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. They require nearly the same kind of food, are exposed to the
same enemies and the same dangers. Hence, if one has ever so slight an
advantage over the other in procuring food or in avoiding danger, in its
rapidity of multiplication or its tenacity of life, it will increase
more rapidly, and by that very fact will cause the other to decrease and
often become altogether extinct. In some cases, no doubt, there is
actual war between the two, the stronger killing the weaker; but this is
by no means necessary, and there may be cases in which the weaker
species, physically, may prevail, by its power of more rapid
multiplication, its better withstanding vicissitudes of climates, or its
greater cunning in escaping the attacks of the common enemies. The same
principle is seen at work in the fact that certain mountain varieties of
sheep will starve out other mountain varieties, so that the two cannot
be kept together. In plants the same thing occurs. If several distinct
varieties of wheat are sown together, and the mixed seed resown, some of
the varieties which best suit the soil and climate, or are naturally the
most fertile, will beat the others and so yield more seed, and will
consequently in a few years supplant the other varieties.

As an effect of this principle, we seldom find closely allied species
of animals or plants living together, but often in distinct though
adjacent districts where the conditions of life are somewhat different.
Thus we may find cowslips (Primula veris) growing in a me