170 DARWINISM chap. 



distinctive marks, and they are, therefore, seldom crossed with 

 these of another colour ; and even Avhen they are so crossed, no 

 notice "would be taken of any slight diminution of fertility, since 

 this is liable to occiu* 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 coloui'ed varieties of wild animals. There are, 

 however, a number of very curious facts sho'wang 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 Ave are discussing, a brief summary of 

 them Avill be here given. 



Correlation of Colour ivith 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 remai'kable is the fact, com- 

 municated to Dar^vin 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.^ 

 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 ; Avhite chickens from the gapes. 

 AATiite-haired horses or cattle are subject to cutaneous 

 diseases from Avhich 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 

 ^ Origin of Species, sixth edition, p. 9. 



