464 POPULAR SCIENCE MONTHLY. 



of an arboreal habit of life. The means of support afforded by the 

 branches is too precarious, the form of locomotion which practical con- 

 ditions impose upon the animal too restricted and interrupted to make 

 the development of such a limb as the human leg possible. The need 

 of supplementary support to which an unstable balance must give rise 

 and the facility with which the arms can come to the aid of the legs as 

 the animal makes its way from tree to tree are likewise factors which 

 retard the development of efficient bipedal locomotion. The freeing 

 of the hands may therefore be regarded as a concomitant of the return 

 of man's progenitor to a terrestrial habitat, in which free, large and 

 continuous movements of locomotion were both possible and necessary. 

 Only on the wide, open spaces of the ground can we conceive the ape- 

 man to have become a swift and sustained runner, holding the body 

 upright and the arms free. 



At the same time with the changes in the leg already described the 

 habit of traveling over the level surface of the ground would tend to 

 produce a closer knitting of the ligaments of the foot and a greater 

 compactness and rigidity in its general structure. The opposition of 

 the great toe, no longer necessary to preserve the animal's equilibrium 

 — since this is sufficiently secured in a lateral direction by the relation 

 of the two legs — becomes a distinct impediment to land travel, owing 

 to its interference with the movements of the fellow limb and its 

 liability to injury by striking upon the objects among which the 

 animal walks. With the further development of the foot, however, 

 whether degenerative or other, we have not here to do. 



As regards the special causes which led to the adoption of a ter- 

 restrial habitat in preference to the earlier arboreal life, it is probable 

 that the change was intimately related to the development of the 

 opposable thumb. The platyrrhine monkeys have the same type of 

 foot as that possessed by the man-ape and do not progress predomi- 

 nately by swinging as do the long-armed apes. Anatomically, there- 

 fore, they differ from the progenitor of man chiefly in the fact that, 

 unlike him, they have retained the parallel-fingered hand. In this 

 differential feature resides their disability. The monkey form of hand 

 is adequate for seizing and clinging to branches, but deficient in adapta- 

 bility to all other mechanical purposes. For grasping and pulling, for 

 digging and tearing, for handling stones and sticks the human hand 

 with its opposable thumb is incomparably superior. Among the uses 

 for which, in virtue of these capacities, it is especially fitted are the 

 employment of weapons, the construction of means of defense from 

 attacks by carnivorous beasts and later the use of tools. 



The relinquishment of an arboreal habit involved the giving up of 

 an important refuge and the assumption of a mode of life assailed by 

 many new and grave dangers. The tree is a place of safety; it affords 

 a secure retreat from some enemies and concealment from many others. 



