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.
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