the rapid
increase of many animals and plants during a succession of peculiar
seasons and when naturalised in new countries. More individuals are born
than can possibly survive. A grain in the balance may determine which
individuals 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. On the other hand, the struggle will often be severe
between beings remote in the scale of Nature. The slightest advantage in
certain individuals, at any age or during any season, over those with
which they come into competition, or better adaptation, in however
slight a degree, to the surrounding physical conditions, will, in the
long run, turn the balance.

With animals having separated sexes, there will be in most cases a
struggle between the males for the possession of the females. The most
vigorous males, or those which have most successfully struggled with
their conditions of life, will generally leave most progeny. But success
will often depend on the males having special weapons, or means of
defence, or charms; and a slight advantage will lead to victory.

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected to find that organic beings have varied
under Nature in the same way as they have varied under domestication.
And if there has been 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 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 everyone admits
that species present individual differences. But, besides such
differences, all naturalists admit that natural varieties exist, which
are considered sufficiently distinct to be worthy of record in
systematic works.

No one has drawn any clear distinction between individual differences
and slight varieties, or be