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 have admitted the existence of varieties, which they think
sufficiently distinct to be worthy of record in systematic works. No one
can draw any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties