quently in a few years quite supplant the other
varieties. To keep up a mixed stock of even such extremely close varieties
as the variously {76} 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
driven before it its great congener. One species of charlock will supplant
another, and so in other cases. We can dimly see why the competition should
be most severe between allied forms, which fill nearly the same place in
the economy of nature; {77} but probably in no one case could we precisely
say why one species has been victorious over another in the great battle of
life.

A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related, in
the most essential yet often hidden manner, to that of all other organic
beings, with which it comes into competition for food or residence, or from
which it has to escape, or on which it preys. This is obvious in the
structure of the tee