But there is also a very
powerful cause of isolation in the mental nature--the likes and
dislikes--of animals; and to this is probably due the fact of the
comparative rarity of hybrids in a state of nature. The differently
coloured herds of cattle in the Falkland Islands, each of which keeps
separate, have been already mentioned; and it may be added, that the
mouse-coloured variety seem to have already developed a physiological
peculiarity in breeding a month earlier than the others. Similar facts
occur, however, among our domestic animals and are well known to
breeders. Professor Low, one of the greatest authorities on our
domesticated animals, says: "The female of the dog, when not under
restraint, makes selection of her mate, the mastiff selecting the
mastiff, the terrier the terrier, and so on." And again: "The Merino
sheep and Heath sheep of Scotland, if two flocks are mixed together,
each will breed with its own variety." Mr. Darwin has collected many
facts illustrating this point. One of the chief pigeon-fanciers in
England informed him that, if free to choose, each breed would prefer
pairing with its own kind.
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