CHAPTER III. STRUGGLE FOR EXISTENCE.
3. GEOMETRICAL RATIO OF INCREASE.
A struggle for existence inevitably follows from the high rate at which all
organic beings tend to increase. Every being, which during its natural
lifetime produces several eggs or seeds, must suffer destruction during
some period of its life, and during some season or occasional 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 less than a 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 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 safest to assume that it begins breeding
when thirty years old, and goes on breeding till ninety years old, bringing
forth six young in the interval, and surviving till one hundred years old;
if this be so, after a period of from 740 to 750 years there would be
nearly nineteen million elephants alive descended from the first pair.