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.
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