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
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