48 GEOMETRICAL RATIO OF INCREASE. [CHAP. III. 



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 tem- 

 porarily increased in any sensible degree. The obvious explanation 

 is that the conditions of life have been highly favourable, 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 extraordinarily rapid in- 

 crease and wide diffusion in their new homes. 



In a state of nature almost every full-grown plant annually pro- 

 duces seed, and amongst 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 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 falling 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 require a few more years to 

 people, under favourable conditions, a whole district, let it be ever 

 so large. The condor 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 believed 

 to be the most numerous bird in the world. One fly deposits 

 hundreds of eggs, and another, like the hippobosca, 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 importance to those species which depend on a 

 fluctuating amount of food, for it allows them rapidly to increase 

 in number. But the real importance of a large number of eggs or 

 seeds is to make up for much destruction at some period of life ; 

 and this period in the great majority of cases is an early one. If 

 an animal can in any way protect its own eggs or young, a small 

 number may be produced, and yet the average stock be fully kept 

 up ; but if many eggs or young are destroyed, many must be pro- 

 duced, or the species will become extinct. It would suffice to keep 

 up the full number of a tree, which lived on an average for a 

 thousand years, if a single seed were produced once in a thousand 

 years, supposing that this seed were never destroyed, and could be 

 ensured to germinate in a fitting place. So that, in all cases, the 



