STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE SAME
SPECIES.
As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them, if they
come into competition with each other, than between the 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. In Australia the imported hive-bee is
rapidly exterminating the small, stingless native bee. One species of
charlock has been known to supplant another species; 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.
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