Among the wild horses in Paraguay those of
the same colour and size associate together; while in Circassia there
are three races of horses which have received special names, and which,
when living a free life, almost always refuse to mingle and cross, and
will even attack one another. On one of the Faroe Islands, not more than
half a mile in diameter, the half-wild native black sheep do not readily
mix with imported white sheep. In the Forest of Dean, and in the New
Forest, the dark and pale coloured herds of fallow deer have never been
known to mingle; and even the curious Ancon sheep of quite modern origin
have been observed to keep together, separating themselves from the rest
of the flock when put into enclosures with other sheep. The same rule
applies to birds, for Darwin was informed by the Rev. W.D. Fox that his
flocks of white and Chinese geese kept distinct.[61]
This constant preference of animals for their like, even in the case of
slightly different varieties of the same species, is evidently a fact
of great importance in considering the origin of species by natural
selection, since it shows us that, so soon as a slight differentiation
of form or colour has been effected, isolation will at once arise by the
selective association of the animals themselves; and thus the great
stumbling-block of "the swamping effects of intercrossing," which has
been so prominently brought forward by many naturalists, will be
completely obviated.
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