We have, also, seen that the species, which are the commonest and most widely-diffused, vary more than do the rare and restricted species.

Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country.

The branching and diverging dotted lines of unequal lengths proceeding from (A), may represent its varying offspring.

The variations are supposed to be extremely slight, but of the most diversified nature; they are not supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed to endure for equal periods.

Only those variations which are in some way profitable will be preserved or naturally selected.