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.

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 favorable 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 incredible. So it is with plants;
cases could be given of introduced plants which have become common
throughout whole islands in a period of less than ten years. Several of
the plants, such as the cardoon and a tall thistle, which are now the
commonest over the wide plains of La Plata, clothing square leagues of
surface almost to the exclusion of every other plant, have been
introduced from Europe; and there are plants which now range in India,
as I hear from Falconer, from Cape Comorin to the Himalaya, which have
been imported from America since its discovery