Discriminate Isolation 



kind of discriminate isolator, since it isolates the 

 fit by destroying all the unfit, and, inasmuch as 

 it kills off all those creatures which it fails to 

 isolate, it differs from other forms of isolation 

 in preventing the inter-breeding of the unisolated 

 forms and their giving rise to a different race. 

 Thus it is clear that natural selection, unless 

 aided by some other form of isolation, can give 

 effect to only monotypic evolution. This is a 

 point on which Romanes rightly insists strongly. 



There are several other forms of discriminate 

 isolation. Sexual selection would be one of 

 these. Suppose, for example, that in any 

 species there are large and small varieties 

 formed, and like tends to breed with like, then 

 the small individuals will breed with other 

 small individuals, while large ones will mate 

 with large ones ; thus two races a large one 

 and a small one will be evolved side by side, 

 provided, of course, natural selection does not 

 step in and destroy one of them. 



Another kind of discriminate isolation may be 

 due to the fact that one variety is ready to pair 

 before the other ; thus two races are likely to 

 arise which breed at different seasons. It is un- 

 necessary for us to discourse further on the 

 subject of discriminate isolation ; those interested 

 in the subject should read vol. iii. of Darwin 

 and after Darwin, by Romanes. 



It is impossible to deny the importance of 



