sposed 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. 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 up; but if many eggs or young are
destroyed, many must be produced or the species will become extinct. It
would suffice 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 animal or plant depends only indirectly on the
number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing
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 falls either on the young or old during each
generation or at recurrent intervals. Lighten any check, mitigate the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount.

NATURE OF THE CHECKS TO INCREASE.

The causes which check the natural tendency of each species to increase
are most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will it tend to increase still further. We
know not exa