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 
zsolation 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 
2A 369 
