when an American forest is cut down, a very different vegetation
springs up; but it has been observed that the trees now growing on the
ancient Indian mounds, in the Southern United States, display the same
beautiful diversity and proportion of kinds as in the surrounding
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 consequently in a
few years quite supplant the other varieties. To keep up a mixed stock
of even such extremely close varieties as the variously coloured
sweet-peas, they must be each year harvested separately, and the seed
then mixed in due proportion,
otherwise the weaker kinds will steadily decrease in numbers and
disappear. So again with the varieties of sheep: it has been asserted
that certain mountain-varieties will starve out other
mountain-varieties, so that they cannot be kept together. The same
result has followed from keeping together different varieties of the
medicinal leech. It may even be doubted whether the varieties of any
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