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.
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