re 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. One large group will slowly conquer another large group,
reduce its number, 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 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 have as yet suffered least
extinction, will, for a long period, continue to increase. But which
groups will ultimately prevail, no man can predict; for we 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 as, according to this view, extremely
few of the more ancient species have transmitted descendants to the
present day, and, as all the descendants of the same species form a
class, we can understand how it is that there exist so few classes in
each main division of the animal and vegetable kingdoms. Although few of
the most ancient species have left modified descendants, yet, at remote
geological periods, t