consider that most of our pure and valued
domestic breeds are characterised by definite colours which constitute
one of their distinctive marks, and they are, therefore, seldom crossed
with these of another colour; and even when they are so crossed, no
notice would be taken of any slight diminution of fertility, since this
is liable to occur from many causes. 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. In the Tarentino, black sheep are not injured by eating the
Hypericum crispum--a species of St. John's-wort--which kills white
sheep. White terriers suffer most from distemper; white chickens from
the gapes. White-haired horses or cattle are subject to cutaneous
diseases from which the dark coloured are free; while, both in Thuringia
and the West Indies, it has been noticed that white or pale coloured
cattle are much more troubled by flies than are those which are brown or
black. The same law even extends to insects, for it is found that
silkworms which produce white coc