Evidences of Physiological Selection. 73 



those on either side of it by the minute and trivial 

 distinctions of a secondary kind, but all the links 

 of which differ from one another in respect of the 

 primary distinction, so that no -one member of the 

 series is perfectly fertile with any other member. Can 

 it be supposed that in every case this constant 

 primary distinction has been superinduced by the 

 secondary distinctions, distributed as they are over 

 different parts of all these kindred organisms, and 

 yet nowhere presenting any but a trifling amount of 

 morphological change ? 



For my own part, I cannot believe any more 

 than Darwin could believe that all these numerous, 

 diverse, and trivial changes have always had the 

 accidental effect of inducing the same peculiar change 

 in the reproductive system, and so producing it with- 

 out any reference to the process of specific divergence. 

 Nor can I believe, as Darwin incidentally and pro- 

 visionally suggested, that prolonged exposure to 

 uniform conditions of life have so generally induced 

 an equally meaningless result. I can only believe 

 that all the closely allied species inhabiting our 

 supposed continent, and differing from one anothor 

 in so many and such divers points of small detail, are 

 merely so many records of the fact that selective 

 fertility has arisen among their ancestry, and has 

 thus given as many opportunities for the occurrence 

 of morphological differentiations as it has furnished 

 cases of efficient isolation. Of course, I do not deny 

 that many, or probably most, of these trivial morpho- 

 logical differentiations have been produced by natural 

 selection on account of their utility : I merely deny 

 that they could have been so produced on this 



