Before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must briefly discuss whether these latter are subject to any variation.

To treat this subject properly, a long catalogue of dry facts ought to be given; but these I shall reserve for a future work.

Nor shall I here discuss the various definitions which have been given of the term species.

No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.