54 ARBOREAL MAN 



very great. There is a ventral meeting of the elements of 

 the pelvic girdle at the pubic symphysis. This meeting 

 of the ventral parts of the pelvic girdle subserves the 

 functional role played by the clavicle in the anterior 

 extremity, and this bone is not represented morpho- 

 logically in the skeleton of the posterior extremity. The 

 hip-joint obviously corresponds to the shoulder. The femur 

 with its muscles recapitulates the features we have noted in 

 the humerus, the knee-joint corresponds to the elbow. 



The tibia and fibula, free to move on the femur at the 

 knee-joint, and free to move upon each other, are the 

 homologues of the radius and ulna (see Fig. 19). The 

 ideal tarsus consists of nine bones : tibiale, intermedium 

 and fibulare constituting the first row, tarsalia I.-V. the 

 second, and an os centra] e is included between the two 

 rows, this condition obviously reproducing that we have 

 already noted in the carpus. Five metacarpals and five 

 digits with their appropriate muscles complete the archi- 

 tecture of the foot. The digits are composed (as in the 

 hand) of three separate phalanges, and all the digits are 

 well developed, with the middle one as the longest member 

 of the series. 



These are the features which we may presume to be 

 present in the primitive hind-limb, and they will become 

 modified, now in this direction and now in that, as func- 

 tion demands it. 



There is a strong presumption that the hind-limb will 

 depart more early than the fore-limb from this primitive 

 condition, and this presumption is strongly borne out by 

 the facts. From what we have seen of the effects of even 

 minimal demands for a supporting function in the case 

 of the fore-limb, we shall not expect to find a thoroughly 

 primitive hind-limb at all far up in the land-living verte- 

 brate stock. The hind-limb was called on, in the land- 

 living Vertebrates, at the very dawn of mammalian 

 specialization, as a support for the body weight. Sta- 

 bility became substituted at the outset of the story for 



