an
uneducated eye. This process of selection has been the great agency in the
production of the most distinct and useful domestic breeds. That many of
the breeds produced by man have to a large extent the character of natural
species, is shown by the inextricable doubts whether very many of them are
varieties or aboriginal species.

There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In the
preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful 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 rapid increase of many animals and plants during a succession of
peculiar seasons, or when naturalised in a new country. 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 {468} 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 un