A history of land mammals in the western hemisphere

by William Berryman Scott

The question of the origin of the Mammalia is still involved in great obscurity, and the most divergent opinions are held concerning it. It remains an unsolved problem whether the mammals were all descended from a common stock, or have been derived from two independent lines of ancestry, or, in technical phrase, whether the class is monophyletic or diphyletic. Assuming, as seems most probable from present knowledge, that the mammals are monophyletic, the question next arises: From what lower vertebrates are they descended? A great controversial literature has grown up around this problem, one party regarding the Amphibia and the other the Reptilia as the parent group. The palæontological evidence, while not conclusive, is decidedly in favour of the latter view. In the Triassic of South Africa is found a group of reptiles which approximated the mammals very much more closely than do any other known representatives of the lower vertebrates. While it is not believed that any of these Triassic reptiles were directly ancestral to the mammals, they did, to a very great extent, bridge the gap between the two classes and show us what the reptilian ancestors of the mammals were probably like.

With perhaps the exception of certain Insectivora, the Paleocene faunas contained few, if any, ancestors of modern mammals. These originated in some region which has not been identified, but may be plausibly conjectured to be central Asia, whence they migrated westward to Europe and eastward to North America, reaching both of those continents in the lower Eocene. From that time onward they increased and multiplied, becoming more and more differentiated through divergent evolution, until the existing state of things was attained. From the lower Eocene we are on firm ground, and, though very much remains to be learned, much has already been accomplished in the way of tracing the history and development of many mammalian orders. It has been my endeavour in the body of this book to sketch the better established and more significant parts of this marvellous story.

CHAPTER XVIII
MODES OF MAMMALIAN EVOLUTION

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.