THE ORIGIX OF SPECIES. 39 



nature, when circumstances have been favorable to tbein dur- 

 ing two or three following seasons. Still more striking is the 

 evidence from our domestic animals of many kinds wliicli 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 authenti- 

 cated, they would have been quite 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 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 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 instances could be given, no one sup- 

 poses that the fertility of these animals or plants has been sud- 

 denly and temporarily increased in any sensible degree. The 

 obvious explanation is, that the conditions of life have been very 

 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. In such cases the geometrical ratio of in- 

 crease, the result of which never fails to be surprising, simply 

 explains the extraordinarily rapid increase and wide diffusion of 

 naturalized productions in their new homes." — (pp. 64, 65.) 



"All plants and animals are tending to increase at a geo- 

 metrical ratio; all would most rapidly stock any station in 

 which they could anyhow exist ; the increase must be checked 

 by destruction at some period of life." — (p. 65.) 



The difference between the most and the least pro- 

 lific species is of no account : 



" The condor lays a couple of eggs, and the ostrich a score; 

 and yet in the same country the condor may be the more numer- 

 ous of the two. The Fulmar petrel lays but one egg, yet it is 

 believed to be the most numerous bird in the world." — (p. 68.) 



" The amount of food gives the extreme limit to which each 



