EXTERNAL PARTS OF BIRDS. — THE FEET. 133 



inner one. In nearly every three-toed bird, all three toes are anterior ; our single exception is 

 in the genus Pico'ides, where the true hind toe is wanting, the outer anterior one being reversed 

 as usual in zygodactyles. No bird has more toes beliind than in front. Birds' toes, and their 

 respective joints, are 



Numbered, in a certain definite order, as follows (see figs. 34, 36) : hind toe = first toe, 

 1^; inner anterior toe =: second toe, 2t ; middle anterior toe =^ third toe, 3t; outer anterior 

 toe = fourth toe, 4:t. Such identification oilt, 2t, 3t, 4:t applies to the ordinary case of three 

 toes in front and one behind. But, obviously, it holds good for any other arrangement of the 

 toes, if we only know which one is changed in position, — a thing always easy to learn, as we 

 shall see at once. In birds with the hind toe reversed, leaving all four in front, the same 

 order is evident, though then 1 i is the inner anterior, 2 t the next, etc. ; for it always happens, 

 when a hind toe turns forward, that it turns on the inner side of the foot. Similarly, in yoke- 

 toed birds (excepting Trogonidee), it is the outer anterior which is turned backward, as above 

 said ; then, evidently, inner hind toe = lt; inner front toe = 2 ^ ; outer front toe = 3 ^ ; outer 

 hind toe = i t. In Trogonidce, with inner front toe reversed, the coiTection of the formula is 

 easily made. Moreover, when the number of toes decreases from four to three or two, the 

 digits are almost always reduced in the same order : thus, in three-toed birds, 1 / is the missing 

 one ; in the two-toed ostrich, 1 1 and 2 t are gone. The only known exceptions to this general- 

 ization are afi"orded by two exotic genera of kingfishers, Ceijx and Alcyone, in which 2 ^ is 

 defective ; and by the anomalous passerine Cholornis of China, in which it is in like case. 

 The rule is proven by the 



Number of Phalanges, or joints, of the digits. The constancy of the joints in birds' 



toes is remarkable, — it is one of the strongest expressions of the highly monomorphic character 



of Aves. In cdl birds, excepting Procellarudcc, 1 1 when present has two joints (not counting, 



of course, the accessory metatarsal). In all birds, 2 t when present has tJiree joints. In nearly 



all birds, 3 t has four joints. In nearly all birds, 4 t has five joints. Thus, any digit has one 



more joint than the number of itself. The exceptions to this regularity consist in the lessening 



of the number of joints of 1 t or 3 thy one, and of 4 ^ by one or two. So when the joints do 



not run 2, 3, 4, 5, for toes 1 to 4, they run either, 1, 3, 4, 5, or 2, 3, 4, 4, or 2, 3, 3, 3. (These 



statements do not regard the anomalous cases of Ceyx, Alcyone, and Cholornis — see above.) 



Tliis variability is nearly confined to certain Picarian birds : our examples of it are in certain 



ryi genera of Cypselince, fig. 40, where the ratio is 2, 3, 3, 3, 



I of Caprimulgince, fig. 41, where it is 2, 3, 4, 4 ; and the petrel 



[J family, with 1, 3, 4, 5. Such admirable conservatism enables 



<^/7^jt. us to tell what toes are missing in any case, or what ones are 



j^ t) out of the regular position. Thus, in Pico'ides, the hind toe, 



a})parently 1 1, is known to be 4 t, because it is five-jointed ; 



iu a troi^on, the inner hind toe is 2 t, beine three-jointed ; in 

 Fig. 40.-Pha- , ° , ' . , , „ ■, , , , 



langes of Cypse- ^ne ostrich, with only two toes, 6t and 4i are seen to be 



line foot, 2, 3, 3, 3. preserved, because they are respectively four- and five-jointed. 



(See fig. 34, where the digits and their phalanges are numbered.) Besides 



tliis interesting numerical ratio, the phalanges have other inter-relations of 



some consequence in classification, resulting from their comparative lengths. langesofCaprimuU 



In some families of birds, one or more of the basal or proximal phalanges ginefoot, 2, 3, 4,4. 



(those next to the foot — opposed to distal, or those at the ends of the digits) of the front toes 



are extremely short, being mere nodules of bono (fig. 40) ; in other and more frequent cases, 



they are the hmgest tif all, as in figs. 34, 41. On the whole, they generally decrease in length 



fniin pri)xinial to distal extremity, and the last one of any toe is quite small, serving merely 



