al 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 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 incredible. So it is with plants: cases could be given of {65}
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, now most numerous over the wide plains of La Plata,
clothing square leagues of surface almost to the exclusion of all other
plants, have been introduced from Europe; and there are plants which now
range in India, as I hear from Dr. Falconer, from Cape Comorin to the
Himalaya, which have been imported from America since its discovery. In
such cases, and endless instances could be given, no one supposes that the
fertility of these animals or plants has been su