Page:Popular Science Monthly Volume 77.djvu/482

475 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 (b) 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 during 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 progressively higher types in a phylum, arriving in successive waves of migration, 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 preserved in any other region would be apparently that of a discontinuous development. It would be not the actual history of its evolution but