Page - 81 - in The Origin of Species

GEOMETRICAL RATIO OF INCREASE 81 some period of life. Our familiarity with the larger domes- tic 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 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 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 hun- dreds of eggs, and another, like the hippobosca, a single one; but this difference does not determine how many indi- viduals of the two species can be supported in a district. A large number of eggs is of some importance to those spe- cies which depend on a fluctuating amount of food, for it allows them rapidly to increase in number. But the real im- portance 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 num- ber 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 suf- fice 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 ani- mal 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 increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably