The importance of isolation is twofold.
In the first place, it leads to a body of individuals of each species being limited in their range and thus subjected to uniform conditions for long spaces of time.

Both the direct action of the environment and the natural selection of such varieties only as are suited to the conditions, will, therefore, be able to produce their full effect.

In the second place, the process of change will not be interfered with by intercrossing with other individuals which are becoming adapted to somewhat different conditions in an adjacent area.

But this question of the swamping effects of intercrossing will be considered in another chapter.
Mr.Darwin was of opinion that, on the whole, the largeness of the area occupied by a species was of more importance than isolation, as a factor in the production of new species, and in this I quite agree with him.

It must, too, be remembered, that isolation will often be produced in a continuous area whenever a species becomes modified in accordance with varied conditions or diverging habits.