24 Darwin y and after Darwin, 



of the highest importance, as we shall immediately 

 perceive. 



In nearly all the other forms of isolation, polytypic 

 or divergent evolution may arise under the influence 

 of that form alone, or without the necessary co- 

 operation of any other form. This we have already 

 seen, for example, in regard to geographical isolation, 

 under which there may be as many different lines 

 of transmutation going on simultaneously as there 

 are different cases of isolation — say, in so many 

 different oceanic islands. Again, in regard to physio- 

 logical isolation the same remark obviously applies ; 

 for it is evident that even upon the same geographical 

 area there may be as many different lines of trans- 

 mutation going on simultaneously as there are cases 

 of this form of isolation. The bar of mutual sterility, 

 whenever and wherever it occurs, must always render 

 polytypic evolution possible. And so it is with almost 

 all the other forms of isolation: that is to say, one 

 form does not necessarily require the assistance of 

 another form in order to create an additional case 

 of isolation. But it is a peculiarity of natural selec- 

 tion, considered as a form of isolation, that it does 

 necessarily require the assistance of some other form 

 before it can give rise to an additional case of isola- 

 tion; and therefore before it can give rise to any 

 divergence of character in ramifying lines, as distin- 

 guished from transformation of characters in a single 

 line. Or, in other words, natural selection, when act- 

 ing alone, can never induce polytypic evolution, but 

 only monotypic. 



That this important conclusion is a necessary 

 deduction from the theory of natural selection itself, 



