Page:Popular Science Monthly Volume 68.djvu/347

Rh unto themselves the whole articular surface of the carpus and tarsus (D).

Like the pig, the little water-deer (Dorcatherium) possesses four distinct functional toes, but in Tragulus, a closely related form, the outer toes are exceedingly slender and do not articulate proximally (E). The upper ends of these small digits have been reduced in the foot of the roebuck (Capreolus carca); in the extremities of the red deer (Cervus elaphus), these digits are represented only by the bones of the phalanges and vestiges of the metacarpals and metatarsals (F). In the foot of the sheep the outer digits are reduced to two small phalanges (G); these are absent in the foot of the ox and the antelope. Finally, the small hoofs, the only vestiges of the second and fifth digits of the ox, disappear in the extremities of the giraffe and the camel (H).

This series of extremities thus shows a reduction from five to two digits. The gradual atrophy of these three toes has been ascribed to the specialization of the foot as an organ of rapid locomotion. Primitive

 A series of arterio-dactyl extremities showing successive reduction of the digits from five to two. A, Ancodus (fossil); B, Hippopotamus; C, Hyopotamus (fossil): D, Sus; K, Tragulus; F, Cervus; G, Ovis; H, Camelus.