Chap. XXV. CORRELATED VARIABILITY. 315 



been seen to exceed the wing-feathers in length, and in one 

 case were actually nine and a half inches long ! As Mr. 

 Blyth has remarked to me, these leg-feathers resemble the 

 primary wing-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 redundancy to the plumage, and then 

 that the law of homologous variation has led to the develop- 

 ment of feathers on the legs, 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. 7 Now, in the wing of the pigeon 

 or of any other bird, the first and fifth digits are 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 

 exterior 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 8 has remarked that the face or head and 

 the limbs usually vary together in general proportions. 

 Compare, for instance, the 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 



7 Naturalists differ with respect Lyceum of Nat. Hist, of New York,' 



to the homologies of the digits of vol. x., 1872, p. 1 tj. 



birds ; but several uphold the view 8 A. Walker on Intermarriage, 



above advanced. See on this subject !^38, p. 160. 

 Dr. E. S. Morse in 'Annals ■£ the 



