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 insured 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.

OF THE NATURE OF THE CHECKS TO INCREASE

From 'The Origin of Species'

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 exactly what the checks are, even in a single instance. Nor
will this surprise any one who reflects how ignorant we are on this
head, even in regard to mankind, although so incomparably better known
than any other animal. This subject of the checks to increase has been
ably treated by several authors, and I hope in a future work to discuss
it 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 chief points. Eggs or very
young animals seem generally to suffer most, but this is not invariably
the case. With plants there is a vast destruction of seeds; but from
some observations which I have made, it appears that the seedlings
suffer most, from germinating in ground already thickly stocked with
other plants. Seedlings also are destroyed in vast numbers by various
enemies: for instance, on a piece of ground three feet long and two
wide, dug and cleared, and where there could be no choking from other
plants, I marked all the seedlings of our native weeds as they came up,
and out of 357 no less than 295 wer