Opinions on Isolation. 119 



Delbceuf, or the cumulative nature of divergence of 

 type when once such divergence begins in a separated 

 section. Therefore, in this all-important and, indeed, 

 essential respect, amixia differs entirely from the 

 principle which has been severally stated by Delbceuf, 

 Gulick, and myself. 



Upon the whole, then, we must say that although 

 Professor Weismann was the first to recognize the 

 diversifying influence of merely indiscriminate isolation 

 per se (apogamy), he did so only in part. He failed 

 to distinguish the true essence of the principle, and by 

 overlaying it with a mass of hypothetical speculation, 

 concealed even more of it than he revealed, 



The general theory of Isolation, as independently 

 worked out by Mr. Gulick and myself, has already 

 been so fully explained, that it will here be sufficient 

 merely to enumerate its more distinguishing features. 

 These are, first, drawing the sharpest possible line 

 between evolution as monotypic and polytypic ; 

 second, showing that while for the former the peculiar 

 kind of isolation which is presented by natural 

 selection suffices of itself to transform a specific type, 

 in order to work for the latter, or to branch a specific 

 type, natural selection must necessarily be assisted by 

 some other kind of isolation ; third, that even in the 

 absence of natural selection, other kinds of isolation 

 may be sufficient to effect specific divergence through 

 independent generation alone ; fourth, that, neverthe- 

 less, natural selection, where present, will always 

 accelerate the process of divergence ; fifth, that 

 monotypic evolution by natural selection depends 

 upon the presence of intercrossing, quite as much as 



