CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
6. EXTINCTION CAUSED BY NATURAL SELECTION.
This subject will be more fully discussed in our chapter on Geology; but it
must here be 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. Owing to
the high geometrical rate of increase of all organic beings, each 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 animals, and varieties of flowers, take the place
of older and inferior kinds. In Yorkshire, it is historically known that
the ancient black cattle were displaced by the long-horns, and that these
"were swept away by the short-horns" (I quote the words of an agricultural
writer) "as if by some murderous pestilence."