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STRUCTURE AND POSITION OF THE TOES. 49 
terior toe (3t) = third toe; outer anterior toe (4t) = fourth toe. In birds. 
with the hind ‘toe reversed, the same order is obvious: only, inner anterior 
toe = lt, ete. In zygodactyli (except Trogonide), inner hind toe =1t; 
inner front toe — 2t; outer front toe — 3t; outer hind toe = 4t. Now 
when the number of toes decreases, the toes are always reduced in the 
same order: thus, in all three-toed birds, 1t is wanting: in the two-toed 
birds 1t and 2t are wanting. This is proven by the 
§ 86. Numeer or Jorts, or number of phalanges (§ 72, f) of the toes. 
The constancy of the joints in birds’ toes is remarkable, one of the strong- 
est expressions we have of the highly monomorphic character of the class 
Aves. In all birds, 1t has éwo joints (not counting the accessory metatar- 
sal). Inall birds, 2t has three joints. In nearly all birds, 3t has four joints. 
In nearly all birds, 4t has five joints. The only exceptions to this, consist 
in the lessening of the joints of 3t by one, and the lessening of the joints 
of 4t by one or two. So in all cases, where the joints do not run 2, 3, 4, 5, 
for the toes from Ist to 4th, they run either 2, 3, 4, 4, or 2, 3, 3, 4, or 2, 
3, 3, 3. This variability in number of the internodes is confined (wholly ?) 
to the order Strisores. Our examples are in the sub-families Cypselince and 
Caprimulginee (which see; see also figs. 119 and 122). This admirable 
conservatism enables us to always determine what toes are missing, in birds 
with less than four; thus, in Picoides, the hind toe, though seemingly lt, 
is evidently 4t, because 5-jointed; in the ostrich, with only two toes, 3t 
and 4t are seen to be preserved, because they are respectively 4- and 5- 
jointed. (In fig. 8, the dotted line 1 indicates the first series of phalanges 
of all the toes; dot-line 2, the second; the correspondence of the remaining 
phalanges is seen at a glance.) 
§ 87. THe posrrion of the toes, other than in respect of their direction, 
is important. In att birds the front toes are on the same level, or so 
nearly so, that the difference is not notable. And the same may be said of 
the hind toes, when there are fwo, as in Scansores. But the hind toe, when 
present and single, varies remarkably in position, and must have special 
notice, as this character is important in taxonomy. The insertion of this 
toe varies, from the very bottom of the tarsus, where it is on a level with 
the front toes, to some distance up the tarsus. When flush with the bases 
of the other toes, so that its whole under surface touches the ground, it is 
said to be incumbent. When just so much raised that its tip only touches 
the ground, it is called znsistent. When so high up that it does not reach 
the ground at all, it is termed remote (amotus). But as the precise position 
varies insensibly, so that the foregoing distinctions are not readily per- 
ceived, it is practically best to recognize only two of these three conditions, 
and say simply, “hind toe elevated,” when it is inserted appreciably above 
the rest, or “hind toe not elevated,” when its insertion is flush with that of 
the other toes. In round terms: it is characteristic of all Jnsessores to have 
the hind toe pown; it is characteristic of all other birds to have the hind toe 
up (when present). The exceptions to the first statement are extremely 
KEY TO N. A. BIRDS. 7 
