58 Darwin, and after Darwin. 



some degree of cross-infertility. But through this 

 association, the segregating influence of both the 

 causes concerned is, as Mr. Gulick has shown, greatly 

 increased. For instance, if the segregating influence 

 of some degree of cross-infertility be associated with 

 that of any othef form of isolation, then, not only 

 will the two segregating influences be added, but 

 multiplied together. And thus, by their mutual 

 action and reaction, divergent evolution is promoted 

 at a rapidly increasing rate. 



I will now summarize the main points of the theory 

 of physiological isolation in a categorical form. 



1. If no other form of isolation be present, specific 

 divergence can only take place when some degree of 

 cross-infertility has previously arisen between two or 

 more sections of a species. 



2. When such cross-infertility has arisen it may 

 cause specific divergence, either (a) by allowing in- 

 dependent variability in each of the physiologically 

 isolated groups ; (b) by becoming associated with any 

 other cause of differentiation already operating ; or 

 (c) by both these means combined. 



3. As some degree of cross-infertility generally 

 obtains between allied species, we are justified in 

 concluding that this has been the most frequent 

 or, at any rate, the most effective kind of isola- 

 tion where the origin of species is concerned ; and 

 therefore the kind with which, in the case of 

 species-formation, natural selection, or any other 

 cause of specific divergence, has been most usually 

 associated. 



4. Where varietal divergence has begun in the 



