One of the chief pigeon-fanciers in
England informed him that, if free to choose, each breed would prefer
pairing with its own kind. 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.

If now we combine with this fact the correlation of colour with
important constitutional peculiarities, and, in some cases, with
infertility; and consider, further, the curious parallelism that has
been shown to exist between the effects of changed conditions and the
intercrossing of varieties in producing either an increase or a decrease
of fertility, we shall have obtained, at all events, a starting-point
for the production of that infertility which is so characteristic a
feature of distinct species when intercrossed. All we need, now, is some
means of increasing or accumulating this initial tendency; and to a
discussion of this problem we will therefore address ourselves.

_The Influence of Natural Selection upon Sterility and Fertility._

It will occur to many persons that, as the infertility or sterility of
incipient species would be useful to