Our familiarity with the larger domestic animals tends, I think, to mislead us; we see no great destruction falling on them, and 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.