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 animals, and varieties of flowers, take the place
of older and inferior kinds.
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