52 THE ORIGIN OF SPECIES 



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 pro- 

 duces 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 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 dis- 

 posed 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 favorable 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 de- 

 posits 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 numbers. But the real importance of a large number of eggs or 

 seed 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 

 insured to germinate in a fitting place; so that, in all cases, the 

 average number of any animal or plant depends only indirectly 

 on the number of its eggs or seeds. 



In looking at Nature, it is most necessary to keep the fore- 

 going considerations always in mind — never to forget that every 

 single organic being may be said to be striving to the utmost to 



