THE EVOLUTION OF MAN 195 



than those provided by the body itself, the possibihtics are without 

 hmit. Inventiveness in the human race has gradually brought us 

 to our present state. We cannot yet see the limits of our powers 

 in this direction, but our attainments are only the gradual accu- 

 mulation of the ages. 



Evolution of Terrestrial Primates. Such diversity of activities 

 and its resulting diverse reactions upon the organism suggest 

 that subsequent physical modification would be slight. With ade- 

 quate locomotion, a free pair of grasping appendages, and the 

 ability to make use of inanimate objects to compensate inherent 

 deficiencies, everything would favor continued development along 

 the same lines, and increasing mental power would be the result. 



One physical modification begun during arboreal life might be 

 expected to continue. Brachiation does not emancipate the fore- 

 limbs to the extent that is possible in terrestrial life, and conse- 

 quently does not favor the development of completely erect pos- 

 ture. Terrestrial life supplies a dependable support on which 

 the hind-limbs are adequate for locomotion. Increase in stability 

 of equilibrium and the consequent freedom of the arms from acces- 

 sory locomotor functions would make possible the maximum 

 maintenance of the erect position, and would favor any changes in 

 structure dependent upon it. 



The Results of Erectness. Man is the only available example 

 of a wholly erect animal. In his body are found all of the modifica- 

 tions which depend upon the change from quadrupedal to erect 

 posture. Such a change involves primarily a shifting of the hori- 

 zontal axis of the body to a vertical position with concomitant 

 changes in anatomy, but Jones logically cautions against accept- 

 ance of this shift as an explanation. It seems to him rather an 

 outcome of "an arboreal apprenticeship." "Walking upright 

 upon the surface of the earth," he points out, "has produced its 

 changes in the human body, of this there is no doubt; but we must 

 be careful to distinguish between these 'finishing touches' and 

 those other changes which are so much older and so much more 

 important — the adaptations to arboreal life." 



These "finishing touches," since they include a fundamental 

 change in the axis of the body, bring about compensating changes 

 in axial structure (Fig. 113). The spinal column of a quadruped is 

 arched between the supporting appendages, and has flexible ante- 

 rior and posterior parts, the neck and tail. Rotation through 90° 



