86 ISOLATION AS A FACTOR 



neither useful nor injurious. Unless we use 

 " Natural Selection " to cover both processes, as 

 Darwin certainly would have done, we must as- 

 sign to selection the preservation and intensifi- 

 cation of adaptive characters, and to segregation 

 the seizing and fixing of the non-useful, usually 

 fluctuating, element. It is, however, a fact well 

 knoAvn to breeders that these indifferent or non- 

 useful characters are often or generally more per- 

 sistent in heredity than the traits which are 

 plainly adaptive. The slight traits which mark the 

 races of men are in themselves, often not ob- 

 viously, valuable in the struggle for existence. 

 They are mostly ineradicable in such selective 

 breeding as history offers. In like manner the 

 dusky face, and other marks of Hampshire sheep, 

 persist after the adaptive traits of the original 

 breed have been enormously modified by selection 

 in the direction we regard as sheep improvement. 

 But fine or coarse, fat or lean, Hampshire sheep 

 are still Hampshires. 



In Darwin's view, isolation or segregation was 

 doubtless a feature of Natural Selection, not to 

 be set off against the latter as a separate factor 

 in descent. It is very plain from Darwin's own 

 words, as well as from the explicit statement of 

 Francis Darwin, that his main contention was for 

 the reasonableness of the idea of the origin of spe- 

 cies through descent with modification. What 

 were the causes of modification, was to him a sec- 

 ondary matter. But he was convinced of the 



