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.