150 DARWINISM chap. 



the isolated portion of the species. The struggle for existence 

 will differ in its severity and in its incidence from that which 

 affects the bulk of the species. The absence of some one 

 insect or other creature inimical to the young animal or plant 

 may cause a vast difference in its conditions of existence, and 

 may necessitate a modification of its external or internal 

 characters in quite a different direction from that which 

 happened to be present in the average of the individuals 

 which were first isolated. 



On the whole, then, we conclude that, while isolation is an 

 important factor in effecting some modification of species, it is 

 so, not on account of any effect produced, or influence exerted 

 by isolation per se, but because it is always and necessarily 

 accompanied by a change of environment, both physical and 

 biological. Natural selection will then begin to act in 

 adapting the isolated portion to its new conditions, and will 

 do this the more quickly and the more effectually because of 

 the isolation. We have, however, seen reason to believe that 

 geographical or local isolation is by no means essential to the 

 differentiation of species, because the same result is brought 

 about by the incipient species acquiring different habits or 

 frequenting a different station ; and also by the fact that 

 different varieties of the same species are known to prefer to 

 pair with their like, and thus to bring about a physiological 

 isolation of the most effective kind. This part of the subject 

 will be again referred to when the very difficult problems 

 presented by hybridity are discussed. 1 



Cases in which Isolation is Ineffective. 



One objection to the views of those who, like Mr. Gulick, 

 believe isolation itself to be a cause of modification of species 

 deserves attention, namely, the entire absence of change where, 



1 In Mr. Gulick's last paper (Journal of Linn. Soc. Zool., vol. xx. pp. 189- 

 274) he discusses the various forms of isolation above referred to, under no 

 less than thirty-eight different divisions and subdivisions, with an elaborate 

 terminology, and he argues that these will frequently bring about divergent 

 evolution without any change in the environment or any action of natural 

 selection. The discussion of the problem here given will, I believe, sufficiently 

 expose the fallacy of his contention ; but his illustration of the varied and 

 often recondite modes by which practical isolation may be brought .about, 

 may help to remove one of the popular difficulties in the way of the action 

 of natural selection in the origination of species. 



