ISOLATION 159 



the same way, then it is easy to see that at some point in their 

 range the breeding seasons might coincide. Or again, two 

 species with different habitat preferences might be brought 

 into close proximity in certain areas where only an intermediate 

 type of habitat was available. We are in great need of accurate 

 analyses of actual concrete examples. 



The most important conclusion in relation to our more 

 general argument as to the course of evolution is that, in so 

 far as the isolation of species from one another depends on the 

 combined effect of several agencies, it is likely that the same 

 agencies produce some degree of isolation between populations 

 within the species. The likelihood that species are much 

 broken up into populations which are to a considerable extent 

 isolated from one another must be fully allowed for in any 

 theory as to the spread of variants. 



