ment of these bones, the distal bones only, or very nearly only, articulating with the corresponding bones in the upper series. In the modern types (cf. Fig. 113) there is, on the other hand, an interlocking, so that the bones of the distal series articulate with
Fig. 111.—Series of metacarpals and metatarsals of Camelidae, to show secular and progressive increase in size. From left to right the species are Protylopus petersoni, Poebrotherium labiatum, Gomphotherium sternbergi, Procamelus occidentalis. F, Fore-foot; H, hind-foot; III, IV, third and fourth metapodials. (After Wortman.)
two of those of the proximal series. By this is produced, as it would appear, a much firmer foot, less liable to "give" under pressure, and thus more fitted for an animal that runs. It is the same principle as that adopted in the laying of bricks. The actual stress and strain of impact has been held responsible for those changes. An equally ingenious and possibly truer explanation of the undoubted facts has lately been advanced by Mr. W. D.
