182 ANATOMY OF THE WOOD RAT 



diverge from the center of the trunk, the humerus and femur 

 converge, while the forearm and lower leg diverge once 

 more. The foot proper is also inclined forward, while the 

 tendency of the toes is to bend rearward. The hand or 

 fore foot seems to present an anomaly to this line of reason- 

 ing, for it, too, is inclined forward, but this is an adaptational 

 modification and its present position was not originally 

 fundamental. It must be remembered that the primal 

 position of the limbs was lateral, with the feet extended 

 laterad instead of craniad. A rearward inclination of the 

 fore foot would obviously constitute a modification im- 

 practical in a quadruped; so the rotation of the forearm and 

 change to a forward inclination of the hand was forced 

 during the evolution of the Mammalia coincident with the 

 abandonment of the purely lateral extension of the limbs. 

 This is illustrated in the development of embryos, for in the 

 early stages the radius and ulna are parallel, while later the 

 hand is pronated and there are changes in the wrist to fit 

 the position of the fore foot which has proved to be struc- 

 turally advantageous. In man and such other mammals 

 as have no rigid articulation of the radius and ulna, the 

 preadaptational position of the hand may be assumed at will, 

 or almost so. It will then be apparent that in this position 

 the muscles which raise the hand towards the arm act as 

 flexors rather than extensors as normally termed, and that 

 considered as counterparts, the flexors of the hand must 

 be treated as corresponding to the extensors of the foot, and 

 vice versa. 



Before one could be qualified to present an authoritative 

 opinion regarding the theory of antero-posterior synometry 

 as affecting the limbs he would be obliged to do a vast 

 amount of research on the subject. Concerning this the 

 writer offers no opinion one way or the other. There are 

 several objections to this thesis but they do not seem to be 

 insurmountable. Even though there be no true homology 



