Opinions on Isolation. 129 



of isolation or segregation. And there is no questioning the 

 fact that it is of great importance. Its importance, indeed, can 

 only be denied by denying the swamping effects of intercrossing, 

 and such denial implies the tacit assumption that interbreeding 

 and interblending are held in check by some form of segregation. 

 The isolation explicitly denied is implicitly assumed '. 



Similarly, and still more recently, Professor 

 Le Conte writes : 



It is evident, then, as Romanes claims, that natural selection 

 alone tends to monotypic evolution. Isolation of some sort 

 seems necessary to polytypic evolution. The tree of evolution 

 under the influence of natural selection alone grows palm-like 

 from its terminal bud. Isolation was necessary to the starting 

 of lateral buds, and thus for the profuse ramification which is its 

 most conspicuous character*. 



In order to complete this historical review, it only 

 remains to consider Mr. Wallace's utterances upon the 

 subject. 



It is needless to say that he stoutly resists the 

 view of Weismann, Delboeuf, Gulick, and myself, that 

 specific divergence can ever be due or, as I under- 

 stand him, even so much as assisted by this prin- 

 ciple of indiscriminate isolation (apogamy). It will be 

 remembered, however, that Mr. Gulick has adduced 

 certain general principles and certain special facts 

 of geographical distribution, in order to prove that 

 apogamy eventually leads to divergence of character, 

 provided that the isolated section of the species does 

 not contain any very large number of individuals. 

 Now, Mr. Wallace, without making any reference to 

 this argument of Mr. Gulick, simply states the reverse 

 namely, that, as a matter of fact, indiscriminate 



1 Animal Life and Intelligence, pp. 98, 99 (1890-1891). 

 1 The Factors of Evolution (1891). 



nt. K 



