196 EVOLUTION AND GENETICS 



from a fixed attachment to the pelvic girdle brings about some 

 curvature immediately above the point of attachment; this lumbar 

 curvature is incipient in monkeys, and well developed in man. 

 The head, supported without change of position upon this new 

 axis, would point upward rather than forward. Its articulation 

 with the vertebral column in quadrupeds allows only partial in- 

 clination, since flexibihty of the neck provides a greater latitude of 

 movement. The vertical position of the axis is compensated by 

 a shift in position of the occipital condyles, which are ventrocaudal 

 in quadrupeds. In arboreal primates, and to a greater degree in 

 man, they have shifted along the formerly ventral surface of the 

 skull to such a degree that this surface has also swung through 90° 

 and become caudal instead of ventral. The face is thus brought 

 forward. 



Changes in the skull are not, however, entirely referable to the 

 changed axis of the body. Reduction of the prognathous form is 

 more directly correlated with the ability of the organism to use 

 its hands for grasping, breaking or tearing, and bringing food to 

 the mouth. The mouth in an animal of even semi-erect form is 

 not its only facility for handling objects, and consequently is not 

 used in the same way as the prognathous mouths of other 

 animals. 



Of the remaining parts of the skeleton the pelvic girdle and limbs 

 alone undergo great change. The pectoral girdle and limbs are 

 more primitive than in many other mammals. In comparison 

 with the horse, for example, the hands of man are still in the 

 primitive pentadactyl state, while the fore-limbs of the horse have 

 lost four digits. The forearm in man contains the primitive bones, 

 the ulna and radius, and they retain their primitive flexibility of 

 movement. In the horse the ulna is reduced to a vestige which is 

 combined with the radius, and the articulations are so modified 

 that the limb moves in one plane, forward and back. Man also 

 retains the clavicle, which is no longer present in the horse. 



The pelvic girdle in man is compact and firmly articulated with 

 the spinal column, as in all bipedal animals. Shift of the body axis 

 places the stress of body weight differently upon its articulation, 

 however, and the result is an elongation of the articular surfaces 

 cranio-caudally, in contrast to the dorso-ventral elongation found 

 in quadrupeds. The shift of stresses also throws the weight of the 

 viscera toward the pelvis instead of toward the ventral body wall, 



