CHAPTER V. 



FURTHER EVIDENCES OF PHYSIOLOGICAL 

 SELECTION. 



Evidence from Topographical Distribution of 

 Varieties. 



IN the last section we have considered the topo- 

 graphical distribution of closely allied species. I now 

 propose to go still further into matters of detail, by 

 considering the case of natural varieties. And here 

 we come upon a branch of our inquiry where we may 

 well expect to meet with the most crucial tests of 

 our theory. For if it should appear that these nascent 

 species more or less resemble fully developed species 

 in presenting the feature of cross-infertility, the theory 

 would be verified in the most direct and conclusive 

 manner possible. These nascent species may be 

 called embryo species, which are actually in course 

 of differentiation from their parent-type ; and there- 

 fore, if they do not exhibit the feature in relation 

 to that type which the present theory infers to be 

 necessary for the purposes of differentiation, the 

 theory must be abandoned. On the other hand, if 

 they do exhibit this feature, it is just the feature 

 which the theory predicted as one that would be 

 found highly characteristic of such embryo types. 



