74 Darwin, and after Darwin. 



common area, but for the sexual isolation with which 

 every distinct set of them is now found to be asso- 

 ciated. 



Evidence from Topographical Distribution of 

 Species. 



By topographical distribution I mean the distri- 

 bution of organisms with reference to comparatively 

 small areas, as distinguished from larger regions with 

 reference to which the term geographical distribution 

 is appropriate. 



It will be at once apparent that a study of the 

 topographical distribution of organic types is of even 

 more importance for us than a study of their geogra- 

 phical distribution. For while the former study is 

 conducted, as it were, with a low power of our 

 observing microscope, the latter is conducted with 

 a high power. The larger facts of geographical 

 distribution yield, indeed, all the general characters 

 which we might expect them to yield, on the theory 

 that divergence of specific types on common areas 

 has been in chief part determined by physiological 

 conditions. But for the purpose of testing this 

 theory in a still more exacting manner, it is of the 

 first importance to consider the more detailed facts 

 of topographical distribution, since we here come to 

 closer quarters with the problem of specific differen- 

 tiation. Therefore, as we have already considered 

 this problem under the most general points of view, 

 we will now consider it under more special points 

 of view. 



It is self-evident, as we have seen in the preceding 



