Chap. XXV. CORRELATED VARIABILITY. 323 



according to the excellent authority of Mr. Hewitt, 6 been seen 

 to exceed the wing-feathers in length, and in one case were 

 actually nine and a half inches in length ! As Mr. Blyth has 

 remarked to me, these leg-feathers resemble the primary win^- 

 feathers, and are totally unlike the fine down which naturally 

 grows on the legs of some birds, such as grouse and owls. Hence 

 it may be suspected that excess of food has first given redun- 

 dancy to the plumage, and then that the law of homologous 

 variation has led to the development of feathers on the le«-s in 

 a position corresponding with those on the wing, namely, on the 

 outside of the tarsi and toes. I am strengthened in this belief 

 by the following curious case of correlation, which for a long 

 time seemed to me utterly inexplicable, namely, that in pigeons 

 of any breed, if the legs are feathered, the two outer toes are 

 partially connected by skin. These two outer toes correspond 

 with our third and fourth toes. Now, in the wing of the pigeon 

 or any other bird, the first and fifth digits are wholly aborted ; 

 the second is rudimentary and carries the so-called " bastard- 

 wing ;" whilst the third and fourth digits are completely united 

 and enclosed by skin, together forming the extremity of the 

 wing. So that in feather-footed pigeons, not only does the ex- 

 terior surface support a row of long feathers, like wing-feathers, 

 but the very same digits which in the wing are completely united 

 by skin become partially united by skin in the feet ; and thus 

 by the law of the correlated variation of homologous parts we 

 can understand the curious connection of feathered legs and 

 membrane between the two outer toes. 



Andrew Knight 7 has remarked that the face or head and the 

 limbs vary together in general proportions. Compare, for instance, 

 the head and limbs of a dray and race-horse, or of a greyhound 

 and mastiff. What a monster a greyhound would appear with 

 the head of a mastiff! The modern bulldog, however, has fine 

 limbs, but this is a recently-selected character. From the 

 measurements given in the sixth chapter, we clearly see that 

 in all the breeds of the pigeon the length of the beak and the 

 size of the feet are correlated. The view which, as before ex- 

 plained, seems the most probable is, that disuse in all cases tends 



6 ' The Poultry Book/ by W. B. Tegetmeiev, 1866, p. 250. 



7 A. Walker on Intermarriage, 1838, p. 160. 



Y 2 



