BONES OF WRIST AND ANKLE 



197 



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 



H 



H 



H 



FIG. 111. Series of metacarpals and nietatarsals of Camelidae, to show secular and 

 progressive increase in size. From left to right the species are Protylopus 

 petersoni, Poebrotherium labiatum, Gomphotherium sternbergi, Procamdus occi- 

 dentalis. F, Fore-foot ; H, hind-foot ; III, IV, third and fourth nietapodials. 

 (After Wortmau.) 



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. 



