30 Darwin, and after Darwin. 



rung of which can be allowed to bifurcate in the 

 presence of the uniformity secured for that generation 

 by the free intercrossing of the most fit. Even 

 though beneficial variations may arise in two or more 

 directions simultaneously, and all be simultaneously 

 selected by survival of the fittest, the effect of free 

 intercrossing (in the absence of any other form of 

 isolation) will be to fuse all these beneficial variations 

 into one common type, and so to end in monotypic 

 evolution as before. In order to secure polytypic 

 evolution, intercrossing between the different bene- 

 ficial variants which may arise must be prevented ; 

 and there is nothing to prevent such intercrossing in 

 the process of natural selection per se. In order that 

 the original group of intergenerants should be divided 

 and sub-divided into two or more groups of inter- 

 generants, some additional form of isolation must 

 necessarily supervene — when, of course, polytypic 

 evolution will i-esult. And, as Mr. Gulick has shown, 

 the conclusion thus established by deductive reason- 

 ing is verified inductively by the facts of geographical 

 distribution. 



How, then, are we to account for the fact that 

 Darwin attributed to natural selection the power to 

 cause divergence of character? The answer is suffi- 

 ciently simple. He does so by tacitly involei^."^ the aid 

 of some otJier form of homogamy in every case. If we 

 carelully read pp. 86-97 of the Origin of Species, where 

 this subject is under consideration^ we shall find that 

 in every one of the arguments and illustrations which 

 are adduced to prove the power cf natural selection to 

 eftect ■' divergence of character," he either pre-supposes 

 or actually names some other form of homogamy as 



