64 
HIGH KATE OF INCKEASE. 
Chap. HI. 
can be no artificial increase of food, and no prudential 
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 pro¬ 
duced 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 fol¬ 
lowing 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 incre¬ 
dible. So it is with plants: cases could be given of 
