Natural selection acts solely through the
preservation of variations in some way advantageous, which
consequently endure. But as from the high geometrical powers of
increase of all organic beings, 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
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.
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