Isolation. 9 



an immensely important part in the differentiation 

 of varieties into species. But meanwhile we have 

 only to observe that wherever such incompatibility is 

 concerned, it is to be regarded as an isolating agency 

 of the very first importance. And as it is of a 

 character purely physiological, I have assigned to it 

 the name Physiological Isolation ; while for the par- 

 ticular case where this general principle is concerned 

 in the origination of specific types, I have reserved 

 the name Physiological Selection. 



The other most important form of discriminate 

 isolation to which I have alluded is Natural Selection. 

 To some evolutionists it has seemed paradoxical 

 thus to regard natural selection as a form of isola- 

 tion ; but a little thought will suffice to show that 

 such is really the most accurate way of regarding it. 

 For, as Mr. Gulick says, " Natural selection is the 

 exclusive breeding of those better adapted to the en- 

 vironment : ... it is a process in which the fittest are 

 prevented from crossing with the less fitted, by the 

 exclusion of the less fitted." Therefore it is, strictly 

 and accurately, a mode of isolation, where the 

 isolation has reference to adaptation, and is secured 

 in the most effectual of possible ways — i.e. by the 

 destruction of all individuals whose intercrossing would 

 interfere with the isolation. Indeed, the very term 

 " natural selection " shows that the principle is tacitly 

 understood to be one of isolation, because this name 

 was assigned to the principle by Darwin for the 

 express purpose of marking the analogy that obtains 

 between it and the intentional isolation which is 

 practised by breeders, fanciers, and horticulturists. 

 The only difference between " natural selection " and 



