424 OUTLINES OF EVOLUTIONARY BIOLOGY 



existing Simiidse, arboreal in habit. Their limbs, whose primi- 

 tive pentad actyl structure indicates their origin from some 

 little-specialized mammalian type, had ceased to be used exclu- 

 sively as organs of locomotion on the ground and become adapted 

 for climbing trees. Hence the opposable toe and thumb, which 

 enabled their possessor to obtain a firm grasp of the branches. 

 In the existing apes hand and foot are very similar, and. both, 

 as regards function, partake as much of the nature of hand as of 

 foot, whence the name " Quadrumana " applied to the group by 

 the older naturalists. In many of the lower apes or monkeys a 

 long prehensile tail, which can be twisted round the branches, is 

 of great assistance to the animals in their arboreal habits, but 

 in the higher forms, such as the chimpanzee, the gorilla and 

 the orang utan, the tail has already disappeared. 



These tailless forms are still mainly arboreal, but when they 

 have occasion to come to the ground they assume a semi-erect 

 posture in locomotion. In walking they usually put their hands 

 to the ground, but generally resting upon the backs of the fingers, 

 instead of upon the palms as in the lower apes. 1 Hence in these 

 large old-world apes the hands and feet are more completely 

 differentiated from one another. 



The next step in the evolution of man was probably the 

 gradual abandoning of arboreal habits and the assumption of a 

 more completely erect attitude during locomotion. This appears 

 to have been the real starting point of his human career. It 

 was this change of habit which led to those structural modifica- 

 tions required to balance the body properly in its new position, 

 and to the further differentiation between hand and foot, the 

 latter being used to the complete exclusion of the former as an 

 organ of locomotion and at the same time losing its prehensile 

 character, so that one of the chief distinguishing features between 

 man and the higher apes is that in man the great toe has ceased 

 to be opposable. 



In this way the hand was completely set at liberty for the 

 purposes of a prehensile organ. It was, however, no longer used, 

 as in the apes, chiefly for laying hold of branches in climbing 

 and for conveying food to the mouth, but more for grasping 

 loose objects and converting them into weapons or tools for 

 different purposes. Hanno observed that the anthropoid apes 

 which he met with in Africa defended themselves with stones, 



1 Vide Beddard, " Mammalia " (Cambridge Natural History, Vol. X), p. 570. 



