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.