Isolation. 25 



a very few words will be enough to show. For. 

 according to the theory, survival of the fittest is 

 a form of isolation which acts through utility, by 

 destroying 1 all the individuals whom it fails to isolate. 

 Hence it follows that survival of the fittest is a form 

 of isolation which, if acting alone, cannot possibly 

 effect divergent evolution. For, in the first place, 

 there is nothing in this form of isolation to ensure 

 that the fitter individuals should fail to interbreed 

 with the less fit which are able to survive ; and, in 

 the second place, in all cases where the less fit are 

 not sufficiently fit to be suffered to breed, they are 

 exterminated i.e. not permitted to form a distinct 

 variety of their own. If it be said that survival of 

 the fittest may develop simultaneously two or more 

 lines of useful change, the answer is that it can 

 only do this if each of the developing varieties is 

 isolated from the others by some additional form 

 of isolation ; for, if not, there can be no commence- 

 ment of utilitarian divergence^ since whatever number 

 of utilitarian changes may be in course of simul- 

 taneous development, they must in this case be all 

 blended together in a single line of specific trans- 

 mutation. Nay, even if specific divergence has 

 actually been commenced by natural selection when 

 associated with some other form of homogamy, if 

 the latter should afterwards be withdrawn, natural 

 selection would then be unable to maintain even so 

 much divergence of character as may already have 

 been attained : free intercrossing between the two 

 collateral, and no longer isolated branches, would 

 ensure their eventual blending into a common stock. 

 Therefore, I repeat, natural selection, when acting 



