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 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.