We have seen that in each country it is the species belonging to the larger genera which oftenest present varieties or incipient species.

This, indeed, might have been expected; for as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common.

Hence, the struggle for the production of new and modified descendants will mainly lie between the larger groups, which are all trying to increase in number.

One large group will slowly conquer another large group, reduce its number, and thus lessen its chance of further variation and improvement.