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 sj^ecies. 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.,\ ol. 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. 
