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 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 introduced
plants which have become common throughout whole islands in a period of
less than ten years.
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