| THE PALEONTOLOGIC RECORD |
By Professor S. W. WILLISTON
THE UNIVERSITY OF CHICAGO
VARIOUS writers, from Le Conte to Smith Woodward, have spoken of critical or rhythmical periods in evolution, periods when evolutionary forces have acted more vigorously than at others, with intervals of relative quiescence. What these forces are and have been we are not yet sure, whether extrinsic, that is, environmental or Lamarckian, or intrinsic, that is, orthogenetic, teleological or what not. Perhaps we shall sometime be more certain of the basal causes of evolution, for the paleontologist at least is not satisfied with the crass ignorance of our Weismannian friends who impute the beginning of all things to mere chance. Perhaps when we do know these fundamental causes we shall understand better why evolution has been rhythmical, if such was really the case, as some of us believe with Woodward.
But, whether there have been internal forces which have had chiefly to do with the rhythm of evolution, or whether such critical periods in the evolution of organic life have been due solely to the larger cosmic forces, I think we shall all admit that there have been critical places of organic evolution, places upon the earth where evolution has advanced with more rapid pace than in others, places perhaps where environmental conditions have conspired to hasten the development of life, or of particular groups, classes or kingdoms of life.
Such a critical period, at least for the higher organisms, it seems to me, was the early Pliocene; such a critical place was central Asia; and both together resulted in the birth of man.
It is a curious fact that nearly all our domestic animals had their origin in Asia. It is also a curious fact that the domestic animals are, almost without exception, the crowning ends of their respective lines of descent, the most highly specialized of their kinds. The genus Bos, the most highly developed of the even-toed ungulates began, to the best of our present knowledge, in the Lower Pliocene of India. And its four distinctive types likewise first appeared there: the Bubalus group, including the domestic buffalo of India, and its untamable kin of Africa; the group that is represented by the domesticated humped oxen of India and their wild relatives of Africa; the bison strain which spread in Pleistocene times almost to the remote corners of the earth;
