OUTLINES OF 
 EVOLUTIONARY BIOLOGY
 
 OUTLINES OF 
 EVOLUTIONARY BIOLOGY 
 
 BY 
 
 ARTHUR BENDY, D.Sc., F.R.S., 
 
 Professor of Zoology in the University of London (King's College) ; 
 
 Zoological Secretary of the Linne'an Society of London ; 
 
 Honorary Member of the New Zealand Institute ; formerly 
 
 Professor of Biology in the Canterbury College 
 
 (University of New Zealand), and Professor of 
 
 Zoology in the South African College, 
 
 Cape Town. 
 
 LONDON 
 
 CONSTABLE & COMPANY LTD 
 
 10 ORANGE STREET LEICESTER SQUARE W.C. 
 1912
 
 PEEFACE 
 
 BIOLOGY, the fundamental science of living things in all their 
 manifold relations, is a study which, at the present time, is but 
 little encouraged by educational authorities in this country. It 
 has no place in the ordinary school curriculum and even in our 
 Universities it has been thrust into the background, partly 
 because University authorities devote so much of their attention 
 nowadays to subjects which are considered more likely to 
 bring in a direct pecuniary reward to the student, and partly 
 because of the immense elaboration of the various branches of 
 biological science, such as Zoology, Botany, Physiology, Com- 
 parative Anatomy and Embryology, that has taken place in 
 recent years, and the claims of these to more or less separate 
 recognition. 
 
 The student, if he studies Biology at all for its own sake, 
 which is seldom the case, usually confines himself almost 
 entirely to one or other of these branches, which he finds 
 treated more or less as an independent science, with an extensive 
 literature of its own, and he runs a grave risk of losing sight of 
 the general principles which underlie all and from which all 
 derive their chief educational value. 
 
 The medical student, it is true, usually takes a year's course 
 in what is called Biology, but his curriculum is, perhaps 
 unavoidably, dominated by the type-system and by what is 
 thought likely to be of direct service to him in his future 
 anatomical and physiological studies, so that in the brief time 
 which the medical authorities allow him to devote to the 
 scientific foundation of his professional work he has but little 
 opportunity for a philosophical treatment of the subject. 
 
 When we at length come to realize the meaning of Man's 
 position as, for the time being, the highest term of a great 
 evolutionary series which stretches far back into the dawn of
 
 vi PEE FACE 
 
 the earth's history, and to appreciate the importance of the fact 
 that he derives his existence from the same ultimate sources and 
 is subject to the same natural laws as all the other living things 
 with which he shares the earth, we shall perhaps see the 
 necessity for making Biology, in the widest sense of the term, 
 one of the foundation stones of our educational system. 
 
 In the meantime those who wish to familiarize themselves 
 with the rapidly accumulating results of biological investigation 
 and the bearing of these results upon human problems ought 
 not to be debarred from so doing by want of the necessary 
 knowledge of fundamental facts and principles, and it is largely 
 with a view to the requirements of such students that the present 
 work is offered to the public. 
 
 That even the elementary study of biological theory should, 
 wherever possible, be preceded by a systematic course in Zoology 
 and Botany, based upon the type-system and including laboratory 
 work, is, no doubt, indisputable. Unfortunately, under existing 
 conditions, regular laboratory work is, for most people, im- 
 possible. We are apt to forget, however, that in reality we all 
 of us spend our lives in a biological laboratory, where we are 
 surrounded by living organisms which we can hardly avoid 
 studying. In this way we learn much of the nature of living- 
 things and are to some extent prepared for the study of biological 
 principles. 
 
 The problems of life, however, cannot be satisfactorily solved 
 if we confine our attention to the higher and more familiar forms 
 of plants and animals. Man, in particular, is far too complex a 
 type to begin with in a philosophical treatment of the subject. 
 The logical method of study is, no doubt, to follow as closely as 
 possible the course which we believe to have been taken in the 
 actual evolution of living things, beginning with the simple and 
 ending with the complex. This method, of course, is attended 
 with certain practical difficulties, mainly due to the microscopic 
 size of the more primitive organisms, but these difficulties are 
 not insurmountable and need not be considered in relation to the 
 present work. 
 
 As I wish this book to be of use to those who have had no 
 special biological training, as well as to students who have taken 
 the ordinary first year's course, I have, in the earlier chapters, 
 dealt in a very elementary manner with the structure and 
 functions of both plants and animals. I have described Amoeba
 
 PREFACE vii 
 
 and Hfematococcus in considerable detail and used these familiar 
 organisms as pegs on which to hang some elementary ideas with 
 regard to the nature of living things and the differences between 
 animals and plants. Otherwise I have as far as possible avoided 
 the type-system as being altogether unsuitable for a work of 
 this kind, though of course I have been obliged to refer to 
 numerous different organisms in illustration of special points. 
 
 It was only by rigidly excluding from the earlier part of the 
 book everything that was not considered essential to the under- 
 standing of general principles that it has been possible to find 
 space for even a brief presentation of the evidence upon which 
 the theory of organic evolution rests, and for a discussion of the 
 principal factors which appear to have co-operated in determining 
 the course of that evolution. 
 
 Although the entire work is intended to be of an elementary 
 character, it has been impossible, in connection with the theory 
 of heredity, to avoid, on the one hand, a considerable amount of 
 cytological detail, and, on the other, some discussion of theoretical 
 speculations of a highly controversial nature. In dealing with 
 these vexed questions, which underlie the whole problem of 
 organic evolution, I have endeavoured to present the views of 
 opposing schools of thought as fairly as possible, but I must 
 confess that I have ventured to lay considerable stress upon 
 ideas which, though widely accepted elsewhere, have not as yet 
 met with much appreciation in this country, though that they 
 will do so in the future can hardly be doubted. 
 
 By way of introduction to the discussion of the factors of 
 organic evolution a chapter has been devoted to the views of 
 Buffon, Erasmus Darwin and Lamarck, and another to those of 
 Charles Darwin, Robert Chambers and Alfred Russel Wallace, 
 and I have endeavoured to present the opinions of these classical 
 authors as far as possible by means of quotations from their 
 own writings. 
 
 In a work such as the present the employment of numerous 
 technical terms is, of course, unavoidable, but it is hoped that 
 the meaning of these is sufficiently explained in the text, and the 
 use of the index should obviate any difficulties in this respect, 
 especially if the book is read systematically from the first chapter 
 onwards. 
 
 It is a pleasure to record my thanks to many colleagues who 
 have ungrudgingly helped me in various ways. Amongst these
 
 viii PREFACE 
 
 I should like especially to mention Professor Poulton, Professor 
 Herbert Jackson, Dr. Stapf, Dr. Daydon Jackson, Dr. Sibly and 
 Professor G. E. Nicholls ; while to Mr. E. W. H. Eow, of the 
 Zoological Department at King's College, I am much indebted 
 for the trouble which he has taken in reading through the 
 proof-sheets and for many valuable suggestions. 
 
 I owe a sincere acknowledgment to my publishers, Messrs. 
 Constable & Co., for their generosity in the matter of illustrations. 
 Most of these are either entirely new or, in some cases, specially 
 re-drawn from original memoirs. I have made extensive use of 
 photomicrography for microscopic subjects, and in the prepara- 
 tion of the photographs at King's College have received much 
 assistance from Dr. Eosenheim, Mr. Eow, and my assistant 
 Mr. Charles Biddolph. 
 
 For the loan of the blocks from which the remainder of the 
 illustrations have been printed, or for permission to copy, I am 
 indebted to the generosity of the following : 
 
 Messrs. George Allen and Son, 
 
 " The American Journal of Science," 
 
 Mr. Edward Arnold, 
 
 Messrs. George Bell and Sons, 
 
 Messrs. A. and C. Black, 
 
 The Cambridge University Press, 
 
 The Clarendon Press, 
 
 Messrs. Constable & Co., 
 
 Messrs. Duckworth & Co., 
 
 Herr W. Engelmann, 
 
 Herr Gustav Fischer, 
 
 " The Journal of Experimental Zoology,' 
 
 Messrs. Kegan Paul, Trench, Triibner & Co., 
 
 The Council of the Linnean Society of London, 
 
 Messrs. Longmans, Green & Co., 
 
 Mr. John Murray, 
 
 " The Quarterly Journal of Microscopical Science," 
 
 The Council of the Eay Society, 
 
 Messrs. Smith, Elder & Co., 
 
 The Trustees of the British Museum, 
 
 Messrs. T. Fisher Unwin, 
 
 Messrs. F. Warne & Co., 
 
 Verlagdes Bibliographischen Instituts, Leipzig und'Wien.
 
 PREFACE ix 
 
 I must also express my gratitude to the numerous authors 
 whose work I have made use of, and whose names are mentioned 
 in the appropriate places. 
 
 I am indebted to the Council of the Royal Society of Arts for 
 permission to make use of the Aldred Lecture which I delivered 
 before the Society in 1909, and which is to a large extent 
 reprinted from their Journal in Chapter XIV. 
 
 ARTHUR DENDY. 
 
 KING'S COLLEGE, LONDON, 
 December, 1911.
 
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 CONTEXTS 
 
 PART L THE STRUCTURE AND FUNCTIONS OF 
 ORGANISMS THE CELL THEORY 
 
 CHAPTER I 
 
 MOT 
 
 Introductory: The nature of life The living organism viewed as a 
 machine The essential functions of the living body The source 
 of energy in living things 1 
 
 CHAPTER n 
 Amoeba as a typical organism The properties of protoplasm . . . 13 
 
 CHAPTER TTT 
 Hjematocoecus The differences between animals and plants . . 27 
 
 CHAPTER IV 
 
 The cell theory Unicellular organisms Differentiation and division 
 of labour Co-operation The transition from the unicellular to 
 the multicellular condition The early development of niulticeilular 
 animals and plants 36 
 
 CHAPTER V 
 
 The cell theory as illustrated by the histologies! structure of the higher 
 animal* and plants Limitations of the cell theory The cell as 
 the physiological unit 51 
 
 CHAPTER YI 
 The multiplication of cells Alitotic and amitotic nuclear division . 69 
 
 PART H. THE EVOLUTION OF SEX 
 
 CHAPTER \TI 
 
 Limitation of the powers of cell-division Rejuvenescence by conjuga- 
 tion of gametes The origin of sex in the Protista ... 81
 
 xii CONTEXTS 
 
 CHAPTER VIII 
 
 PAGE 
 
 Sexual phenomena in multicellular plants The distinction between 
 somatic cells and germ cells Alternation of sexual and asexual 
 generations Suppression of the gametophyte in flowering plants . 95 
 
 CHAPTER IX 
 
 Sexual phenomena in multicellular animals Structure and life history 
 of Hydra and Obelia Alternation of generations The coelomate 
 type of structure Secondary sexual characters The evolution 
 of sex 113 
 
 CHAPTER X 
 
 Origin of the germ cells in multicellular animals Maturation of the 
 germ cells Reduction of the chromosomes Sex determination in 
 insects Different forms of gametes Mutual attraction of the 
 gametes Fertilization and parthenogenesis . . . . .129 
 
 PART III. VARIATION AND HEREDITY 
 CHAPTER XI 
 
 Variation Meristic and substantive variations Fluctuations and 
 mutations Somatogenic and blastogenic variations Origin of 
 blastogenic variations . . . . . . . . .148 
 
 CHAPTER XII 
 
 Heredity General observations Darwin's theory of pangenesis and 
 "VVeismann's theory of the continuity of the genii plasm The 
 nucleus as the bearer of inheritable characters . . . . 161 
 
 CHAPTER XIII 
 
 The inheritance of acquired characters and the mnemic theory of 
 
 heredity 176 
 
 CHAPTER XIV 
 
 The Mendelian experiments in hybridization The doctrine of unit 
 characters and the purity of the gametes Galton's law of 
 inheritance . . . .... ... . 194 
 
 PART IV. THE THEORY AND EVIDENCE OF 
 ORGANIC EVOLUTION: ADAPTATION 
 
 CHAPTER XV 
 
 Organic evolution versus special creation Spontaneous generation and 
 
 biogenesis The origin of living things . . . . . .211
 
 CONTENTS xiii 
 
 CHAPTER XVI 
 
 PAGE 
 
 The continuity of life The conception of species -The principles of 
 
 taxonomy The taxonomic evidence of organic evolution . . 221 
 
 CHAPTER XVII 
 
 Connecting links Homology and analogy - Convergent evolution - 
 
 Change of function Vestigial structures Reversion . . . 232 
 
 CHAPTER XVIII 
 
 Ontogeny The recapitulation hypothesis Interpretation of the onto- 
 
 genetic record Palingenetic and csenogenetic characters . . 263 
 
 CHAPTER XIX 
 
 The stratified rocks Geological periods The age of the habitable earth 
 --The geological record The succession of the great vertebrate 
 groups 282 
 
 CHAPTER XX 
 
 Fossil pedigrees Ancestry of birds, horses, elephants and whales . 305 
 
 CHAPTER XXI 
 
 Geographical distribution Areas of distribution Barriers to migra- 
 tion Means of dispersal Changes in the physical conditions of 
 the earth's surface The evidence afforded by the study of geogra- 
 phical distribution with regard to the theory of organic evolution . 319 
 
 CHAPTER XXII 
 
 Adaptation to environment in animals Deep sea animals The coloura- 
 tion of animals Protective and aggressive resemblances Warning 
 colours Mimicry Epigamic ornamentation ..... 334 
 
 CHAPTER XXIII 
 
 Adaptation to the environment in plants Alpine plants, desert plants 
 and lianes The modification of flowers in relation to insect- 
 fertilization ........... 350 
 
 PART V. FACTORS OF ORGANIC EVOLUTION 
 
 CHAPTER XXIV 
 Views of Buffon , Erasmus Darwin and Lamarck ..... 365 
 
 CHAPTER XXV 
 
 Robert Chambers and the "Vestiges of Creation" Natural selection 
 
 The views of Charles Darwin and Alfred Russel Wallace . . 383
 
 xiv CONTENTS 
 
 CHAPTER XXVI 
 
 PAIJB 
 
 Selection not confined to the organic world Illustrations of the action 
 of natural selection in the struggle for existence Degeneration 
 Flightless birds Extermination of the Morioris Sedentary 
 animals Parasites Co-operation of natural selection and the 
 so-called Lamarckian factors of evolution The influence of 
 internal secretions upon growth Increase in size beyond the 
 limits of utility 395 
 
 CHAPTER XXVII 
 
 Artificial selection Continuous and single selection The mutation 
 theory of the origin of species Mutual adaptation Unit 
 characters Isolation Physiological selection Non-adaptive 
 characters The evolution of man 410 
 
 INDEX . 429
 
 OUTLINES OF EVOLUTIONARY 
 BIOLOGY 
 
 PART I. THE STRUCTURE AND FUNCTIONS 
 OF ORGANISMS THE CELL THEORY 
 
 CHAPTER I 
 
 Introductory : The nature of life The living organism viewed as a machine 
 The essential functions of the living body The source of energy in 
 living things. 
 
 THERE are many ways in which the extremely heterogeneous 
 materials of which the earth is composed may be classified. 
 Physicists often speak of them as solid, liquid or gaseous 
 respectively, but we know that one and the same substance may 
 exist in any of these conditions, changing from one to the other 
 in accordance with changing conditions of temperature and 
 pressure. Chemists, on the other hand, tell us that all sub- 
 stances are made up of certain definite chemical elements, 
 occurring free or in a state of combination with one another, 
 and that the almost infinite variety of gases, liquids and solids 
 with which we are familiar has arisen mainly from this power 
 of combination in very diverse but perfectly definite ways. The 
 tendency of modern research, however, is to show that even the 
 so-called chemical elements are not really so elementary in their 
 nature as has been supposed, and it is quite conceivable that 
 they may all have a common origin. 
 
 Our present purpose is to investigate a condition of matter 
 which altogether transcends the classifications of the chemist 
 and the physicist, a condition in which it exhibits those peculiar 
 phenomena which we regard as manifestations of Life, and the 
 study of which constitutes the science of Biology. It is not
 
 2 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 that living matter differs fundamentally from not-living matter 
 in chemical or physical characters. On the contrary we recog- 
 nize in it exactly the same chemical elements or rather some 
 of them as we find in other constituents of the earth, and 
 these elements appear to obey precisely the same chemical and 
 physical " laws " in both cases, but in the living body they are 
 associated and combined with one another in such a manner as 
 to give rise to substances much more complex than any found 
 outside the animal and vegetable kingdoms, and possessed of 
 peculiar properties which raise them to an altogether higher 
 plane of existence. The complex substances in question con- 
 stitute protoplasm, which is the one essential constituent of 
 every living thing, upon the peculiar properties of which the life 
 of the organism depends. 
 
 We may begin our investigations into the nature of this life 
 by examining the well-worn but none the less valuable analogy 
 of the flame of a candle. Chemists and physicists have taught 
 us that flame consists of incandescent matter, raised to a high 
 temperature by the process of combustion, or chemical union 
 with the oxygen of the atmosphere, and that the flame can exist 
 only so long as the combustion goes on and a sufficiently high 
 temperature is thereby maintained to render the burning matter 
 luminous, or in other words to produce those vibrations of the 
 invisible and intangible ether which we recognize as light. The 
 flame, then, is the outward and visible sign of certain chemical 
 and physical processes, of the action and reaction between the 
 material which is being burnt and the atmosphere which 
 surrounds it. 
 
 Similarly biologists have learnt to recognize life as the expres- 
 sion of the constant interaction which goes on between the living 
 organism and its environment, or, in the words of Herbert 
 Spencer, " the continuous adjustment of internal relations to 
 external relations." So far as we have yet been able to analyze 
 it, this interaction also consists of chemical and physical pro- 
 cesses, amongst which combustion plays a large part ; but the 
 whole business is vastly more complex than the processes 
 involved in the production of a flame, and so far many of its 
 details have defied analysis. 
 
 We may vary our analogy by looking upon the body of a living 
 organism, whether plant or animal, as an extremely elaborate 
 engine or machine, whose existence depends upon a perfect
 
 ORGANISMS AND MACHINES 3 
 
 adaptation to its environment, and whose action consists in 
 continual self -adjustment to changes in that environment. 
 What we call the life of the organism consists of the sum total 
 of all the activities which it thus exhibits. The question at once 
 arises : How, then, does a living organism differ from a mere 
 man-made machine ? and this question is one which it is by no 
 means easy to answer. An organism, however, is not merely a 
 piece of apparatus which has the power of maintaining itself for 
 a longer or shorter period in a state of equilibrium with its 
 environment and thereby preserving itself from destruction, for 
 it also has the power of reproducing its kind by a process of self- 
 multiplication. In the case of an artificial machine, where there 
 is little or no automatic adjustment, the forces of the environ- 
 ment very soon get the upper hand ; the metal work becomes 
 corroded by oxidation, or worn away by friction, and presently 
 the whole affair comes to a standstill. Oxidation and friction, 
 and innumerable other chemical and physical agencies also tend 
 to destroy the machinery of the living body, but for a longer or 
 shorter period they are held in check by automatic processes of 
 repair and renewal, and when the inevitable end does come it is 
 usually not until the organism has produced at least enough off- 
 spring to take its place in the struggle for existence. 
 
 One of the most brilliant writers of the nineteenth century, 
 Samuel Butler, has indulged in the somewhat fantastic sugges- 
 tion that some day the construction of machines might be so 
 perfected that they also would be able to reproduce their kind, and 
 the little steam-engines would be seen playing about the door of 
 the engine shed. It certainly does not seem possible that 
 machines will ever multiply in this way, but should they do so, 
 and should they at the same time be able to feed and grow, it is 
 difficult to see why they should not be as much entitled to be 
 called living organisms as any of the plants and animals which 
 inhabit the earth to-day. They would, however, still be totally 
 different from plants and animals both in structure and 
 composition. 
 
 One of the most remarkable and characteristic features of the 
 living things which inhabit this earth is that they are all com- 
 posed of very similar materials, which are very different in their 
 nature from any which we should be likely to choose in the 
 construction of a machine. In making an engine we select those 
 substances which seem best calculated to resist the destructive 
 
 B 2
 
 4 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 action of the environment; hard and rigid metals which will 
 bear heavy strains, and as far as possible such as will be proof 
 against the chemical action of the atmosphere ; and we do our 
 best by means of oil and paint to protect even these from injurious 
 influences. 
 
 A living body may also have its hard protective structures, as 
 the shell of the oyster and the scales of the fish, or its rigid 
 levers, as the bones in our own limbs, but the really essential 
 part of the organism is built up of just those materials which are 
 most liable to destruction by chemical and physical agencies of 
 that almost liquid and extremely unstable substance which we 
 have already referred to as protoplasm, and of its various 
 derivatives. The experience of every day teaches us how 
 rapidly the bodies of animals and plants decay when they are 
 left exposed to the atmosphere after life has become extinct ; and 
 this decay is simply the destruction caused by the disintegrating 
 forces of the environment. A disabled steamboat may lie 
 rusting on the shore for many years without undergoing much 
 change, but the dead body of a stranded jelly-fish thrown up 
 beside it will become disorganized and disappear in the course of 
 a few hours, and yet the jelly-fish when alive was undoubtedly 
 the more complex and perfect piece of apparatus. 
 
 The body of an organism, moreover, undergoes destruction 
 not only after death ; it is always undergoing destruction, and 
 its very life depends upon its destruction, just as the flame of a 
 candle depends upon the destruction of the candle. But as it is 
 destroyed it is constantly built up again ; new protoplasm is 
 formed and new tissues take the place of those which are worn 
 out. The life of the organism is, in fact, the outcome of the 
 constant struggle between destructive and constructive forces, 
 and the keener the struggle the more vigorous will be the life 
 just as the flame will be brighter or hotter in proportion to the 
 activity of the combustion to which it owes its existence. 
 
 Life, like the flame, is a manifestation of energy, and the 
 living body is, like the steam-engine, a machine for transforming 
 one kind of energy into another. Moreover, the ultimate source 
 of the energy is the same in both cases. That of the steam- 
 engine is derived from the combustion or oxidation of coal, which 
 contains stores of energy derived millions of years ago from the 
 light and heat of the sun by the green plants which flourished 
 in the vast forests of the Carboniferous epoch. The green plants
 
 SOUECE OF ENEEGY OF OEGANISMS 5 
 
 of to-day still derive supplies of energy from the same source 
 and lock up in their leaves and stems what they do not them- 
 selves expend, while the animals in turn derive their energy 
 from the green plants upon which they directly or indirectly 
 feed. Hence both plants and animals are ultimately dependent 
 upon the sun for their existence. 
 
 Even the most superficial examination is sufficient to demon- 
 strate the fact that the body of any of the more familiar animals 
 or plants is, as we have already indicated, an extremely complicated 
 thing. Whatever may be the degree of complexity, however, and 
 however much one organism may differ from another in details 
 of structure, whether it be a microscopic alga or an oak tree, an 
 Amoeba or a man, there are always certain things which have to 
 be done, certain actions or functions which have to be performed, 
 in order that its life may be maintained. 
 
 In the first place, the organism must safeguard itself as far as 
 possible from the destructive influences of its environment. It 
 must not only be able to protect itself from such physical agents 
 as heat and cold, mechanical impact and friction, but it must be 
 able to select a situation where life is possible, and to escape 
 from other organisms by which it is liable to be attacked. All 
 this involves the expenditure of energy in some form or another ; 
 it may be in the manufacture or secretion of protective envelopes 
 or shells, such as we find even in some of the simplest Protozoa, 
 or it may be in actively moving away from the source of danger. 
 Thus it appears that the very first thing necessary for the 
 maintenance of life is the expenditure of energy. This 
 energy, though ultimately derived from the sun, is, as we have 
 already seen, derived immediately from the combustion of 
 fuel, very much as in the case of a steam-engine, but with the 
 important difference that in the living organism the fuel is, at 
 any rate to a large extent, the actual substance of which the 
 bony is composed. In this respect the comparison with a candle 
 is especially apt, for it is the combustion of the actual substance 
 of which the candle is composed that liberates the energy 
 manifested in the light and heat of the flame. 
 
 Now combustion is, of course, simply another name for what 
 chemists term oxidation, or combination with the element 
 oxygen, a process which is often accompanied by the liberation 
 of a considerable amount of energy in the form of heat and 
 light, though this is by no means always the case. In the
 
 6 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 higher animals sufficient heat is evolved to maintain the 
 temperature of the body at a level considerably above that of the 
 surrounding atmosphere, and such animals are accordingly 
 termed " warm-blooded " ; in plants, on the other hand, and in 
 the " cold-blooded " lower animals, the amount of heat evolved 
 is not as a rule sufficient to raise the body temperature to any 
 great extent, if at all. Heat, however, is only one form in which 
 energy may be manifested, and in living organisms it is, as a 
 matter of fact, much more conspicuously manifested in the form 
 of motion, especially in animals, while in not a few cases even a 
 low temperature combustion may liberate energy in the form of 
 light, as in the glow-worm and numerous other luminous 
 animals and plants. 
 
 When a piece of charcoal is burnt in the air it enters into 
 combination with the elementary oxygen gas of the atmosphere, 
 and another invisible gas which we term carbon dioxide, or 
 carbonic acid, is produced, in accordance with the equation 
 
 C + 2 C0 2 
 
 (Carbon) (Oxygen) (Carbon Dioxide). 
 
 In this process energy is set free in the form both of heat and 
 light. We must now inquire a little more carefully whence this 
 energy really comes, for although this is a question primarily 
 for the chemist and physicist it is also clearly one which the 
 biologist cannot afford to leave unanswered. 
 
 In accordance with the principles of the conservation of 
 energy and the indestructibility of matter we believe that the 
 quantities of energy and matter which exist in the universe 
 are fixed and constant. Neither energy nor matter can be 
 created and neither can be destroyed, though each may express 
 itself in a great variety of ways and change more or less readily 
 from one mode of expression to another. Thus, as we have 
 already seen, the energy of the sun's rays may be utilized in 
 building up the bodies of green plants, and locked up, as it were, 
 in the substances of which these are composed. 
 
 We also know that different chemical elements have a very 
 strong " affinity " for one another, their atoms tending to 
 unite and form compound molecules when they are brought 
 within the sphere of each other's attraction. Once united 
 they can only be separated again by the expenditure of energy, 
 and when they unite a corresponding amount of energy is
 
 ENERGY OF CHEMICAL AFFINITY 7 
 
 liberated. We may say, for example, that in the elements 
 carbon and oxygen, so long as they remain separate, a certain 
 amount of energy remains latent. We call this potential 
 energy. When the carbon and oxygen atoms are allowed to 
 come together and unite, this potential energy of chemical 
 affinity is liberated as kinetic energy, and manifested in the 
 form of light and heat. 
 
 It is from the potential energy of chemical affinity that the 
 energy of a living organism is immediately derived. Protoplasm, 
 the fundamental constituent of both plants and animals, contains 
 chemical compounds of extremely complex structure, composed 
 of many elements and containing a large amount of potential 
 energy locked up in them. Moreover, these proteids, as 
 they are termed, are extremely unstable bodies, readily breaking 
 up on oxidation into simpler and more stable combinations and 
 thus liberating energy. It is the presence of these unstable 
 proteids which confers upon protoplasm its peculiar fitness to 
 form what has been so aptly termed by Huxley " The physical 
 basis of life." They play the part of the gunpowder in a 
 cartridge, ready to produce a manifestation of energy as 
 soon as the proper stimulus is applied. 
 
 It is, then, the breaking up of proteids, or of some other 
 complex substances, usually by recombination of their con- 
 stituents with oxygen, which furnishes the constant supply of 
 energy which an organism requires. This process, however, 
 can only go on so long as the supply of combustible matter on 
 the one hand and of oxygen on the other is adequately main- 
 tained, and this brings us to the consideration of two of the 
 most important functions which every living organism must 
 perform, nutrition and respiration. 
 
 Under the head of nutrition we must include all those 
 processes which are concerned in building up the body, in 
 making good the waste of substance necessitated by the expendi- 
 ture of energy and thus providing new stores of fuel for the 
 use of the organism. The first step in nutrition is the taking 
 into the body of suitable food material. In the case of the 
 typical animal this material must contain in some form or 
 other all the necessary supply of potential energy, locked up in 
 more or less complex and unstable chemical compounds such 
 as it can obtain only from the bodies of other organisms. The 
 green plant, on the other hand, by virtue of the chlorophyll
 
 8 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 which it contains, has the power of absorbing energy directly 
 from the sun's rays and using this to build up the complex 
 proteids from very simple constituents. The feeding of the 
 organism, whether plant or animal, is comparable to the stoking 
 of the engine, but with this difference, that the food material, 
 unlike the fuel in the engine furnace, has usually to undergo 
 complex chemical processes, which may actually result in the 
 formation of new protoplasm, before it is available as a source 
 of energy. 
 
 Supplies of energy are, of course, useless unless they can 
 be liberated when required. The fuel must be burnt, and 
 for this purpose, as we have already said, a supply of oxygen 
 gas is necessary, and the function which is concerned in pro- 
 viding this supply we call respiration. The term respiration, 
 however, is one in the employment of which we shall have to 
 exercise a certain amount of care. It is naturally associated 
 in our minds with the mechanical act of breathing which we 
 ourselves perform. We can see a man breathing, but we cannot 
 see an oak tree breathing ; nevertheless the oak tree performs 
 the function of respiration just as efficiently as the man. "We 
 have to learn to dissociate the essential part of this function, 
 which is common to all living things, from the subsidiary com- 
 plications which have been introduced in the case of the higher 
 animals during the course of their evolution from lower forms. 
 
 Respiration, in the scientific acceptance of the term, is simply 
 the exchange by the organism of the carbon dioxide gas which 
 has been formed in the body in the process of combustion for 
 the oxygen gas which is required for that combustion. It is 
 therefore a double function oxygen being taken in and carbon 
 dioxide got rid of by one and the same process. This process 
 is, in its essential features, an extremely simple one, depending 
 upon the physical principle of osmosis or diffusion, in accordance 
 with which two gases of different densities tend to change places, 
 until equilibrium is established, whenever they are placed in 
 the necessary relations with one another, as when they are 
 separated only by some membrane through which both can 
 pass. 
 
 Carbon dioxide, however, is not the only waste product 
 resulting from the breaking up of the complex proteids, for 
 these also contain hydrogen, nitrogen, sulphur and phosphorus, 
 and other products of decomposition are accordingly formed
 
 METABOLISM 9 
 
 which contain amongst them all of these elements. These must 
 also be eliminated from the body, and this process of elimination 
 of waste products constitutes the function of excretion. This 
 function may be performed in a variety of ways and by a variety 
 of organs. In so far as the carbon dioxide is concerned it is, as 
 we have already seen, an essential part of the function of 
 respiration. Urea, on the other hand, a nitrogenous substance 
 which is perhaps the most characteristic waste product in the 
 higher animals, is eliminated by special excretory organs, such 
 as the kidneys. In the case of the higher plants the waste 
 products are for the most part stored up in the leaves and 
 got rid of when these are shed. 
 
 We have thus seen that the supply of energy to a living 
 
 Living Protoplasm. 
 
 Food Material. / \Waste Products. 
 
 FIG. 1. Diagram of Metabolism. 
 
 organism, and therefore also its life, depends upon a series 
 of complex chemical processes which take place within the body. 
 All these processes collectively are spoken of as metabolism, and 
 we may distinguish between two sets of metabolic changes : 
 those which are constructive and lead to the building up of 
 new living protoplasm out of food material, and those which 
 are destructive and lead to the decomposition of the body 
 substance, the liberation of energy and the formation of waste 
 products. The former are termed anabolic and the latter 
 katabolic. 
 
 We may roughly illustrate these elementary conceptions of 
 the chemical processes which take place in the living body by 
 the accompanying diagram, in which a mass of living protoplasm 
 is represented as balanced in a very unstable position on the 
 apex of a triangle. It is constantly undergoing destruction,
 
 10 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 accompanied by the liberation of energy and resulting in the 
 formation of waste products, substances which, in falling down 
 one side of the triangle to a lower level, to a greater or 
 less extent lose their potential energy. On the other side of 
 the triangle, however, complex food material, containing fresh 
 supplies of potential energy, is supposed to be taken in, or, in 
 the case of green plants, actually built up from simple con- 
 stituents by the energy of the sun's rays, and used in repairing 
 the waste of the protoplasmic body. 
 
 If the constructive processes proceed more vigorously and 
 rapidly than the destructive, if the food supply is abundant and 
 the expenditure of energy comparatively low, the body may 
 grow, though, as we shall see presently, only within certain 
 limits. If the reverse is the case and the expenditure exceeds 
 the income, the body may dwindle away and finally die. If it 
 is to remain in a condition of healthy equilibrium a just balance 
 must be maintained between the two sides of the account. 
 
 Perhaps the most characteristic property of living things is, 
 as we have already suggested, the power of reproduction. This is 
 the last resort of the organism in the struggle for existence. The 
 individual, which owes its very life to the perishable nature of 
 its body, always succumbs to the destructive influences of the 
 environment sooner or later, but before yielding to the inevitable 
 it will, under normal conditions, have produced offspring which 
 will carry on the struggle for another generation. 
 
 The phenomenon of reproduction is intimately associated 
 with that of growth, and may be traced back to the division of 
 a simple ancestral mass of protoplasm into two parts whenever 
 its size increased to such an extent that the ratio of surface to 
 volume became too small for the necessary intercourse between 
 the organism and its environment. With this division of the 
 protoplasmic body the proper proportion is restored, and hence 
 reproduction by multiplication of protoplasmic units may be 
 looked upon as primarily the direct consequence of super- 
 abundant nutrition. 
 
 We have now learnt to look upon an animal or a plant as a 
 complex and extremely delicate piece of mechanism, constantly 
 employed in collecting energy directly or indirectly from the 
 sun's rays and in using that energy to maintain an incessant 
 struggle against the destructive forces of its environment. This 
 incessant getting and spending, winning and losing, constitutes
 
 THE NATURE OF LIFE 11 
 
 what we call the life of the organism. In considering what is 
 the meaning of all this we must remember that, primarily at any 
 rate, every living tiling exists for its own benefit, and that 
 living, like virtue, is its own reward. The organism also, 
 however, exists for the benefit of future generations, to which 
 it may hand on the lamp of life before its own flame is finally 
 extinguished. There is a race life as well as an individual life, 
 and we cannot realize too clearly that in the economy of nature 
 the former is of infinitely greater importance than the latter. 
 
 The ideas which we have just been considering are by no 
 means of modern origin. More than three centuries ago the 
 philosopher Descartes endeavoured to explain the human body 
 as a machine, but as a machine under the control of the " soul," 
 which he curiously located in that part of the brain known as 
 the pineal gland. His ideas of physiology, however, were, 
 naturally, of the crudest description, and immense strides have 
 been made in this direction since his time. Chemists and 
 physicists have helped us much towards a correct understanding 
 of the living mechanism, but when they have done their best it 
 may well be that the question " What is Life? " will still remain 
 unanswered, and that we may still have to take refuge in the 
 idea of an unknown " soul " to explain the difference between 
 living and not-living things. The " soul " of Descartes' 
 philosophy corresponds more or less closely with the " vital 
 force " of some more recent writers and the " entelechy " 1 of 
 others, but whatever term we employ it must be rather as a 
 cloak for our ignorance than as an expression of any definite 
 opinion as to what it is that really animates the living body. 
 
 1 Vide Driesch, " The Science and Philosophy of the Organism," (London, 
 A. & C. Black, 1908), Vol. 2, pp. 137, 138.
 
 CHAPTER II 
 
 Amoeba as a typical organism The properties of protoplasm. 
 
 IN illustration of the general principles dealt with in the fore- 
 going chapter we may now consider a definite concrete example 
 of a living organism. Probably none is better suited for this 
 purpose than the familiar Amoeba, which may be regarded as a 
 kind of pocket edition of a typical animal. 
 
 Amoebae may be found creeping about on the mud at the 
 bottom of ponds and ditches. Although of microscopic size and, 
 usually at any rate, invisible to the naked eye, they are by no 
 means the smallest or simplest of living things, but exhibit 
 within the narrow limits of their gelatinous bodies a considerable 
 amount of structural differentiation. 
 
 In general appearance (Fig. 2) an Amoeba resembles nothing 
 so much as an irregular speck of translucent jelly, but if we 
 watch it for a few minutes under the microscope we soon find 
 that it is something more than this. If in an active and 
 healthy condition it never maintains the same shape for long 
 together, but manifests an ever-changing irregularity as it 
 slowly creeps about from place to place, throwing out 
 irregular projections of its body first in one direction and 
 then in another and withdrawing old projections as new ones 
 are put forth. 
 
 The viscid substance of which the entire body is composed is 
 protoplasm, but this protoplasm is not homogeneous throughout ; 
 on the contrary, it exhibits a characteristic differentiation into 
 parts or organs, which can be more or less readily distinguished 
 from one another and each of which has its own duties or 
 functions to perform. 
 
 Inasmuch as it consists of but one protoplasmic unit, however, 
 we may speak of the body of an Amoeba as a single cell. 1 As 
 in all other typical cells, the most fundamental differentiation 
 
 1 The origin and meaning of this term will be discussed more fully in a later 
 chapter.
 
 AMCEBA 13 
 
 which it shows is into cell-body and nucleus. The cell-body 
 forms by far the greater part of the organism, and the 
 
 FIG. 2. Amoeba. 
 
 A, B. The same individual in two phases of active movement, showing change of form. 
 
 C. Another specimen, of a different species. Note the numerous short projections at 
 
 the hinder end due to contraction, while at ect. the commencement of a new pseudo- 
 podium is indicated by a thickening of the ectoplasm. 
 
 D. A specimen with two nuclei. 
 
 E. Diagram of reproduction by simple fission. 
 
 c.v. Contractile vacuole ; ect. ectoplasm or ectosarc; end. endoplasm or endosarc ; f.p. 
 food particles; f.v. food vacuole; nu. nucleus; psd. pseudopodium. (The arrows 
 show the general direction in which the animal is moving.) 
 
 protoplasm of which it is composed is often distinguished as 
 cytoplasm.
 
 14 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 The nucleus (Fig. 2, nu.), composed of a somewhat different 
 variety of protoplasm sometimes known as karyoplasm or 
 nucleoplasm, is a very definite body of more or less rounded 
 form, sometimes shaped like a bun, and easily distinguishable 
 from the cytoplasm even in the living animal by its more highly 
 refractive character. Its position is by no means constant, for 
 it floats about from place to place in the interior of the almost 
 liquid cell-body. 
 
 The cytoplasm is very imperfectly differentiated into inner and 
 outer portions. The former, in which the nucleus is lodged, is 
 often called the endosarc or endoplasm (Fig. 2, end.), the latter 
 the ectosarc or ectoplasm (Fig. 2, ect). The ectoplasm must be 
 regarded as a feebly developed protective layer ; it is the part which 
 comes into direct relation with the surrounding water and through 
 which all intercourse between the Amoeba and its environment 
 must take place. Though soft and gelatinous, it is a good deal 
 firmer and denser than the endoplasm, and it is also more trans- 
 parent, for the endoplasm contains imbedded in it numerous more 
 or less opaque particles of various kinds which give it a coarsely 
 granular character. Most of these particles are minute, but 
 others are generally present of comparatively large size and 
 enclosed in drops of clear liquid. These latter are food particles 
 (Fig. 2, f.p.) undergoing digestion, and they can frequently be 
 identified as the bodies of other organisms which the Amoaba 
 has taken in, microscopic plants or animals smaller than itself. 
 The drops of liquid in which they occur are termed food 
 vacuoles (Fig. 2,/.v.). 
 
 Another spherical drop of liquid (Fig. 2, c.v.) may be observed 
 somewhere near the surface of the cell-body. This is perfectly 
 clear and contains no solid particles ; moreover it undergoes a 
 rhythmical dilatation and contraction, gradually increasing to a 
 maximum size and then suddenly disappearing owing to the 
 discharge of its contents into the surrounding water. If the 
 spot where this " contractile vacuole " disappears be carefully 
 watched another drop of liquid is seen gradually to accumulate 
 there, and the process is repeated. 
 
 We are told that in the early days of chemistry, before the 
 highly specialized apparatus which is now used was thought of, 
 the originator of the atomic theory performed his experiments 
 with the ordinary domestic crockery. So also an Amoeba is able 
 to perform, with the extremely simple apparatus at its disposal,
 
 LOCOMOTION IN AMCEBA 15 
 
 all those actions or functions which are really essential for the 
 maintenance of life. 
 
 In the higher animals the primary differentiation of the body 
 is into an outer protective and an inner digestive layer, each of 
 very complex structure. The Amoeba accomplishes the same 
 end in its own primitive manner by the differentiation into 
 ectoplasm and endoplasm. In most of the higher animals, 
 again, we find very well developed organs of locomotion in 
 the form of limbs. The Amceba has no permanent organs of 
 locomotion at all but merely temporary projections of the body, 
 the so-called pseudopodia (Fig. 2, psd.), which are put forth when 
 required. In both cases, however, movement is effected in 
 essentially the same way, by contraction and expansion of the 
 living protoplasm. In the higher animals this power of con- 
 traction is localized in the muscles, which are highly specialized 
 for the purpose and have no other duties to perform, while in 
 the Amceba any part of the body, or at any rate of the ectoplasm, 
 may contract or expand as occasion requires. 
 
 In the process of formation of a new pseudopodium we see 
 first a thickening and protrusion of the clear ectoplasm (Fig. 2, 
 C., ect.), accompanied by a streaming in of the endoplasm, and 
 the latter seems to bulge out the ectoplasm as it flows forwards. 
 The pseudopodium is withdrawn again by a reversal of the pro- 
 cess, the endoplasm streaming out from it into the central mass 
 of cytoplasm and the ectoplasm contracting after the retreating 
 endoplasm. Thus at the posterior end of an actively creeping 
 Amoeba one frequently sees numerous blunt projections which are 
 the last remnants of retracted pseudopodia (Fig. 2, (7.). The 
 shape of the pseudopodia when fully extended differs very much 
 in different kinds of Amoebae. In some species they are com- 
 paratively short, thick and blunt, as in our illustration, while in 
 others they are very long and slender, radiating outwards from 
 the body of the animal in all directions. They are used by their 
 possessor not only as organs of locomotion but also as tactile 
 organs and, as we shall see directly, for the capture of prey. 
 
 The protrusion and retraction of pseudopodia imply, of course, 
 the expenditure of energy, and this energy must be derived from 
 the combustion of the body of the Amceba. In this way the 
 protoplasm is gradually used up, and, unless the animal is to die 
 of starvation, it must be replaced, which brings us to the 
 consideration of the manner in which an Amceba performs the
 
 1C OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 important function of nutrition. As it crawls slowly about the 
 pseudopodia come into contact with all sorts of solid particles in 
 the surrounding water. Some of these will be inorganic, grains of 
 sand and so forth, others will be the dead or living bodies of other 
 organisms, sometimes much smaller than the Amoeba itself. 
 The Amoeba has the remarkable power of distinguishing amongst 
 these different kinds of particles those which are good for food 
 from those which are not. How it does so we do not know ; we 
 can only say that the presence of a particle which is good for 
 food stimulates the living protoplasm in a way quite different 
 from that in which it is stimulated by the presence of a mere 
 grain of sand. In the latter case the Amoeba will simply pass to 
 one side and avoid the object ; in the former it will put forth 
 pseudopodia which will close around and envelop it. The food 
 particle is thus passed through the ectoplasm into the interior 
 of the body. There is no definite mouth, but food is taken in 
 wherever it happens to come into contact with the surface of the 
 body, and the aperture closes up after it. Thus a temporary 
 mouth is formed as occasion demands. Similarly there is no 
 permanent digestive cavity or stomach, but merely a temporary 
 food vacuole into which a digestive fluid is doubtless secreted 
 by the surrounding protoplasm. 
 
 Digestion, as in higher animals, is essentially a process of 
 solution, whereby those parts of the food which are digestible are 
 dissolved and rendered capable of diffusing from the digestive 
 cavity into the surrounding body. The higher animals make 
 use chiefly of three classes of food material, proteids, carbo- 
 hydrates (e.g., starches and sugars) and fats. It is said that 
 Amoeba can only digest proteids, which of course it must obtain 
 from the protoplasmic bodies of other organisms. When diges- 
 tion is complete a certain amount of insoluble residues from the 
 food will remain over ; these constitute the faces and have to be 
 got rid of. There is, however, no permanent vent or anus, and 
 the faces are cast out through the ectoplasm at the hinder end 
 of the body as the animal crawls along. 
 
 Owing to the minute size of the whole organism there is no 
 need for a complex circulatory system, such as is found in the 
 higher animals, for the distribution of the digested food; it merely 
 soaks into the surrounding protoplasmic body from the food 
 vacuoles, and, by anabolic changes which are not fully under- 
 stood, is converted into new, living protoplasm.
 
 RESPIRATION IN AMCEBA 17 
 
 It is not easy, if it be possible at all, actually to observe the 
 process of respiration in so small an animal as an Amceba, but 
 we know perfectly well from the analogy of higher organisms 
 what must take place. Oxygen is required for the combustion of 
 the protoplasm from which the energy of the organism is 
 derived, and this oxygen occurs in a state of solution in all 
 ordinary water which is exposed to the air. At the same time 
 carbon dioxide, or carbonic acid gas, must be produced as one of 
 the products of the combustion, by oxidation of the carbon in the 
 protoplasm. This waste product (C0 2 ) will first of all be 
 dissolved in the water which forms the greater part of the bulk 
 of the living organism, while at the same time the water by 
 which the animal is surrounded may be regarded as a very 
 dilute solution of oxygen. The outermost layer of the ectoplasm 
 may be looked upon as a thin membrane separating the two 
 solutions. 
 
 We know from experiment that whenever two gases, or solutions 
 of gases, of different density, are separated from each other by a 
 thin organic membrane, they will pass through that membrane 
 in opposite directions until a state of equilibrium is established 
 between the two. This process of osmosis or diffusion is, as we 
 have already seen, the essential feature of respiration in all plants 
 and animals, although probably the purely physical process is 
 controlled to some extent by the living protoplasm. 
 
 In the case of the Amoaba then, the carbon dioxide diffuses 
 out through the ectoplasm into the surrounding water while 
 the oxygen from the surrounding water diffuses in, and the 
 necessary exchange of gases is brought about. No specialized 
 organs of respiration, such as we meet with in the higher 
 animals, are required. The whole surface is a respiratory sur- 
 face, and all parts of the interior are within reach by the simple 
 process of diffusion, aided doubtless by the circulation of the semi- 
 liquid protoplasm which is constantly going on inside the body. 
 
 Other waste products must be formed by the combustion of the 
 protoplasm in addition to carbon dioxide. What these are we 
 do not exactly know in the case of the Amoeba, but it is evident 
 that they must contain nitrogen, which is one of the' essential 
 constituents of all proteids. These waste products must be got 
 rid of by some process of excretion, and it is usually supposed 
 that they are passed in a state of solution to the contractile 
 vacuole and thence periodically expelled to the exterior, so that 
 
 B. c
 
 18 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the contractile vacuole is probably to be regarded as the special 
 excretory organ of the animal. 
 
 In all but the lowest animals there is a more or less specialized 
 nervous system, whose function it is to place the different parts 
 of the body in communication with one another and, through 
 the mediation of the sense organs, with the external world or 
 environment. The action of this system depends primarily 
 upon one of the fundamental properties of living protoplasm, 
 the power of responding to stimuli by some definite change 
 in its own condition. The stimulus is, in the first instance, 
 supplied by some factor of the environment, such as light, heat, 
 electricity or mechanical impact. It may appear to originate in 
 the central nervous system itself, but this is probably secondary. 
 It has the effect of liberating stored energy in those parts of 
 the organism which are sensitive to that particular stimulus, 
 in somewhat the same way that the stimulus of heat may 
 have the effect of liberating the stored energy in a charge of 
 gunpowder. The living molecule has, indeed, actually been 
 described as explosive. In both cases potential energy is 
 converted into kinetic energy, and the effect which is produced 
 may be out of all proportion to the amount of energy represented 
 by the liberating stimulus itself. 
 
 In the higher animals, then, the stimulus received from the 
 external environment, whatever its nature may be, acts 
 primarily upon some special sense organ or receptor, whence it 
 is transmitted along highly specialized tracts of tissue, the 
 nerves, to some part of the central nervous system, where it 
 usually gives rise to what we call a sensation. The central 
 nervous system, again, not only has the power of receiving stimuli 
 through afferent or sensory nerves, but also of sending stimuli 
 through efferent nerves to the various organs of the body, 
 whereby their functions are controlled and regulated. The con- 
 traction of muscles and the secretion of glands are all con- 
 trolled in this manner, and the entire working of the body is 
 co-ordinated by the action of the nervous system. 
 
 In the Amoeba, however, we see no trace of a special nervous 
 system, nor of sense organs, but nevertheless the organism is 
 certainly capable of receiving and responding to stimuli ; in 
 other words it is irritable. Thus the protoplasmic body responds 
 by contraction to the stimuli of mechanical impact, heat, light, 
 electricity and chemical reagents, and, as we have already seen,
 
 VITALISM 19 
 
 the presence of particles which are good for food causes the pro- 
 trusion of pseudopodia in a definite and purposive manner. 
 Probably the whole of at least the ectoplasm is to some extent 
 sensitive to stimuli of certain kinds, and it is also probable that 
 stimuli may be conducted from one part of the body to another 
 without the existence of special nervous tracts. 
 
 One of the most difficult problems in connection with the 
 physiology of Amoeba and indeed of any living organism is 
 that of automatism. Does an Amoeba do anything really auto- 
 matically or spontaneously, or are all its actions the result, 
 direct or indirect, of the application of external stimuli to the 
 explosive molecules of living matter ? Is the organism merely a 
 machine run by the environment, or is it something more ? Here, 
 of course, at the very beginning of our investigations, we are face 
 to face with the old question, already referred to, of the existence 
 of an animating principle or " soul," which exercises some sort of 
 control over the physical and chemical processes upon which the life 
 of the organism depends. This is a question which perhaps falls 
 within the province of the philosopher rather than that of the 
 biologist. The theory of vitalism, by postulating the existence 
 of some such special vital force in all living things, undoubtedly 
 enables us to avoid many difficulties, but it is doubtful if it really 
 explains anything. As a matter of fact the more we study living 
 organisms by actual observation and experiment, the more 
 fully are we able to interpret their behaviour in terms of 
 chemistry and physics, but this is a very different thing from 
 saying that chemistry and physics will ultimately yield a 
 complete explanation of vital phenomena. 
 
 It is quite possible, for example, that the movements of the 
 Amcebamay all ultimately be interpreted in such terms, for 
 Biitschli has shown that they can be closely imitated by minute 
 artificially prepared drops of oil-foam surrounded by water. The 
 substance of which these droplets are composed is of course 
 totally different chemically from protoplasm and is in no sense 
 alive, but it seems highly probable, if not certain, that since 
 purely physical processes (amongst which surface tension seems 
 to play an important part) are capable of producing strikingly 
 amoaboid movements in the oil-foam, they may also be largely, 
 if not solely, responsible for the similar phenomena of movement 
 in the living protoplasm of the Amoaba itself, which seems closely 
 to resemble an oil-foam in its physical properties. 
 
 c 2
 
 20 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Like other organisms, the Amoeba sometimes undergoes a period 
 of rest, during which its various activities are more or less com- 
 pletely suspended. Under these circumstances the pseudopodia 
 are withdrawn, the body is rounded off and a protective envelope 
 or cyst is secreted by the protoplasm. This, however, is only a 
 temporary state, perhaps necessitated by unfavourable conditions 
 of the environment, and sooner or later the organism emerges 
 from its retirement and resumes its activity. 
 
 If in the course of its wanderings the Amoeba meets with an 
 abundant supply of food and takes in more than is actually 
 required to make good the waste of protoplasm ; if, in other 
 words, anabolism preponderates over katabolism, the organism 
 may increase in size by growth, by the addition of new particles 
 of protoplasm in excess of those which are used up. These new 
 particles are deposited, not on the surface, but throughout the 
 whole mass of protoplasm, between those which are already 
 formed. Thus growth takes place, not by accretion, as in a 
 crystal or a snowball, but by intussusception, and we have here 
 a characteristic though by no means absolute distinction between 
 the growth of not-living and that of living matter. 
 
 As in all organisms, however, there is a limit to the size which 
 the body may attain, and this limit varies with different species 
 of Amoeba. It depends primarily, no doubt, upon the necessary 
 relation between surface and volume. As we have seen, all 
 interchange between the organism and its environment has to be 
 maintained through the surface, and a given area of surface 
 cannot supply the wants of more than a certain volume of 
 protoplasm. As the animal grows the volume must necessarily 
 increase in a much higher ratio than the surface, and the pro- 
 portion between the two is rapidly altered. This is probably 
 not the whole explanation of the limitation of growth in an 
 Amceba, the problem being doubtless complicated by other factors, 
 but we may take it as quite certain that increase beyond a certain 
 size, if possible, would inevitably result in death. Such a 
 calamity is avoided by the simple expedient of dividing into two 
 parts whenever the limit of safety is reached. The nucleus 
 divides first and the two halves move away from one another, 
 then the protoplasm constricts into a bridge between the two 
 nuclei, the bridge narrows and finally ruptures, and instead of 
 one Amoeba there are now two, each exactly resembling the 
 parent (Fig. 2, E.).
 
 PHYSICAL PROPERTIES OF PROTOPLASM 21 
 
 This simple fission of a single protoplasmic unit, or cell, forms, 
 as we shall see later on, the essential feature of ordinary repro- 
 duction throughout the animal and vegetable kingdoms. It will 
 be observed that in this process generation succeeds generation 
 without the intervention of anything which we can speak of as 
 death. There is indeed no room for death in the history of 
 these simple organisms, unless it be death by accident, for every 
 time fission takes place the entire body is used up, and nothing is 
 left over to die. Nor is there any distinction to be drawn between 
 parent and offspring, for the two new individuals are in all 
 respects similar to one another, and neither can be said to precede 
 the other in point of time. 
 
 We have now become sufficiently well acquainted with the 
 nature of protoplasm to profit by a more detailed examination of 
 its physical and chemical properties. We have seen that, as it 
 occurs in the body of an Amoeba, it is a viscid, more or less 
 liquid, colourless substance, almost transparent but exhibiting, 
 under moderately high powers of the microscope, a granular 
 appearance due to the presence of numerous minute and more or 
 less opaque particles. These particles may be regarded as 
 impurities, and indeed protoplasm can never be obtained in a 
 perfectly pure state, for it is constantly undergoing chemical 
 change, both constructive and destructive, and the impurities 
 owe their origin partly to the food materials which are taken in 
 and partly to katabolic processes which give rise ultimately to 
 waste products. 
 
 Even apart from these impurities, however, the protoplasm 
 itself never exhibits a perfectly uniform structure. It is by no 
 means homogeneous but shows a more or less distinct differentia- 
 tion into different portions, as, for example, into nucleoplasm and 
 cytoplasm, ectoplasm and endoplasm, and so forth. In other 
 words it is an organized substance. Moreover, it has a character- 
 istic minute structure or texture which can to some extent be 
 recognized under high powers of the microscope and concerning 
 the interpretation of which different observers are as yet by no 
 means all agreed. According to Professor Biitschli it is a kind 
 of microscopic foam, consisting of exceedingly minute drops of 
 a more liquid substance separated by very thin layers of denser 
 material, and thus resembling physically such a foam as can 
 be prepared from a mixture of oil, salts of various kinds, and
 
 22 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 water. If this really be a correct account of the minute structure 
 of living protoplasm it helps us, as we have already seen, to 
 explain its characteristic movements in terms of well known 
 physical phenomena. 
 
 Other competent observers, however, maintain that the appear- 
 ance of foam-structure is a delusion and that what Biitschli 
 interprets as thin sheets separating the droplets from one another 
 are in reality very delicate fibres arranged in a network. These 
 fibrillae are supposed to be contractile and thus to be responsible 
 for the movements of the protoplasm as a whole. But whence 
 comes the contraction of the fibrillae ? 
 
 Various considerations, again, and especially the phenomena 
 of heredity, oblige us to postulate for protoplasm an even more 
 minute fundamental structure than the microscope is capable of 
 revealing to us. It is, in all probability, made up of ultra- 
 microscopic material units, each composed of a group of molecules, 
 which units, or "biophors," must themselves be regarded as living 
 bodies capable of nourishing themselves, growing and multiplying 
 by division. 
 
 It is difficult to form a satisfactory idea of the chemical com- 
 position of protoplasm because it is impossible to analyze it in the 
 living condition ; indeed, in the living condition it is constantly 
 undergoing chemical change, and the moment it dies it ceases to 
 be protoplasm. It is certain, however, that it is not a definite 
 chemical compound, but a mixture of several distinct sub- 
 stances : proteids, mineral salts and water. Moreover, different 
 samples of protoplasm, taken from different organisms or from 
 different parts of the same organism, may differ widely from one 
 another in chemical composition. Thus the difference between 
 nucleoplasm and cytoplasm is largely a chemical one, depending 
 to some extent upon the relatively large amount of phosphorus 
 present in the former. 
 
 By far the most characteristic and important of the chemical 
 constituents of protoplasm are, of course, the proteids. These 
 form a remarkable class of substances which do not occur in 
 nature except in the bodies of plants and animals. They are 
 definite chemical compounds containing the elements carbon, 
 hydrogen, oxygen, nitrogen, sulphur and frequently phosphorus, 
 and they have an extremely complex and unstable constitution, 
 readily splitting up on oxidation into simpler and more stable 
 compounds and thereby liberating kinetic energy. Many
 
 CHEMICAL COMPOSITION OF PROTOPLASM 23 
 
 different kinds of proteids are known to us and have received 
 special names ; such are the albumen which occurs in the white 
 of an egg, the casein which is met with in cheese, the legumin 
 which is characteristic of peas and beans, the gliadin and 
 glutinin of flour, and so forth. 
 
 These proteids are, for the most part at any rate, colloid 
 substances, that is to say they are more or less gelatinous and 
 incapable of diffusing through organic membranes. This may 
 be accounted for if we assume that the highly complex molecules 
 of which they are composed are too large to pass through the 
 very minute pores which occur in such membranes and which 
 readily allow of the passage of the molecules of simpler, 
 crystalloid substances. The colloid nature of the proteid con- 
 stituents of protoplasm plays a very important part in determining 
 its properties and behaviour. Crystalloid mineral salts and other 
 diffusible substances in a state of solution can pass through a 
 cell-wall or membrane by osmosis, and thus the living proto- 
 plasm receives fresh supplies of nutriment, but the colloid 
 proteids are as a rule formed inside the cell and cannot usually 
 pass out again until they have undergone some chemical change 
 whereby they are rendered diffusible. 
 
 The mineral salts which we find in the protoplasm, usually- in 
 a state of solution, are of very various kinds, compounds of 
 sodium, potassium, calcium and other elements with various 
 inorganic" and organic acids, such as sulphuric, hydrochloric, 
 malic and citric acids. 
 
 Finally, water must always be present in living protoplasm and 
 usually forms a very large percentage of the whole mass. 
 
 Whatever view we may take with regard to the question of 
 vitalism, there can be no doubt that the most distinctive property 
 of living protoplasm is its power of controlling chemical and 
 physical processes so as to make them yield results different from 
 those which would be obtained if we were dealing with not-living 
 matter. The various processes upon which depend the functions 
 of movement, nutrition, respiration and excretion all appear to be 
 controlled in this manner, but the general principle is perhaps 
 most beautifully illustrated in the case of many of the lower 
 animals and plants, in which the protoplasm secretes a protective 
 or supporting skeleton of some mineral substance, such as silica, 
 or carbonate of lime. Silica, for example, in the inorganic world, 
 occurs abundantly in a state of solution in water, from which it
 
 24 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 may be deposited in different forms, in shapeless masses as in 
 the case of flints, or in symmetrical crystals as in some specimens 
 
 FIG. 3. Different forms of Kadiolarian Skeletons. In the central figure the 
 protoplasmic psteudopodia are seen coming out from the openings in the 
 shell. (From Haeckel's " Kunstformen der Natur.") 
 
 of quartz, whilst the beautiful opal, chemically speaking, is 
 merely a hydrate of silica, or silicic acid.
 
 SELECTIVE POWERS OF PROTOPLASM 
 
 25 
 
 Many simple unicellular organisms, such as the Radiolaria 
 (Fig. 3) amongst animals, and the diatoms amongst plants, have 
 
 FIG. 4. Different forms of Foraminiferan Skeletons. (From Haeckel's 
 " Kunstformen der Natur.") 
 
 the power of taking up dissolved silica from the water in which 
 they live and using it for building skeletons. These skeletons,
 
 26 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 however, which are really composed of opal, do not consist either 
 of shapeless masses or of geometrical crystals, but assume beauti- 
 fully symmetrical forms which vary with each particular kind of 
 organism and which are wholly different from any forms occurring 
 in the inorganic world. The same is true of those somewhat 
 more highly organized members of the animal kingdom, the 
 siliceous sponges, whose skeletons consist of spicules of opal 
 (Fig. 88), often of most beautiful and characteristic shape, and each 
 one as a rule formed b^ the activity of the living protoplasm 
 within the compass of a single cell. 
 
 Whilst many unicellular organisms and many sponges thus 
 manufacture for themselves skeletons of a siliceous character, 
 others, living perhaps in the same water, secrete skeletons which 
 are composed of carbonate of lime. Such, for example, are the 
 Foraminifera, the dead calcareous shells of which (Fig. 4) 
 accumulate to-day at the bottom of the deep sea in many places 
 in the form of ooze, while in the Cretaceous period of the earth's 
 history they played the principal part in building up the chalk 
 cliffs on the south coast of England. Each of these microscopic 
 shells, which are often of extreme beauty and frequently 
 ornamented with elaborate patterns, is formed as a protective 
 envelope by a simple protoplasmic organism closely resembling 
 an Amoeba. 
 
 It is evident, then, that we must attribute to living proto- 
 plasm a very remarkable power of selection or choice, for it is 
 able, as it were, to pick out certain materials from its environ- 
 ment for its own purposes and to reject others. We have already 
 seen this in the case of the selection of food materials by the 
 Amoeba ; we see it again in the selection of the materials for 
 skeleton formation in other simple organisms. The fact that 
 one organism will select silica while another selects carbonate of 
 lime from the same sample of sea water and for the same purpose, 
 must correspond to some deep-seated difference in the protoplasm 
 of which they are composed, and illustrates very well the diverse 
 potentialities of this remarkable substance.
 
 CHAPTER III 
 
 Haematococcus The differences between animals and plants. 
 
 IN striking contrast to Amoeba, which, though primitive, 
 is nevertheless a very typical example of an animal organism, 
 stands Haematococcus or Sphaerella, which, by botanists at any 
 rate, is regarded as a very simply organized member of the 
 vegetable kingdom. Like Amoeba, this organism is of microscopic 
 size, consisting of only a single cell or protoplasmic unit. 
 
 Hcematococcus pluvialis (also known as Spharetta lacustris) 
 occurs in temporary pools of stagnant rain-water or, in the rest- 
 ing condition, in dried-up mud or dust. Though individually 
 invisible, or barely visible, to the naked eye, it may occur in such 
 dense associations as to give the water a bright red (or some- 
 times green) colour and form a red crust on the sides of the 
 vessel in which it is cultivated. A closely related, if not identical, 
 species (Htematococcus nivalis) is the cause of the red patches 
 which are sometimes observed on the snow-fields in Arctic 
 regions. Cultivation is easy, and the same stock may be kept 
 going for many years and multiplied indefinitely. Twenty 
 years ago or more I had a sample given to me in Australia, 
 descendants of which are now flourishing in full vigour in 
 England. It can be dried up when not required and when 
 wanted again in the active condition needs only to be supplied 
 with fresh rain-water and placed in the sun. 
 
 In the resting condition each individual consists of a spherical 
 protoplasmic body (Fig. 5, A) of a bright red or green colour, or 
 sometimes partly green and partly red, with a more or less 
 centrally placed nucleus (mi.). It differs from an Amceba in the 
 presence of a very distinct, firm cell-wall (c-.w.) on the outside, as 
 well as in its definite shape and characteristic colour. The cell- 
 wall is composed of cellulose, one of a group of chemical com- 
 pounds known as carbohydrates. These compounds are all 
 characterized by the fact that they contain only three elements, 
 carbon, hydrogen and oxygen, the hydrogen and oxygen occurring
 
 28 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 in the same proportions as in water (H 2 0). Thus the chemical 
 formula for cellulose is (C 6 Hi 5 )n, while that for glucose or grape 
 sugar, another carbohydrate, is C 6 H 12 6 . The presence of very 
 definite cell-walls, composed of cellulose and formed as a secretion 
 by the living protoplasm, is very characteristic of vegetable as 
 contrasted with animal organisms. 
 
 The protoplasm which lies inside the cell-wall is, as we have 
 already said, either red, green or parti-coloured. The green 
 
 FlG. 5. Structure and Life-history of HcKinatococcus plnvialis. 
 
 A. Resting stage with thick cell- wall. 
 
 B. Division into four motile zoospores within the old cell-wall. 
 
 C. Free-swimming zoospore. 
 
 D. Division of the resting cell into 32 microzooids or gametes. 
 
 E. Free-swimming gamete. 
 FG. Conjugation of two gametes. 
 
 H. Zygote with four flagella, formed by conjugation. 
 J. Zygote with flagella withdrawn. 
 
 K. Resting cell formed from the zygote by secretion of a thick cell-wall. 
 
 c.w. cell-wall ; fl ' . flagellum ; nu. nucleus ; py. pyrenoid ; vac. vacuole. 
 
 (Figs. D K adapted from Peebles.) 
 
 colour is due to the presence of that extremely characteristic 
 vegetable pigment known as chlorophyll, a remarkable pro- 
 duct of the activity of the living protoplasm with which we are 
 all familiar in the case of ordinary green plants. The red 
 pigment, known us hsematochrome, is but a slight chemical 
 modification of the green chlorophyll, and the one may readily 
 be converted into the other. If some nitrogenous substance, 
 such as a small quantity of manure water, be placed in a 
 vessel containing red Haematococcus, the latter will in a short 
 time assume a bright green colour, whence we may conclude that 
 the red colouration is probably an effect of nitrogen starvation.
 
 H/EMATOCOCCUS 29 
 
 When a dried -up sample of Htematococcus is supplied with 
 fresh rain-water and placed in the sunlight it undergoes a 
 remarkable change. The protoplasmic body within the cell-wall 
 undergoes division, first into two and then into four parts 
 (Fig. 5, B). This is effected by a process of simple fission exactly 
 comparable to that which we have already described in the case 
 of Amoeba. The four parts or daughter cells (sometimes called 
 zoospores) are for a short time kept together within the old cell- 
 wall, but presently this ruptures and they escape into the 
 surrounding water. 
 
 It will now be seen that these so-called zoospores differ greatly 
 in structure from the resting Haematococcus. Instead of being 
 spherical they are more or less oval or pear-shaped in outline 
 (Fig. 5, C). Each has secreted a new cellulose wall of its own 
 (c.w.), but this is separated from the main protoplasmic body by 
 a considerable space, or vacuole, filled with water (vac.), across 
 which stretch delicate threads of colourless protoplasm, which 
 keep the protoplasmic body in position. The main mass of 
 protoplasm is coloured red or green, as before, and contains the 
 nucleus (??%.). At one end it is drawn out into a kind of beak, 
 from which two very long and slender threads of colourless 
 protoplasm (ft.) pass outwards, through minute apertures in the 
 cellulose wall, into the surrounding water. Owing to their 
 whip-like form and characteristic lashing movements, these are 
 termed nagella. 
 
 It is by the very rapid movements of the nagella that the 
 locomotion of the active Hsematococcus is effected. They are 
 carried in front, and the body of the organism is pulled through 
 the water by their action much as a boat is pulled by a pair of 
 sculls, at a rate which, though very slow when judged by our 
 own standards, appears very rapid when considered in relation to 
 the minute size of the organism. The movements of the nagella 
 are somewhat complex and of an undulatory kind. They appear 
 to be entirely automatic, but it seems probable that they must 
 be performed in response to stimuli which we are unable to 
 recognize. The nagella are much more definite and highly 
 specialized structures than the pseudopodia of an Amoeba. Like 
 the latter, hawever, they owe their value as organs of locomo- 
 tion to that inherent power of contraction which is one of the 
 most characteristic features of living protoplasm. It is probable 
 that each really consists of several very slender filaments, lying
 
 30 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 side by side, and that the complex undulatory movements are 
 due to alternating contractions and relaxations of these. 
 
 The presence of a firm cell-wall makes the protrusion of 
 pseudopodia in the case of Haematococcus quite impossible, and 
 at the same time prevents the organism from taking in any 
 solid food, for there is no aperture through which such food 
 could pass. It must therefore depend entirely for its food supply 
 upon substances which are able to pass through the cell-wall 
 in a state of solution. These substances are all very simple 
 chemical compounds, consisting of certain mineral salts and 
 carbon dioxide gas, which, amongst them, contain all the elements 
 necessary for the formation of protoplasm. They are, however, 
 very stable bodies, with little or no affinity for oxygen gas and 
 little potential energy. They cannot, therefore, by themselves 
 supply the energy which the organism requires for its vital 
 activities. Energy has to be supplied from the environment 
 and the simple food materials thereby partially deoxidised and 
 combined together in more complex and less stable molecules 
 containing stores of potential energy which can be liberated by 
 oxidation as required. 
 
 The energy which Haematococcus uses for the building up of 
 its complex molecules is, as we have already observed for green 
 plants in general, the radiant energy of the sun, in the form of 
 light. The process is known as photosynthesis, and can only 
 take place in organisms which possess chlorophyll or some 
 functionally equivalent pigment, such as hacmatochrome. In 
 some way or other the pigment absorbs the energy of the light 
 rays and renders it available for the process of constructive 
 metabolism (which in plants is also spoken of as assimilation). 
 
 The first step in this complex process involves a chemical 
 decomposition, carbon dioxide, obtained in solution from the 
 surrounding water, being decomposed with evolution of free 
 oxygen gas. 
 
 We have already seen that in the combustion of charcoal 
 the reverse of this takes place, carbon and oxygen combining 
 to form carbon dioxide and the act of combination being accom- 
 panied by the liberation of energy. It is obvious that if energy is 
 liberated in the one process a corresponding amount must be 
 absorbed in the other. 
 
 When a glass jar of water containing Haematococcus, or any 
 green aquatic plant, is placed in bright sunlight the decomposition
 
 PHOTOSYNTHESIS 31 
 
 of carbon dioxide in the plant takes place so rapidly that 
 minute bubbles of oxygen may often be seen rising to the surface 
 of the water. If, for example, we cut off a leafy branch of the 
 common Canadian water weed, known as Elodea canadeusis, and 
 fix it under water in such a jar, it is possible to arrange the 
 experiment so that a regular stream of small oxygen bubbles will 
 be given off from the cut end, and it is further possible to adjust 
 the experiment so delicately that the interposition of a dark 
 screen between the jar and the sunlight will cause the immediate 
 cessation of the stream of bubbles, which will start again the 
 instant the screen is removed. This simple and beautiful 
 experiment clearly demonstrates the dependence of the process 
 of decomposition of carbon dioxide upon the presence of 
 sunlight. 
 
 The oxygen liberated in this way is not (with the exception of 
 a relatively small quantity used in respiration) required by the 
 organism, and is accordingly at once discharged into the sur- 
 rounding medium. The carbon, on the other hand, is needed for 
 the manufacture of new protoplasm. It is never actually set free 
 as carbon, but its molecules are probably recombined under the 
 influence of the sunlight with the molecules of water to form the 
 carbohydrate known as glucose or grape sugar. This process 
 may be represented by the equation 
 
 6C0 2 + 6H 2 = 60 2 + C 6 H 12 6 
 
 (Carbon Dioxide) (Water) (Oxygen) (Glucose). 
 
 It is probable that a simpler compound, possibly formaldehyde 
 (CH 2 0), is formed as an intermediate product, while, on the other 
 hand, the glucose appears to be rapidly converted into starch, 
 which is the first visible product of the process of photosynthesis 
 in the plant cell. 
 
 Starch, like glucose and cellulose, is a carbohydrate, and, 
 though differing in many of its chemical and physical properties, 
 has the same general formula as the latter (C 6 Hi 5 ) n . This 
 means simply that the elements carbon, hydrogen and oxygen 
 are present in the same proportions as in cellulose, but they must 
 be linked together differently in the molecule. 
 
 The first step in the actual construction of the proteid molecule 
 is then the combination of carbon with the elements hydrogen 
 and oxygen to form a carbohydrate. In the higher plants starch 
 first appears in the chlorophyll-containing cells of the leaves in
 
 32 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the form of starch grains, which may afterwards be converted 
 into sugar again and then, in solution, transferred to other parts 
 of the plant, where it is redeposited and stored up, once more in the 
 form of starch grains, for future use, as in the potato and in starch- 
 containing seeds such as peas and beans. 
 
 Both starch and chlorophyll are, at any rate usually, formed in 
 the cell in connection with specialized portions of the protoplasm 
 known as plastids. These are regarded as living bodies which 
 are specially concerned in the formation of chlorophyll, starch and 
 other substances. When they contain chlorophyll they are termed 
 chloroplastids, and in the higher plants they take the form of 
 numerous minute "chlorophyll corpuscles" of definite shape, 
 which occur in abundance in the cells of all green parts, and in 
 connection with which the starch grains are formed (vide Fig. 26). 
 In Haematococcus practically the whole central mass of cytoplasm 
 is coloured by the chlorophyll (or haematochrome) and may 
 perhaps be regarded as a single large chloroplastid. The starch, 
 however, is collected around small, specialized, proteid bodies 
 imbedded in the general mass of cytoplasm. These are known 
 as pyrenoids (Fig. 5, C, py.). 
 
 We have thus seen how the green plant obtains the carbon, 
 hydrogen and oxygen which it requires for the manufacture of 
 protoplasm. Other elements, however, have to be combined with 
 the molecules of carbohydrate before proteids can be formed. These 
 are nitrogen, sulphur and, sometimes at any rate, phosphorus, all 
 of which are obtained by green plants by the decomposition of 
 mineral salts nitrates, phosphates and sulphates which exist in 
 solution in the water or damp soil in which the plant grows. In 
 the higher plants these substances are taken up by the root-hairs 
 and transmitted to the leaves by a system of vessels and tracheids 
 analogous to the circulatory system of animals. In such a plant 
 as Haematococcus they simply diffuse into the protoplasmic body 
 from the surrounding water by the process of osmosis. Exactly 
 what happens when they meet with the carbohydrates we do not 
 know, but further chemical combinations must take place 
 under the influence of sunlight, which finally result in the 
 formation of new proteid molecules which are added to the 
 already existing protoplasm. 
 
 Respiration in Haematococcus probably takes place exactly 
 as in Amoeba, but it is more easily studied in the case of the 
 higher plants. In daylight the process is greatly obscured by
 
 CONJUGATION OF GAMETES 33 
 
 the absorption of carbon dioxide and the evolution of oxygen gas 
 which accompany photosynthesis. In darkness, however, the 
 gaseous interchange which forms the essential feature of respira- 
 tion can readily be detected, carbon dioxide, produced by oxidation 
 of the protoplasm, being given off and oxygen taken in. 
 
 No special organ of excretion has been observed in Haemato- 
 coccus, though a contractile vacuole occurs in closely allied forms, 
 and waste products must simply diffuse into the surrounding 
 water through the permeable cell-wall. 
 
 We have already seen that Hsematococcus multiplies itself by 
 simple fission within the old cell-wall. This process usually 
 results immediately in the production of four new individuals. 
 Under favourable circumstances it may be repeated very rapidly, 
 without the organism going through any true resting stage, so 
 that in a short space of time the number of active zoospores 
 may be very largely increased. The individuals thus produced 
 are usually aJl of the same form, and ultimately of the same size, 
 as the parent. Occasionally, however, a somewhat different 
 process of multiplication takes place. Instead of dividing into 
 four relatively large zoospores a resting individual may divide into 
 thirty-two or sixty -four much smaller " microzooids " (Fig. 5, D), 
 which differ from the ordinary active form in the absence of the 
 characteristic cell-wall with its underlying vacuole. 
 
 The microzooids (Fig. 5, E) swim actively about by means of 
 their nagella. Sooner or later, however, they come together in 
 pairs (Fig. 5, F, G), and the members of each pair fuse completely 
 with one another to form a single individual (Fig. 5, H) with 
 four flagella, which presently loses its flagella, secretes around 
 itself a thick cell-wall, and enters upon the resting state 
 (Fig. 5, J, K). From this resting individual new generations will 
 be produced by the ordinary method of division into zoospores. 
 
 We have here an excellent illustration of what is usually 
 termed sexual reproduction, the essential feature of which is the 
 union or conjugation of two sexual cells or gametes (in this case 
 the microzooids) to form a single cell, the zygote, which is the 
 starting point of a fresh series of cell generations. This important 
 process will be discussed more fully in a subsequent chapter. 
 
 We have spoken of Amoaba as an animal, and, as we have seen, 
 many people regard Haematococcus as a plant. We must next 
 endeavour to find out what it is that really differentiates a plant 
 from an animal. Of course amongst the more highly organized
 
 34 OUTLINES OF E VOLUTION AEY BIOLOGY 
 
 members of the animal and vegetable kingdoms we can point to 
 many obvious distinctions. The higher plants are fixed and 
 stationary, while the animals move about from place to place by 
 means of special organs of locomotion. The animals have 
 complex digestive, respiratory, excretory, nervous and sensory 
 organs, which are wanting in the plants. Lastly, the animals 
 have no chlorophyll and cannot therefore, like the green plants, 
 obtain their supplies of energy directly from the sun's rays by 
 photosynthesis, but must depend upon the potential energy con- 
 tained in the complex molecules of their food, which they obtain 
 ready made from the bodies of other organisms. 
 
 Amongst the lower organisms, however, we find that most of 
 these distinctions disappear. Thus many of the lower plants 
 move about actively while many of the lower animals, such as 
 the sponges, hydroids and corals, are fixed and stationary in the 
 adult condition, though still showing their animal nature in other 
 respects, such as their method of nutrition. This mixture of 
 what were once regarded as distinctively animal and vegetable 
 characters in such forms as the corals and hydroids gave rise to 
 the name " zoophytes," or animal-plants, by which these organisms 
 were known to the older naturalists. 
 
 When we descend to forms still lower in the scale of organiza- 
 tion, consisting each of a single cell, we find that every dis- 
 tinction may disappear except that of the presence or absence of 
 chlorophyll and the mode of nutrition immediately dependent 
 thereon : plant-like or holophytic as in Haematococcus, animal- 
 like or holozoic as in Amoeba. 1 
 
 It cannot be maintained, however, that even these characters 
 form an absolute distinction between plants and animals, for, in 
 the first place, many undoubted plants, such as the Fungi, have 
 lost their chlorophyll by degeneration, and, in the second place, 
 while botanists claim Haematococcus and the forms closely related 
 to it as plants, zoologists claim them as animals, chiefly because 
 they are so closely related in structure to other unicellular 
 flagellates which contain no chlorophyll that we cannot refuse to 
 include them in the same group. 
 
 1 The nutrition of any typical green plant is holophytic, that of any typical 
 animal holozoic. The latter term implies the taking in of solid food derived from 
 the bodies of other organisms, and is thus distinguished from the saprophytic type 
 of nutrition met with in many of the lower animals and plants (e.g. Fungi), which 
 consists in the absorption of liquid food derived from the decaying bodies of other 
 organisms.
 
 PLANTS AND ANIMALS 35 
 
 The explanation of the difficulty really lies in the fact that 
 both plants and animals originally sprang from common unicel- 
 lular ancestors which were neither the one thing nor the other. 
 The first appearance of chlorophyll initiated the great cleavage 
 between the animal and vegetable kingdoms. Thenceforward 
 the two great groups developed each along lines of its own. By 
 virtue of their chlorophyll the green plants became the great 
 proteid manufacturers of the world, and the animals became 
 dependent upon them for their food supply. Animals are largely 
 dependent upon green plants in another respect also, for the 
 latter, as we have seen, split up the carbon dioxide, formed as a 
 waste product in the respiration of both groups, and thus set free 
 fresh supplies of the necessary oxygen. 
 
 Closely correlated with the differences in their mode of nutri- 
 tion are the great differences in the mode of life of the higher 
 plants and animals. Plants have no need to move from place to 
 place in search of food, which they obtain from the air and the 
 soil, and the supplies of which are constantly renewed by wind 
 and rain. Highly organized animals, on the other hand, would 
 soon exhaust their supplies if they remained always in the same 
 place, and it is doubtless the necessity for actively seeking out 
 and even contending one with another for fresh supplies that 
 has brought about the wonderful elaboration and perfection of 
 their organization, while the fact that in so many cases they 
 devour one another, instead of remaining directly dependent upon 
 vegetable organisms, must have greatly intensified the struggle 
 for existence and correspondingly increased the rate of progress 
 in their evolution. 
 
 D 2
 
 CHAPTER IV 
 
 The cell theory Unicellular organisms Differentiation and division of 
 labour Co-operation The transition from the unicellular to the 
 mult icellular condition The early development of niulticellular animals 
 and plants. 
 
 WE have seen that, although Haematococcus and Amoeba differ 
 widely from one another in various respects, they nevertheless 
 exhibit a fundamental agreement in structure, for each consists 
 essentially of a single nucleated mass of protoplasm, each is a 
 single cell. 
 
 The history of the term cell is a curious one, and affords a 
 good illustration of the manner in which our scientific conceptions 
 
 
 
 
 
 FIG. 6. Thin Section of a Bottle Cork, showing cellular Structure, X 170. 
 (From a photograph.) 
 
 gradually become modified and improved as our knowledge 
 increases. Nothing was known of cells before the invention of 
 the microscope, but in the latter half of the seventeenth century 
 this invention opened up an entirely new field of research, and 
 enabled tbe earlier microscopists to lay the foundations of the 
 modern science of biology. To Eobert Hooke has been assigned the 
 credit of first observing the cellular structure of vegetable tissues, 
 and his observations, published in 1665, were soon afterwards
 
 ORIGIN OF THE TERM CELL 
 
 37 
 
 confirmed by Nehemiah Grew, who published his great work on 
 the Anatomy of Plants in 1672. 
 
 It is now a matter of common knowledge that many vegetable 
 tissues, such as cork and pith, when seen in section under the 
 microscope, exhibit a honeycomb-like appearance, being composed 
 
 CoPt. 
 
 Fid. 7. Part of a cross Section of a Maize Boot, showing cellular 
 
 Structure, X 84. (From a photograph.) 
 
 Cort., cortex; pi., pith; V., vessels. 
 
 of rectangular or polygonal, or it may be spherical chambers 
 separated from one other by firm walls. This structure is very 
 well shown in Fig. 6, which represents part of a thin section of an 
 ordinary bottle cork, and in Fig. 7, which represents part of a 
 transverse section of a root of maize. It was the resemblance to 
 a honeycomb that led to the application of the term cell to 
 these chambers. The earlier observers naturally attached most
 
 38 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 importance to the cell-walls, which indeed are alone visible in 
 dead tissues such as dry cork and pith. 
 
 It was not until 1846 that von Mohl first gave the name 
 protoplasm to the slimy contents of the cells in living tissues. 
 It then gradually became evident that this protoplasm was the 
 really vital constituent of the cell, and that it was identical in 
 nature with the substance of which minute naked organisms 
 such as the Amoaba are composed, and which was already 
 known by the name sarcode, given to it by Dujardin in 
 1835. 
 
 The cell theory, first propounded in a very imperfect form by 
 Schleiden and Schwann, about the year 1838, rapidly developed, 
 during the course of the nineteenth century, into one of the most 
 fertile generalizations of natural science. At the present day the 
 term cell is extended to protoplasmic units which may have no 
 cell-walls at all, and to which therefore it is etymologically quite 
 inapplicable, and for a long time a cell has been defined simply 
 as a single nucleated mass of protoplasm. 
 
 In accordance with the cell theory such nucleated masses of 
 protoplasm- are the organic units of which the bodies of all living 
 things are built up. The simpler organisms, such as Amoeba 
 and Haematococcus, consist each of a single unit only, and are 
 therefore said to be unicellular, while the more complex forms, 
 both of animals and plants, consist each of many such units united 
 together in a muiticellular body. Moreover, we now know that cells 
 never originate de novo but multiply by division, so that each one 
 is the immediate descendant of a pre-existing cell, a very 
 important fact which was emphasized by Virchow in his often 
 quoted phrase " Omnis cellula e cellula." 
 
 The zoologist includes under the name Protozoa all those 
 unicellular organisms which he claims as members of the animal 
 kingdom, whilst the unicellular plants are relegated to the domain 
 of the botanist under the name Protophyta, but, as we have 
 already seen, it is impossible to draw a rational line of demarcation 
 between these two groups and they are often included together 
 under Haeckel's term Protista. 
 
 The outstanding feature in all these simple forms of life is that 
 the single cell is a complete and self-supporting organism. It 
 has to perform all the necessary vital functions for itself, by means 
 of such simple organs, temporary or permanent, as can be 
 produced by differentiation within the microscopic limits of its
 
 PAKAMCECIUM 39 
 
 protoplasmic body. 1 It is surprising what a high degree of 
 organization, as indicated by complexity of structure, may be 
 attained in such a case. 
 
 Differentiation and division of labour are the results of progressive 
 evolution, and at the same time the means by which further 
 progress is effected. Even in Amoeba and Haematococcus we see 
 clearly enough the operation of these two great principles. Many 
 unicellular organisms, however, exhibit a far higher degree of 
 organization. The Protozoon, Paramcecium (Fig. 8), so common 
 in infusions of decaying vegetable matter, swims actively about by 
 means of innumerable short vibratile cilia which project all over 
 the surface of the body. It has a definite mouth, through which 
 
 ^ , , , 
 MY 
 
 FIG. 8. Paramvecium attrelm, X 300. (From Marshall and Hurst's 
 " Practical Zoology.") 
 
 AV, anterior contractile vacuole (dilated) ; EC, ectoplasm with trichocysts ; EN, endo- 
 
 S'asm ; EP, micronucleus ; FV, food vacuole ; M, mouth ; MY, contractile fibrillae ; 
 , meganucleus ; OG, groove leading to mouth : PV, posterior contractile vacuole 
 (contracted) ; TR, discharged trichocyst threads ; X, cilia. 
 
 solid food particles are taken in, and a less definite anal spot at 
 which faecal matter is ejected. It has special weapons of offence or 
 defence (trichocysts) which can shoot out from the surface of the 
 body long threads when the animal is irritated. It has two 
 contractile vacuoles, each with a system of radiating canals 
 discharging into it, and it has two nuclei, large and small 
 (meganucleus and micronucleus), which appear to fulfil different 
 functions and each of which doubtless has a complex structure of 
 its own. How complex the structure of the nucleus may be we 
 shall be better able to judge when we come to speak of the 
 phenomena of nuclear division in a subsequent chapter. 
 
 1 The organs into which a single cell may be differentiated are sometimes spoken 
 of as organellae, but if we define an organ as any part of an organism which is 
 specialized for the fulfilment of some particular function, it is quite unnecessary to 
 distinguish the organs of a single cell by a special term.
 
 40 OUTLINES OF EVOLUTIONAKY BIOLOGY 
 
 Paramoecium and many other Protozoa, again, have specialized 
 contractile threads of protoplasm lying just beneath the surface 
 of the body, comparable to the muscle fibres of higher animals, 
 which enable them to perform movements of a different kind 
 from those effected by flagella or cilia. By means of such con- 
 tractile fibres, localized mainly in a long stalk, the bell- 
 animalcule, Vorticella (Fig. 9, 10), can instantaneously draw in 
 its ciliated disc and pull itself out of harm's way on the approach 
 of danger. Other examples of these highly organized ciliate 
 Protozoa (Infusoria) are shown in Fig. 9.' These forms should 
 be compared with the Radiolaria and Foraminifera represented in 
 Figs. 3 and 4. 
 
 Another very important factor in the progress of organic 
 evolution is supplied by the principle of co-operation between 
 different organic units. This is illustrated to some extent in the 
 process known as colony-formation met with in certain 
 Protozoa and Protophyta. Carchesium, Epistylis and Zootham- 
 nium (Fig. 9, n 15), for example, are colony-forming Infusoria 
 closely related to Vorticella. Vorticella itself multiplies rapidly 
 by simple longitudinal fission. The bell-shaped protoplasmic 
 body at the end of the stalk divides into two parts, one of which 
 swims away, attaches itself to some foreign object, and develops 
 a new stalk. If, instead of separating, the two daughter cells 
 remained together and went on dividing, and if this division also 
 extended each time to the upper part of the stalk, the organism 
 would presently arrive at a branching, tree-like condition. This 
 is what has happened in Carchesium, Epistylis and Zootham- 
 nium, and also in various other Protozoa. 
 
 This arborescent type of colony-formation, moreover, is by 
 no means the only one met with amongst the Protista. It is 
 characteristic of stalked forms. Other forms, which are not 
 stalked, may give rise to free-swimming or floating, solid or 
 hollow aggregates of plate-like or it may be spherical shape. 
 The development of such colonies is perhaps nowhere better 
 seen than in the small group of unicellular organisms to which 
 Haematococcus belongs, the members of which are known, on 
 account of their plant-like character and their possession of 
 flagella, as Phytoflagellata. 
 
 Hsematococcus, or the closely related Chlamydomonas, may be 
 taken as the starting point of the series. These two are always 
 solitary, the individuals separating completely from one another
 
 CILIATE PROTOZOA 
 
 41 
 
 FIG. 9. Various forms of Ciliate Protozoa (Infusoria), highly magnified. 
 (From Haeckel's " Kunstfonnen der Natur.") 
 
 1 Codonella ; 2, 3, Dictyacysta (shell only) ; 4, Tintinnopsis (shell only) ; 5, Cyttaro- 
 cyclis (shell only); 6, Petalotricha (shell only); 7, 8, Stentor ; 9, Freia ; 10, Vor- 
 t'iccUii ; 11, 12, Carcliesium ; 13, Epistylis ; 14, 15, Zoothamnium.
 
 42 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 after each cell-division. In Pandorina (Fig. 10), however, which 
 is another commo'n fresh-water organism, the individuals pro- 
 duced by fission remain together and form a solid ball, composed 
 of from sixteen to sixty-four cells, enclosed in a gelatinous 
 envelope. These little colonies swim about actively by means of 
 their flagella, which project from the surface in pairs, one pair 
 belonging to each individual. Multiplication is usually effected 
 by the division of each individual cell into sixteen, so that as 
 many daughter colonies are formed as there were cells in the 
 parent colony, and these daughter colonies finally separate 
 
 from one another. Eudorina 
 (Fig. 40) forms slightly larger 
 colonies of a similar kind, but 
 with the component individuals 
 somewhat widely separated from 
 one another by the gelatinous 
 matrix. Finally, in Volvox 
 (Fig. 11), one of the most 
 familiar and beautiful of the 
 microscopic fresh -water organ- 
 isms, we find the individual 
 cells, each one still closely 
 resembling a Haematococcus, 
 arranged side by side in the 
 gelatinous wall of a hollow 
 sphere, with their flagella pro- 
 jecting from the surface, while 
 daughter colonies are frequently seen swimming about freely in 
 the interior of the sphere. 
 
 In all these colonies the gelatinous matrix or ground substance 
 is formed as a secretion by the cells which it serves to hold 
 together. It is noteworthy that the connection between the 
 individual cells is much more intimate in Volvox than it is in the 
 lower types, for they are all united together by extensions of 
 their protoplasmic bodies which give a reticulate appearance to 
 the wall of the sphere. Volvox, moreover, attains a relatively 
 large size, and there may be as many as 22,000 cells in a single 
 colony, though about half that number appears to be more 
 usual. 
 
 We meet with another example of the formation of hollow 
 spherical colonies in the case of the beautiful Radiolarian, 
 
 FlG. 10. Pandorina morum, X 400. 
 (From Vines' " Botany.") 
 
 A, free-swimming colony ; B, conjugation 
 of two gametes.
 
 RESULTS OF COLONY FORMATION 
 
 43 
 
 Sphaerozoum (Fig. 12), which floats about in the surface waters of 
 the open ocean and has in each of its component cells a supporting 
 skeleton of branched siliceous spicules. 
 
 The co-operation of a larger or smaller number of cells to form 
 a colony at once opens up new possibilities with regard to 
 
 FIG. 11. Volvox aureus. (From Weismann's "Evolution Theory," after 
 Klein and Schenck.) 
 
 A, mature colony containing daughter colonies (t) and ova (o) ; B, group of 32 developing 
 spermatozoa seen end on ; C, the same seen sideways ; D, mature spermatozoa, 
 
 differentiation and division of labour. If a sufficiently good 
 understanding, so to speak, can be established between the 
 different members (or zooids) of the colony it will no longer be 
 necessary for each one to do everything for itself. At the expense 
 of becoming mutually dependent upon one another they will be 
 able to specialize in different directions, some identifying them- 
 selves with one necessary duty or function and some with
 
 11 
 
 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 another. As a result of this specialization the various functions 
 may be much more efficiently and economically performed, but a 
 no less important result will be that the different individuals will 
 no longer be able to lead independent lives if separated from 
 one another they will perish because unable to perform for 
 themselves individually all the functions which are necessary for 
 their existence. 
 
 In colonies of Protista we meet with little if any of this 
 differentiation and division of labour ; from the physiological 
 point of view the cell units remain almost if not quite inde- 
 pendent of one another, and that is why they are still classed 
 
 amongst the unicellular organ- 
 isms. There can be no doubt, 
 however, that it was this habit 
 of colony-formation that led to 
 the origin of true multicellular 
 animals and plants Metazoa 
 and Metaphyta from unicel- 
 lular ancestors. The compo- 
 nent cells of a colony gradually 
 became integrated to form an 
 individual of a higher order, 
 and this process was accom- 
 panied by that differentiation 
 and division of labour which 
 in course of time led to the 
 astonishing complexity of 
 structure which characterizes 
 the higher members of both the animal and vegetable kingdoms. 
 In contrast to this complexity of the organism as a whole we 
 shall find that the individual cells of one of the higher animals 
 or plants are usually much simpler in structure than the more 
 highly organized Protista. After what we have said about 
 differentiation and division of labour the reason for this should 
 be sufficiently obvious. 
 
 Perhaps no more convincing demonstration of the applicability 
 of the cell theory to the higher plants and animals could be 
 given than that which is afforded by the study of development, 
 for, however highly organized a plant or an animal may be in the 
 adult condition, it always commences its individual existence as 
 a single cell the fertilized ovum or zygote and attains the 
 
 FIG. 12. A colony of Sphserozoum, 
 cut in half. (After Haeckel, in 
 " Challenger " Eeport.)
 
 SEGMENTATION OF THE OVUM 45 
 
 adult state by means of a longer or shorter series of cell-divisions. 
 The earlier cell-divisions constitute what is termed the segmenta- 
 tion of the ovum, and result in the formation of a number of 
 daughter cells, which exhibit little or no differentiation amongst 
 themselves and are known as blastomeres. Sooner or later, 
 however, differentiation sets in and leads to the formation of 
 more or less highly specialized tissues or cell aggregates. 
 
 In the primitive, fish-like Amphioxus, 1 for example, the ovum 
 (Fig. 13, I) is a spherical, nucleated cell about ^oth inch in 
 diameter. After fertilization by a male gamete or spermatozoon 
 it divides first into two equal and similar embryonic cells or 
 blastomeres (Fig. 13, II) by a vertical cleavage. Another vertical 
 cleavage, at right angles to the first, divides each blastomere 
 into two smaller ones (Fig. 13, III). This is followed by a hori- 
 zontal cleavage, which results in the formation of eight cells, 
 in two tiers of four each, the four upper ones being slightly 
 smaller than the four lower (Fig. 13, IV). The blastomeres go on 
 dividing and presently arrange themselves in the form of a 
 hollow sphere, whose wall is composed of a single layer of cells 
 (Fig. 13, VII). The embryo has now reached the blastula (or 
 blastosphere) stage of its development, a stage which is passed 
 through by all multicellular animals whose life-history follows 
 a typical course unmodified by secondary features. 
 
 If we allow for the fact that the cells all remain together 
 instead of separating from one another after each division, it is 
 obvious that the segmentation of the fertilized ovum into blasto- 
 meres is identical with the process of multiplication by fission in 
 such a protozoon as Amoeba. The fact that each blastomere is 
 equivalent to a single protozoon and multiplies in a similar 
 manner has been experimentally demonstrated in an extremely 
 interesting way by Herbst, whose observations were made upon 
 the development of the common sea urchin (Echinus). He 
 found that, if the eggs are allowed to develop in sea water from 
 which every trace of calcium has been removed, the blastomeres 
 actually do separate after each division and give rise to 808 
 individual cells, which swim about separately like so many 
 flagellate Protozoa instead of remaining united together and 
 co-operating with one another to form the normal blastula. The 
 blastula stage itself, of course, corresponds very closely in general 
 features with such a protozoon colony as we meet with in the case 
 
 1 The external appearance of the adult Amphioxus is represented in Fig. 118.
 
 46 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 FIG. 13. Early Development! of Amphioxus. (Adapted from Ziegler's 
 Models, after Hatschek.) 
 
 I, the fertilized egg ; II, two blastomeres formed by the first cleavage ; III, stage with 
 four blastomeres; IV, stage with eight blastomeres; V, stage with sixteen blasto- 
 meres ; VI, stage with thirty-two blastomeres, cut in half vertically ; VII, blastula or 
 blastosphere stage, cut in half; VIII, early stage of gastrulation, cut in half;
 
 BLASTULA AND GASTEULA 47 
 
 of Volvox or Sphserozoum (compare Figs. 11 and 12), a point to 
 which we shall have occasion to return in a later chapter. 
 
 In the blastula of Amphioxus the cells are still all very much 
 alike, except that those at one pole of the sphere are somewhat 
 larger than the others. Differentiation, however, now sets in in 
 a very marked manner, and the cells thereby become divided into 
 two distinct groups. That portion of the wall of the hollow 
 blastula which is formed by the larger cells becomes pushed 
 inwards or invaginated (Fig. 13, VIII), much as a tennis ball 
 may be pushed in by the pressure of the thumb, until it comes 
 into contact with the inner surface of the remainder of the wall. 
 In this way the original cavity (blastocoel) is obliterated and 
 the embryo takes on the form of a double cup (Fig. 13, IX, X). 
 The cavity of this cup is an entirely new formation. It is the 
 primitive digestive cavity of the animal and is known as the 
 enteron or gastral cavity. Its mouth gradually contracts to a 
 narrow aperture, the blastopore. The outer layer of cells forming 
 the wall of the cup is termed the epiblast and the inner the 
 hypoblast, and the two are continuous with one another all round 
 the blastopore. The stage now reached is spoken of as the 
 gastrula stage. 
 
 In nearly all Metazoa the blastula stage of development is 
 followed by one exhibiting the essential features of the gastrula, 
 or at any rate some indication thereof. The primary differentia- 
 tion of the component cells of the body into an outer epiblast 
 (which becomes the ectoderm of the adult) and an inner 
 hypoblast (which becomes the endoderm of the adult), the one 
 serving for protection and for the maintenance of all the 
 necessary relations with the external environment, and the other 
 surrounding a gastral cavity and concerned with the digestion of 
 the solid food which the animal captures, is closely correlated 
 with the characteristic animal or holozoic method of nutrition. 
 
 In later stages of development, in all animals higher than the 
 
 IX, young gastrula in longitudinal section ; X, older gastrula in longitudinal section ; 
 XI, XII, XIII, transverse sections of older embryos, showing the formation of the 
 coelomic pouches, notochord and neural tube ; XIV, longitudinal section of embryo 
 of about the same age as XII; XV, side view of embryo of same age as XIV with 
 the epiblast stripped off from one side to show the mesoblastic somites formed from 
 the coalomic pouches. 
 
 bp. blastopore ; blc. blastocoel or segmentation cavity; c.p. ccelomic pouch; ent. enteron ; 
 ep. epiblast ; hyp. hypoblast; m.s. mesoblastic somites ; not. uotochord; n.p. neuro- 
 pore or anterior opening of neural tube; n.pl. neural plate; n.t. neural tube (central 
 nervous system, formed by folding of neural plate); o.c.p. openings of coelomic 
 pouches into enteron.
 
 48 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Coelenterata (a group which includes such forms as jelly-fish, sea 
 anemones and corals), a third layer of cells makes its appearance 
 between the first two and forms the mesoblast (giving rise to the 
 mesoderm of the adult). In Amphioxus this mesoblast arises as 
 a series of hollow, pouch-like outgrowths of the hypoblast (Fig. 13, 
 XI, XII, c.p.}, and the cavities which these pouches contain give 
 rise to the body cavity or coelom, while their walls give rise to 
 mesodermal tissues. 
 
 In accordance with what is commonly known as the germ- 
 layer theory, these three layers of cells, epiblast, mesoblast, and 
 hypoblast, can be recognized in the embryos of all the higher 
 animals, and from them all the various parts of the adult body 
 are derived. The epiblast gives rise to the outer skin or 
 epidermis, the nervous system and the essential parts of the 
 sense-organs ; the hypoblast gives rise to the lining epithelium of 
 the alimentary canal and its various outgrowths, including the 
 digestive glands, while from the mesoblast arise the various 
 connective and skeletal tissues, the blood-vessels and the essential 
 organs of reproduction. The cells of which each tissue is com- 
 posed acquire a characteristic structure of their own, and in 
 this way the histological differentiation of the body is gradually 
 completed. 
 
 With the early stages in the development of Amphioxus, 
 described above, we may compare the corresponding processes in 
 the life-history of a flowering plant. Here the ovum, instead of 
 being at once liberated from the parent, undergoes all the 
 earlier stages of its development within the so-called ovule, 
 which, with the contained embryo, will ultimately be set free as 
 the ripe seed (compare Chapter VIII.). 
 
 As a definite example we may take the common weed known as 
 shepherd's purse, Capsella bursa-j)astoris (Fig. 14). The ovum 
 lies in a cavity in the ovule termed the embryo-sac. It is at first 
 a single nucleated mass of protoplasm without any cell-wall. 
 After fertilization it divides into two cells separated by a wall, 
 and the process is repeated by a series of divisions parallel to the 
 first one until we have a row of cells. The cell at one end of 
 this row, known as the basal cell (Fig. 14, A, B, I.e.), is larger 
 than the others and is attached to the wall of the embryo-sac. 
 At the opposite end is a rounded cell, .called the embryonic cell, 
 from which the body of the young plant will be chiefly formed. 
 The remainder of the row, including the basal cell, is known as
 
 EARLY DEVELOPMENT OF CAPSELLA 
 
 49 
 
 the suspensor, and serves for the attachment and nutrition of the 
 embryo, which at first develops entirely at the expense of the 
 parent, having no means of feeding itself. All the cell-divisions 
 
 -b.c. 
 
 FIG. 14. Four stages in the early Development of a Flowering Plant, 
 Capsella bursa-pastoris, X 200. (From Scott's " Structural Botany," 
 after Hanstein.) 
 
 b.c., basal cell of suspensor; ct., cotyledons growing out ; d, dermatogen ; e, embryonic 
 group of cells; ff.p., growing point of stem; h, uppermost cell of suspensor; 
 pe, penblem ; pr, pi, cells of plerome; j s cells derived from h. 
 
 so far have taken place in planes parallel to one another, thus 
 giving rise to a single row of cells, but the embryonic cell now 
 divides into four parts by the formation of two cell-walls at right 
 angles to each other and to the preceding divisions (Fig. 14, 
 A, e), and each of these again divides into two by the formation
 
 50 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 of a wall parallel to the earlier divisions (Fig. 14, B, e). The 
 embryo now consists of eight cells or " octants." The next divi- 
 sions are parallel to its surface and cut off a superficial covering 
 of cells, known as the dermatogen (Fig. 14, C, d), from which 
 the whole of the epidermis of the adult shoot will be derived. 
 The mass of rapidly dividing cells within this soon becomes 
 differentiated into two parts, the plerome (Fig. 14, D,pr,pl), lying 
 in the axis of the embryo, and the periblem (Fig. 14, D, pe), lying 
 between the plerome and the dermatogen. The plerome will 
 give rise to the vascular system of the plant and the tissues asso- 
 ciated therewith, while the periblem will give rise to the cortical 
 tissues of the stem and root and the mesophyll or middle layer of 
 the leaves. The periblem of the root, and the root-cap, are really 
 formed from the uppermost cell of the suspensor (Fig. 14, C, h). 
 By further cell-multiplication and differentiation in the three 
 primary layers dermatogen, periblem and plerome all the 
 various tissues of the adult plant are proc[uced. 
 
 Thus we see that in the higher plants and animals alike the 
 development of the individual from the fertilized egg consists in 
 the first place of a process of cell-division, and in the second place 
 of differentiation 'between the cells thus produced, accompanied 
 by grouping of the differentiated cells to form the more or less 
 sharply denned tissues of the adult.
 
 CHAPTEE V 
 
 The cell theory as illustrated by the histological structure of the higher 
 animals and plants Limitations of the cell theory The cell as the 
 physiological unit. 
 
 IN all the higher animals and plants the constituent cells of 
 the adult body are grouped in more or less well defined tissues, 
 which originate from the fertilized ovum in the manner indicated 
 in the last chapter, and the cells of each tissue co-operate 
 with one another in the fulfilment of some common function. 
 The study of the microscopic structure of tissues is termed 
 histology. 
 
 As examples of animal tissues we may take blood, epithelium, 
 fat, cartilage, muscular tissue and nervous tissue, as met with 
 in typical vertebrates. 
 
 Blood is exceptional in that it is a liquid tissue, a condition 
 which is of course necessary in order that it may circulate through 
 the blood-vessels and perform its functions as the distributor 
 throughout the body of food material and oxygen, and the carrier 
 of carbon dioxide and other waste products from all the various 
 parts of the body to the special organs of respiration and excre- 
 tion. Floating in the liquid portion, or plasma, are found two 
 kinds of cells, the white and red blood-corpuscles. 
 
 The white corpuscles, or leucocytes (Fig. 15, a.), closely 
 resemble AmrebaB. They are colourless, nucleated cells, exhibiting 
 amoeboid movements, and they have the remarkable power of 
 creeping through the thin walls of the blood-capillaries into the 
 surrounding tissues. Like the Amoeba they feed, in part at any 
 rate, by taking in and digesting the bodies of other minute 
 organisms, and they grow and multiply by simple fission. They 
 exhibit a much greater degree of independence than most of the 
 cells of the body and can even live outside the body for a time if 
 kept in suitable culture media and at the proper temperature. 
 Their most important function appears to be to defend the body 
 from the attacks of bacteria and other harmful micro-organisms. 
 
 s 2
 
 52 OUTLINES OF EVOLUTIONAEY BIOLOGY 
 
 If these gain entrance into the tissues at any weak spot they are 
 set upon by the leucocytes and literally devoured. This process 
 is known as phagocytosis, and in this capacity the leucocytes are 
 often spoken of as phagocytes. It is obvious that the health of 
 the body must depend largely upon the activity of the phagocytes 
 and their efficiency in dealing with disease-producing " germs." ^ 
 The white blood-corpuscles, then, differ in no essential parti- 
 cular as regards their structure and mode of life from so many 
 Protozoa. It is true they cannot live permanently outside the 
 
 : v, 
 
 
 FIG. 15. Blood Corpuscles of the Frog, X 326. (From a photograph.) 
 
 a., white corpuscle or leucocyte; b., red corpuscles or hsematids. The nuclei appear 
 light-coloured in the photograph owing to their having been stained blue in the 
 preparation. 
 
 body, but that is also the case with many parasitic Protozoa 
 which live in the blood of other animals. That they are not 
 independent organisms, but form an integral part of the body 
 in which they occur, is, however, obvious from the fact that 
 they have a common origin with all the other tissues from the 
 developing ovum. 
 
 The red corpuscles, or hsematids, are very different bodies. 
 They float passively in the blood-stream and serve as the carriers of 
 oxygen gas from the respiratory organs to the various tissues. 
 Unlike the leucocytes they have definite and constant outlines, 
 though, owing to the flexible nature of the thin cell-membrane by 
 which they are enclosed, they may undergo temporary distortion.
 
 HISTOLOGY OF HIGHER ANIMALS 
 
 53 
 
 Individually of a pale yellow colour, and so small as to be quite 
 invisible to the naked eye, they occur in such vast numbers as to 
 give the blood its characteristic scarlet or purple colour. It is 
 estimated that in a cubic millimetre of human blood there are 
 about five millions of these red corpuscles. 
 
 In the frog the haematids are flattened oval cells about 0'02 mm. 
 in longer diameter, with a centrally placed nucleus (Fig. 15, b.). 
 In man they are a good deal smaller, and circular in out- 
 line, like biscuits, and, as in all the Mammalia, the nucleus 
 
 FIG. 16. Epithelium from the Mesentery of a Frog, x 280. (From a 
 photograph.) 
 
 The underlying tissues are seen indistinctly through the transparent epithelial cells, 
 whose outlines only are visible. 
 
 has entirely disappeared. They owe their red colour, and 
 their power to act as carriers of oxygen, to the presence in them 
 of a peculiar pigment known as haemoglobin, with which the 
 oxygen appears to enter into a state of loose chemical combination 
 from which it is easily liberated again when required by the tissues. 
 They may indeed be regarded as mere bags of haemoglobin, formed 
 from highly specialized cells which have lost all power of indepen- 
 dent existence. They cannot even multiply by division, but, as 
 the old ones are worn out, they are replaced by the formation 
 of new ones from less specialized cells in various parts of the 
 body.
 
 54 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 The term epithelium is applied to any layer of cells covering 
 a free surface. An epithelium is therefore primarily a protective 
 layer, but it frequently becomes modified for other purposes. It 
 may, for example, become glandular, certain of its cells taking on 
 the function of secretion, or it may become sensory, with cells 
 specially adapted for the reception of stimuli. It may consist of 
 a single layer of cells or of several layers one above the other. 
 
 A good example of the single-layered type is found in the 
 peritoneal epithelium which covers the surface of the mesentery 
 or membrane supporting the intestines in the coelom or body 
 cavity. Fig. 16 represents a portion of such an epithelium in 
 
 which the cell-outlines have 
 been rendered very distinct 
 by staining with silver 
 nitrate ; the nuclei, how- 
 ever, are not shown by this 
 method. Each cell has the 
 form of a thin, flat, poly- 
 gonal plate, and they all 
 fit accurately together at 
 their edges. With this 
 Figure should be compared 
 Fig. 28, A, which represents 
 a single-layered epithelium 
 prepared in such a way as 
 to show both nuclei and cell- 
 outlines. 
 
 If we gently scrape the inside of the cheek with some clean, 
 blunt instrument, and examine the milky-looking product under 
 the microscope, we shall find that it contains a number of 
 flattened, scale-like bodies (Fig. 17), either entirely separated 
 from one another or still more or less connected together by their 
 edges, and probably to some extent overlapping. These also are 
 epithelial cells, which have formed part of the special epithelium 
 known as the epidermis, which is derived from the epiblast or 
 external cell-layer of the embryo and covers the outer surface of 
 the body. If we examine our preparation more carefully we shall 
 find that the cells have an irregularly rounded contour and that 
 they measure about 0'08 mm. in diameter. There is a more or 
 less centrally placed nucleus (which appears dark in the figure 
 owing to the manner in 'which it has been stained) and the 
 
 FIG. 17. Five isolated Epithelial Cells 
 from the inner Surface of thehuman 
 Cheek, x 420. (From a photograph.) 
 The nuclei are stained darkly.
 
 HISTOLOGY OF HIGHER ANIMALS 
 
 55 
 
 cytoplasm is granular. They are, however, dead, or at any rate 
 moribund cells. Owing to the constant friction to which the 
 
 epd. 
 
 der.< 
 
 FIG. 18. Vertical Section of Stratified Epithelium from the Mouth of a 
 footal Cat, X 280. (From a photograph.) 
 
 der., dermis; epd., epidermis; s.m., Stratum Malpighii, or layer of actively dividir 
 
 surface of the body is exposed such cells are always being rubbed 
 off, and it is these which, by accumulating in places where the 
 friction is less severe, form the so-called scurf of the hair. 
 
 Fia. 19. Section of Adipose Tissue from which the Fat has been dissolved 
 out, leaving the thin-walled Cells empty and shrivelled, X 175. (From 
 a photograph.) 
 
 As they are worn away their places are taken by other cells 
 which arise from a deeply situated layer, at the lower limit of the
 
 56 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 epidermis, in which cell-division goes on actively throughout life. 
 In this way a many-layered or stratified epithelium is formed, as 
 shown in Fig. 18, which represents a small portion of a thin 
 vertical section through the epidermis (epd.) in the mouth of a 
 foatal cat. At the lower limit of the epithelium, resting imme- 
 diately upon the connective tissue of the dermis (der.), is seen 
 the layer of actively dividing cells (s.m.). The cells cut off from 
 this layer are gradually pushed outwards, becoming flattened and 
 scale-like as they approach the surface. 
 
 Fat, or adipose tissue, consists of an aggregation of more or 
 
 V * 
 
 %' 
 
 I ' ~,* ; 
 
 FIG. 20. Section of Cartilage, showing the Cartilage Cells (c.c.) imbedded in 
 the transparent intercellular Matrix or Ground Substance (in.), X 390. 
 (From a photograph.) 
 
 less globular cells, swollen out by the accumulation within them 
 of drops of oil. If the oil is dissolved out by suitable reagents 
 the empty cells are left with their cell- walls or membranes in a 
 somewhat shrivelled condition, as shown in Fig. 19, and the 
 tissue now bears a curious resemblance, when seen in section, to 
 vegetable parenchyma, such as is seen in sections of pith 
 (compare Fig. 7). 
 
 Cartilage, or gristle, is one of the skeletal tissues, serving for 
 the support of the body and the protection of special organs. 
 In some of the lower vertebrates, such as the dog-fish, it forms 
 practically the whole of the internal skeleton, but in higher forms
 
 HISTOLOGY OF HIGHEE ANIMALS 
 
 67 
 
 it is to a greater or less extent supplemented or even replaced by 
 bone. It consists mainly of a tough, translucent matrix, or 
 intercellular substance, which is formed as a secretion by the 
 cartilage cells and in which the latter are imbedded at wide 
 intervals (Fig. 20). In this respect it differs greatly from the 
 epidermis, in which the cells lie close together and little or no 
 intercellular substance is developed. The cartilage grows by 
 repeated division of the cells which it contains and the secre- 
 tion of additional intercellular matrix between 
 them. The frequent arrangement of the 
 nucleated cells in pairs, as shown in the 
 illustration, is an indication of recent cell- 
 division. Bone is a more complex tissue than 
 cartilage and is further strengthened and 
 hardened by the deposition of calcareous 
 salts, chiefly phosphate of lime, in the matrix. 
 
 Muscular tissue is specialized in a totally 
 different direction from any of the foregoing. 
 Its function is to contract, and by so doing 
 to bring about the various movements of 
 which the higher animals are capable. There 
 are two very distinct kinds of muscular tissue, 
 the one comparatively simple and the other 
 much more complex in structure. The former, 
 which is known as unstriped muscle (Fig. 21), 
 consists of greatly elongated cells, the muscle- 
 fibres, associated in sheets or bundles. Each 
 has a centrally placed nucleus and its cellular 
 nature is at once obvious. The wall of the 
 alimentary canal, outside its lining epithelium, 
 is composed chiefly of muscle-fibres of this 
 kind. Their rhythmical and co-ordinated contraction causes 
 the characteristic peristaltic movement whereby the onward 
 passage of the food is secured. These and similar movements 
 effected in other organs by the action of unstriped muscular 
 tissue take place quite independently of the will whence the 
 term " involuntary " is often applied to this type of muscle. 
 
 Striped or striated muscular tissue is usually under the control 
 of the will, and is hence often spoken of as " voluntary," but it is 
 found in the higher animals wherever very sharp, precise move- 
 ments are required, as for example in the walls of the heart, the 
 
 FIG. 21. Unstriped 
 Muscle Fibres 
 from the Wall of 
 the Eabbit's In- 
 testine, X 300. 
 
 nu., nuclei.
 
 58 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 contraction of which serves to pump the blood through the blood- 
 vessels. It is more especially associated, however, with the 
 movements of the limbs, the bones of which form a system of 
 levers operated by the muscles which are attached to them. It 
 differs greatly in min ute structure from unstriped muscle, though 
 consisting essentially of greatly elongated, nucleated fibres 
 endowed with remarkable powers of contraction. These fibres, 
 and the fibrillse into which they are subdivided, are characterized 
 by a transverse striation of alternate light and dark bands. 
 Their structure is very complex and in the fully developed 
 muscle it is difficult if not impossible to recognize the limits 
 
 n. 
 
 FIG. 22. Striped Muscle-Fibres (m.) from the Tail of a larval Axolotl, 
 showing their nuclei (n.), x 560. (From a photograph.) 
 
 between the constituent cells. Fig. 22 represents a number of 
 striated muscle-fibres from the tail of a larval axolotl, in which 
 each fibre is seen to be provided with several distinct nuclei. 
 
 The nervous system, as we have already pointed out, serves to 
 place the different parts of the body in communication with one 
 another and exercises a controlling and co-ordinating influence 
 over the whole, while through the mediation of the special organs 
 of sense it keeps the organism in close touch with its environ- 
 ment. The tissue of which it is composed (Fig. 23) consists 
 of nerve-cells and nerve-fibres, but the fibres are merely out- 
 growths of the cells. A cell and fibre together form a neuron 
 a single unit of the nervous system. The nerve-cells, or 
 rather their bodies, occur chiefly in the brain and spinal cord, 
 which constitute the central nervous system, but also in small
 
 HISTOLOGY OF HIGHEE ANIMALS 59 
 
 local aggregations, or ganglia, in various parts of the body. 
 The nerve-fibres extend outwards from the central nervous 
 system in long, slender bundles, the nerves, which are distributed 
 to the various organs. 
 
 The body of a nerve-cell contains the nucleus and is usually 
 much branched into slender processes or dendrons (Fig. 24), 
 which are quite distinct from the nerve-fibre and are supposed 
 to afford the means of transmitting impulses between one nerve- 
 cell and another, with the dendrons of which they interlace. 
 
 FIG. 23. --Nervous Tissue, as seen in a thin Section of the Brain (Medulla 
 oblongata) of the Monk Fish. Two large nucleated Nerve Cells are 
 shown imbedded in a Mass of smaller Cells and Fibres, X 168. (From a 
 photograph.) 
 
 Like other higher specialized tissue-cells of the animal body the 
 neurons have lost the power of multiplication by division. More- 
 over there appears to be no provision, in some adult vertebrates, 
 for their renewal when worn out or injured. A certain number 
 are formed in the course of the development of the embryo and 
 these have to serve the animal for the whole of its life. 
 
 All the different kinds of cells met with in the body, a few of 
 which have been thus briefly described, are derived from the 
 apparently simple unicellular ovum by repeated subdivision and 
 gradual differentiation. The functions which in an Amoeba are 
 all performed by a single protoplasmic unit are in one of the
 
 60 
 
 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 higher animals distributed amongst thousands of millions of 
 such units, arranged, so to speak, in regiments and armies, each 
 group with its own duties to perform and all co-operating for the 
 common good under the supervision and control of the central 
 nervous system. The marvellous perfection of the whole 
 machinery is the result of that differentiation and division of 
 labour which was first rendered possible by the union and co- 
 operation of the individuals 
 of a protozoon family to form 
 a multicellular body. 
 
 Turning now to the higher 
 plants, we shall find that, in 
 accordance with their much 
 lower degree of functional 
 activity, their organization 
 is far less elaborate than in 
 the higher animals. In cor- 
 relation with their stationary 
 habit all those organs and 
 tissues which are specially 
 concerned with locomotion 
 are absent, and in further 
 correlation with this character 
 there are no nervous system 
 and no special organs of sense. 
 Of the functions concerned 
 with the life of the individual 
 that is, other than repro- 
 ductive functions that of 
 nutrition is alone highly 
 
 FIG. 24. Diagram of a Neuron, 
 (From Hertwig.) 
 
 Nervenfortsatz, a nerve-fibre coming off 
 from the body of the cell. 
 
 developed. The entire plant 
 is little more than a piece of 
 apparatus for extracting carbon, water and mineral salts from the 
 air and soil, and converting these, with the aid of the sun's rays, 
 into organic substances. Although the higher plants often attain 
 a much larger size than any animals, this does not indicate a 
 higher degree of organization, for it is brought about simply by 
 the repetition of similar parts such as roots, branches and 
 leaves and the accumulation of dead cell-walls in the form of 
 wood and bark. As we have already seen, a green plant, instead 
 of spending the energy which it derives from the sun on its own
 
 HISTOLOGY OF HIGHEE PLANTS 61 
 
 activities, stores most of it up in the complex chemical compounds 
 which it manufactures. 
 
 Nevertheless, though the degree of histological differentiation 
 is not nearly so high as it is in the higher animals, we find in 
 the higher plants also a considerable variety of cells and tissues, 
 derived, as we have already pointed out, from the dermatogen, 
 periblem and plerome of the embryo. 
 
 We may illustrate this point by a study of some of the cells 
 and tissues which occur in the well-known spiderwort of our 
 gardens, Tradescantia virginica. If we examine the flowers of 
 this plant we shall find that the stamens are covered with long 
 
 B. 
 
 FIG. 25. Structure of a Hair from a Stamen of Tradescantia virginica. 
 
 A. End of a hair as seen under a low power of the microscope. The hair is made up of 
 
 a single row of cells. 
 
 B. A single cell more highly magnified. 
 
 c.w. cell-wall; nu. nucleus; p.u. primordial utricle; vac. vacuole filled with coloured 
 or colourless cell-sap. 
 
 slender hairs. It will be convenient to make these hairs the 
 starting point of our inquiry. If we study them first under a low 
 magnifying power we shall see that each hair (Fig. 25, A) is 
 made up of a single row of cells, arranged like the beads in a 
 necklace ; most of the cells are elongated, but towards the apex 
 of the hair they become short and spherical. 
 
 If we now concentrate our attention on one of the larger cells 
 and study it carefully under a moderately high power of the 
 microscope, we shall find that it exhibits the appearance shown 
 in Fig. 25, B. It measures about 0'27 mm. in length by 
 0'08 rum. in breadth and consists of a thin-walled bag filled 
 with living protoplasm. The wall (c.w.) is transparent and 
 colourless and is composed, as in Haematococcus, of cellulose.
 
 62 OUTLINES OF EVOLUTIONAKY BIOLOGY 
 
 The granular, colourless protoplasm does not fill the interior 
 of the cell in a uniform manner, but is arranged partly as a 
 thin lining to the cell-wall, known as the primordial utricle 
 (p.n.), and partly in irregular strings which branch and anas- 
 tomose and stretch across the cavity of the cell in various 
 directions. These strings of protoplasm tend to converge towards 
 an irregular mass in which the nucleus is situated. The nucleus 
 itself (m*.) is a nearly spherical body of denser protoplasm, about 
 0'024 mm. in diameter. The extensive space which lies inside 
 the primordial utricle and between the strands of protoplasm 
 is filled with a more fluid liquid known as the cell-sap. It is 
 to this cell-sap that the flowers of Tradescantia owe their 
 colour; if the flowers are blue the cell-sap will be found to 
 be blue and if they are white it is because the cell-sap is 
 colourless. 
 
 The most striking feature of the cell which we are examining 
 still remains to be noticed. The protoplasm is in constant move- 
 ment. This is at once evident from the characteristic streaming 
 of tho small granules which it contains. Both in the primordial 
 utricle and in the network of threads a constant circulation is 
 kept up, though not in a very definite manner, as the threads 
 themselves are constantly undergoing slow changes in their 
 arrangement. The arrows in Fig. 25, B indicate approximately 
 the course taken by the streaming protoplasm at the time when 
 the drawing was made. 
 
 The streaming of the protoplasm appears at first sight to be 
 an essentially vital phenomenon, but it is probably merely the 
 mechanical result of chemical and physical processes going on 
 in the cell, such as the diffusion of various substances in solution 
 from one cell to another, which must take place in the process of 
 nutrition. If the cell is killed by the addition of alcohol the 
 physical and chemical conditions are at once altered and the 
 movement ceases ; the protoplasm is coagulated and, if we are 
 dealing with a cell containing coloured cell-sap, the nucleus 
 absorbs the colouring matter with great avidity and becomes 
 deeply stained, while the cytoplasm stains only very slightly or 
 not at all. We are thus able to make the cell stain itself 
 differentially, without the aid of any extraneous colouring matter, 
 the cell-sap acting as what is termed a nuclear stain. 
 
 It will be necessary to restrict our further observations on 
 Tradescantia to the structure of the leaf (Fig. 26). The leaves
 
 HISTOLOGY OF HIGHER PLANTS 
 
 68 
 
 of this plant are long and narrow, somewhat resembling those of 
 grasses. There is a pronounced midrib and the so-called veins 
 
 ruz~ 
 
 FIG. 26. Histology of the Leaf of Tradescantia viryinica. 
 
 A. Part of a transverse section of the leaf. 
 
 J5. Piece of the epidermis stripped from the lower surface of the leaf. 
 
 C. Chlorophyll cells from the mesophyll, as seen in a longitudinal section of the leaf. 
 
 D. Portions of four vessels from a vascular bundle, with spiral and annular markings, as 
 
 seen in a longitudinal section of the bundle. 
 
 (All more or less highly magnified.) 
 
 a.c. air-cavity; c.c. chlorophyll cell ; cp. chloroplastids ; ep.c. epidermic cell ; epd. epi- 
 dermis; g.c. guard-cell ; mes. mesophyll ; nu. nucleus ; par. colourless parenchyma ; 
 ,s7/. sheath of vascular bundle, composed of thin-walled cells containing starch grains ; 
 sk. thick-walled skeletal tissue; st. stoma; v.b. vascular bundle ; ves. vessels. 
 
 run parallel with one another from base to apex, as in all typical 
 Monocotyledons. 
 
 If we cut a thin transverse section of a living leaf and 
 examine it under the microscope in a drop of water (Fig. 26, A) we
 
 64 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 shall see at once that it is made up of three principal tissue- 
 systems, the epidermis, the mesophyll and the vascular bundles. 
 The epidermis (epd.) covers the upper and lower surfaces in the 
 form of two single layers of cells, the mesophyll (mes.) occupies the 
 space between these two layers, and the vascular bundles (v.b.) 
 corresponding to the veins of the leaf are imbedded in the 
 mesophyll at fairly regular intervals (only one is shewn in the 
 figure). We must examine each of these tissue-systems sepa- 
 rately, and in order to gain a correct idea of the form and 
 arrangement of the cells of which they are composed it will be 
 necessary to study them from various points of view. 
 
 The epidermis may be stripped off bodily from the surface of 
 the leaf and then exhibits under the microscope the appearance 
 shown in Fig. 26, B. It is composed of elongated cells, placed 
 side by side in a single layer. The amount of protoplasm which 
 these cells contain is very small, but the nucleus (mi.) is 
 frequently conspicuous. Their external walls are specially 
 thickened in relation to their protective function, a feature 
 which can only be seen in sections (Fig. 26, A). At frequent 
 intervals little slit-like openings occur in the epidermis. These 
 are the stomata (st.) which lead into air-spaces in the mesophyll. 
 Each stoma is bounded by a pair of specially modified epidermic 
 cells known as the guard-cells (#.c.) the only epidermic cells 
 containing chlorophyll which have the power of opening and 
 closing the stoma like a pair of lips and thus regulating the 
 amount of aqueous vapour which passes through the stomata in 
 the process of transpiration. On the outer side of each guard- 
 cell lies another epidermic cell of much smaller size than the 
 ordinary kind, and these, together with the guard-cells, form a 
 kind of roof (or floor) to the air-cavity (Fig. 26, A, a.c.). 
 
 The mesophyll contains the chlorophyll-bearing cells, by 
 which the assimilation of carbon dioxide is effected and which 
 are at once recognized by their green colour. They appear more 
 or less round or oval in transverse sections (Fig. 26, A, c.c.), but 
 are really considerably elongated, parallel to the length of the 
 leaf, as shown in Fig. 26, C. They come in contact with one 
 another by numerous short protuberances, between which lie the 
 spaces in which the air, containing carbon dioxide and aqueous 
 vapour, circulates. Each contains a nucleus (wtt.) and numerous 
 small, biscuit-shaped green bodies, the chloroplastids or chloro- 
 phyll corpuscles (cp.), imbedded in the cytoplasm.
 
 HISTOLOGY OF HIGHER PLANTS 65 
 
 In certain places the green mesophyll cells are interrupted by 
 groups of cells containing no chlorophyll. At intervals beneath 
 the upper epidermis we see masses of large, thin-walled, colourless 
 cells forming a parenchyma or ground-tissue (par.}, while beneath 
 the vascular bundles we find bands of very thick- walled cells (sk.) 
 which play an important part in the mechanical support and 
 strengthening of the leaf. 
 
 The vascular bundles are surrounded each by a sheath of thin- 
 walled cells (/<.) containing numerous small starch grains. 
 Within this lie the bast or phloem and the wood or xylem, com- 
 posed of the elongated tubular elements through which the sap 
 circulates. The raw sap consists of water with mineral salts 
 in solution, and ascends through the xylem, while elaborated 
 sap, containing the proteids which have been manufactured in 
 the leaf under the influence of sunlight, descends through the 
 phloem. For our present purposes we may confine our attention 
 to certain of the xylem elements, known as the spiral and 
 annular vessels. These consist really of dead cell-walls, forming 
 long narrow tubes each composed originally of a row of cylindrical 
 cells placed end to end. In the course of their development the 
 transverse dividing walls between these cells are absorbed, the 
 protoplasm disappears, and nothing remains but a long hollow 
 tube whose walls are strengthened by spiral or annular thicken- 
 ings. Portions of these vessels are represented separately in 
 Fig. 26, D ; in A they are seen only in transverse section (res.). 
 
 Such are the principal kinds of tissue met with in a typical 
 flowering plant, and such is the way in which it carries out the 
 principles of co-operation, differentiation and division of labour 
 amongst its constituent cells. 
 
 To most people it will probably appear that the fundamental 
 truth and general applicability of the cell theory are sufficiently 
 firmly established by considerations such as those with which we 
 have been dealing. It has, however, certain undoubted limita- 
 tions, and upon these limitations some biologists are inclined to 
 lay a good deal of stress. 
 
 Thus, from the point of view of the cell theory, we regard the 
 cell as the organic unit; there are, however, units of a lower 
 order, of which the cell itself is composed. The chloroplastids of 
 one of the higher plants, for example, exhibit a good deal of 
 individuality, being capable of independent growth and multi- 
 plication, while, as we shall see later on, it is necessary for 
 
 B. F
 
 06 
 
 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 theoretical reasons to postulate the existence of some such bodies 
 as Professor Weismann's biophors as the primary units of which 
 living protoplasm is built up. 
 
 Then, again, although it may be questioned whether any 
 absolutely unnucleated organisms, such as Haeckel's Monera, 
 really exist, there can be no doubt that the most simply organized 
 living things known to us, the Bacteria, which probably stand a 
 long way below the point where the animal and vegetable king- 
 doms part company, and which are the most abundant of all 
 living organisms, do not show that sharp differentiation into 
 cell body and nucleus which is so characteristic of typical cells, 
 
 FIG. 27. Bacillus saccobranchi. Bacteria from the Blood of a Fish (ftacco- 
 branchus) stained so as to show the distribution of the chromatin 
 (nuclear) material, which is represented in black, and which-may be 
 arranged in small scattered granules throughout the cell, or in an irregular 
 network, or in an irregular, more or less twisted rod, X 2000. (After 
 Dobell in the " Quarterly Journal of Microscopical Science.") 
 
 the nuclear constituents being more or less scattered throughout 
 the cytoplasm (Fig. 27). 
 
 A difficulty of another kind is met with in the fact that in a 
 good many cases the division of the nucleus is not followed at 
 any rate not immediately by corresponding division of the 
 cytoplasm. Some Amosbae constantly have two nuclei, and we some- 
 times get relatively large masses of protoplasm containing many 
 nuclei formed by repeated division. These are termed syncytia. 
 We meet, in fact, with all degrees of separation of the cytoplasm 
 into distinct cells, and a great many of the cells even of highly 
 developed plants and animals may remain connected together 
 throughout life by thin strands of protoplasm. We have already 
 noticed an example of this continuity of the protoplasm in the
 
 CONTINUITY OF LIFE 
 
 67 
 
 case of Volvox ; another is shown in Fig. 28, B, which represents 
 a syncytial epithelium. 
 
 On the other hand, in some of the lower organisms the 
 Myxomycetes, slime- fungi or Mycetozoa, as they are variously 
 called numerous originally separate Amoeba-like individuals 
 may fuse together to form plasmodia, which may continue to 
 feed and grow and undergo nuclear division until they form great 
 sheets of living protoplasm containing perhaps hundreds or 
 thousands of nuclei (Fig. 29). These and similar facts, how- 
 ever, interesting and 
 instructive as they un- 
 doubtedly are, cannot 
 be regarded as con- 
 stituting a serious 
 invalidation of the cell 
 theory. 
 
 There is no more 
 fundamental or more 
 stimulating conception 
 in the domain of bio- 
 logical science than 
 that of the continuity 
 of life as formulated by 
 this theory. We have 
 to imagine the whole FIG. 28. 
 
 Organic World as COn- A- A single-layered epithelium with very distinct cell- 
 LI:^^.. t AK~ i : ~* *.:! /a~,7. 7 
 
 sisting of a continuous 
 stream of living pro- 
 toplasrn, which com- 
 menced to flow many 
 millions of years ago and has continued without interruption 
 ever since. At every cell-division the stream branches and 
 physical continuity is more or less completely interrupted, but 
 this in no way invalidates the conclusion that if all living things 
 did not actually have a common origin in a single primordial 
 protoplasmic unit, they probably at least originated from several 
 such units which themselves arose under unknown conditions 
 from inorganic matter. 
 
 The modern science of cytology, which is contrasted with 
 histology as the study of individual cells rather than that of 
 tissues or cell combinations, and which is yielding such important 
 
 p 2 
 
 outlines, from the brain of a reptile (Sphenodon 
 punctatus), x 750. 
 
 \. syncytial epithelium, without cell-outlines, from 
 another part of the brain of the same animal, 
 x 750. 
 
 mi. nuclei.
 
 68 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 results both for pure biology and for medical science, owes its 
 origin and development entirely to the elaboration of the cell 
 
 FIG. 29. Part of the Plasraodiura of a Mycetozoon (Budhamia utricularis], 
 
 X 50. (From a photograph.) 
 The small dark spots are the very numerous nuclei. 
 
 theory under the influence of improved methods of microscopical 
 investigation. 
 
 There is another point of view with regard to the living cell 
 which we may briefly refer to in this place. It is not only the 
 morphological or structural unit of the body but also the physio- 
 logical or functional unit. All the essential vital processes take 
 place within the bodies of cells, and all the different materials 
 required for and produced by these processes pass in and out of 
 them. Each cell may be looked upon as a microscopic laboratory 
 in which the complex chemical reactions comprised under the 
 term metabolism take place, and although, in the higher 
 organisms, the cells have become mutually dependent upon one 
 another, yet each retains a certain degree of physiological as 
 well as of morphological individuality.
 
 CHAPTER VI 
 
 The multiplication of cells Mitotic and amitotic nuclear division. 
 
 WE have already seen that the possibilities of structural 
 differentiation within the limits of the individual cell are by no 
 means exhausted by the distinction between cytoplasm and 
 nucleus, but it is only when we come to study in detail the pro- 
 cess of cell- division that we begin to gain any adequate concep- 
 lion of the fundamental complexity of the organic unit. We 
 have hitherto spoken of this process, as it occurs for example in 
 Amoeba, as though it were a simple matter, initiated by constric- 
 tion of the nucleus into two parts and concluded by a correspond- 
 ing division of the cytoplasm. The researches of the last forty 
 years, however, rendered possible by the improvements in 
 microscopical apparatus and micro-chemical technique, have 
 taught us that in the vast majority of cases the process of cell- 
 division is one of extreme complexity, accompanied by remark- 
 able phenomena which reveal a previously unsuspected degree of 
 structural differentiation within the nucleus itself. To these 
 phenomena Schleicher in 1878 gave the name karyokinesis, for 
 which Flemming, in 1882, proposed to substitute mitosis. Both 
 these terms are still in common use. 
 
 In a typical cell (Fig. 30) the cytoplasm (cyl.) is a semi-liquid 
 substance usually enclosed in a thin cell-membrane (animal cells) 
 or a thicker cell- wall (plant cells). It exhibits a microscopic 
 structure which is variously interpreted as reticular (fibrillar), 
 alveolar (foam-like), or simply granular, the probability being that 
 the real truth is expressed by a combination of these different 
 views. It may or may not contain plastids of various kinds (e.g. 
 chloroplastids in green plants). 
 
 The nucleus (nn,.), consisting of the so-called nucleoplasm or 
 karyoplasm, is usually a spherical, more or less centrally situated 
 body enclosed in a definite nuclear membrane (n.m.). Within 
 this membrane the karyoplasm is differentiated into various con- 
 stituents. In the first place there is a network or reticulum of
 
 70 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 delicate protoplasmic threads, known as the linin network, the 
 meshes of which are filled with a clear, probably liquid ground- 
 substance known as nuclear sap. These two together seem to 
 differ but little from the cytoplasm which lies outside the 
 nucleus. 
 
 The most characteristic constituent of the nucleus is another 
 substance, to which, on account of the readiness with which it 
 becomes coloured by certain dyes, the name chromatin 1 has been 
 given. This substance usually occurs in the form of small 
 granules (ch.g.) scattered dver the linin network, so that when 
 very close together they appear to form a chromatin network, 
 
 eft 
 
 FIG. 30. Diagram of a typical Cell. 
 
 ch.g., chromatin granules; c.m., cell- membrane ; c.s., centrosomes lying in centrosphere 
 cyt., cytoplasm ; n.m., nuclear membrane; nu., nucleus; nucl., nucleolus. 
 
 while not infrequently a specially large aggregation of chromatin 
 substance forms a nucleolus or karyosome (mid.). 
 
 Owing to the presence of this chromatin the nucleus as a whole, 
 under low powers of the microscope, appears to be deeply coloured 
 by such stains as various preparations of logwood and carmine 
 and the basic aniline dyes. We have already had occasion to 
 observe this staining property in the case of the hair-cells of 
 Tradescantia, where, it will be remembered, the nucleus becomes 
 deeply stained by the coloured cell-sap as soon as the cells are 
 killed by the action of alcohol. Other stains, again, affect the 
 cytoplasm rather than the nucleus, and these various chemical 
 reactions enable us to differentiate fairly sharply between the 
 different constituents of which the cell is composed, though it is 
 a matter of some doubt exactly how far the structure of the 
 
 1 Greek XP"^ " a colour.
 
 MITOTIC DIVISION OF CELLS 71 
 
 living protoplasm really corresponds to the appearances 
 exhibited in our preparations. 
 
 The differences in their staining reactions of course indicate 
 corresponding differences in chemical composition between the 
 chromatin and the cytoplasm, and analysis has shown that the 
 chromatin is characterized by the presence of relatively large 
 quantities of phosphorus. This is contained in the complex 
 nucleinic acid, with which various albuminous bodies may be 
 combined to form the chromatin substance. 
 
 When a typical animal cell is about to divide another structure 
 makes it appearance, usually just outside but occasionally inside 
 the nucleus. This is the centrosome (Fig. 30, c.s.), a very 
 minute body which has peculiar staining properties and which 
 is surrounded by a differentiated area of protoplasm known 
 as the centrosphere or attraction sphere (Fig. 31, A, cspli). 
 It has been questioned whether a centrosome and attraction 
 sphere are always present or whether they make their 
 appearance only when the nucleus is about to undergo 
 mitosis. This process certainly seems to be initiated by the 
 centrosome, which may divide into two parts long before the 
 nucleus itself commences to do so, so that two centrosomes often 
 appear alongside the so-called resting nucleus (Figs. 30; 31, A). 
 Presently the two centrosomes move away from one another, 
 both still keeping close to the nucleus, and each is now seen to be 
 surrounded by its own attraction sphere (Fig. 31, B). Around 
 each attraction sphere delicate threads or fibrillae of the cyto- 
 plasm become radially arranged to form a star or aster, and 
 the rays of the asters which lie between the two centrospheres 
 combine to form a spindle (Fig. 31, C, ksp). 
 
 In the meantime remarkable changes have commenced in the 
 nucleus itself. The chromatin granules, together with the linin 
 by which they are apparently held together, have arranged them- 
 selves in the form of a long coiled thread, the spireme (Fig. 31, B), 
 and presently the nuclear membrane begins to disappear (Fig. 31, 
 C, km), being apparently dissolved in the general protoplasm. 
 In this way the distinction between cytoplasm and nucleoplasm 
 is obliterated. 
 
 The spireme thread breaks up into a number of short 
 lengths known as chromosomes (Fig. 31, C, chrs), the actual 
 number being, with certain exceptions, a constant character for 
 each species of plant or animal. The centrosomes at about this
 
 72 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 time take up their positions at points corresponding to two 
 opposite poles of the original nucleus, with the spindle of fine 
 
 FIG. 31. Diagram of Ihe principal Stages in the mifotic Division of the 
 Nucleus in a typical Animal Cell. (From Weismann's " Evolution 
 Theory," adapted from E. B. Wilson.) 
 
 aeq, equatorial plate ; chr, chromatin ; chrs, chromosomes ; cs, centrosome ; csph, centre- 
 sphere, containing one or two ceiitrosomes ; kk, nucleolus ; km, nuclear mem- 
 brane; kn, nucleus; ksp, sp, nuclear spindle; p, aster; tk, daughter nucleus; 
 in, daughter cell ; zk, cytoplasm forming the cell body. 
 
 protoplasmic fibres stretched between them (Fig. 31, D), and the 
 chromosomes "go on the spindle," arranging themselves in a 
 so-called equatorial plate across its widest part (Fig. 31, D, aeq).
 
 MITOTIC DIVISION OF CELLS 
 
 73 
 
 The shape of the chromosomes varies much in different cases ; 
 they may be more or less spherical, but they are frequently short, 
 rod-like bodies, often shaped like a V (Fig. 32, A). Each one is 
 composed, like the spireme thread from which they are derived, 
 of an aggregation of chromatin granules, held together by a 
 linin basis. The chromatin granules are sometimes arranged 
 like the beads on a necklace (Figs. 32, B ; 77), and are known as 
 chromomeres. The number of the chromosomes also varies 
 greatly, from as low as two in a variety of the horse-worm 
 (Ascaris) to as many as one hundred and sixty-eight in the shrimp 
 Artemia. In cases where the chromosomes are very small each one 
 may perhaps be equivalent to only a single chromomere. 
 
 Either before or after taking up its position in the equatorial 
 
 FIG. ^.-^Sperm-mother- cells of a Salamander, during Mitosis. In A the 
 chromosomes are shown ; in B the spireme thread is split lengthwise, 
 and also shows very clearly the chromomeres of which it is made 
 up. (Prom Weismann's " Evolution Theory," after Hermann and 
 Driiner.) 
 
 c, dividing centrosome ; chr, chromosomes ; Jd, chromomeres ; zk, cytoplasm. 
 
 plate, each chromosome splits longitudinally into two parts 
 (Fig. 31, D), in fact the splitting can sometimes be observed in 
 the spireme thread even before it breaks up transversely into 
 chromosomes (Fig. 32, B). The result of this splitting is that 
 the number of chromosomes is doubled ; but the daughter chromo- 
 somes very soon separate into two equal groups, one of which 
 moves towards each centrosome (Fig. 31, E). Each group contains 
 one of the two halves of each parent chromosome. 
 
 Having migrated to opposite poles of the spindle the two 
 groups of daughter chromosomes there form the foundations of two 
 new nuclei (Fig. 31, F). The chromosomes break up into granules 
 again ; a new nuclear membrane is formed, whereby a portion of 
 the general cytoplasm is separated off to form the linin network 
 and ground-substance of the nucleus ; the asters and nuclear
 
 74 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 spindle more or less completely disappear though the centro- 
 some may certainly persist in some cases if not in all and the 
 newly constituted nucleus (Fig. 31, G, &) enters upon a longer or 
 shorter period of inactivity accompanied by growth. 
 
 In the meantime the cytoplasm which constitutes the cell-body 
 has also divided into two parts in a plane which passes through 
 the middle of the nuclear spindle and at right angles to its 
 length. In animal cells this division is usually effected by a 
 constriction which starts from the outside (Fig. 31, F, G) and 
 in plant cells by the deposition of a cell-plate (Fig. 34, E, c.p.) 
 in the equator of the nuclear spindle. This cell-plate forms the 
 foundation of the double cell-wall which will separate the two 
 daughter cells ; it must not, of course, be confounded with the 
 equatorial plate formed temporarily by the chromosomes. 
 
 Various attempts have been made to explain the dynamics of 
 this remarkable process of mitosis or karyokiriesis, the essential 
 features of which are always much the same though the details 
 vary considerably in different cases. The centrosomes, with 
 their centrospheres, asters and spindle, sometimes spoken of 
 collectively as the achromatic figure, are usually regarded as a 
 special mechanism for bringing about the equitable partition of 
 the chromatin substance between the two daughter nuclei. This 
 substance is evidently so important that no rough arfd ready 
 division will suffice. It is probable, as we shall see later on, that 
 the chromomeres of which each chromosome is composed have 
 different properties, and that it is necessary, in ordinary cell- 
 division, not only that the chromosome as a whole shall be 
 divided into two parts but that each daughter nucleus shall have 
 its share of each individual chromomere (compare Fig. 77). In 
 other words a qualitative as well as a quantitative division of 
 the chromatin material has to be effected, and this is secured by 
 the longitudinal splitting of the chromosomes. A transverse 
 division would only result in the separation of the chromomeres 
 into two groups, but the longitudinal division involves each one. 
 
 According to some observers the fibres of the nuclear spindle 
 are actively contractile and actually pull the two halves of each 
 split chromosome asunder. Others maintain that the centro- 
 somes attract the chromosomes in somewhat the same way as the 
 poles of a horse-shoe magnet attract iron filings sprinkled 
 between them. 
 
 Of late years the electro-magnetic explanation has been coming
 
 MITOTIC DIVISION OF CELLS 75 
 
 more prominently to the front. Thus Gallardo has suggested 
 that the chromatin substance is charged with negative and the 
 cytoplasmic colloids with positive electricity, while the centro- 
 sornes are capable of acquiring a positive potential higher than 
 that of the general cytoplasm. Increase of this potential causes 
 the centrosome to divide and the radiations which form the 
 asters and spindle indicate lines of force in the cytoplasm. The 
 two daughter centrosomes, inasmuch as they bear like charges of 
 electricity, repel one another. In a similar way the chromosomes 
 divide under the influence of their high negative charges and 
 the two halves of each repel one .another and are at the same 
 time attracted by the positive centrosomes. The two new groups 
 of negatively charged chromosomes then attract the positive 
 cytoplasm in opposite directions and thus the division of the cell 
 body follows upon that of the nucleus. 
 
 Whatever may be the physical explanation of these complex 
 phenomena, we must think of them as lying at the root of all 
 normal processes of growth and multiplication in the higher 
 plants and animals. With comparatively rare exceptions, some 
 of which will be mentioned later on, every one of the innumer- 
 able series of cell-divisions initiated by the fertilized ovum, 
 and continued throughout life in the growth and repair of tissues, 
 is accompanied by complicated processes similar to those above 
 described. The process of cell-multiplication, however, is 
 frequently confined in adult organisms to certain regions. 
 Thus, as we have already seen, in the higher animals the growth 
 of the epidermis depends upon cell-divisions which go on only in 
 its deepest layer, the stratum Malpighii (Fig. 18, s.m.). Most of 
 the cells in the body sooner or later lose the power of division, 
 but they are then usually short-lived, as in the case of those 
 cells which form the outer layers of the epidermis and which 
 rapidly become converted into more or less horny scales to be 
 cast off on reaching the surface. 
 
 The majority of the tissues are thus renewed throughout 
 life by the mitotic activity of some unspecialized cell-group, a 
 high degree of specialization in the tissue cells of the higher 
 organisms being always, as we have already seen in the case of 
 red blood corpuscles and nerve cells, accompanied by the loss of 
 the power of multiplication. 
 
 The limitation of cell-multiplication to definite circumscribed 
 regions of the body is perhaps best seen in the case of the higher
 
 76 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 plants, where the various meristematic or actively dividing 
 tissues remain in an undifferentiated embryonic condition and 
 give rise to those additions to the permanent tissues whereby 
 growth is effected. Such actively dividing meristem is found at 
 the growing points of stems and roots, where it serves to bring 
 about growth in length, and in the cambium, which serves, by 
 the addition of new elements to the wood and the bast, to bring 
 about growth in thickness. 
 
 The microscopic appearance of such a meristematic tissue, 
 
 FIG. 33. Part of a longitudinal Section of the actively growing Eoofc of a 
 Hyacinth (Galtonia candicans} showing the Nuclei of the Cells in various 
 stages of mitotic Division, A X 280 ; B X 640. (Prom photographs.) 
 
 when suitably stained and prepared for examination, is shown in 
 Fig. 33, taken from photographs of part of a longitudinal section 
 of the growing point of the root of a hyacinth (Galtonia candicans). 
 The cell-walls are as yet thin and inconspicuous and filled with 
 dense protoplasm, while the conspicuous nuclei exhibit all stages 
 of mitosis, the whole forming a striking contrast to the dead 
 tissues, such as cork and wood, of which the bulk of many plants 
 is made up, and which consists merely of cell-walls without any 
 protoplasmic contents (cf. Figs. 6 and 7). 
 
 Mitosis in the cells of the higher plants is usually, though by
 
 MITOSIS IN PLANT CELLS 
 
 77 
 
 no means always, characterized by the absence of recognizable 
 centrosomes. The actual appearance of some of the principal 
 stages in the process is shown more highly magnified in Fig. 34, 
 
 A, 
 
 D. E. F, 
 
 FIG. 34. - Six selected Stages in the mitotic Division of the Nucleus iu the 
 growing Boot of Galtonia candicans, X 1120. (From photographs.) 
 
 A. Besting nucleus with large nucleolus (Nuls.). 
 
 B. Spireme stage, with coiled chromatin thread. 
 
 C. The spireme thread has broken up into chromosomes which are forming the equa- 
 
 torial plate. 
 
 D. The chromosomes have split longitudinally and the two groups of daughter chromo- 
 
 somes thus formed are passing to opposite poles of the spindle. 
 
 E. Formation of the cell-plate (c.p.) across the equator of the nuclear spindle. 
 
 P. Completion of the cell-division, and disappearance of the individual chromosomes in 
 the daughter nuclei. 
 
 which represents a series of six selected stages arranged in proper 
 sequence, reproduced from photo-micrographs. 
 
 Fig. 34, A represents the so-called resting stage of the nucleus, 
 in which it will be noticed that there is, in addition to 
 the minute, scattered chromatin' granules, a large spherical 
 chromatin nucleolus orkaryosome (Nuls.). B shows the spireme
 
 78 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 stage, with the chromatin granules collected together in a long 
 spirally coiled thread and the nucleolus still very conspicuous. 
 C shows the group of chromosomes formed by transverse breaking 
 
 
 FIG. 35. Mitosis in the segmenting Egg of the Horse-Worm (Ascaria mfyolo- 
 cephala), X 770. (From photographs.) 
 
 A. Lateral view of the egg during the first cleavage; showing the nuclear spindle (sp.), 
 
 the equatorial plate (aeq.), one of the two centrosomes (c.s.), the other being out of 
 focus, and the asters (as.) formed by fine threads of protoplasm radiating from 
 around the centrosomes. Polar bodies (p.b.) are also shown. 
 
 B. The same stage viewed from one pole, showing the four V-shaped chromosomes (chr.) 
 
 in the equatorial plate. 
 
 C. The first division is completed and the nuclei have again passed into the spireme stage. 
 
 A polar body (p.b.) is still visible. 
 
 D. Each of the first two blastomeres has again reached the stage represented in A 
 
 and B. 
 
 up of the spireme thread. The karyosome has now disappeared, 
 having apparently been used up in the formation of the chromo- 
 somes. D shows the two groups of daughter chromosomes 
 formed by longitudinal splitting of the parent chromosomes and 
 retreating towards the two ends of the spindle, which is only
 
 MITOSIS IN ANIMAL CELLS 79 
 
 faintly visible. E shows the commencement of the cell-plate 
 (c.p.) across the middle of the spindle, and F the two young 
 daughter cells each with a new nucleus- in which the chromo- 
 somes have again broken up into granules. 
 
 It is only by the examination of large numbers of examples 
 that all the minute details of the process can be elucidated, but 
 the main features as represented in the above figures can very 
 easily be made out. 
 
 For comparison with the process of mitosis as seen in typical 
 plants such as Galtonia, we may take the first division of the 
 fertilized egg in the horse-worm, Ascaris, a classical subject from 
 the study of which much of our knowledge of nuclear division in 
 animal cells has been derived. In this case there are only four 
 chromosomes, but they are large and conspicuous, and charac- 
 teristically V-shaped when forming the equatorial plate on the 
 spindle. 
 
 Fig. 35 is again taken from actual photographs. In this 
 figure, A represents a side view of the entire egg-cell during 
 the division of the nucleus, with spindle (sp.), asters (as.), 
 centrosomes (one only of which, cs., appears in the photograph, 
 the other being out of focus), and equatorial plate (aeq.). B 
 shows a similar stage viewed from one pole, so that the spindle 
 itself does not appear, but the four chromosomes forming the 
 equatorial plate are distinctly visible. C shows the two daughter 
 cells or blastomeres resulting from the first division of the egg, 
 each with the nucleus preparing for further division, and D 
 represents a later stage in which the nucleus of each daughter 
 cell is again actually in process of division and shows the 
 separate chromosomes very distinctly. 
 
 It must not be supposed that the phenomena of mitosis are by 
 any means confined to the higher animals and plants ; they are 
 observable throughout the animal and vegetable kingdoms, in 
 unicellular as well as multicellular forms. The process has long 
 been known to take place, for example, in at any rate some 
 Amoebae, and it probably occurs wherever there is a clearly 
 differentiated nucleus. The Bacteria and their allies, in which 
 the chromatin granules are scattered throughout the cell body 
 and there is no proper differentiation into cytoplasm and nucleus, 
 apparently form exceptions to the general rule. 
 
 There are, however, even amongst the higher animals, some cases 
 of cell-division which do not exhibit mitotic phenomena, but in
 
 80 
 
 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 which the nucleus appears simply to constrict into two or more 
 parts (Fig. 36). This is known as direct or amitotic nuclear division. 
 
 It is frequently met with in 
 degenerating cells and patho- 
 logical tissues, but it is doubtful 
 if it ever occurs (in the higher 
 organisms at any rate) in cells 
 which are destined to undergo 
 long - continued multiplication. 
 We may therefore regard it as 
 a more or less abnormal process 
 with which we have no need to 
 concern ourselves any further. The phenomena of mitosis, on 
 the other hand, are thoroughly normal and practically universal, 
 and, as we shall see later on, they are of the deepest significance 
 from the point of view of the theories of heredity and variation. 
 
 FIG. 36. Amitotic nuclear Division 
 as seen in Cells from the Cavity 
 of the Parapbysis in Spbenodon, 
 X 1000. 
 
 nu. nuclei.
 
 PART II. THE EVOLUTION OF SEX 
 
 CHAPTER VII 
 
 Limitation of the powers of cell-division Rejuvenescence by conjugation 
 of gametes The origin of sex in the Protista. 
 
 BY the process of cell-division an unbroken continuity has 
 been established in the chain of living things from the earliest 
 appearance of unicellular organisms to the present day. Every 
 cell is the descendant of pre-existing cells and, in accordance 
 with the theory of evolution, all cells which exist to-day, distri- 
 buted amongst the bodies of countless millions of different 
 organisms, could, if our knowledge were sufficiently complete, be 
 traced back to a single ancestral cell. 
 
 It by no means follows from these considerations, however, 
 that there is, under natural conditions, no. limit to the ordinary 
 process of cell-division. On the contrary it is well known that 
 in any cell family, whether belonging to a unicellular or a multi- 
 cellular organism, the power of multiplication tends to become 
 exhausted, and, if that particular cell family is to continue its 
 existence, has to be in some way renewed. 
 
 Take, for example, an ordinary ciliate or flagellate Protozoon, 
 which multiplies by simple fission. If a single individual be 
 isolated and placed in water containing suitable food material, 
 and kept under suitable conditions of temperature, light and so 
 forth, it will multiply very rapidly, until possibly hundreds of 
 generations of separate cells have been produced and the total 
 number increased perhaps to millions. But under ordinary 
 circumstances a time presently arrives when the individuals 
 begin to show signs of exhaustion, accompanied by physical 
 degeneration, and to slack off in their rate of multiplication. 
 They may be stimulated to renewed activity for a time by special 
 feeding or by constantly varying the culture medium, 1 but in a 
 
 1 Mr. L. L. Woodruff has kept a culture of Paramoecium under observation for 
 nearly three and a half years, taking precautions to prevent the possibility of con- 
 jugation, but constantly varying the culture medium. During this time more than 
 two thousand generations of Paramoecium were produced by repeated fission an 
 B. G
 
 82 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 state of nature the chief if not the only means by which the 
 family can be kept from speedy extinction is conjugation, the 
 
 3- 
 
 FIG. 37. Life History of Copromoiias. (From Bourne's "Comparative 
 Anatomy," after Dobell.) 
 
 cv., contractile vacuole ; cph., cell pharynx; cst., cell mouth; fv., food vacuole ; 
 N., nucleus; B., reservoir; tr., flagellum. (For further explanation see text.) 
 
 exhausted individuals approaching one another and finally uniting 
 in pairs. In this way they appear to become rejuvenated and 
 
 average of about one division every fifteen hours and at the end of the period the 
 organisms were still in a perfectly normal condition.
 
 LIFE HTSTOEY OF COPROMONAS 83 
 
 their failing powers of multiplication by cell-division are com- 
 pletely restored. 
 
 For the purpose of studying this process of conjugation in its 
 primitive simplicity we can hardly do better than take the minute 
 flagellate form Copromonas, which is found in water frequented 
 by frogs, from the excrement of which it derives its nutriment. 
 The adult organism (Fig. 37, A) consists of a very minute ovoid 
 mass of protoplasm with a single flagellum (tr.) springing from 
 the narrow end. Alongside the base of the flagellum is a definite 
 cell mouth (cytostome, cst) through which solid particles of food 
 are taken into the interior of the body. Close to this there is 
 a contractile vacuole (cr.), accompanied by a "reservoir" (R) 
 into which it discharges. The nucleus (N) is situated nearer to 
 the broadly rounded hinder end of the body, which may also 
 contain a number of food-vacuoles (/#.) 
 
 If the food supply be abundant the individual Copromonas will 
 grow and presently divide into two by simple longitudinal fission, 
 which commences at the narrow anterior end (Fig. 37, B D). 
 The division of the nucleus is said to be amitotic. The two 
 daughter cells separate, feed, grow and repeat the process, and in 
 this way a whole swarm of monads is produced. In the course of a 
 few days, however, they appear to become exhausted and conjuga- 
 tion sets in, the individuals uniting in pairs (Fig. 37, 2 5). The 
 result of each such union is a single larger individual, which may 
 either undergo a period of rest within the protection of a special 
 envelope or cyst (Fig. 37, 7), or at once assume the ordinary 
 form and begin to multiply with renewed activity (Fig. 37, Y;. 
 For the continued existence of the species it is probably necessary 
 that the encysted monads should at some time or another be 
 swallowed by frogs and passed out again in the faeces, in order 
 that they may be brought in touch with the necessary food 
 supply. 
 
 We have here, as in the case of Haematococcus described in 
 Chapter III., a perfectly typical example of conjugation 1 occurring 
 at longer or shorter intervals in the life cycle of the organism. The 
 whole process consists in the union of two separate cells, known 
 in this connection as gametes, to form a single cell known as the 
 zygote, and it is of the utmost importance to observe that not only 
 is there a union between the cytoplasm of the two gametes but 
 the nuclei also unite to form a single zygote nucleus. Indeed, 
 
 1 Also known as syngamy or zygosis. 
 
 G 2
 
 84 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 as we shall see later on, it is the union of the nuclei which is 
 the really important part of the business, for in some cases (e.g., 
 ParamcBcium) the union of the two cell bodies is a merely 
 temporary affair, a necessary preliminary to an exchange and 
 subsequent union of nuclei. 
 
 In such simple cases as that of Copromonas we see all the 
 essential features of the sexual process which occurs so constantly 
 throughout the animal and vegetable kingdoms. It is evident 
 that in itself conjugation is not a process of reproduction, for its 
 immediate result is to halve the total number of cells instead of 
 doubling it. It has in fact exactly the opposite effect to that of 
 cell-division. It is a process which appears to be necessary for 
 the rejuvenescence, at longer or shorter intervals, of exhausted 
 cells, whereby they are endowed with renewed powers of multi- 
 plication by ordinary cell-division. At the same time it forms 
 the starting point of all those remarkable structural modifications 
 of the organism, whether unicellular or multicellular, w r hich 
 accompany the evolution of sex. 
 
 In Copromonas and in Hsematococcus, although there is a 
 true sexual process, there is apparently no sexual differentiation 
 at all ; there is no distinction between male and female gametes ; 
 the two conjugating cells are exactly alike, and the conjugation 
 is therefore said to be isogamous. In Copromonas, moreover, the 
 gametes or sexual cells are indistinguishable from the ordinary 
 individuals, every individual being at least a potential gamete. 
 Starting from such a case as this we find, even amongst the 
 unicellular plants and animals, every stage in the evolution of highly 
 specialized male and female gametes, differing widely from the 
 ordinary individuals and from each other. Conjugation will then 
 take place between two dissimilar gametes, and is said to be 
 anisogamous. 
 
 The first hint, so to speak, of sexual differentiation is to 
 be observed in the behaviour of the conjugating gametes; it is 
 a physiological rather than a structural or morphological 
 phenomenon, and consists in the fact that one gamete is active 
 while the other remains comparatively passive. We shall find 
 that this distinction lies at the root of all sexual differentiation 
 throughout the animal and vegetable kingdoms. The more active 
 gamete is spoken of as male and the more passive as female. The 
 passivity of the female is intimately associated with and probably 
 to a large extent dependent upon the fact that it contains more
 
 SEXUAL DIFFERENTIATION 85 
 
 cytoplasm and is therefore more heavily weighted than the male 
 gamete. This cytoplasm, moreover, is in many cases densely 
 charged with food material, which constitutes the capital with 
 which the zygote, formed by the union of the two gametes, has 
 to begin its new life cycle. 
 
 It is quite clear that the primary distinction between the sexes 
 is a simple case of division of labour accompanied by a 
 corresponding structural differentiation. Two ends have to be 
 secured by the gametes. They must come together in order that 
 they may conjugate, and therefore one or both must be capable 
 of active locomotion. They must also contain between them 
 sufficient material, either in the form of actual protoplasm or of 
 some substance that can easily be worked up into protoplasm, to 
 give the new individual which results from their union a fair 
 start in life. 
 
 A cell body heavily weighted with food material is, however, 
 clearly incompatible with great activity, so one of the two gametes 
 remains unencumbered and becomes specialized as the active 
 partner, charged with the duty of seeking out its mate 
 and bringing about their union, while the other, more or less 
 burdened with the necessary supplies, passively awaits the event. 
 The conjugation of such differentiated gametes leads to a more 
 satisfactory result than can be attained in cases of isogamy 
 like that of Copromonas, for the new individual will have a better 
 chance in life owing to the greater amount of capital with which it 
 commences. Such sexual differentiation of the gametes finds its 
 most complete expression in the formation of female ova and 
 male spermatozoa, which are especially characteristic of the 
 higher animals (compare Fig. 69), though they also occur in 
 many plants and even in some unicellular forms. The process 
 of conjugation in such a case is often spoken of as the fertiliza- 
 tion of the ovum by the spermatozoon. 
 
 These considerations enable us to understand at once the great 
 difference in size which usually distinguishes the male from the 
 female gamete, whence the general terms microgametes and mega- 
 gametes so often applied to them. The microgamete is as small 
 as possible in order that its activity may not be impaired ; the 
 megagamete is swollen out with nutrient material. 
 
 We may illustrate these general principles by a brief description 
 of a few more cases of conjugation amongst unicellular organisms. 
 
 Bodo, or Heteromita (Fig. 38), is a very minute flagellate monad
 
 86 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 FIG. 38. Life History of Bodo, or the Springing Monad, very highly 
 magnified. (From Dallinger and Drysdale.) 
 
 A. Ordinary individual. 
 
 B., C. Multiplication by longitudinal fission (the nucleus divides and the cell body and 
 
 both flagella split lengthwise). 
 D F. Multiplication by transverse fission (the nucleus and cell body divide, the trailing 
 
 flagellum splits lengthwise and a new anterior flagellum is budded out at one end). 
 G. Two gametes about to conjugate. 
 H. Conjugation. 
 
 3 . Zygote formed by conjugation, with flagella still attached. 
 K. Fully formed zygote. 
 
 L. Escape of spores by rupture of the zygote wall. 
 M. Development of the spores, 
 a./., anterior flagellum; a'.f., new anterior flagellum sprouting out; nu., nucleus; 
 
 sj>., spores ; t.f., trailing flagellum ; zyg.nu., zygote nucleus.
 
 LIFE HISTORY OF BODO 87 
 
 which occurs in long-standing infusions of cod's head. It differs 
 from Copromorias chiefly in the possession of two flagella and in 
 the absence of a cell mouth, all its food being taken in in a state of 
 solution by diffusion through the thin cell membrane. The two 
 flagella both spring from the beak-like anterior extremity. One 
 (A, a./.) extends forwards and by its movements enables the 
 organism to swim actively about, the other (A, t.f.) hangs down 
 and is trailed behind during active locomotion. The monad 
 anchors itself by the trailing flagellum and then, by coiling and 
 uncoiling the latter, executes characteristic springing movements. 
 Asexual reproduction (i.e., reproduction without any sexual 
 process) is effected by simple fission, which may be either 
 longitudinal (B, C) or transverse (D F). 
 
 There are no structurally differentiated gametes or sexual 
 cells, but conjugation (G J) is effected between two apparently 
 similar individuals which are indistinguishable from the ordinary 
 form. It is noteworthy, however, that one of the two gametes at the 
 time of union is anchored, while the other swims actively up to it, 
 and thus we get a slight indication of physiological differentiation 
 into active and passive, or male and female. The male gamete 
 also arises by a somewhat peculiar method of fission. Conjugation 
 of the two gametes produces a zygote which has somewhat the 
 shape of a triangular sac (K). The flagella disappear and in the 
 interior of the sac cell-division goes on with great rapidity, giving 
 rise to an immense number of very minute spores, which ultimately 
 escape from the corners of the sac in the form of very fine duot 
 (L, p.). Each spore no doubt is a minute nucleated cell, but it is 
 so small that the nucleus cannot at first be made out. It grows 
 by absorbing liquid food from the infusion in which it lives, and 
 as it grows the nucleus becomes apparent, flagella are put forth, 
 and the adult form is gradually attained (M). We have here a 
 striking illustration of the fact that the most obvious result of 
 conjugation is an increase of the power of cell-division. 
 
 As a case of complete morphological as well as physiological 
 differentiation between male and female gametes in a unicellular 
 organism we may take that of Coccidium schubergi, which occurs 
 as a parasite in the intestine of a centipede (Lithobius forficatus). 
 The life history of this remarkable protozoon is very com- 
 plicated and it is not necessary for our purposes to describe it 
 in detail. The adult organisms occur in the form of spherical 
 nucleated cells, each actually inside one of the epithelial cells
 
 88 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 which line the intestine of the host and upon which the parasites 
 feed (Fig. 39, A.). The latter increase in number very rapidly 
 by a kind of multiple fission, and successive generations of 
 parasites attack fresh epithelial cells of the host until the 
 epithelium is more or less completely destroyed. After many 
 generations have been produced asexually in this manner a 
 sexual process sets in. Megagainetes and microgametes are pro- 
 duced ; the former by growth of an ordinary individual into a 
 large spherical ovum, or egg-cell (Fig. 39, B., ? gam.), the latter 
 by growth and division of an ordinary individual into a number 
 
 <?.r 
 nu. 
 
 A."* D 
 
 FlG. 39. Coccidium schubergi, highly magnified. (After Scliaudinn.) 
 
 A. A full grown Coccidium lying within an epithelial cell of the host. 
 
 B. Male and female gametes about to conjugate. 
 
 coc., the Coccidium ; c.r., the cone of reception put out by the female gamete ; TIM., nucleus 
 of female gamete ; nu.coc., nucleus of adult Coccidium ; nu.ep., nucleus of epithelial 
 cell of host ; g gam., male gamete or spermatozoon ; J gam., female gamete or ovum. 
 
 of very much smaller spermatozoa or sperm cells (Fig. 39, B., 
 J gam.). 
 
 The ovum is densely filled with food granules and has no 
 power of locomotion. The spermatozoon closely resembles a 
 flagellate monad, being provided with a pair of flagella by means 
 of which it swims actively about ; the body, however, is long and 
 slender and consists almost entirely of nuclear (chromatin) material. 
 It seeks out the ovum, which exercises a peculiar attraction upon 
 it, and the two conjugate, the spermatozoon boring its way into 
 the ovum and their nuclei fusing to form the zygote nucleus. 
 The only trace of activity, apart from nuclear phenomena, which 
 the ovum exhibits is the protrusion of a small "cone of recep- 
 tion " (Fig. 39, B. c.?-.) from the surface of the cell towards the 
 approaching spermatozoon, which seems to indicate that the
 
 ORIGIN OF SEX IN PLANTS 89 
 
 attraction is mutual. The process of conjugation is followed as 
 usual by cell-division on the part of the zygote, which in this 
 case results in the formation of a small number of comparatively 
 large spores, enclosed in tough protective envelopes. From 
 these spores new individuals are produced which, under favour- 
 able circumstances, commence the life cycle afresh. It is 
 extremely interesting to observe that we have here, in a 
 unicellular Prot3zoon, as complete a sexual differentiation of 
 the gametes as we meet with in any of even the most highly 
 organized plants and animals. 
 
 We must now briefly notice the sexual phenomena exhibited 
 by those interesting Protista which we have already had occasion 
 to refer to in Chapter IV. under the name Phytoflagellata. It 
 will be remembered that in Haematococcus (Fig. 5) the process of 
 simple fission sometimes results in the production of a relatively 
 large number (3264) of small individuals instead of the usual four 
 comparatively large ones. These small individuals are specialized 
 gametes, differing from the large ones not only as regards size 
 but also in the absence of the characteristic cell-wall of the latter. 
 There is, however, no differentiation into male and female. 
 Conjugation is of the isogamous type and produces a zygote 
 which grows into an ordinary resting cell which will presently 
 begin to multiply actively by ordinary fission. 
 
 We have also seen that this organism forms the starting point 
 of a series of forms, represented by the genera Haematococcus, 
 Pandorina, Eudorina and Volvox, which illustrate progressive 
 stages in the process of colony formation. The same series also 
 shows us very clearly the differentiation between male and female 
 gametes the origin of sex in the vegetable kingdom. It will be 
 remembered that Pandorina forms colonies of sixteen or thirty- 
 two cells enclosed in a common envelope (Fig. 10, A). A sexual 
 multiplication is effected by each cell of the colony dividing into 
 2, 4, 8, and finally 16 or 82, which form a daughter colony within 
 the parent, to be liberated presently by softening of the parental 
 envelope. Occasionally, however, the individual cells of a colony 
 divide each into eight gametes. These are small cells, each with 
 a pair of flagella, which escape and swim about separately. They 
 exhibit no clear distinction into male and female, but some are 
 comparatively large, some small, and some intermediate in size. 
 They conjugate in pairs (Fig. 10, B), and the two members of a 
 conjugating pair are often, though apparently not always, of
 
 90 
 
 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 different sizes. It is probable that we have here a kind of 
 foreshadowing of that sharp differentiation into large (female) 
 
 megagametes and small (male) microgametes of which we have 
 already spoken. 
 
 In Eudorina, which forms colonies somewhat similar to those of 
 Pandorina, this differentiation is already fully expressed (Fig. 40). 
 The megagametes or ova 1 differ scarcely at all from ordinary 
 
 1 Often termed by botanists oospheres.
 
 SEXUAL DIFFERENTIATION IN PROTOPHYTA 91 
 
 individuals. They are shown imbedded in the gelatinous 
 matrix of the large colony on the left of the figure. The micro- 
 gametes or spermatozoa, 1 on the other hand, are much smaller, 
 club-shaped bodies, having a characteristic yellowish colour and 
 with a pair of flagella at the narrow, pointed end (Fig. 40, M 3 ). 
 They are produced in bundles of sixty-four by repeated divi- 
 sion of a mother cell (Fig. 40, II VI). Thus the male and 
 female gametes, spermatozoa and ova, do not occur together 
 in the same colony, but the colonies, when they consist of 
 gametes and not of ordinary individuals, contain only one or the 
 other kind. Hence the sexual differentiation is in this case 
 extended from the gametes themselves to the colonies which 
 bear them, and we may recognize colonies of three kinds : (1) 
 Asexual, which produce no gametes and reproduce by ordinary 
 fission of all or any of the component cells, (2) male sexual, which 
 produce microgametes or spermatozoa, and (3) female sexual, 
 which produce megagametes or ova. The bundles or colonies of 
 microgametes (Fig. 40, MI, Ma) swim about actively by means of 
 their flagella, apparently in search of the larger and much less 
 active female colonies (Fig. 40, I). Having found such a 
 colony the now separated microgametes (Fig. 40, M 3 ) bore 
 their way in, ultimately conjugating with the megagametes to 
 form zygotes. 
 
 In any one colony of Pandorina or Eudorina the constituent 
 cells are all of one kind, all ordinary asexual cells or all male 
 gametes or all female gametes. In Volvox (Fig. 11) we meet with 
 a further advance. Even in asexual colonies which do not produce 
 gametes at all we find the cells differentiated in so far that some 
 only are capable of giving rise (asexually) to daughter colonies, 
 while in the female colonies only some of the cells form female 
 gametes or ova (Fig. 11, A, o). The male gametes (Fig. 11, B D) 
 are very similar to those of Eudorina. They unite with the 
 female gametes to form zygotes, which, after a period of rest, 
 develop into new Volvox colonies. 
 
 In order to emphasize the fact that the process of conjugation 
 is essentially a nuclear phenomenon we may now turn to the 
 case of ParamcBcium. The general appearance and structure of 
 this protozoon have been described in Chapter IV. (Fig. 8). It 
 multiplies by simple transverse fission, and under favourable 
 conditions continues to do so until exhaustion sets in, when its 
 
 1 Often termed by botanists spermatoKoids.
 
 92 
 
 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 failing powers are restored by conjugation. The conjugation in 
 this case, however, is not quite like that which takes place in 
 the other unicellular organisms which we have been studying, 
 and the term conjugant may be applied, in preference to the 
 term gamete, to the individuals concerned. 
 
 It may easily be observed that the union of the two con- 
 jugants (Fig. 41) is a merely temporary affair. They remain 
 attached together by their mouth-bearing surfaces for a short 
 
 St. 
 
 FIG. 41. Diagram of the Process of Conjugation in Faramoacium. 
 
 gam., gametic nuclei; M., mouth; meg., meganucleus; mic., micronucleus; mig., 
 migratory nucleus; \ p. b., products of the division of the micronucleus which dis- 
 appear (polar bodies); st., stationary nucleus ; zyg., zygote nucleus. (For further 
 explanation see text.) 
 
 time and then separate again and continue their independent 
 lives. Before separating, however, they evidently undergo some 
 kind of rejuvenescence whereby their vigour and power of multi- 
 plication are completely restored. This is accounted for by the 
 fact that during the time of their union certain complex nuclear 
 processes take place, the net result of which is an exchange of 
 chromatfn material between the two conjugants. 
 
 It will be remembered that Paramoecium differs from most 
 Protozoa in the possession of two nuclei, large and small, or 
 meganucleus and micronucleus (Fig. 41, A, meg. and mic.). The
 
 CONJUGATION IN PARAMCECIUM 93 
 
 latter is alone concerned in the process of conjugation, the 
 meganucleus in the meantime breaking up and being absorbed 
 into the cytoplasm, to be replaced in the manner described later 
 on. We may confine our attention, therefore, to the behaviour 
 of the micronucleus. In each conjugant this divides mitotically 
 into two daughter nuclei (Fig. 41, B, mic.' and p.b.) each of which 
 again divides, so that there are now four micronuclei (Fig. 41, C). 
 Of these four three (p.l>-} go to the bad, being apparently absorbed 
 into the cytoplasm, while the remaining one (mic.") divides once 
 more, so that each conjugant has now again two micronuclei 
 (Fig. 41, D). These two, though similar in appearance, differ 
 strikingly in their behaviour, one of them remaining quiescent 
 while the other passes over into the body of the other conjugant. 
 They are therefore known respectively as the stationary (st.) and 
 the migratory (mig.) micronuclei. In this way the migratory 
 micronuclei of the two conjugants change places with one another, 
 as indicated by the arrows, and the sole object of the temporary 
 union of the two conjugants appears to be to enable this inter- 
 change to take place. When it has been effected a true conjuga- 
 tion occurs between the two micronuclei in each cell (Fig. 41, E, 
 gam.), derived one from each conjugant. This is the real sexual 
 process. The migratory and stationary nuclei are gametic nuclei 
 and the result of their union is a zygote nucleus (Fig. 41, F, zyg.\ 
 Moreover, we have here again an evident distinction into 
 male and female gametic nuclei, characterized in the usual 
 way by the activity of the one and the passivity of the other. 
 The two conjugants themselves, however, cannot be distin- 
 guished as male and female, for each produces both male 
 and female gametic nuclei and may therefore be regarded as 
 hermaphrodite. 
 
 After the interchange of gametic nuclei has taken place the two 
 conjugants separate as ex-conjugants (Fig. 41, F). The zygote 
 nucleus in each divides repeatedly by mitosis and from the 
 daughter nuclei thus produced both micronuclei and meganuclei 
 are formed. Presently the ex-conjugants themselves begin 
 to divide once more by fission and the new micronuclei 
 and meganuclei are distributed amongst the new individuals 
 (Fig. 41, G). 
 
 The essential feature of this very complicated process is clearly 
 the same as in the simpler cases which we have examined, and 
 consists in the union of two nuclei belonging to different cells to
 
 94 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 form a single zygote nucleus which has renewed powers of 
 multiplication hy division. As already observed, the inicronucleus 
 alone takes part in the process, the meganucleus being merely 
 concerned in the life of the individual and its asexual multiplication 
 by simple fission.
 
 CHAPTEK VIII 
 
 Sexual phenomena in multicellular plants The distinction between somatic 
 cells and germ cells Alternation of sexual and asexual generations- 
 Suppression of the gametophyte in flowering plants. 
 
 WHEN w r e consider the habit of colony formation which is so 
 common amongst the Protophyta, and which we have discussed 
 in the cases of Pandorina, Eudorina and 
 Volvox, we see at once that it is impossible to 
 draw any strictly logical distinction between 
 such primitive forms and the true multi- 
 cellular plants or Metaphyta. The common " 
 fresh water alga, Spirogyra, for example, str. 
 might be regarded either as a colony of single 
 cells or as a very simple multicellular plant 
 in which the constituent cells exhibit little 
 or no differentiation amongst themselves. 
 In any case it forms a very convenient start- 
 ing point for the consideration of the sexual 
 phenomena met with in Metaphyta, being in 
 this respect actually in a much more primitive 
 condition than either Eudorina or Volvox. 
 
 The fully developed Spirogyra plants con- 
 sist of long green filaments of hair-like 
 dimensions, which float in loose slimy 
 masses in clear fresh water. Each filament 
 consists of a single row of cylindrical cells 
 placed end to end, each cell being enclosed in 
 a thin, transparent wall of cellulose (Fig. 42, 
 c. ?i-.), whereby its protoplasmic contents are 
 completely separated from those of adjacent 
 cells. The cytoplasm forms a thin primordial 
 utricle (p.w.), lining the cell-wall and enclosing 
 a large vacuole (vac.) filled with colourless, watery cell-sap, in 
 which a more or less central mass of cytoplasm, containing the 
 
 FlG. 42. Part of a 
 filament of Spiro- 
 gyra, showing 
 one complete cell 
 and parts of two 
 others ; highly 
 magnified. 
 
 c.w., cell-wall; cr., chro- 
 matophore ; nu., 
 nucleus ; p.u., pri- 
 mordial utricle; 
 Py r -> pyrenoid; sir., 
 strands of proto- 
 plasm; vac., vacuole.
 
 96 
 
 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 nucleus (nu.\ is suspended by slender radiating protoplasmic 
 threads (sir.). So far, both in structure and arrangement of its 
 component cells, the plant closely resembles a single hair of 
 Tradescantia (compare Fig. 25). It differs, however, in the 
 presence in each cell of one or more 
 conspicuous chloroplastids or chroma- 
 tophores (cr.), coloured bright green 
 by chlorophyll and wound spirally 
 round and round inside the cell-wall, 
 like pieces of ribbon. It is from 
 these characteristic structures that 
 the name Spirogyra is derived. 
 
 The filaments increase in length by 
 transverse fission of the component 
 cells. Every cell, however, or to speak 
 more accurately its protoplasmic con- 
 tents, must also be looked upon as 
 a potential gamete. Conjugation in 
 some species takes place between the 
 cells of two filaments which are lying 
 side by side, parallel with one another 
 (Figs. 43, 44). In others it takes place 
 between adjacent cells of the same 
 filament, but we may confine our 
 attention to the former case. The 
 first indication of the process is seen 
 in the formation of a small, hollow r pro- 
 tuberance on the wall of one of a pair 
 of cells which happen to be more or 
 F IG . 43. _ Conjugation in less opposite to each other (Fig. 44, a.). 
 Spirogyra, showing in one This is shortly followed by the forma- 
 Filanient solitary Cells ,. , . ., , , 
 
 (s.c.) which have failed tion of a similar protuberance on 
 to mate, and in the other the wall of the other cell (&.). The 
 Cftf. &&UL) 88 ' two protuberances meet (*.) and fuse 
 together, and the cell-walls at the 
 
 point of union are dissolved away (d.). Thus a hollow canal is 
 formed placing the cavities of the two conjugating cells in free 
 communication with one another. In the meantime changes 
 are going on in the protoplasmic contents of the conjugating 
 cells, essentially similar in the two members of each pair but 
 with one cell still taking the lead and the other lagging somewhat
 
 CONJUGATION IN SPIROGYRA 
 
 97 
 
 behind. The chloroplastid breaks up ; the primordial utricle 
 
 retreats from the cell-wall towards the middle, and the entire 
 
 protoplasmic contents round themselves off inta a compact 
 
 nucleated mass the gamete (gam.). The time has now arrived 
 
 for the all-important event ; the gamete from one cell-chamber 
 
 creeps through the canal into the other chamber and conjugates 
 
 with the gamete which there awaits 
 
 it (Fig. 44, <?.). The result is a 
 
 zygote or zygospore (Figs. 48, 44, 
 
 zyg.) which surrounds itself with a 
 
 thick protective envelope within 
 
 the cell-wall of the parent and 
 
 after a period of rest germinates, 
 
 giving rise by repeated transverse 
 
 division to a new filament. 
 
 In this case it will be observed 
 that the two conjugating gametes 
 are morphologically alike, and we 
 may therefore describe the process 
 of conjugation as isogamous. As in 
 the case of Bodo, however, there is 
 a physiological distinction between 
 the two as regards their activity ; 
 the gamete which takes, the lead 
 throughout the whole process and 
 finally crawls through the canal of 
 communication is obviously to be 
 regarded as male, and its partner 
 as female. Moreover, one entire 
 filament may produce nothing but 
 male gametes and may therefore 
 be regarded as a male plant (Fig. 
 44, <? ), while the other may produce 
 
 only female gametes and therefore be regarded as a female plant 
 (Fig. 44, ? ). In the closely related form, Zygogonium, there is 
 no distinction of any kind between the two sexes, both gametes 
 exhibit the same degree of activity and they meet each other half- 
 way, in the connecting canal between their respective chambers, 
 to form the zygote. 
 
 In Spirogyra, as we have just seen, every cell of the adult plant 
 is a potential gamete or germ cell ; in the case of Volvox, it will 
 
 B. II 
 
 FlG. 44. Diagram of two con- 
 jugating filaments of Spiro- 
 
 a. e., pairs of cells in successive 
 stages of conjugation ; gam., 
 male gametes; J gam., female 
 gametes; S.C., solitary cell;
 
 98 
 
 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 be remembered, this is not so, certain cells only being concerned 
 in the sexual process. We have here a kind of fore-shadowing of 
 that distinction into somatic or body cells and sexual or germ 
 cells which is so characteristic of the higher organisms, both 
 plants and animals. We see a somewhat similar fore-shadowing 
 
 FIG. 45. Fucua vesiculosus, about half nat. size. (From Vines' " Botaiiy.") 
 b, air bladders ; /, fertile branch. 
 
 even in the unicellular Paramcecium, except that the distinction 
 is here confined to the nuclei, the meganucleus being concerned 
 with the life of the individual and the asexual process of ordinary 
 cell-division, while the micronucleus is alone concerned in the 
 sexual phenomena. We can, in this case, distinguish between 
 somatic nucleus and germ nucleus, though not between somatic 
 and germ cells.
 
 GERM CELLS AND SOMATIC CELLS 99 
 
 The loss of the power to act as germ cells or gametes which 
 the vast majority of the constituent cells in a typical multicellular 
 organism usually suffer is undoubtedly one of the penalties 
 which they have to pay for their high degree of speciali- 
 zation. The germ cells themselves, on the other hand, always 
 remain in a more primitive, less specialized condition, and 
 may, in fact, be regarded as so many unicellular Protista 
 enclosed within the multicellular body. They resemble the 
 free-living Protozoa and Protophyta also in that they exhibit a 
 certain degree of independence and are in the majority of cases 
 actually set free from the parent body as unicellular individuals ; 
 
 FIG. 46. Fucus vesiculosus ; Section through a female Conceptacle, X 50. 
 (From Vines' " Botany," after Thuret.) 
 
 short-lived, it is true, unless they happen to meet one another 
 and conjugate, but nevertheless enjoying that liberty which is 
 only possible to independent organisms. 
 
 All this is very beautifully illustrated in the case of the common 
 brown seaweed or bladder-wrack, Fucus vesiculosus (Fig. 45). 
 The entire plant consists of flattened branches, copiously 
 subdivided and attached by a root, usually to some rock between 
 tide-marks. Here and there the branches are swollen out to form 
 air-bladders (I), which serve as organs of flotation. The 
 histological structure of the plant is comparatively simple and 
 need not detain us ; we are concerned only with its reproductive 
 processes. 
 
 Fucux rcisiculosna is dioecious or unisexual, there being distinct 
 
 H 2
 
 100 OUTLINES OF E VOLUTION AKY BIOLOGY 
 
 male and female plants. In both cases certain branches (/), 
 usually described as fertile, are characterized by the presence of 
 numerous minute spherical pits, opening on to the surface by narrow 
 mouths. These pits or conceptacles, one of which is represented 
 in vertical section in Fig. 46, contain the sexual organs, male 
 antheridia or female oogonia, as the case may be, intermingled 
 with hair-like structures known as paraphyses. The antheridia 
 (Fig. 47, a) are attached to the branching paraphyses in the male 
 
 FIG. 47. Fucus vesiculosus. 
 
 a, branching paraphysis from male conceptacle, bearing antheridia ; 6, an oogonium 
 surrounded by unbranehed paraphyses and with its contents divided into eight ova ; 
 c, a discharged ovum surrounded by spermatozoa, one of which will fertilize it ; d, a 
 developing embryo ; all x 160. (From Vines' " Botany," after Thuret.) 
 
 conceptacles in the form of small sacs in which the male gametes 
 (spermatozoa) are produced. These are minute, nucleated, pear- 
 shaped cells, each with two flagella ; they are produced in large 
 numbers in each antheridium and set free by rupture of the wall 
 of the latter to make their way out of the opening of the 
 conceptacle by their own activity. 
 
 The oogonia are oval sacs, a good deal larger than the antheridia, 
 and occur amongst the hair-like paraphyses in the female con- 
 ceptacles (Fig. 46). In each oogonium (Fig. 47, />) eight female 
 gametes (egg cells, ova or oospheres) are formed by cell-division. 
 These are spherical, nucleated cells, very much larger than the
 
 ALTERNATION OF GENERATIONS IN PLANTS 101 
 
 spermatozoa, owing to the great amount of cytoplasm which they 
 contain. They are liberated by rupture of the oogonium and 
 discharged through the opening of the conceptacle on to the 
 surface of the plant. There they are found by the spermatozoa, 
 which swarm around them in large numbers, endeavouring to 
 conjugate (Fig. 47, c). Finally a single spermatozoon succeeds 
 in boring its way into each large egg cell, and fertilization is 
 effected by the union of the male and female nuclei. The zygote, 
 well supplied with food material by the egg cell, begins to undergo 
 cell-division immediately, forming a multicellular embryo 
 (Fig. 47, d) which attaches itself by roots and grows into a plant 
 resembling the parents. 
 
 Here we have a perfectly typical case of differentiation of the 
 gametes or germ cells into large passive female ova and small 
 active male spermatozoa, and conjugation is anisogamous, the 
 ovum being " fertilized " by the spermatozoon. 
 
 In the ferns, mosses and other more highly organized plants 
 a new complication is introduced by the fact that two distinct 
 forms of the plant alternate with one another in the life cycle. 
 In one only of these forms, known accordingly as the gametophyte, 
 does a sexual process occur ; the other, known as the sporophyte, 
 reproduces by means of unicellular spores, which are produced 
 asexually and develop into new individuals without any process 
 of conjugation. The gametes or germ cells, borne on the gameto- 
 phyte, on the other hand, conjugate, and the zygote develops, 
 not into another gametophyte but into a sporophyte, while, 
 conversely, the spores produced by the sporophyte develop into 
 gametophytes. 
 
 This alternation of sexual and asexual generations is a 
 phenomenon of very wide-spread occurrence in the vegetable 
 kingdom, and, as we shall see in our next chapter, something of 
 the same kind occurs also in certain multicellular animals. 
 
 Take, for example, any ordinary fern. The conspicuous plant 
 (Fig. 48) is the sporophyte. It is very highly organized and shows 
 the typical differentiation into root, stem and leaf met with in all 
 the higher groups of the vegetable kingdom. Some or all of the 
 leaves sooner or later produce on their lower surfaces sporangia 
 (Fig. 48, A, C), little sac-shaped structures in which the spores 
 arise -by division of mother cells into fours. These spores are 
 liberated, by rupture of the sporangia, in the form of fine brown 
 dust, which may be carried to considerable distances by the wind.
 
 102 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 FIG. 48. The Sporophyte Generation of a Fern, Aspidium filix mas. (From 
 
 Strasburger.) 
 
 A, section through a sorus or group of sporangia, covered by the indusium (x 20, after 
 Kiiy); B, lower surface of a pinna, showing the indusia covering the sori; C, lower 
 
 Any 
 
 surfi 
 
 ace of a pinna with the sori exposed.
 
 LIFE HISTORY OF A FERN 
 
 103 
 
 ft*. 
 
 If one of them alights on a suitable spot, in a moist and shady 
 situation, it may germinate (Fig. 49). Its thick outer wall rup- 
 tures and a delicate tube is 
 put forth, containing the pro- 
 toplasm and nucleus. Cell- 
 division takes place and results 
 presently in the formation of 
 the gametophyte. 
 
 The gametophyte of the 
 fern (Fig. 50) is known as a FIG. 49. Diagram of a young Prothallus 
 prothallus. It is an indepen- (^A.) formed by Germination of a 
 
 .. Fern Spore. 
 
 dent, Bell-supporting plant, 7 , . . , 
 
 rli., rhizoid or root hair; sp.c., spore coat. 
 
 but much less highly orga- 
 nized than the sporophyte, consisting usually of a green, heart- 
 shaped plate of cells, not more than perhaps a quarter of an 
 inch in diameter, and attached to the substratum by delicate 
 hair-like rhizoids. It develops no vascular system but never- 
 theless obtains its food 
 in the same way as the 
 sporophyte, absorbing 
 water containing dis- 
 solved mineral salts 
 from the soil by means 
 of its rhizoids, and 
 splitting up carbon 
 dioxide, obtained from 
 the air, by aid of its 
 chlorophyll. Such pro- 
 thalli are frequently 
 to be found attached to 
 the surfaces of flower- 
 pots and walls in damp 
 greenhouses and other 
 places where ferns are 
 grown. 
 
 The sexual organs, 
 male antheridia (Fig. 
 50, an) and female archegonia (Fig. 50, ar), are, like the rhizoids, 
 found, on the lower surface of the prothallus, both usually occurring 
 on one and the same plant, which is therefore monoecious or 
 hermaphrodite. The antheridia (Fig. 51, a) are essentially similar 
 
 FIG. oO. The Gametophyte Generation or Pro- 
 thallus of a Fern, Aspidium filix mas, X 8. 
 (From Strasburger.) 
 
 A, lower surf ace of a sexually mature prothallus, showing 
 
 antheridia (an), archegonia (ar), and rhizoids (rh). 
 
 B, an older prothallus with the young sporophyte genera- 
 
 tion or fern plant (b, w) attached to it.
 
 104 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 to those of Fucus, being hollow sacs in which the male gametes 
 are developed. The latter (Fig. 51, s.) are active spermatozoa, 
 each having a spirally coiled body, 
 consisting chiefly of chromatin 
 material, and bearing a bunch of 
 cilia at one end, by the vibration of 
 which the gamete swims actively 
 about in any dew or other moisture 
 which may be deposited on the 
 prothallus. 
 
 The archegonia (Fig. ,52) differ 
 considerably from the oogonia of 
 Fucus, having a characteristic 
 structure which is more or less 
 accurately repeated in the corre- 
 sponding organs of all the higher 
 plants. Each consists of a hollow 
 swollen venter, sunk in the tissue 
 of the prothallus, and a long neck 
 which projects from the surface, 
 and the wall of which is composed 
 of four rows of cells. The venter 
 contains a single relatively large ovum or oosphere (Figs. 52, A, o, 
 and 52, B), above which an axial row of canal cells (A'', K") 
 
 FIG. 51. Antheridiumof aFern, 
 discharging Spermatozoa 
 (Antherozoids) from its 
 opening, highly magnified. 
 (From Vines' " Botany.") 
 a, antheridium ; s, spermatozoon. 
 
 FIG. 52. Archegonia of a Fern, Polypodwm vulyare, X 240. (From 
 Strasburger.) 
 
 A, young, showing ovum(o) and canal cells (K',K"), and with the end of the neck closed 
 
 B, mature, with the end of the neck open. 
 
 extends into the neck. When the ovum is ready for fertilization 
 the canal cells degenerate into mucilage and the cells at the end
 
 CONJUGATION IN FERNS 105 
 
 of the neck separate so as to form an opening (Fig. 52, B). The 
 spermatozoa appear to be attracted to the opening by an acid 
 secretion discharged therefrom. One of them makes its way 
 down the neck to the ovum and fertilizes it by the usual process 
 of conjugation. 
 
 The zygote begins to develop, by cell-division, within the 
 venter of the archegonium, and forms a young sporophyte, which 
 for some time remains attached to the prothallus as shown in 
 Fig. 50, B, drawing nutriment therefrom by means of a special 
 temporary organ known as the foot. Presently, root, stem and 
 leaf are developed and the sporophyte becomes self-supporting. 
 
 One very remarkable fact in connection with the sexual 
 process in the fern remains to be noticed. The gametes, as we 
 have seen, are normally produced in special sexual organs and 
 are themselves perhaps as highly differentiated in relation to 
 the function of conjugation as gametes ever are. It has been 
 found, however, that if the normal sexual union between ova 
 and spermatozoa be prevented a conjugation may take place 
 between nuclei from adjacent vegetative cells of the prothallus, 
 resulting in the formation of an embryo sporophyte by so-called 
 apogamy. In these cases it is obvious that the sexual process 
 is not really suppressed, but simply transferred to ordinary 
 prothallial cells, w r hich, though they do not conjugate under 
 normal circumstances, have retained the power of so doing when 
 occasion arises. It seems probable, however, that in' some cases 
 true apogamy, or suppression of the sexual process, occurs, the 
 embryo sporophyte arising from the prothallus without any 
 conjugation of gametes. 
 
 The gametophyte or prothallus of an ordinary fern is, as we 
 have already seen, produced by the development of a unicellular 
 spore (Fig. 49), and in most cases is monoecious or hermaphro- 
 dite, bearing both male and female sexual organs and male and 
 female gametes. In the less commonly known and not very 
 fern-like " heterosporous " forms, however (Isoetes, Salvinia and 
 Marsilea), the gametophyte is dioecious or unisexual, there being 
 distinct male and female prothalli, and this sexual differentia- 
 tion affects not only the prothalli themselves but the spores from 
 which these are developed. Hence in these forms we find small 
 microspores which produce male prothalli, and large megaspores 
 which produce female prothalli. The spores themselves are set 
 free from the parent sporophyte, but the prothalli are very much
 
 106 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 reduced in size and never become free from the spores ; they 
 nevertheless develop antheridia and archegonia respectively, 
 in which spermatozoa and ova are produced, and from the 
 conjugation of these arise zygotes or fertilized ova which 
 develop into new sporophytes. 
 
 We have briefly noticed these heterosporous ferns because, 
 as regards the sexual phenomena which they exhibit, they 
 constitute a very interesting connecting link between the ordinary 
 (homosporous) ferns, which produce only one kind of spore, and the 
 highest members of the vegetable series, the flowering plants. 
 
 In the flowering plants an alternation of sexual and asexual 
 generations can still be traced, but here the gametophyte is so 
 much reduced in size and has become so degenerate in structure 
 that it is quite inconspicuous, and can only be detected by micro- 
 scopical examination and recognized as constituting a distinct 
 generation in the light of our knowledge of lower forms. 
 
 The flowering plant itself is the sporophyte, and it is hetero- 
 sporous, producing microspores and megaspores. The pollen 
 grains are the microspores, while the megaspores are represented 
 by the embryo sacs enclosed within the ovules or unripe 
 seeds. The microspores, like the spores of ferns, are set free 
 from the parent sporophyte, the megaspores, however, are never 
 set free as such, and in neither case does the gametophyte become 
 free from the spore. 
 
 The terms pollen grain and embryo sac were applied to 
 the structures in question long before their true nature as 
 microspores and megaspores was recognized, and they have 
 become so firmly established that it is hardly possible to avoid 
 using them. 
 
 If we examine any typical, fully developed flower, such as is 
 represented diagrammatically in Fig. 53, we shall find that it 
 consists of four whorls or circlets of specially modified leaves. 
 Beginning at the outside we find first the calyx (AY), composed 
 of a number of sepals, which usually, but by no means always, 
 retain the green colour characteristic of leaves and serve mainly for 
 the protection of the inner parts of the flower while in the bud ; 
 then the corolla (K), composed of petals, which may be brightly 
 coloured and serve to attract insects ; then the androecium, 
 composed of stamens (a,/); and lastly, in the centre of the 
 flower, the gynoecium or pistil (n, g, F), composed of carpels. 
 The stamens and carpels are often spoken of as the essential
 
 STRUCTURE OP A FLOWER 
 
 107 
 
 parts of the flower. They are really to be regarded as spore- 
 bearing leaves or sporophylls. 
 
 Each stamen consists usually of a long stalk or filament (/), 
 bearing an anther (a) at its extremity. The anther is a bilobed 
 structure, and each lobe contains two chambers, or pollen sacs, 
 in which the pollen grains (p) are formed by the division of 
 mother cells into fours, just as the spores of an ordinary fern 
 are developed within the spo- 
 rangia. The pollen sacs are, in 
 fact, nothing but sporangia 
 and microsporangia, because they 
 contain microspores. 
 
 The pistil is formed of a vary- 
 ing number of carpels, which, 
 either singly or united, give rise 
 to a closed chamber below, the 
 so-called ovary 1 (F), surmounted 
 by a longer or shorter style (g), 
 ending above in a rounded viscid 
 surface, the stigma (n). In the 
 interior of the ovary, attached 
 
 to the carpels, are developed the p IG . 53. Diagram of a typical 
 ovules OS'), which are nothing 
 but sporangia (megasporangia) 
 enclosed each in a double enve- 
 lope (i). In each ovule a single 
 embryo sac or megaspore (em) 
 is produced. Only one ovule 
 is represented in the diagram, but there are usually a large 
 number in each ovary. 
 
 Having thus briefly described the parts of the sporophyte 
 with which we are immediately concerned, we must turn our 
 attention for a few moments to the gametophyte. The male 
 gametophyte, which never consists of more than a very small 
 number of cells, is developed from the pollen grain or microspore. 
 This latter is at first a perfectly typical unicellular spore, 
 with a single nucleus surrounded by cytoplasm, and the whole 
 enclosed in a thick protective cell-wall often ornamented with 
 microscopic sculpture of various patterns. The commencement 
 
 1 This is a very unfortunate name because the structure in question is an entirely 
 different kind of organ from the ovary of animals. 
 
 Flower in vertical Section. 
 (From Vines' " Botany.") 
 anther ; em, embryo sac ; E, ovum ; 
 /, filament of stamen; F, wall of 
 ovary; g, style; i, integument of 
 ovule; K, corolla; Ke, calyx; n, 
 stigma ; p, pollen grains ; ^js, pollen 
 tube ; S, ovule.
 
 108 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 of germination of this spore in typical cases (Fig. 54) is 
 marked by the division of the nucleus into two. Around one of 
 these two cytoplasm collects to form a naked "antheridial cell " 
 (m} ; the other, with the remainder of the cytoplasm, constitutes 
 the " vegetative cell " (k), which may or may not divide again. 
 The antheridial cell divides into two " generative cells." The 
 vegetative cell, or cells, represents the last vestige of the 
 body of the male prothallus ; the generative cells are male gametes. 
 The germination of the pollen grain and development of the 
 male prothallus are completed by 
 the putting forth of the pollen tube 
 (Fig. 53, ps, Fig. 54), which takes 
 place if the pollen grain is fortunate 
 enough to alight upon the stigma of 
 a flower of the right kind. The 
 pollen tube forces its way through 
 the loose tissue of the style to the 
 ovary and comes into intimate rela- 
 tions with one of the ovules contained 
 therein. 
 
 The female gametophyte is repre- 
 sented by a few cells formed by 
 
 FIG. 54.-^ermination of the division of the ^gaspore (embryo 
 
 Pollen Grain of Lilium sac), or rather of its nucleus, and to 
 
 martagon, x 375. (From some extent of its cytoplasm, within 
 
 Strasburger, after ,, i mi " 
 
 Guignard.) the ovule. The process is a some- 
 
 *, nuctena of the vegetative ceil of what complicated one, but, without 
 
 Stt^^^^Sa g in g in t details ' we ma y note that 
 by division of the antheridial at the time when the ovule is ready 
 
 for " fertilization " the embryo sac in a 
 
 typical flowering plant (Angiosperm) contains seven cells, one of 
 which is a female gamete (ovum or oosphere), while the others 
 may be taken to represent the female prothallus, including a 
 vestige of an archegoniuni. The arrangement of these cells is 
 shown in Fig. 55 (at, c, k, s). 
 
 The embryo sac or megaspore (E) is surrounded by a cellular 
 layer known as the nucellus (Fig. 55, K), which represents the wall 
 of the sporangium, and this in turn by two other coats (ai and ii), 
 the outer and inner integuments of the ovule, which grow up 
 around the nucellus. The entire ovule is attached to the wall 
 of the ovary by a stalk or funiculus (/), upon which it is
 
 CONJUGATION IN FLOWERING PLANTS 
 
 109 
 
 frequently bent sharply round as shown in Fig. 55. Opposite 
 to the spot where the funiculus is attached to the ovule an 
 aperture is left in the integuments known as the micropyle (m). 
 It is through this micropyle that the tip of the pollen tube 
 usually forces its way in search of the female gamete (e), which 
 lies at the end of the embryo sac close to it. The wall of the 
 pollen tube and that of the embryo sac are absorbed where 
 they come in contact with one 
 another, and thus a passage is 
 opened for the male gamete, 
 which passes down the pollen 
 tube into the embryo sac and 
 there conjugates with the ovum. 
 The zygote thus formed develops 
 within the embryo sac into an 
 embryo sporophyte x ; the integu- 
 ments of the ovule become har- 
 dened to form the seed coat ; 
 reserve food material, such as 
 starch or oil, is stored up 
 either in the embryo itself or 
 in the endosperm 2 around it, and 
 the ovule and its contents 
 separate from the parent sporo- Fia 55. Diagram of the unfer- 
 
 phyte as the ripe seed (Fig. 
 56, A). 
 
 When the seed ripens the 
 
 tilized Ovule of a Flowering 
 Plant (Angiosperm) in longi- 
 tudinal Section. (From Vines' 
 " Botany.") 
 
 development Of the Contained ai, outer integument; at, antipodal cells; 
 , , -, e, ovum ; E, embryo sac ; f, funiculus ; 
 
 embryo is suspended for an 
 
 ii, inner integument ; Jc, central or 
 definitive nucleus of embryo sac ; K, 
 nucellus ; m, micropyle ; s, synergidae. 
 
 indefinite period, to be resumed 
 
 again if and when the seed finds 
 
 a suitable situation in which to germinate. The embryo may then 
 
 continue its development into the adult sporophyte (Fig. 56, B). 
 
 It is interesting to notice that no less than three distinct 
 generations take part in the formation of the seed. The seed 
 coat belongs to the parent sporophyte, while the contents of the 
 
 1 (;,/,, pp. 48 r>0 (Fig. 14). 
 
 2 The second generative nucleus from the pollen tube conjugates with the. nucleus 
 in the middle of the embryo sac (Fig. ;">."> #), and the " endosperm " arises by repeated 
 division of 'the nucleus resulting from this conjugation. The whole process is 
 somewhat complex and it is not, essential to our present purpose to describe it in 
 detail.
 
 110 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 embryo sac at first represent the female gametophyte, which dis- 
 appears as the zygote develops into the embryo of another 
 sporophyte. In this way a very intimate relation is established 
 between each sporophyte generation and the one which precedes 
 it, and the gametophyte is crushed out of existence, as an inde- 
 pendent generation, between the two. 
 
 Accompany ing this almost total suppression of the gametophyte 
 we find a delegation of certain responsibilities connected with the 
 sexual function to the asexual sporophyte, and the development 
 by the latter of what may be termed vicarious sexual characters. 
 
 These characters find 
 their expression in that 
 most remarkable feature 
 of all the flowering 
 plants, the flower itself. 
 Thus the use of the 
 terms male and female 
 may be extended in the 
 first instance to the sta- 
 mens and pistil, though 
 these are really merely 
 the spore-bearing leaves 
 of the asexual genera- 
 tion. Similarly the 
 transference of the pol- 
 len grains from stamens 
 
 A. ripe seed split in half; B. seed germinating. ri -\ 
 
 cot., cotyledons or seed leaves ; pi., plumule or shoot of to stigma IS often spoken 
 
 embryo; rad., radicle or root of embryo; test., Q f ag faQ fertilization of 
 
 testa or seed coat. 
 
 the flower, though it is 
 
 obviously not the true process of fertilization but only a necessary 
 preliminary to the conjugation of the gametes, and is therefore 
 more accurately spoken of as pollination. 
 
 It will have been observed from the foregoing description that 
 not only is the male gametophyte of the flowering plant reduced 
 almost to the point of disappearance, but the male gamete itself 
 has apparently suffered great degeneration. It is no longer an 
 active spermatozoon, swimming about by means of flagella or 
 cilia, as in Eudorina or in the ferns, but a shapeless nucleated 
 mass of protoplasm which has at the most a sort of amoeboid 
 power of locomotion. By far the greater part of the travelling 
 which it has to accomplish in order to reach the ovum is 
 
 rad. 
 
 FIG. 56. Garden Pea (Pimm sativum}.
 
 POLLINATION 111 
 
 effected while it is still enclosed within the pollen grain and at 
 the expense of some external agency, for it is not until the pollen 
 grain has alighted upon the stigma and the pollen tube is put 
 forth that the " generative cell " begins to exercise its own feeble 
 powers of locomotion. 
 
 The transference of the pollen to the stigma is effected usually 
 in one of two ways, either by the action of the wind or by the 
 agency of insects. Flowers which are pollinated in the first of 
 these two ways are said to be anemophilous, and they are usually 
 small and inconspicuous, as in the grasses and plantains, and many 
 forest trees. It is a very extravagant method of pollination, 
 involving the production of enormous quantities of pollen, most 
 of which is wasted, for only a very minute percentage of the 
 pollen grains will ever chance to alight upon stigmas. We 
 realize this when we see the enormous quantities of yellow 
 pollen dust which are blown off the pine trees in spring time. 
 The entomophilous or insect-pollinated flowers, on the other hand, 
 have hit upon a much more economical way of doing the 
 business. The development of nectar or honey as a bait for 
 their insect visitors, and of gaily coloured petals or sepals and 
 sweet scents as a means of attracting them, and in many cases 
 of elaborate mechanical contrivances to secure that the insect 
 shall not obtain its reward without doing the work of pollination, 
 all co-operate in bringing about the desired end, and the study 
 of these various adaptations forms one of the most interesting 
 chapters in biological science. We shall refer to it again when 
 we come to deal with adaptation and natural selection. 
 
 In many cases a complete sexual differentiation is manifested 
 by the entire flowers themselves, some having stamens without 
 carpels and others carpels without stamens, and being spoken of 
 as "male" or "female" flowers accordingly. We see this, for 
 example, in the case of the vegetable marrow, where both kinds 
 of flower are borne on the same plant, while in other cases, such 
 as the weeping willow and the Japanese Aucuba, the whole plant 
 may be either " male " or " female," producing flowers of the one 
 kind only. Thus the terminology which strictly speaking is 
 applicable only to the sexual gametophyte, has, as a matter of 
 convenience, been extended to the asexual sporophyte in order to 
 describe the secondary sexual characters which have been trans- 
 ferred to it in consequence of the suppression of the former. 
 
 The telescoping of successive generations one within the other
 
 112 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the embryo sporophyte into the ovule of the preceding sporo- 
 phyte generation, with the gametophyte crushed in between 
 is the most characteristic feature of the life cycle of the higher 
 plants. 
 
 From the homosporous ferns upwards throughout the vegetable 
 series it is obvious that the gametophyte is not nearly so well 
 adapted in its organization to the conditions under which the 
 higher plants have to live as is the sporophyte. There has 
 apparently been a kind of rivalry between the two alternating 
 generations, in which the gametophyte has had much the worst 
 of it. The sexual function itself, however, is far too important 
 to be altogether abandoned, and so the successful sporophyte has 
 finally taken over many of the responsibilities connected there- 
 with, while the poverty-stricken gametophyte has ultimately 
 become entirely parasitic upon its rival.
 
 CHAPTER IX 
 
 Sexual phenomena in multicellular animals Structure and life history of 
 Hydra and Obelia Alternation of generations The ccelomate type of 
 structure Secondary sexual characters The evolution of sex. 
 
 IN multicellular animals or Metazoa, as in multicellular plants, 
 a sharp distinction can usually be drawn between the somatic cells 
 which build up the various tissues and are concerned with the 
 life of the individual, and the germ cells or gametes which are 
 concerned with the propagation of the race and which alone (in 
 most cases) have the power of separating from the parent soma 
 or body and giving rise to new individuals. 
 
 In nearly all the Metazoa the gametes are sexually differen- 
 tiated into relatively large, passive ova and much more minute, 
 active spermatozoa which swim about by means of flagella. 
 The actual gametes arise by subdivision of undifferentiated 
 primordial germ cells. In the sponges, whose organization has 
 not advanced very much beyond that of complex colonies of 
 Protozoa, the primordial germ cells are merely wandering 
 amo3boid cells, resembling the white blood corpuscles of verte- 
 brates. Some of these round themselves off and give rise to 
 more or less spherical ova, others divide into spermatozoa, and 
 probably the entire sponge itself is in most cases either male or 
 female, producing one kind of gamete only. In the sponge the 
 germ cells are not localized in definite organs but scattered 
 singly or in groups throughout the gelatinous ground-substance 
 of which the body is largely composed. 
 
 In the great majority of Metazoa, on the other hand, the 
 germ cells are segregated in well-defined organs termed gonads. 
 As a rule each gonad produces only ova, when it is known as an 
 ovary ; or spermatozoa, when it is known as a spermary or testis ; 
 only occasionally does it produce both, as in the case of the ovo- 
 testis of the snail. The gonads may accordingly be spoken of as 
 female, male, or hermaphrodite as the case may be, and the same 
 terms are also applied to the animals themselves, a male or a 
 
 B i
 
 L14 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 female animal possessing either male or female gonads, while a 
 hermaphrodite animal may either possess a combined ovo-testis 
 or both ovaries and testes separately. 
 
 In illustration of these points we may briefly describe the 
 structure and life history of the common fresh water polype, 
 
 FIG. 57. The fresh water Polype (Hydra) cut in half longitudinally and 
 
 greatly enlarged. (From Marshall and Hurst's " Practical Zoology.") 
 A, mouth ; B, hypostome ; C, digestive cavity ; D, ectoderm ; E, mesogloea ; F, endoderm ; 
 G, tentacle ; H, testis ; I, ovum in ovary ; K, bud ; L, foot. 
 
 Hydra (Fig. 57), so frequently found attached to aquatic plants 
 in ponds and ditches. Hydra is a member of the great group 
 Coelenterata, which includes the sea -firs, jelly-fish, sea -anemones 
 and corals, and which are distinguished by the fact that they 
 retain throughout life the fundamental features of the gastrula 
 (compare Fig. 13, IX, X). The body of a typical Ccelenterate 
 animal consists essentially of a simple sac (Fig. 57) whose wall
 
 HYDEA 
 
 115 
 
 is composed of two layers of cells. The cavity of the sac (C) is 
 the digestive or gastral cavity (enteron), and it has only a single 
 opening to the exterior, the mouth (A), usually surrounded hy a 
 ring of tentacles (G). The wall of the sac is solid and there is 
 no body cavity or coelom surrounding the digestive tube as in 
 higher animals (Ccelornata). The outer cell layer (D) of the body 
 wall is the ectoderm (epiblast of the embryo), the inner (F) is the 
 
 N, 
 
 FIG. 58. A small portion of a thin longitudinal Section through the Body 
 Wall of Hydra viridi*, x 800. (From Marshall and Hurst's " Practical 
 Zoology.") 
 
 A, a large ectoderm cell ; B, its nucleus ; C, its muscle process ; D, an undischarged 
 thread cell ; E, its trigger process ; F, a thread cell with discharged thread ; G, inter- 
 stitial cells ; H, mesoglcea ; I, endoderm cell ; K, vacuole ; L, nucleus of endoderm 
 cell; M, green algal cells living in the endoderm cells; N, flagellum of endoderm cell. 
 
 endoderm (hypoblast of the embryo) and between the two is a 
 gelatinous, non-cellular supporting lamella, the mesogloea (E). 
 
 Hydra itself is a very small form, but easily recognizable by 
 the naked eye. The body is long and slender or short and thick, 
 according to its state of contraction, and the same is true of the 
 tentacles, which may be visible as mere knobs around the mouth 
 at the unattached end of the animal, or as long slender threads 
 extended through the water like fishing lines and serving for the 
 capture of the minute organisms upon which the Hydra feeds. 
 The mouth is situated on the top of a conical projection, 
 
 i 2
 
 116 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the hypostome (Fig. 57, B), which lies within the circle of 
 tentacles. 
 
 The endoderm, which immediately lines the gastral cavity, is 
 made up of a single layer of relatively large cells (Fig. 58, I) 
 whose function is digestive. The ectoderm is made up of several 
 kinds of cells, some larger than others. The larger ones (A) 
 are much broader at their outer than at their inner ends 
 and the interstices thus left between the latter are filled up 
 by small interstitial cells (G). The endoderm cells and the 
 larger ectoderm cells both send out prolongations of their bodies 
 into the gelatinous mesogloaa (H) which lies between them, and 
 these processes, having the form of elongated fibres (C), are the 
 seat of that power of contraction which Hydra possesses in such 
 a high degree they are in fact muscular. 
 
 Hydra has two very distinct methods of reproduction, asexual 
 and sexual respectively. The former consists in a process of 
 budding, little hollow outgrowths of the body being formed, 
 which elongate, acquire mouth and tentacles, and for a time 
 remain attached to the parent (Fig. 57, K). In this way tem- 
 porary colonies of polypes may be produced, but sooner or later 
 the buds separate and begin to lead independent lives. 
 
 Sexual reproduction is effected by means of ova and sperma- 
 tozoa, which are essentially similar to those of higher animals. 
 They are produced in gonads ovaries and testes and as 
 both kinds of gonad usually occur in the same individual the 
 animal is hermaphrodite. The testes (Fig. 57, H) take the form 
 of little swellings situated at a short distance beneath the ring 
 of tentacles and formed each by an accumulation of interstitial 
 ectoderm cells. These are the primordial germ cells, by the 
 division of which the spermatozoa are formed. The spermatozoon 
 is perfectly typical, resembling, a flagellate protozoon, with an 
 ovoid head consisting almost entirely of chrornatin and a long 
 cytoplasrnic tail or flagellum by means of which it swims 
 actively about when shed into the surrounding water by rupture 
 of the testis. 
 
 There is usually only a single ovary, appearing as a larger 
 projection from the body wall nearer to the attached end of 
 the animal; it consists at first, like the testes, of a heap of 
 primordial germ cells formed by the multiplication of interstitial 
 cells. In each ovary, however, only a single cell develops into a 
 mature ovum (Fig. 57, I), its sister cells, which may all be
 
 HYDRA 
 
 117 
 
 regarded as potential ova, being sacrificed for the benefit of the 
 one ; in fact they are simply devoured by the voracious egg 
 
 FIG. 59. Development of Hydra. (From Bourne's " Comparative Anatomy," 
 partly after Brauer.) 
 
 A, the mature ovum, full of yolk granules and still attached to the body wall of the 
 parent ; B, section of blastula or blastosphere produced by segmentation of the ovum ; 
 C, the embryo becoming solid by migration into the blastocoel of cells cut off from 
 the wall of the blastula to form the hypoblast ; D, solid embryo composed of epiblast 
 and hypoblast and enclosed in a protective shell ; E, embryo flattening itself .out 
 within the shell ; P, embryo emerging from the shell and with the gastral cavity 
 appearing in the endoderm (hypoblast) ; G, empty shell after the escape of the 
 embryo. 
 
 blc, blastocoel ; ec, ectoderm (epiblast) ; en, endoderm (hypoblast) ; i, solid mass of hypo- 
 blast cells ; mg, mesogloea ; sh, shell ; she, outer layer of shell ; ski, inner layer of shell. 
 
 cell, which puts forth pseudopodia and feeds upon them like a 
 hungry Amoeba. In this way the ovum attains a relatively large 
 size and its cytoplasm becomes loaded with yolk corpuscles
 
 118 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 which will serve later on for the nutrition of the developing 
 
 embryo. 
 
 Some of the surrounding cells of the ectoderm at first 
 
 form a covering 
 or envelope for the 
 
 /- 
 
 M 
 
 -hyth. 
 
 growing ovum, 
 but this is pre- 
 sently ruptured 
 and the mature 
 egg is exposed on 
 the surface of the 
 B body of the parent 
 
 Hydra. There it 
 
 is found by a spermatozoon, which is 
 attracted towards it and by its own 
 activity bores its way into the ovum. 
 This act of fertilization is concluded in 
 the usual manner by the fusion of the 
 nucleus of the spermatozoon (male pro- 
 nucleus) with that of the ovum (female 
 pronucleus) to form the zygote nucleus. 
 The fertilized ovum or zygote (Fig. 
 59, A) undergoes segmentation while 
 still remaining attached to the parent 
 Hydra. In this way a single-layered, 
 hollow blastula (B) is formed, which 
 becomes converted into a two-layered 
 embryo by migration of cells into the in- 
 terior to form the at first solid hypoblast 
 or endoderm (C, D, ?'). The epiblast 
 or ectoderm cells (D, ec) now secrete a 
 thick horny protective envelope (sh, 
 shi) around the embryo, w r hich falls 
 off from the parent and undergoes a 
 period of rest at the bottom of the 
 pond. After a time the interrupted 
 development is resumed, the horny envelope is ruptured, and 
 the embryo escapes (F). The gastral cavity appears in the 
 midst of the endoderm cells, the mouth is formed by perforation 
 at one end, and the tentacles bud out. "With the formation of 
 the mouth the gastrula stage is reached, but it will be noted that 
 
 A. 
 
 FIG. 60. Obella yemculata. 
 
 A, part of hydroid colony ; B, 
 free - swimming medusa ; 
 both x 13. (From photo- 
 graphs.) 
 
 bst., blastostyle; gon., gonad; 
 gth., gonotheca ; liyc., hy- 
 drocaulus ; hyd., hydranth ; 
 hyth., hydrotheca ; mn., 
 manubrium ; p.s., perisarc ; 
 r.c., radial canal ; ten., ten- 
 tacles.
 
 OBELIA 119 
 
 this condition is arrived at by a somewhat different route from 
 that which leads to the corresponding stage in Amphioxus (com- 
 pared Fig. 13, I X). 
 
 Closely related to Hydra are a large number of marine Coelen- 
 terates, which, from their obviously animal nature combined 
 with their plant-like mode of growth, were known to the older 
 naturalists as zoophytes. One of the most familiar examples of 
 these is Obelia (Fij. 60), which is frequently found attached to 
 rocks or seaweeds near low water mark. 
 
 Obelia differs from Hydra in several interesting particulars. 
 In the first place the asexual process of multiplication by means 
 of budding takes place in a very regular manner, and the buds, 
 instead of separating from the parent, remain connected together 
 to form permanent colonies (Fig. 60, A) in which the constituent 
 individuals or persons (sometimes called zooids) are arranged 
 in a perfectly definite way. The colony is comparable to an 
 arborescent colony of Protozoa such as Zoothamnium or Epi- 
 stylis (compare Fig. 9, n 15), but the individuals of which it 
 is composed are units of a higher order than single cells. 
 
 In the second place the colony develops a common skeleton, 
 secreted by the ectoderm, which takes the form of a slender 
 tube of horny perisarc (ps.) enclosing all the branches and 
 expanding at the end of each into a little cup or hydrotheca (hyth.), 
 occupied by a single zooid. Lastly the colony is polymorphic, 
 the zooids exhibiting a certain amount of differentiation and 
 division of labour amongst themselves. 
 
 A network of root-like branches at the base of the colony 
 creeps over the substratum and serves for attachment. From 
 this network, which is not shown in the illustration, arises a 
 little forest of vertical stems, each of which has a characteristic 
 zig-zag outline (Fig. 60, A). From each angle of the stem a 
 short branch is given off which terminates in a single hydra-like 
 zooid known as a hydranth (hyiL), enclosed in one of the horny 
 cups or hydrothecae, from the mouth of which its tentacles are 
 extended into the water. 
 
 The structure of the hydranth is similar in all essential 
 respects to that of Hydra. In the middle of the ring of tentacles 
 is the mouth, situated on a projecting hypostome and leading 
 into the digestive cavity, and the different hydranths of the 
 colony are all placed in communication with one another by the 
 tubular hydrocaulus, or common stalk (/</c.), enclosed in the horny
 
 120 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 perisarc. These hydranths are the nutritive individuals of the 
 colony, whose function it is to capture and digest the prey. 
 
 Very often we find, in the angle between a hydranth-hearing 
 branch and the main stem, a short branch bearing an individual 
 or zooid of a different kind, enclosed in a horny cup of totally 
 different shape. These individuals are somewhat club-shaped ; 
 they have no mouth and no tentacles, and they are termed 
 blastostyles (Fig. 60, A, bst.). The horny cup in which each is 
 enclosed is urn-shaped, with constricted mouth, and is distin- 
 guished by the name gonotheca (gth.). The function of the 
 blastostyle is exclusively reproductive and it is entirely dependent 
 for its nutrition upon digested food received from the hydranths 
 through the hydrocaulus. It reproduces by budding, and the 
 buds are often found attached to it in large numbers, within the 
 sheltering gonotheca, as shown in the figure. 
 
 The colony itself increases in size by the formation of new 
 buds in regular succession, alternately on the right and left sides, 
 beneath what is, for the time being, the topmost hydranth of each 
 main stem, each new bud giving rise to a hydranth-bearing 
 branch which overtops its immediate predecessor. The buds 
 formed on the blastostyle. on the other hand, do not develop into 
 hydranths at all, but into another kind of individual known as 
 a medusa, or medusoid person, which presently detaches itself 
 from the parent and escapes through the mouth of the gonotheca 
 as a free-swimming individual. 
 
 The medusa of Obelia (Fig. 60, B), though larger than the 
 hydranths, is still very small, not more than about tuth of an 
 inch in diameter. At first sight it looks very different from 
 a hydroid individual (hydranth), but it can easily be shown to 
 have the same fundamental plan of structure. It consists of a 
 circular disk surrounded by a fringe of tentacles (ten.). From the 
 middle of one surface projects a handle-shaped structure, the 
 manubrium (mn.), corresponding to the hypostome of the hydranth 
 and bearing the mouth at its extremity. This mouth leads into 
 a central digestive cavity from which four radial canals (r.c.) run 
 outwards through the gelatinous ground-substance to a circular 
 marginal canal. The medusa swims actively about by muscular 
 contractions of the disk, and in doing so it assumes a bell-like 
 shape with the manubrium projecting from the convex surface, 
 like the handle of the bell. 
 
 Most medusae the larger of which are familiar to us as
 
 ALTERNATION OF GENERATIONS 121 
 
 "jelly-fish" are more definitely bell-shaped, but the manu- 
 brium lies in the hollow of the bell, like its clapper ; in comparison 
 with these the Obelia medusa may be said to turn itself inside out 
 in the act of swimming. In accordance with their active life the 
 medusae have a fairly well developed nervous system and a number 
 of sense organs around the margin of the bell. The point with 
 which we are immediately concerned, however, is their mode of 
 reproduction. 
 
 Unlike the hydranths and blastostyles of which the Obelia 
 colony is composed the medusae are sexual individuals, bearing 
 sexual organs or gonads from which the gametes are set free. 
 The gonads (Fig. 60, B, gon.) are masses of germ cells lying one 
 beneath each radial canal, on the same side of the disk as the 
 manubrium. The sexes are distinct, the medusae being either 
 male or female, and the gonads accordingly producing either 
 spermatozoa or ova, which, when mature, are discharged into the 
 water by rupture of the gonad. 
 
 The ova are fertilized by the spermatozoa and undergo 
 segmentation much as in the case of Hydra, developing 
 ultimately into a hydroid individual (hydranth), which by 
 budding gives rise to a new Obelia colony. 
 
 It is obvious that we have here a case of alternation of sexual 
 and asexual generations (metagenesis), comparable to that which 
 occurs in the fern, except that the sexual generation arises from a 
 multicellular bud and not from a unicellular spore. The sexual 
 medusoid individual is produced by budding from the asexual 
 hydroid, and itself gives rise, through the conjugation of gametes 
 and the development of the zygote thus produced, to the asexual 
 hydroid again. The process is somewhat complicated by the fact 
 that the hydroid also produces other asexual individuals by bud- 
 ding, but this really amounts to little more than does the forma- 
 tion of numerous branches by the budding of one of the higher 
 plants, for in the latter case also each bud may be regarded as, 
 potentially at any rate, a perfect individual. 
 
 We may draw an even closer comparison between the alterna- 
 tion of hydroid and medusoid generations in the Coalenterata and 
 that of sporophyte and gametophyte in the higher plants, for in the 
 former, as in the latter, w r e see in many instances a more or less 
 strongly developed tendency towards the suppression of the sexual 
 generation (sometimes called the gamobium in animals). 
 
 In Tubularia, for example, the medusoid individual never
 
 122 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 separates from the parent colony, but remains attached to it as a 
 bud, though still showing clear evidence, in its bell-like shape and 
 in the presence of the manubrium and of radial and circular 
 canals, of its medusoid nature. It is in fact merely a degenerate 
 
 ten. 
 
 FlG. 61. A single Hydranth of Tubularia, with medusoid Individuals 
 (Gonophores) budded out between the two Circles of Tentacles ; highly 
 magnified. (After Allman.) 
 
 act., actinula larva enclosed in gonophore ; act. ', actinula larva escaping ; ent., enteron 
 or digestive cavity ; gon., gonophore ; ?., mouth ; per., perisarc ; ten., tentacles. 
 
 medusa or gonophore (Fig. 61, gon.). It still retains its sexual 
 function, producing either ova or spermatozoa, and in this case 
 the fertilized ova actually develop into young hydroids or 
 " actinula larvae " (Fig. 61, act., act.') before escaping from the 
 parent gonophore. We have here a telescoping of the successive 
 generations quite comparable to what we find in a flowering
 
 SUPPRESSION OF MEDUSOID 123 
 
 plant, and again it is the sexual generation which has undergone 
 reduction. 
 
 In other cases the medusoid bud or gonophore more or less 
 completely loses its medusa-like structure and becomes a mere 
 sac containing the gonad, and it has been suggested that in 
 Hydra we may have the last stage in this progressive reduction 
 of the sexual generation, the medusoid having altogether dis- 
 appeared, after having transferred its sexual functions to the 
 hydroid. 
 
 It seems almost incredible that the germ cells themselves 
 which are originally produced by one generation should in this 
 manner be transferred to what is really the preceding generation, 
 but as a matter of fact we see all stages in this process in different 
 genera of Hydrozoa. Even in Obelia and certain other medusae 
 the germ cells do not originate in the gonads beneath the radial 
 canals but in the ectoderm of the manubrium, reaching their 
 final position by a process of migration. In Eudendrium, where 
 the medusoid is reduced to a mere vestige, the germ cells 
 no longer originate in the sexual individual at all, but in the main 
 stem of the hydroid colony, whence they migrate into the 
 medusoid bud. If we imagine them ceasing to migrate from their 
 place of origin in the hydroid, while the medusoid is no longer 
 formed, we reach the condition of Hydra, with a complete trans- 
 ference of the sexual function to what should be the asexual 
 generation. 
 
 In the coelenterates, however, it is not always the asexual 
 (hydroid) generation which predominates, for in some cases this 
 is suppressed more or less completely and the medusa or jelly- 
 fish reproduces its own kind directly without the intervention of 
 a hydroid phase. 
 
 In coelenterate animals such as Hydra and Obelia, as we 
 have already seen, the body consists of a single hollow tube 
 whose wall is made up of only two cell layers, the ectoderm 
 on the surface and the endoderm lining the digestive cavity, with 
 a supporting layer of gelatinous consistency the mesogloea 
 between the two. In the case of the larger jelly-fish 
 this mesogloea attains a great thickness ; it is, however, 
 never a true cell layer like the ectoderm and endoderm, but 
 rather of the nature of intercellular substance secreted by the 
 cells on either side of it. 
 
 In Hydra and Obelia the germ cells arise from the ectoderm,
 
 124 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 and the gonads (Fig. 62, A, yon.) are accordingly situated close 
 to the surface of the body and no special ducts are required for 
 the conveyance of ova or spermatozoa to the exterior. 
 
 In the great group Coelomata, which includes practically all 
 animals higher in the scale of organization than the ccelenterates, 
 we find a different state of affairs. The mesoglcea has been 
 replaced by a true cellular mesoderm (Fig. 62, B, mcs.), formed of 
 cells derived from ectoderm or endoderm, or from both, and in 
 the thickness of this layer a cavity is developed, the ccelom or 
 
 end, 
 
 FIG. 62. Comparison of Ccelenterate and Invertebrate Coaloinate Types of 
 Structure. 
 
 A. Diagram of a transverse section of a ccelenterate animal. 
 
 B. Diagram of a transverse section of an invertebrate coRlomate animal. 
 
 coel. coalom or body cavity; ccel. ep. ccelomic epithelium lining body cavity; d.v. dorsal 
 blood-vessel ; ect. ectoderm ; end. endoderm ; ent. enteron or digestive cavity ; 
 g. a. genital aperture ; g.cl. genital duct (gonoduct) ; gon. gonad ; mes. mesoderm ; 
 mesgl. mesoglcea ; n.c. double nerve cord ; som. somatopleure or body wall ; spl. 
 splanchnopleure or gut wall ; v.v. ventral blood-vessel. 
 
 body cavity (coel.}, which more or less completely surrounds the 
 digestive tube or alimentary canal (compare the development of 
 Amphioxus, Fig. 13, XI XIII) . The outer layer of the mesoderm 
 unites with the ectoderm to form the body wall or somatopleure 
 (som.) , while the inner layer unites with the endoderm to form the 
 gut wall or splanchnopleure (spZ.), and the body thus acquires the 
 form of a double tube. The body cavity is lined by a layer of 
 epithelial cells (coel. ep.), and it is from this coelomic epithelium 
 that the germ cells arise. The gonads (Fig. 62, B, gon.) therefore 
 project into the body cavity and into this cavity the mature germ 
 cells are primarily discharged. In the great majority of cases
 
 SECONDARY SEXUAL CHARACTERS 125 
 
 special genital ducts or gonoducts (g.d.) are developed which 
 pierce the body wall and serve for the passage of the germ cells 
 to the exterior. The sexes are usually distinct in the higher 
 forms (Vertebrata), but many of the invertebrate coelomates 
 (e.g., the earthworm) are hermaphrodite, the same individual 
 bearing both male and female gonads (testes and ovaries) with 
 the corresponding gonoducts (vasa deferentia and oviducts). 
 
 Fertilization of the ova by the spermatozoa, or in other words 
 conjugation of the gametes, may take place either within the 
 body of the parent, as in most terrestrial forms, or externally, as 
 in a very large proportion of aquatic animals. In the former case 
 special organs are developed for the transference of the sperma- 
 tozoa from one individual to another, and such transference 
 usually occurs even in hermaphrodite forms, which, as a rule, are 
 incapable of self-fertilization. Further modifications may arise 
 in connection with the nutrition of the embryo, which may 
 remain within the body of the parent in an enlarged portion of 
 the oviduct known as the uterus until it has reached an 
 advanced stage of development. This takes place more par- 
 ticularly in the females of the higher vertebrates. 
 
 In connection with the sexual differentiation, more 
 especially in the higher animals, numerous secondary sexual 
 characters may arise which are not directly connected with the 
 organs of reproduction. Such are the various ornamental out- 
 growths of hair, feathers and so forth, which distinguish the males 
 of many vertebrates and are .supposed to appeal to the aesthetic 
 sense and thus to contribute towards the mutual attraction 
 between male and female, and the special weapons, such as 
 antlers and spurs, which male animals frequently develop and 
 which are used in combat for the possession of the females. 
 
 Although not directly connected with the gonads these 
 secondary sexual characters seem to depend for their development 
 in some curious way upon the presence of these organs. Thus 
 it is well known that if the testes be removed by castration the 
 secondary sexual characters will not, in most cases at any rate, 
 develop properly. We see an excellent illustration of this in the 
 case of the antlers of the stag, which are confined to the male and 
 do not develop at all if the animal be castrated in early youth, 
 while if the operation be performed after the antlers are fully 
 developed these are prematurely cast off and replaced by 
 imperfect ones.
 
 126 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 How this intimate correlation between gonads and secondary 
 sexual characters is brought about is still uncertain, but there is 
 strong reason to suppose that it is due to the secretion by the 
 gonad of some specific substance (hormone), perhaps of the 
 nature of a ferment, which circulates throughout the body 
 chiefly no doubt in the blood and controls the development 
 of the characters in question. That internal secretions may act 
 in this way upon organs remote from their own place of origin is 
 well known and is strikingly exemplified in the case of the 
 corpora lutea of the mammalian ovary. These structures appear 
 on the surface of the ovary in the places whence ova have been 
 discharged, and are apparently of a glandular nature. As the 
 fertilized ova pass down the oviduct they begin to develop, and 
 on reaching the uterus fix themselves to the wall of the latter, in 
 which they become imbedded. The spot where fixation takes place 
 is far distant from the ovary, but it has been demonstrated that 
 if the corpora lutea on the surface of the latter be destroyed the 
 embryos will not become fixed at all. There is a close correlation, 
 then, between the presence of the corpus luteum and the fixation of 
 the embryo, and this is explained by supposing that the corpus 
 luteum secretes some substance which, circulating in the blood, 
 reaches the uterus and stimulates its epithelial lining to respond 
 to contact with the embryo. Moreover, the reaction, whatever its 
 cause, appears to be mutual, for if the discharged ovum does not 
 get fertilized the corpus luteum does not attain its full develop- 
 ment and soon disappears. 
 
 We have now very briefly traced the evolution of sexual 
 characters from their starting point in the male and female 
 gametes of the Protista to their culmination in the higher plants 
 and animals. We have seen how sexual differentiation, which 
 primarily concerns the gametes themselves, is gradually extended 
 to the colonies or to the multicellular individuals from which the 
 gametes arise, or even to a preceding, originally non-sexual 
 generation. The various structural modifications thus brought 
 about are all directed towards one end, the conjugation of the 
 gametes. The mutual attraction which undoubtedly exists 
 between the gametes themselves is not sufficient, at any rate in 
 the case of the more highly developed and complex organisms, 
 where the distances between their places of origin are relatively 
 very great, to secure their union. 
 
 In the flowering plants their own efforts are supplemented by
 
 THE EVOLUTION OF SEX 127 
 
 all the elaborate devices for securing pollination, and by far tbe 
 most active part in the process is played by external agencies, 
 especially by those insects which have become the vicarious 
 fertilizers of the flowers. 
 
 In the higher animals, on the other hand, the necessity for 
 bringing the gametes into close proximity with one another has 
 led to the development of all those secondary sexual characters, both 
 bodily and mental, which play so conspicuous a part in the drama 
 of life. Throughout the whole course of this remarkable process 
 of evolution, except in the case of certain obviously degenerate 
 forms, we observe that same fundamental distinction between the 
 sexes which we first noticed in the gametes of unicellular 
 organisms, and which in the higher animals is extended with 
 the sexual differentiation itself from the gametes to the complex 
 multicellular body which bears them. The female is the more 
 passive partner and is especially concerned with the nutrition 
 and rearing of the offspring, and her bodily organization is 
 especially adapted to her maternal functions. These functions 
 constitute an inevitable handicap in the struggle for existence, 
 and the females and young of the higher animals are in most 
 cases largely dependent upon the less burdened and consequently 
 more active and vigorous males for their protection. 
 
 The explanation of this progressive sexual differentiation is 
 undoubtedly to be found in the advantages to be derived from 
 division of labour and the accompanying possibilities of special- 
 ization. The origin of conjugation itself, upon which all sexual 
 phenomena are based, is another, and more fundamental, question. 
 At first, as we have seen, the conjugating gametes were apparently 
 exactly alike one another and exhibited no visible sexual differen- 
 tiation at all. The habit of conjugation probably arose from 
 the necessity of making good some disturbance of equilibrium in 
 the protoplasm of the cell. It has been supposed that, as the result 
 of repeated fission, some condition of inequality was gradually 
 set up amongst the daughter cells, whereby some of them came 
 to have too much of one constituent and too little of another, 
 while others were in the opposite condition. In this way the 
 successive unicellular generations gradually became more and 
 more enfeebled as we saw in the case of Paramcecium and, owing 
 perhaps to some sort of polarization, those which had become 
 modified in opposite directions came to exercise an attraction 
 upon one another which resulted in conjugation and restoration
 
 128 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 of the proper equilibrium. It may be that from the very first 
 the inequalities of fission resulted in the accumulation of more 
 active protoplasm in some cells and a greater amount of reserve 
 material in others, and that this was the starting point of the 
 differentiation into male and female. 
 
 Although fusion of the nuclei (karyogamy) of the two 
 gametes appears now to be the most important feature of con- 
 jugation, we must suppose that it was preceded by plastogamy 1 or 
 fusion of the cytoplasm, which obviously constitutes the natural 
 preliminary to karyogamy. In some of the lower Protozoa such 
 plastogamy has been sometimes observed unaccompanied by 
 karyogamy, and it is possible that in some cases plastogamy 
 alone is sufficient to bring about rejuvenescence and renewed 
 activity in cell-division. 
 
 - 1 Sometimes called plasmoganay.
 
 CHAPTER X 
 
 Origin of the gerin cells in inulticellular animals Maturation of the germ 
 cells Reduction of the chromosomes Sex determination in insects 
 Different forms of gametes Mutual attraction of the gametes 
 Fertilization and parthenogenesis. 
 
 IN many inulticellular animals the distinction between somatic 
 cells and germ cells becomes manifest at a very early stage in the 
 development of the individual. An extreme instance of this is 
 seen in the parasitic round- worm of the horse, Ascaris meyalo- 
 ccphala. Here the distinction in question precedes all other 
 histological differentiation. The two cells or blastomeres into 
 which the fertilized ovum first divides (Fig. 85, C, D) are originally 
 similar to one another, but as they prepare for the next mitotic 
 division of the nucleus a remarkable difference is, according to 
 the observations of Professor Boveri, established between them. 
 Both at first (in the case of the variety known as imivalens) 
 exhibit two elongated chromosomes (the variety Uvalens, which is 
 represented in Fig. 35, having four), but in one the thickened 
 ends of the two chromosomes are thrown off into the surrounding 
 cytoplasm, where they degenerate, while the more slender middle 
 portions break up into a number of short pieces. Thus two 
 differentiated cells are produced, one with two large chromosomes 
 arid the other with numerous small ones. The latter gives rise 
 by its subsequent divisions to somatic cells only. The former is 
 a primordial germ cell ; for some five or six times it will divide 
 like its parent cell into a somatic cell and a primordial germ cell, 
 but after these early divisions the primordial germ cells will 
 give rise to their own kind only, until the time comes for the 
 production of the actual gametes. The somatic cells, on the 
 other hand, will gradually become differentiated into all the 
 various tissue cells of the adult. 
 
 Perhaps the most significant part of this remarkable process 
 as observed in Ascaris is the elimination of chromatin material 
 from the nuclei of the somatic cells when these are first 
 
 B. K
 
 130 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 differentiated from the germ cells. The result of this is that the 
 germ cells alone retain the full complement of chromatiu 
 derived from the parents, and their nuclei are accordingly 
 actually much larger than those of the somatic cells. 
 
 The differentiation into somatic cells and germ cells cannot 
 usually be traced so far back in the development of the individual 
 as in Ascaris, but in a great many animals the distinction can 
 be recognized at a very early stage. In certain insects, for 
 example, the primordial germ cells can be traced back to a large 
 " pole-cell " which lies at one end of the segmenting ovum, and 
 
 in the arrow-worm, Sagitta, 
 they can be identified at the 
 gastrula stage (Fig. 63, p.g.c.). 
 As we shall see later on, this 
 early segregation of the germ 
 cells is of very great interest 
 from the point of view of the 
 theory of heredity. It can 
 hardly be said, however, at 
 any rate in the present state 
 of our knowledge, to be a 
 phenomenon of universal or 
 
 FIG. 63. -Section of the Gastrula of an even general occurrence, and 
 Arrow Worm (Sagitta) showing the in the majority of coelomate 
 HeSg 8 ) 1 Gei ' m elIS ' (After0 ' animals the germ cells are first 
 
 6 P . ) blastopore';e^.,enteron;^.,epiblast; ^Cognizable ill the CCelomic 
 
 %P., hypobiast; p.g.c., primordial germ epithelium at a compara- 
 
 tively late stage of develop- 
 
 ment (Fig. 62, B). In plants also, in cases where there is 
 a well developed gametophyte this appears to attain its full 
 development before the germ cells are recognizable, and the 
 entire life of the sporophyte is passed without any distinction 
 between somatic and germ cells manifesting itself. Moreover, 
 the fact that the ordinary cells of a fern prothallus can, on 
 occasion, act as germ cells, 1 prevents us from admitting any 
 absolute distinction between the two categories. 
 
 The primordial germ cells may undergo extensive multiplication 
 by ordinary mitotic division before giving rise to the actual 
 gametes. In multicellular animals the process of gametogenesis 
 (formation of gametes), which is either oogenesis or spermato- 
 
 i Vide, p. 105.
 
 MALE AND FEMALE PRONUCLEI 
 
 131 
 
 genesis according to whether ova or spermatozoa are produced 
 thereby, is accompanied by nuclear phenomena of very great 
 interest, whereby the maturation of the germ cells is effected. 
 Before describing this process we must lay stress upon certain 
 preliminary considerations. 
 
 As we have already seen, each kind of animal or plant is 
 characterized by the appearance of a definite number of chromo- 
 somes in the nuclei of its cells at the time when these are under- 
 going division by mitosis. Although not absolutely constant in 
 all cases the number is usually the same for all the different 
 somatic cells of which the body 
 is composed. It is usually an 
 even number, and (with certain 
 exceptions) it remains the same 
 in successive generations of 
 individuals. 
 
 It will also be remembered 
 that the zygote or fertilized 
 egg from which the individual 
 develops is formed by the con- 
 jugation of two gametes, ovum 
 and spermatozoon, and that in 
 this process the nuclei of the 
 gametes, sometimes called the 
 male and female pronuclei, 
 unite, or at any rate co-operate 
 as a single nucleus. Fig. 64 is 
 taken from an actual photograph of an egg of Ascaris in process 
 of fertilization ; the spermatozoon has already entered the 
 ovum and the male and female pronuclei (pro.) are seen lying 
 side by side in the cytoplasm. Each pronucleus brings with it 
 its own set of chromosomes, and hence the zygote nucleus has 
 double the number of chromosomes possessed by either of the 
 gametes. Thus it appears at first sight that every conjugation 
 or sexual union of gametes (zygosis) must be accompanied 
 by a doubling of the number of chromosomes, and we might 
 therefore expect to find each successive generation with twice as 
 many chromosomes in its nuclei as the preceding one. That 
 this is not actually so depends (in the case of animals) upon the 
 fact that the nuclei of the gametes contain only half the number 
 of chromosomes characteristic of the somatic cells; if the 
 
 K 2 
 
 FIG. 64. Ovum of the Horse Worm 
 (Asmris megolocephala] during 
 the Process of Fertilization, 
 showing the male and female 
 Pronuclei (pro), X 770. (From 
 a photograph.)
 
 132 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 somatic cells have eight the mature ovum or spermatozoon will 
 have only four, and so on. This reduction of the number of 
 chromosomes (nieiosis) is the essential part of the process of 
 maturation which the animal germ cells undergo, and it is 
 
 Spermalogenesis 
 1 
 
 Spermatogonia 
 
 Multiplication of Epithelial 
 Cells (Spermatogonia and 
 Oogonia) in the Gonad 
 (Test is or Ovary) by ordinary 
 Somatic Mitosis 
 
 Oogenesis 
 
 Oogonia 
 
 Primary 
 
 Secondary 
 
 Kytes 
 
 ( 
 
 -3. Synapsis or Pairing ^ 
 ^ of the Chromosomes ^ 
 
 / 
 
 
 \ Reducing Division 
 (Meiosis ) j 
 
 4> < 
 
 4> 4 
 
 \ L^ 
 $ V 
 
 / '. 
 
 \ ; 
 
 A ! 
 
 ^4 
 
 >ii 
 
 i ; '. Polar t&- 
 
 to <s> '"la 
 
 "ifliiii ^ 
 
 1 / J ) / / / ^fonjugatton of / 
 
 /[//(/(/ \Gametes or .' , 
 (I I 1 fertilization^ f 
 
 ^Primary 
 'Oocytes 
 
 ^ Secondary i 
 K <?n</ /"c/d/ 1 fiotf/fi 
 
 W^w^ ?J 
 '<* Polar Bodies 
 
 MJ Mature Ova 
 Female Gametes 
 
 Male Gametes 
 
 &J Zygote or Fertilised Ovum 
 
 C^X) (The figures indicate the numbers 
 Segmentation of ;'; ;; oF Chromosomes present) 
 the Zygote by 
 Ordinary Somatic 0$) 
 Mitosis 00 
 
 I 
 FIG. 65. Diagram of Gametogenesis. 
 
 effected by a modification of the mitotic nuclear division, in 
 which the chromosomes are separated into two groups, half the 
 total number of entire chromosomes going into one daughter 
 cell and half into the other. 
 
 In a typical case of spermatogenesis the first stage is the
 
 GAMETOGENESIS 133 
 
 multiplication of cells (the so-called spermatogonia) derived from 
 the germinal epithelium, in the testis, by ordinary cell-division. 
 Let us suppose the number of chromosomes found in the nuclei 
 of the somatic cells to be eight ; it will, of course, remain the 
 same so long as the character of the mitosis undergoes no 
 change, each chromosome splitting into two at every nuclear 
 division. Presently, however, we find the chromosomes arranged 
 in pairs instead of all appearing separately in the mitotic figure, 
 and the cells, which have increased considerably in size by the 
 absorption of nutriment, may now be termed primary spermato- 
 cytes. This pairing of the chromosomes (synapsis) marks the 
 onset of the "reducing division " ; a nuclear spindle is formed, the 
 paired chromosomes arrange themselves upon it, and the two 
 members of each pair separate and travel towards opposite poles. 
 Thus two new nuclei are formed each with only four chromosomes. 
 The reduction is now complete and the new generation of cells, 
 with reduced nuclei, may be termed secondary spermatocytes. 
 One more mitotic division takes place, this time involving the 
 splitting of each chromosome, so that there is no further reduc- 
 tion in their number, and giving rise to the minute spermatids, 
 each of which develops a long, vibratile, cytoplasmic tail and 
 forms a spermatozoon. Hence we see that each primary 
 spermatocyte gives rise to four spermatozoa, with reduced 
 nuclei containing half the number of chromosomes found in the 
 somatic cells. The essential features of the whole process are 
 represented diagrammatically in Fig. 65. 
 
 The process of oogenesis takes place in essentially the same 
 manner ; the so-called oogonia, 1 derived from the germinal 
 epithelium of the ovary, multiply and give rise to oocytes. 
 Synapsis and reduction in the number of the chromosomes take 
 place as they do in spermatogenesis, but, owing doubtless to the 
 fact that it takes a comparatively large amount of cytoplasm to 
 form the body of an egg, we find that only one perfect ovum 
 arises from each primary oocyte, the other three forming the 
 " polar bodies." Owing to their minute size as compared with 
 the ovum itself, two of the polar bodies (Fig. 35, A, p.b.) appear 
 to be cast out of the latter as it undergoes maturation, while 
 the third is formed by division of the first ; it will be clear, 
 however, from a careful study of Figs. 65 and 68 that the 
 
 1 This term is very unfortunately chosen ;md must not, of course, be confounded 
 with the same term as applied to the female organs of such plants as Fucus.
 
 134 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 FIG. 66. Some Stages in the Spermatogenesis of a Grasshopper (Stenobothrus 
 viridulus). (After Meek.) 
 
 1. A secondary spermatogonium in mitosis, with seventeen chromosomes (eight pairs and 
 
 one accessory). Polar view. 
 
 2. Resting or growth stage following the mitosis of the secondary spermatogonium. The 
 
 ordinary chromosomes have become resolved into a network but the accessory 
 chromosome retains its individuality. 
 
 3. Primary spermatocyte formed by growth of 2, preparing for mitosis. 
 
 4. Mitosis of the primary spermatocyte, showing the pairing (synapsis) of the sixteen 
 
 ordinary chromosomes on the nuclear spindle and the accessory chromosome 
 unpaired. Side view. 
 
 5. Later stage in the mitosis of the primary spermatocyte, showing separation of the 
 
 paired chromosomes. Side view. 
 
 6. Still later stage in the division of the primary spermatocyte into two secondary sper- 
 
 matocytes ; the chromosomes massing at the two poles of the spindle (one mass only 
 will contain the accessory chromosome). Side view. 
 
 7. A secondary spermatocyte in mitosis, showing the reduced number of ordinary chromo- 
 
 somes (8) and the accessory chromosome. (The sister-cell would of course contain no 
 accessory chromosome). Polar view. 
 
 8. Later stage in the mitosis of the secondary spermatocyte ; each chromosome (including 
 
 the accessory one, which lags behind the others) has split into two parts and the two 
 groups are separating. Side view. 
 
 9. A spermatid, formed by division of a secondary spermatocyte with an accessory 
 
 chromosome. It will fonn a single spermatozoon. 
 
 x The accessory chromosome.
 
 SPERMATOGENESIS IN INSECTS 135 
 
 process is merely one of repeated cell-division as in the case of 
 spermatogenesis. The three polar bodies consist almost entirely 
 of chromatin and each of course contains the same reduced 
 number of chromosomes as the ovum itself; they undergo no 
 further development, however, and finally disappear. The truth 
 of the view, now generally held, that the polar bodies are 
 merely ova which have not sufficient cytoplasm to allow of their 
 development, is demonstrated by the fact that in one of the 
 turbellarian flat-worms, according to Francotte, the exceptionally 
 large first polar body may occasionally be fertilized and actually 
 develop as far as the gastrula stage. 
 
 The details of the process of gametogenesis vary very much 
 in different cases, but the above outline may be regarded as 
 generally applicable. 
 
 In a large number of insects it has been found that the male 
 animal possesses an odd number of chromosomes in the somatic 
 nuclei, due to the presence of a single "accessory" chromosome 
 or " monosome," while the female possesses an even number 
 (one more than the male owing to the presence of two 
 " accessory " chromosomes). This leads to a curious complica- 
 tion in the process of spermatogenesis. The accessory chromo- 
 some (Fig. 66, X) can often be distinguished by its appearance 
 from the others, and at the time of synapsis (Fig. 66, 4) it has 
 no mate. Hence in the reducing division the chromosomes are 
 separated into two unequal groups, one of which contains the 
 accessory chromosome while the other does not (Fig. 66, 5). Two 
 kinds of spermatozoa are accordingly produced in equal numbers, 
 one kind with an odd number of chromosomes and the other with 
 an even number. 
 
 The matured ova, on the other hand, all have the same number 
 of chromosomes, because the accessory chromosome has a synaptic 
 mate. Fertilization of an ovum by a spermatozoon containing 
 an accessory chromosome results in the production of a female 
 animal with an even number of chromosomes in its somatic cells ; 
 fertilization by a spermatozoon which has no accessory chromo- 
 some results in the production of a male animal with an odd 
 number of chromosomes in its somatic cells, as shown in Fig. 67. 
 
 Thus it appears that the chromosomes, at any rate in some 
 cases, have a very important influence on the determination of 
 sex, and that the latter is not, as has often been supposed, merely 
 the result of nutritional and other environmental influences upon
 
 186 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the developing organism, but a character of far more deeply 
 seated origin. 
 
 It must not be forgotten that the chromosomes, as such, are 
 only recognizable during the process of mitosis ; in the resting 
 condition of the nucleus they appear to be broken up into larger 
 or smaller granules of chromatin scattered through the linin 
 reticulum. The observations on accessory chromosomes above 
 mentioned, and others to be referred to presently, point to the 
 
 6 (Unreduced Cells) $( Unreduced Cells) 
 
 Reduction 
 
 (4+1) 4 
 
 5 Spermatozoa 
 
 (4 + 1) 
 
 10(9 Zygote) 
 
 9(6 Zygote) 
 
 FIG. 67. Diagram illustrating the Correlation between the Number of 
 Chromosomes and the Sex in certain Insects. (The numbers of chromo- 
 somes given in this diagram are arbitrarily chosen and are obviously 
 different from those which occur in Stenobothi us, as shown in Fig. 66.) 
 
 conclusion, however, that, in spite of this, the different chromo- 
 somes preserve some sort of individuality from one cell-genera- 
 tion to another. In other words, we have reason to believe that 
 the chromosomes which make their appearance at the onset of 
 each mitosis are, taken each as a whole, the same as those which 
 become disintegrated at the close of the preceding mitosis, though 
 it is very possible that the constituent parts of the old chromo- 
 somes (chromatin granules, chromomeres or ids), after absorbing 
 nutriment and increasing in size during the resting period, may 
 come together in new combinations to form the new chromosomes 
 each time division of the nucleus takes place. 1 
 
 1 Vide Farmer, Croonian Lecture, Proc. Itoyal Soc., Ser. B, Vol. 79, 1907.
 
 PAIRING OF CHROMOSOMES 137 
 
 It is also highly probable, though by no means certain, that the 
 chromosomes which are derived from the male parent remain 
 throughout life distinct from those which are contributed by the 
 female parent. According to this view every ordinary somatic 
 cell has two sets of chromosomes, paternal and maternal respec- 
 tively, and this again is strongly supported by the observations 
 on the germ cells of insects above referred to, where all the 
 chromosomes appear to be duplicated, with the exception of the 
 accessory chromosome in the male animal. 
 
 There is reason for believing, therefore, that in ordinary cases 
 every paternal chromosome in an unreduced nucleus has an equiva- 
 lent or " homologous " mate derived from the female parent, and 
 that the phenomenon of synapsis 1 represents a pairing of these 
 homologous paternal and maternal mates. The reducing division 
 which follows on synapsis consists in the separation of the mates 
 once more, one of each pair going to each daughter cell, so that 
 the matured germ cells are left with a single instead of a double 
 set of chromosomes. 
 
 If we assume that, as seems highly probable, the chromo- 
 somes of each paternal or maternal set are not all identical 
 but differentiated amongst themselves a differentiation which 
 in some cases is actually visible, as shown in Fig. 66 and 
 that one of each kind is necessary to make up the full com- 
 plement of the nucleus of the gamete, the importance of 
 the pairing of homologous chromosomes which takes place in 
 synapsis becomes at once evident, for one of each pair goes to 
 each daughter nucleus, which will therefore be certain to receive 
 a chromosome of each kind instead of a chance assemblage. 
 The chromosome of each kind which it receives, however, may be 
 either the paternal or the maternal representative of that kind, 
 and as these, though essentially homologous, may differ from one 
 another to some extent in accordance with individual peculiarities 
 of the parents from which they were derived, it will be seen that 
 the matured gametes may differ widely amongst themselves in 
 their nuclear constitution. This, as we shall see presently, is a 
 very important matter from the point of view of the theories 
 of heredity and variation. 
 
 This somewhat complex subject will be rendered more readily 
 
 1 The paii-int: of the chromosomes, for which we have used the term "synapsis," 
 is spoken of by some writers as " syndesis," and by others as "conjugation." The 
 use of the latter term seems likely to lead to confusion.
 
 138 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 intelligible by a careful study of Fig. 68, which represents in a 
 very diagrammatic manner the formation of the polar bodies and 
 the distribution of the maternal and paternal chromosomes in the 
 maturation of a typical animal ovum. 
 
 Periodic reduction of the number of chromosomes is clearly a 
 necessary consequence of the sexual process, for a doubling of 
 
 FIG. 68.- -Diagram of the Maturation of a typical Animal Ovum, showing 
 the Behaviour of the Maternal and Paternal Chromosomes and the 
 Formation of Polar Bodies. (The somatic number of chromosomes is 
 supposed to be four ; the maternal chromosomes are shaded and the 
 paternal not, and the differences between the two of each set are 
 indicated by their shapes.) 
 
 A, ordinary somatic mitosis in an oogoiiium, eacli chromosome split ; B, daughter 
 oogonium ; C, synapsis in primary oocyte ; D, reducing division, formation of first 
 polar body ; E, commencement of formation of second (or third) polar body by ordinary 
 mitosis and of division of the first ; F, mature ovum with three polar bodies. 
 
 chr., chromosomes ; c.s., centrosome ; n.m. t nuclear membrane ; p.b. 1 3, polar bodies. 
 
 the number at every zygosis or conjugation of gametes could not 
 go on indefinitely without some such compensation. In animals, 
 as we have seen, the reduction takes place during the maturation 
 of the germ cells, but it is by no means necessarily associated with 
 this process. In the ferns, where alternating sexual and asexual 
 generations are represented by independent and well developed 
 organisms, the reduction takes place in the process of spore- 
 formation by the sporophyte. Hence the gametophyte (pro- 
 thallus) to which the spore gives rise has in all its somatic cells
 
 POLAR BODIES IN PROTOZOA 
 
 189 
 
 flead 
 
 nuts. 
 
 the reduced number of chromosomes, while the sporophyte (fern 
 plant proper), owing to the fact that it develops from a fertilized 
 ovum and receives a set of chromosomes from each parent, has the 
 full number, or, perhaps we should say more correctly, the double 
 number. 
 
 Even amongst the Protozoa the phenomenon of nuclear reduc- 
 tion has been observed, and it probably occurs wherever the sexual 
 
 process takes place. Thus, 
 in the case of Copromonas, 
 the life history of which we 
 have already dealt with, 
 the nucleus of each of the 
 similar gametes, previous 
 to uniting with its mate to 
 form the zygote nucleus, 
 gives off two "polar bodies" 
 which undergo degenera- 
 tion in the cytoplasm (Fig. 
 37, 3, 4), and something very 
 similar may be observed in 
 the maturation of the ovum 
 of Coccidium. In the con- 
 jugating Paramcecium, 
 again, the products of divi- 
 sion of the micronucleus 
 (Fig. 41, B, C) which undergo 
 no further development may 
 be regarded as polar bodies. 
 As we have already seen, 
 the maturegametes through- 
 out both the animal and 
 vegetable kingdoms usually 
 sexual dimorphism, which 
 into active 
 
 Tail 
 
 FIG. 69. Diagram of typical Sperma- 
 tozoon and Ovum, the former much 
 more highly magnified than the 
 latter. 
 
 ax., axial filament; e.g., chromatin granules; 
 cs., centrosome; cyt., cytoplasm; m.p., 
 middle piece; n.m., nuclear membrane; 
 nil,., nucleus; mils., nncleolus ; v.m., 
 vitelline membrane ; y-g-, yolk granules. 
 
 very strongly marked 
 
 exhibit 
 
 attains its fullest expression in the differentiation 
 spermatozoon and passive ovum (Fig. 69). 
 
 A typical spermatozoon, as we have also pointed out, closely 
 resembles a flagellate protozoon. It consists of a " head " and 
 a "tail," connected together by a " middle piece " (m.p.). The 
 head contains the nucleus (MM.), which almost entirely fills it, 
 being covered with only a very thin envelope of cytoplasm. The 
 middle piece contains a centrosome (cs.). The tail, or flagellum, is
 
 140 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 sh.m. 
 
 FIG. 70. Diagram of the Structure 
 Bird's Egg. 
 
 a.c., air chamber; alb., albumen; ch., chalazte, 
 twisted cords of dense albumen which serve to 
 keep the " yolk " in position ; g.d., germinal 
 disk; nu., nucleus; sh., shell; sh.m., shell 
 membrane ; v. TO., vitelline membrane ;y k., "yolk." 
 
 the active locomotor organ, by the movements of which the 
 
 spermatozoon swims about ; it contains an axial filament (ax.) 
 
 and may or may not be 
 
 n \" f? , provided with a lateral 
 
 undulating membrane. 
 A typical ovum is a 
 relatively large, spheri- 
 cal cell with a con- 
 spicuous nucleus (mi.) 
 surrounded by a large 
 quantity of cytoplasm 
 (cyt.), and the whole 
 enclosed in a delicate 
 vitelline membrane 
 (v.m.). The actual size 
 of the ovum depends 
 almost entirely on the 
 amount of food material 
 (deutoplasm or yolk), 
 
 which is stored up in the cytoplasm in the form of granules (?/.//.). 
 
 In Amphioxus (Fig. 13, 1) the amount of food material is very 
 
 small, and the egg is only about alo^h 
 
 inch in diameter. 
 
 An extreme contrast to the egg of 
 
 Amphioxus is seen in that of a bird 
 
 (Fig. 70), where the amount of yolk 
 
 is enormously large and the active 
 
 protoplasm is confined to a minute 
 
 " germinal disk " (fj.d.), containing the 
 
 nucleus (nu.), which lies within the 
 
 vitelline membrane (r.m.) on the top 
 
 of the "yolk" (yk.), while the ovum 
 
 proper is entirely enclosed in accessory 
 
 structures the " white " or albumen 
 
 (nib.) and the shell (sh.), with its lining 
 
 membrane (sh.m.). 
 
 The mammalian ovum, on the other 
 
 hand, is, like that of Amphioxus, very 
 
 minute, in the rabbit (Fig. 71) again only about ^ a th inch in 
 
 diameter. It is enclosed in an envelope known as the zona 
 
 radiata (Z), which lies outside the vitelline membrane, and it 
 
 FIG. 71. Ovum of a Babbit, 
 X 200. (From Marshall's 
 " Vertebrate Embry- 
 ology," after Bischoff.) 
 
 MO, spermatozoa which have 
 penetrated the zona radiata ; 
 N, nucleus; NU, nucleolus; 
 Z, zona radiata.
 
 ATTRACTION OF GAMETES 141 
 
 contains very little deutoplasm. This is correlated with the fact 
 that it develops within the body of the parent at the expense of 
 food material derived from the blood of the latter. There is 
 reason to believe, however, as we shall see later on, that the 
 small size of the mammalian egg is a secondary feature. 
 
 The plant egg-cell may also be loaded up with food material, 
 so as to attain a large size, as in the green alga, Chara, 
 where the contrast between the minute flagellate spermatozoon 
 and the relatively gigantic ovum, filled in this case with starch 
 grains, is very striking. In the higher plants, however, where, 
 as in the case of the Mammalia, the developing embryo is not 
 dependent for its nutrition upon food supplies stored in the egg- 
 cell, the latter remains quite small, as, for example, in the fern 
 (Fig. 52) and the flowering plant (Fig. 55, e). 
 
 According to some authorities one of the most important 
 differences between ovum and spermatozoon in animals lies in the 
 fact that the centrosome of the former disappears finally during 
 the process of maturation, the centrosome of the zygote being 
 contributed by the spermatozoon alone. In view of the fact, 
 however, that a definite centrosome is not usually recognizable at 
 all in the higher plants we cannot attribute very great importance 
 to its supposed absence in the animal ovum, and we shall also see 
 presently that centrosomes appear in developing eggs which have 
 not been fertilized by spermatozoa. 
 
 We have already had occasion to refer to the existence of 
 some attractive force whereby the male and female gametes 
 are brought together in conjugation. Many observers main- 
 tain that this is simply a case of positive chemotaxis, or 
 the chemical stimulation of the protoplasm of one gamete by 
 a specific secretion of the other in such a way as to cause 
 them to respond by approaching one another (or by the male 
 gamete approaching the female). 
 
 It is a well known fact that certain spermatozoa are attracted 
 by specific chemical substances. Thus the free-swimming sperma- 
 tozoa of ferns and mosses are attracted by weak solutions of 
 malic acid and cane sugar respectively, and those of Coccidium 
 are attracted by nuclear matter discharged from the ovum in the 
 process of maturation. 
 
 There can be no doubt that, whether the attracting substance 
 be secreted by the germ cells themselves or by some other part 
 of the organism, chemotaxis sometimes plays a very important
 
 142 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 part in bringing the gametes together. So also, of course, do 
 many other factors in various plants and animals, but we must 
 distinguish between factors which act directly upon the gametes, 
 such as chemotaxis, and those which act indirectly through the 
 soma or body of the organism, as for example through the visual 
 and olfactory senses of the higher animals. 
 
 It is probable, however, that chemotaxis itself is but a secondary 
 factor which serves to bring the gametes within the range of one 
 another's direct influence. Thus in Coccidium (Fig. 39) the 
 chemotactic action seems to be exhausted after a certain number 
 of spermatozoa have been attracted to the neighbourhood of the 
 ovum and a fresh attraction appears to be exerted by the ovum 
 itself or by its nucleus. 
 
 The term cytotropism, or cytotaxis, has been applied to the 
 attraction which, according to some observers, is sometimes 
 set up between two adjacent cells, and something of this kind 
 probably comes into play in the mutual attraction of gametes. 
 It can probably act only at very short distances, and hence the 
 necessity for some preliminary means of attraction such as 
 chemotaxis. That chemotaxis alone is not a sufficient explana- 
 tion of the phenomenon in question is suggested by the case of 
 Spirogyra. The conjugation of the gametes in this plant has 
 already been described in Chapter VIII. It will be remembered 
 that the process may take place between the cells of two filaments 
 lying close together, parallel with one another (Figs. 43 and 
 44), and is then inaugurated by those of the male filament. 
 Each of these cells which happens to lie opposite to a cell of the 
 female filament puts forth a hollow protuberance of its wall, which 
 is presently met by a similar protuberance from the wall of the 
 female cell, the two projections uniting to form a canal through 
 which the protoplasmic body of the male gamete creeps inside the 
 cell- wall of the female gamete to conjugate with the latter. It 
 sometimes happens, however, that, owing to inequalities in the 
 sizes of the cells, there may be a cell in one filament which lies 
 between two cells of the opposite filament and for which there is 
 no mate, all the adjacent cells being coupled. In such cases the 
 solitary cell (Figs. 43 and 44, S.C.), if it exhibits any of those re- 
 markable activities which are shown by the conjugating cells on 
 either side of it, merely makes preliminary advances which are 
 prematurely checked, as though there were a competition for 
 partners in which it was unsuccessful. Here it is obvious that
 
 PARTHENOGENESIS 143 
 
 in the case of two cells lying opposite to one another, though not 
 in contact, and though each is enclosed in a firm cell-wall, some 
 stimulus is transmitted from one to the other which calls forth 
 a definite response manifested in the formation of the connecting 
 canals and the conversion of the protoplasmic contents of the 
 cells into gametes. The insufficiency of the principle of chemo- 
 taxis to account for these phenomena appears to be indicated 
 by the fact that cells which have no mates do not form either 
 complete connecting canals or gametes, though exposed equally 
 with their more fortunate neighbours to the influence of any 
 chemical substances dissolved in the surrounding water. The 
 only explanation appears to be that the solitary cell cannot 
 attract, or at any rate retain, the attention of a mate to stimulate 
 it to complete the process of conjugation. 1 
 
 The cytotropic attraction of the gametes, as we have already 
 observed, probably depends upon some difference of polarity 
 between the two. That it is mutual is demonstrated by such 
 cases as that of Zygogonium (see p. 97) and by the fact that even 
 when the ovum is too heavily laden with food material to take any 
 active part in the process of conjugation it yet in many cases puts 
 out a definite " cone of attraction " towards the advancing sper- 
 matozoon, as seen in Coccidium (Fig. 39). What the real nature 
 of this primary attraction between the gametes is we do not know ; 
 it may ultimately be explicable in terms of some force already 
 known to us, or it may be one of those cases where it will be 
 convenient to cloak our ignorance by the assumption of some 
 special vital force of which we know nothing. 
 
 Although as a general rule an egg does not develop unless 
 fertilized by a spermatozoon, this is by no means always the 
 case, and many instances are known of parthenogenesis or the 
 development of unfertilized eggs. This may either be a normal 
 occurrence in the life cycle or it may be artificially induced. 
 
 Natural parthenogenesis occurs chiefly in insects, especially 
 amongst the aphides or plant lice. In these animals males and 
 perfect females appear only in the autumn. Fertilized eggs are 
 then laid which hibernate through the winter and hatch in the 
 spring, producing imperfect viviparous females. In these imperfect 
 females eggs are formed which develop parthenogenetically within 
 the body of the parent and give rise to fresh generations of 
 viviparous forms. This reproduction by means of unfertilized eggs 
 
 1 See, however, the footnote on p. 185).
 
 144 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 is repeated again and again throughout the summer months and 
 thus the aphides multiply with great rapidity. In the autumn, 
 however, males and perfect females are again produced. The 
 viviparous imperfect females, as well as the males, are generally 
 winged, the perfect females are wingless. We have here another 
 kind of alternation of generations, in which forms which repro- 
 duce parthenogenetically alternate with others which exhibit 
 the normal sexual process ; to this type of alternation the 
 term heterogeny is sometimes applied. A very large number 
 of parthenogenetic generations may intervene between two 
 sexual ones. 
 
 Another well-known case of parthenogenesis is that of the hive- 
 bee, where the eggs laid by the queen may either be fertilized or 
 not, in the former case giving rise to females (workers or queens) 
 and in the latter to males (drones). Other instances occur amongst 
 those parasitic flat-worms known as flukes (Trematoda) and in 
 some Crustacea (Cladocera, the so-called water fleas). 
 
 The most remarkable cases of natural parthenogenesis, how- 
 ever, are those to which the special term paedogenesis has been 
 applied, in which the imperfect females do not even wait to 
 attain maturity before producing their offspring, but actually do 
 so in the larval condition, as in Chironomus and some other 
 two-winged flies (Diptera). 
 
 In general we may say that parthenogenesis occurs in cases 
 where it is desirable to take advantage of a brief season of favour- 
 able conditions to multiply the race as rapidly as possible. It is 
 necessary to make hay while the sun shines. When adverse 
 conditions set in, such as the advent of winter in the case of the 
 aphides, or discharge from the body of the host in the case of 
 parasitic flukes, the vast majority of the race will perish, but 
 a sufficient number will be able to protect themselves in some 
 way (like the encysted cercarise of the fluke), or a sufficient 
 number of fertilized and protected eggs will be produced (as in 
 the aphides) to tide over the evil time and form the starting 
 points for fresh generations at the first favourable opportunity. 
 
 It seems possible, also, that in some cases parthenogenesis 
 may be continued indefinitely without fertilization ever occurring, 
 for in certain species of minute rotifers and crustaceans no males 
 have as yet been observed. 
 
 Recent researches have shown that parthenogenesis can be 
 artificially induced in cases where it does not occur naturally at
 
 ARTIFICIAL PABTHENOGENESIS 145 
 
 all, and Professor Loeb 1 goes so far as to maintain that the 
 problem of fertilization is really one of physical chemistry. He 
 holds that the development of the egg is to be regarded as a 
 chemical process which depends mainly on oxidation, and finds 
 that the unfertilized eggs of various animals (sea-urchins and 
 worms) will undergo development (at any rate up to a certain 
 point) after exposure to the action of certain chemical reagents. 
 The unfertilized eggs of a sea-urchin, for example, developed into 
 larvre after being placed for two hours in sea water, the osmotic 
 pressure of which had been raised about 60 % by the addition of 
 some kind of salt or sugar, but this " hypertonic " solution must 
 contain a sufficient quantity of free oxygen. In another case the 
 unfertilized eggs of the worm Chaetopterus were stimulated to 
 develop into larvae by the mere addition of potash and acids, 
 without the osmotic pressure of the sea water being raised. 
 
 Exactly what takes place under these circumstances we do not 
 know, and any speculation on this point is perhaps somewhat 
 premature, but it is quite clear from the experiments of Loeb and 
 other workers in the same field that we can no longer regard 
 fertilization as an indispensable condition of development even in 
 the case of eggs which do not naturally exhibit the phenomenon 
 of parthenogenesis. These experiments may also throw some 
 light upon the process of normal fertilization, especially as 
 regards the nature of the actual stimulus which causes the 
 fertilized egg to begin segmenting. 
 
 The casting out of polar bodies during the maturation of the 
 ovum led many of the earlier observers of this phenomenon to 
 believe that the matured ovum is incapable of development 
 because it has an imperfect nucleus, the importance of the 
 nucleus as taking the lead in cell-division having been established 
 at a comparatively early date. The imperfect nucleus of the 
 matured ovum was termed the female pronucleus, and it was 
 supposed to be converted into a perfect segmentation nucleus by 
 union with the male pronucleus brought into the egg by the 
 spermatozoon (Fig. 64), and the power of cell-division was supposed 
 to result from this completion of the nucleus. The observations 
 upon which this belief was based were perfectly correct, but the 
 conclusions drawn from them have not been sustained by recent 
 investigations, for in cases of artificial parthenogenesis development 
 
 1 "Die chemische Entwicklungserregung cles tierischen Eies" (kiinstliche Par- 
 thenbgenese). Berlin, 190'J. 
 
 B. L
 
 146 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 takes place in spite of the fact that reduction of the nucleus of 
 the ovum has occurred in maturation and has not heen made 
 good by union with the nucleus of a spermatozoon. 
 
 In cases of parthenogenesis it is clear that the developing 
 organism is provided only with maternal chromosomes, but we are 
 now also acquainted with cases which form the exact converse to 
 this, only paternal chromosomes being present. Such cases may 
 arise when an ovum is artificially enucleated, so that all the 
 maternal chromosomes are removed, and then fertilized by a 
 spermatozoon. It has actually been found possible in this way to 
 induce the development of enucleated eggs, and the phenomenon, 
 our knowledge of which we owe mainly to Delage, is known as 
 merogony. Although it is obvious that, as a result either of 
 artificial parthenogenesis or of merogony, the developing organism 
 may start life with only half the normal number of chromosomes, 
 it is possible that, as maintained by Delage, this number may be 
 subsequently doubled in some way. 
 
 It cannot therefore be the union of male and female pronuclei 
 that furnishes the stimulus to development; this union, or 
 amphimixis as Weismann terms it, has another significance, and, 
 as we shall see later on, is most probably connected with the 
 transmission of inherited characters from parent to offspring. 
 
 Boveri, followed by other observers, has put forward the view 
 that the unfertilized egg cannot develop because the centrosome, 
 which is to be regarded as the dynamical centre of the cell, has 
 been eliminated during the process of maturation. It is the 
 spermatozoon that, in the act of fertilization, brings with it 
 the centrosome upon the activity of which the cell-divisions of the 
 fertilized egg depend. This is probably perfectly true in cases 
 of normal fertilization and development in animals, but the view 
 that the centrosome of the spermatozoon supplies the essential 
 stimulus to development seems to be hopelessly negatived by the 
 phenomena of parthenogenesis, in which new centrosomes 
 undoubtedly arise in unfertilized eggs. 
 
 It seems therefore as if we were for the present thrown back 
 upon Loeb's hypothesis of a chemical stimulus, which he main- 
 tains for normal fertilization as well as for artificial partheno- 
 genesis. In the former case the necessary chemical substances 
 are supposed to be brought in by the spermatozoon. Loeb 
 believes that there are two of these substances. One, which he 
 terms a lysin, is supposed to bring about the formation on the
 
 STIMULUS TO DEVELOPMENT 147 
 
 surface of the egg of the " fertilization membrane," while the 
 other is supposed to prevent the cytolysis or disintegration of the 
 ovum, induced by the formation of the fertilization membrane, 
 from going too far. It remains to be seen whether this " lysin 
 theory" will stand the test of time. 
 
 What it is that stimulates the unfertilized ovum to develop in 
 normally occurring parthenogenesis we do not yet know. In 
 most cases of this kind the process of maturation seems to differ 
 more or less from that which takes place in eggs which are 
 destined to be fertilized by a spermatozoon, and it may be that 
 these differences have something to do with the power of the egg 
 to develop parthenogenetically, but the discussion of this very 
 difficult problem is altogether beyond the scope of the present 
 work. 
 
 L 2
 
 PART III. VARIATION AND HEREDITY 
 
 CHAPTER XI 
 
 Variation Merislic and substantive variations Fluctuations and muta- 
 tions Somatogenic and blastogenic variations Origin of blastogenic 
 variations. 
 
 THE term variation is used in more than one sense ; it may 
 be denned in the first instance as the process whereby closely 
 related organisms come to differ amongst themselves. It is a 
 matter of everyday experience that neither animals nor plants 
 exhibit absolutely fixed and constant characters, which are 
 handed on without alteration from parent to offspring. This 
 is very well seen in the case of human families, in which 
 there is rarely any difficulty in distinguishing the different 
 members by more or less pronounced and characteristic 
 individual traits. One may be fair and another dark, one short 
 and another tall, one with brown eyes and another with blue, 
 one clever and another stupid, and so on. In this way they vary 
 amongst themselves and deviate from the common parents of the 
 family often to a very considerable extent. 
 
 We cannot, however, avoid extending the use of the term 
 variation from the process itself to the results of that process, 
 and speaking of organisms as exhibiting variations, but this usage 
 is not likely to cause any confusion. 
 
 Variations are of many kinds and may be classified in different 
 ways according to the point of view from which we regard 
 them. 
 
 Meristic variations, which are variations in the number of the 
 repeated parts of an organism, are sometimes contrasted 1 with 
 substantive variations, which depend upon the structure (in- 
 cluding shape, size and colour) of the organism or its parts. 
 
 Small fluctuating or continuous variations, which fluctuate 
 
 1 Bateson, " Materials for the Study of Variation." (Macmillan & Co., London, 
 1894.)
 
 MERISTIC VARIATIONS 149 
 
 about a mean or average condition, are contrasted with larger 
 discontinuous variations, or mutations. 
 
 Somatogenic variations, which affect the body of the 
 organism and are acquired in the life-time of the individual, are 
 contrasted with blastogenic or germinal variations, which arise as 
 a consequence of some modification in the germ cells. 
 
 These three methods of classification obviously overlap one 
 another. Thus a m eristic variation may be continuous or 
 discontinuous, somatogenic or blastogenic, and so on, but it will 
 be convenient to deal with each group separately. 
 
 Meristic or Numerical Variations. In a very great number of 
 organisms certain parts are repeated with a greater or less 
 degree of regularity and constancy, and it is upon this repetition 
 that the symmetry of the organism very largely depends. An 
 ordinary star-fish, for example, is radially symmetrical, with five 
 similar arms or rays radiating from a common centre ; meristic 
 variation, however, not infrequently gives rise to six-rayed 
 individuals. The common jelly-fish, Aureiiti aurita, again, usually 
 has four principal radii, but specimens are occasionally found with 
 two, three, or six. 
 
 All vertebrate animals, on the other hand, and many inverte- 
 brates, are bilaterally symmetrical, and at the same time rneta- 
 inerically segmented, i.e. with repetition of similar parts in linear 
 series one behind the other. The vertebral column, for example, 
 is made up of a larger or smaller number of morphologically 
 equivalent units, the vertebrae, to which the ribs are attached in 
 linear series on each side, and the limbs are arranged in two pairs, 
 in each of which the same fundamental structure can be traced. 
 Many instances occur in different animals of variation in the 
 number of vertebrae, and in man the occasional occurrence of 
 thirteen ribs instead of the normal twelve on each side is well 
 known. Cases of polydactylism, or the development of extra 
 digits on hand or foot, also come under the head of meristic 
 variation. 
 
 In the vegetable kingdom, where the repetition of similar 
 parts, such as the leaflets of leaves, is even more conspicuous 
 than amongst animals, meristic variation is again a common 
 phenomenon. A clover leaf, for example, normally consists of 
 three leaflets, but the occasional discovery of a specimen with four 
 has been a source of satisfaction to superstitious folk from time 
 immemorial. We meet with similar variations in the number
 
 150 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 of petals in flowers and the number of flowers in inflorescences. 
 An example of this will be given later on in the case of the 
 grape hyacinth. 
 
 Substantive Variations. Examples of this type of variation are 
 seen in the shape, size and colour of the human body and of its 
 various parts. The total height of the individual, the shape of 
 such features as the nose, ears and fingers, the colour of eyes 
 and hair, are all subject to very great variation. The same is 
 true of the shape and size of leaves and the colour of many 
 flowers. Numerous physiological variations, which doubtless 
 depend upon unrecognized and perhaps unrecognizable differences 
 in structure, may also be classed under this head, as for example 
 the variations in the egg-laying powers of fowls and the milk- 
 giving powers of cows, and in the percentage of sugar present 
 in the roots of the sugar beet. 
 
 Fluctuating Variations. The variations which come under this 
 heading are also spoken of as normal, individual or continuous. 
 They may be either meristic or substantive. They are 
 characterized by the fact that they pass into one another by very 
 minute or even insensible gradations, which fluctuate on either 
 side of a mean or average condition, so that the extent to which 
 a given organism (or part of an organism) varies can be 
 graphically represented in the form of a curve. Fluctuating 
 meristic variations lend themselves most readily to this graphic 
 method of treatment, for all we have to do is to count the 
 numerically varying parts in a sufficiently large number of cases 
 and plot our curve accordingly. 
 
 The manner in which this is done is represented in the 
 accompanying diagram (Fig. 72), which expresses graphically the 
 results obtained by counting the number of flowers in 202 in- 
 florescences of the common grape hyacinth (Muscari sp.). The 
 number of flowers in an inflorescence ranged from 20 to 42, and 
 the number of inflorescences showing each of the different numbers 
 of flowers ranged from 1 to 29. 
 
 The base line of the diagram is divided into a number of equal 
 parts (abscissae) corresponding to the number of flowers (from 20 
 to 42). At one side an ordinate is erected on the base line and 
 divided into a number of equal parts corresponding to the 
 number of the individual inflorescences (from 1 to 29) bearing the 
 different numbers of flowers. Lines are then ruled parallel to 
 the base line and to the ordinate respectively, so as to divide the
 
 FLUCTUATING VARIATIONS 
 
 151 
 
 paper into a number of equal squares (or the proceeding may be 
 simplified by working with already squared paper). The diagram 
 is completed by shading in an appropriate square for each 
 inflorescence, the position of the square being determined by the 
 number of flowers which it contains, and by the number of 
 
 20 21 22 25 24 25 26 27 28 29 30 51 32 3 J 34 55 56 57 38 39 40 41 42 
 
 Number of flowers in each Inflorescence 
 
 Fio. 72. Fluctuating Variation in Inflorescences of the 
 Grape Hyacinth. 
 
 similar inflorescences which have preceded it in the enumeration. 
 Thus an inflorescence with twenty-five flowers must be repre- 
 sented by a square added to. the column above the number 25 
 on the base line, and so on. If now we join the middle points of 
 the tops of all the columns by straight lines, we get a zig-zag line 
 which represents the curve of variation as determined by our actual 
 observations. It is obviously a very imperfect result, but its
 
 152 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 imperfection is undoubtedly due in the main to the fact that the 
 number of observations upon which it is based has not been 
 sufficiently large to give a fair average. 
 
 The larger the number of individual cases examined and 
 recorded the more closely will our zig-zag line approximate to 
 a regular curve. Even as it is, with all its imperfections, it 
 shows certain very characteristic features. It rises steeply 
 in the middle and falls away from the highest point, at first 
 suddenly and then more gradually, on either side. This is a 
 graphic representation of the facts, (1) that the most frequently 
 occurring number of flowers in the inflorescence (28) lies not very 
 far from midway between the two extremes, and (2) that the 
 
 Stature fn Inche 
 
 FlG. 73. Fluctuating Variation in Human Stature. (From Lock.) 
 
 more the number of flowers deviates from this mean on either 
 side the smaller is the corresponding number of inflorescences ; 
 or, to put it more generally, the number of individuals exhibiting 
 any given degree of deviation from the mean condition of the 
 species is inversely proportional to the amount of that 
 deviation. 
 
 Fig. 73 is a diagram constructed on the same principles as 
 the above, but based upon totally different material and a very 
 much larger number of observations. It is taken from Mr. R. H. 
 Lock's work on " Variation, Heredity and Evolution," and shows 
 the variation in stature observed amongst 4,426 British members 
 of the University of Cambridge. The dots in this diagram 
 correspond to the middle points of the tops of the vertical columns 
 in the preceding one. The statures are reckoned in the nearest
 
 NORMAL CURVE OF VARIATION 153 
 
 whole numbers of inches. It is clear that a zig-zag line formed 
 by joining the dots in the diagram would approximate fairly 
 closely to the curve actually drawn. 
 
 Such a normal curve of variation agrees very closely with what 
 mathematicians term the curve of frequency of error, which is 
 the graphic representation of the mode of occurrence of chance 
 deviations from a mean or average and may be derived from the 
 theory of probability. 
 
 Such a curve may be experimentally produced by drawing a 
 vertical line on a target and firing a large number of shots at it. 
 There will be a more or less strongly marked tendency for the 
 shots to hit the line, depending upon the skill or otherwise of the 
 marksman. Most of them will probably strike to the right or 
 left of the line and fairly near it, but a- few will probably be 
 very wide of the mark on either side. If the distances of the 
 striking places from the vertical line be measured and tabulated 
 the result may be expressed in the form of a curve which, if the 
 number of shots be large enough, will probably closely resemble 
 the curves described above. 
 
 The fact that the normal curve of fluctuating variation for any 
 kind of organism is practically identical with the mathematical 
 curve of frequency of error suggests very forcibly that the 
 variation in question is due to chance or accident causing each 
 individual in the course of its development to depart more or 
 less from the mean or average condition of the species to which 
 it belongs. These deviations must depend upon numerous factors. 
 They are to some extent, no doubt, due to the direct influence of 
 the environment, such as the effect of nutrition upon the size of 
 the organism, but they may also depend largely upon the vary- 
 ing characters of the germ cells from which the organism 
 develops, and especially upon the permutations and combinations 
 of characters which happen to take place in the maturation of 
 the germ cells and in their sexual union or amphimixis. 
 
 In short, the general tendency is doubtless for each individual 
 to conform to the type of the species to which it belongs, but 
 many accidental circumstances combine to prevent the realization 
 of this tendency and deviations from the type (or mean) take 
 place in accordance with the laws of chance. 
 
 Mutations. The term mutation, or discontinuous variation, is 
 applied to the process whereby new and more or less conspicuous 
 characters appear suddenly and spontaneously, without any
 
 154 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 obvious reason. In extreme cases the organisms exhibiting such 
 mutation are often spoken of as sports or monstrosities ; the 
 latter term, however, is also frequently applied in the case of 
 purely artificial modifications, which must not be included under 
 this heading. Such artificial modifications are not, at any rate 
 as a rule, inherited by future generations, while true mutations 
 usually, if not invariably, are. 
 
 Mutations differ from fluctuating variations not only in that they 
 usually deviate more widely from the type of the species, but 
 also in the much greater rarity of their occurrence and in the 
 fact that they do not fluctuate about the old mean or average 
 condition ; nevertheless it may be questioned whether we can 
 logically draw a hard and fast distinction between the two. 
 
 Mutations may be either meristic or substantive. Human 
 beings with six fingers or toes, in place of the normal five, 
 are not infrequently met with, and this meristic mutation (hexa- 
 dactylism) is well known to be transmitted by heredity. 
 
 The classical instance of the Ancon or otter sheep, on the 
 other hand, affords one of the best known examples of substantive 
 mutation. To quote the words of Huxley, 1 " It appears that one 
 Seth Wright, the proprietor of a farm on the banks of the Charles 
 River, in Massachusetts, possessed a flock of fifteen ewes and a ram 
 of the ordinary kind. In the year 1791, one of the ewes pre- 
 sented her owner with a male lamb, differing, for no assignable 
 reason, from its parents by a proportionally long body and 
 short bandy legs, whence it was unable to emulate its relatives 
 in those sportive leaps over the neighbours' fences, in which they 
 were in the habit of indulging, much to the good farmer's vexa- 
 tion." The inheritance of this peculiarity was so strong that 
 this single individual actually became the starting point of a new 
 breed. 
 
 Mutations, more or less pronounced in character, are also not 
 infrequently met with in the vegetable kingdom. Foxgloves, for 
 example, are sometimes found in which some or all of the petals 
 are converted into stamens, and it has been proved by experi- 
 ment that this peculiarity is handed down from parent to 
 offspring. 
 
 Professor Hugo de Vries has, as is well known, made a special 
 study of mutation amongst plants. In the year 1886 this dis- 
 guished botanist found a large number of specimens of the evening 
 
 v l " Darwiniana," p. 3">.
 
 MUTATIONS 
 
 155 
 
 primrose, (Enotliera lamarckiana, growing in a field near Amster- 
 dam, whither they had made their way from a neighbouring 
 garden. The plants were in a state of intense variability and their 
 seeds gave rise to several quite distinct new forms, which, if they 
 had occurred in a state of nature, would have been considered 
 as separate species. 1 Professor De Vries attributes great import- 
 ance to mutations as the starting points of new species, which he 
 believes to arise in this sudden manner rather than by fluctuating 
 variation. 2 We shall discuss this point in a subsequent chapter. 
 The difference between fluctuating variation and mutation is 
 sometimes illustrated by means of the model known as Gal ton's 
 polygon (Fig. 74). A thick slab of wood is cut into the form of 
 a polygon, with unequal sides and capable of resting in a position 
 of more or less stable equilibrium upon any of its edges, the 
 
 FIG. 74. Model of a Polygon in two Positions, illustrating the DiD'erence 
 between Fluctuating Variation and Mutation. 
 
 degree of stability depending upon the position of the centre of 
 gravity above the edge upon which it rests. 
 
 The polygon may be supposed to represent an organism, or 
 rather a number of successive generations of an organism, 
 whose stability (or adherence to type) tends to be more or less 
 disturbed by the unknown factors which cause variation. If the 
 model be pushed it may be made to rock backwards and forwards 
 on either side of a mean or average position, and if the oscillation 
 does not exceed certain limits it will return to rest in that position. 
 This oscillation may be compared to fluctuating variation. If the 
 disturbing force be sufficiently great, however, the model will topple 
 over into a new position of stability and come to rest on another 
 
 1 Considerable doubt has, however, been thrown upon the interpretation of these 
 observations, the results being conceivably due to the splitting up of some unknown 
 hybrid form, such as is well known to take place in other cases (see Chapter XIV.). 
 
 2 "The Mutation Theory." Trans, by Farmer and Darbishire. London, 1910.
 
 156 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 edge. This may be compared to the process of mutation or discon- 
 tinuous variation, whereby an organism acquires a new type of 
 structure. 
 
 Concerning the forces which bring about mutation in organisms 
 we know little or nothing. It is possible that in some cases the 
 mutation may be gradually prepared within the germ cells long 
 before it manifests itself the final upsetting of the equilibrium 
 only taking place on the addition of the last straw. If such be 
 the case there is no need to suppose that mutations differ 
 essentially in nature from small, fluctuating variations. Tower's 
 observations on the artificial production of mutations in certain 
 beetles, however, indicate very clearly that such modifications 
 may apparently arise quite suddenly as the result of some change 
 in the environment acting directly upon the germ cells of the 
 parent. These observations will be referred to again at the close 
 of the present chapter. 
 
 Somatoyenic Variations. Characters are said to be somatogenic 
 or " acquired " when they arise in the life-time of the individual 
 exhibiting them and owe their origin to the direct influence of 
 the environment upon the soma or body. They are, usually at 
 any rate, not transmitted by heredity to succeeding generations, 
 except perhaps to a very limited and inappreciable extent. 1 
 Under this heading are included the effects of use and disuse 
 of organs, and numerous cases in which modifications of the 
 body are artificially produced, as well as those in which they are 
 due to natural causes. 
 
 Amongst the effects of use and disuse we may mention, on the 
 one hand, the enlargement or the atrophy of special organs con- 
 sequent upon the extent to which they are employed, and, on the 
 other, the effects of education. The muscles of an athlete may 
 be greatly increased in size by constant use, and similarly if one 
 of the two kidneys be removed the other, having more work 
 thrown upon it, becomes enormously enlarged ; but we should not 
 expect these modifications to be handed on to the next generation. 
 Children have been taught to speak ever since man first became 
 differentiated from his speechless ancestors, but every child has to 
 learn the art anew and if brought up amongst foreigners will 
 come to speak a language different from that of its parents. 
 
 The small feet of Chinese ladies and the slender waists of 
 many Europeans are artificially produced somatic modifications 
 
 l This may be, however, a very important exception.
 
 ARTIFICIAL MONSTROSITIES 157 
 
 which are well known not to be inherited. They are merely 
 distortions, effected by easily recognizable mechanical agencies. 
 
 Much more remarkable and difficult to understand are those 
 cases in which the addition of specific chemical reagents to the 
 water in which aquatic larvae are developing produces such 
 definite and extensive modifications of structure as to give rise to 
 veritable monstrosities. The "Lithium larva?" of sea-urchins and 
 frogs have long been known and more recently Stockard has 
 described 1 the "Magnesium larva" of the fish Fundidtis hetero- 
 clitus (Fig. 75). He found that, when the developing embryos 
 of this fish are subjected to the influence of magnesium salts 
 dissolved in sea water, a large percentage of them acquire a 
 " cyclopean " character, with a single median eye in place of the 
 ordinary pair. Such embryos may hatch and swim about in a 
 perfectly normal manner, but it is not known whether they can 
 be reared to the adult condition. These observations seem to 
 indicate that the cyclopean monsters which sometimes occur in 
 man and other mammals may also be somatogenic variations due 
 to some unknown environmental influence. 
 
 Blastogenic or Germinal Variations. In this category are included 
 those variations which are believed to owe their origin to some 
 modification in the germ cells from which the organism 
 exhibiting them has developed. 
 
 It is important to observe that the term congenital, sometimes 
 used in this connection, is not synonymous with blastogenic, for it 
 is obvious that animals which, like the mammalia, remain within 
 the womb of the mother during the early stages of development, 
 may come to develop purely somatogenic characters before birth, 
 due to environmental influences acting in utero (e.g. poisoning of 
 the fffitus due to parental alcoholism). 
 
 It is very doubtful, as we have already said, whether we 
 can really draw any absolute distinction between blastogenic 
 and somatogenic characters, and it seems by no means impos- 
 sible that somatogenic modifications may sooner or later 
 make an impression upon the germ cells and thus ultimately 
 become blastogenic. This point, however, will be discussed 
 later on. 
 
 Blastogenic modifications are from their very nature as 
 attributes of the germ cells handed on by heredity from gene- 
 ration to generation. All true mutations must be regarded as 
 
 1 " Journal of Experimental Zoology," February, 1909.
 
 158 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 B. 
 
 C. 
 
 E. 
 FIG. 75. Free-swimming Larvae of Fundulus heteroclitus. (From Stockard.) 
 
 A. Normal larva, with anteriorly placed mouth (M). 
 
 B. Incompletely cyclopean larva, with the two eyes joined and occupying the position 
 
 usually taken by the mouth. 
 
 C. Completely cyclopean larva, with single antero-median eye. Dorsal aspect. 
 
 D. Lateral aspect of same, showing the ventral mouth (M). 
 
 E. Ventral aspect of same ; ys, yolk sac.
 
 TOWER'S EXPERIMENTS ON BEETLES 159 
 
 blastogenic and many so-called fluctuating variations may also 
 perhaps belong to the same category. 
 
 The modifications of the germ cells by virtue of which the 
 offspring come to differ to a greater or less extent from their 
 parents are, as we have seen, often attributed in large 
 measure to permutations and combinations of different characters 
 which take place in the sexual process (amphimixis) and the 
 preceding nuclear reduction. It has long been suspected, how- 
 ever, that the germ cells themselves, apparently independently of 
 the body in which they are enclosed, may be influenced by the 
 environment to which an animal or plant is exposed, and the 
 observations of Tower 1 upon beetles of the genus Lcptinotarsa 
 may be referred to in this connection. 
 
 This observer considers that all permanent variations in these 
 beetles, so far as can be discovered, arise in the germ cells and 
 are in no wise the results of inherited somatic modifications. He 
 attributes their appearance to the direct action of the environment 
 upon the germ plasm and supports his views by a series of very 
 interesting experiments. He subjected the parents to environ- 
 mental stimuli of various kinds during the growth and matura- 
 tion of their germ cells, and then, after the ova had been fertilized, 
 allowed the development of the young to take place under normal 
 conditions. The parents, having already reached their final state, 
 were not themselves visibly affected by the stimuli, but a large 
 percentage of the offspring showed surprising modifications 
 which were strictly inherited. These modifications appear to be 
 in no way adaptive. They seem to bear no relation to the nature 
 of the stimulus which calls them forth and to be of no value to 
 the organism in the struggle for existence. 
 
 We may cite one example of Tower's experiment's. Four males 
 and four females of Leptinotarsa decemlineata (the potato beetle) 
 were exposed during the earlier part of the laying period (the 
 eggs being matured and laid in successive batches) to extremely 
 hot, dry conditions accompanied by low atmospheric pressure. 
 The eggs were removed as soon as laid and reared under natural 
 conditions. From 506 larvae thus reared 96 adult beetles were 
 obtained, of which 82 were of a form known as pallida, 2 of a form 
 known as immaculothorax, and the remainder unmodified. During 
 
 1 "An Investigation of Evolution in Chrysomeliil Beetles of the Genus Leptino- 
 tarsa," by William Laurence Tower. (Publications of the Carnegie Institution, 
 Washington, 190(5.)
 
 160 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the later part of the laying period the same parents were kept 
 under normal conditions and yielded 319 eggs from which 61 
 normal beetles were obtained and none of the other forms, and 
 these normal beetles continued to breed true for three generations, 
 after which they were killed. 
 
 The two specimens of the immaculothorax form obtained in 
 the earlier part of the experiment unfortunately died from 
 disease, as also did all but two of the pallida. The remaining 
 two, however, both being male, were crossed with normal 
 females, yielding hybrid offspring with the decemlineaia charac- 
 ters dominant, and these hybrids, breeding inter se, gave off- 
 spring which separated out in a characteristic Mendelian fashion 1 
 into pallida, decemlineaia and hybrids again. There can be no 
 question therefore that the pallida characters, first due to modi- 
 fication of the germ cells by the action of changed environment, 
 were strictly heritable. 
 
 It should be observed that the forms pallida and immaculothorax 
 also occur occasionally, but rarely, in a state of nature as sports 
 or mutations, a fact which suggests that sports or mutations in 
 general may owe their existence to the apparently direct action 
 of the environment upon the germ cells. It is, of course, possible, 
 or even probable, that the change in the environment merely acts 
 as a kind of liberating stimulus, whicli enables characters already 
 latent in the germ cells to express themselves in the developing 
 organism, which, under normal conditions, they are unable to do. 
 
 We shall have to return to the question of the origin of blasto- 
 genic variations in future chapters. 
 
 1 See Chapter XIV.
 
 CHAPTER XII 
 
 Heredity General observations Darwin's theory of pangenesis and 
 Weismai m's theory of the continuity of the germ plasm The nucleus 
 as the bearer of inheritable characters. 
 
 WHEN we study the life histories of the unicellular Protista 
 we find ourselves face to face with the problem of heredity in 
 its simplest form. The Amoeba divides into two parts by simple 
 iission, the division of the cell body being preceded by that of 
 the nucleus. The two daughter cells exactly resemble one 
 another, and, except in point of size, also resemble the parent, 
 while the latter ceases to exist as an individual in the very act of 
 reproduction. Here we may suppose that we are dealing with a 
 division which is qualitative as well as quantitative, that every 
 organ possessed by the parent cell is divided into two similar 
 parts and the totdl inheritance thus fairly apportioned between 
 the offspring, which will therefore be exactly similar to one 
 another and will need only to feed and grow in order to become 
 exactly similar to the parent. 
 
 The young Amoebae may be supposed to resemble the parent 
 because they arise by duplication of the parent and of its organs j 1 
 moreover, there is a perfect continuity of the living substance, or 
 protoplasm, of which the body is composed from one generation 
 to the next, and the whole of the body of the parent is used up 
 in providing the bodies of the offspring. Thus, although the 
 individuality of the parent comes to an end, the Amoeba never 
 dies, for there is never anything left over to die, and, barring 
 accident, it goes on multiplying for ever. We have here a 
 typical illustration of the so-called immortality of the Protista. 
 
 The case is very different amongst the higher plants and 
 animals. Here, as we have already seen, each individual starts 
 
 1 We cannot, however, say this of all Protista, for in many cases the division 
 takes place asymmetrically and entirely new organs have to be formed by one or both 
 of the daughter cells, as, for example, in the transverse h'ssion of Bodo (Fig. 38, D F). 
 I n such cases it looks as if the nucleus might be the real seat of the inherited 
 tendencies, and as if it were able, by its influence, to mould the daughter cell into 
 the fiirni of the parent. 
 
 B. M
 
 162 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 its life as a single cell the fertilized ovum or zygote which is 
 strictly comparable to a unicellular protistan. Like the Amoeba 
 it divides (under favourable circumstances) repeatedly, but the 
 products of division, instead of separating from one another and 
 going each its own way as an independent unicellular organism, 
 all remain together and co-operate with one another to form a 
 multicellular body of greater or less complexity. The cells of 
 which this body is composed become differentiated and specialized 
 in various directions. In so doing the vast majority of them 
 lose their faculty for independent existence, and when their powers 
 of division have become exhausted the tissues into which they 
 are combined become worn out and ultimately die. Thus the 
 body or soma, as a whole, must suffer death sooner or later. The 
 only cells which are, even potentially, exempt from this fate are 
 the germ cells. These, instead of becoming highly specialized 
 constituents of the soma, remain in the condition of more or less 
 independent Protista, and have the power of separating sooner 
 or later from the parent body. - Like most of the Protista 
 they have also the habit of conjugating in pairs and thereby 
 renewing their powers of cell-division, and thus arise the zygotes 
 or fertilized eggs from which the new individuals take their 
 origin. 
 
 The eggs from which the individuals of different kinds of 
 plants and animals arise are for the most part extraordinarily 
 similar to one another. The ovum of a rabbit, as we have 
 already seen, is a minute nucleated mass of protoplasm about 
 ^oth inch in diameter, that of 9, human being is a similar but 
 somewhat larger cell, and if the ova of a hundred different kinds 
 of mammals were mixed together it would be an extremely 
 difficult, if not impossible task to sort them all out, even after 
 the most minute microscopical examination. Even where con- 
 spicuous differences exist, as between the eggs of a mammal and 
 those of a bird, these are due almost entirely to the development 
 of accessory features, such as protective envelopes and food-yolk, 
 which have little to do with the nucleated mass of protoplasm 
 which constitutes the really vital part of the egg. Yet each kind 
 of fertilized egg, if it develop, will give rise to an organism 
 resembling the parent from which it was itself derived. More- 
 over, the resemblance will not be merely a general one, it will be 
 specific, and probably even more than specific, for it may include 
 minute individual characters peculiar to one or other of the
 
 PKE-FORMATION AND EPIGENESIS 163 
 
 parents. Everyone is familiar with cases of this kind. It may 
 he some abnormality of fingers or toes, or a lock of white hair in 
 some special situation in a dark-haired man, or even some 
 trifling nervous habit, that is thus indelibly impressed upon the 
 organism and handed on from one generation to another. 
 
 Inasmuch as the only possible connection between parent and 
 offspring is (in most cases) through the germ cells, it follows that 
 there must be something in these germ cells which, so to speak, 
 represents all the inheritable characters of the parents and is 
 capable of giving rise to a repetition of these characters in the 
 course of individual development. 
 
 Two sharply contrasted views as to what takes place in the 
 development of the egg were maintained by the older embryo- 
 logists, and, in a modified form, survive to the present day. The 
 so-called " evolutionists," or " pre-formationists," maintained 
 that the egg contains in itself a complete miniature of the 
 organism into which it develops, and that tbe process of 
 development consists simply in an unfolding (" evolution ") and 
 growth of this miniature. This idea, of course, carried to its 
 logical conclusion, involves the further supposition that every 
 egg contains in miniature the bodies of all future generations, 
 like nests of boxes one within the other. 
 
 In opposition to this view the upholders of the theory of 
 " epigenesis " maintained that there is no pro-formation of organs 
 in the egg but that the different parts of the adult organism 
 become gradually differentiated from the simple undifferentiated 
 ovum during the course of development. This view, which is 
 said to have originated with Aristotle and was strongly supported 
 by the great pioneer embryologist C. F. Wolff about the middle 
 of the eighteenth century, almost entirely superseded the crude 
 ideas of the pre-formationists, but at the present day the latter 
 are being revived to some extent, but in a more refined form, as a 
 result of modern experiments in embryology. It cannot be 
 disputed that in some cases certain parts of the adult organism 
 can be traced back to corresponding portions of the egg, which 
 cannot therefore be entirely undifferentiated, and it is probable 
 that in the end the truth will be found, as in so many other 
 cases, to lie in a compromise between the two extreme views. 
 
 The great problem which has to be solved by any theory of 
 heredity is How do the apparently simple germ cells of a multi- 
 cellular organism come to be representative of all the other 
 
 M 2
 
 164 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 cells of the body, so that when they develop they will give 
 rise to all those different kinds of cells arranged in the same 
 way as in the parent'? We must now briefly consider some 
 of the numerous attempts which have been made to answer 
 this question. 
 
 Darwin, in 1868, put forward his theory of Pangenesis as a 
 provisional hypothesis to explain the facts of heredity, and this 
 theory, though it seems never to have met with any large 
 measure of acceptance, is of considerable historical interest. 1 
 He supposed that all the constituent cells of which the body is 
 composed not only multiply by ordinary cell-division, so as to 
 build up the various tissues, but also, throughout life, give off 
 extremely minute " gemmules " which wander through the body 
 and are collected in vast numbers in the germ cells. The 
 gemmules, although so small as to be invisible even with the 
 highest powers of the' microscope, are supposed to be capable of 
 absorbing nutriment and multiplying by division, and each one 
 is supposed, in some mysterious and unexplained manner, to 
 represent the particular cell of the body from which it was 
 derived, and to be capable, at the proper time and in the proper 
 place, of impressing the character of its parent cell upon a 
 corresponding cell of the new organism which develops from the 
 germ cell. 
 
 In Darwin's own words : 
 
 "The development of each being, including all the forms of 
 metamorphosis and metagenesis, depends on the presence of 
 gemmules thrown off at each period of life, and on their develop- 
 ment, at a corresponding period, in union with preceding cells. 
 Such cells may be said to be fertilized by the gemmules which 
 come next in due order of development. Thus the act of 
 ordinary impregnation and the development of each part in each 
 being are closely analogous processes. The child, strictly 
 speaking, does not grow into the man, but includes germs which 
 slowly and successively become developed and form the man. In 
 the child, as well as in the adult, each part generates the same 
 part. Inheritance must be looked upon as merely a form of 
 growth, like the self-division of a lowly organized unicellular 
 organism. Reversion depends on the transmission from the 
 forefather to his descendants of dormant gemmules, which 
 occasionally become developed under certain known or unknown 
 conditions. Each animal and plant may be compared with a bed 
 
 1 For a complete exposition of the theory see Darwin's "Animals and Plants 
 under Domestication" (Ed. 2, Vol. II., Chapter XXVJI.).
 
 PANGENESIS 165 
 
 of soil full of seeds, some of which soon germinate, some lie 
 dormant for a period, whilst others perish." l 
 
 Darwin did not recognize the modern distinction between 
 somatogenic characters, which are acquired by the body or soma 
 during its individual life-time, and blastogenic or germinal 
 characters, which are supposed to originate in the germ cells ; or 
 rather, in accordance with his theory of pangenesis, he believed 
 that somatogenic modifications might be transferred to the germ 
 cells and thus become blastogenic. In other words, he was a 
 firm believer in the inheritance of acquired characters, a doctrine 
 which, as we shall see presently, is now much discredited, and 
 he endeavoured to explain by means of his theory how such 
 characters may be transmitted from parent to offspring. 
 
 Suppose some part of the body in a particular multicellular 
 individual were to become modified by use or disuse, or by the 
 direct action of the environment. Then the gemmules given off 
 from the modified cells would also be affected in a corresponding 
 manner and would carry information of the change to the germ 
 cells. It would be as if some constituency with many repre- 
 sentatives changed its political opinions and instead of sending 
 conservative members to the House of Commons took to sending 
 liberals. When the proper time came the new representatives 
 would vote according to their instructions ; but we must also 
 suppose that the old ones could never be turned out and that 
 there would be a struggle between the two. At first the old ones 
 would be the more numerous and would outvote the new ones; 
 presently, however, the new ones, being constantly reinforced, 
 would come to outnumber the old ones and perhaps be able to 
 give effect to the altered views of their constituenc}^. As Darwin 
 says, " It is generally necessary that an organism should be 
 exposed during several generations to changed conditions or 
 habits, in order that any modification thus acquired should 
 appear in the offspring." It would probably be more in accord 
 with the facts if, instead of " several generations " we said 
 " a large number of generations." In this sense, we may 
 well believe that acquired characters can be inherited, without 
 expecting to be able to demonstrate such inheritance by cutting 
 off the tails of a few generations of mice. 
 
 The theory of pangenesis certainly explains a great deal, but 
 it involves so many improvable assumptions as to the nature 
 1 Luc. /;/.. !.. :?<)S-.
 
 166 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 and behaviour of the " gemmules " that it cannot be accepted as 
 more than what Darwin himself termed it, a provisional 
 hypothesis or speculation. 
 
 It is interesting to observe that Darwin finds the cause of 
 variation in the direct influence of the environment, including 
 under that term the effects of use and disuse upon the organism. 
 In this respect he agrees with the views of Lamarck and differs 
 widely from those of Weismann and many other modern 
 biologists, who deny, either totally or in part, the possibility of 
 the inheritance of acquired or somatogenic characters. It 
 will have been noticed that the theory of pangenesis is of an 
 essentially pre-formationist character, for it assumes the existence, 
 within the fertilized egg, of an immense number of material 
 particles (gemmules) which in some way or other represent the 
 different inheritable characters of the body. 
 
 The celebrated theory of heredity which we owe to Professor 
 Weismann 1 is based upon what he terms the " Continuity of 
 the Germ Plasm." The general idea of continuity is, of course, 
 by 110 means a new one ; indeed the protoplasmic continuity of 
 parent and offspring, through the germ cells, must form the 
 material basis for the transmission of characters on any theory 
 of heredity. Weismann, however, gives much greater precision 
 to the idea than any of his predecessors. He identifies the 
 chromatin of the nucleus as the actual hereditary substance, the 
 bearer of all inherited tendencies, and draws a very sharp dis- 
 tinction between the somatic cells which, with almost endless 
 diversity of form and function, build up the body of one of the 
 higher plants or animals, and the germ cells, which play little if 
 any part in the life of the individual in which they are lodged 
 but are destined, under favourable circumstances, to give rise to 
 the next generation. 
 
 We have already seen that the germ cells are frequently 
 separated from the somatic cells at a very early stage in develop- 
 ment. It may be that the distinction between the two is actually 
 inaugurated by the very first division of the fertilized ovum, as 
 in the horse worm, Ascariv mecfalocephala (p. 129), or it may be 
 recognizable at the gastrula stage, as in the arrow worm, Sagitta 
 
 1 For full accounts of t his theory see the English translations of Weisniann's two 
 chief works on the subject, "The Oerrn Plasm " (Contemporary Science Series, 
 1893) and "The Evolution Theorv " ( Kdward Arnold, 1<)<>4).
 
 WEISMANN'S THEORY OF HEREDITY 167 
 
 (p. 130), but such cases appear to be very exceptional and the 
 segregation of the germ cells usually takes place much later and at 
 different stages of the development in different species of plants 
 and animals. The exact time at which the separation of the two 
 groups of cells takes place, however, does not seriously affect the 
 argument. In any case the ultimate distinction between germ 
 cells and somatic cells is supposed to lie in the fact that the 
 former retain each a complete sample of the ancestral germ 
 plasm, in which at any rate all the essential characters of the 
 organism are in some way or other represented, while the latter, 
 by a series of differential divisions, gradually undergo a further 
 segregation into the different tissue cells of the adult, each of 
 which contains (in an active condition) only a sample of that 
 part of the ancestral germ plasm which is appropriate to its own 
 particular requirements. 
 
 Hence the germ cells, complete in themselves like so many 
 Protista, alone retain the power of giving rise to new generations 
 of complete individuals. The somatic cells have sacrificed this 
 power to their need for specialization. Certain phenomena, 
 however, such as the regeneration of lost parts in many animals 
 and the propagation of plants by buds and cuttings, necessitate 
 the supposition that some at any rate of the somatic cells must 
 retain a more complete sample of the ancestral germ plasm than 
 is necessary for their own development. For the hereditary sub- 
 stance of any particular cell Weismann adopts Nageli's term 
 " idioplasm," and in order to account for the phenomena just 
 referred to he is obliged to postulate the existence, in the 
 somatic cells in question, of " accessory idioplasm," which is only 
 called into activity under exceptional conditions, as, for example, 
 when it becomes necessary for a crab to regenerate a lost limb 
 or for an earthworm to renew a portion of its body which has 
 been bitten off by a bird or chopped off by a spade. 
 
 Weismann's theory involves the assumption of great complexity 
 of structure in the germ plasm, which, as we have already 
 seen, he identifies with the chromatin substance of the nucleus 
 of the germ cells. He finds it necessary to assume the existence, 
 not only of " determinants," which correspond more or less closely 
 to Darwin's gemmules, and each of which is supposed to be 
 responsible for the development of some special inherited feature 
 of the organism, but also of structural units respectively of a 
 lower and a higher order. Thus each determinant is supposed
 
 1G8 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 to be made up of " biophors," which are themselves the lowest 
 vital units, but each of which is in turn made up of molecules in 
 the chemical sense of the term ; while, on the other hand, the 
 determinants are supposed to be grouped in " ids," each of which 
 is a complete ancestral germ plasm, theoretically sufficient in 
 itself to determine all the different characters of an entire 
 individual. The ids may in some instances correspond to the 
 chromosomes, but these appear generally to be composite bodies 
 (" idants ") made up each of a large number of ids (chromomeres). 
 The determinants, and of course the biophors also, are far below 
 the limits of visibility even with the aid of the most powerful 
 microscope ; the ids, however, frequently appear during the 
 process of mitosis and may give the spireme thread or the chro- 
 mosomes into which it divides a characteristic beaded appearance 
 (Fig. 32, B). Biophors, determinants, ids and idants must all be 
 looked upon as living entities, growing by the absorption of 
 nutriment and multiplying by division. 
 
 In accordance with Weismann's theory the germ cells them- 
 selves may be regarded as so many unicellular organisms, which 
 multiply by fission and periodically, if they chance to meet with 
 mates, conjugate with one another. Their cytoplasm as well as 
 their chromatin is directly continuous from generation to genera- 
 tion just' as it is in a dividing Amoeba, and theoretically there is 
 no reason why the constant succession of germ cells should ever 
 be interrupted by death. The soma, or body, however, stands in 
 a very different position. It may be regarded as a kind of 
 appendage thrown off from the chain of germ cells after each 
 conjugation, and resulting from the fact that most of the cells 
 arising from the segmentation of the zygote not only remain 
 together in intimate association with one another but become 
 specialized in various directions and co-operate with one another 
 to form a complex multicellular individual. Having exhausted 
 its powers of growth and renewal this individual body sooner or 
 later dies ; but the germ cells periodically renew their powers of 
 cell-division by conjugation and, under favourable conditions, go 
 on for ever. 
 
 According to Weismann, inherited characters are transmitted 
 not from soma to soma but from germ cell to germ cell, by virtue 
 f the continuity of the germ plasm. The soma has little if any 
 influence upon the germ cells which it contains beyond that which 
 is involved in supplying them with protection and nourishment.
 
 WEISMANN'S THEORY OF HEREDITY 169 
 
 There is no other means of communication between the soma 
 and the germ cells, and hence somatogenic characters, which 
 are acquired in the life-time of the individual body as the direct 
 result of the action of the environment (including use and disuse 
 of organs), cannot be transmitted to the germ cells and therefore 
 cannot be inherited. The only characters which can be inherited 
 are blastogenic characters, which arise by modification of the 
 germ plasm in the germ cells themselves. 
 
 This denial of the transmission of so-called acquired charac- 
 ters constitutes the most important difference between the 
 theories of heredity propounded by Weismann and Darwin. 
 Both these theories postulate the existence of ultra-microscopical 
 material particles, determinants or gemmules, but Weismann's 
 theory allows of no transference of such particles from soma to 
 germ cell, only from germ cell to germ cell and from germ cell to 
 soma. There is supposed to be no mechanism for the trans- 
 mission of somatogenic characters to the next generation. Accord- 
 ing to the older view the germ cells give rise to the soma and the 
 soma to the germ cells alternately. According to the newer one 
 the germ cells give rise to the soma and at the same time to the 
 next generation of germ cells, while the soma gives rise to nothing 
 but itself and ultimately perishes. 
 
 The contrast between the two views is clearly expressed in the 
 accompanying diagram (Fig. 76), in which, for the sake of sim- 
 plicity, the complication introduced by the process of conjugation 
 of the germ cells has been ignored. 
 
 The inheritance of somatogenic characters being denied, Weis- 
 mann is obliged to seek the origin of variations from some 
 source other than the action of the environment and use and 
 disuse. We shall return to this point presently.. In the meantime 
 we must point out that Weismann's theory harmonizes very well 
 with the phenomena of mitosis, and especially with the remarkable 
 modifications of those phenomena which accompany the matura- 
 tion of the germ cells. 
 
 The entire process of mitosis serves to emphasize the import- 
 ance of the chromatin substance of the nucleus. It is evidently 
 of the utmost consequence that this substance should be accur- 
 ately apportioned between the daughter cells. We accordingly 
 find the elaborate mechanism of centrosomes and nuclear spindle, 
 and a splitting of each individual chromosome, which takes place 
 longitudinally when the chromosomes themselves happen to be
 
 170 OUTLINES OF E VOLUTION AEY BIOLOGY 
 
 elongated in form. If, as Weisrnann maintains, and as can 
 actually be demonstrated in many cases, each elongated chromo- 
 some is made up of a row of chromomeres or ids, which may be 
 supposed to differ to some extent from one another as to the 
 determinants which they contain, it is obvious that longitudinal 
 splitting of the chromosome is the only way in which a qualita- 
 tive as opposed to a mere quantitative division of the hereditary 
 
 FIG. 76. Diagram to illustrate the contrast between Darwin's Theory of 
 Pangenesis and Weismann's Theory of the Contiimity of the Germ 
 Plasm. 
 
 The figures represent an imaginary organism with four processes given off from the soma 
 or body, which is supposed to contain only a single germ cell (dotted). Three genera- 
 tions are represented, and for the sake of simplicity the complication introduced 
 by the periodical conjugation of male and female germ cells is omitted. Figs. A, 
 B, C show how an acquired character the elongation of one of the processes as a 
 result of its use for some special purpose may be supposed to affect, the^germ cells 
 through the migration of gemmules (indicated by the small arrows), and thus be 
 transmitted by heredity in accordance with the theory of Pangenesis. Figs. A', B', 
 C' show how such an acquired character is, in accordance with Weismann's theory, 
 unable to make any impression upon the germ cells and is therefore not transmitted 
 by heredity. In the first case the germ cells of each generation are supposed to arise 
 from the 'soma; in the second case they are supposed to arise directly from the 
 preceding generation of germ cells, which also gives rise to the soma in which they 
 are enclosed, as indicated by the large arrows. 
 
 substance can be brought about. This mode of division, then, 
 otherwise difficult to explain, is fully intelligible in accordance 
 with Weismann's theory. 
 
 It is not necessary to suppose, however, that such division 
 always results in identical daughter chromosomes. We may 
 assume either that all the individual ids divide into similar 
 halves, containing similar determinants (as represented very 
 diagrammatically in Fig. 77, A), in which case the two daughter 
 chromosomes will be exactly alike, or that some or all of the ids
 
 WEISMANN'S THEORY OF HEREDITY 
 
 171 
 
 divide each into two dissimilar halves containing different deter- 
 minants (Fig. 77, B), when the daughter chromosomes will be 
 unlike each other. In the former case the division is said to he 
 integral and in the latter differential. It is by differential division 
 that Weismann believes the histological differentiation of the 
 soma to be brought about. 
 
 When we consider the phenomena of maturation and fer- 
 tilization we find them in no less striking harmony with 
 Weismann's views. We have already pointed out, in Chapter X., 
 that each particular species of plant or 
 animal is, as a general rule, characterized 
 by the appearance of a definite and con- 
 stant number of chromosomes in all the 
 cells of the body during the process of 
 mitosis. At some period in the life-cycle, 
 however, in typical plants during the 
 process of spore-formation and in animals 
 during the maturation of the ova and 
 spermatozoa, this number is reduced to 
 half by separation of the entire chromo- 
 somes into two groups, one of which 
 passes to each of two daughter cells. 
 Thus the mature germ cells have only 
 half the number of chromosomes charac- 
 teristic of the species (or, in the case of 
 typical plants, of the sporophyte genera- 
 tion). The full number is made up again 
 by the union of male and female gametes 
 to form the zygote or fertilized egg. 
 
 To the combination of the maternal and paternal chromosomes 
 in the nucleus of the zygote Weismann has given the name 
 amphimixis, and he sees in this mingling of ancestral germ 
 plasms the cause of that mixture of paternal and maternal 
 characters which we commonly find in animals nnd plants. If, for 
 the sake of simplicity, we imagine that each chromosome consists 
 (as appears to be sometimes the case) of only a single id or 
 chromomere, and that eight of these are present in the nucleus of 
 the mature germ cell, we may represent the effect of repeated 
 amphimixis upon the constitution of the nucleus by means 
 of the diagram (Fig. 78), which shows how the ids must become 
 more and more diversified in each successive generation. In this 
 
 FIG. 77. Diagram of 
 (A) integral and (B) 
 differential Division 
 of a Chromosome con- 
 sisting of five Ids or 
 Chromomeres.
 
 172 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 diagram A represents an unreduced nucleus composed of sixteen 
 ids, eight paternal and eight maternal, all the paternal and all the 
 maternal ids respectively heing supposed to be alike. After 
 reduction and union with another mature germ cell containing 
 also only one kind each of paternal and maternal ids, but both 
 differing in some respect from those of its mate, the nucleus of 
 the next generation will contain four kinds of ids, two paternal 
 
 FIG. 78. Diagram illustrating the Composition of the Germ Plasm 
 (Chromatin Substance of the Nucleus) out of Ancestral Ids, and the 
 Effect thereon of repeated Amphimixis. (From Weismann's " Evolution 
 Theory.") 
 
 A D the unreduced nucleus, containing sixteen chromosomes (each consisting of a 
 single id), of four successive generations. In A the germ plasm consists of only two 
 kinds of ids ; in I? of four ; in C of eight, and in D of sixteen. mJ, pJ, maternal 
 and paternal ids. 
 
 and two maternal, as shown in B, while in the next generation 
 there ma}^ be eight kinds, as shown in C, and in the next sixteen, 
 as in D. 
 
 We must, then, in accordance with the views of Weismann, 
 look upon the germ plasm of any one of the higher organisms as 
 being made up of a larger or smaller number of ids, each one 
 representing the inheritance received from some more or less 
 remote ancestor on the paternal or maternal side, though of course
 
 WEISMANN'S THEORY OF HEKEDITY 173 
 
 it is quite possible that there may be a number of ids of the same 
 kind. When, during the process of maturation, half the total 
 number of chromosomes (each made up of one or more ids) are 
 eliminated from the germ cell, it appears to be largely a matter 
 of chance which shall go and which shall remain, and the nature 
 of the new combination of ids resulting from the process of 
 amphimixis must also be a matter of chance, depending upon 
 what luck the germ cells happen to have in their mating. 
 
 As has been well said, a new shuffling of the cards must take 
 place in each generation. The characters of the organism 
 developed from any zygote will depend upon the hand dealt out 
 to it in the processes of reduction and amphimixis, and as it can 
 rarely, if ever, happen that any two hands will be exactly alike, so 
 it will rarely, if ever, happen that any two organisms, however 
 closely related, will exactly resemble one another in all their 
 characters. Indeed, the only cases in which even an approxima- 
 tion to exact resemblance is known, at any rate amongst the 
 higher animals, are those of " identical " twins, and the explana- 
 tion of these is that they have arisen by an integral division of a 
 single fertilized ovum, followed by separation of the first two 
 daughter cells or blastomeres thus produced. 
 
 We therefore find in the processes of reduction and amphimixis, 
 in the permutation and combination of ancestral characters, an 
 abundant source of variation. This, however, is not supposed 
 to explain fully the origin of variations, and Weismann accord- 
 ingly invokes the aid of another hypothesis, his theory of 
 " Germinal Selection." In accordance with this theory the 
 determinants of which the ids are composed are supposed to be 
 differently situated with regard to their facilities for obtaining the 
 nutriment necessary for their growth and multiplication. There 
 is a kind of struggle for existence going on amongst them. Those 
 which are more successful in obtaining supplies, having once got 
 a start, will tend to supplant those which are less successful. 
 Some will become weaker and some stronger, and thus, as the 
 result of differences in nutrition, variation is set up amongst the 
 determinants themselves. If these vary it follows that their 
 determinates, or the parts which they control in the developing 
 organism, will vary also. 1 
 
 1 There would seem, however, to be a serious objection to the theory of germinal 
 selection in the fact that the nucleus of any given germ cell contains many ids, and 
 that similar determinants must as a rule recur in each id. We can hardly suppose 
 that the corresponding determinants in each id are always subject to precisely the
 
 174 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 In this way Weismann seeks to avoid the necessity of believing 
 in the transmission from parent to offspring of modifications 
 which result from the direct action of the environment upon the 
 body itself, without, however, altogether denying that external 
 influences, and especially nutrition, may act upon the germ plasm 
 through the body and thus cause modifications in the offspring. 
 
 We have seen that Weismann's theory of the continuity of the 
 germ plasm involves the acceptance of the chromatin of the 
 nucleus as the actual material basis of hereditary transmission. 
 There can be no doubt that the phenomena of ordinary mitosis 
 in the case of tissue cells, and those of maturation and fertiliza- 
 tion in the case of the germ cells, strongly support this view, 
 while the simple fact that the spermatozoon, consisting almost 
 entirely of chromatin substance and with a minimum of cyto- 
 plasm, contributes equally with the ovum to the characters of the 
 offspring in normal cases, seems almost conclusive as to the pre- 
 dominating importance of the chromatin in this respect. Some 
 observers, however, still maintain that the cytoplasm plays a very 
 important part in heredity. 
 
 It is probable that much light will be thrown upon this 
 question by the development of that extremely important branch 
 of biological science known as experimental embryology, which is 
 as yet in its infancy. We have already pointed out that in certain 
 cases eggs containing no nucleus may be fertilized by spermatozoa, 
 and may then develop up to a certain point, and that this process 
 is termed merogony. Some years ago Boveri claimed to have 
 fertilized enucleate fragments of the eggs of one genus of sea- 
 urchins (Sphaerechinus) with the sperm of another genus 
 (Echinus), and obtained larvae with only paternal characters. 
 He concluded from this experiment that the nuclear substance is 
 alone responsible for the transmission of inherited characters. 
 Unfortunately it seems that his results are open to a different 
 interpretation, and they have been severely criticized. They 
 certainly cannot be regarded as by any means conclusive. 
 
 More recently Godlewski has succeeded in fertilizing eggs of 
 the common sea-urchin (Echinus) with sperm of the feather star 
 (Antedon), belonging not only to a distinct genus but to a widely 
 
 same advantages or disadvantages of position. It seems much more likely that 
 variations in this res-pect in different ids would tend to neutralize one another, the 
 kind of determinant which is unfavourably situated in one id being favourably 
 situated in another, so that each kind would, on an average, have the same chance 
 of nutrition.
 
 EXPERIMENTS IN HEREDITY 175 
 
 different order of echinoderms. The larvae of these two types 
 are easily distinguished even in early stages of development. The 
 larvae produced by fertilization of normal eggs of Echinus with 
 sperm of Antedon are said to be of the maternal type. This 
 result is in itself very remarkable, but Godlewski was also able to 
 fertilize enucleate egg fragments of the sea-urchin with sperm of 
 the feather star. Eight larvae produced in this way reached the 
 blastula stage, but only four developed as far as the gastrula. 
 These four, however, were again of the maternal type, and could 
 only be distinguished by their size from those of the pure Echinus 
 culture. From these experiments Godlewski concludes that 
 cytoplasm as well as chro matin must be concerned in the trans- 
 mission of hereditary characters, for no maternal chromatin was 
 present in the eggs from which the larvae developed. Whether 
 these results, which are in direct opposition to those of Boveri, 
 will be confirmed or refuted by further observations remains to 
 be seen. Walker, in his recent work on heredity, 1 has accepted 
 Godlewski's conclusions and made use of them in support of the 
 theory that the chromosomes are the bearers of individual 
 characters only, while racial characters may be transmitted by 
 " the whole protoplasm of the cell." 
 
 1 " Hereditary Characters and their Modes of Transmission," by C. E. Walker 
 (London, Edward Arnold, 1910).
 
 CHAPTER XIII 
 
 The inheritance of acquired characters and the mnemic theory of heredity. 
 
 No biological question during the last fifty years has given rise 
 to more acute and vigorous controversy than that of the inherit- 
 ance or non-inheritance of " acquired " characters. The following 
 paragraph may be quoted to show the manner in which Weismann 
 stated the case and endeavoured to give precision to the 
 terminology employed : 
 
 " By acquired characters I mean those which are not preformed 
 in the germ, but which arise only through special influences 
 affecting the body or individual parts of it. They are due to the 
 reaction of these parts to any external influences apart from the 
 necessary conditions for development. I have called them ' somato- 
 genic ' characters, because they are produced by the reaction of 
 the body or soma, and I contrast them with the ' blastogenic ' 
 characters of an individual, or those which originate solely in 
 the primary constituents of the germ (' Keimesanlagen '). It is an 
 inevitable consequence of the theory of the germ-plasm, and of 
 its present elaboration and extension so as to include the doctrine 
 of determinants, that somatogenic variations are not transmissible, 
 and that consequently every permanent variation proceeds from 
 the germ, in which it must be represented by a modification of 
 the primary constituents." l 
 
 As already pointed out, the view here expressed is directly 
 opposed to that of Lamarck and Darwin, who believed that 
 characters acquired in the life-time of the individual, either as a 
 result of the use or disuse of organs or of the direct action of the 
 environment, might be handed on by heredity from parent to 
 offspring, and Darwin's theory of pangenesis was essentially an 
 endeavour to imagine some mechanism by which such trans- 
 ference of acquired characters might be effected. 
 
 We may at once agree with Weismann that blastogenic 
 characters alone are transmitted from parent to offspring, but the 
 real question is Can a somatogenic character be converted into, 
 
 1 " The Germ-Plasm : A Theory of Heredity," by August Weismann (Con- 
 temporary Science Series, 1893), p. 31)2.
 
 INHERITANCE OF ACQUIRED CHARACTERS 177 
 
 or give rise to, a blastogenic one? In other words Can a 
 modification of the body or soma, arising in the life-time of the 
 individual and itself in no way due to inheritance, affect the 
 germ cells in such a way that the offspring developed from 
 them will exhibit a corresponding modification of its soma ? 
 
 It is useless either to deny or to assert the possibility of the 
 inheritance of such characters on any purely a priori grounds. 
 The fact that no satisfactory mechanism for the transference of 
 such characters from parent to offspring has yet been demon- 
 strated does not justify us in denying the possibility of such 
 transference. Our decision must depend upon an unbiassed 
 examination of the evidence which can be brought forward on 
 each side. 
 
 It is, of course, obvious that inasmuch as any organism differs 
 to a greater or less extent from its ancestors, the differences 
 being as a general rule greater in proportion to the remoteness 
 of the particular ancestor with which it is compared, the 
 differentiating characters must have been acquired, in the 
 ordinary sense of the word, during the interval which separates 
 the two generations in question. For example, there can be no 
 reasonable doubt that birds are descended from ancestors which 
 were reptilian in character and had no feathers. Feathers have 
 unquestionably been acquired somehow or other during the pro- 
 gress of the bird's evolution. This, however, is not the sort of 
 acquisition the inheritance of which is in dispute. Weismann and 
 his followers would deny altogether that feathers originated as 
 somatogenic characters ; they would say that certain apparently 
 fortuitous modifications in the constitution of the germ cells 
 themselves were responsible for the first appearance of feathers 
 probably in an extremely rudimentary form and that this new 
 character proving to be of value in the struggle for existence was 
 preserved and fostered by natural selection until after a long 
 process of evolution the elaborate plumage of existing birds was 
 perfected. 
 
 In striking contrast to such a case as the above we have 
 innumerable cases of the more or less sudden appearance of 
 somatic characters during the life-time of an individual as the 
 obvious result of the action of some external or environmental 
 influence, or of the use or disuse of some organ by its possessor, 
 and it is to such cases that Weismann and his followers would, 
 rightly or wrongly, confine the discussion. 
 
 B. N
 
 178 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Artificially or accidentally produced mutilations afford a very 
 good example of such obviously somatogenic characters, and with 
 regard to the inheritance of these Weismann's position is clearly 
 summed up in the following passage : " As far hack as the 
 eighteenth century the great philosopher Kant, and in our own 
 day the anatomist Wilhelm His, gave their verdict decidedly 
 against such allegations, and absolutely denied any inheritance 
 of mutilations ; and now, after a decade or more of lively debate 
 over the pros and cons, combined with detailed anatomical 
 investigations, careful testing of individual cases, and experi- 
 ment, we are in a position to give a decided negative and say 
 there is no inheritance of mutilations." * It is, however, as 
 already pointed out, all a question of evidence, and we may here 
 quote a definite case in order to show the nature of the evidence 
 with which we have to deal : 
 
 " A female (and very prolific) cat, when about half-grown, met 
 with an accident. ' Her fine, long tail was trodden on and had 
 a compound fracture, two vertebra? being so displaced that they 
 ever after formed a short offset between the near and far end of 
 the tail, leaving the two out of line. At first I noticed that out 
 of every litter of kittens some had a tail with a querl in it.' With 
 successive litters the deformity increased, until ' not a kitten of 
 the old cat had a straight tail, and it grew worse in her progeny 
 until now we have not a cat with a normal tail on the premises ' 
 (in a cat-population of six or eight, exclusive of young kittens). 
 ' The tails are now in part mere stumps, some have a semicircular 
 sweep sideways, and some have the original querl. Perhaps the 
 deformity was somewhat aggravated by in-and-in breeding and 
 by artificial selection practised by my Chinaman, who, with the 
 perversity of his race, preferred the crooked tails, and thus 
 preserved them in preference to the normal kittens. There are 
 no other abnormally-tailed cats in the neighbourhood.' " 
 
 Professor Brewer quotes this remarkable case 2 from " that 
 keen observer and eminent scientist, Professor Eugene W. 
 Hilgard of the University of California," along with others of a 
 like kind from various sources. 
 
 It is of course essential to the stability of Weismann's theory 
 as a whole that evidence such as this should be rebutted. I am 
 not aware that he has anywhere criticized this particular case, 
 but his general remarks in somewhat similar cases may be 
 quoted with advantage in this connection : 
 
 i "The Evolution Theory" (London, 1904), Vol. II., p. fiS. 
 
 a V'ule Cope's " Primary Factors of Organic Evolution " (Chicago, 189G), pp. 432-3.
 
 INHERITANCE OF ACQUIRED CHARACTERS 179 
 
 " In the first place, the assertion that congenital stump-tails 
 in dogs and cats depended on inherited mutilation proved to be 
 unfounded. In none of the cases of stump-tails brought forward 
 could it even be proved that the tail of the relevant parent had 
 been torn or cut off, much less that the occurrence, in parents or 
 grandparents, of short tails from internal causes was excluded. 
 At the same time anatomical investigation of such stump-tails as 
 occur in cats in the Isle of Man, and in many Japanese cats, and 
 are frequently found in the most diverse breeds of dogs, showed 
 that these had, in their structure, nothing in common with the 
 remains of a tail that had been cut off, but were spontaneous 
 degenerations of the whole tail, and are thus deformed tails, not 
 shortened ones (Bonnet). 
 
 " Experiments on mice also showed that the cutting off of the 
 tail, even when performed on both parents, does not bring about 
 the slightest diminution in the length of tail in the descendants. 
 I have myself instituted experiments of this kind, and carried 
 them out through twenty-two successive generations, without any 
 positive result. Corroborative results of these experiments on 
 mice have been communicated by Ritzema Bos and, indepen- 
 dently, by Rosenthal, and a corresponding series of experiments 
 on rats, which these two investigators carried out, yielded the 
 same negative results. 
 
 " When we remember that all the cases which have been 
 brought forward in support of an inheritance of mutilations 
 refer to a single injury to one parent, while, in the experiments, 
 the same mutilation was inflicted on both parents through 
 numerous generations, we must regard these experiments as a 
 proof that all earlier statements were based either on a fallacy 
 or on fortuitous coincidence. This conclusion is confirmed by 
 all that we know otherwise of the effects of oft-repeated mutila- 
 tions, as for instance the well-known mutilations and distortions 
 which many peoples have practised for long, sometimes incon- 
 ceivably long, ages on their children, especially circumcision, the 
 breaking of the incisors, the boring of holes in lip, ear, or nose, 
 and so forth. No child of any of these races has ever been 
 brought into the world with one of these marks ; they have to be 
 re-impressed on every generation." * 
 
 It cannot be seriously questioned that in the majority of cases 
 mutilations are not visibly inherited, but the fact that no one has 
 as yet succeeded in producing experimentally an inheritable 
 mutilation does not prove that such never occur accidentally. 
 
 Weismann's arguments can hardly be regarded as conclusive 
 against such strong evidence as that afforded by Professor Hilgard's 
 
 1 \Vei.smann, -'Tlie Evolution Theorj^ v (London, l'J04), Vol. II., pp. <!5-G. 
 
 N 2
 
 180 OUTLINES OP EVOLUTIONARY BIOLOGY 
 
 cats. Moreover, there appears to be considerable difference of 
 opinion as to whether or not the effects of circumcision are ever 
 inherited. Darwin, in discussing this point, 1 admits that it is 
 possible that all the recorded cases of apparent inheritance of 
 such effects may be accidental coincidences, but the fact that 
 there are such recorded cases prevents us from accepting the 
 sweeping generalization which Weismann makes in the last 
 sentence of the paragraph above quoted. 
 
 Darwin himself was convinced that the effects of operations 
 are sometimes inherited by the well known and often quoted case 
 of Dr. Brown-Sequard's guinea-pigs. In these animals not only 
 did epilepsy appear in the offspring of parents which had been 
 rendered epileptic by injury to the nervous system, but certain 
 structural modifications (e.fj. loss of toes) appeared in the 
 offspring when corresponding structural modifications had been 
 produced in the parent (indirectly) by similar injury. Weismann, 
 however, endeavours very ingeniously to explain away these 
 observations, and the correctness of Brown-Sequard's results has 
 been seriously impugned. 
 
 It cannot be denied, however, that the primdfacie evidence for 
 the occasional transmission of such characters as those produced 
 by mutilation is very strong. We may allow much for coinci- 
 dence, but well authenticated cases like that of the cats above 
 referred to, and like the following, are too numerous to be all 
 explained away in this manner : 
 
 " A person, when a boy of ten years, cut the terminal 
 phalange of the little finger of his left hand with a sickle. The 
 joint was not injured, nor was the function of the finger seriously 
 impaired. There was, however, an obvious deformity. The 
 finger was ill-shaped and crooked, and the nail abnormal. He 
 married and had two children, the first a son, with normal 
 fingers, the second a daughter, who had the little finger of the 
 corresponding (the left) hand deformed from birth in the same 
 manner. The function of the finger was not seriously injured, 
 but the deformity was precisely the same in shape, even to the 
 malformation of the finger-nail. She died at thirty, without 
 children, consequently no observation on a succeeding generation 
 could be noted. None of his other kindred had malformed 
 fingers, nor had any ancestor of the child for at least three 
 generations, and there was no knowledge of any such in the 
 more remote ancestry. 
 
 1 - Animals and Plants under Domestication " (2nd Ed., 1882), Vol. I., p. 467.
 
 INHERITANCE OF ACQUIRED CHARACTERS 181 
 
 " (This case was related to me in full detail by the father with 
 the deformed finger, and with whom I was personally acquainted. 
 He was an eminent physician, the president of a large and 
 reputable medical college, and his name is well known to the 
 
 profession.) " l 
 
 It seems only reasonable, then, to admit that suddenly 
 acquired somatogenic characters are occasionally, though probably 
 very rarely, transmitted by heredity in a sufficiently pronounced 
 form to be recognizable, but even if this were not so we should 
 not be justified in concluding that all the characters which an 
 organism exhibits have their origin in the first instance in modi- 
 fications of the germ plasm. It must be remembered that a great 
 many somatogenic modifications are the result of what we may 
 call the normal action of the environment, and that such action 
 may extend over very long periods of time, in which many 
 thousands of generations may be produced. In such cases the 
 stimulus, whatever it may be, to which the organism responds by 
 modification in bodily structure, is repeated a vast number of 
 times, and therefore seems much more likely to produce an 
 inheritable effect than a single accidental or experimental injury. 
 A single drop of water falling on a stone makes no visible 
 impression, but if the dropping goes on for a long time the stone 
 will gradually be worn away, and it is by no means essential to a 
 belief in the heritability of acquired characters that such 
 characters should be immediately inherited in full perfection. 
 It may be merely a question of time. 
 
 It is a well known fact that the " habit " and even the 
 structure of a plant are largely determined by the conditions 
 under which its growth takes place. 2 A plant growing in a hot- 
 house may acquire a very different habit from another of the 
 same species growing in the open air. Many alpine or sub-alpine 
 plants have become adapted to their peculiar environment by 
 various highly characteristic modifications, amongst which a 
 reduction in the size of the leaves is one of the most conspicuous, 
 and such plants may be induced to change their mode of growth 
 by simply removing them to sufficiently warm and sheltered 
 situations, in which their habit comes to approach that of their 
 lowland relatives. In such cases it is difficult to say how far 
 
 1 This is another of the striking cases collet-ted by Professor Brewer and quoted 
 in Cdpc's " Primary Factors of Organic Evolution," pp. 433-4. 
 
 2 The reader should consult the quotations from Lamarck bearing upon this point 
 given in Chapter XXIV., especially the case of Ramtitntliis /t</t/<itilig.
 
 182 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the habit of the plant, whether alpine or lowland, is really 
 inherited, or how far it may he produced afresh in each generation 
 as a direct response to environmental stimuli. 
 
 It appears, however, from the observations of Bordage, 1 that 
 peach trees in the climate of Reunion gradually acquire an 
 almost evergreen habit, and that this character is hereditarily 
 transmitted, being exhibited by seedlings of the modified trees 
 when grown in situations where the peach is usually deciduous. 
 Dr. Bordage considers, and it appears to us that he has good 
 reason for so doing, that his observations and experiments defi- 
 nitely prove, in the case of plants, the hereditary transmission 
 of characters acquired under the influence of a change of climate. 
 
 To take another example, it has been shown that in rats and 
 mice certain modifications in bodily structure are produced as the 
 result of raising or lowering the temperature to which the young 
 animal is exposed during its growth, and such modifications appear 
 to be inherited. Sumner 2 found, as the result of a large number 
 of careful measurements, that mice reared in a warm room (about 
 21 C.) differed considerably from those reared in a cold room 
 (about 5 C.) as regards the mean length of the tail, foot and ear, 
 which were longer in the former than in the latter. The same 
 differences occurred to a recognizable extent in the offspring of 
 the warm room and cold room parents, although these offspring 
 were all reared together in a common room under identical 
 temperature conditions. 
 
 Observations such as these, which are rapidly accumulating, 
 lead us to hope that the question of the inheritance or non- 
 inheritance of somatogenic characters which have undoubtedly 
 arisen in response to the direct action of the environment 
 may, before long, be answered conclusively. They doubtless require 
 confirmation and extension, but they afford very strong evidence 
 in favour of the inheritance of such characters. It has been sug- 
 gested that in such cases the stimulus of changed conditions affects 
 the body and the germ cells simultaneously and in a parallel 
 manner, rather than that the body is modified first and then in- 
 fluences the germ cells, but such a suggestion seems like a last 
 attempt to avoid at all costs the necessity for believing in the 
 
 1 Kdmond Bordage, " A propos do Thorodite dcs caracteres acquis " (Bulletin 
 Scientiriquc dcla France et do la Belgiquc. Tome XL1V, Paris, 1910). 
 
 2 Francis B. Sunnier, '-An Experimental Study of Somaiic Modifications and 
 their Reappearance in the Offspring," (Archiv fur Entwicklungsmechaiiik der 
 Organismen, Bd. 30, 1910).
 
 INHERITANCE OF ACQUIRED CHARACTERS 183 
 
 transmission of " acquired " characters, and is quite as difficult 
 to accept as the latter, while, as Sumner points out, it does not 
 affect the question of the importance of the environment in 
 determining the course of evolution. 
 
 Professor Henslow l believes that the direct action of the 
 environment, in its widest sense, coupled with the responsive 
 power of protoplasm, is the sole and efficient cause of adaptive 
 variations in plants, without any aid from natural selection. 
 This of course implies a firm belief in the inheritance of 
 somatogenic modifications. Although these may be acquired 
 slowly throughout the course of a long series of successive 
 generations, they may become gradually more and more fixed 
 and permanent, until finally they attain a degree of stability 
 which entitles them to be considered as truly blastogenic. 
 
 Professor Eigenmann 2 has arrived at very similar conclusions 
 as a result of his careful study of animals which live in dark 
 caves. A very characteristic feature of such animals is the 
 bleaching which they undergo, due to the loss of pigment. Pro- 
 fessor Eigenmann regards this character as due in the first place 
 to the direct influence of the environment (i.e. the absence of 
 light) upon the individual. He tells us " The bleached condition 
 of animals living in the dark, an individual environmental adap- 
 tation, is transmissible, and finally becomes hereditarily fixed." 
 In Amblyopsis, one of the blind cave fishes, the fixation of this 
 character has become so complete that even when the young are 
 reared in the light they develop without pigment the originally 
 somatogenic character has apparently become blastogenic. In 
 the well known European Proteus a tailed amphibian which 
 lives in subterranean waters on the other hand, it appears that 
 the bleached condition has not yet become hereditarily established, 
 for this animal becomes darker when exposed to the light. Pro- 
 fessor Eigenmann points out that " natural selection cannot 
 have affected the coloration of the cave forms, for it can be of no 
 consequence whether a cave species is white or black." 
 
 We do not know how many generations it may take to effect 
 the fixation of a character acquired slowly under the influence 
 of a constantly repeated or continuous environmental stimulus, 
 but the fact that human beings have not yet learned to speak 
 
 1 "Origin of Plant Structures " (International Scientific Series, Vol. LXXVIL, 
 1895). 
 
 2 " Cave Vertebrates of America. A Study in Degenerative Evolution " (Wash- 
 ington : Carnegie Institution, 11109).
 
 184 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 without being taught shows that the number may, in some cases 
 at any rate, be very large. Speech, however, is a comparatively 
 recent acquisition in the human race and many of the higher 
 animals perform actions without being taught which their ancestors 
 may very well have originally learnt to perform by observation 
 or experience. It would be extremely difficult to explain the 
 nest-building habits of birds, and other so-called " instincts," 
 except as having been originally acquired in this way in the 
 life-time of the individual. 
 
 On the whole, then, the available evidence seems to indicate that 
 suddenly and exceptionally acquired characters, such as mutilations, 
 are occasionally but only rarely inherited to such an extent as to be 
 recognizable, while, on the other hand, characters which are due to 
 the continued action of some external stimulus, extending perhaps 
 over many generations, in the long run become so firmly impressed 
 upon the organism that they affect the germ cells as well as the 
 somatic cells and thus become truly blastogenic. 
 
 We must look upon the germ cells as highly conservative 
 bodies, possessed of great inertia, which can only be induced very 
 slowly in normal circumstances, or by some sudden revolution, 
 of the nature of which we know nothing, in abnormal circum- 
 stances, to alter their constitution. Tins conservatism is no 
 doubt of great value as a check upon the inheritance of accidental 
 and unfavourable modifications. The adaptation of the individual 
 to its particular environment is to a large extent provided for by 
 somatogenic modifications which arise in its own life-time. What 
 suits one individual, however, may not suit its successors, which 
 may have to live under more or less widely different conditions, 
 and for progressive evolution what is needed is an average 
 adaptation of the race to an average environment. Hence the 
 germ plasm in its evolution lags behind the soma and is only 
 very slowly impressed with those characters which experience 
 has shown to be of permanent value to the race. It is a kind of 
 second chamber which acts as a useful check upon too radical 
 tendencies. 
 
 That the germ plasm is to some extent capable of modification by 
 environmental stimuli, however, is clearly shown by the experiments 
 of Tower referred to in Chapter XL, but it is extremely difficult to 
 see how such stimuli as he mentions could affect it except by 
 acting primarily upon the soma in which the germ cells are 
 enclosed. Then the soma might be modified in response to the
 
 INHERITANCE OF ACQUIRED CHARACTERS 185 
 
 stimulus in the first instance, and this modification, whatever its 
 nature (and it might of course be quite unrecognizable by us), 
 might affect the germ cells in which case the germ cells could not 
 be entirely shut off from the influence of the soma. 
 
 The chief obstacle in the way of our belief in the inheritance of 
 acquired characters lies, as we have already seen, in the difficulty 
 of imagining any mechanism adequate to bring about the con- 
 version of somatogenic into blastogenic modifications. If, as 
 Weismann insists, the germ cells are really incapable of being 
 influenced by the body, the difficulty does indeed seem insur- 
 mountable ; but there seems to be no valid reason why we should 
 follow Weismann in this respect. Nor does it seem necessary 
 that we should adopt any theory which postulates the migration 
 of material particles, such as the gemmules of Darwin's pan- 
 genesis, in order to get over the difficulty. 
 
 Herbert Spencer long ago indicated the direction in which the 
 solution of this problem must be sought. " It is," he says, " an 
 unquestionable deduction from the persistence of force, that in 
 every individual organism each new incident force must work its 
 equivalent of change ; and that where it is a constant or recurrent 
 force, the limit of the change it works must be an adaptation of 
 structure such as opposes to the new outer force an equal inner 
 force. The only thing open to question is, whether such re-adjust- 
 ment is inheritable ; and further consideration will, I think, show, 
 that to say it is not inheritable is indirectly to say that force does 
 not persist. If all parts of an organism have their functions 
 co-ordinated into a moving equilibrium, such that every part 
 perpetually influences all other parts, and cannot be changed 
 without initiating changes in all other parts if the limit of 
 change is the establishment of a complete harmony among the 
 movements, molecular and other, of all parts ; then among other 
 parts that are modified, molecularly or otherwise, must be those 
 which cast off the germs of new organisms. The molecules of 
 their produced germs must tend ever to conform the motions of 
 their components, and therefore the arrangements of their com- 
 ponents, to the molecular forces of the organism as a whole ; and 
 if this aggregate of molecular forces is modified in its distribution 
 by a local change of structure, the molecules of the germs must 
 be gradually changed in the motions and arrangements of their 
 components, until they are re-adjusted to the aggregate of mole- 
 cular forces. For to hold that the moving equilibrium of an
 
 180 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 organism may be altered without altering the movements going 
 on in a particular part of it, is to hold that these movements will 
 not be affected by the altered distribution of forces ; and to hold 
 this is to deny the persistence of force." x 
 
 In other words, the whole complex system of forces which 
 determines the constitution of the germ cells must be in a state 
 of equilibrium with the system of forces which determines the 
 constitution of the body, and any disturbance in the latter must 
 be met by a corresponding disturbance in the former. The same 
 idea may be expressed by saying that modifications of the soma 
 act as stimuli, which make more or less permanent impressions or 
 " engrams " upon the germ cells in much the same way that 
 stimuli of various kinds received through the sense organs make 
 impressions upon certain cells in the brain. In the case of the 
 brain cells the original impressions may become dormant and be 
 revived later on as memories, which are a kind of reproduction 
 of the phenomena to which the impressions were originally due. 
 In the case of the germ cells it is supposed that the engrams also 
 become dormant, but are aroused to activity again in the course 
 of the development of these cells into new organisms, exerting 
 their influence in such a manner as to bring about a kind of 
 reflection, in the body of the offspring, of the parental characters. 
 This is the central idea of the " Mnemic " a theory of heredity, 
 associated more especially with the names of Hering, Samuel 
 Butler 3 and Semon. 4 
 
 According to some authorities it is through a continuity of 
 protoplasm from cell to cell, or especially through the nervous 
 system in the case of the higher animals, that stimuli are trans- 
 mitted from the soma to the germ cells, but the assumption of 
 any definite material paths for such transmission seems an 
 altogether superfluous encumbrance of the theory. 
 
 It has long been known that it is possible to send messages, or if 
 we like to call them so, stimuli, from one place to another without 
 any material means of communication. The very existence of 
 life on the earth depends upon stimuli received in the form of 
 
 1 " Principles of Biology " (London, 1884), Vol. II., p. 387. 
 
 2 From the Greek /j.v^fj.rj, memory. 
 
 3 See Butler's "Life and Habit" (2nd Edition, London, 1878) and "Un- 
 conscious Memory " (New Edition, London, 1910), the latter of which contains a 
 translation of Hering's lecture " On Memory as a Universal Function of Organized 
 Matter." 
 
 4 Semon, " Die Mneme als erhaltendes Prinzip im Wechsel des organischen 
 Geschehens," 3rd Ed., Leipzig, 1SM1.
 
 INHERITANCE OF ACQUIRED CHARACTERS 187 
 
 light and heat from the sun across an intervening space of almost 
 inconceivable extent which is supposed to contain nothing but 
 the hypothetical and intangible ether. To account for such 
 transmission we postulate the occurrence of vibrations or undula- 
 tions in this ether, and the truth of this theory may now be 
 regarded as conclusively demonstrated. The stimulation of the 
 sensory cells of the retina by certain of these vibrations gives rise, 
 when transmitted along the optic nerve to certain cells in the brain, 
 to the sensation which we recognize as light, while other 
 vibrations are perceived by us as heat. 
 
 The rapid development of wireless telegraphy in recent years 
 has familiarized us with another type of vibration in the ether, 
 by taking advantage of which we are able to send messages over 
 immense distances from one instrument to another, without the 
 aid of wires or any other material connections. The Rontgen 
 rays, again, can make impressions upon sensitive photographic 
 plates after passing through solid objects which are quite imper- 
 vious to the ordinary light vibrations. 
 
 In view of these facts it seems absurd to deny that the living 
 cells of the sorna, in which doubtless complex vibrations, possibly 
 comparable to those which are responsible for the phenomena 
 of light and electricity, are constantly going on, may conceivably 
 influence the germ cells without our being able to demonstrate 
 the existence of material connections by which the necessary 
 stimuli might be transmitted. 
 
 The different means of communication between one cell and 
 another in the living body may be compared to the different 
 methods by which messages are transmitted between distant 
 members of a civilized human community. The circulation of 
 fluids (such as the blood) distributes throughout the body definite 
 substances secreted by certain cells, which may act as stimu- 
 lants upon other and far distant cells. We may compare this 
 to the transmission of messages by letter post. The nervous 
 system of the higher animals, and probably also the delicate threads 
 of protoplasm which so frequently connect one cell with another 
 both in animals and plants, provide a means of communication 
 which is closely comparable to ordinary telegraphy over conducting 
 wires. Why should we deny the possibility that a third means 
 of communication, analogous to wireless telegraphy, may also 
 exist ? 
 
 Darwin's hypothesis of pangenesis depends upon the
 
 188 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 existence of the first of these means of communication. So also 
 does the modern theory of hormones, chemical substances which 
 are believed to exercise a most important controlling influence 
 over parts of the body that may be far remote from the 
 place where they themselves are secreted, 1 and the aid of 
 which has also been invoked to account for the phenomena 
 of heredity. Others, again, as we have seen, have called in the 
 aid of the second means of communication to account for the 
 transmission of stimuli from somatic to germ cells, but their 
 view is hardly supported by what we know of the arrangement of 
 the nervous system throughout the animal kingdom, and the 
 difficulty of accepting it is still greater in the case of plants, in 
 which no definite nervous system is developed. The third method 
 of communication, however, seems open to no objection, and, 
 whether it may be supplemented by the others or not, seems 
 amply sufficient to account for the facts. 
 
 The faculty of receiving and responding to stimuli of various 
 kinds is one of the most characteristic features of living proto- 
 plasm. In the higher animals this function is more or less 
 concentrated in definite sense cells, each of which is adapted for the 
 reception of stimuli of one particular kind. The visual sense cells 
 are adapted for the reception of stimuli from the light-vibrations 
 of the ether, the auditory sense cells are stimulated by vibrations of 
 the air or water in which the animal lives, the olfactory sense cells 
 are stimulated by the chemical action of material particles, and 
 the tactile cells by the mechanical stimuli of contact and pressure. 
 In such an organism as the unicellular Amoaba, on the other 
 hand, all parts of the superficial protoplasm are probably sensitive 
 to stimuli and no special organs of sense are developed. We may 
 well suppose, therefore, that the undifferentiated germ cells of the 
 multicellular animals and plants are sensitive to stimuli received 
 from the somatic cells, though it is impossible in the present state 
 of our knowledge to determine the nature of these. 
 
 It is not difficult to demonstrate that one cell may actually be 
 stimulated by another without the existence of any protoplasmic 
 connection between the two, as, for example, in the mutual 
 attraction of male and female gametes. Such stimulation, it is 
 true, is usually attributed, mainly at any rate, to the secretion 
 of some specific chemical substance which diffuses into the sur- 
 rounding water, and classed accordingly under the head of 
 
 i Vide p. 126.
 
 INHERITANCE OF ACQUIRED CHARACTERS 189 
 
 chemotaxis. In the case of Spirogyra, however, discussed in 
 Chapter X, 1 it certainly seems as if chemotaxis were not sufficient 
 to account for the phenomena. 
 
 Belief in the possibility of the transmission of acquired 
 characters from somatic to germ cells by no means obliges us to 
 throw over Weismann's theory of determinants. Indeed it seems 
 necessary to postulate the existence in the germ plasm of some 
 such material primordia as the responsible agents in the trans- 
 mission of heritable characters in order to account for the 
 Mendelian phenomena of hybridism, and especially the existence 
 of interchangeable unit characters, which will be dealt with in 
 the following chapter. Moreover, the theory of determinants, as 
 we bave already seen, harmonizes very well with what we know 
 of the microscopic structure of the germ plasm and its behaviour 
 in cell-division and conjugation. 
 
 It may be that it is the determinants themselves that are 
 influenced by stimuli received from the somatic cells. It is even 
 possible that each different kind of determinant ib " tuned " 
 to respond to vibrations of a particular wave-length, emanating 
 from corresponding determinants in the nuclei of somatic cells of 
 a particular kind. Modifications of these somatic cells may then, 
 by altering the character of the vibrations in the determinants of 
 their own nuclei, affect the corresponding determinants in the 
 distant germ cells in a similar manner. By means of some such 
 hypothesis as this, which would be strictly in accordance with 
 modern physical ideas on the subject of the transmission of 
 electrical energy from one group of electrons to another at a dis- 
 tance, any a priori difficulty in accepting the possibility of the 
 inheritance of acquired characters might be removed. 
 
 The case of neuter insects has frequently been adduced as a 
 serious stumbling block in the way of the theory of the origin of 
 permanent, heritable characters from somatogenic modifications, 
 for these neuters possess features of their own which are not 
 shared by the males and perfect females and which, as the 
 neuters do not themselves produce offspring, they cannot trans- 
 mit directly to future generations. According to Weismann's 
 theory these characters are supposed to result from fortuitous 
 favourable variations of the germ plasm of the parents, which are 
 accumulated and intensified under the influence of natural 
 
 1 Pp. 142-8. I have perhaps laid too much stress upon this case; chemotaxis 
 may have more to do with it than appears at first sight.
 
 190 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 selection, the welfare of the insect community as a whole, rather 
 than that of its constituent individuals, being in this case the 
 determining condition through which natural selection operates. 
 The case of such communities, however, is exactly analogous to 
 that of individuals, except that the single cells are replaced by 
 complete and separate multicellular units. The whole community 
 may be looked upon as one individual of a higher order, and the 
 problem of the transference of stimuli from the bodies of the 
 neuters to the germ cells of the perfect insects differs in no 
 essential respect from that of the transference of similar stimuli 
 from somatic cells to germ cells in an ordinary individual. The 
 close association in which members of such communities live 
 may be supposed to facilitate the transfer of such stimuli (arising 
 from modification of the somatic cells of the neuters in response to 
 the environment) to the germ cells of the males and perfect 
 females, without the aid of material conductors. 
 
 In the present state of our knowledge, of course, any suggestions 
 which may be put forward on this subject must be regarded as mere 
 hypotheses, incapable of demonstration, and as such they will 
 doubtless appear to many people to be unwarrantable. If, however, 
 they serve to show that there is no a priori reason for denying the 
 possibility of the transmission of somatogenic characters to the 
 germ cells, and thence to future generations, they will serve a 
 useful purpose. 
 
 We must, however, again point out that, under normal cir- 
 cumstances, it probably takes many generations before any 
 impression produced by modification of the soma upon the germ 
 cells becomes so deeply ingrained as to find full expression in the 
 offspring produced by their development. The impression, once 
 made, appears to be equally difficult to remove, and hence has 
 probably arisen that conservatism or inertia of the germ cells to 
 which we have already alluded. Occasionally, however, it appears 
 that a suddenly acquired somatogenic character at once makes a 
 deep impression upon the germ cells. Why this should be so we do 
 not know, but it is perhaps no more remarkable than the well 
 known fact that the germ cells themselves may occasionally throw 
 aside their conservatism and give rise to sports or mutations. 1 
 
 It is well known that, apart altogether from the highly 
 specialized brain cells, living protoplasm frequently has the power 
 
 1 It is possible, however, that the suddenness of a mutation is apparent rather 
 than real, at any rate in some cases ; compare p. 106.
 
 MNEMIC THEORY OF HEREDITY 191 
 
 not only of receiving and responding immediately to stimuli of 
 various kinds, but also of storing up impressions or engrams for 
 future use. This is demonstrated quite clearly by the phenomena 
 known as " after-effects " in various plants and animals. One of 
 the best known examples of such after-effects is seen in the daily 
 periodicity of plant growth. The light of the sun acts as a 
 restraining or inhibiting stimulus upon the rate of growth of 
 ordinary plants. In consequence of this the plant grows most 
 rapidly in the early hours of the morning after a prolonged 
 exposure to darkness, and most slowly in the afternoon, after 
 long exposure to daylight. It has been shown that this daily 
 periodicity, or variation in rate of growth in correspondence with 
 the periodic variation in environment, is continued when the 
 plant is kept in perpetual darkness, and the direct stimulation of 
 changing environment thereby rendered impossible. In other 
 words, the plant, though entirely devoid of any nervous system 
 in the ordinary sense of the term, establishes a habit, which must 
 depend upon something analogous to memory on its part. 
 
 The mneinic theory of heredity is based, as we have seen, upon a 
 comparison of the phenomena of inheritance with those of memory. 
 The latter are reasonably explained by supposing that impressions 
 received by certain cells of the brain may be stored up as 
 " engrams " in these cells for use on future occasions, when, 
 under appropriate stimulation, they give rise to mental condi- 
 tions corresponding to those produced by the original stimuli. 
 The stimulus which evokes a memory is commonly the 
 repetition of some stimulus which was originally associated 
 with the thing remembered. Thus the sight of a person whom 
 we have not seen for a long time, or even of his portrait, will 
 evoke a whole train of memories connected with that person. 
 
 The degree of accuracy with which we remember any occurrence 
 depends partly upon the nature of the occurrence itself and 
 partly upon the frequency with which it has been brought under 
 our notice. The first time we take a walk we may have to pay 
 attention to every turning and every signpost in order to find our 
 way at all, but if we take the same walk many times we at length 
 become so familiar with it that we may be thinking of other 
 things the whole time and not consciously notice a single land- 
 mark from start to finish. As Samuel Butler has most forcibly 
 pointed out, the more perfect memory becomes the less is it 
 accompanied by consciousness of the things remembered, as when
 
 192 OUTLINES OF EVOLUTIONAKY BIOLOGY 
 
 a skilled musician, playing a composition for perhaps the 
 hundredth time, is quite unconscious of the individual actions 
 which he performs by memory in striking the notes, while a 
 beginner, who has only played the same piece once or twice 
 before, may be acutely conscious of every note which he strikes. 
 The development of an individual organism from the egg is, 
 according to the mnemic theory, merely an unconscious repetition, 
 by memory, of acts which have been performed many thousands 
 of times by its ancestors, and, as in the performance of a piece of 
 music, each successive act constitutes the stimulus which calls 
 forth the next. 
 
 The fertilized ovum may be looked upon as being charged with 
 the latent memories of past generations. The unconscious memory 
 of what it did last time it was a fertilized ovum prompts it to divide 
 into two' cells, this re-arrangement of its constituents supplies 
 another stimulus which prompts it to a further division, and so on 
 through all the stages of ontogenetic development. In the words 
 of Professor Francis Darwin, "the rhythm of ontogeny is actually 
 and literally a habit." l 
 
 Progressive evolution takes place owing to the fact that each 
 successive generation may add a little bit to the record on its own 
 account, this addition being the result of some new experience 
 due to some difference in its environment as compared with the 
 environment of its predecessors. 
 
 There are, then, tw r o distinct sets of factors which determine 
 the individual development of any organism, first the inherited 
 tendencies, or " engrams," and second the stimuli provided by 
 the environment during its own life-time. An egg placed under 
 very unusual conditions, as, for example, too low or too high a 
 temperature, may be unable to develop at all, but if the 
 change of conditions be but slight the organism may adapt 
 itself, educate itself and furnish itself with a new store of 
 experiences, which in course of time may be impressed upon the 
 germ plasm and thus added to the record and handed on to 
 future generations. 
 
 We have already had occasion to notice, in the case of 
 artificially produced cyclopean fish lame, how an alteration of the 
 environment not sufficient altogether to prevent development 
 may so far overcome the inherited tendencies of the organism 
 as to give rise to the production of monstrosities (p. 157). 
 
 1 Presidential Address to the British Association, Dublin, 1908.
 
 ACTION OF THE ENVIRONMENT 193 
 
 Such cases clearly prove the immense influence of the environ- 
 ment in determining the character of the organism. If we admit 
 that inherited tendencies also owe their origin in the first instance 
 to the action of the environment on previous generations, it is 
 difficult to avoid the conclusion that every organism is ultimately 
 indebted for all its characters to the action of external stimuli 
 upon responsive protoplasm.
 
 CHAPTER XIV 
 
 The Mendelian experiments in hybridization The doctrine of unit 
 characters and the purity of the gametes Galton's law of inheritance. 
 
 WHATEVER view we may adopt as to the mechanism of heredity 
 there can be no doubt as to the existence of a material foundation 
 for the transmission of characters from parent to offspring. We 
 know that the protoplasm which forms the material basis of all 
 living things is continuous, by means of the germ cells or gametes, 
 in an unbroken stream from one generation to another. At each 
 conjugation of gametes, however, two such streams are mingled, 
 and the offspring receives part of its initial stock of protoplasm 
 from one parent and part from the other, each part bringing with 
 it all the potentialities which may have been derived from a long 
 line of ancestors. Bearing these facts in mind, we must now 
 direct our attention to some results which have been obtained 
 by the "Mendelian" method of attacking the problem of 
 heredity. 
 
 In the early part of the nineteenth century much attention was 
 paid by horticulturists to experiments on the hybridization of 
 plants. It was found that, within certain limits, it was possible 
 to fertilize the flower of one variety of plant with the pollen 
 of another variety, and in this manner to alter the character 
 of the offspring. Many .crosses or hybrids between different 
 varieties were thus obtained, differing in various ways from 
 the parent plants. It was also found that by repeatedly fertilizing 
 the flowers of one variety with the pollen of another the 
 descendants of the first could be entirely changed into the second. 
 This clearly proved that all the distinctive characters of the male 
 parent were in some way represented in the pollen grains, and 
 could be transmitted by these to future generations, but it also 
 proved something more, it proved that it was possible to 
 eliminate the special characters of the female parent, or at least 
 to prevent them from manifesting themselves in the offspring. 
 
 As a rule it is only possible to bring such experiments to a
 
 MENDEL'S WORK ON HYBRIDIZATION 195 
 
 successful issue when working with closely related species or 
 varieties. The flower of one kind of orchid, for example, may 
 perhaps be fertilized by the pollen of a different orchid, but the 
 pollen of such a plant as a lily would probably not have the 
 slightest effect upon it. 
 
 The first observer to throw a clear light upon the meaning of 
 the remarkable results obtained by hybridization was Gregor 
 Johann Mendel, a native of Austrian Silesia, born in 1822, whose 
 work has recently attracted so much attention. As Abbot of 
 Briinn, he was the happy possessor of a garden, and presumably 
 also of that peace and quiet which are so essential to intellectual 
 work. He was not content with merely casual experiments ; he 
 had time to think about what he was doing and he attacked the 
 problem in the true scientific spirit. Unfortunately for science, 
 however, his seclusion was a little too complete. He was content 
 to publish his results in 1865 and 1869 in the proceedings of a local 
 natural history society, where they remained buried and almost 
 unnoticed for more than thirty years. Thus, by the irony of fate, 
 our own illustrious countryman, Charles Darwin, although a con- 
 temporary of Mendel, probably never heard of those remarkable 
 discoveries which bid fair to solve some of the problems in which 
 he himself was so deeply interested. 
 
 In order to gain an insight into the nature of Mendel's work 
 we cannot do better than turn to his original memoir, entitled 
 " Experiments in Plant Hybridization," of which an excellent 
 translation has been published by Professor Bateson, 1 one of the 
 leading exponents of what is now termed Mendelism. Even fifty 
 years ago, experiments in hybridization were, as we have already 
 seen, no new thing. Mendel had many predecessors in this line 
 of research, but it was reserved for him to introduce exact 
 statistical methods into the work, and it is to these methods that 
 he owed his success. It was already known that hybridization, 
 or the crossing of distinct species and varieties, might result 
 in the production of several distinct types of offspring from 
 the same hybrids. Mendel was not content with this know- 
 ledge : he proceeded by numerous and often repeated experi- 
 ments, extending over several years, to classify the offspring 
 produced by hybridization, to follow out their descendants from 
 generation to generation, and above all to find out the exact 
 numerical proportions in which the different types appeared in 
 
 1 Vide Bateson, " Mendel's Principles of Heredity " (Cambridge, 1909). 
 
 o 2
 
 196 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 successive generations. It was the discovery of these proportions 
 that furnished the clue to the mystery. 
 
 Mendel found the material which he required for his experi- 
 ments chiefly in the numerous garden varieties of the common 
 edible pea. How these varieties first originated we do not know. 
 Possibly they arose as sudden and apparently spontaneous 
 variations, or mutations. No less than twenty-two such varieties 
 were chosen for experiment. 
 
 The next thing was to select certain differentiating characters 
 upon which to concentrate attention, and this is a very important 
 point. Of course, all the varieties agreed with one another in 
 most of their characters ; in other words, they were all edible 
 peas exhibiting the specific characters of the plant known to 
 botanists as Pisum sativum, but they differed in numerous minor 
 features. Of these differentiating characters Mendel selected 
 seven for the purposes of his experiments, amongst which I need 
 mention only two : (1) the form of the ripe seed, which, roughly 
 speaking, may be either round and smooth or angular and 
 wrinkled ; and (2) the difference in colour of the seed-contents or 
 cotyledons, which may be either yellow or green and which usually 
 determine the colour of the seed as a whole. 
 
 The two differentiating characters of each pair were artificially 
 united by cross-fertilization, the flowers of a round-seeded pea 
 being fertilized by the pollen of a wrinkled-seeded pea. those of a 
 green-seeded pea by the pollen of a yellow-seeded pea, and so on. 
 
 In this way a number of hybrid plants, represented in the first 
 instance by seeds, were obtained. 1 These hybrids constituted 
 what modern writers term the F l or "first filial" generation, 
 and the curious fact was observed that every hybrid closely 
 resembled one of the two parents, instead of being, as is frequently 
 the case in other hybrids, intermediate in character between 
 them. Further experiments with the offspring of the hybrids, 
 however, showed that, although only one of the two contrasted 
 characters manifested itself in the hybrid, the other was there 
 also in a latent or dormant condition, and appeared again in 
 
 1 It must be remembered that the pea-seed consists chiefly of the young plant 
 in a dormant condition, and that its characters are therefore, with the exception 
 of those appertaining to the seed-coat, which are not considered in this 
 chapter, those of the sporophyte generation which follows the conjugation of the 
 gametes. The fact that the complete life-cycle of the plant really includes two 
 generations, a well developed sporophyte and a vestigial gametophyte, does not 
 affect the Metidelian conclusions.
 
 HYBRIDIZATION IN PEAS 197 
 
 subsequent generations. The character which appears in the 
 hybrid is said to be dominant, while that which is suppressed is 
 said to be recessive. In the case of these peas, then, one character 
 is always dominant over the other in the hybrid, and, moreover, 
 it is always the same character, and it does not matter whether 
 it is derived from the male or from the female parent. Thus the 
 round form of seed is dominant over the wrinkled, the yellow 
 colour of the seed-contents over the green, and so on. 
 
 Having obtained the hybrids, the next step was to follow the 
 history of the offspring throughout successive generations. For 
 this purpose the flowers of the plants raised from the hybrid 
 seeds were allowed to fertilize themselves with their own pollen, 
 no further crossing being permitted. The seeds thus obtained 
 (constituting the F 2 or " second filial " generation) were now 
 again apparently of two kinds, resembling the two original parent 
 forms from which the hybrid was produced. Moreover, these two 
 kinds occurred in definite proportions, three of the dominant to 
 one of the recessive three round seeds to one wrinkled, three 
 yellow to one green, and so on. Of course, these proportions are 
 only averages, and, in order to eliminate errors of chance, large 
 numbers of observations must be made the greater the number 
 the more reliable the result. To take an actual example from 
 Mendel's work, 7,324 seeds were obtained in the second trial year 
 from the hybrids between round and wrinkled. Of this number 
 5,474 were found to be round, and 1,850 wrinkled showing the 
 ratio of 2*96 to 1. Again, out of 8,023 seeds produced by the 
 hybrids between green and yellow, 6,022 were yellow and 2,001 
 green, the ratio being 3'01 to 1. These experiments have often 
 been repeated during recent years, especially by Mr. A. D. 
 Darbishire, 1 and it is found that the average ratio of three to one 
 is always maintained. 
 
 The significance of this proportion is not at first sight obvious. 
 It is necessary to continue the experiment for at least another 
 generation in order to gain further insight into the matter. We 
 have as the result of the self-fertilization of our hybrids 
 apparently only two kinds of seeds or plants, which we may 
 call D, showing the dominant character, and R, showing the 
 recessive character, in the proportion of 3 D to 1 R. If these 
 
 1 Some beautiful illustrations of Mendelian results in both plants and animals 
 are given in Mr. Darbishire's recent work " Breeding and the Mendelian Discovery " 
 (Cas.sell A: Co., Ld., 1911).
 
 198 OUTLINES OF EVOLUTION AEY BIOLOGY 
 
 plants are again cultivated and allowed to fertilize themselves we 
 find that every one of the R's breeds true, and no matter how 
 many generations we raise they will always remain of the 
 recessive type. These are now called "extracted recessives." 
 When, however, we cultivate the plants showing the dominant 
 character in the same way we soon find that, though similar 
 to one another externally, they are not in reality all alike, 
 for one-third of them will yield nothing but D's, while the 
 remaining two-thirds will yield D's and R's in the same 
 proportion of 3 to 1 as the original hybrid. Moreover, the 
 D's obtained from the one-third will continue to breed true 
 from generation to generation, and do what we will we can 
 never get an R out of them again. They are called " extracted 
 dominants." 
 
 It is obvious then, from these experiments, that the apparent 
 D's are not all true D's, but that two-thirds of them contain the 
 recessive character in a latent or dormant condition, and are, 
 therefore, still hybrid in composition. If we indicate those which 
 we can thus prove by breeding to contain the recessive character 
 by the letters D (R), we can sum the whole story up in the following 
 simple diagram : 
 
 D x R (Original Parents) 
 
 Generation 
 
 D D D D D D D R D D D R R R R R; - - F 3 
 
 extracted dominants Extracted recessiues 
 
 FlG. 79. Monohybridism. 
 
 Thus we see that from generation to generation of the 
 offspring of the hybrid there goes on a constant sorting out into 
 three categories, the two original parental types and the hybrid 
 form being always produced in definite proportions. If we 
 assume that each kind produces on an average an equal number 
 of offspring, we shall see from the diagram that in the course of a 
 few generations of undisturbed reproduction by far the greater 
 number of the plants will have finally reverted, in equal propor- 
 tions, to one or other of the original types, but a certain number
 
 EXPLANATION OF MENDELIAN EESULTS 199 
 
 will always remain in the hybrid condition and will continue 
 to split up as before in the characteristic Mendelian proportions. 
 We see, then, from Mendel's experiments, that in the case of 
 peas, where one of the two contrasted characters is dominant 
 over the other, the offspring of the first ' hybrid appear in the 
 proportion of three apparent dominants to one recessive, but that 
 further analysis shows that the real proportion is 
 
 1 D : 2 D (E) : 1 E. 1 
 
 This proportion has since been observed not only in peas but 
 in a large number of other cases, including animals as well as 
 plants. It is evidently a phenomenon of very common 
 occurrence, though we cannot as yet say that it occurs 
 universally whenever two forms with contrasted characters are 
 crossed. Moreover, as we have already noticed, the phenomenon 
 of dominance is not always shown, and the hybrid may exhibit 
 a character intermediate between those of the two parents or 
 different from either. The occurrence of Mendelian propor- 
 tions, however, is sufficiently frequent to demand explanation, 
 and this explanation we must now seek. 
 
 Suppose we take a large number of black and an equal number 
 of white counters, all alike in shape, size and weight, and after 
 shaking them up thoroughly in a bag, draw them out two at a 
 time, and one on top of the other, without looking. Each pair 
 that we draw out may consist of white over white, black over white, 
 white over black, or black over black. If we draw out a sufficiently 
 large number of pairs entirely at random in this way and then count 
 them, we shall find that they occur in the proportion : 1 W W : 
 1 B W : 1 W B : 1 B B, or, since B W and W B may be looked 
 upon as the same, 1 W W : 2 W B : 1 B B, or one all white, two 
 white and black, and one all black ; this being, of course, only 
 what is to be expected in accordance with the mathematical law 
 of probability. This is also the same as the simple Mendelian 
 proportion, and at once suggests that the latter may, perhaps, be 
 explained in a similar way as the result of random union of 
 characters in accordance with the laws of chance. 
 
 We know that each individual plant or animal produced by 
 sexual reproduction is formed by the union of two germ cells or 
 gametes. Now these gametes are formed in very large numbers 
 
 i This is sometimes written more fully 1 D 1) : 2 D (R) : 1 K R. The reason 
 
 for so doinir will be obvious from what follows.
 
 200 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 in the parent organisms, and if we suppose that each one contains 
 only one of the two contrasted characters with which we are 
 experimenting, and if we further suppose that the two kinds are 
 produced in equal proportions and that they unite at random, 
 then, in accordance with the law of chance, we should expect 
 to get in the offspring the proportion of one with the one 
 character, two with both characters and one with the other 
 character, exactly as Mendel found in his experiments. 1 There 
 seems to be no other possible explanation of the Mendelian 
 phenomena. 
 
 The conclusions to be drawn from these results are of 
 fundamental importance. In the first place, we learn that small 
 individual characters may be separately represented in the 
 germ cells and separately transmitted from parent to offspring. 
 This indicates that the entire organism may be built up of a 
 number of "unit characters," and if we can once establish 
 the general occurrence of such unit characters we shall have 
 taken a long stride towards the understanding of the laws of 
 heredity. 
 
 In the second place, we may conclude from these experiments 
 that, as regards the unit characters with which we are dealing, 
 we have a complete segregation amongst the germ cells or 
 gametes, each of which carries only one of each pair of contrasted 
 characters (technically termed allelomorphs), and in this respect 
 it makes no difference whether the germ cell be male or female. 
 Thus we arrive at what is sometimes termed the doctrine of the 
 " purity of the gametes " purity, that is to say, with regard to 
 each character of any such contrasted pair, and without reference 
 to the other innumerable characters which must be represented 
 in the germ cells in order that the whole complex structure of the 
 organism may be developed from them. 
 
 We may now examine an example of a class of practical 
 problems which may be solved by the application of such simple 
 Mendelian principles. 
 
 Fowl-fanciers are well acquainted with a particular breed of 
 fowl known as the blue Andalusian. It has long been known 
 that this kind of fowl cannot be made to breed true. Some of 
 
 1 A little consideration will show that if the contrasted characters are repre- 
 sented in equal proportions of both male and female gametes, the facts tlint a 
 male must always unite with a female, and that the actual number of sperm cells 
 produced is usually very much larger than the number of ova, will not affect the 
 result.
 
 HYBRIDIZATION IN FOWLS 201 
 
 the offspring will be white and others white with black splashes, 
 the remainder being like the parents. It has been shown, 
 moreover, that the three kinds of chicken occur, on an average, in 
 definite proportions, a quarter being black, a half blue, and a 
 quarter white splashed with black. Here we have the familiar 
 Mendelian proportion 1:2:1, suggesting that the blue Andalusian 
 is really a hybrid, and that the so-called " wasters " are the parent 
 forms. It is easy to prove that this is the case, for if the two 
 kinds of waster are mated we invariably get the blue Andalusian 
 again. The old idea of the so-called practical breeder w r ould have 
 been to go on destroying the wasters and carefully selecting the 
 blues in order to maintain the " purity of the breed." We now 
 know, however, that there is no such thing as a pure blue Andalu- 
 sian breed ; the blue is really a hybrid, and you can get more blues 
 by mating the wasters than by breeding from the blues 
 themselves. This case is also interesting as affording an example 
 of a hybrid which differs in character from either of the parent 
 forms. 
 
 If only a single pair of alternative characters or allelomorphs is 
 dealt with in the experiment the case is termed one of mono- 
 hybridism, and such cases yield the proportion 1:2:1 (or 
 apparently 1 : 3 in cases of dominance) amongst the offspring of 
 the hybrid, i.e. in the F 2 generation. It will frequently happen, 
 however, that the two varieties which are united in the formation 
 of the hybrid will differ from one another as regards more than 
 one pair of contrasted characters. We may illustrate this by a 
 case of dihybridism, in which two pairs of contrasted characters 
 are involved. The case selected is one which shows us how, 
 under certain circumstances, we can obtain a hybrid exhibiting 
 an entirely new combination of characters, which in spite of its 
 hybrid nature will continue to breed true. 
 
 Suppose a gardener had only two kinds of peas, one with round 
 green seeds and the other with wrinkled yellow seeds, and that he 
 wished to obtain peas with wrinkled green seeds. Mendel has 
 shown us how he may get what he desires, both surely and 
 speedily, from the material already in his possession. 
 
 The necessary procedure will be evident from the following 
 scheme : 
 
 Let R = round, W = wrinkled, Y = yellow, and G = green. 
 We know from previous experiments that R is dominant to W, 
 and Y to G.
 
 202 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 R G crossed with W Y gives the hybrid, R (G) (W) Y, which, owing 
 to the dominance of R and Y, will appear in the form of round 
 yellow seeds (the brackets around the G and W indicating the 
 recessive nature of green and wrinkled). That is not what our 
 gardener wants, and if he knew no better he would naturally be 
 
 FIG. 80. An Example of Dihybridism in Peas. 
 
 A, wrinkled yellow peas; B, round green peas; C, hybrid peas (Fi generation, round 
 and yellow in appearance) produced by crossing A and B; D, plant of the FI 
 generation, with seeds of the.F 2 generation, produced by self-fertilization and of the 
 four different kinds, round yellow, round green, wrinkled yellow, and wrinkled green, 
 lying in the pods. (Photographed from a preparation by Mr. Darbishire, who has 
 reproduced the same subject in colours in his work on " Breeding and the Mendeliaii 
 Discovery "). 
 
 much disappointed with the result. If, however, the hybrids be 
 allowed to fertilize themselves and reproduce, then their off- 
 spring will appear in the apparent proportions : 
 
 9 R Y : 3 R G : 3 W Y : 1 W G, 
 
 no account being taken of the non-apparent recessive characters. 
 The meaning of the proportion 9:3:3:1 exhibited in this
 
 DIHYBRIDISM 
 
 203 
 
 case wil be readily understood from the accompanying table 
 (Fig. 81) : 
 
 R Y 
 
 RG 
 
 W Y 
 
 WG 
 
 I 
 
 R Y 
 RG 
 W Y 
 WG 
 
 R Y 
 R Y 
 
 R(G) 
 R Y 
 
 (W)Y 
 R Y 
 
 (W) (G) 
 R Y 
 
 R Y 
 R(G) 
 
 RG 
 RG 
 
 (W)Y 
 R(G) 
 
 (W)G 
 RG 
 
 R Y 
 (W)Y 
 
 R(G) 
 (W) Y 
 
 VV Y 
 W Y 
 
 W(G) 
 W Y 
 
 R Y 
 
 (W) (G) 
 
 RG 
 (W) G 
 
 WY 
 
 W(G) 
 
 WG 
 WG 
 
 FIG. 81. Dihybridism. 
 
 The constitution of the hybrid (in so far as the characters in 
 question are concerned) is E (G) (W) Y. Each of its germ cells, 
 however, contains one character, and one only, from each con- 
 trasted pair. Each may, therefore, contain E Y, E G, W Y or 
 W G. These different kinds of germ cells will occur on the 
 average in equal numbers, and on self-fertilization of the hybrid 
 flowers they will unite in pairs at random. The possible 
 ways in which such union may take place are shown in the table, 
 and if allowance be made for the phenomenon of dominance, in 
 accordance with which G disappears from view whenever it meets 
 Y, and W whenever it meets E (as indicated by the brackets in 
 the table), we get the apparent proportion of 9 round yellow 
 seeds, 3 round green seeds, 3 wrinkled yellow seeds, and 1 wrinkled 
 green seed. The number of wrinkled green seeds will be small at 
 first, but they will breed true, containing as they do only the 
 extracted recessive characters, for if a dominant character were 
 present it would necessarily show itself. 
 
 Thus we see that one way of producing new forms of plants 
 and animals is by the artificial combination of characters 
 which already exist in different varieties. These elementary or 
 unit characters can be brought together by the process of hy- 
 bridization, and new organisms produced in somewhat the same
 
 204 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 way as that in which the chemist is able to produce new compounds 
 by analysis and synthesis from other substances. 
 
 Probably no man has made more successful use of the process 
 of hybridization in the production of new and valuable forms of 
 plant-life than Luther Burbank, at his celebrated California!! 
 nurseries. Mr. Burbank himself is unfortunately not a writer, 
 and for a scientific account of his work we are indebted to 
 Professor Hugo de Vries, who in his book on "Plant Breeding" 1 
 describes what he himself saw and heard during his visits to 
 Mr. Burbank's farms. Some idea of the commercial value of 
 Mr. Burbank's work may be formed from the fact that special 
 companies have been formed for the propagation and sale of 
 some of these wonderful hybrids. White blackberries, stoneless 
 prunes, " plumcots " and thornless cacti are only some amongst 
 the many novelties which he has produced by crossing different 
 varieties by hybridization and thus combining two or more 
 desirable qualities in one plant. 
 
 Take, for instance, the case of the stoneless prune. It had 
 somehow or other come to the knowledge of Mr. Burbank that 
 about 200 years ago there existed in France a plum known as the 
 " Prune sans noyau." He succeeded in obtaining specimens of 
 this fruit, but it proved to be of little or no commercial value 
 owing to its poor quality. Its one valuable feature was its stone- 
 lessness, and Burbank set to work to transfer this character, by 
 hybridization, to a variety of good quality. He succeeded, and 
 there appears to be no reason why the stoneless character should 
 not be similarly implanted upon all the different varieties of 
 plums now in cultivation. 
 
 Similarly, the "plumcots" are hybrids between plums and 
 apricots, which Professor de Vries speaks of as " most delicious 
 and beautiful fruits." 
 
 It must be admitted that Burbank's work is practical rather 
 than scientific. He apparently pays no attention to the law of 
 Mendel in his operations, and just goes on hybridizing until he 
 gets what he wants. In this process enormous numbers of plants 
 which do not fulfil his requirements have to be destroyed for 
 every one that is worth preserving. 
 
 He is not necessarily concerned with the production of pure 
 breeds ; of forms, that is to say, which will breed true from seed, 
 
 1 Hu<ro de Vries, " Plant Breeding " (London, Kegan Paul. Trench, Triibner& Co.. 
 Ld., 11107).
 
 PEACTICAL APPLICATIONS 205 
 
 or, in other words, of forms which will hand on their desirable 
 qualities to future generations by heredity. This does not matter 
 in the case of fruit trees and other plants which can be propa- 
 gated by buds independently of sexual reproduction. As a 
 general rule, a fruit tree cannot be relied upon to come true 
 from seed, the characters which it has received from different 
 ancestors not being permanently combined, but separating out 
 and undergoing fresh combinations in the sexual process, pro- 
 bably in accordance with Mendelian principles. The seedlings 
 will therefore be " degenerate," as horticulturists say, and will no 
 longer exhibit those valuable qualities which depend upon the 
 confluence in one individual of particular lines of ancestry. 
 
 The majority of Burbank's productions could not survive in a 
 state of nature at all ; they are essentially artificial and have to 
 be artificially propagated by means of buds or cuttings. In this 
 respect they are quite different from the pure races which it is 
 possible to produce by hybridization carried out in accordance 
 with Mendelian principles, in which new and permanent com- 
 binations of unit characters, capable of being transmitted by 
 heredity, are effected. 
 
 Professor Bateson, Professor Biffen, Mr. Hurst and others have 
 lately done much to demonstrate the possibilities of progress in 
 this direction, and the value which the application of the 
 Mendelian principles of heredity must have from the economic 
 point of view. We know now that such cereals as wheat and barley 
 obey the Mendelian laws of hybridization as regards various 
 important characters. So also do horses as regards the colour of 
 their coats, and human beings as regards the colour of their eyes, 
 and in some other respects, especially as regards the inheritance 
 of certain diseases. In short, it appears certain that Mendelian 
 principles have a very general application, and the economic 
 value of the knowledge of such principles must be enormous. 
 Professor Biffen has shown, for example, that susceptibility and 
 insusceptibility to that destructive disease in wheat known as 
 " rust," behave as Mendelian characters, and that it is possible 
 by a very simple process of hybridization to confer immunity 
 from this disease upon naturally susceptible varieties. 
 
 From the theoretical point of view the great value of the 
 Mendelian experiments lies in the possibilities which they present, 
 at any rate in certain cases, of analyzing the constitution of the 
 hereditary substance (germ plasm), and thereby gaining some
 
 206 OUTLINES OF E VOLUTION AEY BIOLOGY 
 
 insight into the real nature of heredity. We may regard the 
 existence of " unit characters " as being conclusively demonstrated 
 by these experiments, and the fact that we are able to interchange 
 these characters and to add and subtract particular characters to 
 and from the organism seems to indicate very clearly that the 
 germ plasm must contain material primordia or " factors," as they 
 are often called, which are responsible for the development of 
 these characters. 
 
 In so far as they demonstrate the existence of such factors 
 which evidently correspond very closely with Weismann's hypo- 
 thetical determinants the Mendelian experiments may be taken 
 as affording confirmation of Weismann's theory of the constitu- 
 tion of the germ plasm. Their results also harmonize very well 
 with those of recent cytological investigations. We have seen 
 that there is strong evidence for regarding the chromatin 
 substance of the nucleus as the material basis of heredity. We 
 have further seen that this chromatin is very complex in 
 structure and that the chromosomes, or units of the highest order 
 of which it is composed, can in many cases be optically resolved 
 into chromomeres or units of the next lower order. According 
 to Weismann's theory these chromomeres or ids are in their 
 turn made up of the determinants. Moreover the associa- 
 tions and redistributions of chromosomes which take place 
 in the process of " reduction " and in the conjugation of the 
 germ cells seem to afford ample opportunity for those permuta- 
 tions and combinations of factors the occurrence of which is demon- 
 strated by the Mendelian experiments. As Professor Farmer 
 observes, " The facts of meiosis 3 are seen to fall completely into 
 line with conclusions drawn from experiments on breeding as far 
 as the numerical distribution of characters is concerned." 2 
 
 The doctrine of the "purity of the gametes" teaches us that 
 any given mature germ cell contains only one member of any 
 given pair of alternative factors or primordia (allelomorphs). 
 This may be accounted for by the pairing of the chromosomes 
 and the subsequent halving of their number which take place at 
 some period or other prior to the formation of the gametes. 
 There is good reason to believe that each member of such a pair 
 of chromosomes is homologous with or morphologically equivalent 
 
 1 I.e. the phenomena accompanying the reduction in the number of chromosomes 
 which takes place periodically in all typical organisms (tide Chapter X.). 
 
 2 Croonian Lecture. Proceedings of the Royal Society, B, Vol. 7'J, l'J07.
 
 HOMOZYGOTE AND HETEROZYGOTE 207 
 
 to the other, the difference between them lying in the fact that 
 one is paternal and the other maternal in origin. 1 Reduction (or 
 halving of the total number of chromosomes) is accomplished by 
 the distribution of the two members of each pair to different 
 daughter cells. Hence if the paternal and maternal chromosomes 
 contain different alternative factors (or determinants), derived 
 from different parents, so also will the gametes when these are 
 formed. 
 
 Suppose we represent the two members of any pair of factors 
 by the letters a and b, then any one gamete may contain either 
 a or b, but not both. When the gametes unite in conjugation the 
 zygote will again have both maternal and paternal chromosomes 
 and there will be three possibilities as to its constitution in 
 regard to the characters in question. If a gamete containing a 
 happens to unite with another containing a the zygote will 
 contain the pair a a, and will be termed in Mendelian phraseology 
 a " homozygote." 2 If a gamete containing b happens to unite 
 with another containing b the zygote will contain the pair b b and 
 will again be a homozygote. If, on the other hand, a gamete 
 containing a unites with another containing b the zygote will 
 contain the pair a b, and such a zygote is termed a " heterozygote " 
 or hybrid. 
 
 In accordance with what is termed the " presence or absence 
 hypothesis " one of the two " factors" in an allelomorphic pair may 
 be merely negative in character ; i.e. it may have no real existence 
 as a material prmiordiurn or determinant. In the words of 
 Professor Bateson, "All observations point to a conclusion of great 
 importance, namely that a dominant character is the condition 
 due to the presence of a definite factor, while the corresponding 
 recessive owes its condition to the absence of the same factor. This 
 generalization, which so far as we yet see, is applicable throughout 
 the whole range of Mendelian phenomena, renders invaluable 
 assistance in the interpretation of the phenomena of Heredity. 
 The green pea, for instance, owes its recessive greenness to the 
 absence of the factor which, if present, would turn the colouring 
 matter yellow, and so forth." 3 
 
 It is obvious that if, in our general formula, we take a to 
 represent the dominant factor and b the recessive, we might 
 
 1 Compare Chapter X. 
 
 2 This term has been extended to the fully developed organism which arises from 
 the zygote. 
 
 3 Bateson, " Mendel's Principles of Heredity " (Cambridge, 1909), pp. 5354.
 
 208 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 substitute not a for b without altering the result, and the differences 
 between the paternal and maternal chromosomes of any given pair 
 may depend merely upon the absence from one or the other of 
 one or more factors which are present in its mate. 
 
 Mendelian phenomena are often greatly complicated and 
 rendered very difficult of interpretation by the fact that the 
 activity of a given factor, or its power to influence the development 
 of the zygote, may be dependent upon the presence of another 
 factor belonging to a different " allelomorphic pair." In other 
 words, a given character may depend not merely upon the 
 presence of a single factor in the zygote but upon the co-operation 
 of two or more factors, and if these factors happen to be separated 
 in the process of reduction and do not happen to come 
 together again in the union of the gametes to form the zygote, 
 then the character in question l will not appear in the organism 
 developed from the zygote. In this way it may happen that a 
 character which was present in the ancestors of a particular 
 organism may disappear for many generations and then suddenly 
 re-appear as the result of some cross or hybridization in which 
 the necessary factors happen to be brought together again in the 
 zygote. In this way are explained those cases of " reversion " to 
 ancestral types which Darwin found to occur so frequently as the 
 result of cross-breeding. 
 
 One of the most thoroughly investigated cases of this kind 2 is 
 that of certain white sweet peas belonging to the variety known 
 to horticulturists as "Emily Henderson." Plants of this variety 
 are not really all alike but differ from one another in the shape of 
 their pollen grains, and when the two kinds, distinguishable only 
 in this way, are crossed, the offspring are frequently found to 
 possess purple flowers resembling those of the wild Sicilian pea 
 from which our cultivated varieties have been derived. It is not 
 necessary to go into the somewhat complicated analysis of this 
 case in any detail, but it has been demonstrated by the researches 
 of Professor Bateson and Professor Punnett that the re-appearance 
 of the lost colour in the hybrid is due to the re-union in the zygote 
 of certain factors which had become separated at some period or 
 other in the ancestral history of the white-flowered parents. 
 
 The existence of interactions of this kind between the different 
 factors or determinants in the germ plasm, giving rise often to 
 
 1 Professor Bateson terms such characters " compound characters." 
 
 2 Bateson, op. cit., pp. 8'J et seq.
 
 G ALTON'S LAW OF INHERITANCE 209 
 
 very complex results as regards the characters of the offspring, is 
 a sufficient explanation of the fact that many cases of hybridism 
 are at present incapable of interpretation in terms of the 
 Mendelian theory, and such cases cannot be legitimately used as 
 arguments against the validity of that theory. It is obviously 
 impossible to explain the results of any particular cross until we 
 have some knowledge, not only of the extremely complex con- 
 stitution of the parental germ plasm on both sides, but also of 
 the way in which the factors which meet in the zygote may be 
 expected to influence one another. 
 
 Mendelian phenomena, of course, can occur only as a result 
 of crossing or hybridization at some stage or other of the 
 ancestral history, and it does not seem likely that such crossing 
 has had any very great influence upon the evolution of plants and 
 animals in a state of nature. In cases of monohybridism, as we 
 have seen above, the hybrid form in the course of a few generations 
 would probably be quite swamped by the numerical preponderance 
 of the pure extracted forms which have reverted to one or other 
 of the original parental types, the hybrid itself never being 
 permanently fixed. New and permanent combinations may, of 
 course, sometimes arise naturally through dihybridism, but they 
 would at first be produced in very small numbers and could only 
 be expected to survive in the struggle for existence if they 
 happened to possess some material advantage over the parent 
 forms. Thus it appears that in a state of nature the results of 
 hybridization tend to be automatically eliminated. 
 
 Attention has frequently been called to the alleged discrepancy 
 between the results of the Mendelian experiments and those 
 obtained by the late Sir Francis Galton and others by statistical 
 methods based upon the quantitative estimation of characters in 
 a number of successive generations. As a result of his investiga- 
 tions Galton was able to formulate the following " Law " of 
 inheritance : " The two parents contribute between them on the 
 average one-half, or (0'5) of the total heritage of the offspring ; 
 the four grand-parents, one-quarter, or (0'5) 2 ; the eight great- 
 grandparents, one-eighth, or (0'5) 3 , and so on. Thus the sum 
 of the ancestral contributions is expressed by the series { (0'5) + 
 (0'5) 2 + (0'5) 3 , &c.}, which, being equal to 1, accounts for the 
 whole heritage." 1 
 
 This series has been modified by Professor Karl Pearson for 
 
 1 Proceedings of the Royal Society of London, Vol. LXI.. 1897. p. 402. 
 B. P
 
 210 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 certain reasons into which we cannot enter in this place, but it 
 may be taken as a substantially correct expression of the results 
 obtained by biometrical inquiry. If we remember that Galton's 
 Law merely expresses the average results which may be antici- 
 pated from the interbreeding of a large population, in which 
 hybridization probably plays a very small part, it does not appear 
 in any way antagonistic to the Mendelian theory. It is of course 
 not applicable to individual cases of hybridization, where we are 
 concerned, not with small variations in the same characters, but 
 with the permutations and combinations of alternative character 
 units. 
 
 Intimately bound up with Galton's Law of Inheritance is 
 another important generalization known as the Law of Filial 
 Regression, which we owe to the same distinguished philosopher. 
 The law of ancestral inheritance teaches us that on an average 
 the individual derives half its characteristics from its immediate 
 parents and the remaining half from its more remote ancestry. 
 If the immediate parents, or one of them, happen to depart from 
 the average condition of the race in respect of any character 
 there will be a tendency on the part of the offspring to inherit 
 the deviation in question ; but not to the same extent, for the 
 influence of the more remote ancestry will tend to counteract 
 that of the parents and cause a partial return or regression 
 towards mediocrity. As a result of his statistical investiga- 
 tions Galton estimated that the offspring of parents exhibiting 
 a marked deviation from the average would tend to inherit that 
 deviation to the extent of only one-third of its magnitude in the 
 parents (the mean of the two parental deviations being taken as 
 the standard of comparison). In the w r ords of Professor Karl 
 Pearson, " It is the heavy weight of this mediocre ancestry which 
 causes the son of an exceptional father to regress towards the 
 general population mean ; it is the balance of this sturdy common - 
 placeness which enables the son of a degenerate father to escape 
 the whole burden of the parental ill. Among mankind we trust 
 largely for our exceptional men to extreme variations occurring 
 among the commonplace, but ... if we could remove the 
 drag of the mediocre element in ancestry, were it only for a few 
 generations, we should sensibly eliminate regression or create a 
 stock of exceptional men." l 
 
 1 Grammar of Science, 1900, pp. 456457.
 
 PART IV. THE THEORY AND EVIDENCE OF 
 ORGANIC EVOLUTION: ADAPTATION 
 
 CHAPTER XV 
 
 Organic evolution versus special creation Spontaneous generation and 
 biogenesis The origin of living things. 
 
 AT the present day we see the surface of the earth teeming with 
 hosts of living things, incalculable in number and of endless 
 diversity in form and structure. Every situation where life is 
 possible is occupied by plants or animals of some kind or other, 
 all specially adapted in bodily organization to the conditions 
 under which they have to maintain their existence. From the 
 bleak and inhospitable summits of high mountain ranges to ocean 
 depths which can be measured in miles ; from the perpetually 
 frozen circumpolar regions to the torrid zone on either side of 
 the equator, living things abound. Seas, rivers, lakes, dry land 
 and air have all alike been taken possession of by representatives 
 of the animal and vegetable kingdoms. A single drop of water 
 may contain thousands of organisms, and every chalk-cliff, coal- 
 seam or peat-bog testifies to the countless myriads which have 
 lived in past times and whose remains have contributed in no 
 small measure to the formation of the earth's crust. 
 
 In the animal kingdom alone it is probable that at least a 
 million different kinds or species are living on the earth at the 
 present day. Some half million or so have already been dis- 
 covered, named and more or less imperfectly described, while 
 every exploring expedition brings back many which have never 
 before been seen. 
 
 Nevertheless there must have been a time when no living 
 things whatever existed on the earth. According to the 
 nebular hypothesis our planet is still gradually cooling from a 
 molten state, preceded probably by an incandescent gaseous 
 condition, which must have rendered the existence of any proto- 
 plasmic organism a physical impossibility. Protoplasm becomes 
 
 p 2
 
 212 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 coagulated and thereby destroyed as living substance at com- 
 paratively low temperatures, and cannot have been present on the 
 earth until the surface of the latter had not only solidified but 
 also cooled down to something approaching its present degree 
 of heat. 
 
 Living things must therefore have first put in an appearance 
 at some definite period well advanced in the earth's history- 
 According to the biblical account given in the Book of Genesis 
 their advent was due to a series of special acts of creation per- 
 formed by the Creator at the appropriate time, and this account 
 forms the basis of the doctrine of Special Creation which still 
 survives amongst uneducated people. Upholders of this doctrine 
 maintain that each kind of plant or animal was not only created 
 as a separate kind and in a state of full perfection, but was 
 specially designed to suit the conditions under which it was 
 placed, and in this way the marvellous adaptation of plants and 
 animals to their environment was supposed to be accounted for. 
 The fish was created to swim in the sea, the horse to run on 
 dry land, the monkey to climb in the tree and the bird to fly in 
 the air, and each was constructed in accordance with its foreseen 
 requirements. Moreover, each species was supposed to be immut- 
 able, propagating its own kind by reproduction but never changing 
 into a different kind. 1 
 
 Such a view of the origin of living things could only have arisen 
 in a state of almost complete ignorance of the phenomena which 
 have to be accounted for, and at the present day it has been 
 entirely superseded, as a scientific theory, by the doctrine of 
 Organic Evolution. This doctrine teaches us that species or kinds 
 are not immutable but are subject to changes whereby one may 
 give rise to another, and that all existing species are the more or 
 less modified descendants of pre-existing ones. Modern biologists, 
 moreover, maintain that the further back we trace the ancestral 
 history of living things the less diversity of structure do they 
 show, until finally, if we could trace them back to their origin, 
 we should find all the different lines of descent converging towards 
 a common starting point far back in geological time. We shall 
 see presently that the adaptation to environment which forms 
 such a characteristic feature of all living organisms can be 
 explained just as well in accordance with the theory of organic 
 
 1 Compare the quotation from Linnaeus on p. 222 (1st footnote) and that from 
 Buffon on p. 3.69.
 
 SPECIAL CREATION AND EVOLUTION 
 
 213 
 
 evolution, or transmutation of species, as in accordance with that 
 of special creation. 
 
 The fundamental difference between these two opposing and 
 irreconcilable doctrines can be expressed by means of the 
 accompanying simple diagrams (Fig. 82) : 
 
 v V XX v v 
 
 C' D' 
 
 Special Creation 
 
 FIG. 82. 
 
 The letters A E are supposed to represent five closely related 
 existing species in each case. According to the doctrine of 
 special creation each of these species has persisted without altera- 
 tion from the time when it was first created, so that the 
 ancestral species may be represented by the letters A' B' C' D' 
 and E', and the lines of descent of existing species run parallel 
 with one another. The idea of evolution, on the other hand, 
 is expressed by showing these lines of descent diverging from 
 a remote ancestral species, x, which may have been quite different 
 in character from any of the existing species. 
 
 Any existing species at the present day is made up of a 
 larger or smaller number of individual organisms, and we know 
 as a matter of fact, proved by daily observation, that these
 
 214 OUTLINES OF EVOLUTIONAEY BIOLOGY 
 
 individuals arise by some process of reproduction from pre-exist- 
 ing parent individuals. This is represented in the upper parts 
 of both diagrams, where a 1 8, b 1 7, c 1 4, d 1 5 and 
 e 1 5, on each side, are supposed to represent small groups of 
 individuals of the species A B C D and E respectively, each 
 group being descended from some common ancestor (or pair of 
 ancestors) which itself belonged to the species. 
 
 The study of this diagram is alone sufficient to afford strong 
 presumptive evidence in favour of the view of the evolutionist as 
 against that of the upholder of special creation, for the evolu- 
 tionist in his imagination extends backwards into the past the 
 processes which he sees taking place constantly at the present 
 day, and endeavours to account for the origin of species in 
 accordance with what he knows to be true of the origin of 
 individuals. In other words, the diagram expressing the idea of 
 organic evolution is consistent throughout, whereas that which 
 represents the idea of special creation is made up of two incon- 
 gruous portions, and is therefore less likely to be correct. The 
 great weakness of the doctrine of special creation, however, lies 
 in its failure to explain countless facts of comparative anatomy, 
 embryology, geographical distribution and palaeontology, all of 
 which are readily explicable in terms of evolution. We shall 
 deal with some of these facts in subsequent chapters. 
 
 It is obvious, then, that in order to be logically consistent the 
 evolutionist need not postulate more than a single starting point 
 for the evolution of the whole organic world, though he is not 
 obliged to limit himself in this way should evidence be forth- 
 coming that there has been more than one starting point. This 
 brings us to the consideration of the extremely difficult and at 
 present quite insoluble problem of the origin of the first living 
 organisms. 
 
 Here again we may first inquire whether our experience of what 
 takes place at the present day throws any light upon the problem. 
 Many people in the past have held, and some few still maintain, 
 that living things may, even now, arise from not-living matter. 
 This is known as the doctrine of " Spontaneous Generation " or 
 abiogenesis. One of the earliest expressions of this idea is met 
 with in classical mythology, in the story of the birth of Aphrodite 
 from the sea-foam ; and Vergil, in the Georgics, accepts the prin- 
 ciple involved therein apparently in perfectly good faith. 
 
 Vergil's account of the manner in which a swarm of bees may
 
 SPONTANEOUS GENERATION 215 
 
 be produced from the decomposing body of a dead ox affords a 
 very striking illustration of that want of scientific training in 
 observation and reasoning which has led to so many erroneous 
 beliefs. He tells us, with much elaboration of detail, that if the 
 body of an ox is well beaten and enclosed in a suitable chamber a 
 swarm of bees will arise from it within a certain number of days. 
 We now know perfectly well what may really happen under these 
 circumstances. In the first place the supposed bees are not bees 
 at all, but drone-flies, which superficially resemble bees, though 
 easily recognizable as belonging to a totally different order of 
 insects by the fact that they possess only two wings instead of 
 four, and consequently any husbandman who followed Vergil's 
 instructions must have been grievously disappointed in his 
 expectations of honey. In the second place the drone-flies are not 
 spontaneously generated from the body of the ox, or from any- 
 thing else, but are hatched out, first in the form of maggots, from 
 eggs which were laid by pre-existing flies. These maggots, 
 having fed abundantly on the decaying carcase, presently undergo 
 their metamorphosis and emerge as flies. 
 
 Many similar instances of alleged spontaneous generation, all 
 resting upon gross ignorance of the real facts of the case, might 
 be collected from the writings of ancient and mediaeval authors. 
 The cruder stories, which could be easily disproved by simple 
 observation, were soon cast aside as fables under the influence of 
 modern scientific methods. The invention of the microscope, 
 however, and the consequent revelation of a new world of 
 hitherto invisible organisms, led to a revival of the doctrine of 
 abiogenesis. It was noticed that organic infusions, even after 
 boiling, presently became densely filled with various kinds of 
 micro-organisms, especially Bacteria, and as the boiling was 
 supposed to have killed any organisms that might have been in 
 them at first it was argued that living things arose in them 
 de noi-o, by spontaneous generation. This was, however, merely 
 a revival of Vergil's story of the swarm of bees in a more refined 
 form. Had Vergil's ox been protected from flies none of the 
 alleged bees would have been produced, and the careful experi- 
 ments of such observers as Tyndall and Pasteur have conclusively 
 demonstrated that if the organic infusions are adequately pro- 
 tected from the countless microscopical germs which float in the 
 air they will remain free from living organisms, and consequently 
 from putrefaction, for an indefinite period, provided always that
 
 216 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 they are themselves first properly sterilized. This sterilization, 
 however, cannot always be effected by a single boiling, for the 
 germs of some organisms are extremely resistant to heat and 
 special precautions have to be taken accordingly. 
 
 Every story of alleged spontaneous generation has, so far, 
 failed to stand the test of scientific criticism and experiment, and 
 accordingly biologists are now almost unanimous in maintaining 
 the contrary doctrine of biogenesis, which teaches that, at the 
 present day at any rate, living organisms arise only by repro- 
 duction from pre-existing living organisms. Nevertheless there 
 must, as we have seen, have been a time when there were no 
 living things on the earth, and therefore living things must have 
 either reached the earth from some outside source or have arisen 
 on the earth itself from matter which was previously not-living, 
 or in other words, by spontaneous generation. It seems as if we 
 had got to take our choice between these two alternatives, neither 
 of which is at all easy to accept. 
 
 It has been maintained by some that organic life is eternal and 
 is transferred from one world to another possibly on meteorites 
 in the form of minute germs or " Cosmozoa," but no one has ever 
 seen such Cosmozoa, and it is difficult to imagine any which 
 would be capable of surviving such a journey. It is perhaps less 
 difficult to believe that under certain conditions, of which we at 
 present know nothing and which perhaps no longer exist on the 
 earth's surface, living protoplasm may have arisen from in- 
 organic matter as the result of chemical and physical processes. 
 In this connection we must remember that protoplasm the 
 physical basis of life contains no chemical elements that are 
 not also found in inorganic matter and is, as a matter of fact, 
 constantly being built up from inorganic constituents in the 
 bodies of living organisms, though apparently only by the action 
 of pre-existing protoplasm. 
 
 Whether physical and chemical forces, such as we are 
 acquainted with, alone sufficed to bring about the transformation 
 of not-living, inorganic material into living protoplasm, or 
 whether, as some people suppose, some unknown " vital force " 
 was (and still is) involved in the process, must remain, for the 
 present at any rate, an open question. 
 
 The time has evidently not yet arrived for solving the great 
 problem of the origin of life, but we may, at any rate, legitimately 
 speculate upon the nature of the living organisms which first
 
 THE FIRST LIVING THINGS 
 
 217 
 
 appeared on the earth. In the first place we may safely assume 
 that they were extremely simple in structure, for it is generally 
 agreed that the evolution of the higher forms of life has been 
 accompanied by a gradually increasing complexity of organization. 
 It is also certain that they cannot have been animals, for animals, 
 as we have seen in a previous chapter, are dependent for 
 their food supply upon other living things, being themselves 
 unable to build up the proteid molecule from inorganic constitu- 
 ents. Green plants are the great proteid manufacturers at the 
 present day, but only by virtue of the fact that they contain 
 chlorophyll, which enables them to utilize the energy of the sun's 
 
 FIG. 83. Different Forms of Disease-producing Bacteria, X about 1500. 
 (From Strasburger, after Fischer.) 
 
 a, pus cocci ; b, erysipelas cocci ; c, gonorrhoea cocci ; d, bacilli of splenic fever ; e, bacilli 
 of tetanus; //bacilli of diphtheria; g, tubercle bacilli; h, typhoid bacilli ; i, colon 
 bacilli ; k, cholera bacilli. 
 
 rays in the process of photosynthesis. Now chlorophyll itself is a 
 very complex substance, and we cannot suppose that the first living 
 things already possessed it. If, then, they had no other organisms 
 to feed upon, and if they possessed no chlorophyll, how did they 
 obtain the energy necessary to enable them to maintain life 
 at all? 
 
 The simplest and at the same time the smallest organisms known 
 to us at the present day are the Bacteria (Fig. 83), many of which 
 are so minute as to be hardly visible even under the highest 
 powers of the microscope. These Bacteria are neither plants 
 nor animals, but occupy a position lower than either. They have 
 not even attained to the dignity of perfect cells, for they exhibit 
 no proper differentiation into cell body and nucleus, the chromatin 
 substance, which in a typical cell is aggregated in the nucleus,
 
 218 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 being scattered in minute granules throughout the cytoplasm 
 (vide Fig. 27). They contain no chlorophyll and the colourless 
 protoplasm is enclosed in a very thin and delicate cell wall. In 
 form they vary greatly, being sometimes spherical, sometimes 
 rod-shaped and sometimes corkscrew-shaped, and frequently 
 they occur united together in chains. Some of them swim about 
 by means of cilia or flagella, others are motionless, and they 
 multiply by simple fission and also by the formation of spores, 
 which have extraordinary powers of resistance and thus serve to 
 secure the wide dispersal of these organisms by air and water. 
 
 A great number of different kinds of these Bacteria are already 
 known to us, and doubtless a vast number still remain to be dis- 
 covered. They occur practically everywhere ; earth, air and water 
 are full of them, and they exercise a most 
 profound influence sometimes injurious 
 and sometimes beneficial upon the lives 
 of other living things. Some of them are 
 the active agents in the putrefaction and 
 fermentation which are rapidly set up in 
 dead organisms, and others are responsible 
 for many of the most grievous ills which 
 living flesh is heir to. Thus most of them 
 
 FIG. 84. - Nitrosomonas live ither as saprophytes (upon dead 
 
 europcea, x 600. organic matter) or as parasites (upon 
 
 (From Percival's Jiving organisms) and obtain their supply 
 ' ' Agricultural Bacte- * *. / 
 
 riology.") f energy from already formed proteids 
 
 and other complex chemical compounds. 
 
 Some, however, are said to live free in the soil and to be able 
 to derive their food supply, and with it their energy, from purely 
 inorganic substances. These are called nitrifying Bacteria. One 
 form, Nitrosomonas (Fig. 84), converts salts of ammonia into 
 nitrites, and another, Nitrobacter, converts nitrites into nitrates. 
 In this way they perform an important function in preparing the 
 food material in the soil for the use of green plants, but their chief 
 interest for us lies in the fact that they are able, in some manner 
 which we do not understand, to build up their own proteid mole- 
 cules directly from inorganic constituents without the aid of chloro- 
 phyll. They show us that such a thing is at any rate possible, 
 and that therefore the first living things may have been able to 
 maintain their existence without either possessing chlorophyll 
 or having other organisms to feed upon.
 
 SPONTANEOUS GENEEATION 219 
 
 It seems likely that of all the organisms known to us these 
 Bacteria come nearest to the first living things, and yet they 
 probably stand very far from them and represent a much later 
 stage in the process of evolution. Minute and apparently 
 simple as the Bacteria are, it seems more than probable that the 
 first living things were very much smaller and simpler, so that 
 even if we had them under the highest powers of our micro- 
 scopes we should be unable to recognize them. Weismann has 
 suggested that they may have been single biophors, i.e. vital 
 units of the first order, such as he believes to constitute the 
 ultimate living particles of protoplasm, and he has actually 
 proposed the name " Biophoridae " for these hypothetical free- 
 living primordial organisms. 
 
 These considerations throw a new light upon the question 
 of spontaneous generation, for if living matter is first formed 
 in such ultra-microscopic particles and can only be recog- 
 nized as living matter after it has reached a comparatively 
 high stage of evolution, it is obvious that we are not entitled to 
 say that it is never formed from not-living matter at the present 
 day. We cannot see it being formed, and we probably never 
 shall see it being formed, but it is possible that it is still 
 being "spontaneously generated" all the same. We are not 
 logically obliged, as we said before, to content ourselves with a 
 single starting point for organic evolution, and it would be quite 
 impossible to prove that all the different kinds of Bacteria, the 
 simplest organisms known to us, have descended from a single 
 ancestor. They may equally well have been derived from a 
 number of ancestral protoplasmic units which originated inde- 
 pendently from inorganic, not-living matter. If such an event 
 can have taken place once it may have taken place many times, 
 and may still be taking place around us, though the imperfect 
 means of observation at our disposal will not allow us to demon- 
 strate the fact. We may safely affirm, however, that no living 
 organism which we are at present capable of recognizing as such 
 has arisen, or ever will arise, by spontaneous generation, but that 
 all organisms known to us have been derived from pre-existing 
 organisms by some process of reproduction ; that, in the course 
 of long ages, they have undergone slow changes, whereby they 
 have become more and more diversified and usually more com- 
 plex in structure, and that in this way the evolution of the 
 animal and vegetable kingdoms has been brought about.
 
 220 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 In concluding this chapter it may be well to remind the reader 
 that the idea of organic evolution is no novelty, but can be 
 traced back to the philosophy of ancient Greece. It will be more 
 convenient, however, to postpone what we have to say about the 
 history of the evolution theory to a later chapter and in the 
 meantime to familiarize ourselves with the theory itself and the 
 evidence upon which it is based.
 
 CHAPTER XVI 
 
 The continuity of life The conception of species The principles of 
 taxonomy The taxonomic evidence of organic evolution. 
 
 IN accordance with the theory of evolution we may picture 
 to ourselves the entire animal and vegetable population of the 
 earth, both past and present, as forming one vast, tree-like 
 organism, all parts of which, if we knew enough about their 
 history, could be traced into actual, although of course not simul- 
 taneous, protoplasmic connection with all other parts. This 
 tree commenced its growth far back in geological time, and its 
 branches became ever more and more ramified in succeeding 
 ages, and more and more diversified in character as they 
 diverged from one another. Only the youngest twigs of the 
 tree, however, are still actually alive, being represented by the 
 animal and vegetable population of the earth for the time being. 
 All the older parts have died away, most of them without leaving 
 any traces of their former existence, and thus has arisen the 
 actual discontinuity which is found to occur between the sur- 
 viving groups of organisms at any given period. 
 
 The tendency to structural variation which all organisms 
 exhibit, whatever may be its cause, is responsible for the 
 progressive diversity which is gradually set up between the 
 different branches of the organic tree. The combined action of 
 the forces of heredity and variation bring about " descent with 
 modification." How it is that such modification always leads, 
 in the long run, to a more or less perfect and often very 
 specialized adaptation of both plants and animals to the con- 
 ditions under which they live will be discussed subsequently. 
 At the moment we have to deal with things as we actually find 
 them. 
 
 In any scheme of zoological or botanical classification the 
 lowest unit must, of course, be the individual. The untrained 
 observer, acquainted only with the more familiar plants and 
 animals, sees no difficulty in arranging these in apparently
 
 222 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 sharply circumscribed groups. A child collecting flowers by 
 the wayside will tell you without difficulty how many kinds he 
 has found, and these kinds speaking generally correspond 
 to the different species recognized by the botanist. 
 
 Even amongst the higher and better known plants and 
 animals, however, the so-called species are not always sharply 
 distinguishable from one another, for we not infrequently meet 
 with intermediate forms, or connecting links, between what are 
 usually recognized as distinct species. Linnaeus himself, who 
 pronounced most explicitly in favour of the doctrine of special 
 creation, 1 was forced by his profound knowledge of the animal 
 and vegetable kingdoms to recognize this fact in the case of 
 certain groups of plants 2 ; and when we come to study some of 
 the lower members of the animal kingdom, as, for example, the 
 sponges, the difficulty of distinguishing species sharply from one 
 another frequently becomes so great that the definition of any 
 particular one is little more than a matter of individual 
 opinion. The more we study the animal and vegetable king- 
 doms, in short, the more clearly is the fact impressed upon us that 
 if we could have before us all past and present individuals we 
 should find it impossible, except in an arbitrary manner, to 
 arrange them in species at all, for each kind would be found 
 to be connected with others by a series of small gradations. 
 
 According to Darwin, the differences which separate existing 
 species from one another have, at any rate in most cases, arisen 
 by the gradual accumulation of small successive variations. If 
 this be the case the fact that we are able logically to distinguish 
 such species from one another at all can only be due to that dis- 
 appearance of the older portions of the organic tree whereby 
 discontinuity has arisen between the surviving branches. In 
 any case it is quite certain that such destruction of former 
 connecting links has played a very large part in the separation of 
 those groups of individuals to which the term species is usually 
 applied. This process is clearly illustrated in the accompanying 
 diagram (Fig. 85), in which each enclosed area is supposed 
 
 1 " Species tot sunt, quot diversas formas ab initio prodnxit Infinitum Ens ; quas 
 cleinde formae secundum gencrationis inditas leges produxere plures, at sibi semper 
 similes, ut Species mine nobis non sint plures, quam qua3 fuere ab initio." 
 
 Linnseus. Gen. Plant. (1737). 
 
 2 " Species Rosarum difficillime limitibus circumscribuntur et forte natura vix 
 cos posuit." Liimams. Spec. Plant,, Ed. 2, p. 705 (17(52). 1 am indebted to my 
 friend and colleague at the Linnean Society, Dr. B. Daydon Jackson, for showing 
 me these quotations.
 
 DEFINITION OF SPECIES 
 
 223 
 
 to represent the limits of an existing species. Within each one 
 the individuals differ but slightly from one another, not more, 
 perhaps, than might be expected amongst the offspring of the 
 same parents. The three species represented have, however, 
 become more or less widely separated from one another by the 
 dying away of the branches from which they arose, and which 
 are represented in the diagram by dotted lines. 
 
 Darwin has told us that he looked " at the term species as 
 one arbitrarily given, for the sake of convenience, to a set of 
 individuals closely resembling each other, and that it does not 
 essentially differ from the 
 term variety, which is 
 given to less distinct and 
 more fluctuating forms. 
 The term variety, again, 
 in comparison with mere 
 individual differences, is 
 also applied arbitrarily, for 
 convenience' sake." l 
 
 This, of course, is no defi- 
 nition of the term species 
 and was not intended as 
 such. A definition in ac- 
 cordance with the above 
 views might, however, be 
 given as follows : " A 
 
 species is a group of FiG. 85. Diagram to illustrate the Separa- 
 individuals that closely 
 resemble one another 
 owing to their descent from common ancestors, which has 
 become more or less sharply separated from all other co-existing 
 species by the disappearance of intermediate forms." Such a 
 definition would be in accord with the practice of many systematic 
 naturalists, who are in the habit of uniting species previously 
 considered as distinct whenever intermediate forms are found, 
 the former species being reduced to the rank of varieties of one 
 and the same species. 
 
 It is obvious that the element of time must be taken into 
 consideration in forming such a conception of species. At any 
 
 1 ( )i ijin of Species." Ed. 6, p. 42. The reader should compare the very similar 
 views ( ,f Lamarck on the species question, given in Chapter XXIV. 
 
 tion of Species by the Dying out of 
 connecting Links.
 
 224 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 given time the species living on the earth would form naturally 
 limited groups, which we should be able to define more or less 
 sharply if we had sufficient knowledge of the then existing 
 fauna and flora to enable us to trace their boundaries. If, 
 however, we were to include extinct forms in our survey, it is 
 obvious that the number of existing species which we should 
 recognize would be inversely proportional to the extent of our 
 palaeontological information, for the gaps between the survivors 
 would gradually be filled up by the discovery of intermediate 
 fossil forms. 
 
 Naturalists have long recognized the fact that there is another 
 way in which more or less sharply defined groups of individuals 
 may arise, and that is by the occasional and sudden appearance of 
 " sports," or " mutations " as they are now generally called, 1 which 
 " breed true," handing on their special peculiarities from genera- 
 tion to generation. Darwin was of opinion that such sports differ 
 only in degree from the ordinary variations to the accumulation of 
 which he attributed the chief importance in the evolution of species. 
 At the present day, as we have already had occasion to point out, 
 some observers, and especially Professor Hugo de Vries, 2 consider 
 them to be totally different in kind from fluctuating or small 
 successive variations, and regard them as affording the sole means 
 by which new species originate. 
 
 According to the mutation theory any species may, from time 
 to time, throw off such sports, either singly or in groups, and they 
 are from the first distinguishable by well defined, though it may 
 be minute, characters from the parent form. These mutations 
 are regarded by de Vries as constituting " elementary species," 
 which are already fully characterized and will not change again 
 unless by giving rise to fresh mutations. De Vries maintains 
 that many of the species of flowering plants as defined by 
 Linnaeus are really " aggregate species," each made up of a larger 
 or smaller number of such elementary species. The common 
 weed known as Draba verna, for example, regarded by Linnaeus 
 as constituting a single species, occurs under about 200 more or 
 less distinct forms. All of these so-called elementary species are 
 believed to come true from seed and to have arisen as sudden 
 mutations. 
 
 1 Compare Chapter XI. 
 
 2 Vide "The Mutation Theory," by Hugo de Vries. English translation by 
 Farmer and Darbishire (Kegan Paul, Trench, Triibner & Co. Ld., London, 1910).
 
 CLASSIFICATION 225 
 
 According to this view a newly arisen species, at its first origin, 
 is already sharply distinguished from the parent species and the 
 extinction of intermediate forms is not required to separate the two. 
 
 The elementary species, however, though apparently constant 
 in their characters, are often so similar to one another, and 
 they are moreover so numerous, that for practical purposes of 
 classification it would be undesirable to regard them as distinct 
 species in the ordinary sense of the word. It is far more con- 
 venient to maintain the older view that they are merely varieties 
 or subspecies, and their occurrence in no way invalidates our 
 conception of the slow, tree-like evolution of the organic world. 
 If de Vries be right as regards the importance he attributes to 
 them as the sole means by which species have originated, which 
 is extremely doubtful, it merely means that evolution has taken 
 place in a rather more jerky fashion than we previously supposed. 
 It will be necessary to return to this question when we come to 
 discuss the factors of organic evolution. 1 
 
 It is not sufficient for the purposes of classification to arrange the 
 individual plants or animals with which we are acquainted in their 
 respective species. These species in turn must be arranged in 
 groups of higher order, which are termed genera. Each genus 
 stands to the species included within it in much the same relation 
 that the species themselves stand in with regard to individuals, 
 and related genera, though often sharply definable, are only so by 
 virtue of the disappearance of connecting links. The characters 
 by which genera can be distinguished from one another are of 
 a more deep-seated and fundamental nature than those which 
 distinguish species. Individual opinion, however, differs greatly 
 as to the exact limits which should be assigned to each, and it is a 
 common thing for an old established genus to be subdivided into 
 several as the result of the discovery of new species and the more 
 exhaustive study of old ones. 2 It is all a matter of convenience, 
 and one may justifiably make a separate genus for any group 
 of closely related species which can be distinguished by some 
 common character from all other co-existing groups of species. 
 Many naturalists, however, consider that the mutual relation- 
 ships of species are more accurately expressed by making use of 
 subgenera as intermediate groups between genera and species. 
 
 1 Vide Chapter XXVII. 
 
 2 A striking illustration of this is afforded by the recent history of the old genera 
 Peripatus and Amphioxus, each of which is now subdivided into several. 
 
 B. Q.
 
 226 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Genera in turn are grouped in families, separated from one 
 another by still more fundamental characters, and with sub- 
 families as intermediate groups where necessary. Families are 
 grouped in suborders and orders, orders in subclasses and classes, 
 classes in phyla (or subkingdoms), and phyla in kingdoms. Of 
 kingdoms only two are recognized, animals and plants, and even 
 these cannot be sharply separated from one another, because 
 amongst the lowest forms of life (Protista) the distinction between 
 animals and plants, as we have already seen, ceases to exist. 
 
 The arrangement of individual organisms in species, and the 
 grouping of these to form genera, families, orders, classes, phyla 
 and kingdoms, constitutes the work of the systematist and is 
 usually spoken of as classification, while the study of the 
 principles in accordance with which classification should be 
 carried out forms a special branch of biological science sometimes 
 known as taxonomy. 
 
 The first and greatest of the modern systematists was Karl von 
 Linne, now usually known as Linnaeus, a Swedish naturalist who 
 was born in 1707 and died in 1778. In his great work, the 
 " Systema Naturae," which is now recognized as the starting point 
 of modern systematic zoology and botany, he described all the 
 species of plants and animals then known, and endeavoured to 
 arrange them in a " natural system " in accordance with their 
 mutual resemblances and differences. As an aid to the 
 accomplishment of this formidable task he invented the binomial 
 system of nomenclature, in accordance with which each kind of 
 plant or animal is referred to by the name of the genus as well as 
 by that of the species to which it belongs. The necessity for the 
 constant repetition of longer or shorter descriptions as a means 
 of identification every time there is occasion to refer to a 
 particular species is thus avoided, while at the same time the use 
 of the generic name makes it possible to employ the same 
 specific name in a number of different genera without risk of 
 confusion, a very important point in consideration of the 
 enormous number of species which have to be distinguished from 
 one another. 
 
 The binomial system of nomenclature has proved so well 
 adapted to its purpose that it has survived practically unchanged 
 to the present day, although the number of known species has 
 increased enormously in the interval and the Linnaean system 
 of classification has undergone profound modification at the hands
 
 NOMENCLATURE 227 
 
 of later investigators. Very many of the more familiar plants 
 and animals are, however, still known by the names which 
 Linnaeus gave to them. 
 
 It still frequently happens that newly discovered species have 
 to be named and new genera denned, and various attempts have 
 been made to formulate a code of rules for the guidance of those 
 engaged in this work. This is necessary in order to secure, so 
 far as possible, uniformity of method, and to avoid confusion 
 such as would arise by giving the same name to different genera 
 or different species within a genus, or different names to the same 
 genus or species. The chief points so far agreed upon are that 
 generic and specific names (as well as those of larger groups) 
 should be given in either Latin or Greek and that strict attention 
 should be paid to the law of priority, the name first given 
 taking precedence over any which may be proposed subsequently, 
 as is often the case owing to ignorance of the work of previous 
 writers. 
 
 The greatest latitude has, however, been allowed in the past 
 with regard to the coining of generic and specific names. Even 
 the resources of the Latin and Greek languages have proved hardly 
 sufficient to meet all requirements, and many generic names have 
 been given which, though euphonious enough, have no meaning 
 whatever. It is said that one eminent naturalist (Dr. Gray) 
 used to make up such names by drawing letters out of a hat, and 
 it is certain that Dr. Leach derived the generic names of a whole 
 series of parasitic Crustacea (Cirolana, Conilera, Nerocila, &c.) from 
 anagrams on his wife's name, Caroline. The most satisfactory 
 names, on the other hand, are those which convey some definite 
 information as to the characters of the genus or species concerned 
 or as to the place where it is found. 
 
 If we have to classify a number of inorganic bodies we may 
 set about our work in a variety of ways. We may arrange them 
 according to colour, or size, or weight, or shape, or texture, or 
 chemical composition, or according to the places they come from, 
 the precise method adopted depending upon what it is that we 
 wish to express by means of our classification. 
 
 It is exactly the same with plants and animals, the way in 
 which we classify them will depend upon our point of view. A 
 fisherman classifies fishes largely according to their food-value, and 
 various other animals according to whether or not they are good for 
 bait, and such classification serves its purpose quite satisfactorily. 
 
 Q 2
 
 228 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 The aim which the zoologist or botanist sets before himself in 
 classification is the expression of the " natural affinities " of the 
 plants or animals investigated. The existence of such natural 
 affinities was clearly recognized long before the explanation of 
 them was known, and even by upholders of the doctrine of special 
 
 creation. Thus it was 
 Individual 
 vn Bears 
 
 Individuals 
 
 Species 
 
 Genera 
 
 Families 
 
 Orders 
 
 Classes 
 
 Species A re cos 
 
 t 
 
 Genus Ursus 
 
 Family Ursidae: 
 
 Order Carnivora 
 
 Class Mammalia 
 
 Phylum Vertebrata 
 
 well known to Linnaeus 
 that individuals fall 
 naturally into species, 
 that species may be 
 grouped in genera, 
 genera in families, and 
 so on in ever widening 
 circles. It also soon 
 came to be recognized 
 that natural affinities 
 could be best determined 
 by taking into account 
 as many characters as 
 possible instead of rely- 
 ing merely on one or 
 two. A system of classi- 
 fication based on a small 
 number of characters 
 only is always more 
 or less artificial, and 
 though it may serve its 
 purpose for a time it 
 will have to be amended 
 by future workers who 
 are able to bring a more 
 complete knowledge to 
 bear upon the problem. 
 Thus it comes about 
 that our views on the 
 classification of the animal and vegetable kingdoms are always 
 undergoing change, old systems are constantly being discarded 
 as too artificial and more natural ones proposed in their stead. 
 
 As our knowledge progresses it becomes more and more evident 
 that a natural classification assumes a tree-like form. The 
 division of the whole organic world (with the exception of the 
 
 Phyla 
 
 Kingdoms 
 
 Kingdom Animalia 
 
 The Organic World 
 
 FlG. 86. Diagram, to show the Tree-like Form 
 assumed by a natural Classification.
 
 THE TAXONOMIC TREE 229 
 
 lower Protista) into plants and animals is represented by the first 
 branching of the tree, the division of each of these great king- 
 doms into phyla by the next branching, and the subdivision of 
 phyla into classes, orders, families, genera, species and individuals 
 by the further ramifications, as shown in the accompanying 
 diagram (Fig. 86), which represents a mere fragment of the whole 
 tree selected to show the systematic position of a single species of 
 animal, the common brown bear, Ursus arctos. 
 
 This great truth affords one of the most striking pieces of 
 evidence in favour of the theory of organic evolution, for the 
 tree-like form assumed by a natural classification bears an 
 unmistakable resemblance to the tree-like development of 
 the whole organic world which evolutionists believe to have 
 taken place. The two results represent, indeed, but slightly 
 different aspects of the same truth ; the resemblance between 
 them is no mere coincidence, but the fact that we are able 
 to classify organisms in a tree-like manner indicates very clearly 
 that these organisms have been produced by tree-like evolution. 
 
 The systematist is now able to replace the vague groping after 
 " natural affinities " by a much clearer conception of the aims of 
 taxonomy. What he has to strive after is the elucidation of the 
 actual pedigrees of existing organisms, the unravelling of what is 
 termed their phylogeny or ancestral history. This means neither 
 more nor less than the ultimate reconstruction of the whole vast 
 tree of life. Piece by piece this is gradually being done, and it is 
 the great German biologist, Ernst Haeckel, who has led the way 
 in this particular department of biological investigation. Some 
 of the main branches of the tree have already been reconstructed 
 in a satisfactory manner, but it is a work of immense difficulty 
 and practically unlimited extent, and at every step opinions 
 differ as to the value of conflicting evidence and the interpreta- 
 tion of obscure facts. The greater part of the tree is dead and 
 gone beyond recall, and such fragmentary information concerning 
 extinct forms as may be supplied by the discovery of fossils can 
 only be supplemented by indirect evidence derived from the study 
 of existing organisms. 
 
 We must now inquire a little more fully into the relation 
 between the tree-like form assumed by a natural system of 
 classification, or the taxonomic tree, as we may term it, and the 
 phylogenetic tree, which represents the actual pedigrees of 
 organisms. In the former (Fig. 86) the main trunk of the tree,
 
 230 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 as we have seen, represents the entire organic world; in the 
 latter it represents the remote Protistan ancestors from which 
 both plants and animals have sprung. The first great branching 
 represents in the one case the systematist's main division of 
 organisms into vegetable and animal kingdoms, and in the other 
 the gradual differentiation of the ancestral Protista (which at one 
 
 time constituted the 
 'TL ' *,*, entire organic world) 
 into the primitive 
 unicellular plants or 
 Protophyta on the 
 one hand and the 
 primitive unicellular 
 animals or Protozoa 
 on the other ; and so 
 on with subsequent 
 ramifications. 
 
 The parallelism 
 between the two is 
 sufficiently striking 
 to justify us in believ- 
 ing that it would be 
 complete if only our 
 knowledge of classi- 
 fication and phyto- 
 geny were so ; we 
 should then doubtless 
 see at once that the 
 taxonomic tree and 
 the phylogenetic tree 
 are, after all, one and 
 the same thing, for we should arrange all organisms strictly in 
 accordance with the course of their evolution. 
 
 One point remains to be noticed in connection with the phylo- 
 genetic tree. The branching has been monopodial rather than 
 dichotomous or polychotomous, each branch, in addition to giving 
 off lateral branches, being itself continued on, so to speak, at 
 each forking. The descendants of any given ancestral group 
 did not all undergo modification to the same extent, but some 
 adhered more or less closely to the ancestral condition and have 
 continued to do so to the present day. Even the primitive 
 
 Ancestral CoelenferaL 
 
 Ancestrj/ Protista 
 
 FIG. 87. Diagram illustrating the Eelation 
 between Classification and Phytogeny .
 
 CLASSIFICATION AND PHYLOGENY 231 
 
 ancestral Protista are still represented by descendants which 
 probably differ but little from their remote progenitors and which 
 have not yet reached the level of either plants or animals, and 
 there are still living unicellular forms in abundance which, though 
 they may perhaps be distinguished as plants or animals, have 
 never been able to learn the secret of forming multicellular 
 bodies. So it is with most of the great ancestral groups ; they 
 are represented still by descendants which have undergone com- 
 paratively little change in structure since remote geological 
 periods. The algae are still algae, the ccelenterates are still 
 coelenterates and the fishes are still fishes, though each of these 
 great groups has in past time given rise to descendants which 
 have gradually become modified into higher types. 
 
 Many subordinate groups, such as the trilobites and ammonites 
 and the winged reptiles of the secondary period, have of course 
 become extinct during past geological ages, but the fact remains 
 that even at the present day there still exist on the earth organic 
 types which represent, in a more or less unmodified form, all the 
 principal stages of organic evolution, and thus it is that, even if 
 we had only living plants and animals to consider, our classifica- 
 tion of the organic world would still assume a tree-like form, 
 with the simplest organisms at the bottom and the most complex 
 at the top of the tree. It would be difficult indeed to explain 
 this fact in accordance with the theory of special creation and 
 immutability of species. 
 
 The monopodial branching of the organic tree and the relation 
 which the natural classification of existing animals bears to the 
 phylogenetic or ancestral series is diagrammatically represented 
 in Fig. 87. On the right hand side are shown some of the 
 principal stages in the evolution of vertebrates from ancestral 
 Protista, and at the top of the tree are shown the existing groups 
 by which these stages are actually represented at the present 
 day. Only a few stages are represented and the great majority 
 of the lateral branches of the tree are supposed to have been 
 lopped off.
 
 CHAPTEE XVII 
 
 Connecting links Homology and analogy Convergent evolution 
 Change of function Vestigial structures Eeversion. 
 
 IN presenting the evidence of organic evolution it is convenient 
 to separate that derived from the study of comparative anatomy 
 from that afforded by taxonomy, but the separation is not 
 a logical one, for classification is necessarily based upon com- 
 parative anatomy, and much of the evidence might be equally well 
 dealt with under either heading. 
 
 The mere fact that we are able to arrange existing organisms 
 in progressive series, of which the extremes are connected by 
 intermediate forms, in itself suggests that one form has been 
 derived from another. This gradation is equally obvious whether 
 we study entire organisms or confine our attention to their com- 
 ponent parts or organs. It is illustrated very clearly by the 
 accompanying diagram of the modifications exhibited by the 
 siliceous spicules of certain sponges (Fig. 88). These micro- 
 scopic organs of the sponge, each of which arises within a single 
 cell, exhibit an astonishing diversity and beauty of form. The 
 extreme types are as different from one another as we can well 
 imagine, but in the particular group of sponges from which our 
 illustration is taken (the Tetraxonida) all may be derived from the 
 same four-rayed architype (number 1 in the centre of the 
 figure) and numerous intermediate forms mark out the lines 
 along which evolution has taken place. An analogous diagram, 
 with a triaxonid architype, could easily be constructed for those 
 remarkable deep-sea sponges known as the Triaxonida or 
 Hexactinellida, and another for the calcareous sponges. 
 
 Amongst the higher animals also innumerable illustrations of 
 the same general principle of gradation in structure, even in co- 
 existing types, are met with, and are to be explained in the 
 way indicated in the last chapter, as expressions of phylogenetic 
 relationship. Take, for example, those now widely contrasted 
 groups of vertebrate animals the Reptilia and the Mammalia. The
 
 CONNECTING LINKS 
 
 233 
 
 reptiles as a group are distinguished from the mammals in 
 many ways. Amongst other things they usually have a scaly 
 epidermic covering, but they never have hair; they lay large, 
 
 
 79. 
 
 FIG. 88. Series of tetraxon Sponge Spicules, showing how all may be 
 derived from the same fundamental type, highly magnified. (From 
 Bendy, Article " Sponges " in Encyclopaedia Britannica, Ed. XL) 
 
 heavily -yolked eggs and do not suckle their young ; they 
 have a cloaca into which the urinary and genital ducts, 
 as well as the alimentary canal, discharge ; they have a very 
 complex shoulder girdle (Fig. 89), consisting of a consider- 
 able number of more or less separate bones or cartilages,
 
 234 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 CL. 
 
 <E P C. 
 
 including suprascapula, scapula, coracoid, epicoracoid, clavicle 
 and interclavicle. 
 
 The mammals, on the other hand, are characterized by the 
 possession of an epidermic covering of hairs ; they generally 
 have very minute eggs almost destitute of yolk, and they always 
 suckle their young ; they usually have no cloaca but separate 
 openings for the alimentary canal and urino-genital ducts, and 
 the shoulder girdle (Fig. 90) is very greatly reduced, being often 
 represented by only a single bone, the scapula, with a small 
 
 remnant or vestige of 
 the coracoid completely 
 fused with it, though a 
 slender clavicle is some- 
 times present as well. 
 
 There exists in Aus- 
 tralia and some of the 
 adjacent islands, how- 
 ever, a small but very 
 interesting group of 
 animals known as the 
 Monotremata, repre- 
 sented by the duck-billed 
 Platypus (Ornithorhyn- 
 chus, Fig. 91) and the 
 spiny anteater (Echidna, 
 Fig. 92). The fact that 
 these animals possess 
 hair and suckle their 
 young justifies us in 
 classifying them amongst the Mammalia, but at the same time 
 they exhibit certain characters in respect of which they differ 
 from all typical mammals and agree with the reptiles. Thus they 
 lay large, heavily-yolked eggs, the alimentary canal and urino- 
 genital ducts discharge into a common cloaca, and the shoulder 
 girdle (Fig. 93) is made up of a considerable number of separate 
 bones almost exactly as in reptiles, most remarkable amongst 
 which is the interclavicle, which is found in no other mammals. 
 
 Here then we have animals still existing which undoubtedly 
 occupy an intermediate position between the reptiles and the 
 typical mammals, and the three groups exhibit a progressive series 
 of structural peculiarities. Such definite connecting links as the 
 
 fnt 
 
 FIG. 89. Shoulder Girdle and Sternum of a 
 Lizard ( Varanus) seen from below. 
 
 Cl. Clavicle; Cor. Coracoid; Ep. C. Epicoracoid; 
 Gl. Glenoid cavity ; Int. Interclavicle ; E. Ribs ; 
 Sc. Scapula ; St. Sternum.
 
 LIMBS OF VERTEBRATES 
 
 235 
 
 Monotremata, however, are more usually met with amongst the 
 extinct animals of past geological periods. They indicate the 
 paths along which the more highly organized groups have pro- 
 gressed during their evolution from more lowly organized 
 ancestral groups, which latter may or may not still be represented 
 by surviving forms at the present day. 
 
 It would be difficult to explain the occurrence of such graduated 
 series of organs and organisms by the theory of special creation, 
 and it would be no less difficult to explain in this way those 
 remarkable facts of comparative anatomy which are grouped 
 
 FIG. 90. Shoulder Girdle and part of Sternum of a Rabbit, seen from below. 
 
 Cl. Clavicle; Cor. Coracoid process of Scapula (= Vestige of Coracoid); Gl. Glenoid 
 cavity ; E. Ribs ; Sc. Scapula ; St. Sternum. 
 
 together under the terms homology and analogy (or homoplasy) 
 to which we shall next refer. 
 
 We have already had occasion to point out that all living 
 organisms are more or less perfectly adapted to the conditions under 
 which they exist, and are accordingly provided with organs suitable 
 for their various requirements. Thus all typical vertebrates have 
 organs of locomotion formed from two pairs of limbs, but these 
 differ very greatly in form and structure in accordance with their 
 adaptation to very diverse conditions of life. In terrestrial 
 vertebrates amphibians, reptiles and mammals both pairs of 
 limbs usually take the form of ambulatory legs, adapted for 
 moving the body on dry land. In birds the hind limbs are 
 generally used for the same purpose and similarly constructed, but
 
 236 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the fore limbs are modified to form wings for flying in the air ; 
 and the same is true of the fore limbs of bats and of the extinct 
 
 FIG. 91. The Duck-billed Platypus (Ornithorhynchus anatinus). (From 
 British Museum Guide.) 
 
 flying reptiles known as Pterosauria (pterodactyls). The Cetacea 
 (whales and porpoises), the Sirenia (dugongs and manatees) and 
 the seals, amongst mammals; and the turtles and extinct 
 
 FIG. 92. The Echidna or Spiny Anteater (Echidna aculeata). (From British 
 Museum Guide.) 
 
 Ichthyosauria and Plesiosauria amongst reptiles, on the other 
 hand, have limbs which take the form of paddles and are 
 specially adapted for swimming in water. 
 
 We find, then, three chief types of locomotor organs amongst
 
 LIMBS OF VERTEBRATES 
 
 237 
 
 the air-breathing vertebrates : ambulatory legs, wings and 
 paddles, employed in totally different methods of locomotion and 
 differing widely from one another in general form and appear- 
 ance. Yet when we come to examine these organs closely we very 
 soon discover the remarkable fact that all are constructed on 
 essentially the same plan ; all belong to what is known as the 
 pentadactyl or five-fingered type of appendage. 
 
 We ourselves retain this type of limb-structure in a compara- 
 tively primitive condition (Fig. 94), although our fore limbs have 
 taken on new functions as organs of prehension. The fore limb 
 
 FIG. 93. Shoulder Girdle and part of Sternum of Ornithorhynchus, seen 
 from below. 
 
 Cl. Clavicle ; Cor. Coracoid ; Ep. C. Epicoracoid ; Gl. Glenoid cavity ; Int. Interclavicle ; 
 12. Bibs ; Sc. Scapula ; St. Sternum. 
 
 consists of arm, forearm, wrist and hand (manus) with five 
 fingers ; the hind limb of thigh, leg, ankle and foot (pes) with five 
 toes. Each segment of the limb is supported by an internal 
 skeleton of bone which is clothed with muscle and skin. The bones 
 are articulated with one another at the joints and moved like so 
 many levers by the contraction of the muscles attached to them. 
 The bone of the arm is the humerus and the corresponding bone of 
 the thigh is the femur. In the forearm we find the radius and 
 ulna and these are represented in the leg by the tibia and fibula. 
 The wrist is made up of a number of small carpal bones and the 
 ankle of a number of more or less similar tarsals. These small 
 bones are arranged mainly in two rows, and the distal or far row
 
 238 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 affords support to the five metacarpal bones in the palm of the 
 hand or the five corresponding metatarsal bones in the foot. Each 
 metacarpal or metatarsal bone in turn forms the support of a 
 series of phalanges which constitute the skeleton of the fingers or 
 toes. The thumb and great toe have each two phalanges and 
 each of the other digits has three. 
 
 When the human arm is stretched out at right angles to the 
 long axis of the body, with the thumb towards the head, it is in 
 the primitive position of the pentadactyl limb. The thumb and 
 the radius then lie on what is called the preaxial or anterior border, 
 
 FIG. 94. Skeleton of the fore Limb (A) and hind Limb (B) of Man, showing 
 the pentadactyl Structure. (From photographs.) 
 
 c. carpals ; fe. femur; fi. fibula: hum. humerus; m.c. metacarpals; mt. metatarsals ; 
 ph. phalanges; r. radius; t. tarsals ; ti. tibia; ul. ulna; I V, digits. 
 
 the little finger and the ulna on the postaxial or posterior border 
 of the limb. For convenience of comparison with other types 
 the digits are numbered from in front backwards, the thumb 
 being No. I and the little finger No. V. Similarly in the hind 
 limb, which in man has become permanently twisted out of its 
 primitive position, it is easy to show that the great toe is really 
 the preaxial digit (No. I) and the little toe the postaxial (No. V). 
 In Fig. 94 both limbs are represented in the primitive position. 
 
 Starting from the primitive pentadactyl type let us next inquire 
 more closely how different groups of air-breathing vertebrates have 
 solved the problems of locomotion on land, in air and in water. 
 
 In considering the modifications of the limbs adapted for
 
 LIMBS OF VERTEBEATES 239 
 
 locomotion on dry land we may conveniently confine our attention 
 to the hoofed mammals or Ungulata. In the elephant the limbs 
 remain, as in man, in a primitive condition, with five well 
 developed digits in each. The entire limb is stout and massive 
 and the foot 1 remains very short and affords a broad support for 
 
 EIG. 95. Skeletons of Man and Horse, photographed from a group in the 
 Natural History Museum. (From Lankester's " Extinct Animals.") 
 
 E, Elbow bone (olecranon process of ulna) ; H, Heel bone in ankle (the hock of the 
 horse) ; K, Knee joint (the stifle of the horse) ; P, Hip bone ; Sh, Shoulder bone 
 (scapula) ; T, Vertebrae of tail (coccyx in man) ; W, Wrist or carpus (the so-called 
 knee in the horse's fore leg). 
 
 the heavy body. This type of limb is somewhat clumsy and not 
 adapted for very rapid locomotion. 
 
 In the more typical ungulates we always find a reduction in the 
 number of digits, accompanying an uplifting of the hinder part 
 of the foot from the ground, until finally the animal comes to 
 
 1 The term foot in the case of quadrupeds is commonly used to include both 
 manus and pes.
 
 240 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 walk and rest on the extreme tips of one or two toes only. The 
 assumption of this " unguligrade " condition is accompanied by 
 elongation of the limb and especially of the remaining 
 metapodial bones (metacarpals and metatarsals), so that the wrist 
 and ankle in the horse, for example, are uplifted high above the 
 ground and form the so-called knee and hock respectively (Fig. 95). 
 
 FIG. 96. Skeleton of (A) the right fore Foot and (B) the right hind Foot 
 of a Tapir, X . (From photographs.) 
 
 c. carpals; m.c. metacarpals; m.t. metatarsals; ph. phalanges; t, tarsals; II V, digits. 
 
 It is also accompanied by a reduction of the ulna in the fore limb 
 and of the fibula in the hind limb to mere vestiges. Thus the 
 entire limb becomes long and slender and adapted for rapid 
 locomotion by running on hard open ground. 
 
 The disappearance of digits from the manus and pes in the 
 Mammalia follows a very simple law. The first to disappear is 
 always the preaxial digit (No. I), which is .the shortest of the 
 series (having only two phalanges while the remainder have
 
 LIMBS OF VERTEBRATES 
 
 '241 
 
 I 
 
 X/s. 
 
 \Y- 
 
 three each) and consequently the first to leave the ground as 
 the hinder part of the foot is uplifted. The next to go is the 
 postaxial digit (No. V) from the opposite side, the next No. II 
 and the next No. IV. 
 
 In some ungulates the long axis of the foot passes through 
 the middle digit (No. Ill) and in others between the third and 
 fourth digits. The true ungulates are 
 accordingly divided into two series, odd- 
 toed or perissodactyl and even-toed or 
 artiodactyl. 
 
 To the former belong the tapir 
 (Fig. 96), with four digits in the fore foot 
 and three in the hind ; the rhinoceros, 
 with four or three digits in the fore foot 
 and three in the hind, and the horse 
 (Fig. 97), with only the middle (third) 
 digit remaining in both manus and pes, 
 but with vestiges of Nos. II and IV in 
 the functionless " splint-bones " which 
 lie alongside the greatly elongated meta- 
 podial (= metatarsal or metacarpal) of 
 No. III. 
 
 In the artiodactyl series (Fig. 98) we 
 find the hippopotamus, with four well- 
 developed toes on each foot; the pig, 
 with four toes on each foot but the two 
 outer ones greatly reduced; the deer, 
 with the two outer digits still further 
 reduced ; the sheep and oxen, with the 
 second and fifth digits reduced to small 
 nodules of bone, and the camels and 
 llamas, in which all traces of digits other 
 than the third and fourth have completely disappeared. The 
 gradual reduction of the number of digits in the artiodactyl series is 
 accompanied by fusion of the two remaining metapodials to form 
 a single" cannon bone " (well shown in Fig. 98, deer and camel), 
 whereby the now greatly elongated and uplifted foot acquires 
 much greater strength and rigidity. 
 
 It is very easy to see that all the different varieties of 
 perissodactyl and artiodactyl limbs are modifications of one 
 and the same primitive pentadactyl type. We must next 
 
 B. R 
 
 FIG. 97. Fore and hind 
 Feet of a Horse, show- 
 ing reduction to a single 
 functional Digit (No. 
 Ill) with Vestiges of 
 two others (II and IV) 
 in the form of splint 
 bones, X J-. (From 
 Lull, after Marsh.)
 
 242 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 consider the way in which this type has been adapted to form 
 organs of flight in different vertebrate groups. 
 
 The wing of the extinct pterodactyls (Fig. 99) was formed 
 
 by an expansion of the skin of the body stretched between the 
 fore and hind limbs, and the phalanges of the little finger were 
 enormously elongated so as to aid in the support of its anterior
 
 LIMBS OF VERTEBKATES 
 
 243 
 
 margin. The surface of the flying membrane was probably 
 covered with scales. In the bats (Fig. 99) four of the digits 
 
 of the hand, Nos. II V, are elongated and take part in the 
 support of the flying membrane, the surface of which is either 
 naked or covered with fine hair. In the birds (Fig. 99) the 
 
 B 2
 
 244 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 flying membrane is formed chiefly of feathers, and the distal 
 part of the wing is supported mainly by the second digit ; 
 the first and third digits are greatly reduced, and the 
 other digits are absent. In all these different types of wing 
 the skeleton is again clearly of the pentadactyl type, modified 
 by enlargement, diminution or total suppression of certain of its 
 constituent bones. 
 
 In the paddles of the modified aquatic mammals, such as the 
 sirenians, seals (Fig. 100) and whales (Fig. 101), the whole limb 
 
 FIG. 100. Skeleton of right fore Foot (A) and right hind Foot (B) of a 
 Seal (Otaria hookeri], x j?. (From photographs.) 
 
 c. carpals ; m.c. metacarpals ; m.t. metatarsals ; ph. phalanges ; I V, digits. 
 
 is very much shortened and flattened and the digits are enclosed in 
 a common integument so as to offer a greater resistance to the 
 water, but the skeleton still reveals the essential pentadactyl 
 features, and the same is true of those aquatic reptiles, the 
 turtles, plesiosaurians (Fig. 141) and ichthyosaurians (Fig. 142). 
 In the whales, however, a curious increase in the number of 
 phalanges (hyperphalangy) may often be observed, and in the 
 ichthyosaurians supplementary small bones are developed in 
 such positions that the entire skeleton of the paddle assumes 
 the form of a mosaic pavement.
 
 LIMBS OF VERTEBRATES 
 
 245
 
 246 OUTLINES OF E VOLUTION AEY BIOLOGY 
 
 We can only explain the occurrence of the same type of 
 skeleton and that a very complex type in all these different 
 kinds of organs of locomotion on the assumption that it has been 
 inherited from common ancestors. We cannot believe that one 
 and the same type of skeleton was necessarily the most suitable 
 for all these different cases, including ambulatory legs, wings and 
 paddles, and was therefore specially and independently created for 
 each. We must conclude rather that each organism receives a 
 certain kind of material by inheritance from its ancestors and 
 has to adapt it to its own requirements as best it may ; has, in 
 short, to cut its coat according to its cloth, and, whatever the 
 
 FIG. 102. A swimming Crab (Portttnus depurator), showing jointed ambu- 
 latory appendages and also swimming appendages (s.l.). (From a 
 photograph.) 
 
 shape of the coat may ultimately develop into, the cloth will 
 retain, more or less evidently, traces of the original pattern. 
 
 This conclusion is greatly strengthened when we turn to other 
 groups of animals and see how they have solved the same problems 
 with the aid of different materials. The vertebrates are not the 
 only animals which have organs of locomotion in the form of 
 jointed appendages. Many members of the great group 
 Arthropoda insects, crustaceans and spiders have ambulatory 
 limbs which externally bear considerable resemblance to those 
 of vertebrates. If we examine these limbs, however, we find that 
 they are constructed upon a wholly different plan. In the first 
 place the skeleton is external instead of internal, and is composed
 
 HOMOLOGY AND ANALOGY 247 
 
 of the hard cuticle secreted by the epidermis, there being no true 
 bone. The muscles therefore lie inside the skeleton instead of 
 outside. The number and arrangement of the limb-segments, 
 again, is totally different, and there is, of course, no pentadactyl 
 structure (compare Fig. 102). 
 
 Thus, with different material at their disposal, the arthropods 
 have solved the problem of constructing an ambulatory appendage 
 in a very efficient manner, but quite differently from the 
 vertebrates. Many of them have also solved the problem of 
 locomotion in water by modification of certain of the limbs to 
 form paddles, as in the case of the swimming crab (Fig. 102). 
 Many insects, on the other hand, have acquired the power of 
 flight by means of wings, which, though bearing some external 
 resemblance to those of vertebrates, are totally different in 
 structure and origin not only from the latter but also from the 
 other appendages of the arthropods themselves. 
 
 We are now in a position to define the meaning of the terms 
 homology and analogy as used by biologists. Homologous 
 organs are such as have the same essential structure, 
 which they owe to inheritance from common ancestors, 
 though they may be very differently modified in adaptation to 
 different functions. The pentadactyl limbs of air-breathing 
 vertebrates, however much they may differ amongst themselves, 
 are all homologous organs in so far as their essential pentadactyl 
 structure is concerned. 
 
 Analogous or homoplastic organs, on the other hand, bear only 
 a superficial resemblance to one another, which they owe not to 
 common ancestry but to adaptation of fundamentally different 
 structures along similar lines for similar functions. The 
 ambulatory appendages of arthropods and vertebrates are 
 analogous but not homologous organs, so also are the wings of 
 birds and insects. 
 
 The evolutionary process by which analogous but not 
 homologous structures have come to resemble one another is 
 sometimes spoken of as convergence, and the result may be looked 
 upon as an illustration of the general principle that similar causes 
 tend to produce similar effects. The necessity for the adaptation 
 of different organs and organisms to the same environment and 
 the same mode of life results in a superficial resemblance between 
 the organs and organisms thus adapted. 
 
 One of the most familiar examples of convergent evolution
 
 248 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 is afforded by the whales, dolphins and porpoises (Figs. 101 
 and 161) with their extraordinary external resemblance to 
 fishes. The fore limbs, as we have already seen, are modified 
 to form paddles, but they retain the pentadactyl structure and 
 are thus totally different from the fins of fishes, which have 
 never reached the pentadactyl stage. The hind limbs have 
 completely disappeared, but vestiges of the limb bones and their 
 supporting girdle, which in some cases are found buried deeply 
 beneath the skin in the pelvic region (Fig. 101, f,p), still bear 
 witness to their former presence. The powerful tail fin has 
 obviously no real relationship to the tail fin of a fish, for it 
 
 lies horizontally instead of 
 vertically. There are, of 
 course, no gills, as in fishes, 
 but the animal comes to 
 the surface to breathe air 
 by means of lungs. 
 
 Not only is the whale 
 not a fish, but it belongs 
 to the group of vertebrates 
 most remote from fishes. 
 It is a warm-blooded 
 mammal, suckling its 
 young and exhibiting other 
 FIG. 103. The seven cervical Vertebrae characteristically mamma- 
 
 of a Whale, fused together in one 
 Mass. (From Reynolds " V ertebrate 
 Skeleton.") 
 
 li an features. It has seven 
 
 . , 
 cervical vertebrae, a number 
 
 which is curiously constant 
 throughout the mammalian series, but owing to the extreme 
 shortness of the neck these vertebras are all crushed together to 
 form practically a single bone (Fig. 103). In the giraffe we also 
 find seven cervical vertebrae, but they are all greatly elongated 
 in accordance with the enormous length of the neck (Fig. 104). 
 On the hypothesis of special creation we should certainly have 
 expected the whale to have fewer vertebras in its neck than the 
 giraffe, and we can only suppose that the number seven has 
 been inherited from some common mammalian ancestor. The 
 resemblance of the whale to the fish, in short, is simply due to the 
 fact that both have acquired the external form best adapted for an 
 active aquatic life. 
 
 The winga of pterodactyls, bats and birds are equally good
 
 CONVERGENT EVOLUTION 249 
 
 examples of convergent evolution within much narrower limits. 
 They can only be regarded as homologous to the extent of all being 
 
 FIG. 104. Skeleton of the Giraffe, showing the seven separate and greatly 
 elongated cervical Vertebra. (From Brehm's "Thierleben.") 
 
 pentadactyl. They do not owe their special characters as wings 
 to descent from a common ancestral winged form, but have been
 
 250 
 
 OUTLINES OF EVOLUTIONAEY BIOLOGY 
 
 evolved independently from more primitive pentadactyl types of 
 limb. 
 
 We find analogous cases in other vertebrate groups. Several 
 of the lizards, such as the so-called slow worm or blind worm 
 (Fig. 105) and the " glass snakes " (Ophisaurus), have, by loss 
 of their limbs, come so closely to resemble snakes as to be 
 
 FIG. 105. The Slow Worm, Anguis frayili's, X ^. (From a photograph.) 
 
 indistinguishable by most people. The Coeciliidse (Fig. 106) 
 amongst amphibians, the Amphisbfenidae amongst lizards and 
 the Typhlopidffi amongst snakes have all adapted themselves to a 
 burrowing underground life, like the earthworms. Their limbs 
 have completely disappeared, the body has become cylindrical and 
 worm-like and they have lost the use of their eyes, but though they 
 
 FIG. 106. A worm-like, limbless Amphibian, Uran,typlilus africanus. (From 
 British Museum Guide.) 
 
 have all come to resemble one another closely as the result of 
 convergent evolution they are not in reality at all nearly related. 
 The loss of limbs, the assumption of the worm-like form, &c., 
 have taken place quite independently in each group. 
 
 The mammalian fauna of Australia consists mainly of members 
 of the single order Marsupialia or pouched mammals, but 
 different representatives of this order have come, by convergence, 
 to resemble closely members of widely different orders of
 
 CONVERGENT EVOLUTION 
 
 251 
 
 mammals found in other parts of the world. The so-called 
 marsupial wolf of Tasmania (Thylacinus) closely resembles a 
 typical carnivore in its habits and general structure. Its teeth 
 are adapted for a predaceous life, and the entire skull (Fig. 107, B), 
 with its dentition, bears an extraordinary general resemblance to 
 that of a dog (Fig. 107, A), being distinguishable only by details 
 of structure which would hardly be noticed except by an 
 
 FIG. 107. A. Skull of Dog, side view ; B. Skull of Thylacine, side view. 
 (From photographs.) 
 
 anatomist. These details, however, are quite sufficient to show 
 that there is in reality no close relationship between the two. 
 Thus the dog (Fig. 108, A, AI) has in each upper jaw three 
 incisor teeth (i. 1 i. 3 ), one canine (c.), four premolars (p.m. 1 p.m. 4 ) 
 and two molars (m. 1 m. a ) ; and the dentition of the lower jaw is 
 the same except for the presence of a third molar (m. 3 ), which is, 
 however, in a vestigial condition. The thylacine, on the other 
 hand (Fig. 108, B, BI), has four incisors (i. 1 i. 4 ) in the upper and
 
 252 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 FIG. 108. A, A!, Skull and Mandible of Dog ; B, B t , Skull and Mandible 
 of Thylacine, to .show the arrangement of the teeth, &c. (From 
 photographs.) 
 
 i. 1 . i. 4 , incisors of upper jaw;i.i i. a , incisors of lower jaw; c., canine; p.m. 1 p.m.4, 
 premolars of upper jaw ; p.ra.j p.m. 4) premolars of lower jaw ; m. 1 m.4, molars 
 of upper jaw ; m.j m.^, molars of lower jaw.
 
 CONVERGENT EVOLUTION 25B 
 
 three (i.i i. 3 ) in the lower jaw ; and one canine (c.), three 
 premolars (p.m.i 3) and four molars (ma 4) in each jaw. The 
 thylacine skull is further distinguished from that of the dog by 
 the presence of large vacuities in the hinder part of the bony 
 palate and by the strongly marked inflection of the angles of the 
 mandibles. These two characters, both of which are shown in 
 the illustrations, as well as the arrangement of the cheek teeth 
 and other minor features which it is unnecessary to specify, are 
 fundamental peculiarities of the great group Marsupialia and at 
 once indicate the true affinities of Thylacinus. 
 
 The case of the Australian marsupial mole, Notoryctes, is 
 equally striking. The powerfully built digging limbs, the soft, 
 close fur, the absence of external ears and the loss of sight, as 
 well as the general shape of the body, all in adaptation to its 
 burrowing habits, cause it to assume a wonderfully mole-like 
 aspect, while in reality it comes nowhere near the true moles as 
 regards genetic relationship. 
 
 Perhaps the most remarkable of all known cases of convergent 
 evolution, however, is met with in the Ungulata. We have 
 already seen that amongst the typical ungulate mammals of the 
 present day we can distinguish two series, odd-toed or perisso- 
 dactyl, and even-toed or artiodactyl. In the odd-toed series the 
 reduction of the digits culminates in the horse, with a single 
 perfect digit in each foot and vestiges of two others in the form 
 of splint-bones. This extreme modification of the unguligrade 
 type is so peculiar that it is difficult to believe that precisely the 
 same line of evolution has been followed independently by two 
 different groups of animals. Yet such appears to be actually 
 the case. There is an extinct group of ungulates known as the 
 Litopterna, whose remains have been described by Ameghino 
 from Tertiary beds of Patagonia. The small size of the 
 brain-cavity, the characters of the dentition, cervical vertebrae, 
 carpus and tarsus, indicate that they were more primitive forms 
 than the true Perissodactyla, and, as Dr. Smith Woodward 
 observes, they reached their maximum of specialization at an 
 earlier period than the latter. We find amongst them forms 
 (Theosodon, Fig. 109, A, B) with three well-developed toes as in 
 the rhinoceros, forms (Proterotherium, Fig. 109, C) with one well- 
 developed toe and two small ones as in some of the extinct 
 ancestors of the horse, and forms (Thoatherium, Fig. 109, D, E) 
 with a single toe as in existing horses ; and in all cases the axis
 
 254 OUTLINES OF E VOLUTION AEY BIOLOGY 
 
 IV 
 
 FIG. 109. Feet of Ungulates 
 belonging to the extinct 
 group Litopterna, from 
 Lower Tertiary deposits of 
 Patagonia. (From Smith 
 Woodward's " Vertebrate 
 Palaeontology," after 
 Ameghino.) 
 
 A, B, Theosodon lydekkeri; right fore and hind feet, x J. 
 
 C, Proterotherium intermixtum ; right fore foot, x . 
 
 D, E, Thoatherium crepidatum ; left fore and hind feet,
 
 CONVERGENCE AND CHANGE OF FUNCTION 255 
 
 of the foot passes down the middle of the third digit. If we 
 compare the foot of Thoatherium (Fig. 109, D, E) with that of a 
 horse (Fig. 97) it is hard indeed to imagine that the animals 
 possessing such closely similar and highly specialized limbs are 
 not nearly related to one another. Expert palaeontologists, 
 however, tell us that they are not, and we must believe that the 
 resemblance is due entirely to adaptation to a similar mode of 
 life in the two cases. We shall have something to say as to how 
 this adaptation was brought about in the case of the horse when 
 we come to discuss the ancestral history of that animal in 
 Chapter XIX. 
 
 It will be readily understood from the examples which we have 
 been considering that the phenomena of convergence provide 
 many pitfalls for the unwary biologist, and have led to many 
 other mistakes in classification besides the popular error of 
 placing the whales amongst the fishes. We have already noticed 
 how the limbs of arthropods have come to bear a superficial 
 resemblance to those of vertebrates, though so absolutely different 
 in their essential structure that no anatomist would dream of 
 regarding them as homologous organs. Many aquatic arthropods, 
 belonging to the class Crustacea, like the lobsters, cray-fishes 
 and crabs, breathe by means of gills which bear a superficial 
 resemblance to those of fishes, but are again by no means 
 homologous structures, and there are other resemblances between 
 arthropods and vertebrates, due probably to convergence, which 
 have led more than one observer to conclude that vertebrates are 
 descended from arthropod ancestors, a conclusion which is by no 
 means justified by the facts. 
 
 We may now further consider the process known as " change 
 of function," in which an organ primarily adapted and used for 
 one purpose takes on a new and altogether different duty and 
 becomes modified accordingly. The lungs of air-breathing 
 vertebrates, for example, are generally believed to be homologous 
 with the swim-bladder of fishes, for both arise in the same way as 
 outgrowths of the front part of the alimentary canal, in the 
 region of the throat. The swim-bladder of a fish is a hydrostatic 
 organ ; it is filled with gas, the amount of which can be regulated 
 by suitable means, and assists the animal in maintaining its 
 proper position in the water. In the dipnoan fishes, such as 
 the Australian mud-fish (Neoceratodus, Fig. 110), the South 
 American Lepidosiren and the African Protopterus, the walls of
 
 256 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the swim-bladder have become highly vascular and it serves as 
 an organ of respiration. Air is taken into it through the mouth, 
 so that the blood circulating in its wall is oxygenated, and the 
 arrangement of the heart and blood-vessels has become modified 
 accordingly. Functional gills, however, are still present. 
 
 In the amphibians the swim-bladder has become completely 
 converted into a pair of lungs, and in this way was rendered 
 possible that great step in the evolution of the vertebrates, the 
 migration from water to land. Thus in the air-breathing verte- 
 brates the gills have been entirely supplanted and replaced 
 functionally by the lungs, which are still to be regarded as 
 homologous with, or morphologically equivalent to, the air- 
 bladder of fishes. 
 
 The hand of man affords another beautiful example of change 
 
 FIG. 110.- The Australian Mud-Fish, Neoceratodus Jorsttri, greatly reduced. 
 (From British Museum Guide.) 
 
 of function from locomotion to prehension but it has under- 
 gone surprisingly little structural modification in the process. 
 The proboscis of the elephant is also a very efficient organ of 
 prehension, but it is formed by enormous elongation of the nose, 
 which is primarily an organ for conveying air to and from the 
 olfactory organs and lungs. Still more remarkable is the con- 
 version of muscle fibres in the torpedo and the electric eel 
 (Gymnotus) into powerful electric batteries, capable of giving 
 severe shocks and therefore valuable as weapons of offence and 
 defence. In this case the change of function is accompanied by 
 very profound modifications in structure. 
 
 Wherever we turn we find that novel requirements are met, not 
 by the sudden creation of new organs, but by the gradual modifi- 
 cation of old ones. As we have already said, the organism has 
 to do its best with the material which it has inherited from its 
 ancestors ; and yet the power of living protoplasm to meet every 
 new requirement by a suitable modification of bodily structure
 
 VESTIGIAL ORGANS 257 
 
 appears to be almost unlimited. We must remember, however, 
 that such modifications, in a state of nature at any rate, only 
 take place very slowly and gradually. 
 
 Traces or vestiges of organs which have ceased to be of any 
 use to their possessors persist with astonishing pertinacity in 
 many organisms. Evidently their complete removal must be 
 an extremely slow process. We have already noticed the per- 
 sistence of vestiges of the pelvic girdle and leg bones in the 
 whale (Fig. 101), of the reduced metapodials or splint bones in 
 
 FIG. 111. A New Zealand Kiwi, X &. (From a photograph of a stuffed 
 specimen.) 
 
 the feet of the horse (Fig. 97), and of the coracoid in the typical 
 mammalian shoulder girdle (Fig. 90). Unless we are to believe that 
 such structures have been put where they are on purpose to 
 mislead us we cannot possibly explain their occurrence on the 
 theory of special creation. They are, at any rate in many 
 cases, perfectly useless to their possessors, and the only rational 
 way of accounting for their presence is by supposing them 
 to be inherited from remote ancestors in which they were 
 functional. 
 
 We may now briefly notice a few other instances of such 
 B, s
 
 258 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 vestigial structures. A characteristic feature of oceanic islands 
 is the presence on them of birds which have more or less lost the 
 power of flight. Such were the dodo of Mauritius, the solitaire 
 of Rodriguez, and the gigantic moas of New Zealand. All these 
 forms are now extinct, but we still find in New Zealand several 
 flightless birds surviving. One of the most interesting is the 
 kiwi or Apteryx (Fig. Ill), a moderate-sized bird of nocturnal 
 
 habits, now rapidly be- 
 coming exterminated. 
 The whole body is covered 
 with coarse, hair - like 
 plumage and externally 
 shows no trace of wings. 
 There is, however, a 
 minute remnant of a 
 wing present on each 
 side, completely con- 
 cealed in the plumage 
 and entirely functionless 
 as an organ of flight. 
 Yet it still possesses the 
 pentadactyl skeleton, 
 though in a greatly 
 reduced condition (Fig. 
 112). It is said that 
 when the kiwi goes to 
 sleep it still endeavours 
 to tuck its long beak 
 under its wing after the 
 
 FIG. 112. Skeleton of a Kiwi, showing the manner of other birds, 
 vestigial Wing Bones, behind which a j th ti fc mo t 
 piece of Black Paper has been placed, 
 
 x i. (From a photograph.) which the -kiwis are per- 
 
 haps related, even the 
 
 last vestiges of the wings seem to have disappeared, for amongst 
 the enormous quantities of the remains of these gigantic birds 
 which occur in New Zealand no wing bones have ever been found. 
 There is very strong reason to believe that the remote ancestors 
 of existing vertebrates possessed an additional pair of eyes on 
 the top of the head, behind the still existing lateral eyes. 
 Traces of this second pair the pineal or parietal eyes are yet 
 to be found in lampreys and lizards, and especially in that
 
 VESTIGIAL ORGANS 259 
 
 remarkable New Zealand reptile the tuatara (Sphenodonpunctatus). 
 This animal (Fig. 113) is usually regarded as the oldest surviving 
 
 FIG. 113. The New Zealand Tuitara, X \. (Drawn by Miss V. E. Dondy 
 from a model and photographs.) 
 
 type of terrestrial vertebrate, the order to which it belongs (the 
 llhynchocephalia) dating back to the close of the Palaeozoic period 
 of the earth's history (Permian epoch). 
 
 In the adult tuatara there is still a fairly well developed pineal 
 
 FIG. 114. Section through the Pineal Eye of the Tuatara, X 68 (From 
 
 photograph.) 
 le., lens ; n., nerve ; ret., retina. 
 
 eye (Fig. 114), with retina, lens and nerve, but it is only about 
 5\yth inch in diameter and lies deeply buried beneath the 
 
 s 2
 
 260 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 skin in the parietal foramen in the middle of the roof of the 
 skull. It may to some extent still be functional, though probably 
 of far less importance than in the extinct amphibia and reptiles 
 of the Palaeozoic and Mesozoic periods, in which the presence of 
 a comparatively large parietal foramen indicates that the eye was 
 better developed. In the case of the tuatara it has been demon- 
 strated by anatomical and embryological investigation that the 
 remaining pineal eye is the left-hand member of the original 
 pair. In the lampreys it appears to be the right-hand member. 
 
 Turning now to the frog, we sometimes find in this animal a 
 minute light-coloured spot on top of the head (Fig. 115), between 
 the paired eyes. This marks the position of a little sac-shaped 
 
 vesicle which lies 
 beneath the skin and 
 is known as Stieda's 
 organ, consisting merely 
 of a number of undiffer- 
 entiated cells surround- 
 ing a central cavity and 
 attached to the end of a 
 kind of stalk (Fig. 116). 
 The vesicle represents 
 the functionless vestige 
 of a pineal eye and the 
 stalk probably repre- 
 sents its disappearing 
 nerve. 
 
 In the birds and mammals all trace of eye-like structure has 
 disappeared from this region, but a vestige of one or both of the 
 pineal eyes is probably to be recognized in the so-called pineal 
 gland lying on the roof of the brain, which attained such celebrity 
 in the seventeenth century owing to its identification by the 
 philosopher Descartes as the seat of the soul. 
 
 In studying the mammalian dentition we again meet with 
 plenty of illustrations of vestigial structures. The whale-bone 
 whales (Balaanidffi) have no teeth in the adult at all, but in the 
 tetus vestiges of teeth can be found imbedded in the gums. The 
 young Australian duck-billed Platypus (Ornithorhynchus) has 
 vestigial teeth which are entirely replaced by horny pads in the 
 adult. The third molar in the lower jaw of the dog (Fig. 108, 
 Ai, m. 3 ) is, as we have already seen, evidently in a vestigial 
 
 FIG. 115. Head of Frog (Rana temporaria), 
 showing Stieda's Organ between the lateral 
 Eyes. (From Studnicka, after Stieda.)
 
 VESTIGIAL ORGANS 261 
 
 condition and can be of very little if any use to its possessor, 
 and there is reason to fear that in man himself the teeth are 
 disappearing a good deal more rapidly than we could wish, 
 though in this case the disappearance seems to be chiefly due 
 to disease. 
 
 A better example of a vestigial structure in man is the 
 coccyx at the hinder end of the vertebral column (Fig. 95), which 
 represents the last remnant of the ancestral tail, and is occasion- 
 ally accompanied by vestiges of the muscles by which an ordinary 
 tail is moved. The hair on the chest, again, is a vestige of the 
 
 r.s. 
 
 FIG. 116. Section through the vestigial Pineal Eye of the Frog (Tadpole), 
 
 X 168. (From a photograph.) 
 epd., epidermis ; p.e., pineal eye; r.s., roof of skull; v.n., vestigial stalk and nerve. 
 
 hairy coat which once clothed the entire human body, as it still 
 does that of the apes. 
 
 Examples of vestigial organs might be multiplied to an 
 indefinite extent, but enough has perhaps been said to show that 
 such structures are of very common occurrence and to indicate 
 their value as evidences of organic evolution. They occur, of 
 course, not only in the adult condition but also, as in the case of 
 the embryonic gill-slits of air-breathing vertebrates, at earlier 
 stages of development, but such cases will be more conveniently 
 dealt with in the next chapter. 
 
 Closely akin to the occurrence of vestigial structures is another 
 phenomenon known as atavism or reversion, by which we mean
 
 262 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the sudden and sporadic reappearance of some ancestral structure 
 which had either been completely lost or very greatly reduced. 
 The elder Pliny has placed it on record that Caesar the Dictator 
 had a horse whose fore feet were like those of a man. This 
 statement evidently refers to a more or less complete return to 
 the ancestral pentadactyl condition. Marsh has given a figure 
 of the fore foot of a horse in which the second digit is fairly 
 well developed, with three perfect phalanges, though not so large 
 as the third, while the first is represented by a splint bone, and a 
 very similar case is exhibited in the Natural History Museum at 
 South Kensington. 
 
 In man the occasional excessive development of the canine 
 teeth is regarded as due to reversion to an ancestral condition 
 similar to that of the anthropoid apes, in which the canine teeth 
 are very large and used as weapons. The occasional occurrence 
 in man of vestigial tail muscles, and even of a short tail, may 
 also be regarded as due to reversion. Whether or not all such 
 cases are capable of being explained on Mendelian principles, as 
 suggested in Chapter XIV, it is impossible to say, but this does 
 not affect their importance as evidence of the truth of the theory 
 of organic evolution. 
 
 Although, in the present chapter, we have so far drawn our 
 illustrations entirely from the animal kingdom, it must not be 
 supposed that the same general principles cannot be equally well 
 demonstrated in the case of plants. Indeed the whole chapter 
 might be re-written entirely from the botanical point of view. 
 Change of function is very well shown, for example, in the 
 " pitchers " of Nepenthes and Sarracenia, formed from modified 
 leaves and serving as traps for catching insects. Convergent evolu- 
 tion is illustrated by the strong superficial resemblance which exists 
 amongst the various Alpine cushion plants belonging to widely 
 different orders, all of which have assumed the form best suited 
 for withstanding the peculiar hardships of their environment. 
 The leaves of the parasitic dodder, reduced to tiny scales (Fig. 
 184), and the staminodes or functionless stamens of many flowers, 
 again, might well serve as examples of vestigial structures. 
 Considerations of space, however, forbid us to pursue this fertile 
 line of inquiry any further.
 
 CHAPTER XVIII 
 
 Ontogeny The recapitulation hypothesis Interpretal ion of the onto- 
 genetic record Palingenetic and caenogenetic characters. 
 
 IN dealing with the cell theory, in Chapter IV, we have 
 already laid stress upon the fact that every multicellular animal 
 or plant commences its individual life as a unicellular egg or 
 ovum, and gradually passes by slow stages of cell-division and 
 differentiation into the adult condition. There are, of course, 
 apparent exceptions to this rule in the case of animals and 
 plants which reproduce by budding or by some analogous process, 
 where the bud arises from a group of cells belonging to the 
 parent, but even in these cases reproduction by means of 
 germ cells is resorted to at more or less frequent intervals and 
 the budding must be regarded merely as an additional method of 
 multiplication interpolated in the life-cycle. 
 
 The individual organism, then, does not come into existence 
 fully formed in all its perfection, but passes through a longer or 
 shorter process of development to the adult condition. In fact 
 it undergoes a process of individual evolution which constitutes 
 its individual life-history or ontogeny, and the length and 
 complexity of this process are proportional to the complexity of 
 organization of the adult. Thus the complete life-cycle of many 
 of the lower multicellular animals and plants is passed through 
 in the course of a few months, while a man requires twenty 
 years or more to attain his full development. 
 
 Although, for the sake of convenience, the ontogeny of any 
 given organism may be divided up into a number of stages, yet 
 these stages cannot in reality be sharply distinguished from 
 one another. Even the division or segmentation of the ovum 
 (Figs. 13 and 119), whereby the organism passes from the uni- 
 cellular to the multicellular condition, does not take place 
 suddenly but by means of the slow and complicated process of 
 mitosis or karyokinesis (Fig. 31) ; and in cases where there is an 
 apparently abrupt change, or metamorphosis, from one condition
 
 264 OUTLINES OF EVOLUTIONAKY BIOLOGY 
 
 to another, as, for example, at a later stage, from the chrysalis to 
 the butterfly, the development really progresses quite slowly 
 and gradually internally, although the external appearance may 
 for a long time remain unaltered and give the impression that 
 it is completely at a standstill. 
 
 The fact that the young organism cannot commence its life at 
 the stage reached by its parents, but has to make a fresh start 
 from the beginning and go through a whole series of stages 
 
 FIG. 117. A. Chick Embryo of about 5| days, X 5. B. Rabbit Embryo of 
 about 13 days, X 5. In both cases the foetal membranes and yolk-sac 
 have been removed. (From photographs.) 
 
 before reaching the adult condition, is very significant from the 
 point of view of the evolutionist. Still more significant is the 
 fact that different organisms all commence at the same stage 
 as unicellular eggs and come to diverge further and further 
 from one another in structure as their development progresses. 
 If we examine a number of series of embryos belonging to 
 different vertebrate types, no matter how widely the adult 
 animals may differ from one another, we shall find that as we 
 trace their life-histories backwards they gradually converge until, 
 while still at a relatively advanced stage of development, the 
 different embryos come to resemble one another so closely that
 
 THE RECAPITULATION HYPOTHESIS 265 
 
 in many cases it is doubtful if even an experienced embryologist 
 could distinguish between them. This great generalization 
 appears to have been first reached by the embryologist von Baer, 
 who in 1827 discovered the mammalian ovum. It may be 
 illustrated by the chick and rabbit embryos represented in 
 Fig. 117. 
 
 Von Baer's generalization contained the germ of what is now 
 known as the Recapitulation Hypothesis, or, as Haeckel has 
 termed it, the Biogenetic Law, which states that every organism 
 in its individual life-history recapitulates the various stages 
 through which its ancestors have passed in the course of their 
 evolution. In other words, ontogeny, or the life-history of the 
 individual, is a repetition of phylogeny, or the ancestral history 
 of the race to which the individual belongs. 
 
 We have already referred, in Chapter IV (Fig. 13), to some of 
 the earlier stages in the development of that primitive fish- 
 like animal Amphioxus (Fig. 118). Let us next inquire how 
 these stages may be interpreted in accordance with the recapitu- 
 lation hypothesis. The unicellular ovum (Fig. 13, I) obviously 
 represents the remote protozoon ancestors which were common 
 to the whole animal kingdom, and which are also represented 
 at the present day by independent unicellular organisms such 
 as the Amosba. The segmentation of the ovum into primitive 
 embryonic cells or blastomeres (Fig. 13, II VI) represents the 
 transition from the condition of the simple protozoon to that of 
 the protozoon colony, in which the individual cells, instead of 
 separating, as in the dividing Amoeba, remain together, but still 
 without undergoing any marked differentiation and division of 
 labour. The arrangement of the blastomeres in the form of a 
 hollow sphere, the blastula or blastosphere (Fig. 13, VII), with a 
 single layer of cells surrounding a central cavity, represents the 
 formation of such a protozoon colony as we see in the existing 
 Volvox (Fig. 11) or Sphaerozoum (Fig. 12). The process of 
 gastrulation, whereby the single-layered blastula is converted 
 into a two-layered gastrula (Fig. 13, VIII X), with primitive 
 digestive cavity (enteron) and primitive mouth (blastopore), repre- 
 sents the transition from the protozoon colony to the coelenterate 
 stage of evolution, the latter being still represented at the present 
 day by such forms as Hydra (Fig. 57), Obelia (Fig. 60), the jelly- 
 fish, the corals and all their numerous relations, which retain in 
 their organization all' the essential features of the gastrula,
 
 266 OUTLINES OP EVOLUTIONARY BIOLOGY 
 
 though generally complicated by the development of tentacles, 
 skeleton, &c. The development of coelomic pouches (Fig. 13, 
 XI XIII, c.p.) as outgrowths of the primitive digestive cavity, 
 and the conversion of these into mesoblastic somites (Fig. 13, 
 XV, in.8.), arranged serially or metamerically down each side of 
 the body, mark the transition from the unsegmented and 
 coelenterate condition to the metamerically segmented l and 
 co3lomate condition. The cavities of the ccelomic pouches form 
 the coelom or body cavity, which is at first transversely sub- 
 divided into compartments, as it still is in the adult earthworm, 
 while their walls form the third germ-layer or mesoblast, lying 
 between the epiblast which covers the surface of the body and 
 
 FIG. 1 1 8. The Lancelet (Air.phn.xva lar.ceolatus], X 2. (From a photograph.) 
 
 the hypoblast which lines the digestive cavity. Along the mid- 
 dorsal line of the body a strip of epiblast sinks down (Fig. 13, 
 XI, n.pl.) and becomes folded into the form of a tube (Fig. 13, 
 XII XIV, n.t.), the rudiment of the central nervous system 
 (brain and spinal cord), and beneath this tube a long strip of 
 hypoblast becomes nipped off from the roof of the gut, forming 
 the notochord (Fig. 13, XI XIV, not.) or axial skeletal rod 
 the foundation around which in higher types the vertebral 
 column is built up. A little later the front part of the gut 
 becomes pierced by gill- slits for purposes of respiration and 
 the primitive chordate condition is thus fully attained. 
 
 Amphioxus (Fig. 118) does not progress much beyond this 
 stage in its development. It never acquires any limbs but swims 
 about by lateral undulations of the body caused by the contraction 
 
 1 Metameric segmentation has, of course, nothing to do with the segmentation of 
 the ovum, but is the term applied to the transverse division of the body in many 
 animals into a series of joints or segments arranged longitudinally, each of which 
 typically repeats the essential features of all the others.
 
 THE RECAPITULATION HYPOTHESIS 267 
 
 of the lateral sheets of muscle derived from the mesoblast. No 
 skull and no vertebral column are formed, the notochord 
 remaining throughout life as almost the sole representative of the 
 internal skeleton. The brain does not become distinctly 
 differentiated from the spinal cord and no paired organs of sense 
 are developed, nor does that division of the body into head and 
 trunk, which is so characteristic of typical vertebrates, take 
 place. Amphioxus is a chordate animal but it is not a true 
 vertebrate. It probably, however, represents fairly closely a 
 stage of evolution through which the ancestors of the vertebrates 
 have passed, though it becomes somewhat modified by secondary 
 features in the later stages of its development. 
 
 It is easy enough to interpret the earlier development of Amphi- 
 oxus in terms of the recapitulation hypothesis and to recognize 
 in the ontogeny certain stages of the phylogenetic history. The 
 ontogenetic record, however, is by no means always so readily 
 decipherable. It is generally more or less obscured by secondary 
 features superposed upon it, much as an ancient picture may 
 be concealed by another painted over it. These secondary 
 features are characters which are developed in relation to the 
 requirements of the embryo itself at its various stages, for it is 
 not only the adult organism which becomes modified in structure 
 in the course of evolution but all the stages of its life-history 
 are likewise subject to adaptation. 
 
 In the first place nourishment has to be provided for the 
 developing embryo, and this necessity is present from the very 
 first division of the fertilized ovum. The blastomeres into 
 which it divides cannot go about and seek their own food supplies 
 like so many Amoabae, they have sacrificed the power of leading 
 independent lives for the sake of remaining together and building 
 up a more or less complex multicellular body ; but none the less 
 they require feeding. Hence we find that all eggs contain a 
 larger or smaller quantity of nutrient material stored up in 
 the cytoplasm. This usually takes the form of definite granules 
 or particles of food-yolk (deutoplasm), the amount of which 
 varies immensely in different cases. If there is very little the 
 egg is said to be microlecithal as in Amphioxus; if there is 
 much it is said to be megalecithal, as in the frog and still more 
 so in the bird, and the size of the egg, as we have already had 
 occasion to point out in Chapter X. depends almost entirely upon 
 the amount of food-yolk which it contains.
 
 268 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 The possession of food-yolk is, of course, of immense import- 
 ance to the embryo, for the stage which it is able to reach in its 
 development before it has to begin to provide for itself will 
 depend upon the amount of nutrient material which it has at its 
 disposal. The presence of this food-yolk, however, must also 
 act as a mechanical hindrance to the development, just as the 
 supply of provisions which he carries with him must impede the 
 progress of a traveller, though enabling him ultimately to 
 accomplish a longer journey. 
 
 In Amphioxus the egg contains hardly any food-yolk and the 
 process of segmentation and the formation of the blastula and 
 gastrula take place in an almost perfectly typical manner. 
 There is here practically no hindrance to development and no 
 obliteration of the ontogenetic record. There is, however, just 
 sufficient nutrient material to cause some of the cells those 
 which will invaginate to form the hypoblast of the gastrula to 
 be slightly larger than the remainder, which are destined to form 
 the epiblast (Fig. 13, VII, VIII). 
 
 Let us compare with this the corresponding stages in the 
 development of the frog (Fig. 119). Here the egg is, as we have 
 already indicated, much larger, owing to the greater quantity of 
 food-yolk which it contains. This is chiefly collected in the 
 lower half of the ovum, while the upper half is deeply pigmented 
 and thus readily distinguished. Segmentation begins as in 
 Amphioxus by the formation of two vertical clefts, or cell-divisions, 
 one after the other and at right angles to one another, whereby 
 the four-celled stage is reached (Fig. 119, I III). Each cleft 
 begins at the pigmented pole and throughout the whole of the 
 segmentation the yolk-laden half of the egg lags behind the 
 pigmented half because of the mechanical hindrance which the 
 food-yolk opposes to the process of cell-division. The first 
 horizontal cleft (Fig. 119, IV) divides each of the four blasto- 
 meres into two daughter cells of very unequal size, an upper 
 pigmented cell with little food-yolk and a much larger, lower, 
 yolk-laden cell with little pigment. After this, segmentation 
 continues to progress more rapidly in the upper than in the 
 lower half of the egg (Fig. 119, V), until we reach the blastula 
 stage (Fig. 119, VI), where we find the large yolk-laden cells 
 forming the lower half of the wall of the hollow sphere and pro- 
 jecting into and partly obliterating its cavity, while the small 
 pigmented cells form the upper half.
 
 EARLY DEVELOPMENT OF THE FROG 
 
 269 
 
 The process of gastrulation is profoundly modified by the 
 presence of the food-yolk, and instead of a simple invagination 
 of the lower half of the blastula wall to form the hypoblast, we 
 find the small (epiblast) cells, now arranged in several layers, 
 spreading themselves over the yolk-containing cells until they 
 completely enclose the latter except for a small circular area 
 
 yk.c. 
 
 FIG. 119. Early Stages in the Development of the Frog (partly from 
 Ziegler's models and partly adapted from Marshall.) 
 
 J, the fertilized ovum ; II V, segmentation of the ovum ; VI, blastula ; VII, modified 
 blastula, with wall composed of more than one layer of cells ; VIII, commencement 
 of gastrulation ; IX, the modified gastrula stage (VI IX in vertical section). 
 
 blc., blastocoel or segmentation cavity; bp., blastopore ; ent., enteron; ep., epiblast; 
 hyp., hypoblast; u.l.bp., upper lip of blastopore; yk.c., yolk-containing cells. 
 
 which corresponds to the blastopore or aperture of invagination 
 in Amphioxus blocked up by yolk cells (Fig. 119, VIII, IX). 
 Indeed, practically the whole of the primitive digestive cavity or 
 enteron in the frog's gastrula may be regarded as being filled 
 with yolk cells, from which hypoblast will be derived, and it 
 only gradually opens out later on in development as the yolk is 
 used up. Hence the coelenterate stage of the ancestral history
 
 270 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 (Fig. 119, IX) is very much disguised in the ontogeny, simply 
 owing to the presence of a large quantity of nutrient material 
 in the egg. 
 
 In the development of the bird's egg (and also in that of reptiles, 
 which is closely similar) it is still more difficult to recognize the 
 process of gastrulation. The quantity of yolk (Fig. 70) is so 
 enormous that it entirely prevents cell-division from taking 
 place in the greater "part of the egg, and segmentation is con- 
 sequently confined to a small yolk-free area at one pole of the 
 spherical mass, where it gives rise to a layer of cells known as the 
 blastoderm. This blastoderm gradually spreads over the yolk and 
 from it the embryo is formed by a kind of pinching-off process, 
 the yolk remaining enclosed in a bag the yolk-sac attached to 
 the lower surface of the embryo, until it is all absorbed by means 
 of a special set of blood-vessels developed in the yolk-sac, and 
 used up in the nourishment of the growing embryo. The 
 embryo itself meanwhile becomes provided with special organs 
 for its protection and respiration, the so-called foetal membranes. 
 Of these the amnion, formed by outgrowth of the embryonic 
 body-wall, forms a sac filled with fluid, which surrounds the 
 embryo at a distance and leaves it free room for growth, while 
 the allantois, formed by outgrowth of the embryonic gut-wall, 
 forms a highly vascular organ which comes into relation with 
 the extremely porous shell and thus provides for the necessary 
 gaseous interchange. 
 
 The yolk-sac, tbe amnion and the allantois are all organs 
 which have been developed in accordance with the special 
 requirements of the embryo, and which must therefore be left 
 out of account in reconstructing the direct ontagenetic record. 
 
 In the dogfish also (Fig. 120) we find an excellent illustration 
 of the formation of a special yolk-sac to contain the enormous 
 supply of food material for the developing embryo, but in this 
 case neither amnion nor allantois is developed. 
 
 The Mammalia, on the other hand, have adopted an improved 
 method of nourishing their young. The developing embryos are 
 retained within the body of the parent and obtain their food 
 supply from the blood of the latter by means of a special organ 
 known as the placenta, which is developed partly from the 
 allantois of the embryo or " foetus," and by which the latter is 
 attached to the wall of the uterus. This method seems to have 
 proved much more satisfactory than the provisioning of the egg
 
 INFLUENCE OF FOOD YOLK 271 
 
 with food-yolk, and we accordingly find that the mammalian 
 ovum (Fig. 71) has become reduced in size again until it is 
 no larger than that of Amphioxus, i.e. about ^^h i ncn m 
 diameter. The early stages of segmentation have also gone 
 back to the primitive type, but, strange to say, a yolk-sac is still 
 formed although there is no yolk for it to contain affording an 
 admirable example of a vestigial embryonic organ. Here one 
 record has been superposed upon another in the ontogeny and 
 we have a recapitulation of an embryonic stage of the reptilian 
 ancestors of ths mammals. After birth, of course, the young 
 
 FIG. 120. Embryo of a DogBsh, with Yolk-Sac (Y.S.) attached, X 2. (From 
 a photograph.) 
 
 mammal is nourished by the secretion of milk, and is thereby 
 enabled to reach a very advanced stage of development before it 
 has to begin ta find its own food. The chick, owing to the very 
 large amount of food-yolk with which the egg is supplied, is also 
 able to reach a highly developed condition before it hatches and 
 begins to look after itself. 
 
 With animals whose eggs contain insufficient food-yolk, and 
 which are not provided with any other spscial means of 
 nourishing the developing young, the case is very different. They 
 are obliged to hatch and begin life on their own account while 
 still a long way from the adult condition when only a small 
 part of their ontogenetic journey has been accomplished. 
 Amphioxus, for example, hatches as a free-swimming ciliated 
 embryo shortly after the gastrula stage has been passed through, 
 and completes its development at the expense of food supplies 
 which it obtains for itself.
 
 272 OUTLINES OF E VOLUTION AKY BIOLOGY 
 
 Embryos which, having hatched at a comparatively early stage 
 of their development, lead independent, self-supporting lives, are 
 commonly spoken of as larvae, and amongst such larvae we again 
 find abundant instances of special larval organs developed in 
 relation to their special requirements. 
 
 The tadpole of the frog (Fig. 121, 411) affords an excellent 
 example of a free-living larva. It has certain special organs, 
 such as its horny jaws and the suckers on the under surface of 
 
 FIG. 121. Stages in the Life History of the Frog. (From Brehm's " Thier- 
 leben.") 
 
 1, freshly laid eggs; 2, eggs slightly magnified, adhering together by their gelatinous 
 envelopes to form " spawn " ; 3, young tadpole still enclosed in the jelly ; 4, newly 
 hatched tadpoles adhering by their suckers to a blade of grass ; 5, 6, tadpoles with 
 external gills ; 7, 8, tadpoles with the gills concealed by the operculum ; 9, 10, tad- 
 poles with conspicuous hind legs but with the front legs still concealed beneath the 
 operculum ; 11, tadpole during metamorphosis, with front legs protruded ; 12, young 
 frog with tail not completely absorbed. 
 
 (For microscopic details of the early stages see Fig. 119.) 
 
 the head, which are developed in relation to its larval life, but 
 there can be no question about the phylogenetic significance 
 of the tadpole stage as a. whole. In every essential feature of its 
 organization the tadpole, before its legs develop, is a fish. It 
 swims like a fish by means of its well developed tail, it breathes 
 like a fish by means of its gills and gill-slits, and its internal 
 structure is that of a fish. There can be no doubt that it 
 represents a fish-like stage in the evolution of the frog. 
 
 In the tadpole, owing to its free-living, aquatic mode of life,
 
 ABBREVIATION OF ONTOGENY 
 
 273 
 
 the characteristic piscine organs are fully functional, but even in 
 the embryos of higher air-breathing vertebrates reptiles, birds 
 and mammals while enclosed within the egg-shell or within the 
 womb of the parent, we can still recognize clear traces of a 
 similar fish-like stage in their evolution, for gill-slits are present 
 in all (Fig. 122, g.s.) and the 
 arrangement of the principal veins 
 and arteries is unmistakably fish- 
 like. 
 
 Difficulties in the interpretation of 
 the ontogenetic record arise not only 
 from the addition of new features 
 in the form of embryonic or larval 
 organs but also from the abbreviation 
 of the different stages through which 
 the organism passes. The develop- 
 ment of any of the higher organisms 
 must be looked upon as a kind of 
 compromise, due to the necessity of 
 attaining as perfect a condition as 
 possible in as .short a time as pos- 
 sible, for the sooner the adult condi- 
 tion is reached the sooner will the 
 organism be able to reproduce its 
 kind, and hence any abbreviation of 
 the earlier stages of development will 
 be advantageous to the species and 
 will be favoured by natural selection. 
 For this reason the fish-like stage 
 in the development of reptiles, birds 
 and mammals is passed through very 
 rapidly and the fish-like structure is 
 no longer fully developed. Although gill-slits are present the 
 gills themselves are not, and other piscine characters are only 
 represented by transient vestiges. 
 
 Turning now to some of the lower groups of the animal king- 
 dom, we may briefly notice a beautiful illustration of the law of 
 recapitulation afforded by the life-history of the common feather- 
 star, Antedon (Fig. 123). This animal, found abundantly in 
 comparatively shallow water off the British coast, belongs to the 
 great phylum Echinodermata, which also include? the star-fish, 
 
 FlG. 122. Chick Embryo of 
 about 3 Days, with the 
 Fretal Membranes and 
 Yolk-Sac removed. (Pho- 
 tographed from one of 
 Ziegler's models.) 
 
 au., auditory organ ; e.,eye ; g.s., gill 
 slits; h., heart; m.s., meso- 
 blastic somites ; n., nasal organ.
 
 274 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 sea-urchins and sea-cucumbers. The subdivision to which it 
 belongs the Crinoidea is characteristically a deep-water group. 
 It is also a group of great antiquity, the remains of crinoids 
 such as the well-known " Encrinites " of the Carboniferous period 
 being abundant in certain Palaeozoic formations. 
 
 Both the Palaeozoic crinoids and the surviving deep-sea mem- 
 bers of the group, such as Pentacrinus (Fig. 125), are stalked 
 forms, the " calyx," with its radiating arms, being attached to 
 
 FIG. 123. FIG. 124. 
 
 FlG. 123. The Feather Star (Antedon bi/ida), nat. size. 
 FlG. 124. Pentacrinoid Stage in the development of Antedon, X 14. 
 dr. cirri ; st . stalk. 
 
 the end of a long, jointed, calcareous stem, the lower extremity of 
 which is permanently fixed to the sea-bottom. Antedon, and the 
 other shallow-water crinoids of the present day, on the other 
 hand (Fig. 123), have no stalks but in place thereof a number of 
 slender " cirri " whereby they temporarily attach themselves to 
 seaweed or other objects. 
 
 Now in the course of its development from the egg the feather- 
 star passes first through a free-swimming larval stage and then 
 through a fixed stalked stage (Fig. 124), known, from its resem- 
 blance to the deep-sea genus Pentacrinus, as the pentacrinoid
 
 LARVAL ORGANS 
 
 275 
 
 FIG. 125. A Deep-Sea Crinoid, Penta- 
 crinus (Isocrinus) decorus, about nat. 
 
 stage. Later on the young animal develops cirri and falls off 
 the end of 
 the stalk, 
 assuming 
 once more 
 an active 
 mode of life. 
 Here we 
 recognize 
 very clearly 
 in the pen- 
 tacrinoid 
 stage of the 
 ontogeny 
 a repeti- 
 tion of the 
 stalked 
 condition 
 
 through which the ancestors of the feather-star 
 must have passed. 
 
 We also find amongst invertebrate animals 
 abundant examples of special larval organs. 
 The body of a caterpillar, for instance, is built 
 up very largely of such structures, and it has 
 to undergo an almost complete reconstruction 
 within the protective envelope of the chrysalis 
 before it reaches the adult condition and emerges 
 as a perfect insect. It is therefore extremely 
 difficult to interpret the caterpillar stage in 
 terms of the ancestral history. 
 
 The ophiuroids or brittle stars (Fig. 126), so 
 commonly met with between tide-marks, have 
 remarkable, free-swimming " Pluteus " larvae 
 (Fig. 127), totally different in appearance from 
 the adult. These larvae occur in immense 
 numbers in the surface waters of the ocean. 
 They are provided with long slender arms 
 supported on calcareous rods, which probably 
 serve, in part at any rate, to protect the very 
 delicate body from the attacks of other small animals. Before 
 attaining the adult condition the larva undergoes a complex 
 
 T 2
 
 276 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 metamorphosis, during which the long arms are completely lost 
 and the form of the body entirely changed. The echinoids or 
 sea-urchins pass through a very similar larval stage. 
 
 The free-swimming " Zoaea " larva of the crab (Fig. 128) is also 
 provided with conspicuous defensive spines which are lost in the 
 
 FIG. 126. 
 
 FIG. 127. 
 
 FIG. 126. -A "Brittle Star (Ophiura ciliaris), X f. 
 FIG. 127. The Pluteus Larva of a Brittle Star, X 62. 
 
 adult condition (Fig. 102) and which have doubtless been developed 
 in relation to the special conditions of the larval life. 
 
 In many cases the development of larval organs causes the 
 young animal to differ so widely from the adult that the relation- 
 ship of the two for a long time remained unsuspected, and hence 
 the special names which many larvae have received. In other 
 cases, however, it is the adult animal which has become so pro- 
 foundly modified in relation to special conditions of life as to
 
 INTERPRETATION OF ONTOGENY 
 
 277 
 
 obscure its genetic affinities, which may still be clearly indicated 
 by embryonic or larval stages. 
 
 The common Ascidian, for example (Fig. 129), is a sac-shaped 
 organism, permanently attached to rock or seaweed in the adult 
 condition, which by the uninitiated would hardly be taken for an 
 animal at all. Before their life-history was known the Ascidians 
 were placed by zoologists amongst the " Molluscoida," a group of 
 invertebrates supposed to resemble the mollusca or shell-fish. It 
 was only the discovery that 
 the typical Ascidians pass 
 through an active tadpole 
 stage in their development 
 (Fig. 130), with a muscular 
 tail, a notochord and a central 
 nervous system all formed as 
 in vertebrates, that demon- 
 strated their true position 
 as degenerate members of 
 the great phylum Chordata 
 (= Vertebrata in the widest 
 sense of the term), the tad- 
 pole stage being, of course, 
 . a recapitulation of the ances- 
 tral, fish-like, chordate con- 
 dition. 
 
 It will be readily under- 
 stood from the foregoing 
 illustrations that in endea- 
 vouring to reconstruct the 
 ancestral history of an organ- 
 ism from the stages which it 
 passes through in its indivi- 
 dual development, we have to distinguish very carefully between 
 two sets of characters. In the first place there are characters which 
 are due to inheritance from the adult condition of very remote 
 ancestors and which really indicate stages in what we may call 
 the direct line of evolution. These are termed palingenetic or 
 ancient characters. Such, for example, are the essential features 
 of the gastrula stage, which occurs so generally and which indi- 
 cates a remote ccelenterate ancestor (the Gastrtea of Professor 
 Haeckel) for at any rate the great majority of multicellular animals. 
 
 FIG. 128. The Zosea Larva of a 
 Crab (Portunus), X 30. (From a 
 photograph.)
 
 278 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Such also are the essential characters of the tadpole stage of the 
 frog and of the ascidians, and the vestigial gill-slits of higher 
 vertebrates, which indicate descent from some gill-breathing, fish- 
 like form. 
 
 In the second place there are characters which have been 
 acquired secondarily by the embryo or larva in relation to its 
 
 FIG. 129. A Simple Ascidian (dona 
 inlestinalis), X J. 
 
 a.p., processes for attachment ; in.ap., 
 inhalant apertur 
 aperture. 
 
 re; ex.ap., exhalant 
 
 FIG. 130. Tadpole Larva of a 
 Compound Ascidian (J)iplosoma 
 crystallinum), X 35. (From a 
 photograph.) 
 
 p., papillse for attachment ; t., tail. 
 
 special embryonic or larval requirements. These are termed 
 caenogenetic or modern characters, and it is these which tend 
 more or less to obscure the ontogenetic record. Such are the 
 presence of food-yolk, the total membranes of air-breathing 
 vertebrates and the various special larval organs to which we 
 have above referred. It is difficult, however, to draw a hard and 
 fast line between palingenetic and caenogenetic characters, and 
 we must always remember that embryonic stages, as well as the
 
 PHYLOGENY AND ONTOGENY 
 
 279 
 
 adult, undergo modification during evolution, and that some 
 caenogenetic characters are very much more ancient than others. 
 The relation which exists between the phylogenetic and onto- 
 genetic stages in the evolution of an organism are very diagram- 
 matically expressed in Fig. 131, which is drawn with special 
 reference to the familiar case of the frog. The vertical line is 
 supposed to represent the ancestral history, some of the principal 
 stages of which are indicated. 
 The horizontal line at the top Sc * 9 " ^ 
 
 represents the individual life- 
 history, with the stages indi- 
 cated which represent those 
 selected in the ancestral his- 
 tory. The ovum represents 
 the protozoon ancestor ; the 
 blastula the hollow, spherical 
 protozoon colony; thegastrula 
 the cffilenterate ; the meta- 
 merically segmented embryo 
 the segmented worm, and the 
 tadpole the fish-like stage. 
 The oblique lines represent 
 the lines of evolution of the 
 different ontogenetic stages 
 themselves, in the course of 
 which caenogenetic characters, 
 such as the acquisition of 
 food-yolk by the ovum and of 
 suckers and horny jaws by 
 the tadpole, have arisen. The 
 shorter horizontal lines repre- 
 
 FIG. 131. Diagram illustrating the 
 Eelation between Ontogeny and 
 Phylogeny. 
 
 sent the ontogeny at the different ancestral stages, and their 
 increase in length from below upwards is obviously due to the 
 successive addition of new stages to the ontogenetic record, while 
 the shortness of the horizontal lines as compared with the corre- 
 sponding portions of the vertical line may be taken to indicate, 
 though very imperfectly, the abbreviation of the ontogeny. 
 
 In the vegetable kingdom we find illustrations of the law of 
 recapitulation and of the complication of the ontogenetic record 
 by the development of caenogenetic characters, quite as striking 
 as any which we find amongst animals. Take, for example, the
 
 280 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 life-history of some of the Australian acacias, or wattles. These 
 are leguminous plants and many of them have the pinnately 
 subdivided leaves so characteristic of the order. Others, however, 
 have the pinnate leaves in the adult entirely replaced by narrow 
 " phyllodes," which closely resemble simple undivided leaves 
 like those of some of the eucalypts, but are really only flattened 
 leaf -stalks or petioles. The development of such phyllodes is no 
 doubt to be regarded as an adaptation to the heat and drought of 
 
 the Australian climate, 
 which delicately sub- 
 divided leaves of the 
 ordinary leguminous 
 type are ill-suited to 
 withstand, and the re- 
 semblance of the phyl- 
 lodes to true foliage 
 leaves of the undivided 
 type and especially to 
 those of the eucalypts 
 affords a very good 
 illustration of the 
 phenomenon of conver- 
 gence. If, now, we 
 examine the seedlings 
 of a phyllode-bearing 
 acacia (Fig. 132) we 
 shall find that imme- 
 diately after the coty- 
 ledons or seed - leaves 
 they bear typical pin- 
 nate leaves, and then 
 
 of intermediate form, thus clearly recapitulating the 
 ancestral condition from which the phyllode-bearing forms were 
 derived. A very similar state of things occurs in the common 
 European gorse or furze, another leguminous plant, in which 
 the leaves of the adult have become modified to form spines, 
 while those of the seedling still retain the ancestral ternate 
 condition, somewhat resembling those of the clover. 
 
 The cotyledons or true seed-leaves of flowering plants, on 
 the other hand, afford excellent examples of caenogenetic characters 
 developed in relation to the special requirements of the embryo. 
 
 FIG. 132. Seedling of a phyllode-bearing 
 Acacia (Acacia pycnantha). (From Stras- 
 burger.) (The cotyledons have already 
 been shed.)
 
 RECAPITULATION AND HEREDITY 
 
 281 
 
 They are very commonly modified to form receptacles for a 
 store of food material for the young plant in the form of 
 starch, as in the peas and beans (compare Fig. 56), and may 
 never even leave the seed-coat or emerge above the ground. 
 In such cases they develop no chlorophyll and differ absolutely 
 from ordinary foliage leaves, without, of course, indicating any 
 ancestral stage through which the latter have passed. Even 
 when they emerge above the ground, as in the beech (Fig. 133), 
 and become green, their form is usually widely different from 
 that of the foliage leaves and 
 appears to have been developed 
 largely in adaptation to the neces- 
 sity for close package within the 
 narrow compass of the seed-coat, 
 the shape of the cotyledons being 
 governed, as Lord Avebury has 
 pointed out, by that of the seed 
 within which, in a dormant condi- 
 tion, they pass most of their lives. 
 In short, the embryological 
 investigation of both animal -and 
 vegetable organisms leaves no 
 doubt as to the general truth FIG. ^133. 
 of the recapitulation hypothesis, 
 and must convince any unbiassed 
 observer that, however much modified it may be by abbrevia- 
 tion and by the superposition of secondary features, the life-history 
 of the individual is essentially a condensed epitome of the 
 ancestral history of the race. The law of recapitulation, indeed, 
 may be regarded simply as a logical extension of the law of 
 heredity, for every organism tends to inherit the characters not 
 only of its immediate progenitors but of all its ancestors, and 
 these characters appear in the individual life-history in the same 
 order as that in which they first appeared in the ancestral 
 history in other words, ontogeny is a repetition of phylogeny 
 and can only be explained in terms of organic evolution. 
 
 Seedling of Beech, 
 showing the Cotyledons. 
 (From Lubbock.)
 
 CHAPTER XIX 
 
 The stratified rocks Geological periods The age of the habitable earth 
 The geological record The succession of the great vertebrate groups. 
 
 THE materials of which the solid crnst of the earth is made up 
 are, as it is perhaps hardly necessary to point out, extremely 
 varied, but whatever may be their physical or chemical characters 
 they are all spoken of by geologists as rocks. In accordance 
 with their mode of formation these rocks may be divided into two 
 chief categories, first, those which, like granite and basalt, have 
 been formed directly by cooling of molten matter, of which the 
 earth was perhaps at one time entirely composed, and, second, 
 those which, like sandstone, shale, limestone and coal, have been 
 formed by accumulation of detritus derived from pre-existing 
 rocks or from the remains of dead organisms. The first are 
 commonly termed igneous and the second sedimentary, the 
 latter deriving their name from the fact that they have been 
 deposited under water. Owing to the manner in which they 
 have been laid down, usually in the form of gravel, sand, mud or 
 ooze, which have subsequently become hardened and consolidated 
 by the pressure of more recent deposits, the sedimentary rocks 
 form a series of layers or strata of varying thickness superposed 
 one upon another. The stratification must at first have been 
 horizontal, but, owing to the irregular shrinkage of the earth's 
 crust and to the action of volcanic upheavals, it has usually been 
 more or less disturbed, so that the planes of bedding have been 
 tilted at various angles and frequently exhibit a high degree of 
 curvature or even contortion. Moreover, rocks which were 
 originally laid down on the bed of the sea have in many cases 
 come to form part of the dry land and even to be uplifted far above 
 sea-level in the form of mountain ranges. 
 
 The formation of sedimentary rocks must have been going on 
 ever since the earth became cool enough to admit of the con- 
 densation of aqueous vapour into water, and it is still going on 
 in the same way at the present day. Every river is continually
 
 THE STRATIFIED ROCKS 283 
 
 carrying down to the sea immense quantities of sand and mud, 
 which will ultimately be deposited in the form of new strata ; 
 every cliff by the sea-shore is slowly or rapidly weathering away 
 under the influence of atmospheric agencies rain, frost, tides 
 and wind ; and far out, beyond the reach of the detritus derived 
 from the land, the sea-bottom is being covered by the slow 
 accumulation of ooze or mud derived almost entirely from the 
 remains of marine organisms, while the activities of corals and 
 other reef -building animals in their turn make no small contribu- 
 tion to the grand total. 
 
 The sedimentary rocks naturally contain the remains of vast 
 numbers of organisms which flourished on the earth at more or 
 less remote periods of its history, and the study of these fossils 
 may justly be expected to yield results of the greatest importance 
 from the point of view of the theory of organic evolution. We 
 have here, in fact, the only really direct evidence of the course 
 which the evolution of the animal and vegetable kingdoms has 
 actually taken, and this evidence constitutes what is commonly 
 known as the geological record. 
 
 The relative ages of the different sedimentary rocks and of 
 the different geological formations which they build up are of 
 course determined primarily by their position with regard to one 
 another, each layer or bed being necessarily of more recent date 
 than the one below it. 1 The series of strata met with in any 
 particular locality may, however, be very different from that 
 which occurs in other localities, for while mud may be accumu- 
 lating in one place, sandstone may be forming in another and 
 limestone in a third, and over other areas, which lie above 
 sea-level, rock formation will be replaced by destruction or 
 denudation. Nevertheless, geologists have found it possible to 
 compare the series of stratified rocks found in different parts of 
 the world one with another, and, by the aid of the fossils which 
 they contain, to identify a number of more or less well defined 
 epochs in the geological history of the earth. 
 
 At the bottom of the stratified series lies an immense thickness 
 of rocks which seem to have been originally sedimentary, 
 but many of which have undergone profound changes due to 
 pressure and heat since they were laid down. These rocks 
 indicate a very long period of the earth's history, the 
 
 1 Except in a few cases where contortion has been so extensive as to reverse the 
 relative positions.
 
 284 OUTLINES OF 
 
 GEOLOGICAL TIME-SCALE, 
 
 EVOLUTIONARY BIOLOGY 
 
 AS INDICATED BY THE STRATIFIED ROCKS. 
 
 1 
 
 Epochs, with relative duration 
 indicated by thickness of 
 strati Hed rocks. 
 
 Some representative 
 British Formations. 
 
 Range in Time of 
 Animal Groups. 
 
 |p' 
 
 f ii <. ~ 5 ?= . 
 
 
 Recent tiinl l'ic,istnrciif4iKM) tet-t. 
 
 Alluvium, IViit, boulder Clay, <.Vc. 
 
 
 
 
 Pliocene 13000 feet. 
 
 Norwich and Red Crag, Coralline 
 Crag, &c. 
 
 Ill ; ' 
 
 r Tertia 
 
 Miocene 14000 feet. 
 
 
 
 1 
 
 Oligocene 12000 feet. 
 
 Hamstead, Bembridge, Osborne 
 and Headon Beds. 
 
 
 i 
 
 Eocene 20000 feet. 
 
 Barton and Bagshot Beds, London 
 Clay, Thanet Sands, &c. 
 
 
 c or Secondary. 
 
 Cretaceous 44000 feet. 
 
 Chalk, Greensand, Gault, AVealdcM. 
 fce. 
 
 
 1 
 
 Jurassic 8000 feet. 
 
 Oolites, Lias. 
 
 \ \ H. : 1 
 
 s 
 
 Triassic 17000 feet. 
 
 White Lias (Rhwtic), Elgin Sand- 
 stone, &c. 
 
 
 
 Permian 12000 feet. 
 
 Magnesian Limestone, Red 
 Marls, &c. 
 
 
 
 Carboniferous 29000 feet. 
 
 Coal Measures, Millstone Grit, 
 Mountain Limestone, &c. 
 
 
 or Primar 
 
 Devonian 22000 feet. 
 
 Old Red Sandstone. 
 
 
 1 
 
 1 
 
 Silurian 15000 feet. 
 
 Ludlow and Wenlock Sliales and 
 Limestones, &c. 
 
 
 fi 
 
 Ordovician 17000 feet. 
 
 Slates, Flags, Sandstones, &c., of 
 Bala, Caradoc, Llandeilo, Skid- 
 daw, &c. 
 
 
 
 Cambrian 26000 feet. 
 
 Lingula Flags, Harlech Series, &c. 
 
 
 
 Pre-Cambrian (extent unknown). 
 I 
 
 Archaean Schists, Gneiss and 
 other Crystalline Rocks ; largely 
 luetainorphic. 
 
 t
 
 THE AGE OF THE HABITABLE EARTH 285 
 
 pre-Cambrian epoch, during which organic evolution must have 
 been taking place, though only somewhat doubtful indications 
 of organic remains have been met with. Probably the pre- 
 Cambrian rocks originally contained abundant fossils, which have 
 been destroyed by the treatment to which they have been sub- 
 jected. All the subsequent periods of the earth's history, 
 however, are represented by rocks containing organic remains in 
 greater or less abundance. These deposits indicate a division of 
 the earth's history since pre-Cambrian times into three main 
 eras primary or palaeozoic, secondary or mesozoic and tertiary 
 or cainozoic. Each of these three is subdivided into a series of 
 epochs, as shown in the accompanying table, in which are also 
 mentioned some of the principal rock formations by which the 
 different epochs are represented in the British Islands. 
 
 The thickness of the strata deposited during each period differs 
 greatly in different parts of the world, and in North America the 
 various formations appear to be much better developed than in 
 North Western Europe. It is of the utmost importance that our 
 estimates of this thickness should be as accurate as possible, for 
 they constitute important data from which to calculate the age 
 of the habitable earth and the duration of the different geological 
 periods. 
 
 Sir Archibald Geikie, 1 in 1892, very cautiously estimated the 
 total thickness of the stratified rocks, where most fully developed, 
 from the bottom of the Cambrian onwards, as not less than 
 100,000 feet. He also estimated that it would take from 730 to 
 6,800 years to add a foot to this thickness by accumulation of 
 material derived from the denudation of the land. Of course the 
 actual time must vary very greatly according to the nature of the 
 material deposited and the agencies at work. There is, however, 
 no reason to suppose that denudation and sedimentation took place, 
 on an average, more rapidly in past geological time than at the 
 present day. If, then, we take the average between the most 
 rapid and the slowest rate of denudation and sedimentation, and 
 the thickness of the stratified rocks as calculated by Professor 
 Geikie, we arrive at 376,500,000 years as the age of the habitable 
 earth exclusive of the pre-Cambrian period. 
 
 Sir Archibald Geikie appears, however, to have assumed for 
 the purposes of his calculation that material derived from the 
 wearing away of the land has been spread out over an equal area 
 1 Presidential Address, British Association, Edinburgh, 1892.
 
 286 OUTLINES OF EVOLUTIONABY BIOLOGY 
 
 of sea-bottom, so that every foot in depth worn off the land is 
 taken to indicate a foot in thickness piled up in new deposits. 
 This hardly seems to be a justifiable assumption, for in many 
 localities sediment derived from a large area must be accu- 
 mulated over a small one, and the increase in thickness of the 
 newly forming rock must therefore take place at a much greater 
 rate than the decrease in thickness of that which is being worn 
 away. 
 
 Professor Sollas, in his presidential address to the Geological 
 Society of London in 1909, gives the total maximum thickness of 
 the sedimentary rocks from the bottom of the Cambrian to the top 
 of the most recent formations as being no less than 253,000 feet. 
 I have accepted this estimate in drawing up the geological time- 
 scale on page 284, in which the supposed thickness of the 
 deposits belonging to each of the main subdivisions of the three 
 great eras is indicated both numerically and by the relative depth 
 of the space allotted to each. 
 
 Professor Sollas, however, estimates a much more rapid rate of 
 accumulation of these deposits as high, indeed, as one foot per 
 century which, even with his own increased estimate of thickness, 
 would give only 25,800,000 years as the total lapse of time since 
 the commencement of the Cambrian epoch. But he also points 
 out that there is good reason for believing that the pre- 
 Cambrian epoch must have been as long as all the succeeding 
 epochs put together, and the total lapse of time since the formation 
 of the stratified rocks (including the pre-Cambrian) commenced 
 might accordingly be estimated at 50,600,000 years. 
 
 We must also, of course, make abundant allowance for the fact 
 that denudation has been going on at all times since the deposition 
 of the stratified rocks commenced, and that the total thickness of 
 these rocks has been enormously reduced thereby, the same 
 material having been used over and over again in rock formation. 
 
 Where opinions differ so much, and where there is so much 
 uncertainty as to the value of the data themselves, it is obvious 
 that any estimate of geological time derived from the thickness 
 of the sedimentary rocks can have only a doubtful value, and that 
 it is highly desirable that such an estimate should be checked 
 by making use of other sources of information. 
 
 Several other methods of estimating the age of the earth have 
 been employed. Perhaps the most satisfactory is that which is 
 especially associated with the name of Professor Joly, and which
 
 THE AGE OF THE HABITABLE EARTH 287 
 
 depends upon the amount of sodium present in solution in sea- 
 water. If we knew the total amount of sodium present in the 
 ocean, and if we also knew the rate at which it has been accu- 
 mulating since the ocean was first formed, it would be a simple 
 matter to calculate the age of the ocean and therefore that of 
 the stratified rocks, whose existence must have commenced with 
 that of the ocean itself. The greater part at any rate of the 
 salt contained in the sea must have been derived from the 
 disintegration of the land and carried down by rivers. Estimates 
 have been made, based upon careful analysis, of the amount of 
 sodium contained in the sea and in the water of various rivers, 
 as well as of the amount of water which flows into the sea 
 annually, and Professor Sollas has come to the conclusion that 
 " on a review of all the facts, the most probable estimate of the 
 age of the ocean would appear to lie between 80 and 150 millions 
 of years," which is considerably in excess of the estimate which 
 he arrives at from the consideration of the thickness of the 
 sedimentary rocks. 
 
 We cannot assume, however, that the earth has been habitable 
 during the whole of this period, for when the ocean was first 
 formed both it and the land must have been at far too high a 
 temperature to admit of the existence of protoplasmic organisms. 
 Possibly if we assume 100,000,000 years as the total age of the 
 habitable earth, and therefore as the time which has been available 
 for the evolution of the organic world, we shall be as near as we 
 can get to the truth in the present state of our knowledge. 
 
 We must now turn to the consideration of the geological 
 record itself, and in the first place we may ask, what is to be 
 legitimately expected from such a record ? Have we any right 
 to expect anything like a complete representation, by fossil 
 remains, of the past history of the organic world ? Obviously 
 the answer to this question must be a decided negative. The 
 imperfection of the geological record is due to many causes. It 
 must have been imperfect when first laid down, for the great 
 majority of organisms usually disintegrate and pass away 
 without leaving any recognizable remains behind them at all. 
 In most cases it is only hard skeletal structures that have any 
 chance of being preserved. Bodies composed entirely of soft 
 tissues, such as jelly-fish and many worms, are rarely repre- 
 sented by fossils. Even hard structures, such as animal 
 skeletons, will only be fossilized if they happen by some
 
 288 OUTLINES OF EVOLUTIONAKY BIOLOGY 
 
 fortunate chance to find their way to some place where rock 
 formation of a suitable kind is going on. The remains of land 
 animals may be carried by rivers to some sea or lake and buried 
 in a suitable accumulation of mud or sand, but it is more likely 
 that they will not. Marine animals, provided they have hard 
 skeletons, have, of course, many more opportunities of attaining 
 a lasting monument, and we often find great thicknesses of rock, 
 such as chalk and limestone, composed almost entirely of their 
 remains. 
 
 Even when the record has been successfully established, 
 however, it is liable to destruction by various agencies. The 
 
 A. B. c. 
 
 FIG. 134. Three Species of Trilobites. (From British Museum Guide.) 
 A., Agnostus princeps ; B., Olenus cataractes ; C., Staurocephalus murchisoni. 
 
 rocks containing it may be uplifted above sea-level and planed 
 right away by sub-aerial denudation, or they may be sunk so 
 far beneath later accumulations as to be profoundly altered by 
 the action of heat and pressure, by which means any fossils 
 which they contain may be completely destroyed. Then, again, 
 we must remember that only a relatively small proportion of 
 the earth's crust is accessible for investigation. Deep-lying 
 rocks may be brought to the surface by upheaval, and denudation 
 of overlying strata, or they may be reached by deep mining, but 
 we know practically nothing of the strata which now lie beneath 
 the bed of the sea, and even the dry land has only been tentatively 
 scratched by inquisitive man in a few places. 
 
 Even were the geological record far less perfect than it 
 actually is, therefore, its imperfection could, not legitimately be
 
 THE GEOLOGICAL RECORD 289 
 
 used as an argument against the theory of organic evolution. 
 From the nature of the case it must be imperfect and the 
 surprising thing about it is that it should have yielded so much 
 evidence as it has done. The earlier portions of the record, how- 
 ever, have apparently been completely destroyed. The fact that 
 already in the Cambrian epoch highly organized invertebrate 
 animals, such as trilobites (Fig. 134) and brachiopods, were in 
 existence, shows us that evolution must then have been going on 
 for an immensely long period. This period, as we have already 
 indicated, is represented by the pre-Cambrian rocks, in which 
 the organic remains have been so far destroyed as to be only 
 recognizable in a few doubtful cases. 
 
 The investigation of what remains of the geological record 
 has, however, yielded some very remarkable and conclusive 
 results. In the first place we have learnt that life on the earth 
 has been continuous without a break from the commencement 
 of the Cambrian epoch to the present day, and in the second 
 place we have learnt that the higher groups of animals and 
 plants have appeared on the earth in exactly the order which we 
 should expect on the assumption that each has arisen from some 
 preceding and more lowly organized ancestral group. 
 
 In discussing this point we may confine our attention to the 
 Vertebrata, the evolution of which seerns to have taken place 
 entirely during post-Cambrian times. The most lowly organized 
 group of true vertebrates existing at the present day is that of 
 the Cyclostomata, which includes the lampreys and hag-fishes. 
 These are distinguished from true fishes by the absence of jaws, 
 the mouth being suctorial, and by other primitive features. The 
 skull and vertebral column remain cartilaginous throughout life 
 and there are no paired limbs. Neither is any dermal armour 
 of any kind developed. The only structures which they possess 
 which seem at all likely to be preserved as fossils are the horny 
 teeth. Certain bodies known as conodonts, which occur as 
 fossils in strata ranging from the Lower Silurian to the 
 Carboniferous, may perhaps represent such teeth. If so they 
 are perhaps the earliest vertebrate remains as yet known. 1 
 
 The remarkable group of fishes known as Ostracodermi, 
 however, makes its appearance in the Upper Silurian and 
 continues on to the Upper Devonian. These animals agree 
 
 1 Palaeospondylus, from the Lower Old Red Sandstone of Scotland, is probably 
 a l*o a fyclcstorne.
 
 290 OUTLINES OF EVOLUTIONAEY BIOLOGY 
 
 with the cyclostomes in the absence of true jaws and (in most 
 known forms) of paired fins, and in the extremely primitive 
 character of the vertebral column. They had also, like the 
 cyclostomes, median fins, but they differed from the latter con- 
 spicuously in the presence of a strongly developed dermal armour, 
 
 FIG. 135. Restoration of Cephalaspis murchiwni, from the Lower Old Red 
 Sandstone, X |. (From British Museum Guide, after Smith Wood- 
 ward.) 
 
 to the existence of which their fossil remains owe their preser- 
 vation. As representative genera of this group we may men- 
 tion Cyathaspis (Upper Silurian), Pteraspis (Lower Devonian), 
 
 FIG. l36.Pterichthys milleri, from the Lower Old Red Sandstone; restored 
 by R. H. Traquair. A. Dorsal; B. Ventral; C. Lateral views, x ^. 
 (From Smith Woodward's " Vertebrate Palaeontology.") 
 
 Cephalaspis (Upper Silurian and Lower Devonian, Fig. 135) and 
 Pterichthys (Lower Devonian, Fig. 136). 
 
 The origin of the Ostracodermi is unknown, but their relation- 
 ship to the cyclostomes has led to their inclusion with the latter in 
 one class, the Agnatha (jawless vertebrates), which stands at the
 
 THE GEOLOGICAL RECORD 
 
 291 
 
 very bottom of the vertebrate series and was probably derived from 
 some primitive chordate ancestor, not unlike Amphioxus, which 
 had no hard structures to be handed down to us in a fossil 
 condition. It is true that the ostracoderms bear a curious 
 resemblance to certain extinct contemporaneous arthropods, the 
 Eurypteridffl (Fig. 137), 
 but this resemblance is 
 probably merely super- 
 ficial and due to conver- 
 gence owing to similar 
 conditions of life; it 
 cannot be accepted as 
 evidence of the origin of 
 vertebrates from arthro- 
 podan ancestors, which, 
 on other grounds, is ex- 
 tremely improbable. 
 
 The most primitive 
 group of true jaw-bearing 
 fishes is that of the Elas- 
 mobranchii, including the 
 existing sharks, dog-fishes, 
 skates and rays. These 
 have well developed paired 
 fins and a dermal armour 
 of scales or denticles, but 
 their primitive character 
 is shown by the fact 
 that the internal skeleton 
 remains cartilaginous 
 throughout life. Frag- FIG. ISI.Pterygotus osiliemis, an Upper 
 mentary indications of Silurian Eurypterid, dorsal view, re- 
 
 i i , duced. (From Wood s " Palseontologj', 
 
 elasmobranchs occur after Schmidt.) 
 
 already in the Upper 
 
 Silurian, but the first satisfactory specimens have so far been met 
 with in rocks of Lower Devonian age (Old Red Sandstone). A 
 carboniferous form, Acanthodes, is represented in Fig. 138. 
 
 The remarkable Dipnoi, feebly represented by the mud- 
 fishes or lung-fishes of the present day the Australian Neo- 
 ceratodus (Fig. 110), the African Protopteriis and the South 
 American Lepidosiren seem to have arisen during the 
 
 u 2
 
 292 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Devonian period and to have attained their maximum of 
 development and specialization during that epoch. This group 
 is of especial interest, as we have already had occasion to notice, 
 as indicating how the transition from an aquatic to a terrestrial 
 mode of life was first rendered possible to vertebrate animals by 
 the conversion of the swim-bladder into a pair of lungs. The 
 dipnoids, however, have remained aquatic in habit, and retain 
 their gills for aquatic respiration as well as being able to breathe 
 air directly by means of their lungs. 
 
 The ordinary bony fishes (Teleostei), to which the vast 
 majority of existing species belong, do not attain any prominence 
 as a group before the secondary era, but they had fore-runners 
 in some of the early ganoids which date back to lower 
 Devonian times. The teleosts are the most specialized of all 
 
 FIG. 138. Bestoration of Acanthodes ward!, from the Coal Measures of 
 Staffordshire, X \. (From British Museum Guide.) 
 
 fishes and it is not from such a group that we should expect the 
 next great advance in organization to take its origin. 
 
 There are the strongest anatomical and embryological grounds 
 for believing that the Amphibia, a group which at the present 
 day includes the frogs, toads, newts and salamanders, are 
 the descendants of fish-like ancestors which became adapted to 
 an air-breathing mode of life by the development of lungs. The 
 dipnoid fishes, which, as we have already seen, attained their 
 maximum of development in the Devonian epoch, show us 
 clearly enough how such a transition probably took place, and 
 it was doubtless either from some dipnoid form, or from some 
 other primitive type of fish which had also succeeded in con- 
 verting its swim-bladder into lungs, that the Amphibia arose. 
 The fact that amphibian remains first occur in Lower Carboni- 
 ferous rocks is in complete harmony with this hypothesis as to 
 the origin of the group.
 
 THE GEOLOGICAL RECORD 
 
 293 
 
 The earliest known amphibians are the Stegocephalia, so- 
 called on account of the strongly developed bony armour 
 which covered the head and formed the roof of the skull, as it 
 did already in many of the earlier fishes. These Stegocephalia 
 included both small, salamander-like forms such as Branchio- 
 saurus (Fig. 139) and large crocodile-like creatures the 
 labyrinthodonts. Some of them survived into the jTriassic 
 
 FIG. 139. Branchiosaurus amblystomus ; restoration of Skeleton (A) and 
 ventral Armour (B) by H. Credner, nat. size. Lower Permian. (From 
 Smith Woodward's " Vertebrate Palaeontology.") 
 
 epoch, after which they appear to have become entirely extinct, 
 and although a fair number of Amphibia still exist at the present 
 day, the group must be regarded as having attained its maximum 
 of development in late Palaeozoic and early Mesozoic times. The 
 existing frogs and toads are very highly specialized forms, 
 completely adapted to a terrestrial life in the adult condition but 
 for the most part resorting to water to deposit their eggs, which 
 hatch out in the form of aquatic fish-like larvae or tadpoles,
 
 294 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 breathing by means of gills (Fig. 121). The newts, on the other 
 hand, though developing lungs, retain the fish-like form and the 
 aquatic habit throughout life. 
 
 In one respect the imperfection of the geological record as 
 regards the origin of the Amphibia from fishes is especially to 
 be regretted. One of the chief distinctions between the two 
 groups lies in the fact that whereas the Amphibia always 
 have pentadactyl limbs (except when these have been lost 
 by degeneration), the fishes have only fins, which are not 
 pentadactyl. The conversion of the fin into a pentadactyl limb 
 was no doubt an adaptation to the terrestrial mode of life, but 
 neither comparative anatomy nor embryology nor yet the evidence 
 of the geological record have so far enabled us to discover how the 
 transformation of the one type of limb into the other actually took 
 place. We can only hope that light may yet be thrown upon this 
 difficult problem by the discovery of the fossil remains of inter- 
 mediate forms. 
 
 The reptiles and the amphibians are closely related to one 
 another, but existing reptiles are readily distinguished from 
 existing amphibians in a variety of ways. For example, they 
 produce much larger eggs, within which the young are nourished 
 up to a very advanced stage of development by means of the yolk, 
 while the embryo is provided with the special protective and 
 respiratory foetal membranes known as amnion and allantois, 
 which are not found in amphibians. A reptile, again, never 
 develops gills at any stage of its existence, while an amphibian 
 not only has gills in the larval but sometimes also in the adult 
 state. 
 
 Fossil remains naturally afford no clue as to when the develop- 
 ment of the fcetal membranes and the suppression of the larval 
 gills took place, and we are thrown back entirely upon skeletal 
 peculiarities as a means of distinguishing between extinct 
 members of the two groups. From one point of view this is 
 unsatisfactory, but the very difficulty which we experience in 
 drawing a hard and fast line between the fossil reptiles and 
 amphibians only serves to emphasize the fact that the one group 
 has been derived from the other. 
 
 One of the chief osteological peculiarities which distinguish 
 existing reptiles from amphibians is, as Dr. Smith Woodward 
 observes, " the degeneration of the parasphenoid bone and its 
 functional replacement in the basi-cranial axis by the pterygoids.
 
 THE GEOLOGICAL RECORD 295 
 
 . . . If this feature in the palate has always been distinctive, 
 Paheohatteria, from the Lower Permian of Saxony, is the 
 earliest member of the class Reptilia hitherto discovered ; and it 
 is certain that during the Upper Permian age there were numerous 
 reptiles both in Europe and America, probably -also in South 
 Africa." 1 So far as the evidence goes, therefore, the appearance 
 of the Reptilia succeeds that of the Amphibia in exactly the way 
 demanded by the theory of organic evolution. 
 
 The reptiles as a group attained their maximum of develop- 
 ment in Mesozoic times, and of the nine great orders into 
 which the class has been subdivided only four persist at the 
 present day, and one of these four is represented by only a single 
 
 FIG. 140. Skeleton of Pariasaurus baini, X 3 V (From Smith Woodward, 
 after Seeley.) 
 
 species. This species, Sphenodon (= Hattcria) punctatus (Fig. 113), 
 now confined to certain small islands off the coast of New 
 Zealand, is the sole surviving representative of the order 
 Rhynchocephalia, and not only constitutes the oldest existing type 
 of reptile but also makes a very close approach to the oldest type 
 known from fossil remains the extinct Pala3ohatteria of the Lower 
 Permian. The other existing orders of reptiles are the Chelonia 
 (turtles and tortoises), the Squamata (lizards and snakes) and the 
 Crocodilia (crocodiles and alligators). 
 
 Many of the extinct Secondary reptiles were far more highly 
 specialized and remarkable than any existing forms. Different 
 members of the class were adapted to the most diverse modes of 
 life, the group having taken full possession of sea, land and air. 
 
 1 A. Smith Wooiward, " Outlines of Vertebrate Palaeontology for Students of 
 Zoology " (Cambridge, 1898).
 
 296 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 The earliest and at the same time the least specialized of the 
 extinct orders were the Anomodontia or Theromorpha, which Dr. 
 Smith Woodward tells us were "directly intermediate in skeletal 
 
 Pn 
 c 
 
 i 
 
 I 
 
 ll 
 
 -I 
 
 characters between the highest Labyrinthodonts (Mastodonsanrus 
 and its allies) and the lowest Mammals (Monotremata)." These 
 animals, whose remains have been found only in Permian and 
 Triassic deposits, were terrestrial creatures with limbs modified 
 for walking on dry land, so that they often bore a close general
 
 THE GEOLOGICAL RECORD 297 
 
 resemblance to mammals. One of the best known is the celebrated 
 Pariasaurus (Fig. 140), described by the late Professor Seeley 
 from the Karoo formation of Cape Colony. This animal attained 
 a length of some ten or eleven feet. 
 
 The Sauropterygia or Plesiosauria (Fig. 141) were long- 
 necked lizard-like forms, sometimes of large dimensions, which 
 had become re-adapted to a marine life, and whose limbs, while 
 retaining the typical pentadactyl structure, gradually became 
 modified to form paddles. The group seems to have persisted 
 
 FIG. 143. Skeleton of If/uanodon lernissariensis, from the Wealden of 
 Belgium, x ^V (From British Museum Guide.) 
 
 throughout the whole of the Secondary period and shows a gradual 
 evolution from the Trias onwards. 
 
 The Ichthyopterygia or Ichthyosauria (Fig. 142), which also 
 range throughout the whole of the Secondary period, were more 
 completely adapted to life in the ocean and acquired a remarkably 
 fish-like form, like the whales, dolphins and porpoises amongst 
 existing mammals. 
 
 The Dinosauria or Ornithoscelida formed an enormous gr^up 
 of land reptiles which seems to have arisen in the Triassic period 
 and reached its climax in Jurassic and Cretaceous times. 
 Some of them were carnivorous, others herbivorous; some walked 
 on all fours and others on their hind legs, and they often attained 
 gigantic dimensions. The well known Iguanodon (Fig. 143), from the
 
 298 OUTLINES OF EVOLUTIONARY BIOLOGY
 
 THE GEOLOGICAL RECORD 299 
 
 Wealden deposits of Europe, was a huge herbivore, which seems to 
 have supported its massive body, chiefly at any rate, on its hind 
 limbs, which were much more strongly developed than the front 
 pair. The American Brontosaurus (Fig. 144) and Diplodocus 
 walked on all fours, and the latter attained a length of 80 feet, 
 but a large proportion of this was taken up by the enormously 
 long neck and tail. In both these forms the head was of 
 astonishingly small dimensions in proportion to the rest of the 
 body. Other Dinosaurs, such as Stegosaurus (Fig. 145) and 
 Triceratops (Fig. 146), developed an extraordinary dermal 
 armature of bony plates or spines. 
 
 Perhaps the most remarkable of all the extinct reptiles of 
 the Secondary period, however, were the Ornithosauria (Ptero- 
 sauria or pterodactyls), which at that time occupied the place now 
 
 FIG. 147. Skeleton and Outline of Pteranodon occiilentalis, from the Upper 
 Cretaceous of Kansas, U.S.A. ; X 5%. (From British Museum Guide.) 
 
 filled by the birds and bats. These animals were very perfectly 
 adapted for flight, the wing being formed, as already described in 
 Chapter XVII, by an extension of the skin supported by the arms, 
 legs and tail, and especially by the enormously elongated fifth digit 
 (Fig. 99). Some of these flying reptiles attained a very large 
 size, the total expanse of the wings in Pteranodon (Fig. 147) 
 being about eighteen feet. 
 
 The origin of birds from reptilian ancestors has been quite 
 conclusively demonstrated on anatomical and embryological 
 
 FIG. 144. Skeleton of Brontosaurus excelsus, from the Upper Jurassic of 
 
 Wyoming, U.S.A. ; x T stj. (From British Museum Guide, after O. C. 
 
 Marsh.) 
 FIG. 145. Skeleton of Stegosaurus uitgulalus, from Jurassic of Wyoming ; 
 
 X ff V (From Smith Woodward's "Vertebrate Palaeontology," after 
 
 O. C. Marsh.) 
 FIG. 146. -Skeleton of Triceratops prorsus, from Cretaceous of Wyoming; 
 
 X S V (From British Museum Guide, after 0. C. Marsh.)
 
 300 OUTLINES OF EVOLUTIONAKY BIOLOGY 
 
 grounds, and Professor Huxley included both birds and reptiles 
 in one large group, the Sauropsida. The structure of their 
 wings shows, however, that birds cannot have arisen from 
 pterodactyls but must have sprung from some less specialized 
 form, mainly through the development of a dermal exoskeleton 
 in the form of feathers, which constitutes the chief distinguishing 
 feature of the group. It is, therefore, quite in accordance with 
 expectation that the earliest known bird, Archaeopteryx (Fig. 151), 
 should have been found in the lithographic slates of Solenhofen 
 in Bavaria, which are of Upper Jurassic age, and should exhibit 
 characters intermediate between those of existing birds and 
 reptiles. We shall have occasion to refer to this remarkable 
 connecting link more in detail in our next chapter. 
 
 The Mammalia, so far as existing evidence indicates, appeared 
 at an earlier date than the birds, but the earliest known mammals 
 were far less highly specialized forms than birds, and although 
 what appear to be mammalian remains have been found as low 
 down as the Trias, the group did not attain much importance 
 before the Tertiary epoch, when it replaced the reptiles as the 
 dominant group of vertebrates. Opinions have been a good deal 
 divided as to whether the mammals arose directly from amphibian 
 ancestors or from some primitive group of reptiles. On the 
 whole the evidence seems to be decidedly in favour of the latter 
 view, which fits in very well with the first appearance of the 
 mammals just when the reptiles were beginning to become 
 dominant, and with the fact that, as we have already seen, 
 some of the earlier reptiles were extraordinarily mammalian 
 in aspect. 
 
 The earliest mammals were small forms of doubtful affinities. 
 Dr. Smith Woodward remarks in this connection : " It is as yet 
 impossible to determine at what particular stage in the evolution 
 of the vertebrate skeleton the lung-breathers first acquired the 
 characteristic mammalian circulatory system, the milk-producing 
 glands, and a dermal covering of hair," these being the principal 
 features, apart from the skeleton, which distinguish mammals 
 from reptiles at the present day. The difficulty is increased by 
 the fact that some of the anomodont reptiles had acquired a 
 dentition very similar to that of primitive mammals, while the 
 earliest known mammals are represented by little more than a 
 few fossil jaws and teeth. 
 
 The Mammalia, both living and extinct, may be divided into
 
 THE GEOLOGICAL RECORD 301 
 
 three main subclasses. (1) Prototherid, with the single surviving 
 order Monotremata, represented by the Australian spiny ant- 
 eater (Echidna, Fig. 92) and duck-billed Platypus (Ornithorhyn- 
 chus, Fig. 91), which, as we have already seen, are intermediate 
 both in anatomical structure and in their method of reproduction 
 between the reptiles and the more typical mammals. (2) Meta- 
 theria, with the sole order Marsupialia, represented to-day by 
 the Australian kangaroos, wombats, phalangers, thylacines and 
 numerous other pouched mammals, and by the American opossums 
 (Didelphyidae) and Csenolestes. These are in some respects primi- 
 tive forms, in which the young are born at a very early stage of 
 their development (there being at the most only a very feebly 
 developed placenta) and usually carried about, attached to the 
 teat of the mother, in a marsupium or pouch. They are readily 
 distinguished by osteological features, especially the structure of the 
 juws and teeth (Fig. 108), from the higher mammals. (3) Eutheria, 
 the dominant mammals of the present day, with a large number 
 of orders edentates, whales, sirenians, ungulates, rodents, 
 carnivores, insectivores, bats and primates ; with well developed 
 placenta and retaining the young in the womb until a very 
 advanced stage of development. There can be no doubt that 
 the Eutheria are on the whole more highly organized forms 
 than the Metatheria and the latter than the Prototheria. This 
 is especially indicated by the gradual acquisition and perfection 
 of the characteristically mammalian method of nourishing the 
 young by means of the placenta and mammary glands. 
 
 The order of appearance in time of the principal subdivisions 
 of the class Mammalia, as indicated by the geological record, 
 entirely supports the evolutionary hypothesis. The earliest 
 known fossils which may possibly be referable to the class are of 
 Triassic age. Tritylodon is represented by an imperfect skull 
 from the Karoo formation of South Africa. This has generally 
 been regarded as mammalian, but Dr. Smith Woodward, notwith- 
 standing the fact that it has tuberculated grinding teeth with 
 deeply cleft roots, considers that it probably belonged to an 
 anomodont or theromorph reptile. Microlestes is represented by 
 double-rooted, multituberculate teeth obtained from European 
 Rhaetic formations, and may also possibly be reptilian. 
 
 In deposits of Lower and Upper Jurassic age we find more 
 convincing evidence of the existence of true mammals. The 
 celebrated Stonesfield slate of Oxfordshire (Lower Jurassic) has
 
 302 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 yielded a fragment of a jaw, containing three small multituber- 
 culate grinding teeth, to which the name Stereognathus has been 
 given, and one of the Middle Purbeck beds of Swanage (Upper 
 Jurassic) has contributed the well known jaws of Plagiaulax 
 (Fig. 148). Remains of closely related forms, supposed to have 
 belonged to the same family (Plagiaulacidae), have been discovered 
 in Upper Jurassic and Upper Cretaceous formations of North 
 America, and the family appears to have survived, both in Europe 
 and North America, into early Tertiary times. 
 
 The Plagiaulacidae and related forms have been grouped together 
 in the extinct order Multituberculata, which Dr. Smith Wood- 
 ward places provisionally amongst the Prototheria, side by side 
 with the surviving order Monotremata, no remains of which are 
 p known to occur before Tertiary 
 
 times. It is possible, however, 
 2 ^MS^SSVSSSU ' > ^S that the Multituberculata may be 
 
 metatherian rather than proto- 
 
 therian. 
 
 The remains of undoubted Meta- 
 
 theria (Marsupialia) first occur, so 
 
 FIG. 148. Mandible of Plagiaulax , 
 
 minor, x 4. (From Smith far as we yet know, in the same 
 Woodward's "Vertebrate beds as the earliest Multituber- 
 PalsBontolo^v " al'tsr Fal- j P 
 
 con.6r.) o 
 
 the enigmatical Tritylodon and 
 
 Microlestes). Mandibles of Phascolotherium (Fig. 149) and 
 Amphitherium have been found in the Stonesfield slate, while 
 Triconodon and Spalacotherium are similarly represented in the 
 mammal bed at Durdlestone Bay near Swanage. 
 
 Throughout the whole of the Secondary period the mammals 
 remained of insignificant size, and in a more or less primitive 
 condition, such as is represented at the present day by the 
 surviving monotremes and marsupials. The typical placental 
 mammals (Eutheria) are not known to us from formations of 
 earlier date than the Eocene. Then, all at once, they seem to have 
 branched out in every direction and taken possession of land, sea 
 and air just as the reptiles had done before them ; whales replacing 
 the Ichthyosauria and Plesiosauria, various groups of land 
 mammals replacing the Theromorpha and Dinosauria, and bats 
 sharing with the birds the kingdom of the air which had formerly 
 belonged to the pterodactyls. 
 
 As in the case of their amphibian and reptilian predecessors,
 
 THE GEOLOGICAL RECORD 303 
 
 many of the mammalian groups in Tertiary times have run to 
 great size ; most of the larger forms, such as the primitive 
 ungulate, Tinoceras (Fig. 150), of the American Eocene, and the 
 giant ground sloth, Megatherium, of the American Pleistocene, 
 are already extinct, but it must not be forgotten that the existing 
 whales are amongst the largest animals that have ever lived, and 
 in bulk at any rate will bear comparison with the largest of 
 the great extinct reptiles. 
 
 The last term in the evolutionary series of the Mammalia is 
 man, whose advent, so far as we at present know, dates back only 
 
 Nat. siss. 
 
 pm c i 
 
 \ /A//L 
 \ ^C4^a 
 
 FIG. 149. Mandible of Phascolotheriwn buc&landi; x 3. (From Smith 
 Woodward's "Vertebrate Palaeontology," after Goodrich.) 
 
 to about the commencement of the Pleistocene or end of the 
 Pliocene epoch. 1 
 
 There is one more point that is well worth emphasizing about the 
 evolution of the Vertebrata, as indicated not only by the geological 
 record but also by the facts of comparative anatomy. Each 
 successive great group appears to have arisen, not from the 
 most highly specialized members of some preceding great 
 group, but from comparatively undifferentiated forms. Thus the 
 Amphibia arose, not from bony fishes, but from primitive dipnoids 
 or ganoids ; the reptiles arose, not from frogs or toads, but from 
 primitive stegocephalian amphibia ; the birds arose, not from 
 pterodactyls, but from comparatively unspecialized reptiles ; the 
 mammals also arose from the more primitive reptilian forms, and 
 man himself, whose advent undoubtedly marks the commencement 
 of a fresh line of evolution, belongs to the order Primates, which 
 in respect of bodily organization, as seen, for example, in the 
 
 i Vide Chapter XXV II.
 
 304 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 typical pentadactyl limbs, is far more primitive than many other 
 mammalian groups. 
 
 Each great group seems to have begun in a small way, then 
 developed rapidly, branching out in many directions and becoming 
 the dominant group for the time being, only to dwindle away 
 again and give place to some new and vigorous off-shoot. The 
 dominance of any particular group has often been accompanied 
 by the attainment of enormous size by its individual members, 1 
 
 FIG. 150. Skeleton of Tinocemsingens, from the Middle Eocene of Wyoming, 
 X gV (From British Museum Guide, after O. C. Marsh.) 
 
 and it is not impossible that this may have had something to do 
 with its subsequent decline or complete extinction. 
 
 So far as the animal kingdom is concerned, we may perhaps 
 say without exaggeration that the succession in time of the 
 different groups, as indicated by the geological record, amounts 
 to positive demonstration of the truth of the theory of organic 
 evolution. In the case of plants the record is perhaps not quite 
 so clear, but here also there can be no reasonable doubt that the 
 great groups succeeded one another in a manner consistent with 
 the theory, commencing with the algfe and ending with the 
 flowering plants of the present day. 
 
 1 A possible explanation of this fact is suggested in Chapter XXVI.
 
 CHAPTER XX 
 
 Fossil pedigrees Ancestry of birds, horses, elephants and whales. 
 
 IN our last chapter we gave a brief outline of the general course 
 of evolution amongst vertebrate animals as indicated by the 
 geological record. We may now study in somewhat greater 
 detail certain branches of the great phylogenetic tree which are 
 especially well represented by fossil remains and therefore 
 particularly instructive from the point of view of the evolution 
 theory. 
 
 One of the most highly specialized groups of vertebrates that 
 have ever existed is that of the birds. We have already pointed 
 out that, on anatomical grounds, birds are classed together with 
 reptiles as Sauropsida. They agree with reptiles in their method 
 of reproduction by means of large, heavily yolked eggs, and in 
 the presence, in the embryo, of the characteristic foetal membranes, 
 amnion and allantois, as well as in certain morphological charac- 
 ters of the adult. They differ from reptiles, however, in many 
 striking features. Thus they possess feathers, which almost (but 
 not quite) completely replace the reptilian scales as a protective 
 exo-skeleton. The anterior limbs are modified to form wings, 
 constructed, as we have already seen in Chapter XVII, on an 
 entirely different plan from those of flying reptiles. The digits of 
 the hand are very greatly reduced ; only one of them, and that in 
 a vestigial condition, projects freely from the anterior border of the 
 wing, forming the so-called " ala spuria " (Fig. 99). The true tail 
 is greatly abbreviated and the caudal vertebrae reduced in number 
 and to a large extent fused together to form the " ploughshare 
 bone " which supports the tail feathers. Lastly, all existing birds 
 have completely lost their teeth, which are functionally replaced 
 by the horny beak. 
 
 The remains of the earliest known bird, Archaeopteryx (Fig. 151), 
 have been found in the celebrated lithographic stone of Solenhofen 
 in Bavaria, of Upper Jurassic age, which, owing to its extremely 
 fine grain, is peculiarly well suited for the preservation of even 
 
 B. x
 
 306 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 such delicate structures as feathers. This animal was about the 
 size of a rook and the presence of well developed feathers and 
 wings of the avian type is alone sufficient to show that we are 
 dealing with a true bird. It still exhibits, however, a number of 
 features which are usually met with in reptiles but have dis- 
 appeared in modern birds. The digits of the anterior limb are 
 not nearly so much reduced as in the latter, for three claw-bearing 
 
 FIG. 151. Fossil Eemains of Archceopteryx siemensi, showing the three 
 fingers in each wing, the long tail, feathers, &c. (From Lankester's 
 " Extinct Animals.") 
 
 fingers project from the anterior margin of the wing ; the tail is 
 elongated like that of a lizard and supported by about twenty 
 separate vertebrae each carrying a pair of feathers ; and numerous 
 teeth are present in the beak. 
 
 It is obvious that Archseopteryx represents a stage in the 
 derivation of birds from reptilian ancestors, and this is exactly 
 what we should expect of the earliest birds in accordance with 
 the theory of evolution. Unfortunately, with the exception of a 
 few other toothed birds of Cretaceous date, Archseopteryx is
 
 EVOLUTION OF THE HORSE 
 
 307 
 
 almost the only link in the pedigree of birds which has so far 
 been discovered, and it teaches us nothing as to the origin of 
 those characteristic avian structures, the feathers, which it 
 possesses already in a fully developed condition. 
 
 It will be observed that Archseopteryx occupies a position 
 between reptiles and typical birds exactly comparable with that 
 of the Monotremata between reptiles and typical mammals (see 
 Chapter XVII), the only difference being that the Monotremata 
 still survive side by side with mammals of the most highly 
 advanced type, while Archaeopteryx has long since become extinct. 
 
 One of the most complete fossil pedigrees as yet known to us 
 
 FIG. 152. Skeleton of Phenacodus, a five-toed Eocene Ungulate. (From 
 Lankester's "Extinct Animals.") 
 
 is that of the Equidas or horse family. As we have already seen 
 in Chapter XVII, the study of comparative anatomy indicates very 
 clearly that the highly specialized single-toed limbs of the horse 
 (Fig. 97) must have arisen from some primitive pentadactyl 
 type by gradual suppression of all the digits except the middle 
 one. Amongst the fossil remains of horse-like animals which 
 abound in various tertiary formations of Europe and America we 
 find a very complete series of stages in the evolution of the 
 modern horse, which entirely confirms this conclusion. Our 
 knowledge of this extremely interesting phylogenetic series is due 
 largely to the late Professor 0. C. Marsh and has been admirably 
 summarized by Mr. B. S. Lull in the American Journal of Science. 1 
 
 1 Series IV, Vol. XXIII, 1907. 
 
 x 2
 
 308 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 a 
 
 FIG. 153. Outlines of Horses of different Geological Periods, showing their 
 Eelative Sizes. (From Lull.) 
 
 a, Protorohippus (Eocene) ; b, Orohippus (Eocene) ; c, Mesohippus (Oligocene) ; d, Meryc- 
 hippus (Miocene) ; e, Pliohippus (Pliocene) ; /, Equus (Recent).
 
 EVOLUTION OF THE HORSE 309 
 
 It is generally admitted that the Equidae originated from the 
 Condylarthra, a group of primitive, five-toed, ungulate mammals 
 which made its appearance in early Eocene times, and the best- 
 known representative of which is Phenacodus (Fig. 152). The 
 evolution of the horses appears to have taken place chiefly in 
 America, though occasionally representatives of the group seem to 
 have migrated to or from Europe, doubtless by a former land 
 connection in the neighbourhood of Behring Strait. In com- 
 paratively recent times, however, the family became confined to 
 the old world and was only re-introduced to America by human 
 agency. 
 
 The course of their evolution has evidently been determined by 
 the development of extensive, dry, grass-covered, open plains 
 on the American continent. In adaptation to life on such areas 
 structural modification has proceeded chiefly in two directions. 
 The limbs have become greatly elongated and the foot uplifted 
 from the ground, and thus adapted for rapid flight from pursuing 
 enemies, while the middle digit has become more and more 
 important and the others, together with the ulna and the fibula, 
 have gradually disappeared or become reduced to mere vestiges. 
 At the same time the grazing mechanism has been gradually 
 perfected. The neck and head have become elongated so 
 that the animal is able to reach the ground without bending its 
 legs, and the cheek teeth have acquired complex grinding surfaces 
 and have greatly increased in length to compensate for the 
 increased rate of wear. As in so many other groups, the evolution 
 of these special characters has been accompanied by gradual 
 increase in size (Fig. 153). Thus Eohippus, of lower Eocene 
 times, appears to have been not more than 11 inches high at the 
 shoulder, while existing horses measure about 64 inches, and the 
 numerous intermediate genera for the most part show a regular 
 progress in this respect. 
 
 All these changes have taken place very gradually, and a 
 beautiful series of intermediate forms indicating the different 
 stages from Eohippus to the modern horse (Equus) have been 
 discovered. The sequence of these stages in geological time 
 exactly fits in with the theory that each one has been derived 
 from the one next below it by more perfect adaptation to the 
 conditions of life. Numerous genera have been described, but it 
 is not necessary to mention more than a few. 
 
 The first indisputably horse-like animal appears to have been
 
 310 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Hyracotherium, remains of which have been found in the 
 London Clay (Lower Eocene). Another Lower Eocene genus 
 was Eohippus, which seems to have arisen in Western Europe, 
 possibly from a hyracotherian ancestry, and migrated, by way 
 of Northern Asia, to America, where its remains occur in rocks 
 of the same age. In this animal (Fig. 154) the fore foot had 
 four well developed digits and the thumb was represented by a 
 splint bone ; in the hind foot the great toe had entirely dis- 
 appeared and the fifth digit was represented only by a splint 
 bone. In both fore and hind feet 
 the third or middle digit was already 
 conspicuously larger than any of the 
 others. 
 
 Eohippus was succeeded by Pro- 
 torohippus (Fig. 153, a), which was 
 some 3 inches higher and had lost 
 the vestigial thumb. Then came 
 Orohippus (Fig. 153, b), again a little 
 larger and with closely similar feet 
 (Fig. 155), but with a considerable 
 advance in the evolution of the 
 grinding teeth. The last of the 
 Eocene horses was Epihippus, still 
 " li) /V with four toes in front and three 
 i behind, but with the lateral toes 
 
 b further reduced in size and another 
 
 FIG. 154. a, Fore Foot and distinct advance in tooth structure. 
 
 b, Hind Foot of Eohippus i n Qligocene times there occurred 
 
 after Marsh*) in N rth America Mesohippus and 
 
 Miohippus, and in Europe Anchi- 
 
 therium. Mesohippus (Fig. 153, c) was 18 inches or more in 
 height, with three digits and a vestige of the fifth in the fore 
 foot and three digits only in the hind foot (Fig. 156). Miohippus 
 attained a height of 24 inches and closely resembled Mesohippus 
 in the structure of its feet. Anchitherium is supposed to be 
 a European derivative of Miohippus. 
 
 In the Miocene period the horses appear to have attained their 
 maximum of development as a group, and a number of extinct 
 American genera are distinguishable. Merychippus (Fig. 153, d), 
 Protohippus and Neohipparion were still three-toed horses, though 
 the lateral digits were now greatly reduced (Fig. 157). Pliohippus
 
 EVOLUTION OF THE HORSE 
 
 311 
 
 (Fig. 153, e), which continued on into Pliocene times and attained 
 a height of 48 inches, had the second and fourth digits of each 
 foot represented by mere splint bones as in modern horses, 
 and had therefore already attained to the single-toed condition 
 (Fig. 158). 
 
 In Pliocene times, however, we still find a three- toed horse 
 Hipparion surviving in Europe, but the modern one-toed genus 
 Equus (Fig. 153, /) also makes 
 its appearance both in the old and 
 new worlds, becoming extinct in 
 the new world in Post-Pleistocene 
 times until re-introduced from 
 Europe by the agency of man. 
 
 The time occupied in the evolu- 
 tion of the genus Equus from 
 its remote ancestor Eohippus is 
 estimated by Professor Sollas at 
 five or six millions of years. This 
 period is sufficient to allow of a 
 very slow and gradual change 
 from one condition to the other. 
 Allowing five years for each 
 generation, Sollas arrives at the 
 conclusion that somewhere about 
 a million generations intervene 
 between the two extremes. The 
 total increase in height during 
 this time has been 53 inches, and 
 if this increase were spread fairly 
 uniformly over the whole period 
 it would only mean about 0*00005 inch for each successive 
 generation an amount which would be quite imperceptible to 
 human observers. 
 
 In reconstructing such a pedigree as that of the horse from 
 palaeontological evidence it is of course necessary to bear in 
 mind that the great majority of extinct forms which come to 
 light will almost certainly not be actually in the direct line of 
 descent. Collateral branches will have been given off from the 
 phylogenetic tree in various directions, and it is much more likely 
 that any particular form discovered will belong to one of these 
 branches than that it will belong to the main stem. This fact, 
 
 FIG. 155. a, Fore Foot and b, Hind 
 Foot of OroJiippus agilis, X ! 
 (From Lull, after Marsh.)
 
 312 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 however, by no means vitiates the general argument, for it is usually 
 possible to pick out pretty accurately those which come into or 
 near to the direct line, and even the collaterals afford valuable 
 evidence as to the general course of evolution. We may safely 
 say that the palaontological evidence amounts to a clear 
 
 demonstration of the evolution 
 of the horse from a five-toed 
 ancestor along the lines indi- 
 cated above. 
 
 The ancestry of the elephants 
 is less well known than that 
 of the horses, but recent dis- 
 coveries in the Egyptian Ter- 
 tiary formations, which we owe 
 especially to the investigations 
 of Dr. Andrews, have done 
 much to elucidate the history 
 of this remarkable group of 
 mammals, and there can now 
 be no doubt as to the main 
 line of evolution which has led 
 up to the existing Proboscidea. 
 In some respects the elephants 
 have remained in a some- 
 what primitive condition, as 
 is indicated very clearly by the 
 fact that all the digits remain 
 well developed in both fore and 
 hind feet. It is in the struc- 
 ture of the head that they 
 exhibit a high degree of special- 
 ization, marked particularly by 
 the elongation of the snout to 
 form a long prehensile trunk, by the enormous development of 
 the occipital region of the skull, by the enlargement of the 
 incisor teeth to form great tusks, by the shortening of the jaws 
 and by the increase in size and complexity and the reduction in 
 number of the cheek teeth. These changes have been accom- 
 panied by a huge increase in the size of the entire body, so that 
 most of the elephants are amongst the largest of known land 
 mammals, whether fossil or recent. 
 
 FIG. 156. a, Fore Foot and b, Hind 
 Foot of Mesohippus celer, X i. 
 (From Lull, after Marsh.)
 
 EVOLUTION OF ELEPHANTS 
 
 813 
 
 Like the horses, the elephants probably originated from that 
 primitive ungulate group, the Condylarthra. The earliest known 
 form exhibiting proboscidean characters is Mreritherium, a 
 tapir-like animal whose remains have been found in the Middle 
 and Upper Eocene deposits of 
 the Egyptian Fayum. This was 
 a comparatively small creature, 
 about as large as a Newfoundland 
 dog. It probably differed but little 
 from other primitive ungulates, 
 but the skull (Fig. 159, 1) already 
 shows marked proboscidean ten- 
 dencies. The position of the nasal 
 bones, away back from the tip of 
 the snout, indicates that there 
 was in all likelihood a short pro- 
 boscis. The occipital region of 
 the skull is beginning to grow up 
 and air cells are beginning to 
 develop in the bones. The second 
 pair of incisor teeth in each jaw 
 are enlarged to form small tusks 
 and the hinder cheek teeth are 
 beginning to show an increase in 
 complexity of structure. The total 
 number of teeth however (86) is 
 only eight short of the full typical 
 mammalian dentition. 
 
 The next stage is represented 
 by Palaeomastodon (Fig. 159, 2) 
 from the Upper Eocene of the same 
 region, some species of which were 
 little larger than Moeritherium 
 while others attained almost 
 
 elephantine proportions. In this genus we notice a strong 
 accentuation of the proboscidean characters. The occiput is higher, 
 the nasal opening in the skull further back, the upper tusks 
 better developed, the cheek teeth more complex ; while the canines 
 and all the incisors except the tusks in both jaws have dis- 
 appeared. It will be observed that as yet there is no shortening 
 of the jaws, but, on the contrary, the lower jaw has become 
 
 ivhitneyi, x i- (From Lull.)
 
 314 OUTLINES OF E VOLUTION AKY BIOLOGY 
 
 considerably elongated, apparently serving as a support for the 
 lengthening proboscis. 
 
 In Tetrabelodon angustidens, from European Miocene formations, 
 this elongation of the mandible is much more marked, so that 
 the lower jaw is much longer than the upper one and the short 
 lower tusk comes to project almost as 
 far forward as the long upper one (Fig. 
 159, 3). From this time onwards, how- 
 ever, the chin shortens, thereby allowing 
 greater flexibility to the proboscis, so that 
 in the lower Pliocene we find Tetrabelodon 
 lonnirostris (Fig. 159, 4) with the lower 
 jaw only a little longer than the upper, 
 leading the way to the mastodons and 
 true elephants (Elephas), which also 
 appeared in Pliocene times and in which 
 the tusks have entirely disappeared from 
 the greatly abbreviated mandibles while 
 the cheek teeth have become enormously 
 enlarged and complicated (Fig. 159, 5). 
 
 We have here a wonderfully perfect 
 series of connecting links between the 
 most primitive known ungulate mammals 
 and the elephants. Only forms which 
 appear to lie in or near the direct line of 
 descent have been mentioned in the above 
 brief account. Other modifications of 
 the proboscidean type arose as lateral 
 offshoots from this main stem. One of 
 the most remarkable of these is Dino- 
 therium, with its great, downwardly 
 directed lower tusks (Fig. 160), which 
 appeared in Europe in the Pliocene period. 
 In the case of the Cetacea, a group which includes the whales, 
 porpoises and dolphins, we have as yet only a much more frag- 
 mentary pedigree, but still quite sufficient to justify, on the 
 palaeontological side, the conclusion, already arrived at on ana- 
 tomical grounds, that these extremely aberrant forms are the 
 descendants of typical terrestrial mammals which have become 
 re-adapted to an aquatic life and in accordance therewith have 
 re-acquired a superficial resemblance to their much more remote 
 
 FlG. 158. a, Fore Foot 
 and b, Hind Foot of 
 Pliohippns pernix, X ^. 
 (From Lull.)
 
 EVOLUTION OF ELEPHANTS 
 
 315 
 
 Recent 
 Pleistocene 
 Ufifier Pliocene 
 
 ELEPHAS 
 (short chin) 
 
 Lower Pliocene TETRABELODON 
 
 [LONGIROSTRIS STAGE] 
 Ufijier Miocene (shortening chin) 
 
 Middle Miocene TETRABELODON 
 
 fANCUSTIDENS STAGE] 
 
 Lower Miocene ( long chin) 
 
 T , , Migration from Africa 
 
 UrmerOhqocene . J r 
 
 into turone -Asia 
 
 Lower Oligocene 
 UfijierEocene 
 MiddleEocene 
 Lower Eocene 
 
 I 
 
 \ PAL 
 J (le 
 
 PAL AEO MASTODON 
 gthemng chin) 
 
 MOERITHERIUM 
 
 FIG. 159. Some Stages in the Evolution of the Skull in the Proboscidea 
 (From British Museum Guide.)
 
 .316 OUTLINES OF EVOLUTIONAKY BIOLOGY 
 
 fish-like ancestors. 1 The fore limbs have become converted into 
 paddles while the hind limbs have entirely disappeared externally 
 
 FIG. 160. Skull of Dinotherium giganteum, Lower Pliocene, X T ^. (From 
 Smith Woodward's " Vertebrate Palaeontology," after Kaup.) 
 
 (Fig. 161). The tail has become flattened out into a horizontal 
 fin and there is frequently a well developed dorsal fin. The skull 
 
 FIG. 161. The Dolphin, Delphinm ddpliis, x 
 Guide.) 
 
 (From British Museum 
 
 has undergone very curious changes. The brain case is rounded 
 and strongly arched (Fig. 162) and the nasal apertures or blow- 
 holes lie far back, at or near the highest point of the head 
 
 1 Compare Chapter XVII.
 
 EVOLUTION OF WHALES 
 
 317 
 
 (Fig. 101, />), while the jaws have become greatly elongated. The 
 teeth have in some cases completely disappeared, as in the whale- 
 
 FIG. 162. Skull of the Dolphin, X J. (From British Museum Guide.) 
 
 bone whales (except for fetal vestiges), while in others they are 
 present in large numbers but have lost the typical mammalian 
 
 FIG. 163. Dorsal and lateral views of the Skull of a primitive Whale, 
 Prozeuglodon atrox, X T V (From British Museum Guide.) 
 
 differentiation into incisors, canines, premolars and molars, being 
 represented by a continuous and uniform series, all of which are 
 conical in shape and single-rooted. Such teeth occur in both
 
 318 OUTLINES OF EVOLUTIONARY' BIOLOGY 
 
 jaws of the porpoises and dolphins (Fig. 162) and in the lower 
 jaw only of the sperm whale. 
 
 In the extinct shark-toothed Dolphins (Squalodon-tidse), whose 
 remains have been found in Miocene formations of Europe and 
 America, the teeth are still differentiated into incisors, canines, 
 premolars and molars, and the molars have double roots and 
 compressed crowns with serrated edges. 
 
 Further back, in Eocene times, there existed, widely distributed 
 over the northern hemisphere, a group of still more primitive 
 whale-like animals known as Zeuglodontidae. In these the seven 
 vertebrae of the neck, which in existing whales are more or. 
 less fused together into a solid mass (Fig. 103), are all separate, 
 and the typical dental formula is identical with that of primitive 
 
 ,..31 4 32 
 
 carnivorous land mammals, viz. i , c. n , p.m. - , m. 3 
 
 o 1 4 o 
 
 The genus Prozeuglodon, from the Egyptian Eocene, ap- 
 proaches so closely in the characters of the skull and teeth 
 (Fig. 163) to the primitive carnivores (Creodontia)of about the same 
 period as to leave no reasonable doubt about the derivation of the 
 Cetacea from that group, although it is quite possible that none 
 of the extinct forms so far discovered are actually in the direct 
 line of descent of any of the modern whales.
 
 CHAPTER XXI 
 
 Geographical distribution 1 Areas of distribution Barriers to migration 
 Means of dispersal Changes in the physical conditions of the earth's 
 surface The evidence afforded by the study of geographical distribution 
 with regard to the theory of organic evolution. 
 
 IT is hardly necessary to remind the reader that each species 
 of plant or animal, in a state of nature, is more or less sharply 
 restricted to a certain portion of the earth's surface, the entire 
 region over which it may be found, whether sea or land, being 
 termed its area of distribution. Such areas of specific distribu- 
 tion are nearly always continuous, without any considerable gaps 
 or intervals from which the species is entirely absent. This does 
 not, of course, mean that the species necessarily occurs in all 
 parts of its area of distribution at once, but that it is free to 
 range over the whole of it and may accordingly be found in any 
 suitable part of it at any time. It is necessary to introduce some 
 such qualifying word as " suitable " in this connection, because 
 each species is not only restricted in its range to a more or less 
 well-defined geographical area, but can only live continuously in 
 certain portions of that area, to the special conditions of which it 
 is structurally and physiologically adapted and which constitute 
 its habitat. Thus, for example, a fresh-water snail may perhaps 
 range over an entire continent, but it would be useless to look for 
 it except in fresh water. Individuals of a species may pass with 
 more or less freedom, according to the nature of the case, from 
 one habitat to another within the area of distribution, but it is 
 only on rare and exceptional occasions that they are able to 
 transgress the boundaries of the area itself. 
 
 True discontinuity in areas of specific distribution, as distin- 
 guished from mere discontinuity of habitats, is extremely rare. 
 We have a good example of it, however, in the case of the marsh 
 
 1 The reader is referred to Dr. Wallace's classical volumes on " Island Life " and 
 the "Geographical Distribution of Animals," and to Professor Heilprin's work on 
 the "Distribution of Animals" in the International Scientific Series (Vol. LVIII, 
 
 1887), for further information on this subject.
 
 320 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 tit (Parus palustris), which has two areas of distribution separated 
 from one another by an interval of four thousand miles in 
 Europe and Asia Minor on the one hand and in Northern China 
 on the other. 
 
 The size of the area over which a species may range varies 
 immensely, in some cases comprisinglin entire continent, or even 
 more, and in others only a few square miles. Thus the leopard 
 ranges over the whole of Africa and most of Southern Asia, while 
 the Tuatara (Fig. 113) is confined to certain small islands off the 
 coast of New Zealand, and certain species of humming birds 
 are said to occur only on the volcanic peak of Chimborazo in the 
 equatorial Andes. 
 
 An area of generic distribution is the sum of the areas of 
 distribution of all the species which are comprised within the 
 genus, and thus genera have usually a much wider geographical 
 range than species. Families, again, have a wider range than 
 genera and orders than families, and so on with groups of still 
 higher value. In short, the more comprehensive the group the 
 larger will be its area of distribution, until we find that the sub- 
 kingdoms or phyla are cosmopolitan, ranging more or less over 
 the entire world, wherever a suitable habitat is to be found. 
 
 The reason why species are rarely, if ever, cosmopolitan in 
 their distribution is that they are confined within their own 
 limited areas by the existence of physical conditions which con- 
 stitute what are called barriers to migration. Such barriers 
 either form absolutely insuperable obstacles to the passage of the 
 species in question or they may be surmounted only at rare 
 intervals and by some happy chance. 
 
 The nature of the barriers varies, of course, with tbe species 
 concerned, and wbat is a barrier to one may be a high road 
 to another. In the case of marine animals the principal barriers 
 are continents and temperature conditions, while the deep sea 
 itself acts as a barrier to the distribution of shore-dwelling or 
 littoral forms. For terrestrial animals the chief barriers are 
 seas, rivers, mountain ranges, deserts and climate generally ; for 
 fresh-water animals land and sea. In the case of plants the 
 barriers to migration are very much the same. 
 
 It may be laid down as a general law that every organism, 
 whether animal or vegetable, at some period or other of its 
 existence is specially adapted so as to secure dispersal either by 
 its own exertions or by the action of some external agency. By
 
 DISPERSAL OF PLANTS 321 
 
 some means or another it is able, not only to spread itself over 
 its own area of distribution, but also, when occasion offers, to 
 extend tbat area by surmounting its barriers. 
 
 The lower terrestrial plants, such as fungi, mosses and ferns, 
 are dispersed by means of spores, which, protected by special 
 envelopes, may be widely distributed by the wind. In the 
 higher plants the spores, as agents of dispersal, are replaced by 
 seeds, which, usually still within the fruit, maybe carried on the 
 wind, floated on rivers or ocean currents, carried about entangled 
 in the hair or feathers of animals, or actually eaten and passed 
 out uninjured in the faeces. A great many seeds and fruits are 
 specially modified in structure to secure their distribution in one 
 or other of these ways, and the study of such adaptations consti- 
 tutes one of the most interesting chapters in botanical science. 
 We need only refer here to such fruits as the blackberry, whose 
 succulence tempts the birds to eat them and carry the seeds, 
 safely enclosed in their hard protective envelopes, to long 
 distances ; the various kinds of burs with their hooks for entangle- 
 ment in fur and feathers ; the winged fruits of the maple, elm 
 and ash, and the thistledown of the thistles, adapted for floating 
 on the wind. 
 
 The dispersal of plants is in all cases passive and dependent 
 on external agencies, though sometimes aided by some purely 
 mechanical device in the plant itself ; in the case of animals it 
 may take place either passively or actively, by the exertions of 
 the animals themselves. 
 
 Beginning^ with the marine fauna, we find that the larger 
 forms whales, porpoises, dolphins and fishes -owe their dis- 
 persal mainly to their own active powers of locomotion, while 
 the smaller animals, especially the invertebrates, are largely 
 dependent in this respect upon oceanic currents. 
 
 Even animals which, like the sponges and corals, are firmly 
 fixed to the sea-bottom in the adult condition, have free-swim- 
 ming larval forms (Fig. 164) whose own limited powers of 
 locomotion may, under favourable circumstances, be enormously 
 supplemented by the action of currents. Such larval forms, 
 from the point of view of dispersal, play the part of the spores and 
 seeds of plants, and they occur not only in cases where the adult 
 is strictly sedentary in habit but also where its powers of loco- 
 motion are limited, as in many worms, snails, crabs (Fig. 128), 
 star-fishes, brittle stars (Fig. 127) and sea-urchins. The
 
 322 OUTLINES OF EYOLUTIONAEY BIOLOGY 
 
 dispersal even of large and active fish, like the mackerel, is largely 
 assisted by the action of currents upon the floating eggs, and 
 this factor must be of still greater importance in the case of 
 the comparatively sluggish bottom dwellers, such as the turbot 
 and sole. 
 
 The more superficial waters of the open ocean are densely 
 populated with pelagic animals and plants and with pelagic eggs 
 and larvae in various stages of development, all drifting more or 
 less helplessly wherever the ocean currents may carry them, for 
 their own powers of locomotion are usually quite insufficient to 
 
 enable them to pursue an inde- 
 pendent course. This floating 
 population is technically spoken 
 of as " plankton " and its in- 
 vestigation, which is of great 
 importance for the solution of 
 practical fishery problems, has 
 lately attracted a great deal of 
 attention. 
 
 We must therefore regard all 
 the great ocean currents, such 
 as the Gulf Stream, as highways 
 FIG. 164. Free-swimming Larva of thronged with life of many kinds, 
 hMwanS ia conil ' T68Sa '' including representatives of all 
 
 (The lafvaLTmfby leans of the rapid the mOr6 important groups of 
 
 undulations of the numerous flageiia marine animals, any one of 
 
 with which it is provided.) < > i , 
 
 which may be on its way to 
 
 found a new colony and establish its own particular species in some 
 region far distant from its original home. Some of the wanderers 
 are only immature forms, belonging partly to shore-dwelling 
 species, others are adult. Almost all exhibit some special adap- 
 tation to their pelagic mode of life. The fish-eggs are floated 
 by oil-globules, and the larvae of the crabs, brittle stars and 
 sea-urchins are provided with defensive spines (Figs. 127, 128) ; 
 but the most general and characteristic feature of all the pelagic 
 host is transparency, whereby they are rendered inconspicuous 
 and less likely to become the victims of the numerous enemies 
 which feed upon the plankton. Adult jelly-fish, worms, molluscs 
 and crustaceans, and innumerable larval forms, all exhibit this 
 same peculiarity. 
 
 The effect of ocean currents upon the distribution of marine
 
 DISPERSAL OF ANIMALS 323 
 
 animals is shown in a very interesting manner in the case of the 
 Mediterranean and the Red Sea. In each of these there is a 
 surface current constantly flowing in from the open ocean and 
 bringing in vast numbers of individuals, both larval and adult, 
 which never find their way out again. Hence these almost 
 enclosed seas form a kind of trap for marine animals and we 
 accordingly find them to be inhabited by an exceptionally rich 
 and varied fauna. 
 
 The range of marine species, though sometimes very wide, is 
 usually more or less strictly limited, so that the shores of every 
 continent have their own characteristic fauna and flora. This is 
 no doubt partly accounted for by differences in climatic conditions, 
 food supply and so forth, but it is mainly due to the fact that, 
 in spite of the facilities for travel afforded by ocean currents, 
 the dangers incidental to a long voyage from one continent to 
 another are rarely surmounted, at any rate by shore-dwelling 
 organisms. We know that many such forms flourish quite as well 
 in some other part of the world as in their original home if they 
 can once overcome the initial difficulty of migration. Thus 
 the artificial introduction of the American oyster into British 
 seas has accidentally brought with it the introduction of the 
 remarkable limpet-like Crepidula, which attaches itself to the 
 oyster shells and runs riot over the oyster beds on the Essex 
 coast. That such occurrences may occasionally take place in a 
 state of nature, and a species thereby be enabled to extend its 
 area of distribution, there can be no reasonable doubt, for even 
 American turtles have occasionally been carried by the Gulf 
 Stream to the shores of Great Britain. 
 
 Amongst the higher forms of non-aquatic animals, and 
 especially birds and mammals, their own powers of locomotion 
 constitute the most important means of dispersal. Even these, 
 however, are frequently transported for long distances by those 
 external agencies which are chiefly responsible for the dispersal 
 of less highly organized forms. 
 
 Leaving out of account,- for the moment, the action of man, 
 which has brought about immense changes in the geographical 
 distribution of the existing fauna, the chief agents to be noticed 
 in this connection are wind and water. 
 
 It is to the action of the wind that large numbers of winged 
 animals insects, birds and bats owe the wide distribution which 
 they enjoy. All actively flying land animals are liable to be 
 
 Y 2
 
 324 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 carried out to sea in storms, and although the great majority of 
 these will inevitably perish a few will occasionally manage to 
 reach some distant haven where they may succeed in establish- 
 ing a colony and thus extending the range of the species. 
 
 During westerly winds American birds not infrequently make 
 their appearance on various parts of the coast of Europe, while 
 north of the 58th parallel of latitude the polar winds trend in 
 the opposite direction and with them we find a transference of 
 European birds, by way of Iceland and Greenland, to the 
 American continent. 1 During storms, again, European birds are 
 cast upon the Azores, abotit 1,000 miles from the nearest 
 continental coast, and there is strong reason for believing that 
 the little wax-eye (Zoster ops lateralls) has been transported in 
 this way from Australia to New Zealand, where it has succeeded 
 in establishing itself. 
 
 Water currents may play an important part in the dispersal 
 of two groups of terrestrial animals those which occasionally 
 swim and those which are liable to be carried away on icebergs 
 or on floating masses of vegetation. Most quadrupeds swim well 
 and even if not habitual swimmers may be forced to take to the 
 water in times of flood. In this way they may cross large 
 rivers and even get carried out to sea and perhaps to some 
 neighbouring island, but they cannot cross large stretches 
 of open ocean, and are accordingly never found, except 
 when introduced by man, on islands far remote from any 
 continent. 
 
 In polar regions the floating ice affords a means of dispersal 
 to such animals as wolves and polar bears, while within the 
 tropics floating islands or rafts formed of matted vegetation play 
 the same part. Such islands have been observed floating out to 
 sea from the mouths of large rivers like the Ganges, the Amazon, 
 the Congo and the Orinoco. They serve as a means of transport 
 to many different kinds of terrestrial reptiles, birds and mammals, 
 and countless molluscs, worms and insects, to say nothing of 
 plants. 
 
 " If," says Sir Charles Lyell, " the surface of the deep be 
 calm, and the rafts are carried along by a current, or wafted by 
 some slight breath of air fanning the foliage of the green trees, 
 it may arrive, after a passage of several weeks, at the bay of an 
 island into which its plants and animals may be poured out as 
 
 1 Heilprin, op. cit., p. 47.
 
 DISPERSAL OF ANIMALS 325 
 
 from an ark, and thus a colony of several hundred new species 
 may at once be naturalized." 1 
 
 As a definite example of this kind of dispersal may be men- 
 tioned the fact that in 1827 a large boa constrictor, twisted round 
 the trunk of a tree, was carried by ocean currents from South 
 America to the Island of St. Vincent, where it was destroyed 
 after killing a few sheep. 
 
 A current flows from the North Island of New Zealand 
 southwards to Chatham Island, four hundred miles distant 
 from the nearest point on the New Zealand coast. This current 
 carries considerable quantities of New Zealand timber to the 
 island and its existence probably accounts for the fact that the 
 planarian worm, Geoplana exulans, has been found both in the 
 North Island of New Zealand and on Chatham Island, but, as 
 yet, nowhere else. The land planarians habitually creep into 
 the crevices of decayed timber, and their eggs are enclosed in 
 tough, horny cocoons which may probably occasionally be 
 transported even over wide stretches of sea. 
 
 Small terrestrial animals are, of course, often accidentally 
 dispersed by human agency. Rats, mice and cockroaches have 
 been carried nearly all over the world by ships, and snails, 
 worms and other small creatures may be carried about with 
 timber and earth, especially around the roots of plants. When 
 I was in New Zealand I had some plants sent to me from 
 England in a tightly closed tin box. When they arrived, after 
 a voyage of some five or six weeks, I found an earthworm still 
 alive in th^ tin. Many invertebrates have doubtless been 
 unknowingly dispersed in this manner and great care has to be 
 taken to make due allowance for such possibilities in studying 
 problems of distribution. In exactly the same sort of way the 
 seeds of many plants are accidentally dispersed over the world 
 in ships' ballast, so that the same common European weeds 
 occur in the neighbourhood of the ports along all the great 
 routes of commerce. 
 
 The restrictions placed upon the dispersal of fresh water 
 animals are more severe than in the case of either marine or 
 terrestrial forms. One river system or one lake is separated 
 from another by intervening land or sea which fresh water 
 animals cannot as a rule cross by their own exertions. There 
 are of course exceptions, as in the case of the lampreys and eels, 
 
 1 Lyell's " Principles of Geology," Ed. 5, Vol. III., p. 44.
 
 326 OUTLINES OF EVOLUTIONABY BIOLOGY 
 
 and other fish which go down to the sea periodically, but for the 
 most part the inhabitants of fresh water are largely dependent 
 upon external agencies for their dispersal. Accordingly we find 
 two groups of such animals, widely contrasted with one another 
 as regards their distribution. Those which do not go down to 
 the sea and which are not likely to be carried about by external 
 agencies, such as most of the fishes, have usually restricted areas 
 of specific distribution, and individual mountain lakes sometimes 
 contain peculiar species of fish which are found nowhere else in 
 
 FIG. 165. 
 
 FIG. 166. 
 
 FIG. 165. Gemmule or Statoblast of a Fresh Water Polyzoon, Cristatetta 
 
 mucedo, X 40. (From Sollas.) 
 
 FIG. 166. Two Gemmules of a Fresh Water Sponge, Ephydatia (Spongilla) 
 fluviatilis, x 60. (From a photograph.) 
 
 gem., gemmules ; sp., spicules of the parent sponge. 
 
 the world. Galaxias nigothoruk, for example, is a small fish 
 which occurs abundantly in lake Nigothoruk in Victoria 
 (Australia). This lake is in a very isolated position in a 
 mountainous region and the only outlet is by percolation under- 
 ground. There appears to be no natural means by which the 
 fish could be transferred to any other locality at the present 
 time, and it is not known to occur elsewhere. 
 
 On the other hand many fresh water invertebrates, such as 
 the Polyzoa, hydras and sponges, and above all the microscopic 
 Protozoa, are remarkable for their wide distribution. Identical 
 genera if not identical species of tliese groups occur almost all 
 over the world, and the reason for this is not far to seek, for all
 
 CLIMATIC CHANGES 327 
 
 of them have some special character which enables them to be 
 easily dispersed by external agencies. The fresh water Polyzoa 
 and sponges produce minute buds (statoblasts or gemmules) 
 enclosed in hard protective envelopes (Figs. 165, 166), which are 
 likely to be carried about in the mud on the feet of wading birds 
 and mammals. The embryo of Hydra secretes its own protective 
 envelope (Fig. 59, D G) within which it passes through a period 
 of rest embedded in the mud ; while many of the Protista (e.g. 
 Hsematococcus) are capable of being dried up at some period or 
 other of their life-history and carried about by the wind in the 
 form of dust. Thus a sample of mud, taken from a pond and 
 dried up, may, after an interval of many months, if again placed 
 in water, give rise to an abundant fauna, amongst which even 
 Buch highly organized forms as Crustacea (e.g. Apus and 
 Branchipus, which lay specially protected eggs) will frequently 
 appear. 
 
 We must remember that the present distribution of animals 
 and plants is the outcome not only of the existing physical 
 conditions of the earth's surface but also of conditions which 
 obtained in past geological periods. From time to time these 
 conditions undergo great changes, which may concern not only 
 the climate of particular regions or of the entire world, but also 
 the relative distribution of land and sea. 
 
 The earth has been subject, at various periods of its history, 
 to climatic changes of two chief kinds, (1) cold or even glacial 
 epochs in temperate regions, and (2) mild or warm epochs in 
 arctic or antarctic regions. Probably alternations of these two 
 extremes have been not infrequent, but the case of which we 
 have most complete knowledge occurred in the Pleistocene period 
 and is usually known as the " Glacial Epoch " par excellence. 
 
 There is clear evidence that during a portion of the Pleistocene 
 period a very large part of the northern hemisphere, which now 
 enjoys a temperate climate, was covered with perpetual ice and 
 snow and reduced to a condition resembling that of Greenland 
 at the present time. Scandinavia and the whole of Northern 
 Europe were buried beneath the ice-sheet, and the same is truo 
 of the northern part of North America. 
 
 The glacial epoch in the north must have driven the greater 
 number of the northern plants and animals southwards, causing 
 a keen struggle for existence in which many species were 
 exterminated. Its influence was possibly intensified by the
 
 328 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 fact that the glaciation was not continuous but alternated with a 
 succession of warm periods. The southern hemisphere also 
 experienced a glacial epoch during which warm and cold periods 
 alternated, and astronomers hold that the warm periods in one 
 hemisphere coincided with cold ones in the other. It has been 
 calculated that each warm or cold period lasted for about 
 21,000 years. 
 
 Other important changes in climate occurred long before the 
 great glacial epoch. Thus the fossil remains of a luxuriant 
 vegetation in Greenland and other northern localities indicate 
 the occurrence of a mild arctic climate in Miocene times. Such 
 a climate must have favoured migration between the old and 
 new worlds by way of what is now Behring Strait, which may 
 very well have been dry land at the time. 
 
 Owing to the gradual loss of the earth's heat by radiation and 
 the consequent shrinkage and crumpling of the solid crust, 
 variations in the level of the land are constantly taking place. 
 Areas which are at present separated by sea may have been 
 connected in former times and vice versa, and there can be no 
 doubt that the distribution of plants and animals has been 
 profoundly influenced in this way. Many cases of discontinuity 
 in distribution may be explained by the former existence of land 
 connections which no longer remain. It is necessary, however, 
 to be extremely careful how we invoke the aid of this principle, 
 which, as an easy way out of difficulties, is apt to lead us into 
 all sorts of unjustifiable speculations. 
 
 Those remarkable animals the lemurs, as a group, exhibit a 
 very curious discontinuity in their distribution, occurring in 
 Africa (and especially Madagascar) on the one hand and in 
 Southern Asia on the other. To explain this distribution it has 
 been suggested that in former times a continent Lemuria 
 existed in the Indian Ocean. Similarly, but with perhaps 
 greater justification, it is believed by many people that the 
 antarctic continent at one time extended much further north 
 than at the present day, so as to afford, possibly with the aid of 
 a chain of islands and with the co-operation of a mild antarctic 
 climate, a route along which migration might take place between 
 South America and Australasia. In this way may be explained 
 certain remarkable points of agreement between the fauna and 
 flora of Australia and New Zealand and those of South America. 
 The genus Fuchsia, for example, is typically South American,
 
 CHANGES IN LAND AND SEA 329 
 
 but one or two species occur in New Zealand ; and the same is 
 true of Calceolaria. The evergreen beech forests of New Zealand 
 must be extraordinarily like those which Darwin described in 
 Patagonia. Even the same curious genus of fungus (Cyttaria) 
 is found on the beech trees in South America, New Zealand and 
 Tasmania. A fresh water lamprey, Geotria, also occurs both in 
 New Zealand and South America and similar cases could be 
 quoted from the invertebrate fauna. 
 
 It is very doubtful, however, whether such an extensive change 
 in the configuration of the earth's surface as the submergence of 
 an entire continent has ever taken place. According to 
 Dr. Wallace, who is recognized as tbe greatest authority on 
 tlie subject of geographical distribution, the existing continents 
 and oceans as a whole are permanent features, although their 
 outlines may be greatly affected by oscillations of the earth's 
 crust. 
 
 Perhaps the strongest argument against the former existence 
 of continents where we now have oceans lies in the fact that the 
 average depth of the sea is many times greater than the average 
 height of the land, no less than twelve thousand feet as com- 
 pared with one thousand, for the great depths of the ocean 
 extend over vast areas while the greatest heights of the land 
 are narrow mountain ranges. Hence, although large areas of 
 land might be submerged by a comparatively slight change of 
 level, it would take an enormous movement to bring any extensive 
 tract of the ocean bed to the surface. 
 
 In short, we are only justified in postulating the former 
 existence of land in places where the ocean is comparatively 
 shallow, but even this limitation leaves abundant opportunity 
 for changes in the relative distribution of land and sea which 
 would profoundly aflect the distribution of plants and animals. 
 The actual occurrence of such changes is abundantly proved by 
 geological evidence and they are known to be going on at the 
 present day in many parts of the world. 
 
 There is good reason to believe that the principal groups of 
 terrestrial animals originated in the great northern land masses 
 and that the southern peninsular areas of Africa, Australasia 
 (now, of course, represented by detached islands) and South 
 America have been peopled mainly by successive waves of 
 migration from the north. We find in all these southern areas 
 primitive, ancient forms of life. Marsupials at the present day
 
 330 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 are found only in Australasia and America, but the fossil remains 
 of such animals are widely distributed over the northern hemi- 
 sphere. The Onychophora, again, a small group of extremely 
 primitive arthropods, which until recently were all included by 
 zoologists in the single genus Peripatus (Fig. 167), are almost 
 confined to Australasia, South Africa and South America, in all 
 of which regions they are fairly abundant. It is more reasonable 
 to imagine that the ancestors of the Onychophora migrated 
 from the north, where the group has now become extinct, 
 than to invent imaginary continents 
 across which they may have wandered, 
 or even to suppose that they have 
 been so widely distributed as we now 
 find them by some external agency 
 such as floating timber. 
 
 The geographical distribution of 
 plants and animals would be quite 
 inexplicable on the supposition that 
 they had all been independently 
 created and deposited where they 
 now live. It is, however, easy enough 
 to explain it on the theory that the 
 earth has been peopled by the des- 
 cendants of common ancestors which 
 migrated from place to place as 
 
 FIG. 167. Peripatus capmsis, occasion permitted and at the same 
 
 (rrom C a P p\otogr Jph X ) f ' time underwent modification in many 
 
 different directions. We may now 
 
 briefly summarize the principal facts of distribution which justify 
 us in holding this view. 
 
 (1) The extent of the area of distribution of any group of 
 animals is directly proportional to its means of dispersal. Thus 
 flying animals are much more widely distributed than quadrupeds. 
 Birds occur abundantly on oceanic islands, but the only mammals 
 which occur there in a state of nature are bats and small forms 
 like rats and mice which may be carried on floating timber. 
 Nevertheless we know that when the larger mammals are trans- 
 ported by man to such localities they flourish exceedingly. 
 Many Protozoa, again, which are readily blown about in the form 
 of dust, are almost cosmopolitan even as regards their species. 
 
 (2) The degree of peculiarity of the fauna and flora of any
 
 GEOGEAPHICAL ISOLATION 331 
 
 area is proportional to the length of time for which and the 
 extent to which that area has been isolated from other areas. 
 Thus Australia, which has probably been separated from the 
 Asiatic continent ever since the Cretaceous period, has a most 
 peculiar fauna and flora. We have already referred to the 
 numerous different kinds of marsupials kangaroos, wombats, 
 phalangers, native bears, native cats and so forth which have 
 not as yet been supplanted by the more recently developed 
 groups of mammals found in other parts of the world. Australia 
 is also still the home of those most primitive and reptile-like 
 of all the mammals, the Monotremata (Figs. 91, 92). The 
 Australasian forests, again, are composed principally of eucalypts 
 of many different species, which are found nowhere else in the 
 world. In New Zealand, which is even more isolated than 
 Australia, we find no less peculiar inhabitants, including the 
 wonderful tuatara (Fig. 113), the oldest surviving type of terrestrial 
 vertebrate, together with the kiwi (Fig. Ill) and other remarkable 
 flightless birds. 
 
 The reasons why the degree of peculiarity of the fauna and 
 flora of any region is proportional to the degree of geographical 
 isolation are not difficult to find. On the one hand ancient types, 
 such as the tuatara, the monotrenies and the marsupials, may be 
 preserved from competition with more modern forms long after 
 they have been exterminated elsewhere. On the other hand, indivi- 
 duals accidentally introduced from distant areas at rare intervals 
 will have few opportunities of breeding with others of the same 
 species, and thus whatever variation occurs amongst them will 
 be less liable to be swamped by intercrossing with the parent 
 form. New races and ultimately new species will thus become 
 established more readily in such areas than elsewhere. This 
 principle of geographical isolation as a factor in the production 
 of new species is of great importance and we shall have to refer 
 to it again in a subsequent chapter. 
 
 The zoological or botanical affinities of the inhabitants of any 
 given area, not only with one another but also with those of 
 adjacent areas, are exactly what we should expect in accordance 
 with the views which we are advocating. It is impossible to 
 believe that the existing marsupials were (with the exception of 
 the few American species) all specially created in Australasia 
 when we know perfectly well that marsupials used to exist in 
 Europe in past geological times and can still exist in Europe
 
 332 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 when transported there by human agency, and it is equally 
 impossible to believe that such animals as sheep and rabbits, to 
 which the Australian climate appears to be pre-eminently suited, 
 were specially created in Europe and Asia but never in Australia. 
 The existing condition of the Australian fauna is, however, easily 
 explained on the supposition that it was originally derived from 
 Asia at the time when marsupials and monotremes flourished in 
 the north, and that the island continent became separated from 
 the mainland before the more recent mammalian types, such as 
 sheep and rabbits, had arisen on the latter. Divergent evolution 
 within the limits of this isolated area is then quite sufficient to 
 account for the immense variety of marsupials occurring there at 
 the present day. 
 
 It is very instructive in this 'connection to contrast the con- 
 dition of the fauna of a comparatively recently separated 
 continental island, such as Great Britain, which is not far 
 removed from its parent continent, with that of the fauna of a 
 typical oceanic island which has never formed part of a continent 
 at all and is very widely separated from any other land. The 
 native or indigenous population of continental islands always 
 exhibits a close relationship with that of the adjacent mainland, 
 from which it was originally derived and with which it is still 
 able to keep up a certain amount of intercourse. Such an island 
 will contain indigenous quadrupeds, and the great majority of the 
 species of plants and animals found in it will be identical with 
 those of the mainland. True oceanic islands, on the other hand, 
 such as St. Helena and the Sandwich Islands, are peopled 
 entirely by waifs and strays which have gained access to them at 
 rare intervals in one or other of the ways discussed in the earlier 
 part of this chapter. They never contain large quadrupeds and, 
 owing to their more or less complete isolation, the animals which 
 do occur almost always belong to peculiar species found nowhere 
 else in the world. 
 
 (3) Palaeontological investigations have demonstrated that the 
 present animal population of any tolerably isolated area is closely 
 related to the population of the same area in comparatively 
 recent geological periods. Thus in Australia we not only find 
 that at the present day marsupials are by far the most charac- 
 teristic features of the fauna, but also that the remains of extinct 
 marsupials, many of which belong to genera and species different 
 from any now living, are very abundant in the tertiary deposits
 
 DISCONTINUOUS DISTRIBUTION 333 
 
 of the same region. Similarly in South America at the present 
 time the edentates (sloths, armadillos and ant-eaters) form the 
 most characteristic mammalian group, and the tertiary deposits 
 of that country have yielded the remains of a great number of 
 extinct forms belonging to the same order. It would be very 
 difficult to explain these facts on any theory of special creation, 
 but we can easily understand how a group of animals, having 
 once gained a footing in any area and finding itself secure and 
 more or less cut off from communication with other parts of the 
 world, would increase and vary, producing new species and 
 ultimately becoming the dominant group in that particular 
 region. 
 
 (4) Cases of discontinuous distribution are readily explicable 
 on the theory of evolution and migration. Either individuals 
 of the species in question have occasionally transgressed the 
 barriers to their dispersal and established new and distant 
 colonies, or possibly a large area of distribution has become 
 broken up into a number of smaller ones by geographical or 
 climatic changes rendering portions of it uninhabitable. 
 
 " Thus, for instance," says Romanes, " it is easy to understand 
 that during the last cold epoch the mountain hare would have 
 had a continuous range ; but that as the arctic climate gradually 
 receded to polar regions, the species would be able to survive in 
 southern latitudes only on mountain ranges, and thus would 
 become broken up into many discontinuous patches, correspond- 
 ing with these ranges. In the same way we can explain the 
 occurrence of arctic vegetation on the Alps and Pyrenees 
 namely, as left behind by the retreat of the arctic climate at the 
 close of the glacial period." 1 
 
 1 " Darwin and after Darwin," Vol. I., p. 209.
 
 CHAPTER XXII 
 
 Adaptation to environment in animals Deep sea animals The colouration 
 of animals Protective and aggressive resemblances Warning colours 
 Mimicry Epigamic ornamentation. 
 
 IN the last few chapters we have discussed a number of facts 
 selected from that great and ever increasing mass of evidence 
 which leads us to the inevitable conclusion that the present con- 
 dition of the fauna and flora of the earth, with their almost 
 endless diversity of plants and animals, is the outcome of a long 
 process of organic evolution. It is desirable at this stage of our 
 inquiry to emphasize the fact that this evolution, in the main, 
 has been of a progressive character, and of such a character, 
 moreover, as to maintain a more or less perfect harmony between 
 the organism and its environment. 
 
 Adaptation in bodily organization 'and in corresponding 
 function, whereby each kind of plant or animal is enabled to 
 meet the constant demands made upon it and maintain its 
 existence in the endless warfare of life, is the great outstanding 
 feature of living things. So universal is this adaptation that 
 we are apt to take it for granted, and any want of it is at once 
 recognized as an exception and an anomaly. Anyone, for 
 example, who watches the slow and clumsy movements of a 
 tortoise cannot fail to be struck with the fact that the limbs of 
 this animal are but ill-suited for purposes of locomotion, but 
 even in this case there is compensation in that the tortoise 
 carries its place of refuge about with it and has therefore little 
 need to hurry itself. 
 
 We have seen in an earlier chapter how completely the 
 pentadactyl limbs of air-breathing vertebrates may become 
 modified from their primitive condition in correspondence with 
 changes in the mode of life. The fore limbs, adapted in the 
 first instance for locomotion on land, have become changed in 
 the whales, seals and dugoiigs into paddles ; in the pterodactyls, 
 birds and bats into wings, and in man into organs of prehension.
 
 DEEP SEA ANIMALS 
 
 335 
 
 Indeed, given time enough, the power which an organism 
 possesses of altering its bodily structure in accordance with new 
 demands on the part of the environment seems, as we have 
 already pointed out, to he almost without limits. 
 
 This plasticity is illustrated in the most striking manner in 
 cases where the organism has been removed from what may be 
 regarded as the normal environment of the group to which it 
 belongs, and to which the great majority of the group are adapted, 
 and come to live under new 
 and very different conditions. 
 Thus it is with tbe aquatic 
 and aerial mammals, which, 
 in encroaching upon the 
 domains of the fishes and 
 birds, have, by convergent 
 evolution, come to resemble 
 these in bodily form. 
 
 Wherever we turn we find 
 fresh illustrations of the same 
 principle. At great depths 
 of the ocean the conditions of 
 life are very different from 
 those which obtain in shal- 
 low water, and we find the 
 animals which inhabit these 
 abysses modified accordingly. 
 Fig. 168 represents two deep 
 sea sponges obtained by the 
 " Challenger " expedition ; 
 Cladorhiza lonyi.pinna from a 
 depth of 3000 fatlioms in the North Pacific and Axoniderma mirabile 
 from a depth of 2250 fathoms in the South Pacific. It will be seen 
 at once that the form assumed by these sponges is very unusual 
 and quite unlike tbat exhibited by their shallow water relatives. 
 The great majority of the members of the group of sponges (the 
 Tetraxonida) to which they belong are indeed by no means 
 remarkable for symmetry of shape, but these two are beautifully 
 symmetrical, their form at once suggesting that of a parachute, 
 with a small conical body fringed by long radiating processes 
 surrounding a central root-like projection. This " Crinorhiza 
 form," as it is termed, is obviously an adaptation which serves 
 
 FIG. 168. Two Deep Sea Sponges, 
 exhibiting the Crinorhi/a Form. 
 A. Cladorhiza longipinna, ; B, Axoni- 
 derma mirabile; nat. size. (After 
 Ridley and Dendy in " Challenger " 
 Reports.)
 
 336 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 to prevent the sponge from sinking into and being smothered by 
 the soft mud or ooze which covers the bottom of the ocean at 
 very great depths, and it is interesting to observe that species of 
 several distinct though related genera have adopted the same 
 device, thus affording a beautiful example of the phenomenon of 
 convergence. Other sessile deep sea animals have found different 
 means of overcoming the same difficulty, especially in many 
 cases by the development of long stalks. 
 
 The absence of light at great ocean depths has led to the 
 acquisition on the part of many of the deep sea fishes of brilliant 
 phosphorescent organs, arranged like little lamps on various 
 parts of the body. In some cases at any rate these serve to 
 attract other animals upon wliich these fishes prey. Some of 
 them, again, develop long and delicate feelers by aid of which 
 they grope their way about in the dark. 
 
 In the brilliantly illuminated surface waters of the ocean con- 
 ditions are very different, and here we find that the most favourite 
 device for preserving life amidst a host of enemies is transparency, 
 but we have already alluded to this in the preceding chapter and 
 need not dwell upon it further. It is a phenomenon which falls 
 under the head of protective colouration, of which we shall find 
 better instances elsewhere. 
 
 The significance of the colouration of animals as a means of 
 adaptation to environment is a subject which has in recent years 
 developed into a special branch of biological science, and which 
 already has a copious literature of its own. Professor Poulton, 
 in his well known work on the Colours of Animals, 1 has suggested 
 an elaborate scheme of colour classification from this point of 
 view. He distinguishes, in the first place, between apatetic 
 (deceitful) colours, sematic (warning and signalling) colours, and 
 epigamic colours (displayed in courtship), all of which afford 
 marvellous instances of more or less highly specialized adaptation. 
 We have not space to follow out the details of this classification 
 but we shall presently refer to examples of all the more important 
 types of colouration included therein. 
 
 Every observer of nature must have been struck with the 
 general harmony of colouration which exists between animals 
 and their surroundings. So complete is this harmony that our 
 sense of hearing is frequently a better guide to the whereabouts 
 of an insect, bird or mammal than our sense of sight. I 
 
 1 International Scientific Series, Vol. LXV1II.
 
 PROTECTIVE AND AGGRESSIVE RESEMBLANCE 337 
 
 remember standing with my gun in the midst of a dense patch 
 of scrub in Australia and hearing the pademelons 1 hopping about 
 all around me. For a long time, however, I could see nothing 
 but the trees. My native guide pointed out where I was to aim, 
 but I only fired at a log from the side of which a pademelon 
 hopped away. Again he pointed, and this time at a small white 
 spot which I could just distinguish amongst the trees. I fired 
 once more, aiming at the white spot, and sure enough a pademelon 
 rolled over. It appeared that I had aimed at the white fur which 
 occurs on the breast of the animal and which to the experienced 
 eye of the native told all that he needed to 
 know. It is often supposed that conspicuous 
 patches of this kind serve as recognition marks 
 between individuals of the same species, but it 
 may be questioned how far the advantage of 
 being recognized by a friend compensates for a 
 disturbance of the colour harmony which reveals 
 an animal to its enemies. 
 
 The type of colouration which aids in the 
 concealment of an animal is termed, by Pro- 
 fessor Poulton, cryptic. It belongs, of course, 
 to the apatetic group. Concealment may be 
 desirable either as a means of escape from 
 enemies or for the purpose of ambuscading 
 prey, or possibly for both. In the former case 
 we may speak of it as protective resemblance 
 (procryptic colouration), in the latter as aggres- 
 sive resemblance (anticryptic colouration). 
 
 Protective resernbjance is often of a very highly 
 specialized character, and may be due as much to adaptation 
 in actual form as to adaptation in colour ; frequently these two 
 factors unite in producing the result, and a third may be added, 
 viz., adaptation in habit or instinct. In the common stick 
 caterpillars of the geometer moths we see all three factors 
 co-operating. In colour and shape these caterpillars precisely 
 resemble small twigs. They move about with a characteristic 
 looping action amongst the leaves or branches of the bushes 
 which they frequent, but when at rest they stiffen themselves up 
 and stand out from the branch at the exact angle of a twig, and 
 
 FIG. 169. Larva 
 of the Brim- 
 stone Moth 
 (Rnmia cratae- 
 yata) resting 
 upon a Haw- 
 thorn twig ; 
 nat.size. (From 
 Poulton.) 
 
 A small species of kangaroo.
 
 338 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 in this condition it is extremely difficult to detect them. Professor 
 Poulton remarks : 
 
 "These caterpillars are extremely common, and between two and 
 three hundred species are found in this country ; but the great 
 majority are rarely seen because of their perfect resemblance to 
 the twigs of the plants upon which they feed." 
 
 As will be seen from the illustration (Fig. 169), which represents 
 the larva of the brimstone moth upon its food plant, the hawthorn, 
 
 the caterpillar is enabled to main- 
 tain its position for a long period 
 by attaching its head to a twig by 
 means of a silken thread. 
 
 Numerous moths so closely re- 
 semble in the colour and pattern 
 of the upper surface of their wings 
 the objects upon which they rest 
 in the daytime, such as the bark 
 of trees, that they are almost 
 invisible, but perhaps the most 
 perfect examples of protective 
 resemblance are met with in the 
 wonderful leaf insects. Fig. 170 
 represents an orthopterous insect, 
 Pulchriphyllium crurifoliiim, from 
 Ceylon. The whole insect is of a 
 bright leaf-green colour, and not 
 only are the wings shaped and 
 veined so as to resemble leaves, 
 but even the body and legs exhibit 
 leaf-like outgrowths. 
 
 In the well known Indian leaf 
 butterfly, Kallima (Fig. 171), the resemblance to a leaf is only 
 seen when the insect comes to rest with its wings folded together 
 above the body so as to expose their under surfaces. It is a dry, 
 dead leaf which is imitated this time, and stalk and midrib, veins 
 and colour markings, even down to such minutiae as rust spots, 
 are perfectly represented. 
 
 The Mantidae or praying insects feed upon flies, &c., which 
 they capture with marvellous dexterity with their serrated claws. 
 In some species the uniform green colouration doubtless serves, 
 not only to protect them from their own enemies, but also to 
 
 FIG. 170. A Green Leaf Insect 
 ( P ulchriphylUum crurifoliiim, 
 ?), from Ceylon; x . (From 
 a photograph.)
 
 PROTECTIVE AND AGGRESSIVE RESEMBLANCE 339 
 
 prevent them from being seen by their victims before they have 
 come within range. Other species exhibit even more wonderful 
 adaptations both in form and colour. Thus the South African 
 Harpax tricolor sits amongst the pink and white flowers of the 
 heath, which are imitated by similarly coloured outgrowths of 
 the insect, and there awaits the approach of its unsuspecting 
 victims; while in Mozambique the terrible Idolum diabolicum 
 
 FIG. 171. An Indian Leaf Butterfly (Kallima inacliis); A., with wings 
 expanded ; B., with wings folded ; X f . (From a photograph.) 
 
 \ 
 
 simulates, both in form and colour, a large flower, and thereby 
 deceives and attracts other insects in search of honey. 
 
 It is no doubt amongst the almost innumerable species of the 
 great group Insecta that cases of highly specialized adaptation 
 for purposes of concealment or deception are most frequently 
 met with. They also occur, however, and by no means 
 uncommonly, in other groups of the animal kingdom. A 
 familiar instance is afforded by the common British spider crab, 
 now known as Macropodia rostrata, 1 of which excellent illus- 
 trations (under the name Cancer PJialangium) were given by 
 
 1 I am indebted to my friend, the Rev. T. R. R. Stebbing. F.R.S., for information 
 as to the correct nomenclature, &c., of this species. 
 
 z 2
 
 340 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Dr. Macculloch, in the Transactions of the Linnean Society, as 
 far back as 1801. I am enabled by the courtesy of the Council 
 of the Society to reproduce here, on a reduced scale, Dr. Maccul- 
 loch's original plate (Fig. 172). The crab actually breaks off 
 fronds of seaweed and attaches them to the long hairs of its body, 
 thus disguising itself so effectually as to be quite unrecognizable 
 except by careful examination. Dr. Macculloch was of opinion 
 
 FlG. 172. Reduced Facsimile of Dr. Macculloch's Plate of Macropodia 
 rostrata, in the Transactions of the Linnean Society. On the left is 
 shown a plant of the seaweed in which the crab dresses itself up ; on 
 the right the crab without the seaweed, and at the bottom the crab 
 dressed up. 
 
 that this dressing up of the crab in seaweed was an artifice 
 which assisted it in capturing its food (anticryptic), but it is 
 much more likely that it is protective (procryptic). The late 
 Professor Bell has told us how the slow and sluggish habits of 
 the crab render it an easy prey to fishes, and the stomach of a 
 thornback ray has been found entirely filled with them, so that 
 there appears to be ample reason for them to seek concealment. 
 
 In the case of Macropodia the adaptation for concealment 
 shows itself as an inherited habit or instinct more than in any 
 modification of bodily structure, but such an instinct is probably
 
 PROTECTIVE AND AGGRESSIVE RESEMBLANCE 341 
 
 itself the effect of some structural modification, however impossible 
 to detect, in nervous tissue. In the Australian Phyllopteryx 
 eques, a fish which is closely related to the curious sea-horse 
 (Hippocampus) of our own coasts, we get precisely the same 
 idea, so to speak, carried out in a different manner. Both 
 Hippocampus and Phyllopteryx live amongst seaweed, to which 
 they attach themselves by means of their curious prehensile 
 tails. Hippocampus (Fig. 173) exhibits no special resemblance 
 
 FIG. 174. 
 
 FIG. 173. 
 
 (From 
 
 FIG. 173. A Sea-horse (Hippocampus antiquorum), X 
 
 photograph.) 
 
 FIG. 174. PJiyllnpteryx eques, attached to seaweed. (From Giiuther's 
 "Study of Fishes.") 
 
 to its surroundings, but in Phyllopteryx (Fig. 174) the body is 
 covered with cutaneous outgrowths which float out in the water 
 like fronds of seaweed and doubtless effect a most satisfactory 
 disguise. This is certainly a less troublesome plan than that of 
 dressing up in clothing borrowed from the outside world. 
 
 The well known colour changes of the chameleon and of 
 various flat fishes, not to mention numerous other instances 
 which might be cited from different groups of the animal king- 
 dom, are due to a complex apparatus, controlled by the nervous
 
 342 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 system, whose function it is to bring about a varying adaptation 
 for concealment under varying conditions of the environment. 
 How perfect the adaptation may be will be realized by all who 
 have ever observed with what marvellous accuracy the colour 
 markings of a turbot in an aquarium are made to match the 
 sand or gravel upon which it is lying. 
 
 In striking contrast with the cryptic colouration by which an 
 animal seeks, as it were, to avoid observation, are those numerous 
 cases in which self-advertisement appears to be the main object 
 in view. The British army, which only in recent years has 
 learnt the advantages of khaki clothing when in the field, still 
 exhibits some of the most startling instances of conspicuous 
 colouration met with anywhere in the animal kingdom, though 
 whether these examples should be classed under the head of 
 warning colours, or regarded as belonging to the epigamic 
 category, is perhaps an open question. We must, however, con- 
 fine our attention in this place to a few examples of warning 
 colours met with amongst the lower animals. 
 
 We have seen that both warning and signalling colours, or 
 recognition marks, are spoken of as sematic. The former are 
 further distinguished as aposematic and the latter as episematic. 
 Aposematic colours are exhibited by many animals which possess 
 some special means of defence and find it advantageous to 
 advertize the fact. Wasps and hornets, with their conspicuous 
 orange- and black-banded bodies, are excellent examples. Such 
 animals do not seek to conceal themselves but rely upon their 
 warning colours to remind their enemies that they had better 
 leave them alone. It is not enough that they should possess the 
 power of making themselves disagreeable ; the fact must be 
 clearly recognized, otherwise they would be constantly exposed 
 to experimental attack, and suffer injuries for which any damage 
 which they might inflict upon their pursuers would be but a 
 poor consolation. Orange, red and black, owing to their great 
 conspicuousness, especially when associated with one another, 
 are the colours most frequently met with in this connection, and 
 we find these colours, not only in noxious insects, but in various 
 vertebrate animals, such as poisonous reptiles, toads and sala- 
 manders. The Gila monster (Heloderma suspectum), of Mexico 
 and Arizona, is the only known poisonous lizard, and is con- 
 spicuously coloured in tints of blackish brown, yellow and 
 orange, while other members of the same group are usually
 
 MIMICRY 
 
 343 
 
 coloured so as to harmonize more or less perfectly with their 
 surroundings. 
 
 If it is advantageous for a noxious species to advertize its true 
 character, it is no less so for an innocuous one to advertize a 
 false character, and gain credit for some power of making itself 
 objectionable which it does not really possess. The practice of 
 bluffing is by no means an exclusively human institution. Thus 
 we find many insects, which in themselves are quite inoffensive, 
 taking on the characteristic warning colouration of dangerous 
 species. The drone-fly mimics the bee, and though they belong 
 to widely different orders of insects, 
 the one having only two wings and the 
 other four, the resemblance is so close 
 as to have given rise, as we saw in an 
 earlier chapter, to the ancient belief 
 in the spontaneous generation of bees 
 from the carcases of oxen (on which, of 
 course, drone-flies had deposited their 
 eggs). 
 
 Most moths, as is well known, have 
 opaque wings, covered with microscopic 
 scales, but in the clear-winged moths 
 (Fig. 175, A) the wings have partially 
 lost their scales and beconle transparent, 
 and this anomalous feature, combined 
 with the colouration of the body, enables 
 these perfectly harmless insects to mimic 
 the dangerous hornets (Fig. 175, B). 
 Even a harmless Snake may mimic 
 the warning colouration of a venomous 
 species, and thus secure for itself the respect which is properly 
 due only to the latter. 
 
 It is not necessary that an animal should be capable of 
 inflicting serious injury upon its enemies when attacked for it to 
 secure immunity from pursuit as soon as recognized. Many 
 butterflies and other insects, which are probably merely distasteful 
 or nauseous (or perhaps actually unwholesome) to birds, exhibit 
 aposematic or warning colouration. Amongst these we find 
 curious associations known as synaposematic groups, the 
 members of which, belonging to distinct species and often by 
 no means closely related to one another, seem to have combined 
 
 B 
 
 FIG. 175. A., a clear- 
 winged Moth (Sesia cra- 
 broniformis] mimicking 
 B. , a Hornet ( Vespa 
 crabro) ; both X . (From 
 a photograph.)
 
 314 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 together to share the expenses of a common advertisement and 
 thereby reduce the cost to each. Young birds have to learn 
 by experience which insects are good to eat and which are not. 
 In making their experiments no doubt they themselves suffer, 
 but the subjects of the experiment are probably actually killed. 
 Obviously, then, if one experiment can be made to serve for a 
 number of different species of insects there will be a corre- 
 sponding reduction in the death-rate, and hence it is that we 
 
 FIG. 176. A Synaposematic Group of South American Lepidoptera, all X . 
 (From a photograph.) 
 
 A, Tithorea harmonia ; B, HeHconius etliilla ; C, Perrliybris (Mylothris) malenka, 
 g ; D, Dismorphia praxinoe, J ; E, Pericopis angulosa. 
 
 find these groups of species all adopting the same type of 
 warning colour, and thus coming to resemble one another very 
 closely, although perhaps belonging to totally distinct families. 
 
 We may illustrate this somewhat complex phenomenon by 
 reference to certain South American Lepidoptera which take 
 part in the formation of such a synaposematic group. In 
 Fig. 176 A, B and D represent butterflies belonging to three 
 distinct families, while E is a moth, as may be seen at once by 
 its thick body and the absence of terminal knobs on the antennae.
 
 SYNAPOSEMATIC GROUPS 345 
 
 All of these, in common with numerous other species which 
 inhabit the same area, have adopted the same characteristic 
 scheme of warning colouration, wherein the prevailing tints are 
 black and orange. 
 
 In such a synaposematic group, or mimicry ring, it is usually 
 possible to distinguish between certain species which seem to 
 have led the way in the development of the warning colouration, 
 and others which seem to have followed their example. In 
 the particular case under notice the original " models " belong 
 to the group Ithomiiiiffi, of which Tithorca harmonia (Fig. 176, A) 
 is a representative. These are probably the most distasteful 
 members of the combination to birds. They have been imitated 
 by Heliconinae, such as Heliconius ethilla (Fig. 176, B), Pierinse 
 ("whites"), such as Dismorpliia praxinoe (Fig. 176, D), and 
 Hypsidae (a family of moths), as exemplified by Pericopis anyidosa 
 (Fig. 176, E), all of which may be regarded as mimics of the 
 Ithomiinae. 
 
 The case of the pierine mimics is particularly instructive, 
 and shows very clearly that these forms really imitate other 
 species, for the female is commonly a far more perfect mimic 
 than the male, which often departs little, if at all, from the typical 
 colouration of the group to which it belongs. Fig. 176, C repre- 
 sents a male pierine, Perrkybris (Mylotliris) malenka, which is 
 at once recognizable from its colouration as a " white," although 
 even here, curiously enough, there is a faint trace of the warning 
 colouration on the under surface of the hind wings. 1 The 
 female of the same species has the warning colouration well 
 developed, as it is in both male and female of Dismorpliia 
 praxinoe. ^ 
 
 So different are the males and females of some of these 
 mimicking species that it would be difficult to believe, were it not 
 for breeding experiments, that they are really specifically 
 identical. The explanation of the difference is doubtless to be 
 found in the fact that it is much more important, from the point 
 of view of the species, that the females, heavily laden with the 
 eggs upon which the existence of future generations depends, 
 should be able to warn off the birds, than that the males should 
 do so, for the latter, having once accomplished the fertilization 
 of the eggs, is of no further value to the race. 
 
 1 Doubtless inherited incompletely from female ancestors, as. in the case of the 
 vestigial nipples of man.
 
 346 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 When an unquestionably harmless species mimics the warning 
 colours of an undoubtedly noxious one, the case is sometimes 
 spoken of as one of " Batesian " mimicry, after the distinguished 
 naturalist, H. W. Bates, who added so much to our knowledge of 
 the subject. In the case of a synaposematic group, or mimicry 
 ring, however, it is often impossible to say whether any particular 
 species is edible or not, and it may very well be that in some 
 cases all are more or less inedible, though undoubtedly some, 
 which are presumably the less objectionable forms, mimic others, 
 which are presumably the more objectionable. This kind of 
 mimicry, resulting in the development of a warning colour 
 common to a number of inedible species, is sometimes distin- 
 guished as " Miillerian " mimicry, after the naturalist Fritz 
 Miiller, who first suggested the correct interpretation of the 
 phenomenon. 
 
 Perhaps the most remarkable case of mimicry known 
 amongst butterflies is that of certain species of Papilio found in 
 Africa and Madagascar, which have formed the subject of 
 exhaustive study by Trimen, Poulton and others. In Madagascar 
 occurs Papilio meriones, a non-mimetic species in which the 
 male and female (Fig. 177, A) closely resemble one another and 
 both possess the " tail " on the hind wing which is such a charac- 
 teristic feature of the genus. We may take it, then, that this is a 
 primitive form. On the continent of Africa is found the wide- 
 spread Papilio clardamis, with several subspecies. In these sub- 
 species the male (Fig. 177, BI) retains the ancestral form, but in 
 most of them the female is mimetic ; it has lost the Papilio tail 
 and closely mimics, both in shape and colour markings, some 
 one or another of various species of butterflies belonging to 
 different families which occur in the same region. Nor is this 
 all, for the female is likewise polymorphic, and different 
 individuals of the same subspecies resemble widely different 
 models. Thus the subspecies merope is known to have three 
 forms of female, a hippocoon form (Fig. 177, B 2 ) which mimics 
 the danaine butterfly Amanris niavius (Fig. ~L17,C),atrop1ionii<s 
 form (Fig. 177, B 3 ) which mimics anotber danaine, Limiui* 
 chrysippus (Fig. 177, D), and a jylanemoides form which mimics 
 the acraeine, Planema poggei. Our illustrations, which are 
 reproduced from Mr. Trimen's original memoir, give a good 
 idea of the form and pattern of some of these insects, but they 
 lack the beautiful colouring of the original figures, which is
 
 MIMICRY IN BUTTERFLIES 
 
 347 
 
 A. Papilio merioites $ Bi. Papilio dardarvus (?nerqpe) o* 
 
 C.Amauris niavi-us 
 
 D. Limnas chrysippus. 
 
 Papilio dardarms (merope) % 
 (/i//}joocoof7 form) 
 
 Papilio dardanus (merope) 
 (tropfionius form) 
 
 FIG. 177. Mimicry in Butterflies. (After Trimen, from coloured plates in 
 the Transactions of the Linnean Society, First Series, Vol. XXVI.)
 
 348 OUTLINES OF E VOLUTION AEY BIOLOGY 
 
 necessary in order to give a true idea of the close resemblance 
 between mimics and models. 
 
 Breeding experiments carried out by Mr. G. F. Leigh with the 
 subspecies cenea of Papilio dardamis have shown that the eggs 
 laid by one and the same butterfly may give rise to at least 
 
 FIG. 178. Male and Female of an Australian Lyre-Bird (Menura sitperla). 
 (Photographed from specimens in the British Museum, Natural History.) 
 
 three different forms of mimetic female, as well as, of course, the 
 male. 
 
 It is obvious that we have here the very opposite of a 
 synaposematic group, for, instead of concentrating upon a single 
 warning pattern, different individuals, even of the same species, 
 adopt totally different patterns in imitation of totally different 
 models. This case still requires a great deal of explanation, but 
 concerning the facts there can be no doubt.
 
 EPIGAMIC OKNAMENTATION 349 
 
 Frequenters of any good museum of natural history will be 
 familiar with plenty of examples of epigamic ornamentation. 
 The highly elaborate and gorgeous plumage of many male birds, 
 such as the peacock, the Argus pheasant, the Australian lyre- 
 bird (Fig. 178) and numerous species of humming birds, to say 
 nothing of less conspicuous examples, affords the best illustration 
 of this phenomenon./ In all such cases the function of the 
 ornamentation appears to be to gratify the aesthetic sense of the 
 female during the period of courtship, and render her amenable 
 to the attentions of her mate. There is no doubt that in many 
 cases the cock bird deliberately displays himself to the best 
 advantage before the admiring eyes of the hen, who is credited 
 with a no less deliberate choice of the partner who most nearly 
 approaches her ideal standard of beauty. In all these cases the 
 plumage of the female is comparatively sombre and uninteresting. 
 An elaborate and gaudy tail would be a disadvantage during 
 the lengthy period of incubation, when it is desirable, both 
 for her own sake and that of her offspring, that the female 
 should be as inconspicuous as possible. Thus it is the male bird 
 that has had to adapt his clothing to the requirements of the 
 female, and she herself is unable to follow the fashions which she 
 imposes upon her mate. 
 
 It is obvious that no theory of evolution can be regarded as 
 satisfactory which does not offer some explanation of the origin of 
 such highly specialized and precise adaptations as those which we 
 have been considering in this chapter, and it was in order to 
 emphasize the need for such explanation that we have laid so 
 much stress upon them, ^e shall see in our next chapter that 
 no less remarkable and precise adaptations for special purposes 
 are also met with in the vegetable kingdom, and shall then pass 
 on to seek the necessary explanation.
 
 CHAPTER XXIII 
 
 Adaptation to the environment in plants Alpine plants, desert plants and 
 lianes The modification of flowers in relation to insect-fertilization. 
 
 IN plants no less than in animals we find adaptation to the 
 conditions under which they have to live to be the most striking 
 feature of their organization. We have already noticed, in 
 dealing with the phenomenon of convergent evolution, the 
 manner in which Alpine plants of many kinds tend to assume the 
 compact cushion-like form which seems best suited to withstand 
 the rigorous climate to which they are exposed. Wherever a 
 plant may be found growing in a state of nature the character of 
 the environment is sure to be reflected more or less obviously in 
 its structure arid habit. We see this equally clearly in the 
 water-storing plants of desert regions the cacti of America 
 or the aloes of Africa with their succulent stems or leaves and 
 other structural modifications which enable them to withstand 
 the effect of long-continued drought, and in the climbing lianes 
 of tropical forests, whose one object in life appears to be to reach 
 some elevation where they can expose their foliage to the light 
 and air. Just as we find those air-breathing mammals which 
 have taken to an aquatic life adopting the form and habits of 
 fishes as being best suited to their changed conditions, so in the 
 tropical forests of Queensland we find palms, members of a group 
 which elsewhere are types of self-supporting independence, 
 adopting the form and habit of climbing plants as the only means 
 of coping with the exigencies of the situation. 
 
 If the adaptation amongst plants usually appears less remark- 
 able than is often the case in animals, it is because the 
 relations of a plant to its environment are usually less complex 
 than those of an animal. The greater activity of animals is 
 associated with the development of highly specialized organs of 
 locomotion, sense-organs and nervous system, all of which are 
 alike subject to adaptation. Plants afford much more restricted 
 opportunities for the effects of the environment to show them- 
 selves, and it is in their relations with animals, or to speak more
 
 ADVANTAGE OF CROSS-FERTILIZATION 351 
 
 accurately, in those organs which are immediately concerned 
 with these relations, that adaptation becomes most complex and 
 precise. 
 
 Even amongst animals, however, it would be difficult to find 
 better illustrations of accurate adaptation to highly specialized 
 conditions of the environment than those which we see in the 
 wonderful structural modifications whereby the majority of 
 flowers are adapted for pollination by insect agency. This 
 process of pollination is commonly referred to as the " fertiliza- 
 tion " of the flower, although, as we have seen in a previous 
 chapter, the real act of fertilization takes place inside the 
 so-called ovule and consists in the conjugation of the male and 
 female gametes, the sperm-cells and egg-cells. 
 
 The great majority of flowers produce both pollen and ovules, 
 containing respectively the male and female gametes ; in other 
 words they are hermaphrodite. It is obvious that in such cases 
 we have two possibilities with regard to fertilization, for the 
 flower may either be fertilized by its own pollen or by that of 
 some other flower. It may be either " self-fertilized " or " cross- 
 fertilized," and the cross-fertilization may be effected either by 
 pollen from another flower of the same plant or by pollen from a 
 different plant of the same kind, the latter being the more 
 advantageous. 
 
 It seems at first sight a strange thing that when a flower is 
 capable of self-fertilization such a roundabout and apparently 
 unnecessary proceeding as cross-fertilization should ever take 
 place at all. It has been shown, however, that cross-fertilization, 
 if not absolutely necessary, is at any rate of very great 
 advantage to the plant, or rather to the species, in which it 
 occurs. 
 
 Charles Darwin experimented for eleven years on this subject, 
 and proved conclusively that cross-fertilization yields better 
 results than self-fertilization, both as regards the number of seeds 
 produced and also as regards the quality of the offspring. It 
 would be impossible to give an adequate account of his work in 
 this place, but we may briefly notice one series of experiments 
 and refer for the remainder to his classical volume on the " Cross 
 and Self Fertilization of Plants." He started with the " Con- 
 volvulus major" (Ipomcea purpurea), the flowers of which are 
 hermaphrodite and may be either cross- or self-fertilized in a 
 state of nature. It is an easy matter, by artificially conveying
 
 352 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the pollen on the tip of a feather, to bring about the fertilization 
 in any way which may be desired. Ten flowers were thus self- 
 fertilized with their own pollen, while ten others were cross- 
 fertilized with pollen from a distinct plant. The crossed and 
 self-fertilized seeds thus obtained were carefully cultivated under 
 exactly the same conditions, and it was found that the plants 
 raised from cross-fertilized seeds were taller than those raised 
 from self-fertilized seeds in the proportion of 100 to 76. 
 
 The same experiment was repeated with ten successive 
 generations of the same plants, and always with the same result 
 in favour of the cross-fertilized individuals. Moreover, it was 
 proved at the same time that the fertility of the plants produced 
 by cross-fertilization was greatly superior to that of the self- 
 fertilized plants, a much larger quantity of seed being produced. 
 
 These results, taken in conjunction with many others of the 
 same kind, clearly proved the advantages of cross-fertilization. 
 To quote Darwin's own words : 
 
 " It has been shown that the offspring from the 
 
 union of two distinct individuals, especially if their progenitors 
 have been subjected to very different conditions, have an 
 immense advantage in height, weight, constitutional vigour, and 
 fertility over the self-fertilized offspring from one of the same 
 parents." 
 
 The theoretical explanation of this advantage is not an easy 
 matter, but is probably to be sought in the admixture of two 
 distinct series of hereditary tendencies in the offspring of a 
 cross. 
 
 In a state of nature it is a very rare thing for plants to be 
 exclusively self -fertilized, for, although self-fertilization may take 
 place in some cases to a very large extent, and although some 
 flowers are so constructed that self-fertilization alone is possible, 
 yet there is nearly always at least an occasional cross by the 
 introduction of pollen from a different plant. 
 
 Considering the advantages which arise from cross-fertilization, 
 we need not be surprised to find that a very large number of 
 flowers are provided with special adaptations whereby these 
 advantages may be secured to them. We may notice first certain 
 contrivances by means of which self-fertilization is more or less 
 effectually prevented and the injurious effects of perpetual close 
 inbreeding thereby avoided. These contrivances may be classed 
 under three principal heads.
 
 FERTILIZATION OF FLOWERS. 353 
 
 (1) Separation of the Sexes. In many flowers we find that only 
 stamens or pistil are developed, never both, so that we get 
 distinct male and female flowers, which renders self-fertilization 
 absolutely impossible, though it does not necessarily prevent 
 fertilization by pollen from other flowers on the same plant. 
 
 (2) Physiological Self -sterility. In some flowers, to quote the 
 words of Darwin, " the ovules absolutely refuse to be fertilized 
 by pollen from the same plant, but can be fertilized by pollen 
 from any other individual of the same species." Again, in a 
 large number of plants, although self-fertilization may take place, 
 yet, if the pollen from another individual be brought to the 
 stigma, it takes precedence, so to speak, of the flower's own 
 pollen and renders the latter ineffectual ; it is said to be 
 prepotent. 
 
 (3) Dichogamy. In a large number of flowers, in which both 
 male and female organs are present, the stamens and pistil 
 become mature at different times, so that self-fertilization 
 cannot possibly take place and, physiologically speaking, the 
 flowers are unisexual. Usually in these cases the pollen ripens 
 and is all shed before the stigma is ready to receive it. The 
 flower is then termed protandrous, as for example in the 
 common pink (Dianthus). More rarely the stigma ripens and 
 withers before the pollen is ripe, and such species are termed 
 protogynous, as in the fig-wort (Scroplmlaria nodosa). 
 
 Of course it would be worse than useless to prevent self- 
 fertilization unless some means were provided at the same time 
 for securing cross-fertilization. We saw in Chapter VIII that 
 the principal agents in conveying pollen from one flower to 
 another are wind and insects, and that flowers are accordingly dis- 
 tinguished as anemophilous or wind-fertilized and entomophilous 
 or insect-fertilized. The fact that the former are usually very 
 small and inconspicuous, as for example in the grasses, while the 
 latter are large and brightly coloured, affords strong presumptive 
 evidence that entomophilous flowers have become modified in 
 relation to the insects which visit them in search of food or 
 shelter. A great many different kinds of insects have this habit, 
 bees, flies, butterflies, moths and even beetles ; while in South 
 America some of the humming birds in like manner play the 
 part of pollen carriers. 
 
 We are in the habit of regarding bees as the most important 
 insects concerned in the cross-fertilization of flowers, and in 
 
 B. A A
 
 354 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 at 
 
 countries where they are plentiful this is perhaps the case. 
 They visit the flowers in order to collect both honey and pollen, 
 to be used as food for themselves and their offspring. The honey 
 is usually found at the bottom of a long narrow tube formed by 
 
 the lower part of the corolla, 
 so that in order to reach it 
 the bee requires a corre- 
 spondingly long and narrow 
 instrument. This is pro- 
 vided in the shape of a very 
 complicated proboscis (Fig. 
 179), formed by modifica- 
 tion, especially elongation, 
 of certain of the appendages 
 surrounding the mouth, 
 which in more primitive 
 insects, like the cockroach, 
 remain short and simple. 
 When not in use the pro- 
 boscis is neatly folded away 
 beneath the head, but when 
 a flower is visited it is un- 
 folded and inserted into the 
 tube containing the honey, 
 which is then drawn up into 
 the bee's stomach by means 
 of a special sucking ap- 
 paratus. In butterflies and 
 moths also a somewhat 
 similar proboscis is used for 
 the same purpose, but it 
 differs so much from that of 
 the bee in details of struc- 
 ture as to indicate that it has 
 been independently evolved 
 from the primitive mouth parts of some remote insect ancestor. 
 In both cases the mouth appendages have become specially adapted 
 for the very special purpose of sucking honey, and the necessity 
 for the fulfilment of the same function has led, as usual, to a 
 superficial resemblance between the two types of proboscis. We 
 have here, of course, another illustration of convergent evolution. 
 
 .pi 
 
 FIG. 179.--HeadofaBee,showingthecom- 
 plex Proboscis formed from modified 
 Mouth Parts. (From Weismann's 
 " Evolution Theory.") 
 
 at, antennae; Au, large compound eye; au, 
 ocellus ; la., labruin or upper lip ; le, outer 
 division of second maxilla (paraglossa) ; li, 
 ligule or tongue, formed by fusion of inner 
 divisions of second maxillse ; md, mandible ; 
 nix 1 , nix 2 , first and second maxillae ; pi, 
 labial palp; p.m., maxillary palp.
 
 FERTILIZATION OF FLOWERS. 355 
 
 For collecting pollen the bees make use of their legs, on which 
 special collecting brushes and baskets are formed by the stiff 
 hairs. The pollen is first moistened with honey to make it stick 
 together, and then picked up on the collecting brushes, placed 
 in the baskets on the hind legs and carried to the hive or nest. 
 
 Insects commonly visit a 1 large number of flowers of the same 
 kind in rapid succession, and, whether they intentionally collect 
 pollen for their own purposes or not, it is evident that, after each 
 visit to a flower containing ripe pollen grains, they will uninten- 
 tionally carry away some of these, accidentally attached to various 
 parts of the body. The pollen which is thus unconsciously 
 carried away is equally unconsciously deposited on the stigma of 
 the next flower visited, and thus cross-fertilization is effected. 
 It may, of course, happen sometimes that the pollen is deposited 
 on the wrong kind of flower, but that is of no consequence, for 
 pollen is, with rare exceptions, quite incapable of fertilizing 
 flowers of any but its own particular species. 
 
 Under these circumstances it will obviously be advantageous 
 to a plant to make its flowers as attractive to insects as possible, 
 and, just as it is desirable for a nauseous insect to inform the 
 birds by means of warning colours of the fact of its unpalatability, 
 so also is it desirable for flowers which have honey to offer in 
 exchange for the service of pollination to make that fact known 
 to their insect visitors by means of brilliant colours and strong 
 scents. There can be no reasonable doubt that the colours serve 
 to attract the insects and to enable them to recognize the flowers 
 which they prefer, and that the same is true of the scents is very 
 clearly indicated by the fact that certain flowers, such as various 
 species of Arum and related genera, which are fertilized by 
 carrion-loving flies, have managed to perfume themselves in 
 accordance with the tastes of their visitors. 
 
 It is also desirable to ensure, by means of mechanical arrange- 
 ments, that the insects shall not get their honey without paying 
 for it, that is to say, without effecting fertilization, and this end 
 is usually secured by concealing the honey at the bottom of a 
 long tube, in such a manner that the insect must brush against 
 the stamens and stigma before it can reach.it. Sometimes, how- 
 ever, the insects become a little too clever for the flower and steal 
 the honey by biting a hole through the bottom of the tube with- 
 out ever touching the stamens or stigma at all. In this way the 
 red clover flowers are often robbed of their honey by the humble 
 
 A A 2
 
 356 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 bee, Bombus terrestris, thus affording an example of imperfect 
 adaptation. 
 
 We may now consider a few definite examples of the manner 
 in which flowers may be specially adapted in structure so as to 
 secure the advantages of cross -fertilization by insect agency. 
 
 If a number of plants of the common primrose, the oxlip, the cow- 
 slip or the polyanthus (species of the genus Primula) be examined 
 carefully, it will be seen that in each case two quite different 
 forms of flower occur. In other words the flower is dimorphic. 
 The two forms are known to 
 gardeners as "pin-eyed" and 
 " thrum-eyed " respectively. In the 
 pin-eyed flowers (Fig. 180, A) the 
 style (g) is comparatively long and 
 the stigma (n) appears as a round 
 knob, like the head of a pin, in the 
 centre of the flower, at the entrance 
 to the long tube formed by the 
 lower part of the corolla. The 
 anthers (a) lie much lower down in 
 the tube, so that they are invisible 
 until the flower is cut open. In the 
 thrum-eyed flowers (B) the positions 
 of the stigma and anthers are 
 reversed ; the style being much 
 shorter the stigma lies only half 
 way up the tube, while the anthers 
 appear in the centre of the flower, 
 
 in the mouth of the tube. The two kinds of flower are 
 always found on separate plants, and the long-styled and short- 
 styled plants are said to occur in about equal numbers under 
 natural conditions. The pollen grains also differ in the two 
 kinds of flower, those of the thrum-eyed being larger than those 
 of the pin-eyed and of somewhat different shape. This particular 
 kind of dimorphism, which is sometimes known as heterostylism, 
 is extremely characteristic of the genus Primula, and it has 
 recently been shown that the distinguishing features of the two 
 forms are inherited in a Mendelian fashion. 
 
 The flowers of the primulas are fertilized by the agency of 
 insects such as humble bees, and Darwin found that if insects 
 were carefully excluded by covering the flowers with a net, little 
 
 B 
 
 FIG. 180. Heterostyled Flowers 
 of the Oxlip (Primula elatior) 
 in longitudinal section. 
 (From Vines' " Botany.") 
 
 A, long-styled ; B, short-styled flower, 
 a, anthers ; c, corolla ; f , ovary ; 
 g, style ; k, calyx ; n, stigma.
 
 FERTILIZATION OF FLOWERS. 357 
 
 or no seed was produced. In order to reach the honey, which is 
 secreted at the bottom of the long tube of the corolla, the bee 
 thrusts its proboscis down the tube and in so doing brushes past 
 the stigma and the anthers. If a pin-eyed flower be visited the 
 pollen from the anthers is.deposited comparatively low down on 
 the proboscis. If a thrum-eyed flower be visited the pollen is 
 deposited much higher up, in accordance with the more elevated 
 position of the anthers. Thus the bees carry about two different 
 kinds of pollen on two different parts of the proboscis, a fact which 
 has been established by microscopic examination of the proboscis 
 itself with the attached pollen. 
 
 If it be remembered that in the two different kinds of flower 
 the relative positions of the anthers and stigma are reversed, it 
 will be obvious that when a bee sucks honey from a long-styled 
 flower the stigma will be touched and pollinated by that part of 
 the proboscis which touches the anthers of a short-styled flower, 
 and vice versa. Hence it follows that pin-eyed flowers will be 
 fertilized by pollen from thrum-eyed, and thrum-eyed flowers by 
 pollen from pin-eyed. 
 
 Here, then, we have a very precise adaptation for a special kind 
 of cross-fertilization, and Darwin further proved by his experiments 
 that this is the only kind of fertilization which results in complete 
 fertility. Cross-fertilization in these plants is, however, possible 
 in no less than four distinct ways : (1) A pin-eyed flower may 
 be fertilized by pollen from another pin-eyed ; (2) a thrum-eyed 
 flower may be fertilized by pollen from another thrum-eyed ; 
 (3) a pin-eyed flower may be fertilized by pollen from a thrum- 
 eyed ; (4) a thrum-eyed flower may be fertilized by pollen from 
 a pin-eyed. The first two methods have been termed by Darwin 
 "illegitimate unions" and they result in incomplete fertility; the 
 last two have been termed " legitimate " and result in complete 
 fertility. 
 
 The benefit derived from the existence of the two kinds of 
 flower lies in the intercrossing, not merely of two distinct 
 flowers, but of two distinct plants, for it will be remembered that 
 each plant bears only one kind of flower. Self-fertilization is 
 not absolutely prevented in this case, for the anthers and stigma 
 are mature at the same time in the same flower, but it is not 
 likely to take place, and if it does it results in incomplete fertility 
 and weakly offspring. But even if pollen should accidentally find 
 its way to a stigma of its own plant it need not necessarily
 
 358 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 prevent cross-fertilization, for foreign pollen will be prepotent 
 even if deposited on the stigma some time afterwards. 
 
 " To test this belief," Darwin observes, " I placed on several 
 stigmas of a long-styled cowslip plenty of pollen from the same 
 plant, and after twenty-four hours added some from a short- 
 styled dark-red polyanthus, which is a variety of the cowslip. 
 From the flowers thus treated, thirty seedlings were raised, and 
 all these, without exception, bore reddish flowers, so that the 
 effect of pollen from the same form, though placed on the stigmas 
 
 twenty-four hours pre- 
 viously, was quite des- 
 troyed by that of pollen 
 from a plant belonging to 
 the other form." 
 
 gr' 
 
 U 
 
 FIG. 181.- -Flower of Salviaj.ratensis. (From 
 Weismanu's " Evolution Theory," after 
 H. Miiller.) 
 
 gr' immature stigma; gr", mature stigma; si', 
 anther-lobe concealed in the " helmet " ; st", 
 anther-lobes lowered ; U, lower lip of corolla. 
 
 The flowers of the 
 common sage, and of 
 other species of the genus 
 Sal via (Fig. 181), afford a 
 no less striking example 
 of profound structural 
 modification in adapta- 
 tion to the visits of insects. 
 These flowers are pro- 
 tandrous, the stamens 
 maturing and shedding 
 their pollen before the 
 stigma is ready to receive 
 it, so that self-fertiliza- 
 tion is absolutely prevented. Moreover, the stigma in young 
 flowers (Fig. 181, gr') lies in such a position that it will not 
 be touched by visiting insects, which crawl right under it in 
 order to reach the honey at the bottom of the corolla-tube. In 
 older flowers the style elongates and curves downwards, so that 
 the stigma (Fig. 181, gr") comes to lie in the mouth of the 
 corolla-tube and must be brushed against by insects in search of 
 honey. 
 
 The most remarkable feature of the flower, however, is a 
 special mechanical contrivance for placing the pollen on the back 
 of any insect which attempts to suck honey from it in its young 
 or male condition. There are only two fully developed stamens, 
 and these are most curiously modified in structure. Each anther
 
 FERTILIZATION OF FLOWERS. 359 
 
 consists as usual of two anther-lobes united together by a con- 
 nective ; but the connective is very greatly elongated, so that the 
 anther- lobes are widely separated from one another instead of 
 lying close together at the top of the filament. The anther-lobe 
 at one end of eacli connective is imperfect and produces little or 
 no pollen ; the other is fully developed. The connective is 
 attached to the top of the very short filament by a movable joint, 
 so that it can be swung freely up and down in a vertical plane. 
 It has a long limb which curves upwards inside the " helmet " of 
 the corolla and terminates in the perfect anther-lobe, and a 
 short one which bears the imperfect lobe, as shown in the figure. 
 The two stamens lie side by side in the flower and the imperfect 
 anther-lobes are held downwards in the mouth of the corolla- 
 tube, exactly in the path of a visiting insect. The perfect anther- 
 lobes, on the other hand, are, at first, held up well above the 
 level of the insect's back (Fig. 181, st'). 
 
 When a large insect, such as a bee, visits the flower, it alights 
 upon the large lower lip ( (/), which, as in so many other flowers, 
 affords a convenient platform and has doubtless been specially 
 adapted to that end. The head of the insect is then thrust into the 
 corolla-tube and pushes against the two imperfect anther-lobes. 
 The connectives turn on their pivots like a see-saw and the two 
 ripe anther-lobes (st") are clapped down on the back of the insect 
 and dust it with pollen. This happens when the insect visits a 
 young flower. When an older flower, in the female condition, is 
 visited, the pollen from the back of the insect is deposited on the 
 mature stigma (#/'"), which now hangs directly in front of the 
 entrance to the corolla-tube. 
 
 It is, however, in the highly specialized order Orchidaceae that 
 we find perhaps the most remarkable contrivances for ensuring 
 cross-fertilization that are to be met with in the vegetable 
 kingdom. Many of these have been described and illustrated by 
 Darwin in his well-known work on the " Fertilization of Orchids." 
 
 In the common early orchis of Europe (Orchis mascula, 
 Fig. 182) there are three sepals and three petals, and both sepals 
 and petals are brightly coloured. The lower petal is very large and 
 in front forms a tongue-like projection termed the labellum (r), 
 which serves as a landing platform, while behind it is produced 
 backwards into a hollow spur or nectary (sp, n) in which honey 
 is secreted. The reproductive organs lie just above and partly 
 in front of the entrance to the nectary, so that an insect, in
 
 360 OUTLINES OF EVOLUTIONAEY BIOLOGY 
 
 poking its' proboscis down to get at the honey, cannot fail to touch 
 them. Both male and female organs are very much modified in 
 structure, and united together to form the column (C). There 
 are three stigmas. Of these, however, only two (B, na) are 
 functional, and they lie at the back of the entrance to the 
 nectary. The third is specially modified to form a remarkable 
 organ known as the rostellum (B, C, r), which projects above the 
 
 FIG. 182. Adaptation in the Flower of Orchis mascvla in relation to Insect- 
 Fertilization. (From Weismann's "Evolution Theory.") 
 
 A, side view of flower; B, front view; C, vertical section through the column; D, pol- 
 linia removed on the point of a pencil and still standing erect; E, the same later on, 
 bent forwards. 
 
 ei, entrance to nectary ; n, nectary ; na, stigmatic surface ; p, pollinia ; r, rostellum ; 
 Sm, honey guide ; sp, spur ; st. ovary (twisted) ; U, lower lip of flower (labellum). 
 
 other two in front of the mouth of the nectary. This rostellum, 
 or beak, consists of an exceedingly sticky body covered over by a 
 thin, membranous cap. The cap is so delicately adjusted that 
 the slightest touch is sufficient to push it down and expose the 
 sticky mass beneath, and it is so elastic that when the pressure 
 is removed it springs back into its original position and again 
 covers up the sticky substance. 
 
 There is only one perfect anther, consisting of two sacs which 
 stand up above and behind the rostellum. Each sac contains a 
 single coherent mass of pollen grains instead of the usual loose,
 
 FERTILIZATION OF FLOWERS. 361 
 
 powdery pollen. Each mass of pollen grains, or pollinium 
 (I), C, p), is a pear-shaped body provided with a short stalk or 
 caudicle. The stalk is continued downwards into the rostellum 
 as shown in C, where it ends in a membranous disc attached 
 to the sticky substance. When the pollen is ripe the pollinia are 
 exposed by rupture of the sacs in which they lie, so that in the 
 mature flower they are only attached by means of their stalks or 
 caudicles, loosely fixed by the sticky substance of the rostellum. 
 
 If we take any slender object, such as a pencil point, and poke 
 it gently into the mouth of the nectary, we shall find on with- 
 drawing it again that it will bring with it either one or both of 
 the pollinia, attached to it by sticky cement. The pencil has, of 
 course, touched the rostellum and pushed down the little cap 
 which covers it. The sticky substance of the rostellum, thus 
 exposed, has cemented on to the pencil the disc at the lower end 
 of the caudicle, and thus the pollinium itself, or both of them, is 
 pulled bodily out with the pencil. The cement rapidly hardens 
 on exposure to the air. 
 
 When first-attached to the pencil the pollinium is in a nearly 
 upright position (D), so that if the pencil were inserted into 
 another flower it would simply go back into its old place, without 
 touching the stigma and of course, therefore, without effecting 
 fertilization. After being exposed to the air for a short time, 
 however, the disc by which the pollinium is attached to the 
 pencil begins to contract, and in such a manner that the polli- 
 nium is pulled down until it comes to project straight forwards 
 (E). If noiv the pencil be inserted into a flower it will be found 
 that the pollinium exactly strikes against one of the stigmas and 
 dusts it with pollen. 
 
 In the economy of nature the proboscis of some insect takes the 
 place of our experimental pencil. Bees and moths have fre- 
 quently been observed with the pollinia of various species of 
 Orchis attached to them ; indeed Darwin figures one instance in 
 which no less than fourteen pollinia are attached to the proboscis 
 of a moth. Every time such an insect visits a flower in search 
 of honey it will effect cross-fertilization by pushing the pollinia 
 which it brings with it against the stigmas, and may perhaps 
 carry off another pollinium into the bargain. 
 
 In this case one can hardly fail to be astonished at the number 
 and complexity of the adaptations which have arisen in the 
 flower for the purpose of ensuring cross-fertilization. There is
 
 362 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the conspicuous colour of the sepals and petals, which serves as 
 an advertisement ; the formation of the long spur or nectary, with 
 its secreted honey ; the great development of the lahellum to 
 serve as a landing place, marked with the " honey-guide " (Sin) 
 to point out the way to the visitor; the secretion of sticky 
 cement by the rostellum ; the formation of an elastic cap to keep 
 the cement from being dried up before it is wanted ; the agglu- 
 tination of the pollen into solid masses, which serve to fertilize 
 a large number of flowers in succession, losing a few grains at 
 each contact ; the remarkable form of these pollinia, with their 
 adhesive discs and their peculiar relation to the sticky substance 
 on the rostellum ; and, lastly, the wonderful mechanism by means 
 of which the pollinia become bent downwards after their removal, 
 so as exactly to adjust them for contact with the stigma of 
 another flower ! 
 
 In some species of plants certain parts of the flower are highly 
 sensitive and respond to the slightest touch by a sudden and 
 vigorous movement, going off, so to speak, like a spring trap or 
 a hair trigger. This is well exemplified in a New Zealand orchid, 
 Pterostylis trulli folia, as described by Mr. Cheeseman. In this 
 flower the lower petal, or labellum, is highly sensitive, and when 
 an insect alights upon it springs up and imprisons the visitor 
 in a cage. The parts of the flower are so arranged that the 
 insect can only escape by crawling through a narrow passage, 
 and in such a manner that it must carry away the pollinia and 
 also leave on the stigma some of any pollen which it may chance 
 to bring with it. 
 
 A still more remarkable orchid, Catasetum, actually throws the 
 pollinia at its insect visitors as soon as they touch the flower, 
 and the tiny projectiles attach themselves to the intruder's head. 
 Darwin has described how, on one occasion, when he experi- 
 mentally irritated this flower, the pollinium was thrown for a 
 distance of nearly three feet, when it stuck on to a window 
 pane. 
 
 The presence of irritable structures which aid in cross- 
 fertilization by insect agency is, however, by no means confined 
 to orchids. Candollea (Stylidium) graminifolia is a common 
 Australian wild flower belonging to a totally different order, the 
 Candolleacese. It is found growing on open heaths and sends 
 up tall stems from the midst of a tuft of long, grass-like leaves, 
 each stem bearing a large number of rather small pink flowers
 
 FERTILIZATION OF FLOWERS. 363 
 
 along its length. There are five petals, but only four are fully 
 developed, and these spread themselves out in the form of a 
 cross around the mouth of the tube in which the honey is 
 secreted. The anthers and stigma are borne on the end of a 
 long slender column which springs from the middle of the flower. 
 Close to its base the column is bent downwards at a sharp angle 
 so that it hangs out at one side of the flower, between two of 
 the fully developed petals and resting upon the aborted petal as 
 on a cushion. Near its apex the column is again bent at a sharp 
 angle, this time upwards. The flowers are protandrous, the 
 anthers becoming mature and shedding their pollen before the 
 stigma is fully developed. 
 
 An insect, visiting the flower and poking its proboscis down 
 the tube of the corolla in search of honey, must touch the column 
 at the first bend. This is the irritable spot, and no sooner is it 
 touched than the column springs over to the other side of the 
 flower and brings the anthers or stigma, as the case may be, 
 down on the back of its visitor. In tin's way pollen is deposited 
 upon or removed from the insect's back and cross-fertilization is 
 effected. One of the most curious things about this case is that 
 the column loses its irritability at nightfall and refuses to jump, 
 however much you may tickle it. 
 
 The few examples of entomophilous flowers which we have now 
 studied must suffice to give some idea of how wonderfully minute 
 and perfect may be the adaptation of plants to highly specialized 
 environmental conditions. The dominating factor of the envi- 
 ronment here is, of course, the presence of insects, which can be 
 pressed into service as pollen carriers, and it is obviously in 
 relation to the requirements and tastes of these insects that the 
 various adaptations which we have been describing have arisen. 
 There can be little doubt that the insects select for their visits 
 those flowers which please them best and which are most readily 
 recognizable as the bearers of the coveted honey. 
 
 Although, as we have previously pointed out, some carrion- 
 feeding flies prefer scents which are repulsive to ourselves, yet 
 the vast majority of insect-fertilized flowers have scents and 
 colours which we appreciate perhaps no less than the insects for 
 whose gratification they primarily exist. As we shall see later on, 
 these scents and colours, though now in many cases improved 
 by human selection, doubtless arose in the first instance in 
 response to insect selection. If we pursue this line of thought a
 
 364 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 little further, there seems good reason to suspect that man, who 
 appeared upon the scene at a very much later date and whose 
 ideas of what is beautiful are doubtless largely derived from the 
 contemplation of flowers, may owe much of his aesthetic develop- 
 ment to the fact that he has been educated by flower-loving 
 insects.
 
 PART V. FACTORS OF ORGANIC EVOLUTION 
 
 -\ 
 
 CHAPTER XXIV 
 
 Views of Buffon, Erasmus Darwin and Lamarck. 
 
 WE shall have occasion to point out in a subsequent chapter 
 that many organisms exhibit characters which it is extremely 
 difficult, if not impossible, to bring into any direct relation- 
 ship with the environment, but this fact does not invalidate 
 the generalization that the most striking feature of all living 
 things is adaptation to the conditions under which they have to 
 carry on their existence. In seeking for an explanation of the 
 means whereby organic evolution has been effected, this fact 
 must constantly be borne in mind, and, as we have already said, 
 no theory can be considered adequate which does not take 
 fully into account the phenomena of adaptation, and offer some 
 reasonable explanation of the wonderful harmony which exists 
 between living things and their surroundings. 
 
 We have said before that the theory of organic evolution is no 
 new thing, but can be traced back even to the ancient Greek 
 philosophers. In the middle ages such ideas were thrust into 
 the background, along with other fruits of Greek and Roman 
 intellectual activity, and to a large extent supplanted by the 
 teachings of dogmatic theology. With the revival of scientific 
 inquiry, however, the theory of organic evolution, as opposed to 
 the doctrine of special creation and the immutability of species, 
 again began to occupy a prominent place in the minds of 
 thinking men, and a brief consideration of the views of some of 
 the chief philosophical biologists of the last two centuries will 
 perhaps form the most fitting introduction to this part of our 
 subject. 
 
 The celebrated French naturalist, Buffon (17071788), who 
 held the post of Superintendent of the Jardin des Plantes and, 
 in conjunction with his colleagues, published a large number of 
 volumes of Natural History, was one of those who had at any
 
 366 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 rate strong leanings towards the theory of organic evolution, 
 although, unfortunately, he seems to have found it very difficult 
 to give a frank expression to his views. The following trans- 
 lated extracts will serve to illustrate his position. 
 
 Speaking of the arbitrary character of our systems of classifi- 
 cation he observes : 
 
 " But Nature proceeds by unrecognized gradations and con- 
 sequently she cannot lend herself completely to these divisions, 
 since she passes from one species to another species, and often 
 from one genus to another genus, by imperceptible shades : so 
 that we find a great number of intermediate species and objects 
 which we do not know where to place, and which necessarily 
 upset the plan of the general system." 1 
 
 Buffon certainly appears in this passage as no believer in the 
 immutability of species, and amongst the causes which bring 
 about their modification he attributes great importance to the 
 action of climate : 
 
 " If we again consider each species in different climates, we 
 shall find obvious varieties both as regards size and form ; all 
 are influenced more or less strongly by the climate. These 
 changes only take place slowly and imperceptibly ; the great 
 workman of Nature is Time : he walks always with even strides, 
 uniform and regular, he does nothing by leaps ; but by degrees, 
 by gradations, by succession, he does everything; and these 
 changes, at first imperceptible, little by little become evident, 
 and express themselves at length in results about which we 
 cannot be mistaken." 2 
 
 In dealing with the animals of the old and the new worlds, and 
 after speaking of the extinction of the mammoth, he says : 
 
 " This species was certainly the foremost, the largest and the 
 strongest of all the quadrupeds : inasmuch as it has disappeared, 
 how many other smaller ones, weaker and less remarkable, have 
 had to succumb also, without having left us either witness or 
 evidence of their past existence ? How many other species, having 
 become modified in their nature, that is to say, perfected or 
 degraded by the great vicissitudes of land and sea, by the neglect 
 or the culture of Nature, by the long influence of a climate 
 become contrary or favourable, are no longer the same that they 
 formerly were ? And, moreover, the quadrupeds are, next to 
 man, the beings whose nature is the most fixed and whose form 
 is the most constant: that of the birds and of the fishes varies 
 
 1 Buffon, "Histoire Naturelle," Tom. I, p. 13. 
 
 2 Op. cit., Tom. VI, pp. 59, 60.
 
 VIEWS OF BUFFON. 367 
 
 more ; that of the insects still more, and if we descend to the 
 plants, which we must not exclude from animated nature, we 
 shall be surprised at the promptitude with which the species 
 vary, and at the facility with which they change their nature 
 while taking on new forms. 
 
 " It would not be impossible then, that, even without reversing 
 the order of Nature, all these animals of the new world may have 
 been originally the same as~those of the old, from which they 
 may have been formerly derived ; one might say that having 
 been separated subsequently by immense seas or impassable 
 lands, they would, in the course of time, have received all the 
 impressions, suffered all the effects, of a climate itself altered in 
 character by the same causes which brought about the separa- 
 tion ; that in consequence they would in time have become 
 dwarfed, changed their nature, &c. But that should not prevent 
 us from regarding them to-day as animals of different species : 
 whatever may be the cause of this difference, whether it has been 
 produced by time, climate and country, or whether it be of the 
 same date as the creation, it is none the less real. Nature, I 
 admit, is in a continual state of flux ; but it is enough for man 
 to seize her as she is in his own time, and to glance backwards 
 and forwards in endeavouring to gain some glimpse of what she 
 may have been in former times and of what she may become in 
 the future." l 
 
 This passage also seems to be a sufficiently clear declaration 
 in favour of the theory of organic evolution and the action of 
 the environment in modifying species. Buffon, however, by no 
 means confined himself to the consideration of climate as a 
 factor in the production of such modifications. The following 
 quotation shows that he also realized the importance of the 
 principles of use and disuse and the inheritance of acquired 
 characters, which were destined to take such a prominent place 
 in the subsequent writings of Lamarck : 
 
 " The llama, which, like the camel, passes its life in bearing 
 burdens and only rests by lying down upon its breast, has similar 
 callosities which are perpetuated in the same way by generation. 
 The baboons and the apes, whose most usual attitude, whether 
 awake or asleep, is sitting, have also callosities beneath the region 
 of the buttocks, and this callous skin has even become adherent 
 to the bones against which it is continually pressed by the weight 
 of the body ; but these callosities of the baboons and apes are dry 
 and healthy, because they do not arise from the constraint of 
 trammels or of the weight of a foreign burden, and because they 
 
 1 Op. cit., Tom. IX, pp. 126, 127.
 
 368 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 are merely the effects of the natural habits of the animal, which 
 remains seated more willingly and for a longer time than in any 
 other posture : it is the same with these callosities of the apes as 
 with the double sole of skin which we carry beneath our feet ; 
 this sole is a natural callosity which our constant habit of walking 
 or resting upright renders more or less thick, or more or less 
 hard, according to the amount of friction to which the soles of 
 our feet are exposed." l 
 
 It has been maintained that Buffon not only anticipated 
 Lamarck's views as to the influence of the environment and the 
 principle of use and disuse, but also those of Malthus and 
 Charles Darwin with regard to the importance of the struggle for 
 existence and the process of natural selection. That some such 
 ideas were present in his mind seems sufficiently clear from the 
 following passages. After speaking of the invasions of the Huns 
 and Goths and other peoples he continues : 
 
 " These great events, these conspicuous epochs in the history of 
 the human race, are, however, only trifling vicissitudes in the 
 ordinary course of living nature ; it is in general always constant, 
 always the same ; its movement, always regular, turns on two 
 fixed pivots, the one the unlimited fecundity given to all species, 
 the other the innumerable obstacles which reduce the product of 
 this fecundity to a fixed quantity and at all times leave only 
 approximately the same number of individuals in each species." 2 
 
 " The causes of destruction, of annihilation and sterility follow 
 immediately upon those of excessive multiplication ; and, inde- 
 pendently of contagion, the necessary consequence of too great 
 an accumulation of any living matter in one place, there are in 
 each species special causes of death and destruction, which we 
 shall indicate in the sequel and which alone suffice to compensate 
 for the excess of former generations." 3 
 
 " The least perfect species, the most delicate, the most heavily 
 burdened, the least active, the least well armed, &c.. have already 
 disappeared or will disappear." 4 
 
 Even if, on the strength of this last passage, however, we can 
 claim that Buffon had conceived the idea of the survival of the 
 
 1 Op. cit., Tom. XIV, pp. 325, 326. 
 
 2 Op. cit., Tom. VI, p. 248. 
 
 3 Ibid., p. 251. 
 
 4 I translate this passage, which I have not found in the original, from a quotation 
 given by Osborn in his interesting work " From the Greeks to Darwin " (Columbia 
 University Biological Series). The student should refer to this work for the history 
 of the theory of evolution.
 
 VIEWS OF BUFFON 369 
 
 fittest, the mere question of priority is a matter of small moment. 
 As a matter of fact it is now well known that this idea is very 
 much older than Buffon, and can be traced back, as Osborn 
 remarks, " to Empedocles, six centuries before Christ." l 
 Osborn also points out that : 
 
 " Buffon's ideas regarding the physical basis of heredity are 
 very similar to those of Damocritus, and certainly contain the 
 basis of the conception of the Pangenesis theory of Darwin, for 
 he supposes that the elements of the germ-cells were gathered 
 from all parts of the body." 2 
 
 So far we have presented the views of Buffon as those 
 of a thoroughgoing evolutionist. He had, however, apparently 
 another side to his mind, which is extremely difficult to under- 
 stand in the author of the foregoing passages. In the fourth 
 volume of his Natural History, after discussing the possible 
 modification of species, he continues : 
 
 " But no, it is certain, by revelation, that all animals have 
 participated equally in the grace of creation, that the two first 
 of each species and of all the species came forth complete from 
 the hands of the Creator ; and we must believe that they were 
 then much the same as they are now represented to us by their 
 descendants." 3 
 
 The doctrine of special creation could hardly ba more clearly 
 expressed. 
 
 Of course it is possible, as Samuel Butler suggests in his 
 interesting discussion of Buffon in " Evolution Old and New," 
 that such passages as this may be ironical, but it seems more 
 likely that Buffon vacillated between what was then regarded as 
 religious orthodoxy and the more rational views which he knew 
 so well how to express. Indeed, Butler himself remarks, apropos 
 of another passage : 
 
 " This is Buffon's way. Whenever he has shown us clearly 
 what we ought to think, he stops short suddenly on religious 
 grounds." 4 
 
 1 Op. cit., p. 117. 
 
 2 Ibid., p. 135. 
 
 3 " Histoire Naturelle," Tom. IV, p. 383. 
 
 4 " Evolution Old and New " (New Issue, London, A. & C. Fifield), p. 115. This 
 work contains numerous passages translated from Buffon and Lamarck, and I 
 have found it of great use as a guide to the more salient passages in the voluminous 
 writings of the former. I have, however, in all cases made fresh translations from 
 the French.
 
 370 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Buffon's inability to reconcile the logical consequences of bis 
 own inductions with his religious convictions is perhaps nowhere 
 better illustrated than in the attitude which he adopted with 
 regard to the position of man in the animal kingdom. Such an 
 experienced observer as he was could not fail to realize the close 
 agreement in structure between man and the higher apes : 
 
 " We shall see, in the history of the orang utan that, were we to 
 pay attention only to the form, we might equally well look upon 
 this animal either as the highest of the apes or as the lowest 
 of mankind ; because, with the exception of the soul, he lacks 
 nothing of all that we possess, and because he differs less from 
 man in bodily structure than he differs from other animals to 
 which the name of ape has been given." l 
 
 " I admit that, if one should judge only by form, the ape species 
 might be taken for a variety of the human species : the Creator 
 did not think fit to make for the human body a model absolutely 
 different from that of the animal ; He has included his form, like 
 that of all the animals, in one general plan ; but at the same 
 time that He bestowed upon him this ape-like material form, He 
 penetrated this animal body with His divine breath." 2 
 
 The ape, on the other hand, is a mere animal, and 
 
 " in spite of his resemblance to man, far from being the second 
 in our species, he is not the first in the order of animals, since 
 he is not the most intelligent." 3 
 
 There can be no doubt that the views of Erasmus Darwin 
 (1731 1802) were largely influenced by the writings of Buffon, to 
 which he repeatedly refers. Darwin's " Zoonomia" was published 
 in 1794, 4 but, perhaps because it was mainly a medical work, it 
 received but little attention from professional naturalists. The 
 section dealing with " Generation " contains his views on evolu- 
 tion. A few quotations will suffice to illustrate these : 
 
 " Owing to the imperfection of language the offspring is termed 
 a new animal, but is in truth a branch or elongation of the 
 parent ; since a part of the embryon-animal is, or was, a part of 
 the parent ; and therefore in strict language it cannot be said to 
 
 1 " Histoire Naturelle," Tom. XIV, p. 30. The name "orang utan " was applied 
 by Buffon to the chimpanzee. 
 
 2 IMd., p. 32. 
 s Ibid., p. 37. 
 
 4 My quotations are taken from an edition published by B. Dugdale at Dublin 
 iu 18UU.
 
 VIEWS OF ERASMUS DARWIN 371 
 
 be entirely new at the time of its production ; and therefore it 
 may retain some of the habits of the parent-system." 
 
 Here we have clearly expressed the idea of continuity which 
 plays such an important part in modern theories of heredity. 
 Erasmus Darwin was, however, a " Spermatist," that is to say 
 he believed 
 
 " that the embryon is produced solely by the male, and that the 
 female supplies it with a proper nidus, with sustenance, and 
 with oxygenation ; and that the idea of the semen of the male 
 constituting only a stimulus to the egg of the female, exciting it 
 into life, (as held by some philosophers) has no support from 
 experiment or analogy." 
 
 It must be remembered that he wrote in the days before the 
 cell theory had shed its illuminating rays over the science of 
 embryology : 
 
 "I conceive," says he, "the primordium, or rudiment of the 
 embryon, as secreted from the blood of the parent, to consist of a 
 simple filament as a muscular fibre," 
 
 but this filament was not necessarily thread-like in form, for 
 he adds : 
 
 " I suppose this living filament, of whatever form it may be, 
 whether sphere, cube, or cylinder, to be endued with the capability 
 of being excited into action by certain kinds of stimulus." 
 
 It thus absorbs nutriment and becomes organized by "accre- 
 tion of parts," and this leads to the development of new kinds 
 of " irritability." According to this view the appearance of a 
 new organ precedes its use : 
 
 " the lungs must be previously formed before their exertions to 
 obtain fresh air can exist." 
 
 "From hence I conclude, that with the acquisition of new 
 parts, new sensations, and new desires, as well as new powers, are 
 produced ; and this by accretion to the old ones, and not by 
 distention of them." 
 
 But the exercise of these new powers in turn gives rise to the 
 development of more new parts, which 
 
 " are formed by the irritations and sensations, and consequent 
 exertions of the parts previously existing, and to which the new 
 parts are to be attached." 
 
 " From this account of reproduction it appears, that all animals 
 have a similar origin, viz. from a single living filament ; and 
 
 B 2
 
 372 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 that the difference of their forms and qualities has arisen only 
 from the different irritabilities and sensibilities, or voluntarities, 
 or associabilities, of this original living filament ; and perhaps in 
 some degree from the different forms of the particles of the fluids, 
 by which it has been at first stimulated into activity." 
 
 "From their first rudiment, or primordium, to the termina- 
 tion of their lives, all animals undergo perpetual transformations; 
 which are in part produced by their own exertions inconsequence 
 of their desires and aversions, of their pleasures and their pains, 
 or of irritations, or of associations ; and many of these acquired 
 forms or propensities are transmitted to their posterity." 
 
 Dr. Darwin thus passes from the discussion of what is now 
 termed the ontogeny or development of the individual to that of 
 the phylogeny or development of the race. From consideration 
 of the former he endeavoured to gain some insight into the latter, 
 and it may be fairly claimed that he thus anticipated what is 
 known in modern biology as the Recapitulation Hypothesis : 
 
 "From thus meditating on the great similarity of the struc- 
 ture of the warm-blooded animals, and at the same time of the 
 great changes they undergo both before and after their nativity ; 
 and by considering in how minute a portion of time many of 
 the changes of animals above described have been produced ; 
 would it be too bold to imagine, that in the great length of time, 
 since the earth began to exist, perhaps millions of ages before 
 the commencement of the history of mankind, would it be too 
 bold to imagine, that all warm-blooded animals have arisen from 
 one living filament, which the great First Cause endued with ani- 
 mality, with the power of acquiring new parts, attended with 
 new propensities, directed by irritations, sensations, volitions 
 and associations ; and thus possessing the faculty of continuing 
 to improve by its own inherent activity, and of delivering down 
 those improvements by generation to its posterity, world without 
 end ! " 
 
 Erasmus Darwin was perfectly familiar with the idea of 
 adaptation, as manifested, for example, in the colours of 
 animals : 
 
 " The colours of many animals seem adapted to their purposes 
 of concealing themselves either to avoid danger, or to spring upon 
 their prey. Thus the snake and wild cat, and leopard, are so 
 coloured as to resemble dark leaves and their lighter interstices ; 
 birds resemble the colour of the brown ground, or the green
 
 VIEWS OF ERASMUS DARWIN 378 
 
 hedges, which they frequent; and moths and butterflies are 
 coloured like the flowers which they rob of their honey." 
 
 " A proboscis of admirable structure has been acquired by the 
 bee, the moth, and the humming bird, for the purpose of 
 plundering the nectaries of flowers. All which seem to have 
 been formed by the original living filament, excited into action 
 by the necessities of the creatures, which possess them, and on 
 which their existence depends." 
 
 In the following sentences we have at any rate a very close 
 approach to the idea of natural selection which forms the key- 
 note of the evolutionary theory as advocated by the illustrious 
 grandson of the writer : 
 
 " A great want of one part of the animal world has consisted 
 in the desire of the exclusive possession of the females ; and these 
 have acquired weapons to combat each other for this purpose. . . . 
 So the horns of the stag are sharp to offend his adversary, but 
 are branched for the purpose of parrying or receiving the thrusts 
 of horns similar to his own, and have therefore been formed for 
 the purpose of combating other stags for the exclusive posses- 
 sion of the females ; who are observed, like the ladies in the 
 times of chivalry, to attend the car of the victor." 
 
 " The final cause of this contest amongst the males seems to be, 
 that the strongest and most active animal should propagate the 
 species, which should thence become improved." 
 
 Here the principle of selection seems to be clearly enough 
 recognized, at any rate that form thereof which Charles Darwin 
 afterwards distinguished as sexual selection. 
 
 The great French philosophical biologist, 1 Jean Baptiste 
 Pierre Antoine de Monet, Chevalier de Lamarck, was born in 
 1744 and died in 1829. His most celebrated work, the 
 " Philosophie Zoologique," which contains the fullest expression 
 of his mature views on the theory of organic evolution, was 
 published in 1809, but these views appear to have been first 
 announced to the world in the opening lecture of the Course of 
 Zoology given at the Natural History Museum in Paris in 1800, 
 
 1 Lamarck was the originator of the term Biology for the Science of Living 
 Things.
 
 374 OUTLINES OF EVOLUTIONAEY BIOLOGY 
 
 and published in the following year in the form of a preface to 
 the " Systeme des Animaux sans Vertebres." 1 
 
 It seems impossible to doubt that Lamarck, like Erasmus 
 Darwin, was largely influenced in his views by the writings of 
 his great compatriot Buffon, with whom he was on terms of 
 personal friendship, and to whom he refers in his published 
 work. Whether or not he was acquainted with the works of 
 Erasmus Darwin will probably never be known, but it is evident 
 that both drew part at any rate of their inspiration from the 
 same source. Owing largely to his official position as Professor 
 of Invertebrate Zoology in the National Museum, Lamarck was 
 able to bring forward a very imposing array of facts in support 
 of his opinions, and though these opinions were but the natural 
 development of those enunciated by his predecessors, he was able 
 to place the theory of organic evolution on a much firmer and 
 broader basis than it had previously enjoyed. He also doubtless 
 derived great advantage from the fact that he was an experienced 
 botanist as well as a zoologist, having published many important 
 botanical works before he turned his attention more particularly 
 to the zoological aspect of biology. 
 
 During the earlier part of his life Lamarck appears to have 
 accepted the still prevalent doctrine as to the immutability of 
 species, and it is perhaps significant that his conversion to 
 evolutionary views seems to have followed very rapidly upon the 
 extension of his investigations from the vegetable to the animal 
 kingdom. 
 
 In the " Philosophie Zoologique " he maintains that the first 
 living things arose by a process of spontaneous generation, which 
 may still take place, and that from the starting points thus pro- 
 vided the entire animal and vegetable kingdoms as they now exist 
 have arisen as the result of orderly and progressive evolution. 
 
 He devotes a large amount of space to the question of classifi- 
 cation and the conception of species, and arrives at the following 
 conclusions, which are, on the whole, in singularly close agreement 
 with those of modern biologists : 
 
 " But these classifications, of which several have been so happily 
 imagined by naturalists, and the divisions and sub- divisions 
 
 1 Vide Packard's "Lamarck, the Founder of Evolution, his Life and Work" 
 (Longmans, Green & Co., 1901). This most interesting work contains translations of 
 portions of Lamarck's writings, and has for the first time made the work of the great 
 French philosopher available to the general reader in England and America. I have, 
 however, thought it desirable to give fresh translations from the original French.
 
 VIEWS OF LAMAECK 875 
 
 which they present, are entirely artificial contrivances. Nothing 
 of all that, I repeat, occurs in nature, in spite of the foundation 
 which appears to be given to them by certain portions of the 
 natural series which are known to us, and which have the 
 appearance of being isolated. We may be certain that amongst 
 her productions, nature has really formed neither classes, nor 
 orders, nor families, nor genera, nor constant species, but only 
 individuals which succeed one another and which resemble 
 those which produced them. Now these individuals belong to 
 infinitely diversified races, which shade off under all forms and 
 in all degrees of organization, and each of which maintains 
 itself without change so long as no cause of change acts 
 upon it." 1 
 
 " The name species has been given to every collection of 
 similar individuals which have been produced by other individuals 
 like themselves. 
 
 "This definition is exact; for every individual that enjoys 
 life always resembles very closely that or those from which it 
 sprang. But to this definition has been added the supposition 
 that the individuals which make up a species never vary in 
 their specific character, and that consequently the species has 
 an absolute constancy in nature. 
 
 "It is exactly this supposition that I propose to combat, 
 because the clear evidence obtained by observation shews that 
 it is unfounded. 
 
 " The supposition, almost universally admitted, that living 
 bodies form species constantly distinguished by invariable 
 characters, and that the existence of these species is as old as 
 that of nature herself, was established at a time when obser- 
 vations were insufficient, and when the natural sciences were 
 still almost non-existent. It is always contradicted in the eyes 
 of those who have seen much, who have for a long time followed 
 nature, and who have profitably consulted the great and rich 
 collections of our Museum." 2 
 
 After speaking of the doctrine of special creation Lamarck 
 continues : 
 
 "Without doubt, nothing exists except by the will of the 
 sublime Author of all things. But can we assign to Him laws 
 in the execution of His will, and fix the method which He has 
 followed in this respect ? Has not His infinite power been able 
 to create an order of tJiiiif/s which should give existence 
 
 1 " Philosophic Zoologique," Tom. I, pp. 21, 22. 
 
 2 Of), cit., Tom. I, pp. 54, 55.
 
 376 OUTLINES OF EVOLUTIONAKY BIOLOGY 
 
 successively to everything that we see, as to everything which 
 exists and which is unknown to us? 
 
 " Assuredly, whatever may have been His will, the immensity 
 of His power is always the same ; and in whatever manner this 
 supreme will may have been executed, nothing can diminish 
 its grandeur." 1 
 
 Lamarck seems to have been the first to insist upon the 
 branching character of evolutionary series ; after speaking 
 of such series he goes on : 
 
 " I do not wish to say thereby that existing animals form a 
 very simple series, everywhere equally graduated ; but I say 
 that they form a branching series, irregularly graduated, and 
 which has no discontinuity in its parts, or which, at least, has 
 not always had, if it be true that, in consequence of some species 
 having been lost, such discontinuity occurs anywhere. It 
 results from this that the species which terminate each branch 
 of the general series are connected, at least on one side, with 
 other neighbouring species which shade into them." 2 
 
 Like Buffon, he lays great stress upon the action of the 
 environment in modifying organisms : 
 
 " Many facts teach us that in proportion as the individuals of 
 one of our species change their situation, their climate, their 
 manner of living or their habits, they thereby receive influences 
 which little by little change the consistency and the proportions 
 of their parts, their form, their faculties, even their organization ; 
 so that everything in them participates, in the course of time, in 
 the transformations which they experience. 
 
 " In the same climate, very different situations and exposures 
 at first cause the individuals exposed thereto simply to vary ; 
 but, in the course of time, the continual difference in situation of 
 the individuals of which I am speaking, which live and reproduce 
 themselves successively in the same circumstances, causes in 
 them differences which become, in some way, essential to their 
 existence ; so that, after many generations have succeeded one 
 another, these individuals, which belonged originally to another 
 species, find themselves in the end transformed into a new species, 
 distinct from the other. 
 
 " For example, if the seeds of a grass, or of any other plant 
 natural to a damp meadow, be transported by any chance, first 
 to the slope of a neighbouring hill, where the soil, although more 
 elevated, is still sufficiently cool to permit of the plant main- 
 taining itself, and if afterwards, after having lived there and 
 
 1 Op. cit.. Tom. I, pp. 56, 57. 
 
 2 Ibid., p. 59. 

 
 VIEWS OF LAMARCK 377 
 
 reproduced itself many times, it reaches, by slow degrees, the 
 dry and almost arid soil of a mountainous region ; if the plant 
 succeeds in living tliere, and perpetuates itself for a succession 
 of generations, it will then become so changed that the botanists 
 who come across it will make of it a distinct species." 1 
 
 As Lamarck's views have so frequently been misrepresented it 
 is incumbent upon us to make ourselves thoroughly acquainted 
 with what he really meant by the action of the environment, and 
 on this point, fortunately, he is very precise : 
 
 " Here it becomes necessary for me to explain the meaning 
 which I attach to these expressions : The environment (les 
 circonstanccs) influences the form and organization of animals, or 
 in other words, as it becomes very different it changes, in course 
 of time, the form and organization themselves by proportional 
 modifications. 
 
 " Certainly, if people took these expressions literally they 
 would attribute to me an error ; for, whatever the environment 
 ma}' be, it does not directly effect any modification whatever in 
 the form and organization of animals. 
 
 " But great changes in the environment lead, in the case of 
 animals, to great changes in their requirements (besoins), and 
 such changes in their requirements lead necessarily to actions. 
 Now, if the new requirements become constant or very lasting, 
 the animals then adopt new habits, which are as lasting as the 
 requirements which have called them forth. . . . 
 
 " Now, if a novel environment, having become permanent for 
 a race of animals, has given to these animals new habits, or in 
 other words has led them to new actions which have become 
 habitual, there will have resulted therefrom the use of some 
 particular part [of the body] in preference to some other, and in 
 certain cases the complete disuse of some part which has become 
 useless. 
 
 "None of these statements should be considered as hypo- 
 thetical or as the expression of individual opinion ; they are, on 
 the contrary, truths which need only attention and the observa- 
 tion of facts to render them evident. 
 
 " We shall see immediately, by the citation of known facts 
 which attest it, on the one hand that new requirements having 
 made some part necessary, have really, by a series of efforts, 
 caused this part to arise, and that afterwards its continued 
 employment has little by little strengthened it, developed it, and 
 in the end considerably enlarged it ; on the other hand, we shall 
 see that, in certain cases, the novel environment and new 
 requirements having rendered some part quite useless, the 
 
 1 Op. cit., Tom. I, pp. 62, 63.
 
 378 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 complete want of employment of this part has caused it to 
 gradually cease from developing like the other parts of the 
 animal ; that it has become reduced and attenuated little by 
 little, and that at length, when this want of employment has 
 been complete during a long period, the part in question has in 
 the end disappeared. All this is positive ; I propose to give the 
 most convincing proofs of it. 
 
 " In plants, where there are no actions, and consequently no 
 habits properly so-called, great changes in the environment have 
 none the less led to great differences in the development of their 
 parts ; in such a way that these differences have caused certain 
 of them to appear and develop, while they have caused many 
 others to dwindle away and disappear. But here everything is 
 effected by changes which take place in the nutrition of the 
 plant, in its absorptions and transpirations, in the amount of 
 heat, light, air and moisture, which it then habitually receives ; 
 finally, in the superiority which certain of the various vital 
 movements may acquire over others." 1 
 
 Like his predecessors, and like those who followed him, 
 Lamarck adduces in support of these views the remarkable 
 modifications which have taken place in animals and plants 
 under the influence of domestication : 
 
 " That which nature does with the aid of much time, we do 
 every day by suddenly changing, in relation to some living plant, 
 the conditions under which it and all the individuals of its species, 
 have existed. 
 
 " All botanists know that the plants which they transport from 
 their native place in order to cultivate them in gardens, undergo, 
 little by little, changes which, in the end, make them unrecogniz- 
 able. . . . 
 
 " Is not the cultivated wheat (Triticum sativum) a plant 
 brought by man to the condition in which we now actually see 
 it ? Who will tell me in what country such a plant occurs 
 naturally, that is to say except as the result of its cultivation in 
 some neighbouring place ? 
 
 " Where do we find, in a state of nature, our cabbages, our 
 lettuces, &c., as we now possess them in our kitchen gardens ? 
 Is it not the same with respect to the many animals which 
 domestication has changed or considerably modified?" 2 
 
 We may next consider two examples of the kind of evidence 
 which Lamarck brings forward in proof of the effects of the 
 
 1 Op. cit., Tom. I, pp. 221223. 
 
 2 Ibid., pp. 226227.
 
 VIEWS OF LAMARCK 379 
 
 environment and of the use and disuse of organs upon plants 
 and animals living in a state of nature : 
 
 " The following fact proves, with regard to plants, how much 
 the alteration of any important factor in the environment 
 modifies the parts of these living bodies. 
 
 " So long as Ranunculus aquatilis is sunk beneath the surface 
 of the water, its leaves are all finely divided and have their 
 divisions hair-like ; but when the stems of this plant reach the 
 surface of the water, the leaves which develop in the air 
 become broadened, rounded and simply lobed. If some runners 
 of the same plant succeed in pushing their way into a soil which 
 is merely damp, without being inundated, their stems are then 
 short, and none of their leaves are divided into hair-like 
 segments ; thus arises Ranunculus hederaceus, which botanists 
 regard as a species when they come across it." 1 
 
 " With regard to habits, it is curious to observe the result 
 thereof in the remarkable form and in the stature of the giraffe 
 (Camclo-pardalis) : we know that this animal, the tallest of the 
 mammals, inhabits the interior of Africa, and that it lives in 
 places where the earth, almost always arid and without herbage, 
 obliges it to browse on the foliage of trees and to make con- 
 tinual efforts to reach it. As a result of this habit, maintained 
 for a long time in all the individuals of its race, the fore limbs 
 have become much longer than the hind ones, and the neck has 
 become so much elongated that the giraffe, without standing 
 up on its hind legs, raises its head and reaches a height of 
 six metres (nearly twenty feet)." 2 
 
 The following passage must suffice to give some idea of 
 Lamarck's views on the inheritance of " acquired " characters, 
 which his theory necessarily implies, though not in the 
 exaggerated sense of some modern writers : 
 
 " These familiar facts are surely well suited to prove what is 
 the result of the habitual use by animals of some particular 
 organ or part ; and if, when we observe, in an animal, an organ 
 specially developed, strong and powerful, anyone pretends that 
 its habitual exercise has caused it to gain nothing, that its 
 continued disuse would cause it to lose nothing, and that, in 
 short, this organ has always been as it is since the creation of 
 the species to which the animal belongs, I would ask why our 
 domestic ducks can no longer fly like wild ducks ; in a word, I 
 would cite a multitude of examples relative to ourselves, which 
 bear witness to the differences which result in our own bodies 
 
 1 Op. tit 
 
 2 Ibid., ] 
 
 , Tom. I, p. 230. 
 pp. 256 257.
 
 380 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 from the exercise or the want of exercise of any of our organs, 
 although these differences are not maintained in the individuals 
 of the next generation, for then their results would be much 
 more considerable. 
 
 " I shall show in the second part that when the will determines 
 an animal to some action, the organs which have to execute this 
 action are forthwith stimulated by the affluence of subtle fluids 
 (the nervous fluid) which become the determining cause of the 
 movements which the action in question requires. A multitude 
 of observations establishes this fact, which should no longer be 
 called in question. 
 
 " It results therefrom that numerous repetitions of these acts 
 of organization strengthen, extend, develop and even create 
 the organs which are needed. It is only necessary to observe 
 attentively what is happening everywhere in this respect to 
 convince oneself of the actuality of this cause of organic 
 development and modification. 
 
 " Now, every change acquired in an organ by a habit of use 
 sufficient to have produced it, is maintained afterwards by 
 generation, if it is common to the individuals which have united 
 in the act of fecundation for the reproduction of their species. 
 In short, this modification is propagated, and thus passes to all 
 the individuals which follow and which are subjected to the 
 same environment, without their being obliged to acquire it by 
 the means which really created it. 
 
 " The mingling in reproductive unions, however, between 
 individuals which have different qualities or forms, is necessarily 
 opposed to the constant propagation of those qualities and forms. 
 This is the reason why in man, who is subjected to so many 
 different modifying circumstances, the accidental qualities or 
 defects which he has chanced to acquire are not preserved and 
 propagated by generation. If, when any peculiarities of form or 
 any defects have been acquired, two individuals in this condition 
 should always unite, they would reproduce the same peculiarities, 
 and successive generations confining themselves to similar 
 unions, a special and distinct race would then be formed. But 
 the perpetual mingling between individuals which have not the 
 same peculiarities of form causes all peculiarities acquired as the 
 result of peculiar circumstances of the environment to disappear. 
 Whence one may be certain that if human beings were not 
 separated by the distances of their habitations, the mixed breed- 
 ing would cause the general characters which distinguish the 
 different nations to disappear." l 
 
 In the light of modern knowledge these views on the subject 
 of heredity are, of course, crude and inaccurate enough, but there 
 
 1 Ojt. cit., Tom. I, pp. 258 262.
 
 VI KWS OF LAMARCK 381 
 
 is nothing absurd in them, and at the time when Lamarck wrote 
 it would scarcely have been possible to formulate anything 
 better. 
 
 It is, however, this assumption of the inheritance of somato- 
 genic characters that has probably done more than anything else 
 to prevent many modern biologists from accepting the so-called 
 Lamarckian factors of evolution. Those who hold with Weis- 
 mann that there is no possible mechanism by which a somato- 
 genic character can be converted into a blastogenic one are 
 forced to reject Lamarck's teaching, but the Weismannian 
 assumption that there is no such possibility of inheritance of 
 somatogenic characters rests upon no better foundation than the 
 Lamarckian assumption that there is. We have dealt with this 
 question at some length in an earlier chapter and need only now 
 remember that, if there are many modern biologists who reject 
 the Lamarckian factors because they cannot reconcile them with 
 Weismannism, there are probably quite as many others who 
 accept them because they appeal irresistibly to their common 
 sense, and because they refuse to believe that Weismann's 
 difficulties are really insurmountable. 
 
 It is, of course, easy to ridicule Lamarck's views, and to say, 
 for example, that he maintained that an animal could develop 
 an organ by simply wishing for it, and ridicule of this kind has 
 undoubtedly done much to hinder the due appreciation of his 
 work. Such statements, however, are only made by those who 
 have never paid adequate attention to the writings of the great 
 French biologist. 
 
 It is evident from the passage last quoted that Lamarck also 
 did not fail to perceive the importance of isolation as a factor in 
 organic evolution, necessary to prevent the swampin effects of 
 intercrossing upon newly arisen species or varieties. 
 
 In another place he discusses the influence of the struggle for 
 existence in counteracting the effects of excessive multiplication, 
 and in so doing just misses the idea of Natural Selection : 
 
 " Animals eat one another, except those which live only upon 
 plants ; but the latter are liable to be devoured by carnivores. 
 
 " We know that it is the stronger and the better armed which 
 eat the weaker, and that the large species devour the smaller 
 ones. Nevertheless the individuals of one and the same race 
 rarely eat each other ; they make war on other races." l 
 
 1 Op. tit., Tom. I, p. 99.
 
 382 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Lamarck summed up his views as to the factors which have 
 co-operated in organic evolution, at any rate so far as the animal 
 kingdom is concerned, in a later work, the " Histoire Naturelle 
 des Animaux sans Vertebres," published in 1815, in the form of 
 four " Laws," which may be taken as replacing the two " Laws " 
 of the " Philosophie Zoologique." They are as follows : 
 
 " First Law : Life, by her own forces, tends continually to 
 increase the volume of every body which possesses it, and to 
 extend the dimensions of its parts, up to a limit which she 
 herself imposes." 1 
 
 " Second Law : The production of a new organ in an animal 
 body results from a new requirement which continues to make 
 itself felt, and from a new movement which this requirement 
 begets and maintains." 2 
 
 " Third Law : The development and efficiency of organs are 
 constantly in proportion to the use of these organs." 3 
 
 "Fourth Law: All that has been acquired, traced out or 
 altered in the organization of individuals during the course of 
 their life, is preserved by generation, and transmitted to the 
 new individuals which originate from those which have 
 experienced these modifications." 4 
 
 With regard to the position of man in the animal kingdom 
 Lamarck, unfortunately, does not seem to have been able, any 
 more than Buffon, to divest himself of the fetters of religious 
 orthodoxy. After pointing out at length the numerous ties by 
 which man, in his bodily organization, is united to the lower 
 animals, and especially his close relationship to the apes, he 
 saves himself, so to speak, in the following paragraph : 
 
 " Such would be the reflections which one might make if man, 
 considered here as the pre-eminent race in question, were only 
 distinguished from the animals by the characters of his 
 organization, and if his origin were not different from theirs." 5 
 
 1 " Histoire Naturelle des Animaux sans Vertebres," Tom. I, 1815, p. 182. 
 
 2 IMd., p. 185. 
 
 3 Ibid., p. 189. 
 
 4 Jlid., p. 199. 
 
 5 " Philosophic Zoologique," Tom. I, p. 357.
 
 CHAPTER XXV 
 
 Robert Chambers and the " Vestiges of Creation " Natural Selection 
 The Views of Charles Darwin and Alfred Russel Wallace. 
 
 FOR half a century after the appearance of the " Philosophie 
 Zoologique " the theory of organic evolution made but little 
 progress. The gap, however, was to some extent filled by the 
 publication, at first anonymously, of Robert Chambers' celebrated 
 book, "Vestiges of the Natural History of Creation." This work 
 first appeared in the year 1844 and rapidly passed through a large 
 number of editions. Though the views of its author can hardly 
 be said to mark any advance, but on the whole perhaps rather a 
 retrogression, the work, which gave rise to much controversy, 
 undoubtedly played a very important part in preparing the way 
 for the reception of Charles Darwin's " Origin of Species." 
 Chambers presented the evidence of organic evolution in a very 
 convincing manner, laying great stress upon that afforded by 
 the geological record and the facts of comparative anatomy and 
 embryology, and he included mankind in his general scheme of 
 evolution. 
 
 His views as to the modus opcrandi of organic evolution are 
 probably expressed as clearly as such views could be in the 
 following paragraphs : 
 
 " The proposition determined on after much consideration is, 
 that the several series of animated beings, from the simplest and 
 oldest up to the highest and most recent, are, under the providence 
 of God, the results, first, of an impulse which has been imparted 
 to the forms of life, advancing them, in definite times, by genera- 
 tion, through grades of organization terminating in the highest 
 dicotyledons and vertebrata, these grades being few in number, 
 and generally marked by intervals of organic character which 
 we find to be a practical difficulty in ascertaining affinities ; 
 second, of another impulse connected with the vital forces, 
 tending, in the course of generations, to modify organic structures 
 in accordance with external circumstances, as food, the nature 
 of the habitat and the meteoric agencies, these being the
 
 384 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 " adaptations " of the natural theologian. We may contemplate 
 these phenomena as ordained to take place in every situation, 
 and at every time, where and when the requisite materials and 
 conditions are presented in other orbs as well as in this in 
 any geographical area of this globe which may at any time 
 arise observing only the variations due to difference of 
 materials and of conditions." 1 
 
 " For the history, then, of organic nature, I embrace, not as a 
 proved fact, but as a rational interpretation of things as far as 
 science has revealed them, the idea of Progressive Development. 
 We contemplate the simplest and most primitive types of being, 
 as giving, under a law to which that of like-production is sub- 
 ordinate, birth to a type superior to it in composite ness of 
 organization and endowment of faculties; this again producing 
 the next higher, and so on to the highest. We contemplate, in 
 short, a universal gestation of nature, analogous to that of the 
 individual being ; and attended as little by circumstances of 
 a startling or miraculous kind, as the silent advance of an 
 ordinary mother from one week to another of her pregnancy. 
 We see but the chronicle of one or two great areas, within which 
 the development has reached the highest forms. In some others, 
 as Australia and the islands of the Pacific, development appears 
 to have not yet passed through the whole of its stages, because, 
 owing to the comparatively late uprise of the land, the terrestrial 
 portion of the development was there commenced more recently. 
 It would commence and proceed in any new appropriate area, on 
 this or any other sphere, exactly as it commenced upon our area 
 in the time of the earliest fossiliferous rocks, whichever these 
 are. Nay, it perhaps starts every hour with common infusions, 
 and in similar humble theatres, and might there proceed through 
 all the subsequent stages, granting suitable space and conditions. 
 Thus simple after ages of marvelling appears Organic 
 Creation, while yet the whole phenomena are, in another point 
 of view, wonders of the highest kind, being the undoubted results 
 of ordinances arguing the highest attributes of foresight, skill, 
 and goodness on the part of their Divine Author." 2 
 
 Progressive evolution is here clearly attributed to some inherent 
 tendency implanted in the first living things, and apparently the 
 writer imagines that the same or closely similar results may 
 have been arrived at along many different lines of evolution, each 
 commencing at a distinct starting point and at a different time and 
 
 1 " Vestiges of the Natural History of Creation." 12th ed., 1884, pp. 201, 202. 
 
 2 Ibid., pp.230, 231.
 
 NATURAL SELECTION 385 
 
 place from all the others. The author of this hypothesis evidently 
 considers it a distinct improvement upon the views of Lamarck, 
 which he very briefly discusses. He thinks that Lamarck attributes 
 too much importance to the principle of use and disuse, which he 
 regards as " obviously insufficient to account for the great grades 
 of organization," though he admits that external conditions may 
 have been "a means of producing the exterior characters." 
 There can be little doubt, however, that, in relying upon a system 
 of more or less definite and continuously operating natural 
 causes as factors of organic evolution, Lamarck took up a far 
 more scientific position than Robert Chambers. 
 
 In insisting upon the great importance of Natural Selection as 
 a factor in organic evolution, Charles Darwin and Alfred Russel 
 Wallace made a great advance upon the position of any of their 
 predecessors. 
 
 We have seen in the last chapter that this principle had been 
 hinted at by more than one writer about the close of the eighteenth 
 and the commencement of the nineteenth centuries, and that it 
 can even be traced back to the philosophy of ancient Greece. In 
 the historical sketch which prefaces the later editions of the 
 " Origin of Species," Charles Darwin himself quotes a translation 
 from Aristotle which shows sufficiently clearly that the idea was 
 familiar to the great Greek biologist. In the same sketch he 
 also quotes a passage from Dr. W. C. Wells, from a paper read 
 before the Royal Society in 1813, in which the same principle is 
 recognized in the most explicit and unmistakable manner. 
 
 It was not, however, until the year 1858 that the part played 
 by natural selection in organic evolution began to be gene- 
 rally understood. In that year Sir Charles Lyell and Dr. J. D. 
 Hooker communicated to the Linnean Society certain papers, 1 
 written by Darwin and Wallace, which at once called prominent 
 attention to the importance of this factor and contained the 
 essential parts of the theory of natural selection as subsequently 
 developed by both these writers. 
 
 Darwin's paper consisted of an extract from his as yet 
 unpublished work, together with an abstract of a letter to 
 
 1 These papers have been reprinted by the Linnean Society in the volume pub- 
 lished in connection with the Darwin-Wallace Celebration held on July 1st, 1908, 
 and it is from this volume that the quotations which follow are taken. 
 B. C C
 
 386 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Professor Asa Gray. A few quotations will suffice to indicate 
 his views : 
 
 " De Candolle, in an eloquent passage, Las declared that all 
 nature is at war, one organism with another, or with external 
 nature. ... It is the doctrine of Malthus applied in most 
 cases with tenfold force. . . . Even slow-breeding mankind 
 has doubled in twenty-five years ; and if he could increase his 
 food with greater ease, he would double in less time. But for 
 animals without artificial means, the amount of food for each 
 species must, on an average, be constant, whereas the increase of 
 all organisms tends to be geometrical, and in a vast majority of 
 cases at an enormous ratio. Suppose in a certain spot there 
 are eight pairs of birds, and that only four pairs of them 
 annually (including double hatches) rear only four young, and 
 that these go on rearing their young at the same rate, then at 
 the end of seven years (a short life, excluding violent deaths, for 
 any bird) there will be 2048 birds, instead of the original sixteen. 
 As this increase is quite impossible, we must conclude either 
 that birds do not rear nearly half their young, or that the 
 average life of a bird is, from accident, not nearly seven years. 
 Both checks probably concur. The same kind of calculation 
 applied to all plants and animals affords results more or less 
 striking, but in very few instances more striking than in man." 
 
 " Lighten any check in the least degree, and the geometrical 
 powers of increase in every organism will almost instantly 
 increase the average number of the favoured species. . . . 
 Finally, let it be borne in mind that this average number of 
 individuals (the external conditions remaining the same) in each 
 country is kept up by recurrent struggles against other species 
 or against external nature (as on the borders of the Arctic 
 regions, where the cold checks life), and that ordinarily each 
 individual of every species holds its place, either by its own 
 struggle and capacity of acquiring nourishment in some period 
 of its life, from the egg upwards ; or by the struggle of its 
 parents (in short-lived organisms, when the main check occurs 
 at longer intervals) with other individuals of the same or different 
 species. 
 
 " But let the external conditions of a country alter. If in a 
 small degree, the relative proportions of the inhabitants will in 
 most cases simply be slightly changed ; but let the number of 
 inhabitants be small, as on an island, and free access to it from 
 other countries be circumscribed, and let the change of conditions 
 continue progressing (forming new stations), in such a case the 
 original inhabitants must cease to be as perfectly adapted to the
 
 VIEWS OF CHAELES DARWIN 387 
 
 changed conditions as they were originally. It has been shown 
 in a former part of this work, that such changes of external 
 conditions would, from their acting on the reproductive system, 
 probably cause the organization of those beings which were 
 most affected to become, as under domestication, plastic. Now, 
 can it be doubted, from the struggle each individual has to obtain 
 subsistence, that any minute variation in structure, habits, or 
 instincts, adapting that individual better to the new conditions, 
 would tell upon its vigour and health ? In the struggle it would 
 have a better chance of surviving ; and those of its offspring 
 which inherited the variation, be it ever so slight, would also 
 have a better chance. Yearly more are bred than can survive ; 
 the smallest grain in the balance, in the long run, must tell on 
 which death shall fall, and which shall survive. Let this work 
 of selection on the one hand, and death on the other, go on for 
 a thousand generations, who will pretend to affirm that it would 
 produce no effect, when we remember what, in a few years, 
 Bakewell effected in cattle,, and Western in sheep, by this identical 
 principle of selection ? " 
 
 " In nature we have some slight variation occasionally in all 
 parts-; and I think it can be shewn that changed conditions of 
 existence is the main cause of the child not exactly resembling 
 its parents ; and in nature geology shews us what changes have 
 taken place, and are taking place. We have almost unlimited 
 time ; " 
 
 " Another principle, which may be called the principle of diver- 
 gence, plays, I believe, an important part in the origin of species. 
 The same spot will support more life if occupied by very diverse 
 forms. We see this in the many generic forms in a square yard 
 of turf, and in the plants or insects on any little uniform islet, 
 belonging almost invariably to as many genera and families 
 as species. . . . Now, every organic being, by propagating BO 
 rapidly, may be said to be striving its utmost to increase in 
 numbers. So it will be with the offspring of any species after it 
 has become diversified into varieties, or subspecies, or true 
 species. And it follows, I think, from the foregoing facts, that 
 the varying offspring of each species will try (only few will 
 succeed) to seize on as many and as diverse places in the 
 economy of nature as possible. Each new variety or species, 
 when formed, will generally take the place of, and thus exter- 
 minate its less well-fitted parent. This I believe to be the origin 
 of the classification and affinities of organic beings at all times ; 
 for organic beings always seem to branch and sub-branch like 
 the limbs of a tree from a common trunk, the flourishing and 
 
 c c 2
 
 388 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 diverging twigs destroying the less vigorous the dead and lost 
 branches rudely representing extinct genera and families." 
 
 The same paper also contains a sketch of the supplementary 
 theory of sexual selection, which will be seen to agree very closely 
 with the paragraph from Erasmus Darwin's " Zoonomia " quoted 
 in the last chapter : 
 
 " Besides this natural means of selection, by which those 
 individuals are preserved, whether in their egg, or larval, or 
 mature state, which are best adapted to the place they fill in 
 nature, there is a second agency at work in most unisexual 
 animals, tending to produce the same effect, namely, the struggle 
 of the males for the females. These struggles are generally 
 decided by the law of battle, but in the case of birds, apparently, 
 by the charms of their song, by their beauty or their power of 
 courtship, as in the dancing rock-thrush of Guiana. The most 
 vigorous and healthy males, implying perfect adaptation, must 
 generally gain the victory in their contests. This kind of selec- 
 tion, however, is less rigorous than the other ; it does not require 
 the death of the less successful, but gives to them fewer 
 descendants." 
 
 The publication from which these quotations are taken itself 
 consists of extracts from Charles Darwin's manuscripts, selected 
 with a view to explaining, as clearly as possible, his theory of 
 natural selection, and is therefore especially suitable for citation. 
 In the following year (1859) the author's classical work, " The 
 Origin of Species by means of Natural Selection," made its 
 appearance, the first of that notable series of volumes on 
 philosophical biology which have made his name so famous. 
 In these works both the general theory of Evolution and the sub- 
 sidiary theory of Natural Selection are elaborated and supported 
 by an immense body of evidence drawn from published records 
 and personal observations, and so successfully was this done that 
 in a comparatively few years these theories met with general 
 acceptance on the part, not only of scientific men, but also 
 of the educated public. It must not be forgotten, either, that 
 Charles Darwin applied the doctrine of organic evolution 
 in a fearless and uncompromising manner to the origin of the 
 human race. 
 
 Dr. Wallace's contribution to the Linnean Society symposium 
 of 1858 was entitled " On the Tendency of Varieties to depart
 
 VIEWS OF A. R. WALLACE 389 
 
 indefinitely from the Original Type." The following quotations 
 will serve to show that his views on Natural Selection were closely 
 similar to those of Charles Darwin. 
 
 " The life of wild animals is a struggle for existence. The 
 full exertion of all their faculties and all their energies is 
 required to preserve their own existence and provide for that of 
 their infant offspring. The possibility of procuring food during 
 the least favourable seasons, and of escaping the attacks of their 
 most dangerous enemies, are the primary conditions which 
 determine the existence both of individuals and of entire 
 species." 
 
 " Even the least prolific of animals would increase rapidly if 
 unchecked, whereas it is evident that the animal population of 
 the globe must be stationary, or perhaps, through the influence 
 of man, decreasing. Fluctuations there may be ; but permanent 
 increase, except in restricted localities, is almost impossible. 
 For example, our own observation must convince us that birds 
 do not go on increasing every year in a geometrical ratio, as 
 they would do, were there not some powerful check to their 
 natural increase. Very few birds produce less than two young 
 ones each year, while many have six, eight, or ten ; four will 
 certainly be below the average ; and if we suppose that each pair 
 produce young only four times in their life, that will also be below 
 the average, supposing them not to die either by violence or want 
 of food. Yet at this rate how tremendous would be the increase 
 in a few years from a single pair ! A simple calculation will 
 show that in fifteen years each pair of birds would have increased 
 to nearly ten millions ! Whereas we have no reason to believe 
 that the number of the birds of any country increases at all in 
 fifteen or in one hundred and fifty years. With such powers of 
 increase the population must have reached its limits, and have 
 become stationary, in a very few years after the origin of each 
 species. It is evident, therefore, that each year an immense 
 number of birds must perish as many in fact as are born. . . . 
 It is, as we commenced by remarking, ' a struggle for existence,' 
 in which the weakest and least perfectly organized must always 
 succumb. 
 
 " Now it is clear that what takes place among the individuals 
 of a species must also occur among the several allied species of 
 a group, viz., that those which are best adapted to obtain a 
 regular supply of food, and to defend themselves against the 
 attacks of their enemies and the vicissitudes of the seasons, must 
 necessarily obtain and preserve a superiority in population ; 
 while those species which from some defect of power or organization 
 are the least capable of counteracting the vicissitudes of food
 
 890 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 supply, &c., must diminish in numbers, and, in extreme cases, 
 become altogether extinct." 
 
 " Most or perhaps all the variations from the typical form of 
 a species must have some definite effect, however slight, on the 
 habits or capacities of the individuals. Even a change of colour 
 might, by rendering them more or less distinguishable, affect 
 their safety ; a greater or less development of hair might modify 
 their habits. . . . An antelope with shorter or weaker legs must 
 necessarily suffer more from the attacks of the feline carnivora. 
 ... If, on the other hand, any species should produce a variety 
 having slightly increased powers of preserving existence, that 
 variety must inevitably in time acquire a superiority in numbers. 
 . . . All varieties will therefore fall into two classes those 
 which under the same conditions would never reach the popula- 
 tion of the parent species, and those which would in time obtain 
 and keep a numerical superiority.. Now, let some alteration 
 of physical conditions occur in the district a long period of 
 drought, a destruction of vegetation by locusts, the irruption of 
 some new carnivorous animal seeking ' pastures new ' any 
 change in fact tending to render existence more difficult to the 
 species in question, and tasking its utmost powers to avoid com- 
 plete extermination ; it is evident that, of all the individuals 
 composing the species, those forming the least numerous and 
 most feebly organized variety would suffer first, and, were the 
 pressure severe, must soon become extinct. The same causes 
 continuing in action, the parent species would next suffer, 
 would gradually diminish in numbers, and with a recurrence 
 of similar unfavourable conditions might also become 
 extinct. The superior variety would then alone remain, and 
 on a return to favourable conditions would rapidly increase 
 in numbers and occupy the place of the extinct species and 
 variety. 
 
 " The variety would now have replaced the species, of which it 
 would be a more perfectly developed and more highly organized 
 form. It would be in all respects better adapted to secure its 
 safety, and to prolong its individual existence and that of the 
 race. Such a variety could not return to the original form ; for 
 that form is an inferior one, and could never compete with it 
 for existence. . . . But this new, improved, and populous race 
 might itself, in course of time, give rise to new varieties, 
 exhibiting several diverging modifications of form, any of which, 
 tending to increase the facilities for preserving existence, must, 
 by the same general law, in their turn become predominant. 
 Here, then, we have progression and continued divergence deduced 
 from the general laws which regulate the existence of animals, in
 
 CHARLES DARWIN AND LAMARCK 391 
 
 a state of nature, and from the undisputed fact that varieties do 
 frequently occur." 
 
 It will be evident from the above sketch of the theory of 
 Natural Selection, which I have thought it desirable to give in 
 the actual words of its chief exponents, that adaptation is 
 explained as the logical consequence of certain facts which can 
 at any time be verified by direct observation. (1) All 
 organisms tend to increase in a high geometrical ratio ; (2) there 
 is, partly as a direct result of such increase, a keen struggle for 
 existence, to which all organisms are more or less exposed and in 
 which vast numbers perish without leaving offspring; (3) all 
 organisms tend to vary in many directions ; (4) variations, 
 whether favourable or otherwise, tend to be transmitted by 
 heredity from generation to generation; though, as we have 
 already seen, there is at the present time much dispute as to 
 whether variations x>f a certain kind ought not to be excluded 
 from this generalization. 
 
 It follows inevitably from these premisses that in every genera- 
 tion there will be a more or less strongly pronounced tendency 
 towards the elimination of those individuals which are least well 
 adapted to their environment and a corresponding preservation 
 and encouragement of those which are best adapted, or, in 
 Herbert Spencer's celebrated phrase, a " survival of the fittest." 
 This process, continued from generation to generation for count- 
 less ages, has resulted in that marvellous perfection of adaptation 
 which we have seen to be such a striking feature of both plants 
 and animals. 
 
 Charles Darwin himself, however, was not satisfied with natural 
 selection as the sole factor concerned in bringing about pro- 
 gressive evolution and adaptation. Although, in the historical 
 sketch which he added to the later editions of the " Origin of 
 Species," he remarks : 
 
 " It is curious how largely my grandfather, Dr. Erasmus 
 Darwin, anticipated the views and erroneous grounds of opinion 
 of Lamarck in his ' Zoonomia,' " 
 
 and although he himself at first appears to have attached very 
 little importance to Lamarck's opinions, yet we find in the last 
 chapter of the sixth edition of the " Origin of Species " abundant 
 evidence that he was obliged to admit the efficacy of the chief 
 " Lamarckian " factor, the principle of use and disuse, in
 
 392 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 modifying species, and also, to some extent, that of the direct 
 action of the environment : 
 
 " Disuse, aided sometimes by natural selection, will often have 
 reduced organs when rendered useless under changed habits or 
 conditions of life ; and we can understand on this view the 
 meaning of rudimentary organs. 1 But disuse and selection will 
 generally act on each creature, when it has come to maturity 
 and has to play its full part in the struggle for existence, and 
 will thus have little power on an organ during early life ; hence 
 the organ will not be reduced or rendered rudimentary at this 
 early age. The calf, for instance, has inherited teeth, which 
 never cut through the gums of the upper jaw, from an early pro- 
 genitor having well-developed teeth ; and we may believe, that 
 the teeth in the mature animal were formerly reduced by disuse, 
 owing to the tongue and palate, or lips, having become 
 excellently fitted through natural selection to browse without their 
 aid ; whereas in the calf, the teeth have been left unaffected, and 
 on the principle of inheritance at corresponding ages have been 
 inherited from a remote period to the present day." 2 
 
 " I have now recapitulated the facts and considerations which 
 have thoroughly convinced me that species have been modified, 
 during a long course of descent. This has been effected chiefly 
 through the natural selection of numerous successive, slight, 
 favourable variations ; aided in an important manner by the 
 inherited effects of the use and disuse of parts ; and in an 
 unimportant manner, that is in relation to adaptive structures, 
 whether past or present, by the direct action of external condi- 
 tions, and by variations which seem to us in our ignorance to 
 arise spontaneously. It appears that I formerly underrated the 
 frequency and value of these latter forms of variation, as leading 
 to permanent modifications of structure independently of natural 
 selection. But as my conclusions have lately been much mis- 
 represented, and it has been stated that I attribute the modifica- 
 tion of species exclusively to natural selection, I may be permitted 
 to remark that in the first edition of this work, and subsequently, 
 I placed in a most conspicuous position namely, at the close of 
 the Introduction the following words : ' I am convinced that 
 natural selection has been the main but not the exclusive means 
 of modification.' This has been of no avail. Great is the 
 power of steady misrepresentation ; but the history of science 
 shows that fortunately this power does not long endure." 3 
 
 1 Often now called " vestigial organs." 
 8 " Origin of Species," Ed. vi. p. 420. 
 3 Ibid., p. 421.
 
 A. R. WALLACE AND LAMARCK 393 
 
 It is of the greatest interest to recognize the fact that Darwin 
 himself saw nothing incompatible between the so-called 
 Lamarckian factors of use and disuse and the direct action of 
 the environment, and the principle of natural selection, but, on 
 the other hand, that the one set of factors might supplement the 
 other. 
 
 On the occasion of the unveiling of the statue of Charles 
 Darwin in the Natural History Museum at South Kensington, 
 Professor Huxley found occasion to observe that "science 
 commits suicide when it adopts a creed." 1 This warning, it is 
 to be feared, has not been heeded by all of Darwin's followers. 
 Many of these have departed very far from the moderate and 
 rational position of their leader and, while attributing to natural 
 selection almost every advance which has been made in the 
 evolution of the organic world, are, as we have already seen, 
 obliged to justify their neglect of the " Lamarckian " factors by 
 denying altogether the possibility of the inheritance of acquired 
 characters, which Darwin, of course, freely admitted. Natural 
 selection, in the hands of these enthusiasts, and in spite of 
 Charles Darwin's efforts to maintain a just balance between this 
 and other factors, has indeed become a creed. 
 
 Dr. Wallace from the first adopted an uncompromising 
 attitude towards the opinions of Lamarck. In the Linnean 
 Society paper from which we have already quoted he says : 
 
 " The hypothesis of Lamarck that progressive changes in 
 species have been produced by the attempts of animals to increase 
 the development of their own organs, and thus modify their 
 structure and habits has been repeatedly and easily refuted by all 
 writers on the subject of varieties and species, and it seems to have 
 been considered that when this was done the whole question has 
 been finally settled ; but the view here developed renders such 
 an hypothesis quite unnecessary, by shewing that similar results 
 must be produced by the action of principles constantly at work 
 in nature. The powerful retractile talons of the falcon- and the 
 cat-tribes have not been produced or increased by the volition 
 of those animals ; 2 but among the different varieties which 
 occurred in the earlier and less highly organized forms of these 
 groups, those always survived longest ichich had the greatest facilities 
 for seizing their prey '." 
 
 1 Vide Herbert Spencer's " Factors of Organic Evolution," p. 75. 
 
 2 Who ever said they had, except in the sense that an animal voluntarily uses its 
 claws on appropriate occasions and that constantly repeated use causes them to
 
 394 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 Wallace's views l also differ from those of Darwin in that, at 
 any rate in later years, they have become strongly anthropo- 
 centric, and he now regards the whole of the organic world as 
 having been designed by the Creator for the ultimate reception 
 and benefit of mankind. He does not, it is true, go back to the 
 old idea that species have been separately and specially created 
 as we now find them, but he holds that the entire scheme of 
 evolution was planned out in the mind of the Creator, and even 
 suggests that the working out of this scheme may have been 
 delegated by the Supreme Being to a body of "organizing 
 spirits " : 
 
 " At successive stages of development of the life-world, more 
 and perhaps higher intelligences might be required to direct the 
 main lines of variation in definite directions in accordance with 
 the general design to be worked out, and to guard against a 
 break in the particular line which alone could lead ultimately to 
 the production of the human form." 2 
 
 Such speculations as this would render natural selection and 
 all other natural factors of organic evolution superfluous, but we 
 cannot profitably discuss them in a work like the present. We 
 may point out, however, that they are in essential agreement with 
 the views of the author of the " Vestiges of Creation," to which 
 we have referred in the early part of this chapter, excepting that 
 Robert Chambers did not venture to call in the assistance of 
 subordinate " organizing spirits " to carry out the plans of the 
 Creator. 
 
 increase in size and efficiency ? Lamarck did not suppose that an animal simply 
 willed organs to sprout out of its body ! 
 
 1 For a full exposition of these views the reader should refer to Dr. Wallace's 
 " Darwinism " (London : Macmillan & Co., 1889). 
 
 2 " The World of Life, a Manifestation of Creative Power, Directive Mind and 
 Ultimate Purpose," by Alfred Russel Wallace (London : Chapman and Hall, Ltd., 
 1910), p. 395.
 
 CHAPTER XXVI 
 
 Selection not confined to the organic world Illustrations of the action of 
 natural selection iu the struggle for existence Degeneration -Flight- 
 less birds Extermination of the Morioris Sedentary animals Para- 
 sites Co-operation of natural selection and the so-called Lamarckian 
 factors of evolution: The influence of internal secretions upon growth 
 Increase in size beyond the limits of utility. 
 
 THE principle of selection is, of course, by no means confined 
 to living things. The various bodies which make up the 
 inorganic world owe their actual form and arrangement largely 
 to processes of selection which are constantly going on amongst 
 them. The outline of the sea coast is the result of the selective 
 action of atmospheric and tidal agencies upon the different kinds 
 of rock of which it is composed. The softer parts are destroyed 
 first, leaving the more resistant portions to stand out in the form 
 of bluffs or promontories, and to illustrate in the inanimate 
 world the principle of the survival of the fittest. We might even 
 say that the prominent headlands exhibit adaptation, for if they 
 were not adapted by their peculiar hardness to resist the dis- 
 integrating influences of the environment they would not be 
 there, but would have perished with those portions of the land 
 which formerly occupied the bays and inlets. 
 
 All things, in short, must be subject to the selective action of 
 their environment, and we need not hesitate to attribute to 
 natural selection a very large share in the modelling of the 
 features of the organic world as we now see it. We know what 
 we ourselves, by our so-called artificial selection, are able to do 
 in this way. The chief difference between artificial and natural 
 selection is that man selects for his own purposes and modifies 
 organisms to suit his own ends, while Nature selects to the benefit 
 of the species operated upon, which becomes thereby modified to 
 its own advantage and preservation in the struggle for existence. 
 But we cannot really draw a distinction between the two kinds 
 of selection, for even in a state of nature organisms are often 
 selected and modified to the advantage of other organisms.
 
 396 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 As we saw in a previous chapter, the forms, colours and scents 
 of many flowers are probably the result of unconscious selection 
 by insects, extending over countless generations. It may be said 
 that the advantage gained in this case is mutual ; the insect gets 
 the honey and the flower gets fertilized. This of course is true, 
 but exactly the same is true of human selection. The sheep gets 
 the pasture and man gets the wool. 
 
 It seems impossible to explain on any other hypothesis than 
 that of the natural selection and gradual accumulation of chance, 
 favourable variations, those marvellous adaptations of animals 
 which lead to protective resemblance and mimicry, for although 
 we may admit that an organ which is actively employed may be 
 modified by the efforts of an animal to maintain itself by the 
 use of that organ, we can hardly extend the same principle to 
 such passive features as colour and ornamentation, or the out- 
 growth of leaf -like dermal appendages and so forth. It may be 
 questioned if, even with the aid of natural selection, we can fully 
 account for all the wonderful phenomena of mimicry, for why, if 
 it be an advantage to some species to adopt a common warning 
 colour and band themselves together in synaposematic groups, 
 should it be desirable for others to do just the reverse and split 
 up into a number of differently coloured forms, each of which 
 mimics some particular model ? We can only say that we do 
 not know all the factors of the environment, and that until we do 
 our inability to solve the problem cannot be justly considered as 
 an argument against the efficacy of natural selection. 
 
 It is, of course, extremely difficult, if not impossible, to obtain 
 direct evidence of the action of natural selection in modifying 
 species in a state of nature. Human life is all too brief to 
 admit of our making very satisfactory observations concerning 
 processes which extend perhaps over millions of years. Man 
 has, however, in a comparatively short space of time, so changed 
 the conditions of life for many of the lower animals as to lead, 
 albeit unintentionally, to the more or less complete extermina- 
 tion of many species, and by studying these cases we may hope 
 to arrive at sound conclusions as to what takes place in a state of 
 nature. After all, mankind is a part of nature and we have no 
 just reason for excluding his influence in our consideration of 
 the factors which have brought about the present condition of 
 the organic world. 
 
 It is well known that many of the birds of various remote
 
 FLIGHTLESS BIRDS 397 
 
 islands have lost the power of flight. Such are the kiwi, the 
 kakapo, the weka, the notornis and the already extinct gigantic 
 moas of New Zealand ; the dodo of Mauritius, and the solitaire 
 of Rodriguez. Although belonging to several very distinct 
 families of birds, including ratites, parrots, rails and pigeons, all 
 the forms enumerated have undergone the same curious modifi- 
 cation, resulting in the most extreme cases (the moas) in the 
 complete loss of the wings, and in others in the reduction of those 
 organs to a more or less vestigial condition. 1 
 
 This convergence is clearly due to the similarity of the con- 
 ditions under which these birds have had to live. One of the 
 most characteristic features of oceanic islands is the absence 
 from them of predaceous mammals, the natural enemies of birds, 
 which have never been able to cross the great stretches of open 
 ocean which separate such islands from the continental areas on 
 which the Mammalia have been evolved. Birds, however, and 
 even land birds, by virtue of their powers of flight, have been 
 able to reach these islands at more or less frequent intervals 
 and to establish themselves there. Finding abundance of food, 
 which they could obtain near the ground, and finding themselves 
 no longer under the necessity of constantly using their wings in 
 order to escape from their enemies, some of these birds, though 
 by no means all, gradually gave up flying and their wings under- 
 went a slow process of degeneration in accordance with Lamarck's 
 principle of disuse. No doubt such disuse, if continued only 
 through a single lifetime, could scarcely produce a visible effect 
 upon the next generation, but continued under the same con- 
 ditions throughout thousands of generations it has brought about 
 a permanent deterioration which can no longer be retrieved. 
 
 It is to be noted that this degeneration is the result of the 
 removal of the organism, to a certain extent, from the struggle 
 for existence. Natural selection can only act through the struggle 
 for existence and upon those organs which are of value in the 
 struggle. When the struggle ceases, natural selection ceases and 
 degeneration sets in, for there is no longer any reason why a 
 high standard of perfection should be maintained. All degrees of 
 imperfection now have equal opportunities of propagating them- 
 selves. The inferior individuals are no longer weeded out, and 
 the average condition of the species consequently deteriorates. 
 
 But observe what happens when a degenerate organism is 
 
 1 Compare Chapter XVII, Figs. Ill, 112.
 
 398 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 once more exposed, by some unfortunate change in its environ- 
 ment, to the old struggle from which it had escaped. This has 
 actually taken place in the case of the flightless birds of New 
 Zealand and other remote islands. With the advent of Euro- 
 peans, predaceous mammals of many species dogs, cats, rats, 
 weasels, stoats and ferrets have been let loose upon their 
 helpless victims. These are once more exposed to a keen 
 struggle for existence, while at the same time they have lost 
 those very organs which are necessary to enable them to maintain 
 themselves in that struggle, and natural selection, having 
 regained her power, is rapidly exterminating them. It is not 
 too much to say that in a few years' time there will be no 
 flightless birds left in New Zealand except in special reserves 
 where they are being protected by man. 
 
 It is highly instructive in this connection to contrast the con- 
 dition of such a bird as the flightless parrot, or kakapo, with that 
 of its relative the kea. The kakapo is a large, heavy bird of 
 nocturnal habits and with practically no means of defence ; it 
 haunts the dense forest and is rarely seen except when hunted 
 out by dogs. The kea, on the contrary, is one of the strongest 
 fliers of the parrot tribe. It frequents high and more or less 
 inaccessible mountain regions and since the advent of Europeans 
 has learnt to make use of the sheep which they have introduced 
 as an additional food supply. It is doubtful whether the utmost 
 efforts of the sheep farmers, who annually expend large sums of 
 money for the purpose, will ever enable them to exterminate the 
 kea, and it is equally doubtful whether the efforts of the New 
 Zealand Government to preserve the unique flightless birds will 
 suffice to prevent the complete extermination of the kakapo 
 within the next few years. 
 
 The aboriginal human population of remote islands has of 
 course suffered not less than the lower animals from the in- 
 vasion of their retreats by Europeans, although not always 
 exclusively at the hands of the Europeans themselves. There 
 are, perhaps, few more striking examples of the extermination of 
 a primitive native race than that afforded by the rapid disap- 
 pearance of the Moriori inhabitants of the Chatham Islands, 
 some four hundred miles to the east of New Zealand, during the 
 nineteenth century. 1 At the time of my visit to these islands, in 
 
 1 Compare Dendy,"The Chatham Islands : A Study in Biology " (Memoirs and Pro- 
 ceedings of the Manchester Literary and Philosophical Society, Vol. XLVI., 1902).
 
 EXTERMINATION OF MORIORIS 399 
 
 January, 1901, there were only about a dozen pure-blooded indi- 
 viduals left ; some of these were of great age, while the youngest 
 was a lad of about 16, and they had all, I think, more or less 
 completely adopted European manners and customs. Under 
 these circumstances we are fortunate in possessing any reliable 
 record of this interesting people, and that we do so is largely 
 due to the energy and enthusiasm of Mr. Alexander Shand, who 
 for more than thirty years lived amongst the Morioris and 
 made a special study both of that race and of their Maori 
 conquerors. 
 
 It appears from their language, customs and traditions, as 
 well as from their physical characteristics, that the Morioris are 
 closely related to the New Zealand Maoris. Their ignorance of 
 the art of tattooing, and their very inferior artistic faculties in 
 general, however, point to a very remote separation of the two 
 races. 
 
 Like the Maoris they trace their origin to an unknown father- 
 land called Hawaiki, from which they must have emigrated to 
 Chatham Island in canoes. In their new home they appear to 
 have found the conditions of life remarkably easy, indeed, as the 
 sequel shows, fatally so. With an abundant natural food supply 
 of fruit, shell-fish, &c., and with no enemies to contend with, 
 they multiplied until the islands were thickly populated, while 
 at the same time they doubtless became lazy and effeminate. 
 
 The discovery of the islands by the brig " Chatham," in 1790, 
 may be said to have sealed the fate of the unfortunate Moriori, 
 though it is doubtful whether any serious injury ensued until 
 the advent of the whaling and sealing vessels in 1828. These 
 vessels brought with them many undesirable visitors, and prob- 
 ably were the means of introducing a disease which soon played 
 havoc with the native race. On board some of the ships, moreover, 
 were Maoris from New Zealand, who, on their return, painted 
 such a glowing picture of the land of plenty, that a large number 
 of their fellow-countrymen determined to emigrate to the islands 
 en masse. 
 
 In order to effect this purpose they took possession of the brig 
 " Rodney " at Port Nicholson, in New Zealand, about the 
 beginning of November, 1835. They are said to have seized the 
 crew and compelled the captain to transport them, about 900 in 
 number, to their destination. At the time of the invasion the 
 Morioris are supposed to have numbered about 2000, and had they
 
 400 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 attacked the new-comers on their first arrival, they might have 
 exterminated them with little trouble and prolonged for an 
 indefinite period the life of their own race. Unfortunately for 
 themselves, however, they had lost the art of self-defence. Owing 
 to the absence of competition they had, in this respect at any 
 rate, undergone degeneration. Killing was actually forbidden 
 by their laws, and peace had reigned too long and too securely to 
 give place at once to war when the emergency arose. 
 
 Just as the flightless birds of New Zealand have more or less 
 completely disappeared since the advent of carnivorous mammals, 
 so the Morioris, their happy isolation once broken, fell an easy 
 prey to the more virile Maoris. The latter proceeded to parcel 
 out the conquered country amongst themselves, claiming not 
 only the land but also the inhabitants thereof, many of whom 
 were massacred under circumstances of unutterable atrocity, 
 while the remnant were speedily reduced to the condition of 
 slaves. Under the changed conditions which had suddenly arisen 
 in their environment the Morioris were no longer fit to survive 
 in the struggle for existence, they had become degenerate in a 
 vital respect, and natural selection, as soon as opportunity arose, 
 stepped in and eliminated them. 
 
 It would be easy to multiply illustrations of the great generaliza- 
 tion that when removed from the struggle for existence all 
 organisms tend to become degenerate, the organs or faculties 
 which they no longer require atrophying and gradually dis- 
 appearing for want of employment. We see this very clearly in 
 the case of sedentary animals such as the ascidians (Figs. 129, 
 130). The young ascidian is a highly organized creature which 
 swims actively about by means of a muscular tail, in the same 
 way as the tadpole of a frog. Like the latter it has nervous 
 system, notochord and sense organs though the sense organs 
 are of a type peculiar to itself and is an undoubted chordate. 
 It never, however, progresses further in organization, so as to 
 attain the true vertebrate condition. On the contrary, it gives up 
 its active life and withdraws as far as possible from the struggle 
 for existence by fixing itself to some rock or seaweed and envelop- 
 ing its entire body in a thick protective envelope, within which 
 it undergoes extensive degeneration. The tail and notochord 
 completely disappear, so do the sense organs, none of these 
 being any longer required under the new conditions of life. The
 
 DEGENERATION IN PARASITES 
 
 401 
 
 nervous system dwindles away to a mere ganglion, from which a 
 few nerves come off, and the entire animal is reduced to the 
 condition of a bag, with two openings through which the 
 remaining organs obtain their food supply and communicate 
 with the outside world by means of a stream of water maintained 
 by ciliary action. 
 
 Still more conspicuous is the degeneration undergone by 
 the great majority of parasites, whether animals or plants. 
 Sacculina, for example, in the earlier stages of its existence, is 
 an active crustacean which swims vigorously about by means of 
 well developed appen- 
 dages. It belongs to a 
 group, the barnacles or 
 cirripedes, which are 
 notorious for sedentary 
 habits and consequent 
 degeneration in the adult 
 condition. Sacculina, 
 however, not content with 
 a sedentary life, goes 
 further down hill and 
 becomes parasitic. It 
 attacks crabs, and in the 
 adult state is reduced to 
 the condition of a large, 
 irregularly shaped bag 
 (Fig. 183) fixed to 'the 
 under surface of the crab's 
 abdomen by root-like pro- 
 cesses which penetrate the body of the host and extract nutri- 
 ment therefrom. With the exception of these root-like processes, 
 which are a special, caenogenetic development, adapted for nutri- 
 tion under new conditions of life, the only organs which have 
 not undergone degeneration are those of reproduction, for upon 
 these depends the perpetuation of the race and upon these, 
 therefore, natural selection is still able to retain her hold. 
 
 It is, indeed, a general rule amongst parasitic animals that the 
 reproductive organs are largely developed and very complicated, 
 for the conditions which have become necessary for the existence 
 of these animals are so complex and highly specialized, while 
 the chances of mating between different individuals for purposes. 
 
 B. D D 
 
 FIG. 183. Lower surface of a Swimming 
 Crab (Port units depurator) with a Sac- 
 culina (Sac.) attached to it. (From a 
 photograph.)
 
 402 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 of sexual reproduction are so remote, that the ova and spermatozoa 
 have to be produced in vast numbers to compensate for the 
 immense mortality which must take place amongst them and 
 amongst the young animals to which they may give rise. Thus 
 
 FIG. 184. The Dodder, Cuscuta europcea. Part of a Plant parasitic on a 
 Branch of Willow, with germinating Seedlings on the right and 
 Section of Host and Parasite on the left. (From Strasburger.) 
 
 b, vestigial leaves ; Bl, flowers ; Cus, stem of parasite in section ; H, haustoria of parasite 
 in section; W, stem of host in section, with vascular bundles (v, c) ; t, seedlings. 
 
 one of the most frequent results, or at any rate concomitants, of 
 parasitism, and one which is well exemplified in the case of 
 Sacculina, is hermaphroditism, which affords many more chances 
 for the fertilization of the eggs than the unisexual condition. 
 
 We meet with precisely analogous phenomena in the case of 
 many parasitic plants. In the dodder (Fig. 184) the leaves and
 
 INSUFFICIENCY OF NATURAL SELECTION 403 
 
 roots have disappeared almost completely and no chlorophyll is pro- 
 duced, but special nutritive organs, the sucker-like haustoria, are 
 developed on the slender, twining stems, and serve to extract the 
 necessary food from the host plant. The flowers, however, upon 
 which the perpetuation of the race depends, still remain in a 
 well developed condition. 
 
 Both Sacculina and the dodder have lost the power of inde- 
 pendent existence, and if, for any reason, they were to find 
 themselves suddenly confined to an environment where there were 
 no suitable hosts, their races would inevitably become extinct. 
 Nature would treat them just as she is treating the wingless 
 birds, and make them pay the penalty for the degeneration which 
 they have undergone. 
 
 It has sometimes been pointed out as an objection to the theory 
 of natural selection that it cannot account for the first origin of 
 favourable variations. The theory takes variations for granted 
 and assumes that some will be favourable and some not, that the 
 former will be fostered and accumulated from generation to 
 generation and the latter ruthlessly eliminated. It is further 
 alleged that variations are usually so slight at their first appear- 
 ance that they can have no selective value, and that something 
 is wanted to account for the increase of such variations along 
 apparently definite lines of utility. The theory also takes the 
 inheritance of variations for granted, and many people, as we 
 have seen, consider nowadays that this is not altogether a 
 justifiable proceeding, that while some variations undoubtedly 
 are inherited, others, and amongst them many which would be 
 likely to be of the greatest value to the organism, are not. 
 
 We have, then, to go much deeper than the idea of natural 
 selection before we can reach a satisfactory working hypothesis 
 as to the manner in which organic evolution has taken place. 
 The problems of variation and heredity have already been dealt 
 with in earlier chapters, and it will be unnecessary to discuss the 
 matter now at great length, but there are certain points which 
 we must recapitulate in this connection. 
 
 We have seen that somatogenic or bodily variations in the 
 individual are undoubtedly brought about by the direct action of 
 the environment and by the use and disuse of organs. We have 
 also seen that blastogenic variations, which originate in the germ 
 plasm, may likewise be brought about by the action of the 
 
 D D 2
 
 404 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 environment (as in the case of the potato beetle as demonstrated 
 by Tower), but that they probably also arise from the mingling 
 of different streams of ancestral tendencies in the process of 
 amphimixis or conjugation of gametes, and possibly in yet other 
 ways with which we are not acquainted. 
 
 Of course, natural selection can only influence a species 
 through variations which are capable of being inherited, and it 
 is, as everyone knows, urged by many modern writers that 
 somatogenic variations, due either to the direct action of the 
 environment or to the use and disuse of parts, cannot be 
 inherited and therefore have no significance in evolution, and 
 that natural selection must content herself with such fortuitous 
 and non-adaptive variations as may happen to arise in the germ 
 plasm. This indeed seems an extreme view, and it is just 
 here that the split between the extreme selectionists, who have 
 gone far beyond Charles Darwin in this matter, and the followers 
 of Lamarck arises. The curious thing about the controversy is 
 that there is no inherent incompatibility between the views of 
 the two schools. The theory of the inheritance, to a limited 
 extent, of acquired characters, indeed, appears to be just what is 
 necessary to supply the deficiencies of that of natural selection. 
 
 To say that acquired characters cannot be inherited because 
 we cannot see them being inherited in our own brief lifetimes 1 is 
 like saying that a glacier does not move because we do not see it 
 or feel it moving as we walk over it. I have endeavoured to 
 show in an earlier chapter that it is not difficult to imagine a 
 mechanism by which somatogenic characters may gradually be 
 converted into blastogenic ones, and if this is in any way 
 possible there is no reason why we should deny the possibility 
 of their inheritance. No one, however, would be rash enough 
 to suppose that all that an animal or plant acquires in its 
 individual lifetime is transmitted to its heirs. Nature imposes 
 a heavy death duty and takes away by far the greater part of the 
 capital which has been accumulated by each individual. We 
 may suppose, however, that a fraction remains, however 
 unrecognizable by our limited powers, and that these 
 fractions, accumulating under the same influences throughout 
 thousands of generations, ultimately confer upon the organism 
 as a birthright that adaptation which is essential to its existence. 
 
 1 As a matter of fact, it appears from recent experiments that in some cases we 
 can see them being inherited (vide p. 182).
 
 CO-OPERATION OF FACTORS 405 
 
 Even the individual can do much in its own lifetime to adapt 
 itself to its environment, and when the residua of all the 
 individual adaptations are summed up by inheritance the result 
 is such that we may well wonder how it can have been produced. 
 Throughout the whole process, of course, natural selection must 
 help by constantly weeding out inferiority, but it is probably 
 the direct influence of the environment, including the use and 
 disuse of organs in response to that influence, that is in most 
 cases the determining factor in bringing about adaptation. 
 
 It may well be, however, that there are also cases in which 
 natural selection alone, acting through the occurrence of purely 
 fortuitous variations, has, in the struggle for existence, been 
 sufficient to produce marvellous adaptations. This may have 
 been the case with protective resemblance and mimicry in form 
 and colour, and with the adaptation of flowers for fertilization by 
 insects, in all of which it is difficult to see how the direct action 
 of the environment or the use and disuse of organs could bring 
 about adaptive modifications. But the fact that natural selection 
 alone appears to have been sufficient in some cases must not 
 prevent us from admitting the action of other factors in other 
 cases. The fact that some carriages are pulled by motors affords 
 no justification for asserting that other carriages may not be 
 pulled by horses, or that the same carriage may not at one 
 time be pulled by a motor and at another by a horse, or even 
 by both together. Many factors must have co-operated to 
 bring about such a marvellously complex result as the present 
 condition of the organic world, and no sufficient reason has 
 yet been shown for denying ourselves the assistance of 
 " Lamarckian " factors in our endeavours to discover the processes 
 through which this result has come about. 1 
 
 We may now turn our attention to a group of cases which 
 certainly appear to support the view that the factors at work in 
 determining any particular line of evolution are more complex 
 than might at first sight be supposed. It is a fact well known 
 to palaeontologists that many widely separated groups of the 
 animal kingdom have, during the course of their evolution, and 
 especially towards the end of that course, shown a strongly 
 marked tendency to enormous increase in size. We see this 
 in the extinct eurypterids (Fig. 137), giants amongst the 
 
 1 " To insist on ascribing complex results to single causes is the well-known vice 
 of narrow and untrained minds " (Morley's " Life of Gladstone." Vol. II., p. 68).
 
 406 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 arthropods; in the huge labyrinthodont amphibians; in many 
 groups of reptiles of the Secondary period, some of which 
 attained a length of 80 feet or more, and amongst mammals in 
 the extinct Tinoceras (Fig. 150) and the still surviving elephants 
 and whales. Comparative anatomists are familiar with similar 
 phenomena exhibited by individual organs, such as the extra- 
 ordinary development of horns and spines in many of the 
 extinct reptiles referred to (Fig. 145), the immense tusks of 
 
 FIG. 185. Head of Babirusa alfurus. (From Flower and Lyddeker's 
 " Mammals Living and Extinct.") 
 
 the babirusa (Fig. 185), and the gigantic and grotesque beak 
 and " helmet " of the hornbill (Fig. 186). 
 
 The exuberant development of some organs of this kind may 
 possibly be attributed to the action of sexual selection, and indeed 
 our daily experience of our own species seems to warrant us in 
 believing that there is no limit to the grotesque results which 
 may ensue from the unrestricted exercise of the aesthetic faculties 
 of either sex, but it seems hardly reasonable to attempt to 
 explain all such bizarre and monstrous productions in this 
 manner. 
 
 In all the cases cited, and in many others which could be 
 adduced, either the entire body or some particular organ appears 
 to have acquired some sort of momentum, by virtue of which it 
 continues to grow far beyond the original limits of utility, although 
 perhaps in some cases a new use may be found which will assist 
 the species in maintaining itself in the struggle for existence.
 
 EXTINCTION OF GIANT RACES 407 
 
 An enormous increase of mere bodily size, however, seems in the 
 long run to be always fatal to the race, whose place will be taken 
 by smaller and presumably more active forms. The gigantic 
 amphibians are all extinct, so are the really gigantic reptiles, 
 and of the gigantic mammals only a couple of species of elephants 
 
 FIG. 186. A Hornbill, Buceros rhinoceros, from North- West Borneo. (Drawn 
 from a specimen in the British Museum, Natural History.) 
 
 and a few whales survive, all of which are being rapidly exter- 
 minated in competition with man. 
 
 There is perhaps some justification in recent developments of 
 physiological science for the belief that a race of animals may 
 acquire a momentum of the kind referred to ; that some brake 
 is normally applied to the growth of organisms and organs and 
 that sometimes this brake is removed, leaving the organism to 
 rush onwards to destruction like a car running away down hill. 
 
 Many modern physiologists hold the view that the growth of
 
 408 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the different parts of the animal body is controlled by internal 
 secretions, or hormones, the products of various glands. Thus 
 we know that disease of the pituitary body in man may lead to 
 acromegaly, one of the symptoms of which is great enlargement 
 of certain parts. The most dreadful of all the diseases to which 
 human beings are subject, cancer, is essentially due to an unre- 
 strained multiplication of cells, and consequent abnormal growth 
 of tissue, which may very possibly be correlated with the extent 
 to which some specific controlling secretion is produced in the 
 body. In short, we are justified in supposing that in the individual 
 growth may be normally inhibited or checked by specific secretions, 
 and that in the absence of these it may continue far beyond the 
 ordinary limits. 
 
 It is difficult to see any good reason why we should not apply 
 this principle to the race as well as to the individual, and, 
 paradoxical as it may appear, it even seems possible to explain 
 both the growth of the organism as a whole and that of its 
 various organs, beyond the limits of utility, as an indirect result 
 of natural selection. 
 
 When a useful organ, such as the tusk of a wild boar, is first 
 beginning to develop, or to take on some new function for the 
 execution of which an increase in size will be advantageous, 
 natural selection will favour those individuals in which it grows 
 most rapidly and attains the largest size in the individual lifetime. 
 If growth is normally checked and controlled by some specific 
 secretion, or hormone, natural selection will favour those 
 individuals in which the glands which produce this secretion are 
 least developed, or at any rate least active. The process being 
 repeated from generation to generation these glands (whatever 
 may be their nature, and we use the term " gland " for any cell 
 or group of cells which produces a specific secretion, whether 
 recognizable as a distinct organ or not) may ultimately be 
 eliminated, or at any rate cease altogether to produce the par- 
 ticular hormone in question. Moreover, this elimination may 
 take place long before the organ whose growth is being favoured 
 by natural selection has reached the optimum size. When it 
 has reached this optimum it is certainly desirable that it should 
 grow no larger, but is there now any means by which further 
 growth can be checked ? The inhibiting hormone is no longer 
 produced ; the brake has been removed, and further growth may 
 be supposed to take place irrespective of utility, until, when
 
 EXCESSIVE GROWTH 409 
 
 the size of the organ gets too great to be any longer compatible 
 with the well-being of the race, natural selection again steps in 
 and eliminates the race. The same argument of course applies 
 to the size of the body as a whole as well as to that of its con- 
 stituent parts. 
 
 It may be thought that many of the bizarre and almost 
 monstrous characters under discussion, such, for example, as 
 some of the excrescences of the dermal armature in extinct 
 reptiles (Fig. 145), can never have had any value as adaptations, 
 and that therefore natural selection could never have encouraged 
 them to increase so much in size as to get beyond her control. 
 Here, however, the principle of correlation comes in. Just as 
 many totally different organs are affected by disease of the 
 pituitary body, so the removal of the gland which controlled the 
 development of some undoubtedly useful organ, such as a frontal 
 horn, might at the same time permit the growth of all sorts of 
 excrescences which have no adaptive significance. 
 
 Thus it appears not impossible that, the normal checks to 
 growth being removed along certain lines by the action of natural 
 selection, a definite direction might be given to the course of 
 evolution, which the organism would continue to follow irrespec- 
 tive both of natural selection and of the principle of use and 
 disuse. 1 
 
 In the present state of our knowledge, however, the above 
 suggestions can only be regarded as tentative. They are doubtless 
 open to much criticism, and it is unfortunately impossible to 
 subject them to the crucial test of experiment. 
 
 1 I have discussed this question at somewhat greater length in a paper read 
 before the British Association for the Advancement of Science (vide Report of the 
 Portsmouth Meeting, 1911).
 
 CHAPTER XXVII 
 
 Artificial selection Continuous and single selection The mutation theory 
 of the origin of species Mutual adaptation Unit characters 
 Isolation Physiological selection Non-adaptive characters The 
 evolution of man. 
 
 IT has long been recognized that much light may be thrown 
 upon the problem of the origin of species by the careful 
 study of the methods which mankind has adopted for the 
 improvement of the various races of cultivated plants and 
 domesticated animals. Many such races have been so greatly 
 modified that, did they occur in what is commonly called a state 
 of nature, we should be obliged to regard them as distinct species. 
 The history of some of these is lost in antiquity and we have no 
 positive knowledge of the methods by which the improvement of 
 wild species was first effected. We may assume, however, with 
 some degree of confidence, that the earliest breeders and 
 cultivators would select for cultivation and propagation those 
 individuals which offered them the most valuable qualities, and 
 that they would reject such as exhibited marked signs of 
 inferiority. This process, repeated from generation to generation 
 through thousands of years, and aided in each generation by the 
 direct effects of cultivation, could not fail to bring about con- 
 spicuous results. For an account of what has been effected 
 in this manner the student should consult Charles Darwin's 
 classical work on the " Variation of Animals and Plants under 
 Domestication." 
 
 The almost unconscious efforts of our ancestors have given 
 place in modern times to deliberate and systematic attempts to 
 discover the principles upon which the improvement of cultivated 
 races, both of plants and animals, should be based. 
 
 Perhaps no species of plants have been more improved by 
 man than the various cereals upon which he relies so largely for 
 his food supply. Professor de Vries, in his interesting book on 
 " Plant-Breeding," 1 describes how such improvement has been 
 
 1 Kegan Paul, Trench, Triibner & Co., London, 1907.
 
 CONTINUOUS SELECTION 411 
 
 effected in recent times. In the first place much care and 
 thought have been devoted to carrying out experiments in 
 accordance with the principle of continuous selection : 
 
 " The general custom [in Germany] was to start such experi- 
 ments from the best local or improved varieties by an initial choice 
 of a certain number of typical heads. Such a group of selected 
 plants was called the elite, and this elite had to be ameliorated 
 according to the prevailing demands or even simply in accordance 
 with some ideal model. Year after year, the best ears of the 
 elite group were chosen for the continuance of the strain or 
 family, and slowly, but gradually, its qualities were seen to 
 improve in the desired direction. After some years, such a 
 family might become decidedly better than the variety from 
 which it had been derived. Then its yearly harvest would be 
 divided into two parts, after having been sufficiently purified by 
 the rejection of accidental ears of minor worth. The best ears 
 were carefully sought out and laid aside for the continuance of 
 the elite strain, but the remainder were sown on a distant field 
 in order to be multiplied as fast as possible. By this means, after 
 a multiplication during two or three generations, its product 
 could be used as seed grain for the farm or sold to others for 
 the same purpose. Each year the elite would, of course, give a 
 new and better harvest which could be multiplied and sold in the 
 same manner." 1 
 
 By this method improvement may undoubtedly be effected, 
 but the selection has to be constantly repeated, otherwise the 
 improved strain rapidly deteriorates again. Indeed it may be 
 questioned whether it is possible in this way to effect any per- 
 manent improvement, at any rate in the case of cereals. One 
 reason for this appears to be that we are dealing all the time, 
 not with a single pure race, but with a mixture of distinct races. 
 We must also remember that many of the characters which it is 
 desired to perpetuate and increase may be the direct result of the 
 cultural methods employed, and, as we have already seen, we 
 cannot expect such causes to produce visibly heritable effects in 
 the course of a few generations, whatever they might do in the 
 long run. 
 
 We have had occasion to point out in an earlier chapter that, 
 according to Professor de Vries, new species arise, in a state of 
 nature, not by the accumulation in particular directions of small, 
 fluctuating variations, but by the sudden appearance of those 
 
 1 De Vries, op. cit., p. 58.
 
 412 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 more conspicuous variations known as mutations. De Vries 
 points out that many of the so-called Linnean species, 
 such as Draba verna, are in reality made up of a large 
 number of " elementary species " which have arisen in this 
 manner, and certain results which have been obtained in experi- 
 ments upon the improvement of cereals appear at first sight to 
 afford considerable support to these views. 
 
 It has long been known that an ordinary field of wheat con- 
 tains a larger or smaller number of " types," " mutations," or 
 " elementary species," which can be recognized by the experienced 
 eye, and it has been shown that if a single plant of one of these 
 types be isolated it will produce offspring like itself and continue 
 to breed true for an indefinite number of generations. Of course 
 it is necessary that there should be no crossing with other 
 types, but this is easily avoided, for, although accidental 
 crosses may occur, the cereals, with the exception of rye, are 
 usually self-fertilizing. Upon this knowledge is founded the 
 method of single as opposed to continuous selection, a single 
 selection of a suitable type being enough to establish the desired 
 strain. 
 
 One of the first to make use of the method of single selections 
 was Patrick Shirreff : 
 
 " His first discovery was made in the year 1819. He observed 
 a plant of wheat which surpassed its neighbors by its high 
 degree of branching. It yielded 63 ears with about 2500 
 kernels. He saved the seeds, sowed them on a separate field 
 and at considerable distances apart so as to induce in all the 
 plants the same rich branching. He contrived to multiply it so 
 rapidly that it took only two generations to get seed enough to 
 bring it advantageously into the trade. He gave it the name of 
 Mungoswell's wheat, and it soon became one of the most profit- 
 able varieties of Scotland. It has found its way into England 
 and into France, where it is still considered one of the best sorts 
 of wheat." l 
 
 The same method has been subjected to severe tests and 
 placed upon a thoroughly scientific footing at the Swedish 
 Agricultural Station of Svalof. 
 
 We have seen, in Chapter XIV, that hybridization may 
 occasionally give rise to permanent races or strains exhibiting 
 new combinations of characters, and that this takes place in 
 
 1 De Vries, op. cit., pp. 34, 35.
 
 HYBRIDIZATION AND MUTATION 413 
 
 accordance with Mendelian principles. It cannot be doubted that 
 hybridization occurs occasionally in cultivated cereals, and 
 Professor de Vries is of opinion that the occurrence of the 
 different types or mutations is often the result of hybridization 
 at various periods in the history of the race : 
 
 "Experience, however, shows that in ordinary fields almost 
 all possible combinations may be met with, and it is to be pre- 
 sumed that at least the greater number of them are due to 
 crosses in previous and, perhaps, in long-forgotten years." x 
 
 Some of these combinations, as might be expected, are not 
 stable but split up into a number of varieties in the next 
 generation, but also : 
 
 " We may conclude that some, and perhaps many, of the types 
 which may be selected and isolated in the fields and which prove 
 to be constant races must be of hybrid origin." 2 
 
 De Vries maintains that in the case of the cereals so many of 
 these " types " now lie ready to our hand that all we have to do 
 is to pick out those which we require and cultivate them in 
 isolation from each other and from the remainder. 3 Professor 
 Biffen has shown, however, as we have already pointed out in 
 Chapter XIV, that it is possible by intelligent artificial hybridi- 
 zation to produce yet other stable combinations or hybrids 
 which may surpass in value any which have accidentally arisen 
 in the past. 
 
 It appears, then, that many at any rate of the so-called 
 mutations or types amongst cereals are due to hybridization. 
 How far this applies to mutations in general it is quite impossible 
 to decide. That it is not always so, however, appears to be proved 
 by the occurrence of such mutations or sports as hexadactylism, 
 which are known to be inherited and which cannot have 
 arisen in this way. 
 
 De Vries tells us in another work that : 
 
 " According to the theory of mutation species have not arisen 
 gradually as the result of selection operating for hundreds, or 
 thousands, of years but discontinuously by sudden, however 
 small, changes. In contradistinction to fluctuating variations 
 
 1 De Vries, i>2>. cit., p. 80. 
 
 2 Ibid., loc. cit. 
 
 3 1 bid., p. 50. It seems strange, considering that de Vries admits that many 
 at any rate of the "types' 1 have probably arisen by hybridization in the first 
 instance, that he should attribute so little value to artificial hybridization as a 
 means of improvement.
 
 414 OUTLINES OF E VOLUTION AEY BIOLOGY 
 
 which are merely of a plus or minus character the changes which 
 we call mutations are given off in almost every manner of new 
 direction. They only appear from time to time, their periodicity 
 being probably due to perfectly definite but hitherto undiscovered 
 causes. 
 
 " The theory of the inheritance of acquired characters comes 
 under the heading of fluctuations. Acquired characters have 
 nothing to do with the origin of species. Nor can the theory of 
 descent be applied to the solution of social problems." * 
 
 There is here no suggestion of a hybrid origin for the 
 mutations in question. If, however, as seems probable, a large 
 proportion of so-called mutations are really the result of hybridi- 
 zation, and if, as we showed in Chapter XIV, hybrids tend to be 
 automatically eliminated in a state of nature though of course 
 there is nothing to prevent a constant hybrid from being pre- 
 served if it happens to possess characters peculiarly favourable 
 to its own existence it does not seem likely that such mutations 
 can have played any very great part in organic evolution. In 
 any case there is no need to suppose that the theories of muta- 
 tion and natural selection are mutually exclusive, for, however 
 new characters may arise, they must be subject to the action of 
 natural selection in the struggle for existence. 
 
 Professor de Vries' objection to small, fluctuating variations as 
 the material upon which natural selection operates in the 
 modification of species appears to be based upon the view that 
 such characters are acquired in the lifetime of the individual 
 and cannot be inherited. If, however, we admit that a soma- 
 togenic character may, in the course of many generations 
 and under the continued influence of the same conditions which 
 originally called it forth, become converted into a blastogenic 
 character, this difficulty entirely disappears. 
 
 It is extremely hard to believe that mutations, which, apart 
 from the occurrence of hybridization, seem to occur very rarely 
 and at long intervals, can have afforded sufficient opportunity 
 for the production of all the marvellous adaptations which exist 
 in nature. Take, for example, the mutual adaptations which we 
 see between the length of the nectary in certain flowers and the 
 length of the proboscis in the insects which fertilize them. We 
 cannot suppose that either the elongated nectary or the elongated 
 proboscis arose by sudden mutations, for unless these mutations 
 
 i " The Mutation Theory." English Trans., Vol. I., p. 213.
 
 MUTATION AND ADAPTATION 415 
 
 took place simultaneously in flower and insect, and in the same 
 locality, and in a sufficient number of each, a supposition which, 
 to say the least of it, is wildly improbable, the delicate correla- 
 tion between the two would be thrown out of gear and the 
 individuals exhibiting the mutations .would be eliminated by 
 natural selection because they were no longer sufficiently well 
 adapted to the very special conditions of their environment. 
 
 We can only believe that the increase in length of nectary and 
 proboscis took place so slowly that their reciprocal adaptation 
 was never upset. A slight increase in the length of the nectary 
 obliged the insect to poke further into the flower for the honey 
 and thus increased the chances of fertilization. A slight increase 
 in the length of the proboscis enabled the insect to get more 
 honey and thus gave it a better chance of existence. After 
 perhaps many thousands of generations, under the influence of 
 natural selection, combined in the case of the insect with the 
 effects of use and disuse, the present enormous lengths of 
 proboscis and nectary have been attained, and no one doubts the 
 fact that they have become blastogenic characters. 
 
 The same argument applies to all accurate adaptations to 
 special conditions of the environment. How can we explain the 
 facts of protective resemblance and mimicry except as due to the 
 accumulation under the influence of natural selection of what 
 Charles Darwin called slow successive variations ? How, again, 
 can the theory of mutation be applied to such cases as that of 
 the flightless birds on oceanic islands? Who can doubt that 
 the reduction of the wings and the loss of the power of flight has 
 been brought about slowly and gradually as a result of disuse ? 
 and at the same time who would venture to argue that the 
 flightless birds are not specifically distinct from their actively 
 flying ancestors ? If it be urged that mutations may be so small 
 as to be almost imperceptible, then we must ask how do they 
 differ from fluctuating variations ? and if we are told that they 
 occur very rarely and do not fluctuate, that very answer is 
 sufficient to show that they can hardly have given rise to adaptive 
 modifications. 
 
 De Vries' theory of the origin of species by mutation is 
 supposed to harmonize with the Mendelian principle of unit 
 characters, but we have to ask ourselves, how do new unit 
 characters arise in the first instance? It seems at least as 
 probable that they arise by the gradual accumulation of slight
 
 416 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 fluctuating variations under the control of natural selection as 
 that they originate in any other way that can be suggested in 
 the present state of our knowledge ; and even if these variations 
 are at first purely somatogenic, we may suppose that in the 
 course of many generations they gradually exert a cumulative 
 influence upon the germ plasm until "the latter, so to speak, 
 topples over into some new position of equilibrium and a new 
 unit character arises. We cannot, however, now add to what we 
 have already said on this subject when dealing with the theory 
 of heredity. 
 
 One of the most important factors in bringing about divergent 
 evolution is undoubtedly isolation. However a new character 
 may have arisen it is liable to be swamped by the crossing of 
 the individuals which possess it with others which do not possess 
 it unless by some means or other the two groups are prevented 
 from interbreeding. We cannot fail to see the importance of this 
 principle when we study the fauna and flora of remote islands, 
 and their relationships to those of the nearest continental areas 
 or of other islands. 
 
 Take, for example, the case of the Chatham Islands, which, as we 
 have already seen, lie some 400 miles to the east of New Zealand. 
 There can be very little doubt that these islands were formerly 
 connected with the New Zealand mainland, and this connection 
 probably continued into Pleistocene times, when a great 
 depression took place which caused the two to be separated by 
 a wide tract of ocean. All who have studied the question are 
 agreed that the fauna and flora of the Chatham Islands are 
 simply isolated detachments of those of New Zealand. Many 
 species, especially of the plants, are identical with New Zealand 
 species, but many others, though closely related to those of New 
 Zealand, are considered by systematists to be specifically distinct, 
 and they occur nowhere else in the world. 
 
 We have here an excellent illustration of the effects of 
 geographical isolation, which we shall be able to appreciate 
 better, perhaps, if we confine our attention to a few typical cases. 
 The common New Zealand wood pigeon, Carpophaga (Hemiphaya) 
 novce-zealandids, is represented on the Chathams by a species 
 known as Carpophaga (Hemiphaga) chathamensis, differing but 
 slightly from its new genus congener, and the New Zealand
 
 GEOGRAPHICAL ISOLATION 417 
 
 lizard, Lygosoma moco, is represented on Pitt Island (one of the 
 Chathams) by a very similar form described by Mr. Boulenger 
 under the name Lygosoma dcndi/i. The remarkable New Zealand 
 lance-woods (Pseudopanax crassifolium and P. ferox) are repre- 
 sented on Chatham Island by the closely related Pseudopanax 
 chathamicum, and the Chatham Island ribbon- wood also differs 
 slightly from the common New Zealand species (Platjianthus 
 bctidinus). 
 
 The explanation of these differences is that the two portions 
 into which each of the species mentioned became divided when 
 the Chatham Islands were separated from New Zealand have 
 diverged from one another and followed somewhat different lines 
 of evolution. Owing, perhaps, to slightly different conditions of 
 the environment, or to other causes which it is impossible to 
 specify, one or both has become modified to a greater or less 
 extent in its own particular direction and, owing to the 
 geographical isolation, there has been no interbreeding between 
 the two sections to keep them both in the same average condition. 
 
 The principle of isolation fully explains why the fauna and flora 
 of oceanic islands in general are made up almost entirely of 
 peculiar species found nowhere else in the world. The ancestors 
 of these species were originally derived from some very distant, 
 probably continental area, and their descendants have had few 
 if any opportunities of interbreeding with the parent species, 
 from which they have gradually diverged further and further 
 under their new conditions of life. 
 
 Certain writers, such as Mr. Gulick and Dr. Romanes, have 
 maintained that the mere separation of a species into two or 
 more sections which are prevented from interbreeding would 
 suffice to bring about divergent evolution, irrespective of whether 
 or not the separate sections were exposed to different environ- 
 mental conditions. It would probably be impossible to divide 
 a species into two sections whose average qualities are identical, 
 and : 
 
 "No matter how infmitesimally small the difference maybe 
 between the average qualities of an isolated section of a species 
 compared with the average qualities of the rest of that species, if 
 the isolation continues sufficiently long, differentiation of specific 
 type is necessarily bound to ensue." 1 
 
 i Romanes, " Darwin and after Darwin," Vol. Ill, " Isolation and Physiological 
 Selection," p. 13. 
 
 B. BE
 
 418 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 If isolation is necessary for the establishment of new species 
 by divergent evolution, why, it may be asked, do we find closely 
 related species, which we must suppose to be descended from 
 common ancestors at no very distant period, actually inhabiting 
 the same areas at the present time ? The answer is that there 
 are other means by which groups of organisms can be prevented 
 from interbreeding besides geographical isolation. 
 
 It used to be supposed that one of the best tests of the specific 
 distinctness of any two forms was their incapacity for breeding 
 together and producing fertile offspring. 1 The mule, it is true, 
 is the offspring of parents belonging to two distinct species, the 
 ass and the horse, but the mule is almost always sterile, and most 
 well characterized species 2 are incapable of breeding together at 
 all. This fact has been regarded by many people as a most serious 
 difficulty in the way of comparing the origin of species by natural 
 selection with the results produced amongst domesticated plants 
 and animals by artificial selection, for the products of artificial 
 selection, however much they may differ from one another, if 
 they have been derived from the same parent species will 
 remain capable of breeding together with perfect fertility. 
 
 The solution of this difficulty is to be found in the theory of 
 " Physiological Selection," which we owe to Mr. Gulick, Dr. 
 Eomanes and others. These writers point out that amongst 
 the endless variations to which plants and animals are subject 
 will be variations in the reproductive system, by which certain 
 individuals will be rendered infertile when crossed with others 
 of the same species, while remaining fertile with individuals 
 which have varied in the same manner as themselves. In this way 
 a species may be as effectively divided into two sections as by any 
 geographical barrier, and under these circumstances divergent 
 evolution may be expected to take place. According to this view 
 the mutual sterility which, to a greater or less extent, undoubtedly 
 does characterize distinct species in a state of nature, is the 
 cause and not the result of their distinctness, and cannot be 
 regarded as a reason for supposing that there is any essential 
 difference between the processes of artificial and natural 
 selection. In artificial selection it is merely another kind of 
 
 1 Although Lamarck pointed out a century ago that this is Really no criterion of 
 specific distinction. The idea that it is so is clearly expressed by Buffon (" Histoire 
 Naturelle," Tom. VI, 1756, p. 16). 
 
 2 At any rate amongst the higher animals.
 
 NON-ADAPTIVE CHAKACTERS 419 
 
 isolation that has been employed to prevent the swamping 
 effects of intercrossing ; but it is an isolation that may be 
 broken through at any moment, and if all the different varieties 
 of some domestic plant or animal were turned loose to struggle 
 for existence and interbreed with one another and with the 
 parent species in a state of nature, they would probably in most 
 cases very soon cease to have any separate existence. 
 
 The theory of natural selection, combined with that of the 
 gradual inheritance of the effects of use and disuse and of other 
 modifications brought about by the long-continued influence of 
 the environment, affords a satisfactory explanation of the evolu- 
 tion of adaptive characters. Many if not all organisms, how- 
 ever, exhibit characters to which we can assign no adaptive value, 
 which do not seem to be of any particular use to the organism in 
 the struggle for existence, or which apparently might, so far as 
 utility is concerned, be equally well replaced by any one of a 
 number of alternative characters. 
 
 Amongst the microscopic Protozoa species are frequently 
 distinguished from one another by minute differences in the form 
 or ornamentation of the skeleton (compare Figs. 3 and 4). 
 Different species of the genus Lagena (Fig. 4), 1 amongst the 
 Foraminifera, for example, exhibit different sculptured patterns 
 upon their flask-shaped calcareous shells. Are we to suppose 
 that it is of any consequence to the gelatinous, Amoeba-like 
 inhabitant of the flask whether its shell be ornamented in one 
 way rather than in another ? Does one pattern help a uni- 
 cellular foraminiferan or radiolarian more Lhan another in the 
 struggle for existence ? The same argument applies to the 
 extremely minute siliceous flesh-spicules or microscleres 
 which occur scattered without order through the ground sub- 
 stance of many sponges, and the form of which is regarded by 
 those who have studied the question as by far the most reliable 
 guide, not only in the recognition of species, but also in the 
 grouping of these species in genera and families. Take, for 
 example, the wonderful chelae (Fig. 187), characteristic of the 
 family Desmacidonidse. Different genera and species are dis- 
 tinguished by differences in the size, shape and number of 
 
 1 The two figures below the centre figure and two in the bottom right hand 
 corner represent four species of Lagena. 
 
 E B 2
 
 420 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 the teeth of these microscopic and apparently useless organs 
 useless at any rate so far as their generic and specific characters 
 are concerned, for what can it matter to the sponge whether the 
 number of the teeth be three or more or less, or whether the teeth 
 at the two ends of the spicule be equal or unequal ? Yet, in the 
 course of evolution, such characters as these have become more 
 
 FlG. 187. Siliceous Spicules (Chela)) of Sponges. (After Ridley and 
 Dendy, in " Challenger " Report.) 
 
 A. A', front and side views of chela of Esperella lapidiformis, x 360. 
 
 B. B', front and side views of chela of Esperiopsis pidchella, x 284. 
 
 C. C', front and side views of chela of Cladorhiza (?) tridentata, x 360. 
 
 or less fixed and constant and they are evidently handed down 
 from generation to generation by the ordinary process of heredity. 1 
 We pointed out in an earlier chapter that the external form 
 of the entire sponge is, in some cases at any rate, explicable 
 as an adaptation to peculiar conditions of the environment. 
 We saw this is the case of the curious " Crinorhiza " form 
 (Fig. 168) which prevents the sponge from sinking into the soft 
 mud or ooze on which it rests. Let us glance for a moment, 
 however, at another deep sea sponge, Esperiopsis cltallaujai, 
 dredged up by the "Challenger" Expedition from a depth of 
 
 1 The reader should refer back to Fig. 88 for other forms of! sponge spicules.
 
 NON-ADAPTIYE CHARACTERS 
 
 421 
 
 825 fathoms in the Malay Archipelago. In some respects this is 
 the most remarkable sponge that has ever been discovered. Its 
 form (Fig. 188) is absolutely unique and resembles rather that of 
 some graceful plant than those of other sponges. It belongs to a 
 group of sponges whose members occur 
 mostly in much shallower water and are by 
 no means distinguished by beauty or sym- 
 metry of shape. In spiculation, moreover, 
 and other minute anatomical features, it 
 exhibits no striking peculiarities ; indeed, so 
 closely does it agree with more ordinary 
 species of Esperiopsis that it has not as 
 yet been considered necessary to separate it 
 generically. How then can we account for 
 this wonderful form? Can we say that it 
 is an adaptation to any special conditions of 
 the environment? It hardly seems likely 
 that this is the case, for we know that the 
 relatives of this sponge get on well enough 
 with all sorts of other forms, for the most 
 part more or less irregular and ill-defined, 
 and we know of nothing in the conditions 
 under which it lives to make such a unique and 
 beautiful form especially advantageous. The 
 " Challenger" obtained no less than thirteen 
 specimens of this sponge at the same place, 
 and the form appears to be quite constant. 
 No doubt the undisturbed condition of the 
 sea at great depths favours symmetry of 
 growth in sessile organisms like sponges, 
 but why this particular and absolutely unique 
 shape, so different from anything else that 
 has ever been met with ? 
 
 These are questions which we cannot 
 answer, and it must suffice to point out that such cases can hardly 
 be explained by the theory of natural selection. Many factors must 
 combine in determining the course of evolution of any particular 
 organism, and some of the characters which result from their 
 interaction may perhaps have no more direct relation to the 
 necessities of the organism in the struggle for existence than has 
 the colour of a pebble to its continued existence on the sea-shore. 
 
 FIG. 188. Esperiopsis 
 challenyeri, X |. 
 (After Ridley and 
 Dendy, in " Chal- 
 lenger'' Report.)
 
 422 OUTLINES OF EVOLUTIONAEY BIOLOGY 
 
 It may even be questioned whether any large proportion of 
 specific characters can have arisen through the action of natural 
 selection. The characters by which we are accustomed to sepa- 
 rate one species from another such as minute differences in 
 size, shape and colour are usually so slight that we are hardly 
 justified in attributing to them any adaptive value. They may, 
 however, form the starting points from which, under the influ- 
 ence of natural selection and use and disuse, adaptive characters 
 may subsequently arise. 
 
 The application of the principles of organic evolution to the 
 problem of the origin and progress of the human race cannot be 
 adequately dealt witli in the present volume, while at the same 
 time we cannot altogether ignore it, for not only is it in man 
 himself that we find the most remarkable illustration of what 
 has been accomplished by evolution, but the future progress of 
 mankind must depend in large measure upon the correct 
 understanding of the principles in question. 
 
 Buffon, more than a century ago, pointed out the close resem- 
 blance in anatomical structure between man and the higher 
 apes, and it is clear that both he and Lamarck were only pre- 
 vented by religious scruples from definitely maintaining the 
 origin of the human species from ape-like ancestors, a view 
 which at the present time is universally accepted amongst 
 scientific men. This reluctance to admit the obviously close 
 relationship of the human species with the apes was one of the 
 evil results of the intellectual dishonesty and obscurantism of 
 the middle ages. If we go back to the days of Carthage, we 
 find that the explorer Han no did not hesitate to speak of the 
 "gorillas !>1 which he met with in Africa as hairy men and 
 women of the woods, and although this was doubtless going too 
 far in the opposite direction, it shows not only that he recognized 
 the relationship but also that he approached the question with a 
 mind entirely free from prejudice. 
 
 Man is one of the latest products of organic evolution, and his 
 appearance upon the scene possibly does not date further back than 
 Pliocene times. It is said that flint flakes of human workman- 
 ship have been discovered in early Pliocene deposits of Burmah, 
 
 1 Probably really chimpanzees.
 
 ANTIQUITY OF MAX 423 
 
 but perhaps the earliest actually human fossil so far known is 
 a lower jaw of very massive form which was found in a deposit 
 of late Pliocene or early Pleistocene age near Heidelberg, and 
 described by Schcetensack in 1908 under the name Homo 
 Jicidelbergensis. This name implies that in the opinion of its 
 author the fossil man of Heidelberg was generically, but not 
 specifically, identical with the human beings which now exist. 
 The question of the distinction of species in the genus Homo, 
 however, is, as in most other genera, a very difficult one, and 
 opinions are divided as to whether only one or several species 
 should be recognized amongst the existing races of mankind. 
 
 So close is the anatomical agreement between the genus 
 Homo and the higher apes that there is little room for con- 
 necting links between them, the difficulty being rather to find 
 any definite characters by which they can be separated than to 
 discover reasons for bringing them together. Nevertheless, the 
 gap, small as it is, has been filled by the discovery, by Dr. 
 Dubois in 1894, of the remains of a semi-human, ape-like 
 creature, to wliich the name Pithecanthropus crectus has been 
 given. These remains were found in Pliocene strata in the 
 island of Java and consist of a portion of a cranium, a thigh 
 bone and two molar teeth. 1 
 
 In deposits of Pleistocene age undoubtedly human remains 
 become fairly abundant, and are found associated with the bones 
 of other mammals, of which many, such as the mammoth, the 
 cave bear and the woolly rhinoceros, are now extinct. 
 
 The chief factors which contributed towards the gradual 
 transformation of ape-like into human creatures were doubtless 
 the same as those which have operated in the evolution of other 
 branches of the animal kingdom, namely, the efforts which the 
 ancestral forms were obliged to make in order to maintain them- 
 selves in the struggle for existence and the natural selection of 
 favourable variations. In no group of the animal kingdom do we 
 see better illustrated the importance of Lamarck's principle of 
 the effect of changed habits upon bodily organization. 2 
 
 The anthropoid ancestors of man were undoubtedly, like 
 
 1 Some authorities regard these remains as truly human. The difference of 
 opinion is itself very instructive. 
 
 2 Darwin, notwithstanding what he says in the sixth edition of the " Origin of 
 Species " about the effects of use and disuse (quoted on p. 392), denies, in the 
 " Descent of Man " (Ed. 2, p. 619), that man has risen through his own exertions. 
 I can see no reason for such a pessimistic view.
 
 424 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 existing Simiidse, arboreal in habit. Their limbs, whose primi- 
 tive pentadactyl structure indicates their origin from some 
 little-specialized mammalian type, had ceased to be used exclu- 
 sively as organs of locomotion on the ground and become adapted 
 for climbing trees. Hence the opposable toe and thumb, which 
 enabled their possessor to obtain a firm grasp of the branches. 
 In the existing apes hand and foot are very similar, and both, 
 as regards function, partake as much of the nature of hand as of 
 foot, whence the name " Quadrumana " applied to the group by 
 the older naturalists. In many of the lower apes or monkeys a 
 long prehensile tail, which can be twisted round the branches, is 
 of great assistance to the animals in their arboreal habits, but 
 in the higher forms, such as the chimpanzee, the gorilla and 
 the orang utan, the tail has already disappeared. 
 
 These tailless forms are still mainly arboreal, but when they 
 have occasion to come to the ground they assume a semi-erect 
 posture in locomotion. In walking they usually put their hands 
 to the ground, but generally resting upon the backs of the fingers, 
 instead of upon the palms as in the lower apes. 1 Hence in these 
 large old-world apes the hands and feet are more completely 
 differentiated from one another. 
 
 The next step in the evolution of man was probably the 
 gradual abandoning of arboreal habits and the assumption of a 
 more completely erect attitude during locomotion. This appears 
 to have been the real starting point of his human career. It 
 was this change of habit which led to those structural modifica- 
 tions required to balance the body properly in its new position, 
 and to the further differentiation between hand and foot, the 
 latter being used to the complete exclusion of the former as an 
 organ of locomotion and at the same time losing its prehensile 
 character, so that one of the chief distinguishing features between 
 man and the higher apes is that in man the great toe has ceased 
 to be opposable. 
 
 In this way the hand was completely set at liberty for the 
 purposes of a prehensile organ. It was, however, no longer used, 
 as in the apes, chiefly for laying hold of branches in climbing 
 and for conveying food to the mouth, but more for grasping 
 loose objects and converting them into weapons or tools for 
 different purposes. Hanno observed that the anthropoid apes 
 which he met with in Africa defended themselves with stones, 
 
 1 Vide Beddard, " Mammalia " (Cambridge Natural History, Vol. X), p. 570.
 
 EVOLUTION OF MAN 425 
 
 so that we can hardly say that the use of tools or weapons is an 
 exclusively human attribute ; hut the hand of man is undoubtedly 
 one of his most characteristic features, and by its aid man has 
 been able to make for himself an unlimited number of what are 
 really additional organs, derived not from his own body but from 
 his environment. 
 
 To the experience gained by the exercise of the hands in so 
 many different ways must also be attributed in large measure 
 the extraordinary mental and moral development which,more than 
 anything else, separates mankind from the apes. The constant 
 exploitation of the environment stimulated and exercised the brain, 
 which in turn suggested methods of employing the hands and 
 the tools which they had constructed to ever greater advantage; 
 and thus both hand and brain progressed until they attained their 
 present wonderful state of efficiency. 
 
 The development of the brain has, however, long since taken 
 the lead in human evolution and, considering the immense 
 difference in intellectual capacity, it is surprising that there 
 should be so little structural difference between the brain of 
 man and that of the higher apes. As Huxley has pointed 
 out : 
 
 " It is only in minor characters, such as the greater excavation 
 of the anterior lobes, the constant presence of fissures usually 
 absent in man, and the different disposition and proportions of 
 some convolutions, that the Chimpanzee's or the Orang's brain 
 can be structurally distinguished from Man's." 1 
 
 Intellectual capacity appears, however, to depend mainly upon 
 the size of the cerebral hemispheres, which is doubtless correlated 
 with the number of nerve cells present, and in this respect the 
 human brain is far in advance of that of any ape. 
 
 One of the most important characters which differentiate man- 
 kind from the apes is the faculty of articulate speech, but even this 
 undoubtedly had its beginnings in the inarticulate sounds made 
 by ape-like ancestors, either as spontaneous expressions of their 
 emotions or as a means of communicating more or less definite 
 ideas to one another. The development of speech provided a 
 new means for the transmission of experiences from one genera- 
 tion to the next, and as a consequence knowledge began to 
 accumulate in the minds of the human race. When oral tradition 
 
 1 Huxley, '-Man's Place in Nature," p. 140 (Coll. < ml K-ays. V..1. VII).
 
 426 OUTLINES OF EVOLUTIONARY BIOLOGY 
 
 gave place to the establishment of written records, and methods 
 were invented for the indefinite multiplication of these, the 
 accumulation of knowledge took place at a much more rapid rate 
 and it became possible for every human being, at any rate in 
 civilized communities, to benefit from the experience of all his 
 fellow men. The acceleration of intellectual and moral progress 
 which has been brought about in this way has led to results 
 which may well have deluded man into the belief that he is the 
 centre of the universe and that between himself and the lower 
 animals there is a great gulf fixed. 
 
 Man has indeed acquired a degree of control over his environ- 
 ment and over his own destiny which distinguishes him from any 
 of the lower animals, but at the same time the conditions of his 
 life have become far more complex and the young, at any rate in 
 civilized communities, have to go through a long course of 
 education before they are fit to enter upon the struggle for 
 existence on their own account. Amongst the lower animals all, 
 or almost all, the faculties necessary for existence are directly 
 inherited from the parents, incorporated in the organism itself, 
 but man inherits in this way only a relatively small proportion 
 of the powers which he requires to carry on his life. The greater 
 part of human experience is of too recent origin to have become 
 heritable ; it has to be acquired afresh by education in every 
 generation, and in this respect is strikingly contrasted with the 
 instincts of the lower animals. 
 
 The immense advance which civilized man has made since he 
 parted company with his ape-like progenitors is shown, not only 
 by the fact that he has already to a large extent subjugated the 
 remainder of the organic world and directed the forces of inanimate 
 nature into new channels to serve his own purposes, but also by 
 the intelligent forethought which he exercises for the future 
 welfare of his own race. At the present time this forethought 
 is being exercised in new directions, and a determined effort is 
 being made to apply our knowledge of the principles of organic 
 evolution to the furtherance of human progress. In spite 
 of many differences of opinion, such as that which still 
 prevails with regard to the relative importance of breeding 
 and education, we have undoubtedly arrived already at many 
 results which are of vital significance in this connection, 
 and the intelligent application of scientific principles must 
 here, as always, lead to further progress. We cannot, however,
 
 HUMAN PROGRESS 427 
 
 control the future of the human race until we have familiarized 
 ourselves with the past, and learned to recognize the part 
 played by the numerous different factors which have been con- 
 cerned in the evolution, not only of mankind, but of the whole 
 organic world. 
 
 It must always be remembered that the problem before us is 
 one of extreme complexity, and that we cannot afford to neglect 
 any of the factors involved. Above all, we must avoid dogma- 
 tizing on an insufficient basis of fact. If, for example, the very 
 modern doctrine of the non -inheritance of acquired characters is 
 allowed to influence the actions of men and women, and if, after 
 all, this doctrine should prove to be erroneous, as seems highly 
 probable at the present time, the attempt to apply biological 
 principles to the welfare of humanity may well end in disaster. 
 The penalty which each generation has to pay, in regard to 
 bodily and mental organization, for the mistakes and misfortunes 
 of its ancestors, may, in most cases, be a very small one ; but if 
 there is any penalty at all, and if the mistakes are continued 
 from generation to generation, it will surely be a cumulative one. 
 
 In dealing with problems of this kind a rational conservatism, 
 with a mind always open to conviction, seems the only safe 
 attitude to adopt.
 
 INDEX 
 
 A. 
 
 ABBREVIATION of ontogeny, 273. 
 abiogenesis, 214 216. 
 Acacia pycnantha, seedling, Fig. 132. 
 acacias, life-history, 280. 
 Acanthodes, 291, Fig. 138. 
 accessory chromosomes, 135. 
 accessory idioplasm, 167. 
 accretion, 20. 
 achromatic figure, 74. 
 acquired characters and human 
 progress, 427. 
 
 ,, ,, Bordageon,182. 
 
 ,, ,, Brewer on, 178, 
 
 180. 
 , , , , Brown - Sequard 
 
 on, 180. 
 ,, ,, Buffon on, 367, 
 
 368. 
 ,, ,, Charles Darwin 
 
 on, 180, 393. 
 ,, ,, de Vries on, 
 
 414. 
 ,, ,, Eigenmann on, 
 
 183. 
 
 ,, ,, Erasmus Dar- 
 
 win on, 372. 
 
 Henslowon,183 
 
 ,, Herbert Spencer 
 
 on, 185. 
 
 ,, Hilgardon, 178. 
 
 ,, ,, inheritance of, 
 
 165, 169, 176 
 193, 404. 
 
 ,, ,, Lamarck on, 
 
 379, 380, 382. 
 
 ,, ,, meaning of 
 
 term, 156, 176, 
 177. 
 
 ,, ,, Sumner on, 182. 
 
 ,, ,, Weismann on, 
 
 176, 179. 
 
 actinula larva, 122. 
 activity of male, 84, 85. 
 adaptation, 235, 365. 
 
 and design, 212. 
 ,, ,, fluctuating varia- 
 
 tion, 414, 415. 
 
 adaptation and mutation, 414, 415. 
 ,, ,, natural selection, 
 
 396, 404, 405. 
 ,, average, 184. 
 ,, co-operation of factors in 
 
 bringing about, 405. 
 ,, embryonic, 267 
 ,, Erasmus Darwin on, 
 
 372, 373. 
 ,, in animals, 334 349. 
 
 individual, 184. 
 ,, in inorganic world, 395. 
 ,, in leaves of acacias, 280. 
 in plants, 350363. 
 mutual, 414, 415. 
 ,, origin of, 183. 
 ,, resulting from survival 
 
 of fittest, 391. 
 ,, Robert Chambers on, 
 
 383, 384. 
 
 .adipose tissue, 56, Fig. 19. 
 aesthetic development of man, in- 
 fluenced by insects, 364. 
 aesthetic sense, in courtship, 349. 
 affinities, natural, 228, 229. 
 after-effects, 191. 
 age of habitable earth, 285 287. 
 age of ocean, 287. 
 aggregate species, 224. 
 aggressive resemblance, 337. 
 Agnatha, 290. 
 Agnostns princeps, Fig. 134. 
 air-bladders of Fucus, 99. 
 air-breathing vertebrates, origin, 
 
 256. 
 
 ala spuria, 305. 
 albumen, 23. 
 alcoholism, 157. 
 alimentary canal, 124. 
 allantois, 270. 
 allelomorphs, 200, 206. 
 aloes, 350. 
 
 Alpine plants, 181,262,350. 
 Alps, Arctic vegetation on, 333. 
 alternation of generations, 101. 
 in aphides, 144. 
 in ccelenterates, 121. 
 in ferns, 101106. 
 in flowering plants, 106 112.
 
 430 
 
 INDEX 
 
 Amauris niavius, 346, Fig. 177. 
 Amblyopsis, 183. 
 
 ambulatory appendages of arthro- 
 pods, 246, 247. 
 
 ambulatory legs of vertebrates, 235. 
 Ameghino, 253. 
 amitotic nuclear division, 80, Fig. 36. 
 
 in Copromonas, 83. 
 amnion, 270. 
 Amooba, 1221, Fig. 2. 
 ,, mitosis in, 79. 
 ,, sensitive to stimuli, 188. 
 Amphibia, geological range, 284. 
 
 limbs, 235. 
 origin, 292. 
 
 amphimixis, 146, 171, Fig. 78. 
 ,, and variation, 173. 
 
 Amphioxus, adult, 266, 267, Fig. 118. 
 ,, early development, 45 
 
 48, 265, Fig. 13. 
 ,, free swimming embryo, 
 
 271. 
 
 ovum, 140. 
 
 Amphisbsenidse, 250. 
 Amphitherium, 302. 
 anabolism, 9, Fig. 1. 
 analogy, 235, 247. 
 anatomy, evidence afforded by, 232 
 
 262. 
 
 ancestral history, 229. 
 Anchitherium, 310. 
 Ancon sheep, 154. 
 Andalusian fowls, blue, 200, 201. 
 Andrews, C. W., 312. 
 andrceciurn, 106. 
 anemophilous flowers, 111, 353. 
 Anguisfrtigilis, 250, Fig. 105. 
 animals, compared with plants, 34, 
 
 35. 
 ,, dependent on green plants, 
 
 35. 
 
 anisogamous conjugation, 84. 
 anisogamy in Fucus, 101. 
 Anomodontia, 296. 
 Antarctic continent, 328. 
 anteaters, 333. 
 
 Antedon, hybridization in, 174, 175. 
 ,, recapitulation in develop- 
 ment, 273, 275, Figs. 123, 
 124. 
 antheridia, of Fern, 103, 104, Fig. 51. 
 
 ,, Fucus, 100, Fig. 47. 
 
 antheridial cell of pollen grains, 108, 
 
 Fig. 54. 
 anthers, 107. 
 
 anticryptic coloration, 337. 
 antiquity of man, 422, 423. 
 
 antlers, 125. 
 apatetic colours, 336. 
 apes, Buffon on, 367, 368. 
 ,, and man, Buffon. on, 370. 
 ,, ,, ,, Lamarck on, 382. 
 ,, ,, ,, relationship of, 422 
 
 425. 
 
 aphides, parthenogenesis in, 143. 
 Aphrodite, story of, 214. 
 apogamy in ferns, lOo. 
 aposematic coloration, 342. 
 Apteryx, 258, Figs. Ill, 112. 
 Apus, dispersal of, 327. 
 arborescent colonies, 40. 
 Archseopteryx, 300, 305307, Fig. 
 
 151. 
 archegonium, of Fern, 103, 104, 
 
 Fig. 52. 
 ,, ,, flowering plant, 
 
 108. 
 
 architype, 232. 
 Arctic climate, retreat of, 333. 
 
 ,, vegetation on Alps and Pyre- 
 nees, 333. 
 
 areas of distribution, 319, 320, 330. 
 Argus pheasant, 349. 
 Aristotle, 163, 385. 
 arm, of man, 237, 238, Fig. 94. 
 armadillos, 333. 
 arrow worm, origin of germ cells, 
 
 130. 
 
 Artemia, chromosomes, 73. 
 Arthropoda, limbs, 246, 247. 
 arthropods and vertebrates, 255. 
 artificially produced characters, 156, 
 
 157. 
 artificial parthenogenesis, 145. 
 
 selection, 395, 396, 410 
 
 413. 
 and fertility, 418, 
 
 419. 
 
 artiodactyl limbs, 241, Fig. 98. 
 Arum, fertilization, 355. 
 Ascaris, chromosomes in, 73. 
 
 ,, distinction between somatic 
 
 and germ cells, 166. 
 fertilization of egg, 131, 
 
 Fig. 64. 
 
 ,, mitosis, 79. 
 ,, origin of germ cells, 129. 
 ascidian, adult, 277, Fig. 129. 
 ,, degeneration in, 400. 
 larva, 277, Fig. 130. 
 asexual reproduction, in Eudorina, 
 
 91. 
 
 in Hydra, 
 
 116.
 
 INDEX 
 
 431 
 
 asexual reproduction, in Obelia 
 
 119. 
 ,, ,, inPandorina 
 
 89. 
 
 Aspidiuin, Figs. 48, 50. 
 asters, in mitosis, 71. 
 atavism, 261, 262. 
 attraction of gametes, 143. 
 
 ,, ,, in Coccidium, 
 
 141, 142. 
 ,, ,, ,, in ferns and 
 mosses, 141, 
 142. 
 ,, ,, ,, in Spirogyra, 
 
 ,, ,, sexes, 125 127. 
 ,, sphere, 71. 
 Aucuba, male and female plants, 
 
 111. 
 
 Aurelia, meristic variation in, 149. 
 Australia, fauna and flora of, 250, 
 
 328, 329, 331. 
 Australian climate, adaptation to, 
 
 280. 
 
 automatism, 19. 
 Avebury, Lord, 281. 
 average adaptation, 184. 
 axial filament, of spermatozoon, 140. 
 Axoniderma mirabile, Fig. 168. 
 
 B. 
 
 BABIRUSA, tusks, 406, Fig. 185. 
 baboons, Buffon on, 367, 368. 
 Bacillus sacc* /branch i, 66, Fig. 27. 
 Bacteria, 217, 218, Fig. 83. 
 
 ,, alleged spontaneous genera- 
 tion, 215. 
 origin, 219. 
 structure, 66, Fig. 27. 
 Badhamia, Fig. 29. 
 Bakewell, 387. 
 
 liafana mysticeius, 245, Fig. 101. 
 Balsenidse, teeth, 260. 
 barnacles, degenei-ation, 401. 
 barriers to migration, 320. 
 basal cell, 48. 
 bast, 65. 
 
 Bates, H. W., 346. 
 Batesian mimicry, 346. 
 Eateson, W., 195, 205, 207, 208. 
 bats, 301. 
 
 ,, dispersal, 323. 
 
 ,, wings, 236, 243, Fig. 99. 
 battle, Charles Darwin on law of, 388. 
 
 ,, Erasmus Darwin on law of, 373. 
 beech, evergreen, 329. 
 
 beech, seedling, 281, Fig. 133. 
 bees, alleged spontaneous genera- 
 tion, 214, 215, 343. 
 ,, as pollen carriers, 354, 355, &c. 
 ,, parthenogenesis, 144. 
 ,, proboscis, 354, Fig. 179. 
 beetles, mutation in, 159. 
 Behring Strait, former land connec- 
 tion, 309. 
 ,, ,, former mild climate 
 
 328. 
 
 Bell, 340. 
 
 Biff en, R. H., 205. 
 binomial nomenclature, 226. 
 biogenesis, 216. 
 biogenetic law, 265. 
 Biology, origin of name, 373. 
 
 ,, science of, 1. 
 Biophoridse, 219. 
 biophors, 22, 66, 168, 219. 
 birds, compared with reptiles, 305. 
 dispersal, 323, 324. 
 egg, 140, Fig. 70. 
 evolution, 306, 307. 
 geological range, 284. 
 limbs, 235. 
 origin, 299, 300. 
 wing, 243, 244, Fig. 99. 
 blackberries, white, 204. 
 blastocoel, 47. 
 blastoderm, 270. 
 blastogenic characters, 176. 
 
 ,, ,, inheritance, 
 
 169. 
 ,, ,, origin, 159, 
 
 160. 
 
 ,, variations, 149, 157 160. 
 
 blastomeres, 45, 79. 
 blastopore, 47, 265, 269. 
 blastosphere, 45, 265. 
 blastostyles, 120. 
 blastuk, 45, 265. 
 
 interpretation, 279. 
 
 of Amphioxus, 45, 47, 
 
 Fig. 13. 
 
 of frog, 268, Fig. 119. 
 of Hydra, 118, Fig. 59. 
 blind worm, 250, Fig. 105. 
 blood, 51, Fig. 15. 
 blue Andalusian fowls, 200, 201. 
 3oa constrictor, dispersal, 325. 
 Bodo, life-history, 8587, Fig. 38. 
 body cavity, 48, 124. 
 body wall, 124. 
 Bombus terrestris, robbing clover, 
 
 355, 356. 
 >one, 57.
 
 432 
 
 INDEX 
 
 bony fishes, 292. 
 
 Bordage, E., 182. 
 
 Boveri, T., 146, 174. 
 
 brachiopods, 289. 
 
 brain, in evolution of man, 425. 
 
 ,, of man and apes, 425. 
 branching classification, Charles 
 
 Darwin on, 387. 
 ,, evolutionary series, La- 
 
 marck on, 376. 
 
 Branchiosaurus, 293, Fig. 139. 
 Branchipus, dispersal, 327. 
 breathing, 8. 
 breeding experiments with Papilio, 
 
 348. 
 
 Brewer, 178. 
 
 brimstone moth, larva, Pig. 169. 
 brittle star, 275, Fig. 126. 
 Brontosaurus, 299, Fig. 144. 
 Brown-Sequard, 180. 
 Bnceros rhinoceros, Fig. 407. 
 budding, 263. 
 
 in Hydra, 116. 
 in Obelia, 119, 120. 
 Buffon, on sterility of species when 
 
 crossed, 418. 
 views of, 365370. 
 Burbank, L., 204. 
 Butler, Samuel, 3, 186, 191, 369. 
 Biitschli, 0., 19, 21, 22. 
 butterflies, warning colours, 343 
 
 347. 
 
 butterfly, leaf, 338, Fig. 171. 
 ,, metamorphosis, 264. 
 proboscis, 354. 
 
 C. 
 
 CACTI, 350. 
 
 ,, thornless, 204. 
 caenogenetic characters, 278 281. 
 Csenolestes, 301. 
 Caesar's horse, 262. 
 Cainozoic Era, 284, 285. 
 calcareous skeletons, 26. 
 Calceolaria, distribution of, 328. 
 calcium, influence upon segments 
 
 tion, 45. 
 
 callosities, Buffon on, 367, 368. 
 calyx, 106. 
 cambium, 76. 
 Cambrian Epoch, 284. 
 camel, Buffon on, 367. 
 
 feet, 241, Fig. 98. 
 Camelo-pardalis, Lamarck on, 379. 
 cancer, nature of, 408. 
 Cancer plmlangium, 339. 
 
 candle, analogy of, 2, 5. 
 
 Canflollea gramini folia, fertilization, 
 
 362. 
 cane sugar, attracting spermatozoa 
 
 141. 
 
 canine teeth, reversion in, 262. 
 cannon bone, 241. 
 Capsella bursa-pastoris, development, 
 
 48, Fig. 14. 
 carbohydrates, 27. 
 carbon, 6. 
 
 carbon dioxide, 6, 8. 
 Carboniferous Epoch, 284. 
 Carchesium, 40. 
 carnivores, 301. 
 carpals, 237. 
 carpels, 106. 
 Carpojjhaga chathamensis, 416. 
 
 novw zealanduK, 416. 
 
 carrion flies, 355. 
 cartilage, 56, Fig. 20. 
 casein, 23. 
 
 castration, effect, 125. 
 Catasetum, fertilization, 362. 
 caterpillar, larval organs, 275. 
 
 stick, 337, 338. 
 cats, inheritance of mutilation, 178. 
 
 ,, stump-tailed, 179. 
 caudicle, 361. 
 cave-animals, bleaching inherited, 
 
 183. 
 cell, 12. 
 
 body, 13. 
 definition of, 38. 
 division, 6980. 
 
 ,, limits of, 81. 
 history of term, 36, 37. 
 membrane, 69. 
 plate, 74. 
 sap, 62 
 theory, 38. 
 
 ,, limitations of, 65. 
 typical, 69, 70, Fig. 30. 
 wall, 27, 28, 69. 
 cellular structure, 36. 
 cellulose, 27, 28. 
 centrosome, 71. 
 
 ,, absence in higher plants, 
 
 141. 
 
 >, -- ,, ovum, 141. 
 
 ,, as stimulus to develop- 
 
 ment, 146. 
 ,, in spermatozoon, 139, 
 
 Fig. 69. 
 
 ,, in unfertilized egg, 146. 
 
 ,, origin of, in zygote, 141. 
 
 centrosphere, 71.
 
 INDEX 
 
 433 
 
 Cephalaspi", 290, Fig. 135. 
 Ceratodus (see Neoceratodus). 
 cercarise, 144. 
 cereals, fertilization, 412. 
 
 ,, improvement, 410 413. 
 
 ,, Mendeliaii inheritance in, 
 
 205. 
 cervical vertebrae, of giraffe, 248, 
 
 Fig. 104. 
 whale, 248, 
 318, Fig. 103. 
 Cetacea, evolution of, 314 318. 
 
 limbs, 236. 
 
 Chaetopterus, artificial partheno- 
 genesis, 145. 
 chalk, 26. 
 
 chainaeleon, colour change in, 341. 
 Chambers, Robert, views of, 383 
 
 385. 
 
 change of function, 255, 256, 262. 
 Chara, gametes, 141. 
 characters, compound, 208. 
 " Chatham," H.M.S., 399. 
 Chatham Islands, 325, 398400, 416, 
 
 417. 
 
 chelae, of sponges, 420, Fig. 187. 
 Chelonia, 295. 
 chemical affinity, 6, 7. 
 ,, processes, 9. 
 ,, stimulus to development, 
 
 146. 
 
 chemically modified larvae, 157. 
 chemotaxis, 141, 143, 189. 
 chick, embryo, 265, Figs. 117, 122. 
 chimpanzee, brain, 425. 
 
 Buffon on, 370. 
 Chinese women, small feet, 156. 
 Chironomus, paedogenesis, 144. 
 Chlamydomonas, 40. 
 chlorophyll, 7, 8, 28. 
 cells, 64. 
 
 ,, corpuscles, 32, 64. 
 
 chloroplaslids, 32, 64. 
 
 ,, in Spirogj-ra, 96. 
 
 Chordata, 277. 
 chordate condition, 266. 
 chromatin, 70, 71. 
 
 in heredity, 166, 169, 174,- 
 
 206. 
 
 chromatophores, in Spirogyra, 96. 
 chromomeres, 73, 168, 206. 
 chromosomes, 71, 73, 168. 
 
 and sex, 135, Fig. 67. 
 differential division, 
 
 171, Fig. 77. 
 
 in garnetophyte, 138. 
 
 sporophyte, 139. 
 
 chromosomes, integral division, 171, 
 
 Fig. 77. 
 maternaland paternal, 
 
 137, 207, Fig. 68. 
 ,, pairing, 133, 137, 206, 
 
 Fig. 68. 
 
 reduction, 132, 133, 
 
 137, 138, 206, 207, 
 Fig. 68. 
 chrysalis, 264. 
 cilia, 39. 
 
 dona irdestinaUs, Fig. 129. 
 circumcision, 179, 180. 
 Cirolana, 227. 
 
 cirripedes, degeneration, 401. 
 Cladocera, parthenogenesis, 144 
 Cladorhiza longipihna, Fig. 168. 
 
 ,, (?) tridentuta, spicules, 
 
 Fig. 187. 
 classes, 226. 
 classification, 225-228. 
 
 and phytogeny, 230, 
 
 Fig. 87. 
 ,, artificial, 228. 
 
 Buffon on, 366. 
 ,, Charles Darwin on, 
 
 387. 
 
 ,, Lamarck on, 374, 375. 
 
 natural, 228, Fig. 86. 
 clavicle, 234, Figs. 89, 90, 93. 
 clear-winged moths, mimicry, 343, 
 
 Fig. 175. 
 
 cleavage of ovum, 75. 
 climate, adaptation to, 280. 
 ,, changes in, 327. 
 ,, influence of, Buffon on, 366. 
 ,, ,, ,, Lamarck on, 
 
 376. 
 
 Miocene, 328. 
 
 clover leaf, meristic variation, 149. 
 coal, energy in, 4. 
 
 Coccidium, gametes, 87, 88, Fig. 39. 
 ,, maturation of ovum, 139. 
 coccyx, in man. 261. 
 cockroaches, dispersal of, 325. 
 Codonella, Fig. 9. 
 Cceciliida?, 250, Fig. 106. 
 Ccelenterata, 114. 
 ccelenterate and gastrula, 265, 269, 
 
 270, 277. 
 
 coalom, 48, 124, 266. 
 ccelomate and ccelenterate types, 
 
 124, Fig. 62. 
 coelomic epithelium, 124. 
 
 ,, pouches, 266. 
 cold-blooded animals, 6. 
 colloids, 23. 
 
 F F
 
 434 
 
 INDEX 
 
 colony formation in Hydra, 116. 
 ,, Obelia, 119. 
 Protozoa, 40 44. 
 colour changes in animals, 341, 342. 
 colours of animals, 336, 337. 
 
 ,, ,, ,, Erasmus Darwin 
 
 on, 372. 
 
 ,, ,, flowers, 355. 
 column, of flower, 360, 363. 
 combustion, energy liberated by, 4, 6. 
 
 ,, nature of, 2, 5, 6. 
 
 communication between cells, 187. 
 compound characters, 208. 
 conceptacles of Fucus, 100. 
 Condylarthra, 309, 313. 
 cone of attraction (or reception) in 
 
 Coccidium, 88, 1'43. 
 congenital variations(characters),157. 
 Conilera, 227. 
 conjugants, in Pararncecium, 92, 
 
 Fig. 41. 
 conjugation of chromosomes, 137, 
 
 Fig. 68. 
 
 gametes, 33, 8285, 
 131, 141, 
 206, 207, 
 Fig. 65. 
 
 ,, ,, in Ascaris, 
 
 131, Fig. 
 64. 
 ,, ,, in Bodo, 87, 
 
 Fig. 38. 
 
 ,, ,, in Cocci- 
 
 dium, 88, 
 Fig. 39. 
 
 ,, in Ccelo- 
 
 mata, 125. 
 
 ,, ,, in Copromo- 
 
 nas, 83, 
 Fig. 37. 
 
 ,, ,, inEudorina, 
 
 91, Fig. 40. 
 
 ,, ,, in ferns, 105. 
 
 ,, ,, in flowering 
 
 plants, 
 109. 
 
 ,, ,, in Fucus, 
 
 101, Fig. 
 47. 
 
 ,, in Hsema- 
 
 tococcus, 
 
 33, Fig. 5. 
 
 -, in heredity, 
 
 171. 
 
 .. ,, in Pando- 
 
 rina, 89, 
 Fig. 10. 
 
 conjugation of gametes in Pa ra- 
 ni oceium. 
 9 2 , 9 .'5 , 
 Fig. 11. 
 
 ,, ,, in opiro- 
 
 gyra, 96, 
 97, 142, 
 Figs. 43, 
 44. 
 
 mum, 
 ,, of nuclei in zygote, 
 
 131, Fig. 64. 
 ,, origin of, 127. 
 
 results of, 84, 87. 
 connecting links, 232235, 305 
 
 318. 
 ,, ,, destruction of, 
 
 222. 
 
 conodonts, 289. 
 conservatism of germ cells, 184, 
 
 190. 
 
 continental islands, fauna, 332. 
 continents, permanence, 329. 
 continuity of germ plasm, 166, 168, 
 
 Fig. 76. 
 life, 67. 
 
 ,, ,, parent and offspring, 
 
 Erasmus Darwin on, 
 370. 
 
 ,, ,, protoplasm, 66. 
 
 continuous selection, 411. 
 
 variations, 148, 150. 
 contractile tissue (see muscle fibres). 
 
 vacuole, 14, 18. 
 
 convergence, 247 255. 
 
 in Acacia, 280. 
 flightless birds, 397. 
 plants, 2fi2. 
 ,, ,, proboscis of bees and 
 
 Lepidoptera, 354. 
 " Convolvulus major," fertilization, 
 
 351, 352. 
 co-operation, 40. 
 
 ,, of factors in evolution, 
 
 421. 
 
 Copromonas, life-history, 83, Fig. 37. 
 ,, nuclear reduction, 139. 
 
 coracoid, 234, Figs. 89, 93. 
 
 ,, vestigial, 257, Fig. 90. 
 corals, 114, 25. 
 
 cork, cellular structure, 37, Fig. 6. 
 corolla, 106. 
 corpora lutea, 126. 
 corpuscles, blood, 51, 52, Fig. 15. 
 correlation, 409, 415. 
 cortical tissues, 50.
 
 INDEX 
 
 435 
 
 cosmopolitan distribution, 320. 
 
 Cosmozoa, 216. 
 
 cotyledons, crenogenetic characters, 
 
 280, 281, Figs. 56, 133. 
 cowslip, fertilization, 356. 
 crab, larva, 276, Fig. 128. 
 
 ,, swimming, 247, Fig. 102. 
 creation, Lamarck on, 375, 376. 
 
 special, 212214, Fig. 82. 
 ,, ,, Buffon on, 369. 
 
 ,, Linnaeus on, 222. 
 
 Creodontia, 318. 
 Crepidula, 323. 
 Cretaceous Epoch, 284. 
 Crinoidea, 274. 
 
 Crinorhiza form, 335, 420, Fig. 168. 
 Cristatella mucedo, statoblast, Fig. 
 
 165. 
 
 Crocodilia, 295. 
 cross -fertilization of flowers, 351 
 
 363. 
 
 crossing (see hybridization). 
 Crustacea, fresh water, dispersal, 
 
 327. 
 
 ,, gills, 255. 
 
 cryptic coloration, 337. 
 crystalloids, 23. 
 
 currents, dispersal by, 321325. 
 curve of frequency of error, 153. 
 
 variation, 150, 153, Figs. 72, 
 
 73. 
 
 Cuscuta europma, 402, 403, Fig. 184. 
 cushion plants, 262, 350. 
 Cyathaspis, 290. 
 cyclopean larvae, 157. 
 Cyclostomata, 289. 
 cyst, 20, 83. 
 cytology, 67. 
 cytolysis of ovum, 147. 
 cytoplasm, 13, 69. 
 
 ,, in heredity, 174, 175. 
 cytostome, 83. 
 cytotaxis, 142. 
 cytotropism, 142. 
 
 ,, of gametes, 143. 
 
 Cyttaria, 329. 
 Cyttarocyclis, Fig. 9. 
 
 1). 
 
 DARBISHIRE, A. D., 197. 
 
 Darwin, Charles, views of, 164 167, 
 176, 180, 185, 195, 208, 222 
 224, 329, 351, 353, 356, 357, 
 359, 385388, 391393, 410, 
 415, 423. 
 
 Darwin, Erasmus, views of, 370 373. 
 
 ,, Francis, 192. 
 death, 21, 162, 168. 
 De Candolle, 386. 
 decay, nature of, 4. 
 deep sea animals, adaptation in, 335, 
 
 336. 
 
 deer, feet, 241, Fig. 98. 
 degeneration, 397 403. 
 
 in ascidians, 400. 
 
 flightless birds, 397. 
 
 Morioris, 400. 
 
 results of, 398. 
 
 Delage 
 
 Y., 146. 
 
 Ddphmm delphis, Figs. 161, 162. 
 
 Democritus, 369. 
 
 dendrons, 59. 
 
 dentition of dog and thylacine, 251 
 
 253, Figs. 107, 108. 
 denudation, 285, 288. 
 dermatogen, 50. 
 Descartes, 11, 260. 
 descent with modification, 221. 
 design, doctrine of, 212. 
 Desmacidonidse, spicules, 419, 420, 
 
 Fig. 187. 
 determinants, 167, 206. 
 
 ,, effect of stimuli upon, 
 
 189. 
 
 ,, vibrations in, 189. 
 
 deutoplasm, 140, 267. 
 development, 263281. 
 
 , , and unconscious memory, 
 
 192. 
 ,, Erasmus Darwin on, 
 
 371, 372. 
 
 factors in, 192. 
 ,, of birds and reptiles, 270. 
 ,, of flowering plants, 48 
 
 50, 109, Fig. 14. 
 of Frog, 268270, 272, 
 
 Figs. 119, 121. 
 of Hydra, 118, Fig. 59. 
 Devonian Epoch, 284. 
 De Vries, Hugo, 154, 204, 224, 410 
 
 414. 
 
 Dianthus, 353. 
 diatoms, 25. 
 dichogamy, 353. 
 Dictyocysta, Fig. 9. 
 Didelphyidse, 301. 
 differences between plants and 
 
 animals, 34, 35. 
 differential division of chromosomes, 
 
 177, Fig. 77. 
 
 differentiation, 39, 43, 44, 60, 119. 
 sexual, 85. 
 
 F F 2
 
 436 
 
 INDEX 
 
 diffusion of gases, 8. 
 digestion, in Amoeba, 16. 
 digestive cavity, of Hydra, 115. 
 Medusa, 120. 
 Obelia, 119. 
 digits, reduction, 239 241. 
 dihybridism, 201-203, Figs. 80, 81. 
 dimorphic flowers, 356. 
 dimorphism of gametes, 139. 
 Dinosauria, 297. 
 Dinotherium, 314, Fig. 160. 
 direcious, 99, 105. 
 Diplodocus, 299. 
 Diplosoma crystallinum, larva, Fig. 
 
 130. 
 
 Dipnoi, 255, 291. 
 Diptera, ppedogenesis, 199. 
 direct nuclear division, 80, Fig. 36. 
 discontinuity between species, 222, 
 
 Fig. 85. 
 , , in distribution, 319, 320, 
 
 333. 
 in evolutionary series, 
 
 Lamarck on, 376. 
 ,, in organic world, 221. 
 
 discontinuous variation, 149, 153 
 
 156. 
 
 Dismorphia praxinoe, 345, Fig. 176. 
 dispersal of organisms, 320 327. 
 distribution, geographical, 319 333. 
 disuse (see use and disuse). 
 
 of wings, effects of, 397, 398. 
 divergence in evolution, 213. 
 
 A. E. Wallace on, 390. 
 ,, Charles Darwin on, 387. 
 
 division of labour, 39, 43, 44, 60, 85, 
 
 119. 
 ,, between male and 
 
 female, 127. 
 dodder, 402, 403, Fig. 184. 
 
 , , vestigial leaves, 262, Fig. 184. 
 dodo, 258, 397. 
 dog, skull and dentition, 251253, 
 
 Figs. 107, 108. 
 vestigial teeth, 260. 
 dogfish, embryo, 270, Fig. 120. 
 dolphin, Figs. 161, 162. 
 dolphins, convergence in, 248. 
 shark-toothed, 318. 
 domestication, Charles Darwin on, 
 
 410. 
 
 Lamarck on, 378. 
 
 dominant characters, 197, 207. 
 Draba verna, elementary species in, 
 
 224, 412. 
 
 drone flies, mimicking bees, 343. 
 mistaken for bees, 215. 
 
 Dubois, E., 423. 
 
 Dujardin, 38. 
 
 dynamics of cell-division, 74. 
 
 E. 
 
 ECHIDNA, 234, 301, Fig. 92. 
 echinoids, larval stage, 276. 
 Echinus, blastomeres, 45. 
 
 ., hybridization in, 174. 
 ectoderm, 47. 
 
 ,, of coelomates, 124. 
 
 Hydra, 115, 116, 118. 
 edentates, 301, 333. 
 education, 426. 
 eels, dispersal of, 325, 326. 
 egg cell (see ovum). 
 ,, of Ascaris, mitosis, 79, Fig. 35. 
 ,, bird, 140, Fig. 70. 
 ,, frog, 268. 
 eggs, dispersal of fish, 322. 
 
 ,, similarity in different or- 
 ganisms, 162. 
 size, 267. 
 Eigenmaun, 183. 
 Elasmobranchii, 291. 
 electrical energy, transmission, 189. 
 electric eel, 256. 
 
 ,, organs, 256. 
 electro -magnetic theory of mitosis. 
 
 74, 75. 
 
 elementary species, 224, 412. 
 elephants, ancestry, 312 314, Fig. 
 
 159. 
 
 limbs, 239. 
 Elephas, 314. 
 elite, 411. 
 
 Elodea, evolution of oxygen, 31. 
 embryo, fixation, 126. 
 
 of chick, 265, Figs. 117, 122. 
 ,, ,, Fucus, 101, Fig. 47. 
 ,, ,, mammals, 270. 
 
 ,, rabbit, 265, Fig. 117. 
 embrjology, Erasmus Darwin on, 
 
 371, 372. 
 ,, evidence afforded by, 
 
 261281. 
 embryonic cell, 48. 
 embryo-sac, 48, 106, 107, 108. 
 embryos of birds and reptiles, 270. 
 Emily Henderson, sweet pea, 208. 
 Empedocles, 369. 
 Encrinites, 274. 
 endoderm, 47. 
 
 ,, of coelomates, 1 24. 
 
 of Hydra, 115, 116, 118. 
 endoplasm, 14, 22. 
 endosarc, 14.
 
 INDEX 
 
 437 
 
 endosperm, 109. 
 energy, conservation of, 6. 
 ,, manifested in life, 4, 5. 
 ,, of chemical affinity, 7. 
 ,, source of, 4, 6, 7, 9. 
 ,, ,, ,, in green plants, 30. 
 
 engrains, 186. 
 entelechy, 11. 
 enteron, 47, 265. 
 
 ,, in Hydra, 115. 
 entomophilous flowers, 111, 353. 
 enucleate eggs, fertilization (see 
 
 merogony). 
 environment, control of, 426. 
 
 influence of, 5, 6, 166, 
 
 182, 183. 
 
 A.B.Wallace on, 390. 
 
 Buffon on, 366, 367. 
 
 Charles Darwin on, 
 
 387, 392. 
 
 Lamarck on, 377 
 379. 
 
 upon development, 
 193. 
 
 , , germ plasm, 
 159. 
 
 Eocene Epoch, 284. 
 Eohippus, 309, 310, Fig. 154. 
 Ephydatiaflni-iatiUs, gemmules, Fig. 
 
 166. 
 epiblast, 47, 48, 266. 
 
 of Hydra, 115, 118. 
 epicoracoid, 234, Figs. 89, 93. 
 epidermis, 54, Fig. 17. 
 
 of plants, 50, 64, Fig. 26. 
 epigamic ornamentation, 336, 349. 
 epigenesis, 163. 
 Epihippus, 310. 
 epilepsy, in guinea pigs, 180. 
 episematic coloration, 342. 
 Epistylis, 40. 
 
 epithelium, 54, Figs. 1618, 28. 
 epochs of earth's history, 284, 285. 
 equatorial plate, 72. 
 Equidse, pedigree of, 307 312. 
 Equus, 309, 311, Fig. 153. 
 eras of earth's history, 284, 285. 
 Esperella lapidiformis, spicules, Fig. 
 
 187. 
 Esperiopsis challengeri, 420, 421, Fig. 
 
 188. 
 ,, pukhella, spicules, Fig. 
 
 187. 
 eucalypts, 331. 
 
 leaves, 280. 
 
 Eudendrium, migration of germ 
 cells, 123. 
 
 Eudorina, 42, 90, Fig. 40. 
 Eurypteridse, 291, Fig. 137. 
 
 ,, size of, 405. 
 
 Eutheria, 301. 
 
 ,, first appearance of, 302. 
 evolution, factors of, 365427. 
 in development, 163. 
 individual, 263. 
 of sex, 81 147. 
 progressive, 192, 334. 
 theory and evidence of, 
 
 211364. 
 ,, versus special creation, 
 
 212214, Fig. 82. 
 
 ex-conjugants, in Paramoecium, 93. 
 excretion, 9. 
 
 ,, in Amoeba, 17. 
 experiments in heredity, 174, 175, 
 
 194209. 
 
 explosive character of living mole- 
 cule, 18. 
 extinction of groups, 231. 
 
 ,, ,, species, Buffon on, 366. 
 extracted dominants, 198. 
 recessives, 198. 
 eye-colour, Mendelian inheritance of, 
 
 205. 
 eyes, pineal, 258260. 
 
 F. 
 
 FACTORS, co-operation of, 208. 
 ,, in development, 192. 
 ,, ,, germ plasm, 206, 207. 
 ,, of organic evolution, 365 
 
 427. 
 
 ,, permutations and combina- 
 tions of, 206. 
 fseces, 16. 
 
 families, in classification, 226. 
 Farmer, J. B., 206. 
 fat, 56, Fig. 19. 
 
 feather star, hybridization, 174, 175. 
 ,, recapitulation in life- 
 history, 273275, 
 Figs. 123, 124. 
 feathers, acquisition of, 177. 
 female animal, 114. 
 
 ,, characters, 84, 85, 127. 
 ,, dependence on male, 127. 
 femur, 237. 
 fermentation, 218. 
 fern, life-history, 101105, Figs. 48 
 
 52. 
 
 ferns, dispersal, 321. 
 fertility and cross-fertilization, 352, 
 
 357. 
 test of specific identity, 418
 
 438 
 
 INDEX 
 
 fertilization, adaptation of flowers for, 
 
 351363. 
 ,, chemical stimulus in, 
 
 146. 
 ,, development without, 
 
 145. 
 
 in heredity, 171. 
 
 ,, membrane, 147. 
 
 of flowers, 110, 351 
 
 363. 
 
 ,, ovum, 85, Fig. 65. 
 
 ,, ,, ,, in Ascaris, 131, 
 
 Fig. 64. 
 ,, ,, ,, Coelomata, 
 
 125. 
 
 ,, ferns, 105. 
 
 ,, ,, Fucus, 101. 
 
 ,, ,, Hydra, 118. 
 
 ,, ,, ,, ,, medusae, 121. 
 
 fibrillar structure of protoplasm, 
 
 22. 
 
 fibula, 237. 
 fig-wort, 353. 
 filament of stamen, 107. 
 finger, inheritance of mutilation, 
 
 180. 
 First Cause, Erasmus Darwin on, 
 
 372. 
 
 fish eggs, dispersal, 322. 
 fishes, bony, 292. 
 deep sea, 336. 
 ,, geological range, 284. 
 fish-like stage in ontogeny and evo- 
 lution, 272, 273, 277, 278, 279. 
 fission, in Amoeba, 21. 
 Bodo, 87. 
 ,, ,, Copromonas, 83. 
 ,, ,, Hsematococcus, 29. 
 fixation of embryo in uterus, 126. 
 flagella, in Bodo", 87. 
 ,, Eudorina, 91. 
 ,, ,, Hsematococcus, 29. 
 flame, nature of, 2. 
 Flemming, 69. 
 flightless birds, 258. 
 
 ,, ,, and fluctuating varia- 
 
 tion, 415. 
 ,, ,, natural selection, 
 
 397, 398. 
 flint, 24. 
 
 floating islands, dispersal by, 324. 
 flower, structure of, 106, 107, Fig. 53. 
 flowering plants, life-history, 106 
 
 112. 
 flowers, adaptation for fertilization, 
 
 351363. 
 sexual characters, 110, 111. 
 
 fluctuating variations, 148, 150, 155, 
 Figs. 72, 73. 
 
 ,, ,, and adapta- 
 
 tion, 414, 
 415. 
 
 ,, ,, and natural 
 
 selection, 
 414. 
 ,, De Vries on, 
 
 413, 414. 
 
 flukes, parthenogenesis in, 144. 
 foam structure of protoplasm, 21.' 
 foetal membranes, 270, 278. 
 foetus, in Mammalia, 270. 
 food materials of animals, 16. 
 ,, ,, ,, green plants, 30 
 
 32. 
 
 ,, nature of, 7. 
 
 ,, vacuoles, 14. 
 
 food-yolk, 267, 278. 
 
 influence of, 268, 271. 
 foot, artiodactyl, 241. 
 ,, of apes and monkeys, 424. 
 camel, 241, Fig. 98. 
 ,, deer, 241, Fig. 98. 
 ,, elephant, 239. 
 ,, ,, hippopotamus, 241, Fig. 98. 
 ,, ,, horse, 241, Fig. 97. 
 ,, ,, ,, atavism in, 262. 
 evolution of, 310312, 
 
 Figs. 154158. 
 ,, Litopterna, 253255, Fig. 
 
 109. 
 
 llama, 241. 
 ,, man, 238, Fig. 94. 
 ,, ,, oxen, 241. 
 ,, pig, 241, Fig. 98. 
 ,, ,, rhinoceros, 241. 
 ,, seal, 244, Fig. 100. 
 ,, ,, sheep, 241. 
 ,, tapir, 241, Fig. 96. 
 ,, ungulates, 239241, 253 
 
 255. 
 
 ,, pentadactyl, 238. 
 , , perissodactyl, 24 1 . 
 Foraminifera, 26, Fig. 4. 
 fossilization, 287, 288. 
 fowls, Mendelian inheritance in, 200, 
 
 201. 
 
 foxgloves, mutation in, 154. 
 Francotte, 135. 
 Freia, Fig. 9. 
 
 frequency of error, curve, 153. 
 fresh water animals, dispersal, 325, 
 
 326. 
 
 frog, early development, 268 270, 
 Fig. 119.
 
 INDEX 
 
 439 
 
 frog, life-history, 272, Fig. 121. 
 
 ,, ,, interpretation of, 
 
 279, Fig. 131. 
 
 , , pineal eye in, 260, Figs. 115,116. 
 frogs and toads, 293. 
 fruit trees, propagation, 205. 
 fruits, dispersal, 321. 
 Fuchsia, distribution, 328. 
 Fucus, 99 101, Figs. 45 47. 
 Functions of organisms, 5. 
 Funduhis, cyclopean larvae, 157, 
 
 Fig. 75. 
 Fungi, 34, 321. 
 f uniculus, of ovule, 108. 
 furze, recapitulation in seedlings, 280 
 
 G. 
 
 Galaxias niyothortik, dispersal of, 
 
 326. 
 
 Gallardo, 75. 
 Galton, Francis, 209. 
 Galton's law of inheritance, 209. 
 Galton's polygon, 155. 
 Galtonia, mitosis in, 76, Figs. 33, 34. 
 gametes, 33, 83 (see also germ cells). 
 
 attraction of, 141, 142, 143. 
 ,, conjugation of, 207. 
 ,, evolution of male and female, 
 
 84, 85. 
 ofBodo, 87. 
 fern, 104. 
 flowering plant, 108, 109, 
 
 110. 
 
 ,, ,, Fucus, 100. 
 ,, ,, Pandorina, 89. 
 - ,, Spirogyra, 96, 97, 142, 
 
 143. 
 
 Volvox, 91. 
 purity of, 200, 206. 
 ,, sexual dimorphism of, 113, 
 
 139. 
 
 gametic nuclei in Paramcocium, 93. 
 gametogenesis, 132, Fig. 65. 
 gametophyte, 101. 
 
 ,, chromosomes of, 138 
 
 of fern, 103, Fig. 50. 
 ,, ,, flowering plant, 107, 
 
 108. 
 suppression of, 
 
 112. 
 
 gamobium, 121. 
 ganglion, 59. 
 ganoids, 292. 
 gastrsea, 277. 
 gastral cavity, 47. 
 
 in Hydra, 115, 118. 
 inObelia, 119. 
 
 gastrula, 47, 265, 277. 
 
 ,, interpretation of, 265, 279. 
 of Hydra, 118. 
 ,, Sagitta, 130, Fig. 63. 
 gastr ulation, in birds and reptiles, 
 
 270. 
 
 frog, 269, Fig. 119. 
 Geikie, Sir A., 285. 
 gemmules, dispersal of, 327. 
 ,, in pangenesis, 164. 
 ,, of fresh water sponge, 
 
 327, Fig. 166. 
 genera, 225. 
 generative cells, 103. 
 Genesis, Book of, 212. 
 genital ducts, 125. 
 geographical distribution, 319 333. 
 ,, ,, summary, 330. 
 
 isolation, 416, 417. 
 geological formations, 284. 
 
 history of the earth, 284, 
 
 285. 
 
 periods, 284. 
 range of animal groups, 
 
 284. 
 
 record, 283, 287304. 
 geometer moths, caterpillars, 337, 
 
 338, Fig. 169. 
 
 Geoplana exulans, dispersal, 325. 
 Geotria, distribution, 329. 
 germ cells (see also gametes). 
 
 and somatic cells, 97, 98, 
 99, 113, 129, 166, 167, 
 168. 
 
 ,, conservatism, 184, 190. 
 immortality, 162, 168. 
 ,, independence, 99. 
 maturation, 138. 
 ,, migration in Hydrozoa, 
 
 123. 
 ,, origin in Ascaris, 129. 
 
 Cceloniates, 124, 
 
 130. 
 
 Hydrozoa, 123. 
 
 plants, 130. 
 ,, Sagitta, 130. 
 potentialities, 163. 
 ,, sensitive to stimuli, 188. 
 germinal disk, 140. 
 
 ,, selection, 173. 
 ,, variations, 149, 157160. 
 germination of fern spore, 103, Fig. 
 
 ,, pollen grain, 108, 
 
 Fig. 54. 
 
 seed, 109, Fig. 56. 
 germ layer theory, 48.
 
 440 
 
 INDEX 
 
 germ plasm, complexity, 167. 
 
 ,, composition, 172, Fig. 78. 
 
 constitution, 205, 206. 
 continuity, 166, 168, 
 
 Fig. 76. 
 
 ,, influenced by environ- 
 
 ment, 159. 
 gestation of nature, R. Chambers on, 
 
 384. 
 
 pigantic animals, 303, 304, 405409. 
 Gila monster, 342. 
 gill slits, in Amphioxus, 266. 
 
 embryos, 261, 273, Fig. 
 
 122. 
 
 gills of crustaceans and fishes, 255. 
 giraffe, 248, Fig. 104. 
 
 ,, Lamarck on, 379. 
 glacial periods, 327, 328. 
 glass snakes, 250. 
 gliadin, 23. 
 glucose, 28, 31. 
 glutinin, 23. 
 Godlewski, 174. 
 gonads, 113. 
 
 in Coolomates, 124. 
 ,, Hydrozoa, 124. 
 ,, ,, medusae, 121. 
 gonoducts, 125. 
 gonophores, 122. 
 gonotheca, 120. 
 "gorillas," Hanno on, 422. 
 gorse, recapitulation in seedlings, 280. 
 gradation in nature, Buffon on, 366. 
 ,, structure, 232. 
 ,, of animals, Lamarck on, 
 
 376. 
 Grantia compressa, larva, 322, Fig. 
 
 164. 
 grape hyacinth, curve of variation, 
 
 150, Fig. 72. 
 ,, sugar, 28, 31. 
 grasshopper, spermatogenesis, 134, 
 
 Fig. 66. 
 Gray, Asa, 386. 
 Gray, J. E., 227. 
 grazing mechanism, 309. 
 Great Britain, a continental island, 
 
 332. 
 
 Greenland, former mild climate, 328. 
 Grew, Nehemiah, 37. 
 gristle, 56. 
 growth, 10, 20. 
 
 control of, 408. 
 in Amoeba, 20. 
 ,, animal tissues, 75. 
 plants, 76. 
 
 periodicity of, 191. 
 
 guard cells, 64, Fig. 26. 
 guinea pigs, Brown-Sequard's experi- 
 ments, 180. 
 
 Gulf Stream, dispersal by, 322, 323. 
 Gulick, 417, 418. 
 gut wall, 124. 
 Gymiiotus, 256. 
 gynoccium, 106. 
 
 H. 
 
 HABIT, of plants modified by en- 
 vironment, 181. 
 habitat, 319, 
 habits, adaptation in, 337. 
 
 ,, influence of, Lamarck on, 
 
 377, 378, 379, 380. 
 ,, in plants, 191. 
 Haeckel, Ernst, 66, 229, 265, 277. 
 hsematids, 52, Fig. 15. 
 haematochrome, 28. 
 Hsematococcus, 27 34, Fig. 5. 
 ,, conjugation, 89. 
 
 ,, dispersal, 327. 
 
 haemoglobin, 53. 
 hair, vestigial, 261. 
 hairs of Tradescantia, 61, Fig. 25. 
 hand of man, 256, 424, 425. 
 
 ,, ,, monkeys and apes, 424. 
 Hanno, 422, 424. 
 harmony, in coloration, 336. 
 ffarpax tricolor, 339. 
 Hatteria (see Sphenodon). 
 Hawaiki, 399. 
 
 heat, in living organisms, 6. 
 Heidelberg, fossil man, 423. 
 Heilprin, A., 319. 
 Heliconinse, 345. 
 Heliconius ethi/la, 345, Fig. 176. 
 Heloderma suspectum, 342. 
 Hemiphaya chathamensis, 416. 
 
 ,, novcK zealandicp, 416. 
 Henslow, G., 183. 
 Herbst, 45. 
 
 heredity, 161210 (see also inherit- 
 ance). 
 
 Buffon on, 369. 
 ,, Charles Darwin's theory, 
 
 164166. 
 ,, fertilization experiments, 
 
 174, 175. 
 
 Galton on, 209, 210. 
 ,, in neuter insects, 189, 190. 
 
 Protista, 161. 
 Lamarck on, 380, 381. 
 Mendelian experiments, 
 194209,
 
 INDEX 
 
 441 
 
 heredity, mnemic theory. 186, 191, 
 
 192. 
 
 ,, nature of problem, 163. 
 ,, Pearson on, 210. 
 ,, Weismann's theory, 166 
 
 174. 
 
 Hering, E., 186. 
 
 hermaphrodite, 103, 105, 113, 114. 
 hermaphroditism, 93, 116, 125, 402. 
 heterogamy, 144. 
 Heteroinita, life-history, 85 87 
 
 Fig. 38. 
 
 heterosporous ferns, 105. 
 heterostyled flowers, 356, Fig. 180. 
 heterozygote, 207. 
 hexadactylism, 154, 413. 
 Hiigard, E. W., 178. 
 Hipparion, 311. 
 Hippocampus antiquorum, 341, Fig. 
 
 1 . 3. 
 
 Hippopotamus, feet, 241 , Fig. 98. 
 histological differentiation, 48. 
 histology, 51. 
 His, Wilhelm, 178. 
 hock, of horse, 240, Fig. 95. 
 holophytic nutrition, 34. 
 holozoic nutrition, 34. 
 Homo heidelheryensis, 423. 
 Homo, species of, 423. 
 homologous chromosomes, 137, 206, 
 
 207. 
 
 homology, 235, 247. 
 homoplasy, 235, 247. 
 homosporous ferns, 106. 
 homozygote, 207. 
 honey guide, 362. 
 honey sucking apparatus, 354. 
 Hooke, Eobert, 36. 
 Hooker, J. D., 385. 
 hormones, 126, 188, 408. 
 hornbill, 406, Fig. 186. 
 hornets, colours of, 342, 343. 
 horns, excessive development, 406. 
 horse, evolution of, 307 312, Figs. 
 
 153158. 
 horse's feet (see foot of horse). 
 
 ,, skeleton, Fig. 95. 
 horses, Mendelian inheritance in, 205. 
 horse-worm (see Ascaris). 
 humble bee, robbing clover, 355, 
 
 356. 
 
 humerus, 237. 
 humming birds, as pollen carriers, 
 
 353. 
 
 distribution, 320. 
 
 ,, ,, epiganjic orna- 
 
 mentation, 349. 
 
 hyacinth, mitosis in, 76, Figs. 33, 34. 
 hybridization, 194209. 
 
 and evolution, 209. 
 
 ,, and mutation, 413, 
 
 414. 
 
 in cereals, 413. 
 
 Hydra, 114 118, 265, Figs. 57 59. 
 ,, dispersal, 327. 
 ,, suppression of inedusoid, 123. 
 hydranth, 119. 
 hydrocaulus, 119. 
 hydrotheca, 119. 
 Hydrozoa, 123. 
 hyperphalangy, 244. 
 hypertonic solution, producing par- 
 thenogenesis, 145. 
 hypoblast, 47, 48. 
 
 of Amphioxus, 47, 266. 
 Frog, 269. 
 Hydra, 115, 118. 
 hypostome, 116, 119. 
 Hypsidse, 345. 
 Hyracotherium, 310. 
 
 ICEBERGS, dispersal by, 324. 
 Ichthyopterygia (Ichthyosauria), 
 
 297, Fig. 142. 
 paddles, 236, 244. 
 Ichthyosaurus communis, Fig. 142. 
 idants, 168. 
 identical twins, 173. 
 idioplasm, 167. 
 Idolum diabolicum, 339. 
 ids, 168, 206. 
 igneous rocks, 282. 
 Iguanodon, 297, Fig. 143. 
 illegitimate unions, in Primula, 357. 
 immortality of germ cells, 162, 168. 
 
 Protista, 161. 
 immutability of species, 
 Buff on on, 369. 
 Lamarck on, 374. 
 Linnaeus on, 222. 
 individual adaptation, 184. 
 
 , , characters, transmission 
 
 of, 175. 
 
 ,, variations, 150. 
 individuality of cells, 68. 
 inertia of genii cells, 190. 
 inflorescences, variation in, 150. 
 Infusoria, 40, Fig. 41. 
 inheritance (see heredity). 
 
 ,, of acquired characters, 
 
 165, 176-193. 
 . mutilations, 178, 179.
 
 442 
 
 INDEX 
 
 insect communities, as individuals, 
 
 190. 
 
 insectivores, 301. 
 
 insects, as pollen carriers, 353 355. 
 ,, dispersal of, 323. 
 ,, primordial germ cells, 130. 
 ,, selection by, 363. 
 ,, sex determination, 135, 136, 
 
 Fig. 67. 
 
 ,, spermatogenesis, 135. 
 ,, wings, 247. 
 instincts, adaptation in, 337. 
 
 origin of, 184. 
 integral division of chi'omosomes, 
 
 171, Fig. 77. 
 integration, 44. 
 integuments of ovule, 108. 
 intercellular substance, 57, 123. 
 interclavicle, 234, Figs. 89, 93. 
 intercrossing, swamping effects, 416. 
 interstitial cells, 116. 
 intussusception, 20. 
 invagination, 47. 
 invertebrates, dispersal, 325. 
 
 ,, geological range, 284. 
 
 Ipomcea purpurea, fertilization, 351, 
 
 352. 
 
 irritability, 18. 
 irritable structures in flowers, 362, 
 
 363. 
 
 Isoetes, 105. 
 isogamy, 84, 89, 97. 
 isolation, 331, 416-419. 
 
 Lamarck on, 380, 381. 
 Ithomiinse, 345. 
 
 J. 
 
 JELLY-FISH, 114, 121, 265. 
 
 Joly, 286. 
 
 Jurassic Epoch, 284. 
 
 K. 
 
 KAKAPO, 397. 
 
 Kallima inachis, 338, Fig. 171. 
 
 kangaroos, 301. 
 
 3ant, 178. 
 
 karyogamy, 128. 
 
 karyokinesis, 69 79, Figs. 31 33. 
 
 karyoplasm, 14, 69. 
 
 karyosome, 70, 77. 
 
 katabolism, 9, Fig. 1. 
 
 kea, 398. 
 
 khaki clothing, 342. 
 
 kidney, effect of removal, 156. 
 
 kinetic energy, 7. 
 
 kingdoms, 226. 
 
 kiwi, 258, 331, 397, Figs. Ill, 112. 
 knee, of horse, 240, Fig. 95. 
 
 L. 
 
 LABELLUM, 359. 
 labyrinthodonts, 243, 406. 
 Lagena, non-adaptive characters,419. 
 Lamarck, views of, 176, 373 382, 
 
 418. 
 
 Lamarck's four laws, 382. 
 Lamarckian factors, A. E. Wallace 
 
 on, 393. 
 
 ,, ,, Charles Darwin 
 
 on, 166, 392, 
 393. 
 
 ,, neglect of, 393. 
 
 Eobert Cham- 
 bers on, 385. 
 
 lampreys, dispersal, 325, 326. 
 distribution, 329. 
 
 pineal eye, 258, 260. 
 lance-woods, 417. 
 land connections, former, 328, 329. 
 
 ,, planarians, dispersal, 325. 
 larva of Antedon, 274. 
 
 Ascidian, 277, Fig. 130. 
 crabs, 276, Fig. 128. 
 echinoids, 276. 
 frog, 272, Fig. 121. 
 Grautia, 322, Fig. 164. 
 ophiuroids, 275, Fig. 127. 
 larvae, 272. 
 larval forms, dispersal, 321, 322. 
 
 organs, 272, 275, 278. 
 Leach, 227. 
 leaf insects, 338, 339, Figs. 170, 171. 
 
 structure, 63-65, Fig. 26. 
 leg, of man, 237, 238, Fig. 94. 
 legitimate unions, in Primula, 357. 
 legumin, 23. 
 Leigh, G. F., 348. 
 Lemuria, 328. 
 lemurs, distribution, 328. 
 leopard, distribution, 320. 
 Lepidosiren, 255, 291. 
 Leptinotarsa, mutation in, 159, 160. 
 leucocytes, 51, Fig. 15. 
 level, changes of in land, 328. 
 lianes, 350. 
 
 life-history, 263 (see also ontogeny). 
 life, nature of, 2, 3, 4, 11. 
 Lilium, germination of pollen grain, 
 
 Fig. 54. 
 limbs, of arthropods, 246, 247. 
 
 ,, vertebrates, 235246. 
 Limnas chrysippus, 346, Fig. 177. 
 linin network, 70.
 
 INDEX 
 
 443 
 
 Linnpean species, 412. 
 Linnaeus, 222, 226, 228. 
 Linnean Society, 340, 385. 
 lithium larva?, 157. 
 Lithobius, host of Coccidium, 87. 
 Litopterua, 253, Fig. 109. 
 lizards, pineal eye, 258. 
 
 shoulder girdle, 234, Fig. 89. 
 llama, Buffon on, 367. 
 
 feet, 241. 
 Lock, E. H., 152. 
 locomotion in Amoeba, 15. 
 Loeb, J., 145, 146. 
 Lubbock, Sir John, 281. 
 Lull, E. S., 307. 
 luminous organisms, 6. 
 lung-fishes (see Dipnoi), 
 lungs and swim-bladder, 255, 256. 
 Lyell, Sir Charles, 324, 385. 
 Lyfjosoma dendyi, 417. 
 
 ,, moco, 417. 
 lyre bird, 349, Fig. 178. 
 lysin theory of fertilization, 146, 147. 
 
 M. 
 
 MACCULLOCH, 340. 
 machine, analogy of, 2, 3, 10. 
 Macropodia rostrata, 339, 340, Fig. 1 72. 
 magnesium larvae, 157, Fig. 75. 
 maize root, section, 37, Fig. 7. 
 male animal, 114. 
 
 characters, 84, 85, 127. 
 malic acid, attracting spermatozoa, 
 
 141. 
 
 Malthus, 386. 
 Mammalia, compared with Eeptilia, 
 
 232 235. 
 dispersal, 323. 
 geological history, 284, 
 
 301304. 
 gigantic, 406. 
 limbs, 235. 
 
 nutrition of young, 270. 
 origin, 300. 
 ovum, 140, Fig. 71. 
 mammoth, Buffon on extinction of, 
 
 366. 
 
 man, aesthetic development in- 
 fluenced by insects, 364. 
 ,, and apes, relations, 422, 423, 
 424, 425. 
 ,, ,, ,, Buffon on, 
 
 370. 
 
 ,, ,, Lamarck 
 
 on, 382. 
 antiquity, 284, 303, 422, 423. 
 
 man, control of environment, 426. 
 evolution, 422 -127. 
 ,, Charles Darwin on, 
 
 423. 
 ,, influence on other organisms, 
 
 396. 
 
 limbs, 237, Fig. 94. 
 ,, Mendelian inheritance in, 205. 
 ,, progress of, 422 427. 
 ,, races of, 423. 
 ,, reversion in, 262. 
 vestigial hair, 261. 
 tail, 261. 
 Mantidse, 338, 339. 
 manubrium, 120. 
 Maoris and Morioris, 398 400. 
 marginal canal, 120. 
 marine animals, dispersal, 321. 
 
 fauna and flora, 323. 
 Marsh, O. C., 262, 307. 
 marsh tit, distribution, 319, 320. 
 Marsilea, 105. 
 marsupial mole, 253. 
 wolf, 251. 
 Marsupialia, 250, 301, 331. 
 
 distribution, 329, 330, 
 
 331, 332. 
 extinct, 302, 332. 
 Mastodon, 314. 
 Mastodonsaurus, 296. 
 maternal chromosomes, 137, Fig. 68. 
 functions, a handicap, 127. 
 matter, indestructibility of, 6. 
 maturation of germ cells, 132, 137, 
 
 138, Figs. 65, 68. 
 in heredity, 171. 
 Mauritius, 258. 
 Mediterranean fauna, 323. 
 medusae, 120, 121, 123, Fig. 60. 
 megagametes, 85, 88, 90. 
 megalecithal, 267. 
 meganucleus, 39, 92. 
 megasporangia, 107. 
 megaspores, 105, 106, 107, 108. 
 Megatherium, 303. 
 meiosis, 132, 206. 
 memory, 186, 191, 192. 
 Mendel, G. J., 195. 
 Mendelian experiments, 194209. 
 ,, inheritance in Leptino- 
 
 tarsa, 160. 
 in Primula, 356. 
 ,, principles, application of, 
 
 205. 
 
 ,, proportions, 199. 
 Mendelism, 195. 
 
 ,, and mutations, 415.
 
 444 
 
 INDEX 
 
 Men ura superba, 349, Fig. 178. 
 
 meristem, 76. 
 
 meristic mutations, 154. 
 
 variations, 148, 149. 
 merogony, 146, 174. 
 Merychippus, 310, Fig. 153. 
 mesentery, 54. 
 mesoblast, 48, 266. 
 mesoblastic somites, 266. 
 mesoderm, 48, 124. 
 mesogloea, 115, 116. 
 Mesohippus, 310, Figs. 153, 156. 
 mesophyll, 50, 64, Fig. 26. 
 Mesozoic Era, 284, 285. 
 metabolism, 9, Fig. 1. 
 metacarpals, 238. 
 metagenesis, 121. 
 metameric segmentation, 149, 266, 
 
 279. 
 
 metamorphic rocks, 283. 
 metamorphosis, 263. 
 Metaphyta, 44, 95. 
 metapodials, 240, 241. 
 metatarsals, 238. 
 Metatheria, 301, 302. 
 Metazoa, 44. 
 mice, dispersal, 325. 
 
 ,, experiments in heredity, 179, 
 
 182. 
 
 microgametes, 85, 88, 91. 
 inicrolecithal, 267. 
 Microlestes, 301, 302. 
 micronucleus, 39, 92. 
 micro pyle, 109. 
 microsporangia, 107. 
 microspores, 105, 106, 107. 
 microzooids, 33. 
 migration from north, 329. 
 milk, 271. 
 mimicry, 343348. 
 
 ,, and fluctuating variation, 
 
 415. 
 ,, and natural selection, 396, 
 
 405. 
 
 rings, 345. 
 mineral salts, 23. 
 Miocene Epoch, 284. 
 Miohippus, 310. 
 mitosis, 6979, Figs. 3135. 
 
 ,, in heredity, 169. 
 mnemic theory of heredity, 186, 191, 
 
 192. 
 
 moas, 258, 397. 
 models and mimics, 345, 348. 
 Mceritherium, 313, Fig. 159. 
 moles, marsupial, 253. 
 Molluscoida, 277. 
 
 momentum in evolution, 406 409. 
 monads, 83. 
 Monera, 66. 
 monoecious, 103, 105. 
 monohybridism, 201, Fig. 79. 
 monopodial branching of phyloge- 
 
 netic tree, 230, 231, Fig. 87. 
 monosome, 135. 
 
 Monotremata, 234,296, 301, 302, 331. 
 monstrosities, 154, 157. 
 Morioris, extermination, 398 400. 
 mosses, dispersal, 321. 
 moths, clear- winged, 343, Fig. 175. 
 ,, proboscis, 354. 
 ,, protective resemblance, 338. 
 motion, in living organisms, 6. 
 mountain hare, distribution, 333. 
 mud, dispersal in, 327. 
 mud-fishes, 255, 291. 
 Muller, Fritz, 346. 
 Miillerian mimicry, 346. 
 multicellular, 38. 
 
 ,, organisms, origin of, 
 
 44. 
 
 Multituberculata, 302. 
 multiplication of cells, 6980. 
 Mungoswell's wheat, 412. 
 Muscari, variation in, 150. 
 muscle fibres, 57, 58, 116, Figs. 21, 
 
 22, 58. 
 
 music and memory, 192. 
 mutation and adaptation, 414, 415. 
 evolution, 414. 
 hybridization, 413 
 
 414. 
 
 ,, Mendelism, 415. 
 ,, natural selection, 414. 
 theory, 224. 
 mutations, 149, 153156, 224. 
 ,, meristic, 154. 
 
 origin, 159, 160. 
 ,, of species from, 
 
 411, 412, 413. 
 ,, substantive, 154. 
 mutilations, inheritance of, 178, 179. 
 mutual adaptation, 414, 415. 
 Mycetozoa (Myxoinycetes), 67, Fig. 
 29. 
 
 N. 
 
 NAGELI, 167. 
 
 natural affinities, 228, 229. 
 
 selection, 395409. 
 ,, ,, and mutation, 414. 
 
 A. E. Wallace on, 
 
 389391. 
 Buffon on, 368.
 
 INDEX 
 
 445 
 
 natural selection, Charles Darwin on, 
 
 385388, 392. 
 ,, ,, Erasmus Darwin 
 
 on, 373. 
 ,, ,, insufficiency of, 
 
 404. 
 
 ,, ,, insufficiency of, 
 
 Charles Darwin 
 on, 392. 
 ,, summary of theory, 
 
 391. 
 
 ,, system, 226. 
 nectaries, 359, 360, 414, 415. 
 Neoceratodus, 255, 291, Fig. 110. 
 Neohipparion, 310, Fig. 157. 
 Nepenthes, 262. 
 Nerocila, 227. 
 nerve-cells, 58, Figs. 23, 24. 
 
 fibres, 58. 
 nerves, 59. 
 
 nervous system, 18, 121, 266. 
 neurons, 58, 59. 
 
 neuter insects and heredity, 189, 190. 
 newts, 294. 
 New Zealand, fauna and flora, 258, 
 
 328, 329, 331. 
 nipples, vestigial, 345. 
 nitrifying bacteria, 218. 
 Nitrobacter, 218. 
 Nitrosomonas, 218, Fig. 84. 
 nomenclature, 226, 227. 
 non-adaptive characters, 419 422. 
 normal curve of variation, 153. 
 
 ,, variations, 150. 
 notochord, 266. 
 Notornis, 397. 
 Notoryotes, 253. 
 nucellus, 108. 
 nuclear membrane, 69. 
 nucleinic acid, 71. 
 nucleolus, 70, 77. 
 nucleoplasm, 14, 22, 69. 
 nucleus, 13, 69. 
 
 ,, division (see mitosis and 
 amitotic nuclear division), 
 in heredity, 161, 166, 174. 
 ,, zygote, 83. 
 numerical variations, 149. 
 nutrition, 7 (see also food materials). 
 of embryo in Coelomata, 
 
 O. 
 
 OBELIA, 119, 265, Fig. 60. 
 ocean, age of, 287. 
 
 oceanic islands, fauna, 330, 332, 397. 
 oceans, permanence, 329. 
 
 (Enothera, mutation in, 155. 
 
 oil-foain, 19, 21. 
 
 Olenus cataractes, Fig. 134. 
 
 Oligocene Epoch, 284. 
 
 ontojjenetic record, obscuring of, 
 
 267, 278. 
 ontogeny, 263. 
 
 abbreviation of, 273. 
 
 a habit, 192. 
 and phylogeny, 265, 279, 
 
 Fig. 131. 
 ,, Erasmus Darwin on, 371, 
 
 372. 
 
 ,, interpretation of, 277. 
 Onychophora, distribution, 330. 
 oocytes, 133. 
 oogenesis, 133, Fig. 65. 
 oogonia, of Fucus, 100. 
 
 ,, in oogenesis, 133. 
 oospheres, 90, 100, 104, 108. 
 ooze, 26, 283. 
 opal, 24, 26. 
 Ophisaurus, 250. 
 Ophiiira ciliariSf Fig. 126. 
 ophiuroids, 275, 276. 
 opossums, 301. 
 
 opposable great toe and thumb, 424. 
 orang, brain, 425. 
 " orang utan," Buffon on, 370. 
 Orchidacete, fertilization, 359 362. 
 Orchis mascula, fertilization, 359 
 
 362, Fig. 182. 
 orders, 226. 
 
 Ordovician Epoch, 284. 
 organellse, 39. 
 organic evolution, doctrine of, 212 
 
 (see also evolution). 
 units, 38, 65. 
 organism, 3. 
 
 organisms, first appearance of, 212. 
 nature of first, 217, 219. 
 ,, number and varietj r of, 
 
 211. 
 
 origin of, 214216. 
 organizing spirits, A. E. Wallace on, 
 
 394. 
 
 organs, 12, 38. 
 
 origin of great groups, 303, 304. 
 living things, 214216. 
 sex, 84, 85, 89, 126128. 
 Ornithorhynchus, 234, 260, 301, 
 
 Fig. 91. 
 
 Ornithosauria, 299. 
 Ornithoscelida, 297. 
 Orohippus, 310, Figs. 153, 155. 
 Osborn, 369. 
 osmosis, 8, 17, 23.
 
 446 
 
 INDEX 
 
 osmotic pressure, effect on egg, 145. 
 Ostracodermi, 289, 290, Figs. 135, 
 
 136. 
 
 Otaria hookeri, paddles, Fig. 100. 
 otter sheep, 154. 
 ovary, of animals, 113, 116. 
 
 ,, flower, 107. 
 oviducts, 125. 
 ovotestis, 113. 
 
 ovule, 48, 106, 107, Fig. 55. 
 ovum, 85. 
 
 ,, interpretation, 279. 
 ,, maturation, 133, Figs. 65, 68. 
 ,, of Amphioxus, 45, Fig. 13. 
 ,, Ascaris, 131, Fig. 64. 
 bird, 140, Fig. 70. 
 Chara, 141. 
 
 ,, Coccidium, 88, Fig. 39. 
 ,, Eudorina, 91, Fig. 40. 
 fern, 104, Fig. 52. 
 ,, flowering plant, 108, 109, 
 
 Fig. 55. 
 Fucus, 100, Fig. 47. 
 
 Hydra, 116, Figs. 57, 59. 
 ,, ,, mammals, 71, 271, Fig. 
 
 140. 
 
 ,, ,, medusae, 121. 
 ,, plants, 141. 
 rabbit, Fig. 71. 
 ,, ,, sponges, 113. 
 
 Volvox, 91, Fiir. 11. 
 segmentation of, 263, 265. 
 ,, similarity in different or- 
 ganisms, 162. 
 ,, size of, 267. 
 typical, 140, Fig. 69. 
 oxen, feet, 241. 
 oxidation, 5. 
 oxlip, fertilization, 356. 
 oxygen, 5, 6, 8, 31. 
 oyster, American, introduced, 323. 
 
 P. 
 
 PADDLES, 236, 244, 247. 
 pademelons, coloration, 337. 
 paedogenesis, 144. 
 pairing of chromosomes, 133, 137, 
 
 206, Figs. 65, 66, 68. 
 Palaeohatteria, 295. 
 Palseomastodon, 313, Fig. 159. 
 palaeontology, evidence afforded by, 
 
 287304. 
 
 Palaeospondylus, 289. 
 Palaeozoic Era, 284, 285. 
 palingenetic characters, 277. 
 palms, climbing, 350. 
 
 Pandorina, 42, 89, Fig. 10. 
 pangenesis, 164166, 369, Fig. 76. 
 Papilio, mimicry in, 346348, 
 
 Fig. 177. 
 parallel modification of body and 
 
 germ cells, 182. 
 
 Paramcocium, 39, 98, 139, Fig. 8. 
 paraphyses, of Fucus, 100. 
 parasites, peculiarities of, 401 403. 
 parasitism, of gametophyte, 112. 
 parenchyma, 65. 
 Pariasaurus, 297, Fig. 140. 
 parietal eyes, 258260, Figs. 114 
 
 116. 
 
 foramen, 260. 
 parthenogenesis, 143, 1-14. 
 
 artificial, 144147. 
 Paruspalustris, distribution. 319,320. 
 passivity of female, 84, 85. 
 Pasteur, 215. 
 
 Patagonia, beech forests, 329. 
 paternal chromosomes, 137, Fig. 68. 
 peach trees, inheritance of acquired 
 
 characters, 182. 
 
 peacock, epigamic ornamentation, 349. 
 Pearson', Karl, 210. 
 pea, Sicilian, 208. 
 ,, structure and germination of 
 
 seed, Fig. 56. 
 peas, Mendelian experiments on, 
 
 196199. 
 
 pedigrees, 229, 311. 
 Peebles, Florence, 28. 
 pelagic organisms, 322, 336. 
 pentacrinoid stage of Antedon, 274, 
 
 275, Fig. 124. 
 
 Pentacrinus, 274, Fig. 125. 
 pentadactyl limbs, 237, 294 (see also 
 
 limbs and feet), 
 periblem, 50. 
 
 Pericopis anyulosa, 345, Fig. 176. 
 Peripatus, Fig. 167. 
 
 ,, distribution, 330. 
 perisarc, 119. 
 perissodactyl, 241, 253. 
 peristaltic movement, 57. 
 peritoneal epithelium, 54, Fig. 16. 
 Permian Epoch, 284. 
 Perrhybris malenka, 345, Fig. 176. 
 persons in colony, 119. 
 Petalotricha, Fig. 9. 
 petals, 106. 
 
 phagocytes, phagocytosis, 52. 
 phalangers, 301. 
 phalanges, 238. 
 
 Phascolotherium, 302, Fig. 149. 
 Phenacodus, 309, Fig. 152.
 
 INDEX 
 
 447 
 
 phloem, 65. 
 
 phosphorescent organs, 336. 
 phosphorus, 22, 71. 
 photosynthesis, 30, 31. 
 phyla, "226. 
 
 Pliyllopteryx eques, 341, Fig. 174. 
 phylogenetic tree, 229. 
 phylogeny, 229. 
 
 ,, and classification, 230, 
 
 231, Fig. 87. 
 ,, ontogeny, 265, 279, 
 
 Fig. 131. 
 
 ,, Erasmus Darwin on, 372. 
 physical basis of life, 7. 
 
 ,, conditions of earth, changes 
 
 in, 327. 
 physiological selection, 418. 
 
 unit, the cell, 6$. 
 Phytoflagellata, colony formation, 40, 
 
 42. 
 evolution of sex, 89 
 
 91. 
 
 Pieriiiae, 345. 
 pig, feet, 241, Fig. 98. 
 pineal eyes, 258 260, Figs. 114 116. 
 
 gland, 11, 260. 
 pin-eyed flowers, 356. 
 pistil, 106. 
 
 Pisinn sativiim, varieties, 196. 
 pitcher plants, 262. 
 1'itliecanthropus erect us, 423. 
 pituitary body, 408. 
 placenta, 270. 
 placenta! mammals, appearance of, 
 
 302. 
 
 Playianthiis betulinus, 417. 
 PlagiaulacidjB, 302. 
 Plagiaulax, 302, Fig. 148. 
 I'ldnema pogyei, 346. 
 plankton, 322. 
 plants, compared with animals, 34, 35. 
 
 dispersal of, 321. 
 plasma, 51. 
 plasmodia, 67. 
 plasniogamy, 128. 
 plasticity of organisms, 335. 
 plastids, 32. 
 plastogamy, 128. 
 Platypus (see Oniithorhynchus). 
 Pleistocene Epoch, 284. 
 plerorne, 50. 
 
 Plesiosauria, 236, 244, 297. 
 J'lesiosaiirus macrocep /talus, Fig. 141. 
 Pliny, 262. 
 Pliocene Epoch, 284. 
 Pliohippus, 310, 311, Figs. 153, 158. 
 
 ploughshare bone, 305. 
 
 plumage of birds, 349. 
 
 plumcots, 204. 
 
 pluteus larva), 275, 276, Fig. 127. 
 
 polar bodies, 133, 135, 139, 145, Figs. 
 
 35, 65, 68. 
 
 pole-cell, in insect egg, 130. 
 pollen, collection of, 355. 
 ,, grains, 106, 107. 
 sacs, 107. 
 tube, 108. 
 pollination, 110, 111. 
 
 ,, adaptations for, 35 1363. 
 pollinia, 361. 
 
 polyanthus, pollination, 356. 
 poly dactyli sin, 149. 
 polymorphic colony, 119. 
 polymorphism in Papilio, 346, Fig. 
 
 177. 
 
 polype, fresh water (see Hydra). 
 Polypodium, archegonium, Fig. 52. 
 Poly/oa, dispersal, 327. 
 population, Buff on on, 368. 
 
 ,, Charles Darwin on, 386. 
 ,, Lamarck on, 381. 
 Wallace on, 389. 
 porpoises, convergence in, 248. 
 Portunus, 247, Fig. 102. 
 
 larva, Fig. 128. 
 postaxial, 238. 
 potential energy, 7. 
 Poulton, E. B., 336, 337, 346. 
 preaxial, 238. 
 
 Pre-Cauibrian Epoch, 284, 285. 
 pre-formation in development, 163. 
 prehension, organs of, 256. 
 prepotency, 353, 358. 
 presence or absence hypothesis, 207. 
 Primary Era, 284, 285. 
 Primates, 301, 303. 
 primordia, in germ plasm, 206. 
 primordial germ cells, 113 (see also 
 
 germ cells, origin). 
 ,, utricle, 62, 95. 
 
 Primula, fertilization, 356358. 
 priority in nomenclature, 227. 
 Proboscidea, evolution, 312 314, 
 
 Fig. 159. 
 proboscis, of bee, 354, Fig. 179. 
 
 ,, butterflies and moths, 
 
 354. 
 
 elephants, 256. 
 , , insects, Erasmus Darwin 
 
 on, 373. 
 ,, mutual adaptation, 
 
 414, 415. 
 procryptic coloration, 337 342.
 
 448 
 
 INDEX 
 
 progress, human, 426, 427. 
 progressive development, E. Cham- 
 bers on, 384. 
 ,, evolution, 334. 
 
 ,, cause of, 192. 
 
 Wallace on, 390. 
 ,, series, 232, 234. 
 
 pronuclei, 131, 145, 146, Fig. 64. 
 protandrous, 353, 363. 
 protective resemblance, 337 342. 
 ,, and fluctuating varia- 
 
 tion, 415. 
 ,, ,, natural selection, 
 
 396, 405. 
 proteide, 7, 22. 
 
 Proterotherium, 253, Fig. 109. 
 Proteus, 183. 
 prothallus, of fern, 103. 
 
 ,, ,, flowering plant, 108. 
 
 Protista, 38. 
 
 ,, dispersal of, 327. 
 ,, immortality of, 161. 
 protogynous, 353. 
 Protohippus, 310. 
 Protophyta, 38, 95. 
 
 , , sexual differentiation in, 
 
 91. 
 protoplasm, 2, 38. 
 
 ,, chemical composition, 
 
 22, 23. 
 
 controlling power, 23. 
 energy in, 7. 
 physical properties, 21. 
 selective action, 26. 
 sensitive to stimuli, 188. 
 streaming, 62. 
 Protopterus, 255, 291. 
 Protorohippus, 310, Fig. 153. 
 Prototheria, 301, 302. 
 Protozoa, 38, &c., Figs. 3, 4, 9, &c. 
 ,, non-adaptive characters. 
 
 419. 
 
 ,, nuclear reduction, 139. 
 ,, powers of multiplication, 
 
 81. 
 
 sexual differentiation, 89. 
 Prozeuglodon, 318, Fig. 163. 
 prunes, stoneless, 204. 
 Pseudopanax chathamicum, 417. 
 crussifolium, 417. 
 
 ferox, 417. 
 pseudopodia, 15. 
 Pteranodon, 299, Fig. 147. 
 Pteraspis, 290. 
 Pterichthys, 290, Fig. 136. 
 Pterosaur'ia (pterodactyls), 236, 242, 
 299, Figs, 99, 147. 
 
 Fterostylis trulli folia, fertilization, 
 
 362. 
 
 Pteri/yotus osiUensis, Fig. 137. 
 Palchriphylliam crurifolium, 338, 
 
 Fig. 170. 
 
 Punnett, E. C,, 208. 
 purity of gametes, 200, 206. 
 putrefaction, 218. 
 
 Pyrenees, arctic vegetation on, 333. 
 pyrenoids, 32, Figs. 5, 42. 
 
 Q. 
 
 QUADRUMANA, 424. 
 quartz, 24. 
 
 E. 
 
 EABBIT, embryo, 265, Fig. 117. 
 
 shoulder girdle, 234, Fig. 90. 
 racial characters, transmission, 175. 
 radial canals, 120. 
 Eadiolaria, 25, Fig. 3. 
 radius, 237. 
 
 rafts, dispersal by, 324. 
 range in time of animal groups, 284. 
 Ranunculus aquatilis and Ranunculus 
 
 fiederaceus, Lamarck on, 379. 
 rats, 179, 182, 325. 
 recapitulation hypothesis, 265281. 
 ,, ,, anticipated 
 
 by Erasmus Darwin, 372. 
 receptor, 18. 
 
 recessive characters, 197, 207. 
 recognition marks, 337. 
 red blood corpuscles, 52, 53, Fig. 15. 
 ,, clover, robbed by humble bee, 
 
 355, 356. 
 
 Eed Sea, fauna, 323. 
 red snow, 27. 
 
 reduction of chromosomes (reducing 
 division), 132139, 206, 207, Figs. 
 6568. 
 
 regeneration, 167. 
 regression, filial, 210. 
 rejuvenescence, 92. 
 reproduction, 10. 
 Eeptilia, compared with Amphibia, 
 
 294. 
 ,, compared with Mammalia, 
 
 232, 235. 
 fossil, 295299. 
 geological range, 284. 
 giant, 406. 
 limbs, 235. 
 origin, 294, 295.
 
 INDEX 
 
 449 
 
 respiration, 8. 
 
 ,, ia Amoeba, 17. 
 
 ,, ,, embryos, 270. 
 
 ,, plants, 32, 33. 
 response to stimuli, 18. 
 Reunion, peach trees in, 182. 
 reversion, 261, 262. 
 
 Mendelian explanation, 
 
 208. 
 
 rhinoceros, feet, 241. 
 Rhynchocephalia, 259, 295 (see also 
 
 Sphenodon). 
 ribbon-wood, 417. 
 ribs, meristic variation, 149. 
 right whale, 245, Fig. 101. 
 Ritzema Bos, 179. 
 rock-formation, 282. 
 rodents, 301. 
 " Rodney," brig, 399. 
 Rodriguez, 258. 
 Romanes, G. J., 333, 417, 418. 
 Rontgen rays, 187. 
 root-cap, 50. 
 
 root, growing point, 76, Figs. 33, 34. 
 Rosenthal, 179. 
 rostellum, 360. 
 
 rotifers, parthenogenesis, 144. 
 rudimentary organs (see vestigial 
 
 organs). 
 ,, ,, Charles Darwin 
 
 on, 392. 
 
 Rumia crattryata, larva, Fig. 169. 
 rust, in w>eat, 205. 
 rye, fertilization, 412. 
 
 S. 
 
 SA< CULIXA, degeneration, 401, Fig. 
 
 183. 
 
 sage (see Salvia). 
 Sagitta, origin of germ cells, 130, 
 
 Fig. 63. 
 
 salt, in sea, 287. 
 Salvia, structure of flower and 
 
 fertilization, 358, 359, Fig. 181. 
 Salvinia, 105. 
 
 Sandwich Islands, oceanic, 332. 
 sap, 65. 
 
 ,, nuclear, 70. 
 saprophytic nutrition, 34. 
 sarcode, 38. 
 Sarracenia, 262. 
 Sanropsida, 300. 
 Sauropterygia, 297, Fig. 141. 
 scapula, 234, Figs. 89, 90, 93. 
 scents, of flowers, 355. 
 
 Schleicher, 69. 
 Schleiden, 38. 
 Schoetensack, 423. 
 Schwann, 38. 
 ticrophularia nodosa, 353. 
 scurf, 55. 
 
 sea anemones, 114. 
 ,, coast, selection in, 395. 
 ,, firs, 114. 
 
 ,, urchins, artificial partheno- 
 genesis, 145. 
 
 ,, ,, blastoineres, 45. 
 
 ,, ,, hybridization, 174, 
 
 175. 
 
 larva-, 276. 
 
 seal, paddles, 244, Fig. 100. 
 Secondary Era, 284, 285. 
 
 ,, sexual characters in 
 
 Coelomata, 125. 
 secretions, controlling growth, 408. 
 
 internal, 126. 
 
 sedimentary rocks, 282, 28(5. 
 sedimentation, rate of, 285, 286. 
 seed, 109, Fig. 56. 
 coat, 109. 
 ,, leaves (sfe cotyledons), 
 seedling of Acacia, 280, Fig. 132. 
 
 ,, ,, pea, Fig. 56. 
 seeds, dispersal ,321. 
 Seeley, II. G., 297. 
 segmentation, metameric, 149, 266. 
 segmentation of ovum, 45, 263. 
 in Amphioxus, 45, Fig. 13. 
 ,, birds and reptiles, 270. 
 frog, 268, Fig. 119. 
 Hydra, 118. 
 ,, mammals, 271. 
 interpretation of, 265. 
 segregation of germ cells (see germ 
 
 cells, origin), 
 selection, artificial, 395, 396. 
 
 ,, ,, Charles Darwin 
 
 on, 387. 
 
 ,, by insects, 396. 
 ,, continuous, 411. 
 ,, germinal, 173. 
 ,, natural (see natural selec- 
 tion). 
 ,, not confined to living 
 
 things, 395. 
 
 ,, sexual (see sexual selec- 
 tion). 
 
 single, 412. 
 
 self-fertilization, 351, 352, 353. 
 sematic colours, 336, 342. 
 Seinon, Richard, 186. 
 sensation, 16.
 
 450 
 
 INDEX 
 
 sense cells, stimulation of, 188. 
 
 ,, organs, 18, 121. 
 sepals, 106. 
 
 series, progressive, 232, 23-1. 
 tiesia crabroniformis, Fig. 175. 
 sex determination in insects, 135, 
 
 Pig. 67. 
 sexes, fundamental distinction 
 
 between, 127. 
 
 sexual characters transferred to 
 asexual generation, 110, 
 111. 
 
 ,, differentiation, origin of, 128. 
 sexual phenomena, 84, 85. 
 evolution of, 126, 127. 
 in Bodo, 87, Fig. 38. 
 Coccidium, 88, Fig. 39. 
 Ccelomata, 125. 
 Copromonas, 83, Fig. 37. 
 Eudorina, 91, Fig. 40. 
 ferns, 104, 105, Figs. 51, 52 
 flowering plants, 108, 109, 
 
 Figs. 54, 55. 
 
 Fucus, 100, 101, Fig. 47. 
 Hsematococcus, 33, Fig. 5. 
 Hydra, 116. 
 Obelia, 121. 
 
 Pandorina, 89, Fig. 10. 
 Paramoecium, 93, Fig. 41. 
 Spirogyra, 96, 97, Figs. 43, 44. 
 Volvox, 91, Fig. 11. 
 Zygogonium, 97. 
 sexual reproduction, 33. 
 
 ,, selection, Charles Darwin on, 
 
 388. 
 ,, ,, Erasmus Darwin 
 
 on, 373. 
 
 Shand, Alexander, 399. 
 sharks, fossil, 291. 
 sheep. Ancon or otter, 154. 
 
 feet, 241. 
 
 shepherd's purse, 48, Fig. 14. 
 Shirreff, Patrick, 412. 
 shoulder girdle, of mammals, 234, 
 
 Fig. 90. 
 ,, ,, ,, Ornithorhynchus, 
 
 234, Fig. 93. 
 reptiles, 233, 234, 
 
 Fig. 89. 
 
 Sicilian pea, 208. 
 silica, 23, 24. 
 siliceous skeleton, 25. 
 Silurian Epoch, 284. 
 single selection, 412. 
 Sirenia, 301. 
 
 paddles, 236, 244. 
 size, increase of, 309, 405409. 
 
 skeleton (see feet, limbs, paddles 
 
 and wings). 
 ,, of extinct animals, Figs. 
 
 139152. 
 giraffe, Fig. 104. 
 ,, horse and man, Fig. 95. 
 
 kiwi, Fig. 112. 
 Obelia, 119. 
 Protista, 2326, Figs. 
 
 3,4. 
 
 whale, Fig. 101. 
 skulls of dog and thylacine, 251, 
 
 Figs. 107, 108. 
 slime fungi (see Mycetozoa). 
 sloths, 333. 
 
 slow worm (see Anyuisfragilis}. 
 snails, dispersal, 325. 
 snakes, mimicry in, 343. 
 social problems, de Vries on, 414. 
 sodium, in sea water, 287. 
 sole of foot, Buff on on, 368. 
 solitaire, 258, 397. 
 SoUas, W. J., 286, 287, 311. 
 soma and germ cells contrasted (see 
 
 germ cells and somatic cells). 
 somatic and germ nuclei in Para- 
 
 moacium, 98. 
 
 somatogenic characters, supposed 
 non-inheritance, 169, 176 (see also 
 acquired characters), 
 somatogenic variations, 149, 136 
 
 157. 
 
 somatopleure, 124. 
 somites, 266. 
 soul, 11, 19. 
 South America, fauna and flora, 328, 
 
 329, 333. 
 
 Spalacotherium, 302. 
 special creation, 212 214, 222, Fig. 
 
 82. 
 
 Buff on 011, 369. 
 ,, Lamarck on, 375, 
 
 376. 
 species, aggregate, 224. 
 
 Charles Darwin on, 223. 
 ,, definition of, 223. 
 ,, elementary, 224. 
 ,, Lamarck on, 375. 
 ,, modification of, Buffon on, 
 
 366, 367. 
 
 ,, nature of, 222224. 
 ,, number of living, 211. 
 ,, origin from mutations, 225. 
 ,, supposed immutability of (see 
 
 immutability of species). 
 ,, transformation of, Lamarck 
 on, 376, 377.
 
 INDEX 
 
 451 
 
 specific characters, non-adaptive, 
 
 419422. 
 speech, an acquired character, 156, 
 
 184. 
 
 ,, evolution of, 425. 
 Spencer, Herbert, 2, 185, 391. 
 sperinary, 113. 
 
 spermatids, 133, Figs. 65, 66. 
 spermatist (Erasmus Darwin), 371. 
 spermatocytes, 133, Figs. 65, 66. 
 spermatogenesis, 132, 133, 134, 135, 
 
 Figs. 65, 66. 
 
 spermatogonia, 133, Figs. 65, 66. 
 spermatozoa, 85, 139, Fig. 69. 
 
 ,, chemo taxis in, 141. 
 
 ,, development (see sper- 
 
 matogenesis). 
 of Chara, 141. 
 Coccidium, 88, 141, 
 
 Fig. 39. 
 
 ,, Eudorina,91,Fig. 40. 
 
 fern, 104, 141, Fig. 
 
 51. 
 
 Fucus, 100, Fig. 47. 
 Hydra, 116. 
 medusa?, 121. 
 mosses, 141. 
 sponges, 113. 
 Volvox, 91, Fig. 11. 
 spermatozoids, 91. 
 
 sphserechinus, experiments in hybri- 
 dization, 174. 
 
 Sphserella (see Hsematococcus). 
 Sphserozoum, 43, 265, Fig. 12. 
 tSphenodon pundatm, 259, 260, 295, 
 
 320, 331, Figs. 113, 114. 
 spicules (see sponges, spicules). 
 spider crab, 339, 340, Fig. 172. 
 spindle, nuclear, 71. 
 spines, excessive development, 406. 
 spiny anteater (see Echidna), 
 spireme, 71. 
 
 Spirogyra, 9597, 142, Figs. 4244. 
 splanchnopleure, 124. 
 splint bones, 241, 257. 
 sponges, deep sea, 335. 
 
 ,, dispersal of fresh water, 
 
 327. 
 ,, gemmules of fresh water, 
 
 327, Fig. 166. 
 ,, germ cells, 113. 
 ,, non - adaptive characters, 
 
 419421. 
 spicules, 26, 232, 419, 420, 
 
 Figs. 88, 187. 
 
 ftponyilla fluriatilis, gemmules. Fig. 
 166. 
 
 spontaneous generation, 214 216, 
 
 219. 
 
 Lamarck on, 374. 
 sporangia of fern, 101, Fig. 48. 
 
 flowering plant, 107, 
 
 Fig. 53. 
 spores, 101. 
 
 ,, dispersal, 321. 
 ,, formation, reduction of chro- 
 mosomes, 138. 
 germination, in fern, 103. 
 of Bodo, 87. 
 ,, ,, Coccidium, 89. 
 sporophylls, 107. 
 sporophyte, 101. 
 
 ,, of fern, 105, Fig. 48. 
 
 ,, ,, flowering plants, 106. 
 
 ,, number of chromosomes, 
 
 139. 
 
 sports, 154, 224 (see also mutations). 
 Squalodontidae, 318. 
 Squamata, 295. 
 
 stability of organisms, 155, Fig. 74. 
 stag, antlers, 125. 
 staining reactions, 62, 70. 
 stamens, 106. 
 staminodes, 262. 
 starch, 31. 
 
 star-fish, meristic variation, 149. 
 statoblasts, 327, Fig. 165. 
 stature, variation in human, 152. 
 Staurocephalus murchisoni, Fig. 134. 
 Stebbing, T. R. R, 339. 
 Stegocephalia, 293. 
 Stegosaurus, 299, Fig. 145. 
 Stenobothrus viridulus, spermato- 
 genesis, Fig. 66. 
 Stentor, Fig. 9. 
 Stereognathus, 302. 
 sterility of species when crossed, 
 
 418. 
 
 St. Helena, an oceanic island, 332. 
 stick caterpillars, 337, 338, Fig. 
 
 169. 
 
 Stieda's organ, 260, Figs. 115, 116. 
 stigma, of flower, 107. 
 stimulation of protoplasm and cells, 
 
 188. 
 stimuli, 18. 
 
 ,, to development, 145, 146. 
 stimulus, transmission between cells. 
 
 143. 
 ,, to germ cells, 
 
 186190. 
 Stockard, 157. 
 stomata, 64, Fig. 26. 
 storms, dispersal by, 324.
 
 452 
 
 INDEX 
 
 stratified epithelium, 56, Fig. 18. 
 
 rocks, 282286. 
 striped muscle, 57, Fig. 22. 
 struggle for existence, 
 
 A. E. Wallace ou, 389. 
 Buffon on, 368. 
 Charles Darwin on, 386. 
 importance of, 397, 398, 400. 
 Lamarck on, 381. 
 stump tails, 179. 
 style, of flower, 107. 
 Stylidium graminifolia, fertilization, 
 
 362. 
 
 subclasses, 226. 
 subfamilies, 226. 
 subgenera, 225. 
 subkingdoms, 226. 
 suborders, 226. 
 substantive mutations, 154. 
 
 ,, variations, 148, 150. 
 
 succulent plants, 350. 
 Sunnier, F. B., 182. 
 sun, dependence of life upon, 5. 
 suprascapula, 234. 
 surface, relation to volume, 20. 
 
 ,, tension, 19. 
 survival of fittest, 391. 
 
 Buffon on, 368. 
 ,, Charles Darwin on, 
 
 387. 
 ,, ,, in inorganic world, 
 
 395. 
 
 suspensor, 49. 
 Svalof, 412. 
 
 sweet peas, reversion in, 208. 
 swim bladder and lungs, 255, 256. 
 synaposematic groups, 343 346, 
 
 Fig. 176. 
 
 synapsis, 133, 137, 206, Figs. 65, 66, 
 "68. 
 
 synaptic mates, 135. 
 syncytia, 66. 
 
 syncytial epithelium, 67, Fig. 28. 
 syndesis, 137 (see also synapsis). 
 syngamy, 83 (see also conjugation), 
 systematist, work of, 226. 
 
 T. 
 
 TADPOLE, ascidian, 277, Fig. 130. 
 
 frog, 272, Fig. 121. 
 
 stage, 278, 279. 
 tail, in man, 261, 262, 424. 
 tails, mutilation of, 178, 179. 
 tapir, feet, 241, Fig. 96. 
 tarsals, 237. 
 taxonomic tree, 228, 229, Fig. 86, 
 
 taxonomy, 226. 
 
 aims of, 229. 
 
 teeth, of Cetacea, 317, 318, Figs. 162, 
 163. 
 
 ,, ,, dog and thylacine, 251 
 253, Figs. 107, 108. 
 
 ,, ,, elephants, 314. 
 horses, 309. 
 
 ,, vestigial, 260. 
 Teleostei, 292. 
 telescoping of generations, 111, 112, 
 
 122. 
 
 temperature of body, 6. 
 tentacles, 115, 118. 
 Tertiary Era, 284, 285. 
 testis, 113, 116. 
 Tetrabelodon angustidens, 314, Fig. 
 
 159. 
 ,, longirostris, 314, Fi-. 
 
 159. 
 Tetraxonida, spicules, 232, Figs. 88, 
 
 187. 
 
 Theosodon, 253, Fig. 109. 
 Theromorpha, 296. 
 Thoatherium, 253, Fig. 109. 
 thrum-eyed flowers, 356. 
 thylacines, 301. 
 
 Thylacinus, 251, Figs. 107, 108. 
 tibia, 237. 
 time, geological, 285 287. 
 
 ,, importance in evolution, Buff OH 
 on, 366. 
 
 ,, in evolution of horse, 311. 
 
 ,, scale, geological, 284. 
 Tinoceras, 303, 406, Fig. 150. 
 Tintinnopsis, Fig. 9. 
 tissue-formation, 45, 50, 51, 75 79. 
 tissues, 37, 5165. 
 Tithorea harmonia, 345, Fig. 176. 
 tools, use of, 424, 425. 
 torpedo, 256. 
 tortoise, 331. 
 
 Tower, W. L., 156, 159, 184. 
 Tradescantia, histology of, 61 65, 
 
 Figs. 25, 26. 
 transformation of species (see 
 
 species), 
 transparency, of pelagic animals. 
 
 322, 336. 
 tree-like classification, 228,229, Fi<- 
 
 86. 
 
 evolution, 221. 
 
 Trematoda, parthenogenesis, 141, 
 Triassic Epoch, 284. 
 Triceratops, 299, Fig. 1-46. 
 trichocysts, 39. 
 Triconodon, 302.
 
 INDEX 
 
 453 
 
 trilobites, 289, Fig. 134. 
 
 Trimen, E., 346. 
 
 Triticwn saticum, Lamarck on, 378. 
 
 Tritylodon, 301, 302. 
 
 tuatara (se p . Hphenodon punctatas}. 
 
 Tubularia, 121, Fig. 61. 
 
 turbellarian, fertilization of polar 
 
 body, 135. 
 
 turbot, colour changes, 342. 
 turtles, dispersal, 323 
 
 limbs, 236, 244. 
 twins, identical, 173. 
 types, in wheat, 412. 
 Tyndall, 215. 
 Typhlopida?, 250. 
 
 ULNA, 237. 
 
 unconscious memory, 191, 192. 
 
 ungulates, 301. 
 
 limbs of, 239241. 
 unguligrade, 240. 
 unicellular, 38. 
 unisexual, 99, 105. 
 unit characters, 200, 206. 
 
 ,, ,, origin of, 415, 416. 
 
 unstriped muscle, 57, Fig. 21. 
 Urccotypltlus africanus, Fig. 106. 
 urea, 9. 
 I *rsus arctos, systematic position, 229, 
 
 Fig. 86. 
 use and disuse, effects of, 156, 166. 
 
 Buffon on, 367368. 
 
 Charles Darwin on, 391, 392. 
 
 Lamarck on, 377, 378, 379, 380, 
 382. 
 
 E. Chambers on, 385. 
 uterus, 125. 
 
 VACUOLES, contractile, 39, 83. 
 Ynrunus, shoulder girdle, 234, Fig. 
 
 89. 
 
 variation, 148160. 
 ,, cause of, 166. 
 
 curve of, 150, 153, Figs. 72, 
 
 ., in grape hyacinth, 150, 
 
 Fig. 72. 
 ,, ,, human stature, 152, 
 
 Fig. 73. 
 variations, blastogenic, 149, 157 
 
 160. 
 
 ,, congenital, 157. 
 
 ,, continuous, 148, 150. 
 
 variations, discontinuous, 149, 153 
 
 156. 
 fluctuating, 148, 150, 
 
 155, Figs. 72, 73. 
 germinal, 149, 157 160. 
 individual, 150. 
 inheritance, 403, 404. 
 meristic, 148, 149. 
 normal, 150. 
 origin, 153, 173, 403, 
 
 404. 
 
 selective value, 403, 404. 
 somatogenic, 149, 156, 
 
 157. 
 
 substantive, 148, 150. 
 vasa deferentia, 125. 
 vascular bundles, 65. 
 
 system in plants, 50. 
 vegetable marrow, flowers, 111. 
 vegetative cell in pollen grain, 108. 
 Vergil, 214. 
 
 vertebrae, meristic variation, 149. 
 vertebral column, development, 266. 
 Vertebrata, geological history, 289 
 
 304. 
 origin, 291. 
 Vespd crabro, Fig. 175. 
 vessels, of plants, 65. 
 vestigial organs, 257262, 271, 345, 
 
 397. 
 ,, Charles Darwin on, 
 
 392. 
 
 vibrations, in determinants, 189. 
 Virchow, 38. 
 vital force, 11, 143, 216. 
 vitalism, 19. 
 
 vitelline membrane, 140. 
 volume, increase of, Lamarck on, 
 
 382. 
 
 relation to surface, 20. 
 Volvox, 42, 91, 97, 98, 265, Fig. 11. 
 Von Baer, 265. 
 Von Mohl, 38. 
 \ r orticeJ/<i, 40, Fig. 9. 
 
 WAISTS, slender, 156. 
 
 Walker, C. E., 175. 
 
 Wallace, A. 11., views of, 319, 329 
 
 389391, 393, 394. 
 warm-blooded animals, 6. 
 warning colours, 342 - 3-ls. 
 
 ,, ,, and natural selec 
 
 tion, 396. 
 
 wasps, coloui-s, 342. 
 waste products, 8, 9.
 
 454 
 
 INDEX 
 
 wattles, life-history, 280. 
 weapons, use of, 424, 425. 
 weeds, dispersal, 325. 
 Weismann, views of, 146, 160 174, 
 176, 177, 178, 179, 180, 185, 189, 
 206, 219. 
 weka, 397, 398. 
 Wells, W. C., 385. 
 Western, 387. 
 whales, ancestry, 314318. 
 
 ,, cervical vertebrae, 248, Fig. 
 
 103. 
 
 convergence in, 248. 
 paddles, 244, Fig. 101. 
 size, 303. 
 
 systematic position, 301. 
 vestigial organs in, 257, 260, 
 
 Fig. 101. 
 wheat, improvement of, 411 413. 
 
 ,, Lamarck on, 378. 
 wind, dispersal by, 323. 
 winged animals, dispersal of, 323. 
 wings, 236. 
 
 convergence in, 248 250. 
 of bat, 243, Fig. 99. 
 bird, 243, 244, Fig. 99. 
 ,, insects, 247. 
 
 pterodactyls, 242, Fig. 99. 
 wireless telegraphy, analogy of, 187. 
 Wolff, C. F., 163. 
 wombats, 301. 
 wood, 65. 
 
 Woodward, A. S., 253, 294, 296, 300, 
 
 301, 302. 
 worms, dispersal, 325. 
 
 XYLEM, 65. 
 
 YOLK, 140, 267. 
 
 ,, corpuscles, 117. 
 
 ,, influence on development, 2(58 
 
 270. 
 ,, sac, 270, 271, Fig. 120. 
 
 ZEUGLODONTID^E, 318. 
 
 zorea larva, 276, Fig. 128. 
 
 zona radiata, 140. 
 
 zooids, 43, 119. 
 
 zoophytes, 34, 119. 
 
 zoospores, 29. 
 
 Zoothamnium, 40. 
 
 Zosterofs lateralis, dispersal, 324. 
 
 Zygogonium, conjugation, 97, 143. 
 
 zygosis, 83, 131. 
 
 zygospore, 97. 
 
 zygote, 33. 83, 87, 88, 91, 97, 101, 
 
 105, 109, 118. 
 
 ,, conjugation of nuclei in, 
 131, Fig. 64. 
 
 BRADBURY, AGNEW, & CQ. LD., PRINTERS, CONDON AND TONBRIDGE.
 
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