476 THE POPULAR SCIENCE MONTHLY 
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 eae: 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 (b) in more than one, 
or (c) different animals in different continents may be equally able to 
adapt themselves to the new conditions. 
3. Asa 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.? 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 
2To 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. 
