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.
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