CHAPTER XI 



OTHER AEBOREAL ADAPTATIONS OF THE 

 HIND-LIMB 



BESIDES the features which we have already noted in the 

 arboreal hind-limb there are others of equal, or even 

 greater, importance in the story of the evolution of Man 

 as an arboreal animal. These other changes can only be 

 referred to in outline, since the details of anatomical 

 arrangements connected with them are legion. In pic- 

 turing the early stages of the development of climbing, 

 it was noted how the animal, supporting its body weight 

 temporarily on its hind-limbs, reached out ahead for a 

 new hold for its hands. This was the interval which 

 marked the dawn of specialization of the functions of 

 fore and hind limbs, and in which stability was demanded 

 in some measure from the hind-limb. But in this interval 

 another, and a very important, thing is happening, for 

 as the animal reaches ahead, its body axis is altered, and 

 the support of the hind-limbs is called upon in a very 

 special manner. In this interval of climbing up, the 

 body axis approaches the vertical, and the animal be- 

 comes in this way a temporary orthograde. There are 

 degrees in this development of an orthograde habit, even 

 if it be only a temporary phase, as in the primitive arboreal 

 enterprise we are picturing. An animal may carry its 

 body axis upright as a temporary expedient or as a life 

 habit, while still retaining its thigh at right angles to its 

 trunk; or it may hold its trunk erect upon an extended 

 thigh. 



There are many animals which can maintain the trunk 

 62 



