Isolation 



does not and cannot give rise to differentia- 

 tion and divergence, so long as interbreeding 

 with consequent interblending of characters be 

 freely permitted. Whence it inevitably follows, 

 as a matter of simple logic, that where diver- 

 gence has occurred, intercrossing and interbreed- 

 ing must in some way have been lessened or 

 prevented. 



Thus a new factor is introduced, that of 

 isolation or segregation. And there is no ques- 

 tioning 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." 



This is very sound criticism, and is not very 

 materially affected by the fact that the inter- 

 crossing of varieties does not necessarily imply 

 a blending of their characters in the offspring ; 

 for, as we have seen, some characters do not 

 blend. No matter what form inheritance takes, 

 in order that natural selection may cause poly- 

 typic evolution it must be assisted by isolation 

 in some form or other. 



Thus isolation is an important factor in evolu- 

 tion, though probably not so important as its 

 more extreme advocates would have us believe. 

 Wagner, Romanes, and Gulick have, in insisting 

 2 A 369 



