We have also reason to believe that fertility has been increased by long domestication, in addition to the fact of the original stocks being exceptionally fertile; and no experiments have been made on the differently coloured varieties of wild animals.

There are, however, a number of very curious facts showing that colour in animals, as in plants, is often correlated with constitutional differences of a remarkable kind, and as these have a close relation to the subject we are discussing, a brief summary of them will be here given.
_Correlation of Colour with Constitutional Peculiarities._ The correlation of a white colour and blue eyes in male cats with deafness, and of the tortoise-shell marking with the female sex of the same animal, are two well-known but most extraordinary cases.

Equally remarkable is the fact, communicated to Darwin by Mr.Tegetmeier, that white, yellow, pale blue, or dun pigeons, of all breeds, have the young birds born naked, while in all other colours they are well covered with down.

Here we have a case in which colour seems of more physiological importance than all the varied structural differences between the varieties and breeds of pigeons.

In Virginia there is a plant called the paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours their bones pink, and causes the hoofs of all but the black varieties to drop off; so that black pigs only can be kept in the district.[58] Buckwheat in flower is also said to be injurious to white pigs but not to black.