Page:Popular Science Monthly Volume 77.djvu/483

Rh an approximation to it. The closeness of the approximation would be largely measured by the nearness and accessibility of the region in question to the center of dispersal of the race.

6. If the evolution at the center of dispersal was sharply discontinuous this discontinuity would be merely emphasised elsewhere. If on the other hand it was continuous, we should get a near approach to continuity in a complete evolutionary series from a region not remote from the center of diffusion of the race, while the evolutionary series from the same region, of a race whose center of dispersal was remote, would be sharply discontinuous.

7. Applying these principles to some of our American Tertiary phyla, we find that certain phyla which we can be sure were of North American origin, such as the camels, oreodonts and peccaries, do present a much nearer approach to continuity of development than do other phyla which we can be sure were of old world origin, such as the deer, the antelopes or the proboscideans.

I assume that since the oreodonts and peccaries never reached the old world, and the camels did not reach it till the Pliocene, their centers of dispersal were well to the south of the Bering Sea connection with the old world. I assume that since the horses are represented by a double evolutionary series, one in Europe, a closer one in North America, their center of dispersal lay far enough north to spread into Europe on one hand, North America on the other, but that the latter was nearer or more accessible, i. e. their center of dispersal was northeastern Asia or Alaska. On similar grounds the center of dispersal of most of the Tertiary ruminants might be located in northwest Asia, of proboscideans in central Asia, of tapirs in northeastern Asia, of rhinoceroses northeast Asia and Alaska, of dogs in northwest Canada, and so on—a series of indefinite guesses which a careful study of the present geographic distribution, with these principles and the imperfect geologic data in mind, might serve to fix more definitely.

The point at present to be considered is that in such series as the camels, oreodonts and peccaries, we do have a sufficiently close approach to a continuous series to warrant our believing that the true process of their evolution in the center of their dispersal was a gradual one as regards the evolution of genera and higher groups, but for aught that paleontology tells to the contrary, it may have been partly, though not wholly, discontinuous and saltatory so far as the evolution of new species is concerned. But the larger and more complete the series of specimens studied, the more perfect the record in successive strata, and the nearer is the hypothetic center of dispersal of the race, the closer do we come to a phyletic series whose intergrading stages are well within the limits of observed individual variation in the race. The known facts in vertebrate paleontology are, in my opinion, utterly inadequate