EMANCIPATION OF THE FORE-LIMBS 13 



stage, that much of the interest of the modern study of 

 human morphology is centred. It is the latter view, 

 that the human stock has never been typically prono- 

 grade and four-footed, that is here put forward as the 

 truth. 



In attempting to maintain this view, definite answers 

 must be given to three questions. The first: What was 

 the factor that saved the particular mammalian stock 

 which culminated in Man from becoming four-footed 

 pronogrades?" We will answer by saying at once, " The 

 arboreal habit." The second: "When did this factor 

 come into play in the philogenetic history of the Mam- 

 mals?" We will dismiss this by asserting that it was at the / 



very outset, at the very dawn of mammalian life. The ^ 



anatomical basis for this assertion will be given in detail ' 

 later on. The third: " How did this factor enable that i 

 particular stock to acquire supremacy?" will be answered, \ 

 so far as is possible, by the study of the influence of the 

 arboreal habit upon the animal body. 



We will deal first with the influence of the arboreal 

 habit upon the structure and function of the limbs. We 

 are assuming that the primitive Mammal, new born from 

 the Therapsidian ancestor, possesses limbs such as we 

 have deflned, with but little stability, but with a high 

 degree of mobility, and this mobility includes the power 

 of rotating the second and third segments around the 

 central axis of the limb in the actions of pronation and 

 supination. The effects of mammalian habit upon these 

 limbs will probably be best appreciated by following the 

 story as it is unfolded in animals that directed their 

 newly acquired mammalian possibilities into the natural 

 channel of supremacy in walking and running over the 

 surface of the earth. 



All four limbs of such an animal will become equally 

 developed as organs of support and of progression (see 

 Fig. 5). Mobility at shoulder, elbow and wrist, hip, knee 

 and ankle will be essential, but stability becomes a prime 



