e 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
one of our domestic plants or animals have so exactly the same strength,
habits, and constitution, that the original proportions of a mixed stock
could be kept up for half a dozen generations, if they were allowed to
struggle together, like beings in a state of nature, and if the seed or
young were not annually sorted.

As species of the same genus have usually, though by no means
invariably, some similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between species
of the same genus, when they come into competition with each other, than
between species of distinct genera. We see this in the recent extension
over parts of the United States of one species of swallow having
caused the decrease of another species. The recent increase of the
missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. How frequently we hear of one species of rat taking the
place of another species under the most different climates! In Russia
the small Asiatic cockroach has everywhere dr