Chap. XXV. COEBELATED VARIABILITY. 325 



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. With poultry and certain ducks we have 

 seen that the colour of the plumage stands in some connexion 

 with the colour of the shell of the egg,— that is, with the 

 mucous membrane which secretes the shell. The colour of 

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

 skin, are said 9 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; but 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 in correlation with the 

 elongated and reversed feathers on the back of the neck, which 

 form the hood. 



The hoofs and hair are homologous appendages ; and a careful 

 observer, namely Azara, 10 states that in Paraguay horses of various 

 colours are often born with their hair curled and twisted like that 

 on the head of a negro. This peculiarity is strongly inherited. 

 But what is remarkable is that the hoofs of these horses " are 

 " absolutely like those of a mule." The hair also of the mane and 

 tail is invariably much shorter than usual, being only from four 



9 Godron, ' S ur l'Espece,' torn. ii. p. 217, 

 10 ' Quadrapedes du Paraguay,' torn. ii. p. 333. 



