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; 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 teeth and talons of the tiger; and
in that of the legs and claws of the parasite which clings to the hair
on the tiger s body. But in the beautifully plumed seed of the
dandelion, and in the flattened and fringed legs of the water-beetle,
the relation seems at first confined to the elements of air and water.
Yet the advantage of plumed seeds no doubt stands in the closest
relation to the land being already thickly clothed by other plants; so
that the seeds may be widely distributed and fall on unoccupied ground.
In the water-beetle, the structure of its legs, so well adapted for
diving, allows it to compete with other aquatic insects, to hunt for
its own prey, and to escape serving as prey to other animals.
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