iversity and proportion of kinds as in the
surrounding virgin forests. What a struggle must have gone on during
long centuries between the several kinds of trees, 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, all feeding on each other, or on the
trees, 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 fall to the ground according to definite
laws; but how simple is the problem where each shall fall compared to
that of 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 likewise sometimes 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 will almost invariably
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 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
number 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
of our domestic plants or animals have so exactly the