Chap. XXV. CORRELATED VARIABILITY. 317 



The lopping forwards and downwards of the immense ears 

 of fancy rabbits seems partly due to the disuse of the muscles, 

 and partly to the weight and length of the ears, which have 

 been increased by selection during many generations. Now, 

 with the increased size and changed direction of the ears not 

 only has the bony auditory meatus become changed in outline, 

 direction, and greatly in size, but the whole skull has been 

 slightly modified. This could be clearly seen in " half-lops " 

 — that is, in rabbits with only one ear lopping forward — for 

 the opposite sides of their skulls were not strictly symmetrical. 

 This seems to me a curious instance of correlation, between 

 hard bones and organs so soft and flexible, as well as so 

 unimportant under a physiological point of view, as the 

 external ears. The result no doubt is largely due to mere 

 mechanical action, that is, to the weight of the ears, on the 

 same principle that the skull of a human infant is easily 

 modified by pressure. 



The skin and the appendages of hair, feathers, hoofs, horns, 

 and teeth, are homologous over the whole body. Every one 

 knows that the colour of the skin and that of the hair usually 

 vary together; so that Virgil advises the shepherd to look 

 whether the mouth and tongue of the ram are black, lest the 

 lambs should not be purely white. The colour of the skin 

 and hair, and the odour emitted by the glands of the skin, 

 are said 10 to be connected, even in the same race of men. 

 Generally the hair varies in the same way all over the body 

 in length, fineness, and curliness. The same rule holds good 

 with feathers, as we see with the laced and frizzled breeds 

 both of fowls and pigeons. In the common cock the feathers 

 on the neck and loins are always of a particular shape, called 

 hackles : now in the Polish breed, both sexes are characterised 

 by a tuft of feathers on the head, and through correlation 

 these feathers in the male always assume the form of hackles. 

 The wing and tail-feathers, though arising from parts not 

 homologous, vary in length together ; so that long or short 

 winged pigeons generally have long or short tails. The case 

 of the Jacobin-pigeon is more curious, for the wing and tail 

 feathers are remarkably long ; and this apparently has arisen 



10 Godron, ' Sur l'Espece,' torn. ii. p. 217. 



