204 THE GENESIS OF SPECIES. [CHAP. 



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 development 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. 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 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 connexion of feathered 

 legs and membrane between the outer toes." 



Irregularities in the circulating system are far from 

 uncommon, and sometimes illustrate this homological ten- 

 dency. My friend and colleague Mr. George G. Gascoyen, 

 surgeon at St. Mary's Hospital, has supplied me with two 

 instances of symmetrical affections which have come under 

 his observation. 



In the first of these the brachial artery bifurcated almost 



