of descent in our natural genealogies, by characters of any kind which have long been inherited. rudimentary organs will speak infallibly with respect to the nature of long-lost structures. species and groups of species, which are called aberrant, and which may fancifully be called living fossils, will aid us in forming a picture of the ancient forms of life. embryology will reveal to us the structure, in some degree obscured, of the prototypes of each great class. when we can feel assured that all the individuals of the same species, and all the closely allied species of most genera, have within a not very remote period descended from one parent, and have migrated from some one birthplace; and when we better know the many means of migration, then, by the light which geology now throws, and will continue to throw, on former changes of climate and of the level of the land, we shall surely be enabled to trace in an admirable manner the former migrations of the inhabitants of the whole world. even at present, by comparing the differences of the inhabitants of the sea on the opposite sides of a continent, and the nature of the various inhabitants of that continent in relation to their apparent means of immigration, some light can be thrown on ancient geography. the noble science of geology loses glory from the extreme imperfection of the record. the crust of the earth with its embedded remains must not be looked at as a well-filled museum, but as a poor collection made at hazard and at rare intervals. the accumulation of each great fossiliferous formation will be recognised as having depended on an unusual concurrence of circumstances, and the blank intervals between the successive stages as having been of vast duration. but we shall { } be able to gauge with some security the duration of these intervals by a comparison of the preceding and succeeding organic forms. we must be cautious in attempting to correlate as strictly contemporaneous two formations, which include few identical species, by the general succession of their forms of life. as species are produced and exterminated by slowly acting and still existing causes, and not by miraculous acts of creation and by catastrophes; and as the most important of all causes of organic change is one which is almost independent of altered and perhaps suddenly altered physical conditions, namely, the mutual relation of organism to organism,--the improvement of one being entailing the improvement or the extermination of others; it follows, that the amount of organic change in the fossils of consecutive formations probably serves as a fair measure of the lapse of actual time. a number of species, however, keeping in a body might remain for a long period unchanged, whilst within this same period, several of these species, by migrating into new countries and coming into competition with foreign associates, might become modified; so that we must not overrate the accuracy of organic change as a measure of time. during early periods of the earth's history, when the forms of life were probably fewer and simpler, the rate of change was probably slower; and at the first dawn of life, when very few forms of the simplest structure existed, the rate of change may have been slow in an extreme degree. the whole history of the world, as at present known, although of a length quite incomprehensible by us, will hereafter be recognised as a mere fragment of time, compared with the ages which have elapsed since the first creature, the progenitor of innumerable extinct and living descendants, was created. in the distant future i see open fields for far more { } important researches. psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. light will be thrown on the origin of man and his history. authors of the highest eminence seem to be fully satisfied with the view that each species has been independently created. to my mind it accords better with what we know of the laws impressed on matter by the creator, that the production and extinction of the past and present inhabitants of the world should have been due to secondary causes, like those determining the birth and death of the individual. when i view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the silurian system was deposited, they seem to me to become ennobled. judging from the past, we may safely infer that not one living species will transmit its unaltered likeness to a distant futurity. and of the species now living very few will transmit progeny of any kind to a far distant futurity; for the manner in which all organic beings are grouped, shows that the greater number of species of each genus, and all the species of many genera, have left no descendants, but have become utterly extinct. we can so far take a prophetic glance into futurity as to foretel that it will be the common and widely-spread species, belonging to the larger and dominant groups, which will ultimately prevail and procreate new and dominant species. as all the living forms of life are the lineal descendants of those which lived long before the silurian epoch, we may feel certain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world. hence we may look with some confidence to a secure future of equally inappreciable length. and as natural selection works { } solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection. it is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. these laws, taken in the largest sense, being growth with reproduction; inheritance which is almost implied by reproduction; variability from the indirect and direct action of the external conditions of life, and from use and disuse; a ratio of increase so high as to lead to a struggle for life, and as a consequence to natural selection, entailing divergence of character and the extinction of less-improved forms. thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. there is grandeur in this view of life, with its several powers, having been originally breathed by the creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. * * * * * { } index. a. aberrant groups, . abyssinia, plants of, . acclimatisation, . affinities of extinct species, . ---- of organic beings, . agassiz on amblyopsis, . ---- on groups of species suddenly appearing, , . ---- on embryological succession, . ---- on the glacial period, . ---- on embryological characters, . ---- on the embryos of vertebrata, . ---- on parallelism of embryological development and geological succession, . algæ of new zealand, . alligators, males, fighting, . amblyopsis, blind fish, . america, north, productions allied to those of europe, . --------, boulders and glaciers of, . ----, south, no modern formations on west coast, . ammonites, sudden extinction of, . anagallis, sterility of, . analogy of variations, . ancylus, . animals, not domesticated from being variable, . ----, domestic, descended from several stocks, . --------, acclimatisation of, . ---- of australia, . ---- with thicker fur in cold climates, . ----, blind, in caves, . ----, extinct, of australia, . anomma, . antarctic islands, ancient flora of, . antirrhinum, . ants attending aphides, . ----, slave-making instinct, . ----, neuter, structure of, . aphides, attended by ants, . aphis, development of, . apteryx, . arab horses, . aralo-caspian sea, . archaic, m. de, on the succession of species, . artichoke, jerusalem, . ascension, plants of, . asclepias, pollen of, . asparagus, . aspicarpa, . asses, striped, . ateuchus, . audubon on habits of frigate-bird, . ---- on variation in birds'-nests, . ---- on heron eating seeds, . australia, animals of, . ----. dogs of, . ----, extinct animals of, . ----, european plants in, . azara on flies destroying cattle, . azores, flora of, . b. babington, mr., on british plants, . balancement of growth, . bamboo with hooks, . barberry, flowers of, . barrande, m., on silurian colonies, . ---- on the succession of species, . ---- on parallelism of palæozoic formations, . ---- on affinities of ancient species, . barriers, importance of, . batrachians on islands, . bats, how structure acquired, . ----, distribution of, . bear, catching water-insects, . bee, sting of, . ----, queen, killing rivals, . bees fertilising flowers, . ----, hive, not sucking the red clover, . { } --------, cell-making instinct, . ----, humble, cells of, . ----, parasitic, . beetles, wingless, in madeira, . ---- with deficient tarsi, . bentham, mr., on british plants, . ----, on classification, . berkeley, mr., on seeds in salt-water, . bermuda, birds of, . birds acquiring fear, . ---- annually cross the atlantic, . ----, colour of, on continents, . ----, footsteps and remains of, in secondary rocks, . ----, fossil, in caves of brazil, . ---- of madeira, bermuda, and galapagos, . ----, song of males, . ---- transporting seeds, . ----, waders, . ----, wingless, , . ----, with traces of embryonic teeth, . bizcacha, . ----, affinities of, . bladder for swimming in fish, . blindness of cave animals, . blyth, mr., on distinctness of indian cattle, . ----, on striped hemionus, . ----, on crossed geese, . boar, shoulder-pad of, . borrow, mr., on the spanish pointer, . bory st. vincent on batrachians, . bosquet, m., on fossil chthamalus, . boulders, erratic, on the azores, . branchiæ, . brent, mr., on house-tumblers, . ----, on hawks killing pigeons, . brewer, dr., on american cuckoo, . britain, mammals of, . bronn on duration of specific forms, . brown, robert, on classification, . buckman on variation in plants, . buzareingues on sterility of varieties, . c. cabbage, varieties of, crossed, . calceolaria, . canary-birds, sterility of hybrids, . cape de verde islands, . cape of good hope, plants of, , . carrier-pigeons killed by hawks, . cassini on flowers of compositæ, . catasetum, . cats, with blue eyes, deaf, . ----, variation in habits of, . ---- curling tail when going to spring, . cattle destroying fir-trees, . ---- destroyed by flies in la plata, . ----, breeds of, locally extinct, . ----, fertility of indian and european breeds, . cave, inhabitants of, blind, . centres of creation, . cephalopodæ, development of, . cervulus, . cetacea, teeth and hair, . ceylon, plants of, . chalk formation, . characters, divergence of, . ----, sexual, variable, . ----, adaptive or analogical, . charlock, . checks to increase, . ---- ----, mutual, . chickens, instinctive tameness of, . chthamalinæ, . chthamalus, cretacean species of, . circumstances favourable to selection of domestic products, . ---- ---- to natural selection, . cirripedes capable of crossing, . ----, carapace aborted, . ----, their ovigerous frena, . ----, fossil, . ----, larvæ of, . classification, . clift, mr., on the succession of types, . climate, effects of, in checking increase of beings, . ----, adaptation of, to organisms, . { } cobites, intestine of, . cockroach, . collections, palæontological, poor, . colour, influenced by climate, . ----, in relation to attacks by flies, . columba livia, parent of domestic pigeons, . colymbetes, . compensation of growth, . compositæ, outer and inner florets of, . ----, male flowers of, . conclusion, general, . conditions, slight changes in, favourable to fertility, . coot, . coral-islands, seeds drifted to, . ---- reefs, indicating movements of earth, . corn-crake, . correlation of growth in domestic productions, . ---- of growth, , . cowslip, . creation, single centres of, . crinum, . crosses, reciprocal, . crossing of domestic animals, importance in altering breeds, . ----, advantages of, . ---- unfavourable to selection, . crustacea of new zealand, . crustacean, blind, . cryptocerus, . ctenomys, blind, . cuckoo, instinct of, . currants, grafts of, . currents of sea, rate of, . cuvier on conditions of existence, . ---- on fossil monkeys, . ----, fred., on instinct, . d. dana, prof., on blind cave-animals, . ----, on relations of crustaceans of japan, . ----, on crustaceans of new zealand, . de candolle on struggle for existence, . ---- on umbelliferæ, . ---- on general affinities, . ----, alph., on low plants, widely dispersed, . ----, ----, on widely-ranging plants being variable, . ----, ----, on naturalisation, . ----, ----, on winged seeds, . ----, ----, on alpine species suddenly becoming rare, . ----, ----, on distribution of plants with large seeds, . ----, ----, on vegetation of australia, . ----, ----, on fresh-water plants, . ----, ----, on insular plants, . degradation of coast-rocks, . denudation, rate of, . ---- of oldest rocks, . development of ancient forms, . devonian system, . dianthus, fertility of crosses, . dirt on feet of birds, . dispersal, means of, . ---- during glacial period, . distribution, geographical, . ----, means of, . disuse, effects of, under nature, . divergence of character, . division, physiological, of labour, . dogs, hairless, with imperfect teeth, . ---- descended from several wild stocks, . ----, domestic instincts of, . ----, inherited civilisation of, . ----, fertility of breeds together, . ----, ---- of crosses, . ----, proportions of, when young, . domestication, variation under, . downing, mr., on fruit-trees in america, . downs, north and south, . dragon-flies, intestines of, . drift-timber, . driver-ant, . drones killed by other bees, . duck, domestic, wings of, reduced, . ----, logger-headed, . { } duckweed, . dugong, affinities of, . dung-beetles with deficient tarsi, . dyticus, . e. earl, mr. w., on the malay archipelago, . ears, drooping, in domestic animals, . ----, rudimentary, . earth, seeds in roots of trees, . eciton, . economy of organisation, . edentata, teeth and hair, . ----, fossil species of, . edwards, milne, on physiological divisions of labour, . ----, on gradations of structure, . ----, on embryonical characters, . eggs, young birds escaping from, . electric organs, . elephant, rate of increase, . ---- of glacial period, . embryology, . existence, struggle for, . ----, conditions of, . extinction, as bearing on natural selection, . ---- of domestic varieties, , ----, . eye, structure of, . ----, correction for aberration, . eyes reduced in moles, . f. fabre, m. on parasitic sphex, . falconer, dr., on naturalisation of plants in india, . ---- on fossil crocodile, . ---- on elephants and mastodons, . ---- and cautley on mammals of sub-himalayan beds, . falkland island, wolf of, . faults, . faunas, marine, . fear, instinctive, in birds, . feet of bird, young molluscs adhering to, . fertility of hybrids, . ---- from slight changes in conditions, . ---- of crossed varieties, . fir-trees destroyed by cattle, . ---- ----, pollen of, . fish, flying, . ----, teleostean, sudden appearance of, . ---- eating seeds, , . ----, fresh-water, distribution of, . fishes, ganoid, now confined to fresh water, . ----, electric organs of, . ----, ganoid, living in fresh water, . ---- of southern hemisphere, . flight, powers of, how acquired, . flowers, structure of, in relation to crossing, . ---- of compositæ and umbelliferæ, . forbes, e., on colours of shells, . ---- on abrupt range of shells in depth, . ---- on poorness of palæontological collections, . ---- on continuous succession of genera, . ---- on continental extensions, . ---- on distribution during glacial period, . ---- on parallelism in time and space, . forests, changes in, in america, . formation, devonian, . formations, thickness of, in britain, . ----, intermittent, . formica rufescens, . ---- sanguinea, . ---- flava, neuter of, . frena, ovigerous, of cirripedes, . fresh-water productions, dispersal of, . fries on species in large genera being closely allied to other species, . frigate-bird, . frogs on islands, . fruit-trees, gradual improvement of, . ---- ---- in united states, . ---- ----, varieties of, acclimatised in united states, . { } fuci, crossed, . fur, thicker in cold climates, . furze, . g. galapagos archipelago, birds of, . ----, productions of, , . galeopithecus, . game, increase of, checked by vermin, . gärtner on sterility of hybrids, , . ----, on reciprocal crosses, . ----, on crossed maize and verbascum, . ----, on comparison of hybrids and mongrels, . geese, fertility when crossed, . ----, upland, . genealogy important in classification, . geoffroy st. hilaire on balancement, . ---- ---- on homologous organs, . ---- ----, isidore, on variability of repeated parts, . ---- ----, on correlation in monstrosities, . ---- ----, on correlation, . ---- ----, on variable parts being often monstrous, . geographical distribution, . geography, ancient, . geology, future progress of, . ----, imperfection of the record, . giraffe, tail of, . glacial period, . gmelin on distribution, . gnathodon, fossil, . godwin-austen, mr., on the malay archipelago, . goethe on compensation of growth, . gooseberry, grafts of, . gould, dr. a., on land-shells, . ----, mr., on colours of birds, . ----, on birds of the galapagos, . ----, on distribution of genera of birds, . gourds, crossed, . grafts, capacity of, . grasses, varieties of, . gray, dr. asa, on trees of united states, . ----, on naturalised plants in the united states, . ----, on rarity of intermediate varieties, . ----, on alpine plants, . ----, dr. j. e., on striped mule, . grebe, . groups, aberrant, . grouse, colours of, . ----, red, a doubtful species, . growth, compensation of, . ----, correlation of, in domestic products, . ----, correlation of, . h. habit, effect of, under domestication, . ----, effect of, under nature, . ----, diversified, of same species, . hair and teeth, correlated, . harcourt, mr. e. v., on the birds of madeira, . hartung, m. on boulders in the azores, . hazel-nuts, . hearne on habits of bears, . heath, changes in vegetation, . heer, o., on plants of madeira, . helix pomatia, . helosciadium, . hemionus, striped, . herbert, w., on struggle for existence, . ----, on sterility of hybrids, . hermaphrodites crossing, . heron eating seed, . heron, sir r., on peacocks, . heusinger on white animals not poisoned by certain plants, . hewitt, mr., on sterility of first crosses, . himalaya, glaciers of, . ----, plants of, . hippeastrum, . holly-trees, sexes of, . hollyhock, varieties of, crossed, . hooker, dr., on trees of new zealand, . { } ----, on acclimatisation of himalayan trees, . ----, on flowers of umbelliferæ, . ----, on glaciers of himalaya, . ----, on algæ of new zealand, . ----, on vegetation at the base of the himalaya, . ----, on plants of tierra del fuego, , . ----, on australian plants, , . ----, on relations of flora of south america, . ----, on flora of the antarctic lands, , . ----, on the plants of the galapagos, , . hooks on bamboos, . ---- to seeds on islands, . horner, mr., on the antiquity of egyptians, . horns, rudimentary, . horse, fossil, in la plata, . horses destroyed by flies in la plata, . ----, striped, . ----, proportions of, when young, . horticulturists, selection applied by, . huber on cells of bees, . ----, p., on reason blended with instinct, . ----, on habitual nature of instincts, . ----, on slave-making ants, . ----, on melipona domestica, . humble-bees, cells of, . hunter, j., on secondary sexual characters, . hutton, captain, on crossed geese, . huxley, prof., on structure of hermaphrodites, . ----, on embryological succession, . ----, on homologous organs, . ----, on the development of aphis, . hybrids and mongrels compared, . hybridism, . hydra, structure of, . i. ibla, . icebergs transporting seeds, . increase, rate of, . individuals, numbers favourable to selection, . ----, many, whether simultaneously created, . inheritance, laws of, . ---- at corresponding ages, , . insects, colour of, fitted for habitations, . ----, sea-side, colours of, . ----, blind, in caves, . ----, luminous, . ----, neuter, . instinct, . instincts, domestic, . intercrossing, advantages of, . islands, oceanic, . isolation favourable to selection, . j. japan, productions of, . java, plants of, . jones, mr. j. m., on the birds of bermuda, . jussieu on classification, . k. kentucky, caves of, . kerguelen-land, flora of, , . kidney-bean, acclimatisation of, . kidneys of birds, . kirby on tarsi deficient in beetles, . knight, andrew, on cause of variation, . kölreuter on the barberry, . ---- on sterility of hybrids, . ---- on reciprocal crosses, . ---- on crossed varieties of nicotiana, . ---- on crossing male and hermaphrodite flowers, . l. lamarck on adaptive characters, . land-shells, distribution of, . ---- of madeira, naturalised, . languages, classification of, . lapse, great, of time, . { } larvæ, . laurel, nectar secreted by the leaves, laws of variation, . leech, varieties of, . leguminosæ, nectar secreted by glands, . lepidosiren, , . life, struggle for, . lingula, silurian, . linnæus, aphorism of, . lion, mane of, . ----, young of, striped, . lobelia fulgens, , . lobelia, sterility of crosses, . loess of the rhine, . lowness of structure connected with variability, . lowness, related to wide distribution, . lubbock, mr., on the nerves of coccus, . lucas, dr. p., on inheritance, . ----, on resemblance of child to parent, . lund and clausen on fossils of brazil, . lyell, sir c, on the struggle for existence, . ----, on modern changes of the earth, . ----, on measure of denudation, . ----, on a carboniferous land-shell, . ----, on strata beneath silurian system, . ----, on the imperfection of the geological record, . ----, on the appearance of species, . ----, on barrande's colonies, . ----, on tertiary formations of europe and north america, . ----, on parallelism of tertiary formations, . ----, on transport of seeds by icebergs, . ----, on great alternations of climate, . ----, on the distribution of fresh-water shells, . ----, on land-shells of madeira, . lyell and dawson on fossilized trees in nova scotia, . m. macleay on analogical characters, . madeira, plants of, . ----, beetles of, wingless, . ----, fossil land-shells of, . ----, birds of, . magpie tame in norway, . maize, crossed, . malay archipelago compared with europe, . ----, mammals of, . malpighiaceæ, . mammæ, rudimentary, . mammals, fossil, in secondary formation, . ----, insular, . man, origin of races of, . manatee, rudimentary nails of, . marsupials of australia, . ----, fossil species of, . martens, m., experiment on seeds, . martin, mr. w. c., on striped mules, . matteucci on the electric organs of rays, . matthiola, reciprocal crosses of, . means of dispersal, . melipona domestica, . metamorphism of oldest rocks, . mice destroying bees, . ----, acclimatisation of, . migration, bears on first appearance of fossils, . miller, prof., on the cells of bees, . mirabilis, crosses of, . missel-thrush, . misseltoe, complex relations of, . mississippi, rate of deposition at mouth, . mocking-thrush of the galapagos, . modification of species, how far applicable, . moles, blind, . mongrels, fertility and sterility of, . ---- and hybrids compared, . { } monkeys, fossil, . monocanthus, . mons, van, on the origin of fruit-trees, . moquin-tandon on sea-side plants, . morphology, . mozart, musical powers of, . mud, seeds in, . mules, striped, . müller, dr. f., on alpine australian plants, . murchison, sir r., on the formations of russia, . ----, on azoic formations, . ----, on extinction, . mustela vison, . myanthus, . myrmecocystus, . myrmica, eyes of, . n. nails, rudimentary, . natural history, future progress of, . ---- selection, . ---- system, . naturalisation of forms distinct from the indigenous species, . ---- in new zealand, . nautilus, silurian, . nectar of plants, . nectaries, how formed, . nelumbium luteum, . nests, variation in, . neuter insects, . newman, mr., on humble-bees, . new zealand, productions of, not perfect, . ----, naturalised products of, . ----, fossil birds of, . ----, glacial action in, . ----, crustaceans of, . ----, algæ of, . ----, number of plants of, . ----, flora of, . nicotiana, crossed varieties of, . ----, certain species very sterile, . noble, mr., on fertility of rhododendron, . nodules, phosphatic, in azoic rocks, . o. oak, varieties of, . onites apelles, . orchis, pollen of, . organs of extreme perfection, . ----, electric, of fishes, . ---- of little importance, . ----, homologous, . ----, rudiments of, and nascent, . ornithorhynchus, , . ostrich not capable of flight, . ----, habit of laying eggs together, . ----, american, two species of, . otter, habits of, how acquired, . ouzel, water, . owen, prof., on birds not flying, . ----, on vegetative repetition, . ----, on variable length of arms in ourang-outang, . ----, on the swim-bladder of fishes, . ----, on electric organs, . ----, on fossil horse of la plata, . ----, on relations of ruminants and pachyderms, . ----, on fossil birds of new zealand, . ----, on succession of types, . ----, on affinities of the dugong, . ----, on homologous organs, . ----, on the metamorphosis of cephalopods and spiders, . p. pacific ocean, faunas of, . paley on no organ formed to give pain, . pallas on the fertility of the wild stocks of domestic animals, . paraguay, cattle destroyed by flies, . parasites, . partridge, dirt on feet, . parts greatly developed, variable, . ----, degrees of utility of, . parus major, . passiflora, . peaches in united states, . pear, grafts of, . { } pelargonium, flowers of, . ----, sterility of, . pelvis of women, . peloria, . period, glacial, . petrels, habits of, . phasianus, fertility of hybrids, . pheasant, young, wild, . philippi on tertiary species in sicily, . pictet, prof., on groups of species suddenly appearing, , . ----, on rate of organic change, . ----, on continuous succession of genera, . ----, on close alliance of fossils in consecutive formations, . ----, on embryological succession, . pierce, mr., on varieties of wolves, . pigeons with feathered feet and skin between toes, . ----, breeds described, and origin of, . ----, breeds of, how produced, , . ----, tumbler, not being able to get out of egg, . ----, reverting to blue colour, . ----, instinct of tumbling, . ----, carriers, killed by hawks, . ----, young of, . pistil, rudimentary, . plants, poisonous, not affecting certain coloured animals, . ----, selection applied to, . ----, gradual improvement of, . ---- not improved in barbarous countries, . ---- destroyed by insects, . ----, in midst of range, have to struggle with other plants, . ----, nectar of, . ----, fleshy, on sea-shores, . ----, fresh-water, distribution of, . ----, low in scale, widely distributed, . plumage, laws of change in sexes of birds, . plums in the united states, . pointer dog, origin of, . ----, habits of, . poison not affecting certain coloured animals, . ----, similar effect of, on animals and plants, . pollen of fir-trees, . poole, col., on striped hemionus, . potamogeton, . prestwich, mr., on english and french eocene formations, . primrose, . ----, sterility of, . primula, varieties of, . proteolepas, . proteus, . psychology, future progress of, . q. quagga, striped, . quince, grafts of, . r. rabbit, disposition of young, . races, domestic, characters of, . race-horses, arab, . ----, english, . ramond on plants of pyrenees, . ramsay, prof., on thickness of the british formations, . ----, on faults, . ratio of increase, . rats, supplanting each other, . ----, acclimatisation of, . ----, blind in cave, . rattle-snake, . reason and instinct, . recapitulation, general, . reciprocity of crosses, . record, geological, imperfect, . rengger on flies destroying cattle, . reproduction, rate of, . resemblance to parents in mongrels and hybrids, . reversion, law of inheritance, . ---- in pigeons to blue colour, . rhododendron, sterility of, . richard, prof., on aspicarpa, . richardson, sir j., on structure of squirrels, . ----, on fishes of the southern hemisphere, . robinia, grafts of, . { } rodents, blind, . rudimentary organs, . rudiments important for classification, . s. sagaret on grafts, . salmons, males fighting, and hooked jaws of, . salt-water, how far injurious to seeds, . saurophagus sulphuratus, . schiödte on blind insects, . schlegel on snakes, . sea-water, how far injurious to seeds, . sebright, sir j., on crossed animals, . ----, on selection of pigeons, . sedgwick, prof., on groups of species suddenly appearing, . seedlings destroyed by insects, . seeds, nutriment in, . ----, winged, . ----, power of resisting salt-water, . ---- in crops and intestines of birds, . ---- eaten by fish, , . ---- in mud, . ----, hooked, on islands, . selection of domestic products, . ----, principle not of recent origin, . ----, unconscious, . ----, natural, . ----, sexual, . ----, natural, circumstances favourable to, . sexes, relations of, . sexual characters variable, . ---- selection, . sheep, merino, their selection, . ----, two sub-breeds unintentionally produced, . ----, mountain, varieties of, . shells, colours of, . ----, littoral, seldom embedded, . ----, fresh-water, dispersal of, ---- of madeira, . ----, land, distribution of, . silene, fertility of crosses, . silliman, prof., on blind rat, . skulls of young mammals, , . slave-making instinct, . smith, col. hamilton, on striped horses, . ----, mr. fred., on slave-making ants, . ----, on neuter ants, . ----, mr., of jordan hill, on the degradation of coast-rocks, . snap-dragon, . somerville, lord, on selection of sheep, . sorbus, grafts of, . spaniel, king charles's breed, . species, polymorphic, . ----, common, variable, . ---- in large genera variable, . ----, groups of, suddenly appearing, , . ---- beneath silurian formations, . ---- successively appearing, . ---- changing simultaneously throughout the world, . spencer, lord, on increase in size of cattle, . sphex, parasitic, . spiders, development of, . spitz-dog crossed with fox, . sports in plants, . sprengel, c. c, on crossing, . ----, on ray-florets, . squirrels, gradations in structure, . staffordshire, heath, changes in, . stag-beetles, fighting, . sterility from changed conditions of life, . ---- of hybrids, . ---- ----, laws of, . ---- ----, causes of, . ---- from unfavourable conditions, . ---- of certain varieties, . st. helena, productions of, . st. hilaire, aug., on classification, . st. john, mr., on habits of cats, . sting of bee, . stocks, aboriginal, of domestic animals, . strata, thickness of, in britain, . stripes on horses, . { } structure, degrees of utility of, . struggle for existence, . succession, geological, . succession of types in same areas, . swallow, one species supplanting another, . swim-bladder, . system, natural, . t. tail of giraffe, . ---- of aquatic animals, . ----, rudimentary, . tarsi deficient, . tausch on umbelliferous flowers, . teeth and hair correlated, . ----, embryonic, traces of, in birds, . ----, rudimentary, in embryonic calf, , . tegetmeier, mr., on cells of bees, , . temminck on distribution aiding classification, . thouin on grafts, . thrush, aquatic species of, . ----, mocking, of the galapagos, . ----, young of, spotted, . ----, nest of, . thuret, m., on crossed fuci, . thwaites, mr., on acclimatisation, . tierra del fuego, dogs of, . ----, plants of, , . timber-drift, . time, lapse of, . titmouse, . toads on islands, . tobacco, crossed varieties of, . tomes, mr., on the distribution of bats, . transitions in varieties rare, . trees on islands belong to peculiar orders, . ---- with separated sexes, . trifolium pratense, , . ---- incarnatum, . trigonia, . trilobites, . ----, sudden extinction of, . troglodytes, . tucutucu, blind, . tumbler pigeons, habits of, hereditary, . ----, young of, . turkey-cock, brush of hair on breast, . turkey, naked skin on head, . ----, young, wild, . turnip and cabbage, analogous variations of, . type, unity of, . types, succession of, in same areas, . u. udders enlarged by use, . ----, rudimentary, . ulex, young leaves of, . umbelliferæ, outer and inner florets of, . unity of type, . use, effects of, under domestication, . ----, effects of, in a state of nature, . utility, how far important in the construction of each part, . v. valenciennes on fresh-water fish, . variability of mongrels and hybrids, . variation under domestication, . ---- caused by reproductive system being affected by conditions of life, . ---- under nature, . ----, laws of, . variations appear at corresponding ages, , . ----, analogous in distinct species, . varieties, natural, . ----, struggle between, . ----, domestic, extinction of, . ----, transitional, rarity of, . ----, when crossed, fertile, . ----, when crossed, sterile, . ----, classification of, . verbascum, sterility of, . ----, varieties of, crossed, . verneuil, m. de, on the succession of species, . viola tricolor, . { } volcanic islands, denudation of, . vulture, naked skin on head, . w. wading-birds, . wallace, mr., on origin of species, . ----, on law of geographical distribution, . ----, on the malay archipelago, . wasp, sting of, . water, fresh, productions of, . water-hen, . waterhouse, mr., on australian marsupials, . ----, on greatly developed parts being variable, . ----, on the cells of bees, . ----, on general affinities, . water-ouzel, . watson, mr. h. c, on range of varieties of british plants, . ----, on acclimatisation, . ----, on flora of azores, . ----, on alpine plants, , . ----, on rarity of intermediate varieties, . weald, denudation of, . web of feet in water-birds, . west indian islands, mammals of, . westwood on species in large genera being closely allied to others, . ---- on the tarsi of engidæ, . ---- on the antennæ of hymenopterous insects, . wheat, varieties of, . white mountains, flora of, . wings, reduction of size, . ---- of insects homologous with branchiæ, . ----, rudimentary, in insects, . wolf crossed with dog, . ---- of falkland isles, . wollaston, mr., on varieties of insects, . ----, on fossil varieties of land-shells in madeira, . ----, on colours of insects on sea-shore, . ----, on wingless beetles, . ----, on rarity of intermediate varieties, . ----, on insular insects, . ----, on land-shells of madeira, naturalised, . wolves, varieties of, . woodpecker, habits of, . ----, green colour of, . woodward, mr., on the duration of specific forms, . ----, on the continuous succession of genera, . ----, on the succession of types, . world, species changing simultaneously throughout, . wrens, nest of, . y. youatt, mr., on selection, . ----, on sub-breeds of sheep, . ----, on rudimentary horns in young cattle, . z. zebra, stripes on, . the end. * * * * * london: printed by william clowes and sons, stamford street, and charing cross. * * * * * corrections made to printed original. p. . "the slightest use to a being": 'slighest' in original. p. . "as matteucci asserts": 'matteucei' in original (the index correctly has matteucci). p. . "deposited in the living bodies of other insects": 'depo-sisted' (across page break) in original. p. . "the newly-formed fantail": 'faintail' in original. p. . "the volcanic nature of the soil": 'volanic' in original. p. . "madeira and the adjoining islet": 'maderia' in original; and so in "from porto santo to madeira". p. . "the same individual embryo": 'indivividual' in original. p. . "innumerable species, genera, and families": 'inumerable' in original. p. . "inheritance which is almost implied by reproduction": 'inheritrnce' in original. [illustration: fossil man of mentone. (from popular science monthly, october, .)] was man created? by henry a. mott, jr., e.m., ph.d., etc., _member of the american chemical society, member of the berlin chemical society, member of the new york academy of sciences, member of the american association for the advancement of science, member of the american pharmaceutical association, fellow of the geographical society, etc., etc._ author of the "chemists' manual," "adulteration of milk," "artificial butter," "testing the value of rifles by firing under water," etc., etc. new york: griswold & company, nassau street. . copyright by henry a. mott, jr., . trow's printing and bookbinding co., _ - east th st._, new york. electrotyped by smith & mcdougal, beekman street, n. y. preface. this work was originally written to be delivered as a lecture; but as its pages continued to multiply, it was suggested to the author by numerous friends that it ought to be published in book-form; this, at last, the author concluded to do. this work, therefore, does not claim to be an exhaustive discussion of the various departments of which it treats; but rather it has been the aim of the author to present the more interesting observations in each department in as concise a form as possible. the author has endeavored to give credit in every instance where he has taken advantage of the labors of others. this work is not intended for that class of people who are so absolutely certain of the truth of their religion and of the immortality that it teaches, that they have become unqualified to entertain or even perceive of any scientific objection; for such people may be likened unto those who, "_seeing, they see, but will not perceive; and hearing, they hear, but will not understand._" this work is written for the man of culture who is seeking for truth--believing, as does the author, that all truth is god's truth, and therefore it becomes the duty of every scientific man to accept it; knowing, however, that it will surely modify the popular creeds and methods of interpretation, its final result can only be to the glory of god and to the establishment of a more exalted and purer religion. all facts are truths; it consequently follows that all scientific facts are truths--there is no half-way house--a statement is either a truth or it is not a truth, according to the _law of non-contradiction_. if, therefore, we find tabulated amongst scientific facts (or truths) a statement which is not a fact, it is not science; but all statements which are facts it naturally follows are truths, and as such must be accepted, no matter how repulsive they may at first seem to some of our poetical imaginings and pet theories. we cannot help but sympathize with the feelings which prompted president barnard to write the following lines, still we will see he was too hasty: "much as i love truth in the abstract," he says, "i love my hope of immortality more." * * * he maintained that it is better to close one's eyes to the evidences than to be convinced of the _truth_ of certain doctrines which _he regards_ as subversive of the fundamentals of christian faith. "if this (is all) is the best that science can give me, then i pray no more science. let me live on in my simple ignorance, as my fathers lived before me; and when i shall at length be summoned to my final repose, let me still be able to fold the drapery of my couch about me, and lie down to pleasant, even though they be deceitful, dreams."[ ] the limitations to the acceptance of truth that president barnard makes is wrong; for, as professor winchell has said, "we think it is a higher aspiration to wish to know 'the truth and the whole truth.' at the same time, we have not the slightest apprehension that the whole truth can ever dissipate our faith in a future life."[ ] let us "prove all things and hold fast unto that which is good," recognizing the fact that "the truth-seeker is the only god-seeker." author january , . table of contents. page preface v, vi chart of man's development - protoplasm cells life vital force analysis of man unity of organic and inorganic nature spontaneous generation the coming into existence of man evolution theories of the world's formation the bible kant's cosmogony , nature a perpetual creation laws of evolution survival of the fittest rudimentary organs reproduction by means of eggs double-sexed individuals inheritance artificial monsters acquired qualities geological record ontogeny the attributes of man muscular force thought force the attributes of animals the attributes of a savage language faith true conscience belief in god proof of the existence of god unity of all nature soul the finite senses of man the unseen universe manifestations of god hope of immortality - was man created? haeckel's chart of man's development, arranged by henry a. mott, jr., ph. d. = . americans.= (_indians._) | | esquimaux. | | | hyperboreans. magyars. | | | = . arctic men.= | | | fins. +------+------+ | | tungusians. calmucks. tartars. | samoides. | | | | | | +-----------+-------+----+-------+ +---+--+ | | altaians. uralians. | | +-----------------+-------+ japanese. chinese. siamese. | | | tibet. | | | | | | ural-altaians. coreans. +-------+-------+ | | | | | indo-chinese. | coreo-japanese. | | | | | +----+--------------+-----------------+ | indo-germanians. | semites. basques. | caucasians. | | | | | | +----------+--+--------+------------+ | | | = . mediteranese.= | | | singalese. | fulatians. | | | | | deccans. | dongolese. | | | = . dradidas.= | = . nubians.= | | | | | +----+--+--------+ | polynesians. | | | madagascars. euplocomi. = . negroes.= | | | | | | +-----+---+ | = . kaffirs.= | | | | | | | sundanesians. | +---+----+ | | | | = . mongols= = . malays= | eriocomi. | | | | +------------+--------------+ | promalays. = . hottentots=| | = . papuans.= | | | = . australians.= | | | | | +---+-------+ | +--+--+ | | | | | euthycomi. lophocomi. | | | | | +----+----------+ | | lissotrichi (_straight-haired_) ulotrichi (_woolly-haired_). | | +------------+----------+ | =alali= (_speechless men_). =pithecanthropi= (_ape-like men_). | v | primeval men. | | satyrus engeco gorilla | (_orang_). hylobates (_chimpanzee_). (_gorilla_). | | (_gibbon_). | | | | | +---------------+ +---------+------------+ | | african asiatic (_man-like apes_). (_man-like apes_). | | +-------------------------------------+ | | nasalis anthropoides semnopithecus (_nose apes_). (_man-like apes_). (_tall apes_). | | | | | +-------------+ | | arctopitheci labidocera | cercopithecus cynocephalus (_silk-apes_). (_clutch-tails_). | (_sea-cat_). (_pavian_). | | | | | +----------------+ +--------+---------------+ | | aphyocera catarrhina menocerca (_flap-tails_). (_tailed, narrow-nosed apes_). platyrhinæ catarrhinæ (_flat-nosed apes_). (_narrow-nosed_). | | +--------------------------------+ | simiæ (_apes_). brachytarsi | (_lemurs_). | | +--------------+ proboscidea | pinnipedia (_elephants_). | (_marine animals lamnungia | | of prey_). (_rock-conies_). | | nycterides | | | | (_bats_). carnivora +-------------+ | | (_land animals | | pterocynes of prey_). chelophora | (_flying foxes_). | (_pseudo-hoofed_). | | carnaria | | chiroptera (_animals rodentia | (_flying animals_). of prey_). (_gnawing animals_). | | | | | +------------------+ | leptodactyla | | | (_fingered | insectivora | animals_). | (_insect eaters_). | | | | +-----------+ | | | | | +----------------+------------------+ | prosimiÆ sarcoceta (_true whales_). prosimiÆ (_brought forward_,) | (_semi-apes_). sirenia (_sea-cows_). cetacea (_whales_). | ungulata edentata deciduata (_hoofed animals_). (_poor in teeth_). (_deciduous animals_). | | | +--------+----------------+ | | | indeciduous | (_indeciduata_). | | | +-------------------------------------+--------+ | placentalia (_placental animals_). | marsupialia | marsupialia botanophaga | zoophaga (_herbivorous_ | (_carnivorous_ _marsupials_). | _marsupials_). | | | +--------------------------+-------------+ | ornithostoma marsupialia (_beaked animals_). (_marsupial_). | | +---------------------------+-------+ | promammalia (_glacal animals_). mammalia (_mammals_). aves (_birds_). | | | reptilia (_reptiles_). | | | +---------------+---------+ | teleostei halisauria | (_osseous fish_). (_sea-dragons_). amniota (_amnion animals_). | dipneusta | | | (_mud-fish_). | amphibia (_batrachians_). ganoidei | | | (_ganoid fish_). +----------+-------+--------------+ | | | amphipneumones | (_vertebrate animals, breathing through lungs_). | | +--+------------------------------+ | selachii (_primeval fish_). | pisces (_fishes_). | | amphirrhina cyclostoma (_double nostrils_). (_round-mouthed_). | | +----------------------------------------------+--------+ | monorrhina (_single-nostriled_). craniota (_animals with skulls_). leptocardia | (_tube-hearted_). | | | thaliacea. +--------+--------+ (_sea-barrels_). ascidiæ. | | | acrania +--------+-------+ (_skull-less animals_). | tunicata vertebrata (_tunicate animals_). (_vertebrate animals_). | | +-------------------+---------+ | vermes (_worms_). | zoophytes | (_animal trees_). | | | +-----+-----+ | protozoa (_primeval animals_). animal monera. | | vegetable monera. | neutral monera. | | | +---------------------+-------------------+ | archigonic monera (_pieces of protoplasm which have originated by spontaneous generation._) was man created? what science can answer. "the object of science is not to find out what we like or what we dislike--the object of science is truth." in the discussion of the subject, "_was man created?_" our object will be--not to study the many ways god might have created him, but the way he actually did create him, for all ways would be alike easy to an omnipotent being. let us look at man and ask the question: what is there about him which would need an independent act of creation any more than about the "mountain of granite or the atom of sand"? the answer comes back: besides life, man has many mental attributes. let us direct our attention at first to the grand phenomena of life, and then to man's attributes. to discover the nature of life, to find out what life really is, it would be folly to commence by comparing man, the perfection of living beings, with an inorganic or inanimate substance like a brick, to discover the hidden secret; for, as professor orton says:[ ] "that only is essential to life which is common to all forms of life. our brains, stomach, livers, hands and feet are luxuries. they are necessary to make us human, but not living beings." instead of man, then, it will be necessary for us to take the simplest being which possesses such a phenomena; and such are the little homogeneous specks of protoplasm, constituting the group _monera_, which are entirely destitute of structure, and to which the name "cytode" has been given. in the fresh waters in the neighborhood of jena minute lumps of protoplasm were discovered by haeckel, which, on being examined under the most powerful lens of a microscope, were seen to have no constant form, their outlines being in a state of perpetual change, caused by the protrusion from various parts of their surface of broad lobes and thick finger-like projections, which, after remaining visible for a time, would be withdrawn, to make their appearance again on some other part of the surface. to this little mass of protoplasm haeckel has given the name _protanæba primitiva_. these little lumps multiply by spontaneous division into two pieces, which, on becoming dependent, increase in size and acquire all the characteristics of the parent. from this illustration, it will be seen that "reproduction is a form of nutrition and a growth of the individual to a size beyond that belonging to it as an individual, so that a part is thus elevated into a (new) whole." it is to this simple state of the monera the _fertilized_ egg of any animal is transformed--the germ vesicle; the original egg kernel disappears, and the parent kernel (cytococcus) forms itself anew; and it is in this condition, a non-nucleated ball of protoplasm, a true cytod, a homogeneous, structureless body, without different constituent parts, that the human child, as well as all other living beings, take their first steps in development. no matter how wonderful this may seem, the fact stares us in the face that the entire human child, as well as every animal with all their great future possibilities, are in their first stage a small ball of this complex homogeneous substance. whether we consider "a mere infinitesimal ovoid particle which finds space and duration enough to multiply into countless millions in the body of a living fly, and then of the wealth of foliage, the luxuriance of flower and fruit which lies between this bald sketch of a plant and the gigantic pine of california, towering to the dimensions of a cathedral spire, or the indian fig which covers acres with its profound shadow, and endures while nations and empires come and go around its vast circumference," or we look "at the other half of the world of life, picturing to ourselves the great finner whale, hugest of beasts that live or have lived, disporting his eighty or ninety feet of bone, muscle, and blubber, with easy roll, among the waves in which the stoutest ship that ever left dock-yard would founder hopelessly, and contrast him with the invisible animalcule, mere gelatinous specks, multitudes of which could in fact dance upon the point of a needle with the same ease as the angels of the schoolman could in imagination;--with these images before our minds, it would be strange if we did not ask what community of form or structure is there between the fungus and the fig-tree, the animalcule and the whale? and, _à fortiori_, between all four? notwithstanding these apparent difficulties, a threefold unity--namely, a unity of power or faculty, a unity of form, and a unity of substantial composition--does pervade the whole living world."[ ] and this unit is protoplasm. so we see it is necessary for us to retreat to our protoplasm as a naked formless plasma, if we would find freed from all non-essential complications the agent to which has been assigned the duty of building up structure and of transforming the energy of lifeless matter into the living. even goethe (in ) almost stated this when he said: "plants and animals, regarded in their most imperfect condition, are hardly distinguishable. this much, however, we may say, that from a condition in which plant is hardly to be distinguished from animal, creatures have appeared, gradually perfecting themselves in two opposite directions--the plant is finally glorified into the tree, enduring and motionless; the animal into the human being of the highest mobility and freedom." let us examine for a moment this substance protoplasm, and see in what way it differs from inorganic matter, or in what way the animate differs from the inanimate--the living from the dead. felix dujardin, a french zoologist ( ) pointed out that the only living substance in the body of rhizopods and other inferior primitive animals, is identical with protoplasm. he called it _sarcode_. hugo von mohl ( ) first applied the name protoplasm to the peculiar serus and mobile substance in the interior of vegetable cells; and he perceived its high importance, but was very far from understanding its significance in relation to all organisms. not, however, until ferdinand cohn ( ) and more fully franz unger ( ) had established the identity of the animate and contractile protoplasm in vegetable cells and the sarcode of the lower animals, could max shultz in - elaborate the protoplasm theory of the sarcode so as to proclaim protoplasm to be the most essential and important constituent of all organic cells, and to show that the bag or husk of the cell, the cellular membrane and intercellular substance, are but secondary parts of the cell, and are frequently wanting. in a similar manner lionel beale ( ) gave to protoplasm, including the cellular germ, the name of "germinal matter," and to all the other substance entering into the composition of tissue, being secondary, and produced the name of "formed matter." "wherever there is life there is protoplasm; wherever there is protoplasm, there, too, is life." the physical consistence of protoplasm varies with the amount of water with which it is combined, from the solid form in which we find it in the dormant state to the thin watery state in which it occurs in the leaves of valisneria. as to its composition, chemistry can as yet give but scanty information; it can tell that it is composed of carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus, and it can also tell the percentage of each element, but it cannot give more than a formula that will express it as a whole, giving no information as to the nature of the numerous albuminoid substances which compose it. edward cope, in his article on comparative anatomy,[ ] gives the formula for protoplasm (as a whole), c{ }h{ }n{ }o{ } + s and p, in small quantities under some circumstances. it is therefore, he says, a nitryl of cellulose: c{ }h{ }o{ } + nh{ }. according to mulder the composition of albumen, one of the class of protein substances to which protoplasm belongs, is (c{ }h{ }n{ }o{ }) + s{ }p. protoplasm is identical in both the animal and vegetable kingdom; it behaves the same from whatever source it may be derived towards several re-agents, as also electricity. is it possible, then, that the protoplasm which produces the mould is exactly the same composition as that which produces the human child? the answer is yes, so far as the elements are concerned, but the proportions of carbon, hydrogen, etc., must enter into an infinite number of diverse stratifications and combination in the production of the various forms of life. professor frankland, speaking of protein, for instance, says it is capable of existing under probably at least a thousand isomeric forms. protoplasm may be distinguished under the microscope from other members of the class to which it belongs, on account of the faculty it possesses of combining with certain coloring matters, as carmine and aniline; it is colored dark-red or yellowish-brown by iodine and nitric acid, and it is coagulated by alcohol and mineral acids as well as by heat. it possesses the quality of absorbing water in various quantities, which renders it sometimes extremely soft and nearly liquid, and sometimes hard and firm like leather. its prominent physical properties are excitability and contractility, which kühne and others have especially investigated. the motion of protoplasm in plants was first made known by bonaventure corti a century ago in the charoe plants; but this important fact was forgotten, and it had to be discovered by treviranus in . the regular motion of the protoplasm, forming a perfect current, may be seen in the hairs of the nettle, and weighty evidence exists that similar currents occur in all young vegetable cells. "if such be the case," says huxley, "the wonderful noonday silence of a tropical forest is, after all, due only to the dullness of our hearing, and could our ears catch the murmur of these tiny maelstroms, as they whirl in innumerable myriads of living cells, which constitute each tree, we should be stunned as with a roar of a great city." one step higher in the scale of life than the monera is the vegetable or animal cell, which arose out of the monera by the important process of segregation in their homogeneous viscid bodies, the differentiation of an inner kernel from the surrounding plasma. by this means the great progress from a simple cytod (without kernel) into a real cell (with kernel) was accomplished. some of these cells at an early stage encased themselves by secreting a hardened membrane; they formed the first vegetable cells, while others remaining naked developed into the first aggregate of animal cells. the vegetable cell has usually two concentric coverings--cell-wall and primordial utricle. in animal cells the former is wanting, the membrane representing the utricle. as a general fact, also, animal cells are smaller than vegetable cells. their size[ ] varies greatly, but are generally invisible to the naked eye, ranging from / to / of an inch in diameter. about four thousand of the smallest would be required to cover the dot put over the letter i in writing. the shape of cells varies greatly; the normal form, though, is spheroidal as in the cells of fat, but they often become[ ] many-sided--sometimes flattened as in the cuticle, and sometimes elongated into a simple filament as in fibrous tissue or muscular fibre. the cell, therefore, is extremely interesting, since all animal and vegetable structure is but the multiplication of the cell as a unit, and the whole life of the plant or animal is that of the cells which compose them, and in them or by them all its vital processes are carried on. it may sound paradoxical to speak of an animal or plant being composed of millions of cells; but beyond the momentary shock of the paradox no harm is done. the cell, then, can be regarded as the basis of our physiological idea of the elementary organism; but in the animal as well as in the plant, neither cell-wall nor nucleus is an essential constituent of the cell, inasmuch as bodies which are unquestionably the equivalents of cells--true morphological units--may be mere masses of protoplasm, devoid alike of cell-wall or nucleus. for the whole living world, then, the primary and a mental form of life is merely an individual mass of protoplasm in which no further structure is discernible. well, then, has protoplasm been called the "universal concomitant of every phenomena of life." life is inseparable from this substance, but is dormant unless excited by some external stimulant, such as heat, light, electricity, food, water, and oxygen. although we have seen that the life of the plant as well as of the animal is protoplasm, and that the protoplasm of the plant and that of the animal bear the closest resemblance, yet plants can manufacture protoplasm out of mineral compounds, whereas animals are obliged to procure it ready made, and hence in the end depend on plants. "without plants," says professor orton, "animals would perish; without animals, plants had no need to be." the food of a plant is a matter whose energy is all expended--is a fallen weight. but the plant organism receives it, exposes it to the sun's rays, and in a way mysterious to us converts the actual energy of the sunlight into potential energy within it. it is for this reason that life has been termed "bottled-sunshine." the principal food of the plant consists of carbon united with oxygen to form carbonic acid, hydrogen united with oxygen to form water, and nitrogen united with hydrogen to form ammonia. these elements thus united, which in themselves are perfectly lifeless, the plant is able to convert into living protoplasm. "plants are," says huxley, "the accumulators of the power which animals distribute and disperse." boussengault found long since that peas sown in pure sand, moistened with distilled water and fed by the air, obtained all the carbon necessary for their development, flowering, and fructification. here we see a plant which not only maintains its vigor on these few substances, but grows until it has increased a millionfold or a million-millionfold the quantity of protoplasm it originally possessed, and this protoplasm exhibits the phenomena of life. this and other proof led m. dumas to say: "from the loftiest point of view, and in connection with the physics of the globe, it would be imperative on us to say that in so far as their truly organic elements are concerned, plants and animals are the offspring of the air." schleiden,[ ] speaking of the haymakers of switzerland and the tyrol, says: "he mows his definite amount of grass every year on the alps, inaccessible to cattle, and gives not back the smallest quantity of organic substance to the soil. whence comes the hay, if not from the atmosphere." it has been seen, then, that plants can manufacture protoplasm, a faculty which animals are not possessed of; they at best can only convert dead protoplasm into living protoplasm. thus when vegetable or meat is cooked their protoplasm dies, but is not rendered incompetent of resuming its old functions as a matter of life. "if i," says huxley, "should eat a piece of cooked mutton, which was once the living protoplasm of a sheep, the protoplasm, rendered dead by cooking, will be changed into living protoplasm, and thus i would transubstantiate sheep into man; and were i to return to my own place by sea and undergo shipwreck, the crustacean might and probably would return the compliment, and demonstrate our common nature by turning my protoplasm into living lobster." as has been said before, where there are life manifestations there is protoplasm. life is regarded by one class of thinkers as the principle or cause of organization; and according to the other, life is the product or effect of organization. we must, however, agree with professor orton, who says: "life is the effect of organization, not the result of it. animals do not live because they are organized, but are organized because they are alive." in whatever way it is looked at, life is but a forced condition. "the more advanced thinkers, then, in science to-day," says barker, "therefore look upon the life of the living form as inseparable from its substance, and believe that the former is purely phenomenal and only a manifestation of the latter. during the existence of a special force as such, they retain the term only to express the sum of the phenomena of living beings. the word life must be regarded, then, as only a generalized expression signifying the sum-total of the properties of matter possessing such organization." in what manner, then, does this matter, possessing the phenomena of life, differ from inorganic matter, or in what manner does living matter differ from matter not living? the forces which are at work on the one side are at work on the other. the phenomena of life are all dependent upon the working of the same physical and chemical forces as those which are active in the rest of the world. it may be convenient to use the terms "vitality" and "vital force" to denote the cause of certain groups of natural operations, as we employ the names of "electricity" and "electrical force" to denote others; but it ceases to do so, if such a name implies the absurd assumption that either "electricity" or "vitality" is an entity, playing the part of a sufficient cause of electrical or vital phenomena. a mass of living protoplasm is simply a machine of great complexity, the total result of the work of which, or its vital phenomena, depend on the one hand upon its construction, and on the other upon the energy supplied to it; and to speak of "vitality" as anything but the names of a series of operations is as if one should talk of the "horologity" of a clock.[ ] when hydrogen and oxygen are united by an electrical spark water is produced; certainly there is no parity between the liquid produced and the two gases. at ° f., oxygen and hydrogen are elastic gaseous bodies, whose particles tend to fly away from one another; water at the same temperature is a strong though brittle solid. such changes are called the properties of water. it is not assumed that a certain something called "acquosity" has entered into and taken possession of the oxide of hydrogen as soon as formed, and then guarded the particles in the facets of the crystal or amongst the leaflets of the hoar-frost. on the contrary, it is hoped molecular physics will in time explain the phenomena. "what better philosophical status," says huxley,[ ] "has vitality than acquosity. if the properties of water may be properly said to result from the nature and disposition of its molecules, i can find no intelligible ground for refusing to say that the properties of protoplasm result from the nature and disposition of its molecules." "to distinguish the living from the dead body," herbert spencer says, "the tree that puts out leaves when the spring brings change of temperature, the flower which opens and closes with the rising and setting of the sun, the plant that droops when the soil is dry and re-erects itself when watered, are considered alive because of these produced changes; in common with the zoophyte, which contracts on the passing of a cloud over the sun, the worm that comes to the ground when continually shaken, and the hedgehog which rolls itself up when attacked." "seeds of wheat produced antecedent to the pharaohs," says bastain,[ ] "remaining in egyptian catacombs through century after century display of course no vital manifestations, but nevertheless retain the potentiality of growing into perfect plants whenever they may be brought into contact with suitable external conditions. we must presume that either ( ) during this long lapse of centuries the 'vital principle' of the plant has been imprisoned in the most dreary and impenetrable of dungeons, whither no sister effluence from the general 'soul of nature' could affect it; or else ( ) that the germ of the future living plant is there only in the form of an inherited structure, whose molecular complexities are of such a kind that, after moisture has restored mobility to its atoms, its potential life may pass into actual life. some of the lowest forms of animals and plants have such a tenacity to life that their vital manifestation may be kept in abeyance for five, ten, fifteen, or even twenty years. though not living any more than the wheat, they also retain the potentiality of manifestation of life; and for each alike, in order that this potentiality may pass into actuality, the first requisition is water with which to restore them to that possibility of molecular rearrangement under the influence of incident forces, of which the absence of water had deprived them, and without which, life in any real sense is impossible." analysis of a man. (by prof. miller.) a man feet inches high, weighing pounds. lbs. oz. grs. oxygen hydrogen carbon nitrogen inorganic elements in the ash: phosphorus calcium sulphur chlorine ounce = grains. sodium iron potassium magnesium silica total the quantity of the substances found in a human body weighing pounds: lbs. oz. grs. water gelatin albumen fibrine fat ashes total (from the "chemists' manual.") professor owen[ ] says: "there are organisms (vibrieo, rotifer, macrobiotus, etc.) which we can devitalize and revitalize--devive and revive--many times. as the dried animalcule manifest no phenomena suggesting any idea contributing to form the complex one of 'life' in my mind, i regard it to be as completely lifeless as is the drowned man, whose breath and heat have gone, and whose blood has ceased to circulate. * * * the change of work consequent on drying or drowning forthwith begins to alter relations or compositions, and in time to a degree adverse to resumption of the vital form of force, a longer period being needed for this effect in the rotifer, a shorter one in the man, still shorter it may be in the amoeba." "there is," says dumas,[ ] "an eternal round in which death is quickened and life appears, but in which matter merely changes its place and form." let us now compare the inorganic world with the organic--the inanimate with the animate--and see if there does exist an inseparable boundary between them. the fundamental properties of every natural body are matter, form, and force. one important point to be noticed is, that the elements which compose all animate bodies are the very elements that help to build up the inanimate bodies. no new elements appear in the vegetable or animal world which are not to be found in the inorganic world. the difference between animate and inanimate bodies, therefore, is certainly not in the elements which form them, but in the molecular combination of them; and it is to be hoped that molecular physics will, at some not far distant time, enlighten us as to the peculiar state of aggregation in which the molecules exist in living matter. as to the form, it is impossible to find any essential difference in the external form and inner structure between inorganic and organic bodies--for the simple monad, which is as much a living organism as the most complex being, is nothing but a homogeneous, structureless mass of protoplasm. but just as the inorganic substance, according to well-defined laws, elaborates its structure into a crystal of great beauty, so does the protoplasm elaborate itself into the most beautiful of all structures--the cell unit. just as gold and copper crystallizes in a geometrical form, a cube--bismuth and antimony in a hexagonal, iodine and sulphur in a rhombic form--so we find among radiolaria, and among other protista and lower forms, that they "may be traced to a mathematical, fundamental form, and whose form in its whole, as well as in its parts, is bounded by definite geometrically determinable planes and angles." now, as to the forces of the two different groups of bodies. surely the constructive force of a crystal is due to the chemical composition, and to its material constitution. as the shape of the crystal and its size are influenced by surrounding circumstances, there is, therefore, an external constructive force at work. the only difference between the growth of an organism and that of a crystal is, that in the former case, in consequence of its semi-fluid state of aggregation, the newly added particles penetrate into the interior of the organism (inter-susception), whereas inorganic substances receive homogeneous matter from without, only by opposition or an addition of new particles to the surface. "if we, then, designate the growth and the formation of organisms as a process of life, we may with equal reason apply the same term with the developing crystal." it is for these and other reasons, demonstrating as they do the "unity of organic and inorganic nature," the essential agreement of inorganic and organic bodies in matter, form, and force, which led tyndall[ ] to say: "abandoning all disguise, the confession that i feel bound to make before you is, that i prolong the vision backward across the boundary of experimental evidence, and discern in that matter which we in our ignorance, and notwithstanding our professed reverence for its creator, have hitherto covered with opprobrium, the promise and potency of every form and quality of life." returning now to our protoplasm, let us ask the question: where did it come from? or, how did it come into existence? though chemical synthesis has built up a number of organic substances which have been deemed the product of vitality, yet, up to the present day, the fact stands out before us that no one has ever built up one particle of living matter, however minute, from lifeless elements. the protoplasm of to-day is simply a continuation of the protoplasm of other ages, handed down to us through periods of undefinable and indeterminable time. the question of where protoplasm came from--how it arose--chemistry is unable to answer; but the question is answered, probably, by spontaneous generation. only the merest particle of living protoplasm was necessary to be formed from lifeless matter in the beginning; for, in the eyes of any consistent evolutionist, any further independent formation would be sheer waste, as the hypothesis of evolution postulates the unlimited, though perhaps not, indefinite modifiability of such matter. as we have seen that there exists no absolute barrier between organic and inorganic bodies, it is not so difficult to conceive that the first particle of protoplasm may have originated, under suitable conditions, out of inorganic or lifeless matter. but the causes which have led to the origination of this particle, it may be said, we know absolutely nothing--as in the formation of the crystal and the cell--the ultimate causes remain in both cases concealed from us. at the time in the earth's history when water, in a liquid state, made its appearance on the cooled crust of the earth, the carbon probably existed as carbonic acid dispersed in the atmosphere; and from the very best of grounds, it is reasonable to assume that the density and electric condition of the atmosphere were quite different, as also the chemical and physical nature of the primeval ocean was quite different. in any case, therefore, even[ ] if we do not know anything more about it, there remains the supposition, which can at least not be disputed, that at that time, under conditions quite different from those of to-day, a spontaneous generation, which is now perhaps no longer possible, may have taken place. this point is now conceded by most all of the advanced scientists of the day, and is absolutely necessary for the completion of the hypothesis of evolution. the answer may come to this--well, suppose the first protoplasm did originate by spontaneous generation, where did the elements or force come from which compose it? science has nothing to do with the coming into existence of matter or force, for she proves both to be indestructible; when they disappear, they do so only to reappear in some other form. the coming into existence of matter and force, as also the ultimate cause of all phenomena, is beyond the domain of scientific inquiry. science has only to do with the coming in of the form of matter, not the coming in of its existence. [illustration: fig. i.--a moneron (protamoeba) in act of reproduction; _a_, the whole moneron, which moves like ordinary amoeba, by means of variable processes: _b_, a contraction around its circumference parts it into two halves; _c_, the two halves separate, and each now forms independent individuals. (much enlarged.)--_haeckel._] [illustration: fig. ii.--_a_, is a crawling amoeba (much enlarged).--_haeckel._ the whole organism has the form-value of a naked cell and moves about by means of changeable processes, which are extended from the protoplasmic body and again drawn in. in the inside is the bright-colored, roundish cell-kernel or nucleus. _b_, egg-cell of a chalk sponge (olynthus).--_haeckel._] [illustration: fig. iii.--represents the next higher stage, mulberry-germ or morula (synamoeba).--_haeckel._] the coming into existence of man, by the slow process of development. it is necessary now to take up the little mass of living matter, admitting its coming into existence by spontaneous generation as probable, and so probable that it almost amounts to a certainty, and follow it through the many changes it is about to make under the influence of the laws which govern evolution until it has culminated in man, and these laws still acting on the brain of man, perfecting it, and leading him on to the comprehension of a grander and nobler conception of the almighty and of his works. the start, then, must be made with a homogeneous mass of protoplasm, such as the existing _protamoeba primitiva_ of the present day, which is a structureless organism without organs, and which came into existence during the laurentian period. it is to this simplified condition, as i have previously stated, all fertilized eggs return before they commence to develop. the first process of adaptation effected by the monera must have been the condensation of an external crust, which, as a protecting covering, shut in the softer interior from the hostile influences of the outer world. as soon as, by condensation of the homogeneous moneron, a cell-kernel arose in the interior, and a membrane arose on the surface, all the fundamental parts of the unit were then furnished. such a unit was an organism, similar to the white corpuscle of the blood, and called _amoebæ_. here we have two different stages of evolution; the protoplasma (better plasson) of the cytod undergoes differentiation, and is split up into two kinds of albuminous substances--the inner cell-kernel (nucleus) and the outer cell-substance (protoplasma). edward von benden, in his work upon _gregarinæ_, first clearly pointed out this fact, that we must distinguish thoroughly between the plasson of cytods and the protoplasm of cells. an irrefutable proof that such single-celled primæval animals like the amoeba really existed as the direct ancestors of man, is furnished, according to the fundamental law of biogeny, by the fact that the human egg is nothing more than a simple cell. the next step taken in advance is the division of the cell in two;--there arise from the single germinal spot two new kernel specks, and then, in like manner, out of the germinal vesicle two new cell-kernels. the same process of cell-division now repeats itself several times in succession, and the products of the division form a perfect union. this organism may be called a community of _amoebæ_ (synamoebæ). from the community of amoeba morula, now arose ciliated larvæ. the cells lying on the surface extended hair-like processes or fringes of hair, which, by striking against the water, kept the whole body rotating--the lanceolate animals or amphioxus were thus first produced. here we find from the synamoebæ which crept about slowly at the bottom of the laurentian primeval ocean by means of movements like those of an amoeba, that the newly-formed planæa by the vibrating movements of the cilia, the entire multicellular body acquired a more rapid and stronger motion, and passed over from the creeping to the swimming mode of locomotion. the planæa consisted, then, of two kinds of cells--inner ones like the amoebæ, and external "ciliated cells." the ancestors of man, which possessed the form value of the ciliated larva, is, of course, extinct at the present day. [illustration: fig. i.--the norwegian flimmer-ball (magosphoera planula), swimming by means of its vibratile fringes; seen from the surface.--_haeckel._] [illustration: fig. ii.--the same in section. the pear-shaped cells are seen bound together in the centre of the gelatinous sphere by a thread-like process. each cell contains both a kernel and a contractile vesicle. (planÆa series.)--_haeckel._] [illustration: figs. iii and iv.--represents gastrÆa series. the body consists merely of a simple primitive intestine, the wall of which is formed of two primary germ-layers.--_haeckel._] [illustration: figs. i and ii.--represents the next higher stage (tubularia). fig. i, a simple gliding worm (rhabdocoelum); _m_, mouth; _sd_, throat-epithelium; _sm_, throat-muscles; _d_, stomach-intestine; _nc_, kidney-ducts; _nm_, opening of the kidneys; _au_, eye; _na_, nose-pit. fig. ii, the same gliding worm, showing the remaining organs; _g_, brain; _au_, eye; _na_, nose-pit; _n_, nerves; _h_, testes; [male symbol], male opening; [female symbol], female opening; _e_, ovary; _f_, ciliated outer-skin.--_haeckel._] [illustration: fig. iii.--represents soft worms (scolecida) and is a young acorn worm (balanoglossus), after _agassiz_. _r_, acorn-like proboscis; _h_, collar; _k_, gill-openings and gill-arches of the anterior intestine, in a long row, one behind the other, on each side; _d_, digestive posterior intestine, filling the greater part of the body cavity; _v_, intestinal vessel, lying between two parallel folds of the skin; _a_, anus.] out of the planula, then, develops an exceedingly important animal form--the gastrula (that is, larva with a stomach or intestine), which resembles the planula, but differs essentially in the fact that it encloses a cavity which opens to the outside by a mouth. the wall of the progaster (primary stomach) consists of two layers of cells: an outer layer of smaller ciliated cells (outer skin or ectoderm), and of an inner layer of large non-ciliated cells (inner skin or entoderm). this exceedingly important larval form, the "gastrula," makes its appearance in the ontogenesis of all tribes of animals. these gastræada must have existed during the older primordial period, and they must have also included the ancestors of man. a certain proof of this is furnished by the amphioxus, which, in spite of its blood relationship to man, still passes through the stage of the gastrula with a simple intestine and a double intestinal wall.[ ] by motion of the cilia or fringes of the skin-layer, the gastræa swam freely about in the laurentian ocean. the development of the gastræa now deviated in two directions--one branch of gastræads gave up free locomotion, adhered to the bottom of the sea, and thus, by adopting an adhesive mode of life, gave rise to the proascus, the common primary form of the animal plants (zoophyta). the other branch was originated by the formation of a middle germ-layer or muscular layer, and also by the further differentiation of the internal parts into various organs; more especially, the first formation of a nervous system, the simplest organs of sense, the simplest organs for secretion (kidneys), and generation (sexual organs)--this branch is the prothelmis, the common primary worms (vermes). like the turbellaria of the present day, the whole surface of their body was covered with cilia, and they possessed a simple body of an oval shape, entirely without appendages. these acoelomatous worms did not as yet possess a true body cavity (coelom) nor blood. no member of the next higher animals are in existence, neither are there any fossil remains, owing to the soft nature of their body. they are therefore called soft worms, or scoleceda. they developed out of the turbellaria of the sixth stage by forming a true body cavity (a coelom) and blood in their interior. the nearest still living coelomati is probably the acorn worms (balanoglossus). the form value of this stage must, moreover, have been represented by several different intermediate stages. out of the four different groups of the worm tribe, the four higher tribes of the animal kingdom were developed--the star-fishes (echinoderma) and insects (arthropoda) on the one hand, and the molluscs (mollusca) and vertebrated animals (vertebrata) on the other. out of certain coelomati, the most ancient skull-less vertebrata were directly developed. among the coelomati of the present day, the ascidians are the nearest relatives of this exceedingly remarkable worm, which connect the widely differing classes of invertebrate and vertebrate animals. to these animals have been given the name of sack-worms (himatega). they originated out of the worms of the seventh stage by the formation of a dorsal nerve marrow (medulla tube), and by the formation of the spinal rod (chorda dorsalis) which lies below it. it is just the position of this central spinal rod or axial skeleton, between the dorsal marrow on the dorsal side and the intestinal canal on the ventral side, which is most characteristic of all vertebrate animals, including man, but also of the larvæ of the ascidia. we now come to the second half of the series of human ancestors. the skull-less animal lancelet, which is still living, affords a faint idea of the members of this group (acrania). since this little animal, in its earliest embryonic state, entirely agrees with the ascidia, and in its further development shows itself to be a true vertebrate animal, it forms a direct transition from the vertebrata to the invertebrata. [illustration: fig. i.--appendicularia, seen from the left side, _m_, mouth; _k_, gill intestine; _o_, oesophagus; _v_, stomach; _a_, anus; _n_, nerve ganglia (upper throat-knots); _g_, ear vesicle; _f_, ciliated groove under the gill; _h_, heart; _e_, ovary; _c_, notochord; _s_, tail.--_haeckel._] [illustration: fig. ii.--represents sack worms (himatega), and is the structure of an ascidian, seen from the left. _sb_, gill-sac; _v_, stomach; _i_, large intestine; _c_, heart; _t_, testes; _vd_, seed duct; _o_, ovary; _o'_, matured eggs in the body cavity. after _milne-edwards_.] [illustration: fig. iii.--represents the acrania series. lancelet (amhioxus lanceolatus), twice the actual size, seen from the left. _a_, mouth-opening, surrounded by cilia; _b_, anal-opening; _c_, ventral-opening (porus abdominalis); _d_, gill-body; _e_, stomach; _f_, liver-coecum; _g_, large intestine; _h_, coelum; _i_, notochord (under it the aorta); _k_, arches of the aorta; _l_, main gill-artery; _m_, swellings on its branches; _n_, hollow vein; _o_, intestinal vein.--_haeckel._] [illustration: fig. i.--represents the monorhina series. lamprey (petromyzon americanus) from the atlantic--_orton._] [illustration: fig. ii.--represents the selachii. shark (carcharias vulgaris) from the atlantic--_orton._] [illustration: fig. iii.--represents the mud-fish (dipneusta). lepidosiren annecteus, one-fourth natural size; african rivers.--_orton._ form a link between typical fishes and the amphibians.] at this stage, most probably, the separation of the two sexes began. the simpler and most ancient form of sexual propagation is through double-sexed individuals (hermaphroditismus). it occurs in the great majority of plants, but only in a minority of animals; for example, in the garden-snails, leeches, earth-worms and many other worms. every single individual among hermaphrodites produces within itself materials of both sexes--egg and sperm. in most of the higher plants every blossom contains both the male organs (stamen and anther) and the female organs (style and germ). every garden-snail produces in one part of its sexual gland eggs, and in another sperm. many hermaphrodites can fructify themselves; in others, however, copulation and reciprocal fructification of both hermaphrodites are necessary for causing the development of the eggs. this latter case is evidently a transition to sexual separation (gonoehorismus). out of the members of the last group arose animals with skulls or craniata, having round mouths, and which are divided into hags and lampreys. the hags (myxinoides) have long cylindrical worm-like bodies. the lampreys (petromyxontes) includes those well known "nine eyes" common at the seaside. these single-nostril animals (monorrhina) arose during the primordial period out of the skull-less animals by the anterior end of the dorsal marrow developing into the brain, and the anterior end of the dorsal skull into the skull. by the division of the single nostril of the members of the last group into two lateral halves, by the formation of a sympathetic nervous system, a jaw skeleton, a swimming bladder and two pairs of legs (breast fins or fore-legs, and ventral fins or hind-legs), arose the primæval fish (selachii), which is best represented by the still-living shark (squalacei). out of the primæval fish arose the mud-fish (dipneusta), which is very imperfectly represented by the still-living salamander fish; the primæval fish adapting itself to land, and by the transforming of the swimming bladder into an air-breathing lung, and of the nasal cavity (which was now open into the mouth cavity) into air-passages. their organization _might_, in some respect, be like the ceratodus and proloptems; but this is not certain. the dipneusta is an intermediate stage between the selachii and amphibia. out of the dipneusta arose the class of amphibia, having five toes (the pentadactyla). the gill amphibians are man's most ancient ancestors of the class amphibia. besides possessing lungs as well as the mud-fish, they retain throughout life regular gills like the still-living proteus and axolotl. most gilled batrachia live in north america. the paddle-fins of the dipneusta changed into five-toed legs, which were afterwards transmitted to the higher vertebrata up to man. the gilled amphibia (sozobrachia) of the last group finally lost their gills but retained their tail, and tailed amphibians (sozura) were produced, such as the salamander and newt of the present day. out of the sozura originated the primæval amniota (protamnia) by the complete loss of the gills by the formation of the amnion of the cochlea, and of the round window in the auditory organ, and of the organ of tears. out of the protamnia originated the primary mammals (promammalia). the most closely related were the ornithostoma; they differed through having teeth in their jaws. no fossil remains of the primary mammals have as yet been found, although they lived during the trias period--they possessed a very highly developed jaw. from the primary mammal arose the pouched animals (marsupialia). numerous representatives of this group still exist: kangaroos, pouched rats and pouched dogs. the marsupial animals developed, very probably, in the mesolithic epoch (during the jura) out of the cloacal animals; by the division of the cloaca into the rectum and the urogenital sinus, by the formation of a nipple on the mammary gland, and the partial suppression of the clavicles. [illustration: figs. i and ii.--the ceratodus forsteri occur in the swamps of southern australia. form transition between fishes and amphibia.--_haeckel._] [illustration: fig. i.--represents the gilled amphibians (soyobranchia). the axolotl (siredon pisciforme), after tegetmeier. the ordinary form with persistent branchiæ.] [illustration: fig. ii.--proteus anguinus. europe.--_orton._] [illustration: fig. iii.--represents the tailed amphibians (soyura). great water-newt (triton cristatus), after _bell._] from the marsupialia originated a most interesting small group of semi-apes (prosimiæ), for they are the primary forms of genuine apes and consequently of man. they developed out of handed or ape-footed marsupials (pedumana), of rat-like appearance, by the formation of a placenta, the loss of the marsupium and the marsupial bones, and by the higher development of the commissures of the brain. the still-living short-footed semi-ape (brachytarsi), especially the muki, indie and lori, possess possibly a faint resemblance. out of the semi-apes developed two classes of genuine apes; but as the narrow-nosed or catarrhini class are the only ones related to man, the others will not be considered. these narrow-nosed apes originated by the transformation of the jaw, and by the claws on the toes changing into nails. the still-living long-tail nose-apes and holy apes (semnopithecus) probably resembled the oldest ancestors of this group. the tailed apes by the loss of their tail and some of their hair covering, and by the excessive development of that portion of their brain above the facial portion of the skull, developed into the man-like apes (anthropoides)--such as the gorilla and chimpanzee of africa, and the orang and gibbon of asia. the human ancestors of this group existed during the miocene period. from the anthropoides developed the ape-like men (pithecanthropi) during the tertiary period. the speechless primæval men (alali), then, is the connecting link between the man-like apes and man. the fore-hand of the anthropoides became the human hand, their hinder-hand a foot for walking. they did not possess the articulate human language of words and the higher developments, as consciousness and the formation of ideas must have been very imperfect. out of the pithecanthropi men developed genuine man, by the development of the animal language of sounds into a connected or articulate language of words--the brain also developed higher and higher. this transition took place, probably, at the beginning of the quaternary period, or possibly in the tertiary. we have now very briefly reviewed the principal outlines of the ancestors of man, showing that man has developed from the little mass of protoplasm, as have all animals and plants. he therefore was not _spontaneously_ created, but was developed. the question is often asked by simple-minded people, with much delight, why do we not behold the interesting spectacle of the transformation of a chimpanzee into a man, or conversely of a man by retrogression into an orang?--it only shows that they are not acquainted with the first principles of the doctrine of descent. "not one of the apes," says schmidt, "can revert to the state of his primordial ancestors, except by retrogression--by which a primordial condition is by no means attained--he cannot divest himself of his acquired characters fixed by heredity, nor can he exceed himself and become man; for man does not stand in the direct line of development from the ape. the development of the anthropoid apes has taken a lateral course from the nearest human progenitors, and man can as little be transformed into a gorilla as a squirrel can be changed into a rat." [illustration: fig. i.--salamandra maculata.--_haeckel_. the water newts and salamanders were the next higher stage after the proteus and the axolotl.] [illustration: fig. i.--represents primæval amniota (protamnia). lizard (lacerta), after _orton_.] [illustration: fig. ii.--represents primary mammals (promammalia). amniota series. duck-billed platypus (ornithorhynchus paradoxus).--_haeckel_.] "feeling evidently,"[ ] says haeckel, "rather than understanding, induces most people to combat the theory of their 'descent from apes.' it is simply because the organism of the ape appears a caricature of man, a distorted likeness of ourselves in a not very attractive form; because the customary æsthetic ideas and self-glorification of man are touched by this in so sensitive a point, that most men shrink from recognizing their descent from apes. it seems much pleasanter to be descended from a more highly developed divine being, and hence, as is well known, human vanity has from the earliest times flattered itself by assuming the original descent of the race from gods or demi-gods." evolution. in the last chapter a description was given of the various stages in man's development, from the microscopic monad up. it will be necessary now to describe briefly the various laws which have governed this evolutionary chain from the monad to man. but before proceeding directly to the subject, let us look at the doctrine of evolution as a whole, and trace it first in the formation of the world. the doctrine of evolution is also called the theory of development--it must not, however, be confused with darwinism--for they are not exactly synonymous. darwinism is an attempt to explain the laws or manner of evolution. strictly speaking, only the theory of selection should be called darwinism, which was established in . the theory of descent, or transmutation theory, or doctrine of filiation, should properly be called lamarckism, who for the first time worked out the theory of descent as an independent scientific theory of the first order, and as the philosophical foundation of the whole science of biology. "according to the theory of development (evolution) in its simplest form," says henry hartshorne,[ ] "the universe as it now exists is a result of 'an immense series of changes,' related to and dependent upon each other as successive steps, or rather growths, constituting a progress; analogous to the unfolding or evolving of the parts of a growing organism." herbert spencer defined evolution as consisting in a progress from the homogeneous to the heterogeneous, from general to special, from the simple to the complex; and this process is considered to be traceable in the formation of worlds in space, in the multiplication of the types and species of plants and animals on the globe, in the origination and diversity of languages, literature, arts and sciences, and in all changes of human institutions and society. [illustration: fig. i.--skeleton of platypus.--_haeckel._] [illustration: fig. i.--represents pouched animals (marsupialia). kangaroo. (popular science monthly, feb., .)] let us now apply this theory of evolution to the physical world. no determined opposition by the mass of people is likely to be manifested to the doctrine of evolution as applied to the physical world, or even to the vegetable or animal world up to man; but the minute man is included--then is a voice raised up against it, and it was for this reason that darwin in his first work on the "theory of descent" did not mention man as being included in the evolutionary series. he knew too well the foolish human weakness that existed. in a recent work by prof. challes, he states that he regards the material universe as "a vast and wonderful mechanism of which the least wonderful thing is its being so constructed that we can understand it." the following is a brief description of the various theories of the world's formation: _first theory._--by the first theory the world is supposed to have existed from eternity under its actual form. aristotle embraced this doctrine, and conceived the universe to be the eternal effect of an eternal cause; maintaining that not only the heavens and the earth, but all animate and inanimate beings, are without beginning. to use huxley's illustration: if you can imagine a spectator on the earth, however far back in time, he would have seen a world "essentially similar, though not perhaps in all its details, to that which now exists. the animals which existed would be the ancestors of those which now exist, and like them; the plants in like manner would be such as we have now, and like them; and the supposition is that, at however distant a period of time you place your observer, he would still find mountains, lands, and waters, with animal and vegetable products flourishing upon them and sporting in them just as he finds now." this theory being perfectly inconsistent with facts, had to be abandoned. _second theory._--the second theory considers the universe eternal, but not its form. this was the system of epicurus and most of the ancient philosophers and poets, who imagined the world either to be produced by fortuitous concourse of atoms existing from all eternity, or to have sprung out of the chaotic form which preceded its present state. _third theory._--by this theory the matter and form of the earth is ascribed to the direct agency of a spiritual cause. it is needless to say that this last theory has for its basis the popular account, generally credited to moses in the first chapter of genesis. i say popular, for it certainly is not a scientific account, nor was it the intention of the writer to make it so. the supposed object was to show the relation between the creator and his works. if it had been an ultimate scientific account, the ablest minds of to-day would be unable to comprehend it, as science is progressive and constantly changing; in fifty thousand years to come, it would still appear utterly absurd. it cannot be said for this fact that the account is any the less true because it is not presented in scientific phraseology; for instance, when we remark in popular language "the sun rises," who shall say that though the expression is not astronomically true, we do not, for all practical purposes, utter as important a truth, as when we say, "the earth by its revolution brings us to that point where the sun becomes visible?" the language, also, in which the writer wrote was very imperfect; it had no equivalent to our word "air" or "atmosphere," properly speaking, for they knew not the words. "their nearest approaches," according to j. pye smith, "were with words that denoted watery vapor condensed, and thus rendered visible, whether floating around them or seen in the breathing of animals; and words for smoke from substances burning; and for air in motion, wind, a zephyr whisper or a storm." it must also be remembered, "that the hebrews had no term for the abstract ideas which we express by 'fluid' or 'matter.' if the writer had designed to express the idea, 'in the beginning god created _matter_,' he could not have found words to serve his purpose" (phin). [illustration: fig. i.--skeleton of kangaroo. (popular science monthly.)] [illustration: fig. i.--represents semi-apes (prosimiæ). the slow loris, after _tickel_ and _alp. miln-edwards_. (natural history, by _duncan_.)] it is unnecessary to state how the bible, which contains the so-called mosaic account, is regarded by the different church denominations, as undoubtedly that is familiar to every one. but with respect to the view entertained by the scientist and critical school of biblical scholars, represented chiefly by modern germans, i may state briefly: "they regard the bible as the human record of a divine revelation; not absolutely infallible, since there is no book written in any human language but must partake in a measure of the imperfections of that language. many of this school, while admitting the bible to contain the record of a true supernatural revelation, do not consider it to be without positive error of historical fact, not without false coloring from popular legend and tradition, but nevertheless a record as good as human hands could make a truly divine revelation."[ ] there is, though, a class of thinkers that altogether reject the bible; that is to say, refuse to believe it to be a divine revelation. hume, whom huxley calls "the most acute thinker of the eighteenth century," thus ends one of his essays: "if we take in hand any volume of divinity or school metaphysics, for instance, let us ask, _does it contain any abstract reasoning concerning quantity or number?_ no. _does it contain any experimental reasoning concerning matter of fact and existence?_ no. commit it, then, to the flames, for it can contain nothing but sophistry and illusion." to this huxley says: "permit me to enforce this wise advice, why trouble ourselves about matters of which, however important they may be, we do know nothing, and can know nothing? we live in a world which is full of misery and ignorance, and the plain duty of each and all of us is to try to make the little corner he can influence somewhat less miserable and somewhat less ignorant than it was before he entered it. to do this effectually, it is necessary to be fully possessed of only two beliefs: the first, that the order of nature is ascertainable by our faculties to an extent which is practically unlimited; the second, that our volitions count for something as a condition of the course of events. each of these beliefs can be verified experimentally, as often as we like to try. each, therefore, stands upon the strongest foundation upon which any belief can rest, and forms one of our highest truths." the first words in the mosaic account are:[ ] "in the beginning god created the heaven and the earth."[ ] it is seen, then, that the so-called revelation points to a beginning. the beginning referred to is an absolute beginning, for we find: "in the beginning was the word, and the word was with god, and the word was god."[ ] * * * "all things were made by him; and without him was not anything made that was made."[ ] science points also to a beginning. geology points to a time when man did not inhabit the earth; when for him there was a beginning. so, too, for lower organisms; so, too, for the rocky minerals; so, too, for the round world itself. but the beginning that science points to is not an absolute beginning. science has to start from some point, and that point must have a scientific foundation--the foundation of science is matter, which is inseparable from form and force. natural science teaches that matter is eternal and imperishable; for experience has never shown us that even the smallest particle of matter has come into existence or passed away. "a naturalist," says haeckel, "can no more imagine the coming into existence of matter than he can imagine its disappearance, and he therefore looks upon the existing quantity of matter in the universe as a given fact." "the creation of matter, if, indeed," says haeckel,[ ] "it ever took place, is completely beyond human comprehension, and can therefore never become a subject of scientific inquiry. we can as little imagine a _first beginning_ of the eternal phenomena of the motion of the universe as of its final end."[ ] it is evident, then, that the absolute beginning of the universe and its absolute end are not questions of science, and can be known only as revealed by faith. paul says: "by faith we understand that the world was framed by the word of god, so that things which are seen were not made of things which appeared."[ ] [illustration: fig. i.--represents tailed apes (menocerca). proboscis monkey (presbytes larvatus). (mammalia.)--_louis figuier._ the natives of borneo pretend that these monkeys, or, as sometimes called, kahan, are men who have retired to the woods to avoid paying taxes; and they entertain the greatest respect for a being who has found such ready means of evading the responsibilities of society.--_figuier._] [illustration: gibbon. orang. chimpanzee. gorilla. man. fig. i.--photographically reduced from diagrams of the natural size (except that of the gibbon, which was twice as large as nature), drawn by _waterhouse hawkins_, from specimens in the museum of the royal college of surgeons. (_huxley's_ "man's place in nature.")] if, therefore, science makes the "history of creation" its highest and most difficult and most comprehensible problem, it must deal with "_the coming into being of the form_ of natural bodies." let us look for a minute at kant's cosmogony, or, as haeckel says,[ ] kant's cosmological gas theory: "this wonderful theory," says haeckel, "harmonizes with all the general series of phenomena at present known to us, and stands in no irreconcilable contradiction to any one of them. moreover, it is purely mechanical and monistic, makes use exclusively of the inherent forces of eternal matter, and entirely excludes every supernatural process, every prearranged and conscious action of a personal creator." compare this last statement with the following: "i will, however," says haeckel,[ ] "not deny that kant's grand cosmogony has some weak points." * * * "a great unsolved difficulty lies in the fact that the cosmological gas theory furnishes no starting-point at all in explanation of the first impulse which caused the rotary motion in the gas-filled universe." whewell[ ] has pointed out, that the nebular hypothesis is null without a creative act to produce the inequality of distribution of cosmic matter in space. it is seen, then, that according to kant's theory we are to suppose that millions of years ago there appeared a nebulous mass possessing a rotary motion, and unequally distributed through space. this is what science calls a beginning, and may assert that every physical event of a hundred million of ages existed potentially in that nebulous mass. but this is really no explanation of the ultimate and real cause of anything. reason demands the cause of this beginning, the source that gave to the nebulous mass its rotary motion; the power that distributed the matter in space; the antecedents of the cosmical vapor. in absence of antecedents, what was the cause of this fire-mist--of these forces active in it? reason will never remain satisfied until these questions are answered. but physical science can trace the thread no further back, and must be dumb to all ulterior inquiries. it is true, then, as physicists assert, "that their science does not mount actually to god." [illustration: fig. i.--represents man-like apes (anthropoides). the male gorilla. (natural history, by _duncan_.)] [illustration: fig. ii.--represents ape-like men (pithecanthropi). imaginative. (from scientific american.)] [illustration: fig. iii.--men (homines). from woolly-haired men developed the papuans. (scientific american, march , .)] [illustration: fig. i.--the monkey men of dourga strait. (natural history, by _rev. dr. wood_.)] to god then, in strict accordance with our reason, is to be attributed not only the origination of matter, but all its future developments. when i speak of matter, it must be understood that i mean force; for "if matter were not force, and immediately known as force, it could not be known at all, could not be rationally inferred. the operation of force could furnish no evidence of the existence of forceless matter. if force is not matter, then force can exist and operate without matter; its existence and operation are no evidence of the existence of matter. and as matter is forceless, it can itself give no evidence of its own existence, for that would be an exercise of force. if force cannot exist and operate without matter, then force depends for its existence and operation on the forceless, which destroys itself; or force depends for its existence on matter as some property or force, and so matter and force are identified, and force depends on itself only, as it must."[ ] the idea, then, that force is an attribute of matter and inherent in it, is absurd, for there is not a shadow of evidence that force is or can be an attribute of matter. we have no knowledge of the origin of any force save of that which emanates from human volition. all our knowledge of force presents it as an effort of intelligent will. "we are driven," says winchell, "by the necessary laws of thought, to pronounce those energies styled gravitation, heat, chemical affinity and their correlates, nothing less than intelligent will. but as it is not human will which energizes in whirlwind and the comet, it must be divine will." "in all cases, the creative power of god is an act of power, and the power does not perish with its inception, but continues to operate until the act is reversed and undone; so that everything that god has created constitutes a positive and intrinsic force, though borrowed from him. every incident runs back to god as its originator and real cause. the true philosophical doctrine makes god distinct from all his works, and yet acting in them. this doctrine has been held by the greatest thinkers the world has ever produced, such as descartes, lerbrisky, berkeley, herschel, faraday, and a multitude of others." "it seems to be required," says dr. mccosh, "by that deep law of causation which not only prompts us to seek for a law in everything but an adequate cause, to be found only in an intelligent mind." "our greatest american thinker, jonathan edwards," says dr. mccosh, (whom i can claim as my predecessor,) "maintains that, as an image in a mirror is kept up by a constant succession of rays of light, so nature is sustained by a constant forth-putting of the divine power. in this view nature is a perpetual creation. god is to be seen not only in creation at first, but in the continuance of all things." "they continue to this day according to thine ordinances." returning now to the history of the creation given by moses, haeckel says, "although moses looks upon the results of the great laws of organic development as the direct actions of a constructing creator, yet in his theory there lies hidden the ruling idea of a progressive development and a differentiation of the originally simple matter. we can therefore bestow our just and sincere admiration on the jewish lawgiver's grand insight into nature, without discovering in it a so-called 'divine revelation.' that it cannot be such is clear from the fact that two great fundamental errors are asserted in it, namely, first the _geocentric_ error, that the earth is the fixed central point of the whole universe, round which the sun, moon and stars move; and secondly, the _anthropocentric_ error that man is the premeditated aim of the creation of the world, for whose service alone all the rest of nature is said to have been created. the former of these errors was demolished by copernicus' system of the universe in the beginning of the sixteenth century, the latter by lamarck's doctrine of descent in the beginning of the nineteenth century." [illustration: fig. i.--australian savage.--_orton._] [illustration: fig. ii.--skull of orang-utan (simia satyrus).--_orton._] [illustration: fig. iii.--skull of chimpanzee (troglodytes niger).] [illustration: fig. iv.--skull of gorilla.--_duncan._] [illustration: fig. v.--skull of european.] [illustration: fig. vi.--skull of negro.--_orton._] prof. huxley, in his lecture on "evidences of evolution," spoke of the mosaic account as milton's hypothesis. first, "because," says huxley, "we are now assured upon the authority of the highest critics, and even of dignitaries of the church, that there is no evidence whatever that moses ever wrote this chapter, or knew anything about it;" and second, as this hypothesis is presented in milton's work on "paradise lost," it is appropriate to call it the miltonic hypothesis. "in the miltonic account," says huxley, "the order in which animals should have made their appearance in the stratified rocks would be this: fishes, including the great whale, and birds; after that all the varieties of terrestrial animals. nothing could be further from the facts as we find them. as a matter of fact we know of not the slightest evidence of the existence of birds before the jurassic and perhaps the triassic formations. if there were any parallel between the miltonic account and the circumstantial evidence, we ought to have abundant evidence in the devonian, the silurian, and carboniferous rocks. i need not tell you that this is not the case, and that not a trace of birds makes its appearance until the far later period which i have mentioned. and again, if it be true that all varieties of fishes, and the great whales and the like, made their appearance on the fifth day, then we ought to find the remains of these things in the older rocks--in those which preceded the carboniferous epoch. fishes, it is true, we find, and numerous ones; but the great whales are absent, and the fishes are not such as now live. not one solitary species of fish now in existence is to be found there, and hence you are introduced again to the difficulty, to the dilemma, that either the creatures that were created then, which came into existence the sixth day, were not those which are found at present, or are not the direct and immediate predecessors of those which now exist; but in that case you must either have had a fresh species of which nothing has been said, or else the whole story must be given up as absolutely devoid of any circumstantial evidence." it is for these and many other reasons that i feel bound to omit the mosaic account, no matter how near some portions of it coincide with the facts the earth has opened out to the scientist. kant's cosmogony. it is maintained by kant's cosmogony that every substance, be it solid or liquid, constituting the entire universe, was, inconceivable ages ago, in their homogeneous gaseous or nebulous condition. owing to an impulse being given to the nebulous mass, it acquired a rotary movement, which divided the nebulous mass up into a number of masses which, owing to the rotation, acquired greater density than the remaining gaseous mass, and then acted on the latter as central points of attraction. our solar system was thus a gigantic gaseous or nebulous ball, all the particles of which revolved around a common central point--the solar nucleus. this nebulous ball assumed by its continual rotation a more or less flattened spheroidal form. by the continual revolution of this mass, under the influence of the centripetal and centrifugal forces, a circular nebular ring separated (like the present ring around saturn) from the rotating ball. in time the nebulous ring condensed to a planet, which began to revolve around its own axis. when the centrifugal force became more powerful than the centripetal force in the planet, rings were formed, which, in turn, formed planets which revolved around their axes, as also around their planets, as the latter moved around the sun, and thus arose the moons, only one of which moves around our earth, while four move around jupiter and six around uranus. this order of things was repeated over and over again until thereby arose the different solar systems--the planets rotating around their central suns, and the satellites or moons moving around their planets. by a continuous increasing of refrigeration and condensation, a fiery fluid or molten state occurred in these rotating bodies. they then emitted an enormous amount of heat by rapid condensation, and the rotating bodies--suns, planets, and moons--soon became glowing balls of fire, emitting light and heat. the / part of a pound of magnesium wire, burning in the open air, will give a light which will last during one second, and can be seen at a distance of thirty miles; imagine, then, what the light would be from these huge balls of fire floating through space. the earth forms a small part--nay, even the sun whose mass is equal to , earths like ours, is but an infinitesimal portion of the whole. by the continual emitting of heat, however, these fiery balls had a crust form on the outside, which enclosed a fiery fluid nucleus. the crust for a time must have been a smooth sheet, but afterward very uneven, having protuberances and cavities form over its surface, owing to the molten mass within becoming condensed and contracted; the crust not following this change sufficiently close, must have fallen in, and thus produced the cavities. [illustration: mongolian.] [illustration: malay.] [illustration: ethiopian.] [illustration: american indian.] [illustration: facial angle, by _prof. nelson sizer_. , snake; , dog; , elephant; , ape; , human idiot; , the bushman; , the uncultivated; , the improved; , the civilized; , the enlightened; , the caucasian (highest type).] [illustration: caucasian (after _van evrie_).] [illustration: head of nose-ape (after _brehm_).] [illustration: julia pastrana (photographed by _hintye_).] [illustration: living idiot (on blackwell's island).] all the time, by the condensation, the diameter of the earth was being diminished. the irregular cooling of the crust caused irregular contractions on the surface, and as the diameter of the molten mass within was continually diminishing, many elevations and depressions were caused, which were the foundations of mountains and valleys. after the temperature of the earth had been reduced by the thickening of the crust--when it became sufficiently cool--the water which existed in steam was condensed and precipitated, falling in torrents, washing down the elevations, filling the depressions with the mud carried along, and depositing it in layers. it was not until the earth became covered with water that life was possible in any form, as both animals and plants consist to a very great extent of water. at this stage in the history of the earth, then, the little mass of protoplasm, which we have spoken so much about, came into existence in all probability, as has been stated, by spontaneous generation. laws of evolution. let us now examine some of the laws of evolution, as also some of the connecting links which blend one stage of man's development with another, which at first thought would seem unexplainable. haeckel[ ] summarizes the inductive evidences of darwinism as follows: . paleontological series (phylogeny); . embryological development of the individual (ontogeny); . the correspondence in the terms of these two series; . comparative anatomy (typical forms and structures); . correspondence between comparative anatomy and ontogeny; . rudimentary organs (dipeliology); . the natural system of organisms (classification); . geographical distribution (chorology); . adaptation to the environment (oecology); . the unity of biological phenomena. it will of course be impossible to consider even hastily all of the inductive evidence belonging to the several groups mentioned above, for the scope of this work would not permit of it. only such facts as present themselves most forcibly to the mind will be considered. darwinism, as has already been stated, is not the doctrine of evolution; it is, however, a successful attempt to explain the law or manner of evolution. the _law of natural selection_, pointed out by darwin, is called by herbert spencer, _the struggle for existence_. darwin discovered that natural selection produces fitness between organisms and their circumstances, which explains the law of _the survival of the fittest_. it is a well-known fact that man can, by pursuing a certain method of breeding or cultivation, improve and in various ways modify the character of the different domestic animals and plants. by always selecting the best specimen from which to propagate the race, those features which it is desired to perpetuate become more and more developed; so that what are admitted to be real varieties sometimes acquire, in the course of successive generations, a character as strikingly distinct, to all appearances, from those of the varieties, as one species is from another species of the same genus. it is evident that both natural and artificial selection depends on adaptation and inheritance. the difference between the two forms of selection is that, in the first case, the will of man makes the selection according to a plan, whereas in natural selection the struggle for life and the survival of the fittest acts without a plan other than that the most adaptable organism shall survive which is most fit to contend with the circumstances under which it is placed. natural selection acts, therefore, much more slowly than artificial selection, although it brings about the same end. adaptation in the struggle for life is an absolute necessity. in every act of breeding, a certain amount of protoplasm is transferred from the parents to the child, and along with it there is transferred the individual peculiar molecular motion. adaptation or transmutation depends upon the material influence which organism experiences from its surroundings, or its conditions of existence; while the transmission from inheritance is due to the partial identity of producing and produced organisms. organized beings, as a rule, are gifted with enormous powers of increase. wild plants yield their crop of seed annually, and most wild animals bring forth their young yearly or oftener. should this process go on unchecked, in a short time the earth would be completely overrun with living beings. it has been calculated that if a plant produces fifty seeds (which is far below the reproductive capacity of many plants) the first year, each of these seeds growing up into a plant which produces fifty seeds, or altogether two thousand five hundred seeds the next year, and so on, it would under favorable conditions of growth give rise in nine years to more plants by five hundred trillions than there are square feet of dry land upon the surface of the earth. slow-breeding man has been known to double his number in twenty-five years, and according to euler, this might occur in little over twelve years. but assuming the former rate of increase, and taking the population of the united states at only thirty millions, in six hundred and eighty-five years their living progeny would have each but a square foot to stand upon, were they spread over the entire globe, land and water included. but millions of species are doing the same thing, so that the inevitable result of this strife cannot be a matter of chance. evidently those individuals or varieties having some advantage over their competitors will stand the best chance to live, while those destitute of these advantages will be liable to destruction. nature may be said (metaphorically) to choose (like the will of man in artificial selection) which shall be preserved and which destroyed. that portion of the theory of development which maintains the common descent of all species of animals and plants from the simplest common origin, i have already stated with full justice should be called lamarckism. progress is recognized by all scientists to be a law of nature. some of the more important facts which sustain the theory of development, i propose now to present as briefly as possible. rudimentary organs. one of the strongest arguments in favor of the hypothesis of a genetic connection among all animals (including man), at least among all those belonging to the same great types, is the presence of rudimentary parts. by rudiments in anatomy are meant organs or structures imperfectly developed, so as to be almost or entirely without functional use. "each of them represents in germ, as it were, in one animal (or plant), that which is perfect and useful in another type." for a few examples: the little fold of caruncle at the inner margin of the eye in man, represents the nictitating membrane of birds. eyes which do not see form a striking example. these are found in very many animals which live in the dark, as in caves or underground. their eyes are often perfectly developed but are covered by a membrane, so that no ray of light can enter and they can never see. such eyes, without the function of sight, are found in several species of moles and mice which live underground, in serpents and lizards, in amphibious animals (proteus, cæcilia) and in fishes; also in numerous invertebrate animals which pass their lives in the dark, as do many beetles, crabs, snails, worms, etc. other rudimentary organs are the wings of animals which cannot fly. for example, the wings of the running birds, like the ostrich, emeu, cassowary, etc., the legs of which become exceedingly developed. the muscles which move the ears of animals are still present in man, but of course are of no use; by continual practice persons have been able to move their ears by these muscles. the rudiment of the tail of animals which man possesses in his - tail vertebræ, is another rudimentary part--in the human embryo it stands out prominently during the first two months of its development; it afterwards becomes hidden. "the rudimentary little tail of man is irrefutable proof that he is descended from tailed ancestors." in woman the tail is generally, by one vertebra, longer than in man. there still exists rudimentary muscles in the human tail which formerly moved it. another case of human rudimentary organs, only belonging to the male, and which obtains in like manner in all mammals, is furnished by the mammary glands on the breast, which, as a rule, are active only in the female sex. however, cases of different mammals are known, especially of men, sheep and goats, in which the mammary glands were fully developed in the male sex, and yield milk as food for their offspring. the vermiform appendix of the large intestine in man, is another illustration of a part which has no use, but in one marsupial is three times the length of its body. the rudimentary covering of hair over certain portions of the body, is not without interest. over the body we find but a scanty covering, which is thick only on the head, in the armpits, and on some other parts of the body. the short hairs on the greater part of the body are entirely useless, and are the last scanty remains of the hairy covering of our ape ancestors. both on the upper and lower arm the hairs are directed toward the elbow, where they meet at an obtuse angle--this striking arrangement is only found in man and the anthropoid apes, the gorilla, chimpanzee, orang, and several species of gibbons. the fine short hairs on the body become developed into "thickset, long, and rather coarse dark hairs," when abnormally nourished near old-standing inflamed surfaces.[ ] the fine wool-like hair or so-called lanugo with which the human foetus, during the fifth and sixth months, is thickly covered, offers another proof that man is descended from an animal which was born hairy, and remained so during life. this covering is first developed during the fifth month, on the eyebrows and face, and especially around the mouth, where it is much longer than that on the head. three or four cases have been recorded of persons born with their whole bodies and faces thickly covered with fine long hairs. prof. alex. brandt compared the hair from the face of a man thus characterized, aged thirty-five, with the lanugo of a foetus, and finds it quite similar in texture. eschricht[ ] has devoted great attention to this rudimentary covering, and has thrown much light on the subject. he showed that the female as well as the male foetus possessed this hairy covering, showing that both are descended from progenitors, both sexes of whom were hairy. eschricht also showed, as stated above, that the hair on the face of the fifth month foetus is longer on the face than on the head, which indicates that our semi-human progenitors were not furnished with long tresses, which must therefore have been a late acquisition. the question naturally arises, is there any explanation for the loss of hair covering? [illustration: fig. i.--the hairy-faced burmese family. (from scientific american, feb. , .)] darwin is of the opinion that the absence of hair on the body is, to a certain extent, a secondary sexual character; for, in all parts of the world, women are less hairy than men. he says: "therefore we may reasonably suspect that this character has been gained through sexual selection." as the body in woman is less hairy than in man, and as this character is common to all races, we may conclude that it was our female semi-human ancestors who were first divested of hair. professor grant allen[ ] has given much study to the subject of the loss of hair in the human being; and his investigations are worthy of careful consideration. he shows conclusively that those parts of an animal which are in constant contact with other objects are specially liable to lose their hair. this is noticeable on the under surface of the body of all animals which habitually lie on the stomach. the soles of the feet of all mammals where they touch the ground are quite hairless; the palms of the hands in the quadrumana present the same appearance. the knees of those species which frequently kneel, such as camels and other ruminants, are apt to become bare and hard-skinned. the friction of the water has been the means of removing the hair from many aquatic mammals--the whales, porpoises, dugongs, and manatees are examples. as the back of man forms the specially hairless region of his body, we must conclude that it is in all probability the first part which became entirely denuded of hair. the gorilla, according to professor gervais, is the only mammal which agrees with man in having the hair thinner on the back, where it is partly rubbed off, than on the lower surface. du chaillu states that he has "himself come upon fresh traces of a gorilla's bed on several occasions, and could see that the male had seated himself with his back against a tree-trunk." he also says: "in both male and female the hair is found worn off the back; but this is only found in very old females. this is occasioned, i suppose, by their resting at night against trees, at whose base they sleep." the gorilla has only very partially acquired the erect position, and probably sits but little in the attitude common to man. in man the case is different; in proportion as his progenitors grew more and more erect, he must have lain less and less upon his stomach, and more and more upon his back or sides, and this is seen in the savage man during his lazy hours--who stretches himself on the ground in the sun, with his back propped, where possible, by a slight mound or the wall of his hut. the continual friction of the surface of the back would arrest the growth of hair; for hair grows where there is normally less friction, and _vice versâ_. as man became more and more hairless, especially among savage and naked races, we should conclude that such a modification would be considered a beauty, and women would select such men in preference to more hairy individuals. the new zealand proverb is: "there is no woman for a hairy man." sexual selection, then, would play a very important part; and the difficulty of understanding how man became divested of hair is readily explained. haeckel says: "even if we knew absolutely nothing of the other phenomena of development, we should be obliged to believe in the truth of the theory of descent, solely on the ground of the existence of rudimentary organs." reproduction by means of eggs. it might be thought there existed a missing link between animals which lay eggs and those which do not; this, however, is done away with in many instances--one, for example, is found in our commonest indigenous snake. the ringed snake lays eggs which require three weeks time to develop; but when it is kept in captivity, and no sand is strewn in the cage, it does not lay eggs, but retains them until the young ones are developed. this only shows how powerfully influences affect the habit of animals. double-sexed individuals. another difficulty might be supposed to arise between animals which produce themselves other than by sexual reproduction. this has already been slightly touched upon; and it has been shown that numerous plants and animals propagate themselves through their double-sexed organs. it occurs in a great majority of plants, but only in a minority of animals; for example, the garden-snail, leeches, earth-worms, and many other worms. every garden-snail produces in one part of its sexual gland eggs, and in another part sperm. parthenogenesis offers an interesting form of transition from sexual reproduction to the non-sexual formation of germ-cells (which most resembles it). it has been demonstrated to occur in many cases among insects, especially by seebold's excellent investigations. among the common bees, a male individual (a drone) arises out of the eggs of the queen, if the eggs have not been fructified; a female (a queen or working bee), if the egg has been fructified. gonochorismus or sexual separation, which characterizes the more complicated of the two kinds of sexual reproduction, has evidently been developed from the condition of hermaphroditism at a late period of the organic history of the world. in this case the female individual in both animal and plant produces eggs or egg-cells. in animals, the male individual secretes the fructifying sperm (sperma); in plants, the corpuscles, which correspond to the sperm. inheritance. the remarkable facts of inheritance, extending to the reproduction of unimportant peculiarities of parts or organs (rudimentary parts) mentioned above, and the occasional outbreak of ancestral characters that have been dormant through several generations (some of which i will mention further on), might be thought perfectly unexplainable; but they are readily accounted for by the supposition that each part of an organism contributes its constituent and effective molecules to the germ and sperm particles. mr. sorby made numerous investigations with relation to the number of molecules in the germinal matter of eggs, and the spermatic matter supplied by the male. omitting the alkali, mr. sorby takes the formula, c{ }h{ }n{ }so{ }, as representing the composition of albumen. in a / of an inch cube, he reckons-- albumen , , , , molecules. water , , , , " -------------------------------- , , , , , molecules. or, in a sphere of the same diameter, , , , , of the two components. taking a single mammalian spermatozoon, having a mean diameter of / of an inch; "it might contain two and a half million of such gemmules. if these were lost, destroyed, or fully developed at the rate of one in each second, this number would be exhausted in about one month; but since a number of spermatozoa appears to be necessary to produce perfect fertilization, it is quite easy to understand that the number of gemmules introduced into the ovum may be so great that the influence of the male parent may be very marked, even after having been, as regards particular character, apparently dormant for many years." the germinal vesicle of a mammalian ovum being about / of an inch, mean diameter, might contain five hundred million of gemmules, which, if used up at the rate of one per second, would last more than seventeen years. if the whole ovum, about / in diameter, were all gemmules, the number would be sufficient to last, at this rate, one per second for , years! this, however, is not probable; but mr. sorby's remarks has completely removed all doubt as to its physical possibility from the darwinian theory; "and they prompt us," says slack, "to a wonderful conception of the powers residing in minute quantities of matter." the laws of inheritance are divisible into two series, conservative and progressive transmission; the laws of adaptation to direct (active) or indirect (potential) adaptation. external causes often influence the reproductive system, especially in organism propagating in a sexual way. this can be strikingly shown in artificially produced monstrosities. monstrosities can be produced by subjecting the parental organism to certain extraordinary conditions of life; and curiously enough, such an extraordinary condition of life does not produce a change of the organism itself, but a change in its descendants. the new formation exists in the parental organism only as a possibility (potential); in the descendants it becomes a reality (actual). most commonly, monstrosities with very abnormal forms are sterile, but there are instances where they reproduce their kind and become a species.[ ] geoffroy st. hilaire, who perhaps made the deepest investigations ever conducted into the nature and causes of their production, first conceived the idea of artificially producing them, and to this end he began modifications of the physical conditions of the evolution of the chicken during natural and artificial incubation. he determined the fact that monsters could be produced in this way, but scarcely carried his investigation further. this work has been taken up by m. dareste, and he has lately published a volume in paris which recounts the results of a quarter of a century's experimenting. eggs, he states, were submitted to incubation in a vertical instead of a horizontal position; they were covered with varnish in certain places so as to stop or modify evaporation and respiration. the evolution of the chick was rendered slower by a temperature below that of the normal heat of incubation. finally, eggs were warmed only at one point, so that the young animal, during development, was submitted at different parts to variable temperatures. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] these perturbations resulted in the most curious and unlooked for deformities in the embryo, some being not alone peculiar to the bird, but being similar to those which have been recognized in many other animals, and even in the human species. the data obtained have been deemed so important that m. dareste has recently received the lacaze prize for physiology from the french academy of sciences. it would be impossible to review even a fraction of the many forms of monstrosities which m. dareste has discovered. those that we give will, however, suffice to convey an idea of the wonderful variations produced. fig. is a chick embryo with the encephalon entirely outside the head, the heart, liver, and gizzard outside the umbilical opening, right wing lifted up beside the head, and the development of the left one stopped. in fig. the encephalon is herniated and marked with blood spots, the eye is rudimentary and replaced by a spot of pigment, the upper beak is shorter than the lower one, while the heart, liver, etc., are all outside. in figs. and the head is compressed, eyes well developed, but in the back instead of in the sides of the head; the body is bent, abdominal intestines not closed, heart largely developed and herniated. the literal references to the foregoing are: _am_, amnion; _al_, allantois; _v_, vitellus; _h_, encephalon; _i_, eye; _c_, heart; _f_, liver; _g_, gizzard; _ms_, upper, and _mi_, lower member. the commonest case of monstrosity observed by m. dareste has been that of the head protruding from the navel, and the heart or hearts above the head. this is a most extraordinary and new monster, and, if it persist, a chicken with its heart on its back, like a hump, may be expected. a curious fact discovered is the duplicity of the heart at the beginning of incubation, two hearts, beating separately, being clearly seen. another anomaly consists in heads with a frontal swelling, which is filled by the cerebral hemispheres. m. dareste's artificial monsters are all produced from the single germ or cicatricule (as the white circular spot seen in the yellow of the egg, and from which the embryo springs, is termed). he has not yet been able to determine artificially the production of monsters, the origin of which takes place in a peculiar state of the cicatricule before incubation. but having submitted to incubation some , eggs, he has obtained several remarkable examples of double monstrosities in process of formation, some representations of which are given herewith. fig. shows three embryos, all derived from a single cicatricule. fig. represents three embryos from two cicatricules. on one side of the line of junction are two imperfectly developed embryos, one having no heart. the single embryo on the other side is generally normal, but has a heart on the right side. in fig. are twins, one well formed, the heart circulating colorless blood, the other having no heart and a rudimentary head. fig. exhibits a double monster with lateral union. the heads are separate, and there are three upper and three lower members, those of the latter on the median line belonging equally to each of the pair. acquired qualities. when an organism has been subjected to abnormal conditions in life it can transmit any peculiarity it may have acquired. this is, however, not always possible, otherwise descendants of men who have lost their arm or leg would be born without the corresponding arm or leg--this shows that some acquired qualities are more easily transmitted than others--although there are cases, as, for instance, a race of dogs without tails has been produced by cutting off the tails of both sexes of the dog, during several generations. "a few years ago," says haeckel, "a case occurred on an estate near jena in which, by the careless slamming of a stable-door, the tail of a bull was wrenched off, and the calves begotten by this bull were all born without a tail. this is certainly an exception; but it is very important to note the fact that under certain unknown conditions such violent changes are transmitted in the same manner as many diseases." the transmission of diseases such as consumption, madness, and albinism form examples. albinoes are those individuals who are distinguished by the absence of coloring matter from their skins; they are of frequent occurrence among men, animals and plants. among many animals, such as rabbits and mice, albinoes with white fur and red eyes are so much liked that they are propagated. this would be impossible were it not for the law of the transmission of adaptations. hornless cattle have descended from a single bull born in of horned parents, but whose absence of horns was the result of some unknown cause. the law of interrupted or latent transmission, as illustrated in grandchildren who are like the grandparents, but quite unlike the parents. animals often resume a form which have not existed for many generations. one of the most remarkable instances of this kind of reversion, or "atavism," is the fact that in some horses there sometimes appear singular dark stripes similar to those of the zebra, quagga, and other wild species of african horse. nutrition directly modifies adaptation, as is well illustrated by animals which have been bred for domestic or other purposes. if a farmer is breeding for fine wool he gives much different food to the sheep than he would if he wished to obtain flesh or an abundance of fat. even the bodily form of man is quite different according to its nutrition. food containing much nitrogen produces little fat, that containing little nitrogen produces a great deal of fat. people who by means of banting's system, at present so popular, wish to become thin, eat only meat and eggs--no bread, no potatoes. man can breed for milk in cattle, for feathers in pigeons, for colored flowers in plants, and, in fact, for almost any desirable quality. geological record. _the geological record_ (palæontology) furnishes weighty evidence of man's descent; for the circumstantial evidence derived from this source is written without the possibility of a mistake, with no chance of error, on the stratified rocks. it is true that the geological record must be incomplete, because it can only preserve remains found in certain favorable localities, and under particular conditions; that this valuable record must be destroyed by processes of denudation, and obliterated by processes of metamorphosis, it cannot be doubted. "beds of rock of any thickness, crammed full of organic remains, may yet," says huxley, "by the percolation of water through them, or the influence of subterranean heat (if they descend far enough toward the centre of the earth), lose all trace of these remains, and present the appearance of beds of rock formed under conditions in which there was no trace of living forms. such metamorphic rocks occur in formations of all ages; and we know with perfect certainty, when they do appear, that they have contained organic remains, and that those remains have been absolutely obliterated." if we look at the geological record, we find: the first epoch.--_the archilithic_, or primordial epoch, constitutes the _age of skull-less animals and sea-weed forests_, and is made up of the laurentian, cambrian, and silurian period. the second epoch.--_the palæolithic_, or primary epoch, constitutes the _age of fishes and fern forests_, and is made up of the devonian, coal, and permian period. the third epoch.--_the mesolithic_, or secondary epoch, constitutes the _age of reptiles and pine forests, coniferæ_, and is made up of the triassic, jurassic, and chalk period. the fourth epoch.--_the cænolithic_, or tertiary epoch, constitutes the _age of mammals and leaf forests_, and is made up of the eocene, miocene, and phocene period. the fifth epoch.--the _anthropolithic_, or quaternary epoch, constitutes the _age of man and cultivated forests,_ and is made up of the glacial and postglacial period, and the period of culture. during the archilithic epoch the inhabitants of our planet, as has been already stated, consisted of skull-less animals, or aquatic forms. no remains of terrestrial animals or plants, dated from this period, have as yet been found. the archilithic period was longer than the whole long period between the close of the archilithic and the present time; for if the total thickness of all sedimentary strata be estimated as about one hundred and thirty thousand feet, then seventy thousand feet belong to this epoch. it was during this epoch that the little mass of protoplasm, which has been so often spoken of, came into existence. it has been stated above that palæontology is quite deficient. this is not only true of the record, but of the lack as yet of sufficient investigations. the greatest fields of investigation in this department have never been explored. the whole of the petrifactions accurately known do not probably amount to a hundredth part of those which, by more elaborate explorations, are yet to be discovered. the most ancient of all distinctly preserved petrifactions is the eozoon canadense, which was found in the lowest laurentian strata in the ottawa formation. probably no discovery in palæontology ranks higher than the discovery of the descendants of the horse. the horse, for example, as far as his limbs and teeth go, differs far more from extant graminivora than man differs from the ape. had not fossil ungulates been found, which demonstrate the common origin of the horse with didactyles and multidactyles, some would have deemed the horse a special miraculous creation. but now the links are complete, and the descent of the horse is found to follow exactly what the doctrine of evolution could have predicted. ontogeny. it has been stated that the palæontological record is quite incomplete, owing to many facts, some of which have been mentioned; fortunately, the history of the development of the organic individual, or ontogeny, comes in to fill up many deficiencies. ontogeny is a repetition of the principal forms through which the respective individuals have passed from the beginning of their tribe, and its great advantage is that it reveals a field of information which it was impossible for the rocks to retain; for the petrification of the ancient ancestors of all the different animal and vegetable species, which were soft, tender bodies, was not possible. the annexed plate illustrates the dog, rabbit, and man in their first stages of development. illustrations of a fish, an amphibious animal, a reptile, a bird, or any mammal, could also be given; for all vertebrate animals of the most different classes, in their early stages of development, cannot be distinguished, and the nearer the animal approaches man in the ascending scale, the longer does this similarity continue to exist--when reptiles and birds are distinctly different from mammals, the dog and the man are almost identical. the gill-arches of the fish exist in man, in dogs, in fowls, in reptiles, and in other vertebrate animals during the first stages of their development. man also possesses, in his first stages, a real tail, as well as his nearest kindred--the tailless apes (orang-outang, chimpanzee, gorilla), and vertebrate animals in general. the tail, as has been stated, man still retains, though hidden as a rudiment. [illustration: fig. i.--human embryo.--_ecker._] [illustration: fig. ii.--embryo of dog.--_bischoff._] [illustration: fig. iii.--dog embryo.--_huxley._] [illustration: figs. iv, v, and vi.--embryo of rabbit in three stages of development.--_haeckel._] [illustration: figs. vii, viii, and ix.--embryo of man in three stages of development.--_haeckel._ _v_, fore brain; _z_, twix brain; _m_, middle brain; _h_, hind brain; _n_, after brain; _r_, spinal marrow; _e_, nose; _a_, eye; _o_, ear; _k_, gillarches; _g_, heart; _w_, vertebral column; _f_, fore limbs; _b_, hind limbs; _s_, tail.] "man presents in his earliest stages of embryonic growth, a skeleton of cartilage, like that of the lamprey; also, five origins of the aorta and five slits on the neck, like the _lamprey_ and the _shark_. later, he has but four aortic origins, and a heart now divided into two chambers, like _bony fishes_; the optic lobes of his brain also having a very fish-like predominance in size. three chambers of the heart and three aortic origins follow, presenting a condition permanent in the _batrachia_; then two origins with enlarged hemispheres of the brain, as in _reptiles_. four heart chambers and one aortic root on each side, with slight development of the cerebellum, agree with the characters of the _crocodiles_, and immediately present the special mammalian conditions, single aortic root, and the full development of the cerebellum. later comes that of the cerebrum, also in its higher mammalian or human traits." at no time in the development of the egg, save at the start, do the embryos of the various vertebra assume the _exact_ or _entire_ characteristics of one another, but they assimilate so closely that it requires the eye of the expert to distinguish them; and, as has already been stated, the more closely an animal resembles another, the longer and the more intimately do their embryos resemble one another; so that, for example, the embryo of the snake and of a lizard remain like one another longer than do those of a snake and of a bird; and the embryo of a dog and of a cat remain like one another for a far longer period than do those of a dog and a bird, or a dog and an opossum, or even those of a dog and a monkey. surely it must be admitted that the short brief history given by the development of the egg, is far more wonderful than phylogeny or the long and slow history of the development of the tribe, which has taken thousands of years. compare this time with the time required for the development of the smallest mammals--the harvest mice which develops in three weeks, or the smallest of all birds, the humming-bird, which quits the egg on the twelfth day, or with man who passes through the whole course of his development in forty weeks, or with the rhinoceros who requires - / years, or the elephant who requires ninety weeks. how insignificant are these various periods to the long period originally required; yet in these short periods the whole phylogeny is run through in the ontogeny or the history of the development of the egg. the attributes of man. we must now consider briefly some of the attributes of man, and see if he really possesses attributes which are in no inferior degree possessed by animals. before proceeding directly to the consideration of the attributes of man, it will be best to show the correlation that exists between what are called man's vital forces and the physical forces of nature. to do this let us choose three forms of its manifestation: these shall be heat evolved within the body; muscular energy or motion; and lastly, nervous energy or that form of force which, on the one hand, stimulates a muscle to contract, and on the other appears in forms called mental. it will not take any extensive argument to demonstrate that the heat of the body does not differ from heat from any other source. it is known that the food taken into the body contains potential energy, which is capable of being in part converted into actual heat by oxidation; and since we know that the food taken into the body is oxidized by the oxygen of the air supplied by the lungs, the heat of the body must be due to the slow oxidation of the carbon, perhaps also hydrogen, sulphur, and phosphorus in the food. now since this so-called vital heat is developed by oxidation, is recognized by the same tests and applied to the same purposes as any other heat, it is as truly correlated to the other forces as when it has a purely physical origin. the amoeboid activity of a white blood corpuscle is stimulated within certain limits by heat. hatching of eggs and the germination of seeds may be likewise hastened or retarded by access or deprivation of heat. it was considerations such as these which led to the doctrine of correlation of the vital and physical forces. with respect to the muscular force exerted by an animal, it was supposed that it was created by the animal. dr. frankland[ ] says to this: "an animal can no more generate an amount of force capable of moving a grain of sand, than a stone can fall upwards or a locomotive drive a train without fuel." as the amount of co{ } exhaled by the lungs is increased in the exact ratio of work done by the muscle, it cannot be doubted that the actual force of the muscle is due to the converted potential energy of the food. since every exertion of a muscle and nerves involves the death and decay of those tissues to a certain extent, as shown by the excretions, prof. orton[ ] has been led to say: "an animal begins to die the moment it begins to live." "a muscle," says barker,[ ] "is like a steam-engine, is a machine for converting the potential energy of carbon into motion; but unlike a steam-engine, the muscle accomplishes this conversion directly, the energy not passing through the intermediate stages of heat. for this reason the muscle is the most economical producer of mechanical force known." the muscles which give the downward stroke of the wing of a bird are fastened to the breastbone, and their power in proportion to the weight of the bird is as , to . this great power is needed, for the air is times lighter than water; the hawk being able to travel miles an hour. the last of the so-called vital forces under consideration, is that produced by the nerves and nervous centres. barker says: "in the nerve which stimulates a muscle to contract, this force is undeniably motion, since it is propagated along this nerve from one extremity to the other." this force has been likened unto electricity, the gray or cellular matter being the battery, the white or fibrous matter the conductors. du bois reymond[ ] has demonstrated that this force is not electricity, though by showing that its velocity is only ninety-seven feet a second. the velocity varies, though, in different animals; it is, according to prof. orton,[ ] "more rapid in warm-blooded than in cold-blooded animals, being nearly twice as fast in man as in the frog." wheatstone, by his method, gives the velocity of electricity in copper wire at , geographical miles per second; but as neither fizeau, gould, gonnelle and others could arrive at the same result, the method was shown to be incorrect, and it remained for dr. siemen[ ] to discover the true method, which gives the velocity just one-half that of wheatstone's estimate, or , geographical miles per second. in the opinion of bence jones, the propagation of a nervous impulse is a sort "of successive molecular polarization, like magnetism." but that this agent is a force as analogous to electricity as is magnetism, is shown not only by the fact that the transmission of electricity along a nerve will cause the contraction of a muscle to which it leads, but also by the important fact discovered by marshall, that the contraction of a muscle is excited by diminishing its normal electrical current,[ ] a result which could take place only with a stimulus, says barker, "closely allied to electricity. nerve force must therefore be transmuted potential energy." prof. huxley says,[ ] "the results of recent inquiries into the structure of the nervous system of animals, converge toward the conclusion that the nerve-fibres which we have hitherto regarded as ultimate elements of nervous tissue, are not such, but are simply the visible aggregations of vastly more attenuated filaments, the diameter of which dwindles down to the limits of our present microscopic vision, greatly as these have been extended by modern improvements of the microscope; and that a nerve is, in its essence, nothing but a linear tract of specially modified protoplasm between two points of an organism, one of which is able to affect the other by means of the communication so established. hence it is conceivable that even the simplest living being may possess a nervous system." herbert spencer[ ] says all direct and indirect evidence "justifies us in concluding that the nervous system consists of _one_ kind of matter. in the gray tissue this matter exists in masses containing _corpuscles_, which are soft and have granules dispersed through them, and which, besides being thus unstably composed, are placed so as to be liable to disturbances to the greatest degree. in the white tissue this matter is collected together in extremely slender _threads_ that are denser, that are uniform in texture, and that are shielded in an unusual manner from disturbing forces, except at their two extremities." the last consideration is that form of force (thought power) which appears in manifestations called mental. it must be noticed at the outset, that every external manifestation of thought force is a muscular one, as a word spoken or written, a gesture, or an expression of the face always takes place; hence this force must be intimately correlated to nerve force. it is very certain, then, that thought force is capable in external manifestations of converting itself into actual motion. but here the question arises, can it be manifested inwardly without such a transformation of energy? or is the evolution of thought entirely independent of the matter of the brain? this question can be answered by actual experiment, strange as it may appear. experiments have demonstrated that any change of temperature within the skull was soonest manifested externally in that depression which exists just above the occipital protuberance. here lombard[ ] fastened to the head at this point two little bars, one made of bismuth, the other of an alloy of antimony and zinc, which were connected with a delicate galvanometer;[ ] to neutralize the result of a gradual rise of temperature over the whole body, a second pair of bars, reversed in direction, was attached to the leg or arm, so that if a like increase of heat came to both, the electricity developed by one would be neutralized by the other, and no effect would be produced by the needle unless only one was affected. by long practice it was ascertained that a mental torpor could be induced, lasting for hours, in which the needle remained stationary. but let a person knock on the door outside of the room, or speak a single word, even though the experimenter remained absolutely passive, the reception of the intelligence caused the needle to swing twenty degrees. "in explanation of this production of heat," says barker,[ ] "the analogy of the muscle at once suggests itself. no conversion of energy is complete, and as the heat of muscular action represents force which has escaped conversion into motion, so the heat evolved during the reception of an idea is energy which has escaped conversion into thought, from precisely the same cause." dr. lombard's experiments have shown that the amount of heat developed by the recitation to one's self of emotional poetry, was in every case less when recitation was oral; this is of course accounted for by the muscular expression. chemistry teaches that thought-force, like muscle-force, comes from the food, and demonstrates that the force evolved by the brain, like that produced by the muscle, comes not from the disintegration of its own tissue, but is the converted energy of burning carbon.[ ] "can we longer doubt," says barker,[ ] "that the brain too, is a machine for the conversion of energy? can we longer refuse to believe that even thought force is in some mysterious way correlated to the other natural forces? and this even in the face of the fact that it has never yet been measured.[ ] have we not a right to ask 'why a special force (vital force) should be needed to effect the transformation of physical forces into those modes of energy which are active in the manifestation of living beings, while no peculiar force is deemed necessary to effect the transformation of one mode of physical force into any other mode of physical force?" richard owen says:[ ] "in the endeavor to clearly comprehend and explain the functions of the combination of forces called 'brain,' the physiologist is hindered and troubled by the views of the nature of those cerebral forces which the needs of dogmatic theology have imposed on mankind. * * * religion, pure and undefiled, can best answer how far it is righteous or just to charge a neighbor with being unsound in his principles who holds the term 'life' to be a sound expressing the sum of living phenomena, and who maintains these phenomena to be modes of force into which other forms of force have passed from potential to active states, and reciprocally, through the agency of the sums or combinations of forces impressing the mind with the ideas signified by the terms 'monad,' 'moss,' 'plant,' or 'animal.'" we have now shown that the very forces which give vent to the attributes of man, are correlated to the physical forces. let us now consider his attributes as manifested by his mental powers. there is no doubt the difference between the mental faculties of the ape and that of the lowest savage, who cannot express any number higher than four and who uses hardly any abstract terms for common objects or for the affections,[ ] is still very great and would still be great, says darwin, "even if one of the higher apes had been improved or civilized as much as a dog has been in comparison with its parent form, the wolf or jackal." but when we examine the interval of mental power between one of the lowest fishes, as a lamprey or a lancelet, and one of the higher apes, and recognize the fact that this interval is filled up by numberless gradations, it does not become so difficult to understand the interval between an ape and man, which is not by far so great. as in finding out what is peculiar to a living body in distinction to a body not living, we found it absurd to take man as the perfection of the animal scale--the microscopic monad possessing life as well as him--so in the case of man's mental attributes, which have always been increasing, always perfecting, since the first genuine man came into existence, it would be equally absurd to compare the intellectual man of to-day with an ape to see what attributes he possesses which the ape does not possess; but if we go down in the scale and compare the savage with the ape, the difficulty is not by far so great. it will be found on close examination, though, that man and the higher animals, especially the primates, have many instincts in common. "all," says darwin, "have the same senses, intuitions and sensations; similar passions, affections, and emotions; even the more complex ones, such as jealousy, suspicion, emulation, gratitude and magnanimity; they practice deceit and are revengeful; they are sometimes susceptible to ridicule and even have a sense of humor; they feel wonder and curiosity; they possess the same faculties of imitation, attention, deliberation, choice, memory, imagination, the association of ideas, and reason, though in very different degrees. the individuals of the same species graduate in intellect from absolute imbecility to high excellence; they are also liable to insanity, though far less often than in the case of man."[ ] nevertheless, in the face of these facts, many authors have insisted that man is divided by an inseparable barrier from all the lower animals in his mental faculties. it only shows the improper or imperfect consideration of the subject they have under discussion. it may be thought at first that some of the mental attributes mentioned above are not possessed by animals. i therefore will briefly consider a few of the more complex ones. we can dismiss the consideration of such attributes as happiness, terror, suspicion, courage, timidity, jealousy, shame, and wonder, as well-known attributes. _curiosity_ in animals is often observed. an instance mentioned by brehm will serve to illustrate: brehm gives a curious account of the instinctive dread which his monkeys exhibited for snakes; but their curiosity was so great that they could not desist from occasionally satiating their horror in a most human fashion, by lifting up the lid of the box in which the snakes were kept. _imitation_ is also found among the action of animals, especially among monkeys, which are well known to be ridiculous mockers. it is unnecessary to refer to the faculty of attention, as it is common to almost all animals, and the same may be said of memory as for persons or places. one would hesitate to believe an animal possesses _imagination_, but such is the case. dreaming, it will be admitted, gives us the best notion of this power. now as dogs, cats, horses, and probably all the higher animals, even birds, have vivid dreams--this is shown by their movements and the sounds uttered--"we must admit," says darwin, "they possess some power of imagination. there must be something special which causes dogs to howl in the night, and especially during moonlight, in that remarkable and melancholy manner, called baying. all dogs do not do so; and, according to housyeau,[ ] they do not look at the moon, but at some fixed point near the horizon. housyeau thinks that their imaginations are disturbed by the vague outlines of the surrounding objects, and conjure up before them fantastic images; if this be so, their feelings may almost be called superstitious." the next mental faculty is _reason_, which stands at the summit; but still there are few persons who will deny that animals possess some power of reasoning. a few illustrations will be all that is necessary to satisfy the inquiring mind on this point. reugger, a most careful observer, states that when he first gave eggs to his monkey in paraguay they smashed them, and thus lost much of their contents; afterward they gently hit one end against some hard body, and picked off the bits of shell with their fingers. after cutting themselves _once_ with any sharp tool, they would not touch it again, or would handle it with the greatest caution. lumps of sugar were often given them, wrapped up in paper; and reugger sometimes put a live wasp in the paper, so that in hastily unfolding it they got stung; after this had _once_ happened, they afterward first held the packet to their ears to detect any movement within. the following cases relating to dogs are described by darwin: mr. colquhoun winged two wild ducks, which fell on the farther side of a stream; his retriever tried to bring over both at once, but could not succeed; she then, though never before known to ruffle a feather, deliberately killed one, brought over the other, and returned for the dead bird. colonel hutchinson relates that two partridges were shot at once--one being killed, the other wounded; the latter ran away, and was caught by the retriever, who, on her return, came across the dead bird; "she stopped, evidently greatly puzzled, and after one or two trials, finding she could not take it up without permitting the escape of the winged bird, she considered a moment, then deliberately murdered it by giving it a severe crunch, and afterward brought away both together. this was the only known instance of her ever having wilfully injured any game. here we have reason, though not quite perfect; for the retriever might have brought the wounded bird first, and then returned for the dead one, as in the case of the two wild ducks. i give the above cases as resting on the evidence of two independent witnesses; and because in both instances the retrievers, after deliberation, broke through a habit which was inherited by them (that of not killing the game retrieved), and because they show how strong their reasoning faculty must have been to overcome a fixed habit."[ ] it has often been said that no animal uses any tool, but this can be so easily refuted on reflection, that it is hardly worth while considering; for illustration, though, the chimpanzee in a state of nature cracks nuts with a stone; darwin saw a young orang put a stick in a crevice, slip his hand to the other end, and use it in a proper manner as a lever. the baboons in abyssinia descend in troops from the mountains to plunder fields, and when they meet troops of another species a fight ensues. they commence by rolling great stones at their enemies, as they often do when attacked with fire-arms. the duke of argyll remarks that the fashioning of an implement for a special purpose is absolutely peculiar to man; and he considers this forms an immeasurable gulf between him and the brutes. "this is no doubt," says darwin, "a very important distinction; but there appears to me much truth in sir j. lubbock's suggestion,[ ] that when primeval man first used flint-stones for any purpose, he would have accidentally splintered them, and would then have used the sharp fragments. from this step it would be a small one to break the flints on purpose, and not a very wide step to fashion them rudely. the later advance, however, may have taken long ages, if we may judge by the immense interval of time which elapsed before the men of the neolithic period took to grinding and polishing their stone tools. in breaking the flints, as sir j. lubbock likewise remarks, sparks would have been emitted, and in grinding them heat would have been evolved; thus the two usual methods of 'obtaining fire may have originated.' the nature of fire would have been known in many volcanic regions where lava occasionally flows through forests." it becomes a difficult task to determine how far animals exhibit any traces of such high faculties as _abstraction_, _general conception_, _self-consciousness_, _mental individuality_. there can be no doubt, if the mental faculties of an animal can be improved, that the higher complex faculties such as abstraction and self-consciousness have developed from a combination of the simpler ones; this seems to be well illustrated in the young child, as such faculties are developed by imperceptible degrees. these high faculties are very sparingly possessed by the savage; as buchner[ ] has remarked, how little can the hard-worked wife of a degraded australian savage, who uses very few abstract words and cannot count above four, exert her self-consciousness or reflect on the nature of her own existence. if there exist a class of people so inferior in their mental faculties as these, it is not difficult for us to understand how the educated animal who possesses memory, attention, association, and even some imagination and reason, can become capable of abstraction, &c., in an inferior degree even to the savage. it certainly cannot be doubted that an animal possesses mental individuality--as when a master returns to a dog which he has not seen for years, and the dog recognizes him at once. one of the chief distinctions between man and animals is the faculty of language. let us look at this for a moment. "the essential differences," says prof. whitney, "which separate man's means of communication in kind as well as degree from that of the other animals is that, while the latter is instinctive, the former is in all its parts arbitrary and conventional. no man can become possessed of any language without learning it; no animal (that we know of) has any expression which he learns, which is not the direct gift of nature to him." any child of parents living in a foreign country grows up to speak the foreign speech, unless carefully guarded from doing so; or it speaks both this and the tongue of its parent with equal readiness. a child must learn to observe and distinguish before speech is possible, and every child begins to know things by their name before he begins to call them. "if it were not for the added push," says prof. whitney, "given by the desire of communication, the great and wonderful power of the human soul would never move in this particular direction; but when this leads the way, all the rest follows." no philologist now supposes that any language has been deliberately invented; it has been slowly and unconsciously developed by many steps. there can be no question that language owes its origin to the imitation and modification of various natural sounds, the voices of other animals, and man's own instinctive cries, aided by signs and gestures; and this is the opinion of max müller. and prof. whitney remarks that "spoken language began, we may say, when a cry of pain, formally wrung out by real suffering, and seen to be understood and sympathized with, was repeated in imitation, no longer as a mere instinctive utterance, but for the purpose of intimating to another." darwin says that "the early progenitor of man probably first used his voice in producing true musical cadences, that is, in singing, as do some gibbon-apes at the present day. it is therefore probable that the imitation of musical cries by articulate sounds may have given rise to words expressive of very complex emotions." the nearest approach to language are the sounds uttered by birds. all that sing exert their power instinctively, but the actual song, and even the call notes, are learned from their parents or foster-parents. these sounds are no more innate than language is in man, as has been proved by davies barrington.[ ] the first attempt to sing "may be compared to the imperfect endeavor in a child to babble." prof. whitney says, if the last transition forms of man "could be restored, we should find the transition forms toward our speech to be, not at all a minor provision of natural articulate signs, but an inferior system of conventional signs, in tone, gesture, and grimace. as between these three natural means of expression, it is simply by a kind of process of natural selection and survival of the fittest that the voice has gained the upper hand, and come to be so much the most prominent that we give the name of language (tonguiness) to all expression." a single utterance or two at first had to do the duty of a whole clause; afterward man learned to piece together parts of speech, and thus arose sentences. although no language, as has already been said, has been deliberately invented, "still each word may not be unfitly compared to an invention; it has its own place, mode, and circumstances of devisal, its preparation in the previous habits of speech, its influence in determining the after progress of speech development; but every language in the gross is an institution, on which scores or hundreds of generations and unnumbered thousands of individual workers have labored."[ ] there is no question at all but that the mental powers in the earliest progenitors of man must have been more highly developed than in the ape, before even the most imperfect form of speech could have come into use; but the constant advancement of this power would have reacted on the mind to enable it to carry on longer trains of thought. "a complex train of thought," says darwin, "can no more be carried on without the aid of words, whether spoken or silent, than a long calculation without the use of figures in algebra. it appears also that even an ordinary train of thought almost requires or is greatly facilitated by some form of language; for the dumb, deaf, and blind girl, laura bridgman, was observed to use her fingers while dreaming.[ ] nevertheless a long succession of vivid ideas may pass through the mind, without the aid of any form of language, as we may infer from the movements of dogs during their dreams." the struggle for existence is going on in every language; one after another will be swept out of existence, and the languages best fitted for the practical uses of the masses of people will alone survive. max müller has well remarked: "a struggle for life is constantly going on amongst the words and grammatical forms in each language. the better the shorter; the easier forms are constantly gaining the upper hand, and they owe their success to their own inherent virtue."[ ] it must not be thought for a moment that that which distinguishes a man from the lower animals is the understanding of articulate sounds--for, as every one knows, dogs understand many words and sentences; and darwin says, at this stage they are at the same stage of development as infants, between the ages of ten and twelve months, who understand many words and sentences, but still cannot utter a single word. it is not the mere articulation which is our distinguishing character; for parrots and other birds possess the power. nor is it the mere capacity of connecting definite sounds with definite ideas; for it is certain that some parrots, which have been taught to speak, connect unerringly words with things, and persons with events." the lower animals, as has already been stated, differ from man solely in his almost infinitely larger power of associating together the most diversified sounds and ideas; and this obviously depends on the high development of his mental powers. we now come to the consideration of a very delicate subject--a subject which is certainly at best very unsatisfactory to handle, as far as popular sentiment is concerned; for, no matter how successfully it may be handled, according to one class of thinkers, to another class of more orthodox thinkers it would be entirely at fault. the subject is, _man's moral sense, belief in god, religion, conscience, and hope of immortality_. it has been stated by some writers that where "faith commences science ends." how erroneous is such a statement as this! for, as krauth has said, "the great body of scientific facts is actually the object of knowledge to a few, and is supposed to be a part of the knowledge of the many, only because the many have faith in the statements of the few, though they can neither verify them, nor even understand the processes by which they are reached."[ ] "we believe," says lewes, "that the sensation of violet is produced by the striking of the ethereal waves against the retina more than seven hundred billions of times in a second. * * * these statements are accepted _on trust_ by us who know that there are thinkers for whom they are irresistible conclusions." it is evident that it is to faith that science owes, to a very great extent, her progress and development; for it is impossible for man to prove by experimental demonstration all the facts of science, and since a certain number of facts have got to be accepted before a new experiment can be attempted, he has to accept on faith that such and such a statement is a fact, because such and such a scientist has claimed to have demonstrated it. "we are not _responsible_ for the fact," says krauth, "that under the conditions of knowledge we _know_, or in defect of them do not know; we are responsible if, under the conditions of a well-grounded faith, we disbelieve."[ ] let us look, then, at the belief in god. the question under consideration at first will not be whether there exists a god, the creator and ruler of the universe--for this will be afterward considered--but is there any evidence that man was aboriginally endowed with the ennobling belief in the existence of an omnipotent god. schweinfurth relates that the niam-niam, that highly interesting dwarf people of central africa, have no word for god, and therefore, it must be supposed, no idea; and moritz wagner has given a whole selection of reports on the absence of religious consciousness in inferior nations. the idea that conscience is a sort of permanent inspiration or dwelling of god in the soul, i think, on consideration, any reasonable man will not assume. "it is a purely human faculty," says savage, "like the faculty for art or music; and it gets its authority, as they do by being true, and just in so far as it is true. consciousness is our own knowledge of ourselves and of the relation between our own faculties and powers. conscience is our recognition of the relations, as right or wrong, in which we stand to those about us, god and our fellows. _con-scio_ is to know with, in relation. there is such a thing, of course, as a _false conscience_ and a _true conscience_. all the false "conscientiousness grows out of the fact that men suppose they stand in certain relationships that do not really exist. thus they imagined duties that are not duties at all." the virtues which must be practised by rude men, so that they can hold together in tribes, are of course important. no tribe could hold together if robbery, murder, treachery, etc., were common; in other words, there must be honor among thieves. "a north-american indian is well pleased with himself, and is honored by others, when he scalps a man of another tribe; and a dyak cuts off the head of an unoffending person, and dries it as a trophy. the murder of infants has prevailed on the largest scale throughout the world, and has been met with no reproach; but infanticide, especially of females, has been thought to be good for the tribe, or at least not injurious. suicide during former times was not generally considered as a crime, but rather, from the courage displayed, as an honorable act; and it is still practised by some semi-civilized and savage nations without reproach, for it does not obviously concern others of the tribe. it has been recorded that an indian thug conscientiously regretted that he had not robbed and strangled as many travelers as did his father before him."[ ] see how weak the conscience of even more highly civilized men are in their dealings with the brute creation; how the sportsman delights in hunting-scenes, spanish bull-fights, cock-fights, etc.; how indignant was the sensitive cowper, if any one should "needlessly set foot upon a worm"! the rights of the worm are as sacred in his degree as ours are, and a true conscience will recognize them. what, then, is a true conscience? savage states in a few words, it is "one that knows and is adjusted to the realities of life. when men know the truth about god, about themselves--body and mind and spirit--about the real relations of equity in which they stand to their fellow-men in state and church and society, and when they appreciate these, and adjust their conscience to them, then they will have a true conscience. an absolutely true conscience, of course, cannot exist so long as our knowledge of the reality of things is only partial." it is evident, then, that the conscience of man depends on his education and environments, and therefore is the subject of improvement. it becomes, then, the duty of every man to search for truth, for his conscience is not infallible, and by so doing he will bring it to accord with the real facts of god. "throw away," says savage, "prejudice and conceit, seek to make your conscience like the magnetic needle. the needle ever and naturally seeking the unchanging pole." as conscience, then, is but a faculty capable of development, it is not so difficult to understand a race of people whose conscience was in just the first stages of development; and, finally, a race which did not possess this faculty at all, as in the inferior nations which wagner speaks of. [illustration: fig. i.--butcher's shop of the anziques, anno . (from man's place in nature, by _huxley_.)] what kind of conscience and intelligence had the people near cape lopez, called the anziques, which m. du chaillu describes. they had incredible ferocity; for they ate one another, sparing neither friends nor relations. their butcher-shops were filled with human flesh, instead of that of oxen or sheep, for they ate the enemies they captured in battle. they fattened, slayed, and devoured their slaves also, unless they thought they could get a good price for them; and moreover, for weariness of life or desire for glory (for they thought it a great thing and a sign of a generous soul to despise life), or for love of their rulers, offered themselves up for food. there were, indeed, many cannibals, as in the east indies and brazil and elsewhere, but none such as these, since the others only ate their enemies, but these their own blood relations. there is therefore, combining the fact mentioned by wagner with the fact that some nations have no idea of one or more gods, not even a word to express it (proving that they have no idea), i say, there is therefore no evidence that man was aboriginally endowed with any such belief as the existence of an omnipotent god; and in this assertion almost all the learned men concur. "if, however," says darwin, "we include under the term religion, the belief in unseen or spiritual agencies, the case is wholly different; for this belief seems to be universal with the less civilized races. nor is it difficult to understand how it arose." the savage has a stronger belief in bad spirits than in good ones. "the same high mental faculties which first led man to believe in unseen spiritual agencies, then in fetishism, polytheism, and ultimately in monotheism, would infallibly lead him, as long as his reasoning powers remained poorly developed, to very strange superstitions and customs. many of these are terrible to think of: such as the sacrifice of human beings to a blood-loving god, the trial of innocent persons by the ordeal of poison, of fire, of witchcraft, etc.; yet it is well occasionally to reflect on these superstitions, for they show us what an infinite debt of gratitude we owe to the improvement of our reason, to science, and to our accumulated knowledge."[ ] as sir j. lubbock has well observed: "it is not too much to say that the possible dread of unknown evil hangs like a thick cloud over savage life, and embitters every pleasure. these miserable and indirect consequences of our highest faculties may be compared with the incidental and occasional mistakes of the instincts of the lower animals." the belief, then, of the existence of an omnipotent god came with the development of the mental faculties; and although there does exist such a belief in the minds of men whose conscience is in a normal condition, still there are temptations to unbelief, and these have led men to atheism. i cannot think of an atheist unless i associate in my thoughts the words: "the ruling passion, be it what it may-- the ruling passion conquers reason still." the atheist has decided not to believe in the existence of a god, unless he can see him and understand him; in other words, the finite would comprehend the infinite. following the logical method of reasoning of an atheist, the simple fact of seeing god in no way ought to prove his existence. for when you say you see a person, and that you have not the least doubt about it, i answer, that what you are really conscious of is an affection of your retina. and if you urge that you can check your sight of the person by touching him, i would answer, that you are equally transgressing the limits of fact; for what you are really conscious of is, not that he is there, but that the nerves of your hand have undergone a change. all you hear and see and touch and taste and smell are mere variations of your own condition, beyond which, even to the extent of a hair's-breadth, you cannot go. that anything answering to your impression exists outside of yourself is not a _fact_, but an _inference_, to which all validity would be denied by an idealist like berkeley, or by a skeptic like hume.[ ] thomas cooper[ ] said: "i do not say--there is no god; but this i say--i know not." mr. bradlaugh says: "the atheist does not say, 'there is no god'; but he says, i know not what you mean by god; i am without idea of god; the word 'god' is to me a sound conveying no clear or distinct affirmation. i do not deny god, because i cannot deny that of which i have no conception, and the conception of which, by its affirmer, is so imperfect that he is unable to define it to me." austin holyoake[ ] says: "the only way of proving the fallacy of atheism is by _proving_ the existence of a god." if it is logical proof that is wanted, there is plenty. the following arguments, although not all meeting my approbation, are still of interest: the _ontological argument_ has been presented in different forms. . anselm,[ ] archbishop of canterbury ( - ), states this argument thus: we have an idea of an infinitely perfect being. but real existence is an element of infinite perfection. therefore an infinitely perfect being exists; otherwise the infinitely perfect, as we conceive it, would lack an essential element of perfection. . descartes[ ] ( - ) states the argument thus: the idea of an infinitely perfect being which we possess could not have originated in a finite source, and therefore must have been communicated by an infinitely perfect being. . dr. samuel clark[ ] ( ) argues that time and space are infinite and necessarily existent, but they are not substances. therefore there must exist an eternal and infinite substance of which they are properties. . cousin[ ] maintained that the idea of the finite implies the idea of the infinite as inevitably as the idea of the "me" implies that of the "not me." the _cosmological argument_ may be stated thus: "every new thing and every change in a previously existing thing must have a cause sufficient and pre-existing. the universe consists of a series of changes. therefore the universe must have a cause exterior and anterior to itself. the _teleological argument_, or argument from design or final causes, is as follows: design, or the adaptation of means to effect an end, implies the exercise of intelligence and free choice. the universe is full of traces of design. therefore the "first cause" must have been a personal spirit. the _moral argument_ may be thus stated: "in looking at the works of god there is," says rev. dr. hopkins, "i suppose, evidence enough, especially if interpreted by the moral consciousness, to prove to a candid man the being of god." the educated man is a religious being. the instinct of prayer and worship, the longing for and faith in divine love and help, are inseparable from human nature under normal conditions, as known in history. it is evident from the above that it is not for logical reasoning or arguments that the atheist is led to say, "that up to this moment the world has remained without knowledge of a god."[ ] it is from the folly of his heart; and, as solomon says, that "though you bray him and his false logic in the mortar of reason, among the wheat of facts, with the pestle of argument, yet will not his folly depart from him."[ ] i fully agree with hobbes when he says, "where there is no reason for our belief, there is no reason we should believe," but i think the several arguments given above, which could be greatly expanded, affords sufficient reason for a perfect belief in an infinite god. for-- "god is a being, and that you may see in the fold of the flower, in the leaf of the tree, in the storm-cloud of darkness, in the rainbow of life, in the sunlight at noontide, in the darkness of night, in the wave of the ocean, in the furrow of land, in the mountain of granite, in the atom of sand; gaze where ye may from the sky to the sod-- where can you gaze and not see a god." yes, the infinite god must include all. if he is not in the dust of our streets, in the bricks of our house, in the beat of our hearts, then he is not infinite, but is finite, having boundaries. yes, god's power it was that set the nebulous mass into vibration, and caused the world to be formed; it was his force which first shaped the atoms into molecules, and then into more complex chemical products, till finally "organizable protoplasm" was reached, which, by evolution, climbed up to man. 'tis god we see in the family, in society, in the state, in all religions, up to the highest outflowings of christianity. 'tis him we see in art, literature, and science; and so proclaims evolution. "god is the universal causal law; god is the source of all force and all matter." "for us," says haeckel, "all nature is animated, _i. e._, penetrated with divine spirit, with law, and with necessity." we know of no matter without this divine spirit. the "ultimate repulsion, constituting the extension and impenetrability of the atoms of matter," says dr. samuel brown, "could be conceived of in no other way than as the persistent existence of the will of god himself, in whom we live and move and have our being, and which, if but for an instant withdrawn, the whole material universe and its forces in all their vastness, glory, and beauty, would collapse and sink in a moment into their original nothingness." the advancement of science, instead of depriving man of his god, only deprives men of their earlier and ruder conceptions of deity, only to impart a larger and grander thought of him. "it is true, in the educational process some few minds have lost sight of him altogether, but these are the exceptional, and therefore notable instances; with the great body of men, the conception of god has steadily enlarged with the progress of science."[ ] if science can demonstrate that evolution is true, then it is god's truth, and as such it is man's religious duty to accept it; if he rejects it, superstitiously or unreasonably, he not only defrauds himself but insults the author of truth. what, then, has science demonstrated? science has demonstrated the unity of the forces: light, heat, electricity, magnetism, motion, are all correlated to one another, and are all mutually convertible one into another. heat may be said to produce electricity--electricity to produce heat; magnetism to produce electricity--electricity, magnetism, and so on for the rest. unity of matter and force.--"for if matter were not force, and immediately known as force, it could not be known at all--could not be rationally inferred." unity of the life substance in all organic and animal bodies.--"a unity of power or faculty, a unity of form, and a unity of substantial composition." unity of animate and inanimate nature in matter, form, and force. unity of the laws of development.--hence we can proclaim the unity of all nature and of her laws of development. in the beautiful words of giordano bruno: "a spirit exists in all things, and no body is so small but contains a part of the divine substance within itself, by which it is animated." hence we arrive at the sublime idea, since we can in no other way account for the ultimate cause of anything, that it is god's spirit which pervades and sustains all nature. by this admission we are not led to say: "there is no god but force;" but rather, "there is no force but god." god is infinite, and therefore includes nature; but is nature all? it is all that our finite minds can discover, 'tis true; but can there not exist another nature or world unknown to us; and if so, since god is infinite, he will include that world also. let us look to this and see what science can answer. it will be necessary for us to consider before proceeding, what is meant by the term soul; and this becomes a somewhat difficult task, as the term has been variously applied to signify the principle of life in an organic body, or the first and most undeveloped stages of individualized spiritual being, or finally, all stages of spiritual individuality, incorporeal as well as corporeal.[ ] the popular belief is, that the soul is not material but substantial, a divine gift to the highest alone of god's creatures; but scientific men, such as carl vogt, moleschott, büchner, schmidt, haeckel, consider the phenomena of the soul to be functions of the brain and nerves. schmidt says: "the soul of the new-born infant is, in its manifestations, in no way different from that of the young animal. these are the functions of the infantine nervous system, with this they grow and are developed together with speech." the idea of the immortality of the soul was not aboriginal with mankind, as sir j. lubbock has shown that the barbarous races possess no clear belief of this kind, and rajah brook, at a missionary meeting in liverpool, told his hearers there that the dyaks, a people with whom he was connected, had no knowledge of god, of a soul, or of any future state. darwin remarks, that "man may be excused for feeling some pride at having risen, though not through his own exertions, to the very summit of the organic scale; and the fact of his having thus risen, instead of having been aboriginally placed there, may give hope for a still higher destiny in the distant future." the belief in a future life amongst the civilized race of mankind is almost universally prevalent. the proofs of immortality are various. the desire that man has to live forever and his horror of annihilation is one; the good suffer in this world and the wicked triumph--this would indicate the necessity of future retribution. the infinite perfectibility of the human mind never reaches its full capacity in this life; the faculty of insight which sees in an individual all its past history at a glance is the immortal attribute and is continually on the increase; and it is possible that aristotle was right so far as he stated that the lower faculties of the soul, such as sensation, imagination, feeling, memory, etc., are perishable. no matter if this be so or not, it is certain that in the next life, where all is perfection, only the fittest attributes will exist, the others would have perished. the doctrine of the immortality of the soul has been defended by marhemeke, blasche, weisse, hinnichs, fecham, j. h. fichte, and others. let us look for a moment at the visible universe and see if it is not reasonable, on a scientific basis, to admit of the existence of another universe, although it remains unseen to us. one can not help but be struck with the fact that energy is being dissipated in this visible universe, that the visible universe is apparently very wasteful. look at the sun which pours her vast store of high-class energy into space, at the rate of , miles per second. what will be the result of this? the answer is simple: the inevitable destruction of the visible universe. yes, just as the visible universe had its beginning it will have its end. but there existed a power before the visible universe came into existence, and which is acting in the visible universe as the ultimate cause of all phenomena. "for we are obliged," says herbert spencer in his first principles, "to regard every phenomenon as a manifestation of some power by which we are acted upon; though omnipresence is unthinkable, yet, as experience discloses no bounds to the diffusion of phenomena, we are unable to think of limits to the presence of this power, while the criticisms of science teaches us that this power is incomprehensible." and so we should expect, for a finite cannot comprehend an infinite. it is for this and other reasons one is led to believe that the visible universe is only an infinitesimal part of "that stupendous whole which is alone entitled to be called the universe."[ ] as there existed an invisible universe before the visible one came into existence, we can conclude that there still exists an invisible universe now, and that this invisible universe will still exist when the present visible one has passed away. let us see what light our finite senses can throw on this. it is well known that all our senses have only a certain narrow gauge within which they are able to bring us into sensible contact with the world about us. all outside this range we are unable to reach. for example, we do not see all forms and colors; we do not hear all sounds; we do not smell all odors; we cannot conscientiously touch all substances; we cannot taste all flavors. vision depends on the wave motion of light. the length of a wave of mean red light is about / th of an inch, that of violet / th of an inch. but the number of oscillations of ether in a second, necessary to produce the sensation of red, are , , , , , all of which enter the eye in one second. for the sensation of violet, the eye must receive , , , , oscillations in one second, as light travels , miles in one second. but when waves of light having all possible lengths act on the eye simultaneously, the sensation of white is produced. so, as has been previously stated, without eyes the world would be wrapped in darkness, there being no light and color outside of one's eye. so we see our sense of sight has its limits, and we know how finite these are. that there are vibrations of the ether on each side of our limits of vision cannot be doubted; and if our eyes were acute enough to receive them, we could have the sensation of some color, which must under present conditions remain forever blank. the owl and bat can see when we cannot; their eyes can receive oscillations of ether, which pass by without affecting us. so with sound, which "is a sensation produced when vibrations of a certain character are excited in the auditory apparatus of the ear."[ ] the longest wave which can give an impression has a length of about ft., which is equal to - / vibrations per second; when the wave is reduced to three or four tenths of an inch, equal to from , to , vibrations per second, sound becomes again inaudible. the piano, for instance, only runs between - / vibrations in a second up to , . sound travels about , feet per second, and the human voice can be heard feet away, whilst a rifle can be heard , feet ( . miles), and very strong cannonading , feet, or miles. that there are vibrations above and below - / and , , there is no room to doubt, as there exist ears which can hear them, such as the hare; but to us they are as though they did not exist. of all our senses, the sense of smell far surpasses that of the other sense. valentine has calculated that we are able to perceive about the three one-hundred-millionth of a grain of musk. the minute particle which we perceive by smell, no chemical reaction can detect, and even spectrum analysis, which can recognize fifteen-millionths of a grain, is far surpassed. but this sense in man is far surpassed by the hound. our sense of taste is also limited, and as has been already stated, cannot distinguish all flavors. we can recognize by taste one part of sulphuric acid in parts of water; one drop of this on the tongue would contain / of a grain ( / of a grain) of sulphuric acid. the length of time needed for reaction in sensation has been determined by vintschgau and hougschmied, and in a person whose sense of taste was highly developed, the reaction time was, for common salt, . second; for sugar, . second; for acid, . second; and for quinine, . second. reviewing, then, the above, it is evident there are eyes which can see what we cannot, there are ears which can hear what we cannot, and there are animals who can smell and touch what we cannot. "for anything we know to the contrary, then," says savage, "a refined and spiritualized order of existences may be the inhabitants of another and unseen world all about us." as milton has said: "millions of spiritual creatures walk the earth unseen, both when we wake and when we sleep." if there is a life very much different from and very much higher than our present one, it is not strange we are ignorant of it. it is impossible to make a person understand anything which is entirely unlike all that has ever been seen or heard, for every idea in the world that man has came to him by nature. man[ ] cannot conceive of anything the hint of which has not been received from his surroundings. he can imagine an animal with the hoof of a bison, with the pouch of a kangaroo, with the wings of an eagle, with the beak of a bird, and with the tail of a lion; and yet every point of this monster he borrowed from nature. everything he can think of, everything he can dream of, is borrowed from his surroundings--everything. "so, if an angel should come and tell of another life, it would mean nothing to us, unless we could translate it into terms of our own experience. we could not understand a 'light that never was on land or sea.' our ignorance is not even then a probability against our belief."[ ] as has already been stated, the visible universe must have its doom, must end as it began, by consisting of a single mass of matter; but is there not a more primitive state of matter than the matter such as we know it? yes; and the so-called ether is that matter. it is unlike any of the forms of matter which we can weigh and measure. it is in some respects like unto a fluid, and in some respects like unto a solid. it is both hard and elastic to an almost inconceivable degree. "it fills all material bodies like a sea in which the atoms of the material bodies are as islands, and it occupies the whole of what we call empty space. it is so sensitive that a disturbance in any part of it causes a 'tremor which is felt on the surface of countless worlds.' it exerts frictions; and although the friction is infinitely small, yet as it has an almost infinite time to work in, it will diminish the momentum of the planets, and diminish their ability to maintain their distance from the sun, the consequence of which will be the planets will fall into the sun, and the solar system will end where it begun."[ ] according to sir william thompson, the ultimate atoms of matter are vortex rings, which professor clifford describes as being more closely packed together (finer grained) in ether than in matter. and he says, "whatever may turn out to be the ultimate nature of the ether and of molecules, we know that to some extent at least they obey the same dynamic laws, and that they act on one another in accordance with these laws. until therefore it is absolutely disproved, it must remain the simplest and most probable assumption that they are finally made of the same stuff, that the material molecule is in some kind of knot or coagulation of ether."[ ] the molecule of matter such as we know, then, may have been, and very probably was, produced by evolution from the atoms or vortex rings of ether, according to the theory advanced by the authors of the work called the "unseen universe," which i have referred to. the world of ether is to be regarded in some sort the obverse complement of the world of sensible matter, so that whatever energy is dissipated in the one is by the same act accumulated in the other; or, as fiske describes it, "it is like the negative plate in photography, where light answers to shadow and shadow to light." every act of consciousness is accompanied by molecular displacements in the brain, and these of course are responded to by movements in the ethereal world. views of this kind were long ago entertained by babbage, and they have since recommended themselves to other men of science, and amongst others to jevon, who says: "mr. babbage has pointed out that if we had power to follow and detect the manifest effects of any disturbance, each particle of existing matter must be a register of all that has happened. * * * the air itself is one vast library on whose pages are forever written all that man has ever said or whispered. there in their mutable but unerring characters, mixed with, the earliest as well as the latest sighs of mortality, stand forever recorded vows unredeemed, promises unfulfilled, perpetuating in the united movements of each particle the testimony of man's changeful will."[ ] so thought affects the substance of the present visible universe; it produces a material organ of memory. "but the motions which accompany thought," say the authors,[ ] "will also affect the invisible order of things," and thus it follows that "thought conceived to affect the matter of another universe, simultaneously with this, may explain a future state."[ ] death, then, is for the individual but a transfer from one physical state of existence to another, according to the "authors'"[ ] idea; and so, on the largest scale, the death or final loss of energy by the whole visible universe has its counterpart in the acquirement of a maximum of life, the correlative unseen world. according to this theory, therefore, as the psychical or spiritual phenomena of the visible world only begins to be manifested with some complex aggregate of material phenomena, therefore it is necessary for the continuance of mind in a future state to have some sort of material vehicle also, which the ether is supposed to supply. "the essential weakness of such a theory as this," says fiske, "lies in the fact that it is thoroughly materialistic in character. we have reason for thinking it probable that ether and ordinary matter are alike composed of vortex rings in a quasi-frictionless fluid; but whatever be the fate of this subtle hypothesis, we may be sure that no theory will ever be entertained in which analysis of ether shall require different symbols from that of ordinary matter. in our authors' theory, therefore, the putting on of immortality is in nowise the passage from a material to a spiritual state. it is the passage of one kind of materially conditioned state to another." this theory, dealing with matter, should receive support by actual experience, as matter is a subject of investigation. to accept it, therefore, as being possible without any positive evidence for its support, it remains but a weak speculation, no matter how ingenious it may seem. to support an after life, which is not materially conditioned, i agree with mr. fiske, that although it will be unsupported by any item of experience whatever, it may nevertheless be an impregnable assertion. if all were to agree, what we call matter is really force, as it certainly is, for if matter were not force it would be unthinkable, being force it becomes thinkable; this point i have touched on before, but it may be well to elaborate on it a little just here. the great lesson that berkeley taught mankind was that what we call material phenomena are really the products of consciousness co-operating with some unknown power (not material) existing beyond consciousness. "we do very well to speak of matter," says fiske, "in common parlance, but all that the word really means is a group of qualities which have no existence apart from our minds." the ablest modern thinkers, then, believe that the only real things that exist are the mind and god, and that the universe is only the infinitely varied manifestation of god in the human conscience. it is evident, then, that _matter_, the only thing the materialist concedes real existence, is simply an orderly phantasmagoria; and god and soul, which materialists regard as mere fictions of the imagination, are the only conceptions that answer to real existence.[ ] for instance, let us see what it is we know about a table. you say you can see it; i can respond that all you are conscious of is that the nerves of your eye have undergone a change. you say, i can check my sight of it by touching it; to this i reply, all that you are really conscious of is a sensation, and that something outside of you has produced it. but that all that is outside of me is anything more than the manifestation to me of a power or of god, is an inference and cannot be proven. to constant manifestations of this power, always assuming the same form and characters which can be studied, different names have been given; but that the dust of the street or beat of our heart is anything else but that peculiar manifestation of the infinite god, cannot be contradicted. mr. savage says, "the movement of electricity along a telegraph-line is accompanied by certain molecular changes in the wire itself; but the wire is not electricity, neither does it produce it. thus modern science has found it utterly impossible to explain mind either as a part or a product of matter. it is perfectly reasonable, then, for any man to believe in a purely intellectual and spiritual existence, apart from any material form or substance." to comprehend the immortal life is an impossibility; it transcends any earthly experience of man. the caterpillar probably knows nothing about any life higher than that of his toilsome crawling on the ground; but that is no proof against the fact that we know he is to become a butterfly. the boy knows nothing about manhood, and cannot know. though he sees men and their labors all about him, he has and can have no conception whatever of what it means to be a man; it transcends all experience.[ ] "the existence," says fiske, "of a single soul, or congeries of psychical phenomena, unaccompanied by a material body, would be evidence sufficient to demonstrate this hypothesis. but in the nature of things, even were there a million such souls round about us, we could not become aware of the existence of one of them; for we have no organ or faculty for the perception of soul apart from the material structure and activities in which it has been manifested throughout the whole course of our experience. even our own self-consciousness involves the consciousness of ourselves as partly material bodies. these considerations show that our hypothesis is very different from the ordinary hypothesis with which science deals. _the entire absence of testimony does not raise a negative presumption, except in cases where testimony is accessible._" my object has not been to prove the purely spiritual theory of a future life, but to show that it is a theory that intelligent people can entertain as a foundation for their belief "in the hope of immortality." but that the spiritual life instead of the material life is the state in which we can hope for immortality, i think there can be no question; and such was the opinion of paul[ ] when he wrote: "now this i say, brethren, that flesh and blood cannot inherit the kingdom of god, neither does corruption inherit incorruption.... so when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, 'death is swallowed up in victory.' o death, where is thy sting? o grave, where is thy victory?" footnotes: [ ] the law of disease, in college courant, vol. xiv. [ ] winchell. evolution, p. . [ ] comparative zoology, p. . . [ ] huxley. physical basis of life. [ ] johnson, ency. [ ] comparative anatomy--orton, p. . [ ] analytical anatomy and phys.--cutter, p. . [ ] biography of a plant. [ ] see huxley--invertebrate animals, anatomy of. [ ] phys. basis of life. [ ] beginnings of life, p. , vol. i. [ ] monthly micros. jour., may , ' , p. . [ ] chem. and phys. balance of organic nature, , p. (trans.). [ ] inaugural address, aug. , . [ ] haeckel--hist. of creation. [ ] see haeckel--evol. of man. [ ] evolution of man, vol. ii, p. . [ ] johnson's cyclopedia, article "evolution." [ ] sumner, in johnson's cyc. [ ] christian union, vol. xiii, no. , p. . [ ] gen. i. . [ ] st. john i. . [ ] st. john i. . [ ] hist. of creation, p. . [ ] _ibid._, p. . [ ] heb. xi. . revised english ed. [ ] _loc. cit._, vol. i, p. . [ ] _loc. cit._, vol. i, p. . [ ] indications of the creator. [ ] evolution and progress, p. , rev. wm. i. gill. [ ] natürl. schöpfungsgesch., pp. - . [ ] paget, lectures on surgical pathology, , vol. i, p. . [ ] ueber die richtung der haare am menschlichen körper. [ ] pop. sci. monthly, june, , p. . [ ] see sci. am., may , . [ ] source of muscular power, proc. roy. inst., june , . am. i. sci., ii, xlii, , nov. . [ ] comparative zoology, p. . [ ] correlation of the vital and physical forces, p. . [ ] on the time required for the transmission of volition and sensation through the nerves, proc. roy. inst. [ ] comparative zoology, p. . [ ] sci. amer., nov. , , p. . [ ] marshall, outline of physiology. amer. ed., , p. . [ ] macmillon's magazine, pop. sci. monthly, april, . [ ] "principles of psychology," , no. , p. . [ ] j. s. lombard, n. y. med. jour., vol. v, , june, . [ ] _loc. cit._, p. . [ ] the apparatus employed is illustrated and fully described in brown-sequard's archives de phys., vol. i, , june, . by it the - th of a degree centigrade may be indicated. [ ] l. h. wood, "on the influence of mental activity on the excretion of phosphoric acid by the kidneys." proc. conn. med. soc., nov., , p. . [ ] _loc. cit._, p. . [ ] address of dr. f. a. p. barnard, as retiring president, before the am. ass. for adv. of sci., chicago meeting, aug. . "thought cannot be a physical force, because thought admits of no measure." [ ] derivation hypothesis of life and species, forming fortieth chapter of his anatomy of vertebrates, republished in am. jour. sci., ii, xlvii, , jan. . [ ] prehistoric times, p. , by lubbock. [ ] madness in animals, jour. mental sci., july, . dr. w. l. lindsay. [ ] facultés mentales des animaux, , tom. xi, p. . [ ] primeval man, , pp. - . [ ] prehistoric times, , p. . [ ] "conferences ser les théorie darwinienne," , p. . [ ] philosoph. trans., , p. . [ ] prof. whitney, p. . [ ] phys. and pathol. of mind. dr. maudsley. d ed., , p. . [ ] nature, january , , p. . [ ] problems i. . [ ] johnson's cyc. article "faith." c. p. krauth. [ ] darwin's descent of man, p. . [ ] see descent of man, p. . [ ] see tyndall's belfast address. [ ] purgatory of suicides. [ ] thoughts on atheism, p. . [ ] monologium and proslogium. [ ] meditations de primaphilosophia prop. , p. . [ ] demonstration of the being and attributes of god. [ ] elements of psychology. [ ] thoughts on atheism, by holyoake, p. . [ ] proverbs xvii. . [ ] henry ward beecher. [ ] see w. t. harris. johnson's encyc. "soul." [ ] unseen universe. [ ] rood. "sound," johnson's encyc. [ ] see r. g. ingersoll's lecture on hell. [ ] savage. [ ] "the unseen world." john fiske, p. . [ ] fortnightly review, june , p. . [ ] ninth bridgewater treatise. [ ] of the unseen universe. [ ] anagram. nature, oct. , . [ ] of the unseen universe. [ ] fiske. unseen world, p. . [ ] savage. relig. of evol., p. . [ ] corinthians, xv., verses - (part of). _revised english ed._, . transcriber's notes: passages in italics are indicated by _underscore_. passages in bold are indicated by =bold=. numbers enclosed in {brackets} are subscripted in the original text. additional spacing after some of the quotes is intentional to indicate both the end of a quotation and the beginning of a new paragraph as presented in the original text. images have been moved from the middle of a paragraph to the closest paragraph break. the following misprints have been addressed: "hæckel" standardized to "haeckel" (page ) missing "the" added (page ) "paleontology" standardized to "palæontology" (page ) "cerebelbellum" corrected to "cerebellum" (page ) some quotation marks in the original are not paired. obvious errors have been silently closed, while those requiring interpretation have been left open. other than the corrections listed above, printer's inconsistencies in spelling, punctuation, hyphenation, and ligature usage have been retained. generously made available by the internet archive/canadian libraries) the foundations of the origin of species cambridge university press london: fetter lane, e.c. c. f. clay, manager {illustration} edinburgh: , princes street also london: h. k. lewis, , gower street, w.c. berlin: a. asher and co. leipzig: f. a. brockhaus new york: g. p. putnam's sons bombay and calcutta: macmillan and co., ltd. _all rights reserved_ {illustration: charles darwin from a photograph by maull & fox in } the foundations of the origin of species two essays written in and by charles darwin edited by his son francis darwin honorary fellow of christ's college cambridge: at the university press astronomers might formerly have said that god ordered each planet to move in its particular destiny. in same manner god orders each animal created with certain form in certain country. but how much more simple and sublime power,--let attraction act according to certain law, such are inevitable consequences,--let animal(s) be created, then by the fixed laws of generation, such will be their successors. from darwin's _note book_, , p. . to the master and fellows of christ's college, this book is dedicated by the editor in token of respect and gratitude contents essay of pages introduction xi part i § i. on variation under domestication, and on the principles of selection § ii. on variation in a state of nature and on the natural means of selection § iii. on variation in instincts and other mental attributes part ii §§ iv. and v. on the evidence from geology. (the reasons for combining the two sections are given in the introduction) § vi. geographical distribution § vii. affinities and classification § viii. unity of type in the great classes § ix. abortive organs § x. recapitulation and conclusion essay of part i chapter i - on the variation of organic beings under domestication; and on the principles of selection. variation on the hereditary tendency causes of variation on selection crossing breeds whether our domestic races have descended from one or more wild stocks limits to variation in degree and kind in what consists domestication--summary chapter ii - on the variation of organic beings in a wild state; on the natural means of selection; and on the comparison of domestic races and true species. variation natural means of selection differences between "races" and "species":-first, in their trueness or variability difference between "races" and "species" in fertility when crossed causes of sterility in hybrids infertility from causes distinct from hybridisation points of resemblance between "races" and "species" external characters of hybrids and mongrels summary limits of variation chapter iii - on the variation of instincts and other mental attributes under domestication and in a state of nature; on the difficulties in this subject; and on analogous difficulties with respect to corporeal structures. variation of mental attributes under domestication hereditary habits compared with instincts variation in the mental attributes of wild animals principles of selection applicable to instincts difficulties in the acquirement of complex instincts by selection difficulties in the acquirement by selection of complex corporeal structures part ii on the evidence favourable and opposed to the view that species are naturally formed races, descended from common stocks. chapter iv - on the number of intermediate forms required on the theory of common descent; and on their absence in a fossil state chapter v - gradual appearance and disappearance of species. gradual appearance of species extinction of species chapter vi on the geographical distribution of organic beings in past and present times. section first - distribution of the inhabitants in the different continents relation of range in genera and species distribution of the inhabitants in the same continent insular faunas alpine floras cause of the similarity in the floras of some distant mountains whether the same species has been created more than once on the number of species, and of the classes to which they belong in different regions second section - geographical distribution of extinct organisms changes in geographical distribution summary on the distribution of living and extinct organic beings section third - an attempt to explain the foregoing laws of geographical distribution, on the theory of allied species having a common descent improbability of finding fossil forms intermediate between existing species chapter vii - on the nature of the affinities and classification of organic beings. gradual appearance and disappearance of groups what is the natural system? on the kind of relation between distinct groups classification of races or varieties classification of races and species similar origin of genera and families chapter viii - unity of type in the great classes; and morphological structures. unity of type morphology embryology attempt to explain the facts of embryology on the graduated complexity in each great class modification by selection of the forms of immature animals importance of embryology in classification order in time in which the great classes have first appeared chapter ix - abortive or rudimentary organs. the abortive organs of naturalists the abortive organs of physiologists abortion from gradual disuse chapter x - recapitulation and conclusion. recapitulation why do we wish to reject the theory of common descent? conclusion index portrait _frontispiece_ facsimile _to face_ p. introduction we know from the contents of charles darwin's note book of that he was at that time a convinced evolutionist{ }. nor can there be any doubt that, when he started on board the _beagle_, such opinions as he had were on the side of immutability. when therefore did the current of his thoughts begin to set in the direction of evolution? { } see the extracts in _life and letters of charles darwin_, ii. p. . we have first to consider the factors that made for such a change. on his departure in , henslow gave him vol. i. of lyell's _principles_, then just published, with the warning that he was not to believe what he read{ }. but believe he did, and it is certain (as huxley has forcibly pointed out{ }) that the doctrine of uniformitarianism when applied to biology leads of necessity to evolution. if the extermination of a species is no more catastrophic than the natural death of an individual, why should the birth of a species be any more miraculous than the birth of an individual? it is quite clear that this thought was vividly present to darwin when he was writing out his early thoughts in the note book{ }:-- "propagation explains why modern animals same type as extinct, which is law almost proved. they die, without they change, like golden pippins; it is a _generation of species_ like generation _of individuals_." "if _species_ generate other _species_ their race is not utterly cut off." { } the second volume,--especially important in regard to evolution,--reached him in the autumn of , as prof. judd has pointed out in his most interesting paper in _darwin and modern science_. cambridge, . { } obituary notice of c. darwin, _proc. r. soc._ vol. . reprinted in huxley's _collected essays_. see also _life and letters of c. darwin_, ii. p. . { } see the extracts in the _life and letters_, ii. p. . these quotations show that he was struggling to see in the origin of species a process just as scientifically comprehensible as the birth of individuals. they show, i think, that he recognised the two things not merely as similar but as identical. it is impossible to know how soon the ferment of uniformitarianism began to work, but it is fair to suspect that in he had already begun to see that mutability was the logical conclusion of lyell's doctrine, though this was not acknowledged by lyell himself. there were however other factors of change. in his autobiography{ } he wrote:--"during the voyage of the _beagle_ i had been deeply impressed by discovering in the pampean formation great fossil animals covered with armour like that on the existing armadillos; secondly, by the manner in which closely allied animals replace one another in proceeding southward over the continent; and thirdly, by the south american character of most of the productions of the galapagos archipelago, and more especially by the manner in which they differ slightly on each island of the group; none of the islands appearing to be very ancient in a geological sense. it was evident that such facts as these, as well as many others, could only be explained on the supposition that species gradually become modified; and the subject haunted me." { } _life and letters_, i. p. . again we have to ask: how soon did any of these influences produce an effect on darwin's mind? different answers have been attempted. huxley{ } held that these facts could not have produced their essential effect until the voyage had come to an end, and the "relations of the existing with the extinct species and of the species of the different geographical areas with one another were determined with some exactness." he does not therefore allow that any appreciable advance towards evolution was made during the actual voyage of the _beagle_. { } _obituary notice_, _loc. cit._ professor judd{ } takes a very different view. he holds that november may be given with some confidence as the "date at which darwin commenced that long series of observations and reasonings which eventually culminated in the preparation of the _origin of species_." { } _darwin and modern science._ though i think these words suggest a more direct and continuous march than really existed between fossil-collecting in and writing the _origin of species_ in , yet i hold that it was during the voyage that darwin's mind began to be turned in the direction of evolution, and i am therefore in essential agreement with prof. judd, although i lay more stress than he does on the latter part of the voyage. let us for a moment confine our attention to the passage, above quoted, from the autobiography and to what is said in the introduction to the _origin_, ed. i., viz. "when on board h.m.s. 'beagle,' as naturalist, i was much struck with certain facts in the distribution of the inhabitants of south america, and in the geological relations of the present to the past inhabitants of that continent." these words, occurring where they do, can only mean one thing,--namely that the facts suggested an evolutionary interpretation. and this being so it must be true that his thoughts _began to flow in the direction of descent_ at this early date. i am inclined to think that the "new light which was rising in his mind{ }" had not yet attained any effective degree of steadiness or brightness. i think so because in his pocket book under the date he wrote, "in july opened first note-book on 'transmutation of species.' had been greatly struck _from about month of previous march_{ } on character of south american fossils, and species on galapagos archipelago. these facts origin (_especially latter_), of all my views." but he did not visit the galapagos till and i therefore find it hard to believe that his evolutionary views attained any strength or permanence until at any rate quite late in the voyage. the galapagos facts are strongly against huxley's view, for darwin's attention was "thoroughly aroused{ }" by comparing the birds shot by himself and by others on board. the case must have struck him at once,--without waiting for accurate determinations,--as a microcosm of evolution. { } huxley, _obituary_, p. xi. { } in this citation the italics are mine. { } _journal of researches_, ed. , p. . it is also to be noted, in regard to the remains of extinct animals, that, in the above quotation from his pocket book, he speaks of march as the time at which he began to be "greatly struck on character of south american fossils," which suggests at least that the impression made in required reinforcement before a really powerful effect was produced. we may therefore conclude, i think, that the evolutionary current in my father's thoughts had continued to increase in force from onwards, being especially reinforced at the galapagos in and again in when he was overhauling the results, mental and material, of his travels. and that when the above record in the pocket book was made he unconsciously minimised the earlier beginnings of his theorisings, and laid more stress on the recent thoughts which were naturally more vivid to him. in his letter{ } to otto zacharias ( ) he wrote, "on my return home in the autumn of , i immediately began to prepare my journal for publication, and then saw how many facts indicated the common descent of species." this again is evidence in favour of the view that the later growths of his theory were the essentially important parts of its development. { } f. darwin's _life of charles darwin_ (in one volume), , p. . in the same letter to zacharias he says, "when i was on board the _beagle_ i believed in the permanence of species, but as far as i can remember vague doubts occasionally flitted across my mind." unless prof. judd and i are altogether wrong in believing that late or early in the voyage (it matters little which) a definite approach was made to the evolutionary standpoint, we must suppose that in years such advance had shrunk in his recollection to the dimensions of "vague doubts." the letter to zacharias shows i think some forgetting of the past where the author says, "but i did not become convinced that species were mutable until, i think, two or three years had elapsed." it is impossible to reconcile this with the contents of the evolutionary note book of . i have no doubt that in his retrospect he felt that he had not been "convinced that species were mutable" until he had gained a clear conception of the mechanism of natural selection, _i.e._ in - . but even on this last date there is some room, not for doubt, but for surprise. the passage in the autobiography{ } is quite clear, namely that in october he read malthus's _essay on the principle of population_ and "being well prepared to appreciate the struggle for existence ..., it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. the result of this would be the formation of new species. here then i had at last got a theory by which to work." { } _life and letters_, i. p. . it is surprising that malthus should have been needed to give him the clue, when in the note book of there should occur--however obscurely expressed--the following forecast{ } of the importance of the survival of the fittest. "with respect to extinction, we can easily see that a variety of the ostrich (petise{ }), may not be well adapted, and thus perish out; or on the other hand, like orpheus{ }, being favourable, many might be produced. this requires the principle that the permanent variations produced by confined breeding and changing circumstances are continued and produce<d> according to the adaptation of such circumstances, and therefore that death of species is a consequence (contrary to what would appear in america) of non-adaptation of circumstances." { } _life and letters_, ii. p. . { } avestruz petise, _i.e. rhea darwini_. { } a bird. i can hardly doubt, that with his knowledge of the interdependence of organisms and the tyranny of conditions, his experience would have crystallized out into "a theory by which to work" even without the aid of malthus. in my father's autobiography{ } he writes, "in june i first allowed myself the satisfaction of writing a very brief abstract of my theory in pencil in pages; and this was enlarged during the summer of into one of pages{ }, which i had fairly copied out and still possess." these two essays, of and , are now printed under the title _the foundations of the origin of species_. { } _life and letters_, i. p. . { } it contains as a fact pp. it is a strongly bound folio, interleaved with blank pages, as though for notes and additions. his own ms. from which it was copied contains pp. it will be noted that in the above passage he does not mention the ms. of as being in existence, and when i was at work on _life and letters_ i had not seen it. it only came to light after my mother's death in when the house at down was vacated. the ms. was hidden in a cupboard under the stairs which was not used for papers of any value, but rather as an overflow for matter which he did not wish to destroy. the statement in the autobiography that the ms. was written in agrees with an entry in my fathers diary:-- " . may th went to maer. june th to shrewsbury, and on th to capel curig.... during my stay at maer and shrewsbury (five years after commencement) wrote pencil sketch of my species theory." again in a letter to lyell (june , ) he speaks of his "ms. sketch written out in { }." in the _origin of species_, ed. i. p. , he speaks of beginning his speculations in and of allowing himself to draw up some "short notes" after "five years' work," _i.e._ in . so far there seems no doubt as to being the date of the first sketch; but there is evidence in favour of an earlier date{ }. thus across the table of contents of the bound copy of the ms. is written in my father's hand "this was sketched in ." again in a letter to mr wallace{ } (jan. , ) he speaks of his own contributions to the linnean paper{ } of july , , as "written in , now just twenty years ago." this statement as it stands is undoubtedly incorrect, since the extracts are from the ms. of , about the date of which no doubt exists; but even if it could be supposed to refer to the essay, it must, i think, be rejected. i can only account for his mistake by the supposition that my father had in mind the date ( ) at which the framework of his theory was laid down. it is worth noting that in his autobiography (p. ) he speaks of the time "about , when the theory was clearly conceived." however this may be there can be no doubt that is the correct date. since the publication of _life and letters_ i have gained fresh evidence on this head. a small packet containing pp. of ms. came to light in . on the outside is written "first pencil sketch of species theory. written at maer and shrewsbury during may and june ." it is not however written in pencil, and it consists of a single chapter on _the principles of variation in domestic organisms_. a single unnumbered page is written in pencil, and is headed "maer, may , useless"; it also bears the words "this page was thought of as introduction." it consists of the briefest sketch of the geological evidence for evolution, together with words intended as headings for discussion,--such as "affinity,--unity of type,--foetal state,--abortive organs." { } _life and letters_, ii. p. . { } _life and letters_, ii. p. . { } _life and letters_, ii. p. . { } _j. linn. soc. zool._ iii. p. . the back of this "useless" page is of some interest, although it does not bear on the question of date,--the matter immediately before us. it seems to be an outline of the essay or sketch of , consisting of the titles of the three chapters of which it was to have consisted. "i. the principles of var. in domestic organisms. "ii. the possible and probable application of these same principles to wild animals and consequently the possible and probable production of wild races, analogous to the domestic ones of plants and animals. "iii. the reasons for and against believing that such races have really been produced, forming what are called species." it will be seen that chapter iii as originally designed corresponds to part ii (p. ) of the essay of , which is (p. ) defined by the author as discussing "whether the characters and relations of animated things are such as favour the idea of wild species being races descended from a common stock." again at p. the author asks "what then is the evidence in favour of it (the theory of descent) and what the evidence against it." the generalised section of his essay having been originally chapter iii{ } accounts for the curious error which occurs in pp. and where the second part of the essay is called part iii. { } it is evident that _parts_ and _chapters_ were to some extent interchangeable in the author's mind, for p. (of the ms. we have been discussing) is headed in ink chapter i, and afterwards altered in pencil to part i. the division of the essay into two parts is maintained in the enlarged essay of , in which he writes: "the second part of this work is devoted to the general consideration of how far the general economy of nature justifies or opposes the belief that related species and genera are descended from common stocks." the _origin of species_ however is not so divided. we may now return to the question of the date of the essay. i have found additional evidence in favour of in a sentence written on the back of the table of contents of the ms.--not the copied version but the original in my father's writing: "this was written and enlarged from a sketch in pages{ } in pencil (the latter written in summer of at maer and shrewsbury) in beginning of , and finished it <_sic_> in july; and finally corrected the copy by mr fletcher in the last week in september." on the whole it is impossible to doubt that is the date of the earlier of the two essays. { } on p. of the ms. of the _foundations_ is a reference to the "back of p. bis": this suggests that additional pages had been interpolated in the ms. and that it may once have had in place of pp. the sketch of is written on bad paper with a soft pencil, and is in many parts extremely difficult to read, many of the words ending in mere scrawls and being illegible without context. it is evidently written rapidly, and is in his most elliptical style, the articles being frequently omitted, and the sentences being loosely composed and often illogical in structure. there is much erasure and correction, apparently made at the moment of writing, and the ms. does not give the impression of having been re-read with any care. the whole is more like hasty memoranda of what was clear to himself, than material for the convincing of others. many of the pages are covered with writing on the back, an instance of his parsimony in the matter of paper{ }. this matter consists partly of passages marked for insertion in the text, and these can generally (though by no means always) be placed where he intended. but he also used the back of one page for a preliminary sketch to be rewritten on a clean sheet. these parts of the work have been printed as footnotes, so as to allow what was written on the front of the pages to form a continuous text. a certain amount of repetition is unavoidable, but much of what is written on the backs of the pages is of too much interest to be omitted. some of the matter here given in footnotes may, moreover, have been intended as the final text and not as the preliminary sketch. { } _life and letters_, i. p. . when a word cannot be deciphered, it is replaced by:--<illegible>, the angular brackets being, as already explained, a symbol for an insertion by the editor. more commonly, however, the context makes the interpretation of a word reasonably sure although the word is not strictly legible. such words are followed by an inserted mark of interrogation <?>. lastly, words inserted by the editor, of which the appropriateness is doubtful, are printed thus <variation?>. two kinds of erasure occur in the ms. of . one by vertical lines which seem to have been made when the pp. ms. was being expanded into that of , and merely imply that such a page is done with: and secondly the ordinary erasures by horizontal lines. i have not been quite consistent in regard to these: i began with the intention of printing (in square brackets) all such erasures. but i ultimately found that the confusion introduced into the already obscure sentences was greater than any possible gain; and many such erasures are altogether omitted. in the same way i have occasionally omitted hopelessly obscure and incomprehensible fragments, which if printed would only have burthened the text with a string of <illegible>s and queried words. nor have i printed the whole of what is written on the backs of the pages, where it seemed to me that nothing but unnecessary repetition would have been the result. in the matter of punctuation i have given myself a free hand. i may no doubt have misinterpreted the author's meaning in so doing, but without such punctuation, the number of repellantly crabbed sentences would have been even greater than at present. in dealing with the essay of , i have corrected some obvious slips without indicating such alterations, because the ms. being legible, there is no danger of changing the author's meaning. the sections into which the essay of is divided are in the original merely indicated by a gap in the ms. or by a line drawn across the page. no titles are given except in the case of § viii.; and § ii. is the only section which has a number in the original. i might equally well have made sections of what are now subsections, _e.g. natural selection_ p. , or _extermination_ p. . but since the present sketch is the germ of the essay of , it seemed best to preserve the identity between the two works, by using such of the author's divisions as correspond to the chapters of the enlarged version of . the geological discussion with which part ii begins corresponds to two chapters (iv and v) of the essay. i have therefore described it as §§ iv. and v., although i cannot make sure of its having originally consisted of two sections. with this exception the ten sections of the essay of correspond to the ten chapters of that of . the _origin of species_ differs from the sketch of in not being divided into two parts. but the two volumes resemble each other in general structure. both begin with a statement of what may be called the mechanism of evolution,--variation and selection: in both the argument proceeds from the study of domestic organisms to that of animals and plants in a state of nature. this is followed in both by a discussion of the _difficulties on theory_ and this by a section _instinct_ which in both cases is treated as a special case of difficulty. if i had to divide the _origin_ (first edition) into two parts without any knowledge of earlier ms., i should, i think, make part ii begin with ch. vi, _difficulties on theory_. a possible reason why this part of the argument is given in part i of the essay of may be found in the essay of , where it is clear that the chapter on instinct is placed in part i because the author thought it of importance to show that heredity and variation occur in mental attributes. the whole question is perhaps an instance of the sort of difficulty which made the author give up the division of his argument into two parts when he wrote the _origin_. as matters stand §§ iv. and v. of the essay correspond to the geological chapters, ix and x, in the _origin_. from this point onwards the material is grouped in the same order in both works: geographical distribution; affinities and classification; unity of type and morphology; abortive or rudimentary organs; recapitulation and conclusion. in enlarging the essay of into that of , the author retained the sections of the sketch as chapters in the completer presentment. it follows that what has been said of the relation of the earlier essay to the _origin_ is generally true of the essay. in the latter, however, the geological discussion is, clearly instead of obscurely, divided into two chapters, which correspond roughly with chapters ix and x of the _origin_. but part of the contents of chapter x (_origin_) occurs in chapter vi ( ) on geographical distribution. the treatment of distribution is particularly full and interesting in the essay, but the arrangement of the material, especially the introduction of § iii. p. , leads to some repetition which is avoided in the _origin_. it should be noted that hybridism, which has a separate chapter (viii) in the _origin_, is treated in chapter ii of the essay. finally that chapter xiii (_origin_) corresponds to chapters vii, viii and ix of the work of . the fact that in , seventeen years before the publication of the _origin_, my father should have been able to write out so full an outline of his future work, is very remarkable. in his autobiography{ } he writes of the essay, "but at that time i overlooked one problem of great importance.... this problem is the tendency in organic beings descended from the same stock to diverge in character as they become modified." the absence of the principle of divergence is of course also a characteristic of the sketch of . but at p. , the author is not far from this point of view. the passage referred to is: "if any species, _a_, in changing gets an advantage and that advantage ... is inherited, _a_ will be the progenitor of several genera or even families in the hard struggle of nature. _a_ will go on beating out other forms, it might come that _a_ would people <the> earth,--we may now not have one descendant on our globe of the one or several original creations{ }." but if the descendants of _a_ have peopled the earth by beating out other forms, they must have diverged in occupying the innumerable diverse modes of life from which they expelled their predecessors. what i wrote{ } on this subject in is i think true: "descent with modification implies divergence, and we become so habituated to a belief in descent, and therefore in divergence, that we do not notice the absence of proof that divergence is in itself an advantage." { } _life and letters_, i. p. . { } in the footnotes to the essay of attention is called to similar passages. { } _life and letters_, ii. p. . the fact that there is no set discussion on the principle of divergence in the essay, makes it clear why the joint paper read before the linnean society on july , , included a letter{ } to asa gray, as well as an extract{ } from the essay of . it is clearly because the letter to gray includes a discussion on divergence, and was thus, probably, the only document, including this subject, which could be appropriately made use of. it shows once more how great was the importance attached by its author to the principle of divergence. { } the passage is given in the _life and letters_, ii. p. . { } the extract consists of the section on _natural means of selection_, p. . i have spoken of the hurried and condensed manner in which the sketch of is written; the style of the later essay ( ) is more finished. it has, however, the air of an uncorrected ms. rather than of a book which has gone through the ordeal of proof sheets. it has not all the force and conciseness of the _origin_, but it has a certain freshness which gives it a character of its own. it must be remembered that the _origin_ was an abstract or condensation of a much bigger book, whereas the essay of was an expansion of the sketch of . it is not therefore surprising that in the _origin_ there is occasionally evident a chafing against the author's self-imposed limitation. whereas in the essay there is an air of freedom, as if the author were letting himself go, rather than applying the curb. this quality of freshness and the fact that some questions were more fully discussed in than in , makes the earlier work good reading even to those who are familiar with the _origin_. the writing of this essay "during the summer of ," as stated in the autobiography{ }, and "from memory," as darwin says elsewhere{ }, was a remarkable achievement, and possibly renders more conceivable the still greater feat of the writing of the _origin_ between july and september . { } _life and letters_, i. p. . { } _life and letters_, ii. p. . it is an interesting subject for speculation: what influence on the world the essay of would have exercised, had it been published in place of the origin. the author evidently thought of its publication in its present state as an undesirable expedient, as appears clearly from the following extracts from the _life and letters_, vol. ii. pp. -- : _c. darwin to mrs darwin._ down, _july , _. "... i have just finished my sketch of my species theory. if, as i believe, my theory in time be accepted even by one competent judge, it will be a considerable step in science. "i therefore write this in case of my sudden death, as my most solemn and last request, which i am sure you will consider the same as if legally entered in my will, that you will devote £ to its publication, and further will yourself, or through hensleigh{ }, take trouble in promoting it. i wish that my sketch be given to some competent person, with this sum to induce him to take trouble in its improvement and enlargement. i give to him all my books on natural history, which are either scored or have references at the end to the pages, begging him carefully to look over and consider such passages as actually bearing, or by possibility bearing, on this subject. i wish you to make a list of all such books as some temptation to an editor. i also request that you will hand over <to> him all those scraps roughly divided into eight or ten brown paper portfolios. the scraps, with copied quotations from various works, are those which may aid my editor. i also request that you, or some amanuensis, will aid in deciphering any of the scraps which the editor may think possibly of use. i leave to the editor's judgment whether to interpolate these facts in the text, or as notes, or under appendices. as the looking over the references and scraps will be a long labour, and as the _correcting_ and enlarging and altering my sketch will also take considerable time, i leave this sum of £ as some remuneration, and any profits from the work. i consider that for this the editor is bound to get the sketch published either at a publisher's or his own risk. many of the scraps in the portfolios contain mere rude suggestions and early views, now useless, and many of the facts will probably turn out as having no bearing on my theory. { } mrs darwin's brother. "with respect to editors, mr lyell would be the best if he would undertake it; i believe he would find the work pleasant, and he would learn some facts new to him. as the editor must be a geologist as well as a naturalist, the next best editor would be professor forbes of london. the next best (and quite best in many respects) would be professor henslow. dr hooker would be _very_ good. the next, mr strickland{ }. if none of these would undertake it, i would request you to consult with mr lyell, or some other capable man, for some editor, a geologist and naturalist. should one other hundred pounds make the difference of procuring a good editor, i request earnestly that you will raise £ . { } after mr strickland's name comes the following sentence, which has been erased, but remains legible. "professor owen would be very good; but i presume he would not undertake such a work." "my remaining collections in natural history may be given to any one or any museum where <they> would be accepted...." <the following note seems to have formed part of the original letter, but may have been of later date:> "lyell, especially with the aid of hooker (and of any good zoological aid), would be best of all. without an editor will pledge himself to give up time to it, it would be of no use paying such a sum. "if there should be any difficulty in getting an editor who would go thoroughly into the subject, and think of the bearing of the passages marked in the books and copied out of scraps of paper, then let my sketch be published as it is, stating that it was done several years ago{ }, and from memory without consulting any works, and with no intention of publication in its present form." { } the words "several years ago, and" seem to have been added at a later date. the idea that the sketch of might remain, in the event of his death, as the only record of his work, seems to have been long in his mind, for in august, , when he had finished with the cirripedes, and was thinking of beginning his "species work," he added on the back of the above letter, "hooker by far best man to edit my species volume. august ." i have called attention in footnotes to many points in which the _origin_ agrees with the _foundations_. one of the most interesting is the final sentence, practically the same in the essays of and , and almost identical with the concluding words of the _origin_. i have elsewhere pointed out{ } that the ancestry of this eloquent passage may be traced one stage further back,--to the note book of . i have given this sentence as an appropriate motto for the _foundations_ in its character of a study of general laws. it will be remembered that a corresponding motto from whewell's _bridgewater treatise_ is printed opposite the title-page of the _origin of species_. { } _life and letters_, ii. p. . mr huxley who, about the year , read the essay of , remarked that "much more weight is attached to the influence of external conditions in producing variation and to the inheritance of acquired habits than in the _origin_." in the _foundations_ the effect of conditions is frequently mentioned, and darwin seems to have had constantly in mind the need of referring each variation to a cause. but i gain the impression that the slighter prominence given to this view in the _origin_ was not due to change of opinion, but rather because he had gradually come to take this view for granted; so that in the scheme of that book, it was overshadowed by considerations which then seemed to him more pressing. with regard to the inheritance of acquired characters i am not inclined to agree with huxley. it is certain that the _foundations_ contains strong recognition of the importance of germinal variation, that is of external conditions acting indirectly through the "reproductive functions." he evidently considered this as more important than the inheritance of habit or other acquired peculiarities. another point of interest is the weight he attached in - to "sports" or what are now called "mutations." this is i think more prominent in the _foundations_ than in the first edition of the _origin_, and certainly than in the fifth and sixth editions. among other interesting points may be mentioned the "good effects of crossing" being "possibly analogous to good effects of change in condition,"--a principle which he upheld on experimental grounds in his _cross and self-fertilisation_ in . in conclusion, i desire to express my thanks to mr wallace for a footnote he was good enough to supply: and to professor bateson, sir w. thiselton-dyer, dr gadow, professor judd, dr marr, col. prain and dr stapf for information on various points. i am also indebted to mr rutherford, of the university library, for his careful copy of the manuscript of . cambridge, _june , ._ explanation of signs, &c. [] means that the words so enclosed are erased in the original ms. <> indicates an insertion by the editor. _origin_, ed. vi. refers to the popular edition. part i. § i. <on variation under domestication, and on the principles of selection.> an individual organism placed under new conditions [often] sometimes varies in a small degree and in very trifling respects such as stature, fatness, sometimes colour, health, habits in animals and probably disposition. also habits of life develope certain parts. disuse atrophies. [most of these slight variations tend to become hereditary.] when the individual is multiplied for long periods by buds the variation is yet small, though greater and occasionally a single bud or individual departs widely from its type (example){ } and continues steadily to propagate, by buds, such new kind. { } evidently a memorandum that an example should be given. when the organism is bred for several generations under new or varying conditions, the variation is greater in amount and endless in kind [especially{ } holds good when individuals have long been exposed to new conditions]. the nature of the external conditions tends to effect some definite change in all or greater part of offspring,--little food, small size--certain foods harmless &c. &c. organs affected and diseases--extent unknown. a certain degree of variation (müller's twins){ } seems inevitable effect of process of reproduction. but more important is that simple <?> generation, especially under new conditions [when no crossing] <causes> infinite variation and not direct effect of external conditions, but only in as much as it affects the reproductive functions{ }. there seems to be no part (_beau ideal_ of liver){ } of body, internal or external, or mind or habits, or instincts which does not vary in some small degree and [often] some <?> to a great amount. { } the importance of exposure to new conditions for several generations is insisted on in the _origin_, ed. i. p. , also p. . in the latter passage the author guards himself against the assumption that variations are "due to chance," and speaks of "our ignorance of the cause of each particular variation." these statements are not always remembered by his critics. { } cf. _origin_, ed. i. p. , vi. p. , "young of the same litter, sometimes differ considerably from each other, though both the young and the parents, as müller has remarked, have apparently been exposed to exactly the same conditions of life." { } this is paralleled by the conclusion in the _origin_, ed. i. p. , that "the most frequent cause of variability may be attributed to the male and female reproductive elements having been affected prior to the act of conception." { } the meaning seems to be that there must be some variability in the liver otherwise anatomists would not speak of the 'beau ideal' of that organ. [all such] variations [being congenital] or those very slowly acquired of all kinds [decidedly evince a tendency to become hereditary], when not so become simple variety, when it does a race. each{ } parent transmits its peculiarities, therefore if varieties allowed freely to cross, except by the _chance_ of two characterized by same peculiarity happening to marry, such varieties will be constantly demolished{ }. all bisexual animals must cross, hermaphrodite plants do cross, it seems very possible that hermaphrodite animals do cross,--conclusion strengthened: ill effects of breeding in and in, good effects of crossing possibly analogous to good effects of change in condition <?>{ }. { } the position of the following passage is uncertain. "if individuals of two widely different varieties be allowed to cross, a third race will be formed--a most fertile source of the variation in domesticated animals. <in the _origin_, ed. i. p. the author says that "the possibility of making distinct races by crossing has been greatly exaggerated."> if freely allowed, the characters of pure parents will be lost, number of races thus <illegible> but differences <?> besides the <illegible>. but if varieties differing in very slight respects be allowed to cross, such small variation will be destroyed, at least to our senses,--a variation [clearly] just to be distinguished by long legs will have offspring not to be so distinguished. free crossing great agent in producing uniformity in any breed. introduce tendency to revert to parent form." { } the swamping effect of intercrossing is referred to in the _origin_, ed. i. p. , vi. p. . { } a discussion on the intercrossing of hermaphrodites in relation to knight's views occurs in the _origin_, ed. i. p. , vi. p. . the parallelism between crossing and changed conditions is briefly given in the _origin_, ed. i. p. , vi. p. , and was finally investigated in _the effects of cross and self-fertilisation in the vegetable kingdom_, . therefore if in any country or district all animals of one species be allowed freely to cross, any small tendency in them to vary will be constantly counteracted. secondly reversion to parent form--analogue of _vis medicatrix_{ }. but if man selects, then new races rapidly formed,--of late years systematically followed,--in most ancient times often practically followed{ }. by such selection make race-horse, dray-horse--one cow good for tallow, another for eating &c.--one plant's good lay <illegible> in leaves another in fruit &c. &c.: the same plant to supply his wants at different times of year. by former means animals become adapted, as a direct effect to a cause, to external conditions, as size of body to amount of food. by this latter means they may also be so adapted, but further they may be adapted to ends and pursuits, which by no possibility can affect growth, as existence of tallow-chandler cannot tend to make fat. in such selected races, if not removed to new conditions, and <if> preserved from all cross, after several generations become very true, like each other and not varying. but man{ } selects only <?> what is useful and curious--has bad judgment, is capricious,--grudges to destroy those that do not come up to his pattern,--has no [knowledge] power of selecting according to internal variations,--can hardly keep his conditions uniform,--[cannot] does not select those best adapted to the conditions under which <the> form <?> lives, but those most useful to him. this might all be otherwise. { } there is an article on the _vis medicatrix_ in brougham's _dissertations_, , a copy of which is in the author's library. { } this is the classification of selection into methodical and unconscious given in the _origin_, ed. i. p. , vi. p. . { } this passage, and a similar discussion on the power of the creator (p. ), correspond to the comparison between the selective capacities of man and nature, in the _origin_, ed. i. p. , vi. p. . § ii. <on variation in a state of nature and on the natural means of selection.> let us see how far above principles of variation apply to wild animals. wild animals vary exceedingly little--yet they are known as individuals{ }. british plants, in many genera number quite uncertain of varieties and species: in shells chiefly external conditions{ }. primrose and cowslip. wild animals from different [countries can be recognized]. specific character gives some organs as varying. variations analogous in kind, but less in degree with domesticated animals--chiefly external and less important parts. { } i.e. they are individually distinguishable. { } see _origin_, ed. i. p. , vi. p. . our experience would lead us to expect that any and every one of these organisms would vary if <the organism were> taken away <?> and placed under new conditions. geology proclaims a constant round of change, bringing into play, by every possible <?> change of climate and the death of pre-existing inhabitants, endless variations of new conditions. these <?> generally very slow, doubtful though <illegible> how far the slowness <?> would produce tendency to vary. but geolog<ists> show change in configuration which, together with the accidents of air and water and the means of transportal which every being possesses, must occasionally bring, rather suddenly, organism to new conditions and <?> expose it for several generations. hence <?> we should expect every now and then a wild form to vary{ }; possibly this may be cause of some species varying more than others. { } when the author wrote this sketch he seems not to have been so fully convinced of the general occurrence of variation in nature as he afterwards became. the above passage in the text possibly suggests that at this time he laid more stress on _sports_ or _mutations_ than was afterwards the case. according to nature of new conditions, so we might expect all or majority of organisms born under them to vary in some definite way. further we might expect that the mould in which they are cast would likewise vary in some small degree. but is there any means of selecting those offspring which vary in the same manner, crossing them and keeping their offspring separate and thus producing selected races: otherwise as the wild animals freely cross, so must such small heterogeneous varieties be constantly counter-balanced and lost, and a uniformity of character [kept up] preserved. the former variation as the direct and necessary effects of causes, which we can see can act on them, as size of body from amount of food, effect of certain kinds of food on certain parts of bodies &c. &c.; such new varieties may then become adapted to those external [natural] agencies which act on them. but can varieties be produced adapted to end, which cannot possibly influence their structure and which it is absurd to look <at> as effects of chance. can varieties like some vars of domesticated animals, like almost all wild species be produced adapted by exquisite means to prey on one animal or to escape from another,--or rather, as it puts out of question effects of intelligence and habits, can a plant become adapted to animals, as a plant which cannot be impregnated without agency of insect; or hooked seeds depending on animal's existence: woolly animals cannot have any direct effect on seeds of plant. this point which all theories about climate adapting woodpecker{ } to crawl <?> up trees, <illegible> miseltoe, <sentence incomplete>. but if every part of a plant or animal was to vary <illegible>, and if a being infinitely more sagacious than man (not an omniscient creator) during thousands and thousands of years were to select all the variations which tended towards certain ends ([or were to produce causes <?> which tended to the same end]), for instance, if he foresaw a canine animal would be better off, owing to the country producing more hares, if he were longer legged and keener sight,--greyhound produced{ }. if he saw that aquatic <animal would need> skinned toes. if for some unknown cause he found it would advantage a plant, which <?> like most plants is occasionally visited by bees &c.: if that plant's seed were occasionally eaten by birds and were then carried on to rotten trees, he might select trees with fruit more agreeable to such birds as perched, to ensure their being carried to trees; if he perceived those birds more often dropped the seeds, he might well have selected a bird who would <illegible> rotten trees or [gradually select plants which <he> had proved to live on less and less rotten trees]. who, seeing how plants vary in garden, what blind foolish man has done{ } in a few years, will deny an all-seeing being in thousands of years could effect (if the creator chose to do so), either by his own direct foresight or by intermediate means,--which will represent <?> the creator of this universe. seems usual means. be it remembered i have nothing to say about life and mind and _all_ forms descending from one common type{ }. i speak of the variation of the existing great divisions of the organised kingdom, how far i would go, hereafter to be seen. { } the author may possibly have taken the case of the woodpecker from buffon, _histoire nat. des oiseaux_, t. vii. p. , , where however it is treated from a different point of view. he uses it more than once, see for instance _origin_, ed. i. pp. , , , vi. pp. , , . the passage in the text corresponds with a discussion on the woodpecker and the mistletoe in _origin_, ed. i. p. , vi. p. . { } this illustration occurs in the _origin_, ed. i. pp. , , vi. pp. , . { } see _origin_, ed. i. p. , vi. p. , where the word _creator_ is replaced by _nature_. { } note in the original. "good place to introduce, saying reasons hereafter to be given, how far i extend theory, say to all mammalia--reasons growing weaker and weaker." before considering whether <there> be any natural means of selection, and secondly (which forms the nd part of this sketch) the far more important point whether the characters and relations of animated <things> are such as favour the idea of wild species being races <?> descended from a common stock, as the varieties of potato or dahlia or cattle having so descended, let us consider probable character of [selected races] wild varieties. _natural selection._ de candolle's war of nature,--seeing contented face of nature,--may be well at first doubted; we see it on borders of perpetual cold{ }. but considering the enormous geometrical power of increase in every organism and as <?> every country, in ordinary cases <countries> must be stocked to full extent, reflection will show that this is the case. malthus on man,--in animals no moral [check] restraint <?>--they breed in time of year when provision most abundant, or season most favourable, every country has its seasons,--calculate robins,--oscillating from years of destruction{ }. if proof were wanted let any singular change of climate <occur> here <?>, how astoundingly some tribes <?> increase, also introduced animals{ }, the pressure is always ready,--capacity of alpine plants to endure other climates,--think of endless seeds scattered abroad,--forests regaining their percentage{ },--a thousand wedges{ } are being forced into the oeconomy of nature. this requires much reflection; study malthus and calculate rates of increase and remember the resistance,--only periodical. { } see _origin_, ed. i. pp. , , vi. p. , where similar reference is made to de candolle; for malthus see _origin_, p. . { } this may possibly refer to the amount of destruction going on. see _origin_, ed. i. p. , vi. p. , where there is an estimate of a later date as to death-rate of birds in winter. "calculate robins" probably refers to a calculation of the rate of increase of birds under favourable conditions. { } in the _origin_, ed. i. pp. , , vi. p. , he instances cattle and horses and certain plants in s. america and american species of plants in india, and further on, as unexpected effects of changed conditions, the enclosure of a heath, and the relation between the fertilisation of clover and the presence of cats (_origin_, ed. i. p. , vi. p. ). { } _origin_, ed. i. p. , vi. p. . "it has been observed that the trees now growing on ... ancient indian mounds ... display the same beautiful diversity and proportion of kinds as in the surrounding virgin forests." { } the simile of the wedge occurs in the _origin_, ed. i. p. ; it is deleted in darwin's copy of the first edition: it does not occur in ed. vi. the unavoidable effect of this <is> that many of every species are destroyed either in egg or [young or mature (the former state the more common)]. in the course of a thousand generations infinitesimally small differences must inevitably tell{ }; when unusually cold winter, or hot or dry summer comes, then out of the whole body of individuals of any species, if there be the smallest differences in their structure, habits, instincts [senses], health &c, <it> will on an average tell; as conditions change a rather larger proportion will be preserved: so if the chief check to increase falls on seeds or eggs, so will, in the course of generations or ten thousand, those seeds (like one with down to fly{ }) which fly furthest and get scattered most ultimately rear most plants, and such small differences tend to be hereditary like shades of expression in human countenance. so if one parent <?> fish deposits its egg in infinitesimally different circumstances, as in rather shallower or deeper water &c., it will then <?> tell. { } in a rough summary at the close of the essay, occur the words:--"every creature lives by a struggle, smallest grain in balance must tell." { } cf. _origin_, ed. i. p. , vi. p. . let hares{ } increase very slowly from change of climate affecting peculiar plants, and some other <illegible> rabbit decrease in same proportion [let this unsettle organisation of], a canine animal, who formerly derived its chief sustenance by springing on rabbits or running them by scent, must decrease too and might thus readily become exterminated. but if its form varied very slightly, the long legged fleet ones, during a thousand years being selected, and the less fleet rigidly destroyed must, if no law of nature be opposed to it, alter forms. { } this is a repetition of what is given at p. . remember how soon bakewell on the same principle altered cattle and western, sheep,--carefully avoiding a cross (pigeons) with any breed. we cannot suppose that one plant tends to vary in fruit and another in flower, and another in flower and foliage,--some have been selected for both fruit and flower: that one animal varies in its covering and another not,--another in its milk. take any organism and ask what is it useful for and on that point it will be found to vary,--cabbages in their leaf,--corn in size <and> quality of grain, both in times of year,--kidney beans for young pod and cotton for envelope of seeds &c. &c.: dogs in intellect, courage, fleetness and smell <?>: pigeons in peculiarities approaching to monsters. this requires consideration,--should be introduced in first chapter if it holds, i believe it does. it is hypothetical at best{ }. { } compare _origin_, ed. i. p. , vi. p. . "i have seen it gravely remarked, that it was most fortunate that the strawberry began to vary just when gardeners began to attend closely to this plant. no doubt the strawberry had always varied since it was cultivated, but the slight varieties had been neglected." nature's variation far less, but such selection far more rigid and scrutinising. man's races not [even so well] only not better adapted to conditions than other races, but often not <?> one race adapted to its conditions, as man keeps and propagates some alpine plants in garden. nature lets <an> animal live, till on actual proof it is found less able to do the required work to serve the desired end, man judges solely by his eye, and knows not whether nerves, muscles, arteries, are developed in proportion to the change of external form. besides selection by death, in bisexual animals <illegible> the selection in time of fullest vigour, namely struggle of males; even in animals which pair there seems a surplus <?> and a battle, possibly as in man more males produced than females, struggle of war or charms{ }. hence that male which at that time is in fullest vigour, or best armed with arms or ornaments of its species, will gain in hundreds of generations some small advantage and transmit such characters to its offspring. so in female rearing its young, the most vigorous and skilful and industrious, <whose> instincts <are> best developed, will rear more young, probably possessing her good qualities, and a greater number will thus <be> prepared for the struggle of nature. compared to man using a male alone of good breed. this latter section only of limited application, applies to variation of [specific] sexual characters. introduce here contrast with lamarck,--absurdity of habit, or chance?? or external conditions, making a woodpecker adapted to tree{ }. { } here we have the two types of sexual selection discussed in the _origin_, ed. i. pp. et seq., vi. pp. et seq. { } it is not obvious why the author objects to "chance" or "external conditions making a woodpecker." he allows that variation is ultimately referable to conditions and that the nature of the connexion is unknown, i.e. that the result is fortuitous. it is not clear in the original to how much of the passage the two ? refer. before considering difficulties of theory of selection let us consider character of the races produced, as now explained, by nature. conditions have varied slowly and the organisms best adapted in their whole course of life to the changed conditions have always been selected,--man selects small dog and afterwards gives it profusion of food,--selects a long-backed and short-legged breed and gives it no particular exercise to suit this function &c. &c. in ordinary cases nature has not allowed her race to be contaminated with a cross of another race, and agriculturists know how difficult they find always to prevent this,--effect would be trueness. this character and sterility when crossed, and generally a greater amount of difference, are two main features, which distinguish domestic races from species. [sterility not universal admitted by all{ }. _gladiolus_, _crinum_, _calceolaria_{ } must be species if there be such a thing. races of dogs and oxen: but certainly very general; indeed a gradation of sterility most perfect{ } very general. some nearest species will not cross (crocus, some heath <?>), some genera cross readily (fowls{ } and grouse, peacock &c.). hybrids no ways monstrous quite perfect except secretions{ } hence even the mule has bred,--character of sterility, especially a few years ago <?> thought very much more universal than it now is, has been thought the distinguishing character; indeed it is obvious if all forms freely crossed, nature would be a chaos. but the very gradation of the character, even if it always existed in some degree which it does not, renders it impossible as marks <?> those <?> suppose distinct as species{ }]. will analogy throw any light on the fact of the supposed races of nature being sterile, though none of the domestic ones are? mr herbert <and> koelreuter have shown external differences will not guide one in knowing whether hybrids will be fertile or not, but the chief circumstance is constitutional differences{ }, such as being adapted to different climate or soil, differences which [must] probably affect the whole body of the organism and not any one part. now wild animals, taken out of their natural conditions, seldom breed. i do not refer to shows or to zoological societies where many animals unite, but <do not?> breed, and others will never unite, but to wild animals caught and kept _quite tame_ left loose and well fed about houses and living many years. hybrids produced almost as readily as pure breds. st hilaire great distinction of tame and domestic,--elephants,--ferrets{ }. reproductive organs not subject to disease in zoological garden. dissection and microscope show that hybrid is in exactly same condition as another animal in the intervals of breeding season, or those animals which taken wild and _not bred_ in domesticity, remain without breeding their whole lives. it should be observed that so far from domesticity being unfavourable in itself <it> makes more fertile: [when animal is domesticated and breeds, productive power increased from more food and selection of fertile races]. as far as animals go might be thought <an> effect on their mind and a special case. { } the meaning is "that sterility is not universal is admitted by all." { } see _var. under dom._, ed. , i. p. , where the garden forms of _gladiolus_ and _calceolaria_ are said to be derived from crosses between distinct species. herbert's hybrid _crinums_ are discussed in the _origin_, ed. i. p. , vi. p. . it is well known that the author believed in a multiple origin of domestic dogs. { } the argument from gradation in sterility is given in the _origin_, ed. i. pp. , , vi. pp. , . in the _origin_, i have not come across the cases mentioned, viz. crocus, heath, or grouse and fowl or peacock. for sterility between closely allied species, see _origin_, ed. i. p. , vi. p. . in the present essay the author does not distinguish between fertility between species and the fertility of the hybrid offspring, a point on which he insists in the _origin_, ed. i. p. , vi. p. . { } ackermann (_ber. d. vereins f. naturkunde zu kassel_, , p. ) quotes from gloger that a cross has been effected between a domestic hen and a _tetrao tetrix_; the offspring died when three days old. { } no doubt the sexual cells are meant. i do not know on what evidence it is stated that the mule has bred. { } the sentence is all but illegible. i think that the author refers to forms usually ranked as varieties having been marked as species when it was found that they were sterile together. see the case of the red and blue _anagallis_ given from gärtner in the _origin_, ed. i. p. , vi. p. . { } in the _origin_, ed. i. p. , where the author speaks of constitutional differences in this connexion, he specifies that they are confined to the reproductive system. { } the sensitiveness of the reproductive system to changed conditions is insisted on in the _origin_, ed. i. p. , vi. p. . the ferret is mentioned, as being prolific in captivity, in _var. under dom._, ed. , ii. p. . but turning to plants we find same class of facts. i do not refer to seeds not ripening, perhaps the commonest cause, but to plants not setting, which either is owing to some imperfection of ovule or pollen. lindley says sterility is the [curse] bane of all propagators,--linnæus about alpine plants. american bog plants,--pollen in exactly same state as in hybrids,--same in geraniums. persian and chinese{ } lilac will not seed in italy and england. probably double plants and all fruits owe their developed parts primarily <?> to sterility and extra food thus <?> applied{ }. there is here gradation <in> sterility and then parts, like diseases, are transmitted hereditarily. we cannot assign any cause why the pontic azalea produces plenty of pollen and not american{ }, why common lilac seeds and not persian, we see no difference in healthiness. we know not on what circumstances these facts depend, why ferret breeds, and cheetah{ }, elephant and pig in india will not. { } lindley's remark is quoted in the _origin_, ed. i. p. . linnæus' remark is to the effect that alpine plants tend to be sterile under cultivation (see _var. under dom._, ed. , ii. p. ). in the same place the author speaks of peat-loving plants being sterile in our gardens,--no doubt the american bog-plants referred to above. on the following page (p. ) the sterility of the lilac (_syringa persica_ and _chinensis_) is referred to. { } the author probably means that the increase in the petals is due to a greater food supply being available for them owing to sterility. see the discussion in _var. under dom._, ed. , ii. p. . it must be noted that doubleness of the flower may exist without noticeable sterility. { } i have not come across this case in the author's works. { } for the somewhat doubtful case of the cheetah (_felis jubata_) see _var. under dom._, ed. , ii. p. . i do not know to what fact "pig in india" refers. now in crossing it is certain every peculiarity in form and constitution is transmitted: an alpine plant transmits its alpine tendency to its offspring, an american plant its american-bog constitution, and <with> animals, those peculiarities, on which{ } when placed out of their natural conditions they are incapable of breeding; and moreover they transmit every part of their constitution, their respiration, their pulse, their instinct, which are all suddenly modified, can it be wondered at that they are incapable of breeding? i think it may be truly said it would be more wonderful if they did. but it may be asked why have not the recognised varieties, supposed to have been produced through the means of man, [not refused to breed] have all bred{ }. variation depends on change of condition and selection{ }, as far as man's systematic or unsystematic selection <has> gone; he takes external form, has little power from ignorance over internal invisible constitutional differences. races which have long been domesticated, and have much varied, are precisely those which were capable of bearing great changes, whose constitutions were adapted to a diversity of climates. nature changes slowly and by degrees. according to many authors probably breeds of dogs are another case of modified species freely crossing. there is no variety which <illegible> has been <illegible> adapted to peculiar soil or situation for a thousand years and another rigorously adapted to another, till such can be produced, the question is not tried{ }. man in past ages, could transport into different climates, animals and plants which would freely propagate in such new climates. nature could effect, with selection, such changes slowly, so that precisely those animals which are adapted to submit to great changes have given rise to diverse races,--and indeed great doubt on this head{ }. { } this sentence should run "on which depends their incapacity to breed in unnatural conditions." { } this sentence ends in confusion: it should clearly close with the words "refused to breed" in place of the bracket and the present concluding phrase. { } the author doubtless refers to the change produced by the _summation_ of variation by means of selection. { } the meaning of this sentence is made clear by a passage in the ms. of :--"until man selects two varieties from the same stock, adapted to two climates or to other different external conditions, and confines each rigidly for one or several thousand years to such conditions, always selecting the individuals best adapted to them, he cannot be said to have even commenced the experiment." that is, the attempt to produce mutually sterile domestic breeds. { } this passage is to some extent a repetition of a previous one and may have been intended to replace an earlier sentence. i have thought it best to give both. in the _origin_, ed. i. p. , vi. p. , the author gives his opinion that the power of resisting diverse conditions, seen in man and his domestic animals, is an example "of a very common flexibility of constitution." before leaving this subject well to observe that it was shown that a certain amount of variation is consequent on mere act of reproduction, both by buds and sexually,--is vastly increased when parents exposed for some generations to new conditions{ }, and we now find that many animals when exposed for first time to very new conditions, are <as> incapable of breeding as hybrids. it [probably] bears also on supposed fact of crossed animals when not infertile, as in mongrels, tending to vary much, as likewise seems to be the case, when true hybrids possess just sufficient fertility to propagate with the parent breeds and _inter se_ for some generations. this is koelreuter's belief. these facts throw light on each other and support the truth of each other, we see throughout a connection between the reproductive faculties and exposure to changed conditions of life whether by crossing or exposure of the individuals{ }. { } in the _origin_, ed. i. chs. i. and v., the author does not admit reproduction, apart from environment, as being a cause of variation. with regard to the cumulative effect of new conditions there are many passages in the _origin_, ed. i. e.g. pp. , , vi. pp. , . { } as already pointed out, this is the important principle investigated in the author's _cross and self-fertilisation_. professor bateson has suggested to me that the experiments should be repeated with gametically pure individuals. _difficulties on theory of selection_{ }. it may be objected such perfect organs as eye and ear, could never be formed, in latter less difficulty as gradations more perfect; at first appears monstrous and to <the> end appears difficulty. but think of gradation, even now manifest, (tibia and fibula). everyone will allow if every fossil preserved, gradation infinitely more perfect; for possibility of selection a perfect <?> gradation is required. different groups of structure, slight gradation in each group,--every analogy renders it probable that intermediate forms have existed. be it remembered what strange metamorphoses; part of eye, not directly connected with vision, might come to be [thus used] gradually worked in for this end,--swimming bladder by gradation of structure is admitted to belong to the ear system,--rattlesnake. [woodpecker best adapted to climb.] in some cases gradation not possible,--as vertebræ,--actually vary in domestic animals,--less difficult if growth followed. looking to whole animals, a bat formed not for flight{ }. suppose we had flying fish{ } and not one of our now called flying fish preserved, who would have guessed intermediate habits. woodpeckers and tree-frogs both live in countries where no trees{ }. { } in the _origin_ a chapter is given up to "difficulties on theory": the discussion in the present essay seems slight even when it is remembered how small a space is here available. for _tibia_ &c. see p. . { } this may be interpreted "the general structure of a bat is the same as that of non-flying mammals." { } that is truly winged fish. { } the terrestrial woodpecker of s. america formed the subject of a paper by darwin, _proc. zool. soc._, . see _life and letters_, vol. iii. p. . the gradations by which each individual organ has arrived at its present state, and each individual animal with its aggregate of organs has arrived, probably never could be known, and all present great difficulties. i merely wish to show that the proposition is not so monstrous as it at first appears, and that if good reason can be advanced for believing the species have descended from common parents, the difficulty of imagining intermediate forms of structure not sufficient to make one at once reject the theory. § iii. <on variation in instincts and other mental attributes.> the mental powers of different animals in wild and tame state [present still greater difficulties] require a separate section. be it remembered i have nothing to do with origin of memory, attention, and the different faculties of the mind{ }, but merely with their differences in each of the great divisions of nature. disposition, courage, pertinacity <?>, suspicion, restlessness, ill-temper, sagacity and <the> reverse unquestionably vary in animals and are inherited (cuba wildness dogs, rabbits, fear against particular object as man galapagos{ }). habits purely corporeal, breeding season &c., time of going to rest &c., vary and are hereditary, like the analogous habits of plants which vary and are inherited. habits of body, as manner of movement d^o. and d^o. habits, as pointing and setting on certain occasions d^o. taste for hunting certain objects and manner of doing so,--sheep-dog. these are shown clearly by crossing and their analogy with true instinct thus shown,--retriever. do not know objects for which they do it. lord brougham's definition{ }. origin partly habit, but the amount necessarily unknown, partly selection. young pointers pointing stones and sheep--tumbling pigeons--sheep{ } going back to place where born. instinct aided by reason, as in the taylor-bird{ }. taught by parents, cows choosing food, birds singing. instincts vary in wild state (birds get wilder) often lost{ }; more perfect,--nest without roof. these facts [only clear way] show how incomprehensibly brain has power of transmitting intellectual operations. { } the same proviso occurs in the _origin_, ed. i. p. , vi. p. . { } the tameness of the birds in the galapagos is described in the _journal of researches_ ( ), p. . dogs and rabbits are probably mentioned as cases in which the hereditary fear of man has been lost. in the ms. the author states that the cuban feral dog shows great natural wildness, even when caught quite young. { } in the _origin_, ed. i. p. , vi. p. , he refuses to define instinct. for lord brougham's definition see his _dissertations on subjects of science etc._, , p. . { } see james hogg (the ettrick shepherd), works, , _tales and sketches_, p. . { } this refers to the tailor-bird making use of manufactured thread supplied to it, instead of thread twisted by itself. { } _often lost_ applies to _instinct_: _birds get wilder_ is printed in a parenthesis because it was apparently added as an after-thought. _nest without roof_ refers to the water-ousel omitting to vault its nest when building in a protected situation. faculties{ } distinct from true instincts,--finding [way]. it must i think be admitted that habits whether congenital or acquired by practice [sometimes] often become inherited{ }; instincts, influence, equally with structure, the preservation of animals; therefore selection must, with changing conditions tend to modify the inherited habits of animals. if this be admitted it will be found _possible_ that many of the strangest instincts may be thus acquired. i may observe, without attempting definition, that an inherited habit or trick (trick because may be born) fulfils closely what we mean by instinct. a habit is often performed unconsciously, the strangest habits become associated, d^o. tricks, going in certain spots &c. &c., even against will, is excited by external agencies, and looks not to the end,--a person playing a pianoforte. if such a habit were transmitted it would make a marvellous instinct. let us consider some of the most difficult cases of instincts, whether they could be _possibly_ acquired. i do not say _probably_, for that belongs to our rd part{ }, i beg this may be remembered, nor do i mean to attempt to show exact method. i want only to show that whole theory ought not at once to be rejected on this score. { } in the ms. of is an interesting discussion on _faculty_ as distinct from _instinct_. { } at this date and for long afterwards the inheritance of acquired characters was assumed to occur. { } part ii. is here intended: see the introduction. every instinct must, by my theory, have been acquired gradually by slight changes <illegible> of former instinct, each change being useful to its then species. shamming death struck me at first as remarkable objection. i found none really sham death{ }, and that there is gradation; now no one doubts that those insects which do it either more or less, do it for some good, if then any species was led to do it more, and then <?> escaped &c. &c. { } the meaning is that the attitude assumed in _shamming_ is not accurately like that of death. take migratory instincts, faculty distinct from instinct, animals have notion of time,--like savages. ordinary finding way by memory, but how does savage find way across country,--as incomprehensible to us, as animal to them,--geological changes,--fishes in river,--case of sheep in spain{ }. architectural instincts,--a manufacturer's employee in making single articles extraordinary skill,--often said seem to make it almost <illegible>, child born with such a notion of playing{ },--we can fancy tailoring acquired in same perfection,--mixture of reason,--water-ouzel,--taylor-bird,--gradation of simple nest to most complicated. { } this refers to the _transandantes_ sheep mentioned in the ms. of , as having acquired a migratory instinct. { } in the _origin_, ed. i. p. , vi. p. , mozart's pseudo-instinctive skill in piano-playing is mentioned. see _phil. trans._, , p. . bees again, distinction of faculty,--how they make a hexagon,--waterhouse's theory{ },--the impulse to use whatever faculty they possess,--the taylor-bird has the faculty of sewing with beak, instinct impels him to do it. { } in the discussion on bees' cells, _origin_, ed. i. p. , vi. p. , the author acknowledges that his theory originated in waterhouse's observations. last case of parent feeding young with different food (take case of galapagos birds, gradation from hawfinch to sylvia) selection and habit might lead old birds to vary taste <?> and form, leaving their instinct of feeding their young with same food{ },--or i see no difficulty in parents being forced or induced to vary the food brought, and selection adapting the young ones to it, and thus by degree any amount of diversity might be arrived at. although we can never hope to see the course revealed by which different instincts have been acquired, for we have only present animals (not well known) to judge of the course of gradation, yet once grant the principle of habits, whether congenital or acquired by experience, being inherited and i can see no limit to the [amount of variation] extraordinariness <?> of the habits thus acquired. { } the hawfinch-and _sylvia-_types are figured in the _journal of researches_, p. . the discussion of change of form in relation to change of instinct is not clear, and i find it impossible to suggest a paraphrase. _summing up this division._ if variation be admitted to occur occasionally in some wild animals, and how can we doubt it, when we see [all] thousands <of> organisms, for whatever use taken by man, do vary. if we admit such variations tend to be hereditary, and how can we doubt it when we <remember> resemblances of features and character,--disease and monstrosities inherited and endless races produced ( cabbages). if we admit selection is steadily at work, and who will doubt it, when he considers amount of food on an average fixed and reproductive powers act in geometrical ratio. if we admit that external conditions vary, as all geology proclaims, they have done and are now doing,--then, if no law of nature be opposed, there must occasionally be formed races, [slightly] differing from the parent races. so then any such law{ }, none is known, but in all works it is assumed, in <?> flat contradiction to all known facts, that the amount of possible variation is soon acquired. are not all the most varied species, the oldest domesticated: who <would> think that horses or corn could be produced? take dahlia and potato, who will pretend in years{ } <that great changes might not be effected>: perfectly adapted to conditions and then again brought into varying conditions. think what has been done in few last years, look at pigeons, and cattle. with the amount of food man can produce he may have arrived at limit of fatness or size, or thickness of wool <?>, but these are the most trivial points, but even in these i conclude it is impossible to say we know the limit of variation. and therefore with the [adapting] selecting power of nature, infinitely wise compared to those of man, <i conclude> that it is impossible to say we know the limit of races, which would be true <to their> kind; if of different constitutions would probably be infertile one with another, and which might be adapted in the most singular and admirable manner, according to their wants, to external nature and to other surrounding organisms,--such races would be species. but is there any evidence <that> species <have> been thus produced, this is a question wholly independent of all previous points, and which on examination of the kingdom of nature <we> ought to answer one way or another. { } i should interpret this obscure sentence as follows, "no such opposing law is known, but in all works on the subject a law is (in flat contradiction to all known facts) assumed to limit the possible amount of variation." in the _origin_, the author never limits the power of variation, as far as i know. { } in _var. under dom._ ed. , ii. p. , the _dahlia_ is described as showing sensitiveness to conditions in . all the varieties of the _dahlia_ are said to have arisen since (_ibid._ i. p. ). part ii{ }. { } in the original ms. the heading is: part iii.; but part ii. is clearly intended; for details see the introduction. i have not been able to discover where § iv. ends and § v. begins. §§ iv. & v. <on the evidence from geology.> i may premise, that according to the view ordinarily received, the myriads of organisms peopling this world have been created by so many distinct acts of creation. as we know nothing of the <illegible> will of a creator,--we can see no reason why there should exist any relation between the organisms thus created; or again, they might be created according to any scheme. but it would be marvellous if this scheme should be the same as would result from the descent of groups of organisms from [certain] the same parents, according to the circumstances, just attempted to be developed. with equal probability did old cosmogonists say fossils were created, as we now see them, with a false resemblance to living beings{ }; what would the astronomer say to the doctrine that the planets moved <not> according to the law of gravitation, but from the creator having willed each separate planet to move in its particular orbit? i believe such a proposition (if we remove all prejudices) would be as legitimate as to admit that certain groups of living and extinct organisms, in their distribution, in their structure and in their relations one to another and to external conditions, agreed with the theory and showed signs of common descent, and yet were created distinct. as long as it was thought impossible that organisms should vary, or should anyhow become adapted to other organisms in a complicated manner, and yet be separated from them by an impassable barrier of sterility{ }, it was justifiable, even with some appearance in favour of a common descent, to admit distinct creation according to the will of an omniscient creator; or, for it is the same thing, to say with whewell that the beginnings of all things surpass the comprehension of man. in the former sections i have endeavoured to show that such variation or specification is not impossible, nay, in many points of view is absolutely probable. what then is the evidence in favour of it and what the evidence against it. with our imperfect knowledge of past ages [surely there will be some] it would be strange if the imperfection did not create some unfavourable evidence. { } this passage corresponds roughly to the conclusion of the _origin_, see ed. i. p. , vi. p. . { } a similar passage occurs in the conclusion of the _origin_, ed. i. p. , vi. p. . give sketch of the past,--beginning with facts appearing hostile under present knowledge,--then proceed to geograph. distribution,--order of appearance,--affinities,--morphology &c., &c. our theory requires a very gradual introduction of new forms{ }, and extermination of the old (to which we shall revert). the extermination of old may sometimes be rapid, but never the introduction. in the groups descended from common parent, our theory requires a perfect gradation not differing more than breed<s> of cattle, or potatoes, or cabbages in forms. i do not mean that a graduated series of animals must have existed, intermediate between horse, mouse, tapir{ }, elephant [or fowl and peacock], but that these must have had a common parent, and between horse and this <?> parent &c., &c., but the common parent may possibly have differed more from either than the two do now from each other. now what evidence of this is there? so perfect gradation in some departments, that some naturalists have thought that in some large divisions, if all existing forms were collected, a near approach to perfect gradation would be made. but such a notion is preposterous with respect to all, but evidently so with mammals. other naturalists have thought this would be so if all the specimens entombed in the strata were collected{ }. i conceive there is no probability whatever of this; nevertheless it is certain all the numerous fossil forms fall in<to>, as buckland remarks, _not_ present classes, families and genera, they fall between them: so is it with new discoveries of existing forms. most ancient fossils, that is most separated <by> space of time, are most apt to fall between the classes--(but organisms from those countries most separated by space also fall between the classes <_e.g._> ornithorhyncus?). as far as geological discoveries <go> they tend towards such gradation{ }. illustrate it with net. toxodon,--tibia and fibula,--dog and otter,--but so utterly improbable is <it>, in _ex. gr._ pachydermata, to compose series as perfect as cattle, that if, as many geologists seem to infer, each separate formation presents even an approach to a consecutive history, my theory must be given up. even if it were consecutive, it would only collect series of one district in our present state of knowledge; but what probability is there that any one formation during the _immense_ period which has elapsed during each period will _generally_ present a consecutive history. [compare number living at one period to fossils preserved--look at enormous periods of time.] { } see _origin_, ed. i. p. , vi. p. . { } see _origin_, ed. i. pp. , , vi. p. . the author uses his experience of pigeons for examples for what he means by _intermediate_; the instance of the horse and tapir also occurs. { } the absence of intermediate forms between living organisms (and also as regards fossils) is discussed in the _origin_, ed. i. pp. , , vi. p. . in the above discussion there is no evidence that the author felt this difficulty so strongly as it is expressed in the _origin_, ed. i. p. ,--as perhaps "the most obvious and gravest objection that can be urged against my theory." but in a rough summary written on the back of the penultimate page of the ms. he refers to the geological evidence:--"evidence, as far as it does go, is favourable, exceedingly incomplete,--greatest difficulty on this theory. i am convinced not insuperable." buckland's remarks are given in the _origin_, ed. i. p. , vi. p. . { } that the evidence of geology, as far as it goes, is favourable to the theory of descent is claimed in the _origin_, ed. i. pp. - , vi. pp. - . for the reference to _net_ in the following sentence, see note , p. , {note } of this essay. referring only to marine animals, which are obviously most likely to be preserved, they must live where <?> sediment (of a kind favourable for preservation, not sand and pebble){ } is depositing quickly and over large area and must be thickly capped, <illegible> littoral deposits: for otherwise denudation <will destroy them>,--they must live in a shallow space which sediment will tend to fill up,--as movement is <in?> progress if soon brought <?> up <?> subject to denudation,--[if] as during subsidence favourable, accords with facts of european deposits{ }, but subsidence apt to destroy agents which produce sediment{ }. { } see _origin_, ed. i. p. , vi. p. . "the remains that do become embedded, if in sand and gravel, will, when the beds are upraised, generally be dissolved by the percolation of rain-water." { } the position of the following is not clear:--"think of immense differences in nature of european deposits,--without interposing new causes,--think of time required by present slow changes, to cause, on very same area, such diverse deposits, iron-sand, chalk, sand, coral, clay!" { } the paragraph which ends here is difficult to interpret. in spite of obscurity it is easy to recognize the general resemblance to the discussion on the importance of subsidence given in the _origin_, ed. i. pp. et seq., vi. pp. et seq. i believe safely inferred <that> groups of marine <?> fossils only preserved for future ages where sediment goes on long <and> continuous<ly> and with rapid but not too rapid deposition in <an> area of subsidence. in how few places in any one region like europe will <?> these contingencies be going on? hence <?> in past ages mere [gaps] pages preserved{ }. lyell's doctrine carried to extreme,--we shall understand difficulty if it be asked:--what chance of series of gradation between cattle by <illegible> at age <illegible> as far back as miocene{ }? we know then cattle existed. compare number of living,--immense duration of each period,--fewness of fossils. { } see note , p. . { } compare _origin_, ed. i. p. , vi. p. . "we shall, perhaps, best perceive the improbability of our being enabled to connect species by numerous, fine, intermediate, fossil links, by asking ourselves whether, for instance, geologists at some future period will be able to prove that our different breeds of cattle, sheep, horses, and dogs have descended from a single stock or from several aboriginal stocks." this only refers to consecutiveness of history of organisms of each formation. the foregoing argument will show firstly, that formations are distinct merely from want of fossils <of intermediate beds>, and secondly, that each formation is full of gaps, has been advanced to account for _fewness_ of _preserved_ organisms compared to what have lived on the world. the very same argument explains why in older formations the organisms appear to come on and disappear suddenly,--but in [later] tertiary not quite suddenly{ }, in later tertiary gradually,--becoming rare and disappearing,--some have disappeared within man's time. it is obvious that our theory requires gradual and nearly uniform introduction, possibly more sudden extermination,--subsidence of continent of australia &c., &c. { } the sudden appearance of groups of allied species in the lowest known fossiliferous strata is discussed in the _origin_, ed. i. p. , vi. p. . the gradual appearance in the later strata occurs in the _origin_, ed. i. p. , vi. p. . our theory requires that the first form which existed of each of the great divisions would present points intermediate between existing ones, but immensely different. most geologists believe silurian{ } fossils are those which first existed in the whole world, not those which have chanced to be the oldest not destroyed,--or the first which existed in profoundly deep seas in progress of conversion from sea to land: if they are first they <? we> give up. not so hutton or lyell: if first reptile{ } of red sandstone <?> really was first which existed: if pachyderm{ } of paris was first which existed: fish of devonian: dragon fly of lias: for we cannot suppose them the progenitors: they agree too closely with existing divisions. but geologists consider europe as <?> a passage from sea to island <?> to continent (except wealden, see lyell). these animals therefore, i consider then mere introduction <?> from continents long since submerged. { } compare _origin_, ed. i. p. , vi. p. . { } i have interpreted as _sandstone_ a scrawl which i first read as _sea_; i have done so at the suggestion of professor judd, who points out that "footprints in the red sandstone were known at that time, and geologists were not then particular to distinguish between amphibians and reptiles." { } this refers to cuvier's discovery of _palæotherium_ &c. at montmartre. finally, if views of some geologists be correct, my theory must be given up. [lyell's views, as far as they go, are in _favour_, but they go so little in favour, and so much more is required, that it may <be> viewed as objection.] if geology present us with mere pages in chapters, towards end of <a> history, formed by tearing out bundles of leaves, and each page illustrating merely a small portion of the organisms of that time, the facts accord perfectly with my theory{ }. { } this simile is more fully given in the _origin_, ed. i. p. , vi. p. . "for my part, following out lyell's metaphor, i look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. each word of the slowly-changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely separated formations." professor judd has been good enough to point out to me, that darwin's metaphor is founded on the comparison of geology to history in ch. i. of the _principles of geology_, ed. i. , vol. i. pp. - . professor judd has also called my attention to another passage,--_principles_, ed. i. , vol. iii. p. , when lyell imagines an historian examining "two buried cities at the foot of vesuvius, immediately superimposed upon each other." the historian would discover that the inhabitants of the lower town were greeks while those of the upper one were italians. but he would be wrong in supposing that there had been a sudden change from the greek to the italian language in campania. i think it is clear that darwin's metaphor is partly taken from this passage. see for instance (in the above passage from the _origin_) such phrases as "history ... written in a changing dialect"--"apparently abruptly changed forms of life." the passage within [] in the above paragraph:--"lyell's views as far as they go &c.," no doubt refers, as professor judd points out, to lyell not going so far as darwin on the question of the imperfection of the geological record. _extermination._ we have seen that in later periods the organisms have disappeared by degrees and [perhaps] probably by degrees in earlier, and i have said our theory requires it. as many naturalists seem to think extermination a most mysterious circumstance{ } and call in astonishing agencies, it is well to recall what we have shown concerning the struggle of nature. an exterminating agency is at work with every organism: we scarcely see it: if robins would increase to thousands in ten years how severe must the process be. how imperceptible a small increase: fossils become rare: possibly sudden extermination as australia, but as present means very slow and many means of escape, i shall doubt very sudden exterminations. who can explain why some species abound more,--why does marsh titmouse, or ring-ouzel, now little change,--why is one sea-slug rare and another common on our coasts,--why one species of rhinoceros more than another,--why is <illegible> tiger of india so rare? curious and general sources of error, the place of an organism is instantly filled up. { } on rarity and extinction see _origin_, ed. i. pp. , , vi. pp. , . we know state of earth has changed, and as earthquakes and tides go on, the state must change,--many geologists believe a slow gradual cooling. now let us see in accordance with principles of [variation] specification explained in sect. ii. how species would probably be introduced and how such results accord with what is known. the first fact geology proclaims is immense number of extinct forms, and new appearances. tertiary strata leads to belief, that forms gradually become rare and disappear and are gradually supplied by others. we see some forms now becoming rare and disappearing, we know of no sudden creation: in older periods the forms _appear_ to come in suddenly, scene shifts: but even here devonian, permian &c. [keep on supplying new links in chain]--genera and higher forms come on and disappear, in same way leaving a species on one or more stages below that in which the form abounded. <geographical distribution.> § vi. let us consider the absolute state of distribution of organisms of earth's face. referring chiefly, but not exclusively (from difficulty of transport, fewness, and the distinct characteristics of groups) to mammalia; and first considering the three or four main [regions] divisions; north america, europe, asia, including greater part of e. indian archipelago and africa are intimately allied. africa most distinct, especially most southern parts. and the arctic regions, which unite n. america, asia and europe, only separated (if we travel one way by behring's st.) by a narrow strait, is most intimately allied, indeed forms but one restricted group. next comes s. america,--then australia, madagascar (and some small islands which stand very remote from the land). looking at these main divisions separately, the organisms vary according to changes in condition{ } of different parts. but besides this, barriers of every kind seem to separate regions in a greater degree than proportionally to the difference of climates on each side. thus great chains of mountains, spaces of sea between islands and continents, even great rivers and deserts. in fact the amount <of> difference in the organisms bears a certain, but not invariable relation to the amount of physical difficulties to transit{ }. { } in the _origin_, ed. i. p. , vi. p. , the author begins his discussion on geographical distribution by minimising the effect of physical conditions. he lays great stress on the effect of _barriers_, as in the present essay. { } note in the original, "would it be more striking if we took animals, take rhinoceros, and study their habitats?" there are some curious exceptions, namely, similarity of fauna of mountains of europe and n. america and lapland. other cases just <the> reverse, mountains of eastern s. america, altai <?>, s. india <?>{ }: mountain summits of islands often eminently peculiar. fauna generally of some islands, even when close, very dissimilar, in others very similar. [i am here led to observe one or more centres of creation{ }.] { } note by mr a. r. wallace. "the want of similarity referred to, is, between the mountains of brazil and guiana and those of the andes. also those of the indian peninsula as compared with the himalayas. in both cases there is continuous intervening land. "the islands referred to were, no doubt, the galapagos for dissimilarity from s. america; our own islands as compared with europe, and perhaps java, for similarity with continental asia." { } the arguments against multiple centres of creation are given in the _origin_, ed. i. p. , vi. p. . the simple geologist can explain many of the foregoing cases of distribution. subsidence of a continent in which free means of dispersal, would drive the lowland plants up to the mountains, now converted into islands, and the semi-alpine plants would take place of alpine, and alpine be destroyed, if mountains originally were not of great height. so we may see, during gradual changes{ } of climate on a continent, the propagation of species would vary and adapt themselves to small changes causing much extermination{ }. the mountains of europe were quite lately covered with ice, and the lowlands probably partaking of the arctic climate and fauna. then as climate changed, arctic fauna would take place of ice, and an inundation of plants from different temperate countries <would> seize the lowlands, leaving islands of arctic forms. but if this had happened on an island, whence could the new forms have come,--here the geologist calls in creationists. if island formed, the geologist will suggest <that> many of the forms might have been borne from nearest land, but if peculiar, he calls in creationist,--as such island rises in height &c., he still more calls in creation. the creationist tells one, on a <illegible> spot the american spirit of creation makes _orpheus_ and _tyrannus_ and american doves, and in accordance with past and extinct forms, but no persistent relation between areas and distribution, geologico-geograph.-distribution. { } in the _origin_, ed. i. p. , vi. p. , the author does not give his views on the distribution of alpine plants as original but refers to edward forbes' work (_geolog. survey memoirs_, ). in his autobiography, darwin refers to this. "i was forestalled" he says, "in only one important point, which my vanity has always made me regret." (_life and letters_, i. p. .) { } <the following is written on the back of a page of the ms.> discuss one or more centres of creation: allude strongly to facilities of dispersal and amount of geological change: allude to mountain-summits afterwards to be referred to. the distribution varies, as everyone knows, according to adaptation, explain going from n. to s. how we come to fresh groups of species in the same general region, but besides this we find difference, according to greatness of barriers, in greater proportion than can be well accounted for by adaptation. <on representive species see _origin_, ed. i. p. , vi. p. .> this very striking when we think of cattle of pampas, plants <?> &c. &c. then go into discussion; this holds with or main divisions as well as the endless minor ones in each of these great ones: in these i chiefly refer to mammalia &c. &c. the similarity of type, but not in species, in same continent has been much less insisted on than the dissimilarity of different great regions generically: it is more striking. <i have here omitted an incomprehensible sentence.> galapagos islands, tristan d'acunha, _volcanic_ islands covered with craters we know lately did not support any organisms. how unlike these islands in nature to neighbouring lands. these facts perhaps more striking than almost any others. [geology apt to affect geography therefore we ought to expect to find the above.] geological-geographical distribution. in looking to past times we find australia equally distinct. s. america was distinct, though with more forms in common. n. america its nearest neighbour more in common,--in some respects more, in some less allied to europe. europe we find <?> equally european. for europe is now part of asia though not <illegible>. africa unknown,--examples, elephant, rhinoceros, hippopotamus, hyaena. as geology destroys geography we cannot be surprised in going far back we find marsupials and edentata in europe: but geology destroys geography. now according to analogy of domesticated animals let us see what would result. let us take case of farmer on pampas, where everything approaches nearer to state of nature. he works on organisms having strong tendency to vary: and he knows <that the> only way to make a distinct breed is to select and separate. it would be useless to separate the best bulls and pair with best cows if their offspring run loose and bred with the other herds, and tendency to reversion not counteracted; he would endeavour therefore to get his cows on islands and then commence his work of selection. if several farmers in different _rincons_{ } were to set to work, especially if with different objects, several breeds would soon be produced. so would it be with horticulturist and so history of every plant shows; the number of varieties{ } increase in proportion to care bestowed on their selection and, with crossing plants, separation. now, according to this analogy, change of external conditions, and isolation either by chance landing <of> a form on an island, or subsidence dividing a continent, or great chain of mountains, and the number of individuals not being numerous will best favour variation and selection{ }. no doubt change could be effected in same country without any barrier by long continued selection on one species: even in case of a plant not capable of crossing would easier get possession and solely occupy an island{ }. now we can at once see that <if> two parts of a continent isolated, new species thus generated in them, would have closest affinities, like cattle in counties of england: if barrier afterwards destroyed one species might destroy the other or both keep their ground. so if island formed near continent, let it be ever so different, that continent would supply inhabitants, and new species (like the old) would be allied with that continent. an island generally very different soil and climate, and number and order of inhabitants supplied by chance, no point so favourable for generation of new species{ },--especially the mountains, hence, so it is. as isolated mountains formed in a plain country (if such happens) is an island. as other islands formed, the old species would spread and thus extend and the fauna of distant island might ultimately meet and a continent formed between them. no one doubts continents formed by repeated elevations and depressions{ }. in looking backwards, but not so far that all geographical boundaries are destroyed, we can thus at once see why existing forms are related to the extinct in the same manner as existing ones are in some part of existing continent. by chance we might even have one or two absolute parent fossils. { } _rincon_ in spanish means a _nook_ or _corner_, it is here probably used to mean a small farm. { } the following is written across the page: "no one would expect a set of similar varieties to be produced in the different countries, so species different." { } <the following passage seems to have been meant to follow here.> the parent of an organism, we may generally suppose to be in less favourable condition than the selected offspring and therefore generally in fewer numbers. (this is not borne out by horticulture, mere hypothesis; as an organism in favourable conditions might by selection be adapted to still more favourable conditions.) barrier would further act in preventing species formed in one part migrating to another part. { } <the following notes occur on the back of the page.> number of species not related to capabilities of the country: furthermore not always those best adapted, perhaps explained by creationists by changes and progress. <see p. , note .{note }> although creationists can, by help of geology, explain much, how can he explain the marked relation of past and present in same area, the varying relation in other cases, between past and present, the relation of different parts of same great area. if island, to adjoining continent, if quite different, on mountain summits,--the number of individuals not being related to capabilities, or how &c.--our theory, i believe, can throw much light and all facts accord. { } see _origin_, ed. i. p. , vi. p. . { } on oscillation see _origin_, ed. i. p. , vi. p. . the detection of transitional forms would be rendered more difficult on rising point of land. the distribution therefore in the above enumerated points, even the trivial ones, which on any other <theory?> can be viewed as so many ultimate facts, all follow <in> a simple manner on the theory of the occurrence of species by <illegible> and being adapted by selection to <illegible>, conjoined with their power of dispersal, and the steady geographico-geological changes which are now in progress and which undoubtedly have taken place. ought to state the opinion of the immutability of species and the creation by so many separate acts of will of the creator{ }. { } <from the back of ms.> effect of climate on stationary island and on continent, but continent once island. moreover repeated oscillations fresh diffusion when non-united, then isolation, when rising again immigration prevented, new habitats formed, new species, when united free immigration, hence uniform characters. hence more forms <on?> the island. mountain summits. why not true species. first let us recall in part i, conditions of variation: change of conditions during several generations, and if frequently altered so much better [perhaps excess of food]. secondly, continued selection [while in wild state]. thirdly, isolation in all or nearly all,--as well to recall advantages of. [in continent, if we look to terrestrial animal, long continued change might go on, which would only cause change in numerical number <? proportions>: if continued long enough might ultimately affect all, though to most continents <there is> chance of immigration. some few of whole body of species must be long affected and entire selection working same way. but here isolation absent, without barrier, cut off such <illegible>. we can see advantage of isolation. but let us take case of island thrown up by volcanic agency at some distances, here we should have occasional visitants, only in few numbers and exposed to new conditions and <illegible> more important,--a quite new grouping of organic beings, which would open out new sources of subsistence, or <would> control <?> old ones. the number would be few, can old have the very best opportunity. <the conquest of the indigenes by introduced organisms shows that the indigenes were not perfectly adapted, see _origin_, ed. i. p. .> moreover as the island continued changing,--continued slow changes, river, marshes, lakes, mountains &c. &c., new races as successively formed and a fresh occasional visitant. if island formed continent, some species would emerge and immigrate. everyone admits continents. we can see why galapagos and c. verde differ <see _origin_, ed. i. p. >], depressed and raised. we can see from this repeated action and the time required for a continent, why many more forms than in new zealand <see _origin_, ed. i. p. for a comparison between new zealand and the cape> no mammals or other classes <see however, _origin_, ed. i. p. for the case of the frog>. we can at once see how it comes when there has been an old channel of migration,--cordilleras; we can see why indian asiatic flora,--[why species] having a wide range gives better chance of some arriving at new points and being selected, and adapted to new ends. i need hardly remark no necessity for change. finally, as continent (most extinction <?> during formation of continent) is formed after repeated elevation and depression, and interchange of species we might foretell much extinction, and that the survivor would belong to same type, as the extinct, in same manner as different part of same continent, which were once separated by space as they are by time <see _origin_, ed. i. pp. and >. as all mammals have descended from one stock, we ought to expect that every continent has been at some time connected, hence obliteration of present ranges. i do not mean that the fossil mammifers found in s. america are the lineal successors <ancestors> of the present forms of s. america: for it is highly improbable that more than one or two cases (who will say how many races after plata bones) should be found. i believe this from numbers, who have lived,--mere <?> chance of fewness. moreover in every case from very existence of genera and species only few at one time will leave progeny, under form of new species, to distant ages; and the more distant the ages the fewer the progenitors. an observation may be here appended, bad chance of preservation on rising island, the nurseries of new species, appeal to experience <see _origin_, ed. i. p. >. this observation may be extended, that in all cases, subsiding land must be, in early stages, less favourable to formation of new species; but it will isolate them, and then if land recommences rising how favourable. as preoccupation is bar to diffusion to species, so would it be to a selected variety. but it would not be if that variety was better fitted to some not fully occupied station; so during elevation or the formation of new stations, is scene for new species. but during elevation not favourable to preservation of fossil (except in caverns <?>); when subsidence highly favourable in early stages to preservation of fossils; when subsidence, less sediment. so that our strata, as general rule will be the tomb of old species (not undergoing any change) when rising land the nursery. but if there be vestige will generally be preserved to future ages, the new ones will not be entombed till fresh subsidence supervenes. in this long gap we shall have no record: so that wonderful if we should get transitional forms. i do not mean every stage, for we cannot expect that, as before shown, until geologists will be prepared to say that although under unnaturally favourable condition we can trace in future ages short-horn and herefordshire <see note , p. >. {note } § vii. <affinities and classification.> looking now to the affinities of organisms, without relation to their distribution, and taking all fossil and recent, we see the degrees of relationship are of different degrees and arbitrary,--sub-genera,--genera,--sub-families, families, orders and classes and kingdoms. the kind of classification which everyone feels is most correct is called the natural system, but no can define this. if we say with whewell <that we have an> undefined instinct of the importance of organs{ }, we have no means in lower animals of saying which is most important, and yet everyone feels that some one system alone deserves to be called natural. the true relationship of organisms is brought before one by considering relations of analogy, an otter-like animal amongst mammalia and an otter amongst marsupials. in such cases external resemblance and habit of life and _the final end of whole organization_ very strong, yet no relation{ }. naturalists cannot avoid these terms of relation and affinity though they use them metaphorically. if used in simple earnestness the natural system ought to be a genealogical <one>; and our knowledge of the points which are most easily affected in transmission are those which we least value in considering the natural system, and practically when we find they do vary we regard them of less value{ }. in classifying varieties the same language is used and the same kind of division: here also (in pine-apple){ } we talk of the natural classification, overlooking similarity of the fruits, because whole plant differs. the origin of sub-genera, genera, &c., &c., is not difficult on notion of genealogical succession, and accords with what we know of similar gradations of affinity in domesticated organisms. in the same region the organic beings are <illegible> related to each other and the external conditions in many physical respects are allied{ } and their differences of same kind, and therefore when a new species has been selected and has obtained a place in the economy of nature, we may suppose that generally it will tend to extend its range during geographical changes, and thus, becoming isolated and exposed to new conditions, will slightly alter and its structure by selection become slightly remodified, thus we should get species of a sub-genus and genus,--as varieties of merino-sheep,--varieties of british and indian cattle. fresh species might go on forming and others become extinct and all might become extinct, and then we should have <an> extinct genus; a case formerly mentioned, of which numerous cases occur in palæontology. but more often the same advantages which caused the new species to spread and become modified into several species would favour some of the species being preserved: and if two of the species, considerably different, each gave rise to group of new species, you would have two genera; the same thing will go on. we may look at case in other way, looking to future. according to mere chance every existing species may generate another, but if any species, a, in changing gets an advantage and that advantage (whatever it may be, intellect, &c., &c., or some particular structure or constitution) is inherited{ }, a will be the progenitor of several genera or even families in the hard struggle of nature. a will go on beating out other forms, it might come that a would people earth,--we may now not have one descendant on our globe of the one or several original creations{ }. external conditions air, earth, water being same{ } on globe, and the communication not being perfect, organisms of widely different descent might become adapted to the same end and then we should have cases of analogy{ }, [they might even tend to become numerically representative]. from this often happening each of the great divisions of nature would have their representative eminently adapted to earth, to <air>{ }, to water, and to these in <illegible> and then these great divisions would show numerical relations in their classification. { } after "organs" is inserted, apparently as an afterthought:--"no, and instance metamorphosis, afterwards explicable." { } for analogical resemblances see _origin_, ed. i. p. , vi. p. . { } "practically when naturalists are at work, they do not trouble themselves about the physiological value of the characters.... if they find a character nearly uniform, ... they use it as one of high value," _origin_, ed. i. p. , vi. p. . { } "we are cautioned ... not to class two varieties of the pine-apple together, merely because their fruit, though the most important part, happens to be nearly identical," _origin_, ed. i. p. , vi. p. . { } the whole of this passage is obscure, but the text is quite clear, except for one illegible word. { } <the exact position of the following passage is uncertain:> "just as it is not likely every present breed of fancy birds and cattle will propagate, only some of the best." { } this suggests that the author was not far from the principle of divergence on which he afterwards laid so much stress. see _origin_, ed. i. p. , vi. p. , also _life and letters_, i. p. . { } that is to say the same conditions occurring in different parts of the globe. { } the position of the following is uncertain, "greyhound and racehorse have an analogy to each other." the same comparison occurs in the _origin_, ed. i. p. , vi. p. . { } _air_ is evidently intended; in the ms. _water_ is written twice. § viii. unity [or similarity] of type in the great classes. nothing more wonderful in nat. hist. than looking at the vast number of organisms, recent and fossil, exposed to the most diverse conditions, living in the most distant climes, and at immensely remote periods, fitted to wholely different ends, yet to find large groups united by a similar type of structure. when we for instance see bat, horse, porpoise-fin, hand, all built on same structure{ }, having bones{ } with same name, we see there is some deep bond of union between them{ }, to illustrate this is the foundation and objects <?> <of> what is called the natural system; and which is foundation of distinction <?> of true and adaptive characters{ }. now this wonderful fact of hand, hoof, wing, paddle and claw being the same, is at once explicable on the principle of some parent-forms, which might either be <illegible> or walking animals, becoming through infinite number of small selections adapted to various conditions. we know that proportion, size, shape of bones and their accompanying soft parts vary, and hence constant selection would alter, to almost any purpose <?> the framework of an organism, but yet would leave a general, even closest similarity in it. { } written between the lines occurs:--"extend to birds and other classes." { } written between the lines occurs:--"many bones merely represented." { } in the _origin_, ed. i. p. , vi. p. , the term _morphology_ is taken as including _unity of type_. the paddle of the porpoise and the wing of the bat are there used as instances of morphological resemblance. { } the sentence is difficult to decipher. [we know the number of similar parts, as vertebræ and ribs can vary, hence this also we might expect.] also <if> the changes carried on to a certain point, doubtless type will be lost, and this is case with plesiosaurus{ }. the unity of type in past and present ages of certain great divisions thus undoubtedly receives the simplest explanation. { } in the _origin_, ed. i. p. , vi. p. , the author speaks of the "general pattern" being obscured in the paddles of "extinct gigantic sea-lizards." there is another class of allied and almost identical facts, admitted by the soberest physiologists, [from the study of a certain set of organs in a group of organisms] and refers <? referring> to a unity of type of different organs in the same individual, denominated the science of "morphology." the <? this> discovered by beautiful and regular series, and in the case of plants from monstrous changes, that certain organs in an individual are other organs metamorphosed. thus every botanist considers petals, nectaries, stamens, pistils, germen as metamorphosed leaf. they thus explain, in the most lucid manner, the position and number of all parts of the flower, and the curious conversion under cultivation of one part into another. the complicated double set of jaws and palpi of crustaceans{ }, and all insects are considered as metamorphosed <limbs> and to see the series is to admit this phraseology. the skulls of the vertebrates are undoubtedly composed of three metamorphosed vertebræ; thus we can understand the strange form of the separate bones which compose the casket holding man's brain. these{ } facts differ but slightly from those of last section, if with wing, paddle, hand and hoof, some common structure was yet visible, or could be made out by a series of occasional monstrous conversions, and if traces could be discovered of <the> whole having once existed as walking or swimming instruments, these organs would be said to be metamorphosed, as it is they are only said to exhibit a common type. { } see _origin_, ed. i. p. , vi. p. . { } the following passage seems to have been meant to precede the sentence beginning "these facts":--"it is evident, that when in each individual species, organs are metamorph. a unity of type extends." this distinction is not drawn by physiologists, and is only implied by some by their general manner of writing. these facts, though affecting every organic being on the face of the globe, which has existed, or does exist, can only be viewed by the creationist as ultimate and inexplicable facts. but this unity of type through the individuals of a group, and this metamorphosis of the same organ into other organs, adapted to diverse use, necessarily follows on the theory of descent{ }. for let us take case of vertebrata, which if{ } they descended from one parent and by this theory all the vertebrata have been altered by slow degrees, such as we see in domestic animals. we know that proportions alter, and even that occasionally numbers of vertebræ alter, that parts become soldered, that parts are lost, as tail and toes, but we know <that?> here we can see that possibly a walking organ might <?> be converted into swimming or into a gliding organ and so on to a flying organ. but such gradual changes would not alter the unity of type in their descendants, as parts lost and soldered and vertebræ. but we can see that if this carried to extreme, unity lost,--plesiosaurus. here we have seen the same organ is formed <?> <for> different purposes <ten words illegible>: and if, in several orders of vertebrata, we could trace origin <of> spinous processes and monstrosities &c. we should say, instead of there existing a unity of type, morphology{ }, as we do when we trace the head as being the vertebræ metamorphosed. be it observed that naturalists, as they use terms of affinity without attaching real meaning, here also they are obliged to use metamorphosis, without meaning that any parent of crustacean was really an animal with as many legs as crustacean has jaws. the theory of descent at once explains these wonderful facts. { } this is, i believe, the first place in which the author uses the words "theory of descent." { } the sentence should probably run, "let us take the case of the vertebrata: if we assume them to be descended from one parent, then by this theory they have been altered &c." { } that is "we should call it a morphological fact." now few of the physiologists who use this language really suppose that the parent of insect with the metamorphosed jaw, was an insect with [more] so many legs, or that the parent of flowering plants, originally had no stamens, or pistils or petals, but some other means of propagation,--and so in other cases. now according to our theory during the infinite number of changes, we might expect that an organ used for a purpose might be used for a different one by his descendant, as must have been the case by our theory with the bat, porpoise, horse, &c., which are descended from one parent. and if it so chanced that traces of the former use and structure of the part should be retained, which is manifestly possible if not probable, then we should have the organs, on which morphology is founded and which instead of being metaphorical becomes plain and <and instead of being> utterly unintelligible becomes simple matter of fact{ }. { } in the _origin_, ed. i. p. , vi. p. , the author, referring to the expressions used by naturalists in regard to morphology and metamorphosis, says "on my view these terms may be used literally." <_embryology._> this general unity of type in great groups of organisms (including of course these morphological cases) displays itself in a most striking manner in the stages through which the foetus passes{ }. in early stage, the wing of bat, hoof, hand, paddle are not to be distinguished. at a still earlier <stage> there is no difference between fish, bird, &c. &c. and mammal. it is not that they cannot be distinguished, but the arteries{ } <illegible>. it is not true that one passes through the form of a lower group, though no doubt fish more nearly related to foetal state{ }. { } see _origin_, ed. i. p. , vi. p. . { } in the _origin_, ed. i. p. , vi. p. , the author argues that the "loop-like course of the arteries" in the vertebrate embryo has no direct relation to the conditions of existence. { } the following passages are written across the page:--"they pass through the same phases, but some, generally called the higher groups, are further metamorphosed. ? degradation and complication? no tendency to perfection. ? justly argued against lamarck?" this similarity at the earliest stage is remarkably shown in the course of the arteries which become greatly altered, as foetus advances in life and assumes the widely different course and number which characterize full-grown fish and mammals. how wonderful that in egg, in water or air, or in womb of mother, artery{ } should run in same course. { } an almost identical passage occurs in the _origin_, ed. i. p. , vi. p. . light can be thrown on this by our theory. the structure of each organism is chiefly adapted to the sustension of its life, when full-grown, when it has to feed itself and propagate{ }. the structure of a kitten is quite in secondary degree adapted to its habits, whilst fed by its mother's milk and prey. hence variation in the structure of the full-grown species will _chiefly_ determine the preservation of a species now become ill-suited to its habitat, or rather with a better place opened to it in the economy of nature. it would not matter to the full-grown cat whether in its young state it was more or less eminently feline, so that it become so when full-grown. no doubt most variation, (not depending on habits of life of individual) depends on early change{ } and we must suspect that at whatever time of life the alteration of foetus is effected, it tends to appear at same period. when we <see> a tendency to particular disease in old age transmitted by the male, we know some effect is produced during conception, on the simple cell of ovule, which will not produce its effect till half a century afterwards and that effect is not visible{ }. so we see in grey-hound, bull-dog, in race-horse and cart-horse, which have been selected for their form in full-life, there is much less (?) difference in the few first days after birth{ }, than when full-grown: so in cattle, we see it clearly in cases of cattle, which differ obviously in shape and length of horns. if man were during , years to be able to select, far more diverse animals from horse or cow, i should expect there would be far less differences in the very young and foetal state: and this, i think, throws light on above marvellous fact. in larvæ, which have long life selection, perhaps, does much,--in the pupa not so much{ } there is no object gained in varying form &c. of foetus (beyond certain adaptations to mother's womb) and therefore selection will not further act on it, than in giving to its changing tissues a tendency to certain parts afterwards to assume certain forms. { } the following: "deaths of brothers <when> old by same peculiar disease" which is written between the lines seems to have been a memorandum which is expanded a few lines lower. i believe the case of the brothers came from dr r. w. darwin. { } see the discussion to this effect in the _origin_, ed. i. pp. - , vi. p. . the author there makes the distinction between a cause affecting the germ-cell and the reaction occurring at a late period of life. { } possibly the sentence was meant to end "is not visible till then." { } see _origin_, ed. i. pp. - , vi. p. . the query appended to _much less_ is justified, since measurement was necessary to prove that the greyhound and bulldog puppies had not nearly acquired "their full amount of proportional difference." { } <the following discussion, from the back of the page, is in large measure the same as the text.> i think light can be thrown on these facts. from the following peculiarities being hereditary, [we know that some change in the germinal vesicle is effected, which will only betray itself years after] diseases--man, goitre, gout, baldness, fatness, size, [longevity <illegible> time of reproduction, shape of horns, case of old brothers dying of same disease]. and we know that the germinal vesicle must have been affected, though no effect is apparent or can be apparent till years afterwards,--no more apparent than when these peculiarities appear by the exposure of the full-grown individual. <that is, "the young individual is as apparently free from the hereditary changes which will appear later, as the young is actually free from the changes produced by exposure to certain conditions in adult life."> so that when we see a variety in cattle, even if the variety be due to act of reproduction, we cannot feel sure at what period this change became apparent. it may have been effected during early age of free life <or> foetal existence, as monsters show. from arguments before used, and crossing, we may generally suspect in germ; but i repeat it does not follow, that the change should be apparent till life fully developed; any more than fatness depending on heredity should be apparent during early childhood, still less during foetal existence. in case of horns of cattle, which when inherited must depend on germinal vesicle, obviously no effect till cattle full-grown. practically it would appear that the [hereditary] peculiarities characterising our domestic races, therefore resulting from vesicle, do not appear with their full characters in very early states; thus though two breeds of cows have calves different, they are not so different,--grey-hound and bull-dog. and this is what is <to> be expected, for man is indifferent to characters of young animals and hence would select those full-grown animals which possessed the desirable characteristics. so that from mere chance we might expect that some of the characters would be such only as became fully apparent in mature life. furthermore we may suspect it to be a law, that at whatever time a new character appears, whether from vesicle, or effects of external conditions, it would appear at corresponding time <see _origin_, ed. i. p. >. thus diseases appearing in old age produce children with d^o.,--early maturity,--longevity,--old men, brothers, of same disease--young children of d^o. i said men do not select for quality of young,--calf with big bullocks. silk-worms, peculiarities which, appear in caterpillar state or cocoon state, are transmitted to corresponding states. the effect of this would be that if some peculiarity was born in a young animal, but never exercised, it might be inherited in young animal; but if exercised that part of structure would be increased and would be inherited in corresponding time of life after such training. i have said that man selects in full-life, so would it be in nature. in struggle of existence, it matters nothing to a feline animal, whether kitten eminently feline, as long as it sucks. therefore natural selection would act equally well on character which was fully <developed> only in full age. selection could tend to alter no character in foetus, (except relation to mother) it would alter less in young state (putting on one side larva condition) but alter every part in full-grown condition. look to a foetus and its parent, and again after ages foetus and its <i. e. the above mentioned parents> descendant; the parent more variable <?> than foetus, which explains all.] thus there is no power to change the course of the arteries, as long as they nourish the foetus; it is the selection of slight changes which supervene at any time during <illegible> of life. the less differences of foetus,--this has obvious meaning on this view: otherwise how strange that a [monkey] horse, a man, a bat should at one time of life have arteries, running in a manner, which is only intelligibly useful in a fish! the natural system being on theory genealogical, we can at once see, why foetus, retaining traces of the ancestral form, is of the highest value in classification. § ix. <abortive organs.> there is another grand class of facts relating to what are called abortive organs. these consist of organs which the same reasoning power that shows us how beautifully these organs in some cases are adapted to certain end, declares in other cases are absolutely useless. thus teeth in rhinoceros{ }, whale, narwhal,--bone on tibia, muscles which do not move,--little bone of wing of apteryx,--bone representing extremities in some snake,--little wings within <?> soldered cover of beetles,--men and bulls, mammæ: filaments without anthers in plants, mere scales representing petals in others, in feather-hyacinth whole flower. almost infinitely numerous. no one can reflect on these without astonishment, can anything be clearer than that wings are to fly and teeth <to bite>, and yet we find these organs perfect in every detail in situations where they cannot possibly be of their normal use{ }. { } some of these examples occur in _origin_, ed. i. pp. - , vi. pp. - . { } the two following sentences are written, one down the margin, the other across the page. "abortive organs eminently useful in classification. embryonic state of organs. rudiments of organs." the term abortive organ has been thus applied to above structure (as _invariable_ as all other parts{ }) from their absolute similarity to monstrous cases, where from _accident_, certain organs are not developed; as infant without arms or fingers with mere stump representing them: teeth represented by mere points of ossification: headless children with mere button,--viscera represented by small amorphous masses, &c.,--the tail by mere stump,--a solid horn by minute hanging one{ }. there is a tendency in all these cases, when life is preserved, for such structures to become hereditary. we see it in tailless dogs and cats. in plants we see this strikingly,--in thyme, in _linum flavum_,--stamen in _geranium pyrenaicum_{ }. nectaries abort into petals in columbine <_aquilegia_>, produced from some accident and then become hereditary, in some cases only when propagated by buds, in other cases by seed. these cases have been produced suddenly by accident in early growth, but it is part of law of growth that when any organ is not used it tends to diminish (duck's wing{ }?) muscles of dog's ears, <and of> rabbits, muscles wither, arteries grow up. when eye born defective, optic nerve (tuco tuco) is atrophied. as every part whether useful or not (diseases, double flowers) tends to be transmitted to offspring, the origin of abortive organs whether produced at the birth or slowly acquired is easily understood in domestic races of organisms: [a struggle between the atrophy and hereditariness. abortive organs in domestic races.] there will always be a struggle between atrophy of an organ rendered useless, and hereditariness{ }. because we can understand the origin of abortive organs in certain cases, it would be wrong to conclude absolutely that all must have had same origin, but the strongest analogy is in favour of it. and we can by our theory, for during infinite changes some organ, we might have anticipated, would have become useless. <we can> readily explain the fact, so astounding on any other view, namely that organs possibly useless have been formed often with the same exquisite care as when of vital importance. { } i imagine the meaning to be that abortive organs are specific characters in contrast to monstrosities. { } minute hanging horns are mentioned in the _origin_, ed. i. p. , vi. p. , as occurring in hornless breeds of cattle. { } _linum flavum_ is dimorphic: thyme gynodiæcious. it is not clear what point is referred to under _geranium pyrenaicum_. { } the author's work on duck's wings &c. is in _var. under dom._, ed. , i. p. . { } the words _vis medicatrix_ are inserted after "useless," apparently as a memorandum. our theory, i may remark would permit an organ <to> become abortive with respect to its primary use, to be turned to any other purpose, (as the buds in a cauliflower) thus we can see no difficulty in bones of male marsupials being used as fulcrum of muscles, or style of marygold{ },--indeed in one point of view, the heads of [vertebrated] animal may be said to be abortive vertebræ turned into other use: legs of some crustacea abortive jaws, &c., &c. de candolle's analogy of table covered with dishes{ }. { } in the male florets of certain compositæ the style functions merely as a piston for forcing out the pollen. { } <on the back of the page is the following.> if abortive organs are a trace preserved by hereditary tendency, of organ in ancestor of use, we can at once see why important in natural classification, also why more plain in young animal because, as in last section, the selection has altered the old animal most. i repeat, these wondrous facts, of parts created for no use in past and present time, all can by my theory receive simple explanation; or they receive none and we must be content with some such empty metaphor, as that of de candolle, who compares creation to a well covered table, and says abortive organs may be compared to the dishes (some should be empty) placed symmetrically! <the following passage was possibly intended to be inserted here.> degradation and complication see lamarck: no tendency to perfection: if room, [even] high organism would have greater power in beating lower one, thought <?> to be selected for a degraded end. § x. recapitulation and conclusion. let us recapitulate the whole <?> <of> these latter sections by taking case of the three species of rhinoceros, which inhabit java, sumatra, and mainland of malacca or india. we find these three close neighbours, occupants of distinct but neighbouring districts, as a group having a different aspect from the rhinoceros of africa, though some of these latter inhabit very similar countries, but others most diverse stations. we find them intimately related [scarcely <?> differences more than some breeds of cattle] in structure to the rhinoceros, which for immense periods have inhabited this one, out of three main zoological divisions of the world. yet some of these ancient animals were fitted to very different stations: we find all three <illegible> of the generic character of the rhinoceros, which form a [piece of net]{ } set of links in the broken chain representing the pachydermata, as the chain likewise forms a portion in other and longer chains. we see this wonderfully in dissecting the coarse leg of all three and finding nearly the same bones as in bat's wings or man's hand, but we see the clear mark in solid tibia of the fusion into it of the fibula. in all three we find their heads composed of three altered vertebræ, short neck, same bones as giraffe. in the upper jaws of all three we find small teeth like rabbit's. in dissecting them in foetal state we find at a not very early stage their form exactly alike the most different animals, and even with arteries running as in a fish: and this similarity holds when the young one is produced in womb, pond, egg or spawn. now these three undoubted species scarcely differ more than breeds of cattle, are probably subject to many the same contagious diseases; if domesticated these forms would vary, and they might possibly breed together, and fuse into something{ } different <from> their aboriginal forms; might be selected to serve different ends. { } the author doubtless meant that the complex relationships between organisms can be roughly represented by a net in which the knots stand for species. { } between the lines occurs:--"one <?> form be lost." now the creationist believes these three rhinoceroses were created{ } with their deceptive appearance of true, not <illegible> relationship; as well can i believe the planets revolve in their present courses not from one law of gravity but from distinct volition of creator. { } the original sentence is here broken up by the insertion of:--"out of the dust of java, sumatra, these <?> allied to past and present age and <illegible>, with the stamp of inutility in some of their organs and conversion in others." if real species, sterile one with another, differently adapted, now inhabiting different countries, with different structures and instincts, are admitted to have common descent, we can only legitimately stop where our facts stop. look how far in some case a chain of species will lead us. <this probably refers to the crustacea, where the two ends of the series have "hardly a character in common." _origin_, ed. i. p. .> may we not jump (considering how much extermination, and how imperfect geological records) from one sub-genus to another sub-genus. can genera restrain us; many of the same arguments, which made us give up species, inexorably demand genera and families and orders to fall, and classes tottering. we ought to stop only when clear unity of type, independent of use and adaptation, ceases. be it remembered no naturalist pretends to give test from external characters of species; in many genera the distinction is quite arbitrary{ }. but there remains one other way of comparing species with races; it is to compare the effects of crossing them. would it not be wonderful, if the union of two organisms, produced by two separate acts of creation, blended their characters together when crossed according to the same rules, as two races which have undoubtedly descended from same parent stock; yet this can be shown to be the case. for sterility, though a usual <?>, is not an invariable concomitant, it varies much in degree and has been shown to be probably dependent on causes closely analogous with those which make domesticated organisms sterile. independent of sterility there is no difference between mongrels and hybrids, as can be shown in a long series of facts. it is strikingly seen in cases of instincts, when the minds of the two species or races become blended together{ }. in both cases if the half-breed be crossed with either parent for a few generations, all traces of the one parent form is lost (as kölreuter in two tobacco species almost sterile together), so that the creationist in the case of a species, must believe that one act of creation is absorbed into another! { } between the lines occur the words:--"species vary according to same general laws as varieties; they cross according to same laws." { } "a cross with a bull-dog has affected for many generations the courage and obstinacy of greyhounds," _origin_, ed. i. p. , vi. p. . {illustration: facsimile of the original manuscript of the paragraph on p. .} conclusion. such are my reasons for believing that specific forms are not immutable. the affinity of different groups, the unity of types of structure, the representative forms through which foetus passes, the metamorphosis of organs, the abortion of others cease to be metaphorical expressions and become intelligible facts. we no longer look <an> on animal as a savage does at a ship{ }, or other great work of art, as a thing wholly beyond comprehension, but we feel far more interest in examining it. how interesting is every instinct, when we speculate on their origin as an hereditary or congenital habit or produced by the selection of individuals differing slightly from their parents. we must look at every complicated mechanism and instinct, as the summary of a long history, <as the summing up> of{ } useful contrivances, much like a work of art. how interesting does the distribution of all animals become, as throwing light on ancient geography. [we see some seas bridged over.] geology loses in its glory from the imperfection of its archives{ }, but how does it gain in the immensity of the periods of its formations and of the gaps separating these formations. there is much grandeur in looking at the existing animals either as the lineal descendants of the forms buried under thousand feet of matter, or as the coheirs of some still more ancient ancestor. it accords with what we know of the law impressed on matter by the creator, that the creation and extinction of forms, like the birth and death of individuals should be the effect of secondary [laws] means{ }. it is derogatory that the creator of countless systems of worlds should have created each of the myriads of creeping parasites and [slimy] worms which have swarmed each day of life on land and water <on> [this] one globe. we cease being astonished, however much we may deplore, that a group of animals should have been directly created to lay their eggs in bowels and flesh of other,--that some organisms should delight in cruelty,--that animals should be led away by false instincts,--that annually there should be an incalculable waste of eggs and pollen. from death, famine, rapine, and the concealed war of nature we can see that the highest good, which we can conceive, the creation of the higher animals has directly come. doubtless it at first transcends our humble powers, to conceive laws capable of creating individual organisms, each characterised by the most exquisite workmanship and widely-extended adaptations. it accords better with [our modesty] the lowness of our faculties to suppose each must require the fiat of a creator, but in the same proportion the existence of such laws should exalt our notion of the power of the omniscient creator{ }. there is a simple grandeur in the view of life with its powers of growth, assimilation and reproduction, being originally breathed into matter under one or a few forms, and that whilst this our planet has gone circling on according to fixed laws, and land and water, in a cycle of change, have gone on replacing each other, that from so simple an origin, through the process of gradual selection of infinitesimal changes, endless forms most beautiful and most wonderful have been evolved{ }. { } the simile of the savage and the ship occurs in the _origin_, ed. i. p. , vi. p. . { } in the _origin_, ed. i. p. , vi. p. , the author speaks of the "summing up of many contrivances": i have therefore introduced the above words which make the passage clearer. in the _origin_ the comparison is with "a great mechanical invention,"--not with a work of art. { } see a similar passage in the _origin_, ed. i. p. , vi. p. . { } see the _origin_, ed. i. p. , vi. p. . { } the following discussion, together with some memoranda are on the last page of the ms. "the supposed creative spirit does not create either number or kind which <are> from analogy adapted to site (viz. new zealand): it does not keep them all permanently adapted to any country,--it works on spots or areas of creation,--it is not persistent for great periods,--it creates forms of same groups in same regions, with no physical similarity,--it creates, on islands or mountain summits, species allied to the neighbouring ones, and not allied to alpine nature as shown in other mountain summits--even different on different island of similarly constituted archipelago, not created on two points: never mammifers created on small isolated island; nor number of organisms adapted to locality: its power seems influenced or related to the range of other species wholly distinct of the same genus,--it does not equally effect, in amount of difference, all the groups of the same class." { } this passage is the ancestor of the concluding words in the first edition of the _origin of species_ which have remained substantially unchanged throughout subsequent editions, "there is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved." in the nd edition "by the creator" is introduced after "originally breathed." n.b.--there ought somewhere to be a discussion from lyell to show that external conditions do vary, or a note to lyell's works <work?>. besides other difficulties in ii. part, non-acclimatisation of plants. difficulty when asked _how_ did white and negro become altered from common intermediate stock: no facts. we do not know that species are immutable, on the contrary. what arguments against this theory, except our not perceiving every step, like the erosion of valleys{ }. { } compare the _origin_, ed. i. p. , vi. p. , "the difficulty is the same as that felt by so many geologists, when lyell first insisted that long lines of inland cliffs had been formed, and great valleys excavated, by the slow action of the coast-waves." the essay of part i chapter i on the variation of organic beings under domestication; and on the principles of selection the most favourable conditions for variation seem to be when organic beings are bred for many generations under domestication{ }: one may infer this from the simple fact of the vast number of races and breeds of almost every plant and animal, which has long been domesticated. under certain conditions organic beings even during their individual lives become slightly altered from their usual form, size, or other characters: and many of the peculiarities thus acquired are transmitted to their offspring. thus in animals, the size and vigour of body, fatness, period of maturity, habits of body or consensual movements, habits of mind and temper, are modified or acquired during the life of the individual{ }, and become inherited. there is reason to believe that when long exercise has given to certain muscles great development, or disuse has lessened them, that such development is also inherited. food and climate will occasionally produce changes in the colour and texture of the external coverings of animals; and certain unknown conditions affect the horns of cattle in parts of abyssinia; but whether these peculiarities, thus acquired during individual lives, have been inherited, i do not know. it appears certain that malconformation and lameness in horses, produced by too much work on hard roads,--that affections of the eyes in this animal probably caused by bad ventilation,--that tendencies towards many diseases in man, such as gout, caused by the course of life and ultimately producing changes of structure, and that many other diseases produced by unknown agencies, such as goitre, and the idiotcy resulting from it, all become hereditary. { } the cumulative effect of domestication is insisted on in the _origin_, see _e.g. origin_, ed. i. p. , vi. p. . { } this type of variation passes into what he describes as the direct effect of conditions. since they are due to causes acting during the adult life of the organism they might be called individual variations, but he uses this term for congenital variations, _e.g._ the differences discoverable in plants raised from seeds of the same pod _(origin_, ed. i. p. , vi. p. ). it is very doubtful whether the flowers and leaf-buds, annually produced from the same bulb, root, or tree, can properly be considered as parts of the same individual, though in some respects they certainly seem to be so. if they are parts of an individual, plants also are subject to considerable changes during their _individual_ lives. most florist-flowers if neglected degenerate, that is, they lose some of their characters; so common is this, that trueness is often stated, as greatly enhancing the value of a variety{ }: tulips break their colours only after some years' culture; some plants become double and others single, by neglect or care: these characters can be transmitted by cuttings or grafts, and in some cases by true or seminal propagation. occasionally a single bud on a plant assumes at once a new and widely different character: thus it is certain that nectarines have been produced on peach trees and moss roses on provence roses; white currants on red currant bushes; flowers of a different colour from that of the stock, in chrysanthemums, dahlias, sweet-williams, azaleas, &c., &c.; variegated leaf-buds on many trees, and other similar cases. these new characters appearing in single buds, can, like those lesser changes affecting the whole plant, be multiplied not only by cuttings and such means, but often likewise by true seminal generation. { } <it is not clear where the following note is meant to come>: case of orchis,--most remarkable as not long cultivated by seminal propagation. case of varieties which soon acquire, like _Ægilops_ and carrot (and maize) _a certain general character_ and then go on varying. the changes thus appearing during the lives of individual animals and plants are extremely rare compared with those which are congenital or which appear soon after birth. slight differences thus arising are infinitely numerous: the proportions and form of every part of the frame, inside and outside, appear to vary in very slight degrees: anatomists dispute what is the "beau ideal" of the bones, the liver and kidneys, like painters do of the proportions of the face: the proverbial expression that no two animals or plants are born absolutely alike, is much truer when applied to those under domestication, than to those in a state of nature{ }. besides these slight differences, single individuals are occasionally born considerably unlike in certain parts or in their whole structure to their parents: these are called by horticulturists and breeders "sports"; and are not uncommon except when very strongly marked. such sports are known in some cases to have been parents of some of our domestic races; and such probably have been the parents of many other races, especially of those which in some senses may be called hereditary monsters; for instance where there is an additional limb, or where all the limbs are stunted (as in the ancon sheep), or where a part is wanting, as in rumpless fowls and tailless dogs or cats{ }. the effects of external conditions on the size, colour and form, which can rarely and obscurely be detected during one individual life, become apparent after several generations: the slight differences, often hardly describable, which characterize the stock of different countries, and even of districts in the same country, seem to be due to such continued action. { } here, as in the ms. of , the author is inclined to minimise the variation occurring in nature. { } this is more strongly stated than in the _origin_, ed. i. p. . _on the hereditary tendency._ a volume might be filled with facts showing what a strong tendency there is to inheritance, in almost every case of the most trifling, as well as of the most remarkable congenital peculiarities{ }. the term congenital peculiarity, i may remark, is a loose expression and can only mean a peculiarity apparent when the part affected is nearly or fully developed: in the second part, i shall have to discuss at what period of the embryonic life connatal peculiarities probably first appear; and i shall then be able to show from some evidence, that at whatever period of life a new peculiarity first appears, it tends hereditarily to appear at a corresponding period{ }. numerous though slight changes, slowly supervening in animals during mature life (often, though by no means always, taking the form of disease), are, as stated in the first paragraphs, very often hereditary. in plants, again, the buds which assume a different character from their stock likewise tend to transmit their new peculiarities. there is not sufficient reason to believe that either mutilations{ } or changes of form produced by mechanical pressure, even if continued for hundreds of generations, or that any changes of structure quickly produced by disease, are inherited; it would appear as if the tissue of the part affected must slowly and freely grow into the new form, in order to be inheritable. there is a very great difference in the hereditary tendency of different peculiarities, and of the same peculiarity, in different individuals and species; thus twenty thousand seeds of the weeping ash have been sown and not one come up true;--out of seventeen seeds of the weeping yew, nearly all came up true. the ill-formed and almost monstrous "niata" cattle of s. america and ancon sheep, both when bred together and when crossed with other breeds, seem to transmit their peculiarities to their offspring as truly as the ordinary breeds. i can throw no light on these differences in the power of hereditary transmission. breeders believe, and apparently with good cause, that a peculiarity generally becomes more firmly implanted after having passed through several generations; that is if one offspring out of twenty inherits a peculiarity from its parents, then its descendants will tend to transmit this peculiarity to a larger proportion than one in twenty; and so on in succeeding generations. i have said nothing about mental peculiarities being inheritable for i reserve this subject for a separate chapter. { } see _origin_, ed. i. p. . { } _origin_, ed. i. p. , vi. p. . { } it is interesting to find that though the author, like his contemporaries, believed in the inheritance of acquired characters, he excluded the case of mutilation. _causes of variation._ attention must here be drawn to an important distinction in the first origin or appearance of varieties: when we see an animal highly kept producing offspring with an hereditary tendency to early maturity and fatness; when we see the wild-duck and australian dog always becoming, when bred for one or a few generations in confinement, mottled in their colours; when we see people living in certain districts or circumstances becoming subject to an hereditary taint to certain organic diseases, as consumption or plica polonica,--we naturally attribute such changes to the direct effect of known or unknown agencies acting for one or more generations on the parents. it is probable that a multitude of peculiarities may be thus directly caused by unknown external agencies. but in breeds, characterized by an extra limb or claw, as in certain fowls and dogs; by an extra joint in the vertebræ; by the loss of a part, as the tail; by the substitution of a tuft of feathers for a comb in certain poultry; and in a multitude of other cases, we can hardly attribute these peculiarities directly to external influences, but indirectly to the laws of embryonic growth and of reproduction. when we see a multitude of varieties (as has often been the case, where a cross has been carefully guarded against) produced from seeds matured in the very same capsule{ }, with the male and female principle nourished from the same roots and necessarily exposed to the same external influences; we cannot believe that the endless slight differences between seedling varieties thus produced, can be the effect of any corresponding difference in their exposure. we are led (as müller has remarked) to the same conclusion, when we see in the same litter, produced by the same act of conception, animals considerably different. { } this corresponds to _origin_, ed. i. p. , vi. p. . as variation to the degree here alluded to has been observed only in organic beings under domestication, and in plants amongst those most highly and long cultivated, we must attribute, in such cases, the varieties (although the difference between each variety cannot possibly be attributed to any corresponding difference of exposure in the parents) to the indirect effects of domestication on the action of the reproductive system{ }. it would appear as if the reproductive powers failed in their ordinary function of producing new organic beings closely like their parents; and as if the entire organization of the embryo, under domestication, became in a slight degree plastic{ }. we shall hereafter have occasion to show, that in organic beings, a considerable change from the natural conditions of life, affects, independently of their general state of health, in another and remarkable manner the reproductive system. i may add, judging from the vast number of new varieties of plants which have been produced in the same districts and under nearly the same routine of culture, that probably the indirect effects of domestication in making the organization plastic, is a much more efficient source of variation than any direct effect which external causes may have on the colour, texture, or form of each part. in the few instances in which, as in the dahlia{ }, the course of variation has been recorded, it appears that domestication produces little effect for several generations in rendering the organization plastic; but afterwards, as if by an accumulated effect, the original character of the species suddenly gives way or breaks. { } _origin_, ed. i. p. , vi. p. . { } for _plasticity_ see _origin_, ed. i. pp. , . { } _var. under dom._, ed. ii. i. p. . _on selection._ we have hitherto only referred to the first appearance in individuals of new peculiarities; but to make a race or breed, something more is generally{ } requisite than such peculiarities (except in the case of the peculiarities being the direct effect of constantly surrounding conditions) should be inheritable,--namely the principle of selection, implying separation. even in the rare instances of sports, with the hereditary tendency very strongly implanted, crossing must be prevented with other breeds, or if not prevented the best characterized of the half-bred offspring must be carefully selected. where the external conditions are constantly tending to give some character, a race possessing this character will be formed with far greater ease by selecting and breeding together the individuals most affected. in the case of the endless slight variations produced by the indirect effects of domestication on the action of the reproductive system, selection is indispensable to form races; and when carefully applied, wonderfully numerous and diverse races can be formed. selection, though so simple in theory, is and has been important to a degree which can hardly be overrated. it requires extreme skill, the results of long practice, in detecting the slightest difference in the forms of animals, and it implies some distinct object in view; with these requisites and patience, the breeder has simply to watch for every the smallest approach to the desired end, to select such individuals and pair them with the most suitable forms, and so continue with succeeding generations. in most cases careful selection and the prevention of accidental crosses will be necessary for several generations, for in new breeds there is a strong tendency to vary and especially to revert to ancestral forms: but in every succeeding generation less care will be requisite for the breed will become truer; until ultimately only an occasional individual will require to be separated or destroyed. horticulturalists in raising seeds regularly practise this, and call it "roguing," or destroying the "rogues" or false varieties. there is another and less efficient means of selection amongst animals: namely repeatedly procuring males with some desirable qualities, and allowing them and their offspring to breed freely together; and this in the course of time will affect the whole lot. these principles of selection have been _methodically_ followed for scarcely a century; but their high importance is shown by the practical results, and is admitted in the writings of the most celebrated agriculturalists and horticulturalists;--i need only name anderson, marshall, bakewell, coke, western, sebright and knight. { } selection is here used in the sense of isolation, rather than as implying the summation of small differences. professor henslow in his _heredity of acquired characters in plants_, , p. , quotes from darwin's _var. under dom._, ed. i. ii. p. , a passage in which the author, speaking of the direct action of conditions, says:--"a new sub-variety would thus be produced without the aid of selection." darwin certainly did not mean to imply that such varieties are freed from the action of natural selection, but merely that a new form may appear without _summation_ of new characters. professor henslow is apparently unaware that the above passage is omitted in the second edition of _var. under dom._, ii. p. . even in well-established breeds the individuals of which to an unpractised eye would appear absolutely similar, which would give, it might have been thought, no scope to selection, the whole appearance of the animal has been changed in a few years (as in the case of lord western's sheep), so that practised agriculturalists could scarcely credit that a change had not been effected by a cross with other breeds. breeders both of plants and animals frequently give their means of selection greater scope, by crossing different breeds and selecting the offspring; but we shall have to recur to this subject again. the external conditions will doubtless influence and modify the results of the most careful selection; it has been found impossible to prevent certain breeds of cattle from degenerating on mountain pastures; it would probably be impossible to keep the plumage of the wild-duck in the domesticated race; in certain soils, no care has been sufficient to raise cauliflower seed true to its character; and so in many other cases. but with patience it is wonderful what man has effected. he has selected and therefore in one sense made one breed of horses to race and another to pull; he has made sheep with fleeces good for carpets and other sheep good for broadcloth; he has, in the same sense, made one dog to find game and give him notice when found, and another dog to fetch him the game when killed; he has made by selection the fat to lie mixed with the meat in one breed and in another to accumulate in the bowels for the tallow-chandler{ }; he has made the legs of one breed of pigeons long, and the beak of another so short, that it can hardly feed itself; he has previously determined how the feathers on a bird's body shall be coloured, and how the petals of many flowers shall be streaked or fringed, and has given prizes for complete success;--by selection, he has made the leaves of one variety and the flower-buds of another variety of the cabbage good to eat, at different seasons of the year; and thus has he acted on endless varieties. i do not wish to affirm that the long-and short-wooled sheep, or that the pointer and retriever, or that the cabbage and cauliflower have certainly descended from one and the same aboriginal wild stock; if they have not so descended, though it lessens what man has effected, a large result must be left unquestioned. { } see the essay of , p. . in saying as i have done that man makes a breed, let it not be confounded with saying that man makes the individuals, which are given by nature with certain desirable qualities; man only adds together and makes a permanent gift of nature's bounties. in several cases, indeed, for instance in the "ancon" sheep, valuable from not getting over fences, and in the turnspit dog, man has probably only prevented crossing; but in many cases we positively know that he has gone on selecting, and taking advantage of successive small variations. selection{ } has been _methodically_ followed, as i have said, for barely a century; but it cannot be doubted that occasionally it has been practised from the remotest ages, in those animals completely under the dominion of man. in the earliest chapters of the bible there are rules given for influencing the colours of breeds, and black and white sheep are spoken of as separated. in the time of pliny the barbarians of europe and asia endeavoured by cross-breeding with a wild stock to improve the races of their dogs and horses. the savages of guyana now do so with their dogs: such care shows at least that the characters of individual animals were attended to. in the rudest times of english history, there were laws to prevent the exportation of fine animals of established breeds, and in the case of horses, in henry viii's time, laws for the destruction of all horses under a certain size. in one of the oldest numbers of the _phil. transactions_, there are rules for selecting and improving the breeds of sheep. sir h. bunbury, in , has given rules for selecting the finest seedling plants, with as much precision as the best recent horticulturalist could. even in the most savage and rude nations, in the wars and famines which so frequently occur, the most useful of their animals would be preserved: the value set upon animals by savages is shown by the inhabitants of tierra del fuego devouring their old women before their dogs, which as they asserted are useful in otter-hunting{ }: who can doubt but that in every case of famine and war, the best otter-hunters would be preserved, and therefore in fact selected for breeding. as the offspring so obviously take after their parents, and as we have seen that savages take pains in crossing their dogs and horses with wild stocks, we may even conclude as probable that they would sometimes pair the most useful of their animals and keep their offspring separate. as different races of men require and admire different qualities in their domesticated animals, each would thus slowly, though unconsciously, be selecting a different breed. as pallas has remarked, who can doubt but that the ancient russian would esteem and endeavour to preserve those sheep in his flocks which had the thickest coats. this kind of insensible selection by which new breeds are not selected and kept separate, but a peculiar character is slowly given to the whole mass of the breed, by often saving the life of animals with certain characteristics, we may feel nearly sure, from what we see has been done by the more direct method of separate selection within the last years in england, would in the course of some thousand years produce a marked effect. { } see _origin_, ed. i. p. , vi. p. . the evidence is given in the present essay rather more fully than in the _origin_. { } _journal of researches_, ed. , p. . "doggies catch otters, old women no." _crossing breeds._ when once two or more races are formed, or if more than one race, or species fertile _inter se_, originally existed in a wild state, their crossing becomes a most copious source of new races{ }. when two well-marked races are crossed the offspring in the first generation take more or less after either parent or are quite intermediate between them, or rarely assume characters in some degree new. in the second and several succeeding generations, the offspring are generally found to vary exceedingly, one compared with another, and many revert nearly to their ancestral forms. this greater variability in succeeding generations seems analogous to the breaking or variability of organic beings after having been bred for some generations under domestication{ }. so marked is this variability in cross-bred descendants, that pallas and some other naturalists have supposed that all variation is due to an original cross; but i conceive that the history of the potato, dahlia, scotch rose, the guinea-pig, and of many trees in this country, where only one species of the genus exists, clearly shows that a species may vary where there can have been no crossing. owing to this variability and tendency to reversion in cross-bred beings, much careful selection is requisite to make intermediate or new permanent races: nevertheless crossing has been a most powerful engine, especially with plants, where means of propagation exist by which the cross-bred varieties can be secured without incurring the risk of fresh variation from seminal propagation: with animals the most skilful agriculturalists now greatly prefer careful selection from a well-established breed, rather than from uncertain cross-bred stocks. { } the effects of crossing is much more strongly stated here than in the _origin_. see ed. i. p. , vi. p. , where indeed the opposite point of view is given. his change of opinion may be due to his work on pigeons. the whole of the discussion on crossing corresponds to chapter viii of the _origin_, ed. i. rather than to anything in the earlier part of the book. { } the parallelism between the effects of a cross and the effects of conditions is given from a different point of view in the _origin_, ed. i. p. , vi. p. . see the experimental evidence for this important principle in the author's work on _cross and self-fertilisation_. professor bateson has suggested that the experiments should be repeated with gametically pure plants. although intermediate and new races may be formed by the mingling of others, yet if the two races are allowed to mingle quite freely, so that none of either parent race remain pure, then, especially if the parent races are not widely different, they will slowly blend together, and the two races will be destroyed, and one mongrel race left in its place. this will of course happen in a shorter time, if one of the parent races exists in greater number than the other. we see the effect of this mingling, in the manner in which the aboriginal breeds of dogs and pigs in the oceanic islands and the many breeds of our domestic animals introduced into s. america, have all been lost and absorbed in a mongrel race. it is probably owing to the freedom of crossing, that, in uncivilised countries, where inclosures do not exist, we seldom meet with more than one race of a species: it is only in enclosed countries, where the inhabitants do not migrate, and have conveniences for separating the several kinds of domestic animals, that we meet with a multitude of races. even in civilised countries, want of care for a few years has been found to destroy the good results of far longer periods of selection and separation. this power of crossing will affect the races of all _terrestrial_ animals; for all terrestrial animals require for their reproduction the union of two individuals. amongst plants, races will not cross and blend together with so much freedom as in terrestrial animals; but this crossing takes place through various curious contrivances to a surprising extent. in fact such contrivances exist in so very many hermaphrodite flowers by which an occasional cross may take place, that i cannot avoid suspecting (with mr knight) that the reproductive action requires, at _intervals_, the concurrence of distinct individuals{ }. most breeders of plants and animals are firmly convinced that benefit is derived from an occasional cross, not with another race, but with another family of the same race; and that, on the other hand, injurious consequences follow from long-continued close interbreeding in the same family. of marine animals, many more, than was till lately believed, have their sexes on separate individuals; and where they are hermaphrodite, there seems very generally to be means through the water of one individual occasionally impregnating another: if individual animals can singly propagate themselves for perpetuity, it is unaccountable that no terrestrial animal, where the means of observation are more obvious, should be in this predicament of singly perpetuating its kind. i conclude, then, that races of most animals and plants, when unconfined in the same country, would tend to blend together. { } the so-called knight-darwin law is often misunderstood. see goebel in _darwin and modern science_, , p. ; also f. darwin, _nature_, oct. , . _whether our domestic races have descended from one or more wild stocks._ several naturalists, of whom pallas{ } regarding animals, and humboldt regarding certain plants, were the first, believe that the breeds of many of our domestic animals such as of the horse, pig, dog, sheep, pigeon, and poultry, and of our plants have descended from more than one aboriginal form. they leave it doubtful, whether such forms are to be considered wild races, or true species, whose offspring are fertile when crossed _inter se_. the main arguments for this view consist, firstly, of the great difference between such breeds, as the race-and cart-horse, or the greyhound and bull-dog, and of our ignorance of the steps or stages through which these could have passed from a common parent; and secondly that in the most ancient historical periods, breeds resembling some of those at present most different, existed in different countries. the wolves of n. america and of siberia are thought to be different species; and it has been remarked that the dogs belonging to the savages in these two countries resemble the wolves of the same country; and therefore that they have probably descended from two different wild stocks. in the same manner, these naturalists believe that the horse of arabia and of europe have probably descended from two wild stocks both apparently now extinct. i do not think the assumed fertility of these wild stocks any very great difficulty on this view; for although in animals the offspring of most cross-bred species are infertile, it is not always remembered that the experiment is very seldom fairly tried, except when two near species _both_ breed freely (which does not readily happen, as we shall hereafter see) when under the dominion of man. moreover in the case of the china{ } and common goose, the canary and siskin, the hybrids breed freely; in other cases the offspring from hybrids crossed with either pure parent are fertile, as is practically taken advantage of with the yak and cow; as far as the analogy of plants serves, it is impossible to deny that some species are quite fertile _inter se_; but to this subject we shall recur. { } pallas' theory is discussed in the _origin_, ed. i. pp. , , vi. p. . { } see darwin's paper on the fertility of hybrids from the common and chinese goose in _nature_, jan. , . on the other hand, the upholders of the view that the several breeds of dogs, horses, &c., &c., have descended each from one stock, may aver that their view removes all _difficulty about fertility_, and that the main argument from the high antiquity of different breeds, somewhat similar to the present breeds, is worth little without knowing the date of the domestication of such animals, which is far from being the case. they may also with more weight aver that, knowing that organic beings under domestication do vary in some degree, the argument from the great difference between certain breeds is worth nothing, without we know the limits of variation during a long course of time, which is far from the case. they may argue that almost every county in england, and in many districts of other countries, for instance in india, there are slightly different breeds of the domestic animals; and that it is opposed to all that we know of the distribution of wild animals to suppose that these have descended from so many different wild races or species: if so, they may argue, is it not probable that countries quite separate and exposed to different climates would have breeds not slightly, but considerably, different? taking the most favourable case, on both sides, namely that of the dog; they might urge that such breeds as the bull-dog and turnspit have been reared by man, from the ascertained fact that strictly analogous breeds (namely the niata ox and ancon sheep) in other quadrupeds have thus originated. again they may say, seeing what training and careful selection has effected for the greyhound, and seeing how absolutely unfit the italian greyhound is to maintain itself in a state of nature, is it not probable that at least all greyhounds,--from the rough deerhound, the smooth persian, the common english, to the italian,--have descended from one stock{ }? if so, is it so improbable that the deerhound and long-legged shepherd dog have so descended? if we admit this, and give up the bull-dog, we can hardly dispute the probable common descent of the other breeds. { } _origin_, ed. i. p. , vi. p. . the evidence is so conjectural and balanced on both sides that at present i conceive that no one can decide: for my own part, i lean to the probability of most of our domestic animals having descended from more than one wild stock; though from the arguments last advanced and from reflecting on the slow though inevitable effect of different races of mankind, under different circumstances, saving the lives of and therefore selecting the individuals most useful to them, i cannot doubt but that one class of naturalists have much overrated the probable number of the aboriginal wild stocks. as far as we admit the difference of our races <to be> due to the differences of their original stocks, so much must we give up of the amount of variation produced under domestication. but this appears to me unimportant, for we certainly know in some few cases, for instance in the dahlia, and potato, and rabbit, that a great number of varieties have proceeded from one stock; and, in many of our domestic races, we know that man, by slowly selecting and by taking advantage of sudden sports, has considerably modified old races and produced new ones. whether we consider our races as the descendants of one or several wild stocks, we are in far the greater number of cases equally ignorant what these stocks were. _limits to variation in degree and kind._ man's power in making races deends, in the first instance, on the stock on which he works being variable; but his labours are modified and limited, as we have seen, by the direct effects of the external conditions,--by the deficient or imperfect hereditariness of new peculiarities,--and by the tendency to continual variation and especially to reversion to ancestral forms. if the stock is not variable under domestication, of course he can do nothing; and it appears that species differ considerably in this tendency to variation, in the same way as even sub-varieties from the same variety differ greatly in this respect, and transmit to their offspring this difference in tendency. whether the absence of a tendency to vary is an unalterable quality in certain species, or depends on some deficient condition of the particular state of domestication to which they are exposed, there is no evidence. when the organization is rendered variable, or plastic, as i have expressed it, under domestication, different parts of the frame vary more or less in different species: thus in the breeds of cattle it has been remarked that the horns are the most constant or least variable character, for these often remain constant, whilst the colour, size, proportions of the body, tendency to fatten &c., vary; in sheep, i believe, the horns are much more variable. as a general rule the less important parts of the organization seem to vary most, but i think there is sufficient evidence that every part occasionally varies in a slight degree. even when man has the primary requisite variability he is necessarily checked by the health and life of the stock he is working on: thus he has already made pigeons with such small beaks that they can hardly eat and will not rear their own young; he has made families of sheep with so strong a tendency to early maturity and to fatten, that in certain pastures they cannot live from their extreme liability to inflammation; he has made (_i.e._ selected) sub-varieties of plants with a tendency to such early growth that they are frequently killed by the spring frosts; he has made a breed of cows having calves with such large hinder quarters that they are born with great difficulty, often to the death of their mothers{ }; the breeders were compelled to remedy this by the selection of a breeding stock with smaller hinder quarters; in such a case, however, it is possible by long patience and great loss, a remedy might have been found in selecting cows capable of giving birth to calves with large hinder quarters, for in human kind there <are> no doubt hereditary bad and good confinements. besides the limits already specified, there can be little doubt that the variation of different parts of the frame are connected together by many laws{ }: thus the two sides of the body, in health and disease, seem almost always to vary together: it has been asserted by breeders that if the head is much elongated, the bones of the extremities will likewise be so; in seedling-apples large leaves and fruit generally go together, and serve the horticulturalist as some guide in his selection; we can here see the reason, as the fruit is only a metamorphosed leaf. in animals the teeth and hair seem connected, for the hairless chinese dog is almost toothless. breeders believe that one part of the frame or function being increased causes other parts to decrease: they dislike great horns and great bones as so much flesh lost; in hornless breeds of cattle certain bones of the head become more developed: it is said that fat accumulating in one part checks its accumulation in another, and likewise checks the action of the udder. the whole organization is so connected that it is probable there are many conditions determining the variation of each part, and causing other parts to vary with it; and man in making new races must be limited and ruled by all such laws. { } _var. under dom._, ed. ii. vol. ii. p. . { } this discussion corresponds to the _origin_, ed. i. pp. and , vi. pp. and . _in what consists domestication._ in this chapter we have treated of variation under domestication, and it now remains to consider in what does this power of domestication consist{ }, a subject of considerable difficulty. observing that organic beings of almost every class, in all climates, countries, and times, have varied when long bred under domestication, we must conclude that the influence is of some very general nature{ }. mr knight alone, as far as i know, has tried to define it; he believes it consists of an excess of food, together with transport to a more genial climate, or protection from its severities. i think we cannot admit this latter proposition, for we know how many vegetable products, aborigines of this country, here vary, when cultivated without any protection from the weather; and some of our variable trees, as apricots, peaches, have undoubtedly been derived from a more genial climate. there appears to be much more truth in the doctrine of excess of food being the cause, though i much doubt whether this is the sole cause, although it may well be requisite for the kind of variation desired by man, namely increase of size and vigour. no doubt horticulturalists, when they wish to raise new seedlings, often pluck off all the flower-buds, except a few, or remove the whole during one season, so that a great stock of nutriment may be thrown into the flowers which are to seed. when plants are transported from high-lands, forests, marshes, heaths, into our gardens and greenhouses, there must be a considerable change of food, but it would be hard to prove that there was in every case an excess of the kind proper to the plant. if it be an excess of food, compared with that which the being obtained in its natural state{ }, the effects continue for an improbably long time; during how many ages has wheat been cultivated, and cattle and sheep reclaimed, and we cannot suppose their _amount_ of food has gone on increasing, nevertheless these are amongst the most variable of our domestic productions. it has been remarked (marshall) that some of the most highly kept breeds of sheep and cattle are truer or less variable than the straggling animals of the poor, which subsist on commons, and pick up a bare subsistence{ }. in the case of forest-trees raised in nurseries, which vary more than the same trees do in their aboriginal forests, the cause would seem simply to lie in their not having to struggle against other trees and weeds, which in their natural state doubtless would limit the conditions of their existence. it appears to me that the power of domestication resolves itself into the accumulated effects of a change of all or some of the natural conditions of the life of the species, often associated with excess of food. these conditions moreover, i may add, can seldom remain, owing to the mutability of the affairs, habits, migrations, and knowledge of man, for very long periods the same. i am the more inclined to come to this conclusion from finding, as we shall hereafter show, that changes of the natural conditions of existence seem peculiarly to affect the action of the reproductive system{ }. as we see that hybrids and mongrels, after the first generation, are apt to vary much, we may at least conclude that variability does not altogether depend on excess of food. { } see _origin_, ed. i. p. , vi. p. . { } <note in the original.> "isidore g. st hilaire insists that breeding in captivity essential element. schleiden on alkalies. <see _var. under dom._, ed. ii. vol. ii. p. , note .> what is it in domestication which causes variation?" { } <note in the original.> "it appears that slight changes of condition <are> good for health; that more change affects the generative system, so that variation results in the offspring; that still more change checks or destroys fertility not of the offspring." compare the _origin_, ed. i. p. , vi. p. . what the meaning of "not of the offspring" may be is not clear. { } in the _origin_, ed. i. p. , vi. p. the question is differently treated; it is pointed out that a large stock of individuals gives a better chance of available variations occurring. darwin quotes from marshall that sheep in small lots can never be improved. this comes from marshall's _review of the reports to the board of agriculture_, , p. . in this essay the name marshall occurs in the margin. probably this refers to _loc. cit._ p. , where unshepherded sheep in many parts of england are said to be similar owing to mixed breeding not being avoided. { } see _origin_, ed. i. p. , vi. p. . after these views, it may be asked how it comes that certain animals and plants, which have been domesticated for a considerable length of time, and transported from very different conditions of existence, have not varied much, or scarcely at all; for instance, the ass, peacock, guinea-fowl, asparagus, jerusalem artichoke{ }. i have already said that probably different species, like different sub-varieties, possess different degrees of tendency to vary; but i am inclined to attribute in these cases the want of numerous races less to want of variability than to selection not having been practised on them. no one will take the pains to select without some corresponding object, either of use or amusement; the individuals raised must be tolerably numerous, and not so precious, but that he may freely destroy those not answering to his wishes. if guinea-fowls or peacocks{ } became "fancy" birds, i cannot doubt that after some generations several breeds would be raised. asses have not been worked on from mere neglect; but they differ in _some_ degree in different countries. the insensible selection, due to different races of mankind preserving those individuals most useful to them in their different circumstances, will apply only to the oldest and most widely domesticated animals. in the case of plants, we must put entirely out of the case those exclusively (or almost so) propagated by cuttings, layers or tubers, such as the jerusalem artichoke and laurel; and if we put on one side plants of little ornament or use, and those which are used at so early a period of their growth that no especial characters signify, as asparagus{ } and seakale, i can think of none long cultivated which have not varied. in no case ought we to expect to find as much variation in a race when it alone has been formed, as when several have been formed, for their crossing and recrossing will greatly increase their variability. { } see _origin_, ed. i. p. , vi. p. . { } <note in the original.> there are white peacocks. { } <note in the original.> there are varieties of asparagus. _summary of first chapter._ to sum up this chapter. races are made under domestication: st, by the direct effects of the external conditions to which the species is exposed: nd, by the indirect effects of the exposure to new conditions, often aided by excess of food, rendering the organization plastic, and by man's selecting and separately breeding certain individuals, or introducing to his stock selected males, or often preserving with care the life of the individuals best adapted to his purposes: rd, by crossing and recrossing races already made, and selecting their offspring. after some generations man may relax his care in selection: for the tendency to vary and to revert to ancestral forms will decrease, so that he will have only occasionally to remove or destroy one of the yearly offspring which departs from its type. ultimately, with a large stock, the effects of free crossing would keep, even without this care, his breed true. by these means man can produce infinitely numerous races, curiously adapted to ends, both most important and most frivolous; at the same time that the effects of the surrounding conditions, the laws of inheritance, of growth, and of variation, will modify and limit his labours. chapter ii on the variation of organic beings in a wild state; on the natural means of selection; and on the comparison of domestic races and true species having treated of variation under domestication, we now come to it in a _state of nature_. most organic beings in a state of nature vary exceedingly little{ }: i put out of the case variations (as stunted plants &c., and sea-shells in brackish water{ }) which are directly the effect of external agencies and which we do not _know are in the breed_{ }, or are _hereditary_. the amount of hereditary variation is very difficult to ascertain, because naturalists (partly from the want of knowledge, and partly from the inherent difficulty of the subject) do not all agree whether certain forms are species or races{ }. some strongly marked races of plants, comparable with the decided sports of horticulturalists, undoubtedly exist in a state of nature, as is actually known by experiment, for instance in the primrose and cowslip{ }, in two so-called species of dandelion, in two of foxglove{ }, and i believe in some pines. lamarck has observed that, as long as we confine our attention to one limited country, there is seldom much difficulty in deciding what forms to call species and what varieties; and that it is when collections flow in from all parts of the world that naturalists often feel at a loss to decide the limit of variation. undoubtedly so it is, yet amongst british plants (and i may add land shells), which are probably better known than any in the world, the best naturalists differ very greatly in the relative proportions of what they call species and what varieties. in many genera of insects, and shells, and plants, it seems almost hopeless to establish which are which. in the higher classes there are less doubts; though we find considerable difficulty in ascertaining what deserve to be called species amongst foxes and wolves, and in some birds, for instance in the case of the white barn-owl. when specimens are brought from different parts of the world, how often do naturalists dispute this same question, as i found with respect to the birds brought from the galapagos islands. yarrell has remarked that the individuals of the same undoubted species of birds, from europe and n. america, usually present slight, indefinable though perceptible differences. the recognition indeed of one animal by another of its kind seems to imply some difference. the disposition of wild animals undoubtedly differs. the variation, such as it is, chiefly affects the same parts in wild organisms as in domestic breeds; for instance, the size, colour, and the external and less important parts. in many species the variability of certain organs or qualities is even stated as one of the specific characters: thus, in plants, colour, size, hairiness, the number of the stamens and pistils, and even their presence, the form of the leaves; the size and form of the mandibles of the males of some insects; the length and curvature of the beak in some birds (as in opetiorynchus) are variable characters in some species and quite fixed in others. i do not perceive that any just distinction can be drawn between this recognised variability of certain parts in many species and the more general variability of the whole frame in domestic races. { } in chapter ii of the first edition of the _origin_ darwin insists rather on the presence of variability in a state of nature; see, for instance, p. , ed. vi. p. , "i am convinced that the most experienced naturalist would be surprised at the number of the cases of variability ... which he could collect on good authority, as i have collected, during a course of years." { } see _origin_, ed. i. p. , vi. p. . { } <note in the original.> here discuss _what is a species_, sterility can most rarely be told when crossed.--descent from common stock. { } <note in the original.> give only rule: chain of intermediate forms, and _analogy_; this important. every naturalist at first when he gets hold of new variable type is _quite puzzled_ to know what to think species and what variations. { } the author had not at this time the knowledge of the meaning of dimorphism. { } <note in original.> compare feathered heads in very different birds with spines in echidna and hedgehog. <in _variation under domestication_, ed. ii. vol. ii. p. , darwin calls attention to laced and frizzled breeds occurring in both fowls and pigeons. in the same way a peculiar form of covering occurs in echidna and the hedgehog.> plants under very different climate not varying. digitalis shows jumps <?> in variation, like laburnum and orchis case--in fact hostile cases. variability of sexual characters alike in domestic and wild. although the amount of variation be exceedingly small in most organic beings in a state of nature, and probably quite wanting (as far as our senses serve) in the majority of cases; yet considering how many animals and plants, taken by mankind from different quarters of the world for the most diverse purposes, have varied under domestication in every country and in every age, i think we may safely conclude that all organic beings with few exceptions, if capable of being domesticated and bred for long periods, would vary. domestication seems to resolve itself into a change from the natural conditions of the species [generally perhaps including an increase of food]; if this be so, organisms in a state of nature must _occasionally_, in the course of ages, be exposed to analogous influences; for geology clearly shows that many places must, in the course of time, become exposed to the widest range of climatic and other influences; and if such places be isolated, so that new and better adapted organic beings cannot freely emigrate, the old inhabitants will be exposed to new influences, probably far more varied, than man applies under the form of domestication. although every species no doubt will soon breed up to the full number which the country will support, yet it is easy to conceive that, on an average, some species may receive an increase of food; for the times of dearth may be short, yet enough to kill, and recurrent only at long intervals. all such changes of conditions from geological causes would be exceedingly slow; what effect the slowness might have we are ignorant; under domestication it appears that the effects of change of conditions accumulate, and then break out. whatever might be the result of these slow geological changes, we may feel sure, from the means of dissemination common in a lesser or greater degree to every organism taken conjointly with the changes of geology, which are steadily (and sometimes suddenly, as when an isthmus at last separates) in progress, that occasionally organisms must suddenly be introduced into new regions, where, if the conditions of existence are not so foreign as to cause its extermination, it will often be propagated under circumstances still more closely analogous to those of domestication; and therefore we expect will evince a tendency to vary. it appears to me quite _inexplicable_ if this has never happened; but it can happen very rarely. let us then suppose that an organism by some chance (which might be hardly repeated in years) arrives at a modern volcanic island in process of formation and not fully stocked with the most appropriate organisms; the new organism might readily gain a footing, although the external conditions were considerably different from its native ones. the effect of this we might expect would influence in some small degree the size, colour, nature of covering &c., and from inexplicable influences even special parts and organs of the body. but we might further (and <this> is far more important) expect that the reproductive system would be affected, as under domesticity, and the structure of the offspring rendered in some degree plastic. hence almost every part of the body would tend to vary from the typical form in slight degrees, and in no determinate way, and therefore _without selection_ the free crossing of these small variations (together with the tendency to reversion to the original form) would constantly be counteracting this unsettling effect of the extraneous conditions on the reproductive system. such, i conceive, would be the unimportant result without selection. and here i must observe that the foregoing remarks are equally applicable to that small and admitted amount of variation which has been observed in some organisms in a state of nature; as well as to the above hypothetical variation consequent on changes of condition. let us now suppose a being{ } with penetration sufficient to perceive differences in the outer and innermost organization quite imperceptible to man, and with forethought extending over future centuries to watch with unerring care and select for any object the offspring of an organism produced under the foregoing circumstances; i can see no conceivable reason why he could not form a new race (or several were he to separate the stock of the original organism and work on several islands) adapted to new ends. as we assume his discrimination, and his forethought, and his steadiness of object, to be incomparably greater that those qualities in man, so we may suppose the beauty and complications of the adaptations of the new races and their differences from the original stock to be greater than in the domestic races produced by man's agency: the ground-work of his labours we may aid by supposing that the external conditions of the volcanic island, from its continued emergence and the occasional introduction of new immigrants, vary; and thus to act on the reproductive system of the organism, on which he is at work, and so keep its organization somewhat plastic. with time enough, such a being might rationally (without some unknown law opposed him) aim at almost any result. { } a corresponding passage occurs in _origin_, ed. i. p. , vi. p. , where however nature takes the place of the selecting being. for instance, let this imaginary being wish, from seeing a plant growing on the decaying matter in a forest and choked by other plants, to give it power of growing on the rotten stems of trees, he would commence selecting every seedling whose berries were in the smallest degree more attractive to tree-frequenting birds, so as to cause a proper dissemination of the seeds, and at the same time he would select those plants which had in the slightest degree more and more power of drawing nutriment from rotten wood; and he would destroy all other seedlings with less of this power. he might thus, in the course of century after century, hope to make the plant by degrees grow on rotten wood, even high up on trees, wherever birds dropped the non-digested seeds. he might then, if the organization of the plant was plastic, attempt by continued selection of chance seedlings to make it grow on less and less rotten wood, till it would grow on sound wood{ }. supposing again, during these changes the plant failed to seed quite freely from non-impregnation, he might begin selecting seedlings with a little sweeter <or> differently tasted honey or pollen, to tempt insects to visit the flowers regularly: having effected this, he might wish, if it profited the plant, to render abortive the stamens and pistils in different flowers, which he could do by continued selection. by such steps he might aim at making a plant as wonderfully related to other organic beings as is the mistletoe, whose existence absolutely depends on certain insects for impregnation, certain birds for transportal, and certain trees for growth. furthermore, if the insect which had been induced regularly to visit this hypothetical plant profited much by it, our same being might wish by selection to modify by gradual selection the insect's structure, so as to facilitate its obtaining the honey or pollen: in this manner he might adapt the insect (always presupposing its organization to be in some degree plastic) to the flower, and the impregnation of the flower to the insect; as is the case with many bees and many plants. { } the mistletoe is used as an illustration in _origin_, ed. i. p. , vi. p. , but with less detail. seeing what blind capricious man has actually effected by selection during the few last years, and what in a ruder state he has probably effected without any systematic plan during the last few thousand years, he will be a bold person who will positively put limits to what the supposed being could effect during whole geological periods. in accordance with the plan by which this universe seems governed by the creator, let us consider whether there exists any _secondary_ means in the economy of nature by which the process of selection could go on adapting, nicely and wonderfully, organisms, if in ever so small a degree plastic, to diverse ends. i believe such secondary means do exist{ }. { } <note in original.> the selection, in cases where adult lives only few hours as ephemera, must fall on larva--curious speculation of the effect <which> changes in it would bring in parent. _natural means of selection{ }._ { } this section forms part of the joint paper by darwin and wallace read before the linnean society on july , . de candolle, in an eloquent passage, has declared that all nature is at war, one organism with another, or with external nature. seeing the contented face of nature, this may at first be well doubted; but reflection will inevitably prove it is too true. the war, however, is not constant, but only recurrent in a slight degree at short periods and more severely at occasional more distant periods; and hence its effects are easily overlooked. it is the doctrine of malthus applied in most cases with ten-fold force. as in every climate there are seasons for each of its inhabitants of greater and less abundance, so all annually breed; and the moral restraint, which in some small degree checks the increase of mankind, is entirely lost. even slow-breeding mankind has doubled in years{ }, and if he could increase his food with greater ease, he would double in less time. but for animals, without artificial means, _on an average_ the amount of food for each species must be constant; whereas the increase of all organisms tends to be geometrical, and in a vast majority of cases at an enormous ratio. suppose in a certain spot there are eight pairs of [robins] birds, and that _only_ four pairs of them annually (including double hatches) rear only four young; and that these go on rearing their young at the same rate: then at the end of seven years (a short life, excluding violent deaths, for any birds) there will be robins, instead of the original sixteen; as this increase is quite impossible, so we must conclude either that robins do not rear nearly half their young or that the average life of a robin when reared is from accident not nearly seven years. both checks probably concur. the same kind of calculation applied to all vegetables and animals produces results either more or less striking, but in scarcely a single instance less striking than in man{ }. { } occurs in _origin_, ed. i. p. , vi. p. . { } corresponds approximately with _origin_, ed. i. pp. - , vi. p. . many practical illustrations of this rapid tendency to increase are on record, namely during peculiar seasons, in the extraordinary increase of certain animals, for instance during the years to , in la plata, when from drought, some millions of cattle perished, the whole country _swarmed_ with innumerable mice: now i think it cannot be doubted that during the breeding season all the mice (with the exception of a few males or females in excess) ordinarily pair; and therefore that this astounding increase during three years must be attributed to a greater than usual number surviving the first year, and then breeding, and so on, till the third year, when their numbers were brought down to their usual limits on the return of wet weather. where man has introduced plants and animals into a new country favourable to them, there are many accounts in how surprisingly few years the whole country has become stocked with them. this increase would necessarily stop as soon as the country was fully stocked; and yet we have every reason to believe from what is known of wild animals that _all_ would pair in the spring. in the majority of cases it is most difficult to imagine where the check falls, generally no doubt on the seeds, eggs, and young; but when we remember how impossible even in mankind (so much better known than any other animal) it is to infer from repeated casual observations what the average of life is, or to discover how different the percentage of deaths to the births in different countries, we ought to feel no legitimate surprise at not seeing where the check falls in animals and plants. it should always be remembered that in most cases the checks are yearly recurrent in a small regular degree, and in an extreme degree during occasionally unusually cold, hot, dry, or wet years, according to the constitution of the being in question. lighten any check in the smallest degree, and the geometrical power of increase in every organism will instantly increase the average numbers of the favoured species. nature may be compared to a surface, on which rest ten thousand sharp wedges touching each other and driven inwards by incessant blows{ }. fully to realise these views much reflection is requisite; malthus on man should be studied; and all such cases as those of the mice in la plata, of the cattle and horses when first turned out in s. america, of the robins by our calculation, &c., should be well considered: reflect on the enormous multiplying power _inherent and annually in action_ in all animals; reflect on the countless seeds scattered by a hundred ingenious contrivances, year after year, over the whole face of the land; and yet we have every reason to suppose that the average percentage of every one of the inhabitants of a country will _ordinarily_ remain constant. finally, let it be borne in mind that this average number of individuals (the external conditions remaining the same) in each country is kept up by recurrent struggles against other species or against external nature (as on the borders of the arctic regions{ }, where the cold checks life); and that ordinarily each individual of each species holds its place, either by its own struggle and capacity of acquiring nourishment in some period (from the egg upwards) of its life, or by the struggle of its parents (in short lived organisms, when the main check occurs at long intervals) against and compared with other individuals of the _same_ or _different_ species. { } this simile occurs in _origin_, ed. i. p. , not in the later editions. { } <note in the original.> in case like mistletoe, it may be asked why not more species, no other species interferes; answer almost sufficient, same causes which check the multiplication of individuals. but let the external conditions of a country change; if in a small degree, the relative proportions of the inhabitants will in most cases simply be slightly changed; but let the number of inhabitants be small, as in an island{ }, and free access to it from other countries be circumscribed; and let the change of condition continue progressing (forming new stations); in such case the original inhabitants must cease to be so perfectly adapted to the changed conditions as they originally were. it has been shown that probably such changes of external conditions would, from acting on the reproductive system, cause the organization of the beings most affected to become, as under domestication, plastic. now can it be doubted from the struggle each individual (or its parents) has to obtain subsistence that any minute variation in structure, habits, or instincts, adapting that individual better to the new conditions, would tell upon its vigour and health? in the struggle it would have a better _chance_ of surviving, and those of its offspring which inherited the variation, let it be ever so slight, would have a better _chance_ to survive. yearly more are bred than can survive; the smallest grain in the balance, in the long run, must tell on which death shall fall, and which shall survive{ }. let this work of selection, on the one hand, and death on the other, go on for a thousand generations; who would pretend to affirm that it would produce no effect, when we remember what in a few years bakewell effected in cattle and western in sheep, by this identical principle of selection. { } see _origin_, ed. i. pp. , , vi. pp. , . { } recognition of the importance of minute differences in the struggle occurs in the essay of , p. note .{note } to give an imaginary example, from changes in progress on an island, let the organization{ } of a canine animal become slightly plastic, which animal preyed chiefly on rabbits, but sometimes on hares; let these same changes cause the number of rabbits very slowly to decrease and the number of hares to increase; the effect of this would be that the fox or dog would be driven to try to catch more hares, and his numbers would tend to decrease; his organization, however, being slightly plastic, those individuals with the lightest forms, longest limbs, and best eye-sight (though perhaps with less cunning or scent) would be slightly favoured, let the difference be ever so small, and would tend to live longer and to survive during that time of the year when food was shortest; they would also rear more young, which young would tend to inherit these slight peculiarities. the less fleet ones would be rigidly destroyed. i can see no more reason to doubt but that these causes in a thousand generations would produce a marked effect, and adapt the form of the fox to catching hares instead of rabbits, than that greyhounds can be improved by selection and careful breeding. so would it be with plants under similar circumstances; if the number of individuals of a species with plumed seeds could be increased by greater powers of dissemination within its own area (that is if the check to increase fell chiefly on the seeds), those seeds which were provided with ever so little more down, or with a plume placed so as to be slightly more acted on by the winds, would in the long run tend to be most disseminated; and hence a greater number of seeds thus formed would germinate, and would tend to produce plants inheriting this slightly better adapted down. { } see _origin_, ed. i. p. , vi. p. . besides this natural means of selection, by which those individuals are preserved, whether in their egg or seed or in their mature state, which are best adapted to the place they fill in nature, there is a second agency at work in most bisexual animals tending to produce the same effect, namely the struggle of the males for the females. these struggles are generally decided by the law of battle; but in the case of birds, apparently, by the charms of their song{ }, by their beauty or their power of courtship, as in the dancing rock-thrush of guiana. even in the animals which pair there seems to be an excess of males which would aid in causing a struggle: in the polygamous animals{ }, however, as in deer, oxen, poultry, we might expect there would be severest struggle: is it not in the polygamous animals that the males are best formed for mutual war? the most vigorous males, implying perfect adaptation, must generally gain the victory in their several contests. this kind of selection, however, is less rigorous than the other; it does not require the death of the less successful, but gives to them fewer descendants. this struggle falls, moreover, at a time of year when food is generally abundant, and perhaps the effect chiefly produced would be the alteration of sexual characters, and the selection of individual forms, no way related to their power of obtaining food, or of defending themselves from their natural enemies, but of fighting one with another. this natural struggle amongst the males may be compared in effect, but in a less degree, to that produced by those agriculturalists who pay less attention to the careful selection of all the young animals which they breed and more to the occasional use of a choice male{ }. { } these two forms of sexual selection are given in _origin_, ed. i. p. , vi. p. . the guiana rock-thrush is given as an example of bloodless competition. { } <note in original.> seals? pennant about battles of seals. { } in the linnean paper of july , the final word is _mate_: but the context shows that it should be _male_; it is moreover clearly so written in the ms. _differences between "races" and "species":--first, in their trueness or variability._ races{ } produced by these natural means of selection{ } we may expect would differ in some respects from those produced by man. man selects chiefly by the eye, and is not able to perceive the course of every vessel and nerve, or the form of the bones, or whether the internal structure corresponds to the outside shape. he{ } is unable to select shades of constitutional differences, and by the protection he affords and his endeavours to keep his property alive, in whatever country he lives, he checks, as much as lies in his power, the selecting action of nature, which will, however, go on to a lesser degree with all living things, even if their length of life is not determined by their own powers of endurance. he has bad judgment, is capricious, he does not, or his successors do not, wish to select for the same exact end for hundreds of generations. he cannot always suit the selected form to the properest conditions; nor does he keep those conditions uniform: he selects that which is useful to him, not that best adapted to those conditions in which each variety is placed by him: he selects a small dog, but feeds it highly; he selects a long-backed dog, but does not exercise it in any peculiar manner, at least not during every generation. he seldom allows the most vigorous males to struggle for themselves and propagate, but picks out such as he possesses, or such as he prefers, and not necessarily those best adapted to the existing conditions. every agriculturalist and breeder knows how difficult it is to prevent an occasional cross with another breed. he often grudges to destroy an individual which departs considerably from the required type. he often begins his selection by a form or sport considerably departing from the parent form. very differently does the natural law of selection act; the varieties selected differ only slightly from the parent forms{ }; the conditions are constant for long periods and change slowly; rarely can there be a cross; the selection is rigid and unfailing, and continued through many generations; a selection can _never be made_ without the form be _better_ adapted to the conditions than the parent form; the selecting power goes on without caprice, and steadily for thousands of years adapting the form to these conditions. the selecting power is not deceived by external appearances, it tries the being during its whole life; and if less well <?> adapted than its _congeners_, without fail it is destroyed; every part of its structure is thus scrutinised and proved good towards the place in nature which it occupies. { } in the _origin_ the author would here have used the word _variety_. { } the whole of p. and lines of p. are, in the ms., marked through in pencil with vertical lines, beginning at "races produced, &c." and ending with "to these conditions." { } see _origin_, ed. i. p. , vi. p. . { } in the present essay there is some evidence that the author attributed more to _sports_ than was afterwards the case: but the above passage points the other way. it must always be remembered that many of the minute differences, now considered small mutations, are the small variations on which darwin conceived selection to act. we have every reason to believe that in proportion to the number of generations that a domestic race is kept free from crosses, and to the care employed in continued steady selection with one end in view, and to the care in not placing the variety in conditions unsuited to it; in such proportion does the new race become "true" or subject to little variation{ }. how incomparably "truer" then would a race produced by the above rigid, steady, natural means of selection, excellently trained and perfectly adapted to its conditions, free from stains of blood or crosses, and continued during thousands of years, be compared with one produced by the feeble, capricious, misdirected and ill-adapted selection of man. those races of domestic animals produced by savages, partly by the inevitable conditions of their life, and partly unintentionally by their greater care of the individuals most valuable to them, would probably approach closest to the character of a species; and i believe this is the case. now the characteristic mark of a species, next, if not equal in importance to its sterility when crossed with another species, and indeed almost the only other character (without we beg the question and affirm the essence of a species, is its not having descended from a parent common to any other form), is the similarity of the individuals composing the species, or in the language of agriculturalists their "trueness." { } see _var. under dom._, ed. ii. vol. ii. p. . _difference between "races" and "species" in fertility when crossed._ the sterility of species, or of their offspring, when crossed has, however, received more attention than the uniformity in character of the individuals composing the species. it is exceedingly natural that such sterility{ } should have been long thought the certain characteristic of species. for it is obvious that if the allied different forms which we meet with in the same country could cross together, instead of finding a number of distinct species, we should have a confused and blending series. the fact however of a perfect gradation in the degree of sterility between species, and the circumstance of some species most closely allied (for instance many species of crocus and european heaths) refusing to breed together, whereas other species, widely different, and even belonging to distinct genera, as the fowl and the peacock, pheasant and grouse{ }, azalea and rhododendron, thuja and juniperus, breeding together ought to have caused a doubt whether the sterility did not depend on other causes, distinct from a law, coincident with their creation. i may here remark that the fact whether one species will or will not breed with another is far less important than the sterility of the offspring when produced; for even some domestic races differ so greatly in size (as the great stag-greyhound and lap-dog, or cart-horse and burmese ponies) that union is nearly impossible; and what is less generally known is, that in plants kölreuter has shown by hundreds of experiments that the pollen of one species will fecundate the germen of another species, whereas the pollen of this latter will never act on the germen of the former; so that the simple fact of mutual impregnation certainly has no relation whatever to the distinctness in creation of the two forms. when two species are attempted to be crossed which are so distantly allied that offspring are never produced, it has been observed in some cases that the pollen commences its proper action by exserting its tube, and the germen commences swelling, though soon afterwards it decays. in the next stage in the series, hybrid offspring are produced though only rarely and few in number, and these are absolutely sterile: then we have hybrid offspring more numerous, and occasionally, though very rarely, breeding with either parent, as is the case with the common mule. again, other hybrids, though infertile _inter se_, will breed _quite_ freely with either parent, or with a third species, and will yield offspring generally infertile, but sometimes fertile; and these latter again will breed with either parent, or with a third or fourth species: thus kölreuter blended together many forms. lastly it is now admitted by those botanists who have longest contended against the admission, that in certain families the hybrid offspring of many of the species are sometimes perfectly fertile in the first generation when bred together: indeed in some few cases mr herbert{ } found that the hybrids were decidedly more fertile than either of their pure parents. there is no way to escape from the admission that the hybrids from some species of plants are fertile, except by declaring that no form shall be considered as a species, if it produces with another species fertile offspring: but this is begging the question{ }. it has often been stated that different species of animals have a sexual repugnance towards each other; i can find no evidence of this; it appears as if they merely did not excite each others passions. i do not believe that in this respect there is any essential distinction between animals and plants; and in the latter there cannot be a feeling of repugnance. { } <note in the original.> if domestic animals are descended from several species and _become_ fertile _inter se_, then one can see they gain fertility by becoming adapted to new conditions and certainly domestic animals can withstand changes of climate without loss of fertility in an astonishing manner. { } see suchetet, _l'hybridité dans la nature_, bruxelles, , p. . in _var. under dom._, ed. ii. vol. ii. hybrids between the fowl and the pheasant are mentioned. i can give no information on the other cases. { } _origin_, ed. i. p. , vi. p. . { } this was the position of gärtner and of kölreuter: see _origin_, ed. i. pp. - , vi. pp. - . _causes of sterility in hybrids._ the difference in nature between species which causes the greater or lesser degree of sterility in their offspring appears, according to herbert and kölreuter, to be connected much less with external form, size, or structure, than with constitutional peculiarities; by which is meant their adaptation to different climates, food and situation, &c.: these peculiarities of constitution probably affect the entire frame, and no one part in particular{ }. { } <note in the original.> yet this seems introductory to the case of the heaths and crocuses above mentioned. <herbert observed that crocus does not set seed if transplanted before pollination, but that such treatment after pollination has no sterilising effect. (_var. under dom._, ed. ii. vol. ii. p. .) on the same page is a mention of the ericaceæ being subject to contabescence of the anthers. for _crinum_ see _origin_, ed. i. p. : for _rhododenron_ and _calceolaria_ see p. .> from the foregoing facts i think we must admit that there exists a perfect gradation in fertility between species which when crossed are quite fertile (as in rhododendron, calceolaria, &c.), and indeed in an extraordinary degree fertile (as in crinum), and those species which never produce offspring, but which by certain effects (as the exsertion of the pollen-tube) evince their alliance. hence, i conceive, we must give up sterility, although undoubtedly in a lesser or greater degree of very frequent occurrence, as an unfailing mark by which _species_ can be distinguished from _races_, _i.e._ from those forms which have descended from a common stock. _infertility from causes distinct from hybridisation._ let us see whether there are any analogous facts which will throw any light on this subject, and will tend to explain why the offspring of certain species, when crossed, should be sterile, and not others, without requiring a distinct law connected with their creation to that effect. great numbers, probably a large majority of animals when caught by man and removed from their natural conditions, although taken very young, rendered quite tame, living to a good old age, and apparently quite healthy, seem incapable under these circumstances of breeding{ }. i do not refer to animals kept in menageries, such as at the zoological gardens, many of which, however, appear healthy and live long and unite but do not produce; but to animals caught and left partly at liberty in their native country. rengger{ } enumerates several caught young and rendered tame, which he kept in paraguay, and which would not breed: the hunting leopard or cheetah and elephant offer other instances; as do bears in europe, and the species of hawks, belonging to different genera, thousands of which have been kept for hawking and have lived for long periods in perfect vigour. when the expense and trouble of procuring a succession of young animals in a wild state be borne in mind, one may feel sure that no trouble has been spared in endeavours to make them breed. so clearly marked is this difference in different kinds of animals, when captured by man, that st hilaire makes two great classes of animals useful to man:--the _tame_, which will not breed, and the _domestic_ which will breed in domestication. from certain singular facts we might have supposed that the non-breeding of animals was owing to some perversion of instinct. but we meet with exactly the same class of facts in plants: i do not refer to the large number of cases where the climate does not permit the seed or fruit to ripen, but where the flowers do not "set," owing to some imperfection of the ovule or pollen. the latter, which alone can be distinctly examined, is often manifestly imperfect, as any one with a microscope can observe by comparing the pollen of the persian and chinese lilacs{ } with the common lilac; the two former species (i may add) are equally sterile in italy as in this country. many of the american bog plants here produce little or no pollen, whilst the indian species of the same genera freely produce it. lindley observes that sterility is the bane of the horticulturist{ }: linnæus has remarked on the sterility of nearly all alpine flowers when cultivated in a lowland district{ }. perhaps the immense class of double flowers chiefly owe their structure to an excess of food acting on parts rendered slightly sterile and less capable of performing their true function, and therefore liable to be rendered monstrous, which monstrosity, like any other disease, is inherited and rendered common. so far from domestication being in itself unfavourable to fertility, it is well known that when an organism is once capable of submission to such conditions <its> fertility is increased{ } beyond the natural limit. according to agriculturists, slight changes of conditions, that is of food or habitation, and likewise crosses with races slightly different, increase the vigour and probably the fertility of their offspring. it would appear also that even a great change of condition, for instance, transportal from temperate countries to india, in many cases does not in the least affect fertility, although it does health and length of life and the period of maturity. when sterility is induced by domestication it is of the same kind, and varies in degree, exactly as with hybrids: for be it remembered that the most sterile hybrid is no way monstrous; its organs are perfect, but they do not act, and minute microscopical investigations show that they are in the same state as those of pure species in the intervals of the breeding season. the defective pollen in the cases above alluded to precisely resembles that of hybrids. the occasional breeding of hybrids, as of the common mule, may be aptly compared to the most rare but occasional reproduction of elephants in captivity. the cause of many exotic geraniums producing (although in vigorous health) imperfect pollen seems to be connected with the period when water is given them{ }; but in the far greater majority of cases we cannot form any conjecture on what exact cause the sterility of organisms taken from their natural conditions depends. why, for instance, the cheetah will not breed whilst the common cat and ferret (the latter generally kept shut up in a small box) do,--why the elephant will not whilst the pig will abundantly--why the partridge and grouse in their own country will not, whilst several species of pheasants, the guinea-fowl from the deserts of africa and the peacock from the jungles of india, will. we must, however, feel convinced that it depends on some constitutional peculiarities in these beings not suited to their new condition; though not necessarily causing an ill state of health. ought we then to wonder much that those hybrids which have been produced by the crossing of species with different constitutional tendencies (which tendencies we know to be eminently inheritable) should be sterile: it does not seem improbable that the cross from an alpine and lowland plant should have its constitutional powers deranged, in nearly the same manner as when the parent alpine plant is brought into a lowland district. analogy, however, is a deceitful guide, and it would be rash to affirm, although it may appear probable, that the sterility of hybrids is due to the constitutional peculiarities of one parent being disturbed by being blended with those of the other parent in exactly the same manner as it is caused in some organic beings when placed by man out of their natural conditions{ }. although this would be rash, it would, i think, be still rasher, seeing that sterility is no more incidental to _all_ cross-bred productions than it is to all organic beings when captured by man, to assert that the sterility of certain hybrids proved a distinct creation of their parents. { } <note in original.> animals seem more often made sterile by being taken out of their native condition than plants, and so are more sterile when crossed. we have one broad fact that sterility in hybrids is not closely related to external difference, and these are what man alone gets by selection. { } see _var. under dom._, ed. ii. vol. ii. p. ; for the case of the cheetah see _loc cit._ p. . { } _var. under dom._, ed. ii. vol. ii. p. . { } quoted in the _origin_, ed. i. p. . { } see _var. under dom._, ed. ii. vol. ii. p. . { } _var. under dom._, ed. ii. vol. ii. p. . { } see _var. under dom._, ed. ii. vol. ii. p. . { } _origin_, ed. i. p. , vi. p. . this is the principle experimentally investigated in the author's _cross-and self-fertilisation_. but it may be objected{ } (however little the sterility of certain hybrids is connected with the distinct creations of species), how comes it, if species are only races produced by natural selection, that when crossed they so frequently produce sterile offspring, whereas in the offspring of those races confessedly produced by the arts of man there is no one instance of sterility. there is not much difficulty in this, for the races produced by the natural means above explained will be slowly but steadily selected; will be adapted to various and diverse conditions, and to these conditions they will be rigidly confined for immense periods of time; hence we may suppose that they would acquire different constitutional peculiarities adapted to the stations they occupy; and on the constitutional differences between species their sterility, according to the best authorities, depends. on the other hand man selects by external appearance{ }; from his ignorance, and from not having any test at least comparable in delicacy to the natural struggle for food, continued at intervals through the life of each individual, he cannot eliminate fine shades of constitution, dependent on invisible differences in the fluids or solids of the body; again, from the value which he attaches to each individual, he asserts his utmost power in contravening the natural tendency of the most vigorous to survive. man, moreover, especially in the earlier ages, cannot have kept his conditions of life constant, and in later ages his stock pure. until man selects two varieties from the same stock, adapted to two climates or to other different external conditions, and confines each rigidly for one or several thousand years to such conditions, always selecting the individuals best adapted to them, he cannot be said to have even commenced the experiment. moreover, the organic beings which man has longest had under domestication have been those which were of the greatest use to him, and one chief element of their usefulness, especially in the earlier ages, must have been their capacity to undergo sudden transportals into various climates, and at the same time to retain their fertility, which in itself implies that in such respects their constitutional peculiarities were not closely limited. if the opinion already mentioned be correct, that most of the domestic animals in their present state have descended from the fertile commixture of wild races or species, we have indeed little reason now to expect infertility between any cross of stock thus descended. { } _origin_, ed. i. p. , vi. p. . { } <notes in original.> mere difference of structure no guide to what will or will not cross. first step gained by races keeping apart. <it is not clear where these notes were meant to go.> it is worthy of remark, that as many organic beings, when taken by man out of their natural conditions, have their reproductive system <so> affected as to be incapable of propagation, so, we saw in the first chapter, that although organic beings when taken by man do propagate freely, their offspring after some generations vary or sport to a degree which can only be explained by their reproductive system being <in> some way affected. again, when species cross, their offspring are generally sterile; but it was found by kölreuter that when hybrids are capable of breeding with either parent, or with other species, that their offspring are subject after some generations to excessive variation{ }. agriculturists, also, affirm that the offspring from mongrels, after the first generation, vary much. hence we see that both sterility and variation in the succeeding generations are consequent both on the removal of individual species from their natural states and on species crossing. the connection between these facts may be accidental, but they certainly appear to elucidate and support each other,--on the principle of the reproductive system of all organic beings being eminently sensitive to any disturbance, whether from removal or commixture, in their constitutional relations to the conditions to which they are exposed. { } _origin_, ed. i. p. , vi. p. . _points of resemblance between "races" and "species{ }."_ { } this section seems not to correspond closely with any in the _origin_, ed. i.; in some points it resembles pp. , , also the section on analogous variation in distinct species, _origin_, ed. i. p. , vi. p. . races and reputed species agree in some respects, although differing from causes which, we have seen, we can in some degree understand, in the fertility and "trueness" of their offspring. in the first place, there is no clear sign by which to distinguish races from species, as is evident from the great difficulty experienced by naturalists in attempting to discriminate them. as far as external characters are concerned, many of the races which are descended from the same stock differ far more than true species of the same genus; look at the willow-wrens, some of which skilful ornithologists can hardly distinguish from each other except by their nests; look at the wild swans, and compare the distinct species of these genera with the races of domestic ducks, poultry, and pigeons; and so again with plants, compare the cabbages, almonds, peaches and nectarines, &c. with the species of many genera. st hilaire has even remarked that there is a greater difference in size between races, as in dogs (for he believes all have descended from one stock), than between the species of any one genus; nor is this surprising, considering that amount of food and consequently of growth is the element of change over which man has most power. i may refer to a former statement, that breeders believe the growth of one part or strong action of one function causes a decrease in other parts; for this seems in some degree analogous to the law of "organic compensation{ }," which many naturalists believe holds good. to give an instance of this law of compensation,--those species of carnivora which have the canine teeth greatly developed have certain molar teeth deficient; or again, in that division of the crustaceans in which the tail is much developed, the thorax is little so, and the converse. the points of difference between different races is often strikingly analogous to that between species of the same genus: trifling spots or marks of colour{ } (as the bars on pigeons' wings) are often preserved in races of plants and animals, precisely in the same manner as similar trifling characters often pervade all the species of a genus, and even of a family. flowers in varying their colours often become veined and spotted and the leaves become divided like true species: it is known that the varieties of the same plant never have red, blue and yellow flowers, though the hyacinth makes a very near approach to an exception{ }; and different species of the same genus seldom, though sometimes they have flowers of these three colours. dun-coloured horses having a dark stripe down their backs, and certain domestic asses having transverse bars on their legs, afford striking examples of a variation analogous in character to the distinctive marks of other species of the same genus. { } the law of compensation is discussed in the _origin_, ed. i. p. , vi. p. . { } <note in original.> boitard and corbié on outer edging red in tail of bird,--so bars on wing, white or black or brown, or white edged with black or <illegible>: analogous to marks running through genera but with different colours. tail coloured in pigeons. { } <note in original.> oxalis and gentian. <in gentians blue, yellow and reddish colours occur. in oxalis yellow, purple, violet and pink.> _external characters of hybrids and mongrels._ there is, however, as it appears to me, a more important method of comparison between species and races, namely the character of the offspring{ } when species are crossed and when races are crossed: i believe, in no one respect, except in sterility, is there any difference. it would, i think, be a marvellous fact, if species have been formed by distinct acts of creation, that they should act upon each other in uniting, like races descended from a common stock. in the first place, by repeated crossing one species can absorb and wholly obliterate the characters of another, or of several other species, in the same manner as one race will absorb by crossing another race. marvellous, that one act of creation should absorb another or even several acts of creation! the offspring of species, that is hybrids, and the offspring of races, that is mongrels, resemble each other in being either intermediate in character (as is most frequent in hybrids) or in resembling sometimes closely one and sometimes the other parent; in both the offspring produced by the same act of conception sometimes differ in their degree of resemblance; both hybrids and mongrels sometimes retain a certain part or organ very like that of either parent, both, as we have seen, become in succeeding generations variable; and this tendency to vary can be transmitted by both; in both for many generations there is a strong tendency to reversion to their ancestral form. in the case of a hybrid laburnum and of a supposed mongrel vine different parts of the same plants took after each of their two parents. in the hybrids from some species, and in the mongrel of some races, the offspring differ according as which of the two species, or of the two races, is the father (as in the common mule and hinny) and which the mother. some races will breed together, which differ so greatly in size, that the dam often perishes in labour; so it is with some species when crossed; when the dam of one species has borne offspring to the male of another species, her succeeding offspring are sometimes stained (as in lord morton's mare by the quagga, wonderful as the fact{ } is) by this first cross; so agriculturists positively affirm is the case when a pig or sheep of one breed has produced offspring by the sire of another breed. { } this section corresponds roughly to that on _hybrids and mongrels compared independently of their fertility_, _origin_, ed. i. p. , vi. p. . the discussion on gärtner's views, given in the _origin_, is here wanting. the brief mention of prepotency is common to them both. { } see _animals and plants_, ed. ii. vol. i. p. . the phenomenon of _telegony_, supposed to be established by this and similar cases, is now generally discredited in consequence of ewart's experiments. _summary of second chapter_{ }. { } the section on p. is an appendix to the summary. let us sum up this second chapter. if slight variations do occur in organic beings in a state of nature; if changes of condition from geological causes do produce in the course of ages effects analogous to those of domestication on any, however few, organisms; and how can we doubt it,--from what is actually known, and from what may be presumed, since thousands of organisms taken by man for sundry uses, and placed in new conditions, have varied. if such variations tend to be hereditary; and how can we doubt it,--when we see shades of expression, peculiar manners, monstrosities of the strangest kinds, diseases, and a multitude of other peculiarities, which characterise and form, being inherited, the endless races (there are kinds of cabbages{ }) of our domestic plants and animals. if we admit that every organism maintains its place by an almost periodically recurrent struggle; and how can we doubt it,--when we know that all beings tend to increase in a geometrical ratio (as is instantly seen when the conditions become for a time more favourable); whereas on an average the amount of food must remain constant, if so, there will be a natural means of selection, tending to preserve those individuals with any slight deviations of structure more favourable to the then existing conditions, and tending to destroy any with deviations of an opposite nature. if the above propositions be correct, and there be no law of nature limiting the possible amount of variation, new races of beings will,--perhaps only rarely, and only in some few districts,--be formed. { } i do not know the authority for this statement. _limits of variation._ that a limit to variation does exist in nature is assumed by most authors, though i am unable to discover a single fact on which this belief is grounded{ }. one of the commonest statements is that plants do not become acclimatised; and i have even observed that kinds not raised by seed, but propagated by cuttings, &c., are instanced. a good instance has, however, been advanced in the case of kidney beans, which it is believed are now as tender as when first introduced. even if we overlook the frequent introduction of seed from warmer countries, let me observe that as long as the seeds are gathered promiscuously from the bed, without continual observation and _careful_ selection of those plants which have stood the climate best during their whole growth, the experiment of acclimatisation has hardly been begun. are not all those plants and animals, of which we have the greatest number of races, the oldest domesticated? considering the quite recent progress{ } of systematic agriculture and horticulture, is it not opposed to every fact, that we have exhausted the capacity of variation in our cattle and in our corn,--even if we have done so in some trivial points, as their fatness or kind of wool? will any one say, that if horticulture continues to flourish during the next few centuries, that we shall not have numerous new kinds of the potato and dahlia? but take two varieties of each of these plants, and adapt them to certain fixed conditions and prevent any cross for years, and then again vary their conditions; try many climates and situations; and who{ } will predict the number and degrees of difference which might arise from these stocks? i repeat that we know nothing of any limit to the possible amount of variation, and therefore to the number and differences of the races, which might be produced by the natural means of selection, so infinitely more efficient than the agency of man. races thus produced would probably be very "true"; and if from having been adapted to different conditions of existence, they possessed different constitutions, if suddenly removed to some new station, they would perhaps be sterile and their offspring would perhaps be infertile. such races would be undistinguishable from species. but is there any evidence that the species, which surround us on all sides, have been thus produced? this is a question which an examination of the economy of nature we might expect would answer either in the affirmative or negative{ }. { } in the _origin_ no limit is placed to variation as far as i know. { } <note in original.> history of pigeons shows increase of peculiarities during last years. { } compare an obscure passage in the essay of , p. . { } <note in original.> certainly <two pages in the ms.> ought to be here introduced, viz., difficulty in forming such organ, as eye, by selection. <in the _origin_, ed. i., a chapter on _difficulties on theory_ follows that on _laws of variation_, and precedes that on _instinct_: this was also the arrangement in the essay of ; whereas in the present essay _instinct_ follows _variation_ and precedes _difficulties_.> chapter iii on the variation of instincts and other mental attributes under domestication and in state of nature; on the difficulties in this subject; and on analogous difficulties with respect to corporeal structures _variation of mental attributes under domestication._ i have as yet only alluded to the mental qualities which differ greatly in different species. let me here premise that, as will be seen in the second part, there is no evidence and consequently no attempt to show that _all_ existing organisms have descended from any one common parent-stock, but that only those have so descended which, in the language of naturalists, are clearly related to each other. hence the facts and reasoning advanced in this chapter do not apply to the first origin of the senses{ }, or of the chief mental attributes, such as of memory, attention, reasoning, &c., &c., by which most or all of the great related groups are characterised, any more than they apply to the first origin of life, or growth, or the power of reproduction. the application of such facts as i have collected is merely to the differences of the primary mental qualities and of the instincts in the species{ } of the several great groups. in domestic animals every observer has remarked in how great a degree, in the individuals of the same species, the dispositions, namely courage, pertinacity, suspicion, restlessness, confidence, temper, pugnaciousness, affection, care of their young, sagacity, &c., &c., vary. it would require a most able metaphysician to explain how many primary qualities of the mind must be changed to cause these diversities of complex dispositions. from these dispositions being inherited, of which the testimony is unanimous, families and breeds arise, varying in these respects. i may instance the good and ill temper of different stocks of bees and of horses,--the pugnacity and courage of game fowls,--the pertinacity of certain dogs, as bull-dogs, and the sagacity of others,--for restlessness and suspicion compare a wild rabbit reared with the greatest care from its earliest age with the extreme tameness of the domestic breed of the same animal. the offspring of the domestic dogs which have run wild in cuba{ }, though caught quite young, are most difficult to tame, probably nearly as much so as the original parent-stock from which the domestic dog descended. the habitual "_periods_" of different families of the same species differ, for instance, in the time of year of reproduction, and the period of life when the capacity is acquired, and the hour of roosting (in malay fowls), &c., &c. these periodical habits are perhaps essentially corporeal, and may be compared to nearly similar habits in plants, which are known to vary extremely. consensual movements (as called by müller) vary and are inherited,--such as the cantering and ambling paces in horses, the tumbling of pigeons, and perhaps the handwriting, which is sometimes so similar between father and sons, may be ranked in this class. _manners_, and even tricks which perhaps are only _peculiar_ manners, according to w. hunter and my father, are distinctly inherited in cases where children have lost their parent in early infancy. the inheritance of expression, which often reveals the finest shades of character, is familiar to everyone. { } a similar proviso occurs in the chapter on instinct in _origin_, ed. i. p. , vi. p. . { } the discussion occurs later in chapter vii of the _origin_, ed. i. than in the present essay, where moreover it is fuller in some respects. { } in the margin occurs the name of poeppig. in _var. under dom._, ed. ii. vol. i. p. , the reference to poeppig on the cuban dogs contains no mention of the wildness of their offspring. again the tastes and pleasures of different breeds vary, thus the shepherd-dog delights in chasing the sheep, but has no wish to kill them,--the terrier (see knight) delights in killing vermin, and the spaniel in finding game. but it is impossible to separate their mental peculiarities in the way i have done: the tumbling of pigeons, which i have instanced as a consensual movement, might be called a trick and is associated with a taste for flying in a close flock at a great height. certain breeds of fowls have a taste for roosting in trees. the different actions of pointers and setters might have been adduced in the same class, as might the peculiar _manner_ of hunting of the spaniel. even in the same breed of dogs, namely in fox-hounds, it is the fixed opinion of those best able to judge that the different pups are born with different tendencies; some are best to find their fox in the cover; some are apt to run straggling, some are best to make casts and to recover the lost scent, &c.; and that these peculiarities undoubtedly are transmitted to their progeny. or again the tendency to point might be adduced as a distinct habit which has become inherited,--as might the tendency of a true sheep dog (as i have been assured is the case) to run round the flock instead of directly at them, as is the case with other young dogs when attempted to be taught. the "transandantes" sheep{ } in spain, which for some centuries have been yearly taken a journey of several hundred miles from one province to another, know when the time comes, and show the greatest restlessness (like migratory birds in confinement), and are prevented with difficulty from starting by themselves, which they sometimes do, and find their own way. there is a case on good evidence{ } of a sheep which, when she lambed, would return across a mountainous country to her own birth-place, although at other times of year not of a rambling disposition. her lambs inherited this same disposition, and would go to produce their young on the farm whence their parent came; and so troublesome was this habit that the whole family was destroyed. { } <note in original.> several authors. { } in the margin "hogg" occurs as authority for this fact. for the reference, see p. , note . these facts must lead to the conviction, justly wonderful as it is, that almost infinitely numerous shades of disposition, of tastes, of peculiar movements, and even of individual actions, can be modified or acquired by one individual and transmitted to its offspring. one is forced to admit that mental phenomena (no doubt through their intimate connection with the brain) can be inherited, like infinitely numerous and fine differences of corporeal structure. in the same manner as peculiarities of corporeal structure slowly acquired or lost during mature life (especially cognisant <?> in disease), as well as congenital peculiarities, are transmitted; so it appears to be with the mind. the inherited paces in the horse have no doubt been acquired by compulsion during the lives of the parents: and temper and tameness may be modified in a breed by the treatment which the individuals receive. knowing that a pig has been taught to point, one would suppose that this quality in pointer-dogs was the simple result of habit, but some facts, with respect to the occasional appearance of a similar quality in other dogs, would make one suspect that it originally appeared in a less perfect degree, "_by chance_," that is from a congenital tendency{ } in the parent of the breed of pointers. one cannot believe that the tumbling, and high flight in a compact body, of one breed of pigeons has been taught; and in the case of the slight differences in the manner of hunting in young fox-hounds, they are doubtless congenital. the inheritance of the foregoing and similar mental phenomena ought perhaps to create less surprise, from the reflection that in no case do individual acts of reasoning, or movements, or other phenomena connected with consciousness, appear to be transmitted. an action, even a very complicated one, when from long practice it is performed unconsciously without any effort (and indeed in the case of many peculiarities of manners opposed to the will) is said, according to a common expression, to be performed "instinctively." those cases of languages, and of songs, learnt in early childhood and _quite_ forgotten, being _perfectly_ repeated during the unconsciousness of illness, appear to me only a few degrees less wonderful than if they had been transmitted to a second generation{ }. { } in the _origin_, ed. i., he speaks more decidedly against the belief that instincts are hereditary habits, see for instance pp. , , ed. vi. pp. , . he allows, however, something to habit (p. ). { } a suggestion of hering's and s. butler's views on memory and inheritance. it is not, however, implied that darwin was inclined to accept these opinions. _hereditary habits compared with instincts._ the chief characteristics of true instincts appear to be their invariability and non-improvement during the mature age of the individual animal: the absence of knowledge of the end, for which the action is performed, being associated, however, sometimes with a degree of reason; being subject to mistakes and being associated with certain states of the body or times of the year or day. in most of these respects there is a resemblance in the above detailed cases of the mental qualities acquired or modified during domestication. no doubt the instincts of wild animals are more uniform than those habits or qualities modified or recently acquired under domestication, in the same manner and from the same causes that the corporeal structure in this state is less uniform than in beings in their natural conditions. i have seen a young pointer point as fixedly, the first day it was taken out, as any old dog; magendie says this was the case with a retriever which he himself reared: the tumbling of pigeons is not probably improved by age: we have seen that in the case above given that the young sheep inherited the migratory tendency to their particular birth-place the first time they lambed. this last fact offers an instance of a domestic instinct being associated with a state of body; as do the "transandantes" sheep with a time of year. ordinarily the acquired instincts of domestic animals seem to require a certain degree of education (as generally in pointers and retrievers) to be perfectly developed: perhaps this holds good amongst wild animals in rather a greater degree than is generally supposed; for instance, in the singing of birds, and in the knowledge of proper herbs in ruminants. it seems pretty clear that bees transmit knowledge from generation to generation. lord brougham{ } insists strongly on ignorance of the end proposed being eminently characteristic of true instincts; and this appears to me to apply to many acquired hereditary habits; for instance, in the case of the young pointer alluded to before, which pointed so steadfastly the first day that we were obliged several times to carry him away{ }. this puppy not only pointed at sheep, at large white stones, and at every little bird, but likewise "backed" the other pointers: this young dog must have been as unconscious for what end he was pointing, namely to facilitate his master's killing game to eat, as is a butterfly which lays her eggs on a cabbage, that her caterpillars would eat the leaves. so a horse that ambles instinctively, manifestly is ignorant that he performs that peculiar pace for the ease of man; and if man had never existed, he would never have ambled. the young pointer pointing at white stones appears to be as much a mistake of its acquired instinct, as in the case of flesh-flies laying their eggs on certain flowers instead of putrifying meat. however true the ignorance of the end may generally be, one sees that instincts are associated with some degree of reason; for instance, in the case of the tailor-bird, who spins threads with which to make her nest <yet> will use artificial threads when she can procure them{ }; so it has been known that an old pointer has broken his point and gone round a hedge to drive out a bird towards his master{ }. { } lord brougham's _dissertations on subjects of science_, etc., , p. . { } this case is more briefly given in the _origin_, ed. i. p. , vi. p. . the simile of the butterfly occurs there also. { } "a little dose, as pierre huber expresses it, of judgment or reason, often comes into play." _origin_, ed. i. p. , vi. p. . { } in the margin is written "retriever killing one bird." this refers to the cases given in the _descent of man_, nd ed. (in vol.) p. , of a retriever being puzzled how to deal with a wounded and a dead bird, killed the former and carried both at once. this was the only known instance of her wilfully injuring game. there is one other quite distinct method by which the instincts or habits acquired under domestication may be compared with those given by nature, by a test of a fundamental kind; i mean the comparison of the mental powers of mongrels and hybrids. now the instincts, or habits, tastes, and dispositions of one _breed_ of animals, when crossed with another breed, for instance a shepherd-dog with a harrier, are blended and appear in the same curiously mixed degree, both in the first and succeeding generations, exactly as happens when one _species_ is crossed with another{ }. this would hardly be the case if there was any fundamental difference between the domestic and natural instinct{ }; if the former were, to use a metaphorical expression, merely superficial. { } see _origin_, ed. i. p. , vi. p. . { } <note in original.> give some definition of instinct, or at least give chief attributes. <in _origin_, ed. i. p. , vi. p. , darwin refuses to define instinct.> the term instinct is often used in <a> sense which implies no more than that the animal does the action in question. faculties and instincts may i think be imperfectly separated. the mole has the faculty of scratching burrows, and the instinct to apply it. the bird of passage has the faculty of finding its way and the instinct to put it in action at certain periods. it can hardly be said to have the faculty of knowing the time, for it can possess no means, without indeed it be some consciousness of passing sensations. think over all habitual actions and see whether faculties and instincts can be separated. we have faculty of waking in the night, if an instinct prompted us to do something at certain hour of night or day. savages finding their way. wrangel's account--probably a faculty inexplicable by the possessor. there are besides faculties "_means_," as conversion of larvæ into neuters and queens. i think all this generally implied, anyhow useful. <this discussion, which does not occur in the _origin_, is a first draft of that which follows in the text, p. .> _variation in the mental attributes of wild animals._ with respect to the variation{ } of the mental powers of animals in a wild state, we know that there is a considerable difference in the disposition of different individuals of the same species, as is recognised by all those who have had the charge of animals in a menagerie. with respect to the wildness of animals, that is fear directed particularly against man, which appears to be as true an instinct as the dread of a young mouse of a cat, we have excellent evidence that it is slowly acquired and becomes hereditary. it is also certain that, in a natural state, individuals of the same species lose or do not practice their migratory instincts--as woodcocks in madeira. with respect to any variation in the more complicated instincts, it is obviously most difficult to detect, even more so than in the case of corporeal structure, of which it has been admitted the variation is exceedingly small, and perhaps scarcely any in the majority of species at any one period. yet, to take one excellent case of instinct, namely the nests of birds, those who have paid most attention to the subject maintain that not only certain individuals <? species> seem to be able to build very imperfectly, but that a difference in skill may not unfrequently be detected between individuals{ }. certain birds, moreover, adapt their nests to circumstances; the water-ouzel makes no vault when she builds under cover of a rock--the sparrow builds very differently when its nest is in a tree or in a hole, and the golden-crested wren sometimes suspends its nest below and sometimes places it _on_ the branches of trees. { } a short discussion of a similar kind occurs in the _origin_, ed. i. p. , vi. p. . { } this sentence agrees with the ms., but is clearly in need of correction. _principles of selection applicable to instincts._ as the instincts of a species are fully as important to its preservation and multiplication as its corporeal structure, it is evident that if there be the slightest congenital differences in the instincts and habits, or if certain individuals during their lives are induced or compelled to vary their habits, and if such differences are in the smallest degree more favourable, under slightly modified external conditions, to their preservation, such individuals must in the long run have a better _chance_ of being preserved and of multiplying{ }. if this be admitted, a series of small changes may, as in the case of corporeal structure, work great changes in the mental powers, habits and instincts of any species. { } this corresponds to _origin_, ed. i. p. , vi. p. . _difficulties in the acquirement of complex instincts by selection._ every one will at first be inclined to explain (as i did for a long time) that many of the more complicated and wonderful instincts could not be acquired in the manner here supposed{ }. the second part of this work is devoted to the general consideration of how far the general economy of nature justifies or opposes the belief that related species and genera are descended from common stocks; but we may here consider whether the instincts of animals offer such a _primâ facie_ case of impossibility of gradual acquirement, as to justify the rejection of any such theory, however strongly it may be supported by other facts. i beg to repeat that i wish here to consider not the _probability_ but the _possibility_ of complicated instincts having been acquired by the slow and long-continued selection of very slight (either congenital or produced by habit) modifications of foregoing simpler instincts; each modification being as useful and necessary, to the species practising it, as the most complicated kind. { } this discussion is interesting in differing from the corresponding section of the _origin_, ed. i. p. , vi. p. , to the end of the chapter. in the present essay the subjects dealt with are nest-making instincts, including the egg-hatching habit of the australian bush-turkey. the power of "shamming death." "faculty" in relation to instinct. the instinct of lapse of time, and of direction. bees' cells very briefly given. birds feeding their young on food differing from their own natural food. in the _origin_, ed. i., the cases discussed are the instinct of laying eggs in other birds' nests; the slave-making instinct in ants; the construction of the bee's comb, very fully discussed. first, to take the case of birds'-nests; of existing species (almost infinitely few in comparison with the multitude which must have existed, since the period of the new red sandstone of n. america, of whose habits we must always remain ignorant) a tolerably perfect series could be made from eggs laid on the bare ground, to others with a few sticks just laid round them, to a simple nest like the wood-pigeons, to others more and more complicated: now if, as is asserted, there occasionally exist slight differences in the building powers of an individual, and if, which is at least probable, that such differences would tend to be inherited, then we can see that it is at least _possible_ that the nidificatory instincts may have been acquired by the gradual selection, during thousands and thousands of generations, of the eggs and young of those individuals, whose nests were in some degree better adapted to the preservation of their young, under the then existing conditions. one of the most surprising instincts on record is that of the australian bush-turkey, whose eggs are hatched by the heat generated from a huge pile of fermenting materials, which it heaps together; but here the habits of an allied species show how this instinct _might possibly_ have been acquired. this second species inhabits a tropical district, where the heat of the sun is sufficient to hatch its eggs; this bird, burying its eggs, apparently for concealment, under a lesser heap of rubbish, but of a dry nature, so as not to ferment. now suppose this bird to range slowly into a climate which was cooler, and where leaves were more abundant, in that case, those individuals, which chanced to have their collecting instinct strongest developed, would make a somewhat larger pile, and the eggs, aided during some colder season, under the slightly cooler climate by the heat of incipient fermentation, would in the long run be more freely hatched and would probably produce young ones with the same more highly developed collecting tendencies; of these again, those with the best developed powers would again tend to rear most young. thus this strange instinct might _possibly_ be acquired, every individual bird being as ignorant of the laws of fermentation, and the consequent development of heat, as we know they must be. secondly, to take the case of animals feigning death (as it is commonly expressed) to escape danger. in the case of insects, a perfect series can be shown, from some insects, which momentarily stand still, to others which for a second slightly contract their legs, to others which will remain immovably drawn together for a quarter of an hour, and may be torn asunder or roasted at a slow fire, without evincing the smallest sign of sensation. no one will doubt that the length of time, during which each remains immovable, is well adapted to <favour the insect's> escape <from> the dangers to which it is most exposed, and few will deny the _possibility_ of the change from one degree to another, by the means and at the rate already explained. thinking it, however, wonderful (though not impossible) that the attitude of death should have been acquired by methods which imply no imitation, i compared several species, when feigning, as is said, death, with others of the same species really dead, and their attitudes were in no one case the same. thirdly, in considering many instincts it is useful to _endeavour_ to separate the faculty{ } by which they perform it, and the mental power which urges to the performance, which is more properly called an instinct. we have an instinct to eat, we have jaws &c. to give us the faculty to do so. these faculties are often unknown to us: bats, with their eyes destroyed, can avoid strings suspended across a room, we know not at present by what faculty they do this. thus also, with migratory birds, it is a wonderful instinct which urges them at certain times of the year to direct their course in certain directions, but it is a faculty by which they know the time and find their way. with respect to time{ }, man without seeing the sun can judge to a certain extent of the hour, as must those cattle which come down from the inland mountains to feed on sea-weed left bare at the changing hour of low-water{ }. a hawk (d'orbigny) seems certainly to have acquired a knowledge of a period of every days. in the cases already given of the sheep which travelled to their birth-place to cast their lambs, and the sheep in spain which know their time of march{ }, we may conjecture that the tendency to move is associated, we may then call it instinctively, with some corporeal sensations. with respect to direction we can easily conceive how a tendency to travel in a certain course may possibly have been acquired, although we must remain ignorant how birds are able to preserve any direction whatever in a dark night over the wide ocean. i may observe that the power of some savage races of mankind to find their way, although perhaps wholly different from the faculty of birds, is nearly as unintelligible to us. bellinghausen, a skilful navigator, describes with the utmost wonder the manner in which some esquimaux guided him to a certain point, by a course never straight, through newly formed hummocks of ice, on a thick foggy day, when he with a compass found it impossible, from having no landmarks, and from their course being so extremely crooked, to preserve any sort of uniform direction: so it is with australian savages in thick forests. in north and south america many birds slowly travel northward and southward, urged on by the food they find, as the seasons change; let them continue to do this, till, as in the case of the sheep in spain, it has become an urgent instinctive desire, and they will gradually accelerate their journey. they would cross narrow rivers, and if these were converted by subsidence into narrow estuaries, and gradually during centuries to arms of the sea, still we may suppose their restless desire of travelling onwards would impel them to cross such an arm, even if it had become of great width beyond their span of vision. how they are able to preserve a course in any direction, i have said, is a faculty unknown to us. to give another illustration of the means by which i conceive it _possible_ that the direction of migrations have been determined. elk and reindeer in n. america annually cross, as if they could marvellously smell or see at the distance of a hundred miles, a wide tract of absolute desert, to arrive at certain islands where there is a scanty supply of food; the changes of temperature, which geology proclaims, render it probable that this desert tract formerly supported some vegetation, and thus these quadrupeds might have been annually led on, till they reached the more fertile spots, and so acquired, like the sheep of spain, their migratory powers. { } the distinction between _faculty_ and _instinct_ corresponds in some degree to that between perception of a stimulus and a specific reaction. i imagine that the author would have said that the sensitiveness to light possessed by a plant is _faculty_, while _instinct_ decides whether the plant curves to or from the source of illumination. { } <note in the original in an unknown handwriting.> at the time when corn was pitched in the market instead of sold by sample, the geese in the town fields of newcastle <staffordshire?> used to know market day and come in to pick up the corn spilt. { } <note in original.> macculloch and others. { } i can find no reference to the _transandantes_ sheep in darwin's published work. he was possibly led to doubt the accuracy of the statement on which he relied. for the case of the sheep returning to their birth-place see p. , note .{note } fourthly, with respect to the combs of the hive-bee{ }; here again we must look to some faculty or means by which they make their hexagonal cells, without indeed we view these instincts as mere machines. at present such a faculty is quite unknown: mr waterhouse supposes that several bees are led by their instinct to excavate a mass of wax to a certain thinness, and that the result of this is that hexagons necessarily remain. whether this or some other theory be true, some such means they must possess. they abound, however, with true instincts, which are the most wonderful that are known. if we examine the little that is known concerning the habits of other species of bees, we find much simpler instincts: the humble bee merely fills rude balls of wax with honey and aggregates them together with little order in a rough nest of grass. if we knew the instinct of all the bees, which ever had existed, it is not improbable that we should have instincts of every degree of complexity, from actions as simple as a bird making a nest, and rearing her young, to the wonderful architecture and government of the hive-bee; at least such is _possible_, which is all that i am here considering. { } _origin_, ed. i. p. , vi. p. . finally, i will briefly consider under the same point of view one other class of instincts, which have often been advanced as truly wonderful, namely parents bringing food to their young which they themselves neither like nor partake of{ };--for instance, the common sparrow, a granivorous bird, feeding its young with caterpillars. we might of course look into the case still earlier, and seek how an instinct in the parent, of feeding its young at all, was first derived; but it is useless to waste time in conjectures on a series of gradations from the young feeding themselves and being slightly and occasionally assisted in their search, to their entire food being brought to them. with respect to the parent bringing a different kind of food from its own kind, we may suppose either that the remote stock, whence the sparrow and other congenerous birds have descended, was insectivorous, and that its own habits and structure have been changed, whilst its ancient instincts with respect to its young have remained unchanged; or we may suppose that the parents have been induced to vary slightly the food of their young, by a slight scarcity of the proper kind (or by the instincts of some individuals not being so truly developed), and in this case those young which were most capable of surviving were necessarily most often preserved, and would themselves in time become parents, and would be similarly compelled to alter their food for their young. in the case of those animals, the young of which feed themselves, changes in their instincts for food, and in their structure, might be selected from slight variations, just as in mature animals. again, where the food of the young depends on where the mother places her eggs, as in the case of the caterpillars of the cabbage-butterfly, we may suppose that the parent stock of the species deposited her eggs sometimes on one kind and sometimes on another of congenerous plants (as some species now do), and if the cabbage suited the caterpillars better than any other plant, the caterpillars of those butterflies, which had chosen the cabbage, would be most plentifully reared, and would produce butterflies more apt to lay their eggs on the cabbage than on the other congenerous plants. { } this is an expansion of an obscure passage in the essay of , p. . however vague and unphilosophical these conjectures may appear, they serve, i think, to show that one's first impulse utterly to reject any theory whatever, implying a gradual acquirement of these instincts, which for ages have excited man's admiration, may at least be delayed. once grant that dispositions, tastes, actions or habits can be slightly modified, either by slight congenital differences (we must suppose in the brain) or by the force of external circumstances, and that such slight modifications can be rendered inheritable,--a proposition which no one can reject,--and it will be difficult to put any limit to the complexity and wonder of the tastes and habits which may _possibly_ be thus acquired. _difficulties in the acquirement by selection of complex corporeal structures._ after the past discussion it will perhaps be convenient here to consider whether any particular corporeal organs, or the entire structure of any animals, are so wonderful as to justify the rejection _primâ facie_ of our theory{ }. in the case of the eye, as with the more complicated instincts, no doubt one's first impulse is to utterly reject every such theory. but if the eye from its most complicated form can be shown to graduate into an exceedingly simple state,--if selection can produce the smallest change, and if such a series exists, then it is clear (for in this work we have nothing to do with the first origin of organs in their simplest forms{ }) that it may _possibly_ have been acquired by gradual selection of slight, but in each case, useful deviations{ }. every naturalist, when he meets with any new and singular organ, always expects to find, and looks for, other and simpler modifications of it in other beings. in the case of the eye, we have a multitude of different forms, more or less simple, not graduating into each other, but separated by sudden gaps or intervals; but we must recollect how incomparably greater would the multitude of visual structures be if we had the eyes of every fossil which ever existed. we shall discuss the probable vast proportion of the extinct to the recent in the succeeding part. notwithstanding the large series of existing forms, it is most difficult even to conjecture by what intermediate stages very many simple organs could possibly have graduated into complex ones: but it should be here borne in mind, that a part having originally a wholly different function, may on the theory of gradual selection be slowly worked into quite another use; the gradations of forms, from which naturalists believe in the hypothetical metamorphosis of part of the ear into the swimming bladder in fishes{ }, and in insects of legs into jaws, show the manner in which this is possible. as under domestication, modifications of structure take place, without any continued selection, which man finds very useful, or valuable for curiosity (as the hooked calyx of the teazle, or the ruff round some pigeons' necks), so in a state of nature some small modifications, apparently beautifully adapted to certain ends, may perhaps be produced from the accidents of the reproductive system, and be at once propagated without long-continued selection of small deviations towards that structure{ }. in conjecturing by what stages any complicated organ in a species may have arrived at its present state, although we may look to the analogous organs in other existing species, we should do this merely to aid and guide our imaginations; for to know the real stages we must look only through one line of species, to one ancient stock, from which the species in question has descended. in considering the eye of a quadruped, for instance, though we may look at the eye of a molluscous animal or of an insect, as a proof how simple an organ will serve some of the ends of vision; and at the eye of a fish as a nearer guide of the manner of simplification; we must remember that it is a mere chance (assuming for a moment the truth of our theory) if any existing organic being has preserved any one organ, in exactly the same condition, as it existed in the ancient species at remote geological periods. { } the difficulties discussed in the _origin_, ed. i. p. , vi. p. , are the rarity of transitional varieties, the origin of the tail of the giraffe; the otter-like polecat (_mustela vison_); the flying habit of the bat; the penguin and the logger-headed duck; flying fish; the whale-like habit of the bear; the woodpecker; diving petrels; the eye; the swimming bladder; cirripedes; neuter insects; electric organs. of these, the polecat, the bat, the woodpecker, the eye, the swimming bladder are discussed in the present essay, and in addition some botanical problems. { } in the _origin_, ed. vi. p. , the author replies to mivart's criticisms (_genesis of species_, ), referring especially to that writer's objection "that natural selection is incompetent to account for the incipient stages of useful structures." { } <the following sentence seems to have been intended for insertion here> "and that each eye throughout the animal kingdom is not only most useful, but _perfect_ for its possessor." { } _origin_, ed. i. p. , vi. p. . { } this is one of the most definite statements in the present essay of the possible importance of _sports_ or what would now be called _mutations_. as is well known the author afterwards doubted whether species could arise in this way. see _origin_, ed. v. p. , vi. p. , also _life and letters_, vol. iii. p. . the nature or condition of certain structures has been thought by some naturalists to be of no use to the possessor{ }, but to have been formed wholly for the good of other species; thus certain fruit and seeds have been thought to have been made nutritious for certain animals--numbers of insects, especially in their larval state, to exist for the same end--certain fish to be bright coloured to aid certain birds of prey in catching them, &c. now could this be proved (which i am far from admitting) the theory of natural selection would be quite overthrown; for it is evident that selection depending on the advantage over others of one individual with some slight deviation would never produce a structure or quality profitable only to another species. no doubt one being takes advantage of qualities in another, and may even cause its extermination; but this is far from proving that this quality was produced for such an end. it may be advantageous to a plant to have its seeds attractive to animals, if one out of a hundred or a thousand escapes being digested, and thus aids dissemination: the bright colours of a fish may be of some advantage to it, or more probably may result from exposure to certain conditions in favourable haunts for food, _notwithstanding_ it becomes subject to be caught more easily by certain birds. { } see _origin_, ed. i. p. , vi. p. , where the question is discussed for the case of instincts with a proviso that the same argument applies to structure. it is briefly stated in its general bearing in _origin_, ed. i. p. , vi. p. . if instead of looking, as above, at certain individual organs, in order to speculate on the stages by which their parts have been matured and selected, we consider an individual animal, we meet with the same or greater difficulty, but which, i believe, as in the case of single organs, rests entirely on our ignorance. it may be asked by what intermediate forms could, for instance, a bat possibly have passed; but the same question might have been asked with respect to the seal, if we had not been familiar with the otter and other semi-aquatic carnivorous quadrupeds. but in the case of the bat, who can say what might have been the habits of some parent form with less developed wings, when we now have insectivorous opossums and herbivorous squirrels fitted for merely gliding through the air{ }. one species of bat is at present partly aquatic in its habits{ }. woodpeckers and tree-frogs are especially adapted, as their names express, for climbing trees; yet we have species of both inhabiting the open plains of la plata, where a tree does not exist{ }. i might argue from this circumstance that a structure eminently fitted for climbing trees might descend from forms inhabiting a country where a tree did not exist. notwithstanding these and a multitude of other well-known facts, it has been maintained by several authors that one species, for instance of the carnivorous order, could not pass into another, for instance into an otter, because in its transitional state its habits would not be adapted to any proper conditions of life; but the jaguar{ } is a thoroughly terrestrial quadruped in its structure, yet it takes freely to the water and catches many fish; will it be said that it is _impossible_ that the conditions of its country might become such that the jaguar should be driven to feed more on fish than they now do; and in that case is it impossible, is it not probable, that any the slightest deviation in its instincts, its form of body, in the width of its feet, and in the extension of the skin (which already unites the base of its toes) would give such individuals a better _chance_ of surviving and propagating young with similar, barely perceptible (though thoroughly exercised), deviations{ }? who will say what could thus be effected in the course of ten thousand generations? who can answer the same question with respect to instincts? if no one can, the _possibility_ (for we are not in this chapter considering the _probability_) of simple organs or organic beings being modified by natural selection and the effects of external agencies into complicated ones ought not to be absolutely rejected. { } <note in original.> no one will dispute that the gliding is most useful, probably necessary for the species in question. { } <note in original.> is this the galeopithecus? i forget. <_galeopithecus_ "or the flying lemur" is mentioned in the corresponding discussion in the _origin_, ed. i. p. , vi. p. , as formerly placed among the bats. i do not know why it is described as partly aquatic in its habits.> { } in the _origin_, ed. vi. p. , the author modified the statement that it _never_ climbs trees; he also inserted a sentence quoting mr hudson to the effect that in other districts this woodpecker climbs trees and bores holes. see mr darwin's paper, _zoolog. soc. proc._, , and _life and letters_, iii. p. . { } note by the late alfred newton. richardson in _fauna boreali-americana_, i. p. . { } <note in original.> see richardson a far better case of a polecat animal <_mustela vison_>, which half-year is aquatic. <mentioned in _origin_, ed. i. p. , vi. p. .> part ii{ } on the evidence favourable and opposed to the view that species are naturally formed races, descended from common stocks { } in the _origin_ the division of the work into parts i and ii is omitted. in the ms. the chapters of part ii are numbered afresh, the present being ch. i of pt. ii. i have thought it best to call it ch. iv and there is evidence that darwin had some thought of doing the same. it corresponds to ch. ix of _origin_, ed. i., ch. x in ed. vi. chapter iv on the number of intermediate forms required on the theory of common descent; and on their absence in a fossil state i must here premise that, according to the view ordinarily received, the myriads of organisms, which have during past and present times peopled this world, have been created by so many distinct acts of creation. it is impossible to reason concerning the will of the creator, and therefore, according to this view, we can see no cause why or why not the individual organism should have been created on any fixed scheme. that all the organisms of this world have been produced on a scheme is certain from their general affinities; and if this scheme can be shown to be the same with that which would result from allied organic beings descending from common stocks, it becomes highly improbable that they have been separately created by individual acts of the will of a creator. for as well might it be said that, although the planets move in courses conformably to the law of gravity, yet we ought to attribute the course of each planet to the individual act of the will of the creator{ }. it is in every case more conformable with what we know of the government of this earth, that the creator should have imposed only general laws. as long as no method was known by which races could become exquisitely adapted to various ends, whilst the existence of species was thought to be proved by the sterility{ } of their offspring, it was allowable to attribute each organism to an individual act of creation. but in the two former chapters it has (i think) been shown that the production, under existing conditions, of exquisitely adapted species, is at least _possible_. is there then any direct evidence in favour <of> or against this view? i believe that the geographical distribution of organic beings in past and present times, the kind of affinity linking them together, their so-called "metamorphic" and "abortive" organs, appear in favour of this view. on the other hand, the imperfect evidence of the continuousness of the organic series, which, we shall immediately see, is required on our theory, is against it; and is the most weighty objection{ }. the evidence, however, even on this point, as far as it goes, is favourable; and considering the imperfection of our knowledge, especially with respect to past ages, it would be surprising if evidence drawn from such sources were not also imperfect. { } in the essay of the author uses astronomy in the same manner as an illustration. in the _origin_ this does not occur; the reference to the action of secondary causes is more general, _e.g._ ed. i. p. , vi. p. . { } it is interesting to find the argument from sterility given so prominent a place. in a corresponding passage in the _origin_, ed. i. p. , vi. p. , it is more summarily treated. the author gives, as the chief bar to the acceptance of evolution, the fact that "we are always slow in admitting any great change of which we do not see the intermediate steps"; and goes on to quote lyell on geological action. it will be remembered that the question of sterility remained a difficulty for huxley. { } similar statements occur in the essay of , p. , note , and in the _origin_, ed. i. p. . as i suppose that species have been formed in an analogous manner with the varieties of the domesticated animals and plants, so must there have existed intermediate forms between all the species of the same group, not differing more than recognised varieties differ. it must not be supposed necessary that there should have existed forms exactly intermediate in character between any two species of a genus, or even between any two varieties of a species; but it is necessary that there should have existed every intermediate form between the one species or variety of the common parent, and likewise between the second species or variety, and this same common parent. thus it does not necessarily follow that there ever has existed <a> series of intermediate sub-varieties (differing no more than the occasional seedlings from the same seed-capsule,) between broccoli and common red cabbage; but it is certain that there has existed, between broccoli and the wild parent cabbage, a series of such intermediate seedlings, and again between red cabbage and the wild parent cabbage: so that the broccoli and red cabbage are linked together, but not _necessarily_ by directly intermediate forms{ }. it is of course possible that there _may_ have been directly intermediate forms, for the broccoli may have long since descended from a common red cabbage, and this from the wild cabbage. so on my theory, it must have been with species of the same genus. still more must the supposition be avoided that there has necessarily ever existed (though one _may_ have descended from <the> other) directly intermediate forms between any two genera or families--for instance between the genus _sus_ and the tapir{ }; although it is necessary that intermediate forms (not differing more than the varieties of our domestic animals) should have existed between sus and some unknown parent form, and tapir with this same parent form. the latter may have differed more from sus and tapir than these two genera now differ from each other. in this sense, according to our theory, there has been a gradual passage (the steps not being wider apart than our domestic varieties) between the species of the same genus, between genera of the same family, and between families of the same order, and so on, as far as facts, hereafter to be given, lead us; and the number of forms which must have at former periods existed, thus to make good this passage between different species, genera, and families, must have been almost infinitely great. { } in the _origin_, ed. i. p. , vi. p. he uses his newly-acquired knowledge of pigeons to illustrate this point. { } compare the _origin_, ed. i. p. , vi. p. . what evidence{ } is there of a number of intermediate forms having existed, making a passage in the above sense, between the species of the same groups? some naturalists have supposed that if every fossil which now lies entombed, together with all existing species, were collected together, a perfect series in every great class would be formed. considering the enormous number of species requisite to effect this, especially in the above sense of the forms not being _directly_ intermediate between the existing species and genera, but only intermediate by being linked through a common but often widely different ancestor, i think this supposition highly improbable. i am however far from underrating the probable number of fossilised species: no one who has attended to the wonderful progress of palæontology during the last few years will doubt that we as yet have found only an exceedingly small fraction of the species buried in the crust of the earth. although the almost infinitely numerous intermediate forms in no one class may have been preserved, it does not follow that they have not existed. the fossils which have been discovered, it is important to remark, do tend, the little way they go, to make good the series; for as observed by buckland they all fall into or between existing groups{ }. moreover, those that fall between our existing groups, fall in, according to the manner required by our theory, for they do not directly connect two existing species of different groups, but they connect the groups themselves: thus the pachydermata and ruminantia are now separated by several characters, <for instance> the pachydermata{ } have both a tibia and fibula, whilst ruminantia have only a tibia; now the fossil macrauchenia has a leg bone exactly intermediate in this respect, and likewise has some other intermediate characters. but the macrauchenia does not connect any one species of pachydermata with some one other of ruminantia but it shows that these two groups have at one time been less widely divided. so have fish and reptiles been at one time more closely connected in some points than they now are. generally in those groups in which there has been most change, the more ancient the fossil, if not identical with recent, the more often it falls between existing groups, or into small existing groups which now lie between other large existing groups. cases like the foregoing, of which there are many, form steps, though few and far between, in a series of the kind required by my theory. { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . { } the structure of the pachyderm leg was a favourite with the author. it is discussed in the essay of , p. . in the present essay the following sentence in the margin appears to refer to pachyderms and ruminants: "there can be no doubt, if we banish all fossils, existing groups stand more separate." the following occurs between the lines "the earliest forms would be such as others could radiate from." as i have admitted the high improbability, that if every fossil were disinterred, they would compose in each of the divisions of nature a perfect series of the kind required; consequently i freely admit, that if those geologists are in the right who consider the lowest known formation as contemporaneous with the first appearances of life{ }; or the several formations as at all closely consecutive; or any one formation as containing a nearly perfect record of the organisms which existed during the whole period of its deposition in that quarter of the globe;--if such propositions are to be accepted, my theory must be abandoned. { } _origin_, ed. i. p. , vi. p. . if the palæozoic system is really contemporaneous with the first appearance of life, my theory must be abandoned, both inasmuch as it limits _from shortness of time_ the total number of forms which can have existed on this world, and because the organisms, as fish, mollusca{ } and star-fish found in its lower beds, cannot be considered as the parent forms of all the successive species in these classes. but no one has yet overturned the arguments of hutton and lyell, that the lowest formations known to us are only those which have escaped being metamorphosed <illegible>; if we argued from some considerable districts, we might have supposed that even the cretaceous system was that in which life first appeared. from the number of distant points, however, in which the silurian system has been found to be the lowest, and not always metamorphosed, there are some objections to hutton's and lyell's view; but we must not forget that the now existing land forms only / part of the superficies of the globe, and that this fraction is only imperfectly known. with respect to the fewness of the organisms found in the silurian and other palæozoic formations, there is less difficulty, inasmuch as (besides their gradual obliteration) we can expect formations of this vast antiquity to escape entire denudation, only when they have been accumulated over a wide area, and have been subsequently protected by vast superimposed deposits: now this could generally only hold good with deposits accumulating in a wide and deep ocean, and therefore unfavourable to the presence of many living things. a mere narrow and not very thick strip of matter, deposited along a coast where organisms most abound, would have no chance of escaping denudation and being preserved to the present time from such immensely distant ages{ }. { } <pencil insertion by the author.> the parent-forms of mollusca would probably differ greatly from all recent,--it is not directly that any one division of mollusca would descend from first time unaltered, whilst others had become metamorphosed from it. { } _origin_, ed. i. p. , vi. p. . if the several known formations are at all nearly consecutive in time, and preserve a fair record of the organisms which have existed, my theory must be abandoned. but when we consider the great changes in mineralogical nature and texture between successive formations, what vast and entire changes in the geography of the surrounding countries must generally have been effected, thus wholly to have changed the nature of the deposits on the same area. what time such changes must have required! moreover how often has it not been found, that between two conformable and apparently immediately successive deposits a vast pile of water-worn matter is interpolated in an adjoining district. we have no means of conjecturing in many cases how long a period{ } has elapsed between successive formations, for the species are often wholly different: as remarked by lyell, in some cases probably as long a period has elapsed between two formations as the whole tertiary system, itself broken by wide gaps. { } <note in original.> reflect on coming in of the chalk, extending from iceland to the crimea. consult the writings of any one who has particularly attended to any one stage in the tertiary system (and indeed of every system) and see how deeply impressed he is with the time required for its accumulation{ }. reflect on the years elapsed in many cases, since the latest beds containing only living species have been formed;--see what jordan smith says of the , years since the last bed, which is above the boulder formation in scotland, has been upraised; or of the far longer period since the recent beds of sweden have been upraised feet, what an enormous period the boulder formation must have required, and yet how insignificant are the records (although there has been plenty of elevation to bring up submarine deposits) of the shells, which we know existed at that time. think, then, over the entire length of the tertiary epoch, and think over the probable length of the intervals, separating the secondary deposits. of these deposits, moreover, those consisting of sand and pebbles have seldom been favourable, either to the embedment or to the preservation of fossils{ }. { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. pp. , , vi. pp. , . nor can it be admitted as probable that any one secondary formation contains a fair record even of those organisms which are most easily preserved, namely hard marine bodies. in how many cases have we not certain evidence that between the deposition of apparently closely consecutive beds, the lower one existed for an unknown time as land, covered with trees. some of the secondary formations which contain most marine remains appear to have been formed in a wide and not deep sea, and therefore only those marine animals which live in such situations would be preserved{ }. in all cases, on indented rocky coasts, or any other coast, where sediment is not accumulating, although often highly favourable to marine animals, none can be embedded: where pure sand and pebbles are accumulating few or none will be preserved. i may here instance the great western line of the s. american coast{ }, tenanted by many peculiar animals, of which none probably will be preserved to a distant epoch. from these causes, and especially from such deposits as are formed along a line of coast, steep above and below water, being necessarily of little width, and therefore more likely to be subsequently denuded and worn away, we can see why it is improbable that our secondary deposits contain a fair record of the marine fauna of any one period. the east indian archipelago offers an area, as large as most of our secondary deposits, in which there are wide and shallow seas, teeming with marine animals, and in which sediment is accumulating; now supposing that all the hard marine animals, or rather those having hard parts to preserve, were preserved to a future age, excepting those which lived on rocky shores where no sediment or only sand and gravel were accumulating, and excepting those embedded along the steeper coasts, where only a narrow fringe of sediment was accumulating, supposing all this, how poor a notion would a person at a future age have of the marine fauna of the present day. lyell{ } has compared the geological series to a work of which only the few latter but not consecutive chapters have been preserved; and out of which, it may be added, very many leaves have been torn, the remaining ones only illustrating a scanty portion of the fauna of each period. on this view, the records of anteceding ages confirm my theory; on any other they destroy it. { } <note in original.> neither highest or lowest fish (_i.e._ myxina <?> or lepidosiren) could be preserved in intelligible condition in fossils. { } _origin_, ed. i. p. , vi. p. . { } see _origin_, ed. i. p. , vi. p. for lyell's metaphor. i am indebted to prof. judd for pointing out that darwin's version of the metaphor is founded on the first edition of lyell's _principles_, vol. i. and vol. iii.; see the essay of , p. . finally, if we narrow the question into, why do we not find in some instances every intermediate form between any two species? the answer may well be that the average duration of each specific form (as we have good reason to believe) is immense in years, and that the transition could, according to my theory, be effected only by numberless small gradations; and therefore that we should require for this end a most perfect record, which the foregoing reasoning teaches us not to expect. it might be thought that in a vertical section of great thickness in the same formation some of the species ought to be found to vary in the upper and lower parts{ }, but it may be doubted whether any formation has gone on accumulating without any break for a period as long as the duration of a species; and if it had done so, we should require a series of specimens from every part. how rare must be the chance of sediment accumulating for some or thousand years on the same spot{ }, with the bottom subsiding, so that a proper depth might be preserved for any one species to continue living: what an amount of subsidence would be thus required, and this subsidence must not destroy the source whence the sediment continued to be derived. in the case of terrestrial animals, what chance is there when the present time is become a pleistocene formation (at an earlier period than this, sufficient elevation to expose marine beds could not be expected), what chance is there that future geologists will make out the innumerable transitional sub-varieties, through which the short-horned and long-horned cattle (so different in shape of body) have been derived from the same parent stock{ }? yet this transition has been effected in _the same country_, and in a far _shorter time_, than would be probable in a wild state, both contingencies highly favourable for the future hypothetical geologists being enabled to trace the variation. { } see _more letters_, vol. i. pp. - , for darwin's interest in the celebrated observations of hilgendorf and hyatt. { } this corresponds partly to _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . chapter v gradual appearance and disappearance of species{ } { } this chapter corresponds to ch. x of _origin_, ed. i., vi. ch. xi, "on the geological succession of organic beings." in the tertiary system, in the last uplifted beds, we find all the species recent and living in the immediate vicinity; in rather older beds we find only recent species, but some not living in the immediate vicinity{ }; we then find beds with two or three or a few more extinct or very rare species; then considerably more extinct species, but with gaps in the regular increase; and finally we have beds with only two or three or not one living species. most geologists believe that the gaps in the percentage, that is the sudden increments, in the number of the extinct species in the stages of the tertiary system are due to the imperfection of the geological record. hence we are led to believe that the species in the tertiary system have been gradually introduced; and from analogy to carry on the same view to the secondary formations. in these latter, however, entire groups of species generally come in abruptly; but this would naturally result, if, as argued in the foregoing chapter, these secondary deposits are separated by wide epochs. moreover it is important to observe that, with our increase of knowledge, the gaps between the older formations become fewer and smaller; geologists of a few years standing remember how beautifully has the devonian system{ } come in between the carboniferous and silurian formations. i need hardly observe that the slow and gradual appearance of new forms follows from our theory, for to form a new species, an old one must not only be plastic in its organization, becoming so probably from changes in the conditions of its existence, but a place in the natural economy of the district must [be made,] come to exist, for the selection of some new modification of its structure, better fitted to the surrounding conditions than are the other individuals of the same or other species{ }. { } _origin_, ed. i. p. , vi. p. . { } in the margin the author has written "lonsdale." this refers to w. lonsdale's paper "notes on the age of the limestone of south devonshire," _geolog. soc. trans._, series , vol. v. , p. . according to mr h. b. woodward (_history of the geological society of london_, , p. ) "lonsdale's 'important and original suggestion of the existence of an intermediary type of palæozoic fossils, since called devonian,' led to a change which was then 'the greatest ever made at one time in the classification of our english formations'." mr woodward's quotations are from murchison and buckland. { } <note in original.> better begin with this. if species really, after catastrophes, created in showers over world, my theory false. <in the above passage the author is obviously close to his theory of divergence.> in the tertiary system the same facts, which make us admit as probable that new species have slowly appeared, lead to the admission that old ones have slowly disappeared, not several together, but one after another; and by analogy one is induced to extend this belief to the secondary and palæozoic epochs. in some cases, as the subsidence of a flat country, or the breaking or the joining of an isthmus, and the sudden inroad of many new and destructive species, extinction might be locally sudden. the view entertained by many geologists, that each fauna of each secondary epoch has been suddenly destroyed over the whole world, so that no succession could be left for the production of new forms, is subversive of my theory, but i see no grounds whatever to admit such a view. on the contrary, the law, which has been made out, with reference to distinct epochs, by independent observers, namely, that the wider the geographical range of a species the longer is its duration in time, seems entirely opposed to any universal extermination{ }. the fact of species of mammiferous animals and fish being renewed at a quicker rate than mollusca, though both aquatic; and of these the terrestrial genera being renewed quicker than the marine; and the marine mollusca being again renewed quicker than the infusorial animalcula, all seem to show that the extinction and renewal of species does not depend on general catastrophes, but on the particular relations of the several classes to the conditions to which they are exposed{ }. { } opposite to this passage the author has written "d'archiac, forbes, lyell." { } this passage, for which the author gives as authorities the names of lyell, forbes and ehrenberg, corresponds in part to the discussion beginning on p. of _origin_, ed. i., vi. p. . some authors seem to consider the fact of a few species having survived{ } amidst a number of extinct forms (as is the case with a tortoise and a crocodile out of the vast number of extinct sub-himalayan fossils) as strongly opposed to the view of species being mutable. no doubt this would be the case, if it were presupposed with lamarck that there was some inherent tendency to change and development in all species, for which supposition i see no evidence. as we see some species at present adapted to a wide range of conditions, so we may suppose that such species would survive unchanged and unexterminated for a long time; time generally being from geological causes a correlative of changing conditions. how at present one species becomes adapted to a wide range, and another species to a restricted range of conditions, is of difficult explanation. { } the author gives falconer as his authority: see _origin_, ed. i. p. , vi. p. . _extinction of species._ the extinction of the larger quadrupeds, of which we imagine we better know the conditions of existence, has been thought little less wonderful than the appearance of new species; and has, i think, chiefly led to the belief of universal catastrophes. when considering the wonderful disappearance within a late period, whilst recent shells were living, of the numerous great and small mammifers of s. america, one is strongly induced to join with the catastrophists. i believe, however, that very erroneous views are held on this subject. as far as is historically known, the disappearance of species from any one country has been slow--the species becoming rarer and rarer, locally extinct, and finally lost{ }. it may be objected that this has been effected by man's direct agency, or by his indirect agency in altering the state of the country; in this latter case, however, it would be difficult to draw any just distinction between his agency and natural agencies. but we now know in the later tertiary deposits, that shells become rarer and rarer in the successive beds, and finally disappear: it has happened, also, that shells common in a fossil state, and thought to have been extinct, have been found to be still living species, but very _rare_ ones{ }. if the rule is that organisms become extinct by becoming rarer and rarer, we ought not to view their extinction, even in the case of the larger quadrupeds, as anything wonderful and out of the common course of events. for no naturalist thinks it wonderful that one species of a genus should be rare and another abundant, notwithstanding he be quite incapable of explaining the causes of the comparative rareness{ }. why is one species of willow-wren or hawk or woodpecker common in england, and another extremely rare: why at the cape of good hope is one species of rhinoceros or antelope far more abundant than other species? why again is the same species much more abundant in one district of a country than in another district? no doubt there are in each case good causes: but they are unknown and unperceived by us. may we not then safely infer that as certain causes are acting _unperceived_ around us, and are making one species to be common and another exceedingly rare, that they might equally well cause the final extinction of some species without being perceived by us? we should always bear in mind that there is a recurrent struggle for life in every organism, and that in every country a destroying agency is always counteracting the geometrical tendency to increase in every species; and yet without our being able to tell with certainty at what period of life, or at what period of the year, the destruction falls the heaviest. ought we then to expect to trace the steps by which this destroying power, always at work and scarcely perceived by us, becomes increased, and yet if it continues to increase ever so slowly (without the fertility of the species in question be likewise increased) the average number of the individuals of that species must decrease, and become finally lost. i may give a single instance of a check causing local extermination which might long have escaped discovery{ }; the horse, though swarming in a wild state in la plata, and likewise under apparently the most unfavourable conditions in the scorched and alternately flooded plains of caraccas, will not in a wild state extend beyond a certain degree of latitude into the intermediate country of paraguay; this is owing to a certain fly depositing its eggs on the navels of the foals: as, however, man with a _little_ care can rear horses in a tame state _abundantly_ in paraguay, the problem of its extinction is probably complicated by the greater exposure of the wild horse to occasional famine from the droughts, to the attacks of the jaguar and other such evils. in the falkland islands the check to the _increase_ of the wild horse is said to be loss of the sucking foals{ }, from the stallions compelling the mares to travel across bogs and rocks in search of food: if the pasture on these islands decreased a little, the horse, perhaps, would cease to exist in a wild state, not from the absolute want of food, but from the impatience of the stallions urging the mares to travel whilst the foals were too young. { } this corresponds approximately to _origin_, ed. i. p. , vi. p. . { } the case of _trigonia_, a great secondary genus of shells surviving in a single species in the australian seas, is given as an example in the _origin_, ed. i. p. , vi. p. . { } this point, on which the author laid much stress, is discussed in the _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . { } this case does not occur in the _origin_, ed. from our more intimate acquaintance with domestic animals, we cannot conceive their extinction without some glaring agency; we forget that they would undoubtedly in a state of nature (where other animals are ready to fill up their place) be acted on in some part of their lives by a destroying agency, keeping their numbers on an average constant. if the common ox was known only as a wild s. african species, we should feel no surprise at hearing that it was a very rare species; and this rarity would be a stage towards its extinction. even in man, so infinitely better known than any other inhabitant of this world, how impossible it has been found, without statistical calculations, to judge of the proportions of births and deaths, of the duration of life, and of the increase and decrease of population; and still less of the causes of such changes: and yet, as has so often been repeated, decrease in numbers or rarity seems to be the high-road to extinction. to marvel at the extermination of a species appears to me to be the same thing as to know that illness is the road to death,--to look at illness as an ordinary event, nevertheless to conclude, when the sick man dies, that his death has been caused by some unknown and violent agency{ }. { } an almost identical sentence occurs in the _origin_, ed. i. p. , vi. p. . in a future part of this work we shall show that, as a general rule, groups of allied species{ } gradually appear and disappear, one after the other, on the face of the earth, like the individuals of the same species: and we shall then endeavour to show the probable cause of this remarkable fact. { } _origin_, ed. i. p. , vi. p. . chapter vi on the geographical distribution of organic beings in past and present times for convenience sake i shall divide this chapter into three sections{ }. in the first place i shall endeavour to state the laws of the distribution of existing beings, as far as our present object is concerned; in the second, that of extinct; and in the third section i shall consider how far these laws accord with the theory of allied species having a common descent. { } chapters xi and xii in the _origin_, ed. i., vi. chs. xii and xiii ("on geographical distribution") show signs of having been originally one, in the fact that one summary serves for both. the geological element is not separately treated there, nor is there a separate section on "how far these laws accord with the theory, &c." in the ms. the author has here written in the margin "if same species appear at two spot at once, fatal to my theory." see _origin_, ed. i. p. , vi. p. section first. _distribution of the inhabitants in the different continents._ in the following discussion i shall chiefly refer to terrestrial mammifers, inasmuch as they are better known; their differences in different countries, strongly marked; and especially as the necessary means of their transport are more evident, and confusion, from the accidental conveyance by man of a species from one district to another district, is less likely to arise. it is known that all mammifers (as well as all other organisms) are united in one great system; but that the different species, genera, or families of the same order inhabit different quarters of the globe. if we divide the land{ } into two divisions, according to the amount of difference, and disregarding the numbers of the terrestrial mammifers inhabiting them, we shall have first australia including new guinea; and secondly the rest of the world: if we make a three-fold division, we shall have australia, s. america, and the rest of the world; i must observe that north america is in some respects neutral land, from possessing some s. american forms, but i believe it is more closely allied (as it certainly is in its birds, plants and shells) with europe. if our division had been four-fold, we should have had australia, s. america, madagascar (though inhabited by few mammifers) and the remaining land: if five-fold, africa, especially the southern eastern parts, would have to be separated from the remainder of the world. these differences in the mammiferous inhabitants of the several main divisions of the globe cannot, it is well known, be explained by corresponding differences in their conditions{ }; how similar are parts of tropical america and africa; and accordingly we find some _analogous_ resemblances,--thus both have monkeys, both large feline animals, both large lepidoptera, and large dung-feeding beetles; both have palms and epiphytes; and yet the essential difference between their productions is as great as between those of the arid plains of the cape of good hope and the grass-covered savannahs of la plata{ }. consider the distribution of the marsupialia, which are eminently characteristic of australia, and in a lesser degree of s. america; when we reflect that animals of this division, feeding both on animal and vegetable matter, frequent the dry open or wooded plains and mountains of australia, the humid impenetrable forests of new guinea and brazil; the dry rocky mountains of chile, and the grassy plains of banda oriental, we must look to some other cause, than the nature of the country, for their absence in africa and other quarters of the world. { } this division of the land into regions does not occur in the _origin_, ed. i. { } _origin_, ed. i. p. , vi. p. . { } opposite this passage is written "_not botanically_," in sir j. d. hooker's hand. the word _palms_ is underlined three times and followed by three exclamation marks. an explanatory note is added in the margin "singular paucity of palms and epiphytes in trop. africa compared with trop. america and ind. or." <=east indies>. furthermore it may be observed that _all_ the organisms inhabiting any country are not perfectly adapted to it{ }; i mean by not being perfectly adapted, only that some few other organisms can generally be found better adapted to the country than some of the aborigines. we must admit this when we consider the enormous number of horses and cattle which have run wild during the three last centuries in the uninhabited parts of st domingo, cuba, and s. america; for these animals must have supplanted some aboriginal ones. i might also adduce the same fact in australia, but perhaps it will be objected that or years has not been a sufficient period to test this power of struggling <with> and overcoming the aborigines. we know the european mouse is driving before it that of new zealand, like the norway rat has driven before it the old english species in england. scarcely an island can be named, where casually introduced plants have not supplanted some of the native species: in la plata the cardoon covers square leagues of country on which some s. american plants must once have grown: the commonest weed over the whole of india is an introduced mexican poppy. the geologist who knows that slow changes are in progress, replacing land and water, will easily perceive that even if all the organisms of any country had originally been the best adapted to it, this could hardly continue so during succeeding ages without either extermination, or changes, first in the relative proportional numbers of the inhabitants of the country, and finally in their constitutions and structure. { } this partly corresponds to _origin_, ed. i. p. , vi. p. . inspection of a map of the world at once shows that the five divisions, separated according to the greatest amount of difference in the mammifers inhabiting them, are likewise those most widely separated from each other by barriers{ } which mammifers cannot pass: thus australia is separated from new guinea and some small adjoining islets only by a narrow and shallow strait; whereas new guinea and its adjoining islets are cut off from the other east indian islands by deep water. these latter islands, i may remark, which fall into the great asiatic group, are separated from each other and the continent only by shallow water; and where this is the case we may suppose, from geological oscillations of level, that generally there has been recent union. south america, including the southern part of mexico, is cut off from north america by the west indies, and the great table-land of mexico, except by a mere fringe of tropical forests along the coast: it is owing, perhaps, to this fringe that n. america possesses some s. american forms. madagascar is entirely isolated. africa is also to a great extent isolated, although it approaches, by many promontories and by lines of shallower sea, to europe and asia: southern africa, which is the most distinct in its mammiferous inhabitants, is separated from the northern portion by the great sahara desert and the table-land of abyssinia. that the distribution of organisms is related to barriers, stopping their progress, we clearly see by comparing the distribution of marine and terrestrial productions. the marine animals being different on the two sides of land tenanted by the same terrestrial animals, thus the shells are wholly different on the opposite sides of the temperate parts of south america{ }, as they are (?) in the red sea and the mediterranean. we can at once perceive that the destruction of a barrier would permit two geographical groups of organisms to fuse and blend into one. but the original cause of groups being different on opposite sides of a barrier can only be understood on the hypothesis of each organism having been created or produced on one spot or area, and afterwards migrating as widely as its means of transport and subsistence permitted it. { } on the general importance of barriers, see _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . _relation of range in genera and species._ it is generally{ } found, that where a genus or group ranges over nearly the entire world, many of the species composing the group have wide ranges: on the other hand, where a group is restricted to any one country, the species composing it generally have restricted ranges in that country{ }. thus among mammifers the feline and canine genera are widely distributed, and many of the individual species have enormous ranges [the genus mus i believe, however, is a strong exception to the rule]. mr gould informs me that the rule holds with birds, as in the owl genus, which is mundane, and many of the species range widely. the rule holds also with land and fresh-water mollusca, with butterflies and very generally with plants. as instances of the converse rule, i may give that division of the monkeys which is confined to s. america, and amongst plants, the cacti, confined to the same continent, the species of both of which have generally narrow ranges. on the ordinary theory of the separate creation of each species, the cause of these relations is not obvious; we can see no reason, because many allied species have been created in the several main divisions of the world, that several of these species should have wide ranges; and on the other hand, that species of the same group should have narrow ranges if all have been created in one main division of the world. as the result of such and probably many other unknown relations, it is found that, even in the same great classes of beings, the different divisions of the world are characterised by either merely different species, or genera, or even families: thus in cats, mice, foxes, s. america differs from asia and africa only in species; in her pigs, camels and monkeys the difference is generic or greater. again, whilst southern africa and australia differ more widely in their mammalia than do africa and s. america, they are more closely (though indeed very distantly) allied in their plants. { } <note in original.> the same laws seem to govern distribution of species and genera, and individuals in time and space. <see _origin_, ed. i. p. , vi. p. , also a passage in the last chapter, p. .> { } _origin_, ed. i. p. , vi. p. . _distribution of the inhabitants in the same continent._ if we now look at the distribution of the organisms in any one of the above main divisions of the world, we shall find it split up into many regions, with all or nearly all their species distinct, but yet partaking of one common character. this similarity of type in the subdivisions of a great region is equally well-known with the dissimilarity of the inhabitants of the several great regions; but it has been less often insisted on, though more worthy of remark. thus for instance, if in africa or s. america, we go from south to north{ }, or from lowland to upland, or from a humid to a dryer part, we find wholly different species of those genera or groups which characterise the continent over which we are passing. in these subdivisions we may clearly observe, as in the main divisions of the world, that sub-barriers divide different groups of species, although the opposite sides of such sub-barriers may possess nearly the same climate, and may be in other respects nearly similar: thus it is on the opposite sides of the cordillera of chile, and in a lesser degree on the opposite sides of the rocky mountains. deserts, arms of the sea, and even rivers form the barriers; mere preoccupied space seems sufficient in several cases: thus eastern and western australia, in the same latitude, with very similar climate and soils, have scarcely a plant, and few animals or birds, in common, although all belong to the peculiar genera characterising australia. it is in short impossible to explain the differences in the inhabitants, either of the main divisions of the world, or of these sub-divisions, by the differences in their physical conditions, and by the adaptation of their inhabitants. some other cause must intervene. { } _origin_, ed. i. p. , vi. p. . we can see that the destruction of sub-barriers would cause (as before remarked in the case of the main divisions) two sub-divisions to blend into one; and we can only suppose that the original difference in the species, on the opposite sides of sub-barriers, is due to the creation or production of species in distinct areas, from which they have wandered till arrested by such sub-barriers. although thus far is pretty clear, it may be asked, why, when species in the same main division of the world were produced on opposite sides of a sub-barrier, both when exposed to similar conditions and when exposed to widely different influences (as on alpine and lowland tracts, as on arid and humid soils, as in cold and hot climates), have they invariably been formed on a similar type, and that type confined to this one division of the world? why when an ostrich{ } was produced in the southern parts of america, was it formed on the american type, instead of on the african or on australian types? why when hare-like and rabbit-like animals were formed to live on the savannahs of la plata, were they produced on the peculiar rodent type of s. america, instead of on the true{ } hare-type of north america, asia and africa? why when borrowing rodents, and camel-like animals were formed to tenant the cordillera, were they formed on the same type{ } with their representatives on the plains? why were the mice, and many birds of different species on the opposite sides of the cordillera, but exposed to a very similar climate and soil, created on the same peculiar s. american type? why were the plants in eastern and western australia, though wholly different as species, formed on the same peculiar australian types? the generality of the rule, in so many places and under such different circumstances, makes it highly remarkable and seems to demand some explanation. { } the case of the ostrich (_rhea_) occurs in the _origin_, ed. i. p. , vi. p. . { } <note in original.> there is a hare in s. america,--so bad example. { } see _origin_, ed. i. p. , vi. p. . _insular faunas._ if we now look to the character of the inhabitants of small islands{ }, we shall find that those situated close to other land have a similar fauna with that land{ }, whilst those at a considerable distance from other land often possess an almost entirely peculiar fauna. the galapagos archipelago{ } is a remarkable instance of this latter fact; here almost every bird, its one mammifer, its reptiles, land and sea shells, and even fish, are almost all peculiar and distinct species, not found in any other quarter of the world: so are the majority of its plants. but although situated at the distance of between and miles from the s. american coast, it is impossible to even glance at a large part of its fauna, especially at the birds, without at once seeing that they belong to the american type{ }. hence, in fact, groups of islands thus circumstanced form merely small but well-defined sub-divisions of the larger geographical divisions. but the fact is in such cases far more striking: for taking the galapagos archipelago as an instance; in the first place we must feel convinced, seeing that every island is wholly volcanic and bristles with craters, that in a geological sense the whole is of recent origin comparatively with a continent; and as the species are nearly all peculiar, we must conclude that they have in the same sense recently been produced on this very spot; and although in the nature of the soil, and in a lesser degree in the climate, there is a wide difference with the nearer part of the s. american coast, we see that the inhabitants have been formed on the same closely allied type. on the other hand, these islands, as far as their physical conditions are concerned, resemble closely the cape de verde volcanic group, and yet how wholly unlike are the productions of these two archipelagoes. the cape de verde{ } group, to which may be added the canary islands, are allied in their inhabitants (of which many are peculiar species) to the coast of africa and southern europe, in precisely the same manner as the galapagos archipelago is allied to america. we here clearly see that mere geographical proximity affects, more than any relation of adaptation, the character of species. how many islands in the pacific exist far more like in their physical conditions to juan fernandez than this island is to the coast of chile, distant miles; why then, except from mere proximity, should this island alone be tenanted by two very peculiar species of humming-birds--that form of birds which is so exclusively american? innumerable other similar cases might be adduced. { } for the general problem of oceanic islands, see _origin_, ed. i. p. , vi. p. . { } this is an illustration of the general theory of barriers (_origin_, ed. i. p. , vi. p. ). at i. p. , vi. p. the question is discussed from the point of view of means of transport. between the lines, above the words "with that land," the author wrote "cause, formerly joined, no one doubts after lyell." { } _origin_, ed. i. p. , vi. p. . { } see _origin_, ed. i. p. , vi. p. . { } the cape de verde and galapagos archipelagoes are compared in the _origin_, ed. i. p. , vi. p. . see also _journal of researches_, , p. . the galapagos archipelago offers another, even more remarkable, example of the class of facts we are here considering. most of its genera are, as we have said, american, many of them are mundane, or found everywhere, and some are quite or nearly confined to this archipelago. the islands are of absolutely similar composition, and exposed to the same climate; most of them are in sight of each other; and yet several of the islands are inhabited, each by peculiar species (or in some cases perhaps only varieties) of some of the genera characterising the archipelago. so that the small group of the galapagos islands typifies, and follows exactly the same laws in the distribution of its inhabitants, as a great continent. how wonderful it is that two or three closely similar but distinct species of a mocking-thrush{ } should have been produced on three neighbouring and absolutely similar islands; and that these three species of mocking-thrush should be closely related to the other species inhabiting wholly different climates and different districts of america, and only in america. no similar case so striking as this of the galapagos archipelago has hitherto been observed; and this difference of the productions in the different islands may perhaps be partly explained by the depth of the sea between them (showing that they could not have been united within recent geological periods), and by the currents of the sea sweeping _straight_ between them,--and by storms of wind being rare, through which means seeds and birds could be blown, or drifted, from one island to another. there are however some similar facts: it is said that the different, though neighbouring islands of the east indian archipelago are inhabited by some different species of the same genera; and at the sandwich group some of the islands have each their peculiar species of the same genera of plants. { } in the _origin_, ed. i. p. , a strong point is made of birds which immigrated "with facility and in a body" not having been modified. thus the author accounts for the small percentage of peculiar "marine birds." islands standing quite isolated within the intra-tropical oceans have generally very peculiar floras, related, though feebly (as in the case of st helena{ } where almost every species is distinct), with the nearest continent: tristan d'acunha is feebly related, i believe, in its plants, both to africa and s. america, not by having species in common, but by the genera to which they belong{ }. the floras of the numerous scattered islands of the pacific are related to each other and to all the surrounding continents; but it has been said, that they have more of an indo-asiatic than american character{ }. this is somewhat remarkable, as america is nearer to all the eastern islands, and lies in the direction of the trade-wind and prevailing currents; on the other hand, all the heaviest gales come from the asiatic side. but even with the aid of these gales, it is not obvious on the ordinary theory of creation how the possibility of migration (without we suppose, with extreme improbability, that each species with an indo-asiatic character has actually travelled from the asiatic shores, where such species do not now exist) explains this asiatic character in the plants of the pacific. this is no more obvious than that (as before remarked) there should exist a relation between the creation of closely allied species in several regions of the world, and the fact of many such species having wide ranges; and on the other hand, of allied species confined to one region of the world having in that region narrow ranges. { } "the affinities of the st helena flora are strongly south african." hooker's _lecture on insular floras_ in the _gardeners' chronicle_, jan. . { } it is impossible to make out the precise form which the author intended to give to this sentence, but the meaning is clear. { } this is no doubt true, the flora of the sandwich group however has marked american affinities. _alpine floras._ we will now turn to the floras of mountain-summits which are well known to differ from the floras of the neighbouring lowlands. in certain characters, such as dwarfness of stature, hairiness, &c., the species from the most distant mountains frequently resemble each other,--a kind of analogy like that for instance of the succulency of most desert plants. besides this analogy, alpine plants present some eminently curious facts in their distribution. in some cases the summits of mountains, although immensely distant from each other, are clothed by the same identical species{ } which are likewise the same with those growing on the likewise very distant arctic shores. in other cases, although few or none of the species may be actually identical, they are closely related; whilst the plants of the lowland districts surrounding the two mountains in question will be wholly dissimilar. as mountain-summits, as far as their plants are concerned, are islands rising out of an ocean of land in which the alpine species cannot live, nor across which is there any known means of transport, this fact appears directly opposed to the conclusion which we have come to from considering the general distribution of organisms both on continents and on islands--namely, that the degree of relationship between the inhabitants of two points depends on the completeness and nature of the barriers between those points{ }. i believe, however, this anomalous case admits, as we shall presently see, of some explanation. we might have expected that the flora of a mountain summit would have presented the same relation to the flora of the surrounding lowland country, which any isolated part of a continent does to the whole, or an island does to the mainland, from which it is separated by a rather wide space of sea. this in fact is the case with the plants clothing the summits of _some_ mountains, which mountains it may be observed are particularly isolated; for instance, all the species are peculiar, but they belong to the forms characteristic of the surrounding continent, on the mountains of caraccas, of van dieman's land and of the cape of good hope{ }. on some other mountains, for instance <in> tierra del fuego and in brazil, some of the plants though distinct species are s. american forms; whilst others are allied to or are identical with the alpine species of europe. in islands of which the lowland flora is distinct <from> but allied to that of the nearest continent, the alpine plants are sometimes (or perhaps mostly) eminently peculiar and distinct{ }; this is the case on teneriffe, and in a lesser degree even on some of the mediterranean islands. { } see _origin_, ed. i. p. , vi. p. . the present discussion was written before the publication of forbes' celebrated paper on the same subject; see _life and letters_, vol. i. p. . { } the apparent breakdown of the doctrine of barriers is slightly touched on in the _origin_, ed. i. p. , vi. p. . { } in the _origin_, ed. i. p. , vi. p. , the author points out that on the mountains at the cape of good hope "some few representative european forms are found, which have not been discovered in the inter-tropical parts of africa." { } see hooker's _lecture on insular floras_ in the _gardeners' chronicle_, jan. . if all alpine floras had been characterised like that of the mountain of caraccas, or of van dieman's land, &c., whatever explanation is possible of the general laws of geographical distribution would have applied to them. but the apparently anomalous case just given, namely of the mountains of europe, of some mountains in the united states (dr boott) and of the summits of the himalaya (royle), having many identical species in common conjointly with the arctic regions, and many species, though not identical, closely allied, require a separate explanation. the fact likewise of several of the species on the mountains of tierra del fuego (and in a lesser degree on the mountains of brazil) not belonging to american forms, but to those of europe, though so immensely remote, requires also a separate explanation. _cause of the similarity in the floras of some distant mountains._ now we may with confidence affirm, from the number of the then floating icebergs and low descent of the glaciers, that within a period so near that species of shells have remained the same, the whole of central europe and of north america (and perhaps of eastern asia) possessed a very cold climate; and therefore it is probable that the floras of these districts were the same as the present arctic one,--as is known to have been to some degree the case with then existing sea-shells, and those now living on the arctic shores. at this period the mountains must have been covered with ice of which we have evidence in the surfaces polished and scored by glaciers. what then would be the natural and almost inevitable effects of the gradual change into the present more temperate climate{ }? the ice and snow would disappear from the mountains, and as new plants from the more temperate regions of the south migrated northward, replacing the arctic plants, these latter would crawl{ } up the now uncovered mountains, and likewise be driven northward to the present arctic shores. if the arctic flora of that period was a nearly uniform one, as the present one is, then we should have the same plants on these mountain-summits and on the present arctic shores. on this view the arctic flora of that period must have been a widely extended one, more so than even the present one; but considering how similar the physical conditions must always be of land bordering on perpetual frost, this does not appear a great difficulty; and may we not venture to suppose that the almost infinitely numerous icebergs, charged with great masses of rocks, soil and _brushwood_{ } and often driven high up on distant beaches, might have been the means of widely distributing the seeds of the same species? { } in the margin the author has written "(forbes)." this may have been inserted at a date later than , or it may refer to a work by forbes earlier than his alpine paper. { } see _origin_, ed. i. p. , vi. p. . { } <note in original.> perhaps vitality checked by cold and so prevented germinating. <on the carriage of seeds by icebergs, see _origin_, ed. i. p. , vi. p. .> i will only hazard one other observation, namely that during the change from an extremely cold climate to a more temperate one the conditions, both on lowland and mountain, would be singularly favourable for the diffusion of any existing plants, which could live on land, just freed from the rigour of eternal winter; for it would possess no inhabitants; and we cannot doubt that _preoccupation_{ } is the chief bar to the diffusion of plants. for amongst many other facts, how otherwise can we explain the circumstance that the plants on the opposite, though similarly constituted sides of a wide river in eastern europe (as i was informed by humboldt) should be widely different; across which river birds, swimming quadrupeds and the wind must often transport seeds; we can only suppose that plants already occupying the soil and freely seeding check the germination of occasionally transported seeds. { } a note by the author gives "many authors" apparently as authority for this statement. at about the same period when icebergs were transporting boulders in n. america as far as ° south, where the cotton tree now grows in south america, in latitude ° (where the land is now clothed with forests having an almost tropical aspect with the trees bearing epiphytes and intertwined with canes), the same ice action was going on; is it not then in some degree probable that at this period the whole tropical parts of the two americas possessed{ } (as falconer asserts that india did) a more temperate climate? in this case the alpine plants of the long chain of the cordillera would have descended much lower and there would have been a broad high-road{ } connecting those parts of north and south america which were then frigid. as the present climate supervened, the plants occupying the districts which now are become in both hemispheres temperate and even semi-tropical must have been driven to the arctic and antarctic{ } regions; and only a few of the loftiest points of the cordillera can have retained their former connecting flora. the transverse chain of chiquitos might perhaps in a similar manner during the ice-action period have served as a connecting road (though a broken one) for alpine plants to become dispersed from the cordillera to the highlands of brazil. it may be observed that some (though not strong) reasons can be assigned for believing that at about this same period the two americas were not so thoroughly divided as they now are by the west indies and tableland of mexico. i will only further remark that the present most singularly close similarity in the vegetation of the lowlands of kerguelen's land{ } and of tierra del fuego (hooker), though so far apart, may perhaps be explained by the dissemination of seeds during this same cold period, by means of icebergs, as before alluded to{ }. { } opposite to this passage, in the margin, the author has written:--"too hypothetical." { } the cordillera is described as supplying a great line of invasion in the _origin_, ed. i. p. . { } this is an approximation to the author's views on trans-tropical migration (_origin_, ed. i. pp. - ). see thiselton-dyer's interesting discussion in _darwin and modern science_, p. . { } see hooker's _lecture on insular floras_ in the _gardeners' chronicle_, jan. . { } <note by the author.> similarity of flora of coral islands easily explained. finally, i think we may safely grant from the foregoing facts and reasoning that the anomalous similarity in the vegetation of certain very distant mountain-summits is not in truth opposed to the conclusion of the intimate relation subsisting between proximity in space (in accordance with the means of transport in each class) and the degree of affinity of the inhabitants of any two countries. in the case of several quite isolated mountains, we have seen that the general law holds good. _whether the same species has been created more than once._ as the fact of the same species of plants having been found on mountain-summits immensely remote has been one chief cause of the belief of some species having been contemporaneously produced or created at two different points{ }, i will here briefly discuss this subject. on the ordinary theory of creation, we can see no reason why on two similar mountain-summits two similar species may not have been created; but the opposite view, independently of its simplicity, has been generally received from the analogy of the general distribution of all organisms, in which (as shown in this chapter) we almost always find that great and continuous barriers separate distinct series; and we are naturally led to suppose that the two series have been separately created. when taking a more limited view we see a river, with a quite similar country on both sides, with one side well stocked with a certain animal and on the other side not one (as is the case with the bizcacha{ } on the opposite sides of the plata), we are at once led to conclude that the bizcacha was produced on some one point or area on the western side of the river. considering our ignorance of the many strange chances of diffusion by birds (which occasionally wander to immense distances) and quadrupeds swallowing seeds and ova (as in the case of the flying water-beetle which disgorged the eggs of a fish), and of whirlwinds carrying seeds and animals into strong upper currents (as in the case of volcanic ashes and showers of hay, grain and fish{ }), and of the possibility of species having survived for short periods at intermediate spots and afterwards becoming extinct there{ }; and considering our knowledge of the great changes which _have_ taken place from subsidence and elevation in the surface of the earth, and of our ignorance of the greater changes which _may have_ taken place, we ought to be very slow in admitting the probability of double creations. in the case of plants on mountain-summits, i think i have shown how almost necessarily they would, under the past conditions of the northern hemisphere, be as similar as are the plants on the present arctic shores; and this ought to teach us a lesson of caution. { } on centres of creation see _origin_, ed. i. p. , vi. p. . { } in the _journal of researches_, ed. , p. , the distribution of the bizcacha is described as limited by the river uruguay. the case is not i think given in the _origin_. { } in the _origin_, ed. i. a special section (p. , vi. p. ) is devoted to _means of dispersal_. the much greater prominence given to this subject in the _origin_ is partly accounted for by the author's experiments being of later date, _i.e._ (_life and letters_, vol. ii. p. ). the carriage of fish by whirlwinds is given in the _origin_, ed. i. p. , vi. p. . { } the case of islands serving as halting places is given in the _origin_, ed. i. p. , vi. p. . but here the evidence of this having occurred is supposed to be lost by the subsidence of the islands, not merely by the extinction of the species. but the strongest argument against double creations may be drawn from considering the case of mammifers{ } in which, from their nature and from the size of their offspring, the means of distribution are more in view. there are no cases where the same species is found in _very remote_ localities, except where there is a continuous belt of land: the arctic region perhaps offers the strongest exception, and here we know that animals are transported on icebergs{ }. the cases of lesser difficulty may all receive a more or less simple explanation; i will give only one instance; the nutria{ }, i believe, on the eastern coast of s. america live exclusively in fresh-water rivers, and i was much surprised how they could have got into rivulets, widely apart, on the coast of patagonia; but on the opposite coast i found these quadrupeds living exclusively in the sea, and hence their migration along the patagonian coast is not surprising. there is no case of the same mammifer being found on an island far from the coast, and on the mainland, as happens with plants{ }. on the idea of double creations it would be strange if the same species of several plants should have been created in australia and europe; and no one instance of the same species of mammifer having been created, or aboriginally existing, in two as nearly remote and equally isolated points. it is more philosophical, in such cases, as that of some plants being found in australia and europe, to admit that we are ignorant of the means of transport. i will allude only to one other case, namely, that of the mydas{ }, an alpine animal, found only on the distant peaks of the mountains of java: who will pretend to deny that during the ice period of the northern and southern hemispheres, and when india is believed to have been colder, the climate might not have permitted this animal to haunt a lower country, and thus to have passed along the ridges from summit to summit? mr lyell has further observed that, _as in space, so in time_, there is no reason to believe that after the extinction of a species, the self-same form has ever reappeared{ }. i think, then, we may, notwithstanding the many cases of difficulty, conclude with some confidence that every species has been created or produced on a single point or area. { } "we find no inexplicable cases of the same mammal inhabiting distant points of the world." _origin_, ed. i. p. , vi. p. . see also _origin_, ed. i. p. , vi. p. . { } <note by the author.> many authors. <see _origin_, ed. i. p. , vi. p. .> { } _nutria_ is the spanish for otter, and is now a synonym for _lutra_. the otter on the atlantic coast is distinguished by minute differences from the pacific species. both forms are said to take to the sea. in fact the case presents no especial difficulties. { } in _origin_, ed. i. p. , vi. p. , bats are mentioned as an explicable exception to this statement. { } this reference is doubtless to _mydaus_, a badger-like animal from the mountains of java and sumatra (wallace, _geographical distribution_, ii. p. ). the instance does not occur in the _origin_ but the author remarks (_origin_, ed. i. p. , vi. p. ) that cases, strictly analogous to the distribution of plants, occur among terrestrial mammals. { } see _origin_, ed. i. p. , vi. p. . _on the number of species, and of the classes to which they belong in different regions._ the last fact in geographical distribution, which, as far as i can see, in any way concerns the origin of species, relates to the absolute number and nature of the organic beings inhabiting different tracts of land. although every species is admirably adapted (but not necessarily better adapted than every other species, as we have seen in the great increase of introduced species) to the country and station it frequents; yet it has been shown that the entire difference between the species in distant countries cannot possibly be explained by the difference of the physical conditions of these countries. in the same manner, i believe, neither the number of the species, nor the nature of the great classes to which they belong, can possibly in all cases be explained by the conditions of their country. new zealand{ }, a linear island stretching over about miles of latitude, with forests, marshes, plains and mountains reaching to the limits of eternal snow, has far more diversified habitats than an equal area at the cape of good hope; and yet, i believe, at the cape of good hope there are, of phanerogamic plants, from five to ten times the number of species as in all new zealand. why on the theory of absolute creations should this large and diversified island only have from to (? dieffenbach) phanerogamic plants? and why should the cape of good hope, characterised by the uniformity of its scenery, swarm with more species of plants than probably any other quarter of the world? why on the ordinary theory should the galapagos islands abound with terrestrial reptiles? and why should many equal-sized islands in the pacific be without a single one{ } or with only one or two species? why should the great island of new zealand be without one mammiferous quadruped except the mouse, and that was probably introduced with the aborigines? why should not one island (it can be shown, i think, that the mammifers of mauritius and st iago have all been introduced) in the open ocean possess a mammiferous quadruped? let it not be said that quadrupeds cannot live in islands, for we know that cattle, horses and pigs during a long period have run wild in the west indian and falkland islands; pigs at st helena; goats at tahiti; asses in the canary islands; dogs in cuba; cats at ascension; rabbits at madeira and the falklands; monkeys at st iago and the mauritius; even elephants during a long time in one of the very small sooloo islands; and european mice on very many of the smallest islands far from the habitations of man{ }. nor let it be assumed that quadrupeds are more slowly created and hence that the oceanic islands, which generally are of volcanic formation, are of too recent origin to possess them; for we know (lyell) that new forms of quadrupeds succeed each other quicker than mollusca or reptilia. nor let it be assumed (though such an assumption would be no explanation) that quadrupeds cannot be created on small islands; for islands not lying in mid-ocean do possess their peculiar quadrupeds; thus many of the smaller islands of the east indian archipelago possess quadrupeds; as does fernando po on the west coast of africa; as the falkland islands possess a peculiar wolf-like fox{ }; so do the galapagos islands a peculiar mouse of the s. american type. these two last are the most remarkable cases with which i am acquainted; inasmuch as the islands lie further from other land. it is possible that the galapagos mouse may have been introduced in some ship from the s. american coast (though the species is at present unknown there), for the aboriginal species soon haunts the goods of man, as i noticed in the roof of a newly erected shed in a desert country south of the plata. the falkland islands, though between and miles from the s. american coast, may in one sense be considered as intimately connected with it; for it is certain that formerly many icebergs loaded with boulders were stranded on its southern coast, and the old canoes which are occasionally now stranded, show that the currents still set from tierra del fuego. this fact, however, does not explain the presence of the _canis antarcticus_ on the falkland islands, unless we suppose that it formerly lived on the mainland and became extinct there, whilst it survived on these islands, to which it was borne (as happens with its northern congener, the common wolf) on an iceberg, but this fact removes the anomaly of an island, in appearance effectually separated from other land, having its own species of quadruped, and makes the case like that of java and sumatra, each having their own rhinoceros. { } the comparison between new zealand and the cape is given in the _origin_, ed. i. p. , vi. p. . { } in a corresponding discussion in the _origin_, ed. i. p. , vi. p. , stress is laid on the distribution of batrachians not of reptiles. { } the whole argument is given--more briefly than here--in the _origin_, ed. i. p. , vi. p. . { } see _origin_, ed i. p. , vi. p. . the discussion is much fuller in the present essay. before summing up all the facts given in this section on the present condition of organic beings, and endeavouring to see how far they admit of explanation, it will be convenient to state all such facts in the past geographical distribution of extinct beings as seem anyway to concern the theory of descent. section second. _geographical distribution of extinct organisms._ i have stated that if the land of the entire world be divided into (we will say) three sections, according to the amount of difference of the terrestrial mammifers inhabiting them, we shall have three unequal divisions of ( st) australia and its dependent islands, ( nd) south america, ( rd) europe, asia and africa. if we now look to the mammifers which inhabited these three divisions during the later tertiary periods, we shall find them almost as distinct as at the present day, and intimately related in each division to the existing forms in that division{ }. this is wonderfully the case with the several fossil marsupial genera in the caverns of new south wales and even more wonderfully so in south america, where we have the same peculiar group of monkeys, of a guanaco-like animal, of many rodents, of the marsupial didelphys, of armadilloes and other edentata. this last family is at present very characteristic of s. america, and in a late tertiary epoch it was even more so, as is shown by the numerous enormous animals of the megatheroid family, some of which were protected by an osseous armour like that, but on a gigantic scale, of the recent armadillo. lastly, over europe the remains of the several deer, oxen, bears, foxes, beavers, field-mice, show a relation to the present inhabitants of this region; and the contemporaneous remains of the elephant, rhinoceros, hippopotamus, hyæna, show a relation with the grand africo-asiatic division of the world. in asia the fossil mammifers of the himalaya (though mingled with forms long extinct in europe) are equally related to the existing forms of the africo-asiatic division; but especially to those of india itself. as the gigantic and now extinct quadrupeds of europe have naturally excited more attention than the other and smaller remains, the relation between the past and the present mammiferous inhabitants of europe has not been sufficiently attended to. but in fact the mammifers of europe are at present nearly as much africo-asiatic as they were formerly when europe had its elephants and rhinoceroses, etc.; europe neither now nor then possessed peculiar groups as does australia and s. america. the extinction of certain peculiar forms in one quarter does not make the remaining mammifers of that quarter less related to its own great division of the world: though tierra del fuego possesses only a fox, three rodents, and the guanaco, no one (as these all belong to s. american types, but not to the most characteristic forms) would doubt for one minute <as to> classifying this district with s. america; and if fossil edentata, marsupials and monkeys were to be found in tierra del fuego, it would not make this district more truly s. american than it now is. so it is with europe{ }, and so far as is known with asia, for the lately past and present mammifers all belong to the africo-asiatic division of the world. in every case, i may add, the forms which a country has is of more importance in geographical arrangement than what it has not. { } see _origin_, ed. i. p. , vi. p. . { } in the _origin_, ed. i. p. , vi. p. , which corresponds to this part of the present essay, the author does not make a separate section for such cases as the occurrence of fossil marsupials in europe (_origin_, ed. i. p. , vi. p. ) as he does in the present essay; see the section on _changes in geographical distribution_, p. . we find some evidence of the same general fact in a relation between the recent and the tertiary sea-shells, in the different main divisions of the marine world. this general and most remarkable relation between the lately past and present mammiferous inhabitants of the three main divisions of the world is precisely the same kind of fact as the relation between the different species of the several sub-regions of any one of the main divisions. as we usually associate great physical changes with the total extinction of one series of beings, and its succession by another series, this identity of relation between the past and the present races of beings in the same quarters of the globe is more striking than the same relation between existing beings in different sub-regions: but in truth we have no reason for supposing that a change in the conditions has in any of these cases supervened, greater than that now existing between the temperate and tropical, or between the highlands and lowlands of the same main divisions, now tenanted by related beings. finally, then, we clearly see that in each main division of the world the same relation holds good between its inhabitants in time as over space{ }. { } "we can understand how it is that all the forms of life, ancient and recent, make together one grand system; for all are connected by generation." _origin_, ed. i. p. , vi. p. . _changes in geographical distribution._ if, however, we look closer, we shall find that even australia, in possessing a terrestrial pachyderm, was so far less distinct from the rest of the world than it now is; so was s. america in possessing the mastodon, horse, [hyæna,]{ } and antelope. n. america, as i have remarked, is now, in its mammifers, in some respects neutral ground between s. america and the great africo-asiatic division; formerly, in possessing the horse, mastodon and three megatheroid animals, it was more nearly related to s. america; but in the horse and mastodon, and likewise in having the elephant, oxen, sheep, and pigs, it was as much, if not more, related to the africo-asiatic division. again, northern india was much more closely related (in having the giraffe, hippopotamus, and certain musk-deer) to southern africa than it now is; for southern and eastern africa deserve, if we divide the world into five parts, to make one division by itself. turning to the dawn of the tertiary period, we must, from our ignorance of other portions of the world, confine ourselves to europe; and at that period, in the presence of marsupials{ } and edentata, we behold an _entire_ blending of those mammiferous forms which now eminently characterise australia and s. america{ }. { } the word _hyæna_ is erased. there appear to be no fossil hyænidæ in s. america. { } see note { }, p. , also _origin_, ed. i. p. , vi. p. . { } <note by the author.> and see eocene european mammals in n. america. if we now look at the distribution of sea-shells, we find the same changes in distribution. the red sea and the mediterranean were more nearly related in these shells than they now are. in different parts of europe, on the other hand, during the miocene period, the sea-shells seem to have been more different than at present. in{ } the tertiary period, according to lyell, the shells of n. america and europe were less related than at present, and during the cretaceous still less like; whereas, during this same cretaceous period, the shells of india and europe were more like than at present. but going further back to the carbonaceous period, in n. america and europe, the productions were much more like than they now are{ }. these facts harmonise with the conclusions drawn from the present distribution of organic beings, for we have seen, that from species being created in different points or areas, the formation of a barrier would cause or make two distinct geographical areas; and the destruction of a barrier would permit their diffusion{ }. and as long-continued geological changes must both destroy and make barriers, we might expect, the further we looked backwards, the more changed should we find the present distribution. this conclusion is worthy of attention; because, finding in widely different parts of the same main division of the world, and in volcanic islands near them, groups of distinct, but related, species;--and finding that a singularly analogous relation holds good with respect to the beings of past times, when none of the present species were living, a person might be tempted to believe in some mystical relation between certain areas of the world, and the production of certain organic forms; but we now see that such an assumption would have to be complicated by the admission that such a relation, though holding good for long revolutions of years, is not truly persistent. { } <note by the author.> all this requires much verification. { } this point seems to be less insisted on in the _origin_. { } _origin_, ed. i. p. , vi. p. . i will only add one more observation to this section. geologists finding in the most remote period with which we are acquainted, namely in the silurian period, that the shells and other marine productions{ } in north and south america, in europe, southern africa, and western asia, are much more similar than they now are at these distant points, appear to have imagined that in these ancient times the laws of geographical distribution were quite different than what they now are: but we have only to suppose that great continents were extended east and west, and thus did not divide the inhabitants of the temperate and tropical seas, as the continents now do; and it would then become probable that the inhabitants of the seas would be much more similar than they now are. in the immense space of ocean extending from the east coast of africa to the eastern islands of the pacific, which space is connected either by lines of tropical coast or by islands not very distant from each other, we know (cuming) that many shells, perhaps even as many as , are common to the zanzibar coast, the philippines, and the eastern islands of the low or dangerous archipelago in the pacific. this space equals that from the arctic to the antarctic pole! pass over the space of quite open ocean, from the dangerous archipelago to the west coast of s. america, and every shell is different: pass over the narrow space of s. america, to its eastern shores, and again every shell is different! many fish, i may add, are also common to the pacific and indian oceans. { } <note by the author.> d'orbigny shows that this is not so. _summary on the distribution of living and extinct organic beings._ let us sum up the several facts now given with respect to the past and present geographical distribution of organic beings. in a previous chapter it was shown that species are not exterminated by universal catastrophes, and that they are slowly produced: we have also seen that each species is probably only once produced, on one point or area once in time; and that each diffuses itself, as far as barriers and its conditions of life permit. if we look at any one main division of the land, we find in the different parts, whether exposed to different conditions or to the same conditions, many groups of species wholly or nearly distinct as species, nevertheless intimately related. we find the inhabitants of islands, though distinct as species, similarly related to the inhabitants of the nearest continent; we find in some cases, that even the different islands of one such group are inhabited by species distinct, though intimately related to one another and to those of the nearest continent:--thus typifying the distribution of organic beings over the whole world. we find the floras of distant mountain-summits either very similar (which seems to admit, as shown, of a simple explanation) or very distinct but related to the floras of the surrounding region; and hence, in this latter case, the floras of two mountain-summits, although exposed to closely similar conditions, will be very different. on the mountain-summits of islands, characterised by peculiar faunas and floras, the plants are often eminently peculiar. the dissimilarity of the organic beings inhabiting nearly similar countries is best seen by comparing the main divisions of the world; in each of which some districts may be found very similarly exposed, yet the inhabitants are wholly unlike;--far more unlike than those in very dissimilar districts in the same main division. we see this strikingly in comparing two volcanic archipelagoes, with nearly the same climate, but situated not very far from two different continents; in which case their inhabitants are totally unlike. in the different main divisions of the world, the amount of difference between the organisms, even in the same class, is widely different, each main division having only the species distinct in some families, in other families having the genera distinct. the distribution of aquatic organisms is very different from that of the terrestrial organisms; and necessarily so, from the barriers to their progress being quite unlike. the nature of the conditions in an isolated district will not explain the number of species inhabiting it; nor the absence of one class or the presence of another class. we find that terrestrial mammifers are not present on islands far removed from other land. we see in two regions, that the species though distinct are more or less related, according to the greater or less _possibility_ of the transportal in past and present times of species from one to the other region; although we can hardly admit that all the species in such cases have been transported from the first to the second region, and since have become extinct in the first: we see this law in the presence of the fox on the falkland islands; in the european character of some of the plants of tierra del fuego; in the indo-asiatic character of the plants of the pacific; and in the circumstance of those genera which range widest having many species with wide ranges; and those genera with restricted ranges having species with restricted ranges. finally, we find in each of the main divisions of the land, and probably of the sea, that the existing organisms are related to those lately extinct. looking further backwards we see that the past geographical distribution of organic beings was different from the present; and indeed, considering that geology shows that all our land was once under water, and that where water now extends land is forming, the reverse could hardly have been possible. now these several facts, though evidently all more or less connected together, must by the creationist (though the geologist may explain some of the anomalies) be considered as so many ultimate facts. he can only say, that it so pleased the creator that the organic beings of the plains, deserts, mountains, tropical and temperature forests, of s. america, should all have some affinity together; that the inhabitants of the galapagos archipelago should be related to those of chile; and that some of the species on the similarly constituted islands of this archipelago, though most closely related, should be distinct; that all its inhabitants should be totally unlike those of the similarly volcanic and arid cape de verde and canary islands; that the plants on the summit of teneriffe should be eminently peculiar; that the diversified island of new zealand should have not many plants, and not one, or only one, mammifer; that the mammifers of s. america, australia and europe should be clearly related to their ancient and exterminated prototypes; and so on with other facts. but it is absolutely opposed to every analogy, drawn from the laws imposed by the creator on inorganic matter, that facts, when connected, should be considered as ultimate and not the direct consequences of more general laws. section third. _an attempt to explain the foregoing laws of geographical distribution, on the theory of allied species having a common descent._ first let us recall the circumstances most favourable for variation under domestication, as given in the first chapter--viz. st, a change, or repeated changes, in the conditions to which the organism has been exposed, continued through several seminal (_i.e._ not by buds or divisions) generations: nd, steady selection of the slight varieties thus generated with a fixed end in view: rd, isolation as perfect as possible of such selected varieties; that is, the preventing their crossing with other forms; this latter condition applies to all terrestrial animals, to most if not all plants and perhaps even to most (or all) aquatic organisms. it will be convenient here to show the advantage of isolation in the formation of a new breed, by comparing the progress of two persons (to neither of whom let time be of any consequence) endeavouring to select and form some very peculiar new breed. let one of these persons work on the vast herds of cattle in the plains of la plata{ }, and the other on a small stock of or animals in an island. the latter might have to wait centuries (by the hypothesis of no importance){ } before he obtained a "sport" approaching to what he wanted; but when he did and saved the greater number of its offspring and their offspring again, he might hope that his whole little stock would be in some degree affected, so that by continued selection he might gain his end. but on the pampas, though the man might get his first approach to his desired form sooner, how hopeless would it be to attempt, by saving its offspring amongst so many of the common kind, to affect the whole herd: the effect of this one peculiar "sport{ }" would be quite lost before he could obtain a second original sport of the same kind. if, however, he could separate a small number of cattle, including the offspring of the desirable "sport," he might hope, like the man on the island, to effect his end. if there be organic beings of which two individuals _never_ unite, then simple selection whether on a continent or island would be equally serviceable to make a new and desirable breed; and this new breed might be made in surprisingly few years from the great and geometrical powers of propagation to beat out the old breed; as has happened (notwithstanding crossing) where good breeds of dogs and pigs have been introduced into a limited country,--for instance, into the islands of the pacific. { } this instance occurs in the essay of , p. , but not in the _origin_; though the importance of isolation is discussed (_origin_, ed. i. p. , vi. p. ). { } the meaning of the words within parenthesis is obscure. { } it is unusual to find the author speaking of the selection of _sports_ rather than small variations. let us now take the simplest natural case of an islet upheaved by the volcanic or subterranean forces in a deep sea, at such a distance from other land that only a few organic beings at rare intervals were transported to it, whether borne by the sea{ } (like the seeds of plants to coral-reefs), or by hurricanes, or by floods, or on rafts, or in roots of large trees, or the germs of one plant or animal attached to or in the stomach of some other animal, or by the intervention (in most cases the most probable means) of other islands since sunk or destroyed. it may be remarked that when one part of the earth's crust is raised it is probably the general rule that another part sinks. let this island go on slowly, century after century, rising foot by foot; and in the course of time we shall have instead <of> a small mass of rock{ }, lowland and highland, moist woods and dry sandy spots, various soils, marshes, streams and pools: under water on the sea shore, instead of a rocky steeply shelving coast, we shall have in some parts bays with mud, sandy beaches and rocky shoals. the formation of the island by itself must often slightly affect the surrounding climate. it is impossible that the first few transported organisms could be perfectly adapted to all these stations; and it will be a chance if those successively transported will be so adapted. the greater number would probably come from the lowlands of the nearest country; and not even all these would be perfectly adapted to the new islet whilst it continued low and exposed to coast influences. moreover, as it is certain that all organisms are nearly as much adapted in their structure to the other inhabitants of their country as they are to its physical conditions, so the mere fact that a _few_ beings (and these taken in great degree by chance) were in the first case transported to the islet, would in itself greatly modify their conditions{ }. as the island continued rising we might also expect an occasional new visitant; and i repeat that even one new being must often affect beyond our calculation by occupying the room and taking part of the subsistence of another (and this again from another and so on), several or many other organisms. now as the first transported and any occasional successive visitants spread or tended to spread over the growing island, they would undoubtedly be exposed through several generations to new and varying conditions: it might also easily happen that some of the species _on an average_ might obtain an increase of food, or food of a more nourishing quality{ }. according then to every analogy with what we have seen takes place in every country, with nearly every organic being under domestication, we might expect that some of the inhabitants of the island would "sport," or have their organization rendered in some degree plastic. as the number of the inhabitants are supposed to be few and as all these cannot be so well adapted to their new and varying conditions as they were in their native country and habitat, we cannot believe that every place or office in the economy of the island would be as well filled as on a continent where the number of aboriginal species is far greater and where they consequently hold a more strictly limited place. we might therefore expect on our island that although very many slight variations were of no use to the plastic individuals, yet that occasionally in the course of a century an individual might be born{ } of which the structure or constitution in some slight degree would allow it better to fill up some office in the insular economy and to struggle against other species. if such were the case the individual and its offspring would have a better _chance_ of surviving and of beating out its parent form; and if (as is probable) it and its offspring crossed with the unvaried parent form, yet the number of the individuals being not very great, there would be a chance of the new and more serviceable form being nevertheless in some slight degree preserved. the struggle for existence would go on annually selecting such individuals until a new race or species was formed. either few or all the first visitants to the island might become modified, according as the physical conditions of the island and those resulting from the kind and number of other transported species were different from those of the parent country--according to the difficulties offered to fresh immigration--and according to the length of time since the first inhabitants were introduced. it is obvious that whatever was the country, generally the nearest from which the first tenants were transported, they would show an affinity, even if all had become modified, to the natives of that country and even if the inhabitants of the same source (?) had been modified. on this view we can at once understand the cause and meaning of the affinity of the fauna and flora of the galapagos islands with that of the coast of s. america; and consequently why the inhabitants of these islands show not the smallest affinity with those inhabiting other volcanic islands, with a very similar climate and soil, near the coast of africa{ }. { } this brief discussion is represented in the _origin_, ed. i. by a much fuller one (pp. , , vi. pp. , ). see, however, the section in the present essay, p. . { } on the formation of new stations, see _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. pp. , , vi. pp. , . { } in the ms. _some of the species ... nourishing quality_ is doubtfully erased. it seems clear that he doubted whether such a problematical supply of food would be likely to cause variation. { } at this time the author clearly put more faith in the importance of sport-like variation than in later years. { } _origin_, ed. i. p. , vi. p. . to return once again to our island, if by the continued action of the subterranean forces other neighbouring islands were formed, these would generally be stocked by the inhabitants of the first island, or by a few immigrants from the neighbouring mainland; but if considerable obstacles were interposed to any communication between the terrestrial productions of these islands, and their conditions were different (perhaps only by the number of different species on each island), a form transported from one island to another might become altered in the same manner as one from the continent; and we should have several of the islands tenanted by representative races or species, as is so wonderfully the case with the different islands of the galapagos archipelago. as the islands become mountainous, if mountain-species were not introduced, as could rarely happen, a greater amount of variation and selection would be requisite to adapt the species, which originally came from the lowlands of the nearest continent, to the mountain-summits than to the lower districts of our islands. for the lowland species from the continent would have first to struggle against other species and other conditions on the coast-land of the island, and so probably become modified by the selection of its best fitted varieties, then to undergo the same process when the land had attained a moderate elevation; and then lastly when it had become alpine. hence we can understand why the faunas of insular mountain-summits are, as in the case of teneriffe, eminently peculiar. putting on one side the case of a widely extended flora being driven up the mountain-summits, during a change of climate from cold to temperate, we can see why in other cases the floras of mountain-summits (or as i have called them islands in a sea of land) should be tenanted by peculiar species, but related to those of the surrounding lowlands, as are the inhabitants of a real island in the sea to those of the nearest continent{ }. { } see _origin_, ed. i. p. , vi. p. , where the author speaks of alpine humming birds, rodents, plants, &c. in s. america, all of strictly american forms. in the ms. the author has added between the lines "as world has been getting hotter, there has been radiation from high-lands,--old view?--curious; i presume diluvian in origin." let us now consider the effect of a change of climate or of other conditions on the inhabitants of a continent and of an isolated island without any great change of level. on a continent the chief effects would be changes in the numerical proportion of the individuals of the different species; for whether the climate became warmer or colder, drier or damper, more uniform or extreme, some species are at present adapted to its diversified districts; if for instance it became cooler, species would migrate from its more temperate parts and from its higher land; if damper, from its damper regions, &c. on a small and isolated island, however, with few species, and these not adapted to much diversified conditions, such changes instead of merely increasing the number of certain species already adapted to such conditions, and decreasing the number of other species, would be apt to affect the constitutions of some of the insular species: thus if the island became damper it might well happen that there were no species living in any part of it adapted to the consequences resulting from more moisture. in this case therefore, and still more (as we have seen) during the production of new stations from the elevation of the land, an island would be a far more fertile source, as far as we can judge, of new specific forms than a continent. the new forms thus generated on an island, we might expect, would occasionally be transported by accident, or through long-continued geographical changes be enabled to emigrate and thus become slowly diffused. but if we look to the origin of a continent; almost every geologist will admit that in most cases it will have first existed as separate islands which gradually increased in size{ }; and therefore all that which has been said concerning the probable changes of the forms tenanting a small archipelago is applicable to a continent in its early state. furthermore, a geologist who reflects on the geological history of europe (the only region well known) will admit that it has been many times depressed, raised and left stationary. during the sinking of a continent and the probable generally accompanying changes of climate the effect would be little, _except_ on the numerical proportions and in the extinction (from the lessening of rivers, the drying of marshes and the conversion of high-lands into low &c.) of some or of many of the species. as soon however as the continent became divided into many isolated portions or islands, preventing free immigration from one part to another, the effect of climatic and other changes on the species would be greater. but let the now broken continent, forming isolated islands, begin to rise and new stations thus to be formed, exactly as in the first case of the upheaved volcanic islet, and we shall have equally favourable conditions for the modification of old forms, that is the formation of new races or species. let the islands become reunited into a continent; and then the new and old forms would all spread, as far as barriers, the means of transportal, and the preoccupation of the land by other species, would permit. some of the new species or races would probably become extinct, and some perhaps would cross and blend together. we should thus have a multitude of forms, adapted to all kinds of slightly different stations, and to diverse groups of either antagonist or food-serving species. the oftener these oscillations of level had taken place (and therefore generally the older the land) the greater the number of species <which> would tend to be formed. the inhabitants of a continent being thus derived in the first stage from the same original parents, and subsequently from the inhabitants of one wide area, since often broken up and reunited, all would be obviously related together and the inhabitants of the most _dissimilar_ stations on the same continent would be more closely allied than the inhabitants of two very _similar_ stations on two of the main divisions of the world{ }. { } see the comparison between the malay archipelago and the probable former state of europe, _origin_, ed. i. p. , vi. p. , also _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . the arrangement of the argument in the present essay leads to repetition of statements made in the earlier part of the book: in the _origin_ this is avoided. i need hardly point out that we now can obviously see why the number of species in two districts, independently of the number of stations in such districts, should be in some cases as widely different as in new zealand and the cape of good hope{ }. we can see, knowing the difficulty in the transport of terrestrial mammals, why islands far from mainlands do not possess them{ }; we see the general reason, namely accidental transport (though not the precise reason), why certain islands should, and others should not, possess members of the class of reptiles. we can see why an ancient channel of communication between two distant points, as the cordillera probably was between southern chile and the united states during the former cold periods; and icebergs between the falkland islands and tierra del fuego; and gales, at a former or present time, between the asiatic shores of the pacific and eastern islands in this ocean; is connected with (or we may now say causes) an affinity between the species, though distinct, in two such districts. we can see how the better chance of diffusion, from several of the species of any genus having wide ranges in their own countries, explains the presence of other species of the same genus in other countries{ }; and on the other hand, of species of restricted powers of ranging, forming genera with restricted ranges. { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. pp. , , vi. pp. , . as every one would be surprised if two exactly similar but peculiar varieties{ } of any species were raised by man by long continued selection, in two different countries, or at two very different periods, so we ought not to expect that an exactly similar form would be produced from the modification of an old one in two distinct countries or at two distinct periods. for in such places and times they would probably be exposed to somewhat different climates and almost certainly to different associates. hence we can see why each species appears to have been produced singly, in space and in time. i need hardly remark that, according to this theory of descent, there is no necessity of modification in a species, when it reaches a new and isolated country. if it be able to survive and if slight variations better adapted to the new conditions are not selected, it might retain (as far as we can see) its old form for an indefinite time. as we see that some sub-varieties produced under domestication are more variable than others, so in nature, perhaps, some species and genera are more variable than others. the same precise form, however, would probably be seldom preserved through successive geological periods, or in widely and differently conditioned countries{ }. { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . finally, during the long periods of time and probably of oscillations of level, necessary for the formation of a continent, we may conclude (as above explained) that many forms would become extinct. these extinct forms, and those surviving (whether or not modified and changed in structure), will all be related in each continent in the same manner and degree, as are the inhabitants of any two different sub-regions in that same continent. i do not mean to say that, for instance, the present marsupials of australia or edentata and rodents of s. america have descended from any one of the few fossils of the same orders which have been discovered in these countries. it is possible that, in a very few instances, this may be the case; but generally they must be considered as merely codescendants of common stocks{ }. i believe in this, from the improbability, considering the vast number of species, which (as explained in the last chapter) must by our theory have existed, that the _comparatively_ few fossils which have been found should chance to be the immediate and linear progenitors of those now existing. recent as the yet discovered fossil mammifers of s. america are, who will pretend to say that very many intermediate forms may not have existed? moreover, we shall see in the ensuing chapter that the very existence of genera and species can be explained only by a few species of each epoch leaving modified successors or new species to a future period; and the more distant that future period, the fewer will be the _linear_ heirs of the former epoch. as by our theory, all mammifers must have descended from the same parent stock, so is it necessary that each land now possessing terrestrial mammifers shall at some time have been so far united to other land as to permit the passage of mammifers{ }; and it accords with this necessity, that in looking far back into the earth's history we find, first changes in the geographical distribution, and secondly a period when the mammiferous forms most distinctive of two of the present main divisions of the world were living together{ }. { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . i think then i am justified in asserting that most of the above enumerated and often trivial points in the geographical distribution of past and present organisms (which points must be viewed by the creationists as so many ultimate facts) follow as a simple consequence of specific forms being mutable and of their being adapted by natural selection to diverse ends, conjoined with their powers of dispersal, and the geologico-geographical changes now in slow progress and which undoubtedly have taken place. this large class of facts being thus explained, far more than counterbalances many separate difficulties and apparent objections in convincing my mind of the truth of this theory of common descent. _improbability of finding fossil forms intermediate between existing species._ there is one observation of considerable importance that may be here introduced, with regard to the improbability of the chief transitional forms between any two species being found fossil. with respect to the finer shades of transition, i have before remarked that no one has any cause to expect to trace them in a fossil state, without he be bold enough to imagine that geologists at a future epoch will be able to trace from fossil bones the gradations between the short-horns, herefordshire, and alderney breeds of cattle{ }. i have attempted to show that rising islands, in process of formation, must be the best nurseries of new specific forms, and these points are the least favourable for the embedment of fossils{ }: i appeal, as evidence, to the state of the _numerous_ scattered islands in the several great oceans: how rarely do any sedimentary deposits occur on them; and when present they are mere narrow fringes of no great antiquity, which the sea is generally wearing away and destroying. the cause of this lies in isolated islands being generally volcanic and rising points; and the effects of subterranean elevation is to bring up the surrounding newly-deposited strata within the destroying action of the coast-waves: the strata, deposited at greater distances, and therefore in the depths of the ocean, will be almost barren of organic remains. these remarks may be generalised:--periods of subsidence will always be most favourable to an accumulation of great thicknesses of strata, and consequently to their long preservation; for without one formation be protected by successive strata, it will seldom be preserved to a distant age, owing to the enormous amount of denudation, which seems to be a general contingent of time{ }. i may refer, as evidence of this remark, to the vast amount of subsidence evident in the great pile of the european formations, from the silurian epoch to the end of the secondary, and perhaps to even a later period. periods of elevation on the other hand cannot be favourable to the accumulation of strata and their preservation to distant ages, from the circumstance just alluded to, viz. of elevation tending to bring to the surface the circum-littoral strata (always abounding most in fossils) and destroying them. the bottom of tracts of deep water (little favourable, however, to life) must be excepted from this unfavourable influence of elevation. in the quite open ocean, probably no sediment{ } is accumulating, or at a rate so slow as not to preserve fossil remains, which will always be subject to disintegration. caverns, no doubt, will be equally likely to preserve terrestrial fossils in periods of elevation and of subsidence; but whether it be owing to the enormous amount of denudation, which all land seems to have undergone, no cavern with fossil bones has been found belonging to the secondary period{ }. { } _origin_, ed. i. p. , vi. p. . { } "nature may almost be said to have guarded against the frequent discovery of her transitional or linking forms," _origin_, ed. i. p. . a similar but not identical passage occurs in _origin_, ed. vi. p. . { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . hence many more remains will be preserved to a distant age, in any region of the world, during periods of its subsidence{ }, than of its elevation. { } _origin_, ed. i. p. , vi. p. . but during the subsidence of a tract of land, its inhabitants (as before shown) will from the decrease of space and of the diversity of its stations, and from the land being fully preoccupied by species fitted to diversified means of subsistence, be little liable to modification from selection, although many may, or rather must, become extinct. with respect to its circum-marine inhabitants, although during a change from a continent to a _great_ archipelago, the number of stations fitted for marine beings will be increased, their means of diffusion (an important check to change of form) will be greatly improved; for a continent stretching north and south, or a quite open space of ocean, seems to be to them the only barrier. on the other hand, during the elevation of a small archipelago and its conversion into a continent, we have, whilst the number of stations are increasing, both for aquatic and terrestrial productions, and whilst these stations are not fully preoccupied by perfectly adapted species, the most favourable conditions for the selection of new specific forms; but few of them in their early transitional states will be preserved to a distant epoch. we must wait during an enormous lapse of time, until long-continued subsidence shall have taken the place in this quarter of the world of the elevatory process, for the best conditions of the embedment and the preservation of its inhabitants. generally the great mass of the strata in every country, from having been chiefly accumulated during subsidence, will be the tomb, not of transitional forms, but of those either becoming extinct or remaining unmodified. the state of our knowledge, and the slowness of the changes of level, do not permit us to test the truth of these remarks, by observing whether there are more transitional or "fine" (as naturalists would term them) species, on a rising and enlarging tract of land, than on an area of subsidence. nor do i know whether there are more "fine" species on isolated volcanic islands in process of formation, than on a continent; but i may remark, that at the galapagos archipelago the number of forms, which according to some naturalists are true species, and according to others are mere races, is considerable: this particularly applies to the different species or races of the same genera inhabiting the different islands of this archipelago. furthermore it may be added (as bearing on the great facts discussed in this chapter) that when naturalists confine their attention to any one country, they have comparatively little difficulty in determining what forms to call species and what to call varieties; that is, those which can or cannot be traced or shown to be probably descendants of some other form: but the difficulty increases, as species are brought from many stations, countries and islands. it was this increasing (but i believe in few cases insuperable) difficulty which seems chiefly to have urged lamarck to the conclusion that species are mutable. chapter vii on the nature of the affinities and classification of organic beings{ } { } ch. xiii of the _origin_, ed. i., ch. xiv ed. vi. begins with a similar statement. in the present essay the author adds a note:--"the obviousness of the fact (_i.e._ the natural grouping of organisms) alone prevents it being remarkable. it is scarcely explicable by creationist: groups of aquatic, of vegetable feeders and carnivorous, &c., might resemble each other; but why as it is. so with plants,--analogical resemblance thus accounted for. must not here enter into details." this argument is incorporated with the text in the _origin_, ed. i. _gradual appearance and disappearance of groups._ it has been observed from the earliest times that organic beings fall into groups{ }, and these groups into others of several values, such as species into genera, and then into sub-families, into families, orders, &c. the same fact holds with those beings which no longer exist. groups of species seem to follow the same laws in their appearance and extinction{ }, as do the individuals of any one species: we have reason to believe that, first, a few species appear, that their numbers increase; and that, when tending to extinction, the numbers of the species decrease, till finally the group becomes extinct, in the same way as a species becomes extinct, by the individuals becoming rarer and rarer. moreover, groups, like the individuals of a species, appear to become extinct at different times in different countries. the palæotherium was extinct much sooner in europe than in india: the trigonia{ } was extinct in early ages in europe, but now lives in the seas of australia. as it happens that one species of a family will endure for a much longer period than another species, so we find that some whole groups, such as mollusca, tend to retain their forms, or to remain persistent, for longer periods than other groups, for instance than the mammalia. groups therefore, in their appearance, extinction, and rate of change or succession, seem to follow nearly the same laws with the individuals of a species{ }. { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . { } in the _origin_, ed. i. this preliminary matter is replaced (pp. , , vi. pp. , ) by a discussion in which extinction is also treated, but chiefly from the point of view of the theory of divergence. _what is the natural system?_ the proper arrangement of species into groups, according to the natural system, is the object of all naturalists; but scarcely two naturalists will give the same answer to the question, what is the natural system and how are we to recognise it? the most important characters{ } it might be thought (as it was by the earliest classifiers) ought to be drawn from those parts of the structure which determine its habits and place in the economy of nature, which we may call the final end of its existence. but nothing is further from the truth than this; how much external resemblance there is between the little otter (chironectes) of guiana and the common otter; or again between the common swallow and the swift; and who can doubt that the means and ends of their existence are closely similar, yet how grossly wrong would be the classification, which put close to each other a marsupial and placental animal, and two birds with widely different skeletons. relations, such as in the two latter cases, or as that between the whale and fishes, are denominated "analogical{ }," or are sometimes described as "relations of adaption." they are infinitely numerous and often very singular; but are of no use in the classification of the higher groups. how it comes, that certain parts of the structure, by which the habits and functions of the species are settled, are of no use in classification, whilst other parts, formed at the same time, are of the greatest, it would be difficult to say, on the theory of separate creations. { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . some authors as lamarck, whewell &c., believe that the degree of affinity on the natural system depends on the degrees of resemblance in organs more or less physiologically important for the preservation of life. this scale of importance in the organs is admitted to be of difficult discovery. but quite independent of this, the proposition, as a general rule, must be rejected as false; though it may be partially true. for it is universally admitted that the same part or organ, which is of the highest service in classification in one group, is of very little use in another group, though in both groups, as far as we can see, the part or organ is of equal physiological importance: moreover, characters quite unimportant physiologically, such as whether the covering of the body consists of hair or feathers, whether the nostrils communicated with the mouth{ } &c., &c., are of the highest generality in classification; even colour, which is so inconstant in many species, will sometimes well characterise even a whole group of species. lastly, the fact, that no one character is of so much importance in determining to what great group an organism belongs, as the forms through which the embryo{ } passes from the germ upwards to maturity, cannot be reconciled with the idea that natural classification follows according to the degrees of resemblance in the parts of most physiological importance. the affinity of the common rock-barnacle with the crustaceans can hardly be perceived in more than a single character in its mature state, but whilst young, locomotive, and furnished with eyes, its affinity cannot be mistaken{ }. the cause of the greater value of characters, drawn from the early stages of life, can, as we shall in a succeeding chapter see, be in a considerable degree explained, on the theory of descent, although inexplicable on the views of the creationist. { } these instances occur with others in the _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. pp. , , vi. pp. , . practically, naturalists seem to classify according to the resemblance of those parts or organs which in related groups are most uniform, or vary least{ }: thus the æstivation, or manner in which the petals etc. are folded over each other, is found to afford an unvarying character in most families of plants, and accordingly any difference in this respect would be sufficient to cause the rejection of a species from many families; but in the rubiaceæ the æstivation is a varying character, and a botanist would not lay much stress on it, in deciding whether or not to class a new species in this family. but this rule is obviously so arbitrary a formula, that most naturalists seem to be convinced that something ulterior is represented by the natural system; they appear to think that we only discover by such similarities what the arrangement of the system is, not that such similarities make the system. we can only thus understand linnæus'{ } well-known saying, that the characters do not make the genus; but that the genus gives the characters: for a classification, independent of characters, is here presupposed. hence many naturalists have said that the natural system reveals the plan of the creator: but without it be specified whether order in time or place, or what else is meant by the plan of the creator, such expressions appear to me to leave the question exactly where it was. { } _origin_, ed. i. pp. , , vi. pp. , . { } _origin_, ed. i. p. , vi. p. . some naturalists consider that the geographical position{ } of a species may enter into the consideration of the group into which it should be placed; and most naturalists (either tacitly or openly) give value to the different groups, not solely by their relative differences in structure, but by the number of forms included in them. thus a genus containing a few species might be, and has often been, raised into a family on the discovery of several other species. many natural families are retained, although most closely related to other families, from including a great number of closely similar species. the more logical naturalist would perhaps, if he could, reject these two contingents in classification. from these circumstances, and especially from the undefined objects and criterions of the natural system, the number of divisions, such as genera, sub-families, families, &c., &c., has been quite arbitrary{ }; without the clearest definition, how can it be possible to decide whether two groups of species are of equal value, and of what value? whether they should both be called genera or families; or whether one should be a genus, and the other a family{ }? { } _origin_, ed. i. pp. , , vi. pp. , . { } this is discussed from the point of view of divergence in the _origin_, ed. i. pp. , , vi. pp. , . { } <footnote by the author.> i discuss this because if quinarism true, i false. <the quinary system is set forth in w. s. macleay's _horæ entomologicæ_, .> _on the kind of relation between distinct groups._ i have only one other remark on the affinities of organic beings; that is, when two quite distinct groups approach each other, the approach is _generally_ generic{ } and not special; i can explain this most easily by an example: of all rodents the bizcacha, by certain peculiarities in its reproductive system, approaches nearest to the marsupials; of all marsupials the phascolomys, on the other hand, appears to approach in the form of its teeth and intestines nearest to the rodents; but there is no special relation between these two genera{ }; the bizcacha is no nearer related to the phascolomys than to any other marsupial in the points in which it approaches this division; nor again is the phascolomys, in the points of structure in which it approaches the rodents, any nearer related to the bizcacha than to any other rodent. other examples might have been chosen, but i have given (from waterhouse) this example as it illustrates another point, namely, the difficulty of determining what are analogical or adaptive and what real affinities; it seems that the teeth of the phascolomys though _appearing closely_ to resemble those of a rodent are found to be built on the marsupial type; and it is thought that these teeth and consequently the intestines may have been adapted to the peculiar life of this animal and therefore may not show any real relation. the structure in the bizcacha that connects it with the marsupials does not seem a peculiarity related to its manner of life, and i imagine that no one would doubt that this shows a real affinity, though not more with any one marsupial species than with another. the difficulty of determining what relations are real and what analogical is far from surprising when no one pretends to define the meaning of the term relation or the ulterior object of all classification. we shall immediately see on the theory of descent how it comes that there should be "real" and "analogical" affinities; and why the former alone should be of value in classification--difficulties which it would be i believe impossible to explain on the ordinary theory of separate creations. { } in the corresponding passage in the _origin_, ed. i. p. , vi. p. , the term _general_ is used in place of _generic_, and seems a better expression. in the margin the author gives waterhouse as his authority. { } _origin_, ed. i. p. , vi. p. . _classification of races or varieties._ let us now for a few moments turn to the classification of the generally acknowledged varieties and subdivisions of our domestic beings{ }; we shall find them systematically arranged in groups of higher and higher value. de candolle has treated the varieties of the cabbage exactly as he would have done a natural family with various divisions and subdivisions. in dogs again we have one main division which may be called the _family_ of hounds; of these, there are several (we will call them) _genera_, such as blood-hounds, fox-hounds, and harriers; and of each of these we have different _species_, as the blood-hound of cuba and that of england; and of the latter again we have breeds truly producing their own kind, which may be called races or varieties. here we see a classification practically used which typifies on a lesser scale that which holds good in nature. but amongst true species in the natural system and amongst domestic races the number of divisions or groups, instituted between those most alike and those most unlike, seems to be quite arbitrary. the number of the forms in both cases seems practically, whether or not it ought theoretically, to influence the denomination of groups including them. in both, geographical distribution has sometimes been used as an aid to classification{ }; amongst varieties, i may instance, the cattle of india or the sheep of siberia, which from possessing some characters in common permit a classification of indian and european cattle, or siberian and european sheep. amongst domestic varieties we have even something very like the relations of "analogy" or "adaptation{ }"; thus the common and swedish turnip are both artificial varieties which strikingly resemble each other, and they fill nearly the same end in the economy of the farm-yard; but although the swede so much more resembles a turnip than its presumed parent the field cabbage, no one thinks of putting it out of the cabbages into the turnips. thus the greyhound and racehorse, having been selected and trained for extreme fleetness for short distances, present an analogical resemblance of the same kind, but less striking as that between the little otter (marsupial) of guiana and the common otter; though these two otters are really less related than <are> the horse and dog. we are even cautioned by authors treating on varieties, to follow the _natural_ in contradistinction of an artificial system and not, for instance, to class two varieties of the pine-apple{ } near each other, because their fruits accidentally resemble each other closely (though the fruit may be called _the final end_ of this plant in the economy of its world, the hothouse), but to judge from the general resemblance of the entire plants. lastly, varieties often become extinct; sometimes from unexplained causes, sometimes from accident, but more often from the production of more useful varieties, and the less useful ones being destroyed or bred out. { } in a corresponding passage in the _origin_, ed. i. p. , vi. p. , the author makes use of his knowledge of pigeons. the pseudo-genera among dogs are discussed in _var. under dom._, ed. ii. vol. i. p. . { } _origin_, ed. i. pp. , , vi. pp. , . { } _origin_, ed. i. pp. , , vi. pp. , . { } _origin_, ed. i. p. , vi. p. . i think it cannot be doubted that the main cause of all the varieties which have descended from the aboriginal dog or dogs, or from the aboriginal wild cabbage, not being equally like or unlike--but on the contrary, obviously falling into groups and sub-groups--must in chief part be attributed to different degrees of true relationship; for instance, that the different kinds of blood-hound have descended from one stock, whilst the harriers have descended from another stock, and that both these have descended from a different stock from that which has been the parent of the several kinds of greyhound. we often hear of a florist having some choice variety and breeding from it a whole group of sub-varieties more or less characterised by the peculiarities of the parent. the case of the peach and nectarine, each with their many varieties, might have been introduced. no doubt the relationship of our different domestic breeds has been obscured in an extreme degree by their crossing; and likewise from the slight difference between many breeds it has probably often happened that a "sport" from one breed has less closely resembled its parent breed than some other breed, and has therefore been classed with the latter. moreover the effects of a similar climate{ } may in some cases have more than counterbalanced the similarity, consequent on a common descent, though i should think the similarity of the breeds of cattle of india or sheep of siberia was far more probably due to the community of their descent than to the effects of climate on animals descended from different stocks. { } a general statement of the influence of conditions on variation occurs in the _origin_, ed. i. pp. - , vi. pp. - . notwithstanding these great sources of difficulty, i apprehend every one would admit, that if it were possible, a genealogical classification of our domestic varieties would be the most satisfactory one; and as far as varieties were concerned would be the natural system: in some cases it has been followed. in attempting to follow out this object a person would have to class a variety, whose parentage he did not know, by its external characters; but he would have a distinct ulterior object in view, namely, its descent in the same manner as a regular systematist seems also to have an ulterior but undefined end in all his classifications. like the regular systematist he would not care whether his characters were drawn from more or less important organs as long as he found in the tribe which he was examining that the characters from such parts were persistent; thus amongst cattle he does value a character drawn from the form of the horns more than from the proportions of the limbs and whole body, for he finds that the shape of the horns is to a considerable degree persistent amongst cattle{ }, whilst the bones of the limbs and body vary. no doubt as a frequent rule the more important the organ, as being less related to external influences, the less liable it is to variation; but he would expect that according to the object for which the races had been selected, parts more or less important might differ; so that characters drawn from parts generally most liable to vary, as colour, might in some instances be highly serviceable--as is the case. he would admit that general resemblances scarcely definable by language might sometimes serve to allocate a species by its nearest relation. he would be able to assign a clear reason why the close similarity of the fruit in two varieties of pine-apple, and of the so-called root in the common and swedish turnips, and why the similar gracefulness of form in the greyhound and racehorse, are characters of little value in classification; namely, because they are the result, not of community of descent, but either of selection for a common end, or of the effects of similar external conditions. { } _origin_, ed. i. p. , vi. p. . in the margin marshall is given as the authority. _classification of "races" and species similar._ thus seeing that both the classifiers of species and of varieties{ } work by the same means, make similar distinctions in the value of the characters, and meet with similar difficulties, and that both seem to have in their classification an ulterior object in view; i cannot avoid strongly suspecting that the same cause, which has made amongst our domestic varieties groups and sub-groups, has made similar groups (but of higher values) amongst species; and that this cause is the greater or less propinquity of actual descent. the simple fact of species, both those long since extinct and those now living, being divisible into genera, families, orders &c.--divisions analogous to those into which varieties are divisible--is otherwise an inexplicable fact, and only not remarkable from its familiarity. { } _origin_, ed. i. p. , vi. p. . _origin of genera and families._ let us suppose{ } for example that a species spreads and arrives at six or more different regions, or being already diffused over one wide area, let this area be divided into six distinct regions, exposed to different conditions, and with stations slightly different, not fully occupied with other species, so that six different races or species were formed by selection, each best fitted to its new habits and station. i must remark that in every case, if a species becomes modified in any one sub-region, it is probable that it will become modified in some other of the sub-regions over which it is diffused, for its organization is shown to be capable of being rendered plastic; its diffusion proves that it is able to struggle with the other inhabitants of the several sub-regions; and as the organic beings of every great region are in some degree allied, and as even the physical conditions are often in some respects alike, we might expect that a modification in structure, which gave our species some advantage over antagonist species in one sub-region, would be followed by other modifications in other of the sub-regions. the races or new species supposed to be formed would be closely related to each other; and would either form a new genus or sub-genus, or would rank (probably forming a slightly different section) in the genus to which the parent species belonged. in the course of ages, and during the contingent physical changes, it is probable that some of the six new species would be destroyed; but the same advantage, whatever it may have been (whether mere tendency to vary, or some peculiarity of organization, power of mind, or means of distribution), which in the parent-species and in its six selected and changed species-offspring, caused them to prevail over other antagonist species, would generally tend to preserve some or many of them for a long period. if then, two or three of the six species were preserved, they in their turn would, during continued changes, give rise to as many small groups of species: if the parents of these small groups were closely similar, the new species would form one great genus, barely perhaps divisible into two or three sections: but if the parents were considerably unlike, their species-offspring would, from inheriting most of the peculiarities of their parent-stocks, form either two or more sub-genera or (if the course of selection tended in different ways) genera. and lastly species descending from different species of the newly formed genera would form new genera, and such genera collectively would form a family. { } the discussion here following corresponds more or less to the _origin_, ed. i. pp. , , vi. pp. , ; although the doctrine of divergence is not mentioned in this essay (as it is in the _origin_) yet the present section seems to me a distinct approximation to it. the extermination of species follows from changes in the external conditions, and from the increase or immigration of more favoured species: and as those species which are undergoing modification in any one great region (or indeed over the world) will very often be allied ones from (as just explained) partaking of many characters, and therefore advantages in common, so the species, whose place the new or more favoured ones are seizing, from partaking of a common inferiority (whether in any particular point of structure, or of general powers of mind, of means of distribution, of capacity for variation, &c., &c.), will be apt to be allied. consequently species of the same genus will slowly, one after the other, _tend_ to become rarer and rarer in numbers, and finally extinct; and as each last species of several allied genera fails, even the family will become extinct. there may of course be occasional exceptions to the entire destruction of any genus or family. from what has gone before, we have seen that the slow and successive formation of several new species from the same stock will make a new genus, and the slow and successive formation of several other new species from another stock will make another genus; and if these two stocks were allied, such genera will make a new family. now, as far as our knowledge serves, it is in this slow and gradual manner that groups of species appear on, and disappear from, the face of the earth. the manner in which, according to our theory, the arrangement of species in groups is due to partial extinction, will perhaps be rendered clearer in the following way. let us suppose in any one great class, for instance in the mammalia, that every species and every variety, during each successive age, had sent down one unaltered descendant (either fossil or living) to the present time; we should then have had one enormous series, including by small gradations every known mammiferous form; and consequently the existence of groups{ }, or chasms in the series, which in some parts are in greater width, and in some of less, is solely due to former species, and whole groups of species, not having thus sent down descendants to the present time. { } the author probably intended to write "groups separated by chasms." with respect to the "analogical" or "adaptive" resemblances between organic beings which are not really related{ }, i will only add, that probably the isolation of different groups of species is an important element in the production of such characters: thus we can easily see, in a large increasing island, or even a continent like australia, stocked with only certain orders of the main classes, that the conditions would be highly favourable for species from these orders to become adapted to play parts in the economy of nature, which in other countries were performed by tribes especially adapted to such parts. we can understand how it might happen that an otter-like animal might have been formed in australia by slow selection from the more carnivorous marsupial types; thus we can understand that curious case in the southern hemisphere, where there are no auks (but many petrels), of a petrel{ } having been modified into the external general form so as to play the same office in nature with the auks of the northern hemisphere; although the habits and form of the petrels and auks are normally so wholly different. it follows, from our theory, that two orders must have descended from one common stock at an immensely remote epoch; and we can perceive when a species in either order, or in both, shows some affinity to the other order, why the affinity is usually generic and not particular--that is why the bizcacha amongst rodents, in the points in which it is related to the marsupial, is related to the whole group{ }, and not particularly to the phascolomys, which of all marsupialia is related most to the rodents. for the bizcacha is related to the present marsupialia, only from being related to their common parent-stock; and not to any one species in particular. and generally, it may be observed in the writings of most naturalists, that when an organism is described as intermediate between two _great_ groups, its relations are not to particular species of either group, but to both groups, as wholes. a little reflection will show how exceptions (as that of the lepidosiren, a fish closely related to _particular_ reptiles) might occur, namely from a few descendants of those species, which at a very early period branched out from a common parent-stock and so formed the two orders or groups, having survived, in nearly their original state, to the present time. { } a similar discussion occurs in the _origin_, ed. i. p. , vi. p. . { } _puffinuria berardi_, see _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . finally, then, we see that all the leading facts in the affinities and classification of organic beings can be explained on the theory of the natural system being simply a genealogical one. the similarity of the principles in classifying domestic varieties and true species, both those living and extinct, is at once explained; the rules followed and difficulties met with being the same. the existence of genera, families, orders, &c., and their mutual relations, naturally ensues from extinction going on at all periods amongst the diverging descendants of a common stock. these terms of affinity, relations, families, adaptive characters, &c., which naturalists cannot avoid using, though metaphorically, cease being so, and are full of plain signification. chapter viii unity of type in the great classes; and morphological structures _unity of type_{ }. { } _origin_, ed. i. p. , vi. p. . ch. viii corresponds to a section of ch. xiii in the _origin_, ed. i. scarcely anything is more wonderful or has been oftener insisted on than that the organic beings in each great class, though living in the most distant climes and at periods immensely remote, though fitted to widely different ends in the economy of nature, yet all in their internal structure evince an obvious uniformity. what, for instance, is more wonderful than that the hand to clasp, the foot or hoof to walk, the bat's wing to fly, the porpoise's fin{ } to swim, should all be built on the same plan? and that the bones in their position and number should be so similar that they can all be classed and called by the same names. occasionally some of the bones are merely represented by an apparently useless, smooth style, or are soldered closely to other bones, but the unity of type is not by this destroyed, and hardly rendered less clear. we see in this fact some deep bond of union between the organic beings of the same great classes--to illustrate which is the object and foundation of the natural system. the perception of this bond, i may add, is the evident cause that naturalists make an ill-defined distinction between true and adaptive affinities. { } _origin_, ed. i. p. , vi. p. . in the _origin_, ed. i. these examples occur under the heading _morphology_; the author does not there draw much distinction between this heading and that of _unity of type_. _morphology._ there is another allied or rather almost identical class of facts admitted by the least visionary naturalists and included under the name of morphology. these facts show that in an individual organic being, several of its organs consist of some other organ metamorphosed{ }: thus the sepals, petals, stamens, pistils, &c. of every plant can be shown to be metamorphosed leaves; and thus not only can the number, position and transitional states of these several organs, but likewise their monstrous changes, be most lucidly explained. it is believed that the same laws hold good with the gemmiferous vesicles of zoophytes. in the same manner the number and position of the extraordinarily complicated jaws and palpi of crustacea and of insects, and likewise their differences in the different groups, all become simple, on the view of these parts, or rather legs and all metamorphosed appendages, being metamorphosed legs. the skulls, again, of the vertebrata are composed of three metamorphosed vertebræ, and thus we can see a meaning in the number and strange complication of the bony case of the brain. in this latter instance, and in that of the jaws of the crustacea, it is only necessary to see a series taken from the different groups of each class to admit the truth of these views. it is evident that when in each species of a group its organs consist of some other part metamorphosed, that there must also be a "unity of type" in such a group. and in the cases as that above given in which the foot, hand, wing and paddle are said to be constructed on a uniform type, if we could perceive in such parts or organs traces of an apparent change from some other use or function, we should strictly include such parts or organs in the department of morphology: thus if we could trace in the limbs of the vertebrata, as we can in their ribs, traces of an apparent change from being processes of the vertebræ, it would be said that in each species of the vertebrata the limbs were "metamorphosed spinal processes," and that in all the species throughout the class the limbs displayed a "unity of type{ }." { } see _origin_, ed. i. p. , vi. p. , where the parts of the flower, the jaws and palpi of crustaceans and the vertebrate skull are given as examples. { } the author here brings _unity of type_ and _morphology_ together. these wonderful parts of the hoof, foot, hand, wing, paddle, both in living and extinct animals, being all constructed on the same framework, and again of the petals, stamina, germens, &c. being metamorphosed leaves, can by the creationist be viewed only as ultimate facts and incapable of explanation; whilst on our theory of descent these facts all necessary follow: for by this theory all the beings of any one class, say of the mammalia, are supposed to be descended from one parent-stock, and to have been altered by such slight steps as man effects by the selection of chance domestic variations. now we can see according to this view that a foot might be selected with longer and longer bones, and wider connecting membranes, till it became a swimming organ, and so on till it became an organ by which to flap along the surface or to glide over it, and lastly to fly through the air: but in such changes there would be no tendency to alter the framework of the internal inherited structure. parts might become lost (as the tail in dogs, or horns in cattle, or the pistils in plants), others might become united together (as in the feet of the lincolnshire breed of pigs{ }, and in the stamens of many garden flowers); parts of a similar nature might become increased in number (as the vertebræ in the tails of pigs, &c., &c. and the fingers and toes in six-fingered races of men and in the dorking fowls), but analogous differences are observed in nature and are not considered by naturalists to destroy the uniformity of the types. we can, however, conceive such changes to be carried to such length that the unity of type might be obscured and finally be undistinguishable, and the paddle of the plesiosaurus has been advanced as an instance in which the uniformity of type can hardly be recognised{ }. if after long and gradual changes in the structure of the co-descendants from any parent stock, evidence (either from monstrosities or from a graduated series) could be still detected of the function, which certain parts or organs played in the parent stock, these parts or organs might be strictly determined by their former function with the term "metamorphosed" appended. naturalists have used this term in the same metaphorical manner as they have been obliged to use the terms of affinity and relation; and when they affirm, for instance, that the jaws of a crab are metamorphosed legs, so that one crab has more legs and fewer jaws than another, they are far from meaning that the jaws, either during the life of the individual crab or of its progenitors, were really legs. by our theory this term assumes its literal meaning{ }; and this wonderful fact of the complex jaws of an animal retaining numerous characters, which they would probably have retained if they had really been metamorphosed during many successive generations from true legs, is simply explained. { } the solid-hoofed pigs mentioned in _var. under dom._, ed. ii. vol. ii. p. are not _lincolnshire pigs_. for other cases see bateson, _materials for the study of variation_, , pp. - . { } in the margin c. bell is given as authority, apparently for the statement about plesiosaurus. see _origin_, ed. i. p. , vi. p. , where the author speaks of the "general pattern" being obscured in "extinct gigantic sea lizards." in the same place the suctorial entomostraca are added as examples of the difficulty of recognising the type. { } _origin_, ed. i. p. , vi. p. . _embryology_. the unity of type in the great classes is shown in another and very striking manner, namely, in the stages through which the embryo passes in coming to maturity{ }. thus, for instance, at one period of the embryo, the wings of the bat, the hand, hoof or foot of the quadruped, and the fin of the porpoise do not differ, but consist of a simple undivided bone. at a still earlier period the embryo of the fish, bird, reptile and mammal all strikingly resemble each other. let it not be supposed this resemblance is only external; for on dissection, the arteries are found to branch out and run in a peculiar course, wholly unlike that in the full-grown mammal and bird, but much less unlike that in the full-grown fish, for they run as if to ærate blood by branchiæ{ } on the neck, of which even the slit-like orifices can be discerned. how wonderful it is that this structure should be present in the embryos of animals about to be developed into such different forms, and of which two great classes respire only in the air. moreover, as the embryo of the mammal is matured in the parent's body, and that of the bird in an egg in the air, and that of the fish in an egg in the water, we cannot believe that this course of the arteries is related to any external conditions. in all shell-fish (gasteropods) the embryo passes through a state analogous to that of the pteropodous mollusca: amongst insects again, even the most different ones, as the moth, fly and beetle, the crawling larvæ are all closely analogous: amongst the radiata, the jelly-fish in its embryonic state resembles a polype, and in a still earlier state an infusorial animalcule--as does likewise the embryo of the polype. from the part of the embryo of a mammal, at one period, resembling a fish more than its parent form; from the larvæ of all orders of insects more resembling the simpler articulate animals than their parent insects{ }; and from such other cases as the embryo of the jelly-fish resembling a polype much nearer than the perfect jelly-fish; it has often been asserted that the higher animal in each class passes through the state of a lower animal; for instance, that the mammal amongst the vertebrata passes through the state of a fish{ }: but müller denies this, and affirms that the young mammal is at no time a fish, as does owen assert that the embryonic jelly-fish is at no time a polype, but that mammal and fish, jelly-fish and polype pass through the same state; the mammal and jelly-fish being only further developed or changed. { } _origin_, ed. i. p. , vi. p. . { } the uselessness of the branchial arches in mammalia is insisted on in the _origin_, ed. i. p. , vi. p. . also the uselessness of the spots on the young blackbird and the stripes of the lion-whelp, cases which do not occur in the present essay. { } in the _origin_, ed. i. pp. , , vi. pp. , it is pointed out that in some cases the young form resembles the adult, _e.g._ in spiders; again, that in the aphis there is no "worm-like stage" of development. { } in the _origin_, ed. i. p. , vi. p. , the author speaks doubtfully about the recapitulation theory. as the embryo, in most cases, possesses a less complicated structure than that into which it is to be developed, it might have been thought that the resemblance of the embryo to less complicated forms in the same great class, was in some manner a necessary preparation for its higher development; but in fact the embryo, during its growth, may become less, as well as more, complicated{ }. thus certain female epizoic crustaceans in their mature state have neither eyes nor any organs of locomotion; they consist of a mere sack, with a simple apparatus for digestion and procreation; and when once attached to the body of the fish, on which they prey, they never move again during their whole lives: in their embryonic condition, on the other hand, they are furnished with eyes, and with well articulated limbs, actively swim about and seek their proper object to become attached to. the larvæ, also, of some moths are as complicated and are more active than the wingless and limbless females, which never leave their pupa-case, never feed and never see the daylight. { } this corresponds to the _origin_, ed. i. p. , vi. p. , where, however, the example is taken from the cirripedes. _attempt to explain the facts of embryology._ i think considerable light can be thrown by the theory of descent on these wonderful embryological facts which are common in a greater or less degree to the whole animal kingdom, and in some manner to the vegetable kingdom: on the fact, for instance, of the arteries in the embryonic mammal, bird, reptile and fish, running and branching in the same courses and nearly in the same manner with the arteries in the full-grown fish; on the fact i may add of the high importance to systematic naturalists{ } of the characters and resemblances in the embryonic state, in ascertaining the true position in the natural system of mature organic beings. the following are the considerations which throw light on these curious points. { } _origin_, ed. i. p. , vi. p. . in the economy, we will say of a feline animal{ }, the feline structure of the embryo or of the sucking kitten is of quite secondary importance to it; hence, if a feline animal varied (assuming for the time the possibility of this) and if some place in the economy of nature favoured the selection of a longer-limbed variety, it would be quite unimportant to the production by natural selection of a long-limbed breed, whether the limbs of the embryo and kitten were elongated if they _became_ so _as soon_ as the animal had to provide food for itself. and if it were found after continued selection and the production of several new breeds from one parent-stock, that the successive variations had supervened, not very early in the youth or embryonic life of each breed (and we have just seen that it is quite unimportant whether it does so or not), then it obviously follows that the young or embryos of the several breeds will continue resembling each other more closely than their adult parents{ }. and again, if two of these breeds became each the parent-stock of several other breeds, forming two genera, the young and embryos of these would still retain a greater resemblance to the one original stock than when in an adult state. therefore if it could be shown that the period of the slight successive variations does not always supervene at a very early period of life, the greater resemblance or closer unity in type of animals in the young than in the full-grown state would be explained. before practically{ } endeavouring to discover in our domestic races whether the structure or form of the young has or has not changed in an exactly corresponding degree with the changes of full-grown animals, it will be well to show that it is at least quite _possible_ for the primary germinal vesicle to be impressed with a tendency to produce some change on the growing tissues which will not be fully effected till the animal is advanced in life. { } this corresponds to the _origin_, ed. i. pp. - , vi. p. : the "feline animal" is not used to illustrate the generalisation, but is so used in the essay of , p. . { } _origin_, ed. i. p. , vi. p. . { } in the margin is written "get young pigeons"; this was afterwards done, and the results are given in the _origin_, ed. i. p. , vi. p. . from the following peculiarities of structure being inheritable and appearing only when the animal is full-grown--namely, general size, tallness (not consequent on the tallness of the infant), fatness either over the whole body, or local; change of colour in hair and its loss; deposition of bony matter on the legs of horses; blindness and deafness, that is changes of structure in the eye and ear; gout and consequent deposition of chalk-stones; and many other diseases{ }, as of the heart and brain, &c., &c.; from all such tendencies being i repeat inheritable, we clearly see that the germinal vesicle is impressed with some power which is wonderfully preserved during the production of infinitely numerous cells in the ever changing tissues, till the part ultimately to be affected is formed and the time of life arrived at. we see this clearly when we select cattle with any peculiarity of their horns, or poultry with any peculiarity of their second plumage, for such peculiarities cannot of course reappear till the animal is mature. hence, it is certainly _possible_ that the germinal vesicle may be impressed with a tendency to produce a long-limbed animal, the full proportional length of whose limbs shall appear only when the animal is mature{ }. { } in the _origin_, ed. i. the corresponding passages are at pp. , , , vi. pp. , , . in the _origin_, ed. i. i have not found a passage so striking as that which occurs a few lines lower "that the germinal vesicle is impressed with some power which is wonderfully preserved, &c." in the _origin_ this _preservation_ is rather taken for granted. { } <in the margin is written> aborted organs show, perhaps, something about period <at> which changes supervene in embryo. in several of the cases just enumerated we know that the first cause of the peculiarity, when _not_ inherited, lies in the conditions to which the animal is exposed during mature life, thus to a certain extent general size and fatness, lameness in horses and in a lesser degree blindness, gout and some other diseases are certainly in some degree caused and accelerated by the habits of life, and these peculiarities when transmitted to the offspring of the affected person reappear at a nearly corresponding time of life. in medical works it is asserted generally that at whatever period an hereditary disease appears in the parent, it tends to reappear in the offspring at the same period. again, we find that early maturity, the season of reproduction and longevity are transmitted to corresponding periods of life. dr holland has insisted much on children of the same family exhibiting certain diseases in similar and peculiar manners; my father has known three brothers{ } die in very old age in a _singular_ comatose state; now to make these latter cases strictly bear, the children of such families ought similarly to suffer at corresponding times of life; this is probably not the case, but such facts show that a tendency in a disease to appear at particular stages of life can be transmitted through the germinal vesicle to different individuals of the same family. it is then certainly possible that diseases affecting widely different periods of life can be transmitted. so little attention is paid to very young domestic animals that i do not know whether any case is on record of selected peculiarities in young animals, for instance, in the first plumage of birds, being transmitted to their young. if, however, we turn to silk-worms{ }, we find that the caterpillars and coccoons (which must correspond to a _very early_ period of the embryonic life of mammalia) vary, and that these varieties reappear in the offspring caterpillars and coccoons. { } see p. , note .{note } { } the evidence is given in _var. under dom._, i. p. . i think these facts are sufficient to render it probable that at whatever period of life any peculiarity (capable of being inherited) appears, whether caused by the action of external influences during mature life, or from an affection of the primary germinal vesicle, it _tends_ to reappear in the offspring at the corresponding period of life{ }. hence (i may add) whatever effect training, that is the full employment or action of every newly selected slight variation, has in fully developing and increasing such variation, would only show itself in mature age, corresponding to the period of training; in the second chapter i showed that there was in this respect a marked difference in natural and artificial selection, man not regularly exercising or adapting his varieties to new ends, whereas selection by nature presupposes such exercise and adaptation in each selected and changed part. the foregoing facts show and presuppose that slight variations occur at various periods of life _after birth_; the facts of monstrosity, on the other hand, show that many changes take place before birth, for instance, all such cases as extra fingers, hare-lip and all sudden and great alterations in structure; and these when inherited reappear during the embryonic period in the offspring. i will only add that at a period even anterior to embryonic life, namely, during the _egg_ state, varieties appear in size and colour (as with the hertfordshire duck with blackish eggs{ }) which reappear in the egg; in plants also the capsule and membranes of the seed are very variable and inheritable. { } _origin_, ed. i. p. , vi. p. . { } in _var. under dom._, ed. ii. vol. i. p. , such eggs are said to be laid early in each season by the black labrador duck. in the next sentence in the text the author does not distinguish the characters of the vegetable capsule from those of the ovum. if then the two following propositions are admitted (and i think the first can hardly be doubted), viz. that variation of structure takes place at all times of life, though no doubt far less in amount and seldomer in quite mature life{ } (and then generally taking the form of disease); and secondly, that these variations tend to reappear at a corresponding period of life, which seems at least probable, then we might _a priori_ have expected that in any selected breed the _young_ animal would not partake in a corresponding degree the peculiarities characterising the _full-grown_ parent; though it would in a lesser degree. for during the thousand or ten thousand selections of slight increments in the length of the limbs of individuals necessary to produce a long-limbed breed, we might expect that such increments would take place in different individuals (as we do not certainly know at what period they do take place), some earlier and some later in the embryonic state, and some during early youth; and these increments would reappear in their offspring only at corresponding periods. hence, the entire length of limb in the new long-limbed breed would only be acquired at the latest period of life, when that one which was latest of the thousand primary increments of length supervened. consequently, the foetus of the new breed during the earlier part of its existence would remain much less changed in the proportions of its limbs; and the earlier the period the less would the change be. { } this seems to me to be more strongly stated here than in the _origin_, ed. i. whatever may be thought of the facts on which this reasoning is grounded, it shows how the embryos and young of different species might come to remain less changed than their mature parents; and practically we find that the young of our domestic animals, though differing, differ less than their full-grown parents. thus if we look at the young puppies{ } of the greyhound and bulldog--(the two most obviously modified of the breeds of dog)--we find their puppies at the age of six days with legs and noses (the latter measured from the eyes to the tip) of the same length; though in the proportional thicknesses and general appearance of these parts there is a great difference. so it is with cattle, though the young calves of different breeds are easily recognisable, yet they do not differ so much in their proportions as the full-grown animals. we see this clearly in the fact that it shows the highest skill to select the best forms early in life, either in horses, cattle or poultry; no one would attempt it only a few hours after birth; and it requires great discrimination to judge with accuracy even during their full youth, and the best judges are sometimes deceived. this shows that the ultimate proportions of the body are not acquired till near mature age. if i had collected sufficient facts to firmly establish the proposition that in artificially selected breeds the embryonic and young animals are not changed in a corresponding degree with their mature parents, i might have omitted all the foregoing reasoning and the attempts to explain how this happens; for we might safely have transferred the proposition to the breeds or species naturally selected; and the ultimate effect would necessarily have been that in a number of races or species descended from a common stock and forming several genera and families the embryos would have resembled each other more closely than full-grown animals. whatever may have been the form or habits of the parent-stock of the vertebrata, in whatever course the arteries ran and branched, the selection of variations, supervening after the first formation of the arteries in the embryo, would not tend from variations supervening at corresponding periods to alter their course at that period: hence, the similar course of the arteries in the mammal, bird, reptile and fish, must be looked at as a most ancient record of the embryonic structure of the common parent-stock of these four great classes. { } _origin_, ed. i. p. , vi. p. . a long course of selection might cause a form to become more simple, as well as more complicated; thus the adaptation of a crustaceous{ } animal to live attached during its whole life to the body of a fish, might permit with advantage great simplification of structure, and on this view the singular fact of an embryo being more complex than its parent is at once explained. { } _origin_, ed. i. p. , vi. p. . _on the graduated complexity in each great class._ i may take this opportunity of remarking that naturalists have observed that in most of the great classes a series exists from very complicated to very simple beings; thus in fish, what a range there is between the sand-eel and shark,--in the articulata, between the common crab and the daphnia{ },--between the aphis and butterfly, and between a mite and a spider{ }. now the observation just made, namely, that selection might tend to simplify, as well as to complicate, explains this; for we can see that during the endless geologico-geographical changes, and consequent isolation of species, a station occupied in other districts by less complicated animals might be left unfilled, and be occupied by a degraded form of a higher or more complicated class; and it would by no means follow that, when the two regions became united, the degraded organism would give way to the aboriginally lower organism. according to our theory, there is obviously no power tending constantly to exalt species, except the mutual struggle between the different individuals and classes; but from the strong and general hereditary tendency we might expect to find some tendency to progressive complication in the successive production of new organic forms. { } compare _origin_, ed. i. p. , vi. p. . { } <note in original.> scarcely possible to distinguish between non-development and retrograde development. _modification by selection of the forms of immature animals._ i have above remarked that the feline{ } form is quite of secondary importance to the embryo and to the kitten. of course, during any great and prolonged change of structure in the mature animal, it might, and often would be, indispensable that the form of the embryo should be changed; and this could be effected, owing to the hereditary tendency at corresponding ages, by selection, equally well as in mature age: thus if the embryo tended to become, or to remain, either over its whole body or in certain parts, too bulky, the female parent would die or suffer more during parturition; and as in the case of the calves with large hinder quarters{ }, the peculiarity must be either eliminated or the species become extinct. where an embryonic form has to seek its own food, its structure and adaptation is just as important to the species as that of the full-grown animal; and as we have seen that a peculiarity appearing in a caterpillar (or in a child, as shown by the hereditariness of peculiarities in the milk-teeth) reappears in its offspring, so we can at once see that our common principle of the selection of slight accidental variations would modify and adapt a caterpillar to a new or changing condition, precisely as in the full-grown butterfly. hence probably it is that caterpillars of different species of the lepidoptera differ more than those embryos, at a corresponding early period of life, do which remain inactive in the womb of their parents. the parent during successive ages continuing to be adapted by selection for some one object, and the larva for quite another one, we need not wonder at the difference becoming wonderfully great between them; even as great as that between the fixed rock-barnacle and its free, crab-like offspring, which is furnished with eyes and well-articulated, locomotive limbs{ }. { } see p. , where the same illustration is used. { } _var. under dom._, ed. ii. vol. i. p. . { } _origin_, ed. i. p. , vi. p. . _importance of embryology in classification._ we are now prepared to perceive why the study of embryonic forms is of such acknowledged importance in classification{ }. for we have seen that a variation, supervening at any time, may aid in the modification and adaptation of the full-grown being; but for the modification of the embryo, only the variations which supervene at a very early period can be seized on and perpetuated by selection: hence there will be less power and less tendency (for the structure of the embryo is mostly unimportant) to modify the young: and hence we might expect to find at this period similarities preserved between different groups of species which had been obscured and quite lost in the full-grown animals. i conceive on the view of separate creations it would be impossible to offer any explanation of the affinities of organic beings thus being plainest and of the greatest importance at that period of life when their structure is not adapted to the final part they have to play in the economy of nature. { } _origin_, ed. i. p. , vi. p. . _order in time in which the great classes have first appeared._ it follows strictly from the above reasoning only that the embryos of (for instance) existing vertebrata resemble more closely the embryo of the parent-stock of this great class than do full-grown existing vertebrata resemble their full-grown parent-stock. but it may be argued with much probability that in the earliest and simplest condition of things the parent and embryo must have resembled each other, and that the passage of any animal through embryonic states in its growth is entirely due to subsequent variations affecting _only_ the more mature periods of life. if so, the embryos of the existing vertebrata will shadow forth the full-grown structure of some of those forms of this great class which existed at the earlier periods of the earth's history{ }: and accordingly, animals with a fish-like structure ought to have preceded birds and mammals; and of fish, that higher organized division with the vertebræ extending into one division of the tail ought to have preceded the equal-tailed, because the embryos of the latter have an unequal tail; and of crustacea, entomostraca ought to have preceded the ordinary crabs and barnacles--polypes ought to have preceded jelly-fish, and infusorial animalcules to have existed before both. this order of precedence in time in some of these cases is believed to hold good; but i think our evidence is so exceedingly incomplete regarding the number and kinds of organisms which have existed during all, especially the earlier, periods of the earth's history, that i should put no stress on this accordance, even if it held truer than it probably does in our present state of knowledge. { } _origin_, ed. i. p. , vi. p. . chapter ix abortive or rudimentary organs _the abortive organs of naturalists._ parts of structure are said to be "abortive," or when in a still lower state of development "rudimentary{ }," when the same reasoning power, which convinces us that in some cases similar parts are beautifully adapted to certain ends, declares that in others they are absolutely useless. thus the rhinoceros, the whale{ }, etc., have, when young, small but properly formed teeth, which never protrude from the jaws; certain bones, and even the entire extremities are represented by mere little cylinders or points of bone, often soldered to other bones: many beetles have exceedingly minute but regularly formed wings lying under their wing-cases{ }, which latter are united never to be opened: many plants have, instead of stamens, mere filaments or little knobs; petals are reduced to scales, and whole flowers to buds, which (as in the feather hyacinth) never expand. similar instances are almost innumerable, and are justly considered wonderful: probably not one organic being exists in which some part does not bear the stamp of inutility; for what can be clearer{ }, as far as our reasoning powers can reach, than that teeth are for eating, extremities for locomotion, wings for flight, stamens and the entire flower for reproduction; yet for these clear ends the parts in question are manifestly unfit. abortive organs are often said to be mere representatives (a metaphorical expression) of similar parts in other organic beings; but in some cases they are more than representatives, for they seem to be the actual organ not fully grown or developed; thus the existence of mammæ in the male vertebrata is one of the oftenest adduced cases of abortion; but we know that these organs in man (and in the bull) have performed their proper function and secreted milk: the cow has normally four mammæ and two abortive ones, but these latter in some instances are largely developed and even (??) give milk{ }. again in flowers, the representatives of stamens and pistils can be traced to be really these parts not developed; kölreuter has shown by crossing a diæcious plant (a cucubalus) having a rudimentary pistil{ } with another species having this organ perfect, that in the hybrid offspring the rudimentary part is more developed, though still remaining abortive; now this shows how intimately related in nature the mere rudiment and the fully developed pistil must be. { } in the _origin_, ed. i. p. , vi. p. , the author does not lay stress on any distinction in meaning between the terms _abortive_ and _rudimentary_ organs. { } _origin_, ed. i. p. , vi. p. . { } _ibid._ { } this argument occurs in _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. , on male mammæ. in the _origin_ he speaks certainly of the abortive mammæ of the cow giving milk,--a point which is here queried. { } _origin_, ed. i. p. , vi. p. . abortive organs, which must be considered as useless as far as their ordinary and normal purpose is concerned, are sometimes adapted to other ends{ }: thus the marsupial bones, which properly serve to support the young in the mother's pouch, are present in the male and serve as the fulcrum for muscles connected only with male functions: in the male of the marigold flower the pistil is abortive for its proper end of being impregnated, but serves to sweep the pollen out of the anthers{ } ready to be borne by insects to the perfect pistils in the other florets. it is likely in many cases, yet unknown to us, that abortive organs perform some useful function; but in other cases, for instance in that of teeth embedded in the solid jaw-bone, or of mere knobs, the rudiments of stamens and pistils, the boldest imagination will hardly venture to ascribe to them any function. abortive parts, even when wholly useless to the individual species, are of great signification in the system of nature; for they are often found to be of very high importance in a natural classification{ }; thus the presence and position of entire abortive flowers, in the grasses, cannot be overlooked in attempting to arrange them according to their true affinities. this corroborates a statement in a previous chapter, viz. that the physiological importance of a part is no index of its importance in classification. finally, abortive organs often are only developed, proportionally with other parts, in the embryonic or young state of each species{ }; this again, especially considering the classificatory importance of abortive organs, is evidently part of the law (stated in the last chapter) that the higher affinities of organisms are often best seen in the stages towards maturity, through which the embryo passes. on the ordinary view of individual creations, i think that scarcely any class of facts in natural history are more wonderful or less capable of receiving explanation. { } the case of rudimentary organs adapted to new purposes is discussed in the _origin_, ed. i. p. , vi. p. . { } this is here stated on the authority of sprengel; see also _origin_, ed. i. p. , vi. p. . { } _origin_, ed. i. p. , vi. p. . in the margin r. brown's name is given apparently as the authority for the fact. { } _origin_, ed. i. p. , vi. p. . _the abortive organs of physiologists._ physiologists and medical men apply the term "abortive" in a somewhat different sense from naturalists; and their application is probably the primary one; namely, to parts, which from accident or disease before birth are not developed or do not grow{ }: thus, when a young animal is born with a little stump in the place of a finger or of the whole extremity, or with a little button instead of a head, or with a mere bead of bony matter instead of a tooth, or with a stump instead of a tail, these parts are said to be aborted. naturalists on the other hand, as we have seen, apply this term to parts not stunted during the growth of the embryo, but which are as regularly produced in successive generations as any other most essential parts of the structure of the individual: naturalists, therefore, use this term in a metaphorical sense. these two classes of facts, however, blend into each other{ }; by parts accidentally aborted, during the embryonic life of one individual, becoming hereditary in the succeeding generations: thus a cat or dog, born with a stump instead of a tail, tends to transmit stumps to their offspring; and so it is with stumps representing the extremities; and so again with flowers, with defective and rudimentary parts, which are annually produced in new flower-buds and even in successive seedlings. the strong hereditary tendency to reproduce every either congenital or slowly acquired structure, whether useful or injurious to the individual, has been shown in the first part; so that we need feel no surprise at these truly abortive parts becoming hereditary. a curious instance of the force of hereditariness is sometimes seen in two little loose hanging horns, quite useless as far as the function of a horn is concerned, which are produced in hornless races of our domestic cattle{ }. now i believe no real distinction can be drawn between a stump representing a tail or a horn or the extremities; or a short shrivelled stamen without any pollen; or a dimple in a petal representing a nectary, when such rudiments are regularly reproduced in a race or family, and the true abortive organs of naturalists. and if we had reason to believe (which i think we have not) that all abortive organs had been at some period _suddenly_ produced during the embryonic life of an individual, and afterwards become inherited, we should at once have a simple explanation of the origin of abortive and rudimentary organs{ }. in the same manner as during changes of pronunciation certain letters in a word may become useless{ } in pronouncing it, but yet may aid us in searching for its derivation, so we can see that rudimentary organs, no longer useful to the individual, may be of high importance in ascertaining its descent, that is, its true classification in the natural system. { } _origin_, ed. i. p. , vi. p. . { } in the _origin_, ed. i. p. , vi. p. , the author in referring to semi-monstrous variations adds "but i doubt whether any of these cases throw light on the origin of rudimentary organs in a state of nature." in he was clearly more inclined to an opposite opinion. { } _origin_, ed. i. p. , vi. p. . { } see _origin_, ed. i. p. , vi. p. . the author there discusses monstrosities in relation to rudimentary organs, and comes to the conclusion that disuse is of more importance, giving as a reason his doubt "whether species under nature ever undergo abrupt changes." it seems to me that in the _origin_ he gives more weight to the "lamarckian factor" than he did in . huxley took the opposite view, see the introduction. { } _origin_, ed. i. p. , vi. p. . _abortion from gradual disuse._ there seems to be some probability that continued disuse of any part or organ, and the selection of individuals with such parts slightly less developed, would in the course of ages produce in organic beings under domesticity races with such parts abortive. we have every reason to believe that every part and organ in an individual becomes fully developed only with exercise of its functions; that it becomes developed in a somewhat lesser degree with less exercise; and if forcibly precluded from all action, such part will often become atrophied. every peculiarity, let it be remembered, tends, especially where both parents have it, to be inherited. the less power of flight in the common duck compared with the wild, must be partly attributed to disuse{ } during successive generations, and as the wing is properly adapted to flight, we must consider our domestic duck in the first stage towards the state of the apteryx, in which the wings are so curiously abortive. some naturalists have attributed (and possibly with truth) the falling ears so characteristic of most domestic dogs, some rabbits, oxen, cats, goats, horses, &c., &c., as the effects of the lesser use of the muscles of these flexible parts during successive generations of inactive life; and muscles, which cannot perform their functions, must be considered verging towards abortion. in flowers, again, we see the gradual abortion during successive seedlings (though this is more properly a conversion) of stamens into imperfect petals, and finally into perfect petals. when the eye is blinded in early life the optic nerve sometimes becomes atrophied; may we not believe that where this organ, as is the case with the subterranean mole-like tuco-tuco <_ctenomys_>{ }, is frequently impaired and lost, that in the course of generations the whole organ might become abortive, as it normally is in some burrowing quadrupeds having nearly similar habits with the tuco-tuco? { } _origin_, ed. i. p. , vi. p. , where drooping-ears of domestic animals are also given. { } _origin_, ed. i. p. , vi. p. . in as far then as it is admitted as probable that the effects of disuse (together with occasional true and sudden abortions during the embryonic period) would cause a part to be less developed, and finally to become abortive and useless; then during the infinitely numerous changes of habits in the many descendants from a common stock, we might fairly have expected that cases of organs becom<ing> abortive would have been numerous. the preservation of the stump of the tail, as usually happens when an animal is born tailless, we can only explain by the strength of the hereditary principle and by the period in embryo when affected{ }: but on the theory of disuse gradually obliterating a part, we can see, according to the principles explained in the last chapter (viz. of hereditariness at corresponding periods of life{ }, together with the use and disuse of the part in question not being brought into play in early or embryonic life), that organs or parts would tend not to be utterly obliterated, but to be reduced to that state in which they existed in early embryonic life. owen often speaks of a part in a full-grown animal being in an "embryonic condition." moreover we can thus see why abortive organs are most developed at an early period of life. again, by gradual selection, we can see how an organ rendered abortive in its primary use might be converted to other purposes; a duck's wing might come to serve for a fin, as does that of the penguin; an abortive bone might come to serve, by the slow increment and change of place in the muscular fibres, as a fulcrum for a new series of muscles; the pistil{ } of the marigold might become abortive as a reproductive part, but be continued in its function of sweeping the pollen out of the anthers; for if in this latter respect the abortion had not been checked by selection, the species must have become extinct from the pollen remaining enclosed in the capsules of the anthers. { } these words seem to have been inserted as an afterthought. { } _origin_, ed. i. p. , vi. p. . { } this and similar cases occur in the _origin_, ed. i. p. , vi. p. . finally then i must repeat that these wonderful facts of organs formed with traces of exquisite care, but now either absolutely useless or adapted to ends wholly different from their ordinary end, being present and forming part of the structure of almost every inhabitant of this world, both in long-past and present times--being best developed and often only discoverable at a very early embryonic period, and being full of signification in arranging the long series of organic beings in a natural system--these wonderful facts not only receive a simple explanation on the theory of long-continued selection of many species from a few common parent-stocks, but necessarily follow from this theory. if this theory be rejected, these facts remain quite inexplicable; without indeed we rank as an explanation such loose metaphors as that of de candolle's{ }, in which the kingdom of nature is compared to a well-covered table, and the abortive organs are considered as put in for the sake of symmetry! { } the metaphor of the dishes is given in the essay of , p. , note .{note } chapter x recapitulation and conclusion _recapitulation._ i will now recapitulate the course of this work, more fully with respect to the former parts, and briefly <as to> the latter. in the first chapter we have seen that most, if not all, organic beings, when taken by man out of their natural condition, and bred during several generations, vary; that is variation is partly due to the direct effect of the new external influences, and partly to the indirect effect on the reproductive system rendering the organization of the offspring in some degree plastic. of the variations thus produced, man when uncivilised naturally preserves the life, and therefore unintentionally breeds from those individuals most useful to him in his different states: when even semi-civilised, he intentionally separates and breeds from such individuals. every part of the structure seems occasionally to vary in a very slight degree, and the extent to which all kinds of peculiarities in mind and body, when congenital and when slowly acquired either from external influences, from exercise, or from disuse <are inherited>, is truly wonderful. when several breeds are once formed, then crossing is the most fertile source of new breeds{ }. variation must be ruled, of course, by the health of the new race, by the tendency to return to the ancestral forms, and by unknown laws determining the proportional increase and symmetry of the body. the amount of variation, which has been effected under domestication, is quite unknown in the majority of domestic beings. { } compare however darwin's later view:--"the possibility of making distinct races by crossing has been greatly exaggerated," _origin_, ed. i. p. , vi. p. . the author's change of opinion was no doubt partly due to his experience in breeding pigeons. in the second chapter it was shown that wild organisms undoubtedly vary in some slight degree: and that the kind of variation, though much less in degree, is similar to that of domestic organisms. it is highly probable that every organic being, if subjected during several generations to new and varying conditions, would vary. it is certain that organisms, living in an _isolated_ country which is undergoing geological changes, must in the course of time be so subjected to new conditions; moreover an organism, when by chance transported into a new station, for instance into an island, will often be exposed to new conditions, and be surrounded by a new series of organic beings. if there were no power at work selecting every slight variation, which opened new sources of subsistence to a being thus situated, the effects of crossing, the chance of death and the constant tendency to reversion to the old parent-form, would prevent the production of new races. if there were any selective agency at work, it seems impossible to assign any limit{ } to the complexity and beauty of the adaptive structures, which _might_ thus be produced: for certainly the limit of possible variation of organic beings, either in a wild or domestic state, is not known. { } in the _origin_, ed. i. p. , vi. p. , darwin makes a strong statement to this effect. it was then shown, from the geometrically increasing tendency of each species to multiply (as evidenced from what we know of mankind and of other animals when favoured by circumstances), and from the means of subsistence of each species on an _average_ remaining constant, that during some part of the life of each, or during every few generations, there must be a severe struggle for existence; and that less than a grain{ } in the balance will determine which individuals shall live and which perish. in a country, therefore, undergoing changes, and cut off from the free immigration of species better adapted to the new station and conditions, it cannot be doubted that there is a most powerful means of selection, _tending_ to preserve even the slightest variation, which aided the subsistence or defence of those organic beings, during any part of their whole existence, whose organization had been rendered plastic. moreover, in animals in which the sexes are distinct, there is a sexual struggle, by which the most vigorous, and consequently the best adapted, will oftener procreate their kind. { } "a grain in the balance will determine which individual shall live and which shall die," _origin_, ed. i. p. , vi. p. . a similar statement occurs in the essay, p. , note .{note } a new race thus formed by natural selection would be undistinguishable from a species. for comparing, on the one hand, the several species of a genus, and on the other hand several domestic races from a common stock, we cannot discriminate them by the amount of external difference, but only, first, by domestic races not remaining so constant or being so "true" as species are; and secondly by races always producing fertile offspring when crossed. and it was then shown that a race naturally selected--from the variation being slower--from the selection steadily leading towards the same ends{ }, and from every new slight change in structure being adapted (as is implied by its selection) to the new conditions and being fully exercised, and lastly from the freedom from occasional crosses with other species, would almost necessarily be "truer" than a race selected by ignorant or capricious and short-lived man. with respect to the sterility of species when crossed, it was shown not to be a universal character, and when present to vary in degree: sterility also was shown probably to depend less on external than on constitutional differences. and it was shown that when individual animals and plants are placed under new conditions, they become, without losing their healths, as sterile, in the same manner and to the same degree, as hybrids; and it is therefore conceivable that the cross-bred offspring between two species, having different constitutions, might have its constitution affected in the same peculiar manner as when an individual animal or plant is placed under new conditions. man in selecting domestic races has little wish and still less power to adapt the whole frame to new conditions; in nature, however, where each species survives by a struggle against other species and external nature, the result must be very different. { } thus according to the author what is now known as _orthogenesis_ is due to selection. races descending from the same stock were then compared with species of the same genus, and they were found to present some striking analogies. the offspring also of races when crossed, that is mongrels, were compared with the cross-bred offspring of species, that is hybrids, and they were found to resemble each other in all their characters, with the one exception of sterility, and even this, when present, often becomes after some generations variable in degree. the chapter was summed up, and it was shown that no ascertained limit to the amount of variation is known; or could be predicted with due time and changes of condition granted. it was then admitted that although the production of new races, undistinguishable from true species, is probable, we must look to the relations in the past and present geographical distribution of the infinitely numerous beings, by which we are surrounded--to their affinities and to their structure--for any direct evidence. in the third chapter the inheritable variations in the mental phenomena of domestic and of wild organic beings were considered. it was shown that we are not concerned in this work with the first origin of the leading mental qualities; but that tastes, passions, dispositions, consensual movements, and habits all became, either congenitally or during mature life, modified and were inherited. several of these modified habits were found to correspond in every essential character with true instincts, and they were found to follow the same laws. instincts and dispositions &c. are fully as important to the preservation and increase of a species as its corporeal structure; and therefore the natural means of selection would act on and modify them equally with corporeal structures. this being granted, as well as the proposition that mental phenomena are variable, and that the modifications are inheritable, the possibility of the several most complicated instincts being slowly acquired was considered, and it was shown from the very imperfect series in the instincts of the animals now existing, that we are not justified in _prima facie_ rejecting a theory of the common descent of allied organisms from the difficulty of imagining the transitional stages in the various now most complicated and wonderful instincts. we were thus led on to consider the same question with respect both to highly complicated organs, and to the aggregate of several such organs, that is individual organic beings; and it was shown, by the same method of taking the existing most imperfect series, that we ought not at once to reject the theory, because we cannot trace the transitional stages in such organs, or conjecture the transitional habits of such individual species. in the second part{ } the direct evidence of allied forms having descended from the same stock was discussed. it was shown that this theory requires a long series of intermediate forms between the species and groups in the same classes--forms not directly intermediate between existing species, but intermediate with a common parent. it was admitted that if even all the preserved fossils and existing species were collected, such a series would be far from being formed; but it was shown that we have not _good_ evidence that the oldest known deposits are contemporaneous with the first appearance of living beings; or that the several subsequent formations are nearly consecutive; or that any one formation preserves a nearly perfect fauna of even the hard marine organisms, which lived in that quarter of the world. consequently, we have no reason to suppose that more than a small fraction of the organisms which have lived at any one period have ever been preserved; and hence that we ought not to expect to discover the fossilised sub-varieties between any two species. on the other hand, the evidence, though extremely imperfect, drawn from fossil remains, as far as it does go, is in favour of such a series of organisms having existed as that required. this want of evidence of the past existence of almost infinitely numerous intermediate forms, is, i conceive, much the weightiest difficulty{ } on the theory of common descent; but i must think that this is due to ignorance necessarily resulting from the imperfection of all geological records. { } part ii begins with ch. iv. see the introduction, where the absence of division into two parts (in the _origin_) is discussed. { } in the recapitulation in the last chapter of the _origin_, ed. i. p. , vi. p. , the author does not insist on this point as the weightiest difficulty, though he does so in ed. i. p. . it is possible that he had come to think less of the difficulty in question: this was certainly the case when he wrote the th edition, see p. . in the fifth chapter it was shown that new species gradually{ } appear, and that the old ones gradually disappear, from the earth; and this strictly accords with our theory. the extinction of species seems to be preceded by their rarity; and if this be so, no one ought to feel more surprise at a species being exterminated than at its being rare. every species which is not increasing in number must have its geometrical tendency to increase checked by some agency seldom accurately perceived by us. each slight increase in the power of this unseen checking agency would cause a corresponding decrease in the average numbers of that species, and the species would become rarer: we feel not the least surprise at one species of a genus being rare and another abundant; why then should we be surprised at its extinction, when we have good reason to believe that this very rarity is its regular precursor and cause. { } <the following words:> the fauna changes singly <were inserted by the author, apparently to replace a doubtful erasure>. in the sixth chapter the leading facts in the geographical distribution of organic beings were considered--namely, the dissimilarity in areas widely and effectually separated, of the organic beings being exposed to very similar conditions (as for instance, within the tropical forests of africa and america, or on the volcanic islands adjoining them). also the striking similarity and general relations of the inhabitants of the same great continents, conjoined with a lesser degree of dissimilarity in the inhabitants living on opposite sides of the barriers intersecting it--whether or not these opposite sides are exposed to similar conditions. also the dissimilarity, though in a still lesser degree, in the inhabitants of different islands in the same archipelago, together with their similarity taken as a whole with the inhabitants of the nearest continent, whatever its character may be. again, the peculiar relations of alpine floras; the absence of mammifers on the smaller isolated islands; and the comparative fewness of the plants and other organisms on islands with diversified stations; the connection between the possibility of occasional transportal from one country to another, with an affinity, though not identity, of the organic beings inhabiting them. and lastly, the clear and striking relations between the living and the extinct in the same great divisions of the world; which relation, if we look very far backward, seems to die away. these facts, if we bear in mind the geological changes in progress, all simply follow from the proposition of allied organic beings having lineally descended from common parent-stocks. on the theory of independent creations they must remain, though evidently connected together, inexplicable and disconnected. in the seventh chapter, the relationship or grouping of extinct and recent species; the appearance and disappearance of groups; the ill-defined objects of the natural classification, not depending on the similarity of organs physiologically important, not being influenced by adaptive or analogical characters, though these often govern the whole economy of the individual, but depending on any character which varies least, and especially on the forms through which the embryo passes, and, as was afterwards shown, on the presence of rudimentary and useless organs. the alliance between the nearest species in _distinct_ groups being general and not especial; the close similarity in the rules and objects in classifying domestic races and true species. all these facts were shown to follow on the natural system being a genealogical system. in the eighth chapter, the unity of structure throughout large groups, in species adapted to the most different lives, and the wonderful metamorphosis (used metaphorically by naturalists) of one part or organ into another, were shown to follow simply on new species being produced by the selection and inheritance of successive _small_ changes of structure. the unity of type is wonderfully manifested by the similarity of structure, during the embryonic period, in the species of entire classes. to explain this it was shown that the different races of our domestic animals differ less, during their young state, than when full grown; and consequently, if species are produced like races, the same fact, on a greater scale, might have been expected to hold good with them. this remarkable law of nature was attempted to be explained through establishing, by sundry facts, that slight variations originally appear during all periods of life, and that when inherited they tend to appear at the corresponding period of life; according to these principles, in several species descended from the same parent-stock, their embryos would almost necessarily much more closely resemble each other than they would in their adult state. the importance of these embryonic resemblances, in making out a natural or genealogical classification, thus becomes at once obvious. the occasional greater simplicity of structure in the mature animal than in the embryo; the gradation in complexity of the species in the great classes; the adaptation of the larvæ of animals to independent powers of existence; the immense difference in certain animals in their larval and mature states, were all shown on the above principles to present no difficulty. in the <ninth> chapter, the frequent and almost general presence of organs and parts, called by naturalists abortive or rudimentary, which, though formed with exquisite care, are generally absolutely useless <was considered>. <these structures,> though sometimes applied to uses not normal,--which cannot be considered as mere representative parts, for they are sometimes capable of performing their proper function,--which are always best developed, and sometimes only developed, during a very early period of life,--and which are of admitted high importance in classification,--were shown to be simply explicable on our theory of common descent. _why do we wish to reject the theory of common descent?_ thus have many general facts, or laws, been included under one explanation; and the difficulties encountered are those which would naturally result from our acknowledged ignorance. and why should we not admit this theory of descent{ }? can it be shown that organic beings in a natural state are _all absolutely invariable_? can it be said that the _limit of variation_ or the number of varieties capable of being formed under domestication are known? can any distinct line be drawn _between a race and a species_? to these three questions we may certainly answer in the negative. as long as species were thought to be divided and defined by an impassable barrier of _sterility_, whilst we were ignorant of geology, and imagined that the _world was of short duration_, and the number of its past inhabitants few, we were justified in assuming individual creations, or in saying with whewell that the beginnings of all things are hidden from man. why then do we feel so strong an inclination to reject this theory--especially when the actual case of any two species, or even of any two races, is adduced--and one is asked, have these two originally descended from the same parent womb? i believe it is because we are always slow in admitting any great change of which we do not see the intermediate steps. the mind cannot grasp the full meaning of the term of a million or hundred million years, and cannot consequently add up and perceive the full effects of small successive variations accumulated during almost infinitely many generations. the difficulty is the same with that which, with most geologists, it has taken long years to remove, as when lyell propounded that great valleys{ } were hollowed out [and long lines of inland cliffs had been formed] by the slow action of the waves of the sea. a man may long view a grand precipice without actually believing, though he may not deny it, that thousands of feet in thickness of solid rock once extended over many square miles where the open sea now rolls; without fully believing that the same sea which he sees beating the rock at his feet has been the sole removing power. { } this question forms the subject of what is practically a section of the final chapter of the _origin_ (ed. i. p. , vi. p. ). { } _origin_, ed. i. p. , vi. p. . shall we then allow that the three distinct species of rhinoceros{ } which separately inhabit java and sumatra and the neighbouring mainland of malacca were created, male and female, out of the inorganic materials of these countries? without any adequate cause, as far as our reason serves, shall we say that they were merely, from living near each other, created very like each other, so as to form a section of the genus dissimilar from the african section, some of the species of which section inhabit very similar and some very dissimilar stations? shall we say that without any apparent cause they were created on the same generic type with the ancient woolly rhinoceros of siberia and of the other species which formerly inhabited the same main division of the world: that they were created, less and less closely related, but still with interbranching affinities, with all the other living and extinct mammalia? that without any apparent adequate cause their short necks should contain the same number of vertebræ with the giraffe; that their thick legs should be built on the same plan with those of the antelope, of the mouse, of the hand of the monkey, of the wing of the bat, and of the fin of the porpoise. that in each of these species the second bone of their leg should show clear traces of two bones having been soldered and united into one; that the complicated bones of their head should become intelligible on the supposition of their having been formed of three expanded vertebræ; that in the jaws of each when dissected young there should exist small teeth which never come to the surface. that in possessing these useless abortive teeth, and in other characters, these three rhinoceroses in their embryonic state should much more closely resemble other mammalia than they do when mature. and lastly, that in a still earlier period of life, their arteries should run and branch as in a fish, to carry the blood to gills which do not exist. now these three species of rhinoceros closely resemble each other; more closely than many generally acknowledged races of our domestic animals; these three species if domesticated would almost certainly vary, and races adapted to different ends might be selected out of such variations. in this state they would probably breed together, and their offspring would possibly be quite, and probably in some degree, fertile; and in either case, by continued crossing, one of these specific forms might be absorbed and lost in another. i repeat, shall we then say that a pair, or a gravid female, of each of these three species of rhinoceros, were separately created with deceptive appearances of true relationship, with the stamp of inutility on some parts, and of conversion in other parts, out of the inorganic elements of java, sumatra and malacca? or have they descended, like our domestic races, from the same parent-stock? for my own part i could no more admit the former proposition than i could admit that the planets move in their courses, and that a stone falls to the ground, not through the intervention of the secondary and appointed law of gravity, but from the direct volition of the creator. { } the discussion on the three species of _rhinoceros_ which also occurs in the essay of , p. , was omitted in ch. xiv of the _origin_, ed. i. before concluding it will be well to show, although this has incidentally appeared, how far the theory of common descent can legitimately be extended{ }. if we once admit that two true species of the same genus can have descended from the same parent, it will not be possible to deny that two species of two genera may also have descended from a common stock. for in some families the genera approach almost as closely as species of the same genus; and in some orders, for instance in the monocotyledonous plants, the families run closely into each other. we do not hesitate to assign a common origin to dogs or cabbages, because they are divided into groups analogous to the groups in nature. many naturalists indeed admit that all groups are artificial; and that they depend entirely on the extinction of intermediate species. some naturalists, however, affirm that though driven from considering sterility as the characteristic of species, that an entire incapacity to propagate together is the best evidence of the existence of natural genera. even if we put on one side the undoubted fact that some species of the same genus will not breed together, we cannot possibly admit the above rule, seeing that the grouse and pheasant (considered by some good ornithologists as forming two families), the bull-finch and canary-bird have bred together. { } this corresponds to a paragraph in the _origin_, ed. i. p. , vi. p. , where it is assumed that animals have descended "from at most only four or five progenitors, and plants from an equal or lesser number." in the _origin_, however, the author goes on, ed. i. p. , vi. p. : "analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype." no doubt the more remote two species are from each other, the weaker the arguments become in favour of their common descent. in species of two distinct families the analogy, from the variation of domestic organisms and from the manner of their intermarrying, fails; and the arguments from their geographical distribution quite or almost quite fails. but if we once admit the general principles of this work, as far as a clear unity of type can be made out in groups of species, adapted to play diversified parts in the economy of nature, whether shown in the structure of the embryonic or mature being, and especially if shown by a community of abortive parts, we are legitimately led to admit their community of descent. naturalists dispute how widely this unity of type extends: most, however, admit that the vertebrata are built on one type; the articulata on another; the mollusca on a third; and the radiata on probably more than one. plants also appear to fall under three or four great types. on this theory, therefore, all the organisms _yet discovered_ are descendants of probably less than ten parent-forms. _conclusion._ my reasons have now been assigned for believing that specific forms are not immutable creations{ }. the terms used by naturalists of affinity, unity of type, adaptive characters, the metamorphosis and abortion of organs, cease to be metaphorical expressions and become intelligible facts. we no longer look at an organic being as a savage does at a ship{ } or other great work of art, as at a thing wholly beyond his comprehension, but as a production that has a history which we may search into. how interesting do all instincts become when we speculate on their origin as hereditary habits, or as slight congenital modifications of former instincts perpetuated by the individuals so characterised having been preserved. when we look at every complex instinct and mechanism as the summing up of a long history of contrivances, each most useful to its possessor, nearly in the same way as when we look at a great mechanical invention as the summing up of the labour, the experience, the reason, and even the blunders of numerous workmen. how interesting does the geographical distribution of all organic beings, past and present, become as throwing light on the ancient geography of the world. geology loses glory{ } from the imperfection of its archives, but it gains in the immensity of its subject. there is much grandeur in looking at every existing organic being either as the lineal successor of some form now buried under thousands of feet of solid rock, or as being the co-descendant of that buried form of some more ancient and utterly lost inhabitant of this world. it accords with what we know of the laws impressed by the creator{ } on matter that the production and extinction of forms should, like the birth and death of individuals, be the result of secondary means. it is derogatory that the creator of countless universes should have made by individual acts of his will the myriads of creeping parasites and worms, which since the earliest dawn of life have swarmed over the land and in the depths of the ocean. we cease to be astonished{ } that a group of animals should have been formed to lay their eggs in the bowels and flesh of other sensitive beings; that some animals should live by and even delight in cruelty; that animals should be led away by false instincts; that annually there should be an incalculable waste of the pollen, eggs and immature beings; for we see in all this the inevitable consequences of one great law, of the multiplication of organic beings not created immutable. from death, famine, and the struggle for existence, we see that the most exalted end which we are capable of conceiving, namely, the creation of the higher animals{ }, has directly proceeded. doubtless, our first impression is to disbelieve that any secondary law could produce infinitely numerous organic beings, each characterised by the most exquisite workmanship and widely extended adaptations: it at first accords better with our faculties to suppose that each required the fiat of a creator. there{ } is a [simple] grandeur in this view of life with its several powers of growth, reproduction and of sensation, having been originally breathed into matter under a few forms, perhaps into only one{ }, and that whilst this planet has gone cycling onwards according to the fixed laws of gravity and whilst land and water have gone on replacing each other--that from so simple an origin, through the selection of infinitesimal varieties, endless forms most beautiful and most wonderful have been evolved. { } this sentence corresponds, not to the final section of the _origin_, ed. i. p. , vi. p. , but rather to the opening words of the section already referred to (_origin_, ed. i. p. , vi. p. ). { } this simile occurs in the essay of , p. , and in the _origin_, ed. i. p. , vi. p. , _i.e._ in the final section of ch. xiv (vi. ch. xv). in the ms. there is some erasure in pencil of which i have taken no notice. { } an almost identical sentence occurs in the _origin_, ed. i. p. , vi. p. . the fine prophecy (in the _origin_, ed. i. p. , vi. p. ) on "the almost untrodden field of inquiry" is wanting in the present essay. { } see the last paragraph on p. of the _origin_, ed. i., vi. p. . { } a passage corresponding to this occurs in the sketch of , p. , but not in the last chapter of the _origin_. { } this sentence occurs in an almost identical form in the _origin_, ed. i. p. , vi. p. . it will be noted that man is not named though clearly referred to. elsewhere (_origin_, ed. i. p. ) the author is bolder and writes "light will be thrown on the origin of man and his history." in ed. vi. p. , he writes "much light &c." { } for the history of this sentence (with which the _origin of species_ closes) see the essay of , p. , note {note }: also the concluding pages of the introduction. { } these four words are added in pencil between the lines. index for the names of authors, birds, mammals (including names of classes) and plants, see sub-indexes under _authors_, _birds_, _mammals_ and _plants_. acquired characters, _see_ characters affinities and classification, america, fossils, analogy, resemblance by, , , , , animals, marine, preservation of as fossils, , , ; --marine distribution, , australia, fossils, authors, names of:--ackerman on hybrids, ; bakewell, , ; bateson, w., xxix, _n._, ; bellinghausen, ; boitard and corbié, _n._; brougham, lord, , ; brown, r., ; buckland on fossils, , , _n._; buffon on woodpecker, ; bunbury (_sir_ h.), rules for selection, ; butler, s., _n._; d'archiac, _n._; darwin, c., origin of his evolutionary views, xi-xv; --on forbes' theory, ; --his _journal of researches_ quoted, _n._, _n._; --his _cross-and self-fertilisation_, _n._, _n._; --on crossing chinese and common goose, _n._; darwin, mrs, letter to, xxvi; darwin, f., on knight's law, _n._; darwin, r. w., fact supplied by, _n._, ; darwin and wallace, joint paper by, xxiv, _n._; de candolle, , , , , ; d'orbigny, , _n._; ehrenberg, _n._; ewart on telegony, _n._; falconer, ; forbes, e., xxvii, , _n._, _n._, _n._; gadow, dr, xxix; gärtner, , ; goebel on knight's law, _n._; gould on distribution, ; gray, asa, letter to, publication of in linnean paper explained, xxiv; henslow, g., on evolution without selection, _n._; henslow, j. s., xxvii; herbert on hybrids, , ; --sterility of crocus, _n._; hering, _n._; hogg, _n._; holland, dr, ; hooker, j. d., xxvii, xxviii, _n._; --on insular floras, , , ; huber, p., ; hudson on woodpecker, _n._; humboldt, , ; hunter, w., ; hutton, , ; huxley, _n._; --on darwin, xi, xii, xiv; --on darwin's essay of , xxviii, ; judd, xi, xiii, xxix, , _n._; knight, a., _n._, , ; --on domestication, ; knight-darwin law, _n._; kölreuter, , , , , ; lamarck, _n._, , , , ; --reasons for his belief in mutability, ; lindley, ; linnean society, joint paper, _see_ darwin and wallace; linnæus on sterility of alpine plants, ; --on generic characters, ; lonsdale, _n._; lyell, xxvii, _n._, , and _n._, _n._, , , , ; --his doctrine carried to an extreme, ; --his geological metaphor, _n._, ; --his uniformitarianism, _n._; --his views on imperfection of geological record, ; macculloch, _n._; macleay, w. s., ; magendie, ; malthus, xv, , , ; marr, dr, xxix; marshall, ; --on sheep and cattle, and _n._; --on horns of cattle, ; mivart, criticisms, _n._; mozart as a child, his skill on the piano compared to instinct, _n._; müller on consensual movements, ; --on variation under uniform conditions, ( ), ; --on recapitulation theory, ; murchison, _n._; newton, alfred, _n._; owen, r., xxvii, ; pallas, , ; pennant, _n._; pliny on selection, ; poeppig, _n._; prain, col., xxix; rengger, sterility, ; richardson, _n._; rutherford, h. w., xxix; st hilaire on races of dogs, ; --on sterility of tame and domestic animals, , ; smith, jordan, ; sprengel, ; stapf, dr, xxix; strickland, xxvii; suchetet, _n._; thiselton-dyer, sir w., xxix, ; wallace, xxiv, xxix, , _n._; waterhouse, , ; western, lord, , , ; whewell, xxviii, ; woodward, h. b., _n._; wrangel, _n._; zacharias, darwin's letter to, xv barriers and distribution, , , , bees, , ; combs of hive-bee, , , , beetles, abortive wings of, birds, transporting seeds, ; feeding young with food different to their own, , ; migration, , ; nests, , , , ; of galapagos, , ; rapid increase, ; song, birds, names of:--apteryx, , ; duck, , , , , _n._; fowl, domestic, , _n._, , , , ; goose, ; --periodic habit, _n._; grouse, hybridised, , ; guinea-fowl, ; hawk, sterility, ; --periodic habit, ; opetiorynchus, ; orpheus, ; ostrich, distribution of, ; owl, white barn, ; partridge, infertility of, ; peacock, , , ; penguin, _n._, ; petrel, _n._; pheasant, , ; pigeon, , , _n._, , , , , , ; _see_ wood-pigeon; rhea, ; robins, increase in numbers, , ; rock-thrush of guiana, ; swan, species of, ; tailor-bird, , ; turkey, australian bush-turkey, _n._, ; tyrannus, ; water-ouzel, _n._, ; woodcock, loss of migratory instinct, ; woodpecker, , , _n._, ; --in treeless lands, , ; wood-pigeon, ; wren, gold-crested, ; --willow, , breeds, domestic, parentage of, brothers, death of by same peculiar disease in old age, _n._, _n._, bud variation, ; _see_ sports butterfly, cabbage, catastrophes, geological, , caterpillars, food, , characters, acquired, inheritance of, , , , ; --congenital, ; --fixed by breeding, ; --mental, variation in, , , ; --running through whole groups, ; --useless for classification, cirripedes, , classification, natural system of, , , , ; --by any constant character, ; --relation of, to geography, ; --a law that members of two distinct groups resemble each other not specifically but generally, , ; --of domestic races, ; --rarity and extinction in relation to, compensation, law of, conditions, direct, action of, , _n._, , ; --change of, analogous to crossing, , _n._, ; --accumulated effects of, , ; --affecting reproduction, , , , ; --and geographical distribution, continent originating as archipelago, bearing of on distribution, cordillera, as channel of migration, _n._, correlation, creation, centres of, , crocodile, _cross-and self-fertilisation_, early statement of principles of, , _n._, _n._ crossing, swamping effect of, , , ; --of bisexual animals and hermaphrodite plants, ; --analogous to change in conditions, , , ; --in relation to breeds, ; --in plants, adaptations for, death, feigned by insects, difficulties, on theory of evolution, , , , disease, hereditary, _n._, , distribution, geographical, , , , , ; --in space and time, subject to same laws, ; --occasional means of (seeds, eggs, &c.), disuse, inherited effects of, , divergence, principle of, xxv, _n._, _n._, _n._ domestication, variation under, , ; --accumulated effects of, , ; --analysis of effects of, , ears, drooping, elevation, geological, favouring birth of new species, , _n._, _n._, - ; --alternating with subsidence, importance of for evolution, , ; --bad for preservation of fossils, embryo, branchial arches of, , ; --absence of special adaptation in, , _n._, , ; --less variable than parent, hence importance of embryology for classification, _n._, ; --alike in all vertebrates, , ; --occasionally more complicated than adult, , embryology, , ; its value in classification, , ; law of inheritance at corresponding ages, _n._, ; young of very distinct breeds closely similar, _n._, ephemera, selection falls on larva, _n._ epizoa, essay of , question as to date of, xvi; description of ms., xx; compared with the _origin_, xxii essay of , writing of, xvi; compared with that of and with the _origin_, xxii evolution, theory of, why do we tend to reject it, expression, inheritance of, extinction, , , ; locally sudden, ; continuous with rarity, , extinction and rarity, eye, _n._, , , faculty, in relation to instinct, faunas, alpine, , , ; of galapagos, _n._, , ; insular-alpine very peculiar, ; insular, , fauna and flora, of islands related to nearest land, fear of man, inherited, , fertility, interracial, , fish, colours of, , ; eggs of carried by water-beetle, ; flying, _n._; --transported by whirlwind, floras, alpine, ; of oceanic islands, ; alpine, related to surrounding lowlands, ; alpine, identity of on distant mountains, ; alpine resembling arctic, ; arctic relation to alpine, flower, morphology of, , ; degenerate under domestication if neglected, ; changed by selection, fly, causing extinction, flying, evolution of, , food, causing variations, , , , formation (geological) evidence from tertiary system, ; (geological), groups of species appear suddenly in secondary, , ; palæozoic, if contemporary with beginning of life, author's theory false, formations, most ancient escape denudation in conditions unfavourable to life, , forms, transitional, , _n._, , , ; on rising land, ; indirectly intermediate, , fossils, silurian, not those which first existed in the world, , ; falling into or between existing groups and indirectly intermediate, , ; conditions favourable to preservation, not favourable to existence of much life, , , fruit, attractive to animals, galapagos islands and darwin's views, xiv; physical character of in relation to fauna, _n._, galapagos islands, fauna, _n._, gasteropods, embryology, genera, crosses between, , ; wide ranging, has wide ranging species, ; origin of, geography, in relation to geology, _n._, , geographical distribution, _see_ distribution geology, as producing changed conditions, ; evidence from, , ; "destroys geography," _n._ glacial period, effect of on distribution of alpine and arctic plants, habit in relation to instinct, , , , habits in animals taught by parent, heredity, _see_ inheritance homology of limbs, , homology, serial, , hybrid, fowls and grouse, ; fowl and peacock, ; pheasant and grouse, ; azalea and rhododendron, hybrids, gradation in sterility of, , , ; sterility of not reciprocal, ; variability of, ; compared and contrasted with mongrel, individual, meaning of term, inheritance of acquired characters, _see_ character inheritance, delayed or latent, , _n._, ; of character at a time of life corresponding to that at which it first appeared, , _n._, ; germinal, , , insect, adapted to fertilise flowers, ; feigning death, ; metamorphosis, ; variation in larvæ, instinct, variation in, , ; and faculty, , ; guided by reason, , , ; migratory, ; migratory, loss of by woodcocks, ; migratory, origin of, ; due to germinal variation rather than habit, ; requiring education for perfection, ; characterised by ignorance of end: _e.g._ butterflies laying eggs, , ; butterflies laying eggs on proper plant, , ; instinct, natural selection applicable to, , instinct, for finding the way, ; periodic, _i.e._ for lapse of time, ; comb-making of bee, ; birds feeding young, , ; nest-building, gradation in, , , , ; instincts, complex, difficulty in believing in their evolution, , intermediate forms, _see_ forms island, _see_ elevation, fauna, flora island, upheaved and gradually colonised, islands, nurseries of new species, , _n._, , isolation, , _n._, , , , lepidosiren, _n._, limbs, vertebrate, of one type, , mammals, arctic, transported by icebergs, ; distribution, , , ; distribution of, ruled by barriers, ; introduced by man on islands, ; not found on oceanic islands, ; relations in time and space, similarity of, ; of tertiary period, relation of to existing forms in same region, mammals, names of:-- antelope, ; armadillo, ; ass, , , ; bat, , , _n._, , , ; bear, sterile in captivity, ; --whale-like habit, _n._; bizcacha, , , ; bull, mammæ of, ; carnivora, law of compensation in, ; cats, run wild at ascension, ; --tailless, ; cattle, horns of, , ; --increase in s. america, ; --indian, ; --niata, , ; --suffering in parturition from too large calves, ; cheetah, sterility of, and _n._; chironectes, ; cow, abortive mammæ, ; ctenomys, _see_ tuco-tuco; dog, , ; --in cuba, and _n._; --mongrel breed in oceanic islands, ; --difference in size a bar to crossing, ; --domestic, parentage of, , , ; --drooping ears, ; --effects of selection, ; --inter-fertile, ; --long-legged breed produced to catch hares, , , , ; --of savages, ; --races of resembling genera, , ; --australian, change of colour in, ; --bloodhound, cuban, ; --bull-dog, ; --foxhound, , ; --greyhound and bull-dog, young of resembling each other, , _n._, ; --pointer, , , , , ; --retriever, _n._; --setter, ; --shepherd-dog and harrier crossed, instinct of, , ; --tailless, ; --turnspit, ; echidna, _n._; edentata, fossil and living in s. america, ; elephant, sterility of, , ; elk, ; ferret, fertility of, , ; fox, , , ; galeopithecus, _n._; giraffe, fossil, ; --tail, _n._; goat, run wild at tahiti, ; guanaco, ; guinea-pig, ; hare, s. american, _n._; hedgehog, _n._; horse, , , , , ; --checks to increase, , ; --increase in s. america, ; --malconformations and lameness inherited, ; --parentage, , ; --stripes on, ; --young of cart-horse and racehorse resembling each other, ; hyena, fossil, ; jaguar, catching fish, ; lemur, flying, _n._; macrauchenia, ; marsupials, fossil in europe, _n._, ; --pouch bones, , ; mastodon, ; mouse, , ; --enormous rate of increase, , ; mule, occasionally breeding, , ; musk-deer, fossil, ; _mustela vison_, _n._, _n._; mydas, ; mydaus, ; nutria, _see_ otter; otter, , , ; --marsupial, , , ; pachydermata, ; phascolomys, , ; pig, , ; --in oceanic islands, ; --run wild at st helena, ; pole-cat, aquatic, _n._, _n._; porpoise, paddle of, , ; rabbit, , , ; rat, norway, ; reindeer, ; rhinoceros, ; --abortive teeth of, , ; --three oriental species of, , ; ruminantia, and _n._; seal, _n._, ; sheep, , , , ; --ancon variety, , , ; --inherited habit of returning home to lamb, ; --transandantes of spain, their migratory instinct, , , _n._; squirrel, flying, ; tapir, , ; tuco-tuco, blindness of, , ; whale, rudimentary teeth, , ; wolf, , , ; yak, metamorphosis, literal not metaphorical, , metamorphosis, _e.g._ leaves into petals, migrants to new land, struggle among, , migration, taking the place of variation, monstrosities, as starting-points of breeds, , ; their relation to rudimentary organs, , morphology, , ; terminology of, no longer metaphorically used, , mutation, _see_ sports natural selection, _see_ selection nest, bird's, _see_ instinct ocean, depth of, and fossils, , organisms, gradual introduction of new, , ; extinct related to, existing in the same manner as representative existing ones to each other, , ; introduced, beating indigenes, ; dependent on other organisms rather than on physical surroundings, ; graduated complexity in the great classes, ; immature, how subject to natural selection, , , ; all descended from a few parent-forms, , organs, perfect, objection to their evolution, , ; distinct in adult life, indistinguishable in embryo, , ; rudimentary, , , , ; rudimentary, compared to monstrosities, , ; rudimentary, caused by disuse, , ; rudimentary, adapted to new ends, , orthogenesis, _n._ oscillation of level in relation to continents, , _n._, pallas, on parentage of domestic animals, pampas, imaginary case of farmer on, , perfection, no inherent tendency towards, plants, _see also_ flora; fertilisation, ; migration of, to arctic and antarctic regions, ; alpine and arctic, migration of, , ; alpine, characters common to, ; alpine, sterility of, , plants, names of:--Ægilops, _n._; artichoke (jerusalem), ; ash, weeping, seeds of, ; asparagus, ; azalea, , , ; cabbage, , , ; calceolaria, , ; cardoon, ; carrot, variation of, _n._; chrysanthemum, ; crinum, , ; crocus, , _n._; cucubalus, crossing, ; dahlia, , , , , , ; foxglove, ; gentian, colour of flower, _n._; geranium, ; gladiolus, crossed, ancestry of, ; grass, abortive flowers, ; heath, sterility, ; hyacinth, colours of, ; --feather-hyacinth, ; juniperus, hybridised, ; laburnum, peculiar hybrid, ; lilac, sterility of, , ; marigold, style of, , , ; mistletoe, , , , _n._; nectarines on peach trees, ; oxalis, colour of flowers of, _n._; phaseolus, cultivated form suffers from frost, ; pine-apple, ; poppy, mexican, ; potato, , , ; rhododendron, , ; rose, moss, ; --scotch, ; seakale, ; sweet-william, ; syringa, persica and chinensis, _see_ lilac; teazle, ; thuja, hybridised, ; tulips, "breaking" of, ; turnip, swedish and common, ; vine, peculiar hybrid, ; yew, weeping, seeds of, plasticity, produced by domestication, , plesiosaurus, loss of unity of type in, , pteropods, embryology, quadrupeds, extinction of large, quinary system, race, the word used as equivalent to variety, races, domestic, classification of, rarity, , ; and extinction, , , recapitulation theory, , , , record, geological, imperfection of, , regions, geographical, of the world, , , ; formerly less distinct as judged by fossils, resemblance, analogical, , reversion, , , , "roguing," rudimentary organs, _see_ organs savages, domestic animals of, , , selection, human, , ; references to the practice of, in past times, ; great effect produced by, , ; necessary for the formation of breeds, ; methodical, effects of, , ; unconscious, , selection, natural, xvi, , ; natural compared to human, , , ; of instincts, , ; difficulty of believing, , , selection, sexual, two types of, , silk-worms, variation in larval state, _n._, skull, morphology of, , species, representative, seen in going from n. to s. in a continent, _n._, ; representative in archipelagoes, ; wide-ranging, _n._, ; and varieties, difficulty of distinguishing, , , ; sterility of crosses between, supposed to be criterion, , ; gradual appearance and disappearance of, , ; survival of a few among many extinct, species, not created more than once, , , ; evolution of, compared to birth of individuals, , , ; small number in new zealand as compared to the cape, , ; persistence of, unchanged, , sports, , , , , , , , , , sterility, due to captivity, , _n._, ; of various plants, , ; of species when crossed, , , , , ; produced by conditions, compared to sterility due to crossing, , struggle for life, , , , , subsidence, importance of, in relation to fossils, , _n._, ; of continent leading to isolation of organisms, ; not favourable to birth of new species, swimming bladder, , system, natural, is genealogical, , telegony, tibia and fibula, , time, enormous lapse of, in geological epochs, , tortoise, transitional forms, _see_ forms trigonia, _n._, tree-frogs in treeless regions, type, unity of, , ; uniformity of, lost in plesiosaurus, ; persistence of, in continents, , uniformitarian views of lyell, bearing on evolution, use, inherited effects of, _see_ characters, acquired variability, as specific character, ; produced by change and also by crossing, variation, by sports, _see_ sports; under domestication, , , , ; due to causes acting on reproductive system, _see_ variation, germinal; --germinal, , , , ; individual, _n._; causes of, , , , ; due to crossing, , ; limits of, , , , ; small in state of nature, , _n._, , ; results of _without_ selection, ; --minute, value of, ; analogous in species of same genus, ; of mental attributes, , ; in mature life, , , varieties, minute, in birds, ; resemblance of to species, _n._, , vertebrate skull, morphology of, wildness, hereditary, , cambridge: printed by john clay, m.a. at the university press +--------------------------------------------------------------+ | transcriber's notes & errata | | | | inline transcriber's notes are enclosed in curly brackets. | | | | footnote anchors and labels are enclosed in curly brackets. | | | | the footnotes have been renumbered consecutively. | | | | because of this, the changed footnote numbers are appended | | in curly brackets to the internal cross-references. | | | | superscript letters are denoted by a preceding caret e.g., | | d^o | | | | 'oe' ligatures have been rendered as separate letters. | | | | the following typographical errors have been corrected. | | | | |simplication |simplification | | | |care |case | | | |apparant |apparent | | | | | the following words were found in both hyphenated and | | unhyphenated forms. the figures in parentheses are the | | number of instances of each. | | | | |after-thought ( ) |afterthought ( ) | | | |blood-hound ( ) |bloodhound ( ) | | | |bull-dog ( ) |bulldog ( ) | | | |co-descendants ( ) |codescendants ( ) | | | |feather-hyacinth ( ) |feather hyacinth ( ) | | | |grey-hound ( ) |greyhound ( ) | | | |high-lands ( ) |highlands ( ) | | | |long-legged ( ) |long legged ( ) | | | |race-horse ( ) |racehorse ( ) | | | |shepherd-dog ( ) |shepherd dog ( ) | | | |sub-divisions ( ) |subdivisions ( ) | | | |table-land ( ) |tableland ( ) | | | | +--------------------------------------------------------------+ darwiniana essays and reviews pertaining to darwinism by asa gray fisher professor of natural history (botany) in harvard university new york: . contents darwiniana preface article i the origin of species by means of natural selection views and definitions of species--how darwin's differs from that of agassiz, and from the common view--variation, its causes unknown.--darwin's genealogical tree--darwin and agassiz agree in the capital facts--embryology--physical connection of species compatible with intellectual connection--how to prove transmutation.--known extent of variation--cause of likeness unknown--artificial selection.--reversion--interbreeding--natural selection.--classification tentative.--what darwin assumes.--argument stated.--how natural selection works.--where the argument is weakest.--objections--morphology and teleology harmonized.--theory not atheistical.--conceivable modes of relation of god to nature article ii design versus necessity-- a discussion how design in nature can be shown--design not inconsistent with indirect attainment article iii natural selection not inconsistent with natural theology part i.--premonitions of darwinism.--a proper subject for speculation.--summary of facts and ideas suggestive of hypotheses of derivation part ii--limitations of theory conceded by darwin.--what darwinism explains.--geological argument strong in the tertiary period.-- correspondence between rank and geological succession--difficulties in classification.--nature of affinity.--no absolute distinction between vegetable and animal kingdoms.--individuality.--gradation part iii.--theories contrasted.--early arguments against darwinism.--philosophical and theological objections--theory may be theistic.--final cause not excluded.--cause of variation unknown.--three views of efficient cause compatible with theism.--agassiz's objections of a philosophical nature.--minor objections.--conclusion article iv species as to variation, geographical distribution, and succession alphonse de candolle's study of the oak genus.--variability of the species.--antiquity.--a common origin probable.--dr. falconer on the common origin of elephants--variation and natural selection distinguished.--saporta on the gradation between the vegetable forms of the cretaceous and the tertiary.--hypothesis of derivation more likely to be favored by botanists than by zoologists.--views of agassiz respecting the origin, dispersion, variation, characteristics, and successive creation of species contrasted with those of de candolle and others--definition of species--whether its essence is in the likeness or in the genealogical connection of the individuals composing a species article v sequoia and its history: the relations of north american to northeast asian and to tertiary vegetation age and size of sequoia.--isolation.--decadence.--related genera.-- former distribution.--similarity between the flora of japan and that of the united states, especially on the atlantic side.--former glaciation as explaining the present dispersion of species.--this confirmed by the arctic fossil flora of the tertiary period.--tertiary flora derived from the preceding cretaceous.--order and adaptation in organic nature likened to a flow.--order implies an ordainer article vi the attitude of working naturalists toward darwinism general tendency to acceptance of the derivative hypothesis noted.--lyell, owen, alphonse de candolle, bentham, flower, ailman.-- dr. dawson's "story of the earth and man" examined.--difference between scientific men and general speculators or amateurs in the use of hypotheses article vii evolution and theology writings of henslow, hodges, and le conte examined.--evolution and design compatible.--the admission of a system of nature, with fixed laws, concedes in principle all that the doctrine of evolution requires.--hypotheses, probabilities, and surmises, not to be decried by theologians, who use them, perhaps, more freely and loosely than naturalists.--theologians risk too much in the defense of untenable outposts article viii "what is darwinism?" dr. hodges book with this title criticised.--he declares that darwinism is atheism, yet its founder a theist.--darwinism founded, however, upon orthodox conceptions, and opposed, not to theism, but only to intervention in nature, while the key-note of dr. hedge's system is interference.--views and writings of st. clair, winchell, and kingsley adverted to article ix charles darwin: sketch accompanying a portrait in "nature" darwin's characteristics and work as a naturalist compared with those of robert brown.--his illustration of the principle that "nature abhors close fertilization. "--his impression upon natural history exceeded only by linnaeus.--his service in restoring teleology to natural history article x insectivorous plants classification marks distinctions where nature exhibits gradations.-- recovery of forgotten knowledge and history of what was known of dionzea, drosera, and sarracenia. article xi insectivorous and climbing plants review of darwin's two works upon these subjects--no absolute marks for distinguishing between vegetables and animals.--new observations upon the sundews or droseras.--their sensitiveness, movements, discernment of the presence and appropriation of animal matter.--dionaea, and other plants of the same order.--utricularia and pinguicula.--sarracenia and nepenthes.--climbing plants; the climbing effected through sensitiveness or response to external impression and automatic movement.--capacities inherent in plants generally, and apparently of no service to them, developed and utilized by those which climb.--natural selection not a complete explanation article xii duration and origination of race and species part i.--do varieties in plants wear out, or tend to wear out?--the question considered in the light of facts, and in that of the darwinian theory.--conclusion that races sexually propagated need not die of old age.--this conclusion inferred from the provisions and arrangements in nature to secure cross-fertilization of individuals.-- reference to mr. darwin's development of this view part ii.--do species wear out, and, if not, why not?--implication of the darwinian theory that species are unlimited in existence.--examination of an opposite doctrine maintained by naudin.--evidence that species may die out from inherent causes only indirect and inferential from arrangements to secure wide breeding--physiological import of sexes--doubtful whether sexual reproduction with wide breeding is a preventive or only a palliative of decrepitude in species.-- darwinian hypothesis must suppose the former article xiii evolutionary teleology the opposition between morphology and teleology reconciled by darwinism, and the latter reinstated--character of the new teleology.--purpose and design distinguished--man has no monopoly of the latter.--inference of design from adaptation and utility legitimate; also in hume's opinion irresistible--the principle of design, taken with specific creation, totally insufficient and largely inapplicable; but, taken with the doctrine of the evolution of species in nature, applicable, pertinent, and, moreover, necessary.--illustrations from abortive organs, supposed waste of being, etc.--all nature being of a piece, design must either pervade or be absent from the whole.--its absence not to be inferred because the events take place in nature--illustration of the nature and province of natural selection.--it picks out, but does not originate variations; these not a product of, but a response to, the environment; not physical, but physiological--adaptations in nature not explained by natural selection apart from design or final cause--absurdity of associating design only with miracle--what is meant by nature.--the tradition of the divine in nature, testified to by aristotle, comes down to our day with undiminished value preface these papers are now collected at the request of friends and correspondents, who think that they may be useful; and two new essays are added. most of the articles were written as occasion called for them within the past sixteen years, and contributed to various periodicals, with little thought of their forming a series, and none of ever bringing them together into a volume, although one of them (the third) was once reprinted in a pamphlet form. it is, therefore, inevitable that there should be considerable iteration in the argument, if not in the language. this could not be eliminated except by recasting the whole, which was neither practicable nor really desirable. it is better that they should record, as they do, the writer's freely-expressed thoughts upon the subject at the time; and to many readers there may be some advantage in going more than once, in different directions, over the same ground. if these essays were to be written now, some things might be differently expressed or qualified, but probably not so as to affect materially any important point. accordingly, they are here reprinted unchanged, except by a few merely verbal alterations made in proof-reading, and the striking out of one or two superfluous or immaterial passages. a very few additional notes or references are appended. to the last article but one a second part is now added, and the more elaborate article xiii is wholly new. if it be objected that some of these pages are written in a lightness of vein not quite congruous with the gravity of the subject and the seriousness of its issues, the excuse must be that they were written with perfect freedom, most of them as anonymous contributions to popular journals, and that an argument may not be the less sound or an exposition less effective for being playful. some of the essays, however, dealing with points of speculative scientific interest, may redress the balance, and be thought sufficiently heavy if not solid. to the objection likely to be made, that they cover only a part of the ground, it can only be replied that they do not pretend to be systematic or complete. they are all essays relating in some way or other to the subject which has been, during these years, of paramount interest to naturalists, and not much less so to most thinking people. the first appeared between sixteen and seventeen years ago, immediately after the publication of darwin's "origin of species by means of natural selection," as a review of that volume, which, it was then foreseen, was to initiate a revolution in general scientific opinion. long before our last article was written, it could be affirmed that the general doctrine of the derivation of species (to put it comprehensively) has prevailed over that of specific creation, at least to the extent of being the received and presumably in some sense true conception. far from undertaking any general discussion of evolution, several even of mr. darwin's writings have not been noticed, and topics which have been much discussed elsewhere are not here adverted to. this applies especially to what may be called deductive evolution--a subject which lay beyond the writer's immediate scope, and to which neither the bent of his mind nor the line of his studies has fitted him to do justice. if these papers are useful at all, it will be as showing how these new views of our day are regarded by a practical naturalist, versed in one department only (viz., botany), most interested in their bearings upon its special problems, one accustomed to direct and close dealings with the facts in hand, and disposed to rise from them only to the consideration of those general questions upon which they throw or from which they receive illustration. then as to the natural theological questions which (owing to circumstances needless now to be recalled or explained) are here throughout brought into what most naturalists, and some other readers, may deem undue prominence, there are many who may be interested to know how these increasingly prevalent views and their tendencies are regarded by one who is scientifically, and in his own fashion, a darwinian, philosophically a convinced theist, and religiously an acceptor of the "creed commonly called the nicene," as the exponent of the christian faith. "truth emerges sooner from error than from confusion," says bacon; and clearer views than commonly prevail upon the points at issue regarding "religion and science" are still sufficiently needed to justify these endeavors. botanic garden, cambridge, mass., june, . ______________________________________ i the origin of species by means of natural selection [i- ] (american journal of science and arts, march, ) this book is already exciting much attention. two american editions are announced, through which it will become familiar to many of our readers, before these pages are issued. an abstract of the argument--for "the whole volume is one long argument," as the author states--is unnecessary in such a case; and it would be difficult to give by detached extracts. for the volume itself is an abstract, a prodromus of a detailed work upon which the author has been laboring for twenty years, and which "will take two or three more years to complete." it is exceedingly compact; and although useful summaries are appended to the several chapters, and a general recapitulation contains the essence of the whole, yet much of the aroma escapes in the treble distillation, or is so concentrated that the flavor is lost to the general or even to the scientific reader. the volume itself--the proof-spirit--is just condensed enough for its purpose. it will be far more widely read, and perhaps will make deeper impression, than the elaborate work might have done, with all its full details of the facts upon which the author's sweeping conclusions have been grounded. at least it is a more readable book: but all the facts that can be mustered in favor of the theory are still likely to be needed. who, upon a single perusal, shall pass judgment upon a work like this, to which twenty of the best years of the life of a most able naturalist have been devoted? and who among those naturalists who hold a position that entitles them to pronounce summarily upon the subject, can be expected to divest himself for the nonce of the influence of received and favorite systems? in fact, the controversy now opened is not likely to be settled in an off-hand way, nor is it desirable that it should be. a spirited conflict among opinions of every grade must ensue, which--to borrow an illustration from the doctrine of the book before us--may be likened to the conflict in nature among races in the struggle for life, which mr. darwin describes; through which the views most favored by facts will be developed and tested by "natural selection," the weaker ones be destroyed in the process, and the strongest in the long-run alone survive. the duty of reviewing this volume in the american journal of science would naturally devolve upon the principal editor,' whose wide observation and profound knowledge of various departments of natural history, as well as of geology, particularly qualify him for the task. but he has been obliged to lay aside his pen, and to seek in distant lands the entire repose from scientific labor so essential to the restoration of his health--a consummation devoutly to be wished, and confidently to be expected. interested as mr. dana would be in this volume, he could not be expected to accept this doctrine. views so idealistic as those upon which his "thoughts upon species" [i- ] are grounded, will not harmonize readily with a doctrine so thoroughly naturalistic as that of mr. darwin. though it is just possible that one who regards the kinds of elementary matter, such as oxygen and hydrogen, and the definite compounds of these elementary matters, and their compounds again, in the mineral kingdom, as constituting species, in the same sense, fundamentally, as that of animal and vegetable species, might admit an evolution of one species from another in the latter as well as the former case. between the doctrines of this volume and those of the other great naturalist whose name adorns the title-page of this journal, the widest divergence appears. it is interesting to contrast the two, and, indeed, is necessary to our purpose; for this contrast brings out most prominently, and sets in strongest light and shade, the main features of the theory of the origination of species by means of natural selection. the ordinary and generally-received view assumes the independent, specific creation of each kind of plant and animal in a primitive stock, which reproduces its like from generation to generation, and so continues the species. taking the idea of species from this perennial succession of essentially similar individuals, the chain is logically traceable back to a local origin in a single stock, a single pair, or a single individual, from which all the individuals composing the species have proceeded by natural generation. although the similarity of progeny to parent is fundamental in the conception of species, yet the likeness is by no means absolute; all species vary more or less, and some vary remarkably--partly from the influence of altered circumstances, and partly (and more really) from unknown constitutional causes which altered conditions favor rather than originate. but these variations are supposed to be mere oscillations from a normal state, and in nature to be limited if not transitory; so that the primordial differences between species and species at their beginning have not been effaced, nor largely obscured, by blending through variation. consequently, whenever two reputed species are found to blend in nature through a series of intermediate forms, community of origin is inferred, and all the forms, however diverse, are held to belong to one species. moreover, since bisexuality is the rule in nature (which is practically carried out, in the long-run, far more generally than has been suspected), and the heritable qualities of two distinct individuals are mingled in the offspring, it is supposed that the general sterility of hybrid progeny interposes an effectual barrier against the blending of the original species by crossing. from this generally-accepted view the well-known theory of agassiz and the recent one of darwin diverge in exactly opposite directions. that of agassiz differs fundamentally from the ordinary view only in this, that it discards the idea of a common descent as the real bond of union among the individuals of a species, and also the idea of a local origin--supposing, instead, that each species originated simultaneously, generally speaking, over the whole geographical area it now occupies or has occupied, and in perhaps as many individuals as it numbered at any subsequent period. mr. darwin, on the other hand, holds the orthodox view of the descent of all the individuals of a species not only from a local birthplace, but from a single ancestor or pair; and that each species has extended and established itself, through natural agencies, wherever it could; so that the actual geographical distribution of any species is by no means a primordial arrangement, but a natural result. he goes farther, and this volume is a protracted argument intended to prove that the species we recognize have not been independently created, as such, but have descended, like varieties, from other species. varieties, on this view, are incipient or possible species: species are varieties of a larger growth and a wider and earlier divergence from the parent stock; the difference is one of degree, not of kind. the ordinary view--rendering unto caesar the things that are caesar's--looks to natural agencies for the actual distribution and perpetuation of species, to a supernatural for their origin. the theory of agassiz regards the origin of species and their present general distribution over the world as equally primordial, equally supernatural; that of darwin, as equally derivative, equally natural. the theory of agassiz, referring as it does the phenomena both of origin and distribution directly to the divine will--thus removing the latter with the former out of the domain of inductive science (in which efficient cause is not the first, but the last word)--may be said to be theistic to excess. the contrasted theory is not open to this objection. studying the facts and phenomena in reference to proximate causes, and endeavoring to trace back the series of cause and effect as far as possible, darwin's aim and processes are strictly scientific, and his endeavor, whether successful or futile, must be regarded as a legitimate attempt to extend the domain of natural or physical science. for, though it well may be that "organic forms have no physical or secondary cause," yet this can be proved only indirectly, by the failure of every attempt to refer the phenomena in question to causal laws. but, however originated, and whatever be thought of mr. darwin's arduous undertaking in this respect, it is certain that plants and animals are subject from their birth to physical influences, to which they have to accommodate themselves as they can. how literally they are "born to trouble," and how incessant and severe the struggle for life generally is, the present volume graphically describes. few will deny that such influences must have gravely affected the range and the association of individuals and species on the earth's surface. mr. darwin thinks that, acting upon an inherent predisposition to vary, they have sufficed even to modify the species themselves and produce the present diversity. mr. agassiz believes that they have not even affected the geographical range and the actual association of species, still less their forms; but that every adaptation of species to climate, and of species to species, is as aboriginal, and therefore as inexplicable, as are the organic forms themselves. who shall decide between such extreme views so ably maintained on either hand, and say how much of truth there may be in each? the present reviewer has not the presumption to undertake such a task. having no prepossession in favor of naturalistic theories, but struck with the eminent ability of mr. darwin's work, and charmed with its fairness, our humbler duty will be performed if, laying aside prejudice as much as we can, we shall succeed in giving a fair account of its method and argument, offering by the way a few suggestions, such as might occur to any naturalist of an inquiring mind. an editorial character for this article must in justice be disclaimed. the plural pronoun is employed not to give editorial weight, but to avoid even the appearance of egotism, and also the circumlocution which attends a rigorous adherence to the impersonal style. we have contrasted these two extremely divergent theories, in their broad statements. it must not be inferred that they have no points nor ultimate results in common. in the first place, they practically agree in upsetting, each in its own way, the generally-received definition of species, and in sweeping away the ground of their objective existence in nature. the orthodox conception of species is that of lineal descent: all the descendants of a common parent, and no other, constitute a species; they have a certain identity because of their descent, by which they are supposed to be recognizable. so naturalists had a distinct idea of what they meant by the term species, and a practical rule, which was hardly the less useful because difficult to apply in many cases, and because its application was indirect: that is, the community of origin had to be inferred from the likeness; such degree of similarity, and such only, being held to be con-specific as could be shown or reasonably inferred to be compatible with a common origin. and the usual concurrence of the whole body of naturalists (having the same data before them) as to what forms are species attests the value of the rule, and also indicates some real foundation for it in nature. but if species were created in numberless individuals over broad spaces of territory, these individuals are connected only in idea, and species differ from varieties on the one hand, and from genera, tribes, etc., on the other, only in degree; and no obvious natural reason remains for fixing upon this or that degree as specific, at least no natural standard, by which the opinions of different naturalists may be correlated. species upon this view are enduring, but subjective and ideal. any three or more of the human races, for example, are species or not species, according to the bent of the naturalist's mind. darwin's theory brings us the other way to the same result. in his view, not only all the individuals of a species are descendants of a common parent, but of all the related species also. affinity, relationship, all the terms which naturalists use figuratively to express an underived, unexplained resemblance among species, have a literal meaning upon darwin's system, which they little suspected, namely, that of inheritance. varieties are the latest offshoots of the genealogical tree in "an unlineal" order; species, those of an earlier date, but of no definite distinction; genera, more ancient species, and so on. the human races, upon this view, likewise may or may not be species according to the notions of each naturalist as to what differences are specific; but, if not species already, those races that last long enough are sure to become so. it is only a question of time. how well the simile of a genealogical tree illustrates the main ideas of darwin's theory the following extract from the summary of the fourth chapter shows: "it is a truly wonderful fact--the wonder of which we are apt to overlook from familiarity--that all animals and all plants throughout all time and space should be related to each other in group subordinate to group, in the manner which we everywhere behold--namely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. the several subordinate groups in any class cannot be ranked in a single file, but seem rather to be clustered round points, and these round other points, and so on in almost endless cycles. on the view that each species has been independently created, i can see no explanation of this great fact in the classification of all organic beings; but, to the best of my judgment, it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram. "the affinities of all the beings of the same class have sometimes been represented by a great tree. i believe this simile largely speaks the truth. the green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. at each period of growth all the growing twigs have tried to branch out on all sides, and overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. the limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. from the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera, which have now no living representatives, and which are known to us only from having been found in a fossil state. as we here and there see a thin, straggling branch springing from a fork low down in a tree, and which by some chance has been favored and is still alive on its summit, so we occasionally see an animal like the ornithorhynchus or lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. as buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation i believe it has been with the great tree of life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramification." it may also be noted that there is a significant correspondence between the rival theories as to the main facts employed. apparently every capital fact in the one view is a capital fact in the other. the difference is in the interpretation. to run the parallel ready made to our hands: [i- ] "the simultaneous existence of the most diversified types under identical circumstances . . . the repetition of similar types under the most diversified circumstances . . . the unity of plan in otherwise highly-diversified types of animals . . . the correspondence, now generally known as special homologies, in the details of structure otherwise entirely disconnected, down to the most minute peculiarities . . . the various degrees and different kinds of relationship among animals which (apparently) can have no genealogical connection . . . the simultaneous existence in the earliest geological periods, . . . of representatives of all the great types of the animal kingdom . . . the gradation based upon complications of structure which may be traced among animals built upon the same plan; the distribution of some types over the most extensive range of surface of the globe, while others are limited to particular geographical areas . . . the identity of structures of these types, notwithstanding their wide geographical distribution . . . the community of structure in certain respects of animals otherwise entirely different, but living within the same geographical area . . . the connection by series of special structures observed in animals widely scattered over the surface of the globe . . . the definite relations in which animals stand to the surrounding world, . . . the relations in which individuals of the same species stand to one another . . . the limitation of the range of changes which animals undergo during their growth . . . the return to a definite norm of animals which multiply in various ways . . . the order of succession of the different types of animals and plants characteristic of the different geological epochs, . . . the localization of some types of animals upon the same points of the surface of the globe during several successive geological periods . . . the parallelism between the order of succession of animals and plants in geological times, and the gradation among their living representatives . . . the parallelism between the order of succession of animals in geological times and the changes their living representatives undergo during their embryological growth, [i- ] . . . the combination in many extinct types of characters which in later ages appear disconnected in different types, . . . the parallelism between the gradation among animals and the changes they undergo during their growth, . . . the relations existing between these different series and the geographical distribution of animals, . . . the connection of all the known features of nature into one system--" in a word, the whole relations of animals, etc., to surrounding nature and to each other, are regarded under the one view as ultimate facts, or in the ultimate aspect, and interpreted theologically; under the other as complex facts, to be analyzed and interpreted scientifically. the one naturalist, perhaps too largely assuming the scientifically unexplained to be inexplicable, views the phenomena only in their supposed relation to the divine mind. the other, naturally expecting many of these phenomena to be resolvable under investigation, views them in their relations to one another, and endeavors to explain them as far as he can (and perhaps farther) through natural causes. but does the one really exclude the other? does the investigation of physical causes stand opposed to the theological view and the study of the harmonies between mind and nature? more than this, is it not most presumable that an intellectual conception realized in nature would be realized through natural agencies? mr. agassiz answers these questions affirmatively when he declares that "the task of science is to investigate what has been done, to inquire if possible how it has been done, rather than to ask what is possible for the deity, since we can know that only by what actually exists;" and also when he extends the argument for the intervention in nature of a creative mind to its legitimate application in the inorganic world; which, he remarks, "considered in the same light, would not fail also to exhibit unexpected evidence of thought, in the character of the laws regulating the chemical combinations, the action of physical forces, etc., etc." [i- ] mr. agassiz, however, pronounces that "the connection between the facts is only intellectual"--an opinion which the analogy of the inorganic world, just referred to, does not confirm, for there a material connection between the facts is justly held to be consistent with an intellectual--and which the most analogous cases we can think of in the organic world do not favor; for there is a material connection between the grub, the pupa, and the butterfly, between the tadpole and the frog, or, still better, between those distinct animals which succeed each other in alternate and very dissimilar generations. so that mere analogy might rather suggest a natural connection than the contrary; and the contrary cannot be demonstrated until the possibilities of nature under the deity are fathomed. but, the intellectual connection being undoubted, mr. agassiz properly refers the whole to "the agency of intellect as its first cause." in doing so, however, he is not supposed to be offering a scientific explanation of the phenomena. evidently he is considering only the ultimate why, not the proximate why or how. now the latter is just what mr. darwin is considering. he conceives of a physical connection between allied species; but we suppose he does not deny their intellectual connection, as related to a supreme intelligence. certainly we see no reason why he should, and many reasons why he should not, indeed, as we contemplate the actual direction of investigation and speculation in the physical and natural sciences, we dimly apprehend a probable synthesis of these divergent theories, and in it the ground for a strong stand against mere naturalism. even if the doctrine of the origin of species through natural selection should prevail in our day, we shall not despair; being confident that the genius of an agassiz will be found equal to the work of constructing, upon the mental and material foundations combined, a theory of nature as theistic and as scientific as that which he has so eloquently expounded. to conceive the possibility of "the descent of species from species by insensibly fine gradations" during a long course of time, and to demonstrate its compatibility with a strictly theistic view of the universe, is one thing; to substantiate the theory itself or show its likelihood is quite another thing. this brings us to consider what darwin's theory actually is, and how he supports it. that the existing kinds of animals and plants, or many of them, may be derived from other and earlier kinds, in the lapse of time, is by no means a novel proposition. not to speak of ancient speculations of the sort, it is the well-known lamarckian theory. the first difficulty which such theories meet with is that in the present age, with all its own and its inherited prejudgments, the whole burden of proof is naturally, and indeed properly, laid upon the shoulders of the propounders; and thus far the burden has been more than they could bear. from the very nature of the case, substantive proof of specific creation is not attainable; but that of derivation or transmutation of species may be. he who affirms the latter view is bound to do one or both of two things: . either to assign real and adequate causes, the natural or necessary result of which must be to produce the present diversity of species and their actual relations; or, . to show the general conformity of the whole body of facts to such assumption, and also to adduce instances explicable by it and inexplicable by the received view, so perhaps winning our assent to the doctrine, through its competency to harmonize all the facts, even though the cause of the assumed variation remain as occult as that of the transformation of tadpoles into frogs, or that of coryne into sarzia. the first line of proof, successfully carried out, would establish derivation as a true physical theory; the second, as a sufficient hypothesis. lamarck mainly undertook the first line, in a theory which has been so assailed by ridicule that it rarely receives the credit for ability to which in its day it was entitled, but he assigned partly unreal, partly insufficient causes; and the attempt to account for a progressive change in species through the direct influence of physical agencies, and through the appetencies and habits of animals reacting upon their structure, thus causing the production and the successive modification of organs, is a conceded and total failure. the shadowy author of the "vestiges of the natural history of creation" can hardly be said to have undertaken either line, in a scientific way. he would explain the whole progressive evolution of nature by virtue of an inherent tendency to development, thus giving us an idea or a word in place of a natural cause, a restatement of the proposition instead of an explanation. mr. darwin attempts both lines of proof, and in a strictly scientific spirit; but the stress falls mainly upon the first, for, as he does assign real causes, he is bound to prove their adequacy. it should be kept in mind that, while all direct proof of independent origination is attainable from the nature of the case, the overthrow of particular schemes of derivation has not established the opposite proposition. the futility of each hypothesis thus far proposed to account for derivation may be made apparent, or unanswerable objections may be urged against it; and each victory of the kind may render derivation more improbable, and therefore specific creation more probable, without settling the question either way. new facts, or new arguments and a new mode of viewing the question, may some day change the whole aspect of the case. it is with the latter that mr. darwin now reopens the discussion. having conceived the idea that varieties are incipient species, he is led to study variation in the field where it shows itself most strikingly, and affords the greatest facilities to investigation. thoughtful naturalists have had increasing grounds to suspect that a reexamination of the question of species in zoology and botany, commencing with those races which man knows most about, viz., the domesticated and cultivated races, would be likely somewhat to modify the received idea of the entire fixity of species. this field, rich with various but unsystematized stores of knowledge accumulated by cultivators and breeders, has been generally neglected by naturalists, because these races are not in a state of nature; whereas they deserve particular attention on this very account, as experiments, or the materials for experiments, ready to our hand. in domestication we vary some of the natural conditions of a species, and thus learn experimentally what changes are within the reach of varying conditions in nature. we separate and protect a favorite race against its foes or its competitors, and thus learn what it might become if nature ever afforded it equal opportunities. even when, to subserve human uses, we modify a domesticated race to the detriment of its native vigor, or to the extent of practical monstrosity, although we secure forms which would not be originated and could not be perpetuated in free nature, yet we attain wider and juster views of the possible degree of variation. we perceive that some species are more variable than others, but that no species subjected to the experiment persistently refuses to vary; and that, when it has once begun to vary, its varieties are not the less but the more subject to variation. "no case is on record of a variable being ceasing to be variable under cultivation." it is fair to conclude, from the observation of plants and animals in a wild as well as domesticated state, that the tendency to vary is general, and even universal. mr. darwin does "not believe that variability is an inherent and necessary contingency, under all circumstances, with all organic beings, as some authors have thought." no one supposes variation could occur under all circumstances; but the facts on the whole imply a universal tendency, ready to be manifested under favorable circumstances. in reply to the assumption that man has chosen for domestication animals and plants having an extraordinary inherent tendency to vary, and likewise to withstand diverse climates, it is asked: "how could a savage possibly know, when he first tamed an animal, whether it would vary in succeeding generations and whether it would endure other climates? has the little variability of the ass or guinea-fowl, or the small power of endurance of warmth by the reindeer, or of cold by the common camel, prevented their domestication? i cannot doubt that if other animals and plants, equal in number to our domesticated productions, and belonging to equally diverse classes and countries, were taken from a state of nature, and could be made to breed for an equal number of generations under domestication, they would vary on an average as largely as the parent species of our existing domesticated productions have varied." as to amount of variation, there is the common remark of naturalists that the varieties of domesticated plants or animals often differ more widely than do the individuals of distinct species in a wild state: and even in nature the individuals of some species are known to vary to a degree sensibly wider than that which separates related species. in his instructive section on the breeds of the domestic pigeon, our author remarks that "at least a score of pigeons might be chosen which if shown to an ornithologist, and he were told that they were wild birds, would certainly be ranked by him as well-defined species. moreover, i do not believe that any ornithologist would place the english carrier, the short-faced tumbler, the runt, the barb, pouter, and fantail, in the same genus; more especially as in each of these breeds several truly-inherited sub-breeds, or species, as he might have called them, could be shown him." that this is not a case like that of dogs, in which probably the blood of more than one species is mingled, mr. darwin proceeds to show, adducing cogent reasons for the common opinion that all have descended from the wild rock-pigeon. then follow some suggestive remarks: "i have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because when i first kept pigeons and watched the several kinds, knowing well how true they bred, i felt fully as much difficulty in believing that they could ever have descended from a common parent as any naturalist could in coming to a similar conclusion in regard to many species of finches, or other large groups of birds, in nature. one circumstance has struck me much; namely, that all the breeders of the various domestic animals and the cultivators of plants, with whom i have ever conversed, or whose treatises i have read, are firmly convinced that the several breeds to which each has attended are descended from so many aboriginally distinct species. ask, as i have asked, a celebrated raiser of hereford cattle, whether his cattle might not have descended from long-horns, and he will laugh you to scorn. i have never met a pigeon, or poultry, or duck, or rabbit fancier, who was not fully convinced that each main breed was descended from a distinct species. van mons, in his treatise on pears and apples, shows how utterly he disbelieves that the several sorts, for instance a ribston-pippin or codlin-apple, could ever have proceeded from the seeds of the same tree. innumerable other examples could be given. the explanation, i think, is simple: from long-continued study they arc strongly impressed with the differences between the several races; and though they well know that each race varies slightly, for they win their prizes by selecting such slight differences, yet they ignore all general arguments, and refuse to sum up in their minds slight differences accumulated during many successive generations. may not those naturalists who, knowing far less of the laws of inheritance than does the breeder, and knowing no more than he does of the intermediate links in the long lines of descent, yet admit that many of our domestic races have descended from the same parents--may they not learn a lesson of caution, when they deride the idea of species in a state of nature being lineal descendants of other species?" the actual causes of variation are unknown. mr. darwin favors the opinion of the late mr. knight, the great philosopher of horticulture, that variability tinder domestication is somehow connected with excess of food. he regards the unknown cause as acting chiefly upon the reproductive system of the parents, which system, judging from the effect of confinement or cultivation upon its functions, he concludes to be more susceptible than any other to the action of changed conditions of life. the tendency to vary certainly appears to be much stronger under domestication than in free nature. but we are not sure that the greater variableness of cultivated races is not mainly owing to the far greater opportunities for manifestation and accumulation--a view seemingly all the more favorable to mr. darwin's theory. the actual amount of certain changes, such as size or abundance of fruit, size of udder, stands of course in obvious relation to supply of food. really, we no more know the reason why the progeny occasionally deviates from the parent than we do why it usually resembles it. though the laws and conditions governing variation are known to a certain extent, those governing inheritance are apparently inscrutable. "perhaps," darwin remarks, "the correct way of viewing the whole subject would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly." this, from general and obvious considerations, we have long been accustomed to do. now, as exceptional instances are expected to be capable of explanation, while ultimate laws are not, it is quite possible that variation may be accounted for, while the great primary law of inheritance remains a mysterious fact. the common proposition is, that species reproduce their like; this is a sort of general inference, only a degree closer to fact than the statement that genera reproduce their like. the true proposition, the fact incapable of further analysis, is, that individuals reproduce their like--that characteristics are inheritable. so varieties, or deviations, once originated, are perpetuable, like species. not so likely to be perpetuated, at the outset; for the new form tends to resemble a grandparent and a long line of similar ancestors, as well as to resemble its immediate progenitors. two forces which coincide in the ordinary case, where the offspring resembles its parent, act in different directions when it does not and it is uncertain which will prevail. if the remoter but very potent ancestral influence predominates, the variation disappears with the life of the individual. if that of the immediate parent--feebler no doubt, but closer--the variety survives in the offspring; whose progeny now has a redoubled tendency to produce its own like; whose progeny again is almost sure to produce its like, since it is much the same whether it takes after its mother or its grandmother. in this way races arise, which under favorable conditions may be as hereditary as species. in following these indications, watching opportunities, and breeding only from those individuals which vary most in a desirable direction, man leads the course of variation as he leads a streamlet--apparently at will, but never against the force of gravitation--to a long distance from its source, and makes it more subservient to his use or fancy. he unconsciously strengthens those variations which he prizes when he plants the seed of a favorite fruit, preserves a favorite domestic animal, drowns the uglier kittens of a litter, and allows only the handsomest or the best mousers to propagate. still more, by methodical selection, in recent times almost marvelous results have been produced in new breeds of cattle, sheep, and poultry, and new varieties of fruit of greater and greater size or excellence. it is said that all domestic varieties, if left to run wild, would revert to their aboriginal stocks. probably they would wherever various races of one species were left to commingle. at least the abnormal or exaggerated characteristics induced by high feeding, or high cultivation and prolonged close breeding, would promptly disappear; and the surviving stock would soon blend into a homogeneous result (in a way presently explained), which would naturally be taken for the original form; but we could seldom know if it were so. it is by no means certain that the result would be the same if the races ran wild each in a separate region. dr. hooker doubts if there is a true reversion in the case of plants. mr. darwin's observations rather favor it in the animal kingdom. with mingled races reversion seems well made out in the case of pigeons. the common opinion upon this subject therefore probably has some foundation, but even if we regard varieties as oscillations around a primitive centre or type, still it appears from the readiness with which such varieties originate that a certain amount of disturbance would carry them beyond the influence of the primordial attraction, where they may become new centres of variation. some suppose that races cannot be perpetuated indefinitely even by keeping up the conditions under which they were fixed; but the high antiquity of several, and the actual fixity of many of them, negative this assumption. "to assert that we could not breed our cart and race horses, long and short horned cattle, and poultry of various breeds, for almost an infinite number of generations, would be opposed to all experience." why varieties develop so readily and deviate so widely under domestication, while they are apparently so rare or so transient in free nature, may easily be shown. in nature, even with hermaphrodite plants, there is a vast amount of cross-fertilization among various individuals of the same species. the inevitable result of this (as was long ago explained in this journal [i- ]) is to repress variation, to keep the mass of a species comparatively homogeneous over any area in which it abounds in individuals. starting from a suggestion of the late mr. knight, now so familiar, that close interbreeding diminishes vigor and fertility; [i- ] and perceiving that bisexuality is ever aimed at in nature--being attained physiologically in numerous cases where it is not structurally--mr. darwin has worked out the subject in detail, and shown how general is the concurrence, either habitual or occasional, of two hermaphrodite individuals in the reproduction of their kind; and has drawn the philosophical inference that probably no organic being self-fertilizes indefinitely; but that a cross with another individual is occasionally--perhaps at very long intervals--indispensable. we refer the reader to the section on the intercrossing of individuals (pp. -- ), and also to an article in the gardeners' chronicle a year and a half ago, for the details of a very interesting contribution to science, irrespective of theory. in domestication, this intercrossing may be prevented; and in this prevention lies the art of producing varieties. but "the art itself is nature," since the whole art consists in allowing the most universal of all natural tendencies in organic things (inheritance) to operate uncontrolled by other and obviously incidental tendencies. no new power, no artificial force, is brought into play either by separating the stock of a desirable variety so as to prevent mixture, or by selecting for breeders those individuals which most largely partake of the peculiarities for which the breed is valued. {i- ] we see everywhere around us the remarkable results which nature may be said to have brought about under artificial selection and separation. could she accomplish similar results when left to herself? variations might begin, we know they do begin, in a wild state. but would any of them be preserved and carried to an equal degree of deviation? is there anything in nature which in the long-run may answer to artificial selection? mr. darwin thinks that there is; and natural selection is the key-note of his discourse, as a preliminary, he has a short chapter to show that there is variation in nature, and therefore something for natural selection to act upon. he readily shows that such mere variations as may be directly referred to physical conditions (like the depauperation of plants in a sterile soil, or their dwarfing as they approach an alpine summit, the thicker fur of an animal from far northward, etc.), and also those individual differences which we everywhere recognize but do not pretend to account for, are not separable by any assignable line from more strongly-marked varieties; likewise that there is no clear demarkation between the latter and sub-species, or varieties of the highest grade (distinguished from species not by any known inconstancy, but by the supposed lower importance of their characteristics); nor between these and recognized species. "these differences blend into each other in an insensible series, and the series impresses the mind with an idea of an actual passage." this gradation from species downward is well made out. to carry it one step farther upward, our author presents in a strong light the differences which prevail among naturalists as to what forms should be admitted to the rank of species. some genera (and these in some countries) give rise to far more discrepancy than others; and it is concluded that the large or dominant genera are usually the most variable. in a flora so small as the british, plants, generally reckoned as varieties, have been ranked by some botanists as species. selecting the british genera which include the most polymorphous forms, it appears that babington's flora gives them species, bentham's only , a difference of doubtful forms. these are nearly the extreme views, but they are the views of two most capable and most experienced judges, in respect to one of the best-known floras of the world. the fact is suggestive, that the best-known countries furnish the greatest number of such doubtful cases. illustrations of this kind may be multiplied to a great extent. they make it plain that, whether species in nature are aboriginal and definite or not, our practical conclusions about them, as embodied in systematic works, are not facts but judgments, and largely fallible judgments- how much of the actual coincidence of authorities is owing to imperfect or restricted observation, and to one naturalist's adopting the conclusions of another without independent observation, this is not the place to consider. it is our impression that species of animals are more definitely marked than those of plants; this may arise from our somewhat extended acquaintance with the latter, and our ignorance of the former. but we are constrained by our experience to admit the strong likelihood, in botany, that varieties on the one hand, and what are called closely-related species on the other, do not differ except in degree. whenever this wider difference separating the latter can be spanned by intermediate forms, as it sometimes is, no botanist long resists the inevitable conclusion. whenever, therefore, this wider difference can be shown to be compatible with community of origin, and explained through natural selection or in any other way, we are ready to adopt the probable conclusion; and we see beforehand how strikingly the actual geographical association of related species favors the broader view. whether we should continue to regard the forms in question as distinct species, depends upon what meaning we shall finally attach to that term; and that depends upon how far the doctrine of derivation can be carried back and how well it can be supported. in applying his principle of natural selection to the work in hand, mr. darwin assumes, as we have seen: i. some variability of animals and plants in nature; . the absence of any definite distinction between slight variations, and varieties of the highest grade; . the fact that naturalists do not practically agree, and do not increasingly tend to agree, as to what forms are species and what are strong varieties, thus rendering it probable that there may be no essential and original difference, or no possibility of ascertaining it, at least in many cases; also, . that the most flourishing and dominant species of the larger genera on an average vary most (a proposition which can be substantiated only by extensive comparisons, the details of which are not given); and, . that in large genera the species are apt to be closely but unequally allied together, forming little clusters round certain species--just such clusters as would be formed if we suppose their members once to have been satellites or varieties of a central or parent species, but to have attained at length a wider divergence and a specific character. the fact of such association is undeniable; and the use which mr. darwin makes of it seems fair and natural. the gist of mr. darwin's work is to show that such varieties are gradually diverged into species and genera through natural selection; that natural selection is the inevitable result of the struggle for existence which all living things are engaged in; and that this struggle is an unavoidable consequence of several natural causes, but mainly of the high rate at which all organic beings tend to increase. curiously enough, mr. darwin's theory is grounded upon the doctrine of malthus and the doctrine of hobbes. the elder decandolle had conceived the idea of the struggle for existence, and, in a passage which would have delighted the cynical philosopher of malmesbury, had declared that all nature is at war, one organism with another or with external nature; and lyell and herbert had made considerable use of it. but hobbes in his theory of society, and darwin in his theory of natural history, alone have built their systems upon it. however moralists and political economists may regard these doctrines in their original application to human society and the relation of population to subsistence, their thorough applicability to the great society of the organic world in general is now undeniable. and to mr. darwin belongs the credit of making this extended application, and of working out the immensely diversified results with rare sagacity and untiring patience. he has brought to view real causes which have been largely operative in the establishment of the actual association and geographical distribution of plants and animals. in this he must be allowed to have made a very important contribution to an interesting department of science, even if his theory fails in the endeavor to explain the origin or diversity of species. "nothing is easier," says our author, "than to admit in words the truth of the universal struggle for life, or more difficult--at least i have found it so--than constantly to bear this conclusion in mind. yet, unless it be thoroughly ingrained in the mind, i am convinced that the whole economy of nature, with every fact on distribution, rarity, abundance, extinction, and variation, will be dimly seen or quite misunderstood. we behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind that, though food may be now superabundant, it is not so at all seasons of each recurring year."--(p. .) "there is no exception to the rule that every organic being naturally increases at so high a rate that, if not destroyed, the earth would soon be covered by the progeny of a single pair. even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing-room for his progeny. linnaeus has calculated that if an annual plant produced only two seeds--and there is no plant so unproductive as this--and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. the elephant is reckoned to be the slowest breeder of all known animals, and i have taken some pains to estimate its pro!)able minimum rate of natural increase; it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pairs of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair. "but we have better evidence on this subject than mere theoretical calculations, namely, the numerous recorded cases of the astonishingly rapid increase of various animals in a state of nature, when circumstances have been favorable to them during two or three following seasons. still more striking is the evidence from our domestic animals of many kinds which have run wild in several parts of the world; if the statements of the rate of increase of slow-breeding cattle and horses in south america, and latterly in australia, had not been well authenticated, they would have been quite incredible. so it is with plants: cases could be given of introduced plants which have become common throughout whole islands in a period of less than ten years. several of the plants now most numerous over the wide plains of la plata, clothing square leagues of surface almost to the exclusion of all other plants, have been introduced from europe; and there are plants which now range in india, as i hear from dr. falconer, from cape comorin to the himalaya, which have been imported from america since its discovery. in such cases, and endless instances could be given, no one supposes that the fertility of these animals or plants has been suddenly and temporarily increased in any sensible degree. the obvious explanation is, that the conditions of life have been very favorable, and that there has consequently been less destruction of the old and young, and that nearly all the young have been enabled to breed. in such cases the geometrical ratio of increase, the result of which never fails to be surprising, simply explains the extraordinarily rapid increase and wide diffusion of naturalized productions in their new homes."--(pp. , .) "all plants and animals are tending to increase at a geometrical ratio; all would most rapidly stock any station in which they could anyhow exist; the increase must be checked by destruction at some period of life."--(p. .) the difference between the most and the least prolific species is of no account: "the condor lays a couple of eggs, and the ostrich a score; and yet in the same country the condor may be the more numerous of the two. the fulmar petrel lays but one egg, yet it is believed to be the most numerous bird in the world."--(p. .) "the amount of food gives the extreme limit to which each species can increase; but very frequently it is not the obtaining of food, but the serving as prey to other animals, which determines the average numbers of species."--(p. .) "climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought i believe to be the most effective of all checks. i estimated that the winter of --' destroyed four-fifths of the birds in my own grounds; and this is a tremendous destruction, when we remember that ten per cent, is an extraordinarily severe mortality from epidemics with man. the action of climate seems at first sight to be quite independent of the struggle for existence; but, in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food, even when climate, for instance extreme cold, acts directly, it will be the least vigorous, or those which have got least food through the advancing winter, which will suffer most. when we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing; and, the change of climate being conspicuous, we are tempted to attribute the whole effect to its direct action. but this is a very false view; we forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favored by any slight change of climate, they will increase in numbers, and, as each area is already stocked with inhabitants, the other species will decrease. when we travel southward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in other species being favored as in this one being hurt. so it is when we travel northward, but in a somewhat lesser degree, for the number of species of all kinds, and therefore of competitors, decreases northward; hence, in going northward, or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding southward or in descending a mountain. when we reach the arctic regions, or snow-capped summits, or absolute deserts, the struggle for life is almost exclusively with the elements. "that climate acts in main part indirectly by favoring other species, we may clearly see in the prodigious number of plants in our gardens which can perfectly well endure our climate, but which never become naturalized, for they cannot compete with our native plants, nor resist destruction by our native animals."--(pp. , .) after an instructive instance in which "cattle absolutely determine the existence of the scotch fir," we are referred to cases in which insects determine the existence of cattle: "perhaps paraguay offers the most curious instance of this; for here neither cattle, nor horses, nor dogs, have ever run wild, though they swarm southward and northward in a feral state; and azara and rengger have shown that this is caused by the greater number in paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. the increase of these flies, numerous as they are, must be habitually checked by some means, probably by birds. hence, if certain insectivorous birds (whose numbers are probably regulated by hawks or beasts of prey) were to increase in paraguay, the flies would decrease--then cattle and horses would become feral, and this would certainly greatly alter (as indeed i have observed in parts of south america) the vegetation; this, again, would largely affect the insects; and this, as we have just seen in staffordshire, the insectivorous birds, and so onward in ever-increasing circles of complexity. we began this series by insectivorous birds, and we had ended with them. not that in nature the relations can ever be as simple as this. battle within battle must ever be recurring with varying success; and yet in the long-run the forces are so nicely balanced that the face of nature remains uniform for long periods of time, though assuredly the merest trifle would often give the victory to one organic being over another. nevertheless, so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!"--(pp. , .) "when we look at the plants and bushes clothing an entangled bank, we arc tempted to attribute their proportional numbers and kinds to what we call chance. but how false a view is this! every one has heard that when an american forest is cut down, a very different vegetation springs up; but it has been observed that the trees now growing on the ancient indian mounds, in the southern united states, display the same beautiful diversity and proportion of kinds as in the surrounding virgin forests. what a struggle between the several kinds of trees must here have gone on during long centuries, each annually scattering its seeds by the thousand; what war between insect and insect--between insects, snails, and other animals, with birds and beasts of prey--all striving to increase, and all feeding on each other or on the trees, or their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees! throw up a handful of feathers, and all must fall to the ground according to definite laws; but how simple is this problem compared to the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the proportional numbers and kinds of trees now growing on the old indian ruins!"--(pp. , .) for reasons obvious upon reflection, the competition is often, if not generally, most severe between nearly related species when they are in contact, so that one drives the other before it, as the hanoverian the old english rat, the small asiatic cockroach in russia, its greater congener, etc. and this, when duly considered, explains many curious results; such, for instance, as the considerable number of different genera of plants and animals which are generally found to inhabit any limited area. "the truth of the principle that the greatest amount of life can be supported by great diversification of structure is seen under many natural circumstances. in an extremely small area, especially if freely open to immigration, and where the contest between individual and individual must be severe, we always find great diversity in its inhabitants. for instance, i found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants, and these belonged to eighteen genera, and to eight orders, which showed how much these plants differed from each other. so it is with the plants and insects on small and uniform islets; and so in small ponds of fresh water. farmers find that they can raise most food by a rotation of plants belonging to the most different orders; nature follows what may be called a simultaneous rotation. most of the animals and plants which live close round any small piece of ground could live on it (supposing it not to be in any way peculiar in its nature), and may be said to be striving to the utmost to live there; but it is seen that, where they come into the closest competition with each other, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders."--(p. .) the abundance of some forms, the rarity and final extinction of many others, and the consequent divergence of character or increase of difference among the surviving representatives, are other consequences. as favored forms increase, the less favored must diminish in number, for there is not room for all; and the slightest advantage, at first probably inappreciable to human observation, must decide which shall prevail and which must perish, or be driven to another and for it more favorable locality. we cannot do justice to the interesting chapter upon natural selection by separated extracts. the following must serve to show how the principle is supposed to work: "if during the long course of ages, and under varying conditions of life, organic beings vary at all in the several parts of their organization, and i think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed: then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, i think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. but if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterized. this principle of preservation i have called, for the sake of brevity, natural selection."--(pp. , .) "in order to make it clear how, as i believe, natural selection acts, i must beg permission to give one or two imaginary illustrations. let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf is hardest pressed for food. i can under such circumstances see no reason to doubt that the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or selected--provided always that they retained strength to master their prey at this or at some other period of the year, when they might be compelled to prey on other animals. i can see no more reason to doubt this than that man can improve the fleetness of his greyhounds by careful and methodical selection, or by that unconscious selection which results from each man trying to keep the best dogs without any thought of modifying the breed. "even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might be born with an innate tendency to pursue certain kinds of prey. nor can this be thought very improbable; for we often observe great differences in the natural tendencies of our domestic animals: one cat, for instance, taking to catching rats, another mice; one cat, according to mr. st. john, bringing home winged game, another hares or rabbits, and another hunting on marshy ground!, and almost nightly catching woodcocks or snipes. the tendency to catch rats rather than mice is known to be inherited. now, if any slight innate change of habit or of structure benefited an individual wolf, it would have the best chance of surviving and of leaving offspring. some of its young would probably inherit the same habits or structure, and by the repetition of this process a new variety might be formed which would either supplant or coexist with the parent-form of wolf. or, again, the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to hunt different prey; and from a continued preservation of the individuals best fitted for the two sites, two varieties might slowly be formed. these varieties would cross and blend where they met; but to this subject of intercrossing we shall soon have to return. i may add that, according to mr. pierce, there are two varieties of the wolf inhabiting the catskill mountains in the united states, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flock."--(pp. , .) we eke out the illustration here with a counterpart instance, viz., the remark of dr. bachman that "the deer that reside permanently in the swamps of carolina are taller and longer-legged than those in the higher grounds." [i- ] the limits allotted to this article are nearly reached, yet only four of the fourteen chapters of the volume have been touched. these, however, contain the fundamental principles of the theory, and most of those applications of it which are capable of something like verification, relating as they do to the phenomena now occurring. some of our extracts also show how these principles are thought to have operated through the long lapse of the ages. the chapters from the sixth to the ninth inclusive are designed to obviate difficulties and objections, "some of them so grave that to this day," the author frankly says, he "can never reflect on them without being staggered." we do not wonder at it. after drawing what comfort he can from "the imperfection of the geological record" (chapter ix), which we suspect is scarcely exaggerated, the author considers the geological succession of organic beings (chapter x), to see whether they better accord with the common view of the immutability of species, or with that of their slow and gradual modification. geologists must settle that question. then follow two most interesting and able chapters on the geographical distribution of plants and animals, the summary of which we should be glad to cite; then a fitting chapter upon classification, morphology, embryology, etc., as viewed in the light of this theory, closes the argument; the fourteenth chapter being a recapitulation. the interest for the general reader heightens as the author advances on his perilous way and grapples manfully with the most formidable difficulties. to account, upon these principles, for the gradual elimination and segregation of nearly allied forms--such as varieties, sub-species, and closely-related or representative species--also in a general way for their geographical association and present range, is comparatively easy, is apparently within the bounds of possibility. could we stop here we should be fairly contented. but, to complete the system, to carry out the principles to their ultimate conclusion, and to explain by them many facts in geographical distribution which would still remain anomalous, mr. darwin is equally bound to account for the formation of genera, families, orders, and even classes, by natural selection. he does "not doubt that the theory of descent with modification embraces all the members of the same class," and he concedes that analogy would press the conclusion still further; while he admits that "the more distinct the forms are, the more the arguments fall away in force." to command assent we naturally require decreasing probability to be overbalanced by an increased weight of evidence. an opponent might plausibly, and perhaps quite fairly, urge that the links in the chain of argument are weakest just where the greatest stress falls upon them. to which mr. darwin's answer is, that the best parts of the testimony have been lost. he is confident that intermediate forms must have existed; that in the olden times when the genera, the families, and the orders, diverged from their parent stocks, gradations existed as fine as those which now connect closely related species with varieties. but they have passed and left no sign. the geological record, even if all displayed to view, is a book from which not only many pages, but even whole alternate chapters, have been lost out, or rather which were never printed from the autographs of nature. the record was actually made in fossil lithography only at certain times and under certain conditions (i.e., at periods of slow subsidence and places of abundant sediment); and of these records all but the last volume is out of print; and of its pages only local glimpses have been obtained. geologists, except lyell, will object to this--some of them moderately, others with vehemence. mr. darwin himself admits, with a candor rarely displayed on such occasions, that he should have expected more geological evidence of transition than he finds, and that all the most eminent paleontologists maintain the immutability of species. the general fact, however, that the fossil fauna of each period as a whole is nearly intermediate in character between the preceding and the succeeding faunas, is much relied on. we are brought one step nearer to the desired inference by the similar "fact, insisted on by all paleontologists, that fossils from two consecutive formations are far more closely related to each other than are the fossils of two remote formations. pictet gives a well-known instance--the general resemblance of the organic remains from the several stages of the chalk formation, though the species are distinct at each stage. this fact alone, from its generality, seems to have shaken prof. pictet in his firm belief in the immutability of species" (p. ). what mr. darwin now particularly wants to complete his inferential evidence is a proof that the same gradation may be traced in later periods, say in the tertiary, and between that period and the present; also that the later gradations are finer, so as to leave it doubtful whether the succession is one of species--believed on the one theory to be independent, on the other, derivative--or of varieties, which are confessedly derivative. the proof of the finer gradation appears to be forthcoming. des hayes and lyell have concluded that many of the middle tertiary and a large proportion of the later tertiary mollusca are specifically identical with living species; and this is still the almost universally prevalent view. but mr. agassiz states that, "in every instance where he had sufficient materials, he had found that the species of the two epochs supposed to be identical by des hayes and lyell were in reality distinct, although closely allied species."[i- ] moreover, he is now satisfied, as we understand, that the same gradation is traceable not merely in each great division of the tertiary, but in particular deposits or successive beds, each answering to a great number of years; where what have passed unquestioned as members of one species, upon closer examination of numerous specimens exhibit differences which in his opinion entitle them to be distinguished into two, three, or more species. it is plain, therefore, that whatever conclusions can be fairly drawn from the present animal and vegetable kingdoms in favor of a gradation of varieties into species, or into what may be regarded as such, the same may be extended to the tertiary period. in both cases, what some call species others call varieties; and in the later tertiary shells this difference in judgment affects almost half of the species! we pass to a second difficulty in the way of mr. darwin's theory; to a case where we are perhaps entitled to demand of him evidence of gradation like that which connects the present with the tertiary mollusca. wide, very wide is the gap, anatomically and physiologically (we do not speak of the intellectual) between the highest quadrumana and man; and comparatively recent, if ever, must the line have bifurcated. but where is there the slightest evidence of a common progenitor? perhaps mr. darwin would reply by another question: where are the fossil remains of the men who made the flint knives and arrowheads of the somme valley? we have a third objection, one, fortunately, which has nothing to do with geology. we can only state it here in brief terms. the chapter on hybridism is most ingenious, able, and instructive. if sterility of crosses is a special original arrangement to prevent the confusion of species by mingling, as is generally assumed, then, since varieties cross readily and their offspring is fertile inter se, there is a fundamental distinction between varieties and species. mr. darwin therefore labors to show that it is not a special endowment, but an incidental acquirement. he does show that the sterility of crosses is of all degrees; upon which we have only to say, natura non facit saltum, here any more than elsewhere. but, upon his theory he is bound to show how sterility might be acquired, through natural selection or through something else. and the difficulty is, that, whereas individuals of the very same blood tend to be sterile, and somewhat remoter unions diminish this tendency, and when they have diverged into two varieties the cross-breeds between the two are more fertile than either pure stock--yet when they have diverged only one degree more the whole tendency is reversed, and the mongrel is sterile, either absolutely or relatively. he who explains the genesis of species through purely natural agencies should assign a natural cause for this remarkable result; and this mr. darwin has not done. whether original or derived, however, this arrangement to keep apart those forms which have, or have acquired (as the case may be), a certain moderate amount of difference, looks to us as much designed for the purpose, as does a rachet to prevent reverse motion in a wheel. if species have originated by divergence, this keeps them apart. here let us suggest a possibly attainable test of the theory of derivation, a kind of instance which mr. darwin may be fairly asked to produce--viz., an instance of two varieties, or what may be assumed as such, which have diverged enough to reverse the movement, to bring out some sterility in the crosses. the best marked human races might offer the most likely case. if mulattoes are sterile or tend to sterility, as some naturalists confidently assert, they afford mr. darwin a case in point. if, as others think, no such tendency is made out, the required evidence is wanting. a fourth and the most formidable difficulty is that of the production and specialization of organs. it is well said that all organic beings have been formed on two great laws: unity of type, and adaptation to the conditions of existence.[i- ] the special teleologists, such as paley, occupy themselves with the latter only; they refer particular facts to special design, but leave an overwhelming array of the widest facts inexplicable. the morphologists build on unity of type, or that fundamental agreement in the structure of each great class of beings which is quite independent of their habits or conditions of life; which requires each individual "to go through a certain formality," and to accept, at least for a time, certain organs, whether they are of any use to him or not. philosophical minds form various conceptions for harmonizing the two views theoretically. mr. darwin harmonizes and explains them naturally. adaptation to the conditions of existence is the result of natural selection; unity of type, of unity of descent. accordingly, as he puts his theory, he is bound to account for the origination of new organs, and for their diversity in each great type, for their specialization, and every adaptation of organ to function and of structure to condition, through natural agencies. whenever he attempts this he reminds us of lamarck, and shows us how little light the science of a century devoted to structural investigation has thrown upon the mystery of organization. here purely natural explanations fail. the organs being given, natural selection may account for some improvement; if given of a variety of sorts or grades, natural selection might determine which should survive and where it should prevail. on all this ground the only line for the theory to take is to make the most of gradation and adherence to type as suggestive of derivation, and unaccountable upon any other scientific view--deferring all attempts to explain how such a metamorphosis was effected, until naturalists have explained how the tadpole is metamorphosed into a frog, or one sort of polyp into another. as to why it is so, the philosophy of efficient cause, and even the whole argument from design, would stand, upon the admission of such a theory of derivation, precisely where they stand without it. at least there is, or need be, no ground of difference here between darwin and agassiz. the latter will admit, with owen and every morphologist, that hopeless is the attempt to explain the similarity of pattern in members of the same class by utility or the doctrine of final causes. "on the ordinary view of the independent creation of each being, we can only say that so it is, that it has so pleased the creator to construct each animal and plant." mr. darwin, in proposing a theory which suggests a how that harmonizes these facts into a system, we trust implies that all was done wisely, in the largest sense designedly, and by an intelligent first cause. the contemplation of the subject on the intellectual side, the amplest exposition of the unity of plan in creation, considered irrespective of natural agencies, leads to no other conclusion. we are thus, at last, brought to the question, what would happen if the derivation of species were to be substantiated, either as a true physical theory, or as a sufficient hypothesis? what would come of it? the inquiry is a pertinent one, just now. for, of those who agree with us in thinking that darwin has not established his theory of derivation many will admit with us that he has rendered a theory of derivation much less improbable than before; that such a theory chimes in with the established doctrines of physical science, and is not unlikely to be largely accepted long before it can be proved. moreover, the various notions that prevail--equally among the most and the least religious--as to the relations between natural agencies or phenomena and efficient cause, are seemingly more crude, obscure, and discordant, than they need be. it is not surprising that the doctrine of the book should be denounced as atheistical. what does surprise and concern us is, that it should be so denounced by a scientific man, on the broad assumption that a material connection between the members of a series of organized beings is inconsistent with the idea of their being intellectually connected with one another through the deity, i.e., as products of one mind, as indicating and realizing a preconceived plan. an assumption the rebound of which is somewhat fearful to contemplate, but fortunately one which every natural birth protests against. it would be more correct to say that the theory in itself is perfectly compatible with an atheistic view of the universe. that is true; but it is equally true of physical theories generally. indeed, it is more true of the theory of gravitation, and of the nebular hypothesis, than of the hypothesis in question. the latter merely takes up a particular, proximate cause, or set of such causes, from which, it is argued, the present diversity of species has or may have contingently resulted. the author does not say necessarily resulted; that the actual results in mode and measure, and none other, must have taken place. on the other hand, the theory of gravitation and its extension in the nebular hypothesis assume a universal and ultimate physical cause, from which the effects in nature must necessarily have resulted. now, it is not thought, at least at the present day, that the establishment of the newtonian theory was a step toward atheism or pantheism. yet the great achievement of newton consisted in proving that certain forces (blind forces, so far as the theory is concerned), acting upon matter in certain directions, must necessarily produce planetary orbits of the exact measure and form in which observation shows them to exist--a view which is just as consistent with eternal necessity, either in the atheistic or the pantheistic form, as it is with theism. nor is the theory of derivation particularly exposed to the charge of the atheism of fortuity; since it undertakes to assign real causes for harmonious and systematic results. but, of this, a word at the close. the value of such objections to the theory of derivation may be tested by one or two analogous cases. the common scientific as well as popular belief is that of the original, independent creation of oxygen and hydrogen, iron, gold, and the like. is the speculative opinion now increasingly held, that some or all of the supposed elementary bodies are derivative or compound, developed from some preceding forms of matter, irreligious? were the old alchemists atheists as well as dreamers in their attempts to transmute earth into gold? or, to take an instance from force (power)--which stands one step nearer to efficient cause than form--was the attempt to prove that heat, light, electricity, magnetism, and even mechanical power, are variations or transmutations of one force, atheistical in its tendency? the supposed establishment of this view is reckoned as one of the greatest scientific triumphs of this century. perhaps, however, the objection is brought, not so much against the speculation itself, as against the attempt to show how derivation might have been brought about. then the same objection applies to a recent ingenious hypothesis made to account for the genesis of the chemical elements out of the ethereal medium, and to explain their several atomic weights and some other characteristics by their successive complexity--hydrogen consisting of so many atoms of ethereal substance united in a particular order, and so on. the speculation interested the philosophers of the british association, and was thought innocent, but unsupported by facts. surely mr. darwin's theory is none the worse, morally, for having some foundation in fact. in our opinion, then, it is far easier to vindicate a theistic character for the derivative theory, than to establish the theory itself upon adequate scientific evidence. perhaps scarcely any philosophical objection can be urged against the former to which the nebular hypothesis is not equally exposed. yet the nebular hypothesis finds general scientific acceptance, and is adopted as the basis of an extended and recondite illustration in mr. agassiz's great work.[i- ] how the author of this book harmonizes his scientific theory with his philosophy and theology, he has not informed us. paley in his celebrated analogy with the watch, insists that if the timepiece were so constructed as to produce other similar watches, after a manner of generation in animals, the argument from design would be all the stronger. what is to hinder mr. darwin from giving paley's argument a further a-fortiori extension to the supposed case of a watch which sometimes produces better watches, and contrivances adapted to successive conditions, and so at length turns out a chronometer, a town clock, or a series of organisms of the same type? from certain incidental expressions at the close of the volume, taken in connection with the motto adopted from whewell, we judge it probable that our author regards the whole system of nature as one which had received at its first formation the impress of the will of its author, foreseeing the varied yet necessary laws of its action throughout the whole of its existence, ordaining when and bow each particular of the stupendous plan should be realized in effect, and--with him to whom to will is to do--in ordaining doing it, whether profoundly philosophical or not, a view maintained by eminent philosophical physicists and theologians, such as babbage on the one hand and jowett on the other, will hardly be denounced as atheism. perhaps mr. darwin would prefer to express his idea in a more general way, by adopting the thoughtful words of one of the most eminent naturalists of this or any age, substituting the word action for "thought," since it is the former (from which alone the latter can be inferred) that he has been considering. "taking nature as exhibiting thought for my guide, it appears to me that while human thought is consecutive, divine thought is simultaneous, embracing at the same time and forever, in the past, the present and the future, the most diversified relations among hundreds of thousands of organized beings, each of which may present complications again, which to study and understand even imperfectly--as for instance man himself-- mankind has already spent thousands of years."[i- ] in thus conceiving of the divine power in act as coetaneous with divine thought, and of both as far as may be apart from the human element of time, our author may regard the intervention of the creator either as, humanly speaking, done from all time, or else as doing through all time. in the ultimate analysis we suppose that every philosophical theist must adopt one or the other conception. a perversion of the first view leads toward atheism, the notion of an eternal sequence of cause and effect, for which there is no first cause--a view which few sane persons can long rest in. the danger which may threaten the second view is pantheism. we feel safe from either error, in our profound conviction that there is order in the universe; that order presupposes mind; design, will; and mind or will, personality. thus guarded, we much prefer the second of the two conceptions of causation, as the more philosophical as well as christian view--a view which leaves us with the same difficulties and the same mysteries in nature as in providence, and no other. natural law, upon this view, is the human conception of continued and orderly divine action. we do not suppose that less power, or other power, is required to sustain the universe and carry on its operations, than to bring it into being. so, while conceiving no improbability of "interventions of creative mind in nature," if by such is meant the bringing to pass of new and fitting events at fitting times, we leave it for profounder minds to establish, if they can, a rational distinction in kind between his working in nature carrying on operations, and in initiating those operations. we wished, under the light of such views, to examine more critically the doctrine of this book, especially of some questionable parts; for instance, its explanation of the natural development of organs, and its implication of a "necessary acquirement of mental power" in the ascending scale of gradation. but there is room only for the general declaration that we cannot think the cosmos a series which began with chaos and ends with mind, or of which mind is a result: that, if, by the successive origination of species and organs through natural agencies, the author means a series of events which succeed each other irrespective of a continued directing intelligence--events which mind does not order and shape to destined ends--then he has not established that doctrine, nor advanced toward its establishment, but has accumulated improbabilities beyond all belief. take the formation and the origination of the successive degrees of complexity of eyes as a specimen. the treatment of this subject (pp. i , ), upon one interpretation, is open to all the objections referred to; but, if, on the other hand, we may rightly compare the eye "to a telescope, perfected by the long-continued efforts of the highest human intellects," we could carry out the analogy, and draw satisfactory illustrations and inferences from it. the essential, the directly intellectual thing is the making of the improvements in the telescope or the steam-engine. whether the successive improvements, being small at each step, and consistent with the general type of the instrument, are applied to some of the individual machines, or entire new machines are constructed for each, is a minor matter. though, if machines could engender, the adaptive method would be most economical; and economy is said to be a paramount law in nature. the origination of the improvements, and the successive adaptations to meet new conditions or subserve other ends, are what answer to the supernatural, and therefore remain inexplicable. as to bringing them into use, though wisdom foresees the result, the circumstances and the natural competition will take care of that, in the long-run. the old ones will go out of use fast enough, except where an old and simple machine remains still best adapted to a particular purpose or condition--as, for instance, the old newcomen engine for pumping out coal-pits. if there's a divinity that shapes these ends, the whole is intelligible and reasonable; otherwise, not. we regret that the necessity of discussing philosophical questions has prevented a fuller examination of the theory itself, and of the interesting scientific points which are brought to bear in its favor. one of its neatest points, certainly a very strong one for the local origination of species, and their gradual diffusion under natural agencies, we must reserve for some other convenient opportunity. the work is a scientific one, rigidly restricted to its direct object; and by its science it must stand or fall. its aim is, probably, not to deny creative intervention in nature--for the admission of the independent origination of certain types does away with all antecedent improbability of as much intervention as may be required--but to maintain that natural selection, in explaining the facts, explains also many classes of facts which thousand-fold repeated independent acts of creation do not explain, but leave more mysterious than ever. how far the author has succeeded, the scientific world will in due time be able to pronounce. as these sheets are passing through the press, a copy of the second edition has reached us. we notice with pleasure the insertion of an additional motto on the reverse of the title page, directly claiming the theistic view which we have vindicated for the doctrine. indeed, these pertinent words of the eminently wise bishop butler comprise, in their simplest expression, the whole substance of our later pages: "the only distinct meaning of the word 'natural' is stated, fixed, or settled; since what is natural as much requires and presupposes an intelligent mind to render it so, i.e., to effect it continually or at stated times, as what is supernatural or miraculous does to effect it for once." ii design versus necessity discussion between two readers of darwin's treatise on the origin of species, upon its natural theology (american journal of science and arts, september, ) d.t.--is darwin's theory atheistic or pantheistic? or, does it tend to atheism or pantheism? before attempting any solution of this question, permit me to say a few words tending to obtain a definite conception of necessity and design, as the sources from which events may originate, each independent of the other; and we shall, perhaps, best attain a clear understanding of each, by the illustration of an example in which simple human designers act upon the physical powers of common matter. suppose, then, a square billiard-table to be placed with its corners directed to the four cardinal points. suppose a player, standing at the north corner, to strike a red ball directly to the south, his design being to lodge the ball in the south pocket; which design, if not interfered with, must, of course be accomplished. then suppose another player, standing at the east corner, to direct a white ball to the west corner. this design also, if not interfered with, must be accomplished. next suppose both players to strike their balls at the same instant, with like forces, in the directions before given. in this case the balls would not pass as before, namely, the red ball to the south, and the white ball to the west, but they must both meet and strike each other in the centre of the table, and, being perfectly elastic, the red ball must pass to the west pocket, and the white ball to the south pocket. we may suppose that the players acted wholly without concert with each other, indeed, they may be ignorant of each other' s design, or even of each other's existence; still we know that the events must happen as herein described. now, the first half of the course of these two balls is from an impulse, or proceeds from a power, acting from design. each player has the design of driving his ball across the table in a diagonal line to accomplish its lodgment at the opposite corner of the table. neither designed that his ball should be deflected from that course and pass to another corner of the table. the direction of this second part of the motion must be referred entirely to necessity, which directly interferes with the purpose of him who designed the rectilinear direction. we are not, in this case, to go back to find design in the creation of the powers or laws of inertia and elasticity, after the order of which the deflection, at the instant of collision, necessarily takes place. we know that these powers were inherent in the balls, and were not created to answer this special deflection. we are required, by the hypothesis, to confine attention in point of time, from the instant preceding the impact of the balls, to the time of their arrival at the opposite corners of the table. the cues aremoved by design. the impacts are acts from design. the first half of the motion of each ball is under the direction of design. we mean by this the particular design of each player. but, at the instant of the collision of the balls upon each other, direction from design ceases, and the balls no longer obey the particular designs of the players, the ends or purposes intended by them are not accomplished, but frustrated, by necessity, or by the necessary action of the powers of inertia and elasticity, which are inherent in matter, and are not made by any design of a creator for this special action, or to serve this special purpose, but would have existed in the materials of which the balls were made, although the players had never been born. i have thus stated, by a simple example in physical action, what is meant by design and what by necessity; and that the latter may exist without any dependence upon the former. if i have given the statement with what may be thought, by some, unnecessary prolixity, i have only to say that i have found many minds to have a great difficulty in conceiving of necessity as acting altogether independent of design. let me now trace these principles as sources of action in darwin's work or theory. let us see how much there is of design acting to produce a foreseen end, and thus proving a reasoning and self-conscious creator; and how much of mere blind power acting without rational design, or without a specific purpose or conscious foresight. mr. darwin has specified in a most clear and unmistakable manner the operation of his three great powers, or rather, the three great laws by which the organic power of life acts in the formation of an eye. (see p. .) following the method he has pointed out, we will take a number of animals of the same species, in which the eye is not developed. they may have all the other senses, with the organs of nutrition, circulation, respiration, and locomotion. they all have a brain and nerves, and some of these nerves may be sensitive to light; but have no combination of retina, membranes, humors, etc., by which the distinct image of an object may be formed and conveyed by the optic nerve to the cognizance of the internal perception, or the mind. the animal in this case would be merely sensible of the difference between light and darkness. he would have no power of discriminating form, size, shape, or color, the difference of objects, and to gain from these a knowledge of their being useful or hurtful, friends or enemies. up to this point there is no appearance of necessity upon the scene. the billiard-balls have not yet struck together, and we will suppose that none of the arguments that may be used to prove, from this organism, thus existing, that it could not have come into form and being without a creator acting to this end with intelligence and design, are opposed by anything that can be found in darwin's theory; for, so far, darwin's laws are supposed not to have come into operation. give the animals, thus organized, food and room, and they may go on, from generation to generation, upon the same organic level. those individuals that, from natural variation, are born with light-nerves a little more sensitive to light than their parents, will cross or interbreed with those who have the same organs a little less sensitive, and thus the mean standard will be kept up without any advancement. if our billiard-table were sufficiently extensive, i. e., infinite, the balls rolled from the corners would never meet, and the necessity which we have supposed to deflect them would never act. the moment, however, that the want of space or food commences natural selection begins. here the balls meet, and all future action is governed by necessity. the best forms, or those nerves most sensitive to light, connected with incipient membranes and humors for corneas and lenses, are picked out and preserved by natural selection, of necessity. all cannot live and propagate, and it is a necessity, obvious to all, that the weaker must perish, if the theory be true. working on, in this way, through countless generations, the eye is at last formed in all its beauty and excellence. it must (always assuming that this theory is true) result from this combined action of natural variation, the struggle for life, and natural selection, with as much certainty as the balls, after collision, must pass to corners of the table different from those to which they were directed, and so far forth as the eye is formed by these laws, acting upward from the nerve merely sensitive to light, we can no more infer design, and from design a designer, than we can infer design in the direction of the billiard-balls after the collision. both are sufficiently accounted for by blind powers acting under a blind necessity. take away the struggle for life from the one, and the collision of the balls from the other--and neither of these was designed--and the animal would have gone on without eyes. the balls would have found the corners of the table to which they were first directed. while, therefore, it seems to me clear that one who can find no proof of the existence of an intelligent creator except through the evidence of design in the organic world, can find no evidence of such design in the construction of the eye, if it were constructed under the operation of darwin's laws, i shall not for one moment contend that these laws are incompatible with design and a self-conscious, intelligent creator. such design might, indeed, have coexisted with the necessity or natural selection; and so the billiard-players might have â��designed the collision of their balls; but neither the formation of the eye, nor the path of the balls after collision, furnishes any sufficient proof of such design in either case. one, indeed, who believes, from revelation or any other cause, in the existence of such a creator, the fountain and source of all things in heaven above and in the earth beneath, will see in natural variation, the struggle for life, and natural selection, only the order or mode in which this creator, in his â��own perfect wisdom, sees fit to act. happy is he who can thus see and adore. but how many are there who have no such belief from intuition, or faith in revelation; but who have by careful and elaborate search in the physical, and more especially in the organic world, inferred, by induction, the existence of god from what has seemed to them the wonderful adaptation of the different organs and parts of the animal body to its, apparently, designed ends! imagine a mind of this skeptical character, in all honesty and under its best reason, after finding itself obliged to reject the evidence of revelation, to commence a search after the creator, in the light of natural theology. he goes through the proof for final cause and design, as given in a summary though clear, plain, and convincing form, in the pages of paley and the "bridgewater treatises." the eye and the hand, those perfect instruments of optical and mechanical contrivance and adaptation, without the least waste or surplusage--these, say paley and bell, certainly prove a designing maker as much as the palace or the watch proves an architect or a watchmaker. let this mind, in this state, cross darwin's work, and find that, after a sensitive nerve or a rudimentary hoof or claw, no design is to be found. from this point upward the development is the mere necessary result of natural selection; and let him receive this law of natural selection as true, and where does he find himself? before, he could refer the existence of the eye, for example, only to design, or chance. there was no other alternative. he rejected chance, as impossible. it must then be a design. but darwin brings up another power, namely, natural selection, in place of this impossible chance. this not only may, but, according to darwin, must of necessity produce an eye. it may indeed coexist with design, but it must exist and act and produce its results, even without design. will such a mind, under such circumstances, infer the existence of the designer--god--when he can, at the same time, satisfactorily account for the thing produced, by the operation of this natural selection? it seems to me, therefore, perfectly evident that the substitution of natural selection, by necessity, for design in the formation of the organic world, is a step decidedly atheistical. it is in vain to say that darwin takes the creation of organic life, in its simplest forms, to have been the work of the deity. in giving up design in these highest and most complex forms of organization, which have always been relied upon as the crowning proof of the existence of an intelligent creator, without whose intellectual power they could not have been brought into being, he takes a most decided step to banish a belief in the intelligent action of god from the organic world. the lower organisms will go next. the atheist will say, wait a little. some future darwin will show how the simple forms came necessarily from inorganic matter. this is but another step by which, according to laplace, "the discoveries of science throw final causes further back." a.g.--it is conceded that, if the two players in the supposed case were ignorant of each other's presence, the designs of both were frustrated, and from necessity. thus far it is not needful to inquire whether this necessary consequence is an unconditional or a conditioned necessity, nor to require a more definite statement of the meaning attached to the word necessity as a supposed third alternative. but, if the players knew of each other's presence, we could not infer from the result that the design of both or of either was frustrated. one of them may have intended to frustrate the other's design, and to effect his own. or both may have been equally conversant with the properties of the matter and the relation of the forces concerned (whatever the cause, origin, or nature, of these forces and properties), and the result may have been according to the designs of both. as you admit that they might or might not have designed the collision of their balls and its consequences the question arises whether there is any way of ascertaining which of the two conceptions we may form about it is the true one. now, let it be remarked that design can never be demonstrated. witnessing the act does not make known the design, as we have seen in the case assumed for the basis of the argument. the word of the actor is not proof; and that source of evidence is excluded from the cases in question. the only way left, and the only possible way in cases where testimony is out of the question, is to infer the design from the result, or from arrangements which strike us as adapted or intended to produce a certain result, which affords a presumption of design. the strength of this presumption may be zero, or an even chance, as perhaps it is in the assumed case; but the probability of design will increase with the particularity of the act, the specialty of the arrangement or machinery, and with the number of identical or yet more of similar and analogous instances, until it rises to a moral certainty--i. e., to a conviction which practically we are as unable to resist as we are to deny the cogency of a mathematical demonstration. a single instance, or set of instances, of a comparatively simple arrangement might suffice. for instance, we should not doubt that a pump was designed to raise water by the moving of the handle. of course, the conviction is the stronger, or at least the sooner arrived at, where we can imitate the arrangement, and ourselves produce the result at will, as we could with a pump, and also with the billiard-balls. and here i would suggest that your billiard-table, with the case of collision, answers well to a machine. in both a result is produced by indirection--by applying a force out of line of the ultimate direction. and, as i should feel as confident that a man intended to raise water who was working a pumphandle, as if he were bringing it up in pailfuls from below by means of a ladder, so, after due examination of the billiard-table and its appurtenances, i should probably think it likely that the effect of the rebound was expected and intended no less than that of the immediate impulse. and a similar inspection of arrangements and results in nature would raise at least an equal presumption of design. you allow that the rebound might have been intended, but you require proof that it was. we agree that a single such instance affords no evidence either way. but how would it be if you saw the men doing the same thing over and over? and if they varied it by other arrangements of the balls or of the blow, and these were followed by analogous results? how if you at length discovered a profitable end of the operation, say the winning of a wager? so in the counterpart case of natural selection: must we not infer intention from the arrangements and the results? but i will take another case of the very same sort, though simpler, and better adapted to illustrate natural selection; because the change of direction--your necessity--acts gradually or successively, instead of abruptly. suppose i hit a man standing obliquely in my rear, by throwing forward a crooked stick, called a boomerang. how could he know whether the blow was intentional or not? but suppose i had been known to throw boomerangs before; suppose that, on different occasions, i had before wounded persons by the same, or other indirect and apparently aimless actions; and suppose that an object appeared to be gained in the result--that definite ends were attained--would it not at length be inferred that my assault, though indirect, or apparently indirect, was designed? to make the case more nearly parallel with those it is brought to illustrate, you have only to suppose that, although the boomerang thrown by me went forward to a definite place, and at least appeared to subserve a purpose, and the bystanders, after a while, could get traces of the mode or the empirical law of its flight, yet they could not themselves do anything with it. it was quite beyond their power to use it. would they doubt, or deny my intention, on that account? no: they would insist that design on my part must be presumed from the nature of the results; that, though design may have been wanting in any one case, yet the repetition of the result, and from different positions and under varied circumstances, showed that there must have been design. moreover, in the way your case is stated, it seems to concede the most important half of the question, and so affords a presumption for the rest, on the side of design. for you seem to assume an actor, a designer, accomplishing his design in the first instance. you--a bystander--infer that the player effected his design in sending the first ball to the pocket before him. you infer this from observation alone. must you not from a continuance of the same observation equally infer a common design of the two players in the complex result, or a design of one of them to frustrate the design of the other? if you grant a designing actor, the presumption of design is as strong, or upon continued observation of instances soon becomes as strong, in regard to the deflection of the balls, or variation of the species, as it was for the result of the first impulse or for the production of the original animal, etc. but, in the case to be illustrated, we do not see the player. we see only the movement of the balls. now, if the contrivances and adaptations referred to really do "prove a designer as much as the palace or the watch proves an architect or a watchmaker"--as paley and bell argue, and as your skeptic admits, while the alternative is between design and chance--then they prove it with all the proof the case is susceptible of, and with complete conviction. for we cannot doubt that the watch had a watchmaker. and if they prove it on the supposition that the unseen operator acted immediately--i.e., that the player directly impelled the balls in the directions we see them moving, i insist that this proof is not impaired by our ascertaining that he acted mediately--i.e., that the present state or form of the plants or animals, like the present position of the billiard-balls, resulted from the collision of the individuals with one another, or with the surroundings. the original impulse, which we once supposed was in the line of the observed movement, only proves to have been in a different direction; but the series of movements took place with a series of results, each and all of them none the less determined, none the less designed. wherefore, when, at the close, you quote laplace, that "the discoveries of science throw final causes farther back," the most you can mean is, that they constrain us to look farther back for the impulse. they do not at all throw the argument for design farther back, in the sense of furnishing evidence or presumption that only the primary impulse was designed, and that all the rest followed from chance or necessity. evidence of design, i think you will allow, everywhere is drawn from the observation of adaptations and of results, and has really nothing to do with anything else, except where you can take the word for the will. and in that case you have not argument for design, but testimony. in nature we have no testimony; but the argument is overwhelming. now, note that the argument of the olden time--that of paley, etc., which your skeptic found so convincing--was always the argument for design in the movement of the balls after deflection. for it was drawn from animals produced by generation, not by creation, and through a long succession of generations or deflections. wherefore, if the argument for design is perfect in the case of an animal derived from a long succession of individuals as nearly alike as offspring is generally like parents and grandparents, and if this argument is not weakened when a variation, or series of variations, has occurred in the course, as great as any variations we know of among domestic cattle, how then is it weakened by the supposition, or by the likelihood, that the variations have been twice or thrice as great as we formerly supposed, or because the variations have been "picked out," and a few of them pre served as breeders of still other variations, by natural selection? finally let it be noted that your element of necessity has to do, so far as we know, only with the picking out and preserving of certain changing forms, i. e., with the natural selection. this selection, you may say, must happen under the circumstances. this is a necessary result of the collision of the balls; and these results can be predicted. if the balls strike so and so, they will be deflected so and so. but the variation itself is of the nature of an origination. it answers well to the original impulse of the balls, or to a series of such impulses. we cannot predict what particular new variation will occur from any observation of the past. just as the first impulse was given to the balls at a point out of sight, so the impulse which resulted in the variety or new form was given at a point beyond observation, and is equally mysterious or unaccountable, except on the supposition of an ordaining will. the parent had not the peculiarity of the variety, the progeny has. between the two is the dim or obscure region of the formation of a new individual, in some unknown part of which, and in some wholly unknown way, the difference is intercalated. to introduce necessity here is gratuitous and unscientific; but here you must have it to make your argument valid. i agree that, judging from the past, it is not improbable that variation itself may be hereafter shown to result from physical causes. when it is so shown, you may extend your necessity into this region, but not till then. but the whole course of scientific discovery goes to assure us that the discovery of the cause of variation will be only a resolution of variation into two factors: one, the immediate secondary cause of the changes, which so far explains them; the other an unresolved or unexplained phenomenon, which will then stand just where the product, variation, stands now, only that it will be one step nearer to the efficient cause. this line of argument appears to me so convincing, that i am bound to suppose that it does not meet your case. although you introduced players to illustrate what design is, it is probable that you did not intend, and would not accept, the parallel which your supposed case suggested. when you declare that the proof of design in the eye and the hand, as given by paley and bell, was convincing, you mean, of course, that it was convincing, so long as the question was between design and chance, but that now another alternative is offered, one which obviates the force of those arguments, and may account for the actual results without design. i do not clearly apprehend this third alternative. will you be so good, then, as to state the grounds upon which you conclude that the supposed proof of design from the eye, or the hand, as it stood before darwin's theory was promulgated, would be invalidated by the admission of this new theory? d.t.--as i have ever found you, in controversy, meeting the array of your opponent fairly and directly, without any attempt to strike the body of his argument through an unguarded joint in the phraseology, i was somewhat surprised at the course taken in your answer to my statement on darwin's theory. you there seem to suppose that i instanced the action of the billiard balls and players as a parallel, throughout, to the formation of the organic world. had it occurred to me that such an application might be supposed to follow legitimately from my introduction of this action, i should certainly have stated that i did not intend, and should by no means accede to, that construction. my purpose in bringing the billiard-table upon the scene was to illustrate, by example, design and necessity, as different and independent sources from which results, it might indeed be identical results, may be derived all the conclusions, therefore, that you have arrived at through this misconception or misapplication of my illustration, i cannot take as an answer to the matter stated or intended to be stated by me. again, following this misconception, you suppose the skeptic (instanced by me as revealing through the evidence of design, exhibited in the structure of the eye, for its designer, god) as bringing to the examination a belief in the existence of design in the construction of the animals as they existed up to the moment when the eye was, according to my supposition, added to the heart, stomach, brain, etc. by skeptic i, of course, intended one who doubted the existence of design in every organic structure, or at least required proof of such design. now, as the watch may be instanced as a more complete exhibition of design than a flint knife or an hour-glass, i selected, after the example of paley, the eye, as exhibiting by its complex but harmonious arrangements a higher evidence of design and a designer than is to be found in a nerve sensitive to light, or any mere rudimentary part or organ. i could not mean by skeptic one who believed in design so far as a claw, or a nerve sensitive to light, was concerned, but doubted all above. for one who believes in design at all will not fail to recognize it in a hand or an eye. but i need not extend these remarks, as you acknowledge in the sequel to your argument that you may not have suited it to the case as i have stated it. you now request me to "state the grounds upon which i conclude that the supposed proof of design from the eye and the hand, as it stood before darwin's theory was promulgated, is invalidated by the admission of that theory." it seems to me that a sufficient answer to this question has already been made in the last part of my former paper; but, as you request it, i will go over the leading points as there given, with more minuteness of detail. let us, then, suppose a skeptic, one who is yet considering and doubting of the existence of god, having already concluded that the testimony from any and all revelation is insufficient, and having rejected what is called the a priori arguments brought forward in natural theology, and pertinaciously insisted upon by dr. clark and others, turning as a last resource to the argument from design in the organic world. voltaire tells him that a palace could not exist without an architect to design it. dr. paley tells him that a watch proves the design of a watchmaker. he thinks this very reasonable, and, although he sees a difference between the works of nature and those of mere human art, yet if he can find in any organic body, or part of a body, the same adaptation to its use that he finds in a watch, this truth will go very far toward proving, if it is not entirely conclusive, that, in making it, the powers of life by which it grew were directed by an intelligent, reasoning master. under the guidance of paley he takes an eye, which, although an optical, and not a mechanical instrument like the watch, is as well adapted to testify to design. he sees, first, that the eye is transparent when every other part of the body is opaque. was this the result of a mere epicurean or lucretian "fortuitous concourse" of living "atoms"? he is not yet certain it might not be so. next he sees that it is spherical, and that this convex form alone is capable of changing the direction of the light which proceeds from a distant body, and of collecting it so as to form a distinct image within its globe. next he sees at the exact place where this image must be formed a curtain of nerve-work, ready to receive and convey it, or excite from it, in its own mysterious way, an idea of it in the mind. last of all, he comes to the crystalline lens. now, he has before learned that without this lens an eye would by the aqueous and vitreous humors alone form an image upon the retina, but this image would be indistinct from the light not being sufficiently refracted, and likewise from having a colored fringe round its edges. this last effect is attributable to the refrangibility of light, that is, to some of the colors being more refracted than others. he likewise knows that more than a hundred years ago mr. dollond having found out, after many experiments, that some kinds of glass have the power of dispersing light, for each degree of its refraction, much more than other kinds, and that on the discovery of this fact he contrived to make telescopes in which he passed the light through two object-glasses successively, one of which he made of crown and one of flint glass, so ground and adapted to each other that the greater dispersion produced by the substance of one should be corrected by the smaller dispersion of the other. this contrivance corrected entirely the colored images which had rendered all previous telescopes very imperfect. he finds in this invention all the elements of design, as it appeared in the thought and action of a human designer. first, conjecture of certain laws or facts in optics. then, experiment proving these laws or facts. then, the contrivance and formation of an instrument by which those laws or facts must produce a certain sought result. thus enlightened, our skeptic turns to his crystalline lens to see if he can discover the work of a dollond in this. here he finds that an eye, having a crystalline lens placed between the humors, not only refracts the light more than it would be refracted by the humors alone, but that, in this combination of humors and lens, the colors are as completely corrected as in the combination of dollond's telescope. can it be that there was no design, no designer, directing the powers of life in the formation of this wonderful organ? our skeptic is aware that, in the arts of man, great aid has been, sometimes, given by chance, that is, by the artist or workman observing some fortuitous combination, form, or action, around him. he has heard it said that the chance arrangement of two pairs of spectacles, in the shop of a dutch optician, gave the direction for constructing the first telescope. possibly, in time, say a few geological ages, it might in some optician's shop have brought about a combination of flint and crown glass which, together, should have been achromatic. but the space between the humors of the eye is not an optician' s shop where object-glasses of all kinds, shapes, and sizes, are placed by chance, in all manner of relations and positions. on the hypothesis under which our skeptic is making his examination--the eye having been completed in all but the formation of the lens--the place which the lens occupies when completed was filled with parts of the humors and plane membrane, homogeneous in texture and surface, presenting, therefore, neither the variety of the materials nor forms which are contained in the optician's shop for chance to make its combinations with. how, then, could it be cast of a combination not before used, and fashioned to a shape different from that before known, and placed in exact combination with all the parts before enumerated, with many others not even mentioned? he sees no parallelism of condition, then, by which chance could act in forming a crystalline lens, which answers to the condition of an optician's shop, where it might be possible in many ages for chance to combine existing forms into an achromatic object-glass. considering, therefore, the eye thus completed and placed in its bony case and provided with its muscles, its lids, its tear-ducts, and all its other elaborate and curious appendages, and, a thousand times more wonderful still, without being encumbered with a single superfluous or useless part, can he say that this could be the work of chance? the improbability of this is so great, and consequently the evidence of design is so strong, that he is about to seal his verdict in favor of design, when he opens mr. darwin's book. there he finds that an eye is no more than a vital aggregation or growth, directed, not by design nor chance, but moulded by natural variation and natural selection, through which it must, necessarily, have been developed and formed. particles or atoms being aggregated by the blind powers of life, must become under the given conditions, by natural variation and natural selection, eyes, without design, as certainly as the red billiard-ball went to the west pocket, by the powers of inertia and elasticity, without the design of the hand that put it in motion. (see darwin, p. .) let us lay before our skeptic the way in which we may suppose that darwin would trace the operation of life, or the vital force conforming to these laws. in doing this we need not go through with the formation of the several membranes, humors, etc., but take the crystalline lens as the most curious and nicely arranged and adapted of all the parts, and as giving, moreover, a close parallel, in the end produced, to that produced by design, by a human designer, dollond, in forming his achromatic object-glass. if it can be shown that natural variation and natural selection were capable of forming the crystalline lens, it will not be denied that they were capable of forming the iris, the sclerotica, the aqueous humors, or any and all the other parts. suppose, then, that we have a number of animals, with eyes yet wanting the crystalline. in this state the animals can see, but dimly and imperfectly, as a man sees after having been couched. some of the offspring of these animals have, by natural variation, merely a portion of the membrane which separates the aqueous from the vitreous humor a little thickened in its middle part, a little swelled out. this refracts the light a little more than it would be refracted by a membrane in which no such swelling existed, and not only so, but, in combination with the humors, it corrects the errors of dispersion and makes the image somewhat more colorless. all the young animals that have this swelled membrane see more distinctly than their parents or brethren. they, therefore, have an advantage over them in the struggle for life. they can obtain food more easily; can find their prey, and escape from their enemies with greater facility than their kindred. this thickening and rounding of the membrane goes on from generation to generation by natural variation; natural selection all the while "picking out with unerring skill all the improvements, through countless generations," until at length it is found that the membrane has become a perfect crystalline lens. now, where is the design in all this? the membrane was not thickened and rounded to the end that the image should be more distinct and colorless; but, being thickened and rounded by the operation of natural variation, inherent in generation, natural selection of necessity produced the result that we have seen. the same result was thus produced of necessity, in the eye, that dollond came at, in the telescope, with design, through painful guessing, reasoning, experimenting, and forming. suppose our skeptic to believe in all this power of natural selection; will he now seal up his verdict for design, with the same confidence that he would before he heard of darwin? if not, then "the supposed proof from design is invalidated by darwin's theory." a.g.--waiving incidental points and looking only to the gist of the question, i remark that the argument for design as against chance, in the formation of the eye, is most convincingly stated in your argument. upon this and upon numerous similar arguments the whole question we are discussing turns. so, if the skeptic was about to seal his verdict in favor of design, and a designer, when darwin's book appeared, why should his verdict now be changed or withheld? all the facts about the eye, which convinced him that the organ was designed, remain just as they were. his conviction was not produced through testimony or eyewitness, but design was irresistibly inferred from the evidence of contrivance in the eye itself. now, if the eye as it is, or has become, so convincingly argued design why not each particular step or part of this result? if the production of a perfect crystalline lens in the eye--you know not how--as much indicated design as did the production of a dollond achromatic lens--you understand how--then why does not "the swelling out" of a particular portion of the membrane behind the iris--caused you know not how--which, by "correcting the errors of dispersion and making the image somewhat more colorless," enabled the "young animals to see more distinctly than their parents or brethren," equally indicate design--if not as much as a perfect crystalline, or a dollond compound lens, yet as much as a common spectacle-glass? darwin only assures you that what you may have thought was done directly and at once was done indirectly and successively. but you freely admit that indirection and succession do not invalidate design, and also that paley and all the natural theologians drew the arguments which convinced your skeptic wholly from eyes indirectly or naturally produced. recall a woman of a past generation and show her a web of cloth; ask her how it was made, and she will say that the wool or cotton was carded, spun, and woven by hand. when you tell her it was not made by manual labor, that probably no hand has touched the materials throughout the process, it is possible that she might at first regard your statement as tantamount to the assertion that the cloth was made without design. if she did, she would not credit your statement. if you patiently explained to her the theory of carding-machines, spinning-jennies, and power-looms, would her reception of your explanation weaken her conviction that the cloth was the result of design? it is certain that she would believe in design as firmly as before, and that this belief would be attended by a higher conception and reverent admiration of a wisdom, skill, and power greatly beyond anything she had previously conceived possible. wherefore, we may insist that, for all that yet appears, the argument for design, as presented by the natural theologians, is just as good now, if we accept darwin's theory, as it was before that theory was promulgated; and that the skeptical juryman, who was about to join the other eleven in a unanimous verdict in favor of design, finds no good excuse for keeping the court longer waiting.[ii- ] iii natural selection not inconsistent with natural theology (atlantic monthly for july, august, and october, , reprinted in ) i novelties are enticing to most people; to us they are simply annoying. we cling to a long-accepted theory, just as we cling to an old suit of clothes. a new theory, like a new pair of breeches (the atlantic still affects the older type of nether garment), is sure to have hard-fitting places; or, even when no particular fault can be found with the article, it oppresses with a sense of general discomfort. new notions and new styles worry us, till we get well used to them, which is only by slow degrees. wherefore, in galileo's time, we might have helped to proscribe, or to burn--had he been stubborn enough to warrant cremation--even the great pioneer of inductive research; although, when we had fairly recovered our composure, and bad leisurely excogitated the matter, we might have come to conclude that the new doctrine was better than the old one, after all, at least for those who had nothing to unlearn. such being our habitual state of mind, it may well be believed that the perusal of the new book "on the origin of species by means of natural selection" left an uncomfortable impression, in spite of its plausible and winning ways. we were not wholly unprepared for it, as many of our contemporaries seem to have been. the scientific reading in which we indulge as a relaxation from severer studies had raised dim forebodings. investigations about the succession of species in time, and their actual geographical distribution over the earth's surface, were leading up from all sides and in various ways to the question of their origin. now and then we encountered a sentence, like prof. owen's "axiom of the continuous operation of the ordained becoming of living things," which haunted us like an apparition. for, dim as our conception must needs be as to what such oracular and grandiloquent phrases might really mean, we felt confident that they presaged no good to old beliefs. foreseeing, yet deprecating, the coming time of trouble, we still hoped that, with some repairs and makeshifts, the old views might last out our days. apres nous le deluge. still, not to lag behind the rest of the world, we read the book in which the new theory is promulgated. we took it up, like our neighbors, and, as was natural, in a somewhat captious frame of mind. well, we found no cause of quarrel with the first chapter. here the author takes us directly to the barn-yard and the kitchen-garden. like an honorable rural member of our general court, who sat silent until, near the close of a long session, a bill requiring all swine at large to wear pokes was introduced, when he claimed the privilege of addressing the house, on the proper ground that he had been "brought up among the pigs, and knew all about them"--so we were brought up among cows and cabbages; and the lowing of cattle, the cackle of hens, and the cooing of pigeons, were sounds native and pleasant to our ears. so "variation under domestication" dealt with familiar subjects in a natural way, and gently introduced "variation under nature," which seemed likely enough. then follows "struggle for existence"--a principle which we experimentally know to be true and cogent--bringing the comfortable assurance, that man, even upon leviathan hobbes's theory of society, is no worse than the rest of creation, since all nature is at war, one species with another, and the nearer kindred the more internecine--bringing in thousandfold confirmation and extension of the malthusian doctrine that population tends far to outrun means of subsistence throughout the animal and vegetable world, and has to be kept down by sharp preventive checks; so that not more than one of a hundred or a thousand of the individuals whose existence is so wonderfully and so sedulously provided for ever comes to anything, under ordinary circumstances; so the lucky and the strong must prevail, and the weaker and ill-favored must perish; and then follows, as naturally as one sheep follows another, the chapter on "natural selection," darwin's cheval de bataille, which is very much the napoleonic doctrine that providence favors the strongest battalions--that, since many more individuals are born than can possibly survive, those individuals and those variations which possess any advantage, however slight, over the rest, are in the long-run sure to survive, to propagate, and to occupy the limited field, to the exclusion or destruction of the weaker brethren. all this we pondered, and could not much object to. in fact, we began to contract a liking for a system which at the outset illustrates the advantages of good breeding, and which makes the most "of every creature's best." could we "let by-gones be by-gones," and, beginning now, go on improving and diversifying for the future by natural selection, could we even take up the theory at the introduction of the actually existing species, we should be well content; and so, perhaps, would most naturalists be. it is by no means difficult to believe that varieties are incipient or possible species, when we see what trouble naturalists, especially botanists, have to distinguish between them--one regarding as a true species what another regards as a variety; when the progress of knowledge continually increases, rather than diminishes, the number of doubtful instances; and when there is less agreement than ever among naturalists as to what is the basis in nature upon which our idea of species reposes, or how the word is to be defined. indeed, when we consider the endless disputes of naturalists and ethnologists over the human races, as to whether they belong to one species or to more, and, if to more, whether to three, or five, or fifty, we can â��hardly help fancying that both may be right--or rather, that the uni-humanitarians would have been right many thousand years ago, and the multi-humanitarians will be several thousand years later; while at present the safe thing to say is, that probably there is some truth on both sides. "natural selection," darwin remarks, "leads to divergence of character; for the more living beings can be supported on the same area, the more they diverge in structure, habits, and constitution" (a principle which, by-the-way, is paralleled and illustrated by the diversification of human labor); and also leads to much extinction of intermediate or unimproved forms. now, though this divergence may "steadily tend to increase," yet this is evidently a slow process in nature, and liable to much counteraction wherever man does not interpose, and so not likely to work much harm for the future. and if natural selection, with artificial to help it, will produce better animals and better men than the present, and fit them better to the conditions of existence, why, let it work, say we, to the top of its bent there is still room enough for improvement. only let us hope that it always works for good: if not, the divergent lines on darwin's lithographic diagram of "transmutation made easy," ominously show what small deviations from the straight path may come to in the end. the prospect of the future, accordingly, is on the whole pleasant and encouraging. it is only the backward glance, the gaze up the long vista of the past, that reveals anything alarming. here the lines converge as they recede into the geological ages, and point to conclusions which, upon the theory, are inevitable, but hardly welcome. the very first step backward makes the negro and the hottentot our blood-relations--not that reason or scripture objects to that, though pride may. the next suggests a closer association of our ancestors of the olden time with "our poor relations" of the quadrumanous family than we like to acknowledge. fortunately, however--even if we must account for him scientifically --man with his two feet stands upon a foundation of his own. intermediate links between the bimana and the quadrumana are lacking altogether; so that, put the genealogy of the brutes upon what footing you will, the four-handed races will not serve for our forerunners--at least, not until some monkey, live or fossil, is producible with great-toes, instead of thumbs, upon his nether extremities; or until some lucky geologist turns up the bones of his ancestor and prototype in france or england, who was so busy "napping the chuckie-stanes" and chipping out flint knives and arrow-heads in the time of the drift, very many ages ago--before the british channel existed, says lyell [iii- ]--and until these men of the olden time are shown to have worn their great-toes in the divergent and thumblike fashion. that would be evidence indeed: but, until some testimony of the sort is produced, we must needs believe in the separate and special creation of man, however it may have been with the lower animals and with plants. no doubt, the full development and symmetry of darwin's hypothesis strongly suggest the evolution of the human no less than the lower animal races out of some simple primordial animal--that all are equally "lineal descendants of some few beings which lived long before the first bed of the silurian system was deposited." but, as the author speaks disrespectfully of spontaneous generation, and accepts a supernatural beginning of life on earth, in some form or forms of being which included potentially all that have since existed and are yet to be, he is thereby not warranted to extend his inferences beyond the evidence or the fair probability. there seems as great likelihood that one special origination should be followed by another upon fitting occasion (such as the introduction of man), as that one form should be transmuted into another upon fitting occasion, as, for instance, in the succession of species which differ from each other only in some details. to compare small things with great in a homely illustration: man alters from time to time his instruments or machines, as new circumstances or conditions may require and his wit suggest. minor alterations and improvements he adds to the machine he possesses; he adapts a new rig or a new rudder to an old boat: this answers to variation. "like begets like," being the great rule in nature, if boats could engender, the variations would doubtless be propagated, like those of domestic cattle. in course of time the old ones would be worn out or wrecked; the best sorts would be chosen for each particular use, and further improved upon; and so the primordial boat be developed into the scow, the skiff, the sloop, and other species of water-craft--the very diversification, as well as the successive improvements, entailing the disappearance of intermediate forms, less adapted to any one particular purpose; wherefore these go slowly out of use, and become extinct species: this is natural selection. now, let a great and important advance be made, like that of steam navigation: here, though the engine might be added to the old vessel, yet the wiser and therefore the actual way is to make a new vessel on a modified plan: this may answer to specific creation. anyhow, the one does not necessarily exclude the other. variation and natural selection may play their part, and so may specific creation also. why not? this leads us to ask for the reasons which call for this new theory of transmutation. the beginning of things must needs lie in obscurity, beyond the bounds of proof, though within those of conjecture or of analogical inference. why not hold fast to the customary view, that all species were directly, instead of indirectly, created after their respective kinds, as we now behold them--and that in a manner which, passing our comprehension, we intuitively refer to the supernatural? why this continual striving after "the unattained and dim?" why these anxious endeavors, especially of late years, by naturalists and philosophers of various schools and different tendencies, to penetrate what one of them calls "that mystery of mysteries," the origin of species? to this, in general, sufficient answer may be found in the activity of the human intellect, "the delirious yet divine desire to know," stimulated as it has been by its own success in unveiling the laws and processes of inorganic nature; in the fact that the principal triumphs of our age in physical science have consisted in tracing connections where none were known before, in reducing heterogeneous phenomena to a common cause or origin, in a manner quite analogous to that of the reduction of supposed independently originated species to a common ultimate origin--thus, and in various other ways, largely and legitimately extending the domain of secondary causes. surely the scientific mind of an age which contemplates the solar system as evolved from a common revolving fluid mass--which, through experimental research, has come to regard light, heat, electricity, magnetism, chemical affinity, and mechanical power as varieties or derivative and convertible forms of one force, instead of independent species--which has brought the so-called elementary kinds of matter, such as the metals, into kindred groups, and pertinently raised the question, whether the members of each group may not be mere varieties of one species--and which speculates steadily in the direction of the ultimate unity of matter, of a sort of prototype or simple element which may be to the ordinary species of matter what the protozoa or what the component cells of an organism are to the higher sorts of animals and plants--the mind of such an age cannot be expected to let the old belief about species pass unquestioned. it will raise the question, how the diverse sorts of plants and animals came to be as they are and where they are and will allow that the whole inquiry transcends its powers only when all endeavors have failed granting the origin to be super natural or miraculous even, will not arrest the inquiry all real origination the philosophers will say, is supernatural, their very question is, whether we have yet gone back to the origin and can affirm that the present forms of plants and animals are the primordial, the miraculously created ones. and, even if they admit that, they will still inquire into the order of the phenomena, into the form of the miracle you might as well expect the child to grow up content with what it is told about the advent of its infant brother indeed, to learn that the new comer is the gift of god, far from lulling inquiry, only stimulates speculation as to how the precious gift was bestowed that questioning child is father to the man--is philosopher in short-clothes. since, then questions about the origin of species will be raised, and have been raised--and since the theorizings, however different in particulars, all proceed upon the notion that one species of plant or animal is somehow derived from another, that the different sorts which now flourish are lineal (or unlineal) descendants of other and earlier sorts--it now concerns us to ask, what are the grounds in nature, the admitted facts, which suggest hypotheses of derivation in some :shape or other? reasons there must be, and plausible ones, for the persistent recurrence of theories upon this genetic basis. a study of darwin's book, and a general glance at the present state of the natural sciences, enable us to gather the following as among the most suggestive and influential. we can only enumerate them here, without much indication of their particular bearing. there is-- . the general fact of variability, and the general tendency of the variety to propagate its like--the patent facts that all species vary more or less; that domesticated plants and animals, being in conditions favorable to the production and preservation of varieties, are apt to vary widely; and that, by interbreeding, any variety may be fixed into a race, that is, into a variety which comes true from seed. many such races, it is allowed, differ from each other in structure and appearance as widely as do many admitted species; and it is practically very difficult, even impossible, to draw a clear line between races and species. witness the human races, for instance. wild species also vary, perhaps about as widely as those of domestication, though in different ways. some of them apparently vary little, others moderately, others immoderately, to the great bewilderment of systematic botanists and zoologists, and increasing disagreement as to whether various forms shall be held to be original species or strong varieties. moreover, the degree to which the descendants of the same stock, varying in different directions, may at length diverge, is unknown. all we know is, that varieties are themselves variable, and that very diverse forms have been educed from one stock. . species of the same genus are not distinguished from each other by equal amounts of difference. there is diversity in this respect analogous to that of the varieties of a polymorphous species, some of them slight, others extreme. and in large genera the unequal resemblance shows itself in the clustering of the species around several types or central species, like satellites around their respective planets. obviously suggestive this of the hypothesis that they were satellites, not thrown off by revolution, like the moons of jupiter, saturn, and our own solitary moon, but gradually and peacefully detached by divergent variation. that such closely-related species may be only varieties of higher grade, earlier origin, or more favored evolution, is not a very violent supposition. anyhow, it was a supposition sure to be made. . the actual geographical distribution of species upon the earth's surface tends to suggest the same notion. for, as a general thing, all or most of the species of a peculiar genus or other type are grouped in the same country, or occupy continuous, proximate, or accessible areas. so well does this rule hold, so general is the implication that kindred species are or were associated geographically, that most trustworthy naturalists, quite free from hypotheses of transmutation, are constantly inferring former geographical continuity between parts of the world now widely disjoined, in order to account thereby for certain generic similarities among their inhabitants; just as philologists infer former connection of races, and a parent language, to account for generic similarities among existing languages. yet no scientific explanation has been offered to account for the geographical association of kindred species, except the hypothesis of a common origin. . here the fact of the antiquity of creation, and in particular of the present kinds of the earth's inhabitants, or of a large part of them, comes in to rebut the objection that there has not been time enough for any marked diversification of living things through divergent variation--not time enough for varieties to have diverged into what we call species. so long as the existing species of plants and animals were thought to have originated a few thousand years ago, and without predecessors, there was no room for a theory of derivation of one sort from another, nor time enough even to account for the establishment of the races which are generally believed to have diverged from a common stock. not so much that five or six thousand years was a short allowance for this; but because some of our familiar domesticated varieties of grain, of fowls, and of other animals, were pictured and mummified by the old egyptians more than half that number of years ago, if not earlier. indeed, perhaps the strongest argument for the original plurality of human species was drawn from the identification of some of the present races of men upon these early historical monuments and records. but this very extension of the current chronology, if we may rely upon the archaeologists, removes the difficulty by opening up a longer vista. so does the discovery in europe of remains and implements of prehistoric races of men, to whom the use of metals was unknown--men of the stone age, as the scandinavian archaeologists designate them. and now, "axes and knives of flint, evidently wrought by human skill, are found in beds of the drift at amiens (also in other places, both in france and england), associated with the bones of extinct species of animals." these implements, indeed, were noticed twenty years ago; at a place in suffolk they have been exhumed from time to time for more than a century; but the full confirmation, the recognition of the age of the deposit in which the implements occur, their abundance, and the appreciation of their bearings upon most interesting questions, belong to the present time. to complete the connection of these primitive people with the fossil ages, the french geologists, we are told, have now "found these axes in picardy associated with remains of elephas primigenius, rhinoceros tichorhinus, equus fossilis, and an extinct species of bos."[iii- ] in plain language, these workers in flint lived in the time of the mammoth, of a rhinoceros now extinct, and along with horses and cattle unlike any now existing--specifically different, as naturalists say, from those with which man is now associated. their connection with existing human races may perhaps be traced through the intervening people of the stone age, who were succeeded by the people of the bronze age, and these by workers in iron.[iii- ] now, various evidence carries back the existence of many of the present lower species of animals, and probably of a larger number of plants, to the same drift period. all agree that this was very many thousand years ago. agassiz tells us that the same species of polyps which are now building coral walls around the present peninsula of florida actually made that peninsula, and have been building there for many thousand centuries. . the overlapping of existing and extinct species, and the seemingly gradual transition of the life of the drift period into that of the present, may be turned to the same account. mammoths, mastodons, and irish elks, now extinct, must have lived down to human, if not almost to historic times. perhaps the last dodo did not long outlive his huge new zealand kindred. the aurochs, once the companion of mammoths, still survives, but owes his present and precarious existence to man's care. now, nothing that we know of forbids the hypothesis that some new species have been independently and supernaturally created within the period which other species have survived. some may even believe that man was created in the days of the mammoth, became extinct, and was recreated at a later date. but why not say the same of the aurochs, contemporary both of the old man and of the new? still it is more natural, if not inevitable, to infer that, if the aurochs of that olden time were the ancestors of the aurochs of the lithuanian forests, so likewise were the men of that age the ancestors of the present human races. then, whoever concludes that these primitive makers of rude flint axes and knives were the ancestors of the better workmen of the succeeding stone age, and these again of the succeeding artificers in brass and iron, will also be likely to suppose that the equus and bos of that time, different though they be, were the remote progenitors of our own horses and cattle. in all candor we must at least concede that such considerations suggest a genetic descent from the drift period down to the present, and allow time enough--if time is of any account-- for variation and natural selection to work out some appreciable results in the way of divergence into races, or even into so-called species. whatever might have been thought, when geological time was supposed to be separated from the present era by a clear line, it is now certain that a gradual replacement of old forms by new ones is strongly suggestive of some mode of origination which may still be operative. when species, like individuals, were found to die out one by one, and apparently to come in one by one, a theory for what owen sonorously calls "the continuous operation of the ordained becoming of living things" could not be far off. that all such theories should take the form of a derivation of the new from the old seems to be inevitable, perhaps from our inability to conceive of any other line of secondary causes in this connection. owen himself is apparently in travail with some transmutation theory of his own conceiving, which may yet see the light, although darwin's came first to the birth. different as the two theories will probably be, they cannot fail to exhibit that fundamental resemblance in this respect which betokens a community of origin, a common foundation on the general facts and the obvious suggestions of modern science. indeed--to turn the point of a pungent simile directed against darwin--the difference between the darwinian and the owenian hypotheses may, after all, be only that between homoeopathic and heroic doses of the same drug. if theories of derivation could only stop here, content with explaining the diversification and succession of species between the teritiary period and the present time, through natural agencies or secondary causes still in operation, we fancy they would not be generally or violently objected to by the savants of the present day. but it is hard, if not impossible, to find a stopping-place. some of the facts or accepted conclusions already referred to, and several others, of a more general character, which must be taken into the account, impel the theory onward with accumulated force. vires (not to say virus) acquirit eundo. the theory hitches on wonderfully well to lyell's uniformitarian theory in geology--that the thing that has been is the thing that is and shall be--that the natural operations now going on will account for all geological changes in a quiet and easy way, only give them time enough, so connecting the present and the proximate with the farthest past by almost imperceptible gradations--a view which finds large and increasing, if not general, acceptance in physical geology, and of which darwin's theory is the natural complement. so the darwinian theory, once getting a foothold, marches; boldly on, follows the supposed near ancestors of our present species farther and yet farther back into the dim past, and ends with an analogical inference which "makes the whole world kin." as we said at the beginning, this upshot discomposes us. several features of the theory have an uncanny look. they may prove to be innocent: but their first aspect is suspicious, and high authorities pronounce the whole thing to be positively mischievous. in this dilemma we are going to take advice. following the bent of our prejudices, and hoping to fortify these by new and strong arguments, we are going now to read the principal reviews which undertake to demolish the theory--with what result our readers shall be duly informed. ii "i can entertain no doubt, after the most deliberate study and dispassionate judgment of which i am capable, that the view which most naturalists entertain, and which i formerly entertained, namely, that each species has been independently created, is erroneous. i am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. furthermore, i am convinced that natural selection has been the main, but not exclusive, means of modification." this is the kernel of the new theory, the darwinian creed, as recited at the close of the introduction to the remarkable book under consideration. the questions, "what will he do with it?" and "how far will he carry it?" the author answers at the close of the volume: "i cannot doubt that the theory of descent with modification embraces all the members of the same class." furthermore, "i believe that all animals have descended from at most only four or five progenitors, and plants from an equal or lesser number." seeing that analogy as strongly suggests a further step in the same direction, while he protests that "analogy may be a deceitful guide," yet he follows its inexorable leading to the inference that-- "probably all the organic beings which have ever lived on this ear have descended from some one primordial form, into which life was first breathed."[iii- ] in the first extract we have the thin end of the wedge driven a little way; in the last, the wedge driven home. we have already sketched some of the reasons suggestive of such a theory of derivation of species, reasons which gave it plausibility, and even no small probability, as applied to our actual world and to changes occurring since the latest tertiary period. we are well pleased at this moment to find that the conclusions we were arriving at in this respect are sustained by the very high authority and impartial judgment of pictet, the swiss paleontologist. in his review of darwin's book[iii- ] -- the fairest and most admirable opposing one that has appeared--he freely accepts that ensemble of natural operations which darwin impersonates under the now familiar name of natural selection, allows that the exposition throughout the first chapters seems "a la fois prudent et fort," and is disposed to accept the whole argument in its foundations, that is, so far as it relates to what is now going on, or has taken place in the present geological period--which period he carries back through the diluvial epoch to the borders of the tertiary.[iii- ] pictet accordingly admits that the theory will very well account for the origination by divergence of nearly-related species, whether within the present period or in remoter geological times; a very natural view for him to take, since he appears to have reached and published, several years ago, the pregnant conclusion that there most probably was some material connection between the closely-related species of two successive faunas, and that the numerous close species, whose limits are so difficult to determine, were not all created distinct and independent. but while thus accepting, or ready to accept, the basis of darwin's theory, and all its legitimate direct inferences, he rejects the ultimate conclusions, brings some weighty arguments to bear against them, and is evidently convinced that he can draw a clear line between the sound inferences, which he favors, and the unsound or unwarranted theoretical deductions, which he rejects. we hope he can. this raises the question, why does darwin press his theory to these extreme conclusions? why do all hypotheses of derivation converge so inevitably to one ultimate point? having already considered some of the reasons which suggest or support the theory at its outset--which may carry it as far as such sound and experienced naturalists as pictet allow that it may be true--perhaps as far as darwin himself unfolds it in the introductory proposition cited at the beginning of this article--we may now inquire after the motives which impel the theorist so much farther. here proofs, in the proper sense of the word, are not to be had. we are beyond the region of demonstration, and have only probabilities to consider. what are these probabilities? what work will this hypothesis do to establish a claim to be adopted in its completeness? why should a theory which may plausibly enough account for the diversification of the species of each special type or genus be expanded into a general system for the origination or successive diversification of all species, and all special types or forms, from four or five remote primordial forms, or perhaps from one? we accept the theory of gravitation because it explains all the facts we know, and bears all the tests that we can put it to. we incline to accept the nebular hypothesis, for similar reasons; not because it is proved--thus far it is incapable of proof--but because it is a natural theoretical deduction from accepted physical laws, is thoroughly congruous with the facts, and because its assumption serves to connect and harmonize these into one probable and consistent whole. can the derivative hypothesis be maintained and carried out into a system on similar grounds? if so, however unproved, it would appear to be a tenable hypothesis, which is all that its author ought now to claim. such hypotheses as, from the conditions of the case, can neither be proved nor disproved by direct evidence or experiment, are to be tested only indirectly, and therefore imperfectly, by trying their power to harmonize the known facts, and to account for what is otherwise unaccountable. so the question comes to this: what will an hypothesis of the derivation of species explain which the opposing view leaves unexplained? questions these which ought to be entertained before we take up the arguments which have been advanced against this theory. we can barely glance at some of the considerations which darwin adduces, or will be sure to adduce in the future and fuller exposition which is promised. to display them in such wise as to indoctrinate the unscientific reader would require a volume. merely to refer to them in the most general terms would suffice for those familiar with scientific matters, but would scarcely enlighten those who are not. wherefore let these trust the impartial pictet, who freely admits that, "in the absence of sufficient direct proofs to justify the possibility of his hypothesis, mr. darwin relies upon indirect proofs, the bearing of which is real and incontestable;" who concedes that "his theory accords very well with the great facts of comparative anatomy and zoology--comes in admirably to explain unity of composition of organisms, also to explain rudimentary and representative organs, and the natural series of genera and species--equally corresponds with many paleontological data--agrees well with the specific resemblances which exist between two successive faunas, with the parallelism which is sometimes observed between the series of paleontological succession and of embryonal development," etc.; and finally, although he does not accept the theory in these results, he allows that "it appears to offer the best means of explaining the manner in which organized beings were produced in epochs anterior to our own." what more than this could be said for such an hypothesis? here, probably, is its charm, and its strong hold upon the speculative mind. unproven though it be, and cumbered prima facie with cumulative improbabilities as it proceeds, yet it singularly accords with great classes of facts otherwise insulated and enigmatic, and explains many things which are thus far utterly inexplicable upon any other scientific assumption. we have said that darwin's hypothesis is the natural complement to lyell's uniformitarian theory in physical geology. it is for the organic world what that is for the inorganic; and the accepters of the latter stand in a position from which to regard the former in the most favorable light. wherefore the rumor that the cautious lyell himself has adopted the darwinian hypothesis need not surprise us. the two views are made for each other, and, like the two counterpart pictures for the stereoscope, when brought together, combine into one apparently solid whole. if we allow, with pictet, that darwin's theory will very well serve for all that concerns the present epoch of the world's history--an epoch in which this renowned paleontologist includes the diluvial or quaternary period--then darwin's first and foremost need in his onward course is a practicable road from this into and through the tertiary period, the intervening region between the comparatively near and the far remote past. here lyell's doctrine paves the way, by showing that in the physical geology there is no general or absolute break between the two, probably no greater between the latest tertiary and the quaternary period than between the latter and the present time. so far, the lyellian view is, we suppose, generally concurred in. it is largely admitted that numerous tertiary species have continued down into the quaternary, and many of them to the present time. a goodly percentage of the earlier and nearly half of the later tertiary mollusca, according to des hayes, lye!!, and, if we mistake not, bronn, still live. this identification, however, is now questioned by a naturalist of the very highest authority. but, in its bearings on the new theory, the point here turns not upon absolute identity so much as upon close resemblance. for those who, with agassiz, doubt the specific identity in any of these cases, and those who say, with pictet, that "the later tertiary deposits contain in general the debris of species very nearly related to those which still exist, belonging to the same genera, but specifically different," may also agree with pictet, that the nearly-related species of successive faunas must or may have had "a material connection." but the only material connection that we have an idea of in such a case is a genealogical one. and the supposition of a genealogical connection is surely not unnatural in such cases--is demonstrably the natural one as respects all those tertiary species which experienced naturalists have pronounced to be identical with existing ones, but which others now deem distinct for to identify the two is the same thing as to conclude the one to be the ancestor of the other no doubt there are differences between the tertiary and the present individuals, differences equally noticed by both classes of naturalists, but differently estimated by the one these are deemed quite compatible, by the other incompatible, with community of origin but who can tell us what amount of difference is compatible with community of origin? this is the very question at issue, and one to be settled by observation alone who would have thought that the peach and the nectarine came from one stock? but, this being proved is it now very improbable that both were derived from the almond, or from some common amygdaline progenitor? who would have thought that the cabbage, cauliflower, broccoli kale, and kohlrabi are derivatives of one species, and rape or colza, turnip, and probably ruta-baga, of another species? and who that is convinced of this can long undoubtingly hold the original distinctness of turnips from cabbages as an article of faith? on scientific grounds may not a primordial cabbage or rape be assumed as the ancestor of all the cabbage races, on much the same ground that we assume a common ancestry for the diversified human races? if all our breeds of cattle came from one stock why not this stock from the auroch, which has had all the time between the diluvial and the historic periods in which to set off a variation perhaps no greater than the difference between some sorts of domestic cattle? that considerable differences are often discernible between tertiary individuals and their supposed descendants of the present day affords no argument against darwin's theory, as has been rashly thought, but is decidedly in its favor. if the identification were so perfect that no more differences were observable between the tertiary and the recent shells than between various individuals of either, then darwin's opponents, who argue the immutability of species from the ibises and cats preserved by the ancient egyptians being just like those of the present day, could triumphantly add a few hundred thousand years more to the length of the experiment and to the force of their argument. as the facts stand, it appears that, while some tertiary forms are essentially undistinguishable from existing ones, others are the same with a difference, which is judged not to be specific or aboriginal; and yet others show somewhat greater differences, such as are scientifically expressed by calling them marked varieties, or else doubtful species; while others, differing a little more, are confidently termed distinct, but nearly-related species. now, is not all this a question of degree, of mere gradation of difference? and is it at all likely that these several gradations came to be established in two totally different ways--some of them (though naturalists can't agree which) through natural variation, or other secondary cause, and some by original creation, without secondary cause? we have seen that the judicious pictet answers such questions as darwin would have him do, in affirming that, in all probability, the nearly-related species of two successive faunas were materially connected, and that contemporaneous species, similarly resembling each other, were not all created so, but have become so. this is equivalent to saying that species (using the term as all naturalists do, and must continue to employ the word) have only a relative, not an absolute fixity; that differences fully equivalent to what are held to be specific may arise in the course of time, so that one species may at length be naturally replaced by another species a good deal like it, or may be diversified into two, three, or more species, or forms as different as species. this concedes all that darwin has a right to ask, all that he can directly infer from evidence. we must add that it affords a locus standi, more or less tenable, for inferring more. here another geological consideration comes in to help on this inference. the species of the later tertiary period for the most part not only resembled those of our days--many of them so closely as to suggest an absolute continuity--but also occupied in general the same regions that their relatives occupy now. the same may be said, though less specially, of the earlier tertiary and of the later secondary; but there is less and less localization of forms as we recede, yet some localization even in palaeozoic times. while in the secondary period one is struck with the similarity of forms and the identity of many of the species which flourished apparently at the same time in all or in the most widely-separated parts of the world, in the tertiary epoch, on the contrary, along with the increasing specialization of climates and their approximation to the present state, we find abundant evidence of increasing localization of orders, genera and species, and this localization strikingly accords with the present geographical distribution of the same groups of species where the imputed forefathers lived their relatives and supposed descendants now flourish all the actual classes of the animal and vegetable kingdoms were represented in the tertiary faunas and floras and in nearly the same proportions and the same diversities as at present the faunas of what is now europe, asia america and australia, differed from each other much as they now differ: in fact--according to adolphe brongniart, whose statements we here condense[iii- ]--the inhabitants of these different regions appear for the most part to have acquired, before the close of the tertiary period, the characters which essentially distinguish their existing faunas. the eastern continent had then, as now, its great pachyderms, elephants, rhinoceros, hippopotamus; south america, its armadillos, sloths, and anteaters; australia, a crowd of marsupials; and the very strange birds of new zealand had predecessors of similar strangeness. everywhere the same geographical distribution as now, with a difference in the particular area, as respects the northern portion of the continents, answering to a warmer climate then than ours, such as allowed species of hippopotamus, rhinoceros, and elephant, to range even to the regions now inhabited by the reindeer and the musk-ox, and with the serious disturbing intervention of the glacial period within a comparatively recent time. let it be noted also that those tertiary species which have continued with little change down to our days are the marine animals of the lower grades, especially mollusca. their low organization, moderate sensibility, and the simple conditions of an existence in a medium like the ocean, not subject to great variation and incapable of sudden change, may well account for their continuance; while, on the other hand, the more intense, however gradual, climatic vicissitudes on land, which have driven all tropical and subtropical forms out of the higher latitudes and assigned to them their actual limits, would be almost sure to extinguish such huge and unwieldy animals as mastodons, mammoths, and the like, whose power of enduring altered circumstances must have been small. this general replacement of the tertiary species of a country by others so much like them is a noteworthy fact. the hypothesis of the independent creation of all species, irrespective of their antecedents, leaves this fact just as mysterious as is creation itself; that of derivation undertakes to account for it. whether it satisfactorily does so or not, it must be allowed that the facts well accord with that hypothesis. the same may be said of another conclusion, namely, that the geological succession of animals and plants appears to correspond in a general way with their relative standing or rank in a natural system of classification. it seems clear that, though no one of the grand types of the animal kingdom can be traced back farther than the rest, yet the lower classes long preceded the higher; that there has been on the whole a steady progression within each class and order; and that the highest plants and animals have appeared only in relatively modern times. it is only, however, in a broad sense that this generalization is now thought to hold good. it encounters many apparent exceptions, and sundry real ones. so far as the rule holds, all is as it should be upon an hypothesis of derivation. the rule has its exceptions. but, curiously enough, the most striking class of exceptions, if such they be, seems to us even more favorable to the doctrine of derivation than is the general rule of a pure and simple ascending gradation. we refer to what agassiz calls prophetic and synthetic types; for which the former name may suffice, as the difference between the two is evanescent. "it has been noticed," writes our great zoologist, "that certain types, which are frequently prominent among the representatives of past ages, combine in their structure peculiarities which at later periods are only observed separately in different, distinct types. sauroid fishes before reptiles, pterodactyles before birds, ichthyosauri before dolphins, etc. there are entire families, of nearly every class of animals, which in the state of their perfect development exemplify such prophetic relations. the sauroid fishes of the past geological ages are an example of this kind these fishes which preceded the appearance of reptiles present a combination of ichthyic and reptilian characters not to be found in the true members of this class, which form its bulk at present. the pterodactyles, which preceded the class of birds, and the ichthyosauri, which preceded the cetacea, are other examples of such prophetic types."--(agassiz, "contributions, essay on classification," p. .) now, these reptile-like fishes, of which gar-pikes are the living representatives, though of earlier appearance, are admittedly of higher rank than common fishes. they dominated until reptiles appeared, when they mostly gave place to (or, as the derivationists will insist, were resolved by divergent variation and natural selection into) common fishes, destitute of reptilian characters, and saurian reptiles--the intermediate grades, which, according to a familiar piscine saying, are "neither fish, flesh, nor good red-herring," being eliminated and extinguished by natural consequence of the struggle for existence which darwin so aptly portrays. and so, perhaps, of the other prophetic types. here type and antitype correspond. if these are true prophecies, we need not wonder that some who read them in agassiz's book will read their fulfillment in darwin's. note also, in this connection, that along with a wonderful persistence of type, with change of species, genera, orders, etc., from formation to formation, no species and no higher group which has once unequivocally died out ever afterward reappears. why is this, but that the link of generation has been sundered? why, on the hypothesis of independent originations, were not failing species recreated, either identically or with a difference, in regions eminently adapted to their well-being? to take a striking case. that no part of the world now offers more suitable conditions for wild horses and cattle than the pampas and other plains of south america, is shown by the facility with which they have there run wild and enormously multiplied, since introduced from the old world not long ago. there was no wild american stock. yet in the times of the mastodon and megatherium, at the dawn of the present period, wild-horses--certainly very much like the existing horse--roamed over those plains in abundance. on the principle of original and direct created adaptation of species to climate and other conditions, why were they not reproduced, when, after the colder intervening era, those regions became again eminently adapted to such animals? why, but because, by their complete extinction in south america, the line of descent was there utterly broken? upon the ordinary hypothesis, there is no scientific explanation possible of this series of facts, and of many others like them. upon the new hypothesis, "the succession of the same types of structure within the same areas during the later geological periods ceases to be mysterious, and is simply explained by inheritance." their cessation is failure of issue. along with these considerations the fact (alluded to on page ) should be remembered that, as a general thing, related species of the present age are geographically associated. the larger part of the plants, and still more of the animals, of each separate country are peculiar to it; and, as most species now flourish over the graves of their by-gone relatives of former ages, so they now dwell among or accessibly near their kindred species. here also comes in that general "parallelism between the order of succession of animals and plants in geological times, and the gradation among their living representatives" from low to highly organized, from simple and general to complex and specialized forms; also "the parallelism between the order of succession of animals in geological times and the changes their living representatives undergo during their embryological growth," as if the world were one prolonged gestation. modern science has much insisted on this parallelism, and to a certain extent is allowed to have made it out. all these things, which conspire to prove that the ancient and the recent forms of life "are somehow intimately connected together in one grand system," equally conspire to suggest that the connection is one similar or analogous to generation. surely no naturalist can be blamed for entering somewhat confidently upon a field of speculative inquiry which here opens so invitingly; nor need former premature endeavors and failures utterly dishearten him. all these things, it may naturally be said, go to explain the order, not the mode, of the incoming of species. but they all do tend to bring out the generalization expressed by mr. wallace in the formula that "every species has come into existence coincident both in time and space with preexisting closely-allied species." not, however, that this is proved even of existing species as a matter of general fact. it is obviously impossible to prove anything of the kind. but we must concede that the known facts strongly suggest such an inference. and--since species are only congeries of individuals, since every individual came into existence in consequence of preexisting individuals of the same sort, so leading up to the individuals with which the species began, and since the only material sequence we know of among plants and animals is that from parent to progeny--the presumption becomes exceedingly strong that the connection of the incoming with the preexisting species is a genealogical one. here, however, all depends upon the probability that mr. wallace's inference is really true. certainly it is not yet generally accepted; but a strong current is setting toward its acceptance. so long as universal cataclysms were in vogue, and all life upon the earth was thought to have been suddenly destroyed and renewed many times in succession, such a view could not be thought of. so the equivalent view maintained by agassiz, and formerly, we believe, by d'orbigny, that irrespectively of general and sudden catastrophes, or any known adequate physical cause, there has been a total depopulation at the close of each geological period or formation, say forty or fifty times or more, followed by as many independent great acts of creation, at which alone have species been originated, and at each of which a vegetable and an animal kingdom were produced entire and complete, full-fledged, as flourishing, as wide-spread, and populous, as varied and mutually adapted from the beginning as ever afterward--such a view, of course, supersedes all material connection between successive species, and removes even the association and geographical range of species entirely out of the domain of physical causes and of natural science. this is the extreme opposite of wallace's and darwin's view, and is quite as hypothetical. the nearly universal opinion, if we rightly gather it, manifestly is, that the replacement of the species of successive formations was not complete and simultaneous, but partial and successive; and that along the course of each epoch some species probably were introduced, and some, doubtless, became extinct. if all since the tertiary belongs to our present epoch, this is certainly true of it: if to two or more epochs, then the hypothesis of a total change is not true of them. geology makes huge demands upon time; and we regret to find that it has exhausted ours--that what we meant for the briefest and most general sketch of some geological considerations in favor of darwin's hypothesis has so extended as to leave no room for considering "the great facts of comparative anatomy and zoology" with which darwin's theory "very well accords," nor for indicating how "it admirably serves for explaining the unity of composition of all organisms, the existence of representative and rudimentary organs, and the natural series which genera and species compose." suffice it to say that these are the real strongholds of the new system on its theoretical side; that it goes far toward explaining both the physiological and the structural gradations and relations between the two kingdoms, and the arrangement of all their forms in groups subordinate to groups, all within a few great types; that it reads the riddle of abortive organs and of morphological conformity, of which no other theory has ever offered a scientific explanation, and supplies a ground for harmonizing the two fundamental ideas which naturalists and philosophers conceive to have ruled the organic world, though they could not reconcile them; namely, adaptation to purpose and conditions of existence, and unity of type. to reconcile these two undeniable principles is the capital problem in the philosophy of natural history; and the hypothesis which consistently does so thereby secures a great advantage. we all know that the arm and hand of a monkey, the foreleg and foot of a dog and of a horse, the wing of a bat, and the fin of a porpoise, are fundamentally identical; that the long neck of the giraffe has the same and no more bones than the short one of the elephant; that the eggs of surinam frogs hatch into tadpoles with as good tails for swimming as any of their kindred, although as tadpoles they never enter the water; that the guinea-pig is furnished with incisor teeth which it never uses, as it sheds them before birth; that embryos of mammals and birds have branchial slits and arteries running in loops, in imitation or reminiscence of the arrangement which is permanent in fishes; and that thousands of animals and plants have rudimentary organs which, at least in numerous cases, are wholly useless to their possessors, etc., etc. upon a derivative theory this morphological conformity is explained by community of descent; and it has not been explained in any other way. naturalists are constantly speaking of "related species," of the "affinity" of a genus or other group, and of "family resemblance"--vaguely conscious that these terms of kinship are something more than mere metaphors, but unaware of the grounds of their aptness. mr. darwin assures them that they have been talking derivative doctrine all their lives--as m. jourdain talked prose--without knowing it. if it is difficult and in many cases practically impossible to fix the limits of species, it is still more so to fix those of genera; and those of tribes and families are still less susceptible of exact natural circumscription. intermediate forms occur, connecting one group with another in a manner sadly perplexing to systematists, except to those who have ceased to expect absolute limitations in nature. all this blending could hardly fail to suggest a former material connection among allied forms, such as that which the hypothesis of derivation demands. here it would not be amiss to consider the general principle of gradation throughout organic nature--a principle which answers in a general way to the law of continuity in the inorganic world, or rather is so analogous to it that both may fairly be expressed by the leibnitzian axiom, natura non agit saltatim. as an axiom or philosophical principle, used to test modal laws or hypotheses, this in strictness belongs only to physics. in the investigation of nature at large, at least in the organic world, nobody would undertake to apply this principle as a test of the validity of any theory or supposed law. but naturalists of enlarged views will not fail to infer the principle from the phenomena they investigate--to perceive that the rule holds, under due qualifications and altered forms, throughout the realm of nature; although we do not suppose that nature in the organic world makes no distinct steps, but only short and serial steps--not infinitely fine gradations, but no long leaps, or few of them. to glance at a few illustrations out of many that present themselves. it would be thought that the distinction between the two organic kingdoms was broad and absolute. plants and animals belong to two very different categories, fulfill opposite offices and, as to the mass of them are so unlike that the difficulty of the ordinary observer would be to find points of comparison without entering into details which would fill an article, we may safely say that the difficulty with the naturalist is all the other way--that all these broad differences vanish one by one as we approach the lower confines of the two kingdoms, and that no absolute distinction whatever is now known between them. it is quite possible that the same organism may be both vegetable and animal, or may be first the one and then the other. if some organisms may be said to be at first vegetables and then animals, others, like the spores and other reproductive bodies of many of the lower algae, may equally claim to have first a characteristically animal, and then an unequivocally vegetable existence. nor is the gradation restricted to these simple organisms. it appears in general functions, as in that of reproduction, which is reducible to the same formula in both kingdoms, while it exhibits close approximations in the lower forms; also in a common or similar ground of sensibility in the lowest forms of both, a common faculty of effecting movements tending to a determinate end, traces of which pervade the vegetable kingdom--while, on the other hand, this indefinable principle, this vegetable "animula vagula, blandula, hospes comesque corporis," graduates into the higher sensitiveness of the lower class of animals. nor need we hesitate to recognize the fine gradations from simple sensitiveness and volition to the higher instinctive and to the other psychical manifestations of the higher brute animals. the gradation is undoubted, however we may explain it. again, propagation is of one mode in the higher animals, of two in all plants; but vegetative propagation, by budding or offshoots, extends through the lower grades of animals. in both kingdoms there may be separation of the offshoots, or indifference in this respect, or continued and organic union with the parent stock; and this either with essential independence of the offshoots, or with a subordination of these to a common whole; or finally with such subordination and amalgamation, along with specialization of function, that the same parts, which in other cases can be regarded only as progeny, in these become only members of an individual. this leads to the question of individuality, a subject quite too large and too recondite for present discussion. the conclusion of the whole matter, however, is, that individuality--that very ground of being as distinguished from thing--is not attained in nature at one leap. if anywhere truly exemplified in plants, it is only in the lowest and simplest, where the being is a structural unit, a single cell, member-less and organless, though organic--the same thing as those cells of which all the more complex plants are built up, and with which every plant and (structurally) every animal began its development. in the ascending gradation of the vegetable kingdom individuality is, so to say, striven after, but never attained; in the lower animals it is striven after with greater though incomplete success; it is realized only in animals of so high a rank that vegetative multiplication or offshoots are out of the question, where all parts are strictly members and nothing else, and all subordinated to a common nervous centre--is fully realized only in a conscious person. so, also, the broad distinction between reproduction by seeds or ova and propagation by buds, though perfect in some of the lowest forms of life, becomes evanescent in others; and even the most absolute law we know in the physiology of genuine reproduction--that of sexual cooperation--has its exceptions in both kingdoms in parthenogenesis, to which in the vegetable kingdom a most curious and intimate series of gradations leads. in plants, likewise, a long and finely graduated series of transitions leads from bisexual to unisexual blossoms; and so in various other respects. everywhere we may perceive that nature secures her ends, and makes her distinctions on the whole manifest and real but everywhere without abrupt breaks we need not wonder therefore that gradations between species and varieties should occur; the more so, since genera, tribes, and other groups into which the naturalist collocates species, are far from being always absolutely limited in nature, though they are necessarily represented to be so in systems. >from the necessity of the case, the classifications of the naturalist abruptly define where nature more or less blends. our systems are nothing, if not definite. they express differences, and some of the coarser gradations. but this evinces not their perfection, but their imperfection. even the best of them are to the system of nature what consecutive patches of the seven colors are to the rainbow. now the principle of gradation throughout organic nature may, of course, be interpreted upon other assumptions than those of darwin's hypothesis--certainly upon quite other than those of a materialistic philosophy, with which we ourselves have no sympathy. still we conceive it not only possible, but probable, that this gradation, as it has its natural ground, may yet have its scientific explanation. in any case, there is no need to deny that the general facts correspond well with an hypothesis like darwin's, which is built upon fine gradations. we have contemplated quite long enough the general presumptions in favor of an hypothesis of the derivation of species. we cannot forget, however, while for the moment we overlook, the formidable difficulties which all hypotheses of this class have to encounter, and the serious implications which they seem to involve. we feel, moreover, that darwin's particular hypothesis is exposed to some special objections. it requires no small strength of nerve steadily to conceive, not only of the diversification, but of the formation of the organs of an animal through cumulative variation and natural selection. think of such an organ as the eye, that most perfect of optical instruments, as so produced in the lower animals and perfected in the higher! a friend of ours, who accepts the new doctrine, confesses that for a long while a cold chill came over him whenever he thought of the eye. he has at length got over that stage of the complaint, and is now in the fever of belief, perchance to be succeeded by the sweating stage, during which sundry peccant humors may be eliminated from the system. for ourselves, we dread the chill, and have some misgivings about the consequences of the reaction. we find ourselves in the "singular position" acknowledged by pictet--that is, confronted with a theory which, although it can really explain much, seems inadequate to the heavy task it so boldly assumes, but which, nevertheless, appears better fitted than any other that has been broached to explain, if it be possible to explain, somewhat of the manner in which organized beings may have arisen and succeeded each other. in this dilemma we might take advantage of mr. darwin's candid admission, that he by no means expects to convince old and experienced people, whose minds are stocked with a multitude of facts all regarded during a long course of years from the old point of view. this is nearly our case. so, owning no call to a larger faith than is expected of us, but not prepared to pronounce the whole hypothesis untenable, under such construction as we should put upon it, we naturally sought to attain a settled conviction through a perusal of several proffered refutations of the theory. at least, this course seemed to offer the readiest way of bringing to a head the various objections to which the theory is exposed. on several accounts some of these opposed reviews especially invite examination. we propose, accordingly, to conclude our task with an article upon "darwin and his reviewers." iii the origin of species, like all origination, like the institution of any other natural state or order, is beyond our immediate ken. we see or may learn how things go on; we can only frame hypotheses as to how they began. two hypotheses divide the scientific world, very unequally, upon the origin of the existing diversity of the plants and animals which surround us. one assumes that the actual kinds are primordial; the other, that they are derivative. one, that all kinds originated supernaturally and directly as such, and have continued unchanged in the order of nature; the other, that the present kinds appeared in some sort of genealogical connection with other and earlier kinds, that they became what they now are in the course of time and in the order of nature. or, bringing in the word species, which is well defined as "the perennial succession of individuals," commonly of very like individuals--as a close corporation of individuals perpetuated by generation, instead of election--and reducing the question to mathematical simplicity of statement: species are lines of individuals coming down from the past and running on to the future; lines receding, therefore, from our view in either direction. within our limited observation they appear to be parallel lines, as a general thing neither approaching to nor diverging from each other. the first hypothesis assumes that they were parallel from the unknown beginning and will be to the unknown end. the second hypothesis assumes that the apparent parallelism is not real and complete, at least aboriginally, but approximate or temporary; that we should find the lines convergent in the past, if we could trace them far enough; that some of them, if produced back, would fall into certain fragments of lines, which have left traces in the past, lying not exactly in the same direction, and these farther back into others to which they are equally unparallel. it will also claim that the present lines, whether on the whole really or only approximately parallel, sometimes fork or send off branches on one side or the other, producing new lines (varieties), which run for a while, and for aught we know indefinitely when not interfered with, near and approximately parallel to the parent line. this claim it can establish; and it may also show that these close subsidiary lines may branch or vary again, and that those branches or varieties which are best adapted to the existing conditions may be continued, while others stop or die out. and so we may have the basis of a real theory of the diversification of species and here indeed, there is a real, though a narrow, established ground to build upon but as systems of organic nature, both doctrines are equally hypotheses, are suppositions of what there is no proof of from experience, assumed in order to account for the observed phenomena, and supported by such indirect evidence as can be had. even when the upholders of the former and more popular system mix up revelation with scientific discussion--which we decline to do--they by no means thereby render their view other than hypothetical. agreeing that plants and animals were produced by omnipotent fiat does not exclude the idea of natural order and what we call secondary causes. the record of the fiat--"let the earth bring forth grass, the herb yielding seed," etc., "and it was so;" "let the earth bring forth the living creature after his kind, cattle and creeping thing and beast of the earth after his kind, and it was so"--seems even to imply them. agreeing that they were formed of "the dust of the ground," and of thin air, only leads to the conclusion that the pristine individuals were corporeally constituted like existing individuals, produced through natural agencies. to agree that they were created "after their kinds" determines nothing as to what were the original kinds, nor in what mode, during what time, and in what connections it pleased the almighty to introduce the first individuals of each sort upon the earth. scientifically considered, the two opposing doctrines are equally hypothetical. the two views very unequally divide the scientific world; so that believers in "the divine right of majorities" need not hesitate which side to take, at least for the present. up to a time quite within the memory of a generation still on the stage, two hypotheses about the nature of light very unequally divided the scientific world. but the small minority has already prevailed: the emission theory has gone out; the undulatory or wave theory, after some fluctuation, has reached high tide, and is now the pervading, the fully-established system. there was an intervening time during which most physicists held their opinions in suspense. the adoption of the undulatory theory of light called for the extension of the same theory to heat, and this promptly suggested the hypothesis of a correlation, material connection, and transmutability of heat, light, electricity, magnetism, etc.; which hypothesis the physicists held in absolute suspense until very lately, but are now generally adopting. if not already established as a system, it promises soon to become so. at least, it is generally received as a tenable and probably true hypothesis. parallel to this, however less cogent the reasons, darwin and others, having shown it likely that some varieties of plants or animals have diverged in time into cognate species, or into forms as different as species, are led to infer that all species of a genus may have thus diverged from a common stock, and thence to suppose a higher community of origin in ages still farther back, and so on. following the safe example of the physicists, and acknowledging the fact of the diversification of a once homogeneous species into varieties, we may receive the theory of the evolution of these into species, even while for the present we hold the hypothesis of a further evolution in cool suspense or in grave suspicion. in respect to very many questions a wise man's mind rests long in a state neither of belief nor unbelief. but your intellectually short-sighted people are apt to be preternaturally clear-sighted, and to find their way very plain to positive conclusions upon one side or the other of every mooted question. in fact, most people, and some philosophers, refuse to hold questions in abeyance, however incompetent they may be to decide them. and, curiously enough, the more difficult, recondite, and perplexing, the questions or hypotheses are--such, for instance, as those about organic nature--the more impatient they are of suspense. sometimes, and evidently in the present case, this impatience grows out of a fear that a new hypothesis may endanger cherished and most important beliefs. impatience under such circumstances is not unnatural, though perhaps needless, and, if so, unwise. to us the present revival of the derivative hypothesis, in a more winning shape than it ever before had, was not unexpected. we wonder that any thoughtful observer of the course of investigation and of speculation in science should not have foreseen it, and have learned at length to take its inevitable coming patiently; the more so, as in darwin's treatise it comes in a purely scientific form, addressed only to scientific men. the notoriety and wide popular perusal of this treatise appear to have astonished the author even more than the book itself has astonished the reading world coming as the new presentation does from a naturalist of acknowledged character and ability and marked by a conscientiousness and candor which have not always been reciprocated we have thought it simply right to set forth the doctrine as fairly and as favorably as we could there are plenty to decry it and the whole theory is widely exposed to attack for the arguments on the other side we may look to the numerous adverse publications which darwin s volume has already called out and especially to those reviews which propose directly to refute it. taking various lines and reflecting very diverse modes of thought, these hostile critics may be expected to concentrate and enforce the principal objections which can be brought to bear against the derivative hypothesis in general, and darwin's new exposition of it in particular. upon the opposing side of the question we have read with attention-- . an article in the north american review for april last; . one in the christian examiner, boston, for may; . m. pictet's article in the bibliotheque universelle, which we have already made considerable use of, which seems throughout most able and correct, and which in tone and fairness is admirably in contrast with-- . the article in the edinburgh review for may, attributed--although against a large amount of internal presumptive evidence--to the most distinguished british comparative anatomist; . an article in the north british review for may; . prof. agassiz has afforded an early opportunity to peruse the criticisms he makes in the forthcoming third volume of his great work, by a publication of them in advance in the american journal of science for july. in our survey of the lively discussion which has been raised, it matters little how our own particular opinions may incline. but we may confess to an impression, thus far, that the doctrine of the permanent and complete immutability of species has not been established, and may fairly be doubted. we believe that species vary, and that "natural selection" works; but we suspect that its operation, like every analogous natural operation, may be limited by something else. just as every species by its natural rate of reproduction would soon completely fill any country it could live in, but does not, being checked by some other species or some other condition--so it may be surmised that variation and natural selection have their struggle and consequent check, or are limited by something inherent in the constitution of organic beings. we are disposed to rank the derivative hypothesis in its fullness with the nebular hypothesis, and to regard both as allowable, as not unlikely to prove tenable in spite of some strong objections, but as not therefore demonstrably true. those, if any there be, who regard the derivative hypothesis as satisfactorily proved, must have loose notions as to what proof is. those who imagine it can be easily refuted and cast aside, must, we think, have imperfect or very prejudiced conceptions of the facts concerned and of the questions at issue. we are not disposed nor prepared to take sides for or against the new hypothesis, and so, perhaps, occupy a good position from which to watch the discussion and criticise those objections which are seemingly inconclusive. on surveying the arguments urged by those who have undertaken to demolish the theory, we have been most impressed with a sense of their great inequality. some strike us as excellent and perhaps unanswerable; some, as incongruous with other views of the same writers; others, when carried out, as incompatible with general experience or general beliefs, and therefore as proving too much; still others, as proving nothing at all; so that, on the whole, the effect is rather confusing and disappointing. we certainly expected a stronger adverse case than any which the thoroughgoing opposers of darwin appear to have made out. wherefore, if it be found that the new hypothesis has grown upon our favor as we proceeded, this must be attributed not so much to the force of the arguments of the book itself as to the want of force of several of those by which it has been assailed. darwin's arguments we might resist or adjourn; but some of the refutations of it give us more concern than the book itself did. these remarks apply mainly to the philosophical and theological objections which have been elaborately urged, almost exclusively by the american reviewers. the north british reviewer, indeed, roundly denounces the book as atheistical, but evidently deems the case too clear for argument. the edinburgh reviewer, on the contrary, scouts all such objections--as well he may, since he records his belief in "a continuous creative operation," a constantly operating secondary creational law," through which species are successively produced; and he emits faint, but not indistinct, glimmerings of a transmutation theory of his own;[iii- ] so that he is equally exposed to all the philosophical objections advanced by agassiz, and to most of those urged by the other american critics, against darwin himself. proposing now to criticise the critics, so far as to see what their most general and comprehensive objections amount to, we must needs begin with the american reviewers, and with their arguments adduced to prove that a derivative hypothesis ought not to be true, or is not possible, philosophical, or theistic. it must not be forgotten that on former occasions very confident judgments have been pronounced by very competent persons, which have not been finally ratified. of the two great minds of the seventeenth century, newton and leibnitz, both profoundly religious as well as philosophical, one produced the theory of gravitation, the other objected to that theory that it was subversive of natural religion. the nebular hypothesis--a natural consequence of the theory of gravitation and of the subsequent progress of physical and astronomical discovery--has been denounced as atheistical even down to our own day. but it is now largely adopted by the most theistical natural philosophers as a tenable and perhaps sufficient hypothesis, and where not accepted is no longer objected to, so far as we know, on philosophical or religious grounds. the gist of the philosophical objections urged by the two boston reviewers against an hypothesis of the derivation of species--or at least against darwin's particular hypothesis-- is, that it is incompatible with the idea of any manifestation of design in the universe, that it denies final causes. a serious objection this, and one that demands very serious attention. the proposition, that things and events in nature were not designed to be so, if logically carried out, is doubtless tantamount to atheism. yet most people believe that some were designed and others were not, although they fall into a hopeless maze whenever they undertake to define their position. so we should not like to stigmatize as atheistically disposed a person who regards certain things and events as being what they are through designed laws (whatever that expression means), but as not themselves specially ordained, or who, in another connection, believes in general, but not in particular providence. we could sadly puzzle him with questions; but in return he might equally puzzle us. then, to deny that anything was specially designed to be what it is, is one proposition; while to deny that the designer supernaturally or immediately made it so, is another: though the reviewers appear not to recognize the distinction. also, "scornfully to repudiate" or to "sneer at the idea of any manifestation of design in the material universe,"[iii- ] is one thing; while to consider, and perhaps to exaggerate, the difficulties which attend the practical application of the doctrine of final causes to certain instances, is quite another thing: yet the boston reviewers, we regret to say, have not been duly regardful of the difference. whatever be thought of darwin's doctrine, we are surprised that he should be charged with scorning or sneering at the opinions of others, upon such a subject. perhaps darwin' s view is incompatible with final causes--we will consider that question presently-- but as to the examiner's charge, that he "sneers at the idea of any manifestation of design in the material universe," though we are confident that no misrepresentation was intended, we are equally confident that it is not at all warranted by the two passages cited in support of it. here are the passages: "if green woodpeckers alone had existed, or we did not know that there were many black and pied kinds, i dare say that we should have thought that the green color was a beautiful adaptation to hide this tree-frequenting bird from its enemies." "if our reason leads us to admire with enthusiasm a multitude of inimitable contrivances in nature, this same reason tells us, though we may easily err on both sides, that some contrivances are less perfect. can we consider the sting of the wasp or of the bee as perfect, which, when used against many attacking animals, cannot be withdrawn, owing to the backward serratures, and so inevitably causes the death of the insect by tearing out its viscera?" if the sneer here escapes ordinary vision in the detached extracts (one of them wanting the end of the sentence), it is, if possible, more imperceptible when read with the context. moreover, this perusal inclines us to think that the examiner has misapprehended the particular argument or object, as well as the spirit, of the author in these passages. the whole reads more naturally as a caution against the inconsiderate use of final causes in science, and an illustration of some of the manifold errors and absurdities which their hasty assumption is apt to involve--considerations probably equivalent to those which induced lord bacon to liken final causes to "vestal virgins." so, if any one, it is here bacon that "sitteth in the seat of the scornful." as to darwin, in the section from which the extracts were made, he is considering a subsidiary question, and trying to obviate a particular difficulty, but, we suppose, is wholly unconscious of denying "any manifestation of design in the material universe." he concludes the first sentence: --"and consequently that it was a character of importance, and might have been acquired through natural selection; as it is, i have no doubt that the color is due to some quite distinct cause, probably to sexual selection." after an illustration from the vegetable creation, darwin adds: "the naked skin on the head of a vulture is generally looked at as a direct adaptation for wallowing in putridity; and so it may be, or it may possibly be due to the direct action of putrid matter; but we should be very cautious in drawing any such inference, when we see that the skin on the head of the clean-feeding male turkey is likewise naked. the sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt they facilitate or may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals." all this, simply taken, is beyond cavil, unless the attempt to explain scientifically how any designed result is accomplished savors of impropriety. in the other place, darwin is contemplating the patent fact that "perfection here below" is relative, not absolute--and illustrating this by the circumstance that european animals, and especially plants, are now proving to be better adapted for new zealand than many of the indigenous ones--that "the correction for the aberration of light is said, on high authority, not to be quite perfect even in that most perfect organ, the eye." and then follows the second extract of the reviewer. but what is the position of the reviewer upon his own interpretation of these passages? if he insists that green woodpeckers were specifically created so in order that they might be less liable to capture, must he not equally hold that the black and pied ones were specifically made of these colors in order that they might be more liable to be caught? and would an explanation of the mode in which those woodpeckers came to be green, however complete, convince him that the color was undesigned? as to the other illustration, is the reviewer so complete an optimist as to insist that the arrangement and the weapon are wholly perfect (quoad the insect) the normal use of which often causes the animal fatally to injure or to disembowel itself? either way it seems to us that the argument here, as well as the insect, performs hari-kari. the examiner adds: "we should in like manner object to the word favorable, as implying that some species are placed by the creator under unfavorable circumstances, at least under such as might be advantageously modified." but are not many individuals and some races of men placed by the creator "under unfavorable circumstances, at least under such as might be advantageously modified?" surely these reviewers must be living in an ideal world, surrounded by "the faultless monsters which our world ne'er saw," in some elysium where imperfection and distress were never heard of! such arguments resemble some which we often hear against the bible, holding that book responsible as if it originated certain facts on the shady side of human nature or the apparently darker lines of providential dealing, though the facts are facts of common observation and have to be confronted upon any theory. the north american reviewer also has a world of his own--just such a one as an idealizing philosopher would be apt to devise--that is, full of sharp and absolute distinctions: such, for instance, as the "absolute invariableness of instinct;" an absolute want of intelligence in any brute animal; and a complete monopoly of instinct by the brute animals, so that this "instinct is a great matter" for them only, since it sharply and perfectly distinguishes this portion of organic nature from the vegetable kingdom on the one hand and from man on the other: most convenient views for argumentative purposes, but we suppose not borne out in fact. in their scientific objections the two reviewers take somewhat different lines; but their philosophical and theological arguments strikingly coincide. they agree in emphatically asserting that darwin's hypothesis of the origination of species through variation and natural selection "repudiates the whole doctrine of final causes," and "all indication of design or purpose in the organic world . . . is neither more nor less than a formal denial of any agency beyond that of a blind chance in the developing or perfecting of the organs or instincts of created beings. . . . it is in vain that the apologists of this hypothesis might say that it merely attributes a different mode and time to the divine agency--that all the qualities subsequently appearing in their descendants must have been implanted, and have remained latent in the original pair." such a view, the examiner declares, "is nowhere stated in this book, and would be, we are sure, disclaimed by the author." we should like to be informed of the grounds of this sureness. the marked rejection of spontaneous generation--the statement of a belief that all animals have descended from four or five progenitors, and plants from an equal or lesser number, or, perhaps, if constrained to it by analogy, "from some one primordial form into which life was first breathed"--coupled with the expression, "to my mind it accords better with what we know of the laws impressed on matter by the creator, that the production and extinction of the past and present inhabitants of the world should have been due to secondary causes," than "that each species has been independently created"--these and similar expressions lead us to suppose that the author probably does accept the kind of view which the examiner is sure he would disclaim. at least, we charitably see nothing in his scientific theory to hinder his adoption of lord bacon's "confession of faith" in this regard-- "that, notwithstanding god hath rested and ceased from creating, yet, nevertheless, he doth accomplish and fulfill his divine will in all things, great and small, singular and general, as fully and exactly by providence as he could by miracle and new creation, though his working be not immediate and direct, but by compass; not violating nature, which is his own law upon the creature." however that may be, it is undeniable that mr. darwin has purposely been silent upon the philosophical and theological applications of his theory. this reticence, under the circumstances, argues design, and raises inquiry as to the final cause or reason why. here, as in higher instances, confident as we are that there is a final cause, we must not be overconfident that we can infer the particular or true one. perhaps the author is more familiar with natural-historical than with philosophical inquiries, and, not having decided which particular theory about efficient cause is best founded, he meanwhile argues the scientific questions concerned--all that relates to secondary causes--upon purely scientific grounds, as he must do in any case. perhaps, confident, as he evidently is, that his view will finally be adopted, he may enjoy a sort of satisfaction in hearing it denounced as sheer atheism by the inconsiderate, and afterward, when it takes its place with the nebular hypothesis and the like, see this judgment reversed, as we suppose it would be in such event. whatever mr. darwin's philosophy may be, or whether he has any, is a matter of no consequence at all, compared with the important questions, whether a theory to account for the origination and diversification of animal and vegetable forms through the operation of secondary causes does or does not exclude design; and whether the establishment by adequate evidence of darwin's particular theory of diversification through variation and natural selection would essentially alter the present scientific and philosophical grounds for theistic views of nature. the unqualified affirmative judgment rendered by the two boston reviewers, evidently able and practised reasoners, "must give us pause." we hesitate to advance our conclusions in opposition to theirs. but, after full and serious consideration, we are constrained to say that, in our opinion, the adoption of a derivative hypothesis, and of darwin's particular hypothesis, if we understand it, would leave the doctrines of final causes, utility, and special design, just where they were before. we do not pretend that the subject is not environed with difficulties. every view is so environed; and every shifting of the view is likely, if it removes some difficulties, to bring others into prominence. but we cannot perceive that darwin's theory brings in any new kind of scientific difficulty, that is, any with which philosophical naturalists were not already familiar. since natural science deals only with secondary or natural causes, the scientific terms of a theory of derivation of species--no less than of a theory of dynamics--must needs be the same to the theist as to the atheist. the difference appears only when the inquiry is carried up to the question of primary cause--a question which belongs to philosophy. wherefore, darwin 's reticence about efficient cause does not disturb us. he considers only the scientific questions. as already stated, we think that a theistic view of nature is implied in his book, and we must charitably refrain from suggesting the contrary until the contrary is logically deduced from his premises. if, however, he anywhere maintains that the natural causes through which species are diversified operate without an ordaining and directing intelligence, and that the orderly arrangements and admirable adaptations we see all around us are fortuitous or blind, undesigned results--that the eye, though it came to see, was not designed for seeing, nor the hand for handling--then, we suppose, he is justly chargeable with denying, and very needlessly denying, all design in organic nature; otherwise, we suppose not. why, if darwin's well-known passage about the eye[iii- ] equivocal though some of the language be--does not imply ordaining and directing intelligence, then he refutes his own theory as effectually as any of his opponents are likely to do. he asks: "may we not believe that [under variation proceeding long enough, generation multiplying the better variations times enough, and natural selection securing the improvements] a living optical instrument might be thus formed as superior to one of glass as the works of the creator are to those of man?" this must mean one of two things: either that the living instrument was made and perfected under (which is the same thing as by) an intelligent first cause, or that it was not. if it was, then theism is asserted; and as to the mode of operation, how do we know, and why must we believe, that, fitting precedent forms being in existence, a living instrument (so different from a lifeless manufacture) would be originated and perfected in any other way, or that this is not the fitting way? if it means that it was not, if he so misuses words that by the creator he intends an unintelligent power, undirected force, or necessity, then he has put his case so as to invite disbelief in it. for then blind forces have produced not only manifest adaptions of means to specific ends--which is absurd enough--but better adjusted and more perfect instruments or machines than intellect (that is, human intellect) can contrive and human skill execute--which no sane person will believe. on the other hand, if darwin even admits--we will not say adopts--the theistic view, he may save himself much needless trouble in the endeavor to account for the absence of every sort of intermediate form. those in the line between one species and another supposed to be derived from it he may be bound to provide; but as to "an infinite number of other varieties not intermediate, gross, rude, and purposeless, the unmeaning creations of an unconscious cause," born only to perish, which a relentless reviewer has imposed upon his theory--rightly enough upon the atheistic alternative--the theistic view rids him at once of this "scum of creation." for, as species do not now vary at all times and places and in all directions, nor produce crude, vague, imperfect, and useless forms, there is no reason for supposing that they ever did. good-for-nothing monstrosities, failures of purpose rather than purposeless, indeed, sometimes occur; but these are just as anomalous and unlikely upon darwin's theory as upon any other. for his particular theory is based, and even over-strictly insists, upon the most universal of physiological laws, namely, that successive generations shall differ only slightly, if at all, from their parents; and this effectively excludes crude and impotent forms. wherefore, if we believe that the species were designed, and that natural propagation was designed, how can we say that the actual varieties of the species were not equally designed? have we not similar grounds for inferring design in the supposed varieties of species, that we have in the case of the supposed species of a genus? when a naturalist comes to regard as three closely related species what he before took to be so many varieties of one species how has he thereby strengthened our conviction that the three forms are designed to have the differences which they actually exhibit? wherefore so long as gradatory, orderly, and adapted forms in nature argue design, and at least while the physical cause of variation is utterly unknown and mysterious, we should advise mr. darwin to assume in the philosophy of his hypothesis that variation has been led along certain beneficial lines. streams flowing over a sloping plain by gravitation (here the counterpart of natural selection) may have worn their actual channels as they flowed; yet their particular courses may have been assigned; and where we see them forming definite and useful lines of irrigation, after a manner unaccountable on the laws of gravitation and dynamics, we should believe that the distribution was designed. to insist, therefore, that the new hypothesis of the derivative origin of the actual species is incompatible with final causes and design, is to take a position which we must consider philosophically untenable. we must also regard it as highly unwise and dangerous, in the present state and present prospects of physical and physiological science. we should expect the philosophical atheist or skeptic to take this ground; also, until better informed, the unlearned and unphilosophical believer; but we should think that the thoughtful theistic philosopher would take the other side. not to do so seems to concede that only supernatural events can be shown to be designed, which no theist can admit--seems also to misconceive the scope and meaning of all ordinary arguments for design in nature. this misconception is shared both by the reviewers and the reviewed. at least, mr. darwin uses expressions which imply that the natural forms which surround us, because they have a history or natural sequence, could have been only generally, but not particularly designed--a view at once superficial and contradictory; whereas his true line should be, that his hypothesis concerns the order and not the cause, the how and not the why of the phenomena, and so leaves the question of design just where it was before. to illustrate this from the theist's point of view: transfer the question for a moment from the origination of species to the origination of individuals, which occurs, as we say, naturally. because natural, that is, "stated, fixed, or settled," is it any the less designed on that account? we acknowledge that god is our maker--not merely the originator of the race, but our maker as individuals--and none the less so because it pleased him to make us in the way of ordinary generation. if any of us were born unlike our parents and grandparents, in a slight degree, or in whatever degree, would the case be altered in this regard? the whole argument in natural theology proceeds upon the ground that the inference for a final cause of the structure of the hand and of the valves in the veins is just as valid now, in individuals produced through natural generation, as it would have been in the case of the first man, supernaturally created. why not, then, just as good even on the supposition of the descent of men from chimpanzees and gorillas, since those animals possess these same contrivances? or, to take a more supposable case: if the argument from structure to design is convincing when drawn from a particular animal, say a newfoundland dog, and is not weakened by the knowledge that this dog came from similar parents, would it be at all weakened if, in tracing his genealogy, it were ascertained that he was a remote descendant of the mastiff or some other breed, or that both these and other breeds came (as is suspected) from some wolf? if not, how is the argument for design in the structure of our particular dog affected by the supposition that his wolfish progenitor came from a post-tertiary wolf, perhaps less unlike an existing one than the dog in question is to some other of the numerous existing races of dogs, and that this post-tertiary came from an equally or more different tertiary wolf? and if the argument from structure to design is not invalidated by our present knowledge that our individual dog was developed from a single organic cell, how is it invalidated by the supposition of an analogous natural descent, through a long line of connected forms, from such a cell, or from some simple animal, existing ages before there were any dogs? again, suppose we have two well-known and apparently most decidedly different animals or plants, a and d, both presenting, in their structure and in their adaptations to the conditions of existence, as valid and clear evidence of design as any animal or plant ever presented: suppose we have now discovered two intermediate species, b and c, which make up a series with equable differences from a to d. is the proof of design or final cause in a and d, whatever it amounted to, at all weakened by the discovery of the intermediate forms? rather does not the proof extend to the intermediate species, and go to show that all four were equally designed? suppose, now, the number of intermediate forms to be much increased, and therefore the gradations to be closer yet--as close as those between the various sorts of dogs, or races of men, or of horned cattle: would the evidence of design, as shown in the structure of any of the members of the series, be any weaker than it was in the case of a and d? whoever contends that it would be, should likewise maintain that the origination of individuals by generation is incompatible with design, or an impossibility in nature. we might all have confidently thought the latter, antecedently to experience of the fact of reproduction. let our experience teach us wisdom. these illustrations make it clear that the evidence of design from structure and adaptation is furnished complete by the individual animal or plant itself, and that our knowledge or our ignorance of the history of its formation or mode of production adds nothing to it and takes nothing away. we infer design from certain arrangements and results; and we have no other way of ascertaining it. testimony, unless infallible, cannot prove it, and is out of the question here. testimony is not the appropriate proof of design: adaptation to purpose is. some arrangements in nature appear to be contrivances, but may leave us in doubt. many others, of which the eye and the hand are notable examples, compel belief with a force not appreciably short of demonstration. clearly to settle that such as these must have been designed goes far toward proving that other organs and other seemingly less explicit adaptations in nature must also have been designed, and clinches our belief, from manifold considerations, that all nature is a preconcerted arrangement, a manifested design. a strange contradiction would it be to insist that the shape and markings of certain rude pieces of flint, lately found in drift-deposits, prove design, but that nicer and thousand-fold more complex adaptations to use in animals and vegetables do not a fortiori argue design. we could not affirm that the arguments for design in nature are conclusive to all minds. but we may insist, upon grounds already intimated, that, whatever they were good for before darwin's book appeared, they are good for now. to our minds the argument from design always appeared conclusive of the being and continued operation of an intelligent first cause, the ordainer of nature; and we do not see that the grounds of such belief would be disturbed or shifted by the adoption of darwin's hypothesis. we are not blind to the philosophical difficulties which the thoroughgoing implication of design in nature has to encounter, nor is it our vocation to obviate them it suffices us to know that they are not new nor peculiar difficulties--that, as darwin's theory and our reasonings upon it did not raise these perturbing spirits, they are not bound to lay them. meanwhile, that the doctrine of design encounters the very same difficulties in the material that it does in the moral world is just what ought to be expected. so the issue between the skeptic and the theist is only the old one, long ago argued out--namely, whether organic nature is a result of design or of chance. variation and natural selection open no third alternative; they concern only the question how the results, whether fortuitous or designed, may have been brought about. organic nature abounds with unmistakable and irresistible indications of design, and, being a connected and consistent system, this evidence carries the implication of design throughout the whole. on the other hand, chance carries no probabilities with it, can never be developed into a consistent system, but, when applied to the explanation of orderly or beneficial results, heaps up improbabilities at every step beyond all computation. to us, a fortuitous cosmos is simply inconceivable. the alternative is a designed cosmos. it is very easy to assume that, because events in nature are in one sense accidental, and the operative forces which bring them to pass are themselves blind and unintelligent (physically considered, all forces are), therefore they are undirected, or that he who describes these events as the results of such forces thereby assumes that they are undirected. this is the assumption of the boston reviewers, and of mr. agassiz, who insists that the only alternative to the doctrine, that all organized beings were supernaturally created just as they are, is, that they have arisen spontaneously through the omnipotence of matter.[iii- ] as to all this, nothing is easier than to bring out in the conclusion what you introduce in the premises. if you import atheism into your conception of variation and natural selection, you can readily exhibit it in the result. if you do not put it in, perhaps there need be none to come out. while the mechanician is considering a steamboat or locomotive-engine as a material organism, and contemplating the fuel, water, and steam, the source of the mechanical forces, and how they operate, he may not have occasion to mention the engineer. but, the orderly and special results accomplished, the why the movements are in this or that particular direction, etc., is inexplicable without him. if mr. darwin believes that the events which he supposes to have occurred and the results we behold were undirected and undesigned, or if the physicist believes that the natural forces to which he refers phenomena are uncaused and undirected, no argument is needed to show that such belief is atheism. but the admission of the phenomena and of these natural processes and forces does not necessitate any such belief, nor even render it one whit less improbable than before. surely, too, the accidental element may play its part in nature without negativing design in the theist's view. he believes that the earth's surface has been very gradually prepared for man and the existing animal races, that vegetable matter has through a long series of generations imparted fertility to the soil in order that it may support its present occupants, that even beds of coal have been stored up for man's benefit yet what is more accidental, and more simply the consequence of physical agencies than the accumulation of vegetable matter in a peat bog and its transformation into coal? no scientific person at this day doubts that our solar system is a progressive development, whether in his conception he begins with molten masses, or aeriform or nebulous masses, or with a fluid revolving mass of vast extent, from which the specific existing worlds have been developed one by one what theist doubts that the actual results of the development in the inorganic worlds are not merely compatible with design but are in the truest sense designed re suits? not mr. agassiz, certainly, who adopts a remarkable illustration of design directly founded on the nebular hypothesis drawing from the position and times of the revolution of the world, so originated direct evidence that the physical world has been ordained in conformity with laws which obtain also among living beings but the reader of the interesting exposition[iii- ] will notice that the designed result has been brought to pass through what, speaking after the manner of men, might be called a chapter of accidents. a natural corollary of this demonstration would seem to be, that a material connection between a series of created things--such as the development of one of them from another, or of all from a common stock--is highly compatible with their intellectual connection, namely, with their being designed and directed by one mind. yet upon some ground which is not explained, and which we are unable to conjecture, mr. agassiz concludes to the contrary in the organic kingdoms, and insists that, because the members of such a series have an intellectual connection, "they cannot be the result of a material differentiation of the objects themselves,"[iii- ] that is, they cannot have had a genealogical connection. but is there not as much intellectual connection between the successive generations of any species as there is between the several species of a genus, or the several genera of an order? as the intellectual connection here is realized through the material connection, why may it not be so in the case of species and genera? on all sides, therefore, the implication seems to be quite the other way. returning to the accidental element, it is evident that the strongest point against the compatibility of darwin's hypothesis with design in nature is made when natural selection is referred to as picking out those variations which are improvements from a vast number which are not improvements, but perhaps the contrary, and therefore useless or purposeless, and born to perish. but even here the difficulty is not peculiar; for nature abounds with analogous instances. some of our race are useless, or worse, as regards the improvement of mankind; yet the race may be designed to improve, and may be actually improving. or, to avoid the complication with free agency--the whole animate life of a country depends absolutely upon the vegetation, the vegetation upon the rain. the moisture is furnished by the ocean, is raised by the sun's heat from the ocean's surface, and is wafted inland by the winds. but what multitudes of raindrops fall back into the ocean--are as much without a final cause as the incipient varieties which come to nothing! does it therefore follow that the rains which are bestowed upon the soil with such rule and average regularity were not designed to support vegetable and animal life? consider, likewise, the vast proportion of seeds and pollen, of ova and young--a thousand or more to one--which come to nothing, and are therefore purposeless in the same sense, and only in the same sense, as are darwin's unimproved and unused slight variations. the world is full of such cases; and these must answer the argument--for we cannot, except by thus showing that it proves too much. finally, it is worth noticing that, though natural selection is scientifically explicable, variation is not. thus far the cause of variation, or the reason why the offspring is sometimes unlike the parents, is just as mysterious as the reason why it is generally like the parents. it is now as inexplicable as any other origination; and, if ever explained, the explanation will only carry up the sequence of secondary causes one step farther, and bring us in face of a somewhat different problem, but which will have the same element of mystery that the problem of variation has now. circumstances may preserve or may destroy the variations man may use or direct them but selection whether artificial or natural no more originates them than man originates the power which turns a wheel when he dams a stream and lets the water fall upon it the origination of this power is a question about efficient cause. the tendency of science in respect to this obviously is not toward the omnipotence of matter, as some suppose, but to ward the omnipotence of spirit. so the real question we come to is as to the way in which we are to conceive intelligent and efficient cause to be exerted, and upon what exerted. are we bound to suppose efficient cause in all cases exerted upon nothing to evoke something into existence--and this thousands of times repeated, when a slight change in the details would make all the difference between successive species? why may not the new species, or some of them, be designed diversifications of the old? there are, perhaps, only three views of efficient cause which may claim to be both philosophical and theistic: . the view of its exertion at the beginning of time, endowing matter and created things with forces which do the work and produce the phenomena. . this same view, with the theory of insulated interpositions, or occasional direct action, engrafted upon it--the view that events and operations in general go on in virtue simply of forces communicated at the first, but that now and then, and only now and then, the deity puts his hand directly to the work. . the theory of the immediate, orderly, and constant, however infinitely diversified, action of the intelligent efficient cause. it must be allowed that, while the third is preeminently the christian view, all three are philosophically compatible with design in nature. the second is probably the popular conception. perhaps most thoughtful people oscillate from the middle view toward the first or the third--adopting the first on some occasions, the third on others. those philosophers who like and expect to settle all mooted questions will take one or the other extreme. the examiner inclines toward, the north american reviewer fully adopts, the third view, to the logical extent of maintaining that "the origin of an individual, as well as the origin of a species or a genus, can be explained only by the direct action of an intelligent creative cause." to silence his critics, this is the line for mr. darwin to take; for it at once and completely relieves his scientific theory from every theological objection which his reviewers have urged against it. at present we suspect that our author prefers the first conception, though he might contend that his hypothesis is compatible with either of the three. that it is also compatible with an atheistic or pantheistic conception of the universe, is an objection which, being shared by all physical, and some ethical or moral science, cannot specially be urged against darwin's system. as he rejects spontaneous generation, and admits of intervention at the beginning of organic life, and probably in more than one instance, he is not wholly excluded from adopting the middle view, although the interventions he would allow are few and far back. yet one interposition admits the principle as well as more. interposition presupposes particular necessity or reason for it, and raises the question, when and how often it may have been necessary. it might be the natural supposition, if we had only one set of species to account for, or if the successive inhabitants of the earth had no other connections or resemblances than those which adaptation to similar conditions, which final causes in the narrower sense, might explain. but if this explanation of organic nature requires one to "believe that, at innumerable periods in the earth's history, certain elemental atoms have been commanded suddenly to flash into living tissues," and this when the results are seen to be strictly connected and systematic, we cannot wonder that such interventions should at length be considered, not as interpositions or interferences, but rather--to use the reviewer's own language--as "exertions so frequent and beneficent that we come to regard them as the ordinary action of him who laid the foundation of the earth, and without whom not a sparrow falleth to the ground."[iii- ] what does the difference between mr. darwin and his reviewer now amount to? if we say that according to one view the origination of species is natural, according to the other miraculous, mr. darwin agrees that "what is natural as much requires and presupposes an intelligent mind to render it so-- that is, to effect it continually or at stated times--as what is supernatural does to effect it for once."[iii- ] he merely inquires into the form of the miracle, may remind us that all recorded miracles (except the primal creation of matter) were transformations or actions in and upon natural things, and will ask how many times and how frequently may the origination of successive species be repeated before the supernatural merges in the natural. in short, darwin maintains that the origination of a species, no less than that of an individual, is natural; the reviewer, that the natural origination of an individual, no less than the origination of a species, requires and presupposes divine power. a fortiori, then, the origination of a variety requires and presupposes divine power. and so between the scientific hypothesis of the one and the philosophical conception of the other no contrariety remains. and so, concludes the north american reviewer, "a proper view of the nature of causation places the vital doctrine of the being and the providence of a god on ground that can never be shaken."[iii- ] a worthy conclusion, and a sufficient answer to the denunciations and arguments of the rest of the article, so far as philosophy and natural theology are concerned. if a writer must needs use his own favorite dogma as a weapon with which to give coup de grace to a pernicious theory, he should be careful to seize his edge-tool by the handle, and not by the blade. we can barely glance at a subsidiary philosophical objection of the north american reviewer, which the examiner also raises, though less explicitly. like all geologists, mr. darwin draws upon time in the most unlimited manner. he is not peculiar in this regard. mr. agassiz tells us that the conviction is "now universal, among well-informed naturalists, that this globe has been in existence for innumerable ages, and that the length of time elapsed since it first became inhabited cannot be counted in years;" pictet, that the imagination refuses to calculate the immense number of years and of ages during which the faunas of thirty or more epochs have succeeded one another, and developed their long succession of generations. now, the reviewer declares that such indefinite succession of ages is "virtually infinite," "lacks no characteristic of eternity except its name," at least, that "the difference between such a conception and that of the strictly infinite, if any, is not appreciable." but infinity belongs to metaphysics. therefore, he concludes, darwin supports his theory, not by scientific but by metaphysical evidence; his theory is "essentially and completely metaphysical in character, resting altogether upon that idea of â��the infinite' which the human mind can neither put aside nor comprehend."[iii- ] and so a theory which will be generally regarded as much too physical is transferred by a single syllogism to metaphysics. well, physical geology must go with it: for, even on the soberest view, it demands an indefinitely long time antecedent to the introduction of organic life upon our earth. a fortiori is physical astronomy a branch of metaphysics, demanding, as it does, still larger "instalments of infinity," as the reviewer calls them, both as to time and number. moreover, far the greater part of physical inquiries now relate to molecular actions, which, a distinguished natural philosopher informs us, "we have to regard as the results of an infinite number of in finitely small material particles, acting on each other at infinitely small distances"--a triad of infinities--and so physics becomes the most metaphysical of sciences. verily, if this style of reasoning is to prevail-- "thinking is but an idle waste of thought, and naught is everything, and everything is naught." the leading objection of mr. agassiz is likewise of a philosophical character. it is, that species exist only "as categories of thought"--that, having no material existence, they can have had no material variation, and no material community of origin. here the predication is of species in the subjective sense, the inference in the objective sense. reduced to plain terms, the argument seems to be: species are ideas; therefore the objects from which the idea is derived cannot vary or blend, and cannot have had a genealogical connection. the common view of species is, that, although they are generalizations, yet they have a direct objective ground in nature, which genera, orders, etc., have not. according to the succinct definition of jussieu--and that of linnaeus is identical in meaning--a species is the perennial succession of similar individuals in continued generations. the species is the chain of which the individuals are the links. the sum of the genealogically-connected similar individuals constitutes the species, which thus has an actuality and ground of distinction not shared by genera and other groups which were not supposed to be genealogically connected. how a derivative hypothesis would modify this view, in assigning to species only a temporary fixity, is obvious. yet, if naturalists adopt that hypothesis, they will still retain jussieu's definition, which leaves untouched the question as to how and when the "perennial successions" were established. the practical question will only be, how much difference between two sets of individuals entitles them to rank under distinct species? and that is the practical question now, on whatever theory. the theoretical question is--as stated at the beginning of this article--whether these specific lines were always as distinct as now. mr. agassiz has "lost no opportunity of urging the idea that, while species have no material existence, they yet exist as categories of thought in the same way [and only in the same way] as genera, families, orders, classes," etc. he "has taken the ground that all the natural divisions in the animal kingdom are primarily distinct, founded upon different categories of characters, and that all exist in the same way, that is, as categories of thought, embodied in individual living forms. i have attempted to show that branches in the animal kingdom are founded upon different plans of structure, and for that very reason have embraced from the beginning representatives between which there could be no community of origin; that classes are founded upon different modes of execution of these plans, and therefore they also embrace representatives which could have no community of origin; that orders represent the different degrees of complication in the mode of execution of each class, and therefore embrace representatives which could not have a community of origin any more than the members of different classes or branches; that families are founded upon different patterns of form, and embrace, representatives equally independent in their origin; that genera are founded upon ultimate peculiarities of structure, embracing representatives which, from the very nature of their peculiarities, could have no community of origin; and that, finally, species are based upon relations--and proportions that exclude, as much as all the preceding distinctions, the idea of a common descent. "as the community of characters among the beings belonging to these different categories arises from the intellectual connection which shows them to be categories of thought, they cannot be the result of a gradual material differentiation of the objects themselves. the argument on which these views are founded may be summed up in the following few words: species, genera, families, etc., exist as thoughts, individuals as facts."[iii- ] an ingenious dilemma caps the argument: "it seems to me that there is much confusion of ideas in the general statement of the variability of species so often repeated lately. if species do not exist at all, as the supporters of the transmutation theory maintain, how can they vary? and if individuals alone exist, how can the differences which may be observed among them prove the variability of species?" now, we imagine that mr. darwin need not be dangerously gored by either horn of this curious dilemma. although we ourselves cherish old-fashioned prejudices in favor of the probable permanence, and therefore of a more stable objective ground of species, yet we agree--and mr. darwin will agree fully with mr. agassiz--that species, and he will add varieties, "exist as categories of thought," that is, as cognizable distinctions--which is all that we can make of the phrase here, whatever it may mean in the aristotelian metaphysics. admitting that species are only categories of thought, and not facts or things, how does this prevent the individuals, which are material things, from having varied in the course of time, so as to exemplify the present almost innumerable categories of thought, or embodiments of divine thought in material forms, or--viewed on the human side--in forms marked with such orderly and graduated resemblances and differences as to suggest to our minds the idea of species, genera, orders, etc., and to our reason the inference of a divine original? we have no clear idea how mr. agassiz intends to answer this question, in saying that branches are founded upon different plans of structure, classes upon different mode of execution of these plans, orders on different degrees of complication in the mode of execution, families upon different patterns of form, genera upon ultimate peculiarities of structure, and species upon relations and proportions. that is, we do not perceive how these several "categories of thought" exclude the possibility or the probability that the individuals which manifest or suggest the thoughts had an ultimate community of origin. moreover, mr. darwin might insinuate that the particular philosophy of classification upon which this whole argument reposes is as purely hypothetical and as little accepted as is his own doctrine. if both are pure hypotheses, it is hardly fair or satisfactory to extinguish the one by the other. if there is no real contradiction between them, nothing is gained by the attempt. as to the dilemma propounded, suppose we try it upon that category of thought which we call chair. this is a genus, comprising a common chair (sella vulgaris), arm or easy chair (s. cathedra), the rocking-chair (s. oscillans)--widely distributed in the united states--and some others, each of which has sported, as the gardeners say, into many varieties. but now, as the genus and the species have no material existence, how can they vary? if only individual chairs exist, how can the differences which may be observed among them prove the variability of the species? to which we reply by asking, which does the question refer to, the category of thought, or the individual embodiment? if the former, then we would remark that our categories of thought vary from time to time in the readiest manner. and, although the divine thoughts are eternal, yet they are manifested to us in time and succession, and by their manifestation only can we know them, how imperfectly! allowing that what has no material existence can have had no material connection or variation, we should yet infer that what has intellectual existence and connection might have intellectual variation; and, turning to the individuals, which represent the species, we do not see how all this shows that they may not vary. observation shows us that they do. wherefore, taught by fact that successive individuals do vary, we safely infer that the idea must have varied, and that this variation of the individual representatives proves the variability of the species, whether objectively or subjectively regarded. each species or sort of chair, as we have said, has its varieties, and one species shades off by gradations into another. and--note it well--these numerous and successively slight variations and gradations, far from suggesting an accidental origin to chairs and to their forms, are very proofs of design. again, edifice is a generic category of thought. egyptian, grecian, byzantine, and gothic buildings are well-marked species, of which each individual building of the sort is a material embodiment. now, the question is, whether these categories or ideas may not have been evolved, one from another in succession, or from some primal, less specialized, edificial category. what better evidence for such hypothesis could we have than the variations and grades which connect these species with each other? we might extend the parallel, and get some good illustrations of natural selection from the history of architecture, and the origin of the different styles under different climates and conditions. two considerations may qualify or limit the comparison. one, that houses do not propagate, so as to produce continuing lines of each sort and variety; but this is of small moment on agassiz's view, he holding that genealogical connection is not of the essence of a species at all. the other, that the formation and development of the ideas upon which human works proceed are gradual; or, as the same great naturalist well states it, "while human thought is consecutive, divine thought is simultaneous." but we have no right to affirm this of divine action. we must close here. we meant to review some of the more general scientific objections which we thought not altogether tenable. but, after all, we are not so anxious just now to know whether the new theory is well founded on facts, as whether it would be harmless if it were. besides, we feel quite unable to answer some of these objections, and it is pleasanter to take up those which one thinks he can. among the unanswerable, perhaps the weightiest of the objections, is that of the absence, in geological deposits, of vestiges of the intermediate forms which the theory requires to have existed. here all that mr. darwin can do is to insist upon the extreme imperfection of the geological record and the uncertainty of negative evidence. but, withal, he allows the force of the objection almost as much as his opponents urge it--so much so, indeed, that two of his english critics turn the concession unfairly upon him, and charge him with actually basing his hypothesis upon these and similar difficulties--as if he held it because of the difficulties, and not in spite of them; a handsome return for his candor! as to this imperfection of the geological record, perhaps we should get a fair and intelligible illustration of it by imagining the existing animals and plants of new england, with all their remains and products since the arrival of the mayflower, to be annihilated; and that, in the coming time, the geologists of a new colony, dropped by the new zealand fleet on its way to explore the ruins of london, undertake, after fifty years of examination, to reconstruct in a catalogue the flora and fauna of our day, that is, from the close of the glacial period to the present time. with all the advantages of a surface exploration, what a beggarly account it would be! how many of the land animals and plants which are enumerated in the massachusetts official reports would it be likely to contain? another unanswerable question asked by the boston reviewers is, why, when structure and instinct or habit vary-- as they must have varied, on darwin's hypothesis--they vary together and harmoniously, instead of vaguely? we cannot tell, because we cannot tell why either varies at all. yet, as they both do vary in successive generations--as is seen under domestication--and are correlated, we can only adduce the fact. darwin may be precluded from our answer, but we may say that they vary together because designed to do so. a reviewer says that the chance of their varying together is inconceivably small; yet, if they do not, the variant individuals must all perish. then it is well that it is not left to chance. to refer to a parallel case: before we were born, nourishment and the equivalent to respiration took place in a certain way. but the moment we were ushered into this breathing world, our actions promptly conformed, both as to respiration and nourishment, to the before unused structure and to the new surroundings. "now," says the examiner, "suppose, for instance, the gills of an aquatic animal converted into lungs, while instinct still compelled a continuance under water, would not drowning ensue?" no doubt. but--simply contemplating the facts, instead of theorizing--we notice that young frogs do not keep their heads under water after ceasing to be tadpoles. the instinct promptly changes with the structure, without supernatural interposition--just as darwin would have it, if the development of a variety or incipient species, though rare, were as natural as a metamorphosis. "or if a quadruped, not yet furnished with wings, were suddenly inspired with the instinct of a bird, and precipitated itself from a cliff, would not the descent be hazardously rapid?" doubtless the animal would be no better supported than the objection. but darwin makes very little indeed of voluntary efforts as a cause of change, and even poor lamarck need not be caricatured. he never supposed that an elephant would take such a notion into his wise head, or that a squirrel would begin with other than short and easy leaps; yet might not the length of the leap be increased by practice? the north american reviewer's position, that the higher brute animals have comparatively little instinct and no intelligence, is a heavy blow and great discouragement to dogs, horses, elephants, and monkeys. thus stripped of their all, and left to shift for themselves as they may in this hard world, their pursuit and seeming attainment of knowledge under such peculiar difficulties are interesting to contemplate. however, we are not so sure as is the critic that instinct regularly increases downward and decreases upward in the scale of being. now that the case of the bee is reduced to moderate proportions,[iii- ] we know of nothing in instinct surpassing that of an animal so high as a bird, the talegal, the male of which plumes himself upon making a hot-bed in which to batch his partner's eggs--which he tends and regulates the beat of about as carefully and skillfully as the unplumed biped does an eccaleobion.[iii- ] as to the real intelligence of the higher brutes, it has been ably defended by a far more competent observer, mr. agassiz, to whose conclusions we yield a general assent, although we cannot quite place the best of dogs "in that respect upon a level with a considerable proportion of poor humanity," nor indulge the hope, or indeed the desire, of a renewed acquaintance with the whole animal kingdom in a future life. the assertion that acquired habitudes or instincts, and acquired structures, are not heritable, any breeder or good observer can refute. that "the human mind has become what it is out of a developed instinct," is a statement which mr. darwin nowhere makes, and, we presume, would not accept. that he would have us believe that individual animals acquire their instincts gradually,[iii- ] is a statement which must have been penned in inadvertence both of the very definition of instinct, and of everything we know of in mr. darwin's book. it has been attempted to destroy the very foundation of darwin's hypothesis by denying that there are any wild varieties, to speak of, for natural selection to operate upon. we cannot gravely sit down to prove that wild varieties abound. we should think it just as necessary to prove that snow falls in winter. that variation among plants cannot be largely due to hybridism, and that their variation in nature is not essentially different from much that occurs in domestication, and, in the long-run, probably hardly less in amount, we could show if our space permitted. as to the sterility of hybrids, that can no longer be insisted upon as absolutely true, nor be practically used as a test between species and varieties, unless we allow that hares and rabbits are of one species. that such sterility, whether total or partial, subserves a purpose in keeping species apart, and was so designed, we do not doubt. but the critics fail to perceive that this sterility proves nothing whatever against the derivative origin of the actual species; for it may as well have been intended to keep separate those forms which have reached a certain amount of divergence, as those which were always thus distinct. the argument for the permanence of species, drawn from the identity with those now living of cats, birds, and other animals preserved in egyptian catacombs, was good enough as used by cuvier against st.-hilaire, that is, against the supposition that time brings about a gradual alteration of whole species; but it goes for little against darwin, unless it be proved that species never vary, or that the perpetuation of a variety necessitates the extinction of the parent breed. for darwin clearly maintains--what the facts warrant--that the mass of a species remains fixed so long as it exists at all, though it may set off a variety now and then. the variety may finally supersede the parent form, or it may coexist with it; yet it does not in the least hinder the unvaried stock from continuing true to the breed, unless it crosses with it. the common law of inheritance may be expected to keep both the original and the variety mainly true as long as they last, and none the less so because they have given rise to occasional varieties. the tailless manx cats, like the curtailed fox in the fable, have not induced the normal breeds to dispense with their tails, nor have the dorkings (apparently known to pliny) affected the permanence of the common sort of fowl. as to the objection that the lower forms of life ought, on darwin's theory, to have been long ago improved out of existence, and replaced by higher forms, the objectors forget what a vacuum that would leave below, and what a vast field there is to which a simple organization is best adapted, and where an advance would be no improvement, but the contrary. to accumulate the greatest amount of being upon a given space, and to provide as much enjoyment of life as can be under the conditions, is what nature seems to aim at; and this is effected by diversification. finally, we advise nobody to accept darwin's or any other derivative theory as true. the time has not come for that, and perhaps never will. we also advise against a similar credulity on the other side, in a blind faith that species--that the manifold sorts and forms of existing animals and vegetables--"have no secondary cause." the contrary is already not unlikely, and we suppose will hereafter become more and more probable. but we are confident that, if a derivative hypothesis ever is established, it will be so on a solid theistic ground. meanwhile an inevitable and legitimate hypothesis is on trial--an hypothesis thus far not untenable--a trial just now very useful to science, and, we conclude, not harmful to religion, unless injudicious assailants temporarily make it so. one good effect is already manifest; its enabling the advocates of the hypothesis of a multiplicity of human species to perceive the double insecurity of their ground. when the races of men are admitted to be of one species, the corollary, that they are of one origin, may be expected to follow. those who allow them to be of one species must admit an actual diversification into strongly-marked and persistent varieties, and so admit the basis of fact upon which the darwinian hypothesis is built; while those, on the other hand, who recognize several or numerous human species, will hardly be able to maintain that such species were primordial and supernatural in the ordinary sense of the word. the english mind is prone to positivism and kindred forms of materialistic philosophy, and we must expect the derivative theory to be taken up in that interest. we have no predilection for that school, but the contrary. if we had, we might have looked complacently upon a line of criticism which would indirectly, but effectively, play into the hands of positivists and materialistic atheists generally. the wiser and stronger ground to take is, that the derivative hypothesis leaves the argument for design, and therefore for a designer, as valid as it ever was; that to do any work by an instrument must require, and therefore presuppose, the exertion rather of more than of less power than to do it directly; that whoever would be a consistent theist should believe that design in the natural world is coextensive with providence, and hold as firmly to the one as he does to the other, in spite of the wholly similar and apparently insuperable difficulties which the mind encounters whenever it endeavors to develop the idea into a system, either in the material and organic, or in the moral world. it is enough, in the way of obviating objections, to show that the philosophical difficulties of the one are the same, and only the same, as of the other. iv species as to variation, geographical distribution, and succession (american journal of science and arts, may, ) etude sur l'espece, a l'occasion d'une revision de la famille des cupuliferes, par m. alphonse de candolle.-- this is the title of a paper by m. alph. de candolle, growing out of his study of the oaks. it was published in the november number of the bibliotheque universelle, and separately issued as a pamphlet. a less inspiring task could hardly be assigned to a botanist than the systematic elaboration of the genus quercus and its allies. the vast materials assembled under de candolle's hands, while disheartening for their bulk, offered small hope of novelty. the subject was both extremely trite and extremely difficult. happily it occurred to de candolle that an interest might be imparted to an onerous undertaking, and a work of necessity be turned to good account for science, by studying the oaks in view of the question of species. what this term species means, or should mean, in natural history, what the limits of species, inter se or chronologically, or in geographical distribution, their modifications, actual or probable, their origin, and their destiny--these are questions which surge up from time to time; and now and then in the progress of science they come to assume a new and hopeful interest. botany and zoology, geology, and what our author, feeling the want of a new term proposes to name epiontology, [iv- ] all lead up to and converge into this class of questions, while recent theories shape and point the discussion so we look with eager interest to see what light the study of oaks by a very careful experienced and conservative botanist, particularly conversant with the geographical relations of plants may throw upon the subject. the course of investigation in this instance does not differ from that ordinarily pursued by working botanists nor, in deed are the theoretical conclusions other than those to which a similar study of other orders might not have equally led. the oaks afford a very good occasion for the discussion of questions which press upon our attention, and perhaps they offer peculiarly good materials on account of the number of fossil species. preconceived notions about species being laid aside, the specimens in hand were distributed, according to their obvious resemblances, into groups of apparently identical or nearly identical forms, which were severally examined and compared. where specimens were few, as from countries little explored, the work was easy, but the conclusions, as will be seen, of small value. the fewer the materials, the smaller the likelihood of forms intermediate between any two, and--what does not appear being treated upon the old law-maxim as non-existent--species are readily enough defined. where, however, specimens abound, as in the case of the oaks of europe, of the orient, and of the united states, of which the specimens amounted to hundreds, collected at different ages, in varied localities, by botanists of all sorts of views and predilections--here alone were data fit to draw useful conclusions from. here, as de candolle remarks, he had every advantage, being furnished with materials more complete than any one person could have procured from his own herborizations, more varied than if he had observed a hundred times over the same forms in the same district, and more impartial than if they had all been amassed by one person with his own ideas or predispositions. so that vast herbaria, into which contributions from every source have flowed for years, furnish the best possible data--at least are far better than any practicable amount of personal herborization--or the comparative study of related forms occurring over wide tracts of territory. but as the materials increase, so do the difficulties. forms, which appeared totally distinct, approach or blend through intermediate gradations; characters, stable in a limited number of instances or in a limited district, prove unstable occasionally, or when observed over a wider area; and the practical question is forced upon the investigator, what here is probably fixed and specific, and what is variant, pertaining to individual, variety, or race? in the examination of these rich materials, certain characters were found to vary upon the same branch, or upon the same tree, sometimes according to age or development, sometimes irrespective of such relations or of any assignable reasons. such characters, of course, are not specific, although many of them are such as would have been expected to be constant in the same species, and are such as generally enter into specific definitions. variations of this sort, de candolle, with his usual painstaking, classifies and tabulates, and even expresses numerically their frequency in certain species. the results are brought well to view in a systematic enumeration: . of characters which frequently vary upon the same branch: over a dozen such are mentioned. . of those which sometimes vary upon the same branch: a smaller number of these are mentioned. . those so rare that they might be called monstrosities. then he enumerates characters, ten in number, which he has never found to vary on the same branch, and which, therefore, may better claim to be employed as specific. but, as among them he includes the duration of the leaves, the size of the cupule, and the form and size of its scales, which are by no means quite uniform in different trees of the same species, even these characters must be taken with allowance. in fact, having first brought together, as groups of the lowest order, those forms which varied upon the same stock, he next had to combine similarly various forms which, though not found associated upon the same branch, were thoroughly blended by intermediate degrees: "the lower groups (varieties or races) being thus constituted, i have given the rank of species to the groups next above these, which differ in other respects, i.e., either in characters which were not found united upon certain individuals, or in those which do not show transitions from one individual to another. for the oaks of regions sufficiently known, the species thus formed rest upon satisfactory bases, of which the proof can be furnished. it is quite otherwise with those which are represented in our herbaria by single or few specimens. these are provisional species--species which may hereafter fall to the rank of simple varieties. i have not been inclined to prejudge such questions; indeed, in this regard, i am not disposed to follow those authors whose tendency is, as they say, to reunite species. i never reunite them without proof in each particular case; while the botanists to whom i refer do so on the ground of analogous variations or transitions occurring in the same genus or in the same family. for example resting on the fact that quercus hex, q. coccifera, q. acutifolia, etc., have the leaves sometimes entire and sometimes toothed upon the same branch, or present transitions from one tree to another, i might readily have united my q. tlapuxahuensis to q. sartorii of liebmann, since these two differ only in their entire or their toothed leaves. from the fact that the length of the peduncle varies in q. robur and many other oaks, i might have combined q. seemannii liebm. with q. salicifolia nee. i have not admitted these inductions, but have demanded visible proof in each particular case. many species are thus left as provisional; but, in proceeding thus, the progress of the science will be more regular, and the synonymy less dependent upon the caprice or the theoretical opinions of each author." this is safe and to a certain degree judicious, no doubt, as respects published species. once admitted, they may stand until they are put down by evidence, direct or circumstantial. doubtless a species may rightfully be condemned on good circumstantial evidence. but what course does de candolle pursue in the case--of every-day occurrence to most working botanists, having to elaborate collections from countries not so well explored as europe--when the forms in question, or one of the two, are as yet unnamed? does he introduce as a new species every form which he cannot connect by ocular proof with a near relative, from which it differs only in particulars which he sees are inconstant in better known species of the same group? we suppose not. but, if he does, little improvement for the future upon the state of things revealed in the following quotation can be expected: "in the actual state of our knowledge, after having seen nearly all the original specimens, and in some species as many as two hundred representatives from different localities, i estimate that, out of the three hundred species of cupuliferae which will be enumerated in the prodromus, two-thirds at least are provisional species. in general, when we consider what a multitude of species were described from a single specimen, or from the forms of a single locality, of a single country, or are badly described, it is difficult to believe that above one-third of the actual species in botanical works will remain unchanged." such being the results of the want of adequate knowledge, how is it likely to be when our knowledge is largely increased? the judgment of so practised a botanist as de candolle is important in this regard, and it accords with that of other botanists of equal experience. "they are mistaken," he pointedly asserts, "who repeat that the greater part of our species are clearly limited, and that the doubtful species are in a feeble minority. this seemed to be true, so long as a genus was imperfectly known, and its species were founded upon few specimens, that is to say, were provisional. just as we come to know them better, intermediate forms flow in, and doubts as to specific limits augment." de candolle insists, indeed, in this connection, that the higher the rank of the groups the more definite their limitation, or, in other terms, the fewer the ambiguous or doubtful forms, that genera are more strictly limited than species tribes than genera, orders than tribes, etc. we are not convinced of this often where it has appeared to be so, advancing discovery has brought intermediate forms to light, perplexing to the systematist. "they are mistaken, we think more than one systematic botanist will say, "who repeat that the greater part of our natural orders and tribes are absolutely limited," however we may agree that we will limit them. provisional genera we suppose are proportionally hardly less common than provisional species; and hundreds of genera are kept up on considerations of general propriety or general convenience, although well known to shade off into adjacent ones by complete gradations. somewhat of this greater fixity of higher groups, therefore, is rather apparent than real. on the other hand, that varieties should be less definite than species, follows from the very terms employed. they are ranked as varieties, rather than species, just because of their less definiteness. singular as it may appear, we have heard it denied that spontaneous varieties occur. de candolle makes the important announcement that, in the oak genus, the best known species are just those which present the greatest number of spontaneous varieties and sub-varieties. the maximum is found in q. robur, with twenty-eight varieties, all spontaneous. of q. lusitanica eleven varieties are enumerated, of q. calliprinos ten, of q. coccifera eight, * etc. and he significantly adds that "these very species which offer such numerous modifications are themselves ordinarily surrounded by other forms, provisionally called species, because of the absence of known transitions or variations, but to which some of these will probably have to be joined hereafter." the inference is natural, if not inevitable, that the difference between such species and such varieties is only one of degree, either as to amount of divergence, or of hereditary fixity, or as to the frequency or rarity at the present time of intermediate forms. this brings us to the second section of de candolle's article, in which he passes on, from the observation of the present forms and affinities of cupuliferous plants, to the consideration of their probable history and origin. suffice it to say, that he frankly accepts the inferences derived from the whole course of observation, and contemplates a probable historical connection between congeneric species. he accepts and, by various considerations drawn from the geographical distribution of european cupuliferae, fortifies the conclusion--long ago arrived at by edward forbes--that the present species, and even some of their varieties, date back to about the close of the tertiary epoch, since which time they have been subject to frequent and great changes of habitation or limitation, but without appreciable change of specific form or character; that is, without profounder changes than those within which a species at the present time is known to vary. moreover, he is careful to state that he is far from concluding that the time of the appearance of a species in europe at all indicates the time of its origin. looking back still further into the tertiary epoch, of which the vegetable remains indicate many analogous, but few, if any, identical forms, he concludes, with heer and others, that specific changes of form, as well as changes of station, are to be presumed; and, finally, that "the theory of a succession of forms through the deviation of anterior forms is the most natural hypothesis, and the most accordant with the known facts in palaeontology, geographical botany and zoology, of anatomical structure and classification: but direct proof of it is wanting, and moreover, if true, it must have taken place very slowly; so slowly, indeed, that its effects are discernible only after a lapse of time far longer than our historic epoch." in contemplating the present state of the species of cupuliferae in europe, de candolle comes to the conclusion that, while the beech is increasing, and extending its limits southward and westward (at the expense of coniferae and birches), the common oak, to some extent, and the turkey oak decidedly, are diminishing and retreating, and this wholly irrespective of man's agency. this is inferred of the turkey oak from the great gaps found in its present geographical area, which are otherwise inexplicable, and which he regards as plain indications of a partial extinction. community of descent of all the individuals of species is of course implied in these and all similar reasonings. an obvious result of such partial extinction is clearly enough brought to view the european oaks (like the american species) greatly tend to vary that is they manifest an active disposition to produce new forms every form tends to become hereditary and so to pass from the state of mere variation to that of race and of these competing incipient races some only will survive. quercus robur offers a familiar illustration of the manner in which one form may in the course of time become separated into two or more distinct ones. to linnaeus this common oak of europe was all of one species. but of late years the greater number of european botanists have regarded it as including three species, q. pedunculata, q. sessiliflora, and q. pubescens. de candolle looks with satisfaction to the independent conclusion which he reached from a long and patient study of the forms (and which webb, gay, bentham, and others, had equally reached), that the view of linnaeus was correct, inasmuch as it goes to show that the idea and the practical application of the term species have remained unchanged during the century which has elapsed since the publication of the "species plantarum." but, the idea remaining unchanged, the facts might appear under a different aspect, and the conclusion be different, under a slight and very supposable change of circumstances. of the twenty-eight spontaneous varieties of q. robur, which de candolle recognizes, all but six, he remarks, fall naturally under the three sub-species, pedunculata, sessiliflora, and pubescens, and are therefore forms grouped around these as centres; and, moreover, the few connecting forms are by no means the most common. were these to die out, it is clear that the three forms which have already been so frequently taken for species would be what the group of four or five provisionally admitted species which closely surround q. robur now are. the best example of such a case, as having in all probability occurred through geographical segregation and partial extinction, is that of the cedar, thus separated into the deodar, the lebanon, and the atlantic cedars--a case admirably worked out by dr. hooker two or three years ago. [iv- ] a special advantage of the cupuliferae for determining the probable antiquity of existing species in europe, de candolle finds in the size and character of their fruits. however it may be with other plants (and he comes to the conclusion generally that marine currents and all other means of distant transport have played only a very small part in the actual dispersion of species), the transport of acorns and chestnuts by natural causes across an arm of the sea in a condition to germinate, and much more the spontaneous establishment of a forest of oaks or chestnuts in this way, de candolle conceives to be fairly impossible in itself, and contrary to all experience. from such considerations, i.e., from the actual dispersion of the existing species (with occasional aid from post-tertiary deposits), it is thought to be shown that the principal cupuliferae of the old world attained their actual extension before the present separation of sicily, sardinia and corsica, and of britain, from the european continent. this view once adopted, and this course once entered upon, has to be pursued farther. quercus robur of europe with its bevy of admitted derivatives, and its attending species only provisionally admitted to that rank, is very closely related to certain species of eastern asia, and of oregon and california--so closely that "a view of the specimens by no means forbids the idea that they have all originated from q. robur, or have originated, with the latter, from one or more preceding forms so like the present ones that a naturalist could hardly know whether to call them species or varieties." moreover, there are fossil leaves from diluvian deposits in italy, figured by gaudin, which are hardly distinguishable from those of q. robur on the one hand, and from those of q. douglasii, etc., of california, on the other. no such leaves are found in any tertiary deposit in europe; but such are found of that age, it appears, in northwest america, where their remote descendants still flourish. so that the probable genealogy of q. robur, traceable in europe up to the commencement of the present epoch, looks eastward and far into the past on far-distant shores. quercus ilex, the evergreen oak of southern europe and northern africa, reveals a similar archaeology; but its presence in algeria leads de candolle to regard it as a much more ancient denizen of europe than q. robur; and a tertiary oak, q. ilicoides, from a very old miocene bed in switzerland, is thought to be one of its ancestral forms. this high antiquity once established, it follows almost of course that the very nearly-related species in central asia, in japan, in california, and even our own live-oak with its mexican relatives, may probably enough be regarded as early offshoots from the same stock with q. hex. in brief--not to continue these abstracts and remarks, and without reference to darwin's particular theory (which de candolle at the close very fairly considers)--if existing species, or many of them, are as ancient as they are now generally thought to be, and were subject to the physical and geographical changes (among them the coming and the going of the glacial epoch) which this antiquity implies; if in former times they were as liable to variation as they now are; and if the individuals of the same species may claim a common local origin, then we cannot wonder that "the theory of a succession of forms by deviations of anterior forms" should be regarded as "the most natural hypothesis," nor at the general advance made toward its acceptance. the question being, not, how plants and animals originated, but, how came the existing animals and plants to be just where they are and what they are, it is plain that naturalists interested in such inquiries are mostly looking for the answer in one direction. the general drift of opinion, or at least of expectation, is exemplified by this essay of de candolle; and the set and force of the current are seen by noticing how it carries along naturalists of widely different views and prepossessions--some faster and farther than others--but all in one way. the tendency is, we may say, to extend the law of continuity, or something analogous to it, from inorganic to organic nature, and in the latter to connect the present with the past in some sort of material connection. the generalization may indeed be expressed so as not to assert that the connection is genetic, as in mr. wallace's formula: "every species has come into existence coincident both in time and space with preexisting closely-allied species." edward forbes, who may be called the originator of this whole line of inquiry, long ago expressed a similar view. but the only material sequence we know, or can clearly conceive, in plants and animals, is that from parent to progeny; and, as de candolle implies, the origin of species and that of races can hardly be much unlike, nor governed by other than the same laws, whatever these may be. the progress of opinion upon this subject in one generation is not badly represented by that of de candolle himself, who is by no means prone to adopt new views without much consideration. in an elementary treatise published in the year , he adopted and, if we rightly remember, vigorously maintained, schouw's idea of the double or multiple origin of species, at least of some species--a view which has been carried out to its ultimate development only perhaps by agassiz, in the denial of any necessary genetic connection among the individuals of the same species, or of any original localization more restricted than the area now occupied by the species. but in i , in his "geographic botanique," the multiple hypothesis, although in principle not abandoned, loses its point, in view of the probable high antiquity of existing species. the actual vegetation of the world being now regarded as a continuation, through numerous geological, geographical, and more recently historical changes, of anterior vegetations, the actual distribution of plants is seen to be a consequence of preceding conditions; and geological considerations, and these alone, may be expected to explain all the facts--many of them so curious and extraordinary--of the actual geographical distribution of the species. in the present essay, not only the distribution but the origin of congeneric species is regarded as something derivative; whether derived by slow and very gradual changes in the course of ages, according to darwin, or by a sudden, inexplicable change of their tertiary ancestors, as conceived by heer, de candolle hazards no opinion. it may, however, be inferred that he looks upon "natural selection" as a real, but insufficient cause; while some curious remarks upon the number of monstrosities annually produced, and the possibility of their enduring, may be regarded as favorable to heer's view. as an index to the progress of opinion in the direction referred to, it will be interesting to compare sir charles lyell's well-known chapters of twenty or thirty years ago, in which the permanence of species was ably maintained, with his treatment of the same subject in a work just issued in england, which, however, has not yet reached us. a belief of the derivation of species may be maintained along with a conviction of great persistence of specific characters. this is the idea of the excellent swiss vegetable palaeontologist, heer, who imagines a sudden change of specific type at certain periods, and perhaps is that of pictet. falconer adheres to somewhat similar views in his elaborate paper on elephants, living and fossil, in the natural history review for january last. noting that "there is clear evidence of the true mammoth having existed in america long after the period of the northern drift, when the surface of the country had settled down into its present form, and also in europe so late as to have been a contemporary of the irish elk, and on the other hand that it existed in england so far back as before the deposition of the bowlder clay; also that four well-defined species of fossil elephant are known to have existed in europe; that "a vast number of the remains of three of these species have been exhumed over a large area in europe; and, even in the geological sense, an enormous interval of time has elapsed between the formation of the most ancient and the most recent of these deposits, quite sufficient to test the persistence of specific characters in an elephant," he presents the question, "do, then, the successive elephants occurring in these strata show any signs of a passage from the older form into the newer?" to which the reply is: "if there is one fact which is impressed on the conviction of the observer with more force than any other, it is the persistence and uniformity of the characters of the molar teeth in the earliest known mammoth and his most modern successor . . . assuming the observation to be correct, what strong proof does it not afford of the persistence and constancy, throughout vast intervals of time, of the distinctive characters of those organs which arc most concerned in the existence and habits of the species? if we cast a glance back on the long vista of physical changes which our planet has undergone since the neozoic epoch, we can nowhere detect signs of a revolution more sudden and pronounced, or more important in its results, than the intercalation and sudden disappearance of the glacial period. yet the 'dicyclotherian' mammoth lived before it, and passed through the ordeal of all the hard extremities it involved, bearing his organs of locomotion and digestion all but unchanged. taking the group of four european fossil species above enumerated, do they show any signs in the successive deposits of a transition from the one form into the other? here again the result of my observation, in so far as it has extended over the european area, is, that the specific characters of the molars are constant in each, within a moderate range of variation, and that we nowhere meet with intermediate forms." . . . dr. falconer continues (page ): "the inferences which i draw from these facts are not opposed to one of the leading propositions of darwin's theory. with him, i have no faith in the opinion that the mammoth and other extinct elephants made their appearance suddenly, after the type in which their fossil remains are presented to us. the most rational view seems to be, that they are in some shape the modified descendants of earlier progenitors. but if the asserted facts be correct, they seem clearly to indicate that the older elephants of europe, such as e. meridionalis and e. antiguus, were not the stocks from which the later species, e. primigenius and e. africanus sprung, and that we must look elsewhere for their origin. the nearest affinity, and that a very close one, of the european e. meridionalis is with the miocene e. planifrons of india; and of e. primigenius, with the existing india species. "another reflection is equally strong in my mind--that the means which have been adduced to explain the origin of the species by 'natural selection,' or a process of variation from external influences, are inadequate to account for the phenomena. the law of phyllotaxis, which governs the evolution of leaves around the axis of a plant, is as nearly constant in its manifestation as any of the physical laws connected with the material world. each instance, however different from another, can be shown to be a term of some series of continued fractions. when this is coupled with the geometrical law governing the evolution of form, so manifest in some departments of the animal kingdom, e. g., the spiral shells of the mollusca, it is difficult to believe that there is not, in nature, a deeper-seated and innate principle, to the operation of which natural selection is merely an adjunct. the whole range of the mammalia, fossil and recent, cannot furnish a species which has had a wider geographical distribution, and passed through a longer term of time, and through more extreme changes of climatal conditions, than the mammoth. if species are so unstable, and so susceptible of mutation through such influences, why does that extinct form stand out so signally a monument of stability? by his admirable researches and earnest writings, darwin has, beyond all his contemporaries, given an impulse to the philosophical investigation of the most backward and obscure branch of the biological sciences of his day; he has laid the foundations of a great edifice; but he need not be surprised if, in the progress of erection, the superstructure is altered by his successors, like the duomo of milan from the roman to a different style of architecture." entertaining ourselves the opinion that something more than natural selection is requisite to account for the orderly production and succession of species, we offer two incidental remarks upon the above extract. . we find in it--in the phrase "natural selection, or a process of variation from external influences"--an example of the very common confusion of two distinct things, viz., variation and natural selection. the former has never yet been shown to have its cause in "external influences," nor to occur at random. as we have elsewhere insisted, if not inexplicable, it has never been explained; all we can yet say is, that plants and animals are prone to vary, and that some conditions favor variation. perhaps in this dr. falconer may yet find what he seeks: for "it is difficult to believe that there is not in nature a deeper-seated and innate principle, to the operation of which natural selection is merely an adjunct." the latter, which is the ensemble of the external influences, including the competition of the individuals them selves, picks out certain variations as they arise, but in no proper sense can be said to originate them . although we are not quite sure how dr falconer in tends to apply the law of phyllotaxis to illustrate his idea, we fancy that a pertinent illustration may be drawn from it in this way. there are two species of phyllotaxis, perfectly distinct, and we suppose, not mathematically reducible the one to the other, viz.: ( .) that of alternate leaves, with its vane ties and ( .) that of verticillate leaves, of which opposite leaves present the simplest case that although generally constant a change from one variety of alternate phyllotaxis to an other should occur on the same axis, or on successive axes, is not surprising, the different sorts being terms of a regular series--although indeed we have not the least idea as to how the change from the one to the other comes to pass but it is interesting and in this connection perhaps instructive, to remark that while some dicotyledonous plants hold to the verticillate, i.e., opposite-leaved phyllotaxis throughout, a larger number--through the operation of some deep seated and innate principle which we cannot fathom--change abruptly into the other species at the second or third node, and change back again in the flower, or else effect a synthesis of the two species in a manner which is puzzling to understand. here is a change from one fixed law to another, as unaccountable, if not as great, as from one specific form to another. an elaborate paper on the vegetation of the tertiary period in the southeast of france, by count gaston de saporta, published in the annales des sciences naturelles in , vol. xvi., pp. - --which we have not space to analyze--is worthy of attention from the general inquirer, on account of its analysis of the tertiary flora into its separate types, cretaceous, austral, tropical, and boreal, each of which has its separate and different history--and for the announcement that "the hiatus, which, in the idea of most geologists, intervened between the close of the cretaceous and the beginning of the tertiary, appears to have had no existence, so far as concerns the vegetation; that in general it was not by means of a total overthrow, followed by a complete new emission of species, that the flora has been renewed at each successive period; and that while the plants of southern europe inherited from the cretaceous period more or less rapidly disappeared, as also the austral forms, and later the tropical types (except the laurel, the myrtle, and the chamaerops humilis), the boreal types, coming later, survived all the others, and now compose, either in europe, or in the north of asia, or in north america, the basis of the actual arborescent vegetation. especially "a very considerable number of forms nearly identical with tertiary forms now exist in america, where they have found, more easily than in our soil--less vast and less extended southward--refuge from ulterior revolutions," the extinction of species is attributed to two kinds of causes; the one material or physical, whether slow or rapid; the other inherent in the nature of organic beings, incessant, but slow, in a manner latent, but somehow assigning to the species, as to the individuals, a limited period of existence, and, in some equally mysterious but wholly natural way, connected with the development of organic types: "by type meaning a collection of vegetable forms constructed upon the same plan of organization, of which they reproduce the essential lineaments with certain secondary modifications, and which appear to run back to a common point of departure." in this community of types, no less than in the community of certain existing species, saporta recognizes a prolonged material union between north america and europe in former times. most naturalists and geologists reason in the same way--some more cautiously than others--yet perhaps most of them seem not to perceive how far such inferences imply the doctrine of the common origin of related species. for obvious reasons such doctrines are likely to find more favor with botanists than with zoologists. but with both the advance in this direction is seen to have been rapid and great; yet to us not unexpected. we note, also, an evident disposition, notwithstanding some endeavors to the contrary, to allow derivative hypotheses to stand or fall upon their own merits--to have indeed upon philosophical grounds certain presumptions in their favor--and to be, perhaps, quite as capable of being turned to good account as to bad account in natural theology.[iv- ] among the leading naturalists, indeed, such views--taken in the widest sense--have one and, so far as we are now aware, only one thoroughgoing and thoroughly consistent opponent, viz., mr. agassiz. most naturalists take into their very conception of a species, explicitly or by implication, the notion of a material connection resulting from the descent of the individuals composing it from a common stock, of local origin. agassiz wholly eliminates community of descent from his idea of species, and even conceives a species to have been as numerous in individuals and as wide-spread over space, or as segregated in discontinuous spaces, from the first as at the later period. the station which it inhabits, therefore, is with other naturalists in no wise essential to the species, and may not have been the region of its origin. in agassiz's view the habitat is supposed to mark the origin, and to be a part of the character of the species. the habitat is not merely the place where it is, but a part of what it is. most naturalists recognize varieties of species; and many, like de candolle, have come to conclude that varieties of the highest grade, or races, so far partake of the characteristics of species, and are so far governed by the same laws, that it is often very difficult to draw a clear and certain distinction between the two. agassiz will not allow that varieties or races exist in nature, apart from man's agency. most naturalists believe that the origin of species is supernatural, their dispersion or particular geographical area, natural, and their extinction, when they disappear, also the result of physical causes. in the view of agassiz, if rightly understood, all three are equally independent of physical cause and effect, are equally supernatural. in comparing preceding periods with the present and with each other, most naturalists and palaeontologists now appear to recognize a certain number of species as having survived from one epoch to the next, or even through more than one formation, especially from the tertiary into the post-tertiary period, and from that to the present age. agassiz is understood to believe in total extinctions and total new creations at each successive epoch, and even to recognize no existing species as ever contemporary with extinct ones, except in the case of recent exterminations. these peculiar views if sustained will effectually dispose of every form of derivative hypothesis. returning for a moment to de candolle's article, we are disposed to notice his criticism of linnaeus's "definition" of the term species (philosophia botanica, no. ): "species tot numeramus quot diversae formae in principio sunt creatae"-- which he declares illogical, inapplicable, and the worst that has been propounded. "so, to determine if a form is specific, it is necessary to go back to its origin which is impossible a definition by a character which can never be verified is no definition at all." now as linnaeus practically applied the idea of species with a sagacity which has never been surpassed and rarely equaled and indeed may be said to have fixed its received meaning in natural history, it may well be inferred that in the phrase above cited he did not so much undertake to frame a logical definition, as to set forth the idea which, in his opinion, lay at the foundation of species; on which basis a.l. jussieu did construct a logical definition--"nunc rectius definitur perennis individuorum similium successio continuata generatione renascentium." the fundamental idea of species, we would still maintain, is that of a chain of which genetically-connected individuals are the links. that, in the practical recognition of species, the essential characteristic has to be inferred, is no great objection--the general fact that like engenders like being an induction from a vast number of instances, and the only assumption being that of the uniformity of nature. the idea of gravitation, that of the atomic constitution of matter, and the like, equally have to be verified inferentially. if we still hold to the idea of linnaeus, and of agassiz, that existing species were created independently and essentially all at once at the beginning of the present era, we could not better the propositions of linnaeus and of jussieu. if; on the other hand, the time has come in which we may accept, with de candolle, their successive origination, at the commencement of the present era or before, and even by derivation from other forms, then the "in principio" of linnaeus will refer to that time, whenever it was, and his proposition be as sound and wise as ever. in his "geographie botanique" (ii., - ) de candolle discusses this subject at length, and in the same interest. remarking that of the two great facts of species, viz., likeness among the individuals, and genealogical connection, zoologists have generally preferred the latter,[iv- ] while botanists have been divided in opinion, he pronounces for the former as the essential thing, in the following argumentative statement: "quant a moi, j'ai ete conduit, dans ma definition de l'espece, a mettre decidement la ressemblance au-dessus de caracteres de succession. ce n'est pas seulement a cause des circonstances propres au regne vegetal, dont je m'occupe exclusivement; ce n'est pas non plus afin de sortir ma definition des theories et de la rendre le plus possible utile aux naturalistes descripteurs et nomenclateurs, c'est aussi par un motif philosophique. en toute chose il faut aller au fond des questions, quand on le peut. or, pourquoi la reproduction est-elle possible, habituelle, feconde indefiniment, entre des etres organises que nous dirons de la meme espece? parce qu'ils se ressemblent et uniquement a cause de cela. lorsque deux especes ne peuvent, ou, s'il s'agit d'animaux superieurs, ne peuvent et ne veulent se croiser, c'est qu'elles sont tres differentes. si l'on obtient des croisements, c'est que les individus sont analogues; si ces croisements donnent des produits feconds, c'est que les individus etaient plus analogues; si ces produits euxmemes sont feconds, c'est que la ressemblance etait plus grande; s'ils sont fecond habituellement et indefiniment, c'est que la ressemblance interieure et exterieure etait tres grande. ainsi le degre de ressemblance est le fond; la reproduction en est seulement la manifestation et la mesure, et il est logique de placer la cause au-dessus de l'effet." we are not yet convinced. we still hold that genealogical connection, rather than mutual resemblance, is the fundamental thing--first on the ground of fact, and then from the philosophy of the case. practically, no botanist can say what amount of dissimilarity is compatible with unity of species; in wild plants it is sometimes very great, in cultivated races often enormous. de candolle himself informs us that the different variations which the same oak-tree exhibits arc significant indications of a disposition to set up separate varieties, which becoming hereditary may constitute a race; he evidently looks upon the extreme forms, say of quercus robur, as having thus originated; and on this ground, inferred from transitional forms, and not from their mutual resemblance, he includes them in that species. this will be more apparent should the discovery of transitions, which he leads us to expect, hereafter cause the four provisional species which attend q. robur to be merged in that species. it may rightly be replied that this conclusion would be arrived at from the likeness step by step in the series of forms; but the cause of the likeness here is obvious. and this brings in our "motif philosophique." not to insist that the likeness is after all the variable, not the constant, element--to learn which is the essential thing, resemblance among individuals or their genetic connection--we have only to ask which can be the cause of the other. in hermaphrodite plants (the normal case), and even as the question is ingeniously put by de candolle in the above extract, the former surely cannot be the cause of the latter, though it may, in case of crossing, offer occasion. but, on the ground of the most fundamental of all things in the constitution of plants and animals--the fact incapable of further analysis, that individuals reproduce their like, that characteristics are inheritable--the likeness is a direct natural consequence of the genetic succession; "and it is logical to place the cause above the effect." we are equally disposed to combat a proposition of de candolle's about genera, elaborately argued in the "geographie botanique," and incidentally reaffirmed in his present article, viz., that genera are more natural than species, and more correctly distinguished by people in general, as is shown by vernacular names. but we have no space left in which to present some evidence to the contrary. v sequoia and its history the relations of north american to northeast asian and to tertiary vegetation (a presidential address to the american association for the advancement of science, at dubuque, august, ) the session being now happily inaugurated, your presiding officer of the last year has only one duty to perform before he surrenders the chair to his successor. if allowed to borrow a simile from the language of my own profession, i might liken the president of this association to a biennial plant. he flourishes for the year in which he comes into existence, and performs his appropriate functions as presiding officer. when the second year comes round, he is expected to blossom out in an address and disappear. each president, as he retires, is naturally expected to contribute something from his own investigations or his own line of study, usually to discuss some particular scientific topic. now, although i have cultivated the field of north american botany, with some assiduity, for more than forty years, have reviewed our vegetable hosts, and assigned to no small number of them their names and their place in the ranks, yet, so far as our own wide country is concerned, i have been to a great extent a closet botanist. until this summer i had not seen the mississippi, nor set foot upon a prairie. to gratify a natural interest, and to gain some title for addressing a body of practical naturalists and explorers, i have made a pilgrimage across the continent. i have sought and viewed in their native haunts many a plant and flower which for me had long bloomed unseen, or only in the hortus siccus. i have been able to see for myself what species and what forms constitute the main features of the vegetation of each successive region, and record--as the vegetation unerringly does--the permanent characteristics of its climate. passing on from the eastern district, marked by its equably distributed rainfall, and therefore naturally forest-clad, i have seen the trees diminish in number, give place to wide prairies, restrict their growth to the borders of streams, and then disappear from the boundless drier plains; have seen grassy plains change into a brown and sere desert--desert in the common sense, but hardly anywhere botanically so--have seen a fair growth of coniferous trees adorning the more favored slopes of a mountain-range high enough to compel summer showers; have traversed that broad and bare elevated region shut off on both sides by high mountains from the moisture supplied by either ocean, and longitudinally intersected by sierras which seemingly remain as naked as they were born; and have reached at length the westward slopes of that high mountain-barrier which, refreshed by the pacific, bears the noble forests of the sierra nevada and the coast ranges, and among them trees which are the wonder of the world. as i stood in their shade, in the groves of mariposa and calaveras, and again under the canopy of the commoner redwood, raised on columns of such majestic height and ample girth, it occurred to me that i could not do better than to share with you, upon this occasion, some of the thoughts which possessed my mind. in their development they may, perhaps, lead us up to questions of considerable scientific interest. i shall not detain you with any remarks--which would now be trite--upon the size or longevity of these far-famed sequoia-trees, or of the sugar-pines, incense-cedar, and firs associated with them, of which even the prodigious bulk of the dominating sequoia does not sensibly diminish the grandeur. although no account and no photographic representation of either species of the far-famed sequoia-trees gives any adequate impression of their singular majesty--still less of their beauty--yet my interest in them did not culminate merely or mainly in considerations of their size and age. other trees, in other parts of the world, may claim to be older. certain australian gumtrees (eucalypti) are said to be taller. some, we are told, rise so high that they might even cast a flicker of shadow upon the summit of the pyramid of cheops. yet the oldest of them doubtless grew from seed which was shed long after the names of the pyramid-builders had been forgotten. so far as we can judge from the actual counting of the layers of several trees, no sequoia now alive sensibly antedates the christian era. nor was i much impressed with an attraction of man's adding. that the more remarkable of these trees should bear distinguishing appellations seems proper enough; but the tablets of personal names which are affixed to many of them in the most visited groves--as if the memory of more or less notable people of our day might be made enduring by the juxtaposition--do suggest some incongruity. when we consider that a hand's breadth at the circumference of any one of the venerable trunks so placarded has recorded in annual lines the lifetime of the individual thus associated with it, one may question whether the next hand's breadth may not measure the fame of some of the names thus ticketed for adventitious immortality. whether it be the man or the tree that is honored in the connection, probably either would live as long, in fact and in memory, without it. one notable thing about the sequoia-trees is their isolation. most of the trees associated with them are of peculiar species, and some of them are nearly as local. yet every pine, fir, and cypress of california is in some sort familiar, because it has near relatives in other parts of the world. but the redwoods have none. the redwood--including in that name the two species of "big-trees"--belongs to the general cypress family, but is sui generis. thus isolated systematically, and extremely isolated geographically, and so wonderful in size and port, they more than other trees suggest questions. were they created thus local and lonely, denizens of california only; one in limited numbers in a few choice spots on the sierra nevada, the other along the coast range from the bay of monterey to the frontiers of oregon? are they veritable melchizedeks, without pedigree or early relationship, and possibly fated to be without descent? or are they now coming upon the stage--or rather were they coming but for man's interference--to play a part in the future? or are they remnants, sole and scanty survivors of a race that has played a grander part in the past, but is now verging to extinction? have they had a career, and can that career be ascertained or surmised, so that we may at least guess whence they came, and how, and when? sequoia and its history time was, and not long ago, when such questions as these were regarded as useless and vain--when students of natural history, unmindful of what the name denotes, were content with a knowledge of things as they now are, but gave little heed as to how they came to be so. now such questions are held to be legitimate, and perhaps not wholly unanswerable. it cannot now be said that these trees inhabit their present restricted areas simply because they are there placed in the climate and soil of all the world most congenial to them. these must indeed be congenial, or they would not survive. but when we see how the australian eucalyptus-trees thrive upon the californian coast, and how these very redwoods flourish upon another continent; how the so-called wild-oat (avena sterilis of the old world) has taken full possession of california; how that cattle and horses, introduced by the spaniard, have spread as widely and made themselves as much at home on the plains of la plata as on those of tartary; and that the cardoon-thistle-seeds, and others they brought with them, have multiplied there into numbers probably much exceeding those extant in their native lands; indeed, when we contemplate our own race, and our particular stock, taking such recent but dominating possession of this new world; when we consider how the indigenous flora of islands generally succumbs to the foreigners which come in the train of man; and that most weeds (i.e., the prepotent plants in open soil) of all temperate climates are not "to the manner born," but are self-invited intruders--we must needs abandon the notion of any primordial and absolute adaptation of plants and animals to their habitats, which may stand in lieu of explanation, and so preclude our inquiring any further. the harmony of nature and its admirable perfection need not be regarded as inflexible and changeless. nor need nature be likened to a statue, or a cast in rigid bronze, but rather to an organism, with play and adaptability of parts, and life and even soul informing the whole. under the former view nature would be "the faultless monster which the world ne'er saw," but inscrutable as the sphinx, whom it were vain, or worse, to question of the whence and whither. under the other, the perfection of nature, if relative, is multifarious and ever renewed; and much that is enigmatical now may find explanation in some record of the past. that the two species of redwood we are contemplating originated as they are and where they are, and for the part they are now playing, is, to say the least, not a scientific supposition, nor in any sense a probable one. nor is it more likely that they are destined to play a conspicuous part in the future, or that they would have done so, even if the indian's fires and the white man's axe had spared them. the redwood of the coast (sequoia sempervirens) had the stronger hold upon existence, forming as it did large forests throughout a narrow belt about three hundred miles in length, and being so tenacious of life that every large stump sprouts into a copse. but it does not pass the bay of monterey, nor cross the line of oregon, although so grandly developed not far below it. the more remarkable sequoia gigantea of the sierra exists in numbers so limited that the separate groves may be reckoned upon the fingers, and the trees of most of them have been counted, except near their southern limit, where they are said to be more copious. a species limited in individuals holds its existence by a precarious tenure; and this has a foothold only in a few sheltered spots, of a happy mean in temperature, and locally favored with moisture in summer. even there, for some reason or other, the pines with which they are associated (pinus lambertiana and p. ponderosa), the firs (abies grandis and a. amabilis), and even the incense-cedar (libocedrus decurrens), possess a great advantage, and, though they strive in vain to emulate their size, wholly overpower the sequoias in numbers. "to him that hath shall be given." the force of numbers eventually wins. at least in the commonly-visited groves sequoia gigantea is invested in its sequoia and its history last stronghold, can neither advance into more exposed positions above, nor fall back into drier and barer ground below, nor hold its own in the long-run where it is, under present conditions; and a little further drying of the climate, which must once have been much moister than now, would precipitate its doom. whatever the individual longevity, certain if not speedy is the decline of a race in which a high death-rate afflicts the young. seedlings of the big trees occur not rarely, indeed, but in meagre proportion to those of associated trees; t small indeed is the chance that any of these will attain to "the days of the years of their fathers." "few and evil" are .: the days of all the forest likely to be, while man, both bar-barian and civilized, torments them with fires, fatal at once to seedlings, and at length to the aged also. the forests of california, proud as the state may be of them, are already too scanty and insufficient for her uses. two lines, such as may be drawn with one sweep of a brush over the map, would cover them all. the coast redwood--the most important tree in california, although a million times more numerous than its relative of the sierra--is too good to live long. such is its value for lumber and its accessibility, that, judging the future by the past, it is not likely, in its primeval growth, to outlast its rarer fellow-species. happily man preserves and disseminates as well as destroys. the species will doubtless be preserved to science, and for ornamental and other uses, in its own and other lands; and the more remarkable individuals of the present day are likely to be sedulously cared for, all the more so as they become scarce. our third question remains to be answered: have these famous sequoias played in former times and upon a larger stage a more imposing part, of which the present is but the epilogue? we cannot gaze high up the huge and venerable trunks, which one crosses the continent to behold, without wishing that these patriarchs of the grove were able, like the long-lived antediluvians of scripture, to hand down to us, through a few generations, the traditions of centuries, and so tell us somewhat of the history of their race. fifteen hundred annual layers have been counted, or satisfactorily made out, upon one or two fallen trunks. it is probable that close to the heart of some of the living trees may be found the circle that records the year of our saviour's nativity. a few generations of such trees might carry the history a long way back. but the ground they stand upon, and the marks of very recent geological change and vicissitude in the region around, testify that not very many such generations can have flourished just there, at least in an unbroken series. when their site was covered by glaciers, these sequoias must have occupied other stations, if, as there is reason to believe, they then existed in the land. i have said that the redwoods have no near relatives in the country of their abode, and none of their genus anywhere else. perhaps something may be learned of their genealogy by inquiring of such relatives as they have. there are only two of any particular nearness of kin; and they are far away. one is the bald cypress, our southern cypress, taxodium, inhabiting the swamps of the atlantic coast from maryland to texas, thence extending--with, probably, a specific difference--into mexico. it is well known as one of the largest trees of our atlantic forest-district, and, although it never--except perhaps in mexico, and in rare instances--attains the portliness of its western relatives, yet it may equal them in longevity. the other relative is glyptostrobus, a sort of modified taxodium, being about as much like our bald cypress as one species of redwood is like the other. now, species of the same type, especially when few, and the type peculiar, are, in a general way, associated geographically, i.e., inhabit the same country, or (in a large sense) the same region. where it is not so, where near relatives are separated, there is usually something to be explained. here is an instance. stance. these four trees, sole representatives of their tribe, dwell almost in three separate quarters of the world: the two redwoods in california, the bald cypress in atlantic north america, its near relative, glyptostrobus, in china. it was not always so. in the tertiary period, the geological botanists assure us, our own very taxodium or bald cypress, and a glyptostrobus, exceedingly like the present chinese tree, and more than one sequoia, coexisted in a fourth quarter of the globe, viz., in europe! this brings up the question, is it possible to bridge over these four wide intervals of space and the much vaster interval of time, so as to bring these extraordinarily separated relatives into connection? the evidence which may be brought to bear upon this question is various and widely scattered. i bespeak your patience while i endeavor to bring together, in an abstract, the most important points of it. some interesting facts may come out by comparing generally the botany of the three remote regions, each of which is the sole home of one of these genera, i.e., sequoia in california, taxodium in the atlantic united states,[v- ] and glyptostrobus in china, which compose the whole of the peculiar tribe under consideration. note then, first, that there is another set of three or four peculiar trees, in this case of the yew family, which has just the same peculiar distribution, and which therefore may have the same explanation, whatever that explanation be. the genus torreya, which commemorates our botanical nestor and a former president of this association, dr. torrey, was founded upon a tree rather lately discovered (that is, about thirty-five years ago) in northern florida. it is a noble, yew like tree, and very local, being, so far as known, nearly confined to a few miles along the shores of a single river. it seems as if it had somehow been crowded down out of the alleghanies into its present limited southern quarters; for in cultivation it evinces a northern hardiness. now, another species of torreya is a characteristic tree of japan; and one very like it, if not the same, inhabits the mountains of northern china--belongs, therefore, to the eastern asiatic temperate region, of which northern china is a part, and japan, as we shall see, the portion most interesting to us. there is only one more species of torreya, and that is a companion of the redwoods in california. it is the tree locally known under the name of the california nutmeg. here are three or four near brethren, species of the same genus, known nowhere else than in these three habitats. moreover, the torreya of florida is associated with a yew; and the trees of this grove are the only yew-trees of eastern north america; for the yew of our northern woods is a decumbent shrub. a yew-tree, perhaps the same, is found with taxodium in the temperate parts of mexico. the only other yews in america grow with the redwoods and the other torreya in california, and extend northward into oregon. yews are also associated with torreya in japan; and they extend westward through mantchooria and the himalayas to western europe, and even to the azores islands, where occurs the common yew of the old world. so we have three groups of coniferous trees which agree in this peculiar geographical distribution, with, however, a notable extension of range in the case of the yew: . the redwoods, and their relatives, taxodium and glyptostrobus, which differ so as to constitute a genus for each of the three regions; . the torreyas, more nearly akin, merely a different species in each region; . the yews, still more closely related while more widely disseminated, of which it is yet uncertain whether they constitute seven, five, three, or only one species. opinions differ, and can hardly be brought to any decisive test. however it be determined, it may still be said that the extreme differences among the yews do not surpass those of the recognized variations of the european yew, the cultivated races included. it appears to me that these several instances all raise the very same question, only with different degrees of emphasis, and, if to be explained at all, will have the same kind of explanation. continuing the comparison between the three regions with which we are concerned, we note that each has its own species of pines, firs, larches, etc., and of a few deciduous-leaved trees, such as oaks and maples; all of which have no peculiar significance for the present purpose, because they are of genera which are common all round the northern hemisphere. leaving these out of view, the noticeable point is that the vegetation of california is most strikingly unlike that of the atlantic united states. they possess some plants, and some peculiarly american plants, in common--enough to show, as i imagine, that the difficulty was not in the getting from the one district to the other, or into both from a common source, but in abiding there. the primordially unbroken forest of atlantic north america, nourished by rainfall distributed throughout the year, is widely separated from the western region of sparse and discontinuous tree-belts of the same latitude on the western side of the continent (where summer rain is wanting, or nearly so), by immense treeless plains and plateaux of more or less aridity, traversed by longitudinal mountain-ranges of a similar character. their nearest approach is at the north, in the latitude of lake superior, where, on a more rainy line, trees of the atlantic forest and that of oregon may be said to intermix. the change of species and of the aspect of vegetation in crossing, say on the forty-seventh parallel, is slight in comparison with that on the thirty-seventh or near it. confining our attention to the lower latitude, and under the exceptions already specially noted, we may say that almost every characteristic form in the vegetation of the atlantic states is wanting in california, and the characteristic plants and trees of california are wanting here. california has no magnolia nor tulip trees, nor star-anise tree; no so-called papaw (asimina); no barberry of the common single-leaved sort; no podophyllum or other of the peculiar associated genera; no nelumbo nor white water-lily; no prickly ash nor sumach; no loblolly-bay nor stuartia; no basswood nor linden-trees; neither locust, honey-locust, coffeetrees (gymnocladus) nor yellow-wood (cladrastis); nothing answering to hydrangea or witch-hazel, to gum-trees (nyssa and liquidambar), viburnum or diervilla; it has few asters and golden-rods; no lobelias; no huckleberries and hardly any blueberries; no epigaea, charm of our earliest eastern spring, tempering an icy april wind with a delicious wild fragrance; no kalmia nor clethra, nor holly, nor persimmon; no catalpa-tree, nor trumpet-creeper (tecoma); nothing answering to sassafras, nor to benzoin-tree, nor to hickory; neither mulberry nor elm; no beech, true chestnut, hornbeam, nor iron-wood, nor a proper birch-tree; and the enumeration might be continued very much further by naming herbaceous plants and others familiar only to botanists. in their place california is filled with plants of other types--trees, shrubs, and herbs, of which i will only remark that they are, with one or two exceptions, as different from the plants of the eastern asiatic region with which we are concerned (japan, china, and mantchooria), as they are from those of atlantic north america. their near relatives, when they have any in other lands, are mostly southward, on the mexican plateau, or many as far south as chili. the same may be said of the plants of the intervening great plains, except that northward in the subsaline vegetation there are some close alliances with the flora of the steppes of siberia. and along the crests of high mountain-ranges the arctic-alpine . flora has sent southward more or less numerous representatives through the whole length of the country. if we now compare, as to their flora generally, the atlantic united states with japan, mantchooria, and northern china--i.e., eastern north america with eastern north asia, half the earth's circumference apart--we find an astonishing similarity. the larger part of the genera of our own region, which i have enumerated as wanting in california, are present in japan or mantchooria, along with many other peculiar plants, divided between the two. there are plants enough of the one region which have no representatives in the other. there are types which appear to have reached the atlantic states from the south; and there is a larger infusion of subtropical asiatic types into temperate china and japan; among these there is no relationship between the two countries to speak of. there are also, as i have already said, no small number of genera and some species which, being common all round or partly round the northern temperate zone, have no special significance because of their occurrence in these two antipodal floras, although they have testimony to bear upon the general question of geographical distribution. the point to be remarked is, that many, or even most, of the genera and species which are peculiar to north america as compared with europe, and largely peculiar to atlantic north america as compared with the californian region, are also represented in japan and mantchooria, either by identical or by closely-similar forms! the same rule holds on a more northward line, although not so strikingly. if we compare the plants, say of new england and pennsylvania (latitude _ ), with those of oregon, and then with those of northeastern asia, we shall find many of our own curiously repeated in the latter, while only a small number of them can be traced along the route even so far as the western slope of the rocky mountains. and these repetitions of east american types in japan and neighboring districts are in all degrees of likeness. sometimes the one is undistinguishable from the other; sometimes there is a difference of aspect, but hardly of tangible character; sometimes the two would be termed marked varieties if they grew naturally in the same forest or in the same region; sometimes they are what the botanist calls representative species, the one answering closely to the other, but with some differences regarded as specific; sometimes the two are merely of the same genus, or not quite that, but of a single or very few species in each country; in which case the point which interests us is, that this peculiar limited type should occur in two antipodal places, and nowhere else. it would be tedious, and, except to botanists, abstruse, to enumerate instances; yet the whole strength of the case depends upon the number of such instances. i propose therefore, if the association does me the honor to print this discourse, to append in a note a list of the more remarkable ones.[v- ] but i would here mention certain cases as specimens. our rhus toxicodendron, or poison-ivy, is very exactly repeated in japan, but is found in no other part of the world, although a species much like it abounds in california. our other poisonous rhus (r. venenata), commonly called poison-dogwood, is in no way represented in western america, but has so close an analogue in japan that the two were taken for the same by thunberg and linnaeus, who called them both r. vernix. our northern fox-grape, vitis labrusca, is wholly confined to the atlantic states, except that it reappears in japan and that region. the original wistaria is a woody leguminous climber with showy blossoms, native to the middle atlantic states; the other species, which we so much prize in cultivation, w. sinensis, is from china, as its name denotes, or perhaps only from japan, where it is certainly indigenous. our yellow-wood (cladrastis) inhabits a very limited district on the western slope of the alleghanies. its only and very near relative, maackia, is confined to mantchooria. the hydrangeas have some species in our alleghany region: all the rest belong to the chino-japanese region and its continuation westward. the same may be said of philadelphus, except that there are one or two mostly very similar species in california and oregon. our may-flower (epigaea) and our creeping snowberry, otherwise peculiar to atlantic north america, recur in japan. our blue cohosh (caulophyllum) is confined to the woods of the atlantic states, but has lately been discovered in japan. a peculiar relative of it, diphylleia, confined to the higher alleghanies, is also repeated in japan, with a slight difference, so that it may barely be distinguished as another : species. another relative is our twin-leaf (jeffersonia) of the alleghany region alone: a second species has lately turned up in mantchooria. a relative of this is podophyllum, our mandrake, a common inhabitant of the atlantic united states, but found nowhere else. there is one other species of it, and that is in the himalayas. here are four most peculiar genera of one family, each of a single species in the atlantic united states, which are duplicated on the other side of the world, either in identical or almost identical species, or in an analogous species, while nothing else of the kind is known in any other part of the world. i ought not to omit ginseng, the root so prized by the chinese, which they obtain from their northern provinces and mantchooria, and which is now known to inhabit corea and northern japan. the jesuit fathers identified the plant in canada and the atlantic states, brought over the chinese name by which we know it, and established the trade in it, which was for many years most profitable. the exportation of ginseng to china probably has not yet entirely ceased. whether the asiatic and the atlantic american ginsengs are to be regarded as of the same species or not is somewhat uncertain, but they are hardly, if at all, distinguishable. there is a shrub, elliottia, which is so rare and local that it is known only at two stations on the savannah river in georgia. it is of peculiar structure, and was without near relative until one was lately discovered in japan (tripetaleia), so like it as hardly to be distinguishable except by having the parts of the blossom in threes instead of fours--a difference not uncommon in the same genus, or even in the same species. suppose elliottia had happened to be collected only once, a good while ago, and all knowledge of the limited and obscure locality were lost; and meanwhile the japanese form came to be known. such a case would be parallel with an actual one. a specimen of a peculiar plant (shortia galacifolia) was detected in the herbarium of the elder michaux, who collected it (as his autograph ticket shows) somewhere in the high alleghany mountains, more than eighty years ago. no one has seen the living plant since or knows where to find it, if haply it still flourishes in some secluded spot. at length it is found in japan; and i had the satisfaction of making the identification.[v- ] a relative is also known in japan; and a less near one has just been detected in thibet. whether the japanese and the alleghanian plants are exactly the same or not, it needs complete specimens of the two to settle. so far as we know, they are just alike; and, even if some difference were discerned between them, it would not appreciably alter the question as to how such a result came to pass. each and every one of the analogous cases i have been detailing--and very many more could be mentioned--raises the same question, and would be satisfied with the same answer. these singular relations attracted my curiosity early in the course of my botanical studies, when comparatively few of them were known, and my serious attention in later years, when i had numerous and new japanese plants to study in the collections made, by messrs. williams and morrow, during commodore perry's visit in , and especially, by mr. charles wright, of commodore rodgers's expedition in . i then discussed this subject somewhat fully, and tabulated the facts within my reach.[v- ] this was before heer had developed the rich fossil botany of the arctic zone, before the immense antiquity of existing species of plants was recognized, and before the publication of darwin's now famous volume on the "origin of species" had introduced and familiarized the scientific world with those now current ideas respecting the history and vicissitudes of species with which i attempted to deal in a moderate and feeble way. my speculation was based upon the former glaciation of the northern temperate zone, and the inference of a warmer period preceding and perhaps following. i considered that our own present vegetation, or its proximate ancestry, must have occupied the arctic and subarctic regions in pliocene times, and that it had been gradually pushed southward as the temperature lowered and the glaciation advanced, even beyond its present habitation; that plants of the same stock and kindred, probably ranging round the arctic zone as the present arctic species do, made their forced migration southward upon widely different longitudes, and receded more or less as the climate grew warmer; that the general difference of climate which marks the eastern and the western sides of the continents--the one extreme, the other mean--was doubtless even then established, so that the same species and the same sorts of species would be likely to secure and retain foothold in the similar climates of japan and the atlantic united states, but not in intermediate regions of different distribution of heat and moisture; so that different species of the same genus, as in torreya, or different genera of the same group, as redwood, taxodium, and glyptostrobus, or different associations of forest-trees, might establish themselves each in the region best suited to the particular requirements, while they would fail to do so in any other. these views implied that the sources of our actual vegetation and the explanation of these peculiarities were to be sought in, and presupposed, an ancestry in pliocene or earlier times, occupying the higher northern regions. and it was thought that the occurrence of peculiar north american genera in europe in the tertiary period (such as taxodium, carya, liquidambar, sassafras, negundo, etc.) might be best explained on the assumption of early interchange and diffusion through north asia, rather than by that of the fabled atlantis. the hypothesis supposed a gradual modification of species in different directions under altering conditions, at least to the extent of producing varieties, sub-species, and representative species, as they may be variously regarded; likewise the single and local origination of each type, which is now almost universally taken for granted. the remarkable facts in regard to the eastern american and asiatic floras which these speculations were to explain have since increased in number, especially through the admirable collections of dr. maximowicz in japan and adjacent countries, and the critical comparisons he has made and is still engaged upon. i am bound to state that, in a recent general work[v- ] by a distinguished european botanist, prof. grisebach, of jotting, these facts have been emptied of all special significance, and the relations between the japanese and the atlantic united states flora declared to be no more intimate than might be expected from the situation, climate, and present opportunity of interchange. this extraordinary conclusion is reached by regarding as distinct species all the plants common to both countries between which any differences have been discerned, although such differences would probably count for little if the two inhabited the same country, thus transferring many of my list of identical to that of representative species; and then by simply eliminating from consideration the whole array of representative species, i.e., all cases in which the japanese and the american plant are not exactly alike. as if, by pronouncing the cabalistic word species, the question were settled, or rather the greater part of it remanded out of the domain of science; as if, while complete identity of forms implied community of origin, anything short of it carried no presumption of the kind; so leaving all these singular duplicates to be wondered at, indeed, but wholly beyond the reach of inquiry. now, the only known cause of such likeness is inheritance; and as all transmission of likeness is with some difference in individuals, and as changed conditions have resulted, as is well known, in very considerable differences, it seems to me that, if the high antiquity of our actual vegetation could be rendered probable, not to say certain, and the former habitation of any of our species or of very near relatives of them in high northern regions could be ascertained, my whole case would be made out. the needful facts, of which i was ignorant when my essay was published, have now been for some years made known--thanks, mainly, to the researches of heer upon ample collections of arctic fossil plants. these are confirmed and extended by new investigations, by heer and lesquereux, the results of which have been indicated to me by the latter.[v- ] the taxodium, which everywhere abounds in the miocene formations in europe, has been specifically identified, first by goeppert, then by heer, with our common cypress of the southern states. it has been found fossil in spitzbergen, greenland, and alaska--in the latter country along with the remains of another form, distinguishable, but very like the common species; and this has been identified by lesquereux in the miocene of the rocky mountains. so there is one species of tree which has come down essentially unchanged from the tertiary period, which for a long while inhabited both europe and north america, and also, at some part of the period, the region which geographically connects the two (once doubtless much more closely than now), but which has survived only in the atlantic united states and mexico. the same sequoia which abounds in the same miocene formations in northern europe has been abundantly found in those of iceland, spitzbergen, greenland, mackenzie river, and alaska. it is named s. langsdorfii, but is pronounced to be very much like s. sempervirens, our living redwood of the californian coast, and to be the ancient representative of it. fossil specimens of a similar, if not the same, species have recently been detected in the rocky mountains by hayden, and determined by our eminent palaeontological botanist, lesquereux; and he assures me that he has the common redwood itself from oregon in a deposit of tertiary age. another sequoia (s. sternbergii), discovered in miocene deposits in greenland, is pronounced to be the representative of s. gigantea, the big tree of the californian sierra. if the taxodium of the tertiary time in europe and throughout the arctic regions is the ancestor of our present bald cypress--which is assumed in regarding them as specifically identical-- then i think we may, with our present light, fairly assume that the two redwoods of california are the direct or collateral descendants of the two ancient species which so closely resemble them. the forests of the arctic zone in tertiary times contained at least three other species of sequoia, as determined by their remains, one of which, from spitzbergen, also much resembles the common redwood of california. another, "which appears to have been the commonest coniferous tree on disco," was common in england and some other parts of europe. so the sequoias, now remarkable for their restricted station and numbers, as well as for their extraordinary size, are of an ancient stock; their ancestors and kindred formed a large part of the forests which flourished throughout the polar regions, now desolate and ice-clad, and which extended into low latitudes in europe. on this continent one species, at least, had reached to the vicinity of its present habitat before the glaciation of the region. among the fossil specimens already found in california, but which our trustworthy palaeontological botanist has not yet had time to examine, we may expect to find evidence of the early arrival of these two redwoods upon the ground which they now, after much vicissitude, scantily occupy. differences of climate, or circumstances of migration, or both, must have determined the survival of sequoia upon the pacific, and of taxodium upon the atlantic coast. and still the redwoods will not stand in the east, nor could our taxodium find a congenial station in california. both have probably had their opportunity in the olden time, and failed. as to the remaining near relative of sequoia, the chinese glyptostrobus, a species of it, and its veritable representative, was contemporaneous with sequoia and taxodium, not only in temperate europe, but throughout the arctic regions from greenland to alaska. according to newberry, it was abundantly represented in the miocene flora of the temperate zone of our own continent, from nebraska to the pacific. very similar would seem to have been the fate of a more familiar gymnospermous tree, the gingko or salisburia. it is now indigenous to japan only. its ancestor, as we may fairly call it--since, according to heer, "it corresponds so entirely with the living species that it can scarcely be separated from it"--once inhabited northern europe and the whole arctic region round to alaska, and had even a representative farther south, in our rocky mountain district. for some reason, this and glyptostrobus survive only on the shores of eastern asia. libocedrus, on the other hand, appears to have cast in its lot with the sequoias. two species, according to heer, were with them in spitzbergen. l. decurrens, the incense cedar, is one of the noblest associates of the present redwoods. but all the rest are in the southern hemisphere, two at the southern extremity of the andes, two in the south-sea islands. it is only by bold and far-reaching suppositions that they can be geographically associated. the genealogy of the torreyas is still wholly obscure; yet it is not unlikely that the yew-like trees, named taxites, which flourished with the sequoias in the tertiary arctic forests, are the remote ancestors of the three species of torreya, now severally in florida, in california, and in japan. as to the pines and firs, these were more numerously associated with the ancient sequoias of the polar forests than with their present representatives, but in different species, apparently more like those of eastern than of western north america. they must have encircled the polar zone then, as they encircle the present temperate zone now. i must refrain from all enumeration of the angiospermous or ordinary deciduous trees and shrubs, which are now known, by their fossil remains, to have flourished throughout the polar regions when greenland better deserved its name and enjoyed the present climate of new england and new jersey. then greenland and the rest of the north abounded with oaks, representing the several groups of species which now inhabit both our eastern and western forest districts; several poplars, one very like our balsam poplar or balm-of-gilead tree; more beeches than there are now, a hornbeam, and a hop-hornbeam, some birches, a persimmon, and a planer-tree, near representatives of those of the old world, at least of asia, as well as of atlantic north america, but all wanting in california; one juglans like the walnut of the old world, and another like our black walnut; two or three grapevines, one near our southern fox grape or muscadine, another near our northern frostgrape; a tilia, very like our basswood of the atlantic states only; a liquidambar; a magnolia, which recalls our m. grandiflora; a liriodendron, sole representative of our tulip-tree; and a sassafras, very like the living tree. most of these, it will be noticed, have their nearest or their only living representatives in the atlantic states, and when elsewhere, mainly in eastern asia. several of them, or of species like them, have been detected in our tertiary deposits, west of the mississippi, by newberry and lesquereux. herbaceous plants, as it happens, are rarely preserved in a fossil state, else they would probably supply additional testimony to the antiquity of our existing vegetation, its wide diffusion over the northern and now frigid zone, and its enforced migration under changes of climate.[v- ] concluding, then, as we must, that our existing vegetation is a continuation of that of the tertiary period, may we suppose that it absolutely originated then? evidently not. the preceding cretaceous period has furnished to carruthers in europe a fossil fruit like that of the sequoia gigantea of the famous groves, associated with pines of the same character as those that accompany the present tree; has furnished to heer, from greenland, two more sequoias, one of them identical with a tertiary species, and one nearly allied to sequoia langsdorfii, which in turn is a probable ancestor of the common california redwood; has furnished to newberry and lesquereux in north america the remains of another ancient sequoia, a glyptostrobus, a liquidambar which well represents our sweet-gum-tree, oaks analogous to living ones, leaves of a plane-tree, which are also in the tertiary, and are scarcely distinguishable from our own platanus occidentalis, of a magnolia and a tulip-tree, and "of a sassafras undistinguishable from our living species." i need not continue the enumeration. suffice it to say that the facts justify the conclusion which lesquereux--a scrupulous investigator--has already announced: that "the essential types of our actual flora are marked in the cretaceous period, and have come to us after passing, without notable changes, through the tertiary formations of our continent." according to these views, as regards plants at least, the adaptation to successive times and changed conditions has been maintained, not by absolute renewals, but by gradual modifications. i, for one, cannot doubt that the present existing species are the lineal successors of those that garnished the earth in the old time before them, and that they were as well adapted to their surroundings then, as those which flourish and bloom around us are to their conditions now. order and exquisite adaptation did not wait for man's coming, nor were they ever stereotyped. organic nature--by which i mean the system and totality of living things, and their adaptation to each other and to the world--with all its apparent and indeed real stability, should be likened, not to the ocean, which varies only by tidal oscillations from a fixed level to which it is always returning, but rather to a river, so vast that we can neither discern its shores nor reach its sources, whose onward flow is not less actual because too slow to be observed by the ephemerae which hover over its surface, or are borne upon its bosom. such ideas as these, though still repugnant to some, and not long since to many, have so possessed the minds of the naturalists of the present day that hardly a discourse can be pronounced or an investigation prosecuted without reference to them. i suppose that the views here taken are little, if at all, in advance of the average scientific mind of the day. i cannot regard them as less noble than those which they are succeeding. an able philosophical writer, miss frances power cobbe, has recently and truthfully said:[v- ] "it is a singular fact that, when we can find out how anything is done, our first conclusion seems to be that god did not do it. no matter how wonderful, how beautiful, how intimately complex and delicate has been the machinery which has worked, perhaps for centuries, perhaps for millions of ages, to bring about some beneficent result, if we can but catch a glimpse of the wheels its divine character disappears." i agree with the writer that this first conclusion is premature and unworthy--i will add, deplorable. through what faults or infirmities of dogmatism on the one hand, and skepticism on the other, it came to be so thought, we need not here consider. let us hope, and i confidently expect, that it is not to last; that the religious faith which survived without a shock the notion of the fixity of the earth itself may equally outlast the notion of the fixity of the species which inhabit it; that, in the future even more than in the past, faith in an order, which is the basis of science, will not--as it cannot reasonably--be dissevered from faith in an ordainer, which is the basis of religion. vi the attitude of working naturalists toward darwinism [vi- ] (the nation, october , ) that homely adage, "what is one man's meat is another man's poison," comes to mind when we consider with what different eyes different naturalists look upon the hypothesis of the derivative origin of actual specific forms, since mr. darwin gave it vogue and vigor and a raison d'être for the present day. this latter he did, not only by bringing forward a vera causa in the survival of the fittest under changing circumstances--about which the question among naturalists mainly is how much it will explain, some allowing it a restricted, others an unlimited operation--but also by showing that the theory may be made to do work, may shape and direct investigations, the results of which must in time tell us whether the theory is likely to hold good or not. if the hypothesis of natural selection and the things thereto appertaining had not been capable of being put to useful work, although, like the "vestiges of the natural history of creation," it might have made no little noise in the world, it would hardly have engaged the attention of working naturalists as it has done. we have no idea even of opening the question as to what work the darwinian theory has incited, and in what way the work done has reacted upon the theory; and least of all do we like to meddle with the polemical literature of the subject, already so voluminous that the german bibliographers and booksellers make a separate class of it. but two or three treatises before us, of a minor or incidental sort, suggest a remark or two upon the attitude of mind toward evolutionary theories taken by some of the working naturalists. mr. darwin's own expectation, that his new presentation of the subject would have little or no effect upon those who had already reached middle-age, has--out of paris--not been fulfilled. there are, indeed, one or two who have thought it their duty to denounce the theory as morally dangerous, as well as scientifically baseless; a recent instance of the sort we may have to consider further on. others, like the youth at the river's bank, have been waiting in confident expectation of seeing the current run itself dry. on the other hand, a notable proportion of the more active-minded naturalists had already come to doubt the received doctrine of the entire fixity of species, and still more that of their independent and supernatural origination. while their systematic work all proceeded implicitly upon the hypothesis of the independence and entire permanence of species, they were perceiving more or less clearly that the whole question was inevitably to be mooted again, and so were prepared to give the alternative hypothesis a dispassionate consideration. the veteran lyell set an early example, and, on a reconsideration of the whole question, wrote anew his famous chapter and reversed his former and weighty opinion. owen, still earlier, signified his adhesion to the doctrine of derivation in some form, but apparently upon general, speculative grounds; for he repudiated natural selection, and offered no other natural solution of the mystery of the orderly incoming of cognate forms. as examples of the effect of darwin's "origin of species" upon the minds of naturalists who are no longer young, and whose prepossessions, even more than lyell's, were likely to bias them against the new doctrine, two from the botanical side are brought to our notice through recent miscellaneous writings which are now before us.[vi- ] before the publication of darwin's first volume, m. alphonse de candolle had summed up the result of his studies in this regard, in the final chapter of his classical "geographie botanique raisonnee," in the conclusion, that existing vegetation must be regarded as the continuation, through many geological and geographical changes, of the anterior vegetations of the world; and that, consequently, the present distribution of species is explicable only in the light of their geological history. he surmised that, notwithstanding the general stability of forms, certain species or quasi-species might have originated through diversification under geographical isolation. but, on the other hand, he was still disposed to admit that even the same species might have originated independently in two or more different regions of the world; and he declined, as unpractical and unavailing, all attempts to apply hypotheses to the elucidation of the origin of species. soon after darwin's book appeared, de candolle had occasion to study systematically a large and wide-spread genus-- that of the oak. investigating it under the new light of natural selection, he came to the conclusion that the existing oaks are all descendants of earlier forms, and that no clear line can be drawn between the diversification whic h has resulted in species and that which is exhibited in races and minor varieties. and now, in the introductory chapter of the volume of essays before us, he informs us that the idea which pervades them all, and in some sort connects very diverse topics, is that of considering this principle of selection. of the principle itself, he remarks that it is neither a theory nor an hypothesis, but the expression of a necessary fact; that to deny it is very much like denying that round stones will roll downhill faster and farther than flat ones; and that the question of the present day in natural history is not whether there be natural selection, or even whether forms are derived from other forms, but to comprehend how, in what proportions, and by what means hereditary deviations take place, and in what ways an inevitable selection takes effect upon these. in two of these essays natural selection is directly discussed in its application to the human race; the larger one dealing ably with the whole subject, and with results at first view seemingly in a great degree negative, but yet showing that the supposed "failure of natural selection in the case of man" was an unwarrantable conclusion from too limited a view of a very complicated question. the article abounds in acute and fertile suggestions, and its closing chapter, "on the probable future of the human species" under the laws of selection, is highly interesting and noteworthy. the other and shorter essay discusses a special point, and brings out a corollary of the law of heredity which may not have been thought of before, but which is perfectly clear as soon as it is stated. it explains at once why contagious or epidemic diseases are most fatal at their first appearance, and less so afterward: not by the dying out of a virus--for, when the disease reaches a new population, it is as virulent as ever (as, for instance, the smallpox among the indians)--but by the selection of a race less subject to attack through the destruction of those that were more so, and the inheritance of the comparative immunity by the children and the grandchildren of the survivors; and how this immunity itself, causing the particular disease to become rare, paves the way to a return of the original fatality; for the mass of such population, both in the present and the immediately preceding generation, not having been exposed to the infection, or but little exposed, has not undergone selection, and so in time the proportion liable to attack, or to fatal attack, gets to be as large as ever. the greater the fatality, especially in the population under marriageable age, the more favorable the condition of the survivors; and, by the law of heredity, their children should share in the immunity. this explanation of the cause, or of one cause, of the return of pests at intervals no less applies to the diminution of the efficacy of remedies, and of preventive means, such as vaccination. when jenner introduced vaccination, the small-pox in europe and european colonies must have lost somewhat of its primitive intensity by the vigorous weeding out of the more susceptible through many generations. upon the residue, vaccination was almost complete protection, and, being generally practised, small-pox consequently became rare. selection thus ceasing to operate, a population arises which has not been exposed to the contagion, and of which a considerable proportion, under the common law of atavism, comes to be very much in the condition of a people invaded for the first time by the disease. to these, as we might expect, vaccination would prove a less safeguard than to their progenitors three or four generations before. mr. bentham is a veteran systematic botanist of the highest rank and widest knowledge. he had not, so far as we know, touched upon questions of origination in the ante-darwinian era. the dozen of presidential addresses delivered at anniversary meetings of the linnean society, from his assumption of the chair in the year down to the current year--each devoted to some topic of interest--and his recent "memoir on compositae," summing up the general results of a revision of an order to which a full tenth of all higher plants belong, furnish apt examples both of cautious criticism, conditional assent (as becomes the inaugurator of the quantification of the predicate), and of fruitful application of the new views to various problems concerning the classification and geographical distribution of plants. in his hands the hypothesis is turned at once to practical use as an instrument of investigation, as a means of interrogating nature. in the result, no doubt seems to be left upon the author's mind that the existing species of plants are the result of the differentiation of previous species, or at least that the derivative hypothesis is to be adopted as that which offers the most natural, if not the only, explanation of the problems concerned. similar conclusions reached in this country, from a study of the relations of its present flora with that which in earlier ages occupied the arctic zone, might also be referred to. (see preceding article.) an excellent instance of the way in which the derivative hypothesis is practically applied in these days, by a zoologist, is before us in prof. flower's modest and admirable paper on the ungulata, or hoofed animals, and their geological history. we refer to it here, not so much for the conclusions it reaches or suggests, as to commend the clearness and the impartiality of the handling, and the sobriety and moderation of the deductions. confining himself "within the region of the known, it is shown that, at least in one group of animals, the facts which we have as yet acquired point to the former existence of various intermediate forms, so numerous that they go far to discredit the view of the sudden introduction of new species. . . . the modern forms are placed along lines which converge toward a common centre." the gaps between the existing forms of the odd-toed group of ungulates (of which horses, rhinoceroses, and tapirs, are the principal representatives) are most bridged over by palaeontology, and somewhat the same may be said of the even-toed group, to which the ruminants and the porcine genus belong. "moreover, the lines of both groups to a certain extent approximate, but, within the limits of our knowledge, they do not meet. . - . was the order according to which the introduction of new forms seems to have taken place since the eocene then entirely changed, or did it continue as far back as the period when these lines would have been gradually fused in a common centre?" facts like these, which suggest grave diversification under long lapse of time, are well supplemented by those which essentially demonstrate a slighter diversification of many species over a wide range of space; whether into species or races depends partly upon how the naturalist uses these terms, partly upon the extent of the observations, or luck in getting together intermediate forms. the researches of prof. baird upon the birds of this continent afford a good illustration. a great number of our birds which have been, and must needs have been, regarded as very distinct species, each mainly with its own geographical area, are found to mingle their characters along bordering lines; and the same kinds of differences (of coloration, form, or other) are found to prevail through the species of each region, thus impressing upon them a geographical facies. upon a submergence of the continent, reducing these several regions to islands sufficiently separated, these forms would be unquestioned species. considerations such as these, of which a few specimens have now been adduced (not general speculations, as the unscientific are apt to suppose), and trials of the new views to see how far they will explain the problems or collocate the facts they are severally dealing with, are what have mainly influenced working naturalists in the direction of the provisional acceptance of the derivative hypothesis. they leave to polemical speculators the fruitless discussion of the question whether all species came from one or two, or more; they are trying to grasp the thing by the near, not by the farther end, and to ascertain, first of all, whether it is probable or provable that present species are descendants of former ones which were like them, but less and less like them the farther back we go. and it is worth noting that they all seem to be utterly unconscious of wrong-doing. their repugnance to novel hypotheses is only the natural and healthy one. a change of a wonted line of thought is not made without an effort, nor need be made without adequate occasion. some courage was required of the man who first swallowed an oyster from its shell; and of most of us the snail would still demand more. as the unaccustomed food proves to be good and satisfying, and also harmless, we may come to like it. that, however, which many good and eminent naturalists find to be healthful and reasonable, and others innocuous, a few still regard as most unreasonable and harmful. at present, we call to mind only two who not only hold to the entire fixity of species as an axiom or a confirmed principle, but also as a dogma, and who maintain, either expressly or implicitly, that the logical antithesis to the creation of species as they are, is not by law (which implies intention), but by chance. a recent book by one of these naturalists, or rather, by a geologist of eminence, the "story of the earth and man," by dr. dawson, is now before us. the title is too near that of guyot's "earth and man," with the publication of which popular volume that distinguished physical naturalist commenced his career in this country; and such catch-titles are a sort of trade-mark. as to the nature and merits of dr. dawson's work, we have left ourselves space only to say: . that it is addressed ad populum, which renders it rather the more than less amenable to the criticisms we may be disposed to make upon it. . that the author is thoroughly convinced that no species or form deserving the name was ever derived from another, or originated from natural causes; and he maintains this doctrine with earnestness, much variety of argument and illustration, and no small ability; so that he may be taken as a representative of the view exactly opposed to that which is favored by those naturalists whose essays we have been considering--to whom, indeed, he stands in marked contrast in spirit and method, being greatly disposed to argue the question from the remote rather than the near end. . and finally, he has a conviction that the evolutionary doctrines of the day are not only untrue, but thoroughly bad and irreligious. this belief, and the natural anxiety with which he contemplates their prevalence, may excuse a certain vehemence and looseness of statement which were better avoided, as where the geologists of the day are said to be "broken up into bands of specialists, little better than scientific banditti, liable to be beaten in detail, and prone to commit outrages on common-sense and good taste which bring their otherwise good cause into disrepute;" and where he despairingly suggests that the prevalence of the doctrines he deprecates "seems to indicate that the accumulated facts of our age have gone altogether beyond its capacity for generalization, and, but for the vigor which one sees everywhere, might be taken as an indication that the human mind has fallen into a state of senility." this is droll reading, when one considers that the "evolutionist" is the only sort of naturalist who has much occasion to employ his "capacity for generalization" upon "the accumulated facts" in their bearing upon the problem of the origin of species; since the "special creationist," who maintains that they were supernaturally originated just as they are, by the very terms of his doctrine places them out of the reach of scientific explanation. again, when one reflects upon the new impetus which the derivative hypothesis has given to systematic natural history, and reads the declaration of a master in this department (the president of the linnean society) that mr. darwin "has in this nineteenth century brought about as great a revolution in the philosophic study of organic nature as that which was effected in the previous century by the immortal swede," it sounds oddly to hear from dr. dawson that "it obliterates the fine perception of differences from the mind of the naturalist, . . . . destroys the possibility of a philosophical classification, reducing all things to a mere series, and leads to a rapid decay in systematic zoology and botany, which is already very manifest among the disciples of spencer and darwin in england." so, also, "it removes from the study of nature the ideas of final cause and purpose"--a sentence which reads curiously in the light of darwin's special investigations, such as those upon the climbing of plants, the agency of insects in the fertilization of blossoms, and the like, which have brought back teleology to natural science, wedded to morphology and already fruitful of discoveries. the difficulty with dr. dawson here is (and it need not be underrated) that apparently he cannot as yet believe an adaptation, act, or result, to be purposed the apparatus of which is perfected or evolved in the course of nature--a common but a crude state of mind on the part of those who believe that there is any originating purpose in the universe, and one which, we are sure, dr. dawson does not share as respects the material world until he reaches the organic kingdoms, and there, possibly, because he sees man at the head of them--of them, while above them. however that may be, the position which dr. dawson chooses to occupy is not left uncertain. after concluding, substantially, that those "evolutionists" who exclude design from nature thereby exclude theism, which nobody will deny, he proceeds (on page ) to give his opinion that the "evolutionism which professes to have a creator somewhere behind it . . . . is practically atheistic," and, "if possible, more unphilosophical than that which professes to set out from absolute and eternal nonentity," etc. there are some sentences which might lead one to suppose that dr. dawson himself admitted of an evolution "with a creator somewhere behind it." he offers it (page ) as a permissible alternative that even man "has been created mediately by the operation of forces also concerned in the production of other animals;" concedes that a just theory "does not even exclude evolution or derivation, to a certain extent" (page ); and that "a modern man of science" may safely hold "that all things have been produced by the supreme creative will, acting either directly or through the agency of the forces and materials of his own production." well, if this be so, why denounce the modern man of science so severely upon the other page merely for accepting the permission? at first sight, it might be thought that our author is exposing himself in one paragraph to a share of the condemnation which he deals out in the other. but the permitted views are nowhere adopted as his own; the evolution is elsewhere restricted within specific limits; and as to "mediate creation," although we cannot divine what is here meant by the term, there is reason to think it does not imply that the several species of a genus were mediately created, in a natural way, through the supernatural creation of a remote common ancestor. so that his own judgment in the matter is probably more correctly gathered from the extract above referred to and other similar deliverances, such as that in which he warns those who "endeavor to steer a middle course, and to maintain that the creator has proceeded by way of evolution," that "the bare, hard logic of spencer, the greatest english authority on evolution, leaves no place for this compromise, and shows that the theory, carried out to its legitimate consequences, excludes the knowledge of a creator and the possibility of his work." now, this is a dangerous line to take. those defenders of the faith are more zealous than wise who must needs fire away in their catapults the very bastions of the citadel, in the defense of outposts that have become untenable. it has been and always will be possible to take an atheistic view of nature, but far more reasonable from science and philosophy only to take a theistic view. voltaire's saying here holds true: that if there were no god known, it would be necessary to invent one. it is the best, if not the only, hypothesis for the explanation of the facts. whether the philosophy of herbert spencer (which is not to our liking) is here fairly presented, we have little occasion and no time to consider. in this regard, the close of his article no. in the contemporary review shows, at least, his expectation of the entire permanence of our ideas of cause, origin, and religion, and predicts the futility of the expectation that the "religion of humanity" will be the religion of the future, or "can ever more than temporarily shut out the thought of a power, of which humanity is but a small and fugitive product, which was in its course of ever-changing manifestation before humanity was, and will continue through other manifestations when humanity has ceased to be." if, on the one hand, the philosophy of the unknowable of the infinite may be held in a merely quasi-theistic or even atheistic way, were not its ablest expounders and defenders hamilton and dean mansel? one would sup-pose that dr. dawson might discern at least as much of a divine foundation to nature as herbert spencer and matthew arnold; might recognize in this power that "something not ourselves that makes" for order as well as "for righteousness," and which he fitly terms supreme creative will; and, resting in this, endure with more complacency and faith the inevitable prevalence of evolutionary views which he is powerless to hinder. although he cannot arrest the stream, he might do something toward keeping it in safe channels. we wished to say something about the way in which scientific men, worthy of the name, hold hypotheses and theories, using them for the purpose of investigation and the collocation of facts, yielding or withholding assent in degrees or provisionally, according to the amount of verification or likelihood, or holding it long in suspense; which is quite in contrast to that of amateurs and general speculators (not that we reckon dr. dawson in this class), whose assent or denial seldom waits, or endures qualification. with them it must on all occasions be yea or nay only, according to the letter of the scriptural injunction, and whatsoever is less than this, or between the two, cometh of evil. vii evolution and theology [vii- ] (the nation, january , ) the attitude of theologians toward doctrines of evolution, from the nebular hypothesis down to "darwinism," is no less worthy of consideration, and hardly less diverse, than that of naturalists. but the topic, if pursued far, leads to questions too wide and deep for our handling here, except incidentally, in the brief notice which it falls in our way to take of the rev. george henslow's recent volume on "the theory of evolution of living things." this treatise is on the side of evolution, "considered as illustrative of the wisdom and beneficence of the almighty." it was submitted for and received one of the actonian prizes recently awarded by the royal institution of great britain. we gather that the staple of a part of it is worked up anew from some earlier discourses of the author upon "genesis and geology," "science and scripture not antagonistic," etc. in coupling with it a chapter of the second volume of dr. hodge's "systematic theology (part ii, anthropology)," we call attention to a recent essay, by an able and veteran writer, on the other side of the question. as the two fairly enough represent the extremes of christian thought upon the subject, it is convenient to review them in connection. theologians have a short and easy, if not wholly satisfactory, way of refuting scientific doctrines which they object to, by pitting the authority or opinion of one savant against another. already, amid the currents and eddies of modern opinion, the savants may enjoy the same advantage at the expense of the divines-- we mean, of course, on the scientific arena; for the mutual refutation of conflicting theologians on their own ground is no novelty. it is not by way of offset, however, that these divergent or contradictory views are here referred to, but only as an illustration of the fact that the divines are by no means all arrayed upon one side of the question in hand. and indeed, in the present transition period, until some one goes much deeper into the heart of the subject, as respects the relations of modern science to the foundations of religious belief, than either of these writers has done, it is as well that the weight of opinion should be distributed, even if only according to prepossessions, rather than that the whole stress should bear upon a single point, and that perhaps the authority of an interpretation of scripture. a consensus of opinion upon dr. hodge's ground, for instance (although better guarded than that of dr. dawson), if it were still possible, would--to say the least--probably not at all help to reconcile science and religion. therefore, it is not to be regretted that the diversities of view among accredited theologians and theological naturalists are about as wide and as equably distributed between the extremes (and we may add that the views themselves are quite as hypothetical) as those which prevail among the various naturalists and natural philosophers of the day. as a theologian, mr. henslow doubtless is not to be compared with the veteran professor at princeton. on the other hand, he has the advantage of being a naturalist, and the son of a naturalist, as well as a clergyman: consequently he feels the full force of an array of facts in nature, and of the natural inferences from them, which the theological professor, from his biblical standpoint, and on his implicit assumption that the old testament must needs teach true science, can hardly be expected to appreciate. accordingly, a naturalist would be apt to say of dr. hodge's exposition of "theories of the universe" and kindred topics--and in no captious spirit-- that whether right or wrong on particular points, he is not often right or wrong in the way of a man of science. probably from the lack of familiarity with prevalent ideas and their history, the theologians are apt to suppose that scientific men of the present day are taking up theories of evolution in pure wantonness or mere superfluity of naughtiness; that it would have been quite possible, as well as more proper, to leave all such matters alone. quieta non movere is doubtless a wise rule upon such subjects, so long as it is fairly applicable. but the time for its application in respect to questions of the origin and relations of existing species has gone by. to ignore them is to imitate the foolish bird that seeks security by hiding its head in the sand. moreover, the naturalists did not force these questions upon the world; but the world they study forced them upon the naturalists. how these questions of derivation came naturally and inevitably to be revived, how the cumulative probability that the existing are derived from preexisting forms impressed itself upon the minds of many naturalists and thinkers, mr. henslow has briefly explained in the introduction and illustrated in the succeeding chapters of the first part of his book. science, he declares, has been compelled to take up the hypothesis of the evolution of living things as better explaining all the phenomena. in his opinion, it has become "infinitely more probable that all living and extinct beings have been developed or evolved by natural laws of generation from preexisting forms, than that they, with all their innumerable races and varieties, should owe their existences severally to creative fiats." this doctrine, which even dr. hodge allows may possibly be held in a theistic sense, and which, as we suppose, is so held or viewed by a great proportion of the naturalists of our day, mr. henslow maintains is fully compatible with dogmatic as well as natural theology; that it explains moral anomalies, and accounts for the mixture of good and evil in the world, as well as for the merely relative perfection of things; and, finally, that "the whole scheme which god has framed for man's existence, from the first that was created to all eternity, collapses if the great law of evolution be suppressed." the second part of his book is occupied with a development of this line of argument. by this doctrine of evolution he does not mean the darwinian hypothesis, although he accepts and includes this, looking upon natural selection as playing an important though not an unlimited part. he would be an evolutionist with mivart and owen and argyll, even if he had not the vera causa which darwin contributed to help him on. and, on rising to man, he takes ground with wallace, saying: "i would wish to state distinctly that i do not at present see any evidence for believing in a gradual development of man from the lower animals by ordinary natural laws; that is, without some special interference, or, if it be preferred, some exceptional conditions which have thereby separated him from all other creatures, and placed him decidedly in advance of them all. on the other hand, it would be absurd to regard him as totally severed from them. it is the great degree of difference i would insist upon, bodily, mental, and spiritual, which precludes the idea of his having been evolved by exactly the same processes, and with the same limitations, as, for example, the horse from the palaeotherium." in illustrating this view, he reproduces wallace's well-known points, and adds one or two of his own. we need not follow up his lines of argument. the essay, indeed, adds nothing material to the discussion of evolution, although it states one side of the case moderately well, as far as it goes. dr. hodge approaches the subject from the side of systematic theology, and considers it mainly in its bearing upon the origin and original state of man. under each head he first lays down "the scriptural doctrine," and then discusses "anti-scriptural theories," which latter, under the first head, are the heathen doctrine of spontaneous generation, the modern doctrine of spontaneous generation, theories of development, specially that of darwin, the atheistic character of the theory, etc. although he admits "that there is a theistic and an atheistic form of the nebular hypothesis as to the origin of the universe, so there may be a theistic interpretation of the darwinian theory," yet he contends that "the system is thoroughly atheistic," notwithstanding that the author "expressly acknowledges the existence of god." curiously enough, the atheistic form of evolutionary hypotheses, or what he takes for such, is the only one which dr. hodge cares to examine. even the "reign of law" theory, owen's "purposive route of development and chance . . . . by virtue of inherent tendencies thereto," as well as other expositions of the general doctrine on a theistic basis, are barely mentioned without a word of comment, except, perhaps, a general "protest against the arraying of probabilities against the teachings of scripture." now, all former experience shows that it is neither safe nor wise to pronounce a whole system "thoroughly atheistic" which it is conceded may be held theistically, and which is likely to be largely held, if not to prevail, on scientific grounds. it may be well to remember that, "of the two great minds of the seventeenth century, newton and leibnitz, both profoundly religious as well as philosophical, one produced the theory of gravitation, the other objected to that theory that it was subversive of natural religion; also that the nebular hypothesis--a natural consequence of the theory of gravitation and of the subsequent progress of physical and astronomical discovery--has been denounced as atheistical even down to our day." it has now outlived anathema. it is undeniable that mr. darwin lays himself open to this kind of attack. the propounder of natural selection might be expected to make the most of the principle, and to overwork the law of parsimony in its behalf. and a system in which exquisite adaptation of means to ends, complicated inter-dependencies, and orderly sequences, appear as results instead of being introduced as factors, and in which special design is ignored in the particulars, must needs be obnoxious, unless guarded as we suppose mr. darwin might have guarded his. ground if he had chosen to do so. our own opinion, after long consideration, is, that mr. darwin has no atheistical intent; and that, as respects the test question of design in nature, his view may be made clear to the theological mind by likening it to that of the "believer in general but not in particular providence." there is no need to cull passages in support of this interpretation from his various works while the author--the most candid of men--retains through all the editions of the "origin of species" the two mottoes from whewell and bishop butler.[vii- ] the gist of the matter lies in the answer that should be rendered to the questions-- . do order and useful-working collocation, pervading a system throughout all its parts, prove design? and, . is such evidence negatived or invalidated by the probability that these particular collocations belong to lineal series of such in time, and diversified in the course of nature--grown up, so to say, step by step? we do not use the terms "adaptation, "arrangement of means to ends," and the like, because they beg the question in stating it. finally, ought not theologians to consider whether they have not already, in principle, conceded to the geologists and physicists all that they are asked to concede to the evolutionists; whether, indeed, the main natural theological difficulties which attend the doctrine of evolution--serious as they may be--are not virtually contained in the admission that there is a system of nature with fixed laws. this, at least, we may say, that, under a system in which so much is done "by the establishment of general laws," it is legitimate for any one to prove, if he can, that any particular thing in the natural world is so done; and it is the proper business of scientific men to push their enquiries in this direction. it is beside the point for dr. hodge to object that, "from the nature of the case, what concerns the origin of things cannot be known except by a supernatural revelation;" that "science has to do with the facts and laws of nature: here the question concerns the origin of such facts." for the very object of the evolutionists, and of mr. darwin in particular, is to remove these subjects from the category of origination, and to bring them under the domain of science by treating them as questions about how things go on, not how they began. whether the succession of living forms on the earth is or is not among the facts and laws of nature, is the very matter in controversy. moreover, adds dr. hodge, it has been conceded that in this matter "proofs, in the proper sense of the word, are not to be had; we are beyond the region of demonstration, and have only probabilities to consider." wherefore "christians have a right to protest against the arraying of probabilities against the clear teachings of scripture." the word is italicized, as if to intimate that probabilities have no claims which a theologian is bound to respect. as to arraying them against scripture, there is nothing whatever in the essay referred to that justifies the statement. indeed, no occasion offered; for the writer was discussing evolution in its relations to theism, not to biblical theology, and probably would not be disposed to intermix arguments so different in kind as those from natural science and those from revelation. to pursue each independently, according to its own method, and then to compare the results, is thought to be the better mode of proceeding. the weighing of probabilities we had regarded as a proper exercise of the mind preparatory to forming an opinion. probabilities, hypotheses, and even surmises, whatever they may be worth, are just what, as it seems to us, theologians ought not to be foremost in decrying, particularly those who deal with the reconciliation of science with scripture, genesis with geology, and the like. as soon as they go beyond the literal statements even of the english text, and enter into the details of the subject, they find ample occasion and display a special aptitude for producing and using them, not always with very satisfactory results. it is not, perhaps, for us to suggest that the theological army in the past has been too much encumbered with impedimenta for effective aggression in the conflict against atheistic tendencies in modern science; and that in resisting attack it has endeavored to hold too much ground, so wasting strength in the obstinate defense of positions which have become unimportant as well as untenable. some of the arguments, as well as the guns, which well served a former generation, need to be replaced by others of longer range and greater penetration. if the theologians are slow to discern the signs and exigencies of the times, the religious philosophical naturalists must be looked to. since the above remarks were written, prof. le conte's "religion and science," just issued, has come to our hands. it is a series of nineteen sunday lectures on the relation of natural and revealed religion, prepared in the first instance for a bible-class of young men, his pupils in the university of south carolina, repeated to similar classes at the university of california, and finally delivered to a larger and general audience. they are printed, the preface states, from a verbatim report, with only verbal alterations and corrections of some redundancies consequent upon extemporaneous delivery. they are not, we find, lectures on science under a religious aspect, but discourses upon christian theology and its foundations from a scientific layman's point of view, with illustrations from his own lines of study. as the headings show, they cover, or, more correctly speaking, range over, almost the whole field of theological thought, beginning with the personality of deity as revealed in nature, the spiritual nature and attributes of deity, and the incarnation; discussing by the way the general relations of theology to science, man, and his place in nature; and ending with a discussion of predestination and free-will, and of prayer in relation to invariable law--all in a volume of three hundred and twenty-four duodecimo pages! and yet the author remarks that many important subjects have been omitted because he felt unable to present them in a satisfactory manner from a scientific point of view. we note, indeed, that one or two topics which would naturally come in his way--such, especially, as the relation of evolution to the human race--are somewhat conspicuously absent. that most of the momentous subjects which he takes up are treated discursively, and not exhaustively, is all the better for his readers. what they and we most want to know is, how these serious matters are viewed by an honest, enlightened, and devout scientific man. to solve the mysteries of the universe, as the french lady required a philosopher to explain his new system, "dans un mot," is beyond rational expectation. all that we have time and need to say of this little book upon great subjects relates to its spirit and to the view it takes of evolution. its theology is wholly orthodox; its tone devotional, charitable, and hopeful; its confidence in religious truth, as taught both in nature and revelation, complete; the illustrations often happy, but often too rhetorical; the science, as might be expected from this author, unimpeachable as regards matters of fact, discreet as to matters of opinion. the argument from design in the first lecture brings up the subject of the introduction of species. of this, considered "as a question of history, there is no witness on the stand except geology." "the present condition of geological evidence is undoubtedly in favor of some degree of suddenness--is against infinite gradations. the evidence may be meagre . . . but whether meagre or not, it is all the evidence we have. . . . now, the evidence of geology to-day is, that species seem to come in suddenly and in full perfection, remain substantially unchanged during the term of their existence, and pass away in full perfection. other species take their place apparently by substitution, not by transmutation. but you will ask me, 'do you, then, reject the doctrine of evolution? do you accept the creation of species directly and without secondary agencies and processes?' i answer, no! science knows nothing of phenomena which do not take place by secondary causes and processes. she does not deny such occurrence, for true science is not dogmatic, and she knows full well that, tracing up the phenomena from cause to cause, we must somewhere reach the more direct agency of a first cause. . . . it is evident that, however species were introduced, whether suddenly or gradually, it is the duty of science ever to strive to understand the means and processes by which species originated. . . . now, of the various conceivable secondary causes and processes, by some of which we must believe species originated, by far the most probable is certainly that of evolution from other species." (we might interpose the remark that the witness on the stand, if subjected to cross-examination by a biologist, might be made to give a good deal of testimony in favor of transmutation rather than substitution.) after referring to different ideas as to the cause or mode of evolution, he concludes that it can make no difference, so far as the argument of design in nature is concerned, whether there be evolution or not, or whether, in the case of evolution, the change be paroxysmal or uniform. we may infer even that he accepts the idea that "physical and chemical forces are changed into vital force, and vice versa." physicists incline more readily to this than physiologists; and if what is called vital force be a force in the physicists' sense, then it is almost certainly so. but the illustration on page touches this point only seemingly. it really concerns only the storing and the using of physical force in a living organism. if, for want of a special expression, we continue to use the term vital force to designate that intangible something which directs and governs the accumulation and expenditure of physical force in organisms, then there is as yet no proof and little likelihood that this is correlate with physical force. "a few words upon the first chapter of genesis and the mosaic cosmogony, and i am done," says prof. le conte, and so are we: "it might be expected by many that, after speaking of schemes of reconciliation, i should give mine also. my christian friends, these schemes of reconciliation become daily more and more distasteful to me. i have used them in times past; but now the deliberate construction of such schemes seems to me almost like trifling with the words of scripture and the teachings of nature. they seem to me almost irreverent, and quite foreign to the true, humble, liberal spirit of christianity; they are so evidently artificial, so evidently mere ingenious human devices. it seems to me that if we will only regard the two books in the philosophical spirit which i have endeavored to describe, and then simply wait and possess our souls in patience, the questions in dispute will soon adjust themselves as other similar questions have already done." viii what is darwinism? [viii- ] the nation, may , ) the question which dr. hodge asks he promptly and decisively answers: "what is darwinism? it is atheism." leaving aside all subsidiary and incidental matters, let us consider-- . what the darwinian doctrine is, and . how it is proved to be atheistic. dr. hodge's own statement of it cannot be very much bettered: "his [darwin's] work on the 'origin of species' does not purport to be philosophical. in this aspect it is very different from the cognate works of mr. spencer. darwin does not speculate on the origin of the universe, on the nature of matter or of force. he is simply a naturalist, a careful and laborious observer, skillful in his descriptions, and singularly candid in dealing with the difficulties in the way of his peculiar doctrine. he set before himself a single problem--namely, how are the fauna and flora of our earth to be accounted for? . . . to account for the existence of matter and life, mr. darwin admits a creator. this is done explicitly and repeatedly. . . . he assumes the efficiency of physical causes, showing no disposition to resolve them into mind-force or into the efficiency of the first cause. . . . he assumes, also, the existence of life in the form of one or more primordial germs. . . . how all living things on earth, including the endless variety of plants and all the diversity of animals, . . . have descended from the primordial animalcule, he thinks, may be accounted for by the operation of the following natural laws, viz.: first, the law of heredity, or that by which like begets like--the offspring are like the parent. second, the law of variation; that is, while the offspring are in all essential characteristics like their immediate progenitor, they nevertheless vary more or less within narrow limits from their parent and from each other. some of these variations are indifferent, some deteriorations, some improvements--that is, such as enable the plant or animal to exercise its functions to greater advantage. third, the law of over-production. all plants and animals tend to increase in a geometrical ratio, and therefore tend to overrun enormously the means of support. if all the seeds of a plant, all the spawn of a fish, were to arrive at maturity, in a very short time the world could not contain them. hence, of necessity, arises a struggle for life. only a few of the myriads born can possibly live. fourth, here comes in the law of natural selection, or the survival of the fittest; that is, if any individual of a given species of plant or animal happens to have a slight deviation from the normal type favorable to its success in the struggle for life, it will survive. this variation, by the law of heredity, will be transmitted to its offspring, and by them again to theirs. soon these favored ones gain the ascendency, and the less favored perish, and the modification becomes established in the species. after a time, another and another of such favorable variations occur, with like results. thus, very gradually, great changes of structure are introduced, and not only species, but genera, families, and orders, in the vegetable and animal world, are produced" (pp. - ). now, the truth or the probability of darwin's hypothesis is not here the question, but only its congruity or incongruity with theism. we need take only one exception to this abstract of it, but that is an important one for the present investigation. it is to the sentence which we have italicized in the earlier part of dr. hodge's own statement of what darwinism is. with it begins our inquiry as to how he proves the doctrine to be atheistic. first, if we rightly apprehend it, a suggestion of atheism is infused into the premises in a negative form: mr. darwin shows no disposition to resolve the efficiency of physical causes into the efficiency of the first cause. next (on page ) comes the positive charge that "mr. darwin, although himself a theist," maintains that "the contrivances manifested in the organs of plants and animals . . . are not due to the continued cooperation and control of the divine mind, nor to the original purpose of god in the constitution of the universe." as to the negative statement, it might suffice to recall dr. hodge's truthful remark that darwin "is simply a naturalist," and that "his work on the origin of species does not purport to be philosophical." in physical and physiological treatises, the most religious men rarely think it necessary to postulate the first cause, nor are they misjudged by the omission. but surely mr. darwin does show the disposition which our author denies him, not only by implication in many instances, but most explicitly where one would naturally look for it, namely--at the close of the volume in question: "to my mind, it accords better with what we know of the laws impressed on matter by the creator," etc. if that does not refer the efficiency of physical causes to the first cause, what form of words could do so? the positive charge appears to be equally gratuitous. in both dr. hodge must have overlooked the beginning as well as the end of the volume which he judges so hardly. just as mathematicians and physicists, in their systems, are wont to postulate the fundamental and undeniable truths they are concerned with, or what they take for such and require to be taken for granted, so mr. darwin postulates, upon the first page of his notable work, and in the words of whewell and bishop butler: . the establishment by divine power of general laws, according to which, rather than by insulated interpositions in each particular case, events are brought about in the material world; and . that by the word ':natural" is meant "stated, fixed, or settled," by this same power, "since what is natural as much requires and presupposes an intelligent agent to render it so--i.e., to effect it continually or at stated times--as what is supernatural or miraculous does to effect it for once.[viii- ] so when mr. darwin makes such large and free use of "natural as antithetical to supernatural" causes, we are left in no doubt as to the ultimate source which he refers them to. rather let us say there ought to be no doubt, unless there are other grounds for it to rest upon. such ground there must be, or seem to be, to justify or excuse a veteran divine and scholar like dr. hodge in his deduction of pure atheism from a system produced by a confessed theist, and based, as we have seen, upon thoroughly orthodox fundamental conceptions. even if we may not hope to reconcile the difference between the theologian and the naturalist, it may be well to ascertain where their real divergence begins, or ought to begin, and what it amounts to. seemingly, it is in their proximate, not in their ultimate, principles, as dr. hodge insists when he declares that the whole drift of darwinism is to prove that everything "may be accounted for by the blind operation of natural causes, without any intention, purpose, or cooperation of god." "why don't he say," cries the theologian, "that the complicated organs of plants and animals are the product of the divine intelligence? if god made them, it makes no difference, so far as the question of design is concerned, how he made them, whether at once or by process of evolution." but, as we have seen, mr. darwin does say that, and he over and over implies it when he refers the production of species "to secondary causes," and likens their origination to the origination of individuals; species being series of individuals with greater difference. it is not for the theologian to object that the power which made individual men and other animals, and all the differences which the races of mankind exhibit, through secondary causes, could not have originated congeries of more or less greatly differing individuals through the same causes. clearly, then, the difference between the theologian and the naturalist is not fundamental, and evolution may be as profoundly and as particularly theistic as it is increasingly probable. the taint of atheism which, in dr. hodge's view, leavens the whole lump, is not inherent in the original grain of darwinism--in the principles posited--but has somehow been introduced in the subsequent treatment. possibly, when found, it may be eliminated. perhaps there is mutual misapprehension growing out of some ambiguity in the use of terms. "without any intention, purpose, or cooperation of god."- these are sweeping and effectual words. how came they to be applied to natural selection by a divine who professes that god ordained whatsoever cometh to pass? in this wise: "the point to be proved is, that it is the distinctive doctrine of mr. darwin that species owe their origin-- . not to the original intention of the divine mind; . not to special acts of creation calling new forms into existence at certain epochs; . not to the constant and everywhere operative efficiency of god guiding physical causes in the production of intended effects; but . to the gradual accumulation of unintended variations of structure and instinct securing some advantage to their subjects." then dr. hodge adduces "darwin's own testimony," to the purport that natural selection denotes the totality of natural causes and their interactions, physical and physiological, reproduction, variation, birth, struggle, extinction--in short, all that is going on in nature; that the variations which in this interplay are picked out for survival are not intentionally guided; that "nothing can be more hopeless than the attempt to explain this similarity of pattern in members of the same class by utility or the doctrine of final causes" (which dr. hodge takes to be the denial of any such thing as final causes); and that the interactions and processes going on which constitute natural selection may suffice to account for the present diversity of animals and plants (primordial organisms being postulated and time enough given) with all their structures and adaptations--that is, to account for them scientifically, as science accounts for other things. a good deal may be made of this, but does it sustain the indictment? moreover, the counts of the indictment may be demurred to. it seems to us that only one of the three points which darwin is said to deny is really opposed to the fourth, which he is said to maintain, except as concerns the perhaps ambiguous word unintended. otherwise, the origin of species through the gradual accumulation of variations--i.e., by the addition of a series of small differences--is surely not incongruous with their origin through "the original intention of the divine mind" or through "the constant and everywhere operative efficiency of god."- one or both of these mr. darwin (being, as dr. hodge says, a theist) must needs hold to in some form or other; wherefore he may be presumed to hold the fourth proposition in such wise as not really to contradict the first or the third. the proper antithesis is with the second proposition only, and the issue comes to this: have the multitudinous forms of living creatures, past and present, been produced by as many special and independent acts of creation at very numerous epochs? or have they originated under causes as natural as reproduction and birth, and no more so, by the variation and change of preceding into succeeding species? those who accept the latter alternative are evolutionists. and dr. hodge fairly allows that their views, although clearly wrong, may be genuinely theistic. surely they need not become the less so by the discovery or by the conjecture of natural operations through which this diversification and continued adaptation of species to conditions is brought about. now, mr. darwin thinks--and by this he is distinguished. from most evolutionists--that he can assign actual natural causes, adequate to the production of the present out of the preceding state of the animal and vegetable world, and so on backward--thus uniting, not indeed the beginning but the far past with the present in one coherent system of nature. but in assigning actual natural causes and processes, and applying them to the explanation of the whole case, mr. dar-win assumes the obligation of maintaining their general sufficiency--a task from which the numerous advocates and acceptors of evolution on the general concurrence of probabilities and its usefulness as a working hypothesis (with or without much conception of the manner how) are happily free. having hit upon a modus operandi which all who understand it admit will explain something, and many that it will explain very much, it is to be expected that mr. darwin will make the most of it. doubtless he is far from pretending to know all the causes and operations at work; he has already added some and restricted the range of others; he probably looks for additions to their number and new illustrations of their efficiency; but he is bound to expect them all to fall within the category of what he calls natural selection (a most expansible principle), or to be congruous with it--that is, that they shall be natural causes. also--and this is the critical point--he is bound to maintain their sufficiency without intervention. here, at length, we reach the essential difference between darwin, as we understand him, and dr. hodge. the terms which darwin sometimes uses, and doubtless some of the ideas they represent, are not such as we should adopt or like to defend; and we may say once for all--aside though it be from the present issue--that, in our opinion, the adequacy of the assigned causes to the explanation of the phenomena has not been made out. but we do not understand him to deny "purpose, intention, or the cooperation of god" in nature. this would be as gratuitous as unphilosophical, not to say unscientific. when he speaks of this or that particular or phase in the course of events or the procession of organic forms as not intended, he seems to mean not specially and disjunctively intended and not brought about by intervention. purpose in the whole, as we suppose, is not denied but implied. and when one considers how, under whatever view of the case, the designed and the contingent lie inextricably commingled in this world of ours, past man's disentanglement, and into what metaphysical dilemmas the attempt at unraveling them leads, we cannot greatly blame the naturalist for relegating such problems to the philosopher and the theologian. if charitable, these will place the most favorable construction upon attempts to extend and unify the operation of known secondary causes, this being the proper business of the naturalist and physicist; if wise, they will be careful not to predicate or suggest the absence of intention from what comes about by degrees through the continuous operation of physical causes, even in the organic world, lest, in their endeavor to retain a probable excess of supernaturalism in that realm of nature, they cut away the grounds for recognizing it at all in inorganic nature, and so fall into the same condemnation that some of them award to the darwinian. moreover, it is not certain that mr. darwin would very much better his case, dr. hodge being judge, if he did propound some theory of the nexus of divine causation and natural laws, or even if he explicitly adopted the one or the other of the views which he is charged with rejecting. either way he might meet a procrustean fate; and, although a saving amount of theism might remain, he would not be sound or comfortable. for, if he predicates "the constant and everywhere operative efficiency of god," he may "lapse into the same doctrine" that the duke of argyll and sir john herschel "seem inclined to," the latter of whom is blamed for thinking "it but reasonable to regard the force of gravitation as the direct or indirect result of a consciousness or will existing somewhere," and the former for regarding "it unphilosophical 'to think or speak as if the forces of nature were either independent of or even separate from the creator's power' ": while if he falls back upon an "original intention of the divine mind," endowing matter with forces which he foresaw and intended should produce such results as these contrivances in nature, he is told that this banishes god from the world, and is inconsistent with obvious facts. and that because of its implying that "he never interferes to guide the operation of physical causes. we italicize the word, for interference proves to be the keynote of dr. hodge's system. interference with a divinely ordained physical nature for the accomplishment of natural results! an unorthodox friend has just imparted to us, with much misgiving and solicitude lest he should be thought irreverent, his tentative hypothesis, which is, that even the creator may be conceived to have improved with time and experience! never before was this theory so plainly and barely put before us. we were obliged to say that, in principle and by implication, it was not wholly original. but in such matters, which are far too high for us, no one is justly to be held responsible for the conclusions which another may draw from his principles or assumptions. dr. hodge's particular view should be gathered from his own statement of it: "in the external world there is always and everywhere indisputable evidence of the activity of two kinds of force, the one physical, the other mental. the physical belongs to matter, and is due to the properties with which it has been endowed; the other is the everywhere present and ever-acting mind of god. to the latter are to be referred all the manifestations of design in nature, and the ordering of events in providence. this doctrine does not ignore the efficiency of second causes; it simply asserts that god overrules and controls them. thus the psalmist says: 'i am fearfully and wonderfully made. my substance was not hid from thee when i was made in secret, and curiously wrought (or embroidered) in the lower parts of the earth. . . . god makes the grass to grow, and herbs for the children of men.'- he sends rain, frost, and snow. he controls the winds and the waves. he determines the casting of the lot, the flight of an arrow, and the falling of a sparrow." far be it from us to object to this mode of conceiving divine causation, although, like the two other theistic conceptions referred to, it has its difficulties, and perhaps the difficulties of both. but, if we understand it, it draws an unusually hard and fast line between causation in organic and inorganic nature, seems to look for no manifestation of design in the latter except as "god overrules and controls" second causes, and, finally, refers to this overruling and controlling (rather than to a normal action through endowment) all embryonic development, the growth of vegetables, and the like. he even adds, without break or distinction, the sending of rain, frost, and snow, the flight of an arrow, and the falling of a sparrow. somehow we must have misconceived the bearing of the statement; but so it stands as one of "the three ways," and the right way, of "accounting for contrivances in nature; the other two being-- . their reference to the blind operation of natural causes; and, . that they were foreseen and purposed by god, who endowed matter with forces which he foresaw and intended should produce such results, but never interferes to guide their operation. in animadverting upon this latter view, dr. hodge brings forward an argument against evolution, with the examination of which our remarks must close: "paley, indeed, says that if the construction of a watch be an undeniable evidence of design, it would be a still more wonderful manifestation of skill if a watch could be made to produce other watches, and, it may be added, not only other watches, but all kinds of timepieces, in endless variety. so it has been asked, if a man can make a telescope, why cannot god make a telescope which produces others like itself? this is simply asking whether matter can be made to do the work of mind. the idea involves a contradiction. for a telescope to make a telescope supposes it to select copper and zinc in due proportions, and fuse them into brass; to fashion that brass into inter-entering tubes; to collect and combine the requisite materials for the different kinds of glass needed; to melt them, grind, fashion, and polish them, adjust their densities, focal distances, etc., etc. a man who can believe that brass can do all this might as well believe in god" (pp. , ). if dr. hodge's meaning is, that matter unconstructed cannot do the work of mind, he misses the point altogether; for original construction by an intelligent mind is given in the premises. if he means that the machine cannot originate the power that operates it, this is conceded by all except believers in perpetual motion, and it equally misses the point; for the operating power is given in the case of the watch, and implied in that of the reproductive telescope. but if he means that matter cannot be made to do the work of mind in constructions, machines, or organisms, he is surely wrong. "sovitur ambulando," vel scribendo; he confuted his argument in the act of writing the sentence. that is just what machines and organisms are for; and a consistent christian theist should maintain that is what all matter is for. finally, if, as we freely suppose, he means none of these, he must mean (unless we are much mistaken) that organisms originated by the almighty creator could not be endowed with the power of producing similar organisms, or slightly dissimilar organisms, without successive interventions. then he begs the very question in dispute, and that, too, in the face of the primal command, "be fruitful and multiply," and its consequences in every natural birth. if the actual facts could be ignored, how nicely the parallel would run! "the idea involves a contradiction." for an animal to make an animal, or a plant to make a plant, supposes it to select carbon, hydrogen, oxygen, and nitrogen, to combine these into cellulose and protoplasm, to join with these some phosphorus, lime, etc., to build them into structures and usefully-adjusted organs. a man who can believe that plants and animals can do this (not, indeed, in the crude way suggested, but in the appointed way) "might as well believe in god." yes, verily, and so he probably will, in spite of all that atheistical philosophers have to offer, if not harassed and confused by such arguments and statements as these. there is a long line of gradually-increasing divergence from the ultra-orthodox view of dr. hodge through those of such men as sir william thomson, herschel, argyll, owen, mivart, wallace, and darwin, down to those of strauss, vogt, and buchner. to strike the line with telling power and good effect, it is necessary to aim at the right place. excellent as the present volume is in motive and clearly as it shows that darwinism may bear an atheistic as well as a theistic interpretation, we fear that it will not contribute much to the reconcilement of science and religion. the length of the analysis of the first book on our list precludes the notices which we intended to take of the three others. they are all the production of men who are both scientific and religious, one of them a celebrated divine and writer unusually versed in natural history. they all look upon theories of evolution either as in the way of being established or as not unlikely to prevail, and they confidently expect to lose thereby no solid ground for theism or religion. mr. st. clair, a new writer, in his "darwinism and design; or, creation by evolution," takes his ground in the following succinct statement of his preface: "it is being assumed by our scientific guides that the design-argument has been driven out of the field by the doctrine of evolution. it seems to be thought by our theological teachers that the best defense of the faith is to deny evolution in toto, and denounce it as anti-biblical. my volume endeavors to show that, if evolution be true, all is not lost; but, on the contrary, something is gained: the design-argument remains unshaken, and the wisdom and beneficence of god receive new illustration." of his closing remark, that, so far as he knows, the subject has never before been handled in the same way for the same purpose, we will only say that the handling strikes us as mainly sensible rather than as substantially novel. he traverses the whole ground of evolution, from that of the solar system to "the origin of moral species." he is clearly a theistic darwinian without misgiving, and the arguments for that hypothesis and for its religious aspects obtain from him their most favorable presentation, while he combats the dysteleology of hackel, buchner, etc., not, however, with any remarkable strength. dr. winchell, chancellor of the new university at syracuse, in his volume just issued upon the "doctrine of evolution," adopts it in the abstract as "clearly as the law of universal intelligence under which complex results are brought into existence" (whatever that may mean), accepts it practically for the inorganic world as a geologist should, hesitates as to the organic world, and sums up the arguments for the origin of species by diversification unfavorably for the darwinians, regarding it mainly from the geological side. as some of our zoologists and palaeontologists may have somewhat to say upon this matter, we leave it for their consideration. we are tempted to develop a point which dr. winchell incidentally refers to--viz., how very modern the idea of the independent creation and fixity of species is, and how well the old divines got on without it. dr. winchell reminds us that st. augustine and st. thomas aquinas were model evolutionists; and, where authority is deferred to, this should count for something. mr. kingsley's eloquent and suggestive "westminster sermons," in which he touches here and there upon many of the topics which evolution brings up, has incorporated into the preface a paper which he read in i to a meeting of london clergy at sion college, upon certain problems of natural theology as affected by modern theories in science. we may hereafter have occasion to refer to this volume. meanwhile, perhaps we may usefully conclude this article with two or three short extracts from it: "the god who satisfies our conscience ought more or less to satisfy our reason also. to teach that was butler's mission; and he fulfilled it well. but it is a mission which has to be refulfilled again and again, as human thought changes, and human science develops, for if, in any age or country, the god who seems to be revealed by nature seems also different from the god who is revealed by the then-popular religion, then that god and the religion which tells of that god will gradually cease to be believed in. "for the demands of reason--as none knew better than good bishop butler--must be and ought to be satisfied. and, therefore, when a popular war arises between the reason of any generation and its theology, then it behooves the ministers of religion to inquire, with all humility and godly fear, on whose side lies the fault; whether the theology which they expound is all that it should be, or whether the reason of those who impugn it is all that it should be." pronouncing it to be the duty of the naturalist to find out the how of things, and of the natural theologian to find out the why, mr. kingsley continues: "but if it be said, 'after all, there is no why; the doctrine of evolution, by doing away with the theory of creation, does away with that of final causes,' let us answer boldly, 'not in the least.' we might accept all that mr. darwin, all that prof. huxley, all that other most able men have so learnedly and acutely written on physical science, and yet preserve our natural theology on the same basis as that on which butler and paley left it. that we should have to develop it i do not deny. "let us rather look with calmness, and even with hope and good-will, on these new theories; they surely mark a tendency toward a more, not a less, scriptural view of nature. "of old it was said by him, without whom nothing is made, 'my father worketh hitherto, and i work.' shall we quarrel with science if she should show how these words are true? what, in one word, should we have to say but this: 'we know of old that god was so wise that he could make all things; but, behold, he is so much wiser than even that, that he can make all things make themselves?' " charles darwin: a sketch (nature, june , , accompanying a portrait) two british naturalists, robert brown and charles darwin, have, more than any others, impressed their influence upon science in this nineteenth century. unlike as these men and their works were and are, we may most readily subserve the present purpose in what we are called upon to say of the latter by briefly comparing and contrasting the two. robert brown died sixteen years ago, full of years and scientific honors, and he seems to have finished, several years earlier, all the scientific work that he had undertaken. to the other, charles darwin, a fair number of productive years may yet remain, and are earnestly hoped for. both enjoyed the great advantage of being all their lives long free from exacting professional duties or cares, and so were able in the main to apply themselves to research without distraction and according to their bent. both, at the beginning of their career, were attached to expeditions of exploration in the southern hemisphere, where they amassed rich stores of observation and materials, and probably struck out, while in the field, some of the best ideas which they subsequently developed. they worked in different fields and upon different methods; only in a single instance, so far as we know, have they handled the same topic; and in this the more penetrating insight of the younger naturalist into an interesting general problem may be appealed to in justification of a comparison which some will deem presumptuous. be this as it may, there will probably be little dissent from the opinion that the characteristic trait common to the two is an unrivaled scientific sagacity. in this these two naturalists seem to us, each in his way, preeminent. there is a characteristic likeness, too--underlying much difference--in their admirable manner of dealing with facts closely, and at first hand, without the interposition of the formal laws, vague ideal conceptions, or "glittering generalities" which some philosophical naturalists make large use of. a likeness may also be discerned in the way in which the work or contributions of predecessors and contemporaries are referred to. the brief historical summaries prefixed to many of mr. brown's papers are models of judicial conscientiousness. and mr. darwin's evident delight at discovering that some one else has "said his good things before him," or has been on the verge of uttering them, seemingly equals that of making the discovery himself. it reminds one of goethe's insisting that his views in morphology must have been held before him and must be somewhere on record, so obvious did they appear to him. considering the quiet and retired lives led by both these men, and the prominent place they are likely to occupy in the history of science, the contrast between them as to contemporary and popular fame is very remarkable. while mr. brown was looked up to with the greatest reverence by all the learned botanists, he was scarcely heard of by any one else; and out of botany he was unknown to science except as the discoverer of the brownian motion of minute particles, which discovery was promulgated in a privately-printed pamphlet that few have ever seen. although mr. darwin had been for twenty years well and widely known for his "naturalist's journal," his works on "coral islands," on "volcanic islands, and especially for his researches on the barnacles, it was not till about fifteen years ago that his name became popularly famous. ever since no scientific name has been so widely spoken. many others have had hypotheses or systems named after them, but no one else that we know of a department of bibliography. the nature of his latest researches accounts for most of the difference, but not for all, the origin of species is a fascinating topic, having interests and connections with every branch of science, natural and moral. the investigation of recondite affinities is very dry and special; its questions, processes, and results alike--although in part generally presentable in the shape of morphology--are mainly, like the higher mathematics, unintelligible except to those who make them a subject of serious study. they are especially so when presented in mr. brown's manner. perhaps no naturalist ever recorded the results of his investigations in fewer words and with greater precision than robert brown: certainly no one ever took more pains to state nothing beyond the precise point in question. indeed, we have sometimes fancied that he preferred to enwrap rather than to explain his meaning; to put it into such a form that, unless you follow solomon's injunction and dig for the wisdom as for hid treasure, you may hardly apprehend it until you have found it all out for yourself, when you will have the satisfaction of perceiving that mr. brown not only knew all about it, but had put it upon record. very different from this is the way in which mr. darwin takes his readers into his confidence, freely displays to them the sources of his information, and the working of his mind, and even shares with them all his doubts and misgivings, while in a clear exposition he sets forth the reasons which have guided him to his conclusions. these you may hesitate or decline to adopt, but you feel sure that they have been presented with perfect fairness; and if you think of arguments against them you may be confident that they have all been duly considered before. the sagacity which characterizes these two naturalists is seen in their success in finding decisive instances, and their sure insight into the meaning of things. as an instance of the latter on mr. darwin's part, and a justification of our venture to compare him with the facile princeps botanicorum, we will, in conclusion, allude to the single instance in which they took the same subject in hand. in his papers on the organs and modes of fecundation in orchideae and asclepiadeae, mr. brown refers more than once to c.k. sprengel's almost forgotten work, shows how the structure of the flowers in these orders largely requires the agency of insects for their fecundation, and is aware that "in asclepiadeae . . . the insect so readily passes from one corolla to another that it not unfrequently visits every flower of the umbel." he must also have contemplated the transport of pollen from plant to plant by wind and insects; and we know from another source that he looked upon sprengel's ideas as far from fantastic. yet, instead of taking the single forward step which now seems so obvious, he even hazarded the conjecture that the insect-forms of some orchideous flowers are intended to deter rather than to attract insects. and so the explanation of all these and other extraordinary structures, as well as of the arrangement of blossoms in general, and even the very meaning and need of sexual propagation, were left to be supplied by mr. darwin. the aphorism "nature abhors a vacuum" is a characteristic specimen of the science of the middle ages. the aphorism "nature abhors close fertilization," and the demonstration of the principle, belong to our age, and to mr. darwin. to have originated this, and also the principle of natural selection--the truthfulness and importance of which are evident the moment it is apprehended--and to have applied these principles to the system of nature in such a manner as to make, within a dozen years, a deeper impression upon natural history than has been made since linnaeus, is ample title for one man's fame. there is no need of our giving any account or of estimating the importance of such works as the "origin of species by means of natural selection," the "variation of animals and plants under domestication," the "descent of man, and selection in relation to sex," and the "expression of the emotions in men and animals"--a series to which we may hope other volumes may in due time be added. we would rather, if space permitted, attempt an analysis of the less known, but not less masterly, subsidiary essays, upon the various arrangements for insuring cross-fertilization in flowers, for the climbing of plants, and the like. these, as we have heard, may before long be reprinted in a volume, and supplemented by some long-pending but still unfinished investigations upon the action of dionaea and drosera--a capital subject for mr. darwin's handling. a propos to these papers, which furnish excellent illustrations of it, let us recognize darwin's great service to natural science in bringing back to it teleology; so that, instead of morphology versus teleology, we shall have morphology wedded to teleology. to many, no doubt, evolutionary teleology comes in such a questionable shape as to seem shorn of all its goodness; but they will think better of it in time, when their ideas become adjusted, and they see what an impetus the new doctrines have given to investigation. they are much mistaken who suppose that darwinism is only of speculative importance, and perhaps transient interest. in its working applications it has proved to be a new power, eminently practical and fruitful. and here, again, we are bound to note a striking contrast to mr. brown, greatly as we revere his memory. he did far less work than was justly to be expected from him. mr. darwin not only points out the road, but labors upon it indefatigably and unceasingly. a most commendable noblesse oblige assures us that he will go on while strength (would we could add health) remains. the vast amount of such work he has already accomplished might overtax the powers of the strongest. that it could have been done at all under constant infirm health is most wonderful. x insectivorous plants (the nation, april and , ) that animals should feed upon plants is natural and normal, and the reverse seems impossible. but the adage, "natura non agit saltatim," has its application even here. it is the naturalist, rather than nature, that draws hard and fast lines everywhere, and marks out abrupt boundaries where she shades off with gradations. however opposite the parts which animals and vegetables play in the economy of the world as the two opposed kingdoms of organic nature, it is becoming more and more obvious that they are not only two contiguous kingdoms, but are parts of one whole--antithetical and complementary to each other, indeed; but such "thin partitions do the bounds divide" that no definitions yet framed hold good without exception. this is a world of transition in more senses than is commonly thought; and one of the lessons which the philosophical naturalist learns, or has to learn, is, that differences the most wide and real in the main, and the most essential, may nevertheless be here and there connected or bridged over by gradations. there is a limbo filled with organisms which never rise high enough in the scale to be manifestly either animal or plant, unless it may be said of some of them that they are each in turn and neither long. there are undoubted animals which produce the essential material of vegetable fabric, or build up a part of their structure of it, or elaborate the characteristic leaf-green which, under solar light, assimilates inorganic into organic matter, the most distinguishing function of vegetation. on the other hand, there are plants--microscopic, indeed, but unquestionable--which move spontaneously and freely around and among animals that are fixed and rooted. and, to come without further parley to the matter in hand, while the majority of animals feed directly upon plants, "for 'tis their nature to," there are plants which turn the tables and feed upon them. some, being parasitic upon living animals, feed insidiously and furtively; these, although really cases in point, are not so extraordinary, and, as they belong to the lower orders, they are not much regarded, except for the harm they do. there are others, and those of the highest orders, which lure or entrap animals in ways which may well excite our special wonder--all the more so since we are now led to conclude that they not only capture but consume their prey. as respects the two or three most notable instances, the conclusions which have been reached are among the very recent acquisitions of physiological science. curiously enough, however, now that they are made out, it appears that they were in good part long ago attained, recorded, and mainly forgotten. the earlier observations and surmises shared the common fate of discoveries made before the time, or by those who were not sagacious enough to bring out their full meaning or importance. vegetable morphology, dimly apprehended by linnaeus, initiated by casper frederick wolff, and again, independently in successive generations, by goethe and by de candolle, offers a parallel instance. the botanists of goethe's day could not see any sense, advantage, or practical application, to be made of the proposition that the parts of a blossom answer to leaves; and so the study of homologies had long to wait. until lately it appeared to be of no consequence whatever (except, perhaps, to the insects) whether drosera and sarracenia caught flies or not; and even dionaea excited only unreflecting wonder as a vegetable anomaly. as if there were real anomalies in nature, and some one plant possessed extraordinary powers denied to all others, and (as was supposed) of no importance to itself! that most expert of fly-catchers, dionaea, of which so much has been written and so little known until lately, came very near revealing its secret to solander and ellis a hundred years ago, and doubtless to john bartram, our botanical pioneer, its c probable discoverer, who sent it to europe. ellis, in his published letter to linnaeus, with which the history begins, described the structure and action of the living trap correctly; noticed that the irritability which called forth the quick movement closing the trap, entirely resided in the few small bristles of its upper face; that this whole surface was studded c with glands, which probably secreted a liquid; and that the trap did not open again when an insect was captured, even upon the death of the captive, although it opened very soon when nothing was caught, or when the irritation was caused by a bit of straw, or any such substance. it was linnaeus who originated the contrary and erroneous statement, which has long prevailed in the books, that the trap reopened when the fatigued captive became quiet, and let it go; as if the plant caught flies in mere play and pastime! linnaeus also omitted all allusion to a secreted liquid--which was justifiable, as. ellis does not state that he had actually seen any; and, if he did see it, quite mistook its use, supposing it to be, like the nectar of flowers, a lure for insects, a bait for the trap. whereas, in fact, the lure, if there be any, must be an odor (although nothing is perceptible to the human olfactories); for the liquid secreted by the glands never appears until the trap has closed upon some insect, and held it at least for some hours a prisoner. within twenty-four or forty-eight hours this glairy liquid is abundant, bathing and macerating the body of the perished insect. its analogue is not the nectar of flowers, but the saliva or the gastric juice! the observations which compel such an inference are re-cent, and the substance of them may be briefly stated. the late rev. dr. m. a. curtis (by whose death, two years ago, we lost one of our best botanists, and the master in his especial line, mycology), forty years and more ago resided at wilmington, north carolina, in the midst of the only district to which the dionaea is native; and he published, in , in the first volume of the "journal of the boston society of natural history," by far the best account of this singular plant which had then appeared. he remarks that "the little prisoner is not crushed and suddenly destroyed, as is sometimes supposed," for he had often liberated "captive flies and spiders, which sped away as fast as fear or joy could hasten them." but he neglected to state, although he must have noticed the fact, that the two sides of the trap, at first concave to the contained insect, at length flatten and close down firmly upon the prey, exerting no inconsiderable pressure, and insuring the death of any soft-bodied insect, if it had not already succumbed to the confinement and salivation. this last dr. curtis noticed, and first discerned its import, although he hesitated to pronounce upon its universality. that the captured insects were in some way "made subservient to the nourishment of the plant" had been conjectured from the first. dr. curtis "at times (and he might have always at the proper time) found them enveloped in a fluid of mucilaginous consistence, which seems to act as a solvent, the insects being more or less consumed in it." this was verified and the digestive character of the liquid well-nigh demonstrated six or seven years ago by mr. canby, of wilmington, delaware, who, upon a visit to the sister-town of north carolina, and afterward at his home, followed up dr. curtis's suggestions with some capital observations and experiments. these were published at philadelphia in the tenth volume of meehan's gardeners' monthly, august, i ; but they do not appear to have attracted the attention which they merited. the points which mr. canby made out are, that this fluid is always poured out around the captured insect in due time, "if the leaf is in good condition and the prey suitable;" that it comes from the leaf itself, and not from the decomposing insect (for, when the trap caught a plum-curculio, the fluid was poured out while he was still alive, though very weak, and endeavoring, ineffectively, to eat his way out); that bits of raw beef, although sometimes rejected after a while, were generally acted upon in the same manner--i.e., closed down upon tightly, salvered with the liquid, dissolved mainly, and absorbed; so that, in fine, the fluid may well be said to be analogous to the gastric juice of animals, dissolving the prey and rendering it fit for absorption by the leaf. many leaves remain inactive or slowly die away after one meal; others reopen for a second and perhaps even a third capture, and are at least capable of digesting a second meal. before mr. canby's experiments had been made, we were aware that a similar series had been made in england by mr. darwin, with the same results, and with a small but highly-curious additional one--namely, that the fluid secreted in the trap of dionaea, like the gastric juice, has an acid reaction. having begun to mention unpublished results (too long allowed to remain so), it may be well, under the circumstances, to refer to a still more remarkable experiment by the same most sagacious investigator. by a prick with a sharp lancet at a certain point, he has been able to paralyze one-half of the leaf-trap, so that it remained motionless under the stimulus to which the other half responded. such high and sensitive organization entails corresponding ailments. mr. canby tells us that he gave to one of his dionaea-subjects a fatal dyspepsia by feeding it with cheese; and under mr. darwin's hands another suffers from paraplegia. finally, dr. burdon-sanderson's experiments, detailed at the last meeting of the british association for the advancement of science, show that the same electrical currents are developed upon the closing of the dionaea-trap as in the contraction of a muscle. if the venus's fly-trap stood alone, it would be doubly marvelous--first, on account of its carnivorous propensities, and then as constituting a real anomaly in organic nature, to which nothing leads up. before acquiescing in such a conclusion, the modern naturalist would scrutinize its relatives. now, the nearest relatives of our vegetable wonder are the sundews. while dionaea is as local in habitation as it is singular in structure and habits, the droseras or sundews are widely diffused over the world and numerous in species. the two whose captivating habits have attracted attention abound in bogs all around the northern hemisphere. that flies are caught by them is a matter of common observation; but this was thought to be purely accidental. they spread out from the root a circle of small leaves, the upper face of which especially is beset and the margin fringed with stout bristles (or what seem to be such, although the structure is more complex), tipped by a secreting gland, which produces, while in vigorous state, a globule of clear liquid like a drop of dew-- whence the name, both greek and english. one expects these seeming dew-drops to be dissipated by the morning sun; but they remain unaffected. a touch shows that the glistening drops are glutinous and extremely tenacious, as flies learn to their cost on alighting, perhaps to sip the tempting liquid, which acts first as a decoy and then like birdlime. a small fly is held so fast, and in its struggles comes in contact with so many of these glutinous globules, that it seldom escapes. the result is much the same to the insect, whether captured in the trap of dionaea or stuck fast to the limed bristles of drosera. as there are various plants upon whose glandular hairs or glutinous surfaces small insects are habitually caught and perish, it might be pure coincidence that the most effectual arrangement of the kind happens to occur in the nearest relatives of dionaea. roth, a keen german botanist of the eighteenth century, was the first to detect, or at least to record, some evidence of intention in drosera, and to compare its action with that of dionaea, which, through ellis's account, had shortly before been made known in europe. he noticed the telling fact that not only the bristles which the unfortunate insect had come in contact with, but also the surrounding rows, before widely spreading, curved inward one by one, although they had not been touched, so as within a few hours to press their glutinous tips likewise against the body of the captive insect--thus doubling or quadrupling the bonds of the victim and (as we may now suspect) the surfaces through which some part of the animal substance may be imbibed. for roth surmised that both these plants were, in their way, predaceous. he even observed that the disk of the drosera-leaf itself often became concave and enveloped the prey. these facts, although mentioned now and then in some succeeding works, were generally forgotten, except that of the adhesion of small insects to the leaves of sundews, which must have been observed in every generation. up to and even within a few years past, if any reference was made to these asserted movements (as by such eminent physiologists as meyen and treviranus) it was to discredit them. not because they are difficult to verify, but because, being naturally thought improbable, it was easier to deny or ignore them. so completely had the knowledge of almost a century ago died out in later years that, when the subject was taken up anew in our days by mr. darwin, he had, as we remember, to advertise for it, by sending a "note and query" to the magazines, asking where any account of the fly-catching of the leaves of sundew was recorded. when mr. darwin takes a matter of this sort in hand, he is not likely to leave it where he found it. he not only confirmed all roth's observations as to the incurving of the bristles toward and upon an insect entangled on any part of the disk of the leaf, but also found that they responded similarly to a bit of muscle or other animal substance, while to any particles of inorganic matter they were nearly indifferent. to minute fragments of carbonate of ammonia, however, they were more responsive. as these remarkable results, attained (as we are able to attest) half a dozen years ago, remained unpublished (being portions of an investigation not yet completed), it would have been hardly proper to mention them, were it not that independent observers were beginning to bring out the same or similar facts. mrs. treat, of new jersey, noticed the habitual infolding of the leaf in the longer-leaved species of sundew (american journal of science for november, ), as was then thought for the first time--roth's and withering's observations not having been looked up. in recording this, the next year, in a very little book, entitled "how plants behave," the opportunity was taken to mention, in the briefest way, the capital discovery of mr. darwin that the leaves of drosera act differently when different objects are placed upon them, the bristles closing upon a particle of raw meat as upon a living insect, while to a particle of chalk or wood they are nearly inactive. the same facts were independently brought out by mr. a. w. bennett at the last year's meeting of the british association for the advancement of science, and have been mentioned in the journals. if to these statements, which we may certify, were added some far more extraordinary ones, communicated to the french academy of science in may last by m. zeigler, a stranger story of discrimination on the part of sundew-bristles would be told. but it is safer to wait for the report of the committee to which these marvels were referred, and conclude this sufficiently "strange eventful history" with some details of experiments made last summer by mrs. treat, of new jersey, and published in the december number of the american naturalist. it is well to note that mrs. treat selects for publication the observations of one particular day in july, when the sundew-leaves were unusually active; for their moods vary with the weather, and also in other unaccountable ways, although in general the sultrier days are the most appetizing: "at fifteen minutes past ten of the same day i placed bits of raw beef on some of the most vigorous leaves of drosera longifolia. ten minutes past twelve, two of the leaves had folded around the beef, hiding it from sight. half-past eleven of the same day, i placed living flies on the leaves of d. longifolia. at and minutes one of the leaves had folded entirely around its victim, the other leaves had partially folded, and the flies had ceased to struggle. by and minutes four leaves had each folded around a fly. . . . i tried mineral substances--bits of dry chalk, magnesia, and pebbles. in twenty-four hours, neither the leaves nor their bristles had made any move like clasping these articles. i wet a piece of chalk in water, and in less than an hour the bristles were curving about it, but soon unfolded again, leaving the chalk free on the blade of the leaf. parallel experiments made on d. rotundifolia, with bits of beef and of chalk, gave the same results as to the action of the bristles; while with a piece of raw apple, after eleven hours, "part of the bristles were clasping it, but not so closely as the beef," and in twenty-four hours "nearly all the bristles were curved toward it, but not many of the glands were touching it." to make such observations is as easy as it is interesting. throughout the summer one has only to transfer plants of drosera from the bogs into pots or pans filled with wet moss--if need be, allowing them to become established in the somewhat changed conditions, or even to put out fresh leaves--and to watch their action or expedite it by placing small flies upon the disk of the leaves. the more common round-leaved sundew acts as well as the other by its bristles, and the leaf itself is sometimes almost equally prehensile, although in a different way, infolding the whole border instead of the summit only. very curious, and even somewhat painful, is the sight when a fly, alighting upon the central dew-tipped bristles, is held as fast as by a spider's web; while the efforts to escape not only entangle the insect more hopelessly as they exhaust its strength, but call into action the surrounding bristles, which, one by one, add to the number of the bonds, each by itself apparently feeble, but in their combination so effectual that the fly may be likened to the sleeping gulliver made fast in the tiny but multitudinous toils of the liliputians. anybody who can believe that such an apparatus was not intended to capture flies might say the same of a spider's web. is the intention here to be thought any the less real because there are other species of drosera which are not so perfectly adapted for fly-catching, owing to the form of their leaves and the partial or total want of cooperation of their scattered bristles? one such species, d. filiformis, the thread-leaved sundew, is not uncommon in this country, both north and south of the district that dionaea locally inhabits. its leaves are long and thread-shaped, beset throughout with glutinous gland-tipped bristles, but wholly destitute of a blade. flies, even large ones, and even moths and butterflies, as mrs. treat and mr. canby affirm (in the american naturalist), get stuck fast to these bristles, whence they seldom escape. accidental as such captures are, even these thread-shaped leaves respond more or less to the contact, somewhat in the manner of their brethren. in mr. canby's recent and simple experiment, made at mr. darwin's suggestion, when a small fly alights upon a leaf a little below its slender apex, or when a bit of crushed fly is there affixed, within a few hours the tip of the leaf bends at the point of contact, and curls over or around the body in question; and mrs. treat even found that when living flies were pinned at half an inch in distance from the leaves, these in forty minutes had bent their tips perceptibly toward the flies, and in less than two hours reached them! if this be confirmed--and such a statement needs ample confirmation--then it may be suspected that these slender leaves not only incurve after prolonged contact, just as do the leaf-stalks of many climbers, but also make free and independent circular sweeps, in the manner of twining stems and of many tendrils. correlated movements like these indicate purpose. when performed by climbing plants, the object and the advantage are obvious. that the apparatus and the actions of dionaea and drosera are purposeless and without advantage to the plants themselves, many have been believed in former days, when it was likewise conceived that abortive and functionless organs were specially created "for the sake of symmetry" and to display a plan; but this is not according to the genius of modern science. in the cases of insecticide next to be considered, such evidence of intent is wanting, but other and circumstantial evidence may be had, sufficient to warrant convictions. sarracenias have hollow leaves in the form of pitchers or trumpet-shaped tubes, containing water, in which flies and other insects are habitually drowned. they are all natives of the eastern side of north america, growing in bogs or low ground, so that they cannot be supposed to need the water as such. indeed, they secrete a part if not all of it. the commonest species, and the only one at the north, which ranges from newfoundland to florida, has a broad-mouthed pitcher with an upright lid, into which rain must needs fall more or less. the yellow sarracenia, with long tubular leaves, called "trumpets in the southern states, has an arching or partly upright lid, raised well above the orifice, so that some water may rain in; but a portion is certainly secreted there, and may be seen bedewing the sides and collected at the bottom before the mouth opens. in other species, the orifice is so completely overarched as essentially to prevent the access of water from without. in these tubes, mainly in the water, flies and other insects accumulate, perish, and decompose. flies thrown into the open-mouthed tube of the yellow sarracenia, even when free from water, are unable to get out--one hardly sees why, except that they cannot fly directly upward; and microscopic chevaux-de-frise of fine, sharp-pointed bristles which line most of the interior, pointing strictly downward, may be a more effectual obstacle to crawling up the sides than one would think possible. on the inside of the lid or hood of the purple northern species, the bristles are much stronger; but an insect might escape by the front without encountering these. in this species, the pitchers, however, are so well supplied with water that the insects which somehow are most abundantly attracted thither are effectually drowned, and the contents all summer long are in the condition of a rich liquid manure. that the tubes or pitchers of the southern species are equally attractive and fatal to flies is well known. indeed, they are said to be taken into houses and used as fly-traps. there is no perceptible odor to draw insects, except what arises from the decomposition of macerated victims; nor is any kind of lure to be detected at the mouth of the pitcher of the common purple-flowered species. some incredulity was therefore natural when it was stated by a carolinian correspondent (mr. b.f. grady) that in the long-leaved, yellow-flowered species the lid just above the mouth of the tubular pitcher habitually secretes drops of a sweet and viscid liquid, which attracts flies and apparently intoxicates them, since those that sip it soon become unsteady in gait and mostly fall irretrievably into the well beneath. but upon cultivating plants of this species, obtained for the purpose, the existence of this lure was abundantly verified; and, although we cannot vouch for its inebriating quality, we can no longer regard it as unlikely. no sooner was it thus ascertained that at least one species of sarracenia allures flies to their ruin than it began to appear that--just as in the case of drosera--most of this was a mere revival of obsolete knowledge. the "insect-destroying process" was known and well described sixty years ago, the part played by the sweet exudation indicated, and even the intoxication perhaps hinted at, although evidently little thought of in those ante-temperance days. dr. james macbride, of south carolina--the early associate of elliott in his "botany of south carolina and georgia," and to whose death, at the age of thirty-three, cutting short a life of remarkable promise, the latter touchingly alludes in the preface to his second volume--sent to sir james edward smith an account of his observations upon this subject, made in and the following years. this was read to the linnaean society in , and published in the twelfth volume of its "transactions." from this forgotten paper (to which attention has lately been recalled) we cull the following extracts, premising that the observations mostly relate to a third species, sarracenia adunca, alias variolaris, which is said to be the most efficient fly-catcher of the kind: "if, in the months of may, june, or july, when the leaves of those plants perform their extraordinary functions in the greatest perfection, some of them be removed to a house and fixed in an erect position, it will soon be perceived that flies are attracted by them. these insects immediately approach the fauces of the leaves, and, leaning over their edges, appear to sip with eagerness something from their internal surfaces. in this position they linger; but at length, allured as it would seem by the pleasure of taste, they enter the tubes. the fly which has thus changed its situation will be seen to stand unsteadily; it totters for a few seconds, slips, and falls to the bottom of the tube, where it is either drowned or attempts in vain to ascend against the points of the hairs. the fly seldom takes wing in its fall and escapes. . . . in a house much infested with flies, this entrapment goes on so rapidly that a tube is filled in a few hours, and it becomes necessary to add water, the natural quantity being insufficient to drown the imprisoned insects. the leaves of s. adunca and rubra might well be employed as fly-catchers; indeed, i am credibly informed they are in some neighborhoods. the leaves of the s. flava [the species to which our foregoing remarks mainly relate], although they are very capacious, and often grow to the height of three feet or more, are never found to contain so many insects as those of the species above mentioned. "the cause which attracts flies is evidently a sweet, viscid substance resembling honey, secreted by or exuding from the internal surface of the tube . . . from the margin, where it commences, it does not extend lower than one-fourth of an inch. "the falling of the insect as soon as it enters the tube is wholly attributable to the downward or inverted position of the hairs of the internal surface of the leaf. at the bottom of a tube split open, the hairs are plainly discernible pointing downward; as the eye ranges upward, they gradually become shorter and attenuated, till at or just below the surface covered by the bait they are no longer perceptible to the naked eye nor to the most delicate touch. it is here that the fly cannot take a hold sufficiently strong to support itself, but falls. the in. ability of insects to crawl up against the points of the hairs i have often tested in the most satisfactory manner." from the last paragraph it may be inferred that dr. macbride did not suspect any inebriating property in the nectar, and in a closing note there is a conjecture of an impalpable loose powder in s. flava, at the place where the fly stands so unsteadily, and from which it is supposed to slide. we incline to take mr. grady's view of the case. the complete oblivion into which this paper and the whole subject had fallen is the more remarkable when it is seen that both are briefly but explicitly referred to in elliott's book, with which botanists are familiar. it is not so wonderful that the far earlier allusion to these facts by the younger bartram should have been overlooked or disregarded. with the genuine love of nature and fondness for exploration, 'william bartram did not inherit the simplicity of his father, the earliest native botanist of this country. fine writing was his foible; and the preface to his well-known "travels" (published at philadelphia in ) is its full-blown illustration, sometimes perhaps deserving the epithet which he applies to the palms of florida--that of pomposity. in this preface he declares that "all the sarracenias are insect-catchers, and so is the drosera rotundifolia. whether the insects caught in their leaves, and which dissolve and mix with the fluid, serve for aliment or support to these kind of plants is doubtful," he thinks, but he should be credited with the suggestion. in one sentence he speaks of the quantities of insects which, "being invited down to sip the mellifluous exuvia from the interior surface of the tube, where they inevitably perish," being prevented from returning by the stiff hairs all pointing downward. this, if it refers to the sweet secretion, would place it below, and not, as it is, above the bristly surface, while the liquid below, charged with decomposing insects, is declared in an earlier sentence to be "cool and animating, limpid as the morning dew." bartram was evidently writing from memory; and it is very doubtful if he ever distinctly recognized the sweet exudation which entices insects. why should these plants take to organic food more than others? if we cannot answer the question, we may make a probable step toward it. for plants that are not parasitic, these, especially the sundews, have much less than the ordinary amount of chlorophyll--that is, of the universal leaf-green upon which the formation of organic matter out of inorganic materials depends. these take it instead of making it, to a certain extent. what is the bearing of these remarkable adaptations and operations upon doctrines of evolution? there seems here to be a field on which the specific creationist, the evolutionist with design, and the necessary evolutionist, may fight out an interesting, if not decisive, "triangular duel." xi insectivorous and climbing plants [xi- ] (the nation, january and , ) "minerals grow; vegetables grow and live; animals grow, live, and feel;" this is the well-worn, not to say out-worn, diagnosis of the three kingdoms by linnaeus. it must be said of it that the agreement indicated in the first couplet is unreal, and that the distinction declared in the second is evanescent. crystals do not grow at all in the sense that plants and animals grow. on the other hand, if a response to external impressions by special movements is evidence of feeling, vegetables share this endowment with animals; while, if conscious feeling is meant, this can be affirmed only of the higher animals. what appears to remain true is, that the difference is one of successive addition. that the increment in the organic world is of many steps; that in the long series no absolute lines separate, or have always separated, organisms which barely respond to impressions from those which more actively and variously respond, and even from those that consciously so respond--this, as we all know, is what the author of the works before us has undertaken to demonstrate. without reference here either to that part of the series with which man is connected, and in some sense or other forms a part of, or to that lower limbo where the two organic kingdoms apparently merge--or whence, in evolutionary phrase, they have emerged--mr. darwin, in the present volumes, directs our attention to the behavior of the highest plants alone. he shows that some (and he might add that all) of them execute movements for their own advantage, and that some capture and digest living prey. when plants are seen to move and to devour, what faculties are left that are distinctively animal? as to insectivorous or otherwise carnivorous plants, we have so recently here discussed this subject--before it attained to all this new popularity--that a brief account of mr. darwin's investigation may suffice.[xi- ] it is full of interest as a physiological research, and is a model of its kind, as well for the simplicity and directness of the means employed as for the clearness with which the results are brought out--results which any one may verify now that the way to them is pointed out, and which, surprising as they are, lose half their wonder in the ease and sureness with which they seem to have been reached. rather more than half the volume is devoted to one subject, the round-leaved sundew (drosera rotundifolia), a rather common plant in the northern temperate zone. that flies stick fast to its leaves, being limed by the tenacious seeming dew-drops which stud its upper face and margins, had long been noticed in europe and in this country. we have heard hunters and explorers in our northern woods refer with satisfaction to the fate which in this way often befalls one of their plagues, the black fly of early summer. and it was known to some observant botanists in the last century, although forgotten or discredited in this, that an insect caught on the viscid glands it has happened to alight upon is soon fixed by many more--not merely in consequence of its struggles, but by the spontaneous incurvation of the stalks of surrounding and untouched glands; and even the body of the leaf had been observed to incurve or become cup-shaped so as partly to involve the captive insect. mr. darwin's peculiar investigations not only confirm all this, but add greater wonders. they relate to the sensitiveness of these tentacles, as he prefers to call them, and the mode in which it is manifested; their power of absorption; their astonishing discernment of the presence of animal or other soluble azotized matter, even in quantities so minute as to rival the spectroscope--that most exquisite instrument of modern research--in delicacy; and, finally, they establish the fact of a true digestion, in all essential respects similar to that of the stomach of animals. first as to sensitiveness and movement. sensitiveness is manifested by movement or change of form in response to an external impression. the sensitiveness in the sundew is all in the gland which surmounts the tentacle. to incite movement or other action, it is necessary that the gland itself should be reached. anything laid on the surface of the viscid drop, the spherule of clear, glairy liquid which it secretes, produces no effect unless it sinks through to the gland; or unless the substance is soluble and reaches it in solution, which, in the case of certain substances, has the same effect. but the glands themselves do not move, nor does any neighboring portion of the tentacle. the outer and longer tentacles bend inward (toward the centre of the leaf) promptly, when the gland is irritated or stimulated, sweeping through an arc of or less, or more--the quickness and the extent of the inflection depending, in equally vigorous leaves, upon the amount of irritation or stimulation, and also upon its kind. a tentacle with a particle of raw meat on its gland sometimes visibly begins to bend in ten seconds, becomes strongly incurved in five minutes, and its tip reaches the centre of the leaf in half an hour; but this is a case of extreme rapidity. a particle of cinder, chalk, or sand, will also incite the bending, if actually brought in contact with the gland, not merely resting on the drop; but the inflection is then much less pronounced and more transient. even a bit of thin human hair, only / of an inch in length, weighing only the / of a grain, and largely supported by the viscid secretion, suffices to induce movement; but, on the other hand, one or two momentary, although rude, touches with a hard object produce no effect, although a repeated touch or the slightest pressure, such as that of a gnat's foot, prolonged for a short time, causes bending. the seat of the movement is wholly or nearly confined to a portion of the lower part of the tentacle, above the base, where local irritation produces not the slightest effect. the movement takes place only in response to some impression made upon its own gland at the distant extremity, or upon other glands far more remote. for if one of these members suffers irritation the others sympathize with it. very noteworthy is the correlation between the central tentacles, upon which an insect is most likely to alight, and these external and larger ones, which, in proportion to their distance from the centre, take the larger share in the movement. the shorter central ones do not move at all when a bit of meat, or a crushed fly, or a particle of a salt of ammonia, or the like, is placed upon them; but they transmit their excitation across the leaf to the surrounding tentacles on all sides; and they, although absolutely untouched, as they successively receive the mysterious impulse, bend strongly inward, just as they do when their own glands are excited. whenever a tentacle bends in obedience to an impulse from its own gland, the movement is always toward the centre of the leaf; and this also takes place, as we have seen, when an exciting object is lodged at the centre. but when the object is placed upon either half of the leaf, the impulse radiating thence causes all the surrounding untouched tentacles to bend with precision toward the point of excitement, even the central tentacles, which are motionless when themselves charged, now responding to the call. the inflection which follows mechanical irritation or the presence of any inorganic or insoluble body is transient; that which follows the application of organic matter lasts longer, more or less, according to its nature and the amount; but sooner or later the tentacles resume their former position, their glands glisten anew with fresh secretion, and they are ready to act again. as to how the impulse is originated and propagated, and how the movements are made, comparatively simple as the structure is, we know as little as we do of the nature of nervous impulse and muscular motion. but two things mr. darwin has wellnigh made out, both of them by means and observations so simple and direct as to command our confidence, although they are contrary to the prevalent teaching. first, the transmission is through the ordinary cellular tissue, and not through what are called the fibrous or vascular bundles. second, the movement is a vital one, and is effected by contraction on the side toward which the bending takes place, rather than by turgescent tension of the opposite side. the tentacle is pulled over rather than pushed over. so far all accords with muscular action. the operation of this fly-catching apparatus, in any case, is plain. if the insect alights upon the disk of the leaf, the viscid secretion holds it fast--at least, an ordinary fly is unable to escape--its struggles only increase the number of glands involved and the amount of excitement; this is telegraphed to the surrounding and successively longer tentacles, which bent over in succession, so that within ten to thirty hours, if the leaf is active and the fly large enough, every one of the glands (on the average, nearly two hundred in number) will be found applied to the body of the insect. if the insect is small, and the lodgment toward one side, only the neighboring tentacles may take part in the capture. if two or three of the strong marginal tentacles are first encountered, their prompt inflection carries the intruder to the centre, and presses it down upon the glands which thickly pave the floor; these notify all the surrounding tentacles of the capture, that they may share the spoil, and the fate of that victim is even as of the first. a bit of meat or a crushed insect is treated in the same way. this language implies that the animal matter is in some way or other discerned by the tentacles, and is appropriated. formerly there was only a presumption of this, on the general ground that such an organization could hardly be purposeless. yet, while such expressions were natural, if not unavoidable, they generally were used by those familiar with the facts in a half-serious, half-metaphorical sense. thanks to mr. darwin's investigations, they may now be used in simplicity and seriousness. that the glands secrete the glairy liquid of the drop is evident, not only from its nature, but from its persistence through a whole day's exposure to a summer sun, as also from its renewal after it has been removed, dried up, or absorbed. that they absorb as well as secrete, and that the whole tentacle may be profoundly affected thereby, are proved by the different effects, in kind and degree, which follow the application of different substances. drops of rain-water, like single momentary touches of a solid body, produce no effect, as indeed they could be of no advantage; but a little carbonate of ammonia in the water, or an infusion of meat, not only causes inflection, but promptly manifests its action upon the contents of the cells of which the tentacle is constructed. these cells are sufficiently transparent to be viewed under the microscope without dissection or other interference; and the change which takes place in the fluid contents of these cells, when the gland above has been acted upon, is often visible through a weak lens, or sometimes even by the naked eye, although higher powers are required to discern what actually takes place. this change, which mr. darwin discovered, and turns to much account in his researches, he terms "aggregation of the protoplasm." when untouched and quiescent, the contents appear as an homogeneous purple fluid. when the gland is acted upon, minute purple particles appear, suspended in the now colorless or almost colorless fluid; and this change appears first in the cells next the gland, and then in those next beneath, traveling down the whole length of the tentacle. when the action is slight, this appearance does not last long; the particles of "aggregated protoplasm redissolved, the process of redissolution traveling upward from the base of the tentacle to the gland in a reverse direction to that of the aggregation. whenever the action is more prolonged or intense, as when a bit of meat or crushed fly, or a fitting solution, is left upon the gland, the aggregation proceeds further, so that the whole protoplasm of each cell condenses into one or two masses, or into a single mass which will often separate into two, which afterward reunite; indeed, they incessantly change their forms and positions, being never at rest, although their movements are rather slow. in appearance and movements they are very like amoebae and the white corpuscles of the blood. their motion, along with the streaming movement of rotation in the layer of white granular protoplasm that flows along the walls of the cell, under the high powers of the microscope "presents a wonderful scene of vital activity." this continues while the tentacle is inflected or the gland fed by animal matter, but vanishes by dissolution when the work is over and the tentacle straightens. that absorption takes place, and matter is conveyed from cell to cell, is well made out, especially by the experiments with carbonate of ammonia. nevertheless, this aggregation is not dependent upon absorption, for it equally occurs from mechanical irritation of the gland, and always accompanies inflection, however caused, though it may take place without it. this is also apparent from the astonishingly minute quantity of certain substances which suffices to produce sensible inflection and aggregation--such, for instance, as the / or even the / of a grain of phosphate or nitrate of ammonia! by varied experiments it was found that the nitrate of ammonia was more powerful than the carbonate, and the phosphate more powerful than the nitrate, this result being intelligible from the difference in the amount of nitrogen in the first two salts, and from the presence of phosphorus in the third. there is nothing surprising in the absorption of such extremely dilute solutions by a gland. as our author remarks: "all physiologists admit that the roots of plants absorb the salts of ammonia brought to them by the rain; and fourteen gallons of rain-water contain a grain of ammonia; therefore, only a little more than twice as much as in the weakest solution employed by me. the fact which appears truly wonderful is that the / of a grain of the phosphate of ammonia, including less than / of efficient matter (if the water of crystallization is deducted), when absorbed by a gland, should induce some change in it which leads to a motor impulse being transmitted down the whole length of the tentacle, causing its basal part to bend, often through an angle of degrees." but odoriferous particles which act upon the nerves of animals must be infinitely smaller, and by these a dog a quarter of a mile to the leeward of a deer perceives his presence by some change in the olfactory nerves transmitted through them to the brain. when mr. darwin obtained these results, fourteen years ago, he could claim for drosera a power and delicacy in the detection of minute quantities of a substance far beyond the resources of the most skillful chemist; but in a foot-note he admits that "now the spectroscope has altogether beaten drosera; for, according to bunsen and kirchhoff, probably less than the / of a grain of sodium can be thus detected." finally, that this highly-sensitive and active living organism absorbs, will not be doubted when it is proved to digest, that is, to dissolve otherwise insoluble animal matter by the aid of special secretions. that it does this is now past doubting. in the first place, when the glands are excited they pour forth an increased amount of the ropy secretion. this occurs directly when a bit of meat is laid upon the central glands; and the influence which they transmit to the long-stalked marginal glands causes them, while incurving their tentacles, to secrete more copiously long before they have themselves touched anything. the primary fluid, secreted without excitation, does not of itself digest. but the secretion under excitement changes in nature and becomes acid. so, according to schiff, mechanical irritation excites the glands of the stomach to secrete an acid. in both this acid appears to be necessary to, but of itself insufficient for, digestion. the requisite solvent, a kind of ferment called pepsin, which acts only in the presence of the acid, is poured forth by the glands of the stomach only after they have absorbed certain soluble nutritive substances of the food; then this pepsin promptly dissolves muscle, fibrine, coagulated albumen, cartilage, and the like. similarly it appears that drosera-glands, after irritation by particles of glass, did not act upon little cubes of albumen. but when moistened with saliva, or replaced by bits of roast-meat or gelatine, or even cartilage, which supply some soluble peptone-matter to initiate the process, these substances are promptly acted upon, and dissolved or digested; whence it is inferred that the analogy with the stomach holds good throughout, and that a ferment similar to pepsin is poured out under the stimulus of some soluble animal matter. but the direct evidence of this is furnished only by the related carnivorous plant, dionaea, from which the secretions, poured out when digestion is about to begin, may be collected in quantity sufficient for chemical examination. in short, the experiments show "that there is a remarkable accordance in the power of digestion between the gastric juice of animals, with its pepsin and hydrochloric acid, and the secretion of drosera, with its ferment and acid belonging to the acetic series. we can, therefore, hardly doubt that the ferment in both cases is closely similar, if not identically the same. that a plant and an animal should pour forth the same, or nearly the same, complex secretion, adapted for the same purpose of digestion, is a new and wonderful fact in physiology." there are one or two other species of sundew--one of them almost as common in europe and north america as the ordinary round-leaved species--which act in the same way, except that, having their leaves longer in proportion to their breadth, their sides never curl inward, but they are much disposed to aid the action of their tentacles by incurving the tip of the leaf, as if to grasp the morsel. there are many others, with variously less efficient and less advantageously arranged insectivorous apparatus, which, in the language of the new science, may be either on the way to acquire something better, or of losing what they may have had, while now adapting themselves to a proper vegetable life. there is one member of the family (drosophyllum lusitanicum), an almost shrubby plant, which grows on dry and sunny hills in portugal and morocco--which the villagers call "the flycatcher," and hang up in their cottages for the purpose--the glandular tentacles of which have wholly lost their powers of movement, if they ever had any, but which still secrete, digest, and absorb, being roused to great activity by the contact of any animal matter. a friend of ours once remarked that it was fearful to contemplate the amount of soul that could be called forth in a dog by the sight of a piece of meat. equally wonderful is the avidity for animal food manifested by these vegetable tentacles, that can "only stand and wait" for it. only a brief chapter is devoted to dionaea of north carolina, the venus's fly-trap, albeit, "from the rapidity and force of its movements, one of the most wonderful in the world." it is of the same family as the sundew; but the action is transferred from tentacles on the leaf to the body of the leaf itself, which is transformed into a spring-trap, closing with a sudden movement over the alighted insect. no secretion is provided beforehand either for allurement or detention; but after the captive is secured, microscopic glands within the surface of the leaf pour out an abundant gastric juice to digest it. mrs. glass's classical directions in the cook-book, "first catch your hare," are implicitly followed. avoiding here all repetition or recapitulation of our former narrative, suffice it now to mention two interesting recent additions to our knowledge, for which we are indebted to mr. darwin. one is a research, the other an inspiration. it is mainly his investigations which have shown that the glairy liquid, which is poured upon and macerates the captured insect, accomplishes a true digestion; that, like the gastric juice of animals, it contains both a free acid and pepsin or its analogue, these two together dissolving albumen, meat, and the like. the other point relates to the significance of a peculiarity in the process of capture. when the trap suddenly incloses an insect which has betrayed its presence by touching one of the internal sensitive bristles, the closure is at first incomplete. for the sides approach in an arching way, surrounding a considerable cavity, and the marginal spine-like bristles merely intercross their tips, leaving intervening spaces through which one may look into the cavity beneath. a good idea may be had of it by bringing the two palms near together to represent the sides of the trap, and loosely interlocking the fingers to represent the marginal bristles or bars. after remaining some time in this position the closure is made complete by the margins coming into full contact, and the sides finally flattening down so as to press firmly upon the insect within; the secretion excited by contact is now poured out, and digestion begins. why these two stages? why should time be lost by this preliminary and incomplete closing? the query probably was never distinctly raised before, no one noticing anything here that needed explanation. darwinian teleology, however, raises questions like this, and mr. darwin not only propounded the riddle but solved it. the object of the partial closing is to permit small insects to escape through the meshes, detaining only those plump enough to be worth the trouble of digesting. for naturally only one insect is caught at a time, and digestion is a slow business with dionaeas, as with anacondas, requiring ordinarily a fortnight. it is not worth while to undertake it with a gnat when larger game may be had. to test this happy conjecture, mr. canby was asked, on visiting the dionaeas in their native habitat, to collect early in the season a good series of leaves in the act of digesting naturally-caught insects. upon opening them it was found that ten out of fourteen were engaged upon relatively large prey, and of the remaining four three had insects as large as ants, and one a rather small fly. "there be land-rats and water-rats" in this carnivorous sun-dew family. aldrovanda, of the warmer parts of europe and of india, is an aquatic plant, with bladdery leaves, which were supposed to be useful in rendering the herbage buoyant in water. but it has recently been found that the bladder is composed of two lobes, like the trap of its relative dionaea, or the valves of a mussel-shell; that these open when the plant is in an active state, are provided with some sensitive bristles within, and when these are touched close with a quick movement. these water-traps are manifestly adapted for catching living creatures; and the few incomplete investigations that have already been made render it highly probably that they appropriate their prey for nourishment; whether by digestion or by mere absorption of decomposing animal matter, is uncertain. it is certainly most remarkable that this family of plants, wherever met with, and under the most diverse conditions and modes of life, should always in some way or other be predaceous and carnivorous. if it be not only surprising but somewhat confounding to our classifications that a whole group of plants should subsist partly by digesting animal matter and partly in the normal way of decomposing carbonic acid and producing the basis of animal matter, we have, as mr. darwin remarks, a counterpart anomaly in the animal kingdom. while some plants have stomachs, some animals have roots. "the rhizocephalous crustaceans do not feed like other animals by their mouths, for they are destitute of an alimentary canal, but they live by absorbing through root-like processes the juices of the animals on which they are parasitic." to a naturalist of our day, imbued with those ideas of the solidarity of organic nature which such facts as those we have been considering suggest, the greatest anomaly of all would be that they are really anomalous or unique. reasonably supposing, therefore, that the sundew did not stand alone, mr. darwin turned his attention to other groups of plants; and, first, to the bladderworts, which have no near kinship with the sundews, but, like the aquatic representative of that family, are provided with bladdery sacs, under water. in the common species of utricularia or bladderwort, these little sacs, hanging from submerged leaves or branches, have their orifice closed by a lid which opens inwardly--a veritable trapdoor. it had been noticed in england and france that they contained minute crustacean animals. early in the summer of , mr. darwin ascertained the mechanism for their capture and the great success with which it is used. but before his account was written out, prof. cohn published an excellent paper on the subject in germany; and mrs. treat, of vineland, new jersey, a still earlier one in this country--in the new york tribune in the autumn of . of the latter, mr. darwin remarks that she "has been more successful than any other observer in witnessing the actual entrance of these minute creatures." they never come out, but soon perish in their prison, which receives a continued succession of victims, but little, if any, fresh air to the contained water. the action of the trap is purely mechanical, without evident irritability in the opening or shutting. there is no evidence nor much likelihood of proper digestion; indeed, mr. darwin found evidence to the contrary. but the more or less decomposed and dissolved animal matter is doubtless absorbed into the plant; for the whole interior of the sac is lined with peculiar, elongated and four-armed very thin-walled processes, which contain active protoplasm, and which were proved by experiment to "have the power of absorbing matter from weak solutions of certain salts of ammonia and urea, and from a putrid infusion of raw meat." although the bladderworts "prey on garbage," their terrestrial relatives "live cleanly," as nobler plants should do, and have a good and true digestion. pinguicula, or butterwort, is the representative of this family upon land. it gets both its latin and its english name from the fatty or greasy appearance of the upper face of its broad leaves; and this appearance is due to a dense coat or pile of short-stalked glands, which secrete a colorless and extremely viscid liquid. by this small flies, or whatever may alight or fall upon the leaf, are held fast. these waifs might be useless or even injurious to the plant. probably mr. darwin was the first to ask whether they might be of advantage. he certainly was the first to show that they probably are so. the evidence from experiment, shortly summed up, is, that insects alive or dead, and also other nitrogenous bodies, excite these glands to increased secretion; the secretion then becomes acid, and acquires the power of dissolving solid animal substances--that is, the power of digestion in the manner of drosera and dionaea. and the stalks of their glands under the microscope give the same ocular evidence of absorption. the leaves of the butterwort are apt to have their margins folded inward, like a rim or hem. taking young and vigorous leaves to which hardly anything had yet adhered, and of which the margins were still flat, mr. darwin set within one margin a row of small flies. fifteen hours afterward this edge was neatly turned inward, partly covering the row of flies, and the surrounding glands were secreting copiously. the other edge remained flat and unaltered. then he stuck a fly to the middle of the leaf just below its tip, and soon both margins infolded, so as to clasp the object. many other and varied experiments yielded similar results. even pollen, which would not rarely be lodged upon these leaves, as it falls from surrounding wind-fertilized plants, also small seeds, excited the same action, and showed signs of being acted upon. "we may therefore conclude," with mr. darwin, "that pinguicula vulgaris, with its small roots, is not only supported to a large extent by the extraordinary number of insects which it habitually captures, but likewise draws some nourishment from the pollen, leaves, and seeds, of other plants which often adhere to its leaves. it is, therefore, partly a vegetable as well as an animal feeder." what is now to be thought of the ordinary glandular hairs which render the surface of many and the most various plants extremely viscid? their number is legion. the chinese primrose of common garden and house culture is no extraordinary instance; but mr. francis darwin, counting those on a small space measured by the micrometer, estimated them at , to the square inch of foliage, taking in both surfaces of the leaf, or two or three millions on a moderate-sized specimen of this small herb. glands of this sort were loosely regarded as organs for excretion, without much consideration of the question whether, in vegetable life, there could be any need to excrete, or any advantage gained by throwing off such products; and, while the popular name of catch-fly, given to several common species of silene, indicates long familiarity with the fact, probably no one ever imagined that the swarms of small insects which perish upon these sticky surfaces were ever turned to account by the plant. in many such cases, no doubt they perish as uselessly as when attracted into the flame of a candle. in the tobacco-plant, for instance, mr. darwin could find no evidence that the glandular hairs absorb animal matter. but darwinian philosophy expects all gradations between casualty and complete adaptation. it is most probable that any thin-walled vegetable structure which secretes may also be capable of absorbing under favorable conditions. the myriads of exquisitely-constructed glands of the chinese primrose are not likely to be functionless. mr. darwin ascertained by direct experiment that they promptly absorb carbonate of ammonia, both in watery solution and in vapor. so, since rain-water usually contains a small percentage of ammonia, a use for these glands becomes apparent--one completely congruous with that of absorbing any animal matter, or products of its decomposition, which may come in their way through the occasional entanglement of insects in their viscid secretion. in several saxifrages--not very distant relatives of drosera--the viscid glands equally manifested the power of absorption. to trace a gradation between a simply absorbing hair with a glutinous tip, through which the plant may perchance derive slight contingent advantage, and the tentacles of a sundew, with their exquisite and associated adaptations, does not much lessen the wonder nor explain the phenomena. after all, as mr. darwin modestly concludes, "we see how little has been made out in comparison with what remains unexplained and unknown." but all this must be allowed to be an important contribution to the doctrine of the gradual acquirement of uses and functions, and hardly to find conceivable explanation upon any other hypothesis. there remains one more mode in which plants of the higher grade are known to prey upon animals; namely, by means of pitchers, urns, or tubes, in which insects and the like are drowned or confined, and either macerated or digested. to this mr. darwin barely alludes on the last page of the present volume. the main facts known respecting the american pitcher-plants have, as was natural, been ascertained in this country; and we gave an abstract, two years ago, of our then incipient knowledge. much has been learned since, although all the observations have been of a desultory character. if space permitted, an instructive narrative might be drawn up, as well of the economy of the sarracenias as of how we came to know what we do of it. but the very little we have room for will be strictly supplementary to our former article. the pitchers of our familiar northern sarracenia, which is likewise southern, are open-mouthed; and, although they certainly secrete some liquid when young, must derive most of the water they ordinarily contain from rain. how insects are attracted is unknown, but the water abounds with their drowned bodies and decomposing remains. in the more southern s. flava, the long and trumpet-shaped pitchers evidently depend upon the liquid which they themselves secrete, although at maturity, when the hood becomes erect, rain may somewhat add to it. this species, as we know, allures insects by a peculiar sweet exudation within the orifice; they fall in and perish, though seldom by drowning, yet few are able to escape; and their decomposing remains accumulate in the narrow bottom of the vessel. two other long-tubed species of the southern states are similar in these respects. there is another, s. psittacina, the parrot-headed species, remarkable for the cowl-shaped hood so completely inflexed over the mouth of the small pitcher that no rain can possibly enter. little is known, however, of the efficiency of this species as a fly-catcher; but its conformation has a morphological interest, leading up, as it does, to the californian type of pitcher presently to be mentioned. but the remaining species, s. variolaris, is the most wonderful of our pitcher-plants in its adaptations for the capture of insects. the inflated and mottled lid or hood overarches the ample orifice of the tubular pitcher sufficiently to ward off the rain, but not to obstruct the free access of flying insects. flies, ants, and most insects, glide and fall from the treacherous smooth throat into the deep well below, and never escape. they are allured by a sweet secretion just within the orifice-- which was discovered and described long ago, and the knowledge of it wellnigh forgotten until recently. and, finally, dr. mellichamp, of south carolina, two years ago made the capital discovery that, during the height of the season, this lure extends from the orifice down nearly to the ground, a length of a foot or two, in the form of a honeyed line or narrow trail on the edge of the wing-like border which is conspicuous in all these species, although only in this one, so far as known, turned to such account. here, one would say, is a special adaptation to ants and such terrestrial and creeping insects. well, long before this sweet trail was known, it was remarked by the late prof. wyman and others that the pitchers of this species, in the savannahs of georgia and florida, contain far more ants than they do of all other insects put together. finally, all this is essentially repeated in the peculiar californian pitcher-plant (darlingtonia), a genus of the same natural family, which captures insects in great variety, enticing them by a sweetish secretion over the whole inside of the inflated hood and that of a curious forked appendage, resembling a fish-tail, which overhangs the orifice. this orifice is so concealed that it can be seen and approached only from below, as if--the casual observer might infer--to escape visitation. but dead insects of all kinds, and their decomposing remains, crowd the cavity and saturate the liquid therein contained, enticed, it is said, by a peculiar odor, as well as by the sweet lure which is at some stages so abundant as to drip from the tips of the overhanging appendage. the principal observations upon this pitcher-plant in its native habitat have been made by mrs. austin, and only some of the earlier ones have thus far been published by mr. canby. but we are assured that in this, as in the sarracenia variolaris, the sweet exudation extends at the proper season from the orifice down the wing nearly to the ground, and that ants follow this honeyed pathway to their destruction. also, that the watery liquid in the pitcher, which must be wholly a secretion, is much increased in quantity after the capture of insects. it cannot now well be doubted that the animal matter is utilized by the plant in all these cases, although most probably only after maceration or decomposition. in some of them even digestion, or at least the absorption of undecomposed soluble animal juices, may be suspected; but there is no proof of it. but, if pitchers of the sarracenia family are only macerating vessels, those of nepenthes--the pitchers of the indian archipelago, familiar in conservatories--seem to be stomachs. the investigations of the president of the royal society, dr. hooker, although incomplete, wellnigh demonstrate that these not only allure insects by a sweet secretion at the rim and upon the lid of the cup, but also that their capture, or the presence of other partly soluble animal matter, produces an increase and an acidulation of the contained watery liquid, which thereupon becomes capable of acting in the manner of that of drosera and dionaea, dissolving flesh, albumen, and the like. after all, there never was just ground for denying to vegetables the use of animal food. the fungi are by far the most numerous family of plants, and they all live upon organic matter, some upon dead and decomposing, some upon living, some upon both; and the number of those that feed upon living animals is large. whether these carnivorous propensities of higher plants which so excite our wonder be regarded as survivals of ancestral habits, or as comparatively late acquirements, or even as special endowments, in any case what we have now learned of them goes to strengthen the conclusion that the whole organic world is akin. the volume upon "the movements and habits of climbing plants" is a revised and enlarged edition of a memoir communicated to the linnaean society in , and published in the ninth volume of its journal. there was an extra impression, but, beyond the circle of naturalists, it can hardly have been much known at first-hand. even now, when it is made a part of the general darwinian literature, it is unlikely to be as widely read as the companion volume which we have been reviewing; although it is really a more readable book, and well worthy of far more extended notice at our hands than it can now receive. the reason is obvious. it seems as natural that plants should climb as it does unnatural that any should take animal food. most people, knowing that some plants "twine with the sun," and others "against the sun," have an idea that the sun in some way causes the twining; indeed, the notion is still fixed in the popular mind that the same species twines in opposite directions north and south of the equator. readers of this fascinating treatise will learn, first of all, that the sun has no influence over such movements directly, and that its indirect influence is commonly adverse or disturbing, except the heat, which quickens vegetable as it does animal life. also, that climbing is accomplished by powers and actions as unlike those generally predicated of the vegetable kingdom as any which have been brought to view in the preceding volume. climbing plants "feel" as well as "grow and live;" and they also manifest an automatism which is perhaps more wonderful than a response by visible movement to an external irritation. nor do plants grow up their supports, as is unthinkingly supposed; for, although only growing or newly-grown parts act in climbing, the climbing and the growth are entirely distinct. to this there is one exception--an instructive one, as showing how one action passes into another, and how the same result may be brought about in different ways--that of stems which climb by rootlets, such as of ivy and trumpet-creeper. here the stem ascends by growth alone, taking upward direction, and is fixed by root-lets as it grows. there is no better way of climbing walls, precipices, and large tree-trunks. but small stems and similar supports are best ascended by twining; and this calls out powers of another and higher order. the twining stem does not grow around its support, but winds around it, and it does this by a movement the nature of which is best observed in stems which have not yet reached their support, or have overtopped it and stretched out beyond it. then it may be seen that the extending summit, reaching farther and farther as it grows, is making free circular sweeps, by night as well as by day, and irrespective of external circumstances, except that warmth accelerates the movement, and that the general tendency of young stems to bend toward the light may, in case of lateral illumination, accelerate one-half the circuit while it equally retards the other. the arrest of the revolution where the supporting body is struck, while the portion beyond continues its movement, brings about the twining. as to the proximate cause of this sweeping motion, a few simple experiments prove that it results from the bowing or bending of the free summit of the stem into a more or less horizontal position (this bending being successively to every point of the compass, through an action which circulates around the stem in the direction of the sweep), and of the consequent twining, i.e., "with the sun," or with the movement of the hands of a watch, in the hop, or in the opposite direction in pole-beans and most twiners. twining plants, therefore, ascend trees or other stems by an action and a movement of their own, from which they derive advantage. to plants liable to be overshadowed by more robust companions, climbing is an economical method of obtaining a freer exposure to light and air with the smallest possible expenditure of material. but twiners have one disadvantage: to rise ten feet they must produce fifteen feet of stem or thereabouts, according to the diameter of the support, and the openness or closeness of the coil. a rootlet-climber saves much in this respect, but has a restricted range of action, and other disadvantages. there are two other modes, which combine the utmost economy of material with freer range of action. there are, in the first place, leaf-climbers of various sorts, agreeing only in this, that the duty of laying hold is transferred to the leaves, so that the stem may rise in a direct line. sometimes the blade or leaflets, or some of them, but more commonly their slender stalks, undertake the work, and the plant rises as a boy ascends a tree, grasping first with one hand or arm, then with the other. indeed, the comparison, like the leaf-stalk, holds better than would be supposed; for the grasping of the latter is not the result of a blind groping in all directions by a continuous movement, but of a definite sensitiveness which acts only upon the occasion. most leaves make no regular sweeps; but when the stalks of a leaf-climbing species come into prolonged contact with any fitting extraneous body, they slowly incurve and make a turn around it, and then commonly thicken and harden until they attain a strength which may equal that of the stem itself. here we have the faculty of movement to a definite end, upon external irritation, of the same nature with that displayed by dionaea and drosera, although slower for the most part than even in the latter. but the movement of the hour-hand of the clock is not different in nature or cause from that of the second-hand. finally--distribution of office being, on the whole, most advantageous and economical, and this, in the vegetable kingdom, being led up to by degrees--we reach, through numerous gradations, the highest style of climbing plants in the tendril-climber. a tendril morphologically, is either a leaf or branch of stem, or a portion of one, specially organized for climbing. some tendrils simply turn away from light, as do those of grape-vines, thus taking the direction in which some supporting object is likely to be encountered; most are indifferent to light; and many revolve in the manner of the summit of twining stems. as the stems which bear these highly-endowed tendrils in many cases themselves also revolve more or less, though they seldom twine, their reach is the more extensive; and to this endowment of automatic movement most tendrils add the other faculty, that of incurving and coiling upon prolonged touch, or even brief contact, in the highest degree. some long tendrils, when in their best condition, revolve so rapidly that the sweeping movement may be plainly seen; indeed, we have seen a quarter-circuit in a passiflora sicyoides accomplished in less than a minute, and the half-circuit in ten minutes; but the other half (for a reason alluded to in the next paragraph) takes a much longer time. then, as to the coiling upon contact, in the case first noticed in this country,[xi- ] in the year , which mr. darwin mentions as having led him into this investigation, the tendril of sicyos was seen to coil within half a minute after a stroke with the hand, and to make a full turn or more within the next minute; furnishing ocular evidence that tendrils grasp and coil in virtue of sensitiveness to contact, and, one would suppose, negativing sachs's recent hypothesis that all these movements are owing "to rapid growth on the side opposite to that which becomes concave"--a view to which mr. darwin objects, but not so strongly as he might. the tendril of this sort, on striking some fitting object, quickly curls round and firmly grasps it; then, after some hours, one side shortening or remaining short in proportion to the other, it coils into a spire, dragging the stem up to its support, and enabling the next tendril above to secure a readier hold. in revolving tendrils perhaps the most wonderful adaptation is that by which they avoid attachment to, or winding themselves upon, the ascending summit of the stem that bears them. this they would inevitably do if they continued their sweep horizontally. but when in its course it nears the parent stem the tendril moves slowly, as if to gather strength, then c.~ stiffens and rises into an erect position parallel with it, and c so passes by the dangerous point; after which it comes rapidly down to the horizontal position, in which it moves until it again approaches and again avoids the impending obstacle. climbing plants are distributed throughout almost all the natural orders. in some orders climbing is the rule, in most it is the exception, occurring only in certain genera. the tendency of stems to move in circuits--upon which climbing more commonly depends, and out of which it is conceived to have been educed--is manifested incipiently by many a plant which does not climb. of those that do there are all degrees, from the feeblest to the most efficient, from those which have no special adaptation to those which have exquisitely-endowed special organs for climbing. the conclusion reached is, that the power "is inherent, though undeveloped, in almost every plant;" "that climbing plants have utilized and perfected a widely-distributed and incipient capacity, which, as far as we can see, is of no service to ordinary plants." inherent powers and incipient manifestations, useless to their possessors but useful to their successors--this, doubtless, is according to the order of nature; but it seems to need something more than natural selection to account for it. xii duration and origination of race and species-- import of sexual reproduction i do varieties wear out, or tend to wear out? (new york tribune, and american journal of science and the arts, february, ) this question has been argued from time to time for more than half a century, and is far from being settled yet. indeed, it is not to be settled either way so easily as is sometimes thought. the result of a prolonged and rather lively discussion of the topic about forty years ago in england, in which lindley bore a leading part on the negative side, was, if we rightly remember, that the nays had the best of the argument. the deniers could fairly well explain away the facts adduced by the other side, and evade the force of the reasons then assigned to prove that varieties were bound to die out in the course of time. but if the case were fully re-argued now, it is by no means certain that the nays would win it. the most they could expect would be the scotch verdict, "not proven." and this not because much, if any, additional evidence of the actual wearing out of any variety has turned up since, but because a presumption has been raised under which the evidence would take a bias the other way. there is now in the minds of scientific men some reason to expect that certain varieties would die out in the long run, and this might have an important influence upon the interpretation of the facts. curiously enough, however, the recent discussions to which our attention has been called seem, on both sides, to have overlooked this. but, first of all, the question needs to be more specifically stated. there are varieties and varieties. they may, some of them, disappear or deteriorate, but yet not wear out--not come to an end from any inherent cause. one might even say, the younger they are the less the chance of survival unless well cared for. they may be smothered out by the adverse force of superior numbers; they are even more likely to be bred out of existence by unprevented cross-fertilization, or to disappear from mere change of fashion. the question, however, is not so much about reversion to an ancestral state, or the falling off of a high-bred stock into an inferior condition. of such cases it is enough to say that, when a variety or strain, of animal or vegetable, is led up to unusual fecundity or of size or product of any organ, for our good, and not for the good of the plant or animal itself, it can be kept so only by high feeding and exceptional care; and that with high feeding and artificial appliances comes vastly increased liability to disease, which may practically annihilate the race. but then the race, like the bursted boiler, could not be said to wear out, while if left to ordinary conditions, and allowed to degenerate back into a more natural if less useful state, its hold on life would evidently be increased rather than diminished. as to natural varieties or races under normal conditions, sexually propagated, it could readily be shown that they are neither more nor less likely to disappear from any inherent cause than the species from which they originated. whether species wear out, i.e., have their rise, culmination, and decline, from any inherent cause, is wholly a geological and very speculative problem, upon which, indeed, only vague conjectures can be offered. the matter actually under discussion concerns cultivated domesticated varieties only, and, as to plants, is covered by two questions. first, will races propagated by seed, being so fixed that they come true to seed, and purely bred (not crossed with any other sort), continue so indefinitely, or will they run out in time--not die out, perhaps, but lose their distinguishing characters? upon this, all we are able to say is that we know no reason why they should wear out or deteriorate from any inherent cause. the transient existence or the deterioration and disappearance of many such races are sufficiently accounted for otherwise; as in the case of extraordinarily exuberant varieties, such as mammoth fruits or roots, by increased liability to disease, already adverted to, or by the failure of the high feeding they demand. a common cause, in ordinary cases, is cross-breeding, through the agency of wind or insects, which is difficult to guard against. or they go out of fashion and are superseded by others thought to be better, and so the old ones disappear. or, finally, they may revert to an ancestral form. as offspring tend to resemble grandparents almost as much as parents, and as a line of close-bred ancestry is generally prepotent, so newly-originated varieties have always a tendency to reversion. this is pretty sure to show itself in some of the progeny of the earlier generations, and the breeder has to guard against it by rigid selection. but the older the variety is--that is, the longer the series of generations in which it has come true from seed--the less the chance of reversion: for now, to be like the immediate parents, is also to be like a long line of ancestry; and so all the influences concerned--- that is, both parental and ancestral heritability--act in one and the same direction. so, since the older a race is the more reason it has to continue true, the presumption of the unlimited permanence of old races is very strong. of course the race itself may give off new varieties; but that is no interference with the vitality of the original stock. if some of the new varieties supplant the old, that will not be because the unvaried stock is worn out or decrepit with age, but because in wild nature the newer forms are better adapted to the surroundings, or, under man's care, better adapted to his wants or fancies. the second question, and one upon which the discussion about the wearing out of varieties generally turns, is, will varieties propagated from buds, i.e., by division, grafts, bulbs, tubers, and the like, necessarily deteriorate and die out? first, do they die out as a matter of fact? upon this, the testimony has all along been conflicting. andrew knight was sure that they do, and there could hardly be a more trustworthy witness. "the fact," he says, fifty years ago, "that certain varieties of some species of fruit which have been long cultivated cannot now be made to grow in the same soils and under the same mode of management, which was a century ago so perfectly successful, is placed beyond the reach of controversy. every experiment which seemed to afford the slightest prospect of success was tried by myself and others to propagate the old varieties of the apple and pear which formerly constituted the orchards of herefordshire, without a single healthy or efficient tree having been obtained; and i believe all attempts to propagate these varieties have, during some years, wholly ceased to be made." to this it was replied, in that and the next generation, that cultivated vines have been transmitted by perpetual division from the time of the romans, and that several of the sorts, still prized and prolific, are well identified, among them the ancient graecula, considered to be the modern corinth or currant grape, which has immemorially been seedless; that the old nonpareil apple was known in the time of queen elizabeth; that the white beurre pears of france have been propagated from the earliest times; and that golden pippins, st. michael pears, and others said to have run out, were still to be had in good condition. coming down to the present year, a glance through the proceedings of pomological societies, and the debates of farmers' clubs, brings out the same difference of opinion. the testimony is nearly equally divided. perhaps the larger number speak of the deterioration and failure of particular old sorts; but when the question turns on "wearing out," the positive evidence of vigorous trees and sound fruits is most telling. a little positive testimony outweighs a good deal of negative. this cannot readily be explained away, while the failures may be, by exhaustion of soil, incoming of disease, or alteration of climate or circumstances. on the other hand, it may be urged that, if a variety of this sort is fated to become decrepit and die out, it is not bound to die out all at once, and everywhere at the same time. it would be expected first to give way wherever it is weakest, from whatever cause. this consideration has an important bearing upon the final question, are old varieties of this kind on the way to die out on account of their age or any inherent limit of vitality? here, again, mr. knight took an extreme view. in his essay in the "philosophical transactions," published in the year , he propounded the theory, not merely of a natural limit to varieties from grafts and cuttings, but even that they would not survive the natural term of the life of the seedling trees from which they were originally taken. whatever may have been his view of the natural term of the life of a tree, and of a cutting being merely a part of the individual that produced it, there is no doubt that he laid himself open to the effective replies which were made from all sides at the time, and have lost none of their force since. weeping-willows, bread-fruits, bananas, sugar-cane, tiger-lilies, jerusalem artichokes, and the like, have been propagated for a long while in this way, without evident decadence. moreover, the analogy upon which his hypothesis is founded will not hold. whether or not one adopts the present writer's conception, that individuality is not actually reached or maintained in the vegetable world, it is clear enough that a common plant or tree is not an individual in the sense that a horse or man, or any one of the higher animals, is--that it is an individual only in the sense that a branching zoophyte or mass of coral is. solvitur crescendo: the tree and the branch equally demonstrate that they are not individuals, by being divided with impunity and advantage, with no loss of life, but much increase. it looks odd enough to see a writer like mr. sisley reproducing the old hypothesis in so bare a form as this: "i am prepared to maintain that varieties are individuals, and that as they are born they must die, like other individuals . . . we know that oaks, sequoias, and other trees, live several centuries, but how many we do not exactly know. but that they must die, no one in his senses will dispute." now, what people in their senses do dispute is, not that the tree will die, but that other trees, established from its cuttings, will die with it. but does it follow from this that non-sexually-propagated varieties are endowed with the same power of unlimited duration that is possessed by varieties and species propagated sexually--i.e., by seed? those who think so jump too soon at their conclusion. for, as to the facts, it is not enough to point out the diseases or the trouble in the soil or the atmosphere to which certain old fruits are succumbing, nor to prove that a parasitic fungus (peronospora infestans) is what is the matter with potatoes. for how else would constitutional debility, if such there be, more naturally manifest itself than in such increased liability or diminished resistance to such attacks? and if you say that, anyhow, such varieties do not die of old age--meaning that each individual attacked does not die of old age, but of manifest disease--it may be asked in return, what individual man ever dies of old age in any other sense than of a similar inability to resist invasions which in earlier years would have produced no noticeable effect? aged people die of a slight cold or a slight accident, but the inevitable weakness that attends old age is what makes these slight attacks fatal. finally, there is a philosophical argument which tells strongly for some limitation of the duration of non-sexually propagated forms, one that probably knight never thought of, but which we should not have expected recent writers to overlook. when mr. darwin announced the principle that cross-fertilization between the individuals of a species is the plan of nature, and is practically so universal that it fairly sustains his inference that no hermaphrodite species continually self-fertilized would continue to exist, he made it clear to all who apprehend and receive the principle that a series of plants propagated by buds only must have weaker hold of life than a series reproduced by seed. for the former is the closest possible kind of close breeding. upon this ground such varieties may be expected ultimately to die out; but "the mills of the gods grind so exceeding slow" that we cannot say that any particular grist has been actually ground out under human observation. if it be asked how the asserted principle is proved or made probable, we can here merely say that the proof is wholly inferential. but the inference is drawn from such a vast array of facts that it is wellnigh irresistible. it is the legitimate explanation of those arrangements in nature to secure cross-fertilization in the species, either constantly or occasionally, which are so general, so varied and diverse, and, we may add, so exquisite and wonderful, that, once propounded, we see that it must be true.* what else, indeed, is the meaning and * here an article would be in place, explaining the arrangements in nature for cross-fertilization, or wide-breeding, in plants, through the agency, sometimes of the winds, but more commonly of insects; the more so, since the development of the principle, the appreciation of its importance, and its confirmation by abundant facts, are mainly due to mr. darwin. but our reviews and notices of his early work "on the contrivances in nature for the fertilization of orchids by means of insects, in , and his various subsequent papers upon other parts of this subject, are either too technical or too fragmentary or special to be here reproduced. indeed, a popular essay is now hardly needed, since the topic has been fully presented, of late years, in the current popular and scientific journals, and in common educational works and text-books, so that it is in the way of becoming a part--and a most inviting part--of ordinary botanical instruction. use of sexual reproduction? not simply increase of numbers; for that is otherwise effectually provided for by budding propagation in plants and many of the lower animals. there are plants, indeed, of the lower sort (such as diatoms), in which the whole multiplication takes place in this way, and with great rapidity. these also have sexual reproduction; but in it two old individuals are always destroyed to make a single new one! here propagation diminishes the number of individuals fifty per cent. who can suppose that such a costly process as this, and that all the exquisite arrangements for cross-fertilization in hermaphrodite plants, do not subserve some most important purpose? how and why the union of two organisms, or generally of two very minute portions of them, should reenforce vitality, we do not know, and can hardly conjecture. but this must be the meaning of sexual reproduction. the conclusion of the matter, from the scientific point of view, is, that sexually-propagated varieties or races, although liable to disappear through change, need not be expected to wear out, and there is no proof that they do; but, that non-sexually propagated varieties, though not especially liable to change, may theoretically be expected to wear out, but to be a very long time about it. ii do species wear out? and if not, why not? the question we have just been considering was merely whether races are, or may be, as enduring as species. as to the inherently unlimited existence of species themselves, or the contrary, this, as we have said, is a geological and very speculative problem. not a few geologists and naturalists, however, have concluded, or taken for granted, that species have a natural term of existence--that they culminate, decline, and disappear through exhaustion of specific vitality, or some equivalent internal cause. as might be expected from the nature of the inquiry, the facts which bear upon the question are far from decisive. if the fact that species in general have not been interminable, but that one after another in long succession has become extinct, would seem to warrant this conclusion, the persistence through immense periods of no inconsiderable number of the lower forms of vegetable and animal life, and of a few of the higher plants from the tertiary period to the present, tells even more directly for the limitless existence of species. the disappearance is quite compatible with the latter view; while the persistence of any species is hardly explicable upon any other. so that, even under the common belief of the entire stability and essential inflexibility of species, extinction is more likely to have been accidental than predetermined, and the doctrine of inherent limitation is unsupported by positive evidence. on the other hand, it is an implication of the darwinian doctrine that species are essentially unlimited in existence. when they die out--as sooner or later any species may--the verdict must be accidental death, under stress of adverse circumstances, not exhaustion of vitality; and, commonly, when the species seems to die out, it will rather have suffered change. for the stock of vitality which enables it to vary and. survive in changed forms under changed circumstances must be deemed sufficient for a continued unchanged existence under unaltered conditions. and, indeed, the advancement from simpler to more complex, which upon the theory must have attended the diversification, would warrant or require the supposition of increase instead of diminution of power from age to age. the only case we call to mind which, under the darwinian view, might be interpreted as a dying out from inherent causes, is that of a species which refuses to vary, and thus lacks the capacity of adaptation to altering conditions. under altering conditions, this lack would be fatal. but this would be the fatality of some species or form in particular, not of species or forms generally, which, for the most part, may and do vary sufficiently, and in varying survive, seemingly none the worse, but rather the better, for their long tenure of life. the opposite idea, however, is maintained by m. naudin,[xii- ] in a detailed exposition of his own views of evolution, which differ widely from those of darwin in most respects, and notably in excluding that which, in our day, gives to the subject its first claim to scientific (as distinguished from purely speculative) attention; namely, natural selection. instead of the causes or operations collectively personified under this term, and which are capable of exact or probable appreciation, m. naudin invokes "the two principles of rhythm and of the decrease of forces in nature." he is a thorough evolutionist, starting from essentially the same point with darwin; for he conceives of all the forms or species of animals and plants "comme tire tout entier d'un protoplasma primordial, uniform, instable, eminemment plastique." also in "l'integration croissante de la force evolutive a mesure qu'elle se partage dans les formes produites, et la decroissance proportionelle de la plasticite de ces formes a mesure qu'elles s'eloignent davantage de leur origine, et qu'elles sont mieux arretees." as they get older, they gain in fixity through the operation of the fundamental law of inheritance; but the species, like the individual, loses plasticity and vital force. to continue in the language of the original: "c'est dire qu'il y a eu, pour l'ensemble du monde organique, une periode de formation ou tout etait changeant et mobile, une phase analogue a la vie embryonnaire et a la jeunesse de chaque etre particulier; et qu'a cet age de mobilite et de croissance a succede une periode de stabilite, au moins relative, une sorte d'age adulte, ou la force evolutive, ayant acheve son oeuvre, n'est plus occupee qu'a la maintenir, sans pouvoir produire d'organismes nouveaux. limitee en quantite, comme toutes les forces en jeu dans une planete ou dans un systeme sideral tout entier, cette force n'a pu accomplir qu'un travail limite; et du meme qu'un organisme, animal ou vegetal, ne croit pas indefiniment et qu'il s'arrete a des proportions que rien ne peut faire depasser, de meme aussi l'organisme total de la nature s'est arrete a un etat d'equilibre, dont la duree, selon toutes vraisemblances, doit etre beaucoup plus longue que celle de la phase de developpement et de croissance. a fixed amount of "evolutive force" is given, to begin with. at first enormous, because none has been used up in work, it is necessarily enfeebled in the currents into which the stream divides, and the narrower and narrower channels in which it flows with slowly-diminishing power. hence the limited although very unequal duration of all individuals, of all species, and of all types of organization. a multitude of forms have disappeared already, and the number of species, far from increasing, as some have believed, must, on the contrary, be diminishing. some species, no doubt, have suffered death by violence or accident, by geological changes, local alteration of the conditions, or the direct or indirect attacks of other species; but these have only anticipated their fate, for m. naudin contends that most of the extinct species have died a natural death from exhaustion of force, and that all the survivors are on the way to it. the great timepiece of nature was wound up at the beginning, and is running down. in the earlier stages of great plasticity and exuberant power, diversification took place freely, but only in definite lines, and species and types multiplied. as the power of survival is inherently limited, still more the power of change: this diminishes in time, if we rightly apprehend the idea, partly through the waning of vital force, partly through the fixity acquired by heredity--like producing like, the more certainly in proportion to the length and continuity of the ancestral chain and so the small variations of species which we behold are the feeble remnants of the pristine plasticity and an exhausted force.[xii- ] this force of variation or origination of forms has acted rhythmically or intermittently, because each movement was the result of the rupture of an equilibrium, the liberation of a force which till then was retained in a potential state by some opposing force or obstacle, overcoming which it passes to a new equilibrium and so on hence alternations of dynamic activity and static repose, of origination of species and types, alternated with periods of stability or fixity. the timepiece does not run down regularly, but "la force procede par saccades; et . . . par pulsations d'autant plus energiques que la nature etait plus pres de son commencement." such is the hypothesis. for a theory of evolution, this is singularly unlike darwin's in most respects, and particularly in the kind of causes invoked and speculations indulged in. but we are not here to comment upon it beyond the particular point under consideration, namely, its doctrine of the inherently limited duration of species. this comes, it will be noticed, as a deduction from the modern physical doctrine of the equivalence of force. the reasoning is ingenious, but, if we mistake not, fallacious. to call that "evolutive force" which produces the change of one kind of plant or animal into another, is simple and easy, but of little help by way of explanation. to homologize it with physical force, as m. naudin's argument requires, is indeed a step, and a hardy one; but it quite invalidates the argument. for, if the "evolutive force" is a part of the physical force of the universe, of which, as he reminds us, the sum is fixed and the tendency is toward a stable equilibrium in which all change is to end, then this evolutive was derived from the physical force; and why not still derivable from it? what is to prevent its replenishment in vegetation, pari passu with that great operation in which physical force is stored up in vegetable organisms, and by the expenditure or transformation of which their work, and that of all animals, is carried on? whatever be the cause (if any there be) which determines the decadence and death of species, one cannot well believe that it is a consequence of a diminution of their proper force by plant-development and division; for instance, that the sum of what is called vital force in a full-grown tree is not greater, instead of less, than that in the seeding, and in the grove greater than in the single parental tree. this power, if it be properly a force, is doubtless as truly derived from the sunbeam as is the power which the plant and animal expend in work. here, then, is a source of replenishment as lasting as the sun itself, and a ground--so far as a supply of force is concerned--for indefinite duration. for all that any one can mean by the indefinite existence of species is, that they may (for all that yet appears) continue while the external conditions of their being or well-being continue. perhaps, however, m. naudin does not mean that "evolutive force," or the force of vitality, is really homologous with common physical force, but only something which may be likened to it. in that case the parallel has only a metaphorical value, and the reason why variation must cease and species die out is still to seek. in short, if that which continues the series of individuals in propagation, whether like or unlike the parents, be a force in the physical sense of the term, then there is abundant provision in nature for its indefinite replenishment. if, rather, it be a part or phase of that something which directs and determines the expenditure of force, then it is not subject to the laws of the latter, and there is no ground for inferring its exhaustibility. the limited vitality is an unproved and unprovable conjecture. the evolutive force, dying out in the using, is either the same conjecture repeated, or a misapplied analogy. after all--apart from speculative analogies--the only evidences we possess which indicate a tendency in species to die out, are those to which mr. darwin has called attention. these are, first, the observed deterioration which results, at least in animals, from continued breeding in and in, which may possibly be resolvable into cumulative heritable disease; and, secondly, as already stated (p. ), what may be termed the sedulous and elaborate pains everywhere taken in nature to prevent close breeding--arrangements which are particularly prominent in plants, the greater number of which bear hermaphrodite blossoms. the importance of this may be inferred from the universality, variety, and practical perfection of the arrangements which secure the end; and the inference may fairly be drawn that this is the physiological import of sexes. it follows from this that there is a tendency, seemingly inherent, in species as in individuals, to die out; but that this tendency is counteracted or checked by sexual wider breeding, which is, on the whole, amply secured in nature, and which in some way or other reenforces vitality to such an extent as to warrant darwin's inference that "some unknown great good is derived from the union of individuals which have been kept distinct for many generations." whether this reenforcement is a complete preventive of decrepitude in species, or only a palliative, is more than we can determine. if the latter, then existing species and their derivatives must perish in time, and the earth may be growing poorer in species, as m. naudin supposes, through mere senility. if the former, then the earth, if not even growing richer, may be expected to hold its own, and extant species or their derivatives should last as long as the physical world lasts and affords favorable conditions. general analogies seem to favor the former view. such facts as we possess, and the darwinian hypothesis, favor the latter. xiii evolutionary teleology when cuvier spoke of the "combination of organs in such order that they may be in consistence with the part which the animal has to play in nature," his opponent, geoffroy st.-hilaire, rejoined, "i know nothing of animals which have to play a part in nature." the discussion was a notable one in its day. from that time to this, the reaction of morphology against "final causes" has not rarely gone to the extent of denying the need and the propriety of assuming ends in the study of animal and vegetable organizations. especially in our day, when it became apparent that the actual use of an organ might not be the fundamental reason of its existence-- that one and the same organ, morphologically considered, was modified in different cases to the most diverse uses, while intrinsically different organs subserved identical functions, and consequently that use was a fallacious and homology the surer guide to correct classification--it was not surprising that teleological ideas nearly disappeared from natural history. probably it is still generally thought that the school of cuvier and that of st.-hilaire have neither common ground nor capability of reconcilement. in a review of darwin's volume on the "fertilization of orchids" * (too technical and too detailed for reproduction here), and later in a brief sketch of the character of his scientific work (art. ix, p. ), we expressed our sense of the great gain to science from his having brought back teleology to natural history. in darwinism, usefulness and purpose come to the front again as working principles of the first order; upon them, indeed, the whole system rests. to most, this restoration of teleology has come from an unexpected quarter, and in an unwonted guise; so that the first look of it is by no means reassuring to the minds of those who cherish theistic views of nature. adaptations irresistibly suggesting purpose had their supreme application in natural theology. being manifold, particular, and exquisite, and evidently inwrought into the whole system of the organic world, they were held to furnish irrefragable as well as independent proof of a personal designer, a divine originator of nature. by a confusion of thought, now obvious, but at the time not unnatural, they were also regarded as proof of a direct execution of the contriver's purpose in the creation of each organ and organism, as it were, in the manner man contrives and puts together a machine--an idea which has been set up as the orthodox doctrine, but which to st. augustine and other learned christian fathers would have savored of heterodoxy. in the doctrine of the origination of species through natural selection, these adaptations appear as the outcome rather than as the motive, as final results rather than final causes. adaptation to use, although the very essence of darwinism, is not a fixed and inflexible adaptation, realized once for all at the outset; it includes a long progression and succession of modifications, adjusting themselves to changing circumstances, under which they may be more and more diversified, specialized, and in a just sense perfected. now, the question is, does this involve the destruction or only the reconstruction of our consecrated ideas of teleology? is it compatible with our seemingly inbore conception of nature as an ordered system? furthermore, and above all, can the darwinian theory itself dispense with the idea of purpose, in the ordinary sense of the word, as tantamount to design? from two opposing sides we hear the first two questions answered in the negative. and an affirmative response to the third is directly implied in the following citation: "the word purpose has been used in a sense to which it is, perhaps, worth while to call attention. adaptation of means to an end may be provided in two ways that we at present know of: by processes of natural selection, and by the agency of an intelligence in which an image or idea of the end preceded the use of the means. in both cases the existence of the adaptation is accounted for by the necessity or utility of the end. it seems to me convenient to use the word purpose as meaning generally the end to which certain means are adapted, both in these two cases and in any other that may hereafter become known, provided only that the adaptation is accounted for by the necessity or utility of the end. and there seems no objection to the use of the phrase 'final cause' in this wider sense, if it is to be kept at all. the word 'design' might then be kept for the special case of adaptation by an intelligence. and we may then say that, since the process of natural selection has been understood, purpose has ceased to suggest design to instructed people, except in cases where the agency of man is independently probable."--p.c.w., in the contemporary review for september, , p. . the distinction made by this anonymous writer is convenient and useful, and his statement clear. we propose to adopt this use of the terms purpose and design, and to examine the allegation. the latter comes to this: "processes of natural selection" exclude "the agency of an intelligence in which the image or idea of the end precedes the use of the means;" and since the former have been understood "purpose has ceased to suggest design to instructed people, except in cases where the agency of man is independently probable." the maxim "l'homme propose, dieu dispose," under this reading means that the former has the monopoly of design, while the latter accomplishes without designing. man's works alone suggest design. but it is clear to us that this monopoly is shared with certain beings of inferior grade. granting that quite possibly the capture of flies for food by dionaea and the sundews may be attributed to purpose apart from design (if it be practicable in the last resort to maintain this now convenient distinction), still their capture by a spider's-web, and by a swallow on the wing, can hardly "cease to suggest design to instructed people." and surely, in coming at his master's call, the dog fulfills his own design as well as that of his master; and so of other actions and constructions of brute animals. without doubt so acute a writer has a clear and sensible meaning; so we conclude that he regards brutes as automata, and was thinking of design as coextensive merely with general conceptions. not concerning ourselves with the difficulty he may have in drawing a line between the simpler judgments and affections of man and those of the highest-endowed brutes, we subserve our immediate ends by remarking that the automatic theory would seem to be one which can least of all dispense with design, since, either in the literal or current sense of the word, undesigned automatism is, as near as may be, a contradiction in terms. as the automaton man constructs manifests the designs of its maker and mover, so the more efficient automata which man did not construct would not legitimately suggest less than human intelligence. and so all adaptations in the animal and vegetable world which irresistibly suggest purpose (in the sense now accepted) would also suggest design, and, under the law of parsimony, claim to be thus interpreted, unless some other hypothesis will better account for the facts. we will consider, presently, if any other does so. we here claim only that some beings other than men design, and that the adaptations of means to ends in the structure of animals and plants, in so far as they carry the marks of purpose, carry also the implication of having been designed. also, that the idea or hypothesis of a designing mind, as the author of nature--however we came by it--having possession of the field, and being one which man, himself a designer, seemingly must needs form, cannot be rivaled except by some other equally adequate for explanation, or displaced except by showing the illegitimacy of the inference. as to the latter, is the common apprehension and sense of mankind in this regard well grounded? can we rightly reason from our own intelligence and powers to a higher or a supreme intelligence ordering and shaping the system of nature? a very able and ingenious writer upon "the evidences of design in nature," in the westminster review for july, , maintains the negative. his article may be taken as the argument in support of the position assumed by "p.c.w.," in the contemporary review above cited. it opens with the admission that the orthodox view is the most simple and apparently convincing, has had for centuries the unhesitating assent of an immense majority of thinkers, and that the latest master-writer upon the subject disposed to reject it, namely, mill, comes to the conclusion that, "in the present state of our knowledge, the adaptations in nature afford a large balance of probability in favor of creation by intelligence." it proceeds to attack not so much the evidence in favor of design as the foundation upon which the whole doctrine rests, and closes with the prediction that sooner or later the superstructure must fall. and, truly, if his reasonings are legitimate, and his conclusions just, "science has laid the axe to the tree." "given a set of marks which we look upon in human productions as unfailing indications of design," he asks, "is not the inference equally legitimate when we recognize these marks in nature? to gaze on such a universe as this, to feel our hearts exult within us in the fullness of existence, and to offer in explanation of such beneficent provision no other word but chance, seems as unthankful and iniquitous as it seems absurd. chance produces nothing in the human sphere; nothing, at least, that can be relied upon for good. design alone engenders harmony, consistency; and chance not only never is the parent, but is constantly the enemy of these. how, then, can we suppose chance to be the author of a system in which everything is as regular as clockwork? . . . the hypothesis of chance is inadmissible." there is, then, in nature, an order; and, in "p.c.w.'s" sense of the word, a manifest purpose. some sort of conception as to the cause of it is inevitable, that of design first and foremost. "why"--the westminster reviewer repeats the question--"why, if the marks of utility and adaptation are conclusive in the works of man, should they not be considered equally conclusive in the works of nature?" his answer appears to us more ingenious than sound. because, referring to paley's watch,-- "the watch-finder is not guided solely in his inference by marks of adaptation and utility; he would recognize design in half a watch, in a mere fragment of a watch, just as surely as in a whole time-keeper . . . two cog-wheels, grasping each other, will be thought conclusive evidence of design, quite independently of any use attaching to them. and the inference, indeed, is perfectly correct; only it is an inference, not from a mark of design, properly so called, but from a mark of human workmanship . . . no more is needed for the watch-finder, since all the works of man are, at the same time, products of design; but a great deal more is requisite for us, who are called upon by paley to recognize design in works in which this stamp, this label of human workmanship, is wanting. the mental operation required in the one case is radically different from that performed in the other; there is no parallel, and paley's demonstration is totally irrelevant."[xiii- ] but, surely, all human doings are not "products of design;" many are contingent or accidental. and why not suppose that the finder of the watch, or of the watch-wheel, infers both design and human workmanship? the two are mutually exclusive only on the supposition that man alone is a designer, which is simply begging the question in discussion. if the watch-finder's attention had been arrested by a different object, such as a spider's web, he would have inferred both design and non-human workmanship. of some objects he might be uncertain whether they were of human origin or not, with-out ever doubting they were designed, while of others this might remain doubtful. nor is man's recognition of human workmanship, or of any other, dependent upon his comprehending how it was done, or what particular ends it subserves. such considerations make it clear that "the label of human workmanship" is not the generic stamp from which man infers design. it seems equally clear that "the mental operation required in the one case" is not so radically or materially "different from that performed in the other" as this writer would have us suppose. the judgment respecting a spider's web, or a trap-door spider's dwelling, would be the very same in this regard if it preceded, as it occasionally might, all knowledge of whether the object met with were of human or animal origin. a dam across a stream, and the appearance of the stumps of trees which entered into its formation, would suggest design quite irrespective of and antecedent to the considerable knowledge or experience which would enable the beholder to decide whether this was the work of men or of beavers. why, then, should the judgment that any particular structure is a designed work be thought illegitimate when attributed to a higher instead of a lower intelligence than that of man? it might, indeed, be so if the supposed observer had no conception of a power and intelligence superior to his own. but it would then be more than "irrelevant;" it would be impossible, except on the supposition that the phenomena would of themselves give rise to such an inference. that it is now possible to make the inference, and, indeed, hardly possible not to make it, is sufficient warrant of its relevancy. it may, of course, be rejoined that, if this important factor is given, the inference yields no independent argument of a divine creator; and it may also be reasonably urged that the difference between things that are made under our observation and comprehension, and things that grow, but have originated beyond our comprehension, is too wide for a sure inference from the one to the other. but the present question involves neither of these. it is simply whether the argument for design from adaptations in nature is relevant, not whether it is independent or sure. it is conceded that the argument is analogical, and the parallel incomplete. but the gist is in the points that are parallel or similar. pulleys, valves, and suchlike elaborate mechanical adaptations, cannot differ greatly in meaning, wherever met with. the opposing argument is repeated and passed in another form: "the evidence of design afforded by the marks of adaptation in works of human competence is null and void in the case of creation itself . . . nature is full of adaptations; but these are valueless to us as traces of design, unless we know something of the rival adaptations among which an intelligent being might have chosen. to assert that in nature no such rival adaptations existed, and that in every case the useful function in question could be established by no other instrument but one, is simply to reason in a circle, since it is solely from what we find existing that our notions of possibility and impossibility are drawn. . . . we cannot imagine ourselves in the position of the creator before his work began, nor examine the materials among which he had to choose, nor count the laws which limited his operations. here all is dark, and the inference we draw from the seeming perfections of the existing instruments or means is a measure of nothing but our ignorance." but the question is not about the perfection of these adaptations, or whether others might have been instituted in their place. it is simply whether observed adaptations of intricate sorts, admirably subserving uses, do or do not legitimately suggest to one designing mind that they are the product of some other. if so, no amount of ignorance, or even inconceivability, of the conditions and mode of production could affect the validity of the inference, nor could it be affected by any misunderstanding on our part as to what the particular use or function was; a statement which would have been deemed superfluous, except for the following: "there is not an organ in our bodies but what has passed, and is still passing, through a series of different and often contradictory interpretations. our lungs, for instance, were anciently conceived to be a kind of cooling apparatus, a refrigerator; at the close of the last century they were supposed to be a centre of combustion; and nowadays both these theories have been abandoned for a third . . . have these changes modified in the slightest degree the supposed evidence of design?" we have not the least idea why they should. so, also, of complicated processes, such as human digestion, being replaced by other and simpler ones in lower animals, or even in certain plants. if "we argue the necessity of every adaptation solely from the fact that it exists," and that "we cannot mutilate it grossly without injury to the function," we do not "announce triumphantly that digestion is impossible in any way but this," etc., but see equal wisdom and no impugnment of design in any number of simpler adaptations accomplishing equivalent purposes in lower animals. finally, adaptation and utility being the only marks of design in nature which we possess, and adaptation only as subservient to usefulness, the westminster reviewer shows us how: "the argument from utility may be equally refuted another way. we found in our discussion of the mark of adaptation that the positive evidence of design afforded by the mechanisms of the human frame was never accompanied by the possibility of negative evidence. we regarded this as a suspicious circumstance, just as the fox, invited to attend the lion in his den, was deterred from his visit by observing that all the foottracks lay in one direction. the same suspicious circumstance warns us now. if positive evidence of design be afforded by the presence of a faculty, negative evidence of design ought to be afforded by the absence of a faculty. this, however, is not the case." [then follows the account of a butterfly, which, from the wonderful power of the males to find the females at a great distance, is conceived to possess a sixth sense.] "do we consider the deficiency of this sixth sense in man as the slightest evidence against design? should we be less apt to infer creative wisdom if we had only four senses instead of five, or three instead of four? no, the case would stand precisely as it does now. we value our senses simply because we have them, and because our conception of life as we desire it is drawn from them. but to reason from such value to the origin of our endowment, to argue that our senses must have been given to us by a deity because we prize them, is evidently to move round and round in a vicious circle. "the same rejoinder is easily applicable to the argument from beauty, which indeed is only a particular aspect of the argument from utility. it is certainly improbable that a random daubing of colors on a canvas will produce a tolerable painting, even should the experiment be continued for thousands of years. our conception of beauty being given, it is utterly improbable that chance should select, out of the infinity of combinations which form and color may afford, the precise combination which that conception will approve. but the universe is not posterior to our sense of beauty, but antecedent to it: our sense of beauty grows out of what we see; and hence the conformance of our world to our aesthetical conceptions is evidence, not of the world's origin, but of our own." we are accustomed to hear design doubted on account of certain failures of provision, waste of resources, or functionless condition of organs; but it is refreshingly new to have the very harmony itself of man with his surroundings, and the completeness of provision for his wants and desires, brought up as a refutation of the validity of the argument for design. it is hard, indeed, if man must be out of harmony with nature in order to judge anything respecting it, or his relations with it; if he must have experience of chaos before he can predicate anything of order. but is it true that man has all that he conceives of, or thinks would be useful, and has no "negative evidence of design afforded by the absence of a faculty" to set against the positive evidence afforded by its presence? he notes that he lacks the faculty of flight, sometimes wants it, and in dreams imagines that he has it, yet as thoroughly believes that he was designed not to have it as that he was designed to have the faculties and organs which he possesses. he notes that some animals lack sight, and so, with this negative side of the testimony to the value of vision, he is "apt to infer creative wisdom" both in what he enjoys and in what the lower animal neither needs nor wants. that man does not miss that which he has no conception of, and is by this limitation disqualified from judging rightly of what he can conceive and know, is what the westminster reviewer comes to, as follows: "we value the constitution of our world because we live by it, and because we cannot conceive ourselves as living otherwise. our conceptions of possibility, of law, of regularity, of logic, are all derived from the same source; and as we are constantly compelled to work with these conceptions, as in our increasing endeavors to better our condition and increase our provision we are constantly compelled to guide ourselves by nature's regulations, we accustom ourselves to look upon these regularities and conceptions as antecedent to all work, even to a creator's, and to judge of the origin of nature as we judge of the origin of inventions and utilities ascribable to man. this explains why the argument of design has enjoyed such universal popularity. but that such popularity is no criterion of the argument's worth, and that, indeed, it is no evidence of anything save of an unhappy weakness in man's mental constitution, is abundantly proved by the explanation itself." well, the constitution and condition of man being such that he always does infer design in nature, what stronger presumption could there possibly be of the relevancy of the inference? we do not say of its correctness: that is another thing, and is not the present point. at the last, as has well been said, the whole question resolves itself into one respecting the ultimate veracity of nature, or of the author of nature, if there be any. passing from these attempts to undermine the foundation of the doctrine--which we judge to be unsuccessful--we turn to the consideration of those aimed at the superstructure. evidences of design may be relevant, but not cogent. they may, as mill thought, preponderate, or the wavering balance may incline the other way. there are two lines of argument: one against the sufficiency, the other against the necessity, of the principle of design. design has been denied on the ground that it squares with only one part of the facts, and fails to explain others; it may be superseded by showing that all the facts are in the way of being explained without it. the things which the principle of design does not explain are many and serious. some are in their nature inexplicable, at least are beyond the power and province of science. others are of matters which scientific students have to consider, and upon which they may form opinions, more or less well grounded. as to biological science--with which alone we are concerned--it is getting to be generally thought that this principle, as commonly understood, is weighted with much more than it can carry. this statement will not be thought exaggerated by those most familiar with the facts and the ideas of the age, and accustomed to look them in the face. design is held to, no doubt, by most, and by a sure instinct; not, however, as always offering an explanation of the facts, but in spite of the failure to do so. the stumbling-blocks are various, and they lie in every path: we can allude only to one or two as specimens. adaptation and utility are the marks of design. what, then, are organs not adapted to use marks of? functionless organs of some sort are the heritage of almost every species. we have ways of seeming to account for them--and of late one which may really account for them--but they are unaccountable on the principle of design. some, shutting their eyes to the difficulty, deny that we know them to be functionless, and prefer to believe they must have a use because they exist, and are more or less connected with organs which are correlated to obvious use; but only blindfolded persons care to tread the round of so narrow a circle. of late some such abortive organs in flowers and fruits are found to have a use, though not the use of their kind. but unwavering believers in design should not trust too much to instances of this sort. there is an old adage that, if anything be kept long enough, a use will be found for it. if the following up of this line, when it comes in our way, should bring us round again to a teleological principle, it will not be one which conforms to the prevalent ideas now attacked. it is commonly said that abortive and useless organs exist for the sake of symmetry, or as parts of a plan. to say this, and stop there, is a fine instance of mere seeming to say something. for, under the principle of design, what is the sense of introducing useless parts into a useful organism, and what shadow of explanation does "symmetry" give? to go further and explain the cause of the symmetry and how abortive organs came to be, is more to the purpose, but it introduces quite another principle than that of design. the difficulty recurs in a somewhat different form when an organ is useful and of exquisite perfection in some species, but functionless in another. an organ, such as an eye, strikes us by its exquisite and, as we may, perfect adaptation and utility in some animal; it is found repeated, still useful but destitute of many of its adaptations, in some animal of lower grade; in some one lower still it is rudimentary and useless. it is asked, if the first was so created for its obvious and actual use, and the second for such use as it has, what was the design of the third? one more case, in which use after all is well subserved, we cite from the article already much quoted from: "it is well known that certain fishes (pleuronecta) display the singularity of having both eyes on the same side of their head, one eye being placed a little higher than the other. this arrangement has its utility; for the pleuronecta, swimming on their side quite near the bottom of the sea, have little occasion for their eyesight except to observe what is going on above them. but the detail to which we would call notice is, that the original position of the eyes is symmetrical in these fishes, and that it is only at a certain point of their development that the anomaly is manifested, one of the eyes passing to the other side of the head. it is almost inconceivable that an intelligent being should have selected such an arrangement; and that, intending the eyes to be used only on one side of the head, he should have placed them originally on different sides." then the waste of being is enormous, far beyond the common apprehension. seeds, eggs, and other germs, are designed to be plants and animals, but not one of a thousand or of a million achieves its destiny. those that fall into fitting places and in fitting numbers find beneficent provision, and, if they were to wake to consciousness, might argue design from the adaptation of their surroundings to their well-being. but what of the vast majority that perish? as of the light of the sun, sent forth in all directions, only a minute portion is intercepted by the earth or other planets where some of it may be utilized for present or future life, so of potential organisms, or organisms begun, no larger proportion attain the presumed end of their creation. "destruction, therefore, is the rule; life is the exception. we notice chiefly the exception--namely, the lucky prize-winner in the lottery-- and take but little thought about the losers, who vanish from our field of observation, and whose number it is often impossible to estimate. but, in this question of design, the losers are important witnesses. if the maxim 'audi alteram partem' is applicable anywhere, it is applicable here. we must hear both sides, and the testimony of the seed fallen on good ground must be corrected by the testimony of that which falls by the wayside, or on the rocks. when we find, as we have seen above, that the sowing is a scattering at random, and that, for one being provided for and living, ten thousand perish unprovided for, we must allow that the existing order would be accounted as the worst disorder in any human sphere of action." it is urged, moreover, that all this and much more applies equally to the past stages of our earth and its immensely long and varied succession of former inhabitants, different from, yet intimately connected with, the present. it is not one specific creation that the question has to deal with--as was thought not very many years ago--but a series of creations through countless ages, and of which the beginning is unknown. these references touch a few out of many points, and merely allude to some of the difficulties which the unheeding pass by, but which, when brought before the mind, are seen to be stupendous. somewhat may be justly, or at least plausibly, said in reply to all this from the ordinary standpoint, but probably not to much effect. there were always insuperable difficulties, which, when they seemed to be few, might be regarded as exceptional; but, as they increase in number and variety, they seem to fall into a system. no doubt we may still insist that, "in the present state of our knowledge, the adaptations in nature afford a large balance of probability in favor of creation by intelligence," as mill concluded; and probability must needs be the guide of reason through these dark places. still, the balancing of irreconcilable facts is not a satisfying occupation, nor a wholly hopeful one, while fresh weights are from time to time dropping into the lighter side of the balance. strong as our convictions are, they may be overborne by evidence. we cannot rival the fabled woman of ephesus, who, beginning by carrying her calf from the day of its birth, was still able to do so when it became an ox. the burden which our fathers carried comfortably, with some adventitious help, has become too heavy for our shoulders. seriously, there must be something wrong in the position, some baleful error mixed with the truth, to which this contradiction of our inmost convictions may be attributed. the error, as we suppose, lies in the combination of the principle of design with the hypothesis of the immutability and isolated creation of species. the latter hypothesis, in its nature un-provable, has, on scientific grounds, become so far improbable that few, even of the anti-darwinian naturalists, now hold to it; and, whatever may once have been its religious claims, it is at present a hinderance rather than a help to any just and consistent teleology. by the adoption of the darwinian hypothesis, or something like it, which we incline to favor, many of the difficulties are obviated, and others diminished. in the comprehensive and far-reaching teleology which may take the place of the former narrow conceptions, organs and even faculties, useless to the individual, find their explanation and reason of being. either they have done service in the past, or they may do service in the future. they may have been essentially useful in one way in a past species, and, though now functionless, they may be turned to useful account in some very different way hereafter. in botany several cases come to our mind which suggest such interpretation. under this view, moreover, waste of life and material in organic nature ceases to be utterly inexplicable, because it ceases to be objectless. it is seen to be a part of the general "economy of nature," a phrase which has a real meaning. one good illustration of it is furnished by the pollen of flowers. the seeming waste of this in a pine-forest is enormous. it gives rise to the so-called "showers of sulphur," which every one has heard of. myriads upon myriads of pollen-grains (each an elaborate organic structure) are wastefully dispersed by the winds to one which reaches a female flower and fertilizes a seed. contrast this with one of the close-fertilized flowers of a violet, in which there are not many times more grains of pollen produced than there are of seeds to be fertilized; or with an orchis-flower, in which the proportion is not widely different. these latter are certainly the more economical; but there is reason to believe that the former arrangement is not wasteful. the plan in the violet-flower assures the result with the greatest possible saving of material and action; but this result, being close-fertilization or breeding in and in, would, without much doubt, in the course of time, defeat the very object of having seeds at all.[xiii- ] so the same plant produces other flowers also, provided with a large surplus of pollen, and endowed (as the others are not) with color, fragrance, and nectar, attractive to certain insects, which are thereby induced to convey this pollen from blossom to blossom, that it may fulfill its office. in such blossoms, and in the great majority of flowers, the fertilization and consequent perpetuity of which are committed to insects, the likelihood that much pollen may be left behind or lost in the transit is sufficient reason for the apparent superfluity. so, too, the greater economy in orchis-flowers is accounted for by the fact that the pollen is packed in coherent masses, all attached to a common stalk, the end of which is expanded into a sort of button, with a glutinous adhesive face (like a bit of sticking-plaster), and this is placed exactly where the head of a moth or butterfly will be pressed against it when it sucks nectar from the flower, and so the pollen will be bodily conveyed from blossom to blossom, with small chance of waste or loss. the floral world is full of such contrivances; and while they exist the doctrine of purpose or final cause is not likely to die out. now, in the contrasted case, that of pine-trees, the vast superabundance of pollen would be sheer waste if the intention was to fertilize the seeds of the same tree, or if there were any provision for insect-carriage; but with wide-breeding as the end, and the wind which "bloweth where it listeth" as the means, no one is entitled to declare that pine-pollen is in wasteful excess. the cheapness of wind-carriage may be set against the overproduction of pollen. similar considerations may apply to the mould-fungi and other very low organisms, with spores dispersed through the air in countless myriads, but of which only an infinitesimal portion find opportunity for development. the myriads perish. the exceptional one, falling into a fit medium, is imagined by the westminster reviewer to argue design from the beneficial provision it finds itself enjoying, in happy ignorance of the perishing or latent multitude. but, in view of the large and important part they play (as the producers of all fermentation and as the omnipresent scavenger-police of nature), no good ground appears for arguing either wasteful excess or absence of design from the vast disparity between their potential and their actual numbers. the reserve and the active members of the force should both be counted in, ready as they always and everywhere are for service. considering their ubiquity, persistent vitality, and promptitude of action upon fitting occasion, the suggestion would rather be that, while ". . . thousands at his bidding speed, and post o'er land and ocean without rest, they also serve [which] only stand and wait." finally, darwinian teleology has the special advantage of accounting for the imperfections and failures as well as for successes. it not only accounts for them, but turns them to practical account. it explains the seeming waste as being part and parcel of a great economical process. without the competing multitude, no struggle for life; and without this, no natural selection and survival of the fittest, no continuous adaptation to changing surroundings, no diversification and improvement, leading from lower up to higher and nobler forms. so the most puzzling things of all to the old-school teleologists are the principia of the darwinian. in this system the forms and species, in all their variety, are not mere ends in themselves, but the whole a series of means and ends, in the contemplation of which we may obtain higher and more comprehensive, and perhaps worthier, as well as more consistent, views of design in nature than heretofore. at least, it would appear that in darwinian evolution we may have a theory that accords with if it does not explain the principal facts, and a teleology that is free from the common objections. but is it a teleology, or rather--to use the new-fangled term--a dysteleology? that depends upon how it is held. darwinian evolution (whatever may be said of other kinds) is neither theistical nor nontheistical. its relations to the question of design belong to the natural theologian, or, in the larger sense, to the philosopher. so long as the world lasts it will probably be open to any one to hold consistently, in the last resort, either of the two hypotheses, that of a divine mind, or that of no divine mind. there is no way that we know of c by which the alternative may be excluded. viewed philosophically, the question only is, which is the better supported hypothesis of the two? we have only to say that the darwinian system, as we understand it, coincides well with the theistic view of nature. it not only acknowledges purpose (in the contemporary reviewer's sense), but builds upon it; and if purpose in this sense does not of itself imply design, it is certainly compatible with it, and suggestive of it. difficult as it may be to conceive and impossible to demonstrate design in a whole of which the series of parts appear to be contingent, the alternative may be yet more difficult and less satisfactory. if all nature is of a piece--as modern physical philosophy insists-- then it seems clear that design must in some way, and in some sense, pervade the system, or be wholly absent from it. of the alternatives, the predication of design--special, general, or universal, as the case may be--is most natural to the mind; while the exclusion of it throughout, because some utilities may happen, many adaptations may be contingent results, and no organic maladaptations could continue, runs counter to such analogies as we have to guide us, and leads to a conclusion which few men ever rested in. it need not much trouble us that we are incapable of drawing clear lines of demarkation between mere utilities, contingent adaptations, and designed contrivances in nature; for we are in much the same condition as respects human affairs and those of lower animals. what results are comprehended in a plan, and what are incidental, is often more than we can readily determine in matters open to observation. and in plans executed mediately or indirectly, and for ends comprehensive and far-reaching, many purposed steps must appear to us incidental or meaningless. but the higher the intelligence, the more fully will the incidents enter into the plan, and the more universal and interconnected may the ends be. trite as the remark is, it would seem still needful to insist that the failure of a finite being to compass the designs of an infinite mind should not invalidate its conclusions respecting proximate ends which he can understand. it is just as in physical science, where, as our knowledge and grasp increase, and happy discoveries are made, wider generalizations are formed, which commonly comprehend, rather than destroy, the earlier and partial ones. so, too, the "sterility" of the old doctrine of final causes in science, and the presumptuous uses made of them, when it was supposed that every adapted arrangement or structure existed for this or that direct and special end, and for no other, can hardly be pressed to the conclusion that there are no final causes, i.e., ultimate reasons of things.[xiii- ] design in nature is distinguished from that in human affairs--as it fittingly should be--by all comprehensiveness and system. its theological synonym is providence. its application in particular is surrounded by similar insoluble difficulties; nevertheless, both are bound up with theism. probably few at the present day will maintain that darwinian evolution is incompatible with the principle of design; but some insist that the theory can dispense with, and in fact supersedes, this principle. the westminster reviewer cleverly expounds how it does so. the exposition is too long to quote, and an abstract is unnecessary, for the argument adverse to design is, as usual, a mere summation or illustration of the facts and assumptions of the hypothesis itself, by us freely admitted. simplest forms began; variations occurred among them; under the competition consequent upon the arithmetical or geometrical progression in numbers, only the fittest for the conditions survive and propagate, vary further, and are similarly selected; and so on. "progress having once begun by the establishment of species, the laws of atavism and variability will suffice to tell the remainder of the story. the colonies gifted with the faculty of forming others in their likeness will soon by their increase become sole masters of the field; but the common enemy being thus destroyed, the struggle for life will be renewed among the conquerors. the saying that 'a house divided against itself cannot stand,' receives in nature its flattest contradiction. civil war is here the very instrument of progress; it brings about the survival of the fittest. original differences in the cell-colonies, however slight, will bring about differences of life and action; the latter, continued through successive generations, will widen the original differences of structure; innumerable species will thus spring up, branching forth in every direction from the original stock; and the competition of these species among each other for the ground they occupy, or the food they seek, will bring out and develop the powers of the rivals. one chief cause of superiority will lie in the division of labor instituted by each colony; or, in other words, in the localization of the colony's functions. in the primitive associations (as in the lowest organisms existing now), each cell performed much the same work as its neighbor, and the functions necessary to the existence of the whole (alimentation, digestion, respiration, etc.) were exercised by every colonist in his own behalf. social life, however, acting upon the cells as it acts upon the members of a human family, soon created differences among them--differences ever deepened by continuance, and which, by narrowing the limits of each colonist's activity, and increasing his dependence on the rest, rendered him fitter for his special task. each function was thus gradually monopolized; but it came to be the appanage of a single group of cells, or organ; and so excellent did this arrangement prove, so greatly were the powers of each commonwealth enhanced by the division of its labor, that the more organs a colony possessed, the more likely it was to succeed in its struggle for life. . . we shall go no further, for the reader will easily fill out the remainder of the picture for himself. man is but an immense colony of cells, in which the division of labor, together with the centralization of the nervous system, has reached its highest limit. it is chiefly to this that his superiority is due; a superiority so great, as regards certain functions of the brain, that he may be excused for having denied his humbler relatives, and dreamed that, standing alone in the centre of the universe, sun, moon, and stars, were made for him." let us learn from the same writer how both eyes of the flounder get, quite unintentionally, on the same side of the head. the writer makes much of this case (see p. ), and we are not disposed to pass it by: "a similar application may be made to the pleuronecta. presumably, these fishes had adopted their peculiar mode of swimming long before the position of their eyes became adapted to it. a spontaneous variation occurred, consisting in the passage of one eye to the opposite side of the head; and this variation afforded its possessors such increased facilities of sight that in the course of time the exception became the rule. but the remarkable point is, that the law of heredity not only preserved the variation itself, but the date of its occurrence; and that, although for thousands of years the adult pleuronecta have had both eyes on the same side, the young still continue during their earlier development to exhibit the contrary arrangement, just as if the variation still occurred spontaneously." here a wonderful and one would say unaccountable transference takes place in a short time. as steenstrup showed, one eye actually passes through the head while the young fish is growing. we ask how this comes about; and we are told, truly enough, that it takes place in each generation because it did so in the parents and in the whole line of ancestors. why offspring should be like parent is more than any one can explain; but so it is, in a manner so nearly fixed and settled that we can count on it; yet not from any absolute necessity that we know of, and, indeed, with sufficiently striking difference now and then to demonstrate that it might have been otherwise, or is so in a notable degree. this transference of one eye through the head, from the side where it would be nearly useless to that in which it may help the other, bears all the marks of purpose, and so carries the implication of design. the case is adduced as part of the evidence that darwinian evolution supersedes design. but how? not certainly in the way this goes on from generation to generation; therefore, doubtless in the way it began. so we look for the explanation of how it came about at the first unintentionally or accidentally; how, under known or supposed conditions, it must have happened, or at least was likely to happen. and we read, "a spontaneous variation occurred, consisting in the passage of one eye to the opposite side of the head." that is all; and we suppose there is nothing more to be said. in short, this surprising thing was undesigned because it took place, and has taken place ever since! the writer presumes, moreover (but this is an obiter dictum), that the peculiarity originated long after flounders had fixed the habit of swimming on one side (and in this particular case it is rather difficult to see how the two may have gone on pari passu), and so he cuts away all obvious occasion for the alteration through the summation of slight variations in one direction, each bringing some advantage. this is a strongly-marked case; but its features, although unusually prominent, are like those of the general run of the considerations by which evolution is supposed to exclude design. those of the penultimate citation and its context are all of the same stamp. the differences which begin as variations are said to be spontaneous--a metaphorical word of wide meanings--are inferred to be casual (whereas we only know them to be occult), or to be originated by surrounding agencies (which is not in a just sense true); they are legitimately inferred to be led on by natural selection, wholly new structures or organs appear, no one can say how, certainly no one can show that they are necessary outcomes of what preceded; and these two are through natural selection kept in harmony with the surroundings, adapted to different ones, diversified, and perfected; purposes are all along subserved through exquisite adaptations; and yet the whole is thought to be undesigned, not because of any assigned reason why this or that must have been thus or so, but simply because they all occurred in nature! the darwinian theory implies that the birth and development of a species are as natural as those of an individual, are facts of the same kind in a higher order. the alleged proof of the absence of design from it amounts to a simple reiteration of the statement, with particulars. now, the marks of contrivance in the structure of animals used not to be questioned because of their coming in the way of birth and development. it is curious that a further extension of this birth and development should be held to disprove them. it appears to us that all this is begging the question against design in nature, instead of proving that it may be dispensed with. two things have helped on this confusion. one is the notion of the direct and independent creation of species, with only an ideal connection between them, to question which was thought to question the principle of design. the other is a wrong idea of the nature and province of natural selection. in former papers we have over and over explained the darwinian doctrine in this respect. it may be briefly illustrated thus: natural selection is not the wind which propels the vessel, but the rudder which, by friction, now on this side and now on that, shapes the course. the rudder acts while the vessel is in motion, effects nothing when it is at rest. variation answers to the wind: "thou hearest the sound thereof, but canst not tell when it cometh and whither it goeth." its course is controlled by natural selection, the action of which, at any given moment, is seemingly small or insensible; but the ultimate results are great. this proceeds mainly through outward influences. but we are more and more convinced that variation, and therefore the ground of adaptation, is not a product of, but a response to, the action of the environment. variations, in other words, the differences between individual plants and animals, however originated, are evidently not from without but from within--not physical but physiological. we cannot here assign particularly the reasons for this opinion. but we notice that the way in which varieties make their appearance strongly suggests it. the variations of plants which spring up in a seed-bed, for instance, seem to be in no assignable relation to the external conditions. they arise, as we say, spontaneously, and either with decided characters from the first, or with obvious tendencies in one or few directions. the occult power, whatever it be, does not seem in any given case to act vaguely, producing all sorts of variations from a common centre, to be reduced by the struggle for life to fewness and the appearance of order; there are, rather, orderly indications from the first. the variations of which we speak, as originating in no obvious casual relation to the external conditions, do not include dwarfed or starved, and gigantesque or luxuriant forms, and those drawn up or expanded on the one hand, or contracted and hardened on the other, by the direct difference in the supply of food and moisture, light and heat. here the action of the environment is both obvious and direct. but such cases do not account for much in evolution. moreover, while we see how the mere struggle and interplay among occurring forms may improve them and lead them on, we cannot well imagine how the adaptations which arrest our attention are thereby secured. our difficulty, let it be understood, is not about the natural origination of organs. to the triumphant outcry, "how can an organ, such as an eye, be formed under nature?" we would respond with a parallel question, how can a complex and elaborate organ, such as a nettle-sting, be formed under nature? but it is so formed. in the same species some individuals have these exquisitely-constructed organs and some have not. and so of other glands, the structure and adaptation of which, when looked into, appear to be as wonderful as anything in nature. the impossibility lies in conceiving how the obvious purpose was effectuated under natural selection alone. this, under our view, any amount of gradation in a series of forms goes a small way in explaining. the transit of a young flounder's eye across the head is a capital instance of a wonderful thing done under nature, and done unaccountably. but simpler correlations are involved in similar difficulty. the superabundance of the pollen of pine-trees above referred to, and in oak-trees, is correlated with chance fertilization under the winds. in the analogous instance of willows a diminished amount of pollen is correlated with direct transportation by insects. even in so simple a case as this it is not easy to see how this difference in the conveyance would reduce the quantity of pollen produced. it is, we know, in the very alphabet of darwinism that if a male willow-tree should produce a smaller amount of pollen, and if this pollen communicated to the offspring of the female flowers it fertilized a similar tendency (as it might), this male progeny would secure whatever advantage might come from the saving of a certain amount of work and material; but why should it begin to produce less pollen? but this is as nothing compared with the arrangements in orchid-flowers, where new and peculiar structures are introduced--structures which, once originated and then set into variation, may thereupon be selected, and thereby led on to improvement and diversification. but the origination, and even the variation, still remains unexplained either by the action of insects or by any of the processes which collectively are personified by the term natural selection. we really believe that these exquisite adaptations have come to pass in the course of nature, and under natural selection, but not that natural selection alone explains or in a just sense originates them. or rather, if this term is to stand for sufficient cause and rational explanation, it must denote or include that inscrutable something which produces--as well as that which results in the survival of--"the fittest." we have been considering this class of questions only as a naturalist might who sought for the proper or reasonable interpretation of the problem before him, unmingled with considerations from any other source. weightier arguments in the last resort, drawn from the intellectual and moral constitution of man, lie on a higher plane, to which it was unnecessary for our particular purpose to rise, however indispensable this be to a full presentation of the evidence of mind in nature. to us the evidence, judged as impartially as we are capable of judging, appears convincing. but, whatever view one unconvinced may take, it cannot remain doubtful what position a theist ought to occupy. if he cannot recognize design in nature because of evolution, he may be ranked with those of whom it was said, "except ye see signs and wonders ye will not believe." how strange that a convinced theist should be so prone to associate design only with miracle! all turns, however, upon what is meant by this nature, to which it appears more and more probable that the being and becoming--no less than the well-being and succession--of species and genera, as well as of individuals, are committed. to us it means "the world of force and movement in time and space," as aristotle defined it--the system and totality of things in the visible universe. what is generally called nature prof. tyndall names matter--a peculiar nomenclature, requiring new definitions (as he avers), inviting misunderstanding, and leaving the questions we are concerned with just where they were. for it is still to ask: whence this rich endowment of matter? whence comes that of which all we see and know is the outcome? that to which potency may in the last resort be ascribed, prof. tyndall, suspending further judgment, calls mystery--using the word in one of its senses, namely, something hidden from us which we are not to seek to know. but there are also mysteries proper to be inquired into and to be reasoned about; and, although it may not be given unto us to know the mystery of causation, there can hardly be a more legitimate subject of philosophical inquiry. most scientific men have thought themselves intellectually authorized to have an opinion about it. "for, by the primitive and very ancient men, it has been handed down in the form of myths, and thus left to later generations, that the divine it is which holds together all nature;" and this tradition, of which aristotle, both naturalist and philosopher, thus nobly speaks[xiii- ]--continued through succeeding ages, and illuminated by the light which has come into the world--may still express the worthiest thoughts of the modern scientific investigator and reasoner. footnotes: i- . "on the origin of species by means of natural selection, or the preservation of favored races in the struggle for life," by charles darwin, m.a., fellow of the royal, geological, linnaean, etc., societies, author of "journal of researches during h. m. s. beagle's voyage round the world." london: john murray. . pp., post vo. i- . article in this journal, vol. xxiv., p. . i- . "species tot sunt, quot diversas formas ab initio produxit infinitum ens; quae formae secundum generationis inditas leges, produxere plures, at sibi semper similes."--linn. phil. bot., , . i- . agassiz, "essay on classification; contributions to natural history," p. , et seq. i- . as to this, darwin remarks that he can only hope to see the law hereafter proved true (p. ); and p. : "agassiz insists that ancient animals resemble to a certain extent the embryos of recent animals of the same classes; or that the geological succession of extinct forms is in some degree parallel to the embryological development of recent forms. i must follow pictet and huxley in thinking that the truth of this doctrine is very far from proved. yet i fully expect to see it hereafter confirmed, at least in regard to subordinate groups, which have branched off from each other within comparatively recent times. for this doctrine of agassiz accords well with the theory of natural selection." i- . op. cit., p. .--one or two bridgewater treatises, and most modern works upon natural theology, should have rendered the evidences of thought in inorganic nature not "unexpected." i- . volume xvii. ( ), , p. . i- . we suspect that this is not an ultimate fact, but a natural consequence of inheritance--the inheritance of disease or of tendency to disease, which close interbreeding perpetuates and accumulates, but wide breeding may neutralize or eliminate. i- . the rules and processes of breeders of animals, and their results, are so familiar that they need not be particularized. less is popularly known about the production of vegetable races. we refer our readers back to this journal, vol. xxvii., pp. -- (may, ), for an abstract of the papers of m. vilmorin upon this subject. i- . quadrupeds of america," vol. ii., p. . i- . "proceedings of the american academy of arts and sciences," vol. iv., p. . i- . owen adds a third, viz., vegetative repetition; but this, in the vegetable kingdom, is simply unity of type. i- . "contributions to natural history of america," vol. i., pp. -- . i- . op. cit., p. . ii- . to parry an adversary's thrust at a vulnerable part, or to show that it need not be fatal, is an incomplete defense. if the discussion had gone on, it might, perhaps, have been made to appear that the darwinian hypothesis, so far from involving the idea of necessity (except in the sense that everything is of necessity), was based upon the opposite idea, that of contingency. iii- . vide "proceedings of the british association for the advancement of science," , and london athenoeum, passim. it appears to be conceded that these "celts" or stone knives are artificial productions, and apparently of the age of the mammoth, the fossil rhinoceros, etc. iii- . see "correspondence of m. nickles," in american journal of science and arts, for march, . iii- . see morlot, "some general views on archaeology," in american journal of science and arts, for january, o, translated from "bulletin de la societe vaudoise," . iii- . page , english edition. in the new american edition (vide supplement, pp. , ) the principal analogies which suggest the extreme view are referred to, and the remark is appended: "but this inference is chiefly grounded on analogy, and it is immaterial whether or not it be accepted. the case is different with the members of each great class, as the vertebrata or articulata; for here we have in the laws of homology, embryology, etc., some distinct evidence that all have descended from a single primordial parent." iii- . in bibliotheque universelle de geneve, march, . iii- . this we learn from his very interesting article, "de la question de l'homme fossile," in the same (march) number of the biblioteque universelle. (see, also, the same author's "note sur la periode quaternaire ou diluvienne, consideree dans ses rapports avec l'epoque actuelle," in the number for august, , of the same periodical.) iii- . in comptes rendus, academie des sciences, february , . iii- . whatever it may be, it is not "the homoeopathic form of the transmutative hypothesis," as darwin's is said to be (p. , american reprint), so happily that the prescription is repeated in the second (p. ) and third (p. ) dilutions, no doubt, on hahnemann's famous principle, of an increase of potency at each dilution. probably the supposed transmutation is per saltus. "homoeopathic doses of transmutation," indeed! well, if we really must swallow transmutation in some form or other, as this reviewer intimates, we might prefer the mild homoeopathic doses of darwin's formula to the allopathic bolus which the edinburgh general practitioner appears to be compounding. iii- . vide north american review, for april, , p. , and christian examiner, for may, p. . iii- . page , english edition. iii- . in american journal of science, july, , pp. -- . iii- . in "contributions to the natural history of the united states," vol. i., p. , . iii- . contributions to the natural history of the united states," vol. , p. ; and american journal of science, july, , p. . iii- . north american review for april , p. . iii- . vide motto from butler, prefixed to the second edition of darwin's work. iii- . north american review, loc. cit., p. . iii- . north american review, loc. cit., p. , et passim. iii- . in american journal of science, july, , p. . iii- . vide article by mr. c. wright, in the mathematical monthly for may last. iii- . vide edinburgh review for january, , article on "acclimatization," etc. iii- . american journal of science, july, , p. . iv- . a name which, at the close of his article, de candolle proposes for the study of the succession of organized beings, to comprehend, therefore, palaeontology and all included under what is called geographical botany and zoology--the whole forming a science parallel to geology--the latter devoted to the history of unorganized bodies, the former, to that of organized beings, as respects origin, distribution, and succession. we are not satisfied with the word, notwithstanding the precedent of palaeontology; since ontology, the science of being, has an established meaning as referring to mental existence--i.e., is a synonym for a department of metaphysics. iv- . natural history review, january, iv- . what the rev. principal tulloch remarks in respect to the philosophy of miracles has a pertinent application here. we quote at second hand: "the stoutest advocates of interference can mean nothing more than that the supreme will has so moved the hidden springs of nature that a new issue arises on given circumstances. the ordinary issue is supplanted by a higher issue. the essential facts before us are a certain set of phenomena, and a higher will moving them. how moving them? is a question for human definition; the answer to which does not and cannot affect the divine meaning of the change. yet when we reflect that this higher will is every. where reason and wisdom, it seems a juster as well as a more comprehensive view to regard it as operating by subordination and evolution, rather than by interference or violation." iv- . particularly citing flourens: "la ressemblance n'est qu'une condition secondaire; la condition essentielle est la descendance: ce n'est pas la ressemblance, c'est la succession des individus, qui fait l'espece." v- . the phrase "atlantic united states" is here used throughout in contradistinction to pacific united states: to the former of course belong, botanically and geographically, the valley of the mississippi and its tributaries up to the eastern border of the great woodless plains, which constitute an intermediate region. v- . the tabulated list referred to was printed as an appendix to the official edition of this discourse, but is here omitted. v- . american journal of science, , p. ; "proceedings of american academy," vol. viii., p. . v- . "memoirs of american academy," vol. vi., pp. -- ( ) v- . die vegetation der erde nach ihrer kilmatischen anordnung," . v- . reference should also be made to the extensive researches of newberry upon the tertiary and cretaceous floras of the western united states. see especially prof. newberry's paper in the boston journal of natural history, vol. vii., no. , describing fossil plants of vancouver's island, etc.; his "notes on the later extinct floras of north america," etc., in "annals of the lyceum of natural history," vol. ix., april, ; "report on the cretaceous and tertiary plants collected in raynolds and hayden's yellowstone and missouri exploring expedition, -- ," published in ; and an interesting article entitled "the ancient lakes of western america, their deposits and drainage," published in the american naturalist, january, . the only document i was able to consult was lesquereux's "report on the fossil plants," in hayden's report of . v- . there is, at least, one instance so opportune to the present argument that it should not pass unnoticed, although i had overlooked the record until now. onoclea sensibilis is a fern peculiar to the atlantic united states (where it is common and wide-spread) and to japan. prof. newberry identified it several years ago in a collection, obtained by dr. hayden, of miocene fossil plants of dakota territory, which is far beyond its present habitat. he moreover regards it as probably identical with a fossil specimen "described by the late prof. e. forbes, under the name of filicites hebridicus, and obtained by the duke of argyll from the island of mull." v- . "darwinism in morals," in theological review, april, . vi- . "histoire des sciences et des sevants depuis deux siecles, suivie d'autres etudes sur des sujets scientifiques, en particulier sur la selection dans 'espèce humaine, par alphonse de candolle." geneve: h. georg. . "addresses of george bentham, president, read at the anniversary meetings of the linnaean society, -- ." "notes on the classification, history, and geographical distribution of compositae. by george bentham." separate issue from the journal of the linnean society. vol. xiii. london. . "on palaeontological evidence of gradual modification of animal forms, read at the royal institution of great britain, april , . by prof. w.h. flower." (journal of the royal institution, pp. .) "the distribution and migration of birds. memoir presented to the national academy of sciences, january, , abstracted in the american journal of science and the arts. , etc. by spencer f. baird." "the story of the earth and man. by j.w. dawson, ll.d., f.r.s., f.g.s., principal and vice-chancellor of mcgill university, montreal. london: hodder & stoughton; new york: harper & brothers. . pp. , mo. vi- . since this article was in type, noteworthy examples of appreciative scientific judgment of the derivative hypothesis have come to hand: . in the opening address to the geological section of the british association, at its recent meeting, by its president, the veteran phillips, perhaps the oldest surviving geologist after lyell; and, . that of prof. allman, president of the biological section. the first touches the subject briefly, but in the way of favorable suggestion; the second is a full and discriminating exposition of the reasons which seem to assure at least the provisional acceptance of the hypothesis, as a guide in all biological studies, "a key to the order and hidden forces of the world of life." vii- . "the theory of evolution of living things, and the application of the principles of evolution to religion, considered as illustrative of the 'wisdom and beneficence of the almighty.' by the rev. george henslow, m.a., f.l.s., f.g.s., etc." new york: macmillan & co. . mo, pp. . "systematic theology. by charles hodge, d.d., professor in the theological seminary, princeton, new jersey. vol. ii. (part ii, anthropology.") new york: charles scribner & co. . "religion and science: a series of sunday lectures on the relation of natural and revealed religion, or the truths revealed in nature and scripture. by joseph le conte, professor of geology and natural history in the university of california." new york: d. appleton & co. . mo, pp. . vii- . "but with regard to the material world, we can at least go so far as this-- we can perceive that events are brought about, not by insulated interpositions of divine power, exerted in each particular case, but by the establishment of general laws.--whewell's bridgewater treatise. "the only distinct meaning of the world 'natural' is stated, fixed, or settled; since what is natural as much requires and presupposes an intelligent agent to render it so--i.e., to effect it continually or at stated times--as what is supernatural or miraculous does to effect it for once."--butler's analogy. viii- . "what is darwinism? by charles hodge, princeton, n.j." new york: scribner, armstrong & co. . "the doctrine of evolution. by alexander winchell, ll.d., etc. new york: harper & brothers. . "darwinism and design; or, creation by evolution. by george st. clair." london: hodder & stoughton. . "westminster sermons. by the rev. charles kingsley, f.l.s., f.g.s., canon of westminster, etc." london and new york: macmillan & co. . viii- . these two postulate-mottoes are quoted in full in a previous article, in no. of the nation. xi- . "insectivorous plants. by charles darwin, m.a., f.r.s." with illustrations. london: john murray. . pp. . new york: d. appleton & co. "the movements and habits of climbing plants. by charles darwin, m.a., f.r.s., etc." second edition, revised, with illustrations. london: john murray. . pp. . new york: d. appleton & co. xi- . the nation, nos. , , . it was in these somewhat light and desultory, but substantially serious, articles that some account of mr. darwin's observations upon the digestive powers of drosera and dionaea first appeared; in fact, their leading motive was to make sufficient reference to his then unpublished discoveries to guard against expected or possible claims to priority. dr. burdon-sanderson's lecture, and the report in nature, which first made them known in england, appeared later. a mistake on our part in the reading of a somewhat ambiguous sentence in a letter led to the remark, at the close of the first of those articles, that the leaf-trap of dionaea had been paralyzed on one side in consequence of a dexterous puncture. what was communicated really related to drosera. xi- . a. gray, in "proceedings of the american academy of arts and sciences," vol. iv., p. ; and american journal of science and the arts, march, , p. . xii- . "les especes affines et la theorie de l'evolution," par charles naudin, membre de l'institut, in bulletin de la societe botanique de france, tome xxi., pp. - , . see also comptes rendus, september and october , , reproduced in "annales des sciences naturelles," , pp. - . xii- . in noticing m. naudin's paper in the comptes rendus, now reprinted in the "annales des sciences naturelles," entitled "variation desordonnee des plantes hybrides et deductions qu'on peut en tirer," we were at a loss to conceive why he attributed all present variation of species to atavism, i.e., to the reappearance of ancestral characters (american journal of science, february, ). his anterior paper was not then known to us; from which it now appears that this view comes in as a part of the hypothesis of extreme plasticity and variability at the first, subsiding at length into entire fixity and persistence of character. according to which, it is assumed that the species of our time have lost all power of original variation, but can still reproduce some old ones--some reminiscences, as it were, of youthful vagaries--in the way of atavism. xiii- . london, . xiii- . hume, in his "essays," anticipated this argument. but he did not rest on it. his matured convictions appear to be expressed in statements such as the following, here cited at second hand from jackson's "philosophy of natural theology," a volume to which a friend has just called our attention: "though the stupidity of men," writes hume, "barbarous and uninstructed, be so great that they may not see a sovereign author in the more obvious works of nature, to which they are so much familiarized, yet it scarce seems possible that any one of good understanding should reject that idea, when once it is suggested to him. a purpose, an intention, a design, is evident in everything; and when our comprehension is so far enlarged as to contemplate the first rise of this visible system, we must adopt, with the strongest conviction, the idea of some intelligent cause or author. the uniform maxims, too, which prevail throughout the whole frame of the universe, naturally, if not necessarily, lead us to conceive this intelligence as single and undivided, where the prejudices of education oppose not so reasonable a theory. even the contrarieties of nature, by discovering themselves everywhere, become proofs of some consistent plan, and establish one single purpose or intention, however inexplicable and incomprehensible."---("natural history of religion," xv.) "in many views of the universe, and of its parts, particularly the latter, the beauty and fitness of final causes strike us with such irresistible force that all objections appear (what i believe they really are) mere cavils and sophisms."-- ("dialogues concerning natural religion," part x.) "the order and arrangement of nature, the curious adjustment of final causes, the plain use and intention of every part and organ, all these bespeak in the clearest language an intelligent cause or author."--(ibid., part iv.) xiii- . see section i, chapter . xiii- . "no single and limited good can be assigned by us as the final cause of any contrivance in nature. the real final cause . . . is the sum of all the uses to which it is ever to be put. any use to which a contrivance of nature is put, we may be sure, is a part of its final cause."--(g. f. wright, in the new-englander, october, .) xiii- . "no single and limited good can be assigned by us as the final cause of any contrivance in nature. the real final cause . . . is the sum of all the uses to which it is ever to be put. any use to which a contrivance of nature is put, we may be sure, is a part of its final cause."--(g. f. wright, in the new-englander, october, .) the origin of species by means of natural selection; or, the preservation of favoured races in the struggle for life. by charles darwin, m.a., f.r.s., author of "the descent of man," etc., etc. sixth london edition, with all additions and corrections. the th edition is often considered the definitive edition. also see project gutenberg etext # for the first edition. "but with regard to the material world, we can at least go so far as this--we can perceive that events are brought about not by insulated interpositions of divine power, exerted in each particular case, but by the establishment of general laws."--whewell: "bridgewater treatise". "the only distinct meaning of the word 'natural' is stated, fixed or settled; since what is natural as much requires and presupposes an intelligent agent to render it so, i.e., to effect it continually or at stated times, as what is supernatural or miraculous does to effect it for once."--butler: "analogy of revealed religion". "to conclude, therefore, let no man out of a weak conceit of sobriety, or an ill-applied moderation, think or maintain, that a man can search too far or be too well studied in the book of god's word, or in the book of god's works; divinity or philosophy; but rather let men endeavour an endless progress or proficience in both."--bacon: "advancement of learning". an historical sketch of the progress of opinion on the origin of species, previously to the publication of the first edition of this work. i will here give a brief sketch of the progress of opinion on the origin of species. until recently the great majority of naturalists believed that species were immutable productions, and had been separately created. this view has been ably maintained by many authors. some few naturalists, on the other hand, have believed that species undergo modification, and that the existing forms of life are the descendants by true generation of pre existing forms. passing over allusions to the subject in the classical writers (aristotle, in his "physicae auscultationes" (lib. , cap. , s. ), after remarking that rain does not fall in order to make the corn grow, any more than it falls to spoil the farmer's corn when threshed out of doors, applies the same argument to organisation; and adds (as translated by mr. clair grece, who first pointed out the passage to me), "so what hinders the different parts (of the body) from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. and in like manner as to other parts in which there appears to exist an adaptation to an end. wheresoever, therefore, all things together (that is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity; and whatsoever things were not thus constituted, perished and still perish." we here see the principle of natural selection shadowed forth, but how little aristotle fully comprehended the principle, is shown by his remarks on the formation of the teeth.), the first author who in modern times has treated it in a scientific spirit was buffon. but as his opinions fluctuated greatly at different periods, and as he does not enter on the causes or means of the transformation of species, i need not here enter on details. lamarck was the first man whose conclusions on the subject excited much attention. this justly celebrated naturalist first published his views in ; he much enlarged them in in his "philosophie zoologique", and subsequently, , in the introduction to his "hist. nat. des animaux sans vertebres". in these works he up holds the doctrine that all species, including man, are descended from other species. he first did the eminent service of arousing attention to the probability of all change in the organic, as well as in the inorganic world, being the result of law, and not of miraculous interposition. lamarck seems to have been chiefly led to his conclusion on the gradual change of species, by the difficulty of distinguishing species and varieties, by the almost perfect gradation of forms in certain groups, and by the analogy of domestic productions. with respect to the means of modification, he attributed something to the direct action of the physical conditions of life, something to the crossing of already existing forms, and much to use and disuse, that is, to the effects of habit. to this latter agency he seems to attribute all the beautiful adaptations in nature; such as the long neck of the giraffe for browsing on the branches of trees. but he likewise believed in a law of progressive development, and as all the forms of life thus tend to progress, in order to account for the existence at the present day of simple productions, he maintains that such forms are now spontaneously generated. (i have taken the date of the first publication of lamarck from isidore geoffroy saint-hilaire's ("hist. nat. generale", tom. ii. page , ) excellent history of opinion on this subject. in this work a full account is given of buffon's conclusions on the same subject. it is curious how largely my grandfather, dr. erasmus darwin, anticipated the views and erroneous grounds of opinion of lamarck in his "zoonomia" (vol. i. pages - ), published in . according to isid. geoffroy there is no doubt that goethe was an extreme partisan of similar views, as shown in the introduction to a work written in and , but not published till long afterward; he has pointedly remarked ("goethe als naturforscher", von dr. karl meding, s. ) that the future question for naturalists will be how, for instance, cattle got their horns and not for what they are used. it is rather a singular instance of the manner in which similar views arise at about the same time, that goethe in germany, dr. darwin in england, and geoffroy saint-hilaire (as we shall immediately see) in france, came to the same conclusion on the origin of species, in the years - .) geoffroy saint-hilaire, as is stated in his "life", written by his son, suspected, as early as , that what we call species are various degenerations of the same type. it was not until that he published his conviction that the same forms have not been perpetuated since the origin of all things. geoffroy seems to have relied chiefly on the conditions of life, or the "monde ambiant" as the cause of change. he was cautious in drawing conclusions, and did not believe that existing species are now undergoing modification; and, as his son adds, "c'est donc un probleme a reserver entierement a l'avenir, suppose meme que l'avenir doive avoir prise sur lui." in dr. w.c. wells read before the royal society "an account of a white female, part of whose skin resembles that of a negro"; but his paper was not published until his famous "two essays upon dew and single vision" appeared in . in this paper he distinctly recognises the principle of natural selection, and this is the first recognition which has been indicated; but he applies it only to the races of man, and to certain characters alone. after remarking that negroes and mulattoes enjoy an immunity from certain tropical diseases, he observes, firstly, that all animals tend to vary in some degree, and, secondly, that agriculturists improve their domesticated animals by selection; and then, he adds, but what is done in this latter case "by art, seems to be done with equal efficacy, though more slowly, by nature, in the formation of varieties of mankind, fitted for the country which they inhabit. of the accidental varieties of man, which would occur among the first few and scattered inhabitants of the middle regions of africa, some one would be better fitted than others to bear the diseases of the country. this race would consequently multiply, while the others would decrease; not only from their in ability to sustain the attacks of disease, but from their incapacity of contending with their more vigorous neighbours. the colour of this vigorous race i take for granted, from what has been already said, would be dark. but the same disposition to form varieties still existing, a darker and a darker race would in the course of time occur: and as the darkest would be the best fitted for the climate, this would at length become the most prevalent, if not the only race, in the particular country in which it had originated." he then extends these same views to the white inhabitants of colder climates. i am indebted to mr. rowley, of the united states, for having called my attention, through mr. brace, to the above passage of dr. wells' work. the hon. and rev. w. herbert, afterward dean of manchester, in the fourth volume of the "horticultural transactions", , and in his work on the "amaryllidaceae" ( , pages , ), declares that "horticultural experiments have established, beyond the possibility of refutation, that botanical species are only a higher and more permanent class of varieties." he extends the same view to animals. the dean believes that single species of each genus were created in an originally highly plastic condition, and that these have produced, chiefly by inter-crossing, but likewise by variation, all our existing species. in professor grant, in the concluding paragraph in his well-known paper ("edinburgh philosophical journal", vol. xiv, page ) on the spongilla, clearly declares his belief that species are descended from other species, and that they become improved in the course of modification. this same view was given in his fifty-fifth lecture, published in the "lancet" in . in mr. patrick matthew published his work on "naval timber and arboriculture", in which he gives precisely the same view on the origin of species as that (presently to be alluded to) propounded by mr. wallace and myself in the "linnean journal", and as that enlarged in the present volume. unfortunately the view was given by mr. matthew very briefly in scattered passages in an appendix to a work on a different subject, so that it remained unnoticed until mr. matthew himself drew attention to it in the "gardeners' chronicle", on april , . the differences of mr. matthew's views from mine are not of much importance: he seems to consider that the world was nearly depopulated at successive periods, and then restocked; and he gives as an alternative, that new forms may be generated "without the presence of any mold or germ of former aggregates." i am not sure that i understand some passages; but it seems that he attributes much influence to the direct action of the conditions of life. he clearly saw, however, the full force of the principle of natural selection. the celebrated geologist and naturalist, von buch, in his excellent "description physique des isles canaries" ( , page ), clearly expresses his belief that varieties slowly become changed into permanent species, which are no longer capable of intercrossing. rafinesque, in his "new flora of north america", published in , wrote (page ) as follows: "all species might have been varieties once, and many varieties are gradually becoming species by assuming constant and peculiar characters;" but further on (page ) he adds, "except the original types or ancestors of the genus." in - professor haldeman ("boston journal of nat. hist. u. states", vol. iv, page ) has ably given the arguments for and against the hypothesis of the development and modification of species: he seems to lean toward the side of change. the "vestiges of creation" appeared in . in the tenth and much improved edition ( ) the anonymous author says (page ): "the proposition determined on after much consideration is, that the several series of animated beings, from the simplest and oldest up to the highest and most recent, are, under the providence of god, the results, first, of an impulse which has been imparted to the forms of life, advancing them, in definite times, by generation, through grades of organisation terminating in the highest dicotyledons and vertebrata, these grades being few in number, and generally marked by intervals of organic character, which we find to be a practical difficulty in ascertaining affinities; second, of another impulse connected with the vital forces, tending, in the course of generations, to modify organic structures in accordance with external circumstances, as food, the nature of the habitat, and the meteoric agencies, these being the 'adaptations' of the natural theologian." the author apparently believes that organisation progresses by sudden leaps, but that the effects produced by the conditions of life are gradual. he argues with much force on general grounds that species are not immutable productions. but i cannot see how the two supposed "impulses" account in a scientific sense for the numerous and beautiful coadaptations which we see throughout nature; i cannot see that we thus gain any insight how, for instance, a woodpecker has become adapted to its peculiar habits of life. the work, from its powerful and brilliant style, though displaying in the early editions little accurate knowledge and a great want of scientific caution, immediately had a very wide circulation. in my opinion it has done excellent service in this country in calling attention to the subject, in removing prejudice, and in thus preparing the ground for the reception of analogous views. in the veteran geologist m.j. d'omalius d'halloy published in an excellent though short paper ("bulletins de l'acad. roy. bruxelles", tom. xiii, page ) his opinion that it is more probable that new species have been produced by descent with modification than that they have been separately created: the author first promulgated this opinion in . professor owen, in ("nature of limbs", page ), wrote as follows: "the archetypal idea was manifested in the flesh under diverse such modifications, upon this planet, long prior to the existence of those animal species that actually exemplify it. to what natural laws or secondary causes the orderly succession and progression of such organic phenomena may have been committed, we, as yet, are ignorant." in his address to the british association, in , he speaks (page li) of "the axiom of the continuous operation of creative power, or of the ordained becoming of living things." further on (page xc), after referring to geographical distribution, he adds, "these phenomena shake our confidence in the conclusion that the apteryx of new zealand and the red grouse of england were distinct creations in and for those islands respectively. always, also, it may be well to bear in mind that by the word 'creation' the zoologist means 'a process he knows not what.'" he amplifies this idea by adding that when such cases as that of the red grouse are "enumerated by the zoologist as evidence of distinct creation of the bird in and for such islands, he chiefly expresses that he knows not how the red grouse came to be there, and there exclusively; signifying also, by this mode of expressing such ignorance, his belief that both the bird and the islands owed their origin to a great first creative cause." if we interpret these sentences given in the same address, one by the other, it appears that this eminent philosopher felt in his confidence shaken that the apteryx and the red grouse first appeared in their respective homes "he knew not how," or by some process "he knew not what." this address was delivered after the papers by mr. wallace and myself on the origin of species, presently to be referred to, had been read before the linnean society. when the first edition of this work was published, i was so completely deceived, as were many others, by such expressions as "the continuous operation of creative power," that i included professor owen with other palaeontologists as being firmly convinced of the immutability of species; but it appears ("anat. of vertebrates", vol. iii, page ) that this was on my part a preposterous error. in the last edition of this work i inferred, and the inference still seems to me perfectly just, from a passage beginning with the words "no doubt the type-form," etc.(ibid., vol. i, page xxxv), that professor owen admitted that natural selection may have done something in the formation of a new species; but this it appears (ibid., vol. iii. page ) is inaccurate and without evidence. i also gave some extracts from a correspondence between professor owen and the editor of the "london review", from which it appeared manifest to the editor as well as to myself, that professor owen claimed to have promulgated the theory of natural selection before i had done so; and i expressed my surprise and satisfaction at this announcement; but as far as it is possible to understand certain recently published passages (ibid., vol. iii. page ) i have either partially or wholly again fallen into error. it is consolatory to me that others find professor owen's controversial writings as difficult to understand and to reconcile with each other, as i do. as far as the mere enunciation of the principle of natural selection is concerned, it is quite immaterial whether or not professor owen preceded me, for both of us, as shown in this historical sketch, were long ago preceded by dr. wells and mr. matthews. m. isidore geoffroy saint-hilaire, in his lectures delivered in (of which a resume appeared in the "revue et mag. de zoolog.", jan., ), briefly gives his reason for believing that specific characters "sont fixes, pour chaque espece, tant qu'elle se perpetue au milieu des memes circonstances: ils se modifient, si les circonstances ambiantes viennent a changer. en resume, l'observation des animaux sauvages demontre deja la variabilite limitee des especes. les experiences sur les animaux sauvages devenus domestiques, et sur les animaux domestiques redevenus sauvages, la demontrent plus clairment encore. ces memes experiences prouvent, de plus, que les differences produites peuvent etre de valeur generique." in his "hist. nat. generale" (tom. ii, page , ) he amplifies analogous conclusions. from a circular lately issued it appears that dr. freke, in ("dublin medical press", page ), propounded the doctrine that all organic beings have descended from one primordial form. his grounds of belief and treatment of the subject are wholly different from mine; but as dr. freke has now ( ) published his essay on the "origin of species by means of organic affinity", the difficult attempt to give any idea of his views would be superfluous on my part. mr. herbert spencer, in an essay (originally published in the "leader", march, , and republished in his "essays", in ), has contrasted the theories of the creation and the development of organic beings with remarkable skill and force. he argues from the analogy of domestic productions, from the changes which the embryos of many species undergo, from the difficulty of distinguishing species and varieties, and from the principle of general gradation, that species have been modified; and he attributes the modification to the change of circumstances. the author ( ) has also treated psychology on the principle of the necessary acquirement of each mental power and capacity by gradation. in m. naudin, a distinguished botanist, expressly stated, in an admirable paper on the origin of species ("revue horticole", page ; since partly republished in the "nouvelles archives du museum", tom. i, page ), his belief that species are formed in an analogous manner as varieties are under cultivation; and the latter process he attributes to man's power of selection. but he does not show how selection acts under nature. he believes, like dean herbert, that species, when nascent, were more plastic than at present. he lays weight on what he calls the principle of finality, "puissance mysterieuse, indeterminee; fatalite pour les uns; pour les autres volonte providentielle, dont l'action incessante sur les etres vivantes determine, a toutes les epoques de l'existence du monde, la forme, le volume, et la duree de chacun d'eux, en raison de sa destinee dans l'ordre de choses dont il fait partie. c'est cette puissance qui harmonise chaque membre a l'ensemble, en l'appropriant a la fonction qu'il doit remplir dans l'organisme general de la nature, fonction qui est pour lui sa raison d'etre." (from references in bronn's "untersuchungen uber die entwickelungs-gesetze", it appears that the celebrated botanist and palaeontologist unger published, in , his belief that species undergo development and modification. dalton, likewise, in pander and dalton's work on fossil sloths, expressed, in , a similar belief. similar views have, as is well known, been maintained by oken in his mystical "natur-philosophie". from other references in godron's work "sur l'espece", it seems that bory st. vincent, burdach, poiret and fries, have all admitted that new species are continually being produced. i may add, that of the thirty-four authors named in this historical sketch, who believe in the modification of species, or at least disbelieve in separate acts of creation, twenty-seven have written on special branches of natural history or geology.) in a celebrated geologist, count keyserling ("bulletin de la soc. geolog.", nd ser., tom. x, page ), suggested that as new diseases, supposed to have been caused by some miasma have arisen and spread over the world, so at certain periods the germs of existing species may have been chemically affected by circumambient molecules of a particular nature, and thus have given rise to new forms. in this same year, , dr. schaaffhausen published an excellent pamphlet ("verhand. des naturhist. vereins der preuss. rheinlands", etc.), in which he maintains the development of organic forms on the earth. he infers that many species have kept true for long periods, whereas a few have become modified. the distinction of species he explains by the destruction of intermediate graduated forms. "thus living plants and animals are not separated from the extinct by new creations, but are to be regarded as their descendants through continued reproduction." a well-known french botanist, m. lecoq, writes in ("etudes sur geograph." bot. tom. i, page ), "on voit que nos recherches sur la fixite ou la variation de l'espece, nous conduisent directement aux idees emises par deux hommes justement celebres, geoffroy saint-hilaire et goethe." some other passages scattered through m. lecoq's large work make it a little doubtful how far he extends his views on the modification of species. the "philosophy of creation" has been treated in a masterly manner by the rev. baden powell, in his "essays on the unity of worlds", . nothing can be more striking than the manner in which he shows that the introduction of new species is "a regular, not a casual phenomenon," or, as sir john herschel expresses it, "a natural in contradistinction to a miraculous process." the third volume of the "journal of the linnean society" contains papers, read july , , by mr. wallace and myself, in which, as stated in the introductory remarks to this volume, the theory of natural selection is promulgated by mr. wallace with admirable force and clearness. von baer, toward whom all zoologists feel so profound a respect, expressed about the year (see prof. rudolph wagner, "zoologisch-anthropologische untersuchungen", , s. ) his conviction, chiefly grounded on the laws of geographical distribution, that forms now perfectly distinct have descended from a single parent-form. in june, , professor huxley gave a lecture before the royal institution on the "persistent types of animal life". referring to such cases, he remarks, "it is difficult to comprehend the meaning of such facts as these, if we suppose that each species of animal and plant, or each great type of organisation, was formed and placed upon the surface of the globe at long intervals by a distinct act of creative power; and it is well to recollect that such an assumption is as unsupported by tradition or revelation as it is opposed to the general analogy of nature. if, on the other hand, we view "persistent types" in relation to that hypothesis which supposes the species living at any time to be the result of the gradual modification of pre-existing species, a hypothesis which, though unproven, and sadly damaged by some of its supporters, is yet the only one to which physiology lends any countenance; their existence would seem to show that the amount of modification which living beings have undergone during geological time is but very small in relation to the whole series of changes which they have suffered." in december, , dr. hooker published his "introduction to the australian flora". in the first part of this great work he admits the truth of the descent and modification of species, and supports this doctrine by many original observations. the first edition of this work was published on november , , and the second edition on january , . contents. introduction chapter i. variation under domestication. causes of variability--effects of habit and the use or disuse of parts--correlated variation--inheritance--character of domestic varieties--difficulty of distinguishing between varieties and species--origin of domestic varieties from one or more species--domestic pigeons, their differences and origin--principles of selection, anciently followed, their effects--methodical and unconscious selection--unknown origin of our domestic productions--circumstances favourable to man's power of selection. chapter ii. variation under nature. variability--individual differences--doubtful species--wide ranging, much diffused, and common species, vary most--species of the larger genera in each country vary more frequently than the species of the smaller genera--many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges. chapter iii. struggle for existence. its bearing on natural selection--the term used in a wide sense--geometrical ratio of increase--rapid increase of naturalised animals and plants--nature of the checks to increase--competition universal--effects of climate--protection from the number of individuals--complex relations of all animals and plants throughout nature--struggle for life most severe between individuals and varieties of the same species; often severe between species of the same genus--the relation of organism to organism the most important of all relations. chapter iv. natural selection; or the survival of the fittest. natural selection--its power compared with man's selection--its power on characters of trifling importance--its power at all ages and on both sexes--sexual selection--on the generality of intercrosses between individuals of the same species--circumstances favourable and unfavourable to the results of natural selection, namely, intercrossing, isolation, number of individuals--slow action--extinction caused by natural selection--divergence of character, related to the diversity of inhabitants of any small area and to naturalisation--action of natural selection, through divergence of character and extinction, on the descendants from a common parent--explains the grouping of all organic beings--advance in organisation--low forms preserved--convergence of character--indefinite multiplication of species--summary. chapter v. laws of variation. effects of changed conditions--use and disuse, combined with natural selection; organs of flight and of vision--acclimatisation--correlated variation--compensation and economy of growth--false correlations--multiple, rudimentary, and lowly organised structures variable--parts developed in an unusual manner are highly variable; specific characters more variable than generic; secondary sexual characters variable--species of the same genus vary in an analogous manner--reversions to long-lost characters--summary. chapter vi. difficulties of the theory. difficulties of the theory of descent with modification--absence or rarity of transitional varieties--transitions in habits of life--diversified habits in the same species--species with habits widely different from those of their allies--organs of extreme perfection--modes of transition--cases of difficulty--natura non facit saltum--organs of small importance--organs not in all cases absolutely perfect--the law of unity of type and of the conditions of existence embraced by the theory of natural selection. chapter vii. miscellaneous objections to the theory of natural selection. longevity--modifications not necessarily simultaneous--modifications apparently of no direct service--progressive development--characters of small functional importance, the most constant--supposed incompetence of natural selection to account for the incipient stages of useful structures--causes which interfere with the acquisition through natural selection of useful structures--gradations of structure with changed functions--widely different organs in members of the same class, developed from one and the same source--reasons for disbelieving in great and abrupt modifications. chapter viii. instinct. instincts comparable with habits, but different in their origin--instincts graduated--aphides and ants--instincts variable--domestic instincts, their origin--natural instincts of the cuckoo, molothrus, ostrich, and parasitic bees--slave-making ants--hive-bee, its cell-making instinct--changes of instinct and structure not necessarily simultaneous--difficulties on the theory of the natural selection of instincts--neuter or sterile insects--summary. chapter ix. hybridism. distinction between the sterility of first crosses and of hybrids--sterility various in degree, not universal, affected by close interbreeding, removed by domestication--laws governing the sterility of hybrids--sterility not a special endowment, but incidental on other differences, not accumulated by natural selection--causes of the sterility of first crosses and of hybrids--parallelism between the effects of changed conditions of life and of crossing--dimorphism and trimorphism--fertility of varieties when crossed and of their mongrel offspring not universal--hybrids and mongrels compared independently of their fertility--summary. chapter x. on the imperfection of the geological record. on the absence of intermediate varieties at the present day--on the nature of extinct intermediate varieties; on their number--on the lapse of time, as inferred from the rate of denudation and of deposition--on the lapse of time as estimated in years--on the poorness of our palaeontological collections--on the intermittence of geological formations--on the denudation of granitic areas--on the absence of intermediate varieties in any one formation--on the sudden appearance of groups of species--on their sudden appearance in the lowest known fossiliferous strata--antiquity of the habitable earth. chapter xi. on the geological succession of organic beings. on the slow and successive appearance of new species--on their different rates of change--species once lost do not reappear--groups of species follow the same general rules in their appearance and disappearance as do single species--on extinction--on simultaneous changes in the forms of life throughout the world--on the affinities of extinct species to each other and to living species--on the state of development of ancient forms--on the succession of the same types within the same areas--summary of preceding and present chapter. chapter xii. geographical distribution. present distribution cannot be accounted for by differences in physical conditions--importance of barriers--affinity of the productions of the same continent--centres of creation--means of dispersal by changes of climate and of the level of the land, and by occasional means--dispersal during the glacial period--alternate glacial periods in the north and south. chapter xiii. geographical distribution--continued. distribution of fresh-water productions--on the inhabitants of oceanic islands--absence of batrachians and of terrestrial mammals--on the relation of the inhabitants of islands to those of the nearest mainland--on colonisation from the nearest source with subsequent modification--summary of the last and present chapter. chapter xiv. mutual affinities of organic beings: morphology--embryology--rudimentary organs. classification, groups subordinate to groups--natural system--rules and difficulties in classification, explained on the theory of descent with modification--classification of varieties--descent always used in classification--analogical or adaptive characters--affinities, general, complex and radiating--extinction separates and defines groups--morphology, between members of the same class, between parts of the same individual--embryology, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age--rudimentary organs; their origin explained--summary. chapter xv. recapitulation and conclusion. recapitulation of the objections to the theory of natural selection--recapitulation of the general and special circumstances in its favour--causes of the general belief in the immutability of species--how far the theory of natural selection may be extended--effects of its adoption on the study of natural history--concluding remarks. glossary of scientific terms. index. origin of species. introduction. when on board h.m.s. beagle, as naturalist, i was much struck with certain facts in the distribution of the organic beings inhabiting south america, and in the geological relations of the present to the past inhabitants of that continent. these facts, as will be seen in the latter chapters of this volume, seemed to throw some light on the origin of species--that mystery of mysteries, as it has been called by one of our greatest philosophers. on my return home, it occurred to me, in , that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. after five years' work i allowed myself to speculate on the subject, and drew up some short notes; these i enlarged in into a sketch of the conclusions, which then seemed to me probable: from that period to the present day i have steadily pursued the same object. i hope that i may be excused for entering on these personal details, as i give them to show that i have not been hasty in coming to a decision. my work is now ( ) nearly finished; but as it will take me many more years to complete it, and as my health is far from strong, i have been urged to publish this abstract. i have more especially been induced to do this, as mr. wallace, who is now studying the natural history of the malay archipelago, has arrived at almost exactly the same general conclusions that i have on the origin of species. in he sent me a memoir on this subject, with a request that i would forward it to sir charles lyell, who sent it to the linnean society, and it is published in the third volume of the journal of that society. sir c. lyell and dr. hooker, who both knew of my work--the latter having read my sketch of --honoured me by thinking it advisable to publish, with mr. wallace's excellent memoir, some brief extracts from my manuscripts. this abstract, which i now publish, must necessarily be imperfect. i cannot here give references and authorities for my several statements; and i must trust to the reader reposing some confidence in my accuracy. no doubt errors may have crept in, though i hope i have always been cautious in trusting to good authorities alone. i can here give only the general conclusions at which i have arrived, with a few facts in illustration, but which, i hope, in most cases will suffice. no one can feel more sensible than i do of the necessity of hereafter publishing in detail all the facts, with references, on which my conclusions have been grounded; and i hope in a future work to do this. for i am well aware that scarcely a single point is discussed in this volume on which facts cannot be adduced, often apparently leading to conclusions directly opposite to those at which i have arrived. a fair result can be obtained only by fully stating and balancing the facts and arguments on both sides of each question; and this is here impossible. i much regret that want of space prevents my having the satisfaction of acknowledging the generous assistance which i have received from very many naturalists, some of them personally unknown to me. i cannot, however, let this opportunity pass without expressing my deep obligations to dr. hooker, who, for the last fifteen years, has aided me in every possible way by his large stores of knowledge and his excellent judgment. in considering the origin of species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that species had not been independently created, but had descended, like varieties, from other species. nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species, inhabiting this world have been modified, so as to acquire that perfection of structure and coadaptation which justly excites our admiration. naturalists continually refer to external conditions, such as climate, food, etc., as the only possible cause of variation. in one limited sense, as we shall hereafter see, this may be true; but it is preposterous to attribute to mere external conditions, the structure, for instance, of the woodpecker, with its feet, tail, beak, and tongue, so admirably adapted to catch insects under the bark of trees. in the case of the mistletoe, which draws its nourishment from certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by the effects of external conditions, or of habit, or of the volition of the plant itself. it is, therefore, of the highest importance to gain a clear insight into the means of modification and coadaptation. at the commencement of my observations it seemed to me probable that a careful study of domesticated animals and of cultivated plants would offer the best chance of making out this obscure problem. nor have i been disappointed; in this and in all other perplexing cases i have invariably found that our knowledge, imperfect though it be, of variation under domestication, afforded the best and safest clue. i may venture to express my conviction of the high value of such studies, although they have been very commonly neglected by naturalists. from these considerations, i shall devote the first chapter of this abstract to variation under domestication. we shall thus see that a large amount of hereditary modification is at least possible; and, what is equally or more important, we shall see how great is the power of man in accumulating by his selection successive slight variations. i will then pass on to the variability of species in a state of nature; but i shall, unfortunately, be compelled to treat this subject far too briefly, as it can be treated properly only by giving long catalogues of facts. we shall, however, be enabled to discuss what circumstances are most favourable to variation. in the next chapter the struggle for existence among all organic beings throughout the world, which inevitably follows from the high geometrical ratio of their increase, will be considered. this is the doctrine of malthus, applied to the whole animal and vegetable kingdoms. as many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. from the strong principle of inheritance, any selected variety will tend to propagate its new and modified form. this fundamental subject of natural selection will be treated at some length in the fourth chapter; and we shall then see how natural selection almost inevitably causes much extinction of the less improved forms of life, and leads to what i have called divergence of character. in the next chapter i shall discuss the complex and little known laws of variation. in the five succeeding chapters, the most apparent and gravest difficulties in accepting the theory will be given: namely, first, the difficulties of transitions, or how a simple being or a simple organ can be changed and perfected into a highly developed being or into an elaborately constructed organ; secondly the subject of instinct, or the mental powers of animals; thirdly, hybridism, or the infertility of species and the fertility of varieties when intercrossed; and fourthly, the imperfection of the geological record. in the next chapter i shall consider the geological succession of organic beings throughout time; in the twelfth and thirteenth, their geographical distribution throughout space; in the fourteenth, their classification or mutual affinities, both when mature and in an embryonic condition. in the last chapter i shall give a brief recapitulation of the whole work, and a few concluding remarks. no one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and varieties, if he make due allowance for our profound ignorance in regard to the mutual relations of the many beings which live around us. who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare? yet these relations are of the highest importance, for they determine the present welfare and, as i believe, the future success and modification of every inhabitant of this world. still less do we know of the mutual relations of the innumerable inhabitants of the world during the many past geological epochs in its history. although much remains obscure, and will long remain obscure, i can entertain no doubt, after the most deliberate study and dispassionate judgment of which i am capable, that the view which most naturalists until recently entertained, and which i formerly entertained--namely, that each species has been independently created--is erroneous. i am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. furthermore, i am convinced that natural selection has been the most important, but not the exclusive, means of modification. chapter i. variation under domestication. causes of variability--effects of habit and the use and disuse of parts--correlated variation--inheritance--character of domestic varieties--difficulty of distinguishing between varieties and species--origin of domestic varieties from one or more species--domestic pigeons, their differences and origin--principles of selection, anciently followed, their effects--methodical and unconscious selection--unknown origin of our domestic productions--circumstances favourable to man's power of selection. causes of variability. when we compare the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us is, that they generally differ more from each other than do the individuals of any one species or variety in a state of nature. and if we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treatment, we are driven to conclude that this great variability is due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent species had been exposed under nature. there is, also, some probability in the view propounded by andrew knight, that this variability may be partly connected with excess of food. it seems clear that organic beings must be exposed during several generations to new conditions to cause any great amount of variation; and that, when the organisation has once begun to vary, it generally continues varying for many generations. no case is on record of a variable organism ceasing to vary under cultivation. our oldest cultivated plants, such as wheat, still yield new varieties: our oldest domesticated animals are still capable of rapid improvement or modification. as far as i am able to judge, after long attending to the subject, the conditions of life appear to act in two ways--directly on the whole organisation or on certain parts alone and in directly by affecting the reproductive system. with respect to the direct action, we must bear in mind that in every case, as professor weismann has lately insisted, and as i have incidently shown in my work on "variation under domestication," there are two factors: namely, the nature of the organism and the nature of the conditions. the former seems to be much the more important; for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform. the effects on the offspring are either definite or in definite. they may be considered as definite when all or nearly all the offspring of individuals exposed to certain conditions during several generations are modified in the same manner. it is extremely difficult to come to any conclusion in regard to the extent of the changes which have been thus definitely induced. there can, however, be little doubt about many slight changes, such as size from the amount of food, colour from the nature of the food, thickness of the skin and hair from climate, etc. each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same cause were to act uniformly during a long series of generations on many individuals, all probably would be modified in the same manner. such facts as the complex and extraordinary out growths which variably follow from the insertion of a minute drop of poison by a gall-producing insect, shows us what singular modifications might result in the case of plants from a chemical change in the nature of the sap. in definite variability is a much more common result of changed conditions than definite variability, and has probably played a more important part in the formation of our domestic races. we see in definite variability in the endless slight peculiarities which distinguish the individuals of the same species, and which cannot be accounted for by inheritance from either parent or from some more remote ancestor. even strongly-marked differences occasionally appear in the young of the same litter, and in seedlings from the same seed-capsule. at long intervals of time, out of millions of individuals reared in the same country and fed on nearly the same food, deviations of structure so strongly pronounced as to deserve to be called monstrosities arise; but monstrosities cannot be separated by any distinct line from slighter variations. all such changes of structure, whether extremely slight or strongly marked, which appear among many individuals living together, may be considered as the in definite effects of the conditions of life on each individual organism, in nearly the same manner as the chill effects different men in an in definite manner, according to their state of body or constitution, causing coughs or colds, rheumatism, or inflammation of various organs. with respect to what i have called the in direct action of changed conditions, namely, through the reproductive system of being affected, we may infer that variability is thus induced, partly from the fact of this system being extremely sensitive to any change in the conditions, and partly from the similarity, as kolreuter and others have remarked, between the variability which follows from the crossing of distinct species, and that which may be observed with plants and animals when reared under new or unnatural conditions. many facts clearly show how eminently susceptible the reproductive system is to very slight changes in the surrounding conditions. nothing is more easy than to tame an animal, and few things more difficult than to get it to breed freely under confinement, even when the male and female unite. how many animals there are which will not breed, though kept in an almost free state in their native country! this is generally, but erroneously attributed to vitiated instincts. many cultivated plants display the utmost vigour, and yet rarely or never seed! in some few cases it has been discovered that a very trifling change, such as a little more or less water at some particular period of growth, will determine whether or not a plant will produce seeds. i cannot here give the details which i have collected and elsewhere published on this curious subject; but to show how singular the laws are which determine the reproduction of animals under confinement, i may mention that carnivorous animals, even from the tropics, breed in this country pretty freely under confinement, with the exception of the plantigrades or bear family, which seldom produce young; whereas, carnivorous birds, with the rarest exception, hardly ever lay fertile eggs. many exotic plants have pollen utterly worthless, in the same condition as in the most sterile hybrids. when, on the one hand, we see domesticated animals and plants, though often weak and sickly, breeding freely under confinement; and when, on the other hand, we see individuals, though taken young from a state of nature perfectly tamed, long-lived, and healthy (of which i could give numerous instances), yet having their reproductive system so seriously affected by unperceived causes as to fail to act, we need not be surprised at this system, when it does act under confinement, acting irregularly, and producing offspring somewhat unlike their parents. i may add that as some organisms breed freely under the most unnatural conditions--for instance, rabbits and ferrets kept in hutches--showing that their reproductive organs are not easily affected; so will some animals and plants withstand domestication or cultivation, and vary very slightly--perhaps hardly more than in a state of nature. some naturalists have maintained that all variations are connected with the act of sexual reproduction; but this is certainly an error; for i have given in another work a long list of "sporting plants;" as they are called by gardeners; that is, of plants which have suddenly produced a single bud with a new and sometimes widely different character from that of the other buds on the same plant. these bud variations, as they may be named, can be propagated by grafts, offsets, etc., and sometimes by seed. they occur rarely under nature, but are far from rare under culture. as a single bud out of many thousands produced year after year on the same tree under uniform conditions, has been known suddenly to assume a new character; and as buds on distinct trees, growing under different conditions, have sometimes yielded nearly the same variety--for instance, buds on peach-trees producing nectarines, and buds on common roses producing moss-roses--we clearly see that the nature of the conditions is of subordinate importance in comparison with the nature of the organism in determining each particular form of variation; perhaps of not more importance than the nature of the spark, by which a mass of combustible matter is ignited, has in determining the nature of the flames. effects of habit and of the use or disuse of parts; correlated variation; inheritance. changed habits produce an inherited effect as in the period of the flowering of plants when transported from one climate to another. with animals the increased use or disuse of parts has had a more marked influence; thus i find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild duck; and this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parents. the great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with these organs in other countries, is probably another instance of the effects of use. not one of our domestic animals can be named which has not in some country drooping ears; and the view which has been suggested that the drooping is due to disuse of the muscles of the ear, from the animals being seldom much alarmed, seems probable. many laws regulate variation, some few of which can be dimly seen, and will hereafter be briefly discussed. i will here only allude to what may be called correlated variation. important changes in the embryo or larva will probably entail changes in the mature animal. in monstrosities, the correlations between quite distinct parts are very curious; and many instances are given in isidore geoffroy st. hilaire's great work on this subject. breeders believe that long limbs are almost always accompanied by an elongated head. some instances of correlation are quite whimsical; thus cats which are entirely white and have blue eyes are generally deaf; but it has been lately stated by mr. tait that this is confined to the males. colour and constitutional peculiarities go together, of which many remarkable cases could be given among animals and plants. from facts collected by heusinger, it appears that white sheep and pigs are injured by certain plants, while dark-coloured individuals escape: professor wyman has recently communicated to me a good illustration of this fact; on asking some farmers in virginia how it was that all their pigs were black, they informed him that the pigs ate the paint-root (lachnanthes), which coloured their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the "crackers" (i.e. virginia squatters) added, "we select the black members of a litter for raising, as they alone have a good chance of living." hairless dogs have imperfect teeth; long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet. hence if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation. the results of the various, unknown, or but dimly understood laws of variation are infinitely complex and diversified. it is well worth while carefully to study the several treatises on some of our old cultivated plants, as on the hyacinth, potato, even the dahlia, etc.; and it is really surprising to note the endless points of structure and constitution in which the varieties and sub-varieties differ slightly from each other. the whole organisation seems to have become plastic, and departs in a slight degree from that of the parental type. any variation which is not inherited is unimportant for us. but the number and diversity of inheritable deviations of structure, both those of slight and those of considerable physiological importance, are endless. dr. prosper lucas' treatise, in two large volumes, is the fullest and the best on this subject. no breeder doubts how strong is the tendency to inheritance; that like produces like is his fundamental belief: doubts have been thrown on this principle only by theoretical writers. when any deviation of structure often appears, and we see it in the father and child, we cannot tell whether it may not be due to the same cause having acted on both; but when among individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in the parent--say, once among several million individuals--and it reappears in the child, the mere doctrine of chances almost compels us to attribute its reappearance to inheritance. every one must have heard of cases of albinism, prickly skin, hairy bodies, etc., appearing in several members of the same family. if strange and rare deviations of structure are truly inherited, less strange and commoner deviations may be freely admitted to be inheritable. perhaps the correct way of viewing the whole subject would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly. the laws governing inheritance are for the most part unknown; no one can say why the same peculiarity in different individuals of the same species, or in different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characteristics to its grandfather or grandmother or more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not exclusively to the like sex. it is a fact of some importance to us, that peculiarities appearing in the males of our domestic breeds are often transmitted, either exclusively or in a much greater degree, to the males alone. a much more important rule, which i think may be trusted, is that, at whatever period of life a peculiarity first appears, it tends to reappear in the offspring at a corresponding age, though sometimes earlier. in many cases this could not be otherwise; thus the inherited peculiarities in the horns of cattle could appear only in the offspring when nearly mature; peculiarities in the silk-worm are known to appear at the corresponding caterpillar or cocoon stage. but hereditary diseases and some other facts make me believe that the rule has a wider extension, and that, when there is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent. i believe this rule to be of the highest importance in explaining the laws of embryology. these remarks are of course confined to the first appearance of the peculiarity, and not to the primary cause which may have acted on the ovules or on the male element; in nearly the same manner as the increased length of the horns in the offspring from a short-horned cow by a long-horned bull, though appearing late in life, is clearly due to the male element. having alluded to the subject of reversion, i may here refer to a statement often made by naturalists--namely, that our domestic varieties, when run wild, gradually but invariably revert in character to their aboriginal stocks. hence it has been argued that no deductions can be drawn from domestic races to species in a state of nature. i have in vain endeavoured to discover on what decisive facts the above statement has so often and so boldly been made. there would be great difficulty in proving its truth: we may safely conclude that very many of the most strongly marked domestic varieties could not possibly live in a wild state. in many cases we do not know what the aboriginal stock was, and so could not tell whether or not nearly perfect reversion had ensued. it would be necessary, in order to prevent the effects of intercrossing, that only a single variety should be turned loose in its new home. nevertheless, as our varieties certainly do occasionally revert in some of their characters to ancestral forms, it seems to me not improbable that if we could succeed in naturalising, or were to cultivate, during many generations, the several races, for instance, of the cabbage, in very poor soil--in which case, however, some effect would have to be attributed to the definite action of the poor soil--that they would, to a large extent, or even wholly, revert to the wild aboriginal stock. whether or not the experiment would succeed is not of great importance for our line of argument; for by the experiment itself the conditions of life are changed. if it could be shown that our domestic varieties manifested a strong tendency to reversion--that is, to lose their acquired characters, while kept under the same conditions and while kept in a considerable body, so that free intercrossing might check, by blending together, any slight deviations in their structure, in such case, i grant that we could deduce nothing from domestic varieties in regard to species. but there is not a shadow of evidence in favour of this view: to assert that we could not breed our cart and race-horses, long and short-horned cattle, and poultry of various breeds, and esculent vegetables, for an unlimited number of generations, would be opposed to all experience. character of domestic varieties; difficulty of distinguishing between varieties and species; origin of domestic varieties from one or more species. when we look to the hereditary varieties or races of our domestic animals and plants, and compare them with closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species. domestic races often have a somewhat monstrous character; by which i mean, that, although differing from each other and from other species of the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when compared one with another, and more especially when compared with the species under nature to which they are nearest allied. with these exceptions (and with that of the perfect fertility of varieties when crossed--a subject hereafter to be discussed), domestic races of the same species differ from each other in the same manner as do the closely allied species of the same genus in a state of nature, but the differences in most cases are less in degree. this must be admitted as true, for the domestic races of many animals and plants have been ranked by some competent judges as the descendants of aboriginally distinct species, and by other competent judges as mere varieties. if any well marked distinction existed between a domestic race and a species, this source of doubt would not so perpetually recur. it has often been stated that domestic races do not differ from each other in characters of generic value. it can be shown that this statement is not correct; but naturalists differ much in determining what characters are of generic value; all such valuations being at present empirical. when it is explained how genera originate under nature, it will be seen that we have no right to expect often to find a generic amount of difference in our domesticated races. in attempting to estimate the amount of structural difference between allied domestic races, we are soon involved in doubt, from not knowing whether they are descended from one or several parent species. this point, if it could be cleared up, would be interesting; if, for instance, it could be shown that the greyhound, bloodhound, terrier, spaniel and bull-dog, which we all know propagate their kind truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many closely allied natural species--for instance, of the many foxes--inhabiting the different quarters of the world. i do not believe, as we shall presently see, that the whole amount of difference between the several breeds of the dog has been produced under domestication; i believe that a small part of the difference is due to their being descended from distinct species. in the case of strongly marked races of some other domesticated species, there is presumptive or even strong evidence that all are descended from a single wild stock. it has often been assumed that man has chosen for domestication animals and plants having an extraordinary inherent tendency to vary, and likewise to withstand diverse climates. i do not dispute that these capacities have added largely to the value of most of our domesticated productions; but how could a savage possibly know, when he first tamed an animal, whether it would vary in succeeding generations, and whether it would endure other climates? has the little variability of the ass and goose, or the small power of endurance of warmth by the reindeer, or of cold by the common camel, prevented their domestication? i cannot doubt that if other animals and plants, equal in number to our domesticated productions, and belonging to equally diverse classes and countries, were taken from a state of nature, and could be made to breed for an equal number of generations under domestication, they would on an average vary as largely as the parent species of our existing domesticated productions have varied. in the case of most of our anciently domesticated animals and plants, it is not possible to come to any definite conclusion, whether they are descended from one or several wild species. the argument mainly relied on by those who believe in the multiple origin of our domestic animals is, that we find in the most ancient times, on the monuments of egypt, and in the lake-habitations of switzerland, much diversity in the breeds; and that some of these ancient breeds closely resemble, or are even identical with, those still existing. but this only throws far backward the history of civilisation, and shows that animals were domesticated at a much earlier period than has hitherto been supposed. the lake-inhabitants of switzerland cultivated several kinds of wheat and barley, the pea, the poppy for oil and flax; and they possessed several domesticated animals. they also carried on commerce with other nations. all this clearly shows, as heer has remarked, that they had at this early age progressed considerably in civilisation; and this again implies a long continued previous period of less advanced civilisation, during which the domesticated animals, kept by different tribes in different districts, might have varied and given rise to distinct races. since the discovery of flint tools in the superficial formations of many parts of the world, all geologists believe that barbarian men existed at an enormously remote period; and we know that at the present day there is hardly a tribe so barbarous as not to have domesticated at least the dog. the origin of most of our domestic animals will probably forever remain vague. but i may here state that, looking to the domestic dogs of the whole world, i have, after a laborious collection of all known facts, come to the conclusion that several wild species of canidae have been tamed, and that their blood, in some cases mingled together, flows in the veins of our domestic breeds. in regard to sheep and goats i can form no decided opinion. from facts communicated to me by mr. blyth, on the habits, voice, constitution and structure of the humped indian cattle, it is almost certain that they are descended from a different aboriginal stock from our european cattle; and some competent judges believe that these latter have had two or three wild progenitors, whether or not these deserve to be called species. this conclusion, as well as that of the specific distinction between the humped and common cattle, may, indeed, be looked upon as established by the admirable researches of professor rutimeyer. with respect to horses, from reasons which i cannot here give, i am doubtfully inclined to believe, in opposition to several authors, that all the races belong to the same species. having kept nearly all the english breeds of the fowl alive, having bred and crossed them, and examined their skeletons, it appears to me almost certain that all are the descendants of the wild indian fowl, gallus bankiva; and this is the conclusion of mr. blyth, and of others who have studied this bird in india. in regard to ducks and rabbits, some breeds of which differ much from each other, the evidence is clear that they are all descended from the common duck and wild rabbit. the doctrine of the origin of our several domestic races from several aboriginal stocks, has been carried to an absurd extreme by some authors. they believe that every race which breeds true, let the distinctive characters be ever so slight, has had its wild prototype. at this rate there must have existed at least a score of species of wild cattle, as many sheep, and several goats, in europe alone, and several even within great britain. one author believes that there formerly existed eleven wild species of sheep peculiar to great britain! when we bear in mind that britain has now not one peculiar mammal, and france but few distinct from those of germany, and so with hungary, spain, etc., but that each of these kingdoms possesses several peculiar breeds of cattle, sheep, etc., we must admit that many domestic breeds must have originated in europe; for whence otherwise could they have been derived? so it is in india. even in the case of the breeds of the domestic dog throughout the world, which i admit are descended from several wild species, it cannot be doubted that there has been an immense amount of inherited variation; for who will believe that animals closely resembling the italian greyhound, the bloodhound, the bull-dog, pug-dog, or blenheim spaniel, etc.--so unlike all wild canidae--ever existed in a state of nature? it has often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal species; but by crossing we can only get forms in some degree intermediate between their parents; and if we account for our several domestic races by this process, we must admit the former existence of the most extreme forms, as the italian greyhound, bloodhound, bull-dog, etc., in the wild state. moreover, the possibility of making distinct races by crossing has been greatly exaggerated. many cases are on record showing that a race may be modified by occasional crosses if aided by the careful selection of the individuals which present the desired character; but to obtain a race intermediate between two quite distinct races would be very difficult. sir j. sebright expressly experimented with this object and failed. the offspring from the first cross between two pure breeds is tolerably and sometimes (as i have found with pigeons) quite uniform in character, and every thing seems simple enough; but when these mongrels are crossed one with another for several generations, hardly two of them are alike, and then the difficulty of the task becomes manifest. breeds of the domestic pigeon, their differences and origin. believing that it is always best to study some special group, i have, after deliberation, taken up domestic pigeons. i have kept every breed which i could purchase or obtain, and have been most kindly favoured with skins from several quarters of the world, more especially by the hon. w. elliot from india, and by the hon. c. murray from persia. many treatises in different languages have been published on pigeons, and some of them are very important, as being of considerable antiquity. i have associated with several eminent fanciers, and have been permitted to join two of the london pigeon clubs. the diversity of the breeds is something astonishing. compare the english carrier and the short-faced tumbler, and see the wonderful difference in their beaks, entailing corresponding differences in their skulls. the carrier, more especially the male bird, is also remarkable from the wonderful development of the carunculated skin about the head, and this is accompanied by greatly elongated eyelids, very large external orifices to the nostrils, and a wide gape of mouth. the short-faced tumbler has a beak in outline almost like that of a finch; and the common tumbler has the singular inherited habit of flying at a great height in a compact flock, and tumbling in the air head over heels. the runt is a bird of great size, with long, massive beak and large feet; some of the sub-breeds of runts have very long necks, others very long wings and tails, others singularly short tails. the barb is allied to the carrier, but, instead of a long beak, has a very short and broad one. the pouter has a much elongated body, wings, and legs; and its enormously developed crop, which it glories in inflating, may well excite astonishment and even laughter. the turbit has a short and conical beak, with a line of reversed feathers down the breast; and it has the habit of continually expanding, slightly, the upper part of the oesophagus. the jacobin has the feathers so much reversed along the back of the neck that they form a hood, and it has, proportionally to its size, elongated wing and tail feathers. the trumpeter and laugher, as their names express, utter a very different coo from the other breeds. the fantail has thirty or even forty tail-feathers, instead of twelve or fourteen, the normal number in all the members of the great pigeon family: these feathers are kept expanded and are carried so erect that in good birds the head and tail touch: the oil-gland is quite aborted. several other less distinct breeds might be specified. in the skeletons of the several breeds, the development of the bones of the face, in length and breadth and curvature, differs enormously. the shape, as well as the breadth and length of the ramus of the lower jaw, varies in a highly remarkable manner. the caudal and sacral vertebrae vary in number; as does the number of the ribs, together with their relative breadth and the presence of processes. the size and shape of the apertures in the sternum are highly variable; so is the degree of divergence and relative size of the two arms of the furcula. the proportional width of the gape of mouth, the proportional length of the eyelids, of the orifice of the nostrils, of the tongue (not always in strict correlation with the length of beak), the size of the crop and of the upper part of the oesophagus; the development and abortion of the oil-gland; the number of the primary wing and caudal feathers; the relative length of the wing and tail to each other and to the body; the relative length of the leg and foot; the number of scutellae on the toes, the development of skin between the toes, are all points of structure which are variable. the period at which the perfect plumage is acquired varies, as does the state of the down with which the nestling birds are clothed when hatched. the shape and size of the eggs vary. the manner of flight, and in some breeds the voice and disposition, differ remarkably. lastly, in certain breeds, the males and females have come to differ in a slight degree from each other. altogether at least a score of pigeons might be chosen, which, if shown to an ornithologist, and he were told that they were wild birds, would certainly be ranked by him as well-defined species. moreover, i do not believe that any ornithologist would in this case place the english carrier, the short-faced tumbler, the runt, the barb, pouter, and fantail in the same genus; more especially as in each of these breeds several truly-inherited sub-breeds, or species, as he would call them, could be shown him. great as are the differences between the breeds of the pigeon, i am fully convinced that the common opinion of naturalists is correct, namely, that all are descended from the rock-pigeon (columba livia), including under this term several geographical races or sub-species, which differ from each other in the most trifling respects. as several of the reasons which have led me to this belief are in some degree applicable in other cases, i will here briefly give them. if the several breeds are not varieties, and have not proceeded from the rock-pigeon, they must have descended from at least seven or eight aboriginal stocks; for it is impossible to make the present domestic breeds by the crossing of any lesser number: how, for instance, could a pouter be produced by crossing two breeds unless one of the parent-stocks possessed the characteristic enormous crop? the supposed aboriginal stocks must all have been rock-pigeons, that is, they did not breed or willingly perch on trees. but besides c. livia, with its geographical sub-species, only two or three other species of rock-pigeons are known; and these have not any of the characters of the domestic breeds. hence the supposed aboriginal stocks must either still exist in the countries where they were originally domesticated, and yet be unknown to ornithologists; and this, considering their size, habits and remarkable characters, seems improbable; or they must have become extinct in the wild state. but birds breeding on precipices, and good flyers, are unlikely to be exterminated; and the common rock-pigeon, which has the same habits with the domestic breeds, has not been exterminated even on several of the smaller british islets, or on the shores of the mediterranean. hence the supposed extermination of so many species having similar habits with the rock-pigeon seems a very rash assumption. moreover, the several above-named domesticated breeds have been transported to all parts of the world, and, therefore, some of them must have been carried back again into their native country; but not one has become wild or feral, though the dovecot-pigeon, which is the rock-pigeon in a very slightly altered state, has become feral in several places. again, all recent experience shows that it is difficult to get wild animals to breed freely under domestication; yet on the hypothesis of the multiple origin of our pigeons, it must be assumed that at least seven or eight species were so thoroughly domesticated in ancient times by half-civilized man, as to be quite prolific under confinement. an argument of great weight, and applicable in several other cases, is, that the above-specified breeds, though agreeing generally with the wild rock-pigeon in constitution, habits, voice, colouring, and in most parts of their structure, yet are certainly highly abnormal in other parts; we may look in vain through the whole great family of columbidae for a beak like that of the english carrier, or that of the short-faced tumbler, or barb; for reversed feathers like those of the jacobin; for a crop like that of the pouter; for tail-feathers like those of the fantail. hence it must be assumed, not only that half-civilized man succeeded in thoroughly domesticating several species, but that he intentionally or by chance picked out extraordinarily abnormal species; and further, that these very species have since all become extinct or unknown. so many strange contingencies are improbable in the highest degree. some facts in regard to the colouring of pigeons well deserve consideration. the rock-pigeon is of a slaty-blue, with white loins; but the indian sub-species, c. intermedia of strickland, has this part bluish. the tail has a terminal dark bar, with the outer feathers externally edged at the base with white. the wings have two black bars. some semi-domestic breeds, and some truly wild breeds, have, besides the two black bars, the wings chequered with black. these several marks do not occur together in any other species of the whole family. now, in every one of the domestic breeds, taking thoroughly well-bred birds, all the above marks, even to the white edging of the outer tail-feathers, sometimes concur perfectly developed. moreover, when birds belonging to two or more distinct breeds are crossed, none of which are blue or have any of the above-specified marks, the mongrel offspring are very apt suddenly to acquire these characters. to give one instance out of several which i have observed: i crossed some white fantails, which breed very true, with some black barbs--and it so happens that blue varieties of barbs are so rare that i never heard of an instance in england; and the mongrels were black, brown and mottled. i also crossed a barb with a spot, which is a white bird with a red tail and red spot on the forehead, and which notoriously breeds very true; the mongrels were dusky and mottled. i then crossed one of the mongrel barb-fantails with a mongrel barb-spot, and they produced a bird of as beautiful a blue colour, with the white loins, double black wing-bar, and barred and white-edged tail-feathers, as any wild rock-pigeon! we can understand these facts, on the well-known principle of reversion to ancestral characters, if all the domestic breeds are descended from the rock-pigeon. but if we deny this, we must make one of the two following highly improbable suppositions. either, first, that all the several imagined aboriginal stocks were coloured and marked like the rock-pigeon, although no other existing species is thus coloured and marked, so that in each separate breed there might be a tendency to revert to the very same colours and markings. or, secondly, that each breed, even the purest, has within a dozen, or at most within a score, of generations, been crossed by the rock-pigeon: i say within a dozen or twenty generations, for no instance is known of crossed descendants reverting to an ancestor of foreign blood, removed by a greater number of generations. in a breed which has been crossed only once the tendency to revert to any character derived from such a cross will naturally become less and less, as in each succeeding generation there will be less of the foreign blood; but when there has been no cross, and there is a tendency in the breed to revert to a character which was lost during some former generation, this tendency, for all that we can see to the contrary, may be transmitted undiminished for an indefinite number of generations. these two distinct cases of reversion are often confounded together by those who have written on inheritance. lastly, the hybrids or mongrels from between all the breeds of the pigeon are perfectly fertile, as i can state from my own observations, purposely made, on the most distinct breeds. now, hardly any cases have been ascertained with certainty of hybrids from two quite distinct species of animals being perfectly fertile. some authors believe that long-continued domestication eliminates this strong tendency to sterility in species. from the history of the dog, and of some other domestic animals, this conclusion is probably quite correct, if applied to species closely related to each other. but to extend it so far as to suppose that species, aboriginally as distinct as carriers, tumblers, pouters, and fantails now are, should yield offspring perfectly fertile, inter se, seems to me rash in the extreme. from these several reasons, namely, the improbability of man having formerly made seven or eight supposed species of pigeons to breed freely under domestication--these supposed species being quite unknown in a wild state, and their not having become anywhere feral--these species presenting certain very abnormal characters, as compared with all other columbidae, though so like the rock-pigeon in most other respects--the occasional reappearance of the blue colour and various black marks in all the breeds, both when kept pure and when crossed--and lastly, the mongrel offspring being perfectly fertile--from these several reasons, taken together, we may safely conclude that all our domestic breeds are descended from the rock-pigeon or columba livia with its geographical sub-species. in favour of this view, i may add, firstly, that the wild c. livia has been found capable of domestication in europe and in india; and that it agrees in habits and in a great number of points of structure with all the domestic breeds. secondly, that although an english carrier or a short-faced tumbler differs immensely in certain characters from the rock-pigeon, yet that by comparing the several sub-breeds of these two races, more especially those brought from distant countries, we can make, between them and the rock-pigeon, an almost perfect series; so we can in some other cases, but not with all the breeds. thirdly, those characters which are mainly distinctive of each breed are in each eminently variable, for instance, the wattle and length of beak of the carrier, the shortness of that of the tumbler, and the number of tail-feathers in the fantail; and the explanation of this fact will be obvious when we treat of selection. fourthly, pigeons have been watched and tended with the utmost care, and loved by many people. they have been domesticated for thousands of years in several quarters of the world; the earliest known record of pigeons is in the fifth aegyptian dynasty, about b.c., as was pointed out to me by professor lepsius; but mr. birch informs me that pigeons are given in a bill of fare in the previous dynasty. in the time of the romans, as we hear from pliny, immense prices were given for pigeons; "nay, they are come to this pass, that they can reckon up their pedigree and race." pigeons were much valued by akber khan in india, about the year ; never less than , pigeons were taken with the court. "the monarchs of iran and turan sent him some very rare birds;" and, continues the courtly historian, "his majesty, by crossing the breeds, which method was never practised before, has improved them astonishingly." about this same period the dutch were as eager about pigeons as were the old romans. the paramount importance of these considerations in explaining the immense amount of variation which pigeons have undergone, will likewise be obvious when we treat of selection. we shall then, also, see how it is that the several breeds so often have a somewhat monstrous character. it is also a most favourable circumstance for the production of distinct breeds, that male and female pigeons can be easily mated for life; and thus different breeds can be kept together in the same aviary. i have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because when i first kept pigeons and watched the several kinds, well knowing how truly they breed, i felt fully as much difficulty in believing that since they had been domesticated they had all proceeded from a common parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches, or other groups of birds, in nature. one circumstance has struck me much; namely, that nearly all the breeders of the various domestic animals and the cultivators of plants, with whom i have conversed, or whose treatises i have read, are firmly convinced that the several breeds to which each has attended, are descended from so many aboriginally distinct species. ask, as i have asked, a celebrated raiser of hereford cattle, whether his cattle might not have descended from long-horns, or both from a common parent-stock, and he will laugh you to scorn. i have never met a pigeon, or poultry, or duck, or rabbit fancier, who was not fully convinced that each main breed was descended from a distinct species. van mons, in his treatise on pears and apples, shows how utterly he disbelieves that the several sorts, for instance a ribston-pippin or codlin-apple, could ever have proceeded from the seeds of the same tree. innumerable other examples could be given. the explanation, i think, is simple: from long-continued study they are strongly impressed with the differences between the several races; and though they well know that each race varies slightly, for they win their prizes by selecting such slight differences, yet they ignore all general arguments, and refuse to sum up in their minds slight differences accumulated during many successive generations. may not those naturalists who, knowing far less of the laws of inheritance than does the breeder, and knowing no more than he does of the intermediate links in the long lines of descent, yet admit that many of our domestic races are descended from the same parents--may they not learn a lesson of caution, when they deride the idea of species in a state of nature being lineal descendants of other species? principles of selection anciently followed, and their effects. let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species. some effect may be attributed to the direct and definite action of the external conditions of life, and some to habit; but he would be a bold man who would account by such agencies for the differences between a dray and race-horse, a greyhound and bloodhound, a carrier and tumbler pigeon. one of the most remarkable features in our domesticated races is that we see in them adaptation, not indeed to the animal's or plant's own good, but to man's use or fancy. some variations useful to him have probably arisen suddenly, or by one step; many botanists, for instance, believe that the fuller's teasel, with its hooks, which can not be rivalled by any mechanical contrivance, is only a variety of the wild dipsacus; and this amount of change may have suddenly arisen in a seedling. so it has probably been with the turnspit dog; and this is known to have been the case with the ancon sheep. but when we compare the dray-horse and race-horse, the dromedary and camel, the various breeds of sheep fitted either for cultivated land or mountain pasture, with the wool of one breed good for one purpose, and that of another breed for another purpose; when we compare the many breeds of dogs, each good for man in different ways; when we compare the game-cock, so pertinacious in battle, with other breeds so little quarrelsome, with "everlasting layers" which never desire to sit, and with the bantam so small and elegant; when we compare the host of agricultural, culinary, orchard, and flower-garden races of plants, most useful to man at different seasons and for different purposes, or so beautiful in his eyes, we must, i think, look further than to mere variability. we can not suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; indeed, in many cases, we know that this has not been their history. the key is man's power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. in this sense he may be said to have made for himself useful breeds. the great power of this principle of selection is not hypothetical. it is certain that several of our eminent breeders have, even within a single lifetime, modified to a large extent their breeds of cattle and sheep. in order fully to realise what they have done it is almost necessary to read several of the many treatises devoted to this subject, and to inspect the animals. breeders habitually speak of an animal's organisation as something plastic, which they can model almost as they please. if i had space i could quote numerous passages to this effect from highly competent authorities. youatt, who was probably better acquainted with the works of agriculturalists than almost any other individual, and who was himself a very good judge of animals, speaks of the principle of selection as "that which enables the agriculturist, not only to modify the character of his flock, but to change it altogether. it is the magician's wand, by means of which he may summon into life whatever form and mould he pleases." lord somerville, speaking of what breeders have done for sheep, says: "it would seem as if they had chalked out upon a wall a form perfect in itself, and then had given it existence." in saxony the importance of the principle of selection in regard to merino sheep is so fully recognised, that men follow it as a trade: the sheep are placed on a table and are studied, like a picture by a connoisseur; this is done three times at intervals of months, and the sheep are each time marked and classed, so that the very best may ultimately be selected for breeding. what english breeders have actually effected is proved by the enormous prices given for animals with a good pedigree; and these have been exported to almost every quarter of the world. the improvement is by no means generally due to crossing different breeds; all the best breeders are strongly opposed to this practice, except sometimes among closely allied sub-breeds. and when a cross has been made, the closest selection is far more indispensable even than in ordinary cases. if selection consisted merely in separating some very distinct variety and breeding from it, the principle would be so obvious as hardly to be worth notice; but its importance consists in the great effect produced by the accumulation in one direction, during successive generations, of differences absolutely inappreciable by an uneducated eye--differences which i for one have vainly attempted to appreciate. not one man in a thousand has accuracy of eye and judgment sufficient to become an eminent breeder. if gifted with these qualities, and he studies his subject for years, and devotes his lifetime to it with indomitable perseverance, he will succeed, and may make great improvements; if he wants any of these qualities, he will assuredly fail. few would readily believe in the natural capacity and years of practice requisite to become even a skilful pigeon-fancier. the same principles are followed by horticulturists; but the variations are here often more abrupt. no one supposes that our choicest productions have been produced by a single variation from the aboriginal stock. we have proofs that this is not so in several cases in which exact records have been kept; thus, to give a very trifling instance, the steadily increasing size of the common gooseberry may be quoted. we see an astonishing improvement in many florists' flowers, when the flowers of the present day are compared with drawings made only twenty or thirty years ago. when a race of plants is once pretty well established, the seed-raisers do not pick out the best plants, but merely go over their seed-beds, and pull up the "rogues," as they call the plants that deviate from the proper standard. with animals this kind of selection is, in fact, likewise followed; for hardly any one is so careless as to breed from his worst animals. in regard to plants, there is another means of observing the accumulated effects of selection--namely, by comparing the diversity of flowers in the different varieties of the same species in the flower-garden; the diversity of leaves, pods, or tubers, or whatever part is valued, in the kitchen-garden, in comparison with the flowers of the same varieties; and the diversity of fruit of the same species in the orchard, in comparison with the leaves and flowers of the same set of varieties. see how different the leaves of the cabbage are, and how extremely alike the flowers; how unlike the flowers of the heartsease are, and how alike the leaves; how much the fruit of the different kinds of gooseberries differ in size, colour, shape, and hairiness, and yet the flowers present very slight differences. it is not that the varieties which differ largely in some one point do not differ at all in other points; this is hardly ever--i speak after careful observation--perhaps never, the case. the law of correlated variation, the importance of which should never be overlooked, will ensure some differences; but, as a general rule, it cannot be doubted that the continued selection of slight variations, either in the leaves, the flowers, or the fruit, will produce races differing from each other chiefly in these characters. it may be objected that the principle of selection has been reduced to methodical practice for scarcely more than three-quarters of a century; it has certainly been more attended to of late years, and many treatises have been published on the subject; and the result has been, in a corresponding degree, rapid and important. but it is very far from true that the principle is a modern discovery. i could give several references to works of high antiquity, in which the full importance of the principle is acknowledged. in rude and barbarous periods of english history choice animals were often imported, and laws were passed to prevent their exportation: the destruction of horses under a certain size was ordered, and this may be compared to the "roguing" of plants by nurserymen. the principle of selection i find distinctly given in an ancient chinese encyclopaedia. explicit rules are laid down by some of the roman classical writers. from passages in genesis, it is clear that the colour of domestic animals was at that early period attended to. savages now sometimes cross their dogs with wild canine animals, to improve the breed, and they formerly did so, as is attested by passages in pliny. the savages in south africa match their draught cattle by colour, as do some of the esquimaux their teams of dogs. livingstone states that good domestic breeds are highly valued by the negroes in the interior of africa who have not associated with europeans. some of these facts do not show actual selection, but they show that the breeding of domestic animals was carefully attended to in ancient times, and is now attended to by the lowest savages. it would, indeed, have been a strange fact, had attention not been paid to breeding, for the inheritance of good and bad qualities is so obvious. unconscious selection. at the present time, eminent breeders try by methodical selection, with a distinct object in view, to make a new strain or sub-breed, superior to anything of the kind in the country. but, for our purpose, a form of selection, which may be called unconscious, and which results from every one trying to possess and breed from the best individual animals, is more important. thus, a man who intends keeping pointers naturally tries to get as good dogs as he can, and afterwards breeds from his own best dogs, but he has no wish or expectation of permanently altering the breed. nevertheless we may infer that this process, continued during centuries, would improve and modify any breed, in the same way as bakewell, collins, etc., by this very same process, only carried on more methodically, did greatly modify, even during their lifetimes, the forms and qualities of their cattle. slow and insensible changes of this kind could never be recognised unless actual measurements or careful drawings of the breeds in question have been made long ago, which may serve for comparison. in some cases, however, unchanged, or but little changed, individuals of the same breed exist in less civilised districts, where the breed has been less improved. there is reason to believe that king charles' spaniel has been unconsciously modified to a large extent since the time of that monarch. some highly competent authorities are convinced that the setter is directly derived from the spaniel, and has probably been slowly altered from it. it is known that the english pointer has been greatly changed within the last century, and in this case the change has, it is believed, been chiefly effected by crosses with the foxhound; but what concerns us is, that the change has been effected unconsciously and gradually, and yet so effectually that, though the old spanish pointer certainly came from spain, mr. borrow has not seen, as i am informed by him, any native dog in spain like our pointer. by a similar process of selection, and by careful training, english race-horses have come to surpass in fleetness and size the parent arabs, so that the latter, by the regulations for the goodwood races, are favoured in the weights which they carry. lord spencer and others have shown how the cattle of england have increased in weight and in early maturity, compared with the stock formerly kept in this country. by comparing the accounts given in various old treatises of the former and present state of carrier and tumbler pigeons in britain, india, and persia, we can trace the stages through which they have insensibly passed, and come to differ so greatly from the rock-pigeon. youatt gives an excellent illustration of the effects of a course of selection which may be considered as unconscious, in so far that the breeders could never have expected, or even wished, to produce the result which ensued--namely, the production of the distinct strains. the two flocks of leicester sheep kept by mr. buckley and mr. burgess, as mr. youatt remarks, "have been purely bred from the original stock of mr. bakewell for upwards of fifty years. there is not a suspicion existing in the mind of any one at all acquainted with the subject that the owner of either of them has deviated in any one instance from the pure blood of mr. bakewell's flock, and yet the difference between the sheep possessed by these two gentlemen is so great that they have the appearance of being quite different varieties." if there exist savages so barbarous as never to think of the inherited character of the offspring of their domestic animals, yet any one animal particularly useful to them, for any special purpose, would be carefully preserved during famines and other accidents, to which savages are so liable, and such choice animals would thus generally leave more offspring than the inferior ones; so that in this case there would be a kind of unconscious selection going on. we see the value set on animals even by the barbarians of tierra del fuego, by their killing and devouring their old women, in times of dearth, as of less value than their dogs. in plants the same gradual process of improvement through the occasional preservation of the best individuals, whether or not sufficiently distinct to be ranked at their first appearance as distinct varieties, and whether or not two or more species or races have become blended together by crossing, may plainly be recognised in the increased size and beauty which we now see in the varieties of the heartsease, rose, pelargonium, dahlia, and other plants, when compared with the older varieties or with their parent-stocks. no one would ever expect to get a first-rate heartsease or dahlia from the seed of a wild plant. no one would expect to raise a first-rate melting pear from the seed of a wild pear, though he might succeed from a poor seedling growing wild, if it had come from a garden-stock. the pear, though cultivated in classical times, appears, from pliny's description, to have been a fruit of very inferior quality. i have seen great surprise expressed in horticultural works at the wonderful skill of gardeners in having produced such splendid results from such poor materials; but the art has been simple, and, as far as the final result is concerned, has been followed almost unconsciously. it has consisted in always cultivating the best known variety, sowing its seeds, and, when a slightly better variety chanced to appear, selecting it, and so onwards. but the gardeners of the classical period, who cultivated the best pears which they could procure, never thought what splendid fruit we should eat; though we owe our excellent fruit in some small degree to their having naturally chosen and preserved the best varieties they could anywhere find. a large amount of change, thus slowly and unconsciously accumulated, explains, as i believe, the well-known fact, that in a number of cases we cannot recognise, and therefore do not know, the wild parent-stocks of the plants which have been longest cultivated in our flower and kitchen gardens. if it has taken centuries or thousands of years to improve or modify most of our plants up to their present standard of usefulness to man, we can understand how it is that neither australia, the cape of good hope, nor any other region inhabited by quite uncivilised man, has afforded us a single plant worth culture. it is not that these countries, so rich in species, do not by a strange chance possess the aboriginal stocks of any useful plants, but that the native plants have not been improved by continued selection up to a standard of perfection comparable with that acquired by the plants in countries anciently civilised. in regard to the domestic animals kept by uncivilised man, it should not be overlooked that they almost always have to struggle for their own food, at least during certain seasons. and in two countries very differently circumstanced, individuals of the same species, having slightly different constitutions or structure, would often succeed better in the one country than in the other, and thus by a process of "natural selection," as will hereafter be more fully explained, two sub-breeds might be formed. this, perhaps, partly explains why the varieties kept by savages, as has been remarked by some authors, have more of the character of true species than the varieties kept in civilised countries. on the view here given of the important part which selection by man has played, it becomes at once obvious, how it is that our domestic races show adaptation in their structure or in their habits to man's wants or fancies. we can, i think, further understand the frequently abnormal character of our domestic races, and likewise their differences being so great in external characters, and relatively so slight in internal parts or organs. man can hardly select, or only with much difficulty, any deviation of structure excepting such as is externally visible; and indeed he rarely cares for what is internal. he can never act by selection, excepting on variations which are first given to him in some slight degree by nature. no man would ever try to make a fantail till he saw a pigeon with a tail developed in some slight degree in an unusual manner, or a pouter till he saw a pigeon with a crop of somewhat unusual size; and the more abnormal or unusual any character was when it first appeared, the more likely it would be to catch his attention. but to use such an expression as trying to make a fantail is, i have no doubt, in most cases, utterly incorrect. the man who first selected a pigeon with a slightly larger tail, never dreamed what the descendants of that pigeon would become through long-continued, partly unconscious and partly methodical, selection. perhaps the parent bird of all fantails had only fourteen tail-feathers somewhat expanded, like the present java fantail, or like individuals of other and distinct breeds, in which as many as seventeen tail-feathers have been counted. perhaps the first pouter-pigeon did not inflate its crop much more than the turbit now does the upper part of its oesophagus--a habit which is disregarded by all fanciers, as it is not one of the points of the breed. nor let it be thought that some great deviation of structure would be necessary to catch the fancier's eye: he perceives extremely small differences, and it is in human nature to value any novelty, however slight, in one's own possession. nor must the value which would formerly have been set on any slight differences in the individuals of the same species, be judged of by the value which is now set on them, after several breeds have fairly been established. it is known that with pigeons many slight variations now occasionally appear, but these are rejected as faults or deviations from the standard of perfection in each breed. the common goose has not given rise to any marked varieties; hence the toulouse and the common breed, which differ only in colour, that most fleeting of characters, have lately been exhibited as distinct at our poultry-shows. these views appear to explain what has sometimes been noticed, namely, that we know hardly anything about the origin or history of any of our domestic breeds. but, in fact, a breed, like a dialect of a language, can hardly be said to have a distinct origin. a man preserves and breeds from an individual with some slight deviation of structure, or takes more care than usual in matching his best animals, and thus improves them, and the improved animals slowly spread in the immediate neighbourhood. but they will as yet hardly have a distinct name, and from being only slightly valued, their history will have been disregarded. when further improved by the same slow and gradual process, they will spread more widely, and will be recognised as something distinct and valuable, and will then probably first receive a provincial name. in semi-civilised countries, with little free communication, the spreading of a new sub-breed will be a slow process. as soon as the points of value are once acknowledged, the principle, as i have called it, of unconscious selection will always tend--perhaps more at one period than at another, as the breed rises or falls in fashion--perhaps more in one district than in another, according to the state of civilisation of the inhabitants--slowly to add to the characteristic features of the breed, whatever they may be. but the chance will be infinitely small of any record having been preserved of such slow, varying, and insensible changes. circumstances favourable to man's power of selection. i will now say a few words on the circumstances, favourable or the reverse, to man's power of selection. a high degree of variability is obviously favourable, as freely giving the materials for selection to work on; not that mere individual differences are not amply sufficient, with extreme care, to allow of the accumulation of a large amount of modification in almost any desired direction. but as variations manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be much increased by a large number of individuals being kept. hence number is of the highest importance for success. on this principle marshall formerly remarked, with respect to the sheep of part of yorkshire, "as they generally belong to poor people, and are mostly in small lots, they never can be improved." on the other hand, nurserymen, from keeping large stocks of the same plant, are generally far more successful than amateurs in raising new and valuable varieties. a large number of individuals of an animal or plant can be reared only where the conditions for its propagation are favourable. when the individuals are scanty all will be allowed to breed, whatever their quality may be, and this will effectually prevent selection. but probably the most important element is that the animal or plant should be so highly valued by man, that the closest attention is paid to even the slightest deviations in its qualities or structure. unless such attention be paid nothing can be effected. i have seen it gravely remarked, that it was most fortunate that the strawberry began to vary just when gardeners began to attend to this plant. no doubt the strawberry had always varied since it was cultivated, but the slight varieties had been neglected. as soon, however, as gardeners picked out individual plants with slightly larger, earlier, or better fruit, and raised seedlings from them, and again picked out the best seedlings and bred from them, then (with some aid by crossing distinct species) those many admirable varieties of the strawberry were raised which have appeared during the last half-century. with animals, facility in preventing crosses is an important element in the formation of new races--at least, in a country which is already stocked with other races. in this respect enclosure of the land plays a part. wandering savages or the inhabitants of open plains rarely possess more than one breed of the same species. pigeons can be mated for life, and this is a great convenience to the fancier, for thus many races may be improved and kept true, though mingled in the same aviary; and this circumstance must have largely favoured the formation of new breeds. pigeons, i may add, can be propagated in great numbers and at a very quick rate, and inferior birds may be freely rejected, as when killed they serve for food. on the other hand, cats, from their nocturnal rambling habits, can not be easily matched, and, although so much valued by women and children, we rarely see a distinct breed long kept up; such breeds as we do sometimes see are almost always imported from some other country. although i do not doubt that some domestic animals vary less than others, yet the rarity or absence of distinct breeds of the cat, the donkey, peacock, goose, etc., may be attributed in main part to selection not having been brought into play: in cats, from the difficulty in pairing them; in donkeys, from only a few being kept by poor people, and little attention paid to their breeding; for recently in certain parts of spain and of the united states this animal has been surprisingly modified and improved by careful selection; in peacocks, from not being very easily reared and a large stock not kept; in geese, from being valuable only for two purposes, food and feathers, and more especially from no pleasure having been felt in the display of distinct breeds; but the goose, under the conditions to which it is exposed when domesticated, seems to have a singularly inflexible organisation, though it has varied to a slight extent, as i have elsewhere described. some authors have maintained that the amount of variation in our domestic productions is soon reached, and can never afterward be exceeded. it would be somewhat rash to assert that the limit has been attained in any one case; for almost all our animals and plants have been greatly improved in many ways within a recent period; and this implies variation. it would be equally rash to assert that characters now increased to their utmost limit, could not, after remaining fixed for many centuries, again vary under new conditions of life. no doubt, as mr. wallace has remarked with much truth, a limit will be at last reached. for instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by the friction to be overcome, the weight of the body to be carried, and the power of contraction in the muscular fibres. but what concerns us is that the domestic varieties of the same species differ from each other in almost every character, which man has attended to and selected, more than do the distinct species of the same genera. isidore geoffroy st. hilaire has proved this in regard to size, and so it is with colour, and probably with the length of hair. with respect to fleetness, which depends on many bodily characters, eclipse was far fleeter, and a dray-horse is comparably stronger, than any two natural species belonging to the same genus. so with plants, the seeds of the different varieties of the bean or maize probably differ more in size than do the seeds of the distinct species in any one genus in the same two families. the same remark holds good in regard to the fruit of the several varieties of the plum, and still more strongly with the melon, as well as in many other analogous cases. to sum up on the origin of our domestic races of animals and plants. changed conditions of life are of the highest importance in causing variability, both by acting directly on the organisation, and indirectly by affecting the reproductive system. it is not probable that variability is an inherent and necessary contingent, under all circumstances. the greater or less force of inheritance and reversion determine whether variations shall endure. variability is governed by many unknown laws, of which correlated growth is probably the most important. something, but how much we do not know, may be attributed to the definite action of the conditions of life. some, perhaps a great, effect may be attributed to the increased use or disuse of parts. the final result is thus rendered infinitely complex. in some cases the intercrossing of aboriginally distinct species appears to have played an important part in the origin of our breeds. when several breeds have once been formed in any country, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of crossing has been much exaggerated, both in regard to animals and to those plants which are propagated by seed. with plants which are temporarily propagated by cuttings, buds, etc., the importance of crossing is immense; for the cultivator may here disregard the extreme variability both of hybrids and of mongrels, and the sterility of hybrids; but plants not propagated by seed are of little importance to us, for their endurance is only temporary. over all these causes of change, the accumulative action of selection, whether applied methodically and quickly, or unconsciously and slowly, but more efficiently, seems to have been the predominant power. chapter ii. variation under nature. variability--individual differences--doubtful species--wide ranging, much diffused, and common species, vary most--species of the larger genera in each country vary more frequently than the species of the smaller genera--many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges. before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must briefly discuss whether these latter are subject to any variation. to treat this subject properly, a long catalogue of dry facts ought to be given; but these i shall reserve for a future work. nor shall i here discuss the various definitions which have been given of the term species. no one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. generally the term includes the unknown element of a distinct act of creation. the term "variety" is almost equally difficult to define; but here community of descent is almost universally implied, though it can rarely be proved. we have also what are called monstrosities; but they graduate into varieties. by a monstrosity i presume is meant some considerable deviation of structure, generally injurious, or not useful to the species. some authors use the term "variation" in a technical sense, as implying a modification directly due to the physical conditions of life; and "variations" in this sense are supposed not to be inherited; but who can say that the dwarfed condition of shells in the brackish waters of the baltic, or dwarfed plants on alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for at least a few generations? and in this case i presume that the form would be called a variety. it may be doubted whether sudden and considerable deviations of structure, such as we occasionally see in our domestic productions, more especially with plants, are ever permanently propagated in a state of nature. almost every part of every organic being is so beautifully related to its complex conditions of life that it seems as improbable that any part should have been suddenly produced perfect, as that a complex machine should have been invented by man in a perfect state. under domestication monstrosities sometimes occur which resemble normal structures in widely different animals. thus pigs have occasionally been born with a sort of proboscis, and if any wild species of the same genus had naturally possessed a proboscis, it might have been argued that this had appeared as a monstrosity; but i have as yet failed to find, after diligent search, cases of monstrosities resembling normal structures in nearly allied forms, and these alone bear on the question. if monstrous forms of this kind ever do appear in a state of nature and are capable of reproduction (which is not always the case), as they occur rarely and singly, their preservation would depend on unusually favourable circumstances. they would, also, during the first and succeeding generations cross with the ordinary form, and thus their abnormal character would almost inevitably be lost. but i shall have to return in a future chapter to the preservation and perpetuation of single or occasional variations. individual differences. the many slight differences which appear in the offspring from the same parents, or which it may be presumed have thus arisen, from being observed in the individuals of the same species inhabiting the same confined locality, may be called individual differences. no one supposes that all the individuals of the same species are cast in the same actual mould. these individual differences are of the highest importance for us, for they are often inherited, as must be familiar to every one; and they thus afford materials for natural selection to act on and accumulate, in the same manner as man accumulates in any given direction individual differences in his domesticated productions. these individual differences generally affect what naturalists consider unimportant parts; but i could show, by a long catalogue of facts, that parts which must be called important, whether viewed under a physiological or classificatory point of view, sometimes vary in the individuals of the same species. i am convinced that the most experienced naturalist would be surprised at the number of the cases of variability, even in important parts of structure, which he could collect on good authority, as i have collected, during a course of years. it should be remembered that systematists are far from being pleased at finding variability in important characters, and that there are not many men who will laboriously examine internal and important organs, and compare them in many specimens of the same species. it would never have been expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species; it might have been thought that changes of this nature could have been effected only by slow degrees; yet sir j. lubbock has shown a degree of variability in these main nerves in coccus, which may almost be compared to the irregular branching of the stem of a tree. this philosophical naturalist, i may add, has also shown that the muscles in the larvae of certain insects are far from uniform. authors sometimes argue in a circle when they state that important organs never vary; for these same authors practically rank those parts as important (as some few naturalists have honestly confessed) which do not vary; and, under this point of view, no instance will ever be found of an important part varying; but under any other point of view many instances assuredly can be given. there is one point connected with individual differences which is extremely perplexing: i refer to those genera which have been called "protean" or "polymorphic," in which species present an inordinate amount of variation. with respect to many of these forms, hardly two naturalists agree whether to rank them as species or as varieties. we may instance rubus, rosa, and hieracium among plants, several genera of insects, and of brachiopod shells. in most polymorphic genera some of the species have fixed and definite characters. genera which are polymorphic in one country seem to be, with a few exceptions, polymorphic in other countries, and likewise, judging from brachiopod shells, at former periods of time. these facts are very perplexing, for they seem to show that this kind of variability is independent of the conditions of life. i am inclined to suspect that we see, at least in some of these polymorphic genera, variations which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter to be explained. individuals of the same species often present, as is known to every one, great differences of structure, independently of variation, as in the two sexes of various animals, in the two or three castes of sterile females or workers among insects, and in the immature and larval states of many of the lower animals. there are, also, cases of dimorphism and trimorphism, both with animals and plants. thus, mr. wallace, who has lately called attention to the subject, has shown that the females of certain species of butterflies, in the malayan archipelago, regularly appear under two or even three conspicuously distinct forms, not connected by intermediate varieties. fritz muller has described analogous but more extraordinary cases with the males of certain brazilian crustaceans: thus, the male of a tanais regularly occurs under two distinct forms; one of these has strong and differently shaped pincers, and the other has antennae much more abundantly furnished with smelling-hairs. although in most of these cases, the two or three forms, both with animals and plants, are not now connected by intermediate gradations, it is possible that they were once thus connected. mr. wallace, for instance, describes a certain butterfly which presents in the same island a great range of varieties connected by intermediate links, and the extreme links of the chain closely resemble the two forms of an allied dimorphic species inhabiting another part of the malay archipelago. thus also with ants, the several worker-castes are generally quite distinct; but in some cases, as we shall hereafter see, the castes are connected together by finely graduated varieties. so it is, as i have myself observed, with some dimorphic plants. it certainly at first appears a highly remarkable fact that the same female butterfly should have the power of producing at the same time three distinct female forms and a male; and that an hermaphrodite plant should produce from the same seed-capsule three distinct hermaphrodite forms, bearing three different kinds of females and three or even six different kinds of males. nevertheless these cases are only exaggerations of the common fact that the female produces offspring of two sexes which sometimes differ from each other in a wonderful manner. doubtful species. the forms which possess in some considerable degree the character of species, but which are so closely similar to other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank them as distinct species, are in several respects the most important for us. we have every reason to believe that many of these doubtful and closely allied forms have permanently retained their characters for a long time; for as long, as far as we know, as have good and true species. practically, when a naturalist can unite by means of intermediate links any two forms, he treats the one as a variety of the other, ranking the most common, but sometimes the one first described as the species, and the other as the variety. but cases of great difficulty, which i will not here enumerate, sometimes arise in deciding whether or not to rank one form as a variety of another, even when they are closely connected by intermediate links; nor will the commonly assumed hybrid nature of the intermediate forms always remove the difficulty. in very many cases, however, one form is ranked as a variety of another, not because the intermediate links have actually been found, but because analogy leads the observer to suppose either that they do now somewhere exist, or may formerly have existed; and here a wide door for the entry of doubt and conjecture is opened. hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgment and wide experience seems the only guide to follow. we must, however, in many cases, decide by a majority of naturalists, for few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges. that varieties of this doubtful nature are far from uncommon cannot be disputed. compare the several floras of great britain, of france, or of the united states, drawn up by different botanists, and see what a surprising number of forms have been ranked by one botanist as good species, and by another as mere varieties. mr. h.c. watson, to whom i lie under deep obligation for assistance of all kinds, has marked for me british plants, which are generally considered as varieties, but which have all been ranked by botanists as species; and in making this list he has omitted many trifling varieties, but which nevertheless have been ranked by some botanists as species, and he has entirely omitted several highly polymorphic genera. under genera, including the most polymorphic forms, mr. babington gives species, whereas mr. bentham gives only --a difference of doubtful forms! among animals which unite for each birth, and which are highly locomotive, doubtful forms, ranked by one zoologist as a species and by another as a variety, can rarely be found within the same country, but are common in separated areas. how many of the birds and insects in north america and europe, which differ very slightly from each other, have been ranked by one eminent naturalist as undoubted species, and by another as varieties, or, as they are often called, geographical races! mr. wallace, in several valuable papers on the various animals, especially on the lepidoptera, inhabiting the islands of the great malayan archipelago, shows that they may be classed under four heads, namely, as variable forms, as local forms, as geographical races or sub-species, and as true representative species. the first or variable forms vary much within the limits of the same island. the local forms are moderately constant and distinct in each separate island; but when all from the several islands are compared together, the differences are seen to be so slight and graduated that it is impossible to define or describe them, though at the same time the extreme forms are sufficiently distinct. the geographical races or sub-species are local forms completely fixed and isolated; but as they do not differ from each other by strongly marked and important characters, "there is no possible test but individual opinion to determine which of them shall be considered as species and which as varieties." lastly, representative species fill the same place in the natural economy of each island as do the local forms and sub-species; but as they are distinguished from each other by a greater amount of difference than that between the local forms and sub-species, they are almost universally ranked by naturalists as true species. nevertheless, no certain criterion can possibly be given by which variable forms, local forms, sub species and representative species can be recognised. many years ago, when comparing, and seeing others compare, the birds from the closely neighbouring islands of the galapagos archipelago, one with another, and with those from the american mainland, i was much struck how entirely vague and arbitrary is the distinction between species and varieties. on the islets of the little madeira group there are many insects which are characterized as varieties in mr. wollaston's admirable work, but which would certainly be ranked as distinct species by many entomologists. even ireland has a few animals, now generally regarded as varieties, but which have been ranked as species by some zoologists. several experienced ornithologists consider our british red grouse as only a strongly marked race of a norwegian species, whereas the greater number rank it as an undoubted species peculiar to great britain. a wide distance between the homes of two doubtful forms leads many naturalists to rank them as distinct species; but what distance, it has been well asked, will suffice if that between america and europe is ample, will that between europe and the azores, or madeira, or the canaries, or between the several islets of these small archipelagos, be sufficient? mr. b.d. walsh, a distinguished entomologist of the united states, has described what he calls phytophagic varieties and phytophagic species. most vegetable-feeding insects live on one kind of plant or on one group of plants; some feed indiscriminately on many kinds, but do not in consequence vary. in several cases, however, insects found living on different plants, have been observed by mr. walsh to present in their larval or mature state, or in both states, slight, though constant differences in colour, size, or in the nature of their secretions. in some instances the males alone, in other instances, both males and females, have been observed thus to differ in a slight degree. when the differences are rather more strongly marked, and when both sexes and all ages are affected, the forms are ranked by all entomologists as good species. but no observer can determine for another, even if he can do so for himself, which of these phytophagic forms ought to be called species and which varieties. mr. walsh ranks the forms which it may be supposed would freely intercross, as varieties; and those which appear to have lost this power, as species. as the differences depend on the insects having long fed on distinct plants, it cannot be expected that intermediate links connecting the several forms should now be found. the naturalist thus loses his best guide in determining whether to rank doubtful forms as varieties or species. this likewise necessarily occurs with closely allied organisms, which inhabit distinct continents or islands. when, on the other hand, an animal or plant ranges over the same continent, or inhabits many islands in the same archipelago, and presents different forms in the different areas, there is always a good chance that intermediate forms will be discovered which will link together the extreme states; and these are then degraded to the rank of varieties. some few naturalists maintain that animals never present varieties; but then these same naturalists rank the slightest difference as of specific value; and when the same identical form is met with in two distant countries, or in two geological formations, they believe that two distinct species are hidden under the same dress. the term species thus comes to be a mere useless abstraction, implying and assuming a separate act of creation. it is certain that many forms, considered by highly competent judges to be varieties, resemble species so completely in character that they have been thus ranked by other highly competent judges. but to discuss whether they ought to be called species or varieties, before any definition of these terms has been generally accepted, is vainly to beat the air. many of the cases of strongly marked varieties or doubtful species well deserve consideration; for several interesting lines of argument, from geographical distribution, analogical variation, hybridism, etc., have been brought to bear in the attempt to determine their rank; but space does not here permit me to discuss them. close investigation, in many cases, will no doubt bring naturalists to agree how to rank doubtful forms. yet it must be confessed that it is in the best known countries that we find the greatest number of them. i have been struck with the fact that if any animal or plant in a state of nature be highly useful to man, or from any cause closely attracts his attention, varieties of it will almost universally be found recorded. these varieties, moreover, will often be ranked by some authors as species. look at the common oak, how closely it has been studied; yet a german author makes more than a dozen species out of forms, which are almost universally considered by other botanists to be varieties; and in this country the highest botanical authorities and practical men can be quoted to show that the sessile and pedunculated oaks are either good and distinct species or mere varieties. i may here allude to a remarkable memoir lately published by a. de candolle, on the oaks of the whole world. no one ever had more ample materials for the discrimination of the species, or could have worked on them with more zeal and sagacity. he first gives in detail all the many points of structure which vary in the several species, and estimates numerically the relative frequency of the variations. he specifies above a dozen characters which may be found varying even on the same branch, sometimes according to age or development, sometimes without any assignable reason. such characters are not of course of specific value, but they are, as asa gray has remarked in commenting on this memoir, such as generally enter into specific definitions. de candolle then goes on to say that he gives the rank of species to the forms that differ by characters never varying on the same tree, and never found connected by intermediate states. after this discussion, the result of so much labour, he emphatically remarks: "they are mistaken, who repeat that the greater part of our species are clearly limited, and that the doubtful species are in a feeble minority. this seemed to be true, so long as a genus was imperfectly known, and its species were founded upon a few specimens, that is to say, were provisional. just as we come to know them better, intermediate forms flow in, and doubts as to specific limits augment." he also adds that it is the best known species which present the greatest number of spontaneous varieties and sub-varieties. thus quercus robur has twenty-eight varieties, all of which, excepting six, are clustered round three sub-species, namely q. pedunculata, sessiliflora and pubescens. the forms which connect these three sub-species are comparatively rare; and, as asa gray again remarks, if these connecting forms which are now rare were to become totally extinct the three sub-species would hold exactly the same relation to each other as do the four or five provisionally admitted species which closely surround the typical quercus robur. finally, de candolle admits that out of the species, which will be enumerated in his prodromus as belonging to the oak family, at least two-thirds are provisional species, that is, are not known strictly to fulfil the definition above given of a true species. it should be added that de candolle no longer believes that species are immutable creations, but concludes that the derivative theory is the most natural one, "and the most accordant with the known facts in palaeontology, geographical botany and zoology, of anatomical structure and classification." when a young naturalist commences the study of a group of organisms quite unknown to him he is at first much perplexed in determining what differences to consider as specific and what as varietal; for he knows nothing of the amount and kind of variation to which the group is subject; and this shows, at least, how very generally there is some variation. but if he confine his attention to one class within one country he will soon make up his mind how to rank most of the doubtful forms. his general tendency will be to make many species, for he will become impressed, just like the pigeon or poultry fancier before alluded to, with the amount of difference in the forms which he is continually studying; and he has little general knowledge of analogical variation in other groups and in other countries by which to correct his first impressions. as he extends the range of his observations he will meet with more cases of difficulty; for he will encounter a greater number of closely-allied forms. but if his observations be widely extended he will in the end generally be able to make up his own mind; but he will succeed in this at the expense of admitting much variation, and the truth of this admission will often be disputed by other naturalists. when he comes to study allied forms brought from countries not now continuous, in which case he cannot hope to find intermediate links, he will be compelled to trust almost entirely to analogy, and his difficulties will rise to a climax. certainly no clear line of demarcation has as yet been drawn between species and sub-species--that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species; or, again, between sub-species and well-marked varieties, or between lesser varieties and individual differences. these differences blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage. hence i look at individual differences, though of small interest to the systematist, as of the highest importance for us, as being the first step towards such slight varieties as are barely thought worth recording in works on natural history. and i look at varieties which are in any degree more distinct and permanent, as steps toward more strongly marked and permanent varieties; and at the latter, as leading to sub-species, and then to species. the passage from one stage of difference to another may, in many cases, be the simple result of the nature of the organism and of the different physical conditions to which it has long been exposed; but with respect to the more important and adaptive characters, the passage from one stage of difference to another may be safely attributed to the cumulative action of natural selection, hereafter to be explained, and to the effects of the increased use or disuse of parts. a well-marked variety may therefore be called an incipient species; but whether this belief is justifiable must be judged by the weight of the various facts and considerations to be given throughout this work. it need not be supposed that all varieties or incipient species attain the rank of species. they may become extinct, or they may endure as varieties for very long periods, as has been shown to be the case by mr. wollaston with the varieties of certain fossil land-shells in madeira, and with plants by gaston de saporta. if a variety were to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the species as the variety; or it might come to supplant and exterminate the parent species; or both might co-exist, and both rank as independent species. but we shall hereafter return to this subject. from these remarks it will be seen that i look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. the term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience sake. wide-ranging, much diffused, and common species vary most. guided by theoretical considerations, i thought that some interesting results might be obtained in regard to the nature and relations of the species which vary most, by tabulating all the varieties in several well-worked floras. at first this seemed a simple task; but mr. h.c. watson, to whom i am much indebted for valuable advice and assistance on this subject, soon convinced me that there were many difficulties, as did subsequently dr. hooker, even in stronger terms. i shall reserve for a future work the discussion of these difficulties, and the tables of the proportional numbers of the varying species. dr. hooker permits me to add that after having carefully read my manuscript, and examined the tables, he thinks that the following statements are fairly well established. the whole subject, however, treated as it necessarily here is with much brevity, is rather perplexing, and allusions cannot be avoided to the "struggle for existence," "divergence of character," and other questions, hereafter to be discussed. alphonse de candolle and others have shown that plants which have very wide ranges generally present varieties; and this might have been expected, as they are exposed to diverse physical conditions, and as they come into competition (which, as we shall hereafter see, is a far more important circumstance) with different sets of organic beings. but my tables further show that, in any limited country, the species which are the most common, that is abound most in individuals, and the species which are most widely diffused within their own country (and this is a different consideration from wide range, and to a certain extent from commonness), oftenest give rise to varieties sufficiently well-marked to have been recorded in botanical works. hence it is the most flourishing, or, as they may be called, the dominant species--those which range widely, are the most diffused in their own country, and are the most numerous in individuals--which oftenest produce well-marked varieties, or, as i consider them, incipient species. and this, perhaps, might have been anticipated; for, as varieties, in order to become in any degree permanent, necessarily have to struggle with the other inhabitants of the country, the species which are already dominant will be the most likely to yield offspring, which, though in some slight degree modified, still inherit those advantages that enabled their parents to become dominant over their compatriots. in these remarks on predominence, it should be understood that reference is made only to the forms which come into competition with each other, and more especially to the members of the same genus or class having nearly similar habits of life. with respect to the number of individuals or commonness of species, the comparison of course relates only to the members of the same group. one of the higher plants may be said to be dominant if it be more numerous in individuals and more widely diffused than the other plants of the same country, which live under nearly the same conditions. a plant of this kind is not the less dominant because some conferva inhabiting the water or some parasitic fungus is infinitely more numerous in individuals, and more widely diffused. but if the conferva or parasitic fungus exceeds its allies in the above respects, it will then be dominant within its own class. species of the larger genera in each country vary more frequently than the species of the smaller genera. if the plants inhabiting a country as described in any flora, be divided into two equal masses, all those in the larger genera (i.e., those including many species) being placed on one side, and all those in the smaller genera on the other side, the former will be found to include a somewhat larger number of the very common and much diffused or dominant species. this might have been anticipated, for the mere fact of many species of the same genus inhabiting any country, shows that there is something in the organic or inorganic conditions of that country favourable to the genus; and, consequently, we might have expected to have found in the larger genera, or those including many species, a larger proportional number of dominant species. but so many causes tend to obscure this result, that i am surprised that my tables show even a small majority on the side of the larger genera. i will here allude to only two causes of obscurity. fresh water and salt-loving plants generally have very wide ranges and are much diffused, but this seems to be connected with the nature of the stations inhabited by them, and has little or no relation to the size of the genera to which the species belong. again, plants low in the scale of organisation are generally much more widely diffused than plants higher in the scale; and here again there is no close relation to the size of the genera. the cause of lowly-organised plants ranging widely will be discussed in our chapter on geographical distribution. from looking at species as only strongly marked and well-defined varieties, i was led to anticipate that the species of the larger genera in each country would oftener present varieties, than the species of the smaller genera; for wherever many closely related species (i.e., species of the same genus) have been formed, many varieties or incipient species ought, as a general rule, to be now forming. where many large trees grow, we expect to find saplings. where many species of a genus have been formed through variation, circumstances have been favourable for variation; and hence we might expect that the circumstances would generally still be favourable to variation. on the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few. to test the truth of this anticipation i have arranged the plants of twelve countries, and the coleopterous insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the species on the side of the larger genera presented varieties, than on the side of the smaller genera. moreover, the species of the large genera which present any varieties, invariably present a larger average number of varieties than do the species of the small genera. both these results follow when another division is made, and when all the least genera, with from only one to four species, are altogether excluded from the tables. these facts are of plain signification on the view that species are only strongly marked and permanent varieties; for wherever many species of the same genus have been formed, or where, if we may use the expression, the manufactory of species has been active, we ought generally to find the manufactory still in action, more especially as we have every reason to believe the process of manufacturing new species to be a slow one. and this certainly holds true if varieties be looked at as incipient species; for my tables clearly show, as a general rule, that, wherever many species of a genus have been formed, the species of that genus present a number of varieties, that is, of incipient species, beyond the average. it is not that all large genera are now varying much, and are thus increasing in the number of their species, or that no small genera are now varying and increasing; for if this had been so, it would have been fatal to my theory; inasmuch as geology plainly tells us that small genera have in the lapse of time often increased greatly in size; and that large genera have often come to their maxima, declined, and disappeared. all that we want to show is, that where many species of a genus have been formed, on an average many are still forming; and this certainly holds good. many of the species included within the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges. there are other relations between the species of large genera and their recorded varieties which deserve notice. we have seen that there is no infallible criterion by which to distinguish species and well-marked varieties; and when intermediate links have not been found between doubtful forms, naturalists are compelled to come to a determination by the amount of difference between them, judging by analogy whether or not the amount suffices to raise one or both to the rank of species. hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties. now fries has remarked in regard to plants, and westwood in regard to insects, that in large genera the amount of difference between the species is often exceedingly small. i have endeavoured to test this numerically by averages, and, as far as my imperfect results go, they confirm the view. i have also consulted some sagacious and experienced observers, and, after deliberation, they concur in this view. in this respect, therefore, the species of the larger genera resemble varieties, more than do the species of the smaller genera. or the case may be put in another way, and it may be said, that in the larger genera, in which a number of varieties or incipient species greater than the average are now manufacturing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by a less than the usual amount of difference. moreover, the species of the larger genera are related to each other, in the same manner as the varieties of any one species are related to each other. no naturalist pretends that all the species of a genus are equally distinct from each other; they may generally be divided into sub-genera, or sections, or lesser groups. as fries has well remarked, little groups of species are generally clustered like satellites around other species. and what are varieties but groups of forms, unequally related to each other, and clustered round certain forms--that is, round their parent-species. undoubtedly there is one most important point of difference between varieties and species, namely, that the amount of difference between varieties, when compared with each other or with their parent-species, is much less than that between the species of the same genus. but when we come to discuss the principle, as i call it, of divergence of character, we shall see how this may be explained, and how the lesser differences between varieties tend to increase into the greater differences between species. there is one other point which is worth notice. varieties generally have much restricted ranges. this statement is indeed scarcely more than a truism, for if a variety were found to have a wider range than that of its supposed parent-species, their denominations would be reversed. but there is reason to believe that the species which are very closely allied to other species, and in so far resemble varieties, often have much restricted ranges. for instance, mr. h.c. watson has marked for me in the well-sifted london catalogue of plants ( th edition) sixty-three plants which are therein ranked as species, but which he considers as so closely allied to other species as to be of doubtful value: these sixty-three reputed species range on an average over . of the provinces into which mr. watson has divided great britain. now, in this same catalogue, fifty-three acknowledged varieties are recorded, and these range over . provinces; whereas, the species to which these varieties belong range over . provinces. so that the acknowledged varieties have very nearly the same restricted average range, as have the closely allied forms, marked for me by mr. watson as doubtful species, but which are almost universally ranked by british botanists as good and true species. summary. finally, varieties cannot be distinguished from species--except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them; for two forms, if differing very little, are generally ranked as varieties, notwithstanding that they cannot be closely connected; but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined. in genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties. in large genera the species are apt to be closely but unequally allied together, forming little clusters round other species. species very closely allied to other species apparently have restricted ranges. in all these respects the species of large genera present a strong analogy with varieties. and we can clearly understand these analogies, if species once existed as varieties, and thus originated; whereas, these analogies are utterly inexplicable if species are independent creations. we have also seen that it is the most flourishing or dominant species of the larger genera within each class which on an average yield the greatest number of varieties, and varieties, as we shall hereafter see, tend to become converted into new and distinct species. thus the larger genera tend to become larger; and throughout nature the forms of life which are now dominant tend to become still more dominant by leaving many modified and dominant descendants. but, by steps hereafter to be explained, the larger genera also tend to break up into smaller genera. and thus, the forms of life throughout the universe become divided into groups subordinate to groups. chapter iii. struggle for existence. its bearing on natural selection--the term used in a wide sense--geometrical ratio of increase--rapid increase of naturalised animals and plants--nature of the checks to increase--competition universal--effects of climate--protection from the number of individuals--complex relations of all animals and plants throughout nature--struggle for life most severe between individuals and varieties of the same species: often severe between species of the same genus--the relation of organism to organism the most important of all relations. before entering on the subject of this chapter i must make a few preliminary remarks to show how the struggle for existence bears on natural selection. it has been seen in the last chapter that among organic beings in a state of nature there is some individual variability: indeed i am not aware that this has ever been disputed. it is immaterial for us whether a multitude of doubtful forms be called species or sub-species or varieties; what rank, for instance, the two or three hundred doubtful forms of british plants are entitled to hold, if the existence of any well-marked varieties be admitted. but the mere existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in nature. how have all those exquisite adaptations of one part of the organisation to another part, and to the conditions of life and of one organic being to another being, been perfected? we see these beautiful co-adaptations most plainly in the woodpecker and the mistletoe; and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world. again, it may be asked, how is it that varieties, which i have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? how do those groups of species, which constitute what are called distinct genera and which differ from each other more than do the species of the same genus, arise? all these results, as we shall more fully see in the next chapter, follow from the struggle for life. owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. the offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. i have called this principle, by which each slight variation, if useful, is preserved, by the term natural selection, in order to mark its relation to man's power of selection. but the expression often used by mr. herbert spencer, of the survival of the fittest, is more accurate, and is sometimes equally convenient. we have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of nature. but natural selection, we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man's feeble efforts, as the works of nature are to those of art. we will now discuss in a little more detail the struggle for existence. in my future work this subject will be treated, as it well deserves, at greater length. the elder de candolle and lyell have largely and philosophically shown that all organic beings are exposed to severe competition. in regard to plants, no one has treated this subject with more spirit and ability than w. herbert, dean of manchester, evidently the result of his great horticultural knowledge. nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult--at least i found it so--than constantly to bear this conclusion in mind. yet unless it be thoroughly engrained in the mind, the whole economy of nature, with every fact on distribution, rarity, abundance, extinction, and variation, will be dimly seen or quite misunderstood. we behold the face of nature bright with gladness, we often see superabundance of food; we do not see or we forget that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that, though food may be now superabundant, it is not so at all seasons of each recurring year. the term, struggle for existence, used in a large sense. i should premise that i use this term in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. two canine animals, in a time of dearth, may be truly said to struggle with each other which shall get food and live. but a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. a plant which annually produces a thousand seeds, of which only one of an average comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. the mistletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it languishes and dies. but several seedling mistletoes, growing close together on the same branch, may more truly be said to struggle with each other. as the mistletoe is disseminated by birds, its existence depends on them; and it may metaphorically be said to struggle with other fruit-bearing plants, in tempting the birds to devour and thus disseminate its seeds. in these several senses, which pass into each other, i use for convenience sake the general term of struggle for existence. geometrical ratio of increase. a struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. it is the doctrine of malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them. there is no exception to the rule that every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair. even slow-breeding man has doubled in twenty-five years, and at this rate, in less than a thousand years, there would literally not be standing room for his progeny. linnaeus has calculated that if an annual plant produced only two seeds--and there is no plant so unproductive as this--and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. the elephant is reckoned the slowest breeder of all known animals, and i have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from to years there would be nearly nineteen million elephants alive descended from the first pair. but we have better evidence on this subject than mere theoretical calculations, namely, the numerous recorded cases of the astonishingly rapid increase of various animals in a state of nature, when circumstances have been favourable to them during two or three following seasons. still more striking is the evidence from our domestic animals of many kinds which have run wild in several parts of the world; if the statements of the rate of increase of slow-breeding cattle and horses in south america, and latterly in australia, had not been well authenticated, they would have been incredible. so it is with plants; cases could be given of introduced plants which have become common throughout whole islands in a period of less than ten years. several of the plants, such as the cardoon and a tall thistle, which are now the commonest over the wide plains of la plata, clothing square leagues of surface almost to the exclusion of every other plant, have been introduced from europe; and there are plants which now range in india, as i hear from dr. falconer, from cape comorin to the himalaya, which have been imported from america since its discovery. in such cases, and endless others could be given, no one supposes that the fertility of the animals or plants has been suddenly and temporarily increased in any sensible degree. the obvious explanation is that the conditions of life have been highly favourable, and that there has consequently been less destruction of the old and young and that nearly all the young have been enabled to breed. their geometrical ratio of increase, the result of which never fails to be surprising, simply explains their extraordinarily rapid increase and wide diffusion in their new homes. in a state of nature almost every full-grown plant annually produces seed, and among animals there are very few which do not annually pair. hence we may confidently assert that all plants and animals are tending to increase at a geometrical ratio--that all would rapidly stock every station in which they could any how exist, and that this geometrical tendency to increase must be checked by destruction at some period of life. our familiarity with the larger domestic animals tends, i think, to mislead us; we see no great destruction falling on them, and we do not keep in mind that thousands are annually slaughtered for food, and that in a state of nature an equal number would have somehow to be disposed of. the only difference between organisms which annually produce eggs or seeds by the thousand, and those which produce extremely few, is, that the slow breeders would require a few more years to people, under favourable conditions, a whole district, let it be ever so large. the condor lays a couple of eggs and the ostrich a score, and yet in the same country the condor may be the more numerous of the two. the fulmar petrel lays but one egg, yet it is believed to be the most numerous bird in the world. one fly deposits hundreds of eggs, and another, like the hippobosca, a single one. but this difference does not determine how many individuals of the two species can be supported in a district. a large number of eggs is of some importance to those species which depend on a fluctuating amount of food, for it allows them rapidly to increase in number. but the real importance of a large number of eggs or seeds is to make up for much destruction at some period of life; and this period in the great majority of cases is an early one. if an animal can in any way protect its own eggs or young, a small number may be produced, and yet the average stock be fully kept up; but if many eggs or young are destroyed, many must be produced or the species will become extinct. it would suffice to keep up the full number of a tree, which lived on an average for a thousand years, if a single seed were produced once in a thousand years, supposing that this seed were never destroyed and could be ensured to germinate in a fitting place; so that, in all cases, the average number of any animal or plant depends only indirectly on the number of its eggs or seeds. in looking at nature, it is most necessary to keep the foregoing considerations always in mind--never to forget that every single organic being may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old during each generation or at recurrent intervals. lighten any check, mitigate the destruction ever so little, and the number of the species will almost instantaneously increase to any amount. nature of the checks to increase. the causes which check the natural tendency of each species to increase are most obscure. look at the most vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still further. we know not exactly what the checks are even in a single instance. nor will this surprise any one who reflects how ignorant we are on this head, even in regard to mankind, although so incomparably better known than any other animal. this subject of the checks to increase has been ably treated by several authors, and i hope in a future work to discuss it at considerable length, more especially in regard to the feral animals of south america. here i will make only a few remarks, just to recall to the reader's mind some of the chief points. eggs or very young animals seem generally to suffer most, but this is not invariably the case. with plants there is a vast destruction of seeds, but from some observations which i have made it appears that the seedlings suffer most from germinating in ground already thickly stocked with other plants. seedlings, also, are destroyed in vast numbers by various enemies; for instance, on a piece of ground three feet long and two wide, dug and cleared, and where there could be no choking from other plants, i marked all the seedlings of our native weeds as they came up, and out of no less than were destroyed, chiefly by slugs and insects. if turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown plants; thus out of twenty species grown on a little plot of mown turf (three feet by four) nine species perished, from the other species being allowed to grow up freely. the amount of food for each species, of course, gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average number of a species. thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin. if not one head of game were shot during the next twenty years in england, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually shot. on the other hand, in some cases, as with the elephant, none are destroyed by beasts of prey; for even the tiger in india most rarely dares to attack a young elephant protected by its dam. climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought seem to be the most effective of all checks. i estimated (chiefly from the greatly reduced numbers of nests in the spring) that the winter of - destroyed four-fifths of the birds in my own grounds; and this is a tremendous destruction, when we remember that ten per cent. is an extraordinarily severe mortality from epidemics with man. the action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food. even when climate, for instance, extreme cold, acts directly, it will be the least vigorous individuals, or those which have got least food through the advancing winter, which will suffer the most. when we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing; and the change of climate being conspicuous, we are tempted to attribute the whole effect to its direct action. but this is a false view; we forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers; and as each area is already fully stocked with inhabitants, the other species must decrease. when we travel southward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in other species being favoured, as in this one being hurt. so it is when we travel northward, but in a somewhat lesser degree, for the number of species of all kinds, and therefore of competitors, decreases northward; hence in going northward, or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding southward or in descending a mountain. when we reach the arctic regions, or snow-capped summits, or absolute deserts, the struggle for life is almost exclusively with the elements. that climate acts in main part indirectly by favouring other species we clearly see in the prodigious number of plants which in our gardens can perfectly well endure our climate, but which never become naturalised, for they cannot compete with our native plants nor resist destruction by our native animals. when a species, owing to highly favourable circumstances, increases inordinately in numbers in a small tract, epidemics--at least, this seems generally to occur with our game animals--often ensue; and here we have a limiting check independent of the struggle for life. but even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through facility of diffusion among the crowded animals, been disproportionally favoured: and here comes in a sort of struggle between the parasite and its prey. on the other hand, in many cases, a large stock of individuals of the same species, relatively to the numbers of its enemies, is absolutely necessary for its preservation. thus we can easily raise plenty of corn and rape-seed, etc., in our fields, because the seeds are in great excess compared with the number of birds which feed on them; nor can the birds, though having a superabundance of food at this one season, increase in number proportionally to the supply of seed, as their numbers are checked during the winter; but any one who has tried knows how troublesome it is to get seed from a few wheat or other such plants in a garden; i have in this case lost every single seed. this view of the necessity of a large stock of the same species for its preservation, explains, i believe, some singular facts in nature such as that of very rare plants being sometimes extremely abundant, in the few spots where they do exist; and that of some social plants being social, that is abounding in individuals, even on the extreme verge of their range. for in such cases, we may believe, that a plant could exist only where the conditions of its life were so favourable that many could exist together, and thus save the species from utter destruction. i should add that the good effects of intercrossing, and the ill effects of close interbreeding, no doubt come into play in many of these cases; but i will not here enlarge on this subject. complex relations of all animals and plants to each other in the struggle for existence. many cases are on record showing how complex and unexpected are the checks and relations between organic beings, which have to struggle together in the same country. i will give only a single instance, which, though a simple one, interested me. in staffordshire, on the estate of a relation, where i had ample means of investigation, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five years previously and planted with scotch fir. the change in the native vegetation of the planted part of the heath was most remarkable, more than is generally seen in passing from one quite different soil to another: not only the proportional numbers of the heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath. the effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the heath; and the heath was frequented by two or three distinct insectivorous birds. here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception of the land having been enclosed, so that cattle could not enter. but how important an element enclosure is, i plainly saw near farnham, in surrey. here there are extensive heaths, with a few clumps of old scotch firs on the distant hill-tops: within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. when i ascertained that these young trees had not been sown or planted i was so much surprised at their numbers that i went to several points of view, whence i could examine hundreds of acres of the unenclosed heath, and literally i could not see a single scotch fir, except the old planted clumps. but on looking closely between the stems of the heath, i found a multitude of seedlings and little trees, which had been perpetually browsed down by the cattle. in one square yard, at a point some hundred yards distant from one of the old clumps, i counted thirty-two little trees; and one of them, with twenty-six rings of growth, had, during many years tried to raise its head above the stems of the heath, and had failed. no wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs. yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually searched it for food. here we see that cattle absolutely determine the existence of the scotch fir; but in several parts of the world insects determine the existence of cattle. perhaps paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state; and azara and rengger have shown that this is caused by the greater number in paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. the increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. hence, if certain insectivorous birds were to decrease in paraguay, the parasitic insects would probably increase; and this would lessen the number of the navel-frequenting flies--then cattle and horses would become feral, and this would certainly greatly alter (as indeed i have observed in parts of south america) the vegetation: this again would largely affect the insects; and this, as we have just seen in staffordshire, the insectivorous birds, and so onwards in ever-increasing circles of complexity. not that under nature the relations will ever be as simple as this. battle within battle must be continually recurring with varying success; and yet in the long-run the forces are so nicely balanced that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would give the victory to one organic being over another. nevertheless, so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life! i am tempted to give one more instance showing how plants and animals, remote in the scale of nature, are bound together by a web of complex relations. i shall hereafter have occasion to show that the exotic lobelia fulgens is never visited in my garden by insects, and consequently, from its peculiar structure, never sets a seed. nearly all our orchidaceous plants absolutely require the visits of insects to remove their pollen-masses and thus to fertilise them. i find from experiments that humble-bees are almost indispensable to the fertilisation of the heartsease (viola tricolor), for other bees do not visit this flower. i have also found that the visits of bees are necessary for the fertilisation of some kinds of clover; for instance twenty heads of dutch clover (trifolium repens) yielded , seeds, but twenty other heads, protected from bees, produced not one. again, heads of red clover (t. pratense) produced , seeds, but the same number of protected heads produced not a single seed. humble bees alone visit red clover, as other bees cannot reach the nectar. it has been suggested that moths may fertilise the clovers; but i doubt whether they could do so in the case of the red clover, from their weight not being sufficient to depress the wing petals. hence we may infer as highly probable that, if the whole genus of humble-bees became extinct or very rare in england, the heartsease and red clover would become very rare, or wholly disappear. the number of humble-bees in any district depends in a great measure upon the number of field-mice, which destroy their combs and nests; and colonel newman, who has long attended to the habits of humble-bees, believes that "more than two-thirds of them are thus destroyed all over england." now the number of mice is largely dependent, as every one knows, on the number of cats; and colonel newman says, "near villages and small towns i have found the nests of humble-bees more numerous than elsewhere, which i attribute to the number of cats that destroy the mice." hence it is quite credible that the presence of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district! in the case of every species, many different checks, acting at different periods of life, and during different seasons or years, probably come into play; some one check or some few being generally the most potent, but all will concur in determining the average number, or even the existence of the species. in some cases it can be shown that widely-different checks act on the same species in different districts. when we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. but how false a view is this! every one has heard that when an american forest is cut down, a very different vegetation springs up; but it has been observed that ancient indian ruins in the southern united states, which must formerly have been cleared of trees, now display the same beautiful diversity and proportion of kinds as in the surrounding virgin forests. what a struggle must have gone on during long centuries between the several kinds of trees, each annually scattering its seeds by the thousand; what war between insect and insect--between insects, snails, and other animals with birds and beasts of prey--all striving to increase, all feeding on each other, or on the trees, their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees. throw up a handful of feathers, and all fall to the ground according to definite laws; but how simple is the problem where each shall fall compared to that of the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the proportional numbers and kinds of trees now growing on the old indian ruins! the dependency of one organic being on another, as of a parasite on its prey, lies generally between beings remote in the scale of nature. this is likewise sometimes the case with those which may strictly be said to struggle with each other for existence, as in the case of locusts and grass-feeding quadrupeds. but the struggle will almost invariably be most severe between the individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers. in the case of varieties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon decided: for instance, if several varieties of wheat be sown together, and the mixed seed be resown, some of the varieties which best suit the soil or climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years supplant the other varieties. to keep up a mixed stock of even such extremely close varieties as the variously coloured sweet-peas, they must be each year harvested separately, and the seed then mixed in due proportion, otherwise the weaker kinds will steadily decrease in number and disappear. so again with the varieties of sheep: it has been asserted that certain mountain-varieties will starve out other mountain-varieties, so that they cannot be kept together. the same result has followed from keeping together different varieties of the medicinal leech. it may even be doubted whether the varieties of any of our domestic plants or animals have so exactly the same strength, habits, and constitution, that the original proportions of a mixed stock (crossing being prevented) could be kept up for half-a-dozen generations, if they were allowed to struggle together, in the same manner as beings in a state of nature, and if the seed or young were not annually preserved in due proportion. struggle for life most severe between individuals and varieties of the same species. as the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera. we see this in the recent extension over parts of the united states of one species of swallow having caused the decrease of another species. the recent increase of the missel-thrush in parts of scotland has caused the decrease of the song-thrush. how frequently we hear of one species of rat taking the place of another species under the most different climates! in russia the small asiatic cockroach has everywhere driven before it its great congener. in australia the imported hive-bee is rapidly exterminating the small, stingless native bee. one species of charlock has been known to supplant another species; and so in other cases. we can dimly see why the competition should be most severe between allied forms, which fill nearly the same place in the economy of nature; but probably in no one case could we precisely say why one species has been victorious over another in the great battle of life. a corollary of the highest importance may be deduced from the foregoing remarks, namely, that the structure of every organic being is related, in the most essential yet often hidden manner, to that of all other organic beings, with which it comes into competition for food or residence, or from which it has to escape, or on which it preys. this is obvious in the structure of the teeth and talons of the tiger; and in that of the legs and claws of the parasite which clings to the hair on the tiger's body. but in the beautifully plumed seed of the dandelion, and in the flattened and fringed legs of the water-beetle, the relation seems at first confined to the elements of air and water. yet the advantage of the plumed seeds no doubt stands in the closest relation to the land being already thickly clothed with other plants; so that the seeds may be widely distributed and fall on unoccupied ground. in the water-beetle, the structure of its legs, so well adapted for diving, allows it to compete with other aquatic insects, to hunt for its own prey, and to escape serving as prey to other animals. the store of nutriment laid up within the seeds of many plants seems at first sight to have no sort of relation to other plants. but from the strong growth of young plants produced from such seeds, as peas and beans, when sown in the midst of long grass, it may be suspected that the chief use of the nutriment in the seed is to favour the growth of the seedlings, whilst struggling with other plants growing vigorously all around. look at a plant in the midst of its range! why does it not double or quadruple its numbers? we know that it can perfectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges into slightly hotter or colder, damper or drier districts. in this case we can clearly see that if we wish in imagination to give the plant the power of increasing in numbers, we should have to give it some advantage over its competitors, or over the animals which prey on it. on the confines of its geographical range, a change of constitution with respect to climate would clearly be an advantage to our plant; but we have reason to believe that only a few plants or animals range so far, that they are destroyed exclusively by the rigour of the climate. not until we reach the extreme confines of life, in the arctic regions or on the borders of an utter desert, will competition cease. the land may be extremely cold or dry, yet there will be competition between some few species, or between the individuals of the same species, for the warmest or dampest spots. hence we can see that when a plant or animal is placed in a new country, among new competitors, the conditions of its life will generally be changed in an essential manner, although the climate may be exactly the same as in its former home. if its average numbers are to increase in its new home, we should have to modify it in a different way to what we should have had to do in its native country; for we should have to give it some advantage over a different set of competitors or enemies. it is good thus to try in imagination to give any one species an advantage over another. probably in no single instance should we know what to do. this ought to convince us of our ignorance on the mutual relations of all organic beings; a conviction as necessary, as it is difficult to acquire. all that we can do is to keep steadily in mind that each organic being is striving to increase in a geometrical ratio; that each, at some period of its life, during some season of the year, during each generation, or at intervals, has to struggle for life and to suffer great destruction. when we reflect on this struggle we may console ourselves with the full belief that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply. chapter iv. natural selection; or the survival of the fittest. natural selection--its power compared with man's selection--its power on characters of trifling importance--its power at all ages and on both sexes--sexual selection--on the generality of intercrosses between individuals of the same species--circumstances favourable and unfavourable to the results of natural selection, namely, intercrossing, isolation, number of individuals--slow action--extinction caused by natural selection--divergence of character, related to the diversity of inhabitants of any small area and to naturalisation--action of natural selection, through divergence of character and extinction, on the descendants from a common parent--explains the grouping of all organic beings--advance in organisation--low forms preserved--convergence of character--indefinite multiplication of species--summary. how will the struggle for existence, briefly discussed in the last chapter, act in regard to variation? can the principle of selection, which we have seen is so potent in the hands of man, apply under nature? i think we shall see that it can act most efficiently. let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary tendency. under domestication, it may truly be said that the whole organisation becomes in some degree plastic. but the variability, which we almost universally meet with in our domestic productions is not directly produced, as hooker and asa gray have well remarked, by man; he can neither originate varieties nor prevent their occurrence; he can only preserve and accumulate such as do occur. unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues; but similar changes of conditions might and do occur under nature. let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life. can it then be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations? if such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? on the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. this preservation of favourable individual differences and variations, and the destruction of those which are injurious, i have called natural selection, or the survival of the fittest. variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions. several writers have misapprehended or objected to the term natural selection. some have even imagined that natural selection induces variability, whereas it implies only the preservation of such variations as arise and are beneficial to the being under its conditions of life. no one objects to agriculturists speaking of the potent effects of man's selection; and in this case the individual differences given by nature, which man for some object selects, must of necessity first occur. others have objected that the term selection implies conscious choice in the animals which become modified; and it has even been urged that, as plants have no volition, natural selection is not applicable to them! in the literal sense of the word, no doubt, natural selection is a false term; but who ever objected to chemists speaking of the elective affinities of the various elements?--and yet an acid cannot strictly be said to elect the base with which it in preference combines. it has been said that i speak of natural selection as an active power or deity; but who objects to an author speaking of the attraction of gravity as ruling the movements of the planets? every one knows what is meant and is implied by such metaphorical expressions; and they are almost necessary for brevity. so again it is difficult to avoid personifying the word nature; but i mean by nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us. with a little familiarity such superficial objections will be forgotten. we shall best understand the probable course of natural selection by taking the case of a country undergoing some slight physical change, for instance, of climate. the proportional numbers of its inhabitants will almost immediately undergo a change, and some species will probably become extinct. we may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independently of the change of climate itself, would seriously affect the others. if the country were open on its borders, new forms would certainly immigrate, and this would likewise seriously disturb the relations of some of the former inhabitants. let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. but in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up if some of the original inhabitants were in some manner modified; for, had the area been open to immigration, these same places would have been seized on by intruders. in such cases, slight modifications, which in any way favoured the individuals of any species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would have free scope for the work of improvement. we have good reason to believe, as shown in the first chapter, that changes in the conditions of life give a tendency to increased variability; and in the foregoing cases the conditions the changed, and this would manifestly be favourable to natural selection, by affording a better chance of the occurrence of profitable variations. unless such occur, natural selection can do nothing. under the term of "variations," it must never be forgotten that mere individual differences are included. as man can produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily from having incomparably longer time for action. nor do i believe that any great physical change, as of climate, or any unusual degree of isolation, to check immigration, is necessary in order that new and unoccupied places should be left for natural selection to fill up by improving some of the varying inhabitants. for as all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modifications in the structure or habits of one species would often give it an advantage over others; and still further modifications of the same kind would often still further increase the advantage, as long as the species continued under the same conditions of life and profited by similar means of subsistence and defence. no country can be named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could be still better adapted or improved; for in all countries, the natives have been so far conquered by naturalised productions that they have allowed some foreigners to take firm possession of the land. and as foreigners have thus in every country beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted the intruders. as man can produce, and certainly has produced, a great result by his methodical and unconscious means of selection, what may not natural selection effect? man can act only on external and visible characters: nature, if i may be allowed to personify the natural preservation or survival of the fittest, cares nothing for appearances, except in so far as they are useful to any being. she can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. man selects only for his own good; nature only for that of the being which she tends. every selected character is fully exercised by her, as is implied by the fact of their selection. man keeps the natives of many climates in the same country. he seldom exercises each selected character in some peculiar and fitting manner; he feeds a long and a short-beaked pigeon on the same food; he does not exercise a long-backed or long-legged quadruped in any peculiar manner; he exposes sheep with long and short wool to the same climate; does not allow the most vigorous males to struggle for the females; he does not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his productions. he often begins his selection by some half-monstrous form, or at least by some modification prominent enough to catch the eye or to be plainly useful to him. under nature, the slightest differences of structure or constitution may well turn the nicely-balanced scale in the struggle for life, and so be preserved. how fleeting are the wishes and efforts of man! how short his time, and consequently how poor will be his results, compared with those accumulated by nature during whole geological periods! can we wonder, then, that nature's productions should be far "truer" in character than man's productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship? it may metaphorically be said that natural selection is daily and hourly scrutinising, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. we see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long-past geological ages that we see only that the forms of life are now different from what they formerly were. in order that any great amount of modification should be effected in a species, a variety, when once formed must again, perhaps after a long interval of time, vary or present individual differences of the same favourable nature as before; and these must again be preserved, and so onward, step by step. seeing that individual differences of the same kind perpetually recur, this can hardly be considered as an unwarrantable assumption. but whether it is true, we can judge only by seeing how far the hypothesis accords with and explains the general phenomena of nature. on the other hand, the ordinary belief that the amount of possible variation is a strictly limited quantity, is likewise a simple assumption. although natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance, may thus be acted on. when we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather, we must believe that these tints are of service to these birds and insects in preserving them from danger. grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey,--so much so that on parts of the continent persons are warned not to keep white pigeons, as being the most liable to destruction. hence natural selection might be effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant. nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect; we should remember how essential it is in a flock of white sheep to destroy a lamb with the faintest trace of black. we have seen how the colour of hogs, which feed on the "paint-root" in virginia, determines whether they shall live or die. in plants, the down on the fruit and the colour of the flesh are considered by botanists as characters of the most trifling importance; yet we hear from an excellent horticulturist, downing, that in the united states smooth-skinned fruits suffer far more from a beetle, a curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums; whereas another disease attacks yellow-fleshed peaches far more than those with other coloured flesh. if, with all the aids of art, these slight differences make a great difference in cultivating the several varieties, assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of enemies, such differences would effectually settle which variety, whether a smooth or downy, a yellow or a purple-fleshed fruit, should succeed. in looking at many small points of difference between species, which, as far as our ignorance permits us to judge, seem quite unimportant, we must not forget that climate, food, etc., have no doubt produced some direct effect. it is also necessary to bear in mind that, owing to the law of correlation, when one part varies and the variations are accumulated through natural selection, other modifications, often of the most unexpected nature, will ensue. as we see that those variations which, under domestication, appear at any particular period of life, tend to reappear in the offspring at the same period; for instance, in the shape, size and flavour of the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult; so in a state of nature natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations profitable at that age, and by their inheritance at a corresponding age. if it profit a plant to have its seeds more and more widely disseminated by the wind, i can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees. natural selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern the mature insect; and these modifications may affect, through correlation, the structure of the adult. so, conversely, modifications in the adult may affect the structure of the larva; but in all cases natural selection will ensure that they shall not be injurious: for if they were so, the species would become extinct. natural selection will modify the structure of the young in relation to the parent and of the parent in relation to the young. in social animals it will adapt the structure of each individual for the benefit of the whole community; if the community profits by the selected change. what natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another species; and though statements to this effect may be found in works of natural history, i cannot find one case which will bear investigation. a structure used only once in an animal's life, if of high importance to it, might be modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, used exclusively for opening the cocoon--or the hard tip to the beak of unhatched birds, used for breaking the eggs. it has been asserted, that of the best short-beaked tumbler-pigeons a greater number perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching. now, if nature had to make the beak of a full-grown pigeon very short for the bird's own advantage, the process of modification would be very slow, and there would be simultaneously the most rigorous selection of all the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish: or, more delicate and more easily broken shells might be selected, the thickness of the shell being known to vary like every other structure. it may be well here to remark that with all beings there must be much fortuitous destruction, which can have little or no influence on the course of natural selection. for instance, a vast number of eggs or seeds are annually devoured, and these could be modified through natural selection only if they varied in some manner which protected them from their enemies. yet many of these eggs or seeds would perhaps, if not destroyed, have yielded individuals better adapted to their conditions of life than any of those which happened to survive. so again a vast number of mature animals and plants, whether or not they be the best adapted to their conditions, must be annually destroyed by accidental causes, which would not be in the least degree mitigated by certain changes of structure or constitution which would in other ways be beneficial to the species. but let the destruction of the adults be ever so heavy, if the number which can exist in any district be not wholly kept down by such causes--or again let the destruction of eggs or seeds be so great that only a hundredth or a thousandth part are developed--yet of those which do survive, the best adapted individuals, supposing that there is any variability in a favourable direction, will tend to propagate their kind in larger numbers than the less well adapted. if the numbers be wholly kept down by the causes just indicated, as will often have been the case, natural selection will be powerless in certain beneficial directions; but this is no valid objection to its efficiency at other times and in other ways; for we are far from having any reason to suppose that many species ever undergo modification and improvement at the same time in the same area. sexual selection. inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, so no doubt it will be under nature. thus it is rendered possible for the two sexes to be modified through natural selection in relation to different habits of life, as is sometimes the case; or for one sex to be modified in relation to the other sex, as commonly occurs. this leads me to say a few words on what i have called sexual selection. this form of selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. the result is not death to the unsuccessful competitor, but few or no offspring. sexual selection is, therefore, less rigorous than natural selection. generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. but in many cases victory depends not so much on general vigour, but on having special weapons, confined to the male sex. a hornless stag or spurless cock would have a poor chance of leaving numerous offspring. sexual selection, by always allowing the victor to breed, might surely give indomitable courage, length of spur, and strength to the wing to strike in the spurred leg, in nearly the same manner as does the brutal cockfighter by the careful selection of his best cocks. how low in the scale of nature the law of battle descends i know not; male alligators have been described as fighting, bellowing, and whirling round, like indians in a war-dance, for the possession of the females; male salmons have been observed fighting all day long; male stag-beetles sometimes bear wounds from the huge mandibles of other males; the males of certain hymenopterous insects have been frequently seen by that inimitable observer m. fabre, fighting for a particular female who sits by, an apparently unconcerned beholder of the struggle, and then retires with the conqueror. the war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons. the males of carnivorous animals are already well armed; though to them and to others, special means of defence may be given through means of sexual selection, as the mane of the lion, and the hooked jaw to the male salmon; for the shield may be as important for victory as the sword or spear. among birds, the contest is often of a more peaceful character. all those who have attended to the subject, believe that there is the severest rivalry between the males of many species to attract, by singing, the females. the rock-thrush of guiana, birds of paradise, and some others, congregate, and successive males display with the most elaborate care, and show off in the best manner, their gorgeous plumage; they likewise perform strange antics before the females, which, standing by as spectators, at last choose the most attractive partner. those who have closely attended to birds in confinement well know that they often take individual preferences and dislikes: thus sir r. heron has described how a pied peacock was eminently attractive to all his hen birds. i cannot here enter on the necessary details; but if man can in a short time give beauty and an elegant carriage to his bantams, according to his standard of beauty, i can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect. some well-known laws, with respect to the plumage of male and female birds, in comparison with the plumage of the young, can partly be explained through the action of sexual selection on variations occurring at different ages, and transmitted to the males alone or to both sexes at corresponding ages; but i have not space here to enter on this subject. thus it is, as i believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection: that is, by individual males having had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms; which they have transmitted to their male offspring alone. yet, i would not wish to attribute all sexual differences to this agency: for we see in our domestic animals peculiarities arising and becoming attached to the male sex, which apparently have not been augmented through selection by man. the tuft of hair on the breast of the wild turkey-cock cannot be of any use, and it is doubtful whether it can be ornamental in the eyes of the female bird; indeed, had the tuft appeared under domestication it would have been called a monstrosity. illustrations of the action of natural selection, or the survival of the fittest. in order to make it clear how, as i believe, natural selection acts, i must beg permission to give one or two imaginary illustrations. let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf was hardest pressed for food. under such circumstances the swiftest and slimmest wolves have the best chance of surviving, and so be preserved or selected, provided always that they retained strength to master their prey at this or some other period of the year, when they were compelled to prey on other animals. i can see no more reason to doubt that this would be the result, than that man should be able to improve the fleetness of his greyhounds by careful and methodical selection, or by that kind of unconscious selection which follows from each man trying to keep the best dogs without any thought of modifying the breed. i may add that, according to mr. pierce, there are two varieties of the wolf inhabiting the catskill mountains, in the united states, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks. even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might be born with an innate tendency to pursue certain kinds of prey. nor can this be thought very improbable; for we often observe great differences in the natural tendencies of our domestic animals; one cat, for instance, taking to catch rats, another mice; one cat, according to mr. st. john, bringing home winged game, another hares or rabbits, and another hunting on marshy ground and almost nightly catching woodcocks or snipes. the tendency to catch rats rather than mice is known to be inherited. now, if any slight innate change of habit or of structure benefited an individual wolf, it would have the best chance of surviving and of leaving offspring. some of its young would probably inherit the same habits or structure, and by the repetition of this process, a new variety might be formed which would either supplant or coexist with the parent-form of wolf. or, again, the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to hunt different prey; and from the continued preservation of the individuals best fitted for the two sites, two varieties might slowly be formed. these varieties would cross and blend where they met; but to this subject of intercrossing we shall soon have to return. i may add, that, according to mr. pierce, there are two varieties of the wolf inhabiting the catskill mountains in the united states, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks. it should be observed that in the above illustration, i speak of the slimmest individual wolves, and not of any single strongly marked variation having been preserved. in former editions of this work i sometimes spoke as if this latter alternative had frequently occurred. i saw the great importance of individual differences, and this led me fully to discuss the results of unconscious selection by man, which depends on the preservation of all the more or less valuable individuals, and on the destruction of the worst. i saw, also, that the preservation in a state of nature of any occasional deviation of structure, such as a monstrosity, would be a rare event; and that, if at first preserved, it would generally be lost by subsequent intercrossing with ordinary individuals. nevertheless, until reading an able and valuable article in the "north british review" ( ), i did not appreciate how rarely single variations, whether slight or strongly marked, could be perpetuated. the author takes the case of a pair of animals, producing during their lifetime two hundred offspring, of which, from various causes of destruction, only two on an average survive to pro-create their kind. this is rather an extreme estimate for most of the higher animals, but by no means so for many of the lower organisms. he then shows that if a single individual were born, which varied in some manner, giving it twice as good a chance of life as that of the other individuals, yet the chances would be strongly against its survival. supposing it to survive and to breed, and that half its young inherited the favourable variation; still, as the reviewer goes onto show, the young would have only a slightly better chance of surviving and breeding; and this chance would go on decreasing in the succeeding generations. the justice of these remarks cannot, i think, be disputed. if, for instance, a bird of some kind could procure its food more easily by having its beak curved, and if one were born with its beak strongly curved, and which consequently flourished, nevertheless there would be a very poor chance of this one individual perpetuating its kind to the exclusion of the common form; but there can hardly be a doubt, judging by what we see taking place under domestication, that this result would follow from the preservation during many generations of a large number of individuals with more or less strongly curved beaks, and from the destruction of a still larger number with the straightest beaks. it should not, however, be overlooked that certain rather strongly marked variations, which no one would rank as mere individual differences, frequently recur owing to a similar organisation being similarly acted on--of which fact numerous instances could be given with our domestic productions. in such cases, if the varying individual did not actually transmit to its offspring its newly-acquired character, it would undoubtedly transmit to them, as long as the existing conditions remained the same, a still stronger tendency to vary in the same manner. there can also be little doubt that the tendency to vary in the same manner has often been so strong that all the individuals of the same species have been similarly modified without the aid of any form of selection. or only a third, fifth, or tenth part of the individuals may have been thus affected, of which fact several instances could be given. thus graba estimates that about one-fifth of the guillemots in the faroe islands consist of a variety so well marked, that it was formerly ranked as a distinct species under the name of uria lacrymans. in cases of this kind, if the variation were of a beneficial nature, the original form would soon be supplanted by the modified form, through the survival of the fittest. to the effects of intercrossing in eliminating variations of all kinds, i shall have to recur; but it may be here remarked that most animals and plants keep to their proper homes, and do not needlessly wander about; we see this even with migratory birds, which almost always return to the same spot. consequently each newly-formed variety would generally be at first local, as seems to be the common rule with varieties in a state of nature; so that similarly modified individuals would soon exist in a small body together, and would often breed together. if the new variety were successful in its battle for life, it would slowly spread from a central district, competing with and conquering the unchanged individuals on the margins of an ever-increasing circle. it may be worth while to give another and more complex illustration of the action of natural selection. certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap: this is effected, for instance, by glands at the base of the stipules in some leguminosae, and at the backs of the leaves of the common laurel. this juice, though small in quantity, is greedily sought by insects; but their visits do not in any way benefit the plant. now, let us suppose that the juice or nectar was excreted from the inside of the flowers of a certain number of plants of any species. insects in seeking the nectar would get dusted with pollen, and would often transport it from one flower to another. the flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, as can be fully proved, gives rise to vigorous seedlings, which consequently would have the best chance of flourishing and surviving. the plants which produced flowers with the largest glands or nectaries, excreting most nectar, would oftenest be visited by insects, and would oftenest be crossed; and so in the long-run would gain the upper hand and form a local variety. the flowers, also, which had their stamens and pistils placed, in relation to the size and habits of the particular insect which visited them, so as to favour in any degree the transportal of the pollen, would likewise be favoured. we might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nectar; and as pollen is formed for the sole purpose of fertilisation, its destruction appears to be a simple loss to the plant; yet if a little pollen were carried, at first occasionally and then habitually, by the pollen-devouring insects from flower to flower, and a cross thus effected, although nine-tenths of the pollen were destroyed it might still be a great gain to the plant to be thus robbed; and the individuals which produced more and more pollen, and had larger anthers, would be selected. when our plant, by the above process long continued, had been rendered highly attractive to insects, they would, unintentionally on their part, regularly carry pollen from flower to flower; and that they do this effectually i could easily show by many striking facts. i will give only one, as likewise illustrating one step in the separation of the sexes of plants. some holly-trees bear only male flowers, which have four stamens producing a rather small quantity of pollen, and a rudimentary pistil; other holly-trees bear only female flowers; these have a full-sized pistil, and four stamens with shrivelled anthers, in which not a grain of pollen can be detected. having found a female tree exactly sixty yards from a male tree, i put the stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without exception, there were a few pollen-grains, and on some a profusion. as the wind had set for several days from the female to the male tree, the pollen could not thus have been carried. the weather had been cold and boisterous and therefore not favourable to bees, nevertheless every female flower which i examined had been effectually fertilised by the bees, which had flown from tree to tree in search of nectar. but to return to our imaginary case; as soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence. no naturalist doubts the advantage of what has been called the "physiological division of labour;" hence we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in another flower or on another plant. in plants under culture and placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent; now if we suppose this to occur in ever so slight a degree under nature, then, as pollen is already carried regularly from flower to flower, and as a more complete separation of the sexes of our plant would be advantageous on the principle of the division of labour, individuals with this tendency more and more increased, would be continually favoured or selected, until at last a complete separation of the sexes might be effected. it would take up too much space to show the various steps, through dimorphism and other means, by which the separation of the sexes in plants of various kinds is apparently now in progress; but i may add that some of the species of holly in north america are, according to asa gray, in an exactly intermediate condition, or, as he expresses it, are more or less dioeciously polygamous. let us now turn to the nectar-feeding insects; we may suppose the plant of which we have been slowly increasing the nectar by continued selection, to be a common plant; and that certain insects depended in main part on its nectar for food. i could give many facts showing how anxious bees are to save time: for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which with a very little more trouble they can enter by the mouth. bearing such facts in mind, it may be believed that under certain circumstances individual differences in the curvature or length of the proboscis, etc., too slight to be appreciated by us, might profit a bee or other insect, so that certain individuals would be able to obtain their food more quickly than others; and thus the communities to which they belonged would flourish and throw off many swarms inheriting the same peculiarities. the tubes of the corolla of the common red or incarnate clovers (trifolium pratense and incarnatum) do not on a hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of the common red clover, which is visited by humble-bees alone; so that whole fields of the red clover offer in vain an abundant supply of precious nectar to the hive-bee. that this nectar is much liked by the hive-bee is certain; for i have repeatedly seen, but only in the autumn, many hive-bees sucking the flowers through holes bitten in the base of the tube by humble bees. the difference in the length of the corolla in the two kinds of clover, which determines the visits of the hive-bee, must be very trifling; for i have been assured that when red clover has been mown, the flowers of the second crop are somewhat smaller, and that these are visited by many hive-bees. i do not know whether this statement is accurate; nor whether another published statement can be trusted, namely, that the ligurian bee, which is generally considered a mere variety of the common hive-bee, and which freely crosses with it, is able to reach and suck the nectar of the red clover. thus, in a country where this kind of clover abounded, it might be a great advantage to the hive-bee to have a slightly longer or differently constructed proboscis. on the other hand, as the fertility of this clover absolutely depends on bees visiting the flowers, if humble-bees were to become rare in any country, it might be a great advantage to the plant to have a shorter or more deeply divided corolla, so that the hive-bees should be enabled to suck its flowers. thus i can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted to each other in the most perfect manner, by the continued preservation of all the individuals which presented slight deviations of structure mutually favourable to each other. i am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open to the same objections which were first urged against sir charles lyell's noble views on "the modern changes of the earth, as illustrative of geology;" but we now seldom hear the agencies which we see still at work, spoken of as trifling and insignificant, when used in explaining the excavation of the deepest valleys or the formation of long lines of inland cliffs. natural selection acts only by the preservation and accumulation of small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure. on the intercrossing of individuals. i must here introduce a short digression. in the case of animals and plants with separated sexes, it is of course obvious that two individuals must always (with the exception of the curious and not well understood cases of parthenogenesis) unite for each birth; but in the case of hermaphrodites this is far from obvious. nevertheless there is reason to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur for the reproduction of their kind. this view was long ago doubtfully suggested by sprengel, knight and kolreuter. we shall presently see its importance; but i must here treat the subject with extreme brevity, though i have the materials prepared for an ample discussion. all vertebrate animals, all insects and some other large groups of animals, pair for each birth. modern research has much diminished the number of supposed hermaphrodites and of real hermaphrodites a large number pair; that is, two individuals regularly unite for reproduction, which is all that concerns us. but still there are many hermaphrodite animals which certainly do not habitually pair, and a vast majority of plants are hermaphrodites. what reason, it may be asked, is there for supposing in these cases that two individuals ever concur in reproduction? as it is impossible here to enter on details, i must trust to some general considerations alone. in the first place, i have collected so large a body of facts, and made so many experiments, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a general law of nature that no organic being fertilises itself for a perpetuity of generations; but that a cross with another individual is occasionally--perhaps at long intervals of time--indispensable. on the belief that this is a law of nature, we can, i think, understand several large classes of facts, such as the following, which on any other view are inexplicable. every hybridizer knows how unfavourable exposure to wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully exposed to the weather! if an occasional cross be indispensable, notwithstanding that the plant's own anthers and pistil stand so near each other as almost to ensure self-fertilisation, the fullest freedom for the entrance of pollen from another individual will explain the above state of exposure of the organs. many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family; but these almost invariably present beautiful and curious adaptations in relation to the visits of insects. so necessary are the visits of bees to many papilionaceous flowers, that their fertility is greatly diminished if these visits be prevented. now, it is scarcely possible for insects to fly from flower to flower, and not to carry pollen from one to the other, to the great good of the plant. insects act like a camel-hair pencil, and it is sufficient, to ensure fertilisation, just to touch with the same brush the anthers of one flower and then the stigma of another; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species; for if a plant's own pollen and that from another species are placed on the same stigma, the former is so prepotent that it invariably and completely destroys, as has been shown by gartner, the influence of the foreign pollen. when the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards it, the contrivance seems adapted solely to ensure self-fertilisation; and no doubt it is useful for this end: but the agency of insects is often required to cause the stamens to spring forward, as kolreuter has shown to be the case with the barberry; and in this very genus, which seems to have a special contrivance for self-fertilisation, it is well known that, if closely-allied forms or varieties are planted near each other, it is hardly possible to raise pure seedlings, so largely do they naturally cross. in numerous other cases, far from self-fertilisation being favoured, there are special contrivances which effectually prevent the stigma receiving pollen from its own flower, as i could show from the works of sprengel and others, as well as from my own observations: for instance, in lobelia fulgens, there is a really beautiful and elaborate contrivance by which all the infinitely numerous pollen-granules are swept out of the conjoined anthers of each flower, before the stigma of that individual flower is ready to receive them; and as this flower is never visited, at least in my garden, by insects, it never sets a seed, though by placing pollen from one flower on the stigma of another, i raise plenty of seedlings. another species of lobelia, which is visited by bees, seeds freely in my garden. in very many other cases, though there is no special mechanical contrivance to prevent the stigma receiving pollen from the same flower, yet, as sprengel, and more recently hildebrand and others have shown, and as i can confirm, either the anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that these so-named dichogamous plants have in fact separated sexes, and must habitually be crossed. so it is with the reciprocally dimorphic and trimorphic plants previously alluded to. how strange are these facts! how strange that the pollen and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of self-fertilisation, should be in so many cases mutually useless to each other! how simply are these facts explained on the view of an occasional cross with a distinct individual being advantageous or indispensable! if several varieties of the cabbage, radish, onion, and of some other plants, be allowed to seed near each other, a large majority of the seedlings thus raised turn out, as i found, mongrels: for instance, i raised seedling cabbages from some plants of different varieties growing near each other, and of these only were true to their kind, and some even of these were not perfectly true. yet the pistil of each cabbage-flower is surrounded not only by its own six stamens but by those of the many other flowers on the same plant; and the pollen of each flower readily gets on its stigma without insect agency; for i have found that plants carefully protected from insects produce the full number of pods. how, then, comes it that such a vast number of the seedlings are mongrelized? it must arise from the pollen of a distinct variety having a prepotent effect over the flower's own pollen; and that this is part of the general law of good being derived from the intercrossing of distinct individuals of the same species. when distinct species are crossed the case is reversed, for a plant's own pollen is always prepotent over foreign pollen; but to this subject we shall return in a future chapter. in the case of a large tree covered with innumerable flowers, it may be objected that pollen could seldom be carried from tree to tree, and at most only from flower to flower on the same tree; and flowers on the same tree can be considered as distinct individuals only in a limited sense. i believe this objection to be valid, but that nature has largely provided against it by giving to trees a strong tendency to bear flowers with separated sexes. when the sexes are separated, although the male and female flowers may be produced on the same tree, pollen must be regularly carried from flower to flower; and this will give a better chance of pollen being occasionally carried from tree to tree. that trees belonging to all orders have their sexes more often separated than other plants, i find to be the case in this country; and at my request dr. hooker tabulated the trees of new zealand, and dr. asa gray those of the united states, and the result was as i anticipated. on the other hand, dr. hooker informs me that the rule does not hold good in australia: but if most of the australian trees are dichogamous, the same result would follow as if they bore flowers with separated sexes. i have made these few remarks on trees simply to call attention to the subject. turning for a brief space to animals: various terrestrial species are hermaphrodites, such as the land-mollusca and earth-worms; but these all pair. as yet i have not found a single terrestrial animal which can fertilise itself. this remarkable fact, which offers so strong a contrast with terrestrial plants, is intelligible on the view of an occasional cross being indispensable; for owing to the nature of the fertilising element there are no means, analogous to the action of insects and of the wind with plants, by which an occasional cross could be effected with terrestrial animals without the concurrence of two individuals. of aquatic animals, there are many self-fertilising hermaphrodites; but here the currents of water offer an obvious means for an occasional cross. as in the case of flowers, i have as yet failed, after consultation with one of the highest authorities, namely, professor huxley, to discover a single hermaphrodite animal with the organs of reproduction so perfectly enclosed that access from without, and the occasional influence of a distinct individual, can be shown to be physically impossible. cirripedes long appeared to me to present, under this point of view, a case of great difficulty; but i have been enabled, by a fortunate chance, to prove that two individuals, though both are self-fertilising hermaphrodites, do sometimes cross. it must have struck most naturalists as a strange anomaly that, both with animals and plants, some species of the same family and even of the same genus, though agreeing closely with each other in their whole organisation, are hermaphrodites, and some unisexual. but if, in fact, all hermaphrodites do occasionally intercross, the difference between them and unisexual species is, as far as function is concerned, very small. from these several considerations and from the many special facts which i have collected, but which i am unable here to give, it appears that with animals and plants an occasional intercross between distinct individuals is a very general, if not universal, law of nature. circumstances favourable for the production of new forms through natural selection. this is an extremely intricate subject. a great amount of variability, under which term individual differences are always included, will evidently be favourable. a large number of individuals, by giving a better chance within any given period for the appearance of profitable variations, will compensate for a lesser amount of variability in each individual, and is, i believe, a highly important element of success. though nature grants long periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors it will be exterminated. unless favourable variations be inherited by some at least of the offspring, nothing can be effected by natural selection. the tendency to reversion may often check or prevent the work; but as this tendency has not prevented man from forming by selection numerous domestic races, why should it prevail against natural selection? in the case of methodical selection, a breeder selects for some definite object, and if the individuals be allowed freely to intercross, his work will completely fail. but when many men, without intending to alter the breed, have a nearly common standard of perfection, and all try to procure and breed from the best animals, improvement surely but slowly follows from this unconscious process of selection, notwithstanding that there is no separation of selected individuals. thus it will be under nature; for within a confined area, with some place in the natural polity not perfectly occupied, all the individuals varying in the right direction, though in different degrees, will tend to be preserved. but if the area be large, its several districts will almost certainly present different conditions of life; and then, if the same species undergoes modification in different districts, the newly formed varieties will intercross on the confines of each. but we shall see in the sixth chapter that intermediate varieties, inhabiting intermediate districts, will in the long run generally be supplanted by one of the adjoining varieties. intercrossing will chiefly affect those animals which unite for each birth and wander much, and which do not breed at a very quick rate. hence with animals of this nature, for instance birds, varieties will generally be confined to separated countries; and this i find to be the case. with hermaphrodite organisms which cross only occasionally, and likewise with animals which unite for each birth, but which wander little and can increase at a rapid rate, a new and improved variety might be quickly formed on any one spot, and might there maintain itself in a body and afterward spread, so that the individuals of the new variety would chiefly cross together. on this principle nurserymen always prefer saving seed from a large body of plants, as the chance of intercrossing is thus lessened. even with animals which unite for each birth, and which do not propagate rapidly, we must not assume that free intercrossing would always eliminate the effects of natural selection; for i can bring forward a considerable body of facts showing that within the same area two varieties of the same animal may long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from the individuals of each variety preferring to pair together. intercrossing plays a very important part in nature by keeping the individuals of the same species, or of the same variety, true and uniform in character. it will obviously thus act far more efficiently with those animals which unite for each birth; but, as already stated, we have reason to believe that occasional intercrosses take place with all animals and plants. even if these take place only at long intervals of time, the young thus produced will gain so much in vigour and fertility over the offspring from long-continued self-fertilisation, that they will have a better chance of surviving and propagating their kind; and thus in the long run the influence of crosses, even at rare intervals, will be great. with respect to organic beings extremely low in the scale, which do not propagate sexually, nor conjugate, and which cannot possibly intercross, uniformity of character can be retained by them under the same conditions of life, only through the principle of inheritance, and through natural selection which will destroy any individuals departing from the proper type. if the conditions of life change and the form undergoes modification, uniformity of character can be given to the modified offspring, solely by natural selection preserving similar favourable variations. isolation also is an important element in the modification of species through natural selection. in a confined or isolated area, if not very large, the organic and inorganic conditions of life will generally be almost uniform; so that natural selection will tend to modify all the varying individuals of the same species in the same manner. intercrossing with the inhabitants of the surrounding districts, will also be thus prevented. moritz wagner has lately published an interesting essay on this subject, and has shown that the service rendered by isolation in preventing crosses between newly-formed varieties is probably greater even than i supposed. but from reasons already assigned i can by no means agree with this naturalist, that migration and isolation are necessary elements for the formation of new species. the importance of isolation is likewise great in preventing, after any physical change in the conditions, such as of climate, elevation of the land, etc., the immigration of better adapted organisms; and thus new places in the natural economy of the district will be left open to be filled up by the modification of the old inhabitants. lastly, isolation will give time for a new variety to be improved at a slow rate; and this may sometimes be of much importance. if, however, an isolated area be very small, either from being surrounded by barriers, or from having very peculiar physical conditions, the total number of the inhabitants will be small; and this will retard the production of new species through natural selection, by decreasing the chances of favourable variations arising. the mere lapse of time by itself does nothing, either for or against natural selection. i state this because it has been erroneously asserted that the element of time has been assumed by me to play an all-important part in modifying species, as if all the forms of life were necessarily undergoing change through some innate law. lapse of time is only so far important, and its importance in this respect is great, that it gives a better chance of beneficial variations arising and of their being selected, accumulated, and fixed. it likewise tends to increase the direct action of the physical conditions of life, in relation to the constitution of each organism. if we turn to nature to test the truth of these remarks, and look at any small isolated area, such as an oceanic island, although the number of the species inhabiting it is small, as we shall see in our chapter on geographical distribution; yet of these species a very large proportion are endemic,--that is, have been produced there and nowhere else in the world. hence an oceanic island at first sight seems to have been highly favourable for the production of new species. but we may thus deceive ourselves, for to ascertain whether a small isolated area, or a large open area like a continent, has been most favourable for the production of new organic forms, we ought to make the comparison within equal times; and this we are incapable of doing. although isolation is of great importance in the production of new species, on the whole i am inclined to believe that largeness of area is still more important, especially for the production of species which shall prove capable of enduring for a long period, and of spreading widely. throughout a great and open area, not only will there be a better chance of favourable variations, arising from the large number of individuals of the same species there supported, but the conditions of life are much more complex from the large number of already existing species; and if some of these many species become modified and improved, others will have to be improved in a corresponding degree, or they will be exterminated. each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into competition with many other forms. moreover, great areas, though now continuous, will often, owing to former oscillations of level, have existed in a broken condition, so that the good effects of isolation will generally, to a certain extent, have concurred. finally, i conclude that, although small isolated areas have been in some respects highly favourable for the production of new species, yet that the course of modification will generally have been more rapid on large areas; and what is more important, that the new forms produced on large areas, which already have been victorious over many competitors, will be those that will spread most widely, and will give rise to the greatest number of new varieties and species. they will thus play a more important part in the changing history of the organic world. in accordance with this view, we can, perhaps, understand some facts which will be again alluded to in our chapter on geographical distribution; for instance, the fact of the productions of the smaller continent of australia now yielding before those of the larger europaeo-asiatic area. thus, also, it is that continental productions have everywhere become so largely naturalised on islands. on a small island, the race for life will have been less severe, and there will have been less modification and less extermination. hence, we can understand how it is that the flora of madeira, according to oswald heer, resembles to a certain extent the extinct tertiary flora of europe. all fresh water basins, taken together, make a small area compared with that of the sea or of the land. consequently, the competition between fresh water productions will have been less severe than elsewhere; new forms will have been more slowly produced, and old forms more slowly exterminated. and it is in fresh water basins that we find seven genera of ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world, as the ornithorhynchus and lepidosiren, which, like fossils, connect to a certain extent orders at present widely separated in the natural scale. these anomalous forms may be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having been exposed to less varied, and therefore less severe, competition. to sum up, as far as the extreme intricacy of the subject permits, the circumstances favourable and unfavourable for the production of new species through natural selection. i conclude that for terrestrial productions a large continental area, which has undergone many oscillations of level, will have been the most favourable for the production of many new forms of life, fitted to endure for a long time and to spread widely. while the area existed as a continent the inhabitants will have been numerous in individuals and kinds, and will have been subjected to severe competition. when converted by subsidence into large separate islands there will still have existed many individuals of the same species on each island: intercrossing on the confines of the range of each new species will have been checked: after physical changes of any kind immigration will have been prevented, so that new places in the polity of each island will have had to be filled up by the modification of the old inhabitants; and time will have been allowed for the varieties in each to become well modified and perfected. when, by renewed elevation, the islands were reconverted into a continental area, there will again have been very severe competition; the most favoured or improved varieties will have been enabled to spread; there will have been much extinction of the less improved forms, and the relative proportional numbers of the various inhabitants of the reunited continent will again have been changed; and again there will have been a fair field for natural selection to improve still further the inhabitants, and thus to produce new species. that natural selection generally act with extreme slowness i fully admit. it can act only when there are places in the natural polity of a district which can be better occupied by the modification of some of its existing inhabitants. the occurrence of such places will often depend on physical changes, which generally take place very slowly, and on the immigration of better adapted forms being prevented. as some few of the old inhabitants become modified the mutual relations of others will often be disturbed; and this will create new places, ready to be filled up by better adapted forms; but all this will take place very slowly. although all the individuals of the same species differ in some slight degree from each other, it would often be long before differences of the right nature in various parts of the organisation might occur. the result would often be greatly retarded by free intercrossing. many will exclaim that these several causes are amply sufficient to neutralise the power of natural selection. i do not believe so. but i do believe that natural selection will generally act very slowly, only at long intervals of time, and only on a few of the inhabitants of the same region. i further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at which the inhabitants of the world have changed. slow though the process of selection may be, if feeble man can do much by artificial selection, i can see no limit to the amount of change, to the beauty and complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may have been effected in the long course of time through nature's power of selection, that is by the survival of the fittest. extinction caused by natural selection. this subject will be more fully discussed in our chapter on geology; but it must here be alluded to from being intimately connected with natural selection. natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants, and it follows from this, that as the favoured forms increase in number, so, generally, will the less favoured decrease and become rare. rarity, as geology tells us, is the precursor to extinction. we can see that any form which is represented by few individuals will run a good chance of utter extinction, during great fluctuations in the nature or the seasons, or from a temporary increase in the number of its enemies. but we may go further than this; for as new forms are produced, unless we admit that specific forms can go on indefinitely increasing in number, many old forms must become extinct. that the number of specific forms has not indefinitely increased, geology plainly tells us; and we shall presently attempt to show why it is that the number of species throughout the world has not become immeasurably great. we have seen that the species which are most numerous in individuals have the best chance of producing favourable variations within any given period. we have evidence of this, in the facts stated in the second chapter, showing that it is the common and diffused or dominant species which offer the greatest number of recorded varieties. hence, rare species will be less quickly modified or improved within any given period; they will consequently be beaten in the race for life by the modified and improved descendants of the commoner species. from these several considerations i think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct. the forms which stand in closest competition with those undergoing modification and improvement, will naturally suffer most. and we have seen in the chapter on the struggle for existence that it is the most closely-allied forms,--varieties of the same species, and species of the same genus or related genera,--which, from having nearly the same structure, constitution and habits, generally come into the severest competition with each other. consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them. we see the same process of extermination among our domesticated productions, through the selection of improved forms by man. many curious instances could be given showing how quickly new breeds of cattle, sheep and other animals, and varieties of flowers, take the place of older and inferior kinds. in yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these "were swept away by the short-horns" (i quote the words of an agricultural writer) "as if by some murderous pestilence." divergence of character. the principle, which i have designated by this term, is of high importance, and explains, as i believe, several important facts. in the first place, varieties, even strongly-marked ones, though having somewhat of the character of species--as is shown by the hopeless doubts in many cases how to rank them--yet certainly differ far less from each other than do good and distinct species. nevertheless according to my view, varieties are species in the process of formation, or are, as i have called them, incipient species. how, then, does the lesser difference between varieties become augmented into the greater difference between species? that this does habitually happen, we must infer from most of the innumerable species throughout nature presenting well-marked differences; whereas varieties, the supposed prototypes and parents of future well-marked species, present slight and ill-defined differences. mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a greater degree; but this alone would never account for so habitual and large a degree of difference as that between the species of the same genus. as has always been my practice, i have sought light on this head from our domestic productions. we shall here find something analogous. it will be admitted that the production of races so different as short-horn and hereford cattle, race and cart horses, the several breeds of pigeons, etc., could never have been effected by the mere chance accumulation of similar variations during many successive generations. in practice, a fancier is, for instance, struck by a pigeon having a slightly shorter beak; another fancier is struck by a pigeon having a rather longer beak; and on the acknowledged principle that "fanciers do not and will not admire a medium standard, but like extremes," they both go on (as has actually occurred with the sub-breeds of the tumbler-pigeon) choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks. again, we may suppose that at an early period of history, the men of one nation or district required swifter horses, while those of another required stronger and bulkier horses. the early differences would be very slight; but, in the course of time, from the continued selection of swifter horses in the one case, and of stronger ones in the other, the differences would become greater, and would be noted as forming two sub-breeds. ultimately after the lapse of centuries, these sub-breeds would become converted into two well-established and distinct breeds. as the differences became greater, the inferior animals with intermediate characters, being neither very swift nor very strong, would not have been used for breeding, and will thus have tended to disappear. here, then, we see in man's productions the action of what may be called the principle of divergence, causing differences, at first barely appreciable, steadily to increase, and the breeds to diverge in character, both from each other and from their common parent. but how, it may be asked, can any analogous principle apply in nature? i believe it can and does apply most efficiently (though it was a long time before i saw how), from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers. we can clearly discern this in the case of animals with simple habits. take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. if its natural power of increase be allowed to act, it can succeed in increasing (the country not undergoing any change in conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations, climbing trees, frequenting water, and some perhaps becoming less carnivorous. the more diversified in habits and structure the descendants of our carnivorous animals become, the more places they will be enabled to occupy. what applies to one animal will apply throughout all time to all animals--that is, if they vary--for otherwise natural selection can effect nothing. so it will be with plants. it has been experimentally proved, that if a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbage can be raised in the latter than in the former case. the same has been found to hold good when one variety and several mixed varieties of wheat have been sown on equal spaces of ground. hence, if any one species of grass were to go on varying, and the varieties were continually selected which differed from each other in the same manner, though in a very slight degree, as do the distinct species and genera of grasses, a greater number of individual plants of this species, including its modified descendants, would succeed in living on the same piece of ground. and we know that each species and each variety of grass is annually sowing almost countless seeds; and is thus striving, as it may be said, to the utmost to increase in number. consequently, in the course of many thousand generations, the most distinct varieties of any one species of grass would have the best chance of succeeding and of increasing in numbers, and thus of supplanting the less distinct varieties; and varieties, when rendered very distinct from each other, take the rank of species. the truth of the principle that the greatest amount of life can be supported by great diversification of structure, is seen under many natural circumstances. in an extremely small area, especially if freely open to immigration, and where the contest between individual and individual must be very severe, we always find great diversity in its inhabitants. for instance, i found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants, and these belonged to eighteen genera and to eight orders, which shows how much these plants differed from each other. so it is with the plants and insects on small and uniform islets: also in small ponds of fresh water. farmers find that they can raise more food by a rotation of plants belonging to the most different orders: nature follows what may be called a simultaneous rotation. most of the animals and plants which live close round any small piece of ground, could live on it (supposing its nature not to be in any way peculiar), and may be said to be striving to the utmost to live there; but, it is seen, that where they come into the closest competition, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders. the same principle is seen in the naturalisation of plants through man's agency in foreign lands. it might have been expected that the plants which would succeed in becoming naturalised in any land would generally have been closely allied to the indigenes; for these are commonly looked at as specially created and adapted for their own country. it might also, perhaps, have been expected that naturalised plants would have belonged to a few groups more especially adapted to certain stations in their new homes. but the case is very different; and alph. de candolle has well remarked, in his great and admirable work, that floras gain by naturalisation, proportionally with the number of the native genera and species, far more in new genera than in new species. to give a single instance: in the last edition of dr. asa gray's "manual of the flora of the northern united states," naturalised plants are enumerated, and these belong to genera. we thus see that these naturalised plants are of a highly diversified nature. they differ, moreover, to a large extent, from the indigenes, for out of the naturalised genera, no less than genera are not there indigenous, and thus a large proportional addition is made to the genera now living in the united states. by considering the nature of the plants or animals which have in any country struggled successfully with the indigenes, and have there become naturalised, we may gain some crude idea in what manner some of the natives would have had to be modified in order to gain an advantage over their compatriots; and we may at least infer that diversification of structure, amounting to new generic differences, would be profitable to them. the advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that of the physiological division of labour in the organs of the same individual body--a subject so well elucidated by milne edwards. no physiologist doubts that a stomach by being adapted to digest vegetable matter alone, or flesh alone, draws most nutriment from these substances. so in the general economy of any land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will a greater number of individuals be capable of there supporting themselves. a set of animals, with their organisation but little diversified, could hardly compete with a set more perfectly diversified in structure. it may be doubted, for instance, whether the australian marsupials, which are divided into groups differing but little from each other, and feebly representing, as mr. waterhouse and others have remarked, our carnivorous, ruminant, and rodent mammals, could successfully compete with these well-developed orders. in the australian mammals, we see the process of diversification in an early and incomplete stage of development. the probable effects of the action of natural selection through divergence of character and extinction, on the descendants of a common ancestor. after the foregoing discussion, which has been much compressed, we may assume that the modified descendants of any one species will succeed so much the better as they become more diversified in structure, and are thus enabled to encroach on places occupied by other beings. now let us see how this principle of benefit being derived from divergence of character, combined with the principles of natural selection and of extinction, tends to act. the accompanying diagram will aid us in understanding this rather perplexing subject. let a to l represent the species of a genus large in its own country; these species are supposed to resemble each other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the letters standing at unequal distances. i have said a large genus, because as we saw in the second chapter, on an average more species vary in large genera than in small genera; and the varying species of the large genera present a greater number of varieties. we have, also, seen that the species, which are the commonest and most widely-diffused, vary more than do the rare and restricted species. let (a) be a common, widely-diffused, and varying species, belonging to a genus large in its own country. the branching and diverging dotted lines of unequal lengths proceeding from (a), may represent its varying offspring. the variations are supposed to be extremely slight, but of the most diversified nature; they are not supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed to endure for equal periods. only those variations which are in some way profitable will be preserved or naturally selected. and here the importance of the principle of benefit derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection. when a dotted line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to form it into a fairly well-marked variety, such as would be thought worthy of record in a systematic work. the intervals between the horizontal lines in the diagram, may represent each a thousand or more generations. after a thousand generations, species (a) is supposed to have produced two fairly well-marked varieties, namely a and m . these two varieties will generally still be exposed to the same conditions which made their parents variable, and the tendency to variability is in itself hereditary; consequently they will likewise tend to vary, and commonly in nearly the same manner as did their parents. moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages which made their parent (a) more numerous than most of the other inhabitants of the same country; they will also partake of those more general advantages which made the genus to which the parent-species belonged, a large genus in its own country. and all these circumstances are favourable to the production of new varieties. if, then, these two varieties be variable, the most divergent of their variations will generally be preserved during the next thousand generations. and after this interval, variety a is supposed in the diagram to have produced variety a , which will, owing to the principle of divergence, differ more from (a) than did variety a . variety m is supposed to have produced two varieties, namely m and s , differing from each other, and more considerably from their common parent (a). we may continue the process by similar steps for any length of time; some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modified condition, some producing two or three varieties, and some failing to produce any. thus the varieties or modified descendants of the common parent (a), will generally go on increasing in number and diverging in character. in the diagram the process is represented up to the ten-thousandth generation, and under a condensed and simplified form up to the fourteen-thousandth generation. but i must here remark that i do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular, nor that it goes on continuously; it is far more probable that each form remains for long periods unaltered, and then again undergoes modification. nor do i suppose that the most divergent varieties are invariably preserved: a medium form may often long endure, and may or may not produce more than one modified descendant; for natural selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely complex relations. but as a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will increase. in our diagram the line of succession is broken at regular intervals by small numbered letters marking the successive forms which have become sufficiently distinct to be recorded as varieties. but these breaks are imaginary, and might have been inserted anywhere, after intervals long enough to allow the accumulation of a considerable amount of divergent variation. as all the modified descendants from a common and widely-diffused species, belonging to a large genus, will tend to partake of the same advantages which made their parent successful in life, they will generally go on multiplying in number as well as diverging in character: this is represented in the diagram by the several divergent branches proceeding from (a). the modified offspring from the later and more highly improved branches in the lines of descent, will, it is probable, often take the place of, and so destroy, the earlier and less improved branches: this is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines. in some cases no doubt the process of modification will be confined to a single line of descent, and the number of modified descendants will not be increased; although the amount of divergent modification may have been augmented. this case would be represented in the diagram, if all the lines proceeding from (a) were removed, excepting that from a to a . in the same way the english racehorse and english pointer have apparently both gone on slowly diverging in character from their original stocks, without either having given off any fresh branches or races. after ten thousand generations, species (a) is supposed to have produced three forms, a , f , and m , which, from having diverged in character during the successive generations, will have come to differ largely, but perhaps unequally, from each other and from their common parent. if we suppose the amount of change between each horizontal line in our diagram to be excessively small, these three forms may still be only well-marked varieties; but we have only to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into doubtful or at least into well-defined species: thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species. by continuing the same process for a greater number of generations (as shown in the diagram in a condensed and simplified manner), we get eight species, marked by the letters between a and m , all descended from (a). thus, as i believe, species are multiplied and genera are formed. in a large genus it is probable that more than one species would vary. in the diagram i have assumed that a second species (i) has produced, by analogous steps, after ten thousand generations, either two well-marked varieties (w and z ) or two species, according to the amount of change supposed to be represented between the horizontal lines. after fourteen thousand generations, six new species, marked by the letters n to z , are supposed to have been produced. in any genus, the species which are already very different in character from each other, will generally tend to produce the greatest number of modified descendants; for these will have the best chance of seizing on new and widely different places in the polity of nature: hence in the diagram i have chosen the extreme species (a), and the nearly extreme species (i), as those which have largely varied, and have given rise to new varieties and species. the other nine species (marked by capital letters) of our original genus, may for long but unequal periods continue to transmit unaltered descendants; and this is shown in the diagram by the dotted lines unequally prolonged upwards. but during the process of modification, represented in the diagram, another of our principles, namely that of extinction, will have played an important part. as in each fully stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms, there will be a constant tendency in the improved descendants of any one species to supplant and exterminate in each stage of descent their predecessors and their original progenitor. for it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution and structure. hence all the intermediate forms between the earlier and later states, that is between the less and more improved states of a the same species, as well as the original parent-species itself, will generally tend to become extinct. so it probably will be with many whole collateral lines of descent, which will be conquered by later and improved lines. if, however, the modified offspring of a species get into some distinct country, or become quickly adapted to some quite new station, in which offspring and progenitor do not come into competition, both may continue to exist. if, then, our diagram be assumed to represent a considerable amount of modification, species (a) and all the earlier varieties will have become extinct, being replaced by eight new species (a to m ); and species (i) will be replaced by six (n to z ) new species. but we may go further than this. the original species of our genus were supposed to resemble each other in unequal degrees, as is so generally the case in nature; species (a) being more nearly related to b, c, and d than to the other species; and species (i) more to g, h, k, l, than to the others. these two species (a and i), were also supposed to be very common and widely diffused species, so that they must originally have had some advantage over most of the other species of the genus. their modified descendants, fourteen in number at the fourteen-thousandth generation, will probably have inherited some of the same advantages: they have also been modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many related places in the natural economy of their country. it seems, therefore, extremely probable that they will have taken the places of, and thus exterminated, not only their parents (a) and (i), but likewise some of the original species which were most nearly related to their parents. hence very few of the original species will have transmitted offspring to the fourteen-thousandth generation. we may suppose that only one (f) of the two species (e and f) which were least closely related to the other nine original species, has transmitted descendants to this late stage of descent. the new species in our diagram, descended from the original eleven species, will now be fifteen in number. owing to the divergent tendency of natural selection, the extreme amount of difference in character between species a and z will be much greater than that between the most distinct of the original eleven species. the new species, moreover, will be allied to each other in a widely different manner. of the eight descendants from (a) the three marked a , q , p , will be nearly related from having recently branched off from a ; b and f , from having diverged at an earlier period from a , will be in some degree distinct from the three first-named species; and lastly, o , e , and m , will be nearly related one to the other, but, from having diverged at the first commencement of the process of modification, will be widely different from the other five species, and may constitute a sub-genus or a distinct genus. the six descendants from (i) will form two sub-genera or genera. but as the original species (i) differed largely from (a), standing nearly at the extreme end of the original genus, the six descendants from (i) will, owing to inheritance alone, differ considerably from the eight descendants from (a); the two groups, moreover, are supposed to have gone on diverging in different directions. the intermediate species, also (and this is a very important consideration), which connected the original species (a) and (i), have all become, except (f), extinct, and have left no descendants. hence the six new species descended from (i), and the eight descendants from (a), will have to be ranked as very distinct genera, or even as distinct sub-families. thus it is, as i believe, that two or more genera are produced by descent with modification, from two or more species of the same genus. and the two or more parent-species are supposed to be descended from some one species of an earlier genus. in our diagram this is indicated by the broken lines beneath the capital letters, converging in sub-branches downwards towards a single point; this point represents a species, the supposed progenitor of our several new sub-genera and genera. it is worth while to reflect for a moment on the character of the new species f , which is supposed not to have diverged much in character, but to have retained the form of (f), either unaltered or altered only in a slight degree. in this case its affinities to the other fourteen new species will be of a curious and circuitous nature. being descended from a form that stood between the parent-species (a) and (i), now supposed to be extinct and unknown, it will be in some degree intermediate in character between the two groups descended from these two species. but as these two groups have gone on diverging in character from the type of their parents, the new species (f ) will not be directly intermediate between them, but rather between types of the two groups; and every naturalist will be able to call such cases before his mind. in the diagram each horizontal line has hitherto been supposed to represent a thousand generations, but each may represent a million or more generations; it may also represent a section of the successive strata of the earth's crust including extinct remains. we shall, when we come to our chapter on geology, have to refer again to this subject, and i think we shall then see that the diagram throws light on the affinities of extinct beings, which, though generally belonging to the same orders, families, or genera, with those now living, yet are often, in some degree, intermediate in character between existing groups; and we can understand this fact, for the extinct species lived at various remote epochs when the branching lines of descent had diverged less. i see no reason to limit the process of modification, as now explained, to the formation of genera alone. if, in the diagram, we suppose the amount of change represented by each successive group of diverging dotted lines to be great, the forms marked a to p , those marked b and f , and those marked o to m , will form three very distinct genera. we shall also have two very distinct genera descended from (i), differing widely from the descendants of (a). these two groups of genera will thus form two distinct families, or orders, according to the amount of divergent modification supposed to be represented in the diagram. and the two new families, or orders, are descended from two species of the original genus; and these are supposed to be descended from some still more ancient and unknown form. we have seen that in each country it is the species belonging to the larger genera which oftenest present varieties or incipient species. this, indeed, might have been expected; for as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common. hence, the struggle for the production of new and modified descendants will mainly lie between the larger groups, which are all trying to increase in number. one large group will slowly conquer another large group, reduce its number, and thus lessen its chance of further variation and improvement. within the same large group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. small and broken groups and sub-groups will finally disappear. looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which have as yet suffered least extinction, will, for a long period, continue to increase. but which groups will ultimately prevail, no man can predict; for we know that many groups, formerly most extensively developed, have now become extinct. looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that, of the species living at any one period, extremely few will transmit descendants to a remote futurity. i shall have to return to this subject in the chapter on classification, but i may add that as, according to this view, extremely few of the more ancient species have transmitted descendants to the present day, and, as all the descendants of the same species form a class, we can understand how it is that there exist so few classes in each main division of the animal and vegetable kingdoms. although few of the most ancient species have left modified descendants, yet, at remote geological periods, the earth may have been almost as well peopled with species of many genera, families, orders and classes, as at the present day. on the degree to which organisation tends to advance. natural selection acts exclusively by the preservation and accumulation of variations, which are beneficial under the organic and inorganic conditions to which each creature is exposed at all periods of life. the ultimate result is that each creature tends to become more and more improved in relation to its conditions. this improvement inevitably leads to the gradual advancement of the organisation of the greater number of living beings throughout the world. but here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organisation. among the vertebrata the degree of intellect and an approach in structure to man clearly come into play. it might be thought that the amount of change which the various parts and organs pass through in their development from embryo to maturity would suffice as a standard of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva. von baer's standard seems the most widely applicable and the best, namely, the amount of differentiation of the parts of the same organic being, in the adult state, as i should be inclined to add, and their specialisation for different functions; or, as milne edwards would express it, the completeness of the division of physiological labour. but we shall see how obscure this subject is if we look, for instance, to fishes, among which some naturalists rank those as highest which, like the sharks, approach nearest to amphibians; while other naturalists rank the common bony or teleostean fishes as the highest, inasmuch as they are most strictly fish-like, and differ most from the other vertebrate classes. we see still more plainly the obscurity of the subject by turning to plants, among which the standard of intellect is of course quite excluded; and here some botanists rank those plants as highest which have every organ, as sepals, petals, stamens and pistils, fully developed in each flower; whereas other botanists, probably with more truth, look at the plants which have their several organs much modified and reduced in number as the highest. if we take as the standard of high organisation, the amount of differentiation and specialisation of the several organs in each being when adult (and this will include the advancement of the brain for intellectual purposes), natural selection clearly leads towards this standard: for all physiologists admit that the specialisation of organs, inasmuch as in this state they perform their functions better, is an advantage to each being; and hence the accumulation of variations tending towards specialisation is within the scope of natural selection. on the other hand, we can see, bearing in mind that all organic beings are striving to increase at a high ratio and to seize on every unoccupied or less well occupied place in the economy of nature, that it is quite possible for natural selection gradually to fit a being to a situation in which several organs would be superfluous or useless: in such cases there would be retrogression in the scale of organisation. whether organisation on the whole has actually advanced from the remotest geological periods to the present day will be more conveniently discussed in our chapter on geological succession. but it may be objected that if all organic beings thus tend to rise in the scale, how is it that throughout the world a multitude of the lowest forms still exist; and how is it that in each great class some forms are far more highly developed than others? why have not the more highly developed forms every where supplanted and exterminated the lower? lamarck, who believed in an innate and inevitable tendency towards perfection in all organic beings, seems to have felt this difficulty so strongly that he was led to suppose that new and simple forms are continually being produced by spontaneous generation. science has not as yet proved the truth of this belief, whatever the future may reveal. on our theory the continued existence of lowly organisms offers no difficulty; for natural selection, or the survival of the fittest, does not necessarily include progressive development--it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life. and it may be asked what advantage, as far as we can see, would it be to an infusorian animalcule--to an intestinal worm--or even to an earth-worm, to be highly organised. if it were no advantage, these forms would be left, by natural selection, unimproved or but little improved, and might remain for indefinite ages in their present lowly condition. and geology tells us that some of the lowest forms, as the infusoria and rhizopods, have remained for an enormous period in nearly their present state. but to suppose that most of the many now existing low forms have not in the least advanced since the first dawn of life would be extremely rash; for every naturalist who has dissected some of the beings now ranked as very low in the scale, must have been struck with their really wondrous and beautiful organisation. nearly the same remarks are applicable, if we look to the different grades of organisation within the same great group; for instance, in the vertebrata, to the co-existence of mammals and fish--among mammalia, to the co-existence of man and the ornithorhynchus--among fishes, to the co-existence of the shark and the lancelet (amphioxus), which latter fish in the extreme simplicity of its structure approaches the invertebrate classes. but mammals and fish hardly come into competition with each other; the advancement of the whole class of mammals, or of certain members in this class, to the highest grade would not lead to their taking the place of fishes. physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aerial respiration; so that warm-blooded mammals when inhabiting the water lie under a disadvantage in having to come continually to the surface to breathe. with fishes, members of the shark family would not tend to supplant the lancelet; for the lancelet, as i hear from fritz muller, has as sole companion and competitor on the barren sandy shore of south brazil, an anomalous annelid. the three lowest orders of mammals, namely, marsupials, edentata, and rodents, co-exist in south america in the same region with numerous monkeys, and probably interfere little with each other. although organisation, on the whole, may have advanced and be still advancing throughout the world, yet the scale will always present many degrees of perfection; for the high advancement of certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of those groups with which they do not enter into close competition. in some cases, as we shall hereafter see, lowly organised forms appear to have been preserved to the present day, from inhabiting confined or peculiar stations, where they have been subjected to less severe competition, and where their scanty numbers have retarded the chance of favourable variations arising. finally, i believe that many lowly organised forms now exist throughout the world, from various causes. in some cases variations or individual differences of a favourable nature may never have arisen for natural selection to act on and accumulate. in no case, probably, has time sufficed for the utmost possible amount of development. in some few cases there has been what we must call retrogression or organisation. but the main cause lies in the fact that under very simple conditions of life a high organisation would be of no service--possibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and injured. looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest structure, how, it has been asked, could the first step in the advancement or differentiation of parts have arisen? mr. herbert spencer would probably answer that, as soon as simple unicellular organisms came by growth or division to be compounded of several cells, or became attached to any supporting surface, his law "that homologous units of any order become differentiated in proportion as their relations to incident forces become different" would come into action. but as we have no facts to guide us, speculation on the subject is almost useless. it is, however, an error to suppose that there would be no struggle for existence, and, consequently, no natural selection, until many forms had been produced: variations in a single species inhabiting an isolated station might be beneficial, and thus the whole mass of individuals might be modified, or two distinct forms might arise. but, as i remarked towards the close of the introduction, no one ought to feel surprise at much remaining as yet unexplained on the origin of species, if we make due allowance for our profound ignorance on the mutual relations of the inhabitants of the world at the present time, and still more so during past ages. convergence of character. mr. h.c. watson thinks that i have overrated the importance of divergence of character (in which, however, he apparently believes), and that convergence, as it may be called, has likewise played a part. if two species belonging to two distinct though allied genera, had both produced a large number of new and divergent forms, it is conceivable that these might approach each other so closely that they would have all to be classed under the same genus; and thus the descendants of two distinct genera would converge into one. but it would in most cases be extremely rash to attribute to convergence a close and general similarity of structure in the modified descendants of widely distinct forms. the shape of a crystal is determined solely by the molecular forces, and it is not surprising that dissimilar substances should sometimes assume the same form; but with organic beings we should bear in mind that the form of each depends on an infinitude of complex relations, namely on the variations which have arisen, these being due to causes far too intricate to be followed out--on the nature of the variations which have been preserved or selected, and this depends on the surrounding physical conditions, and in a still higher degree on the surrounding organisms with which each being has come into competition--and lastly, on inheritance (in itself a fluctuating element) from innumerable progenitors, all of which have had their forms determined through equally complex relations. it is incredible that the descendants of two organisms, which had originally differed in a marked manner, should ever afterwards converge so closely as to lead to a near approach to identity throughout their whole organisation. if this had occurred, we should meet with the same form, independently of genetic connection, recurring in widely separated geological formations; and the balance of evidence is opposed to any such an admission. mr. watson has also objected that the continued action of natural selection, together with divergence of character, would tend to make an indefinite number of specific forms. as far as mere inorganic conditions are concerned, it seems probable that a sufficient number of species would soon become adapted to all considerable diversities of heat, moisture, etc.; but i fully admit that the mutual relations of organic beings are more important; and as the number of species in any country goes on increasing, the organic conditions of life must become more and more complex. consequently there seems at first no limit to the amount of profitable diversification of structure, and therefore no limit to the number of species which might be produced. we do not know that even the most prolific area is fully stocked with specific forms: at the cape of good hope and in australia, which support such an astonishing number of species, many european plants have become naturalised. but geology shows us, that from an early part of the tertiary period the number of species of shells, and that from the middle part of this same period, the number of mammals has not greatly or at all increased. what then checks an indefinite increase in the number of species? the amount of life (i do not mean the number of specific forms) supported on an area must have a limit, depending so largely as it does on physical conditions; therefore, if an area be inhabited by very many species, each or nearly each species will be represented by few individuals; and such species will be liable to extermination from accidental fluctuations in the nature of the seasons or in the number of their enemies. the process of extermination in such cases would be rapid, whereas the production of new species must always be slow. imagine the extreme case of as many species as individuals in england, and the first severe winter or very dry summer would exterminate thousands on thousands of species. rare species, and each species will become rare if the number of species in any country becomes indefinitely increased, will, on the principal often explained, present within a given period few favourable variations; consequently, the process of giving birth to new specific forms would thus be retarded. when any species becomes very rare, close interbreeding will help to exterminate it; authors have thought that this comes into play in accounting for the deterioration of the aurochs in lithuania, of red deer in scotland and of bears in norway, etc. lastly, and this i am inclined to think is the most important element, a dominant species, which has already beaten many competitors in its own home, will tend to spread and supplant many others. alph. de candolle has shown that those species which spread widely tend generally to spread very widely, consequently they will tend to supplant and exterminate several species in several areas, and thus check the inordinate increase of specific forms throughout the world. dr. hooker has recently shown that in the southeast corner of australia, where, apparently, there are many invaders from different quarters of the globe, the endemic australian species have been greatly reduced in number. how much weight to attribute to these several considerations i will not pretend to say; but conjointly they must limit in each country the tendency to an indefinite augmentation of specific forms. summary of chapter. if under changing conditions of life organic beings present individual differences in almost every part of their structure, and this cannot be disputed; if there be, owing to their geometrical rate of increase, a severe struggle for life at some age, season or year, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of life, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, it would be a most extraordinary fact if no variations had ever occurred useful to each being's own welfare, in the same manner as so many variations have occurred useful to man. but if variations useful to any organic being ever do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance, these will tend to produce offspring similarly characterised. this principle of preservation, or the survival of the fittest, i have called natural selection. it leads to the improvement of each creature in relation to its organic and inorganic conditions of life; and consequently, in most cases, to what must be regarded as an advance in organisation. nevertheless, low and simple forms will long endure if well fitted for their simple conditions of life. natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed, or young as easily as the adult. among many animals sexual selection will have given its aid to ordinary selection by assuring to the most vigorous and best adapted males the greatest number of offspring. sexual selection will also give characters useful to the males alone in their struggles or rivalry with other males; and these characters will be transmitted to one sex or to both sexes, according to the form of inheritance which prevails. whether natural selection has really thus acted in adapting the various forms of life to their several conditions and stations, must be judged by the general tenour and balance of evidence given in the following chapters. but we have already seen how it entails extinction; and how largely extinction has acted in the world's history, geology plainly declares. natural selection, also, leads to divergence of character; for the more organic beings diverge in structure, habits and constitution, by so much the more can a large number be supported on the area, of which we see proof by looking to the inhabitants of any small spot, and to the productions naturalised in foreign lands. therefore, during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified the descendants become, the better will be their chance of success in the battle for life. thus the small differences distinguishing varieties of the same species, steadily tend to increase, till they equal the greater differences between species of the same genus, or even of distinct genera. we have seen that it is the common, the widely diffused, and widely ranging species, belonging to the larger genera within each class, which vary most; and these tend to transmit to their modified offspring that superiority which now makes them dominant in their own countries. natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life. on these principles, the nature of the affinities, and the generally well defined distinctions between the innumerable organic beings in each class throughout the world, may be explained. it is a truly wonderful fact--the wonder of which we are apt to overlook from familiarity--that all animals and all plants throughout all time and space should be related to each other in groups, subordinate to groups, in the manner which we everywhere behold--namely, varieties of the same species most closely related, species of the same genus less closely and unequally related, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. the several subordinate groups in any class cannot be ranked in a single file, but seem clustered round points, and these round other points, and so on in almost endless cycles. if species had been independently created, no explanation would have been possible of this kind of classification; but it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram. the affinities of all the beings of the same class have sometimes been represented by a great tree. i believe this simile largely speaks the truth. the green and budding twigs may represent existing species; and those produced during former years may represent the long succession of extinct species. at each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have at all times overmastered other species in the great battle for life. the limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear the other branches; so with the species which lived during long-past geological periods, very few have left living and modified descendants. from the first growth of the tree, many a limb and branch has decayed and dropped off; and these fallen branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state. as we here and there see a thin, straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the ornithorhynchus or lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. as buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation i believe it has been with the great tree of life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications. chapter v. laws of variation. effects of changed conditions--use and disuse, combined with natural selection; organs of flight and of vision--acclimatisation--correlated variation--compensation and economy of growth--false correlations--multiple, rudimentary, and lowly organised structures variable--parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable--species of the same genus vary in an analogous manner--reversions to long-lost characters--summary. i have hitherto sometimes spoken as if the variations--so common and multiform with organic beings under domestication, and in a lesser degree with those under nature--were due to chance. this, of course is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation. some authors believe it to be as much the function of the reproductive system to produce individual differences, or slight deviations of structure, as to make the child like its parents. but the fact of variations and monstrosities occurring much more frequently under domestication than under nature, and the greater variability of species having wide ranges than of those with restricted ranges, lead to the conclusion that variability is generally related to the conditions of life to which each species has been exposed during several successive generations. in the first chapter i attempted to show that changed conditions act in two ways, directly on the whole organisation or on certain parts alone, and indirectly through the reproductive system. in all cases there are two factors, the nature of the organism, which is much the most important of the two, and the nature of the conditions. the direct action of changed conditions leads to definite or indefinite results. in the latter case the organisation seems to become plastic, and we have much fluctuating variability. in the former case the nature of the organism is such that it yields readily, when subjected to certain conditions, and all, or nearly all, the individuals become modified in the same way. it is very difficult to decide how far changed conditions, such as of climate, food, etc., have acted in a definite manner. there is reason to believe that in the course of time the effects have been greater than can be proved by clear evidence. but we may safely conclude that the innumerable complex co-adaptations of structure, which we see throughout nature between various organic beings, cannot be attributed simply to such action. in the following cases the conditions seem to have produced some slight definite effect: e. forbes asserts that shells at their southern limit, and when living in shallow water, are more brightly coloured than those of the same species from further north or from a greater depth; but this certainly does not always hold good. mr. gould believes that birds of the same species are more brightly coloured under a clear atmosphere, than when living near the coast or on islands; and wollaston is convinced that residence near the sea affects the colours of insects. moquin-tandon gives a list of plants which, when growing near the sea-shore, have their leaves in some degree fleshy, though not elsewhere fleshy. these slightly varying organisms are interesting in as far as they present characters analogous to those possessed by the species which are confined to similar conditions. when a variation is of the slightest use to any being, we cannot tell how much to attribute to the accumulative action of natural selection, and how much to the definite action of the conditions of life. thus, it is well known to furriers that animals of the same species have thicker and better fur the further north they live; but who can tell how much of this difference may be due to the warmest-clad individuals having been favoured and preserved during many generations, and how much to the action of the severe climate? for it would appear that climate has some direct action on the hair of our domestic quadrupeds. instances could be given of similar varieties being produced from the same species under external conditions of life as different as can well be conceived; and, on the other hand, of dissimilar varieties being produced under apparently the same external conditions. again, innumerable instances are known to every naturalist, of species keeping true, or not varying at all, although living under the most opposite climates. such considerations as these incline me to lay less weight on the direct action of the surrounding conditions, than on a tendency to vary, due to causes of which we are quite ignorant. in one sense the conditions of life may be said, not only to cause variability, either directly or indirectly, but likewise to include natural selection, for the conditions determine whether this or that variety shall survive. but when man is the selecting agent, we clearly see that the two elements of change are distinct; variability is in some manner excited, but it is the will of man which accumulates the variations in certain direction; and it is this latter agency which answers to the survival of the fittest under nature. effects of the increased use and disuse of parts, as controlled by natural selection. from the facts alluded to in the first chapter, i think there can be no doubt that use in our domestic animals has strengthened and enlarged certain parts, and disuse diminished them; and that such modifications are inherited. under free nature we have no standard of comparison by which to judge of the effects of long-continued use or disuse, for we know not the parent-forms; but many animals possess structures which can be best explained by the effects of disuse. as professor owen has remarked, there is no greater anomaly in nature than a bird that cannot fly; yet there are several in this state. the logger-headed duck of south america can only flap along the surface of the water, and has its wings in nearly the same condition as the domestic aylesbury duck: it is a remarkable fact that the young birds, according to mr. cunningham, can fly, while the adults have lost this power. as the larger ground-feeding birds seldom take flight except to escape danger, it is probable that the nearly wingless condition of several birds, now inhabiting or which lately inhabited several oceanic islands, tenanted by no beasts of prey, has been caused by disuse. the ostrich indeed inhabits continents, and is exposed to danger from which it cannot escape by flight, but it can defend itself, by kicking its enemies, as efficiently as many quadrupeds. we may believe that the progenitor of the ostrich genus had habits like those of the bustard, and that, as the size and weight of its body were increased during successive generations, its legs were used more and its wings less, until they became incapable of flight. kirby has remarked (and i have observed the same fact) that the anterior tarsi, or feet, of many male dung-feeding beetles are often broken off; he examined seventeen specimens in his own collection, and not one had even a relic left. in the onites apelles the tarsi are so habitually lost that the insect has been described as not having them. in some other genera they are present, but in a rudimentary condition. in the ateuchus or sacred beetle of the egyptians, they are totally deficient. the evidence that accidental mutilations can be inherited is at present not decisive; but the remarkable cases observed by brown-sequard in guinea-pigs, of the inherited effects of operations, should make us cautious in denying this tendency. hence, it will perhaps be safest to look at the entire absence of the anterior tarsi in ateuchus, and their rudimentary condition in some other genera, not as cases of inherited mutilations, but as due to the effects of long-continued disuse; for as many dung-feeding beetles are generally found with their tarsi lost, this must happen early in life; therefore the tarsi cannot be of much importance or be much used by these insects. in some cases we might easily put down to disuse modifications of structure which are wholly, or mainly due to natural selection. mr. wollaston has discovered the remarkable fact that beetles, out of the species (but more are now known) inhabiting madeira, are so far deficient in wings that they cannot fly; and that, of the twenty-nine endemic genera, no less than twenty-three have all their species in this condition! several facts, namely, that beetles in many parts of the world are very frequently blown to sea and perish; that the beetles in madeira, as observed by mr. wollaston, lie much concealed, until the wind lulls and the sun shines; that the proportion of wingless beetles is larger on the exposed desertas than in madeira itself; and especially the extraordinary fact, so strongly insisted on by mr. wollaston, that certain large groups of beetles, elsewhere excessively numerous, which absolutely require the use of their wings, are here almost entirely absent. these several considerations make me believe that the wingless condition of so many madeira beetles is mainly due to the action of natural selection, combined probably with disuse. for during many successive generations each individual beetle which flew least, either from its wings having been ever so little less perfectly developed or from indolent habit, will have had the best chance of surviving from not being blown out to sea; and, on the other hand, those beetles which most readily took to flight would oftenest have been blown to sea, and thus destroyed. the insects in madeira which are not ground-feeders, and which, as certain flower-feeding coleoptera and lepidoptera, must habitually use their wings to gain their subsistence, have, as mr. wollaston suspects, their wings not at all reduced, but even enlarged. this is quite compatible with the action of natural selection. for when a new insect first arrived on the island, the tendency of natural selection to enlarge or to reduce the wings, would depend on whether a greater number of individuals were saved by successfully battling with the winds, or by giving up the attempt and rarely or never flying. as with mariners shipwrecked near a coast, it would have been better for the good swimmers if they had been able to swim still further, whereas it would have been better for the bad swimmers if they had not been able to swim at all and had stuck to the wreck. the eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered by skin and fur. this state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. in south america, a burrowing rodent, the tuco-tuco, or ctenomys, is even more subterranean in its habits than the mole; and i was assured by a spaniard, who had often caught them, that they were frequently blind. one which i kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. as frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would aid the effects of disuse. it is well known that several animals, belonging to the most different classes, which inhabit the caves of carniola and kentucky, are blind. in some of the crabs the foot-stalk for the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. as it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, their loss may be attributed to disuse. in one of the blind animals, namely, the cave-rat (neotoma), two of which were captured by professor silliman at above half a mile distance from the mouth of the cave, and therefore not in the profoundest depths, the eyes were lustrous and of large size; and these animals, as i am informed by professor silliman, after having been exposed for about a month to a graduated light, acquired a dim perception of objects. it is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar climate; so that, in accordance with the old view of the blind animals having been separately created for the american and european caverns, very close similarity in their organisation and affinities might have been expected. this is certainly not the case if we look at the two whole faunas; with respect to the insects alone, schiodte has remarked: "we are accordingly prevented from considering the entire phenomenon in any other light than something purely local, and the similarity which is exhibited in a few forms between the mammoth cave (in kentucky) and the caves in carniola, otherwise than as a very plain expression of that analogy which subsists generally between the fauna of europe and of north america." on my view we must suppose that american animals, having in most cases ordinary powers of vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the kentucky caves, as did european animals into the caves of europe. we have some evidence of this gradation of habit; for, as schiodte remarks: "we accordingly look upon the subterranean faunas as small ramifications which have penetrated into the earth from the geographically limited faunas of the adjacent tracts, and which, as they extended themselves into darkness, have been accommodated to surrounding circumstances. animals not far remote from ordinary forms, prepare the transition from light to darkness. next follow those that are constructed for twilight; and, last of all, those destined for total darkness, and whose formation is quite peculiar." these remarks of schiodte's it should be understood, apply not to the same, but to distinct species. by the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have effected other changes, such as an increase in the length of the antennae or palpi, as a compensation for blindness. notwithstanding such modifications, we might expect still to see in the cave-animals of america, affinities to the other inhabitants of that continent, and in those of europe to the inhabitants of the european continent. and this is the case with some of the american cave-animals, as i hear from professor dana; and some of the european cave-insects are very closely allied to those of the surrounding country. it would be difficult to give any rational explanation of the affinities of the blind cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation. that several of the inhabitants of the caves of the old and new worlds should be closely related, we might expect from the well-known relationship of most of their other productions. as a blind species of bathyscia is found in abundance on shady rocks far from caves, the loss of vision in the cave species of this one genus has probably had no relation to its dark habitation; for it is natural that an insect already deprived of vision should readily become adapted to dark caverns. another blind genus (anophthalmus) offers this remarkable peculiarity, that the species, as mr. murray observes, have not as yet been found anywhere except in caves; yet those which inhabit the several caves of europe and america are distinct; but it is possible that the progenitors of these several species, while they were furnished with eyes, may formerly have ranged over both continents, and then have become extinct, excepting in their present secluded abodes. far from feeling surprise that some of the cave-animals should be very anomalous, as agassiz has remarked in regard to the blind fish, the amblyopsis, and as is the case with the blind proteus, with reference to the reptiles of europe, i am only surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the scanty inhabitants of these dark abodes will have been exposed. acclimatisation. habit is hereditary with plants, as in the period of flowering, in the time of sleep, in the amount of rain requisite for seeds to germinate, etc., and this leads me to say a few words on acclimatisation. as it is extremely common for distinct species belonging to the same genus to inhabit hot and cold countries, if it be true that all the species of the same genus are descended from a single parent-form, acclimatisation must be readily effected during a long course of descent. it is notorious that each species is adapted to the climate of its own home: species from an arctic or even from a temperate region cannot endure a tropical climate, or conversely. so again, many succulent plants cannot endure a damp climate. but the degree of adaptation of species to the climates under which they live is often overrated. we may infer this from our frequent inability to predict whether or not an imported plant will endure our climate, and from the number of plants and animals brought from different countries which are here perfectly healthy. we have reason to believe that species in a state of nature are closely limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates. but whether or not this adaptation is in most cases very close, we have evidence with some few plants, of their becoming, to a certain extent, naturally habituated to different temperatures; that is, they become acclimatised: thus the pines and rhododendrons, raised from seed collected by dr. hooker from the same species growing at different heights on the himalayas, were found to possess in this country different constitutional powers of resisting cold. mr. thwaites informs me that he has observed similar facts in ceylon; analogous observations have been made by mr. h.c. watson on european species of plants brought from the azores to england; and i could give other cases. in regard to animals, several authentic instances could be adduced of species having largely extended, within historical times, their range from warmer to colder latitudes, and conversely; but we do not positively know that these animals were strictly adapted to their native climate, though in all ordinary cases we assume such to be the case; nor do we know that they have subsequently become specially acclimatised to their new homes, so as to be better fitted for them than they were at first. as we may infer that our domestic animals were originally chosen by uncivilised man because they were useful, and because they bred readily under confinement, and not because they were subsequently found capable of far-extended transportation, the common and extraordinary capacity in our domestic animals of not only withstanding the most different climates, but of being perfectly fertile (a far severer test) under them, may be used as an argument that a large proportion of other animals now in a state of nature could easily be brought to bear widely different climates. we must not, however, push the foregoing argument too far, on account of the probable origin of some of our domestic animals from several wild stocks: the blood, for instance, of a tropical and arctic wolf may perhaps be mingled in our domestic breeds. the rat and mouse cannot be considered as domestic animals, but they have been transported by man to many parts of the world, and now have a far wider range than any other rodent; for they live under the cold climate of faroe in the north and of the falklands in the south, and on many an island in the torrid zones. hence adaptation to any special climate may be looked at as a quality readily grafted on an innate wide flexibility of constitution, common to most animals. on this view, the capacity of enduring the most different climates by man himself and by his domestic animals, and the fact of the extinct elephant and rhinoceros having formerly endured a glacial climate, whereas the living species are now all tropical or sub-tropical in their habits, ought not to be looked at as anomalies, but as examples of a very common flexibility of constitution, brought, under peculiar circumstances, into action. how much of the acclimatisation of species to any peculiar climate is due to mere habit, and how much to the natural selection of varieties having different innate constitutions, and how much to both means combined, is an obscure question. that habit or custom has some influence, i must believe, both from analogy and from the incessant advice given in agricultural works, even in the ancient encyclopaedias of china, to be very cautious in transporting animals from one district to another. and as it is not likely that man should have succeeded in selecting so many breeds and sub-breeds with constitutions specially fitted for their own districts, the result must, i think, be due to habit. on the other hand, natural selection would inevitably tend to preserve those individuals which were born with constitutions best adapted to any country which they inhabited. in treatises on many kinds of cultivated plants, certain varieties are said to withstand certain climates better than others; this is strikingly shown in works on fruit-trees published in the united states, in which certain varieties are habitually recommended for the northern and others for the southern states; and as most of these varieties are of recent origin, they cannot owe their constitutional differences to habit. the case of the jerusalem artichoke, which is never propagated in england by seed, and of which, consequently, new varieties have not been produced, has even been advanced, as proving that acclimatisation cannot be effected, for it is now as tender as ever it was! the case, also, of the kidney-bean has been often cited for a similar purpose, and with much greater weight; but until some one will sow, during a score of generations, his kidney-beans so early that a very large proportion are destroyed by frost, and then collect seed from the few survivors, with care to prevent accidental crosses, and then again get seed from these seedlings, with the same precautions, the experiment cannot be said to have been even tried. nor let it be supposed that differences in the constitution of seedling kidney-beans never appear, for an account has been published how much more hardy some seedlings are than others; and of this fact i have myself observed striking instances. on the whole, we may conclude that habit, or use and disuse, have, in some cases, played a considerable part in the modification of the constitution and structure; but that the effects have often been largely combined with, and sometimes overmastered by, the natural selection of innate variations. correlated variation. i mean by this expression that the whole organisation is so tied together, during its growth and development, that when slight variations in any one part occur and are accumulated through natural selection, other parts become modified. this is a very important subject, most imperfectly understood, and no doubt wholly different classes of facts may be here easily confounded together. we shall presently see that simple inheritance often gives the false appearance of correlation. one of the most obvious real cases is, that variations of structure arising in the young or larvae naturally tend to affect the structure of the mature animal. the several parts which are homologous, and which, at an early embryonic period, are identical in structure, and which are necessarily exposed to similar conditions, seem eminently liable to vary in a like manner: we see this in the right and left sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and limbs, varying together, for the lower jaw is believed by some anatomists to be homologous with the limbs. these tendencies, i do not doubt, may be mastered more or less completely by natural selection: thus a family of stags once existed with an antler only on one side; and if this had been of any great use to the breed, it might probably have been rendered permanent by natural selection. homologous parts, as has been remarked by some authors, tend to cohere; this is often seen in monstrous plants: and nothing is more common than the union of homologous parts in normal structures, as in the union of the petals into a tube. hard parts seem to affect the form of adjoining soft parts; it is believed by some authors that with birds the diversity in the shape of the pelvis causes the remarkable diversity in the shape of the kidneys. others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child. in snakes, according to schlegel, the shape of the body and the manner of swallowing determine the position and form of several of the most important viscera. the nature of the bond is frequently quite obscure. m. is. geoffroy st. hilaire has forcibly remarked that certain malconformations frequently, and that others rarely, coexist without our being able to assign any reason. what can be more singular than the relation in cats between complete whiteness and blue eyes with deafness, or between the tortoise-shell colour and the female sex; or in pigeons, between their feathered feet and skin betwixt the outer toes, or between the presence of more or less down on the young pigeon when first hatched, with the future colour of its plumage; or, again, the relation between the hair and the teeth in the naked turkish dog, though here no doubt homology comes into play? with respect to this latter case of correlation, i think it can hardly be accidental that the two orders of mammals which are most abnormal in their dermal covering, viz., cetacea (whales) and edentata (armadilloes, scaly ant-eaters, etc.), are likewise on the whole the most abnormal in their teeth, but there are so many exceptions to this rule, as mr. mivart has remarked, that it has little value. i know of no case better adapted to show the importance of the laws of correlation and variation, independently of utility, and therefore of natural selection, than that of the difference between the outer and inner flowers in some compositous and umbelliferous plants. everyone is familiar with the difference between the ray and central florets of, for instance, the daisy, and this difference is often accompanied with the partial or complete abortion of the reproductive organs. but in some of these plants the seeds also differ in shape and sculpture. these differences have sometimes been attributed to the pressure of the involucra on the florets, or to their mutual pressure, and the shape of the seeds in the ray-florets of some compositae countenances this idea; but with the umbelliferae it is by no means, as dr. hooker informs me, the species with the densest heads which most frequently differ in their inner and outer flowers. it might have been thought that the development of the ray-petals, by drawing nourishment from the reproductive organs causes their abortion; but this can hardly be the sole case, for in some compositae the seeds of the outer and inner florets differ, without any difference in the corolla. possibly these several differences may be connected with the different flow of nutriment towards the central and external flowers. we know, at least, that with irregular flowers those nearest to the axis are most subject to peloria, that is to become abnormally symmetrical. i may add, as an instance of this fact, and as a striking case of correlation, that in many pelargoniums the two upper petals in the central flower of the truss often lose their patches of darker colour; and when this occurs, the adherent nectary is quite aborted, the central flower thus becoming peloric or regular. when the colour is absent from only one of the two upper petals, the nectary is not quite aborted but is much shortened. with respect to the development of the corolla, sprengel's idea that the ray-florets serve to attract insects, whose agency is highly advantageous, or necessary for the fertilisation of these plants, is highly probable; and if so, natural selection may have come into play. but with respect to the seeds, it seems impossible that their differences in shape, which are not always correlated with any difference in the corolla, can be in any way beneficial; yet in the umbelliferae these differences are of such apparent importance--the seeds being sometimes orthospermous in the exterior flowers and coelospermous in the central flowers--that the elder de candolle founded his main divisions in the order on such characters. hence modifications of structure, viewed by systematists as of high value, may be wholly due to the laws of variation and correlation, without being, as far as we can judge, of the slightest service to the species. we may often falsely attribute to correlated variation structures which are common to whole groups of species, and which in truth are simply due to inheritance; for an ancient progenitor may have acquired through natural selection some one modification in structure, and, after thousands of generations, some other and independent modification; and these two modifications, having been transmitted to a whole group of descendants with diverse habits, would naturally be thought to be in some necessary manner correlated. some other correlations are apparently due to the manner in which natural selection can alone act. for instance, alph. de candolle has remarked that winged seeds are never found in fruits which do not open; i should explain this rule by the impossibility of seeds gradually becoming winged through natural selection, unless the capsules were open; for in this case alone could the seeds, which were a little better adapted to be wafted by the wind, gain an advantage over others less well fitted for wide dispersal. compensation and economy of growth. the elder geoffroy and goethe propounded, at about the same time, their law of compensation or balancement of growth; or, as goethe expressed it, "in order to spend on one side, nature is forced to economise on the other side." i think this holds true to a certain extent with our domestic productions: if nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is difficult to get a cow to give much milk and to fatten readily. the same varieties of the cabbage do not yield abundant and nutritious foliage and a copious supply of oil-bearing seeds. when the seeds in our fruits become atrophied, the fruit itself gains largely in size and quality. in our poultry, a large tuft of feathers on the head is generally accompanied by a diminished comb, and a large beard by diminished wattles. with species in a state of nature it can hardly be maintained that the law is of universal application; but many good observers, more especially botanists, believe in its truth. i will not, however, here give any instances, for i see hardly any way of distinguishing between the effects, on the one hand, of a part being largely developed through natural selection and another and adjoining part being reduced by the same process or by disuse, and, on the other hand, the actual withdrawal of nutriment from one part owing to the excess of growth in another and adjoining part. i suspect, also, that some of the cases of compensation which have been advanced, and likewise some other facts, may be merged under a more general principle, namely, that natural selection is continually trying to economise in every part of the organisation. if under changed conditions of life a structure, before useful, becomes less useful, its diminution will be favoured, for it will profit the individual not to have its nutriment wasted in building up a useless structure. i can thus only understand a fact with which i was much struck when examining cirripedes, and of which many other instances could be given: namely, that when a cirripede is parasitic within another cirripede and is thus protected, it loses more or less completely its own shell or carapace. this is the case with the male ibla, and in a truly extraordinary manner with the proteolepas: for the carapace in all other cirripedes consists of the three highly important anterior segments of the head enormously developed, and furnished with great nerves and muscles; but in the parasitic and protected proteolepas, the whole anterior part of the head is reduced to the merest rudiment attached to the bases of the prehensile antennae. now the saving of a large and complex structure, when rendered superfluous, would be a decided advantage to each successive individual of the species; for in the struggle for life to which every animal is exposed, each would have a better chance of supporting itself, by less nutriment being wasted. thus, as i believe, natural selection will tend in the long run to reduce any part of the organisation, as soon as it becomes, through changed habits, superfluous, without by any means causing some other part to be largely developed in a corresponding degree. and conversely, that natural selection may perfectly well succeed in largely developing an organ without requiring as a necessary compensation the reduction of some adjoining part. multiple, rudimentary, and lowly-organised structures are variable. it seems to be a rule, as remarked by is. geoffroy st. hilaire, both with varieties and species, that when any part or organ is repeated many times in the same individual (as the vertebrae in snakes, and the stamens in polyandrous flowers) the number is variable; whereas the number of the same part or organ, when it occurs in lesser numbers, is constant. the same author as well as some botanists, have further remarked that multiple parts are extremely liable to vary in structure. as "vegetative repetition," to use professor owen's expression, is a sign of low organisation; the foregoing statements accord with the common opinion of naturalists, that beings which stand low in the scale of nature are more variable than those which are higher. i presume that lowness here means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why natural selection should not have preserved or rejected each little deviation of form so carefully as when the part has to serve for some one special purpose. in the same way that a knife which has to cut all sorts of things may be of almost any shape; whilst a tool for some particular purpose must be of some particular shape. natural selection, it should never be forgotten, can act solely through and for the advantage of each being. rudimentary parts, as is generally admitted, are apt to be highly variable. we shall have to recur to this subject; and i will here only add that their variability seems to result from their uselessness, and consequently from natural selection having had no power to check deviations in their structure. a part developed in any species in an extraordinary degree or manner, in comparison with the same part in allied species, tends to be highly variable. several years ago i was much struck by a remark to the above effect made by mr. waterhouse. professor owen, also, seems to have come to a nearly similar conclusion. it is hopeless to attempt to convince any one of the truth of the above proposition without giving the long array of facts which i have collected, and which cannot possibly be here introduced. i can only state my conviction that it is a rule of high generality. i am aware of several causes of error, but i hope that i have made due allowances for them. it should be understood that the rule by no means applies to any part, however unusually developed, unless it be unusually developed in one species or in a few species in comparison with the same part in many closely allied species. thus, the wing of the bat is a most abnormal structure in the class of mammals; but the rule would not apply here, because the whole group of bats possesses wings; it would apply only if some one species had wings developed in a remarkable manner in comparison with the other species of the same genus. the rule applies very strongly in the case of secondary sexual characters, when displayed in any unusual manner. the term, secondary sexual characters, used by hunter, relates to characters which are attached to one sex, but are not directly connected with the act of reproduction. the rule applies to males and females; but more rarely to females, as they seldom offer remarkable secondary sexual characters. the rule being so plainly applicable in the case of secondary sexual characters, may be due to the great variability of these characters, whether or not displayed in any unusual manner--of which fact i think there can be little doubt. but that our rule is not confined to secondary sexual characters is clearly shown in the case of hermaphrodite cirripedes; i particularly attended to mr. waterhouse's remark, whilst investigating this order, and i am fully convinced that the rule almost always holds good. i shall, in a future work, give a list of all the more remarkable cases. i will here give only one, as it illustrates the rule in its largest application. the opercular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures, and they differ extremely little even in distinct genera; but in the several species of one genus, pyrgoma, these valves present a marvellous amount of diversification; the homologous valves in the different species being sometimes wholly unlike in shape; and the amount of variation in the individuals of the same species is so great that it is no exaggeration to state that the varieties of the same species differ more from each other in the characters derived from these important organs, than do the species belonging to other distinct genera. as with birds the individuals of the same species, inhabiting the same country, vary extremely little, i have particularly attended to them; and the rule certainly seems to hold good in this class. i cannot make out that it applies to plants, and this would have seriously shaken my belief in its truth, had not the great variability in plants made it particularly difficult to compare their relative degrees of variability. when we see any part or organ developed in a remarkable degree or manner in a species, the fair presumption is that it is of high importance to that species: nevertheless it is in this case eminently liable to variation. why should this be so? on the view that each species has been independently created, with all its parts as we now see them, i can see no explanation. but on the view that groups of species are descended from some other species, and have been modified through natural selection, i think we can obtain some light. first let me make some preliminary remarks. if, in our domestic animals, any part or the whole animal be neglected, and no selection be applied, that part (for instance, the comb in the dorking fowl) or the whole breed will cease to have a uniform character: and the breed may be said to be degenerating. in rudimentary organs, and in those which have been but little specialised for any particular purpose, and perhaps in polymorphic groups, we see a nearly parallel case; for in such cases natural selection either has not or cannot come into full play, and thus the organisation is left in a fluctuating condition. but what here more particularly concerns us is, that those points in our domestic animals, which at the present time are undergoing rapid change by continued selection, are also eminently liable to variation. look at the individuals of the same breed of the pigeon; and see what a prodigious amount of difference there is in the beak of tumblers, in the beak and wattle of carriers, in the carriage and tail of fantails, etc., these being the points now mainly attended to by english fanciers. even in the same sub-breed, as in that of the short-faced tumbler, it is notoriously difficult to breed nearly perfect birds, many departing widely from the standard. there may truly be said to be a constant struggle going on between, on the one hand, the tendency to reversion to a less perfect state, as well as an innate tendency to new variations, and, on the other hand, the power of steady selection to keep the breed true. in the long run selection gains the day, and we do not expect to fail so completely as to breed a bird as coarse as a common tumbler pigeon from a good short-faced strain. but as long as selection is rapidly going on, much variability in the parts undergoing modification may always be expected. now let us turn to nature. when a part has been developed in an extraordinary manner in any one species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of modification since the period when the several species branched off from the common progenitor of the genus. this period will seldom be remote in any extreme degree, as species rarely endure for more than one geological period. an extraordinary amount of modification implies an unusually large and long-continued amount of variability, which has continually been accumulated by natural selection for the benefit of the species. but as the variability of the extraordinarily developed part or organ has been so great and long-continued within a period not excessively remote, we might, as a general rule, still expect to find more variability in such parts than in other parts of the organisation which have remained for a much longer period nearly constant. and this, i am convinced, is the case. that the struggle between natural selection on the one hand, and the tendency to reversion and variability on the other hand, will in the course of time cease; and that the most abnormally developed organs may be made constant, i see no reason to doubt. hence, when an organ, however abnormal it may be, has been transmitted in approximately the same condition to many modified descendants, as in the case of the wing of the bat, it must have existed, according to our theory, for an immense period in nearly the same state; and thus it has come not to be more variable than any other structure. it is only in those cases in which the modification has been comparatively recent and extraordinarily great that we ought to find the generative variability, as it may be called, still present in a high degree. for in this case the variability will seldom as yet have been fixed by the continued selection of the individuals varying in the required manner and degree, and by the continued rejection of those tending to revert to a former and less modified condition. specific characters more variable than generic characters. the principle discussed under the last heading may be applied to our present subject. it is notorious that specific characters are more variable than generic. to explain by a simple example what is meant: if in a large genus of plants some species had blue flowers and some had red, the colour would be only a specific character, and no one would be surprised at one of the blue species varying into red, or conversely; but if all the species had blue flowers, the colour would become a generic character, and its variation would be a more unusual circumstance. i have chosen this example because the explanation which most naturalists would advance is not here applicable, namely, that specific characters are more variable than generic, because they are taken from parts of less physiological importance than those commonly used for classing genera. i believe this explanation is partly, yet only indirectly, true; i shall, however, have to return to this point in the chapter on classification. it would be almost superfluous to adduce evidence in support of the statement, that ordinary specific characters are more variable than generic; but with respect to important characters, i have repeatedly noticed in works on natural history, that when an author remarks with surprise that some important organ or part, which is generally very constant throughout a large group of species, differs considerably in closely-allied species, it is often variable in the individuals of the same species. and this fact shows that a character, which is generally of generic value, when it sinks in value and becomes only of specific value, often becomes variable, though its physiological importance may remain the same. something of the same kind applies to monstrosities: at least is. geoffroy st. hilaire apparently entertains no doubt, that the more an organ normally differs in the different species of the same group, the more subject it is to anomalies in the individuals. on the ordinary view of each species having been independently created, why should that part of the structure, which differs from the same part in other independently created species of the same genus, be more variable than those parts which are closely alike in the several species? i do not see that any explanation can be given. but on the view that species are only strongly marked and fixed varieties, we might expect often to find them still continuing to vary in those parts of their structure which have varied within a moderately recent period, and which have thus come to differ. or to state the case in another manner: the points in which all the species of a genus resemble each other, and in which they differ from allied genera, are called generic characters; and these characters may be attributed to inheritance from a common progenitor, for it can rarely have happened that natural selection will have modified several distinct species, fitted to more or less widely different habits, in exactly the same manner: and as these so-called generic characters have been inherited from before the period when the several species first branched off from their common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight degree, it is not probable that they should vary at the present day. on the other hand, the points in which species differ from other species of the same genus are called specific characters; and as these specific characters have varied and come to differ since the period when the species branched off from a common progenitor, it is probable that they should still often be in some degree variable--at least more variable than those parts of the organisation which have for a very long period remained constant. secondary sexual characters variable. i think it will be admitted by naturalists, without my entering on details, that secondary sexual characters are highly variable. it will also be admitted that species of the same group differ from each other more widely in their secondary sexual characters, than in other parts of their organisation; compare, for instance, the amount of difference between the males of gallinaceous birds, in which secondary sexual characters are strongly displayed, with the amount of difference between the females. the cause of the original variability of these characters is not manifest; but we can see why they should not have been rendered as constant and uniform as others, for they are accumulated by sexual selection, which is less rigid in its action than ordinary selection, as it does not entail death, but only gives fewer offspring to the less favoured males. whatever the cause may be of the variability of secondary sexual characters, as they are highly variable, sexual selection will have had a wide scope for action, and may thus have succeeded in giving to the species of the same group a greater amount of difference in these than in other respects. it is a remarkable fact, that the secondary differences between the two sexes of the same species are generally displayed in the very same parts of the organisation in which the species of the same genus differ from each other. of this fact i will give in illustration the first two instances which happen to stand on my list; and as the differences in these cases are of a very unusual nature, the relation can hardly be accidental. the same number of joints in the tarsi is a character common to very large groups of beetles, but in the engidae, as westwood has remarked, the number varies greatly and the number likewise differs in the two sexes of the same species. again in the fossorial hymenoptera, the neuration of the wings is a character of the highest importance, because common to large groups; but in certain genera the neuration differs in the different species, and likewise in the two sexes of the same species. sir j. lubbock has recently remarked, that several minute crustaceans offer excellent illustrations of this law. "in pontella, for instance, the sexual characters are afforded mainly by the anterior antennae and by the fifth pair of legs: the specific differences also are principally given by these organs." this relation has a clear meaning on my view: i look at all the species of the same genus as having as certainly descended from the same progenitor, as have the two sexes of any one species. consequently, whatever part of the structure of the common progenitor, or of its early descendants, became variable; variations of this part would, it is highly probable, be taken advantage of by natural and sexual selection, in order to fit the several places in the economy of nature, and likewise to fit the two sexes of the same species to each other, or to fit the males to struggle with other males for the possession of the females. finally, then, i conclude that the greater variability of specific characters, or those which distinguish species from species, than of generic characters, or those which are possessed by all the species; that the frequent extreme variability of any part which is developed in a species in an extraordinary manner in comparison with the same part in its congeners; and the slight degree of variability in a part, however extraordinarily it may be developed, if it be common to a whole group of species; that the great variability of secondary sexual characters and their great difference in closely allied species; that secondary sexual and ordinary specific differences are generally displayed in the same parts of the organisation, are all principles closely connected together. all being mainly due to the species of the same group being the descendants of a common progenitor, from whom they have inherited much in common, to parts which have recently and largely varied being more likely still to go on varying than parts which have long been inherited and have not varied, to natural selection having more or less completely, according to the lapse of time, overmastered the tendency to reversion and to further variability, to sexual selection being less rigid than ordinary selection, and to variations in the same parts having been accumulated by natural and sexual selection, and thus having been adapted for secondary sexual, and for ordinary purposes. distinct species present analogous variations, so that a variety of one species often assumes a character proper to an allied species, or reverts to some of the characters of an early progenitor. these propositions will be most readily understood by looking to our domestic races. the most distinct breeds of the pigeon, in countries widely apart, present sub-varieties with reversed feathers on the head, and with feathers on the feet, characters not possessed by the aboriginal rock-pigeon; these then are analogous variations in two or more distinct races. the frequent presence of fourteen or even sixteen tail-feathers in the pouter may be considered as a variation representing the normal structure of another race, the fantail. i presume that no one will doubt that all such analogous variations are due to the several races of the pigeon having inherited from a common parent the same constitution and tendency to variation, when acted on by similar unknown influences. in the vegetable kingdom we have a case of analogous variation, in the enlarged stems, or as commonly called roots, of the swedish turnip and ruta-baga, plants which several botanists rank as varieties produced by cultivation from a common parent: if this be not so, the case will then be one of analogous variation in two so-called distinct species; and to these a third may be added, namely, the common turnip. according to the ordinary view of each species having been independently created, we should have to attribute this similarity in the enlarged stems of these three plants, not to the vera causa of community of descent, and a consequent tendency to vary in a like manner, but to three separate yet closely related acts of creation. many similar cases of analogous variation have been observed by naudin in the great gourd family, and by various authors in our cereals. similar cases occurring with insects under natural conditions have lately been discussed with much ability by mr. walsh, who has grouped them under his law of equable variability. with pigeons, however, we have another case, namely, the occasional appearance in all the breeds, of slaty-blue birds with two black bars on the wings, white loins, a bar at the end of the tail, with the outer feathers externally edged near their bases with white. as all these marks are characteristic of the parent rock-pigeon, i presume that no one will doubt that this is a case of reversion, and not of a new yet analogous variation appearing in the several breeds. we may, i think, confidently come to this conclusion, because, as we have seen, these coloured marks are eminently liable to appear in the crossed offspring of two distinct and differently coloured breeds; and in this case there is nothing in the external conditions of life to cause the reappearance of the slaty-blue, with the several marks, beyond the influence of the mere act of crossing on the laws of inheritance. no doubt it is a very surprising fact that characters should reappear after having been lost for many, probably for hundreds of generations. but when a breed has been crossed only once by some other breed, the offspring occasionally show for many generations a tendency to revert in character to the foreign breed--some say, for a dozen or even a score of generations. after twelve generations, the proportion of blood, to use a common expression, from one ancestor, is only in ; and yet, as we see, it is generally believed that a tendency to reversion is retained by this remnant of foreign blood. in a breed which has not been crossed, but in which both parents have lost some character which their progenitor possessed, the tendency, whether strong or weak, to reproduce the lost character might, as was formerly remarked, for all that we can see to the contrary, be transmitted for almost any number of generations. when a character which has been lost in a breed, reappears after a great number of generations, the most probable hypothesis is, not that one individual suddenly takes after an ancestor removed by some hundred generations, but that in each successive generation the character in question has been lying latent, and at last, under unknown favourable conditions, is developed. with the barb-pigeon, for instance, which very rarely produces a blue bird, it is probable that there is a latent tendency in each generation to produce blue plumage. the abstract improbability of such a tendency being transmitted through a vast number of generations, is not greater than that of quite useless or rudimentary organs being similarly transmitted. a mere tendency to produce a rudiment is indeed sometimes thus inherited. as all the species of the same genus are supposed to be descended from a common progenitor, it might be expected that they would occasionally vary in an analogous manner; so that the varieties of two or more species would resemble each other, or that a variety of one species would resemble in certain characters another and distinct species, this other species being, according to our view, only a well-marked and permanent variety. but characters exclusively due to analogous variation would probably be of an unimportant nature, for the preservation of all functionally important characters will have been determined through natural selection, in accordance with the different habits of the species. it might further be expected that the species of the same genus would occasionally exhibit reversions to long-lost characters. as, however, we do not know the common ancestor of any natural group, we cannot distinguish between reversionary and analogous characters. if, for instance, we did not know that the parent rock-pigeon was not feather-footed or turn-crowned, we could not have told, whether such characters in our domestic breeds were reversions or only analogous variations; but we might have inferred that the blue colour was a case of reversion from the number of the markings, which are correlated with this tint, and which would not probably have all appeared together from simple variation. more especially we might have inferred this from the blue colour and the several marks so often appearing when differently coloured breeds are crossed. hence, although under nature it must generally be left doubtful, what cases are reversions to formerly existing characters, and what are new but analogous variations, yet we ought, on our theory, sometimes to find the varying offspring of a species assuming characters which are already present in other members of the same group. and this undoubtedly is the case. the difficulty in distinguishing variable species is largely due to the varieties mocking, as it were, other species of the same genus. a considerable catalogue, also, could be given of forms intermediate between two other forms, which themselves can only doubtfully be ranked as species; and this shows, unless all these closely allied forms be considered as independently created species, that they have in varying assumed some of the characters of the others. but the best evidence of analogous variations is afforded by parts or organs which are generally constant in character, but which occasionally vary so as to resemble, in some degree, the same part or organ in an allied species. i have collected a long list of such cases; but here, as before, i lie under the great disadvantage of not being able to give them. i can only repeat that such cases certainly occur, and seem to me very remarkable. i will, however, give one curious and complex case, not indeed as affecting any important character, but from occurring in several species of the same genus, partly under domestication and partly under nature. it is a case almost certainly of reversion. the ass sometimes has very distinct transverse bars on its legs, like those on the legs of a zebra. it has been asserted that these are plainest in the foal, and from inquiries which i have made, i believe this to be true. the stripe on the shoulder is sometimes double, and is very variable in length and outline. a white ass, but not an albino, has been described without either spinal or shoulder stripe; and these stripes are sometimes very obscure, or actually quite lost, in dark-coloured asses. the koulan of pallas is said to have been seen with a double shoulder-stripe. mr. blyth has seen a specimen of the hemionus with a distinct shoulder-stripe, though it properly has none; and i have been informed by colonel poole that foals of this species are generally striped on the legs and faintly on the shoulder. the quagga, though so plainly barred like a zebra over the body, is without bars on the legs; but dr. gray has figured one specimen with very distinct zebra-like bars on the hocks. with respect to the horse, i have collected cases in england of the spinal stripe in horses of the most distinct breeds, and of all colours; transverse bars on the legs are not rare in duns, mouse-duns, and in one instance in a chestnut; a faint shoulder-stripe may sometimes be seen in duns, and i have seen a trace in a bay horse. my son made a careful examination and sketch for me of a dun belgian cart-horse with a double stripe on each shoulder and with leg-stripes. i have myself seen a dun devonshire pony, and a small dun welsh pony has been carefully described to me, both with three parallel stripes on each shoulder. in the northwest part of india the kattywar breed of horses is so generally striped, that, as i hear from colonel poole, who examined this breed for the indian government, a horse without stripes is not considered as purely bred. the spine is always striped; the legs are generally barred; and the shoulder-stripe, which is sometimes double and sometimes treble, is common; the side of the face, moreover, is sometimes striped. the stripes are often plainest in the foal; and sometimes quite disappear in old horses. colonel poole has seen both gray and bay kattywar horses striped when first foaled. i have also reason to suspect, from information given me by mr. w.w. edwards, that with the english race-horse the spinal stripe is much commoner in the foal than in the full-grown animal. i have myself recently bred a foal from a bay mare (offspring of a turkoman horse and a flemish mare) by a bay english race-horse. this foal, when a week old, was marked on its hinder quarters and on its forehead with numerous very narrow, dark, zebra-like bars, and its legs were feebly striped. all the stripes soon disappeared completely. without here entering on further details i may state that i have collected cases of leg and shoulder stripes in horses of very different breeds in various countries from britain to eastern china; and from norway in the north to the malay archipelago in the south. in all parts of the world these stripes occur far oftenest in duns and mouse-duns; by the term dun a large range of colour is included, from one between brown and black to a close approach to cream colour. i am aware that colonel hamilton smith, who has written on this subject, believes that the several breeds of the horse are descended from several aboriginal species, one of which, the dun, was striped; and that the above-described appearances are all due to ancient crosses with the dun stock. but this view may be safely rejected, for it is highly improbable that the heavy belgian cart-horse, welsh ponies, norwegian cobs, the lanky kattywar race, etc., inhabiting the most distant parts of the world, should have all have been crossed with one supposed aboriginal stock. now let us turn to the effects of crossing the several species of the horse genus. rollin asserts that the common mule from the ass and horse is particularly apt to have bars on its legs; according to mr. gosse, in certain parts of the united states, about nine out of ten mules have striped legs. i once saw a mule with its legs so much striped that any one might have thought that it was a hybrid zebra; and mr. w.c. martin, in his excellent treatise on the horse, has given a figure of a similar mule. in four coloured drawings, which i have seen, of hybrids between the ass and zebra, the legs were much more plainly barred than the rest of the body; and in one of them there was a double shoulder-stripe. in lord morton's famous hybrid, from a chestnut mare and male quagga, the hybrid and even the pure offspring subsequently produced from the same mare by a black arabian sire, were much more plainly barred across the legs than is even the pure quagga. lastly, and this is another most remarkable case, a hybrid has been figured by dr. gray (and he informs me that he knows of a second case) from the ass and the hemionus; and this hybrid, though the ass only occasionally has stripes on his legs and the hemionus has none and has not even a shoulder-stripe, nevertheless had all four legs barred, and had three short shoulder-stripes, like those on the dun devonshire and welsh ponies, and even had some zebra-like stripes on the sides of its face. with respect to this last fact, i was so convinced that not even a stripe of colour appears from what is commonly called chance, that i was led solely from the occurrence of the face-stripes on this hybrid from the ass and hemionus to ask colonel poole whether such face-stripes ever occurred in the eminently striped kattywar breed of horses, and was, as we have seen, answered in the affirmative. what now are we to say to these several facts? we see several distinct species of the horse genus becoming, by simple variation, striped on the legs like a zebra, or striped on the shoulders like an ass. in the horse we see this tendency strong whenever a dun tint appears--a tint which approaches to that of the general colouring of the other species of the genus. the appearance of the stripes is not accompanied by any change of form, or by any other new character. we see this tendency to become striped most strongly displayed in hybrids from between several of the most distinct species. now observe the case of the several breeds of pigeons: they are descended from a pigeon (including two or three sub-species or geographical races) of a bluish colour, with certain bars and other marks; and when any breed assumes by simple variation a bluish tint, these bars and other marks invariably reappear; but without any other change of form or character. when the oldest and truest breeds of various colours are crossed, we see a strong tendency for the blue tint and bars and marks to reappear in the mongrels. i have stated that the most probable hypothesis to account for the reappearance of very ancient characters, is--that there is a tendency in the young of each successive generation to produce the long-lost character, and that this tendency, from unknown causes, sometimes prevails. and we have just seen that in several species of the horse genus the stripes are either plainer or appear more commonly in the young than in the old. call the breeds of pigeons, some of which have bred true for centuries, species; and how exactly parallel is the case with that of the species of the horse genus! for myself, i venture confidently to look back thousands on thousands of generations, and i see an animal striped like a zebra, but perhaps otherwise very differently constructed, the common parent of our domestic horse (whether or not it be descended from one or more wild stocks) of the ass, the hemionus, quagga, and zebra. he who believes that each equine species was independently created, will, i presume, assert that each species has been created with a tendency to vary, both under nature and under domestication, in this particular manner, so as often to become striped like the other species of the genus; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus. to admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown cause. it makes the works of god a mere mockery and deception; i would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore. summary. our ignorance of the laws of variation is profound. not in one case out of a hundred can we pretend to assign any reason why this or that part has varied. but whenever we have the means of instituting a comparison, the same laws appear to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus. changed conditions generally induce mere fluctuating variability, but sometimes they cause direct and definite effects; and these may become strongly marked in the course of time, though we have not sufficient evidence on this head. habit in producing constitutional peculiarities, and use in strengthening, and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects. homologous parts tend to vary in the same manner, and homologous parts tend to cohere. modifications in hard parts and in external parts sometimes affect softer and internal parts. when one part is largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of the structure which can be saved without detriment will be saved. changes of structure at an early age may affect parts subsequently developed; and many cases of correlated variation, the nature of which we are unable to understand, undoubtedly occur. multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialised for any particular function, so that their modifications have not been closely checked by natural selection. it follows probably from this same cause, that organic beings low in the scale are more variable than those standing higher in the scale, and which have their whole organisation more specialised. rudimentary organs, from being useless, are not regulated by natural selection, and hence are variable. specific characters--that is, the characters which have come to differ since the several species of the same genus branched off from a common parent--are more variable than generic characters, or those which have long been inherited, and have not differed within this same period. in these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ; but we have also seen in the second chapter that the same principle applies to the whole individual; for in a district where many species of a genus are found--that is, where there has been much former variation and differentiation, or where the manufactory of new specific forms has been actively at work--in that district and among these species, we now find, on an average, most varieties. secondary sexual characters are highly variable, and such characters differ much in the species of the same group. variability in the same parts of the organisation has generally been taken advantage of in giving secondary sexual differences to the two sexes of the same species, and specific differences to the several species of the same genus. any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. but when a species with an extraordinarily developed organ has become the parent of many modified descendants--which on our view must be a very slow process, requiring a long lapse of time--in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed. species inheriting nearly the same constitution from a common parent, and exposed to similar influences, naturally tend to present analogous variations, or these same species may occasionally revert to some of the characters of their ancient progenitors. although new and important modifications may not arise from reversion and analogous variation, such modifications will add to the beautiful and harmonious diversity of nature. whatever the cause may be of each slight difference between the offspring and their parents--and a cause for each must exist--we have reason to believe that it is the steady accumulation of beneficial differences which has given rise to all the more important modifications of structure in relation to the habits of each species. chapter vi. difficulties of the theory. difficulties of the theory of descent with modification--absence or rarity of transitional varieties--transitions in habits of life--diversified habits in the same species--species with habits widely different from those of their allies--organs of extreme perfection--modes of transition--cases of difficulty--natura non facit saltum--organs of small importance--organs not in all cases absolutely perfect--the law of unity of type and of the conditions of existence embraced by the theory of natural selection. long before the reader has arrived at this part of my work, a crowd of difficulties will have occurred to him. some of them are so serious that to this day i can hardly reflect on them without being in some degree staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, i think, fatal to the theory. these difficulties and objections may be classed under the following heads: first, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? why is not all nature in confusion, instead of the species being, as we see them, well defined? secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some other animal with widely different habits and structure? can we believe that natural selection could produce, on the one hand, an organ of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, an organ so wonderful as the eye? thirdly, can instincts be acquired and modified through natural selection? what shall we say to the instinct which leads the bee to make cells, and which has practically anticipated the discoveries of profound mathematicians? fourthly, how can we account for species, when crossed, being sterile and producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired? the two first heads will be here discussed; some miscellaneous objections in the following chapter; instinct and hybridism in the two succeeding chapters. on the absence or rarity of transitional varieties. as natural selection acts solely by the preservation of profitable modifications, each new form will tend in a fully-stocked country to take the place of, and finally to exterminate, its own less improved parent-form and other less-favoured forms with which it comes into competition. thus extinction and natural selection go hand in hand. hence, if we look at each species as descended from some unknown form, both the parent and all the transitional varieties will generally have been exterminated by the very process of the formation and perfection of the new form. but, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth? it will be more convenient to discuss this question in the chapter on the imperfection of the geological record; and i will here only state that i believe the answer mainly lies in the record being incomparably less perfect than is generally supposed. the crust of the earth is a vast museum; but the natural collections have been imperfectly made, and only at long intervals of time. but it may be urged that when several closely allied species inhabit the same territory, we surely ought to find at the present time many transitional forms. let us take a simple case: in travelling from north to south over a continent, we generally meet at successive intervals with closely allied or representative species, evidently filling nearly the same place in the natural economy of the land. these representative species often meet and interlock; and as the one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other. but if we compare these species where they intermingle, they are generally as absolutely distinct from each other in every detail of structure as are specimens taken from the metropolis inhabited by each. by my theory these allied species are descended from a common parent; and during the process of modification, each has become adapted to the conditions of life of its own region, and has supplanted and exterminated its original parent-form and all the transitional varieties between its past and present states. hence we ought not to expect at the present time to meet with numerous transitional varieties in each region, though they must have existed there, and may be embedded there in a fossil condition. but in the intermediate region, having intermediate conditions of life, why do we not now find closely-linking intermediate varieties? this difficulty for a long time quite confounded me. but i think it can be in large part explained. in the first place we should be extremely cautious in inferring, because an area is now continuous, that it has been continuous during a long period. geology would lead us to believe that most continents have been broken up into islands even during the later tertiary periods; and in such islands distinct species might have been separately formed without the possibility of intermediate varieties existing in the intermediate zones. by changes in the form of the land and of climate, marine areas now continuous must often have existed within recent times in a far less continuous and uniform condition than at present. but i will pass over this way of escaping from the difficulty; for i believe that many perfectly defined species have been formed on strictly continuous areas; though i do not doubt that the formerly broken condition of areas now continuous, has played an important part in the formation of new species, more especially with freely-crossing and wandering animals. in looking at species as they are now distributed over a wide area, we generally find them tolerably numerous over a large territory, then becoming somewhat abruptly rarer and rarer on the confines, and finally disappearing. hence the neutral territory between two representative species is generally narrow in comparison with the territory proper to each. we see the same fact in ascending mountains, and sometimes it is quite remarkable how abruptly, as alph. de candolle has observed, a common alpine species disappears. the same fact has been noticed by e. forbes in sounding the depths of the sea with the dredge. to those who look at climate and the physical conditions of life as the all-important elements of distribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly. but when we bear in mind that almost every species, even in its metropolis, would increase immensely in numbers, were it not for other competing species; that nearly all either prey on or serve as prey for others; in short, that each organic being is either directly or indirectly related in the most important manner to other organic beings--we see that the range of the inhabitants of any country by no means exclusively depends on insensibly changing physical conditions, but in large part on the presence of other species, on which it lives, or by which it is destroyed, or with which it comes into competition; and as these species are already defined objects, not blending one into another by insensible gradations, the range of any one species, depending as it does on the range of others, will tend to be sharply defined. moreover, each species on the confines of its range, where it exists in lessened numbers, will, during fluctuations in the number of its enemies or of its prey, or in the nature of the seasons, be extremely liable to utter extermination; and thus its geographical range will come to be still more sharply defined. as allied or representative species, when inhabiting a continuous area, are generally distributed in such a manner that each has a wide range, with a comparatively narrow neutral territory between them, in which they become rather suddenly rarer and rarer; then, as varieties do not essentially differ from species, the same rule will probably apply to both; and if we take a varying species inhabiting a very large area, we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone. the intermediate variety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser area; and practically, as far as i can make out, this rule holds good with varieties in a state of nature. i have met with striking instances of the rule in the case of varieties intermediate between well-marked varieties in the genus balanus. and it would appear from information given me by mr. watson, dr. asa gray, and mr. wollaston, that generally, when varieties intermediate between two other forms occur, they are much rarer numerically than the forms which they connect. now, if we may trust these facts and inferences, and conclude that varieties linking two other varieties together generally have existed in lesser numbers than the forms which they connect, then we can understand why intermediate varieties should not endure for very long periods: why, as a general rule, they should be exterminated and disappear, sooner than the forms which they originally linked together. for any form existing in lesser numbers would, as already remarked, run a greater chance of being exterminated than one existing in large numbers; and in this particular case the intermediate form would be eminently liable to the inroads of closely allied forms existing on both sides of it. but it is a far more important consideration, that during the process of further modification, by which two varieties are supposed to be converted and perfected into two distinct species, the two which exist in larger numbers, from inhabiting larger areas, will have a great advantage over the intermediate variety, which exists in smaller numbers in a narrow and intermediate zone. for forms existing in larger numbers will have a better chance, within any given period, of presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers. hence, the more common forms, in the race for life, will tend to beat and supplant the less common forms, for these will be more slowly modified and improved. it is the same principle which, as i believe, accounts for the common species in each country, as shown in the second chapter, presenting on an average a greater number of well-marked varieties than do the rarer species. i may illustrate what i mean by supposing three varieties of sheep to be kept, one adapted to an extensive mountainous region; a second to a comparatively narrow, hilly tract; and a third to the wide plains at the base; and that the inhabitants are all trying with equal steadiness and skill to improve their stocks by selection; the chances in this case will be strongly in favour of the great holders on the mountains or on the plains improving their breeds more quickly than the small holders on the intermediate narrow, hilly tract; and consequently the improved mountain or plain breed will soon take the place of the less improved hill breed; and thus the two breeds, which originally existed in greater numbers, will come into close contact with each other, without the interposition of the supplanted, intermediate hill variety. to sum up, i believe that species come to be tolerably well-defined objects, and do not at any one period present an inextricable chaos of varying and intermediate links: first, because new varieties are very slowly formed, for variation is a slow process, and natural selection can do nothing until favourable individual differences or variations occur, and until a place in the natural polity of the country can be better filled by some modification of some one or more of its inhabitants. and such new places will depend on slow changes of climate, or on the occasional immigration of new inhabitants, and, probably, in a still more important degree, on some of the old inhabitants becoming slowly modified, with the new forms thus produced and the old ones acting and reacting on each other. so that, in any one region and at any one time, we ought to see only a few species presenting slight modifications of structure in some degree permanent; and this assuredly we do see. secondly, areas now continuous must often have existed within the recent period as isolated portions, in which many forms, more especially among the classes which unite for each birth and wander much, may have separately been rendered sufficiently distinct to rank as representative species. in this case, intermediate varieties between the several representative species and their common parent, must formerly have existed within each isolated portion of the land, but these links during the process of natural selection will have been supplanted and exterminated, so that they will no longer be found in a living state. thirdly, when two or more varieties have been formed in different portions of a strictly continuous area, intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will generally have had a short duration. for these intermediate varieties will, from reasons already assigned (namely from what we know of the actual distribution of closely allied or representative species, and likewise of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they tend to connect. from this cause alone the intermediate varieties will be liable to accidental extermination; and during the process of further modification through natural selection, they will almost certainly be beaten and supplanted by the forms which they connect; for these, from existing in greater numbers will, in the aggregate, present more varieties, and thus be further improved through natural selection and gain further advantages. lastly, looking not to any one time, but at all time, if my theory be true, numberless intermediate varieties, linking closely together all the species of the same group, must assuredly have existed; but the very process of natural selection constantly tends, as has been so often remarked, to exterminate the parent forms and the intermediate links. consequently evidence of their former existence could be found only among fossil remains, which are preserved, as we shall attempt to show in a future chapter, in an extremely imperfect and intermittent record. on the origin and transition of organic beings with peculiar habits and structure. it has been asked by the opponents of such views as i hold, how, for instance, could a land carnivorous animal have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted? it would be easy to show that there now exist carnivorous animals presenting close intermediate grades from strictly terrestrial to aquatic habits; and as each exists by a struggle for life, it is clear that each must be well adapted to its place in nature. look at the mustela vison of north america, which has webbed feet, and which resembles an otter in its fur, short legs, and form of tail; during summer this animal dives for and preys on fish, but during the long winter it leaves the frozen waters, and preys, like other polecats on mice and land animals. if a different case had been taken, and it had been asked how an insectivorous quadruped could possibly have been converted into a flying bat, the question would have been far more difficult to answer. yet i think such difficulties have little weight. here, as on other occasions, i lie under a heavy disadvantage, for, out of the many striking cases which i have collected, i can give only one or two instances of transitional habits and structures in allied species; and of diversified habits, either constant or occasional, in the same species. and it seems to me that nothing less than a long list of such cases is sufficient to lessen the difficulty in any particular case like that of the bat. look at the family of squirrels; here we have the finest gradation from animals with their tails only slightly flattened, and from others, as sir j. richardson has remarked, with the posterior part of their bodies rather wide and with the skin on their flanks rather full, to the so-called flying squirrels; and flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to glide through the air to an astonishing distance from tree to tree. we cannot doubt that each structure is of use to each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey, or to collect food more quickly, or, as there is reason to believe, to lessen the danger from occasional falls. but it does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive under all possible conditions. let the climate and vegetation change, let other competing rodents or new beasts of prey immigrate, or old ones become modified, and all analogy would lead us to believe that some, at least, of the squirrels would decrease in numbers or become exterminated, unless they also become modified and improved in structure in a corresponding manner. therefore, i can see no difficulty, more especially under changing conditions of life, in the continued preservation of individuals with fuller and fuller flank-membranes, each modification being useful, each being propagated, until, by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was produced. now look at the galeopithecus or so-called flying lemur, which was formerly ranked among bats, but is now believed to belong to the insectivora. an extremely wide flank-membrane stretches from the corners of the jaw to the tail, and includes the limbs with the elongated fingers. this flank-membrane is furnished with an extensor muscle. although no graduated links of structure, fitted for gliding through the air, now connect the galeopithecus with the other insectivora, yet there is no difficulty in supposing that such links formerly existed, and that each was developed in the same manner as with the less perfectly gliding squirrels; each grade of structure having been useful to its possessor. nor can i see any insuperable difficulty in further believing it possible that the membrane-connected fingers and fore-arm of the galeopithecus might have been greatly lengthened by natural selection; and this, as far as the organs of flight are concerned, would have converted the animal into a bat. in certain bats in which the wing-membrane extends from the top of the shoulder to the tail and includes the hind-legs, we perhaps see traces of an apparatus originally fitted for gliding through the air rather than for flight. if about a dozen genera of birds were to become extinct, who would have ventured to surmise that birds might have existed which used their wings solely as flappers, like the logger headed duck (micropterus of eyton); as fins in the water and as front legs on the land, like the penguin; as sails, like the ostrich; and functionally for no purpose, like the apteryx? yet the structure of each of these birds is good for it, under the conditions of life to which it is exposed, for each has to live by a struggle: but it is not necessarily the best possible under all possible conditions. it must not be inferred from these remarks that any of the grades of wing-structure here alluded to, which perhaps may all be the result of disuse, indicate the steps by which birds actually acquired their perfect power of flight; but they serve to show what diversified means of transition are at least possible. seeing that a few members of such water-breathing classes as the crustacea and mollusca are adapted to live on the land; and seeing that we have flying birds and mammals, flying insects of the most diversified types, and formerly had flying reptiles, it is conceivable that flying-fish, which now glide far through the air, slightly rising and turning by the aid of their fluttering fins, might have been modified into perfectly winged animals. if this had been effected, who would have ever imagined that in an early transitional state they had been inhabitants of the open ocean, and had used their incipient organs of flight exclusively, so far as we know, to escape being devoured by other fish? when we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we should bear in mind that animals displaying early transitional grades of the structure will seldom have survived to the present day, for they will have been supplanted by their successors, which were gradually rendered more perfect through natural selection. furthermore, we may conclude that transitional states between structures fitted for very different habits of life will rarely have been developed at an early period in great numbers and under many subordinate forms. thus, to return to our imaginary illustration of the flying-fish, it does not seem probable that fishes capable of true flight would have been developed under many subordinate forms, for taking prey of many kinds in many ways, on the land and in the water, until their organs of flight had come to a high stage of perfection, so as to have given them a decided advantage over other animals in the battle for life. hence the chance of discovering species with transitional grades of structure in a fossil condition will always be less, from their having existed in lesser numbers, than in the case of species with fully developed structures. i will now give two or three instances, both of diversified and of changed habits, in the individuals of the same species. in either case it would be easy for natural selection to adapt the structure of the animal to its changed habits, or exclusively to one of its several habits. it is, however, difficult to decide and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed habits; both probably often occurring almost simultaneously. of cases of changed habits it will suffice merely to allude to that of the many british insects which now feed on exotic plants, or exclusively on artificial substances. of diversified habits innumerable instances could be given: i have often watched a tyrant flycatcher (saurophagus sulphuratus) in south america, hovering over one spot and then proceeding to another, like a kestrel, and at other times standing stationary on the margin of water, and then dashing into it like a kingfisher at a fish. in our own country the larger titmouse (parus major) may be seen climbing branches, almost like a creeper; it sometimes, like a shrike, kills small birds by blows on the head; and i have many times seen and heard it hammering the seeds of the yew on a branch, and thus breaking them like a nuthatch. in north america the black bear was seen by hearne swimming for hours with widely open mouth, thus catching, almost like a whale, insects in the water. as we sometimes see individuals following habits different from those proper to their species and to the other species of the same genus, we might expect that such individuals would occasionally give rise to new species, having anomalous habits, and with their structure either slightly or considerably modified from that of their type. and such instances occur in nature. can a more striking instance of adaptation be given than that of a woodpecker for climbing trees and seizing insects in the chinks of the bark? yet in north america there are woodpeckers which feed largely on fruit, and others with elongated wings which chase insects on the wing. on the plains of la plata, where hardly a tree grows, there is a woodpecker (colaptes campestris) which has two toes before and two behind, a long-pointed tongue, pointed tail-feathers, sufficiently stiff to support the bird in a vertical position on a post, but not so stiff as in the typical wood-peckers, and a straight, strong beak. the beak, however, is not so straight or so strong as in the typical woodpeckers but it is strong enough to bore into wood. hence this colaptes, in all the essential parts of its structure, is a woodpecker. even in such trifling characters as the colouring, the harsh tone of the voice, and undulatory flight, its close blood-relationship to our common woodpecker is plainly declared; yet, as i can assert, not only from my own observations, but from those of the accurate azara, in certain large districts it does not climb trees, and it makes its nest in holes in banks! in certain other districts, however, this same woodpecker, as mr. hudson states, frequents trees, and bores holes in the trunk for its nest. i may mention as another illustration of the varied habits of this genus, that a mexican colaptes has been described by de saussure as boring holes into hard wood in order to lay up a store of acorns. petrels are the most aerial and oceanic of birds, but, in the quiet sounds of tierra del fuego, the puffinuria berardi, in its general habits, in its astonishing power of diving, in its manner of swimming and of flying when made to take flight, would be mistaken by any one for an auk or a grebe; nevertheless, it is essentially a petrel, but with many parts of its organisation profoundly modified in relation to its new habits of life; whereas the woodpecker of la plata has had its structure only slightly modified. in the case of the water-ouzel, the acutest observer, by examining its dead body, would never have suspected its sub-aquatic habits; yet this bird, which is allied to the thrush family, subsists by diving,--using its wings under water and grasping stones with its feet. all the members of the great order of hymenopterous insects are terrestrial, excepting the genus proctotrupes, which sir john lubbock has discovered to be aquatic in its habits; it often enters the water and dives about by the use not of its legs but of its wings, and remains as long as four hours beneath the surface; yet it exhibits no modification in structure in accordance with its abnormal habits. he who believes that each being has been created as we now see it, must occasionally have felt surprise when he has met with an animal having habits and structure not in agreement. what can be plainer than that the webbed feet of ducks and geese are formed for swimming? yet there are upland geese with webbed feet which rarely go near the water; and no one except audubon, has seen the frigate-bird, which has all its four toes webbed, alight on the surface of the ocean. on the other hand, grebes and coots are eminently aquatic, although their toes are only bordered by membrane. what seems plainer than that the long toes, not furnished with membrane, of the grallatores, are formed for walking over swamps and floating plants. the water-hen and landrail are members of this order, yet the first is nearly as aquatic as the coot, and the second is nearly as terrestrial as the quail or partridge. in such cases, and many others could be given, habits have changed without a corresponding change of structure. the webbed feet of the upland goose may be said to have become almost rudimentary in function, though not in structure. in the frigate-bird, the deeply scooped membrane between the toes shows that structure has begun to change. he who believes in separate and innumerable acts of creation may say, that in these cases it has pleased the creator to cause a being of one type to take the place of one belonging to another type; but this seems to me only restating the fact in dignified language. he who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being varies ever so little, either in habits or structure, and thus gains an advantage over some other inhabitant of the same country, it will seize on the place of that inhabitant, however different that may be from its own place. hence it will cause him no surprise that there should be geese and frigate-birds with webbed feet, living on the dry land and rarely alighting on the water, that there should be long-toed corncrakes, living in meadows instead of in swamps; that there should be woodpeckers where hardly a tree grows; that there should be diving thrushes and diving hymenoptera, and petrels with the habits of auks. organs of extreme perfection and complication. to suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, i freely confess, absurd in the highest degree. when it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of vox populi, vox dei, as every philosopher knows, cannot be trusted in science. reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. how a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but i may remark that, as some of the lowest organisms in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility. in searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its lineal progenitors; but this is scarcely ever possible, and we are forced to look to other species and genera of the same group, that is to the collateral descendants from the same parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted in an unaltered or little altered condition. but the state of the same organ in distinct classes may incidentally throw light on the steps by which it has been perfected. the simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells and covered by translucent skin, but without any lens or other refractive body. we may, however, according to m. jourdain, descend even a step lower and find aggregates of pigment-cells, apparently serving as organs of vision, without any nerves, and resting merely on sarcodic tissue. eyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness. in certain star-fishes, small depressions in the layer of pigment which surrounds the nerve are filled, as described by the author just quoted, with transparent gelatinous matter, projecting with a convex surface, like the cornea in the higher animals. he suggests that this serves not to form an image, but only to concentrate the luminous rays and render their perception more easy. in this concentration of the rays we gain the first and by far the most important step towards the formation of a true, picture-forming eye; for we have only to place the naked extremity of the optic nerve, which in some of the lower animals lies deeply buried in the body, and in some near the surface, at the right distance from the concentrating apparatus, and an image will be formed on it. in the great class of the articulata, we may start from an optic nerve simply coated with pigment, the latter sometimes forming a sort of pupil, but destitute of lens or other optical contrivance. with insects it is now known that the numerous facets on the cornea of their great compound eyes form true lenses, and that the cones include curiously modified nervous filaments. but these organs in the articulata are so much diversified that muller formerly made three main classes with seven subdivisions, besides a fourth main class of aggregated simple eyes. when we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and graduated range of structure in the eyes of the lower animals; and when we bear in mind how small the number of all living forms must be in comparison with those which have become extinct, the difficulty ceases to be very great in believing that natural selection may have converted the simple apparatus of an optic nerve, coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the articulata class. he who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume that large bodies of facts, otherwise inexplicable, can be explained by the theory of modification through natural selection; he ought to admit that a structure even as perfect as an eagle's eye might thus be formed, although in this case he does not know the transitional states. it has been objected that in order to modify the eye and still preserve it as a perfect instrument, many changes would have to be effected simultaneously, which, it is assumed, could not be done through natural selection; but as i have attempted to show in my work on the variation of domestic animals, it is not necessary to suppose that the modifications were all simultaneous, if they were extremely slight and gradual. different kinds of modification would, also, serve for the same general purpose: as mr. wallace has remarked, "if a lens has too short or too long a focus, it may be amended either by an alteration of curvature, or an alteration of density; if the curvature be irregular, and the rays do not converge to a point, then any increased regularity of curvature will be an improvement. so the contraction of the iris and the muscular movements of the eye are neither of them essential to vision, but only improvements which might have been added and perfected at any stage of the construction of the instrument." within the highest division of the animal kingdom, namely, the vertebrata, we can start from an eye so simple, that it consists, as in the lancelet, of a little sack of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other apparatus. in fishes and reptiles, as owen has remarked, "the range of gradation of dioptric structures is very great." it is a significant fact that even in man, according to the high authority of virchow, the beautiful crystalline lens is formed in the embryo by an accumulation of epidermic cells, lying in a sack-like fold of the skin; and the vitreous body is formed from embryonic subcutaneous tissue. to arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but i have felt the difficulty far to keenly to be surprised at others hesitating to extend the principle of natural selection to so startling a length. it is scarcely possible to avoid comparing the eye with a telescope. we know that this instrument has been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. but may not this inference be presumptuous? have we any right to assume that the creator works by intellectual powers like those of man? if we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. further we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers; and carefully preserving each which, under varied circumstances, in any way or degree, tends to produce a distincter image. we must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better is produced, and then the old ones to be all destroyed. in living bodies, variation will cause the slight alteration, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the creator are to those of man? modes of transition. if it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. but i can find out no such case. no doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, around which, according to the theory, there has been much extinction. or again, if we take an organ common to all the members of a class, for in this latter case the organ must have been originally formed at a remote period, since which all the many members of the class have been developed; and in order to discover the early transitional grades through which the organ has passed, we should have to look to very ancient ancestral forms, long since become extinct. we should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind. numerous cases could be given among the lower animals of the same organ performing at the same time wholly distinct functions; thus in the larva of the dragon-fly and in the fish cobites the alimentary canal respires, digests, and excretes. in the hydra, the animal may be turned inside out, and the exterior surface will then digest and the stomach respire. in such cases natural selection might specialise, if any advantage were thus gained, the whole or part of an organ, which had previously performed two functions, for one function alone, and thus by insensible steps greatly change its nature. many plants are known which regularly produce at the same time differently constructed flowers; and if such plants were to produce one kind alone, a great change would be effected with comparative suddenness in the character of the species. it is, however, probable that the two sorts of flowers borne by the same plant were originally differentiated by finely graduated steps, which may still be followed in some few cases. again, two distinct organs, or the same organ under two very different forms, may simultaneously perform in the same individual the same function, and this is an extremely important means of transition: to give one instance--there are fish with gills or branchiae that breathe the air dissolved in the water, at the same time that they breathe free air in their swim-bladders, this latter organ being divided by highly vascular partitions and having a ductus pneumaticus for the supply of air. to give another instance from the vegetable kingdom: plants climb by three distinct means, by spirally twining, by clasping a support with their sensitive tendrils, and by the emission of aerial rootlets; these three means are usually found in distinct groups, but some few species exhibit two of the means, or even all three, combined in the same individual. in all such cases one of the two organs might readily be modified and perfected so as to perform all the work, being aided during the progress of modification by the other organ; and then this other organ might be modified for some other and quite distinct purpose, or be wholly obliterated. the illustration of the swim-bladder in fishes is a good one, because it shows us clearly the highly important fact that an organ originally constructed for one purpose, namely flotation, may be converted into one for a widely different purpose, namely respiration. the swim-bladder has, also, been worked in as an accessory to the auditory organs of certain fishes. all physiologists admit that the swim-bladder is homologous, or "ideally similar" in position and structure with the lungs of the higher vertebrate animals: hence there is no reason to doubt that the swim-bladder has actually been converted into lungs, or an organ used exclusively for respiration. according to this view it may be inferred that all vertebrate animals with true lungs are descended by ordinary generation from an ancient and unknown prototype which was furnished with a floating apparatus or swim-bladder. we can thus, as i infer from professor owen's interesting description of these parts, understand the strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachea, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. in the higher vertebrata the branchiae have wholly disappeared--but in the embryo the slits on the sides of the neck and the loop-like course of the arteries still mark their former position. but it is conceivable that the now utterly lost branchiae might have been gradually worked in by natural selection for some distinct purpose: for instance, landois has shown that the wings of insects are developed from the trachea; it is therefore highly probable that in this great class organs which once served for respiration have been actually converted into organs for flight. in considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another, that i will give another instance. pedunculated cirripedes have two minute folds of skin, called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until they are hatched within the sack. these cirripedes have no branchiae, the whole surface of the body and of the sack, together with the small frena, serving for respiration. the balanidae or sessile cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at the bottom of the sack, within the well-enclosed shell; but they have, in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunae of the sack and body, and which have been considered by all naturalists to act as branchiae. now i think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiae of the other family; indeed, they graduate into each other. therefore it need not be doubted that the two little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly aided in the act of respiration, have been gradually converted by natural selection into branchiae, simply through an increase in their size and the obliteration of their adhesive glands. if all pedunculated cirripedes had become extinct, and they have suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiae in this latter family had originally existed as organs for preventing the ova from being washed out of the sack? there is another possible mode of transition, namely, through the acceleration or retardation of the period of reproduction. this has lately been insisted on by professor cope and others in the united states. it is now known that some animals are capable of reproduction at a very early age, before they have acquired their perfect characters; and if this power became thoroughly well developed in a species, it seems probable that the adult stage of development would sooner or later be lost; and in this case, especially if the larva differed much from the mature form, the character of the species would be greatly changed and degraded. again, not a few animals, after arriving at maturity, go on changing in character during nearly their whole lives. with mammals, for instance, the form of the skull is often much altered with age, of which dr. murie has given some striking instances with seals. every one knows how the horns of stags become more and more branched, and the plumes of some birds become more finely developed, as they grow older. professor cope states that the teeth of certain lizards change much in shape with advancing years. with crustaceans not only many trivial, but some important parts assume a new character, as recorded by fritz muller, after maturity. in all such cases--and many could be given--if the age for reproduction were retarded, the character of the species, at least in its adult state, would be modified; nor is it improbable that the previous and earlier stages of development would in some cases be hurried through and finally lost. whether species have often or ever been modified through this comparatively sudden mode of transition, i can form no opinion; but if this has occurred, it is probable that the differences between the young and the mature, and between the mature and the old, were primordially acquired by graduated steps. special difficulties of the theory of natural selection. although we must be extremely cautious in concluding that any organ could not have been produced by successive, small, transitional gradations, yet undoubtedly serious cases of difficulty occur. one of the most serious is that of neuter insects, which are often differently constructed from either the males or fertile females; but this case will be treated of in the next chapter. the electric organs of fishes offer another case of special difficulty; for it is impossible to conceive by what steps these wondrous organs have been produced. but this is not surprising, for we do not even know of what use they are. in the gymnotus and torpedo they no doubt serve as powerful means of defence, and perhaps for securing prey; yet in the ray, as observed by matteucci, an analogous organ in the tail manifests but little electricity, even when the animal is greatly irritated; so little that it can hardly be of any use for the above purposes. moreover, in the ray, besides the organ just referred to, there is, as dr. r. mcdonnell has shown, another organ near the head, not known to be electrical, but which appears to be the real homologue of the electric battery in the torpedo. it is generally admitted that there exists between these organs and ordinary muscle a close analogy, in intimate structure, in the distribution of the nerves, and in the manner in which they are acted on by various reagents. it should, also, be especially observed that muscular contraction is accompanied by an electrical discharge; and, as dr. radcliffe insists, "in the electrical apparatus of the torpedo during rest, there would seem to be a charge in every respect like that which is met with in muscle and nerve during the rest, and the discharge of the torpedo, instead of being peculiar, may be only another form of the discharge which attends upon the action of muscle and motor nerve." beyond this we cannot at present go in the way of explanation; but as we know so little about the uses of these organs, and as we know nothing about the habits and structure of the progenitors of the existing electric fishes, it would be extremely bold to maintain that no serviceable transitions are possible by which these organs might have been gradually developed. these organs appear at first to offer another and far more serious difficulty; for they occur in about a dozen kinds of fish, of which several are widely remote in their affinities. when the same organ is found in several members of the same class, especially if in members having very different habits of life, we may generally attribute its presence to inheritance from a common ancestor; and its absence in some of the members to loss through disuse or natural selection. so that, if the electric organs had been inherited from some one ancient progenitor, we might have expected that all electric fishes would have been specially related to each other; but this is far from the case. nor does geology at all lead to the belief that most fishes formerly possessed electric organs, which their modified descendants have now lost. but when we look at the subject more closely, we find in the several fishes provided with electric organs, that these are situated in different parts of the body, that they differ in construction, as in the arrangement of the plates, and, according to pacini, in the process or means by which the electricity is excited--and lastly, in being supplied with nerves proceeding from different sources, and this is perhaps the most important of all the differences. hence in the several fishes furnished with electric organs, these cannot be considered as homologous, but only as analogous in function. consequently there is no reason to suppose that they have been inherited from a common progenitor; for had this been the case they would have closely resembled each other in all respects. thus the difficulty of an organ, apparently the same, arising in several remotely allied species, disappears, leaving only the lesser yet still great difficulty: namely, by what graduated steps these organs have been developed in each separate group of fishes. the luminous organs which occur in a few insects, belonging to widely different families, and which are situated in different parts of the body, offer, under our present state of ignorance, a difficulty almost exactly parallel with that of the electric organs. other similar cases could be given; for instance in plants, the very curious contrivance of a mass of pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently the same in orchis and asclepias, genera almost as remote as is possible among flowering plants; but here again the parts are not homologous. in all cases of beings, far removed from each other in the scale of organisation, which are furnished with similar and peculiar organs, it will be found that although the general appearance and function of the organs may be the same, yet fundamental differences between them can always be detected. for instance, the eyes of cephalopods or cuttle-fish and of vertebrate animals appear wonderfully alike; and in such widely sundered groups no part of this resemblance can be due to inheritance from a common progenitor. mr. mivart has advanced this case as one of special difficulty, but i am unable to see the force of his argument. an organ for vision must be formed of transparent tissue, and must include some sort of lens for throwing an image at the back of a darkened chamber. beyond this superficial resemblance, there is hardly any real similarity between the eyes of cuttle-fish and vertebrates, as may be seen by consulting hensen's admirable memoir on these organs in the cephalopoda. it is impossible for me here to enter on details, but i may specify a few of the points of difference. the crystalline lens in the higher cuttle-fish consists of two parts, placed one behind the other like two lenses, both having a very different structure and disposition to what occurs in the vertebrata. the retina is wholly different, with an actual inversion of the elemental parts, and with a large nervous ganglion included within the membranes of the eye. the relations of the muscles are as different as it is possible to conceive, and so in other points. hence it is not a little difficult to decide how far even the same terms ought to be employed in describing the eyes of the cephalopoda and vertebrata. it is, of course, open to any one to deny that the eye in either case could have been developed through the natural selection of successive slight variations; but if this be admitted in the one case it is clearly possible in the other; and fundamental differences of structure in the visual organs of two groups might have been anticipated, in accordance with this view of their manner of formation. as two men have sometimes independently hit on the same invention, so in the several foregoing cases it appears that natural selection, working for the good of each being, and taking advantage of all favourable variations, has produced similar organs, as far as function is concerned, in distinct organic beings, which owe none of their structure in common to inheritance from a common progenitor. fritz muller, in order to test the conclusions arrived at in this volume, has followed out with much care a nearly similar line of argument. several families of crustaceans include a few species, possessing an air-breathing apparatus and fitted to live out of the water. in two of these families, which were more especially examined by muller, and which are nearly related to each other, the species agree most closely in all important characters: namely in their sense organs, circulating systems, in the position of the tufts of hair within their complex stomachs, and lastly in the whole structure of the water-breathing branchiae, even to the microscopical hooks by which they are cleansed. hence it might have been expected that in the few species belonging to both families which live on the land, the equally important air-breathing apparatus would have been the same; for why should this one apparatus, given for the same purpose, have been made to differ, while all the other important organs were closely similar, or rather, identical. fritz muller argues that this close similarity in so many points of structure must, in accordance with the views advanced by me, be accounted for by inheritance from a common progenitor. but as the vast majority of the species in the above two families, as well as most other crustaceans, are aquatic in their habits, it is improbable in the highest degree that their common progenitor should have been adapted for breathing air. muller was thus led carefully to examine the apparatus in the air-breathing species; and he found it to differ in each in several important points, as in the position of the orifices, in the manner in which they are opened and closed, and in some accessory details. now such differences are intelligible, and might even have been expected, on the supposition that species belonging to distinct families had slowly become adapted to live more and more out of water, and to breathe the air. for these species, from belonging to distinct families, would have differed to a certain extent, and in accordance with the principle that the nature of each variation depends on two factors, viz., the nature of the organism and that of the surrounding conditions, their variability assuredly would not have been exactly the same. consequently natural selection would have had different materials or variations to work on, in order to arrive at the same functional result; and the structures thus acquired would almost necessarily have differed. on the hypothesis of separate acts of creation the whole case remains unintelligible. this line of argument seems to have had great weight in leading fritz muller to accept the views maintained by me in this volume. another distinguished zoologist, the late professor claparede, has argued in the same manner, and has arrived at the same result. he shows that there are parasitic mites (acaridae), belonging to distinct sub-families and families, which are furnished with hair-claspers. these organs must have been independently developed, as they could not have been inherited from a common progenitor; and in the several groups they are formed by the modification of the fore legs, of the hind legs, of the maxillae or lips, and of appendages on the under side of the hind part of the body. in the foregoing cases, we see the same end gained and the same function performed, in beings not at all or only remotely allied, by organs in appearance, though not in development, closely similar. on the other hand, it is a common rule throughout nature that the same end should be gained, even sometimes in the case of closely related beings, by the most diversified means. how differently constructed is the feathered wing of a bird and the membrane-covered wing of a bat; and still more so the four wings of a butterfly, the two wings of a fly, and the two wings with the elytra of a beetle. bivalve shells are made to open and shut, but on what a number of patterns is the hinge constructed, from the long row of neatly interlocking teeth in a nucula to the simple ligament of a mussel! seeds are disseminated by their minuteness, by their capsule being converted into a light balloon-like envelope, by being embedded in pulp or flesh, formed of the most diverse parts, and rendered nutritious, as well as conspicuously coloured, so as to attract and be devoured by birds, by having hooks and grapnels of many kinds and serrated awns, so as to adhere to the fur of quadrupeds, and by being furnished with wings and plumes, as different in shape as they are elegant in structure, so as to be wafted by every breeze. i will give one other instance: for this subject of the same end being gained by the most diversified means well deserves attention. some authors maintain that organic beings have been formed in many ways for the sake of mere variety, almost like toys in a shop, but such a view of nature is incredible. with plants having separated sexes, and with those in which, though hermaphrodites, the pollen does not spontaneously fall on the stigma, some aid is necessary for their fertilisation. with several kinds this is effected by the pollen-grains, which are light and incoherent, being blown by the wind through mere chance on to the stigma; and this is the simplest plan which can well be conceived. an almost equally simple, though very different plan occurs in many plants in which a symmetrical flower secretes a few drops of nectar, and is consequently visited by insects; and these carry the pollen from the anthers to the stigma. from this simple stage we may pass through an inexhaustible number of contrivances, all for the same purpose and effected in essentially the same manner, but entailing changes in every part of the flower. the nectar may be stored in variously shaped receptacles, with the stamens and pistils modified in many ways, sometimes forming trap-like contrivances, and sometimes capable of neatly adapted movements through irritability or elasticity. from such structures we may advance till we come to such a case of extraordinary adaptation as that lately described by dr. cruger in the coryanthes. this orchid has part of its labellum or lower lip hollowed out into a great bucket, into which drops of almost pure water continually fall from two secreting horns which stand above it; and when the bucket is half-full, the water overflows by a spout on one side. the basal part of the labellum stands over the bucket, and is itself hollowed out into a sort of chamber with two lateral entrances; within this chamber there are curious fleshy ridges. the most ingenious man, if he had not witnessed what takes place, could never have imagined what purpose all these parts serve. but dr. cruger saw crowds of large humble-bees visiting the gigantic flowers of this orchid, not in order to suck nectar, but to gnaw off the ridges within the chamber above the bucket; in doing this they frequently pushed each other into the bucket, and their wings being thus wetted they could not fly away, but were compelled to crawl out through the passage formed by the spout or overflow. dr. cruger saw a "continual procession" of bees thus crawling out of their involuntary bath. the passage is narrow, and is roofed over by the column, so that a bee, in forcing its way out, first rubs its back against the viscid stigma and then against the viscid glands of the pollen-masses. the pollen-masses are thus glued to the back of the bee which first happens to crawl out through the passage of a lately expanded flower, and are thus carried away. dr. cruger sent me a flower in spirits of wine, with a bee which he had killed before it had quite crawled out, with a pollen-mass still fastened to its back. when the bee, thus provided, flies to another flower, or to the same flower a second time, and is pushed by its comrades into the bucket and then crawls out by the passage, the pollen-mass necessarily comes first into contact with the viscid stigma, and adheres to it, and the flower is fertilised. now at last we see the full use of every part of the flower, of the water-secreting horns of the bucket half-full of water, which prevents the bees from flying away, and forces them to crawl out through the spout, and rub against the properly placed viscid pollen-masses and the viscid stigma. the construction of the flower in another closely allied orchid, namely, the catasetum, is widely different, though serving the same end; and is equally curious. bees visit these flowers, like those of the coryanthes, in order to gnaw the labellum; in doing this they inevitably touch a long, tapering, sensitive projection, or, as i have called it, the antenna. this antenna, when touched, transmits a sensation or vibration to a certain membrane which is instantly ruptured; this sets free a spring by which the pollen-mass is shot forth, like an arrow, in the right direction, and adheres by its viscid extremity to the back of the bee. the pollen-mass of the male plant (for the sexes are separate in this orchid) is thus carried to the flower of the female plant, where it is brought into contact with the stigma, which is viscid enough to break certain elastic threads, and retain the pollen, thus effecting fertilisation. how, it may be asked, in the foregoing and in innumerable other instances, can we understand the graduated scale of complexity and the multifarious means for gaining the same end. the answer no doubt is, as already remarked, that when two forms vary, which already differ from each other in some slight degree, the variability will not be of the same exact nature, and consequently the results obtained through natural selection for the same general purpose will not be the same. we should also bear in mind that every highly developed organism has passed through many changes; and that each modified structure tends to be inherited, so that each modification will not readily be quite lost, but may be again and again further altered. hence, the structure of each part of each species, for whatever purpose it may serve, is the sum of many inherited changes, through which the species has passed during its successive adaptations to changed habits and conditions of life. finally, then, although in many cases it is most difficult even to conjecture by what transitions organs could have arrived at their present state; yet, considering how small the proportion of living and known forms is to the extinct and unknown, i have been astonished how rarely an organ can be named, towards which no transitional grade is known to lead. it is certainly true, that new organs appearing as if created for some special purpose rarely or never appear in any being; as indeed is shown by that old, but somewhat exaggerated, canon in natural history of "natura non facit saltum." we meet with this admission in the writings of almost every experienced naturalist; or, as milne edwards has well expressed it, "nature is prodigal in variety, but niggard in innovation." why, on the theory of creation, should there be so much variety and so little real novelty? why should all the parts and organs of many independent beings, each supposed to have been separately created for its own proper place in nature, be so commonly linked together by graduated steps? why should not nature take a sudden leap from structure to structure? on the theory of natural selection, we can clearly understand why she should not; for natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by the short and sure, though slow steps. organs of little apparent importance, as affected by natural selection. as natural selection acts by life and death, by the survival of the fittest, and by the destruction of the less well-fitted individuals, i have sometimes felt great difficulty in understanding the origin or formation of parts of little importance; almost as great, though of a very different kind, as in the case of the most perfect and complex organs. in the first place, we are much too ignorant in regard to the whole economy of any one organic being to say what slight modifications would be of importance or not. in a former chapter i have given instances of very trifling characters, such as the down on fruit and the colour of its flesh, the colour of the skin and hair of quadrupeds, which, from being correlated with constitutional differences, or from determining the attacks of insects, might assuredly be acted on by natural selection. the tail of the giraffe looks like an artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better fitted, for so trifling an object as to drive away flies; yet we should pause before being too positive even in this case, for we know that the distribution and existence of cattle and other animals in south america absolutely depend on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great advantage. it is not that the larger quadrupeds are actually destroyed (except in some rare cases) by flies, but they are incessantly harassed and their strength reduced, so that they are more subject to disease, or not so well enabled in a coming dearth to search for food, or to escape from beasts of prey. organs now of trifling importance have probably in some cases been of high importance to an early progenitor, and, after having been slowly perfected at a former period, have been transmitted to existing species in nearly the same state, although now of very slight use; but any actually injurious deviations in their structure would of course have been checked by natural selection. seeing how important an organ of locomotion the tail is in most aquatic animals, its general presence and use for many purposes in so many land animals, which in their lungs or modified swim-bladders betray their aquatic origin, may perhaps be thus accounted for. a well-developed tail having been formed in an aquatic animal, it might subsequently come to be worked in for all sorts of purposes, as a fly-flapper, an organ of prehension, or as an aid in turning, as in the case of the dog, though the aid in this latter respect must be slight, for the hare, with hardly any tail, can double still more quickly. in the second place, we may easily err in attributing importance to characters, and in believing that they have been developed through natural selection. we must by no means overlook the effects of the definite action of changed conditions of life, of so-called spontaneous variations, which seem to depend in a quite subordinate degree on the nature of the conditions, of the tendency to reversion to long-lost characters, of the complex laws of growth, such as of correlation, comprehension, of the pressure of one part on another, etc., and finally of sexual selection, by which characters of use to one sex are often gained and then transmitted more or less perfectly to the other sex, though of no use to the sex. but structures thus indirectly gained, although at first of no advantage to a species, may subsequently have been taken advantage of by its modified descendants, under new conditions of life and newly acquired habits. if green woodpeckers alone had existed, and we did not know that there were many black and pied kinds, i dare say that we should have thought that the green colour was a beautiful adaptation to conceal this tree-frequenting bird from its enemies; and consequently that it was a character of importance, and had been acquired through natural selection; as it is, the colour is probably in chief part due to sexual selection. a trailing palm in the malay archipelago climbs the loftiest trees by the aid of exquisitely constructed hooks clustered around the ends of the branches, and this contrivance, no doubt, is of the highest service to the plant; but as we see nearly similar hooks on many trees which are not climbers, and which, as there is reason to believe from the distribution of the thorn-bearing species in africa and south america, serve as a defence against browsing quadrupeds, so the spikes on the palm may at first have been developed for this object, and subsequently have been improved and taken advantage of by the plant, as it underwent further modification and became a climber. the naked skin on the head of a vulture is generally considered as a direct adaptation for wallowing in putridity; and so it may be, or it may possibly be due to the direct action of putrid matter; but we should be very cautious in drawing any such inference, when we see that the skin on the head of the clean-feeding male turkey is likewise naked. the sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt they facilitate, or may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals. we are profoundly ignorant of the cause of each slight variation or individual difference; and we are immediately made conscious of this by reflecting on the differences between the breeds of our domesticated animals in different countries, more especially in the less civilized countries, where there has been but little methodical selection. animals kept by savages in different countries often have to struggle for their own subsistence, and are exposed to a certain extent to natural selection, and individuals with slightly different constitutions would succeed best under different climates. with cattle susceptibility to the attacks of flies is correlated with colour, as is the liability to be poisoned by certain plants; so that even colour would be thus subjected to the action of natural selection. some observers are convinced that a damp climate affects the growth of the hair, and that with the hair the horns are correlated. mountain breeds always differ from lowland breeds; and a mountainous country would probably affect the hind limbs from exercising them more, and possibly even the form of the pelvis; and then by the law of homologous variation, the front limbs and the head would probably be affected. the shape, also, of the pelvis might affect by pressure the shape of certain parts of the young in the womb. the laborious breathing necessary in high regions tends, as we have good reason to believe, to increase the size of the chest; and again correlation would come into play. the effects of lessened exercise, together with abundant food, on the whole organisation is probably still more important, and this, as h. von nathusius has lately shown in his excellent treatise, is apparently one chief cause of the great modification which the breeds of swine have undergone. but we are far too ignorant to speculate on the relative importance of the several known and unknown causes of variation; and i have made these remarks only to show that, if we are unable to account for the characteristic differences of our several domestic breeds, which nevertheless are generally admitted to have arisen through ordinary generation from one or a few parent-stocks, we ought not to lay too much stress on our ignorance of the precise cause of the slight analogous differences between true species. utilitarian doctrine, how far true: beauty, how acquired. the foregoing remarks lead me to say a few words on the protest lately made by some naturalists against the utilitarian doctrine that every detail of structure has been produced for the good of its possessor. they believe that many structures have been created for the sake of beauty, to delight man or the creator (but this latter point is beyond the scope of scientific discussion), or for the sake of mere variety, a view already discussed. such doctrines, if true, would be absolutely fatal to my theory. i fully admit that many structures are now of no direct use to their possessors, and may never have been of any use to their progenitors; but this does not prove that they were formed solely for beauty or variety. no doubt the definite action of changed conditions, and the various causes of modifications, lately specified, have all produced an effect, probably a great effect, independently of any advantage thus gained. but a still more important consideration is that the chief part of the organisation of every living creature is due to inheritance; and consequently, though each being assuredly is well fitted for its place in nature, many structures have now no very close and direct relation to present habits of life. thus, we can hardly believe that the webbed feet of the upland goose, or of the frigate-bird, are of special use to these birds; we cannot believe that the similar bones in the arm of the monkey, in the fore leg of the horse, in the wing of the bat, and in the flipper of the seal, are of special use to these animals. we may safely attribute these structures to inheritance. but webbed feet no doubt were as useful to the progenitor of the upland goose and of the frigate-bird, as they now are to the most aquatic of living birds. so we may believe that the progenitor of the seal did not possess a flipper, but a foot with five toes fitted for walking or grasping; and we may further venture to believe that the several bones in the limbs of the monkey, horse and bat, were originally developed, on the principle of utility, probably through the reduction of more numerous bones in the fin of some ancient fish-like progenitor of the whole class. it is scarcely possible to decide how much allowance ought to be made for such causes of change, as the definite action of external conditions, so-called spontaneous variations, and the complex laws of growth; but with these important exceptions, we may conclude that the structure of every living creature either now is, or was formerly, of some direct or indirect use to its possessor. with respect to the belief that organic beings have been created beautiful for the delight of man--a belief which it has been pronounced is subversive of my whole theory--i may first remark that the sense of beauty obviously depends on the nature of the mind, irrespective of any real quality in the admired object; and that the idea of what is beautiful, is not innate or unalterable. we see this, for instance, in the men of different races admiring an entirely different standard of beauty in their women. if beautiful objects had been created solely for man's gratification, it ought to be shown that before man appeared there was less beauty on the face of the earth than since he came on the stage. were the beautiful volute and cone shells of the eocene epoch, and the gracefully sculptured ammonites of the secondary period, created that man might ages afterwards admire them in his cabinet? few objects are more beautiful than the minute siliceous cases of the diatomaceae: were these created that they might be examined and admired under the higher powers of the microscope? the beauty in this latter case, and in many others, is apparently wholly due to symmetry of growth. flowers rank among the most beautiful productions of nature; but they have been rendered conspicuous in contrast with the green leaves, and in consequence at the same time beautiful, so that they may be easily observed by insects. i have come to this conclusion from finding it an invariable rule that when a flower is fertilised by the wind it never has a gaily-coloured corolla. several plants habitually produce two kinds of flowers; one kind open and coloured so as to attract insects; the other closed, not coloured, destitute of nectar, and never visited by insects. hence, we may conclude that, if insects had not been developed on the face of the earth, our plants would not have been decked with beautiful flowers, but would have produced only such poor flowers as we see on our fir, oak, nut and ash trees, on grasses, spinach, docks and nettles, which are all fertilised through the agency of the wind. a similar line of argument holds good with fruits; that a ripe strawberry or cherry is as pleasing to the eye as to the palate--that the gaily-coloured fruit of the spindle-wood tree and the scarlet berries of the holly are beautiful objects--will be admitted by everyone. but this beauty serves merely as a guide to birds and beasts, in order that the fruit may be devoured and the matured seeds disseminated. i infer that this is the case from having as yet found no exception to the rule that seeds are always thus disseminated when embedded within a fruit of any kind (that is within a fleshy or pulpy envelope), if it be coloured of any brilliant tint, or rendered conspicuous by being white or black. on the other hand, i willingly admit that a great number of male animals, as all our most gorgeous birds, some fishes, reptiles, and mammals, and a host of magnificently coloured butterflies, have been rendered beautiful for beauty's sake. but this has been effected through sexual selection, that is, by the more beautiful males having been continually preferred by the females, and not for the delight of man. so it is with the music of birds. we may infer from all this that a nearly similar taste for beautiful colours and for musical sounds runs through a large part of the animal kingdom. when the female is as beautifully coloured as the male, which is not rarely the case with birds and butterflies, the cause apparently lies in the colours acquired through sexual selection having been transmitted to both sexes, instead of to the males alone. how the sense of beauty in its simplest form--that is, the reception of a peculiar kind of pleasure from certain colours, forms and sounds--was first developed in the mind of man and of the lower animals, is a very obscure subject. the same sort of difficulty is presented if we enquire how it is that certain flavours and odours give pleasure, and others displeasure. habit in all these cases appears to have come to a certain extent into play; but there must be some fundamental cause in the constitution of the nervous system in each species. natural selection cannot possibly produce any modification in a species exclusively for the good of another species; though throughout nature one species incessantly takes advantage of, and profits by the structures of others. but natural selection can and does often produce structures for the direct injury of other animals, as we see in the fang of the adder, and in the ovipositor of the ichneumon, by which its eggs are deposited in the living bodies of other insects. if it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection. although many statements may be found in works on natural history to this effect, i cannot find even one which seems to me of any weight. it is admitted that the rattlesnake has a poison-fang for its own defence and for the destruction of its prey; but some authors suppose that at the same time it is furnished with a rattle for its own injury, namely, to warn its prey. i would almost as soon believe that the cat curls the end of its tail when preparing to spring, in order to warn the doomed mouse. it is a much more probable view that the rattlesnake uses its rattle, the cobra expands its frill and the puff-adder swells while hissing so loudly and harshly, in order to alarm the many birds and beasts which are known to attack even the most venomous species. snakes act on the same principle which makes the hen ruffle her feathers and expand her wings when a dog approaches her chickens. but i have not space here to enlarge on the many ways by which animals endeavour to frighten away their enemies. natural selection will never produce in a being any structure more injurious than beneficial to that being, for natural selection acts solely by and for the good of each. no organ will be formed, as paley has remarked, for the purpose of causing pain or for doing an injury to its possessor. if a fair balance be struck between the good and evil caused by each part, each will be found on the whole advantageous. after the lapse of time, under changing conditions of life, if any part comes to be injurious, it will be modified; or if it be not so, the being will become extinct, as myriads have become extinct. natural selection tends only to make each organic being as perfect as, or slightly more perfect than the other inhabitants of the same country with which it comes into competition. and we see that this is the standard of perfection attained under nature. the endemic productions of new zealand, for instance, are perfect, one compared with another; but they are now rapidly yielding before the advancing legions of plants and animals introduced from europe. natural selection will not produce absolute perfection, nor do we always meet, as far as we can judge, with this high standard under nature. the correction for the aberration of light is said by muller not to be perfect even in that most perfect organ, the human eye. helmholtz, whose judgment no one will dispute, after describing in the strongest terms the wonderful powers of the human eye, adds these remarkable words: "that which we have discovered in the way of inexactness and imperfection in the optical machine and in the image on the retina, is as nothing in comparison with the incongruities which we have just come across in the domain of the sensations. one might say that nature has taken delight in accumulating contradictions in order to remove all foundation from the theory of a pre-existing harmony between the external and internal worlds." if our reason leads us to admire with enthusiasm a multitude of inimitable contrivances in nature, this same reason tells us, though we may easily err on both sides, that some other contrivances are less perfect. can we consider the sting of the bee as perfect, which, when used against many kinds of enemies, cannot be withdrawn, owing to the backward serratures, and thus inevitably causes the death of the insect by tearing out its viscera? if we look at the sting of the bee, as having existed in a remote progenitor, as a boring and serrated instrument, like that in so many members of the same great order, and that it has since been modified but not perfected for its present purpose, with the poison originally adapted for some other object, such as to produce galls, since intensified, we can perhaps understand how it is that the use of the sting should so often cause the insect's own death: for if on the whole the power of stinging be useful to the social community, it will fulfil all the requirements of natural selection, though it may cause the death of some few members. if we admire the truly wonderful power of scent by which the males of many insects find their females, can we admire the production for this single purpose of thousands of drones, which are utterly useless to the community for any other purpose, and which are ultimately slaughtered by their industrious and sterile sisters? it may be difficult, but we ought to admire the savage instinctive hatred of the queen-bee, which urges her to destroy the young queens, her daughters, as soon as they are born, or to perish herself in the combat; for undoubtedly this is for the good of the community; and maternal love or maternal hatred, though the latter fortunately is most rare, is all the same to the inexorable principles of natural selection. if we admire the several ingenious contrivances by which orchids and many other plants are fertilised through insect agency, can we consider as equally perfect the elaboration of dense clouds of pollen by our fir-trees, so that a few granules may be wafted by chance on to the ovules? summary: the law of unity of type and of the conditions of existence embraced by the theory of natural selection. we have in this chapter discussed some of the difficulties and objections which may be urged against the theory. many of them are serious; but i think that in the discussion light has been thrown on several facts, which on the belief of independent acts of creation are utterly obscure. we have seen that species at any one period are not indefinitely variable, and are not linked together by a multitude of intermediate gradations, partly because the process of natural selection is always very slow, and at any one time acts only on a few forms; and partly because the very process of natural selection implies the continual supplanting and extinction of preceding and intermediate gradations. closely allied species, now living on a continuous area, must often have been formed when the area was not continuous, and when the conditions of life did not insensibly graduate away from one part to another. when two varieties are formed in two districts of a continuous area, an intermediate variety will often be formed, fitted for an intermediate zone; but from reasons assigned, the intermediate variety will usually exist in lesser numbers than the two forms which it connects; consequently the two latter, during the course of further modification, from existing in greater numbers, will have a great advantage over the less numerous intermediate variety, and will thus generally succeed in supplanting and exterminating it. we have seen in this chapter how cautious we should be in concluding that the most different habits of life could not graduate into each other; that a bat, for instance, could not have been formed by natural selection from an animal which at first only glided through the air. we have seen that a species under new conditions of life may change its habits, or it may have diversified habits, with some very unlike those of its nearest congeners. hence we can understand, bearing in mind that each organic being is trying to live wherever it can live, how it has arisen that there are upland geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with the habits of auks. although the belief that an organ so perfect as the eye could have been formed by natural selection, is enough to stagger any one; yet in the case of any organ, if we know of a long series of gradations in complexity, each good for its possessor, then under changing conditions of life, there is no logical impossibility in the acquirement of any conceivable degree of perfection through natural selection. in the cases in which we know of no intermediate or transitional states, we should be extremely cautious in concluding that none can have existed, for the metamorphoses of many organs show what wonderful changes in function are at least possible. for instance, a swim-bladder has apparently been converted into an air-breathing lung. the same organ having performed simultaneously very different functions, and then having been in part or in whole specialised for one function; and two distinct organs having performed at the same time the same function, the one having been perfected whilst aided by the other, must often have largely facilitated transitions. we have seen that in two beings widely remote from each other in the natural scale, organs serving for the same purpose and in external appearance closely similar may have been separately and independently formed; but when such organs are closely examined, essential differences in their structure can almost always be detected; and this naturally follows from the principle of natural selection. on the other hand, the common rule throughout nature is infinite diversity of structure for gaining the same end; and this again naturally follows from the same great principle. in many cases we are far too ignorant to be enabled to assert that a part or organ is so unimportant for the welfare of a species, that modifications in its structure could not have been slowly accumulated by means of natural selection. in many other cases, modifications are probably the direct result of the laws of variation or of growth, independently of any good having been thus gained. but even such structures have often, as we may feel assured, been subsequently taken advantage of, and still further modified, for the good of species under new conditions of life. we may, also, believe that a part formerly of high importance has frequently been retained (as the tail of an aquatic animal by its terrestrial descendants), though it has become of such small importance that it could not, in its present state, have been acquired by means of natural selection. natural selection can produce nothing in one species for the exclusive good or injury of another; though it may well produce parts, organs, and excretions highly useful or even indispensable, or highly injurious to another species, but in all cases at the same time useful to the possessor. in each well-stocked country natural selection acts through the competition of the inhabitants and consequently leads to success in the battle for life, only in accordance with the standard of that particular country. hence the inhabitants of one country, generally the smaller one, often yield to the inhabitants of another and generally the larger country. for in the larger country there will have existed more individuals, and more diversified forms, and the competition will have been severer, and thus the standard of perfection will have been rendered higher. natural selection will not necessarily lead to absolute perfection; nor, as far as we can judge by our limited faculties, can absolute perfection be everywhere predicated. on the theory of natural selection we can clearly understand the full meaning of that old canon in natural history, "natura non facit saltum." this canon, if we look to the present inhabitants alone of the world, is not strictly correct; but if we include all those of past times, whether known or unknown, it must on this theory be strictly true. it is generally acknowledged that all organic beings have been formed on two great laws--unity of type, and the conditions of existence. by unity of type is meant that fundamental agreement in structure which we see in organic beings of the same class, and which is quite independent of their habits of life. on my theory, unity of type is explained by unity of descent. the expression of conditions of existence, so often insisted on by the illustrious cuvier, is fully embraced by the principle of natural selection. for natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them during past periods of time: the adaptations being aided in many cases by the increased use or disuse of parts, being affected by the direct action of external conditions of life, and subjected in all cases to the several laws of growth and variation. hence, in fact, the law of the conditions of existence is the higher law; as it includes, through the inheritance of former variations and adaptations, that of unity of type. chapter vii. miscellaneous objections to the theory of natural selection. longevity--modifications not necessarily simultaneous--modifications apparently of no direct service--progressive development--characters of small functional importance, the most constant--supposed incompetence of natural selection to account for the incipient stages of useful structures--causes which interfere with the acquisition through natural selection of useful structures--gradations of structure with changed functions--widely different organs in members of the same class, developed from one and the same source--reasons for disbelieving in great and abrupt modifications. i will devote this chapter to the consideration of various miscellaneous objections which have been advanced against my views, as some of the previous discussions may thus be made clearer; but it would be useless to discuss all of them, as many have been made by writers who have not taken the trouble to understand the subject. thus a distinguished german naturalist has asserted that the weakest part of my theory is, that i consider all organic beings as imperfect: what i have really said is, that all are not as perfect as they might have been in relation to their conditions; and this is shown to be the case by so many native forms in many quarters of the world having yielded their places to intruding foreigners. nor can organic beings, even if they were at any one time perfectly adapted to their conditions of life, have remained so, when their conditions changed, unless they themselves likewise changed; and no one will dispute that the physical conditions of each country, as well as the number and kinds of its inhabitants, have undergone many mutations. a critic has lately insisted, with some parade of mathematical accuracy, that longevity is a great advantage to all species, so that he who believes in natural selection "must arrange his genealogical tree" in such a manner that all the descendants have longer lives than their progenitors! cannot our critics conceive that a biennial plant or one of the lower animals might range into a cold climate and perish there every winter; and yet, owing to advantages gained through natural selection, survive from year to year by means of its seeds or ova? mr. e. ray lankester has recently discussed this subject, and he concludes, as far as its extreme complexity allows him to form a judgment, that longevity is generally related to the standard of each species in the scale of organisation, as well as to the amount of expenditure in reproduction and in general activity. and these conditions have, it is probable, been largely determined through natural selection. it has been argued that, as none of the animals and plants of egypt, of which we know anything, have changed during the last three or four thousand years, so probably have none in any part of the world. but, as mr. g.h. lewes has remarked, this line of argument proves too much, for the ancient domestic races figured on the egyptian monuments, or embalmed, are closely similar or even identical with those now living; yet all naturalists admit that such races have been produced through the modification of their original types. the many animals which have remained unchanged since the commencement of the glacial period, would have been an incomparably stronger case, for these have been exposed to great changes of climate and have migrated over great distances; whereas, in egypt, during the last several thousand years, the conditions of life, as far as we know, have remained absolutely uniform. the fact of little or no modification having been effected since the glacial period, would have been of some avail against those who believe in an innate and necessary law of development, but is powerless against the doctrine of natural selection or the survival of the fittest, which implies that when variations or individual differences of a beneficial nature happen to arise, these will be preserved; but this will be effected only under certain favourable circumstances. the celebrated palaeontologist, bronn, at the close of his german translation of this work, asks how, on the principle of natural selection, can a variety live side by side with the parent species? if both have become fitted for slightly different habits of life or conditions, they might live together; and if we lay on one side polymorphic species, in which the variability seems to be of a peculiar nature, and all mere temporary variations, such as size, albinism, etc., the more permanent varieties are generally found, as far as i can discover, inhabiting distinct stations, such as high land or low land, dry or moist districts. moreover, in the case of animals which wander much about and cross freely, their varieties seem to be generally confined to distinct regions. bronn also insists that distinct species never differ from each other in single characters, but in many parts; and he asks, how it always comes that many parts of the organisation should have been modified at the same time through variation and natural selection? but there is no necessity for supposing that all the parts of any being have been simultaneously modified. the most striking modifications, excellently adapted for some purpose, might, as was formerly remarked, be acquired by successive variations, if slight, first in one part and then in another; and as they would be transmitted all together, they would appear to us as if they had been simultaneously developed. the best answer, however, to the above objection is afforded by those domestic races which have been modified, chiefly through man's power of selection, for some special purpose. look at the race and dray-horse, or at the greyhound and mastiff. their whole frames, and even their mental characteristics, have been modified; but if we could trace each step in the history of their transformation--and the latter steps can be traced--we should not see great and simultaneous changes, but first one part and then another slightly modified and improved. even when selection has been applied by man to some one character alone--of which our cultivated plants offer the best instances--it will invariably be found that although this one part, whether it be the flower, fruit, or leaves, has been greatly changed, almost all the other parts have been slightly modified. this may be attributed partly to the principle of correlated growth, and partly to so-called spontaneous variation. a much more serious objection has been urged by bronn, and recently by broca, namely, that many characters appear to be of no service whatever to their possessors, and therefore cannot have been influenced through natural selection. bronn adduces the length of the ears and tails in the different species of hares and mice--the complex folds of enamel in the teeth of many animals, and a multitude of analogous cases. with respect to plants, this subject has been discussed by nageli in an admirable essay. he admits that natural selection has effected much, but he insists that the families of plants differ chiefly from each other in morphological characters, which appear to be quite unimportant for the welfare of the species. he consequently believes in an innate tendency towards progressive and more perfect development. he specifies the arrangement of the cells in the tissues, and of the leaves on the axis, as cases in which natural selection could not have acted. to these may be added the numerical divisions in the parts of the flower, the position of the ovules, the shape of the seed, when not of any use for dissemination, etc. there is much force in the above objection. nevertheless, we ought, in the first place, to be extremely cautious in pretending to decide what structures now are, or have formerly been, of use to each species. in the second place, it should always be borne in mind that when one part is modified, so will be other parts, through certain dimly seen causes, such as an increased or diminished flow of nutriment to a part, mutual pressure, an early developed part affecting one subsequently developed, and so forth--as well as through other causes which lead to the many mysterious cases of correlation, which we do not in the least understand. these agencies may be all grouped together, for the sake of brevity, under the expression of the laws of growth. in the third place, we have to allow for the direct and definite action of changed conditions of life, and for so-called spontaneous variations, in which the nature of the conditions apparently plays a quite subordinate part. bud-variations, such as the appearance of a moss-rose on a common rose, or of a nectarine on a peach-tree, offer good instances of spontaneous variations; but even in these cases, if we bear in mind the power of a minute drop of poison in producing complex galls, we ought not to feel too sure that the above variations are not the effect of some local change in the nature of the sap, due to some change in the conditions. there must be some efficient cause for each slight individual difference, as well as for more strongly marked variations which occasionally arise; and if the unknown cause were to act persistently, it is almost certain that all the individuals of the species would be similarly modified. in the earlier editions of this work i underrated, as it now seems probable, the frequency and importance of modifications due to spontaneous variability. but it is impossible to attribute to this cause the innumerable structures which are so well adapted to the habits of life of each species. i can no more believe in this than that the well-adapted form of a race-horse or greyhound, which before the principle of selection by man was well understood, excited so much surprise in the minds of the older naturalists, can thus be explained. it may be worth while to illustrate some of the foregoing remarks. with respect to the assumed inutility of various parts and organs, it is hardly necessary to observe that even in the higher and best-known animals many structures exist, which are so highly developed that no one doubts that they are of importance, yet their use has not been, or has only recently been, ascertained. as bronn gives the length of the ears and tail in the several species of mice as instances, though trifling ones, of differences in structure which can be of no special use, i may mention that, according to dr. schobl, the external ears of the common mouse are supplied in an extraordinary manner with nerves, so that they no doubt serve as tactile organs; hence the length of the ears can hardly be quite unimportant. we shall, also, presently see that the tail is a highly useful prehensile organ to some of the species; and its use would be much influence by its length. with respect to plants, to which on account of nageli's essay i shall confine myself in the following remarks, it will be admitted that the flowers of the orchids present a multitude of curious structures, which a few years ago would have been considered as mere morphological differences without any special function; but they are now known to be of the highest importance for the fertilisation of the species through the aid of insects, and have probably been gained through natural selection. no one until lately would have imagined that in dimorphic and trimorphic plants the different lengths of the stamens and pistils, and their arrangement, could have been of any service, but now we know this to be the case. in certain whole groups of plants the ovules stand erect, and in others they are suspended; and within the same ovarium of some few plants, one ovule holds the former and a second ovule the latter position. these positions seem at first purely morphological, or of no physiological signification; but dr. hooker informs me that within the same ovarium the upper ovules alone in some cases, and in others the lower ones alone are fertilised; and he suggests that this probably depends on the direction in which the pollen-tubes enter the ovarium. if so, the position of the ovules, even when one is erect and the other suspended within the same ovarium, would follow the selection of any slight deviations in position which favoured their fertilisation, and the production of seed. several plants belonging to distinct orders habitually produce flowers of two kinds--the one open, of the ordinary structure, the other closed and imperfect. these two kinds of flowers sometimes differ wonderfully in structure, yet may be seen to graduate into each other on the same plant. the ordinary and open flowers can be intercrossed; and the benefits which certainly are derived from this process are thus secured. the closed and imperfect flowers are, however, manifestly of high importance, as they yield with the utmost safety a large stock of seed, with the expenditure of wonderfully little pollen. the two kinds of flowers often differ much, as just stated, in structure. the petals in the imperfect flowers almost always consist of mere rudiments, and the pollen-grains are reduced in diameter. in ononis columnae five of the alternate stamens are rudimentary; and in some species of viola three stamens are in this state, two retaining their proper function, but being of very small size. in six out of thirty of the closed flowers in an indian violet (name unknown, for the plants have never produced with me perfect flowers), the sepals are reduced from the normal number of five to three. in one section of the malpighiaceae the closed flowers, according to a. de jussieu, are still further modified, for the five stamens which stand opposite to the sepals are all aborted, a sixth stamen standing opposite to a petal being alone developed; and this stamen is not present in the ordinary flowers of this species; the style is aborted; and the ovaria are reduced from three to two. now although natural selection may well have had the power to prevent some of the flowers from expanding, and to reduce the amount of pollen, when rendered by the closure of the flowers superfluous, yet hardly any of the above special modifications can have been thus determined, but must have followed from the laws of growth, including the functional inactivity of parts, during the progress of the reduction of the pollen and the closure of the flowers. it is so necessary to appreciate the important effects of the laws of growth, that i will give some additional cases of another kind, namely of differences in the same part or organ, due to differences in relative position on the same plant. in the spanish chestnut, and in certain fir-trees, the angles of divergence of the leaves differ, according to schacht, in the nearly horizontal and in the upright branches. in the common rue and some other plants, one flower, usually the central or terminal one, opens first, and has five sepals and petals, and five divisions to the ovarium; while all the other flowers on the plant are tetramerous. in the british adoxa the uppermost flower generally has two calyx-lobes with the other organs tetramerous, while the surrounding flowers generally have three calyx-lobes with the other organs pentamerous. in many compositae and umbelliferae (and in some other plants) the circumferential flowers have their corollas much more developed than those of the centre; and this seems often connected with the abortion of the reproductive organs. it is a more curious fact, previously referred to, that the achenes or seeds of the circumference and centre sometimes differ greatly in form, colour and other characters. in carthamus and some other compositae the central achenes alone are furnished with a pappus; and in hyoseris the same head yields achenes of three different forms. in certain umbelliferae the exterior seeds, according to tausch, are orthospermous, and the central one coelospermous, and this is a character which was considered by de candolle to be in other species of the highest systematic importance. professor braun mentions a fumariaceous genus, in which the flowers in the lower part of the spike bear oval, ribbed, one-seeded nutlets; and in the upper part of the spike, lanceolate, two-valved and two-seeded siliques. in these several cases, with the exception of that of the well-developed ray-florets, which are of service in making the flowers conspicuous to insects, natural selection cannot, as far as we can judge, have come into play, or only in a quite subordinate manner. all these modifications follow from the relative position and inter-action of the parts; and it can hardly be doubted that if all the flowers and leaves on the same plant had been subjected to the same external and internal condition, as are the flowers and leaves in certain positions, all would have been modified in the same manner. in numerous other cases we find modifications of structure, which are considered by botanists to be generally of a highly important nature, affecting only some of the flowers on the same plant, or occurring on distinct plants, which grow close together under the same conditions. as these variations seem of no special use to the plants, they cannot have been influenced by natural selection. of their cause we are quite ignorant; we cannot even attribute them, as in the last class of cases, to any proximate agency, such as relative position. i will give only a few instances. it is so common to observe on the same plant, flowers indifferently tetramerous, pentamerous, etc., that i need not give examples; but as numerical variations are comparatively rare when the parts are few, i may mention that, according to de candolle, the flowers of papaver bracteatum offer either two sepals with four petals (which is the common type with poppies), or three sepals with six petals. the manner in which the petals are folded in the bud is in most groups a very constant morphological character; but professor asa gray states that with some species of mimulus, the aestivation is almost as frequently that of the rhinanthideae as of the antirrhinideae, to which latter tribe the genus belongs. aug. st. hilaire gives the following cases: the genus zanthoxylon belongs to a division of the rutaceae with a single ovary, but in some species flowers may be found on the same plant, and even in the same panicle, with either one or two ovaries. in helianthemum the capsule has been described as unilocular or tri-locular; and in h. mutabile, "une lame plus ou moins large, s'etend entre le pericarpe et le placenta." in the flowers of saponaria officinalis dr. masters has observed instances of both marginal and free central placentation. lastly, st. hilaire found towards the southern extreme of the range of gomphia oleaeformis two forms which he did not at first doubt were distinct species, but he subsequently saw them growing on the same bush; and he then adds, "voila donc dans un meme individu des loges et un style qui se rattachent tantot a un axe verticale et tantot a un gynobase." we thus see that with plants many morphological changes may be attributed to the laws of growth and the inter-action of parts, independently of natural selection. but with respect to nageli's doctrine of an innate tendency towards perfection or progressive development, can it be said in the case of these strongly pronounced variations, that the plants have been caught in the act of progressing towards a higher state of development? on the contrary, i should infer from the mere fact of the parts in question differing or varying greatly on the same plant, that such modifications were of extremely small importance to the plants themselves, of whatever importance they may generally be to us for our classifications. the acquisition of a useless part can hardly be said to raise an organism in the natural scale; and in the case of the imperfect, closed flowers, above described, if any new principle has to be invoked, it must be one of retrogression rather than of progression; and so it must be with many parasitic and degraded animals. we are ignorant of the exciting cause of the above specified modifications; but if the unknown cause were to act almost uniformly for a length of time, we may infer that the result would be almost uniform; and in this case all the individuals of the species would be modified in the same manner. from the fact of the above characters being unimportant for the welfare of the species, any slight variations which occurred in them would not have been accumulated and augmented through natural selection. a structure which has been developed through long-continued selection, when it ceases to be of service to a species, generally becomes variable, as we see with rudimentary organs; for it will no longer be regulated by this same power of selection. but when, from the nature of the organism and of the conditions, modifications have been induced which are unimportant for the welfare of the species, they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise modified, descendants. it cannot have been of much importance to the greater number of mammals, birds, or reptiles, whether they were clothed with hair, feathers or scales; yet hair has been transmitted to almost all mammals, feathers to all birds, and scales to all true reptiles. a structure, whatever it may be, which is common to many allied forms, is ranked by us as of high systematic importance, and consequently is often assumed to be of high vital importance to the species. thus, as i am inclined to believe, morphological differences, which we consider as important--such as the arrangement of the leaves, the divisions of the flower or of the ovarium, the position of the ovules, etc., first appeared in many cases as fluctuating variations, which sooner or later became constant through the nature of the organism and of the surrounding conditions, as well as through the intercrossing of distinct individuals, but not through natural selection; for as these morphological characters do not affect the welfare of the species, any slight deviations in them could not have been governed or accumulated through this latter agency. it is a strange result which we thus arrive at, namely, that characters of slight vital importance to the species, are the most important to the systematist; but, as we shall hereafter see when we treat of the genetic principle of classification, this is by no means so paradoxical as it may at first appear. although we have no good evidence of the existence in organic beings of an innate tendency towards progressive development, yet this necessarily follows, as i have attempted to show in the fourth chapter, through the continued action of natural selection. for the best definition which has ever been given of a high standard of organisation, is the degree to which the parts have been specialised or differentiated; and natural selection tends towards this end, inasmuch as the parts are thus enabled to perform their functions more efficiently. a distinguished zoologist, mr. st. george mivart, has recently collected all the objections which have ever been advanced by myself and others against the theory of natural selection, as propounded by mr. wallace and myself, and has illustrated them with admirable art and force. when thus marshalled, they make a formidable array; and as it forms no part of mr. mivart's plan to give the various facts and considerations opposed to his conclusions, no slight effort of reason and memory is left to the reader, who may wish to weigh the evidence on both sides. when discussing special cases, mr. mivart passes over the effects of the increased use and disuse of parts, which i have always maintained to be highly important, and have treated in my "variation under domestication" at greater length than, as i believe, any other writer. he likewise often assumes that i attribute nothing to variation, independently of natural selection, whereas in the work just referred to i have collected a greater number of well-established cases than can be found in any other work known to me. my judgment may not be trustworthy, but after reading with care mr. mivart's book, and comparing each section with what i have said on the same head, i never before felt so strongly convinced of the general truth of the conclusions here arrived at, subject, of course, in so intricate a subject, to much partial error. all mr. mivart's objections will be, or have been, considered in the present volume. the one new point which appears to have struck many readers is, "that natural selection is incompetent to account for the incipient stages of useful structures." this subject is intimately connected with that of the gradation of the characters, often accompanied by a change of function, for instance, the conversion of a swim-bladder into lungs, points which were discussed in the last chapter under two headings. nevertheless, i will here consider in some detail several of the cases advanced by mr. mivart, selecting those which are the most illustrative, as want of space prevents me from considering all. the giraffe, by its lofty stature, much elongated neck, fore legs, head and tongue, has its whole frame beautifully adapted for browsing on the higher branches of trees. it can thus obtain food beyond the reach of the other ungulata or hoofed animals inhabiting the same country; and this must be a great advantage to it during dearths. the niata cattle in south america show us how small a difference in structure may make, during such periods, a great difference in preserving an animal's life. these cattle can browse as well as others on grass, but from the projection of the lower jaw they cannot, during the often recurrent droughts, browse on the twigs of trees, reeds, etc., to which food the common cattle and horses are then driven; so that at these times the niatas perish, if not fed by their owners. before coming to mr. mivart's objections, it may be well to explain once again how natural selection will act in all ordinary cases. man has modified some of his animals, without necessarily having attended to special points of structure, by simply preserving and breeding from the fleetest individuals, as with the race-horse and greyhound, or as with the game-cock, by breeding from the victorious birds. so under nature with the nascent giraffe, the individuals which were the highest browsers and were able during dearths to reach even an inch or two above the others, will often have been preserved; for they will have roamed over the whole country in search of food. that the individuals of the same species often differ slightly in the relative lengths of all their parts may be seen in many works of natural history, in which careful measurements are given. these slight proportional differences, due to the laws of growth and variation, are not of the slightest use or importance to most species. but it will have been otherwise with the nascent giraffe, considering its probable habits of life; for those individuals which had some one part or several parts of their bodies rather more elongated than usual, would generally have survived. these will have intercrossed and left offspring, either inheriting the same bodily peculiarities, or with a tendency to vary again in the same manner; while the individuals less favoured in the same respects will have been the most liable to perish. we here see that there is no need to separate single pairs, as man does, when he methodically improves a breed: natural selection will preserve and thus separate all the superior individuals, allowing them freely to intercross, and will destroy all the inferior individuals. by this process long-continued, which exactly corresponds with what i have called unconscious selection by man, combined, no doubt, in a most important manner with the inherited effects of the increased use of parts, it seems to me almost certain that an ordinary hoofed quadruped might be converted into a giraffe. to this conclusion mr. mivart brings forward two objections. one is that the increased size of the body would obviously require an increased supply of food, and he considers it as "very problematical whether the disadvantages thence arising would not, in times of scarcity, more than counterbalance the advantages." but as the giraffe does actually exist in large numbers in africa, and as some of the largest antelopes in the world, taller than an ox, abound there, why should we doubt that, as far as size is concerned, intermediate gradations could formerly have existed there, subjected as now to severe dearths. assuredly the being able to reach, at each stage of increased size, to a supply of food, left untouched by the other hoofed quadrupeds of the country, would have been of some advantage to the nascent giraffe. nor must we overlook the fact, that increased bulk would act as a protection against almost all beasts of prey excepting the lion; and against this animal, its tall neck--and the taller the better--would, as mr. chauncey wright has remarked, serve as a watch-tower. it is from this cause, as sir s. baker remarks, that no animal is more difficult to stalk than the giraffe. this animal also uses its long neck as a means of offence or defence, by violently swinging its head armed with stump-like horns. the preservation of each species can rarely be determined by any one advantage, but by the union of all, great and small. mr. mivart then asks (and this is his second objection), if natural selection be so potent, and if high browsing be so great an advantage, why has not any other hoofed quadruped acquired a long neck and lofty stature, besides the giraffe, and, in a lesser degree, the camel, guanaco and macrauchenia? or, again, why has not any member of the group acquired a long proboscis? with respect to south africa, which was formerly inhabited by numerous herds of the giraffe, the answer is not difficult, and can best be given by an illustration. in every meadow in england, in which trees grow, we see the lower branches trimmed or planed to an exact level by the browsing of the horses or cattle; and what advantage would it be, for instance, to sheep, if kept there, to acquire slightly longer necks? in every district some one kind of animal will almost certainly be able to browse higher than the others; and it is almost equally certain that this one kind alone could have its neck elongated for this purpose, through natural selection and the effects of increased use. in south africa the competition for browsing on the higher branches of the acacias and other trees must be between giraffe and giraffe, and not with the other ungulate animals. why, in other quarters of the world, various animals belonging to this same order have not acquired either an elongated neck or a proboscis, cannot be distinctly answered; but it is as unreasonable to expect a distinct answer to such a question as why some event in the history of mankind did not occur in one country while it did in another. we are ignorant with respect to the conditions which determine the numbers and range of each species, and we cannot even conjecture what changes of structure would be favourable to its increase in some new country. we can, however, see in a general manner that various causes might have interfered with the development of a long neck or proboscis. to reach the foliage at a considerable height (without climbing, for which hoofed animals are singularly ill-constructed) implies greatly increased bulk of body; and we know that some areas support singularly few large quadrupeds, for instance south america, though it is so luxuriant, while south africa abounds with them to an unparalleled degree. why this should be so we do not know; nor why the later tertiary periods should have been much more favourable for their existence than the present time. whatever the causes may have been, we can see that certain districts and times would have been much more favourable than others for the development of so large a quadruped as the giraffe. in order that an animal should acquire some structure specially and largely developed, it is almost indispensable that several other parts should be modified and coadapted. although every part of the body varies slightly, it does not follow that the necessary parts should always vary in the right direction and to the right degree. with the different species of our domesticated animals we know that the parts vary in a different manner and degree, and that some species are much more variable than others. even if the fitting variations did arise, it does not follow that natural selection would be able to act on them and produce a structure which apparently would be beneficial to the species. for instance, if the number of individuals existing in a country is determined chiefly through destruction by beasts of prey--by external or internal parasites, etc.--as seems often to be the case, then natural selection will be able to do little, or will be greatly retarded, in modifying any particular structure for obtaining food. lastly, natural selection is a slow process, and the same favourable conditions must long endure in order that any marked effect should thus be produced. except by assigning such general and vague reasons, we cannot explain why, in many quarters of the world, hoofed quadrupeds have not acquired much elongated necks or other means for browsing on the higher branches of trees. objections of the same nature as the foregoing have been advanced by many writers. in each case various causes, besides the general ones just indicated, have probably interfered with the acquisition through natural selection of structures, which it is thought would be beneficial to certain species. one writer asks, why has not the ostrich acquired the power of flight? but a moment's reflection will show what an enormous supply of food would be necessary to give to this bird of the desert force to move its huge body through the air. oceanic islands are inhabited by bats and seals, but by no terrestrial mammals; yet as some of these bats are peculiar species, they must have long inhabited their present homes. therefore sir c. lyell asks, and assigns certain reasons in answer, why have not seals and bats given birth on such islands to forms fitted to live on the land? but seals would necessarily be first converted into terrestrial carnivorous animals of considerable size, and bats into terrestrial insectivorous animals; for the former there would be no prey; for the bats ground-insects would serve as food, but these would already be largely preyed on by the reptiles or birds, which first colonise and abound on most oceanic islands. gradations of structure, with each stage beneficial to a changing species, will be favoured only under certain peculiar conditions. a strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted into an animal so thoroughly aquatic as to brave the open ocean. but seals would not find on oceanic islands the conditions favourable to their gradual reconversion into a terrestrial form. bats, as formerly shown, probably acquired their wings by at first gliding through the air from tree to tree, like the so-called flying squirrels, for the sake of escaping from their enemies, or for avoiding falls; but when the power of true flight had once been acquired, it would never be reconverted back, at least for the above purposes, into the less efficient power of gliding through the air. bats, might, indeed, like many birds, have had their wings greatly reduced in size, or completely lost, through disuse; but in this case it would be necessary that they should first have acquired the power of running quickly on the ground, by the aid of their hind legs alone, so as to compete with birds or other ground animals; and for such a change a bat seems singularly ill-fitted. these conjectural remarks have been made merely to show that a transition of structure, with each step beneficial, is a highly complex affair; and that there is nothing strange in a transition not having occurred in any particular case. lastly, more than one writer has asked why have some animals had their mental powers more highly developed than others, as such development would be advantageous to all? why have not apes acquired the intellectual powers of man? various causes could be assigned; but as they are conjectural, and their relative probability cannot be weighed, it would be useless to give them. a definite answer to the latter question ought not to be expected, seeing that no one can solve the simpler problem, why, of two races of savages, one has risen higher in the scale of civilisation than the other; and this apparently implies increased brain power. we will return to mr. mivart's other objections. insects often resemble for the sake of protection various objects, such as green or decayed leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds, and living insects; but to this latter point i shall hereafter recur. the resemblance is often wonderfully close, and is not confined to colour, but extends to form, and even to the manner in which the insects hold themselves. the caterpillars which project motionless like dead twigs from the bushes on which they feed, offer an excellent instance of a resemblance of this kind. the cases of the imitation of such objects as the excrement of birds, are rare and exceptional. on this head, mr. mivart remarks, "as, according to mr. darwin's theory, there is a constant tendency to indefinite variation, and as the minute incipient variations will be in all directions, they must tend to neutralize each other, and at first to form such unstable modifications that it is difficult, if not impossible, to see how such indefinite oscillations of infinitesimal beginnings can ever build up a sufficiently appreciable resemblance to a leaf, bamboo, or other object, for natural selection to seize upon and perpetuate." but in all the foregoing cases the insects in their original state no doubt presented some rude and accidental resemblance to an object commonly found in the stations frequented by them. nor is this at all improbable, considering the almost infinite number of surrounding objects and the diversity in form and colour of the hosts of insects which exist. as some rude resemblance is necessary for the first start, we can understand how it is that the larger and higher animals do not (with the exception, as far as i know, of one fish) resemble for the sake of protection special objects, but only the surface which commonly surrounds them, and this chiefly in colour. assuming that an insect originally happened to resemble in some degree a dead twig or a decayed leaf, and that it varied slightly in many ways, then all the variations which rendered the insect at all more like any such object, and thus favoured its escape, would be preserved, while other variations would be neglected and ultimately lost; or, if they rendered the insect at all less like the imitated object, they would be eliminated. there would indeed be force in mr. mivart's objection, if we were to attempt to account for the above resemblances, independently of natural selection, through mere fluctuating variability; but as the case stands there is none. nor can i see any force in mr. mivart's difficulty with respect to "the last touches of perfection in the mimicry;" as in the case given by mr. wallace, of a walking-stick insect (ceroxylus laceratus), which resembles "a stick grown over by a creeping moss or jungermannia." so close was this resemblance, that a native dyak maintained that the foliaceous excrescences were really moss. insects are preyed on by birds and other enemies whose sight is probably sharper than ours, and every grade in resemblance which aided an insect to escape notice or detection, would tend towards its preservation; and the more perfect the resemblance so much the better for the insect. considering the nature of the differences between the species in the group which includes the above ceroxylus, there is nothing improbable in this insect having varied in the irregularities on its surface, and in these having become more or less green-coloured; for in every group the characters which differ in the several species are the most apt to vary, while the generic characters, or those common to all the species, are the most constant. the greenland whale is one of the most wonderful animals in the world, and the baleen, or whalebone, one of its greatest peculiarities. the baleen consists of a row, on each side of the upper jaw, of about plates or laminae, which stand close together transversely to the longer axis of the mouth. within the main row there are some subsidiary rows. the extremities and inner margins of all the plates are frayed into stiff bristles, which clothe the whole gigantic palate, and serve to strain or sift the water, and thus to secure the minute prey on which these great animals subsist. the middle and longest lamina in the greenland whale is ten, twelve, or even fifteen feet in length; but in the different species of cetaceans there are gradations in length; the middle lamina being in one species, according to scoresby, four feet, in another three, in another eighteen inches, and in the balaenoptera rostrata only about nine inches in length. the quality of the whalebone also differs in the different species. with respect to the baleen, mr. mivart remarks that if it "had once attained such a size and development as to be at all useful, then its preservation and augmentation within serviceable limits would be promoted by natural selection alone. but how to obtain the beginning of such useful development?" in answer, it may be asked, why should not the early progenitors of the whales with baleen have possessed a mouth constructed something like the lamellated beak of a duck? ducks, like whales, subsist by sifting the mud and water; and the family has sometimes been called criblatores, or sifters. i hope that i may not be misconstrued into saying that the progenitors of whales did actually possess mouths lamellated like the beak of a duck. i wish only to show that this is not incredible, and that the immense plates of baleen in the greenland whale might have been developed from such lamellae by finely graduated steps, each of service to its possessor. the beak of a shoveller-duck (spatula clypeata) is a more beautiful and complex structure than the mouth of a whale. the upper mandible is furnished on each side (in the specimen examined by me) with a row or comb formed of thin, elastic lamellae, obliquely bevelled so as to be pointed, and placed transversely to the longer axis of the mouth. they arise from the palate, and are attached by flexible membrane to the sides of the mandible. those standing towards the middle are the longest, being about one-third of an inch in length, and they project fourteen one-hundredths of an inch beneath the edge. at their bases there is a short subsidiary row of obliquely transverse lamellae. in these several respects they resemble the plates of baleen in the mouth of a whale. but towards the extremity of the beak they differ much, as they project inward, instead of straight downward. the entire head of the shoveller, though incomparably less bulky, is about one-eighteenth of the length of the head of a moderately large balaenoptera rostrata, in which species the baleen is only nine inches long; so that if we were to make the head of the shoveller as long as that of the balaenoptera, the lamellae would be six inches in length, that is, two-thirds of the length of the baleen in this species of whale. the lower mandible of the shoveller-duck is furnished with lamellae of equal length with these above, but finer; and in being thus furnished it differs conspicuously from the lower jaw of a whale, which is destitute of baleen. on the other hand, the extremities of these lower lamellae are frayed into fine bristly points, so that they thus curiously resemble the plates of baleen. in the genus prion, a member of the distinct family of the petrels, the upper mandible alone is furnished with lamellae, which are well developed and project beneath the margin; so that the beak of this bird resembles in this respect the mouth of a whale. from the highly developed structure of the shoveller's beak we may proceed (as i have learned from information and specimens sent to me by mr. salvin), without any great break, as far as fitness for sifting is concerned, through the beak of the merganetta armata, and in some respects through that of the aix sponsa, to the beak of the common duck. in this latter species the lamellae are much coarser than in the shoveller, and are firmly attached to the sides of the mandible; they are only about fifty in number on each side, and do not project at all beneath the margin. they are square-topped, and are edged with translucent, hardish tissue, as if for crushing food. the edges of the lower mandible are crossed by numerous fine ridges, which project very little. although the beak is thus very inferior as a sifter to that of a shoveller, yet this bird, as every one knows, constantly uses it for this purpose. there are other species, as i hear from mr. salvin, in which the lamellae are considerably less developed than in the common duck; but i do not know whether they use their beaks for sifting the water. turning to another group of the same family. in the egyptian goose (chenalopex) the beak closely resembles that of the common duck; but the lamellae are not so numerous, nor so distinct from each other, nor do they project so much inward; yet this goose, as i am informed by mr. e. bartlett, "uses its bill like a duck by throwing the water out at the corners." its chief food, however, is grass, which it crops like the common goose. in this latter bird the lamellae of the upper mandible are much coarser than in the common duck, almost confluent, about twenty-seven in number on each side, and terminating upward in teeth-like knobs. the palate is also covered with hard rounded knobs. the edges of the lower mandible are serrated with teeth much more prominent, coarser and sharper than in the duck. the common goose does not sift the water, but uses its beak exclusively for tearing or cutting herbage, for which purpose it is so well fitted that it can crop grass closer than almost any other animal. there are other species of geese, as i hear from mr. bartlett, in which the lamellae are less developed than in the common goose. we thus see that a member of the duck family, with a beak constructed like that of a common goose and adapted solely for grazing, or even a member with a beak having less well-developed lamellae, might be converted by small changes into a species like the egyptian goose--this into one like the common duck--and, lastly, into one like the shoveller, provided with a beak almost exclusively adapted for sifting the water; for this bird could hardly use any part of its beak, except the hooked tip, for seizing or tearing solid food. the beak of a goose, as i may add, might also be converted by small changes into one provided with prominent, recurved teeth, like those of the merganser (a member of the same family), serving for the widely different purpose of securing live fish. returning to the whales. the hyperoodon bidens is destitute of true teeth in an efficient condition, but its palate is roughened, according to lacepede, with small unequal, hard points of horn. there is, therefore, nothing improbable in supposing that some early cetacean form was provided with similar points of horn on the palate, but rather more regularly placed, and which, like the knobs on the beak of the goose, aided it in seizing or tearing its food. if so, it will hardly be denied that the points might have been converted through variation and natural selection into lamellae as well-developed as those of the egyptian goose, in which case they would have been used both for seizing objects and for sifting the water; then into lamellae like those of the domestic duck; and so onward, until they became as well constructed as those of the shoveller, in which case they would have served exclusively as a sifting apparatus. from this stage, in which the lamellae would be two-thirds of the length of the plates of baleen in the balaenoptera rostrata, gradations, which may be observed in still-existing cetaceans, lead us onward to the enormous plates of baleen in the greenland whale. nor is there the least reason to doubt that each step in this scale might have been as serviceable to certain ancient cetaceans, with the functions of the parts slowly changing during the progress of development, as are the gradations in the beaks of the different existing members of the duck-family. we should bear in mind that each species of duck is subjected to a severe struggle for existence, and that the structure of every part of its frame must be well adapted to its conditions of life. the pleuronectidae, or flat-fish, are remarkable for their asymmetrical bodies. they rest on one side--in the greater number of species on the left, but in some on the right side; and occasionally reversed adult specimens occur. the lower, or resting-surface, resembles at first sight the ventral surface of an ordinary fish; it is of a white colour, less developed in many ways than the upper side, with the lateral fins often of smaller size. but the eyes offer the most remarkable peculiarity; for they are both placed on the upper side of the head. during early youth, however, they stand opposite to each other, and the whole body is then symmetrical, with both sides equally coloured. soon the eye proper to the lower side begins to glide slowly round the head to the upper side; but does not pass right through the skull, as was formerly thought to be the case. it is obvious that unless the lower eye did thus travel round, it could not be used by the fish while lying in its habitual position on one side. the lower eye would, also, have been liable to be abraded by the sandy bottom. that the pleuronectidae are admirably adapted by their flattened and asymmetrical structure for their habits of life, is manifest from several species, such as soles, flounders, etc., being extremely common. the chief advantages thus gained seem to be protection from their enemies, and facility for feeding on the ground. the different members, however, of the family present, as schiodte remarks, "a long series of forms exhibiting a gradual transition from hippoglossus pinguis, which does not in any considerable degree alter the shape in which it leaves the ovum, to the soles, which are entirely thrown to one side." mr. mivart has taken up this case, and remarks that a sudden spontaneous transformation in the position of the eyes is hardly conceivable, in which i quite agree with him. he then adds: "if the transit was gradual, then how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is, indeed, far from clear. it seems, even, that such an incipient transformation must rather have been injurious." but he might have found an answer to this objection in the excellent observations published in by malm. the pleuronectidae, while very young and still symmetrical, with their eyes standing on opposite sides of the head, cannot long retain a vertical position, owing to the excessive depth of their bodies, the small size of their lateral fins, and to their being destitute of a swim-bladder. hence, soon growing tired, they fall to the bottom on one side. while thus at rest they often twist, as malm observed, the lower eye upward, to see above them; and they do this so vigorously that the eye is pressed hard against the upper part of the orbit. the forehead between the eyes consequently becomes, as could be plainly seen, temporarily contracted in breadth. on one occasion malm saw a young fish raise and depress the lower eye through an angular distance of about seventy degrees. we should remember that the skull at this early age is cartilaginous and flexible, so that it readily yields to muscular action. it is also known with the higher animals, even after early youth, that the skull yields and is altered in shape, if the skin or muscles be permanently contracted through disease or some accident. with long-eared rabbits, if one ear flops forward and downward, its weight drags forward all the bones of the skull on the same side, of which i have given a figure. malm states that the newly-hatched young of perches, salmon, and several other symmetrical fishes, have the habit of occasionally resting on one side at the bottom; and he has observed that they often then strain their lower eyes so as to look upward; and their skulls are thus rendered rather crooked. these fishes, however, are soon able to hold themselves in a vertical position, and no permanent effect is thus produced. with the pleuronectidae, on the other hand, the older they grow the more habitually they rest on one side, owing to the increasing flatness of their bodies, and a permanent effect is thus produced on the form of the head, and on the position of the eyes. judging from analogy, the tendency to distortion would no doubt be increased through the principle of inheritance. schiodte believes, in opposition to some other naturalists, that the pleuronectidae are not quite symmetrical even in the embryo; and if this be so, we could understand how it is that certain species, while young, habitually fall over and rest on the left side, and other species on the right side. malm adds, in confirmation of the above view, that the adult trachypterus arcticus, which is not a member of the pleuronectidae, rests on its left side at the bottom, and swims diagonally through the water; and in this fish, the two sides of the head are said to be somewhat dissimilar. our great authority on fishes, dr. gunther, concludes his abstract of malm's paper, by remarking that "the author gives a very simple explanation of the abnormal condition of the pleuronectoids." we thus see that the first stages of the transit of the eye from one side of the head to the other, which mr. mivart considers would be injurious, may be attributed to the habit, no doubt beneficial to the individual and to the species, of endeavouring to look upward with both eyes, while resting on one side at the bottom. we may also attribute to the inherited effects of use the fact of the mouth in several kinds of flat-fish being bent towards the lower surface, with the jaw bones stronger and more effective on this, the eyeless side of the head, than on the other, for the sake, as dr. traquair supposes, of feeding with ease on the ground. disuse, on the other hand, will account for the less developed condition of the whole inferior half of the body, including the lateral fins; though yarrel thinks that the reduced size of these fins is advantageous to the fish, as "there is so much less room for their action than with the larger fins above." perhaps the lesser number of teeth in the proportion of four to seven in the upper halves of the two jaws of the plaice, to twenty-five to thirty in the lower halves, may likewise be accounted for by disuse. from the colourless state of the ventral surface of most fishes and of many other animals, we may reasonably suppose that the absence of colour in flat-fish on the side, whether it be the right or left, which is under-most, is due to the exclusion of light. but it cannot be supposed that the peculiar speckled appearance of the upper side of the sole, so like the sandy bed of the sea, or the power in some species, as recently shown by pouchet, of changing their colour in accordance with the surrounding surface, or the presence of bony tubercles on the upper side of the turbot, are due to the action of the light. here natural selection has probably come into play, as well as in adapting the general shape of the body of these fishes, and many other peculiarities, to their habits of life. we should keep in mind, as i have before insisted, that the inherited effects of the increased use of parts, and perhaps of their disuse, will be strengthened by natural selection. for all spontaneous variations in the right direction will thus be preserved; as will those individuals which inherit in the highest degree the effects of the increased and beneficial use of any part. how much to attribute in each particular case to the effects of use, and how much to natural selection, it seems impossible to decide. i may give another instance of a structure which apparently owes its origin exclusively to use or habit. the extremity of the tail in some american monkeys has been converted into a wonderfully perfect prehensile organ, and serves as a fifth hand. a reviewer, who agrees with mr. mivart in every detail, remarks on this structure: "it is impossible to believe that in any number of ages the first slight incipient tendency to grasp could preserve the lives of the individuals possessing it, or favour their chance of having and of rearing offspring." but there is no necessity for any such belief. habit, and this almost implies that some benefit great or small is thus derived, would in all probability suffice for the work. brehm saw the young of an african monkey (cercopithecus) clinging to the under surface of their mother by their hands, and at the same time they hooked their little tails round that of their mother. professor henslow kept in confinement some harvest mice (mus messorius) which do not possess a structurally prehensive tail; but he frequently observed that they curled their tails round the branches of a bush placed in the cage, and thus aided themselves in climbing. i have received an analogous account from dr. gunther, who has seen a mouse thus suspend itself. if the harvest mouse had been more strictly arboreal, it would perhaps have had its tail rendered structurally prehensile, as is the case with some members of the same order. why cercopithecus, considering its habits while young, has not become thus provided, it would be difficult to say. it is, however, possible that the long tail of this monkey may be of more service to it as a balancing organ in making its prodigious leaps, than as a prehensile organ. the mammary glands are common to the whole class of mammals, and are indispensable for their existence; they must, therefore, have been developed at an extremely remote period, and we can know nothing positively about their manner of development. mr. mivart asks: "is it conceivable that the young of any animal was ever saved from destruction by accidentally sucking a drop of scarcely nutritious fluid from an accidentally hypertrophied cutaneous gland of its mother? and even if one was so, what chance was there of the perpetuation of such a variation?" but the case is not here put fairly. it is admitted by most evolutionists that mammals are descended from a marsupial form; and if so, the mammary glands will have been at first developed within the marsupial sack. in the case of the fish (hippocampus) the eggs are hatched, and the young are reared for a time, within a sack of this nature; and an american naturalist, mr. lockwood, believes from what he has seen of the development of the young, that they are nourished by a secretion from the cutaneous glands of the sack. now, with the early progenitors of mammals, almost before they deserved to be thus designated, is it not at least possible that the young might have been similarly nourished? and in this case, the individuals which secreted a fluid, in some degree or manner the most nutritious, so as to partake of the nature of milk, would in the long run have reared a larger number of well-nourished offspring, than would the individuals which secreted a poorer fluid; and thus the cutaneous glands, which are the homologues of the mammary glands, would have been improved or rendered more effective. it accords with the widely extended principle of specialisation, that the glands over a certain space of the sack should have become more highly developed than the remainder; and they would then have formed a breast, but at first without a nipple, as we see in the ornithorhyncus, at the base of the mammalian series. through what agency the glands over a certain space became more highly specialised than the others, i will not pretend to decide, whether in part through compensation of growth, the effects of use, or of natural selection. the development of the mammary glands would have been of no service, and could not have been affected through natural selection, unless the young at the same time were able to partake of the secretion. there is no greater difficulty in understanding how young mammals have instinctively learned to suck the breast, than in understanding how unhatched chickens have learned to break the egg-shell by tapping against it with their specially adapted beaks; or how a few hours after leaving the shell they have learned to pick up grains of food. in such cases the most probable solution seems to be, that the habit was at first acquired by practice at a more advanced age, and afterwards transmitted to the offspring at an earlier age. but the young kangaroo is said not to suck, only to cling to the nipple of its mother, who has the power of injecting milk into the mouth of her helpless, half-formed offspring. on this head mr. mivart remarks: "did no special provision exist, the young one must infallibly be choked by the intrusion of the milk into the wind-pipe. but there is a special provision. the larynx is so elongated that it rises up into the posterior end of the nasal passage, and is thus enabled to give free entrance to the air for the lungs, while the milk passes harmlessly on each side of this elongated larynx, and so safely attains the gullet behind it." mr. mivart then asks how did natural selection remove in the adult kangaroo (and in most other mammals, on the assumption that they are descended from a marsupial form), "this at least perfectly innocent and harmless structure?" it may be suggested in answer that the voice, which is certainly of high importance to many animals, could hardly have been used with full force as long as the larynx entered the nasal passage; and professor flower has suggested to me that this structure would have greatly interfered with an animal swallowing solid food. we will now turn for a short space to the lower divisions of the animal kingdom. the echinodermata (star-fishes, sea-urchins, etc.) are furnished with remarkable organs, called pedicellariae, which consist, when well developed, of a tridactyle forceps--that is, of one formed of three serrated arms, neatly fitting together and placed on the summit of a flexible stem, moved by muscles. these forceps can seize firmly hold of any object; and alexander agassiz has seen an echinus or sea-urchin rapidly passing particles of excrement from forceps to forceps down certain lines of its body, in order that its shell should not be fouled. but there is no doubt that besides removing dirt of all kinds, they subserve other functions; and one of these apparently is defence. with respect to these organs, mr. mivart, as on so many previous occasions, asks: "what would be the utility of the first rudimentary beginnings of such structures, and how could such insipient buddings have ever preserved the life of a single echinus?" he adds, "not even the sudden development of the snapping action would have been beneficial without the freely movable stalk, nor could the latter have been efficient without the snapping jaws, yet no minute, nearly indefinite variations could simultaneously evolve these complex co-ordinations of structure; to deny this seems to do no less than to affirm a startling paradox." paradoxical as this may appear to mr. mivart, tridactyle forcepses, immovably fixed at the base, but capable of a snapping action, certainly exist on some star-fishes; and this is intelligible if they serve, at least in part, as a means of defence. mr. agassiz, to whose great kindness i am indebted for much information on the subject, informs me that there are other star-fishes, in which one of the three arms of the forceps is reduced to a support for the other two; and again, other genera in which the third arm is completely lost. in echinoneus, the shell is described by m. perrier as bearing two kinds of pedicellariae, one resembling those of echinus, and the other those of spatangus; and such cases are always interesting as affording the means of apparently sudden transitions, through the abortion of one of the two states of an organ. with respect to the steps by which these curious organs have been evolved, mr. agassiz infers from his own researches and those of mr. muller, that both in star-fishes and sea-urchins the pedicellariae must undoubtedly be looked at as modified spines. this may be inferred from their manner of development in the individual, as well as from a long and perfect series of gradations in different species and genera, from simple granules to ordinary spines, to perfect tridactyle pedicellariae. the gradation extends even to the manner in which ordinary spines and the pedicellariae, with their supporting calcareous rods, are articulated to the shell. in certain genera of star-fishes, "the very combinations needed to show that the pedicellariae are only modified branching spines" may be found. thus we have fixed spines, with three equi-distant, serrated, movable branches, articulated to near their bases; and higher up, on the same spine, three other movable branches. now when the latter arise from the summit of a spine they form, in fact, a rude tridactyle pedicellariae, and such may be seen on the same spine together with the three lower branches. in this case the identity in nature between the arms of the pedicellariae and the movable branches of a spine, is unmistakable. it is generally admitted that the ordinary spines serve as a protection; and if so, there can be no reason to doubt that those furnished with serrated and movable branches likewise serve for the same purpose; and they would thus serve still more effectively as soon as by meeting together they acted as a prehensile or snapping apparatus. thus every gradation, from an ordinary fixed spine to a fixed pedicellariae, would be of service. in certain genera of star-fishes these organs, instead of being fixed or borne on an immovable support, are placed on the summit of a flexible and muscular, though short, stem; and in this case they probably subserve some additional function besides defence. in the sea-urchins the steps can be followed by which a fixed spine becomes articulated to the shell, and is thus rendered movable. i wish i had space here to give a fuller abstract of mr. agassiz's interesting observations on the development of the pedicellariae. all possible gradations, as he adds, may likewise be found between the pedicellariae of the star-fishes and the hooks of the ophiurians, another group of the echinodermata; and again between the pedicellariae of sea-urchins and the anchors of the holothuriae, also belonging to the same great class. certain compound animals, or zoophytes, as they have been termed, namely the polyzoa, are provided with curious organs called avicularia. these differ much in structure in the different species. in their most perfect condition they curiously resemble the head and beak of a vulture in miniature, seated on a neck and capable of movement, as is likewise the lower jaw or mandible. in one species observed by me, all the avicularia on the same branch often moved simultaneously backwards and forwards, with the lower jaw widely open, through an angle of about degrees, in the course of five seconds; and their movement caused the whole polyzoary to tremble. when the jaws are touched with a needle they seize it so firmly that the branch can thus be shaken. mr. mivart adduces this case, chiefly on account of the supposed difficulty of organs, namely the avicularia of the polyzoa and the pedicellariae of the echinodermata, which he considers as "essentially similar," having been developed through natural selection in widely distinct divisions of the animal kingdom. but, as far as structure is concerned, i can see no similarity between tridactyle pedicellariae and avicularia. the latter resembles somewhat more closely the chelae or pincers of crustaceans; and mr. mivart might have adduced with equal appropriateness this resemblance as a special difficulty, or even their resemblance to the head and beak of a bird. the avicularia are believed by mr. busk, dr. smitt and dr. nitsche--naturalists who have carefully studied this group--to be homologous with the zooids and their cells which compose the zoophyte, the movable lip or lid of the cell corresponding with the lower and movable mandible of the avicularium. mr. busk, however, does not know of any gradations now existing between a zooid and an avicularium. it is therefore impossible to conjecture by what serviceable gradations the one could have been converted into the other, but it by no means follows from this that such gradations have not existed. as the chelae of crustaceans resemble in some degree the avicularia of polyzoa, both serving as pincers, it may be worth while to show that with the former a long series of serviceable gradations still exists. in the first and simplest stage, the terminal segment of a limb shuts down either on the square summit of the broad penultimate segment, or against one whole side, and is thus enabled to catch hold of an object, but the limb still serves as an organ of locomotion. we next find one corner of the broad penultimate segment slightly prominent, sometimes furnished with irregular teeth, and against these the terminal segment shuts down. by an increase in the size of this projection, with its shape, as well as that of the terminal segment, slightly modified and improved, the pincers are rendered more and more perfect, until we have at last an instrument as efficient as the chelae of a lobster. and all these gradations can be actually traced. besides the avicularia, the polyzoa possess curious organs called vibracula. these generally consist of long bristles, capable of movement and easily excited. in one species examined by me the vibracula were slightly curved and serrated along the outer margin, and all of them on the same polyzoary often moved simultaneously; so that, acting like long oars, they swept a branch rapidly across the object-glass of my microscope. when a branch was placed on its face, the vibracula became entangled, and they made violent efforts to free themselves. they are supposed to serve as a defence, and may be seen, as mr. busk remarks, "to sweep slowly and carefully over the surface of the polyzoary, removing what might be noxious to the delicate inhabitants of the cells when their tentacula are protruded." the avicularia, like the vibracula, probably serve for defence, but they also catch and kill small living animals, which, it is believed, are afterwards swept by the currents within reach of the tentacula of the zooids. some species are provided with avicularia and vibracula, some with avicularia alone and a few with vibracula alone. it is not easy to imagine two objects more widely different in appearance than a bristle or vibraculum, and an avicularium like the head of a bird; yet they are almost certainly homologous and have been developed from the same common source, namely a zooid with its cell. hence, we can understand how it is that these organs graduate in some cases, as i am informed by mr. busk, into each other. thus, with the avicularia of several species of lepralia, the movable mandible is so much produced and is so like a bristle that the presence of the upper or fixed beak alone serves to determine its avicularian nature. the vibracula may have been directly developed from the lips of the cells, without having passed through the avicularian stage; but it seems more probable that they have passed through this stage, as during the early stages of the transformation, the other parts of the cell, with the included zooid, could hardly have disappeared at once. in many cases the vibracula have a grooved support at the base, which seems to represent the fixed beak; though this support in some species is quite absent. this view of the development of the vibracula, if trustworthy, is interesting; for supposing that all the species provided with avicularia had become extinct, no one with the most vivid imagination would ever have thought that the vibracula had originally existed as part of an organ, resembling a bird's head, or an irregular box or hood. it is interesting to see two such widely different organs developed from a common origin; and as the movable lip of the cell serves as a protection to the zooid, there is no difficulty in believing that all the gradations, by which the lip became converted first into the lower mandible of an avicularium, and then into an elongated bristle, likewise served as a protection in different ways and under different circumstances. in the vegetable kingdom mr. mivart only alludes to two cases, namely the structure of the flowers of orchids, and the movements of climbing plants. with respect to the former, he says: "the explanation of their origin is deemed thoroughly unsatisfactory--utterly insufficient to explain the incipient, infinitesimal beginnings of structures which are of utility only when they are considerably developed." as i have fully treated this subject in another work, i will here give only a few details on one alone of the most striking peculiarities of the flowers of orchids, namely, their pollinia. a pollinium, when highly developed, consists of a mass of pollen-grains, affixed to an elastic foot-stalk or caudicle, and this to a little mass of extremely viscid matter. the pollinia are by this means transported by insects from one flower to the stigma of another. in some orchids there is no caudicle to the pollen-masses, and the grains are merely tied together by fine threads; but as these are not confined to orchids, they need not here be considered; yet i may mention that at the base of the orchidaceous series, in cypripedium, we can see how the threads were probably first developed. in other orchids the threads cohere at one end of the pollen-masses; and this forms the first or nascent trace of a caudicle. that this is the origin of the caudicle, even when of considerable length and highly developed, we have good evidence in the aborted pollen-grains which can sometimes be detected embedded within the central and solid parts. with respect to the second chief peculiarity, namely, the little mass of viscid matter attached to the end of the caudicle, a long series of gradations can be specified, each of plain service to the plant. in most flowers belonging to other orders the stigma secretes a little viscid matter. now, in certain orchids similar viscid matter is secreted, but in much larger quantities by one alone of the three stigmas; and this stigma, perhaps in consequence of the copious secretion, is rendered sterile. when an insect visits a flower of this kind, it rubs off some of the viscid matter, and thus at the same time drags away some of the pollen-grains. from this simple condition, which differs but little from that of a multitude of common flowers, there are endless gradations--to species in which the pollen-mass terminates in a very short, free caudicle--to others in which the caudicle becomes firmly attached to the viscid matter, with the sterile stigma itself much modified. in this latter case we have a pollinium in its most highly developed and perfect condition. he who will carefully examine the flowers of orchids for himself will not deny the existence of the above series of gradations--from a mass of pollen-grains merely tied together by threads, with the stigma differing but little from that of the ordinary flowers, to a highly complex pollinium, admirably adapted for transportal by insects; nor will he deny that all the gradations in the several species are admirably adapted in relation to the general structure of each flower for its fertilisation by different insects. in this, and in almost every other case, the enquiry may be pushed further backwards; and it may be asked how did the stigma of an ordinary flower become viscid, but as we do not know the full history of any one group of beings, it is as useless to ask, as it is hopeless to attempt answering, such questions. we will now turn to climbing plants. these can be arranged in a long series, from those which simply twine round a support, to those which i have called leaf-climbers, and to those provided with tendrils. in these two latter classes the stems have generally, but not always, lost the power of twining, though they retain the power of revolving, which the tendrils likewise possess. the gradations from leaf-climbers to tendril bearers are wonderfully close, and certain plants may be differently placed in either class. but in ascending the series from simple twiners to leaf-climbers, an important quality is added, namely sensitiveness to a touch, by which means the foot-stalks of the leaves or flowers, or these modified and converted into tendrils, are excited to bend round and clasp the touching object. he who will read my memoir on these plants will, i think, admit that all the many gradations in function and structure between simple twiners and tendril-bearers are in each case beneficial in a high degree to the species. for instance, it is clearly a great advantage to a twining plant to become a leaf-climber; and it is probable that every twiner which possessed leaves with long foot-stalks would have been developed into a leaf-climber, if the foot-stalks had possessed in any slight degree the requisite sensitiveness to a touch. as twining is the simplest means of ascending a support, and forms the basis of our series, it may naturally be asked how did plants acquire this power in an incipient degree, afterwards to be improved and increased through natural selection. the power of twining depends, firstly, on the stems while young being extremely flexible (but this is a character common to many plants which are not climbers); and, secondly, on their continually bending to all points of the compass, one after the other in succession, in the same order. by this movement the stems are inclined to all sides, and are made to move round and round. as soon as the lower part of a stem strikes against any object and is stopped, the upper part still goes on bending and revolving, and thus necessarily twines round and up the support. the revolving movement ceases after the early growth of each shoot. as in many widely separated families of plants, single species and single genera possess the power of revolving, and have thus become twiners, they must have independently acquired it, and cannot have inherited it from a common progenitor. hence, i was led to predict that some slight tendency to a movement of this kind would be found to be far from uncommon with plants which did not climb; and that this had afforded the basis for natural selection to work on and improve. when i made this prediction, i knew of only one imperfect case, namely, of the young flower-peduncles of a maurandia which revolved slightly and irregularly, like the stems of twining plants, but without making any use of this habit. soon afterwards fritz muller discovered that the young stems of an alisma and of a linum--plants which do not climb and are widely separated in the natural system--revolved plainly, though irregularly, and he states that he has reason to suspect that this occurs with some other plants. these slight movements appear to be of no service to the plants in question; anyhow, they are not of the least use in the way of climbing, which is the point that concerns us. nevertheless we can see that if the stems of these plants had been flexible, and if under the conditions to which they are exposed it had profited them to ascend to a height, then the habit of slightly and irregularly revolving might have been increased and utilised through natural selection, until they had become converted into well-developed twining species. with respect to the sensitiveness of the foot-stalks of the leaves and flowers, and of tendrils, nearly the same remarks are applicable as in the case of the revolving movements of twining plants. as a vast number of species, belonging to widely distinct groups, are endowed with this kind of sensitiveness, it ought to be found in a nascent condition in many plants which have not become climbers. this is the case: i observed that the young flower-peduncles of the above maurandia curved themselves a little towards the side which was touched. morren found in several species of oxalis that the leaves and their foot-stalks moved, especially after exposure to a hot sun, when they were gently and repeatedly touched, or when the plant was shaken. i repeated these observations on some other species of oxalis with the same result; in some of them the movement was distinct, but was best seen in the young leaves; in others it was extremely slight. it is a more important fact that according to the high authority of hofmeister, the young shoots and leaves of all plants move after being shaken; and with climbing plants it is, as we know, only during the early stages of growth that the foot-stalks and tendrils are sensitive. it is scarcely possible that the above slight movements, due to a touch or shake, in the young and growing organs of plants, can be of any functional importance to them. but plants possess, in obedience to various stimuli, powers of movement, which are of manifest importance to them; for instance, towards and more rarely from the light--in opposition to, and more rarely in the direction of, the attraction of gravity. when the nerves and muscles of an animal are excited by galvanism or by the absorption of strychnine, the consequent movements may be called an incidental result, for the nerves and muscles have not been rendered specially sensitive to these stimuli. so with plants it appears that, from having the power of movement in obedience to certain stimuli, they are excited in an incidental manner by a touch, or by being shaken. hence there is no great difficulty in admitting that in the case of leaf-climbers and tendril-bearers, it is this tendency which has been taken advantage of and increased through natural selection. it is, however, probable, from reasons which i have assigned in my memoir, that this will have occurred only with plants which had already acquired the power of revolving, and had thus become twiners. i have already endeavoured to explain how plants became twiners, namely, by the increase of a tendency to slight and irregular revolving movements, which were at first of no use to them; this movement, as well as that due to a touch or shake, being the incidental result of the power of moving, gained for other and beneficial purposes. whether, during the gradual development of climbing plants, natural selection has been aided by the inherited effects of use, i will not pretend to decide; but we know that certain periodical movements, for instance the so-called sleep of plants, are governed by habit. i have now considered enough, perhaps more than enough, of the cases, selected with care by a skilful naturalist, to prove that natural selection is incompetent to account for the incipient stages of useful structures; and i have shown, as i hope, that there is no great difficulty on this head. a good opportunity has thus been afforded for enlarging a little on gradations of structure, often associated with strange functions--an important subject, which was not treated at sufficient length in the former editions of this work. i will now briefly recapitulate the foregoing cases. with the giraffe, the continued preservation of the individuals of some extinct high-reaching ruminant, which had the longest necks, legs, etc., and could browse a little above the average height, and the continued destruction of those which could not browse so high, would have sufficed for the production of this remarkable quadruped; but the prolonged use of all the parts, together with inheritance, will have aided in an important manner in their co-ordination. with the many insects which imitate various objects, there is no improbability in the belief that an accidental resemblance to some common object was in each case the foundation for the work of natural selection, since perfected through the occasional preservation of slight variations which made the resemblance at all closer; and this will have been carried on as long as the insect continued to vary, and as long as a more and more perfect resemblance led to its escape from sharp-sighted enemies. in certain species of whales there is a tendency to the formation of irregular little points of horn on the palate; and it seems to be quite within the scope of natural selection to preserve all favourable variations, until the points were converted, first into lamellated knobs or teeth, like those on the beak of a goose--then into short lamellae, like those of the domestic ducks--and then into lamellae, as perfect as those of the shoveller-duck--and finally into the gigantic plates of baleen, as in the mouth of the greenland whale. in the family of the ducks, the lamellae are first used as teeth, then partly as teeth and partly as a sifting apparatus, and at last almost exclusively for this latter purpose. with such structures as the above lamellae of horn or whalebone, habit or use can have done little or nothing, as far as we can judge, towards their development. on the other hand, the transportal of the lower eye of a flat-fish to the upper side of the head, and the formation of a prehensile tail, may be attributed almost wholly to continued use, together with inheritance. with respect to the mammae of the higher animals, the most probable conjecture is that primordially the cutaneous glands over the whole surface of a marsupial sack secreted a nutritious fluid; and that these glands were improved in function through natural selection, and concentrated into a confined area, in which case they would have formed a mamma. there is no more difficulty in understanding how the branched spines of some ancient echinoderm, which served as a defence, became developed through natural selection into tridactyle pedicellariae, than in understanding the development of the pincers of crustaceans, through slight, serviceable modifications in the ultimate and penultimate segments of a limb, which was at first used solely for locomotion. in the avicularia and vibracula of the polyzoa we have organs widely different in appearance developed from the same source; and with the vibracula we can understand how the successive gradations might have been of service. with the pollinia of orchids, the threads which originally served to tie together the pollen-grains, can be traced cohering into caudicles; and the steps can likewise be followed by which viscid matter, such as that secreted by the stigmas of ordinary flowers, and still subserving nearly but not quite the same purpose, became attached to the free ends of the caudicles--all these gradations being of manifest benefit to the plants in question. with respect to climbing plants, i need not repeat what has been so lately said. it has often been asked, if natural selection be so potent, why has not this or that structure been gained by certain species, to which it would apparently have been advantageous? but it is unreasonable to expect a precise answer to such questions, considering our ignorance of the past history of each species, and of the conditions which at the present day determine its numbers and range. in most cases only general reasons, but in some few cases special reasons, can be assigned. thus to adapt a species to new habits of life, many co-ordinated modifications are almost indispensable, and it may often have happened that the requisite parts did not vary in the right manner or to the right degree. many species must have been prevented from increasing in numbers through destructive agencies, which stood in no relation to certain structures, which we imagine would have been gained through natural selection from appearing to us advantageous to the species. in this case, as the struggle for life did not depend on such structures, they could not have been acquired through natural selection. in many cases complex and long-enduring conditions, often of a peculiar nature, are necessary for the development of a structure; and the requisite conditions may seldom have concurred. the belief that any given structure, which we think, often erroneously, would have been beneficial to a species, would have been gained under all circumstances through natural selection, is opposed to what we can understand of its manner of action. mr. mivart does not deny that natural selection has effected something; but he considers it as "demonstrably insufficient" to account for the phenomena which i explain by its agency. his chief arguments have now been considered, and the others will hereafter be considered. they seem to me to partake little of the character of demonstration, and to have little weight in comparison with those in favour of the power of natural selection, aided by the other agencies often specified. i am bound to add, that some of the facts and arguments here used by me, have been advanced for the same purpose in an able article lately published in the "medico-chirurgical review." at the present day almost all naturalists admit evolution under some form. mr. mivart believes that species change through "an internal force or tendency," about which it is not pretended that anything is known. that species have a capacity for change will be admitted by all evolutionists; but there is no need, as it seems to me, to invoke any internal force beyond the tendency to ordinary variability, which through the aid of selection, by man has given rise to many well-adapted domestic races, and which, through the aid of natural selection, would equally well give rise by graduated steps to natural races or species. the final result will generally have been, as already explained, an advance, but in some few cases a retrogression, in organisation. mr. mivart is further inclined to believe, and some naturalists agree with him, that new species manifest themselves "with suddenness and by modifications appearing at once." for instance, he supposes that the differences between the extinct three-toed hipparion and the horse arose suddenly. he thinks it difficult to believe that the wing of a bird "was developed in any other way than by a comparatively sudden modification of a marked and important kind;" and apparently he would extend the same view to the wings of bats and pterodactyles. this conclusion, which implies great breaks or discontinuity in the series, appears to me improbable in the highest degree. everyone who believes in slow and gradual evolution, will of course admit that specific changes may have been as abrupt and as great as any single variation which we meet with under nature, or even under domestication. but as species are more variable when domesticated or cultivated than under their natural conditions, it is not probable that such great and abrupt variations have often occurred under nature, as are known occasionally to arise under domestication. of these latter variations several may be attributed to reversion; and the characters which thus reappear were, it is probable, in many cases at first gained in a gradual manner. a still greater number must be called monstrosities, such as six-fingered men, porcupine men, ancon sheep, niata cattle, etc.; and as they are widely different in character from natural species, they throw very little light on our subject. excluding such cases of abrupt variations, the few which remain would at best constitute, if found in a state of nature, doubtful species, closely related to their parental types. my reasons for doubting whether natural species have changed as abruptly as have occasionally domestic races, and for entirely disbelieving that they have changed in the wonderful manner indicated by mr. mivart, are as follows. according to our experience, abrupt and strongly marked variations occur in our domesticated productions, singly and at rather long intervals of time. if such occurred under nature, they would be liable, as formerly explained, to be lost by accidental causes of destruction and by subsequent intercrossing; and so it is known to be under domestication, unless abrupt variations of this kind are specially preserved and separated by the care of man. hence, in order that a new species should suddenly appear in the manner supposed by mr. mivart, it is almost necessary to believe, in opposition to all analogy, that several wonderfully changed individuals appeared simultaneously within the same district. this difficulty, as in the case of unconscious selection by man, is avoided on the theory of gradual evolution, through the preservation of a large number of individuals, which varied more or less in any favourable direction, and of the destruction of a large number which varied in an opposite manner. that many species have been evolved in an extremely gradual manner, there can hardly be a doubt. the species and even the genera of many large natural families are so closely allied together that it is difficult to distinguish not a few of them. on every continent, in proceeding from north to south, from lowland to upland, etc., we meet with a host of closely related or representative species; as we likewise do on certain distinct continents, which we have reason to believe were formerly connected. but in making these and the following remarks, i am compelled to allude to subjects hereafter to be discussed. look at the many outlying islands round a continent, and see how many of their inhabitants can be raised only to the rank of doubtful species. so it is if we look to past times, and compare the species which have just passed away with those still living within the same areas; or if we compare the fossil species embedded in the sub-stages of the same geological formation. it is indeed manifest that multitudes of species are related in the closest manner to other species that still exist, or have lately existed; and it will hardly be maintained that such species have been developed in an abrupt or sudden manner. nor should it be forgotten, when we look to the special parts of allied species, instead of to distinct species, that numerous and wonderfully fine gradations can be traced, connecting together widely different structures. many large groups of facts are intelligible only on the principle that species have been evolved by very small steps. for instance, the fact that the species included in the larger genera are more closely related to each other, and present a greater number of varieties than do the species in the smaller genera. the former are also grouped in little clusters, like varieties round species; and they present other analogies with varieties, as was shown in our second chapter. on this same principle we can understand how it is that specific characters are more variable than generic characters; and how the parts which are developed in an extraordinary degree or manner are more variable than other parts of the same species. many analogous facts, all pointing in the same direction, could be added. although very many species have almost certainly been produced by steps not greater than those separating fine varieties; yet it may be maintained that some have been developed in a different and abrupt manner. such an admission, however, ought not to be made without strong evidence being assigned. the vague and in some respects false analogies, as they have been shown to be by mr. chauncey wright, which have been advanced in favour of this view, such as the sudden crystallisation of inorganic substances, or the falling of a facetted spheroid from one facet to another, hardly deserve consideration. one class of facts, however, namely, the sudden appearance of new and distinct forms of life in our geological formations supports at first sight the belief in abrupt development. but the value of this evidence depends entirely on the perfection of the geological record, in relation to periods remote in the history of the world. if the record is as fragmentary as many geologists strenuously assert, there is nothing strange in new forms appearing as if suddenly developed. unless we admit transformations as prodigious as those advocated by mr. mivart, such as the sudden development of the wings of birds or bats, or the sudden conversion of a hipparion into a horse, hardly any light is thrown by the belief in abrupt modifications on the deficiency of connecting links in our geological formations. but against the belief in such abrupt changes, embryology enters a strong protest. it is notorious that the wings of birds and bats, and the legs of horses or other quadrupeds, are undistinguishable at an early embryonic period, and that they become differentiated by insensibly fine steps. embryological resemblances of all kinds can be accounted for, as we shall hereafter see, by the progenitors of our existing species having varied after early youth, and having transmitted their newly-acquired characters to their offspring, at a corresponding age. the embryo is thus left almost unaffected, and serves as a record of the past condition of the species. hence it is that existing species during the early stages of their development so often resemble ancient and extinct forms belonging to the same class. on this view of the meaning of embryological resemblances, and indeed on any view, it is incredible that an animal should have undergone such momentous and abrupt transformations as those above indicated, and yet should not bear even a trace in its embryonic condition of any sudden modification, every detail in its structure being developed by insensibly fine steps. he who believes that some ancient form was transformed suddenly through an internal force or tendency into, for instance, one furnished with wings, will be almost compelled to assume, in opposition to all analogy, that many individuals varied simultaneously. it cannot be denied that such abrupt and great changes of structure are widely different from those which most species apparently have undergone. he will further be compelled to believe that many structures beautifully adapted to all the other parts of the same creature and to the surrounding conditions, have been suddenly produced; and of such complex and wonderful co-adaptations, he will not be able to assign a shadow of an explanation. he will be forced to admit that these great and sudden transformations have left no trace of their action on the embryo. to admit all this is, as it seems to me, to enter into the realms of miracle, and to leave those of science. chapter viii. instinct. instincts comparable with habits, but different in their origin--instincts graduated--aphides and ants--instincts variable--domestic instincts, their origin--natural instincts of the cuckoo, molothrus, ostrich, and parasitic bees--slave-making ants--hive-bee, its cell-making instinct--changes of instinct and structure not necessarily simultaneous--difficulties of the theory of the natural selection of instincts--neuter or sterile insects--summary. many instincts are so wonderful that their development will probably appear to the reader a difficulty sufficient to overthrow my whole theory. i may here premise, that i have nothing to do with the origin of the mental powers, any more than i have with that of life itself. we are concerned only with the diversities of instinct and of the other mental faculties in animals of the same class. i will not attempt any definition of instinct. it would be easy to show that several distinct mental actions are commonly embraced by this term; but every one understands what is meant, when it is said that instinct impels the cuckoo to migrate and to lay her eggs in other birds' nests. an action, which we ourselves require experience to enable us to perform, when performed by an animal, more especially by a very young one, without experience, and when performed by many individuals in the same way, without their knowing for what purpose it is performed, is usually said to be instinctive. but i could show that none of these characters are universal. a little dose of judgment or reason, as pierre huber expresses it, often comes into play, even with animals low in the scale of nature. frederick cuvier and several of the older metaphysicians have compared instinct with habit. this comparison gives, i think, an accurate notion of the frame of mind under which an instinctive action is performed, but not necessarily of its origin. how unconsciously many habitual actions are performed, indeed not rarely in direct opposition to our conscious will! yet they may be modified by the will or reason. habits easily become associated with other habits, with certain periods of time and states of the body. when once acquired, they often remain constant throughout life. several other points of resemblance between instincts and habits could be pointed out. as in repeating a well-known song, so in instincts, one action follows another by a sort of rhythm; if a person be interrupted in a song, or in repeating anything by rote, he is generally forced to go back to recover the habitual train of thought: so p. huber found it was with a caterpillar, which makes a very complicated hammock; for if he took a caterpillar which had completed its hammock up to, say, the sixth stage of construction, and put it into a hammock completed up only to the third stage, the caterpillar simply re-performed the fourth, fifth, and sixth stages of construction. if, however, a caterpillar were taken out of a hammock made up, for instance, to the third stage, and were put into one finished up to the sixth stage, so that much of its work was already done for it, far from deriving any benefit from this, it was much embarrassed, and, in order to complete its hammock, seemed forced to start from the third stage, where it had left off, and thus tried to complete the already finished work. if we suppose any habitual action to become inherited--and it can be shown that this does sometimes happen--then the resemblance between what originally was a habit and an instinct becomes so close as not to be distinguished. if mozart, instead of playing the pianoforte at three years old with wonderfully little practice, had played a tune with no practice at all, be might truly be said to have done so instinctively. but it would be a serious error to suppose that the greater number of instincts have been acquired by habit in one generation, and then transmitted by inheritance to succeeding generations. it can be clearly shown that the most wonderful instincts with which we are acquainted, namely, those of the hive-bee and of many ants, could not possibly have been acquired by habit. it will be universally admitted that instincts are as important as corporeal structures for the welfare of each species, under its present conditions of life. under changed conditions of life, it is at least possible that slight modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so little, then i can see no difficulty in natural selection preserving and continually accumulating variations of instinct to any extent that was profitable. it is thus, as i believe, that all the most complex and wonderful instincts have originated. as modifications of corporeal structure arise from, and are increased by, use or habit, and are diminished or lost by disuse, so i do not doubt it has been with instincts. but i believe that the effects of habit are in many cases of subordinate importance to the effects of the natural selection of what may be called spontaneous variations of instincts;--that is of variations produced by the same unknown causes which produce slight deviations of bodily structure. no complex instinct can possibly be produced through natural selection, except by the slow and gradual accumulation of numerous, slight, yet profitable, variations. hence, as in the case of corporeal structures, we ought to find in nature, not the actual transitional gradations by which each complex instinct has been acquired--for these could be found only in the lineal ancestors of each species--but we ought to find in the collateral lines of descent some evidence of such gradations; or we ought at least to be able to show that gradations of some kind are possible; and this we certainly can do. i have been surprised to find, making allowance for the instincts of animals having been but little observed, except in europe and north america, and for no instinct being known among extinct species, how very generally gradations, leading to the most complex instincts, can be discovered. changes of instinct may sometimes be facilitated by the same species having different instincts at different periods of life, or at different seasons of the year, or when placed under different circumstances, etc.; in which case either the one or the other instinct might be preserved by natural selection. and such instances of diversity of instinct in the same species can be shown to occur in nature. again, as in the case of corporeal structure, and conformably to my theory, the instinct of each species is good for itself, but has never, as far as we can judge, been produced for the exclusive good of others. one of the strongest instances of an animal apparently performing an action for the sole good of another, with which i am acquainted, is that of aphides voluntarily yielding, as was first observed by huber, their sweet excretion to ants: that they do so voluntarily, the following facts show. i removed all the ants from a group of about a dozen aphides on a dock-plant, and prevented their attendance during several hours. after this interval, i felt sure that the aphides would want to excrete. i watched them for some time through a lens, but not one excreted; i then tickled and stroked them with a hair in the same manner, as well as i could, as the ants do with their antennae; but not one excreted. afterwards, i allowed an ant to visit them, and it immediately seemed, by its eager way of running about to be well aware what a rich flock it had discovered; it then began to play with its antennae on the abdomen first of one aphis and then of another; and each, as soon as it felt the antennae, immediately lifted up its abdomen and excreted a limpid drop of sweet juice, which was eagerly devoured by the ant. even the quite young aphides behaved in this manner, showing that the action was instinctive, and not the result of experience. it is certain, from the observations of huber, that the aphides show no dislike to the ants: if the latter be not present they are at last compelled to eject their excretion. but as the excretion is extremely viscid, it is no doubt a convenience to the aphides to have it removed; therefore probably they do not excrete solely for the good of the ants. although there is no evidence that any animal performs an action for the exclusive good of another species, yet each tries to take advantage of the instincts of others, as each takes advantage of the weaker bodily structure of other species. so again certain instincts cannot be considered as absolutely perfect; but as details on this and other such points are not indispensable, they may be here passed over. as some degree of variation in instincts under a state of nature, and the inheritance of such variations, are indispensable for the action of natural selection, as many instances as possible ought to be given; but want of space prevents me. i can only assert that instincts certainly do vary--for instance, the migratory instinct, both in extent and direction, and in its total loss. so it is with the nests of birds, which vary partly in dependence on the situations chosen, and on the nature and temperature of the country inhabited, but often from causes wholly unknown to us. audubon has given several remarkable cases of differences in the nests of the same species in the northern and southern united states. why, it has been asked, if instinct be variable, has it not granted to the bee "the ability to use some other material when wax was deficient?" but what other natural material could bees use? they will work, as i have seen, with wax hardened with vermilion or softened with lard. andrew knight observed that his bees, instead of laboriously collecting propolis, used a cement of wax and turpentine, with which he had covered decorticated trees. it has lately been shown that bees, instead of searching for pollen, will gladly use a very different substance, namely, oatmeal. fear of any particular enemy is certainly an instinctive quality, as may be seen in nestling birds, though it is strengthened by experience, and by the sight of fear of the same enemy in other animals. the fear of man is slowly acquired, as i have elsewhere shown, by the various animals which inhabit desert islands; and we see an instance of this, even in england, in the greater wildness of all our large birds in comparison with our small birds; for the large birds have been most persecuted by man. we may safely attribute the greater wildness of our large birds to this cause; for in uninhabited islands large birds are not more fearful than small; and the magpie, so wary in england, is tame in norway, as is the hooded crow in egypt. that the mental qualities of animals of the same kind, born in a state of nature, vary much, could be shown by many facts. several cases could also be adduced of occasional and strange habits in wild animals, which, if advantageous to the species, might have given rise, through natural selection, to new instincts. but i am well aware that these general statements, without the facts in detail, can produce but a feeble effect on the reader's mind. i can only repeat my assurance, that i do not speak without good evidence. inherited changes of habit or instinct in domesticated animals. the possibility, or even probability, of inherited variations of instinct in a state of nature will be strengthened by briefly considering a few cases under domestication. we shall thus be enabled to see the part which habit and the selection of so-called spontaneous variations have played in modifying the mental qualities of our domestic animals. it is notorious how much domestic animals vary in their mental qualities. with cats, for instance, one naturally takes to catching rats, and another mice, and these tendencies are known to be inherited. one cat, according to mr. st. john, always brought home game birds, another hares or rabbits, and another hunted on marshy ground and almost nightly caught woodcocks or snipes. a number of curious and authentic instances could be given of various shades of disposition and taste, and likewise of the oddest tricks, associated with certain frames of mind or periods of time. but let us look to the familiar case of the breeds of dogs: it cannot be doubted that young pointers (i have myself seen striking instances) will sometimes point and even back other dogs the very first time that they are taken out; retrieving is certainly in some degree inherited by retrievers; and a tendency to run round, instead of at, a flock of sheep, by shepherd-dogs. i cannot see that these actions, performed without experience by the young, and in nearly the same manner by each individual, performed with eager delight by each breed, and without the end being known--for the young pointer can no more know that he points to aid his master, than the white butterfly knows why she lays her eggs on the leaf of the cabbage--i cannot see that these actions differ essentially from true instincts. if we were to behold one kind of wolf, when young and without any training, as soon as it scented its prey, stand motionless like a statue, and then slowly crawl forward with a peculiar gait; and another kind of wolf rushing round, instead of at, a herd of deer, and driving them to a distant point, we should assuredly call these actions instinctive. domestic instincts, as they may be called, are certainly far less fixed than natural instincts; but they have been acted on by far less rigorous selection, and have been transmitted for an incomparably shorter period, under less fixed conditions of life. how strongly these domestic instincts, habits, and dispositions are inherited, and how curiously they become mingled, is well shown when different breeds of dogs are crossed. thus it is known that a cross with a bull-dog has affected for many generations the courage and obstinacy of greyhounds; and a cross with a greyhound has given to a whole family of shepherd-dogs a tendency to hunt hares. these domestic instincts, when thus tested by crossing, resemble natural instincts, which in a like manner become curiously blended together, and for a long period exhibit traces of the instincts of either parent: for example, le roy describes a dog, whose great-grandfather was a wolf, and this dog showed a trace of its wild parentage only in one way, by not coming in a straight line to his master, when called. domestic instincts are sometimes spoken of as actions which have become inherited solely from long-continued and compulsory habit, but this is not true. no one would ever have thought of teaching, or probably could have taught, the tumbler-pigeon to tumble--an action which, as i have witnessed, is performed by young birds, that have never seen a pigeon tumble. we may believe that some one pigeon showed a slight tendency to this strange habit, and that the long-continued selection of the best individuals in successive generations made tumblers what they now are; and near glasgow there are house-tumblers, as i hear from mr. brent, which cannot fly eighteen inches high without going head over heels. it may be doubted whether any one would have thought of training a dog to point, had not some one dog naturally shown a tendency in this line; and this is known occasionally to happen, as i once saw, in a pure terrier: the act of pointing is probably, as many have thought, only the exaggerated pause of an animal preparing to spring on its prey. when the first tendency to point was once displayed, methodical selection and the inherited effects of compulsory training in each successive generation would soon complete the work; and unconscious selection is still in progress, as each man tries to procure, without intending to improve the breed, dogs which stand and hunt best. on the other hand, habit alone in some cases has sufficed; hardly any animal is more difficult to tame than the young of the wild rabbit; scarcely any animal is tamer than the young of the tame rabbit; but i can hardly suppose that domestic rabbits have often been selected for tameness alone; so that we must attribute at least the greater part of the inherited change from extreme wildness to extreme tameness, to habit and long-continued close confinement. natural instincts are lost under domestication: a remarkable instance of this is seen in those breeds of fowls which very rarely or never become "broody," that is, never wish to sit on their eggs. familiarity alone prevents our seeing how largely and how permanently the minds of our domestic animals have been modified. it is scarcely possible to doubt that the love of man has become instinctive in the dog. all wolves, foxes, jackals and species of the cat genus, when kept tame, are most eager to attack poultry, sheep and pigs; and this tendency has been found incurable in dogs which have been brought home as puppies from countries such as tierra del fuego and australia, where the savages do not keep these domestic animals. how rarely, on the other hand, do our civilised dogs, even when quite young, require to be taught not to attack poultry, sheep, and pigs! no doubt they occasionally do make an attack, and are then beaten; and if not cured, they are destroyed; so that habit and some degree of selection have probably concurred in civilising by inheritance our dogs. on the other hand, young chickens have lost wholly by habit, that fear of the dog and cat which no doubt was originally instinctive in them, for i am informed by captain hutton that the young chickens of the parent stock, the gallus bankiva, when reared in india under a hen, are at first excessively wild. so it is with young pheasants reared in england under a hen. it is not that chickens have lost all fear, but fear only of dogs and cats, for if the hen gives the danger chuckle they will run (more especially young turkeys) from under her and conceal themselves in the surrounding grass or thickets; and this is evidently done for the instinctive purpose of allowing, as we see in wild ground-birds, their mother to fly away. but this instinct retained by our chickens has become useless under domestication, for the mother-hen has almost lost by disuse the power of flight. hence, we may conclude that under domestication instincts have been acquired and natural instincts have been lost, partly by habit and partly by man selecting and accumulating, during successive generations, peculiar mental habits and actions, which at first appeared from what we must in our ignorance call an accident. in some cases compulsory habit alone has sufficed to produce inherited mental changes; in other cases compulsory habit has done nothing, and all has been the result of selection, pursued both methodically and unconsciously; but in most cases habit and selection have probably concurred. special instincts. we shall, perhaps, best understand how instincts in a state of nature have become modified by selection by considering a few cases. i will select only three, namely, the instinct which leads the cuckoo to lay her eggs in other birds' nests; the slave-making instinct of certain ants; and the cell-making power of the hive-bee: these two latter instincts have generally and justly been ranked by naturalists as the most wonderful of all known instincts. instincts of the cuckoo. it is supposed by some naturalists that the more immediate cause of the instinct of the cuckoo is that she lays her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on her own eggs, those first laid would have to be left for some time unincubated or there would be eggs and young birds of different ages in the same nest. if this were the case the process of laying and hatching might be inconveniently long, more especially as she migrates at a very early period; and the first hatched young would probably have to be fed by the male alone. but the american cuckoo is in this predicament, for she makes her own nest and has eggs and young successively hatched, all at the same time. it has been both asserted and denied that the american cuckoo occasionally lays her eggs in other birds' nests; but i have lately heard from dr. merrill, of iowa, that he once found in illinois a young cuckoo, together with a young jay in the nest of a blue jay (garrulus cristatus); and as both were nearly full feathered, there could be no mistake in their identification. i could also give several instances of various birds which have been known occasionally to lay their eggs in other birds' nests. now let us suppose that the ancient progenitor of our european cuckoo had the habits of the american cuckoo, and that she occasionally laid an egg in another bird's nest. if the old bird profited by this occasional habit through being enabled to emigrate earlier or through any other cause; or if the young were made more vigorous by advantage being taken of the mistaken instinct of another species than when reared by their own mother, encumbered as she could hardly fail to be by having eggs and young of different ages at the same time, then the old birds or the fostered young would gain an advantage. and analogy would lead us to believe that the young thus reared would be apt to follow by inheritance the occasional and aberrant habit of their mother, and in their turn would be apt to lay their eggs in other birds' nests, and thus be more successful in rearing their young. by a continued process of this nature, i believe that the strange instinct of our cuckoo has been generated. it has, also recently been ascertained on sufficient evidence, by adolf muller, that the cuckoo occasionally lays her eggs on the bare ground, sits on them and feeds her young. this rare event is probably a case of reversion to the long-lost, aboriginal instinct of nidification. it has been objected that i have not noticed other related instincts and adaptations of structure in the cuckoo, which are spoken of as necessarily co-ordinated. but in all cases, speculation on an instinct known to us only in a single species, is useless, for we have hitherto had no facts to guide us. until recently the instincts of the european and of the non-parasitic american cuckoo alone were known; now, owing to mr. ramsay's observations, we have learned something about three australian species, which lay their eggs in other birds' nests. the chief points to be referred to are three: first, that the common cuckoo, with rare exceptions, lays only one egg in a nest, so that the large and voracious young bird receives ample food. secondly, that the eggs are remarkably small, not exceeding those of the skylark--a bird about one-fourth as large as the cuckoo. that the small size of the egg is a real case of adaptation we may infer from the fact of the mon-parasitic american cuckoo laying full-sized eggs. thirdly, that the young cuckoo, soon after birth, has the instinct, the strength and a properly shaped back for ejecting its foster-brothers, which then perish from cold and hunger. this has been boldly called a beneficent arrangement, in order that the young cuckoo may get sufficient food, and that its foster-brothers may perish before they had acquired much feeling! turning now to the australian species: though these birds generally lay only one egg in a nest, it is not rare to find two and even three eggs in the same nest. in the bronze cuckoo the eggs vary greatly in size, from eight to ten lines in length. now, if it had been of an advantage to this species to have laid eggs even smaller than those now laid, so as to have deceived certain foster-parents, or, as is more probable, to have been hatched within a shorter period (for it is asserted that there is a relation between the size of eggs and the period of their incubation), then there is no difficulty in believing that a race or species might have been formed which would have laid smaller and smaller eggs; for these would have been more safely hatched and reared. mr. ramsay remarks that two of the australian cuckoos, when they lay their eggs in an open nest, manifest a decided preference for nests containing eggs similar in colour to their own. the european species apparently manifests some tendency towards a similar instinct, but not rarely departs from it, as is shown by her laying her dull and pale-coloured eggs in the nest of the hedge-warbler with bright greenish-blue eggs. had our cuckoo invariably displayed the above instinct, it would assuredly have been added to those which it is assumed must all have been acquired together. the eggs of the australian bronze cuckoo vary, according to mr. ramsay, to an extraordinary degree in colour; so that in this respect, as well as in size, natural selection might have secured and fixed any advantageous variation. in the case of the european cuckoo, the offspring of the foster-parents are commonly ejected from the nest within three days after the cuckoo is hatched; and as the latter at this age is in a most helpless condition, mr. gould was formerly inclined to believe that the act of ejection was performed by the foster-parents themselves. but he has now received a trustworthy account of a young cuckoo which was actually seen, while still blind and not able even to hold up its own head, in the act of ejecting its foster-brothers. one of these was replaced in the nest by the observer, and was again thrown out. with respect to the means by which this strange and odious instinct was acquired, if it were of great importance for the young cuckoo, as is probably the case, to receive as much food as possible soon after birth, i can see no special difficulty in its having gradually acquired, during successive generations, the blind desire, the strength, and structure necessary for the work of ejection; for those cuckoos which had such habits and structure best developed would be the most securely reared. the first step towards the acquisition of the proper instinct might have been mere unintentional restlessness on the part of the young bird, when somewhat advanced in age and strength; the habit having been afterwards improved, and transmitted to an earlier age. i can see no more difficulty in this than in the unhatched young of other birds acquiring the instinct to break through their own shells; or than in young snakes acquiring in their upper jaws, as owen has remarked, a transitory sharp tooth for cutting through the tough egg-shell. for if each part is liable to individual variations at all ages, and the variations tend to be inherited at a corresponding or earlier age--propositions which cannot be disputed--then the instincts and structure of the young could be slowly modified as surely as those of the adult; and both cases must stand or fall together with the whole theory of natural selection. some species of molothrus, a widely distinct genus of american birds, allied to our starlings, have parasitic habits like those of the cuckoo; and the species present an interesting gradation in the perfection of their instincts. the sexes of molothrus badius are stated by an excellent observer, mr. hudson, sometimes to live promiscuously together in flocks, and sometimes to pair. they either build a nest of their own or seize on one belonging to some other bird, occasionally throwing out the nestlings of the stranger. they either lay their eggs in the nest thus appropriated, or oddly enough build one for themselves on the top of it. they usually sit on their own eggs and rear their own young; but mr. hudson says it is probable that they are occasionally parasitic, for he has seen the young of this species following old birds of a distinct kind and clamouring to be fed by them. the parasitic habits of another species of molothrus, the m. bonariensis, are much more highly developed than those of the last, but are still far from perfect. this bird, as far as it is known, invariably lays its eggs in the nests of strangers; but it is remarkable that several together sometimes commence to build an irregular untidy nest of their own, placed in singular ill-adapted situations, as on the leaves of a large thistle. they never, however, as far as mr. hudson has ascertained, complete a nest for themselves. they often lay so many eggs--from fifteen to twenty--in the same foster-nest, that few or none can possibly be hatched. they have, moreover, the extraordinary habit of pecking holes in the eggs, whether of their own species or of their foster parents, which they find in the appropriated nests. they drop also many eggs on the bare ground, which are thus wasted. a third species, the m. pecoris of north america, has acquired instincts as perfect as those of the cuckoo, for it never lays more than one egg in a foster-nest, so that the young bird is securely reared. mr. hudson is a strong disbeliever in evolution, but he appears to have been so much struck by the imperfect instincts of the molothrus bonariensis that he quotes my words, and asks, "must we consider these habits, not as especially endowed or created instincts, but as small consequences of one general law, namely, transition?" various birds, as has already been remarked, occasionally lay their eggs in the nests of other birds. this habit is not very uncommon with the gallinaceae, and throws some light on the singular instinct of the ostrich. in this family several hen birds unite and lay first a few eggs in one nest and then in another; and these are hatched by the males. this instinct may probably be accounted for by the fact of the hens laying a large number of eggs, but, as with the cuckoo, at intervals of two or three days. the instinct, however, of the american ostrich, as in the case of the molothrus bonariensis, has not as yet been perfected; for a surprising number of eggs lie strewed over the plains, so that in one day's hunting i picked up no less than twenty lost and wasted eggs. many bees are parasitic, and regularly lay their eggs in the nests of other kinds of bees. this case is more remarkable than that of the cuckoo; for these bees have not only had their instincts but their structure modified in accordance with their parasitic habits; for they do not possess the pollen-collecting apparatus which would have been indispensable if they had stored up food for their own young. some species of sphegidae (wasp-like insects) are likewise parasitic; and m. fabre has lately shown good reason for believing that, although the tachytes nigra generally makes its own burrow and stores it with paralysed prey for its own larvae, yet that, when this insect finds a burrow already made and stored by another sphex, it takes advantage of the prize, and becomes for the occasion parasitic. in this case, as with that of the molothrus or cuckoo, i can see no difficulty in natural selection making an occasional habit permanent, if of advantage to the species, and if the insect whose nest and stored food are feloniously appropriated, be not thus exterminated. slave-making instinct. this remarkable instinct was first discovered in the formica (polyerges) rufescens by pierre huber, a better observer even than his celebrated father. this ant is absolutely dependent on its slaves; without their aid, the species would certainly become extinct in a single year. the males and fertile females do no work of any kind, and the workers or sterile females, though most energetic and courageous in capturing slaves, do no other work. they are incapable of making their own nests, or of feeding their own larvae. when the old nest is found inconvenient, and they have to migrate, it is the slaves which determine the migration, and actually carry their masters in their jaws. so utterly helpless are the masters, that when huber shut up thirty of them without a slave, but with plenty of the food which they like best, and with their larvae and pupae to stimulate them to work, they did nothing; they could not even feed themselves, and many perished of hunger. huber then introduced a single slave (f. fusca), and she instantly set to work, fed and saved the survivors; made some cells and tended the larvae, and put all to rights. what can be more extraordinary than these well-ascertained facts? if we had not known of any other slave-making ant, it would have been hopeless to speculate how so wonderful an instinct could have been perfected. another species, formica sanguinea, was likewise first discovered by p. huber to be a slave-making ant. this species is found in the southern parts of england, and its habits have been attended to by mr. f. smith, of the british museum, to whom i am much indebted for information on this and other subjects. although fully trusting to the statements of huber and mr. smith, i tried to approach the subject in a sceptical frame of mind, as any one may well be excused for doubting the existence of so extraordinary an instinct as that of making slaves. hence, i will give the observations which i made in some little detail. i opened fourteen nests of f. sanguinea, and found a few slaves in all. males and fertile females of the slave-species (f. fusca) are found only in their own proper communities, and have never been observed in the nests of f. sanguinea. the slaves are black and not above half the size of their red masters, so that the contrast in their appearance is great. when the nest is slightly disturbed, the slaves occasionally come out, and like their masters are much agitated and defend the nest: when the nest is much disturbed, and the larvae and pupae are exposed, the slaves work energetically together with their masters in carrying them away to a place of safety. hence, it is clear that the slaves feel quite at home. during the months of june and july, on three successive years, i watched for many hours several nests in surrey and sussex, and never saw a slave either leave or enter a nest. as, during these months, the slaves are very few in number, i thought that they might behave differently when more numerous; but mr. smith informs me that he has watched the nests at various hours during may, june and august, both in surrey and hampshire, and has never seen the slaves, though present in large numbers in august, either leave or enter the nest. hence, he considers them as strictly household slaves. the masters, on the other hand, may be constantly seen bringing in materials for the nest, and food of all kinds. during the year , however, in the month of july, i came across a community with an unusually large stock of slaves, and i observed a few slaves mingled with their masters leaving the nest, and marching along the same road to a tall scotch-fir tree, twenty-five yards distant, which they ascended together, probably in search of aphides or cocci. according to huber, who had ample opportunities for observation, the slaves in switzerland habitually work with their masters in making the nest, and they alone open and close the doors in the morning and evening; and, as huber expressly states, their principal office is to search for aphides. this difference in the usual habits of the masters and slaves in the two countries, probably depends merely on the slaves being captured in greater numbers in switzerland than in england. one day i fortunately witnessed a migration of f. sanguinea from one nest to another, and it was a most interesting spectacle to behold the masters carefully carrying their slaves in their jaws instead of being carried by them, as in the case of f. rufescens. another day my attention was struck by about a score of the slave-makers haunting the same spot, and evidently not in search of food; they approached and were vigorously repulsed by an independent community of the slave species (f. fusca); sometimes as many as three of these ants clinging to the legs of the slave-making f. sanguinea. the latter ruthlessly killed their small opponents and carried their dead bodies as food to their nest, twenty-nine yards distant; but they were prevented from getting any pupae to rear as slaves. i then dug up a small parcel of the pupae of f. fusca from another nest, and put them down on a bare spot near the place of combat; they were eagerly seized and carried off by the tyrants, who perhaps fancied that, after all, they had been victorious in their late combat. at the same time i laid on the same place a small parcel of the pupae of another species, f. flava, with a few of these little yellow ants still clinging to the fragments of their nest. this species is sometimes, though rarely, made into slaves, as has been described by mr. smith. although so small a species, it is very courageous, and i have seen it ferociously attack other ants. in one instance i found to my surprise an independent community of f. flava under a stone beneath a nest of the slave-making f. sanguinea; and when i had accidentally disturbed both nests, the little ants attacked their big neighbours with surprising courage. now i was curious to ascertain whether f. sanguinea could distinguish the pupae of f. fusca, which they habitually make into slaves, from those of the little and furious f. flava, which they rarely capture, and it was evident that they did at once distinguish them; for we have seen that they eagerly and instantly seized the pupae of f. fusca, whereas they were much terrified when they came across the pupae, or even the earth from the nest, of f. flava, and quickly ran away; but in about a quarter of an hour, shortly after all the little yellow ants had crawled away, they took heart and carried off the pupae. one evening i visited another community of f. sanguinea, and found a number of these ants returning home and entering their nests, carrying the dead bodies of f. fusca (showing that it was not a migration) and numerous pupae. i traced a long file of ants burdened with booty, for about forty yards back, to a very thick clump of heath, whence i saw the last individual of f. sanguinea emerge, carrying a pupa; but i was not able to find the desolated nest in the thick heath. the nest, however, must have been close at hand, for two or three individuals of f. fusca were rushing about in the greatest agitation, and one was perched motionless with its own pupa in its mouth on the top of a spray of heath, an image of despair over its ravaged home. such are the facts, though they did not need confirmation by me, in regard to the wonderful instinct of making slaves. let it be observed what a contrast the instinctive habits of f. sanguinea present with those of the continental f. rufescens. the latter does not build its own nest, does not determine its own migrations, does not collect food for itself or its young, and cannot even feed itself: it is absolutely dependent on its numerous slaves. formica sanguinea, on the other hand, possesses much fewer slaves, and in the early part of the summer extremely few. the masters determine when and where a new nest shall be formed, and when they migrate, the masters carry the slaves. both in switzerland and england the slaves seem to have the exclusive care of the larvae, and the masters alone go on slave-making expeditions. in switzerland the slaves and masters work together, making and bringing materials for the nest: both, but chiefly the slaves, tend and milk as it may be called, their aphides; and thus both collect food for the community. in england the masters alone usually leave the nest to collect building materials and food for themselves, their slaves and larvae. so that the masters in this country receive much less service from their slaves than they do in switzerland. by what steps the instinct of f. sanguinea originated i will not pretend to conjecture. but as ants which are not slave-makers, will, as i have seen, carry off pupae of other species, if scattered near their nests, it is possible that such pupae originally stored as food might become developed; and the foreign ants thus unintentionally reared would then follow their proper instincts, and do what work they could. if their presence proved useful to the species which had seized them--if it were more advantageous to this species, to capture workers than to procreate them--the habit of collecting pupae, originally for food, might by natural selection be strengthened and rendered permanent for the very different purpose of raising slaves. when the instinct was once acquired, if carried out to a much less extent even than in our british f. sanguinea, which, as we have seen, is less aided by its slaves than the same species in switzerland, natural selection might increase and modify the instinct--always supposing each modification to be of use to the species--until an ant was formed as abjectly dependent on its slaves as is the formica rufescens. cell-making instinct of the hive-bee. i will not here enter on minute details on this subject, but will merely give an outline of the conclusions at which i have arrived. he must be a dull man who can examine the exquisite structure of a comb, so beautifully adapted to its end, without enthusiastic admiration. we hear from mathematicians that bees have practically solved a recondite problem, and have made their cells of the proper shape to hold the greatest possible amount of honey, with the least possible consumption of precious wax in their construction. it has been remarked that a skilful workman, with fitting tools and measures, would find it very difficult to make cells of wax of the true form, though this is effected by a crowd of bees working in a dark hive. granting whatever instincts you please, it seems at first quite inconceivable how they can make all the necessary angles and planes, or even perceive when they are correctly made. but the difficulty is not nearly so great as at first appears: all this beautiful work can be shown, i think, to follow from a few simple instincts. i was led to investigate this subject by mr. waterhouse, who has shown that the form of the cell stands in close relation to the presence of adjoining cells; and the following view may, perhaps, be considered only as a modification of his theory. let us look to the great principle of gradation, and see whether nature does not reveal to us her method of work. at one end of a short series we have humble-bees, which use their old cocoons to hold honey, sometimes adding to them short tubes of wax, and likewise making separate and very irregular rounded cells of wax. at the other end of the series we have the cells of the hive-bee, placed in a double layer: each cell, as is well known, is an hexagonal prism, with the basal edges of its six sides bevelled so as to join an inverted pyramid, of three rhombs. these rhombs have certain angles, and the three which form the pyramidal base of a single cell on one side of the comb, enter into the composition of the bases of three adjoining cells on the opposite side. in the series between the extreme perfection of the cells of the hive-bee and the simplicity of those of the humble-bee, we have the cells of the mexican melipona domestica, carefully described and figured by pierre huber. the melipona itself is intermediate in structure between the hive and humble bee, but more nearly related to the latter: it forms a nearly regular waxen comb of cylindrical cells, in which the young are hatched, and, in addition, some large cells of wax for holding honey. these latter cells are nearly spherical and of nearly equal sizes, and are aggregated into an irregular mass. but the important point to notice is, that these cells are always made at that degree of nearness to each other that they would have intersected or broken into each other if the spheres had been completed; but this is never permitted, the bees building perfectly flat walls of wax between the spheres which thus tend to intersect. hence, each cell consists of an outer spherical portion, and of two, three, or more flat surfaces, according as the cell adjoins two, three or more other cells. when one cell rests on three other cells, which, from the spheres being nearly of the same size, is very frequently and necessarily the case, the three flat surfaces are united into a pyramid; and this pyramid, as huber has remarked, is manifestly a gross imitation of the three-sided pyramidal base of the cell of the hive-bee. as in the cells of the hive-bee, so here, the three plane surfaces in any one cell necessarily enter into the construction of three adjoining cells. it is obvious that the melipona saves wax, and what is more important, labour, by this manner of building; for the flat walls between the adjoining cells are not double, but are of the same thickness as the outer spherical portions, and yet each flat portion forms a part of two cells. reflecting on this case, it occurred to me that if the melipona had made its spheres at some given distance from each other, and had made them of equal sizes and had arranged them symmetrically in a double layer, the resulting structure would have been as perfect as the comb of the hive-bee. accordingly i wrote to professor miller, of cambridge, and this geometer has kindly read over the following statement, drawn up from his information, and tells me that it is strictly correct:-- if a number of equal spheres be described with their centres placed in two parallel layers; with the centre of each sphere at the distance of radius x sqrt( ) or radius x . (or at some lesser distance), from the centres of the six surrounding spheres in the same layer; and at the same distance from the centres of the adjoining spheres in the other and parallel layer; then, if planes of intersection between the several spheres in both layers be formed, there will result a double layer of hexagonal prisms united together by pyramidal bases formed of three rhombs; and the rhombs and the sides of the hexagonal prisms will have every angle identically the same with the best measurements which have been made of the cells of the hive-bee. but i hear from professor wyman, who has made numerous careful measurements, that the accuracy of the workmanship of the bee has been greatly exaggerated; so much so, that whatever the typical form of the cell may be, it is rarely, if ever, realised. hence we may safely conclude that, if we could slightly modify the instincts already possessed by the melipona, and in themselves not very wonderful, this bee would make a structure as wonderfully perfect as that of the hive-bee. we must suppose the melipona to have the power of forming her cells truly spherical, and of equal sizes; and this would not be very surprising, seeing that she already does so to a certain extent, and seeing what perfectly cylindrical burrows many insects make in wood, apparently by turning round on a fixed point. we must suppose the melipona to arrange her cells in level layers, as she already does her cylindrical cells; and we must further suppose, and this is the greatest difficulty, that she can somehow judge accurately at what distance to stand from her fellow-labourers when several are making their spheres; but she is already so far enabled to judge of distance, that she always describes her spheres so as to intersect to a certain extent; and then she unites the points of intersection by perfectly flat surfaces. by such modifications of instincts which in themselves are not very wonderful--hardly more wonderful than those which guide a bird to make its nest--i believe that the hive-bee has acquired, through natural selection, her inimitable architectural powers. but this theory can be tested by experiment. following the example of mr. tegetmeier, i separated two combs, and put between them a long, thick, rectangular strip of wax: the bees instantly began to excavate minute circular pits in it; and as they deepened these little pits, they made them wider and wider until they were converted into shallow basins, appearing to the eye perfectly true or parts of a sphere, and of about the diameter of a cell. it was most interesting to observe that, wherever several bees had begun to excavate these basins near together, they had begun their work at such a distance from each other that by the time the basins had acquired the above stated width (i.e. about the width of an ordinary cell), and were in depth about one sixth of the diameter of the sphere of which they formed a part, the rims of the basins intersected or broke into each other. as soon as this occurred, the bees ceased to excavate, and began to build up flat walls of wax on the lines of intersection between the basins, so that each hexagonal prism was built upon the scalloped edge of a smooth basin, instead of on the straight edges of a three-sided pyramid as in the case of ordinary cells. i then put into the hive, instead of a thick, rectangular piece of wax, a thin and narrow, knife-edged ridge, coloured with vermilion. the bees instantly began on both sides to excavate little basins near to each other, in the same way as before; but the ridge of wax was so thin, that the bottoms of the basins, if they had been excavated to the same depth as in the former experiment, would have broken into each other from the opposite sides. the bees, however, did not suffer this to happen, and they stopped their excavations in due time; so that the basins, as soon as they had been a little deepened, came to have flat bases; and these flat bases, formed by thin little plates of the vermilion wax left ungnawed, were situated, as far as the eye could judge, exactly along the planes of imaginary intersection between the basins on the opposite side of the ridge of wax. in some parts, only small portions, in other parts, large portions of a rhombic plate were thus left between the opposed basins, but the work, from the unnatural state of things, had not been neatly performed. the bees must have worked at very nearly the same rate in circularly gnawing away and deepening the basins on both sides of the ridge of vermilion wax, in order to have thus succeeded in leaving flat plates between the basins, by stopping work at the planes of intersection. considering how flexible thin wax is, i do not see that there is any difficulty in the bees, whilst at work on the two sides of a strip of wax, perceiving when they have gnawed the wax away to the proper thinness, and then stopping their work. in ordinary combs it has appeared to me that the bees do not always succeed in working at exactly the same rate from the opposite sides; for i have noticed half-completed rhombs at the base of a just-commenced cell, which were slightly concave on one side, where i suppose that the bees had excavated too quickly, and convex on the opposed side where the bees had worked less quickly. in one well-marked instance, i put the comb back into the hive, and allowed the bees to go on working for a short time, and again examined the cell, and i found that the rhombic plate had been completed, and had become perfectly flat: it was absolutely impossible, from the extreme thinness of the little plate, that they could have effected this by gnawing away the convex side; and i suspect that the bees in such cases stand in the opposed cells and push and bend the ductile and warm wax (which as i have tried is easily done) into its proper intermediate plane, and thus flatten it. from the experiment of the ridge of vermilion wax we can see that, if the bees were to build for themselves a thin wall of wax, they could make their cells of the proper shape, by standing at the proper distance from each other, by excavating at the same rate, and by endeavouring to make equal spherical hollows, but never allowing the spheres to break into each other. now bees, as may be clearly seen by examining the edge of a growing comb, do make a rough, circumferential wall or rim all round the comb; and they gnaw this away from the opposite sides, always working circularly as they deepen each cell. they do not make the whole three-sided pyramidal base of any one cell at the same time, but only that one rhombic plate which stands on the extreme growing margin, or the two plates, as the case may be; and they never complete the upper edges of the rhombic plates, until the hexagonal walls are commenced. some of these statements differ from those made by the justly celebrated elder huber, but i am convinced of their accuracy; and if i had space, i could show that they are conformable with my theory. huber's statement, that the very first cell is excavated out of a little parallel-sided wall of wax, is not, as far as i have seen, strictly correct; the first commencement having always been a little hood of wax; but i will not here enter on details. we see how important a part excavation plays in the construction of the cells; but it would be a great error to suppose that the bees cannot build up a rough wall of wax in the proper position--that is, along the plane of intersection between two adjoining spheres. i have several specimens showing clearly that they can do this. even in the rude circumferential rim or wall of wax round a growing comb, flexures may sometimes be observed, corresponding in position to the planes of the rhombic basal plates of future cells. but the rough wall of wax has in every case to be finished off, by being largely gnawed away on both sides. the manner in which the bees build is curious; they always make the first rough wall from ten to twenty times thicker than the excessively thin finished wall of the cell, which will ultimately be left. we shall understand how they work, by supposing masons first to pile up a broad ridge of cement, and then to begin cutting it away equally on both sides near the ground, till a smooth, very thin wall is left in the middle; the masons always piling up the cut-away cement, and adding fresh cement on the summit of the ridge. we shall thus have a thin wall steadily growing upward but always crowned by a gigantic coping. from all the cells, both those just commenced and those completed, being thus crowned by a strong coping of wax, the bees can cluster and crawl over the comb without injuring the delicate hexagonal walls. these walls, as professor miller has kindly ascertained for me, vary greatly in thickness; being, on an average of twelve measurements made near the border of the comb, / of an inch in thickness; whereas the basal rhomboidal plates are thicker, nearly in the proportion of three to two, having a mean thickness, from twenty-one measurements, of / of an inch. by the above singular manner of building, strength is continually given to the comb, with the utmost ultimate economy of wax. it seems at first to add to the difficulty of understanding how the cells are made, that a multitude of bees all work together; one bee after working a short time at one cell going to another, so that, as huber has stated, a score of individuals work even at the commencement of the first cell. i was able practically to show this fact, by covering the edges of the hexagonal walls of a single cell, or the extreme margin of the circumferential rim of a growing comb, with an extremely thin layer of melted vermilion wax; and i invariably found that the colour was most delicately diffused by the bees--as delicately as a painter could have done it with his brush--by atoms of the coloured wax having been taken from the spot on which it had been placed, and worked into the growing edges of the cells all round. the work of construction seems to be a sort of balance struck between many bees, all instinctively standing at the same relative distance from each other, all trying to sweep equal spheres, and then building up, or leaving ungnawed, the planes of intersection between these spheres. it was really curious to note in cases of difficulty, as when two pieces of comb met at an angle, how often the bees would pull down and rebuild in different ways the same cell, sometimes recurring to a shape which they had at first rejected. when bees have a place on which they can stand in their proper positions for working--for instance, on a slip of wood, placed directly under the middle of a comb growing downwards, so that the comb has to be built over one face of the slip--in this case the bees can lay the foundations of one wall of a new hexagon, in its strictly proper place, projecting beyond the other completed cells. it suffices that the bees should be enabled to stand at their proper relative distances from each other and from the walls of the last completed cells, and then, by striking imaginary spheres, they can build up a wall intermediate between two adjoining spheres; but, as far as i have seen, they never gnaw away and finish off the angles of a cell till a large part both of that cell and of the adjoining cells has been built. this capacity in bees of laying down under certain circumstances a rough wall in its proper place between two just-commenced cells, is important, as it bears on a fact, which seems at first subversive of the foregoing theory; namely, that the cells on the extreme margin of wasp-combs are sometimes strictly hexagonal; but i have not space here to enter on this subject. nor does there seem to me any great difficulty in a single insect (as in the case of a queen-wasp) making hexagonal cells, if she were to work alternately on the inside and outside of two or three cells commenced at the same time, always standing at the proper relative distance from the parts of the cells just begun, sweeping spheres or cylinders, and building up intermediate planes. as natural selection acts only by the accumulation of slight modifications of structure or instinct, each profitable to the individual under its conditions of life, it may reasonably be asked, how a long and graduated succession of modified architectural instincts, all tending towards the present perfect plan of construction, could have profited the progenitors of the hive-bee? i think the answer is not difficult: cells constructed like those of the bee or the wasp gain in strength, and save much in labour and space, and in the materials of which they are constructed. with respect to the formation of wax, it is known that bees are often hard pressed to get sufficient nectar; and i am informed by mr. tegetmeier that it has been experimentally proved that from twelve to fifteen pounds of dry sugar are consumed by a hive of bees for the secretion of a pound of wax; so that a prodigious quantity of fluid nectar must be collected and consumed by the bees in a hive for the secretion of the wax necessary for the construction of their combs. moreover, many bees have to remain idle for many days during the process of secretion. a large store of honey is indispensable to support a large stock of bees during the winter; and the security of the hive is known mainly to depend on a large number of bees being supported. hence the saving of wax by largely saving honey, and the time consumed in collecting the honey, must be an important element of success any family of bees. of course the success of the species may be dependent on the number of its enemies, or parasites, or on quite distinct causes, and so be altogether independent of the quantity of honey which the bees can collect. but let us suppose that this latter circumstance determined, as it probably often has determined, whether a bee allied to our humble-bees could exist in large numbers in any country; and let us further suppose that the community lived through the winter, and consequently required a store of honey: there can in this case be no doubt that it would be an advantage to our imaginary humble-bee if a slight modification of her instincts led her to make her waxen cells near together, so as to intersect a little; for a wall in common even to two adjoining cells would save some little labour and wax. hence, it would continually be more and more advantageous to our humble-bees, if they were to make their cells more and more regular, nearer together, and aggregated into a mass, like the cells of the melipona; for in this case a large part of the bounding surface of each cell would serve to bound the adjoining cells, and much labour and wax would be saved. again, from the same cause, it would be advantageous to the melipona, if she were to make her cells closer together, and more regular in every way than at present; for then, as we have seen, the spherical surfaces would wholly disappear and be replaced by plane surfaces; and the melipona would make a comb as perfect as that of the hive-bee. beyond this stage of perfection in architecture, natural selection could not lead; for the comb of the hive-bee, as far as we can see, is absolutely perfect in economising labour and wax. thus, as i believe, the most wonderful of all known instincts, that of the hive-bee, can be explained by natural selection having taken advantage of numerous, successive, slight modifications of simpler instincts; natural selection having, by slow degrees, more and more perfectly led the bees to sweep equal spheres at a given distance from each other in a double layer, and to build up and excavate the wax along the planes of intersection. the bees, of course, no more knowing that they swept their spheres at one particular distance from each other, than they know what are the several angles of the hexagonal prisms and of the basal rhombic plates; the motive power of the process of natural selection having been the construction of cells of due strength and of the proper size and shape for the larvae, this being effected with the greatest possible economy of labour and wax; that individual swarm which thus made the best cells with least labour, and least waste of honey in the secretion of wax, having succeeded best, and having transmitted their newly-acquired economical instincts to new swarms, which in their turn will have had the best chance of succeeding in the struggle for existence. objections to the theory of natural selection as applied to instincts: neuter and sterile insects. it has been objected to the foregoing view of the origin of instincts that "the variations of structure and of instinct must have been simultaneous and accurately adjusted to each other, as a modification in the one without an immediate corresponding change in the other would have been fatal." the force of this objection rests entirely on the assumption that the changes in the instincts and structure are abrupt. to take as an illustration the case of the larger titmouse, (parus major) alluded to in a previous chapter; this bird often holds the seeds of the yew between its feet on a branch, and hammers with its beak till it gets at the kernel. now what special difficulty would there be in natural selection preserving all the slight individual variations in the shape of the beak, which were better and better adapted to break open the seeds, until a beak was formed, as well constructed for this purpose as that of the nuthatch, at the same time that habit, or compulsion, or spontaneous variations of taste, led the bird to become more and more of a seed-eater? in this case the beak is supposed to be slowly modified by natural selection, subsequently to, but in accordance with, slowly changing habits or taste; but let the feet of the titmouse vary and grow larger from correlation with the beak, or from any other unknown cause, and it is not improbable that such larger feet would lead the bird to climb more and more until it acquired the remarkable climbing instinct and power of the nuthatch. in this case a gradual change of structure is supposed to lead to changed instinctive habits. to take one more case: few instincts are more remarkable than that which leads the swift of the eastern islands to make its nest wholly of inspissated saliva. some birds build their nests of mud, believed to be moistened with saliva; and one of the swifts of north america makes its nest (as i have seen) of sticks agglutinated with saliva, and even with flakes of this substance. is it then very improbable that the natural selection of individual swifts, which secreted more and more saliva, should at last produce a species with instincts leading it to neglect other materials and to make its nest exclusively of inspissated saliva? and so in other cases. it must, however, be admitted that in many instances we cannot conjecture whether it was instinct or structure which first varied. no doubt many instincts of very difficult explanation could be opposed to the theory of natural selection--cases, in which we cannot see how an instinct could have originated; cases, in which no intermediate gradations are known to exist; cases of instincts of such trifling importance, that they could hardly have been acted on by natural selection; cases of instincts almost identically the same in animals so remote in the scale of nature that we cannot account for their similarity by inheritance from a common progenitor, and consequently must believe that they were independently acquired through natural selection. i will not here enter on these several cases, but will confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to the whole theory. i allude to the neuters or sterile females in insect communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind. the subject well deserves to be discussed at great length, but i will here take only a single case, that of working or sterile ants. how the workers have been rendered sterile is a difficulty; but not much greater than that of any other striking modification of structure; for it can be shown that some insects and other articulate animals in a state of nature occasionally become sterile; and if such insects had been social, and it had been profitable to the community that a number should have been annually born capable of work, but incapable of procreation, i can see no especial difficulty in this having been effected through natural selection. but i must pass over this preliminary difficulty. the great difficulty lies in the working ants differing widely from both the males and the fertile females in structure, as in the shape of the thorax, and in being destitute of wings and sometimes of eyes, and in instinct. as far as instinct alone is concerned, the wonderful difference in this respect between the workers and the perfect females would have been better exemplified by the hive-bee. if a working ant or other neuter insect had been an ordinary animal, i should have unhesitatingly assumed that all its characters had been slowly acquired through natural selection; namely, by individuals having been born with slight profitable modifications, which were inherited by the offspring, and that these again varied and again were selected, and so onwards. but with the working ant we have an insect differing greatly from its parents, yet absolutely sterile; so that it could never have transmitted successively acquired modifications of structure or instinct to its progeny. it may well be asked how it is possible to reconcile this case with the theory of natural selection? first, let it be remembered that we have innumerable instances, both in our domestic productions and in those in a state of nature, of all sorts of differences of inherited structure which are correlated with certain ages and with either sex. we have differences correlated not only with one sex, but with that short period when the reproductive system is active, as in the nuptial plumage of many birds, and in the hooked jaws of the male salmon. we have even slight differences in the horns of different breeds of cattle in relation to an artificially imperfect state of the male sex; for oxen of certain breeds have longer horns than the oxen of other breeds, relatively to the length of the horns in both the bulls and cows of these same breeds. hence, i can see no great difficulty in any character becoming correlated with the sterile condition of certain members of insect communities; the difficulty lies in understanding how such correlated modifications of structure could have been slowly accumulated by natural selection. this difficulty, though appearing insuperable, is lessened, or, as i believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. breeders of cattle wish the flesh and fat to be well marbled together. an animal thus characterized has been slaughtered, but the breeder has gone with confidence to the same stock and has succeeded. such faith may be placed in the power of selection that a breed of cattle, always yielding oxen with extraordinarily long horns, could, it is probable, be formed by carefully watching which individual bulls and cows, when matched, produced oxen with the longest horns; and yet no one ox would ever have propagated its kind. here is a better and real illustration: according to m. verlot, some varieties of the double annual stock, from having been long and carefully selected to the right degree, always produce a large proportion of seedlings bearing double and quite sterile flowers, but they likewise yield some single and fertile plants. these latter, by which alone the variety can be propagated, may be compared with the fertile male and female ants, and the double sterile plants with the neuters of the same community. as with the varieties of the stock, so with social insects, selection has been applied to the family, and not to the individual, for the sake of gaining a serviceable end. hence, we may conclude that slight modifications of structure or of instinct, correlated with the sterile condition of certain members of the community, have proved advantageous; consequently the fertile males and females have flourished, and transmitted to their fertile offspring a tendency to produce sterile members with the same modifications. this process must have been repeated many times, until that prodigious amount of difference between the fertile and sterile females of the same species has been produced which we see in many social insects. but we have not as yet touched on the acme of the difficulty; namely, the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an almost incredible degree, and are thus divided into two or even three castes. the castes, moreover, do not generally graduate into each other, but are perfectly well defined; being as distinct from each other as are any two species of the same genus, or rather as any two genera of the same family. thus, in eciton, there are working and soldier neuters, with jaws and instincts extraordinarily different: in cryptocerus, the workers of one caste alone carry a wonderful sort of shield on their heads, the use of which is quite unknown: in the mexican myrmecocystus, the workers of one caste never leave the nest; they are fed by the workers of another caste, and they have an enormously developed abdomen which secretes a sort of honey, supplying the place of that excreted by the aphides, or the domestic cattle as they may be called, which our european ants guard and imprison. it will indeed be thought that i have an overweening confidence in the principle of natural selection, when i do not admit that such wonderful and well-established facts at once annihilate the theory. in the simpler case of neuter insects all of one caste, which, as i believe, have been rendered different from the fertile males and females through natural selection, we may conclude from the analogy of ordinary variations, that the successive, slight, profitable modifications did not first arise in all the neuters in the same nest, but in some few alone; and that by the survival of the communities with females which produced most neuters having the advantageous modification, all the neuters ultimately came to be thus characterized. according to this view we ought occasionally to find in the same nest neuter-insects, presenting gradations of structure; and this we do find, even not rarely, considering how few neuter-insects out of europe have been carefully examined. mr. f. smith has shown that the neuters of several british ants differ surprisingly from each other in size and sometimes in colour; and that the extreme forms can be linked together by individuals taken out of the same nest: i have myself compared perfect gradations of this kind. it sometimes happens that the larger or the smaller sized workers are the most numerous; or that both large and small are numerous, while those of an intermediate size are scanty in numbers. formica flava has larger and smaller workers, with some few of intermediate size; and, in this species, as mr. f. smith has observed, the larger workers have simple eyes (ocelli), which, though small, can be plainly distinguished, whereas the smaller workers have their ocelli rudimentary. having carefully dissected several specimens of these workers, i can affirm that the eyes are far more rudimentary in the smaller workers than can be accounted for merely by their proportionately lesser size; and i fully believe, though i dare not assert so positively, that the workers of intermediate size have their ocelli in an exactly intermediate condition. so that here we have two bodies of sterile workers in the same nest, differing not only in size, but in their organs of vision, yet connected by some few members in an intermediate condition. i may digress by adding, that if the smaller workers had been the most useful to the community, and those males and females had been continually selected, which produced more and more of the smaller workers, until all the workers were in this condition; we should then have had a species of ant with neuters in nearly the same condition as those of myrmica. for the workers of myrmica have not even rudiments of ocelli, though the male and female ants of this genus have well-developed ocelli. i may give one other case: so confidently did i expect occasionally to find gradations of important structures between the different castes of neuters in the same species, that i gladly availed myself of mr. f. smith's offer of numerous specimens from the same nest of the driver ant (anomma) of west africa. the reader will perhaps best appreciate the amount of difference in these workers by my giving, not the actual measurements, but a strictly accurate illustration: the difference was the same as if we were to see a set of workmen building a house, of whom many were five feet four inches high, and many sixteen feet high; but we must in addition suppose that the larger workmen had heads four instead of three times as big as those of the smaller men, and jaws nearly five times as big. the jaws, moreover, of the working ants of the several sizes differed wonderfully in shape, and in the form and number of the teeth. but the important fact for us is that, though the workers can be grouped into castes of different sizes, yet they graduate insensibly into each other, as does the widely-different structure of their jaws. i speak confidently on this latter point, as sir j. lubbock made drawings for me, with the camera lucida, of the jaws which i dissected from the workers of the several sizes. mr. bates, in his interesting "naturalist on the amazons," has described analogous cases. with these facts before me, i believe that natural selection, by acting on the fertile ants or parents, could form a species which should regularly produce neuters, all of large size with one form of jaw, or all of small size with widely different jaws; or lastly, and this is the greatest difficulty, one set of workers of one size and structure, and simultaneously another set of workers of a different size and structure; a graduated series having first been formed, as in the case of the driver ant, and then the extreme forms having been produced in greater and greater numbers, through the survival of the parents which generated them, until none with an intermediate structure were produced. an analogous explanation has been given by mr. wallace, of the equally complex case, of certain malayan butterflies regularly appearing under two or even three distinct female forms; and by fritz muller, of certain brazilian crustaceans likewise appearing under two widely distinct male forms. but this subject need not here be discussed. i have now explained how, i believe, the wonderful fact of two distinctly defined castes of sterile workers existing in the same nest, both widely different from each other and from their parents, has originated. we can see how useful their production may have been to a social community of ants, on the same principle that the division of labour is useful to civilised man. ants, however, work by inherited instincts and by inherited organs or tools, while man works by acquired knowledge and manufactured instruments. but i must confess, that, with all my faith in natural selection, i should never have anticipated that this principle could have been efficient in so high a degree, had not the case of these neuter insects led me to this conclusion. i have, therefore, discussed this case, at some little but wholly insufficient length, in order to show the power of natural selection, and likewise because this is by far the most serious special difficulty which my theory has encountered. the case, also, is very interesting, as it proves that with animals, as with plants, any amount of modification may be effected by the accumulation of numerous, slight, spontaneous variations, which are in any way profitable, without exercise or habit having been brought into play. for peculiar habits, confined to the workers of sterile females, however long they might be followed, could not possibly affect the males and fertile females, which alone leave descendants. i am surprised that no one has advanced this demonstrative case of neuter insects, against the well-known doctrine of inherited habit, as advanced by lamarck. summary. i have endeavoured in this chapter briefly to show that the mental qualities of our domestic animals vary, and that the variations are inherited. still more briefly i have attempted to show that instincts vary slightly in a state of nature. no one will dispute that instincts are of the highest importance to each animal. therefore, there is no real difficulty, under changing conditions of life, in natural selection accumulating to any extent slight modifications of instinct which are in any way useful. in many cases habit or use and disuse have probably come into play. i do not pretend that the facts given in this chapter strengthen in any great degree my theory; but none of the cases of difficulty, to the best of my judgment, annihilate it. on the other hand, the fact that instincts are not always absolutely perfect and are liable to mistakes; that no instinct can be shown to have been produced for the good of other animals, though animals take advantage of the instincts of others; that the canon in natural history, of "natura non facit saltum," is applicable to instincts as well as to corporeal structure, and is plainly explicable on the foregoing views, but is otherwise inexplicable--all tend to corroborate the theory of natural selection. this theory is also strengthened by some few other facts in regard to instincts; as by that common case of closely allied, but distinct, species, when inhabiting distant parts of the world and living under considerably different conditions of life, yet often retaining nearly the same instincts. for instance, we can understand, on the principle of inheritance, how it is that the thrush of tropical south america lines its nest with mud, in the same peculiar manner as does our british thrush; how it is that the hornbills of africa and india have the same extraordinary instinct of plastering up and imprisoning the females in a hole in a tree, with only a small hole left in the plaster through which the males feed them and their young when hatched; how it is that the male wrens (troglodytes) of north america, build "cock-nests," to roost in, like the males of our kitty-wrens,--a habit wholly unlike that of any other known bird. finally, it may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers, ants making slaves, the larvae of ichneumonidae feeding within the live bodies of caterpillars, not as specially endowed or created instincts, but as small consequences of one general law leading to the advancement of all organic beings--namely, multiply, vary, let the strongest live and the weakest die. chapter ix. hybridism. distinction between the sterility of first crosses and of hybrids--sterility various in degree, not universal, affected by close interbreeding, removed by domestication--laws governing the sterility of hybrids--sterility not a special endowment, but incidental on other differences, not accumulated by natural selection--causes of the sterility of first crosses and of hybrids--parallelism between the effects of changed conditions of life and of crossing--dimorphism and trimorphism--fertility of varieties when crossed and of their mongrel offspring not universal--hybrids and mongrels compared independently of their fertility--summary. the view commonly entertained by naturalists is that species, when intercrossed, have been specially endowed with sterility, in order to prevent their confusion. this view certainly seems at first highly probable, for species living together could hardly have been kept distinct had they been capable of freely crossing. the subject is in many ways important for us, more especially as the sterility of species when first crossed, and that of their hybrid offspring, cannot have been acquired, as i shall show, by the preservation of successive profitable degrees of sterility. it is an incidental result of differences in the reproductive systems of the parent-species. in treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded; namely, the sterility of species when first crossed, and the sterility of the hybrids produced from them. pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the formative organs themselves are perfect in structure, as far as the microscope reveals. in the first case the two sexual elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed. this distinction is important, when the cause of the sterility, which is common to the two cases, has to be considered. the distinction probably has been slurred over, owing to the sterility in both cases being looked on as a special endowment, beyond the province of our reasoning powers. the fertility of varieties, that is of the forms known or believed to be descended from common parents, when crossed, and likewise the fertility of their mongrel offspring, is, with reference to my theory, of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species. degrees of sterility. first, for the sterility of species when crossed and of their hybrid offspring. it is impossible to study the several memoirs and works of those two conscientious and admirable observers, kolreuter and gartner, who almost devoted their lives to this subject, without being deeply impressed with the high generality of some degree of sterility. kolreuter makes the rule universal; but then he cuts the knot, for in ten cases in which he found two forms, considered by most authors as distinct species, quite fertile together, he unhesitatingly ranks them as varieties. gartner, also, makes the rule equally universal; and he disputes the entire fertility of kolreuter's ten cases. but in these and in many other cases, gartner is obliged carefully to count the seeds, in order to show that there is any degree of sterility. he always compares the maximum number of seeds produced by two species when first crossed, and the maximum produced by their hybrid offspring, with the average number produced by both pure parent-species in a state of nature. but causes of serious error here intervene: a plant, to be hybridised, must be castrated, and, what is often more important, must be secluded in order to prevent pollen being brought to it by insects from other plants. nearly all the plants experimented on by gartner were potted, and were kept in a chamber in his house. that these processes are often injurious to the fertility of a plant cannot be doubted; for gartner gives in his table about a score of cases of plants which he castrated, and artificially fertilised with their own pollen, and (excluding all cases such as the leguminosae, in which there is an acknowledged difficulty in the manipulation) half of these twenty plants had their fertility in some degree impaired. moreover, as gartner repeatedly crossed some forms, such as the common red and blue pimpernels (anagallis arvensis and coerulea), which the best botanists rank as varieties, and found them absolutely sterile, we may doubt whether many species are really so sterile, when intercrossed, as he believed. it is certain, on the one hand, that the sterility of various species when crossed is so different in degree and graduates away so insensibly, and, on the other hand, that the fertility of pure species is so easily affected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins. i think no better evidence of this can be required than that the two most experienced observers who have ever lived, namely kolreuter and gartner, arrived at diametrically opposite conclusions in regard to some of the very same forms. it is also most instructive to compare--but i have not space here to enter on details--the evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties, with the evidence from fertility adduced by different hybridisers, or by the same observer from experiments made during different years. it can thus be shown that neither sterility nor fertility affords any certain distinction between species and varieties. the evidence from this source graduates away, and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences. in regard to the sterility of hybrids in successive generations; though gartner was enabled to rear some hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten generations, yet he asserts positively that their fertility never increases, but generally decreases greatly and suddenly. with respect to this decrease, it may first be noticed that when any deviation in structure or constitution is common to both parents, this is often transmitted in an augmented degree to the offspring; and both sexual elements in hybrid plants are already affected in some degree. but i believe that their fertility has been diminished in nearly all these cases by an independent cause, namely, by too close interbreeding. i have made so many experiments and collected so many facts, showing on the one hand that an occasional cross with a distinct individual or variety increases the vigour and fertility of the offspring, and on the other hand that very close interbreeding lessens their vigour and fertility, that i cannot doubt the correctness of this conclusion. hybrids are seldom raised by experimentalists in great numbers; and as the parent-species, or other allied hybrids, generally grow in the same garden, the visits of insects must be carefully prevented during the flowering season: hence hybrids, if left to themselves, will generally be fertilised during each generation by pollen from the same flower; and this would probably be injurious to their fertility, already lessened by their hybrid origin. i am strengthened in this conviction by a remarkable statement repeatedly made by gartner, namely, that if even the less fertile hybrids be artificially fertilised with hybrid pollen of the same kind, their fertility, notwithstanding the frequent ill effects from manipulation, sometimes decidedly increases, and goes on increasing. now, in the process of artificial fertilisation, pollen is as often taken by chance (as i know from my own experience) from the anthers of another flower, as from the anthers of the flower itself which is to be fertilised; so that a cross between two flowers, though probably often on the same plant, would be thus effected. moreover, whenever complicated experiments are in progress, so careful an observer as gartner would have castrated his hybrids, and this would have insured in each generation a cross with pollen from a distinct flower, either from the same plant or from another plant of the same hybrid nature. and thus, the strange fact of an increase of fertility in the successive generations of artificially fertilised hybrids, in contrast with those spontaneously self-fertilised, may, as i believe, be accounted for by too close interbreeding having been avoided. now let us turn to the results arrived at by a third most experienced hybridiser, namely, the hon. and rev. w. herbert. he is as emphatic in his conclusion that some hybrids are perfectly fertile--as fertile as the pure parent-species--as are kolreuter and gartner that some degree of sterility between distinct species is a universal law of nature. he experimented on some of the very same species as did gartner. the difference in their results may, i think, be in part accounted for by herbert's great horticultural skill, and by his having hot-houses at his command. of his many important statements i will here give only a single one as an example, namely, that "every ovule in a pod of crinum capense fertilised by c. revolutum produced a plant, which i never saw to occur in a case of its natural fecundation." so that here we have perfect, or even more than commonly perfect fertility, in a first cross between two distinct species. this case of the crinum leads me to refer to a singular fact, namely, that individual plants of certain species of lobelia, verbascum and passiflora, can easily be fertilised by the pollen from a distinct species, but not by pollen from the same plant, though this pollen can be proved to be perfectly sound by fertilising other plants or species. in the genus hippeastrum, in corydalis as shown by professor hildebrand, in various orchids as shown by mr. scott and fritz muller, all the individuals are in this peculiar condition. so that with some species, certain abnormal individuals, and in other species all the individuals, can actually be hybridised much more readily than they can be fertilised by pollen from the same individual plant! to give one instance, a bulb of hippeastrum aulicum produced four flowers; three were fertilised by herbert with their own pollen, and the fourth was subsequently fertilised by the pollen of a compound hybrid descended from three distinct species: the result was that "the ovaries of the three first flowers soon ceased to grow, and after a few days perished entirely, whereas the pod impregnated by the pollen of the hybrid made vigorous growth and rapid progress to maturity, and bore good seed, which vegetated freely." mr. herbert tried similar experiments during many years, and always with the same result. these cases serve to show on what slight and mysterious causes the lesser or greater fertility of a species sometimes depends. the practical experiments of horticulturists, though not made with scientific precision, deserve some notice. it is notorious in how complicated a manner the species of pelargonium, fuchsia, calceolaria, petunia, rhododendron, etc., have been crossed, yet many of these hybrids seed freely. for instance, herbert asserts that a hybrid from calceolaria integrifolia and plantaginea, species most widely dissimilar in general habit, "reproduces itself as perfectly as if it had been a natural species from the mountains of chile." i have taken some pains to ascertain the degree of fertility of some of the complex crosses of rhododendrons, and i am assured that many of them are perfectly fertile. mr. c. noble, for instance, informs me that he raises stocks for grafting from a hybrid between rhod. ponticum and catawbiense, and that this hybrid "seeds as freely as it is possible to imagine." had hybrids, when fairly treated, always gone on decreasing in fertility in each successive generation, as gartner believed to be the case, the fact would have been notorious to nurserymen. horticulturists raise large beds of the same hybrid, and such alone are fairly treated, for by insect agency the several individuals are allowed to cross freely with each other, and the injurious influence of close interbreeding is thus prevented. any one may readily convince himself of the efficiency of insect agency by examining the flowers of the more sterile kinds of hybrid rhododendrons, which produce no pollen, for he will find on their stigmas plenty of pollen brought from other flowers. in regard to animals, much fewer experiments have been carefully tried than with plants. if our systematic arrangements can be trusted, that is, if the genera of animals are as distinct from each other as are the genera of plants, then we may infer that animals more widely distinct in the scale of nature can be crossed more easily than in the case of plants; but the hybrids themselves are, i think, more sterile. it should, however, be borne in mind that, owing to few animals breeding freely under confinement, few experiments have been fairly tried: for instance, the canary-bird has been crossed with nine distinct species of finches, but, as not one of these breeds freely in confinement, we have no right to expect that the first crosses between them and the canary, or that their hybrids, should be perfectly fertile. again, with respect to the fertility in successive generations of the more fertile hybrid animals, i hardly know of an instance in which two families of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of close interbreeding. on the contrary, brothers and sisters have usually been crossed in each successive generation, in opposition to the constantly repeated admonition of every breeder. and in this case, it is not at all surprising that the inherent sterility in the hybrids should have gone on increasing. although i know of hardly any thoroughly well-authenticated cases of perfectly fertile hybrid animals, i have reason to believe that the hybrids from cervulus vaginalis and reevesii, and from phasianus colchicus with p. torquatus, are perfectly fertile. m. quatrefages states that the hybrids from two moths (bombyx cynthia and arrindia) were proved in paris to be fertile inter se for eight generations. it has lately been asserted that two such distinct species as the hare and rabbit, when they can be got to breed together, produce offspring, which are highly fertile when crossed with one of the parent-species. the hybrids from the common and chinese geese (a. cygnoides), species which are so different that they are generally ranked in distinct genera, have often bred in this country with either pure parent, and in one single instance they have bred inter se. this was effected by mr. eyton, who raised two hybrids from the same parents, but from different hatches; and from these two birds he raised no less than eight hybrids (grandchildren of the pure geese) from one nest. in india, however, these cross-bred geese must be far more fertile; for i am assured by two eminently capable judges, namely mr. blyth and captain hutton, that whole flocks of these crossed geese are kept in various parts of the country; and as they are kept for profit, where neither pure parent-species exists, they must certainly be highly or perfectly fertile. with our domesticated animals, the various races when crossed together are quite fertile; yet in many cases they are descended from two or more wild species. from this fact we must conclude either that the aboriginal parent-species at first produced perfectly fertile hybrids, or that the hybrids subsequently reared under domestication became quite fertile. this latter alternative, which was first propounded by pallas, seems by far the most probable, and can, indeed, hardly be doubted. it is, for instance, almost certain that our dogs are descended from several wild stocks; yet, with perhaps the exception of certain indigenous domestic dogs of south america, all are quite fertile together; but analogy makes me greatly doubt, whether the several aboriginal species would at first have freely bred together and have produced quite fertile hybrids. so again i have lately acquired decisive evidence that the crossed offspring from the indian humped and common cattle are inter se perfectly fertile; and from the observations by rutimeyer on their important osteological differences, as well as from those by mr. blyth on their differences in habits, voice, constitution, etc., these two forms must be regarded as good and distinct species. the same remarks may be extended to the two chief races of the pig. we must, therefore, either give up the belief of the universal sterility of species when crossed; or we must look at this sterility in animals, not as an indelible characteristic, but as one capable of being removed by domestication. finally, considering all the ascertained facts on the intercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal. laws governing the sterility of first crosses and of hybrids. we will now consider a little more in detail the laws governing the sterility of first crosses and of hybrids. our chief object will be to see whether or not these laws indicate that species have been specially endowed with this quality, in order to prevent their crossing and blending together in utter confusion. the following conclusions are drawn up chiefly from gartner's admirable work on the hybridisation of plants. i have taken much pains to ascertain how far they apply to animals, and, considering how scanty our knowledge is in regard to hybrid animals, i have been surprised to find how generally the same rules apply to both kingdoms. it has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from zero to perfect fertility. it is surprising in how many curious ways this gradation can be shown; but only the barest outline of the facts can here be given. when pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust. from this absolute zero of fertility, the pollen of different species applied to the stigma of some one species of the same genus, yields a perfect gradation in the number of seeds produced, up to nearly complete or even quite complete fertility; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond that which the plant's own pollen produces. so in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of the pure parents, a single fertile seed: but in some of these cases a first trace of fertility may be detected, by the pollen of one of the pure parent-species causing the flower of the hybrid to wither earlier than it otherwise would have done; and the early withering of the flower is well known to be a sign of incipient fertilisation. from this extreme degree of sterility we have self-fertilised hybrids producing a greater and greater number of seeds up to perfect fertility. the hybrids raised from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus produced--two classes of facts which are generally confounded together--is by no means strict. there are many cases, in which two pure species, as in the genus verbascum, can be united with unusual facility, and produce numerous hybrid offspring, yet these hybrids are remarkably sterile. on the other hand, there are species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. even within the limits of the same genus, for instance in dianthus, these two opposite cases occur. the fertility, both of first crosses and of hybrids, is more easily affected by unfavourable conditions, than is that of pure species. but the fertility of first crosses is likewise innately variable; for it is not always the same in degree when the same two species are crossed under the same circumstances; it depends in part upon the constitution of the individuals which happen to have been chosen for the experiment. so it is with hybrids, for their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same capsule and exposed to the same conditions. by the term systematic affinity is meant, the general resemblance between species in structure and constitution. now the fertility of first crosses, and of the hybrids produced from them, is largely governed by their systematic affinity. this is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility. but the correspondence between systematic affinity and the facility of crossing is by no means strict. a multitude of cases could be given of very closely allied species which will not unite, or only with extreme difficulty; and on the other hand of very distinct species which unite with the utmost facility. in the same family there may be a genus, as dianthus, in which very many species can most readily be crossed; and another genus, as silene, in which the most persevering efforts have failed to produce between extremely close species a single hybrid. even within the limits of the same genus, we meet with this same difference; for instance, the many species of nicotiana have been more largely crossed than the species of almost any other genus; but gartner found that n. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised, by no less than eight other species of nicotiana. many analogous facts could be given. no one has been able to point out what kind or what amount of difference, in any recognisable character, is sufficient to prevent two species crossing. it can be shown that plants most widely different in habit and general appearance, and having strongly marked differences in every part of the flower, even in the pollen, in the fruit, and in the cotyledons, can be crossed. annual and perennial plants, deciduous and evergreen trees, plants inhabiting different stations and fitted for extremely different climates, can often be crossed with ease. by a reciprocal cross between two species, i mean the case, for instance, of a female-ass being first crossed by a stallion, and then a mare by a male-ass: these two species may then be said to have been reciprocally crossed. there is often the widest possible difference in the facility of making reciprocal crosses. such cases are highly important, for they prove that the capacity in any two species to cross is often completely independent of their systematic affinity, that is of any difference in their structure or constitution, excepting in their reproductive systems. the diversity of the result in reciprocal crosses between the same two species was long ago observed by kolreuter. to give an instance: mirabilis jalapa can easily be fertilised by the pollen of m. longiflora, and the hybrids thus produced are sufficiently fertile; but kolreuter tried more than two hundred times, during eight following years, to fertilise reciprocally m. longiflora with the pollen of m. jalapa, and utterly failed. several other equally striking cases could be given. thuret has observed the same fact with certain sea-weeds or fuci. gartner, moreover, found that this difference of facility in making reciprocal crosses is extremely common in a lesser degree. he has observed it even between closely related forms (as matthiola annua and glabra) which many botanists rank only as varieties. it is also a remarkable fact that hybrids raised from reciprocal crosses, though of course compounded of the very same two species, the one species having first been used as the father and then as the mother, though they rarely differ in external characters, yet generally differ in fertility in a small, and occasionally in a high degree. several other singular rules could be given from gartner: for instance, some species have a remarkable power of crossing with other species; other species of the same genus have a remarkable power of impressing their likeness on their hybrid offspring; but these two powers do not at all necessarily go together. there are certain hybrids which, instead of having, as is usual, an intermediate character between their two parents, always closely resemble one of them; and such hybrids, though externally so like one of their pure parent-species, are with rare exceptions extremely sterile. so again among hybrids which are usually intermediate in structure between their parents, exceptional and abnormal individuals sometimes are born, which closely resemble one of their pure parents; and these hybrids are almost always utterly sterile, even when the other hybrids raised from seed from the same capsule have a considerable degree of fertility. these facts show how completely the fertility of a hybrid may be independent of its external resemblance to either pure parent. considering the several rules now given, which govern the fertility of first crosses and of hybrids, we see that when forms, which must be considered as good and distinct species, are united, their fertility graduates from zero to perfect fertility, or even to fertility under certain conditions in excess; that their fertility, besides being eminently susceptible to favourable and unfavourable conditions, is innately variable; that it is by no means always the same in degree in the first cross and in the hybrids produced from this cross; that the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent; and lastly, that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other. this latter statement is clearly proved by the difference in the result of reciprocal crosses between the same two species, for, according as the one species or the other is used as the father or the mother, there is generally some difference, and occasionally the widest possible difference, in the facility of effecting an union. the hybrids, moreover, produced from reciprocal crosses often differ in fertility. now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature? i think not. for why should the sterility be so extremely different in degree, when various species are crossed, all of which we must suppose it would be equally important to keep from blending together? why should the degree of sterility be innately variable in the individuals of the same species? why should some species cross with facility and yet produce very sterile hybrids; and other species cross with extreme difficulty, and yet produce fairly fertile hybrids? why should there often be so great a difference in the result of a reciprocal cross between the same two species? why, it may even be asked, has the production of hybrids been permitted? to grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related to the facility of the first union between their parents, seems a strange arrangement. the foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility, both of first crosses and of hybrids, is simply incidental or dependent on unknown differences in their reproductive systems; the differences being of so peculiar and limited a nature, that, in reciprocal crosses between the same two species, the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction. it will be advisable to explain a little more fully, by an example, what i mean by sterility being incidental on other differences, and not a specially endowed quality. as the capacity of one plant to be grafted or budded on another is unimportant for their welfare in a state of nature, i presume that no one will suppose that this capacity is a specially endowed quality, but will admit that it is incidental on differences in the laws of growth of the two plants. we can sometimes see the reason why one tree will not take on another from differences in their rate of growth, in the hardness of their wood, in the period of the flow or nature of their sap, etc.; but in a multitude of cases we can assign no reason whatever. great diversity in the size of two plants, one being woody and the other herbaceous, one being evergreen and the other deciduous, and adaptation to widely different climates, does not always prevent the two grafting together. as in hybridisation, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft together trees belonging to quite distinct families; and, on the other hand, closely allied species and varieties of the same species, can usually, but not invariably, be grafted with ease. but this capacity, as in hybridisation, is by no means absolutely governed by systematic affinity. although many distinct genera within the same family have been grafted together, in other cases species of the same genus will not take on each other. the pear can be grafted far more readily on the quince, which is ranked as a distinct genus, than on the apple, which is a member of the same genus. even different varieties of the pear take with different degrees of facility on the quince; so do different varieties of the apricot and peach on certain varieties of the plum. as gartner found that there was sometimes an innate difference in different individuals of the same two species in crossing; so sagaret believes this to be the case with different individuals of the same two species in being grafted together. as in reciprocal crosses, the facility of effecting an union is often very far from equal, so it sometimes is in grafting. the common gooseberry, for instance, cannot be grafted on the currant, whereas the currant will take, though with difficulty, on the gooseberry. we have seen that the sterility of hybrids which have their reproductive organs in an imperfect condition, is a different case from the difficulty of uniting two pure species, which have their reproductive organs perfect; yet these two distinct classes of cases run to a large extent parallel. something analogous occurs in grafting; for thouin found that three species of robinia, which seeded freely on their own roots, and which could be grafted with no great difficulty on a fourth species, when thus grafted were rendered barren. on the other hand, certain species of sorbus, when grafted on other species, yielded twice as much fruit as when on their own roots. we are reminded by this latter fact of the extraordinary cases of hippeastrum, passiflora, etc., which seed much more freely when fertilised with the pollen of a distinct species than when fertilised with pollen from the same plant. we thus see that, although there is a clear and great difference between the mere adhesion of grafted stocks and the union of the male and female elements in the act of reproduction, yet that there is a rude degree of parallelism in the results of grafting and of crossing distinct species. and as we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems, so i believe that the still more complex laws governing the facility of first crosses are incidental on unknown differences in their reproductive systems. these differences in both cases follow, to a certain extent, as might have been expected, systematic affinity, by which term every kind of resemblance and dissimilarity between organic beings is attempted to be expressed. the facts by no means seem to indicate that the greater or lesser difficulty of either grafting or crossing various species has been a special endowment; although in the case of crossing, the difficulty is as important for the endurance and stability of specific forms as in the case of grafting it is unimportant for their welfare. origin and causes of the sterility of first crosses and of hybrids. at one time it appeared to me probable, as it has to others, that the sterility of first crosses and of hybrids might have been slowly acquired through the natural selection of slightly lessened degrees of fertility, which, like any other variation, spontaneously appeared in certain individuals of one variety when crossed with those of another variety. for it would clearly be advantageous to two varieties or incipient species if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate. in the first place, it may be remarked that species inhabiting distinct regions are often sterile when crossed; now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection; but it may perhaps be argued, that, if a species was rendered sterile with some one compatriot, sterility with other species would follow as a necessary contingency. in the second place, it is almost as much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one form should have been rendered utterly impotent on a second form, while at the same time the male element of this second form is enabled freely to fertilise the first form; for this peculiar state of the reproductive system could hardly have been advantageous to either species. in considering the probability of natural selection having come into action, in rendering species mutually sterile, the greatest difficulty will be found to lie in the existence of many graduated steps, from slightly lessened fertility to absolute sterility. it may be admitted that it would profit an incipient species, if it were rendered in some slight degree sterile when crossed with its parent form or with some other variety; for thus fewer bastardised and deteriorated offspring would be produced to commingle their blood with the new species in process of formation. but he who will take the trouble to reflect on the steps by which this first degree of sterility could be increased through natural selection to that high degree which is common with so many species, and which is universal with species which have been differentiated to a generic or family rank, will find the subject extraordinarily complex. after mature reflection, it seems to me that this could not have been effected through natural selection. take the case of any two species which, when crossed, produced few and sterile offspring; now, what is there which could favour the survival of those individuals which happened to be endowed in a slightly higher degree with mutual infertility, and which thus approached by one small step towards absolute sterility? yet an advance of this kind, if the theory of natural selection be brought to bear, must have incessantly occurred with many species, for a multitude are mutually quite barren. with sterile neuter insects we have reason to believe that modifications in their structure and fertility have been slowly accumulated by natural selection, from an advantage having been thus indirectly given to the community to which they belonged over other communities of the same species; but an individual animal not belonging to a social community, if rendered slightly sterile when crossed with some other variety, would not thus itself gain any advantage or indirectly give any advantage to the other individuals of the same variety, thus leading to their preservation. but it would be superfluous to discuss this question in detail: for with plants we have conclusive evidence that the sterility of crossed species must be due to some principle, quite independent of natural selection. both gartner and kolreuter have proved that in genera including numerous species, a series can be formed from species which when crossed yield fewer and fewer seeds, to species which never produce a single seed, but yet are affected by the pollen of certain other species, for the germen swells. it is here manifestly impossible to select the more sterile individuals, which have already ceased to yield seeds; so that this acme of sterility, when the germen alone is effected, cannot have been gained through selection; and from the laws governing the various grades of sterility being so uniform throughout the animal and vegetable kingdoms, we may infer that the cause, whatever it may be, is the same or nearly the same in all cases. we will now look a little closer at the probable nature of the differences between species which induce sterility in first crosses and in hybrids. in the case of first crosses, the greater or less difficulty in effecting a union and in obtaining offspring apparently depends on several distinct causes. there must sometimes be a physical impossibility in the male element reaching the ovule, as would be the case with a plant having a pistil too long for the pollen-tubes to reach the ovarium. it has also been observed that when the pollen of one species is placed on the stigma of a distantly allied species, though the pollen-tubes protrude, they do not penetrate the stigmatic surface. again, the male element may reach the female element, but be incapable of causing an embryo to be developed, as seems to have been the case with some of thuret's experiments on fuci. no explanation can be given of these facts, any more than why certain trees cannot be grafted on others. lastly, an embryo may be developed, and then perish at an early period. this latter alternative has not been sufficiently attended to; but i believe, from observations communicated to me by mr. hewitt, who has had great experience in hybridising pheasants and fowls, that the early death of the embryo is a very frequent cause of sterility in first crosses. mr. salter has recently given the results of an examination of about eggs produced from various crosses between three species of gallus and their hybrids; the majority of these eggs had been fertilised; and in the majority of the fertilised eggs, the embryos had either been partially developed and had then perished, or had become nearly mature, but the young chickens had been unable to break through the shell. of the chickens which were born, more than four-fifths died within the first few days, or at latest weeks, "without any obvious cause, apparently from mere inability to live;" so that from the eggs only twelve chickens were reared. with plants, hybridized embryos probably often perish in a like manner; at least it is known that hybrids raised from very distinct species are sometimes weak and dwarfed, and perish at an early age; of which fact max wichura has recently given some striking cases with hybrid willows. it may be here worth noticing that in some cases of parthenogenesis, the embryos within the eggs of silk moths which had not been fertilised, pass through their early stages of development and then perish like the embryos produced by a cross between distinct species. until becoming acquainted with these facts, i was unwilling to believe in the frequent early death of hybrid embryos; for hybrids, when once born, are generally healthy and long-lived, as we see in the case of the common mule. hybrids, however, are differently circumstanced before and after birth: when born and living in a country where their two parents live, they are generally placed under suitable conditions of life. but a hybrid partakes of only half of the nature and constitution of its mother; it may therefore, before birth, as long as it is nourished within its mother's womb, or within the egg or seed produced by the mother, be exposed to conditions in some degree unsuitable, and consequently be liable to perish at an early period; more especially as all very young beings are eminently sensitive to injurious or unnatural conditions of life. but after all, the cause more probably lies in some imperfection in the original act of impregnation, causing the embryo to be imperfectly developed, rather than in the conditions to which it is subsequently exposed. in regard to the sterility of hybrids, in which the sexual elements are imperfectly developed, the case is somewhat different. i have more than once alluded to a large body of facts showing that, when animals and plants are removed from their natural conditions, they are extremely liable to have their reproductive systems seriously affected. this, in fact, is the great bar to the domestication of animals. between the sterility thus superinduced and that of hybrids, there are many points of similarity. in both cases the sterility is independent of general health, and is often accompanied by excess of size or great luxuriance. in both cases the sterility occurs in various degrees; in both, the male element is the most liable to be affected; but sometimes the female more than the male. in both, the tendency goes to a certain extent with systematic affinity, for whole groups of animals and plants are rendered impotent by the same unnatural conditions; and whole groups of species tend to produce sterile hybrids. on the other hand, one species in a group will sometimes resist great changes of conditions with unimpaired fertility; and certain species in a group will produce unusually fertile hybrids. no one can tell till he tries, whether any particular animal will breed under confinement, or any exotic plant seed freely under culture; nor can he tell till he tries, whether any two species of a genus will produce more or less sterile hybrids. lastly, when organic beings are placed during several generations under conditions not natural to them, they are extremely liable to vary, which seems to be partly due to their reproductive systems having been specially affected, though in a lesser degree than when sterility ensues. so it is with hybrids, for their offspring in successive generations are eminently liable to vary, as every experimentalist has observed. thus we see that when organic beings are placed under new and unnatural conditions, and when hybrids are produced by the unnatural crossing of two species, the reproductive system, independently of the general state of health, is affected in a very similar manner. in the one case, the conditions of life have been disturbed, though often in so slight a degree as to be inappreciable by us; in the other case, or that of hybrids, the external conditions have remained the same, but the organisation has been disturbed by two distinct structures and constitutions, including of course the reproductive systems, having been blended into one. for it is scarcely possible that two organisations should be compounded into one, without some disturbance occurring in the development, or periodical action, or mutual relations of the different parts and organs one to another or to the conditions of life. when hybrids are able to breed inter se, they transmit to their offspring from generation to generation the same compounded organisation, and hence we need not be surprised that their sterility, though in some degree variable, does not diminish; it is even apt to increase, this being generally the result, as before explained, of too close interbreeding. the above view of the sterility of hybrids being caused by two constitutions being compounded into one has been strongly maintained by max wichura. it must, however, be owned that we cannot understand, on the above or any other view, several facts with respect to the sterility of hybrids; for instance, the unequal fertility of hybrids produced from reciprocal crosses; or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent. nor do i pretend that the foregoing remarks go to the root of the matter: no explanation is offered why an organism, when placed under unnatural conditions, is rendered sterile. all that i have attempted to show is, that in two cases, in some respects allied, sterility is the common result--in the one case from the conditions of life having been disturbed, in the other case from the organisation having been disturbed by two organisations being compounded into one. a similar parallelism holds good with an allied yet very different class of facts. it is an old and almost universal belief, founded on a considerable body of evidence, which i have elsewhere given, that slight changes in the conditions of life are beneficial to all living things. we see this acted on by farmers and gardeners in their frequent exchanges of seed, tubers, etc., from one soil or climate to another, and back again. during the convalescence of animals, great benefit is derived from almost any change in their habits of life. again, both with plants and animals, there is the clearest evidence that a cross between individuals of the same species, which differ to a certain extent, gives vigour and fertility to the offspring; and that close interbreeding continued during several generations between the nearest relations, if these be kept under the same conditions of life, almost always leads to decreased size, weakness, or sterility. hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings, and on the other hand, that slight crosses, that is, crosses between the males and females of the same species, which have been subjected to slightly different conditions, or which have slightly varied, give vigour and fertility to the offspring. but, as we have seen, organic beings long habituated to certain uniform conditions under a state of nature, when subjected, as under confinement, to a considerable change in their conditions, very frequently are rendered more or less sterile; and we know that a cross between two forms that have become widely or specifically different, produce hybrids which are almost always in some degree sterile. i am fully persuaded that this double parallelism is by no means an accident or an illusion. he who is able to explain why the elephant, and a multitude of other animals, are incapable of breeding when kept under only partial confinement in their native country, will be able to explain the primary cause of hybrids being so generally sterile. he will at the same time be able to explain how it is that the races of some of our domesticated animals, which have often been subjected to new and not uniform conditions, are quite fertile together, although they are descended from distinct species, which would probably have been sterile if aboriginally crossed. the above two parallel series of facts seem to be connected together by some common but unknown bond, which is essentially related to the principle of life; this principle, according to mr. herbert spencer, being that life depends on, or consists in, the incessant action and reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this tendency is slightly disturbed by any change, the vital forces gain in power. reciprocal dimorphism and trimorphism. this subject may be here briefly discussed, and will be found to throw some light on hybridism. several plants belonging to distinct orders present two forms, which exist in about equal numbers and which differ in no respect except in their reproductive organs; one form having a long pistil with short stamens, the other a short pistil with long stamens; the two having differently sized pollen-grains. with trimorphic plants there are three forms likewise differing in the lengths of their pistils and stamens, in the size and colour of the pollen-grains, and in some other respects; and as in each of the three forms there are two sets of stamens, the three forms possess altogether six sets of stamens and three kinds of pistils. these organs are so proportioned in length to each other that half the stamens in two of the forms stand on a level with the stigma of the third form. now i have shown, and the result has been confirmed by other observers, that in order to obtain full fertility with these plants, it is necessary that the stigma of the one form should be fertilised by pollen taken from the stamens of corresponding height in another form. so that with dimorphic species two unions, which may be called legitimate, are fully fertile; and two, which may be called illegitimate, are more or less infertile. with trimorphic species six unions are legitimate, or fully fertile, and twelve are illegitimate, or more or less infertile. the infertility which may be observed in various dimorphic and trimorphic plants, when they are illegitimately fertilised, that is by pollen taken from stamens not corresponding in height with the pistil, differs much in degree, up to absolute and utter sterility; just in the same manner as occurs in crossing distinct species. as the degree of sterility in the latter case depends in an eminent degree on the conditions of life being more or less favourable, so i have found it with illegitimate unions. it is well known that if pollen of a distinct species be placed on the stigma of a flower, and its own pollen be afterwards, even after a considerable interval of time, placed on the same stigma, its action is so strongly prepotent that it generally annihilates the effect of the foreign pollen; so it is with the pollen of the several forms of the same species, for legitimate pollen is strongly prepotent over illegitimate pollen, when both are placed on the same stigma. i ascertained this by fertilising several flowers, first illegitimately, and twenty-four hours afterwards legitimately, with pollen taken from a peculiarly coloured variety, and all the seedlings were similarly coloured; this shows that the legitimate pollen, though applied twenty-four hours subsequently, had wholly destroyed or prevented the action of the previously applied illegitimate pollen. again, as in making reciprocal crosses between the same two species, there is occasionally a great difference in the result, so the same thing occurs with trimorphic plants; for instance, the mid-styled form of lythrum salicaria was illegitimately fertilised with the greatest ease by pollen from the longer stamens of the short-styled form, and yielded many seeds; but the latter form did not yield a single seed when fertilised by the longer stamens of the mid-styled form. in all these respects, and in others which might be added, the forms of the same undoubted species, when illegitimately united, behave in exactly the same manner as do two distinct species when crossed. this led me carefully to observe during four years many seedlings, raised from several illegitimate unions. the chief result is that these illegitimate plants, as they may be called, are not fully fertile. it is possible to raise from dimorphic species, both long-styled and short-styled illegitimate plants, and from trimorphic plants all three illegitimate forms. these can then be properly united in a legitimate manner. when this is done, there is no apparent reason why they should not yield as many seeds as did their parents when legitimately fertilised. but such is not the case. they are all infertile, in various degrees; some being so utterly and incurably sterile that they did not yield during four seasons a single seed or even seed-capsule. the sterility of these illegitimate plants, when united with each other in a legitimate manner, may be strictly compared with that of hybrids when crossed inter se. if, on the other hand, a hybrid is crossed with either pure parent-species, the sterility is usually much lessened: and so it is when an illegitimate plant is fertilised by a legitimate plant. in the same manner as the sterility of hybrids does not always run parallel with the difficulty of making the first cross between the two parent-species, so that sterility of certain illegitimate plants was unusually great, while the sterility of the union from which they were derived was by no means great. with hybrids raised from the same seed-capsule the degree of sterility is innately variable, so it is in a marked manner with illegitimate plants. lastly, many hybrids are profuse and persistent flowerers, while other and more sterile hybrids produce few flowers, and are weak, miserable dwarfs; exactly similar cases occur with the illegitimate offspring of various dimorphic and trimorphic plants. altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids. it is hardly an exaggeration to maintain that illegitimate plants are hybrids, produced within the limits of the same species by the improper union of certain forms, while ordinary hybrids are produced from an improper union between so-called distinct species. we have also already seen that there is the closest similarity in all respects between first illegitimate unions and first crosses between distinct species. this will perhaps be made more fully apparent by an illustration; we may suppose that a botanist found two well-marked varieties (and such occur) of the long-styled form of the trimorphic lythrum salicaria, and that he determined to try by crossing whether they were specifically distinct. he would find that they yielded only about one-fifth of the proper number of seed, and that they behaved in all the other above specified respects as if they had been two distinct species. but to make the case sure, he would raise plants from his supposed hybridised seed, and he would find that the seedlings were miserably dwarfed and utterly sterile, and that they behaved in all other respects like ordinary hybrids. he might then maintain that he had actually proved, in accordance with the common view, that his two varieties were as good and as distinct species as any in the world; but he would be completely mistaken. the facts now given on dimorphic and trimorphic plants are important, because they show us, first, that the physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction; secondly, because we may conclude that there is some unknown bond which connects the infertility of illegitimate unions with that of their illegitimate offspring, and we are led to extend the same view to first crosses and hybrids; thirdly, because we find, and this seems to me of especial importance, that two or three forms of the same species may exist and may differ in no respect whatever, either in structure or in constitution, relatively to external conditions, and yet be sterile when united in certain ways. for we must remember that it is the union of the sexual elements of individuals of the same form, for instance, of two long-styled forms, which results in sterility; while it is the union of the sexual elements proper to two distinct forms which is fertile. hence the case appears at first sight exactly the reverse of what occurs, in the ordinary unions of the individuals of the same species and with crosses between distinct species. it is, however, doubtful whether this is really so; but i will not enlarge on this obscure subject. we may, however, infer as probable from the consideration of dimorphic and trimorphic plants, that the sterility of distinct species when crossed and of their hybrid progeny, depends exclusively on the nature of their sexual elements, and not on any difference in their structure or general constitution. we are also led to this same conclusion by considering reciprocal crosses, in which the male of one species cannot be united, or can be united with great difficulty, with the female of a second species, while the converse cross can be effected with perfect facility. that excellent observer, gartner, likewise concluded that species when crossed are sterile owing to differences confined to their reproductive systems. fertility of varieties when crossed, and of their mongrel offspring, not universal. it may be urged as an overwhelming argument that there must be some essential distinction between species and varieties inasmuch as the latter, however much they may differ from each other in external appearance, cross with perfect facility, and yield perfectly fertile offspring. with some exceptions, presently to be given, i fully admit that this is the rule. but the subject is surrounded by difficulties, for, looking to varieties produced under nature, if two forms hitherto reputed to be varieties be found in any degree sterile together, they are at once ranked by most naturalists as species. for instance, the blue and red pimpernel, which are considered by most botanists as varieties, are said by gartner to be quite sterile when crossed, and he consequently ranks them as undoubted species. if we thus argue in a circle, the fertility of all varieties produced under nature will assuredly have to be granted. if we turn to varieties, produced, or supposed to have been produced, under domestication, we are still involved in some doubt. for when it is stated, for instance, that certain south american indigenous domestic dogs do not readily unite with european dogs, the explanation which will occur to everyone, and probably the true one, is that they are descended from aboriginally distinct species. nevertheless the perfect fertility of so many domestic races, differing widely from each other in appearance, for instance, those of the pigeon, or of the cabbage, is a remarkable fact; more especially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed. several considerations, however, render the fertility of domestic varieties less remarkable. in the first place, it may be observed that the amount of external difference between two species is no sure guide to their degree of mutual sterility, so that similar differences in the case of varieties would be no sure guide. it is certain that with species the cause lies exclusively in differences in their sexual constitution. now the varying conditions to which domesticated animals and cultivated plants have been subjected, have had so little tendency towards modifying the reproductive system in a manner leading to mutual sterility, that we have good grounds for admitting the directly opposite doctrine of pallas, namely, that such conditions generally eliminate this tendency; so that the domesticated descendants of species, which in their natural state probably would have been in some degree sterile when crossed, become perfectly fertile together. with plants, so far is cultivation from giving a tendency towards sterility between distinct species, that in several well-authenticated cases already alluded to, certain plants have been affected in an opposite manner, for they have become self-impotent, while still retaining the capacity of fertilising, and being fertilised by, other species. if the pallasian doctrine of the elimination of sterility through long-continued domestication be admitted, and it can hardly be rejected, it becomes in the highest degree improbable that similar conditions long-continued should likewise induce this tendency; though in certain cases, with species having a peculiar constitution, sterility might occasionally be thus caused. thus, as i believe, we can understand why, with domesticated animals, varieties have not been produced which are mutually sterile; and why with plants only a few such cases, immediately to be given, have been observed. the real difficulty in our present subject is not, as it appears to me, why domestic varieties have not become mutually infertile when crossed, but why this has so generally occurred with natural varieties, as soon as they have been permanently modified in a sufficient degree to take rank as species. we are far from precisely knowing the cause; nor is this surprising, seeing how profoundly ignorant we are in regard to the normal and abnormal action of the reproductive system. but we can see that species, owing to their struggle for existence with numerous competitors, will have been exposed during long periods of time to more uniform conditions, than have domestic varieties; and this may well make a wide difference in the result. for we know how commonly wild animals and plants, when taken from their natural conditions and subjected to captivity, are rendered sterile; and the reproductive functions of organic beings which have always lived under natural conditions would probably in like manner be eminently sensitive to the influence of an unnatural cross. domesticated productions, on the other hand, which, as shown by the mere fact of their domestication, were not originally highly sensitive to changes in their conditions of life, and which can now generally resist with undiminished fertility repeated changes of conditions, might be expected to produce varieties, which would be little liable to have their reproductive powers injuriously affected by the act of crossing with other varieties which had originated in a like manner. i have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed. but it is impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases, which i will briefly abstract. the evidence is at least as good as that from which we believe in the sterility of a multitude of species. the evidence is also derived from hostile witnesses, who in all other cases consider fertility and sterility as safe criterions of specific distinction. gartner kept, during several years, a dwarf kind of maize with yellow seeds, and a tall variety with red seeds growing near each other in his garden; and although these plants have separated sexes, they never naturally crossed. he then fertilised thirteen flowers of the one kind with pollen of the other; but only a single head produced any seed, and this one head produced only five grains. manipulation in this case could not have been injurious, as the plants have separated sexes. no one, i believe, has suspected that these varieties of maize are distinct species; and it is important to notice that the hybrid plants thus raised were themselves perfectly fertile; so that even gartner did not venture to consider the two varieties as specifically distinct. girou de buzareingues crossed three varieties of gourd, which like the maize has separated sexes, and he asserts that their mutual fertilisation is by so much the less easy as their differences are greater. how far these experiments may be trusted, i know not; but the forms experimented on are ranked by sagaret, who mainly founds his classification by the test of infertility, as varieties, and naudin has come to the same conclusion. the following case is far more remarkable, and seems at first incredible; but it is the result of an astonishing number of experiments made during many years on nine species of verbascum, by so good an observer and so hostile a witness as gartner: namely, that the yellow and white varieties when crossed produce less seed than the similarly coloured varieties of the same species. moreover, he asserts that, when yellow and white varieties of one species are crossed with yellow and white varieties of a distinct species, more seed is produced by the crosses between the similarly coloured flowers, than between those which are differently coloured. mr. scott also has experimented on the species and varieties of verbascum; and although unable to confirm gartner's results on the crossing of the distinct species, he finds that the dissimilarly coloured varieties of the same species yield fewer seeds, in the proportion of eighty-six to , than the similarly coloured varieties. yet these varieties differ in no respect, except in the colour of their flowers; and one variety can sometimes be raised from the seed of another. kolreuter, whose accuracy has been confirmed by every subsequent observer, has proved the remarkable fact that one particular variety of the common tobacco was more fertile than the other varieties, when crossed with a widely distinct species. he experimented on five forms which are commonly reputed to be varieties, and which he tested by the severest trial, namely, by reciprocal crosses, and he found their mongrel offspring perfectly fertile. but one of these five varieties, when used either as the father or mother, and crossed with the nicotiana glutinosa, always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with n. glutinosa. hence the reproductive system of this one variety must have been in some manner and in some degree modified. from these facts it can no longer be maintained that varieties when crossed are invariably quite fertile. from the great difficulty of ascertaining the infertility of varieties in a state of nature, for a supposed variety, if proved to be infertile in any degree, would almost universally be ranked as a species; from man attending only to external characters in his domestic varieties, and from such varieties not having been exposed for very long periods to uniform conditions of life; from these several considerations we may conclude that fertility does not constitute a fundamental distinction between varieties and species when crossed. the general sterility of crossed species may safely be looked at, not as a special acquirement or endowment, but as incidental on changes of an unknown nature in their sexual elements. hybrids and mongrels compared, independently of their fertility. independently of the question of fertility, the offspring of species and of varieties when crossed may be compared in several other respects. gartner, whose strong wish it was to draw a distinct line between species and varieties, could find very few, and, as it seems to me, quite unimportant differences between the so-called hybrid offspring of species, and the so-called mongrel offspring of varieties. and, on the other hand, they agree most closely in many important respects. i shall here discuss this subject with extreme brevity. the most important distinction is, that in the first generation mongrels are more variable than hybrids; but gartner admits that hybrids from species which have long been cultivated are often variable in the first generation; and i have myself seen striking instances of this fact. gartner further admits that hybrids between very closely allied species are more variable than those from very distinct species; and this shows that the difference in the degree of variability graduates away. when mongrels and the more fertile hybrids are propagated for several generations, an extreme amount of variability in the offspring in both cases is notorious; but some few instances of both hybrids and mongrels long retaining a uniform character could be given. the variability, however, in the successive generations of mongrels is, perhaps, greater than in hybrids. this greater variability in mongrels than in hybrids does not seem at all surprising. for the parents of mongrels are varieties, and mostly domestic varieties (very few experiments having been tried on natural varieties), and this implies that there has been recent variability; which would often continue and would augment that arising from the act of crossing. the slight variability of hybrids in the first generation, in contrast with that in the succeeding generations, is a curious fact and deserves attention. for it bears on the view which i have taken of one of the causes of ordinary variability; namely, that the reproductive system, from being eminently sensitive to changed conditions of life, fails under these circumstances to perform its proper function of producing offspring closely similar in all respects to the parent-form. now, hybrids in the first generation are descended from species (excluding those long cultivated) which have not had their reproductive systems in any way affected, and they are not variable; but hybrids themselves have their reproductive systems seriously affected, and their descendants are highly variable. but to return to our comparison of mongrels and hybrids: gartner states that mongrels are more liable than hybrids to revert to either parent form; but this, if it be true, is certainly only a difference in degree. moreover, gartner expressly states that the hybrids from long cultivated plants are more subject to reversion than hybrids from species in their natural state; and this probably explains the singular difference in the results arrived at by different observers. thus max wichura doubts whether hybrids ever revert to their parent forms, and he experimented on uncultivated species of willows, while naudin, on the other hand, insists in the strongest terms on the almost universal tendency to reversion in hybrids, and he experimented chiefly on cultivated plants. gartner further states that when any two species, although most closely allied to each other, are crossed with a third species, the hybrids are widely different from each other; whereas if two very distinct varieties of one species are crossed with another species, the hybrids do not differ much. but this conclusion, as far as i can make out, is founded on a single experiment; and seems directly opposed to the results of several experiments made by kolreuter. such alone are the unimportant differences which gartner is able to point out between hybrid and mongrel plants. on the other hand, the degrees and kinds of resemblance in mongrels and in hybrids to their respective parents, more especially in hybrids produced from nearly related species, follow, according to gartner the same laws. when two species are crossed, one has sometimes a prepotent power of impressing its likeness on the hybrid. so i believe it to be with varieties of plants; and with animals, one variety certainly often has this prepotent power over another variety. hybrid plants produced from a reciprocal cross generally resemble each other closely, and so it is with mongrel plants from a reciprocal cross. both hybrids and mongrels can be reduced to either pure parent form, by repeated crosses in successive generations with either parent. these several remarks are apparently applicable to animals; but the subject is here much complicated, partly owing to the existence of secondary sexual characters; but more especially owing to prepotency in transmitting likeness running more strongly in one sex than in the other, both when one species is crossed with another and when one variety is crossed with another variety. for instance, i think those authors are right who maintain that the ass has a prepotent power over the horse, so that both the mule and the hinny resemble more closely the ass than the horse; but that the prepotency runs more strongly in the male than in the female ass, so that the mule, which is an offspring of the male ass and mare, is more like an ass than is the hinny, which is the offspring of the female-ass and stallion. much stress has been laid by some authors on the supposed fact, that it is only with mongrels that the offspring are not intermediate in character, but closely resemble one of their parents; but this does sometimes occur with hybrids, yet i grant much less frequently than with mongrels. looking to the cases which i have collected of cross-bred animals closely resembling one parent, the resemblances seem chiefly confined to characters almost monstrous in their nature, and which have suddenly appeared--such as albinism, melanism, deficiency of tail or horns, or additional fingers and toes; and do not relate to characters which have been slowly acquired through selection. a tendency to sudden reversions to the perfect character of either parent would, also, be much more likely to occur with mongrels, which are descended from varieties often suddenly produced and semi-monstrous in character, than with hybrids, which are descended from species slowly and naturally produced. on the whole, i entirely agree with dr. prosper lucas, who, after arranging an enormous body of facts with respect to animals, comes to the conclusion that the laws of resemblance of the child to its parents are the same, whether the two parents differ little or much from each other, namely, in the union of individuals of the same variety, or of different varieties, or of distinct species. independently of the question of fertility and sterility, in all other respects there seems to be a general and close similarity in the offspring of crossed species, and of crossed varieties. if we look at species as having been specially created, and at varieties as having been produced by secondary laws, this similarity would be an astonishing fact. but it harmonises perfectly with the view that there is no essential distinction between species and varieties. summary of chapter. first crosses between forms, sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile. the sterility is of all degrees, and is often so slight that the most careful experimentalists have arrived at diametrically opposite conclusions in ranking forms by this test. the sterility is innately variable in individuals of the same species, and is eminently susceptible to action of favourable and unfavourable conditions. the degree of sterility does not strictly follow systematic affinity, but is governed by several curious and complex laws. it is generally different, and sometimes widely different in reciprocal crosses between the same two species. it is not always equal in degree in a first cross and in the hybrids produced from this cross. in the same manner as in grafting trees, the capacity in one species or variety to take on another, is incidental on differences, generally of an unknown nature, in their vegetative systems, so in crossing, the greater or less facility of one species to unite with another is incidental on unknown differences in their reproductive systems. there is no more reason to think that species have been specially endowed with various degrees of sterility to prevent their crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent their inarching in our forests. the sterility of first crosses and of their hybrid progeny has not been acquired through natural selection. in the case of first crosses it seems to depend on several circumstances; in some instances in chief part on the early death of the embryo. in the case of hybrids, it apparently depends on their whole organisation having been disturbed by being compounded from two distinct forms; the sterility being closely allied to that which so frequently affects pure species, when exposed to new and unnatural conditions of life. he who will explain these latter cases will be able to explain the sterility of hybrids. this view is strongly supported by a parallelism of another kind: namely, that, firstly, slight changes in the conditions of life add to the vigour and fertility of all organic beings; and secondly, that the crossing of forms, which have been exposed to slightly different conditions of life, or which have varied, favours the size, vigour and fertility of their offspring. the facts given on the sterility of the illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny, perhaps render it probable that some unknown bond in all cases connects the degree of fertility of first unions with that of their offspring. the consideration of these facts on dimorphism, as well as of the results of reciprocal crosses, clearly leads to the conclusion that the primary cause of the sterility of crossed species is confined to differences in their sexual elements. but why, in the case of distinct species, the sexual elements should so generally have become more or less modified, leading to their mutual infertility, we do not know; but it seems to stand in some close relation to species having been exposed for long periods of time to nearly uniform conditions of life. it is not surprising that the difficulty in crossing any two species, and the sterility of their hybrid offspring, should in most cases correspond, even if due to distinct causes: for both depend on the amount of difference between the species which are crossed. nor is it surprising that the facility of effecting a first cross, and the fertility of the hybrids thus produced, and the capacity of being grafted together--though this latter capacity evidently depends on widely different circumstances--should all run, to a certain extent, parallel with the systematic affinity of the forms subjected to experiment; for systematic affinity includes resemblances of all kinds. first crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not, as is so often stated, invariably fertile. nor is this almost universal and perfect fertility surprising, when it is remembered how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences, and that they have not been long exposed to uniform conditions of life. it should also be especially kept in mind, that long-continued domestication tends to eliminate sterility, and is therefore little likely to induce this same quality. independently of the question of fertility, in all other respects there is the closest general resemblance between hybrids and mongrels, in their variability, in their power of absorbing each other by repeated crosses, and in their inheritance of characters from both parent-forms. finally, then, although we are as ignorant of the precise cause of the sterility of first crosses and of hybrids as we are why animals and plants removed from their natural conditions become sterile, yet the facts given in this chapter do not seem to me opposed to the belief that species aboriginally existed as varieties. chapter x. on the imperfection of the geological record. on the absence of intermediate varieties at the present day--on the nature of extinct intermediate varieties; on their number--on the lapse of time, as inferred from the rate of denudation and of deposition number--on the lapse of time as estimated by years--on the poorness of our palaeontological collections--on the intermittence of geological formations--on the denudation of granitic areas--on the absence of intermediate varieties in any one formation--on the sudden appearance of groups of species--on their sudden appearance in the lowest known fossiliferous strata--antiquity of the habitable earth. in the sixth chapter i enumerated the chief objections which might be justly urged against the views maintained in this volume. most of them have now been discussed. one, namely, the distinctness of specific forms and their not being blended together by innumerable transitional links, is a very obvious difficulty. i assigned reasons why such links do not commonly occur at the present day under the circumstances apparently most favourable for their presence, namely, on an extensive and continuous area with graduated physical conditions. i endeavoured to show, that the life of each species depends in a more important manner on the presence of other already defined organic forms, than on climate, and, therefore, that the really governing conditions of life do not graduate away quite insensibly like heat or moisture. i endeavoured, also, to show that intermediate varieties, from existing in lesser numbers than the forms which they connect, will generally be beaten out and exterminated during the course of further modification and improvement. the main cause, however, of innumerable intermediate links not now occurring everywhere throughout nature depends, on the very process of natural selection, through which new varieties continually take the places of and supplant their parent-forms. but just in proportion as this process of extermination has acted on an enormous scale, so must the number of intermediate varieties, which have formerly existed, be truly enormous. why then is not every geological formation and every stratum full of such intermediate links? geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against my theory. the explanation lies, as i believe, in the extreme imperfection of the geological record. in the first place, it should always be borne in mind what sort of intermediate forms must, on the theory, have formerly existed. i have found it difficult, when looking at any two species, to avoid picturing to myself forms directly intermediate between them. but this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all its modified descendants. to give a simple illustration: the fantail and pouter pigeons are both descended from the rock-pigeon; if we possessed all the intermediate varieties which have ever existed, we should have an extremely close series between both and the rock-pigeon; but we should have no varieties directly intermediate between the fantail and pouter; none, for instance, combining a tail somewhat expanded with a crop somewhat enlarged, the characteristic features of these two breeds. these two breeds, moreover, have become so much modified, that, if we had no historical or indirect evidence regarding their origin, it would not have been possible to have determined from a mere comparison of their structure with that of the rock-pigeon, c. livia, whether they had descended from this species or from some other allied species, such as c. oenas. so with natural species, if we look to forms very distinct, for instance to the horse and tapir, we have no reason to suppose that links directly intermediate between them ever existed, but between each and an unknown common parent. the common parent will have had in its whole organisation much general resemblance to the tapir and to the horse; but in some points of structure may have differed considerably from both, even perhaps more than they differ from each other. hence, in all such cases, we should be unable to recognise the parent-form of any two or more species, even if we closely compared the structure of the parent with that of its modified descendants, unless at the same time we had a nearly perfect chain of the intermediate links. it is just possible, by the theory, that one of two living forms might have descended from the other; for instance, a horse from a tapir; and in this case direct intermediate links will have existed between them. but such a case would imply that one form had remained for a very long period unaltered, whilst its descendants had undergone a vast amount of change; and the principle of competition between organism and organism, between child and parent, will render this a very rare event; for in all cases the new and improved forms of life tend to supplant the old and unimproved forms. by the theory of natural selection all living species have been connected with the parent-species of each genus, by differences not greater than we see between the natural and domestic varieties of the same species at the present day; and these parent-species, now generally extinct, have in their turn been similarly connected with more ancient forms; and so on backwards, always converging to the common ancestor of each great class. so that the number of intermediate and transitional links, between all living and extinct species, must have been inconceivably great. but assuredly, if this theory be true, such have lived upon the earth. on the lapse of time, as inferred from the rate of deposition and extent of denudation. independently of our not finding fossil remains of such infinitely numerous connecting links, it may be objected that time cannot have sufficed for so great an amount of organic change, all changes having been effected slowly. it is hardly possible for me to recall to the reader who is not a practical geologist, the facts leading the mind feebly to comprehend the lapse of time. he who can read sir charles lyell's grand work on the principles of geology, which the future historian will recognise as having produced a revolution in natural science, and yet does not admit how vast have been the past periods of time, may at once close this volume. not that it suffices to study the principles of geology, or to read special treatises by different observers on separate formations, and to mark how each author attempts to give an inadequate idea of the duration of each formation, or even of each stratum. we can best gain some idea of past time by knowing the agencies at work; and learning how deeply the surface of the land has been denuded, and how much sediment has been deposited. as lyell has well remarked, the extent and thickness of our sedimentary formations are the result and the measure of the denudation which the earth's crust has elsewhere undergone. therefore a man should examine for himself the great piles of superimposed strata, and watch the rivulets bringing down mud, and the waves wearing away the sea-cliffs, in order to comprehend something about the duration of past time, the monuments of which we see all around us. it is good to wander along the coast, when formed of moderately hard rocks, and mark the process of degradation. the tides in most cases reach the cliffs only for a short time twice a day, and the waves eat into them only when they are charged with sand or pebbles; for there is good evidence that pure water effects nothing in wearing away rock. at last the base of the cliff is undermined, huge fragments fall down, and these remaining fixed, have to be worn away atom by atom, until after being reduced in size they can be rolled about by the waves, and then they are more quickly ground into pebbles, sand, or mud. but how often do we see along the bases of retreating cliffs rounded boulders, all thickly clothed by marine productions, showing how little they are abraded and how seldom they are rolled about! moreover, if we follow for a few miles any line of rocky cliff, which is undergoing degradation, we find that it is only here and there, along a short length or round a promontory, that the cliffs are at the present time suffering. the appearance of the surface and the vegetation show that elsewhere years have elapsed since the waters washed their base. we have, however, recently learned from the observations of ramsay, in the van of many excellent observers--of jukes, geikie, croll and others, that subaerial degradation is a much more important agency than coast-action, or the power of the waves. the whole surface of the land is exposed to the chemical action of the air and of the rainwater, with its dissolved carbonic acid, and in colder countries to frost; the disintegrated matter is carried down even gentle slopes during heavy rain, and to a greater extent than might be supposed, especially in arid districts, by the wind; it is then transported by the streams and rivers, which, when rapid deepen their channels, and triturate the fragments. on a rainy day, even in a gently undulating country, we see the effects of subaerial degradation in the muddy rills which flow down every slope. messrs. ramsay and whitaker have shown, and the observation is a most striking one, that the great lines of escarpment in the wealden district and those ranging across england, which formerly were looked at as ancient sea-coasts, cannot have been thus formed, for each line is composed of one and the same formation, while our sea-cliffs are everywhere formed by the intersection of various formations. this being the case, we are compelled to admit that the escarpments owe their origin in chief part to the rocks of which they are composed, having resisted subaerial denudation better than the surrounding surface; this surface consequently has been gradually lowered, with the lines of harder rock left projecting. nothing impresses the mind with the vast duration of time, according to our ideas of time, more forcibly than the conviction thus gained that subaerial agencies, which apparently have so little power, and which seem to work so slowly, have produced great results. when thus impressed with the slow rate at which the land is worn away through subaerial and littoral action, it is good, in order to appreciate the past duration of time, to consider, on the one hand, the masses of rock which have been removed over many extensive areas, and on the other hand the thickness of our sedimentary formations. i remember having been much struck when viewing volcanic islands, which have been worn by the waves and pared all round into perpendicular cliffs of one or two thousand feet in height; for the gentle slope of the lava-streams, due to their formerly liquid state, showed at a glance how far the hard, rocky beds had once extended into the open ocean. the same story is told still more plainly by faults--those great cracks along which the strata have been upheaved on one side, or thrown down on the other, to the height or depth of thousands of feet; for since the crust cracked, and it makes no great difference whether the upheaval was sudden, or, as most geologists now believe, was slow and effected by many starts, the surface of the land has been so completely planed down that no trace of these vast dislocations is externally visible. the craven fault, for instance, extends for upward of thirty miles, and along this line the vertical displacement of the strata varies from to , feet. professor ramsay has published an account of a downthrow in anglesea of , feet; and he informs me that he fully believes that there is one in merionethshire of , feet; yet in these cases there is nothing on the surface of the land to show such prodigious movements; the pile of rocks on either side of the crack having been smoothly swept away. on the other hand, in all parts of the world the piles of sedimentary strata are of wonderful thickness. in the cordillera, i estimated one mass of conglomerate at ten thousand feet; and although conglomerates have probably been accumulated at a quicker rate than finer sediments, yet from being formed of worn and rounded pebbles, each of which bears the stamp of time, they are good to show how slowly the mass must have been heaped together. professor ramsay has given me the maximum thickness, from actual measurement in most cases, of the successive formations in different parts of great britain; and this is the result:-- feet palaeozoic strata (not including igneous beds).. , secondary strata................................ , tertiary strata.................................. , --making altogether , feet; that is, very nearly thirteen and three-quarters british miles. some of these formations, which are represented in england by thin beds, are thousands of feet in thickness on the continent. moreover, between each successive formation we have, in the opinion of most geologists, blank periods of enormous length. so that the lofty pile of sedimentary rocks in britain gives but an inadequate idea of the time which has elapsed during their accumulation. the consideration of these various facts impresses the mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity. nevertheless this impression is partly false. mr. croll, in an interesting paper, remarks that we do not err "in forming too great a conception of the length of geological periods," but in estimating them by years. when geologists look at large and complicated phenomena, and then at the figures representing several million years, the two produce a totally different effect on the mind, and the figures are at once pronounced too small. in regard to subaerial denudation, mr. croll shows, by calculating the known amount of sediment annually brought down by certain rivers, relatively to their areas of drainage, that , feet of solid rock, as it became gradually disintegrated, would thus be removed from the mean level of the whole area in the course of six million years. this seems an astonishing result, and some considerations lead to the suspicion that it may be too large, but if halved or quartered it is still very surprising. few of us, however, know what a million really means: mr. croll gives the following illustration: take a narrow strip of paper, eighty-three feet four inches in length, and stretch it along the wall of a large hall; then mark off at one end the tenth of an inch. this tenth of an inch will represent one hundred years, and the entire strip a million years. but let it be borne in mind, in relation to the subject of this work, what a hundred years implies, represented as it is by a measure utterly insignificant in a hall of the above dimensions. several eminent breeders, during a single lifetime, have so largely modified some of the higher animals, which propagate their kind much more slowly than most of the lower animals, that they have formed what well deserves to be called a new sub-breed. few men have attended with due care to any one strain for more than half a century, so that a hundred years represents the work of two breeders in succession. it is not to be supposed that species in a state of nature ever change so quickly as domestic animals under the guidance of methodical selection. the comparison would be in every way fairer with the effects which follow from unconscious selection, that is, the preservation of the most useful or beautiful animals, with no intention of modifying the breed; but by this process of unconscious selection, various breeds have been sensibly changed in the course of two or three centuries. species, however, probably change much more slowly, and within the same country only a few change at the same time. this slowness follows from all the inhabitants of the same country being already so well adapted to each other, that new places in the polity of nature do not occur until after long intervals, due to the occurrence of physical changes of some kind, or through the immigration of new forms. moreover, variations or individual differences of the right nature, by which some of the inhabitants might be better fitted to their new places under the altered circumstance, would not always occur at once. unfortunately we have no means of determining, according to the standard of years, how long a period it takes to modify a species; but to the subject of time we must return. on the poorness of palaeontological collections. now let us turn to our richest museums, and what a paltry display we behold! that our collections are imperfect is admitted by every one. the remark of that admirable palaeontologist, edward forbes, should never be forgotten, namely, that very many fossil species are known and named from single and often broken specimens, or from a few specimens collected on some one spot. only a small portion of the surface of the earth has been geologically explored, and no part with sufficient care, as the important discoveries made every year in europe prove. no organism wholly soft can be preserved. shells and bones decay and disappear when left on the bottom of the sea, where sediment is not accumulating. we probably take a quite erroneous view, when we assume that sediment is being deposited over nearly the whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil remains. throughout an enormously large proportion of the ocean, the bright blue tint of the water bespeaks its purity. the many cases on record of a formation conformably covered, after an immense interval of time, by another and later formation, without the underlying bed having suffered in the interval any wear and tear, seem explicable only on the view of the bottom of the sea not rarely lying for ages in an unaltered condition. the remains which do become embedded, if in sand or gravel, will, when the beds are upraised, generally be dissolved by the percolation of rain water charged with carbonic acid. some of the many kinds of animals which live on the beach between high and low water mark seem to be rarely preserved. for instance, the several species of the chthamalinae (a sub-family of sessile cirripedes) coat the rocks all over the world in infinite numbers: they are all strictly littoral, with the exception of a single mediterranean species, which inhabits deep water and this has been found fossil in sicily, whereas not one other species has hitherto been found in any tertiary formation: yet it is known that the genus chthamalus existed during the chalk period. lastly, many great deposits, requiring a vast length of time for their accumulation, are entirely destitute of organic remains, without our being able to assign any reason: one of the most striking instances is that of the flysch formation, which consists of shale and sandstone, several thousand, occasionally even six thousand feet in thickness, and extending for at least miles from vienna to switzerland; and although this great mass has been most carefully searched, no fossils, except a few vegetable remains, have been found. with respect to the terrestrial productions which lived during the secondary and palaeozoic periods, it is superfluous to state that our evidence is fragmentary in an extreme degree. for instance, until recently not a land-shell was known belonging to either of these vast periods, with the exception of one species discovered by sir c. lyell and dr. dawson in the carboniferous strata of north america; but now land-shells have been found in the lias. in regard to mammiferous remains, a glance at the historical table published in lyell's manual, will bring home the truth, how accidental and rare is their preservation, far better than pages of detail. nor is their rarity surprising, when we remember how large a proportion of the bones of tertiary mammals have been discovered either in caves or in lacustrine deposits; and that not a cave or true lacustrine bed is known belonging to the age of our secondary or palaeozoic formations. but the imperfection in the geological record largely results from another and more important cause than any of the foregoing; namely, from the several formations being separated from each other by wide intervals of time. this doctrine has been emphatically admitted by many geologists and palaeontologists, who, like e. forbes, entirely disbelieve in the change of species. when we see the formations tabulated in written works, or when we follow them in nature, it is difficult to avoid believing that they are closely consecutive. but we know, for instance, from sir r. murchison's great work on russia, what wide gaps there are in that country between the superimposed formations; so it is in north america, and in many other parts of the world. the most skilful geologist, if his attention had been confined exclusively to these large territories, would never have suspected that during the periods which were blank and barren in his own country, great piles of sediment, charged with new and peculiar forms of life, had elsewhere been accumulated. and if, in every separate territory, hardly any idea can be formed of the length of time which has elapsed between the consecutive formations, we may infer that this could nowhere be ascertained. the frequent and great changes in the mineralogical composition of consecutive formations, generally implying great changes in the geography of the surrounding lands, whence the sediment was derived, accord with the belief of vast intervals of time having elapsed between each formation. we can, i think, see why the geological formations of each region are almost invariably intermittent; that is, have not followed each other in close sequence. scarcely any fact struck me more when examining many hundred miles of the south american coasts, which have been upraised several hundred feet within the recent period, than the absence of any recent deposits sufficiently extensive to last for even a short geological period. along the whole west coast, which is inhabited by a peculiar marine fauna, tertiary beds are so poorly developed that no record of several successive and peculiar marine faunas will probably be preserved to a distant age. a little reflection will explain why, along the rising coast of the western side of south america, no extensive formations with recent or tertiary remains can anywhere be found, though the supply of sediment must for ages have been great, from the enormous degradation of the coast rocks and from the muddy streams entering the sea. the explanation, no doubt, is that the littoral and sub-littoral deposits are continually worn away, as soon as they are brought up by the slow and gradual rising of the land within the grinding action of the coast-waves. we may, i think, conclude that sediment must be accumulated in extremely thick, solid, or extensive masses, in order to withstand the incessant action of the waves, when first upraised and during subsequent oscillations of level, as well as the subsequent subaerial degradation. such thick and extensive accumulations of sediment may be formed in two ways; either in profound depths of the sea, in which case the bottom will not be inhabited by so many and such varied forms of life as the more shallow seas; and the mass when upraised will give an imperfect record of the organisms which existed in the neighbourhood during the period of its accumulation. or sediment may be deposited to any thickness and extent over a shallow bottom, if it continue slowly to subside. in this latter case, as long as the rate of subsidence and supply of sediment nearly balance each other, the sea will remain shallow and favourable for many and varied forms, and thus a rich fossiliferous formation, thick enough, when upraised, to resist a large amount of denudation, may be formed. i am convinced that nearly all our ancient formations, which are throughout the greater part of their thickness rich in fossils, have thus been formed during subsidence. since publishing my views on this subject in , i have watched the progress of geology, and have been surprised to note how author after author, in treating of this or that great formation, has come to the conclusion that it was accumulated during subsidence. i may add, that the only ancient tertiary formation on the west coast of south america, which has been bulky enough to resist such degradation as it has as yet suffered, but which will hardly last to a distant geological age, was deposited during a downward oscillation of level, and thus gained considerable thickness. all geological facts tell us plainly that each area has undergone numerous slow oscillations of level, and apparently these oscillations have affected wide spaces. consequently, formations rich in fossils and sufficiently thick and extensive to resist subsequent degradation, will have been formed over wide spaces during periods of subsidence, but only where the supply of sediment was sufficient to keep the sea shallow and to embed and preserve the remains before they had time to decay. on the other hand, as long as the bed of the sea remained stationary, thick deposits cannot have been accumulated in the shallow parts, which are the most favourable to life. still less can this have happened during the alternate periods of elevation; or, to speak more accurately, the beds which were then accumulated will generally have been destroyed by being upraised and brought within the limits of the coast-action. these remarks apply chiefly to littoral and sublittoral deposits. in the case of an extensive and shallow sea, such as that within a large part of the malay archipelago, where the depth varies from thirty or forty to sixty fathoms, a widely extended formation might be formed during a period of elevation, and yet not suffer excessively from denudation during its slow upheaval; but the thickness of the formation could not be great, for owing to the elevatory movement it would be less than the depth in which it was formed; nor would the deposit be much consolidated, nor be capped by overlying formations, so that it would run a good chance of being worn away by atmospheric degradation and by the action of the sea during subsequent oscillations of level. it has, however, been suggested by mr. hopkins, that if one part of the area, after rising and before being denuded, subsided, the deposit formed during the rising movement, though not thick, might afterwards become protected by fresh accumulations, and thus be preserved for a long period. mr. hopkins also expresses his belief that sedimentary beds of considerable horizontal extent have rarely been completely destroyed. but all geologists, excepting the few who believe that our present metamorphic schists and plutonic rocks once formed the primordial nucleus of the globe, will admit that these latter rocks have been stripped of their covering to an enormous extent. for it is scarcely possible that such rocks could have been solidified and crystallised while uncovered; but if the metamorphic action occurred at profound depths of the ocean, the former protecting mantle of rock may not have been very thick. admitting then that gneiss, mica-schist, granite, diorite, etc., were once necessarily covered up, how can we account for the naked and extensive areas of such rocks in many parts of the world, except on the belief that they have subsequently been completely denuded of all overlying strata? that such extensive areas do exist cannot be doubted: the granitic region of parime is described by humboldt as being at least nineteen times as large as switzerland. south of the amazon, boue colours an area composed of rocks of this nature as equal to that of spain, france, italy, part of germany, and the british islands, all conjoined. this region has not been carefully explored, but from the concurrent testimony of travellers, the granitic area is very large: thus von eschwege gives a detailed section of these rocks, stretching from rio de janeiro for geographical miles inland in a straight line; and i travelled for miles in another direction, and saw nothing but granitic rocks. numerous specimens, collected along the whole coast, from near rio de janeiro to the mouth of the plata, a distance of , geographical miles, were examined by me, and they all belonged to this class. inland, along the whole northern bank of the plata, i saw, besides modern tertiary beds, only one small patch of slightly metamorphosed rock, which alone could have formed a part of the original capping of the granitic series. turning to a well-known region, namely, to the united states and canada, as shown in professor h.d. rogers' beautiful map, i have estimated the areas by cutting out and weighing the paper, and i find that the metamorphic (excluding the "semi-metamorphic") and granite rocks exceed, in the proportion of to . , the whole of the newer palaeozoic formations. in many regions the metamorphic and granite rocks would be found much more widely extended than they appear to be, if all the sedimentary beds were removed which rest unconformably on them, and which could not have formed part of the original mantle under which they were crystallised. hence, it is probable that in some parts of the world whole formations have been completely denuded, with not a wreck left behind. one remark is here worth a passing notice. during periods of elevation the area of the land and of the adjoining shoal parts of the sea will be increased and new stations will often be formed--all circumstances favourable, as previously explained, for the formation of new varieties and species; but during such periods there will generally be a blank in the geological record. on the other hand, during subsidence, the inhabited area and number of inhabitants will decrease (excepting on the shores of a continent when first broken up into an archipelago), and consequently during subsidence, though there will be much extinction, few new varieties or species will be formed; and it is during these very periods of subsidence that the deposits which are richest in fossils have been accumulated. on the absence of numerous intermediate varieties in any single formation. from these several considerations it cannot be doubted that the geological record, viewed as a whole, is extremely imperfect; but if we confine our attention to any one formation, it becomes much more difficult to understand why we do not therein find closely graduated varieties between the allied species which lived at its commencement and at its close. several cases are on record of the same species presenting varieties in the upper and lower parts of the same formation. thus trautschold gives a number of instances with ammonites, and hilgendorf has described a most curious case of ten graduated forms of planorbis multiformis in the successive beds of a fresh-water formation in switzerland. although each formation has indisputably required a vast number of years for its deposition, several reasons can be given why each should not commonly include a graduated series of links between the species which lived at its commencement and close, but i cannot assign due proportional weight to the following considerations. although each formation may mark a very long lapse of years, each probably is short compared with the period requisite to change one species into another. i am aware that two palaeontologists, whose opinions are worthy of much deference, namely bronn and woodward, have concluded that the average duration of each formation is twice or thrice as long as the average duration of specific forms. but insuperable difficulties, as it seems to me, prevent us from coming to any just conclusion on this head. when we see a species first appearing in the middle of any formation, it would be rash in the extreme to infer that it had not elsewhere previously existed. so again, when we find a species disappearing before the last layers have been deposited, it would be equally rash to suppose that it then became extinct. we forget how small the area of europe is compared with the rest of the world; nor have the several stages of the same formation throughout europe been correlated with perfect accuracy. we may safely infer that with marine animals of all kinds there has been a large amount of migration due to climatal and other changes; and when we see a species first appearing in any formation, the probability is that it only then first immigrated into that area. it is well known, for instance, that several species appear somewhat earlier in the palaeozoic beds of north america than in those of europe; time having apparently been required for their migration from the american to the european seas. in examining the latest deposits, in various quarters of the world, it has everywhere been noted, that some few still existing species are common in the deposit, but have become extinct in the immediately surrounding sea; or, conversely, that some are now abundant in the neighbouring sea, but are rare or absent in this particular deposit. it is an excellent lesson to reflect on the ascertained amount of migration of the inhabitants of europe during the glacial epoch, which forms only a part of one whole geological period; and likewise to reflect on the changes of level, on the extreme change of climate, and on the great lapse of time, all included within this same glacial period. yet it may be doubted whether, in any quarter of the world, sedimentary deposits, including fossil remains, have gone on accumulating within the same area during the whole of this period. it is not, for instance, probable that sediment was deposited during the whole of the glacial period near the mouth of the mississippi, within that limit of depth at which marine animals can best flourish: for we know that great geographical changes occurred in other parts of america during this space of time. when such beds as were deposited in shallow water near the mouth of the mississippi during some part of the glacial period shall have been upraised, organic remains will probably first appear and disappear at different levels, owing to the migrations of species and to geographical changes. and in the distant future, a geologist, examining these beds, would be tempted to conclude that the average duration of life of the embedded fossils had been less than that of the glacial period, instead of having been really far greater, that is, extending from before the glacial epoch to the present day. in order to get a perfect gradation between two forms in the upper and lower parts of the same formation, the deposit must have gone on continuously accumulating during a long period, sufficient for the slow process of modification; hence, the deposit must be a very thick one; and the species undergoing change must have lived in the same district throughout the whole time. but we have seen that a thick formation, fossiliferous throughout its entire thickness, can accumulate only during a period of subsidence; and to keep the depth approximately the same, which is necessary that the same marine species may live on the same space, the supply of sediment must nearly counterbalance the amount of subsidence. but this same movement of subsidence will tend to submerge the area whence the sediment is derived, and thus diminish the supply, whilst the downward movement continues. in fact, this nearly exact balancing between the supply of sediment and the amount of subsidence is probably a rare contingency; for it has been observed by more than one palaeontologist that very thick deposits are usually barren of organic remains, except near their upper or lower limits. it would seem that each separate formation, like the whole pile of formations in any country, has generally been intermittent in its accumulation. when we see, as is so often the case, a formation composed of beds of widely different mineralogical composition, we may reasonably suspect that the process of deposition has been more or less interrupted. nor will the closest inspection of a formation give us any idea of the length of time which its deposition may have consumed. many instances could be given of beds, only a few feet in thickness, representing formations which are elsewhere thousands of feet in thickness, and which must have required an enormous period for their accumulation; yet no one ignorant of this fact would have even suspected the vast lapse of time represented by the thinner formation. many cases could be given of the lower beds of a formation having been upraised, denuded, submerged, and then re-covered by the upper beds of the same formation--facts, showing what wide, yet easily overlooked, intervals have occurred in its accumulation. in other cases we have the plainest evidence in great fossilised trees, still standing upright as they grew, of many long intervals of time and changes of level during the process of deposition, which would not have been suspected, had not the trees been preserved: thus sir c. lyell and dr. dawson found carboniferous beds , feet thick in nova scotia, with ancient root-bearing strata, one above the other, at no less than sixty-eight different levels. hence, when the same species occurs at the bottom, middle, and top of a formation, the probability is that it has not lived on the same spot during the whole period of deposition, but has disappeared and reappeared, perhaps many times, during the same geological period. consequently if it were to undergo a considerable amount of modification during the deposition of any one geological formation, a section would not include all the fine intermediate gradations which must on our theory have existed, but abrupt, though perhaps slight, changes of form. it is all-important to remember that naturalists have no golden rule by which to distinguish species and varieties; they grant some little variability to each species, but when they meet with a somewhat greater amount of difference between any two forms, they rank both as species, unless they are enabled to connect them together by the closest intermediate gradations; and this, from the reasons just assigned, we can seldom hope to effect in any one geological section. supposing b and c to be two species, and a third, a, to be found in an older and underlying bed; even if a were strictly intermediate between b and c, it would simply be ranked as a third and distinct species, unless at the same time it could be closely connected by intermediate varieties with either one or both forms. nor should it be forgotten, as before explained, that a might be the actual progenitor of b and c, and yet would not necessarily be strictly intermediate between them in all respects. so that we might obtain the parent-species and its several modified descendants from the lower and upper beds of the same formation, and unless we obtained numerous transitional gradations, we should not recognise their blood-relationship, and should consequently rank them as distinct species. it is notorious on what excessively slight differences many palaeontologists have founded their species; and they do this the more readily if the specimens come from different sub-stages of the same formation. some experienced conchologists are now sinking many of the very fine species of d'orbigny and others into the rank of varieties; and on this view we do find the kind of evidence of change which on the theory we ought to find. look again at the later tertiary deposits, which include many shells believed by the majority of naturalists to be identical with existing species; but some excellent naturalists, as agassiz and pictet, maintain that all these tertiary species are specifically distinct, though the distinction is admitted to be very slight; so that here, unless we believe that these eminent naturalists have been misled by their imaginations, and that these late tertiary species really present no difference whatever from their living representatives, or unless we admit, in opposition to the judgment of most naturalists, that these tertiary species are all truly distinct from the recent, we have evidence of the frequent occurrence of slight modifications of the kind required. if we look to rather wider intervals of time, namely, to distinct but consecutive stages of the same great formation, we find that the embedded fossils, though universally ranked as specifically different, yet are far more closely related to each other than are the species found in more widely separated formations; so that here again we have undoubted evidence of change in the direction required by the theory; but to this latter subject i shall return in the following chapter. with animals and plants that propagate rapidly and do not wander much, there is reason to suspect, as we have formerly seen, that their varieties are generally at first local; and that such local varieties do not spread widely and supplant their parent-form until they have been modified and perfected in some considerable degree. according to this view, the chance of discovering in a formation in any one country all the early stages of transition between any two forms, is small, for the successive changes are supposed to have been local or confined to some one spot. most marine animals have a wide range; and we have seen that with plants it is those which have the widest range, that oftenest present varieties, so that, with shells and other marine animals, it is probable that those which had the widest range, far exceeding the limits of the known geological formations in europe, have oftenest given rise, first to local varieties and ultimately to new species; and this again would greatly lessen the chance of our being able to trace the stages of transition in any one geological formation. it is a more important consideration, leading to the same result, as lately insisted on by dr. falconer, namely, that the period during which each species underwent modification, though long as measured by years, was probably short in comparison with that during which it remained without undergoing any change. it should not be forgotten, that at the present day, with perfect specimens for examination, two forms can seldom be connected by intermediate varieties, and thus proved to be the same species, until many specimens are collected from many places; and with fossil species this can rarely be done. we shall, perhaps, best perceive the improbability of our being enabled to connect species by numerous, fine, intermediate, fossil links, by asking ourselves whether, for instance, geologists at some future period will be able to prove that our different breeds of cattle, sheep, horses, and dogs are descended from a single stock or from several aboriginal stocks; or, again, whether certain sea-shells inhabiting the shores of north america, which are ranked by some conchologists as distinct species from their european representatives, and by other conchologists as only varieties, are really varieties, or are, as it is called, specifically distinct. this could be effected by the future geologist only by his discovering in a fossil state numerous intermediate gradations; and such success is improbable in the highest degree. it has been asserted over and over again, by writers who believe in the immutability of species, that geology yields no linking forms. this assertion, as we shall see in the next chapter, is certainly erroneous. as sir j. lubbock has remarked, "every species is a link between other allied forms." if we take a genus having a score of species, recent and extinct, and destroy four-fifths of them, no one doubts that the remainder will stand much more distinct from each other. if the extreme forms in the genus happen to have been thus destroyed, the genus itself will stand more distinct from other allied genera. what geological research has not revealed, is the former existence of infinitely numerous gradations, as fine as existing varieties, connecting together nearly all existing and extinct species. but this ought not to be expected; yet this has been repeatedly advanced as a most serious objection against my views. it may be worth while to sum up the foregoing remarks on the causes of the imperfection of the geological record under an imaginary illustration. the malay archipelago is about the size of europe from the north cape to the mediterranean, and from britain to russia; and therefore equals all the geological formations which have been examined with any accuracy, excepting those of the united states of america. i fully agree with mr. godwin-austen, that the present condition of the malay archipelago, with its numerous large islands separated by wide and shallow seas, probably represents the former state of europe, while most of our formations were accumulating. the malay archipelago is one of the richest regions in organic beings; yet if all the species were to be collected which have ever lived there, how imperfectly would they represent the natural history of the world! but we have every reason to believe that the terrestrial productions of the archipelago would be preserved in an extremely imperfect manner in the formations which we suppose to be there accumulating. not many of the strictly littoral animals, or of those which lived on naked submarine rocks, would be embedded; and those embedded in gravel or sand would not endure to a distant epoch. wherever sediment did not accumulate on the bed of the sea, or where it did not accumulate at a sufficient rate to protect organic bodies from decay, no remains could be preserved. formations rich in fossils of many kinds, and of thickness sufficient to last to an age as distant in futurity as the secondary formations lie in the past, would generally be formed in the archipelago only during periods of subsidence. these periods of subsidence would be separated from each other by immense intervals of time, during which the area would be either stationary or rising; whilst rising, the fossiliferous formations on the steeper shores would be destroyed, almost as soon as accumulated, by the incessant coast-action, as we now see on the shores of south america. even throughout the extensive and shallow seas within the archipelago, sedimentary beds could hardly be accumulated of great thickness during the periods of elevation, or become capped and protected by subsequent deposits, so as to have a good chance of enduring to a very distant future. during the periods of subsidence, there would probably be much extinction of life; during the periods of elevation, there would be much variation, but the geological record would then be less perfect. it may be doubted whether the duration of any one great period of subsidence over the whole or part of the archipelago, together with a contemporaneous accumulation of sediment, would exceed the average duration of the same specific forms; and these contingencies are indispensable for the preservation of all the transitional gradations between any two or more species. if such gradations were not all fully preserved, transitional varieties would merely appear as so many new, though closely allied species. it is also probable that each great period of subsidence would be interrupted by oscillations of level, and that slight climatical changes would intervene during such lengthy periods; and in these cases the inhabitants of the archipelago would migrate, and no closely consecutive record of their modifications could be preserved in any one formation. very many of the marine inhabitants of the archipelago now range thousands of miles beyond its confines; and analogy plainly leads to the belief that it would be chiefly these far-ranging species, though only some of them, which would oftenest produce new varieties; and the varieties would at first be local or confined to one place, but if possessed of any decided advantage, or when further modified and improved, they would slowly spread and supplant their parent-forms. when such varieties returned to their ancient homes, as they would differ from their former state in a nearly uniform, though perhaps extremely slight degree, and as they would be found embedded in slightly different sub-stages of the same formation, they would, according to the principles followed by many palaeontologists, be ranked as new and distinct species. if then there be some degree of truth in these remarks, we have no right to expect to find, in our geological formations, an infinite number of those fine transitional forms, which, on our theory, have connected all the past and present species of the same group into one long and branching chain of life. we ought only to look for a few links, and such assuredly we do find--some more distantly, some more closely, related to each other; and these links, let them be ever so close, if found in different stages of the same formation, would, by many palaeontologists, be ranked as distinct species. but i do not pretend that i should ever have suspected how poor was the record in the best preserved geological sections, had not the absence of innumerable transitional links between the species which lived at the commencement and close of each formation, pressed so hardly on my theory. on the sudden appearance of whole groups of allied species. the abrupt manner in which whole groups of species suddenly appear in certain formations, has been urged by several palaeontologists--for instance, by agassiz, pictet, and sedgwick, as a fatal objection to the belief in the transmutation of species. if numerous species, belonging to the same genera or families, have really started into life at once, the fact would be fatal to the theory of evolution through natural selection. for the development by this means of a group of forms, all of which are descended from some one progenitor, must have been an extremely slow process; and the progenitors must have lived long before their modified descendants. but we continually overrate the perfection of the geological record, and falsely infer, because certain genera or families have not been found beneath a certain stage, that they did not exist before that stage. in all cases positive palaeontological evidence may be implicitly trusted; negative evidence is worthless, as experience has so often shown. we continually forget how large the world is, compared with the area over which our geological formations have been carefully examined; we forget that groups of species may elsewhere have long existed, and have slowly multiplied, before they invaded the ancient archipelagoes of europe and the united states. we do not make due allowance for the enormous intervals of time which have elapsed between our consecutive formations, longer perhaps in many cases than the time required for the accumulation of each formation. these intervals will have given time for the multiplication of species from some one parent-form: and in the succeeding formation, such groups or species will appear as if suddenly created. i may here recall a remark formerly made, namely, that it might require a long succession of ages to adapt an organism to some new and peculiar line of life, for instance, to fly through the air; and consequently that the transitional forms would often long remain confined to some one region; but that, when this adaptation had once been effected, and a few species had thus acquired a great advantage over other organisms, a comparatively short time would be necessary to produce many divergent forms, which would spread rapidly and widely throughout the world. professor pictet, in his excellent review of this work, in commenting on early transitional forms, and taking birds as an illustration, cannot see how the successive modifications of the anterior limbs of a supposed prototype could possibly have been of any advantage. but look at the penguins of the southern ocean; have not these birds their front limbs in this precise intermediate state of "neither true arms nor true wings?" yet these birds hold their place victoriously in the battle for life; for they exist in infinite numbers and of many kinds. i do not suppose that we here see the real transitional grades through which the wings of birds have passed; but what special difficulty is there in believing that it might profit the modified descendants of the penguin, first to become enabled to flap along the surface of the sea like the logger-headed duck, and ultimately to rise from its surface and glide through the air? i will now give a few examples to illustrate the foregoing remarks, and to show how liable we are to error in supposing that whole groups of species have suddenly been produced. even in so short an interval as that between the first and second editions of pictet's great work on palaeontology, published in - and in - , the conclusions on the first appearance and disappearance of several groups of animals have been considerably modified; and a third edition would require still further changes. i may recall the well-known fact that in geological treatises, published not many years ago, mammals were always spoken of as having abruptly come in at the commencement of the tertiary series. and now one of the richest known accumulations of fossil mammals belongs to the middle of the secondary series; and true mammals have been discovered in the new red sandstone at nearly the commencement of this great series. cuvier used to urge that no monkey occurred in any tertiary stratum; but now extinct species have been discovered in india, south america and in europe, as far back as the miocene stage. had it not been for the rare accident of the preservation of footsteps in the new red sandstone of the united states, who would have ventured to suppose that no less than at least thirty different bird-like animals, some of gigantic size, existed during that period? not a fragment of bone has been discovered in these beds. not long ago, palaeontologists maintained that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of professor owen, that a bird certainly lived during the deposition of the upper greensand; and still more recently, that strange bird, the archeopteryx, with a long lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws, has been discovered in the oolitic slates of solenhofen. hardly any recent discovery shows more forcibly than this how little we as yet know of the former inhabitants of the world. i may give another instance, which, from having passed under my own eyes has much struck me. in a memoir on fossil sessile cirripedes, i stated that, from the large number of existing and extinct tertiary species; from the extraordinary abundance of the individuals of many species all over the world, from the arctic regions to the equator, inhabiting various zones of depths, from the upper tidal limits to fifty fathoms; from the perfect manner in which specimens are preserved in the oldest tertiary beds; from the ease with which even a fragment of a valve can be recognised; from all these circumstances, i inferred that, had sessile cirripedes existed during the secondary periods, they would certainly have been preserved and discovered; and as not one species had then been discovered in beds of this age, i concluded that this great group had been suddenly developed at the commencement of the tertiary series. this was a sore trouble to me, adding, as i then thought, one more instance of the abrupt appearance of a great group of species. but my work had hardly been published, when a skilful palaeontologist, m. bosquet, sent me a drawing of a perfect specimen of an unmistakable sessile cirripede, which he had himself extracted from the chalk of belgium. and, as if to make the case as striking as possible, this cirripede was a chthamalus, a very common, large, and ubiquitous genus, of which not one species has as yet been found even in any tertiary stratum. still more recently, a pyrgoma, a member of a distinct subfamily of sessile cirripedes, has been discovered by mr. woodward in the upper chalk; so that we now have abundant evidence of the existence of this group of animals during the secondary period. the case most frequently insisted on by palaeontologists of the apparently sudden appearance of a whole group of species, is that of the teleostean fishes, low down, according to agassiz, in the chalk period. this group includes the large majority of existing species. but certain jurassic and triassic forms are now commonly admitted to be teleostean; and even some palaeozoic forms have thus been classed by one high authority. if the teleosteans had really appeared suddenly in the northern hemisphere at the commencement of the chalk formation, the fact would have been highly remarkable; but it would not have formed an insuperable difficulty, unless it could likewise have been shown that at the same period the species were suddenly and simultaneously developed in other quarters of the world. it is almost superfluous to remark that hardly any fossil-fish are known from south of the equator; and by running through pictet's palaeontology it will be seen that very few species are known from several formations in europe. some few families of fish now have a confined range; the teleostean fishes might formerly have had a similarly confined range, and after having been largely developed in some one sea, have spread widely. nor have we any right to suppose that the seas of the world have always been so freely open from south to north as they are at present. even at this day, if the malay archipelago were converted into land, the tropical parts of the indian ocean would form a large and perfectly enclosed basin, in which any great group of marine animals might be multiplied; and here they would remain confined, until some of the species became adapted to a cooler climate, and were enabled to double the southern capes of africa or australia, and thus reach other and distant seas. from these considerations, from our ignorance of the geology of other countries beyond the confines of europe and the united states, and from the revolution in our palaeontological knowledge effected by the discoveries of the last dozen years, it seems to me to be about as rash to dogmatize on the succession of organic forms throughout the world, as it would be for a naturalist to land for five minutes on a barren point in australia, and then to discuss the number and range of its productions. on the sudden appearance of groups of allied species in the lowest known fossiliferous strata. there is another and allied difficulty, which is much more serious. i allude to the manner in which species belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks. most of the arguments which have convinced me that all the existing species of the same group are descended from a single progenitor, apply with equal force to the earliest known species. for instance, it cannot be doubted that all the cambrian and silurian trilobites are descended from some one crustacean, which must have lived long before the cambrian age, and which probably differed greatly from any known animal. some of the most ancient animals, as the nautilus, lingula, etc., do not differ much from living species; and it cannot on our theory be supposed, that these old species were the progenitors of all the species belonging to the same groups which have subsequently appeared, for they are not in any degree intermediate in character. consequently, if the theory be true, it is indisputable that before the lowest cambrian stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the cambrian age to the present day; and that during these vast periods the world swarmed with living creatures. here we encounter a formidable objection; for it seems doubtful whether the earth, in a fit state for the habitation of living creatures, has lasted long enough. sir w. thompson concludes that the consolidation of the crust can hardly have occurred less than twenty or more than four hundred million years ago, but probably not less than ninety-eight or more than two hundred million years. these very wide limits show how doubtful the data are; and other elements may have hereafter to be introduced into the problem. mr. croll estimates that about sixty million years have elapsed since the cambrian period, but this, judging from the small amount of organic change since the commencement of the glacial epoch, appears a very short time for the many and great mutations of life, which have certainly occurred since the cambrian formation; and the previous one hundred and forty million years can hardly be considered as sufficient for the development of the varied forms of life which already existed during the cambrian period. it is, however, probable, as sir william thompson insists, that the world at a very early period was subjected to more rapid and violent changes in its physical conditions than those now occurring; and such changes would have tended to induce changes at a corresponding rate in the organisms which then existed. to the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the cambrian system, i can give no satisfactory answer. several eminent geologists, with sir r. murchison at their head, were until recently convinced that we beheld in the organic remains of the lowest silurian stratum the first dawn of life. other highly competent judges, as lyell and e. forbes, have disputed this conclusion. we should not forget that only a small portion of the world is known with accuracy. not very long ago m. barrande added another and lower stage, abounding with new and peculiar species, beneath the then known silurian system; and now, still lower down in the lower cambrian formation, mr hicks has found south wales beds rich in trilobites, and containing various molluscs and annelids. the presence of phosphatic nodules and bituminous matter, even in some of the lowest azotic rocks, probably indicates life at these periods; and the existence of the eozoon in the laurentian formation of canada is generally admitted. there are three great series of strata beneath the silurian system in canada, in the lowest of which the eozoon is found. sir w. logan states that their "united thickness may possibly far surpass that of all the succeeding rocks, from the base of the palaeozoic series to the present time. we are thus carried back to a period so remote, that the appearance of the so-called primordial fauna (of barrande) may by some be considered as a comparatively modern event." the eozoon belongs to the most lowly organised of all classes of animals, but is highly organised for its class; it existed in countless numbers, and, as dr. dawson has remarked, certainly preyed on other minute organic beings, which must have lived in great numbers. thus the words, which i wrote in , about the existence of living beings long before the cambrian period, and which are almost the same with those since used by sir w. logan, have proved true. nevertheless, the difficulty of assigning any good reason for the absence of vast piles of strata rich in fossils beneath the cambrian system is very great. it does not seem probable that the most ancient beds have been quite worn away by denudation, or that their fossils have been wholly obliterated by metamorphic action, for if this had been the case we should have found only small remnants of the formations next succeeding them in age, and these would always have existed in a partially metamorphosed condition. but the descriptions which we possess of the silurian deposits over immense territories in russia and in north america, do not support the view that the older a formation is the more invariably it has suffered extreme denudation and metamorphism. the case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained. to show that it may hereafter receive some explanation, i will give the following hypothesis. from the nature of the organic remains which do not appear to have inhabited profound depths, in the several formations of europe and of the united states; and from the amount of sediment, miles in thickness, of which the formations are composed, we may infer that from first to last large islands or tracts of land, whence the sediment was derived, occurred in the neighbourhood of the now existing continents of europe and north america. this same view has since been maintained by agassiz and others. but we do not know what was the state of things in the intervals between the several successive formations; whether europe and the united states during these intervals existed as dry land, or as a submarine surface near land, on which sediment was not deposited, or as the bed of an open and unfathomable sea. looking to the existing oceans, which are thrice as extensive as the land, we see them studded with many islands; but hardly one truly oceanic island (with the exception of new zealand, if this can be called a truly oceanic island) is as yet known to afford even a remnant of any palaeozoic or secondary formation. hence, we may perhaps infer, that during the palaeozoic and secondary periods, neither continents nor continental islands existed where our oceans now extend; for had they existed, palaeozoic and secondary formations would in all probability have been accumulated from sediment derived from their wear and tear; and would have been at least partially upheaved by the oscillations of level, which must have intervened during these enormously long periods. if, then, we may infer anything from these facts, we may infer that, where our oceans now extend, oceans have extended from the remotest period of which we have any record; and on the other hand, that where continents now exist, large tracts of land have existed, subjected, no doubt, to great oscillations of level, since the cambrian period. the coloured map appended to my volume on coral reefs, led me to conclude that the great oceans are still mainly areas of subsidence, the great archipelagoes still areas of oscillations of level, and the continents areas of elevation. but we have no reason to assume that things have thus remained from the beginning of the world. our continents seem to have been formed by a preponderance, during many oscillations of level, of the force of elevation. but may not the areas of preponderant movement have changed in the lapse of ages? at a period long antecedent to the cambrian epoch, continents may have existed where oceans are now spread out, and clear and open oceans may have existed where our continents now stand. nor should we be justified in assuming that if, for instance, the bed of the pacific ocean were now converted into a continent we should there find sedimentary formations, in recognisable condition, older than the cambrian strata, supposing such to have been formerly deposited; for it might well happen that strata which had subsided some miles nearer to the centre of the earth, and which had been pressed on by an enormous weight of superincumbent water, might have undergone far more metamorphic action than strata which have always remained nearer to the surface. the immense areas in some parts of the world, for instance in south america, of naked metamorphic rocks, which must have been heated under great pressure, have always seemed to me to require some special explanation; and we may perhaps believe that we see in these large areas the many formations long anterior to the cambrian epoch in a completely metamorphosed and denuded condition. the several difficulties here discussed, namely, that, though we find in our geological formations many links between the species which now exist and which formerly existed, we do not find infinitely numerous fine transitional forms closely joining them all together. the sudden manner in which several groups of species first appear in our european formations, the almost entire absence, as at present known, of formations rich in fossils beneath the cambrian strata, are all undoubtedly of the most serious nature. we see this in the fact that the most eminent palaeontologists, namely, cuvier, agassiz, barrande, pictet, falconer, e. forbes, etc., and all our greatest geologists, as lyell, murchison, sedgwick, etc., have unanimously, often vehemently, maintained the immutability of species. but sir charles lyell now gives the support of his high authority to the opposite side, and most geologists and palaeontologists are much shaken in their former belief. those who believe that the geological record is in any degree perfect, will undoubtedly at once reject my theory. for my part, following out lyell's metaphor, i look at the geological record as a history of the world imperfectly kept and written in a changing dialect. of this history we possess the last volume alone, relating only to two or three countries. of this volume, only here and there a short chapter has been preserved, and of each page, only here and there a few lines. each word of the slowly-changing language, more or less different in the successive chapters, may represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to have been abruptly introduced. on this view the difficulties above discussed are greatly diminished or even disappear. chapter xi. on the geological succession of organic beings. on the slow and successive appearance of new species--on their different rates of change--species once lost do not reappear--groups of species follow the same general rules in their appearance and disappearance as do single species--on extinction--on simultaneous changes in the forms of life throughout the world--on the affinities of extinct species to each other and to living species--on the state of development of ancient forms--on the succession of the same types within the same areas--summary of preceding and present chapters. let us now see whether the several facts and laws relating to the geological succession of organic beings accord best with the common view of the immutability of species, or with that of their slow and gradual modification, through variation and natural selection. new species have appeared very slowly, one after another, both on the land and in the waters. lyell has shown that it is hardly possible to resist the evidence on this head in the case of the several tertiary stages; and every year tends to fill up the blanks between the stages, and to make the proportion between the lost and existing forms more gradual. in some of the most recent beds, though undoubtedly of high antiquity if measured by years, only one or two species are extinct, and only one or two are new, having appeared there for the first time, either locally, or, as far as we know, on the face of the earth. the secondary formations are more broken; but, as bronn has remarked, neither the appearance nor disappearance of the many species embedded in each formation has been simultaneous. species belonging to different genera and classes have not changed at the same rate, or in the same degree. in the older tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms. falconer has given a striking instance of a similar fact, for an existing crocodile is associated with many lost mammals and reptiles in the sub-himalayan deposits. the silurian lingula differs but little from the living species of this genus; whereas most of the other silurian molluscs and all the crustaceans have changed greatly. the productions of the land seem to have changed at a quicker rate than those of the sea, of which a striking instance has been observed in switzerland. there is some reason to believe that organisms high in the scale, change more quickly than those that are low: though there are exceptions to this rule. the amount of organic change, as pictet has remarked, is not the same in each successive so-called formation. yet if we compare any but the most closely related formations, all the species will be found to have undergone some change. when a species has once disappeared from the face of the earth, we have no reason to believe that the same identical form ever reappears. the strongest apparent exception to this latter rule is that of the so-called "colonies" of m. barrande, which intrude for a period in the midst of an older formation, and then allow the pre-existing fauna to reappear; but lyell's explanation, namely, that it is a case of temporary migration from a distinct geographical province, seems satisfactory. these several facts accord well with our theory, which includes no fixed law of development, causing all the inhabitants of an area to change abruptly, or simultaneously, or to an equal degree. the process of modification must be slow, and will generally affect only a few species at the same time; for the variability of each species is independent of that of all others. whether such variations or individual differences as may arise will be accumulated through natural selection in a greater or less degree, thus causing a greater or less amount of permanent modification, will depend on many complex contingencies--on the variations being of a beneficial nature, on the freedom of intercrossing, on the slowly changing physical conditions of the country, on the immigration of new colonists, and on the nature of the other inhabitants with which the varying species come into competition. hence it is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, should change in a less degree. we find similar relations between the existing inhabitants of distinct countries; for instance, the land-shells and coleopterous insects of madeira have come to differ considerably from their nearest allies on the continent of europe, whereas the marine shells and birds have remained unaltered. we can perhaps understand the apparently quicker rate of change in terrestrial and in more highly organised productions compared with marine and lower productions, by the more complex relations of the higher beings to their organic and inorganic conditions of life, as explained in a former chapter. when many of the inhabitants of any area have become modified and improved, we can understand, on the principle of competition, and from the all-important relations of organism to organism in the struggle for life, that any form which did not become in some degree modified and improved, would be liable to extermination. hence, we see why all the species in the same region do at last, if we look to long enough intervals of time, become modified; for otherwise they would become extinct. in members of the same class the average amount of change, during long and equal periods of time, may, perhaps, be nearly the same; but as the accumulation of enduring formations, rich in fossils, depends on great masses of sediment being deposited on subsiding areas, our formations have been almost necessarily accumulated at wide and irregularly intermittent intervals of time; consequently the amount of organic change exhibited by the fossils embedded in consecutive formations is not equal. each formation, on this view, does not mark a new and complete act of creation, but only an occasional scene, taken almost at hazard, in an ever slowly changing drama. we can clearly understand why a species when once lost should never reappear, even if the very same conditions of life, organic and inorganic, should recur. for though the offspring of one species might be adapted (and no doubt this has occurred in innumerable instances) to fill the place of another species in the economy of nature, and thus supplant it; yet the two forms--the old and the new--would not be identically the same; for both would almost certainly inherit different characters from their distinct progenitors; and organisms already differing would vary in a different manner. for instance, it is possible, if all our fantail-pigeons were destroyed, that fanciers might make a new breed hardly distinguishable from the present breed; but if the parent rock-pigeon were likewise destroyed, and under nature we have every reason to believe that parent forms are generally supplanted and exterminated by their improved offspring, it is incredible that a fantail, identical with the existing breed, could be raised from any other species of pigeon, or even from any other well established race of the domestic pigeon, for the successive variations would almost certainly be in some degree different, and the newly-formed variety would probably inherit from its progenitor some characteristic differences. groups of species, that is, genera and families, follow the same general rules in their appearance and disappearance as do single species, changing more or less quickly, and in a greater or lesser degree. a group, when it has once disappeared, never reappears; that is, its existence, as long as it lasts, is continuous. i am aware that there are some apparent exceptions to this rule, but the exceptions are surprisingly few, so few that e. forbes, pictet, and woodward (though all strongly opposed to such views as i maintain) admit its truth; and the rule strictly accords with the theory. for all the species of the same group, however long it may have lasted, are the modified descendants one from the other, and all from a common progenitor. in the genus lingula, for instance, the species which have successively appeared at all ages must have been connected by an unbroken series of generations, from the lowest silurian stratum to the present day. we have seen in the last chapter that whole groups of species sometimes falsely appear to have been abruptly developed; and i have attempted to give an explanation of this fact, which if true would be fatal to my views. but such cases are certainly exceptional; the general rule being a gradual increase in number, until the group reaches its maximum, and then, sooner or later, a gradual decrease. if the number of the species included within a genus, or the number of the genera within a family, be represented by a vertical line of varying thickness, ascending through the successive geological formations, in which the species are found, the line will sometimes falsely appear to begin at its lower end, not in a sharp point, but abruptly; it then gradually thickens upwards, often keeping of equal thickness for a space, and ultimately thins out in the upper beds, marking the decrease and final extinction of the species. this gradual increase in number of the species of a group is strictly conformable with the theory; for the species of the same genus, and the genera of the same family, can increase only slowly and progressively; the process of modification and the production of a number of allied forms necessarily being a slow and gradual process, one species first giving rise to two or three varieties, these being slowly converted into species, which in their turn produce by equally slow steps other varieties and species, and so on, like the branching of a great tree from a single stem, till the group becomes large. on extinction. we have as yet only spoken incidentally of the disappearance of species and of groups of species. on the theory of natural selection, the extinction of old forms and the production of new and improved forms are intimately connected together. the old notion of all the inhabitants of the earth having been swept away by catastrophes at successive periods is very generally given up, even by those geologists, as elie de beaumont, murchison, barrande, etc., whose general views would naturally lead them to this conclusion. on the contrary, we have every reason to believe, from the study of the tertiary formations, that species and groups of species gradually disappear, one after another, first from one spot, then from another, and finally from the world. in some few cases, however, as by the breaking of an isthmus and the consequent irruption of a multitude of new inhabitants into an adjoining sea, or by the final subsidence of an island, the process of extinction may have been rapid. both single species and whole groups of species last for very unequal periods; some groups, as we have seen, have endured from the earliest known dawn of life to the present day; some have disappeared before the close of the palaeozoic period. no fixed law seems to determine the length of time during which any single species or any single genus endures. there is reason to believe that the extinction of a whole group of species is generally a slower process than their production: if their appearance and disappearance be represented, as before, by a vertical line of varying thickness the line is found to taper more gradually at its upper end, which marks the progress of extermination, than at its lower end, which marks the first appearance and the early increase in number of the species. in some cases, however, the extermination of whole groups, as of ammonites, towards the close of the secondary period, has been wonderfully sudden. the extinction of species has been involved in the most gratuitous mystery. some authors have even supposed that, as the individual has a definite length of life, so have species a definite duration. no one can have marvelled more than i have done at the extinction of species. when i found in la plata the tooth of a horse embedded with the remains of mastodon, megatherium, toxodon and other extinct monsters, which all co-existed with still living shells at a very late geological period, i was filled with astonishment; for, seeing that the horse, since its introduction by the spaniards into south america, has run wild over the whole country and has increased in numbers at an unparalleled rate, i asked myself what could so recently have exterminated the former horse under conditions of life apparently so favourable. but my astonishment was groundless. professor owen soon perceived that the tooth, though so like that of the existing horse, belonged to an extinct species. had this horse been still living, but in some degree rare, no naturalist would have felt the least surprise at its rarity; for rarity is the attribute of a vast number of species of all classes, in all countries. if we ask ourselves why this or that species is rare, we answer that something is unfavourable in its conditions of life; but what that something is, we can hardly ever tell. on the supposition of the fossil horse still existing as a rare species, we might have felt certain, from the analogy of all other mammals, even of the slow-breeding elephant, and from the history of the naturalisation of the domestic horse in south america, that under more favourable conditions it would in a very few years have stocked the whole continent. but we could not have told what the unfavourable conditions were which checked its increase, whether some one or several contingencies, and at what period of the horse's life, and in what degree they severally acted. if the conditions had gone on, however slowly, becoming less and less favourable, we assuredly should not have perceived the fact, yet the fossil horse would certainly have become rarer and rarer, and finally extinct--its place being seized on by some more successful competitor. it is most difficult always to remember that the increase of every living creature is constantly being checked by unperceived hostile agencies; and that these same unperceived agencies are amply sufficient to cause rarity, and finally extinction. so little is this subject understood, that i have heard surprise repeatedly expressed at such great monsters as the mastodon and the more ancient dinosaurians having become extinct; as if mere bodily strength gave victory in the battle of life. mere size, on the contrary, would in some cases determine, as has been remarked by owen, quicker extermination, from the greater amount of requisite food. before man inhabited india or africa, some cause must have checked the continued increase of the existing elephant. a highly capable judge, dr. falconer, believes that it is chiefly insects which, from incessantly harassing and weakening the elephant in india, check its increase; and this was bruce's conclusion with respect to the african elephant in abyssinia. it is certain that insects and blood-sucking bats determine the existence of the larger naturalised quadrupeds in several parts of south america. we see in many cases in the more recent tertiary formations that rarity precedes extinction; and we know that this has been the progress of events with those animals which have been exterminated, either locally or wholly, through man's agency. i may repeat what i published in , namely, that to admit that species generally become rare before they become extinct--to feel no surprise at the rarity of a species, and yet to marvel greatly when the species ceases to exist, is much the same as to admit that sickness in the individual is the forerunner of death--to feel no surprise at sickness, but, when the sick man dies, to wonder and to suspect that he died by some deed of violence. the theory of natural selection is grounded on the belief that each new variety and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of less-favoured forms almost inevitably follows. it is the same with our domestic productions: when a new and slightly improved variety has been raised, it at first supplants the less improved varieties in the same neighbourhood; when much improved it is transported far and near, like our short-horn cattle, and takes the place of other breeds in other countries. thus the appearance of new forms and the disappearance of old forms, both those naturally and artificially produced, are bound together. in flourishing groups, the number of new specific forms which have been produced within a given time has at some periods probably been greater than the number of the old specific forms which have been exterminated; but we know that species have not gone on indefinitely increasing, at least during the later geological epochs, so that, looking to later times, we may believe that the production of new forms has caused the extinction of about the same number of old forms. the competition will generally be most severe, as formerly explained and illustrated by examples, between the forms which are most like each other in all respects. hence the improved and modified descendants of a species will generally cause the extermination of the parent-species; and if many new forms have been developed from any one species, the nearest allies of that species, i.e. the species of the same genus, will be the most liable to extermination. thus, as i believe, a number of new species descended from one species, that is a new genus, comes to supplant an old genus, belonging to the same family. but it must often have happened that a new species belonging to some one group has seized on the place occupied by a species belonging to a distinct group, and thus have caused its extermination. if many allied forms be developed from the successful intruder, many will have to yield their places; and it will generally be the allied forms, which will suffer from some inherited inferiority in common. but whether it be species belonging to the same or to a distinct class, which have yielded their places to other modified and improved species, a few of the sufferers may often be preserved for a long time, from being fitted to some peculiar line of life, or from inhabiting some distant and isolated station, where they will have escaped severe competition. for instance, some species of trigonia, a great genus of shells in the secondary formations, survive in the australian seas; and a few members of the great and almost extinct group of ganoid fishes still inhabit our fresh waters. therefore, the utter extinction of a group is generally, as we have seen, a slower process than its production. with respect to the apparently sudden extermination of whole families or orders, as of trilobites at the close of the palaeozoic period, and of ammonites at the close of the secondary period, we must remember what has been already said on the probable wide intervals of time between our consecutive formations; and in these intervals there may have been much slow extermination. moreover, when, by sudden immigration or by unusually rapid development, many species of a new group have taken possession of an area, many of the older species will have been exterminated in a correspondingly rapid manner; and the forms which thus yield their places will commonly be allied, for they will partake of the same inferiority in common. thus, as it seems to me, the manner in which single species and whole groups of species become extinct accords well with the theory of natural selection. we need not marvel at extinction; if we must marvel, let it be at our presumption in imagining for a moment that we understand the many complex contingencies on which the existence of each species depends. if we forget for an instant that each species tends to increase inordinately, and that some check is always in action, yet seldom perceived by us, the whole economy of nature will be utterly obscured. whenever we can precisely say why this species is more abundant in individuals than that; why this species and not another can be naturalised in a given country; then, and not until then, we may justly feel surprise why we cannot account for the extinction of any particular species or group of species. on the forms of life changing almost simultaneously throughout the world. scarcely any palaeontological discovery is more striking than the fact that the forms of life change almost simultaneously throughout the world. thus our european chalk formation can be recognised in many distant regions, under the most different climates, where not a fragment of the mineral chalk itself can be found; namely, in north america, in equatorial south america, in tierra del fuego, at the cape of good hope, and in the peninsula of india. for at these distant points, the organic remains in certain beds present an unmistakable resemblance to those of the chalk. it is not that the same species are met with; for in some cases not one species is identically the same, but they belong to the same families, genera, and sections of genera, and sometimes are similarly characterised in such trifling points as mere superficial sculpture. moreover, other forms, which are not found in the chalk of europe, but which occur in the formations either above or below, occur in the same order at these distant points of the world. in the several successive palaeozoic formations of russia, western europe and north america, a similar parallelism in the forms of life has been observed by several authors; so it is, according to lyell, with the european and north american tertiary deposits. even if the few fossil species which are common to the old and new worlds were kept wholly out of view, the general parallelism in the successive forms of life, in the palaeozoic and tertiary stages, would still be manifest, and the several formations could be easily correlated. these observations, however, relate to the marine inhabitants of the world: we have not sufficient data to judge whether the productions of the land and of fresh water at distant points change in the same parallel manner. we may doubt whether they have thus changed: if the megatherium, mylodon, macrauchenia, and toxodon had been brought to europe from la plata, without any information in regard to their geological position, no one would have suspected that they had co-existed with sea-shells all still living; but as these anomalous monsters co-existed with the mastodon and horse, it might at least have been inferred that they had lived during one of the later tertiary stages. when the marine forms of life are spoken of as having changed simultaneously throughout the world, it must not be supposed that this expression relates to the same year, or even to the same century, or even that it has a very strict geological sense; for if all the marine animals now living in europe, and all those that lived in europe during the pleistocene period (a very remote period as measured by years, including the whole glacial epoch) were compared with those now existing in south america or in australia, the most skilful naturalist would hardly be able to say whether the present or the pleistocene inhabitants of europe resembled most closely those of the southern hemisphere. so, again, several highly competent observers maintain that the existing productions of the united states are more closely related to those which lived in europe during certain late tertiary stages, than to the present inhabitants of europe; and if this be so, it is evident that fossiliferous beds now deposited on the shores of north america would hereafter be liable to be classed with somewhat older european beds. nevertheless, looking to a remotely future epoch, there can be little doubt that all the more modern marine formations, namely, the upper pliocene, the pleistocene and strictly modern beds of europe, north and south america, and australia, from containing fossil remains in some degree allied, and from not including those forms which are found only in the older underlying deposits, would be correctly ranked as simultaneous in a geological sense. the fact of the forms of life changing simultaneously in the above large sense, at distant parts of the world, has greatly struck those admirable observers, mm. de verneuil and d'archiac. after referring to the parallelism of the palaeozoic forms of life in various parts of europe, they add, "if struck by this strange sequence, we turn our attention to north america, and there discover a series of analogous phenomena, it will appear certain that all these modifications of species, their extinction, and the introduction of new ones, cannot be owing to mere changes in marine currents or other causes more or less local and temporary, but depend on general laws which govern the whole animal kingdom." m. barrande has made forcible remarks to precisely the same effect. it is, indeed, quite futile to look to changes of currents, climate, or other physical conditions, as the cause of these great mutations in the forms of life throughout the world, under the most different climates. we must, as barrande has remarked, look to some special law. we shall see this more clearly when we treat of the present distribution of organic beings, and find how slight is the relation between the physical conditions of various countries and the nature of their inhabitants. this great fact of the parallel succession of the forms of life throughout the world, is explicable on the theory of natural selection. new species are formed by having some advantage over older forms; and the forms, which are already dominant, or have some advantage over the other forms in their own country, give birth to the greatest number of new varieties or incipient species. we have distinct evidence on this head, in the plants which are dominant, that is, which are commonest and most widely diffused, producing the greatest number of new varieties. it is also natural that the dominant, varying and far-spreading species, which have already invaded, to a certain extent, the territories of other species, should be those which would have the best chance of spreading still further, and of giving rise in new countries to other new varieties and species. the process of diffusion would often be very slow, depending on climatal and geographical changes, on strange accidents, and on the gradual acclimatization of new species to the various climates through which they might have to pass, but in the course of time the dominant forms would generally succeed in spreading and would ultimately prevail. the diffusion would, it is probable, be slower with the terrestrial inhabitants of distinct continents than with the marine inhabitants of the continuous sea. we might therefore expect to find, as we do find, a less strict degree of parallelism in the succession of the productions of the land than with those of the sea. thus, as it seems to me, the parallel, and, taken in a large sense, simultaneous, succession of the same forms of life throughout the world, accords well with the principle of new species having been formed by dominant species spreading widely and varying; the new species thus produced being themselves dominant, owing to their having had some advantage over their already dominant parents, as well as over other species; and again spreading, varying, and producing new forms. the old forms which are beaten and which yield their places to the new and victorious forms, will generally be allied in groups, from inheriting some inferiority in common; and, therefore, as new and improved groups spread throughout the world, old groups disappear from the world; and the succession of forms everywhere tends to correspond both in their first appearance and final disappearance. there is one other remark connected with this subject worth making. i have given my reasons for believing that most of our great formations, rich in fossils, were deposited during periods of subsidence; and that blank intervals of vast duration, as far as fossils are concerned, occurred during the periods when the bed of the sea was either stationary or rising, and likewise when sediment was not thrown down quickly enough to embed and preserve organic remains. during these long and blank intervals i suppose that the inhabitants of each region underwent a considerable amount of modification and extinction, and that there was much migration from other parts of the world. as we have reason to believe that large areas are affected by the same movement, it is probable that strictly contemporaneous formations have often been accumulated over very wide spaces in the same quarter of the world; but we are very far from having any right to conclude that this has invariably been the case, and that large areas have invariably been affected by the same movements. when two formations have been deposited in two regions during nearly, but not exactly, the same period, we should find in both, from the causes explained in the foregoing paragraphs, the same general succession in the forms of life; but the species would not exactly correspond; for there will have been a little more time in the one region than in the other for modification, extinction, and immigration. i suspect that cases of this nature occur in europe. mr. prestwich, in his admirable memoirs on the eocene deposits of england and france, is able to draw a close general parallelism between the successive stages in the two countries; but when he compares certain stages in england with those in france, although he finds in both a curious accordance in the numbers of the species belonging to the same genera, yet the species themselves differ in a manner very difficult to account for considering the proximity of the two areas, unless, indeed, it be assumed that an isthmus separated two seas inhabited by distinct, but contemporaneous faunas. lyell has made similar observations on some of the later tertiary formations. barrande, also, shows that there is a striking general parallelism in the successive silurian deposits of bohemia and scandinavia; nevertheless he finds a surprising amount of difference in the species. if the several formations in these regions have not been deposited during the same exact periods--a formation in one region often corresponding with a blank interval in the other--and if in both regions the species have gone on slowly changing during the accumulation of the several formations and during the long intervals of time between them; in this case the several formations in the two regions could be arranged in the same order, in accordance with the general succession of the forms of life, and the order would falsely appear to be strictly parallel; nevertheless the species would not all be the same in the apparently corresponding stages in the two regions. on the affinities of extinct species to each other, and to living forms. let us now look to the mutual affinities of extinct and living species. all fall into a few grand classes; and this fact is at once explained on the principle of descent. the more ancient any form is, the more, as a general rule, it differs from living forms. but, as buckland long ago remarked, extinct species can all be classed either in still existing groups, or between them. that the extinct forms of life help to fill up the intervals between existing genera, families, and orders, is certainly true; but as this statement has often been ignored or even denied, it may be well to make some remarks on this subject, and to give some instances. if we confine our attention either to the living or to the extinct species of the same class, the series is far less perfect than if we combine both into one general system. in the writings of professor owen we continually meet with the expression of generalised forms, as applied to extinct animals; and in the writings of agassiz, of prophetic or synthetic types; and these terms imply that such forms are, in fact, intermediate or connecting links. another distinguished palaeontologist, m. gaudry, has shown in the most striking manner that many of the fossil mammals discovered by him in attica serve to break down the intervals between existing genera. cuvier ranked the ruminants and pachyderms as two of the most distinct orders of mammals; but so many fossil links have been disentombed that owen has had to alter the whole classification, and has placed certain pachyderms in the same sub-order with ruminants; for example, he dissolves by gradations the apparently wide interval between the pig and the camel. the ungulata or hoofed quadrupeds are now divided into the even-toed or odd-toed divisions; but the macrauchenia of south america connects to a certain extent these two grand divisions. no one will deny that the hipparion is intermediate between the existing horse and certain other ungulate forms. what a wonderful connecting link in the chain of mammals is the typotherium from south america, as the name given to it by professor gervais expresses, and which cannot be placed in any existing order. the sirenia form a very distinct group of the mammals, and one of the most remarkable peculiarities in existing dugong and lamentin is the entire absence of hind limbs, without even a rudiment being left; but the extinct halitherium had, according to professor flower, an ossified thigh-bone "articulated to a well-defined acetabulum in the pelvis," and it thus makes some approach to ordinary hoofed quadrupeds, to which the sirenia are in other respects allied. the cetaceans or whales are widely different from all other mammals, but the tertiary zeuglodon and squalodon, which have been placed by some naturalists in an order by themselves, are considered by professor huxley to be undoubtedly cetaceans, "and to constitute connecting links with the aquatic carnivora." even the wide interval between birds and reptiles has been shown by the naturalist just quoted to be partially bridged over in the most unexpected manner, on the one hand, by the ostrich and extinct archeopteryx, and on the other hand by the compsognathus, one of the dinosaurians--that group which includes the most gigantic of all terrestrial reptiles. turning to the invertebrata, barrande asserts, a higher authority could not be named, that he is every day taught that, although palaeozoic animals can certainly be classed under existing groups, yet that at this ancient period the groups were not so distinctly separated from each other as they now are. some writers have objected to any extinct species, or group of species, being considered as intermediate between any two living species, or groups of species. if by this term it is meant that an extinct form is directly intermediate in all its characters between two living forms or groups, the objection is probably valid. but in a natural classification many fossil species certainly stand between living species, and some extinct genera between living genera, even between genera belonging to distinct families. the most common case, especially with respect to very distinct groups, such as fish and reptiles, seems to be that, supposing them to be distinguished at the present day by a score of characters, the ancient members are separated by a somewhat lesser number of characters, so that the two groups formerly made a somewhat nearer approach to each other than they now do. it is a common belief that the more ancient a form is, by so much the more it tends to connect by some of its characters groups now widely separated from each other. this remark no doubt must be restricted to those groups which have undergone much change in the course of geological ages; and it would be difficult to prove the truth of the proposition, for every now and then even a living animal, as the lepidosiren, is discovered having affinities directed towards very distinct groups. yet if we compare the older reptiles and batrachians, the older fish, the older cephalopods, and the eocene mammals, with the recent members of the same classes, we must admit that there is truth in the remark. let us see how far these several facts and inferences accord with the theory of descent with modification. as the subject is somewhat complex, i must request the reader to turn to the diagram in the fourth chapter. we may suppose that the numbered letters in italics represent genera, and the dotted lines diverging from them the species in each genus. the diagram is much too simple, too few genera and too few species being given, but this is unimportant for us. the horizontal lines may represent successive geological formations, and all the forms beneath the uppermost line may be considered as extinct. the three existing genera, a , q , p , will form a small family; b and f , a closely allied family or subfamily; and o , i , m , a third family. these three families, together with the many extinct genera on the several lines of descent diverging from the parent form (a) will form an order; for all will have inherited something in common from their ancient progenitor. on the principle of the continued tendency to divergence of character, which was formerly illustrated by this diagram, the more recent any form is the more it will generally differ from its ancient progenitor. hence, we can understand the rule that the most ancient fossils differ most from existing forms. we must not, however, assume that divergence of character is a necessary contingency; it depends solely on the descendants from a species being thus enabled to seize on many and different places in the economy of nature. therefore it is quite possible, as we have seen in the case of some silurian forms, that a species might go on being slightly modified in relation to its slightly altered conditions of life, and yet retain throughout a vast period the same general characteristics. this is represented in the diagram by the letter f . all the many forms, extinct and recent, descended from (a), make, as before remarked, one order; and this order, from the continued effects of extinction and divergence of character, has become divided into several sub-families and families, some of which are supposed to have perished at different periods, and some to have endured to the present day. by looking at the diagram we can see that if many of the extinct forms supposed to be embedded in the successive formations, were discovered at several points low down in the series, the three existing families on the uppermost line would be rendered less distinct from each other. if, for instance, the genera a , a , a , f , m , m , m , were disinterred, these three families would be so closely linked together that they probably would have to be united into one great family, in nearly the same manner as has occurred with ruminants and certain pachyderms. yet he who objected to consider as intermediate the extinct genera, which thus link together the living genera of three families, would be partly justified, for they are intermediate, not directly, but only by a long and circuitous course through many widely different forms. if many extinct forms were to be discovered above one of the middle horizontal lines or geological formations--for instance, above no. vi.--but none from beneath this line, then only two of the families (those on the left hand a , etc., and b , etc.) would have to be united into one; and there would remain two families which would be less distinct from each other than they were before the discovery of the fossils. so again, if the three families formed of eight genera (a to m ), on the uppermost line, be supposed to differ from each other by half-a-dozen important characters, then the families which existed at a period marked vi would certainly have differed from each other by a less number of characters; for they would at this early stage of descent have diverged in a less degree from their common progenitor. thus it comes that ancient and extinct genera are often in a greater or less degree intermediate in character between their modified descendants, or between their collateral relations. under nature the process will be far more complicated than is represented in the diagram; for the groups will have been more numerous; they will have endured for extremely unequal lengths of time, and will have been modified in various degrees. as we possess only the last volume of the geological record, and that in a very broken condition, we have no right to expect, except in rare cases, to fill up the wide intervals in the natural system, and thus to unite distinct families or orders. all that we have a right to expect is, that those groups which have, within known geological periods, undergone much modification, should in the older formations make some slight approach to each other; so that the older members should differ less from each other in some of their characters than do the existing members of the same groups; and this by the concurrent evidence of our best palaeontologists is frequently the case. thus, on the theory of descent with modification, the main facts with respect to the mutual affinities of the extinct forms of life to each other and to living forms, are explained in a satisfactory manner. and they are wholly inexplicable on any other view. on this same theory, it is evident that the fauna during any one great period in the earth's history will be intermediate in general character between that which preceded and that which succeeded it. thus the species which lived at the sixth great stage of descent in the diagram are the modified offspring of those which lived at the fifth stage, and are the parents of those which became still more modified at the seventh stage; hence they could hardly fail to be nearly intermediate in character between the forms of life above and below. we must, however, allow for the entire extinction of some preceding forms, and in any one region for the immigration of new forms from other regions, and for a large amount of modification during the long and blank intervals between the successive formations. subject to these allowances, the fauna of each geological period undoubtedly is intermediate in character, between the preceding and succeeding faunas. i need give only one instance, namely, the manner in which the fossils of the devonian system, when this system was first discovered, were at once recognised by palaeontologists as intermediate in character between those of the overlying carboniferous and underlying silurian systems. but each fauna is not necessarily exactly intermediate, as unequal intervals of time have elapsed between consecutive formations. it is no real objection to the truth of the statement that the fauna of each period as a whole is nearly intermediate in character between the preceding and succeeding faunas, that certain genera offer exceptions to the rule. for instance, the species of mastodons and elephants, when arranged by dr. falconer in two series--in the first place according to their mutual affinities, and in the second place according to their periods of existence--do not accord in arrangement. the species extreme in character are not the oldest or the most recent; nor are those which are intermediate in character, intermediate in age. but supposing for an instant, in this and other such cases, that the record of the first appearance and disappearance of the species was complete, which is far from the case, we have no reason to believe that forms successively produced necessarily endure for corresponding lengths of time. a very ancient form may occasionally have lasted much longer than a form elsewhere subsequently produced, especially in the case of terrestrial productions inhabiting separated districts. to compare small things with great; if the principal living and extinct races of the domestic pigeon were arranged in serial affinity, this arrangement would not closely accord with the order in time of their production, and even less with the order of their disappearance; for the parent rock-pigeon still lives; and many varieties between the rock-pigeon and the carrier have become extinct; and carriers which are extreme in the important character of length of beak originated earlier than short-beaked tumblers, which are at the opposite end of the series in this respect. closely connected with the statement, that the organic remains from an intermediate formation are in some degree intermediate in character, is the fact, insisted on by all palaeontologists, that fossils from two consecutive formations are far more closely related to each other, than are the fossils from two remote formations. pictet gives as a well-known instance, the general resemblance of the organic remains from the several stages of the chalk formation, though the species are distinct in each stage. this fact alone, from its generality, seems to have shaken professor pictet in his belief in the immutability of species. he who is acquainted with the distribution of existing species over the globe, will not attempt to account for the close resemblance of distinct species in closely consecutive formations, by the physical conditions of the ancient areas having remained nearly the same. let it be remembered that the forms of life, at least those inhabiting the sea, have changed almost simultaneously throughout the world, and therefore under the most different climates and conditions. consider the prodigious vicissitudes of climate during the pleistocene period, which includes the whole glacial epoch, and note how little the specific forms of the inhabitants of the sea have been affected. on the theory of descent, the full meaning of the fossil remains from closely consecutive formations, being closely related, though ranked as distinct species, is obvious. as the accumulation of each formation has often been interrupted, and as long blank intervals have intervened between successive formations, we ought not to expect to find, as i attempted to show in the last chapter, in any one or in any two formations, all the intermediate varieties between the species which appeared at the commencement and close of these periods: but we ought to find after intervals, very long as measured by years, but only moderately long as measured geologically, closely allied forms, or, as they have been called by some authors, representative species; and these assuredly we do find. we find, in short, such evidence of the slow and scarcely sensible mutations of specific forms, as we have the right to expect. on the state of development of ancient compared with living forms. we have seen in the fourth chapter that the degree of differentiation and specialisation of the parts in organic beings, when arrived at maturity, is the best standard, as yet suggested, of their degree of perfection or highness. we have also seen that, as the specialisation of parts is an advantage to each being, so natural selection will tend to render the organisation of each being more specialised and perfect, and in this sense higher; not but that it may leave many creatures with simple and unimproved structures fitted for simple conditions of life, and in some cases will even degrade or simplify the organisation, yet leaving such degraded beings better fitted for their new walks of life. in another and more general manner, new species become superior to their predecessors; for they have to beat in the struggle for life all the older forms, with which they come into close competition. we may therefore conclude that if under a nearly similar climate the eocene inhabitants of the world could be put into competition with the existing inhabitants, the former would be beaten and exterminated by the latter, as would the secondary by the eocene, and the palaeozoic by the secondary forms. so that by this fundamental test of victory in the battle for life, as well as by the standard of the specialisation of organs, modern forms ought, on the theory of natural selection, to stand higher than ancient forms. is this the case? a large majority of palaeontologists would answer in the affirmative; and it seems that this answer must be admitted as true, though difficult of proof. it is no valid objection to this conclusion, that certain brachiopods have been but slightly modified from an extremely remote geological epoch; and that certain land and fresh-water shells have remained nearly the same, from the time when, as far as is known, they first appeared. it is not an insuperable difficulty that foraminifera have not, as insisted on by dr. carpenter, progressed in organisation since even the laurentian epoch; for some organisms would have to remain fitted for simple conditions of life, and what could be better fitted for this end than these lowly organised protozoa? such objections as the above would be fatal to my view, if it included advance in organisation as a necessary contingent. they would likewise be fatal, if the above foraminifera, for instance, could be proved to have first come into existence during the laurentian epoch, or the above brachiopods during the cambrian formation; for in this case, there would not have been time sufficient for the development of these organisms up to the standard which they had then reached. when advanced up to any given point, there is no necessity, on the theory of natural selection, for their further continued process; though they will, during each successive age, have to be slightly modified, so as to hold their places in relation to slight changes in their conditions. the foregoing objections hinge on the question whether we really know how old the world is, and at what period the various forms of life first appeared; and this may well be disputed. the problem whether organisation on the whole has advanced is in many ways excessively intricate. the geological record, at all times imperfect, does not extend far enough back to show with unmistakable clearness that within the known history of the world organisation has largely advanced. even at the present day, looking to members of the same class, naturalists are not unanimous which forms ought to be ranked as highest: thus, some look at the selaceans or sharks, from their approach in some important points of structure to reptiles, as the highest fish; others look at the teleosteans as the highest. the ganoids stand intermediate between the selaceans and teleosteans; the latter at the present day are largely preponderant in number; but formerly selaceans and ganoids alone existed; and in this case, according to the standard of highness chosen, so will it be said that fishes have advanced or retrograded in organisation. to attempt to compare members of distinct types in the scale of highness seems hopeless; who will decide whether a cuttle-fish be higher than a bee--that insect which the great von baer believed to be "in fact more highly organised than a fish, although upon another type?" in the complex struggle for life it is quite credible that crustaceans, not very high in their own class, might beat cephalopods, the highest molluscs; and such crustaceans, though not highly developed, would stand very high in the scale of invertebrate animals, if judged by the most decisive of all trials--the law of battle. beside these inherent difficulties in deciding which forms are the most advanced in organisation, we ought not solely to compare the highest members of a class at any two periods--though undoubtedly this is one and perhaps the most important element in striking a balance--but we ought to compare all the members, high and low, at two periods. at an ancient epoch the highest and lowest molluscoidal animals, namely, cephalopods and brachiopods, swarmed in numbers; at the present time both groups are greatly reduced, while others, intermediate in organisation, have largely increased; consequently some naturalists maintain that molluscs were formerly more highly developed than at present; but a stronger case can be made out on the opposite side, by considering the vast reduction of brachiopods, and the fact that our existing cephalopods, though few in number, are more highly organised than their ancient representatives. we ought also to compare the relative proportional numbers, at any two periods, of the high and low classes throughout the world: if, for instance, at the present day fifty thousand kinds of vertebrate animals exist, and if we knew that at some former period only ten thousand kinds existed, we ought to look at this increase in number in the highest class, which implies a great displacement of lower forms, as a decided advance in the organisation of the world. we thus see how hopelessly difficult it is to compare with perfect fairness, under such extremely complex relations, the standard of organisation of the imperfectly-known faunas of successive periods. we shall appreciate this difficulty more clearly by looking to certain existing faunas and floras. from the extraordinary manner in which european productions have recently spread over new zealand, and have seized on places which must have been previously occupied by the indigenes, we must believe, that if all the animals and plants of great britain were set free in new zealand, a multitude of british forms would in the course of time become thoroughly naturalized there, and would exterminate many of the natives. on the other hand, from the fact that hardly a single inhabitant of the southern hemisphere has become wild in any part of europe, we may well doubt whether, if all the productions of new zealand were set free in great britain, any considerable number would be enabled to seize on places now occupied by our native plants and animals. under this point of view, the productions of great britain stand much higher in the scale than those of new zealand. yet the most skilful naturalist, from an examination of the species of the two countries, could not have foreseen this result. agassiz and several other highly competent judges insist that ancient animals resemble to a certain extent the embryos of recent animals belonging to the same classes; and that the geological succession of extinct forms is nearly parallel with the embryological development of existing forms. this view accords admirably well with our theory. in a future chapter i shall attempt to show that the adult differs from its embryo, owing to variations having supervened at a not early age, and having been inherited at a corresponding age. this process, whilst it leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more difference to the adult. thus the embryo comes to be left as a sort of picture, preserved by nature, of the former and less modified condition of the species. this view may be true, and yet may never be capable of proof. seeing, for instance, that the oldest known mammals, reptiles, and fishes strictly belong to their proper classes, though some of these old forms are in a slight degree less distinct from each other than are the typical members of the same groups at the present day, it would be vain to look for animals having the common embryological character of the vertebrata, until beds rich in fossils are discovered far beneath the lowest cambrian strata--a discovery of which the chance is small. on the succession of the same types within the same areas, during the later tertiary periods. mr. clift many years ago showed that the fossil mammals from the australian caves were closely allied to the living marsupials of that continent. in south america, a similar relationship is manifest, even to an uneducated eye, in the gigantic pieces of armour, like those of the armadillo, found in several parts of la plata; and professor owen has shown in the most striking manner that most of the fossil mammals, buried there in such numbers, are related to south american types. this relationship is even more clearly seen in the wonderful collection of fossil bones made by mm. lund and clausen in the caves of brazil. i was so much impressed with these facts that i strongly insisted, in and , on this "law of the succession of types,"--on "this wonderful relationship in the same continent between the dead and the living." professor owen has subsequently extended the same generalisation to the mammals of the old world. we see the same law in this author's restorations of the extinct and gigantic birds of new zealand. we see it also in the birds of the caves of brazil. mr. woodward has shown that the same law holds good with sea-shells, but, from the wide distribution of most molluscs, it is not well displayed by them. other cases could be added, as the relation between the extinct and living land-shells of madeira; and between the extinct and living brackish water-shells of the aralo-caspian sea. now, what does this remarkable law of the succession of the same types within the same areas mean? he would be a bold man who, after comparing the present climate of australia and of parts of south america, under the same latitude, would attempt to account, on the one hand through dissimilar physical conditions, for the dissimilarity of the inhabitants of these two continents; and, on the other hand through similarity of conditions, for the uniformity of the same types in each continent during the later tertiary periods. nor can it be pretended that it is an immutable law that marsupials should have been chiefly or solely produced in australia; or that edentata and other american types should have been solely produced in south america. for we know that europe in ancient times was peopled by numerous marsupials; and i have shown in the publications above alluded to, that in america the law of distribution of terrestrial mammals was formerly different from what it now is. north america formerly partook strongly of the present character of the southern half of the continent; and the southern half was formerly more closely allied, than it is at present, to the northern half. in a similar manner we know, from falconer and cautley's discoveries, that northern india was formerly more closely related in its mammals to africa than it is at the present time. analogous facts could be given in relation to the distribution of marine animals. on the theory of descent with modification, the great law of the long enduring, but not immutable, succession of the same types within the same areas, is at once explained; for the inhabitants of each quarter of the world will obviously tend to leave in that quarter, during the next succeeding period of time, closely allied though in some degree modified descendants. if the inhabitants of one continent formerly differed greatly from those of another continent, so will their modified descendants still differ in nearly the same manner and degree. but after very long intervals of time, and after great geographical changes, permitting much intermigration, the feebler will yield to the more dominant forms, and there will be nothing immutable in the distribution of organic beings. it may be asked in ridicule whether i suppose that the megatherium and other allied huge monsters, which formerly lived in south america, have left behind them the sloth, armadillo, and anteater, as their degenerate descendants. this cannot for an instant be admitted. these huge animals have become wholly extinct, and have left no progeny. but in the caves of brazil there are many extinct species which are closely allied in size and in all other characters to the species still living in south america; and some of these fossils may have been the actual progenitors of the living species. it must not be forgotten that, on our theory, all the species of the same genus are the descendants of some one species; so that, if six genera, each having eight species, be found in one geological formation, and in a succeeding formation there be six other allied or representative genera, each with the same number of species, then we may conclude that generally only one species of each of the older genera has left modified descendants, which constitute the new genera containing the several species; the other seven species of each old genus having died out and left no progeny. or, and this will be a far commoner case, two or three species in two or three alone of the six older genera will be the parents of the new genera: the other species and the other old genera having become utterly extinct. in failing orders, with the genera and species decreasing in numbers as is the case with the edentata of south america, still fewer genera and species will leave modified blood-descendants. summary of the preceding and present chapters. i have attempted to show that the geological record is extremely imperfect; that only a small portion of the globe has been geologically explored with care; that only certain classes of organic beings have been largely preserved in a fossil state; that the number both of specimens and of species, preserved in our museums, is absolutely as nothing compared with the number of generations which must have passed away even during a single formation; that, owing to subsidence being almost necessary for the accumulation of deposits rich in fossil species of many kinds, and thick enough to outlast future degradation, great intervals of time must have elapsed between most of our successive formations; that there has probably been more extinction during the periods of subsidence, and more variation during the periods of elevation, and during the latter the record will have been least perfectly kept; that each single formation has not been continuously deposited; that the duration of each formation is probably short compared with the average duration of specific forms; that migration has played an important part in the first appearance of new forms in any one area and formation; that widely ranging species are those which have varied most frequently, and have oftenest given rise to new species; that varieties have at first been local; and lastly, although each species must have passed through numerous transitional stages, it is probable that the periods, during which each underwent modification, though many and long as measured by years, have been short in comparison with the periods during which each remained in an unchanged condition. these causes, taken conjointly, will to a large extent explain why--though we do find many links--we do not find interminable varieties, connecting together all extinct and existing forms by the finest graduated steps. it should also be constantly borne in mind that any linking variety between two forms, which might be found, would be ranked, unless the whole chain could be perfectly restored, as a new and distinct species; for it is not pretended that we have any sure criterion by which species and varieties can be discriminated. he who rejects this view of the imperfection of the geological record, will rightly reject the whole theory. for he may ask in vain where are the numberless transitional links which must formerly have connected the closely allied or representative species, found in the successive stages of the same great formation? he may disbelieve in the immense intervals of time which must have elapsed between our consecutive formations; he may overlook how important a part migration has played, when the formations of any one great region, as those of europe, are considered; he may urge the apparent, but often falsely apparent, sudden coming in of whole groups of species. he may ask where are the remains of those infinitely numerous organisms which must have existed long before the cambrian system was deposited? we now know that at least one animal did then exist; but i can answer this last question only by supposing that where our oceans now extend they have extended for an enormous period, and where our oscillating continents now stand they have stood since the commencement of the cambrian system; but that, long before that epoch, the world presented a widely different aspect; and that the older continents, formed of formations older than any known to us, exist now only as remnants in a metamorphosed condition, or lie still buried under the ocean. passing from these difficulties, the other great leading facts in palaeontology agree admirably with the theory of descent with modification through variation and natural selection. we can thus understand how it is that new species come in slowly and successively; how species of different classes do not necessarily change together, or at the same rate, or in the same degree; yet in the long run that all undergo modification to some extent. the extinction of old forms is the almost inevitable consequence of the production of new forms. we can understand why, when a species has once disappeared, it never reappears. groups of species increase in numbers slowly, and endure for unequal periods of time; for the process of modification is necessarily slow, and depends on many complex contingencies. the dominant species belonging to large and dominant groups tend to leave many modified descendants, which form new sub-groups and groups. as these are formed, the species of the less vigorous groups, from their inferiority inherited from a common progenitor, tend to become extinct together, and to leave no modified offspring on the face of the earth. but the utter extinction of a whole group of species has sometimes been a slow process, from the survival of a few descendants, lingering in protected and isolated situations. when a group has once wholly disappeared, it does not reappear; for the link of generation has been broken. we can understand how it is that dominant forms which spread widely and yield the greatest number of varieties tend to people the world with allied, but modified, descendants; and these will generally succeed in displacing the groups which are their inferiors in the struggle for existence. hence, after long intervals of time, the productions of the world appear to have changed simultaneously. we can understand how it is that all the forms of life, ancient and recent, make together a few grand classes. we can understand, from the continued tendency to divergence of character, why the more ancient a form is, the more it generally differs from those now living. why ancient and extinct forms often tend to fill up gaps between existing forms, sometimes blending two groups, previously classed as distinct into one; but more commonly bringing them only a little closer together. the more ancient a form is, the more often it stands in some degree intermediate between groups now distinct; for the more ancient a form is, the more nearly it will be related to, and consequently resemble, the common progenitor of groups, since become widely divergent. extinct forms are seldom directly intermediate between existing forms; but are intermediate only by a long and circuitous course through other extinct and different forms. we can clearly see why the organic remains of closely consecutive formations are closely allied; for they are closely linked together by generation. we can clearly see why the remains of an intermediate formation are intermediate in character. the inhabitants of the world at each successive period in its history have beaten their predecessors in the race for life, and are, in so far, higher in the scale, and their structure has generally become more specialised; and this may account for the common belief held by so many palaeontologists, that organisation on the whole has progressed. extinct and ancient animals resemble to a certain extent the embryos of the more recent animals belonging to the same classes, and this wonderful fact receives a simple explanation according to our views. the succession of the same types of structure within the same areas during the later geological periods ceases to be mysterious, and is intelligible on the principle of inheritance. if, then, the geological record be as imperfect as many believe, and it may at least be asserted that the record cannot be proved to be much more perfect, the main objections to the theory of natural selection are greatly diminished or disappear. on the other hand, all the chief laws of palaeontology plainly proclaim, as it seems to me, that species have been produced by ordinary generation: old forms having been supplanted by new and improved forms of life, the products of variation and the survival of the fittest. chapter xii. geographical distribution. present distribution cannot be accounted for by differences in physical conditions--importance of barriers--affinity of the productions of the same continent--centres of creation--means of dispersal by changes of climate and of the level of the land, and by occasional means--dispersal during the glacial period--alternate glacial periods in the north and south. in considering the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be wholly accounted for by climatal and other physical conditions. of late, almost every author who has studied the subject has come to this conclusion. the case of america alone would almost suffice to prove its truth; for if we exclude the arctic and northern temperate parts, all authors agree that one of the most fundamental divisions in geographical distribution is that between the new and old worlds; yet if we travel over the vast american continent, from the central parts of the united states to its extreme southern point, we meet with the most diversified conditions; humid districts, arid deserts, lofty mountains, grassy plains, forests, marshes, lakes and great rivers, under almost every temperature. there is hardly a climate or condition in the old world which cannot be paralleled in the new--at least so closely as the same species generally require. no doubt small areas can be pointed out in the old world hotter than any in the new world; but these are not inhabited by a fauna different from that of the surrounding districts; for it is rare to find a group of organisms confined to a small area, of which the conditions are peculiar in only a slight degree. notwithstanding this general parallelism in the conditions of old and new worlds, how widely different are their living productions! in the southern hemisphere, if we compare large tracts of land in australia, south africa, and western south america, between latitudes and degrees, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. or, again, we may compare the productions of south america south of latitude degrees with those north of degrees, which consequently are separated by a space of ten degrees of latitude, and are exposed to considerably different conditions; yet they are incomparably more closely related to each other than they are to the productions of australia or africa under nearly the same climate. analogous facts could be given with respect to the inhabitants of the sea. a second great fact which strikes us in our general review is, that barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions. we see this in the great difference in nearly all the terrestrial productions of the new and old worlds, excepting in the northern parts, where the land almost joins, and where, under a slightly different climate, there might have been free migration for the northern temperate forms, as there now is for the strictly arctic productions. we see the same fact in the great difference between the inhabitants of australia, africa, and south america under the same latitude; for these countries are almost as much isolated from each other as is possible. on each continent, also, we see the same fact; for on the opposite sides of lofty and continuous mountain-ranges, and of great deserts and even of large rivers, we find different productions; though as mountain chains, deserts, etc., are not as impassable, or likely to have endured so long, as the oceans separating continents, the differences are very inferior in degree to those characteristic of distinct continents. turning to the sea, we find the same law. the marine inhabitants of the eastern and western shores of south america are very distinct, with extremely few shells, crustacea, or echinodermata in common; but dr. gunther has recently shown that about thirty per cent of the fishes are the same on the opposite sides of the isthmus of panama; and this fact has led naturalists to believe that the isthmus was formerly open. westward of the shores of america, a wide space of open ocean extends, with not an island as a halting-place for emigrants; here we have a barrier of another kind, and as soon as this is passed we meet in the eastern islands of the pacific with another and totally distinct fauna. so that three marine faunas range northward and southward in parallel lines not far from each other, under corresponding climate; but from being separated from each other by impassable barriers, either of land or open sea, they are almost wholly distinct. on the other hand, proceeding still further westward from the eastern islands of the tropical parts of the pacific, we encounter no impassable barriers, and we have innumerable islands as halting-places, or continuous coasts, until, after travelling over a hemisphere, we come to the shores of africa; and over this vast space we meet with no well-defined and distinct marine faunas. although so few marine animals are common to the above-named three approximate faunas of eastern and western america and the eastern pacific islands, yet many fishes range from the pacific into the indian ocean, and many shells are common to the eastern islands of the pacific and the eastern shores of africa on almost exactly opposite meridians of longitude. a third great fact, partly included in the foregoing statement, is the affinity of the productions of the same continent or of the same sea, though the species themselves are distinct at different points and stations. it is a law of the widest generality, and every continent offers innumerable instances. nevertheless, the naturalist, in travelling, for instance, from north to south, never fails to be struck by the manner in which successive groups of beings, specifically distinct, though nearly related, replace each other. he hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. the plains near the straits of magellan are inhabited by one species of rhea (american ostrich), and northward the plains of la plata by another species of the same genus; and not by a true ostrich or emu, like those inhabiting africa and australia under the same latitude. on these same plains of la plata we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits, and belonging to the same order of rodents, but they plainly display an american type of structure. we ascend the lofty peaks of the cordillera, and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or muskrat, but the coypu and capybara, rodents of the south american type. innumerable other instances could be given. if we look to the islands off the american shore, however much they may differ in geological structure, the inhabitants are essentially american, though they may be all peculiar species. we may look back to past ages, as shown in the last chapter, and we find american types then prevailing on the american continent and in the american seas. we see in these facts some deep organic bond, throughout space and time, over the same areas of land and water, independently of physical conditions. the naturalist must be dull who is not led to inquire what this bond is. the bond is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like each other, or, as we see in the case of varieties, nearly alike. the dissimilarity of the inhabitants of different regions may be attributed to modification through variation and natural selection, and probably in a subordinate degree to the definite influence of different physical conditions. the degrees of dissimilarity will depend on the migration of the more dominant forms of life from one region into another having been more or less effectually prevented, at periods more or less remote--on the nature and number of the former immigrants--and on the action of the inhabitants on each other in leading to the preservation of different modifications; the relation of organism to organism in the struggle for life being, as i have already often remarked, the most important of all relations. thus the high importance of barriers comes into play by checking migration; as does time for the slow process of modification through natural selection. widely-ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely-extended homes, will have the best chance of seizing on new places, when they spread out into new countries. in their new homes they will be exposed to new conditions, and will frequently undergo further modification and improvement; and thus they will become still further victorious, and will produce groups of modified descendants. on this principle of inheritance with modification we can understand how it is that sections of genera, whole genera, and even families, are confined to the same areas, as is so commonly and notoriously the case. there is no evidence, as was remarked in the last chapter, of the existence of any law of necessary development. as the variability of each species is an independent property, and will be taken advantage of by natural selection, only so far as it profits each individual in its complex struggle for life, so the amount of modification in different species will be no uniform quantity. if a number of species, after having long competed with each other in their old home, were to migrate in a body into a new and afterwards isolated country, they would be little liable to modification; for neither migration nor isolation in themselves effect anything. these principles come into play only by bringing organisms into new relations with each other and in a lesser degree with the surrounding physical conditions. as we have seen in the last chapter that some forms have retained nearly the same character from an enormously remote geological period, so certain species have migrated over vast spaces, and have not become greatly or at all modified. according to these views, it is obvious that the several species of the same genus, though inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they are descended from the same progenitor. in the case of those species which have undergone, during whole geological periods, little modification, there is not much difficulty in believing that they have migrated from the same region; for during the vast geographical and climatical changes which have supervened since ancient times, almost any amount of migration is possible. but in many other cases, in which we have reason to believe that the species of a genus have been produced within comparatively recent times, there is great difficulty on this head. it is also obvious that the individuals of the same species, though now inhabiting distant and isolated regions, must have proceeded from one spot, where their parents were first produced: for, as has been explained, it is incredible that individuals identically the same should have been produced from parents specifically distinct. single centres of supposed creation. we are thus brought to the question which has been largely discussed by naturalists, namely, whether species have been created at one or more points of the earth's surface. undoubtedly there are many cases of extreme difficulty in understanding how the same species could possibly have migrated from some one point to the several distant and isolated points, where now found. nevertheless the simplicity of the view that each species was first produced within a single region captivates the mind. he who rejects it, rejects the vera causa of ordinary generation with subsequent migration, and calls in the agency of a miracle. it is universally admitted, that in most cases the area inhabited by a species is continuous; and that when a plant or animal inhabits two points so distant from each other, or with an interval of such a nature, that the space could not have been easily passed over by migration, the fact is given as something remarkable and exceptional. the incapacity of migrating across a wide sea is more clear in the case of terrestrial mammals than perhaps with any other organic beings; and, accordingly, we find no inexplicable instances of the same mammals inhabiting distant points of the world. no geologist feels any difficulty in great britain possessing the same quadrupeds with the rest of europe, for they were no doubt once united. but if the same species can be produced at two separate points, why do we not find a single mammal common to europe and australia or south america? the conditions of life are nearly the same, so that a multitude of european animals and plants have become naturalised in america and australia; and some of the aboriginal plants are identically the same at these distant points of the northern and southern hemispheres? the answer, as i believe, is, that mammals have not been able to migrate, whereas some plants, from their varied means of dispersal, have migrated across the wide and broken interspaces. the great and striking influence of barriers of all kinds, is intelligible only on the view that the great majority of species have been produced on one side, and have not been able to migrate to the opposite side. some few families, many subfamilies, very many genera, a still greater number of sections of genera, are confined to a single region; and it has been observed by several naturalists that the most natural genera, or those genera in which the species are most closely related to each other, are generally confined to the same country, or if they have a wide range that their range is continuous. what a strange anomaly it would be if a directly opposite rule were to prevail when we go down one step lower in the series, namely to the individuals of the same species, and these had not been, at least at first, confined to some one region! hence, it seems to me, as it has to many other naturalists, that the view of each species having been produced in one area alone, and having subsequently migrated from that area as far as its powers of migration and subsistence under past and present conditions permitted, is the most probable. undoubtedly many cases occur in which we cannot explain how the same species could have passed from one point to the other. but the geographical and climatical changes which have certainly occurred within recent geological times, must have rendered discontinuous the formerly continuous range of many species. so that we are reduced to consider whether the exceptions to continuity of range are so numerous, and of so grave a nature, that we ought to give up the belief, rendered probable by general considerations, that each species has been produced within one area, and has migrated thence as far as it could. it would be hopelessly tedious to discuss all the exceptional cases of the same species, now living at distant and separated points; nor do i for a moment pretend that any explanation could be offered of many instances. but, after some preliminary remarks, i will discuss a few of the most striking classes of facts, namely, the existence of the same species on the summits of distant mountain ranges, and at distant points in the arctic and antarctic regions; and secondly (in the following chapter), the wide distribution of fresh water productions; and thirdly, the occurrence of the same terrestrial species on islands and on the nearest mainland, though separated by hundreds of miles of open sea. if the existence of the same species at distant and isolated points of the earth's surface can in many instances be explained on the view of each species having migrated from a single birthplace; then, considering our ignorance with respect to former climatical and geographical changes, and to the various occasional means of transport, the belief that a single birthplace is the law seems to me incomparably the safest. in discussing this subject we shall be enabled at the same time to consider a point equally important for us, namely, whether the several species of a genus which must on our theory all be descended from a common progenitor, can have migrated, undergoing modification during their migration from some one area. if, when most of the species inhabiting one region are different from those of another region, though closely allied to them, it can be shown that migration from the one region to the other has probably occurred at some former period, our general view will be much strengthened; for the explanation is obvious on the principle of descent with modification. a volcanic island, for instance, upheaved and formed at the distance of a few hundreds of miles from a continent, would probably receive from it in the course of time a few colonists, and their descendants, though modified, would still be related by inheritance to the inhabitants of that continent. cases of this nature are common, and are, as we shall hereafter see, inexplicable on the theory of independent creation. this view of the relation of the species of one region to those of another, does not differ much from that advanced by mr. wallace, who concludes that "every species has come into existence coincident both in space and time with a pre-existing closely allied species." and it is now well known that he attributes this coincidence to descent with modification. the question of single or multiple centres of creation differs from another though allied question, namely, whether all the individuals of the same species are descended from a single pair, or single hermaphrodite, or whether, as some authors suppose, from many individuals simultaneously created. with organic beings which never intercross, if such exist, each species, must be descended from a succession of modified varieties, that have supplanted each other, but have never blended with other individuals or varieties of the same species, so that, at each successive stage of modification, all the individuals of the same form will be descended from a single parent. but in the great majority of cases, namely, with all organisms which habitually unite for each birth, or which occasionally intercross, the individuals of the same species inhabiting the same area will be kept nearly uniform by intercrossing; so that many individuals will go on simultaneously changing, and the whole amount of modification at each stage will not be due to descent from a single parent. to illustrate what i mean: our english race-horses differ from the horses of every other breed; but they do not owe their difference and superiority to descent from any single pair, but to continued care in the selecting and training of many individuals during each generation. before discussing the three classes of facts, which i have selected as presenting the greatest amount of difficulty on the theory of "single centres of creation," i must say a few words on the means of dispersal. means of dispersal. sir c. lyell and other authors have ably treated this subject. i can give here only the briefest abstract of the more important facts. change of climate must have had a powerful influence on migration. a region now impassable to certain organisms from the nature of its climate, might have been a high road for migration, when the climate was different. i shall, however, presently have to discuss this branch of the subject in some detail. changes of level in the land must also have been highly influential: a narrow isthmus now separates two marine faunas; submerge it, or let it formerly have been submerged, and the two faunas will now blend together, or may formerly have blended. where the sea now extends, land may at a former period have connected islands or possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other. no geologist disputes that great mutations of level have occurred within the period of existing organisms. edward forbes insisted that all the islands in the atlantic must have been recently connected with europe or africa, and europe likewise with america. other authors have thus hypothetically bridged over every ocean, and united almost every island with some mainland. if, indeed, the arguments used by forbes are to be trusted, it must be admitted that scarcely a single island exists which has not recently been united to some continent. this view cuts the gordian knot of the dispersal of the same species to the most distant points, and removes many a difficulty; but to the best of my judgment we are not authorized in admitting such enormous geographical changes within the period of existing species. it seems to me that we have abundant evidence of great oscillations in the level of the land or sea; but not of such vast changes in the position and extension of our continents, as to have united them within the recent period to each other and to the several intervening oceanic islands. i freely admit the former existence of many islands, now buried beneath the sea, which may have served as halting places for plants and for many animals during their migration. in the coral-producing oceans such sunken islands are now marked by rings of coral or atolls standing over them. whenever it is fully admitted, as it will some day be, that each species has proceeded from a single birthplace, and when in the course of time we know something definite about the means of distribution, we shall be enabled to speculate with security on the former extension of the land. but i do not believe that it will ever be proved that within the recent period most of our continents which now stand quite separate, have been continuously, or almost continuously united with each other, and with the many existing oceanic islands. several facts in distribution--such as the great difference in the marine faunas on the opposite sides of almost every continent--the close relation of the tertiary inhabitants of several lands and even seas to their present inhabitants--the degree of affinity between the mammals inhabiting islands with those of the nearest continent, being in part determined (as we shall hereafter see) by the depth of the intervening ocean--these and other such facts are opposed to the admission of such prodigious geographical revolutions within the recent period, as are necessary on the view advanced by forbes and admitted by his followers. the nature and relative proportions of the inhabitants of oceanic islands are likewise opposed to the belief of their former continuity of continents. nor does the almost universally volcanic composition of such islands favour the admission that they are the wrecks of sunken continents; if they had originally existed as continental mountain ranges, some at least of the islands would have been formed, like other mountain summits, of granite, metamorphic schists, old fossiliferous and other rocks, instead of consisting of mere piles of volcanic matter. i must now say a few words on what are called accidental means, but which more properly should be called occasional means of distribution. i shall here confine myself to plants. in botanical works, this or that plant is often stated to be ill adapted for wide dissemination; but the greater or less facilities for transport across the sea may be said to be almost wholly unknown. until i tried, with mr. berkeley's aid, a few experiments, it was not even known how far seeds could resist the injurious action of sea-water. to my surprise i found that out of eighty-seven kinds, sixty-four germinated after an immersion of twenty-eight days, and a few survived an immersion of days. it deserves notice that certain orders were far more injured than others: nine leguminosae were tried, and, with one exception, they resisted the salt-water badly; seven species of the allied orders, hydrophyllaceae and polemoniaceae, were all killed by a month's immersion. for convenience sake i chiefly tried small seeds without the capsules or fruit; and as all of these sank in a few days, they could not have been floated across wide spaces of the sea, whether or not they were injured by salt water. afterwards i tried some larger fruits, capsules, etc., and some of these floated for a long time. it is well known what a difference there is in the buoyancy of green and seasoned timber; and it occurred to me that floods would often wash into the sea dried plants or branches with seed-capsules or fruit attached to them. hence i was led to dry the stems and branches of ninety-four plants with ripe fruit, and to place them on sea-water. the majority sank quickly, but some which, whilst green, floated for a very short time, when dried floated much longer; for instance, ripe hazel-nuts sank immediately, but when dried they floated for ninety days, and afterwards when planted germinated; an asparagus plant with ripe berries floated for twenty-three days, when dried it floated for eighty-five days, and the seeds afterwards germinated: the ripe seeds of helosciadium sank in two days, when dried they floated for above ninety days, and afterwards germinated. altogether, out of the ninety-four dried plants, eighteen floated for above twenty-eight days; and some of the eighteen floated for a very much longer period. so that as / kinds of seeds germinated after an immersion of twenty-eight days; and as / distinct species with ripe fruit (but not all the same species as in the foregoing experiment) floated, after being dried, for above twenty-eight days, we may conclude, as far as anything can be inferred from these scanty facts, that the seeds of / kinds of plants of any country might be floated by sea-currents during twenty-eight days, and would retain their power of germination. in johnston's physical atlas, the average rate of the several atlantic currents is thirty-three miles per diem (some currents running at the rate of sixty miles per diem); on this average, the seeds of / plants belonging to one country might be floated across miles of sea to another country; and when stranded, if blown by an inland gale to a favourable spot, would germinate. subsequently to my experiments, m. martens tried similar ones, but in a much better manner, for he placed the seeds in a box in the actual sea, so that they were alternately wet and exposed to the air like really floating plants. he tried ninety-eight seeds, mostly different from mine, but he chose many large fruits, and likewise seeds, from plants which live near the sea; and this would have favoured both the average length of their flotation and their resistance to the injurious action of the salt-water. on the other hand, he did not previously dry the plants or branches with the fruit; and this, as we have seen, would have caused some of them to have floated much longer. the result was that / of his seeds of different kinds floated for forty-two days, and were then capable of germination. but i do not doubt that plants exposed to the waves would float for a less time than those protected from violent movement as in our experiments. therefore, it would perhaps be safer to assume that the seeds of about / plants of a flora, after having been dried, could be floated across a space of sea miles in width, and would then germinate. the fact of the larger fruits often floating longer than the small, is interesting; as plants with large seeds or fruit which, as alph. de candolle has shown, generally have restricted ranges, could hardly be transported by any other means. seeds may be occasionally transported in another manner. drift timber is thrown up on most islands, even on those in the midst of the widest oceans; and the natives of the coral islands in the pacific procure stones for their tools, solely from the roots of drifted trees, these stones being a valuable royal tax. i find that when irregularly shaped stones are embedded in the roots of trees, small parcels of earth are very frequently enclosed in their interstices and behind them, so perfectly that not a particle could be washed away during the longest transport: out of one small portion of earth thus completely enclosed by the roots of an oak about fifty years old, three dicotyledonous plants germinated: i am certain of the accuracy of this observation. again, i can show that the carcasses of birds, when floating on the sea, sometimes escape being immediately devoured; and many kinds of seeds in the crops of floating birds long retain their vitality: peas and vetches, for instance, are killed by even a few days' immersion in sea-water; but some taken out of the crop of a pigeon, which had floated on artificial sea-water for thirty days, to my surprise nearly all germinated. living birds can hardly fail to be highly effective agents in the transportation of seeds. i could give many facts showing how frequently birds of many kinds are blown by gales to vast distances across the ocean. we may safely assume that under such circumstances their rate of flight would often be thirty-five miles an hour; and some authors have given a far higher estimate. i have never seen an instance of nutritious seeds passing through the intestines of a bird; but hard seeds of fruit pass uninjured through even the digestive organs of a turkey. in the course of two months, i picked up in my garden twelve kinds of seeds, out of the excrement of small birds, and these seemed perfect, and some of them, which were tried, germinated. but the following fact is more important: the crops of birds do not secrete gastric juice, and do not, as i know by trial, injure in the least the germination of seeds; now, after a bird has found and devoured a large supply of food, it is positively asserted that all the grains do not pass into the gizzard for twelve or even eighteen hours. a bird in this interval might easily be blown to the distance of five hundred miles, and hawks are known to look out for tired birds, and the contents of their torn crops might thus readily get scattered. some hawks and owls bolt their prey whole, and after an interval of from twelve to twenty hours, disgorge pellets, which, as i know from experiments made in the zoological gardens, include seeds capable of germination. some seeds of the oat, wheat, millet, canary, hemp, clover, and beet germinated after having been from twelve to twenty-one hours in the stomachs of different birds of prey; and two seeds of beet grew after having been thus retained for two days and fourteen hours. fresh-water fish, i find, eat seeds of many land and water plants; fish are frequently devoured by birds, and thus the seeds might be transported from place to place. i forced many kinds of seeds into the stomachs of dead fish, and then gave their bodies to fishing-eagles, storks, and pelicans; these birds, after an interval of many hours, either rejected the seeds in pellets or passed them in their excrement; and several of these seeds retained the power of germination. certain seeds, however, were always killed by this process. locusts are sometimes blown to great distances from the land. i myself caught one miles from the coast of africa, and have heard of others caught at greater distances. the rev. r.t. lowe informed sir c. lyell that in november, , swarms of locusts visited the island of madeira. they were in countless numbers, as thick as the flakes of snow in the heaviest snowstorm, and extended upward as far as could be seen with a telescope. during two or three days they slowly careered round and round in an immense ellipse, at least five or six miles in diameter, and at night alighted on the taller trees, which were completely coated with them. they then disappeared over the sea, as suddenly as they had appeared, and have not since visited the island. now, in parts of natal it is believed by some farmers, though on insufficient evidence, that injurious seeds are introduced into their grass-land in the dung left by the great flights of locusts which often visit that country. in consequence of this belief mr. weale sent me in a letter a small packet of the dried pellets, out of which i extracted under the microscope several seeds, and raised from them seven grass plants, belonging to two species, of two genera. hence a swarm of locusts, such as that which visited madeira, might readily be the means of introducing several kinds of plants into an island lying far from the mainland. although the beaks and feet of birds are generally clean, earth sometimes adheres to them: in one case i removed sixty-one grains, and in another case twenty-two grains of dry argillaceous earth from the foot of a partridge, and in the earth there was a pebble as large as the seed of a vetch. here is a better case: the leg of a woodcock was sent to me by a friend, with a little cake of dry earth attached to the shank, weighing only nine grains; and this contained a seed of the toad-rush (juncus bufonius) which germinated and flowered. mr. swaysland, of brighton, who during the last forty years has paid close attention to our migratory birds, informs me that he has often shot wagtails (motacillae), wheatears, and whinchats (saxicolae), on their first arrival on our shores, before they had alighted; and he has several times noticed little cakes of earth attached to their feet. many facts could be given showing how generally soil is charged with seeds. for instance, professor newton sent me the leg of a red-legged partridge (caccabis rufa) which had been wounded and could not fly, with a ball of hard earth adhering to it, and weighing six and a half ounces. the earth had been kept for three years, but when broken, watered and placed under a bell glass, no less than eighty-two plants sprung from it: these consisted of twelve monocotyledons, including the common oat, and at least one kind of grass, and of seventy dicotyledons, which consisted, judging from the young leaves, of at least three distinct species. with such facts before us, can we doubt that the many birds which are annually blown by gales across great spaces of ocean, and which annually migrate--for instance, the millions of quails across the mediterranean--must occasionally transport a few seeds embedded in dirt adhering to their feet or beaks? but i shall have to recur to this subject. as icebergs are known to be sometimes loaded with earth and stones, and have even carried brushwood, bones, and the nest of a land-bird, it can hardly be doubted that they must occasionally, as suggested by lyell, have transported seeds from one part to another of the arctic and antarctic regions; and during the glacial period from one part of the now temperate regions to another. in the azores, from the large number of plants common to europe, in comparison with the species on the other islands of the atlantic, which stand nearer to the mainland, and (as remarked by mr. h.c. watson) from their somewhat northern character, in comparison with the latitude, i suspected that these islands had been partly stocked by ice-borne seeds during the glacial epoch. at my request sir c. lyell wrote to m. hartung to inquire whether he had observed erratic boulders on these islands, and he answered that he had found large fragments of granite and other rocks, which do not occur in the archipelago. hence we may safely infer that icebergs formerly landed their rocky burdens on the shores of these mid-ocean islands, and it is at least possible that they may have brought thither the seeds of northern plants. considering that these several means of transport, and that other means, which without doubt remain to be discovered, have been in action year after year for tens of thousands of years, it would, i think, be a marvellous fact if many plants had not thus become widely transported. these means of transport are sometimes called accidental, but this is not strictly correct: the currents of the sea are not accidental, nor is the direction of prevalent gales of wind. it should be observed that scarcely any means of transport would carry seeds for very great distances; for seeds do not retain their vitality when exposed for a great length of time to the action of sea water; nor could they be long carried in the crops or intestines of birds. these means, however, would suffice for occasional transport across tracts of sea some hundred miles in breadth, or from island to island, or from a continent to a neighbouring island, but not from one distant continent to another. the floras of distant continents would not by such means become mingled; but would remain as distinct as they now are. the currents, from their course, would never bring seeds from north america to britain, though they might and do bring seeds from the west indies to our western shores, where, if not killed by their very long immersion in salt water, they could not endure our climate. almost every year, one or two land-birds are blown across the whole atlantic ocean, from north america to the western shores of ireland and england; but seeds could be transported by these rare wanderers only by one means, namely, by dirt adhering to their feet or beaks, which is in itself a rare accident. even in this case, how small would be the chance of a seed falling on favourable soil, and coming to maturity! but it would be a great error to argue that because a well-stocked island, like great britain, has not, as far as is known (and it would be very difficult to prove this), received within the last few centuries, through occasional means of transport, immigrants from europe or any other continent, that a poorly-stocked island, though standing more remote from the mainland, would not receive colonists by similar means. out of a hundred kinds of seeds or animals transported to an island, even if far less well-stocked than britain, perhaps not more than one would be so well fitted to its new home, as to become naturalised. but this is no valid argument against what would be effected by occasional means of transport, during the long lapse of geological time, whilst the island was being upheaved, and before it had become fully stocked with inhabitants. on almost bare land, with few or no destructive insects or birds living there, nearly every seed which chanced to arrive, if fitted for the climate, would germinate and survive. dispersal during the glacial period. the identity of many plants and animals, on mountain-summits, separated from each other by hundreds of miles of lowlands, where alpine species could not possibly exist, is one of the most striking cases known of the same species living at distant points, without the apparent possibility of their having migrated from one point to the other. it is indeed a remarkable fact to see so many plants of the same species living on the snowy regions of the alps or pyrenees, and in the extreme northern parts of europe; but it is far more remarkable, that the plants on the white mountains, in the united states of america, are all the same with those of labrador, and nearly all the same, as we hear from asa gray, with those on the loftiest mountains of europe. even as long ago as , such facts led gmelin to conclude that the same species must have been independently created at many distinct points; and we might have remained in this same belief, had not agassiz and others called vivid attention to the glacial period, which, as we shall immediately see, affords a simple explanation of these facts. we have evidence of almost every conceivable kind, organic and inorganic, that, within a very recent geological period, central europe and north america suffered under an arctic climate. the ruins of a house burnt by fire do not tell their tale more plainly than do the mountains of scotland and wales, with their scored flanks, polished surfaces, and perched boulders, of the icy streams with which their valleys were lately filled. so greatly has the climate of europe changed, that in northern italy, gigantic moraines, left by old glaciers, are now clothed by the vine and maize. throughout a large part of the united states, erratic boulders and scored rocks plainly reveal a former cold period. the former influence of the glacial climate on the distribution of the inhabitants of europe, as explained by edward forbes, is substantially as follows. but we shall follow the changes more readily, by supposing a new glacial period slowly to come on, and then pass away, as formerly occurred. as the cold came on, and as each more southern zone became fitted for the inhabitants of the north, these would take the places of the former inhabitants of the temperate regions. the latter, at the same time would travel further and further southward, unless they were stopped by barriers, in which case they would perish. the mountains would become covered with snow and ice, and their former alpine inhabitants would descend to the plains. by the time that the cold had reached its maximum, we should have an arctic fauna and flora, covering the central parts of europe, as far south as the alps and pyrenees, and even stretching into spain. the now temperate regions of the united states would likewise be covered by arctic plants and animals and these would be nearly the same with those of europe; for the present circumpolar inhabitants, which we suppose to have everywhere travelled southward, are remarkably uniform round the world. as the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by the productions of the more temperate regions. and as the snow melted from the bases of the mountains, the arctic forms would seize on the cleared and thawed ground, always ascending, as the warmth increased and the snow still further disappeared, higher and higher, whilst their brethren were pursuing their northern journey. hence, when the warmth had fully returned, the same species, which had lately lived together on the european and north american lowlands, would again be found in the arctic regions of the old and new worlds, and on many isolated mountain-summits far distant from each other. thus we can understand the identity of many plants at points so immensely remote as the mountains of the united states and those of europe. we can thus also understand the fact that the alpine plants of each mountain-range are more especially related to the arctic forms living due north or nearly due north of them: for the first migration when the cold came on, and the re-migration on the returning warmth, would generally have been due south and north. the alpine plants, for example, of scotland, as remarked by mr. h.c. watson, and those of the pyrenees, as remarked by ramond, are more especially allied to the plants of northern scandinavia; those of the united states to labrador; those of the mountains of siberia to the arctic regions of that country. these views, grounded as they are on the perfectly well-ascertained occurrence of a former glacial period, seem to me to explain in so satisfactory a manner the present distribution of the alpine and arctic productions of europe and america, that when in other regions we find the same species on distant mountain-summits, we may almost conclude, without other evidence, that a colder climate formerly permitted their migration across the intervening lowlands, now become too warm for their existence. as the arctic forms moved first southward and afterwards backward to the north, in unison with the changing climate, they will not have been exposed during their long migrations to any great diversity of temperature; and as they all migrated in a body together, their mutual relations will not have been much disturbed. hence, in accordance with the principles inculcated in this volume, these forms will not have been liable to much modification. but with the alpine productions, left isolated from the moment of the returning warmth, first at the bases and ultimately on the summits of the mountains, the case will have been somewhat different; for it is not likely that all the same arctic species will have been left on mountain ranges far distant from each other, and have survived there ever since; they will also, in all probability, have become mingled with ancient alpine species, which must have existed on the mountains before the commencement of the glacial epoch, and which during the coldest period will have been temporarily driven down to the plains; they will, also, have been subsequently exposed to somewhat different climatical influences. their mutual relations will thus have been in some degree disturbed; consequently they will have been liable to modification; and they have been modified; for if we compare the present alpine plants and animals of the several great european mountain ranges, one with another, though many of the species remain identically the same, some exist as varieties, some as doubtful forms or sub-species and some as distinct yet closely allied species representing each other on the several ranges. in the foregoing illustration, i have assumed that at the commencement of our imaginary glacial period, the arctic productions were as uniform round the polar regions as they are at the present day. but it is also necessary to assume that many sub-arctic and some few temperate forms were the same round the world, for some of the species which now exist on the lower mountain slopes and on the plains of north america and europe are the same; and it may be asked how i account for this degree of uniformity of the sub-arctic and temperate forms round the world, at the commencement of the real glacial period. at the present day, the sub-arctic and northern temperate productions of the old and new worlds are separated from each other by the whole atlantic ocean and by the northern part of the pacific. during the glacial period, when the inhabitants of the old and new worlds lived further southwards than they do at present, they must have been still more completely separated from each other by wider spaces of ocean; so that it may well be asked how the same species could then or previously have entered the two continents. the explanation, i believe, lies in the nature of the climate before the commencement of the glacial period. at this, the newer pliocene period, the majority of the inhabitants of the world were specifically the same as now, and we have good reason to believe that the climate was warmer than at the present day. hence, we may suppose that the organisms which now live under latitude degrees, lived during the pliocene period further north, under the polar circle, in latitude - degrees; and that the present arctic productions then lived on the broken land still nearer to the pole. now, if we look at a terrestrial globe, we see under the polar circle that there is almost continuous land from western europe through siberia, to eastern america. and this continuity of the circumpolar land, with the consequent freedom under a more favourable climate for intermigration, will account for the supposed uniformity of the sub-arctic and temperate productions of the old and new worlds, at a period anterior to the glacial epoch. believing, from reasons before alluded to, that our continents have long remained in nearly the same relative position, though subjected to great oscillations of level, i am strongly inclined to extend the above view, and to infer that during some earlier and still warmer period, such as the older pliocene period, a large number of the same plants and animals inhabited the almost continuous circumpolar land; and that these plants and animals, both in the old and new worlds, began slowly to migrate southwards as the climate became less warm, long before the commencement of the glacial period. we now see, as i believe, their descendants, mostly in a modified condition, in the central parts of europe and the united states. on this view we can understand the relationship with very little identity, between the productions of north america and europe--a relationship which is highly remarkable, considering the distance of the two areas, and their separation by the whole atlantic ocean. we can further understand the singular fact remarked on by several observers that the productions of europe and america during the later tertiary stages were more closely related to each other than they are at the present time; for during these warmer periods the northern parts of the old and new worlds will have been almost continuously united by land, serving as a bridge, since rendered impassable by cold, for the intermigration of their inhabitants. during the slowly decreasing warmth of the pliocene period, as soon as the species in common, which inhabited the new and old worlds, migrated south of the polar circle, they will have been completely cut off from each other. this separation, as far as the more temperate productions are concerned, must have taken place long ages ago. as the plants and animals migrated southward, they will have become mingled in the one great region with the native american productions, and would have had to compete with them; and in the other great region, with those of the old world. consequently we have here everything favourable for much modification--for far more modification than with the alpine productions, left isolated, within a much more recent period, on the several mountain ranges and on the arctic lands of europe and north america. hence, it has come, that when we compare the now living productions of the temperate regions of the new and old worlds, we find very few identical species (though asa gray has lately shown that more plants are identical than was formerly supposed), but we find in every great class many forms, which some naturalists rank as geographical races, and others as distinct species; and a host of closely allied or representative forms which are ranked by all naturalists as specifically distinct. as on the land, so in the waters of the sea, a slow southern migration of a marine fauna, which, during the pliocene or even a somewhat earlier period, was nearly uniform along the continuous shores of the polar circle, will account, on the theory of modification, for many closely allied forms now living in marine areas completely sundered. thus, i think, we can understand the presence of some closely allied, still existing and extinct tertiary forms, on the eastern and western shores of temperate north america; and the still more striking fact of many closely allied crustaceans (as described in dana's admirable work), some fish and other marine animals, inhabiting the mediterranean and the seas of japan--these two areas being now completely separated by the breadth of a whole continent and by wide spaces of ocean. these cases of close relationship in species either now or formerly inhabiting the seas on the eastern and western shores of north america, the mediterranean and japan, and the temperate lands of north america and europe, are inexplicable on the theory of creation. we cannot maintain that such species have been created alike, in correspondence with the nearly similar physical conditions of the areas; for if we compare, for instance, certain parts of south america with parts of south africa or australia, we see countries closely similar in all their physical conditions, with their inhabitants utterly dissimilar. alternate glacial periods in the north and south. but we must return to our more immediate subject. i am convinced that forbes's view may be largely extended. in europe we meet with the plainest evidence of the glacial period, from the western shores of britain to the ural range, and southward to the pyrenees. we may infer from the frozen mammals and nature of the mountain vegetation, that siberia was similarly affected. in the lebanon, according to dr. hooker, perpetual snow formerly covered the central axis, and fed glaciers which rolled , feet down the valleys. the same observer has recently found great moraines at a low level on the atlas range in north africa. along the himalaya, at points miles apart, glaciers have left the marks of their former low descent; and in sikkim, dr. hooker saw maize growing on ancient and gigantic moraines. southward of the asiatic continent, on the opposite side of the equator, we know, from the excellent researches of dr. j. haast and dr. hector, that in new zealand immense glaciers formerly descended to a low level; and the same plants, found by dr. hooker on widely separated mountains in this island tell the same story of a former cold period. from facts communicated to me by the rev. w.b. clarke, it appears also that there are traces of former glacial action on the mountains of the south-eastern corner of australia. looking to america: in the northern half, ice-borne fragments of rock have been observed on the eastern side of the continent, as far south as latitude and degrees, and on the shores of the pacific, where the climate is now so different, as far south as latitude degrees. erratic boulders have, also, been noticed on the rocky mountains. in the cordillera of south america, nearly under the equator, glaciers once extended far below their present level. in central chile i examined a vast mound of detritus with great boulders, crossing the portillo valley, which, there can hardly be a doubt, once formed a huge moraine; and mr. d. forbes informs me that he found in various parts of the cordillera, from latitude to degrees south, at about the height of , feet, deeply-furrowed rocks, resembling those with which he was familiar in norway, and likewise great masses of detritus, including grooved pebbles. along this whole space of the cordillera true glaciers do not now exist even at much more considerable heights. further south, on both sides of the continent, from latitude degrees to the southernmost extremity, we have the clearest evidence of former glacial action, in numerous immense boulders transported far from their parent source. from these several facts, namely, from the glacial action having extended all round the northern and southern hemispheres--from the period having been in a geological sense recent in both hemispheres--from its having lasted in both during a great length of time, as may be inferred from the amount of work effected--and lastly, from glaciers having recently descended to a low level along the whole line of the cordillera, it at one time appeared to me that we could not avoid the conclusion that the temperature of the whole world had been simultaneously lowered during the glacial period. but now, mr. croll, in a series of admirable memoirs, has attempted to show that a glacial condition of climate is the result of various physical causes, brought into operation by an increase in the eccentricity of the earth's orbit. all these causes tend towards the same end; but the most powerful appears to be the indirect influence of the eccentricity of the orbit upon oceanic currents. according to mr. croll, cold periods regularly recur every ten or fifteen thousand years; and these at long intervals are extremely severe, owing to certain contingencies, of which the most important, as sir c. lyell has shown, is the relative position of the land and water. mr. croll believes that the last great glacial period occurred about , years ago, and endured, with slight alterations of climate, for about , years. with respect to more ancient glacial periods, several geologists are convinced, from direct evidence, that such occurred during the miocene and eocene formations, not to mention still more ancient formations. but the most important result for us, arrived at by mr. croll, is that whenever the northern hemisphere passes through a cold period the temperature of the southern hemisphere is actually raised, with the winters rendered much milder, chiefly through changes in the direction of the ocean currents. so conversely it will be with the northern hemisphere, while the southern passes through a glacial period. this conclusion throws so much light on geographical distribution that i am strongly inclined to trust in it; but i will first give the facts which demand an explanation. in south america, dr. hooker has shown that besides many closely allied species, between forty and fifty of the flowering plants of tierra del fuego, forming no inconsiderable part of its scanty flora, are common to north america and europe, enormously remote as these areas in opposite hemispheres are from each other. on the lofty mountains of equatorial america a host of peculiar species belonging to european genera occur. on the organ mountains of brazil some few temperate european, some antarctic and some andean genera were found by gardner which do not exist in the low intervening hot countries. on the silla of caraccas the illustrious humboldt long ago found species belonging to genera characteristic of the cordillera. in africa, several forms characteristic of europe, and some few representatives of the flora of the cape of good hope, occur on the mountains of abyssinia. at the cape of good hope a very few european species, believed not to have been introduced by man, and on the mountains several representative european forms are found which have not been discovered in the intertropical parts of africa. dr. hooker has also lately shown that several of the plants living on the upper parts of the lofty island of fernando po, and on the neighbouring cameroon mountains, in the gulf of guinea, are closely related to those on the mountains of abyssinia, and likewise to those of temperate europe. it now also appears, as i hear from dr. hooker, that some of these same temperate plants have been discovered by the rev. r.t. lowe on the mountains of the cape verde islands. this extension of the same temperate forms, almost under the equator, across the whole continent of africa and to the mountains of the cape verde archipelago, is one of the most astonishing facts ever recorded in the distribution of plants. on the himalaya, and on the isolated mountain ranges of the peninsula of india, on the heights of ceylon, and on the volcanic cones of java, many plants occur either identically the same or representing each other, and at the same time representing plants of europe not found in the intervening hot lowlands. a list of the genera of plants collected on the loftier peaks of java, raises a picture of a collection made on a hillock in europe. still more striking is the fact that peculiar australian forms are represented by certain plants growing on the summits of the mountains of borneo. some of these australian forms, as i hear from dr. hooker, extend along the heights of the peninsula of malacca, and are thinly scattered on the one hand over india, and on the other hand as far north as japan. on the southern mountains of australia, dr. f. muller has discovered several european species; other species, not introduced by man, occur on the lowlands; and a long list can be given, as i am informed by dr. hooker, of european genera, found in australia, but not in the intermediate torrid regions. in the admirable "introduction to the flora of new zealand," by dr. hooker, analogous and striking facts are given in regard to the plants of that large island. hence, we see that certain plants growing on the more lofty mountains of the tropics in all parts of the world, and on the temperate plains of the north and south, are either the same species or varieties of the same species. it should, however, be observed that these plants are not strictly arctic forms; for, as mr. h.c. watson has remarked, "in receding from polar toward equatorial latitudes, the alpine or mountain flora really become less and less arctic." besides these identical and closely allied forms, many species inhabiting the same widely sundered areas, belong to genera not now found in the intermediate tropical lowlands. these brief remarks apply to plants alone; but some few analogous facts could be given in regard to terrestrial animals. in marine productions, similar cases likewise occur; as an example, i may quote a statement by the highest authority, prof. dana, that "it is certainly a wonderful fact that new zealand should have a closer resemblance in its crustacea to great britain, its antipode, than to any other part of the world." sir j. richardson, also, speaks of the reappearance on the shores of new zealand, tasmania, etc., of northern forms of fish. dr. hooker informs me that twenty-five species of algae are common to new zealand and to europe, but have not been found in the intermediate tropical seas. from the foregoing facts, namely, the presence of temperate forms on the highlands across the whole of equatorial africa, and along the peninsula of india, to ceylon and the malay archipelago, and in a less well-marked manner across the wide expanse of tropical south america, it appears almost certain that at some former period, no doubt during the most severe part of a glacial period, the lowlands of these great continents were everywhere tenanted under the equator by a considerable number of temperate forms. at this period the equatorial climate at the level of the sea was probably about the same with that now experienced at the height of from five to six thousand feet under the same latitude, or perhaps even rather cooler. during this, the coldest period, the lowlands under the equator must have been clothed with a mingled tropical and temperate vegetation, like that described by hooker as growing luxuriantly at the height of from four to five thousand feet on the lower slopes of the himalaya, but with perhaps a still greater preponderance of temperate forms. so again in the mountainous island of fernando po, in the gulf of guinea, mr. mann found temperate european forms beginning to appear at the height of about five thousand feet. on the mountains of panama, at the height of only two thousand feet, dr. seemann found the vegetation like that of mexico, "with forms of the torrid zone harmoniously blended with those of the temperate." now let us see whether mr. croll's conclusion that when the northern hemisphere suffered from the extreme cold of the great glacial period, the southern hemisphere was actually warmer, throws any clear light on the present apparently inexplicable distribution of various organisms in the temperate parts of both hemispheres, and on the mountains of the tropics. the glacial period, as measured by years, must have been very long; and when we remember over what vast spaces some naturalised plants and animals have spread within a few centuries, this period will have been ample for any amount of migration. as the cold became more and more intense, we know that arctic forms invaded the temperate regions; and from the facts just given, there can hardly be a doubt that some of the more vigorous, dominant and widest-spreading temperate forms invaded the equatorial lowlands. the inhabitants of these hot lowlands would at the same time have migrated to the tropical and subtropical regions of the south, for the southern hemisphere was at this period warmer. on the decline of the glacial period, as both hemispheres gradually recovered their former temperature, the northern temperate forms living on the lowlands under the equator, would have been driven to their former homes or have been destroyed, being replaced by the equatorial forms returning from the south. some, however, of the northern temperate forms would almost certainly have ascended any adjoining high land, where, if sufficiently lofty, they would have long survived like the arctic forms on the mountains of europe. they might have survived, even if the climate was not perfectly fitted for them, for the change of temperature must have been very slow, and plants undoubtedly possess a certain capacity for acclimatisation, as shown by their transmitting to their offspring different constitutional powers of resisting heat and cold. in the regular course of events the southern hemisphere would in its turn be subjected to a severe glacial period, with the northern hemisphere rendered warmer; and then the southern temperate forms would invade the equatorial lowlands. the northern forms which had before been left on the mountains would now descend and mingle with the southern forms. these latter, when the warmth returned, would return to their former homes, leaving some few species on the mountains, and carrying southward with them some of the northern temperate forms which had descended from their mountain fastnesses. thus, we should have some few species identically the same in the northern and southern temperate zones and on the mountains of the intermediate tropical regions. but the species left during a long time on these mountains, or in opposite hemispheres, would have to compete with many new forms and would be exposed to somewhat different physical conditions; hence, they would be eminently liable to modification, and would generally now exist as varieties or as representative species; and this is the case. we must, also, bear in mind the occurrence in both hemispheres of former glacial periods; for these will account, in accordance with the same principles, for the many quite distinct species inhabiting the same widely separated areas, and belonging to genera not now found in the intermediate torrid zones. it is a remarkable fact, strongly insisted on by hooker in regard to america, and by alph. de candolle in regard to australia, that many more identical or slightly modified species have migrated from the north to the south, than in a reversed direction. we see, however, a few southern forms on the mountains of borneo and abyssinia. i suspect that this preponderant migration from the north to the south is due to the greater extent of land in the north, and to the northern forms having existed in their own homes in greater numbers, and having consequently been advanced through natural selection and competition to a higher stage of perfection, or dominating power, than the southern forms. and thus, when the two sets became commingled in the equatorial regions, during the alternations of the glacial periods, the northern forms were the more powerful and were able to hold their places on the mountains, and afterwards migrate southward with the southern forms; but not so the southern in regard to the northern forms. in the same manner, at the present day, we see that very many european productions cover the ground in la plata, new zealand, and to a lesser degree in australia, and have beaten the natives; whereas extremely few southern forms have become naturalised in any part of the northern hemisphere, though hides, wool, and other objects likely to carry seeds have been largely imported into europe during the last two or three centuries from la plata and during the last forty or fifty years from australia. the neilgherrie mountains in india, however, offer a partial exception; for here, as i hear from dr. hooker, australian forms are rapidly sowing themselves and becoming naturalised. before the last great glacial period, no doubt the intertropical mountains were stocked with endemic alpine forms; but these have almost everywhere yielded to the more dominant forms generated in the larger areas and more efficient workshops of the north. in many islands the native productions are nearly equalled, or even outnumbered, by those which have become naturalised; and this is the first stage towards their extinction. mountains are islands on the land; and their inhabitants have yielded to those produced within the larger areas of the north, just in the same way as the inhabitants of real islands have everywhere yielded and are still yielding to continental forms naturalised through man's agency. the same principles apply to the distribution of terrestrial animals and of marine productions, in the northern and southern temperate zones, and on the intertropical mountains. when, during the height of the glacial period, the ocean-currents were widely different to what they now are, some of the inhabitants of the temperate seas might have reached the equator; of these a few would perhaps at once be able to migrate southwards, by keeping to the cooler currents, while others might remain and survive in the colder depths until the southern hemisphere was in its turn subjected to a glacial climate and permitted their further progress; in nearly the same manner as, according to forbes, isolated spaces inhabited by arctic productions exist to the present day in the deeper parts of the northern temperate seas. i am far from supposing that all the difficulties in regard to the distribution and affinities of the identical and allied species, which now live so widely separated in the north and south, and sometimes on the intermediate mountain ranges, are removed on the views above given. the exact lines of migration cannot be indicated. we cannot say why certain species and not others have migrated; why certain species have been modified and have given rise to new forms, while others have remained unaltered. we cannot hope to explain such facts, until we can say why one species and not another becomes naturalised by man's agency in a foreign land; why one species ranges twice or thrice as far, and is twice or thrice as common, as another species within their own homes. various special difficulties also remain to be solved; for instance, the occurrence, as shown by dr. hooker, of the same plants at points so enormously remote as kerguelen land, new zealand, and fuegia; but icebergs, as suggested by lyell, may have been concerned in their dispersal. the existence at these and other distant points of the southern hemisphere, of species, which, though distinct, belong to genera exclusively confined to the south, is a more remarkable case. some of these species are so distinct, that we cannot suppose that there has been time since the commencement of the last glacial period for their migration and subsequent modification to the necessary degree. the facts seem to indicate that distinct species belonging to the same genera have migrated in radiating lines from a common centre; and i am inclined to look in the southern, as in the northern hemisphere, to a former and warmer period, before the commencement of the last glacial period, when the antarctic lands, now covered with ice, supported a highly peculiar and isolated flora. it may be suspected that before this flora was exterminated during the last glacial epoch, a few forms had been already widely dispersed to various points of the southern hemisphere by occasional means of transport, and by the aid, as halting-places, of now sunken islands. thus the southern shores of america, australia, and new zealand may have become slightly tinted by the same peculiar forms of life. sir c. lyell in a striking passage has speculated, in language almost identical with mine, on the effects of great alternations of climate throughout the world on geographical distribution. and we have now seen that mr. croll's conclusion that successive glacial periods in the one hemisphere coincide with warmer periods in the opposite hemisphere, together with the admission of the slow modification of species, explains a multitude of facts in the distribution of the same and of the allied forms of life in all parts of the globe. the living waters have flowed during one period from the north and during another from the south, and in both cases have reached the equator; but the stream of life has flowed with greater force from the north than in the opposite direction, and has consequently more freely inundated the south. as the tide leaves its drift in horizontal lines, rising higher on the shores where the tide rises highest, so have the living waters left their living drift on our mountain summits, in a line gently rising from the arctic lowlands to a great latitude under the equator. the various beings thus left stranded may be compared with savage races of man, driven up and surviving in the mountain fastnesses of almost every land, which serves as a record, full of interest to us, of the former inhabitants of the surrounding lowlands. chapter xiii. geographical distribution--continued. distribution of fresh-water productions--on the inhabitants of oceanic islands--absence of batrachians and of terrestrial mammals--on the relation of the inhabitants of islands to those of the nearest mainland--on colonisation from the nearest source with subsequent modification--summary of the last and present chapters. fresh-water productions. as lakes and river-systems are separated from each other by barriers of land, it might have been thought that fresh-water productions would not have ranged widely within the same country, and as the sea is apparently a still more formidable barrier, that they would never have extended to distant countries. but the case is exactly the reverse. not only have many fresh-water species, belonging to different classes, an enormous range, but allied species prevail in a remarkable manner throughout the world. when first collecting in the fresh waters of brazil, i well remember feeling much surprise at the similarity of the fresh-water insects, shells, etc., and at the dissimilarity of the surrounding terrestrial beings, compared with those of britain. but the wide ranging power of fresh-water productions can, i think, in most cases be explained by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream, within their own countries; and liability to wide dispersal would follow from this capacity as an almost necessary consequence. we can here consider only a few cases; of these, some of the most difficult to explain are presented by fish. it was formerly believed that the same fresh-water species never existed on two continents distant from each other. but dr. gunther has lately shown that the galaxias attenuatus inhabits tasmania, new zealand, the falkland islands and the mainland of south america. this is a wonderful case, and probably indicates dispersal from an antarctic centre during a former warm period. this case, however, is rendered in some degree less surprising by the species of this genus having the power of crossing by some unknown means considerable spaces of open ocean: thus there is one species common to new zealand and to the auckland islands, though separated by a distance of about miles. on the same continent fresh-water fish often range widely, and as if capriciously; for in two adjoining river systems some of the species may be the same and some wholly different. it is probable that they are occasionally transported by what may be called accidental means. thus fishes still alive are not very rarely dropped at distant points by whirlwinds; and it is known that the ova retain their vitality for a considerable time after removal from the water. their dispersal may, however, be mainly attributed to changes in the level of the land within the recent period, causing rivers to flow into each other. instances, also, could be given of this having occurred during floods, without any change of level. the wide differences of the fish on the opposite sides of most mountain-ranges, which are continuous and consequently must, from an early period, have completely prevented the inosculation of the river systems on the two sides, leads to the same conclusion. some fresh-water fish belong to very ancient forms, and in such cases there will have been ample time for great geographical changes, and consequently time and means for much migration. moreover, dr. gunther has recently been led by several considerations to infer that with fishes the same forms have a long endurance. salt-water fish can with care be slowly accustomed to live in fresh water; and, according to valenciennes, there is hardly a single group of which all the members are confined to fresh water, so that a marine species belonging to a fresh-water group might travel far along the shores of the sea, and could, it is probable, become adapted without much difficulty to the fresh waters of a distant land. some species of fresh-water shells have very wide ranges, and allied species which, on our theory, are descended from a common parent, and must have proceeded from a single source, prevail throughout the world. their distribution at first perplexed me much, as their ova are not likely to be transported by birds; and the ova, as well as the adults, are immediately killed by sea-water. i could not even understand how some naturalised species have spread rapidly throughout the same country. but two facts, which i have observed--and many others no doubt will be discovered--throw some light on this subject. when ducks suddenly emerge from a pond covered with duck-weed, i have twice seen these little plants adhering to their backs; and it has happened to me, in removing a little duck-weed from one aquarium to another, that i have unintentionally stocked the one with fresh-water shells from the other. but another agency is perhaps more effectual: i suspended the feet of a duck in an aquarium, where many ova of fresh-water shells were hatching; and i found that numbers of the extremely minute and just-hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. these just-hatched molluscs, though aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and if blown across the sea to an oceanic island, or to any other distant point, would be sure to alight on a pool or rivulet. sir charles lyell informs me that a dyticus has been caught with an ancylus (a fresh-water shell like a limpet) firmly adhering to it; and a water-beetle of the same family, a colymbetes, once flew on board the "beagle," when forty-five miles distant from the nearest land: how much farther it might have been blown by a favouring gale no one can tell. with respect to plants, it has long been known what enormous ranges many fresh-water, and even marsh-species, have, both over continents and to the most remote oceanic islands. this is strikingly illustrated, according to alph. de candolle, in those large groups of terrestrial plants, which have very few aquatic members; for the latter seem immediately to acquire, as if in consequence, a wide range. i think favourable means of dispersal explain this fact. i have before mentioned that earth occasionally adheres in some quantity to the feet and beaks of birds. wading birds, which frequent the muddy edges of ponds, if suddenly flushed, would be the most likely to have muddy feet. birds of this order wander more than those of any other; and are occasionally found on the most remote and barren islands of the open ocean; they would not be likely to alight on the surface of the sea, so that any dirt on their feet would not be washed off; and when gaining the land, they would be sure to fly to their natural fresh-water haunts. i do not believe that botanists are aware how charged the mud of ponds is with seeds: i have tried several little experiments, but will here give only the most striking case: i took in february three tablespoonfuls of mud from three different points, beneath water, on the edge of a little pond; this mud when dry weighed only and / ounces; i kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were of many kinds, and were altogether in number; and yet the viscid mud was all contained in a breakfast cup! considering these facts, i think it would be an inexplicable circumstance if water-birds did not transport the seeds of fresh-water plants to unstocked ponds and streams, situated at very distant points. the same agency may have come into play with the eggs of some of the smaller fresh-water animals. other and unknown agencies probably have also played a part. i have stated that fresh-water fish eat some kinds of seeds, though they reject many other kinds after having swallowed them; even small fish swallow seeds of moderate size, as of the yellow water-lily and potamogeton. herons and other birds, century after century, have gone on daily devouring fish; they then take flight and go to other waters, or are blown across the sea; and we have seen that seeds retain their power of germination, when rejected many hours afterwards in pellets or in the excrement. when i saw the great size of the seeds of that fine water-lily, the nelumbium, and remembered alph. de candolle's remarks on the distribution of this plant, i thought that the means of its dispersal must remain inexplicable; but audubon states that he found the seeds of the great southern water-lily (probably according to dr. hooker, the nelumbium luteum) in a heron's stomach. now this bird must often have flown with its stomach thus well stocked to distant ponds, and, then getting a hearty meal of fish, analogy makes me believe that it would have rejected the seeds in the pellet in a fit state for germination. in considering these several means of distribution, it should be remembered that when a pond or stream is first formed, for instance on a rising islet, it will be unoccupied; and a single seed or egg will have a good chance of succeeding. although there will always be a struggle for life between the inhabitants of the same pond, however few in kind, yet as the number even in a well-stocked pond is small in comparison with the number of species inhabiting an equal area of land, the competition between them will probably be less severe than between terrestrial species; consequently an intruder from the waters of a foreign country would have a better chance of seizing on a new place, than in the case of terrestrial colonists. we should also remember that many fresh-water productions are low in the scale of nature, and we have reason to believe that such beings become modified more slowly than the high; and this will give time for the migration of aquatic species. we should not forget the probability of many fresh-water forms having formerly ranged continuously over immense areas, and then having become extinct at intermediate points. but the wide distribution of fresh-water plants, and of the lower animals, whether retaining the same identical form, or in some degree modified, apparently depends in main part on the wide dispersal of their seeds and eggs by animals, more especially by fresh-water birds, which have great powers of flight, and naturally travel from one piece of water to another. on the inhabitants of oceanic islands. we now come to the last of the three classes of facts, which i have selected as presenting the greatest amount of difficulty with respect to distribution, on the view that not only all the individuals of the same species have migrated from some one area, but that allied species, although now inhabiting the most distant points, have proceeded from a single area, the birthplace of their early progenitors. i have already given my reasons for disbelieving in continental extensions within the period of existing species on so enormous a scale that all the many islands of the several oceans were thus stocked with their present terrestrial inhabitants. this view removes many difficulties, but it does not accord with all the facts in regard to the productions of islands. in the following remarks i shall not confine myself to the mere question of dispersal, but shall consider some other cases bearing on the truth of the two theories of independent creation and of descent with modification. the species of all kinds which inhabit oceanic islands are few in number compared with those on equal continental areas: alph. de candolle admits this for plants, and wollaston for insects. new zealand, for instance, with its lofty mountains and diversified stations, extending over miles of latitude, together with the outlying islands of auckland, campbell and chatham, contain altogether only kinds of flowering plants; if we compare this moderate number with the species which swarm over equal areas in southwestern australia or at the cape of good hope, we must admit that some cause, independently of different physical conditions, has given rise to so great a difference in number. even the uniform county of cambridge has plants, and the little island of anglesea , but a few ferns and a few introduced plants are included in these numbers, and the comparison in some other respects is not quite fair. we have evidence that the barren island of ascension aboriginally possessed less than half-a-dozen flowering plants; yet many species have now become naturalised on it, as they have in new zealand and on every other oceanic island which can be named. in st. helena there is reason to believe that the naturalised plants and animals have nearly or quite exterminated many native productions. he who admits the doctrine of the creation of each separate species, will have to admit that a sufficient number of the best adapted plants and animals were not created for oceanic islands; for man has unintentionally stocked them far more fully and perfectly than did nature. although in oceanic islands the species are few in number, the proportion of endemic kinds (i.e. those found nowhere else in the world) is often extremely large. if we compare, for instance, the number of endemic land-shells in madeira, or of endemic birds in the galapagos archipelago, with the number found on any continent, and then compare the area of the island with that of the continent, we shall see that this is true. this fact might have been theoretically expected, for, as already explained, species occasionally arriving, after long intervals of time in the new and isolated district, and having to compete with new associates, would be eminently liable to modification, and would often produce groups of modified descendants. but it by no means follows that, because in an island nearly all the species of one class are peculiar, those of another class, or of another section of the same class, are peculiar; and this difference seems to depend partly on the species which are not modified having immigrated in a body, so that their mutual relations have not been much disturbed; and partly on the frequent arrival of unmodified immigrants from the mother-country, with which the insular forms have intercrossed. it should be borne in mind that the offspring of such crosses would certainly gain in vigour; so that even an occasional cross would produce more effect than might have been anticipated. i will give a few illustrations of the foregoing remarks: in the galapagos islands there are twenty-six land birds; of these twenty-one (or perhaps twenty-three) are peculiar; whereas of the eleven marine birds only two are peculiar; and it is obvious that marine birds could arrive at these islands much more easily and frequently than land-birds. bermuda, on the other hand, which lies at about the same distance from north america as the galapagos islands do from south america, and which has a very peculiar soil, does not possess a single endemic land bird; and we know from mr. j.m. jones's admirable account of bermuda, that very many north american birds occasionally or even frequently visit this island. almost every year, as i am informed by mr. e.v. harcourt, many european and african birds are blown to madeira; this island is inhabited by ninety-nine kinds, of which one alone is peculiar, though very closely related to a european form; and three or four other species are confined to this island and to the canaries. so that the islands of bermuda and madeira have been stocked from the neighbouring continents with birds, which for long ages have there struggled together, and have become mutually co-adapted. hence, when settled in their new homes, each kind will have been kept by the others to its proper place and habits, and will consequently have been but little liable to modification. any tendency to modification will also have been checked by intercrossing with the unmodified immigrants, often arriving from the mother-country. madeira again is inhabited by a wonderful number of peculiar land-shells, whereas not one species of sea-shell is peculiar to its shores: now, though we do not know how sea-shells are dispersed, yet we can see that their eggs or larvae, perhaps attached to seaweed or floating timber, or to the feet of wading birds, might be transported across three or four hundred miles of open sea far more easily than land-shells. the different orders of insects inhabiting madeira present nearly parallel cases. oceanic islands are sometimes deficient in animals of certain whole classes, and their places are occupied by other classes; thus in the galapagos islands reptiles, and in new zealand gigantic wingless birds, take, or recently took, the place of mammals. although new zealand is here spoken of as an oceanic island, it is in some degree doubtful whether it should be so ranked; it is of large size, and is not separated from australia by a profoundly deep sea; from its geological character and the direction of its mountain ranges, the rev. w.b. clarke has lately maintained that this island, as well as new caledonia, should be considered as appurtenances of australia. turning to plants, dr. hooker has shown that in the galapagos islands the proportional numbers of the different orders are very different from what they are elsewhere. all such differences in number, and the absence of certain whole groups of animals and plants, are generally accounted for by supposed differences in the physical conditions of the islands; but this explanation is not a little doubtful. facility of immigration seems to have been fully as important as the nature of the conditions. many remarkable little facts could be given with respect to the inhabitants of oceanic islands. for instance, in certain islands not tenanted by a single mammal, some of the endemic plants have beautifully hooked seeds; yet few relations are more manifest than that hooks serve for the transportal of seeds in the wool or fur of quadrupeds. but a hooked seed might be carried to an island by other means; and the plant then becoming modified would form an endemic species, still retaining its hooks, which would form a useless appendage, like the shrivelled wings under the soldered wing-covers of many insular beetles. again, islands often possess trees or bushes belonging to orders which elsewhere include only herbaceous species; now trees, as alph. de candolle has shown, generally have, whatever the cause may be, confined ranges. hence trees would be little likely to reach distant oceanic islands; and an herbaceous plant, which had no chance of successfully competing with the many fully developed trees growing on a continent, might, when established on an island, gain an advantage over other herbaceous plants by growing taller and taller and overtopping them. in this case, natural selection would tend to add to the stature of the plant, to whatever order it belonged, and thus first convert it into a bush and then into a tree. absence of batrachians and terrestrial mammals on oceanic islands. with respect to the absence of whole orders of animals on oceanic islands, bory st. vincent long ago remarked that batrachians (frogs, toads, newts) are never found on any of the many islands with which the great oceans are studded. i have taken pains to verify this assertion, and have found it true, with the exception of new zealand, new caledonia, the andaman islands, and perhaps the solomon islands and the seychelles. but i have already remarked that it is doubtful whether new zealand and new caledonia ought to be classed as oceanic islands; and this is still more doubtful with respect to the andaman and solomon groups and the seychelles. this general absence of frogs, toads and newts on so many true oceanic islands cannot be accounted for by their physical conditions; indeed it seems that islands are peculiarly fitted for these animals; for frogs have been introduced into madeira, the azores, and mauritius, and have multiplied so as to become a nuisance. but as these animals and their spawn are immediately killed (with the exception, as far as known, of one indian species) by sea-water, there would be great difficulty in their transportal across the sea, and therefore we can see why they do not exist on strictly oceanic islands. but why, on the theory of creation, they should not have been created there, it would be very difficult to explain. mammals offer another and similar case. i have carefully searched the oldest voyages, and have not found a single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by the natives) inhabiting an island situated above miles from a continent or great continental island; and many islands situated at a much less distance are equally barren. the falkland islands, which are inhabited by a wolf-like fox, come nearest to an exception; but this group cannot be considered as oceanic, as it lies on a bank in connection with the mainland at a distance of about miles; moreover, icebergs formerly brought boulders to its western shores, and they may have formerly transported foxes, as now frequently happens in the arctic regions. yet it cannot be said that small islands will not support at least small mammals, for they occur in many parts of the world on very small islands, when lying close to a continent; and hardly an island can be named on which our smaller quadrupeds have not become naturalised and greatly multiplied. it cannot be said, on the ordinary view of creation, that there has not been time for the creation of mammals; many volcanic islands are sufficiently ancient, as shown by the stupendous degradation which they have suffered, and by their tertiary strata: there has also been time for the production of endemic species belonging to other classes; and on continents it is known that new species of mammals appear and disappear at a quicker rate than other and lower animals. although terrestrial mammals do not occur on oceanic islands, aerial mammals do occur on almost every island. new zealand possesses two bats found nowhere else in the world: norfolk island, the viti archipelago, the bonin islands, the caroline and marianne archipelagoes, and mauritius, all possess their peculiar bats. why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands? on my view this question can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across. bats have been seen wandering by day far over the atlantic ocean; and two north american species, either regularly or occasionally, visit bermuda, at the distance of miles from the mainland. i hear from mr. tomes, who has specially studied this family, that many species have enormous ranges, and are found on continents and on far distant islands. hence, we have only to suppose that such wandering species have been modified in their new homes in relation to their new position, and we can understand the presence of endemic bats on oceanic islands, with the absence of all other terrestrial mammals. another interesting relation exists, namely, between the depth of the sea separating islands from each other, or from the nearest continent, and the degree of affinity of their mammalian inhabitants. mr. windsor earl has made some striking observations on this head, since greatly extended by mr. wallace's admirable researches, in regard to the great malay archipelago, which is traversed near celebes by a space of deep ocean, and this separates two widely distinct mammalian faunas. on either side, the islands stand on a moderately shallow submarine bank, and these islands are inhabited by the same or by closely allied quadrupeds. i have not as yet had time to follow up this subject in all quarters of the world; but as far as i have gone, the relation holds good. for instance, britain is separated by a shallow channel from europe, and the mammals are the same on both sides; and so it is with all the islands near the shores of australia. the west indian islands, on the other hand, stand on a deeply submerged bank, nearly one thousand fathoms in depth, and here we find american forms, but the species and even the genera are quite distinct. as the amount of modification which animals of all kinds undergo partly depends on the lapse of time, and as the islands which are separated from each other, or from the mainland, by shallow channels, are more likely to have been continuously united within a recent period than the islands separated by deeper channels, we can understand how it is that a relation exists between the depth of the sea separating two mammalian faunas, and the degree of their affinity, a relation which is quite inexplicable on the theory of independent acts of creation. the foregoing statements in regard to the inhabitants of oceanic islands, namely, the fewness of the species, with a large proportion consisting of endemic forms--the members of certain groups, but not those of other groups in the same class, having been modified--the absence of certain whole orders, as of batrachians and of terrestrial mammals, notwithstanding the presence of aerial bats, the singular proportions of certain orders of plants, herbaceous forms having been developed into trees, etc., seem to me to accord better with the belief in the efficiency of occasional means of transport, carried on during a long course of time, than with the belief in the former connection of all oceanic islands with the nearest continent; for on this latter view it is probable that the various classes would have immigrated more uniformly, and from the species having entered in a body, their mutual relations would not have been much disturbed, and consequently, they would either have not been modified, or all the species in a more equable manner. i do not deny that there are many and serious difficulties in understanding how many of the inhabitants of the more remote islands, whether still retaining the same specific form or subsequently modified, have reached their present homes. but the probability of other islands having once existed as halting-places, of which not a wreck now remains, must not be overlooked. i will specify one difficult case. almost all oceanic islands, even the most isolated and smallest, are inhabited by land-shells, generally by endemic species, but sometimes by species found elsewhere striking instances of which have been given by dr. a.a. gould in relation to the pacific. now it is notorious that land-shells are easily killed by sea-water; their eggs, at least such as i have tried, sink in it and are killed. yet there must be some unknown, but occasionally efficient means for their transportal. would the just-hatched young sometimes adhere to the feet of birds roosting on the ground and thus get transported? it occurred to me that land-shells, when hybernating and having a membranous diaphragm over the mouth of the shell, might be floated in chinks of drifted timber across moderately wide arms of the sea. and i find that several species in this state withstand uninjured an immersion in sea-water during seven days. one shell, the helix pomatia, after having been thus treated, and again hybernating, was put into sea-water for twenty days and perfectly recovered. during this length of time the shell might have been carried by a marine country of average swiftness to a distance of geographical miles. as this helix has a thick calcareous operculum i removed it, and when it had formed a new membranous one, i again immersed it for fourteen days in sea-water, and again it recovered and crawled away. baron aucapitaine has since tried similar experiments. he placed land-shells, belonging to ten species, in a box pierced with holes, and immersed it for a fortnight in the sea. out of the hundred shells twenty-seven recovered. the presence of an operculum seems to have been of importance, as out of twelve specimens of cyclostoma elegans, which is thus furnished, eleven revived. it is remarkable, seeing how well the helix pomatia resisted with me the salt-water, that not one of fifty-four specimens belonging to four other species of helix tried by aucapitaine recovered. it is, however, not at all probable that land-shells have often been thus transported; the feet of birds offer a more probable method. on the relations of the inhabitants of islands to those of the nearest mainland. the most striking and important fact for us is the affinity of the species which inhabit islands to those of the nearest mainland, without being actually the same. numerous instances could be given. the galapagos archipelago, situated under the equator, lies at a distance of between and miles from the shores of south america. here almost every product of the land and of the water bears the unmistakable stamp of the american continent. there are twenty-six land birds. of these twenty-one, or perhaps twenty-three, are ranked as distinct species, and would commonly be assumed to have been here created; yet the close affinity of most of these birds to american species is manifest in every character in their habits, gestures, and tones of voice. so it is with the other animals, and with a large proportion of the plants, as shown by dr. hooker in his admirable flora of this archipelago. the naturalist, looking at the inhabitants of these volcanic islands in the pacific, distant several hundred miles from the continent, feels that he is standing on american land. why should this be so? why should the species which are supposed to have been created in the galapagos archipelago, and nowhere else, bear so plainly the stamp of affinity to those created in america? there is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which closely resembles the conditions of the south american coast. in fact, there is a considerable dissimilarity in all these respects. on the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in the climate, height, and size of the islands, between the galapagos and cape verde archipelagos: but what an entire and absolute difference in their inhabitants! the inhabitants of the cape verde islands are related to those of africa, like those of the galapagos to america. facts, such as these, admit of no sort of explanation on the ordinary view of independent creation; whereas, on the view here maintained, it is obvious that the galapagos islands would be likely to receive colonists from america, whether by occasional means of transport or (though i do not believe in this doctrine) by formerly continuous land, and the cape verde islands from africa; such colonists would be liable to modification--the principle of inheritance still betraying their original birthplace. many analogous facts could be given: indeed it is an almost universal rule that the endemic productions of islands are related to those of the nearest continent, or of the nearest large island. the exceptions are few, and most of them can be explained. thus, although kerguelen land stands nearer to africa than to america, the plants are related, and that very closely, as we know from dr. hooker's account, to those of america: but on the view that this island has been mainly stocked by seeds brought with earth and stones on icebergs, drifted by the prevailing currents, this anomaly disappears. new zealand in its endemic plants is much more closely related to australia, the nearest mainland, than to any other region: and this is what might have been expected; but it is also plainly related to south america, which, although the next nearest continent, is so enormously remote, that the fact becomes an anomaly. but this difficulty partially disappears on the view that new zealand, south america, and the other southern lands, have been stocked in part from a nearly intermediate though distant point, namely, from the antarctic islands, when they were clothed with vegetation, during a warmer tertiary period, before the commencement of the last glacial period. the affinity, which, though feeble, i am assured by dr. hooker is real, between the flora of the south-western corner of australia and of the cape of good hope, is a far more remarkable case; but this affinity is confined to the plants, and will, no doubt, some day be explained. the same law which has determined the relationship between the inhabitants of islands and the nearest mainland, is sometimes displayed on a small scale, but in a most interesting manner, within the limits of the same archipelago. thus each separate island of the galapagos archipelago is tenanted, and the fact is a marvellous one, by many distinct species; but these species are related to each other in a very much closer manner than to the inhabitants of the american continent, or of any other quarter of the world. this is what might have been expected, for islands situated so near to each other would almost necessarily receive immigrants from the same original source, and from each other. but how is it that many of the immigrants have been differently modified, though only in a small degree, in islands situated within sight of each other, having the same geological nature, the same height, climate, etc? this long appeared to me a great difficulty: but it arises in chief part from the deeply-seated error of considering the physical conditions of a country as the most important; whereas it cannot be disputed that the nature of the other species with which each has to compete, is at least as important, and generally a far more important element of success. now if we look to the species which inhabit the galapagos archipelago, and are likewise found in other parts of the world, we find that they differ considerably in the several islands. this difference might indeed have been expected if the islands have been stocked by occasional means of transport--a seed, for instance, of one plant having been brought to one island, and that of another plant to another island, though all proceeding from the same general source. hence, when in former times an immigrant first settled on one of the islands, or when it subsequently spread from one to another, it would undoubtedly be exposed to different conditions in the different islands, for it would have to compete with a different set of organisms; a plant, for instance, would find the ground best-fitted for it occupied by somewhat different species in the different islands, and would be exposed to the attacks of somewhat different enemies. if, then, it varied, natural selection would probably favour different varieties in the different islands. some species, however, might spread and yet retain the same character throughout the group, just as we see some species spreading widely throughout a continent and remaining the same. the really surprising fact in this case of the galapagos archipelago, and in a lesser degree in some analogous cases, is that each new species after being formed in any one island, did not spread quickly to the other islands. but the islands, though in sight of each other, are separated by deep arms of the sea, in most cases wider than the british channel, and there is no reason to suppose that they have at any former period been continuously united. the currents of the sea are rapid and deep between the islands, and gales of wind are extraordinarily rare; so that the islands are far more effectually separated from each other than they appear on a map. nevertheless, some of the species, both of those found in other parts of the world and of those confined to the archipelago, are common to the several islands; and we may infer from the present manner of distribution that they have spread from one island to the others. but we often take, i think, an erroneous view of the probability of closely allied species invading each other's territory, when put into free intercommunication. undoubtedly, if one species has any advantage over another, it will in a very brief time wholly or in part supplant it; but if both are equally well fitted for their own places, both will probably hold their separate places for almost any length of time. being familiar with the fact that many species, naturalised through man's agency, have spread with astonishing rapidity over wide areas, we are apt to infer that most species would thus spread; but we should remember that the species which become naturalised in new countries are not generally closely allied to the aboriginal inhabitants, but are very distinct forms, belonging in a large proportion of cases, as shown by alph. de candolle, to distinct genera. in the galapagos archipelago, many even of the birds, though so well adapted for flying from island to island, differ on the different islands; thus there are three closely allied species of mocking-thrush, each confined to its own island. now let us suppose the mocking-thrush of chatham island to be blown to charles island, which has its own mocking-thrush; why should it succeed in establishing itself there? we may safely infer that charles island is well stocked with its own species, for annually more eggs are laid and young birds hatched than can possibly be reared; and we may infer that the mocking-thrush peculiar to charles island is at least as well fitted for its home as is the species peculiar to chatham island. sir c. lyell and mr. wollaston have communicated to me a remarkable fact bearing on this subject; namely, that madeira and the adjoining islet of porto santo possess many distinct but representative species of land-shells, some of which live in crevices of stone; and although large quantities of stone are annually transported from porto santo to madeira, yet this latter island has not become colonised by the porto santo species: nevertheless, both islands have been colonised by some european land-shells, which no doubt had some advantage over the indigenous species. from these considerations i think we need not greatly marvel at the endemic species which inhabit the several islands of the galapagos archipelago not having all spread from island to island. on the same continent, also, pre-occupation has probably played an important part in checking the commingling of the species which inhabit different districts with nearly the same physical conditions. thus, the south-east and south-west corners of australia have nearly the same physical conditions, and are united by continuous land, yet they are inhabited by a vast number of distinct mammals, birds, and plants; so it is, according to mr. bates, with the butterflies and other animals inhabiting the great, open, and continuous valley of the amazons. the same principle which governs the general character of the inhabitants of oceanic islands, namely, the relation to the source whence colonists could have been most easily derived, together with their subsequent modification, is of the widest application throughout nature. we see this on every mountain-summit, in every lake and marsh. for alpine species, excepting in as far as the same species have become widely spread during the glacial epoch, are related to those of the surrounding lowlands; thus we have in south america, alpine humming-birds, alpine rodents, alpine plants, etc., all strictly belonging to american forms; and it is obvious that a mountain, as it became slowly upheaved, would be colonised from the surrounding lowlands. so it is with the inhabitants of lakes and marshes, excepting in so far as great facility of transport has allowed the same forms to prevail throughout large portions of the world. we see the same principle in the character of most of the blind animals inhabiting the caves of america and of europe. other analogous facts could be given. it will, i believe, be found universally true, that wherever in two regions, let them be ever so distant, many closely allied or representative species occur, there will likewise be found some identical species; and wherever many closely-allied species occur, there will be found many forms which some naturalists rank as distinct species, and others as mere varieties; these doubtful forms showing us the steps in the process of modification. the relation between the power and extent of migration in certain species, either at the present or at some former period, and the existence at remote points of the world of closely allied species, is shown in another and more general way. mr. gould remarked to me long ago, that in those genera of birds which range over the world, many of the species have very wide ranges. i can hardly doubt that this rule is generally true, though difficult of proof. among mammals, we see it strikingly displayed in bats, and in a lesser degree in the felidae and canidae. we see the same rule in the distribution of butterflies and beetles. so it is with most of the inhabitants of fresh water, for many of the genera in the most distinct classes range over the world, and many of the species have enormous ranges. it is not meant that all, but that some of the species have very wide ranges in the genera which range very widely. nor is it meant that the species in such genera have, on an average, a very wide range; for this will largely depend on how far the process of modification has gone; for instance, two varieties of the same species inhabit america and europe, and thus the species has an immense range; but, if variation were to be carried a little further, the two varieties would be ranked as distinct species, and their range would be greatly reduced. still less is it meant, that species which have the capacity of crossing barriers and ranging widely, as in the case of certain powerfully-winged birds, will necessarily range widely; for we should never forget that to range widely implies not only the power of crossing barriers, but the more important power of being victorious in distant lands in the struggle for life with foreign associates. but according to the view that all the species of a genus, though distributed to the most remote points of the world, are descended from a single progenitor, we ought to find, and i believe as a general rule we do find, that some at least of the species range very widely. we should bear in mind that many genera in all classes are of ancient origin, and the species in this case will have had ample time for dispersal and subsequent modification. there is also reason to believe, from geological evidence, that within each great class the lower organisms change at a slower rate than the higher; consequently they will have had a better chance of ranging widely and of still retaining the same specific character. this fact, together with that of the seeds and eggs of most lowly organised forms being very minute and better fitted for distant transportal, probably accounts for a law which has long been observed, and which has lately been discussed by alph. de candolle in regard to plants, namely, that the lower any group of organisms stands the more widely it ranges. the relations just discussed--namely, lower organisms ranging more widely than the higher--some of the species of widely-ranging genera themselves ranging widely--such facts, as alpine, lacustrine, and marsh productions being generally related to those which live on the surrounding low lands and dry lands--the striking relationship between the inhabitants of islands and those of the nearest mainland--the still closer relationship of the distinct inhabitants of the islands of the same archipelago--are inexplicable on the ordinary view of the independent creation of each species, but are explicable if we admit colonisation from the nearest or readiest source, together with the subsequent adaptation of the colonists to their new homes. summary of the last and present chapters. in these chapters i have endeavoured to show that if we make due allowance for our ignorance of the full effects of changes of climate and of the level of the land, which have certainly occurred within the recent period, and of other changes which have probably occurred--if we remember how ignorant we are with respect to the many curious means of occasional transport--if we bear in mind, and this is a very important consideration, how often a species may have ranged continuously over a wide area, and then have become extinct in the intermediate tracts--the difficulty is not insuperable in believing that all the individuals of the same species, wherever found, are descended from common parents. and we are led to this conclusion, which has been arrived at by many naturalists under the designation of single centres of creation, by various general considerations, more especially from the importance of barriers of all kinds, and from the analogical distribution of subgenera, genera, and families. with respect to distinct species belonging to the same genus, which on our theory have spread from one parent-source; if we make the same allowances as before for our ignorance, and remember that some forms of life have changed very slowly, enormous periods of time having been thus granted for their migration, the difficulties are far from insuperable; though in this case, as in that of the individuals of the same species, they are often great. as exemplifying the effects of climatical changes on distribution, i have attempted to show how important a part the last glacial period has played, which affected even the equatorial regions, and which, during the alternations of the cold in the north and the south, allowed the productions of opposite hemispheres to mingle, and left some of them stranded on the mountain-summits in all parts of the world. as showing how diversified are the means of occasional transport, i have discussed at some little length the means of dispersal of fresh-water productions. if the difficulties be not insuperable in admitting that in the long course of time all the individuals of the same species, and likewise of the several species belonging to the same genus, have proceeded from some one source; then all the grand leading facts of geographical distribution are explicable on the theory of migration, together with subsequent modification and the multiplication of new forms. we can thus understand the high importance of barriers, whether of land or water, in not only separating but in apparently forming the several zoological and botanical provinces. we can thus understand the concentration of related species within the same areas; and how it is that under different latitudes, for instance, in south america, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are linked together in so mysterious a manner, and are likewise linked to the extinct beings which formerly inhabited the same continent. bearing in mind that the mutual relation of organism to organism is of the highest importance, we can see why two areas, having nearly the same physical conditions, should often be inhabited by very different forms of life; for according to the length of time which has elapsed since the colonists entered one of the regions, or both; according to the nature of the communication which allowed certain forms and not others to enter, either in greater or lesser numbers; according or not as those which entered happened to come into more or less direct competition with each other and with the aborigines; and according as the immigrants were capable of varying more or less rapidly, there would ensue in the to or more regions, independently of their physical conditions, infinitely diversified conditions of life; there would be an almost endless amount of organic action and reaction, and we should find some groups of beings greatly, and some only slightly modified; some developed in great force, some existing in scanty numbers--and this we do find in the several great geographical provinces of the world. on these same principles we can understand, as i have endeavoured to show, why oceanic islands should have few inhabitants, but that of these, a large proportion should be endemic or peculiar; and why, in relation to the means of migration, one group of beings should have all its species peculiar, and another group, even within the same class, should have all its species the same with those in an adjoining quarter of the world. we can see why whole groups of organisms, as batrachians and terrestrial mammals, should be absent from oceanic islands, whilst the most isolated islands should possess their own peculiar species of aerial mammals or bats. we can see why, in islands, there should be some relation between the presence of mammals, in a more or less modified condition, and the depth of the sea between such islands and the mainland. we can clearly see why all the inhabitants of an archipelago, though specifically distinct on the several islets, should be closely related to each other, and should likewise be related, but less closely, to those of the nearest continent, or other source whence immigrants might have been derived. we can see why, if there exist very closely allied or representative species in two areas, however distant from each other, some identical species will almost always there be found. as the late edward forbes often insisted, there is a striking parallelism in the laws of life throughout time and space; the laws governing the succession of forms in past times being nearly the same with those governing at the present time the differences in different areas. we see this in many facts. the endurance of each species and group of species is continuous in time; for the apparent exceptions to the rule are so few that they may fairly be attributed to our not having as yet discovered in an intermediate deposit certain forms which are absent in it, but which occur above and below: so in space, it certainly is the general rule that the area inhabited by a single species, or by a group of species, is continuous, and the exceptions, which are not rare, may, as i have attempted to show, be accounted for by former migrations under different circumstances, or through occasional means of transport, or by the species having become extinct in the intermediate tracts. both in time and space species and groups of species have their points of maximum development. groups of species, living during the same period of time, or living within the same area, are often characterised by trifling features in common, as of sculpture or colour. in looking to the long succession of past ages, as in looking to distant provinces throughout the world, we find that species in certain classes differ little from each other, whilst those in another class, or only in a different section of the same order, differ greatly from each other. in both time and space the lowly organised members of each class generally change less than the highly organised; but there are in both cases marked exceptions to the rule. according to our theory, these several relations throughout time and space are intelligible; for whether we look to the allied forms of life which have changed during successive ages, or to those which have changed after having migrated into distant quarters, in both cases they are connected by the same bond of ordinary generation; in both cases the laws of variation have been the same, and modifications have been accumulated by the same means of natural selection. chapter xiv. mutual affinities of organic beings: morphology--embryology--rudimentary organs. classification, groups subordinate to groups--natural system--rules and difficulties in classification, explained on the theory of descent with modification--classification of varieties--descent always used in classification--analogical or adaptive characters--affinities, general, complex and radiating--extinction separates and defines groups--morphology, between members of the same class, between parts of the same individual--embryology, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age--rudimentary organs; their origin explained--summary. classification. from the most remote period in the history of the world organic beings have been found to resemble each other in descending degrees, so that they can be classed in groups under groups. this classification is not arbitrary like the grouping of the stars in constellations. the existence of groups would have been of simple significance, if one group had been exclusively fitted to inhabit the land, and another the water; one to feed on flesh, another on vegetable matter, and so on; but the case is widely different, for it is notorious how commonly members of even the same subgroup have different habits. in the second and fourth chapters, on variation and on natural selection, i have attempted to show that within each country it is the widely ranging, the much diffused and common, that is the dominant species, belonging to the larger genera in each class, which vary most. the varieties, or incipient species, thus produced, ultimately become converted into new and distinct species; and these, on the principle of inheritance, tend to produce other new and dominant species. consequently the groups which are now large, and which generally include many dominant species, tend to go on increasing in size. i further attempted to show that from the varying descendants of each species trying to occupy as many and as different places as possible in the economy of nature, they constantly tend to diverge in character. this latter conclusion is supported by observing the great diversity of forms, which, in any small area, come into the closest competition, and by certain facts in naturalisation. i attempted also to show that there is a steady tendency in the forms which are increasing in number and diverging in character, to supplant and exterminate the preceding, less divergent and less improved forms. i request the reader to turn to the diagram illustrating the action, as formerly explained, of these several principles; and he will see that the inevitable result is, that the modified descendants proceeding from one progenitor become broken up into groups subordinate to groups. in the diagram each letter on the uppermost line may represent a genus including several species; and the whole of the genera along this upper line form together one class, for all are descended from one ancient parent, and, consequently, have inherited something in common. but the three genera on the left hand have, on this same principle, much in common, and form a subfamily, distinct from that containing the next two genera on the right hand, which diverged from a common parent at the fifth stage of descent. these five genera have also much in common, though less than when grouped in subfamilies; and they form a family distinct from that containing the three genera still further to the right hand, which diverged at an earlier period. and all these genera, descended from (a), form an order distinct from the genera descended from (i). so that we here have many species descended from a single progenitor grouped into genera; and the genera into subfamilies, families and orders, all under one great class. the grand fact of the natural subordination of organic beings in groups under groups, which, from its familiarity, does not always sufficiently strike us, is in my judgment thus explained. no doubt organic beings, like all other objects, can be classed in many ways, either artificially by single characters, or more naturally by a number of characters. we know, for instance, that minerals and the elemental substances can be thus arranged. in this case there is of course no relation to genealogical succession, and no cause can at present be assigned for their falling into groups. but with organic beings the case is different, and the view above given accords with their natural arrangement in group under group; and no other explanation has ever been attempted. naturalists, as we have seen, try to arrange the species, genera and families in each class, on what is called the natural system. but what is meant by this system? some authors look at it merely as a scheme for arranging together those living objects which are most alike, and for separating those which are most unlike; or as an artificial method of enunciating, as briefly as possible, general propositions--that is, by one sentence to give the characters common, for instance, to all mammals, by another those common to all carnivora, by another those common to the dog-genus, and then, by adding a single sentence, a full description is given of each kind of dog. the ingenuity and utility of this system are indisputable. but many naturalists think that something more is meant by the natural system; they believe that it reveals the plan of the creator; but unless it be specified whether order in time or space, or both, or what else is meant by the plan of the creator, it seems to me that nothing is thus added to our knowledge. expressions such as that famous one by linnaeus, which we often meet with in a more or less concealed form, namely, that the characters do not make the genus, but that the genus gives the characters, seem to imply that some deeper bond is included in our classifications than mere resemblance. i believe that this is the case, and that community of descent--the one known cause of close similarity in organic beings--is the bond, which, though observed by various degrees of modification, is partially revealed to us by our classifications. let us now consider the rules followed in classification, and the difficulties which are encountered on the view that classification either gives some unknown plan of creation, or is simply a scheme for enunciating general propositions and of placing together the forms most like each other. it might have been thought (and was in ancient times thought) that those parts of the structure which determined the habits of life, and the general place of each being in the economy of nature, would be of very high importance in classification. nothing can be more false. no one regards the external similarity of a mouse to a shrew, of a dugong to a whale, of a whale to a fish, as of any importance. these resemblances, though so intimately connected with the whole life of the being, are ranked as merely "adaptive or analogical characters;" but to the consideration of these resemblances we shall recur. it may even be given as a general rule, that the less any part of the organisation is concerned with special habits, the more important it becomes for classification. as an instance: owen, in speaking of the dugong, says, "the generative organs, being those which are most remotely related to the habits and food of an animal, i have always regarded as affording very clear indications of its true affinities. we are least likely in the modifications of these organs to mistake a merely adaptive for an essential character." with plants how remarkable it is that the organs of vegetation, on which their nutrition and life depend, are of little signification; whereas the organs of reproduction, with their product the seed and embryo, are of paramount importance! so again, in formerly discussing certain morphological characters which are not functionally important, we have seen that they are often of the highest service in classification. this depends on their constancy throughout many allied groups; and their constancy chiefly depends on any slight deviations not having been preserved and accumulated by natural selection, which acts only on serviceable characters. that the mere physiological importance of an organ does not determine its classificatory value, is almost proved by the fact, that in allied groups, in which the same organ, as we have every reason to suppose, has nearly the same physiological value, its classificatory value is widely different. no naturalist can have worked at any group without being struck with this fact; and it has been fully acknowledged in the writings of almost every author. it will suffice to quote the highest authority, robert brown, who, in speaking of certain organs in the proteaceae, says their generic importance, "like that of all their parts, not only in this, but, as i apprehend in every natural family, is very unequal, and in some cases seems to be entirely lost." again, in another work he says, the genera of the connaraceae "differ in having one or more ovaria, in the existence or absence of albumen, in the imbricate or valvular aestivation. any one of these characters singly is frequently of more than generic importance, though here even, when all taken together, they appear insufficient to separate cnestis from connarus." to give an example among insects: in one great division of the hymenoptera, the antennae, as westwood has remarked, are most constant in structure; in another division they differ much, and the differences are of quite subordinate value in classification; yet no one will say that the antennae in these two divisions of the same order are of unequal physiological importance. any number of instances could be given of the varying importance for classification of the same important organ within the same group of beings. again, no one will say that rudimentary or atrophied organs are of high physiological or vital importance; yet, undoubtedly, organs in this condition are often of much value in classification. no one will dispute that the rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary bones of the leg, are highly serviceable in exhibiting the close affinity between ruminants and pachyderms. robert brown has strongly insisted on the fact that the position of the rudimentary florets is of the highest importance in the classification of the grasses. numerous instances could be given of characters derived from parts which must be considered of very trifling physiological importance, but which are universally admitted as highly serviceable in the definition of whole groups. for instance, whether or not there is an open passage from the nostrils to the mouth, the only character, according to owen, which absolutely distinguishes fishes and reptiles--the inflection of the angle of the lower jaw in marsupials--the manner in which the wings of insects are folded--mere colour in certain algae--mere pubescence on parts of the flower in grasses--the nature of the dermal covering, as hair or feathers, in the vertebrata. if the ornithorhynchus had been covered with feathers instead of hair, this external and trifling character would have been considered by naturalists as an important aid in determining the degree of affinity of this strange creature to birds. the importance, for classification, of trifling characters, mainly depends on their being correlated with many other characters of more or less importance. the value indeed of an aggregate of characters is very evident in natural history. hence, as has often been remarked, a species may depart from its allies in several characters, both of high physiological importance, and of almost universal prevalence, and yet leave us in no doubt where it should be ranked. hence, also, it has been found that a classification founded on any single character, however important that may be, has always failed; for no part of the organisation is invariably constant. the importance of an aggregate of characters, even when none are important, alone explains the aphorism enunciated by linnaeus, namely, that the characters do not give the genus, but the genus gives the character; for this seems founded on the appreciation of many trifling points of resemblance, too slight to be defined. certain plants, belonging to the malpighiaceae, bear perfect and degraded flowers; in the latter, as a. de jussieu has remarked, "the greater number of the characters proper to the species, to the genus, to the family, to the class, disappear, and thus laugh at our classification." when aspicarpa produced in france, during several years, only these degraded flowers, departing so wonderfully in a number of the most important points of structure from the proper type of the order, yet m. richard sagaciously saw, as jussieu observes, that this genus should still be retained among the malpighiaceae. this case well illustrates the spirit of our classifications. practically, when naturalists are at work, they do not trouble themselves about the physiological value of the characters which they use in defining a group or in allocating any particular species. if they find a character nearly uniform, and common to a great number of forms, and not common to others, they use it as one of high value; if common to some lesser number, they use it as of subordinate value. this principle has been broadly confessed by some naturalists to be the true one; and by none more clearly than by that excellent botanist, aug. st. hilaire. if several trifling characters are always found in combination, though no apparent bond of connexion can be discovered between them, especial value is set on them. as in most groups of animals, important organs, such as those for propelling the blood, or for aerating it, or those for propagating the race, are found nearly uniform, they are considered as highly serviceable in classification; but in some groups all these, the most important vital organs, are found to offer characters of quite subordinate value. thus, as fritz muller has lately remarked, in the same group of crustaceans, cypridina is furnished with a heart, while in two closely allied genera, namely cypris and cytherea, there is no such organ; one species of cypridina has well-developed branchiae, while another species is destitute of them. we can see why characters derived from the embryo should be of equal importance with those derived from the adult, for a natural classification of course includes all ages. but it is by no means obvious, on the ordinary view, why the structure of the embryo should be more important for this purpose than that of the adult, which alone plays its full part in the economy of nature. yet it has been strongly urged by those great naturalists, milne edwards and agassiz, that embryological characters are the most important of all; and this doctrine has very generally been admitted as true. nevertheless, their importance has sometimes been exaggerated, owing to the adaptive characters of larvae not having been excluded; in order to show this, fritz muller arranged, by the aid of such characters alone, the great class of crustaceans, and the arrangement did not prove a natural one. but there can be no doubt that embryonic, excluding larval characters, are of the highest value for classification, not only with animals but with plants. thus the main divisions of flowering plants are founded on differences in the embryo--on the number and position of the cotyledons, and on the mode of development of the plumule and radicle. we shall immediately see why these characters possess so high a value in classification, namely, from the natural system being genealogical in its arrangement. our classifications are often plainly influenced by chains of affinities. nothing can be easier than to define a number of characters common to all birds; but with crustaceans, any such definition has hitherto been found impossible. there are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the articulata. geographical distribution has often been used, though perhaps not quite logically, in classification, more especially in very large groups of closely allied forms. temminck insists on the utility or even necessity of this practice in certain groups of birds; and it has been followed by several entomologists and botanists. finally, with respect to the comparative value of the various groups of species, such as orders, suborders, families, subfamilies, and genera, they seem to be, at least at present, almost arbitrary. several of the best botanists, such as mr. bentham and others, have strongly insisted on their arbitrary value. instances could be given among plants and insects, of a group first ranked by practised naturalists as only a genus, and then raised to the rank of a subfamily or family; and this has been done, not because further research has detected important structural differences, at first overlooked, but because numerous allied species, with slightly different grades of difference, have been subsequently discovered. all the foregoing rules and aids and difficulties in classification may be explained, if i do not greatly deceive myself, on the view that the natural system is founded on descent with modification--that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, all true classification being genealogical--that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike. but i must explain my meaning more fully. i believe that the arrangement of the groups within each class, in due subordination and relation to each other, must be strictly genealogical in order to be natural; but that the amount of difference in the several branches or groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification which they have undergone; and this is expressed by the forms being ranked under different genera, families, sections or orders. the reader will best understand what is meant, if he will take the trouble to refer to the diagram in the fourth chapter. we will suppose the letters a to l to represent allied genera existing during the silurian epoch, and descended from some still earlier form. in three of these genera (a, f, and i) a species has transmitted modified descendants to the present day, represented by the fifteen genera (a to z ) on the uppermost horizontal line. now, all these modified descendants from a single species are related in blood or descent in the same degree. they may metaphorically be called cousins to the same millionth degree, yet they differ widely and in different degrees from each other. the forms descended from a, now broken up into two or three families, constitute a distinct order from those descended from i, also broken up into two families. nor can the existing species descended from a be ranked in the same genus with the parent a, or those from i with parent i. but the existing genus f may be supposed to have been but slightly modified, and it will then rank with the parent genus f; just as some few still living organisms belong to silurian genera. so that the comparative value of the differences between these organic beings, which are all related to each other in the same degree in blood, has come to be widely different. nevertheless, their genealogical arrangement remains strictly true, not only at the present time, but at each successive period of descent. all the modified descendants from a will have inherited something in common from their common parent, as will all the descendants from i; so will it be with each subordinate branch of descendants at each successive stage. if, however, we suppose any descendant of a or of i to have become so much modified as to have lost all traces of its parentage in this case, its place in the natural system will be lost, as seems to have occurred with some few existing organisms. all the descendants of the genus f, along its whole line of descent, are supposed to have been but little modified, and they form a single genus. but this genus, though much isolated, will still occupy its proper intermediate position. the representation of the groups as here given in the diagram on a flat surface, is much too simple. the branches ought to have diverged in all directions. if the names of the groups had been simply written down in a linear series the representation would have been still less natural; and it is notoriously not possible to represent in a series, on a flat surface, the affinities which we discover in nature among the beings of the same group. thus, the natural system is genealogical in its arrangement, like a pedigree. but the amount of modification which the different groups have undergone has to be expressed by ranking them under different so-called genera, subfamilies, families, sections, orders, and classes. it may be worth while to illustrate this view of classification, by taking the case of languages. if we possessed a perfect pedigree of mankind, a genealogical arrangement of the races of man would afford the best classification of the various languages now spoken throughout the world; and if all extinct languages, and all intermediate and slowly changing dialects, were to be included, such an arrangement would be the only possible one. yet it might be that some ancient languages had altered very little and had given rise to few new languages, whilst others had altered much owing to the spreading, isolation and state of civilisation of the several co-descended races, and had thus given rise to many new dialects and languages. the various degrees of difference between the languages of the same stock would have to be expressed by groups subordinate to groups; but the proper or even the only possible arrangement would still be genealogical; and this would be strictly natural, as it would connect together all languages, extinct and recent, by the closest affinities, and would give the filiation and origin of each tongue. in confirmation of this view, let us glance at the classification of varieties, which are known or believed to be descended from a single species. these are grouped under the species, with the subvarieties under the varieties; and in some cases, as with the domestic pigeon, with several other grades of difference. nearly the same rules are followed as in classifying species. authors have insisted on the necessity of arranging varieties on a natural instead of an artificial system; we are cautioned, for instance, not to class two varieties of the pine-apple together, merely because their fruit, though the most important part, happens to be nearly identical; no one puts the swedish and common turnip together, though the esculent and thickened stems are so similar. whatever part is found to be most constant, is used in classing varieties: thus the great agriculturist marshall says the horns are very useful for this purpose with cattle, because they are less variable than the shape or colour of the body, etc.; whereas with sheep the horns are much less serviceable, because less constant. in classing varieties, i apprehend that if we had a real pedigree, a genealogical classification would be universally preferred; and it has been attempted in some cases. for we might feel sure, whether there had been more or less modification, that the principle of inheritance would keep the forms together which were allied in the greatest number of points. in tumbler pigeons, though some of the subvarieties differ in the important character of the length of the beak, yet all are kept together from having the common habit of tumbling; but the short-faced breed has nearly or quite lost this habit; nevertheless, without any thought on the subject, these tumblers are kept in the same group, because allied in blood and alike in some other respects. with species in a state of nature, every naturalist has in fact brought descent into his classification; for he includes in his lowest grade, that of species, the two sexes; and how enormously these sometimes differ in the most important characters is known to every naturalist: scarcely a single fact can be predicated in common of the adult males and hermaphrodites of certain cirripedes, and yet no one dreams of separating them. as soon as the three orchidean forms, monachanthus, myanthus, and catasetum, which had previously been ranked as three distinct genera, were known to be sometimes produced on the same plant, they were immediately considered as varieties; and now i have been able to show that they are the male, female, and hermaphrodite forms of the same species. the naturalist includes as one species the various larval stages of the same individual, however much they may differ from each other and from the adult; as well as the so-called alternate generations of steenstrup, which can only in a technical sense be considered as the same individual. he includes monsters and varieties, not from their partial resemblance to the parent-form, but because they are descended from it. as descent has universally been used in classing together the individuals of the same species, though the males and females and larvae are sometimes extremely different; and as it has been used in classing varieties which have undergone a certain, and sometimes a considerable amount of modification, may not this same element of descent have been unconsciously used in grouping species under genera, and genera under higher groups, all under the so-called natural system? i believe it has been unconsciously used; and thus only can i understand the several rules and guides which have been followed by our best systematists. as we have no written pedigrees, we are forced to trace community of descent by resemblances of any kind. therefore, we choose those characters which are the least likely to have been modified, in relation to the conditions of life to which each species has been recently exposed. rudimentary structures on this view are as good as, or even sometimes better than other parts of the organisation. we care not how trifling a character may be--let it be the mere inflection of the angle of the jaw, the manner in which an insect's wing is folded, whether the skin be covered by hair or feathers--if it prevail throughout many and different species, especially those having very different habits of life, it assumes high value; for we can account for its presence in so many forms with such different habits, only by inheritance from a common parent. we may err in this respect in regard to single points of structure, but when several characters, let them be ever so trifling, concur throughout a large group of beings having different habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor; and we know that such aggregated characters have especial value in classification. we can understand why a species or a group of species may depart from its allies, in several of its most important characteristics, and yet be safely classed with them. this may be safely done, and is often done, as long as a sufficient number of characters, let them be ever so unimportant, betrays the hidden bond of community of descent. let two forms have not a single character in common, yet, if these extreme forms are connected together by a chain of intermediate groups, we may at once infer their community of descent, and we put them all into the same class. as we find organs of high physiological importance--those which serve to preserve life under the most diverse conditions of existence--are generally the most constant, we attach especial value to them; but if these same organs, in another group or section of a group, are found to differ much, we at once value them less in our classification. we shall presently see why embryological characters are of such high classificatory importance. geographical distribution may sometimes be brought usefully into play in classing large genera, because all the species of the same genus, inhabiting any distinct and isolated region, are in all probability descended from the same parents. analogical resemblances. we can understand, on the above views, the very important distinction between real affinities and analogical or adaptive resemblances. lamarck first called attention to this subject, and he has been ably followed by macleay and others. the resemblance in the shape of the body and in the fin-like anterior limbs between dugongs and whales, and between these two orders of mammals and fishes, are analogical. so is the resemblance between a mouse and a shrew-mouse (sorex), which belong to different orders; and the still closer resemblance, insisted on by mr. mivart, between the mouse and a small marsupial animal (antechinus) of australia. these latter resemblances may be accounted for, as it seems to me, by adaptation for similarly active movements through thickets and herbage, together with concealment from enemies. among insects there are innumerable instances; thus linnaeus, misled by external appearances, actually classed an homopterous insect as a moth. we see something of the same kind even with our domestic varieties, as in the strikingly similar shape of the body in the improved breeds of the chinese and common pig, which are descended from distinct species; and in the similarly thickened stems of the common and specifically distinct swedish turnip. the resemblance between the greyhound and race-horse is hardly more fanciful than the analogies which have been drawn by some authors between widely different animals. on the view of characters being of real importance for classification, only in so far as they reveal descent, we can clearly understand why analogical or adaptive characters, although of the utmost importance to the welfare of the being, are almost valueless to the systematist. for animals, belonging to two most distinct lines of descent, may have become adapted to similar conditions, and thus have assumed a close external resemblance; but such resemblances will not reveal--will rather tend to conceal their blood-relationship. we can thus also understand the apparent paradox, that the very same characters are analogical when one group is compared with another, but give true affinities when the members of the same group are compared together: thus the shape of the body and fin-like limbs are only analogical when whales are compared with fishes, being adaptations in both classes for swimming through the water; but between the the several members of the whale family, the shape of the body and the fin-like limbs offer characters exhibiting true affinity; for as these parts are so nearly similar throughout the whole family, we cannot doubt that they have been inherited from a common ancestor. so it is with fishes. numerous cases could be given of striking resemblances in quite distinct beings between single parts or organs, which have been adapted for the same functions. a good instance is afforded by the close resemblance of the jaws of the dog and tasmanian wolf or thylacinus--animals which are widely sundered in the natural system. but this resemblance is confined to general appearance, as in the prominence of the canines, and in the cutting shape of the molar teeth. for the teeth really differ much: thus the dog has on each side of the upper jaw four pre-molars and only two molars; while the thylacinus has three pre-molars and four molars. the molars also differ much in the two animals in relative size and structure. the adult dentition is preceded by a widely different milk dentition. any one may, of course, deny that the teeth in either case have been adapted for tearing flesh, through the natural selection of successive variations; but if this be admitted in the one case, it is unintelligible to me that it should be denied in the other. i am glad to find that so high an authority as professor flower has come to this same conclusion. the extraordinary cases given in a former chapter, of widely different fishes possessing electric organs--of widely different insects possessing luminous organs--and of orchids and asclepiads having pollen-masses with viscid discs, come under this same head of analogical resemblances. but these cases are so wonderful that they were introduced as difficulties or objections to our theory. in all such cases some fundamental difference in the growth or development of the parts, and generally in their matured structure, can be detected. the end gained is the same, but the means, though appearing superficially to be the same, are essentially different. the principle formerly alluded to under the term of analogical variation has probably in these cases often come into play; that is, the members of the same class, although only distantly allied, have inherited so much in common in their constitution, that they are apt to vary under similar exciting causes in a similar manner; and this would obviously aid in the acquirement through natural selection of parts or organs, strikingly like each other, independently of their direct inheritance from a common progenitor. as species belonging to distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances--to inhabit, for instance, the three elements of land, air and water--we can perhaps understand how it is that a numerical parallelism has sometimes been observed between the subgroups of distinct classes. a naturalist, struck with a parallelism of this nature, by arbitrarily raising or sinking the value of the groups in several classes (and all our experience shows that their valuation is as yet arbitrary), could easily extend the parallelism over a wide range; and thus the septenary, quinary, quaternary and ternary classifications have probably arisen. there is another and curious class of cases in which close external resemblance does not depend on adaptation to similar habits of life, but has been gained for the sake of protection. i allude to the wonderful manner in which certain butterflies imitate, as first described by mr. bates, other and quite distinct species. this excellent observer has shown that in some districts of south america, where, for instance, an ithomia abounds in gaudy swarms, another butterfly, namely, a leptalis, is often found mingled in the same flock; and the latter so closely resembles the ithomia in every shade and stripe of colour, and even in the shape of its wings, that mr. bates, with his eyes sharpened by collecting during eleven years, was, though always on his guard, continually deceived. when the mockers and the mocked are caught and compared, they are found to be very different in essential structure, and to belong not only to distinct genera, but often to distinct families. had this mimicry occurred in only one or two instances, it might have been passed over as a strange coincidence. but, if we proceed from a district where one leptalis imitates an ithomia, another mocking and mocked species, belonging to the same two genera, equally close in their resemblance, may be found. altogether no less than ten genera are enumerated, which include species that imitate other butterflies. the mockers and mocked always inhabit the same region; we never find an imitator living remote from the form which it imitates. the mockers are almost invariably rare insects; the mocked in almost every case abounds in swarms. in the same district in which a species of leptalis closely imitates an ithomia, there are sometimes other lepidoptera mimicking the same ithomia: so that in the same place, species of three genera of butterflies and even a moth are found all closely resembling a butterfly belonging to a fourth genus. it deserves especial notice that many of the mimicking forms of the leptalis, as well as of the mimicked forms, can be shown by a graduated series to be merely varieties of the same species; while others are undoubtedly distinct species. but why, it may be asked, are certain forms treated as the mimicked and others as the mimickers? mr. bates satisfactorily answers this question by showing that the form which is imitated keeps the usual dress of the group to which it belongs, while the counterfeiters have changed their dress and do not resemble their nearest allies. we are next led to enquire what reason can be assigned for certain butterflies and moths so often assuming the dress of another and quite distinct form; why, to the perplexity of naturalists, has nature condescended to the tricks of the stage? mr. bates has, no doubt, hit on the true explanation. the mocked forms, which always abound in numbers, must habitually escape destruction to a large extent, otherwise they could not exist in such swarms; and a large amount of evidence has now been collected, showing that they are distasteful to birds and other insect-devouring animals. the mocking forms, on the other hand, that inhabit the same district, are comparatively rare, and belong to rare groups; hence, they must suffer habitually from some danger, for otherwise, from the number of eggs laid by all butterflies, they would in three or four generations swarm over the whole country. now if a member of one of these persecuted and rare groups were to assume a dress so like that of a well-protected species that it continually deceived the practised eyes of an entomologist, it would often deceive predaceous birds and insects, and thus often escape destruction. mr. bates may almost be said to have actually witnessed the process by which the mimickers have come so closely to resemble the mimicked; for he found that some of the forms of leptalis which mimic so many other butterflies, varied in an extreme degree. in one district several varieties occurred, and of these one alone resembled, to a certain extent, the common ithomia of the same district. in another district there were two or three varieties, one of which was much commoner than the others, and this closely mocked another form of ithomia. from facts of this nature, mr. bates concludes that the leptalis first varies; and when a variety happens to resemble in some degree any common butterfly inhabiting the same district, this variety, from its resemblance to a flourishing and little persecuted kind, has a better chance of escaping destruction from predaceous birds and insects, and is consequently oftener preserved; "the less perfect degrees of resemblance being generation after generation eliminated, and only the others left to propagate their kind." so that here we have an excellent illustration of natural selection. messrs. wallace and trimen have likewise described several equally striking cases of imitation in the lepidoptera of the malay archipelago and africa, and with some other insects. mr. wallace has also detected one such case with birds, but we have none with the larger quadrupeds. the much greater frequency of imitation with insects than with other animals, is probably the consequence of their small size; insects cannot defend themselves, excepting indeed the kinds furnished with a sting, and i have never heard of an instance of such kinds mocking other insects, though they are mocked; insects cannot easily escape by flight from the larger animals which prey on them; therefore, speaking metaphorically, they are reduced, like most weak creatures, to trickery and dissimulation. it should be observed that the process of imitation probably never commenced between forms widely dissimilar in colour. but, starting with species already somewhat like each other, the closest resemblance, if beneficial, could readily be gained by the above means, and if the imitated form was subsequently and gradually modified through any agency, the imitating form would be led along the same track, and thus be altered to almost any extent, so that it might ultimately assume an appearance or colouring wholly unlike that of the other members of the family to which it belonged. there is, however, some difficulty on this head, for it is necessary to suppose in some cases that ancient members belonging to several distinct groups, before they had diverged to their present extent, accidentally resembled a member of another and protected group in a sufficient degree to afford some slight protection, this having given the basis for the subsequent acquisition of the most perfect resemblance. on the nature of the affinities connecting organic beings. as the modified descendants of dominant species, belonging to the larger genera, tend to inherit the advantages which made the groups to which they belong large and their parents dominant, they are almost sure to spread widely, and to seize on more and more places in the economy of nature. the larger and more dominant groups within each class thus tend to go on increasing in size, and they consequently supplant many smaller and feebler groups. thus, we can account for the fact that all organisms, recent and extinct, are included under a few great orders and under still fewer classes. as showing how few the higher groups are in number, and how widely they are spread throughout the world, the fact is striking that the discovery of australia has not added an insect belonging to a new class, and that in the vegetable kingdom, as i learn from dr. hooker, it has added only two or three families of small size. in the chapter on geological succession i attempted to show, on the principle of each group having generally diverged much in character during the long-continued process of modification, how it is that the more ancient forms of life often present characters in some degree intermediate between existing groups. as some few of the old and intermediate forms having transmitted to the present day descendants but little modified, these constitute our so-called osculant or aberrant groups. the more aberrant any form is, the greater must be the number of connecting forms which have been exterminated and utterly lost. and we have evidence of aberrant groups having suffered severely from extinction, for they are almost always represented by extremely few species; and such species as do occur are generally very distinct from each other, which again implies extinction. the genera ornithorhynchus and lepidosiren, for example, would not have been less aberrant had each been represented by a dozen species, instead of as at present by a single one, or by two or three. we can, i think, account for this fact only by looking at aberrant groups as forms which have been conquered by more successful competitors, with a few members still preserved under unusually favourable conditions. mr. waterhouse has remarked that when a member belonging to one group of animals exhibits an affinity to a quite distinct group, this affinity in most cases is general and not special: thus, according to mr. waterhouse, of all rodents, the bizcacha is most nearly related to marsupials; but in the points in which it approaches this order, its relations are general, that is, not to any one marsupial species more than to another. as these points of affinity are believed to be real and not merely adaptive, they must be due in accordance with our view to inheritance from a common progenitor. therefore, we must suppose either that all rodents, including the bizcacha, branched off from some ancient marsupial, which will naturally have been more or less intermediate in character with respect to all existing marsupials; or that both rodents and marsupials branched off from a common progenitor, and that both groups have since undergone much modification in divergent directions. on either view we must suppose that the bizcacha has retained, by inheritance, more of the character of its ancient progenitor than have other rodents; and therefore it will not be specially related to any one existing marsupial, but indirectly to all or nearly all marsupials, from having partially retained the character of their common progenitor, or of some early member of the group. on the other hand, of all marsupials, as mr. waterhouse has remarked, the phascolomys resembles most nearly, not any one species, but the general order of rodents. in this case, however, it may be strongly suspected that the resemblance is only analogical, owing to the phascolomys having become adapted to habits like those of a rodent. the elder de candolle has made nearly similar observations on the general nature of the affinities of distinct families of plants. on the principle of the multiplication and gradual divergence in character of the species descended from a common progenitor, together with their retention by inheritance of some characters in common, we can understand the excessively complex and radiating affinities by which all the members of the same family or higher group are connected together. for the common progenitor of a whole family, now broken up by extinction into distinct groups and subgroups, will have transmitted some of its characters, modified in various ways and degrees, to all the species; and they will consequently be related to each other by circuitous lines of affinity of various lengths (as may be seen in the diagram so often referred to), mounting up through many predecessors. as it is difficult to show the blood-relationship between the numerous kindred of any ancient and noble family, even by the aid of a genealogical tree, and almost impossible to do so without this aid, we can understand the extraordinary difficulty which naturalists have experienced in describing, without the aid of a diagram, the various affinities which they perceive between the many living and extinct members of the same great natural class. extinction, as we have seen in the fourth chapter, has played an important part in defining and widening the intervals between the several groups in each class. we may thus account for the distinctness of whole classes from each other--for instance, of birds from all other vertebrate animals--by the belief that many ancient forms of life have been utterly lost, through which the early progenitors of birds were formerly connected with the early progenitors of the other and at that time less differentiated vertebrate classes. there has been much less extinction of the forms of life which once connected fishes with batrachians. there has been still less within some whole classes, for instance the crustacea, for here the most wonderfully diverse forms are still linked together by a long and only partially broken chain of affinities. extinction has only defined the groups: it has by no means made them; for if every form which has ever lived on this earth were suddenly to reappear, though it would be quite impossible to give definitions by which each group could be distinguished, still a natural classification, or at least a natural arrangement, would be possible. we shall see this by turning to the diagram: the letters, a to l, may represent eleven silurian genera, some of which have produced large groups of modified descendants, with every link in each branch and sub-branch still alive; and the links not greater than those between existing varieties. in this case it would be quite impossible to give definitions by which the several members of the several groups could be distinguished from their more immediate parents and descendants. yet the arrangement in the diagram would still hold good and would be natural; for, on the principle of inheritance, all the forms descended, for instance from a, would have something in common. in a tree we can distinguish this or that branch, though at the actual fork the two unite and blend together. we could not, as i have said, define the several groups; but we could pick out types, or forms, representing most of the characters of each group, whether large or small, and thus give a general idea of the value of the differences between them. this is what we should be driven to, if we were ever to succeed in collecting all the forms in any one class which have lived throughout all time and space. assuredly we shall never succeed in making so perfect a collection: nevertheless, in certain classes, we are tending toward this end; and milne edwards has lately insisted, in an able paper, on the high importance of looking to types, whether or not we can separate and define the groups to which such types belong. finally, we have seen that natural selection, which follows from the struggle for existence, and which almost inevitably leads to extinction and divergence of character in the descendants from any one parent-species, explains that great and universal feature in the affinities of all organic beings, namely, their subordination in group under group. we use the element of descent in classing the individuals of both sexes and of all ages under one species, although they may have but few characters in common; we use descent in classing acknowledged varieties, however different they may be from their parents; and i believe that this element of descent is the hidden bond of connexion which naturalists have sought under the term of the natural system. on this idea of the natural system being, in so far as it has been perfected, genealogical in its arrangement, with the grades of difference expressed by the terms genera, families, orders, etc., we can understand the rules which we are compelled to follow in our classification. we can understand why we value certain resemblances far more than others; why we use rudimentary and useless organs, or others of trifling physiological importance; why, in finding the relations between one group and another, we summarily reject analogical or adaptive characters, and yet use these same characters within the limits of the same group. we can clearly see how it is that all living and extinct forms can be grouped together within a few great classes; and how the several members of each class are connected together by the most complex and radiating lines of affinities. we shall never, probably, disentangle the inextricable web of the affinities between the members of any one class; but when we have a distinct object in view, and do not look to some unknown plan of creation, we may hope to make sure but slow progress. professor haeckel in his "generelle morphologie" and in another works, has recently brought his great knowledge and abilities to bear on what he calls phylogeny, or the lines of descent of all organic beings. in drawing up the several series he trusts chiefly to embryological characters, but receives aid from homologous and rudimentary organs, as well as from the successive periods at which the various forms of life are believed to have first appeared in our geological formations. he has thus boldly made a great beginning, and shows us how classification will in the future be treated. morphology. we have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organisation. this resemblance is often expressed by the term "unity of type;" or by saying that the several parts and organs in the different species of the class are homologous. the whole subject is included under the general term of morphology. this is one of the most interesting departments of natural history, and may almost be said to be its very soul. what can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions? how curious it is, to give a subordinate though striking instance, that the hind feet of the kangaroo, which are so well fitted for bounding over the open plains--those of the climbing, leaf-eating koala, equally well fitted for grasping the branches of trees--those of the ground-dwelling, insect or root-eating, bandicoots--and those of some other australian marsupials--should all be constructed on the same extraordinary type, namely with the bones of the second and third digits extremely slender and enveloped within the same skin, so that they appear like a single toe furnished with two claws. notwithstanding this similarity of pattern, it is obvious that the hind feet of these several animals are used for as widely different purposes as it is possible to conceive. the case is rendered all the more striking by the american opossums, which follow nearly the same habits of life as some of their australian relatives, having feet constructed on the ordinary plan. professor flower, from whom these statements are taken, remarks in conclusion: "we may call this conformity to type, without getting much nearer to an explanation of the phenomenon;" and he then adds "but is it not powerfully suggestive of true relationship, of inheritance from a common ancestor?" geoffroy st. hilaire has strongly insisted on the high importance of relative position or connexion in homologous parts; they may differ to almost any extent in form and size, and yet remain connected together in the same invariable order. we never find, for instance, the bones of the arm and forearm, or of the thigh and leg, transposed. hence the same names can be given to the homologous bones in widely different animals. we see the same great law in the construction of the mouths of insects: what can be more different than the immensely long spiral proboscis of a sphinx-moth, the curious folded one of a bee or bug, and the great jaws of a beetle? yet all these organs, serving for such widely different purposes, are formed by infinitely numerous modifications of an upper lip, mandibles, and two pairs of maxillae. the same law governs the construction of the mouths and limbs of crustaceans. so it is with the flowers of plants. nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same class, by utility or by the doctrine of final causes. the hopelessness of the attempt has been expressly admitted by owen in his most interesting work on the "nature of limbs." on the ordinary view of the independent creation of each being, we can only say that so it is; that it has pleased the creator to construct all the animals and plants in each great class on a uniform plan; but this is not a scientific explanation. the explanation is to a large extent simple, on the theory of the selection of successive slight modifications, each being profitable in some way to the modified form, but often affecting by correlation other parts of the organisation. in changes of this nature, there will be little or no tendency to alter the original pattern, or to transpose the parts. the bones of a limb might be shortened and flattened to any extent, becoming at the same time enveloped in thick membrane, so as to serve as a fin; or a webbed hand might have all its bones, or certain bones, lengthened to any extent, with the membrane connecting them increased, so as to serve as a wing; yet all these modifications would not tend to alter the framework of the bones or the relative connexion of the parts. if we suppose that an early progenitor--the archetype, as it may be called--of all mammals, birds and reptiles, had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain signification of the homologous construction of the limbs throughout the class. so with the mouths of insects, we have only to suppose that their common progenitor had an upper lip, mandibles, and two pairs of maxillae, these parts being perhaps very simple in form; and then natural selection will account for the infinite diversity in structure and function of the mouths of insects. nevertheless, it is conceivable that the general pattern of an organ might become so much obscured as to be finally lost, by the reduction and ultimately by the complete abortion of certain parts, by the fusion of other parts, and by the doubling or multiplication of others, variations which we know to be within the limits of possibility. in the paddles of the gigantic extinct sea-lizards, and in the mouths of certain suctorial crustaceans, the general pattern seems thus to have become partially obscured. there is another and equally curious branch of our subject; namely, serial homologies, or the comparison of the different parts or organs in the same individual, and not of the same parts or organs in different members of the same class. most physiologists believe that the bones of the skull are homologous--that is, correspond in number and in relative connexion--with the elemental parts of a certain number of vertebrae. the anterior and posterior limbs in all the higher vertebrate classes are plainly homologous. so it is with the wonderfully complex jaws and legs of crustaceans. it is familiar to almost every one, that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves, arranged in a spire. in monstrous plants, we often get direct evidence of the possibility of one organ being transformed into another; and we can actually see, during the early or embryonic stages of development in flowers, as well as in crustaceans and many other animals, that organs, which when mature become extremely different are at first exactly alike. how inexplicable are the cases of serial homologies on the ordinary view of creation! why should the brain be enclosed in a box composed of such numerous and such extraordinarily shaped pieces of bone apparently representing vertebrae? as owen has remarked, the benefit derived from the yielding of the separate pieces in the act of parturition by mammals, will by no means explain the same construction in the skulls of birds and reptiles. why should similar bones have been created to form the wing and the leg of a bat, used as they are for such totally different purposes, namely flying and walking? why should one crustacean, which has an extremely complex mouth formed of many parts, consequently always have fewer legs; or conversely, those with many legs have simpler mouths? why should the sepals, petals, stamens, and pistils, in each flower, though fitted for such distinct purposes, be all constructed on the same pattern? on the theory of natural selection, we can, to a certain extent, answer these questions. we need not here consider how the bodies of some animals first became divided into a series of segments, or how they became divided into right and left sides, with corresponding organs, for such questions are almost beyond investigation. it is, however, probable that some serial structures are the result of cells multiplying by division, entailing the multiplication of the parts developed from such cells. it must suffice for our purpose to bear in mind that an indefinite repetition of the same part or organ is the common characteristic, as owen has remarked, of all low or little specialised forms; therefore the unknown progenitor of the vertebrata probably possessed many vertebrae; the unknown progenitor of the articulata, many segments; and the unknown progenitor of flowering plants, many leaves arranged in one or more spires. we have also formerly seen that parts many times repeated are eminently liable to vary, not only in number, but in form. consequently such parts, being already present in considerable numbers, and being highly variable, would naturally afford the materials for adaptation to the most different purposes; yet they would generally retain, through the force of inheritance, plain traces of their original or fundamental resemblance. they would retain this resemblance all the more, as the variations, which afforded the basis for their subsequent modification through natural selection, would tend from the first to be similar; the parts being at an early stage of growth alike, and being subjected to nearly the same conditions. such parts, whether more or less modified, unless their common origin became wholly obscured, would be serially homologous. in the great class of molluscs, though the parts in distinct species can be shown to be homologous, only a few serial homologies; such as the valves of chitons, can be indicated; that is, we are seldom enabled to say that one part is homologous with another part in the same individual. and we can understand this fact; for in molluscs, even in the lowest members of the class, we do not find nearly so much indefinite repetition of any one part as we find in the other great classes of the animal and vegetable kingdoms. but morphology is a much more complex subject than it at first appears, as has lately been well shown in a remarkable paper by mr. e. ray lankester, who has drawn an important distinction between certain classes of cases which have all been equally ranked by naturalists as homologous. he proposes to call the structures which resemble each other in distinct animals, owing to their descent from a common progenitor with subsequent modification, "homogenous"; and the resemblances which cannot thus be accounted for, he proposes to call "homoplastic". for instance, he believes that the hearts of birds and mammals are as a whole homogenous--that is, have been derived from a common progenitor; but that the four cavities of the heart in the two classes are homoplastic--that is, have been independently developed. mr. lankester also adduces the close resemblance of the parts on the right and left sides of the body, and in the successive segments of the same individual animal; and here we have parts commonly called homologous which bear no relation to the descent of distinct species from a common progenitor. homoplastic structures are the same with those which i have classed, though in a very imperfect manner, as analogous modifications or resemblances. their formation may be attributed in part to distinct organisms, or to distinct parts of the same organism, having varied in an analogous manner; and in part to similar modifications, having been preserved for the same general purpose or function, of which many instances have been given. naturalists frequently speak of the skull as formed of metamorphosed vertebrae; the jaws of crabs as metamorphosed legs; the stamens and pistils in flowers as metamorphosed leaves; but it would in most cases be more correct, as professor huxley has remarked, to speak of both skull and vertebrae, jaws and legs, etc., as having been metamorphosed, not one from the other, as they now exist, but from some common and simpler element. most naturalists, however, use such language only in a metaphorical sense: they are far from meaning that during a long course of descent, primordial organs of any kind--vertebrae in the one case and legs in the other--have actually been converted into skulls or jaws. yet so strong is the appearance of this having occurred that naturalists can hardly avoid employing language having this plain signification. according to the views here maintained, such language may be used literally; and the wonderful fact of the jaws, for instance, of a crab retaining numerous characters, which they probably would have retained through inheritance, if they had really been metamorphosed from true though extremely simple legs, is in part explained. development and embryology. this is one of the most important subjects in the whole round of natural history. the metamorphoses of insects, with which every one is familiar, are generally effected abruptly by a few stages; but the transformations are in reality numerous and gradual, though concealed. a certain ephemerous insect (chloeon) during its development, moults, as shown by sir j. lubbock, above twenty times, and each time undergoes a certain amount of change; and in this case we see the act of metamorphosis performed in a primary and gradual manner. many insects, and especially certain crustaceans, show us what wonderful changes of structure can be effected during development. such changes, however, reach their acme in the so-called alternate generations of some of the lower animals. it is, for instance, an astonishing fact that a delicate branching coralline, studded with polypi, and attached to a submarine rock, should produce, first by budding and then by transverse division, a host of huge floating jelly-fishes; and that these should produce eggs, from which are hatched swimming animalcules, which attach themselves to rocks and become developed into branching corallines; and so on in an endless cycle. the belief in the essential identity of the process of alternate generation and of ordinary metamorphosis has been greatly strengthened by wagner's discovery of the larva or maggot of a fly, namely the cecidomyia, producing asexually other larvae, and these others, which finally are developed into mature males and females, propagating their kind in the ordinary manner by eggs. it may be worth notice that when wagner's remarkable discovery was first announced, i was asked how was it possible to account for the larvae of this fly having acquired the power of a sexual reproduction. as long as the case remained unique no answer could be given. but already grimm has shown that another fly, a chironomus, reproduces itself in nearly the same manner, and he believes that this occurs frequently in the order. it is the pupa, and not the larva, of the chironomus which has this power; and grimm further shows that this case, to a certain extent, "unites that of the cecidomyia with the parthenogenesis of the coccidae;" the term parthenogenesis implying that the mature females of the coccidae are capable of producing fertile eggs without the concourse of the male. certain animals belonging to several classes are now known to have the power of ordinary reproduction at an unusually early age; and we have only to accelerate parthenogenetic reproduction by gradual steps to an earlier and earlier age--chironomus showing us an almost exactly intermediate stage, viz., that of the pupa--and we can perhaps account for the marvellous case of the cecidomyia. it has already been stated that various parts in the same individual, which are exactly alike during an early embryonic period, become widely different and serve for widely different purposes in the adult state. so again it has been shown that generally the embryos of the most distinct species belonging to the same class are closely similar, but become, when fully developed, widely dissimilar. a better proof of this latter fact cannot be given than the statement by von baer that "the embryos of mammalia, of birds, lizards and snakes, probably also of chelonia, are in the earliest states exceedingly like one another, both as a whole and in the mode of development of their parts; so much so, in fact, that we can often distinguish the embryos only by their size. in my possession are two little embryos in spirit, whose names i have omitted to attach, and at present i am quite unable to say to what class they belong. they may be lizards or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals. the extremities, however, are still absent in these embryos. but even if they had existed in the earliest stage of their development we should learn nothing, for the feet of lizards and mammals, the wings and feet of birds, no less than the hands and feet of man, all arise from the same fundamental form." the larvae of most crustaceans, at corresponding stages of development, closely resemble each other, however different the adults may become; and so it is with very many other animals. a trace of the law of embryonic resemblance occasionally lasts till a rather late age: thus birds of the same genus, and of allied genera, often resemble each other in their immature plumage; as we see in the spotted feathers in the young of the thrush group. in the cat tribe, most of the species when adult are striped or spotted in lines; and stripes or spots can be plainly distinguished in the whelp of the lion and the puma. we occasionally, though rarely, see something of the same kind in plants; thus the first leaves of the ulex or furze, and the first leaves of the phyllodineous acacias, are pinnate or divided like the ordinary leaves of the leguminosae. the points of structure, in which the embryos of widely different animals within the same class resemble each other, often have no direct relation to their conditions of existence. we cannot, for instance, suppose that in the embryos of the vertebrata the peculiar loop-like courses of the arteries near the branchial slits are related to similar conditions--in the young mammal which is nourished in the womb of its mother, in the egg of the bird which is hatched in a nest, and in the spawn of a frog under water. we have no more reason to believe in such a relation than we have to believe that the similar bones in the hand of a man, wing of a bat, and fin of a porpoise, are related to similar conditions of life. no one supposes that the stripes on the whelp of a lion, or the spots on the young blackbird, are of any use to these animals. the case, however, is different when an animal, during any part of its embryonic career, is active, and has to provide for itself. the period of activity may come on earlier or later in life; but whenever it comes on, the adaptation of the larva to its conditions of life is just as perfect and as beautiful as in the adult animal. in how important a manner this has acted, has recently been well shown by sir j. lubbock in his remarks on the close similarity of the larvae of some insects belonging to very different orders, and on the dissimilarity of the larvae of other insects within the same order, according to their habits of life. owing to such adaptations the similarity of the larvae of allied animals is sometimes greatly obscured; especially when there is a division of labour during the different stages of development, as when the same larva has during one stage to search for food, and during another stage has to search for a place of attachment. cases can even be given of the larvae of allied species, or groups of species, differing more from each other than do the adults. in most cases, however, the larvae, though active, still obey, more or less closely, the law of common embryonic resemblance. cirripedes afford a good instance of this: even the illustrious cuvier did not perceive that a barnacle was a crustacean: but a glance at the larva shows this in an unmistakable manner. so again the two main divisions of cirripedes, the pedunculated and sessile, though differing widely in external appearance, have larvae in all their stages barely distinguishable. the embryo in the course of development generally rises in organisation. i use this expression, though i am aware that it is hardly possible to define clearly what is meant by organisation being higher or lower. but no one probably will dispute that the butterfly is higher than the caterpillar. in some cases, however, the mature animal must be considered as lower in the scale than the larva, as with certain parasitic crustaceans. to refer once again to cirripedes: the larvae in the first stage have three pairs of locomotive organs, a simple single eye, and a probosciformed mouth, with which they feed largely, for they increase much in size. in the second stage, answering to the chrysalis stage of butterflies, they have six pairs of beautifully constructed natatory legs, a pair of magnificent compound eyes, and extremely complex antennae; but they have a closed and imperfect mouth, and cannot feed: their function at this stage is, to search out by their well-developed organs of sense, and to reach by their active powers of swimming, a proper place on which to become attached and to undergo their final metamorphosis. when this is completed they are fixed for life: their legs are now converted into prehensile organs; they again obtain a well-constructed mouth; but they have no antennae, and their two eyes are now reconverted into a minute, single, simple eye-spot. in this last and complete state, cirripedes may be considered as either more highly or more lowly organised than they were in the larval condition. but in some genera the larvae become developed into hermaphrodites having the ordinary structure, or into what i have called complemental males; and in the latter the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time and is destitute of mouth, stomach, and every other organ of importance, excepting those for reproduction. we are so much accustomed to see a difference in structure between the embryo and the adult, that we are tempted to look at this difference as in some necessary manner contingent on growth. but there is no reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all their parts in proper proportion, as soon as any part became visible. in some whole groups of animals and in certain members of other groups this is the case, and the embryo does not at any period differ widely from the adult: thus owen has remarked in regard to cuttle-fish, "there is no metamorphosis; the cephalopodic character is manifested long before the parts of the embryo are completed." land-shells and fresh-water crustaceans are born having their proper forms, while the marine members of the same two great classes pass through considerable and often great changes during their development. spiders, again, barely undergo any metamorphosis. the larvae of most insects pass through a worm-like stage, whether they are active and adapted to diversified habits, or are inactive from being placed in the midst of proper nutriment, or from being fed by their parents; but in some few cases, as in that of aphis, if we look to the admirable drawings of the development of this insect, by professor huxley, we see hardly any trace of the vermiform stage. sometimes it is only the earlier developmental stages which fail. thus, fritz muller has made the remarkable discovery that certain shrimp-like crustaceans (allied to penoeus) first appear under the simple nauplius-form, and after passing through two or more zoea-stages, and then through the mysis-stage, finally acquire their mature structure: now in the whole great malacostracan order, to which these crustaceans belong, no other member is as yet known to be first developed under the nauplius-form, though many appear as zoeas; nevertheless muller assigns reasons for his belief, that if there had been no suppression of development, all these crustaceans would have appeared as nauplii. how, then, can we explain these several facts in embryology--namely, the very general, though not universal, difference in structure between the embryo and the adult; the various parts in the same individual embryo, which ultimately become very unlike, and serve for diverse purposes, being at an early period of growth alike; the common, but not invariable, resemblance between the embryos or larvae of the most distinct species in the same class; the embryo often retaining, while within the egg or womb, structures which are of no service to it, either at that or at a later period of life; on the other hand, larvae which have to provide for their own wants, being perfectly adapted to the surrounding conditions; and lastly, the fact of certain larvae standing higher in the scale of organisation than the mature animal into which they are developed? i believe that all these facts can be explained as follows. it is commonly assumed, perhaps from monstrosities affecting the embryo at a very early period, that slight variations or individual differences necessarily appear at an equally early period. we have little evidence on this head, but what we have certainly points the other way; for it is notorious that breeders of cattle, horses and various fancy animals, cannot positively tell, until some time after birth, what will be the merits and demerits of their young animals. we see this plainly in our own children; we cannot tell whether a child will be tall or short, or what its precise features will be. the question is not, at what period of life any variation may have been caused, but at what period the effects are displayed. the cause may have acted, and i believe often has acted, on one or both parents before the act of generation. it deserves notice that it is of no importance to a very young animal, as long as it is nourished and protected by its parent, whether most of its characters are acquired a little earlier or later in life. it would not signify, for instance, to a bird which obtained its food by having a much-curved beak whether or not while young it possessed a beak of this shape, as long as it was fed by its parents. i have stated in the first chapter, that at whatever age any variation first appears in the parent, it tends to reappear at a corresponding age in the offspring. certain variations can only appear at corresponding ages; for instance, peculiarities in the caterpillar, cocoon, or imago states of the silk-moth; or, again, in the full-grown horns of cattle. but variations which, for all that we can see might have appeared either earlier or later in life, likewise tend to reappear at a corresponding age in the offspring and parent. i am far from meaning that this is invariably the case, and i could give several exceptional cases of variations (taking the word in the largest sense) which have supervened at an earlier age in the child than in the parent. these two principles, namely, that slight variations generally appear at a not very early period of life, and are inherited at a corresponding not early period, explain, as i believe, all the above specified leading facts in embryology. but first let us look to a few analogous cases in our domestic varieties. some authors who have written on dogs maintain that the greyhound and bull-dog, though so different, are really closely allied varieties, descended from the same wild stock, hence i was curious to see how far their puppies differed from each other. i was told by breeders that they differed just as much as their parents, and this, judging by the eye, seemed almost to be the case; but on actually measuring the old dogs and their six-days-old puppies, i found that the puppies had not acquired nearly their full amount of proportional difference. so, again, i was told that the foals of cart and race-horses--breeds which have been almost wholly formed by selection under domestication--differed as much as the full-grown animals; but having had careful measurements made of the dams and of three-days-old colts of race and heavy cart-horses, i find that this is by no means the case. as we have conclusive evidence that the breeds of the pigeon are descended from a single wild species, i compared the young pigeons within twelve hours after being hatched. i carefully measured the proportions (but will not here give the details) of the beak, width of mouth, length of nostril and of eyelid, size of feet and length of leg, in the wild parent species, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. now, some of these birds, when mature, differ in so extraordinary a manner in the length and form of beak, and in other characters, that they would certainly have been ranked as distinct genera if found in a state of nature. but when the nestling birds of these several breeds were placed in a row, though most of them could just be distinguished, the proportional differences in the above specified points were incomparably less than in the full-grown birds. some characteristic points of difference--for instance, that of the width of mouth--could hardly be detected in the young. but there was one remarkable exception to this rule, for the young of the short-faced tumbler differed from the young of the wild rock-pigeon, and of the other breeds, in almost exactly the same proportions as in the adult stage. these facts are explained by the above two principles. fanciers select their dogs, horses, pigeons, etc., for breeding, when nearly grown up. they are indifferent whether the desired qualities are acquired earlier or later in life, if the full-grown animal possesses them. and the cases just given, more especially that of the pigeons, show that the characteristic differences which have been accumulated by man's selection, and which give value to his breeds, do not generally appear at a very early period of life, and are inherited at a corresponding not early period. but the case of the short-faced tumbler, which when twelve hours old possessed its proper characters, proves that this is not the universal rule; for here the characteristic differences must either have appeared at an earlier period than usual, or, if not so, the differences must have been inherited, not at a corresponding, but at an earlier age. now, let us apply these two principles to species in a state of nature. let us take a group of birds, descended from some ancient form and modified through natural selection for different habits. then, from the many slight successive variations having supervened in the several species at a not early age, and having been inherited at a corresponding age, the young will have been but little modified, and they will still resemble each other much more closely than do the adults, just as we have seen with the breeds of the pigeon. we may extend this view to widely distinct structures and to whole classes. the fore-limbs, for instance, which once served as legs to a remote progenitor, may have become, through a long course of modification, adapted in one descendant to act as hands, in another as paddles, in another as wings; but on the above two principles the fore-limbs will not have been much modified in the embryos of these several forms; although in each form the fore-limb will differ greatly in the adult state. whatever influence long continued use or disuse may have had in modifying the limbs or other parts of any species, this will chiefly or solely have affected it when nearly mature, when it was compelled to use its full powers to gain its own living; and the effects thus produced will have been transmitted to the offspring at a corresponding nearly mature age. thus the young will not be modified, or will be modified only in a slight degree, through the effects of the increased use or disuse of parts. with some animals the successive variations may have supervened at a very early period of life, or the steps may have been inherited at an earlier age than that at which they first occurred. in either of these cases the young or embryo will closely resemble the mature parent-form, as we have seen with the short-faced tumbler. and this is the rule of development in certain whole groups, or in certain sub-groups alone, as with cuttle-fish, land-shells, fresh-water crustaceans, spiders, and some members of the great class of insects. with respect to the final cause of the young in such groups not passing through any metamorphosis, we can see that this would follow from the following contingencies: namely, from the young having to provide at a very early age for their own wants, and from their following the same habits of life with their parents; for in this case it would be indispensable for their existence that they should be modified in the same manner as their parents. again, with respect to the singular fact that many terrestrial and fresh-water animals do not undergo any metamorphosis, while marine members of the same groups pass through various transformations, fritz muller has suggested that the process of slowly modifying and adapting an animal to live on the land or in fresh water, instead of in the sea, would be greatly simplified by its not passing through any larval stage; for it is not probable that places well adapted for both the larval and mature stages, under such new and greatly changed habits of life, would commonly be found unoccupied or ill-occupied by other organisms. in this case the gradual acquirement at an earlier and earlier age of the adult structure would be favoured by natural selection; and all traces of former metamorphoses would finally be lost. if, on the other hand, it profited the young of an animal to follow habits of life slightly different from those of the parent-form, and consequently to be constructed on a slightly different plan, or if it profited a larva already different from its parent to change still further, then, on the principle of inheritance at corresponding ages, the young or the larvae might be rendered by natural selection more and more different from their parents to any conceivable extent. differences in the larva might, also, become correlated with successive stages of its development; so that the larva, in the first stage, might come to differ greatly from the larva in the second stage, as is the case with many animals. the adult might also become fitted for sites or habits, in which organs of locomotion or of the senses, etc., would be useless; and in this case the metamorphosis would be retrograde. from the remarks just made we can see how by changes of structure in the young, in conformity with changed habits of life, together with inheritance at corresponding ages, animals might come to pass through stages of development, perfectly distinct from the primordial condition of their adult progenitors. most of our best authorities are now convinced that the various larval and pupal stages of insects have thus been acquired through adaptation, and not through inheritance from some ancient form. the curious case of sitaris--a beetle which passes through certain unusual stages of development--will illustrate how this might occur. the first larval form is described by m. fabre, as an active, minute insect, furnished with six legs, two long antennae, and four eyes. these larvae are hatched in the nests of bees; and when the male bees emerge from their burrows, in the spring, which they do before the females, the larvae spring on them, and afterwards crawl on to the females while paired with the males. as soon as the female bee deposits her eggs on the surface of the honey stored in the cells, the larvae of the sitaris leap on the eggs and devour them. afterwards they undergo a complete change; their eyes disappear; their legs and antennae become rudimentary, and they feed on honey; so that they now more closely resemble the ordinary larvae of insects; ultimately they undergo a further transformation, and finally emerge as the perfect beetle. now, if an insect, undergoing transformations like those of the sitaris, were to become the progenitor of a whole new class of insects, the course of development of the new class would be widely different from that of our existing insects; and the first larval stage certainly would not represent the former condition of any adult and ancient form. on the other hand it is highly probable that with many animals the embryonic or larval stages show us, more or less completely, the condition of the progenitor of the whole group in its adult state. in the great class of the crustacea, forms wonderfully distinct from each other, namely, suctorial parasites, cirripedes, entomostraca, and even the malacostraca, appear at first as larvae under the nauplius-form; and as these larvae live and feed in the open sea, and are not adapted for any peculiar habits of life, and from other reasons assigned by fritz muller, it is probable that at some very remote period an independent adult animal, resembling the nauplius, existed, and subsequently produced, along several divergent lines of descent, the above-named great crustacean groups. so again, it is probable, from what we know of the embryos of mammals, birds, fishes and reptiles, that these animals are the modified descendants of some ancient progenitor, which was furnished in its adult state with branchiae, a swim-bladder, four fin-like limbs, and a long tail, all fitted for an aquatic life. as all the organic beings, extinct and recent, which have ever lived, can be arranged within a few great classes; and as all within each class have, according to our theory, been connected together by fine gradations, the best, and, if our collections were nearly perfect, the only possible arrangement, would be genealogical; descent being the hidden bond of connexion which naturalists have been seeking under the term of the natural system. on this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. in two or more groups of animals, however much they may differ from each other in structure and habits in their adult condition, if they pass through closely similar embryonic stages, we may feel assured that they are all descended from one parent-form, and are therefore closely related. thus, community in embryonic structure reveals community of descent; but dissimilarity in embryonic development does not prove discommunity of descent, for in one of two groups the developmental stages may have been suppressed, or may have been so greatly modified through adaptation to new habits of life as to be no longer recognisable. even in groups, in which the adults have been modified to an extreme degree, community of origin is often revealed by the structure of the larvae; we have seen, for instance, that cirripedes, though externally so like shell-fish, are at once known by their larvae to belong to the great class of crustaceans. as the embryo often shows us more or less plainly the structure of the less modified and ancient progenitor of the group, we can see why ancient and extinct forms so often resemble in their adult state the embryos of existing species of the same class. agassiz believes this to be a universal law of nature; and we may hope hereafter to see the law proved true. it can, however, be proved true only in those cases in which the ancient state of the progenitor of the group has not been wholly obliterated, either by successive variations having supervened at a very early period of growth, or by such variations having been inherited at an earlier age than that at which they first appeared. it should also be borne in mind, that the law may be true, but yet, owing to the geological record not extending far enough back in time, may remain for a long period, or for ever, incapable of demonstration. the law will not strictly hold good in those cases in which an ancient form became adapted in its larval state to some special line of life, and transmitted the same larval state to a whole group of descendants; for such larval state will not resemble any still more ancient form in its adult state. thus, as it seems to me, the leading facts in embryology, which are second to none in importance, are explained on the principle of variations in the many descendants from some one ancient progenitor, having appeared at a not very early period of life, and having been inherited at a corresponding period. embryology rises greatly in interest, when we look at the embryo as a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class. rudimentary, atrophied, and aborted organs. organs or parts in this strange condition, bearing the plain stamp of inutility, are extremely common, or even general, throughout nature. it would be impossible to name one of the higher animals in which some part or other is not in a rudimentary condition. in the mammalia, for instance, the males possess rudimentary mammae; in snakes one lobe of the lungs is rudimentary; in birds the "bastard-wing" may safely be considered as a rudimentary digit, and in some species the whole wing is so far rudimentary that it cannot be used for flight. what can be more curious than the presence of teeth in foetal whales, which when grown up have not a tooth in their heads; or the teeth, which never cut through the gums, in the upper jaws of unborn calves? rudimentary organs plainly declare their origin and meaning in various ways. there are beetles belonging to closely allied species, or even to the same identical species, which have either full-sized and perfect wings, or mere rudiments of membrane, which not rarely lie under wing-covers firmly soldered together; and in these cases it is impossible to doubt, that the rudiments represent wings. rudimentary organs sometimes retain their potentiality: this occasionally occurs with the mammae of male mammals, which have been known to become well developed and to secrete milk. so again in the udders of the genus bos, there are normally four developed and two rudimentary teats; but the latter in our domestic cows sometimes become well developed and yield milk. in regard to plants, the petals are sometimes rudimentary, and sometimes well developed in the individuals of the same species. in certain plants having separated sexes kolreuter found that by crossing a species, in which the male flowers included a rudiment of a pistil, with an hermaphrodite species, having of course a well-developed pistil, the rudiment in the hybrid offspring was much increased in size; and this clearly shows that the rudimentary and perfect pistils are essentially alike in nature. an animal may possess various parts in a perfect state, and yet they may in one sense be rudimentary, for they are useless: thus the tadpole of the common salamander or water-newt, as mr. g.h. lewes remarks, "has gills, and passes its existence in the water; but the salamandra atra, which lives high up among the mountains, brings forth its young full-formed. this animal never lives in the water. yet if we open a gravid female, we find tadpoles inside her with exquisitely feathered gills; and when placed in water they swim about like the tadpoles of the water-newt. obviously this aquatic organisation has no reference to the future life of the animal, nor has it any adaptation to its embryonic condition; it has solely reference to ancestral adaptations, it repeats a phase in the development of its progenitors." an organ, serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose, and remain perfectly efficient for the other. thus, in plants, the office of the pistil is to allow the pollen-tubes to reach the ovules within the ovarium. the pistil consists of a stigma supported on the style; but in some compositae, the male florets, which of course cannot be fecundated, have a rudimentary pistil, for it is not crowned with a stigma; but the style remains well developed and is clothed in the usual manner with hairs, which serve to brush the pollen out of the surrounding and conjoined anthers. again, an organ may become rudimentary for its proper purpose, and be used for a distinct one: in certain fishes the swim-bladder seems to be rudimentary for its proper function of giving buoyancy, but has become converted into a nascent breathing organ or lung. many similar instances could be given. useful organs, however little they may be developed, unless we have reason to suppose that they were formerly more highly developed, ought not to be considered as rudimentary. they may be in a nascent condition, and in progress towards further development. rudimentary organs, on the other hand, are either quite useless, such as teeth which never cut through the gums, or almost useless, such as the wings of an ostrich, which serve merely as sails. as organs in this condition would formerly, when still less developed, have been of even less use than at present, they cannot formerly have been produced through variation and natural selection, which acts solely by the preservation of useful modifications. they have been partially retained by the power of inheritance, and relate to a former state of things. it is, however, often difficult to distinguish between rudimentary and nascent organs; for we can judge only by analogy whether a part is capable of further development, in which case alone it deserves to be called nascent. organs in this condition will always be somewhat rare; for beings thus provided will commonly have been supplanted by their successors with the same organ in a more perfect state, and consequently will have become long ago extinct. the wing of the penguin is of high service, acting as a fin; it may, therefore, represent the nascent state of the wing: not that i believe this to be the case; it is more probably a reduced organ, modified for a new function: the wing of the apteryx, on the other hand, is quite useless, and is truly rudimentary. owen considers the simple filamentary limbs of the lepidosiren as the "beginnings of organs which attain full functional development in higher vertebrates;" but, according to the view lately advocated by dr. gunther, they are probably remnants, consisting of the persistent axis of a fin, with the lateral rays or branches aborted. the mammary glands of the ornithorhynchus may be considered, in comparison with the udders of a cow, as in a nascent condition. the ovigerous frena of certain cirripedes, which have ceased to give attachment to the ova and are feebly developed, are nascent branchiae. rudimentary organs in the individuals of the same species are very liable to vary in the degree of their development and in other respects. in closely allied species, also, the extent to which the same organ has been reduced occasionally differs much. this latter fact is well exemplified in the state of the wings of female moths belonging to the same family. rudimentary organs may be utterly aborted; and this implies, that in certain animals or plants, parts are entirely absent which analogy would lead us to expect to find in them, and which are occasionally found in monstrous individuals. thus in most of the scrophulariaceae the fifth stamen is utterly aborted; yet we may conclude that a fifth stamen once existed, for a rudiment of it is found in many species of the family, and this rudiment occasionally becomes perfectly developed, as may sometimes be seen in the common snap-dragon. in tracing the homologies of any part in different members of the same class, nothing is more common, or, in order fully to understand the relations of the parts, more useful than the discovery of rudiments. this is well shown in the drawings given by owen of the leg bones of the horse, ox, and rhinoceros. it is an important fact that rudimentary organs, such as teeth in the upper jaws of whales and ruminants, can often be detected in the embryo, but afterwards wholly disappear. it is also, i believe, a universal rule, that a rudimentary part is of greater size in the embryo relatively to the adjoining parts, than in the adult; so that the organ at this early age is less rudimentary, or even cannot be said to be in any degree rudimentary. hence rudimentary organs in the adult are often said to have retained their embryonic condition. i have now given the leading facts with respect to rudimentary organs. in reflecting on them, every one must be struck with astonishment; for the same reasoning power which tells us that most parts and organs are exquisitely adapted for certain purposes, tells us with equal plainness that these rudimentary or atrophied organs are imperfect and useless. in works on natural history, rudimentary organs are generally said to have been created "for the sake of symmetry," or in order "to complete the scheme of nature." but this is not an explanation, merely a restatement of the fact. nor is it consistent with itself: thus the boa-constrictor has rudiments of hind limbs and of a pelvis, and if it be said that these bones have been retained "to complete the scheme of nature," why, as professor weismann asks, have they not been retained by other snakes, which do not possess even a vestige of these same bones? what would be thought of an astronomer who maintained that the satellites revolve in elliptic courses round their planets "for the sake of symmetry," because the planets thus revolve round the sun? an eminent physiologist accounts for the presence of rudimentary organs, by supposing that they serve to excrete matter in excess, or matter injurious to the system; but can we suppose that the minute papilla, which often represents the pistil in male flowers, and which is formed of mere cellular tissue, can thus act? can we suppose that rudimentary teeth, which are subsequently absorbed, are beneficial to the rapidly growing embryonic calf by removing matter so precious as phosphate of lime? when a man's fingers have been amputated, imperfect nails have been known to appear on the stumps, and i could as soon believe that these vestiges of nails are developed in order to excrete horny matter, as that the rudimentary nails on the fin of the manatee have been developed for this same purpose. on the view of descent with modification, the origin of rudimentary organs is comparatively simple; and we can understand to a large extent the laws governing their imperfect development. we have plenty of cases of rudimentary organs in our domestic productions, as the stump of a tail in tailless breeds, the vestige of an ear in earless breeds of sheep--the reappearance of minute dangling horns in hornless breeds of cattle, more especially, according to youatt, in young animals--and the state of the whole flower in the cauliflower. we often see rudiments of various parts in monsters; but i doubt whether any of these cases throw light on the origin of rudimentary organs in a state of nature, further than by showing that rudiments can be produced; for the balance of evidence clearly indicates that species under nature do not undergo great and abrupt changes. but we learn from the study of our domestic productions that the disuse of parts leads to their reduced size; and that the result is inherited. it appears probable that disuse has been the main agent in rendering organs rudimentary. it would at first lead by slow steps to the more and more complete reduction of a part, until at last it became rudimentary--as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced by beasts of prey to take flight, and have ultimately lost the power of flying. again, an organ, useful under certain conditions, might become injurious under others, as with the wings of beetles living on small and exposed islands; and in this case natural selection will have aided in reducing the organ, until it was rendered harmless and rudimentary. any change in structure and function, which can be effected by small stages, is within the power of natural selection; so that an organ rendered, through changed habits of life, useless or injurious for one purpose, might be modified and used for another purpose. an organ might, also, be retained for one alone of its former functions. organs, originally formed by the aid of natural selection, when rendered useless may well be variable, for their variations can no longer be checked by natural selection. all this agrees well with what we see under nature. moreover, at whatever period of life either disuse or selection reduces an organ, and this will generally be when the being has come to maturity and to exert its full powers of action, the principle of inheritance at corresponding ages will tend to reproduce the organ in its reduced state at the same mature age, but will seldom affect it in the embryo. thus we can understand the greater size of rudimentary organs in the embryo relatively to the adjoining parts, and their lesser relative size in the adult. if, for instance, the digit of an adult animal was used less and less during many generations, owing to some change of habits, or if an organ or gland was less and less functionally exercised, we may infer that it would become reduced in size in the adult descendants of this animal, but would retain nearly its original standard of development in the embryo. there remains, however, this difficulty. after an organ has ceased being used, and has become in consequence much reduced, how can it be still further reduced in size until the merest vestige is left; and how can it be finally quite obliterated? it is scarcely possible that disuse can go on producing any further effect after the organ has once been rendered functionless. some additional explanation is here requisite which i cannot give. if, for instance, it could be proved that every part of the organisation tends to vary in a greater degree towards diminution than toward augmentation of size, then we should be able to understand how an organ which has become useless would be rendered, independently of the effects of disuse, rudimentary and would at last be wholly suppressed; for the variations towards diminished size would no longer be checked by natural selection. the principle of the economy of growth, explained in a former chapter, by which the materials forming any part, if not useful to the possessor, are saved as far as is possible, will perhaps come into play in rendering a useless part rudimentary. but this principle will almost necessarily be confined to the earlier stages of the process of reduction; for we cannot suppose that a minute papilla, for instance, representing in a male flower the pistil of the female flower, and formed merely of cellular tissue, could be further reduced or absorbed for the sake of economising nutriment. finally, as rudimentary organs, by whatever steps they may have been degraded into their present useless condition, are the record of a former state of things, and have been retained solely through the power of inheritance--we can understand, on the genealogical view of classification, how it is that systematists, in placing organisms in their proper places in the natural system, have often found rudimentary parts as useful as, or even sometimes more useful than, parts of high physiological importance. rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue for its derivation. on the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the old doctrine of creation, might even have been anticipated in accordance with the views here explained. summary. in this chapter i have attempted to show that the arrangement of all organic beings throughout all time in groups under groups--that the nature of the relationships by which all living and extinct organisms are united by complex, radiating, and circuitous lines of affinities into a few grand classes--the rules followed and the difficulties encountered by naturalists in their classifications--the value set upon characters, if constant and prevalent, whether of high or of the most trifling importance, or, as with rudimentary organs of no importance--the wide opposition in value between analogical or adaptive characters, and characters of true affinity; and other such rules--all naturally follow if we admit the common parentage of allied forms, together with their modification through variation and natural selection, with the contingencies of extinction and divergence of character. in considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, dimorphic forms, and acknowledged varieties of the same species, however much they may differ from each other in structure. if we extend the use of this element of descent--the one certainly known cause of similarity in organic beings--we shall understand what is meant by the natural system: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms, varieties, species, genera, families, orders, and classes. on this same view of descent with modification, most of the great facts in morphology become intelligible--whether we look to the same pattern displayed by the different species of the same class in their homologous organs, to whatever purpose applied, or to the serial and lateral homologies in each individual animal and plant. on the principle of successive slight variations, not necessarily or generally supervening at a very early period of life, and being inherited at a corresponding period, we can understand the leading facts in embryology; namely, the close resemblance in the individual embryo of the parts which are homologous, and which when matured become widely different in structure and function; and the resemblance of the homologous parts or organs in allied though distinct species, though fitted in the adult state for habits as different as is possible. larvae are active embryos, which have become specially modified in a greater or less degree in relation to their habits of life, with their modifications inherited at a corresponding early age. on these same principles, and bearing in mind that when organs are reduced in size, either from disuse or through natural selection, it will generally be at that period of life when the being has to provide for its own wants, and bearing in mind how strong is the force of inheritance--the occurrence of rudimentary organs might even have been anticipated. the importance of embryological characters and of rudimentary organs in classification is intelligible, on the view that a natural arrangement must be genealogical. finally, the several classes of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera and families, with which this world is peopled, are all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that i should without hesitation adopt this view, even if it were unsupported by other facts or arguments. chapter xv. recapitulation and conclusion. recapitulation of the objections to the theory of natural selection--recapitulation of the general and special circumstances in its favour--causes of the general belief in the immutability of species--how far the theory of natural selection may be extended--effects of its adoption on the study of natural history--concluding remarks. as this whole volume is one long argument, it may be convenient to the reader to have the leading facts and inferences briefly recapitulated. that many and serious objections may be advanced against the theory of descent with modification through variation and natural selection, i do not deny. i have endeavoured to give to them their full force. nothing at first can appear more difficult to believe than that the more complex organs and instincts have been perfected, not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor. nevertheless, this difficulty, though appearing to our imagination insuperably great, cannot be considered real if we admit the following propositions, namely, that all parts of the organisation and instincts offer, at least individual differences--that there is a struggle for existence leading to the preservation of profitable deviations of structure or instinct--and, lastly, that gradations in the state of perfection of each organ may have existed, each good of its kind. the truth of these propositions cannot, i think, be disputed. it is, no doubt, extremely difficult even to conjecture by what gradations many structures have been perfected, more especially among broken and failing groups of organic beings, which have suffered much extinction; but we see so many strange gradations in nature, that we ought to be extremely cautious in saying that any organ or instinct, or any whole structure, could not have arrived at its present state by many graduated steps. there are, it must be admitted, cases of special difficulty opposed to the theory of natural selection; and one of the most curious of these is the existence in the same community of two or three defined castes of workers or sterile female ants; but i have attempted to show how these difficulties can be mastered. with respect to the almost universal sterility of species when first crossed, which forms so remarkable a contrast with the almost universal fertility of varieties when crossed, i must refer the reader to the recapitulation of the facts given at the end of the ninth chapter, which seem to me conclusively to show that this sterility is no more a special endowment than is the incapacity of two distinct kinds of trees to be grafted together; but that it is incidental on differences confined to the reproductive systems of the intercrossed species. we see the truth of this conclusion in the vast difference in the results of crossing the same two species reciprocally--that is, when one species is first used as the father and then as the mother. analogy from the consideration of dimorphic and trimorphic plants clearly leads to the same conclusion, for when the forms are illegitimately united, they yield few or no seed, and their offspring are more or less sterile; and these forms belong to the same undoubted species, and differ from each other in no respect except in their reproductive organs and functions. although the fertility of varieties when intercrossed, and of their mongrel offspring, has been asserted by so many authors to be universal, this cannot be considered as quite correct after the facts given on the high authority of gartner and kolreuter. most of the varieties which have been experimented on have been produced under domestication; and as domestication (i do not mean mere confinement) almost certainly tends to eliminate that sterility which, judging from analogy, would have affected the parent-species if intercrossed, we ought not to expect that domestication would likewise induce sterility in their modified descendants when crossed. this elimination of sterility apparently follows from the same cause which allows our domestic animals to breed freely under diversified circumstances; and this again apparently follows from their having been gradually accustomed to frequent changes in their conditions of life. a double and parallel series of facts seems to throw much light on the sterility of species, when first crossed, and of their hybrid offspring. on the one side, there is good reason to believe that slight changes in the conditions of life give vigour and fertility to all organic beings. we know also that a cross between the distinct individuals of the same variety, and between distinct varieties, increases the number of their offspring, and certainly gives to them increased size and vigour. this is chiefly owing to the forms which are crossed having been exposed to somewhat different conditions of life; for i have ascertained by a labourious series of experiments that if all the individuals of the same variety be subjected during several generations to the same conditions, the good derived from crossing is often much diminished or wholly disappears. this is one side of the case. on the other side, we know that species which have long been exposed to nearly uniform conditions, when they are subjected under confinement to new and greatly changed conditions, either perish, or if they survive, are rendered sterile, though retaining perfect health. this does not occur, or only in a very slight degree, with our domesticated productions, which have long been exposed to fluctuating conditions. hence when we find that hybrids produced by a cross between two distinct species are few in number, owing to their perishing soon after conception or at a very early age, or if surviving that they are rendered more or less sterile, it seems highly probable that this result is due to their having been in fact subjected to a great change in their conditions of life, from being compounded of two distinct organisations. he who will explain in a definite manner why, for instance, an elephant or a fox will not breed under confinement in its native country, whilst the domestic pig or dog will breed freely under the most diversified conditions, will at the same time be able to give a definite answer to the question why two distinct species, when crossed, as well as their hybrid offspring, are generally rendered more or less sterile, while two domesticated varieties when crossed and their mongrel offspring are perfectly fertile. turning to geographical distribution, the difficulties encountered on the theory of descent with modification are serious enough. all the individuals of the same species, and all the species of the same genus, or even higher group, are descended from common parents; and therefore, in however distant and isolated parts of the world they may now be found, they must in the course of successive generations have travelled from some one point to all the others. we are often wholly unable even to conjecture how this could have been effected. yet, as we have reason to believe that some species have retained the same specific form for very long periods of time, immensely long as measured by years, too much stress ought not to be laid on the occasional wide diffusion of the same species; for during very long periods there will always have been a good chance for wide migration by many means. a broken or interrupted range may often be accounted for by the extinction of the species in the intermediate regions. it cannot be denied that we are as yet very ignorant as to the full extent of the various climatical and geographical changes which have affected the earth during modern periods; and such changes will often have facilitated migration. as an example, i have attempted to show how potent has been the influence of the glacial period on the distribution of the same and of allied species throughout the world. we are as yet profoundly ignorant of the many occasional means of transport. with respect to distinct species of the same genus, inhabiting distant and isolated regions, as the process of modification has necessarily been slow, all the means of migration will have been possible during a very long period; and consequently the difficulty of the wide diffusion of the species of the same genus is in some degree lessened. as according to the theory of natural selection an interminable number of intermediate forms must have existed, linking together all the species in each group by gradations as fine as our existing varieties, it may be asked, why do we not see these linking forms all around us? why are not all organic beings blended together in an inextricable chaos? with respect to existing forms, we should remember that we have no right to expect (excepting in rare cases) to discover directly connecting links between them, but only between each and some extinct and supplanted form. even on a wide area, which has during a long period remained continuous, and of which the climatic and other conditions of life change insensibly in proceeding from a district occupied by one species into another district occupied by a closely allied species, we have no just right to expect often to find intermediate varieties in the intermediate zones. for we have reason to believe that only a few species of a genus ever undergo change; the other species becoming utterly extinct and leaving no modified progeny. of the species which do change, only a few within the same country change at the same time; and all modifications are slowly effected. i have also shown that the intermediate varieties which probably at first existed in the intermediate zones, would be liable to be supplanted by the allied forms on either hand; for the latter, from existing in greater numbers, would generally be modified and improved at a quicker rate than the intermediate varieties, which existed in lesser numbers; so that the intermediate varieties would, in the long run, be supplanted and exterminated. on this doctrine of the extermination of an infinitude of connecting links, between the living and extinct inhabitants of the world, and at each successive period between the extinct and still older species, why is not every geological formation charged with such links? why does not every collection of fossil remains afford plain evidence of the gradation and mutation of the forms of life? although geological research has undoubtedly revealed the former existence of many links, bringing numerous forms of life much closer together, it does not yield the infinitely many fine gradations between past and present species required on the theory, and this is the most obvious of the many objections which may be urged against it. why, again, do whole groups of allied species appear, though this appearance is often false, to have come in suddenly on the successive geological stages? although we now know that organic beings appeared on this globe, at a period incalculably remote, long before the lowest bed of the cambrian system was deposited, why do we not find beneath this system great piles of strata stored with the remains of the progenitors of the cambrian fossils? for on the theory, such strata must somewhere have been deposited at these ancient and utterly unknown epochs of the world's history. i can answer these questions and objections only on the supposition that the geological record is far more imperfect than most geologists believe. the number of specimens in all our museums is absolutely as nothing compared with the countless generations of countless species which have certainly existed. the parent form of any two or more species would not be in all its characters directly intermediate between its modified offspring, any more than the rock-pigeon is directly intermediate in crop and tail between its descendants, the pouter and fantail pigeons. we should not be able to recognise a species as the parent of another and modified species, if we were to examine the two ever so closely, unless we possessed most of the intermediate links; and owing to the imperfection of the geological record, we have no just right to expect to find so many links. if two or three, or even more linking forms were discovered, they would simply be ranked by many naturalists as so many new species, more especially if found in different geological substages, let their differences be ever so slight. numerous existing doubtful forms could be named which are probably varieties; but who will pretend that in future ages so many fossil links will be discovered, that naturalists will be able to decide whether or not these doubtful forms ought to be called varieties? only a small portion of the world has been geologically explored. only organic beings of certain classes can be preserved in a fossil condition, at least in any great number. many species when once formed never undergo any further change but become extinct without leaving modified descendants; and the periods during which species have undergone modification, though long as measured by years, have probably been short in comparison with the periods during which they retained the same form. it is the dominant and widely ranging species which vary most frequently and vary most, and varieties are often at first local--both causes rendering the discovery of intermediate links in any one formation less likely. local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they have spread, and are discovered in a geological formation, they appear as if suddenly created there, and will be simply classed as new species. most formations have been intermittent in their accumulation; and their duration has probably been shorter than the average duration of specific forms. successive formations are in most cases separated from each other by blank intervals of time of great length, for fossiliferous formations thick enough to resist future degradation can, as a general rule, be accumulated only where much sediment is deposited on the subsiding bed of the sea. during the alternate periods of elevation and of stationary level the record will generally be blank. during these latter periods there will probably be more variability in the forms of life; during periods of subsidence, more extinction. with respect to the absence of strata rich in fossils beneath the cambrian formation, i can recur only to the hypothesis given in the tenth chapter; namely, that though our continents and oceans have endured for an enormous period in nearly their present relative positions, we have no reason to assume that this has always been the case; consequently formations much older than any now known may lie buried beneath the great oceans. with respect to the lapse of time not having been sufficient since our planet was consolidated for the assumed amount of organic change, and this objection, as urged by sir william thompson, is probably one of the gravest as yet advanced, i can only say, firstly, that we do not know at what rate species change, as measured by years, and secondly, that many philosophers are not as yet willing to admit that we know enough of the constitution of the universe and of the interior of our globe to speculate with safety on its past duration. that the geological record is imperfect all will admit; but that it is imperfect to the degree required by our theory, few will be inclined to admit. if we look to long enough intervals of time, geology plainly declares that species have all changed; and they have changed in the manner required by the theory, for they have changed slowly and in a graduated manner. we clearly see this in the fossil remains from consecutive formations invariably being much more closely related to each other than are the fossils from widely separated formations. such is the sum of the several chief objections and difficulties which may justly be urged against the theory; and i have now briefly recapitulated the answers and explanations which, as far as i can see, may be given. i have felt these difficulties far too heavily during many years to doubt their weight. but it deserves especial notice that the more important objections relate to questions on which we are confessedly ignorant; nor do we know how ignorant we are. we do not know all the possible transitional gradations between the simplest and the most perfect organs; it cannot be pretended that we know all the varied means of distribution during the long lapse of years, or that we know how imperfect is the geological record. serious as these several objections are, in my judgment they are by no means sufficient to overthrow the theory of descent with subsequent modification. now let us turn to the other side of the argument. under domestication we see much variability, caused, or at least excited, by changed conditions of life; but often in so obscure a manner, that we are tempted to consider the variations as spontaneous. variability is governed by many complex laws, by correlated growth, compensation, the increased use and disuse of parts, and the definite action of the surrounding conditions. there is much difficulty in ascertaining how largely our domestic productions have been modified; but we may safely infer that the amount has been large, and that modifications can be inherited for long periods. as long as the conditions of life remain the same, we have reason to believe that a modification, which has already been inherited for many generations, may continue to be inherited for an almost infinite number of generations. on the other hand we have evidence that variability, when it has once come into play, does not cease under domestication for a very long period; nor do we know that it ever ceases, for new varieties are still occasionally produced by our oldest domesticated productions. variability is not actually caused by man; he only unintentionally exposes organic beings to new conditions of life and then nature acts on the organisation and causes it to vary. but man can and does select the variations given to him by nature, and thus accumulates them in any desired manner. he thus adapts animals and plants for his own benefit or pleasure. he may do this methodically, or he may do it unconsciously by preserving the individuals most useful or pleasing to him without any intention of altering the breed. it is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be inappreciable except by an educated eye. this unconscious process of selection has been the great agency in the formation of the most distinct and useful domestic breeds. that many breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether many of them are varieties or aboriginally distinct species. there is no reason why the principles which have acted so efficiently under domestication should not have acted under nature. in the survival of favoured individuals and races, during the constantly recurrent struggle for existence, we see a powerful and ever-acting form of selection. the struggle for existence inevitably follows from the high geometrical ratio of increase which is common to all organic beings. this high rate of increase is proved by calculation--by the rapid increase of many animals and plants during a succession of peculiar seasons, and when naturalised in new countries. more individuals are born than can possibly survive. a grain in the balance may determine which individuals shall live and which shall die--which variety or species shall increase in number, and which shall decrease, or finally become extinct. as the individuals of the same species come in all respects into the closest competition with each other, the struggle will generally be most severe between them; it will be almost equally severe between the varieties of the same species, and next in severity between the species of the same genus. on the other hand the struggle will often be severe between beings remote in the scale of nature. the slightest advantage in certain individuals, at any age or during any season, over those with which they come into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will, in the long run, turn the balance. with animals having separated sexes, there will be in most cases a struggle between the males for the possession of the females. the most vigorous males, or those which have most successfully struggled with their conditions of life, will generally leave most progeny. but success will often depend on the males having special weapons or means of defence or charms; and a slight advantage will lead to victory. as geology plainly proclaims that each land has undergone great physical changes, we might have expected to find that organic beings have varied under nature, in the same way as they have varied under domestication. and if there has been any variability under nature, it would be an unaccountable fact if natural selection had not come into play. it has often been asserted, but the assertion is incapable of proof, that the amount of variation under nature is a strictly limited quantity. man, though acting on external characters alone and often capriciously, can produce within a short period a great result by adding up mere individual differences in his domestic productions; and every one admits that species present individual differences. but, besides such differences, all naturalists admit that natural varieties exist, which are considered sufficiently distinct to be worthy of record in systematic works. no one has drawn any clear distinction between individual differences and slight varieties; or between more plainly marked varieties and subspecies and species. on separate continents, and on different parts of the same continent, when divided by barriers of any kind, and on outlying islands, what a multitude of forms exist, which some experienced naturalists rank as varieties, others as geographical races or sub species, and others as distinct, though closely allied species! if, then, animals and plants do vary, let it be ever so slightly or slowly, why should not variations or individual differences, which are in any way beneficial, be preserved and accumulated through natural selection, or the survival of the fittest? if man can by patience select variations useful to him, why, under changing and complex conditions of life, should not variations useful to nature's living products often arise, and be preserved or selected? what limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure, and habits of each creature, favouring the good and rejecting the bad? i can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life. the theory of natural selection, even if we look no further than this, seems to be in the highest degree probable. i have already recapitulated, as fairly as i could, the opposed difficulties and objections: now let us turn to the special facts and arguments in favour of the theory. on the view that species are only strongly marked and permanent varieties, and that each species first existed as a variety, we can see why it is that no line of demarcation can be drawn between species, commonly supposed to have been produced by special acts of creation, and varieties which are acknowledged to have been produced by secondary laws. on this same view we can understand how it is that in a region where many species of a genus have been produced, and where they now flourish, these same species should present many varieties; for where the manufactory of species has been active, we might expect, as a general rule, to find it still in action; and this is the case if varieties be incipient species. moreover, the species of the larger genera, which afford the greater number of varieties or incipient species, retain to a certain degree the character of varieties; for they differ from each other by a less amount of difference than do the species of smaller genera. the closely allied species also of a larger genera apparently have restricted ranges, and in their affinities they are clustered in little groups round other species--in both respects resembling varieties. these are strange relations on the view that each species was independently created, but are intelligible if each existed first as a variety. as each species tends by its geometrical rate of reproduction to increase inordinately in number; and as the modified descendants of each species will be enabled to increase by as much as they become more diversified in habits and structure, so as to be able to seize on many and widely different places in the economy of nature, there will be a constant tendency in natural selection to preserve the most divergent offspring of any one species. hence during a long-continued course of modification, the slight differences characteristic of varieties of the same species, tend to be augmented into the greater differences characteristic of the species of the same genus. new and improved varieties will inevitably supplant and exterminate the older, less improved and intermediate varieties; and thus species are rendered to a large extent defined and distinct objects. dominant species belonging to the larger groups within each class tend to give birth to new and dominant forms; so that each large group tends to become still larger, and at the same time more divergent in character. but as all groups cannot thus go on increasing in size, for the world would not hold them, the more dominant groups beat the less dominant. this tendency in the large groups to go on increasing in size and diverging in character, together with the inevitable contingency of much extinction, explains the arrangement of all the forms of life in groups subordinate to groups, all within a few great classes, which has prevailed throughout all time. this grand fact of the grouping of all organic beings under what is called the natural system, is utterly inexplicable on the theory of creation. as natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modifications; it can act only by short and slow steps. hence, the canon of "natura non facit saltum," which every fresh addition to our knowledge tends to confirm, is on this theory intelligible. we can see why throughout nature the same general end is gained by an almost infinite diversity of means, for every peculiarity when once acquired is long inherited, and structures already modified in many different ways have to be adapted for the same general purpose. we can, in short, see why nature is prodigal in variety, though niggard in innovation. but why this should be a law of nature if each species has been independently created no man can explain. many other facts are, as it seems to me, explicable on this theory. how strange it is that a bird, under the form of a woodpecker, should prey on insects on the ground; that upland geese, which rarely or never swim, would possess webbed feet; that a thrush-like bird should dive and feed on sub-aquatic insects; and that a petrel should have the habits and structure fitting it for the life of an auk! and so in endless other cases. but on the view of each species constantly trying to increase in number, with natural selection always ready to adapt the slowly varying descendants of each to any unoccupied or ill-occupied place in nature, these facts cease to be strange, or might even have been anticipated. we can to a certain extent understand how it is that there is so much beauty throughout nature; for this may be largely attributed to the agency of selection. that beauty, according to our sense of it, is not universal, must be admitted by every one who will look at some venomous snakes, at some fishes, and at certain hideous bats with a distorted resemblance to the human face. sexual selection has given the most brilliant colours, elegant patterns, and other ornaments to the males, and sometimes to both sexes of many birds, butterflies and other animals. with birds it has often rendered the voice of the male musical to the female, as well as to our ears. flowers and fruit have been rendered conspicuous by brilliant colours in contrast with the green foliage, in order that the flowers may be easily seen, visited and fertilised by insects, and the seeds disseminated by birds. how it comes that certain colours, sounds and forms should give pleasure to man and the lower animals, that is, how the sense of beauty in its simplest form was first acquired, we do not know any more than how certain odours and flavours were first rendered agreeable. as natural selection acts by competition, it adapts and improves the inhabitants of each country only in relation to their co-inhabitants; so that we need feel no surprise at the species of any one country, although on the ordinary view supposed to have been created and specially adapted for that country, being beaten and supplanted by the naturalised productions from another land. nor ought we to marvel if all the contrivances in nature be not, as far as we can judge, absolutely perfect; as in the case even of the human eye; or if some of them be abhorrent to our ideas of fitness. we need not marvel at the sting of the bee, when used against the enemy, causing the bee's own death; at drones being produced in such great numbers for one single act, and being then slaughtered by their sterile sisters; at the astonishing waste of pollen by our fir-trees; at the instinctive hatred of the queen-bee for her own fertile daughters; at ichneumonidae feeding within the living bodies of caterpillars; and at other such cases. the wonder, indeed, is, on the theory of natural selection, that more cases of the want of absolute perfection have not been detected. the complex and little known laws governing the production of varieties are the same, as far as we can judge, with the laws which have governed the production of distinct species. in both cases physical conditions seem to have produced some direct and definite effect, but how much we cannot say. thus, when varieties enter any new station, they occasionally assume some of the characters proper to the species of that station. with both varieties and species, use and disuse seem to have produced a considerable effect; for it is impossible to resist this conclusion when we look, for instance, at the logger-headed duck, which has wings incapable of flight, in nearly the same condition as in the domestic duck; or when we look at the burrowing tucu-tucu, which is occasionally blind, and then at certain moles, which are habitually blind and have their eyes covered with skin; or when we look at the blind animals inhabiting the dark caves of america and europe. with varieties and species, correlated variation seems to have played an important part, so that when one part has been modified other parts have been necessarily modified. with both varieties and species, reversions to long-lost characters occasionally occur. how inexplicable on the theory of creation is the occasional appearance of stripes on the shoulders and legs of the several species of the horse-genus and of their hybrids! how simply is this fact explained if we believe that these species are all descended from a striped progenitor, in the same manner as the several domestic breeds of the pigeon are descended from the blue and barred rock-pigeon! on the ordinary view of each species having been independently created, why should specific characters, or those by which the species of the same genus differ from each other, be more variable than the generic characters in which they all agree? why, for instance, should the colour of a flower be more likely to vary in any one species of a genus, if the other species possess differently coloured flowers, than if all possessed the same coloured flowers? if species are only well-marked varieties, of which the characters have become in a high degree permanent, we can understand this fact; for they have already varied since they branched off from a common progenitor in certain characters, by which they have come to be specifically distinct from each other; therefore these same characters would be more likely again to vary than the generic characters which have been inherited without change for an immense period. it is inexplicable on the theory of creation why a part developed in a very unusual manner in one species alone of a genus, and therefore, as we may naturally infer, of great importance to that species, should be eminently liable to variation; but, on our view, this part has undergone, since the several species branched off from a common progenitor, an unusual amount of variability and modification, and therefore we might expect the part generally to be still variable. but a part may be developed in the most unusual manner, like the wing of a bat, and yet not be more variable than any other structure, if the part be common to many subordinate forms, that is, if it has been inherited for a very long period; for in this case it will have been rendered constant by long-continued natural selection. glancing at instincts, marvellous as some are, they offer no greater difficulty than do corporeal structures on the theory of the natural selection of successive, slight, but profitable modifications. we can thus understand why nature moves by graduated steps in endowing different animals of the same class with their several instincts. i have attempted to show how much light the principle of gradation throws on the admirable architectural powers of the hive-bee. habit no doubt often comes into play in modifying instincts; but it certainly is not indispensable, as we see in the case of neuter insects, which leave no progeny to inherit the effects of long-continued habit. on the view of all the species of the same genus having descended from a common parent, and having inherited much in common, we can understand how it is that allied species, when placed under widely different conditions of life, yet follow nearly the same instincts; why the thrushes of tropical and temperate south america, for instance, line their nests with mud like our british species. on the view of instincts having been slowly acquired through natural selection, we need not marvel at some instincts being not perfect and liable to mistakes, and at many instincts causing other animals to suffer. if species be only well-marked and permanent varieties, we can at once see why their crossed offspring should follow the same complex laws in their degrees and kinds of resemblance to their parents--in being absorbed into each other by successive crosses, and in other such points--as do the crossed offspring of acknowledged varieties. this similarity would be a strange fact, if species had been independently created and varieties had been produced through secondary laws. if we admit that the geological record is imperfect to an extreme degree, then the facts, which the record does give, strongly support the theory of descent with modification. new species have come on the stage slowly and at successive intervals; and the amount of change after equal intervals of time, is widely different in different groups. the extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows from the principle of natural selection; for old forms are supplanted by new and improved forms. neither single species nor groups of species reappear when the chain of ordinary generation is once broken. the gradual diffusion of dominant forms, with the slow modification of their descendants, causes the forms of life, after long intervals of time, to appear as if they had changed simultaneously throughout the world. the fact of the fossil remains of each formation being in some degree intermediate in character between the fossils in the formations above and below, is simply explained by their intermediate position in the chain of descent. the grand fact that all extinct beings can be classed with all recent beings, naturally follows from the living and the extinct being the offspring of common parents. as species have generally diverged in character during their long course of descent and modification, we can understand why it is that the more ancient forms, or early progenitors of each group, so often occupy a position in some degree intermediate between existing groups. recent forms are generally looked upon as being, on the whole, higher in the scale of organisation than ancient forms; and they must be higher, in so far as the later and more improved forms have conquered the older and less improved forms in the struggle for life; they have also generally had their organs more specialised for different functions. this fact is perfectly compatible with numerous beings still retaining simple and but little improved structures, fitted for simple conditions of life; it is likewise compatible with some forms having retrograded in organisation, by having become at each stage of descent better fitted for new and degraded habits of life. lastly, the wonderful law of the long endurance of allied forms on the same continent--of marsupials in australia, of edentata in america, and other such cases--is intelligible, for within the same country the existing and the extinct will be closely allied by descent. looking to geographical distribution, if we admit that there has been during the long course of ages much migration from one part of the world to another, owing to former climatical and geographical changes and to the many occasional and unknown means of dispersal, then we can understand, on the theory of descent with modification, most of the great leading facts in distribution. we can see why there should be so striking a parallelism in the distribution of organic beings throughout space, and in their geological succession throughout time; for in both cases the beings have been connected by the bond of ordinary generation, and the means of modification have been the same. we see the full meaning of the wonderful fact, which has struck every traveller, namely, that on the same continent, under the most diverse conditions, under heat and cold, on mountain and lowland, on deserts and marshes, most of the inhabitants within each great class are plainly related; for they are the descendants of the same progenitors and early colonists. on this same principle of former migration, combined in most cases with modification, we can understand, by the aid of the glacial period, the identity of some few plants, and the close alliance of many others, on the most distant mountains, and in the northern and southern temperate zones; and likewise the close alliance of some of the inhabitants of the sea in the northern and southern temperate latitudes, though separated by the whole intertropical ocean. although two countries may present physical conditions as closely similar as the same species ever require, we need feel no surprise at their inhabitants being widely different, if they have been for a long period completely sundered from each other; for as the relation of organism to organism is the most important of all relations, and as the two countries will have received colonists at various periods and in different proportions, from some other country or from each other, the course of modification in the two areas will inevitably have been different. on this view of migration, with subsequent modification, we see why oceanic islands are inhabited by only few species, but of these, why many are peculiar or endemic forms. we clearly see why species belonging to those groups of animals which cannot cross wide spaces of the ocean, as frogs and terrestrial mammals, do not inhabit oceanic islands; and why, on the other hand, new and peculiar species of bats, animals which can traverse the ocean, are often found on islands far distant from any continent. such cases as the presence of peculiar species of bats on oceanic islands and the absence of all other terrestrial mammals, are facts utterly inexplicable on the theory of independent acts of creation. the existence of closely allied representative species in any two areas, implies, on the theory of descent with modification, that the same parent-forms formerly inhabited both areas; and we almost invariably find that wherever many closely allied species inhabit two areas, some identical species are still common to both. wherever many closely allied yet distinct species occur, doubtful forms and varieties belonging to the same groups likewise occur. it is a rule of high generality that the inhabitants of each area are related to the inhabitants of the nearest source whence immigrants might have been derived. we see this in the striking relation of nearly all the plants and animals of the galapagos archipelago, of juan fernandez, and of the other american islands, to the plants and animals of the neighbouring american mainland; and of those of the cape de verde archipelago, and of the other african islands to the african mainland. it must be admitted that these facts receive no explanation on the theory of creation. the fact, as we have seen, that all past and present organic beings can be arranged within a few great classes, in groups subordinate to groups, and with the extinct groups often falling in between the recent groups, is intelligible on the theory of natural selection with its contingencies of extinction and divergence of character. on these same principles we see how it is that the mutual affinities of the forms within each class are so complex and circuitous. we see why certain characters are far more serviceable than others for classification; why adaptive characters, though of paramount importance to the beings, are of hardly any importance in classification; why characters derived from rudimentary parts, though of no service to the beings, are often of high classificatory value; and why embryological characters are often the most valuable of all. the real affinities of all organic beings, in contradistinction to their adaptive resemblances, are due to inheritance or community of descent. the natural system is a genealogical arrangement, with the acquired grades of difference, marked by the terms, varieties, species, genera, families, etc.; and we have to discover the lines of descent by the most permanent characters, whatever they may be, and of however slight vital importance. the similar framework of bones in the hand of a man, wing of a bat, fin of the porpoise, and leg of the horse--the same number of vertebrae forming the neck of the giraffe and of the elephant--and innumerable other such facts, at once explain themselves on the theory of descent with slow and slight successive modifications. the similarity of pattern in the wing and in the leg of a bat, though used for such different purpose--in the jaws and legs of a crab--in the petals, stamens, and pistils of a flower, is likewise, to a large extent, intelligible on the view of the gradual modification of parts or organs, which were aboriginally alike in an early progenitor in each of these classes. on the principle of successive variations not always supervening at an early age, and being inherited at a corresponding not early period of life, we clearly see why the embryos of mammals, birds, reptiles, and fishes should be so closely similar, and so unlike the adult forms. we may cease marvelling at the embryo of an air-breathing mammal or bird having branchial slits and arteries running in loops, like those of a fish which has to breathe the air dissolved in water by the aid of well-developed branchiae. disuse, aided sometimes by natural selection, will often have reduced organs when rendered useless under changed habits or conditions of life; and we can understand on this view the meaning of rudimentary organs. but disuse and selection will generally act on each creature, when it has come to maturity and has to play its full part in the struggle for existence, and will thus have little power on an organ during early life; hence the organ will not be reduced or rendered rudimentary at this early age. the calf, for instance, has inherited teeth, which never cut through the gums of the upper jaw, from an early progenitor having well-developed teeth; and we may believe, that the teeth in the mature animal were formerly reduced by disuse owing to the tongue and palate, or lips, having become excellently fitted through natural selection to browse without their aid; whereas in the calf, the teeth have been left unaffected, and on the principle of inheritance at corresponding ages have been inherited from a remote period to the present day. on the view of each organism with all its separate parts having been specially created, how utterly inexplicable is it that organs bearing the plain stamp of inutility, such as the teeth in the embryonic calf or the shrivelled wings under the soldered wing-covers of many beetles, should so frequently occur. nature may be said to have taken pains to reveal her scheme of modification, by means of rudimentary organs, of embryological and homologous structures, but we are too blind to understand her meaning. i have now recapitulated the facts and considerations which have thoroughly convinced me that species have been modified, during a long course of descent. this has been effected chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner, that is, in relation to adaptive structures, whether past or present, by the direct action of external conditions, and by variations which seem to us in our ignorance to arise spontaneously. it appears that i formerly underrated the frequency and value of these latter forms of variation, as leading to permanent modifications of structure independently of natural selection. but as my conclusions have lately been much misrepresented, and it has been stated that i attribute the modification of species exclusively to natural selection, i may be permitted to remark that in the first edition of this work, and subsequently, i placed in a most conspicuous position--namely, at the close of the introduction--the following words: "i am convinced that natural selection has been the main but not the exclusive means of modification." this has been of no avail. great is the power of steady misrepresentation; but the history of science shows that fortunately this power does not long endure. it can hardly be supposed that a false theory would explain, in so satisfactory a manner as does the theory of natural selection, the several large classes of facts above specified. it has recently been objected that this is an unsafe method of arguing; but it is a method used in judging of the common events of life, and has often been used by the greatest natural philosophers. the undulatory theory of light has thus been arrived at; and the belief in the revolution of the earth on its own axis was until lately supported by hardly any direct evidence. it is no valid objection that science as yet throws no light on the far higher problem of the essence or origin of life. who can explain what is the essence of the attraction of gravity? no one now objects to following out the results consequent on this unknown element of attraction; notwithstanding that leibnitz formerly accused newton of introducing "occult qualities and miracles into philosophy." i see no good reasons why the views given in this volume should shock the religious feelings of any one. it is satisfactory, as showing how transient such impressions are, to remember that the greatest discovery ever made by man, namely, the law of the attraction of gravity, was also attacked by leibnitz, "as subversive of natural, and inferentially of revealed, religion." a celebrated author and divine has written to me that "he has gradually learned to see that it is just as noble a conception of the deity to believe that he created a few original forms capable of self-development into other and needful forms, as to believe that he required a fresh act of creation to supply the voids caused by the action of his laws." why, it may be asked, until recently did nearly all the most eminent living naturalists and geologists disbelieve in the mutability of species? it cannot be asserted that organic beings in a state of nature are subject to no variation; it cannot be proved that the amount of variation in the course of long ages is a limited quantity; no clear distinction has been, or can be, drawn between species and well-marked varieties. it cannot be maintained that species when intercrossed are invariably sterile and varieties invariably fertile; or that sterility is a special endowment and sign of creation. the belief that species were immutable productions was almost unavoidable as long as the history of the world was thought to be of short duration; and now that we have acquired some idea of the lapse of time, we are too apt to assume, without proof, that the geological record is so perfect that it would have afforded us plain evidence of the mutation of species, if they had undergone mutation. but the chief cause of our natural unwillingness to admit that one species has given birth to other and distinct species, is that we are always slow in admitting any great changes of which we do not see the steps. the difficulty is the same as that felt by so many geologists, when lyell first insisted that long lines of inland cliffs had been formed, and great valleys excavated, by the agencies which we still see at work. the mind cannot possibly grasp the full meaning of the term of even a million years; it cannot add up and perceive the full effects of many slight variations, accumulated during an almost infinite number of generations. although i am fully convinced of the truth of the views given in this volume under the form of an abstract, i by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. it is so easy to hide our ignorance under such expressions as the "plan of creation," "unity of design," etc., and to think that we give an explanation when we only restate a fact. any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject the theory. a few naturalists, endowed with much flexibility of mind, and who have already begun to doubt the immutability of species, may be influenced by this volume; but i look with confidence to the future, to young and rising naturalists, who will be able to view both sides of the question with impartiality. whoever is led to believe that species are mutable will do good service by conscientiously expressing his conviction; for thus only can the load of prejudice by which this subject is overwhelmed be removed. several eminent naturalists have of late published their belief that a multitude of reputed species in each genus are not real species; but that other species are real, that is, have been independently created. this seems to me a strange conclusion to arrive at. they admit that a multitude of forms, which till lately they themselves thought were special creations, and which are still thus looked at by the majority of naturalists, and which consequently have all the external characteristic features of true species--they admit that these have been produced by variation, but they refuse to extend the same view to other and slightly different forms. nevertheless, they do not pretend that they can define, or even conjecture, which are the created forms of life, and which are those produced by secondary laws. they admit variation as a vera causa in one case, they arbitrarily reject it in another, without assigning any distinction in the two cases. the day will come when this will be given as a curious illustration of the blindness of preconceived opinion. these authors seem no more startled at a miraculous act of creation than at an ordinary birth. but do they really believe that at innumerable periods in the earth's history certain elemental atoms have been commanded suddenly to flash into living tissues? do they believe that at each supposed act of creation one individual or many were produced? were all the infinitely numerous kinds of animals and plants created as eggs or seed, or as full grown? and in the case of mammals, were they created bearing the false marks of nourishment from the mother's womb? undoubtedly some of these same questions cannot be answered by those who believe in the appearance or creation of only a few forms of life or of some one form alone. it has been maintained by several authors that it is as easy to believe in the creation of a million beings as of one; but maupertuis' philosophical axiom "of least action" leads the mind more willingly to admit the smaller number; and certainly we ought not to believe that innumerable beings within each great class have been created with plain, but deceptive, marks of descent from a single parent. as a record of a former state of things, i have retained in the foregoing paragraphs, and elsewhere, several sentences which imply that naturalists believe in the separate creation of each species; and i have been much censured for having thus expressed myself. but undoubtedly this was the general belief when the first edition of the present work appeared. i formerly spoke to very many naturalists on the subject of evolution, and never once met with any sympathetic agreement. it is probable that some did then believe in evolution, but they were either silent or expressed themselves so ambiguously that it was not easy to understand their meaning. now, things are wholly changed, and almost every naturalist admits the great principle of evolution. there are, however, some who still think that species have suddenly given birth, through quite unexplained means, to new and totally different forms. but, as i have attempted to show, weighty evidence can be opposed to the admission of great and abrupt modifications. under a scientific point of view, and as leading to further investigation, but little advantage is gained by believing that new forms are suddenly developed in an inexplicable manner from old and widely different forms, over the old belief in the creation of species from the dust of the earth. it may be asked how far i extend the doctrine of the modification of species. the question is difficult to answer, because the more distinct the forms are which we consider, by so much the arguments in favour of community of descent become fewer in number and less in force. but some arguments of the greatest weight extend very far. all the members of whole classes are connected together by a chain of affinities, and all can be classed on the same principle, in groups subordinate to groups. fossil remains sometimes tend to fill up very wide intervals between existing orders. organs in a rudimentary condition plainly show that an early progenitor had the organ in a fully developed condition, and this in some cases implies an enormous amount of modification in the descendants. throughout whole classes various structures are formed on the same pattern, and at a very early age the embryos closely resemble each other. therefore i cannot doubt that the theory of descent with modification embraces all the members of the same great class or kingdom. i believe that animals are descended from at most only four or five progenitors, and plants from an equal or lesser number. analogy would lead me one step further, namely, to the belief that all animals and plants are descended from some one prototype. but analogy may be a deceitful guide. nevertheless all living things have much in common, in their chemical composition, their cellular structure, their laws of growth, and their liability to injurious influences. we see this even in so trifling a fact as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree. with all organic beings, excepting perhaps some of the very lowest, sexual reproduction seems to be essentially similar. with all, as far as is at present known, the germinal vesicle is the same; so that all organisms start from a common origin. if we look even to the two main divisions--namely, to the animal and vegetable kingdoms--certain low forms are so far intermediate in character that naturalists have disputed to which kingdom they should be referred. as professor asa gray has remarked, "the spores and other reproductive bodies of many of the lower algae may claim to have first a characteristically animal, and then an unequivocally vegetable existence." therefore, on the principle of natural selection with divergence of character, it does not seem incredible that, from some such low and intermediate form, both animals and plants may have been developed; and, if we admit this, we must likewise admit that all the organic beings which have ever lived on this earth may be descended from some one primordial form. but this inference is chiefly grounded on analogy, and it is immaterial whether or not it be accepted. no doubt it is possible, as mr. g.h. lewes has urged, that at the first commencement of life many different forms were evolved; but if so, we may conclude that only a very few have left modified descendants. for, as i have recently remarked in regard to the members of each great kingdom, such as the vertebrata, articulata, etc., we have distinct evidence in their embryological, homologous, and rudimentary structures, that within each kingdom all the members are descended from a single progenitor. when the views advanced by me in this volume, and by mr. wallace or when analogous views on the origin of species are generally admitted, we can dimly foresee that there will be a considerable revolution in natural history. systematists will be able to pursue their labours as at present; but they will not be incessantly haunted by the shadowy doubt whether this or that form be a true species. this, i feel sure and i speak after experience, will be no slight relief. the endless disputes whether or not some fifty species of british brambles are good species will cease. systematists will have only to decide (not that this will be easy) whether any form be sufficiently constant and distinct from other forms, to be capable of definition; and if definable, whether the differences be sufficiently important to deserve a specific name. this latter point will become a far more essential consideration than it is at present; for differences, however slight, between any two forms, if not blended by intermediate gradations, are looked at by most naturalists as sufficient to raise both forms to the rank of species. hereafter we shall be compelled to acknowledge that the only distinction between species and well-marked varieties is, that the latter are known, or believed to be connected at the present day by intermediate gradations, whereas species were formerly thus connected. hence, without rejecting the consideration of the present existence of intermediate gradations between any two forms, we shall be led to weigh more carefully and to value higher the actual amount of difference between them. it is quite possible that forms now generally acknowledged to be merely varieties may hereafter be thought worthy of specific names; and in this case scientific and common language will come into accordance. in short, we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations made for convenience. this may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species. the other and more general departments of natural history will rise greatly in interest. the terms used by naturalists, of affinity, relationship, community of type, paternity, morphology, adaptive characters, rudimentary and aborted organs, etc., will cease to be metaphorical and will have a plain signification. when we no longer look at an organic being as a savage looks at a ship, as something wholly beyond his comprehension; when we regard every production of nature as one which has had a long history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, in the same way as any great mechanical invention is the summing up of the labour, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting--i speak from experience--does the study of natural history become! a grand and almost untrodden field of inquiry will be opened, on the causes and laws of variation, on correlation, on the effects of use and disuse, on the direct action of external conditions, and so forth. the study of domestic productions will rise immensely in value. a new variety raised by man will be a far more important and interesting subject for study than one more species added to the infinitude of already recorded species. our classifications will come to be, as far as they can be so made, genealogies; and will then truly give what may be called the plan of creation. the rules for classifying will no doubt become simpler when we have a definite object in view. we possess no pedigree or armorial bearings; and we have to discover and trace the many diverging lines of descent in our natural genealogies, by characters of any kind which have long been inherited. rudimentary organs will speak infallibly with respect to the nature of long-lost structures. species and groups of species which are called aberrant, and which may fancifully be called living fossils, will aid us in forming a picture of the ancient forms of life. embryology will often reveal to us the structure, in some degree obscured, of the prototypes of each great class. when we can feel assured that all the individuals of the same species, and all the closely allied species of most genera, have, within a not very remote period descended from one parent, and have migrated from some one birth-place; and when we better know the many means of migration, then, by the light which geology now throws, and will continue to throw, on former changes of climate and of the level of the land, we shall surely be enabled to trace in an admirable manner the former migrations of the inhabitants of the whole world. even at present, by comparing the differences between the inhabitants of the sea on the opposite sides of a continent, and the nature of the various inhabitants of that continent in relation to their apparent means of immigration, some light can be thrown on ancient geography. the noble science of geology loses glory from the extreme imperfection of the record. the crust of the earth, with its embedded remains, must not be looked at as a well-filled museum, but as a poor collection made at hazard and at rare intervals. the accumulation of each great fossiliferous formation will be recognised as having depended on an unusual occurrence of favourable circumstances, and the blank intervals between the successive stages as having been of vast duration. but we shall be able to gauge with some security the duration of these intervals by a comparison of the preceding and succeeding organic forms. we must be cautious in attempting to correlate as strictly contemporaneous two formations, which do not include many identical species, by the general succession of the forms of life. as species are produced and exterminated by slowly acting and still existing causes, and not by miraculous acts of creation; and as the most important of all causes of organic change is one which is almost independent of altered and perhaps suddenly altered physical conditions, namely, the mutual relation of organism to organism--the improvement of one organism entailing the improvement or the extermination of others; it follows, that the amount of organic change in the fossils of consecutive formations probably serves as a fair measure of the relative, though not actual lapse of time. a number of species, however, keeping in a body might remain for a long period unchanged, whilst within the same period, several of these species, by migrating into new countries and coming into competition with foreign associates, might become modified; so that we must not overrate the accuracy of organic change as a measure of time. in the future i see open fields for far more important researches. psychology will be securely based on the foundation already well laid by mr. herbert spencer, that of the necessary acquirement of each mental power and capacity by gradation. much light will be thrown on the origin of man and his history. authors of the highest eminence seem to be fully satisfied with the view that each species has been independently created. to my mind it accords better with what we know of the laws impressed on matter by the creator, that the production and extinction of the past and present inhabitants of the world should have been due to secondary causes, like those determining the birth and death of the individual. when i view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the cambrian system was deposited, they seem to me to become ennobled. judging from the past, we may safely infer that not one living species will transmit its unaltered likeness to a distinct futurity. and of the species now living very few will transmit progeny of any kind to a far distant futurity; for the manner in which all organic beings are grouped, shows that the greater number of species in each genus, and all the species in many genera, have left no descendants, but have become utterly extinct. we can so far take a prophetic glance into futurity as to foretell that it will be the common and widely spread species, belonging to the larger and dominant groups within each class, which will ultimately prevail and procreate new and dominant species. as all the living forms of life are the lineal descendants of those which lived long before the cambrian epoch, we may feel certain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world. hence, we may look with some confidence to a secure future of great length. and as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection. it is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. these laws, taken in the largest sense, being growth with reproduction; inheritance which is almost implied by reproduction; variability from the indirect and direct action of the conditions of life, and from use and disuse; a ratio of increase so high as to lead to a struggle for life, and as a consequence to natural selection, entailing divergence of character and the extinction of less improved forms. thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. there is grandeur in this view of life, with its several powers, having been originally breathed by the creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved. glossary of the principal scientific terms used in the present volume. (i am indebted to the kindness of mr. w.s. dallas for this glossary, which has been given because several readers have complained to me that some of the terms used were unintelligible to them. mr. dallas has endeavoured to give the explanations of the terms in as popular a form as possible.) aberrant.--forms or groups of animals or plants which deviate in important characters from their nearest allies, so as not to be easily included in the same group with them, are said to be aberrant. aberration (in optics).--in the refraction of light by a convex lens the rays passing through different parts of the lens are brought to a focus at slightly different distances--this is called spherical aberration; at the same time the coloured rays are separated by the prismatic action of the lens and likewise brought to a focus at different distances--this is chromatic aberration. abnormal.--contrary to the general rule. aborted.--an organ is said to be aborted, when its development has been arrested at a very early stage. albinism.--albinos are animals in which the usual colouring matters characteristic of the species have not been produced in the skin and its appendages. albinism is the state of being an albino. algae.--a class of plants including the ordinary sea-weeds and the filamentous fresh-water weeds. alternation of generations.--this term is applied to a peculiar mode of reproduction which prevails among many of the lower animals, in which the egg produces a living form quite different from its parent, but from which the parent-form is reproduced by a process of budding, or by the division of the substance of the first product of the egg. ammonites.--a group of fossil, spiral, chambered shells, allied to the existing pearly nautilus, but having the partitions between the chambers waved in complicated patterns at their junction with the outer wall of the shell. analogy.--that resemblance of structures which depends upon similarity of function, as in the wings of insects and birds. such structures are said to be analogous, and to be analogues of each other. animalcule.--a minute animal: generally applied to those visible only by the microscope. annelids.--a class of worms in which the surface of the body exhibits a more or less distinct division into rings or segments, generally provided with appendages for locomotion and with gills. it includes the ordinary marine worms, the earth-worms, and the leeches. antennae.--jointed organs appended to the head in insects, crustacea and centipedes, and not belonging to the mouth. anthers.--the summits of the stamens of flowers, in which the pollen or fertilising dust is produced. aplacentalia, aplacentata or aplacental mammals.--see mammalia. archetypal.--of or belonging to the archetype, or ideal primitive form upon which all the beings of a group seem to be organised. articulata.--a great division of the animal kingdom characterised generally by having the surface of the body divided into rings called segments, a greater or less number of which are furnished with jointed legs (such as insects, crustaceans and centipedes). asymmetrical.--having the two sides unlike. atrophied.--arrested in development at a very early stage. balanus.--the genus including the common acorn-shells which live in abundance on the rocks of the sea-coast. batrachians.--a class of animals allied to the reptiles, but undergoing a peculiar metamorphosis, in which the young animal is generally aquatic and breathes by gills. (examples, frogs, toads, and newts.) boulders.--large transported blocks of stone generally embedded in clays or gravels. brachiopoda.--a class of marine mollusca, or soft-bodied animals, furnished with a bivalve shell, attached to submarine objects by a stalk which passes through an aperture in one of the valves, and furnished with fringed arms, by the action of which food is carried to the mouth. branchiae.--gills or organs for respiration in water. branchial.--pertaining to gills or branchiae. cambrian system.--a series of very ancient palaeozoic rocks, between the laurentian and the silurian. until recently these were regarded as the oldest fossiliferous rocks. canidae.--the dog-family, including the dog, wolf, fox, jackal, etc. carapace.--the shell enveloping the anterior part of the body in crustaceans generally; applied also to the hard shelly pieces of the cirripedes. carboniferous.--this term is applied to the great formation which includes, among other rocks, the coal-measures. it belongs to the oldest, or palaeozoic, system of formations. caudal.--of or belonging to the tail. cephalopods.--the highest class of the mollusca, or soft-bodied animals, characterised by having the mouth surrounded by a greater or less number of fleshy arms or tentacles, which, in most living species, are furnished with sucking-cups. (examples, cuttle-fish, nautilus.) cetacea.--an order of mammalia, including the whales, dolphins, etc., having the form of the body fish-like, the skin naked, and only the fore limbs developed. chelonia.--an order of reptiles including the turtles, tortoises, etc. cirripedes.--an order of crustaceans including the barnacles and acorn-shells. their young resemble those of many other crustaceans in form; but when mature they are always attached to other objects, either directly or by means of a stalk, and their bodies are enclosed by a calcareous shell composed of several pieces, two of which can open to give issue to a bunch of curled, jointed tentacles, which represent the limbs. coccus.--the genus of insects including the cochineal. in these the male is a minute, winged fly, and the female generally a motionless, berry-like mass. cocoon.--a case usually of silky material, in which insects are frequently enveloped during the second or resting-stage (pupa) of their existence. the term "cocoon-stage" is here used as equivalent to "pupa-stage." coelospermous.--a term applied to those fruits of the umbelliferae which have the seed hollowed on the inner face. coleoptera.--beetles, an order of insects, having a biting mouth and the first pair of wings more or less horny, forming sheaths for the second pair, and usually meeting in a straight line down the middle of the back. column.--a peculiar organ in the flowers of orchids, in which the stamens, style and stigma (or the reproductive parts) are united. compositae or compositous plants.--plants in which the inflorescence consists of numerous small flowers (florets) brought together into a dense head, the base of which is enclosed by a common envelope. (examples, the daisy, dandelion, etc.) confervae.--the filamentous weeds of fresh water. conglomerate.--a rock made up of fragments of rock or pebbles, cemented together by some other material. corolla.--the second envelope of a flower usually composed of coloured, leaf-like organs (petals), which may be united by their edges either in the basal part or throughout. correlation.--the normal coincidence of one phenomenon, character, etc., with another. corymb.--a bunch of flowers in which those springing from the lower part of the flower stalks are supported on long stalks so as to be nearly on a level with the upper ones. cotyledons.--the first or seed-leaves of plants. crustaceans.--a class of articulated animals, having the skin of the body generally more or less hardened by the deposition of calcareous matter, breathing by means of gills. (examples, crab, lobster, shrimp, etc.) curculio.--the old generic term for the beetles known as weevils, characterised by their four-jointed feet, and by the head being produced into a sort of beak, upon the sides of which the antennae are inserted. cutaneous.--of or belonging to the skin. degradation.--the wearing down of land by the action of the sea or of meteoric agencies. denudation.--the wearing away of the surface of the land by water. devonian system or formation.--a series of palaeozoic rocks, including the old red sandstone. dicotyledons, or dicotyledonous plants.--a class of plants characterised by having two seed-leaves, by the formation of new wood between the bark and the old wood (exogenous growth) and by the reticulation of the veins of the leaves. the parts of the flowers are generally in multiples of five. differentation.--the separation or discrimination of parts or organs which in simpler forms of life are more or less united. dimorphic.--having two distinct forms.--dimorphism is the condition of the appearance of the same species under two dissimilar forms. dioecious.--having the organs of the sexes upon distinct individuals. diorite.--a peculiar form of greenstone. dorsal.--of or belonging to the back. edentata.--a peculiar order of quadrupeds, characterised by the absence of at least the middle incisor (front) teeth in both jaws. (examples, the sloths and armadillos.) elytra.--the hardened fore-wings of beetles, serving as sheaths for the membranous hind-wings, which constitute the true organs of flight. embryo.--the young animal undergoing development within the egg or womb. embryology.--the study of the development of the embryo. endemic.--peculiar to a given locality. entomostraca.--a division of the class crustacea, having all the segments of the body usually distinct, gills attached to the feet or organs of the mouth, and the feet fringed with fine hairs. they are generally of small size. eocene.--the earliest of the three divisions of the tertiary epoch of geologists. rocks of this age contain a small proportion of shells identical with species now living. ephemerous insects.--insects allied to the may-fly. fauna.--the totality of the animals naturally inhabiting a certain country or region, or which have lived during a given geological period. felidae.--the cat-family. feral.--having become wild from a state of cultivation or domestication. flora.--the totality of the plants growing naturally in a country, or during a given geological period. florets.--flowers imperfectly developed in some respects, and collected into a dense spike or head, as in the grasses, the dandelion, etc. foetal.--of or belonging to the foetus, or embryo in course of development. foraminifera.--a class of animals of very low organisation and generally of small size, having a jelly-like body, from the surface of which delicate filaments can be given off and retracted for the prehension of external objects, and having a calcareous or sandy shell, usually divided into chambers and perforated with small apertures. fossiliferous.--containing fossils. fossorial.--having a faculty of digging. the fossorial hymenoptera are a group of wasp-like insects, which burrow in sandy soil to make nests for their young. frenum (pl. frena).--a small band or fold of skin. fungi (sing. fungus).--a class of cellular plants, of which mushrooms, toadstools, and moulds, are familiar examples. furcula.--the forked bone formed by the union of the collar-bones in many birds, such as the common fowl. gallinaceous birds.--an order of birds of which the common fowl, turkey, and pheasant, are well-known examples. gallus.--the genus of birds which includes the common fowl. ganglion.--a swelling or knot from which nerves are given off as from a centre. ganoid fishes.--fishes covered with peculiar enamelled bony scales. most of them are extinct. germinal vesicle.--a minute vesicle in the eggs of animals, from which the development of the embryo proceeds. glacial period.--a period of great cold and of enormous extension of ice upon the surface of the earth. it is believed that glacial periods have occurred repeatedly during the geological history of the earth, but the term is generally applied to the close of the tertiary epoch, when nearly the whole of europe was subjected to an arctic climate. gland.--an organ which secretes or separates some peculiar product from the blood or sap of animals or plants. glottis.--the opening of the windpipe into the oesophagus or gullet. gneiss.--a rock approaching granite in composition, but more or less laminated, and really produced by the alteration of a sedimentary deposit after its consolidation. grallatores.--the so-called wading-birds (storks, cranes, snipes, etc.), which are generally furnished with long legs, bare of feathers above the heel, and have no membranes between the toes. granite.--a rock consisting essentially of crystals of felspar and mica in a mass of quartz. habitat.--the locality in which a plant or animal naturally lives. hemiptera.--an order or sub-order of insects, characterised by the possession of a jointed beak or rostrum, and by having the fore-wings horny in the basal portion and membranous at the extremity, where they cross each other. this group includes the various species of bugs. hermaphrodite.--possessing the organs of both sexes. homology.--that relation between parts which results from their development from corresponding embryonic parts, either in different animals, as in the case of the arm of man, the fore-leg of a quadruped, and the wing of a bird; or in the same individual, as in the case of the fore and hind legs in quadrupeds, and the segments or rings and their appendages of which the body of a worm, a centipede, etc., is composed. the latter is called serial homology. the parts which stand in such a relation to each other are said to be homologous, and one such part or organ is called the homologue of the other. in different plants the parts of the flower are homologous, and in general these parts are regarded as homologous with leaves. homoptera.--an order or sub-order of insects having (like the hemiptera) a jointed beak, but in which the fore-wings are either wholly membranous or wholly leathery, the cicadae, frog-hoppers, and aphides, are well-known examples. hybrid.--the offspring of the union of two distinct species. hymenoptera.--an order of insects possessing biting jaws and usually four membranous wings in which there are a few veins. bees and wasps are familiar examples of this group. hypertrophied.--excessively developed. ichneumonidae.--a family of hymenopterous insects, the members of which lay their eggs in the bodies or eggs of other insects. imago.--the perfect (generally winged) reproductive state of an insect. indigenes.--the aboriginal animal or vegetable inhabitants of a country or region. inflorescence.--the mode of arrangement of the flowers of plants. infusoria.--a class of microscopic animalcules, so called from their having originally been observed in infusions of vegetable matters. they consist of a gelatinous material enclosed in a delicate membrane, the whole or part of which is furnished with short vibrating hairs (called cilia), by means of which the animalcules swim through the water or convey the minute particles of their food to the orifice of the mouth. insectivorous.--feeding on insects. invertebrata, or invertebrate animals.--those animals which do not possess a backbone or spinal column. lacunae.--spaces left among the tissues in some of the lower animals and serving in place of vessels for the circulation of the fluids of the body. lamellated.--furnished with lamellae or little plates. larva (pl. larvae).--the first condition of an insect at its issuing from the egg, when it is usually in the form of a grub, caterpillar, or maggot. larynx.--the upper part of the windpipe opening into the gullet. laurentian.--a group of greatly altered and very ancient rocks, which is greatly developed along the course of the st. laurence, whence the name. it is in these that the earliest known traces of organic bodies have been found. leguminosae.--an order of plants represented by the common peas and beans, having an irregular flower in which one petal stands up like a wing, and the stamens and pistil are enclosed in a sheath formed by two other petals. the fruit is a pod (or legume). lemuridae.--a group of four-handed animals, distinct from the monkeys and approaching the insectivorous quadrupeds in some of their characters and habits. its members have the nostrils curved or twisted, and a claw instead of a nail upon the first finger of the hind hands. lepidoptera.--an order of insects, characterised by the possession of a spiral proboscis, and of four large more or less scaly wings. it includes the well-known butterflies and moths. littoral.--inhabiting the seashore. loess.--a marly deposit of recent (post-tertiary) date, which occupies a great part of the valley of the rhine. malacostraca.--the higher division of the crustacea, including the ordinary crabs, lobsters, shrimps, etc., together with the woodlice and sand-hoppers. mammalia.--the highest class of animals, including the ordinary hairy quadrupeds, the whales and man, and characterised by the production of living young which are nourished after birth by milk from the teats (mammae, mammary glands) of the mother. a striking difference in embryonic development has led to the division of this class into two great groups; in one of these, when the embryo has attained a certain stage, a vascular connection, called the placenta, is formed between the embryo and the mother; in the other this is wanting, and the young are produced in a very incomplete state. the former, including the greater part of the class, are called placental mammals; the latter, or aplacental mammals, include the marsupials and monotremes (ornithorhynchus). mammiferous.--having mammae or teats (see mammalia). mandibles.--in insects, the first or uppermost pair of jaws, which are generally solid, horny, biting organs. in birds the term is applied to both jaws with their horny coverings. in quadrupeds the mandible is properly the lower jaw. marsupials.--an order of mammalia in which the young are born in a very incomplete state of development, and carried by the mother, while sucking, in a ventral pouch (marsupium), such as the kangaroos, opossums, etc. (see mammalia). maxillae.--in insects, the second or lower pair of jaws, which are composed of several joints and furnished with peculiar jointed appendages called palpi, or feelers. melanism.--the opposite of albinism; an undue development of colouring material in the skin and its appendages. metamorphic rocks.--sedimentary rocks which have undergone alteration, generally by the action of heat, subsequently to their deposition and consolidation. mollusca.--one of the great divisions of the animal kingdom, including those animals which have a soft body, usually furnished with a shell, and in which the nervous ganglia, or centres, present no definite general arrangement. they are generally known under the denomination of "shellfish"; the cuttle-fish, and the common snails, whelks, oysters, mussels, and cockles, may serve as examples of them. monocotyledons, or monocotyledonous plants.--plants in which the seed sends up only a single seed-leaf (or cotyledon); characterised by the absence of consecutive layers of wood in the stem (endogenous growth), by the veins of the leaves being generally straight, and by the parts of the flowers being generally in multiples of three. (examples, grasses, lilies, orchids, palms, etc.) moraines.--the accumulations of fragments of rock brought down by glaciers. morphology.--the law of form or structure independent of function. mysis-stage.--a stage in the development of certain crustaceans (prawns), in which they closely resemble the adults of a genus (mysis) belonging to a slightly lower group. nascent.--commencing development. natatory.--adapted for the purpose of swimming. nauplius-form.--the earliest stage in the development of many crustacea, especially belonging to the lower groups. in this stage the animal has a short body, with indistinct indications of a division into segments, and three pairs of fringed limbs. this form of the common fresh-water cyclops was described as a distinct genus under the name of nauplius. neuration.--the arrangement of the veins or nervures in the wings of insects. neuters.--imperfectly developed females of certain social insects (such as ants and bees), which perform all the labours of the community. hence, they are also called workers. nictitating membrane.--a semi-transparent membrane, which can be drawn across the eye in birds and reptiles, either to moderate the effects of a strong light or to sweep particles of dust, etc., from the surface of the eye. ocelli.--the simple eyes or stemmata of insects, usually situated on the crown of the head between the great compound eyes. oesophagus.--the gullet. oolitic.--a great series of secondary rocks, so called from the texture of some of its members, which appear to be made up of a mass of small egg-like calcareous bodies. operculum.--a calcareous plate employed by many molluscae to close the aperture of their shell. the opercular valves of cirripedes are those which close the aperture of the shell. orbit.--the bony cavity for the reception of the eye. organism.--an organised being, whether plant or animal. orthospermous.--a term applied to those fruits of the umbelliferae which have the seed straight. osculant.--forms or groups apparently intermediate between and connecting other groups are said to be osculant. ova.--eggs. ovarium or ovary (in plants).--the lower part of the pistil or female organ of the flower, containing the ovules or incipient seeds; by growth after the other organs of the flower have fallen, it usually becomes converted into the fruit. ovigerous.--egg-bearing. ovules (of plants).--the seeds in the earliest condition. pachyderms.--a group of mammalia, so called from their thick skins, and including the elephant, rhinoceros, hippopotamus, etc. palaeozoic.--the oldest system of fossiliferous rocks. palpi.--jointed appendages to some of the organs of the mouth in insects and crustacea. papilionaceae.--an order of plants (see leguminosae), the flowers of these plants are called papilionaceous, or butterfly-like, from the fancied resemblance of the expanded superior petals to the wings of a butterfly. parasite.--an animal or plant living upon or in, and at the expense of, another organism. parthenogenesis.--the production of living organisms from unimpregnated eggs or seeds. pedunculated.--supported upon a stem or stalk. the pedunculated oak has its acorns borne upon a footstool. peloria or pelorism.--the appearance of regularity of structure in the flowers of plants which normally bear irregular flowers. pelvis.--the bony arch to which the hind limbs of vertebrate animals are articulated. petals.--the leaves of the corolla, or second circle of organs in a flower. they are usually of delicate texture and brightly coloured. phyllodineous.--having flattened, leaf-like twigs or leafstalks instead of true leaves. pigment.--the colouring material produced generally in the superficial parts of animals. the cells secreting it are called pigment-cells. pinnate.--bearing leaflets on each side of a central stalk. pistils.--the female organs of a flower, which occupy a position in the centre of the other floral organs. the pistil is generally divisible into the ovary or germen, the style and the stigma. placentalia, placentata.--or placental mammals, see mammalia. plantigrades.--quadrupeds which walk upon the whole sole of the foot, like the bears. plastic.--readily capable of change. pleistocene period.--the latest portion of the tertiary epoch. plumule (in plants).--the minute bud between the seed-leaves of newly-germinated plants. plutonic rocks.--rocks supposed to have been produced by igneous action in the depths of the earth. pollen.--the male element in flowering plants; usually a fine dust produced by the anthers, which, by contact with the stigma effects the fecundation of the seeds. this impregnation is brought about by means of tubes (pollen-tubes) which issue from the pollen-grains adhering to the stigma, and penetrate through the tissues until they reach the ovary. polyandrous (flowers).--flowers having many stamens. polygamous plants.--plants in which some flowers are unisexual and others hermaphrodite. the unisexual (male and female) flowers, may be on the same or on different plants. polymorphic.--presenting many forms. polyzoary.--the common structure formed by the cells of the polyzoa, such as the well-known seamats. prehensile.--capable of grasping. prepotent.--having a superiority of power. primaries.--the feathers forming the tip of the wing of a bird, and inserted upon that part which represents the hand of man. processes.--projecting portions of bones, usually for the attachment of muscles, ligaments, etc. propolis.--a resinous material collected by the hivebees from the opening buds of various trees. protean.--exceedingly variable. protozoa.--the lowest great division of the animal kingdom. these animals are composed of a gelatinous material, and show scarcely any trace of distinct organs. the infusoria, foraminifera, and sponges, with some other forms, belong to this division. pupa (pl. pupae).--the second stage in the development of an insect, from which it emerges in the perfect (winged) reproductive form. in most insects the pupal stage is passed in perfect repose. the chrysalis is the pupal state of butterflies. radicle.--the minute root of an embryo plant. ramus.--one half of the lower jaw in the mammalia. the portion which rises to articulate with the skull is called the ascending ramus. range.--the extent of country over which a plant or animal is naturally spread. range in time expresses the distribution of a species or group through the fossiliferous beds of the earth's crust. retina.--the delicate inner coat of the eye, formed by nervous filaments spreading from the optic nerve, and serving for the perception of the impressions produced by light. retrogression.--backward development. when an animal, as it approaches maturity, becomes less perfectly organised than might be expected from its early stages and known relationships, it is said to undergo a retrograde development or metamorphosis. rhizopods.--a class of lowly organised animals (protozoa), having a gelatinous body, the surface of which can be protruded in the form of root-like processes or filaments, which serve for locomotion and the prehension of food. the most important order is that of the foraminifera. rodents.--the gnawing mammalia, such as the rats, rabbits, and squirrels. they are especially characterised by the possession of a single pair of chisel-like cutting teeth in each jaw, between which and the grinding teeth there is a great gap. rubus.--the bramble genus. rudimentary.--very imperfectly developed. ruminants.--the group of quadrupeds which ruminate or chew the cud, such as oxen, sheep, and deer. they have divided hoofs, and are destitute of front teeth in the upper jaw. sacral.--belonging to the sacrum, or the bone composed usually of two or more united vertebrae to which the sides of the pelvis in vertebrate animals are attached. sarcode.--the gelatinous material of which the bodies of the lowest animals (protozoa) are composed. scutellae.--the horny plates with which the feet of birds are generally more or less covered, especially in front. sedimentary formations.--rocks deposited as sediments from water. segments.--the transverse rings of which the body of an articulate animal or annelid is composed. sepals.--the leaves or segments of the calyx, or outermost envelope of an ordinary flower. they are usually green, but sometimes brightly coloured. serratures.--teeth like those of a saw. sessile.--not supported on a stem or footstalk. silurian system.--a very ancient system of fossiliferous rocks belonging to the earlier part of the palaeozoic series. specialisation.--the setting apart of a particular organ for the performance of a particular function. spinal cord.--the central portion of the nervous system in the vertebrata, which descends from the brain through the arches of the vertebrae, and gives off nearly all the nerves to the various organs of the body. stamens.--the male organs of flowering plants, standing in a circle within the petals. they usually consist of a filament and an anther, the anther being the essential part in which the pollen, or fecundating dust, is formed. sternum.--the breast-bone. stigma.--the apical portion of the pistil in flowering plants. stipules.--small leafy organs placed at the base of the footstalks of the leaves in many plants. style.--the middle portion of the perfect pistil, which rises like a column from the ovary and supports the stigma at its summit. subcutaneous.--situated beneath the skin. suctorial.--adapted for sucking. sutures (in the skull).--the lines of junction of the bones of which the skull is composed. tarsus (pl. tarsi).--the jointed feet of articulate animals, such as insects. teleostean fishes.--fishes of the kind familiar to us in the present day, having the skeleton usually completely ossified and the scales horny. tentacula or tentacles.--delicate fleshy organs of prehension or touch possessed by many of the lower animals. tertiary.--the latest geological epoch, immediately preceding the establishment of the present order of things. trachea.--the windpipe or passage for the admission of air to the lungs. tridactyle.--three-fingered, or composed of three movable parts attached to a common base. trilobites.--a peculiar group of extinct crustaceans, somewhat resembling the woodlice in external form, and, like some of them, capable of rolling themselves up into a ball. their remains are found only in the palaeozoic rocks, and most abundantly in those of silurian age. trimorphic.--presenting three distinct forms. umbelliferae.--an order of plants in which the flowers, which contain five stamens and a pistil with two styles, are supported upon footstalks which spring from the top of the flower stem and spread out like the wires of an umbrella, so as to bring all the flowers in the same head (umbel) nearly to the same level. (examples, parsley and carrot.) ungulata.--hoofed quadrupeds. unicellular.--consisting of a single cell. vascular.--containing blood-vessels. vermiform.--like a worm. vertebrata or vertebrate animals.--the highest division of the animal kingdom, so called from the presence in most cases of a backbone composed of numerous joints or vertebrae, which constitutes the centre of the skeleton and at the same time supports and protects the central parts of the nervous system. whorls.--the circles or spiral lines in which the parts of plants are arranged upon the axis of growth. workers.--see neuters. zoea-stage.--the earliest stage in the development of many of the higher crustacea, so called from the name of zoea applied to these young animals when they were supposed to constitute a peculiar genus. zooids.--in many of the lower animals (such as the corals, medusae, etc.) reproduction takes place in two ways, namely, by means of eggs and by a process of budding with or without separation from the parent of the product of the latter, which is often very different from that of the egg. the individuality of the species is represented by the whole of the form produced between two sexual reproductions; and these forms, which are apparently individual animals, have been called zooide. index. aberrant groups abyssinia, plants of acclimatisation adoxa affinities of extinct species --of organic beings agassiz on amblyopsis --on groups of species suddenly appearing --on prophetic forms --on embryological succession --on the glacial period --on embryological characters --on the latest tertiary forms --on parallelism of embryological development and geological succession --alex., on pedicellariae algae of new zealand alligators, males, fighting alternate generations amblyopsis, blind fish america, north, productions allied to those of europe --boulders and glaciers of --south, no modern formations on west coast ammonites, sudden extinction of anagallis, sterility of analogy of variations andaman islands inhabited by a toad ancylus animals, not domesticated from being variable --domestic; descended from several stocks --acclimatisation of animals of australia --with thicker fur in cold climates --blind, in caves --extinct, of australia anomma antarctic islands, ancient flora of antechinus ants attending aphides --slave-making instinct --neuters, structure of apes, not having acquired intellectual powers aphides attended by ants aphis, development of apteryx arab horses aralo-caspian sea archeopteryx archiac, m. de, on the succession of species artichoke, jerusalem ascension, plants of asclepias, pollen of asparagus aspicarpa asses, striped --improved by selection ateuchus aucapitaine, on land-shells audubon, on habits of frigate-bird --on variation in birds' nests --on heron eating seeds australia, animals of --dogs of --extinct animals of --european plants in --glaciers of azara, on flies destroying cattle azores, flora of babington, mr., on british plants baer, von, standard of highness --comparison of bee and fish --embryonic similarity of the vertebrata baker, sir s., on the giraffe balancement of growth baleen barberry, flowers of barrande, m., on silurian colonies --on the succession of species --on parallelism of palaeozoic formations --on affinities of ancient species barriers, importance of bates, mr., on mimetic butterflies batrachians on islands bats, how structure acquired --distribution of bear, catching water-insects beauty, how acquired bee, sting of --queen, killing rivals --australian, extermination of bees, fertilizing flowers --hive, not sucking the red clover --hive, cell-making instinct --ligurian --variation in habits bees, parasitic --humble, cells of beetles, wingless, in madeira --with deficient tarsi bentham, mr., on british plants --on classification berkeley, mr., on seeds in salt-water bermuda, birds of birds acquiring fear --beauty of --annually cross the atlantic --colour of, on continents --footsteps, and remains of, in secondary rocks --fossil, in caves of brazil --of madeira, bermuda, and galapagos --song of males --transporting seeds --waders --wingless bizcacha, affinities of bladder for swimming, in fish blindness of cave animals blyth, mr., on distinctness of indian cattle --on striped hemionus --on crossed geese borrow, mr., on the spanish pointer bory st. vincent, on batrachians bosquet, m., on fossil chthamalus boulders, erratic, on the azores branchiae --of crustaceans braun, prof., on the seeds of fumariaceae brent, mr., on house-tumblers britain, mammals of broca, prof., on natural selection bronn, prof., on duration of specific forms --various objections by brown, robert, on classification brown-sequard, on inherited mutilations busk, mr., on the polyzoa butterflies, mimetic buzareingues, on sterility of varieties cabbage, varieties of, crossed calceolaria canary-birds, sterility of hybrids cape de verde islands, productions of --plants of, on mountains cape of good hope, plants of carpenter, dr., on foraminifera carthemus catasetum cats, with blue eyes, deaf --variation in habits of --curling tail when going to spring cattle destroying fir-trees --destroyed by flies in paraguay --breeds of, locally extinct --fertility of indian and european breeds --indian cave, inhabitants of, blind cecidomyia celts, proving antiquity of man centres of creation cephalopodae, structures of eyes --development of cercopithecus, tail of ceroxylus laceratus cervulus cetacea, teeth and hair --development of the whalebone cetaceans ceylon, plants of chalk formation characters, divergence of --sexual, variable --adaptive or analogical charlock checks to increase --mutual chelae of crustaceans chickens, instinctive tameness of chironomus, its asexual reproduction chthamalinae chthamalus, cretacean species of circumstances favourable to selection of domestic products --to natural selection cirripedes capable of crossing --carapace aborted --their ovigerous frena --fossil --larvae of claparede, prof., on the hair-claspers of the acaridae clarke, rev. w.b., on old glaciers in australia classification clift, mr., on the succession of types climate, effects of, in checking increase of beings --adaptation of, to organisms climbing plants --development of clover visited by bees cobites, intestine of cockroach collections, palaeontological, poor colour, influenced by climate --in relation to attacks by flies columba livia, parent of domestic pigeons colymbetes compensation of growth compositae, flowers and seeds of --outer and inner florets of --male flowers of conclusion, general conditions, slight changes in, favourable to fertility convergence of genera coot cope, prof., on the acceleration or retardation of the period of reproduction coral-islands, seeds drifted to --reefs, indicating movements of earth corn-crake correlated variation in domestic productions coryanthes creation, single centres of crinum croll, mr., on subaerial denudation --on the age of our oldest formations --on alternate glacial periods in the north and south crosses, reciprocal crossing of domestic animals, importance in altering breeds --advantages of --unfavourable to selection cruger, dr., on coryanthes crustacea of new zealand crustacean, blind air-breathers crustaceans, their chelae cryptocerus ctenomys, blind cuckoo, instinct of cunningham, mr., on the flight of the logger-headed duck currants, grafts of currents of sea, rate of cuvier on conditions of existence --on fossil monkeys cuvier, fred., on instinct cyclostoma, resisting salt water dana, prof., on blind cave-animals --on relations of crustaceans of japan --on crustaceans of new zealand dawson, dr., on eozoon de candolle, aug. pyr., on struggle for existence --on umbelliferae --on general affinities de candolle, alph., on the variability of oaks --on low plants, widely dispersed --on widely-ranging plants being variable --on naturalisation --on winged seeds --on alpine species suddenly becoming rare --on distribution of plants with large seeds --on vegetation of australia --on fresh-water plants --on insular plants degradation of rocks denudation, rate of --of oldest rocks --of granite areas development of ancient forms devonian system dianthus, fertility of crosses dimorphism in plants dirt on feet of birds dispersal, means of --during glacial period distribution, geographical --means of disuse, effect of, under nature diversification of means for same general purpose division, physiological, of labour divergence of character dog, resemblance of jaw to that of the thylacinus dogs, hairless, with imperfect teeth --descended from several wild stocks --domestic instincts of --inherited civilisation of --fertility of breeds together --of crosses --proportions of body in different breeds, when young domestication, variation under double flowers downing, mr., on fruit-trees in america dragon-flies, intestines of drift-timber driver-ant drones killed by other bees duck, domestic, wings of, reduced --beak of --logger-headed duckweed dugong, affinities of dung-beetles with deficient tarsi dyticus earl, mr., w., on the malay archipelago ears, drooping, in domestic animals --rudimentary earth, seeds in roots of trees --charged with seeds echinodermata, their pedicellariae eciton economy of organisation edentata, teeth and hair --fossil species of edwards, milne, on physiological division of labour --on gradations of structure edwards, on embryological characters eggs, young birds escaping from egypt, productions of, not modified electric organs elephant, rate of increase --of glacial period embryology eozoon canadense epilipsy inherited existence, struggle for --condition of extinction, as bearing on natural selection --of domestic varieties eye, structure of --correction for aberration eyes, reduced, in moles fabre, m., on hymenoptera fighting --on parasitic sphex --on sitaris falconer, dr., on naturalisation of plants in india --on elephants and mastodons --and cautley on mammals of sub-himalayan beds falkland islands, wolf of faults faunas, marine fear, instinctive, in birds feet of birds, young molluscs adhering to fertilisation variously effected fertility of hybrids --from slight changes in conditions --of crossed varieties fir-trees destroyed by cattle --pollen of fish, flying --teleostean, sudden appearance of --eating seeds --fresh-water, distribution of fishes, ganoid, now confined to fresh water --ganoid, living in fresh water --electric organs of --of southern hemisphere flight, powers of, how acquired flint-tools, proving antiquity of man flower, prof., on the larynx --on halitherium --on the resemblance between the jaws of the dog and thylacinus --on the homology of the feet of certain marsupials flowers, structure of --in relation to crossing --of composite and umbelliferae --beauty of --double flysch formation, destitute of organic remains forbes, mr. d., on glacial action in the andes forbes, e., on colours of shells --on abrupt range of shells in depth --on poorness of palaeontological collections --on continuous succession of genera --on continental extensions --on distribution during glacial period --on parallelism in time and space forests, changes in, in america formation, devonian --cambrian --intermittent --thickness of, in britain formica --rufescens --sanguinea --flava, neuter of forms, lowly organised, long enduring frena, ovigerous, of cirripedes fresh-water productions, dispersal of fries on species in large genera being closely allied to other species frigate-bird frogs on islands fruit-trees, gradual improvement of --in united states --varieties of, acclimatised in united states fuci, crossed fur, thicker in cold climates furze galapagos archipelago, birds of --productions of galaxias, its wide range galeopithecus game, increase of, checked by vermin gartner on sterility of hybrids --on reciprocal crosses --on crossed maize and verbascum --on comparison of hybrids and mongrels gaudry, prof., on intermediate genera of fossil mammals in attica geese, fertility when crossed --upland geikie, mr., on subaerial denudation genealogy, important in classification generations, alternate geoffroy st. hilaire, on balancement --on homologous organs geoffroy st. hilaire, isidore, on variability of repeated parts --on correlation, in monstrosities --on correlation --on variable parts being often monstrous geographical distribution geography, ancient geology, future progress of --imperfection of the record gervais, prof., on typotherium giraffe, tail of --structure of glacial period --affecting the north and south glands, mammary gmelin, on distribution godwin-austin, mr., on the malay archipelago goethe, on compensation of growth gomphia gooseberry, grafts of gould, dr. aug. a., on land-shells gould, mr., on colours of birds --on instincts of cuckoo --on distribution of genera of birds gourds, crossed graba, on the uria lacrymans grafting, capacity of granite, areas of denuded grasses, varieties of gray, dr. asa, on the variability of oaks --on man not causing variability --on sexes of the holly --on trees of the united states --on naturalised plants in the united states --on aestivation --on rarity of intermediate varieties --on alpine plants gray, dr. j.e., on striped mule grebe grimm, on asexual reproduction groups, aberrant grouse, colours of --red, a doubtful species growth, compensation of gunther, dr., on flat-fish --on prehensile tails --on the fishes of panama --on the range of fresh-water fishes --on the limbs of lepidosiren haast, dr., on glaciers of new zealand habit, effect of, under domestication --effect of, under nature --diversified, of same species hackel, prof., on classification and the lines of descent hair and teeth, correlated halitherium harcourt, mr. e.v., on the birds of madeira hartung, m., on boulders in the azores hazel-nuts hearne, on habits of bears heath, changes in vegetation hector, dr., on glaciers of new zealand heer, oswald, on ancient cultivated plants --on plants of madeira helianthemum helix pomatia, resisting salt water helmholtz, m., on the imperfection of the human eye helosciadium hemionus, striped hensen, dr., on the eyes of cephalopods herbert, w., on struggle for existence --on sterility of hybrids hermaphrodites crossing heron eating seed heron, sir r., on peacocks heusinger, on white animals poisoned by certain plants hewitt, mr., on sterility of first crosses hildebrand, prof., on the self-sterility of corydalis hilgendorf, on intermediate varieties himalaya, glaciers of --plants of hippeastrum hippocampus hofmeister, prof., on the movements of plants holly-trees, sexes of hooker, dr., on trees of new zealand --on acclimatisation of himalayan trees --on flowers of umbelliferae --on the position of ovules --on glaciers of himalaya --on algae of new zealand --on vegetation at the base of the himalaya --on plants of tierra del fuego --on australian plants --on relations of flora of america --on flora of the antarctic lands --on the plants of the galapagos --on glaciers of the lebanon --on man not causing variability --on plants of mountains of fernando po hooks on palms --on seeds, on islands hopkins, mr., on denudation hornbill, remarkable instinct of horns, rudimentary horse, fossil in la plata --proportions of, when young horses destroyed by flies in paraguay --striped horticulturists, selection applied by huber on cells of bees huber, p., on reason blended with instinct --on habitual nature of instincts --on slave-making ants --on melipona domestica hudson, mr., on the ground-woodpecker of la plata --on the molothrus humble-bees, cells of hunter, j., on secondary sexual characters hutton, captain, on crossed geese huxley, prof., on structure of hermaphrodites --on the affinities of the sirenia --on forms connecting birds and reptiles --on homologous organs --on the development of aphis hybrids and mongrels compared hybridism hydra, structure of hymenoptera, fighting hymenopterous insect, diving hyoseris ibla icebergs transporting seeds increase, rate of individuals, numbers favourable to selection --many, whether simultaneously created inheritance, laws of --at corresponding ages insects, colour of, fitted for their stations --sea-side, colours of --blind, in caves --luminous --their resemblance to certain objects --neuter instinct, not varying simultaneously with structure instincts, domestic intercrossing, advantages of islands, oceanic isolation favourable to selection japan, productions of java, plants of jones, mr. j.m., on the birds of bermuda jordain, m., on the eye-spots of star fishes jukes, prof., on subaerial denudation jussieu on classification kentucky, caves of kerguelen-land, flora of kidney-bean, acclimatisation of kidneys of birds kirby, on tarsi deficient in beetles knight, andrew, on cause of variation kolreuter, on intercrossing --on the barberry --on sterility of hybrids --on reciprocal crosses --on crossed varieties of nicotiana --on crossing male and hermaphrodite flowers lamarck, on adaptive characters lancelet, eyes of landois, on the development of the wings of insects land-shells, distribution of --of madeira, naturalised --resisting salt water languages, classification of lankester, mr. e. ray, on longevity --on homologies lapse, great, of time larvae laurel, nectar secreted by the leaves laurentian formation laws of variation leech, varieties of leguminosae, nectar secreted by glands leibnitz, attack on newton lepidosiren, limbs in a nascent condition lewes, mr. g.h., on species not having changed in egypt --on the salamandra atra --on many forms of life having been at first evolved life, struggle for lingula, silurian linnaeus, aphorism of lion, mane of --young of, striped lobelia fulgens lobelia, sterility of crosses lockwood, mr., on the ova of the hippocampus locusts transporting seeds logan, sir w., on laurentian formation lowe, rev. r.t., on locusts visiting madeira lowness, of structure connected with variability --related to wide distribution lubbock, sir j., on the nerves of coccus --on secondary sexual characters --on a diving hymenopterous insect --on affinities --on metamorphoses lucas, dr. p., on inheritance --on resemblance of child to parent lund and clausen, on fossils of brazil lyell, sir c., on the struggle for existence --on modern changes of the earth --on terrestrial animals not having been developed on islands --on a carboniferous land-shell --on strata beneath silurian system --on the imperfection of the geological record --on the appearance of species --on barrande's colonies --on tertiary formations of europe and north america --on parallelism of tertiary formations --on transport of seeds by icebergs --on great alternations of climate --on the distribution of fresh-water shells --on land-shells of madeira lyell and dawson, on fossilized trees in nova scotia lythrum salicaria, trimorphic macleay, on analogical characters macrauchenia mcdonnell, dr., on electric organs madeira, plants of --beetles of, wingless --fossil land-shells of --birds of magpie tame in norway males, fighting maize, crossed malay archipelago, compared with europe --mammals of malm, on flat-fish malpighiaceae, small imperfect flowers of mammae, their development --rudimentary mammals, fossil, in secondary formation --insular man, origin of manatee, rudimentary nails of marsupials, fossil species of marsupials of australia, structure of their feet martens, m., experiment on seeds martin, mr. w.c., on striped mules masters, dr., on saponaria matteucci, on the electric organs of rays matthiola, reciprocal crosses of maurandia means of dispersal melipona domestica merrill, dr., on the american cuckoo metamorphism of oldest rocks mice destroying bees --acclimatisation of --tails of miller, prof., on the cells of bees mirabilis, crosses of missel-thrush mistletoe, complex relations of mivart, mr., on the relation of hair and teeth --on the eyes of cephalopods --various objections to natural selection --on abrupt modifications --on the resemblance of the mouse and antechinus mocking-thrush of the galapagos modification of species, not abrupt moles, blind molothrus, habits of mongrels, fertility and sterility of --and hybrids compared monkeys, fossil monachanthus mons, van, on the origin of fruit-trees monstrosities moquin-tandon, on sea-side plants morphology morren, on the leaves of oxalis moths, hybrid mozart, musical powers of mud, seeds in mules, striped muller, adolph, on the instincts of the cuckoo muller, dr. ferdinand, on alpine australian plants muller, fritz, on dimorphic crustaceans --on the lancelet --on air-breathing crustaceans --on the self-sterility of orchids --on embryology in relation to classification --on the metamorphoses of crustaceans --on terrestrial and fresh-water organisms not undergoing any metamorphosis --on climbing plants multiplication of species not indefinite murchison, sir, r., on the formations of russia --on azoic formations --on extinction murie, dr., on the modification of the skull in old age murray, mr. a., on cave-insects mustela vison myanthus myrmecocystus myrmica, eyes of nageli, on morphological characters nails, rudimentary nathusius, von, on pigs natural history, future progress of --selection --system naturalisation of forms distinct from the indigenous species --in new zealand naudin, on analagous variations in gourds --on hybrid gourds --on reversion nautilus, silurian nectar of plants nectaries, how formed nelumbium luteum nests, variation in neuter insects new zealand, productions of, not perfect --naturalised products of --fossil birds of --glaciers of --crustaceans of --algae of --number of plants of --flora of newman, col., on humble-bees newton, prof., on earth attached to a partridge's foot newton, sir i., attacked for irreligion nicotiana, crossed varieties of --certain species very sterile nitsche, dr., on the polyzoa noble, mr., on fertility of rhododendron nodules, phosphatic, in azoic rocks oak, varieties of onites apelles orchids, fertilisation of --the development of their flowers --forms of orchis, pollen of organisation, tendency to advance organs of extreme perfection --electric, of fishes --of little importance --homologous --rudiments of, and nascent ornithorhynchus, mammae of ostrich not capable of flight --habit of laying eggs together --american, two species of otter, habits of, how acquired ouzel, water owen, prof., on birds not flying --on vegetative repetition --on variability of unusually developed parts --on the eyes of fishes --on the swim-bladder of fishes --on fossil horse of la plata --on generalised form --on relation of ruminants and pachyderms --on fossil birds of new zealand --on succession of types --on affinities of the dugong --on homologous organs --on the metamorphosis of cephalopods pacific ocean, faunas of pacini, on electric organs paley, on no organ formed to give pain pallas, on the fertility of the domesticated descendants of wild stocks palm with hooks papaver bracteatum paraguay, cattle destroyed by flies parasites partridge, with ball of dirt attached to foot parts greatly developed, variable parus major passiflora peaches in united states pear, grafts of pedicellariae pelargonium, flowers of --sterility of peloria pelvis of women period, glacial petrels, habits of phasianus, fertility of hybrids pheasant, young, wild pictet, prof., on groups of species suddenly appearing --on rate of organic change --on continuous succession of genera --on close alliance of fossils in consecutive formations --on change in latest tertiary forms --on early transitional links pierce, mr., on varieties of wolves pigeons with feathered feet and skin between toes --breeds described, and origin of --breeds of, how produced --tumbler, not being able to get out of egg --reverting to blue colour --instinct of tumbling --young of pigs, black, not affected by the paint-root --modified by want of exercise pistil, rudimentary plants, poisonous, not affecting certain coloured animals --selection, applied to --gradual improvement of --not improved in barbarous countries --dimorphic --destroyed by insects --in midst of range, have to struggle with other plants --nectar of --fleshy, on sea-shores --climbing --fresh-water, distribution of --low in scale, widely distributed pleuronectidae, their structure plumage, laws of change in sexes of birds plums in the united states pointer dog, origin of --habits of poison not affecting certain coloured animals poison, similar effect of, on animals and plants pollen of fir-trees --transported by various means pollinia, their development polyzoa, their avicularia poole, col., on striped hemionus potemogeton pouchet, on the colours of flat-fish prestwich, mr., on english and french eocene formations proctotrupes proteolepas proteus psychology, future progress of pyrgoma, found in the chalk quagga, striped quatrefages, m., on hybrid moths quercus, variability of quince, grafts of rabbits, disposition of young races, domestic, characters of race-horses, arab --english radcliffe, dr., the electrical organs of the torpedo ramond, on plants of pyrenees ramsay, prof., on subaerial denudation --on thickness of the british formations --on faults ramsay, mr., on instincts of cuckoo ratio of increase rats, supplanting each other --acclimatisation of --blind, in cave rattle-snake reason and instinct recapitulation, general reciprocity of crosses record, geological, imperfect rengger, on flies destroying cattle reproduction, rate of resemblance, protective, of insects --to parents in mongrels and hybrids reversion, law of inheritance --in pigeons, to blue colour rhododendron, sterility of richard, prof., on aspicarpa richardson, sir j., on structure of squirrels --on fishes of the southern hemisphere robinia, grafts of rodents, blind rogers, prof., map of n. america rudimentary organs rudiments important for classification rutimeyer, on indian cattle sageret, on grafts salamandra atra saliva used in nests salmons, males fighting, and hooked jaws of salt-water, how far injurious to seeds --not destructive to land-shells salter, mr., on early death of hybrid embryos salvin, mr., on the beaks of ducks saurophagus sulphuratus schacht, prof., on phyllotaxy schiodte, on blind insects --on flat-fish schlegel, on snakes schobl, dr., on the ears of mice scott, mr. j., on the self-sterility of orchids --on the crossing of varieties of verbascum sea-water, how far injurious to seeds --not destructive to land-shells sebright, sir j., on crossed animals sedgwick, prof., on groups of species suddenly appearing seedlings destroyed by insects seeds, nutriment in --winged --means of dissemination --power of resisting salt-water --in crops and intestines of birds --eaten by fish --in mud --hooked, on islands selection of domestic products --principle not of recent origin --unconscious --natural --sexual --objections to term --natural, has not induced sterility sexes, relations of sexual characters variable --selection sheep, merino, their selection --two sub-breeds, unintentionally produced --mountain, varieties of shells, colours of, littoral --hinges of --seldom embedded shells, fresh-water, long retain the same forms --fresh-water, dispersal of --of madeira --land, distribution of --land, resisting salt water shrew-mouse silene, infertility of crosses silliman, prof., on blind rat sirenia, their affinities sitaris, metamorphosis of skulls of young mammals slave-making instinct smith, col. hamilton, on striped horses smith, dr., on the polyzoa smith, mr. fred., on slave-making ants --on neuter ants snake with tooth for cutting through egg-shell somerville, lord, on selection of sheep sorbus, grafts of sorex spaniel, king charles' breed specialisation of organs species, polymorphic --dominant --common, variable --in large genera variable --groups of, suddenly appearing --beneath silurian formations --successively appearing --changing simultaneously throughout the world spencer, lord, on increase in size of cattle spencer, mr. herbert, on the first steps in differentiation --on the tendency to an equilibrium in all forces sphex, parasitic spiders, development of sports in plants sprengel, c.c., on crossing --on ray-florets squalodon squirrels, gradations in structure staffordshire, heath, changes in stag-beetles, fighting star fishes, eyes of --their pedicellariae sterility from changed conditions of life --of hybrids --laws of --causes of --from unfavourable conditions --not induced through natural selection st. helena, productions of st. hilaire, aug., on variability of certain plants --on classification st. john, mr., on habits of cats sting of bee stocks, aboriginal, of domestic animals strata, thickness of, in britain stripes on horses structure, degrees of utility of struggle for existence succession, geological --of types in same areas swallow, one species supplanting another swaysland, mr., on earth adhering to the feet of migratory birds swifts, nests of swim-bladder switzerland, lake inhabitants of system, natural tail of giraffe --of aquatic animals --prehensile --rudimentary tanais, dimorphic tarsi deficient tausch, dr., on umbelliferae teeth and hair correlated --rudimentary, in embryonic calf tegetmeier, mr., on cells of bees temminck, on distribution aiding classification tendrils, their development thompson, sir w., on the age of the habitable world --on the consolidation of the crust of the earth thouin, on grafts thrush, aquatic species of --mocking, of the galapagos --young of, spotted --nest of thuret, m., on crossed fuci thwaites, mr., on acclimatisation thylacinus tierra del fuego, dogs of --plants of timber-drift time, lapse of --by itself not causing modification titmouse toads on islands tobacco, crossed varieties of tomes, mr., on the distribution of bats transitions in varieties rare traquair, dr., on flat-fish trautschold, on intermediate varieties trees on islands belong to peculiar orders --with separated sexes trifolium pratense --incarnatum trigonia trilobites --sudden extinction of trimen, mr., on imitating-insects trimorphism in plants troglodytes tuco-tuco, blind tumbler pigeons, habits of, hereditary --young of turkey-cock, tuft of hair on breast turkey, naked skin on head --young of, instinctively wild turnip and cabbage, analogous variations of type, unity of types, succession of, in same areas typotherium udders enlarged by use --rudimentary ulex, young leaves of umbelliferae, flowers and seeds of --outer and inner florets of unity of type uria lacrymans use, effects of --under domestication --in a state of nature utility, how far important in the construction of each part valenciennes, on fresh-water fish variability of mongrels and hybrids variation, under domestication --caused by reproductive system being affected by conditions of life --under nature --laws of --correlated variations appear at corresponding ages --analogous in distinct species varieties, natural --struggle between --domestic, extinction of --transitional, rarity of varieties, when crossed --fertile --sterile --classification of verbascum, sterility of --varieties of, crossed verlot, m., on double stocks verneuil, m. de, on the succession of species vibracula of the polyzoa viola, small imperfect flowers of --tricolor virchow, on the structure of the crystalline lens virginia, pigs of volcanic islands, denudation of vulture, naked skin on head wading-birds wagner, dr., on cecidomyia wagner, moritz, on the importance of isolation wallace, mr., on origin of species --on the limit of variation under domestication --on dimorphic lepidoptera --on races in the malay archipelago --on the improvement of the eye --on the walking-stick insect --on laws of geographical distribution --on the malay archipelago --on mimetic animals walsh, mr. b.d., on phytophagic forms --on equal variability water, fresh, productions of water-hen waterhouse, mr., on australian marsupials --on greatly developed parts being variable --on the cells of bees --on general affinities water-ouzel watson, mr. h.c., on range of varieties of british plants --on acclimatisation --on flora of azores --on rarity of intermediate varieties --on alpine plants --on convergence --on the indefinite multiplication of species weale, mr., on locusts transporting seeds web of feet in water-birds weismann, prof., on the causes of variability --on rudimentary organs west indian islands, mammals of westwood, on species in large genera being closely allied to others --on the tarsi of engidae --on the antennae of hymenopterous insects whales wheat, varieties of white mountains, flora of whittaker, mr., on lines of escarpment wichura, max, on hybrids wings, reduction of size --of insects homologous with branchiae --rudimentary, in insects wolf crossed with dog --of falkland isles wollaston, mr., on varieties of insects --on fossil varieties of shells in madeira wollaston, mr., on colours of insects on sea-shore --on wingless beetles --on rarity of intermediate varieties --on insular insects --on land-shells of madeira naturalised wolves, varieties of woodcock with earth attached to leg woodpecker, habits of --green colour of woodward, mr., on the duration of specific forms --on pyrgoma --on the continuous succession of genera --on the succession of types world, species changing simultaneously throughout wright, mr. chauncey, on the giraffe --on abrupt modifications wrens, nest of wyman, prof., on correlation of colour and effects of poison --on the cells of the bee youatt, mr., on selection --on sub-breeds of sheep --on rudimentary horns in young cattle zanthoxylon zebra, stripes on zeuglodons public domain works from the university of michigan digital libraries.) _contributions to_ the theory of natural selection. a series of essays. by alfred russel wallace, author of "the malay archipelago," etc., etc. _second edition, with corrections and additions._ new york: macmillan and co. . [_the right of translation and reproduction is reserved._] london: printed by head, hole & co., farringdon street, and ivy lane, e.c. preface. the present volume consists of essays which i have contributed to various periodicals, or read before scientific societies during the last fifteen years, with others now printed for the first time. the two first of the series are printed without alteration, because, having gained me the reputation of being an independent originator of the theory of "natural selection," they may be considered to have some historical value. i have added to them one or two very short explanatory notes, and have given headings to subjects, to make them uniform with the rest of the book. the other essays have been carefully corrected, often considerably enlarged, and in some cases almost rewritten, so as to express more fully and more clearly the views which i hold at the present time; and as most of them originally appeared in publications which have a very limited circulation, i believe that the larger portion of this volume will be new to many of my friends and to most of my readers. i now wish to say a few words on the reasons which have led me to publish this work. the second essay, especially when taken in connection with the first, contains an outline sketch of the theory of the origin of species (by means of what was afterwards termed by mr. darwin--"natural selection,") as conceived by me before i had the least notion of the scope and nature of mr. darwin's labours. they were published in a way not likely to attract the attention of any but working naturalists, and i feel sure that many who have heard of them, have never had the opportunity of ascertaining how much or how little they really contain. it therefore happens, that, while some writers give me more credit than i deserve, others may very naturally class me with dr. wells and mr. patrick matthew, who, as mr. darwin has shown in the historical sketch given in the th and th editions of the "origin of species," certainly propounded the fundamental principle of "natural selection" before himself, but who made no further use of that principle, and failed to see its wide and immensely important applications. the present work will, i venture to think, prove, that i both saw at the time the value and scope of the law which i had discovered, and have since been able to apply it to some purpose in a few original lines of investigation. but here my claims cease. i have felt all my life, and i still feel, the most sincere satisfaction that mr. darwin had been at work long before me, and that it was not left for me to attempt to write "the origin of species." i have long since measured my own strength, and know well that it would be quite unequal to that task. far abler men than myself may confess, that they have not that untiring patience in accumulating, and that wonderful skill in using, large masses of facts of the most varied kind,--that wide and accurate physiological knowledge,--that acuteness in devising and skill in carrying out experiments,--and that admirable style of composition, at once clear, persuasive and judicial,--qualities, which in their harmonious combination mark out mr. darwin as the man, perhaps of all men now living, best fitted for the great work he has undertaken and accomplished. my own more limited powers have, it is true, enabled me now and then to seize on some conspicuous group of unappropriated facts, and to search out some generalization which might bring them under the reign of known law; but they are not suited to that more scientific and more laborious process of elaborate induction, which in mr. darwin's hands has led to such brilliant results. another reason which has led me to publish this volume at the present time is, that there are some important points on which i differ from mr. darwin, and i wish to put my opinions on record in an easily accessible form, before the publication of his new work, (already announced,) in which i believe most of these disputed questions will be fully discussed. i will now give the date and mode of publication of each of the essays in this volume, as well as the amount of alteration they have undergone. i.--on the law which has regulated the introduction of new species. first published in the "annals and magazine of natural history," september, . reprinted without alteration of the text. ii.--on the tendency of varieties to depart indefinitely from the original type. first published in the "journal of the proceedings of the linnæan society," august, . reprinted without alteration of the text, except one or two grammatical emendations. iii.--mimicry and other protective resemblances among animals. first published in the "westminster review," july, . reprinted with a few corrections and some important additions, among which i may especially mention mr. jenner weir's observations and experiments on the colours of the caterpillars eaten or rejected by birds. iv.--the malayan papilionidÆ, or swallow-tailed butterflies, as illustrative of the theory of natural selection. first published in the "transactions of the linnæan society," vol. xxv. (read march, ), under the title, "on the phenomena of variation and geographical distribution, as illustrated by the papilionidæ of the malayan region." the introductory part of this essay is now reprinted, omitting tables, references to plates, &c., with some additions, and several corrections. owing to the publication of dr. felder's "voyage of the novara" (lepidoptera) in the interval between the reading of my paper and its publication, several of my new species must have their names changed for those given to them by dr. felder, and this will explain the want of agreement in some cases between the names used in this volume and those of the original paper. v.--on instinct in man and animals. not previously published. vi.--the philosophy of birds' nests. first published in the "intellectual observer," july, . reprinted with considerable emendations and additions. vii.--a theory of birds' nests; showing the relation of certain differences of colour in birds to their mode of nidification. first published in the "journal of travel and natural history" (no. ), . now reprinted with considerable emendations and additions, by which i have endeavoured more clearly to express, and more fully to illustrate, my meaning in those parts which have been misunderstood by my critics. viii.--creation by law. first published in the "quarterly journal of science," october, . now reprinted with a few alterations and additions. ix.--the development of human races under the law of natural selection. first published in the "anthropological review," may, . now reprinted with a few important alterations and additions. i had intended to have considerably extended this essay, but on attempting it i found that i should probably weaken the effect without adding much to the argument. i have therefore preferred to leave it as it was first written, with the exception of a few ill-considered passages which never fully expressed my meaning. as it now stands, i believe it contains the enunciation of an important truth. x.--the limits of natural selection as applied to man. this is the further development of a few sentences at the end of an article on "geological time and the origin of species," which appeared in the "quarterly review," for april, . i have here ventured to touch on a class of problems which are usually considered to be beyond the boundaries of science, but which, i believe, will one day be brought within her domain. * * * * * for the convenience of those who are acquainted with any of my essays in their original form, i subjoin references to the more important additions and alterations now made to them. _additions and corrections to the essays as originally published_. essays i. and ii. are unaltered, but short notes are added at pp. , , , and . iii.--_mimicry, and other protective resemblances among animals._ page additional illustration of protective colouring in the case of the wood-dove and the robin. on moths resembling bird's dung and mortar. correction of some names of african papilios and a reference to mr. trimen's observations. mr. jenner weir's observation on birds which refused to eat _spilosoma menthrasti_. an additional case of snake mimicry in _oxyrhopus trigeminus_. mr. salvin's case of mimicry among hawks. name, _diadema anomala_, added. to . use of gay colours in caterpillars, with an account of mr. jenner weir's and mr. butler's observations. iv.--_the malayan papilionidæ or swallow-tailed butterflies, as illustrative of the theory of natural selection._ to . additions to the discussion on the rank of the papilionidæ, and on the principles which determine the comparative rank of groups in the animal kingdom. illustration of variability from mr. baker's revision of the british roses. additional facts, on local variations of colour. additional genus of birds (ceycopsis) peculiar to celebes. , . concluding remarks. vi.--_the philosophy of birds' nests._ on nesting of terns and gulls, rewritten. to . daines barrington, and others, on the song of birds. on young birds learning to build, by memory and imitation. levaillant, on mode of nest-building. on imperfect adaptation in birds' nests. vii.--_a theory of birds' nests._ , . introductory passages modified, with some omissions. how modifications of organization would affect the form of the nest. illustration from the habits of children and savages. , . objection to term "hereditary habit" answered. passage rewritten, on more or less variable characters in relation to nidification. on males choosing or rejecting females, and on the various modes in which colour may be acquired by female birds. on probable ancestral colours of female birds. protective colouring of the waxwing. viii.--_creation by law._ amount of variation in dogs. , . the "times" on natural selection. to . on intermediate or generalized forms of extinct animals as an indication of transmutation or development. tabular demonstration of the origin of species by natural selection. ix.--_the development of human races, under the law of natural selection._ on colour as perhaps correlated with immunity from disease in man. , . on the probable future development of man. concluding paragraph rewritten. _london, march, ._ preface to the second edition. the flattering reception of my essays by the public and the press having led to a second edition being called for within a year of its first publication, i have taken the opportunity to make a few necessary corrections. i have also added a few passages to the th and th essays, and have given two notes, explanatory of some portions of the last chapter which appear to have been not always understood. these additions are as follows:-- +-------------------------------------------------------------------+ | _to avoid altering the paging the additional pages now given have | | been lettered._ | +---------+---------+-----------------------------------------------+ | st ed. | nd ed. | | +---------+---------+ | | | | additional facts as to birds acquiring | | | | the song of other species. | | | | | | | a } | mr. spruce's remarks on young birds | | | b } | pairing with old. | | | | | | | a } | pouchet's observations on a change | | | b } | in the nests of swallows. | | | | | | | -- | passage omitted about nest of golden | | | | crested warbler, which had been | | | | inserted on rennie's authority, but | | | | has not been confirmed by any later | | | | observers. | | | | | | | | daines barrington, on importance of | | | | protection to the female bird. | | | | | | | | note a. | | | | | | | b | note b. | +---------+---------+-----------------------------------------------+ contents. i.--_on the law which has regulated the introduction of new species._ pp. - geographical distribution dependent on geologic changes a law deduced from well-known geographical and geological facts the form of a true system of classification determined by this law geographical distribution of organisms geological distribution of the forms of life high organization of very ancient animals consistent with this law objections to forbes' theory of polarity rudimentary organs conclusion ii.--_on the tendency of varieties to depart indefinitely from the original type._ pp. - instability of varieties supposed to prove the permanent distinctness of species the struggle for existence the law of population of species the abundance or rarity of a species dependent upon its more or less perfect adaptation to the conditions of existence useful variations will tend to increase, useless or hurtful variations to diminish superior varieties will ultimately extirpate the original species the partial reversion of domesticated varieties explained lamarck's hypothesis very different from that now advanced conclusion iii.--_mimicry, and other protective resemblances among animals._ pp. - test of true and false theories importance of the principle of utility popular theories of colour in animals importance of concealment as influencing colour special modifications of colour theory of protective colouring objection that colour as being dangerous should not exist in nature mimicry mimicry among lepidoptera lepidoptera mimicking other insects mimicry among beetles beetles mimicking other insects insects mimicking species of other orders cases of mimicry among the vertebrata mimicry among snakes mimicry among birds mimicry among mammals objections to mr. bates' theory of mimicry mimicry by female insects only cause of the dull colours of female birds use of the gaudy colours of many caterpillars summary general deductions as to colour in nature conclusion iv.--_the malayan papilionidæ, or swallow-tailed butterflies, as illustrative of the theory of natural selection._ pp. - special value of the diurnal lepidoptera for inquiries of this nature question of the rank of the papilionidæ distribution of the papilionidæ definition of the word species laws and modes of variation simple variability polymorphism or dimorphism local form or variety co-existing variety race or subspecies species variation as specially influenced by locality local variation of size local variation of form local variations of colour remarks on the facts of local variation mimicry concluding remarks on variation in lepidoptera arrangement and geographical distribution of the malayan papilionidæ arrangement geographical distribution range of the groups of malayan papilionidæ remarkable peculiarities of the island of celebes concluding remarks v.--_on instinct in man and animals._ pp. - how instinct may be best studied definition of instinct does man possess instincts? how indians travel through unknown and trackless forests vi.--_the philosophy of birds' nests._ pp. - instinct or reason in the construction of birds' nests do men build by reason or by imitation? why does each bird build a peculiar kind of nest? how do young birds learn to build their first nest? do birds sing by instinct or by imitation? man's works mainly imitative how young birds may learn to build nests. birds do alter and improve their nests when altered conditions require it conclusion vii.--_a theory of birds' nests; showing the relation of certain differences of colour in female birds to their mode of nidification._ pp. - changed conditions and persistent habits as influencing nidification classification of nests sexual differences of colour in birds the law which connects the colours of female birds with the mode of nidification what the facts teach us colour more variable than structure or habits, and therefore the character which has generally been modified exceptional cases confirmatory of the above explanation real or apparent exceptions to the law stated at p. various modes of protection of animals females of some groups require and obtain more protection than the males conclusion viii.--_creation by law._ pp. - laws from which the origin of species may be deduced mr. darwin's metaphors liable to misconception a case of orchis-structure explained by natural selection adaptation brought about by general laws beauty in nature how new forms are produced by variation and selection the objection that there are limits to variation objection to the argument from classification the _times_ on natural selection intermediate or generalized forms of extinct animals an indication of transmutation or development conclusion a demonstration of the origin of species ix.--_the development of human races under the law of natural selection_. pp. - wide difference of opinion as to man's origin outline of the theory of natural selection different effects of natural selection on animals and on man influence of external nature in the development of the human mind extinction of lower races the origin of the races of man the bearing of these views on the antiquity of man their bearing on the dignity and supremacy of man their bearing on the future development of man summary conclusion x.--_the limits of natural selection as applied to man._ pp. -- what natural selection can not do the brain of the savage shown to be larger than he needs it to be size of brain an important element of mental power comparison of the brains of man and of anthropoid apes range of intellectual power in man intellect of savages and of animals compared the use of the hairy covering of mammalia the constant absence of hair from certain parts of man's body a remarkable phenomenon savage man feels the want of this hairy covering man's naked skin could not have been produced by natural selection feet and hands of man considered as difficulties on the theory of natural selection the origin of some of man's mental faculties, by the preservation of useful variations, not possible difficulty as to the origin of the moral sense summary of the argument as to the insufficiency of natural selection to account for the development of man the origin of consciousness the nature of matter matter is force all force is probably will-force conclusion i. on the law which has regulated the introduction of new species.[a] +--------------------------------------------------------------+ | [a] written at sarawak in february, , and published in | | the "annals and magazine of natural history," september, | | . | +--------------------------------------------------------------+ _geographical distribution dependent on geologic changes._ every naturalist who has directed his attention to the subject of the geographical distribution of animals and plants, must have been interested in the singular facts which it presents. many of these facts are quite different from what would have been anticipated, and have hitherto been considered as highly curious, but quite inexplicable. none of the explanations attempted from the time of linnæus are now considered at all satisfactory; none of them have given a cause sufficient to account for the facts known at the time, or comprehensive enough to include all the new facts which have since been, and are daily being added. of late years, however, a great light has been thrown upon the subject by geological investigations, which have shown that the present state of the earth and of the organisms now inhabiting it, is but the last stage of a long and uninterrupted series of changes which it has undergone, and consequently, that to endeavour to explain and account for its present condition without any reference to those changes (as has frequently been done) must lead to very imperfect and erroneous conclusions. the facts proved by geology are briefly these:--that during an immense, but unknown period, the surface of the earth has undergone successive changes; land has sunk beneath the ocean, while fresh land has risen up from it; mountain chains have been elevated; islands have been formed into continents, and continents submerged till they have become islands; and these changes have taken place, not once merely, but perhaps hundreds, perhaps thousands of times:--that all these operations have been more or less continuous, but unequal in their progress, and during the whole series the organic life of the earth has undergone a corresponding alteration. this alteration also has been gradual, but complete; after a certain interval not a single species existing which had lived at the commencement of the period. this complete renewal of the forms of life also appears to have occurred several times:--that from the last of the geological epochs to the present or historical epoch, the change of organic life has been gradual: the first appearance of animals now existing can in many cases be traced, their numbers gradually increasing in the more recent formations, while other species continually die out and disappear, so that the present condition of the organic world is clearly derived by a natural process of gradual extinction and creation of species from that of the latest geological periods. we may therefore safely infer a like gradation and natural sequence from one geological epoch to another. now, taking this as a fair statement of the results of geological inquiry, we see that the present geographical distribution of life upon the earth must be the result of all the previous changes, both of the surface of the earth itself and of its inhabitants. many causes, no doubt, have operated of which we must ever remain in ignorance, and we may, therefore, expect to find many details very difficult of explanation, and in attempting to give one, must allow ourselves to call into our service geological changes which it is highly probable may have occurred, though we have no direct evidence of their individual operation. the great increase of our knowledge within the last twenty years, both of the present and past history of the organic world, has accumulated a body of facts which should afford a sufficient foundation for a comprehensive law embracing and explaining them all, and giving a direction to new researches. it is about ten years since the idea of such a law suggested itself to the writer of this essay, and he has since taken every opportunity of testing it by all the newly-ascertained facts with which he has become acquainted, or has been able to observe himself. these have all served to convince him of the correctness of his hypothesis. fully to enter into such a subject would occupy much space, and it is only in consequence of some views having been lately promulgated, he believes, in a wrong direction, that he now ventures to present his ideas to the public, with only such obvious illustrations of the arguments and results as occur to him in a place far removed from all means of reference and exact information. _a law deduced from well-known geographical and geological facts._ the following propositions in organic geography and geology give the main facts on which the hypothesis is founded. geography. . large groups, such as classes and orders, are generally spread over the whole earth, while smaller ones, such as families and genera, are frequently confined to one portion, often to a very limited district. . in widely distributed families the genera are often limited in range; in widely distributed genera, well marked groups of species are peculiar to each geographical district. . when a group is confined to one district, and is rich in species, it is almost invariably the case that the most closely allied species are found in the same locality or in closely adjoining localities, and that therefore the natural sequence of the species by affinity is also geographical. . in countries of a similar climate, but separated by a wide sea or lofty mountains, the families, genera and species of the one are often represented by closely allied families, genera and species peculiar to the other. geology. . the distribution of the organic world in time is very similar to its present distribution in space. . most of the larger and some small groups extend through several geological periods. . in each period, however, there are peculiar groups, found nowhere else, and extending through one or several formations. . species of one genus, or genera of one family occurring in the same geological time are more closely allied than those separated in time. . as generally in geography no species or genus occurs in two very distant localities without being also found in intermediate places, so in geology the life of a species or genus has not been interrupted. in other words, no group or species has come into existence twice. . the following law may be deduced from these facts:--_every species has come into existence coincident both in space and time with a pre-existing closely allied species._ this law agrees with, explains and illustrates all the facts connected with the following branches of the subject:-- st. the system of natural affinities. nd. the distribution of animals and plants in space. rd. the same in time, including all the phænomena of representative groups, and those which professor forbes supposed to manifest polarity. th. the phænomena of rudimentary organs. we will briefly endeavour to show its bearing upon each of these. _the form of a true system of classification determined by this law._ if the law above enunciated be true, it follows that the natural series of affinities will also represent the order in which the several species came into existence, each one having had for its immediate antitype a closely allied species existing at the time of its origin. it is evidently possible that two or three distinct species may have had a common antitype, and that each of these may again have become the antitypes from which other closely allied species were created. the effect of this would be, that so long as each species has had but one new species formed on its model, the line of affinities will be simple, and may be represented by placing the several species in direct succession in a straight line. but if two or more species have been independently formed on the plan of a common antitype, then the series of affinities will be compound, and can only be represented by a forked or many branched line. now, all attempts at a natural classification and arrangement of organic beings show, that both these plans have obtained in creation. sometimes the series of affinities can be well represented for a space by a direct progression from species to species or from group to group, but it is generally found impossible so to continue. there constantly occur two or more modifications of an organ or modifications of two distinct organs, leading us on to two distinct series of species, which at length differ so much from each other as to form distinct genera or families. these are the parallel series or representative groups of naturalists, and they often occur in different countries, or are found fossil in different formations. they are said to have an analogy to each other when they are so far removed from their common antitype as to differ in many important points of structure, while they still preserve a family resemblance. we thus see how difficult it is to determine in every case whether a given relation is an analogy or an affinity, for it is evident that as we go back along the parallel or divergent series, towards the common antitype, the analogy which existed between the two groups becomes an affinity. we are also made aware of the difficulty of arriving at a true classification, even in a small and perfect group;--in the actual state of nature it is almost impossible, the species being so numerous and the modifications of form and structure so varied, arising probably from the immense number of species which have served as antitypes for the existing species, and thus produced a complicated branching of the lines of affinity, as intricate as the twigs of a gnarled oak or the vascular system of the human body. again, if we consider that we have only fragments of this vast system, the stem and main branches being represented by extinct species of which we have no knowledge, while a vast mass of limbs and boughs and minute twigs and scattered leaves is what we have to place in order, and determine the true position each originally occupied with regard to the others, the whole difficulty of the true natural system of classification becomes apparent to us. we shall thus find ourselves obliged to reject all these systems of classification which arrange species or groups in circles, as well as these which fix a definite number for the divisions of each group. the latter class have been very generally rejected by naturalists, as contrary to nature, notwithstanding the ability with which they have been advocated; but the circular system of affinities seems to have obtained a deeper hold, many eminent naturalists having to some extent adopted it. we have, however, never been able to find a case in which the circle has been closed by a direct and close affinity. in most cases a palpable analogy has been substituted, in others the affinity is very obscure or altogether doubtful. the complicated branching of the lines of affinities in extensive groups must also afford great facilities for giving a show of probability to any such purely artificial arrangements. their death-blow was given by the admirable paper of the lamented mr. strickland, published in the "annals of natural history," in which he so clearly showed the true synthetical method of discovering the natural system. _geographical distribution of organisms._ if we now consider the geographical distribution of animals and plants upon the earth, we shall find all the facts beautifully in accordance with, and readily explained by, the present hypothesis. a country having species, genera, and whole families peculiar to it, will be the necessary result of its having been isolated for a long period, sufficient for many series of species to have been created on the type of pre-existing ones, which, as well as many of the earlier-formed species, have become extinct, and thus made the groups appear isolated. if in any case the antitype had an extensive range, two or more groups of species might have been formed, each varying from it in a different manner, and thus producing several representative or analogous groups. the sylviadæ of europe and the sylvicolidæ of north america, the heliconidæ of south america and the euploeas of the east, the group of trogons inhabiting asia, and that peculiar to south america, are examples that may be accounted for in this manner. such phænomena as are exhibited by the galapagos islands, which contain little groups of plants and animals peculiar to themselves, but most nearly allied to those of south america, have not hitherto received any, even a conjectural explanation. the galapagos are a volcanic group of high antiquity, and have probably never been more closely connected with the continent than they are at present. they must have been first peopled, like other newly-formed islands, by the action of winds and currents, and at a period sufficiently remote to have had the original species die out, and the modified prototypes only remain. in the same way we can account for the separate islands having each their peculiar species, either on the supposition that the same original emigration peopled the whole of the islands with the same species from which differently modified prototypes were created, or that the islands were successively peopled from each other, but that new species have been created in each on the plan of the pre-existing ones. st. helena is a similar case of a very ancient island having obtained an entirely peculiar, though limited, flora. on the other hand, no example is known of an island which can be proved geologically to be of very recent origin (late in the tertiary, for instance), and yet possesses generic or family groups, or even many species peculiar to itself. when a range of mountains has attained a great elevation, and has so remained during a long geological period, the species of the two sides at and near their bases will be often very different, representative species of some genera occurring, and even whole genera being peculiar to one side only, as is remarkably seen in the case of the andes and rocky mountains. a similar phænomenon occurs when an island has been separated from a continent at a very early period. the shallow sea between the peninsula of malacca, java, sumatra and borneo was probably a continent or large island at an early epoch, and may have become submerged as the volcanic ranges of java and sumatra were elevated. the organic results we see in the very considerable number of species of animals common to some or all of these countries, while at the same time a number of closely allied representative species exist peculiar to each, showing that a considerable period has elapsed since their separation. the facts of geographical distribution and of geology may thus mutually explain each other in doubtful cases, should the principles here advocated be clearly established. in all those cases in which an island has been separated from a continent, or raised by volcanic or coralline action from the sea, or in which a mountain-chain has been elevated in a recent geological epoch, the phænomena of peculiar groups or even of single representative species will not exist. our own island is an example of this, its separation from the continent being geologically very recent, and we have consequently scarcely a species which is peculiar to it; while the alpine range, one of the most recent mountain elevations, separates faunas and floras which scarcely differ more than may be due to climate and latitude alone. the series of facts alluded to in proposition ( ), of closely allied species in rich groups being found geographically near each other, is most striking and important. mr. lovell reeve has well exemplified it in his able and interesting paper on the distribution of the bulimi. it is also seen in the humming-birds and toucans, little groups of two or three closely allied species being often found in the same or closely adjoining districts, as we have had the good fortune of personally verifying. fishes give evidence of a similar kind: each great river has its peculiar genera, and in more extensive genera its groups of closely allied species. but it is the same throughout nature; every class and order of animals will contribute similar facts. hitherto no attempt has been made to explain these singular phenomena, or to show how they have arisen. why are the genera of palms and of orchids in almost every case confined to one hemisphere? why are the closely allied species of brown-backed trogons all found in the east, and the green-backed in the west? why are the macaws and the cockatoos similarly restricted? insects furnish a countless number of analogous examples;--the goliathi of africa, the ornithopteræ of the indian islands, the heliconidæ of south america, the danaidæ of the east, and in all, the most closely allied species found in geographical proximity. the question forces itself upon every thinking mind,--why are these things so? they could not be as they are had no law regulated their creation and dispersion. the law here enunciated not merely explains, but necessitates the facts we see to exist, while the vast and long-continued geological changes of the earth readily account for the exceptions and apparent discrepancies that here and there occur. the writer's object in putting forward his views in the present imperfect manner is to submit them to the test of other minds, and to be made aware of all the facts supposed to be inconsistent with them. as his hypothesis is one which claims acceptance solely as explaining and connecting facts which exist in nature, he expects facts alone to be brought to disprove it, not _à priori_ arguments against its probability. _geological distribution of the forms of life._ the phænomena of geological distribution are exactly analogous to those of geography. closely allied species are found associated in the same beds, and the change from species to species appears to have been as gradual in time as in space. geology, however, furnishes us with positive proof of the extinction and production of species, though it does not inform us how either has taken place. the extinction of species, however, offers but little difficulty, and the _modus operandi_ has been well illustrated by sir c. lyell in his admirable "principles." geological changes, however gradual, must occasionally have modified external conditions to such an extent as to have rendered the existence of certain species impossible. the extinction would in most cases be effected by a gradual dying-out, but in some instances there might have been a sudden destruction of a species of limited range. to discover how the extinct species have from time to time been replaced by new ones down to the very latest geological period, is the most difficult, and at the same time the most interesting problem in the natural history of the earth. the present inquiry, which seeks to eliminate from known facts a law which has determined, to a certain degree, what species could and did appear at a given epoch, may, it is hoped, be considered as one step in the right direction towards a complete solution of it. _high organization of very ancient animals consistent with this law._ much discussion has of late years taken place on the question, whether the succession of life upon the globe has been from a lower to a higher degree of organization. the admitted facts seem to show that there has been a general, but not a detailed progression. mollusca and radiata existed before vertebrata, and the progression from fishes to reptiles and mammalia, and also from the lower mammals to the higher, is indisputable. on the other hand, it is said that the mollusca and radiata of the very earliest periods were more highly organized than the great mass of those now existing, and that the very first fishes that have been discovered are by no means the lowest organised of the class. now it is believed the present hypothesis will harmonize with all these facts, and in a great measure serve to explain them; for though it may appear to some readers essentially a theory of progression, it is in reality only one of gradual change. it is, however, by no means difficult to show that a real progression in the scale of organization is perfectly consistent with all the appearances, and even with apparent retrogression, should such occur. returning to the analogy of a branching tree, as the best mode of representing the natural arrangement of species and their successive creation, let us suppose that at an early geological epoch any group (say a class of the mollusca) has attained to a great richness of species and a high organization. now let this great branch of allied species, by geological mutations, be completely or partially destroyed. subsequently a new branch springs from the same trunk, that is to say, new species are successively created, having for their antitypes the same lower organized species which had served as the antitypes for the former group, but which have survived the modified conditions which destroyed it. this new group being subject to these altered conditions, has modifications of structure and organization given to it, and becomes the representative group of the former one in another geological formation. it may, however, happen, that though later in time, the new series of species may never attain to so high a degree of organization as those preceding it, but in its turn become extinct, and give place to yet another modification from the same root, which may be of higher or lower organization, more or less numerous in species, and more or less varied in form and structure than either of those which preceded it. again, each of these groups may not have become totally extinct, but may have left a few species, the modified prototypes of which have existed in each succeeding period, a faint memorial of their former grandeur and luxuriance. thus every case of apparent retrogression may be in reality a progress, though an interrupted one: when some monarch of the forest loses a limb, it may be replaced by a feeble and sickly substitute. the foregoing remarks appear to apply to the case of the mollusca, which, at a very early period, had reached a high organization and a great development of forms and species in the testaceous cephalopoda. in each succeeding age modified species and genera replaced the former ones which had become extinct, and as we approach the present æra, but few and small representatives of the group remain, while the gasteropods and bivalves have acquired an immense preponderance. in the long series of changes the earth has undergone, the process of peopling it with organic beings has been continually going on, and whenever any of the higher groups have become nearly or quite extinct, the lower forms which have better resisted the modified physical conditions have served as the antitypes on which to found the new races. in this manner alone, it is believed, can the representative groups at successive periods, and the risings and fallings in the scale of organization, be in every case explained. _objections to forbes' theory of polarity._ the hypothesis of polarity, recently put forward by professor edward forbes to account for the abundance of generic forms at a very early period and at present, while in the intermediate epochs there is a gradual diminution and impoverishment, till the minimum occurred at the confines of the palæozoic and secondary epochs, appears to us quite unnecessary, as the facts may be readily accounted for on the principles already laid down. between the palæozoic and neozoic periods of professor forbes, there is scarcely a species in common, and the greater part of the genera and families also disappear to be replaced by new ones. it is almost universally admitted that such a change in the organic world must have occupied a vast period of time. of this interval we have no record; probably because the whole area of the early formations now exposed to our researches was elevated at the end of the palæozoic period, and remained so through the interval required for the organic changes which resulted in the fauna and flora of the secondary period. the records of this interval are buried beneath the ocean which covers three-fourths of the globe. now it appears highly probable that a long period of quiescence or stability in the physical conditions of a district would be most favourable to the existence of organic life in the greatest abundance, both as regards individuals and also as to variety of species and generic group, just as we now find that the places best adapted to the rapid growth and increase of individuals also contain the greatest profusion of species and the greatest variety of forms,--the tropics in comparison with the temperate and arctic regions. on the other hand, it seems no less probable that a change in the physical conditions of a district, even small in amount if rapid, or even gradual if to a great amount, would be highly unfavourable to the existence of individuals, might cause the extinction of many species, and would probably be equally unfavourable to the creation of new ones. in this too we may find an analogy with the present state of our earth, for it has been shown to be the violent extremes and rapid changes of physical conditions, rather than the actual mean state in the temperate and frigid zones, which renders them less prolific than the tropical regions, as exemplified by the great distance beyond the tropics to which tropical forms penetrate when the climate is equable, and also by the richness in species and forms of tropical mountain regions which principally differ from the temperate zone in the uniformity of their climate. however this may be, it seems a fair assumption that during a period of geological repose the new species which we know to have been created would have appeared; that the creations would then exceed in number the extinctions, and therefore the number of species would increase. in a period of geological activity, on the other hand, it seems probable that the extinctions might exceed the creations, and the number of species consequently diminish. that such effects did take place in connexion with the causes to which we have imputed them, is shown in the case of the coal formation, the faults and contortions of which show a period of great activity and violent convulsions, and it is in the formation immediately succeeding this that the poverty of forms of life is most apparent. we have then only to suppose a long period of somewhat similar action during the vast unknown interval at the termination of the palæozoic period, and then a decreasing violence or rapidity through the secondary period, to allow for the gradual repopulation of the earth with varied forms, and the whole of the facts are explained.[b] we thus have a clue to the increase of the forms of life during certain periods, and their decrease during others, without recourse to any causes but those we know to have existed, and to effects fairly deducible from them. the precise manner in which the geological changes of the early formations were effected is so extremely obscure, that when we can explain important facts by a retardation at one time and an acceleration at another of a process which we know from its nature and from observation to have been unequal,--a cause so simple may surely be preferred to one so obscure and hypothetical as polarity. +--------------------------------------------------------------+ | [b] professor ramsay has since shown that a glacial epoch | | probably occurred at the time of the permian formation, | | which will more satisfactorily account for the comparative | | poverty of species. | +--------------------------------------------------------------+ i would also venture to suggest some reasons against the very nature of the theory of professor forbes. our knowledge of the organic world during any geological epoch is necessarily very imperfect. looking at the vast numbers of species and groups that have been discovered by geologists, this may be doubted; but we should compare their numbers not merely with those that now exist upon the earth, but with a far larger amount. we have no reason for believing that the number of species on the earth at any former period was much less than at present; at all events the aquatic portion, with which geologists have most acquaintance, was probably often as great or greater. now we know that there have been many complete changes of species; new sets of organisms have many times been introduced in place of old ones which have become extinct, so that the total amount which have existed on the earth from the earliest geological period must have borne about the same proportion to those now living, as the whole human race who have lived and died upon the earth, to the population at the present time. again, at each epoch, the whole earth was no doubt, as now, more or less the theatre of life, and as the successive generations of each species died, their exuviæ and preservable parts would be deposited over every portion of the then existing seas and oceans, which we have reason for supposing to have been more, rather than less, extensive than at present. in order then to understand our possible knowledge of the early world and its inhabitants, we must compare, not the area of the whole field of our geological researches with the earth's surface, but the area of the examined portion of each formation separately with the whole earth. for example, during the silurian period all the earth was silurian, and animals were living and dying, and depositing their remains more or less over the whole area of the globe, and they were probably (the species at least) nearly as varied in different latitudes and longitudes as at present. what proportion do the silurian districts bear to the whole surface of the globe, land and sea (for far more extensive silurian districts probably exist beneath the ocean than above it), and what portion of the known silurian districts has been actually examined for fossils? would the area of rock actually laid open to the eye be the thousandth or the ten-thousandth part of the earth's surface? ask the same question with regard to the oolite or the chalk, or even to particular beds of these when they differ considerably in their fossils, and you may then get some notion of how small a portion of the whole we know. but yet more important is the probability, nay almost the certainty, that whole formations containing the records of vast geological periods are entirely buried beneath the ocean, and for ever beyond our reach. most of the gaps in the geological series may thus be filled up, and vast numbers of unknown and unimaginable animals, which might help to elucidate the affinities of the numerous isolated groups which are a perpetual puzzle to the zoologist, may there be buried, till future revolutions may raise them in their turn above the waters, to afford materials for the study of whatever race of intelligent beings may then have succeeded us. these considerations must lead us to the conclusion, that our knowledge of the whole series of the former inhabitants of the earth is necessarily most imperfect and fragmentary,--as much so as our knowledge of the present organic world would be, were we forced to make our collections and observations only in spots equally limited in area and in number with those actually laid open for the collection of fossils. now, the hypothesis of professor forbes is essentially one that assumes to a great extent the completeness of our knowledge of the whole series of organic beings which have existed on the earth. this appears to be a fatal objection to it, independently of all other considerations. it may be said that the same objections exist against every theory on such a subject, but this is not necessarily the case. the hypothesis put forward in this paper depends in no degree upon the completeness of our knowledge of the former condition of the organic world, but takes what facts we have as fragments of a vast whole, and deduces from them something of the nature and proportions of that whole which we can never know in detail. it is founded upon isolated groups of facts, recognizes their isolation, and endeavours to deduce from them the nature of the intervening portions. _rudimentary organs._ another important series of facts, quite in accordance with, and even necessary deductions from, the law now developed, are those of rudimentary organs. that these really do exist, and in most cases have no special function in the animal oeconomy, is admitted by the first authorities in comparative anatomy. the minute limbs hidden beneath the skin in many of the snake-like lizards, the anal hooks of the boa constrictor, the complete series of jointed finger-bones in the paddle of the manatus and whale, are a few of the most familiar instances. in botany a similar class of facts has been long recognised. abortive stamens, rudimentary floral envelopes and undeveloped carpels, are of the most frequent occurrence. to every thoughtful naturalist the question must arise, what are these for? what have they to do with the great laws of creation? do they not teach us something of the system of nature? if each species has been created independently, and without any necessary relations with pre-existing species, what do these rudiments, these apparent imperfections mean? there must be a cause for them; they must be the necessary results of some great natural law. now, if, as it has been endeavoured to be shown, the great law which has regulated the peopling of the earth with animal and vegetable life is, that every change shall be gradual; that no new creature shall be formed widely differing from anything before existing; that in this, as in everything else in nature, there shall be gradation and harmony,--then these rudimentary organs are necessary, and are an essential part of the system of nature. ere the higher vertebrata were formed, for instance, many steps were required, and many organs had to undergo modifications from the rudimental condition in which only they had as yet existed. we still see remaining an antitypal sketch of a wing adapted for flight in the scaly flapper of the penguin, and limbs first concealed beneath the skin, and then weakly protruding from it, were the necessary gradations before others should be formed fully adapted for locomotion.[c] many more of these modifications should we behold, and more complete series of them, had we a view of all the forms which have ceased to live. the great gaps that exist between fishes, reptiles, birds, and mammals would then, no doubt, be softened down by intermediate groups, and the whole organic world would be seen to be an unbroken and harmonious system. +--------------------------------------------------------------+ | [c] the theory of natural selection has now taught us that | | these are not the steps by which limbs have been formed; and | | that most rudimentary organs have been produced by abortion, | | owing to disuse, as explained by mr. darwin. | +--------------------------------------------------------------+ _conclusion._ it has now been shown, though most briefly and imperfectly, how the law that "_every species has come into existence coincident both in time and space with a pre-existing closely allied species_," connects together and renders intelligible a vast number of independent and hitherto unexplained facts. the natural system of arrangement of organic beings, their geographical distribution, their geological sequence, the phænomena of representative and substituted groups in all their modifications, and the most singular peculiarities of anatomical structure, are all explained and illustrated by it, in perfect accordance with the vast mass of facts which the researches of modern naturalists have brought together, and, it is believed, not materially opposed to any of them. it also claims a superiority over previous hypotheses, on the ground that it not merely explains, but necessitates what exists. granted the law, and many of the most important facts in nature could not have been otherwise, but are almost as necessary deductions from it, as are the elliptic orbits of the planets from the law of gravitation. ii. on the tendency of varieties to depart indefinitely from the original type.[d] +--------------------------------------------------------------+ | [d] written at ternate, february, ; and published in the | | journal of the proceedings of the linnæan society for | | august, . | +--------------------------------------------------------------+ _instability of varieties supposed to prove the permanent distinctness of species._ one of the strongest arguments which have been adduced to prove the original and permanent distinctness of species is, that _varieties_ produced in a state of domesticity are more or less unstable, and often have a tendency, if left to themselves, to return to the normal form of the parent species; and this instability is considered to be a distinctive peculiarity of all varieties, even of those occurring among wild animals in a state of nature, and to constitute a provision for preserving unchanged the originally created distinct species. in the absence or scarcity of facts and observations as to _varieties_ occurring among wild animals, this argument has had great weight with naturalists, and has led to a very general and somewhat prejudiced belief in the stability of species. equally general, however, is the belief in what are called "permanent or true varieties,"--races of animals which continually propagate their like, but which differ so slightly (although constantly) from some other race, that the one is considered to be a _variety_ of the other. which is the _variety_ and which the original _species_, there is generally no means of determining, except in those rare cases in which the one race has been known to produce an offspring unlike itself and resembling the other. this, however, would seem quite incompatible with the "permanent invariability of species," but the difficulty is overcome by assuming that such varieties have strict limits, and can never again vary further from the original type, although they may return to it, which, from the analogy of the domesticated animals, is considered to be highly probable, if not certainly proved. it will be observed that this argument rests entirely on the assumption, that _varieties_ occurring in a state of nature are in all respects analogous to or even identical with those of domestic animals, and are governed by the same laws as regards their permanence or further variation. but it is the object of the present paper to show that this assumption is altogether false, that there is a general principle in nature which will cause many _varieties_ to survive the parent species, and to give rise to successive variations departing further and further from the original type; and which also produces, in domesticated animals, the tendency of varieties to return to the parent form. _the struggle for existence._ the life of wild animals is a struggle for existence. the full exertion of all their faculties and all their energies is required to preserve their own existence and provide for that of their infant offspring. the possibility of procuring food during the least favourable seasons, and of escaping the attacks of their most dangerous enemies, are the primary conditions which determine the existence both of individuals and of entire species. these conditions will also determine the population of a species; and by a careful consideration of all the circumstances we may be enabled to comprehend, and in some degree to explain, what at first sight appears so inexplicable--the excessive abundance of some species, while others closely allied to them are very rare. _the law of population of species._ the general proportion that must obtain between certain groups of animals is readily seen. large animals cannot be so abundant as small ones; the carnivora must be less numerous than the herbivora; eagles and lions can never be so plentiful as pigeons and antelopes; and the wild asses of the tartarian deserts cannot equal in numbers the horses of the more luxuriant prairies and pampas of america. the greater or less fecundity of an animal is often considered to be one of the chief causes of its abundance or scarcity; but a consideration of the facts will show us that it really has little or nothing to do with the matter. even the least prolific of animals would increase rapidly if unchecked, whereas it is evident that the animal population of the globe must be stationary, or perhaps, through the influence of man, decreasing. fluctuations there may be; but permanent increase, except in restricted localities, is almost impossible. for example, our own observation must convince us that birds do not go on increasing every year in a geometrical ratio, as they would do, were there not some powerful check to their natural increase. very few birds produce less than two young ones each year, while many have six, eight, or ten; four will certainly be below the average; and if we suppose that each pair produce young only four times in their life, that will also be below the average, supposing them not to die either by violence or want of food. yet at this rate how tremendous would be the increase in a few years from a single pair! a simple calculation will show that in fifteen years each pair of birds would have increased to nearly ten millions![e] whereas we have no reason to believe that the number of the birds of any country increases at all in fifteen or in one hundred and fifty years. with such powers of increase the population must have reached its limits, and have become stationary, in a very few years after the origin of each species. it is evident, therefore, that each year an immense number of birds must perish--as many in fact as are born; and as on the lowest calculation the progeny are each year twice as numerous as their parents, it follows that, whatever be the average number of individuals existing in any given country, _twice that number must perish annually_,--a striking result, but one which seems at least highly probable, and is perhaps under rather than over the truth. it would therefore appear that, as far as the continuance of the species and the keeping up the average number of individuals are concerned, large broods are superfluous. on the average all above _one_ become food for hawks and kites, wild cats or weasels, or perish of cold and hunger as winter comes on. this is strikingly proved by the case of particular species; for we find that their abundance in individuals bears no relation whatever to their fertility in producing offspring. +--------------------------------------------------------------+ | [e] this is under estimated. the number would really amount | | to more than two thousand millions! | +--------------------------------------------------------------+ perhaps the most remarkable instance of an immense bird population is that of the passenger pigeon of the united states, which lays only one, or at most two eggs, and is said to rear generally but one young one. why is this bird so extraordinarily abundant, while others producing two or three times as many young are much less plentiful? the explanation is not difficult. the food most congenial to this species, and on which it thrives best, is abundantly distributed over a very extensive region, offering such differences of soil and climate, that in one part or another of the area the supply never fails. the bird is capable of a very rapid and long-continued flight, so that it can pass without fatigue over the whole of the district it inhabits, and as soon as the supply of food begins to fail in one place is able to discover a fresh feeding-ground. this example strikingly shows us that the procuring a constant supply of wholesome food is almost the sole condition requisite for ensuring the rapid increase of a given species, since neither the limited fecundity, nor the unrestrained attacks of birds of prey and of man are here sufficient to check it. in no other birds are these peculiar circumstances so strikingly combined. either their food is more liable to failure, or they have not sufficient power of wing to search for it over an extensive area, or during some season of the year it becomes very scarce, and less wholesome substitutes have to be found; and thus, though more fertile in offspring, they can never increase beyond the supply of food in the least favourable seasons. many birds can only exist by migrating, when their food becomes scarce, to regions possessing a milder, or at least a different climate, though, as these migrating birds are seldom excessively abundant, it is evident that the countries they visit are still deficient in a constant and abundant supply of wholesome food. those whose organization does not permit them to migrate when their food becomes periodically scarce, can never attain a large population. this is probably the reasons why woodpeckers are scarce with us, while in the tropics they are among the most abundant of solitary birds. thus the house sparrow is more abundant than the redbreast, because its food is more constant and plentiful,--seeds of grasses being preserved during the winter, and our farm-yards and stubble-fields furnishing an almost inexhaustible supply. why, as a general rule, are aquatic, and especially sea birds, very numerous in individuals? not because they are more prolific than others, generally the contrary; but because their food never fails, the sea-shores and river-banks daily swarming with a fresh supply of small mollusca and crustacea. exactly the same laws will apply to mammals. wild cats are prolific and have few enemies; why then are they never as abundant as rabbits? the only intelligible answer is, that their supply of food is more precarious. it appears evident, therefore, that so long as a country remains physically unchanged, the numbers of its animal population cannot materially increase. if one species does so, some others requiring the same kind of food must diminish in proportion. the numbers that die annually must be immense; and as the individual existence of each animal depends upon itself, those that die must be the weakest--the very young, the aged, and the diseased--while those that prolong their existence can only be the most perfect in health and vigour--those who are best able to obtain food regularly, and avoid their numerous enemies. it is, as we commenced by remarking, "a struggle for existence," in which the weakest and least perfectly organized must always succumb. _the abundance or rarity of a species dependent upon its more or less perfect adaptation to the conditions of existence._ it seems evident that what takes place among the individuals of a species must also occur among the several allied species of a group,--viz., that those which are best adapted to obtain a regular supply of food, and to defend themselves against the attacks of their enemies and the vicissitudes of the seasons, must necessarily obtain and preserve a superiority in population; while those species which from some defect of power or organization are the least capable of counteracting the vicissitudes of food-supply, &c., must diminish in numbers, and, in extreme cases, become altogether extinct. between these extremes the species will present various degrees of capacity for ensuring the means of preserving life; and it is thus we account for the abundance or rarity of species. our ignorance will generally prevent us from accurately tracing the effects to their causes; but could we become perfectly acquainted with the organization and habits of the various species of animals, and could we measure the capacity of each for performing the different acts necessary to its safety and existence under all the varying circumstances by which it is surrounded, we might be able even to calculate the proportionate abundance of individuals which is the necessary result. if now we have succeeded in establishing these two points-- st, _that the animal population of a country is generally stationary, being kept down by a periodical deficiency of food, and other checks_; and, nd, _that the comparative abundance or scarcity of the individuals of the several species is entirely due to their organization and resulting habits, which, rendering it more difficult to procure a regular supply of food and to provide for their personal safety in some cases than in others, can only be balanced by a difference in the population which have to exist in a given area_--we shall be in a condition to proceed to the consideration of _varieties_, to which the preceding remarks have a direct and very important application. _useful variations will tend to increase; useless or hurtful variations to diminish._ most or perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individuals. even a change of colour might, by rendering them more or less distinguishable, affect their safety; a greater or less development of hair might modify their habits. more important changes, such as an increase in the power or dimensions of the limbs or any of the external organs, would more or less affect their mode of procuring food or the range of country which they could inhabit. it is also evident that most changes would affect, either favourably or adversely, the powers of prolonging existence. an antelope with shorter or weaker legs must necessarily suffer more from the attacks of the feline carnivora; the passenger pigeon with less powerful wings would sooner or later be affected in its powers of procuring a regular supply of food; and in both cases the result must necessarily be a diminution of the population of the modified species. if, on the other hand, any species should produce a variety having slightly increased powers of preserving existence, that variety must inevitably in time acquire a superiority in numbers. these results must follow as surely as old age, intemperance, or scarcity of food produce an increased mortality. in both cases there may be many individual exceptions; but on the average the rule will invariably be found to hold good. all varieties will therefore fall into two classes--those which under the same conditions would never reach the population of the parent species, and those which would in time obtain and keep a numerical superiority. now, let some alteration of physical conditions occur in the district--a long period of drought, a destruction of vegetation by locusts, the irruption of some new carnivorous animal seeking "pastures new"--any change in fact tending to render existence more difficult to the species in question, and tasking its utmost powers to avoid complete extermination; it is evident that, of all the individuals composing the species, those forming the least numerous and most feebly organized variety would suffer first, and, were the pressure severe, must soon become extinct. the same causes continuing in action, the parent species would next suffer, would gradually diminish in numbers, and with a recurrence of similar unfavourable conditions might also become extinct. tho superior variety would then alone remain, and on a return to favourable circumstances would rapidly increase in numbers and occupy the place of the extinct species and variety. _superior varieties will ultimately extirpate the original species._ the _variety_ would now have replaced the _species_, of which it would be a more perfectly developed and more highly organized form. it would be in all respects better adapted to secure its safety, and to prolong its individual existence and that of the race. such a variety _could not_ return to the original form; for that form is an inferior one, and could never compete with it for existence. granted, therefore, a "tendency" to reproduce the original type of the species, still the variety must ever remain preponderant in numbers, and under adverse physical conditions _again alone survive_. but this new, improved, and populous race might itself, in course of time, give rise to new varieties, exhibiting several diverging modifications of form, any of which, tending to increase the facilities for preserving existence, must, by the same general law, in their turn become predominant. here, then, we have _progression and continued divergence_ deduced from the general laws which regulate the existence of animals in a state of nature, and from the undisputed fact that varieties do frequently occur. it is not, however, contended that this result would be invariable; a change of physical conditions in the district might at times materially modify it, rendering the race which had been the most capable of supporting existence under the former conditions now the least so, and even causing the extinction of the newer and, for a time, superior race, while the old or parent species and its first inferior varieties continued to flourish. variations in unimportant parts might also occur, having no perceptible effect on the life-preserving powers; and the varieties so furnished might run a course parallel with the parent species, either giving rise to further variations or returning to the former type. all we argue for is, that certain varieties have a tendency to maintain their existence longer than the original species, and this tendency must make itself felt; for though the doctrine of chances or averages can never be trusted to on a limited scale, yet, if applied to high numbers, the results come nearer to what theory demands, and, as we approach to an infinity of examples, become strictly accurate. now the scale on which nature works is so vast--the numbers of individuals and the periods of time with which she deals approach so near to infinity, than any cause, however slight, and however liable to be veiled and counteracted by accidental circumstances, must in the end produce its full legitimate results. _the partial reversion of domesticated varieties explained._ let us now turn to domesticated animals, and inquire how varieties produced among them are affected by the principles here enunciated. the essential difference in the condition of wild and domestic animals is this,--that among the former, their well-being and very existence depend upon the full exercise and healthy condition of all their senses and physical powers, whereas, among the latter, these are only partially exercised, and in some cases are absolutely unused. a wild animal has to search, and often to labour, for every mouthful of food--to exercise sight, hearing, and smell in seeking it, and in avoiding dangers, in procuring shelter from the inclemency of the seasons, and in providing for the subsistence and safety of its offspring. there is no muscle of its body that is not called into daily and hourly activity; there is no sense or faculty that is not strengthened by continual exercise. the domestic animal, on the other hand, has food provided for it, is sheltered, and often confined, to guard it against the vicissitudes of the seasons, is carefully secured from the attacks of its natural enemies, and seldom even rears its young without human assistance. half of its senses and faculties become quite useless, and the other half are but occasionally called into feeble exercise, while even its muscular system is only irregularly brought into action. now when a variety of such an animal occurs, having increased power or capacity in any organ or sense, such increase is totally useless, is never called into action, and may even exist without the animal ever becoming aware of it. in the wild animal, on the contrary, all its faculties and powers being brought into full action for the necessities of existence, any increase becomes immediately available, is strengthened by exercise, and must even slightly modify the food, the habits, and the whole economy of the race. it creates as it were a new animal, one of superior powers, and which will necessarily increase in numbers and outlive those which are inferior to it. again, in the domesticated animal all variations have an equal chance of continuance; and those which would decidedly render a wild animal unable to compete with its fellows and continue its existence are no disadvantage whatever in a state of domesticity. our quickly fattening pigs, short-legged sheep pouter pigeons, and poodle dogs could never have come into existence in a state of nature, because the very first step towards such inferior forms would have led to the rapid extinction of the race; still less could they now exist in competition with their wild allies. the great speed but slight endurance of the race horse, the unwieldy strength of the ploughman's team, would both be useless in a state of nature. if turned wild on the pampas, such animals would probably soon become extinct, or under favourable circumstances might each gradually lose those extreme qualities which would never be called into action, and in a few generations revert to a common type, which must be that in which the various powers and faculties are so proportioned to each other as to be best adapted to procure food and secure safety,--that in which by the full exercise of every part of its organisation the animal can alone continue to live. domestic varieties, when turned wild, _must_ return to something near the type of the original wild stock, _or become altogether extinct_.[f] +--------------------------------------------------------------+ | [f] that is, they will vary, and the variations which tend | | to adapt them to the wild state, and therefore approximate | | them to wild animals, will be preserved. those individuals | | which do not vary sufficiently will perish. | +--------------------------------------------------------------+ we see, then, that no inferences as to the permanence of varieties in a state of nature can be deduced from the observations of those occurring among domestic animals. the two are so much opposed to each other in every circumstance of their existence, that what applies to the one is almost sure not to apply to the other. domestic animals are abnormal, irregular, artificial; they are subject to variations which never occur and never can occur in a state of nature: their very existence depends altogether on human care; so far are many of them removed from that just proportion of faculties, that true balance of organisation, by means of which alone an animal left to its own resources can preserve its existence and continue its race. _lamarck's hypothesis very different from that now advanced._ the hypothesis of lamarck--that progressive changes in species have been produced by the attempts of animals to increase the development of their own organs, and thus modify their structure and habits--has been repeatedly and easily refuted by all writers on the subject of varieties and species, and it seems to have been considered that when this was done the whole question has been finally settled; but the view here developed renders such hypothesis quite unnecessary, by showing that similar results must be produced by the action of principles constantly at work in nature. the powerful retractile talons of the falcon-and the cat-tribes have not been produced or increased by the volition of those animals; but among the different varieties which occurred in the earlier and less highly organized forms of these groups, _those always survived longest which had the greatest facilities for seizing their prey_. neither did the giraffe acquire its long neck by desiring to reach the foliage of the more lofty shrubs, and constantly stretching its neck for the purpose, but because any varieties which occurred among its antitypes with a longer neck than usual _at once secured a fresh range of pasture over the same ground as their shorter-necked companions, and on the first scarcity of food were thereby enabled to outlive them_. even the peculiar colours of many animals, more especially of insects, so closely resembling the soil or leaves or bark on which they habitually reside, are explained on the same principle; for though in the course of ages varieties of many tints may have occurred, _yet those races having colours best adapted to concealment from their enemies would inevitably survive the longest_. we have also here an acting cause to account for that balance so often observed in nature,--a deficiency in one set of organs always being compensated by an increased development of some others--powerful wings accompanying weak feet, or great velocity making up for the absence of defensive weapons; for it has been shown that all varieties in which an unbalanced deficiency occurred could not long continue their existence. the action of this principle is exactly like that of the centrifugal governor of the steam engine, which checks and corrects any irregularities almost before they become evident; and in like manner no unbalanced deficiency in the animal kingdom can ever reach any conspicuous magnitude, because it would make itself felt at the very first step, by rendering existence difficult and extinction almost sure soon to follow. an origin such as is here advocated will also agree with the peculiar character of the modifications of form and structure which obtain in organized beings--the many lines of divergence from a central type, the increasing efficiency and power of a particular organ through a succession of allied species, and the remarkable persistence of unimportant parts, such as colour, texture of plumage and hair, form of horns or crests, through a series of species differing considerably in more essential characters. it also furnishes us with a reason for that "more specialized structure" which professor owen states to be a characteristic of recent compared with extinct forms, and which would evidently be the result of the progressive modification of any organ applied to a special purpose in the animal economy. _conclusion._ we believe we have now shown that there is a tendency in nature to the continued progression of certain classes of _varieties_ further and further from the original type--a progression to which there appears no reason to assign any definite limits--and that the same principle which produces this result in a state of nature will also explain why domestic varieties have a tendency, when they become wild, to revert to the original type. this progression, by minute steps, in various directions, but always checked and balanced by the necessary conditions, subject to which alone existence can be preserved, may, it is believed, be followed out so as to agree with all the phænomena presented by organized beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct and habits which they exhibit. iii. mimicry, and other protective resemblances among animals. there is no more convincing proof of the truth of a comprehensive theory, than its power of absorbing and finding a place for new facts, and its capability of interpreting phænomena which had been previously looked upon as unaccountable anomalies. it is thus that the law of universal gravitation and the undulatory theory of light have become established and universally accepted by men of science. fact after fact has been brought forward as being apparently inconsistent with them, and one after another these very facts have been shown to be the consequences of the laws they were at first supposed to disprove. a false theory will never stand this test. advancing knowledge brings to light whole groups of facts which it cannot deal with, and its advocates steadily decrease in numbers, notwithstanding the ability and scientific skill with which it may have been supported. the great name of edward forbes did not prevent his theory of "polarity in the distribution of organic beings in time" from dying a natural death; but the most striking illustration of the behaviour of a false theory is to be found in the "circular and quinarian system" of classification propounded by macleay, and developed by swainson, with an amount of knowledge and ingenuity that have rarely been surpassed. this theory was eminently attractive, both from its symmetry and completeness, and from the interesting nature of the varied analogies and affinities which it brought to light and made use of. the series of natural history volumes in "lardner's cabinet cyclopædia," in which mr. swainson developed it in most departments of the animal kingdom, made it widely known; and in fact for a long time these were the best and almost the only popular text-books for the rising generation of naturalists. it was favourably received too by the older school, which was perhaps rather an indication of its unsoundness. a considerable number of well-known naturalists either spoke approvingly of it, or advocated similar principles, and for a good many years it was decidedly in the ascendent. with such a favourable introduction, and with such talented exponents, it must have become established if it had had any germ of truth in it; yet it quite died out in a few short years, its very existence is now a matter of history; and so rapid was its fall that its talented creator, swainson, perhaps lived to be the last man who believed in it. such is the course of a false theory. that of a true one is very different, as may be well seen by the progress of opinion on the subject of natural selection. in less than eight years "the origin of species" has produced conviction in the minds of a majority of the most eminent living men of science. new facts, new problems, new difficulties as they arise are accepted, solved or removed by this theory; and its principles are illustrated by the progress and conclusions of every well established branch of human knowledge. it is the object of the present essay to show how it has recently been applied to connect together and explain a variety of curious facts which had long been considered as inexplicable anomalies. _importance of the principle of utility._ perhaps no principle has ever been announced so fertile in results as that which mr. darwin so earnestly impresses upon us, and which is indeed a necessary deduction from the theory of natural selection, namely--that none of the definite facts of organic nature, no special organ, no characteristic form or marking, no peculiarities of instinct or of habit, no relations between species or between groups of species--can exist, but which must now be or once have been _useful_ to the individuals or the races which possess them. this great principle gives us a clue which we can follow out in the study of many recondite phænomena, and leads us to seek a meaning and a purpose of some definite character in minutiæ which we should be otherwise almost sure to pass over as insignificant or unimportant. _popular theories of colour in animals._ the adaptation of the external colouring of animals to their conditions of life has long been recognised, and has been imputed either to an originally created specific peculiarity, or to the direct action of climate, soil, or food. where the former explanation has been accepted, it has completely checked inquiry, since we could never get any further than the fact of the adaptation. there was nothing more to be known about the matter. the second explanation was soon found to be quite inadequate to deal with all the varied phases of the phænomena, and to be contradicted by many well-known facts. for example, wild rabbits are always of grey or brown tints well suited for concealment among grass and fern. but when these rabbits are domesticated, without any change of climate or food, they vary into white or black, and these varieties may be multiplied to any extent, forming white or black races. exactly the same thing has occurred with pigeons; and in the case of rats and mice, the white variety has not been shown to be at all dependent on alteration of climate, food, or other external conditions. in many cases the wings of an insect not only assume the exact tint of the bark or leaf it is accustomed to rest on, but the form and veining of the leaf or the exact rugosity of the bark is imitated; and these detailed modifications cannot be reasonably imputed to climate or to food, since in many cases the species does not feed on the substance it resembles, and when it does, no reasonable connexion can be shown to exist between the supposed cause and the effect produced. it was reserved for the theory of natural selection to solve all these problems, and many others which were not at first supposed to be directly connected with them. to make these latter intelligible, it will be necessary to give a sketch of the whole series of phænomena which may be classed under the head of useful or protective resemblances. _importance of concealment as influencing colour._ concealment, more or less complete, is useful to many animals, and absolutely essential to some. those which have numerous enemies from which they cannot escape by rapidity of motion, find safety in concealment. those which prey upon others must also be so constituted as not to alarm them by their presence or their approach, or they would soon die of hunger. now it is remarkable in how many cases nature gives this boon to the animal, by colouring it with such tints as may best serve to enable it to escape from its enemies or to entrap its prey. desert animals as a rule are desert-coloured. the lion is a typical example of this, and must be almost invisible when crouched upon the sand or among desert rocks and stones. antelopes are all more or less sandy-coloured. the camel is pre-eminently so. the egyptian cat and the pampas cat are sandy or earth-coloured. the australian kangaroos are of the same tints, and the original colour of the wild horse is supposed to have been a sandy or clay-colour. the desert birds are still more remarkably protected by their assimilative hues. the stonechats, the larks, the quails, the goatsuckers and the grouse, which abound in the north african and asiatic deserts, are all tinted and mottled so as to resemble with wonderful accuracy the average colour and aspect of the soil in the district they inhabit. the rev. h. tristram, in his account of the ornithology of north africa in the st volume of the "ibis," says: "in the desert, where neither trees, brush-wood, nor even undulation of the surface afford the slightest protection to its foes, a modification of colour which shall be assimilated to that of the surrounding country, is absolutely necessary. hence _without exception_ the upper plumage of _every bird_, whether lark, chat, sylvain, or sand-grouse, and also the fur of _all the smaller mammals_, and the skin of _all the snakes and lizards_, is of one uniform isabelline or sand colour." after the testimony of so able an observer it is unnecessary to adduce further examples of the protective colours of desert animals. almost equally striking are the cases of arctic animals possessing the white colour that best conceals them upon snowfields and icebergs. the polar bear is the only bear that is white, and it lives constantly among snow and ice. the arctic fox, the ermine and the alpine hare change to white in winter only, because in summer white would be more conspicuous than any other colour, and therefore a danger rather than a protection; but the american polar hare, inhabiting regions of almost perpetual snow, is white all the year round. other animals inhabiting the same northern regions do not, however, change colour. the sable is a good example, for throughout the severity of a siberian winter it retains its rich brown fur. but its habits are such that it does not need the protection of colour, for it is said to be able to subsist on fruits and berries in winter, and to be so active upon the trees as to catch small birds among the branches. so also the woodchuck of canada has a dark-brown fur; but then it lives in burrows and frequents river banks, catching fish and small animals that live in or near the water. among birds, the ptarmigan is a fine example of protective colouring. its summer plumage so exactly harmonizes with the lichen-coloured stones among which it delights to sit, that a person may walk through a flock of them without seeing a single bird; while in winter its white plumage is an almost equal protection. the snow-bunting, the jer-falcon, and the snowy owl are also white-coloured birds inhabiting the arctic regions, and there can be little doubt but that their colouring is to some extent protective. nocturnal animals supply us with equally good illustrations. mice, rats, bats, and moles possess the least conspicuous of hues, and must be quite invisible at times when any light colour would be instantly seen. owls and goatsuckers are of those dark mottled tints that will assimilate with bark and lichen, and thus protect them during the day, and at the same time be inconspicuous in the dusk. it is only in the tropics, among forests which never lose their foliage, that we find whole groups of birds whose chief colour is green. the parrots are the most striking example, but we have also a group of green pigeons in the east; and the barbets, leaf-thrushes, bee-eaters, white-eyes, turacos, and several smaller groups, have so much green in their plumage as to tend greatly to conceal them among the foliage. _special modifications of colour._ the conformity of tint which has been so far shown to exist between animals and their habitations is of a somewhat general character; we will now consider the cases of more special adaptation. if the lion is enabled by his sandy colour readily to conceal himself by merely crouching down upon the desert, how, it may be asked, do the elegant markings of the tiger, the jaguar, and the other large cats agree with this theory? we reply that these are generally cases of more or less special adaptation. the tiger is a jungle animal, and hides himself among tufts of grass or of bamboos, and in these positions the vertical stripes with which his body is adorned must so assimilate with the vertical stems of the bamboo, as to assist greatly in concealing him from his approaching prey. how remarkable it is that besides the lion and tiger, almost all the other large cats are arboreal in their habits, and almost all have ocellated or spotted skins, which must certainly tend to blend them with the background of foliage; while the one exception, the puma, has an ashy brown uniform fur, and has the habit of clinging so closely to a limb of a tree while waiting for his prey to pass beneath as to be hardly distinguishable from the bark. among birds, the ptarmigan, already mentioned, must be considered a remarkable case of special adaptation. another is a south-american goatsucker (caprimulgus rupestris) which rests in the bright sunshine on little bare rocky islets in the upper rio negro, where its unusually light colours so closely resemble those of the rock and sand, that it can scarcely be detected till trodden upon. the duke of argyll, in his "reign of law," has pointed out the admirable adaptation of the colours of the woodcock to its protection. the various browns and yellows and pale ash-colour that occur in fallen leaves are all reproduced in its plumage, so that when according to its habit it rests upon the ground under trees, it is almost impossible to detect it. in snipes the colours are modified so as to be equally in harmony with the prevalent forms and colours of marshy vegetation. mr. j. m. lester, in a paper read before the rugby school natural history society, observes:--"the wood-dove, when perched amongst the branches of its favourite _fir_, is scarcely discernible; whereas, were it among some lighter foliage, the blue and purple tints in its plumage would far sooner betray it. the robin redbreast too, although it might be thought that the red on its breast made it much easier to be seen, is in reality not at all endangered by it, since it generally contrives to get among some russet or yellow fading leaves, where the red matches very well with the autumn tints, and the brown of the rest of the body with the bare branches." reptiles offer us many similar examples. the most arboreal lizards, the iguanas, are as green as the leaves they feed upon, and the slender whip-snakes are rendered almost invisible as they glide among the foliage by a similar colouration. how difficult it is sometimes to catch sight of the little green tree-frogs sitting on the leaves of a small plant enclosed in a glass case in the zoological gardens; yet how much better concealed must they be among the fresh green damp foliage of a marshy forest. there is a north-american frog found on lichen-covered rocks and walls, which is so coloured as exactly to resemble them, and as long as it remains quiet would certainly escape detection. some of the geckos which cling motionless on the trunks of trees in the tropics, are of such curiously marbled colours as to match exactly with the bark they rest upon. in every part of the tropics there are tree-snakes that twist among boughs and shrubs, or lie coiled up on the dense masses of foliage. these are of many distinct groups, and comprise both venomous and harmless genera; but almost all of them are of a beautiful green colour, sometimes more or less adorned with white or dusky bands and spots. there can be little doubt that this colour is doubly useful to them, since it will tend to conceal them from their enemies, and will lead their prey to approach them unconscious of danger. dr. gunther informs me that there is only one genus of true arboreal snakes (dipsas) whose colours are rarely green, but are of various shades of black, brown, and olive, and these are all nocturnal reptiles, and there can be little doubt conceal themselves during the day in holes, so that the green protective tint would be useless to them, and they accordingly retain the more usual reptilian hues. fishes present similar instances. many flat fish, as for example the flounder and the skate, are exactly the colour of the gravel or sand on which they habitually rest. among the marine flower gardens of an eastern coral reef the fishes present every variety of gorgeous colour, while the river fish even of the tropics rarely if ever have gay or conspicuous markings. a very curious case of this kind of adaptation occurs in the sea-horses (hippocampus) of australia, some of which bear long foliaceous appendages resembling seaweed, and are of a brilliant red colour; and they are known to live among seaweed of the same hue, so that when at rest they must be quite invisible. there are now in the aquarium of the zoological society some slender green pipe-fish which fasten themselves to any object at the bottom by their prehensile tails, and float about with the current, looking exactly like some simple cylindrical algæ. it is, however, in the insect world that this principle of the adaptation of animals to their environment is most fully and strikingly developed. in order to understand how general this is, it is necessary to enter somewhat into details, as we shall thereby be better able to appreciate the significance of the still more remarkable phenomena we shall presently have to discuss. it seems to be in proportion to their sluggish motions or the absence of other means of defence, that insects possess the protective colouring. in the tropics there are thousands of species of insects which rest during the day clinging to the bark of dead or fallen trees; and the greater portion of these are delicately mottled with gray and brown tints, which though symmetrically disposed and infinitely varied, yet blend so completely with the usual colours of the bark, that at two or three feet distance they are quite undistinguishable. in some cases a species is known to frequent only one species of tree. this is the case with the common south american long-horned beetle (onychocerus scorpio) which, mr. bates informed me, is found only on a rough-barked tree, called tapiribá, on the amazon. it is very abundant, but so exactly does it resemble the bark in colour and rugosity, and so closely does it cling to the branches, that until it moves it is absolutely invisible! an allied species (o. concentricus) is found only at pará, on a distinct species of tree, the bark of which it resembles with equal accuracy. both these insects are abundant, and we may fairly conclude that the protection they derive from this strange concealment is at least one of the causes that enable the race to flourish. many of the species of cicindela, or tiger beetle, will illustrate this mode of protection. our common cicindela campestris frequents grassy banks, and is of a beautiful green colour, while c. maritima, which is found only on sandy sea-shores, is of a pale bronzy yellow, so as to be almost invisible. a great number of the species found by myself in the malay islands are similarly protected. the beautiful cicindela gloriosa, of a very deep velvety green colour, was only taken upon wet mossy stones in the bed of a mountain stream, where it was with the greatest difficulty detected. a large brown species (c. heros) was found chiefly on dead leaves in forest paths; and one which was never seen except on the wet mud of salt marshes was of a glossy olive so exactly the colour of the mud as only to be distinguished when the sun shone, by its shadow! where the sandy beach was coralline and nearly white, i found a very pale cicindela; wherever it was volcanic and black, a dark species of the same genus was sure to be met with. there are in the east small beetles of the family buprestidæ which generally rest on the midrib of a leaf, and the naturalist often hesitates before picking them off, so closely do they resemble pieces of bird's dung. kirby and spence mention the small beetle onthophilus sulcatus as being like the seed of an umbelliferous plant; and another small weevil, which is much persecuted by predatory beetles of the genus harpalus, is of the exact colour of loamy soil, and was found to be particularly abundant in loam pits. mr. bates mentions a small beetle (chlamys pilula) which was undistinguishable by the eye from the dung of caterpillars, while some of the cassidæ, from their hemispherical forms and pearly gold colour, resemble glittering dew-drops upon the leaves. a number of our small brown and speckled weevils at the approach of any object roll off the leaf they are sitting on, at the same time drawing in their legs and antennæ, which fit so perfectly into cavities for their reception that the insect becomes a mere oval brownish lump, which it is hopeless to look for among the similarly coloured little stones and earth pellets among which it lies motionless. the distribution of colour in butterflies and moths respectively is very instructive from this point of view. the former have all their brilliant colouring on the upper surface of all four wings, while the under surface is almost always soberly coloured, and often very dark and obscure. the moths on the contrary have generally their chief colour on the hind wings only, the upper wings being of dull, sombre, and often imitative tints, and these generally conceal the hind wings when the insects are in repose. this arrangement of the colours is therefore eminently protective, because the butterfly always rests with his wings raised so as to conceal the dangerous brilliancy of his upper surface. it is probable that if we watched their habits sufficiently we should find the under surface of the wings of butterflies very frequently imitative and protective. mr. t. w. wood has pointed out that the little orange-tip butterfly often rests in the evening on the green and white flower heads of an umbelliferous plant, and that when observed in this position the beautiful green and white mottling of the under surface completely assimilates with the flower heads and renders the creature very difficult to be seen. it is probable that the rich dark colouring of the under side of our peacock, tortoiseshell, and red-admiral butterflies answers a similar purpose. two curious south american butterflies that always settle on the trunks of trees (gynecia dirce and callizona acesta) have the under surface curiously striped and mottled, and when viewed obliquely must closely assimilate with the appearance of the furrowed bark of many kinds of trees. but the most wonderful and undoubted case of protective resemblance in a butterfly which i have ever seen, is that of the common indian kallima inachis, and its malayan ally, kallima paralekta. the upper surface of these insects is very striking and showy, as they are of a large size, and are adorned with a broad band of rich orange on a deep bluish ground. the under side is very variable in colour, so that out of fifty specimens no two can be found exactly alike, but every one of them will be of some shade of ash or brown or ochre, such as are found among dead, dry, or decaying leaves. the apex of the upper wings is produced into an acute point, a very common form in the leaves of tropical shrubs and trees, and the lower wings are also produced into a short narrow tail. between these two points runs a dark curved line exactly representing the midrib of a leaf, and from this radiate on each side a few oblique lines, which serve to indicate the lateral veins of a leaf. these marks are more clearly seen on the outer portion of the base of the wings, and on the inner side towards the middle and apex, and it is very curious to observe how the usual marginal and transverse striæ of the group are here modified and strengthened so as to become adapted for an imitation of the venation of a leaf. we come now to a still more extraordinary part of the imitation, for we find representations of leaves in every stage of decay, variously blotched and mildewed and pierced with holes, and in many cases irregularly covered with powdery black dots gathered into patches and spots, so closely resembling the various kinds of minute fungi that grow on dead leaves that it is impossible to avoid thinking at first sight that the butterflies themselves have been attacked by real fungi. but this resemblance, close as it is, would be of little use if the habits of the insect did not accord with it. if the butterfly sat upon leaves or upon flowers, or opened its wings so as to expose the upper surface, or exposed and moved its head and antennæ as many other butterflies do, its disguise would be of little avail. we might be sure, however, from the analogy of many other cases, that the habits of the insect are such as still further to aid its deceptive garb; but we are not obliged to make any such supposition, since i myself had the good fortune to observe scores of kallima paralekta, in sumatra, and to capture many of them, and can vouch for the accuracy of the following details. these butterflies frequent dry forests and fly very swiftly. they were never seen to settle on a flower or a green leaf, but were many times lost sight of in a bush or tree of dead leaves. on such occasions they were generally searched for in vain, for while gazing intently at the very spot where one had disappeared, it would often suddenly dart out, and again vanish twenty or fifty yards further on. on one or two occasions the insect was detected reposing, and it could then be seen how completely it assimilates itself to the surrounding leaves. it sits on a nearly upright twig, the wings fitting closely back to back, concealing the antennæ and head, which are drawn up between their bases. the little tails of the hind wing touch the branch, and form a perfect stalk to the leaf, which is supported in its place by the claws of the middle pair of feet, which are slender and inconspicuous. the irregular outline of the wings gives exactly the perspective effect of a shrivelled leaf. we thus have size, colour, form, markings, and habits, all combining together to produce a disguise which may be said to be absolutely perfect; and the protection which it affords is sufficiently indicated by the abundance of the individuals that possess it. the rev. joseph greene has called attention to the striking harmony between the colours of those british moths which are on the wing in autumn and winter, and the prevailing tints of nature at those seasons. in autumn various shades of yellow and brown prevail, and he shows that out of fifty-two species that fly at this season, no less than forty-two are of corresponding colours. orgyia antiqua, o. gonostigma, the genera xanthia, glæa, and ennomos are examples. in winter, gray and silvery tints prevail, and the genus chematobia and several species of hybernia which fly during this season are of corresponding hues. no doubt if the habits of moths in a state of nature were more closely observed, we should find many cases of special protective resemblance. a few such have already been noticed. agriopis aprilina, acronycta psi, and many other moths which rest during the day on the north side of the trunks of trees can with difficulty be distinguished from the grey and green lichens that cover them. the lappet moth (gastropacha querci) closely resembles both in shape and colour a brown dry leaf; and the well-known buff-tip moth, when at rest is like the broken end of a lichen-covered branch. there are some of the small moths which exactly resemble the dung of birds dropped on leaves, and on this point mr. a. sidgwick, in a paper read before the rugby school natural history society, gives the following original observation:--"i myself have more than once mistaken cilix compressa, a little white and grey moth, for a piece of bird's dung dropped upon a leaf, and _vice versâ_ the dung for the moth. bryophila glandifera and perla are the very image of the mortar walls on which they rest; and only this summer, in switzerland, i amused myself for some time in watching a moth, probably larentia tripunctaria, fluttering about quite close to me, and then alighting on a wall of the stone of the district which it so exactly matched as to be quite invisible a couple of yards off." there are probably hosts of these resemblances which have not been observed, owing to the difficulty of finding many of the species in their stations of natural repose. caterpillars are also similarly protected. many exactly resemble in tint the leaves they feed upon; others are like little brown twigs, and many are so strangely marked or humped, that when motionless they can hardly be taken to be living creatures at all. mr. andrew murray has remarked how closely the larva of the peacock moth (saturnia pavonia-minor) harmonizes in its ground colour with that of the young buds of heather on which it feeds, and that the pink spots with which it is decorated correspond with the flowers and flower-buds of the same plant. the whole order of orthoptera, grasshoppers, locusts, crickets, &c., are protected by their colours harmonizing with that of the vegetation or the soil on which they live, and in no other group have we such striking examples of special resemblance. most of the tropical mantidæ and locustidæ are of the exact tint of the leaves on which they habitually repose, and many of them in addition have the veinings of their wings modified so as exactly to imitate that of a leaf. this is carried to the furthest possible extent in the wonderful genus, phyllium, the "walking leaf," in which not only are the wings perfect imitations of leaves in every detail, but the thorax and legs are flat, dilated, and leaf-like; so that when tho living insect is resting among the foliage on which it feeds, the closest observation is often unable to distinguish between the animal and the vegetable. the whole family of the phasmidæ, or spectres, to which this insect belongs, is more or less imitative, and a great number of the species are called "walking-stick insects," from their singular resemblance to twigs and branches. some of these are a foot long and as thick as one's finger, and their whole colouring, form, rugosity, and the arrangement of the head, legs, and antennæ, are such as to render them absolutely identical in appearance with dead sticks. they hang loosely about shrubs in the forest, and have the extraordinary habit of stretching out their legs unsymmetrically, so as to render the deception more complete. one of these creatures obtained by myself in borneo (ceroxylus laceratus) was covered over with foliaceous excrescences of a clear olive green colour, so as exactly to resemble a stick grown over by a creeping moss or jungermannia. the dyak who brought it me assured me it was grown over with moss although alive, and it was only after a most minute examination that i could convince myself it was not so. we need not adduce any more examples to show how important are the details of form and of colouring in animals, and that their very existence may often depend upon their being by these means concealed from their enemies. this kind of protection is found apparently in every class and order, for it has been noticed wherever we can obtain sufficient knowledge of the details of an animal's life-history. it varies in degree, from the mere absence of conspicuous colour or a general harmony with the prevailing tints of nature, up to such a minute and detailed resemblance to inorganic or vegetable structures as to realize the talisman of the fairy tale, and to give its possessor the power of rendering itself invisible. _theory of protective colouring._ we will now endeavour to show how these wonderful resemblances have most probably been brought about. returning to the higher animals, let us consider the remarkable fact of the rarity of white colouring in the mammalia or birds of the temperate or tropical zones in a state of nature. there is not a single white land-bird or quadruped in europe, except the few arctic or alpine species, to which white is a protective colour. yet in many of these creatures there seems to be no inherent tendency to avoid white, for directly they are domesticated white varieties arise, and appear to thrive as well as others. we have white mice and rats, white cats, horses, dogs, and cattle, white poultry, pigeons, turkeys, and ducks, and white rabbits. some of these animals have been domesticated for a long period, others only for a few centuries; but in almost every case in which an animal has been thoroughly domesticated, parti-coloured and white varieties are produced and become permanent. it is also well known that animals in a state of nature produce white varieties occasionally. blackbirds, starlings, and crows are occasionally seen white, as well as elephants, deer, tigers, hares, moles, and many other animals; but in no case is a permanent white race produced. now there are no statistics to show that the normal-coloured parents produce white offspring oftener under domestication than in a state of nature, and we have no right to make such an assumption if the facts can be accounted for without it. but if the colours of animals do really, in the various instances already adduced, serve for their concealment and preservation, then white or any other conspicuous colour must be hurtful, and must in most cases shorten an animal's life. a white rabbit would be more surely the prey of hawk or buzzard, and the white mole, or field mouse, could not long escape from the vigilant owl. so, also, any deviation from those tints best adapted to conceal a carnivorous animal would render the pursuit of its prey much more difficult, would place it at a disadvantage among its fellows, and in a time of scarcity would probably cause it to starve to death. on the other hand, if an animal spreads from a temperate into an arctic district, the conditions are changed. during a large portion of the year, and just when the struggle for existence is most severe, white is the prevailing tint of nature, and dark colours will be the most conspicuous. the white varieties will now have an advantage; they will escape from their enemies or will secure food, while their brown companions will be devoured or will starve; and as "like produces like" is the established rule in nature, the white race will become permanently established, and dark varieties, when they occasionally appear, will soon die out from their want of adaptation to their environment. in each case the fittest will survive, and a race will be eventually produced adapted to the conditions in which it lives. we have here an illustration of the simple and effectual means by which animals are brought into harmony with the rest of nature. that slight amount of variability in every species, which we often look upon as something accidental or abnormal, or so insignificant as to be hardly worthy of notice, is yet the foundation of all those wonderful and harmonious resemblances which play such an important part in the economy of nature. variation is generally very small in amount, but it is all that is required, because the change in the external conditions to which an animal is subject is generally very slow and intermittent. when these changes have taken place too rapidly, the result has often been the extinction of species; but the general rule is, that climatal and geological changes go on slowly, and the slight but continual variations in the colour, form, and structure of all animals, has furnished individuals adapted to these changes, and who have become the progenitors of modified races. rapid multiplication, incessant slight variation, and survival of the fittest--these are the laws which ever keep the organic world in harmony with the inorganic, and with itself. these are the laws which we believe have produced all the cases of protective resemblance already adduced, as well as those still more curious examples we have yet to bring before our readers. it must always be borne in mind that the more wonderful examples, in which there is not only a general but a special resemblance--as in the walking leaf, the mossy phasma, and the leaf-winged butterfly--represent those few instances in which the process of modification has been going on during an immense series of generations. they all occur in the tropics, where the conditions of existence are the most favourable, and where climatic changes have for long periods been hardly perceptible. in most of them favourable variations both of colour, form, structure, and instinct or habit, must have occurred to produce the perfect adaptation we now behold. all these are known to vary, and favourable variations when not accompanied by others that were unfavourable, would certainly survive. at one time a little step might be made in this direction, at another time in that--a change of conditions might sometimes render useless that which it had taken ages to produce--great and sudden physical modifications might often produce the extinction of a race just as it was approaching perfection, and a hundred checks of which we can know nothing may have retarded the progress towards perfect adaptation; so that we can hardly wonder at there being so few cases in which a completely successful result has been attained as shown by the abundance and wide diffusion of the creatures so protected. _objection that colour, as being dangerous, should not exist in nature._ it is as well here to reply to an objection that will no doubt occur to many readers--that if protection is so useful to all animals, and so easily brought about by variation and survival of the fittest, there ought to be no conspicuously-coloured creatures; and they will perhaps ask how we account for the brilliant birds, and painted snakes, and gorgeous insects, that occur abundantly all over the world. it will be advisable to answer this question rather fully, in order that we may be prepared to understand the phenomena of "mimicry," which it is the special object of this paper to illustrate and explain. the slightest observation of the life of animals will show us, that they escape from their enemies and obtain their food in an infinite number of ways; and that their varied habits and instincts are in every case adapted to the conditions of their existence. the porcupine and the hedgehog have a defensive armour that saves them from the attacks of most animals. the tortoise is not injured by the conspicuous colours of his shell, because that shell is in most cases an effectual protection to him. the skunks of north america find safety in their power of emitting an unbearably offensive odour; the beaver in its aquatic habits and solidly constructed abode. in some cases the chief danger to an animal occurs at one particular period of its existence, and if that is guarded against its numbers can easily be maintained. this is the case with many birds, the eggs and young of which are especially obnoxious to danger, and we find accordingly a variety of curious contrivances to protect them. we have nests carefully concealed, hung from the slender extremities of grass or boughs over water, or placed in the hollow of a tree with a very small opening. when these precautions are successful, so many more individuals will be reared than can possibly find food during the least favourable seasons, that there will always be a number of weakly and inexperienced young birds who will fall a prey to the enemies of the race, and thus render necessary for the stronger and healthier individuals no other safeguard than their strength and activity. the instincts most favourable to the production and rearing of offspring will in these cases be most important, and the survival of the fittest will act so as to keep up and advance those instincts, while other causes which tend to modify colour and marking may continue their action almost unchecked. it is perhaps in insects that we may best study the varied means by which animals are defended or concealed. one of the uses of the phosphorescence with which many insects are furnished, is probably to frighten away their enemies; for kirby and spence state that a ground beetle (carabus) has been observed running round and round a luminous centipede as if afraid to attack it. an immense number of insects have stings, and some stingless ants of the genus polyrachis are armed with strong and sharp spines on the back, which must render them unpalatable to many of the smaller insectivorous birds. many beetles of the family curculionidæ have the wing cases and other external parts so excessively hard, that they cannot be pinned without first drilling a hole to receive the pin, and it is probable that all such find a protection in this excessive hardness. great numbers of insects hide themselves among the petals of flowers, or in the cracks of bark and timber; and finally, extensive groups and even whole orders have a more or less powerful and disgusting smell and taste, which they either possess permanently, or can emit at pleasure. the attitudes of some insects may also protect them, as the habit of turning up the tail by the harmless rove-beetles (staphylindidæ) no doubt leads other animals besides children to the belief that they can sting. the curious attitude assumed by sphinx caterpillars is probably a safeguard, as well as the blood-red tentacles which can suddenly be thrown out from the neck, by the caterpillars of all the true swallow-tailed butterflies. it is among the groups that possess some of these varied kinds of protection in a high degree, that we find the greatest amount of conspicuous colour, or at least the most complete absence of protective imitation. the stinging hymenoptera, wasps, bees, and hornets, are, as a rule, very showy and brilliant insects, and there is not a single instance recorded in which any one of them is coloured so as to resemble a vegetable or inanimate substance. the chrysididæ, or golden wasps, which do not sting, possess as a substitute the power of rolling themselves up into a ball, which is almost as hard and polished as if really made of metal,--and they are all adorned with the most gorgeous colours. the whole order hemiptera (comprising the bugs) emit a powerful odour, and they present a very large proportion of gay-coloured and conspicuous insects. the lady-birds (coccinellidæ) and their allies the eumorphidæ, are often brightly spotted, as if to attract attention; but they can both emit fluids of a very disagreeable nature, they are certainly rejected by some birds, and are probably never eaten by any. the great family of ground beetles (carabidæ) almost all possess a disagreeable and some a very pungent smell, and a few, called bombardier beetles, have the peculiar faculty of emitting a jet of very volatile liquid, which appears like a puff of smoke, and is accompanied by a distinct crepitating explosion. it is probably because these insects are mostly nocturnal and predacious that they do not present more vivid hues. they are chiefly remarkable for brilliant metallic tints or dull red patches when they are not wholly black, and are therefore very conspicuous by day, when insect-eaters are kept off by their bad odour and taste, but are sufficiently invisible at night when it is of importance that their prey should not become aware of their proximity. it seems probable that in some cases that which would appear at first to be a source of danger to its possessor may really be a means of protection. many showy and weak-flying butterflies have a very broad expanse of wing, as in the brilliant blue morphos of brazilian forests, and the large eastern papilios; yet these groups are tolerably plentiful. now, specimens of these butterflies are often captured with pierced and broken wings, as if they had been seized by birds from whom they had escaped; but if the wings had been much smaller in proportion to the body, it seems probable that the insect would be more frequently struck or pierced in a vital part, and thus the increased expanse of the wings may have been indirectly beneficial. in other cases the capacity of increase in a species is so great that however many of the perfect insect may be destroyed, there is always ample means for the continuance of the race. many of the flesh flies, gnats, ants, palm-tree weevils and locusts are in this category. the whole family of cetoniadæ or rose chafers, so full of gaily-coloured species, are probably saved from attack by a combination of characters. they fly very rapidly with a zigzag or waving course; they hide themselves the moment they alight, either in the corolla of flowers, or in rotten wood, or in cracks and hollows of trees, and they are generally encased in a very hard and polished coat of mail which may render them unsatisfactory food to such birds as would be able to capture them. the causes which lead to the development of colour have been here able to act unchecked, and we see the result in a large variety of the most gorgeously-coloured insects. here, then, with our very imperfect knowledge of the life-history of animals, we are able to see that there are widely varied modes by which they may obtain protection from their enemies or concealment from their prey. some of those seem to be so complete and effectual as to answer all the wants of the race, and lead to the maintenance of the largest possible population. when this is the case, we can well understand that no further protection derived from a modification of colour can be of the slightest use, and the most brilliant hues may be developed without any prejudicial effect upon the species. on some of the laws that determine the development of colour something may be said presently. it is now merely necessary to show that concealment by obscure or imitative tints is only one out of very many ways by which animals maintain their existence; and having done this we are prepared to consider the phenomena of what has been termed "mimicry." it is to be particularly observed, however, that the word is not here used in the sense of voluntary imitation, but to imply a particular kind of resemblance--a resemblance not in internal structure but in external appearance--a resemblance in those parts only that catch the eye--a resemblance that deceives. as this kind of resemblance has the same effect as voluntary imitation or mimicry, and as we have no word that expresses the required meaning, "mimicry" was adopted by mr. bates (who was the first to explain the facts), and has led to some misunderstanding; but there need be none, if it is remembered that both "mimicry" and "imitation" are used in a metaphorical sense, as implying that close external likeness which causes things unlike in structure to be mistaken for each other. _mimicry._ it has been long known to entomologists that certain insects bear a strange external resemblance to others belonging to distinct genera, families, or even orders, and with which they have no real affinity whatever. the fact, however, appears to have been generally considered as dependent upon some unknown law of "analogy"--some "system of nature," or "general plan," which had guided the creator in designing the myriads of insect forms, and which we could never hope to understand. in only one case does it appear that the resemblance was thought to be useful, and to have been designed as a means to a definite and intelligible purpose. the flies of the genus volucella enter the nests of bees to deposit their eggs, so that their larvæ may feed upon the larvæ of the bees, and these flies are each wonderfully like the bee on which it is parasitic. kirby and spence believed that this resemblance or "mimicry" was for the express purpose of protecting the flies from the attacks of the bees, and the connection is so evident that it was hardly possible to avoid this conclusion. the resemblance, however, of moths to butterflies or to bees, of beetles to wasps, and of locusts to beetles, has been many times noticed by eminent writers; but scarcely ever till within the last few years does it appear to have been considered that these resemblances had any special purpose, or were of any direct benefit to the insects themselves. in this respect they were looked upon as accidental, as instances of the "curious analogies" in nature which must be wondered at but which could not be explained. recently, however, these instances have been greatly multiplied; the nature of the resemblances has been more carefully studied, and it has been found that they are often carried out into such details as almost to imply a purpose of deceiving the observer. the phenomena, moreover, have been shown to follow certain definite laws, which again all indicate their dependence on the more general law of the "survival of the fittest," or "the preservation of favoured races in the struggle for life." it will, perhaps, be as well here to state what these laws or general conclusions are, and then to give some account of the facts which support them. the first law is, that in an overwhelming majority of cases of mimicry, the animals (or the groups) which resemble each other inhabit the same country, the same district, and in most cases are to be found together on the very same spot. the second law is, that these resemblances are not indiscriminate, but are limited to certain groups, which in every case are abundant in species and individuals, and can often be ascertained to have some special protection. the third law is, that the species which resemble or "mimic" these dominant groups, are comparatively less abundant in individuals, and are often very rare. these laws will be found to hold good, in all the cases of true mimicry among various classes of animals to which we have now to call the attention of our readers. _mimicry among lepidoptera._ as it is among butterflies that instances of mimicry are most numerous and most striking, an account of some of the more prominent examples in this group will first be given. there is in south america an extensive family of these insects, the heliconidæ, which are in many respects very remarkable. they are so abundant and characteristic in all the woody portions of the american tropics, that in almost every locality they will be seen more frequently than any other butterflies. they are distinguished by very elongate wings, body, and antennæ, and are exceedingly beautiful and varied in their colours; spots and patches of yellow, red, or pure white upon a black, blue, or brown ground, being most general. they frequent the forests chiefly, and all fly slowly and weakly; yet although they are so conspicuous, and could certainly be caught by insectivorous birds more easily than almost any other insects, their great abundance all over the wide region they inhabit shows that they are not so persecuted. it is to be especially remarked also, that they possess no adaptive colouring to protect them during repose, for the under side of their wings presents the same, or at least an equally conspicuous colouring as the upper side; and they may be observed after sunset suspended at the end of twigs and leaves where they have taken up their station for the night, fully exposed to the attacks of enemies if they have any. these beautiful insects possess, however, a strong pungent semi-aromatic or medicinal odour, which seems to pervade all the juices of their system. when the entomologist squeezes the breast of one of them between his fingers to kill it, a yellow liquid exudes which stains the skin, and the smell of which can only be got rid of by time and repeated washings. here we have probably the cause of their immunity from attack, since there is a great deal of evidence to show that certain insects are so disgusting to birds that they will under no circumstances touch them. mr. stainton has observed that a brood of young turkeys greedily devoured all the worthless moths he had amassed in a night's "sugaring," yet one after another seized and rejected a single white moth which happened to be among them. young pheasants and partridges which eat many kinds of caterpillars seem to have an absolute dread of that of the common currant moth, which they will never touch, and tomtits as well as other small birds appear never to eat the same species. in the case of the heliconidæ, however, we have some direct evidence to the same effect. in the brazilian forests there are great numbers of insectivorous birds--as jacamars, trogons, and puffbirds--which catch insects on the wing, and that they destroy many butterflies is indicated by the fact that the wings of these insects are often found on the ground where their bodies have been devoured. but among these there are no wings of heliconidæ, while those of the large showy nymphalidæ, which have a much swifter flight, are often met with. again, a gentleman who had recently returned from brazil stated at a meeting of the entomological society that he once observed a pair of puffbirds catching butterflies, which they brought to their nest to feed their young; yet during half an hour they never brought one of the heliconidæ, which were flying lazily about in great numbers, and which they could have captured more easily than any others. it was this circumstance that led mr. belt to observe them so long, as he could not understand why the most common insects should be altogether passed by. mr. bates also tells us that he never saw them molested by lizards or predacious flies, which often pounce on other butterflies. if, therefore, we accept it as highly probable (if not proved) that the heliconidæ are very greatly protected from attack by their peculiar odour and taste, we find it much more easy to understand their chief characteristics--their great abundance, their slow flight, their gaudy colours, and the entire absence of protective tints on their under surfaces. this property places them somewhat in the position of those curious wingless birds of oceanic islands, the dodo, the apteryx, and the moas, which are with great reason supposed to have lost the power of flight on account of the absence of carnivorous quadrupeds. our butterflies have been protected in a different way, but quite as effectually; and the result has been that as there has been nothing to escape from, there has been no weeding out of slow flyers, and as there has been nothing to hide from, there has been no extermination of the bright-coloured varieties, and no preservation of such as tended to assimilate with surrounding objects. now let us consider how this kind of protection must act. tropical insectivorous birds very frequently sit on dead branches of a lofty tree, or on those which overhang forest paths, gazing intently around, and darting off at intervals to seize an insect at a considerable distance, which they generally return to their station to devour. if a bird began by capturing the slow-flying, conspicuous heliconidæ, and found them always so disagreeable that it could not eat them, it would after a very few trials leave off catching them at all; and their whole appearance, form, colouring, and mode of flight is so peculiar, that there can be little doubt birds would soon learn to distinguish them at a long distance, and never waste any time in pursuit of them. under these circumstances, it is evident that any other butterfly of a group which birds were accustomed to devour, would be almost equally well protected by closely resembling a heliconia externally, as if it acquired also the disagreeable odour; always supposing that there were only a few of them among a great number of the heliconias. if the birds could not distinguish the two kinds externally, and there were on the average only one eatable among fifty uneatable, they would soon give up seeking for the eatable ones, even if they knew them to exist. if, on the other hand, any particular butterfly of an eatable group acquired the disagreeable taste of the heliconias while it retained the characteristic form and colouring of its own group, this would be really of no use to it whatever; for the birds would go on catching it among its eatable allies (compared with which it would rarely occur), it would be wounded and disabled, even if rejected, and its increase would thus be as effectually checked as if it were devoured. it is important, therefore, to understand that if any one genus of an extensive family of eatable butterflies were in danger of extermination from insect-eating birds, and if two kinds of variation were going on among them, some individuals possessing a slightly disagreeable taste, others a slight resemblance to the heliconidæ, this latter quality would be much more valuable than the former. the change in flavour would not at all prevent the variety from being captured as before, and it would almost certainly be thoroughly disabled before being rejected. the approach in colour and form to the heliconidæ, however, would be at the very first a positive, though perhaps a slight advantage; for although at short distances this variety would be easily distinguished and devoured, yet at a longer distance it might be mistaken for one of the uneatable group, and so be passed by and gain another day's life, which might in many cases be sufficient for it to lay a quantity of eggs and leave a numerous progeny, many of which would inherit the peculiarity which had been the safeguard of their parent. now, this hypothetical case is exactly realized in south america. among the white butterflies forming the family pieridæ (many of which do not greatly differ in appearance from our own cabbage butterflies) is a genus of rather small size (leptalis), some species of which are white like their allies, while the larger number exactly resemble the heliconidæ in the form and colouring of the wings. it must always be remembered that these two families are as absolutely distinguished from each other by structural characters as are the carnivora and the ruminants among quadrupeds, and that an entomologist can always distinguish the one from the other by the structure of the feet, just as certainly as a zoologist can tell a bear from a buffalo by the skull or by a tooth. yet the resemblance of a species of the one family to another species in the other family was often so great, that both mr. bates and myself were many times deceived at the time of capture, and did not discover the distinctness of the two insects till a closer examination detected their essential differences. during his residence of eleven years in the amazon valley, mr. bates found a number of species or varieties of leptalis, each of which was a more or less exact copy of one of the heliconidæ of the district it inhabited; and the results of his observations are embodied in a paper published in the linnean transactions, in which he first explained the phenomena of "mimicry" as the result of natural selection, and showed its identity in cause and purpose with protective resemblance to vegetable or inorganic forms. the imitation of the heliconidæ by the leptalides is carried out to a wonderful degree in form as well as in colouring. the wings have become elongated to the same extent, and the antennæ and abdomen have both become lengthened, to correspond with the unusual condition in which they exist in the former family. in colouration there are several types in the different genera of heliconidæ. the genus mechanitis is generally of a rich semi-transparent brown, banded with black and yellow; methona is of large size, the wings transparent like horn, and with black transverse bands; while the delicate ithomias are all more or less transparent, with black veins and borders, and often with marginal and transverse bands of orange red. these different forms are all copied by the various species of leptalis, every band and spot and tint of colour, and the various degrees of transparency, being exactly reproduced. as if to derive all the benefit possible from this protective mimicry, the habits have become so modified that the leptalides generally frequent the very same spots as their models, and have the same mode of flight; and as they are always very scarce (mr. bates estimating their numbers at about one to a thousand of the group they resemble), there is hardly a possibility of their being found out by their enemies. it is also very remarkable that in almost every case the particular ithomias and other species of heliconidæ which they resemble, are noted as being very common species, swarming in individuals, and found over a wide range of country. this indicates antiquity and permanence in the species, and is exactly the condition most essential both to aid in the development of the resemblance, and to increase its utility. but the leptalides are not the only insects who have prolonged their existence by imitating the great protected group of heliconidæ;--a genus of quite another family of most lovely small american butterflies, the erycinidæ, and three genera of diurnal moths, also present species which often mimic the same dominant forms, so that some, as ithomia ilerdina of st. paulo, for instance, have flying with them a few individuals of three widely different insects, which are yet disguised with exactly the same form, colour, and markings, so as to be quite undistinguishable when upon the wing. again, the heliconidæ are not the only group that are imitated, although they are the most frequent models. the black and red group of south american papilios, and the handsome erycinian genus stalachtis, have also a few who copy them; but this fact offers no difficulty, since these two groups are almost as dominant as the heliconidæ. they both fly very slowly, they are both conspicuously coloured, and they both abound in individuals; so that there is every reason to believe that they possess a protection of a similar kind to the heliconidæ, and that it is therefore equally an advantage to other insects to be mistaken for them. there is also another extraordinary fact that we are not yet in a position clearly to comprehend: some groups of the heliconidæ themselves mimic other groups. species of heliconia mimic mechanitis, and every species of napeogenes mimics some other heliconideous butterfly. this would seem to indicate that the distasteful secretion is not produced alike by all members of the family, and that where it is deficient protective imitation comes into play. it is this, perhaps, that has caused such a general resemblance among the heliconidæ, such a uniformity of type with great diversity of colouring, since any aberration causing an insect to cease to look like one of the family would inevitably lead to its being attacked, wounded, and exterminated, even although it was not eatable. in other parts of the world an exactly parallel series of facts have been observed. the danaidæ and the acræidæ of the old world tropics form in fact one great group with the heliconidæ. they have the same general form, structure, and habits: they possess the same protective odour, and are equally abundant in individuals, although not so varied in colour, blue and white spots on a black ground being the most general pattern. the insects which mimic these are chiefly papilios, and diadema, a genus allied to our peacock and tortoiseshell butterflies. in tropical africa there is a peculiar group of the genus danais, characterized by dark-brown and bluish-white colours, arranged in bands or stripes. one of these, danais niavius, is exactly imitated both by papilio hippocoon and by diadema anthedon; another, danais echeria, by papilio cenea; and in natal a variety of the danais is found having a white spot at the tip of wings, accompanied by a variety of the papilio bearing a corresponding white spot. acræa gea is copied in its very peculiar style of colouration by the female of papilio cynorta, by panopæa hirce, and by the female of elymnias phegea. acræa euryta of calabar has a female variety of panopea hirce from the same place which exactly copies it; and mr. trimen, in his paper on mimetic analogies among african butterflies, published in the transactions of the linnæan society for , gives a list of no less than sixteen species and varieties of diadema and its allies, and ten of papilio, which in their colour and markings are perfect mimics of species or varieties of danais or acræa which inhabit the same districts. passing on to india, we have danais tytia, a butterfly with semi-transparent bluish wings and a border of rich reddish brown. this remarkable style of colouring is exactly reproduced in papilio agestor and in diadema nama, and all three insects not unfrequently come together in collections made at darjeeling. in the philippine islands the large and curious idea leuconöe with its semi-transparent white wings, veined and spotted with black, is copied by the rare papilio idæoides from the same islands. in the malay archipelago the very common and beautiful euploea midamus is so exactly mimicked by two rare papilios (p. paradoxa and p. ænigma) that i generally caught them under the impression that they were the more common species; and the equally common and even more beautiful euploea rhadamanthus, with its pure white bands and spots on a ground of glossy blue and black, is reproduced in the papilio caunus. here also there are species of diadema imitating the same group in two or three instances; but we shall have to adduce these further on in connexion with another branch of the subject. it has been already mentioned that in south america there is a group of papilios which have all the characteristics of a protected race, and whose peculiar colours and markings are imitated by other butterflies not so protected. there is just such a group also in the east, having very similar colours and the same habits, and these also are mimicked by other species in the same genus not closely allied to them, and also by a few of other families. papilio hector, a common indian butterfly of a rich black colour spotted with crimson, is so closely copied by papilio romulus, that the latter insect has been thought to be its female. a close examination shows, however, that it is essentially different, and belongs to another section of the genus. papilio antiphus and p. diphilus, black swallow-tailed butterflies with cream-coloured spots, are so well imitated by varieties of p. theseus, that several writers have classed them as the same species. papilio liris, found only in the island of timor, is accompanied there by p. ænomaus, the female of which so exactly resembles it that they can hardly be separated in the cabinet, and on the wing are quite undistinguishable. but one of the most curious cases is the fine yellow-spotted papilio cöon, which is unmistakeably imitated by the female tailed form of papilio memnon. these are both from sumatra; but in north india p. cöon is replaced by another species, which has been named p. doubledayi, having red spots instead of yellow; and in the same district the corresponding female tailed form of papilio androgeus, sometimes considered a variety of p. memnon, is similarly red-spotted. mr. westwood has described some curious day-flying moths (epicopeia) from north india, which have the form and colour of papilios of this section, and two of these are very good imitations of papilio polydorus and papilio varuna, also from north india. almost all these cases of mimicry are from the tropics, where the forms of life are more abundant, and where insect development especially is of unchecked luxuriance; but there are also one or two instances in temperate regions. in north america, the large and handsome red and black butterfly danais erippus is very common; and the same country is inhabited by limenitis archippus, which closely resembles the danais, while it differs entirely from every species of its own genus. the only case of probable mimicry in our own country is the following:--a very common white moth (spilosoma menthastri) was found by mr. stainton to be rejected by young turkeys among hundreds of other moths on which they greedily fed. each bird in succession took hold of this moth and threw it down again, as if too nasty to eat. mr. jenner weir also found that this moth was refused by the bullfinch, chaffinch, yellow hammer, and red bunting, but eaten after much hesitation by the robin. we may therefore fairly conclude that this species would be disagreeable to many other birds, and would thus have an immunity from attack, which may be the cause of its great abundance and of its conspicuous white colour. now it is a curious thing that there is another moth, diaphora mendica, which appears about the same time, and whose female only is white. it is about the same size as spilosoma menthastri, and sufficiently resembles it in the dusk, and this moth is much less common. it seems very probable, therefore, that these species stand in the same relation to each other as the mimicking butterflies of various families do to the heliconidæ and danaidæ. it would be very interesting to experiment on all white moths, to ascertain if those which are most common are generally rejected by birds. it may be anticipated that they would be so, because white is the most conspicuous of all colours for nocturnal insects, and had they not some other protection would certainly be very injurious to them. _lepidoptera mimicking other insects._ in the preceding cases we have found lepidoptera imitating other species of the same order, and such species only as we have good reason to believe were free from the attacks of many insectivorous creatures; but there are other instances in which they altogether lose the external appearance of the order to which they belong, and take on the dress of bees or wasps--insects which have an undeniable protection in their stings. the sesiidæ and Ægeriidæ, two families of day-flying moths, are particularly remarkable in this respect, and a mere inspection of the names given to the various species shows how the resemblance has struck everyone. we have apiformis, vespiforme, ichneumoniforme, scoliæforme, sphegiforme (bee-like, wasp-like, ichneumon-like, &c.) and many others, all indicating a resemblance to stinging hymenoptera. in britain we may particularly notice sesia bombiliformis, which very closely resembles the male of the large and common humble bee, bombus hortorum; sphecia craboniforme, which is coloured like a hornet, and is (on the authority of mr. jenner weir) much more like it when alive than when in the cabinet, from the way in which it carries its wings; and the currant clear-wing, trochilium tipuliforme, which resembles a small black wasp (odynerus sinuatus) very abundant in gardens at the same season. it has been so much the practice to look upon these resemblances as mere curious analogies playing no part in the economy of nature, that we have scarcely any observations of the habits and appearance when alive of the hundreds of species of these groups in various parts of the world, or how far they are accompanied by hymenoptera, which they specifically resemble. there are many species in india (like those figured by professor westwood in his "oriental entomology") which have the hind legs very broad and densely hairy, so as exactly to imitate the brush-legged bees (scopulipedes) which abound in the same country. in this case we have more than mere resemblance of colour, for that which is an important functional structure in the one group is imitated in another whose habits render it perfectly useless. _mimicry among beetles._ it may fairly be expected that if these imitations of one creature by another really serve as a protection to weak and decaying species, instances of the same kind will be found among other groups than the lepidoptera; and such is the case, although they are seldom so prominent and so easily recognised as those already pointed out as occurring in that order. a few very interesting examples may, however, be pointed out in most of the other orders of insects. the coleoptera or beetles that imitate other coleoptera of distinct groups are very numerous in tropical countries, and they generally follow the laws already laid down as regulating these phenomena. the insects which others imitate always have a special protection, which leads them to be avoided as dangerous or uneatable by small insectivorous animals; some have a disgusting taste (analogous to that of the heliconidæ); others have such a hard and stony covering that they cannot be crushed or digested; while a third set are very active, and armed with powerful jaws, as well as having some disagreeable secretion. some species of eumorphidæ and hispidæ, small flat or hemispherical beetles which are exceedingly abundant, and have a disagreeable secretion, are imitated by others of the very distinct group of longicornes (of which our common musk-beetle may be taken as an example). the extraordinary little cyclopeplus batesii, belongs to the same sub-family of this group as the onychocerus scorpio and o. concentricus, which have already been adduced as imitating with such wonderful accuracy the bark of the trees they habitually frequent; but it differs totally in outward appearance from every one of its allies, having taken upon itself the exact shape and colouring of a globular corynomalus, a little stinking beetle with clubbed antennæ. it is curious to see how these clubbed antennæ are imitated by an insect belonging to a group with long slender antennæ. the sub-family anisocerinæ, to which cyclopeplus belongs, is characterised by all its members possessing a little knob or dilatation about the middle of the antennæ. this knob is considerably enlarged in c. batesii, and the terminal portion of the antennæ beyond it is so small and slender as to be scarcely visible, and thus an excellent substitute is obtained for the short clubbed antennæ of the corynomalus. erythroplatis corallifer is another curious broad flat beetle, that no one would take for a longicorn, since it almost exactly resembles cephalodonta spinipes, one of the commonest of the south american hispidæ; and what is still more remarkable, another longicorn of a distinct group, streptolabis hispoides, was found by mr. bates, which resembles the same insect with equal minuteness,--a case exactly parallel to that among butterflies, where species of two or three distinct groups mimicked the same heliconia. many of the soft-winged beetles (malacoderms) are excessively abundant in individuals, and it is probable that they have some similar protection, more especially as other species often strikingly resemble them. a longicorn beetle, pæciloderma terminale, found in jamaica, is coloured exactly in the same way as a lycus (one of the malacoderms) from the same island. eroschema poweri, a longicorn from australia, might certainly be taken for one of the same group, and several species from the malay islands are equally deceptive. in the island of celebes i found one of this group, having the whole body and elytra of a rich deep blue colour, with the head only orange; and in company with it an insect of a totally different family (eucnemidæ) with identically the same colouration, and of so nearly the same size and form as to completely puzzle the collector on every fresh occasion of capturing them. i have been recently informed by mr. jenner weir, who keeps a variety of small birds, that none of them will touch our common "soldiers and sailors" (species of malacoderms), thus confirming my belief that they were a protected group, founded on the fact of their being at once very abundant, of conspicuous colours, and the objects of mimicry. there are a number of the larger tropical weevils which have the elytra and the whole covering of the body so hard as to be a great annoyance to the entomologist, because in attempting to transfix them the points of his pins are constantly turned. i have found it necessary in these cases to drill a hole very carefully with the point of a sharp penknife before attempting to insert a pin. many of the fine long-antennæd anthribidæ (an allied group) have to be treated in the same way. we can easily understand that after small birds have in vain attempted to eat these insects, they should get to know them by sight, and ever after leave them alone, and it will then be an advantage for other insects which are comparatively soft and eatable, to be mistaken for them. we need not be surprised, therefore, to find that there are many longicorns which strikingly resemble the "hard beetles" of their own district. in south brazil, acanthotritus dorsalis is strikingly like a curculio of the hard genus heiliplus, and mr. bates assures me that he found gymnocerus cratosomoides (a longicorn) on the same tree with a hard cratosomus (a weevil), which it exactly mimics. again, the pretty longicorn, phacellocera batesii, mimics one of the hard anthribidæ of the genus ptychoderes, having long slender antennæ. in the moluccas we find cacia anthriboides, a small longicorn which might be easily mistaken for a very common species of anthribidæ found in the same districts; and the very rare capnolymma stygium closely imitates the common mecocerus gazella, which abounded where it was taken. doliops curculionoides and other allied longicorns from the philippine islands most curiously resemble, both in form and colouring, the brilliant pachyrhynchi,--curculionidæ, which are almost peculiar to that group of islands. the remaining family of coleoptera most frequently imitated is the cicindelidæ. the rare and curious longicorn, collyrodes lacordairei, has exactly the form and colouring of the genus collyris, while an undescribed species of heteromera is exactly like a therates, and was taken running on the trunks of trees, as is the habit of that group. there is one curious example of a longicorn mimicking a longicorn, like the papilios and heliconidæ which mimic their own allies. agnia fasciata, belonging to the sub-family hypselominæ, and nemophas grayi, belonging to the lamiinæ, were taken in amboyna on the same fallen tree at the same time, and were supposed to be the same species till they were more carefully examined, and found to be structurally quite different. the colouring of these insects is very remarkable, being rich steel-blue black, crossed by broad hairy bands of orange buff, and out of the many thousands of known species of longicorns they are probably the only two which are so coloured. the nemophas grayi is the larger, stronger, and better armed insect, and belongs to a more widely spread and dominant group, very rich in species and individuals, and is therefore most probably the subject of mimicry by the other species. _beetles mimicking other insects._ we will now adduce a few cases in which beetles imitate other insects, and insects of other orders imitate beetles. charis melipona, a south american longicorn of the family necydalidæ, has been so named from its resemblance to a small bee of the genus melipona. it is one of the most remarkable cases of mimicry, since the beetle has the thorax and body densely hairy like the bee, and the legs are tufted in a manner most unusual in the order coleoptera. another longicorn, odontocera odyneroides, has the abdomen banded with yellow, and constricted at the base, and is altogether so exactly like a small common wasp of the genus odynerus, that mr. bates informs us he was afraid to take it out of his net with his fingers for fear of being stung. had mr. bates's taste for insects been less omnivorous than it was, the beetle's disguise might have saved it from his pin, as it had no doubt often done from the beak of hungry birds. a larger insect, sphecomorpha chalybea, is exactly like one of the large metallic blue wasps, and like them has the abdomen connected with the thorax by a pedicel, rendering the deception most complete and striking. many eastern species of longicorns of the genus oberea, when on the wing exactly resemble tenthredinidæ, and many of the small species of hesthesis run about on timber, and cannot be distinguished from ants. there is one genus of south american longicorns that appears to mimic the shielded bugs of the genus scutellera. the gymnocerous capucinus is one of these, and is very like pachyotris fabricii, one of the scutelleridæ. the beautiful gymnocerous dulcissimus is also very like the same group of insects, though there is no known species that exactly corresponds to it; but this is not to be wondered at, as the tropical hemiptera have been comparatively so little cared for by collectors. _insects mimicking species of other orders._ the most remarkable case of an insect of another order mimicking a beetle is that of the condylodera tricondyloides, one of the cricket family from the philippine islands, which is so exactly like a tricondyla (one of the tiger beetles), that such an experienced entomologist as professor westwood placed it among them in his cabinet, and retained it there a long time before he discovered his mistake! both insects run along the trunks of trees, and whereas tricondylas are very plentiful, the insect that mimics it is, as in all other cases, very rare. mr. bates also informs us that he found at santarem on the amazon, a species of locust which mimicked one of the tiger beetles of the genus odontocheila, and was found on the same trees which they frequented. there are a considerable number of diptera, or two-winged flies, that closely resemble wasps and bees, and no doubt derive much benefit from the wholesome dread which those insects excite. the midas dives, and other species of large brazilian flies, have dark wings and metallic blue elongate bodies, resembling the large stinging sphegidæ of the same country; and a very large fly of the genus asilus has black-banded wings and the abdomen tipped with rich orange, so as exactly to resemble the fine bee euglossa dimidiata, and both are found in the same parts of south america. we have also in our own country species of bombylius which are almost exactly like bees. in these cases the end gained by the mimicry is no doubt freedom from attack, but it has sometimes an altogether different purpose. there are a number of parasitic flies whose larvæ feed upon the larvæ of bees, such as the british genus volucella and many of the tropical bombylii, and most of these are exactly like the particular species of bee they prey upon, so that they can enter their nests unsuspected to deposit their eggs. there are also bees that mimic bees. the cuckoo bees of the genus nomada are parasitic on the andrenidæ, and they resemble either wasps or species of andrena; and the parasitic humble-bees of the genus apathus almost exactly resemble the species of humble-bees in whose nests they are reared. mr. bates informs us that he found numbers of these "cuckoo" bees and flies on the amazon, which all wore the livery of working bees peculiar to the same country. there is a genus of small spiders in the tropics which feed on ants, and they are exactly like ants themselves, which no doubt gives them more opportunity of seizing their prey; and mr. bates found on the amazon a species of mantis which exactly resembled the white ants which it fed upon, as well as several species of crickets (scaphura), which resembled in a wonderful manner different sand-wasps of large size, which are constantly on the search for crickets with which to provision their nests. perhaps the most wonderful case of all is the large caterpillar mentioned by mr. bates, which startled him by its close resemblance to a small snake. the first three segments behind the head were dilatable at the will of the insect, and had on each side a large black pupillated spot, which resembled the eye of the reptile. moreover, it resembled a poisonous viper, not a harmless species of snake, as was proved by the imitation of keeled scales on the crown produced by the recumbent feet, as the caterpillar threw itself backward! the attitudes of many of the tropical spiders are most extraordinary and deceptive, but little attention has been paid to them. they often mimic other insects, and some, mr. bates assures us, are exactly like flower buds, and take their station in the axils of leaves, where they remain motionless waiting for their prey. _cases of mimicry among the vertebrata._ having thus shown how varied and extraordinary are the modes in which mimicry occurs among insects, we have now to enquire if anything of the same kind is to be observed among vertebrated animals. when we consider all the conditions necessary to produce a good deceptive imitation, we shall see at once that such can very rarely occur in the higher animals, since they possess none of those facilities for the almost infinite modifications of external form which exist in the very nature of insect organization. the outer covering of insects being more or less solid and horny, they are capable of almost any amount of change of form and appearance without any essential modification internally. in many groups the wings give much of the character, and these organs may be much modified both in form and colour without interfering with their special functions. again, the number of species of insects is so great, and there is such diversity of form and proportion in every group, that the chances of an accidental approximation in size, form, and colour, of one insect to another of a different group, are very considerable; and it is these chance approximations that furnish the basis of mimicry, to be continually advanced and perfected by the survival of those varieties only which tend in the right direction. in the vertebrata, on the contrary, the skeleton being internal the external form depends almost entirely on the proportions and arrangement of that skeleton, which again is strictly adapted to the functions necessary for the well-being of the animal. the form cannot therefore be rapidly modified by variation, and the thin and flexible integument will not admit of the development of such strange protuberances as occur continually in insects. the number of species of each group in the same country is also comparatively small, and thus the chances of that first accidental resemblance which is necessary for natural selection to work upon are much diminished. we can hardly see the possibility of a mimicry by which the elk could escape from the wolf, or the buffalo from the tiger. there is, however, in one group of vertebrata such a general similarity of form, that a very slight modification, if accompanied by identity of colour, would produce the necessary amount of resemblance; and at the same time there exist a number of species which it would be advantageous for others to resemble, since they are armed with the most fatal weapons of offence. we accordingly find that reptiles furnish us with a very remarkable and instructive case of true mimicry. _mimicry among snakes._ there are in tropical america a number of venomous snakes of the genus elaps, which are ornamented with brilliant colours disposed in a peculiar manner. the ground colour is generally bright red, on which are black bands of various widths and sometimes divided into two or three by yellow rings. now, in the same country are found several genera of harmless snakes, having no affinity whatever with the above, but coloured exactly the same. for example, the poisonous elaps fulvius often occurs in guatemala with simple black bands on a coral-red ground; and in the same country is found the harmless snake pliocerus equalis, coloured and banded in identically the same manner. a variety of elaps corallinus has the black bands narrowly bordered with yellow on the same red ground colour, and a harmless snake, homalocranium semicinctum, has exactly the same markings, and both are found in mexico. the deadly elaps lemniscatus has the black bands very broad, and each of them divided into three by narrow yellow rings; and this again is exactly copied by a harmless snake, pliocerus elapoides, which is found along with its model in mexico. but, more remarkable still, there is in south america a third group of snakes, the genus oxyrhopus, doubtfully venomous, and having no immediate affinity with either of the preceding, which has also the same curious distribution of colours, namely, variously disposed rings of red, yellow, and black; and there are some cases in which species of all three of these groups similarly marked inhabit the same district. for example, elaps mipartitus has single black rings very close together. it inhabits the west side of the andes, and in the same districts occur pliocerus euryzonus and oxyrhopus petolarius, which exactly copy its pattern. in brazil elaps lemniscatus is copied by oxyrhopus trigeminus, both having black rings disposed in threes. in elaps hemiprichii the ground colour appears to be black, with alternations of two narrow yellow bands and a broader red one; and of this pattern again we have an exact double in oxyrhopus formosus, both being found in many localities of tropical south america. what adds much to the extraordinary character of these resemblances is the fact, that nowhere in the world but in america are there any snakes at all which have this style of colouring. dr. gunther, of the british museum, who has kindly furnished some of the details here referred to, assures me that this is the case; and that red, black, and yellow rings occur together on no other snakes in the world but on elaps and the species which so closely resemble it. in all these cases, the size and form as well as the colouration, are so much alike, that none but a naturalist would distinguish the harmless from the poisonous species. many of the small tree-frogs are no doubt also mimickers. when seen in their natural attitudes, i have been often unable to distinguish them from beetles or other insects sitting upon leaves, but regret to say i neglected to observe what species or groups they most resembled, and the subject does not yet seem to have attracted the attention of naturalists abroad. _mimicry among birds._ in the class of birds there are a number of cases that make some approach to mimicry, such as the resemblance of the cuckoos, a weak and defenceless group of birds, to hawks and gallinaceæ. there is, however, one example which goes much further than this, and seems to be of exactly the same nature as the many cases of insect mimicry which have been already given. in australia and the moluccas there is a genus of honeysuckers called tropidorhynchus, good sized birds, very strong and active, having powerful grasping claws and long, curved, sharp beaks. they assemble together in groups and small flocks, and they have a very loud bawling note, which can be heard at a great distance, and serves to collect a number together in time of danger. they are very plentiful and very pugnacious, frequently driving away crows, and even hawks, which perch on a tree where a few of them are assembled. they are all of rather dull and obscure colours. now in the same countries there is a group of orioles, forming the genus mimeta, much weaker birds, which have lost the gay colouring of their allies the golden orioles, being usually olive-green or brown; and in several cases these most curiously resemble the tropidorhynchus of the same island. for example, in the island of bouru is found the tropidorhynchus bouruensis, of a dull earthy colour, and the mimeta bouruensis, which resembles it in the following particulars:--the upper and under surfaces of the two birds are exactly of the same tints of dark and light brown; the tropidorhynchus has a large bare black patch round the eyes; this is copied in the mimeta by a patch of black feathers. the top of the head of the tropidorhynchus has a scaly appearance from the narrow scale-formed feathers, which are imitated by the broader feathers of the mimeta having a dusky line down each. the tropidorhynchus has a pale ruff formed of curious recurved feathers on the nape (which has given the whole genus the name of friar birds); this is represented in the mimeta by a pale band in the same position. lastly, the bill of the tropidorhynchus is raised into a protuberant keel at the base, and the mimeta has the same character, although it is not a common one in the genus. the result is, that on a superficial examination the birds are identical, although they have important structural differences, and cannot be placed near each other in any natural arrangement. as a proof that the resemblance is really deceptive, it may be mentioned that the mimeta is figured and described as a honeysucker in the costly "voyage de l'astrolabe," under the name of philedon bouruensis! passing to the island of ceram, we find allied species of both genera. the tropidorhynchus subcornutus is of an earthy brown colour washed with yellow ochre, with bare orbits, dusky cheeks, and the usual pale recurved nape-ruff. the mimeta forsteni is absolutely identical in the tints of every part of the body, the details of which are imitated in the same manner as in the bouru birds already described. in two other islands there is an approximation towards mimicry, although it is not so perfect as in the two preceding cases. in timor the tropidorhynchus timoriensis is of the usual earthy brown above, with the nape-ruff very prominent, the cheeks black, the throat nearly white, and the whole under surface pale whitish brown. these various tints are all well reproduced in mimeta virescens, the chief want of exact imitation being that the throat and breast of the tropidorhynchus has a very scaly appearance, being covered with rigid pointed feathers which are not imitated in the mimeta, although there are signs of faint dusky spots which may easily furnish the groundwork of a more exact imitation by the continued survival of favourable variations in the same direction. there is also a large knob at the base of the bill of the tropidorhynchus which is not at all imitated by the mimeta. in the island of morty (north of gilolo) there exists the tropidorhynchus fuscicapillus, of a dark sooty brown colour, especially on the head, while the under parts are rather lighter, and the characteristic ruff of the nape is wanting. now it is curious that in the adjacent island of gilolo should be found the mimeta phæochromus, the upper surface of which is of exactly the same dark sooty tint as the tropidorhynchus, and is the only known species that is of such a dark colour. the under side is not quite light enough, but it is a good approximation. this mimeta is a rare bird, and may very probably exist in morty, though not yet found there; or, on the other hand, recent changes in physical geography may have led to the restriction of the tropidorhynchus to that island, where it is very common. here, then, we have two cases of perfect mimicry and two others of good approximation, occurring between species of the same two genera of birds; and in three of these cases the pairs that resemble each other are found together in the same island, and to which they are peculiar. in all these cases the tropidorhynchus is rather larger than the mimeta, but the difference is not beyond the limits of variation in species, and the two genera are somewhat alike in form and proportion. there are, no doubt, some special enemies by which many small birds are attacked, but which are afraid of the tropidorhynchus (probably some of the hawks), and thus it becomes advantageous for the weak mimeta to resemble the strong, pugnacious, noisy, and very abundant tropidorhynchus. my friend, mr. osbert salvin, has given me another interesting case of bird mimicry. in the neighbourhood of rio janeiro is found an insect-eating hawk (harpagus diodon), and in the same district a bird-eating hawk (accipiter pileatus) which closely resembles it. both are of the same ashy tint beneath, with the thighs and under wing-coverts reddish brown, so that when on the wing and seen from below they are undistinguishable. the curious point, however, is that the accipiter has a much wider range than the harpagus, and in the regions where the insect-eating species is not found it no longer resembles it, the under wing-coverts varying to white; thus indicating that the red-brown colour is kept true by its being useful to the accipiter to be mistaken for the insect-eating species, which birds have learnt not to be afraid of. _mimicry among mammals._ among the mammalia the only case which may be true mimicry is that of the insectivorous genus cladobates, found in the malay countries, several species of which very closely resemble squirrels. the size is about the same, the long bushy tail is carried in the same way, and the colours are very similar. in this case the use of the resemblance must be to enable the cladobates to approach the insects or small birds on which it feeds, under the disguise of the harmless fruit-eating squirrel. _objections to mr. bates' theory of mimicry._ having now completed our survey of the most prominent and remarkable cases of mimicry that have yet been noticed, we must say something of the objections that have been made to the theory of their production given by mr. bates, and which we have endeavoured to illustrate and enforce in the preceding pages. three counter explanations have been proposed. professor westwood admits the fact of the mimicry and its probable use to the insect, but maintains that each species was created a mimic for the purpose of the protection thus afforded it. mr. andrew murray, in his paper on the "disguises of nature," inclines to the opinion that similar conditions of food and of surrounding circumstances have acted in some unknown way to produce the resemblances; and when the subject was discussed before the entomological society of london, a third objection was added--that heredity or the reversion to ancestral types of form and colouration, might have produced many of the cases of mimicry. against the special creation of mimicking species there are all the objections and difficulties in the way of special creation in other cases, with the addition of a few that are peculiar to it. the most obvious is, that we have gradations of mimicry and of protective resemblance--a fact which is strongly suggestive of a natural process having been at work. another very serious objection is, that as mimicry has been shown to be useful only to those species and groups which are rare and probably dying out, and would cease to have any effect should the proportionate abundance of the two species be reversed, it follows that on the special-creation theory the one species must have been created plentiful, the other rare; and, notwithstanding the many causes that continually tend to alter the proportions of species, these two species must have always been specially maintained at their respective proportions, or the very purpose for which they each received their peculiar characteristics would have completely failed. a third difficulty is, that although it is very easy to understand how mimicry may be brought about by variation and the survival of the fittest, it seems a very strange thing for a creator to protect an animal by making it imitate another, when the very assumption of a creator implies his power to create it so as to require no such circuitous protection. these appear to be fatal objections to the application of the special-creation theory to this particular case. the other two supposed explanations, which may be shortly expressed as the theories of "similar conditions" and of "heredity," agree in making mimicry, where it exists, an adventitious circumstance not necessarily connected with the well-being of the mimicking species. but several of the most striking and most constant facts which have been adduced, directly contradict both those hypotheses. the law that mimicry is confined to a few groups only is one of these, for "similar conditions" must act more or less on all groups in a limited region, and "heredity" must influence all groups related to each other in an equal degree. again, the general fact that those species which mimic others are rare, while those which are imitated are abundant, is in no way explained by either of these theories, any more than is the frequent occurrence of some palpable mode of protection in the imitated species. "reversion to an ancestral type" no way explains why the imitator and the imitated always inhabit the very same district, whereas allied forms of every degree of nearness and remoteness generally inhabit different countries, and often different quarters of the globe; and neither it, nor "similar conditions," will account for the likeness between species of distinct groups being superficial only--a disguise, not a true resemblance; for the imitation of bark, of leaves, of sticks, of dung; for the resemblance between species in different orders, and even different classes and sub-kingdoms; and finally, for the graduated series of the phenomena, beginning with a general harmony and adaptation of tint in autumn and winter moths and in arctic and desert animals, and ending with those complete cases of detailed mimicry which not only deceive predacious animals, but puzzle the most experienced insect collectors and the most learned entomologists. _mimicry by female insects only._ but there is yet another series of phenomena connected with this subject, which considerably strengthens the view here adopted, while it seems quite incompatible with either of the other hypotheses; namely, the relation of protective colouring and mimicry to the sexual differences of animals. it will be clear to every one that if two animals, which as regards "external conditions" and "hereditary descent," are exactly alike, yet differ remarkably in colouration, one resembling a protected species and the other not, the resemblance that exists in one only can hardly be imputed to the influence of external conditions or as the effect of heredity. and if, further, it can be proved that the one requires protection more than the other, and that in several cases it is that one which mimics the protected species, while the one that least requires protection never does so, it will afford very strong corroborative evidence that there is a real connexion between the necessity for protection and the phenomenon of mimicry. now the sexes of insects offer us a test of the nature here indicated, and appear to furnish one of the most conclusive arguments in favour of the theory that the phenomena termed "mimicry" are produced by natural selection. the comparative importance of the sexes varies much in different classes of animals. in the higher vertebrates, where the number of young produced at a birth is small and the same individuals breed many years in succession, the preservation of both sexes is almost equally important. in all the numerous cases in which the male protects the female and her offspring, or helps to supply them with food, his importance in the economy of nature is proportionately increased, though it is never perhaps quite equal to that of the female. in insects the case is very different; they pair but once in their lives, and the prolonged existence of the male is in most cases quite unnecessary for the continuance of the race. the female, however, must continue to exist long enough to deposit her eggs in a place adapted for the development and growth of the progeny. hence there is a wide difference in the need for protection in the two sexes; and we should, therefore, expect to find that in some cases the special protection given to the female was in the male less in amount or altogether wanting. the facts entirely confirm this expectation. in the spectre insects (phasmidæ) it is often the females alone that so strikingly resemble leaves, while the males show only a rude approximation. the male diadema misippus is a very handsome and conspicuous butterfly, without a sign of protective or imitative colouring, while the female is entirely unlike her partner, and is one of the most wonderful cases of mimicry on record, resembling most accurately the common danais chrysippus, in whose company it is often found. so in several species of south american pieris, the males are white and black, of a similar type of colouring to our own "cabbage" butterflies, while the females are rich yellow and buff, spotted and marked so as exactly to resemble species of heliconidæ with which they associate in the forest. in the malay archipelago is found a diadema which had always been considered a male insect on account of its glossy metallic-blue tints, while its companion of sober brown was looked upon as the female. i discovered, however, that the reverse is the case, and that the rich and glossy colours of the female are imitative and protective, since they cause her exactly to resemble the common euploea midamus of the same regions, a species which has been already mentioned in this essay as mimicked by another butterfly, papilio paradoxa. i have since named this interesting species diadema anomala (see the transactions of the entomological society, , p. ). in this case, and in that of diadema misippus, there is no difference in the habits of the two sexes, which fly in similar localities; so that the influence of "external conditions" cannot be invoked here as it has been in the case of the south american pieris pyrrha and allies, where the white males frequent open sunny places, while the heliconia-like females haunt the shades of the forest. we may impute to the same general cause (the greater need of protection for the female, owing to her weaker flight, greater exposure to attack, and supreme importance)--the fact of the colours of female insects being so very generally duller and less conspicuous than those of the other sex. and that it is chiefly due to this cause rather than to what mr. darwin terms "sexual selection" appears to be shown by the otherwise inexplicable fact, that in the groups which have a protection of any kind independent of concealment, sexual differences of colour are either quite wanting or slightly developed. the heliconidæ and danaidæ, protected by a disagreeable flavour, have the females as bright and conspicuous as the males, and very rarely differing at all from them. the stinging hymenoptera have the two sexes equally well coloured. the carabidæ, the coccinellidæ, chrysomelidæ, and the telephori have both sexes equally conspicuous, and seldom differing in colours. the brilliant curculios, which are protected by their hardness, are brilliant in both sexes. lastly, the glittering cetoniadæ and buprestidæ, which seem to be protected by their hard and polished coats, their rapid motions, and peculiar habits, present few sexual differences of colour, while sexual selection has often manifested itself by structural differences, such as horns, spines, or other processes. _cause of the dull colours of female birds._ the same law manifests itself in birds. the female while sitting on her eggs requires protection by concealment to a much greater extent than the male; and we accordingly find that in a large majority of the cases in which the male birds are distinguished by unusual brilliancy of plumage, the females are much more obscure, and often remarkably plain-coloured. the exceptions are such as eminently to prove the rule, for in most cases we can see a very good reason for them. in particular, there are a few instances among wading and gallinaceous birds in which the female has decidedly more brilliant colours than the male; but it is a most curious and interesting fact that in most if not all these cases the males sit upon the eggs; so that this exception to the usual rule almost demonstrates that it is because the process of incubation is at once very important and very dangerous, that the protection of obscure colouring is developed. the most striking example is that of the gray phalarope (phalaropus fulicarius). when in winter plumage, the sexes of this bird are alike in colouration, but in summer the female is much the most conspicuous, having a black head, dark wings, and reddish-brown back, while the male is nearly uniform brown, with dusky spots. mr. gould in his "birds of great britain" figures the two sexes in both winter and summer plumage, and remarks on the strange peculiarity of the usual colours of the two sexes being reversed, and also on the still more curious fact that the "male alone sits on the eggs," which are deposited on the bare ground. in another british bird, the dotterell, the female is also larger and more brightly-coloured than the male; and it seems to be proved that the males assist in incubation even if they do not perform it entirely, for mr. gould tells us, "that they have been shot with the breast bare of feathers, caused by sitting on the eggs." the small quail-like birds forming the genus turnix have also generally large and bright-coloured females, and we are told by mr. jerdon in his "birds of india" that "the natives report that during the breeding season the females desert their eggs and associate in flocks while the males are employed in hatching the eggs." it is also an ascertained fact, that the females are more bold and pugnacious than the males. a further confirmation of this view is to be found in the fact (not hitherto noticed) that in a large majority of the cases in which bright colours exist in both sexes incubation takes place in a dark hole or in a dome-shaped nest. female kingfishers are often equally brilliant with the male, and they build in holes in banks. bee-eaters, trogons, motmots, and toucans, all build in holes, and in none is there any difference in the sexes, although they are, without exception, showy birds. parrots build in holes in trees, and in the majority of cases they present no marked sexual difference tending to concealment of the female. woodpeckers are in the same category, since though the sexes often differ in colour, the female is not generally less conspicuous than the male. wagtails and titmice build concealed nests, and the females are nearly as gay as their mates. the female of the pretty australian bird pardalotus punctatus, is very conspicuously spotted on the upper surface, and it builds in a hole in the ground. the gay-coloured hang-nests (icterinæ) and the equally brilliant tanagers may be well contrasted; for the former, concealed in their covered nests, present little or no sexual difference of colour--while the open-nested tanagers have the females dull-coloured and sometimes with almost protective tints. no doubt there are many individual exceptions to the rule here indicated, because many and various causes have combined to determine both the colouration and the habits of birds. these have no doubt acted and re-acted on each other; and when conditions have changed one of these characters may often have become modified, while the other, though useless, may continue by hereditary descent an apparent exception to what otherwise seems a very general rule. the facts presented by the sexual differences of colour in birds and their mode of nesting, are on the whole in perfect harmony with that law of protective adaptation of colour and form, which appears to have checked to some extent the powerful action of sexual selection, and to have materially influenced the colouring of female birds, as it has undoubtedly done that of female insects. _use of the gaudy colours of many caterpillars._ since this essay was first published a very curious difficulty has been cleared up by the application of the general principle of protective colouring. great numbers of caterpillars are so brilliantly marked and coloured as to be very conspicuous even at a considerable distance, and it has been noticed that such caterpillars seldom hide themselves. other species, however, are green or brown, closely resembling the colours of the substances on which they feed, while others again imitate sticks, and stretch themselves out motionless from a twig so as to look like one of its branches. now, as caterpillars form so large a part of the food of birds, it was not easy to understand why any of them should have such bright colours and markings as to make them specially visible. mr. darwin had put the case to me as a difficulty from another point of view, for he had arrived at the conclusion that brilliant colouration in the animal kingdom is mainly due to sexual selection, and this could not have acted in the case of sexless larvæ. applying here the analogy of other insects, i reasoned, that since some caterpillars were evidently protected by their imitative colouring, and others by their spiny or hairy bodies, the bright colours of the rest must also be in some way useful to them. i further thought that as some butterflies and moths were greedily eaten by birds while others were distasteful to them, and these latter were mostly of conspicuous colours, so probably these brilliantly coloured caterpillars were distasteful, and therefore never eaten by birds. distastefulness alone would however be of little service to caterpillars, because their soft and juicy bodies are so delicate, that if seized and afterwards rejected by a bird they would almost certainly be killed. some constant and easily perceived signal was therefore necessary to serve as a warning to birds never to touch these uneatable kinds, and a very gaudy and conspicuous colouring with the habit of fully exposing themselves to view becomes such a signal, being in strong contrast with the green or brown tints and retiring habits of the eatable kinds. the subject was brought by me before the entomological society (see proceedings, march th, ), in order that those members having opportunities for making observations might do so in the following summer; and i also wrote a letter to the _field_ newspaper, begging that some of its readers would co-operate in making observations on what insects were rejected by birds, at the same time fully explaining the great interest and scientific importance of the problem. it is a curious example of how few of the country readers of that paper are at all interested in questions of simple natural history, that i only obtained one answer from a gentleman in cumberland, who gave me some interesting observations on the general dislike and abhorrence of all birds to the "gooseberry caterpillar," probably that of the magpie-moth (abraxas grossulariata). neither young pheasants, partridges, nor wild-ducks could be induced to eat it, sparrows and finches never touched it, and all birds to whom he offered it rejected it with evident dread and abhorrence. it will be seen that these observations are confirmed by those of two members of the entomological society to whom we are indebted for more detailed information. in march, , mr. j. jenner weir communicated a valuable series of observations made during many years, but more especially in the two preceding summers, in his aviary, containing the following birds of more or less insectivorous habits:--robin, yellow-hammer, reed-bunting, bullfinch, chaffinch, crossbill, thrush, tree-pipit, siskin, and redpoll. he found that hairy caterpillars were uniformly rejected; five distinct species were quite unnoticed by all his birds, and were allowed to crawl about the aviary for days with impunity. the spiny caterpillars of the tortoiseshell and peacock butterflies were equally rejected; but in both these cases mr. weir thinks it is the taste, not the hairs or spines, that are disagreeable, because some very young caterpillars of a hairy species were rejected although no hairs were developed, and the smooth pupæ of the above-named butterflies were refused as persistently as the spined larvæ. in these cases, then, both hairs and spines would seem to be mere signs of uneatableness. his next experiments were with those smooth gaily-coloured caterpillars which never conceal themselves, but on the contrary appear to court observation. such are those of the magpie moth (abraxas grossulariata), whose caterpillar is conspicuously white and black spotted--the diloba coeruleocephala, whose larvæ is pale yellow with a broad blue or green lateral band--the cucullia verbasci, whose larvæ is greenish white with yellow bands and black spots, and anthrocera filipendulæ (the six spot burnet moth), whose caterpillar is yellow with black spots. these were given to the birds at various times, sometimes mixed with other kinds of larvæ which were greedily eaten, but they were in every case rejected apparently unnoticed, and were left to crawl about till they died. the next set of observations were on the dull-coloured and protected larvæ, and the results of numerous experiments are thus summarised by mr. weir. "all caterpillars whose habits are nocturnal, which are dull coloured, with fleshy bodies and smooth skins, are eaten with the greatest avidity. every species of green caterpillar is also much relished. all geometræ, whose larvæ resemble twigs as they stand out from the plant on their anal prolegs, are invariably eaten." at the same meeting mr. a. g. butler, of the british museum, communicated the results of his observations with lizards, frogs, and spiders, which strikingly corroborate those of mr. weir. three green lizards (lacerta viridis) which he kept for several years, were very voracious, eating all kinds of food, from a lemon cheesecake to a spider, and devouring flies, caterpillars, and humble bees; yet there were some caterpillars and moths which they would seize only to drop immediately. among these the principal were the caterpillar of the magpie moth (abraxas grossulariata) and the perfect six spot burnet moth (anthrocera filipendulæ). these would be first seized but invariably dropped in disgust, and afterwards left unmolested. subsequently frogs were kept and fed with caterpillars from the garden, but two of these--that of the before-mentioned magpie moth, and that of the v. moth (halia wavaria), which is green with conspicuous white or yellow stripes and black spots--were constantly rejected. when these species were first offered, the frogs sprang at them eagerly and licked them into their mouths; no sooner, however, had they done so than they seemed to be aware of the mistake that they had made, and sat with gaping mouths, rolling their tongues about until they had got quit of the nauseous morsels. with spiders the same thing occurred. these two caterpillars were repeatedly put into the webs both of the geometrical and hunting spiders (epeira diadema and lycosa sp.), but in the former case they were cut out and allowed to drop; in the latter, after disappearing in the jaws of their captor down his dark silken funnel, they invariably reappeared, either from below or else taking long strides up the funnel again. mr. butler has observed lizards fight with and finally devour humble bees, and a frog sitting on a bed of stone-crop leap up and catch the bees which flew over his head, and swallow them, in utter disregard of their stings. it is evident, therefore, that the possession of a disagreeable taste or odour is a more effectual protection to certain conspicuous caterpillars and moths, than would be even the possession of a sting. the observations of these two gentlemen supply a very remarkable confirmation of the hypothetical solution of the difficulty which i had given two years before. and as it is generally acknowledged that the best test of the truth and completeness of a theory is the power which it gives us of prevision, we may i think fairly claim this as a case in which the power of prevision has been successfully exerted, and therefore as furnishing a very powerful argument in favour of the truth of the theory of natural selection. _summary._ i have now completed a brief, and necessarily very imperfect, survey of the various ways in which the external form and colouring of animals is adapted to be useful to them, either by concealing them from their enemies or from the creatures they prey upon. it has, i hope, been shown that the subject is one of much interest, both as regard a true comprehension of the place each animal fills in the economy of nature, and the means by which it is enabled to maintain that place; and also as teaching us how important a part is played by the minutest details in the structure of animals, and how complicated and delicate is the equilibrium of the organic world. my exposition of the subject having been necessarily somewhat lengthy and full of details, it will be as well to recapitulate its main points. there is a general harmony in nature between the colours of an animal and those of its habitation. arctic animals are white, desert animals are sand-coloured; dwellers among leaves and grass are green; nocturnal animals are dusky. these colours are not universal, but are very general, and are seldom reversed. going on a little further, we find birds, reptiles, and insects, so tinted and mottled as exactly to match the rock, or bark, or leaf, or flower, they are accustomed to rest upon,--and thereby effectually concealed. another step in advance, and we have insects which are formed as well as coloured so as exactly to resemble particular leaves, or sticks, or mossy twigs, or flowers; and in these cases very peculiar habits and instincts come into play to aid in the deception and render the concealment more complete. we now enter upon a new phase of the phenomena, and come to creatures whose colours neither conceal them nor make them like vegetable or mineral substances; on the contrary, they are conspicuous enough, but they completely resemble some other creature of a quite different group, while they differ much in outward appearance from those with which all essential parts of their organization show them to be really closely allied. they appear like actors or masqueraders dressed up and painted for amusement, or like swindlers endeavouring to pass themselves off for well-known and respectable members of society. what is the meaning of this strange travestie? does nature descend to imposture or masquerade? we answer, she does not. her principles are too severe. there is a use in every detail of her handiwork. the resemblance of one animal to another is of exactly the same essential nature as the resemblance to a leaf, or to bark, or to desert sand, and answers exactly the same purpose. in the one case the enemy will not attack the leaf or the bark, and so the disguise is a safeguard; in the other case it is found that for various reasons the creature resembled is passed over, and not attacked by the usual enemies of its order, and thus the creature that resembles it has an equally effectual safeguard. we are plainly shown that the disguise is of the same nature in the two cases, by the occurrence in the same group of one species resembling a vegetable substance, while another resembles a living animal of another group; and we know that the creatures resembled, possess an immunity from attack, by their being always very abundant, by their being conspicuous and not concealing themselves, and by their having generally no visible means of escape from their enemies; while, at the same time, the particular quality that makes them disliked is often very clear, such as a nasty taste or an indigestible hardness. further examination reveals the fact that, in several cases of both kinds of disguise, it is the female only that is thus disguised; and as it can be shown that the female needs protection much more than the male, and that her preservation for a much longer period is absolutely necessary for the continuance of the race, we have an additional indication that the resemblance is in all cases subservient to a great purpose--the preservation of the species. in endeavouring to explain these phenomena as having been brought about by variation and natural selection, we start with the fact that white varieties frequently occur, and when protected from enemies show no incapacity for continued existence and increase. we know, further, that varieties of many other tints occasionally occur; and as "the survival of the fittest" must inevitably weed out those whose colours are prejudicial and preserve those whose colours are a safeguard, we require no other mode of accounting for the protective tints of arctic and desert animals. but this being granted, there is such a perfectly continuous and graduated series of examples of every kind of protective imitation, up to the most wonderful cases of what is termed "mimicry," that we can find no place at which to draw the line, and say,--so far variation and natural selection will account for the phenomena, but for all the rest we require a more potent cause. the counter theories that have been proposed, that of the "special creation" of each imitative form, that of the action of "similar conditions of existence" for some of the cases, and of the laws of "hereditary descent and the reversion to ancestral forms" for others,--have all been shown to be beset with difficulties, and the two latter to be directly contradicted by some of the most constant and most remarkable of the facts to be accounted for. _general deductions as to colour in nature._ the important part that "protective resemblance" has played in determining the colours and markings of many groups of animals, will enable us to understand the meaning of one of the most striking facts in nature, the uniformity in the colours of the vegetable as compared with the wonderful diversity of the animal world. there appears no good reason why trees and shrubs should not have been adorned with as many varied hues and as strikingly designed patterns as birds and butterflies, since the gay colours of flowers show that there is no incapacity in vegetable tissues to exhibit them. but even flowers themselves present us with none of those wonderful designs, those complicated arrangements of stripes and dots and patches of colour, that harmonious blending of hues in lines and bands and shaded spots, which are so general a feature in insects. it is the opinion of mr. darwin that we owe much of the beauty of flowers to the necessity of attracting insects to aid in their fertilisation, and that much of the development of colour in the animal world is due to "sexual selection," colour being universally attractive, and thus leading to its propagation and increase; but while fully admitting this, it will be evident from the facts and arguments here brought forward, that very much of the _variety_ both of colour and markings among animals is due to the supreme importance of concealment, and thus the various tints of minerals and vegetables have been directly reproduced in the animal kingdom, and again and again modified as more special protection became necessary. we shall thus have two causes for the development of colour in the animal world, and shall be better enabled to understand how, by their combined and separate action, the immense variety we now behold has been produced. both causes, however, will come under the general law of "utility," the advocacy of which, in its broadest sense, we owe almost entirely to mr. darwin. a more accurate knowledge of the varied phenomena connected with this subject may not improbably give us some information both as to the senses and the mental faculties of the lower animals. for it is evident that if colours which please us also attract them, and if the various disguises which have been here enumerated are equally deceptive to them as to ourselves, then both their powers of vision and their faculties of perception and emotion, must be essentially of the same nature as our own--a fact of high philosophical importance in the study of our own nature and our true relations to the lower animals. _conclusion._ although such a variety of interesting facts have been already accumulated, the subject we have been discussing is one of which comparatively little is really known. the natural history of the tropics has never yet been studied on the spot with a full appreciation of "what to observe" in this matter. the varied ways in which the colouring and form of animals serve for their protection, their strange disguises as vegetable or mineral substances, their wonderful mimicry of other beings, offer an almost unworked and inexhaustible field of discovery for the zoologist, and will assuredly throw much light on the laws and conditions which have resulted in the wonderful variety of colour, shade, and marking which constitutes one of the most pleasing characteristics of the animal world, but the immediate causes of which it has hitherto been most difficult to explain. if i have succeeded in showing that in this wide and picturesque domain of nature, results which have hitherto been supposed to depend either upon those incalculable combinations of laws which we term chance or upon the direct volition of the creator, are really due to the action of comparatively well-known and simple causes, i shall have attained my present purpose, which has been to extend the interest so generally felt in the more striking facts of natural history to a large class of curious but much neglected details; and to further, in however slight a degree, our knowledge of the subjection of the phenomena of life to the "reign of law." iv. the malayan papilionidÆ or swallow-tailed butterflies, as illustrative of the theory of natural selection. _special value of the diurnal lepidoptera for enquiries of this nature._ when the naturalist studies the habits, the structure, or the affinities of animals, it matters little to which group he especially devotes himself; all alike offer him endless materials for observation and research. but, for the purpose of investigating the phenomena of geographical distribution and of local, sexual, or general variation, the several groups differ greatly in their value and importance. some have too limited a range, others are not sufficiently varied in specific forms, while, what is of most importance, many groups have not received that amount of attention over the whole region they inhabit, which could furnish materials sufficiently approaching to completeness to enable us to arrive at any accurate conclusions as to the phenomena they present as a whole. it is in those groups which are, and have long been, favourites with collectors, that the student of distribution and variation will find his materials the most satisfactory, from their comparative completeness. pre-eminent among such groups are the diurnal lepidoptera or butterflies, whose extreme beauty and endless diversity have led to their having been assiduously collected in all parts of the world, and to the numerous species and varieties having been figured in a series of magnificent works, from those of cramer, the contemporary of linnæus, down to the inimitable productions of our own hewitson.[g] but, besides their abundance, their universal distribution, and the great attention that has been paid to them, these insects have other qualities that especially adapt them to elucidate the branches of inquiry already alluded to. these are, the immense development and peculiar structure of the wings, which not only vary in form more than those of any other insects, but offer on both surfaces an endless variety of pattern, colouring, and texture. the scales, with which they are more or less completely covered, imitate the rich hues and delicate surfaces of satin or of velvet, glitter with metallic lustre, or glow with the changeable tints of the opal. this delicately painted surface acts as a register of the minutest differences of organization--a shade of colour, an additional streak or spot, a slight modification of outline continually recurring with the greatest regularity and fixity, while the body and all its other members exhibit no appreciable change. the wings of butterflies, as mr. bates has well put it, "serve as a tablet on which nature writes the story of the modifications of species;" they enable us to perceive changes that would otherwise be uncertain and difficult of observation, and exhibit to us on an enlarged scale the effects of the climatal and other physical conditions which influence more or less profoundly the organization of every living thing. +--------------------------------------------------------------+ | [g] w. c. hewitson, esq., of oatlands, walton-on-thames, | | author of "exotic butterflies" and several other works, | | illustrated by exquisite coloured figures drawn by himself; | | and owner of the finest collection of butterflies in the | | world. | +--------------------------------------------------------------+ a proof that this greater sensibility to modifying causes is not imaginary may, i think, be drawn from the consideration, that while the lepidoptera as a whole are of all insects the least essentially varied in form, structure, or habits, yet in the number of their specific forms they are not much inferior to those orders which range over a much wider field of nature, and exhibit more deeply seated structural modifications. the lepidoptera are all vegetable-feeders in their larva-state, and suckers of juices or other liquids in their perfect form. in their most widely separated groups they differ but little from a common type, and offer comparatively unimportant modifications of structure or of habits. the coleoptera, the diptera, or the hymenoptera, on the other hand, present far greater and more essential variations. in either of these orders we have both vegetable and animal-feeders, aquatic, and terrestrial, and parasitic groups. whole families are devoted to special departments in the economy of nature. seeds, fruits, bones, carcases, excrement, bark, have each their special and dependent insect tribes from among them; whereas the lepidoptera are, with but few exceptions, confined to the one function of devouring the foliage of living vegetation. we might therefore anticipate that their species--population would be only equal to that of sections of the other orders having a similar uniform mode of existence; and the fact that their numbers are at all comparable with those of entire orders, so much more varied in organization and habits, is, i think, a proof that they are in general highly susceptible of specific modification. _question of the rank of the papilionidæ._ the papilionidæ are a family of diurnal lepidoptera which have hitherto, by almost universal consent, held the first rank in the order; and though this position has recently been denied them, i cannot altogether acquiesce in the reasoning by which it has been proposed to degrade them to a lower rank. in mr. bates's most excellent paper on the heliconidæ, (published in the transactions of the linnæan society, vol. xxiii., p. ) he claims for that family the highest position, chiefly because of the imperfect structure of the fore legs, which is there carried to an extreme degree of abortion, and thus removes them further than any other family from the hesperidæ and heterocera, which all have perfect legs. now it is a question whether any amount of difference which is exhibited merely in the imperfection or abortion of certain organs, can establish in the group exhibiting it a claim to a high grade of organization, still less can this be allowed when another group along with perfection of structure in the same organs, exhibits modifications peculiar to it, together with the possession of an organ which in the remainder of the order is altogether wanting. this is, however, the position of the papilionidæ. the perfect insects possess two characters quite peculiar to them. mr. edward doubleday, in his "genera of diurnal lepidoptera," says, "the papilionidæ may be known by the apparently four-branched median nervule and the spur on the anterior tibiæ, characters found in no other family." the four-branched median nervule is a character so constant, so peculiar, and so well marked, as to enable a person to tell, at a glance at the wings only of a butterfly, whether it does or does not belong to this family; and i am not aware that any other group of butterflies, at all comparable to this in extent and modifications of form, possesses a character in its neuration to which the same degree of certainty can be attached. the spur on the anterior tibiæ is also found in some of the hesperidæ, and is therefore supposed to show a direct affinity between the two groups: but i do not imagine it can counterbalance the differences in neuration and in every other part of their organization. the most characteristic feature of the papilionidæ, however, and that on which i think insufficient stress has been laid, is undoubtedly the peculiar structure of the larvæ. these all possess an extraordinary organ situated on the neck, the well-known y-shaped tentacle, which is entirely concealed in a state of repose, but which is capable of being suddenly thrown out by the insect when alarmed. when we consider this singular apparatus, which in some species is nearly half an inch long, the arrangement of muscles for its protrusion and retraction, its perfect concealment during repose, its blood-red colour, and the suddenness with which it can be thrown out, we must, i think, be led to the conclusion that it serves as a protection to the larva, by startling and frightening away some enemy when about to seize it, and is thus one of the causes which has led to the wide extension and maintained the permanence of this now dominant group. those who believe that such peculiar structures can only have arisen by very minute successive variations, each one advantageous to its possessor, must see, in the possession of such an organ by one group, and its complete absence in every other, a proof of a very ancient origin and of very long-continued modification. and such a positive structural addition to the organization of the family, subserving an important function, seems to me alone sufficient to warrant us in considering the papilionidæ as the most highly developed portion of the whole order, and thus in retaining it in the position which the size, strength, beauty, and general structure of the perfect insects have been generally thought to deserve. in mr. trimen's paper on "mimetic analogies among african butterflies," in the transactions of the linnæan society, for , he has argued strongly in favour of mr. bates' views as to the higher position of the danaidæ and the lower grade of the papilionidæ, and has adduced, among other facts, the undoubted resemblance of the pupa of parnassius, a genus of papilionidæ, to that of some hesperidæ and moths. i admit, therefore, that he has proved the papilionidæ to have retained several characters of the nocturnal lepidoptera which the danaidæ have lost, but i deny that they are therefore to be considered lower in the scale of organization. other characters may be pointed out which indicate that they are farther removed from the moths even than the danaidæ. the club of the antennæ is the most prominent and most constant feature by which butterflies may be distinguished from moths, and of all butterflies the papilionidæ have the most beautiful and most perfectly developed clubbed antennæ. again, butterflies and moths are broadly characterised by their diurnal and nocturnal habits respectively, and the papilionidæ, with their close allies the pieridæ, are the most pre-eminently diurnal of butterflies, most of them lovers of sunshine, and not presenting a single crepuscular species. the great group of the nymphalidæ, on the other hand (in which mr. bates includes the danaidæ and heliconidæ as sub-families), contains an entire sub-family (brassolidæ) and a number of genera, such as thaumantis, zeuxidia, pavonia, &c., of crepuscular habits, while a large proportion of the satyridæ and many of the danaidæ are shade-loving butterflies. this question, of what is to be considered the highest type of any group of organisms, is one of such general interest to naturalists that it will be well to consider it a little further, by a comparison of the lepidoptera with some groups of the higher animals. mr. trimen's argument, that the lepidopterous type, like that of birds, being pre-eminently aërial, "therefore a diminution of the ambulatory organs, instead of being a sign of inferiority, may very possibly indicate a higher, because a more thoroughly aërial form," is certainly unsound, for it would imply that the most aërial of birds (the swift and the frigate-birds, for example) are the highest in the scale of bird-organization, and the more so on account of their feet being very ill adapted for walking. but no ornithologist has ever so classed them, and the claim to the highest rank among birds is only disputed between three groups, all very far removed from these. they are-- st. the falcons, on account of their general perfection, their rapid flight, their piercing vision, their perfect feet armed with retractile claws, the beauty of their forms, and the ease and rapidity of their motions; nd. the parrots, whose feet, though ill-fitted for walking, are perfect as prehensile organs, and which possess large brains with great intelligence, though but moderate powers of flight; and, rd. the thrushes or crows, as typical of the perching birds, on account of the well-balanced development of their whole structure, in which no organ or function has attained an undue prominence. turning now to the mammalia, it might be argued that as they are pre-eminently the terrestrial type of vertebrates, to walk and run well is essential to the typical perfection of the group; but this would give the superiority to the horse, the deer, or the hunting leopard, instead of to the quadrumana. we seem here to have quite a case in point, for one group of quadrumana, the lemurs, is undoubtedly nearer to the low insectivora and marsupials than the carnivora or the ungulata, as shown among other characters by the opossums possessing a hand with perfect opposable thumb, closely resembling that of some of the lemurs; and by the curious galeopithecus, which is sometimes classed as a lemur, and sometimes with the insectivora. again, the implacental mammals, including the ornithodelphia and the marsupials, are admitted to be lower than the placental series. but one of the distinguishing characters of the marsupials is that the young are born blind and exceedingly imperfect, and it might therefore be argued that those orders in which the young are born most perfect are the highest, because farthest from the low marsupial type. this would make the ruminants and ungulata higher than the quadrumana or the carnivora. but the mammalia offer a still more remarkable illustration of the fallacy of this mode of reasoning, for if there is one character more than another which is essential and distinctive of the class, it is that from which it derives its name, the possession of mammary glands and the power of suckling the young. what more reasonable, apparently, than to argue that the group in which this important function is most developed, that in which the young are most dependent upon it, and for the longest period, must be the highest in the mammalian scale of organization? yet this group is the marsupial, in which the young commence suckling in a foetal condition, and continue to do so till they are fully developed, and are therefore for a long time absolutely dependent on this mode of nourishment. these examples, i think, demonstrate that we cannot settle the rank of a group by a consideration of the degree in which certain characters resemble or differ from those in what is admitted to be a lower group; and they also show that the highest group of a class may be more closely connected to one of the lowest, than some other groups which have developed laterally and diverged farther from the parent type, but which yet, owing to want of balance or too great specialization in their structure, have never reached a high grade of organization. the quadrumana afford a very valuable illustration, because, owing to their undoubted affinity with man, we feel certain that they are really higher than any other order of mammalia, while at the same time they are more distinctly allied to the lowest groups than many others. the case of the papilionidæ seems to me so exactly parallel to this, that, while i admit all the proofs of affinity with the undoubtedly lower groups of hesperidæ and moths, i yet maintain that, owing to the complete and even development of every part of their organization, these insects best represent the highest perfection to which the butterfly type has attained, and deserve to be placed at its head in every system of classification. _distribution of the papilionidæ._ the papilionidæ are pretty widely distributed over the earth, but are especially abundant in the tropics, where they attain their maximum of size and beauty, and the greatest variety of form and colouring. south america, north india, and the malay islands are the regions where these fine insects occur in the greatest profusion, and where they actually become a not unimportant feature in the scenery. in the malay islands in particular, the giant ornithopteræ may be frequently seen about the borders of the cultivated and forest districts, their large size, stately flight, and gorgeous colouring rendering them even more conspicuous than the generality of birds. in the shady suburbs of the town of malacca two large and handsome papilios (memnon and nephelus) are not uncommon, flapping with irregular flight along the roadways, or, in the early morning, expanding their wings to the invigorating rays of the sun. in amboyna and other towns of the moluccas, the magnificent deiphobus and severus, and occasionally even the azure-winged ulysses, frequent similar situations, fluttering about the orange-trees and flower-beds, or sometimes even straying into the narrow bazaars or covered markets of the city. in java the golden-dusted arjuna may often be seen at damp places on the roadside in the mountain districts, in company with sarpedon, bathycles, and agamemnon, and less frequently the beautiful swallow-tailed antiphates. in the more luxuriant parts of these islands one can hardly take a morning's walk in the neighbourhood of a town or village without seeing three or four species of papilio, and often twice that number. no less than species of the family are now known to inhabit the archipelago, and of these ninety-six were collected by myself. thirty species are found in borneo, being the largest number in any one island, twenty-three species having been obtained by myself in the vicinity of sarawak; java has twenty-eight species; celebes twenty-four, and the peninsula of malacca, twenty-six species. further east the numbers decrease; batchian producing seventeen, and new guinea only fifteen, though this number is certainly too small, owing to our present imperfect knowledge of that great island. _definition of the word species._ in estimating these numbers i have had the usual difficulty to encounter, of determining what to consider species and what varieties. the malayan region, consisting of a large number of islands of generally great antiquity, possesses, compared to its actual area, a great number of distinct forms, often indeed distinguished by very slight characters, but in most cases so constant in large series of specimens, and so easily separable from each other, that i know not on what principle we can refuse to give them the name and rank of species. one of the best and most orthodox definitions is that of pritchard, the great ethnologist, who says, that "_separate origin and distinctness of race, evinced by a constant transmission of some characteristic peculiarity of organization_," constitutes a species. now leaving out the question of "origin," which we cannot determine, and taking only the proof of separate origin, "_the constant transmission of some characteristic peculiarity of organization_," we have a definition which will compel us to neglect altogether the _amount_ of difference between any two forms, and to consider only whether the differences that present themselves are _permanent_. the rule, therefore, i have endeavoured to adopt is, that when the difference between two forms inhabiting separate areas seems quite constant, when it can be defined in words, and when it is not confined to a single peculiarity only, i have considered such forms to be species. when, however, the individuals of each locality vary among themselves, so as to cause the distinctions between the two forms to become inconsiderable and indefinite, or where the differences, though constant, are confined to one particular only, such as size, tint, or a single point of difference in marking or in outline, i class one of the forms as a variety of the other. i find as a general rule that the constancy of species is in an inverse ratio to their range. those which are confined to one or two islands are generally very constant. when they extend to many islands, considerable variability appears; and when they have an extensive range over a large part of the archipelago, the amount of unstable variation is very large. these facts are explicable on mr. darwin's principles. when a species exists over a wide area, it must have had, and probably still possesses, great powers of dispersion. under the different conditions of existence in various portions of its area, different variations from the type would be selected, and, were they completely isolated, would soon become distinctly modified forms; but this process is checked by the dispersive powers of the whole species, which leads to the more or less frequent intermixture of the incipient varieties, which thus become irregular and unstable. where, however, a species has a limited range, it indicates less active powers of dispersion, and the process of modification under changed conditions is less interfered with. the species will therefore exist under one or more permanent forms according as portions of it have been isolated at a more or less remote period. _laws and modes of variation._ what is commonly called variation consists of several distinct phenomena which have been too often confounded. i shall proceed to consider these under the heads of-- st, simple variability; nd, polymorphism; rd, local forms; th, co-existing varieties; th, races or subspecies; and th, true species. . _simple variability._--under this head i include all those cases in which the specific form is to some extent unstable. throughout the whole range of the species, and even in the progeny of individuals, there occur continual and uncertain differences of form, analogous to that variability which is so characteristic of domestic breeds. it is impossible usefully to define any of these forms, because there are indefinite gradations to each other form. species which possess these characteristics have always a wide range, and are more frequently the inhabitants of continents than of islands, though such cases are always exceptional, it being far more common for specific forms to be fixed within very narrow limits of variation. the only good example of this kind of variability which occurs among the malayan papilionidæ is in papilio severus, a species inhabiting all the islands of the moluccas and new guinea, and exhibiting in each of them a greater amount of individual difference than often serves to distinguish well-marked species. almost equally remarkable are the variations exhibited in most of the species of ornithoptera, which i have found in some cases to extend even to the form of the wing and the arrangement of the nervures. closely allied, however, to these variable species are others which, though differing slightly from them, are constant and confined to limited areas. after satisfying oneself, by the examination of numerous specimens captured in their native countries, that the one set of individuals are variable and the others are not, it becomes evident that by classing all alike as varieties of one species we shall be obscuring an important fact in nature; and that the only way to exhibit that fact in its true light is to treat the invariable local form as a distinct species, even though it does not offer better distinguishing characters than do the extreme forms of the variable species. cases of this kind are the ornithoptera priamus, which is confined to the islands of ceram and amboyna, and is very constant in both sexes, while the allied species inhabiting new guinea and the papuan islands is exceedingly variable; and in the island of celebes is a species closely allied to the variable p. severus, but which, being exceedingly constant, i have described as a distinct species under the name of papilio pertinax. . _polymorphism or dimorphism._--by this term i understand the co-existence in the same locality of two or more distinct forms, not connected by intermediate gradations, and all of which are occasionally produced from common parents. these distinct forms generally occur in the female sex only, and their offspring, instead of being hybrids, or like the two parents, appear to reproduce all the distinct forms in varying proportions. i believe it will be found that a considerable number of what have been classed as _varieties_ are really cases of polymorphism. albinoism and melanism are of this character, as well as most of those cases in which well-marked varieties occur in company with the parent species, but without any intermediate forms. if these distinct forms breed independently, and are never reproduced from a common parent, they must be considered as separate species, contact without intermixture being a good test of specific difference. on the other hand, intercrossing without producing an intermediate race is a test of dimorphism. i consider, therefore, that under any circumstances the term "variety" is wrongly applied to such cases. the malayan papilionidæ exhibit some very curious instances of polymorphism, some of which have been recorded as varieties, others as distinct species; and they all occur in the female sex. papilio memnon is one of the most striking, as it exhibits the mixture of simple variability, local and polymorphic forms, all hitherto classed under the common title of varieties. the polymorphism is strikingly exhibited by the females, one set of which resemble the males in form, with a variable paler colouring; the others have a large spatulate tail to the hinder wings and a distinct style of colouring, which causes them closely to resemble p. coon, a species having the two sexes alike and inhabiting the same countries, but with which they have no direct affinity. the tailless females exhibit simple variability, scarcely two being found exactly alike even in the same locality. the males of the island of borneo exhibit constant differences of the under surface, and may therefore be distinguished as a local form, while the continental specimens, as a whole, offer such large and constant differences from those of the islands, that i am inclined to separate them as a distinct species, to which the name p. androgeus (cramer) may be applied. we have here, therefore, distinct species, local forms, polymorphism, and simple variability, which seem to me to be distinct phenomena, but which have been hitherto all classed together as varieties. i may mention that the fact of these distinct forms being one species is doubly proved. the males, the tailed and tailless females, have all been bred from a single group of the larvæ, by messrs. payen and bocarmé, in java, and i myself captured, in sumatra, a male p. memnon, and a tailed female p. achates, under circumstances which led me to class them as the same species. papilio pammon offers a somewhat similar case. the female was described by linnæus as p. polytes, and was considered to be a distinct species till westermann bred the two from the same larvæ (see boisduval, "species général des lépidoptères," p. ). they were therefore classed as sexes of one species by mr. edward doubleday, in his "genera of diurnal lepidoptera," in . later, female specimens were received from india closely resembling the male insect, and this was held to overthrow the authority of m. westermann's observation, and to re-establish p. polytes as a distinct species; and as such it accordingly appears in the british museum list of papilionidæ in , and in the catalogue of the east india museum in . this discrepancy is explained by the fact of p. pammon having two females, one closely resembling the male, while the other is totally different from it. a long familiarity with this insect (which replaced by local forms or by closely allied species, occurs in every island of the archipelago) has convinced me of the correctness of this statement; for in every place where a male allied to p. pammon is found, a female resembling p. polytes also occurs, and sometimes, though less frequently than on the continent, another female closely resembling the male: while not only has no male specimen of p. polytes yet been discovered, but the female (polytes) has never yet been found in localities to which the male (pammon) does not extend. in this case, as in the last, distinct species, local forms, and dimorphic specimens, have been confounded under the common appellation of varieties. but, besides the true p. polytes, there are several allied forms of females to be considered, namely, p. theseus (cramer), p. molanides (de haan), p. elyros (g. r. gray), and p. romulus (linnæus). the dark female figured by cramer as p. theseus seems to be the common and perhaps the only form in sumatra, whereas in java, borneo, and timor, along with males quite identical with those of sumatra, occur females of the polytes form, although a single specimen of the true p. theseus taken at lombock would seem to show that the two forms do occur together. in the allied species found in the philippine islands (p. alphenor, cramer = p. ledebouria, eschscholtz, the female of which is p. elyros, g. r. gray,) forms corresponding to these extremes occur, along with a number of intermediate varieties, as shown by a fine series in the british museum. we have here an indication of how dimorphism may be produced; for let the extreme philippine forms be better suited to their conditions of existence than the intermediate connecting links, and the latter will gradually die out, leaving two distinct forms of the same insect, each adapted to some special conditions. as these conditions are sure to vary in different districts, it will often happen, as in sumatra and java, that the one form will predominate in the one island, the other in the adjacent one. in the island of borneo there seems to be a third form; for p. melanides (de haan) evidently belongs to this group, and has all the chief characteristics of p. theseus, with a modified colouration of the hind wings. i now come to an insect which, if i am correct, offers one of the most interesting cases of variation yet adduced. papilio romulus, a butterfly found over a large part of india and ceylon, and not uncommon in collections, has always been considered a true and independent species, and no suspicions have been expressed regarding it. but a male of this form does not, i believe, exist. i have examined the fine series in the british museum, in the east india company's museum, in the hope museum at oxford, in mr. hewitson's and several other private collections, and can find nothing but females; and for this common butterfly no male partner can be found except the equally common p. pammon, a species already provided with two wives, and yet to whom we shall be forced, i believe, to assign a third. on carefully examining p. romulus, i find that in all essential characters--the form and texture of the wings, the length of the antennæ, the spotting of the head and thorax, and even the peculiar tints and shades with which it is ornamented--it corresponds exactly with the other females of the pammon group; and though, from the peculiar marking of the fore wings, it has at first sight a very different aspect, yet a closer examination shows that every one of its markings could be produced by slight and almost imperceptible modifications of the various allied forms. i fully believe, therefore, that i shall be correct in placing p. romulus as a third indian form of the female p. pammon, corresponding to p. melanides, the third form of the malayan p. theseus. i may mention here that the females of this group have a superficial resemblance to the polydorus group of papilios, as shown by p. theseus having been considered to be the female of p. antiphus, and by p. romulus being arranged next to p. hector. there is no close affinity between these two groups of papilio, and i am disposed to believe that we have here a case of mimicry, brought about by the same causes which mr. bates has so well explained in his account of the heliconidæ, and which has led to the singular exuberance of polymorphic forms in this and allied groups of the genus papilio. i shall have to devote a section of my essay to the consideration of this subject. the third example of polymorphism i have to bring forward is papilio ormenus, which is closely allied to the well-known p. erechtheus, of australia. the most common form of the female also resembles that of p. erechtheus; but a totally different-looking insect was found by myself in the aru islands, and figured by mr. hewitson under the name of p. onesimus, which subsequent observation has convinced me is a second form of the female of p. ormenus. comparison of this with boisduval's description of p. amanga, a specimen of which from new guinea is in the paris museum, shows the latter to be a closely similar form; and two other specimens were obtained by myself, one in the island of goram and the other in waigiou, all evidently local modifications of the same form. in each of these localities males and ordinary females of p. ormenus were also found. so far there is no evidence that these light-coloured insects are not females of a distinct species, the males of which have not been discovered. but two facts have convinced me this is not the case. at dorey, in new guinea, where males and ordinary females closely allied to p. ormenus occur (but which seem to me worthy of being separated as a distinct species), i found one of these light-coloured females closely followed in her flight by three males, exactly in the same manner as occurs (and, i believe, occurs only) with the sexes of the same species. after watching them a considerable time, i captured the whole of them, and became satisfied that i had discovered the true relations of this anomalous form. the next year i had corroborative proof of the correctness of this opinion by the discovery in the island of batchian of a new species allied to p. ormenus, all the females of which, either seen or captured by me, were of one form, and much more closely resembling the abnormal light-coloured females of p. ormenus and p. pandion than the ordinary specimens of that sex. every naturalist will, i think, agree that this is strongly confirmative of the supposition that both forms of female are of one species; and when we consider, further, that in four separate islands, in each of which i resided for several months, the two forms of female were obtained and only one form of male ever seen, and that about the same time, m. montrouzier in woodlark island, at the other extremity of new guinea (where he resided several years, and must have obtained all the large lepidoptera of the island), obtained females closely resembling mine, which, in despair at finding no appropriate partners for them, he mates with a widely different species--it becomes, i think, sufficiently evident this is another case of polymorphism of the same nature as those already pointed out in p. pammon and p. memnon. this species, however, is not only dimorphic, but trimorphic; for, in the island of waigiou, i obtained a third female quite distinct from either of the others, and in some degree intermediate between the ordinary female and the male. the specimen is particularly interesting to those who believe, with mr. darwin, that extreme difference of the sexes has been gradually produced by what he terms sexual selection, since it may be supposed to exhibit one of the intermediate steps in that process, which has been accidentally preserved in company with its more favoured rivals, though its extreme rarity (only one specimen having been seen to many hundreds of the other form) would indicate that it may soon become extinct. the only other case of polymorphism in the genus papilio, at all equal in interest to those i have now brought forward, occurs in america; and we have, fortunately, accurate information about it. papilio turnus is common over almost the whole of temperate north america; and the female resembles the male very closely. a totally different-looking insect both in form and colour, papilio glaucus, inhabits the same region; and though, down to the time when boisduval published his "species général," no connexion was supposed to exist between the two species, it is now well ascertained that p. glaucus is a second female form of p. turnus. in the "proceedings of the entomological society of philadelphia," jan., , mr. walsh gives a very interesting account of the distribution of this species. he tells us that in the new england states and in new york all the females are yellow, while in illinois and further south all are black; in the intermediate region both black and yellow females occur in varying proportions. lat. ° is approximately the southern limit of the yellow form, and ° the northern limit of the black form; and, to render the proof complete, both black and yellow insects have been bred from a single batch of eggs. he further states that, out of thousands of specimens, he has never seen or heard of intermediate varieties between these forms. in this interesting example we see the effects of latitude in determining the proportions in which the individuals of each form should exist. the conditions are _here_ favourable to the one form, _there_ to the other; but we are by no means to suppose that these conditions consist in climate alone. it is highly probable that the existence of enemies, and of competing forms of life, may be the main determining influences; and it is much to be wished that such a competent observer as mr. walsh would endeavour to ascertain what are the adverse causes which are most efficient in keeping down the numbers of each of these contrasted forms. dimorphism of this kind in the animal kingdom does not seem to have any direct relations to the reproductive powers, as mr. darwin has shown to be the case in plants, nor does it appear to be very general. one other case only is known to me in another family of my eastern lepidoptera, the pieridæ; and but few occur in the lepidoptera of other countries. the spring and autumn broods of some european species differ very remarkably; and this must be considered as a phenomenon of an analogous though not of an identical nature, while the araschnia prorsa, of central europe, is a striking example of this alternate or seasonal dimorphism. among our nocturnal lepidoptera, i am informed, many analogous cases occur; and as the whole history of many of these has been investigated by breeding successive generations from the egg, it is to be hoped that some of our british lepidopterists will give us a connected account of all the abnormal phenomena which they present. among the coleoptera mr. pascoe has pointed out the existence of two forms of the male sex in seven species of the two genera xenocerus and mecocerus belonging to the family anthribidæ, (proc. ent. soc. lond., ); and no less than six european water-beetles, of the genus dytiscus, have females of two forms, the most common having the elytra deeply sulcate, the rarer smooth as in the males. the three, and sometimes four or more, forms under which many hymenopterous insects (especially ants) occur, must be considered as a related phenomenon, though here each form is specialized to a distinct function in the economy of the species. among the higher animals, albinoism and melanism may, as i have already stated, be considered as analogous facts; and i met with one case of a bird, a species of lory (eos fuscata), clearly existing under two differently coloured forms, since i obtained both sexes of each from a single flock, while no intermediate specimens have yet been found. the fact of the two sexes of one species differing very considerably is so common, that it attracted but little attention till mr. darwin showed how it could in many cases be explained by the principle of sexual selection. for instance, in most polygamous animals the males fight for the possession of the females, and the victors, always becoming the progenitors of the succeeding generation, impress upon their male offspring their own superior size, strength, or unusually developed offensive weapons. it is thus that we can account for the spurs and the superior strength and size of the males in gallinaceous birds, and also for the large canine tusks in the males of fruit-eating apes. so the superior beauty of plumage and special adornments of the males of so many birds can be explained by supposing (what there are many facts to prove) that the females prefer the most beautiful and perfect-plumaged males, and that thus, slight accidental variations of form and colour have been accumulated, till they have produced the wonderful train of the peacock and the gorgeous plumage of the bird of paradise. both these causes have no doubt acted partially in insects, so many species possessing horns and powerful jaws in the male sex only, and still more frequently the males alone rejoicing in rich colours or sparkling lustre. but there is here another cause which has led to sexual differences, viz., a special adaptation of the sexes to diverse habits or modes of life. this is well seen in female butterflies (which are generally weaker and of slower flight), often having colours better adapted to concealment; and in certain south american species (papilio torquatus) the females, which inhabit the forests, resemble the Æneas group of papilios which abound in similar localities, while the males, which frequent the sunny open river-banks, have a totally different colouration. in these cases, therefore, natural selection seems to have acted independently of sexual selection; and all such cases may be considered as examples of the simplest dimorphism, since the offspring never offer intermediate varieties between the parent forms. the phenomena of dimorphism and polymorphism may be well illustrated by supposing that a blue-eyed, flaxen-haired saxon man had two wives, one a black-haired, red-skinned indian squaw, the other a woolly-headed, sooty-skinned negress--and that instead of the children being mulattoes of brown or dusky tints, mingling the separate characteristics of their parents in varying degrees, all the boys should be pure saxon boys like their father, while the girls should altogether resemble their mothers. this would be thought a sufficiently wonderful fact; yet the phenomena here brought forward as existing in the insect-world are still more extraordinary; for each mother is capable not only of producing male offspring like the father, and female like herself, but also of producing other females exactly like her fellow-wife, and altogether differing from herself. if an island could be stocked with a colony of human beings having similar physiological idiosyncrasies with papilio pammon or papilio ormenus, we should see white men living with yellow, red, and black women, and their offspring always reproducing the same types; so that at the end of many generations the men would remain pure white, and the women of the same well-marked races as at the commencement. the distinctive character therefore of dimorphism is this, that the union of these distinct forms does not produce intermediate varieties, but reproduces the distinct forms unchanged. in simple varieties, on the other hand, as well as when distinct local forms or distinct species are crossed, the offspring never resembles either parent exactly, but is more or less intermediate between them. dimorphism is thus seen to be a specialized result of variation, by which new physiological phenomena have been developed; the two should therefore, whenever possible, be kept separate. . _local form, or variety._--this is the first step in the transition from variety to species. it occurs in species of wide range, when groups of individuals have become partially isolated in several points of its area of distribution, in each of which a characteristic form has become more or less completely segregated. such forms are very common in all parts of the world, and have often been classed by one author as varieties, by another as species. i restrict the term to those cases where the difference of the forms is very slight, or where the segregation is more or less imperfect. the best example in the present group is papilio agamemnon, a species which ranges over the greater part of tropical asia, the whole of the malay archipelago, and a portion of the australian and pacific regions. the modifications are principally of size and form, and, though slight, are tolerably constant in each locality. the steps, however, are so numerous and gradual that it would be impossible to define many of them, though the extreme forms are sufficiently distinct. papilio sarpedon presents somewhat similar but less numerous variations. . _co-existing variety._--this is a somewhat doubtful case. it is when a slight but permanent and hereditary modification of form exists in company with the parent or typical form, without presenting those intermediate gradations which would constitute it a case of simple variability. it is evidently only by direct evidence of the two forms breeding separately that this can be distinguished from dimorphism. the difficulty occurs in papilio jason, and p. evemon, which inhabit the same localities, and are almost exactly alike in form, size, and colouration, except that the latter always wants a very conspicuous red spot on the under surface, which is found not only in p. jason, but in all the allied species. it is only by breeding the two insects that it can be determined whether this is a case of a co-existing variety or of dimorphism. in the former case, however, the difference being constant and so very conspicuous and easily defined, i see not how we could escape considering it as a distinct species. a true case of co-existing forms would, i consider, be produced, if a slight variety had become fixed as a local form, and afterwards been brought into contact with the parent species, with little or no intermixture of the two; and such instances do very probably occur. . _race or subspecies._--these are local forms completely fixed and isolated; and there is no possible test but individual opinion to determine which of them shall be considered as species and which varieties. if stability of form and "_the constant transmission of some characteristic peculiarity of organization_" is the test of a species (and i can find no other test that is more certain than individual opinion) then every one of these fixed races, confined as they almost always are to distinct and limited areas, must be regarded as a species; and as such i have in most cases treated them. the various modifications of papilio ulysses, p. peranthus, p. codrus, p. eurypilus, p. helenus, &c., are excellent examples; for while some present great and well-marked, others offer slight and inconspicuous differences, yet in all cases these differences seem equally fixed and permanent. if, therefore, we call some of these forms species, and others varieties, we introduce a purely arbitrary distinction, and shall never be able to decide where to draw the line. the races of papilio ulysses, for example, vary in amount of modification from the scarcely differing new guinea form to those of woodlark island and new caledonia, but all seem equally constant; and as most of these had already been named and described as species, i have added the new guinea form under the name of p. autolycus. we thus get a little group of ulyssine papilios, the whole comprised within a very limited area, each one confined to a separate portion of that area, and, though differing in various amounts, each apparently constant. few naturalists will doubt that all these may and probably have been derived from a common stock, and therefore it seems desirable that there should be a unity in our method of treating them; either call them all _varieties_ or all _species_. varieties, however, continually get overlooked; in lists of species they are often altogether unrecorded; and thus we are in danger of neglecting the interesting phenomena of variation and distribution which they present. i think it advisable, therefore, to name all such forms; and those who will not accept them as species may consider them as subspecies or races. . _species._--species are merely those strongly marked races or local forms which when in contact do not intermix, and when inhabiting distinct areas are generally believed to have had a separate origin, and to be incapable of producing a fertile hybrid offspring. but as the test of hybridity cannot be applied in one case in ten thousand, and even if it could be applied would prove nothing, since it is founded on an assumption of the very question to be decided--and as the test of separate origin is in every case inapplicable--and as, further, the test of non-intermixture is useless, except in those rare cases where the most closely allied species are found inhabiting the same area, it will be evident that we have no means whatever of distinguishing so-called "true species" from the several modes of variation here pointed out, and into which they so often pass by an insensible gradation. it is quite true that, in the great majority of cases, what we term "species" are so well marked and definite that there is no difference of opinion about them; but as the test of a true theory is, that it accounts for, or at the very least is not inconsistent with, the whole of the phenomena and apparent anomalies of the problem to be solved, it is reasonable to ask that those who deny the origin of species by variation and selection should grapple with the facts in detail, and show how the doctrine of the distinct origin and permanence of species will explain and harmonize them. it has been recently asserted by dr. j. e. gray (in the proceedings of the zoological society for , page ), that the difficulty of limiting species is in proportion to our ignorance, and that just as groups or countries are more accurately known and studied in greater detail the limits of species become settled. this statement has, like many other general assertions, its portion of both truth and error. there is no doubt that many uncertain species, founded on few or isolated specimens, have had their true nature determined by the study of a good series of examples: they have been thereby established as species or as varieties; and the number of times this has occurred is doubtless very great. but there are other, and equally trustworthy cases, in which, not single species, but whole groups have, by the study of a vast accumulation of materials, been proved to have no definite specific limits. a few of these must be adduced. in dr. carpenter's "introduction to the study of the foraminifera," he states that "_there is not a single specimen of plant or animal of which the range of variation has been studied by the collocation and comparison of so large a number of specimens as have passed under the review of messrs. williamson, parker, rupert jones, and myself, in our studies of the types of this group_;" and the result of this extended comparison of specimens is stated to be, "_the range of variation is so great among the foraminifera as to include not merely those differential characters which have been usually accounted_ specific, _but also those upon which the greater part of the_ genera _of this group have been founded, and even in some instances those of its_ orders" (foraminifera, preface, x). yet this same group had been divided by d'orbigny and other authors into a number of clearly defined _families_, _genera_, and _species_, which these careful and conscientious researches have shown to have been almost all founded on incomplete knowledge. professor decandolle has recently given the results of an extensive review of the species of cupuliferæ. he finds that the best-known species of oaks are those which produce most varieties and subvarieties; that they are often surrounded by provisional species; and, with the fullest materials at his command, two-thirds of the species he considers more or less doubtful. his general conclusion is, that "_in botany the lowest series of groups,_ subvarieties, varieties, _and_ races _are very badly limited; these can be grouped into_ species _a little less vaguely limited, which again can be formed into sufficiently precise_ genera." this general conclusion is entirely objected to by the writer of the article in the "natural history review," who, however, does not deny its applicability to the particular order under discussion, while this very difference of opinion is another proof that difficulties in the determination of species do not, any more than in the higher groups, vanish with increasing materials and more accurate research. another striking example of the same kind is seen in the genera rubus and rosa, adduced by mr. darwin himself; for though the amplest materials exist for a knowledge of these groups, and the most careful research has been bestowed upon them, yet the various species have not thereby been accurately limited and defined so as to satisfy the majority of botanists. in mr. baker's revision of the british roses, just published by the linnæan society, the author includes under the single species rosa canina, no less than twenty-eight named _varieties_, distinguished by more or less constant characters and often confined to special localities; and to these are referred about seventy of the _species_ of continental and british botanists. dr. hooker seems to have found the same thing in his study of the arctic flora. for though he has had much of the accumulated materials of his predecessors to work upon, he continually expresses himself as unable to do more than group the numerous and apparently fluctuating forms into more or less imperfectly defined species. in his paper on the "distribution of arctic plants," (trans. linn. soc. xxiii., p. ) dr. hooker says:--"the most able and experienced descriptive botanists vary in their estimate of the value of the 'specific term' to a much greater extent than is generally supposed." ... "i think i may safely affirm that the 'specific term' has three different standard values, all current in descriptive botany, but each more or less confined to one class of observers." ... "this is no question of what is right or wrong as to the real value of the specific term; i believe each is right according to the standard he assumes as the specific." lastly, i will adduce mr. bates's researches on the amazons. during eleven years he accumulated vast materials, and carefully studied the variation and distribution of insects. yet he has shown that many species of lepidoptera, which before offered no special difficulties, are in reality most intricately combined in a tangled web of affinities, leading by such gradual steps from the slightest and least stable variations to fixed races and well-marked species, that it is very often impossible to draw those sharp dividing-lines which it is supposed that a careful study and full materials will always enable us to do. these few examples show, i think, that in every department of nature there occur instances of the instability of specific form, which the increase of materials aggravates rather than diminishes. and it must be remembered that the naturalist is rarely likely to err on the side of imputing greater indefiniteness to species than really exists. there is a completeness and satisfaction to the mind in defining and limiting and naming a species, which leads us all to do so whenever we conscientiously can, and which we know has led many collectors to reject vague intermediate forms as destroying the symmetry of their cabinets. we must therefore consider these cases of excessive variation and instability as being thoroughly well established; and to the objection that, after all, these cases are but few compared with those in which species can be limited and defined, and are therefore merely exceptions to a general rule, i reply that a true law embraces all apparent exceptions, and that to the great laws of nature there are no real exceptions--that what appear to be such are equally results of law, and are often (perhaps indeed always) those very results which are most important as revealing the true nature and action of the law. it is for such reasons that naturalists now look upon the study of _varieties_ as more important than that of well-fixed species. it is in the former that we see nature still at work, in the very act of producing those wonderful modifications of form, that endless variety of colour, and that complicated harmony of relations, which gratify every sense and give occupation to every faculty of the true lover of nature. _variation as specially influenced by locality._ the phenomena of variation as influenced by locality have not hitherto received much attention. botanists, it is true, are acquainted with the influences of climate, altitude, and other physical conditions, in modifying the forms and external characteristics of plants; but i am not aware that any peculiar influence has been traced to locality, independent of climate. almost the only case i can find recorded is mentioned in that repertory of natural-history facts, "the origin of species," viz. that herbaceous groups have a tendency to become arboreal in islands. in the animal world, i cannot find that any facts have been pointed out as showing the special influence of locality in giving a peculiar _facies_ to the several disconnected species that inhabit it. what i have to adduce on this matter will therefore, i hope, possess some interest and novelty. on examining the closely allied species, local forms, and varieties distributed over the indian and malayan regions, i find that larger or smaller districts, or even single islands, give a special character to the majority of their papilionidæ. for instance: . the species of the indian region (sumatra, java, and borneo) are almost invariably smaller than the allied species inhabiting celebes and the moluccas; . the species of new guinea and australia are also, though in a less degree, smaller than the nearest species or varieties of the moluccas; . in the moluccas themselves the species of amboyna are the largest; . the species of celebes equal or even surpass in size those of amboyna; . the species and varieties of celebes possess a striking character in the form of the anterior wings, different from that of the allied species and varieties of all the surrounding islands; . tailed species in india or the indian region become tailless as they spread eastward through the archipelago; . in amboyna and ceram the females of several species are dull-coloured, while in the adjacent islands they are more brilliant. _local variation of size._--having preserved the finest and largest specimens of butterflies in my own collection, and having always taken for comparison the largest specimens of the same sex, i believe that the tables i now give are sufficiently exact. the differences of expanse of wings are in most cases very great, and are much more conspicuous in the specimens themselves than on paper. it will be seen that no less than fourteen papilionidæ inhabiting celebes and the moluccas are from one-third to one-half greater in extent of wing than the allied species representing them in java, sumatra, and borneo. six species inhabiting amboyna are larger than the closely allied forms of the northern moluccas and new guinea by about one-sixth. these include almost every case in which closely allied species can be compared. species of papilionidæ of the closely allied species of java and moluccas and celebes (large). the indian region (small). expanse. expanse. inches. inches. ornithoptera (helena { o. pompeus · amboyna) · { o. amphrisius · papilio adamantius } (celebes) · } p. lorquinianus } p. peranthus · (moluccas) · } p. blumei (celebes) · p. brama · p. alphenor (celebes) · p. theseus · p. gigon (celebes) · p. demolion · p. deucalion (celebes) · p. macareus · p. agamemnon, var. (celebes) · p. agamemnon, var. · p. eurypilus (moluccas) · } p. jason · p. telephus (celebes) · } p. Ægisthus (moluccas) · p. rama · p. milon (celebes) · p. sarpedon · p. androcles (celebes) · p. antiphates · p. polyphontes (celebes) · p. diphilus · leptocircus ennius (celebes) · l. meges · species inhabiting amboyna allied species of new guinea and (large). the north moluccas (smaller). papilio ulysses · { p. autolycus · { p. telegonus · p. polydorus · p. leodamas · p. deiphobus · p. deiphontes · p. gambrisius · { p. ormenus · { p. tydeus · p. codrus · p. codrus, var. papuensis · ornithoptera priamus, ornithoptera poseidon, (male) · (male) · _local variation of form._--the differences of form are equally clear. papilio pammon everywhere on the continent is tailed in both sexes. in java, sumatra, and borneo, the closely allied p. theseus has a very short tail, or tooth only, in the male, while in the females the tail is retained. further east, in celebes and the south moluccas, the hardly separable p. alphenor has quite lost the tail in the male, while the female retains it, but in a narrower and less spatulate form. a little further, in gilolo, p. nicanor has completely lost the tail in both sexes. papilio agamemnon exhibits a somewhat similar series of changes. in india it is always tailed; in the greater part of the archipelago it has a very short tail; while far east, in new guinea and the adjacent islands, the tail has almost entirely disappeared. in the polydorus-group two species, p. antiphus and p. diphilus, inhabiting india and the indian region, are tailed, while the two which take their place in the moluccas, new guinea, and australia, p. polydorus and p. leodamas, are destitute of tail, the species furthest east having lost this ornament the most completely. western species, tailed. allied eastern species not tailed. papilio pammon (india) p. thesus (islands) minute tail. p. agamemnon, var. (india) p. agamemnon, var. (islands). p. antiphus (india, java) p. polydorus (moluccas). p. diphilus (india, java) p. leodamas (new guinea). the most conspicuous instance of local modification of form, however, is exhibited in the island of celebes, which in this respect, as in some others, stands alone and isolated in the whole archipelago. almost every species of papilio inhabiting celebes has the wings of a peculiar shape, which distinguishes them at a glance from the allied species of every other island. this peculiarity consists, first, in the upper wings being generally more elongate and falcate; and secondly, in the costa or anterior margin being much more curved, and in most instances exhibiting near the base an abrupt bend or elbow, which in some species is very conspicuous. this peculiarity is visible, not only when the celebesian species are compared with their small-sized allies of java and borneo, but also, and in an almost equal degree, when the large forms of amboyna and the moluccas are the objects of comparison, showing that this is quite a distinct phenomenon from the difference of size which has just been pointed out. in the following table i have arranged the chief papilios of celebes in the order in which they exhibit this characteristic form most prominently. papilios of celebes, having the closely allied papilios of the wings falcate or with abruptly surrounding islands, with less curved costa. wings and slightly falcate curved costa. . p. gigon p. demolion (java). . p. pamphylus p. jason (sumatra). . p. milon p. sarpedon (moluccas, java). . p. agamemnon, var. p. agamemnon, var. (borneo). . p. adamantius p. peranthus (java). . p. ascalaphus p. deiphontes (gilolo). . p. sataspes p. helenus (java). . p. blumei p. brama (sumatra). . p. androcles p. antiphates (borneo). . p. rhesus p. aristæus (moluccas). . p. theseus, var. (male) p. thesus (male) (java). . p. codrus, var. p. codrus (moluccas). . p. encelades p. leucothoë (malacca). it thus appears that every species of papilio exhibits this peculiar form in a greater or less degree, except one, p. polyphontes, allied to p. diphilus of india and p. polydorus of the moluccas. this fact i shall recur to again, as i think it helps us to understand something of the causes that may have brought about the phenomenon we are considering. neither do the genera ornithoptera and leptocircus exhibit any traces of this peculiar form. in several other families of butterflies this characteristic form reappears in a few species. in the pieridæ the following species, all peculiar to celebes, exhibit it distinctly:-- . pieris eperia compared with p. coronis (java). . thyca zebuda " " thyca descombesi (india). . t. rosenbergii " " t. hyparete (java). . tachyris hombronii " " t. lyncida. . t. lycaste " " t. lyncida. . t. zarinda " " t. nero (malacca). . t. ithome " " t. nephele. . eronia tritæa " " eronia valeria (java). . iphias glaucippe, var. " " iphias glaucippe (java). the species of terias, one or two pieris, and the genus callidryas do not exhibit any perceptible change of form. in the other families there are but few similar examples. the following are all that i can find in my collection:-- cethosia Æole compared with cethosia biblis (java). eurhinia megalonice " " eurhinia polynice (borneo). limenitis limire " " limenitis procris (java). cynthia arsinoë, var. " " cynthia arsinoë (java, sumatra, borneo) all these belong to the family of the nymphalidæ. many other genera of this family, as diadema, adolias, charaxes, and cyrestis, as well as the entire families of the danaidæ, satyridæ, lycænidæ, and hesperidæ, present no examples of this peculiar form of the upper wing in the celebesian species. _local variations of colour._--in amboyna and ceram the female of the large and handsome ornithoptera helena has the large patch on the hind wings constantly of a pale dull ochre or buff colour, while in the scarcely distinguishable varieties from the adjacent islands of bouru and new guinea, it is of a golden yellow, hardly inferior in brilliancy to its colour in the male sex. the female of ornithoptera priamus (inhabiting amboyna and ceram exclusively) is of a pale dusky brown tint, while in all the allied species the same sex is nearly black with contrasted white markings. as a third example, the female of papilio ulysses has the blue colour obscured by dull and dusky tints, while in the closely allied species from the surrounding islands, the females are of almost as brilliant an azure blue as the males. a parallel case to this is the occurrence, in the small islands of goram, matabello, ké, and aru, of several distinct species of euploea and diadema, having broad bands or patches of white, which do not exist in any of the allied species from the larger islands. these facts seem to indicate some local influence in modifying colour, as unintelligible and almost as remarkable as that which has resulted in the modifications of form previously described. _remarks on the facts of local variation._ the facts now brought forward seem to me of the highest interest. we see that almost all the species in two important families of the lepidoptera (papilionidæ and pieridæ) acquire, in a single island, a characteristic modification of form distinguishing them from the allied species and varieties of all the surrounding islands. in other equally extensive families no such change occurs, except in one or two isolated species. however we may account for these phenomena, or whether we may be quite unable to account for them, they furnish, in my opinion, a strong corroborative testimony in favour of the doctrine of the origin of species by successive small variations; for we have here slight varieties, local races, and undoubted species, all modified in exactly the same manner, indicating plainly a common cause producing identical results. on the generally received theory of the original distinctness and permanence of species, we are met by this difficulty: one portion of these curiously modified forms are admitted to have been produced by variation and some natural action of local conditions; whilst the other portion, differing from the former only in degree, and connected with them by insensible gradations, are said to have possessed this peculiarity of form at their first creation, or to have derived it from unknown causes of a totally distinct nature. is not the _à priori_ evidence in favour of an identity of the causes that have produced such similar results? and have we not a right to call upon our opponents for some proofs of their own doctrine, and for an explanation of its difficulties, instead of their assuming that they are right, and laying upon us the burthen of disproof? let us now see if the facts in question do not themselves furnish some clue to their explanation. mr. bates has shown that certain groups of butterflies have a defence against insectivorous animals, independent of swiftness of motion. these are generally very abundant, slow, and weak fliers, and are more or less the objects of mimicry by other groups, which thus gain an advantage in a freedom from persecution similar to that enjoyed by those they resemble. now the only papilios which have not in celebes acquired the peculiar form of wing, belong to a group which is imitated both by other species of papilio and by moths of the genus epicopeia. this group is of weak and slow flight; and we may therefore fairly conclude that it possesses some means of defence (probably in a peculiar odour or taste) which saves it from attack. now the arched costa and falcate form of wing is generally supposed to give increased powers of flight, or, as seems to me more probable, greater facility in making sudden turnings, and thus baffling a pursuer. but the members of the polydorus-group (to which belongs the only unchanged celebesian papilio), being already guarded against attack, have no need of this increased power of wing; and "natural selection" would therefore have no tendency to produce it. the whole family of danaidæ are in the same position: they are slow and weak fliers; yet they abound in species and individuals, and are the objects of mimicry. the satyridæ have also probably a means of protection--perhaps their keeping always near the ground and their generally obscure colours; while the lycænidæ and hesperidæ may find security in their small size and rapid motions. in the extensive family of the nymphalidæ, however, we find that several of the larger species, of comparatively feeble structure, have their wings modified (cethosia, limenitis, junonia, cynthia), while the large-bodied powerful species, which have all an excessively rapid flight, have exactly the same form of wing in celebes as in the other islands. on the whole, therefore, we may say that all the butterflies of rather large size, conspicuous colours, and not very swift flight have been affected in the manner described, while the smaller sized and obscure groups, as well as those which are the objects of mimicry, and also those of exceedingly swift flight have remained unaffected. it would thus appear as if there must be (or once have been) in the island of celebes, some peculiar enemy to these larger-sized butterflies which does not exist, or is less abundant, in the surrounding islands. increased powers of flight, or rapidity of turning, was advantageous in baffling this enemy; and the peculiar form of wing necessary to give this would be readily acquired by the action of "natural selection" on the slight variations of form that are continually occurring. such an enemy one would naturally suppose to be an insectivorous bird; but it is a remarkable fact that most of the genera of fly-catchers of borneo and java on the one side (muscipeta, philentoma,) and of the moluccas on the other (monarcha, rhipidura), are almost entirely absent from celebes. their place seems to be supplied by the caterpillar-catchers (graucalus, campephaga, &c.), of which six or seven species are known from celebes and are very numerous in individuals. we have no positive evidence that these birds pursue butterflies on the wing, but it is highly probable that they do so when other food is scarce. mr. bates has suggested to me that the larger dragonflies (Æshna, &c.) prey upon butterflies; but i did not notice that they were more abundant in celebes than elsewhere. however this may be, the fauna of celebes is undoubtedly highly peculiar in every department of which we have any accurate knowledge; and though we may not be able satisfactorily to trace how it has been effected, there can, i think, be little doubt that the singular modification in the wings of so many of the butterflies of that island is an effect of that complicated action and reaction of all living things upon each other in the struggle for existence, which continually tends to readjust disturbed relations, and to bring every species into harmony with the varying conditions of the surrounding universe. but even the conjectural explanation now given fails us in the other cases of local modification. why the species of the western islands should be smaller than those further east,--why those of amboyna should exceed in size those of gilolo and new guinea--why the tailed species of india should begin to lose that appendage in the islands, and retain no trace of it on the borders of the pacific,--and why, in three separate cases, the females of amboyna species should be less gaily attired than the corresponding females of the surrounding islands,--are questions which we cannot at present attempt to answer. that they depend, however, on some general principle is certain, because analogous facts have been observed in other parts of the world. mr. bates informs me that, in three distinct groups, papilios which on the upper amazon and in most other parts of south america have spotless upper wings obtain pale or white spots at pará and on the lower amazon; and also that the Æneas-group of papilios never have tails in the equatorial regions and the amazons valley, but gradually acquire tails in many cases as they range towards the northern or southern tropic. even in europe we have somewhat similar facts; for the species and varieties of butterflies peculiar to the island of sardinia are generally smaller and more deeply coloured than those of the mainland, and the same has recently been shown to be the case with the common tortoiseshell butterfly in the isle of man; while papilio hospiton, peculiar to the former island, has lost the tail, which is a prominent feature of the closely allied p. machaon. facts of a similar nature to those now brought forward would no doubt be found to occur in other groups of insects, were local faunas carefully studied in relation to those of the surrounding countries; and they seem to indicate that climate and other physical causes have, in some cases, a very powerful effect in modifying specific form and colour, and thus directly aid in producing the endless variety of nature. _mimicry._ having fully discussed this subject in the preceding essay, i have only to adduce such illustrations of it, as are furnished by the eastern papilionidæ, and to show their bearing upon the phenomena of variation already mentioned. as in america, so in the old world, species of danaidæ are the objects which the other families most often imitate. but besides these, some genera of morphidæ and one section of the genus papilio are also less frequently copied. many species of papilio mimic other species of these three groups so closely that they are undistinguishable when on the wing; and in every case the pairs which resemble each other inhabit the same locality. the following list exhibits the most important and best marked cases of mimicry which occur among the papilionidæ of the malayan region and india:-- mimickers. species mimicked. common habitat. danaidÆ. . papilio paradoxa euploea midamus } sumatra, &c. (male & female) (male & female) } . p. caunus e. rhadamanthus borneo and sumatra. . p. thule danais sobrina new guinea. . p. macareus d. aglaia malacca, java . papilio agestor danais tytia northern india. . p. idæoides hestia leuconoë philippines. . p. delessertii ideopsis daos penang. morphidÆ. . p. pandion drusilla bioculata new guinea (female) papilio (polydorus- and coon-groups). . p. pammon (romulus, papilio hector india. female) . p. theseus, var. p. antiphus sumatra, borneo. (female) . p. theseus, var. p. diphilus sumatra, java. (female) . p. memnon, var. p. coon sumatra. (achates, female) . p. androgeus, var. p. doubledayi northern india. (achates, female) . p. oenomaus p. liris timor. (female) we have, therefore, fourteen species or marked varieties of papilio, which so closely resemble species of other groups in their respective localities, that it is not possible to impute the resemblance to accident. the first two in the list (papilio paradoxa and p. caunus) are so exactly like euploea midamus and e. rhadamanthus on the wing, that although they fly very slowly, i was quite unable to distinguish them. the first is a very interesting case, because the male and female differ considerably, and each mimics the corresponding sex of the euploea. a new species of papilio which i discovered in new guinea resembles danais sobrina, from the same country, just as papilio marcareus resembles danais aglaia in malacca, and (according to dr. horsfield's figure) still more closely in java. the indian papilio agestor closely imitates danais tytia, which has quite a different style of colouring from the preceding; and the extraordinary papilio idæoides from the philippine islands, must, when on the wing, perfectly resemble the hestia leuconoë of the same region, as also does the papilio delessertii imitate the ideopsis daos from penang. now in every one of these cases the papilios are very scarce, while the danaidæ which they resemble are exceedingly abundant--most of them swarming so as to be a positive nuisance to the collecting entomologist by continually hovering before him when he is in search of newer and more varied captures. every garden, every roadside, the suburbs of every village are full of them, indicating very clearly that their life is an easy one, and that they are free from persecution by the foes which keep down the population of less favoured races. this superabundant population has been shown by mr. bates to be a general characteristic of all american groups and species which are objects of mimicry; and it is interesting to find his observations confirmed by examples on the other side of the globe. the remarkable genus drusilla, a group of pale-coloured butterflies, more or less adorned with ocellate spots, is also the object of mimicry by three distinct genera (melanitis, hyantis, and papilio). these insects, like the danaidæ, are abundant in individuals, have a very weak and slow flight, and do not seek concealment, or appear to have any means of protection from insectivorous creatures. it is natural to conclude, therefore, that they have some hidden property which saves them from attack; and it is easy to see that when any other insects, by what we call accidental variation, come more or less remotely to resemble them, the latter will share to some extent in their immunity. an extraordinary dimorphic form of the female of papilio ormenus has come to resemble the drusillas sufficiently to be taken for one of that group at a little distance; and it is curious that i captured one of these papilios in the aru islands hovering along the ground, and settling on it occasionally, just as it is the habit of the drusillas to do. the resemblance in this case is only general; but this form of papilio varies much, and there is therefore material for natural selection to act upon, so as ultimately to produce a copy as exact as in the other cases. the eastern papilios allied to polydorus, coon, and philoxenus, form a natural section of the genus resembling, in many respects, the Æneas-group of south america, which they may be said to represent in the east. like them, they are forest insects, have a low and weak flight, and in their favourite localities are rather abundant in individuals; and like them, too, they are the objects of mimicry. we may conclude, therefore, that they possess some hidden means of protection, which makes it useful to other insects to be mistaken for them. the papilios which resemble them belong to a very distinct section of the genus, in which the sexes differ greatly; and it is those females only which differ most from the males, and which have already been alluded to as exhibiting instances of dimorphism, which resemble species of the other group. the resemblance of p. romulus to p. hector is, in some specimens, very considerable, and has led to the two species being placed following each other in the british museum catalogues and by mr. e. doubleday. i have shown, however, that p. romulus is probably a dimorphic form of the female p. pammon, and belongs to a distinct section of the genus. the next pair, papilio theseus, and p. antiphus, have been united as one species both by de haan and in the british museum catalogues. the ordinary variety of p. theseus found in java almost as nearly resembles p. diphilus, inhabiting the same country. the most interesting case, however, is the extreme female form of p. memnon (figured by cramer under the name of p. achates), which has acquired the general form and markings of p. coon, an insect which differs from the ordinary male p. memnon, as much as any two species which can be chosen in this extensive and highly varied genus; and, as if to show that this resemblance is not accidental, but is the result of law, when in india we find a species closely allied to p. coon, but with red instead of yellow spots (p. doubledayi), the corresponding variety of p. androgeus (p. achates, cramer, , a, b,) has acquired exactly the same peculiarity of having red spots instead of yellow. lastly, in the island of timor, the female of p. oenomaus (a species allied to p. memnon) resembles so closely p. liris (one of the polydorus-group), that the two, which were often seen flying together, could only be distinguished by a minute comparison after being captured. the last six cases of mimicry are especially instructive, because they seem to indicate one of the processes by which dimorphic forms have been produced. when, as in these cases, one sex differs much from the other, and varies greatly itself, it may happen that occasionally individual variations will occur having a distant resemblance to groups which are the objects of mimicry, and which it is therefore advantageous to resemble. such a variety will have a better chance of preservation; the individuals possessing it will be multiplied; and their accidental likeness to the favoured group will be rendered permanent by hereditary transmission, and, each successive variation which increases the resemblance being preserved, and all variations departing from the favoured type having less chance of preservation, there will in time result those singular cases of two or more isolated and fixed forms, bound together by that intimate relationship which constitutes them the sexes of a single species. the reason why the females are more subject to this kind of modification than the males is, probably, that their slower flight, when laden with eggs, and their exposure to attack while in the act of depositing their eggs upon leaves, render it especially advantageous for them to have some additional protection. this they at once obtain by acquiring a resemblance to other species which, from whatever cause, enjoy a comparative immunity from persecution. _concluding remarks on variation in lepidoptera._ this summary of the more interesting phenomena of variation presented by the eastern papilionidæ is, i think, sufficient to substantiate my position, that the lepidoptera are a group that offer especial facilities for such inquiries; and it will also show that they have undergone an amount of special adaptive modification rarely equalled among the more highly organized animals. and, among the lepidoptera, the great and pre-eminently tropical families of papilionidæ and danaidæ seem to be those in which complicated adaptations to the surrounding organic and inorganic universe have been most completely developed, offering in this respect a striking analogy to the equally extraordinary, though totally different, adaptations which present themselves in the orchideæ, the only family of plants in which mimicry of other organisms appears to play any important part, and the only one in which cases of conspicuous polymorphism occur; for as such we must class the male, female, and hermaphrodite forms of catasetum tridentatum, which differ so greatly in form and structure that they were long considered to belong to three distinct genera. _arrangement and geographical distribution of the malayan papilionidæ_. _arrangement._--although the species of papilionidæ inhabiting the malayan region are very numerous, they all belong to three out of the nine genera into which the family is divided. one of the remaining genera (eurycus) is restricted to australia, and another (teinopalpus) to the himalayan mountains, while no less than four (parnassius, doritis, thais, and sericinus) are confined to southern europe and to the mountain-ranges of the palæarctic region. the genera ornithoptera and leptocircus are highly characteristic of malayan entomology, but are uniform in character and of small extent. the genus papilio, on the other hand, presents a great variety of forms, and is so richly represented in the malay islands, that more than one-fourth of all the known species are found there. it becomes necessary, therefore, to divide this genus into natural groups before we can successfully study its geographical distribution. owing principally to dr. horsfield's observations in java, we are acquainted with a considerable number of the larvæ of papilios; and these furnish good characters for the primary division of the genus into natural groups. the manner in which the hinder wings are plaited or folded back at the abdominal margin, the size of the anal valves, the structure of the antennæ, and the form of the wings are also of much service, as well as the character of the flight and the style of colouration. using these characters, i divide the malayan papilios into four sections, and seventeen groups, as follows:-- genus ornithoptera. a. priamus-group. } black and green. c. brookeanus-group.} b. pompeus-group. black and yellow. genus papilio. a. larvæ short, thick, with numerous fleshy tubercles; of a purplish colour. a. nox-group. abdominal fold in male very large; anal valves small, but swollen; antennæ moderate; wings entire, or tailed; includes the indian philoxenus-group. b. coon-group. abdominal fold in male small; anal valves small, but swollen; antennæ moderate; wings tailed. c. polydorus-group. abdominal fold in male small, or none; anal valves small or obsolete, hairy; wings tailed or entire. b. larvæ with third segment swollen, transversely or obliquely banded; pupa much bent. imago with abdominal margin in male plaited, but not reflexed; body weak; antennæ long; wings much dilated, often tailed. d. ulysses-group. {protenor-group (indian) is e. peranthus-group. {somewhat intermediate between f. memnon-group. {these, and is nearest {to the nox-group. g. helenus-group. h. erectheus-group. i. pammon-group. k. demolion-group. c. larvæ subcylindrical, variously coloured. imago with abdominal margin in male plaited, but not reflexed; body weak; antennæ short, with a thick curved club; wings entire. l. erithonius-group. sexes alike, larva and pupa something like those of p. demolion. m. paradoxa-group. sexes different. n. dissimilis-group. sexes alike; larva bright-coloured; pupa straight, cylindric. d. larvæ elongate, attenuate behind, and often bifid, with lateral and oblique pale stripes, green. imago with the abdominal margin in male reflexed, woolly or hairy within; anal valves small, hairy; antennæ short, stout; body stout. o. macareus-group. hind wings entire. p. antiphates-group. hind wings much tailed (swallow-tails). q. eurypylus-group. hind wings elongate or tailed. genus leptocircus. making, in all, twenty distinct groups of malayan papilionidæ. the first section of the genus papilio (a) comprises insects which, though differing considerably in structure, having much general resemblance. they all have a weak, low flight, frequent the most luxuriant forest-districts, seem to love the shade, and are the objects of mimicry by other papilios. section b consists of weak-bodied, large-winged insects, with an irregular wavering flight, and which, when resting on foliage, often expand the wings, which the species of the other sections rarely or never do. they are the most conspicuous and striking of eastern butterflies. section c consists of much weaker and slower-flying insects, often resembling in their flight, as well as in their colours, species of danaidæ. section d contains the strongest-bodied and most swift-flying of the genus. they love sunlight, and frequent the borders of streams and the edges of puddles, where they gather together in swarms consisting of several species, greedily sucking up the moisture, and, when disturbed, circling round in the air, or flying high and with great strength and rapidity. _geographical distribution._--one hundred and thirty species of malayan papilionidæ are now known within the district extending from the malay peninsula, on the north-west, to woodlark island, near new guinea, on the south-east. the exceeding richness of the malayan region in these fine insects is seen by comparing the number of species found in the different tropical regions of the earth. from all africa only species of papilio are known; but as several are still undescribed in collections, we may raise their number to about . in all tropical asia there are at present described only species, and i have seen in collections but two or three which have not yet been named. in south america, south of panama, there are species, or about one-seventh more than are yet known from the malayan region; but the area of the two countries is very different; for while south america (even excluding patagonia) contains , , square miles, a line encircling the whole of the malayan islands would only include an area of , , square miles, of which the land-area would be about , , square miles. this superior richness is partly real and partly apparent. the breaking up of a district into small isolated portions, as in an archipelago, seems highly favourable to the segregation and perpetuation of local peculiarities in certain groups; so that a species which on a continent might have a wide range, and whose local forms, if any, would be so connected together that it would be impossible to separate them, may become by isolation reduced to a number of such clearly defined and constant forms that we are obliged to count them as species. from this point of view, therefore, the greater proportionate number of malayan species may be considered as apparent only. its true superiority is shown, on the other hand, by the possession of three genera and twenty groups of papilionidæ against a single genus and eight groups in south america, and also by the much greater average size of the malayan species. in most other families, however, the reverse is the case, the south american nymphalidæ, satyridæ, and erycinidæ far surpassing those of the east in number, variety, and beauty. the following list, exhibiting the range and distribution of each group, will enable us to study more easily their internal and external relations. _range of the groups of malayan papilionidæ._ ornithoptera. . priamus-group. moluccas to woodlark island species. . pompeus-group. himalayas to new guinea, (celebes, maximum) " . brookeana-group. sumatra and borneo " papilio. . nox-group. north india, java, and philippines species . coon-group. north india to java " . polydorus-group. india to new guinea and pacific " . ulysses-group. celebes to new caledonia " . peranthus-group. india to timor and moluccas (india, maximum) " . memnon-group. india to timor and moluccas (java, maximum) " . helenus-group. africa and india to new guinea " . pammon-group. india to pacific and australia " . erectheus-group. celebes to australia " . demolion-group. india to celebes " . erithonius-group. africa, india, australia " . paradoxa-group. india to java (borneo, maximum) " . dissimilis-group. india to timor (india, maximum) " . macareus-group. india to new guinea " . antiphates-group. widely distributed " . eurypylus-group. india to australia " leptocircus. . leptocircus-group. india to celebes " this table shows the great affinity of the malayan with the indian papilionidæ, only three out of the twenty groups ranging beyond, into africa, europe, or america. the limitation of groups to the indo-malayan or austro-malayan divisions of the archipelago, which is so well marked in the higher animals, is much less conspicuous in insects, but is shown in some degree by the papilionidæ. the following groups are either almost or entirely restricted to one portion of the archipelago:-- _indo-malayan region._ _austro-malayan region._ nox-group. priamus-group. coon-group. ulysses-group. macareus-group (nearly). erechtheus-group. paradoxa-group. dissimilis-group (nearly). brookeanus-group. leptocircus (genus). the remaining groups, which range over the whole archipelago, are, in many cases, insects of very powerful flight, or they frequent open places and the sea-beach, and are thus more likely to get blown from island to island. the fact that three such characteristic groups as those of priamus, ulysses, and erechtheus are strictly limited to the australian region of the archipelago, while five other groups are with equal strictness confined to the indian region, is a strong corroboration of that division which has been founded almost entirely on the distribution of mammalia and birds. if the various malayan islands have undergone recent changes of level, and if any of them have been more closely united within the period of existing species than they are now, we may expect to find indications of such changes in community of species between islands now widely separated; while those islands which have long remained isolated would have had time to acquire peculiar forms by a slow and natural process of modification. an examination of the relations of the species of the adjacent islands, will thus enable us to correct opinions formed from a mere consideration of their relative positions. for example, looking at a map of the archipelago, it is almost impossible to avoid the idea that java and sumatra have been recently united; their present proximity is so great, and they have such an obvious resemblance in their volcanic structure. yet there can be little doubt that this opinion is erroneous, and that sumatra has had a more recent and more intimate connexion with borneo than it has had with java. this is strikingly shown by the mammals of these islands--very few of the species of java and sumatra being identical, while a considerable number are common to sumatra and borneo. the birds show a somewhat similar relationship; and we shall find that the distribution of the papilionidæ tells exactly the same tale. thus:-- sumatra has species } borneo " " } sp. common to both islands; sumatra " " } java " " } sp. common to both islands; borneo " " } java " " } sp. common to both islands; showing that both sumatra and java have a much closer relationship to borneo than they have to each other--a most singular and interesting result, when we consider the wide separation of borneo from them both, and its very different structure. the evidence furnished by a single group of insects would have had but little weight on a point of such magnitude if standing alone; but coming as it does to confirm deductions drawn from whole classes of the higher animals, it must be admitted to have considerable value. we may determine in a similar manner the relations of the different papuan islands to new guinea. of thirteen species of papilionidæ obtained in the aru islands, six were also found in new guinea, and seven not. of nine species obtained at waigiou, six were new guinea, and three not. the five species found at mysol were all new guinea species. mysol, therefore, has closer relations to new guinea than the other islands; and this is corroborated by the distribution of the birds, of which i will only now give one instance. the paradise bird found in mysol is the common new guinea species, while the aru islands and waigiou have each a species peculiar to themselves. the large island of borneo, which contains more species of papilionidæ than any other in the archipelago, has nevertheless only three peculiar to itself; and it is quite possible, and even probable, that one of these may be found in sumatra or java. the last-named island has also three species peculiar to it; sumatra has not one, and the peninsula of malacca only two. the identity of species is even greater than in birds or in most other groups of insects, and points very strongly to a recent connexion of the whole with each other and the continent. _remarkable peculiarities of the island of celebes._ if we now pass to the next island (celebes), separated from those last mentioned by a strait not wider than that which divides them from each other, we have a striking contrast; for with a total number of species less than either borneo or java, no fewer than eighteen are absolutely restricted to it. further east, the large islands of ceram and new guinea have only three species peculiar to each, and timor has five. we shall have to look, not to single islands, but to whole groups, in order to obtain an amount of individuality comparable with that of celebes. for example, the extensive group comprising the large islands of java, borneo, and sumatra, with the peninsula of malacca, possessing altogether species, has about , or just half, peculiar to it; the numerous group of the philippines possess species, of which are peculiar; the seven chief islands of the moluccas have , of which are peculiar; and the whole of the papuan islands, with an equal number of species, have peculiar. comparable with the most isolated of these groups is celebes, with its species, of which the large proportion of are peculiar. we see, therefore, that the opinion i have elsewhere expressed, of the high degree of isolation and the remarkable distinctive features of this interesting island, is fully borne out by the examination of this conspicuous family of insects. a single straggling island with a few small satellites, it is zoologically of equal importance with extensive groups of islands many times as large as itself; and standing in the very centre of the archipelago, surrounded on every side with islets connecting it with the larger groups, and which seem to afford the greatest facilities for the migration and intercommunication of their respective productions, it yet stands out conspicuous with a character of its own in every department of nature, and presents peculiarities which are, i believe, without a parallel in any similar locality on the globe. briefly to summarize these peculiarities, celebes possesses three genera of mammals (out of the very small number which inhabit it) which are of singular and isolated forms, viz., cynopithecus, a tailless ape allied to the baboons; anoa, a straight-horned antelope of obscure affinities, but quite unlike anything else in the whole archipelago or in india: and babirusa, an altogether abnormal wild pig. with a rather limited bird population, celebes has an immense preponderance of species confined to it, and has also six remarkable genera (meropogon, ceycopsis, streptocitta, enodes, scissirostrum, and megacephalon) entirely restricted to its narrow limits, as well as two others (prioniturus and basilornis) which only range to a single island beyond it. mr. smith's elaborate tables of the distribution of malayan hymenoptera (see "proc. linn. soc." zool. vol. vii.) show that out of the large number of species collected in celebes, (or nearly two-thirds) are absolutely restricted to it, although borneo on one side, and the various islands of the moluccas on the other, were equally well explored by me; and no less than twelve of the genera are not found in any other island of the archipelago. i have shown in the present essay that, in the papilionidæ, it has far more species of its own than any other island, and a greater proportion of peculiar species than many of the large groups of islands in the archipelago--and that it gives to a large number of the species and varieties which inhabit it, st, an increase of size, and, nd, a peculiar modification in the form of the wings, which stamp upon the most dissimilar insects a mark distinctive of their common birth-place. what, i would ask, are we to do with phenomena such as these? are we to rest content with that very simple, but at the same time very unsatisfying explanation, that all these insects and other animals were created exactly _as_ they are, and originally placed exactly _where_ they are, by the inscrutable will of their creator, and that we have nothing to do but to register the facts and wonder? was this single island selected for a fantastic display of creative power, merely to excite a childlike and unreasoning admiration? is all this appearance of gradual modification by the action of natural causes--a modification the successive steps of which we can almost trace--all delusive? is this harmony between the most diverse groups, all presenting analogous phenomena, and indicating a dependence upon physical changes of which we have independent evidence, all false testimony? if i could think so, the study of nature would have lost for me its greatest charm. i should feel as would the geologist, if you could convince him that his interpretation of the earth's past history was all a delusion--that strata were never formed in the primeval ocean, and that the fossils he so carefully collects and studies are no true record of a former living world, but were all created just as they now are, and in the rocks where he now finds them. i must here express my own belief that none of these phenomena, however apparently isolated or insignificant, can ever stand alone--that not the wing of a butterfly can change in form or vary in colour, except in harmony with, and as a part of the grand march of nature. i believe, therefore, that all the curious phenomena i have just recapitulated, are immediately dependent on the last series of changes, organic and inorganic, in these regions; and as the phenomena presented by the island of celebes differ from those of all the surrounding islands, it can, i conceive, only be because the past history of celebes has been, to some extent, unique and different from theirs. we must have much more evidence to determine exactly in what that difference has consisted. at present, i only see my way clear to one deduction, viz., that celebes represents one of the oldest parts of the archipelago; that it has been formerly more completely isolated both from india and from australia than it is now, and that amid all the mutations it has undergone, a relic or substratum of the fauna and flora of some more ancient land has been here preserved to us. it is only since my return home, and since i have been able to compare the productions of celebes side by side with those of the surrounding islands, that i have been fully impressed with their peculiarity, and the great interest that attaches to them. the plants and the reptiles are still almost unknown; and it is to be hoped that some enterprising naturalist may soon devote himself to their study. the geology of the country would also be well worth exploring, and its newer fossils would be of especial interest as elucidating the changes which have led to its present anomalous condition. this island stands, as it were, upon the boundary-line between two worlds. on one side is that ancient australian fauna, which preserves to the present day the facies of an early geological epoch; on the other is the rich and varied fauna of asia, which seems to contain, in every class and order, the most perfect and highly organised animals. celebes has relations to both, yet strictly belongs to neither: it possesses characteristics which are altogether its own; and i am convinced that no single island upon the globe would so well repay a careful and detailed research into its past and present history. _concluding remarks._ in writing this essay it has been my object to show how much may, under favourable circumstances, be learnt by the study of what may be termed the external physiology of a small group of animals, inhabiting a limited district. this branch of natural history had received little attention till mr. darwin showed how important an adjunct it may become towards a true interpretation of the history of organized beings, and attracted towards it some small share of that research which had before been almost exclusively devoted to internal structure and physiology. the nature of species, the laws of variation, the mysterious influence of locality on both form and colour, the phenomena of dimorphism and of mimicry, the modifying influence of sex, the general laws of geographical distribution, and the interpretation of past changes of the earth's surface, have all been more or less fully illustrated by the very limited group of the malayan papilionidæ; while, at the same time, the deductions drawn therefrom have been shown to be supported by analogous facts, occurring in other and often widely-separated groups of animals. v. on instinct in man and animals. the most perfect and most striking examples of what is termed instinct, those in which reason or observation appear to have the least influence, and which seem to imply the possession of faculties farthest removed from our own, are to be found among insects. the marvellous constructive powers of bees and wasps, the social economy of ants, the careful provision for the safety of a progeny they are never to see manifested by many beetles and flies, and the curious preparations for the pupa state by the larvæ of butterflies and moths, are typical examples of this faculty, and are supposed to be conclusive as to the existence of some power or intelligence, very different from that which we derive from our senses or from our reason. _how instinct may be best studied._ whatever we may define instinct to be, it is evidently some form of mental manifestation, and as we can only judge of mind by the analogy of our own mental functions and by observation of the results of mental action in other men and in animals, it is incumbent on us, first, to study and endeavour to comprehend the minds of infants, of savage men, and of animals not very far removed from ourselves, before we pronounce positively as to the nature of the mental operations in creatures so radically different from us as insects. we have not yet even been able to ascertain what are the senses they possess, or what relation their powers of seeing, hearing, and feeling have to ours. their sight may far exceed ours both in delicacy and in range, and may possibly give them knowledge of the internal constitution of bodies analogous to that which we obtain by the spectroscope; and that their visual organs do possess some powers which ours do not, is indicated by the extraordinary crystalline rods radiating from the optic ganglion to the facets of the compound eye, which rods vary in form and thickness in different parts of their length, and possess distinctive characters in each group of insects. this complex apparatus, so different from anything in the eyes of vertebrates, may subserve some function quite inconceivable by us, as well as that which we know as vision. there is reason to believe that insects appreciate sounds of extreme delicacy, and it is supposed that certain minute organs, plentifully supplied with nerves, and situated in the subcostal vein of the wing in most insects, are the organs of hearing. but besides these, the orthoptera (such as grasshoppers, &c.) have what are supposed to be ears on their fore legs, and mr. lowne believes that the little stalked balls, which are the sole remnants of the hind wings in flies, are also organs of hearing or of some analogous sense. in flies, too, the third joint of the antennæ contains thousands of nerve-fibres, which terminate in small open cells, and this mr. lowne believes to be the organ of smell, or of some other, perhaps new, sense. it is quite evident, therefore, that insects may possess senses which give them a knowledge of that which we can never perceive, and enable them to perform acts which to us are incomprehensible. in the midst of this complete ignorance of their faculties and inner nature, is it wise for us to judge so boldly of their powers by a comparison with our own? how can we pretend to fathom the profound mystery of their mental nature, and decide what, and how much, they can perceive or remember, reason or reflect! to leap at one bound from our own consciousness to that of an insect's, is as unreasonable and absurd as if, with a pretty good knowledge of the multiplication table, we were to go straight to the study of the calculus of functions, or as if our comparative anatomists should pass from the study of man's bony structure to that of the fish, and, without any knowledge of the numerous intermediate forms, were to attempt to determine the homologies between these distant types of vertebrata. in such a case would not error be inevitable, and would not continued study in the same direction only render the erroneous conclusions more ingrained and more irremovable. _definition of instinct._ before going further into this subject, we must determine what we mean by the term instinct. it has been variously defined as--"disposition operating without the aid of instruction or experience," "a mental power totally independent of organization," or "a power enabling an animal to do that which, in those things man can do, results from a chain of reasoning, and in things which man cannot do, is not to be explained by any efforts of the intellectual faculties." we find, too, that the word instinct is very frequently applied to acts which are evidently the result either of organization or of habit. the colt or calf is said to walk instinctively, almost as soon as it is born; but this is solely due to its organization, which renders walking both possible and pleasurable to it. so we are said instinctively to hold out our hands to save ourselves from falling, but this is an acquired habit, which the infant does not possess. it appears to me that instinct should be defined as--"the performance by an animal of complex acts, absolutely without instruction or previously-acquired knowledge." thus, acts are said to be performed by birds in building their nests, by bees in constructing their cells, and by many insects in providing for the future wants of themselves or their progeny, without ever having seen such acts performed by others, and without any knowledge of why they perform them themselves. this is expressed by the very common term "blind instinct." but we have here a number of assertions of matters of fact, which, strange to say, have never been proved to be facts at all. they are thought to be so self-evident that they may be taken for granted. no one has ever yet obtained the eggs of some bird which builds an elaborate nest, hatched these eggs by steam or under a quite distinct parent, placed them afterwards in an extensive aviary or covered garden, where the situation and the materials of a nest similar to that of the parent birds may be found, and then seen what kind of nest these birds would build. if under these rigorous conditions they choose the same materials, the same situation, and construct the nest in the same way and as perfectly as their parents did, instinct would be proved in their case; now it is only assumed, and assumed, as i shall show further on, without any sufficient reason. so, no one has ever carefully taken the pupæ of a hive of bees out of the comb, removed them from the presence of other bees, and loosed them in a large conservatory with plenty of flowers and food, and observed what kind of cells they would construct. but till this is done, no one can say that bees build without instruction, no one can say that with every new swarm there are no bees older than those of the same year, who may be the teachers in forming the new comb. now, in a scientific inquiry, a point which can be proved should not be assumed, and a totally unknown power should not be brought in to explain facts, when known powers may be sufficient. for both these reasons i decline to accept the theory of instinct in any case where all other possible modes of explanation have not been exhausted. _does man possess instincts._ many of the upholders of the instinctive theory maintain, that man has instincts exactly of the same nature as those of animals, but more or less liable to be obscured by his reasoning powers; and as this is a case more open to our observation than any other, i will devote a few pages to its consideration. infants are said to suck by instinct, and afterwards to walk by the same power, while in adult man the most prominent case of instinct is supposed to be, the powers possessed by savage races to find their way across a trackless and previously unknown wilderness. let us take first the case of the infant's sucking. it is sometimes absurdly stated that the new-born infant "seeks the breast," and this is held to be a wonderful proof of instinct. no doubt it would be if true, but unfortunately for the theory it is totally false, as every nurse and medical man can testify. still, the child undoubtedly sucks without teaching, but this is one of those _simple_ acts dependent upon organization, which cannot properly be termed instinct, any more than breathing or muscular motion. any object of suitable size in the mouth of an infant excites the nerves and muscles so as to produce the act of suction, and when at a little later period, the will comes into play, the pleasurable sensations consequent on the act lead to its continuance. so, walking is evidently dependent on the arrangement of the bones and joints, and the pleasurable exertion of the muscles, which lead to the vertical posture becoming gradually the most agreeable one; and there can be little doubt that an infant would learn of itself to walk, even if suckled by a wild beast. _how indians travel through unknown and trackless forests._ let us now consider the fact, of indians finding their way through forests they have never traversed before. this is much misunderstood, for i believe it is only performed under such special conditions, as at once to show that instinct has nothing to do with it. a savage, it is true, can find his way through his native forests in a direction in which he has never traversed them before; but this is because from infancy he has been used to wander in them, and to find his way by indications which he has observed himself or learnt from others. savages make long journeys in many directions, and, their whole faculties being directed to the subject, they gain a wide and accurate knowledge of the topography, not only of their own district, but of all the regions round about. every one who has travelled in a new direction communicates his knowledge to those who have travelled less, and descriptions of routes and localities, and minute incidents of travel, form one of the main staples of conversation round the evening fire. every wanderer or captive from another tribe adds to the store of information, and as the very existence of individuals and of whole families and tribes, depends upon the completeness of this knowledge, all the acute perceptive faculties of the adult savage are devoted to acquiring and perfecting it. the good hunter or warrior thus comes to know the bearing of every hill and mountain range, the directions and junctions of all the streams, the situation of each tract characterized by peculiar vegetation, not only within the area he has himself traversed, but for perhaps a hundred miles around it. his acute observation enables him to detect the slightest undulations of the surface, the various changes of subsoil and alterations in the character of the vegetation, that would be quite imperceptible to a stranger. his eye is always open to the direction in which he is going; the mossy side of trees, the presence of certain plants under the shade of rocks, the morning and evening flight of birds, are to him indications of direction, almost as sure as the sun in the heavens. now, if such a savage is required to find his way across this country in a direction in which he has never been before, he is quite equal to the task. by however circuitous a route he has come to the point he is to start from, he has observed all the bearings and distances so well, that he knows pretty nearly where he is, the direction of his own home and that of the place he is required to go to. he starts towards it, and knows that by a certain time he must cross an upland or a river, that the streams should flow in a certain direction, and that he should cross some of them at a certain distance from their sources. the nature of the soil throughout the whole region is known to him, as well as all the great features of the vegetation. as he approaches any tract of country he has been in or near before, many minute indications guide him, but he observes them so cautiously that his white companions cannot perceive by what he has directed his course. every now and then he slightly changes his direction, but he is never confused, never loses himself, for he always feels at home; till at last he arrives at a well-known country, and directs his course so as to reach the exact spot desired. to the europeans whom he guides, he seems to have come without trouble, without any special observation, and in a nearly straight unchanging course. they are astonished, and ask if he has ever been the same route before, and when he answers "no," conclude that some unerring instinct could alone have guided him. but take this same man into another country very similar to his own, but with other streams and hills, another kind of soil, with a somewhat different vegetation and animal life; and after bringing him by a circuitous route to a given point, ask him to return to his starting place, by a straight line of fifty miles through the forest, and he will certainly decline to attempt it, or, attempting it, will more or less completely fail. his supposed instinct does not act out of his own country. a savage, even in a new country, has, however, undoubted advantages, from his familiarity with forest life, his entire fearlessness of being lost, his accurate perception of direction and of distance, and he is thus able very soon to acquire a knowledge of the district that seems marvellous to a civilized man; but my own observation of savages in forest countries has convinced me, that they find their way by the use of no other faculties than those which we ourselves possess. it appears to me, therefore, that to call in the aid of a new and mysterious power to account for savages being able to do that which, under similar conditions, we could almost all of us perform, although perhaps less perfectly, is almost ludicrously unnecessary. in the next essay i shall attempt to show, that much of what has been attributed to instinct in birds, can be also very well explained by crediting them with those faculties of observation, memory, and imitation, and with that limited amount of reason, which they undoubtedly exhibit. vi. the philosophy of birds' nests. _instinct or reason in the construction of birds' nests._ birds, we are told, build their nests by _instinct_, while man constructs his dwelling by the exercise of _reason_. birds never change, but continue to build for ever on the self-same plan; man alters and improves his houses continually. reason advances; instinct is stationary. this doctrine is so very general that it may almost be said to be universally adopted. men who agree on nothing else, accept this as a good explanation of the facts. philosophers and poets, metaphysicians and divines, naturalists and the general public, not only agree in believing this to be probable, but even adopt it as a sort of axiom that is so self-evident as to need no proof, and use it as the very foundation of their speculations on instinct and reason. a belief so general, one would think, must rest on indisputable facts, and be a logical deduction from them. yet i have come to the conclusion that not only is it very doubtful, but absolutely erroneous; that it not only deviates widely from the truth, but is in almost every particular exactly opposed to it. i believe, in short, that birds do _not_ build their nests by instinct; that man does _not_ construct his dwelling by reason; that birds do change and improve when affected by the same causes that make men do so; and that mankind neither alter nor improve when they exist under conditions similar to those which are almost universal among birds. _do men build by reason or by imitation?_ let us first consider the theory of reason, as alone determining the domestic architecture of the human race. man, as a reasonable animal, it is said, continually alters and improves his dwelling. this i entirely deny. as a rule, he neither alters nor improves, any more than the birds do. what have the houses of most savage tribes improved from, each as invariable as the nest of a species of bird? the tents of the arab are the same now as they were two or three thousand years ago, and the mud villages of egypt can scarcely have improved since the time of the pharaohs. the palm-leaf huts and hovels of the various tribes of south america and the malay archipelago, what have they improved from since those regions were first inhabited? the patagonian's rude shelter of leaves, the hollowed bank of the south african earthmen, we cannot even conceive to have been ever inferior to what they now are. even nearer home, the irish turf cabin and the highland stone shelty can hardly have advanced much during the last two thousand years. now, no one imputes this stationary condition of domestic architecture among these savage tribes to instinct, but to simple imitation from one generation to another, and the absence of any sufficiently powerful stimulus to change or improvement. no one imagines that if an infant arab could be transferred to patagonia, or to the highlands, it would, when it grew up, astonish its foster-parents by constructing a tent of skins. on the other hand, it is quite clear that physical conditions, combined with the degree of civilization arrived at, almost necessitate certain types of structure. the turf, or stones, or snow--the palm-leaves, bamboo, or branches, which are the materials of houses in various countries, are used because nothing else is so readily to be obtained. the egyptian peasant has none of these, not even wood. what, then, can he use but mud? in tropical forest-countries, the bamboo and the broad palm-leaves are the natural material for houses, and the form and mode of structure will be decided in part by the nature of the country, whether hot or cool, whether swampy or dry, whether rocky or plain, whether frequented by wild beasts, or whether subject to the attacks of enemies. when once a particular mode of building has been adopted, and has become confirmed by habit and by hereditary custom, it will be long retained, even when its utility has been lost through changed conditions, or through migration into a very different region. as a general rule, throughout the whole continent of america, native houses are built directly upon the ground--strength and security being given by thickening the low walls and the roof. in almost the whole of the malay islands, on the contrary, the houses are raised on posts, often to a great height, with an open bamboo floor; and the whole structure is exceedingly slight and thin. now, what can be the reason of this remarkable difference between countries, many parts of which are strikingly similar in physical conditions, natural productions, and the state of civilization of their inhabitants? we appear to have some clue to it in the supposed origin and migrations of their respective populations. the indigenes of tropical america are believed to have immigrated from the north--from a country where the winters are severe, and raised houses with open floors would be hardly habitable. they moved southwards by land along the mountain ranges and uplands, and in an altered climate continued the mode of construction of their forefathers, modified only by the new materials they met with. by minute observations of the indians of the amazon valley, mr. bates arrived at the conclusion that they were comparatively recent immigrants from a colder climate. he says:--"no one could live long among the indians of the upper amazon without being struck with their constitutional dislike to the heat ... their skin is hot to the touch, and they perspire little ... they are restless and discontented in hot, dry weather, but cheerful on cool days, when the rain is pouring down their naked backs." and, after giving many other details, he concludes, "how different all this is with the negro, the true child of tropical climes! the impression gradually forced itself on my mind that the red indian lives as an immigrant or stranger in these hot regions, and that his constitution was not originally adapted, and has not since become perfectly adapted, to the climate." the malay races, on the other hand, are no doubt very ancient inhabitants of the hottest regions, and are particularly addicted to forming their first settlements at the mouths of rivers or creeks, or in land-locked bays and inlets. they are a pre-eminently maritime or semi-aquatic people, to whom a canoe is a necessary of life, and who will never travel by land if they can do so by water. in accordance with these tastes, they have built their houses on posts in the water, after the manner of the lake-dwellers of ancient europe; and this mode of construction has become so confirmed, that even those tribes who have spread far into the interior, on dry plains and rocky mountains, continue to build in exactly the same manner, and find safety in the height to which they elevate their dwellings above the ground. _why does each bird build a peculiar kind of nest?_ these general characteristics of the abode of savage man will be found to be exactly paralleled by the nests of birds. each species uses the materials it can most readily obtain, and builds in situations most congenial to its habits. the wren, for example, frequenting hedgerows and low thickets, builds its nest generally of _moss_, a material always found where it lives, and among which it probably obtains much of its insect food; but it varies sometimes, using hay or feathers when these are at hand. rooks dig in pastures and ploughed fields for grubs, and in doing so must continually encounter _roots_ and _fibres_. these are used to line its nest. what more natural! the crow feeding on carrion, dead rabbits, and lambs, and frequenting sheep-walks and warrens, chooses _fur_ and _wool_ to line its nest. the lark frequents cultivated fields, and makes its nest, on the ground, of grass lined with _horsehair_--materials the most easy to meet with, and the best adapted to its needs. the kingfisher makes its nest of the _bones_ of the fish which it has eaten. swallows use clay and mud from the margins of the ponds and rivers over which they find their insect food. the materials of birds' nests, like those used by savage man for his house, are, then, those which come first to hand; and it certainly requires no more special instinct to select them in one case than in the other. but, it will be said, it is not so much the materials as the form and structure of nests, that vary so much, and are so wonderfully adapted to the wants and habits of each species; how are these to be accounted for except by instinct? i reply, they may be in a great measure explained by the general habits of the species, the nature of the tools they have to work with, and the materials they can most easily obtain, with the very simplest adaptations of means to an end, quite within the mental capacities of birds. the delicacy and perfection of the nest will bear a direct relation to the size of the bird, its structure and habits. that of the wren or the humming-bird is perhaps not finer or more beautiful in proportion than that of the blackbird, the magpie, or the crow. the wren, having a slender beak, long legs, and great activity, is able with great ease to form a well-woven nest of the finest materials, and places it in thickets and hedgerows which it frequents in its search for food. the titmouse, haunting fruit-trees and walls, and searching in cracks and crannies for insects, is naturally led to build in holes where it has shelter and security; while its great activity, and the perfection of its tools (bill and feet), enable it readily to form a beautiful receptacle for its eggs and young. pigeons having heavy bodies and weak feet and bills (imperfect tools for forming a delicate structure) build rude, flat nests of sticks, laid across strong branches which will bear their weight and that of their bulky young. they can do no better. the caprimulgidæ have the most imperfect tools of all, feet that will not support them except on a flat surface (for they cannot truly perch) and a bill excessively broad, short, and weak, and almost hidden by feathers and bristles. they cannot build a nest of twigs or fibres, hair or moss, like other birds, and they therefore generally dispense with one altogether, laying their eggs on the bare ground, or on the stump or flat limb of a tree. the clumsy hooked bills, short necks and feet, and heavy bodies of parrots, render them quite incapable of building a nest like most other birds. they cannot climb up a branch without using both bill and feet; they cannot even turn round on a perch without holding on with their bill. how, then, could they inlay, or weave, or twist the materials of a nest? consequently, they all lay in holes of trees, the tops of rotten stumps, or in deserted ants' nests, the soft materials of which they can easily hollow out. many terns and sandpipers lay their eggs on the bare sand of the sea-shore, and no doubt the duke of argyll is correct when he says, that the cause of this habit is not that they are unable to form a nest, but that, in such situations, any nest would be conspicuous and lead to the discovery of the eggs. the choice of _place_ is, however, evidently determined by the habits of the birds, who, in their daily search for food, are continually roaming over extensive tide-washed flats. gulls vary considerably in their mode of nesting, but it is always in accordance with their structure and habits. the situation is either on a bare rock or on ledges of sea-cliffs, in marshes or on weedy shores. the materials are sea-weed, tufts of grass or rushes, or the _débris_ of the shore, heaped together with as little order and constructive art as might be expected from the webbed feet and clumsy bill of these birds, the latter better adapted for seizing fish than for forming a delicate nest. the long-legged, broad-billed flamingo, who is continually stalking over muddy flats in search of food, heaps up the mud into a conical stool, on the top of which it lays its eggs. the bird can thus sit upon them conveniently, and they are kept dry, out of reach of the tides. now i believe that throughout the whole class of birds the same general principles will be found to hold good, sometimes distinctly, sometimes more obscurely apparent, according as the habits of the species are more marked, or their structure more peculiar. it is true that, among birds differing but little in structure or habits, we see considerable diversity in the mode of nesting, but we are now so well assured that important changes of climate and of surface have occurred within the period of existing species, that it is by no means difficult to see how such differences have arisen. simple habits are known to be hereditary, and as the area now occupied by each species is different from that of every other, we may be sure that such changes would act differently upon each, and would often bring together species which had acquired their peculiar habits in distinct regions and under different conditions. _how do young birds learn to build their first nest?_ but it is objected, birds do not _learn_ to make their nest as man does to build, for all birds will make exactly the same nest as the rest of their species, even if they have never seen one, and it is instinct alone that can enable them to do this. no doubt this would be instinct if it were true, and i simply ask for proof of the fact. this point, although so important to the question at issue, is always assumed without proof, and even against proof, for what facts there are, are opposed to it. birds brought up from the egg in cages do not make the characteristic nest of their species, even though the proper materials are supplied them, and often make no nest at all, but rudely heap together a quantity of materials; and the experiment has never been fairly tried, of turning out a pair of birds so brought up, into an enclosure covered with netting, and watching the result of their untaught attempts at nest-making. with regard to the songs of birds, however, which is thought to be equally instinctive, the experiment has been tried, and it is found that young birds never have the song peculiar to their species if they have not heard it, whereas they acquire very easily the song of almost any other bird with which they are associated. _do birds sing by instinct or by imitation?_ the hon. daines barrington was of opinion that "notes in birds are no more innate than language is in man, and depend entirely on the master under which they are bred, _as far as their organs will enable them to imitate_ the sounds which they have frequent opportunities of hearing." he has given an account of his experiments in the "philosophical transactions" for (vol. ); he says: "i have educated nestling linnets under the three best singing larks--the skylark, woodlark, and titlark, every one of which, instead of the linnet's song, adhered entirely to that of their respective instructors. when the note of the titlark linnet was thoroughly fixed, i hung the bird in a room with two common linnets for a quarter of a year, which were full in song; the titlark linnet, however, did not borrow any passage from the linnet's song, but adhered stedfastly to that of the titlark." he then goes on to say that birds taken from the nest at two or three weeks old have already learnt the call-note of their species. to prevent this the birds must be taken from the nest when a day or two old, and he gives an account of a goldfinch which he saw at knighton in radnorshire, and which sang exactly like a wren, without any portion of the proper note of its species. this bird had been taken from the nest at two or three days old, and had been hung at a window opposite a small garden, where it had undoubtedly acquired the notes of the wren without having any opportunity of learning even the call of the goldfinch. he also saw a linnet, which had been taken from the nest when only two or three days old, and which, not having any other sounds to imitate, had learnt almost to articulate, and could repeat the words "pretty boy," and some other short sentences. another linnet was educated by himself under a _vengolina_ (a small african finch, which he says sings better than any foreign bird but the american mocking bird), and it imitated its african master so exactly that it was impossible to distinguish the one from the other. still more extraordinary was the case of a common house sparrow, which only chirps in a wild state, but which learnt the song of the linnet and goldfinch by being brought up near those birds. the rev. w. h. herbert made similar observations, and states that the young whinchat and wheatear, which have naturally little variety of song, are ready in confinement to learn from other species, and become much better songsters. the bullfinch, whose natural notes are weak, harsh, and insignificant, has nevertheless a wonderful musical faculty, since it can be taught to whistle complete tunes. the nightingale, on the other hand, whose natural song is so beautiful, is exceedingly apt in confinement to learn that of other birds instead. bechstein gives an account of a redstart which had built under the eaves of his house, which imitated the song of a caged chaffinch in a window underneath, while another in his neighbour's garden repeated some of the notes of a blackcap, which had a nest close by. these facts, and many others which might be quoted, render it certain that the peculiar notes of birds are acquired by imitation, as surely as a child learns english or french, not by instinct, but by hearing the language spoken by its parents. it is especially worthy of remark that, for young birds to acquire a new song correctly, they must be taken out of hearing of their parents very soon, for in the first three or four days they have already acquired some knowledge of the parent notes, which they will afterwards imitate. this shows that very young birds can both hear and remember, and it would be very extraordinary if, after they could see, they could neither observe nor recollect, and could live for days and weeks in a nest and know nothing of its materials and the manner of its construction. during the time they are learning to fly and return often to the nest, they must be able to examine it inside and out in every detail, and as we have seen that their daily search for food invariably leads them among the materials of which it is constructed, and among places similar to that in which it is placed, is it so very wonderful that when they want one themselves they should make one like it? how else, in fact, should they make it? would it not be much more remarkable if they went out of their way to get materials quite different from those used in the parent nest, if they arranged them in a way they had seen no example of, and formed the whole structure differently from that in which they themselves were reared, and which we may fairly presume is that which their whole organization is best adapted to put together with celerity and ease? it has, however, been objected that observation, imitation, or memory, can have nothing to do with a bird's architectural powers, because the young birds, which in england are born in may or june, will proceed in the following april or may to build a nest as perfect and as beautiful as that in which it was hatched, although it could never have seen one built. but surely the young birds _before_ they left the nest had ample opportunities of observing its _form_, its _size_, its _position_, the _materials_ of which it was constructed, and the manner in which those materials were arranged. memory would retain these observations till the following spring, when the materials would come in their way during their daily search for food, and it seems highly probable that the older birds would begin building first, and that those born the preceding summer would follow their example, learning from them how the foundations of the nest are laid and the materials put together.[h] +--------------------------------------------------------------+ | [h] it has been very pertinently remarked by a friend, that, | | if young birds did observe the nest they were reared in, | | they would consider it to be a natural production like the | | leaves and branches and matted twigs that surrounded it, and | | could not possibly conclude that their parents had | | constructed the one and not the other. this may be a valid | | objection, and, if so, we shall have to depend on the mode | | of instruction described in the succeeding paragraphs, but | | the question can only be finally decided by a careful set of | | experiments. | +--------------------------------------------------------------+ again, we have no right to assume that young birds generally pair together. it seems probable that in each pair there is most frequently only one bird born the preceding summer, who would be guided, to some extent, by its partner. my friend, mr. richard spruce, the well-known traveller and botanist, thinks this is the case, and has kindly allowed me to publish the following observations, which he sent me after reading my book. _how young birds may learn to build nests._ "among the indians of peru and ecuador, many of whose customs are relics of the semi-civilisation that prevailed before the spanish conquest, it is usual for the young men to marry old women, and the young women old men. a young man, they say, accustomed to be tended by his mother, would fare ill if he had only an ignorant young girl to take care of him; and the girl herself would be better off with a man of mature years, capable of supplying the place of a father to her. "something like this custom prevails among many animals. a stout old buck can generally fight his way to the doe of his choice, and indeed of as many does as he can manage; but a young buck 'of his first horns,' must either content himself with celibacy, or with some dame well-stricken in years. "compare the nearly parallel case of the domestic cock and of many other birds. then consider the consequences amongst birds that pair, if an old cock sorts with a young hen and an old hen with a young cock, as i think is certainly the case with blackbirds and others that are known to fight for the youngest and handsomest females. one of each pair being already an 'old bird,' will be competent to instruct its younger partner (not only in the futility of 'chaff,' but) in the selection of a site for a nest and how to build it; then, how eggs are hatched and young birds reared. "such, in brief, is my idea of how a bird on its first espousals may be taught the whole duty of the married state." on this difficult point i have sought for information from some of our best field ornithologists, but without success, as it is in most cases impossible to distinguish old from young birds after the first year. i am informed, however, that the males of blackbirds, sparrows, and many other kinds fight furiously, and the conqueror of course has the choice of a mate. mr. spruce's view is at least as probable as the contrary one (that young birds, _as a rule_, pair together), and it is to some extent supported by the celebrated american observer, wilson, who strongly insists on the variety in the nests of birds of the same species, some being so much better finished than others; and he believes _that the less perfect nests are built by the younger, the more perfect by the older, birds_. at all events, till the crucial experiment is made, and a pair of birds raised from the egg without ever seeing a nest are shown to be capable of making one exactly of the parental type, i do not think we are justified in calling in the aid of an unknown and mysterious faculty to do that which is so strictly analogous to the house-building of savage man. again, we always assume that because a nest appears to us delicately and artfully built, that it therefore requires much special knowledge and acquired skill (or their substitute, instinct) in the bird who builds it. we forget that it is formed twig by twig and fibre by fibre, rudely enough at first, but crevices and irregularities, which must seem huge gaps and chasms in the eyes of the little builders, are filled up by twigs and stalks pushed in by slender beak and active foot, and that the wool, feathers, or horsehair are laid thread by thread, so that the result seems a marvel of ingenuity to us, just as would the rudest iinand hut to a native of brobdignag. levaillant has given an account of the process of nest-building by a little african warbler, which sufficiently shows that a very beautiful structure may be produced with very little art. the foundation was laid of moss and flax interwoven with grass and tufts of cotton, and presented a rude mass, five or six inches in diameter, and four inches thick. this was pressed and trampled down repeatedly, so as at last to make it into a kind of felt. the birds pressed it with their bodies, turning round upon them in every direction, so as to get it quite firm and smooth before raising the sides. these were added bit by bit, trimmed and beaten with the wings and feet, so as to felt the whole together, projecting fibres being now and then worked in with the bill. by these simple and apparently inefficient means, the inner surface of the nest was rendered almost as smooth and compact as a piece of cloth. _man's works mainly imitative._ but look at civilised man! it is said; look at grecian, and egyptian, and roman, and gothic, and modern architecture! what advance! what improvement! what refinements! this is what reason leads to, whereas birds remain for ever stationary. if, however, such advances as these are required, to prove the effects of reason as contrasted with instinct, then all savage and many half-civilized tribes have no reason, but build instinctively quite as much as birds do. man ranges over the whole earth, and exists under the most varied conditions, leading necessarily to equally varied habits. he migrates--he makes wars and conquests--one race mingles with another--different customs are brought into contact--the habits of a migrating or conquering race are modified by the different circumstances of a new country. the civilized race which conquered egypt must have developed its mode of building in a forest country where timber was abundant, for it is not probable, that the idea of cylindrical columns originated in a country destitute of trees. the pyramids might have been built by an indigenous race, but not the temples of el uksor and karnak. in grecian architecture, almost every characteristic feature can be traced to an origin in wooden buildings. the columns, the architrave, the frieze, the fillets, the cantelevers, the form of the roof, all point to an origin in some southern forest-clad country, and strikingly corroborate the view derived from philology, that greece was colonised from north-western india. but to erect columns and span them with huge blocks of stone, or marble, is not an act of reason, but one of pure unreasoning imitation. the arch is the only true and reasonable mode of covering over wide spaces with stone, and therefore, grecian architecture, however exquisitely beautiful, is false in principle, and is by no means a good example of the application of reason to the art of building. and what do most of us do at the present day but imitate the buildings of those that have gone before us? we have not even been able to discover or develope any definite style of building best suited for us. we have no characteristic national style of architecture, and to that extent are even below the birds, who have each their characteristic form of nest, exactly adapted to their wants and habits. _birds do alter and improve their nests when altered conditions require it._ the great uniformity in the architecture of each species of bird which has been supposed to prove a nest-building instinct, we may, therefore, fairly impute to the uniformity of the conditions under which each species lives. their range is often very limited, and they very seldom permanently change their country, so as to be placed in new conditions. when, however, new conditions do occur, they take advantage of them just as freely and wisely as man could do. the chimney and house-swallows are a standing proof of a change of habit since chimneys and houses were built, and in america this change has taken place within about three hundred years. thread and worsted are now used in many nests instead of wool and horsehair, and the jackdaw shows an affection for the church steeple which can hardly be explained by instinct. in the more thickly populated parts of the united states, the baltimore oriole uses all sorts of pieces of string, skeins of silk, or the gardener's bass, to weave into its fine pensile nest, instead of the single hairs and vegetable fibres it has painfully to seek in wilder regions; and wilson, a most careful observer, believes that it improves in nest-building by practice--the older birds making the best nests. the purple martin takes possession of empty gourds or small boxes, stuck up for its reception in almost every village and farm in america; and several of the american wrens will also build in cigar boxes, with a small hole cut in them, if placed in a suitable situation. the orchard oriole of the united states offers us an excellent example of a bird which modifies its nest according to circumstances. when built among firm and stiff branches the nest is very shallow, but if, as is often the case, it is suspended from the slender twigs of the weeping willow, it is made much deeper, so that when swayed about violently by the wind the young may not tumble out. it has been observed also, that the nests built in the warm southern states are much slighter and more porous in texture than those in the colder regions of the north. our own house-sparrow equally well adapts himself to circumstances. when he builds in trees, as he, no doubt, always did originally, he constructs a well-made domed nest, perfectly fitted to protect his young ones; but when he can find a convenient hole in a building or among thatch, or in any well-sheltered place, he takes much less trouble, and forms a very loosely-built nest. a curious example of a recent change of habits has occurred in jamaica. previous to , the palm swift (tachornis phænicobea) inhabited exclusively the palm trees in a few districts in the island. a colony then established themselves in two cocoa-nut palms in spanish town, and remained there till , when one tree was blown down, and the other stripped of its foliage. instead of now seeking out other palm trees, the swifts drove out the swallows who built in the piazza of the house of assembly, and took possession of it, building their nests on the tops of the end walls and at the angles formed by the beams and joists, a place which they continue to occupy in considerable numbers. it is remarked that here they form their nest with much less elaboration than when built in the palms, probably from being less exposed. a still more curious example of change and improvement in nest building was published by mr. f. a. pouchet, in the tenth number of the _comptes rendus_ for , just as the first edition of this work appeared. forty years ago m. pouchet had himself collected nests of the house-martin or window-swallow (_hirundo urbica_) from old buildings at rouen, and deposited them in the museum of that city. on recently obtaining some more nests he was surprised, on comparing them with the old ones, to find that they exhibited a decided change of form and structure. this led him to investigate the matter more closely. the changed nests had been obtained from houses in a newly erected quarter of the city, and he found that all the nests in the newly-built streets were of the new form. but on visiting the churches and older buildings, and some rocks where these birds build, he found many nests of the old type along with some of the new pattern. he then examined all the figures and descriptions of the older naturalists, and found that they invariably represented the older form only. the difference between the two forms he states to be as follows. in the old form the nest is a portion of a globe--when situated in the upper angle of a window one-fourth of a hemisphere--and the opening is very small and circular, being of a size just sufficient to allow the body of the bird to pass. in the new form the nest is much wider in proportion to its height, being a segment of a depressed spheroid, and the aperture is very wide and shallow, and close to the horizontal surface to which the nest is attached above. m. pouchet thinks that the new form is an undoubted improvement on the old. the nest has a wider bottom and must allow the young ones to have more freedom of motion than in the old narrower, and deeper nests, and its wide aperture allows the young birds to peep out and breathe the fresh air. this is so wide as to serve as a sort of balcony for them, and two young ones can often be seen on it without interfering with the passage in and out of the old birds. at the same time, by being so close to the roof, it is a better protection against rain, against cold, and against enemies, than the small round hole of the old nests. here, then, we have an improvement in nest building, as well marked as any improvement that takes place in human dwellings in so short a time. but perfection of structure and adaptation to purpose, are not universal characteristics of birds' nests, since there are decided imperfections in the nesting of many birds which are quite compatible with our present theory, but are hardly so with that of instinct, which is supposed to be infallible. the passenger pigeon of america often crowds the branches with its nests till they break, and the ground is strewn with shattered nests, eggs, and young birds. rooks' nests are often so imperfect that during high winds the eggs fall out; but the window-swallow is the most unfortunate in this respect, for white, of selborne, informs us that he has seen them build, year after year, in places where their nests are liable to be washed away by a heavy rain and their young ones destroyed. _conclusion._ a fair consideration of all these facts will, i think, fully support the statement with which i commenced, and show, that the mental faculties exhibited by birds in the construction of their nests, are the same in kind as those manifested by mankind in the formation of their dwellings. these are, essentially, imitation, and a slow and partial adaptation to new conditions. to compare the work of birds with the highest manifestations of human art and science, is totally beside the question. i do not maintain that birds are gifted with reasoning faculties at all approaching in variety and extent to those of man. i simply hold that the phenomena presented by their mode of building their nests, when fairly compared with those exhibited by the great mass of mankind in building their houses, indicate no essential difference in the kind or nature of the mental faculties employed. if instinct means anything, it means the capacity to perform some complex act without teaching or experience. it implies innate ideas of a very definite kind, and, if established, would overthrow mr. mill's sensationalism and all the modern philosophy of experience. that the existence of true instinct may be established in other cases is not impossible, but in the particular instance of birds' nests, which is usually considered one of its strongholds, i cannot find a particle of evidence to show the existence of anything beyond those lower reasoning and imitative powers, which animals are universally admitted to possess. vii. a theory of birds' nests; showing the relation of certain differences of colour in female birds, to their mode of nidification. the habit of forming a more or less elaborate structure for the reception of their eggs and young, must undoubtedly be looked upon as one of the most remarkable and interesting characteristics of the class of birds. in other classes of vertebrate animals, such structures are few and exceptional, and never attain to the same degree of completeness and beauty. birds' nests have, accordingly, attracted much attention, and have furnished one of the stock arguments to prove the existence of a blind but unerring instinct in the lower animals. the very general belief that every bird is enabled to build its nest, not by the ordinary faculties of observation, memory, and imitation, but by means of some innate and mysterious impulse, has had the bad effect of withdrawing attention from the very evident relation that exists between the structure, habits, and intelligence of birds, and the kind of nests they construct. in the preceding essay i have detailed several of these relations, and they teach us, that a consideration of the structure, the food, and other specialities of a bird's existence, will give a clue, and sometimes a very complete one, to the reason why it builds its nest of certain materials, in a definite situation, and in a more or less elaborate manner. i now propose to consider the question from a more general point of view, and to discuss its application to some important problems in the natural history of birds. _changed conditions and persistent habits as influencing nidification._ besides the causes above alluded to, there are two other factors whose effect in any particular case we can only vaguely guess at, but which must have had an important influence in determining the existing details of nidification. these are--changed conditions of existence, whether internal or external, and the influence of hereditary or imitative habit; the first inducing alterations in accordance with changes of organic structure, of climate, or of the surrounding fauna and flora; the other preserving the peculiarities so produced, even when changed conditions render them no longer necessary. many facts have been already given which show that birds do adapt their nests to the situations in which they place them, and the adoption of eaves, chimneys, and boxes, by swallows, wrens, and many other birds, shows that they are always ready to take advantage of changed conditions. it is probable, therefore, that a permanent change of climate would cause many birds to modify the form or materials of their abodes, so as better to protect their young. the introduction of new enemies to eggs or young birds, might introduce many alterations tending to their better concealment. a change in the vegetation of a country, would often necessitate the use of new materials. so, also, we may be sure, that as a species slowly became modified in any external or internal characters, it would necessarily change in some degree its mode of building. this effect would be produced by modifications of the most varied nature; such as the power and rapidity of flight, which must often determine the distance to which a bird will go to obtain materials for its nest; the capacity of sustaining itself almost motionless in the air, which must sometimes determine the position in which a nest can be built; the strength and grasping power of the foot in relation to the weight of the bird, a power absolutely essential to the constructor of a delicately-woven and well-finished nest; the length and fineness of the beak, which has to be used like a needle in building the best textile nests; the length and mobility of the neck, which is needful for the same purpose; the possession of a salivary secretion like that used in the nests of many of the swifts and swallows, as well as that of the song-thrush--peculiarities of habits, which ultimately depend on structure, and which often determine the material most frequently met with or most easily to be obtained. modifications in any of these characters would necessarily lead, either to a change in the materials of the nest, or in the mode of combining them in the finished structure, or in the form or position of that structure. during all these changes, however, certain specialities of nest-building would continue, for a shorter or a longer time after the causes which had necessitated them had passed away. such records of a vanished past meet us everywhere, even in man's works, notwithstanding his boasted reason. not only are the main features of greek architecture, mere reproductions in stone of what were originally parts of a wooden building, but our modern copyists of gothic architecture often build solid buttresses capped with weighty pinnacles, to support a wooden roof which has no outward thrust to render them necessary; and even think they ornament their buildings by adding sham spouts of carved stone, while modern waterpipes, stuck on without any attempt at harmony, do the real duty. so, when railways superseded coaches, it was thought necessary to build the first-class carriages to imitate a number of coach-bodies joined together; and the arm-loops for each passenger to hold on by, which were useful when bad roads made every journey a succession of jolts and lurches, were continued on our smooth macadamised mail-routes, and, still more absurdly, remain to this day in our railway carriages, the relic of a kind of locomotion we can now hardly realize. another good example is to be seen in our boots. when elastic sides came into fashion we had been so long used to fasten them with buttons or laces, that a boot without either looked bare and unfinished, and accordingly the makers often put on a row of useless buttons or imitation laces, because habit rendered the appearance of them necessary to us. it is universally admitted that the habits of children and of savages give us the best clue to the habits and mode of thought of animals; and every one must have observed how children at first imitate the actions of their elders, without any regard to the use or applicability of the particular acts. so, in savages, many customs peculiar to each tribe are handed down from father to son merely by the force of habit, and are continued long after the purpose which they originally served has ceased to exist. with these and a hundred similar facts everywhere around us, we may fairly impute much of what we cannot understand in the details of bird-architecture to an analogous cause. if we do not do so, we must assume, either that birds are guided in every action by pure reason to a far greater extent than men are, or that an infallible instinct leads them to the same result by a different road. the first theory has never, that i am aware of, been maintained by any author, and i have already shown that the second, although constantly assumed, has never been proved, and that a large body of facts is entirely opposed to it. one of my critics has, indeed, maintained that i admit "instinct" under the term "hereditary habit;" but the whole course of my argument shows that i do not do so. hereditary habit is, indeed, the same as instinct when the term is applied to some simple action dependent upon a peculiarity of structure which is hereditary; as when the descendants of tumbler pigeons tumble, and the descendants of pouter pigeons pout. in the present case, however, i compare it strictly to the hereditary, or more properly, persistent or imitative, habits of savages, in building their houses as their fathers did. imitation is a lower faculty than invention. children and savages imitate before they originate; birds, as well as all other animals, do the same. the preceding observations are intended to show, that the exact mode of nidification of each species of bird is probably the result of a variety of causes, which have been continually inducing changes in accordance with changed organic or physical conditions. the most important of these causes seem to be, in the first place, the structure of the species, and, in the second, its environment or conditions of existence. now we know, that every one of the characters or conditions included under these two heads is variable. we have seen that, on the large scale, the main features of the nest built by each group of birds, bears a relation to the organic structure of that group, and we have, therefore, a right to infer, that as structure varies, the nest will vary also in some particular corresponding to the changes of structure. we have seen also, that birds change the position, the form, and the construction of their nest, whenever the available materials or the available situations, vary naturally or have been altered by man; and we have, therefore, a right to infer that similar changes have taken place, when, by a natural process, external conditions have become in any way permanently altered. we must remember, however, that all these factors are very stable during many generations, and only change at a rate commensurate with those of the great physical features of the earth as revealed to us by geology; and we may, therefore, infer that the form and construction of nests, which we have shown to be dependent on them, are equally stable. if, therefore, we find less important and more easily modified characters than these, so correlated with peculiarities of nidification as to indicate that one is probably the cause of the other, we shall be justified in concluding that these variable characters are dependent on the mode of nidification, and not that the form of the nest has been determined by these variable characters. such a correlation i am now about to point out. _classification of nests._ for the purpose of this inquiry it is necessary to group nests into two great classes, without any regard to their most obvious differences or resemblances, but solely looking to the fact of whether the contents (eggs, young, or sitting bird) are hidden or exposed to view. in the first class we place all those in which the eggs and young are completely hidden, no matter whether this is effected by an elaborate covered structure, or by depositing the eggs in some hollow tree or burrow underground. in the second, we group all in which the eggs, young, and sitting bird are exposed to view, no matter whether there is the most beautifully formed nest, or none at all. kingfishers, which build almost invariably in holes in banks; woodpeckers and parrots, which build in hollow trees; the icteridæ of america, which all make beautiful covered and suspended nests; and our own wren, which builds a domed nest, are examples of the former; while our thrushes, warblers, and finches, as well as the crowshrikes, chatterers, and tanagers of the tropics, together with all raptorial birds and pigeons, and a vast number of others in every part of the world, all adopt the latter mode of building. it will be seen that this division of birds according to their nidification, bears little relation to the character of the nest itself. it is a functional not a structural classification. the most rude and the most perfect specimens of bird-architecture are to be found in both sections. it has, however, a certain relation to natural affinities, for large groups of birds, undoubtedly allied, fall into one or the other division exclusively. the species of a genus or of a family are rarely divided between the two primary classes, although they are frequently divided between the two very distinct modes of nidification that exist in the first of them. all the scansorial or climbing, and most of the fissirostral or wide-gaped birds, for example, build concealed nests; and, in the latter group, the two families which build open nests, the swifts and the goat-suckers, are undoubtedly very widely separated from the other families with which they are associated in our classifications. the tits vary much in their mode of nesting, some making open nests concealed in a hole, while others build domed or even pendulous covered nests, but they all come under the same class. starlings vary in a similar way. the talking mynahs, like our own starlings, build in holes, the glossy starlings of the east (of the genus calornis) form a hanging covered nest, while the genus sturnopastor builds in a hollow tree. one of the most striking cases in which one family of birds is divided between the two classes, is that of the finches; for while most of the european species build exposed nests, many of the australian finches make them dome-shaped. _sexual differences of colour in birds._ turning now from the nests to the creatures who make them, let us consider birds themselves from a somewhat unusual point of view, and form them into separate groups, according as both sexes, or the males only, are adorned with conspicuous colours. the sexual differences of colour and plumage in birds are very remarkable, and have attracted much attention; and, in the case of polygamous birds, have been well explained by mr. darwin's principle of sexual selection. we can, to a great extent, understand how male pheasants and grouse have acquired their more brilliant plumage and greater size, by the continual rivalry of the males both in strength and beauty; but this theory does not throw any light on the causes which have made the female toucan, bee-eater, parroquet, macaw and tit, in almost every case as gay and brilliant as the male, while the gorgeous chatterers, manakins, tanagers, and birds of paradise, as well as our own blackbird, have mates so dull and inconspicuous that they can hardly be recognised as belonging to the same species. _the law which connects the colours of female birds with the mode of nidification._ the above-stated anomaly can, however, now be explained by the influence of the mode of nidification, since i find that, with but very few exceptions, it is the rule--_that when both sexes are of strikingly gay and conspicuous colours, the nest is of the first class, or such as to conceal the sitting bird; while, whenever there is a striking contrast of colours, the male being gay and conspicuous, the female dull and obscure, the nest is open and the sitting bird exposed to view_. i will now proceed to indicate the chief facts that support this statement, and will afterwards explain the manner in which i conceive the relation has been brought about. we will first consider those groups of birds in which the female is gaily or at least conspicuously coloured, and is in most cases exactly like the male. . kingfishers (alcedinidæ). in some of the most brilliant species of this family the female exactly resembles the male; in others there is a sexual difference, but it rarely tends to make the female less conspicuous. in some, the female has a band across the breast, which is wanting in the male, as in the beautiful halcyon diops of ternate. in others the band is rufous in the female, as in several of the american species; while in dacelo gaudichaudii, and others of the same genus, the tail of the female is rufous, while that of the male is blue. in most kingfishers the nest is in a deep hole in the ground; in tanysiptera it is said to be in a hole in the nests of termites, or sometimes in crevices under overhanging rocks. . motmots (momotidæ). in these showy birds the sexes are exactly alike, and the nest in a hole under ground. . puff-birds (bucconidæ). these birds are often gaily coloured; some have coral-red bills; the sexes are exactly alike, and the nest is in a hole in sloping ground. . trogons (trogonidæ). in these magnificent birds the females are generally less brightly coloured than the males, but are yet often gay and conspicuous. the nest is in a hole of a tree. . hoopoes (upupidæ). the barred plumage and long crests of these birds render them conspicuous. the sexes are exactly alike, and the nest is in a hollow tree. . hornbills (bucerotidæ). these large birds have enormous coloured bills, which are generally quite as well coloured and conspicuous in the females. their nests are always in hollow trees, where the female is entirely concealed. . barbets (capitonidæ). these birds are all very gaily-coloured, and, what is remarkable, the most brilliant patches of colour are disposed about the head and neck, and are very conspicuous. the sexes are exactly alike, and the nest is in a hole of a tree. . toucans (rhamphastidæ). these fine birds are coloured in the most conspicuous parts of their body, especially on the large bill, and on the upper and lower tail coverts, which are crimson, white, or yellow. the sexes are exactly alike, and they always build in a hollow tree. . plaintain-eaters (musophagidæ). here again the head and bill are most brilliantly coloured in both sexes, and the nest is in a hole of a tree. . ground cuckoos (centropus). these birds are often of conspicuous colours, and are alike in both sexes. they build a domed nest. . woodpeckers (picidæ). in this family the females often differ from the males, in having a yellow or white, instead of a crimson crest, but are almost as conspicuous. they all nest in holes in trees. . parrots (psittaci). in this great tribe, adorned with the most brilliant and varied colours, the rule is, that the sexes are precisely alike, and this is the case in the most gorgeous families, the lories, the cockatoos, and the macaws; but in some there is a sexual difference of colour to a slight extent. all build in holes, mostly in trees, but sometimes in the ground, or in white ants' nests. in the single case in which the nest is exposed, that of the australian ground parrot, pezoporus formosus, the bird has lost the gay colouring of its allies, and is clothed in sombre and completely protective tints of dusky green and black. . gapers (eurylæmidæ). in these beautiful eastern birds, somewhat allied to the american chatterers, the sexes are exactly alike, and are adorned with the most gay and conspicuous markings. the nest is a woven structure, _covered over_, and suspended from the extremities of branches over water. . pardalotus (ampelidæ). in these australian birds the females differ from the males, but are often very conspicuous, having brightly-spotted heads. their nests are sometimes dome-shaped, sometimes in holes of trees, or in burrows in the ground. . tits (paridæ). these little birds are always pretty, and many (especially among the indian species) are very conspicuous. they always have the sexes alike, a circumstance very unusual among the smaller gaily-coloured birds of our own country. the nest is always covered over or concealed in a hole. . nuthatches (sitta). often very pretty birds, the sexes alike, and the nest in a hole. .---- (sittella). the female of these australian nuthatches is often the most conspicuous, being white-and black-marked. the nest is, according to gould, "completely concealed among upright twigs connected together." . creepers (climacteris). in these australian creepers the sexes are alike, or the female most conspicuous, and the nest is in a hole of a tree. . estrelda, amadina. in these genera of eastern and australian finches the females, although more or less different from the males, are still very conspicuous having a red rump, or being white spotted. they differ from most others of the family in building domed nests. . certhiola. in these pretty little american creepers the sexes are alike, and they build a domed nest. . mynahs (sturnidæ). these showy eastern starlings have the sexes exactly alike. they build in holes of trees. . calornis (sturnidæ). these brilliant metallic starlings have no sexual differences. they build a pensile covered nest. . hangnests (icteridæ). the red or yellow and black plumage of most of these birds is very conspicuous, and is exactly alike in both sexes. they are celebrated for their fine purse-shaped pensile nests. it will be seen that this list comprehends six important families of fissirostres, four of scansores, the psittaci, and several genera, with three entire families of passeres, comprising about twelve hundred species, or about one-seventh of all known birds. * * * * * the cases in which, whenever the male is gaily coloured, the female is much less gay or quite inconspicuous, are exceedingly numerous, comprising, in fact, almost all the bright-coloured passeres, except those enumerated in the preceding class. the following are the most remarkable:-- . chatterers (cotingidæ). these comprise some of the most gorgeous birds in the world, vivid blues, rich purples, and bright reds, being the most characteristic colours. the females are always obscurely tinted, and are often of a greenish hue, not easily visible among the foliage. . manakins (pipridæ). these elegant birds, whose caps or crests are of the most brilliant colours, are usually of a sombre green in the female sex. . tanagers (tanagridæ). these rival the chatterers in the brilliancy of their colours, and are even more varied. the females are generally of plain and sombre hues, and always less conspicuous than the males. in the extensive families of the warblers (sylviadæ), thrushes (turdidæ), flycatchers (muscicapidæ), and shrikes (laniadæ), a considerable proportion of the species are beautifully marked with gay and conspicuous tints, as is also the case in the pheasants and grouse; but in every case the females are less gay, and are most frequently of the very plainest and least conspicuous hues. now, throughout _the whole of these families the nest is open_, and i am not aware of a single instance in which any one of these birds builds a _domed nest_, or places it in a _hole of a tree_, or _underground_, or in any place where it is effectually concealed. in considering the question we are now investigating, it is not necessary to take into account the larger and more powerful birds, because these seldom depend much on concealment to secure their safety. in the raptorial birds bright colours are as a rule absent; and their structure and habits are such as not to require any special protection for the female. the larger waders are sometimes very brightly coloured in both sexes; but they are probably little subject to the attacks of enemies, since the scarlet ibis, the most conspicuous of birds, exists in immense quantities in south america. in game birds and water-fowl, however, the females are often very plainly coloured, when the males are adorned with brilliant hues; and the abnormal family of the megapodidæ offers us the interesting fact of an identity in the colours of the sexes (which in megacephalon and talegalla are somewhat conspicuous), in conjunction with the habit of not sitting on the eggs at all. _what the facts teach us._ taking the whole body of evidence here brought forward, embracing as it does almost every group of bright-coloured birds, it will, i think, be admitted that the relation between the two series of facts in the colouring and nidification of birds has been sufficiently established. there are, it is true, a few apparent and some real exceptions, which i shall consider presently; but they are too few and unimportant to weigh much against the mass of evidence on the other side, and may for the present be neglected. let us then consider what we are to do with this unexpected set of correspondences between groups of phenomena which, at first sight, appear so disconnected. do they fall in with any other groups of natural phenomena? do they teach us anything of the way in which nature works, and give us any insight into the causes which have brought about the marvellous variety, and beauty, and harmony of living things? i believe we can answer these questions in the affirmative; and i may mention, as a sufficient proof that these are not isolated facts, that i was first led to see their relation to each other by the study of an analogous though distinct set of phenomena among insects, that of protective resemblance and "mimicry." on considering this remarkable series of corresponding facts, the first thing we are taught by them seems to be, that there is no incapacity in the female sex among birds, to receive the same bright hues and strongly contrasted tints with which their partners are so often decorated, since whenever they are _protected and concealed_ during the period of incubation _they are similarly adorned_. the fair inference is, that it is chiefly due to the absence of protection or concealment during this important epoch, that gay and conspicuous tints are withheld or left undeveloped. the mode in which this has been effected is very intelligible, if we admit the action of natural and sexual selection. it would appear from the numerous cases in which both sexes are adorned with equally brilliant colours (while both sexes are rarely armed with equally developed offensive and defensive weapons when not required for individual safety), that the normal action of "sexual selection" is to develop colour and beauty in both sexes, by the preservation and multiplication of all varieties of colour in either sex which are pleasing to the other. several very close observers of the habits of animals have assured me, that male birds and quadrupeds do often take very strong likes and dislikes to individual females, and we can hardly believe that the one sex (the female) can have a general taste for colour while the other has no such taste. however this may be, the fact remains, that in a vast number of cases the female acquires as brilliant and as varied colours as the male, and therefore most probably acquires them in the same way as the male does; that is, either because the colour is useful to it, or is correlated with some useful variation, or is pleasing to the other sex. the only remaining supposition is that it is transmitted from the other sex, without being of any use. from the number of examples above adduced of bright colours in the female, this would imply that colour-characters acquired by one sex are generally (but not necessarily) transmitted to the other. if this be the case it will, i think, enable us to explain the phenomena, even if we do not admit that the male bird is ever influenced in the choice of a mate by her more gay or perfect plumage. the female bird, while sitting on her eggs in an uncovered nest, is much exposed to the attacks of enemies, and any modification of colour which rendered her more conspicuous would often lead to her destruction and that of her offspring. all variations of colour in this direction in the female, would therefore sooner or later be eliminated, while such modifications as rendered her inconspicuous, by assimilating her to surrounding objects, as the earth or the foliage, would, on the whole, survive the longest, and thus lead to the attainment of those brown or green and inconspicuous tints, which form the colouring (of the upper surface at least), of the vast majority of female birds which sit upon open nests. this does not imply, as some have thought, that all female birds were once as brilliant as the males. the change has been a very gradual one, generally dating from the origin of genera or of larger groups, but there can be no doubt that the remote ancestry of birds having great sexual differences of colour, were nearly or quite alike, sometimes (perhaps in most cases) more nearly resembling the female, but occasionally perhaps being nearer what the male is now. the young birds (which usually resemble the females) will probably give some idea of this ancestral type, and it is well known that the young of allied species and of different sexes are often undistinguishable. _colour more variable than structure or habits, and therefore the character which has generally been modified._ at the commencement of this essay, i have endeavoured to prove, that the characteristic differences and the essential features of birds' nests, are dependent on the structure of the species and upon the present and past conditions of their existence. both these factors are more important and less variable than colour; and we must therefore conclude that in most cases the mode of nidification (dependent on structure and environment) has been the cause, and not the effect, of the similarity or differences of the sexes as regards colour. when the confirmed habit of a group of birds, was to build their nests in holes of trees like the toucans, or in holes in the ground like the kingfishers, the protection the female thus obtained, during the important and dangerous time of incubation, placed the two sexes on an equality as regards exposure to attack, and allowed "sexual selection," or any other cause, to act unchecked in the development of gay colours and conspicuous markings in both sexes. when, on the other hand (as in the tanagers and flycatchers), the habit of the whole group was to build open cup-shaped nests in more or less exposed situations, the production of colour and marking in the female, by whatever cause, was continually checked by its rendering her too conspicuous, while in the male it had free play, and developed in him the most gorgeous hues. this, however, was not perhaps universally the case; for where there was more than usual intelligence and capacity for change of habits, the danger the female was exposed to by a partial brightness of colour or marking might lead to the construction of a concealed or covered nest, as in the case of the tits and hangnests. when this occurred, a special protection to the female would be no longer necessary; so that the acquisition of colour and the modification of the nest, might in some cases act and react on each other and attain their full development together. _exceptional cases confirmatory of the above explanation._ there exist a few very curious and anomalous facts in the natural history of birds, which fortunately serve as crucial tests of the truth of this mode of explaining the inequalities of sexual colouration. it has been long known, that in some species the males either assisted in, or wholly performed, the act of incubation. it has also been often noticed, that in certain birds the usual sexual differences were reversed, the male being the more plainly coloured, the female more gay and often larger. i am not, however, aware that these two anomalies had ever been supposed to stand to each other in the relation of cause and effect, till i adduced them in support of my views of the general theory of protective adaptation. yet it is undoubtedly the fact, that in the best known cases in which the female bird is more conspicuously coloured than the male, it is either positively ascertained that the latter performs the duties of incubation, or there are good reasons for believing such to be the case. the most satisfactory example is that of the gray phalarope (phalaropus fulicarius), the sexes of which are alike in winter, while in summer the female instead of the male takes on a gay and conspicuous nuptial plumage; but the male performs the duties of incubation, sitting upon the eggs, which are laid upon the bare ground. in the dotterell (eudromias morinellus) the female is larger and more brightly coloured than the male; and here, also, it is almost certain that the latter sits upon the eggs. the turnices of india also, have the female larger and often more brightly coloured; and mr. jerdon states, in his "birds of india," that the natives report, that, during the breeding season, the females desert their eggs and associate in flocks, while the males are employed in hatching the eggs. in the few other cases in which the females are more brightly coloured, the habits are not accurately known. the case of the ostriches and emeus will occur to many as a difficulty, for here the male incubates, but is not less conspicuous than the female; but there are two reasons why the case does not apply;--the birds are too large to derive any safety from concealment, from enemies which would devour the eggs they can defend themselves by force, while to escape from their personal foes they trust to speed. we find, therefore, that a very large mass of facts relating to the sexual colouration and the mode of nidification of birds, including some of the most extraordinary anomalies to be found in their natural history, can be shown to have an interdependent relation to each other, on the simple principle of the need of greater protection to that parent which performs the duties of incubation. considering the very imperfect knowledge we possess of the habits of most extra-european birds, the exceptions to the prevalent rule are few, and generally occur in isolated species or in small groups; while several apparent exceptions can be shown to be really confirmations of the law. _real or apparent exceptions to the law stated at page ._ the only marked exceptions i have been able to discover are the following:-- . king crows (dicrourus). these birds are of a glossy black colour with long forked tails. the sexes present no difference, and they build open nests. this apparent exception may probably be accounted for by the fact that these birds do not need the protection of a less conspicuous colour. they are very pugnacious, and often attack and drive away crows, hawks, and kites; and as they are semi-gregarious in their habits, the females are not likely to be attacked while incubating. . orioles (oriolidæ). the true orioles are very gay birds; the sexes are, in many eastern species, either nearly or quite alike, and the nests are open. this is one of the most serious exceptions, but it is one that to some extent proves the rule; for in this case it has been noticed, that the parent birds display excessive care and solicitude in concealing the nest among thick foliage, and in protecting their offspring by incessant and anxious watching. this indicates that the want of protection consequent on the bright colour of the female makes itself felt, and is obviated by an increased development of the mental faculties. . ground thrushes (pittidæ). these elegant and brilliantly-coloured birds are generally alike in both sexes, and build an open nest. it is curious, however, that this is only an apparent exception, for almost all the bright colours are on the under surface, the back being usually olive green or brown, and the head black, with brown or whitish stripes, all which colours would harmonize with the foliage, sticks, and roots which surround the nest, built on or near the ground, and thus serve as a protection to the female bird. . grallina australis. this australian bird is of strongly contrasted black and white colours. the sexes are exactly alike, and it builds an open clay nest in an exposed situation on a tree. this appears to be a most striking exception, but i am by no means sure that it is so. we require to know what tree it usually builds on, the colour of the bark or of the lichens that grow upon it, the tints of the ground, or of other surrounding objects, before we can say that the bird, when sitting on its nest, is really conspicuous. it has been remarked that small patches of white and black blend at a short distance to form grey, one of the commonest tints of natural objects. . sunbirds (nectarineidæ). in these beautiful little birds the males only are adorned with brilliant colours, the females being quite plain, yet they build covered nests in all the cases in which the nidification is known. this is a negative rather than a positive exception to the rule, since there may be other causes besides the need for protection, which prevent the female acquiring the gay colours of her mate, and there is one curious circumstance which tends to elucidate it. the male of leptocoma zeylanica is said to assist in incubation. it is possible, therefore, that the group may originally have used open nests, and some change of conditions, leading the male bird to sit, may have been followed by the adoption of a domed nest. this is, however, the most serious exception i have yet found to the general rule. . superb warblers (maluridæ). the males of these little birds are adorned with the most gorgeous colours, while the females are very plain, yet they make domed nests. it is to be observed, however, that the male plumage is nuptial merely, and is retained for a very short time; the rest of the year both sexes are plain alike. it is probable, therefore, that the domed nest is for the protection of these delicate little birds against the rain, and that there is some unknown cause which has led to the development of colour in the males only. there is one other case which at first sight looks like an exception, but which is far from being one in reality, and deserves to be mentioned. in the beautiful waxwing, (bombycilla garrula,) the sexes are very nearly alike, and the elegant red wax tips to the wing-feathers are nearly, and sometimes quite, as conspicuous in the female as in the male. yet it builds an open nest, and a person looking at the bird would say it ought according to my theory to cover its nest. but it is, in reality, as completely protected by its colouration as the most plainly coloured bird that flies. it breeds only in very high latitudes, and the nest, placed in fir-trees, is formed chiefly of lichens. now the delicate gray and ashy and purplish hues of the head and back, together with the yellow of the wings and tail, are tints that exactly harmonize with the colours of various species of lichens, while the brilliant red wax tips exactly represent the crimson fructification of the common lichen, cladonia coccifera. when sitting on its nest, therefore, the female bird will exhibit no colours that are not common to the materials of which it is constructed; and the several tints are distributed in about the same proportions as they occur in nature. at a short distance the bird would be indistinguishable from the nest it is sitting on, or from a natural clump of lichens, and will thus be completely protected. i think i have now noticed all exceptions of any importance to the law of dependence of sexual colour on nidification. it will be seen that they are very few in number, compared with those which support the generalization; and in several cases there are circumstances in the habits or structure of the species that sufficiently explain them. it is remarkable also that i have found scarcely any _positive_ exceptions, that is, cases of very brilliant or conspicuous female birds in which the nest was not concealed. much less can there be shown any group of birds, in which the females are all of decidedly conspicuous colours on the upper surface, and yet sit in open nests. the many cases in which birds of dull colours in both sexes make domed or concealed nests, do not, of course, affect this theory one way or the other; since its purpose is only to account for the fact, that brilliant females of brilliant males are _always_ found to have covered or hidden nests, while obscure females of brilliant males _almost always_ have open and exposed nests. the fact that all classes of nests occur with dull coloured birds in both sexes merely shows, as i have strongly maintained, that in most cases the character of the nest determines the colouration of the female, and not _vice versâ_. if the views here advocated are correct, as to the various influences that have determined the specialities of every bird's nest, and the general colouration of female birds, with their action and reaction on each other, we can hardly expect to find evidence more complete than that here set forth. nature is such a tangled web of complex relations, that a series of correspondences running through hundreds of species, genera, and families, in every part of the system, can hardly fail to indicate a true casual connexion; and when, of the two factors in the problem, one can be shown to be dependent on the most deeply seated and the most stable facts of structure and conditions of life, while the other is a character universally admitted to be superficial and easily modified, there can be little doubt as to which is cause and which effect. _various modes of protection of animals._ but the explanation of the phenomenon here attempted does not rest alone on the facts i have been able now to adduce. in the essay on "mimicry," it is shown how important a part the necessity for protection has played, in determining the external form and colouration, and sometimes even the internal structure of animals. as illustrating this latter point, i may refer to the remarkable hooked, branched, or star-like spiculæ in many sponges, which are believed to have the function chiefly, of rendering them unpalatable to other creatures. the holothuridæ or sea-cucumbers possess a similar protection, many of them having anchor-shaped spicules embedded in their skin, as the synapta; while others (cuviera squamata) are covered with a hard calcareous pavement. many of these are of a bright red or purple colour, and are very conspicuous, while the allied trepang, or beche-de-mer (holothuria edulis), which is not armed with any such defensive weapons, is of a dull sand-or mud-colour, so as hardly to be distinguished from the sea bed on which it reposes. many of the smaller marine animals are protected by their almost invisible transparency, while those that are most brightly coloured will be often found to have a special protection, either in stinging tentacles like physalia, or in a hard calcareous crust, as in the star fishes. _females of some groups require and obtain more protection than the males._ in the struggle for existence incessantly going on, protection or concealment is one of the most general and most effectual means of maintaining life; and it is by modifications of colour that this protection can be most readily obtained, since no other character is subject to such numerous and rapid variations. the case i have now endeavoured to illustrate is exactly analogous to what occurs among butterflies. as a general rule, the female butterfly is of dull and inconspicuous colours, even when the male is most gorgeously arrayed; but when the species is protected from attack by a disagreeable odour, as in the heliconidæ, danaidæ and acroeidæ, both sexes display the same or equally brilliant hues. among the species which gain a protection by imitating these, the very weak and slow-flying leptalides resemble them in both sexes, because both sexes alike require protection, while in the more active and strong-winged genera--papilio, pieris, and diadema--it is generally the females only that mimic the protected groups, and in doing so often become actually more gay and more conspicuous than the males, thus reversing the usual and in fact almost universal characters of the sexes. so, in the wonderful eastern leaf-insects of the genus phyllium, it is the female only that so marvellously imitates a green leaf; and in all these cases the difference can be traced to the greater need of protection for the female, on whose continued existence, while depositing her eggs, the safety of the race depends. in mammalia and in reptiles, however brilliant the colours may be, there is rarely any difference between that of the sexes, because the female is not necessarily more exposed to attack than the male. it may, i think, be looked upon as a confirmation of this view, that no single case is known either in the above-named genera--papilio, pieris, and diadema--or in any other butterfly, of a male _alone_, mimicking one of the danaidæ or heliconidæ. yet the necessary colour is far more abundant in the males, and variations always seem ready for any useful purpose. this seems to depend on the general law, that each species and each sex can only be modified just as far as is absolutely necessary for it to maintain itself in the struggle for existence, not a step further. a male insect by its structure and habits is less exposed to danger, and also requires less protection than the female. it cannot, therefore, alone acquire any further protection through the agency of natural selection. but the female requires some extra protection, to balance the greater danger to which she is exposed, and her greater importance to the existence of the species; and this she always acquires, in one way or another, through the action of natural selection. in his "origin of species," fourth edition, p. , mr. darwin recognises the necessity for protection as sometimes being a cause of the obscure colours of female birds; but he does not seem to consider it so very important an agent in modifying colour as i am disposed to do. in the same paragraph (p. ), he alludes to the fact of female birds and butterflies being sometimes very plain, sometimes as gay as the males; but, apparently, considers this mainly due to peculiar laws of inheritance, which sometimes continue acquired colour in the line of one sex only, sometimes in both. without denying the action of such a law (which mr. darwin informs me he has facts to support), i impute the difference, in the great majority of cases, to the greater or less need of protection in the female sex in these groups of animals. this need was seen to exist a century ago by the hon. daines barrington, who, in the article already quoted (see p. ), after alluding to the fact that singing birds are all small, and suggesting (but i think erroneously) that this may have arisen from the difficulty larger birds would have in concealing themselves if they called the attention of their enemies by loud notes, goes on thus:--"i should rather conceive it is for the same reason no hen bird sings, because this talent would be still more dangerous during incubation, which _may possibly also account for the inferiority in point of plumage_." this is a curious anticipation of the main idea on which this essay is founded. it has been unnoticed for near a century, and my attention was only recently called to it by mr. darwin himself. _conclusion._ to some persons it will perhaps appear, that the causes to which i impute so much of the external aspect of nature are too simple, too insignificant, and too unimportant for such a mighty work. but i would ask them to consider, that the great object of all the peculiarities of animal structure is to preserve the life of the individual, and to maintain the existence of the species. colour has hitherto been too often looked upon as something adventitious and superficial, something given to an animal not to be useful to itself, but solely to gratify man or even superior beings--to add to the beauty and ideal harmony of nature. if this were the case, then, it is evident that the colours of organised beings would be an exception to most other natural phenomena. they would not be the product of general laws, or determined by ever-changing external conditions; and we must give up all enquiry into their origin and causes, since (by the hypothesis) they are dependent on a will whose motives must ever be unknown to us. but, strange to say, no sooner do we begin to examine and classify the colours of natural objects, than we find that they are intimately related to a variety of other phenomena, and are, like them, strictly subordinated to general laws. i have here attempted to elucidate some of these laws in the case of birds, and have shown how the mode of nidification has affected the colouring of the female sex in this group. i have before shown to how great an extent, and in how many ways, the need of protection has determined the colours of insects, and of some groups of reptiles and mammalia, and i would now call particular attention to the fact that the gay tints of flowers, so long supposed to be a convincing proof that colour has been bestowed for other purposes than the good of its possessor, have been shown by mr. darwin to follow the same great law of utility. flowers do not often need protection, but very often require the aid of insects to fertilize them, and maintain their reproductive powers in the greatest vigour. their gay colours attract insects, as do also their sweet odours and honeyed secretions; and that this is the main function of colour in flowers is shown by the striking fact, that those flowers which can be perfectly fertilized by the wind, and do not need the aid of insects, _rarely or never have gaily-coloured flowers_. this wide extension of the general principle of utility to the colours of such varied groups, both in the animal and vegetable kingdoms, compels us to acknowledge that the "reign of law" has been fairly traced into this stronghold of the advocates of special creation. and to those who oppose the explanation i have given of the facts adduced in this essay, i would again respectfully urge that they must grapple with the whole of the facts, not one or two of them only. it will be admitted that, on the theory of evolution and natural selection, a wide range of facts with regard to colour in nature have been co-ordinated and explained. until at least an equally wide range of facts can be shown to be in harmony with any other theory, we can hardly be expected to abandon that which has already done such good service, and which has led to the discovery of so many interesting and unexpected harmonies among the most common (but hitherto most neglected and least understood), of the phenomena presented by organised beings. viii. creation by law. among the various criticisms that have appeared on mr. darwin's celebrated "origin of species," there is, perhaps, none that will appeal to so large a number of well educated and intelligent persons, as that contained in the duke of argyll's "reign of law." the noble author represents the feelings and expresses the ideas of that large class, who take a keen interest in the progress of science in general, and especially that of natural history, but have never themselves studied nature in detail, or acquired that personal knowledge of the structure of closely allied forms,--the wonderful gradations from species to species and from group to group, and the infinite variety of the phenomena of "variation" in organic beings,--which are absolutely necessary for a full appreciation of the facts and reasonings contained in mr. darwin's great work. nearly half of the duke's book is devoted to an exposition of his idea of "creation by law," and he expresses so clearly what are his difficulties and objections as regards the theory of "natural selection," that i think it advisable that they should be fairly answered, and that his own views should be shown to lead to conclusions, as hard to accept as any which he imputes to mr. darwin. the point on which the duke of argyll lays most stress, is, that proofs of mind everywhere meet us in nature, and are more especially manifest wherever we find "contrivance" or "beauty." he maintains that this indicates the constant supervision and direct interference of the creator, and cannot possibly be explained by the unassisted action of any combination of laws. now, mr. darwin's work has for its main object, to show, that all the phenomena of living things,--all their wonderful organs and complicated structures, their infinite variety of form, size, and colour, their intricate and involved relations to each other,--may have been produced by the action of a few general laws of the simplest kind, laws which are in most cases mere statements of admitted facts. the chief of these laws or facts are the following:-- . _the law of multiplication in geometrical progression._--all organized beings have enormous powers of multiplication. even man, who increases slower than all other animals, could under the most favourable circumstances double his numbers every fifteen years, or a hundred-fold in a century. many animals and plants could increase their numbers from ten to a thousand-fold every year. . _the law of limited populations._--the number of living individuals of each species in any country, or in the whole globe, is practically stationary; whence it follows that the whole of this enormous increase must die off almost as fast as produced, except only those individuals for whom room is made by the death of parents. as a simple but striking example, take an oak forest. every oak will drop annually thousands or millions of acorns, but till an old tree falls, not one of these millions can grow up into an oak. they must die at various stages of growth. . _the law of heredity, or likeness of offspring to their parents._--this is a universal, but not an absolute law. all creatures resemble their parents in a high degree, and in the majority of cases very accurately; so that even individual peculiarities, of whatever kind, in the parents, are almost always transmitted to some of the offspring. . _the law of variation._--this is fully expressed by the lines:-- "no being on this earthly ball, is like another, all in all." offspring resemble their parents very much, but not wholly--each being possesses its individuality. this "variation" itself varies in amount, but it is always present, not only in the whole being, but in every part of every being. every organ, every character, every feeling is individual; that is to say, _varies_ from the same organ, character, or feeling in every other individual. . _the law of unceasing change of physical conditions upon the surface of the earth._--geology shows us that this change has always gone on in times past, and we also know that it is now everywhere going on. . _the equilibrium or harmony of nature._--when a species is well adapted to the conditions which environ it, it flourishes; when imperfectly adapted it decays; when ill-adapted it becomes extinct. if _all_ the conditions which determine an organism's well-being are taken into consideration, this statement can hardly be disputed. * * * * * this series of facts or laws, are mere statements of what is the condition of nature. they are facts or inferences which are generally known, generally admitted--but in discussing the subject of the "origin of species"--as generally forgotten. it is from these universally admitted facts, that the origin of all the varied forms of nature may be deduced by a logical chain of reasoning, which, however, is at every step verified and shown to be in strict accord with facts; and, at the same time, many curious phenomena which can by no other means be understood, are explained and accounted for. it is probable, that these primary facts or laws are but results of the very nature of life, and of the essential properties of organized and unorganized matter. mr. herbert spencer, in his "first principles" and his "biology" has, i think, made us able to understand how this may be; but at present we may accept these simple laws without going further back, and the question then is--whether the variety, the harmony, the contrivance, and the beauty we perceive in organic beings, can have been produced by the action of these laws alone, or whether we are required to believe in the incessant interference and direct action of the mind and will of the creator. it is simply a question of how the creator has worked. the duke (and i quote him as having well expressed the views of the more intelligent of mr. darwin's opponents) maintains, that he has personally applied general laws to produce effects, which those laws are not in themselves capable of producing; that the universe alone, with all its laws intact, would be a sort of chaos, without variety, without harmony, without design, without beauty; that there is not (and therefore we may presume that there could not be) any self-developing power in the universe. i believe, on the contrary, that the universe is so constituted as to be self-regulating; that as long as it contains life, the forms under which that life is manifested have an inherent power of adjustment to each other and to surrounding nature; and that this adjustment necessarily leads to the greatest amount of variety and beauty and enjoyment, because it does depend on general laws, and not on a continual supervision and re-arrangement of details. as a matter of feeling and religion, i hold this to be a far higher conception of the creator and of the universe than that which may be called the "continual interference" hypothesis; but it is not a question to be decided by our feelings or convictions, it is a question of facts and of reason. could the change, which geology shows us has ever taken place in the forms of life, have been produced by general laws, or does it imperatively require the incessant supervision of a creative mind? this is the question for us to consider, and our opponents have the difficult task of proving a negative, if we show that there are both facts and analogies in our favour. _mr. darwin's metaphors liable to misconception._ mr. darwin has laid himself open to much misconception, and has given to his opponents a powerful weapon against himself, by his continual use of metaphor in describing the wonderful co-adaptations of organic beings. "it is curious," says the duke of argyll, "to observe the language which this most advanced disciple of pure naturalism instinctively uses, when he has to describe the complicated structure of this curious order of plants (the orchids). 'caution in ascribing intentions to nature,' does not seem to occur to him as possible. intention is the one thing which he does see, and which, when he does not see, he seeks for diligently until he finds it. he exhausts every form of words and of illustration, by which intention or mental purpose can be described. 'contrivance'--'curious contrivance,'--'beautiful contrivance,'--these are expressions which occur over and over again. here is one sentence describing the parts of a particular species: 'the labellum is developed into a long nectary, _in order_ to attract lepidoptera, and we shall presently give reason for suspecting that the nectar is _purposely_ so lodged, that it can be sucked only slowly _in order_ to give time for the curious chemical quality of the viscid matter setting hard and dry.'" many other examples of similar expressions are quoted by the duke, who maintains that no explanation of these "contrivances" has been or can be given, except on the supposition of a personal contriver, specially arranging the details of each case, although causing them to be produced by the ordinary processes of growth and reproduction. now there is a difficulty in this view of the origin of the structure of orchids which the duke does not allude to. the majority of flowering plants are fertilized, either without the agency of insects or, when insects are required, without any very important modification of the structure of the flower. it is evident, therefore, that flowers might have been formed as varied, fantastic, and beautiful as the orchids, and yet have been fertilized without more complexity of structure than is found in violets, or clover, or primroses, or a thousand other flowers. the strange springs and traps and pitfalls found in the flowers of orchids cannot be necessary _per se_, since exactly the same end is gained in ten thousand other flowers which do not possess them. is it not then an extraordinary idea, to imagine the creator of the universe _contriving_ the various complicated parts of these flowers, as a mechanic might contrive an ingenious toy or a difficult puzzle? is it not a more worthy conception that they are some of the results of those general laws which were so co-ordinated at the first introduction of life upon the earth as to result necessarily in the utmost possible development of varied forms? but let us take one of the simpler cases adduced and see if our general laws are unable to account for it. _a case of orchis-structure explained by natural selection._ there is a madagascar orchis--the angræcum sesquipedale--with an immensely long and deep nectary. how did such an extraordinary organ come to be developed? mr. darwin's explanation is this. the pollen of this flower can only be removed by the base of the proboscis of some very large moths, when trying to get at the nectar at the bottom of the vessel. the moths with the longest probosces would do this most effectually; they would be rewarded for their long tongues by getting the most nectar; whilst on the other hand, the flowers with the deepest nectaries would be the best fertilized by the largest moths preferring them. consequently, the deepest nectaried orchids and the longest tongued moths would each confer on the other an advantage in the battle of life. this would tend to their respective perpetuation, and to the constant lengthening of nectaries and probosces. now let it be remembered, that what we have to account for, is only the unusual length of this organ. a nectary is found in many orders of plants and is especially common in the orchids, but in this one case only is it more than a foot long. how did this arise? we begin with the fact, proved experimentally by mr. darwin, that moths do visit orchids, do thrust their spiral trunks into the nectaries, and do fertilize them by carrying the pollinia of one flower to the stigma of another. he has further explained the exact mechanism by which this is effected, and the duke of argyll admits the accuracy of his observations. in our british species, such as orchis pyramidalis, it is not necessary that there should be any exact adjustment between the length of the nectary and that of the proboscis of the insect; and thus a number of insects of various sizes are found to carry away the pollinia and aid in the fertilization. in the angræcum sesquipedale, however, it is necessary that the proboscis should be forced into a particular part of the flower, and this would only be done by a large moth burying its proboscis to the very base, and straining to drain the nectar from the bottom of the long tube, in which it occupies a depth of one or two inches only. now let us start from the time when the nectary was only half its present length or about six inches, and was chiefly fertilized by a species of moth which appeared at the time of the plant's flowering, and whose proboscis was of the same length. among the millions of flowers of the angræcum produced every year, some would always be shorter than the average, some longer. the former, owing to the structure of the flower, would not get fertilized, because the moths could get all the nectar without forcing their trunks down to the very base. the latter would be well fertilized, and the longest would on the average be the best fertilized of all. by this process alone the average length of the nectary would annually increase, because, the short-nectaried flowers being sterile and the long ones having abundant offspring, exactly the same effect would be produced as if a gardener destroyed the short ones and sowed the seed of the long ones only; and this we know by experience would produce a regular increase of length, since it is this very process which has increased the size and changed the form of our cultivated fruits and flowers. but this would lead in time to such an increased length of the nectary that many of the moths could only just reach the surface of the nectar, and only the few with exceptionally long trunks be able to suck up a considerable portion. this would cause many moths to neglect these flowers because they could not get a satisfying supply of nectar, and if these were the only moths in the country the flowers would undoubtedly suffer, and the further growth of the nectary be checked by exactly the same process which had led to its increase. but there are an immense variety of moths, of various lengths of proboscis, and as the nectary became longer, other and larger species would become the fertilizers, and would carry on the process till the largest moths became the sole agents. now, if not before, the moth would also be affected, for those with the longest probosces would get most food, would be the strongest and most vigorous, would visit and fertilize the greatest number of flowers, and would leave the largest number of descendants. the flowers most completely fertilized by these moths being those which had the longest nectaries, there would in each generation be on the average an increase in the length of the nectaries, and also an average increase in the length of the probosces of the moths; and this would be a _necessary result_ from the fact that nature ever fluctuates about a mean, or that in every generation there would be flowers with longer and shorter nectaries, and moths with longer and shorter probosces than the average. no doubt there are a hundred causes that might have checked this process before it had reached the point of development at which we find it. if, for instance, the variation in the quantity of nectar had been at any stage greater than the variation in the length of the nectary, then smaller moths could have reached it and have effected the fertilization. or if the growth of the probosces of the moths had from other causes increased quicker than that of the nectary, or if the increased length of proboscis had been injurious to them in any way, or if the species of moth with the longest proboscis had become much diminished by some enemy or other unfavourable conditions, then, in any of these cases, the shorter nectaried flowers, which would have attracted and could have been fertilized by the smaller kinds of moths, would have had the advantage. and checks of a similar nature to these no doubt have acted in other parts of the world, and have prevented such an extraordinary development of nectary as has been produced by favourable conditions in madagascar only, and in one single species of orchid. i may here mention that some of the large sphinx moths of the tropics have probosces nearly as long as the nectary of angræcum sesquipedale. i have carefully measured the proboscis of a specimen of macrosila cluentius from south america, in the collection of the british museum, and find it to be nine inches and a quarter long! one from tropical africa (macrosila morganii) is seven inches and a half. a species having a proboscis two or three inches longer could reach the nectar in the largest flowers of angræcum sesquipedale, whose nectaries vary in length from ten to fourteen inches. that such a moth exists in madagascar may be safely predicted; and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet neptune,--and i venture to predict they will be equally successful! now, instead of this beautiful self-acting adjustment, the opposing theory is, that the creator of the universe, by a direct act of his will, so disposed the natural forces influencing the growth of this one species of plant as to cause its nectary to increase to this enormous length; and at the same time, by an equally special act, determined the flow of nourishment in the organization of the moth, so as to cause its proboscis to increase in exactly the same proportion, having previously so constructed the angræcum that it could only be maintained in existence by the agency of this moth. but what proof is given or suggested that this was the mode by which the adjustment took place? none whatever, except a feeling that there is an adjustment of a delicate kind, and an inability to see how known causes could have produced such an adjustment. i believe i have shown, however, that such an adjustment is not only possible but inevitable, unless at some point or other we deny the action of those simple laws which we have already admitted to be but the expressions of existing facts. _adaptation brought about by general laws._ it is difficult to find anything like parallel cases in inorganic nature, but that of a river may perhaps illustrate the subject in some degree. let us suppose a person totally ignorant of modern geology to study carefully a great river system. he finds in its lower part, a deep broad channel filled to the brim, flowing slowly through a flat country and carrying out to the sea a quantity of fine sediment. higher up it branches into a number of smaller channels, flowing alternately through flat valleys and between high banks; sometimes he finds a deep rocky bed with perpendicular walls, carrying the water through a chain of hills; where the stream is narrow he finds it deep, where wide shallow. further up still, he comes to a mountainous region, with hundreds of streams and rivulets, each with its tributary rills and gullies, collecting the water from every square mile of surface, and every channel adapted to the water that it has to carry. he finds that the bed of every branch, and stream, and rivulet, has a steeper and steeper slope as it approaches its sources, and is thus enabled to carry off the water from heavy rains, and to bear away the stones and pebbles and gravel, that would otherwise block up its course. in every part of this system he would see exact adaptation of means to an end. he would say, that this system of channels must have been designed, it answers its purpose so effectually. nothing but a mind could have so exactly adapted the slopes of the channels, their capacity, and frequency, to the nature of the soil and the quantity of the rainfall. again, he would see special adaptation to the wants of man, in broad quiet navigable rivers flowing through fertile plains that support a large population, while the rocky streams and mountain torrents, were confined to those sterile regions suitable only for a small population of shepherds and herdsmen. he would listen with incredulity to the geologist, who assured him, that the adaptation and adjustment he so admired was an inevitable result of the action of general laws. that the rains and rivers, aided by subterranean forces, had modelled the country, had formed the hills and valleys, had scooped out the river beds, and levelled the plains;--and it would only be after much patient observation and study, after having watched the minute changes produced year by year, and multiplying them by thousands and ten thousands, after visiting the various regions of the earth and seeing the changes everywhere going on, and the unmistakable signs of greater changes in past times,--that he could be made to understand that the surface of the earth, however beautiful and harmonious it may appear, is strictly due in every detail to the action of forces which are demonstrably self-adjusting. moreover, when he had sufficiently extended his inquiries, he would find, that every evil effect which he would imagine must be the result of non-adjustment does somewhere or other occur, only it is not always evil. looking on a fertile valley, he would perhaps say--"if the channel of this river were not well adjusted, if for a few miles it sloped the wrong way, the water could not escape, and all this luxuriant valley, full of human beings, would become a waste of waters." well, there are hundreds of such cases. every lake is a valley "wasted by water," and in some cases (as the dead sea) it is a positive evil, a blot upon the harmony and adaptation of the surface of the earth. again, he might say--"if rain did not fall here, but the clouds passed over us to some other regions, this verdant and highly cultivated plain would become a desert." and there are such deserts over a large part of the earth, which abundant rains would convert into pleasant dwelling-places for man. or he might observe some great navigable river, and reflect how easily rocks, or a steeper channel in places, might render it useless to man;--and a little inquiry would show him hundreds of rivers in every part of the world, which are thus rendered useless for navigation. exactly the same thing occurs in organic nature. we see some one wonderful case of adjustment, some unusual development of an organ, but we pass over the hundreds of cases in which that adjustment and development do not occur. no doubt when one adjustment is absent another takes its place, because no organism can continue to exist that is not adjusted to its environment; and unceasing variation with unlimited powers of multiplication, in most cases, furnish the means of self-adjustment. the world is so constituted, that by the action of general laws there is produced the greatest possible variety of surface and of climate; and by the action of laws equally general, the greatest possible variety of organisms have been produced, adapted to the varied conditions of every part of the earth. tho objector would probably himself admit, that the varied surface of the earth--the plains and valleys, the hills and mountains, the deserts and volcanoes, the winds and currents, the seas and lakes and rivers, and the various climates of the earth--are all the results of general laws acting and re-acting during countless ages; and that the creator does not appear to guide and control the action of these laws--here determining the height of a mountain, there altering the channel of a river--here making the rains more abundant, there changing the direction of a current. he would probably admit that the forces of inorganic nature are self-adjusting, and that the result necessarily fluctuates about a given mean condition (which is itself slowly changing), while within certain limits the greatest possible amount of variety is produced. if then a "contriving mind" is not necessary at every step of the process of change eternally going on in the inorganic world, why are we required to believe in the continual action of such a mind in the region of organic nature? true, the laws at work are more complex, the adjustments more delicate, the appearance of special adaptation more remarkable; but why should we measure the creative mind by our own? why should we suppose the machine too complicated, to have been designed by the creator so complete that it would necessarily work out harmonious results? the theory of "continual interference" is a limitation of the creator's power. it assumes that he could not work by pure law in the organic, as he has done in the inorganic world; it assumes that he could not foresee the consequences of the laws of matter and mind combined--that results would continually arise which are contrary to what is best, and that he has to change what would otherwise be the course of nature, in order to produce that beauty, and variety, and harmony, which even we, with our limited intellects, can conceive to be the result of self-adjustment in a universe governed by unvarying law. if we could not conceive the world of nature to be self-adjusting and capable of endless development, it would even then be an unworthy idea of a creator, to impute the incapacity of our minds to him; but when many human minds can conceive, and can even trace out in detail some of the adaptations in nature as the necessary results of unvarying law, it seems strange that, in the interests of religion, any one should seek to prove that the system of nature, instead of being above, is far below our highest conceptions of it. i, for one, cannot believe that the world would come to chaos if left to law alone. i cannot believe that there is in it no inherent power of developing beauty or variety, and that the direct action of the deity is required to produce each spot or streak on every insect, each detail of structure in every one of the millions of organisms that live or have lived upon the earth. for it is impossible to draw a line. if any modifications of structure could be the result of law, why not all? if some self-adaptations could arise, why not others? if any varieties of colour, why not all the varieties we see? no attempt is made to explain this, except by reference to the fact that "purpose" and "contrivance" are everywhere visible, and by the illogical deduction that they could only have arisen from the direct action of some mind, because the direct action of our minds produces similar "contrivances"; but it is forgotten that adaptation, however produced, must have the appearance of design. the channel of a river looks as if made _for_ the river, although it is made _by_ it; the fine layers and beds in a deposit of sand, often look as if they had been sorted, and sifted, and levelled, designedly; the sides and angles of a crystal exactly resemble similar forms designed by man; but we do not therefore conclude that these effects have, in each individual case, required the directing action of a creative mind, or see any difficulty in their being produced by natural law. _beauty in nature._ let us, however, leave this general argument for a while, and turn to another special case, which has been appealed to as conclusive against mr. darwin's views. "beauty" is, to some persons, as great a stumbling-block as "contrivance." they cannot conceive a system of the universe, so perfect, as necessarily to develop every form of beauty, but suppose that when anything specially beautiful occurs, it is a step beyond what that system could have produced, something which the creator has added for his own delectation. speaking of the humming birds, the duke of argyll says: "in the first place, it is to be observed of the whole group, that there is no connection which can be traced or conceived, between the splendour of the humming birds and any function essential to their life. if there were any such connection, that splendour could not be confined, as it almost exclusively is, to only one sex. the female birds are, of course, not placed at any disadvantage in the struggle for existence by their more sombre colouring." and after describing the various ornaments of these birds, he says: "mere ornament and variety of form, and these for their own sake, is the only principle or rule with reference to which creative power seems to have worked in these wonderful and beautiful birds.... a crest of topaz is no better in the struggle for existence than a crest of sapphire. a frill ending in spangles of the emerald is no better in the battle of life than a frill ending in spangles of the ruby. a tail is not affected for the purposes of flight, whether its marginal or its central feathers are decorated with white.... mere beauty and mere variety, for their own sake, are objects which we ourselves seek when we can make the forces of nature subordinate to the attainment of them. there seems to be no conceivable reason why we should doubt or question, that these are ends and aims also in the forms given to living organisms" ("reign of law," p. ). here the statement that "no connection can be conceived between the splendour of the humming birds and any function essential to their life," is met by the fact, that mr. darwin has not only conceived but has shown, both by observation and reasoning, how beauty of colour and form may have a direct influence on the most important of all the functions of life, that of reproduction. in the variations to which birds are subject, any more brilliant colour than usual would be attractive to the females, and would lead to the individuals so adorned leaving more than the average number of offspring. experiment and observation have shown, that this kind of sexual selection does actually take place; and the laws of inheritance would necessarily lead to the further development of any individual peculiarity that was attractive, and thus the splendour of the humming birds is directly connected with their very existence. it is true that "a crest of topaz may be no better than a crest of sapphire," but either of these may be much better than no crest at all; and the different conditions under which the parent form must have existed in different parts of its range, will have determined different variations of tint, either of which were advantageous. the reason why female birds are not adorned with equally brilliant plumes is sufficiently clear; they would be injurious, by rendering their possessors too conspicuous during incubation. survival of the fittest, has therefore favoured the development of those dark green tints on the upper surface of so many female humming birds, which are most conducive to their protection while the important functions of hatching and rearing the young are being carried on. keeping in mind the laws of multiplication, variation, and survival of the fittest, which are for ever in action, these varied developments of beauty and harmonious adjustments to conditions, are not only conceivable but demonstrable results. the objection i am now combating is solely founded on the supposed analogy of the creator's mind to ours, as regards the love of beauty for its own sake; but if this analogy is to be trusted, then there ought to be no natural objects which are disagreeable or ungraceful in our eyes. and yet it is undoubtedly the fact that there are many such. just as surely as the horse and deer are beautiful and graceful, the elephant, rhinoceros, hippopotamus, and camel are the reverse. the majority of monkeys and apes are not beautiful; the majority of birds have no beauty of colour; a vast number of insects and reptiles are positively ugly. now, if the creator's mind is like ours, whence this ugliness? it is useless to say "that is a mystery we cannot explain," because we have attempted to explain one-half of creation by a method that will not apply to the other half. we know that a man with the highest taste and with unlimited wealth, practically does abolish all ungraceful and disagreeable forms and colours from his own domains. if the beauty of creation is to be explained by the creator's love of beauty, we are bound to ask why he has not banished deformity from the earth, as the wealthy and enlightened man does from his estate and from his dwelling; and if we can get no satisfactory answer, we shall do well to reject the explanation offered. again, in the case of flowers, which are always especially referred to, as the surest evidence of beauty being an end of itself in creation, the whole of the facts are never fairly met. at least half the plants in the world have not bright-coloured or beautiful flowers; and mr. darwin has lately arrived at the wonderful generalization, that flowers have become beautiful solely to attract insects to assist in their fertilization. he adds, "i have come to this conclusion from finding it an invariable rule, that when a flower is fertilized by the wind it never has a gaily-coloured corolla." here is a most wonderful case of beauty being _useful_, when it might be least expected. but much more is proved; for when beauty is of no use to the plant it is not given. it cannot be imagined to do any harm. it is simply not necessary, and is therefore withheld! we ought surely to have been told how this fact is consistent with beauty being "an end in itself," and with the statement of its being given to natural objects "for its own sake." _how new forms are produced by variation and selection._ let us now consider another of the popular objections which the duke of argyll thus sets forth:-- "mr. darwin does not pretend to have discovered any law or rule, according to which new forms have been born from old forms. he does not hold that outward conditions, however changed, are sufficient to account for them.... his theory seems to be far better than a mere theory--to be an established scientific truth--in so far as it accounts, in part at least, for the success and establishment and spread of new forms _when they have arisen_. but it does not even suggest the law under which, or by or according to which, such new forms are introduced. natural selection can do nothing, except with the materials presented to its hands. it cannot select except among the things open to selection.... strictly speaking, therefore, mr. darwin's theory is not a theory on the origin of species at all, but only a theory on the causes which lead to the relative success or failure of such new forms as may be born into the world." ("reign of law," p. .) in this, and many other passages in his work, the duke of argyll sets forth his idea of creation as a "creation by birth," but maintains that each birth of a new form from parents differing from itself, has been produced by a special interference of the creator, in order to direct the process of development into certain channels; that each new species is in fact a "special creation," although brought into existence through the ordinary laws of reproduction. he maintains therefore, that the laws of multiplication and variation cannot furnish the right kinds of materials at the right times for natural selection to work on. i believe, on the contrary, that it can be logically _proved_ from the six axiomatic laws before laid down, that such materials would be furnished; but i prefer to show there are abundance of _facts_ which demonstrate that they are furnished. the experience of all cultivators of plants and breeders of animals shows, that when a sufficient number of individuals are examined, variations of any required kind can always be met with. on this depends the possibility of obtaining breeds, races, and fixed varieties of animals and plants; and it is found, that any one form of variation may be accumulated by selection, without materially affecting the other characters of the species; each _seems_ to vary in the one required direction only. for example, in turnips, radishes, potatoes, and carrots, the root or tuber varies in size, colour, form, and flavour, while the foliage and flowers seem to remain almost stationary; in the cabbage and lettuce, on the contrary, the foliage can be modified into various forms and modes of growth, the root, flower, and fruit remaining little altered; in the cauliflower and brocoli the flower heads vary; in the garden pea the pod only changes. we get innumerable forms of fruit in the apple and pear, while the leaves and flowers remain undistinguishable; the same occurs in the gooseberry and garden currant. directly however, (in the very same genus) we want the flower to vary in the ribes sanguineum, it does so, although mere cultivation for hundreds of years has not produced marked differences in the flowers of ribes grossularia. when fashion demands any particular change in the form or size, or colour of a flower, sufficient variation always occurs in the right direction, as is shown by our roses, auriculas, and geraniums; when, as recently, ornamental leaves come into fashion sufficient variation is found to meet the demand, and we have zoned pelargoniums, and variegated ivy, and it is discovered that a host of our commonest shrubs and herbaceous plants have taken to vary in this direction just when we want them to do so! this rapid variation is not confined to old and well-known plants subjected for a long series of generations to cultivation, but the sikim rhododendrons, the fuchsias, and calceolarias from the andes, and the pelargoniums from the cape are equally accommodating, and vary just when and where and how we require them. turning to animals we find equally striking examples. if we want any special quality in any animal we have only to breed it in sufficient quantities and watch carefully, and the required variety is _always_ found, and can be increased to almost any desired extent. in sheep, we get flesh, fat, and wool; in cows, milk; in horses, colour, strength, size, and speed; in poultry, we have got almost any variety of colour, curious modifications of plumage, and the capacity of perpetual egg-laying. in pigeons we have a still more remarkable proof of the universality of variation, for it has been at one time or another the fancy of breeders to change the form of every part of these birds, and they have never found the required variations absent. the form, size, and shape of bill and feet, have been changed to such a degree as is found only in distinct genera of wild birds; the number of tail feathers has been increased, a character which is generally one of the most permanent nature, and is of high importance in the classification of birds; and the size, the colour, and the habits, have been also changed to a marvellous extent. in dogs, the degree of modification and the facility with which it is effected, is almost equally apparent. look at the constant amount of variation in opposite directions that must have been going on, to develop the poodle and the greyhound from the same original stock! instincts, habits, intelligence, size, speed, form, and colour, have always varied, so as to produce the very races which the wants or fancies or passions of men may have led them to desire. whether they wanted a bull-dog to torture another animal, a greyhound to catch a hare, or a bloodhound to hunt down their oppressed fellow-creatures, the required variations have always appeared. now this great mass of facts, of which a mere sketch has been here given, are fully accounted for by the "law of variation" as laid down at the commencement of this paper. universal variability--small in amount but in every direction, ever fluctuating about a mean condition until made to advance in a given direction by "election," natural or artificial,--is the simple basis for the indefinite modification of the forms of life;--partial, unbalanced, and consequently unstable modifications being produced by man, while those developed under the unrestrained action of natural laws, are at every step self-adjusted to external conditions by the dying out of all unadjusted forms, and are therefore stable and comparatively permanent. to be consistent in their views, our opponents must maintain that every one of the variations that have rendered possible the changes produced by man, have been determined at the right time and place by the will of the creator. every race produced by the florist or the breeder, the dog or the pigeon fancier, the ratcatcher, the sporting man, or the slave-hunter, must have been provided for by varieties occurring when wanted; and as these variations were never withheld, it would prove, that the sanction of an all-wise and all-powerful being, has been given to that which the highest human minds consider to be trivial, mean, or debasing. this appears to be a complete answer to the theory, that variation sufficient in amount to be accumulated in a given direction must be the direct act of the creative mind, but it is also sufficiently condemned by being so entirely unnecessary. the facility with which man obtains new races, depends chiefly upon the number of individuals he can procure to select from. when hundreds of florists or breeders are all aiming at the same object, the work of change goes on rapidly. but a common species in nature contains a thousand-or a million-fold more individuals than any domestic race; and survival of the fittest must unerringly preserve all that vary in the right direction, not only in obvious characters but in minute details, not only in external but in internal organs; so that if the materials are sufficient for the needs of man, there can be no want of them to fulfil the grand purpose of keeping up a supply of modified organisms, exactly adapted to the changed conditions that are always occurring in the inorganic world. _the objection that there are limits to variation._ having now, i believe, fairly answered the chief objections of the duke of argyll, i proceed to notice one or two of those adduced in an able and argumentative essay on the "origin of species" in the _north british review_ for july, . the writer first attempts to prove that there are strict limits to variation. when we begin to select variations in any one direction, the process is comparatively rapid, but after a considerable amount of change has been effected it becomes slower and slower, till at length its limits are reached and no care in breeding and selection can produce any further advance. the race-horse is chosen as an example. it is admitted that, with any ordinary lot of horses to begin with, careful selection would in a few years make a great improvement, and in a comparatively short time the standard of our best racers might be reached. but that standard has not for many years been materially raised, although unlimited wealth and energy are expended in the attempt. this is held to prove that there are definite limits to variation in any special direction, and that we have no reason to suppose that mere time, and the selective process being carried on by natural law, could make any material difference. but the writer does not perceive that this argument fails to meet the real question, which is, not whether indefinite and unlimited change in any or all directions is possible, but whether such differences as do occur in nature could have been produced by the accumulation of variations by selection. in the matter of speed, a limit of a definite kind as regards land animals does exist in nature. all the swiftest animals--deer, antelopes, hares, foxes, lions, leopards, horses, zebras, and many others, have reached very nearly the same degree of speed. although the swiftest of each must have been for ages preserved, and the slowest must have perished, we have no reason to believe there is any advance of speed. the possible limit under existing conditions, and perhaps under possible terrestrial conditions, has been long ago reached. in cases, however, where this limit had not been so nearly reached as in the horse, we have been enabled to make a more marked advance and to produce a greater difference of form. the wild dog is an animal that hunts much in company, and trusts more to endurance than to speed. man has produced the greyhound, which differs much more from the wolf or the dingo than the racer does from the wild arabian. domestic dogs, again, have varied more in size and in form than the whole family of canidæ in a state of nature. no wild dog, fox, or wolf, is either so small as some of the smallest terriers and spaniels, or so large as the largest varieties of hound or newfoundland dog. and, certainly, no two wild animals of the family differ so widely in form and proportions as the chinese pug and the italian greyhound, or the bulldog and the common greyhound. the known range of variation is, therefore, more than enough for the derivation of all the forms of dogs, wolves, and foxes from a common ancestor. again, it is objected that the pouter or the fan-tail pigeon cannot be further developed in the same direction. variation seems to have reached its limits in these birds. but so it has in nature. the fan-tail has not only more tail feathers than any of the three hundred and forty existing species of pigeons, but more than any of the eight thousand known species of birds. there is, of course, some limit to the number of feathers of which a tail useful for flight can consist, and in the fan-tail we have probably reached that limit. many birds have the oesophagus or the skin of the neck more or less dilatable, but in no known bird is it so dilatable as in the pouter pigeon. here again the possible limit, compatible with a healthy existence, has probably been reached. in like manner the differences in the size and form of the beak in the various breeds of the domestic pigeon, is greater than that between the extreme forms of beak in the various genera and sub-families of the whole pigeon tribe. from these facts, and many others of the same nature, we may fairly infer, that if rigid selection were applied to any organ, we could in a comparatively short time produce a much greater amount of change than that which occurs between species and species in a state of nature, since the differences which we do produce are often comparable with those which exist between distinct genera or distinct families. the facts adduced by the writer of the article referred to, of the definite limits to variability in certain directions in domesticated animals, are, therefore, no objection whatever to the view, that all the modifications which exist in nature have been produced by the accumulation, by natural selection, of small and useful variations, since those very modifications have equally definite and very similar limits. _objection to the argument from classification._ to another of this writer's objections--that by professor thomson's calculations the sun can only have existed in a solid state , , of years, and that therefore _time_ would not suffice for the slow process of development of all living organisms--it is hardly necessary to reply, as it cannot be seriously contended, even if this calculation has claims to approximate accuracy, that the process of change and development may not have been sufficiently rapid to have occurred within that period. his objection to the classification argument is, however, more plausible. the uncertainty of opinion among naturalists as to which are species and which varieties, is one of mr. darwin's very strong arguments that these two names cannot belong to things quite distinct in nature and origin. the reviewer says that this argument is of no weight, because the works of man present exactly the same phenomena; and he instances patent inventions, and the excessive difficulty of determining whether they are new or old. i accept the analogy though it is a very imperfect one, and maintain that such as it is, it is all in favour of mr. darwin's views. for are not all inventions of the same kind directly affiliated to a common ancestor? are not improved steam engines or clocks the lineal descendants of some existing steam engine or clock? is there ever a new creation in art or science any more than in nature? did ever patentee absolutely originate any complete and entire invention, no portion of which was derived from anything that had been made or described before? it is therefore clear that the difficulty of distinguishing the various classes of inventions which claim to be new, is of the same nature as the difficulty of distinguishing varieties and species, because neither are absolute new creations, but both are alike descendants of pre-existing forms, from which and from each other they differ by varying and often imperceptible degrees. it appears, then, that however plausible this writer's objections may seem, whenever he descends from generalities to any specific statement, his supposed difficulties turn out to be in reality strongly confirmatory of mr. darwin's view. _the "times," on natural selection._ the extraordinary misconception of the whole subject by popular writers and reviewers, is well shown by an article which appeared in the _times_ newspaper on "the reign of law." alluding to the supposed economy of nature, in the adaptation of each species to its own place and its special use, the reviewer remarks: "to this universal law of the greatest economy, the law of natural selection stands in direct antagonism as the law of 'greatest possible waste' of time and of creative power. to conceive a duck with webbed feet and a spoon-shaped bill, living by suction, to pass naturally into a gull with webbed feet and a knife-like bill, living on flesh, in the longest possible time and in the most laborious possible way, we may conceive it to pass from the one to the other state by natural selection. the battle of life the ducks will have to fight will increase in peril continually as they cease (with the change of their bill) to be ducks, and attain a _maximum_ of danger in the condition in which they begin to be gulls; and ages must elapse and whole generations must perish, and countless generations of the one species be created and sacrificed, to arrive at one single pair of the other." in this passage the theory of natural selection is so absurdly misrepresented that it would be amusing, did we not consider the misleading effect likely to be produced by this kind of teaching in so popular a journal. it is assumed that the duck and the gull are essential parts of nature, each well fitted for its place, and that if one had been produced from the other by a gradual metamorphosis, the intermediate forms would have been useless, unmeaning, and unfitted for any place, in the system of the universe. now, this idea can only exist in a mind ignorant of the very foundation and essence of the theory of natural selection, which is, the preservation of _useful_ variations only, or, as has been well expressed, in other words, the "survival of the fittest." every intermediate form which could possibly have arisen during the transition from the duck to the gull, so far from having an unusually severe battle to fight for existence, or incurring any "_maximum_ of danger," would necessarily have been as accurately adjusted to the rest of nature, and as well fitted to maintain and to enjoy its existence, as the duck or the gull actually are. if it were not so, it never could have been produced under the law of natural selection. _intermediate or generalized forms of extinct animals, an indication of transmutation or development._ the misconception of this writer illustrates another point very frequently overlooked. it is an essential part of mr. darwin's theory, that one existing animal has not been derived from any other existing animal, but that both are the descendants of a common ancestor, which was at once different from either, but, in essential characters, intermediate between them both. the illustration of the duck and the gull is therefore misleading; one of these birds has not been derived from the other, but both from a common ancestor. this is not a mere supposition invented to support the theory of natural selection, but is founded on a variety of indisputable facts. as we go back into past time, and meet with the fossil remains of more and more ancient races of extinct animals, we find that many of them actually are intermediate between distinct groups of existing animals. professor owen continually dwells on this fact: he says in his "palæontology," p. : "a more generalized vertebrate structure is illustrated, in the extinct reptiles, by the affinities to ganoid fishes, shown by ganocephala, labyrinthodontia, and icthyopterygia; by the affinities of the pterosauria to birds, and by the approximation of the dinosauria to mammals. (these have been recently shown by professor huxley to have more affinity to birds.) it is manifested by the combination of modern crocodilian, chelonian, and lacertian characters in the cryptodontia and the dicnyodontia, and by the combined lacertian and crocodilian characters in the thecodontia and sauropterygia." in the same work he tells us that, "the anoplotherium, in several important characters resembled the embryo ruminant, but retained throughout life those marks of adhesion to a generalized mammalian type;"--and assures us that he has "never omitted a proper opportunity for impressing the results of observations showing the more generalized structures of extinct as compared with the more specialized forms of recent animals." modern palæontologists have discovered hundreds of examples of these more generalized or ancestral types. in the time of cuvier, the ruminants and the pachyderms were looked upon as two of the most distinct orders of animals; but it is now demonstrated that there once existed a variety of genera and species, connecting by almost imperceptible grades such widely different animals as the pig and the camel. among living quadrupeds we can scarcely find a more isolated group than the genus equus, comprising the horses, asses, and zebras; but through many species of paloplotherium, hippotherium, and hipparion, and numbers of extinct forms of equus found in europe, india, and america, an almost complete transition is established with the eocene anoplothorium and paleotherium, which are also generalized or ancestral types of the tapir and rhinoceros. the recent researches of m. gaudry in greece have furnished much new evidence of the same character. in the miocene beds of pikermi he has discovered the group of the simocyonidæ intermediate between bears and wolves; the genus hyænictis which connects the hyænas with the civets; the ancylotherium, which is allied both to the extinct mastodon and to the living pangolin or scaly ant-eater; and the helladotherium, which connects the now isolated giraffe with the deer and antelopes. between reptiles and fishes an intermediate type has been found in the archegosaurus of the coal formation; while the labyrinthodon of the trias combined characters of the batrachia with those of crocodiles, lizards, and ganoid fishes. even birds, the most apparently isolated of all living forms, and the most rarely preserved in a fossil state, have been shown to possess undoubted affinities with reptiles; and in the oolitic archæopteryx, with its lengthened tail, feathered on each side, we have one of the connecting links from the side of birds; while professor huxley has recently shown that the entire order of dinosaurians have remarkable affinities to birds, and that one of them, the compsognathus, makes a nearer approach to bird organisation than does archæopteryx to that of reptiles. analogous facts to those occur in other classes of animals, as an example of which we have the authority of a distinguished paleontologist, m. barande, quoted by mr. darwin, for the statement, that although the palæozoic invertebrata can certainly be classed under existing groups, yet at this ancient period the groups were not so distinctly separated from each other as they are now; while mr. scudder tells us, that some of the fossil insects discovered in the coal formation of america offer characters intermediate between those of existing orders. agassiz, again, insists strongly that the more ancient animals resemble the embryonic forms of existing species; but as the embryos of distinct groups are known to resemble each other more than the adult animals (and in fact to be undistinguishable at a very early age), this is the same as saying that the ancient animals are exactly what, on darwin's theory, the ancestors of existing animals ought to be; and this, it must be remembered, is the evidence of one of the strongest opponents of the theory of natural selection. _conclusion._ i have thus endeavoured to meet fairly, and to answer plainly, a few of the most common objections to the theory of natural selection, and i have done so in every case by referring to admitted facts and to logical deductions from those facts. as an indication and general summary of the line of argument i have adopted, i here give a brief demonstration in a tabular form of the origin of species by means of natural selection, referring for the _facts_ to mr. darwin's works, and to the pages in this volume, where they are more or less fully treated. _a demonstration of the origin of species by natural selection_. ___________________________________________________________________ | | | |_proved facts_. |_necessary consequences_ | | |(_afterwards taken as proved | | |facts_). | |_________________________________|_________________________________| | | | |rapid increase of organisms, | | |pp. , ; ("origin |struggle for existence, | |of species," p. , th ed.) |the deaths equalling the | | |births on the average, p. ; | |total number of individuals |("origin of species," chap. | |stationary, pp. , |iii.) | | . | | |_________________________________|_________________________________| | | | |struggle for existence. |survival of the fittest, | | |or natural selection; meaning | |heredity with variation, |simply, that on the | |or general likeness with |whole those die who are | |individual differences of parents|least fitted to maintain their | |and offspring, pp. |existence; ("origin of species," | | , - , ; ("origin |chap. iv.) | |of species," chap. i., ii., v.) | | |_________________________________|_________________________________| | | | |survival of the fittest. |changes of organic forms, | | |to keep them in harmony | |change of external conditions, |with the changed conditions; | |universal and unceasing.--see |and as the changes | |"lyell's |of conditions are permanent | |principles of geology." |changes, in the sense | | |of not reverting back to | | |identical previous conditions, | | |the changes of organic | | |forms must be in the | | |same sense permanent, and | | |thus originate species. | |_________________________________|_________________________________| ix. the development of human races under the law of natural selection. among the most advanced students of man, there exists a wide difference of opinion on some of the most vital questions respecting his nature and origin. anthropologists are now, indeed, pretty well agreed that man is not a recent introduction into the earth. all who have studied the question, now admit that his antiquity is very great; and that, though we have to some extent ascertained the minimum of time during which he _must_ have existed, we have made no approximation towards determining that far greater period during which he _may_ have, and probably _has_ existed. we can with tolerable certainty affirm that man must have inhabited the earth a thousand centuries ago, but we cannot assert that he positively did not exist, or that there is any good evidence against his having existed, for a period of ten thousand centuries. we know positively, that he was contemporaneous with many now extinct animals, and has survived changes of the earth's surface fifty or a hundred times greater than any that have occurred during the historical period; but we cannot place any definite limit to the number of species he may have outlived, or to the amount of terrestrial change he may have witnessed. _wide differences of opinion as to man's origin._ but while on this question of man's antiquity there is a very general agreement,--and all are waiting eagerly for fresh evidence to clear up those points which all admit to be full of doubt,--on other, and not less obscure and difficult questions, a considerable amount of dogmatism is exhibited; doctrines are put forward as established truths, no doubt or hesitation is admitted, and it seems to be supposed that no further evidence is required, or that any new facts can modify our convictions. this is especially the case when we inquire,--are the various forms under which man now exists primitive, or derived from pre-existing forms; in other words, is man of one or many species? to this question we immediately obtain distinct answers diametrically opposed to each other: the one party positively maintaining, that man is a _species_ and is essentially _one_--that all differences are but local and temporary variations, produced by the different physical and moral conditions by which he is surrounded; the other party maintaining with equal confidence, that man is a genus of _many species_, each of which is practically unchangeable, and has ever been as distinct, or even more distinct, than we now behold them. this difference of opinion is somewhat remarkable, when we consider that both parties are well acquainted with the subject; both use the same vast accumulation of facts; both reject those early traditions of mankind which profess to give an account of his origin; and both declare that they are seeking fearlessly after truth alone; yet each will persist in looking only at the portion of truth on his own side of the question, and at the error which is mingled with his opponent's doctrine. it is my wish to show how the two opposing views can be combined, so as to eliminate the error and retain the truth in each, and it is by means of mr. darwin's celebrated theory of "natural selection" that i hope to do this, and thus to harmonise the conflicting theories of modern anthropologists. let us first see what each party has to say for itself. in favour of the unity of mankind it is argued, that there are no races without transitions to others; that every race exhibits within itself variations of colour, of hair, of feature, and of form, to such a degree as to bridge over, to a large extent, the gap that separates it from other races. it is asserted that no race is homogeneous; that there is a tendency to vary; that climate, food, and habits produce, and render permanent, physical peculiarities, which, though slight in the limited periods allowed to our observation, would, in the long ages during which the human race has existed, have sufficed to produce all the differences that now appear. it is further asserted that the advocates of the opposite theory do not agree among themselves; that some would make three, some five, some fifty or a hundred and fifty species of man; some would have had each species created in pairs, while others require nations to have at once sprung into existence, and that there is no stability or consistency in any doctrine but that of one primitive stock. the advocates of the original diversity of man, on the other hand, have much to say for themselves. they argue that proofs of change in man have never been brought forward except to the most trifling amount, while evidence of his permanence meets us everywhere. the portuguese and spaniards, settled for two or three centuries in south america, retain their chief physical, mental, and moral characteristics; the dutch boers at the cape, and the descendants of the early dutch settlers in the moluccas, have not lost the features or the colour of the germanic races; the jews, scattered over the world in the most diverse climates, retain the same characteristic lineaments everywhere; the egyptian sculptures and paintings show us that, for at least or years, the strongly contrasted features of the negro and the semitic races have remained altogether unchanged; while more recent discoveries prove, that the mound-builders of the mississippi valley, and the dwellers on brazilian mountains, had, even in the very infancy of the human race, some traces of the same peculiar and characteristic type of cranial formation that now distinguishes them. if we endeavour to decide impartially on the merits of this difficult controversy, judging solely by the evidence that each party has brought forward, it certainly seems that the best of the argument is on the side of those who maintain the primitive diversity of man. their opponents have not been able to refute the permanence of existing races as far back as we can trace them, and have failed to show, in a single case, that at any former epoch the well marked varieties of mankind approximated more closely than they do at the present day. at the same time this is but negative evidence. a condition of immobility for four or five thousand years, does not preclude an advance at an earlier epoch, and--if we can show that there are causes in nature which would check any further physical change when certain conditions were fulfilled--does not even render such an advance improbable, if there are any general arguments to be adduced in its favour. such a cause, i believe, does exist; and i shall now endeavour to point out its nature and its mode of operation. _outline of the theory of natural selection._ in order to make my argument intelligible, it is necessary for me to explain very briefly the theory of "natural selection" promulgated by mr. darwin, and the power which it possesses of modifying the forms of animals and plants. the grand feature in the multiplication of organic life is, that close general resemblance is combined with more or less individual variation. the child resembles its parents or ancestors more or less closely in all its peculiarities, deformities, or beauties; it resembles them in general more than it does any other individuals; yet children of the same parents are not all alike, and it often happens that they differ very considerably from their parents and from each other. this is equally true, of man, of all animals, and of all plants. moreover, it is found that individuals do not differ from their parents in certain particulars only, while in all others they are exact duplicates of them. they differ from them and from each other, in every particular: in form, in size, in colour; in the structure of internal as well as of external organs; in those subtle peculiarities which produce differences of constitution, as well as in those still more subtle ones which lead to modifications of mind and character. in other words, in every possible way, in every organ and in every function, individuals of the same stock vary. now, health, strength, and long life, are the results of a harmony between the individual and the universe that surrounds it. let us suppose that at any given moment this harmony is perfect. a certain animal is exactly fitted to secure its prey, to escape from its enemies, to resist the inclemencies of the seasons, and to rear a numerous and healthy offspring. but a change now takes place. a series of cold winters, for instance, come on, making food scarce, and bringing an immigration of some other animals to compete with the former inhabitants of the district. the new immigrant is swift of foot, and surpasses its rivals in the pursuit of game; the winter nights are colder, and require a thicker fur as a protection, and more nourishing food to keep up the heat of the system. our supposed perfect animal is no longer in harmony with its universe; it is in danger of dying of cold or of starvation. but the animal varies in its offspring. some of these are swifter than others--they still manage to catch food enough; some are hardier and more thickly furred--they manage in the cold nights to keep warm enough; the slow, the weak, and the thinly clad soon die off. again and again, in each succeeding generation, the same thing takes place. by this natural process, which is so inevitable that it cannot be conceived not to act, those best adapted to live, live; those least adapted, die. it is sometimes said that we have no direct evidence of the action of this selecting power in nature. but it seems to me we have better evidence than even direct observation would be, because it is more universal, viz., the evidence of necessity. it must be so; for, as all wild animals increase in a geometrical ratio, while their actual numbers remain on the average stationary, it follows, that as many die annually as are born. if, therefore, we deny natural selection, it can only be by asserting that, in such a case as i have supposed, the strong, the healthy, the swift, the well clad, the well organised animals in every respect, have no advantage over,--do not on the average live longer than, the weak, the unhealthy, the slow, the ill-clad, and the imperfectly organised individuals; and this no sane man has yet been found hardy enough to assert. but this is not all; for the offspring on the average resemble their parents, and the selected portion of each succeeding generation will therefore be stronger, swifter, and more thickly furred than the last; and if this process goes on for thousands of generations, our animal will have again become thoroughly in harmony with the new conditions in which it is placed. but it will now be a different creature. it will be not only swifter and stronger, and more furry, it will also probably have changed in colour, in form, perhaps have acquired a longer tail, or differently shaped ears; for it is an ascertained fact, that when one part of an animal is modified, some other parts almost always change, as it were in sympathy with it. mr. darwin calls this "correlation of growth," and gives as instances, that hairless dogs have imperfect teeth; white cats, when blue-eyed, are deaf; small feet accompany short beaks in pigeons; and other equally interesting cases. grant, therefore, the premises: st. that peculiarities of every kind are more or less hereditary. nd. that the offspring of every animal vary more or less in all parts of their organization. rd. that the universe in which these animals live, is not absolutely invariable;--none of which propositions can be denied; and then consider, that the animals in any country (those at least which are not dying out) must at each successive period be brought into harmony with the surrounding conditions; and we have all the elements for a change of form and structure in the animals, keeping exact pace with changes of whatever nature in the surrounding universe. such changes must be slow, for the changes in the universe are very slow; but just as these slow changes become important, when we look at results after long periods of action, as we do when we perceive the alterations of the earth's surface during geological epochs; so the parallel changes in animal form become more and more striking, in proportion as the time they have been going on is great; as we see when we compare our living animals with those which we disentomb from each successively older geological formation. this is, briefly, the theory of "natural selection," which explains the changes in the organic world as being parallel with, and in part dependent on, those in the inorganic. what we now have to inquire is,--can this theory be applied in any way to the question of the origin of the races of man? or is there anything in human nature that takes him out of the category of those organic existences, over whose successive mutations it has had such powerful sway? _different effects of natural selection on animals and on man._ in order to answer these questions, we must consider why it is that "natural selection" acts so powerfully upon animals; and we shall, i believe, find, that its effect depends mainly upon their self-dependence and individual isolation. a slight injury, a temporary illness, will often end in death, because it leaves the individual powerless against its enemies. if an herbivorous animal is a little sick and has not fed well for a day or two, and the herd is then pursued by a beast of prey, our poor invalid inevitably falls a victim. so, in a carnivorous animal, the least deficiency of vigour prevents its capturing food, and it soon dies of starvation. there is, as a general rule, no mutual assistance between adults, which enables them to tide over a period of sickness. neither is there any division of labour; each must fulfil _all_ the conditions of its existence, and, therefore, "natural selection" keeps all up to a pretty uniform standard. but in man, as we now behold him, this is different. he is social and sympathetic. in the rudest tribes the sick are assisted, at least with food; less robust health and vigour than the average does not entail death. neither does the want of perfect limbs, or other organs, produce the same effects as among animals. some division of labour takes place; the swiftest hunt, the less active fish, or gather fruits; food is, to some extent, exchanged or divided. the action of natural selection is therefore checked; the weaker, the dwarfish, those of less active limbs, or less piercing eyesight, do not suffer the extreme penalty which falls upon animals so defective. in proportion as these physical characteristics become of less importance, mental and moral qualities will have increasing influence on the well-being of the race. capacity for acting in concert for protection, and for the acquisition of food and shelter; sympathy, which leads all in turn to assist each other; the sense of right, which checks depredations upon our fellows; the smaller development of the combative and destructive propensities; self-restraint in present appetites; and that intelligent foresight which prepares for the future, are all qualities, that from their earliest appearance must have been for the benefit of each community, and would, therefore, have become the subjects of "natural selection." for it is evident that such qualities would be for the well-being of man; would guard him against external enemies, against internal dissensions, and against the effects of inclement seasons and impending famine, more surely than could any merely physical modification. tribes in which such mental and moral qualities were predominant, would therefore have an advantage in the struggle for existence over other tribes in which they were less developed, would live and maintain their numbers, while the others would decrease and finally succumb. again, when any slow changes of physical geography, or of climate, make it necessary for an animal to alter its food, its clothing, or its weapons, it can only do so by the occurrence of a corresponding change in its own bodily structure and internal organization. if a larger or more powerful beast is to be captured and devoured, as when a carnivorous animal which has hitherto preyed on antelopes is obliged from their decreasing numbers to attack buffaloes, it is only the strongest who can hold,--those with most powerful claws, and formidable canine teeth, that can struggle with and overcome such an animal. natural selection immediately comes into play, and by its action these organs gradually become adapted to their new requirements. but man, under similar circumstances, does not require longer nails or teeth, greater bodily strength or swiftness. he makes sharper spears, or a better bow, or he constructs a cunning pitfall, or combines in a hunting party to circumvent his new prey. the capacities which enable him to do this are what he requires to be strengthened, and these will, therefore, be gradually modified by "natural selection," while the form and structure of his body will remain unchanged. so, when a glacial epoch comes on, some animals must acquire warmer fur, or a covering of fat, or else die of cold. those best clothed by nature are, therefore, preserved by natural selection. man, under the same circumstances, will make himself warmer clothing, and build better houses; and the necessity of doing this will react upon his mental organization and social condition--will advance them while his natural body remains naked as before. when the accustomed food of some animal becomes scarce or totally fails, it can only exist by becoming adapted to a new kind of food, a food perhaps less nourishing and less digestible. "natural selection" will now act upon the stomach and intestines, and all their individual variations will be taken advantage of, to modify the race into harmony with its new food. in many cases, however, it is probable that this cannot be done. the internal organs may not vary quick enough, and then the animal will decrease in numbers, and finally become extinct. but man guards himself from such accidents by superintending and guiding the operations of nature. he plants the seed of his most agreeable food, and thus procures a supply, independent of the accidents of varying seasons or natural extinction. he domesticates animals, which serve him either to capture food or for food itself, and thus, changes of any great extent in his teeth or digestive organs are rendered unnecessary. man, too, has everywhere the use of fire, and by its means can render palatable a variety of animal and vegetable substances, which he could hardly otherwise make use of; and thus obtains for himself a supply of food far more varied and abundant than that which any animal can command. thus man, by the mere capacity of clothing himself, and making weapons and tools, has taken away from nature that power of slowly but permanently changing the external form and structure, in accordance with changes in the external world, which she exercises over all other animals. as the competing races by which they are surrounded, the climate, the vegetation, or the animals which serve them for food, are slowly changing, they must undergo a corresponding change in their structure, habits, and constitution, to keep them in harmony with the new conditions--to enable them to live and maintain their numbers. but man does this by means of his intellect alone, the variations of which enable him, with an unchanged body, still to keep in harmony with the changing universe. there is one point, however, in which nature will still act upon him as it does on animals, and, to some extent, modify his external characters. mr. darwin has shown, that the colour of the skin is correlated with constitutional peculiarities both in vegetables and animals, so that liability to certain diseases or freedom from them is often accompanied by marked external characters. now, there is every reason to believe that this has acted, and, to some extent, may still continue to act, on man. in localities where certain diseases are prevalent, those individuals of savage races which were subject to them would rapidly die off; while those who were constitutionally free from the disease would survive, and form the progenitors of a new race. these favoured individuals would probably be distinguished by peculiarities of _colour_, with which again peculiarities in the texture or the abundance of _hair_ seem to be correlated, and thus may have been brought about those racial differences of colour, which seem to have no relation to mere temperature or other obvious peculiarities of climate. from the time, therefore, when the social and sympathetic feelings came into active operation, and the intellectual and moral faculties became fairly developed, man would cease to be influenced by "natural selection" in his physical form and structure. as an animal he would remain almost stationary, the changes of the surrounding universe ceasing to produce in him that powerful modifying effect which they exercise over other parts of the organic world. but from the moment that the form of his body became stationary, his mind would become subject to those very influences from which his body had escaped; every slight variation in his mental and moral nature which should enable him better to guard against adverse circumstances, and combine for mutual comfort and protection, would be preserved and accumulated; the better and higher specimens of our race would therefore increase and spread, the lower and more brutal would give way and successively die out, and that rapid advancement of mental organization would occur, which has raised the very lowest races of man so far above the brutes (although differing so little from some of them in physical structure), and, in conjunction with scarcely perceptible modifications of form, has developed the wonderful intellect of the european races. _influence of external nature in the development of the human mind._ but from the time when this mental and moral advance commenced, and man's physical character became fixed and almost immutable, a new series of causes would come into action, and take part in his mental growth. the diverse aspects of nature would now make themselves felt, and profoundly influence the character of the primitive man. when the power that had hitherto modified the body had its action transferred to the mind, then races would advance and become improved, merely by the harsh discipline of a sterile soil and inclement seasons. under their influence, a hardier, a more provident, and a more social race would be developed, than in those regions where the earth produces a perennial supply of vegetable food, and where neither foresight nor ingenuity are required to prepare for the rigours of winter. and is it not the fact that in all ages, and in every quarter of the globe, the inhabitants of temperate have been superior to those of hotter countries? all the great invasions and displacements of races have been from north to south, rather than the reverse; and we have no record of there ever having existed, any more than there exists to-day, a solitary instance of an indigenous inter-tropical civilization. the mexican civilization and government came from the north, and, as well as the peruvian, was established, not in the rich tropical plains, but on the lofty and sterile plateaux of the andes. the religion and civilization of ceylon were introduced from north india; the successive conquerors of the indian peninsula came from the north-west; the northern mongols conquered the more southern chinese; and it was the bold and adventurous tribes of the north that overran and infused new life into southern europe. _extinction of lower races._ it is the same great law of "the preservation of favoured races in the struggle for life," which leads to the inevitable extinction of all those low and mentally undeveloped populations with which europeans come in contact. the red indian in north america, and in brazil; the tasmanian, australian, and new zealander in the southern hemisphere, die out, not from any one special cause, but from the inevitable effects of an unequal mental and physical struggle. the intellectual and moral, as well as the physical, qualities of the european are superior; the same powers and capacities which have made him rise in a few centuries from the condition of the wandering savage with a scanty and stationary population, to his present state of culture and advancement, with a greater average longevity, a greater average strength, and a capacity of more rapid increase,--enable him when in contact with the savage man, to conquer in the struggle for existence, and to increase at his expense, just as the better adapted, increase at the expense of the less adapted varieties in the animal and vegetable kingdoms,--just as the weeds of europe overrun north america and australia, extinguishing native productions by the inherent vigour of their organization, and by their greater capacity for existence and multiplication. _the origin of the races of man._ if these views are correct; if in proportion as man's social, moral, and intellectual faculties became developed, his physical structure would cease to be affected by the operation of "natural selection," we have a most important clue to the origin of races. for it will follow, that those great modifications of structure and of external form, which resulted in the development of man out of some lower type of animal, must have occurred before his intellect had raised him above the condition of the brutes, at a period when he was gregarious, but scarcely social, with a mind perceptive but not reflective, ere any sense of _right_ or feelings of _sympathy_ had been developed in him. he would be still subject, like the rest of the organic world, to the action of "natural selection," which would retain his physical form and constitution in harmony with the surrounding universe. he was probably at a very early period a dominant race, spreading widely over the warmer regions of the earth as it then existed, and in agreement with what we see in the case of other dominant species, gradually becoming modified in accordance with local conditions. as he ranged farther from his original home, and became exposed to greater extremes of climate, to greater changes of food, and had to contend with new enemies, organic and inorganic, slight useful variations in his constitution would be selected and rendered permanent, and would, on the principle of "correlation of growth," be accompanied by corresponding external physical changes. thus might have arisen those striking characteristics and special modifications which still distinguish the chief races of mankind. the red, black, yellow, or blushing white skin; the straight, the curly, the woolly hair; the scanty or abundant beard; the straight or oblique eyes; the various forms of the pelvis, the cranium, and other parts of the skeleton. but while these changes had been going on, his mental development had, from some unknown cause, greatly advanced, and had now reached that condition in which it began powerfully to influence his whole existence, and would therefore become subject to the irresistible action of "natural selection." this action would quickly give the ascendency to mind: speech would probably now be first developed, leading to a still further advance of the mental faculties; and from that moment man, as regards the form and structure of most parts of his body, would remain almost stationary. the art of making weapons, division of labour, anticipation of the future, restraint of the appetites, moral, social, and sympathetic feelings, would now have a preponderating influence on his well being, and would therefore be that part of his nature on which "natural selection" would most powerfully act; and we should thus have explained that wonderful persistence of mere physical characteristics, which is the stumbling-block of those who advocate the unity of mankind. we are now, therefore, enabled to harmonise the conflicting views of anthropologists on this subject. man may have been, indeed i believe must have been, once a homogeneous race; but it was at a period of which we have as yet discovered no remains, at a period so remote in his history, that he had not yet acquired that wonderfully developed brain, the organ of the mind, which now, even in his lowest examples, raises him far above the highest brutes;--at a period when he had the form but hardly the nature of man, when he neither possessed human speech, nor those sympathetic and moral feelings which in a greater or less degree everywhere now distinguish the race. just in proportion as these truly human faculties became developed in him, would his physical features become fixed and permanent, because the latter would be of less importance to his well being; he would be kept in harmony with the slowly changing universe around him, by an advance in mind, rather than by a change in body. if, therefore, we are of opinion that he was not really man till these higher faculties were fully developed, we may fairly assert that there were many originally distinct races of men; while, if we think that a being closely resembling us in form and structure, but with mental faculties scarcely raised above the brute, must still be considered to have been human, we are fully entitled to maintain the common origin of all mankind. _the bearing of these views on the antiquity of man._ these considerations, it will be seen, enable us to place the origin of man at a much more remote geological epoch than has yet been thought possible. he may even have lived in the miocene or eocene period, when not a single mammal was identical in form with any existing species. for, in the long series of ages during which these primeval animals were being slowly changed into the species which now inhabit the earth, the power which acted to modify them would only affect the mental organization of man. his brain alone would have increased in size and complexity, and his cranium have undergone corresponding changes of form, while the whole structure of lower animals was being changed. this will enable us to understand how the fossil crania of denise and engis agree so closely with existing forms, although they undoubtedly existed in company with large mammalia now extinct. the neanderthal skull may be a specimen of one of the lowest races then existing, just as the australians are the lowest of our modern epoch. we have no reason to suppose that mind and brain and skull modification, could go on quicker than that of the other parts of the organization; and we must therefore look back very far in the past, to find man in that early condition in which his mind was not sufficiently developed, to remove his body from the modifying influence of external conditions and the cumulative action of "natural selection." i believe, therefore, that there is no _à priori_ reason against our finding the remains of man or his works in the tertiary deposits. the absence of all such remains in the european beds of this age has little weight, because, as we go further back in time, it is natural to suppose that man's distribution over the surface of the earth was less universal than at present. besides, europe was in a great measure submerged during the tertiary epoch; and though its scattered islands may have been uninhabited by man, it by no means follows that he did not at the same time exist in warm or tropical continents. if geologists can point out to us the most extensive land in the warmer regions of the earth, which has not been submerged since eocene or miocene times, it is there that we may expect to find some traces of the very early progenitors of man. it is there that we may trace back the gradually decreasing brain of former races, till we come to a time when the body also begins materially to differ. then we shall have reached the starting point of the human family. before that period, he had not mind enough to preserve his body from change, and would, therefore, have been subject to the same comparatively rapid modifications of form as the other mammalia. _their bearing on the dignity and supremacy of man._ if the views i have here endeavoured to sustain have any foundation, they give us a new argument for placing man apart, as not only the head and culminating point of the grand series of organic nature, but as in some degree a new and distinct order of being. from those infinitely remote ages, when the first rudiments of organic life appeared upon the earth, every plant, and every animal has been subject to one great law of physical change. as the earth has gone through its grand cycles of geological, climatal, and organic progress, every form of life has been subject to its irresistible action, and has been continually, but imperceptibly moulded into such new shapes as would preserve their harmony with the ever-changing universe. no living thing could escape this law of its being; none (except, perhaps, the simplest and most rudimentary organisms), could remain unchanged and live, amid the universal change around it. at length, however, there came into existence a being in whom that subtle force we term _mind_, became of greater importance than his mere bodily structure. though with a naked and unprotected body, _this_ gave him clothing against the varying inclemencies of the seasons. though unable to compete with the deer in swiftness, or with the wild bull in strength, _this_ gave him weapons with which to capture or overcome both. though less capable than most other animals of living on the herbs and the fruits that unaided nature supplies, this wonderful faculty taught him to govern and direct nature to his own benefit, and make her produce food for him, when and where he pleased. from the moment when the first skin was used as a covering, when the first rude spear was formed to assist in the chase, when fire was first used to cook his food, when the first seed was sown or shoot planted, a grand revolution was effected in nature, a revolution which in all the previous ages of the earth's history had had no parallel, for a being had arisen who was no longer necessarily subject to change with the changing universe--a being who was in some degree superior to nature, inasmuch as he knew how to control and regulate her action, and could keep himself in harmony with her, not by a change in body, but by an advance of mind. here, then, we see the true grandeur and dignity of man. on this view of his special attributes, we may admit, that even those who claim for him a position as an order, a class, or a sub-kingdom by himself, have some show of reason on their side. he is, indeed, a being apart, since he is not influenced by the great laws which irresistibly modify all other organic beings. nay more; this victory which he has gained for himself, gives him a directing influence over other existences. man has not only escaped "natural selection" himself, but he is actually able to take away some of that power from nature which before his appearance she universally exercised. we can anticipate the time when the earth will produce only cultivated plants and domestic animals; when man's selection shall have supplanted "natural selection;" and when the ocean will be the only domain in which that power can be exerted, which for countless cycles of ages ruled supreme over all the earth. _their bearing on the future development of man._ we now find ourselves enabled to answer those who maintain, that if mr. darwin's theory of the origin of species is true, man too must change in form, and become developed into some other animal as different from his present self as he is from the gorilla or the chimpanzee; and who speculate on what this form is likely to be. but it is evident that such will not be the case; for no change of conditions is conceivable, which will render any important alteration of his form and organization so universally useful and necessary to him, as to give those possessing it always the best chance of surviving, and thus lead to the development of a new species, genus, or higher group of man. on the other hand, we know that far greater changes of conditions and of his entire environment have been undergone by man, than any other highly organized animal could survive unchanged, and have been met by mental, not corporeal adaptation. the difference of habits, of food, clothing, weapons, and enemies, between savage and civilized man, is enormous. difference in bodily form and structure there is practically none, except a slightly increased size of brain, corresponding to his higher mental development. we have every reason to believe, then, that man may have existed and may continue to exist, through a series of geological periods which shall see all other forms of animal life again and again changed; while he himself remains unchanged, except in the two particulars already specified--the head and face, as immediately connected with the organ of the mind and as being the medium of expressing the most refined emotions of his nature,--and to a slight extent in colour, hair, and proportions, so far as they are correlated with constitutional resistance to disease. _summary._ briefly to recapitulate the argument;--in two distinct ways has man escaped the influence of those laws which have produced unceasing change in the animal world. . by his superior intellect he is enabled to provide himself with clothing and weapons, and by cultivating the soil to obtain a constant supply of congenial food. this renders it unnecessary for his body, like those of the lower animals, to be modified in accordance with changing conditions--to gain a warmer natural covering, to acquire more powerful teeth or claws, or to become adapted to obtain and digest new kinds of food, as circumstances may require. . by his superior sympathetic and moral feelings, he becomes fitted for the social state; he ceases to plunder the weak and helpless of his tribe; he shares the game which he has caught with less active or less fortunate hunters, or exchanges it for weapons which even the weak or the deformed can fashion; he saves the sick and wounded from death; and thus the power which leads to the rigid destruction of all animals who cannot in every respect help themselves, is prevented from acting on him. this power is "natural selection;" and, as by no other means can it be shown, that individual variations can ever become accumulated and rendered permanent so as to form well-marked races, it follows that the differences which now separate mankind from other animals, must have been produced before he became possessed of a human intellect or human sympathies. this view also renders possible, or even requires, the existence of man at a comparatively remote geological epoch. for, during the long periods in which other animals have been undergoing modification in their whole structure, to such an amount as to constitute distinct genera and families, man's _body_ will have remained generically, or even specifically, the same, while his _head_ and _brain_ alone will have undergone modification equal to theirs. we can thus understand how it is that, judging from the head and brain, professor owen places man in a distinct sub-class of mammalia, while as regards the bony structure of his body, there is the closest anatomical resemblance to the anthropoid apes, "every tooth, every bone, strictly homologous--which makes the determination of the difference between _homo_ and _pithecus_ the anatomist's difficulty." the present theory fully recognises and accounts for these facts; and we may perhaps claim as corroborative of its truth, that it neither requires us to depreciate the intellectual chasm which separates man from the apes, nor refuses full recognition of the striking resemblances to them, which exist in other parts of his structure. _conclusion._ in concluding this brief sketch of a great subject, i would point out its bearing upon the future of the human race. if my conclusions are just, it must inevitably follow that the higher--the more intellectual and moral--must displace the lower and more degraded races; and the power of "natural selection," still acting on his mental organization, must ever lead to the more perfect adaptation of man's higher faculties to the conditions of surrounding nature, and to the exigencies of the social state. while his external form will probably ever remain unchanged, except in the development of that perfect beauty which results from a healthy and well organized body, refined and ennobled by the highest intellectual faculties and sympathetic emotions, his mental constitution may continue to advance and improve, till the world is again inhabited by a single nearly homogeneous race, no individual of which will be inferior to the noblest specimens of existing humanity. our progress towards such a result is very slow, but it still seems to be a progress. we are just now living at an abnormal period of the world's history, owing to the marvellous developments and vast practical results of science, having been given to societies too low morally and intellectually, to know how to make the best use of them, and to whom they have consequently been curses as well as blessings. among civilized nations at the present day, it does not seem possible for natural selection to act in any way, so as to secure the permanent advancement of morality and intelligence; for it is indisputably the mediocre, if not the low, both as regards morality and intelligence, who succeed best in life and multiply fastest. yet there is undoubtedly an advance--on the whole a steady and a permanent one--both in the influence on public opinion of a high morality, and in the general desire for intellectual elevation; and as i cannot impute this in any way to "survival of the fittest," i am forced to conclude that it is due, to the inherent progressive power of those glorious qualities which raise us so immeasurably above our fellow animals, and at the same time afford us the surest proof that there are other and higher existences than ourselves, from whom these qualities may have been derived, and towards whom we may be ever tending. x. the limits of natural selection as applied to man. throughout this volume i have endeavoured to show, that the known laws of variation, multiplication, and heredity, resulting in a "struggle for existence" and the "survival of the fittest," have probably sufficed to produce all the varieties of structure, all the wonderful adaptations, all the beauty of form and of colour, that we see in the animal and vegetable kingdoms. to the best of my ability i have answered the most obvious and the most often repeated objections to this theory, and have, i hope, added to its general strength, by showing how colour--one of the strongholds of the advocates of special creation--may be, in almost all its modifications, accounted for by the combined influence of sexual selection and the need of protection. i have also endeavoured to show, how the same power which has modified animals has acted on man; and have, i believe, proved that, as soon as the human intellect became developed above a certain low stage, man's body would cease to be materially affected by natural selection, because the development of his mental faculties would render important modifications of its form and structure unnecessary. it will, therefore, probably excite some surprise among my readers, to find that i do not consider that all nature can be explained on the principles of which i am so ardent an advocate; and that i am now myself going to state objections, and to place limits, to the power of "natural selection." i believe, however, that there are such limits; and that just as surely as we can trace the action of natural laws in the development of organic forms, and can clearly conceive that fuller knowledge would enable us to follow step by step the whole process of that development, so surely can we trace the action of some unknown higher law, beyond and independent of all those laws of which we have any knowledge. we can trace this action more or less distinctly in many phenomena, the two most important of which are--the origin of sensation or consciousness, and the development of man from the lower animals. i shall first consider the latter difficulty as more immediately connected with the subjects discussed in this volume. _what natural selection can not do._ in considering the question of the development of man by known natural laws, we must ever bear in mind the first principle of "natural selection," no less than of the general theory of evolution, that all changes of form or structure, all increase in the size of an organ or in its complexity, all greater specialization or physiological division of labour, can only be brought about, in as much as it is for the good of the being so modified. mr. darwin himself has taken care to impress upon us, that "natural selection" has no power to produce absolute perfection but only relative perfection, no power to advance any being much beyond his follow beings, but only just so much beyond them as to enable it to survive them in the struggle for existence. still less has it any power to produce modifications which are in any degree injurious to its possessor, and mr. darwin frequently uses the strong expression, that a single case of this kind would be fatal to his theory. if, therefore, we find in man any characters, which all the evidence we can obtain goes to show would have been actually injurious to him on their first appearance, they could not possibly have been produced by natural selection. neither could any specially developed organ have been so produced if it had been merely useless to him, or if its use were not proportionate to its degree of development. such cases as these would prove, that some other law, or some other power, than "natural selection" had been at work. but if, further, we could see that these very modifications, though hurtful or useless at the time when they first appeared, became in the highest degree useful at a much later period, and are now essential to the full moral and intellectual development of human nature, we should then infer the action of mind, foreseeing the future and preparing for it, just as surely as we do, when we see the breeder set himself to work with the determination to produce a definite improvement in some cultivated plant or domestic animal. i would further remark that this enquiry is as thoroughly scientific and legitimate as that into the origin of species itself. it is an attempt to solve the inverse problem, to deduce the existence of a new power of a definite character, in order to account for facts which according to the theory of natural selection ought not to happen. such problems are well known to science, and the search after their solution has often led to the most brilliant results. in the case of man, there are facts of the nature above alluded to, and in calling attention to them, and in inferring a cause for them, i believe that i am as strictly within the bounds of scientific investigation as i have been in any other portion of my work. _the brain of the savage shown to be larger than he needs it to be._ _size of brain an important element of mental power._--the brain is universally admitted to be the organ of the mind; and it is almost as universally admitted, that size of brain is one of the most important of the elements which determine mental power or capacity. there seems to be no doubt that brains differ considerably in quality, as indicated by greater or less complexity of the convolutions, quantity of grey matter, and perhaps unknown peculiarities of organization; but this difference of quality seems merely to increase or diminish the influence of quantity, not to neutralize it. thus, all the most eminent modern writers see an intimate connection between the diminished size of the brain in the lower races of mankind, and their intellectual inferiority. the collections of dr. j. b. davis and dr. morton give the following as the average internal capacity of the cranium in the chief races:--teutonic family, cubic inches; esquimaux, cubic inches; negroes, cubic inches; australians and tasmanians, cubic inches; bushmen, cubic inches. these last numbers, however, are deduced from comparatively few specimens, and may be below the average, just as a small number of finns and cossacks give cubic inches, or considerably more than that of the german races. it is evident, therefore, that the absolute bulk of the brain is not necessarily much less in savage than in civilised man, for esquimaux skulls are known with a capacity of inches, or hardly less than the largest among europeans. but what is still more extraordinary, the few remains yet known of pre-historic man do not indicate any material diminution in the size of the brain case. a swiss skull of the stone age, found in the lake dwelling of meilen, corresponded exactly to that of a swiss youth of the present day. the celebrated neanderthal skull had a larger circumference than the average, and its capacity, indicating actual mass of brain, is estimated to have been not less than cubic inches, or nearly the average of existing australian crania. the engis skull, perhaps the oldest known, and which, according to sir john lubbock, "there seems no doubt was really contemporary with the mammoth and the cave bear," is yet, according to professor huxley, "a fair average skull, which might have belonged to a philosopher, or might have contained the thoughtless brains of a savage." of the cave men of les eyzies, who were undoubtedly contemporary with the reindeer in the south of france, professor paul broca says (in a paper read before the congress of pre-historic archæology in )--"the great capacity of the brain, the development of the frontal region, the fine elliptical form of the anterior part of the profile of the skull, are incontestible characteristics of superiority, such as we are accustomed to meet with in civilised races;" yet the great breadth of the face, the enormous development of the ascending ramus of the lower jaw, the extent and roughness of the surfaces for the attachment of the muscles, especially of the masticators, and the extraordinary development of the ridge of the femur, indicate enormous muscular power, and the habits of a savage and brutal race. these facts might almost make us doubt whether the size of the brain is in any direct way an index of mental power, had we not the most conclusive evidence that it is so, in the fact that, whenever an adult male european has a skull less than nineteen inches in circumference, or has less than sixty-five cubic inches of brain, he is invariably idiotic. when we join with this the equally undisputed fact, that great men--those who combine acute perception with great reflective power, strong passions, and general energy of character, such as napoleon, cuvier, and o'connell, have always heads far above the average size, we must feel satisfied that volume of brain is one, and perhaps the most important, measure of intellect; and this being the case, we cannot fail to be struck with the apparent anomaly, that many of the lowest savages should have as much brains as average europeans. the idea is suggested of a surplusage of power; of an instrument beyond the needs of its possessor. _comparison of the brains of man and of anthropoid apes._--in order to discover if there is any foundation for this notion, let us compare the brain of man with that of animals. the adult male orang-utan is quite as bulky as a small sized man, while the gorilla is considerably above the average size of man, as estimated by bulk and weight; yet the former has a brain of only cubic inches, the latter, one of , or, in the largest specimen yet known, of ½ cubic inches. we have seen that the average cranial capacity of the lowest savages is probably not less than _five-sixths_ of that of the highest civilized races, while the brain of the anthropoid apes scarcely amounts to _one-third_ of that of man, in both cases taking the average; or the proportions may be more clearly represented by the following figures--anthropoid apes, ; savages, ; civilized man, . but do these figures at all approximately represent the relative intellect of the three groups? is the savage really no farther removed from the philosopher, and so much removed from the ape, as these figures would indicate? in considering this question, we must not forget that the heads of savages vary in size, almost as much as those of civilized europeans. thus, while the largest teutonic skull in dr. davis' collection is · cubic inches, there is an araucanian of · , an esquimaux of · , a marquesan of o· , a negro of · , and even an australian of · cubic inches. we may, therefore, fairly compare the savage with the highest european on the one side, and with the orang, chimpanzee, or gorilla, on the other, and see whether there is any relative proportion between brain and intellect. _range of intellectual power in man._--first, let us consider what this wonderful instrument, the brain, is capable of in its higher developments. in mr. galton's interesting work on "hereditary genius," he remarks on the enormous difference between the intellectual power and grasp of the well-trained mathematician or man of science, and the average englishman. the number of marks obtained by high wranglers, is often more than thirty times as great as that of the men at the bottom of the honour list, who are still of fair mathematical ability; and it is the opinion of skilled examiners, that even this does not represent the full difference of intellectual power. if, now, we descend to those savage tribes who only count to three or five, and who find it impossible to comprehend the addition of two and three without having the objects actually before them, we feel that the chasm between them and the good mathematician is so vast, that a thousand to one will probably not fully express it. yet we know that the mass of brain might be nearly the same in both, or might not differ in a greater proportion than as to ; whence we may fairly infer that the savage possesses a brain capable, if cultivated and developed, of performing work of a kind and degree far beyond what he ever requires it to do. again, let us consider the power of the higher or even the average civilized man, of forming abstract ideas, and carrying on more or less complex trains of reasoning. our languages are full of terms to express abstract conceptions. our business and our pleasures involve the continual foresight of many contingencies. our law, our government, and our science, continually require us to reason through a variety of complicated phenomena to the expected result. even our games, such as chess, compel us to exercise all these faculties in a remarkable degree. compare this with the savage languages, which contain no words for abstract conceptions; the utter want of foresight of the savage man beyond his simplest necessities; his inability to combine, or to compare, or to reason on any general subject that does not immediately appeal to his senses. so, in his moral and æsthetic faculties, the savage has none of those wide sympathies with all nature, those conceptions of the infinite, of the good, of the sublime and beautiful, which are so largely developed in civilized man. any considerable development of these would, in fact, be useless or even hurtful to him, since they would to some extent interfere with the supremacy of those perceptive and animal faculties on which his very existence often depends, in the severe struggle he has to carry on against nature and his fellow-man. yet the rudiments of all these powers and feelings undoubtedly exist in him, since one or other of them frequently manifest themselves in exceptional cases, or when some special circumstances call them forth. some tribes, such as the santals, are remarkable for as pure a love of truth as the most moral among civilized men. the hindoo and the polynesian have a high artistic feeling, the first traces of which are clearly visible in the rude drawings of the palæolithic men who were the contemporaries in france of the reindeer and the mammoth. instances of unselfish love, of true gratitude, and of deep religious feeling, sometimes occur among most savage races. on the whole, then, we may conclude, that the general moral and intellectual development of the savage, is not less removed from that of civilized man than has been shown to be the case in the one department of mathematics; and from the fact that all the moral and intellectual faculties do occasionally manifest themselves, we may fairly conclude that they are always latent, and that the large brain of the savage man is much beyond his actual requirements in the savage state. _intellect of savages and of animals compared._--let us now compare the intellectual wants of the savage, and the actual amount of intellect he exhibits, with those of the higher animals. such races as the andaman islanders, the australians, and the tasmanians, the digger indians of north america, or the natives of fuegia, pass their lives so as to require the exercise of few faculties not possessed in an equal degree by many animals. in the mode of capture of game or fish, they by no means surpass the ingenuity or forethought of the jaguar, who drops saliva into the water, and seizes the fish as they come to eat it; or of wolves and jackals, who hunt in packs; or of the fox, who buries his surplus food till he requires it. the sentinels placed by antelopes and by monkeys, and the various modes of building adopted by field mice and beavers, as well as the sleeping place of the orang-utan, and the tree-shelter of some of the african anthropoid apes, may well be compared with the amount of care and forethought bestowed by many savages in similar circumstances. his possession of free and perfect hands, not required for locomotion, enable man to form and use weapons and implements which are beyond the physical powers of brutes; but having done this, he certainly does not exhibit more mind in using them than do many lower animals. what is there in the life of the savage, but the satisfying of the cravings of appetite in the simplest and easiest way? what thoughts, ideas, or actions are there, that raise him many grades above the elephant or the ape? yet he possesses, as we have seen, a brain vastly superior to theirs in size and complexity; and this brain gives him, in an undeveloped state, faculties which he never requires to use. and if this is true of existing savages, how much more true must it have been of the men whose sole weapons were rudely chipped flints, and some of whom, we may fairly conclude, were lower than any existing race; while the only evidence yet in our possession shows them to have had brains fully as capacious as those of the average of the lower savage races. we see, then, that whether we compare the savage with the higher developments of man, or with the brutes around him, we are alike driven to the conclusion that in his large and well-developed brain he possesses an organ quite disproportionate to his actual requirements--an organ that seems prepared in advance, only to be fully utilized as he progresses in civilization. a brain slightly larger than that of the gorilla would, according to the evidence before us, fully have sufficed for the limited mental development of the savage; and we must therefore admit, that the large brain he actually possesses could never have been solely developed by any of those laws of evolution, whose essence is, that they lead to a degree of organization exactly proportionate to the wants of each species, never beyond those wants--that no preparation can be made for the future development of the race--that one part of the body can never increase in size or complexity, except in strict co-ordination to the pressing wants of the whole. the brain of pre-historic and of savage man seems to me to prove the existence of some power, distinct from that which has guided the development of the lower animals through their ever-varying forms of being. _the use of the hairy covering of mammalia._ let us now consider another point in man's organization, the bearing of which has been almost entirely overlooked by writers on both sides of this question. one of the most general external characters of the terrestrial mammalia is the hairy covering of the body, which, whenever the skin is flexible, soft, and sensitive, forms a natural protection against the severities of climate, and particularly against rain. that this is its most important function, is well shown by the manner in which the hairs are disposed so as to carry off the water, by being invariably directed downwards from the most elevated parts of the body. thus, on the under surface the hair is always less plentiful, and, in many cases, the belly is almost bare. the hair lies downwards, on the limbs of all walking mammals, from the shoulder to the toes, but in the orang-utan it is directed from the shoulder to the elbow, and again from the wrist to the elbow, in a reverse direction. this corresponds to the habits of the animal, which, when resting, holds its long arms upwards over its head, or clasping a branch above it, so that the rain would flow down both the arm and fore-arm to the long hair which meets at the elbow. in accordance with this principle, the hair is always longer or more dense along the spine or middle of the back from the nape to the tail, often rising into a crest of hair or bristles on the ridge of the back. this character prevails through the entire series of the mammalia, from the marsupials to the quadrumana, and by this long persistence it must have acquired such a powerful hereditary tendency, that we should expect it to reappear continually even after it had been abolished by ages of the most rigid selection; and we may feel sure that it never could have been completely abolished under the law of natural selection, unless it had become so positively injurious as to lead to the almost invariable extinction of individuals possessing it. _the constant absence of hair from certain parts of man's body a remarkable phenomenon._ in man the hairy covering of the body has almost totally disappeared, and, what is very remarkable, it has disappeared more completely from the back than from any other part of the body. bearded and beardless races alike have the back smooth, and even when a considerable quantity of hair appears on the limbs and breast, the back, and especially the spinal region, is absolutely free, thus completely reversing the characteristics of all other mammalia. the ainos of the kurile islands and japan are said to be a hairy race; but mr. bickmore, who saw some of them, and described them in a paper read before the ethnological society, gives no details as to where the hair was most abundant, merely stating generally, that "their chief peculiarity is their great abundance of hair, not only on the head and face, but over the whole body." this might very well be said of any man who had hairy limbs and breast, unless it was specially stated that his back was hairy, which is not done in this case. the hairy family in birmah have, indeed, hair on the back rather longer than on the breast, thus reproducing the true mammalian character, but they have still longer hair on the face, forehead, and inside the ears, which is quite abnormal; and the fact that their teeth are all very imperfect, shows that this is a case of monstrosity rather than one of true reversion to the ancestral type of man before he lost his hairy covering. _savage man feels the want of this hairy covering._ we must now enquire if we have any evidence to show, or any reason to believe, that a hairy covering to the back would be in any degree hurtful to savage man, or to man in any stage of his progress from his lower animal form; and if it were merely useless, could it have been so entirely and completely removed as not to be continually reappearing in mixed races? let us look to savage man for some light on these points. one of the most common habits of savages is to use some covering for the back and shoulders, even when they have none on any other part of the body. the early voyagers observed with surprise, that the tasmanians, both men and women, wore the kangaroo-skin, which was their only covering, not from any feeling of modesty, but over the shoulders to keep the back dry and warm. a cloth over the shoulders was also the national dress of the maories. the patagonians wear a cloak or mantle over the shoulders, and the fuegians often wear a small piece of skin on the back, laced on, and shifted from side to side as the wind blows. the hottentots also wore a somewhat similar skin over the back, which they never removed, and in which they were buried. even in the tropics most savages take precautions to keep their backs dry. the natives of timor use the leaf of a fan palm, carefully stitched up and folded, which they always carry with them, and which, held over the back, forms an admirable protection from the rain. almost all the malay races, as well as the indians of south america, make great palm-leaf hats, four feet or more across, which they use during their canoe voyages to protect their bodies from heavy showers of rain; and they use smaller hats of the same kind when travelling by land. we find, then, that so far from there being any reason to believe that a hairy covering to the back could have been hurtful or even useless to pre-historic man, the habits of modern savages indicate exactly the opposite view, as they evidently feel the want of it, and are obliged to provide substitutes of various kinds. the perfectly erect posture of man, may be supposed to have something to do with the disappearance of the hair from his body, while it remains on his head; but when walking, exposed to rain and wind, a man naturally stoops forwards, and thus exposes his back; and the undoubted fact, that most savages feel the effects of cold and wet most severely in that part of the body, sufficiently demonstrates that the hair could not have ceased to grow there merely because it was useless, even if it were likely that a character so long persistent in the entire order of mammalia, could have so completely disappeared, under the influence of so weak a selective power as a diminished usefulness. _man's naked skin could not have been produced by natural selection._ it seems to me, then, to be absolutely certain, that "natural selection" could not have produced man's hairless body by the accumulation of variations from a hairy ancestor. the evidence all goes to show that such variations could not have been useful, but must, on the contrary, have been to some extent hurtful. if even, owing to an unknown correlation with other hurtful qualities, it had been abolished in the ancestral tropical man, we cannot conceive that, as man spread into colder climates, it should not have returned under the powerful influence of reversion to such a long persistent ancestral type. but the very foundation of such a supposition as this is untenable; for we cannot suppose that a character which, like hairiness, exists throughout the whole of the mammalia, can have become, in one form only, so constantly correlated with an injurious character, as to lead to its permanent suppression--a suppression so complete and effectual that it never, or scarcely ever, reappears in mongrels of the most widely different races of man. two characters could hardly be wider apart, than the size and development of man's brain, and the distribution of hair upon the surface of his body; yet they both lead us to the same conclusion--that some other power than natural selection has been engaged in his production. _feet and hands of man, considered as difficulties on the theory of natural selection._ there are a few other physical characteristics of man, that may just be mentioned as offering similar difficulties, though i do not attach the same importance to them as to those i have already dwelt on. the specialization and perfection of the hands and feet of man seems difficult to account for. throughout the whole of the quadrumana the foot is prehensile; and a very rigid selection must therefore have been needed to bring about that arrangement of the bones and muscles, which has converted the thumb into a great toe, so completely, that the power of opposability is totally lost in every race, whatever some travellers may vaguely assert to the contrary. it is difficult to see why the prehensile power should have been taken away. it must certainly have been useful in climbing, and the case of the baboons shows that it is quite compatible with terrestrial locomotion. it may not be compatible with perfectly easy erect locomotion; but, then, how can we conceive that early man, _as an animal_, gained anything by purely erect locomotion? again, the hand of man contains latent capacities and powers which are unused by savages, and must have been even less used by palæolithic man and his still ruder predecessors. it has all the appearance of an organ prepared for the use of civilized man, and one which was required to render civilization possible. apes make little use of their separate fingers and opposable thumbs. they grasp objects rudely and clumsily, and look as if a much less specialized extremity would have served their purpose as well. i do not lay much stress on this, but, if it be proved that some intelligent power has guided or determined the development of man, then we may see indications of that power, in facts which, by themselves, would not serve to prove its existence. _the voice of man._--the same remark will apply to another peculiarly human character, the wonderful power, range, flexibility, and sweetness, of the musical sounds producible by the human larynx, especially in the female sex. the habits of savages give no indication of how this faculty could have been developed by natural selection; because it is never required or used by them. the singing of savages is a more or less monotonous howling, and the females seldom sing at all. savages certainly never choose their wives for fine voices, but for rude health, and strength, and physical beauty. sexual selection could not therefore have developed this wonderful power, which only comes into play among civilized people. it seems as if the organ had been prepared in anticipation of the future progress of man, since it contains latent capacities which are useless to him in his earlier condition. the delicate correlations of structure that give it such marvellous powers, could not therefore have been acquired by means of natural selection. _the origin of some of man's mental faculties, by the preservation of useful variations, not possible._ turning to the mind of man, we meet with many difficulties in attempting to understand, how those mental faculties, which are especially human, could have been acquired by the preservation of useful variations. at first sight, it would seem that such feelings as those of abstract justice and benevolence could never have been so acquired, because they are incompatible with the law of the strongest, which is the essence of natural selection. but this is, i think, an erroneous view, because we must look, not to individuals but to societies; and justice and benevolence, exercised towards members of the same tribe, would certainly tend to strengthen that tribe, and give it a superiority over another in which the right of the strongest prevailed, and where consequently the weak and the sickly were left to perish, and the few strong ruthlessly destroyed the many who were weaker. but there is another class of human faculties that do not regard our fellow men, and which cannot, therefore, be thus accounted for. such are the capacity to form ideal conceptions of space and time, of eternity and infinity--the capacity for intense artistic feelings of pleasure, in form, colour, and composition--and for those abstract notions of form and number which render geometry and arithmetic possible. how were all or any of these faculties first developed, when they could have been of no possible use to man in his early stages of barbarism? how could "natural selection," or survival of the fittest in the struggle for existence, at all favour the development of mental powers so entirely removed from the material necessities of savage men, and which even now, with our comparatively high civilization, are, in their farthest developments, in advance of the age, and appear to have relation rather to the future of the race than to its actual status? _difficulty as to the origin of the moral sense._ exactly the same difficulty arises, when we endeavour to account for the development of the moral sense or conscience in savage man; for although the _practice_ of benevolence, honesty, or truth, may have been useful to the tribe possessing these virtues, that does not at all account for the peculiar _sanctity_, attached to actions which each tribe considers right and moral, as contrasted with the very different feelings with which they regard what is merely _useful_. the utilitarian hypothesis (which is the theory of natural selection applied to the mind) seems inadequate to account for the development of the moral sense. this subject has been recently much discussed, and i will here only give one example to illustrate my argument. the utilitarian sanction for truthfulness is by no means very powerful or universal. few laws enforce it. no very severe reprobation follows untruthfulness. in all ages and countries, falsehood has been thought allowable in love, and laudable in war; while, at the present day, it is held to be venial by the majority of mankind, in trade, commerce, and speculation. a certain amount of untruthfulness is a necessary part of politeness in the east and west alike, while even severe moralists have held a lie justifiable, to elude an enemy or prevent a crime. such being the difficulties with which this virtue has had to struggle, with so many exceptions to its practice, with so many instances in which it brought ruin or death to its too ardent devotee, how can we believe that considerations of utility could ever invest it with the mysterious sanctity of the highest virtue,--could ever induce men to value truth for its own sake, and practice it regardless of consequences? yet, it is a fact, that such a mystical sense of wrong does attach to untruthfulness, not only among the higher classes of civilized people, but among whole tribes of utter savages. sir walter elliott tells us (in his paper "on the characteristics of the population of central and southern india," published in the journal of the ethnological society of london, vol. i., p. ) that the kurubars and santals, barbarous hill-tribes of central india, are noted for veracity. it is a common saying that "a kurubar _always_ speaks the truth;" and major jervis says, "the santals are the most truthful men i ever met with." as a remarkable instance of this quality the following fact is given. a number of prisoners, taken during the santal insurrection, were allowed to go free on parole, to work at a certain spot for wages. after some time cholera attacked them and they were obliged to leave, but every man of them returned and gave up his earnings to the guard. two hundred savages with money in their girdles, walked thirty miles back to prison rather than break their word! my own experience among savages has furnished me with similar, although less severely tested, instances; and we cannot avoid asking, how is it, that in these few cases "experiences of utility" have left such an overwhelming impression, while in so many others they have left none? the experiences of savage men as regards the utility of truth, must, in the long run, be pretty nearly equal. how is it, then, that in some cases the result is a sanctity which overrides all considerations of personal advantage, while in others there is hardly a rudiment of such a feeling? the intuitional theory, which i am now advocating, explains this by the supposition, that there is a feeling--a sense of right and wrong--in our nature, antecedent to and independent of experiences of utility. where free play is allowed to the relations between man and man, this feeling attaches itself to those acts of universal utility or self-sacrifice, which are the products of our affections and sympathies, and which we term moral; while it may be, and often is, perverted, to give the same sanction to acts of narrow and conventional utility which are really immoral,--as when the hindoo will tell a lie, but will sooner starve than eat unclean food; and looks upon the marriage of adult females as gross immorality. the strength of the moral feeling will depend upon individual or racial constitution, and on education and habit;--the acts to which its sanctions are applied, will depend upon how far the simple feelings and affections of our nature, have been modified by custom, by law, or by religion. it is difficult to conceive that such an intense and mystical feeling of right and wrong, (so intense as to overcome all ideas of personal advantage or utility), could have been developed out of accumulated ancestral experiences of utility; and still more difficult to understand, how feelings developed by one set of utilities, could be transferred to acts of which the utility was partial, imaginary, or altogether absent. but if a moral sense is an essential part of our nature, it is easy to see, that its sanction may often be given to acts which are useless or immoral; just as the natural appetite for drink, is perverted by the drunkard into the means of his destruction. _summary of the argument as to the insufficiency of natural selection to account for the development of man._ briefly to resume my argument--i have shown that the brain of the lowest savages, and, as far as we yet know, of the pre-historic races, is little inferior in size to that of the highest types of man, and immensely superior to that of the higher animals; while it is universally admitted that quantity of brain is one of the most important, and probably the most essential, of the elements which determine mental power. yet the mental requirements of savages, and the faculties actually exercised by them, are very little above those of animals. the higher feelings of pure morality and refined emotion, and the power of abstract reasoning and ideal conception, are useless to them, are rarely if ever manifested, and have no important relations to their habits, wants, desires, or well-being. they possess a mental organ beyond their needs. natural selection could only have endowed savage man with a brain a little superior to that of an ape, whereas he actually possesses one very little inferior to that of a philosopher. the soft, naked, sensitive skin of man, entirely free from that hairy covering which is so universal among other mammalia, cannot be explained on the theory of natural selection. the habits of savages show that they feel the want of this covering, which is most completely absent in man exactly where it is thickest in other animals. we have no reason whatever to believe, that it could have been hurtful, or even useless to primitive man; and, under these circumstances, its complete abolition, shown by its never reverting in mixed breeds, is a demonstration of the agency of some other power than the law of the survival of the fittest, in the development of man from the lower animals. other characters show difficulties of a similar kind, though not perhaps in an equal degree. the structure of the human foot and hand seem unnecessarily perfect for the needs of savage man, in whom they are as completely and as humanly developed as in the highest races. the structure of the human larynx, giving the power of speech and of producing musical sounds, and especially its extreme development in the female sex, are shown to be beyond the needs of savages, and from their known habits, impossible to have been acquired either by sexual selection, or by survival of the fittest. the mind of man offers arguments in the same direction, hardly less strong than those derived from his bodily structure. a number of his mental faculties have no relation to his fellow men, or to his material progress. the power of conceiving eternity and infinity, and all those purely abstract notions of form, number, and harmony, which play so large a part in the life of civilised races, are entirely outside of the world of thought of the savage, and have no influence on his individual existence or on that of his tribe. they could not, therefore, have been developed by any preservation of useful forms of thought; yet we find occasional traces of them amidst a low civilization, and at a time when they could have had no practical effect on the success of the individual, the family, or the race; and the development of a moral sense or conscience by similar means is equally inconceivable. but, on the other hand, we find that every one of these characteristics is necessary for the full development of human nature. the rapid progress of civilization under favourable conditions, would not be possible, were not the organ of the mind of man prepared in advance, fully developed as regards size, structure, and proportions, and only needing a few generations of use and habit to co-ordinate its complex functions. the naked and sensitive skin, by necessitating clothing and houses, would lead to the more rapid development of man's inventive and constructive faculties; and, by leading to a more refined feeling of personal modesty, may have influenced, to a considerable extent, his moral nature. the erect form of man, by freeing the hands from all locomotive uses, has been necessary for his intellectual advancement; and the extreme perfection of his hands, has alone rendered possible that excellence in all the arts of civilization which raises him so far above the savage, and is perhaps but the forerunner of a higher intellectual and moral advancement. the perfection of his vocal organs has first led to the formation of articulate speech, and then to the development of those exquisitely toned sounds, which are only appreciated by the higher races, and which are probably destined for more elevated uses and more refined enjoyment, in a higher condition than we have yet attained to. so, those faculties which enable us to transcend time and space, and to realize the wonderful conceptions of mathematics and philosophy, or which give us an intense yearning for abstract truth, (all of which were occasionally manifested at such an early period of human history as to be far in advance of any of the few practical applications which have since grown out of them), are evidently essential to the perfect development of man as a spiritual being, but are utterly inconceivable as having been produced through the action of a law which looks only, and can look only, to the immediate material welfare of the individual or the race. the inference i would draw from this class of phenomena is, that a superior intelligence has guided the development of man in a definite direction, and for a special purpose, just as man guides the development of many animal and vegetable forms. the laws of evolution alone would, perhaps, never have produced a grain so well adapted to man's use as wheat and maize; such fruits as the seedless banana and bread-fruit; or such animals as the guernsey milch cow, or the london dray-horse. yet these so closely resemble the unaided productions of nature, that we may well imagine a being who had mastered the laws of development of organic forms through past ages, refusing to believe that any new power had been concerned in their production, and scornfully rejecting the theory (as my theory will be rejected by many who agree with me on other points), that in these few cases a controlling intelligence had directed the action of the laws of variation, multiplication, and survival, for his own purposes. we know, however, that this has been done; and we must therefore admit the possibility that, if we are not the highest intelligences in the universe, some higher intelligence may have directed the process by which the human race was developed, by means of more subtle agencies than we are acquainted with. at the same time i must confess, that this theory has the disadvantage of requiring the intervention of some distinct individual intelligence, to aid in the production of what we can hardly avoid considering as the ultimate aim and outcome of all organized existence--intellectual, ever-advancing, spiritual man. it therefore implies, that the great laws which govern the material universe were insufficient for his production, unless we consider (as we may fairly do) that the controlling action of such higher intelligences is a necessary part of those laws, just as the action of all surrounding organisms is one of the agencies in organic development. but even if my particular view should not be the true one, the difficulties i have put forward remain, and i think prove, that some more general and more fundamental law underlies that of "natural selection." the law of "unconscious intelligence" pervading all organic nature, put forth by dr. laycock and adopted by mr. murphy, is such a law; but to my mind it has the double disadvantage of being both unintelligible and incapable of any kind of proof. it is more probable, that the true law lies too deep for us to discover it; but there seems to me, to be ample indications that such a law does exist, and is probably connected with the absolute origin of life and organization. (_note a._) _the origin of consciousness._ the question of the origin of sensation and of thought can be but briefly discussed in this place, since it is a subject wide enough to require a separate volume for its proper treatment. no physiologist or philosopher has yet ventured to propound an intelligible theory, of how sensation may possibly be a product of organization; while many have declared the passage from matter to mind to be inconceivable. in his presidential address to the physical section of the british association at norwich, in , professor tyndall expressed himself as follows:-- "the passage from the physics of the brain to the corresponding facts of consciousness is unthinkable. granted that a definite thought, and a definite molecular action in the brain occur simultaneously, we do not possess the intellectual organ, nor apparently any rudiment of the organ, which would enable us to pass by a process of reasoning from the one phenomenon to the other. they appear together, but we do not know why. were our minds and senses so expanded, strengthened, and illuminated as to enable us to see and feel the very molecules of the brain; were we capable of following all their motions, all their groupings, all their electric discharges, if such there be, and were we intimately acquainted with the corresponding states of thought and feeling, we should be as far as ever from the solution of the problem, 'how are these physical processes connected with the facts of consciousness?' the chasm between the two classes of phenomena would still remain intellectually impassable." in his latest work ("an introduction to the classification of animals,") published in , professor huxley unhesitatingly adopts the "well founded doctrine, that life is the cause and not the consequence of organization." in his celebrated article "on the physical basis of life," however, he maintains, that life is a property of protoplasm, and that protoplasm owes its properties to the nature and disposition of its molecules. hence he terms it "the matter of life," and believes that all the physical properties of organized beings are due to the physical properties of protoplasm. so far we might, perhaps, follow him, but he does not stop here. he proceeds to bridge over that chasm which professor tyndall has declared to be "intellectually impassable," and, by means which he states to be logical, arrives at the conclusion, that our "_thoughts are the expression of molecular changes in that matter of life which is the source of our other vital phenomena_." not having been able to find any clue in professor huxley's writings, to the steps by which he passes from those vital phenomena, which consist only, in their last analysis, of movements of particles of matter, to those other phenomena which we term thought, sensation, or consciousness; but, knowing that so positive an expression of opinion from him will have great weight with many persons, i shall endeavour to show, with as much brevity as is compatible with clearness, that this theory is not only incapable of proof, but is also, as it appears to me, inconsistent with accurate conceptions of molecular physics. to do this, and in order further to develop my views, i shall have to give a brief sketch of the most recent speculations and discoveries, as to the ultimate nature and constitution of matter. _the nature of matter._ it has been long seen by the best thinkers on the subject, that atoms,--considered as minute solid bodies from which emanate the attractive and repulsive forces which give what we term matter its properties,--could serve no purpose whatever; since it is universally admitted that the supposed atoms never touch each other, and it cannot be conceived that these homogeneous, indivisible, solid units, are themselves the ultimate _cause_ of the forces that emanate from their centres. as, therefore, none of the properties of matter can be due to the atoms themselves, but only to the forces which emanate from the points in space indicated by the atomic centres, it is logical continually to diminish their size till they vanish, leaving only localized centres of force to represent them. of the various attempts that have been made to show how the properties of matter may be due to such modified atoms (considered as mere centres of force), the most successful, because the simplest and the most logical, is that of mr. bayma, who, in his "molecular mechanics," has demonstrated how, from the simple assumption of such centres having attractive and repulsive forces (both varying according to the same law of the inverse squares as gravitation), and by grouping them in symmetrical figures, consisting of a repulsive centre, an attractive nucleus, and one or more repulsive envelopes, we may explain all the general properties of matter; and, by more and more complex arrangements, even the special chemical, electrical, and magnetic properties of special forms of matter.[i] each chemical element will thus consist of a molecule formed of simple atoms, (or as mr. bayma terms them to avoid confusion, "material elements") in greater or less number and of more or less complex arrangement; which molecule is in stable equilibrium, but liable to be changed in form by the attractive or repulsive influences of differently constituted molecules, constituting the phenomena of chemical combination, and resulting in new forms of molecule of greater complexity and more or less stability. +--------------------------------------------------------------+ | [i] mr. bayma's work, entitled "the elements of molecular | | mechanics," was published in , and has received less | | attention than it deserves. it is characterised by great | | lucidity, by logical arrangement, and by comparatively | | simple geometrical and algebraical demonstrations, so that | | it may be understood and appreciated with a very moderate | | knowledge of mathematics. it consists of a series of | | propositions, deduced from the known properties of matter; | | from these are derived a number of theorems, by whose help | | the more complicated problems are solved. nothing is taken | | for granted throughout the work, and the only valid mode of | | escaping from its conclusions is, by either disproving the | | fundamental propositions, or by detecting fallacies in the | | subsequent reasoning. | +--------------------------------------------------------------+ those organic compounds of which organized beings are built up, consist, as is well known, of matter of an extreme complexity. and great instability; whence result the changes of form to which it is continually subject. this view enables us to comprehend the _possibility_, of the phenomena of vegetative life being due to an almost infinite complexity of molecular combinations, subject to definite changes under the stimuli of heat, moisture, light, electricity, and probably some unknown forces. but this greater and greater complexity, even if carried to an infinite extent, cannot, of itself, have the slightest tendency to originate consciousness in such molecules or groups of molecules. if a material element, or a combination of a thousand material elements in a molecule, are alike unconscious, it is impossible for us to believe, that the mere addition of one, two, or a thousand other material elements to form a more complex molecule, could in any way tend to produce a self-conscious existence. the things are radically distinct. to say that mind is a product or function of protoplasm, or of its molecular changes, is to use words to which we can attach no clear conception. you cannot have, in the whole, what does not exist in any of the parts; and those who argue thus should put forth a definite conception of matter, with clearly enunciated properties, and show, that the necessary result of a certain complex arrangement of the elements or atoms of that matter, will be the production of self-consciousness. there is no escape from this dilemma,--either all matter is conscious, or consciousness is something distinct from matter, and in the latter case, its presence in material forms is a proof of the existence of conscious beings, outside of, and independent of, what we term matter. (_note b._) _matter is force._--the foregoing considerations lead us to the very important conclusion, that matter is essentially force, and nothing but force; that matter, as popularly understood, does not exist, and is, in fact, philosophically inconceivable. when we touch matter, we only really experience sensations of resistance, implying repulsive force; and no other sense can give us such apparently solid proofs of the reality of matter, as touch does. this conclusion, if kept constantly present in the mind, will be found to have a most important bearing on almost every high scientific and philosophical problem, and especially on such as relate to our own conscious existence. _all force is probably will-force._--if we are satisfied that force or forces are all that exist in the material universe, we are next led to enquire what is force? we are acquainted with two radically distinct or apparently distinct kinds of force--the first consists of the primary forces of nature, such as gravitation, cohesion, repulsion, heat, electricity, &c.; the second is our own will-force. many persons will at once deny that the latter exists. it will be said, that it is a mere transformation of the primary forces before alluded to; that the correlation of forces includes those of animal life, and that _will_ itself is but the result of molecular change in the brain. i think, however, that it can be shown, that this latter assertion has neither been proved, nor even been proved to be possible; and that in making it, a great leap in the dark has been taken from the known to the unknown. it may be at once admitted that the _muscular force_ of animals and men, is merely the transformed energy derived from the primary forces of nature. so much has been, if not rigidly proved, yet rendered highly probable, and it is in perfect accordance with all our knowledge of natural forces and natural laws. but it cannot be contended that the physiological balance-sheet has ever been so accurately struck, that we are entitled to say, not one-thousandth part of a grain more of force has been exerted by any organized body or in any part of it, than has been derived from the known primary forces of the material world. if that were so, it would absolutely negative the existence of will; for if will is anything, it is a power that _directs_ the action of the forces stored up in the body, and it is not conceivable that this _direction_ can take place, without the exercise of some force in some part of the organism. however delicately a machine may be constructed, with the most exquisitely contrived detents to release a weight or spring by the exertion of the smallest possible amount of force, _some_ external force will always, be required; so, in the animal machine, however minute may be the changes required in the cells or fibres of the brain, to set in motion the nerve currents which loosen or excite the pent up forces of certain muscles, _some force_ must be required to effect those changes. if it is said, "those changes are automatic, and are set in motion by external causes," then one essential part of our consciousness, a certain amount of freedom in willing, is annihilated; and it is inconceivable how or why there should have arisen any consciousness or any apparent will, in such purely automatic organisms. if this were so, our apparent will would be a delusion, and professor huxley's belief--"that our volition counts for something as a condition of the course of events," would be fallacious, since our volition would then be but one link in the chain of events, counting for neither more nor less than any other link whatever. if, therefore, we have traced one force, however minute, to an origin in our own will, while we have no knowledge of any other primary cause of force, it does not seem an improbable conclusion that all force may be will-force; and thus, that the whole universe, is not merely dependent on, but actually _is_, the will of higher intelligences or of one supreme intelligence. it has been often said that the true poet is a seer; and in the noble verse of an american poetess, we find expressed, what may prove to be the highest fact of science, the noblest truth of philosophy: god of the granite and the rose! soul of the sparrow and the bee! the mighty tide of being flows through countless channels, lord, from thee. it leaps to life in grass and flowers, through every grade of being runs, while from creation's radiant towers its glory flames in stars and suns. _conclusion._ these speculations are usually held to be far beyond the bounds of science; but they appear to me to be more legitimate deductions from the facts of science, than those which consist in reducing the whole universe, not merely to matter, but to matter conceived and defined so as to be philosophically inconceivable. it is surely a great step in advance, to get rid of the notion that _matter_ is a thing of itself, which can exist _per se_, and must have been eternal, since it is supposed to be indestructible and uncreated,--that force, or the forces of nature, are another thing, given or added to matter, or else its necessary properties,--and that mind is yet another thing, either a product of this matter and its supposed inherent forces, or distinct from and co-existent with it;--and to be able to substitute for this complicated theory, which leads to endless dilemmas and contradictions, the far simpler and more consistent belief, that matter, as an entity distinct from force, does not exist; and that force is a product of mind. philosophy had long demonstrated our incapacity to prove the existence of matter, as usually conceived; while it admitted the demonstration to each of us of our own self-conscious, ideal existence. science has now worked its way up to the same result, and this agreement between them should give us some confidence in their combined teaching. the view we have now arrived at seems to me more grand and sublime, as well as far simpler, than any other. it exhibits the universe, as a universe of intelligence and will-power; and by enabling us to rid ourselves of the impossibility of thinking of mind, but as connected with our old notions of matter, opens up infinite possibilities of existence, connected with infinitely varied manifestations of force, totally distinct from, yet as real as, what we term matter. the grand law of continuity which we see pervading our universe, would lead us to infer infinite gradations of existence, and to people all space with intelligence and will-power; and, if so, we have no difficulty in believing that for so noble a purpose as the progressive development of higher and higher intelligences, those primal and general will-forces, which have sufficed for the production of the lower animals, should have been guided into new channels and made to converge in definite directions. and if, as seems to me probable, this has been done, i cannot admit that it in any degree affects the truth or generality of mr. darwin's great discovery. it merely shows, that the laws of organic development have been occasionally used for a special end, just as man uses them for his special ends; and, i do not see that the law of "natural selection" can be said to be disproved, if it can be shown that man does not owe his entire physical and mental development to its unaided action, any more than it is disproved by the existence of the poodle or the pouter pigeon, the production of which may have been equally beyond its undirected power. the objections which in this essay i have taken, to the view,--that the same law which appears to have sufficed for the development of animals, has been alone the cause of man's superior physical and mental nature,--will, i have no doubt, be over-ruled and explained away. but i venture to think they will nevertheless maintain their ground, and that they can only be met by the discovery of new facts or new laws, of a nature very different from any yet known to us. i can only hope that my treatment of the subject, though necessarily very meagre, has been clear and intelligible; and that it may prove suggestive, both to the opponents and to the upholders of the theory of natural selection. notes. _note a._ (_page_ .) some of my critics seem quite to have misunderstood my meaning in this part of the argument. they have accused me of unnecessarily and unphilosophically appealing to "first causes" in order to get over a difficulty--of believing that "our brains are made by god and our lungs by natural selection;" and that, in point of fact, "man is god's domestic animal." an eminent french critic, m. claparède, makes me continually call in the aid of--"_une force supérieure_," the capital f, meaning i imagine that this "higher force" is the deity. i can only explain this misconception by the incapacity of the modern cultivated mind to realise the existence of any higher intelligence between itself and deity. angels and archangels, spirits and demons, have been so long banished from our belief as to have become actually unthinkable as actual existences, and nothing in modern philosophy takes their place. yet the grand law of "continuity," the last outcome of modern science, which seems absolute throughout the realms of matter, force, and mind, so far as we can explore them, cannot surely fail to be true beyond the narrow sphere of our vision, and leave an infinite chasm between man and the great mind of the universe. such a supposition seems to me in the highest degree improbable. now, in referring to the origin of man, and its possible determining causes, i have used the words "some other power"--"some intelligent power"--"a superior intelligence"--"a controlling intelligence," and only in reference to the origin of universal forces and laws have i spoken of the will or power of "one supreme intelligence." these are the only expressions i have used in alluding to the power which i believe has acted in the case of man, and they were purposely chosen to show, that i reject the hypothesis of "first causes" for any and every _special_ effect in the universe, except in the same sense that the action of man or of any other intelligent being is a first cause. in using such terms i wished to show plainly, that i contemplated the possibility that the development of the essentially human portions of man's structure and intellect may have been determined by the directing influence of some higher intelligent beings, acting through natural and universal laws. a belief of this nature may or may not have a foundation, but it is an intelligible theory, and is not, _in its nature_, incapable of proof; and it rests on facts and arguments of an exactly similar kind to those, which would enable a sufficiently powerful intellect to deduce, from the existence on the earth of cultivated plants and domestic animals, the presence of some intelligent being of a higher nature than themselves. _note b._ (_page_ .) a friend has suggested that i have not here explained myself sufficiently, and objects, that _life_ does not exist in matter any more than _consciousness_, and if the one can be produced by the laws of matter, why may not the other? i reply, that there is a radical difference between the two. organic or vegetative life consists essentially in chemical transformations and molecular motions, occurring under certain conditions and in a certain order. the matter, and the forces which act upon it, are for the most part known; and if there are any forces engaged in the manifestation of vegetative life yet undiscovered (which is a moot question), we can conceive them as analogous to such forces as heat, electricity, or chemical affinity, with which we are already acquainted. we can thus clearly _conceive_ of the transition from dead matter to living matter. a complex mass which suffers decomposition or decay is dead, but if this mass has the power of attracting to itself, from the surrounding medium, matter like that of which it is composed, we have the first rudiment of vegetative life. if the mass can do this for a considerable time, and if its absorption of new matter more than replaces that lost by decomposition, and if it is of such a nature as to resist the mechanical or chemical forces to which it is usually exposed, and to retain a tolerably constant form, we term it a living organism. we can _conceive_ an organism to be so constituted, and we can further conceive that any fragments, which may be accidentally broken from it, or which may fall away when its bulk has become too great for the cohesion of all its parts, may begin to increase anew and run the same course as the parent mass. this is growth and reproduction in their simplest forms; and from such a simple beginning it is possible to conceive a series of slight modifications of composition, and of internal and external forces, which should ultimately lead to the development of more complex organisms. the life of such an organism may, perhaps, be nothing added to it, but merely the name we give to the result of a balance of internal and external forces in maintaining the permanence of the form and structure of the individual. the simplest conceivable form of such life would be the dewdrop, which owes its existence to the balance between the condensation of aqueous vapour in the atmosphere and the evaporation of its substance. if either is in excess, it soon ceases to maintain an individual existence. i do not maintain that vegetative life _is_ wholly due to such a complex balance of forces, but only that it is _conceivable_ as such. with consciousness the case is very different. its phenomena are not comparable with those of any kind of _matter_ subjected to any of the known or conceivable _forces_ of nature; and we cannot _conceive_ a gradual transition from absolute unconsciousness to consciousness, from an unsentient organism to a sentient being. the merest rudiment of sensation or self-consciousness is infinitely removed from absolutely non-sentient or unconscious matter. we can conceive of no physical addition to, or modification of, an unconscious mass which should create consciousness; no step in the series of changes organised matter may undergo, which should bring in sensation where there was no sensation or power of sensation at the preceding step. it is because the things are utterly incomparable and incommensurable that we can only conceive of _sensation_ coming to matter from without, while _life_ may be conceived as merely a specific combination and co-ordination of the matter and the forces that compose the universe, and with which we are separately acquainted. we may admit with professor huxley that _protoplasm_ is the "matter of life" and the cause of organisation, but we cannot admit or conceive that _protoplasm_ is the primary source of sensation and consciousness, or that it can ever of itself become _conscious_ in the same way as we may perhaps conceive that it may become _alive_. index. _abraxas grossulariata_, . _acanthotritus dorsalis_, . _accipiter pileatus_, . acrÆidÆ, the subjects of mimicry, , . _acronycta psi_, protective colouring of, . adaptation brought about by general laws, ; looks like design, . ÆgeriidÆ mimic hymenoptera, . agassiz, or embryonic character of ancient animals, . _agnia fasciata_, mimics another longicorn, . _agriopis aprilina_, protective colouring of, . alcedinidÆ, sexual colouring and nidification of, . amadina, sexual colouring and nidification of, . ampelidÆ, sexual colouring and nidification of, . ancylotherium, . andrenidÆ, . _angræcum sesquipedale_, ; its fertilization by a large moth, . animals, senses and faculties of, ; intellect of, compared with that of savages, . anisocerinÆ, . anoa, . anoplotherium, . anthribidÆ, mimicry of, ; dimorphism in, . _anthrocera filipendulæ_, . anthropologists, wide difference of opinion among, as to origin of human races, ; conflicting views of, harmonized, . antiquity of man, , . apathus, . apparent exceptions to law of colour and nidification, . aquatic birds, why abundant, . _araschnia prorsa_, . archegosaurus, . archÆopteryx, . architecture of most nations derivative, ; grecian, false in principle, . arctic animals, white colour of, , . argyll, duke of, on colours of woodcock, ; on mind in nature, ; criticism on darwin's works, ; on humming birds ; on creation by birth, . asilus, . aspects of nature as influencing man's development, . babirusa, . balance in nature, . barrington, hon. daines, on song of birds, . basilornis, . bates, mr., first adopted the word "mimicry," ; his observations on leptalis and heliconidæ, ; his paper explaining the theory of mimicry, ; objections to his theory, ; on variation, ; on recent immigration of amazonian indians, . bayma, mr., on "molecular mechanics," , . beauty in nature, ; not universal, ; of flowers useful to them, ; not given for its own sake, . birds, possible rapid increase of, ; numbers that die annually, ; mimicry among, ; dull colour of females, ; nidification as affecting colour of females, ; refusing the gooseberry caterpillar, ; the highest in rank and organization, ; dimorphism in, ; why peculiar nest built by each species, - ; build more perfect nests as they grow older, , ; alter and improve their nests, ; sexual differences of colour in, . _bombus hortorum_, . _bombycilla, garrula_, colours and nidification of, . bombylius, . brain of the savage but slightly less than that of civilized man, ; size of, an important element of mental power, ; of savage races larger than their needs require, , ; of man and of anthropoid apes compared, . broca, professor paul, on the fine crania of the cave men, . _bryophila glandifera_ and _b. perla_ protectively coloured, . bucerotidÆ, sexual colouring and nidification of, . bucconidÆ, sexual colouring and nidification of, . buff-tip moth, resembles a broken stick, . buildings of various races do not change, . buprestidÆ, resembling bird's dung, ; similar colours in two sexes, . butterflies, value of, in studying "natural selection," ; varieties of, in sardinia and isle of man, . _cacia anthriboides_, . _callizona acesta_, protective colouring of, . calornis, . capitonidÆ, sexual colouring and nidification of, . _capnolymma stygium_, . carabidÆ, special protection among, ; similar colouring of two sexes, . cassidÆ, resemble dew drops, . caterpillars, mimicking a poisonous snake, ; gaudy colours of, ; various modes of protection of, ; gooseberry caterpillar, ; mr. jenner weir's observations on, ; mr. a. g. butler's observations on, . celebes, local modifications of form in, ; probable cause of these, ; remarkable zoological peculiarities of, - . centropus, sexual colouring and nidification of, . _cephalodonta spinipes_, . _ceroxylus laceratus_, imitates a moss-covered stick, . certhiola, sexual colouring and nidification of, . _cethosia æole_, ; _biblis_, . cetoniadÆ, how protected, ; similar colours of two sexes, . ceycopsis, . _charis melipona_, . chematobia, wintry colours of this genus, . _chlamys pilula_, resembles dung of caterpillars, . chrysididÆ, how protected, . chrysomelidÆ, similar colouring of two sexes, . cicindela, adaptive colour of various species of, . _cilix compressa_, resembles bird's dung, . cladobates, mimicking squirrels, . classification, form of true, ; circular, inadmissible, ; quinarian and circular, of swainson, ; argument from, against mr. darwin, . climacteris, sexual colouring and nidification of, . coccinellidÆ, how protected, ; similar colouring of sexes, . coexisting varieties, . _collyrodes lacordairei_, . colour, in animals, popular theories of, ; frequent variations of, in domesticated animals, ; influenced by need of concealment, ; in deserts, , ; in arctic regions, , ; nocturnal, ; tropical, ; special modifications of, ; different distribution of, in butterflies and moths, ; of autumnal and winter moths, ; white, generally dangerous and therefore eliminated, ; why it exists so abundantly although often injurious, ; influenced by need of protection, ; of female birds, ; in relation to nidification of birds, ; gaudy colours of many caterpillars, ; in nature, general causes of, ; local variations of, ; sexual differences of, in birds, ; in female birds, how connected with their nidification, , ; more variable than structure or habits, and therefore more easily modified, ; of flowers, as explained by mr. darwin, ; often correlated with disease, . compsognathus, . _condylodera tricondyloides_, . consciousness, origin of, ; professor tyndall on, ; not a product of complex organization, . correlation of growth, . _corynomalus sp._, . cotingidÆ, sexual colouring and nidification of, . cratosomus, a hard weevil, . crickets mimicking sand wasps, . cryptodontia, . _cucullia verbasci_, . curculionidÆ, often protected by hard covering, ; similar colours of two sexes, . _cuviera squamata_, . _cyclopeplus batesii_, . cynopithecus, . _cynthia arsinoë_, . danaidÆ, the subjects of mimicry, , . _danais erippus_, ; _chysippus_, ; _sobrina_, ; _aglaia_, ; _tytia_, . darwin, mr., his principle of utility, ; on cause of colour in flowers, , ; on colours of caterpillars, ; on sexual colouration, ; his metaphors liable to misconception, ; criticism of, in _north british review_, . desert animals, colours of, , . diadema, species of, mimic danaidæ, , ; female with male colouration, . _diadema misippus_, ; _d. anomala_, . _diaphora mendica_, . dicnyodontia, . dicrourus, . _diloba coeruleocephala_, . dimorphism, ; in beetles, ; in birds, ; illustrated, . dinosauria, . diptera mimicking wasps and bees, . _doliops curculionides_, . domesticated animals, their essential difference from wild ones, - . dotterell, . drusilla, mimicked by three genera, . _drusilla bioculata_, . dytiscus, dimorphism in, . egyptian architecture, introduced, . _elaps fulvius_, _e. corallinus_, _e. lemniscatus_, ; _e. mipartitus_, _e. lemniscatus_, _e. hemiprichii_, . enodes, . ennomus, autumnal colours of this genus, . _eos fuscata_, dimorphism of, . equus, . _eronia tritæa_, ; _valeria_, . _eroschema poweri_, . erycinidÆ mimic heliconidæ, . _erythroplatis corallifer_, . estrelda, sexual colouring and nidification of, . eucnemidÆ, mimicking a malacoderm, . _eudromias morinellus_, . _euglossa dimidiata_, . eumorphidÆ, a protected group ; imitated by longicorns, . euploea, local modifications of colour in, . _euploea midamus_, - , ; _e. rhadamanthus_, , . _eurhinia megalonice_, ; _polynice_, . eurylÆmidÆ, sexual colouring and nidification of, . extinct animals, intermediate forms of, . extinction of lower races, . female birds, colours of, ; sometimes connected with their mode of nidification, ; more exposed to enemies than the males, . female butterflies generally dull-coloured, . female insects, mimicry by, , ; colours of, . female sex, has no incapacity for as brilliant colouration as the male, ; in some groups requires more protection than the male, . fishes, protective colouring of, . fissirostral birds, nests of, . flowers, causes of colour in, . flycatchers, genera of, absent from celebes, . forbes, edward, objections to his theory of polarity, - . force is probably all will-force, . galapagos, . galton, mr., on range of intellectual power, . ganocephala, . _gastropacha querci_, protective colour and form of, . gaudry, m., on fossil mammals of greece, . geographical distribution, dependent on geologic changes, ; its agreement with law of introduction of new species, ; of allied species and groups, . geological distribution analogous to geographical, . geology, facts proved by, - . giraffe, how it acquired its long neck, . glÆa, autumnal colours of this genus, . gould, mr., on sexual plumage of gray phalarope, ; on incubation by male dotterell, . _grallina australis_, . green birds almost confined to the tropics, . _gymnocerus cratosomoides_, . _gymnocerous capucinus_, . _gymnocerous dulcissimus_, . gunther, dr., on arboreal snakes, ; on colouring of snakes, . _gynecia dirce_, . habits, often persistent when use of them has ceased, ; of children and savages analogous to those of animals, ; if persistent and imitative may be termed hereditary, , . hairy covering of mammalia, use of, ; absence of, in man remarkable, ; the want of it felt by savages, ; could not have been abolished by natural selection, . _harpagus diodon_, . heiliplus, a hard genus of curculionidæ, . heliconidÆ, the objects of mimicry, ; their secretions, ; not attacked by birds, ; sometimes mimicked by other heliconidæ, . helladotherium, . hemiptera, protected by bad odour, . herbert, rev. w., on song of birds, . hesperidÆ, probable means of protection of, . hesthesis, longicorns resembling ants, . _hestia leuconoë_, . hewitson, mr., . hipparion, . hippotherium, . hispidÆ, imitated by longicorns, . holothuridÆ, . _homalocranium semicinctum_, . hooker, dr., on the value of the "specific term," . houses of american and malay races contrasted, . huxley, professor, on "physical basis of life," ; on volition, . hyÆnictis, . hybernia, wintry colours of this genus, . hymenoptera, large number of, peculiar to celebes, . icteridÆ, sexual colouring and nidification of, . icthyopterygia, . _ideopsis daos_, . imitation, the effects of, in man's works, . indians, how they travel through trackless forests, . insects, protective colouring of, ; mimicking species of other orders, ; senses of, perhaps different from ours, , . instinct, how it may be best studied, ; definition of, ; in many cases assumed without proof, ; if possessed by man, ; supposed, of indians, ; supposed to be shown in the construction of birds' nests, . intellect of savages compared with that of animals, . intellectual power, range of, in man, . _iphias glaucippe_, . ithomia, mimicked by leptalis, . _ithomia ilerdina_, mimicked by four groups of lepidoptera, . java, relations of, to sumatra and borneo, . jamaica swift altering position of nest, . jerdon, mr., on incubation by males in turnix, . _kallima inachis_ and _kallima paralekta_, wonderful resemblance of, to leaves, - . labyrinthodontia, , . lakes as cases of imperfect adaptation, . laniadÆ, sexual colouring and nidification of, . lamarck's hypothesis very different from the author's, . _larentia tripunctaria_, . law which has regulated the introduction of new species, ; confirmed by geographical distribution, ; high organization of ancient animals consistent with, ; of multiplication in geometrical progression, ; of limited populations, ; of heredity, ; of variation, ; of change of physical conditions, ; of the equilibrium of nature, ; as opposed to continual interference, . laycock, dr., on law of "unconscious intelligence," . leaf butterfly, appearance and habits of, - . lepidoptera, especially subject to variation, . leptalis, species of mimic heliconidæ, ; gain a protection thereby, . lester, mr. j. m., on wood-dove and robin, . levaillant, on formation of a nest, . _limenitis archippus_, . _limenitis limire_, ; _procris_, . lizards refusing certain moths and caterpillars, ; devouring bees, . local forms, . local variation of form, ; of colour, ; general remarks on, ; in celebesian butterflies, probable use of, . locustidÆ, adaptive colouring of, . luminousness of some insects a protection, . lycÆnidÆ, probable means of protection of, . mammals, mimicry among, . man, does he build by reason or imitation, ; his works mainly imitative, ; antiquity of, , ; difference of opinion as to his origin, ; unity or plurality of species, ; persistence of type of, ; importance of mental and moral characters, ; his dignity and supremacy, ; his influence on nature, ; his future development, ; range of intellectual power in, ; rudiments of all the higher faculties in savage, ; his feet and hands, difficulties on the theory of natural selection, ; his voice, ; his mental faculties, ; difficulty as to the origin of the moral sense in, ; development of, probably directed by a superior intelligence, . mantidÆ, adaptive colouring of, ; mimicking white ants, . malacoderms, a protected group, . maluridÆ, . matter, the nature of, ; mr. bayma on, ; is force, . mechanitis and methona, mimicked by _leptalis_, . mecocerus, dimorphism of, . _mecocerus gazella_, . megacephalon, . megapodidÆ, sexual colouring and nidification of, . meropogon, . _midas dives_, . mimeta, mimicking tropidorhynchus, . mimicry, meaning of the word, ; theory of, ; among lepidoptera, ; how it acts as a protection, , ; of other insects by lepidoptera, ; among beetles, ; of other insects by beetles, ; of insects by species of other orders, ; among the vertebrata, ; among snakes, ; among tree frogs, ; among birds, ; among mammals, ; objections to the theory of, ; by female insects, ; among papilionidæ, ; never occurs in the male only, . momotidÆ, sexual colouring and nidification of, . montrouzier, m., on butterflies of woodlark island, . moral sense, difficulty as to the origin of, . morphos, how protected, . murray, mr. andrew, objections to theory of mimicry, . muscicapidÆ, sexual colouring and nidification of, . musophagidÆ, sexual colouring and nidification of, . napeogenes, all the species are mimickers, . natural selection, the principle stated, - ; general acceptance of the theory of, ; tabular demonstration of, ; outline of theory of, ; its effects on man and animals different, ; hardly acts among civilized societies, ; what it can not do, ; cannot produce injurious or useless modifications, . nectarineidÆ, . necydalidÆ, mimic hymenoptera, . _nemophas grayi_, a longicorn mimicked by a longicorn, . nests of birds, why different, ; of young birds, how built, ; construction of, described by levaillant, ; imperfections in, ; influenced by changed conditions and persistent habits, ; classification of, according to function, . new forms, how produced by variation and selection, . new guinea, relation of the several papuan islands to, . nocturnal animals, colours of, . nomada, . oberea, species resemble tenthredinidæ, . _odontocera odyneroides_, . odontocheila, . _odyncrus sinuatus_, . _onthophilus sulcatus_, like a seed, . _onychocerus scorpio_, resembles bark, . orange-tip butterfly, protective colouring of, . orchis, structure of an, explained by natural selection, . _orgyia antiqua_ and _o. gonostigma_, autumnal colours of, . oriolidÆ, . _ornithoptera priamus_, , ; _o. helena_, . _oxyrhopus petolarius_, _o. trigeminus_, _o. formosus_, . owen, professor, on more generalized structure of extinct animals, . _pachyotris fabricii_, . pachyrhynchi, weevils mimicked by longicorns, . paleotherium, . paloplotherium, . papilio, black and red group imitated, . _papilio achates_, ; _p. adamantius_, ; _p. ænigma_, ; _p. agamemnon_, , , , ; _p. agestor_, ; _p. alphenor_, , ; _p. amanga_, ; _p. androcles_, ; _p. androgeus_, , , , ; _p. antiphates_, , ; _p. antiphus_, , , , , ; _p. aristæus_, ; _p. arjuna_, ; _p. ascalaphus_, ; _p. autolycus_, ; _p. bathycles_, ; _p. blumei_, ; _p. brama_, ; _p. caunus_, , ; _p. codrus_, , ; _p. cöon_, , , , ; _p. deiphobus_, ; _p. deiphontes_, ; _p. delessertii_, ; _p. demolion_, ; _p. diphilus_, , , , ; _p. doubledayi_, , ; _p. elyros_, ; _p. encelades_, ; _p. erectheus_, ; _p. euripilus_, ; _p. evemon_, ; _p. gigon_, ; _p. glaucus_, ; _p. hector_, , , , ; _p. helenus_, , ; _p. hospiton_, ; _p. idæoides_, ; _p. jason_, , ; _p. ledebouria_, ; _p. leucothoë_, ; _p. leodamas_, ; _p. liris_, , , ; _p. macareus_, ; _p. machaon_, ; _p. melanides_, , ; _p. memnon_, , , , , , , ; _p. milon_, ; _p. nephelus_, ; _p. nicanor_, ; _p. oenomaus_, , , ; _p. onesimus_, ; _p. ormenus_, , , ; _p. pammon_, , , , ; _p. pamphylus_, ; _p. pandion_, , ; _p. paradoxa_, , ; _p. peranthus_, , ; _p. pertinax_, ; _p. philoxenus_, ; _p. polydorus_, , , ; _p. polytes_, , ; _p. rhesus_, ; _p. romulus_, , , , ; _p. sarpedon_, , , ; _p. sataspes_, ; _p. severus_, , ; _p. theseus_, , , , , , , , ; _p. thule_, ; _p. torquatus_, ; _p. turnus_, ; _p. ulysses_, , , ; _p. varuna_, . papilionidÆ, the question of their rank, ; peculiar characters possessed by, ; peculiarly diurnal, ; compared with groups of mammalia, ; distribution of, ; large forms of celebes and moluccas, ; large forms of amboyna, ; local variation of form, ; arrangement of, ; geographical distribution of, ; of indo-malay and austro-malay regions, ; of java, sumatra, and borneo, . paridÆ, sexual colouring and nidification of, . passenger pigeon, cause of its great numbers, . patent inventions, as illustrating classification, . _phacellocera batesii_, mimics one of the anthribidæ, . _phalaropus fulicarius_, , . phasmidÆ, imitate sticks and twigs, ; females resembling leaves, . phyllium, wonderful protective colour and form of, . physalia, . pieridÆ, local modification of form in, . pieris, females only imitating heliconidæ, . _pieris coronis_, ; _eperia_, . _pieris pyrrha_, . picidÆ, sexual colouring and nidification of, . pipridÆ, sexual colouring and nidification of, . pittidÆ, . _pliocerus equalis_, ; _p. elapoides, p. euryzonus_, . _pæciloderma terminale_, . polarity, forbes' theory of, , . polymorphism, ; illustration of, . population of species, law of, ; does not permanently increase, ; not determined by abundance of offspring, ; checks to, ; difference in the case of cats and rabbits explained, . prevision, a case of, . prioniturus, . protection, various modes in which animals obtain it, - , ; greater need of, in female insects and birds, . protective colouring, theory of, . psittaci (parrots), sexual colouring and nidification of, . pterosauria, . ptychoderes, . races, or subspecies, ; of man, origin of, . redbreast and woodpigeon, protective colouring of, , . representative groups, ; of trogons, butterflies, &c., . reptiles, protective colouring of, . rhamphastidÆ, sexual colouring and nidification of, . rhinoceros, . river system, as illustrating self-adaptation, . roses, mr. baker on varieties of, . rudimentary organs, . salvin, mr. osbert, on a case of bird mimicry, . _saturnia pavonia-minor_, protective colouring of larva of, . satyridÆ, probable means of protection of, . sauropterygia, . savages, why they become extinct, ; undeveloped intellect of, , ; intellect of, compared with that of animals, , ; protect their backs from rain, . scansorial birds, nests of, . scaphura, . scissirostrum, . scopulipedes, brush-legged bees, . scudder, mr., on fossil insects, . scutelleridÆ, mimicked by longicorns, . _sesia bombiliformis_, . sesiidÆ, mimic hymenoptera, . sexes, comparative importance of, in different classes of animals, ; diverse habits of, . sexual selection, ; its normal action to develop colour in both sexes, ; among birds, . sidgwick, mr. a., on protective colouring of moths, . simocyonidÆ, . sitta, sexual colouring and nidification of, . sittella, sexual colouring and nidification of, . snakes, mimicry among, . song of birds, instinctive or imitative, . species, law of population of, ; abundance or rarity of, dependent on the adaptation to conditions, ; definition of, , ; the range and constancy of, ; extreme variation in, , . speed of animals, limits of, . _sphecia craboniforme_, . _sphecomorpha chalybea_, . sphegidÆ, mimicked by flies, . spiders, which mimic ants, ; and flower buds, . _spilosoma menthastri_, . stainton, mr., on moths rejected by turkeys, , . stalachtis, a genus of erycinidæ, the object of mimicry, . stinging insects generally conspicuously coloured, . streptocitta, . sturnidÆ, sexual colouring and nidification of, . sturnopastor, . st. helena, . _streptolabis hispoides_, . struggle for existence, , . survival of the fittest, law of, stated, ; its action in determining colour, . swainson's circular and quinarian theory, . sylviadÆ, sexual colouring and nidification of, . synapta, . _tachornis phoenicobea_, . _tachyris hombronii_, ; _ithome_, ; _lycaste_, ; _lyncida_, ; _nephele_, ; _nero_, ; _zarinda_, . tanagridÆ, sexual colouring and nidification of, . tapir, . telephori, similar colouring of two sexes, . temperate and cold climates favourable to civilization, . thecodontia, . therates, mimicked by heteromera, . _thyca descombesi_, ; _hyparete_, ; _rosenbergii_, ; _zebuda_, . tiger, adaptive colouring of, . times newspaper on natural selection, . tools, importance of, to man, . tree frogs, probable mimicry by, . tricondyla, . trimen, mr., on rank of the papilionidæ, . tristram, rev. h., on colours of desert animals, . _trochilium tipuliforme_, . trogonidÆ, sexual colouring and nidification of, . tropical birds often green, . tropics, most favourable to production of perfect adaptation among animals, ; not favourable to growth of civilization, . tropidorhynchus mimicked by orioles, . truthfulness of some savages, ; not to be explained on utilitarian hypothesis, . turdidÆ, sexual colouring and nidification of, . turnix, , . tyndall, professor, on origin of consciousness, . upupidÆ, sexual colouring and nidification of, . useful and useless variations, . utility, importance of the principle of, , . variability, simple, . variations, useful and useless, ; laws of, , ; as influenced by locality, ; of size, ; universality of, - ; are there limits to, ; of domestic dogs, ; of pigeons, . varieties, instability of, supposed to prove the permanent distinctness of species, ; if superior will extirpate original species, ; its reversion then impossible, ; of domesticated animals may partially revert, , ; inconvenience of using the term, . vertebrata, mimicry among, . voice of man, not explained by natural selection, . volucella, species of mimic bees, , . walsh, mr., on dimorphism, of _papilio turnus_, . weapons and tools, how they affect man's progress, . weevils often resemble small lumps of earth, . weir, mr. jenner, on a moth refused by birds, ; on beetles refused by birds, ; on caterpillars eaten and rejected by birds, . westwood, professor, objections to theory of mimicry, . white colour in domesticated and wild animals, . wild and domesticated animals, essential differences of, - . will really exerts force, ; probably the primary source of force, . wood, mr. t. w., on orange-tip butterfly, . woodcocks and snipes, protective colouring of, . woodpeckers, why scarce in england, . _xanthia_, autumnal colours of these moths, . zebras, . +--------------------------------------------------------------+ | transcriber's notes & errata | | | | the following entries were added to the table of contents. | | | | in chapter iv.--_the malayan papilionidæ, or swallow-tailed | | butterflies, as illustrative of the theory of natural | | selection._: | | | | arrangement and geographical distribution of the malayan | | papilionidæ | | | | range of the groups of malayan papilionidæ | | | | | | in chapter vi.--_the philosophy of birds' nests._: | | | | how young birds may learn to build nests. | | | | | | missing page number supplied for the entry "_phacellocera | | batesii_, mimics one of the anthribidæ," in the index. | | | | the following words were found in both hyphenated and | | unhyphenated forms (incidence in parentheses). | | | | |co-existing ( ) |coexisting ( ) | | | |fly-catcher ( ) |flycatcher ( ) | | | |sea-weed ( ) |seaweed ( ) | | | |bull-dog ( ) |bulldog ( ) | | | | | the following typographical errors have been corrected: | | | | |error |correction | | | | | | | | |sparrrow |sparrow | | | |unwieldly |unwieldy | | | |it |its | | | |perphaps |perhaps | | | |confimation |confirmation | | | |pharoahs |pharaohs | | | |receptable |receptacle | | | |occured |occurred | | | |that that |than that | | +--------------------------------------------------------------+