to
play; some one check or some few being generally the most potent, but all
concur in determining the average number or even the existence of the
species. In some cases it can be shown that widely-different checks act on
the same species in different districts. When we look at the plants and
bushes clothing an entangled bank, we are tempted to attribute their
proportional numbers and kinds to what we call chance. But how false a view
is this! Every one has heard that when an American forest is cut down, a
very different vegetation springs up; but it has been observed that ancient
Indian ruins in the Southern United States, which must formerly have been
cleared of trees, now display the same beautiful diversity and proportion
of kinds as in the surrounding {75} virgin forests. What a struggle between
the several kinds of trees must here have gone on during long centuries,
each annually scattering its seeds by the thousand; what war between insect
and insect--between insects, snails, and other animals with birds and
beasts of prey--all striving to increase, and all feeding on each other or
on the trees or their seeds and seedlings, or on the other plants which
first clothed the ground and thus checked the growth of the trees! Throw up
a handful of feathers, and all must fall to the ground according to
definite laws; but how simple is this problem compared to the action and
reaction of the innumerable plants and animals which have determined, in
the course of centuries, the proportional numbers and kinds of trees now
growing on the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This is
often the case with those which may strictly be said to struggle with each
other for existence, as in the case of locusts and grass-feeding
quadrupeds. But the struggle almost invariably will be most severe between
the individuals of the same species, for they frequent the same districts,
require the same food, and are exposed to the same dangers. In the case of
varieties of the same species, the struggle will generally be almost
equally severe, and we sometimes see the contest soon decided; for
instance, if several varieties of wheat be sown together, and the mixed
seed be resown, some of the varieties which best suit the soil or climate,
or are naturally the most fertile, will beat the others and so yield more
seed, and will conse