80 ORIGIN' OF SPECIES 



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 in- 

 credible. So it is with plants: cases could be given of intro- 

 duced 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, cloth- 

 ing square leagues of surface almost to the exclusion of 

 everv 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 discover)-. In 

 such cases, and endless others could be given, no one sup- 

 poses, that the fertility of the animals or plants has been 

 suddenly and tem.porarily increased in any sensible degree. 

 The obvious explanation is that the conditions of life have 

 been higlilv favourable, and that there has constantlv 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, 

 simplv explains their extraordinarily rapid increase and wide 

 diffusion in their new homes. 



In a state of nature almost ever}' full-gro\sTi plant annually 

 produces seed, and amongst animals there are very few 

 which do not annually pair. Hence we may confidently as- 

 sert, that all plants and animals are tending to increase at a 

 geometrical ratio, — that all would rapidly stock ever}- station 

 in which they could anyhow exist, — and that this geomet- 

 rical tendencv to increase must be checked bv destruction at 



