Isolation. 29 



other two principles be granted, the whole theory 

 of descent resolves itself into an inquiry touching the 

 causes, forms, and degrees of Homogamy. 



Save in cases where very large populations are 

 concerned, apogamy must sooner or later give rise 

 per se to homogamy, owing to the Law of Delbceuf 

 which is the principle that I have called Indepen- 

 dent Variability, and Gulick has called Independent 

 Generation. But of course this does not hinder that 

 under apogamy various other causes of homogamy 

 are likely to arise — in particular natural selection. 



That natural selection differs from most of the other 

 forms of isolation in not being capable of causing 

 divergent or polytypic evolution must at once become 

 evident, if we remember that the only way in which 

 isolation of any form can cause such evolution is by 

 partitioning a given group of intergenerants into two 

 or more groups, each of which is able to survive as 

 thus separated from the other, and so to carry on the 

 evolution in divergent lines. But the distinguishing 

 peculiarity of natural selection, considered as a form 

 of isolation, is that it effects the isolation by killing 

 off all the individuals zvhich it fails to isolate : con- 

 sequently, this form of isolation differs from other 

 forms in prohibiting the possibility of any ramification 

 of a single group of intergenerants into two or more 

 groups, for the purpose of carrying on the evolution 

 in divergent lines. Therefore, under this form of 

 isolation alone, evolution must proceed, palm-like, in 

 a single line of growth. So to speak, the successive 

 generations continuously ascend to higher things on 

 the steps supplied by their own " dead selves " ; but 

 in doing so they must climb a single ladder, no 



