Individual differences are of the highest importance for us, for they are often inherited; they thus afford materials for natural selection to act on . . . . It is the most flourishing species of the larger genera which on the average yield the greatest number of varieties, and varieties, as we shall hereafter see, tend to be converted into new and distinct species. Thus, the larger genera tend to become larger; and throughout nature the forms of life which are now dominant tend to become still more dominant by leaving many more modified and dominant descendants. . .