SELECTION 161 



changes which have taken place in domesticated 

 animals, nor their exposure to greatly changed con- 

 ditions of life owing to man having taken them to 

 different parts of the world as neither of these 

 things have produced infertility, Darwin had to add 

 that, to bring it about, a very long time was required, 

 together with exposure to uniform conditions. But 

 these suggestions, which rest on no evidence, are 

 quite insufficient to account for so important and so 

 wide-spread a phenomenon ; and they imply that 

 all incipient species or varieties are quite fertile 

 among themselves, a conclusion which Darwin him- 

 self says is not correct. 



In the later editions of the ' ' Origin of Species ' ' 

 he is still more explicit. He says : " After mature 

 reflection it seems to me that this (sterility) could 

 not have been effected through natural selection;" 

 and, after giving some illustrations, he adds : " But 

 it would be superfluous to discuss this question in 

 detail ; for with plants we have conclusive evidence 

 that the sterility of crossed species must be due to 

 some principle, quite independent of natural selec- 

 tion, and, from the laws governing the various grades 

 of sterility being so uniform throughout the animal 

 and vegetable kingdoms, we may infer that the 

 cause, whatever it may be, is the same or nearly 

 the same in all cases." 47 



Preferential Selection. Under this name I in- 

 clude all those cases where selection is a voluntary 

 act on the part of the selector. By its means 



""Origin of Species," 6th edition, p. 248. See also 

 " Life and Letters," vol. iii., p. 80. 



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