8o THE PALEONTOLOGIC RECORD 



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 discon- 

 tinuous this discontinuity would be merely emphasized 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 north- 

 eastern 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 rhi- 

 noceroses 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 speci- 

 mens 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 



