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.