As an example of what is meant, Darwin
states that the recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush.[12] The black rat
(Mus rattus) was the common rat of Europe till, in the beginning of the
eighteenth century, the large brown rat (Mus decumanus) appeared on the
Lower Volga, and thence spread more or less rapidly till it overran all
Europe, and generally drove out the black rat, which in most parts is
now comparatively rare or quite extinct. This invading rat has now been
carried by commerce all over the world, and in New Zealand has
completely extirpated a native rat, which the Maoris allege they brought
with them from their home in the Pacific; and in the same country a
native fly is being supplanted by the European house-fly. In Russia the
small Asiatic cockroach has driven away a larger native species; and in
Australia the imported hive-bee is exterminating the small stingless
native bee.
The reason why this kind of struggle goes on is apparent if we consider
that the allied species fill nearly the same place in the economy of
nature.
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