A history of land mammals in the western hemisphere

Throughout this book the theory of evolution has been taken for granted, as it seemed superfluous to present an outline of the evidence upon which that theory rests. “Descent with modification” is now accepted among naturalists with almost complete unanimity, but, unfortunately or otherwise, this general agreement does not extend beyond the point of believing that the present organic world has arisen by descent from simpler and simpler forms. The application of the theory to concrete cases is beset with grave difficulties and gives rise to the most divergent views. The uninitiated reader who takes up a treatise upon some animal group may well be surprised to see the apparently minute accuracy with which the genealogy of the series is set forth and the complex relationships of its members marshalled in orderly array. Another treatise on the same subject, however, while agreeing perfectly with the first as to the facts, will contradict its conclusions in almost every particular. Indeed, so notorious did this become, that “phylogenetic trees” were rather a laughing-stock, and most naturalists lost interest in the problems of phylogeny and turned to fields that seemed more promising.

To some extent, this almost hopeless divergence is inherent in the very nature of the problem, which deals with the value of evidence and the balancing of probabilities, as to which men must be expected to differ; but there is another and more potent cause of the discrepancy. When the contradictory schemes are analyzed, it is seen that each is founded upon certain assumptions regarding the evolutionary process, assumptions which are generally implicit and often apparently unconscious. In the present state of knowledge, these postulates are, for the most part, matters of judgment, incapable of definite proof, and they appeal with very different force to different minds; what to one seems almost self-evident, another regards as all but impossible. It will, however, be of service to examine such of these postulates as are involved in mammalian history.

It is quite impracticable to construct a genetic series without making certain assumptions as to the manner in which the developmental process operated and the kinds of modification that actually did occur. In the preceding chapters, which deal with the evolutionary history of various mammalian groups, it was repeatedly stated that, of two contemporary genera, one was to be taken as the ancestor of some later form and the other regarded as a collateral branch, but it was also pointed out that in certain cases, palæontologists differed more or less decidedly as to the proper interpretation of the facts; it is just this lack of agreement as to the modes and processes of change that forms the root of the difficulty.

There are instructive analogies between the history, aims and methods of comparative philology, on the one hand, and zoölogy, on the other. In both sciences the attempt is made to trace the development of the modern from the ancient, to demonstrate the common origin of things which are now widely separated and differ in all obvious characteristics, and to determine the manner in which these cumulative modifications have been effected. At the present time zoölogy is still far behind the science of language with regard to the solution of many of these kindred problems and has hardly advanced beyond the stage which called forth Voltaire’s famous sneer: “L’étymologie est une science ou les voyelles ne font rien et les consonnes fort peu de chose.” Many of the animal genealogies which have been proposed have no better foundation than the “guessing etymologies” of the eighteenth century, and for exactly the same reason. Just as the old etymologists made their derivations upon the basis of a likeness of sound and meaning in the words compared, so the modern zoölogist, in attempting to trace the relationships of animals, must proceed by balancing their similarities and differences of structure. The etymologist had no sure test for distinguishing a true derivation from a plausible but false one, and the zoölogist finds himself in the same predicament. How much weight should be allowed to a given likeness and how far it is offset by an accompanying difference, there are no certain means of determining, and we are still in search of those laws of organic change which shall render such service to zoölogy as Grimm’s law did to the study of the Indo-European languages. Doubtless, the analogy may be pushed still farther, and it may be confidently assumed that, just as sound principles of etymology were established by tracing the changes of words step by step from their modern forms to their ancient origins, so the existing animal forms must be traced back through the intermediate gradations to their distant ancestors, before the modes of organic development can be deduced from well-ascertained facts.

The evolutionary problem has been attacked by the aid of several distinct methods, each of which has its particular advantages and its peculiar limitations and drawbacks. Most of the methods suffer from the fact that they deal only with the present order of things, and thus resemble the attempt to work out the derivations of languages that have no literature to register their changes.

(1) Of necessity, the oldest of these methods is Comparative Anatomy, which had made great advances in pre-Darwinian days. It is the indispensable foundation of the whole inquiry, for an accurate knowledge of Comparative Anatomy is absolutely necessary to the use of the other methods; in the hands of the great masters it has registered many notable triumphs in determining the mutual relationships of animal groups; but finality cannot be reached by this method, because it deals only with existing forms and possesses no sure criterion for determining the value of similarities. It is thus unable to distinguish with certainty between those resemblances which are due to inheritance from a common ancestry and those which have been independently acquired. It is a very frequent fallacy to assume that, because two allied groups, B and C, possess a certain structure, their common ancestor, A, must also have possessed it. This may or may not have been the case, and Comparative Anatomy offers no assured means of deciding between those alternatives or of confidently distinguishing primitive characters from degenerative or retrograde changes.

(2) Embryology, which is the study of the development of the individual animal from the unfertilized egg to the adult condition, was long regarded as the infallible test of theoretical views in zoölogy. This was on the assumption that individual development (ontogeny) is a recapitulation in abbreviated form of the ancestral history (phylogeny) of the species, and was called by Haeckel “the fundamental biogenetic law.” It was soon learned, however, that the “recapitulation theory” was not to be implicitly trusted, for structural features which could not possibly be a part of ancestral history were imposed upon or substituted for those due to phylogenetic inheritance. Now the whole theory is strongly questioned, and the absence of any universally accepted rules of interpretation, by which the contradictory embryological data may be harmonized into a consistent whole, has deprived the method of that authoritative character once so generally ascribed to it. It is like dealing with a literature which has been vitiated with many forgeries, only the grossest of which can be readily detected. Embryology has rendered many great services in the solution of zoölogical problems and will no doubt render many more, but it cannot, of itself, reach final conclusions.

(3) Experimental Zoölogy, especially that part known as “Genetics,” one of the newest and most promising provinces of the science, has already taught us much concerning the laws of inheritance and the manner in which new characters arise, and no one can venture to fix the limits of its possible results. On the other hand, it does not seem likely that the larger problems of relationship and classification can be solved by this method, because of the brief time which the shortness of human life allows for the experiments.

(4) Palæontology suffers from the drawback that much of the past history of life is irretrievably lost, and even when the record is remarkably complete, as it is for certain chapters of the history, the material is but partially preserved. With such rare exceptions as are of little practical importance, only the hard parts, bones, teeth, etc., are retained and the soft parts completely destroyed. Nevertheless, Palæontology has the preëminent advantage of offering to the student the actual stages of development, and thus, to recur to the simile of language, has preserved original documents and in the true order of succession. It is true that it is well-nigh impossible to reconstruct a phylogenetic series of ancestor and descendant, unaffected by theoretical preconceptions, and the differences which arise in the interpretation of undisputed facts are caused by divergent beliefs concerning the actual course of the evolutionary process. If final and definitive results are ever to be reached, it must be through the coöperation of all the methods of research, and such results must be able to stand the tests applied by every sound method. On the other hand, the study of those phylogenetic series which are generally accepted as well established, should furnish us with some fairly definite information as to the modes in which development has operated in the past, since the order of succession in time fixes a limit to the rearrangement of related series. Some of the conclusions thus suggested may be stated here.

I. One of the most fundamental problems concerning the course of development is that which deals with parallel and convergent evolution. The term parallelism implies that forms having a common origin may independently run through a similar course of development and arrive at similar results. Illustrations of this principle are given by the many phyla of horses, rhinoceroses and camels, which persisted side by side through several geological stages, following independent, but parallel, courses of change. An even more striking case is that of the two subfamilies of the cats, the true felines and the †sabre-tooths. Whatever view may be taken of the relationships of these two groups, it is clear that, at least from the upper Oligocene to the Pleistocene, they were separate, but kept remarkably even pace with each other in their advance and specialization.

By convergence is meant a similar result which is reached by two or more independent lines having different starting points, so that the descendants are more alike than were the ancestors, and is thus the opposite of divergence, the result of which is to make the descendants of common ancestors less and less alike with each succeeding stage. Either parallelism or convergence may be involved in the independent acquisition of similar characters, of which these are so many examples. It is obvious that this problem is fundamental and that little real progress is possible until a solution is reached. As to the correct solution, there is much difference of opinion among naturalists. Some deny altogether the reality and importance of these modes of development, but such are almost exclusively concerned with the modern world; others go to the opposite extreme, and looking upon every large group as polyphyletic, consider parallel and convergent development to be the rule of evolution. Few palæontologists are disposed to doubt that these modes of evolution are very frequent; their difficulty is to determine what limits can be drawn, and this difficulty can be removed only by much wider and more exact knowledge than we now possess.

So far as single structures are concerned, the fossils demonstrate unequivocally that they have been independently acquired in a great many cases. The resultant similarity may be attained through the loss, the acquisition or the modification of parts. The reduction of toes from the primitive number of five to four, three, two, or even one, has happened over and over again in the most diverse groups. There is good reason to believe that all the early and primitive placental mammals had the third trochanter on the femur and the epicondylar foramen on the humerus, but in most of the modern groups these structures are lost; and the list of such similar reductions of parts might be almost indefinitely extended.

Of much greater significance is the independent similar modification of parts and acquisition of new structures. Innumerable examples of this kind of parallel and convergent development might be given, but a few will be sufficient to illustrate the principle. (1) The odontoid process of the axis (second vertebra of the neck) was primitively a bluntly conical peg, a form which is still retained in the great majority of mammals, but in the true ruminants, the camels, the horses and the tapirs, the process is spout-shaped, concave on the upper side, convex on the lower. By tracing the development of those groups, it has been conclusively demonstrated that the change of form took place independently in each of the four. (2) The ruminants have molar teeth composed of four crescentic cusps arranged in two transverse pairs, the pattern called selenodont. The evidence is very strong that this highly characteristic molar pattern has been several times independently repeated, as in the true ruminants, the camels, the †oreodonts and probably other groups also. (3) The family †Macrauchenidæ of the extinct †Litopterna shares with the camel tribe the remarkable peculiarity of having the canal for the vertebral artery running through the neural arches of the neck-vertebræ. (4) A very striking instance is afforded by the three widely separated groups of hoofed animals, members of which had their hoofs transformed into claws; the †chalicotheres arose from the normal perissodactyls (p. 356), the †agriochœrids from the †oreodonts and the †Entelonychia from the †toxodonts. From time to time attempts have been made to unite two or more of these groups, but in each case better material and fuller knowledge have demonstrated the unnatural character of such association and the separate origin of the peculiar structure.

Admitting the reality and frequency of these modes of development, a far more difficult problem is to determine the extent to which such independent acquisition of similar structures has actually been carried, and it is at this point that the widest divergences of opinion are to be found. As yet, our knowledge is far too imperfect to permit the making of positive statements, but there is no evidence which would justify the conclusion that the same genus, family or order of mammals ever arose independently from radically different ancestors. We have no reason to believe that identical groups of mammals were ever separately developed in land areas which through long periods of time had no means of intercommunication. If such a thing ever happened, it must have been the rarest of exceptions. On the other hand, parallelism, by which related forms pass through similar stages of development, would seem to have been so exceedingly common, as fairly to deserve being called a normal method of evolution. As more and better material has been gathered, it has grown increasingly clear that almost every large group of generic, family or higher rank, whose history is known in any adequate measure, consists of several distinct, though related phyla, which pursued more or less closely parallel courses of modification, though diverging from one another sufficiently to make the distinction of them comparatively easy. The parallelism was thus not exact, however perfect it may have been in particular structures, and the longer the phyla persisted, the more distinctly did they diverge.

A typical problem, which involves these principles, is afforded by the very curious and interesting group of South American hoofed animals known as the †Litopterna (Chap. XIII). The many remarkable resemblances between these ungulates and the perissodactyls and, more specifically, between the family †Proterotheriidæ and the horses, have been very differently interpreted by palæontologists. Some have insisted that the †Litopterna should be merged in the Perissodactyla, on the ground that such a degree of likeness could not have been independently acquired. Others hold that this is a remarkable case of parallelism or convergence, and the latter is, in my opinion, much the more probable view. Until the ancestry of both groups, Perissodactyla and †Litopterna, shall have been definitely ascertained, it will not be practicable to make a final decision between these alternatives, nor, if the similarities were really independently acquired, to determine whether parallel or convergent evolution is involved. It is quite possible that both groups were rooted in the common ground of the †Condylarthra, and, if so, their relation is one of parallelism; but no such common ancestry has been proved, and it is equally possible that their ancestry was totally distinct. In the latter case the resemblances were due to convergence.

Assuming that the remarkable resemblances between the †Proterotheriidæ and the horses were separately acquired, it should be emphasized that these similarities nowhere amount to identity. The likenesses are not confined to a few structures, but are general throughout the skeleton and may be noted in the teeth, skull, trunk, limbs and feet, but in every single one of these parts the similarities are offset by differences of great significance. No competent anatomist would mistake any of the bones of the †proterotheres for the corresponding parts of the horses, whatever view he might hold as to the relationship between the two groups. The case is thus one of a very instructive kind, as tending to show that identity of structure in so highly complex creatures as mammals is not independently attained by widely separated or entirely unrelated forms. Probable as this conclusion is made by all the available evidence, it cannot be regarded as demonstrated; it is proverbially impossible to prove a negative.

On the other hand, it is equally probable that nearly related forms do very frequently, perhaps normally, pass through separate, but closely similar, courses of development. It is likely that a new species is usually formed through similar and simultaneous modification of many individuals, rather than from a single individual or pair. It may be the general rule, as almost certainly has often happened, that a new genus arises by the separate assumption of the new character by several species of the ancestral genus, rather than through the rapid diversification of a single species, though, no doubt, parallel and divergent modification are both very frequent and important processes. Dr. Eigenmann concludes from his study of South American fresh-water fishes that a certain new genus is even now in process of origin through the transformation of several species of an older genus, which in different parts of the continent are simultaneously, but independently, taking on the new character.

Sometimes it is possible to assign a definite reason for the independent origin of similar structures in different groups of mammals. Except for the head, there is much similarity of appearance among the very massive hoofed animals, such as the elephants, rhinoceroses, tapirs and hippopotamuses of the present time, a fact which induced Cuvier to unite them in one order, the “Pachydermata,” a term which has passed into vernacular, if metaphorical, usage. No doubt also, several extinct groups, such as the †Amblypoda and the perissodactyl family of the †Titanotheriidæ, would have been included, had they been known in Cuvier’s day. In the largest and heaviest of these animals, the elephants, †amblypods and †titanotheres, there are many close correspondences in all parts of the skeleton, which are clearly due to the mechanical necessities imposed by the support of immense weight, and the developmental history of each group shows that the smaller and lighter ancestors were less similar than the larger and more massive descendants. Such subsequently acquired likenesses are thus obvious examples of convergence and were caused by adaptation to similar needs.

Fürbringer has shown that among birds size and weight of body determine many resemblances between unrelated families, the largest forms displaying a more advanced grade of specialization.

It is thus extremely probable that evolution is a highly complex process, in which divergent, parallel and convergent modes of development are normally concerned. This complexity greatly increases the difficulty of determining phylogenies, which would be very much easier could every notable resemblance be at once accepted as proof of relationship. It often renders impossible the classification of some isolated group, which seems to have several incompatible affinities. It emphasizes the necessity of founding schemes of classification upon the totality of structure and the importance of determining the value of characters, whether they are primitive or advanced, divergent, parallel or convergent, before attempting to use them in classification.

In looking over the field of mammalian evolution, so far as that is recorded by the fossils, the general impression received is that the most important process is divergent development, one line branching out into several. This process became especially vigorous and rapid at times of important change in the character of the environment, what Osborn has called “adaptive radiation.” As we have repeatedly observed in the history of particular groups, e.g. the rhinoceroses, horses and camels, numerous parallel phyla of the same family existed together in certain geological stages, but as these phyla were traced back in time, they were found to draw together and display themselves as branches of a single stem. This favours the inference that the mammalian orders, so far as they are truly natural groups and not arbitrary assemblages, are each of single, or monophyletic, origin, and that the parallel and convergent modes of development, while very frequent and important, are subordinate to divergence.

II. A second problem is whether development among mammals is always by means of reduction in the number of parts, or whether that number may not be increased. With this is involved the so-called law of the “irreversibility of evolution,” according to which organs once lost, or reduced to a vestigial condition, are never regained, or reëstablished in function. There can be no question that the usual mode of mammalian development is by reduction in the number of parts and the enlargement and elaboration of those which are retained, as, for example, in the reduction of five toes to one in the series of the horses; but there are cases which require a different explanation. The very numerous teeth of the porpoises and dolphins and of the Giant Armadillo are not a primitive feature, but must have arisen by a process of multiplication. In the very curious Large-eared Wolf (Otocyon) of South Africa the number of molar teeth 3/4 exceeds that found in any other placental mammal. This feature has been interpreted as a proof of marsupial relationship, but, as the creature is a typical dog in all other respects, such a relationship would involve a degree of convergence in development that is quite inadmissible without the most cogent evidence. Until something is learned regarding the descent of Otocyon, no positive statement can be made as to the significance of its exceptional dentition, but much the most likely supposition is that additional teeth have been developed in an otherwise normal canid. However that may be, the testimony of the fossils is unequivocally to the effect that the usual mode of development among mammals is by a reduction in the number of parts, accompanied by enlargement and specialization in those which are retained.

It is equally clear that the “law of irreversibility” holds good in a very large number of cases, but whether it is always valid is very doubtful. In the Guinea Pig, as in all its family (Caviidæ), there are four toes in the front foot, three in the hind; but Professor Castle has lately succeeded in producing a race with four toes in the hind foot. To call this a “monstrosity” or “abnormality” explains nothing; the fact remains that the four-toed race has been established and no reason can be assigned why the same thing might not happen in nature. If Dr. Matthew’s view concerning the origin of the American deer from †Leptomeryx (p. 409), should prove to be well founded, another example of the same kind would be furnished. In †Leptomeryx of the Oligocene the upper canine was reduced to minute, almost vestigial proportions, while in the ancestral deer, †Blastomeryx of the lower Miocene, it was a large, scimitar-like tusk. While I am unable to accept this derivation of the deer, it may be true nevertheless and, if so, will be a most interesting example of the rehabilitation of a vestigial organ. Decision must await the discovery of the intermediate forms. Many such cases and instances of the addition of parts may be so far undetected, but the phylogenetic series, as we have them before us, point decidedly to the conclusion that such rehabilitation or new addition is exceptional.

III. So far as we are able to follow it by the aid of the fossils, development among the mammals would appear to be a remarkably direct and unswerving process. When any long-lived phylum, made up of numerous well-preserved members, is studied, the observer cannot fail to be impressed by the straightforward course of the evolutionary process, as though the animals were consciously making for a predetermined goal, which, needless to say, they were not. A minute cusp makes its appearance on a tooth, enlarges steadily in each succeeding genus, and ultimately becomes a very important element in the pattern; and in this series of changes there is no oscillation backward and forward. In the perissodactyls and a few other groups, the premolars in each family gradually and steadily assumed the size and complexity of molars; beginning at the hinder end of the series, these teeth one by one become molariform, not in irregular and haphazard fashion, but by perfectly graded stages. The same gradual and direct process was maintained in the oft-recurring reduction of digits among the hoofed animals, differing for each group according to the symmetry of the foot. In the horses, for example, the first digit became vestigial and disappeared, and then the fifth followed, leaving a three-toed foot, in which the median digit was notably the largest and bore most of the weight. Throughout the Oligocene and Miocene epochs the horses were all tridactyl, but there was a gradual enlargement of the median digit and dwindling of the laterals, until these became mere dew-claws, not touching the ground, and the weight was carried entirely upon the median one. Finally, the laterals lost their phalanges and were farther reduced to splints, which is the modern condition. In the same gradual and unswerving manner the higher artiodactyls went through a process of digital reduction from five to two, and numberless other instances of similar sort might be adduced.

On the other hand, the direction of change long followed may be departed from, the deviation being due to the introduction of a new factor. In the earliest deer the males were hornless, but they developed effective weapons of defence by the enlargement of the upper canine teeth into long and sharp, sabre-like tusks. When antlers appeared, the work of defence was transferred to them, and the tusks began to dwindle, being eventually suppressed in those deer which had large and complex antlers, though persisting to the present time in the hornless Musk Deer and in the small-antlered Muntjaks, which can defend themselves with their sharp tusks.

It would be inaccurate to say that fluctuations in the size and effectiveness of parts never occurred; on the contrary, there is evidence that such fluctuations in details were not infrequent, and may have been even more common than we suppose. To give one instance, the very early camels of the upper Eocene and lower Oligocene had small canines, which though not at all functionless or vestigial, were yet little larger than incisors. Though the ancestral camels of the middle and lower Eocene are not yet definitely known, there is strong reason to believe that in them, as in all of their contemporaries among the ungulates, the canines were enlarged and fang-like. If so, the canine teeth in the camels underwent decided fluctuations in size, being first larger, then smaller and again enlarging. If Dr. Matthew’s interesting theory as to the origin of the true felines from primitive †sabre-tooth cats (see p. 540) should be confirmed, it would furnish a very striking example of fluctuating development. The acceptance of the theory involves the admission of the following changes: (1) The upper canine was enlarged and changed into a thin, recurved, scimitar-like tusk; (2) the lower canine was much reduced, becoming little larger than the incisors; (3) the lower jaw developed a flange on each side from its inferior border, against which the inner side of the upper canine rested, when the mouth was closed, and the chin was nearly flat, meeting the outer surface of the jaw at a right angle. After these peculiarities had been fully established, the stock divided into two series; in one, the †machairodonts, the specialization continued along the same lines, assuming more and more exaggerated forms, while in the true cats it was reversed. The upper canine grew shorter and thicker, the lower canine was very greatly enlarged, the lower jaw lost its flange, and its external and anterior surfaces no longer met at a right angle, but curved gradually into each other. As previously stated, such a reversal strikes me as improbable and not to be accepted without very much more complete evidence than we now have, but it is perfectly possible that such evidence may be forthcoming.

Making the fullest allowance for all such cases of fluctuation, it remains true that in the great majority of the phyla whose history may be followed in some detail, development has been remarkably direct and unswerving. Plasticity of organization and capacity for differentiation of structure in widely different directions would seem to be limited in the mammals, especially among the more advanced groups.

IV. A question that has been much debated and is still a centre of controversy deals with continuity and discontinuity in development. In other words, does evolution proceed by the cumulative effects of minutely graded modifications, or is it a succession of leaps and sudden changes? The difference is illustrated by many breeds and races of animals and plants under domestication, the history of which is known. Some have arisen from “sports,” sudden and marked deviations from the parent stock, which “breed true” from the beginning. Of this character was the Ancon breed of sheep, which was derived from a single short-legged ram that was born of normal parents in 1791 and transmitted his peculiarities to his offspring. Professor Castle’s race of four-toed Guinea Pig originated from one four-toed individual, which suddenly appeared in a litter of normal ones. Other breeds have been formed by the careful and long-continued selection of minute individual variations. Which of these methods is the one that has been followed under natural conditions? or has now one method been used and now another, according to circumstances? The problem is one that has a profound and far-reaching importance for the whole of evolutionary philosophy, which largely hinges upon it.

Unfortunately, palæontology is not well fitted to give a decisive answer to these questions, for, however complete the record of any given series may be, we never can be sure that it actually is so, and interruptions in the continuity of development might be due either to progress by abrupt changes, or to a failure to preserve all the gradations. For that reason different observers have put divergent interpretations upon the facts as we have them. The general impression that is made by the study of a well-preserved mammalian phylum is that of continuity, but a closer analysis reveals numerous small breaks, and suggests, so far as the record may be trusted, that the advance was made by separate steps, though very short ones. Indeed, it has been objected that so completely recorded a phylum as that of the horses must be illusory, because there is not perfect continuity between the successive genera, it being taken for granted that such continuity is the normal mode of development.

Dr. Schlosser, on the other hand, is a disbeliever in perfect continuity. “I am of the opinion that we must reckon with development per saltum more frequently than is usually done. We have been decidedly spoiled by the phylogenetic series of quiet successive development, such as we meet with in the Oligocene and Miocene of North America in the titanotheres, oreodonts, camels, etc., and in the upper Eocene of Europe in Palæotherium, Paloplotherium, etc., as well as from the Oligocene into the Pleistocene, e.g., in the rhinoceroses, cervids, suillines, amphicyonids. Even here we often make for ourselves artificial difficulties by balancing, with an exaggerated scrupulousness, the individual forms one against another, to see whether they really are exactly fitted to fill up any gaps. It is not the lack of suitable intermediate forms which so often renders difficult the establishment of genetic series, but, quite on the contrary, the abundance of the forms at our disposal, among which we must make a choice. If, however, the development of phyla did not take place in the same region and under constant climatic and topographical conditions, we must necessarily find apparent gaps, for adaptation to a new environment occasions rapid changes of organization, so that the immediate descendant will often deviate considerably from its ancestor. But that must not mislead us into denying the connection between such forms.”[22]

Better adapted to a solution of this problem than mammals are the fossil shells of Mollusca, the development of which may often be traced through a thick series of strata, each step of modification being represented by innumerable individuals. In very many instances it appears that each species in a series of successive modifications had many contemporary fluctuating variations, but the change from one species to the next succeeding one was by a small though abrupt mutation. The difference between two successive species may be no greater than that between two contemporary variants of the same species, but it was a constant and not a fluctuating difference. There is much reason to believe that such is at least a frequent mode of development, namely, that from species to species and genus to genus the transition has been by slight and sudden changes. The possibility that such abrupt changes, however slight, are illusory and due to small gaps in the record, must be admitted, and though this does not seem to be a very likely explanation, it is given plausibility by the almost perfect continuity between successive species which may sometimes be observed.

The extremely important and significant distinction between contemporary, fluctuating variations and successive, constant mutations was first drawn by Waagen, who says of them: “One must therefore distinguish strictly between varieties in space and those in time. To describe the former, the long-used name ‘variety’ will suffice, for the latter, on the other hand, I would propose, for the sake of brevity, a new term, ‘mutation.’ A species as such, with reference to its connection with earlier or later forms, may be conceived and regarded as a mutation. But also in regard to the value of these two concepts, just established (variety and mutation), an entirely different value is displayed on closer consideration. While the former appears extremely vacillating, of small systematic value, the latter, even though in minute characteristics, is extremely constant and always to be recognized with certainty.”[23]

The same conception was adopted and elaborated by Neumayr: “Still other characteristics appear, which mark mutations as something different from varieties, especially that, as a rule, there is a definite direction of mutation in each series, the same characters changing in the same sense through a considerable succession of strata.”[24]

Whether development was continuous or discontinuous, there is no reason to suppose that the amount and rate of modification were always constant. On the contrary, there is strong evidence that at times of great climatic or geographical changes, or when a region was invaded by a horde of immigrants, widespread readjustments were accomplished with comparative rapidity. Indeed, such periods of relatively quick changes have long seemed to be implied by the facts of the palæontological records.

It is only too clear that the principles as to the modes of mammalian development which can be deduced from the history of the various groups must, for the most part, be stated in a cautious and tentative manner, so as not to give an undue appearance of certainty to preliminary conclusions, which should be held as subject to revision with the advance of knowledge. Much has, however, been already learned, and there is every reason to hope that Experimental Zoölogy and Palæontology, by combining their resources, will eventually shed full light upon a subject of such exceptional difficulty.