And the two new families, or orders,
will have descended from two species of the original genus; and these
two species are supposed to have descended from one species of a still
more ancient and unknown genus.
We have seen that in each country it is the species of 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 numbers, and thus lessen its chance of
further variation and improvement. Within the same large group, the
later and more highly perfected sub-groups, from branching out and
seizing on many new places in the polity of Nature, will constantly
tend to supplant and destroy the earlier and less improved sub-groups.
Small and broken groups and sub-groups will finally tend to disappear.
Looking to the future, we can predict that the groups of organic
beings which are now large and triumphant, and which are least broken
up, that is, which as yet have suffered least extinction, will for a
long period continue to increase.
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