In large genera the species are apt to be closely but unequally allied together, forming little clusters round other species.

Species very closely allied to other species apparently have restricted ranges.

In all these respects the species of large genera present a strong analogy with varieties.

And we can clearly understand these analogies, if species once existed as varieties, and thus originated; whereas, these analogies are utterly inexplicable if species are independent creations.
We have also seen that it is the most flourishing or dominant species of the larger genera within each class which on an average yield the greatest number of varieties, and varieties, as we shall hereafter see, tend to become 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 modified and dominant descendants.