Opinions on Isolation. 109 



not be " indispensable." Had he gone a step further, 

 he must have seen that separation, in some form 

 or another, is " indispensable " to poly ty pic evolution. 

 Instead of taking this further step, however, two years 

 later he wrote to Semper as follows : 



I went as far as I could, perhaps too far, in agreement with 

 Wagner [i. e. in the last edition of the Origin of Species} ; since 

 that time I have seen no reason to change my mind ; but then 

 I must add that my attention has been absorbed on other 

 subjects *. 



And he seems to have ended by still failing to 

 perceive that the explanation which he gives of 

 " divergence of character" in the Origin of Species, 

 can only hold on the unexpressed assumption that 

 free intercrossing is in some way prevented at the 

 commencement, and throughout the development, of 

 each diverging type. 



Lastly, we have to consider Darwin's opinion touching 

 the important principle of " Independent Variability." 

 This, it will be remembered, is the principle which 

 ensures that when a portion (not too large) of a 

 species is prevented from interbreeding with the rest 

 of the species, sooner or later a divergence of type 

 will result, owing to the fact that the average qualities 

 of the separated portion at the time of its separation 

 cannot have been exactly the same as the average 

 qualities of the specific type as a whole. Thus the 

 state of Amixia, being a state of what Mr. Gulick 

 calls Independent Generation, will of itself i. e. even 

 if unassisted by natural selection induce divergence 

 of type, in a ratio that has been mathematically 

 calculated by Delboeuf. 



1 Life and Letters, vol. iii. p. 159. 



