If, in the diagram, we suppose the amount of change represented by each successive group of diverging dotted lines to be great, the forms marked a14 to p14, those marked b14 and f14, and those marked o14 to m14, will form three very distinct genera.

We shall also have two very distinct genera descended from (I), differing widely from the descendants of (A).

These two groups of genera will thus form two distinct families, or orders, according to the amount of divergent modification supposed to be represented in the diagram.

And the two new families, or orders, are descended from two species of the original genus; and these are supposed to be descended from some still more ancient and unknown form.
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.