each area is already fully stocked with inhabitants, it follows
that as each selected and favoured form increases in number, so will the
less favoured forms decrease and become rare. Rarity, as geology tells us,
is the precursor to extinction. We can, also, see that any form represented
by few individuals will, during fluctuations in the seasons or in the
number of its enemies, run a good chance of utter extinction. But we may go
further than this; for as new forms are continually and slowly being
produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably must
become extinct. That the number of specific forms has not indefinitely
increased, geology shows us plainly; and indeed we can see reason why they
should not have thus increased, for the number of places in the polity of
nature is not indefinitely great,--not that we {110} have any means of
knowing that any one region has as yet got its maximum of species. Probably
no region is as yet fully stocked, for at the Cape of Good Hope, where more
species of plants are crowded together than in any other quarter of the
world, some foreign plants have become naturalised, without causing, as far
as we know, the extinction of any natives.

Furthermore, the species which are most numerous in individuals will have
the best chance of producing within any given period favourable variations.
We have evidence of this, in the facts given in the second chapter, showing
that it is the common species which afford the greatest number of recorded
varieties, or incipient species. Hence, rare species will be less quickly
modified or improved within any given period, and they will consequently be
beaten in the race for life by the modified descendants of the commoner
species.

From these several considerations I think it inevitably follows, that as
new species in the course of time are formed through natural selection,
others will become rarer and rarer, and finally extinct. The forms which
stand in closest competition with those undergoing modification and
improvement, will naturally suffer most. And we have seen in the chapter on
the Struggle for Existence that it is the most closely-allied
forms,--varieties of the same species, and species of the same genus or of
related genera,--which, from having nearly the same structure,
constitution, and habits, generally come into the severest competition with
each other.