ach area
is already fully stocked with inhabitants, and it follows from this,
that as the favoured forms increase in number, so, generally, will the
less favoured decrease and become rare. Rarity, as geology tells us,
is the precursor to extinction. We can see that any form which
is represented by few individuals will run a good chance of utter
extinction, during great fluctuations in the nature or the seasons, or
from a temporary increase in the number of its enemies. But we may go
further than this; for as new forms are produced, unless we admit that
specific forms can go on indefinitely increasing in number, many old
forms must become extinct. That the number of specific forms has not
indefinitely increased, geology plainly tells us; and we shall presently
attempt to show why it is that the number of species throughout the
world has not become immeasurably great.

We have seen that the species which are most numerous in individuals
have the best chance of producing favourable variations within any given
period. We have evidence of this, in the facts stated in the second
chapter, showing that it is the common and diffused or dominant species
which offer the greatest number of recorded varieties. Hence, rare
species will be less quickly modified or improved within any given
period; they will consequently be beaten in the race for life by the
modified and improved 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 related genera,--which, from having nearly the same
structure, constitution and habits, generally come into the severest
competition with each other. Consequently, each new variety or species,
during the progress of its formation, will generally press hardest
on its nearest kindred, and tend to exterminate them. We see the same
process of extermination among our domesticated productions, through
the selection of improved forms by man. Many curious instances could be
given showing how quickly new breeds of cattle, sheep and other animal