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.
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