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.
Linn us 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 plants which have
become common throughout whole islands in a period of less than ten
years. Several
of the plants 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
suddenly and temporarily increased in any sensible degree. The obvious
explanation is that the conditions of life have been very favourable,
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