332 Darivin, and after Darwin. 



birds," appears to indicate that he had considered the question 

 of utility with special reference to Mr. Wallace's opinion. 

 That opinion was then, as now, the avowed result of a theo- 

 retical prepossession ; and this prepossession, as the above 

 quotations sufficiently show, was expressly repudiated by 

 Darwin. 



Lastly, this is not the only occasion on which Darwin 

 expressly repudiates Mr. Wallace's opinion on the point 

 in question. For it is notorious that these co-authors of 

 the theory of natural selection have expressed diveri^^ent 

 opinions concerning the origin by natural selection of the 

 most general of all specific characters — cross-sterility. 

 Although allowing that cross-sterility between allied species 

 may be of adaptive value in " keeping incipient species from 

 blending," Darwin persistently refused to be influenced by 

 Wallace's belief that it is due to natural selection ; i. e. the 

 belief on which alone can be founded the " necessary de- 

 duction " with which we have been throughout concerned. 



