THUMBS AND BIG TOES 



monkeys of the genus Semnopithecus, which includes the 

 sacred Langurs, the thumb is small but still opposable, 

 and in the allied African genus Colubus it is reduced to a 

 mere tubercle, or is altogether absent. The well-developed 

 big toe is present in all (see Fig. 26). In the Anthropoids, 

 as a rule, the thumb and the big toe are well developed 

 and opposable. 



We may therefore say that among the scattered and 

 very diverse living members of the Primates the develop- 

 ment of the thumb and big toe shows 

 both from the point of view of mere size, 

 and as opposable digits, some striking 

 irregularities. But it is not to be 

 doubted that the underlying principle 

 is clear enough, that the arboreal habit 

 develops the specialized and opposable 

 thumb and big toe, and that peculiar 

 habits of climbing account for the actual 

 condition present in the hand and foot 

 of any individual species. A freakish 

 development of tree-climbing, or an 

 overdoing of the pure ability to climb, 

 may lead to secondary specializations 

 away from the simple condition. The 

 stock from which Man has sprung 

 shows in this, as in so many other 

 features, a tempered adaptation to the 

 arboreal habit without the development 

 of any secondary specializations. 



We may imagine that, from some 

 early stage in which both thumb and 

 big toe were equally specialized for pre- 

 hension, the human stock cultivated especially the hand 

 as the grasping organ, and so retained and perfected the 

 opposable thumb. Some other members of the Primate 

 stock depended more upon the grasp of the foot, and so 

 have retained and specialized an opposable big toe, at 



26. THE 

 LEFT FOOT OF 

 Macacus N ernes - 

 trinus AS SEEN 

 FROM ITS PLAN- 

 TAR ASPECT. 



