. In such cases--and
endless others could be given--no one supposes that the fertility of the
animals or plants has been suddenly and temporarily increased in any
sensible degree. The obvious explanation is that the conditions of life
have been highly favorable, and that there has consequently been less
destruction of the old and young, and that nearly all the young have
been enabled to breed. Their geometrical ratio of increase, the result
of which never fails to be surprising, simply explains their
extraordinarily rapid increase and wide diffusion in their new homes.

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 anyhow exist,--and that this 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, 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 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
favorable 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 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