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VARIATION UNDER NATURE.
Variability--Individual differences--Doubtful species--Wide ranging, much diffused, and common species, vary most--Species of the larger genera in each country vary more frequently than the species of the smaller genera--Many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges.
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.