64 HIGH EATE OF INCREASE. Chap. III. 



be now increasing, more or less rapidly, in numbers, all 

 cannot do so, for trie 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 then seedlings next year pro- 

 duced 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 elej)hants, 

 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 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 



