io8 Darwin, and after Darwin. 



is the very means whereby natural selection is in 

 this case enabled to operate; or, on the other 

 hand, that, in the case of polytypic evolution, the 

 " difficulty '' in question is so absolute as to render 

 such evolution, by natural selection alone, absolutely 

 impossible. Hence, although in one sentence of the 

 Origin of Species he mentions three forms of isolation 

 (besides the geographical form) as serving in some 

 cases to assist natural selection in causing "diver- 

 gence of character" (i.e. polytypic evolution 1 ), on 

 account of not perceiving how great and how sharp 

 is the distinction between the two kinds or " cases " 

 of evolution, he never realized that, where "two or 

 more new species" are in course of differentiation. 

 some form of isolation other than natural selection 

 must necessarily be present, whether or not natural 

 selection be likewise so. The nearest approach which 

 he ever made to perceiving this necessity was in one 

 of his letters to Wagner above quoted, where, after 

 again appealing to the erroneous analogy between 

 monotypic evolution and " unconscious selection," he 

 says : " But I admit that by this process (i. e. un- 

 conscious selection) two or more new species could 

 hardly be formed within the same limited area : some 

 degree of separation, if not indispensable, would be 

 highly advantageous ; and here your facts and views 

 will be of great value." But even in this passage the 

 context shows that by "separation" he is thinking 

 exclusively of geographical separation, which he rightly 

 enough concludes (as against Wagner) need certainly 



1 Page 81. The three forms of isolation mentioned are, "from 

 haunting different stations, from breeding at slightly different seasons, or 

 from the individuals of each variety preferring to pair together." 



