STRUGGLE FOR EXISTENCE 103 



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, de- 

 scended from the first pair. 



But we have better evidence on this subject than 

 mere theoretical calculations; namely, the numerous re- 

 corded cases of the astonishingly rapid increase oi various 

 animals in a state of nature, when circumstances have 

 been favorable 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 dis- 

 covery. In such cases, and endless others could be 

 given, no one supposes that the fertility of the animals 

 or plants has been sudden4y and temporarily increased in 

 any sensible degree. The obvious explanation is that the 

 conditions of life have been highly favorable, and that 

 there has consequently been less destruction of the old 

 and young, and that nearly all the joung have been 



