aracter and
from inheritance from a different parent, will differ widely from the three
genera descended from (A), the two little groups of genera will form two
distinct families, or even orders, according to the amount of divergent
modification supposed to be represented in the diagram. 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 {126} 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 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. But
which groups will ultimately prevail, no man can predict; for we well know
that many groups, formerly most extensively developed, have now become
extinct. Looking still more remotely to the future, we may predict that,
owing to the continued and steady increase of the larger groups, a
multitude of smaller groups will become utterly extinct, and leave no
modified descendants; and consequently that of the species living at any
one period, extremely few will transmit descendants to a remote futurity. I
shall have to return to this subject in the chapter on Classification, but
I may add that on this view of extremely few of the more ancient sp