Opinions on Isolation. 113 



is but one of the many ways whereby intercrossing 

 may be prevented, he failed to perceive that, even 

 as regards the case of polytypic evolution, he greatly 

 erred in representing this one form of isolation as 

 being universally a necessary condition to the process. 

 The necessary condition to this process is. indeed, the 

 prevention of intercrossing by some means or another ; 

 but his unfortunate insistence on geographical separa- 

 tion as the only possible means to this end especially 

 when coupled with his no less unfortunate disregard 

 of monotypic evolution caused him to hinder rather 

 than to advance a generalization which he had only 

 grasped in part. And this generalization is, as now 

 so repeatedly stated, that while the form of isolation 

 which we know as natural selection depends for its 

 action upon the intercrossing of all the individuals 

 which it isolates (i. e. selects), when acting alone 

 it can produce only monotypic evolution ; but that 

 when it is supplemented by any of the other 

 numerous forms of isolation, it is furnished with 

 the necessary condition to producing polytypic 

 evolution and this in as many lines of divergent 

 change as there may be cases of this efficient 

 separation. 



Nevertheless, while we must lament these short- 

 comings on the part of Wagner, we ought to re- 

 member that he rendered important services in the 

 way of calling attention to the swamping effects of 

 free intercrossing, and, still more, in that of showing 

 the high importance of geographical isolation as a 

 factor of organic evolution. Therefore, although in an 

 elaborate criticism of his views Weismann was easily 

 able to dispose of his generalizations in the imperfect 



in. i 



