Man does not actually produce variability; he only unintentionally exposes organic beings to new conditions of life, and then nature acts on the organisation, and causes variability.  But man can and does select the variations given to him by nature, and thus accumulate them in any desired manner.  He thus adapts animals and plants for his own benefit or pleasure.  He may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed.  It is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be quite inappreciable by 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 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.