196 THE GENESIS OF SPECIES. [CHAP. 



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 abort- 

 ed ; the second is rudimentary, and carries the so-called 

 'bastard wing;' while 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 be- 

 come 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 outer toes." 



Irregularities in the circulating system are far from un- 

 common, and sometimes illustrate this homological ten- 

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

 assistant 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 al- 

 most at its origin, the two halves reuniting at the elbow- 

 joint, and then dividing into the radial and ulnar arteries 

 in the usual manner. In the second case an aberrant ar- 

 tery was given off from the radial side of the brachial 

 artery, again almost at its origin. This aberrant artery 



