170 
DARWINISM 
CHAP. 
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 domestica¬ 
tion, 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, com¬ 
municated 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. 1 
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. Jolm’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 Hies than are 
those which are brown or black. The same law even extends 
1 Origin of Species, sixth edition, p. 9. 
