chap, xiii.] DIGITS: BIRDS. 391 



tarsus bears two digits instead of one. The morphological nature of 

 these digits is obscure. Some have judged that one of them is a 

 "prse-hallux ; " Cowper ' sees in the internal toe the true hallux, and 

 argues that the digit commonly called the hallux is really the index ; 

 Howes and Hill 2 consider that the normal hallux has split into these 

 two digits. The diversity of these views comes partly from an insuffi- 

 ciency of the area of fact over which the inquiry has been extended, 

 for it will be found that the conditions are very various and shade off 

 imperceptibly in several directions. As in all cases of Meristic Series, 

 the first question relates to the position of these digits in the system of 

 Symmetry of the limb. Are they in a Successive Series with the 

 other digits, or do they balance them 1 Are they in Succession to each 

 other or do they balance each other as images ? 



Turning to the facts with these considerations in view it will be 

 seen that no general answer can be given, but that the condition is 

 sometimes of the one kind and sometimes of the other. For there are 

 not merely two conditions, a four-toed and a five-toed, but there is a 

 whole series of conditions and according to the cases chosen so may the 

 question be answered. By examining a few score of fowls' feet many 

 sorts may be seen. 



>90. (1) The most usual five-toed foot is that figured by Cowper 

 (I.e., p. 249), in which the metatarsal of the hallux bears two digits, an 

 outer one of two phalanges and an inner of three phalanges. For pur- 

 poses of description let us call the outer the hallux. In this foot then 

 the hallux is the least digit, and the members of the digital series 

 increase in size on either side of it. 



91. (2) But not rarely is found a state like the last save that the 

 inner digit is borne by the proximal phalanx of the hallux. This is 

 very common. The two digits may then be about equal in size, or 

 more often the hallux is the smaller. 



>92. (3) Hallux more or less perfectly divided into two digits with a 

 common base, having (a) two, or (b) three phalanges (as in Howes' case 

 Fig. 5). This state is practically that of the human "double-thumb" 

 (see p. 350), and, just as in that phenomenon, the duplicity may be of 

 various extent, often affecting only the nail and distal phalanx. Be- 

 tween the two parts of such a double digit there is often that relation as 

 of optical images found in human double-thumb, the curvatures of the 

 two parts being equal and opposite. But if both digits are of good size 

 and are separate up to the metatarsal this equality is rarely if ever found, 

 and one of the digits, generally the innermost, is the larger. In this 

 condition therefore there is a Succession from the hallux to the inner 

 digit just as in (1). So the condition of double-hallux, that is to say 

 the representation of one member of a series by two members in bi- 

 lateral symmetry, shades off imperceptibly into the condition in which 

 a new member is formed in Succession to the terminal member. 



It should be noted that this case presents a remarkable difference 

 from that seen in the like cases of variation on the radial side of the 

 hand of Man. In Man the states of true double-thumb are just as in 

 the Fowl ; but if there is a difference or Succession between the two parts 



1 Jour. Anat. Phi/s., xx. p. 593; and xxm. p. 242. 



2 Ibid., xxvi. p. 395, figs. 



