The objection as to the "right" or "beneficial" variation occurring when required, seems therefore to have no weight in view of the actual facts of variation.
_Isolation to prevent Intercrossing._ Most writers on the subject consider the isolation of a portion of a species a very important factor in the formation of new species, while others maintain it to be absolutely essential.

This latter view has arisen from an exaggerated opinion as to the power of intercrossing to keep down any variety or incipient species, and merge it in the parent stock.

But it is evident that this can only occur with varieties which are not useful, or which, if useful, occur in very small numbers; and from this kind of variations it is clear that new species do not arise.
Complete isolation, as in an oceanic island, will no doubt enable natural selection to act more rapidly, for several reasons.

In the first place, the absence of competition will for some time allow the new immigrants to increase rapidly till they reach the limits of subsistence.

They will then struggle among themselves, and by survival of the fittest will quickly become adapted to the new conditions of their environment.