EVIDENCE FROM COMPARATIVE ANATOMY 43 



grasping. In all of them, the upper arm has a single 

 bone, the humerus, and the forearm two, the ulna 

 and radius; in all there is a wrist, formed of an as- 

 semblage of small polygonal bones, followed by the 

 long bones of the hand and, finally, by the fingers. 

 Modification is brought about by the reduction and 

 even the suppression of certain parts, the enlarge- 

 ment of other parts and, very much more rarely, by 

 the addition of new parts. Compare, for example, 

 the human arm and hand with the fore leg of a horse. 

 The human hand is a grasping organ and rotates 

 freely on the arm; there are five digits, one of which, 

 the thumb, is opposable to the others, and each digit 

 is tipped with a flat nail. The wrist-joint is composed 

 of eight bones, arranged in two transverse rows; the 

 two bones of the forearm are complete, entirely 

 separate and movable on each other, the power of 

 rotation of the hand being dependent upon the char- 

 acter of the elbow joint. The upper arm-bone forms 

 a ball and socket joint with the shoulder blade and 

 rotates easily in almost any direction. In the horse, 

 on the contrary, the fore limb is exclusively an organ 

 of locomotion and cannot be used for grasping; as a 

 whole, the limb moves only backward and forward in 

 a plane parallel with that of the backbone and each 

 of the joints is movable only in the same fore-and-aft 

 direction, there being no power of rotating the hand 

 upon the arm, or of the fore limb as a whole upon the 

 shoulder. There is but a single functional digit, the 

 third of the original five, the bones of which are so 



