the herds from blending.

Certain domestic races seem to prefer breeding with their own kind; and
this is a fact of some importance, for it is a step towards that
instinctive feeling which helps to keep closely allied species in a state
of nature distinct. We have now abundant evidence that, if it were not for
this feeling, many more hybrids would be naturally produced than is the
case. We have seen in the first chapter that the alco dog of Mexico
dislikes dogs of other breeds; and the hairless dog of Paraguay mixes less
readily with the European races, than the latter do with each other. In
Germany the female Spitz-dog is said to receive the fox more readily than
will other dogs; a female Australian Dingo in England attracted the wild
male foxes. But these differences in the sexual instinct and attractive
power of the various breeds may be wholly due to their descent from
distinct species. In Paraguay the horses have much freedom, and an
excellent observer[217] believes that the native horses of the same colour
and size prefer associating with each other, and that the horses which have
been imported from Entre Rios and Banda Oriental into Paraguay likewise
prefer associating together. In Circassia six sub-races of the horse are
known and have received distinct names; and a native proprietor of
rank[218] asserts that horses of three of these races, whilst living a free
life, almost always refuse to mingle and cross, and will even attack each
other.

It has been observed, in a district stocked with heavy Lincolnshire and
light Norfolk sheep, that both kinds, though bred together, when turned
out, "in a short time separate to a sheep;" the Lincolnshires drawing off
to the rich soil, and the Norfolks to their own dry light soil; and as long
as there is plenty of grass, "the two breeds keep themselves as distinct as
rooks and pigeons." In this case different habits of {103} life tend to
keep the races distinct. On one of the Faroe islands, not more than half a
mile in diameter, the half-wild native black sheep are said not to have
readily mixed with the imported white sheep. It is a more curious fact that
the semi-monstrous ancon sheep of modern origin "have been observed to keep
together, separating themselves from the rest of the flock, when put into
enclosures with other sheep."[219] With respect to fallow deer, which live
in a semi-domesticated condition, Mr. Bennett[220] states that the dark and
pale coloured herds,