io Darwin, and after Darwin. 



" artificial selection " consists in this that under the 

 former process the excluded individuals must neces- 

 sarily perish, while under the latter they need not do 

 so. But clearly this difference is accidental : it is in 

 no way essential to the process considered as a process 

 of discriminate isolation. For, as far as homogamous 

 breeding is concerned, it can matter nothing whether 

 the exclusion of the dissimilar individuals is effected 

 by separation or by death. 



Natural selection, then, is thus unquestionably 

 a form of isolation of the discriminate kind ; and 

 therefore, notwithstanding its unique importance in 

 certain respects, considered as a principle of organic 

 evolution it is less fundamental and also less ex- 

 tensive than the principle of isolation in general. In 

 other words, it is but a part of a much larger whole. 

 It is but a particular form of a general principle, 

 which, as just shown, presents many other forms, not 

 only of the discriminate, but likewise of the indiscri- 

 minate kind. Or, reverting to the terminology of 

 logic, it is a sub-species of the species Homogamy, 

 which in its turn is but a constituent part of the 

 genus Isolation. 



So much then for homogamy, or isolation of the 

 discriminate order. Passing on now to apogamy, or 

 isolation of the indiscriminate kind, we may well be 

 disposed, at first sight, to conclude that this kind of 

 isolation can count for nothing in the process of evo- 

 lution. For if the fundamental importance of isola- 

 tion in the production of organic forms be due to its 

 segregation of like with like, does it not follow 

 that any form of isolation which is indiscriminate 

 must fail to supply the very condition on which all 



