casional
year, otherwise, on the principle of geometrical increase, its numbers
would quickly become so inordinately great that no country could support
the product. Hence, as more individuals are produced than can possibly
survive, there must in every case be a struggle for existence, either
one individual with another of the same species, or with the individuals
of distinct species, or with the physical conditions of life. It is the
doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial increase
of food, and no prudential restraint from marriage. Although some
species may be now increasing, more or less rapidly, in numbers, all
cannot do so, for the world would not hold them.

There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would soon
be covered by the progeny of a single pair. Even slow-breeding man has
doubled in twenty-five years, and at this rate, in a few thousand years,
there would literally not be standing room for his progeny. Linnaeus has
calculated that if an annual plant produced only two seeds--and there is
no plant so unproductive as this--and their seedlings next year produced
two, and so on, then in twenty years there would be a million plants.
The elephant is reckoned to be the slowest breeder of all known animals,
and I have taken some pains to estimate its probable minimum rate of
natural increase: it will be under the mark to assume that it breeds
when thirty years old, and goes on breeding till ninety years old,
bringing forth three pair of young in this interval; if this be so,
at the end of the fifth century there would be alive fifteen million
elephants, descended from the first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three following
seasons. Still more striking is the evidence from our domestic animals
of many kinds which have run wild in several parts of the world: if the
statements of the rate of increase of slow-breeding cattle and horses
in South America, and latterly in Australia, had not been well
authenticated, they would have been quite incredible. So it is with
plants: cases could be given of introduced pl