60 ON THE ORIGIN OF SPECIES 



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 

 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, 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 intro- 

 duced 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 instances could be given, no one supposes that 

 the fertility of these animals or plants has been suddenly 

 and temporarily increased in any sensible degree. The 

 obvious explanation is that the conditions of life have 

 been very 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. In 

 such cases the geometrical ratio of increase, the 

 result of which never fails to be surprising, simply 



