42 ARBOREAL MAN 



their entire morphological separation. I do not think 

 the facts justify us in regarding the human M. pronator 

 quadratus as a degenerated portion of any muscle present 

 in quadrupedal Mammals, but I imagine that this muscle, 

 which produces rotation of the fore-limb bones, is absent 

 in them, and is replaced by a muscle which braces the 

 immobile bones firmly together. The human M. pronator 

 quadratus finds its parallel in the same forms as does the 

 deep head of the M. pronator radii teres, and I imagine 

 that their story is the same, and that their retention is 

 due to the same primitive nature of the forearm in these 

 types. 



It is probable that when all power of rotation of the 

 forearm bones is lost, the ulnar head of the M. pronator 

 radii teres, being useless under the circumstances, shifts 

 its origin to the humerus, and joining into the superficial 

 mass, acts with it as a flexor of the elbow-joint. 



Regarded in this way, and solely from this point of 

 view, the forearm of Man is more primitive than that of 

 any living Primate except the Orang-utan; but it finds 

 its match in the generalized Insectivora, in the Proto- 

 theria (in part), and in the unspecialized Reptiles and 

 Amphibians, and this is a story very like that told by the 

 bones themselves. 



