The proportional numbers of its inhabitants will almost immediately undergo a change, and some species will probably become extinct.

We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independently of the change of climate itself, would seriously affect the others.

If the country were open on its borders, new forms would certainly immigrate, and this would likewise seriously disturb the relations of some of the former inhabitants.

Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be.