58 GEOMETRICAL RATIO OF INCREASE. 



eral 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 ex 

 elusion 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 Coniorin 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 o 

 plants has been suddenly and temporarily increased in an 

 sensible degree. The obvious explanation is that the con 

 ditions 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 surprising, simply explains their extraor- 

 dinarily rapid increase and wide diffusion in their new homes. 



In a state of nature almost every full-grown plant an- 

 nually produces seed, and among animals there are very 

 few which do not annually pair. Hence we may confi- 

 dently assert that all plants and animals are tending to in- 

 crease at a geometrical ratio — that all would rapidly stock 

 every station in which they could anyhow exist — and that 

 this geometrical tendency 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 failing on them, but we do not 

 keep in mind that thousands are annually slaughtered for 

 food, and that in a state of nature an equal number would 

 have somehow to be disposed of. 



The only difference between organisms which annually 

 produce eggs or seeds by the thousand, and those which 

 produce extremely few, is, that the slow breeders would re- 

 quire a few more years to people, under favorable condi- 

 tions, a whole district, let it be ever so large. The condoi 

 lays a couple of eggs and the ostrich a score, and yet in the 

 same country the condor may be the more numerous of the 

 two. The Fulmar petrel lays but one egg, yet it is be- 

 lieved to be the most numerous bird in the world. One 

 fly deposits hundreds of eggs, and another, like the hippo- 

 bosca, a single one. But this difference does not determine 

 how many individuals of the two species can be supported 

 in a district. A large number of eggs is of some impor- 

 tance to those species which depend on a fluctuating amount 



