AND ITS INHABITANTS 141 



reaching its highest development in the modern gibbon. On 

 the ground, the gibbon walks erect, either touching the knuckles 

 of the hands to the ground or with the arms held above the 

 head. The gait is quick, waddling, with no elasticity of step, 

 and they are soon overtaken. But in the trees they are virtu- 

 ally transformed, for their hand leaps are prodigious, twelve, 

 eighteen, one authority says no less than forty feet being 

 cleared, and that for hours at a time. Fully to appreciate 

 what this means one should compare it with the precise 

 mechanical stride of a racehorse, for whereas in the horse 

 there is a practical uniformity of conditions stride after stride, 

 with the gibbon no two hand leaps can be the same, and each 

 time such a thing is essayed a problem must be solved, for in 

 order neither to over- nor under-shoot the mark, the right 

 amount of force must be used, and this varies with the distance 

 and the ability of the objective branch or branches to bear 

 the creature's weight. Thus, as with a gun pointer, the aim, 

 distance, trajectory, windage, and in addition the varying 

 force, all must be taken into consideration; in the present 

 instance, moreover, the problem must be solved and its prac- 

 tical application brought about instantly, and the penalty for 

 error either in solution or application may be death. Even 

 though the process becomes automatic, the result of much 

 experience, there is, nevertheless, a high premium placed upon 

 mental acuteness, and the weeding out of the unfit is ruthless. 

 Add to this the fact that the hands may be used to bring objects 

 before the face for examination, thus inciting the powers of 

 observation, and two great stimuli to higher mentality are 

 attained. There is reason to believe that the human precursor, 

 before leaving the sheltered life of an arboreal primate, pro- 

 gressed and acted much as do the gibbons, with a consequent 

 quickening of intellect as time went on. 



Descent from the trees. But while tree-life had much to do 

 with the prehuman evolution of our ancestors, the arboreal 



