STRUGGLE FOR EXISTENCE 1469 



which have run wild in several parts of the world; 

 if the statements of the rate of increase of slow-breed- 

 ing 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 pe- 

 riod of less than ten years. In such cases, and end- 

 less others could be given, no one supposes that the 

 fertility of the animals or plants has been suddenly 

 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 young have been en- 

 abled to breed. Their geometrical ratio of increase, 

 the result of which never fails to be surprising, 

 simply explains their extraordinarily rapid increase 

 and wide diffusion in their new homes. 



In a state of nature almost every full-grown plant 

 annually produces seed, and among animals there 

 are very few which do not annually pair. Hence we 

 may confidently assert that all plants and animals 

 are tending to increase at a geometrical ratio that 

 all would rapidly stock every station in which they 

 could anyhow exist and that this geometrical ten- 

 dency to increase must be checked by destruction at 

 some period of life. Our familiarity with the larger 

 domestic animals tends, I think, to mislead us: we 

 see no great destruction falling on them, but we do 

 not keep in mind that thousands are annually 

 slaughtered for food, and that in a state of nature 



