52 GEOMETRICAL RATIO OF INCREASE. [Ciur. III. 



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

 produces 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 any 

 how exist, — and that this geometrical tendency to increase must be 

 checked by destruction at some period of life. Our familiarity with 

 the larger domestio animals tends, I think, to mislead us : we see 

 no great destruction falling on them, hut 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 fluctua- 

 ting 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 

 produced, 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 

 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 foregoing 

 considerations always in mind — never to forget that every single 

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

 in numbers ; that each lives by a struggle at some period of its life ; 

 that neavy destruction inevitably falls either on the young or old, 

 during each generation or at recurrent intervals. Lighten any 



