. 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 {67} 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 causes which check the natural tendency of each species to increase in
number are 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 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, I believe that it is the seedlings which 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 the 357 no less than 295 were
destroyed, chiefly by slugs and insects. If turf which has long been mown,
and the case would be the same with turf closely browsed by quadru