32 Darwin, and after Darwin. 



procal fertility, and so on this change, whatever it 

 may have been, must clearly have been antecedent to 

 any operation of natural selection through the benefit 

 which arose from the change. Therefore the change 

 must in all cases have been due, in the first instance, 

 to some other form of isolation than the superadded 

 form which afterwards arose from superior fitness 

 in the possession of superior benefit although, so 

 long as the prior form of isolation endured, or con- 

 tinued to furnish the necessary condition to the co- 

 operation of survival of the fittest, survival of the 

 fittest would have continued to increase the divergence 

 of character in as many ramifying lines as there were 

 thus given to its action separate cases of isolation 

 by other means. 



In short, as divergence of character must in all cases 

 be due to a prevention of intercrossing, and as in the 

 process of natural selection there is, ex hypothesi, 

 nothing to prevent the intercrossing until the diver- 

 gence has already arisen, to suppose that natural 

 selection alone can have caused the divergence, is to 

 suppose that natural selection can have caused the 

 conditions of its own activity, which is absurd. 



Seeing, then, that even in cases where any" benefit" 

 arises from divergence of character, such benefit can 

 arise only after the divergence has already commenced, 

 and seeing that on this as on other accounts previously 

 mentioned it is plainly impossible to attribute the 

 origin of such divergence to natural selection, we find 

 that natural selection must be in all cases assisted 

 by some other form of isolation, if it is to be con- 

 cerned in polytypic as distinguished from monotypic 

 evolution. But this does not hinder that, when it 



