THE LIBRARY 
 
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 THE UNIVERSITY 
 OF CALIFORNIA 
 
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 SOUTHERN BRANCH, 
 
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 U)S ANGELES, CALIF.
 
 RICHARD WESTBROOK LECTURES OF THE WAGNER FREE INSTITUTE 
 OK SCIENCE, PHILADELPHIA. 
 
 THE THEORY OF EVOLUTION
 
 THE MACMILLAN COMPANY 
 
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 THE 
 THEORY OF EVOLUTION 
 
 WITH SPECIAL REFERENCE TO THE EVIDENCE 
 UPON WHICH IT IS FOUNDED 
 
 BY 
 WILLIAM BERRYMAN SCOTT 
 
 I.D. (HEIDELBERG), HON. D.SC. (HARVARD & OXFORD), LL.D. 
 
 (UNIVERSITY OF PENNSYLVANIA), BLAIR PROFESSOR OF 
 
 GEOLOGY & PALEONTOLOGY IN PRINCETON UNIVERSITY 
 
 Nnu fork 
 THE MACMILLAN COMPANY 
 
 1921 
 
 All rights reserved 
 
 39510
 
 COPYBIGHT, 1917 
 
 BY THE MACMILLAN COMPANY 
 Set up and electrotyped. Published February, 1917.
 
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 iim 36 ^ 
 
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 1917 
 
 ;>7* : \. co p. 2j 
 
 PREFACE 
 
 A new book on evolution, which can lay little 
 claim to novelty of fact or treatment, certainly 
 demands an explanation, if not an apology. My 
 choice of subject for the Westbrook lectures of 1914 
 was determined by the very general misapprehension 
 in the public mind concerning the present status of 
 the evolutionary theory among men of science. It 
 is widely believed that the theory is an outworn 
 device, which naturalists are beginning to discard 
 and that soon it will have a merely historical in- 
 terest. This misunderstanding, for such it is, has 
 arisen from the debates among zoologists and botan- 
 ists as to the manner in which evolution has actually 
 occurred and the efficient causes which have brought 
 it about, and, further, from the ambiguous way in 
 which the term "Darwinism" is often employed. 
 Frequently, the term is made a synonym of evolu- 
 tion, but it ought properly to be restricted to Dar- 
 win's explanation of evolution by natural selection. 
 
 It seemed that a useful service might be rendered 
 by making an outline review of the evidence upon 
 which the doctrine of evolution is founded, for the 
 nature and scope of this evidence are but little under- 
 stood by the educated, though non-scientific pub- 
 lic. The interest displayed in these topics by the
 
 vi PREFACE 
 
 audiences at the Wagner Institute encourages me 
 to hope that a larger and more widely dispersed 
 audience may share the same interest. The annoy- 
 ing barrier of technical language has been evaded 
 so far as possible, but the lack of suitable vernacular 
 terms is such that it is not practicable to escape 
 technicalities altogether. The effort to use ordinary 
 speech and the want of numerous illustrations have 
 caused a certain prolixity of description, which is 
 regrettable, but less so, perhaps, than the alternative 
 of obscurity. 
 
 My experience with graduate students of biology 
 has shown me that usually their training has so com- 
 pletely taken for granted the truth of the evolu- 
 tionary doctrine, that they have but a vague con- 
 ception of the testimony by which that doctrine 
 is supported. This book may be useful in directing 
 their attention to the character of the evidence and, 
 though, of course, the meagre sketch contained in 
 these lectures is entirely inadequate in itself, it does 
 provide an outline which students can easily fill in 
 from their own reading. 
 
 In the winter of 1910-11 an admirable series of 
 popular lectures by several men of high distinction 
 was delivered in the Natural Science Society of 
 Munich, as a tribute to the centennial of Charles 
 Darwin's birth. These lectures were published in 
 1911 by G. Fischer of Jena with the title Die Abstam- 
 mungslehre. To this most excellent compendium 
 of the evidences of evolution I am under great ob-
 
 PREFACE vii 
 
 ligations and I wish that it might soon be translated 
 into English. I take pleasure also in expressing my 
 thanks to my colleagues, Professors Conklin and 
 Smyth and Dr. Mayer for valuable suggestions in 
 the difficulties of treatment and to Dr. W. J. Sinclair 
 for his care in making the figures. 
 
 W. B. S. 
 
 PRINCETON. N. J., December, 1916.
 
 CONTENTS 
 
 LECTURE I 
 
 Present Status of the Question . . 
 
 LECTURE II 
 
 Evidences for the Theory Classification, Domestica- 
 tion and Comparative Anatomy 27 
 
 LECTURE III 
 
 Evidence from Embryology and Blood Tests ... 57 
 
 LECTURE IV 
 
 Evidence from Palaeontology 82 
 
 LECTURE V 
 
 Evidence from Geographical Distribution .... 120 
 
 LECTURE VI 
 Evidence from Experiment Conclusion 149
 
 THE THEORY OF EVOLUTION
 
 THE THEORY OF EVOLUTION 
 
 LECTURE I 
 PRESENT STATUS OF THE QUESTION 
 
 Among that public which, though educated and 
 intelligent, is yet not professionally scientific, there 
 has been, of late, a widespread belief that naturalists 
 have become very doubtful as to the truth of the 
 theory of evolution and are casting about for some 
 more satisfactory substitute, which shall better ex- 
 plain the infinitely varied and manifold character of 
 the organic world. This belief is an altogether mis- 
 taken one, for never before have the students of 
 animals and plants been so nearly unanimous in 
 their acceptance of the theory as they are to-day. 
 It is true that there are still some dissentient voices, 
 as there have been ever since the publication of 
 Darwin's "Origin of Species," but the whole trend 
 of scientific opinion is strongly in favour of the 
 evolutionary hypothesis. 
 
 Whatever may be the private opinion of a greater 
 or less number of naturalists on this question, almost 
 the only zoologist of recognized standing who has 
 taken a pronounced and positive position against the 
 theory, is Professor Fleischmann, of Erlangen. He
 
 2 THE THEORY OF EVOLUTION 
 
 writes: "The more deeply I pursued the alleged evi- 
 dence for it [the theory] and sought to gain, through 
 special investigations, some essential proof of the 
 genetic relationships of animals, the more clearly I 
 recognized that the theory is a seductive romance, 
 which deceptively pretends to give results and ex- 
 planations, rather than a doctrine built upon positive 
 foundations." 1 In another passage he speaks of the 
 "collapse of the theory." This opinion of Professor 
 Fleischmann's stands almost entirely alone in modern 
 biological literature, yet is not altogether surprising, 
 for, from the very nature of the case, such complete 
 and indubitable proof as Professor Fleischmann de- 
 mands is unattainable. 
 
 What, then, has led to the belief, mentioned at 
 the outset, that naturalists have so largely grown 
 sceptical and are inclined to abandon the theory? 
 This belief is principally due to the fact that there 
 is no agreement among men of science as to the 
 manner in which evolution operates (modes) or as 
 to its efficient causes (factors) and, further, to the 
 often fierce controversies which long have been and 
 still are held concerning these problems of modes 
 and factors. Some biologists maintain that these 
 questions are unanswerable in the present state of 
 knowledge and have taken up other lines of investi- 
 gation, but that is very far from being an abandon- 
 ment of the theory of evolution itself. It is one 
 thing to accept a fact as substantially proved and 
 
 1 Albert Fleischmann: Die Descendenztheorie. Leipzig, 1901. p. iii.
 
 PRESENT STATUS OF THE QUESTION 3 
 
 quite another thing to devise a satisfactory explana- 
 tion of that fact. 
 
 A few quotations from distinguished investigators, 
 who are representatives of different schools of 
 thought, will serve to show just what the most strik- 
 ing divergences of opinion are. Professor E. B. Wil- 
 son, in his address as President of the American As- 
 sociation for the Advancement of Science (1914), 
 says: "Biologists turned aside from general theories 
 of evolution and their deductive application to spe- 
 cial problems of descent, in order to take up objective 
 experiments on variation and heredity for their own 
 sake. This was not due to any doubts concerning the 
 reality of evolution or to any lack of interest in its 
 problems. It was a policy of masterly inactivity de- 
 liberately adopted; for further discussions concerning 
 the causes of evolution had clearly become futile until 
 a more adequate and critical view of existing genetic 
 phenomena had been attained" [Italics mine.] 
 
 Professor Gustav Steinmann, of Freiburg, who 
 occupies a very isolated position regarding the actual 
 lines of descent and relationship between the various 
 groups of plants and animals, and whose views on 
 these topics have been very severely criticised, has 
 no hesitation in declaring his unreserved acceptance 
 of the evolutionary theory. His words are: "The 
 theory of development, as it was revived by Darwin 
 nearly half a century ago, is in its modern form pre- 
 vailingly unhistorical. True, it has forced beneath 
 its sceptre the methods of investigation of all the
 
 4 THE THEORY OF EVOLUTION 
 
 sciences which deal with the living world and to-day 
 almost completely controls scientific thought. . . . 
 And yet science does not sincerely rejoice in its con- 
 quests. Only a few incorrigible and uncritically dis- 
 posed optimists steadfastly proclaim what glorious 
 progress we have made; otherwise, in scientific as 
 in lay circles, there prevails a widespread feeling of 
 uncertainty and doubt. Not as though the correctness 
 of the principle of descent were seriously questioned; 
 rather does the conviction steadily grow that it is in- 
 dispensable for the comprehension of living nature, 
 indeed self-evident. [Italics mine.] But never before 
 has it become so notorious as in the last decade, how 
 little there is in this theory that is universally ac- 
 cepted, as appears when the most obvious questions 
 are asked regarding the course of development and 
 its efficient causes. No one can supply simple and 
 satisfactory answers. Should any one ask, by what 
 steps the placental mammals have proceeded from 
 lower quadrupeds, he is informed that some inves- 
 tigators derive them from the reptiles by way of 
 the marsupials, others directly from certain Permian 
 reptiles, while others again push back their origin 
 to the most ancient periods and derive them from 
 entirely unknown ancestors, or, should somebody 
 wish to know why the many remarkable animals 
 and plants of preceding geological periods are no 
 longer living, and what causes led to their extinction, 
 he would receive anything but a clear and unam- 
 biguous answer. Who would venture to-day to
 
 PRESENT STATUS OF THE QUESTION 5 
 
 report anything but the personal conception of in- 
 dividual investigators, if he were asked for infor- 
 mation regarding the hereditary transmission of 
 acquired characters, the significance of natural selec- 
 tion, or many other details of the theory of evolution? 
 For what to one is the corner stone of the theory is 
 to another a factor of quite subordinate importance, 
 while a third regards it as the greatest aberration 
 of the past century." 1 Lastly, may be cited Mr. 
 Bateson, one of the foremost exponents of Mendelism, 
 who rejects Darwinism, one might say, with con- 
 tumely. " The many converging lines of evidence point 
 so clearly to the central fact of the origin of the forms 
 of life by an evolutionary process that we are compelled 
 to accept this deduction, but as to almost all the essential 
 features, whether of cause or of mode, by which specific 
 diversity has become what we perceive it to be, we have 
 to confess an ignorance nearly total. [Italics mine.] 
 The transformation of masses of population by im- 
 perceptible steps guided by selection, is, as most of 
 us now see, so inapplicable to the facts, whether of 
 variation or specificity, that we can only marvel both 
 at the want of penetration displayed by the advocates 
 of such a proposition, and at the forensic skill by 
 which it was made to appear acceptable even for 
 a time." 2 
 
 These quotations might be much extended and 
 diversified, but they cover the principal categories 
 
 1 Gustav Steinmann: Die Abstammungslehre, Leipzig, 1908, pp. 1, 2. 
 
 2 William Bateson: Problems of Genetics, New Haven, 1913, p. 248.
 
 6 THE THEORY OF EVOLUTION 
 
 of modern biological opinions, excepting that of the 
 strictly Darwinian school. They suffice, however, to 
 show how little support such opinions offer to those 
 who would cast aside the theory of evolution as a 
 worn-out hypothesis. It is clear that the differences 
 of belief are almost wholly concerned with the 
 problems of modes and factors, not with the theory 
 itself. The duality of meaning attached to the term 
 "Darwinism" is largely responsible for the popular 
 misapprehension as to the present status of the theory 
 among naturalists. Darwinism is frequently em- 
 ployed as synonymous with evolution, but this is an 
 inexact usage, and the term should be restricted to 
 Darwin's explanation of the evolutionary process. 
 Owing to this inaccurate use of the word, many 
 people, when they hear that such and such prominent 
 biologists have rejected and attacked Darwinism, 
 jump to the entirely false conclusion that the attack 
 is directed against the theory of development. 
 
 The theory of organic evolution made its appear- 
 ance very early in the history of Greek philosophy 
 and reappeared from time to time in Roman and 
 mediaeval writers, because of the charm which the 
 conception of uniformity and continuity in nature 
 exercises over many minds. But these early sugges- 
 tions were merely speculative; no attempt was made 
 to support the theory by evidence, or to gather a 
 systematic body of observed facts which demanded 
 explanation. Each writer was content to quote from 
 such predecessors as appealed to his fancy or his
 
 PRESENT STATUS OF THE QUESTION 7 
 
 judgment, without undertaking to test the conclu- 
 sions handed down to him. Indeed, until the sciences 
 of Zoology and Botany had been gradually built up 
 and a great body of observed facts had been gathered 
 and arranged in an orderly manner, no other pro- 
 cedure was possible; this did not take place until 
 the end of the 18th and beginning of the 19th century. 
 Even then, however, little real progress was made; 
 scientific opinion was not ripe for such a bold gener- 
 alization and the teachings of Lamarck attracted few 
 followers, especially as the overwhelming authority 
 of Cuvier was exerted against these teachings. 
 
 It is of interest to note that Lamarck (1744-1829) 
 arrived at the evolutionary conception, just as Dar- 
 win did nearly half a century later, through a study 
 of the problem of species. Species was first em- 
 ployed as a term in logic and was given a definite 
 application to animals and plants by John Ray (1628- 
 1705), who indicated by it a group of organisms with 
 marked characteristics in common and freely inter- 
 breeding. The concept received a more precise mean- 
 ing and definition from the great Swedish naturalist 
 Linnaeus (1707-1778), who devised the modern 
 scheme of the classification and nomenclature of 
 animals and plants. Linnaeus was somewhat incon- 
 sistent in his expressions on the subject, but the 
 doctrine which was accepted as his by nearly all of 
 his contemporaries and successors was that species 
 were fixed entities which had been separately created. 
 The Linnaean dogma, which prevailed down to the
 
 8 THE THEORY OF EVOLUTION 
 
 time of Darwin, is that "there are as many species 
 as God created in the beginning." Lamarck, who 
 had accomplished an immense amount of systematic 
 work in zoology and botany, felt compelled to deny 
 the Linnsean doctrine and to take up the alternative 
 explanation of evolutionary descent. 
 
 Lamarck's best known work is his "Philosophic 
 Zoblogique," which is very abstruse and speculative 
 and contains but little evidence for his conclusions, 
 other than a reference to his wide experience as a 
 systematist, i. e., a student of classification. He 
 maintained that the species was an artificial con- 
 ception and that nature knew only individuals in 
 graduated steps of relationship. Most of the book 
 is devoted to a consideration of the causes which 
 have led to the transformation of animals and the 
 conclusion is that adaptation to environment is the 
 most efficient factor. This adaptation is to be ex- 
 plained by the fact that the 'conditions of life deter- 
 mine the manner in which animals make use of their 
 organs; these are developed and perfected by use, 
 but dwindle and are atrophied through disuse. An- 
 imals which live in the dark, as in caves or in the 
 abysses of the ocean, are blind, while their organs 
 of feeling are highly developed to take the place of 
 sight. As the environment is subject to constant 
 change, adaptation requires that living beings should 
 be correspondingly, though gradually, transformed. 
 
 Modern accounts of Lamarck's views are almost 
 always taken from the "Zoological Philosophy," but,
 
 PRESENT STATUS OF THE QUESTION 9 
 
 as Giard has pointed out, a clearer conception may 
 be obtained from an examination of the successive 
 introductory lectures of the courses in zoology at the 
 Paris Museum. Giard says : " It was the attentive and 
 minute study of the innumerable species of plants 
 which Lamarck, the botanist, had had to describe 
 and classify in the Flore francaise and Encyclopedic 
 methodique; it was the necessity of beginning, at the 
 age of fifty, a work of the same kind for the lower an- 
 imals . . . ; it was finally the need of crowning with 
 a synthesis thirty years of analytical labours, which 
 led the great naturalist, till then a partisan of the 
 stability of the species, to demonstrate its variability 
 and to search for the causes of the transformation of 
 types." He then gives an extensive quotation from 
 the introductory lecture of 1806, from which the 
 following passage may be taken: 
 
 "All the observations which I have gathered on 
 this important subject, even the difficulty which I 
 know, from my own experience, is now felt in dis- 
 tinguishing species in genera where we are already 
 very rich, a difficulty which is daily increased as the 
 researches of naturalists enlarge our collections, every- 
 thing has convinced me that our species have but a 
 limited existence and, for the most part, differ from 
 neighbouring ones only by shades which are difficult 
 to express. Those who have observed much and 
 have examined great collections, have been able to 
 convince themselves that, as the circumstances of 
 habitat, of exposure, of climate, of nourishment, of
 
 10 THE THEORY OF EVOLUTION 
 
 manner of life, are changed, the characters of stature, 
 form, proportion of parts, colour, consistency, dura- 
 tion, agility and industry in animals are proportion- 
 ally changed. 
 
 "They could see that in animals the more frequent 
 and sustained use of any organ gradually strengthens 
 and develops that organ, enlarges it and gives it a 
 power in proportion to the duration of that use; 
 while constant disuse of such an organ insensibly 
 weakens it, deteriorates it, progressively diminishes 
 its capacities, tends to destroy it. 
 
 "Finally, they could note that everything which 
 nature causes individuals to acquire or to lose through 
 the sustained influence of circumstances to which the 
 race has long been exposed, she transmits by genera- 
 tion to the new individuals which spring from them. 
 These truths are constant and cannot be mistaken 
 save by those who have never observed and followed 
 nature in her operations." 1 
 
 Lamarck, as already noted, had but few disciples 
 in his own or immediately succeeding times, largely 
 because of the opposition of Cuvier, who wielded a 
 despotic authority over scientific opinion in France. 
 In Germany also the evolutionary theory fell into 
 complete discredit, but for another reason; in that 
 country the theory had been advocated chiefly by 
 the so-called "Natural Philosophers," whose wild 
 and fantastic speculations finally disgusted sober- 
 minded men and turned their labors into more ob- 
 
 1 Alfred Giard: Controversies Transformistes, Paris, 1904, pp. 13, 14.
 
 PRESENT STATUS OF THE QUESTION 11 
 
 jective channels. The Linnaean doctrine of the 
 fixity and immutability of species held the field so 
 exclusively for the next half century, that Darwin 
 tells us that he had never met and conversed with 
 any naturalist who did not hold such a belief. 
 
 The way for the great revolution effected by Dar- 
 win was prepared indirectly and in a science which 
 seemed to be very remote from the field of contro- 
 versy, viz., Geology, and the pioneer who opened this 
 new road was Sir Charles Lyell (1797-1875). Before 
 Lyell began his great work, the interpretation of the 
 earth's history was under the sway of Cuvier's theory 
 of "Catastrophism." According to this theory, the 
 history of the earth consisted of long periods of 
 tranquil geographical development, interrupted at 
 intervals by utterly indescribable and unimaginable 
 cataclysms, when elemental forces were unchained, 
 devastating the earth, breaking up and displacing 
 the rocks and destroying every living thing; of these 
 periods of destruction, d'Orbigny enumerated 27. 
 When tranquillity was restored, renewed acts of crea- 
 tion supplied animals and plants for the depopulated 
 earth and thus, as it were, the curtain rose upon a 
 new scene, with a new set of actors, the old having 
 been completely swept away. It was observed that 
 each fresh creation produced animals and plants of 
 a higher type, successively approximating the present 
 order of things. Cuvier was too cautious and too 
 critical to declare dogmatically that there had ac- 
 tually been many successive acts of creation and he
 
 12 THE THEORY OF EVOLUTION 
 
 suggested alternative hypotheses to explain the ob- 
 served facts. On the other hand, he seems to have 
 himself believed in the creations and his contem- 
 poraries and successors accepted that doctrine as 
 his. 
 
 Lyell reverted to the neglected teachings of the 
 Scotchman, James Hutton (1728-1797), that the key 
 to the past history of the earth was to be found in 
 the work of agencies which are now in operation. 
 Like Hutton, he would not admit that any process 
 which is not still at work could be called upon to 
 explain events in the past and he even went so far as 
 to make these existing processes the measure and 
 standard of former operations. In the sharpest con- 
 trast to Cuvier's Catastrophism, LyelPs theory was 
 that of Uniformitarianism, which insisted upon the 
 uniformity and complete continuity of the earth's 
 history and of the agencies which had wrought such 
 profound changes upon and within the globe. In the 
 early editions of his famous "Principles of Geology," 
 Lyell was evidently on the point of accepting the 
 theory of organic evolution, but later he receded 
 from this position, because he was not satisfied with 
 the evidence for it. Lyell 's influence upon geology 
 was rapid and profound in England and Darwin, who 
 became his intimate friend, was conspicuously under 
 that influence, as he himself repeatedly testified. 
 One might almost say that Darwin's work largely 
 consisted in the application of Lyell's principle to 
 the world of living beings.
 
 PRESENT STATUS OF THE QUESTION 13 
 
 Unquestionably, the most important and influen- 
 tial figure in the history of the evolutionary theory 
 is that of Charles Darwin (1809-1882), who effected 
 one of the most remarkable and far-reaching revolu- 
 tions in all departments of human thought of which 
 we have any record. By a curious coincidence, Dar- 
 win and Alfred Russell Wallace (1823-1913) inde- 
 pendently reached the same solution of the problem 
 concerning the origin of new species, but, with a 
 fine absence of jealousy and self-seeking, none too 
 common in the history of science, they cooperated 
 in the production of a preliminary paper, which ap- 
 peared in 1858; this, however, attracted little at- 
 tention. 
 
 The publication of Darwin's "Origin of Species" 
 in 1859 is one of the great landmarks in the history 
 of science and brought about, though gradually, an 
 extraordinary revolution in opinion. That Darwin's 
 work did not share the fate of that of Lamarck and 
 other early evolutionists was due in part to the 
 preparatory labours of Lyell, who had familiarized 
 men's minds with the conception of uniformity, 
 continuity and orderly progress in the development 
 of the earth, but chiefly to Darwin's method of 
 gathering and presenting the evidence. Though 
 greeted at first with a storm of criticism, not un- 
 mingled with abuse, the sheer weight of evidence, 
 presented with such transparent simplicity and 
 candour, inevitably forced conviction even upon un- 
 willing minds. But here it is again necessary to
 
 14 THE THEORY OF EVOLUTION 
 
 emphasize the distinction between the two aspects 
 of Darwin's work. His argument, supported by the 
 amazing mass of proof, which had been gathered 
 through twenty laborious years, gradually con- 
 vinced scientific men that the theory of evolution 
 offered by far the most probable and satisfactory 
 solution of the problems of the living world. On 
 the other hand, Darwin's explanation of the evolu- 
 tionary process, commonly known as the theory of 
 "Natural Selection," won no such universal accept- 
 ance and has been a subject of controversy, more 
 or less acrimonious, from the time of its first pro- 
 mulgation to the present day. On the other hand, it 
 must be recognized that Natural Selection played 
 a very important part in bringing about the general 
 acceptance of evolution, for it was the first sugges- 
 tion of a reasonable and intelligible factor in pro- 
 ducing the transformation of species. Huxley's 
 account of his own attitude of mind is well adapted 
 to explain this. He tells us that before 1858, he 
 had no definite opinions on the subject and this 
 for two reasons: "firstly, that up to that time, the 
 evidence in favour of transmutation was wholly 
 insufficient; and, secondly, that no suggestion re- 
 specting the causes of the transmutation assumed, 
 which had been made, was in any way adequate 
 to explain the phenomena." l Huxley was never 
 fully satisfied with the doctrine of natural selection, 
 but, nevertheless, that doctrine had a large share 
 
 1 Life and Letters, Vol. II.
 
 PRESENT STATUS OF THE QUESTION 15 
 
 in making him the convinced and thoroughgoing 
 evolutionist that he was. 
 
 It is interesting to retrace some of the more signifi- 
 cant steps by which Darwin reached his conclusions, 
 a course which he has made easy by his autobiograph- 
 ical sketch. The famous voyage in the naval brig 
 Beagle, which lasted nearly five years (1831-1836) 
 was, he tells us, "by far the most important event 
 in my life and has determined my whole career." 
 Much of this period was spent in long journeys in 
 the interior of South America, and the wealth of 
 observations thus gathered forced upon him the 
 problem of transmutation. There were three classes 
 of facts by which he was deeply impressed. (1) In 
 the surface deposits of the Argentine pampas he 
 found the remains of gigantic extinct animals, dif- 
 fering from, yet fundamentally resembling those 
 which still inhabit the same region. (2) "The 
 manner in which closely allied animals replace one 
 another in proceeding southward over the conti- 
 nent." Such a succession of closely allied forms 
 is what would be expected on the evolutionary 
 theory; a single species in ranging over a very wide 
 area would become modified so as to give rise to 
 several other species, while from the doctrine of 
 special creation no explanation is possible. (3) 
 What had the greatest weight to his mind was af- 
 forded by his observations in the Galapagos archi- 
 pelago, a group of volcanic islands 500 miles west 
 of the Ecuador coast. As a whole, the animals and
 
 16 THE THEORY OF EVOLUTION 
 
 plants of these islands are of distinctly American 
 type, yet most of the species are peculiar to the 
 group and each island has a surprising number of 
 species peculiar to itself. " Reviewing the facts here 
 given, one is astonished at the amount of creative 
 force if such an expression may be used displayed 
 on these small, barren, rocky islands; and still more 
 so at its diverse and yet analogous action on points 
 so near each other." 
 
 The growth of his belief in evolution is thus de- 
 scribed by Darwin in a letter written in 1877: "When 
 I was on board the 'Beagle,' I believed in the per- 
 manence of species, but, as far as I can remember, 
 vague doubts occasionally flitted across my mind. 
 On my return home in the autumn of 1836, I im- 
 mediately began to prepare my journal for publica- 
 tion, and then saw how many facts indicated the 
 common descent of species. . . . But I did not 
 become convinced that species were mutable until 
 I think two or three years had elapsed." Even after 
 he had gained his conviction of the mutability of 
 species, he could imagine no adequate cause of the 
 change and, consciously adopting the methods which 
 Lyell had used in Geology, he made a great collec- 
 tion of facts, gathered from breeders, fanciers, gar- 
 deners, as well as from his own observations, bearing 
 on the changes and modifications which men had 
 brought about in the various races and breeds of 
 domesticated animals and plants. "I soon perceived 
 that selection was the key-stone of man's success
 
 PRESENT STATUS OF THE QUESTION 17 
 
 in making useful races of animals and plants. But 
 how selection could be applied to organisms living 
 in a state of nature remained for some time a mystery 
 to me." The key to this mystery was suggested to 
 him by reading Malthus' famous essay on Population. 
 In his own words: "Being well prepared to appre- 
 ciate the struggle for existence which everywhere 
 goes on from long continued observation of the 
 habits of animals and plants, it at once struck me 
 that under these circumstances favourable variations 
 would tend to be preserved, and unfavourable ones 
 to be destroyed. The result of this would be the 
 formation of new species. Here then I had a theory 
 by which to work." 
 
 Time fails to trace farther the stages of growth 
 in Darwin's opinions. All that can be here at- 
 tempted is a brief outline of the theory which, after 
 nearly twenty strenuous years, he gave to the world 
 in the "Origin of Species," published in 1859. Un- 
 like most other explanations of evolution, Darwin's 
 theory is a chain of reasoning, every link of which is 
 an undisputed fact, the only question being, does 
 the theory afford a sufficient and adequate explana- 
 tion of the phenomena? The attempts to give a 
 definite and conclusive answer, affirmative or nega- 
 tive, to this question have given rise to the many 
 debates of which we have by no means heard the 
 last word. 
 
 (1) The first of the undisputed facts upon which 
 Darwin's theory is founded is that of variability. It is
 
 18 THE THEORY OF EVOLUTION 
 
 a matter of the commonest observation that no two 
 individuals of a species are exactly alike and, when 
 a very large number of specimens are studied, the 
 range of variation is often found to be exceedingly 
 wide. Such variations are fluctuating and repeated 
 in every generation. 
 
 (2) The second fact is the struggle for existence. 
 Every species in a state of nature produces vastly 
 more young than can possibly survive to maturity; 
 if every egg of the herring should develop to an 
 adult fish and reproduce in its turn, it would not be 
 long before the Atlantic Ocean would fail to contain 
 them. A statistical study of many species leads 
 to the conclusion that, so long as the conditions of 
 life remain the same, the number of individuals of 
 a given species is a substantially constant quantity, 
 subject of course to fluctuations of increase and de- 
 crease. Thus, for every young animal or plant that 
 reaches maturity, an old one must die to make place 
 for it. The struggle for existence is not to be thought 
 of as a combat to slay and devour, but as a competi- 
 tion for food, light and the other necessities of life, 
 as well as resistance or escape from the attacks of 
 predatory enemies, parasites, diseases and the like. 
 Not only is there intense competition between mem- 
 bers of the same species, but also between allied 
 species with similar habits and needs. 
 
 (3) The third fact is natural selection, or, in Herbert 
 Spencer's happier phrase, the survival of the fittest. 
 Survival to the age of reproduction is not a matter
 
 PRESENT STATUS OF THE QUESTION 19 
 
 of chance, but those individuals persist which are 
 the most vigorous and any variation from the average 
 of the species which confers an advantage in the 
 struggle will tend to be preserved and to increase 
 in successive generations. Darwin, as he tells us, 
 very early perceived that in making new breeds of 
 domesticated animals and plants, selection of the 
 parents according to the characteristics which it was 
 desired to perpetuate and improve, played the im- 
 portant role and he concluded that advantageous 
 variations, by enabling their possessors to survive 
 in the struggle, would furnish the material for the 
 formation of new species, while any disadvantageous 
 variation would be promptly extinguished. 
 
 (4) Finally, there is the fact of heredity. It is a 
 commonplace that individuals are like their parents 
 and should the parents have from birth some ad- 
 vantageous peculiarity, this would tend to reappear 
 in the offspring. A repetition of this process from 
 generation to generation would have a cumulative 
 effect, resulting in very extensive changes. 
 
 While Darwin always maintained that natural 
 selection was by far the most important and effi- 
 cient cause in producing transmutation, he admitted 
 the action of other, though minor factors. Thus, 
 he makes frequent appeal to the effects of use and 
 disuse of organs, the agency which Lamarck had 
 regarded as of such primary importance, and he 
 advanced the theory of sexual selection to account 
 for the brilliant colouring and elaborate patterns
 
 20 THE THEORY OF EVOLUTION 
 
 of very many animals, especially of butterflies and 
 birds. Inasmuch, however, as the effects of use 
 and disuse as an evolutionary factor involve the 
 assumption that characters acquired during the 
 lifetime of the parent are or may be transmitted to 
 the offspring, an assumption which is now vigorously 
 denied, modern disciples of Darwin reject this factor 
 and attribute little importance to sexual selection. 
 They accept Weismann's dictum that natural se- 
 lection is the sole and all-powerful agent of evolu- 
 tion. 
 
 Darwin's book, reenforced by his subsequent 
 works, brought about the astonishing revolution 
 in scientific opinion which has already been de- 
 scribed, but, while the evolutionary theory thus 
 gained a complete and almost universal victory, 
 Darwin's particular theory, that of natural selection, 
 was not so fortunate. True, it was and still is very 
 widely accepted, but there has always been a large 
 body of opinion which rejected it as vague, inade- 
 quate and unsatisfactory and there have been many 
 attempts to supplement it, or to substitute some 
 more convincing explanation for it. Thus, the 
 eminent German botanist Carl von Nageli (1817- 
 1891) propounded an elaborate theory of develop- 
 ment, in which he attempted to show that natural 
 selection was an insufficient explanation of evolution 
 and that a "principle of perfection" must be as- 
 sumed, which drives organisms to take on higher 
 and higher forms. He compares the living world
 
 PRESENT STATUS OF THE QUESTION 21 
 
 to a garden full of shrubs, which are continually 
 sending out new branches, the tips of which repre- 
 sent the plants and animals now existing; they grow 
 by their own internal force, but are kept within 
 limits by the action of natural selection, which 
 plays the part of the gardener with his pruning- 
 shears. Were it not for this pruning, the shrubbery 
 would speedily degenerate into a wild and formless 
 thicket. 
 
 One of the earliest and most obvious objections 
 made against Darwin's theory was that a slight 
 favourable variation, arising in a few individuals 
 would be speedily swamped by cross-breeding with 
 the vastly more numerous individuals which did 
 not display that variation. To avoid this objection, 
 Moritz Wagner propounded his theory of the "origin 
 of species by separation in space," according to 
 which geographical separation of groups of indi- 
 viduals, by the prevention of interbreeding, pro- 
 duced new forms through a process of divergent 
 development. Everything that we know indicates 
 that geographical separation has been an important 
 factor in bringing about the diversity which char- 
 acterizes the living world, but few would attribute 
 to this factor the significance which Wagner gave 
 to it. 
 
 Professor August Weismann of Freiburg (1834- 
 1914) was a most influential figure in the biological 
 controversies which raged in the latter part of the 
 nineteenth century, especially by his elaborate the-
 
 22 THE THEORY OF EVOLUTION 
 
 ories of heredity and the constitution of the germ- 
 plasm, which have been remarkably confirmed by the 
 extensive studies which have grown out of the re- 
 searches in the exact mechanism of heredity made by 
 Mendel. Weismann was the first writer who strongly 
 questioned the hereditary transmission of charac- 
 teristics which had been acquired during the life- 
 time of the parent, maintaining that all new struc- 
 tures must first arise in the germ-plasm. He thus 
 rejected altogether the effects of the use and disuse 
 of organs, which Darwin had admitted and to which 
 Lamarck had attributed the primary role in the 
 work of transformation, and regarded natural selec- 
 tion alone as the all-important factor. It would 
 require far more time than is at our disposal to give 
 even an outline sketch of Weismann's theories and 
 it must suffice to point out that his views are still 
 largely in the ascendant and that the great number 
 of naturalists who constitute the so-called "Neo- 
 Darwinian School" look upon Weismann as their 
 foremost spokesman. 
 
 Palaeontologists, whose studies deal with the fossil 
 remains of the ancient plants and animals which 
 once inhabited the earth, but are now altogether ex- 
 tinct, have not, as a rule, been satisfied with the 
 theory of natural selection as an adequate explana- 
 tion of organic evolution. It was a palaeontologist, 
 von Waagen, who, in 1867, first pointed out a dis- 
 tinction, which may yet prove to be the first step 
 toward the formulation of an entirely acceptable
 
 PRESENT STATUS OF THE QUESTION 23 
 
 theory. This distinction was between variations, 
 on the one hand, and mutations, on the other. By 
 variations, Waagen understood the fluctuating dif- 
 ferences between contemporary individuals of the 
 same species, differences which are repeated in every 
 generation and are entirely inconstant. Mutations 
 are successive changes, which, however slight they 
 may be, are yet constant in character, and a series of 
 mutations forms successive connected steps of modi- 
 fication in a definite direction. The eminent Dutch 
 botanist de Vries adopted this conception and, with 
 certain changes, erected it into an elaborate theory, 
 which has formed the subject of much controversy. 
 De Vries points out that Darwin did not distinguish 
 clearly between the two kinds of modifications which 
 have, or may have, furnished the starting-points for 
 the various races and breeds of domesticated animals 
 and plants. One kind is the ordinary difference 
 between individuals and the other is the "sport," or 
 sudden and notable change in one or more charac- 
 teristics, a change which is constant and transmis- 
 sible to the offspring. An often cited "sport" is the 
 Ancon sheep, a breed which arose from a single 
 short-legged ram, born of normal parents in 1791 on 
 a New Hampshire farm. This ram transmitted his 
 peculiarities to his descendants and soon a new kind 
 of sheep, popular because of their inability to jump 
 fences, was established. Such a sudden, unheralded 
 and transmissible change of structure de Vries calls 
 a mutation and he believes that mutation is the
 
 24 THE THEORY OF EVOLUTION 
 
 normal manner in which new species originate in 
 nature and that the struggle for existence is not so 
 much between individuals as a competition between 
 mutants or incipient species. 
 
 Interesting and important as the mutation theory 
 undoubtedly is, it offers no explanation of the phe- 
 nomena, for the cause of such sudden changes re- 
 mains a complete mystery. The seat of the change, 
 it can hardly be questioned, must be sought in the 
 germ-plasm of one or both parents, but we have not 
 the least inkling of how such modification is brought 
 about. 
 
 Finally, should be mentioned the work of Johann 
 Gregor Mendel (1822-1882) an Austrian monk, who 
 made experiments in crossing different varieties of 
 peas and subjected his results to a mathematical 
 analysis. His paper was published in 1866 and at- 
 tracted no attention, but long after his death it was 
 rediscovered, his results having been independently 
 and almost simultaneously attained by three botan- 
 ists, who published them in 1900, de Vries, von 
 Tschermak in Austria and Correns in Germany. It 
 is pathetic to contrast the complete neglect with 
 which Mendel was treated during his lifetime with 
 the honour which is paid to him now. His work has 
 been developed into a new branch of biological 
 science, called "Genetics," an exact analysis of the 
 mechanism of heredity, and will be more fully dealt 
 with in the last lecture of the course. Mendel's name 
 is mentioned here merely to complete the list of those
 
 PRESENT STATUS OF THE QUESTION 25 
 
 who have been the foremost pioneers in clearing the 
 way for the modern conception of evolution. 
 
 From the quotations which have been read in the 
 foregoing part of the lecture, it will be only too evi- 
 dent that, while naturalists are all but unanimous 
 in accepting the theory of evolution as an established 
 truth, there is every possible divergence in their 
 views as to the causes of development and diversi- 
 fication. Could a census of opinions be taken among 
 the zoologists and botanists of the world, it is very 
 probable that a substantial majority of Darwinians 
 would be found. By Darwinians is to be understood 
 those who are convinced that the doctrine of natural 
 selection is the true explanation of organic evolution. 
 Whether or not a majority of all naturalists are to 
 be ranked with the Darwinians, it can hardly be 
 doubted that there are very many more who accept 
 natural selection than there are of those who have 
 adopted any alternative view. Personally, I have 
 never been satisfied that Darwin's explanation is the 
 rightful one; to one who approaches the problem 
 from the study of fossils, the doctrine of natural 
 selection does not appear to offer an adequate ex- 
 planation of the observed facts. The doctrine, in 
 its application to concrete cases, is vague, elastic, 
 unconvincing and seems to leave the whole process 
 to chance. To be sure, this difficulty is, to a great 
 extent, inherent in the nature of the problem, for 
 direct observation of the long course of evolutionary 
 development is impossible; no one ever saw the birth
 
 26 THE THEORY OF EVOLUTION 
 
 of a species and thus we are shut up to the drawing of 
 inferences from what may be learned by comparison 
 and experiment. 
 
 On the other hand, if Darwin's hypothesis be 
 rejected, there is, it must be frankly admitted, no 
 satisfactory alternative to take its place. Granting 
 that de Vries is right in holding that species originate 
 by sudden mutation, the process is not thereby 
 explained and made intelligible and it does not seem 
 likely that such an explanation could ever be found, 
 though it is rash to set a limit to future discovery. 
 In short, while the evolutionary theory is buttressed 
 by such a mass of evidence that nearly all men of 
 science are convinced of its truth, no satisfactory 
 and acceptable explanation of its causation has yet 
 been devised. To repeat Prof. E. B. Wilson's words: 
 "Further discussions concerning the causes of evolu- 
 tion had clearly become futile until a more adequate 
 and critical view of existing genetic phenomena had 
 been attained."
 
 LECTURE II 
 
 EVIDENCES FOR THE THEORY CLASSIFICATION, 
 DOMESTICATION AND COMPARATIVE ANATOMY 
 
 Before attempting to set before you an outline 
 sketch of the evidence which has so firmly convinced 
 zoologists and botanists that the theory of evolution 
 is true, it is necessary to begin with a warning not to 
 expect too much; that is to say, more complete proof 
 than, in the nature of things, it is possible to obtain. 
 Evolution has been a long, historical process, which 
 has gone on with extreme slowness through un- 
 imaginable ages of time, when no man was there to 
 see and observe. Complete demonstration, such as 
 may be given for a physical or chemical law, is there- 
 fore not to be expected. Even in so exact a science 
 as astronomy, the attempt to work out historical 
 problems is beset with the gravest difficulties. No 
 astronomer, for example, doubts that the solar sys- 
 tem is a unit, which has been developed from some 
 originally continuous body of matter, such as a 
 nebula, but there is no general agreement as to the 
 manner in which this development was brought 
 about. Any solution yet proposed is face to face 
 with unsolved problems. Clearly, therefore, the 
 evidence for organic evolution must be indirect, since 
 direct proof is unattainable, yet it need not be un- 
 
 27
 
 28 THE THEORY OF EVOLUTION 
 
 convincing for that reason, because in science, as 
 in the practical affairs of life, probability must be 
 the guide, whenever certainty is not reached and 
 certainty is very rarely reached, if ever. 
 
 Speaking of plants, Dr. D. H. Scott says: "Our 
 ideas of the course of descent must of necessity be 
 diagrammatic; the process, as it actually went on, 
 during ages of inconceivable duration, was doubtless 
 infinitely too complex for the mind to grasp, even 
 were the whole evidence lying open before us. We 
 see an illustration, on a small scale, of the complexity 
 of the problem, in the case of domesticated forms, 
 evolved under the influence of man. Though we 
 know that our cultivated plants, for instance, have 
 been developed from wild species within the human 
 period and often within quite recent years, yet noth- 
 ing is more difficult to trace, in any given instance, 
 than the true history of a field-crop or garden plant, 
 or even, in many cases, to fix its origin with cer- 
 tainty.'* * Under the infinitely more complex con- 
 ditions and longer time, in which natural evolution 
 is believed to have taken place, it is no reason for 
 surprise that the evidence should be not only in- 
 direct, but also general, even sometimes vague, rather 
 than specific in character. On the other hand, what 
 lends great cogency to this evidence is the fact that 
 several quite distinct and independent lines of proof 
 all converge in a common conclusion. While per- 
 haps no one of these lines is altogether complete or 
 
 D. H. Scott: Studies in Fossil Botany, London, 1900, pp. 524-5.
 
 EVIDENCES FOR THE THEORY 29 
 
 convincing, yet the harmony of all of them, all point- 
 ing in the same direction, greatly strengthens the 
 proof and raises the theory to a very high order of 
 probability, despite the many unanswered questions 
 and unsolved problems. Classification, comparative 
 anatomy, embryology, palaeontology, the geographi- 
 cal distribution of animals and plants, all tell the 
 same story. It is because in every department of 
 zoology and botany the evolutionary hypothesis 
 offers the simplest, the most rational and the most 
 satisfactory explanation of the observed facts, that 
 naturalists accept it almost unanimously. 
 
 In order to explain the character of the living world 
 and what is known of its history, but two alternative 
 hypotheses have been propounded. The first, called 
 the doctrine of Special Creation, holds that each 
 kind of animal and plant was created in its present 
 form and is substantially immutable. This doctrine, 
 formulated by Linnaeus, was very widely adopted 
 and held the field until the publication of Darwin's 
 book. The second is the hypothesis of evolution, or 
 "descent with modification," according to which all 
 animals and plants, as we know them, have been 
 developed from primordial germs, leaving open the 
 question as to how life on the earth originated. If 
 any one declines to accept either one of these hy- 
 potheses, it only remains to admit that the problem 
 is a complete mystery, for which no solution has been 
 found, the position taken by Fleischmann. In mak- 
 ing a choice between these alternatives, it often
 
 80 THE THEORY OF EVOLUTION 
 
 seems to be taken for granted that the burden of 
 proof rests entirely upon the evolutionary theory, 
 but this is a mistake, for there is and can be no evi- 
 dence for that of special creation. The only thing 
 which could justify us in holding that doctrine would 
 be to find that the facts could be explained in no 
 other way. 
 
 It has been objected that the gradations between 
 the various forms of life, to which evolutionists 
 attach so much importance, is in itself no proof of a 
 genetic connection between those forms, and the 
 gradual development of human productions is ad- 
 duced as showing similar relations without any 
 genetic connections. For example, houses may be 
 traced from their earliest beginning in the rude hut 
 of the savage, through various stages to the most 
 extensive and elaborate modern dwelling, yet no one 
 would suggest that houses were evolved by descent 
 with modification. Here the relationship is ideal, 
 not material, and the development is that of a plan; 
 why should the same thing not be true of the living 
 world? In reply, it may be said that this objection 
 might have some weight, if houses naturally repro- 
 duced their kind, as all living things do, and each 
 individual structure were not the work of men's 
 hands. Then, too, in order to hold the evolutionary 
 hypothesis, it is not necessary to deny the ideal re- 
 lationships between the successive gradations of 
 living beings, or to exclude belief in a creative 
 plan, which has been worked out by the method
 
 EVIDENCE FROM CLASSIFICATION 31 
 
 of evolution. This, however, is a metaphysical, 
 rather than a scientific question, and has no im- 
 mediate bearing upon the evidences for the theory 
 of evolution. 
 
 As has already been pointed out, the evidence for 
 the theory consists of several distinct and independ- 
 ent lines of testimony, so to speak, and it will con- 
 duce to clearness of treatment, if each of these lines 
 is dealt with separately, though their action and 
 reaction upon one another must not be overlooked. 
 In the first place, we may take up the argument from 
 classification, which is the most obvious one and has 
 the additional historic interest of being the line along 
 which Lamarck and Darwin approached and reached 
 their primary generalizations. It was, in short, the 
 problem of species which led those great naturalists 
 to abandon the current belief in the fixity and im- 
 mutability of living types and to adopt the hypothesis 
 of development. To repeat, in part, the quotation 
 from Lamarck given in the first lecture: "The diffi- 
 culty which I know, from my own experience, is now 
 felt in distinguishing species . . . has convinced me 
 that our species have but a limited existence and, 
 for the most part, differ from neighbouring ones only 
 by shades which it is difficult to express." (See 
 ante, p. 9.) 
 
 The object of classification is to arrange the ex- 
 traordinarily manifold assemblages of animals and 
 plants in groups of ascending comprehensiveness, 
 which shall express, so nearly as the present state of
 
 32 THE THEORY OF EVOLUTION 
 
 knowledge permits, the degrees of relationship be- 
 tween them. In this elaborate scheme, which was 
 developed by Linnaeus about the middle of the 
 eighteenth century, the unit, so to speak, is the 
 species, the storm-centre of many controversies. Lin- 
 naeus believed that species were real, objective en- 
 tities, which it was the business of the naturalist to 
 distinguish, describe and name, and for a very long 
 time the principal occupation of zoologists and botan- 
 ists was the discrimination of species, furnishing them 
 with names and arranging them in genera, families, 
 orders and other groups of higher rank. This pro- 
 cedure may best be explained by an example. The 
 European wolf is a species, lupus, which, with other 
 typical wolves, is included in the genus Canis and is 
 therefore designated as Canis lupus, for the name of 
 a species requires that of its genus to identify it and 
 hence the Linnaean scheme of nomenclature is said 
 to be binomial. The true wolves, jackals, foxes, 
 (genus Vulpes) and many other genera of differing 
 dog-like forms are grouped together in the family 
 CanidcB, which, associated with all the other ter- 
 restrial beasts of prey, cats, bears, raccoons, weasels, 
 otters, etc., etc., are included in the sub-order Fis- 
 sipedia. The latter are joined with the marine forms, 
 seals, sea-lions, walruses, etc., to constitute the order 
 Carnivora. Omitting certain intermediate groups, 
 we find that the order Carnivora belongs in the class 
 Mammalia, or warm-blooded quadrupeds, which, in 
 turn, is a member of the phylum Vertebrata, or
 
 EVIDENCE FROM CLASSIFICATION 33 
 
 animals with internal skeleton, which also includes 
 the fishes, amphibians, reptiles and birds. 
 
 The principle in accordance with which these 
 groups of an ascending order of comprehensiveness 
 are constructed, is simply the greater or less degree 
 of likeness in structural characteristics. All mam- 
 mals agree in fundamental features, however much 
 they may differ in size, form, appearance and habits, 
 and thus such different-looking creatures as a whale, 
 a bat, a horse, a mole and a monkey are all referred 
 to the same class. Under the Linnsean conception, 
 which prevailed down to Darwin's time, the relation- 
 ship between the various species of a genus, or be- 
 tween the genera of a family, the families of an order, 
 etc., was regarded as purely ideal. If each species 
 represented a distinct act of creation and was so 
 fixed as to be incapable of transformation into an- 
 other species, then obviously no actual genetic re- 
 lationship, or blood-kinship between different species 
 was possible. According to the evolutionary hy- 
 pothesis, on the other hand, the relationship is truly 
 genetic and the differences are due to divergent de- 
 velopment, acting under different conditions and cir- 
 cumstances, while the deep-seated, fundamental like- 
 nesses are explained as inheritances from a common 
 ancestry. 
 
 As Lamarck so emphatically pointed out, this dis- 
 tinction of species from one another is often exceed- 
 ingly difficult and has frequently given rise to great 
 differences of opinion and practice among systema-
 
 34 THE THEORY OF EVOLUTION 
 
 lists, because of the manner in which the different 
 individuals of a species vary among themselves and, 
 in many instances, of the way in which the species 
 of a genus grade into each other. No two individuals, 
 belonging to the same species, are exactly alike in all 
 respects and when a very large number of such in- 
 dividuals are compared, they are found, as a rule, 
 to cover a remarkably wide range of variation, the 
 extremes of which often differ more than do the in- 
 dividuals which are referred to distinct species. The 
 species of a large, thriving and vigorous genus are 
 often so unstable and so connected by intergrada- 
 tions, that there is sometimes a radical difference of 
 opinion as to how many separate species should be 
 recognized. "Nothing is brought out more clearly 
 by ecological studies in New Zealand than the ex- 
 treme * plasticity' of many species and structures, 
 and their rapid response to a change of environment. 
 This is so great in numerous instances that the idea of 
 'normal' loses its meaning." l 
 
 For example, the botanists differ greatly in regard 
 to the American oaks and into how many species 
 they are properly divisible; the same is true of many 
 other groups. To evade this difficulty, at least in 
 part, several groups, subordinate to species, are recog- 
 nized and named in systematic botany and zoology, 
 such as geographical races, varieties, subspecies, etc., 
 each of these minor groups being relatively constant 
 and recognizable, yet not differing sufficiently from 
 
 1 L. Cockayne in Trans. New Zealand Institute. Vol. XLIV, pp. 13, 14.
 
 EVIDENCE FROM CLASSIFICATION 35 
 
 the others to require separation as a distinct species. 
 Yet if the natural origin of these minor subdivisions 
 be admitted, there can be no valid reason for denying 
 the natural origin of species, genera and the higher 
 groups of classification, for the difference is one of 
 degree, not of kind, and the intergradation is often so 
 perfect, that it is a matter of personal judgment and 
 experience, whether the systematist shall regard a 
 given group of individuals as a variety, a subspecies, 
 or a species, whence the many differences and con- 
 flicts in practice. 
 
 If a species, say of birds, having a very wide 
 geographical range, be carefully examined and com- 
 pared, it very frequently happens that specimens 
 taken from the extreme points of that range differ 
 so notably from one another, that the systematist 
 would not hesitate to separate them as distinct 
 species, were it not that in the intervening area all 
 the intergradations between the extreme forms can 
 be found. The recognition of species is thus often a 
 purely arbitrary procedure, as to which different 
 writers diverge widely in their judgment. All this 
 flux and inconstancy and intergradation between 
 very distinct extremes are hard to comprehend, if 
 we adopt the hypothesis of special creation, while, 
 on the other hand, they are precisely what would 
 be anticipated under the hypothesis of evolution. 
 
 A study of the races of domesticated animals and 
 plants, which Darwin l investigated so thoroughly, 
 
 1 The Variation of Animab and Plants under Domestication, 2 vols., 1868.
 
 36 THE THEORY OF EVOLUTION 
 
 leads to a similar conclusion. Under the conditions 
 of domestication, these creatures have been so pro- 
 foundly modified that, in many instances, the orig- 
 inal, wild progenitor of the race or breed cannot be 
 determined. The various breeds of horses, cattle, 
 sheep, pigeons, fowls, not to mention the garden and 
 field-plants, differ among themselves far more than 
 do many wild species from each other. Were the 
 pointer, terrier and spaniel dogs, the fan-tail, pouter 
 and tumbler pigeons found in a state of nature, no 
 zoologist would hesitate for a moment to recognize 
 them as well-defined species, yet we know that all the 
 breeds of pigeons have been derived from a single 
 wild form. Dogs are all domesticated wolves, but 
 the history of the various breeds has not been pre- 
 served and it is not known whether all the familiar 
 races are descended from a single wild species or 
 from several, or what that wild progenitor or pro- 
 genitors were. All this diversity has been produced 
 by human agency, acting sometimes with intelligent 
 and conscious purpose, sometimes with no definite 
 end in view, by taking advantage of variability and 
 individual differences. If man can accomplish such 
 results in a relatively short time, it is difficult to see 
 why nature, with illimitable aeons at her disposal, 
 should not bring about correspondingly greater trans- 
 formations. The phenomena of domestication make 
 the dogma of the fixity and immutability of species 
 seem very ill-founded and improbable. This is a 
 very large subject and it has been necessary, on ac-
 
 EVIDENCE FROM DOMESTICATION 37 
 
 count of the limitations of time, to compress into a 
 few sentences, what Darwin has given in two large 
 volumes. To these fascinating volumes may be re- 
 ferred the reader who is interested in following out 
 this line of evidence. 
 
 Even under conditions which cannot be called 
 domestication, certain remarkable transformations of 
 animals have been observed. An interesting case of 
 this sort is that of the Porto Santo Rabbit, as de- 
 scribed by Darwin. At a date variously given as 
 1418, 1419 and 1420, the Portuguese navigator Zarco 
 set free a doe and newly born litter of young rabbits 
 upon the small island of Porto Santo, not far from 
 Madeira. From the fact that the doe littered on ship- 
 board, it is evident that she belonged to one of the 
 domestic races, all of which have been derived from 
 the European wild Rabbit (Lepus cuniculus). The 
 absence of any carnivorous creatures, which would 
 have preyed upon the rabbits, or of any other land 
 mammal, which might have competed with them for 
 food, led to a very rapid multiplication, so that in 
 less than 40 years they are described as "innumer- 
 able." As a result of four and a half centuries of 
 isolation under these novel conditions, the Porto 
 Santo Rabbit has become so different from any of the 
 domestic races and from its wild progenitor, that 
 Haeckel has described it as a distinct species, Lepus 
 huxleyi. The new form is much smaller than the 
 European wild Rabbit, weighing but little more than 
 half as much as the latter, and differs considerably in
 
 38 THE THEORY OF EVOLUTION 
 
 colour. The fur is redder, with far fewer black-tipped 
 hairs; the throat and belly are gray or lead-coloured, 
 instead of pure white, and the tips of the ears and 
 upper surface of the tail lack the blackish-gray fur 
 so characteristic of the European wild form. 
 
 "The two little Porto Santo rabbits, whilst alive 
 in the Zoological Gardens, had a remarkably dif- 
 ferent appearance from the common kind. They 
 were extraordinarily wild and active, so that many 
 persons exclaimed on seeing them that they were 
 more like large rats than rabbits. They were noc- 
 turnal to an unusual degree in their habits, and their 
 wildness was never in the least subdued. . . . Lastly, 
 and this is a highly remarkable fact, Mr. Bartlett 
 could never succeed in getting these two rabbits, 
 which were both males, to associate or breed with 
 the females of several breeds which were repeatedly 
 placed with them. 
 
 "If the history of these Porto Santo rabbits had 
 not been known, most naturalists, on observing their 
 much reduced size, their colour, reddish above and 
 gray beneath, their tails and ears not tipped with 
 black, would have ranked them as a distinct species. 
 They would have been strongly confirmed in this 
 view by seeing them alive in the Zoological Gardens, 
 and hearing that they refused to couple with other 
 rabbits. Yet this rabbit, which there can be little 
 doubt would thus have been ranked as a distinct 
 species, has certainly originated since the year 1420. 
 Finally, from the three cases of the rabbits which
 
 EVIDENCE FROM DOMESTICATION 39 
 
 have run wild in Porto Santo, Jamaica, and the 
 Falkland Islands, we see that these animals do not, 
 under new conditions of life, revert to or retain their 
 aboriginal character, as is so generally asserted to 
 be the case by most authors." l 
 
 Another instance of transformation, in this case 
 very rapid, under "new conditions of life" is that of 
 the Lunar Moth (Saturnia lund) when transported 
 from Texas to Switzerland. In the year 1870, the 
 entomologist Boll brought to Switzerland a number 
 of cocoons of this large and beautiful moth and in 
 May of the following year the full-grown moths 
 emerged from the cocoons and differed in no par- 
 ticular from the ordinary Texas form. From these 
 moths several hundred fertilized eggs were obtained, 
 from which, in the course of a few weeks, small cater- 
 pillars were hatched. In Texas the caterpillars of 
 this species feed upon the leaves of hickory and black 
 walnut trees, but, as such leaves were not to be had 
 in Switzerland, those of the European walnut were 
 substituted. The substitution was entirely accept- 
 able to the caterpillars, which ate greedily and 
 formed their cocoons at the end of June, the adult 
 moths emerging early in August. Much to the sur- 
 prise of all observers, this second generation of 
 moths, the caterpillars of which were fed upon a plant 
 new to them, differed so much in form, colour and 
 markings from the parent Texan species, that any 
 
 Variation of Animals and Plants under Domestication, 
 Vol. I. pp. 119-20.
 
 40 THE THEORY OF EVOLUTION 
 
 entomologist would have regarded it as a new and 
 distinct species, had its origin not been known. 
 Nevertheless, Dr. Gemminger, a distinguished ento- 
 mologist, recognized it as a distinct species and 
 named it " Saturnia bolli." 
 
 Moritz Wagner, from whom this account is taken, 
 gives the following description of the new form: "At 
 the first glance, the connoisseur is surprised by the 
 striking change of form. In the new species the shape 
 of the body, as of the wings, is somewhat larger and 
 heavier, while the feathery antennae are slightly 
 narrower and less luxuriant. From the longer hind- 
 body of the new species the carmine-red, longitudinal 
 stripes, which the parent species bears, have com- 
 pletely disappeared. The front wings have a less 
 sinuous form, but are somewhat broader. This 
 change of form is to be more decidedly observed in 
 the tail-like prolongation of the hind wings. Not 
 less striking than the differences of shape are those 
 of colour. In the ancestral species the colour is a 
 yellowish green, while that of the new species is a 
 beautiful lemon-yellow. The carmine-red marginal 
 stripe, with whitish inner border, borne on the ante- 
 rior wings of Saturnia luna, has quite disappeared 
 in Saturnia bolli and is indicated only by a very 
 narrow, dark yellowish colouring of the outermost 
 margin. 
 
 "Most remarkable in this new species is the ap- 
 pearance of a new marking on the anterior wings, 
 which appears as a transverse stripe, with somewhat
 
 EVIDENCE FROM DOMESTICATION 41 
 
 zig-zag outer edge, but is entirely absent from the 
 anterior wings of the parent species." 1 
 
 Other similar, if less striking, examples of change 
 might be described, did time permit; all that can be 
 attempted here is to give an outline sketch of the 
 evidence with a few illustrations, chosen from the 
 many that are available. To present anything like 
 an exhaustive display of the evidence would require, 
 not one course of lectures, but many. The instances 
 selected suffice to show that, when placed under new 
 conditions of climate, food-supply, attacks of enemies 
 and the like, animals and plants, whether domes- 
 ticated or free, may experience very marked changes 
 of size, form and appearance and that species are, 
 in very many cases, if not in all, far from immutable. 
 Often the metamorphosis is so great, that the wild 
 progenitor of many domesticated animals and cul- 
 tivated plants cannot be determined with any degree 
 of certainty. 
 
 The other lines of evidence which are relied upon 
 to prove the theory of evolution are comparative 
 anatomy; embryology, the study of individual devel- 
 opment; palaeontology, the study of extinct plants 
 and animals which formerly existed on the earth; the 
 geographical distribution of organisms, animal and 
 vegetable; finally, experimental investigation. Each 
 one of these methods of research has its particular 
 advantages, as well as its special limitations and 
 
 1 M. Wagner: Die Entstehung der Arten dutch raumliche Sonderung, 
 pp. 30^-10.
 
 42 THE THEORY OF EVOLUTION 
 
 drawbacks; it is the harmonious result of all of them, 
 pointing in the same direction, which gives great 
 weight to their combined testimony. 
 ^fomparative anatomy, as the name implies, is the 
 comparison of structure and tissues in the whole 
 range of animal life. Such comparison leads to the 
 conclusion that in the animal kingdom there are 
 several distinct types of structure, characterizing the 
 grand divisions of the kingdom. Within any one of 
 these grand divisions may be found almost endless 
 diversities of structure, all of them obviously different 
 modifications of the same fundamental plan. How- 
 ever great the modification, the identity of the plan is 
 clearly to be perceived throughout, just as is the 
 theme of a musical composition, which may be 
 traced through all the variations into which the 
 composer has woven it. A few concrete instances will 
 make clear the application of this principle. Super- 
 ficially examined, the following structures would 
 seem to have little or nothing in common, but rather 
 to be constructed on totally distinct plans for al- 
 together different purposes: the fore leg of a lizard, 
 the wing of a bird and of a bat, the burrowing shovel 
 of a mole, the flipper of a whale, the fore leg of a 
 horse, the human arm and hand. When, however, 
 all of these structures are carefully dissected and the 
 bones, muscles and nerves are compared, it becomes 
 immediately apparent that the plan of structured 
 the same throughout, modified to serve different 
 uses, of running, flying, burrowing, swimming and
 
 : 
 
 EVIDENCE FROM COMPARATIVE ANATOMY 43 
 
 : grasping. In all of them, the upper arm has a single 
 bone, the humerus, and the forearm two, thejiiaa 
 and radius; in all there is a wrist, formed of an as- 
 semblage of small polygonal bones, followed by the 
 long bones of the hand and, finally, by the fingers. 
 Modification is brought about by the reduction and 
 even the suppression of certain parts, the enlarge- 
 ment of other parts and, very much more rarely, by 
 the addition of new parts. Compare, for example, 
 the human arm and hand with the fore leg of a horse. 
 The human hand is a grasping organ and rotates 
 \freely on the arm; there are five digits, one of which, 
 the thumb, is opposable to the others, and each digit 
 is tipped with a flat nail. The wrist-joint is composed 
 of eight bones, arranged in two transverse rows; the 
 two bones of the forearm are complete, entirely 
 separate and movable on each other, the power of 
 rotation of the hand being dependent upon the char- 
 acter of the elbow joint. The upper arm-bone forms 
 a ball and socket joint with the shoulder blade and 
 rotates easily in almost any direction. In the horse, 
 on the contrary, the fore limb is exclusively an organ 
 ^_ of locomotion and cannot Be used for grasping; as a 
 *" wholel'the limb ^yn rJy bnrl-mrrl and forward in 
 a plane parallel with that of the backbone and each 
 of the joints is movable only in the same fore-and-aft 
 direction, there being no power of rotating the hand 
 upon the arm, or of the fore limb as a whole upon the 
 shoulder. There is but a single functional digit, the 
 third of the original five, the bones of which are so
 
 44 
 
 THE THEORY OF EVOLUTION 
 
 enlarged and strengthened as to support the whole 
 weight of the body; thejiajLis-greatly expanded^-to 
 form a hoof, which encloses as in a box the whole 
 
 fcu 
 
 m. a. 2X. m. 
 
 FIG. 1. Right fore limbs of Man (left); Horse (middle); Ox (right). 
 H. humerus. R. = radius. U. ulna. C. = carpus. Me. = 
 metacarpals. Digits are numbered with Roman numerals. 
 
 terminal joint of the digit. Beside the single func- 
 tional digit, there are remnants, orjrudiments.oi two 
 others, the second and fourth of the original series, 
 which are not visible externally and consist only of 
 the long bones of the hand (metacarpals) without 
 free joints or hoofs, and are commonly known as
 
 EVIDENCE FROM COMPARATIVE ANATOMY 45 
 
 "splint-bones." The wrist has seven bones, one of 
 the lower row being suppressed as needless, in corre- 
 lation with the reduction in the number of digits. 
 One of the forearm bones, the ulna, is very greatly 
 reduced, so that only the two ends remain and these 
 are firmly co-ossified with the radium, which is cor- 
 respondingly enlarged and bears the whole body- 
 weight. 
 
 The fore leg of the ox, like that of the horse, is 
 exclusively locomotive in function and can move 
 only backward and forward, there being no power of 
 rotation of the hand on the forearm or of the whole 
 leg on the shoulder. Jn the forearm the enlarged 
 radius carries all the weight and the two ends of the 
 reduced ulna are co-ossified with it. The most 
 striking difference from the horse is to be found in the 
 fact that the foot has two functional digits, the third 
 and fourth, between which the weight is equally 
 divided, and the two hoofs are so shaped as to have a 
 split appearance, which is expressed in the term 
 "cloven-hoofed." While accurately descriptive of 
 the appearance, this term is erroneous if taken to 
 mean that an originally single hoof has been divided. 
 The long bones of the hand, or metacarpals, are 
 completely fused together into a compound "cannon 
 bone" and, in addition to the two functional digits, 
 there are two rudimentary, externally visible ones, 
 the "dew-claws," which are small, nodular hoofs be- 
 hind the principal ones. In the horse and ox, then, 
 we see two slightly divergent modes of converting
 
 46 THE THEORY OF EVOLUTION 
 
 the primitive, five-toed and mobile anterior limb into 
 structures which are purely locomotive in function. 
 
 It is evident that the same elements are present 
 in the human hand and arm as in the fore leg of the 
 horse and ox, but in each case characteristically 
 modified to serve different ends. Man's hand, for- 
 tunately for us, is really a very primitive and un- 
 differentiated structure and can be put to a great 
 many different uses. Had it been highly specialized 
 for a single purpose, human progress and civilization 
 would have been impossible, for these have always 
 depended upon the co-ordination of hand, eye and 
 brain. In the horse everything has been sacrificed 
 to speed, making the animal a "cursorial machine," 
 and, to this end, the functional digits have been 
 reduced from five to one, the proportionate lengths 
 of the various segments of the limb adjusted to the 
 best advantage and the feet raised from the ground, 
 so that the horse, as also the ox, walks upon the very 
 tips of the toes. What is ordinarily called the * ' knee ' ' 
 of a horse is the exact homologue of the human wrist, 
 while the elbow joint is concealed in the muscles of 
 the shoulder. 
 
 Again, the flipper of a whale is, to all external ap- 
 pearances, totally different from the human arm 
 and hand, as from the fore leg of the horse, but dis- 
 section shows that the plan of structure is the same, 
 identical parts being greatly modified in form and 
 function in accordance with the exclusively marine 
 life of the animal. Except at the shoulder, there is
 
 EVIDENCE FROM COMPARATIVE ANATOMY 4? 
 
 no true joint between any of the bones, which ar- 
 rangement allows but a very limited motion, merely 
 enough for the necessary flexibility of the paddle. 
 The arm-bones are short, mas- 
 sive and simple and, what is 
 extraordinarily rare among mam- 
 mals, the number of joints in 
 the digits is much increased, 
 to give the needed length 
 and support to the flipper. 
 Clearly, the whale's paddle is of 
 precisely the same type as the 
 fore limb of a land quadruped, 
 but transformed and adapted to 
 a purely marine habit of life. 
 
 The comparison of a bat's 
 wing with that of a bird brings 
 to light two quite different modes 
 of adaptation of similar elements 
 to the purposes of flight, and in 
 both groups we find certain dif- 
 ferences in the completeness of 
 adaptation. In the bat the 
 bones of the arm are very slender 
 and greatly elongated; the ulna 
 is much reduced, only the upper 
 third remaining and this is co-ossified with the radius. 
 The first digit, or thumb, is divaricated from the 
 others, is not included in the wing-membrane and 
 bears a curved, hook-like claw. The bones of the 
 
 FIG. 2. Bones of left 
 flipper of Greenland 
 Right Whale. Letters 
 as in Pig. 1. (From 
 Men.)
 
 48 
 
 THE THEORY OF EVOLUTION 
 
 other digits are extremely long and slender and 
 usually the terminal joints, with their claws, have 
 been suppressed, but in the fruit-eating bats, or 
 "flying foxes," the second 
 digit retains the terminal 
 joint and claw and in cer- 
 tain other families the third 
 digit (but not the sec- 
 ond) has the terminal joint, 
 though without a claw. The 
 wing is a fold of skin, which 
 is attached along the outer 
 side of the body and the 
 under side of the arm and 
 forearm and is stretched be- 
 tween the bones of the digits, 
 which support it in much the 
 same fashion as the ribs of 
 an umbrella sustain the cloth. 
 The membrane is also at- 
 tached to the legs and ex- 
 tends between the two thighs 
 and the short tail. 
 
 In the wing of a flying 
 bird the problem of making 
 an organ of strong and steady flight is solved in an 
 entirely different manner. The effective air-plane 
 is formed by large quill-feathers, which are inserted 
 along the outer side of the bones of the arm and 
 hand, so there is no wing-membrane and the stiffness 
 
 FIG. 3. Skeleton of right wing 
 of Leaf-nosed Bat. Letters 
 as in Fig. 1.
 
 EVIDENCE FROM COMPARATIVE ANATOMY 49 
 
 of the feathers renders unnecessary any stretching 
 between bony supports. These supports are there- 
 fore developed according to another plan. The 
 forearm bones are compar- 
 atively short and strong, 
 complete and separate from 
 each other; the structure 
 of the hand varies some- 
 what in the different groups 
 of birds and it will there- 
 fore be simpler to take as a 
 type a particular bird, the 
 common raven. Here, the 
 bones of the wrist are re- 
 duced to two, partly by sup- 
 pression and partly by co- 
 ossification of the originally 
 more numerous elements. 
 There are three digits, of 
 which the first, or thumb, 
 is free, while the long bones 
 
 . FIG. 4. Skeleton of left wing of 
 (metacarpals) OI the Second the American Raven. Let- 
 
 and third are co-ossified; Sjy faIli>L (AfterShu ' 
 the fingers are represented 
 
 by one or two free joints in each digit. In many 
 groups of birds the first digit has a claw, as, for ex- 
 ample, the spur on a swan's wing, and several birds 
 have claws on both the first and second digits. 
 
 In the flightless birds, which, there is every reason 
 to believe, are descended from flying ancestors, the
 
 50 THE THEORY OF EVOLUTION 
 
 wing-bones may be like those of the flying birds in 
 form and number, but are altogether too small to 
 be of any functional importance. This is the case 
 in the ostrich, for example, while in the cassowary 
 the digits are reduced to a single one, which is pre- 
 sumably the third. In the penguins, which are in- 
 capable of flight, the wing is not useless, but is 
 adapted to a new purpose and has been transformed 
 into a flipper or swimming paddle. This change of 
 function does not involve much modification of the 
 wing-bones, except that those of the hand are much 
 elongated and flattened; the transformation is most 
 marked in the feathers. It might almost be said 
 that these most curious birds have been adapted to 
 flight in a new medium, water instead of air. 
 
 Still a third type of wing is exemplified by the 
 remarkable group of extinct flying reptiles known 
 as pterodactyls. In these creatures, some of which 
 attained imffiense size, the wing was most like that 
 of the bats, being a membrane or fold of skin, naked 
 and without hair, scales or feathers. Only one digit, 
 the fifth, or little finger, was elongated and strength- 
 ened to support the wing-membrane along its outer 
 border, while the other digits were free and provided 
 with claws. In all three groups, bats, birds and 
 pterodactyls, a vertical keel grows upon the breast- 
 bone (as any one may see in carving the breast of a 
 fowl), in order to furnish a sufficient bony attach- 
 ment to the powerful and greatly developed breast- 
 muscles, which are the principal muscles of flight.
 
 EVIDENCE FROM COMPARATIVE ANATOMY 51 
 
 The size and prominence of this keel are excellent 
 indications as to the power of prolonged and steady 
 flight. 
 
 Many other examples might be given in illustration 
 of the principle that, among the Vertebrates, we are 
 dealing with countless modifications of a single type, 
 with lower and higher grades of organization and 
 adaptation to a great variety of habits of life, ter- 
 restrial, burrowing, climbing, swimming, flying, etc., 
 etc. The hypothesis of evolution offers the simplest 
 and most natural explanation of the facts. 
 
 Similar considerations apply to the other great 
 types of animal structure. Among the Crustacea, 
 for example, we may take the common lobster, 
 which is one of the most advanced of these animals. 
 There is a large anterior shield, or carapace, which 
 is a single piece and, behind this is a number of 
 movable rings or segments, which compose the 
 abdomen or tail. To each segment is attached a 
 pair of jointed appendages, which are the swimmerets, 
 each of which is divided into external and internal 
 portions. The last pair of the swimmerets is very 
 much expanded and forms with the terminal seg- 
 ment a broad tail-fin. When the lower surface of 
 the lobster is examined, it is seen that the region 
 covered by the carapace is likewise composed of 
 segments, each with its pair of jointed appendages 
 but in this region the segments are immovably fixed 
 together and the appendages are altogether different 
 in appearance from those of the tail. The four
 
 52 THE THEORY OF EVOLUTION 
 
 posterior pairs are walking legs, which correspond 
 to the internal division of the swimmerets. In front 
 of the legs is the pair of great claws, which are com- 
 posed of the same number of joints, arranged in the 
 same way, only very much larger, heavier and more 
 powerful. In front of the claws come three pairs 
 of appendages called "foot-jaws" (maxillipeds) 
 which are legs with their basal joints forming organs 
 of mastication; the hindmost pair of the foot- jaws 
 differs but little from the walking legs, while in the 
 foremost pair it is difficult to make out the corre- 
 sponding parts. Next in order come three pairs of 
 jaws, still more modified, then the very long and 
 many jointed feelers (antennae) and the anterior 
 pair of much shorter feelers (antennules) . The ap- 
 pendages of the head, body and tail are thus seen 
 to be modifications of a common plan and grade into 
 each other almost imperceptibly. 
 
 If representatives of the many families and orders 
 of the Crustacea be compared, it is seen that there 
 is great variety in the number of segments which 
 are united to form the head and trunk, as well as 
 in the form and function of the appendages. The 
 appendages of the same segment, counting from the 
 anterior end, are in one group a pair of jaws, in 
 another foot-jaws and in a third walking legs. The 
 comparison immediately suggests the derivation of 
 all the Crustacea from ancestors in which all the 
 segments, except the head, were similar and were 
 provided with appendages which were similar
 
 EVIDENCE FROM COMPARATIVE ANATOMY 53 
 
 throughout. As this group or that advanced in 
 differentiation certain appendages became specialized 
 for the better performance of particular operations. 
 
 In the crabs may be observed a higher grade of 
 organization than in the lobsters, for the crabs, at 
 first sight, appear to have lost the tail entirely. 
 Turning a crab on its back, however, the segmented 
 tail is seen to be folded under the body and lying 
 in a groove on the ventral surface; the appendages 
 of the tail have been almost completely suppressed. 
 
 Even more remarkable is the series of modifica- 
 tions brought to light by a comparative study of 
 the mouth-parts among the insects, in which far- 
 reaching modifications have been brought about in 
 adaptation to different kinds of food and different 
 habits of life. Fundamentally, the parts are the 
 same throughout, but the transformation is fre- 
 quently so profound as to obscure this fact. In 
 insects, such as the cockroach, which live upon solid 
 food, there are masticating jaws; in the bumble bee 
 a licking apparatus; in the butterflies and moths a 
 long, tubular proboscis for sucking the nectar from 
 flowers; in the mosquito a most elaborate arrange- 
 ment for piercing the skin and sucking the blood of 
 its victim. "But always it is the same individual 
 parts, on the same definite segments, which are re- 
 peated according to rule as regards number and 
 form, but with a different mode of use; a pair of 
 upper jaws, a first and second pair of lower jaws, but 
 the latter may be long drawn out and rolled up in
 
 54 THE THEORY OF EVOLUTION 
 
 tubular form, and the former may be transformed 
 from jaws into piercing lancets. This arrangement 
 according to a definite rule, despite the differences 
 of form and use, finds its most probable explanation 
 in community of descent." l 
 
 Examples of the same kind, drawn from all the 
 great divisions of the animal kingdom, might be in- 
 definitely multiplied, but such multiplication would 
 serve no useful purpose, even were it feasible within 
 our limits of time. The principle of numerous modi- 
 fications of a common plan, adapted to a great 
 variety of habits of life is made sufficiently clear by 
 the few instances already given. 
 
 The objection has been frequently urged, just as in 
 the case of classification, that this diversification of 
 a single type of structure is no proof of a genetic 
 connection, or community of descent, but that the 
 connection is purely ideal, the manifestation of a 
 creative plan. The answer is the same in both 
 cases; acceptance of the theory of evolution by no 
 means excludes belief in a creative plan, but that 
 theory offers the most satisfactory solution of the 
 problem. Another and perhaps weightier objection 
 is that comparative anatomy gives us no means of 
 connecting animals of fundamentally different types 
 or plans of structure. It is impossible to derive a 
 fish from a lobster, or a starfish from an oyster and 
 thus the different structural types would seem to be 
 separated by impassable barriers. Evolution within 
 
 1 Otto Maas, in Die Abstammungslehre, Jena, 1911, pp. 258-9.
 
 EVIDENCE FROM COMPARATIVE ANATOMY 55 
 
 the type might be admitted, without conceding the 
 possibility of deriving one type from another. 
 
 In reply, attention must be called to the fact that 
 comparative anatomy deals only with the animal 
 world as it now exists and that is demonstrably but 
 a very small part of the countless horde of animal 
 forms that formerly inhabited the earth, but have 
 become extinct. An oft-used illustration of the re- 
 lations between living and extinct organisms is that 
 of a great tree buried in the earth, so that only the 
 outermost twigs are exposed to view. Such twigs 
 would form scattered groups, more or less widely 
 separated from one another, the branches which con- 
 nect them being buried out of sight. If it were 
 possible to dig away the earth and expose the whole 
 tree, the continuity of all its ramifications would be 
 fully revealed. To a certain extent, this excavation 
 is accomplished by the science of palaeontology, 
 which deals with the remains of extinct animals and 
 plants as preserved in the rocks of the earth's crust. 
 Another lecture (No. IV) will be devoted to the tes- 
 timony of this science to the evolutionary hypothesis. 
 Another illustration may be drawn from a study of 
 modern languages, as it would be if that study were 
 deprived of the help given by books and manuscripts, 
 which register past changes, and confined to an ex- 
 amination of the spoken tongues. This examina- 
 tion would suffice to make clear the interrelationships 
 of most European languages; French, Italian, Por- 
 tuguese, Spanish, etc., the Romance languages, are
 
 56 THE THEORY OF EVOLUTION 
 
 evidently derived from a common source. Another 
 group of common origin is that of the Teutonic 
 languages, German, Danish, Swedish, Dutch, Eng- 
 lish, etc., while a third group, that of the Slavonic 
 tongues, is a natural assemblage of related languages, 
 as is that of the Celtic group. A wider comparison 
 indicates that all of these European languages, to- 
 gether with Persian, Hindustani, Lithuanian, etc., 
 form a related group, the Indo-European, in con- 
 tradistinction to the so-called Semitic and Turanian 
 families. It is an obvious conclusion that all of the 
 Indo-European languages had a common origin at a 
 very ancient date and that this common speech split 
 up into a number of branches, each of which, in 
 turn, became divided into many languages and 
 dialects. 
 
 The argument from comparative anatomy in fa- 
 vour of the evolutionary hypothesis would be notably 
 strengthened by a consideration of rudimentary 
 organs, but this line of evidence can best be dealt 
 with in connection with embryology.
 
 LECTURE IH 
 EVIDENCE FROM EMBRYOLOGY AND BLOOD TESTS 
 
 JEmbryology .is the- study of the development of 
 
 the individual organsim from its beginning in the 
 egg to the attainment of the adult condition. This 
 individual development is called ontq^er^/ and flip 
 question of the relation of ontogeny to the ancestral j- 
 history of the species, or ?>ht/7sKiftiy t jyj|irtitfftTT?m : ' 
 of the main problems of embryology. Around this 
 problem many controversies have raged, contro- 
 versies which have by no means arrived at a definite 
 solution, even to-day. Thirty years ago the "re- 
 capitulation theory" was well-nigh universally ac- 
 cepted, according to which the individual develop- 
 ment, or ontogeny^ was regarded as an abbreviated 
 repetition of the ancestral history of the species, or 
 phylogeny. Haeckel called this theory the "fun- 
 damental biogenetic law" and upon it he established 
 his whole "History of Creation." Nowadays, that 
 "fundamental law" is very seriously questioned and 
 by some high authorities is altogether denied. How- 
 ever, even those who take this extreme position con- 
 cerning the recapitulation theory see in the facts of 
 embryology one of the strongest supports of the 
 doctrine of evolution. 
 
 57
 
 58 THE THEORY OF EVOLUTION 
 
 It was very early recognized that the recapitula- 
 tion theory could not be applied with literal exact- 
 ness, but was subject to certain important excep- 
 tions and qualifications. 
 
 _-4!}Jla_Ltlie} history must have been enormously 
 abbreviated. After three weeks of incubation the 
 tiny speck of protoplasm, which forms a circular 
 mark on the yolk of a hen's egg, is developed into a 
 fully formed chick, ready for hatching and able in 
 large degree to take care of itself. On the other 
 hand, the evolution of birds from their invertebrate 
 ancestors, through the fishes, amphibians, and rep- 
 tiles, the separation of the gallinaceous stock from 
 other birds and the differentiation of this particular 
 species were extremely slow processes, extending 
 through unnumbered millions of years. Admitting 
 recapitulation to the fullest extent, it is evidently 
 a physical impossibility that it should be a perfect 
 repetition of phylogeny; very much of the long story 
 must of necessity be omitted. 
 
 - fl&^Ehrough alL_the stages of development the 
 embryo must be rendered able to live and grow and 
 thrive through adaptation to its surroundings and 
 changes in its environment. .In_sQEae.j3ji.imals L jde- 
 velopment takes place within the body of the mother; 
 in others the embryo is protected by the hard egg- 
 shell, as in birds, while the eggs of certain fishes and 
 many invertebrates float freely in the sea and are 
 almost without protection. Such differences in en- 
 vironment necessitate differences in the mode of
 
 EVIDENCE FROM EMBRYOLOGY 59 
 
 development, while the presence or absence of a 
 large amount of inert food-material, or yolk, exerts 
 a great influence in determining the steps of on- 
 togeny. 
 
 (3) Many animals pass through a larval stage of 
 development, in which the immature young leads 
 an independent and self-sustaining existence, during 
 which it is very different in appearance and structure 
 from its adult parents. Familiar instances of this 
 mode of development are to be found in the tadpole, 
 which is the larva of thelrog, and the caterpillar the 
 larva of a Tnitterfly. Larvae are fully subject to 
 the struggle for existence and must adapt themselves 
 to their environment and to changes in that environ- 
 ment, exactly as do adults, if they are to survive. In 
 this way many changes are introduced into the 
 ontogeny which can have no phylogenetic signifi- 
 cance. It is found in several known instances, that 
 nearly allied species, living under different condi- 
 tions, have quite different modes of ontogeny, though 
 their ancestral history must have been substantially 
 identical. In one and the same species of marine 
 worms, for example, which inhabits both the warm 
 Mediterranean and the cold waters of the North 
 Sea, the larva of the northern form is quite distinct 
 from that of the southern. In attempting to in- 
 terpret the meaning of embryological facts, it is thus 
 necessary to distinguish sharply between j&fiS.,iga- 
 tures which are derived from a long inheritances 
 and are therefore called \palingenetic, from those
 
 60 THE THEORY OF EVOLUTION 
 
 which have been secondarily introduced in re- 
 sponse to the changing needs of embryonic or larval 
 life. These secondary features are termed cen- 
 ogenetic. 
 
 "If we are compelled to admit that cenogenetic 
 characters are intermingled with palingenetic, then 
 we cannot regard ontogeny as a pure source of evi- 
 dence regarding phyletic relationships. Ontogeny 
 accordingly becomes a field in which an active imag- 
 ination has full scope for its dangerous play, but in 
 which positive results are by no means everywhere 
 to be obtained. To attain such results, the palin- 
 genetic and cenogenetic phenomena must be sifted 
 apart, an operation which requires more than one 
 critical grain of salt. On what grounds shall this 
 critique be based? assuredly not by way of a vicious 
 circle on the ontogeny again; for if cenogenetic 
 characters are present in one case, who will guarantee 
 that a second case, used for a comparison with the 
 first, does not likewise appear in cenogenetic dis- 
 guise? If it once be admitted that not everything in 
 development is palingenetic, that not every onto- 
 genetic fact can be accepted at its face value, so to 
 speak, it follows that nothing in ontogeny is imme- 
 diately available for the critique of embryonic 
 development. The necessary critique must be drawn 
 from another source." l 
 
 These remarks of Gegenbaur's were called forth 
 by the state of wild speculation into which embryo- 
 
 1 C. Gegenbaur, Morphologisches Jahrbuch, Vol. XV, p. 5.
 
 EVIDENCE FROM EMBRYOLOGY 61 
 
 logical work had fallen. As there were no generally 
 accepted canons of interpretation for the facts of 
 embryological development, different writers inter- 
 preted these facts in the most divergent and con- 
 tradictory manner, resulting in a chaotic confusion, 
 which led to a strong reaction against the whole 
 method, though there can be little doubt that this 
 reaction has gone too far. 
 
 "It must be evident to any candid observer, not 
 only that the embryological method is open to 
 criticism, but that the whole fabric of morphology, 
 so far as it rests upon embryological evidence, stands 
 in urgent need of reconstruction. For twenty years 
 embryological research has been largely dominated 
 by the recapitulation theory; and unquestionably 
 this theory has illuminated many dark places and 
 has solved many a perplexing problem that without 
 its aid might have remained a standing riddle to the 
 pure anatomist. But while fully recognizing the real 
 and substantial fruits of that theory, we should not 
 close our eyes to the undeniable fact that it, like 
 many another fruitful theory, has been pushed be- 
 yond its legitimate limits. It is largely to an over- 
 weening confidence in the validity of the embryo- 
 logical evidence that we owe the vast number of the 
 elaborate hypothetical phylogenies which confront 
 the modern student in such bewildering confusion. 
 The inquiries of such a student regarding the origin 
 of any of the great principal types of animals involve 
 him in a labyrinth of speculation and hypothesis in
 
 62 THE THEORY OF EVOLUTION 
 
 which he seeks in vain for conclusions of even an 
 approximate certainty." 1 
 
 Many other equally vigorous and well-deserved 
 criticisms of the embryological method might be 
 cited, but it should be emphasized that these crit- 
 icisms are all directed against the application of the 
 method to the solution of definite and concrete 
 problems of descent and relationship. None of them 
 denies and many strongly affirm that embryology 
 affords some of the strongest and most convincing 
 evidence in favor of the evolutionary theory. 
 
 Let us examine some of this evidence. To begin 
 with, it should be noted that, in following out the 
 ontogeny or individual development, the observer 
 witnesses the formation of something new, not 
 merely the enlargement and unfolding of a pre- 
 existing organism, though the theory of preformationjjLLf 
 which was widely accepted in the eighteenth century, 
 looked upon ontogeny precisely in that way, as the 
 growth of a germ which was the miniature of the 
 parent. Such a theory was possible only before the 
 development of microscopic technique had enabled 
 the observer to detect the actual successive steps of 
 change. The egg is a single cell, with the nucleus and 
 all the parts of other undifferentiated cells, though it 
 may be enormously enlarged by the presence of food- 
 yolk, jn the hen's egg this food-yolk is quite inert 
 and the activity of development isconfined to the 
 minute disc of protoplasm on the outside of the yolk, 
 
 1 E. B. Wilson, in Wood's Hole Biological Lectures, 1894, p. 103.
 
 EVIDENCE FROM EMBRYOLOGY 
 
 63 
 
 while i i the frog's egg the yolk is disseminated, 
 though not uniformly, throughout the egg and in the 
 mammalian egg, which is microscopic in size, there is 
 
 FIG. 5. Embryos in corresponding stage of development of 
 Shark (A), Fowl (B), and Man (C). g, gill-slits. 
 
 no yolk. It is a very remarkable fact that all of the 
 vertebrated animals, fishes, amphibians, reptiles, 
 birds and mammals, however different their habits 
 and modes of life, have a mode of ontogeny which is 
 of even more characteristically and unmistakably 
 the same plan than is the type of their adult struc-
 
 64 THE THEORY OF EVOLUTION 
 
 ture, which was described in the last lecture^ _ The_ 
 egg, or the active portion of it. divides in a definite 
 and regular manner into a very large number of cells, 
 which arrange themselves in definite layers, an outer 
 and an inner, and within these layers cell-aggregates 
 form incipient organs, which, step by step, take on 
 the adult condition. Not only is the plan and type of 
 development essentially similar throughout the whole 
 phylum of the vertebrates, but, in accordance with 
 the recapitulation theory, many structural features 
 which are permanent in lower forms appear in the 
 embryos of higher and more advanced types. In 
 the latter, however, these features are transitory and, 
 in the course of development, they either disappear, 
 or are so modified as to be very different, sometimes 
 unrecognizable, in the adulj:^. ^ u IVT .. t v 
 
 j\i a pprtain stagp of the pnfogftny tTip pmhryo of a 
 
 mammal has gill-pouches like a fish, the skeletal 
 supports of the gill-pouches, the arteries and veins 
 which supply them with blood, the structure of the 
 heart, in short, the entire plan of the circulatory 
 system is fish-like. At a later stage most of the gillz 
 pouches have been obliterated, but one is retained 
 and converted into the Eustachian^anal. which 
 connects the throat with the middle ear, inside of the 
 ear-drum. Similarly, the embryological evidence 
 shows that the 
 
 ^_ 
 
 derived from the swim-bladder of fishes, a conclusion 
 which had already been reached by comparative 
 anatomy, for in a remarkable group, known as the
 
 EVIDENCE FROM EMBRYOLOGY 65 
 
 Dipnoi or lung-fishes, the air-bladder is utilized for 
 purposes of respiration. 
 
 It has been objected that, while embryology may 
 prove relationship within a single type, it fails to 
 demonstrate any connection between different types, 
 but this is not altogether true. The Tunicata, a 
 curious group of marine animals once referred to the 
 Mollusca, are shown by their ontogeny to be related 
 to the vertebrates and the same is true of certain 
 marine worms (Balanoglossus) . Indeed, most mod- 
 ern zoologists have adopted a scheme of classifica- 
 tion, in which the type Chordata includes not only 
 the true vertebrates, but also the Lancelet (Am- 
 phioxus), the tunicates and Balanoglossus; this 
 scheme is founded upon the embryological evidence. 
 Among the invertebrates even more remarkable ex- 
 amples have been observed. Such radically differ- 
 ent types as the segmented worms and the shell-fish 
 (Mollusca) are brought into relationship by their 
 ontogeny and their closely similar types of larvae, as 
 are also, though less distinctly, the brachiopods or 
 lamp-shells, and the Bryozoa. The Horseshoe-crab, 
 or King-crab, so abundant along our Atlantic coast, 
 was long of uncertain affinities; originally referred to 
 the Crustacea, largely because of its marine habits 
 of life, embryology makes much more probable 
 its relationship to the air-breathing scorpions and 
 spiders, a result which will be examined again from 
 another point of view in connection with blood-tests. 
 
 Even before the publication of Darwin's "Origin of
 
 66 THE THEORY OF EVOLUTION 
 
 Species" one of the great stumbling blocks in the 
 way of the theory of special creation was the existence 
 in a great many animals of rudimentary organs, or 
 such as are so far reduced and atrophied as to be of 
 no service to their possessors. An analogy employed 
 by my lamented friend, Mr. Richard Lydekker, may 
 be advantageously repeated here. Let us suppose 
 that a screw-steamer, with longitudinal shaft lead- 
 ing aft from the engine-room to the stern, where it 
 carries the propeller, should, on close examination, 
 reveal many signs that it had originally been a "side 
 wheeler," or paddle-boat. Recognizable remnants of 
 paddle-boxes, of bearings for a transverse shaft, and 
 the like, are found; what would be the inevitable 
 conclusion? No one would maintain that a naval 
 architect, in possession of his senses, in constructing a 
 screw-steamer would deliberately introduce features 
 which are useful and appropriate only in a paddle- 
 boat. The only reasonable explanation would be 
 that the vessel had originally been built as a paddle- 
 boat and had subsequently been converted into a 
 screw-steamer and in the conversion it had not been 
 found necessary completely to eradicate all traces of 
 the original construction. Obviously, the same rea- 
 soning applies to rudimentary organs. The only 
 satisfactory explanation of such useless remnants is 
 that their possessors are descendants of ancestors in 
 which those organs were fully functional. It seems 
 quite absurd to assume that, in a separately and 
 specially created animal, useless structures, reminis-
 
 EVIDENCE FROM EMBRYOLOGY 67 
 
 cent of other animals in which the same structures 
 are useful and valuable, should be included, merely 
 to indicate ideal relationships and community of 
 plan. 
 
 It was sought to break the force of this very serious 
 objection to the theory of special creation by saying 
 that apparently useless organs may nevertheless 
 have- functions which are still unknown to us and 
 may be revealed by future discovery. In certain 
 cases, like that of the thyroid gland in the neck, this 
 contention has been justified, but there are many 
 others to which it does not apply. For example, 
 in the great and varied whale-tribe (order Cetacea) 
 which includes the right, or whalebone, whales, the 
 sperm-whales, the porpoises, dolphins, etc., the fore- 
 limbs have been converted into swimming paddles, 
 but the hind limbs appear to have vanished com- 
 pletely, leaving no externally visible trace. In- 
 ternally, however, recognizable remnants of the hind 
 limb-bones may be found in various stages of reduc- 
 tion, which differ in the different members of the 
 order. In the Greenland Right Whale the hip-bone, 
 thigh-bone and shin-bone are indicated; in the Fin- 
 whale only the hip-bones and a minute rudiment of 
 the thigh-bone are to be found; in the toothed whales 
 only an almost unrecognizable remnant of the hip- 
 bone is left and in one of the dolphins even that has 
 disappeared. Similarly, the snakes have lost their 
 limbs completely, so far as external appearance is 
 concerned, and in most members of the group no
 
 68 THE THEORY OF EVOLUTION 
 
 trace of limbs is to be found on dissection, but in 
 certain snakes the rudiments of limbs are to be 
 detected. Leaving aside all preconceptions, which 
 is the more probable explanation of such phenomena, 
 the theory of special creation or the theory of evolu- 
 tion? 
 
 Even if it were admitted that all rudimentary 
 organs and structures found in the adult have a 
 certain unknown use and value, no one could main- 
 tain this with regard to the countless instances of 
 structures which are developed in the embryo, but 
 disappear entirely before birth. It is possible to 
 mention but a very few of such instances out of the 
 great number that have already been observed and re- 
 corded, but these few will suffice to illustrate the 
 principle involved. 
 
 ^Ryamptefr-trf- this may be cited from the most 
 widely different gmnpg- in tl^ ^mhryo O f insects^ 
 especially of beetles, pairs of legs are formed within 
 the_egg, not only on the head and thorax, but also 
 on the abdomen, but while those on the head are 
 transformed into mouth-parts, those on the thorax 
 are farther developed in their jointing and muscula- 
 ture to be locomotive legs, those on the abdomen 
 are again resorbed. In many fresh-water worms, the 
 eggs of which are laid in a cocoon, from which they 
 are hatched as a finished, minute, crawling worm, 
 larval organs are nevertheless formed, which recall 
 those of the Trochophore, the larva of the original 
 worms, which swims freely in the sea. However,
 
 EVIDENCE FROM EMBRYOLOGY 69 
 
 these larval organs . . . are never properly func- 
 tional, since no actually free-swimming larva is 
 developed, but the embryo merely floats in the al- 
 buminous fluid of the cocoon. 
 
 "A particularly beautiful example is offered by 
 the whales in their embryological development, which 
 has been thoroughly studied by Kiikenthal. In the 
 adult condition they show only the anterior ex- 
 tremities, but in the embryo the posterior pair, with 
 their skeletal parts, are formed, but are afterwards 
 completely atrophied. Although they are mammals, 
 in the adult condition they have absolutely no 
 covering of hair, since in their aquatic life another 
 and more effective pfotection against loss of heat is 
 given by means of a thick layer of blubber; only a 
 few coarse bristles, partly with particular functions, 
 have persisted on a few parts of the, body. But in 
 the embryo a dense covering of hair is formed, which 
 is later transformed in a peculiar manner and 
 atrophied. Further, a series of whales have no 
 _te^th_in_jiie_jELdult_^ondition, but only the well- 
 known, eel-trap like, horny plates, from which whale- 
 bone is produced. Nevertheless, in the embryo there 
 is a dentition of numerous teeth, which are, however, 
 resorbed, without ever piercing the gum." 1 
 
 Throughout the great group of the ruminants, 
 which includes the oxen, buffaloes, bison, sheep, 
 goats, antelopes, deer and giraffes, the collar-bone is 
 invariably lacking, since it is superfluous on account 
 
 1 Otto Maas, in Die Abstammungslehre, pp. 273-4.
 
 70 THE THEORY OF EVOLUTION 
 
 of the exclusively locomotive manner in which the 
 fore legs are employed. Tn the embryo sheep J:he 
 collar-bone is established and even, to some extent, 
 ossified, but is subsequently resorbed and disappears 
 entirely. No doubt, the collar-bone will be found 
 in many other embryo ruminants, when the proper 
 examination shall have been made, but its demon- 
 strated presence in the foetal sheep is sufficiently 
 striking. In the higher mammals the number of 
 teeth was originally 44, or 11 on each side of both 
 upper and lower jaws, but in most of the modern or 
 existing groups of these higher mammals this number 
 has been very considerably reduced through the 
 suppression of certain teeth. We have every reason 
 to believe that the ancestors of the forms with re- 
 duced dentition possessed teeth in full numbers and 
 that there has actually been a loss of teeth in the 
 course of descent. This conclusion is abundantly 
 confirmed by the facts of embryology. Take, for 
 example, the great group of the gnawing mammals, 
 or Rodentia, in which the front teeth or incisors, 
 above and below are reduced to one on each side, 
 except in the rabbits. The incisors are chisel-shaped 
 and are faced with hard enamel, so that the action 
 of the upper teeth upon the lower keeps the cutting 
 edges extremely sharp; these teeth do not form 
 roots, but continue to grow throughout the lifetime 
 of the animal. Between the chisel-like incisors and 
 the grinding teeth, there is a long, toothless gap, 
 which, we assume, was, in the ancestors of the
 
 EVIDENCE FROM EMBRYOLOGY 71 
 
 rodents, occupied by the second and third incisors, 
 the canine and two or more grinders. This conclu- 
 sion is justified by the facts of embryology; for in- 
 stance, hi the embryo of the squirrelseveral of the 
 -jnjssing teeth are be^un as distinct tl wth -ff p nTT fi hl1T " 
 
 and are resorbed 
 
 j>efore birth. 
 
 All available evidence points to the conclusion 
 that birds are descended from reptiles, a conclusion 
 which is especially strengthened by the facts of 
 palaeontology and will be examined more at length 
 in the following lecture. Such a descent explains 
 many otherwise puzzling features in the ontogeny 
 of birds, in which reptilian characteristics appear in 
 transitory fashion and are either modified so as to 
 take on typically bird-like character, or are sup- 
 pressed altogether. A remarkable example of this 
 is the formation of rudimentary teeth in certain 
 embryonic birds, followed by their resorption and 
 disappearance before hatching. 
 ^J[tcanhardly_be contended that these rudimentary 
 structures, which are confined to the embryonic 
 stages of development and of which no trace remains 
 in the adult, are so indispensable to the processes 
 of ontogeny, that they were specially created to 
 serve this temporary purpose. For such a conten- 
 tion there is not a particle of evidence and the 
 theory of evolution, which regards these structures 
 as useless remnants, due to inheritance from an- 
 cestors in which the structures were functional,
 
 72 THE THEORY OF EVOLUTION 
 
 offers much the most satisfactory solution of the 
 problem that has yet been suggested. 
 
 Embryology further shows that evolution is not 
 invariably an advance from lower and simpler to 
 higher and more complex types, but may be by way 
 of degeneration, and degradation. The adoption of 
 a parasitic mode of life is very apt to cause such 
 degradation, and some very remarkable instances of 
 the degeneration of parasites have been observed. 
 An instructive example that may be cited is that of 
 Sacculina, a nondescript creature that is parasitic 
 on certain species of crabs. The parasite is attached 
 to the body of its victim, underneatTT~the tail, by 
 means of root-like fibres which penetrate and ramify 
 throughout the interior of the crab. The root-like 
 fibres absorb nutriment and convey it to the body 
 of the parasite, which is reduced to a mere bag, 
 without appendages, muscles, nervous system, sen- 
 sory apparatus, digestive tract, or any determinable 
 organs save those of reproduction. The creature 
 has the power of assimilating the nutritive juices 
 which are conveyed to it by the root-like filaments 
 from the body of its host, and the power of reproduc- 
 tion, and it must have some respiratory and excretory 
 capacity, though there are neither gills nor glands. 
 From an examination of the adult parasite alone, 
 it would be quite impossible to classify it and deter- 
 mine the type and class to which it should be re- 
 ferred, but embryology solves the problem. From 
 the egg is hatched a free-swimming larva, which
 
 EVIDENCE FROM EMBRYOLOGY 73 
 
 has jointed appendages, nervous, muscular and 
 digestive systems and, in short, clearly belongs to 
 that group of the Crustacea which includes the 
 barnacles. This is degeneration carried nearly to 
 the utmost possible extreme and yet the individual 
 development shows the derivation of this otherwise 
 problematical parasite and the steps through which 
 it passed in its deterioration. 
 
 It was stated above that several distinguished nat- 
 uralists altogether reject the recapitulation theory as 
 a means of interpreting the facts of embryology. 
 They do this on the ground that, inasmuch as changes 
 and innovations in form or structure must arise in the 
 germ-plasm, at the very beginning of ontogeny, 
 there is no reason why such changes might not in- 
 volve the whole course of embryological develop- 
 ment. To my mind this a priori objection to the 
 recapitulation theory is quite without force in view 
 of the great body of observed facts, but there is no 
 time to enter upon a discussion of such an abstruse 
 and difficult problem. For our present purpose, 
 however, it is important to note that these objectors 
 are staunch evolutionists and find in the community 
 of mode in ontogeny between different classes of 
 organisms one of the strongest arguments in support 
 of the evolutionary doctrine. 
 
 Here may be conveniently considered the very in- 
 teresting and significant blood tests which have 
 been made in the last fifteen years by various physi- 
 ologists and especially by Dr. George H. F. Nuttall,
 
 74 THE THEORY OF EVOLUTION 
 
 of the University of Cambridge. Though there are 
 several methods of making these tests, the "precipi- 
 tation method" employed by Dr. Nuttall will be 
 quite sufficient for the ends sought in these lectures. 
 The method and significance of the tests can best be 
 explained by taking as an example human blood, 
 which, of course, has been most extensively and 
 minutely studied, because of its legal importance as 
 well as its scientific interest. Ordinary chemical 
 analysis is unable to determine the differences in 
 blood-composition between various animals, but 
 that there were important differences had long been 
 understood. This was shown by the fact that, in 
 performing the operation for the transfusion of blood, 
 it was not practicable to substitute animal for hu- 
 man blood, since the former might cause serious 
 injury to the patient. 
 
 The precipitation method of making blood tests 
 is as follows: Freshly drawn human blood is allowed 
 to coagulate or clot, which it will do in a few min- 
 utes, if left standing in a dish, and then the serum 
 is drained away from the clot. Blood-serum is the 
 watery, almost colourless part of the blood, which 
 remains after coagulation. Small quantities of this 
 serum are injected, at intervals of one or two days, 
 into the veins of a rabbit and cause the formation 
 in the rabbit's blood of an anti-body, analogous to 
 the anti-toxin which is produced in the blood of a 
 horse by the injection of diphtheria virus. After 
 the last injection the rabbit is allowed to live for
 
 EVIDENCE FROM BLOOD TESTS 75 
 
 several days and is then killed and bled, the blood 
 is left until it clots and the serum drained off and 
 preserved. The serum obtained thus from a rabbit 
 is called "anti-human" serum and is an exceedingly 
 delicate test for human blood, not only when the 
 latter is fresh, but also when it is in the form of old 
 and dried blood-stains, or even when the blood is 
 putrid. Stains, for example, are soaked in a very 
 weak solution of common salt and, if necessary, the 
 blood solution is filtered until it is quite limpid and 
 clear. Into the blood solution a few drops of the 
 anti-human serum are conveyed and, if the stains 
 are of human blood, a white precipitate is formed 
 and thrown down, but if the stains are of the blood 
 of some domestic animal, such as a pig, sheep, or 
 fowl, no such reaction follows. In the same manner 
 as above described, we may prepare anti-pig, anti- 
 horse, anti-fowl, etc., etc., sera by injecting the 
 fresh-drawn serum of a pig, horse, fowl, or any other 
 animal into the rabbit, instead of human blood- 
 serum. In some countries, notably in Germany and 
 Austria, this test has already been adopted by the 
 courts of justice and has been found extremely useful 
 in the detection of crime. 
 
 Further investigation showed that these blood 
 tests might be employed to determine the degrees 
 of relationship between different animals, for, al- 
 though a prompt and strong reaction is usually 
 obtained only from the blood of the same species as 
 that from which the original injection into the rabbit
 
 76 THE THEORY OF EVOLUTION 
 
 was taken, the blood of nearly allied species, such as 
 the horse and donkey, for example, gives a weaker 
 and slower precipitation. By using stronger solu- 
 tions and allowing more time, quite distant relation- 
 ships may be brought out. Nuttall and his collabo- 
 rator, Graham-Smith, made many thousands of such 
 experiments bearing upon the problems of relation- 
 ship and classification and it is of great significance 
 to note that their highly interesting and important 
 results contain few surprises, but, in almost all cases, 
 merely serve to confirm the conclusions previously 
 reached by other methods, such as comparative 
 anatomy and palaeontology. It will be instructive 
 to quote some of these results, the quotations being 
 taken from "Blood Immunity and Blood Relation- 
 ship by G. H. F. Nuttall, including Original Re- 
 searches by G. L. Graham-Smith and T. S. P. 
 Strangeways;" Cambridge, 1904. 
 
 "In the absence of palseontological evidence the 
 question of the interrelationship amongst animals 
 is based upon similarities of structure in existing 
 forms. In judging of these similarities, the subjec- 
 tive element may largely enter" (p. 1). "The very 
 interesting observations upon the eye made by 
 Johnson also demonstrate the close relationships 
 between the Old World forms and man, the macula 
 lutea tending to disappear as we descend in the scale 
 of New World monkeys and being absent in the 
 Lemurs. The results which I published upon my 
 tests with precipitins directly supported this evi-
 
 EVIDENCE FROM BLOOD TESTS 77 
 
 dence, for the reactions obtained with the bloods of 
 Simiidse [i. e., Man-like Apes] closely resemble those 
 obtained with human blood, the bloods of Cercopi- 
 thecidse [Old World Monkeys] came next, followed 
 by those of Cebidae and Hapalidee [New World 
 Monkeys and Marmosets] which gave but slight 
 reactions with anti-human serum, whilst the blood 
 of Lemuroidea gave no indication of blood-relation- 
 ship." (p. 2). "A perusal of the pages relating to 
 the tests made upon the many bloods I have exam- 
 ined by means of precipitating anti-sera, will very 
 clearly show that this method of investigation per- 
 mits of our drawing certain definite conclusions. It 
 is a remarkable fact . . . that a common property 
 has persisted in the bloods of certain groups of ani- 
 mals throughout the ages which have elapsed during 
 their evolution from a common ancestor, and this in 
 spite of differences of food and habits of life. The 
 persistence of the chemical blood-relationship be- 
 tween the various groups of animals serves to carry 
 us back into geological times, and I believe we have 
 but begun the work along these lines, and that it will 
 lead to valuable results in the study of various prob- 
 lems of evolution" (pp. 2-^4). 
 
 The general conclusions on interrelationships, so 
 far as they are of particular interest for our purpose, 
 reached by Nuttall and Graham-Smith as the result 
 of many thousands of blood tests, may be sum- 
 marized as follows: 
 
 (1) If sufficiently strong solutions be used and
 
 78 THE THEORY OF EVOLUTION 
 
 time enough be allowed, a relationship between the 
 bloods of all mammals is made evident. 
 
 (2) The degrees of relationship between man, 
 apes and monkeys have already been noted. 
 
 (3) Anti-carnivore sera show "a preponderance 
 of large reactions amongst the bloods of Carnivora, 
 as distinguished from other Mammalia; the maxi- 
 mum reactions usually take place amongst the more 
 closely related forms in the sense of descriptive 
 zoology." 
 
 (4) Anti-pig serum gives maximum reactions only 
 with the bloods of other species of the same family, 
 moderate reactions with those of ruminants and 
 camels, and moderate or slight reactions with those 
 of whales. Anti-llama serum gives a moderate reac- 
 tion with the blood of the camel, and the close rela- 
 tionship between the deer family and the great host 
 of antelopes, sheep, goats and oxen is clearly demon- 
 strated. 
 
 (5) Anti-whale serum gives maximum reactions 
 only with the bloods of other whales and slight reac- 
 tions with those of pigs and ruminants. 
 
 (6) A close relationship is shown to exist between 
 all Marsupials, with the exception of the Thylacine, 
 or so-called Tasmanian Wolf. 
 
 (7) Strong anti-turtle serum gives maximum reac- 
 tions only with the bloods of turtles and crocodiles, 
 with those of lizards and snakes the results are almost 
 negative. With the egg-albumins of reptiles and 
 birds a moderate reaction is given.
 
 EVIDENCE FROM BLOOD TESTS 79 
 
 (8) Anti-lizard serum produces maximum results 
 with the bloods of lizards and reacts well with those 
 of snakes. 
 
 (9) These experiments indicate that there is a 
 close relationship between lizards and snakes, on the 
 one hand, turtles and crocodiles, on the other. They 
 further indicate that birds are more nearly allied 
 with the turtle-crocodile series than with the lizard- 
 snake series, results for which palseontogical studies 
 had already prepared us. 
 
 (10) "Tests were made by means of anti-sera for 
 the fowl and ostrich upon 792 and 649 bloods respec- 
 tively. They demonstrate a similarity in blood con- 
 stitution of all birds, which was in sharp contrast to 
 what had been observed with mammalian bloods, 
 when acted upon by anti-mammalian sera. Differ- 
 ences in the degree of reaction were observed, but 
 did not permit of drawing any conclusions." 
 
 (11) I have already called attention to the fact 
 that the problematical Horseshoe-crab is indicated 
 by its embryology to be related to the air-breathing 
 spiders and scorpions rather than to the marine 
 Crustacea. It is of exceptional interest to learn 
 that embryology is supported by the results of the 
 blood-tests. 
 
 It must not be supposed that there is any exact 
 mathematical ratio between the degrees of relation- 
 ship indicated by the blood tests and those which 
 are shown by anatomical and palaeontological evi- 
 dence. Any supposition of the kind would be imme-
 
 80 THE THEORY OF EVOLUTION 
 
 diately negatived by the contrast between the blood 
 of mammals and that of birds. It could hardly be 
 maintained that an ostrich and a parrot are more 
 nearly allied than a wolf and a hyena and yet that 
 would be the inference from the blood tests. Like 
 all other anatomical and physiological characters, 
 the chemical composition of the blood is subject to 
 change in the course of evolution and these develop- 
 mental changes do not keep equal pace in all parts 
 of the organism. It is the rule rather than the ex- 
 ception to find that one part of the structure ad- 
 vances much more rapidly than other parts, such as 
 the teeth, the skull, or the feet. The human body 
 is, fortunately for us, of rather a primitive kind, 
 while the development of the brain is far superior 
 to that of any other mammal and this great brain- 
 development has necessitated a remodeling of the 
 skull. On the other hand, the skeleton, limbs, 
 hands and feet are but slightly specialized. In 
 the elephant tribe, so far as we can trace them 
 back in time, there has been little change, save in 
 size, in the structure of the body or limbs, while the 
 teeth and skull have passed through a series of re- 
 markable changes. It is for this reason that it is 
 unsafe to found a scheme of classification, which is 
 meant to be a brief expression of relationship, upon 
 a single character, for the result is almost invariably 
 misleading. The results of blood tests must be 
 critically examined and checked by a comparison 
 with the results obtained by other methods of inves-
 
 EVIDENCE FROM BLOOD TESTS 81 
 
 tigation, but after every allowance has been made, 
 these tests are very remarkable. 
 
 The blood tests have brought very strong con- 
 firmation to the theory of evolution and from an 
 entirely unexpected quarter; they come as near to 
 giving a definite demonstration of the theory as we 
 are likely to find, until experimental zoology and 
 botany shall have been improved and perfected far 
 beyond their present state.
 
 LECTURE IV 
 EVIDENCE FROM PALAEONTOLOGY 
 
 Palaeontology is the science which deals with the 
 fossils of animals and plants entombed in the rocks 
 of the earth's crust. This science has certain pre- 
 eminent advantages for the investigation of evolu- 
 tionary problems, because it has to do with the 
 recognizable remains of the actual organisms which 
 formerly inhabited the surface of the earth and which 
 are themselves the links in the chain of development. 
 If there be any truth in the theory of evolution, 
 palaeontology ought to be one of its strongest supports 
 and offer some of the most convincing testimony in 
 its favour, and, on the other hand, if it be false, the 
 fossils should be able to expose its fallacies in an over- 
 whelming manner. 
 
 Here, again, it is necessary to utter a warning not 
 to expect too much, for this method of inquiry suffers 
 from certain very serious difficulties and disad- 
 vantages. 
 
 (1) Many types of animals and plants are not 
 capable of preservation in the fossil state, or else are 
 so extremely rare and are found separated by such 
 vast intervals of time, that their remains are of no 
 practical assistance in our inquiry. It is particularly
 
 EVIDENCE FROM PALAEONTOLOGY 83 
 
 unfortunate that many of these almost or entirely 
 missing types are precisely those which would be 
 most useful in the investigation. For any organism 
 to be preserved and discovered, it is almost essential 
 that it should have been fairly common and abund- 
 ant when living and that is the probable reason why 
 the beginning of so many genealogical lines should be 
 lost in obscurity. 
 
 (2) Of the innumerable fossil forms which have 
 been made known, nearly all are but partially pre- 
 served, only the hard parts, bones, teeth, shells, etc., 
 remaining, and the interpretation of these incom- 
 plete remains is often difficult, sometimes impossible 
 in the present state of knowledge. Exceptions to 
 this almost universal rule are so rare and occur so late 
 in geological time, as to be of little real assistance. 
 
 (3) In Darwin's "Origin of Species" is a famous 
 chapter devoted to an examination of "the imperfec- 
 tion of the geological record," in which he points out 
 the accidental and haphazard way in which or- 
 ganisms have been fossilized and the great gaps which 
 occur in the history of life on the earth, so far as that 
 history has been discovered and deciphered. "I look 
 at the geological record as a history of the world im- 
 perfectly kept and written in a changing dialect; of 
 this history we possess the last volume alone, relating 
 only to two or three countries. Of this volume only 
 here and there a short chapter has been preserved; 
 and of each page only here and there a few lines. 
 Each word of the slowly changing language, more or
 
 84 THE THEORY OF EVOLUTION 
 
 less different in the successive chapters, may repre- 
 sent the forms of life which are entombed in our 
 successive formations and which falsely appear to us 
 to have been abruptly introduced." * 
 
 An excellent example of the imperfectly recorded 
 facts is afforded by the long and narrow, but very 
 thick belt of rocks, which forms the surface of the 
 Connecticut Valley and runs unbrokenly from the 
 Hudson River across New Jersey, Pennsylvania and 
 Maryland into Virginia; the whole formation is 
 referred to what geologists call the Triassic period. 
 These rocks, which were not laid down in the sea, 
 but on land and in various bodies of fresh water, 
 are remarkably barren of fossils and one may search 
 through great thicknesses and over weary miles with- 
 out finding a trace of ancient life. In several local- 
 ities, notably in western Massachusetts, were exten- 
 sive mud-flats, which have preserved countless 
 foot-prints of the great variety of reptiles then 
 inhabiting North America; in size, these impressions 
 range from the prints made by tiny creatures, no 
 larger than a sparrow, to monsters which left tracks 
 each eighteen inches long. From the diversity in 
 form of the tracks it is obvious that a very large 
 variety of animals must have made them. Yet of all 
 this host, bones of only two or three have been found, 
 in localities scattered from Pennsylvania to Massa- 
 chusetts and, when it is remembered that these 
 rocks are quarried very extensively for building stone, 
 
 1 Origin of Species, 6th Ed., London, 1872, p. 289.
 
 EVIDENCE FROM PALAEONTOLOGY 85 
 
 the paucity of fossils is all the more evident. It 
 needs no argument to prove that in this case the 
 record is lamentably incomplete and it is typical of a 
 great many others. In a sense, the foot-prints them- 
 selves constitute a record, but it is one that we do 
 not know how to decipher. 
 
 On the other hand, matters have greatly improved 
 since Darwin wrote his oft-cited Chapter X; many 
 lands then geologically unknown have been explored 
 and many of the missing chapters and paragraphs in 
 the history of life have been brought to light. The 
 most ancient biologically intelligible period of the 
 earth's history is called the Cambrian and, compared 
 with the succeeding periods, the Cambrian has al- 
 ways been poor in fossils, great areas and thicknesses 
 of rocks being entirely barren. No one could doubt 
 that our knowledge of Cambrian life was most in- 
 complete and inadequate. A few years ago Dr. C. D. 
 Walcott, Secretary of the Smithsonian Institution, 
 discovered in the Canadian Rockies a most marvel- 
 lous series of Cambrian fossils of an incredible del- 
 icacy and beauty of preservation, which have thrown 
 a flood of new and unexpected light into very dark 
 places. It is clear that the Cambrian seas swarmed 
 with a great variety and profusion of life, but that in 
 only a few places, so far known to us, were the condi- 
 tions such that these delicate creatures could be 
 preserved. It is not possible to say how far the 
 difficulty caused by the imperfection of the geological 
 record will be removed by the progress of discovery.
 
 86 THE THEORY OF EVOLUTION 
 
 Even as matters stand to-day, the astonishing fact is 
 that so much has been preserved, rather than that the 
 story is so incomplete. Notwithstanding all the 
 difficulties, the palseontological method remains one 
 of the most valuable means of testing the theory of 
 evolution, because certain chapters in the history of 
 life have been recorded with a minuteness that is 
 really very surprising. 
 
 In the brief time at my disposal it is quite im- 
 practicable to make clear all the geological pre- 
 liminaries which are needed to explain the facts of 
 palaeontology. It may seem incredible that a fossil 
 found a thousand feet below the surface is the re- 
 mains of a creature that once swam in the sea and 
 that it was buried in what was the sea-bottom at the 
 time of its death. Yet such is the fact and the ex- 
 planation is very simple. I must also ask you to 
 take for granted the possibility of arranging the 
 fossils which are buried in the rocks in the chrono- 
 logical order of their succession. This is no assump- 
 tion made to bolster up any theory, for, in its main 
 outlines, the scheme of chronology which we now use 
 had been worked out long before the publication of 
 Darwin's revolutionary book, when the theory of 
 special creation had full possession of the field. Any 
 of my hearers who may be interested to learn how 
 this chronological succession of the forms of life has 
 been ascertained, will find the explanation in any of 
 the standard text-books of geology. 
 
 This arrangement in the order of succession in time
 
 EVIDENCE FROM PALAEONTOLOGY 87 
 
 is essential to a comprehension of the huge mass of 
 facts which have already been gathered. It is further 
 possible to make an approximate determination of 
 the relative chronology of the rocks and fossils in 
 different regions and continents, a determination 
 which is of the utmost importance, as will appear 
 later. In the following sketch of palseontological 
 evidence, it is not practicable to avoid all mention of 
 the technical terms used in designating the divisions 
 of geological time and so a table of these terms is 
 appended. 
 
 Cenozoic era 
 
 Mesozoic era 
 
 Permian period 
 
 Carboniferous period 
 
 Devonian period 
 Palaeozoic era 1 Silurian period 
 
 Ordovician period 
 [ Cambrian period 
 Pre-Cambrian eras. 
 
 N. B. The arrangement of the table is such that the most ancient 
 times are at the bottom and the latest at the top; the epochs of the 
 Mesozoic and Palaeozoic eras are omitted as unnecessary. 
 
 If, neglecting all details, we survey the great and 
 ever-changing procession of life, as it is recorded in 
 
 Quaternary period 
 
 Recent epoch 
 Pleistocene epoch 
 
 
 Pliocene epoch 
 
 
 Miocene epoch 
 
 Tertiary period 
 
 Oligocene epoch 
 
 
 Eocene epoch 
 
 
 Paleocene epoch 
 
 1 Cretaceous period 
 
 j Jurassic period 
 
 [ Triassic period
 
 88 THE THEORY OF EVOLUTION 
 
 the rocks, the immediate impression received is that 
 of constant, though not always uniform advance and 
 progress from the most ancient times, and, the more 
 nearly the present is approached in time, the closer is 
 the approximation to the modern animal and vege- 
 table worlds. This most obvious and striking fact 
 impressed itself upon the earlier palaeontologists, like 
 Cuvier and Agassiz, who upheld the theory of special 
 creation and the immutability of species, and they 
 interpreted it as the carrying out of a systematic 
 creative plan, an interpretation which is not at all 
 invalidated by the acceptance of the evolutionary 
 theory. 
 
 The latest of the eras, the Cenozoic, in which we 
 are now living, is characterized by a land vegetation 
 consisting chiefly of the flowering plants, such as the 
 trees, bushes and herbs, with which we are all famil- 
 iar, also palms, lilies and grasses. The conifers, or 
 "evergreens," pines, spruces, firs, etc., also belong to 
 the flowering plants, but to a lower grade, but of the 
 non-flowering or cryptogamic plants, only the ferns, 
 mosses and lichens can be called abundant; the fungi 
 and microscopic plants are left out of account, as so 
 little is known of their history. Certain groups which 
 contain but few species are of interest as being the 
 last survivors of what were once great and dominant 
 assemblages; the Cycads, or erroneously so-called 
 "Sago Palms," are now confined to the warmer parts 
 of the earth and, among the cryptogamic plants, the 
 little ground-pines, or lycopods, and the horse-tails
 
 EVIDENCE FROM PALAEONTOLOGY 89 
 
 are remnants of a more ancient vegetation. Among 
 animals, the most conspicuous groups are the mam- 
 mals, or warm-blooded quadrupeds, and the birds; 
 the reptiles occupy a very subordinate position and 
 only five orders of them are found, lizards and 
 snakes, turtles and crocodiles, including a single 
 lizard-like creature which is confined to New Zealand. 
 The overwhelming majority of Cenozoic fishes be- 
 long to the sub-class of the bony fishes or teleosts, 
 and of the remainder almost all are sharks. But 
 there are, almost exclusively in fresh waters and 
 consisting of very few forms, two groups of very 
 ancient date, the ganoids, and the lung-fishes of the 
 southern hemisphere. Of the vast host of Cenozoic 
 invertebrates it is impossible to make the most hasty 
 survey and nothing more can be attempted than to 
 mention a few of the most conspicuous kinds. There 
 are countless insects, especially of the higher orders, 
 beetles (Coleoptera), moths and butterflies (Lepi- 
 doptera), ants, bees, wasps (Hymenoptera) and flies 
 (Diptera). There is a great profusion of marine 
 life, crustaceans, molluscs, star-fishes, sea-urchins, 
 and others innumerable; survivals of formerly abun- 
 dant groups are the lamp-shells, or brachiopods, and 
 the sea-lilies or crinoids. 
 
 While this general statement will serve to give 
 some conception of Cenozoic life as a whole, the 
 periods and epochs of that era are each char- 
 acterized by stages of constant progress, which 
 may sometimes, when conditions are favourable,
 
 90 THE THEORY OF EVOLUTION 
 
 be followed out in surprising detail. Indeed, the 
 changes within the eras and periods are as marked 
 as those which distinguish these divisions from one 
 another. 
 
 The vegetation of the latest part of the Mesozoic 
 era is substantially the same as that of the Cenozoic, 
 the great revolution in plant life taking place within 
 the Cretaceous period, but the characteristic Meso- 
 zoic plants were conifers and cycads, the latter in 
 great variety and spread all over the earth, even 
 to the Antarctic continent. Birds are rare and far 
 more primitive than those of the Cenozoic, and the 
 same is true of the mammals, all of which were tiny 
 creatures of very low grade. The conspicuous and 
 all-dominating group was that of the reptiles, so 
 that the Mesozoic is appropriately called the "Age 
 of Reptiles." These creatures swarmed in the sea, on 
 the land and in the air and many attained gigantic 
 size, incomparably the largest land animals that 
 ever lived. The contrast between the Cenozoic and 
 Mesozoic is well summarized by the fact that while 
 in the Cenozoic only five orders of reptiles have 
 been found, in the Mesozoic there were no less 
 than twenty. Like the modern type of vegetation, 
 the bony fishes came in in the Cretaceous, while 
 before that period the fish-fauna was almost en- 
 tirely composed of ganoids and sharks. Many of 
 the Mesozoic invertebrates were of modern type, 
 but there were several highly characteristic groups. 
 Among the Mollusca the bivalves and gastro-
 
 EVIDENCE FROM PALAEONTOLOGY 91 
 
 pods were less abundant and varied than in the 
 Cenozoic, the beautiful chambered shells called Am- 
 monites were incredibly numerous and diversified. 
 In the successive forms of these shells many 
 striking genealogical series have been made out. 
 The curious Belemnites are almost exclusively 
 Mesozoic. 
 
 In all the recorded history of life there was no such 
 radical and far-reaching revolution as that between 
 the Palaeozoic and Mesozoic eras. Could a naturalist 
 be transported into the former he would find him- 
 self in an unfamiliar world, in which almost every 
 animal and plant that he saw would be strange to 
 him. The land vegetation was made up of gigantic, 
 tree-like cryptogams, especially the lycopods and 
 horse-tails, and the abundant ferns were both in 
 arboreal and herbaceous forms. The flowering plants 
 were represented but scantily and by curious trees 
 allied to the cycads and conifers, but of such plants 
 in the ordinary use of the word, there was no trace, 
 nor was any mammal or bird in existence. The vast 
 majority of Palaeozoic fossils belong to marine in- 
 vertebrates, for, except in the latter part of the era, 
 the only vertebrates were fishes and fish-like forms, 
 sharks, ganoids, lung-fishes and certain bizarre, 
 mailed animals, which were below the fishes in the 
 scale of organization. The seas were swarming with 
 a profusion of invertebrate life, many of which, 
 though referable to types and classes which are still 
 existing, were much more primitive in structure
 
 92 THE THEORY OF EVOLUTION 
 
 than any living representatives of those groups, 
 and groups which now are very rare were then ex- 
 tremely abundant, and many were dominant in the 
 Palaeozoic. For example, molluscs were less com- 
 mon and brachiopods and sea-lilies vastly more so 
 then than now. Confined to the Palaeozoic were 
 the remarkable Trilobites, an extinct sub-class of 
 the Crustacea, while the higher groups of the 
 latter were wanting. Spiders and scorpions were 
 followed by insects belonging to the lower and more 
 primitive divisions, such as cockroaches and dragon- 
 flies. 
 
 The species of Palaeozoic fossils already described 
 are numbered by thousands and the number is 
 constantly growing, as new discoveries are made. 
 Any attempt to survey this vast assemblage is a 
 highly technical problem and cannot be made here. 
 The few salient features which have been selected 
 are enough to show that the farther back in time 
 we explore the history of life, the greater do the 
 differences from the present order of things become 
 and that not in any haphazard way, but by changes 
 in a well-defined serial order. The history of life, 
 both animal and vegetable, is a story of progress and 
 differentiation, of advance continued through mil- 
 lions of years to modern conditions from far-off 
 beginnings, which were of radically different charac- 
 ter. In itself, this story is a striking testimony to 
 the theory of evolution and the testimony is much 
 strengthened, when the material at hand permits a
 
 EVIDENCE FROM PALEONTOLOGY 93 
 
 close study of the genealogy of some restricted 
 group. 
 
 It has often been objected that the palseontological 
 record, as we actually have it, is irreconcilable with 
 the evolutionary conception, because of the many 
 cases of the sudden and unheralded appearance of 
 new kinds of organisms, such as the remarkable 
 outburst of modern types of plants and fishes in 
 the upper Cretaceous period. This objection over- 
 looks the phenomena of migration, the importance 
 of which has only lately come to be recognized; the 
 many geographical changes in the connections be- 
 tween the different continents and between the 
 seas, which have demonstrably occurred in the 
 past, have from time to time altered the possibilities 
 and directions of migration. Hence, the appear- 
 ance in any given area of new forms may either be 
 through the modification of older forms which pre- 
 viously inhabited that area, or by immigration from 
 some other region. It is therefore not to be taken 
 for granted that any group originated in the region 
 where we happen first to find it; quite as frequently 
 it arose in some other region and spread gradually 
 from that. A very instructive illustration of this 
 principle is afforded by the history of the elephants, 
 which appeared suddenly (lower Miocene) in Europe 
 and somewhat later (middle Miocene) in North 
 America. In neither continent has anything been 
 found in any preceding formation, which could plausi- 
 bly be regarded as ancestral to these ancient and
 
 94 THE THEORY OF EVOLUTION 
 
 primitive elephants, which, to all appearances, might 
 have risen up out of the ground, as Buffon fancied 
 that they actually did. However, on evolutionary 
 principles, they must have had ancestors and the 
 obvious inference was that they had immigrated 
 into the northern continents from some southern 
 land. By a process of elimination, the conclusion 
 was reached that Africa must have been the original 
 home of these animals, a result which has been 
 amply confirmed by discoveries made not long ago 
 in Egypt, which carry the history of the order much 
 farther back in time than had previously been pos- 
 sible. 
 
 Another objection which has frequently been 
 brought against the palseontological argument for 
 the theory of evolution is that, while the genealogical 
 series made out within certain families may be ad- 
 mitted as proving development within relatively 
 small groups, the fossil record fails to connect the 
 larger and more widely separated types, thus indi- 
 cating that evolution, while real enough, is of strictly 
 limited possibilities. This, it will be remembered, 
 is the same objection as has been urged against the 
 argument from embryology, and the answer is in 
 both cases the same. In part, the objection is 
 founded upon an assumption which is not true and 
 is being gradually refuted by the progress of dis- 
 covery and, where this is not likely to happen, there 
 are obvious reasons why we cannot expect to find 
 the connecting links. The most ancient known
 
 EVIDENCE FROM PALAEONTOLOGY 95 
 
 fossiliferous rocks show that most of the principal 
 types of animal life were already differentiated at 
 the time when those rocks were formed and hence 
 there is little hope of finding the beginnings of those 
 types. It must not be forgotten that there is abun- 
 dant indirect evidence that life had existed on the 
 earth through unimaginably long ages before the 
 formation of the earliest fossiliferous rocks; that is 
 what Darwin meant when he said that the geological 
 record contained only the last volume of the world's 
 history. 
 
 An admirable illustration of the manner in which 
 the evidence of fossils may serve to show the con- 
 nection between types which are very widely sepa- 
 rated in the modern world, is afforded by the vegeta- 
 tion of the Carboniferous period, the last but one 
 of the Palaeozoic divisions. This luxuriant vegeta- 
 tion was composed almost entirely of non-flowering 
 or cryptogamic plants and, in addition to a great 
 variety of ferns, giant lycopods and horse-tails, there 
 were two groups of especial interest to evolutionists: 
 (1) the Sphenophyllales, very slender, probably 
 trailing and climbing plants, which are intermediate 
 in structure between the two cryptogamic classes, 
 the lycopods and horse-tails. Doubtless, they were 
 the survivors in Carboniferous times of more ancient 
 plants which were, in turn, the common ancestors 
 of the two classes named. (2) A proverbially im- 
 passable gulf yawns between the flowering and the 
 cryptogamic plants and yet the gulf is at least par-
 
 96 THE THEORY OF EVOLUTION 
 
 tially bridged by the Carboniferous group of the 
 Cycadofilices, which, as the name indicates, serve 
 to connect the cycads with the ferns; in ap- 
 pearance of stem and foliage the Cycadofilices most 
 resembled ferns. In view of such facts, it is idle 
 to say, as is still so often said, that the fossils 
 have never afforded transitions between groups 
 which are now radically distinct and widely sepa- 
 rated. 
 
 Another very interesting and significant transition 
 is that displayed by certain fossil birds, which clearly 
 indicate their derivation from reptiles. The relation- 
 ship between birds and reptiles is shown by com- 
 parative anatomy and embryology and is strongly 
 confirmed by the blood tests which were described 
 in the preceding lecture. To this mass of testimony 
 from three independent lines of inquiry, palaeontology 
 adds its quota. At the famous quarries of litho- 
 graphic stone at Solenhofen, in Bavaria, has been 
 found a marvellously preserved record of the later 
 Jurassic life of Europe and here were discovered two 
 nearly complete skeletons of the most ancient known 
 bird, belonging to the genus Archceopteryx. Though 
 an unmistakable bird, Archceopteryx yet retained 
 many characteristics of its reptilian ancestry, char- 
 acters which are repeated in no existing bird, or 
 appear only as transitory features in the course of 
 embryological development. There is no horny 
 beak, but the jaws are provided with numerous 
 small teeth, which explains the significance of the
 
 EVIDENCE FROM PALAEONTOLOGY 97 
 
 formation of tooth-germs in certain embryo birds, 
 as was pointed out in Lecture III (see p. 71). The 
 joints of the backbone lack the characteristic fea- 
 tures of modern birds, and are much more reptilian 
 in structure. A very remarkable feature is the long, 
 lizard-like tail, com- 
 posed of many vertebrae, 
 to each of which a pair 
 of quill feathers is at- 
 tached. The hand is 
 most un-birdlike; its 
 four fingers are all sepa- 
 rate and free from one 
 another and are com- 
 pletely developed, with 
 the same number of 
 joints as the correspond- 
 ing fingers of a lizard, 
 and each one is pro- 
 vided With a Claw. FlG 6 R estoration of Archaopteryx, 
 There are feathers On Jurassic of Bavaria. (After An- 
 ., . ., j dreae). 
 
 the wings, tail and 
 
 legs, but the head, neck and body are mostly 
 
 naked. We may well hope that the ancestors of 
 
 Archceopteryx, fully intermediate between birds 
 
 and reptiles, will yet be discovered in Triassic 
 
 rocks. 
 
 The ancestry of mammals is still a question of 
 much uncertainty and zoologists continue to debate 
 whether they were derived from reptiles or amphib-
 
 98 THE THEORY OF EVOLUTION 
 
 ians, a state of affairs which Professor Steinmann 
 regards as so scandalous to science (see p. 4), but 
 there are good reasons for the uncertainty. Mesozoic 
 mammals are very rare and, though they have been 
 found in three continents, the specimens are all so 
 incomplete that they contribute but little to a solu- 
 tion of the problem. Certain reptiles obtained from 
 the Permian rocks of South Africa decidedly sug- 
 gest that they were closely allied to the long sought 
 ancestors of the mammals, but the gap is still too 
 great for any final decision. 
 
 The number of genealogical or phylogenetic series 
 within narrower limits, which has already been made 
 out, would fill many volumes. These genealogies 
 have been determined in a great many different 
 types of animals, both vertebrate and invertebrate, 
 but I shall confine attention to the vertebrates, as 
 it is easier to make these intelligible, and can select 
 only a few of the most conspicuous and best known 
 instances. So long as it is clearly understood that 
 but a small number of illustrative examples are here 
 set forth, it will serve no good purpose to pile up a 
 great number of more or less similar cases. Some 
 of the best preserved and most complete of the 
 phylogenetic series are among the families of mam- 
 mals, the changes in which may be traced through 
 the successive stages of the Tertiary period. The 
 most complete of all the series so far discovered is 
 that which displays the development of the horses 
 in North America. One cannot but hesitate to tell
 
 EVIDENCE FROM PALAEONTOLOGY 99 
 
 again this oft-told tale, which has been reiterated 
 until it has become a hackneyed commonplace. 
 Friends and foes of evolution are alike given to 
 sneering at it and calling it the "parade horse," the 
 "hobby horse," the "stalking horse," and other 
 opprobrious names, seeming almost to imagine that, 
 if testimony be very often repeated, it loses all 
 validity. Logically, this is on a par with the pro- 
 cedure of the ship's crew in the "Hunting of the 
 Snark," who believed that, by repeating a state- 
 ment three times, they proved it. On the other 
 hand, no sketch, however slight, which purports to 
 give an outline of the evidences of evolution, can in 
 fairness omit all mention of this remarkable case. 
 
 The Recent and Pleistocene horses, including in 
 that term not only the true horses, but also the 
 asses, zebras, etc., have a skeleton which is wonder- 
 fully adapted to swift running, while their teeth are 
 highly specialized and elaborated for the masticating 
 of grasses, which are among the most abrasive of 
 plants. The front teeth (incisors) used for cropping, 
 continue to grow for a long period and have on their 
 biting surfaces a deep pit, lined with enamel, which 
 horsemen call the "mark." The grinding teeth are 
 very high-crowned, growing persistently in height, 
 as they are worn down, to an advanced period of 
 life, when the formation of roots puts an end to 
 growth; they have masticating surfaces which are 
 kept rough by the different hardness of the three 
 substances exposed on these surfaces, dentine,
 
 100 THE THEORY OF EVOLUTION 
 
 enamel and cement, which wear unequally. The 
 arrangement of the three tooth-substances is in a 
 highly characteristic and very complex pattern, and 
 the anterior grinding teeth (premolars) are similar 
 in form and size and in the pattern of the masticat- 
 ing surface to the posterior teeth (molars). The 
 facial portion of the skull is very long and the jaws 
 high vertically, to provide room for the very deep 
 tooth-sockets. The eye-socket is completely en- 
 circled in bone and displaced backward behind the 
 teeth, which otherwise would invade the socket and 
 compress the eyeball. 
 
 The neck is long and its vertebrae are much special- 
 ized to secure the greatest freedom of movement in 
 combination with strength. The limbs and es- 
 pecially the feet are elongate and their bones have 
 undergone nearly the utmost possible reduction in 
 number, either by complete suppression and loss, or 
 by co-ossification, such concentration being favour- 
 able to rapidity of movement; we find it repeated in 
 other swift runners, such as antelopes and deer. 
 The external bone of the forearm (ulna) is greatly 
 reduced in thickness, the middle portion of its shaft 
 has been lost and its two ends are co-ossified with 
 the enlarged radius, which carries the whole weight 
 imposed upon the fore leg. Similarly, in the hind 
 leg only the shin-bone (tibia) is left in the lower 
 segment, while the fibula seems to have completely 
 disappeared, but if the skeleton of a young colt be 
 examined, it is seen that, while the whole shaft of
 
 EVIDENCE FROM PALAEONTOLOGY 101 
 
 the fibula has been suppressed, its two extreme ends 
 remain and in the adult are indistinguishably fused 
 with the tibia. The feet are very long and have but 
 a single functional toe each, which is greatly en- 
 larged and strengthened to bear the heavy weight. 
 This remaining toe is the median one, or third, of 
 the original five, and the splint-bones are remnants 
 of the second and fourth, but have no joints or 
 hoofs attached to them, and are not visible in the 
 living animal. The horses walk on the very tips 
 of their toes and the curious, box-like hoof, which 
 encloses the terminal joint of the single functional 
 toe, is highly characteristic. 
 
 In surveying the history of the changes which 
 lead back from the Recent horses to their far distant 
 ancestors of the lower Eocene, it will be necessary 
 to omit most of the stages, for lack of time, and 
 mention only the most obvious. Passing back to 
 the lower Pliocene and upper Miocene, we find that 
 the horses of those times, while unmistakably equine, 
 differed in certain very significant ways from their 
 modern descendants. They are decidedly smaller 
 and of more slender and deer-like proportions; their 
 grinding teeth are hardly more than half as high 
 proportionately and with masticating surfaces of 
 similar, but less complicated pattern. The feet are 
 three-toed, the lateral toes (second and fourth) 
 having a full complement of joints and carrying 
 hoofs, but were more dew-claws, not reaching the 
 ground, and could have been of service only in soft
 
 102 
 
 THE THEORY OF EVOLUTION 
 
 mud. A small, nodular rudiment of the fifth toe is 
 also present. The horses of the middle and lower 
 
 FIG. 7. Four stages in the evolution of the equine skull. A, Eohippus, 
 lower Eocene. B, Mesohippus, lower Oligocene. C, Protohippua, 
 upper Miocene. D, Equus, Pleistocene and Recent. 
 
 Miocene display the transition between the high- 
 crowned, cement-covered grinding teeth and those
 
 EVIDENCE FROM PALEONTOLOGY 103 
 
 with low crowns, which were incapable of growth 
 after they had cut the gum, and have no cement on 
 the crown, but only on the roots. 
 
 It is very interesting to note that this adoption of 
 a high-crowned, persistently growing type of grind- 
 ing teeth was by no means confined to the horses, 
 but was repeated in several other families of hoofed 
 animals and gnawers (Rodentia), and came about 
 during the Miocene epoch. This transformation of 
 the teeth indicates a change from the habit of 
 browsing upon leaves and other soft vegetable sub- 
 stances to grazing, or feeding upon the hard, abrasive 
 grasses, and it is highly suggestive that this coincided 
 with, or quickly followed the spread of grassy plains, 
 which afforded a new and abundant supply of nu- 
 tritious food. 
 
 Again omitting intermediate stages and taking 
 another long backward step, we come to the horses 
 of the lower Oligocene, in which are notable changes 
 in all parts of the skeleton, as well as a distinct reduc- 
 tion in size, for these animals are no larger than sheep. 
 The teeth are low-crowned and devoid of cement, 
 save on the roots, and the sharp-edged incisors have 
 no mark. The facial portion of the skull is shorter 
 and the eye-socket farther forward, placed over 
 the teeth, and is partially open behind; the jaws are 
 shallow and slender, as there is no need of providing 
 for very deep sockets. The neck is only moderately 
 elongate and its vertebrae are much less specialized. 
 In the forearm the ulna is already much reduced in
 
 104 THE THEORY OF EVOLUTION 
 
 thickness, but is complete and separate from the 
 radius, and in the lower hind leg, the fibula is separate 
 and has a very slender shaft, but it is entire and 
 uninterrupted. The feet are still three-toed, but 
 the lateral toes are longer and more useful in carry- 
 ing weight. 
 
 Once more omitting many intermediate forms, 
 we may examine the most ancient known horses, 
 those of the lower Eocene, which were little creatures, 
 not exceeding a fox in size. The grinding teeth have 
 a very simple and primitive pattern, though the 
 beginnings of the complicated horse-pattern may be 
 discerned in them; the premolars are all smaller and 
 simpler than the molars. The neck and limbs and 
 especially the feet are relatively short; there are four 
 functional toes in the front foot and three in the 
 hind, but the latter also has splint-like rudiments 
 of the first and fifth, plainly indicating derivation 
 from a five- toed foot. 
 
 Here the line breaks off and cannot at present be 
 traced farther back, the preceding Paleocene of 
 North America having yielded nothing that can be 
 regarded as ancestral to this family. The lower 
 Eocene horses in Europe are of the same type and 
 there also they appeared unheralded by any pre- 
 decessors in the Paleocene. The obvious conclusion 
 from these facts is that the family arose in some 
 region, as yet unidentified, but which not improbably 
 was central Asia, and thence migrated into Europe 
 and North America; the latter continent was made
 
 FIG 8 Four stages in the evolution of the equine fore foot. A, Eohippua, 
 lower Eocene. B, Mesohippus, lower Oligocene. C, Protohippus, upper 
 Miocene,. D, Equus, Pleistocene and Recent.
 
 106 THE THEORY OF EVOLUTION 
 
 accessible by a junction with northeastern Asia, 
 where now are Bering Strait and Sea. 
 
 Though the equine family did not originate in 
 North America, its principal development took place 
 here and went on unbrokenly through the long ages of 
 the Tertiary period. After they had arrived at a 
 high stage of advance and differentiation, the horses 
 spread to other continents and eventually reached all 
 of them except Australia. Invading South America 
 in the late Miocene or early Pliocene, after the two 
 continents had been joined by the upheaval of the 
 Isthmus of Panama, they there gave rise to a number 
 of peculiar and characteristic forms. In the Pleis- 
 tocene epoch great herds of horses, belonging to no 
 less than ten distinct species, ranged through the 
 forests and over the plains of North America, extend- 
 ing from the Atlantic to the Pacific and from Alaska 
 through Mexico. In size, they varied from a Shet- 
 land pony to species exceeding the largest modern 
 draught-horses in stature. Then, for some unknown 
 reason, they died out completely all over the western 
 hemisphere, a fate which was by no means confined 
 to the horses or to the Americas; the close of the 
 Pleistocene witnessed over three-fifths of the land- 
 surface of the globe, the extinction of a multitude of 
 the largest and most conspicuous mammals, but the 
 causes of such a tremendous mortality can only be 
 conjectured. 
 
 In addition to the main line of equine descent, 
 leading up to the modern horses, there were several
 
 EVIDENCE FROM PALAEONTOLOGY 107 
 
 side-branches of the equine stock, which, after a 
 longer or shorter duration, died out without leaving 
 descendants. One such branch was that of the 
 browsing horses, which were abundant through the 
 Miocene and early Pliocene of North America; they 
 retained the low-crowned teeth and the three-toed 
 foot until the end. The peculiar South American 
 forms were likewise side-branches, given off from the 
 main stem at a later date; one of these was a small 
 mountain horse, with very short feet, especially 
 adapted to climbing, like several of the existing 
 mountain antelopes. A very early branch, which 
 probably originated in Asia, diverged so widely from 
 the equine series that it is included in a different 
 family, is that of the Palseotheres, which was abun- 
 dant in the Eocene of Europe, but did not extend its 
 range to the western hemisphere. 
 
 We have thus traced the horses back to little 
 animals, whose structure is but remotely equine and 
 which are in all respects so unlike the modern species, 
 that hardly any palaeontologist would be bold enough 
 to connect them, were it not for the many inter- 
 mediate stages, which join these extremes in almost 
 unbroken continuity. No more illuminating example 
 of evolutionary development can be found among the 
 geological records of mammalian life than the history 
 of the horses, just because well-preserved specimens, 
 exemplifying all the important steps of change, have 
 been discovered. In turn, this was probably due to 
 the fact that, from the beginning to the end of their
 
 108 
 
 THE THEORY OF EVOLUTION 
 
 separate history, the horses have been individually 
 abundant and thus have had a much better chance of 
 
 Me. 
 
 m. 
 
 FIG. 9. Fore foot of Plio- 
 hippus, Pliocene, hind- 
 er view, showing the 
 very long splint-bones 
 (Me. II and IV) and 
 rudiment of fifth digit 
 (Mc.V)(froinTroxell). 
 
 -HC.JT. 
 
 FIG. 10. Fore foot of Horse (Equus) 
 posterior view, showing short 
 splint-bones. Wrist- and toe-bones 
 omitted. 
 
 being preserved as fossils than those of their con- 
 temporaries which were less common. But there are 
 several other families whose history is only less com-
 
 EVIDENCE FROM PALEONTOLOGY 109 
 
 pletely known than that of the horses and which 
 illustrate similar principles. In several instances we 
 are embarrassed with riches, finding it difficult, or 
 even impracticable, to select from the many possi- 
 bilities the forms which are actually and properly 
 referable to a given series. Indeed, there is every 
 reason to believe that, did we possess well-preserved 
 specimens of every mammal that ever lived on the 
 earth, we should be entirely unable to find a clue that 
 would guide us through the complex maze. 
 
 A family that has been traced from the Pleistocene 
 to the Eocene of North America is that of the tapirs, 
 but, for the most part, by the aid of rare, fragmentary 
 and therefore unsatisfactory materials. This, how- 
 ever, is of less consequence in the case of the tapirs, 
 for they are a very conservative family and have 
 undergone relatively little change. There has been a 
 moderate increase in size and bulk in the successive 
 stages, a few changes in the teeth and skull may be 
 noted, especially the modifications of the latter 
 caused by the development of a proboscis, but other- 
 wise the bony structure of an Eocene tapir differs but 
 little from that of its modern descendants. In the 
 Pleistocene, tapirs extended from Pennsylvania to 
 Argentina and to eastern and southern Asia, but 
 they had vanished from Europe, where they had lived 
 in the Miocene and Pliocene. Thus, at one time or 
 another, they ranged all over the northern hemi- 
 sphere; the significance of this fact will appear in the 
 next lecture.
 
 110 THE THEORY OF EVOLUTION 
 
 Another family, the history of which is far more 
 fully recorded, but much more difficult to decipher 
 and understand, is that of the rhinoceroses. The 
 difficulty arises from the number of divergent 
 branches and parallel series which developed within 
 the family and the local forms which arose in the 
 various regions to which they migrated, for, with the 
 exception of South America and Australia, every 
 continent has had its rhinoceroses. Strange to say, 
 the available evidence points to North America as 
 the region of their origin, though the proof of this is 
 not entirely conclusive. At all events, the family has 
 been traced farther back in time here than in any 
 other region. At a very early stage, the family 
 divided into three strongly divergent branches, two 
 of which were short-lived, lasting only from the 
 middle Eocene through the lower Oligocene, while 
 the third, that of the true rhinoceroses, has persisted 
 to the present day, but is confined to the warmer 
 parts of Africa and southern Asia. The brief series 
 of the cursorial rhinoceroses was exclusively North 
 American and differed from the other branches of the 
 family in being quite defenceless, having neither 
 horns nor tusks, and entirely dependent upon speed 
 for their safety. They were lightly built and slender, 
 long-necked, long-legged and narrow-footed, with 
 three-toed feet before and behind. There is much 
 about these curious animals that suggests relation- 
 ship to the horses, but the likeness is superficial 
 and due to cursorial habits, for every tooth and every
 
 EVIDENCE FROM PALEONTOLOGY 111 
 
 bone in the skeleton declares its affinities with the 
 rhinoceroses. 
 
 The second branch, though of American origin, 
 spread to the Old World and has been found in 
 France. The short, low and very broad skull had no 
 horns, but large tusks provided effective weapons of 
 defence. In marked contrast to the cursorial type, 
 these animals were presumably aquatic in habit and 
 though rather low, were very massive in build and 
 had four toes in the front-foot; aside from the head, 
 they must have been quite hippopotamus-like in 
 appearance. 
 
 The third branch, the true rhinoceroses, has had a 
 far more complicated history, because of the number 
 of distinct series within the group, which passed 
 through parallel courses of development; Professor 
 Osborn recognizes no less than seven of these series in 
 the Old and New Worlds. The terminal members of 
 three series are the modern African, Indian and 
 Sumatran species. While the structure of the true 
 rhinoceroses underwent very considerable changes in 
 that part of their history of which the record has been 
 discovered, the changes are much less radical than 
 those which took place in the horses in the same 
 length of time and, for the most part, they affected 
 the skull and dentition. The true rhinoceroses are 
 characterized by the peculiar development of the 
 front teeth (incisors and canines); in the modern 
 African species these teeth have been entirely sup- 
 pressed, but in the Indian and Sumatran and the
 
 112 THE THEORY OF EVOLUTION 
 
 great majority of the extinct species, the peculiarity 
 above mentioned may be observed. The upper jaw 
 has but one incisor on each side and no canine; the 
 remaining incisor is chisel-shaped, with a sharp 
 cutting edge; in the lower jaw are two incisors on 
 each side, but no canine. The first lower incisor is 
 very small and can be of little or no real use, but the 
 second is a fairly large tusk, which points directly 
 forward and the Indian species, at least, uses them 
 effectively as weapons. All of the existing species, 
 as well as most of the fossil forms, are provided with 
 solid horns, which have no bony core; with an excep- 
 tion, to be noted below, the horns are not paired, but, 
 if more than one is present, they are placed one be- 
 hind the other in the middle line of the skull. The 
 skull is much modified to form a strong support for 
 the horns and its upper profile is convex in front, 
 concave behind. The neck is short, the body very 
 large, the legs and feet short and very heavy, with 
 three toes in each foot. It is quite impossible to take 
 up here the many series of the true rhinoceroses; all 
 that can be done is to indicate very briefly the 
 changes that have been noted in the group as a whole. 
 Going back at once to the most ancient known forms 
 which can be positively identified as true rhinoceroses, 
 we find in the lowest Oligocene of North America very 
 interesting species which help to connect the rhinoc- 
 eroses with other families of their order (Perisso- 
 dactyla). In these far-off animals the first upper 
 incisor had already assumed its trenchant shape,
 
 EVIDENCE FROM PALAEONTOLOGY 113 
 
 but is smaller than in later forms, and is followed by 
 two other incisors and a canine. The lower jaw also 
 has three incisors on each side, but has lost the 
 canine. The skull is without horns and the skeleton 
 is much lighter and more tapir-like in its proportions 
 and the whole animal decidedly smaller than the 
 existing rhinoceroses; the front foot has four toes, but 
 the external one (the fifth of the original five), is 
 already much reduced in size and evidently dwindling 
 away. From the Oligocene to the Pliocene, inclusive, 
 North America had an abundance of rhinoceroses, 
 both horned and hornless, but finally they disap- 
 peared from the continent entirely. 
 
 In the uppermost part of the lower Oligocene occur 
 rhinoceroses which have the beginnings of a trans- 
 verse pair of horns on the nose; in the upper Oligo- 
 cene these have increased to full size and this gives 
 us the exceptional series of paired-horn rhinoceroses 
 referred to above. In the lower Miocene the series 
 was represented by its terminal member, a very small 
 animal, of which immense herds lived on the Great 
 Plains. Near Agate, Nebraska, is a bed, perhaps an 
 ancient quicksand, which contains the bones of this 
 little rhinoceros in incredible quantities. 
 
 The story of the rhinoceroses, their development 
 and their wanderings, is a fascinating one, but, un- 
 fortunately, only this bald sketch can be given, from 
 which the most interesting details have been omitted. 
 Their testimony, so far as it goes, is just the same as 
 in the other genealogical series, gradual differentia-
 
 114 THE THEORY OF EVOLUTION 
 
 tion, with diversification and adaptation to different 
 climates, food-plants and habits of life. 
 
 There remains one family of mammals with which 
 it is necessary to deal and that is the camel tribe. 
 This family has two well-defined subdivisions, the 
 camels of the Old World and the llamas, guanacos, 
 etc., of South America. For a very long time, the 
 family was entirely confined to North America and 
 did not reach its present homes until the Pliocene 
 epoch of the Tertiary period. The skeleton of a 
 Patagonian Guanaco may be taken as the starting- 
 point of our inquiry. In this animal the third incisor 
 and the canine are retained in the upper jaw, all the 
 incisors and the canine in the lower. The anterior 
 two grinding teeth have been lost and the others are 
 moderately high-crowned. The skull is broad and 
 capacious behind, narrow and tapering in front. 
 The neck is long and its vertebrae very curiously 
 modified. The limbs are long and slender and have 
 undergone nearly the same modifications as in the 
 horses; the ulna is greatly reduced, interrupted in 
 the middle and its separated portions are fused with 
 the radius. In the hind leg the shaft of the fibula 
 has been completely suppressed; the upper end fuses 
 with the tibia, while the lower remains as a small 
 separate bone, wedged in between the tibia and the 
 heel-bone. The feet are very long and slender, with 
 two toes in each; the long bones of the foot are co- 
 ossified to form a "cannon-bone," the very young 
 skeleton showing that this co-ossification does ac-
 
 EVIDENCE FROM PALAEONTOLOGY 115 
 
 FIG. 11. Four stages in the evolution of the cameline skull. A, Proty- 
 lopus, upper Eocene. B, Poebrotherium, lower Oligocene. C, Pro 
 camelus, upper Miocene. D, Guanaco, Recent.
 
 116 THE THEORY OF EVOLUTION 
 
 tually take place. The toes proper are free, giving 
 the "cloven hoof," but the hoofs are very small and 
 the weight is carried upon a soft thick pad. 
 
 Were there time enough to do so, we might trace 
 the development of this family backward, step by 
 step, through all the many stages between the Pleis- 
 tocene and the upper Eocene in quite as unbroken 
 sequence and in as full detail as can be done for the 
 horses. We must, however, pass over all the inter- 
 mediate steps and consider the ancestral camels of 
 the upper Eocene. These were very little animals, 
 hardly larger than a jack rabbit, which had the full 
 complement of teeth, 44 in total number and all with 
 very low crowns. The limbs and especially the feet, 
 are relatively short, the ulna is complete and sepa- 
 rate, as is also the fibula; there are four toes in each 
 foot, though the lateral pair of the hind foot are ex- 
 tremely slender, and there is no co-ossification to 
 form cannon-bones. The hoofs are well developed, 
 in form like those of an antelope, so that there 
 can have been no pad. For the present, the line 
 cannot be carried back of the upper Eocene, the 
 probable ancestors from the middle and lower Eocene 
 being, as yet, represented only by fragmentary 
 specimens. 
 
 In addition to this main stem of cameline descent, 
 which resulted in the modern species, there were 
 two short-lived side branches which should be men- 
 tioned. One, ending in the lower Miocene, was the 
 series descriptively called "gazelle-camels," small
 
 FIG. 12. Four stages in the evolution of the cameline fore foot. A, 
 Protylopus, upper Eocene. B, Poebrotherium, lower Oligocene. C, 
 Procamelus, upper Miocene. D, Guanaco, Recent.
 
 118 THE THEORY OF EVOLUTION 
 
 animals with very long and slender legs, evidently 
 swift runners. The other series, the so-called "gi- 
 raffe-camels," terminated in the upper Miocene; 
 these were browsers and display an increasing stat- 
 ure, especially in the length of the neck and fore 
 limbs. They adapted themselves to the growing 
 aridity of the western plains. 
 
 Finally, may be mentioned an example from the 
 reptiles. The pre-eminently Mesozoic group of 
 reptiles known as the Dinosaurs has been made 
 quite familiar to the public by our great museums, 
 notably the American Museum in New York, the 
 National Museum in Washington, and the Carnegie 
 Museum in Pittsburgh. In the latter part of the 
 Cretaceous period, North America had a remarkable 
 family of Dinosaurs, harmless, plant-eating and 
 quadrupedal monsters. Though inoffensive, they 
 were well provided for defence, having three long 
 and sharp horns, one on the nose, like that of a rhi- 
 noceros, and one over each eye. The neck was pro- 
 tected by an enormous frill of solid bone, growing 
 out from the skull and reaching back to the 
 shoulders. The family may be traced far back 
 through the Cretaceous, the animals becoming 
 smaller and the horns shorter as we go back in 
 time, and the neck-shield becoming a mere outline 
 framework. When sufficient well-preserved material, 
 leading through long periods of time, can be ob- 
 tained, the result is always suggestive of a gradual 
 development.
 
 EVIDENCE FROM PALAEONTOLOGY 115 
 
 E 
 
 FIG. 13. Skulls of Horned Dinosaurs. A, Torosaurus. B, Triceratops. 
 C, Monoclonius. D, Styracosaurus. E. Ceratops. F, Brachycera- 
 tops. These do not form a single genealogical series, merely sev- 
 eral stages in the development of the skull. (From Matthew, in 
 part).
 
 LECTURE V 
 EVIDENCE FROM GEOGRAPHICAL DISTRIBUTION 
 
 The geographical distribution of animals and 
 plants over the surface of the globe was a subject of 
 interest and importance even in pre-Darwinian days 
 and great attention was paid to it as to so much 
 statistical information. Under the then prevailing 
 theory of special creation and the immutability of 
 species, no explanation of the facts could be expected 
 or even looked for; organisms were created and 
 placed where we find them and that, if true, was an 
 ultimate fact, of which no explanation was possible. 
 As was pointed out in the first lecture, Darwin tells 
 us in his autobiography that it was the facts of dis- 
 tribution which he observed in South America and, 
 above all, in the Galapagos Islands, that first turned 
 his thoughts in the channel of his great work, be- 
 cause these facts seemed to demand an evolutionary 
 explanation. 
 
 Every one knows that different animals and plants 
 are to be found in different parts of the world; the 
 polar bears and musk-oxen occur in the treeless 
 wastes of the Arctic lands, lions and tigers, elephants 
 and rhinoceroses and tall palms must be sought in 
 warm countries and, on hasty consideration, it might 
 120
 
 EVIDENCE FROM DISTRIBUTION 121 
 
 seem that distribution were wholly a matter of 
 climate. But this explanation fails on closer exam- 
 ination. The tropical parts of Australia, Africa and 
 South America have very similar climates and yet 
 it would be difficult to select three regions of the 
 earth which are more radically distinct from the 
 zoological point of view. The fact that a given 
 animal or plant does not occur in a particular region, 
 is no conclusive proof that it is not fitted to live 
 there. The rabbits introduced into Australia have 
 multiplied to such an extent that they have become 
 a very serious pest, as have the European Gypsy and 
 Brown-tail Moths in Massachusetts. The acclima- 
 tization of the Indian Mongoose in Jamaica has 
 upset the whole zoological equilibrium of the island, 
 while domesticated horses and cattle, running wild, 
 grew into vast herds, which overspread the plains of 
 North and South America. European weeds are 
 among the most difficult and annoying enemies 
 with which the American farmer has to contend, 
 and the American cactus, incautiously brought into 
 South Africa, has become a nuisance and taken pos- 
 session of large areas. Cactuses and the Agave (or 
 Century Plant, also American) have taken so kindly 
 to the Mediterranean lands, that many artists have 
 painted them in scenes of classical antiquity, which 
 to the botanical geographer is amusing. These 
 and many similar facts sufficiently show that the 
 introduction of a new animal or plant may be a very 
 serious undertaking, fraught with great possibilities
 
 122 THE THEORY OF EVOLUTION 
 
 of mischief and, further, how excellent are the reasons 
 for the modern legislation which forbids the impor- 
 tation of live animals into the United States, except 
 by permission of the Secretary of Agriculture. No 
 one can tell in advance what plague he may be 
 bringing in. 
 
 These facts also prove definitely that animals 
 and plants may flourish in many regions beside their 
 native homes and that the absence of any organism 
 is not to be explained by assuming its unfitness. 
 Such considerations might easily lead us to conclude 
 that the whole subject of distribution was a meaning- 
 less hodge-podge of uncorrelated facts, which defied 
 explanation and, yet, if there be any truth in the 
 theory of evolution, it must afford an explanation, 
 or else its claims upon our acceptance will be very 
 greatly weakened. The fundamental postulate of 
 the theory is the unbroken continuity of life from its 
 first appearance on the earth and therefore the 
 present arrangement of living things must be the 
 outcome of an unimaginably long series of past 
 changes, not only those produced by the evolutionary 
 process, but also the countless changes of climate 
 and geography which have continually altered the 
 possibility of migration in this or that direction. 
 The great extinctions, which have from time to time 
 devastated whole regions, have also played a part in 
 bringing about the present order of things. At the 
 end of the Pleistocene epoch, immediately preceding 
 our own, North America lost a host of large mam-
 
 EVIDENCE FROM DISTRIBUTION 123 
 
 mals, the bones of which are preserved in such 
 natural museums as the wonderful asphalt pits near 
 Los Angeles: horses, tapirs, llamas, elephants and 
 mastodons, giant ground-sloths, huge wolves, lions, 
 the terrible sabre-tooth tigers, and countless others. 
 Here is a zoological revolution, which equally af- 
 fected the northern part of the Old World. 
 
 We have very clear proof that the form, size and 
 mutual connections of the various continents have 
 undergone a long succession of changes. In the 
 cases where two continents, now connected, were 
 formerly separated by the sea, the proof of this 
 separation almost always remains in the form of 
 marine deposits, including the fossils of marine 
 animals. In this manner the ancient disjunction of 
 North and South America, the separation of the 
 Spanish peninsula from Europe, of north Africa 
 from the remainder of the continent and of India 
 from Asia, may be readily demonstrated. On the 
 other hand, in the case of those lands which were 
 formerly connected, but now are separated by the 
 sea, the direct proof is wanting, because the connec- 
 tion is covered by the sea, but soundings generally 
 indicate it, for most of the former connections which, 
 for one reason or other, we presume to have existed, 
 are marked out by areas of shoal water in very sharp 
 contrast to the profound depths of the oceanic 
 abysses. Such former connections are indicated 
 between Alaska and northeastern Asia, Great 
 Britain and the continent of Europe; Spain and
 
 124 THE THEORY OF EVOLUTION 
 
 Morocco; Italy, Sicily, Malta and Tunis; the Greek 
 islands and the mainland; Japan, Formosa, Borneo, 
 Java, etc., and Asia, and many others. The evi- 
 dences of climatic changes, though of an entirely 
 different kind, are equally clear. 
 
 In brief, the distribution of existing animals and 
 plants constitutes a series of historical problems, 
 the solution of which is to be sought in the facts of 
 geology and palaeontology. On evolutionary prin- 
 ciples, we ought to find that, the longer time any 
 two regions have been separated, the greater is the 
 difference between the animals and plants of those 
 regions, and this is precisely what we do find. So 
 well established is this principle, that we may invert 
 it and use it to distinguish those separations which 
 are earlier from those which are later in geological 
 date, in cases where direct geological evidence is 
 unattainable. The separation of Great Britain from 
 the continent must have taken place very recently, 
 geologically speaking, because there is no important 
 difference between their animals or plants, while the 
 separation of Madagascar from Africa must have 
 been effected at a much earlier time because of the 
 radical differences between the faunas of island and 
 continent. Of course, such instances cannot fairly 
 be used as arguments in favour of evolution, because 
 they assume the truth of the thing to be proved, 
 except in so far as the evolutionary explanation is 
 the only one that will account for the facts. 
 
 Before attempting to explain any of the problems
 
 EVIDENCE FROM DISTRIBUTION 125 
 
 of distribution, it will be necessary to consider the 
 factors which determine distribution under present 
 conditions and, in order to keep the whole great 
 subject within manageable limits, we must confine 
 our attention to a few groups of organisms. For 
 our purposes by far the most suitable examples are 
 those furnished by the mammals. All the evidence 
 at our disposal goes to show that every species orig- 
 inated in some one particular area and has spread 
 from that starting-point as far as circumstances 
 would permit. This is called the "doctrine of 
 specific centres." In the case of more comprehen- 
 sive groups also, there is no reason to believe that, 
 among the higher animals, at least, the same genus, 
 family, order, class, etc., ever arose twice independ- 
 ently in disconnected land areas. This being the 
 case, the possession of identical, or closely similar, 
 animals by two regions now separated is strong evi- 
 dence that these regions were once joined and at 
 no very distant date. The spread of new forms from 
 their centres of origin may be quick or slow, as cir- 
 cumstances are or are not favourable; very rapid 
 spread was shown by the horses which, set free 
 about 1537, on the abandonment of the first settle- 
 ment of Buenos Aires, within forty-three years ap- 
 peared on the Straits of Magellan, nearly a thousand 
 miles away. 
 
 The nature of the barriers which limit the spread 
 of any species differs with the kinds of organisms, 
 but for land mammals the most important are wide
 
 126 THE THEORY OF EVOLUTION 
 
 arms of the sea; no such animal can pass the English 
 Channel or Bering Strait. Next in importance is 
 climate and especially temperature, though hu- 
 midity also plays an effective part. As we have seen, 
 similarity of climate is, of itself, entirely powerless 
 to bring about similarity of animals, but as a passive 
 barrier, climatic differences are most effective in 
 limiting the spread of organisms, many of which are 
 utterly unable to pass an isothermal line. In minor 
 degree, large rivers, mountain-chains and the other 
 great topographic features frequently act as barriers. 
 Still another kind of barrier is pre-occupation by 
 some other species, it may be a competing species 
 which leaves no place in the economy of nature 
 which the newcomer can fill, or it may be an enemy 
 or infectious disease which destroys the immigrants 
 as fast as they can arrive. The Tse-tse flies of Africa 
 close the area of their range against the entrance of 
 most of the domestic animals by infecting them with 
 a fatal disease. 
 
 Many attempts have been made to divide the 
 lands of the globe into zoological regions, which 
 shall express the degrees of likeness and difference 
 in their animal inhabitants. This cannot be satis- 
 factorily done by using the same scheme for all 
 kinds of organisms, for the geological dates of the 
 origin and dispersal of the various types of land 
 animals are so far apart in time, that each found 
 such very different geographical arrangements, and 
 frequently also climatic conditions, that the possi-
 
 EVIDENCE FROM DISTRIBUTION 127 
 
 bilities of migration had been greatly changed. For 
 the mammals a very satisfactory working arrange- 
 ment has been gradually reached, which is well 
 justified by the data of geology and palaeontology. 
 This plan first of all divides the continents into two 
 groups of very unequal size, (1) Australia and its 
 adjoining islands, Tasmania, New Guinea, etc., 
 (2) the rest of the world. This division recognizes 
 the fact that Australia is, zoologically speaking, by 
 far the most isolated region of the earth; aside from 
 the bats and a few mice, its mammals are Mono- 
 tremes and Marsupials. Of the monotreme egg- 
 laying mammals Australia has all existing species 
 and of the marsupials all except the opossums of the 
 western hemisphere, with the addition in South 
 America of one interesting genus, the survivor of a 
 group that was abundant in the Tertiary of that 
 continent. 
 
 Neglecting Australia, the other continents likewise 
 fall into two very unequal groups, (1) South America, 
 including Central America and the West Indies, 
 and (2) the other continents. South America is, 
 after Australia, much the most peculiar part of the 
 globe, a fact which is explained by its geological 
 history. The remaining continents, North America, 
 Europe, Asia and Africa, with great local differences 
 among themselves, have a certain unmistakable 
 similarity in their mammals. The local differences 
 are expressed by dividing this vast land-area into a 
 number of regions, with the curious and unexpected
 
 128 THE THEORY OF EVOLUTION 
 
 result that, except Europe, none of the continents 
 falls into a single region. North America, for ex- 
 ample, contains more or less of three regions, Asia 
 and Africa each two. This striking lack of coinci- 
 dence between geographical and zoological bound- 
 aries is, at first sight, so anomalous as to make the 
 whole scheme appear very doubtful, but, here again, 
 past geographical and present climatic conditions 
 offer an adequate explanation of the anomaly. 
 
 Finer distinctions are expressed by dividing the 
 regions into subregkms, provinces, faunas, etc., but 
 it will not be necessary to consider these subdivi- 
 sions here. The following table, which omits Aus- 
 tralia, gives the arrangement of geographical realms 
 and regions which is now very generally adopted by 
 students of the mammals, past and present. 
 
 I. REALM OF NEOG.EA. 
 
 1. Neotropical Region, South and Central America; low- 
 kinds of Mexico; southern tips of Florida and Lower 
 California; West Indies. 
 
 1. Malagasy Region, Madagascar. 
 
 2. Ethiopian Region, Africa south of the Sahara. 
 
 3. Oriental Region, Southern peninsulas of Asia; Malay 
 
 Archipelago. 
 
 4. Holarctic Region, N. Africa; Europe; Asia north of the 
 
 Oriental Region, Japan; boreal N. America. 
 
 5. Sonoran Region, Remainder of N. America. 
 
 n . 
 
 OF 
 
 ABCTOG.EA. 
 
 A kind of distribution which has seemed very diffi- 
 cult to explain is that known as "discontinuous," 
 where the areas inhabited by a given group, large 
 or small, are separated by a very wide intervening
 
 EVIDENCE FROM DISTRIBUTION 129 
 
 space from which it is absent. Two or three ex- 
 amples, of which history affords the solution, may 
 be cited. At the present time the family of the 
 tapirs is found in the Oriental region and in South 
 America, with almost half the circumference of the 
 earth between them. In the latter part of the Ter- 
 tiary period the tapirs ranged all over the northern 
 hemisphere, but became extinct in Europe before 
 the Pleistocene; at the close of the latter epoch they 
 vanished from eastern Asia and from North America, 
 persisting in southern Asia and in Central and South 
 America to the present day. We have no means of 
 telling why they became extinct in the areas from 
 which they vanished, but the significant fact remains 
 that the vast stretch of land between their modern 
 homes was formerly covered by them. According 
 to the doctrine of special creation this fact explains 
 nothing, because the American species are different 
 from the Asiatic, but this is the almost inevitable 
 result, if evolution be true. The extremes of such a 
 wide ranging genus would, in the great majority of 
 cases, diverge to such an extent that it would be 
 necessary to refer them to distinct species. 
 / A second case is very similar. The camel family is 
 * divided into two well-defined genera, the true camels, 
 which are native to Asia, and the guanacos, llamas, 
 7 etc., of South America. This family, during much 
 the longest part of its history, was exclusively North 
 American in distribution, but in the Pliocene epoch 
 true camels had appeared in India and llamas had
 
 \ 
 
 130 THE THEORY OF EVOLUTION 
 
 extended their range to Argentina. The explanation 
 of the discontinuity is the same as in the tapirs, the 
 former occupation of the intervening lands by the 
 family in question and their eventual extinction in 
 those lands. 
 
 iird case is rather different. On the tops of the 
 White Mountains in New Hampshire grows an 
 assemblage of plants, to find which in the lowlands, 
 one must go hundreds of miles northward to Labra- 
 dor. The probable reason for this strange distribu- 
 tion is given by the glacial phenomena of the Pleis- 
 tocene; when the ice had buried the northern part of 
 the continent, Arctic animals and plants were driven 
 far to the South, the walrus coming down to the 
 coast of Georgia and the musk-ox to Kentucky and 
 Oklahoma. With the amelioration of the climate and 
 retreat of the ice, the Arctic plants and animals once 
 more went northward and ascended the mountains, 
 as these were freed from the ice. When present 
 climatic conditions had been attained, the Arctic 
 forms had disappeared from the foot of the moun- 
 tains, but remained on their cold summits as colonies 
 of the great Arctic invasion. The same thing hap- 
 pened in Europe, where Arctic plants still flourish in 
 the high Alps. Thus, in all cases where the history 
 of the animals or plants involved can be obtained, 
 the explanation of discontinuous distribution is the 
 same, former occupation of the intervening area. 
 The striking contrast between North and South 
 America is expressed in the scheme of classification;
 
 EVIDENCE FROM DISTRIBUTION 131 
 
 North America is a zoological patchwork, consisting 
 of one entire region, the Sonoran, a large area of a 
 second, the Holarctic, and small portions of the 
 third, the Neotropical, while, on the other hand, the 
 southern continent is all included in the Neotropical 
 region. This contrast is explained by the different 
 geological history of the two continents. During the 
 earlier and longer moiety of the Tertiary period 
 North and South America were separated by a broad 
 sea which swept over the Isthmus of Panama and 
 most of Central America and throughout this time of 
 separation the northern and southern continents 
 developed faunas which were most radically different. 
 The student who is familiar with the Tertiary mam- 
 mals of the northern hemisphere, American, Euro- 
 pean and Asiatic, finds himself in a new and strange 
 world, when he takes up those of South America. 
 On the other hand, North America was repeatedly 
 joined to the Old World and as often severed from it, 
 chiefly by way of Alaska and northeastern Asia, but 
 probably also by a land-bridge from Greenland to 
 Scandinavia. It is almost always easy to distinguish 
 the times of separation from those of union, for the 
 former are marked by a rapid divergence from the 
 Old World forms, while the latter, because of the 
 intermigration of mammals, always possess a certain 
 number of families and genera which are common to 
 both hemispheres. The latest of these invasions 
 from Asia took place in the Pleistocene and brought 
 in a crowd of Old World mammals, bears and wol-
 
 132 THE THEORY OF EVOLUTION 
 
 verenes, sheep, antelopes, bison, the wapiti (erro- 
 neously called "elk") the moose, the caribous and 
 many others, and these continue to inhabit the 
 northern half of the continent and have given to it 
 that Old World stamp which is recognized by in- 
 cluding this boreal area in the great Holarctic region. 
 Many species, which are now extinct, accompanied 
 this Pleistocene invasion, the most conspicuous of 
 which was the Siberian mammoth, or hairy elephant, 
 which ranged from the south of France to Kamchatka 
 and, on the American side, from Alaska to New 
 England. Systematic naturalists still debate the 
 question as to whether these immigrants have suffi- 
 ciently changed since their arrival in America to 
 deserve to rank as species distinct from their Old 
 World relatives. Whether the American and Euro- 
 pean bison, the Scandinavian and American moose, 
 the wapiti of America, that of Asia and the European 
 red stag, the caribou and the reindeer, are valid 
 species or merely geographical races of the same 
 species, need not be discussed here. Whichever view 
 we take, the Old World character of the northern 
 half of our continent is not affected. 
 
 Between this "boreal zone" and the Sonoran 
 region, there is a "transition zone" where the two 
 faunas mingle. The Sonoran region, which, roughly 
 speaking, includes the United States, except the 
 high mountain ranges, which carry the northern 
 fauna far to the South, and the plateau of Mexico, 
 is the characteristically North American region. Its
 
 EVIDENCE FROM DISTRIBUTION 133 
 
 deer are all different from the northern genera of the 
 family, which came in during the Pleistocene; they 
 too were ultimately derived from the Old World, but 
 at a very much earlier date, probably as far back as 
 the lower Miocene epoch. This Sonoran fauna was 
 greatly impoverished by the Pleistocene extinctions, 
 which put an end to so many of the ancient American 
 groups, but it retains a large number of characteristic 
 forms, chiefly rodents and carnivores, or beasts of prey. 
 
 Besides the immigration from the eastern hemi- 
 sphere, there were successive waves of invaders from 
 the Neotropical region; the earliest one of which we 
 have found a trace arrived in the Miocene and the 
 latest in the Pleistocene, when North America had 
 several characteristic Neotropical forms, the most 
 striking of which were the giant ground sloths, and 
 the huge, armoured, armadillo-like glyptodonts, both 
 now extinct. Of this southern invasion little now 
 remains; the Canada porcupine is certainly one 
 survivor from the invasion and probably, though not 
 certainly, the opossum is another. The influence of 
 climate upon present distribution is very well shown 
 in Mexico; the high table-land, with its moderate and 
 relatively dry climate, is Sonoran, while the hot, low- 
 lying coast-lands are Neotropical. 
 
 When the existing mammals of South America are 
 reviewed, they are plainly separable into two cat- 
 egories. In the first are included those which are 
 evidently indigenous and are, for the most part, very 
 unlike those of any other continent. The monkeys
 
 134 THE THEORY OF EVOLUTION 
 
 and marmosets are but remotely related to the mon- 
 keys and apes of the Old World; the horde of mos1 
 curious creatures which we call ant-eaters, sloths 
 and armadillos, are altogether peculiar to the Neo- 
 tropical region; the extremely large and varied assem- 
 blage of porcupine-like rodents are either peculiai 
 or, in a few cases, related to those of Africa. The 
 many species of opossums are related, though no1 
 very closely, to the marsupials of Australia. Aus- 
 tralia excepted, no other continent has so manj 
 different kinds of mammals peculiar to itself as 
 South America, and the birds are almost equally 
 characteristic. The second category contains those 
 mammals which are nearly related, often belonging 
 to the same genera, to those which now live, 01 
 formerly lived, as shown by the fossils, in North 
 America. This section includes all the beasts oi 
 prey, the wolves, large and small cats, weasels, 
 otters, skunks, raccoons; all the hoofed animals, the 
 tapirs, peccaries (native swine) llamas, deer (all oi 
 which belong to the Sonoran group), the rats and 
 mice, squirrels and rabbits. 
 
 This twofold division immediately suggests that 
 the mammals belonging to the second category are 
 immigrants from the North and the suggestion is 
 very strongly confirmed by the testimony of the 
 fossils. The geology of the Isthmian region proves 
 that South America was formerly cut off from the 
 northern continent and if we examine the fossil 
 remains of the mammals which lived there during
 
 EVIDENCE FROM DISTRIBUTION 135 
 
 that time of isolation, we find, as already stated, 
 that they are totally distinct from their contem- 
 poraries of the north. The rocks of Patagonia and 
 Argentina have proved to be quite as marvellous a 
 museum of vanished mammalian life as those of the 
 western United States and they tell the story of 
 origins and migrations with the greatest clearness. 
 The older Miocene rocks of Patagonia have already 
 yielded to the zealous collector a great host of 
 beautifully preserved mammalian remains, among 
 which cannot be found a single one that was ancestral 
 to any of the mammals of the second category, 
 which, on other grounds, we concluded to have been 
 derived from the north. On the other hand, we find 
 the ancestors of nearly all the peculiarly South 
 American forms, except those which, there is reason 
 to believe, never ranged so far south as Patagonia. 
 There was a great abundance of hoofed animals, 
 but they all belonged to orders which have long been 
 extinct, and there were many beasts of prey, large 
 and small, but they were not true Carnivora, but 
 rather resembled the predaceous marsupials of 
 Australia. South America suffered even more from 
 the Pleistocene extinctions than did North America 
 and lost not only many indigenous orders, such as 
 the ground sloths and glyptodonts, of which there 
 was an incredible variety, and the native hoofed 
 animals, but also several of the immigrants as well, 
 such as the mastodons, horses and antelopes, the 
 great sabre-tooth tigers and short-faced bears.
 
 136 THE THEORY OF EVOLUTION 
 
 There is a remarkable contrast between the fates 
 that befell the northern invaders of the Neotropical 
 region, and the southern invaders of the Sonoran 
 region. The former, in most cases, conquered a 
 permanent footing in South America and their de- 
 scendants, more or less modified, are living there 
 now, while the latter could not maintain themselves 
 and only one, or at most two, has survived to the 
 present time. The reason for this very striking 
 difference is probably to be found in the fact that 
 the northern animals were of higher type and more 
 advanced grade than the southern. North America 
 was a part of the vast land-area of Arctogsea, a con- 
 dition much more favourable to the development of 
 the higher forms than was the insular state of South 
 America. Such a conclusion might seem to be con- 
 tradicted by Central America, which is Neotropical 
 not Sonoran, but here the dominant factor is ob- 
 viously climatic, as is shown by the conditions in 
 Mexico. The native and immigrant mammals of 
 North America are Arctic and Temperate and most 
 of them are not adapted to life in the Tropics, yet 
 in spite of this, many have permanently established 
 themselves in South America, while the southern 
 immigrants failed to keep their footing in the north. 
 
 The zoological realms and regions which were estab- 
 lished only to express the present order of things, 
 thus prove themselves to be an expression of past 
 changes. Even were nothing known of the geology 
 and palseontology of the western hemisphere, but
 
 EVIDENCE FROM DISTRIBUTION 137 
 
 merely the existing distribution of its mammals, it 
 would be entirely possible to deduce from that dis- 
 tribution the main outlines of the history which 
 geology reveals. The geological dates could not be 
 so definitely fixed, but the principal events and the 
 order in which they occurred could be made out, 
 assuming the truth of the evolutionary theory. The 
 very fact that such an assumption is necessary, if 
 the deductions are to harmonize with the facts re- 
 vealed by geology and palaeontology, is strong evi- 
 dence that the assumption is true. On the theory 
 of special creation and the immutability of species, 
 the facts are meaningless. We find that a certain 
 proportion of South American forms is extremely 
 peculiar, having no close allies in any other conti- 
 nent, and a certain proportion is of mammals clearly 
 related to those which dwell in Arctogsea. But the 
 latter all belong to peculiar species and many of 
 them to peculiar genera, which, on this theory means 
 that they were separately created and therefore that 
 their relationships with North American and other 
 Arctogaean forms are purely ideal. It is highly sig- 
 nificant that the history, so far as it has been recov- 
 ered and deciphered, is seen to lead gradually through 
 a long series of changes to the present order, which 
 is so plainly the inevitable outcome of those changes, 
 provided only that evolution be accepted. 
 
 It may well be asked why no such interchange of 
 mammals between the Americas is going on now, but 
 the question cannot be definitely answered. It is
 
 138 THE THEORY OF EVOLUTION 
 
 probable, however, that a state of equilibrium has 
 been reached after the readjustments made neces- 
 sary by the last geographical and climatic changes. 
 On the other hand, a slow movement may actually 
 be in progress, as is suggested by the geographical 
 races of the Virginia Deer from Mexico to Guiana. 
 Central America contains a considerable number of 
 Sonoran mammals, which have not yet passed the 
 Isthmus, but might perhaps eventually have done 
 so, had not the construction of the canal intervened. 
 
 The elevation of Central America and the Isthmus 
 cut off the waters of the Caribbean Sea from those 
 of the Pacific and the separation led to divergent 
 development of marine organisms in the divided 
 waters. A careful comparative study has been made 
 of the sea-fishes on the two sides of the Isthmus, with 
 the result that nearly all the species are different, 
 while the genera and families are, for the most part, 
 the same. The amount of difference is what might 
 have been expected d priori on the evolutionary 
 hypothesis and, indeed, the ichthyologists had ap- 
 proximated the true date of the separation before 
 the geological evidence had been found. 
 
 There is no time to go into the highly complex 
 history of the vast aggregation of land in the Old 
 World, further than to point out that as late as the 
 Pleistocene epoch, when primitive men were already 
 living in Europe, that continent underwent a re- 
 markable series of climatic and geographical changes. 
 At the time of greatest elevation, the Adriatic and
 
 EVIDENCE FROM DISTRIBUTION 139 
 
 yEgean Seas were land; the junction of Italy and 
 Spain with Africa cut the Mediterranean into two 
 land-locked basins; the North Sea was land and the 
 British Islands were joined to each other and to the 
 continent. This, taken in connection with the 
 barrier made by the Desert of Sahara, explains why 
 North Africa belongs now to the great Holarctic 
 region and not to the Ethiopian, which covers the 
 continent south of the desert. 
 
 I do not wish to give the impression that all the 
 problems of distribution have been solved and all the 
 difficulties removed, for such an impression would 
 be far from the truth. What can fairly be said is 
 that the main outline has been satisfactorily ex- 
 plained on the evolutionary theory and no other 
 theory even pretends to account for the facts. Just 
 in proportion to the completeness with which the 
 past history of any mammalian group is known, is 
 it easy to account for its present distribution. In 
 the same way, the more fully the geographical and 
 biological history of any continent has been ascer- 
 tained, the more readily can the arrangement and 
 relationships of its animals and plants be under- 
 stood. It is largely because the history is incom- 
 plete, that difficulties still remain. 
 
 It is often very puzzling to explain why groups 
 that range together in the region of their origin 
 should behave so differently when new land-con- 
 nections are established and new regions invaded; 
 of this there are very many instances. The mas-
 
 140 THE THEORY OF EVOLUTION 
 
 todons and the true elephants lived together all over 
 North America, but only the mastodons entered 
 South America. A possible explanation of this may 
 be that the species of elephants were cold-country 
 animals which had come in from the north and were 
 unable to cross the Tropics, but no such considera- 
 tions will apply to the Siberian rhinoceros and mam- 
 moth. Both were well protected against the cold 
 and both extended their range in the Pleistocene 
 from southwestern France to Kamchatka; the mam- 
 moth passed over to Alaska and crossed North 
 America to New England, but the rhinoceros has 
 never been found on the American side. Deer mi- 
 grated repeatedly from the Old World to the New, 
 but the giraffes, which were once very numerous and 
 varied in all the Mediterranean lands, did not accom- 
 pany them. Wolves, foxes, cats, bears migrated back 
 and forth, but America never had any hyenas or 
 civets and the raccoon family has always been Amer- 
 ican, though not quite exclusively so. The large, 
 spotted cats, called jaguars, do not extend north of 
 Texas or south of the Argentine Republic, yet the 
 very nearly allied pumas range from Pennsylvania 
 far into Patagonia. These are closely allied species 
 and yet their adaptability to climate is very differ- 
 ent. Deer and antelopes both invaded South Amer- 
 ica, presumably together, yet the deer maintained 
 themselves, while the antelopes failed to do so. 
 Then, too, there is much mystery about the extinc- 
 tion of species and larger groups; hi a general way
 
 EVIDENCE FROM DISTRIBUTION 141 
 
 We can understand why this should have happened, 
 but, in a given case, it is not possible to do more 
 than conjecture the reasons. 
 
 Another series of observations and deductions, 
 which depend for their cogency little or not at all 
 upon the data of geology and palaeontology, is that 
 afforded by the phenomena of the distribution of 
 animals and plants on islands. Darwin's attention 
 was strongly arrested by what he observed in the 
 Galapagos, and A. R. Wallace, who, it will be remem- 
 bered, independently reached the same conclusions 
 as Darwin, was led to his results largely by his studies 
 in the East Indian islands. To the end of his life 
 Wallace was fascinated by this topic and his two de- 
 lightful books, "The Malay Archipelago" and "Is- 
 land Life," testify to his unfailing interest in its 
 problems. The problems of insular life are much 
 simpler and more comprehensible than those of 
 continental distribution; they are like experiments 
 performed upon a small scale and with relatively 
 few factors involved. 
 
 From the point of view of animal and plant dis- 
 tribution there are two clearly distinguished classes 
 of islands: (1) Continental and (2) Oceanic. Con- 
 tinental islands are, as their name implies, detached 
 fragments of a continent, with which they were at 
 one time joined; their geological structure is that of 
 the continent and usually they are not far from the 
 parent continent and are surrounded by shoal water. 
 It should be explained that the line between shallow
 
 142 THE THEORY OF EVOLUTION 
 
 and deep water is commonly taken as the 100-fathom 
 depth; this is not an arbitrary selection, but the 
 reasons for the choice cannot be discussed here. 
 Several of the best known of the continental islands, 
 such as Great Britain and Ireland, Borneo, Sumatra, 
 Java, Madagascar, have been mentioned in this lec- 
 ture. Their animals and plants are those of a con- 
 tinent and the degree of difference from the neigh- 
 bouring mainland is an indication of the date of 
 disjunction. In a few cases continental islands rise 
 from great depths of water, as is true of the Greater 
 Antilles, and some are far from any land, like New 
 Zealand and the Seychelles. Oceanic islands, on 
 the other hand, are all either volcanic and have been 
 built up from the sea-floor by the material ejected 
 in the eruptions of submarine volcanoes, or they are 
 coral reefs. As a rule they rise from the profoundest 
 depths of the sea and are far removed from any land 
 and they appear never to have formed part of any 
 continent. Examples of this class are the Azores, 
 Madeira, the Canaries, Cape Verdes, Hawaiian 
 Islands and the thousands which dot the South 
 Pacific. 
 
 The two classes of islands differ as radically in 
 their animals as they do in structure. The con- 
 tinental islands may and frequently do contain 
 everything that an equal area of the parent main- 
 land had at the time of separation, though in this 
 respect there is naturally a difference between large 
 and small islands. Sometimes it is difficult to un-
 
 EVIDENCE FROM DISTRIBUTION 143 
 
 derstand why a large continental island should be 
 so poor in mammals. The contrast between the 
 large islands of the East and West Indies is very re- 
 markable; Borneo, Sumatra and Java are extremely 
 rich in mammals, while Cuba, Haiti and Porto Rico 
 have very few. The oceanic islands have peculiarly 
 limited faunas; on the theory of special creation, we 
 should expect to find in them as large a variety of 
 animals as their area, their vegetation and their 
 climate would enable them to support. On the 
 theory of evolution, they must have received only 
 such plants and animals as could be carried to them 
 by the wind, or currents of the sea, carried on drift- 
 wood or floating trees, or transported by birds. 
 When an oceanic island is very far from the nearest 
 land, the accession of a new animal or plant must be 
 a very rare event. 
 
 That islands are actually, not merely hypothetic- 
 ally, stocked in this manner is clearly shown by the 
 case of Krakatoa, a volcanic island not far from 
 Java. The great eruption of 1883 blew a great part 
 of the island out of existence and buried the re- 
 mainder so deeply under volcanic debris, that every 
 living thing perished. In a surprisingly short time 
 the island was re-vegetated and abundant insect 
 life arrived, all borne by the winds and the sea from 
 Java and Sumatra. Professor Selenka writes of a 
 visit to Krakatoa: "Under the shade of a Casuarina, 
 amidst cocoa-nut palms and thickets as high as my 
 head, I found here, to my astonishment, an active
 
 144 THE THEORY OF EVOLUTION 
 
 animal life of spiders, flies, bugs, beetles, butterflies; 
 even lizards more than two feet long animated the 
 peaceful picture." In this instance, however, the 
 distance from land was not very great, or else the 
 restocking would have required a very much longer 
 time. 
 
 The oceanic islands have no land mammals, except 
 bats, which like birds are often carried immense dis- 
 tances by strong gales. That this lack of mammals 
 is due to the impossibility of reaching the islands 
 and not to any unfitness to support the mammals is 
 made clear by the artificially introduced rabbits, 
 rats, mice, pigs, goats and cattle, which have, on 
 many such islands, thriven and multiplied exceed- 
 ingly. They also lack Amphibia, for frogs and 
 toads, newts and salamanders, their eggs and larvae, 
 cannot long endure immersion in sea- water; real 
 fresh-water fishes, crayfishes and shells, as distin- 
 guished from those which readily enter rivers from 
 the sea, are likewise wanting. Birds and insects 
 are brought by the winds, reptiles, especially lizards, 
 and snails are carried on driftwood, and it is only 
 such groups that occur in the oceanic islands. 
 
 The observations which Darwin made in the 
 Galapagos, a volcanic group about 500 miles from 
 the coast of Ecuador, will serve as a type of what 
 may be learned from the life of oceanic islands. 
 Leaving out of account the sea-birds, which can 
 traverse the widest expanses of sea, the birds, rep- 
 tiles and plants of the Galapagos for the most part
 
 EVIDENCE FROM DISTRIBUTION 145 
 
 belong to peculiar species, but all are of South 
 American type and have their nearest relatives in 
 that continent. Each of the larger islands of the 
 group has species peculiar to itself, but almost all 
 the species distributed on the various islands belong 
 to the same or nearly allied genera, and thus to- 
 gether form a homogeneous and closed group. The 
 explanation which Darwin gave to the phenomena 
 still remains the most satisfactory that has been 
 offered. The distance from the mainland is so great, 
 that for any bird or lizard to reach the islands must 
 always have been a rare event, but a few species did 
 arrive and were thus practically cut off from their 
 relatives of the continent. Communication be- 
 tween the different islands is likewise rare and so 
 each became the seat of a little colony which gradu- 
 ally changed from its former condition to form races, 
 varieties and eventually species. Thus, the evolu- 
 tionary theory accounts for the facts in a simple 
 and adequate manner, while the rival theory offers 
 no explanation. The Cape Verde Islands, off the 
 west coast of Africa are a volcanic group quite like 
 the Galapagos, but their animals are of African 
 type modified through isolation and the Galapagos 
 kind of distribution is not the exception, but the 
 rule under similar conditions. 
 
 Whether new species, say of birds, will arise in 
 an oceanic island, depends on the frequency with 
 which mainland birds can visit it and thus by con- 
 stant crossing maintain the continental character-
 
 146 THE THEORY OF EVOLUTION 
 
 istics. Such islands as Bermuda and Madeira are 
 annually visited by birds from the adjoining con- 
 tinents and they have hardly any peculiar forms, 
 while St. Helena and the Hawaiian group have 
 almost all their birds and reptiles peculiar to them- 
 selves and so far changed, that it is difficult to make 
 out their geographical relationships. If species are 
 immutable, why should this factor of inaccessibility 
 have any influence upon the peculiarity of the species 
 found in a given island? Another curious series of 
 facts is displayed by the family of birds known as 
 the Rails, which are distributed all over the world 
 except the polar regions. The family includes fifty- 
 five genera, twenty-five of which inhabit islands 
 and of these ten have lost the power of flight. A 
 genus, with only one or two species, will be confined 
 to a single island, while the genera of the continents 
 have more numerous species and all of them can fly. 
 It is an obvious inference that the loss of flight and 
 the separation as distinct genus and species must 
 have taken place after the original progenitors had 
 reached a given oceanic island, for the birds could 
 not have crossed wide seas by swimming, nor could 
 the flightless ones have arrived in any other way. 
 
 "If we briefly resume these surveys of island 
 faunas, the result for our problem is as follows: 
 The islands must have received their animals in 
 some fashion from the mainland, the continental 
 islands when they were a part of it, the oceanic not 
 till after their appearance above the surface of the
 
 EVIDENCE FROM DISTRIBUTION 147 
 
 sea. This may be directly proved for young oceanic 
 islands. Now, since we have all stages from the 
 youngest to the oldest islands and see how the 
 degree of specialization of the animals increases in 
 just such a graduated way, and how their number 
 and kind are different in correspondence with the 
 distance of the island and the animals' power of 
 dispersal and how they everywhere display a rela- 
 tionship with those of the nearest lands, the conclu- 
 sion is cogent that even on the most ancient islands 
 the animals were not created, but immigrated after 
 the islands had been formed. If this be admitted, 
 the further conclusion cannot be escaped, that the 
 greater or less difference of the insular forms from 
 those of their former home must be ascribed to a 
 mutability and capacity of change on the part of 
 the animals." l 
 
 As in the case of all the topics dealt with in the 
 preceding lectures, I cannot but lament the inade- 
 quacy of treatment which is made necessary by the 
 demands of brevity. Nobody would pretend for a 
 moment that we understand all the phenomena of 
 distribution and can account for every seemingly 
 anomalous fact, or that all difficulties have been 
 removed. We are attempting to reconstruct a long 
 history of past changes, and under such circum- 
 stances, partial success is all that can be hoped for. 
 But if, in looking over the vast array of data that 
 have been gathered illustrating distribution and 
 
 1 A. Brauer. iu Die Abstammungslehre, pp. 161-2.
 
 148 THE THEORY OF EVOLUTION 
 
 comparing these with the results of geology and 
 palaeontology which bear upon the same problems, 
 we ask whether the theory of evolution or that of 
 special creation offers the most satisfactory explana- 
 tion, an unprejudiced judge would not long hesitate 
 in giving his decision in favour of evolution.
 
 LECTURE VI 
 EVIDENCE FROM EXPERIMENT CONCLUSION 
 
 Experiment is the method used in physics and 
 chemistry, to determine the laws of matter and force; 
 such experiments can be repeated at will and any 
 competent person can obtain the same results. It 
 is highly desirable that the experimental method 
 should be applied in biology, because of the exacti- 
 tude which can thus be secured, affording an in- 
 valuable complement to the results of observation. 
 Darwin always insisted upon the indispensable char- 
 acter of experiments and performed a great many 
 himself. One might have anticipated that the prob- 
 lems of evolution could be thus attacked with suc- 
 cess and subjected to the tests of a precise analysis. 
 But such hopes have left out of account the difficul- 
 ties inherent in the application of the method; human 
 life is very brief and experiments, to be conclu- 
 sive, must often be continued without interruption 
 through a very long series of generations. Then, 
 too, it is impracticable to make conditions in the 
 laboratory sufficiently like those hi the open to give 
 definite answers to many questions. 
 
 In a sense, the long continued improvement of 
 domesticated animals and plants by careful breed- 
 
 149
 
 150 THE THEORY OF EVOLUTION 
 
 ing and selection has been by a great series of experi- 
 ments, but, unfortunately, the conditions under 
 which these were carried out are too inexact and the 
 records, which are usually lacking altogether, are, 
 even when preserved, too vague to be of much service 
 in a modern investigation. Domestication, as was 
 pointed out at some length in a previous lecture 
 (p. 36), shows how surprisingly animals and plants 
 may be changed by human agency, but tells us little 
 of what we are now seeking to learn. 
 
 The changes which are experimentally produced 
 must be transmissible to the offspring, if they are 
 to have any evolutionary significance. Plants, in 
 particular, are very susceptible to extensive modifica- 
 tion through external influences, such as the chemical 
 constitution of the soil, heat, light, moisture, eleva- 
 tion above sea-level, and the like, and these modifica- 
 tions will continue as long as the external conditions 
 remain the same, but disappear when the plants 
 are once more placed under the original circum- 
 stances. The French botanist Bouvier took shoots 
 or layers from several kinds of common plants and 
 set out, from the same individual, some shoots in 
 the lowlands, others high up in the mountains. The 
 differences in the results were remarkable; the moun- 
 tain plants had more vigorous roots, but the part 
 above ground was much smaller, with more delicate 
 stem and leaves; the flowers were fewer in number, 
 but individually larger and more intensely coloured. 
 When taken back to the lowlands, the plants re-
 
 EVIDENCE FROM EXPERIMENT 151 
 
 turned to their original type. The same species of 
 Ranunculus may be grown on land or in the water, 
 with totally different forms of leaf in the two media. 
 The land individuals have broad, tripartite leaves, 
 with serrate edges, while in the water plants the 
 leaves are subdivided into long, thread-like fila- 
 ments and the whole appearance of the plant is 
 that of a different species. "In all these cases, the 
 plant has not inherited a definite form from its 
 parents, but, within the limits of the hereditary 
 fundamental plan of structure, it has received cer- 
 tain possibilities of development, which did not 
 appear in the parents. Which of these possibilities 
 shall be realized in the development of the indi- 
 vidual, is determined by external conditions." l 
 
 This must not to be taken to mean that it is not 
 possible to produce hereditable modifications in 
 plants, the contrary is true, but merely that very 
 striking changes may be of no lasting importance 
 because they are not transmissible. 
 
 Of late years, a host of experiments have been 
 performed upon animals, the larger number of them 
 with the object of determining whether new char- 
 acters, acquired during the lifetime of the parents, 
 can, under any circumstances, be transmitted to the 
 offspring. This is one of the most hotly disputed 
 questions of modern biology and our whole concep- 
 tion of the efficient factors which have brought about 
 evolution hinges upon the answer to this question. 
 
 1 Karl Giesenhagen, in Die Abstammungslehre, pp. 307-8.
 
 152 THE THEORY OF EVOLUTION 
 
 The same experiments are interpreted in diamet- 
 rically opposite senses by different writers according 
 to their predisposition and general point of view. 
 At the present time, it is probable that a very con- 
 siderable majority of zoologists and botanists, espe- 
 cially in this country, are inclined to deny the 
 hereditary transmission of characters acquired in the 
 post-embryonic life of the parents, but the problem is 
 still far from definite solution. Important as this 
 problem is in any attempt to explain evolution and 
 the manner in which it has been effected, it has no 
 bearing upon the question which I have been en- 
 deavouring to answer in these lectures, as to the 
 probable truth of the evolutionary theory. That 
 theory is held quite as strongly by those who deny 
 as by those who affirm the transmission of acquired 
 characters. Whatever interpretation be put upon 
 the significance of the experiments, presently to be 
 mentioned, as to the problem of acquired characters, 
 they do, at all events, show that hereditarily trans- 
 missible modifications may be artificially produced 
 in both animals and plants. 
 
 We are again, as in every one of the preceding 
 lectures, confronted by the difficulty of making an 
 instructive and convincing selection from a great and 
 ever growing mass of facts, but it is impossible to 
 present more than a few typical examples. Among 
 the older experiments may be mentioned those made 
 by Schmankewitsch upon the brine-shrimp. By 
 increasing the proportion of salt dissolved in the
 
 EVIDENCE FROM EXPERIMENT 153 
 
 water, the little creatures passed, in the course of 
 several generations, into quite a different form, 
 identical with what had previously been regarded as a 
 distinct species. On the other hand, by diminishing 
 the quantity of salt, and keeping many successive 
 generations in progressively freshened water, the 
 brine-shrimp gradually assumed the characters of 
 quite a distinct genus, which lives in fresh water. A 
 very important distinction between these results and 
 those obtained by Bouvier with plants, is that in the 
 latter case the change was effected immediately and 
 went no farther in succeeding generations, but could 
 be at once reversed by restoring the plant to its 
 original place of growth. With the brine-shrimps, on 
 the contrary, the changes were gradual and cumula- 
 tive, many generations being required to bring them 
 to full development. 
 
 By keeping water-fleas in the dark, Kapterew 
 caused them to grow blind, the eyes losing their 
 regular outline, and their black pigment began to 
 wander over the head and body. At first this dete- 
 rioration appeared sporadically, but after a year and 
 a quarter all the individuals were similarly affected 
 and the blindness appeared to be hereditary. 
 
 Many experiments have been made by Weismann, 
 Standfuss, Fischer, Schroder, Pictet and others upon 
 the caterpillars and cocoons of moths and butterflies, 
 changing their food-plants, in some cases, and in 
 others hatching the full-grown insects from cocoons 
 which had been exposed to abnormal conditions of
 
 154 THE THEORY OF EVOLUTION 
 
 temperature. Some very striking changes were 
 produced in this manner and the remarkable case of 
 the Lunar Moth, taken from Texas to Switzerland, 
 has already been described in another connection. 
 (See p. 39.) Cocoons of one species of butterfly 
 and one moth, when exposed to a low temperature, 
 considerably below the freezing point, yielded mature 
 insects which, especially in the males, were darker in 
 colour and had a different wing-pattern from those 
 which were hatched at normal temperatures. The 
 second generation, derived from these modified 
 parents, but hatched from cocoons exposed to the 
 normal temperature of the season, had a consider- 
 able number of insects which resembled their dark 
 parents, while others had reverted to the ordinary 
 condition of the species. In another moth a very 
 decided darkening of the wings and a change of 
 pattern was produced by the opposite method of 
 keeping the cocoons in an atmosphere much hotter 
 than that of the hatching season out of doors. Here 
 also a considerable number of moths in the second 
 generation inherited the characters of their modified 
 parents, though they themselves had been hatched 
 at the normal temperature. The larvae of a beetle, 
 which feed upon a smooth-leaved willow, were 
 transferred to another species with densely hairy 
 leaves. The descendants freely chose the hairy 
 leaves, when both kinds were accessible and the 
 number so choosing increased with each successive 
 generation, the effect being cumulative.
 
 EVIDENCE FROM EXPERIMENT 155 
 
 A very remarkable series of experiments upon the 
 Colorado Potato-beetle were made by Professor 
 Tower, of Chicago, under conditions of the greatest 
 exactitude and rigorous control, as much so as in 
 any chemical or physical laboratory; the results are 
 famous throughout the biological world. In these 
 experiments the larvae and the beetles were subjected 
 to varying conditions of heat and moisture and at 
 different stages of their development, and thus 
 several distinct races were established, differing in 
 size, colour, markings and, in one case, in breeding 
 habits; the new characters are hereditary and trans- 
 mitted from generation to generation. A remarkable 
 feature of these experiments was the discovery that 
 the beetles had a sensitive period in their develop- 
 ment, when they are particularly susceptible to the in- 
 fluence of changes in the environment. When larvae 
 in their sensitive period are exposed to altered condi- 
 tions, the adult beetles display corresponding changes, 
 but these are not hereditary and do not reappear 
 in the next generation. When, however, the beetles, 
 at the time when the eggs are maturing and getting 
 ready for fertilization, are so exposed, the beetles 
 themselves show no visible change, but the offspring 
 do and these modifications are hereditary. Tower in- 
 terprets this as being the action of external condi- 
 tions directly upon the reproductive cells, or "germ- 
 plasm," but this interpretation is disputed by others. 
 
 Kammerer has made numbers of experiments on 
 amphibians and has succeeded in producing hered-
 
 156 THE THEORY OF EVOLUTION 
 
 itary modifications in several of them, of which a 
 few may be mentioned here. A salamander, black 
 with yellow spots, which lives in damp woods, is 
 viviparous and produces sixty to seventy gill-bearing 
 larvae, which are deposited in water and develop 
 there for several months, leaving the water for the 
 land at the time of metamorphosis, when the larvae 
 become adult, air-breathing salamanders. If captive 
 animals are deprived of water in which to deposit 
 their young, a very few are produced, but these are 
 not larvae, but little salamanders like their parents. 
 When this second generation breeds, if it is allowed 
 access to water, it will once more produce larvae, but 
 very large ones, which leave the water in a few days, 
 instead of remaining in it for several months. If 
 adult salamanders of this species are kept for years 
 on yellow soil, the yellow spots and blotches greatly 
 enlarge, encroaching on the black ground, and the 
 effects of this treatment are much increased in the 
 second generation, showing a cumulative action. 
 The larvae of a toad were, by means of darkness, 
 cold water, and other unnatural conditions, made to 
 delay their metamorphosis and became sexually ma- 
 ture and able to breed while in the tadpole stage. 
 These tadpoles attained great size and their offspring 
 did not advance beyond the stage in which the hind- 
 legs are developed, remaining in that condition for 
 years and showing no tendency to metamorphosis. 
 Other experimenters have failed to repeat these re- 
 sults, which are strongly questioned.
 
 EVIDENCE FROM EXPERIMENT 157 
 
 All of these experiments, and many more that 
 might be cited, would seem to prove that it is pos- 
 sible to effect lasting and hereditary modifications in 
 animals by abrupt changes in their environment, and 
 the same result has been obtained with plants. By 
 injecting various substances into the young seed- 
 capsule, when the germs are becoming ready for fer- 
 tilization by the pollen, MacDougal effected some 
 very remarkable changes, which were hereditary 
 through four or more generations and remained sta- 
 ble, the plants exhibiting no tendency to return to 
 the parent type. Gager also produced hereditary 
 modifications in plants by exposing the seed-capsule 
 before fertilization to the action of radium, but these 
 results are not yet conclusive. 
 
 Thus, the experimental proof goes to show that the 
 species of animals and plants are not fixed and 
 immutable entities, but are capable of extensive 
 modification even in the short time which is at the 
 command of the experimenter. 
 
 In the first lecture it was stated that the experi- 
 ments of the Austrian monk, Johann Gregor Mendel, 
 published in 1866, had attracted no attention, because 
 they were too far ahead of their time and their far- 
 reaching significance was not perceived. Redis- 
 covered in 1900, Mendel's results have been enor- 
 mously extended and have led to the development of 
 the new science of Genetics. This is an extremely 
 complex subject, which would require more than one 
 course of lectures for its elucidation, not merely the
 
 158 THE THEORY OF EVOLUTION 
 
 fraction of a single lecture, in which only a hasty and 
 most inadequate sketch can be attempted. On the 
 other hand, it is not possible to omit all mention of 
 Mendel's work from a review of the modern state of 
 opinion on the subject of evolution, for this work has 
 thrown great light upon the mechanism of heredity 
 and, as has been repeatedly insisted, heredity is an 
 all-important part of evolution. 
 
 Mendel crossed two varieties of peas, one of which 
 constantly had white flowers and the other violet 
 flowers. The first generation of hybrid plants all 
 had violet blossoms, which Mendel explained by 
 saying that in this particular cross the violet colour 
 was "dominant" over the white, which was "re- 
 cessive." The second generation of the hybrids, 
 however, showed a change, only three-fourths of the 
 plants had violet blossoms and one-fourth had white; 
 the latter, in all subsequent generations, gave only 
 white flowers; the plants had "reverted" to the origi- 
 nal white parent. Of the violet-flowered plants in the 
 next and all following generations, one-third re- 
 mained constantly violet and two-thirds split in the 
 next generation into three-fourths violet and one- 
 fourth white. From the mere inspection of a violet- 
 flowered plant, it was impossible to say whether it 
 would give rise only to violet offspring, or to those 
 which were partly violet and partly white. These 
 and thousands of other experiments indicate that an 
 organism is a mosaic of "unit characters," which are 
 transmitted independently of one another to the
 
 EVIDENCE FROM EXPERIMENT 159 
 
 offspring, and that in each generation of hybrids a 
 process of segregation goes on, by which a definite 
 proportion of plants is separated out from the mass of 
 cross-bred individuals and returns to the character of 
 one or other original parent. When this segregation 
 has once been effected, all the subsequent progeny is 
 constant to type; the descendants of the segregated 
 white flowers are invariably white. 
 
 The process is made simpler and more intelligible, 
 when the somewhat confusing effects of dominance 
 are absent, as they are in the Jalap, or Wonder of 
 Peru, experimented on by Correns. If the white- 
 flowered race of this plant is crossed with the red- 
 flowered race, all the plants of the first hybrid gener- 
 ation have uniformly pink flowers, because neither 
 color is dominant over the other and the two seem 
 to blend, though they do not actually do so. When 
 the second generation is produced, either by self- 
 fertilization or by crossing the hybrids with each 
 other, one-fourth of this second generation is red, 
 one-fourth white and one-half pink, and each suc- 
 ceeding generation acts in the same way. 
 
 This behaviour is explained by assuming that 
 every unit character is represented in the reproduc- 
 tive cells, or germ, by something which causes the 
 development of that character in the full-grown 
 plant and this something is called the "factor" for 
 the character. If we symbolize the factor for the 
 red colour by R and that for the white by W, then 
 the germs of the red-flowering race contain the fac-
 
 160 THE THEORY OF EVOLUTION 
 
 tor R and those of the white-flowering race the factor 
 W. The female hybrids produce egg-cells, one-half of 
 which contain R and one-half W; likewise the male 
 hybrids produce pollen-cells, one-half of which contain 
 R and one-half W. Fertilization consists in the fusion 
 of the nucleus of the male cell with that of the female 
 cell, as has been very often directly observed in all 
 kinds of animals and plants which have the sexual 
 method of reproduction. After fertilization the germ, 
 which is now ready to begin its development, contains 
 factors derived from both parents and, according to 
 the mathematical laws of probability and combina- 
 tion, there would be four combinations: RR, WR, 
 RW, and WW. The factor from the male parent is put 
 first, so that the combination RW means that the 
 red factor is derived from the male and the white 
 factor from the female; in WR this is reversed, 
 though, in practice, the result is usually the same in 
 either case. In the Jalap, where neither white nor 
 red is dominant, both RW and WR give the pink 
 flowers and, obviously in the proportion of one-half 
 the entire number of plants in the second hybrid gen- 
 eration. The germs having the factors RR give the 
 segregated red flowers and, as there is no W, it is 
 plain that all the descendants of the RR seeds, 
 generation after generation, if crossing is avoided, 
 will produce only red flowers. For the same reason, 
 the WW germs and all their pure-bred descendants 
 can give only white flowers. If the unit-characters 
 do not actually blend and become inseparable, it is
 
 EVIDENCE FROM EXPERIMENT 161 
 
 easy to see why segregation must occur in each gener- 
 ation of the hybrids. 
 
 The complexity of the calculation rises very rap- 
 idly, when, instead of experimenting with only a 
 single pair of alternative characters, as in the cases 
 previously cited, combinations of several such pairs 
 are effected. Mendel himself used as many as seven 
 pairs, combined in many different ways. Yet, if 
 regard be paid only to the behaviour of a single pair 
 of alternatives, it makes no difference how many 
 other unit-characters are associated in the combina- 
 tion, as they all act independently. When the be- 
 haviour of all the characters is analyzed, it is found 
 that the mathematical laws of probability and com- 
 bination are followed as closely as when but a single 
 pair is dealt with. Indeed, the correspondence be- 
 tween the calculated and the experimental results 
 is highly remarkable. 
 
 Mendel's own experiments were made with plants, 
 but the same laws of heredity apply to animals. 
 Lang experimented with the garden-snail of Europe, 
 one race of which has a yellow, unstriped shell and 
 another has the shell with spiral stripes of black. 
 Crossing these, the unstriped yellow proved to be 
 dominant, all the individuals of the first hybrid 
 generation having shells of that kind. In the second 
 generation segregation began and in just the same 
 proportion as in the violet and white peas; one-fourth 
 of the shells were striped, three-fourths unstriped. 
 All the descendants of the former were constantly
 
 162 THE THEORY OF EVOLUTION 
 
 striped, but the yellow forms again split in the next 
 generation. Inasmuch as the sexual method of re- 
 production was independently acquired in both the 
 vegetable and the animal kingdoms, not inherited 
 from an ancestor common to both, this agree- 
 ment indicates that the Mendelian laws of inheri- 
 tance are of universal validity, save under such cir- 
 cumstances as will be mentioned later. 
 
 The Mendelian laws further explain how new 
 races may arise through the sudden loss of one or 
 more factors by mutation. The common house- 
 mouse in its wild state has a characteristic gray 
 colour, but there are several fancy breeds of mice, 
 white, brown, black and blotched, all of which were 
 derived from the wild form. Many experiments in 
 the cross-breeding of these domesticated varieties 
 with one another and with their wild ancestor, in all 
 sorts of combinations, have shown that the varieties 
 have arisen through the suppression of factors. 
 Many factors are involved in these cross-breedings, 
 but, simplifying the matter to the utmost, they may 
 be reduced to four. (1) A colour-factor, which must 
 be present if any colour at all is to be developed in 
 the hairs of the coat. (2) A factor which determines 
 that the colour shall be distributed over the whole 
 body. (3) A pigment-factor, which conditions the 
 occurrence of black in the hairs, which, without it, 
 would be brown. (4) A distribution-factor, which 
 controls the arrangement of black and brown in each 
 hair, so that the combined effect is the characteristic
 
 EVIDENCE FROM EXPERIMENT 163 
 
 mouse-colour. If this last factor should be elim- 
 inated, the result would be the production of black 
 mice, an explanation which is strongly supported by 
 crossing black mice with the normal animal and 
 making a mathematical analysis of the proportion- 
 ate numbers of hybrid forms. If the pigment- 
 factor, which determines the formation of the black 
 colour, should disappear in black mice, brown ani- 
 mals would be produced and, if the factor control- 
 ling the distribution of colour should be lost, then 
 blotched mice would appear. Finally, the suppres- 
 sion of the general colour-factor would lead to the 
 development of white or albino mice. We may 
 thus witness the formation of new varieties through 
 the loss of factors, but the addition of new factors 
 has not yet been observed, unless the additional toe 
 in Castle's four-toed race of guinea pigs should be 
 so interpreted. 
 
 Interesting and profoundly important as are the 
 results of the Mendelian investigations, it must be 
 admitted that, so far, they have rendered but little 
 assistance in making the evolutionary processes 
 more intelligible and, instead of removing difficul- 
 ties, they have rather increased them. That the 
 Mendelian laws of inheritance are immensely valu- 
 able to breeders, is not to be questioned, but it re- 
 mains to be determined just how far they are oper- 
 ative in nature and what bearing they have upon 
 the problems of evolution. The experimenter can, 
 if he so desires, preserve all of his plants or animals
 
 164 THE THEORY OF EVOLUTION 
 
 through many successive generations, deriving 
 thousands of individuals from a single pair of pro- 
 genitors, but, under natural conditions, matters are 
 very different. A single plant may produce thou- 
 sands of seeds in a season, though but a very few, or, 
 perhaps only one, can arrive at maturity and repro- 
 duce in its turn. Unless it possesses some very 
 notable advantage, the determination of which in- 
 dividual seedling shall survive, is far more likely to 
 be conditioned by the spot where it happened to 
 germinate, with reference to light, moisture, etc., 
 than by its germinal constitution. It is difficult to 
 see how the laws of proportionate segregation can 
 have any effect under such conditions. Some stu- 
 dents of genetics go so far as to maintain that all 
 evolutionary changes are made by the elimination 
 of factors, and others that species are immutable, ex- 
 cept as new combinations are formed through hybrid- 
 ization. 
 
 It is quite possible that the Mendelian laws are 
 those of ordinary, conservative inheritance, by 
 means of which species are maintained in their 
 normal condition, and yet may not be applicable, 
 when new characters arise through the action of the 
 environment. This is suggested by Tower's experi- 
 ments on potato-beetles, cited above. He says: 
 "In what way is the constitution of the germ cell 
 modified so that the organism shows in subsequent 
 generations a permanent change in its coloration? 
 It has been pointed out to me by Professor Morgan
 
 EVIDENCE FROM EXPERIMENT 165 
 
 that in those experiments the behavior of the first 
 generation is difficult of interpretation. In these 
 experiments the male cells and the female cells, and 
 sometimes both, have been subjected to conditions 
 of experiment at a susceptible stage. 
 
 "Morgan has raised the question, why do indi- 
 viduals, developed from eggs which have been sub- 
 jected to conditions of experiment and fertilized 
 with normal sperm, not give a subsequent hybrid 
 behavior? No hybrid splitting has ever been found 
 in any of my experiments, or in those of MacDougal 
 or Gager. The resulting modification reproduces 
 itself true to type, and does not give subsequent 
 splittings suggestive of the combination of different 
 factors or unit-characters. If there are unit-char- 
 acters, it is logical to expect that in experiments of 
 this kind the experiment would modify the unit- 
 character in the germ-plasm, and that this modified 
 unit-character would then behave, when crossed 
 with its normal homologue, exactly as hybrids do in 
 other cultures. The total lack of this behavior in 
 my experiments, and those of MacDougal, Gager, 
 and others, might be considered good evidence that 
 there are no such things as unit-characters, nor in 
 the germ cells any potentiality capable of individual 
 removal or behavior. Any such deduction, however, 
 is unwarranted and contrary to known facts, and, 
 furthermore, these modified characters themselves 
 show that after establishment [italics mine] they are 
 alternative and capable in many instances of re-
 
 166 THE THEORY OF EVOLUTION 
 
 placement and recombination in full conformity 
 with established principles of heredity behavior." l 
 
 These and many similar questions must be left 
 to future investigations for reply; it is, in any event, 
 a great advance that we should be able to approach 
 these problems by means of exact experiment and 
 mathematical analysis. 
 
 This concludes our sketch of the various lines of 
 testimony which have been adduced in favour of 
 the theory of evolution and I can readily imagine 
 that some of you, at least, are saying to yourselves: 
 "Is that all? I don't think that amounts to much!" 
 For such lack of conviction, if such there be, there 
 are obvious reasons. It has been necessary, because 
 of the very brief time at our disposal, only six hours 
 all told, to make a very limited selection out of the 
 great mass of evidence that might otherwise have 
 been brought forward. However candidly and skil- 
 fully the selection may be made, it cannot produce 
 the same impression, as when the whole body of 
 testimony can be presented, a principle which is 
 well understood in the courts of justice. I have 
 said very little with regard to plants, picking out 
 only a few conspicuous examples, yet the evidence 
 in their case is the same as for animals, fuller and 
 clearer in some respects, less so in others. Professor 
 Giesenhagen sums up for plant evolution thus: "The 
 evidence disclosed by palaeontology shows us that 
 
 1 Wm. Laurence Tower, in Heredity and Eugenics, Chicago, 1912, 
 pp. 2~~3.
 
 EVIDENCE FROM EXPERIMENT 167 
 
 the species of plants which inhabit the earth to-day 
 have proceeded from the different plant-forms of 
 the earth's earlier epochs. Experimental investiga- 
 tion and observation upon heredity in existing or- 
 ganisms prove that the specific characters of plants 
 are changeable, that especially by hybridization and 
 mutation new species of plants may arise from those 
 already present. The comparative study of struc- 
 ture and course of development in plants justifies 
 us in concluding that the species of plants, under 
 the continually acting influence of the external 
 conditions of life, lose certain possibilities of develop- 
 ment and gain new possibilities and may thereby 
 undergo a genealogical development." 1 
 
 I must again remind you that what we are en- 
 deavouring to prove is a great historical process, 
 which has gone on through vast and unnumbered 
 aeons of time, when no observer was present to 
 record the forward steps in the magnificent proces- 
 sion of life. The details of evolutionary change 
 must have been of an unimaginable complexity and, 
 of necessity, the evidence for that change must be 
 chiefly indirect and circumstantial, to appreciate 
 the full force of which requires a certain degree of 
 technical training. The Indian hunter swiftly fol- 
 lows a trail which is invisible to the white man's 
 eye, and the experienced detective finds clues where 
 the average man sees nothing of importance, so only 
 one who has gained some first-hand knowledge of 
 
 1 Karl Giesenhagen, in Die AbstaHKOungslehre, p 320.
 
 168 THE THEORY OF EVOLUTION 
 
 living things is in a position to estimate the strength 
 and the weakness of such testimony as has passed 
 before us. That does not for a moment imply that 
 we are here dealing with esoteric mysteries which the 
 laity are unable to comprehend and must accept on 
 faith. The detective's work must convince the 
 jury and in this case, the jury is the body of intelli- 
 gent, non-professional opinion. I merely wish to 
 point out that such an exposition as I have been 
 able to make, in elementary and non-technical 
 fashion, is less convincing than the whole body of 
 known facts is to those who are familiar with it. 
 
 From the very nature of the case, complete de- 
 monstration is impossible; we can only determine 
 which one of alternative explanations is most in 
 harmony with observation and best explains all the 
 facts, which one is therefore the most probable. 
 What gives great weight to the evidence in support 
 of the evolutionary theory is the harmonious con- 
 currence of so many independent lines of testimony. 
 Whether we deal with classification, or the results 
 of domestication, with comparative anatomy, em- 
 bryology, blood tests, palaeontology, geographical 
 distribution, or experimental investigation, we find 
 in every instance that the simplest, most satisfactory 
 and least forced interpretation is that which is 
 offered by the theory of evolution. The probability 
 rises in geometrical ratio with each additional, in- 
 dependent class of evidence. 
 
 The pathway of science is strewn with the wrecks
 
 EVIDENCE FROM EXPERIMENT 169 
 
 of hypotheses and theories, which have served their 
 purpose and have been worn out and discarded. 
 How are we to distinguish between those theories 
 which will be permanent and those which are 
 doomed to decay and oblivion? A theory is only 
 an attempt to explain and interpret a body of facts 
 and, for a time, this may be equally well done by a 
 true theory and a false one; both may seem equally 
 plausible and promising. It is the advance of dis- 
 covery, bringing to light many new facts, which 
 were neither known nor anticipated when the rival 
 theories were first propounded, that puts the theories 
 to the severest test. From this point of view, the 
 doctrine of evolution has stood the test remarkably 
 well. Darwin's book put new life into all depart- 
 ments of biology; every one, friends and foes alike, 
 was eager to find proofs which should establish or 
 overthrow this new and innovating belief, so that 
 all lines of inquiry were pushed forward with re- 
 newed activity and enthusiasm. This zealous labour 
 has resulted in the accumulation of a mass of data 
 such as Darwin never dreamed of. The existence 
 of the marvellous treasures which have been disin- 
 terred from the rocks of western North America 
 was hardly suspected when the " Origin of Species " 
 appeared, yet it is there that were found some of the 
 most complete genealogical series yet made known. 
 Though Darwin himself was one of the pioneers in 
 the discovery of South American fossil mammals, 
 it was not until after his death that the immense
 
 170 THE THEORY OF EVOLUTION 
 
 wealth of the southern continent was brought to 
 light. The long isolation of South America made it 
 the arena of a vast experiment in evolution, which 
 may be followed in the most gratifying manner and 
 which has solved some of the most puzzling prob- 
 lems of modern distribution. South Africa, Russia, 
 China and the Malay Archipelago have also yielded 
 to the explorer much of which Darwin knew nothing, 
 but which he would have cordially welcomed as 
 strongly supporting his belief. It is the same in 
 other fields of research, new and unexpected dis- 
 coveries are continually being made, but evolution 
 still affords the best explanation of them. A re- 
 markable instance of this is the blood tests, of which 
 an account was given in the third lecture. Here 
 was a totally new field, the existence of which was 
 altogether unsuspected, yet, when the field was 
 opened, it afforded some of the most cogent evidence 
 in favour of the evolutionary conception that has 
 anywhere been found. Not that new difficulties and 
 perplexities have not arisen, but in spite of these, 
 the probability of the theory remains unshaken after 
 more than hah* a century of unceasingly active 
 investigation carried on all over the world. The 
 doctrine seems stronger now and is upheld by a 
 greater proportion of naturalists than when it made 
 its early conquests of opinion. 
 
 Another searching test of a theory is when it can 
 be made the basis of prediction or deduction. That 
 an astronomer should be able accurately to predict
 
 EVIDENCE FROM EXPERIMENT 171 
 
 an eclipse is strong proof that the theory of lunar 
 and terrestrial motions, from which he makes his 
 calculations, is sound. From irregularities in the 
 motion of Uranus, the outermost planet then visible, 
 the astronomers predicted the discovery of another 
 planet beyond, and the prediction was verified when 
 Neptune was discovered with the telescope. This 
 was rightly accepted as convincing proof that 
 the theory of the solar system and of planetary 
 motions, from which these distinguished men had 
 worked, was true. Similarly, predictions and deduc- 
 tions have been made and subsequently verified con- 
 cerning "missing links" in genealogical series. I my- 
 self had the great pleasure of finding in the rocks of 
 northern Utah a fossil animal with a type of denti- 
 tion which no man had ever seen, but which had been 
 predicted some time before by Dr. Max Schlosser 
 of Munich. That such predictions can be and have 
 been successfully made, is very strong evidence that 
 the theory of evolution, which is assumed as the 
 basis of deduction, is well founded. 
 
 Admitting fully all that has been said, we must 
 yet beware of erecting evolution into a sacred dogma 
 which no one shall dare to criticise or doubt. Huxley 
 has rightly said that "Science commits suicide when 
 she adopts a creed,'* and it is a duty to keep the 
 mind ever open for new and revolutionary discover- 
 ies, which may change the aspect and significance of 
 all previously acquired knowledge. While I believe 
 that the evolutionary conception of nature is one
 
 172 THE THEORY OF EVOLUTION 
 
 of our permanent possessions and that it will in the 
 future continue to direct and condition all lines of 
 intellectual inquiry and advance, I can understand 
 that half a century hence the question may possibly 
 have assumed a very different aspect. Once more 
 quoting from Professor Giesenhagen: "As soon as 
 direct experience brings to light unambiguous nega- 
 tive examples, which can in no way be harmonized 
 with our conclusions from analogy and induction, 
 we must be ready to make a corresponding change 
 in our doctrines and bring them into harmony with 
 the newly recognized truth, for infallibility in mat- 
 ters of science is given to no man. Up till now, 
 however, no such negative examples are known." l 
 
 1 K. Giesenhagen, loc. cit.
 
 INDEX 
 
 N. B. Entries in quotation marks are titles of books. Technical 
 names of genera and species are in italics; names of authors in small 
 capitals. 
 
 Acquired characters, 20, 151 
 
 Adriatic Sea, 138 
 
 ,Egean Sea, 139 
 
 Africa, 94, 98, 110, 121, 123, 124, 
 
 126, 127, 128, 134, 139, 145, 170 
 AGASSIZ, 88 
 Agate, Neb., 113 
 Agave, 121 
 
 Alaska. 106, 123, 131, 132, 140 
 Alps, 130 
 Americas, 137 
 Ammonites, 91 
 Amphibia, 33, 144, 155 
 Amphioxus, 65 
 Ancon sheep, 23 
 ANDREW, 97 
 Antarctic continent, 90 
 Ant-eaters, 134 
 Antelopes, 132, 135, 140 
 
 blood of, 78 
 
 mountain, 107 
 Anti-body, 74 
 Anti-toxin, 74 
 Antilles, 142 
 Ants, 89 
 Apes, blood of, 77 
 
 Old World, 134 
 Archaopteryx. 96, 97 
 
 Arctic animals and plants, 130 
 
 lands, 120 
 
 Arctogsea, 128, 136, 137 
 Argentina, 109, 130, 135 
 Armadillos, 134 
 Asia, 104, 106, 107, 109, 110, 123, 
 
 124, 127, 128, 129, 131, 132 
 Asphalt pits, 123 
 
 Australia, 106, 110, 121, 127. 128, 
 134,135 
 
 Austria, 24 
 Azores, 142 
 
 Balanoglossus, 65 
 Barriers, 125 
 BABTLETT, 88 
 Bat, 33 
 
 wing of, 42, 47 
 BATESON, 5 
 Bats, 127, 144 
 Bavaria, 96 
 
 Beagle, H. M. S., 15, 16 
 Bears, 32, 131, 140 
 
 polar, 120 
 
 short-faced, 135 
 
 Beetle, larvse of, 154 
 
 178
 
 174 
 
 INDEX 
 
 Beetles, Cenozoic, 89 
 
 embryo, 68 
 Belemnites, 91 
 Bering Sea, 106, 126 
 
 Strait, 106, 126 
 Bermuda, birds of, 146 
 Binomial nomenclature, 32 
 Biogenetic law, 57 
 Bird, wing of, 42, 48 
 Birds, 33, 144 
 
 absence of from Palaeozoic, 91 
 
 blood of, 79, 80 
 
 Cenozoic, 89 
 
 derivation from reptiles, 96 
 
 descent of, 71 
 
 Mesozoic, 90 
 ontogeny of, 71 
 
 Bison, 132 
 
 Bivalves, 90 
 
 "Blood Immunity and Blood Re- 
 lationship," 76 
 
 Blood tests, 73, 170 
 
 BOLL, 39 
 
 Boreal zone, 132 
 
 Borneo, 124, 142, 143 
 
 BOUVIER, 150, 153 
 
 Brachiopoda, Cenozoic, 89 
 ontogeny of, 65 
 Palaeozoic, 92 
 
 Brachyceratops, 119 
 
 BRAUER, 147 
 
 Brine-shrimp, 152 
 
 British Islands, 139 
 
 Bryozoa, 65 
 
 Buenos Aires, 125 
 
 BTJFFON, 94 
 
 Bumble bee, 53 
 
 Butterflies, 153 
 
 Butterflies, Cenozoic, 89 
 larva, 59 
 mouth-parts of, 53 
 
 Cactus, 121 
 California, Lower, 128 
 Cambrian fossils, 85 
 
 period, 85, 87 
 Camels, 114 
 
 blood of, 78 
 
 distribution of, 129 
 
 evolution of, 114 
 
 true, 129 
 
 Canadian Rockies, 85 
 Canidae, 32 
 Canis lupus, 32 
 Cannon-bone, 45, 114 
 Cape Verde Ids., 142, 145 
 Carboniferous period, 87, 95, 
 
 96 
 
 Caribbean Sea, 138 
 Caribou, 132 
 Carnivore, 32, 133, 135 
 
 blood of, 78 
 Carpus, 44 
 Cassowary, 50 
 CASTLE, 163 
 Casuarina, 143 
 Catastrophism, 11, 12 
 Caterpillar, 59, 153 
 Cats, 32, 134, 140 
 Cattle, 36, 144 
 
 wild, 121 
 Cebidae, 77 
 Cenogenetic, 60 
 Cenozoic era, 87 
 
 life, 88 
 Central America, 127, 128, 129, 
 
 131, 136, 138
 
 INDEX 
 
 175 
 
 Century plant, 121 
 Ceratops, 119 
 Cercopithecidae, 77 
 Cetacea, 67 
 China, 170 
 Chordata, 65 
 Civets, 140 
 Class, 32, 125 
 Classification, 31 
 Cloven-hoofed, 45 
 COCKAYNE, 34 
 
 Cockroach, mouth-parts of, 53 
 Cockroaches, Palaeozoic. 92 
 Cocoons, 153 
 Coleoptera, 89 
 Comparative anatomy, 42 
 Conifers, 90 
 Connecticut valley, 84 
 Continental islands, 141 
 CORRENS, 24, 159 
 Crabs, 53 
 Crayfishes, 144 
 Creative plan, 30, 88 
 Cretaceous period, 87, 118 
 
 vegetation, 90 
 Crinoids, 89 
 Crocodiles, blood of, 78, 79 
 
 Cenozoic, 89 
 Crustacea, 51, 52 
 
 Cenozoic, 89 
 
 Palaeozoic, 92 
 Cryptogams, 91 
 Cuba, 143 
 
 CUVIER, 7, 10, 11, 88 
 Cycadofilices, 96 
 Cycads, Cenozoic, 88 
 
 Mesozoic, 90 
 
 Palaeozoic, 91 
 
 DARWIN, 1, 3, 7, 8, 11, 12, 13, 14, 
 
 15, 16, 17, 19, 20, 21, 22, 23, 25, 
 
 26, 29, 31, 33, 35, 37, 39, 65, 83, 
 
 85, 86, 95, 120, 141, 144, 145, 
 
 149, 169, 170 
 Darwinians, 6, 25 
 Darwinism, 5, 6 
 Deer, 140 
 
 blood of, 78 
 
 Sonoran, 133 
 
 South American, 138 
 
 Virginia, 138 
 Degeneration, 72 
 Devonian period, 87 
 Dinosaurs, 118 
 Dipnoi, 65 
 Distribution, discontinuous, 128 
 
 geographical, 120 
 Dogs, 36 
 Dolphins, 67 
 Domesticated animals and plant*, 
 
 35 
 
 Domestication, 35, 150 
 Dominant characters, 158 
 Dragon-flies, 92 
 
 East Indies, 141 
 
 mammals of, 143 
 Eastern hemisphere, 133 
 Egg, 62, 64 
 Egg-albumins, 78 
 Egypt, 94 
 
 Elephant, hairy, 132 
 Elephants, 80, 120, 123 
 
 history of, 93 
 
 true, 140 
 Elk, 132 
 Embryology, 57 
 English Channel, 126
 
 176 
 
 INDEX 
 
 Eocene epoch, 87, 104, 105, 109, 
 
 110, 117 
 
 Eohippus, 102, 105 
 Equine family, 106 
 Equus, 102, 105, 108 
 Ethiopian region, 128, 139 
 Europe, 93, 104, 109, 123, 127, 
 
 128, 129, 130, 131, 132, 138 
 Eustachian Canal, 64 
 Experiment, 149 
 Extinction, 122, 140 
 
 Pleistocene, 106, 133, 135 
 
 Factor, 159, 162, 163, 165 
 Family, 32, 125 
 Fan-tail, 36 
 Faunas, island, 146 
 Ferns, 96 
 
 Cenozoic, 88 
 
 Palaeozoic, 91 
 Fertilization, 160 
 Fibula, 100, 104. 114, 116 
 Fin-whale, 67 
 FISCHER, 153 
 Fishes, 33 
 
 Cenozoic, 89 
 
 Cretaceous, 90, 93 
 
 fresh-water, 144 
 
 Mesozoic, 90 
 
 of Panama, 138 
 
 Palaeozoic, 91 
 Fish-like forms, 91 
 Fissipedia. 32 
 FLEISCHMANN. 1, 2, 29 
 Florida, 128 
 Flies. 89 
 
 Flowering plants, 91 
 Flying foxes, 48 
 Foot-prints, fossil, 84 
 
 Formosa, 124 
 Fossils, 85, 92, 123 
 Fowl, blood of, 79 
 
 embryo of, 63 
 Fowls, 36 
 Foxes, 32, 140 
 France, 10, 111, 132, 140 
 Frog, egg of, 63 
 
 larva of, 59 
 Frogs, 144 
 
 GAGER, 157, 165 
 
 Galapagos Ids., 15, 120, 144, 145 
 
 Ganoids, Cenozoic, 89 
 
 Mesozoic, 90 
 
 Palaeozoic, 91 
 Gastropods, 90 
 Gazelle-camels, 116 
 GEGENBAUR, 60 
 GEMMINGER, 40 
 Genetics, 24, 157 
 Genus, 32, 125 
 Geological record, imperfection of, 
 
 83 
 
 Georgia, 130 
 Germany, 10, 24 
 GIARD, 9, 10 
 
 GIESENHAGEN, 151, 166, 167, 172 
 Gill-pouches, embryonic, 64 
 Giraffe-camels, 118 
 Giraffes, 140 
 Glyptodonts, 133, 135 
 Gnawers, 70, 103 
 Goats, 144 
 
 blood of, 78 
 GRAHAM-SMITH, 76, 77 
 Grasses, 88, 103 
 Great Britain, 123, 124, 142 
 
 Plains, 113
 
 INDEX 
 
 177 
 
 Greek islands, 124 
 Greenland, 131 
 Ground-pines, 88 
 Ground-sloths, 123, 133, 135 
 Guanaco, 114, 115, 117, 129 
 Guiana, 138 
 Guinea pigs, 163 
 
 RECKEL, 37, 57 
 
 Haiti, 143 
 
 Hapalidse, 77 
 
 Hawaiian Ids., 142, 146 
 
 Hen, egg of, 62 
 
 Heredity, 19 
 
 "History of Creation," 57 
 
 Holarctic region, 128, 131, 132, 139 
 
 Hoofed animals, 103, 134, 135 
 
 Horse, 33, 46 
 
 blood of, 76 
 
 fore leg of, 42, 43, 44, 45, 46 
 
 mountain, 107 
 Horses, 36, 123, 135 
 
 browsing, 107 
 
 evolution of, 98 
 
 Eocene, 104 
 
 Miocene, 101, 102 
 
 Oligocene, 103 
 
 Pleistocene, 99, 106 
 
 Pliocene, 101 
 
 Recent, 99 
 
 South American, 106, 107 
 
 spread of, 125 
 
 wild, 121 
 Horse-shoe crab, blood of, 79 
 
 ontogeny of, 65 
 Horse-tails, 95 
 
 Cenozoic, 88 
 
 Palaeozoic, 91 
 Houses, development of, 30 
 
 Humerus, 43 
 BUTTON, 12 
 HUXLEY, 14, 171 
 Hybridization, 164, 167 
 Hyenas, 140 
 Hymenoptera, 89 
 
 Immutability of species, 11, 29, 31, 
 
 36, 88, 120, 137 
 India, 123, 129 
 Insects, 144 
 
 Cenozoic, 89 
 
 embryo, 68 
 
 mouth-parts of, 53 
 
 Palaeozoic, 92 
 Invertebrates, Cenozoic, 89 
 
 Mesozoic, 90 
 
 Palaeozoic, 91 
 Ireland, 142 
 "Island Life," 141 
 Islands, animals and plants of, 141 
 
 continental, 141 
 
 oceanic, 141, 144 
 Italy, 124, 139 
 
 Jackals, 32 
 Jaguar, 140 
 Jalap, 159, 160 
 Jamaica, 39, 121 
 Japan, 124, 128 
 Java, 124, 142, 143 
 Jurassic period, 87, 96, 97 
 
 Kamchatka, 132, 140 
 KAMMERER, 155 
 KAPTEREW, 153 
 Kentucky, 130 
 Krakatoa, 143 
 
 KUKENTHAL, 69
 
 178 
 
 INDEX 
 
 Labrador, 130 
 
 LAMARCK, 7, 8, 9, 10, 13, 19, 22, 31, 
 
 33 
 
 Lamp-shells, 65, 89 
 Lancelet, 65 
 LANG, 161 
 Languages, 55 
 Larvse, 59 
 Lemurs, blood of, 77 
 
 eye of, 76 
 Lepidoptera, 89 
 Lepus cuniculus, 37 
 
 huxleyi, 37 
 Lichens, 88 
 Lilies, 88 
 
 LINNAEUS, 7, 11, 29, 32, S3 
 Lions, 120, 123 
 Lizard, fore leg of, 42 
 Lizards, 144 
 
 blood of, 78, 79 
 
 Cenozoic, 89 
 
 Llama, 114, 123, 129, 134 
 Lobster, 51 
 Los Angeles, 123 
 Lung-fishes, 65, 89 
 Lungs, derivation of, 64 
 Lycopods, 95 
 
 Cenozoic, 88 
 
 Palaeozoic, 91 
 LYUEKKER, 66 
 LTELL, 11, 12, 13, 16 
 
 MAA8.54 
 
 MAcDouoAL, 157, 165 
 Macula lutea, 76 
 Madagascar, 124, 128, 142 
 Madeira, 142 
 
 birds of, 146 
 Malagasy region, 128 
 
 Malay Archipelago, 128, 170 
 "Malay Archipelago, The," 141 
 Malta, 124 
 MALTHUS, 17 
 Mammalia, 32, 33 
 Mammals, absent from Palaeozoic, 
 91 
 
 ancestry of, 4, 97 
 
 blood of, 78, 80 
 
 Cenozoic, 89 
 
 Mesozoic, 90, 98 
 
 Tertiary, 98, 131 
 Mammoth, Siberian, 132, 140 
 Man, arm and hand of, 42, 43, 
 44,46 
 
 blood of, 74 
 
 body of, 80 
 
 embryo of, 63 
 
 eye of, 76 
 .Pleistocene, 138 
 Marmosets, 134 
 
 blood of, 77 
 Marsupials, 127 
 
 Australian, 134, 135 
 
 blood of, 78 
 Maryland, 84 
 Massachusetts, 84, 121 
 Mastodons, 123, 135, 139 
 Mediterranean lands, 121, 140 
 
 Sea, 59, 139 
 
 MENDEL, 22, 24, 157, 158, 161 
 Mendelian laws, 162, 163, 164 
 Mendelism, 5 
 Mesohippus, 102, 105 
 Mesozoic era, 87, 90, 98, 118 
 Metacarpal, 44, 45 
 Mexico, 106, 128, 132, 133, 136, 138 
 Mice, 127, 134, 144 
 
 albino, 163
 
 INDEX 
 
 179 
 
 Mice, fancy, 162 
 
 Migration, 93 
 
 Miocene epoch, 87, 93, 101, 102, 
 
 106, 107, 109, 113. 116. 133. 135 
 Missing links, 171 
 Mole, 33 
 
 fore leg of, 42 
 Mollusca, Cenozoic, 89 
 
 Mesozoic, 90 
 
 ontogeny of, 65 
 
 Palaeozoic, 92 
 Mongoose, 121 
 Monkey, 33 
 Monkeys, blood of, 77 
 
 eye of, 76 
 
 Old World, 76, 77, 134 
 
 South American, 76, 77, 133 
 Moose, 132 
 MORGAN, 164, 165 
 Morocco, 124 
 Mosquito, 53 
 
 Monoclonius, 119 
 Monotremes, 127 
 Moth, Brown-tail, 121 
 
 Gypsy, 121 
 
 Lunar, 39, 154 
 Moths, 153 
 
 Cenozoic, 89 
 
 mouth-parts of, 53 
 Mouse, 162 
 Musk-ox, 120, 130 
 Mutation, 162, 167 
 
 Theory, 23 
 Mutations, 23 
 
 NAGEIJ, 20 
 
 Natural Philosophers, 10 
 Selection, 14, 18, 20, 25 
 
 Nebraska, 113 
 
 Neo-Darwinian school, 22 
 
 Neogsea, 128 
 
 Neotropical region, 128. 131, 133, 
 
 134, 136 
 Neptune, 171 
 New England, 140 
 
 Guinea, 127 
 
 Hampshire, 130 
 
 Jersey, 84 
 
 World, 111, 140 
 
 Zealand, 34, 89, 142 
 Newts, 144 
 
 North America, 93, 104, 107, 
 109, 110, 112, 114, 121, 122, 
 123, 127, 128, 129, 131, 132, 
 133, 134, 135, 136, 137, 140, 
 169 
 
 horses of, 98, 106 
 North Sea, 59, 139 
 Northern hemisphere, 129, 131 
 NUTTALL, 73, 74, 76, 77 
 
 Oaks, American, 34 
 
 Oceanic islands, 141, 144 
 
 Oklahoma, 130 
 
 Old World, 111, 114, 123, 131, 132, 
 
 133, 134, 138, 140 
 Oligocene epoch, 87, 102, 103, 105, 
 
 110, 112, 113, 115, 117 
 Ontogeny, 57, 62 
 Opossum, 127, 133, 134 
 ORBIGNT, A. D', 11 
 Order, 32, 125 
 Ordovician period, 87 
 Oriental region, 128, 129 
 "Origin of Species," 1, 17, 65, 83, 
 
 169 
 
 OSBORN, 111
 
 180 
 
 INDEX 
 
 Ostrich, blood of, 79 
 
 wing of, 50 
 Otters, 32, 134 
 Ox, blood of, 78 
 
 fore leg of, 44, 45, 46 
 
 Pacific, islands of South, 142 
 Palaeontology, 55, 82 
 Palaeotheres, 107 
 Palaeozoic era, 87 
 
 life, 91 
 
 Paleocene epoch, 87, 104 
 Palingenetic, 59, 60 
 Palms, 88, 120 
 
 cocoa-nut, 143 
 Pampas, 15 
 Panama, Isthmus of, 131, 134, 138 
 
 marine fishes of, 138 
 Parasites, 72 
 Patagonia, 135 
 Peas, 158 
 Peccaries, 134 
 Penguins, 50 
 Pennsylvania, 84, 109 
 Perissodactyla, 112 
 Permian period, 87, 98 
 "Philosophic Zoblogique," 8 
 Phylogeny, 57 
 Phylum, 32 
 PICTET, 153 
 Pigeons, 36 
 Pigs, 144 
 Plants, Carboniferous, 95 
 
 Cenozoic, 88 
 
 Cretaceous, 93 
 
 evolution of, 166 
 
 modification of, 150 
 
 of White Mts., 130 
 
 origin of cultivated, 28 
 
 Pleistocene epoch, 87, 99, 106, 109, 
 116, 122, 129, 130, 131, 182, 
 133, 138, 140 
 
 Pliocene epoch, 87, 101, 106, 107, 
 108, 109, 113, 129 
 
 Pliohippus, 108 
 
 Poebrotherium, 115, 117 
 
 Pointer, 36 
 
 Porcupine, Canada, 133 
 
 Porto Rico, 143 
 
 Porto Santo, 37, 39 
 
 Potato-beetle, 155, 164 
 
 Pouter, 36 
 
 Pre-Cambrian eras, 87 
 
 Prefonnation, 62 
 
 Procamdus, 115, 117 
 
 Protohippus, 102, 105 
 
 Protylopus, 115, 117 
 
 Province, zoological, 128 
 
 Pterodactyls, 50 
 
 Quadrupeds, 32, 89 
 Quaternary period, 87 
 
 Rabbit, European wild, 37 
 
 Falkland Id., 39 
 
 Jamaica, 39 
 
 Porto Santo, 37 
 Rabbits, 121, 134, 144 
 Raccoons, 32, 134, 140 
 Races, geographical, 34 
 Radius, 43, 44, 45, 100, 104, 114 
 Rails, 146 
 Ranunculus, 151 
 Rats, 134, 144 
 Raven, 49 
 RAT, 7 
 
 Realm, zoological, 136 
 Recapitulation theory, 57, 61, 64, 
 
 73
 
 INDEX 
 
 181 
 
 Recent epoch, 87, 99, 101 
 
 Saturnia bolli, 40 
 
 Recessive characters, 158 
 
 luna, 39, 40 
 
 v Record, geological, 95 
 
 Scandinavia, 181, 132 
 
 palaeontological, 93 
 
 SCHLOSSER, 171 
 
 Region, zoological, 1526, 197, 
 
 SCHMANKEWTTSCH, 152 
 
 136 
 
 SCHHODEH, 153 
 
 Reindeer, 132 
 
 Scorpions, 65, 79, 92 
 
 Reptiles, 33, 144 
 
 SCOTT, 28 
 
 age of, 90 
 
 Sea-lilies, Cenozoic, 89 
 
 Cenozoic, 89, 90 
 
 Palaeozoic, 92 
 
 Mesozoic, 90 
 
 Sea-lions, 32 
 
 Permian, 98 
 
 Seals, 32 
 
 Rhinoceros, African, 111 
 
 Segregation, 159 
 
 Indian, 111, 112 
 
 SELENKA, 143 
 
 Siberian, 140 
 
 Sera, anti-carnivore, 78 
 
 Sumatran, 111 
 
 Serum, 74 
 
 Rhinoceroses, 120 
 
 anti-fowl, 75 
 
 aquatic, 110, 111 
 
 anti-horse, 75 
 
 cursorial, 110 
 
 anti-human, 75, 77 
 
 evolution of, 110 
 
 anti-lizard, 79 
 
 paired-horn, 113 
 
 anti-llama, 78 
 
 true, 110, 111 
 
 anti-pig, 75, 78 
 
 Rodentia, see Rodents. 
 
 anti-turtle, 78 
 
 Rodents, 103, 133 
 
 Sexual selection, 19 
 
 rudimentary teeth in, 70 
 
 Seychelles, 142 
 
 porcupine-like, 134 
 
 Shark, embryo of, 63 
 
 Rudimentary organs, 56, 66 
 
 Sharks, Cenozoic, 89 
 
 Rudiments, 44 
 
 Mesozoic, 90 
 
 Ruminants, blood of, 78 
 
 Palaeozoic, 91 
 
 ontogeny of, 69 
 
 Sheep, 36, 132 
 
 Russia, 170 
 
 blood of, 78 
 
 
 embryo of, 70 
 
 Sabre-tooth tigers, 123, 135 
 
 Shells, fresh-water, 144 
 
 Sacculina, 72 
 
 Sicily, 124 
 
 Sago palms, 88 
 
 Silurian period, 87 
 
 Sahara, 128, 139 
 
 Simiidse, 77 
 
 St. Helena, birds and reptiles of, 
 
 Skunks, 134 
 
 146 
 
 Sloths, 134 
 
 Salamander, 144, 156 
 
 Snail, garden, 161
 
 182 
 
 INDEX 
 
 Snails. 144 
 
 Snakes, blood of, 78, 79 
 
 Cenozoic, 89 
 
 limbs of, 67 
 Solenhofen, 96 
 Sonoran region, 128, 131, 132, 133, 
 
 134, 136, 138 
 
 South America, 15, 110, 114, 120, 
 121, 123, 127, 128, 129, 130, 
 131, 133, 134, 135, 136, 137, 
 140, 145, 169, 170 
 
 birds of, 134 
 
 horses of, 106 
 
 mammals of, 130, 133 
 
 Tertiary mammals, 131, 169 
 Spain, 123, 139 
 Spaniel, 36 
 Special creation, 7, 29, 30, 35, 66, 
 
 67, 86, 88, 120, 129, 137. 148 
 Specific centres, 125 
 SPENCER, 18 
 Sphenophyllales, 95 
 Spiders, 65, 79, 144 
 
 Palaeozoic, 92 
 Splint-bones, 45 
 Sports, 23 
 Squirrels, 134 
 
 rudimentary teeth of, 71 
 Stag, red. 132 
 STANDFUSS, 153 
 STEINMANN, 3, 98 
 Straits of Magellan, 125 
 STRANGEWAYS, 76 
 Struggle for existence, 18, 24 
 Styracosaurus, 119 
 Sub-order, 32 
 Subregion, 128 
 Subspecies, 34 
 Sumatra, 142, 143 
 
 Survival of the fittest, 18 
 Switzerland, 39, 154 
 
 Tadpole, 59, 156 
 Tapirs, 123, 134 
 
 distribution of. 129. 130 
 
 evolution, 109 
 Tasmania, 127 
 
 Tasmanian Wolf, blood of, 78 
 Teleosts, 89 
 Terrier, 36 
 Tertiary period, 87, 98, 106, 127. 
 
 129, 131 
 Texas, 39, 154 
 Thylacine, blood of, 78 
 Tigers, 120 
 Toad, larvae of, 156 
 Toads, 144 
 Torosaurus, 119 
 TOWER, 155, 164, 166 
 Transitional zone, 132 
 Triassic period, 84, 87. 97 
 Triceratops, 119 
 Trilobites, 92 
 Trochophore, 68 
 TROXELL, 108 
 TSCHERMAK, 24 
 Tse-ste flies, 126 
 Tumbler, 36 
 Tunicata, 65 
 Tunis, 124 
 Turtles, blood of. 78. 79 
 
 Cenozoic, 89 
 Type of structure, 42, 54, 55 
 
 Ulna, 43, 45, 100, 103, 114. 116 
 Uniformitarianism, 12 
 Unit characters, 158, 165 
 United States, 132, 135
 
 INDEX 
 
 183 
 
 Uranus, 171 
 Utah, 171 
 
 Variability. 17 
 
 Variations, 23 
 
 Varieties, 34 
 
 Vegetation, Carboniferous, 95 
 
 Cenozoic, 88 
 
 Mesozoic, 90 
 
 Palaeozoic, 91 
 Vertebrata, 32 
 Vertebrates, ontogeny of, 63 
 
 Palaeozoic, 91 
 Virginia, 84 
 
 VRIES, H. DE, 23, 24, 26 
 VuLpes, 32 
 
 WAAGEN, 22, 23 
 WAGNER, 21, 40, 41 
 WALCOTT, 85 
 WALLACE, 13. 141 
 Walrus, 32, 130 
 Wapiti, 132 
 Wasps, 89 
 Water-fleas, 153 
 
 Weasels, 32, 134 
 Weeds, European, 121 
 WEISMANN, 20, 21, 22, 153 
 West Indies, 127, 128 
 
 mammals of, 143 
 Western hemisphere, 127, 136 
 Whale, 33 
 
 flipper of, 42, 46, 47 
 
 Greenland Right, 47, 67 
 Whales, blood of, 78 
 
 development of, 69 
 
 limbs of, 67 
 
 toothed, 67 
 White Mts., 130 
 WILSON, 3, 26, 62 
 Wolf, 32 
 Wolverenes, 131 
 Wolves, 32, 123, 134, 140 
 Wonder of Peru, 159 
 Worms, embryo of, 68 
 
 ontogeny of, 59, 65 
 Wrist-joint, 43 
 
 ZARCO, 37 
 Zebras, 99 
 
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