Origin of Species Variorum

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

There is no obvious reason why the principles which have acted so efficiently under domestication should not act under nature. In the survival of favoured individuals and races, during the constantly-recurrent Struggle for Existence, we see a powerful and ever-acting form 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, and when naturalised in a new country. 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 very 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 turn the balance.