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. 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 around us 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.
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