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 
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