to insects, for it is found that silkworms which produce white cocoons resist the fungus disease much better than do those which produce yellow cocoons.[1] 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.[2]
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
- ↑ In the Medico-Chirurgical Transactions, vol. liii. (1870), Dr. Ogle has adduced some curious physiological facts bearing on the presence or absence of white colours in the higher animals. He states that a dark pigment in the olfactory region of the nostrils is essential to perfect smell, and that this pigment is rarely deficient except when the whole animal is pure white, and the creature is then almost without smell or taste. He observes that there is no proof that, in any of the cases given above, the black animals actually eat the poisonous root or plant; and that the facts are readily understood if the senses of smell and taste are dependent on a pigment which is absent in the white animals, who therefore eat what those gifted with normal senses avoid. This explanation however hardly seems to cover the facts. We cannot suppose that almost all the sheep in the world (which are mostly white) are without smell or taste. The cutaneous disease on the white patches of hair on horses, the special liability of white terriers to distemper, of white chickens to the gapes, and of silkworms which produce yellow silk to the fungus, are not explained by it. The analogous facts in plants also indicate a real constitutional relation with colour, not an affection of the sense of smell and taste only.
- ↑ For all these facts, see Animals and Plants under Domestication, vol. ii. pp. 335-338.
