Over all these causes of change, the accumulative action of selection, whether applied methodically and quickly, or unconsciously and slowly, but more efficiently, seems to have been the predominant power.

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.