mber 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. Lighten any check, mitigate
the destruction ever so little, and the number of the species will
almost instantaneously increase to any amount. The face of Nature may
be compared to a yielding surface, with ten thousand sharp wedges packed
close together and driven inwards by incessant blows, sometimes one
wedge being struck, and then another with greater force.

What checks the natural tendency of each species to increase in number
is most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will its tendency to increase be still
further increased. We know not exactly what the checks are in even
one single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, so incomparably
better known than any other animal. This subject has been ably treated
by several authors, and I shall, in my future work, discuss some of the
checks at considerable length, more especially in regard to the feral
animals of South America. Here I will make only a few remarks, just to
recall to the reader's mind some of the ch