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. 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 Dr. Falconer, from Cape Comorin to the
Himalaya, which have been imported from America since its discovery. In
such cases, and endless others could be given, no one supposes that the
fertility of the animals or plants has been suddenly and temporarily
increased in any sensible degree. The obvious explanation is that the
conditions of life have been highly favourable, and that there has
consequently been less destruction of the old and young and that nearly all
the young have been enabled to breed. Their geometrical ratio of increase,
the result of which never fails to be surprising, simply explains their
extraordinarily rapid increase and wide diffusion in their new homes.