ARBOREAL ADAPTATIONS OF HIND-LIMB 65 



The trunk is first suspended upright from the arms upon 

 the extended legs (as in the existing Gibbons) (see Fig. 

 23), next its weight is partially borne 

 upon the extended legs (as in the 

 existing Giant Apes), afterwards it is 

 entirely borne and balanced upon the 

 fully extended legs in all the ordinary 

 activities of the animal (as in Man). 



The anatomical adaptations which 

 accompany these changes, as they are 

 seen in existing Primates, are practically 

 continuous and harmonious ; but this is 

 not equivalent to saying that the evo- 

 lution of the process is seen in progress 

 among existing types. 



In Man, the fascial insertion and the 

 great increase in size of the M. gluteus 

 maximus, the extended fascial inser- 

 tions of other leg muscles, the modifi- 

 cations of the calf muscles (M. gastro- 

 cnemius and M. soleus), and, above all, 

 the development of the M. peroneus ter- 

 tius, are all instances of the speciali- 

 zations of muscles for the balancing of 

 an upright body upon an extended leg. 



These are later changes produced by 

 terrestrial bipedal orthograde habits, in 

 which the suspending assistance of the 

 hands is entirely dispensed with, but 

 all these things had their beginnings 

 in purely arboreal life. 



Only one other change we need 

 notice here, and that is the finishing 

 touch of the eyersion of the foot in Man. 

 When the arboreal hind-limb has been 

 perfected as a supporting organ in an 

 extended position, it is still a purely 



FIG. 23. DIAGRAM 

 OF A GIBBON (Hy- 

 lobates lar) SUS- 

 PENDED BY THE 

 GRASP OF ITS 

 HANDS. 



From a drawing 

 made from a pho- 

 tograph by Prof. 

 Arthur Keith. 

 5 



