Isolation. 31 



the originating cause of the diversity that is afterwards 

 presented to natural selection for further intensification. 

 To give only one example. At the starting-point of 

 the whole discussion the priority of such other forms 

 of homogamy is assumed in the following words : 



But how, it may be asked, can any analogous principle 

 [to that of diversity caused by artificial selection] apply in 

 nature? I believe it can and does apply most efficiently 

 (though it was a long time before I saw how), from the 

 simple circumstance that the more diversified the descendants 

 from any one species become in structure, constitution, and 

 habits, by so much will they be better enabled to seize on 

 many and widely diversified places in the polity of nature, 

 and so be enabled to increase in numbers. 



Now, without question, so soon as segregate 

 breeding in two or more lines of homogamy has been 

 in any sufficient degree determined by some " change 

 of structure, constitution, or habits " natural selection 

 will forthwith proceed to increase the divergence in 

 as many different lines as there are thus yielded dis- 

 criminately isolated sections of the species. And this 

 fact it must have been that Darwin really had before 

 his mind when he argued that diversification of char- 

 acter is caused by natural selection, through the benefit 

 gained by the diversified forms being thus " enabled 

 to increase in number." Nevertheless he does not ex- 

 pressly state the essential point, that although diversi- 

 fication of character, when once begun^ is thus promoted 

 by natural selection, which forthwith proceeds to cul- 

 tivate each of the resulting branches, yet diversifica- 

 tion of character can never be originated by natural 

 selection. The change of "structure," of "constitution," 

 of " habits," of " station," of geographical area, of red- 



