l
conditions of life, which may be effected in the long course of time by
nature's power of selection.

EXTINCTION.

This subject will be more fully discussed in our chapter on Geology; but
it must be here alluded to from being intimately connected with natural
selection. 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.

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, a