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 in this 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 (16/4. Rengger 'Saugethiere von Paraguay'
s. 336.) 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 have received distinct
names; and a native proprietor of rank (16/5. See a memoir by MM. Lherbette
and De Quatrefages in 'Bull. Soc. d'Acclimat.' tome 8 July 1861 page 312.)
asserts that horses of three of these races, whilst living a free life, almost
always refuse to mingle and cross, and will even attack one another.

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 life tend to keep the races
distinct. On one of the Faroe islands, not more than half a mile in diameter,