CiiAP. III. GEOMETRICAL RATIO OF' INCREASE. 73 



La Plata, clothing square leagues of surface almost to the ex- 

 clusion 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 tlie Himalaya, which have 

 been imported from xVmerica since its discovery. In such 

 cases, and endless instances could be given, no one supposes 

 that the fertility of these animals or phuits has been suddenly 

 and temporarily increased in any sensil)le degree. The obvious 

 explanation is that the conditions of lifeliave been very favor- 

 able, and that there has consequently been less destruction of 

 the old and 3'oung, 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. 



In a state of nature almost every plant 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 

 most rapidly stock every station in which they could anjdiow 

 exist — and that the 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 : 

 Ave see no great destruction falling on them, and we forget 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 pro- 

 duce eggs or seeds by the thousand, and those which produce 

 extremely few, is, that the slow-breeders would require a few 

 more years to people, under favorable conditions, a Avliole dis- 

 trict, let it be ever so large. The condor lays a couple of eggs 

 and tlic 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 o<i;g, yet it is believed to be the most numerous bird in 

 the world. One fly deposits hundreds of eggs, and another, 

 like the hippolwsca, a single one; but this dillerence does not 

 det(^nnine how many individuals of the two species can be sup- 

 ported in a district. A large nmnber of eggs is of some im- 

 j)ortance to those species which depend on a rapidly-fluctuating 

 amount of food, for it allows them rapidly to increase in num- 

 ber. 15ut the real importance of a large number of eggs or 

 seeds is to make up for nuich destruction at some period of life : 

 4 



