Pix 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 
F 
Fic. 111.—Series of metacarpals and metatarsals of Camelidae, to show secular and 
progressive increase in size. From left to right the species are Protylopus 
petersont, Poebrotherium labiatum, Gomphotheriwm sternbergi, Procamelus occt- 
dentalis. 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. 
