STRUGGLE FOR EXISTENCE 51 



in numbers, all cannot do so, for the world would not hold them. 



There is no exception to the rule that every organic being natu- 

 rally 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 less than a 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 unpro- 

 ductive as this — and their seedlings next year produced 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 safest to assume that it begins breed- 

 ing when thirty years old, and goes on breeding till ninety years 

 old, bringing forth six young in 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 theo- 

 retical calculations, namely, the numerous recorded cases of the 

 astonishingly rapid increase of 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 discovery. In such 

 cases, and endless others could be given, no one supposes that the 

 fertility of the animals or plants has been suddenly and tempo- 

 rarily 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 young have been enabled to breed. Their 

 geometrical ratio of increase, the result of which never fails to be 



