

THE PALEONTOLOGIC RECORD 79 



right whatsoever to make either of these assumptions. And without 

 them the argument from paleontology for discontinuous development 

 is almost or quite worthless. 



If we consider the general conditions controlling evolution and 

 migration among land mammals, it will be evident, I think, that 



1. The external conditions favoring the evolution and progress of a 

 given phylum will not be uniformly developed all over the world or 

 all over one continent, but will appear first, and be at all times more 

 advanced, in some circumscribed region in one or another continent, or 

 simultaneously in limited areas of two or more continents, similarly 

 situated as to climate, temperature, etc. 



2. The animal best able to take advantage of these conditions will 

 be existing at the time (a) in one continent or (&) in more than one, 

 or (c) different animals in different continents may be equally able to 

 adapt themselves to the new conditions. 



3. As a result, the new stages of any progressive race will first appear 

 in a limited area and will spread out from that region as the favoring 

 environment spreads, the race at the same time continuing its progress 

 further within that area. This area will be the center of dispersal of 

 the race. Its location will be conditioned by two factors, the early 

 appearance of the new environmental conditions, and the existence of 

 species most able to take advantage of these conditions. Parallelism 

 and convergence in racial evolution will be conditioned by 2b and 2c 



4. Progressive change from uniformly warm to zonal climates dur- 

 ing the Tertiary must needs have been a great factor in controlling the 

 progress and distribution of Tertiary mammals. As the new conditions 

 appeared first at the poles, the chief centers of dispersal of the animals 

 adapted to them must have been in the northern parts of one or another 

 of the great northern continents. 2 The exact location of the dispersal 

 center for each race would be variously decided by the complex of 

 environmental and faunal relations of each, and might be shifted from 

 time to time by changes in these relations. 



5. In the regions distant from the center of dispersal the geological 

 record, if complete, should show the successive appearance of progres- 

 sively higher types in a phylum, arriving in successive waves of migra- 

 tion, and each new type suddenly or gradually displacing the previous 

 stages. Whether the evolution of a race at its center of diffusion was 

 continuous or discontinuous, the geological record of its progress pre- 

 served in any other region would be apparently that of a discontinuous 

 development. It would be not the actual history of its evolution but 



2 To a minor extent in the southern parts of the southern continents, whose 

 restricted area and isolation prevailed in the writer's opinion throughout the 

 Tertiary. There is some evidence, however, along the lines indicated in para- 

 graphs 5 and 6, that Patagonia was the chief center of dispersal of South 

 American Tertiary mammals. 



