EXTEBNAL PARTS OF BIBDS. — THE FEET. 127 



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

 in the genus Picdides, where the true hind toe is wanting, the outer anterior one being reversed 

 as usual in zygodactyles. No bird has more toes behind 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. 

 It; inner anterior toe = second toe, 3t; middle anterior toe = third toe, St; outer anterior 

 toe ^fowrth toe, 4t. Such identification of 1 1, 2 *, 3t, 4 * 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 vnth the hind toe reversed, leaving aU four in front, the same 

 order is evident, though then 1< is the inner anterior, 2 1 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 Trogonida), it is the outei- anterior which is turned backward, as above 

 said ; then, evidently, inner hind toe = 1 1 ; inner front toe = 2 * ; outer front toe ^ 3 * ; outer 

 hind toe = 4 t. In Trogonida, with inner front toe reversed, the correction 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. It is the missing 

 one ; in the two-toed ostrich, 1 1 and 3 1 are gone. The only known exceptions to this general- 

 ization are afibrded by two exotic genera of kingfishers, Ceyx and Alcyone, in which 2 t is 

 defective ; and by the anomalous passerine Cholornis of China, in which 4 i 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 all birds, excepting Procellariidts, 1 1 when present has two joints (not counting, 

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

 all birds, 3 1 has fimr joints. In nemrly all birds, 4 1 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 1 or 3 1 by one, and of 4 t 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 3, 3, 3, 3. (These 

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

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

 genera of Gypseli/ntB, fig. 40, where the ratio is 3, 3, 3, 3, 

 of Caprimulgints, fig. 41, where it is 3, 3, 4, 4 ; and the petrel 

 famUy, with 1, 3, 4, 5. Such admirable conservatism enables 

 us to teU what toes are missing in any case, or what ones are 

 out of the regular position. Thus, in Picoiides, the hind toe, yO'^^^' 



apparently 1 *, is known to be 4 *, because it is five-jointed; <i'\// [I i 



in a trogon, the inner hind toe is 3 *, being three-jointed ; in fM Jt 

 langei) of '^ypse- the ostrich, with only two toes, . 3 * and 4 * are seen to he ^ (/ 

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

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

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

 some consequence in classification, resulting fi'om their comparative lengths. langesofCaprimul- 

 In some families of birds, one or more of the hasal or proximal phalanges gine foot, 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 bone (fig. 40) ; in other and more frequent cases, 

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

 from proximal to distal extremity, and the last one of any toe is quite small, serving merely 



