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.[50] _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, if this were a _vera causa_, we should expect to find it.