ddenly and temporarily
increased in any sensible degree. The obvious explanation is that the
conditions of life have been very favourable, and that there has
consequently been less destruction of the old and young, and that nearly
all the young have been enabled to breed. In such cases the geometrical
ratio of increase, the result of which never fails to be surprising, simply
explains the extraordinarily rapid increase and wide diffusion of
naturalised productions in their new homes.

In a state of nature almost every plant 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 most rapidly stock every station in which
they could any how exist, and that the 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, and we forget 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 {66} 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 rapidly fluctuating 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