240 HISTORY OF THE HUMAN BODY 



The set of supinators and pronators may next be considered, 

 and in Necturus, in which in each limb there is a single 

 supinator on the extensor side, and a single pronator on the 

 flexor side, the homologies are evident. In all cases they 

 extend from a more proximal origin upon the outer side (ulnar 

 or fibular) to a more dorsal insertion upon the inner (radial or 

 tibial). In the human arm there are two pronators, teres and 

 quadratus, but as these are continuous in many marsupials and 

 carnivores, they may be considered as derivatives of the single 

 urodele muscle. Their homologue in the leg is undoubtedly 

 the popliteus. Upon the dorsal side most works on human 

 anatomy record two supinators,, longus and brevis, but as the 

 longus [=brachioradialis BNA] is really a portion of M. 

 brachialis, and belongs with the upper arm, the only true 

 supinator is the one designated brevis [supinator, BNA], 

 undoubtedly the same as that in the urodeles. Its homologue 

 in the leg seems to have disappeared. 



The remaining muscles, those controlling the action of the 

 separate digits, are still more in accord, and that too in spite of 

 the great difference in use between the hand and foot, especially 

 in civilized man, suggesting the conservatism of these parts, 

 and the fact that it is easier to keep a complicated structure, 

 when once obtained, even when not used in all its parts, than 

 to replace it with a simple structure without unnecessary parts, 

 provided only that the more complicated structure is in no case 

 detrimental to the effective working of the organ in its simpli- 

 fied function. It is often presumed that the reason why such 

 changes as these have not occurred is that the time has been 

 insufficient to effect it, but this is not the case. The only 

 reason for an adaptation lies, not in the lapse of time, which 

 in itself is powerless to effect even the slightest change, but in 

 the question of expediency for the animal, that is, whether the 

 part comes within the power of natural selection or not. In 

 the present case the foot of man and his immediate ancestors 

 has borne its present shape from pre-glacial times, a period 

 given by conservative estimates at 50,000 to 100,000 years, and 

 has as yet undergone but little change along the line of reduc- 



