oons resist the fungus disease much
better than do those which produce yellow cocoons.[59] Among plants, we
have in North America green and yellow-fruited plums not affected by a
disease that attacked the purple-fruited varieties. Yellow-fleshed
peaches suffer more from disease than white-fleshed kinds. In Mauritius,
white sugar-canes were attacked by a disease from which the red canes
were free. White onions and verbenas are most liable to mildew; and
red-flowered hyacinths were more injured by the cold during a severe
winter in Holland than any other kinds.[60]

These curious and inexplicable correlations of colour with
constitutional peculiarities, both in animals and plants, render it
probable that the correlation of colour with infertility, which has been
detected in several cases in plants, may also extend to animals in a
state of nature; and if so, the fact is of the highest importance as
throwing light on the origin of the infertility of many allied species.
This will be better understood after considering the facts which will be
now described.

_The Isolation of Varieties by Selective Association._

In the last chapter I have shown that the importance of geographical
isolation for the formation of new species by natural selection has been
greatly exaggerated, because the very change of conditions, which is
the initial power in starting such new forms, leads also to a local or
stational segregation of the forms acted upon. 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.