14 ARBOREAL MAN 



necessity, and the rotation of the parallel bones of the 

 second segment is a hindrance to perfect stability. Little 

 by little this power of rotation becomes lost; the muscles 

 which produce the movements of pronation and supina- 

 tion disappear, or change their action, the joints between 

 the two parallel bones become less perfect; finally the 



FIG. 5. DIAGRAMMATIC DRAWING TO SHOW THE CONDITION OF 

 LlMBS WHICH HAVE BECOME THOROUGHLY STABLE, AND 

 FUNCTION BOTH AS PROPELLING AND SUPPORTING ORGANS. 



two bones fuse together, and one member of the pair 

 practically ceases to exist. Again, the digits, except by 

 virtue of the nails or claws which they bear, cease to be 

 of great individual importance, and some of them soon 

 become reduced to the condition of mere rudiments. 

 The final stage of this process is exemplified in the horse, 

 where one functional digit alone remains, upon the nail 

 (hoof) of which the animal is supported. These four 

 limbs are now stable props which, capable of very definite 

 and specialized movements, support the animal and 



