and ever-acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which is
common to all organic beings. This high rate of increase is proved by
calculation, by the effects of a succession of peculiar seasons, and by
the results of naturalisation, as explained in the third chapter. More
individuals are born than can possibly survive. A grain in the balance
will determine which individual shall live and which shall die, which
variety or species shall increase in number, and which shall decrease,
or finally become extinct. As the individuals
of the same species come in all respects into the closest competition
with each other, the struggle will generally be most severe between
them; it will be almost equally severe between the varieties of the
same species, and next in severity between the species of the same
genus. But the struggle will often be very severe between beings most
remote in the scale of nature. The slightest advantage in one being, at
any age or during any season, over those with which it comes into
competition, or better adaptation in however slight a degree to the
surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on having special weapons or means of
defence, or on the charms of the males; and the slightest advantage
will lead to victory.
As geology plainly proclaims that each land has undergone great
physical changes, we might have expected that organic beings would have
varied under nature, in the same way as they generally have varied
under the changed conditions of domestication. And if there be any
variability under nature, it would be an unaccountable fact if natural
selection had not come into play. It has often been asserted, but the
assertion is quite incapable of proof, that the amount of variation
under nature is a strictly limited quantity. Man, though acting on
external characters alone and often capriciously, can produce within a
short period a great result by adding up mere individual differences in
his domestic productions; and every one admits that there are at least
individual differences in species under nature. But, besides such
differences, all naturalists
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