ai THE ORIGIN OF SPECIES VOL. II 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., LL. D., F. R.S. WITH ADDITIONS AND CORRECTIONS FROM SIXTH AND LAST ENGLISH EDITION IN TWO VOLUMES VOL. II NEW YORK D. APPLETON AND COMPANY 1808 Authorized Edition. CONTENTS OF VOL. II. CHAPTER IX. HYBRIDISM. Distinction between the sterility of first crosses and of hybrids- Sterility various in degree, not universal, affected by close in- terbreeding, 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 uni- versalHybrids and mongrels compared independently of their fertility Summary Page 1 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 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 .... 48 vi CONTENTS. CHAPTER XL 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 ap- pearance 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 . . Page 89 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 . . 129 CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION Continued. Distribution of fresh-water productions On the inhabitants of oceanic islands Absence of Batrachians and of terrestrial Mam- mals 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 171 CONTENTS. 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 De- scent always used in classification Analogical or adaptive char- actersAffinities, 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 super- vening at an early age, and being inherited at a correspond- ing age RUDIMENTARY ORGANS : their origin explained Sum- mary Page 202 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 ex- tendedEffects of its adoption on the study of Natural History Concluding remarks 267 GLOSSARY OF SCIENTIFIC TERMS 307 INDEX . . 323 ORIGIN OF SPECIES. CHAPTER IX. HYBRIDISM. Distinction between the sterility of first crosses and of hybrids- Sterility various in degree, not universal, affected by close in- terbreeding, 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 uni- versalHybrids and mongrels compared independently of their fertility Summary. THE view commonly entertained by naturalists is that species, when intercrossed, have been specially en- dowed 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 dis- tinct had they been capable of freely crossing. The subject is in many ways important for us, more especial- ly 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 1 2 HYBRIDISM. [CHAP. IX. degrees of sterility. It is an incidental result of dif- ferences 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 reproduc- tion 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 per-' feet; in the second case they are either not at all de- veloped, or are imperfectly developed. This distinc- tion 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 en- dowment, 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 off- spring, is, with reference to my theory, of equal im- portance 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 spe- CHAP. IX.] DEGREES OP STERILITY. , 3 cies when crossed and of their hybrid offspring. It is impossible to study the several memoirs and works of those two conscientious and admirable observers, Kol- reuter 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 to- gether, he unhesitatingly ranks them as varieties. Gart- ner, also, makes the rule equally universal; and he dis- putes 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 (ex- cluding all cases such as the Leguminosa?, 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 4 HYBRIDISM. [CHAP. IX. crossed some forms, such as the common red and blue pimpernels (Anagallis arvensis and ccerulea), 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 be- lieved. 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 ex- perienced observers who have ever lived, namely K61- reuter 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 con- stitutional and structural differences. In regard to the sterility of hybrids in successive generations; though Gartner was enabled io rear some hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for CHAP. IX.] DEGREES OP STERILITY. 5 ten generations, yet he asserts positively that their fer- tility 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 constitu- tion 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 di- minished 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, show- ing on the one hand that an occasional cross with a dis- tinct individual or variety increases the vigour and fer- tility 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 fertilisa- tion, pollen is as often taken by chance (as I know from my own experience) from the anthers of another flower, 6 HYBRIDISM. [CHAP. IX. as from the anthers of the flower itself which is to be fer- tilised; 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 ensured 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 genera- tions 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 ex- ample, 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 fecun- dation." 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 CHAP. IX.] DEGREES OF STERILITY. 7 singular fact, namely, that individual plants of certain species of Lobelia, Verbascum and Passiflora, can easily be fertilised by 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 Cory- dalis as shown by Professor Hildebrand, in various or- chids as shown by Mr. Scott and Fritz Miiller, 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 vege- tated freely." Mr. Herbert tried similar experiments during many years, and always with the same result. These cases serve to shoAV on what slight and mysterious causes the lesser or greater fertility of a species some- times 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, Rhodo- dendron, &c., have been crossed, yet many of these hybrids seed freely. For instance, Herbert asserts that 8 HYBRIDISM. [CHAP. IX. a hybrid from Calceolaria integrifolia and plantaginea, species most widely dissimilar in general habit, "re- produces itself as perfectly as if it had been a natural species from the mountains of Chili." 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 nursery- men. Horticulturists raise large beds of the same hy- brid, 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 hy- brid 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 CHAP. IX.] DEGREES OF STERILITY. 9 confinement, few experiments have been fairly tried: for instance, the canary-bird has been crossed with nine dis- tinct 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 in- stance 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 con- stantly 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 increas- ing. 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 cyn- thia and arrindia) were proved in Paris to be fertile inter se for eight generations. It has lately been asserted that two such distinct specieg as the hare and rabbit, when they can be got to breed together, produce off- spring, 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 27 10 HYBRIDISM. [CHAP. IX. 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 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 abo- riginal parent-species at first produced perfectly fertile hybrids, or that the hybrids subsequently reared under domestication became quite fertile. .This latter alter- native, 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 sev- eral 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 ob- servations by Riitimeyer on their important osteological differences, as well as from those by Mr. Blyth on their differences in habits, voice, constitution, &c., these two CHAP. IX.] DEGREES OP STERILITY. H 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 con- sidered 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 ad- mirable 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 king- doms. 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 12 LAWS GOVERNING THE STERILITY [CHAP. IX. 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 spe- cies 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 with- ering of the flower is well known to be a sign of in- cipient 'fertilisation. From this extreme degree of sterility we have self-fertilised hybrids producing a greater and greater number of seeds up to perfect fer- tility. The hybrids raised from two species which are very difficult to cross, and which rarely produce any off- spring, 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- CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 13 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 condi- tions. By flie term systematic affinity is meant, the general resemblance between species in structure and constitu- tion. 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 hy- brids 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 syste- matic 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 dis- tinct species which unite with the utmost facility. In the same family there may be a genus, as Dianthus, in 14 LAWS GOVERNING THE STERILITY [CHAP. IX. which very many species can most readily be crossed; and another ge*nus, as Silene, in which the most perse- vering 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 differ- ences in every part of the flower, even in the pollen, in the fruit, and in the cotyledons, can be crossed. An- nual and perennial plants, deciduous and evergreen trees, plants inhabiting different stations and fitted for ex- tremely 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 independ- ent of their systematic affinity, that is of any differ- ence in their structure or constitution, excepting in CHAP. IX.] OP FIRST CROSSES AND OF HYBRIDS. 15 their reproductive systems. The diversity of the result in reciprocal crosses between the same two species was long ago observed by Kb'lreuter. To give an instance: Mirabilis jalapa can easily be fertilised by the pollen of M. longiflora, and the hybrids thus produced are suffi- ciently fertile; but Kolreuter tried more than two hun- dred times, during eight following years, to fertilise reciprocally M. longifiora 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 recipro- cal crosses is extremely common in a lesser degree. He has observed it even between closely related forms (as Matthiola annua and gilabra) 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 spe- cies having first been used as the father and then as the mother, though -they rarely differ in external char- acters, yet generally differ in fertility in a small, and oc- casionally in a high degree. Several other singular rules could be given from Giirtner: 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 16 LAWS GOVERNING THE STERILITY [CHAP. IX. 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 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 unfa- vourable 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 syste- matic affinity or degree of resemblance to each other. This latter statement is clearly proved by the differ- ence 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 gen- erally soine difference, and occasionally the widest pos- sible 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 CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 17 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 im- portant to keep from blending together? Why should the degree of sterility be innately variable in the in- dividuals of the same species? Why should some spe- cies 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 re- ciprocal 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 lim- ited 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 ad- visable 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 un- important for their welfare in a state of nature, I pre- 18 LAWS GOVERNING THE STERILITY [CHAP. IX. sume that no one will suppose that this capacity is a specially endowed quality, but will admit that it is inci- dental 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 di- versity 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, do 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 ap- ple, which is a member of the same genus. Even dif- ferent varieties of the pear take with different de- grees of facility on the quince; so do different varie- ties 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 Sageret believes this to be the case with different individuals of the same two spe- CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 19 cies 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 goose- berry, for instance, cannot be grafted on the current, whereas the current will take, though with difficulty, on the gooseberry. We have seen that the sterility of hybrids, which have their reproductive organs in an imperfect con- dition, 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, &c., which seed much more freely when fertil- ised 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 dis- tinct 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 differ- ences in their vegetative systems, so I believe that the still more complex laws governing the facility of first 20 CAUSES OP THE STERILITY [CHAP. IX. 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 resem- blance and dissimilarity between organic beings is at- tempted 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 diffi- culty is as important for the endurance and stability of specific forms, as in the case of grafting it is unimpor- tant 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 cer- tain individuals of one variety when crossed with those of another variety. For it would clearly be advantage- ous 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 sepa- rated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection; but it may perhaps be argued, CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 21 that, if a species was rendered sterile with some one com- patriot, 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, whilst 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 ad- vantageous 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 pro- duced 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 22 CAUSES OF THE STERILITY [CHAP. IX. 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 modifica- tions 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 so- cial 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 in- cluding 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 affected, cannot have been gained through selection; and from the laws governing the various grades of sterility being so uniform through- out the animal and vegetable kingdoms, we may infer CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 23 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 na- ture 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 an 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 spe- cies 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 communi- cated 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 500 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 per- 24 CAUSES OP THE STERILITY [CHAP. IX. ished, 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, " with- out any obvious cause, apparently from mere inability to liv.e; " so that from the 500 eggs only twelve chick- ens were reared. With plants, hybridised embryos prob- ably often perish in a like manner; at least it is known that hybrids raised from very distinct species are some- times 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 pro- duced by a cross between distinct species. Until becom- ing acquainted with these facts, I was unwilling to be- lieve 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. Hy- brids, 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 par- takes 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 condi- tions 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 CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 25 more probably lies in some imperfection in the original act of impregnation, causing the embryo to be imper- fectly 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 inde- pendent 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 some- times the female more than the male. In both, the tendency goes to a certain extent with systematic affin- ity, 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 fer- tility; and certain species in a group will produce un- usually fertile hybrids. No one can tell, till he tries, whether any particular animal will breed under confine- ment, 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, 26 CAUSES OF THE STERILITY [CHAP. IX. 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 dis- turbed, though often in so slight a degree as to be in- appreciable 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 off- spring from generation to generation the same com- pounded organisation, and hence we need not be sur- prised that their sterility, though in some degree varia- ble, 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 com- pounded into one has been strongly maintained by Max Wichura. CHAP. IX.] OF FIRST CROSSES AND OF HYBRIDS. 27 It must, however, be owned that we cannot under- stand, 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 recipro- cal crosses; or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent. Nor do I pretend that the fore- going remarks go to the root of the matter; no explana- tion is offered why an organism, when placed under un- natural 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 dis- turbed 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 evi- dence, 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, &c., from one soil or climate to another, and back again. During the con- valescence of animals, great benefit is derived from al- most 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 fer- tility to the offspring; and that close interbreeding continued during several generations between the near- est relations, if these be kept under the same conditions of life, almost always leads to decreased size, weakness, or sterility. 23 STERILITY OP HYBRIDS. [CHAP. IX. Hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic be- ings, and on the other hand, that slight crosses, that is crosses between the males and females of the same spe- cies, 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, or- ganic beings long habituated to certain uniform condi- tions under a state of nature, when subjected, as under confinement, to a considerable change in their condi- tions, 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 gen- erally sterile. He will at the same time be able to ex- plain 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, al- though 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 con- nected 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 CHAP. IX.] DIMORPHISM AND TRIMORPHISM. 29 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 dif- ferently 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 pro- portioned in length to each other, that half the sta- mens 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 cor- responding height in another form. So that with di- morphic species two unions, which may be called legiti- mate, are fully fertile; and two, which may be called illegitimate, are more or less infertile. With trimor- phic species six unions are legitimate, or fully fer- tile, and twelve are illegitimate, or more or less infer- tile. 30 RECIPROCAL DIMORPHISM [CHAP. IX. The infertility which may be observed in various dimorphic and trimorphic plants, when they are il- legitimately fertilised, that is by pollen taken from stamens not corresponding in height with the pistil, differs much in degree, up to absolute and utter steril- ity; just in the same manner as occurs in crossing dis- tinct 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 anni- hilates the effect of theforeign 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 il- legitimately, and twenty-four hours afterwards legiti- mately with the 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 il- legitimate pollen. Again, as in making reciprocal crosses between the same two species, there is occasion- ally 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 fer- tilised 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 CHAP. IX.] AND TRIMORPHISM. 31 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 re- sult 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 the sterility of certain illegitimate plants was unusually great, whilst 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 32 RECIPROCAL DIMORPHISM [CHAP. IX. variable, so it is in a marked manner with illegitimate plants. Lastly, many hybrids are profuse and persistent flowerers, whilst other and more sterile hybrids pro- duce few flowers, and are weak, miserable dwarfs; exactly similar cases occur with the illegitimate off- spring 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, whilst 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 re- spects 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 seeds, 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 main- tain 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. CHAP. IX.] AND TRIMORPHISM. 33 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 in- fertility of illegitimate unions with that of their illegiti- mate 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; whilst 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, doubt- ful whether this is really so; but I will not enlarge on this obscure subject. We may, however, infer as probable from the con- sideration 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 34 FERTILITY OF VARIETIES [C HAP. IX. a second species, whilst the converse cross can be effected with perfect facility. That excellent observer, Gart- ner, likewise concluded that species when crossed are sterile owing to differences confined to their reproduc- tive 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 sub- ject is surrounded by difficulties, for, looking to varie- ties produced under nature, if two forms hitherto re- puted to be varieties be found in any degree sterile to- gether, 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 subsequently 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 in- volved in some doubt. For when it is stated, for in- stance, that certain South American indigenous domes- tic dogs do not readily unite with European dogs, the explanation which will occur to every one, and probably CHAP. IX.] WHEN CROSSED. 35 the true one, is that they are descended from aborigi- nally distinct species. Nevertheless the perfect fertil- ity 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 es- pecially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed. Several considera- tions, 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 steril- ity, 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 modify- ing the reproductive system in a manner leading to mutual sterility, that we have good grounds for admit- ting 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 fer- tile 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 op- posite manner, for they have become self-impotent whilst still retaining the capacity of fertilising, and being fertilised by, other species. If the Pallasian doc- trine of the elimination of sterility through long-con- 36 FERTILITY OF VARIETIES [CHAP. IX. tinued 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 ex- istence with numerous competitors, will have been ex- posed 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 man- ner be eminently sensitive to the influence of an un- natural cross. Domesticated productions, on the other hand, which, as shown by the mere fact of their domesti- cation, were not originally highly sensitive to changes CHAP. IX.] WHEN CROSSED. 37 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 sepa- rated sexes. No one, I believe, has suspected that these varieties of maize are distinct species; and it is impor- tant to notice that the bjbrid 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 38 FERTILITY OF VARIETIES [CHAP. IX. 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 Sageret, 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 varie- ties 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 Gart- ner'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 86 to 100, 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 CHAP. IX.] WHEN CROSSED. 39 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 attend- ing 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 fer- tility does not constitute a fundamental distinction be- tween varieties and species when crossed. The gen- eral sterility of crossed species may safely be looked at, not as a special acquirement or endowment, but as in- cidental on changes of an unknown nature in their sex- ual elements. Hybrids and Mongrels compared, independently of their fertility. Independently of the question of fertility, the off- spring 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 spe- cies and varieties, could find very few, and, as it seems 40 HYBRIDS AND MONGRELS COMPARED. [CHAP. IX to ine, 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 variabil- ity 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 charac- ter 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 CHAP. IX.] HYBRIDS AND MONGRELS COMPARED. 41 ordinary variability; namely, that the reproductive system from being eminently sensitive to changed con- ditions of life, fails under these circumstances to per- form its proper function of producing offspring closely similar in all respects to the parent-form. Now hy- brids in the first generation are descended from spe- cies (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 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; whilst 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 42 HYBRIDS AND MONGRELS COMPARED. [CHAP. IX. 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 mon- grel plants. On the other hand, the degrees and kinds of resemblance in mongrels and in hybrids to their respective parents, more especially in hybrids pro- duced 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 cer- tainly 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 char- acters; 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 pre- potency runs more strongly in the male than in the female ass, so that the mule, which is the offspring of the male ass and mare, is more like an ass, than is CHAP. IX.] HYBRIDS AND MONGRELS COMPARED. 43 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 sud- den reversions to the perfect character of either parent would, also, be much more likely to occur with mon- grels, which are descended from varieties often sud- denly 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 steril- ity, 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 44 SUMMARY. [CHAP. IX. 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 gen- erally, 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 spe- cies, and is eminently susceptible to the action of fa- vourable 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 re- ciprocal 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 capac- ity in one species or variety to take on another, is inci- dental 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 in- cidental 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 na- ture, than to think that trees have been specially en- dowed with various and somewhat analogous degrees of CHAP. IX.] SUMMARY. 45 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 se- lection. 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 organi- sation 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 fer- tility of their offspring. The facts given on the steril- ity of the illegitimate unions of dimorphic and trimor- phic 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 46 SUMMARY. [CHAP. IX. 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 sur- prising 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 circum- stances should all run, to a certain extent, parallel with the systematic affinity of the forms subjected to experiment; for systematic affinity includes resem- blances 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 there- fore little likely to induce this same quality. Inde- pendently of the question of fertility, in all other re- spects there is the closest general resemblance between hybrids and mongrels, in their variability, in their CHAP. IX.] SUMMARY. 47 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 varie- ties. 48 IMPERFECTION OF TEE . [CHAP. X. 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 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 objec- tions which might be justly urged against the views maintained in this volume. Most of them have now been discussed. One, namely the distinctness of spe- cific forms, and their not being blended together by in- numerable transitional links, is a very obvious difficulty. I assigned reasons why such links do not commonly oc- cur at the present day under the circumstances ap- parently most favourable for their presence, namely, on an extensive and continuous area with graduated phys- ical 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 govern- ing conditions of life do not graduate away quite insen- sibly like heat or moisture. I endeavoured, also, to CHAP. X.] GEOLOGICAL RECORD. 49 show that intermediate varieties, from existing in lesser numbers than the forms which they connect, will gen- erally 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 sup- plant 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 stra- tum full of such intermediate links? Geology assured- ly does not reveal any such finely-graduated organic chain; and this, perhaps, is the most obvious and seri- ous objection which can be urged against the theory. The explanation lies, as I believe, in the extreme imper- fection 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 my- self 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 gen- erally have differed in some respects from all its modi- fied 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- 50 IMPERFECTION OF THE [CHAP. X. pigeon; but we should have no varieties directly in- termediate 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 form, such as C. cenas. 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 re- semblance 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 spe- cies, 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 in- termediate 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 re- mained for a very long period unaltered, whilst its de- scendants had undergone a vast amount of change; CHAP.X.] GEOLOGICAL RECORD. 51 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 gen- erally extinct, have in their turn been similarly con- nected 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 spe- cies, must have been inconceivably great. But as- suredly, 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 his- torian will recognise as having produced a revolution in natural science, and yet does not admit how vast have 5$ THE LAPSE OP TIME. [CHAP, X. 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 ob- servers on separate formations, and to mark how each author attempts to give an inadequate idea of the dura- tion 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 super- imposed 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 degra- dation. 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 noth- ing 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, CHAP. X.] THE LAPSE OP TIME. 53 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 deg- radation, 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 learnt from the obser- vations of Ramsay, in the van of many excellent ob- servers of Jukes, Geikie, Croll, and others, that sub- aerial 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 rain-water with its dissolved car- bonic acid, and in colder countries to frost; the disin- tegrated 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 undulat- ing 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 escarp- ment 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, whilst 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 54. THE LAPSE OF TIME. [CHAP. X* origin in chief part to the rocks of which they are com- posed 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 sub- aerial 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 dura- tion 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 sedi- mentary formations. I remember having been much struck when viewing volcanic islands, which have been worn by the waves and pared all round into perpen- dicular 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 up- heaved 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 sur- face 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 CUAP. X.] THE LAPSE OF TIME. 55 upwards of 30 miles, and along this line the vertical displacement of the strata varies from 600 to 3000 feet. Professor Eamsay has published an account of a down- throw in Anglesea of 2300 feet; and he informs me that he fully believes that there is one in Merioneth- shire of 12,000 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) 57,154 Secondary strata 13,190 Tertiary strata 2,240 making altogether 72,584 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 Con- tinent. 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 56 THE LAPSE OF TIME. [CHAP. X idea of the time which has elapsed during their accumu- lation. The consideration of these various facts im- presses 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 phe- nomena, 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 1000 feet of solid rock, as it became gradually disintegrated, would thus be re- moved 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 even if halved or quartered it is still very surprising. Few of us, however, know what a million really means: Mr. Croll gives the fol- lowing illustration: take a narrow strip of paper, 83 feet 4 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 CHAP. X.] THE LAPSE OF TIME. 5? 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 repre- sents 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 guid- ance 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 uncon- scious 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 circumstances, 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. 58 THE POORNESS OF OUR [CHAP. X. On the Poorness of Palceontological Collections. Now let us turn to our richest geological museums, and what a paltry display we behold! That our col- lections are imperfect is admitted by every one. The remark of that admirable paleontologist, 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 dis- coveries made every year in Europe prove. No organ- ism wholly soft can be preserved. Shells and bones de- cay 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 re- mains. 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 con- formably 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 oy the percolation of rain-water charged with carbolic 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 CHAP. X.] PAL^EONTOLOGICAL COLLECTIONS. 59 several species of the Chthamalinas (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 in- habits 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 300 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 Xorth America; but now land-shells have been found in the lias. In regard to mammiferous remains, a glance at the historical table published in LyelPs 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 60 THE POORNESS OF OUR [CHAP. X. deposits; and that not a cave or true lacustrine bed is known belonging to the age of our secondary or palaeo- zoic 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 forma- tions 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. Murchi son's great work on Russia, what wide gaps there are in that coun- try 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 each separate territory, hardly any idea can be formed of the length of time which has elapsed between the consecu- tive formations, we may infer that this could nowhere be ascertained. The frequent and great changes in the mineralogical composition of consecutive forma- tions, 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 CHAP. X.] PAL^ONTOLOGICAL COLLECTIONS. 61 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 de- posits 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 ter- tiary 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, 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 successive 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 pro- found 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 up- raised will give an imperfect record of the organisms 62 THE POORNESS OF OUR [CHAP. X. 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 the 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 up- raised, to resist a large amount of denudation, may be formed. I am convinced that nearly all our ancient forma- tions, 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 1845, 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 subsi- dence. I may add, that the only ancient tertiary forma- tion 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 dis- tant geological age, was deposited during a downward oscillation of level, and thus gained considerable thick- 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 CHAP. X.] PAL^IONTOLOGICAL COLLECTIONS. 63 and preserve the remains before they had time to de- cay. On the other hand, as long as the bed of the sea remains stationary, thick deposits cannot have been ac- cumulated 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 accumu- lated will generally have been destroyed by being upraised and brought within the limits of the coast- action. These remarks apply chiefly to littoral and sub-lit- toral deposits. In the case of an extensive and shallow sea, such as that within a large part of the Malay Archi- pelago, where the depth varies from 30 or 40 to 60 fathoms, a widely extended formation might be formed during a period of elevation, and yet not suffer exces- sively 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 de- posit 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 ac- tion 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 ris- ing movement, though not thick, might afterwards be- come protected by fresh accumulations, and thus be preserved for a long period. Mr. Hopkins also expresses his belief that sedimen- tary beds of considerable horizontal extent have rarely been completely destroyed. But all geologists, except- 6-t THE POORNESS OF OUR [CHAP. X. ing 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 stript of their covering to an enormous ex- tent. For it is scarcely possible that such rocks could have been solidified and crystallized whilst 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, &c., 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 testi- mony of travellers, the granitic area is very large: thus, Von Eschwege gives a detailed section of these rocks, stretching from Eio de Janeiro for 260 geographical miles inland in a straight line; and I travelled for 150 miles in another direction, and saw nothing but granitic rocks. Numerous specimens, collected along the whole coast from near Rio Janeiro to the mouth of the Plata, a distance of 1100 geographical miles, were ex- amined by me, and they all belonged to this class. In- land, along the whole northern bank of the Plata I saw, besides modern tertiary beds, only one small patch CHAP. X.] PAL^ONTOLOGICAL COLLECTIONS. 65> 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's beautiful map, I have estimated the areas by cutting out and weighing the paper, and I find that the metamorphic (excluding " the semi-meta- " morphic ") and granitic rocks exceed, in the propor- tion of 19 to 12.5, the whole of the newer Paleozoic formations. In many regions the metamorphic and granitic rocks would be found much more widely ex- tended 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 crystallized. Hence it is probable that in some parts of the world whole forma- tions 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 ad- joining shoal parts of the sea will be increased, and new stations will often be formed: all circumstances favour- able, 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 varie- ties 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. 66 ABSENCE OF INTERMEDIATE VARIETIES [CnAP.X. 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 gradu- ated 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 Am- monites; 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 Swit- zerland. Although each formation has indisputably required a vast number of years for its deposition, sev- eral 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 "Wood- ward, have concluded that the average duration of each formation is twice or thrice as long as the average dura- tion 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 CHAP.X.] IN ANY SINGLE FORMATION. 67 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 sev- eral 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 probabil- ity is that it only then first immigrated into that area. It is well-known, for instance, that several species ap- pear somewhat earlier in the paleozoic beds of North America than in those of Europe; time having appa- rently 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, con- versely, that some are now abundant in the neighbour- ing 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 dur- ing 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, in- 68 ABSENCE OF INTERMEDIATE VARIETIES [CHAP. X. 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 de- posited 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 prob- ably first appear and disappear at different levels, ow- ing to the migrations of species and to geographical changes. And in the distant future, a geologist, ex- amining 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 hav- ing been really far greater, that is, extending from be- fore 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 forma- tion, the deposit must have gone on continuously ac- cumulating 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 ac- cumulate only during a period of subsidence: and to keep the depth approximately the same, which is neces- sary that the same marine species may live on the same space, the supply of sediment must nearly counterbal- ance the amount of subsidence. But this same move- CHAP. X.] IN ANY SINGLE FORMATION. 69 ment 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 paleontologist, 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 else- where thousands of feet in thickness, and which must have required an enormous period for their accumula- tion; 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, de- nuded, 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 ac- cumulation. In other cases we have the plainest evi- dence 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 70 ABSENCE OF INTERMEDIATE VARIETIES [CHAP. X. not have been suspected, had not the trees been pre- served: thus Sir C. Lyell and Dr. Dawson found carbon- iferous beds 1400 feet thick in Nova Scotia, with an- cient 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 interme- diate 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 CHAP.X.] IN ANY SINGLE FORMATION. 71 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 un- less 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 natu- ralists 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 natu- ralists have been misled by their imaginations, and that these late tertiary species really present no dif- ference whatever from their living representatives, or unless we admit, in opposition to the judgment of most naturalists, that these tertiary species are all truly dis- tinct 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 72 ABSENCE OF INTERMEDIATE VARIETIES [CHAP. X. 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-forms until they have been modified and perfected in some. considerable de- gree. According to this view, the chance of discov- ering 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 ani- mals 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 spe- cies; 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, name- ly, that the period during which each species under- went 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 CHAP.X.] IN ANY SINGLE FORMATION. ?3 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 per- ceive the improbability of our being enabled to con- nect 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 abori- ginal stocks; or, again, whether certain sea-shells in- habiting the shores of North America, which are ranked by some conchologists as distinct species from their European representatives, and by other con- chologists 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 discover- ing in a fossil state numerous intermediate grada- tions; and such success is improbable in the highest de- gree. 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 de- stroy four-fifths of them, no one doubts that the re- mainder 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 31 74 ABSENCE OF INTERMEDIATE VARIETIES [CHAP. X. numerous gradations, as fine as existing varieties, con- necting 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 geo- logical 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 forma- tions 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, prob- ably represents the former state of Europe, whilst 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 ter- restrial productions of the archipelago would be pre- served in an extremely imperfect manner in the forma- tions 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 en- dure 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. CHAP. X.] IN ANY SINGLE FORMATION. 75 Formations rich in fossils of many kinds, and of thickness sufficient to last to an age as distant in futu- rity as the secondary formations He 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 in- tervals of time, during which the area would be either stationary or rising; whilst rising, the fossiliferous for- mations on the steeper shores would be destroyed, al- most 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 with- in the archipelago, sedimentary beds could hardly be accumulated of great thickness during the periods of elevation, or become capped and protected by subse- quent deposits, so as to have a good chance of enduring to a very distant future. During the periods of sub- sidence, 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 accumu- lation of sediment, would exceed the average duration of the same specific forms; and these contingencies are in- dispensable 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 os- cillations of level, and that slight climatal changes 76 ABSENCE OF INTERMEDIATE VARIETIES [CHAP. X. 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 archi- pelago now range thousands of miles beyond its con- fines; 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 varie- ties; and the varieties would at first be local or con- fined to one place, but if possessed of any decided ad- vantage, 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, accord- ing to the principles followed by many palaeontologists, be ranked as new and distinct species. If then there be some degree of truth in these re- marks, we have no right to expect to find, in our geo- logical 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 paleontologists, 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 CHAP.X.] IN ANY SINGLE FORMATION. 77 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 spe- cies suddenly appear in certain formations, has been urged by several palaeontologists for instance, by Agas- siz, Pictet, and Sedgwick as a fatal objection to the be- lief in the transmutation of species. If numerous spe- cies, 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 progeni- tors must have lived long before their modified descen- dants. But we continually overrate the perfection of the geological record, and falsely infer, because certain genera or families have not been found beneath a cer- tain stage, that they did not exist before that stage. In all cases positive palasontological 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 else- where 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 78 SUDDEN APPEARANCE OF [CHAP. X. for the 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 con- fined to some one region; but that, when this adapta- tion 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 an- terior 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 sup- pose 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 CHAP. X]. GROUPS OF ALLIED SPECIES. 79 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 Pietet's great work on Palaeontology, published in 1844-46 and in 1853-57, the conclusions on the first appearance and disappearance of several groups of animals have been considerably modified; and a third edition would re- quire 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 ac- cumulations of fossil mammals belongs to the middle of the secondary series; and true mammals have been dis- covered in the new red sandstone at nearly the com- mencement 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. HadMt not been for the rare accident of the pres- ervation of the 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 80 SUDDEN APPEARANCE OP [CHAP. X. 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 spe- cies; from the extraordinary abundance of the indi- viduals of many species t all over the world, from the Arctic regions to the equator, inhabiting various zones of depths from the upper tidal limits to 50 fathoms; from the perfect manner in which specimens are pre- served 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 cirri- pedes 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 ter- tiary series. This was a sore trouble to me, adding as I then thought one more instance of the abrupt ap- pearance 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 him- self extracted from the chalk of Belgium. And, as if to make the case as striking as possible, this cirripede CHAP. X.] GROUPS OF ALLIED SPECIES. 81 was a Chthamalus, a very common, large, and ubiqui- tous genus, of which not one species has as yet been found even in any tertiary stratum. Still more re- cently, a Pyrgoma, a member of a distinct sub-family of sessile cirripedes, has been discovered by Mr. Wood- ward in the upper chalk; so that we now have abun- dant evidence of the existence of this group of animals during the secondary period. The case most frequently insisted on by palaeonto- logists 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 com- monly admitted to be teleostean; and even some palaeo- zoic 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 simultane- ously developed in other qu'arters of the world. It is almost superfluous to remark that hardly any fossil- fish are known from south of the equator; and by run- ning 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 con- fined 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 82 ' GROUPS OF ALLIED SPECIES [CHAP. X. 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 enable to double the Southern capes of Af- rica or Australia, and thus reach other and distant 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 palasontological knowledge effected by the dis- coveries 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 natura- list to land for five minutes on a barren point in Aus- tralia, 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 fossili- ferous rocks. Most of the arguments which have con- vinced 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 in- stance, it cannot be doubted that all the Cambrian and CHAP. X.] IN LOWEST FOSSILIFEROUS STRATA. 33 Silurian trilobites are descended from some one crusta- cean, 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, &c., 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 sub- sequently appeared, for they are not in any degree in- termediate 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 en- counter 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 20 or more than 400 million years ago, but probably not less than 98 or more than 200 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 60 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 for- mation; and the previous 140 million years can hardly be considered as sufficient for the development of the 84: GROUPS OF ALLIED SPECIES [CHAP. X. varied forms of life which already existed during the Cambrian period. It is, however, probable, as Sir Wil- liam 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 K. Mur- chison 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 dis- puted 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 in 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 azoic rocks, probably indi- cates life at these periods; and the existence of the Eozoon in the Laurentian formation of Canada is gener- ally admitted. There are three great series of strata be- neath 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 palao- CHAP. X.] IN LOWEST FOSSILIFEROUS STRATA. 5 " zoic 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 1859, 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 diffi- culty 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 ob- literated 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 hy- pothesis. From the nature of the organic remains which do not appear to have inhabited profound depths, 86 GROUPS OF ALLIED SPECIES [CHAP. X. 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 be- tween 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 sec- ondary periods, neither continents nor continental is- lands 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 these would have been at least partially upheaved by the oscillations of level, which must have intervened during these enor- mously 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 CHAP. X.] IN LOWEST FOSSILIFEROUS STRATA. 87 land have existed, subjected no doubt to great oscilla- tions of level, since the Cambrian period. The col- oured map appended to my volume on Coral Eeefs, 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 eleva- tion. 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 preponder- ance, during many oscillations of level, of the force of elevation; but may not the areas of preponderant move- ment 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 a 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 nearerto 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 al- ways 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 per- haps believe that we see in these large areas, the many formations long anterior to the Cambrian epoch in a completely metamorphosed and denuded condition. S3 IMPERFECTION OF GEOLOGICAL RECORD. [CHAP. X. 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 transi- tional 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 fos- sils beneath the Cambrian strata, are all undoubtedly of the most serious nature. We see this in the fact that the most eminent paleontologists, namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E. Forbes, &c., and all our greatest geologists, as Lyell, Murchison, Sedgwick, &c., have unanimously, often vehemently, maintained the immutability of species. But Sir Charles Lyell now gives the support of his high authority to the op- posite side; and most geologists and paleontologists are much shaken in their former belief. Those who believe that the geological record is in any degree perfect, will undoubtedly at once reject the 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 pos- sess 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 chap- ters, may represent the forms of life, which are en- tombed in our consecutive formations, and which falsely appear to have been abruptly introduced. On this view, the difficulties above discussed are greatly dimin- ished, or even disappear. CHAP. XL] SUCCESSION OF ORGANIC BEINGS. 89 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 ap- pearance 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. 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 modi- fication, 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 exist- ing 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; 90 THE GEOLOGICAL SUCCESSION [CHAP. XI. 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 or- ganic 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 spe- cies 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, CHAP. XI.] OP ORGANIC BEINGS. 91 which includes no fixed law of development, causing all the inhabitants of an area to change, abruptly, or simul- taneously, or to an equal degree. The process of modi- fication must be slow, and will generally effect 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 sur- prising that one species should retain the same identi- cal form much longer than others; or, if changing, should change in a less degree. We find similar rela- tions between the existing inhabitants of distinct coun- tries; for instance, the land-shells and coleopterous in- sects of Madeira have come to differ considerably from their nearesf 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 produc- tions, 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 im- proved, we can understand, on the principle of compe- tition, and from the all-important relations of organ- 92 THE GEOLOGICAL SUCCESSION [CHAP. XL ism to organism in the struggle for life, that any form which did not become in some degree modified and im- proved, 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 accumu- lated at wide and irregularly intermittent intervals of time; consequently the amount of organic change ex- hibited by the fossils embedded in consecutive forma- tions 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 con- ditions 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 econ- omy 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 organ- isms 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 CHAP. XI.] OF ORGANIC BEINGS. 93 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 dif- ferent, and the newly-formed variety would probably in- herit from its progenitor some characteristic differences. Groups of species, that is, genera and families, fol- low the same general rules in their appearance and dis- appearance 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 abrupt- ly developed; and I have attempted to give an explana- tion of this fact, which if true would be fatal to my views. But such cases are certainly exceptional; the 91 EXTINCTION. [CHAP. XI. 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 vary- ing thickness, ascending through the successive geologi- cal 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 pro- cess, one species first giving rise to two or three varie- ties, 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 dis- appearance 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, Mur- CHAP. XL] EXTINCTION. 95 chison, Barrande, &c., whose general views would natu- rally 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 isth- mus and the consequent irruption of a multitude of new inhabitants into an adjoining sea, or by the final subsi- dence 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 paleozoic 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 pro- duction: if their appearance and disappearance be rep- resented, as before, by a vertical line of varying thick- ness the line is found to taper more gradually at its up- per end, which marks the progress of extermination, than at its lower end, which marks the first appearance and the e*arly 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 sup- posed that, as the individual has a definite length of life, so have species a definite duration. N"o one can have marvelled more than I have done at the extinction 96 EXTINCTION. [CHAP. XL of species. When I found in La Plata the tooth of a horse embedded with the remains of Mastodon, Mega- therium, Toxodon, and other extinct monsters, which all co-existed with still living shells at a very late geo- logical period, I was filled with astonishment; for, see- ing that the horse, since its introduction by the Span- iards into South America, has run wild over the whole country and has increased in numbers at an unparal- leled 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, be- longed 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 cer- tain, 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 condi- tions 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 CHAP. XL] EXTINCTION. 97 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 in- crease of every 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 under- stood, 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 S. America. We see in many cases in the more recent tertiary for- mations, 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 1845, 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 mar- vel greatly when the species ceases to exist, is much the 98 EXTINCTION. [CHAP. XI. 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 competi- tion; and the consequent extinction of the less-fa- voured 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 those artificially produced, are bound to- gether. In flourishing groups, the number of new spe- cific 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 ex- terminated; but we know that species have not gone on indefinitely increasing, at least during the "later geo- logical 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 spe- cies will generally cause the extermination of the parent- species; and if many new forms have been developed CHAP. XL] EXTINCTION. 99 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 be- longing to the same or to a distinct class, which have yielded their places to other modified and improved spe- cies, 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 extermina- tion 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 unusu- 100 FORMS OF LIFE CHANGING [CHAP. XI. ally 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 correspond- ingly rapid manner; and the forms which thus yield their places will commonly be allied, for they will par- take 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 own presumption in imagining for a mo- ment that we understand the many complex contin- gencies 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 econ- omy 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 an- other can be naturalised in a given country; then, and not until then, we may justly feel surprise why we can- not account for the extinction of any particular spe- cies or group of species. On the Forms of Life changing almost simultaneously throughout the World. Scarcely any palaontological 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, CHAP. XL] THROUGHOUT THE WORLD. 101 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 unmis- takeable 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 be- long 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 be- low, occur in the same order at these distant points of the world. In the several successive palaeozoic for- mations of Russia, Western Europe, and North America, a similar parallelism in the forms of life has been ob- served by several authors; so it is, according to Lyell, with the European and North American tertiary de- posits. Even if the few fossil species which are com- mon 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 102 FORMS OF LIFE CHANGING [CHAP. XI. 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 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 gla- cial epoch) were compared with those now existing in South America or in Australia, the most skilful natu- ralist 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 inhabi- tants of Europe; and if this be so, it is evident that fos- siliferous 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 Aus- tralia, 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 simultaneous- CHAP. XL] THROUGHOUT THE WORLD. 103 ly, in the above large sense, at distant parts of the world, has greatly struck these admirable observers, MM. de Verneuil and d'Archiac. After referring to the parallel- ism of the paleozoic 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 con- ditions, as the cause of these great mutations in the forms of life throughout the world, under the most dif- ferent 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 104 FORMS OF LIFE CHANGING [CHAP. XI. 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 acci- dents, and on the gradual acclimatisation 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 ulti- mately prevail. The diffusion would, it is probable, be slower with the terrestrial inhabitants of distinct conti- nents than with the marine inhabitants of the continu- ous sea. We might therefore expect to find, as we do find, a less strict degree of parallelism in the succesion 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 domi- nant species spreading widely and varying; the new species thus produced being themselves dominant, ow- ing to their having had some advantage over their al- ready 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 CHAP. XL] THROUGHOUT THE WORLD. 105 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 sedi- ment 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 be- lieve that large areas are affected by the same move- ment, it is probable that strictly contemporaneous for- mations 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 dur- ing nearly, but not exactly, the same period, we should find in both, from the causes explained in the fore- going paragraphs, the same general succession in the forms of life; but the species would not exactly cor- respond; 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. Preshvich, in his admirable Memoirs on the eocene deposits of England and France, is able to draw a close 106 AFFINITIES OF EXTINCT SPECIES. [CHAP. XL 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 them- selves 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; never- theless 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 accu- mulation 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 be all 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 CHAP. XL] AFFINITIES OF EXTINCT SPECIES. 107 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 in- tervals 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 sys- tem. In the writings of Professor Owen we continu- ally 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 paleontolo- gist, 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 exist- ing genera. Cuvier ranked the Ruminants and Pachy- derms 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 Mac- rauchenia of S. America connects to a certain extent these two grand divisions. No one will deny that the Hipparion is intermediate between the existing horse 108 AFFINITIES OF EXTINCT SPECIES. [CHAP. XL and certain older ungulate forms. What a wonderful connecting link in the chain of mammals is the Typo- therium from S. 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 dis- tinct group of mammals, and one of the most remark- able peculiarities in the existing dugong and lamentin is the entire absence of hind limbs without even a rudi- ment being left; but the extinct Halitherium had, ac- cording 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 natu- ralists in an order by themselves, are considered by Pro- fessor 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 Archeo- pteryx, 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, and a higher author- ity 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, CHAP. XL] AFFINITIES OF EXTINCT SPECIES. 109 or group of species, being considered as intermediate between any two living species, or groups of species. If by this term ifc is meant that an extinct form is di- rectly intermediate in all its characters between two living forms or groups, the objection is probably valid. But in a natural classification many fossil species cer- tainly stand between living species, and some extinct genera between living genera, even between genera be- longing 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 charac- ters, 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 dis- covered having affinities directed towards very distinct groups. Yet if we compare the older Eeptiles 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 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 HO AFFINITIES OF EXTINCT SPECIES. [CHAP. XL 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 be- ing given, but this is unimportant for us. The hori- zontal lines may represent successive geological forma- tions, and all the forms beneath the uppermost line may be considered as extinct. The three existing genera a 14 , ? 14 > P 14 > will form a small family; 6 14 and / 14 a closely allied family or sub-family; and o 14 , CHAP. XIV.] DEVELOPMENT AND EMBRYOLOGY. 247 tain 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 first appeared either earlier or later in life, likewise tend to reappear at a corre- sponding 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 su- pervened 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, ex- plain, as I believe, all the above specified leading facts in embryology. But first let us look to a few analo- gous cases in our domestic varieties. Some authors who have written on Dogs, maintain that the greyhound and bulldog, though so different, are really closely al- lied 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 meas- uring 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 248 DEVELOPMENT AND EMBRYOLOGY. [CHAP. XIV. 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 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 extraordi- nary 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 dis- tinguished, 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 remark- able 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 state. These facts are explained by the above two principles. Fanciers select their dogs, horses, pigeons, &c., for breed- ing, when nearly grown up: they are indifferent whether the desired qualities are acqiiired 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 CHAP. XIV.] DEVELOPMENT AND EMBRYOLOGY. 249 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 in- icrited 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 ex- tend 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 ma- ture, when it was compelled to use its full powers to gain its own living; and the effects thus produced will 42 250 DEVELOPMENT AND EMBRYOLOGY. [CHAP. XIV. have been transmitted to the offspring at a correspond- ing 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 ma- ture 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 under- go any metamorphosis, whilst marine members of the same groups pass through various transformations, Fritz Miiller has suggested that the process of slowly modify- ing and adapting an animal to live on the land or in fresh water, instead of in the sea, would be greatly sim- plified 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 CHAP. XIV.] DEVELOPMENT AND EMBRYOLOGY. 251 changed habits of life, would commonly be found unoc- cupied 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 selec- tion; 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 con- structed 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 corre- sponding ages, the young or the Iarva3 might be ren- dered 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, &c., 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 cor- responding ages, animals might come to pass through stages of development, perfectly distinct from the pri- mordial 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 '252 DEVELOPMENT AND EMBRYOLOGY. [CHAP. XIV. 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 anten- nas, and four eyes. These larva? 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 larva? spring on them, and afterwards crawl on to the females whilst 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 ordi- nary 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, entomo- straca, 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 CHAP. XIV.] DEVELOPMENT AND EMBRYOLOGY. 253 peculiar habits of life, and from other reasons assigned by Fritz Mtiller, it is probable that at some very remote period an independent adult animal, resembling the Nauplius, existed, and subsequently produced, along sev- eral 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 descen- dants 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; de- ecent being the hidden bond of connexion which natural- ists 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 em- bryo is even more important for classification than that of the adult. In two or more groups of animals, how- ever 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 all are descended from one parent-form, and are therefore closely related. Thus, community in em- bryonic 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 254 DEVELOPMENT AND EMBRYOLOGY. [CHAP. XIV. 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 re- vealed 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 larva? to belong to the great class of crustaceans. As the embryo often shows us more or less plainly the structure of the less modi- fied 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 in- herited 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 larva? will not resemble any still more ancient form in its adult state. Thus, as it seems to me, the leading facts in embry- ology, which are second to none in importance, are ex- plained on the principle of variations in the many de- CHAP. XIV.] RUDIMENTARY ORGANS. 255 scendants from some one ancient progenitor, having ap- peared 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 mam- malia, for instance, the males possess rudimentary mam- mae; 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 fcetal 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? Eudimentary 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 repre- sent wings. Eudimentary organs sometimes retain their potentiality: this occasionally occurs with the mam- 256 RUDIMENTARY, ATROPHIED. [CHAP. XIV. mas of male mammals, which have been known to be- come well developed and to secrete milk. So again in the udders in 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 some- times rudimentary, and sometimes well-developed in the individuals of the same species. In certain plants hav- ing 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 hy- brid offspring was much increased in size; and this clear- ly shows that the rudimentary and perfect pistils are es- sentially 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 f ull- " 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 progeni- " tors." An organ, serving for two purposes, may become rudimentary or utterly aborted for one, even the more CHAP. XIV.] AND ABORTED ORGANS. 257 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 a style; but in some Composite, 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 man- ner with hairs, which serve to brush the pollen out of the surrounding and conjoined anthers. Again, an or- gan 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 for- merly more highly developed, ought not to be consid- ered as rudimentary. Th2y may be in a nascent condi- tion, 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 in- heritance, and relate to a former state of things. It is, however, often difficult to distinguish between rudimen- tary and nascent organs; for we can judge only by analogy 258 RUDIMENTARY, ATROPHIED, [CHAP. XIV. whether a part is capable of further development, in which case alone it deserves to be called nascent. Or- gans in this condition will always be somewhat rare; for beings thus provided will commonly have been sup- planted 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, modi- fied 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 Lepi- dosiren as the " beginnings of organs which attain full functional development in higher vertebrates; " but, ac- cording to the view lately advocated by Dr. Giinther, they are probably remnants, consisting of the persist- ent axis of a fin, with the lateral rays or branches abort- ed. 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 cer- tain cirripedes, which have ceased to give attachment to the ova and are feebly developed, are nascent bran- chiae. 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 or- gans may be utterly aborted; and this implies, that in certain animals or plants, parts are entirely absent which CHAP. XIV.] AND ABORTED ORGANS. 259 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 occasion- ally 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 un- derstand 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 rela- tively 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 reason- ing 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 2GO RUDIMENTARY, ATROPHIED, [CHAP. XIV. " to complete the scheme of nature." But this is not an explanation, merely a re-statement 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 the 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 de- veloped 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 CHAP. XIV.] AND ABORTED ORGANS. 261 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 pro- duced; 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 re- duced 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 condi- tions, 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 rudi- mentary. 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 func- 262 RUDIMENTARY, ATROPHIED, [CHAP. XIV. tions. 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 dis- use or selection reduces an organ, and this will generally be when the being has come to maturity and has to exert its full powers of action, the principle of inherit- ance at corresponding ages will tend to reproduce the organ in its reduced state at the same mature age, but will seldom effect it in the embryo. Thus we can un- derstand 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 con- sequence much reduced, how can it be still further re- duced in size until the merest vestige is left; and how can it be finally quite obliterated? It is scarcely pos- sible 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 towards augmentation of size, then we should be able to understand how an organ which has become useless would be rendered, in- CHAP. XIV.] AND ABORTED ORGANS. 263 dependency 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 pos- sessor, are saved as far as 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 re- duced 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 inherit- ance, 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 physiologi- cal importance. Eudimentary 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 condi- tion, 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. 2G4 SUMMARY. [CHAP. XIV. 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 relation- ships 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 con- stant and prevalent, whether of high or of the most tri- fling importance, or, as with rudimentary organs, of no importance, the wide opposition in value between an- alogical or adaptive characters, and characters of true affinity; and other such rules; all naturally follow if we admit the common parentage of allied forms, to- gether 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 ele- ment of descent has been universally used in ranking to- gether the sexes, ages, dimorphic forms, and acknowl- edged 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 CHAP. XIV.] SUMMARY. 265 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 Embry- ology; 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. Larva? are active embryos, which have been 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 rudi- mentary 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 fami- lies, with which this world is peopled, are all descended, 43 266 SUMMARY. [CHAP. XIV. each within its own class or group, from common parents, and have all been modified in the course of de- scent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments. CHAP. XV.] RECAPITULATION. 267 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 ex- tended 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 per- fected, 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 devia- tions of structure or instinct and, lastly, that grada- 268 RECAPITULATION. [CHAP. XV. tions 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 amongst 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 denned 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 recapitula- tion 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 repro- ductive 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 CHAP. XV.] RECAPITULATION. 269 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 re- spect 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 varie- ties which have been experimented on have been pro- duced 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 gradu- ally 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 con- ditions 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; 270 RECAPITULATION. [CHAP. XV. for I have ascertained by a laborious series of experi- ments 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 sur- vive, are rendered sterile, though retaining perfect health. This does not occur, or only in a very slight de- gree, 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 dis- tinct 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, whilst 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 CEAP. XV.] RECAPITULATION. 271 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 re- tained 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 diffu- sion 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 climatal 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 dis- tribution 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 9 very long period; and con- sequently 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 272 RECAPITULATION. [CHAP. XV. existed, linking together all the species in each group by gradations as fine as are 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 be- coming 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 modi- fications 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 sup- planted by the allied forms on either hand; for the lat- ter, 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 infini- tude 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 CHAP. XV.] RECAPITULATION. 273 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 un- doubtedly revealed the former existence of many links, bringing numerous forms of life much closer together, it does not yield the infinitely many fine gradations be- tween 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 geologi- cal stages? Although we now know that organic be- ings appeared on this globe, at a period incalculably re- mote, 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 pro- genitors 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 im- perfect 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 char- acters directly intermediate between its modified off- spring, any more than the rock-pigeon is directly inter- mediate 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 modi- 274 RECAPITULATION. {CHAP. XV. fied species, if we were to examine the two ever so close- ly, 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 dis- covered, they would simply be ranked by many natu- ralists as so many new species, more especially if found in different geological sub-stages, 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? *0nly 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 retain 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, ajid are discovered in a geological forma- tion, 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 dura- CIIAF. XV.] RECAPITULATION. 275 tion has probably been shorter than the average dura- tion 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 encrmous period in nearly their present relative posi- tions, 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 suffi- cient 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 ma'ny philosophers are not as yet willing to admit that we know enough of the constitu- tion 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 276 RECAPITULATION. [CHAP. XV. 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 be justly 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 ques- tions 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 dur- ing the long lapse of years, or that we know how im- perfect is the Geological Record. Serious as these sev- eral objections are, in my judgment they are by no means sufficient to overthrow the theory of descent with sub- sequent 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, compen- sation, 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 CHAP. XV.] RECAPITULATION. 277 productions have been modified; but we may safely in- fer that the amount has been large, and that modifica- tions 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 condi- tions 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, indi- vidual differences so slight as to be inappreciable except by an educated eye. This unconscious process of selec- tion 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 inextric- able doubts whether many of them are varieties or aboriginally distinct species. There is no reason why the principles which have 278 RECAPITULATION. [CHAP. XV> acted so efficiently under domestication should not have acted under nature. In the survival of favoured indi- viduals 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 Avhich shall die, which variety or species shall increase in number, and which shall decrease, or finally become extinct. As the indi- viduals 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 al- most 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 pos- session 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 spe- CHAP. XV.] RECAPITULATION. 279 cial 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 ex- pected to find that organic beings have varied under nature, in the same way as they have varied under do- mestication. And if there has been any variability under nature, it would be an unaccountable fact if natu- ral 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 dif- ferences in his domestic productions; and every one ad- mits that species present individual differences. But, besides such differences, all naturalists admit that natu- ral varieties exist, which are considered sufficiently dis- tinct 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 sub-species, and species. On sepa- rate continents, and on different parts of the same conti- nent when divided by barriers of any kind, and on out- lying 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 indi- vidual 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 280 RECAPITULATION. [CHAP. XV. 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 far- ther than this, seems to be in the highest degree prob- able. 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 Avhy it is that no line of demarcation can be drawn between species, commonly supposed to have been produced by special acts of crea- tion, 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 varie- ties; for where the manufactory of species has been ac- tive, we might expect, as a general rule, to find it still in action; and this is the case if varieties be incipient spe- cies. Moreover, the species of the larger genera, which afford the greater number of varieties or incipient spe- cies, retain to a certain degree the character of varieties; for they differ from each other by a less amount of differ- ence than do the species of smaller genera. The closely CHAP. XV.] RECAPITULATION. 281 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 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 repro- duction 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 spe- cies. Hence, during a long-continued course of modi- fication, the slight differences characteristic of varie- ties of the same species, tend to be augmented into the greater differences characteristic of the species of the same genus. Xew 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 char- acter, 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 44 2S2 RECAPITULATION. [CHAP. XV. 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 in- explicable 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. Hpw 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, should 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 unoccu- pied 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 CHAP. XV.] RECAPITULATION. 283 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 univer- sal, 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 con- spicuous by brilliant colours in contrast with the green foliage, in order that the flowers may be readily seen, visited and fertilised by insects, and the seeds dissem- inated 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 per- fect, 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 284 RECAPITULATION. [CHAP. XV. an enemy, causing the bee's own death; at drones be- ing 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 the ichneumonidaB feeding within the liv- ing bodies of caterpillars; or 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 produc- tion of distinct species. In both cases physical condi- tions seem to have produced some direct and definite effect, but how much we cannot say. Thus, when varie- ties 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 in- habiting 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 modi- fied. With both varieties and species, reversions to long- lost characters occasionally occur. How inexplicable on CHAP. XV.] RECAPITULATION. 285 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 man- ner as the several domestic breeds of the pigeon are de- scended 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 generic char- acters 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 under- gone, 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 286 KECAPITULATION. [CHAP. XV. 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 con- stant 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 under- stand 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 archi- tectural 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 in- sects, 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 under- stand 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 tem- perate 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 varie- ties, we can at once see why their crossed offspring should follow the same complex laws in their degrees and kinds CHAP. XV.] RECAPITULATION. 287 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 slow- ly and at successive intervals; and the amount of change, after equal intervals of time, is widely different in dif- ferent 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 under- stand why it is that the more ancient forms, or early 288 RECAPITULATION. [CHAP. XV. 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 dif- ferent 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 retro- graded 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 climatal and geographical changes and to the many occasional and unknown means of dispersal, then we can understand, on the theory of descent with modi- fication, 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 modifica- tion have been the same. We see the full meaning of the wonderful fact, which has struck every traveller CHAP. XV.] RECAPITULATION. 2S9 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 inhabit- ants 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 under- stand, 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 alli- ance of some of the inhabitants of the sea in the north- ern and southern temperate latitudes, though separated by the whole intertropical ocean. Although two coun- tries 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 modifica- tion, 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 dis- 290 RECAPITULATION. [CHAP. XV. tant from any continent. Such cases as the presence of peculiar species of bats on oceanic islands and the ab- sence of all other terrestrial mammals, are facts utterly inexplicable on the theory of independent acts of crea- tion. The existence of closely allied or representative spe- cies 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. Where- ever many closely allied yet distinct species occur, doubt- ful 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 archi- pelago, 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 mu- tual affinities of the forms within each class are so com- plex and circuitous. We see why certain characters CHAP. XV.] RECAPITULATION. 291 are far more serviceable than others for classification; why adaptive characters, though of paramount import- ance 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 char- acters 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 gene- alogical arrangement, with the acquired grades of dif- ference, marked by the terms, varieties, species, genera, families, &c.; and we have to discover the lines of de- scent 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 vertebras 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 pis- tils of a flower, is likewise, to a large extent, intelligible on the view of the gradual modification of parts or or- gans, which were aboriginally alike in an early progeni- tor in each of these classes. On the principle of succes- sive 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 292 RECAPITULATION. [CHAP. XV. 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 under- stand on this view the meaning of rudimentary organs. But disuse and selection will generally act on each crea- ture, 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 un- affected, and on the principle of inheritance at corre- sponding 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. CHAP. XV.] CONCLUSION. 293 I have now recapitulated the facts and considerations which nave 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 man- ner, that is in relation to adaptive structures, whether past or present, by the direct action of external condi- tions, and by variations which seem to us in our ignor- ance 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 per- mitted to remark that in the first edition of this work, and subsequently, I placed in a most conspicuous posi- tion 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 modi- fication." 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 29 CONCLUSION. [CHAP. XV. 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; nowithstanding that Leibnitz formerly ac- cused Newton of introducing " occult qualities and miracles into philosophy." I see no good reason why the views given in this vol- ume should shock the religious feelings of any one. It is satisfactory, as showing how transient such impres- sions are, to remember that the greatest discovery ever made by man, namely, the law of the attraction of grav- ity, was also attacked by Leibnitz, " as subversive of natural, and inferentially of revealed, religion." A cele- brated author and divine has written to me that " he has ." gradually learnt to see that it is just as noble a concep- " tion of the Deity to believe that He created a few ori- " ginal 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 dis- believe in the mutability of species. It cannot be as- serted that organic beings in a state of nature are sub- ject to no variation; it cannot be proved that the amount of variation in the course of long ages is a lim- ited 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 CIIAP. XV.] CONCLUSION. 295 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 mu- tation. But the chief cause of our natural unwillingness to admit that one species has given birth to clear and dis- tinct species, is that we are always slow in admitting 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 see still 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," &c., and to think that we give an explanation when we only re-state a fact. Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a 296 CONCLUSION. [CHAP. XV. 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 immu- tability 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 be- lieve 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 over- whelmed 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 character- istic 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 vcra 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 blind- ness 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 innu- CHAP. XV.] CONCLUSION. 297 merable periods in the earth's history certain elemental atoms have been commanded suddenly to flash into liv- ing 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 re- tained in the foregoing paragraphs, and elsewhere, sev- eral 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 un- doubtedly this was the general belief when the first edi- tion 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. 45 298 CONCLUSION. [CHAP. XV. 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 op- posed to the admission of great and abrupt modifications. Under a scientific point of view, and as leading to fur- ther investigation, but little advantage is gained by be- lieving that new forms are suddenly developed -in an in- explicable 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 sub- ordinate 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. Through- out 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 CHAP. XV.] CONCLUSION. 299 five progenitors, and plants from an equal or lesser number. Analogy would lead me one step farther, 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 in- jurious 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 production seems to be es- sentially similar. With all, as far as is at present known, the germinal vesicle is the same; so that all organ- isms 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 inter- mediate in character that naturalists have disputed to which kingdom they should be referred. As Professor Asa Gray has remarked, " the spores and other repro- " ductive 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 char- acter, 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 analog}-, and it is immaterial whether or 300 CONCLUSION. [CHAP. XV. 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 Ver- tebrata, Articulata, &c., we have distinct evidence in their embryological, homologous, and rudimentary struc- tures, 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 la- bours 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 ex- perience, 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 differ- ences, 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 CHAP. XV.] CONCLUSION. 301 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 exist- ence 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 acknowl- edged 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 free 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, rudi- mentary and aborted organs, &c., will cease to be meta- phorical, 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 sum- ming 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 302 CONCLUSION. [CHAP. XV. 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 cor- relation, 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 classi- fying will no doubt become simpler when we have a defi- nite 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. Specie.s and groups of species which are called aberrant, and which may fancifully be called living fossils, will aid us in form- ing a picture of the ancient forms of life. p]mbryology will often reveal to us the structure, in some degree ob- scured, 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, CHAP. XV.] CONCLUSION. 393 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 differ- ences between the inhabitants of the sea on the opposite sides of a continent, and the nature of the various in- habitants on 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 imbedded remains must not be looked at as a well-filled museum, but as a poor collection made at hii/ard and at rare intervals. The accumulation of each great fossiliferous formation will be recognised as having depended on an unusual concurrence of favourable cir- cumstances, and the blank intervals between the suc- cessive 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 at- tempting to correlate as strictly contemporaneous two formations, which do not include many identical species, by the general succession of the forms of life. As spe- cies 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 improve- ment 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 304 CONCLUSION. [CHAP. XV. 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 AVC 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 in- dependently 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 descend- ants 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 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 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 CHAP. XV.] CONCLUSION. 305 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 Cam- brian epoch, we may feel certain that the ordinary suc- cession 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 per- fection. 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 Eeproduction; Inherit- ance which is almost implied by reproduction; Varia- bility from the indirect and direct action of the condi- tions of life, and from use and disuse: a Eatio of In- crease 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 con- ceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, 306 CONCLUSION. [CHAP. XV. 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. GLOSSARY PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.* 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. ALG^E. A class of plants including the ordinary sea-weeds and the filamentous fresh-water weeds. * I am indebted to the kindness of Mr. W. S. Dallas for this Glossary, which has been given because several readers have com- plained 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. 807 308 GLOSSAEY. ALTERNATION" OF GENERATIONS. This terra 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 differ- ent from its parent, but from which the parent-form is repro- duced 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 be- tween 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 ex- hibits 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 earthworms, and the leeches. ANTENN.E. Jointed organs appended to the head in Insects, Crus- tacea 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 Mam- malia. ARCHETYPAL. Of or belonging to the Archetype, or ideal primi- tive form upon which all the beings of a group seem to be organised. ARTICULATA. A great division of the Animal Kingdom character- ised 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. BATBACHIANS. A class of animals allied to the Reptiles, but GLOSSARY. 309 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 imbedded 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. BRANCHLE. 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. CANID.E. The Dog-family, including the Dog, Wolf, Fox, Jackal, &c. 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 be- longs 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. (Ex- amples, Cuttle-fish, Nautilus.) CETACEA. An order of Mammalia, including the Whales, Dolphins, &c., 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, &c. 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 at- tached 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. 310 GLOSSARY. 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." CCELOSPERMOUS. A term applied to those fruits of the Umbellif- enu 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. COMPOSITE or COMPOSITOUS PLANTS. Plants in which the inflores- cence consists of numerous small flowers (florets) brought to- gether into a dense head, the base of which is enclosed by a common envelope. (Examples, the Daisy, Dandelion, &c.) CONFERVA. 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, char- acter, &c., with another. CORYMB. A bunch of flowers in which those springing from the lower part of the flower stalk 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, &c.) CURCULIO. The old generic term for the Beetles known as Wee- vils, 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. GLOSSARY. 3H 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, in- cluding the Old Red Sandstone. DICOTYLEDONS or DICOTYLEDONOUS PLANTS. A class of plants characterised by having two seed-leaves, by the formation of new wood t 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. DIFFERENTIATION. 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 con- dition of the appearance of the same species under two dis- similar forms. DICECIOUS. Having the organs of the sexes upon distinct indi- viduals. 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 egc 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. EPIIEMEROUS INSECTS. Insects allied to the May-fly. 312 GLOSSARY. FAUNA. The totality of the animals naturally inhabiting a cer- tain country or region, or which have lived during a given geological period. FELID^:. The Cat-family. FERAL. Having become wild from a state of cultivation or domes- tication. 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, &c. FffiTAL. Of or belonging to the foetus, or embryo in course of de- velopment. FORAMINIFERA. A class of animals of very low organisation, and generally of small size, having a jelly-like body, from the sur- face 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 Hymen- optera are a group of Wasp-like Insects, which burrow in sandy soil to make nests for their young. FRENUM (pi. FRENA). A small band or fold of skin. FUNGI (sing. FUNGUS). A class of cellular plants, of which Mush- rooms, 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 development of the embryo proceeds. GLACIAL PERIOD. A period of great cold and of enormous exten- sion of ice upon the surface of the earth. It is believed that glacial periods have occurred repeatedly during the geological GLOSSARY. 313 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 prod- uct 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 sedi- mentary deposit after its consolidation. GRALLATORES. The so-called Wading-birds (Storks, Cranes, Snipes, &c.), 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 in- dividual, 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, &c., is composed. The latter is called serial homology. The parts which stand in such a rela- tion 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 Cicada, Frog-hoppers, and Aphid*-*, arc well-known examples. HYBRID. The offspring of the union of two distinct species. 40 314: GLOSSARY. 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. HYPEETBOPHIED. Excessively developed. ICHNEUMONID^E. A family of Hymenopterous insects, the mem- bers of which lay their eggs in the bodies or eggs of other insects. IMAGO. The perfect (generally winged) reproductive state of an insect. INDIGENS. 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 par- ticles 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. LACUNA. Spaces left among the tissues in some of the lower ani- mals, and serving in place of vessels for the circulation of the fluids of the body. LAMELLATED. Furnished with lamella} or little plates. LARVA (pi. LARVAE). The first condition of an insect at its issuing from the egg, when it is usually in the form of a grub, cater- pillar, 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. Lau- rence, whence the name. It is in these that the earliest known traces of organic bodies have been found. LEGUMINOS^. 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 pheath formed by two other petals. The fruit is a pod (or legume). GLOSSARY. 315 LEMUIUD^K 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 oi' a nail upon the first finger of the hind hands.. LEPIDOPTERA. An order of Insects, characterised by the posses- sion 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, &c., 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 (Mammce, 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 vas- cular 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 mamma? 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 Quadru- peds 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, &c. (see MAMMALIA). MAXILLA, 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. 316 GLOSSAKY. MELANISM. The opposite of albinism ; an undue development of colouring material in the skin and its appendages. METAMOBPHIC 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 " shell-fish ; " 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 gener- ally in multiples of three. (Examples, Grasses, Lilies, Orchids, Palms, &c.) 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 (JUysis) 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 GLOSSARY. 317 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, &c., 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. (ESOPHAGUS. 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 Mollusca to close the aperture of their shell. The opercular valves of Cir- ripedes 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 Umbel- liferae 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, &c. PALAEOZOIC. The oldest system of fossiliferous rocks. PALPI. Jointed appendages to some of the organs of the mouth in Insects and Crustacea. PAPILIONACE^:. An order of Plants (see LEGUMINOS^;). 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 ex- pense of, another organism. PARTHENOGENESIS. The production of living organisms from un- impregnated eggs or seeds. 318 GLOSSARY. PEDUNCULATED. Supported upon a stem or stalk. The peduncu- lated 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 super- ficial parts of animals. The cells secreting it are called pig- ment-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 MAM- MALIA. 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 igne- ous 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. POLTANDROUS (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 Sea-mats. GLOSSARY. 319 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 attach- ment of muscles, ligaments, &c. PROPOLIS. A resinous material collected by the Hive-Bees 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 (pi. PUP^E). The second stage in the development of an Insect, from which it emerges in the perfect (winged) repro- ductive 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 relation- ships, it is said to undergo a retrograde development or meta- morphosis. 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 Forarainifera. 320 GLOSSARY. RODENTS. The gnawing Mammalia, such as the Eats, Rabbits, and Squirrels. They are especially characterised by the pos- session 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. SCUTELL^E. The horny plates with which the feet of birds are gen- erally 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 enve- lope of an ordinary flower. They are usually green, but some- times 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 CHORD. The central portion of the nervous system in the Vertebrata, which descends from the brain through the arches of the vertebral, and gives off nearly all the nerves to the va- rious 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 brea'st-bone. STIGMA. The apical portion of the pistil in flowering plants. GLOSSARY. 321 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 (pi. 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. TRIMORPIIIC. Presenting three distinct forms. UMBELLIFER^E. 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. 322 GLOSSARY. 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. ZofiA-STAOE. 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, &c.) 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 be- tween two sexual reproductions ; and these forms, which are apparently individual animals, have been called zooids. INDEX. AIir.URANT. BALANCEMENT. A. Antarctic islands, ancient flora of, ii. 190. ABERRANT groups, ii. 227. Abyssinia, plants of, ii. 167. Acclimatisation, i. 173. Antechinus, ii. 219. Ants attending aphides, i. 323. , slave-making instinct, i. 336. Adoxa, i. 270. , neuters, structure of, i. 859. Affinities of extinct species, ii. 106. Apes, not having acquired intel- of organic beings, ii. 225. Agassiz, on Amblyopsis, i. 173. , on groups of species suddenly lectual powers, i. 282. Aphides, attended by ante, i. 323. Aphis, development of, ii. 245. appearing, ii. 88. Apteryx, i. 218. , on prophetic forms, ii. 107. Arab horses, i. 40. , on emoryological succession, Aralo-Caspian Sea, ii. 121. ii. 120. Archeopteryx, ii. 80. , on the Glacial period, ii. 151. Archiac, M. de, on the succession of , on embryological characters, species, ii. 103. ii. 210. Artichoke, Jerusalem, i. 176. , on the latest tertiary forms, ii. Ascension, plants of, ii. 178. 71. . on parallelism of embryologi- cal development and geological Asclepias, pollen of, i. 236. Asparagus, ii. 143. Aspicarpa, ii. 209. succession, ii. 254. Asses, striped, i. 198. , Alex., on pedicellarise, i. 298. Algie of New Zealand, ii. 164. , improved by selection, i. 48. Ateuchus, i. 168. Alligators, males, fighting, i. 108. Aucapitaine, on land-shells, ii. 187. Alternate generations, ii. 239. Amblyopsis, blind fish, i. 173. America, North, productions allied Audubon, on habits of frigate-bird, i. 222. , on variation in birds' nests, i. to those of Europe, ii. 156. 324. ' , , boulders and glaciers of, , on heron eating seeds, ii. 176. ii. 159. Australia, animals of, i. 140. , South, no modern formations , dogs of, i. 328. on west coast, ii. 61. , extinct animals of, ii. 121. Ammonites, sudden extinction of, , European plants in, ii. 163. ii. 99. , glaciers of, ii. 159. Anagallis, sterility of, ii. 4. Azara, on flies destroying cattle, i. Analogy of variations, i. 197. 89. Ancylus, ii. 174. Azores, flora of, ii. 149. Andaman Islands inhabited by a toad, ii. 182. > Animals, not domesticated from being variable, i. 19. , domestic, descended from seve- ral stocks, i. 21. Babington, Mr., on British plants, i. 58. Baer, Von, standard of Highness, i. t , acclimatisation of, i. 175. 151. of Australia, 1. 140. , comparison of bee and fish, ii. with thicker fur in cold cli- 118. mates, i. 166. , embryonic similarity of the , blind, in caves, i. 172. Vertebrata, ii. 241. , extinct, of Australia, ii. 121. Baker, Sir S., on the giraffe, i. 278. Anomina, i. 361. Balancement of growth, i. 182. 323 324- INDEX. Baleen, i. 285. Bizcacha, ii. 133. Barberrv, flowers of, i. 121. Barrantfe, M., on Silurian colonies, , affinities of, ii. 227. Bladder for swimming, in fish, i. 230. ii. 90. Blindness of cave animals, i. 170. , on the succession of species, Blyth, Mr., on distinctness of Indian ii. 103. cattle, i. 21. , on parallelism of palaeozoic , on striped hemionus, i. 199. formations, ii. 106. , on affinities of ancient species, , on crossed geese, ii. 10. Borrow, Mr., on the Spanish pointer, ii. 108. i. 40. Barriers, importance of, ii. 130. Bory St. Vincent, on Batrachians, Bates, Mr., on mimetic butterflies, ii. 182. ii. 222, 223, 224. Bosquet, M., on fossil Chthamalus, Batrachiaus on islands, ii. 182. ii. 80. Bats, how structure acquired, i. Boulders, erratic, on the Azores, ii. 218. 149. , distribution of, ii. 184. Branchiae,!. 231,232. Bear, catching water-insects, i. 220. of crustaceans, i. 238. Beauty, how acquired, i. 249; ii. Braun, Prof., on the seeds of Fuma- 283. _ riaceae, i. 271. Bee, sting of, i. 255. Brent, Mr., on house-tumblers, i. , queen, killing rivals, i. 256. 326. , Australian, extermination of, Britain, mammals of, ii. 185. i. 93. Broca, Prof., on Natural Selection, Bees fertilising flowers, i. 90. , hive, not sucking the red i. 265. Bronn, Prof., on duration of specific clover, i. 117. forms, ii. 66. , Ligurian, i. 117. , hive, cell-making instinct, i. , various objections by, i. 265. Brown, Kobert, on classification, ii. 342. 207. , variation in habits, i. 324. , Se"quard, on inherited muti- , parasitic, i. 336. lations, i. 168. , humble, cells of, i. 343. Beetles, wingless, in Madeira, i. 169. Busk, Mr., on the Polyzoa, i. 301. Butterflies, mimetic, ii. 222, 223, with deficient tarsi, i. 168. 224. Bentham, Mr., on British plants, i. Buzarcingues, on sterility of varie- 58. ties, ii. 38. , on classification, ii. 211. Berkeley, Mr., on seeds in salt C. water, ii. 142. Bermuda, birds of, ii. 180. Cabbage, varieties of, crossed, i. 122. Calceolaria, ii. 7, 8. Birds acquiring fear, i. 325. Canary-birds, sterility of hybrids, , beautv of, i. 252. ii. 9. annually cross the Atlantic, ii. Cape do Verde islands, productions 150. of, ii. 189. , colour of, on continents, i. , plants of, on mountains, ii. 165. 162. , footsteps and remains of, in secondary rocks, ii. 79. Cape of Good Hope, plants of, i. 158; ii. 178. , fossil, in caves of Brazil, ii. 121. Carpenter, Dr., on foraminifera, ii. 117. , of Madeira, Bermuda, and Carthamus, i. 271. Galapagos, ii. 179, 180. Catasetum, i. 243; ii. 216. , song of males, i. 109. Cats, with blue eyes, deaf, i. 13. transporting seeds, ii. 148. , variation in habits of, i. 325. , wadern, ii. 175. curling tail when going to , wingless, i. 167, 218. spring, i. 254. INDEX. 325 Cattle destroying fir-trees, i. 88. destroyed by flies in Paraguay, i. 89. Climbing plants, i. 230. , development of, i. 305. Clover visited bv bees, i. 117. , breeds of, locally extinct, i. 134. Cobites, intestine of, i. 229. , fertility of Indian and Euro- Cockroach, i. 93. pean breeds, ii. 10. Collections, palseontologieal, poor, . Indian, i. 21 ; ii. 10. ii. 58. Cave, inhabitants of, blind, i. 170. Colour, influenced by climate, i. 165. Ceeidomyia, ii. 239. , in relation to attack by flies, Celts, proving antiquity of man, i. 248. i. 21. Columba livia, parent of domestic Centres of Creation, ii. 135. pigeons, i. 26. Cephalopoda;, structures of eyes, i. Colymbetes, ii. 174 236. , development of, ii. 244. Cercopithecus, tail of, i. 294. Compensation of growth, i. 182. Composite, flowers and seeds of, i. 179. Ceroxylus laceratus, i. 284. , outer and inner florets of, i. Cervulus, ii. 9. 270. Cetacea, teeth and hair, 1. 179. , male flowers of, ii. 257. , development of the whale- Conclusion, general, ii. 293. bone, i. 285. Conditions, slight changes in, fa- Cetaceans, i. 285. vourable to fertility, ii. 27. Ceylon, plants of, ii. 164 Chalk formation, ii. 100. Convergence of genera, i. 156. Coot i. 222. Characters, divergence of, i. 134 Cope, Prof., on the acceleration or , sexual, variable, i. 185, 191. , adaptive or analogical, ii. 218. retardation of the period of repro- duction, i. 232. Charlock, i. 94. Coral-islands, seeds drifted to, ii. Checks, to increase, i. 83. 145. , mutual, i. 86. Chelae of Crustaceans, i. 300. reefs, indicating movements of earth, ii. 145. Chickens, instinctive tameness of, Corn-crake, i. 223. i. 329. Correlated variation in domestic Chironomus, its asexual reproduc- productions, i. 13. tion, ii. 240. CoryaiUhes, i. 241. Chthamalina;, ii. 59. Chthamalus, cretacean species of, Creation, single centres of, ii. 135. Crinum, ii. 6. ii. 81. Croll, Mr., on subaerial denudation, Circumstances favourable to selec- ii. 53, 56. tion of domestic products, i. 46. , on the age of our oldest for- to natural selection, i. 124. mations, ii. 83. Cirri pedes capable of crossing, i. 124 , carapace aborted, i. 184". , on alternate Glacial periods in the North and South, ii. 160. , their ovitrcrous frena, i. 232. , fossil, ii. 80. Crosses, reciprocal, ii. 14. Crossing of domestic animals, im- , larva; of, ii. 243. Claparede, Prof., on the hair-clasp- portance in altering breeds, i. 23. , advantages of, j. 119, 120. ers of the Acarido?, i. 239. , unfavourable to selection, i. Clarke, Kev. W. B., on old glaciers 125. in Australia, ii. 159. Cniger, Dr., on Coryanthes, i. 241. Classification, ii. 202. Crustacea of New Zealand, ii. 164. Clift, Mr., on the succession of types, Crustacean, blind, i. 171. ii. 121. air-breathers, i. 238. Climate, effects of, in checking in- Crustaceans, their chela;, i. 300. crease of beings, i. 84. Cryptocerus, i. 359. , adaptation of, to organisms, i. Ctcnomys, blind, i. 170. 174 Cuckoo, instinct of, i. 319, 330. 326 CUXXIXGHAM. INDEX. Cunningham, Mr., on the flight of the logger-headed duck, i. 167. Currants, grafts of, ii. 19. Currents of sea, rate of, ii. 144. Diversification of means for same general purpose, i. 240. Division, physiological, of labour, i. 139. Cuvier, on conditions of existence, i. 320. Dog, resemblance of jaw to that of the Thylacinus, ii. 220. Cuvier, on fossil monkeys, ii. 79. Dogs, hairless, with imperfect teeth, , Fred., on instinct, i. 320. i. 14 Cvclostoma, resisting salt water, ii. descended from several wild '187. stocks, i. 22. , domestic instincts 6f, i. 327. D. , inherited civilisation of. i. 327. , fertility of breeds together, ii. Dana, Prof., on blind cave-animals, 10. i. 172. , of crosses, ii. 35. , on relations of crustaceans of Japan, ii. 158. , proportions of body in differ- ent breeds, when young, ii. 247. , on crustaceans of New Zea- land, ii. 164. Domestication, variation under, i. 7. Double flowers, i. 358. Dawson, Dr., on eozoon, ii. 85. Downing, Mr., on fruit-trees in De Candolle, Aug. Pyr., on struggle for existence, i. 77. America, i. 104 Dragon flies, intestines of, i. 229. , on nrnbelliferee, i. 181. Drift-timber, ii. 145. , on general affinities, ii. 228. Driver-ant, i. 361. , Alph., on the variability of oaks, i. 62. Drones killed by other bees, i. 256. Duck, domestic, wings of, reduced, , on low plants, widely dis- i. 12. persed, ii. 196. , beak of, i. 285. , on widely-ranging plants be- ing variable, i. 67. , logger-headed, i. 218. Duckweed, ii. 173. , on naturalisation, i. 139. Dugong, affinities of, ii. 206. , on winged seeds, i. 181. Dung-beetles with deficient tarsi, L , on Alpine species suddenly becoming rare, i. 210. 168. Dytiscus, ii. 174 , on distribution of plants with large seeds, ii. 145. , on vegetation of Australia, ii. 167. Earl, Mr. W., on the Malay Archi- , on fresh- water plants, ii. 174 pelago, ii. 185. , on insular plants, ii. 178. Degradation of rocks, ii. 52. Ears, drooping, in domestic ani- mals, i. 13. Denudation, rate of, ii. 54 , rudimentary, ii. 261. of oldest rocks, ii. 85. Earth, seeds in roots of trees, ii. of granitic areas, ii. 64. 145. Development of ancient forms, ii. charged with seeds, ii. 148. 116. Devonian svstem, ii. 113. Echinodermata, their pedicel ariae, i. 297. Dianthus, fertility of crosses, ii. 13. Eciton, i. 359. Dimorphism in plants, i. 55; ii. 29. Dirt on feet of birds, ii. 148. Economy of organisation, i. 182. Edentata, teeth and hair, i. 179. Dispersal, means of, ii. 140. , fossil species of, ii. 288. during Glacial period, ii. 151. Distribution, geographical, ii. 129. Edwards, Milne, on physiological division of labour, i. 139. , means of, ii. 140. Disuse, effect of, under nature, i. , on gradations of structure, i. 244. 167. Divergence of character, L 134 , on embryological characters, ii. 210. INDEX. FRIGATE-BIRD. 327 Eggs, young birds escaping from, i. Fishes, of southern hemisphere, ii. 106. 164. Egypt, productions of, not modified, i. 263. Flat-fish, their structure, i. 290. Flight, powers of, how acquired, i. Electric organs, i. 234. Elephant, rate of increase, i. 80. , of Glacial period, i. 176. Flint-tools, proving antiquity of man, i. 21. Embryology, ii. 239 ;> Flower, Prof., on the Larynx, i. 297. Eozoon Canadense, ii. 84. , on Halitherium, ii. 108. Epilepsy inherited, i. 167. , on the resemblance between Existence, struggle for, i. 75. , condition of, i. 261. the jaws of the dog and Thyla- cinus, ii. 220. Extinction, as bearing on natural selection, i. 150. , on the homology of the feet of certain marsupials, ii. 232. of domestic varieties, L 145. Flowers, structure of, in relation to , ii. 94. crossing, i. 114. Eye, structure of, i. 225. * , correction for aberration, i. 255. , of composite and umbelli- iferffi, i. 179, 270. Eyes, reduction in moles, i. 170. , beauty of, i. 252. , double, i. 358. F. Flysch formation, destitute of or- ganic remains, ii. 59. Fabrc, M., on hymenoptera fight- Forbes, Mr. D., on glacial action in ing, i. 108. , the Andes, ii. 160. , on parasitic sphex, i. 336. , on Sitaris, ii. 252. , E., on colours of shells, i. 165. , on abrupt range of shells in Falconer, Dr., on naturalisation of depth, i. 210. plants in India, i. 80. , on elephants and mastodons, , on poorness of palueontological collections, ii. 58. ii. 113. , on continuous succession of and Cautley, on mammals of genera, ii. 93. sub-Himalayan beds, ii. 122. , on continental extensions, ii. Falkland Islands, wolf of, ii. 183. Faults, ii. 54. 140. 141. , on distribution during Glacial Faunas, marine, ii. 131. period, ii. 152. Fear, instinctive, in birds, i. 329. , on parallelism in time and Feet of birds, young molluscs ad- hering to, ii. 174. space, ii. 200. Forests, changes in, in America, i. Fertilisation variously effected, i. 91. 241, 252. Formation, Devonian, ii. 113. Fertility of hybrids, ii. 6. ' , from slight changes in condi- , Cambrian, ii. 84. Formations, thickness of, in Britain, tions, ii 28. ii. 55. of crossed varieties, ii. 34. Fir-trees destroyed by cattle, i. 88. , pollen of, i. 257. , intermittent, ii. 69. Formica, rufescens, i. 336. , Bangui nea, i. 338. Fish, flying, i. 218. , flava. neuter of, i. 360. , te'leostean, sudden appearance Forms, lowly organised, long en- of, ii. 81. during, i. 154. , eating seeds, ii. 14fi, 175. , fresh-water, distribution of, Frena, ovigerous, of cirripedes, i. 232. ii. 17'2. Fresh-water productions, dispersal Fishes, ganoid, now confined to of, ii. 171. fresh water, i. 130. Fries, on species in large genera , electric organs of, i. 234. being closely allied to other spe- , ganoid, living in fresh water, cies, i. 71. ii. 99. Frigate-bird, i. 222. IXDEX. HABIT. Frogs on islands, ii. 182. Glacial period, affecting the North Fruit-trees, gradual improvement of, i. 42. and South, ii. 158. Glands, mammary, i. 295. in United States, i. 104. Gmelin, on distribution, ii. 151. , varieties of, acclimatised in Godwin- Austen, Mr., on the Malay United States, i. 176. Archipelago, ii. 74. Fuci, crossed, ii. 15, 23. Fur, thicker in cold climates, i. 166. Goethe, on compensation of growth, i. 182. Furze, ii. 241. Gomphia, i. 272. Gooseberry, grafts of, ii. 19. G. Gould, Dr. Aug. A., on land-shells, ii. 186. Galapagos Archipelago, birds of, ii. 179. , Mr., on colours of birds, i. 165. , on instincts of cuckoo, i. 333. , productions of, ii. 188, 190. Galaxias, its wide range, ii. 172. Galeopithecus, i. 217. Game, increase of, checked by ver- min, i. 86. , on distribution of genera of birds, ii. 195. Gourds, crossed, ii. 38. Graba, on the Uria lacrymas, i. 113. Grafting, capacity of, ii. 18, 19, 20. Gartner, on sterility of hybrids, ii. 3, 4, 11. Granite, areas of denuded, ii. 64. Grasses, varieties of, i. 137. , on reciprocal crosses, ii. 15. , on crossed maize and verbas- Gray, Dr. Asa, on the variability of oaks, i. 62. cum, ii. 37. , on man not causing variabil- , on comparison of hybrids and mongrels, ii. 40, 41, 42. Gaudry, Prof., on intermediate ge- itv, i. 98. -, on sexes of the holly, i. 116. , on trees of the United States, nera of fossil mammals in Attica, i. 123. ii. 107. , on naturalised plants in the Geese, fertility when crossed, ii. 9, United States, i. 139. 10. , on aestivation, i. 272. , upland, i. 222. , on Alpine plants, ii. 151. Geikie, Mr., on subaerial denuda- , on rarity of intermediate va- tion, ii. 53. rieties, i. 212. Genealogy, important in classifica- , Dr. J. E., on striped mule, i. tion, ii. 212. 199. Generations, alternate, ii. 239. Grebe, i. 221. Geoffroy St. Hilaire, on balance- Grimm, on asexual reproduction, ii. merit, i. 182. 240. , on homologous organs, ii. 233. , Isidore, on variability of re- peated parts, i. 184 , or correlation, in monstrosi- ties, i. 13. Groups, aberrant, ii. 227. Grouse, colours of, i. 104. , red, a doubtful species, i. 59. Growth, compensation of, i. 182. Giinther, Dr., on flat-fish, i. 292. , on correlation, i. 179. , on variable parts being often , on prehensile tails, i. 295. , on the fishes of Panama, ii. monstrous, i. 190. 131. Geographical distribution, ii. 129. , on the range of fresh-water Geography, ancient, ii. 303. Geology, future progress of, ii. 302. , imperfection of the record, ii. fishes, ii. 172. , on the limbs of Lepidosiren, ii. 258. 303. II. Gervais, Prof., on Typotherium, ii. 108. Haast, Dr., on glaciers of New Zea- Giraffe, tail of, i. 245. land, ii. 159. . structure of, i. 276. Habit, efl'ect of, under domestica- Glacial period, ii. 151. tion, i. 12. INDEX. 329 Habit, effect of, under nature, i. 168. , diversified, of same species, i. 219. Hiickel, Prof., on classification and the lines of descent, ii. 231. Hair and teeth, correlated, i. 179. Halitherium, ii. 108. Harcourt, Mr. E. V., on the birds of Madeira, ii. 180. Ilartung, M., on boulders in the Azores, ii. 149. Hazel-nuts, ii. 143. Hearne, on habits of bears, i. 220. Heath, changes in vegetation, i. 87. Hector, Dr., on glaciers of New Zea- land, ii. 159. Heer, Oswald, on ancient cultivated plants, i. 20. -, on plants of Madeira, i. 130. Helianthemum, i. 272, Helix pomatia, ii. 187. , resisting .salt water, ii. 187. Helmholtz, M., on the imperfection of the human eye, i. 255. Helosciadium, ii. 143. Hemionus, striped, i. 202. Hensen, Dr., on the eyes of Cepha- lopoda, i. 237. Herbert, W., on struggle for exist- ence, i. 77. , on sterility of hybrids, ii. 6. Hermaphrodites cross'iug, i. 119. Heron eating seed, ii. 176. Heron, Sir K., on peacocks, i. 109. Heusinger, on white animals poi- soned by certain plants, i. 13. Hewitt, ftlr., on sterility of first crosses, ii. 23. Hildebrand, Prof., on the self-ste- rility of Corydalis, ii. 7. Hilgendorf, on intermediate varie- ties, ii. 66. Himalaya, glaciers of, ii. 159, , plants of, ii. Ifi2. Hippeastrum, ii. 7. Hippocampus, i. 295. Hofmeister, Prof., on the move- ments of plants, i. 308. Holly-trees, sexes of, i. 115. Hooker, Dr., on trees of New Zea- land, i. 123. , on acclimatisation of Hima- layan trees, i. 174. ", on flowers of urnbelliferse, i. 180. , on the position of ovules, i. 268. 47 Hooker, Dr.. on glaciers of Himala- ya, ii. 159. , on algse of New Zealand, ii. 164. , on vegetation at the base of the Himalaya, ii. 164. , on plants of Tierra del Fuego, ii.lGl. , on Australian plants, ii. 163, 190. , on relations of flora of Amer- ica, ii. 167. , on flora of the Antarctic lands, ii. 169,189. , on the plants of the Gala- pagos, ii. 181, 188. , on glaciers of the Lebanon, ii. 159. , on man not causing variabil- ity, i. 97. , on plants of mountains of Fernando Po, ii. 162. Hooks on palms, i. 247. on seeds, on islands, ii. 181. Hopkins, Mr., on denudation, ii. 63. Hornbill, remarkable instinct of, i. 364. Horns, rudimentary, ii. 261. Horse, fossil, in La Plata, ii. 96. , proportions of, when young, ii. 247. Horses destroyed by flies in Para- guay, i. 89. . striped, i. 199. Horticulturists, selection applied by, i. 37. Huber, on cells of bees, i. 349. ' , P., on reason blended with instinct, i. 320. , on habitual nature of instincts, i. 320. - , on slave-making ante, i. 336. -. , on Melipona domestica, i. 343. Hudson, Mr., on the Ground- Wood- pecker of La Plata, i. 221. , on the Molothrus, i. 334. Humble-bees, cells of, i. 343. Hunter, J., on secondary sexual characters, i. 185. Hutton, Captain, on crossed geese, ii. 10. Huxley, Prof., on structure of her- maphrodites, i. 124. , on the affinities of the Sirenia, ii. 108. , on forms connecting birds and reptiles, ii. 108. 330 INDEX. LOBELIA FULGEXS.' Huxley, Prof., on homologous or- Kidney-bean, acclimatisation of, i. gans, ii. 233. , on the development of aphis, ii. 245. 177. Kidneys of birds, i. 178. Kirby, on tarsi deficient in beetles, Hybrids and mongrels compared, ii. 39. i. 163. Knight, Andrew, on cause of varia- Hybridism, ii. 1. tion, i. 8. Hydra, structure of, i. 229. Hvmenoptera, fighting, i. 108. Hvmenopterous msect,diviug, i. 222. Hyoseris, i. 271. Kolreuter, on Intercrossing, i. 119. , on the barberrv, i. 121. , on sterility of "hybrids, ii. 3, 4. , on reciprocal crosses, ii. 15. , on crossed varieties of nico- _ tiana, ii. 38. , on crossing male and herma- Ibla, i. 183. phrodite flowers, ii. 256. Icebergs transporting seeds, ii. 148. Increase, rate of, i. 79. L. Individuals, numbers favourable to selection, i. 124. , many, whether simultaneously Lamarck, on adaptive characters, ii. 218. created, ii. 139. Lancelot, i. 154 Inheritance, laws of, i. 15. , eyes of, i. 227. , at corresponding ages, L 15, Landois, on the development of the 105. wings of insects, i. 231. Insects, colour of, fitted for their Land-shells, distribution of, ii. 186. stations, i. 103. , of Madeira, naturalised, ii. , sea-side, colours of, i. 165. 193. , blind, in caves, i. 171. , resisting salt water, ii. 1 87. , luminous, i. 236. , their resemblance to certain Languages, classification of, ii. 214. Lankester. Mr. E. Kay, on Longe- objects, i. 283. vity, i. 263. , neuter, i. 359. ,*on homologies, ii. 237. Instinct, i. 319. , not varying simultaneously Lapse, great, of time, ii. 51. Larvae, ii. 241, 242, 243. with structure, i. 357. Laurel, nectar secreted by the leaves, Instincts, domestic, i. 325. i. 114. Intercrossing, advantages of, i. 149, Laurentian formation, ii. 84. ii. 27. Laws of variation, i. 164. Islands, oceanic, ii. 177. Leech, varieties of, i. 93. Isolation favourable to selection, i. Leguminosse, nectar secreted by 127. glands, i. 114. J. Leibnitz 1 attack on Newton, ii. 294. Lepidosiren, i. 130 ; ii. 109. Japan, productions of, ii. 158. , limbs in a nascent condition, Java, plants of, ii. 162. ii. 258. Jones, Mr. J. M., on the birds of Lewes, Mr. G. II., on species not Bermuda, ii. 180. Jourdain, M., on the eye-spots of star-fishes, i. 225. Jukes. Prof., on subaerial denuda- having changed in Egypt, i. 263. , on the Salamandra atra, ii. 256. , on many forms of life having been at first evolved, ii. 300. tion, ii. 53. Life, strutrjrle for, i. 77. Jussieu, on classification, ii. 209. K. Lingula. Silurian, ii. 83. Linmeus, aphorism of, ii. 205. Lion, mane of, i. 109. , vouns of, striped, ii. 241. Kentucky, caves of, i. 172. Kerguelcn-land, flora of, ii. 169, 189. Lobefia fulgens, i. 90, 121. , sterility of crosses, ii. 7. LOCKWOOD. INDEX. 331 Lockwood, Mr., on the ova of the Hippocampus, i. 295. . Locusts transporting seeds, ii. 147. Logan, Sir W., oil Laurentian for- Mncleay, on analogical characters, ii. 218. mution, ii. 84. Macrauchenia, ii. 107. Lowe, Kev. K. T., on locusts visiting Madeira, ii. 147. M'l )<>nnell, Dr., on electric organs, i. _':;. Lowness of structure connected with Madeira, plants of. i. 130. variability, i. 1S4. , related to wide distribution, ii. , beetles of, wingless, i. 169. , fossil land-shells of, ii. 121. 19H. , birds of, ii. 180. Lubbock, Sir J., on the nerves of Magpie tame in Norway, i. 325. coccus, i. 54. , on secondary sexual charac- Males lighting, i. 108. Maize, crossed, ii. 37. ters, i. 193. , on a diving hymenopterous Malay Archipelago compared with Europe, ii. 74. insect, i. 222. , mammals of, ii. 185. , on affinities, ii. 73. Malm, on flat-fish, i. 291. , on metamorphoses, ii. 239, 242. Lucas, Dr. P., on inheritance, i. 14. Malpighiaceee, small imperfect flow- ers of, i. 269. , on resemblance of child to , ii. 209. parent, ii. 43. Lund and Clausen, on fossils of Mammae, their development, i. 295. , rudimentary, ii. 255. Brazil, ii. 121. Mammals, fossil, in secondary for- Lyell, Sir C., on the struggle for mation, ii. 79. existence, i. 77. , insular, ii. 188. , on modern changes of the Man, origin of, ii. 304. earth, i. 118. Manatee, rudimentary nails of, ii. , on terrestrial animals not hav- 260. ing been developed on islands, i. Marsupials of Australia, i. 140. 281. , structure of their feet, ii. 232. , on a carboniferous land-shell. , fossil species of, ii. 121. ii. 59. Martens, M., experiment on seeds, , on strata beneath Silurian sys- ii. 144. tem, ii. 84. Martin, Mr. W. C., on striped mules, , on the imperfection of the geo- i. 201. logical record, ii. 88. Masters, Dr., on Saponaria, i. 272. , on the appearance of species, Matteucci, on the electric organs of ii. 88. , on Barrande's colonies, ii. 90. rays, i. 234. Matthiola, reciprocal crosses of, ii. , on tertiary formations of 15. Europe and North America, ii. Maurandia, i. 307. 101. , on parallelism of tertiary for- Means of dispersal, ii. 140. Melipona domestica, i. 343. mations, ii. 106. Merrcll, Dr., on the American , on transport of seeds by ice- cuckoo, i. 330. bergs, ii. 148. , on great alterations of climate, ii. 170. Metamorphism of oldest rocks, ii. 85. Mice destroying bees, i. 90. , acclimatisation of, i. 175. , on the distribution of fresh- , tails of, i. 294. water shells, ii. 174. Miller, Prof., on the cells of bees, i. , on land-shells of Madeira, ii. 344, 350. 193. Mirabilm, crosses of, ii. 15. Lyell and Dawson, on fossilized trees Missel-thrush, i. 93. 'in Nova Scotia, ii. 7<. Lvthrum salicaria, trimorphic, ii. Mistletoe, complex relations of. i. 3. Mivart, Mr., on the relation ol hair 32. and teeth, i. 179. 332 INDEX. Mivart, Mr., on the eyes of.cephalo- Murray, Mr. A., on cave-insects, pods, i. 237. i. 173. .various objections to Natural Mustela vision, i. 216. Selection, i. 275. Myanthus, ii. 216. , on abrupt modifications, i. 313. , on the resemblance of the Myrmecocystus, i. 359. Myrmica, eyes of, i. 361. mouse and antechinus, ii. 218. Mocking-thrush of the Galapagos, N. ii. 193. Modification of species not abrupt, Nageli, on morphological characters, ii. 298. i. 266. Moles, blind, i. 170. Molothrus, habits of, i. 334. Nails, rudimentary, ii. 260. Nathusius, Von, on pigs, i. 249. Mongrels, fertility and sterility of, Natural history, future progress of, ii. 34. ii. 301. and hybrids compared, ii. 39. Monkeys, fossil, ii. 79. selection, i. 97. system, ii. 201. Monachanthus, ii. 216. Naturalisation of forms distinct Mons, Van, on the origin of fruit- trees, i. 33. from the indigenous species, i. 138. Naturalisation in New Zealand, i. Monstrosities, i. 51. 255. Moquin-Tandon, on sea-side plants, Naudin, on analogous variations in i. 166. gourds, i. 195. Morphology, ii. 231. , on hybrid gourds, ii. 38. Morren, on the leaves of Oxalis, i. , on reversion, ii. 4i. 308. Moths, hybrid, ii. 9. Nautilus, Silurian, ii. 83. Nectar of plants, i. 114. Mozart, musical powers of, i. 321. Mud, seeds in, ii. 175. Nectaries, how formed, i. 114. Nelumbium luteum, ii. 176. Mules, striped, i. 201. Nests, variations in, i. 324, 355, 364. Muller, Adolf, on the instincts of the Neuter insects, i. 359, 360. cuckoo, i. 331. Newman, Col., on humble-bees, i. 90. Muller, Dr. Ferdinand, on Alpine New Zealand, productions of, not Australian plants, ii. 163. Muller, Fritz, on dimorphic crusta- ceans, i. 55, 362. perfect, i. 255. , naturalised products of, ii. 119. , fossil birds of, ii. 121. , on the lancelet, i. 154. , glaciers of, ii. 159. , on air-breathing crustaceans, , crustaceans of, ii. 164. i. 238. , algae of, ii. 1(54. , on climbing plants, i. 307. , on the self-sterility of orchids, , number of plants of, ii. 178. , flora of, ii. 189. ii. 7. Newton, Sir I., attacked for irre- , on embryology in relation to ligion, ii. 294. classification, ii. 210. , Prof., on earth attached to a , on the metamorphoses of crus- taceans, ii. 245, 253. partridge's foot, ii. 148. Nicotiana, crossed varieties of, ii. 39. , on terrestrial and fresh-water , certain species very sterile, ii. organisms not undergoing any metamorphosis, ii. 250. Multiplication of species not indefi - 14. Nitsche, Dr., on the Polyzoa, i. 301. Noble, Mr., on fertility of Rhodo- nite, i. 157. dendron, ii. 8. Murchison, Sir R., on the forma- tions of Russia, ii. 60. Nodules, phosphatic, in azoic rocks, ii. 84. , on azoic formations, ii. 84. , on extinction, ii. 94. . Murio, Dr., on the modification of the skull in old age, i. 233. Oaks, variability of, i. 62. Onites, appelles, i. 108. INDEX. PLANTS. 333 Ouonis, small imperfect flowers of, Palm with hooks, i. 247. i. 209. Papaper bructc-atum, i. -J72. Orohids, fertilisation of, i. 241. Paraguay, cattle destroyed by flies, , the development of their i. 89. flowers, i. 303. Parasites, i. 334. , forms of, ii. 216. Partridge, with ball of earth at- Orchis, pollen of, i. 236. tached to foot, ii. 148. Organisation, tendency to ad vance, Parts greatly developed, variable, i. 151. L 185. Organs of extreme perfection, i. 223. , electric, of tislies, i. 234. Parus major, L 220. Passiflora, ii. 7. of little importance, i. 245. Peaches in United States, i. 104. , homologous, ii. 233. Pear, grafts of, ii. 18. , rudiments of, and nascent, ii. Pedicellaria, i. 298. 255. < )rnithorhynchus, i. 130; ii. 208. Pelagornium, flowers of, L 180. , sterility of, ii. 7. , mamma of, i. 296. Pelvis of women, i. 178. Ostrich not capable of flight, i. 281. , habit of laying eggs together, i. 335. Peloria, i. 180. Period, glacial, ii. 151. Petrels, habits of, i. 221. , American, two species of, ii. Phasianus, fertility of hybrids, ii. 9. 132. Pheasant, young, wild, i. 329. Otter, habits of, how acquired, i. Pictet, Prof., on groups of species 216. Ouzel, water, i. 222. suddenly appearing, ii. 77. , on rate ot'orgauic change, ii. 90. Owen, Prof., on birds not flying, L , on continuous succe.ssion of 167. genera, ii. 93. , on vegetative repetition, i. , on change in latest tertiary 184. forms, ii. 71. , on variability of unusually , on close alliance of fossils in developed parts, i. 185. , on the eyes of fishes, i. 227. consecutive formations, ii. 114. , on early transitional links, ii. , on the swim-bladder of fishes, 78. i. 231. Pierce, Mr., on varieties of wolves, , on fossil horse of La Plata, ii. i. 111. 96. , on generalized form, ii. 107. Pigeons with feathered feet and skin between toes, i. 14. , on relation of ruminanta and , breeds described, and origin pachyderms, ii. 107. , on fossil birds of New Zea- of, i. 23. , breeds of, how produced, i. 44, land, ii. 121. 47. , on succession of types, ii. 121. , on affinities of the dugong, ii. 206. , on homologous organs, ii. 233. , tumbler, not being able to get out of egg, i. 106. , reverting to blue colour, i. 197. , instinct of tumbling, i. 327. , on the metamorphosis of ce- , voung of, ii. 248. phalopods, ii. 244. Pigs, 'black, not affected by the paint-root, i. 13. P. , modified by want of exercise, i. 24ft. Pacific Ocean, faunas of, ii. 131. Pistil, rudimentary, ii. 256. Pacini, on electric organs, i. 235. Paley, on no organ formed to give Plants, poisonous, not affecting cer- tain coloured animals, i. 13. pain, i. 254. , selection, applied to, i. 41. Pallas, on the fertility of the domes- ticated descendants of wild stocks, , gradual improvement of, i. 42. , not improved in barbarous ii.10. countries, i. 43. 334 INDEX. SALT WATER. Plants, dimorphic, i. 55 ; ii. 29. Ramond, on plants of Pyrenees, ii. , destroyed by insects, i. 83. 153. , in midst of range, have to Ramsav, Prof., on subaerial denu- struggle with other plants, i. 95. datio'n, ii. 53. , nectar of, i. 114. , on thickness of the British , fleshy, on sea-shores, i. 166. formations, ii. 55, 56. , climbing, i. 230, 305. , on faults, ii. 55. , fresh-water, distribution of, ii. Ramsay, Mr., on instincts of cuckoo, 174. i. 333. , low in scale, widely distri- Ratio of increase, i. 79. buted, ii. 196. Pleuronectidffi, their structure, i. Rats supplanting each other, i. 93. , acclimatisation of, i. 175. 290. , blind, in cave, i. 171. Plumage, laws of change in sexes Rattle-snake, i. 254. of birds, i. 109. Plums in the United States, L 104. Pointer dog, origin of, i. 40. , habits of, i. 327. Reason and instinct, 5. 319. Recapitulation, general, ii. 267. Reciprocity of crosses, ii. 14. Record, geological, imperfect, ii. 48. Poison not affecting certain coloured Rengger, on flies destroying cattle, animals, i. 13. i. 89. , similar effect of, on animals Reproduction, rate of, i. 79. and plants, ii. 299. Resemblance, protective, of insects, Pollen of fir-trees, i. 257. i. 283. transported by various means, to parents in mongrels and i. 241, 252. hybrids, ii. 41. Pollinia, their development, i. 304. Reversion, law of inheritance, i. Polyzoa, their avicularia, i. 301. 16. Poole, Col., on striped hcmiouus, i. , in pigeons, to blue colour, i. 202. 198. Potemogeton, ii. 175. Pouchet, on the colours of flat-fish, Rhododendron, sterility of, ii. 7, 8. Richard, Prof., on Aspicarps, ii. 209. i. 293. Richardson, Sir J., on structure of Prestwich, Mr., on English and squirrels, i. 216. French eocene formations, ii. 105. , on fishes of the southern hemi- Proctotrupcs, i. 222. sphere, ii. 164. Proteolepas, i. 183. Proteus, i. 173. Robinia, grafts of, ii. 19. Rodents, blind, i. 170. Psychology, future progress of, ii. Rogers, Prof., Map of N. America, 304. ii. 65. Pyrgoma, found in the chalk, ii. 81. Rudimentary organs, ii. 255. Rudiments important for classifica- tion, ii. 207. Q. Riitimeyer, on Indian cattle, i. 21 ; Quagga, striped, i. 201. ii. 10. Quatrefages, M., on hybrid moths, S. ii. 9. Quercus, variability of, i. 62. Quince, grafts of, ii. 18. Salamandra atra, ii. 256. Saliva used in nests, i. 355. Salvin, Mr., on the beaks of ducks, R. i. 287. Sageret. on grafts, ii. 18. Rabbits, disposition of young, i. 328- Races, domestic, characters of, i. 18. Salmons, males lighting, and hooked jaws of, i. 108. Race-horses, Arab, i. 40. Salt water, how far injurious to , English, ii. 140. seeds, ii. 142. Radcliffe, Dr., the electrical organs not destructive to land-shells, of the torpedo, i. 234. ii. 187. INDEX. STAR-FISHES. 335 Salter, Mr., on early death of hybrid Shells, fresh-water, dispersal of, ii. embryos, ii. -.ri. 173. Saurophagus sulphuratus, i. 220. Schacnt, Prof., oil Phyllotuxy, i. , of Madeira, ii. 180. , land, distribution of, ii. 180. 270. , land, resisting salt water, ii. Schiodte. on blind insects, i. 172. m. , on flat-fish, i. 290. Shrew-mouse, ii. 218. Sehlegel, on snakes, i. 178. Silene, infertility of crosses, ii. 14. SchObI, Dr., on the ears of mice, i. Silliman, Prof., on blind rat, i. 171. 268. Sireiiiu, their affinities, ii. 108. Scott, J., Mr., on the self-sterility of Sitaris, metamorphosis of, ii. 252. orchids, ii. 7. Skulls of young mammals, i. 248: , on the crossing of varieties of verbascum, ii. 38. ii. 235. Slave-making instinct, i. 336. Sea-water, how far injurious to Smith, Col. Hamilton, on btriped seeds, IK 14:>. horses, i. 200. not destructive to land-shells, , Mr. Fred., on slave-making ii. 187. ants, i. 337. Sebright, Sir J., on crossed animals, i. 23. , on neuter ants, i. 360. Smitt, Dr., on the Polyzoa, i.*301. Sedgwick, Prof., on groups of spe- cies suddenly appearing, ii. 77. Seedlings destroyed by insects, i. Snake with tooth for cutting through egg-shell, i. 334. Somerville, Lord, on selection of 83. sheep, i. 35. Seeds, nutriment in, i. 94. Sorbus, grafts of, ii. 19. , winged, i. 181. Sorex, ii. 218. , means of dissemination, i. Spaniel, King Charles's breed, i. 40. 240,252; ii. 146. , power of resisting salt water, Specialisation of organs, i. 152. Species, polymorphic, i. 54. ii. 143. , dominant, 1. '17. , in crops and intestines ot , common, variable, i. 66. birds, ii. 146. in large genera variable, i. 6!>. , eaten by fish, ii. 146, 176. , in mud, ii. 175. , groups of, suddenly appear- ing, ii. 77, 82. , hooked, on islands, ii. 181. beneath Silurian formations, Selection of domestic products, i. "4. ii. 84. , principle not of recent origin, successively appearing, ii. 89. i. 3'J. changing simultaneously , unconscious, i. 39. throughout the world, ii. 100. , natural, i. 97. Spencer, Lord, on increase in size of , sexual, i. 107. cattle, i. 40. , objections to term, i. 99. natural, has not induced steri- , Herbert, Mr., on the first steps in differentiation, i. 155. lity, ii. 20. , on the tendency to an equili- Sexes, relations of, i. 108. brium in all forces, ii. 29. Sexual characters variable, i. 191. selection, i. 107. Sphex, parasitic, i. 336. Spiders, development of, ii. 245. Sheep, Merino, their selection, i. 36. Sports in plants, i. 11. , two sub- breeds, unintention- ally produced, i. 41. , mountain varieties of, i. 93. Sprengel, C. C., on crossing, i. 119. , on ray-florets, i. 180. Squalodon, ii. 108. Shells, colours of, i. 165. Squirrels, gradations in structure, i. , hinges of, i. 240. SU. , littoral, seldom embedded, ii. Staffordshire, heath, changes in, i. 58. 87. , fresh -water, long retain the Stag-beetles, fighting, i. 108. Barne forms, ii. 117. Star-fishes, eyes of, i. 225. 336 STAR-FISHES. INDEX. Star-Fishes, their pedicellarias, i. 299. Sterility from changed conditions of life, i. 10. Thouin, on grafts, ii. 19. Thrush, aquatic species of, i. 222. , mocking, of the Galapagos, ii. of hybrids, ii. 3. 192. , laws of, ii. 11. , causes of, ii. iiO. , young of, spotted, ii. 241. , nest of, i. 364. , from unfavourable conditions, Tliuret, M., on crossed fuci, ii. 15. ii. 26. Thwaites, Mr. on acclimatisation, not induced through natural i. 174. selection, ii. 21. Thylacinus, ii. 220. St. Helena, productions of, ii. 178. Tierra del Fuego, dogs of, i. 328. St. Hilaire, Aug., on variability of certain plants, i. 272. , plants of, ii. 169. Timber-drift, ii. 145. .on classification, ii. 209. Time, lapse of, ii. 51. St. John, Mr., on habits of cats, i. by itself sot causing modifica- 325. tion, i. 126! Sting of bee, i. 256. Titmouse, i. 220. Stocks, aboriginal, of domestic ani- Toads on islands, ii. 182. mals, i. 22. Tobacco, crossed varieties of, ii. 38. Strata, thickness of, in Britain, ii. 55. Tomes, Mr., on the distribution of Stripes on horses, i. 199. Structure, degreesof utility of, i. 249. bats, ii. 184. Transitions in varieties rare, i. 208. Struggle for existence, i. 75. Succession, geological, ii. 89. Traquair, Dr., on flat-fish, i. 293. Trautschold,on intermediate varie- of types in same areas, ii 121. ties, ii. 66. Swallow, one species supplanting another, i. 93. Trees on islands belong to peculiar orders, ii. 182. Swaysland, Mr., on earth adhering to the feet of migratory birds, ii. with separated sexes, i. 123. Trifolium pratense, i. 90, 117. 148. incarnatum, i. 117. Swifts, nests of, i. 355. Trigonia, ii. 99. Swim-bladder, i. 230. Trilo bites, ii. 83. Switzerland, lake habitations of, i. , sudden extinction of, ii. 99. 20. Trimen, Mr., on imitatiug-insects, System, natural, ii. 204. ii. 224. T. Trimorphism in plants, i. 55 ; ii. 29. Troglodytes, i. 864. Tuco-tuco, blind, i. 170. Tail of giraffe, i. 245. Tumbler pigeons, habits of, heredi- of aquatic animals, i. 246. tary, i. 327. , prehensile, i. 294. , young of, ii. 248. , rudimentary, ii. 260. Turkey-cock, tuft of hair on breast, Tanais, dimorphic, i. 55. i. lib. Tarsi, deficient, i. 168. , naked skin on head, i. 248. Tausch, Dr., on umbelliferfe, i. 271. . voung of, instinctively wild, Teeth and hair correlated, i. 179. i.329. , rudimentary, in embryonic, calf, ii. 255, 292. Turnip and cabbage, analogous variations of, i. 1 95. Tegetmeier, Mr., on cells of bees, i. Type, unity of, i. 260, 261. 346, 352. Types, succession of, in same areas, Temminck, on distribution aiding ink classification, ii. 211. Typotherium, ii. 108. Tendrils, their development, i. 305. Thompson, Sir W., on the age of U 1 the habitable world, ii. 83. . , on the consolidation of the Udders enlarged bv use, i. 12. crust of the earth, ii. 275. , rudimentary, ii. 256. INDEX. 337 Ulex, young leaves of, ii. 241. Umbelliferaj, flowers and seeds of, Wagner, Moritz, on the importance of isolation, i. 1 L'7. i. 180. ^ Wallace, Mr., on origin of species, , outer and inner florets of, i. '270. i. 2. I'nity of type, i. 260,261. , on the limit of variation under I'ria'laorymans, i. 113. domestication, i. 48. I'M-, eiteete of, under domestication, , on dimorphic lepidoptcra, i. i. 12. 55, 862. , effects of, in a state of nature, , on races in the Malay Archi- i. 167. pelago, i. 58. Utility, how far important in the , on the improvement of the construction of each part, i. 2-iy. eye, i. 227. , on the walking-stick insect, i. 284. , on laws of geographical dis- Valenciennes, on fresh-water fish, tribution, ii. 139. ii. 173. , on the Malay Archipelago, ii. Variability of mongrels and hy- 185. l.rids, ii. 39. , on mimetic animals, ii. 224. Variation under domestication, i. 8. Walsh, Mr. B. D., on phytophagic caused by reproductive system being affected by conditions of life, i. 10. forms, i. 60. , on equal variability, i. 195. Water, fresh, productions of, ii. 171. under nature, i. 51. Water-hen, i. 222. ,laws of, i. 104. Waterhouse, Mr., on Australian , correlated, i. 13, 177, 248. marsupials, i. 140. Variations appear at corresponding , on greatly developed parts ages, i. 16, 105. being variable, i. 185. analogous in distinct species, , on the cells of bees, i. 343. i. 193. Varieties, natural, i. 50. , on general affinities, ii. 227. Water-ouzel, i. 222. , struggle between, i. 93. , domestic, extinction of, i. 134. Watson, Mr. H. C., on range of varieties of British plants, i. 57, , transitional, rarity of, i. 208. , when crossed, fertile, ii. 34. 73. , on acclimatisation, i. 134. Varieties, when crossed, sterile, ii.37. , on flora of Azores, ii. 149. , classification of, ii. 215. , on Alpine plants, ii. 153. Verbascum, sterility of, ii. 7. , varieties of crossed, ii. 38. , on rarity of intermediate va- rieties, i. 212. Verlot. M.. on double stocks, i. 358. , on convergence, i. 156. Verneuil, M. de, on the succession , on the indefinite multiplica- of species, ii. 103. tion of species, i. 157. Vibracula of the Polyzoa, i. 301. Weale, Mr., on locusts transporting Viola, small imperfect flowers of, seeds, ii. 147. i. 2(59. Web of feet in water-birds, i. 223. . tricolor, i. 90. Weismann, Prof, on the causes of Virchow, on the structure of the crystalline lens, i. 227. Virginia, pigs of, i. 104. variability, i. 8. , on rudimentary organs, ii. 260. West Indian Island's, mammals of. Volcanic islands, denudation of, ii. ii. 185. 54. Vulture, naked skin on head, i. 247. West wood, on species in large gen- era being closely allied to others, i. 71. W. , on the tarsi of Engidfe, i. 192. , on the antennse of hymeno- Wadincr-birds, ii. 175. pterous insects, ii. 207. Wagner, Dr., on Cecidomyia, ii. 239. Whales, i. 285. 338 INDEX. ZEUGLODOX. Wheat, varieties of, i. 137. Woodward, Mr., on the duration of White Mountains, flora of, ii. 151. specific forms, ii. 66. Whittaker, Mr., ou lines of escarp- , ou Fyrgoma, ii. 81. . ment, ii. 53. , on the continuous succession Wichura, Max, on hybrids, ii. 24, of genera, ii. 93. 27, 41. , on the succession of types, ii. Wings, reduction of size, i. 169. of insects homologous with 121. World, species changing simultane- branchiae, i. 231. ously throughout, ii. 100. , rudimentary, in insects, ii. W reus, nest of, i. 304. 255. Wright, Mr. Chauncey, on the gi- Wolf crossed with dog, i. 327. ratfe, i. 278. of Falkland Isles, ii. 1S3. , on abrupt modifications, i. Wollastou, Mr., on varieties of in- 310. sects, i. 59. Wyman, Prof., on correlation of , on fossil varieties of shells in Madeira, i. 65. colour and effects of poison, i. 13. , on the cells of the bee, i. 345. , on colours of insects on sea- shore, i. 165. Y. , on wingless beetles, i. 109. , on rarity of intermediate va- Youatt, Mr., on selection, i. 35. rieties, i. 212. , on sub-breeds of sheep, i. 41. , on insular insects, ii. 178. , on rudimentary horns in , on land-shells of Madeira nat- young cattle, ii. 261. uralised, ii. 193. Wolves, varieties of, i. 111. Z. Woodcock with earth attached to leg, ii. 148. Zanthoxylon, i. 272. Woodpecker, habits of, i. 220. , green colour of, i. 247. Zebra, stripes ou, i. 199. Zeuglodon, ii. 108. THE END. 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