R. S. Lull — The Evolution of the Horse Family. 165 



Changes in the Feet and Limbs. — The adaptation for speed 

 includes an actual lengthening of the limb, more especially in 

 the lower portions, the thigh and upper arm remaining com- 

 paratively short. The muscles which actuate the legs are 

 near the body, their power being transmitted by long slender 

 tendons. This not alone produces a more graceful member, 

 but, as the center of gravity is high, the limb though long 

 moves quickly, like a short pendulum, combining rapidity of 

 movement with a lengthened stride. 



The foot changes from its primitive posture, in which more 

 or less of the sole rests on the ground, to that in which the 

 weight is borne on the tips of the toes, the claw, or nail, 

 becoming modified into a hoof. The reduction of the lateral 



Fig. 4. Fore feet of a, Orohipjms (Eocene) : 6, Miohippus (Miocene) ; 

 c, Hipparion (Pliocene) ; d, Equus (Quaternary). (After Marsh.) 



digits invariably follows such a change of posture, especially 

 if the animal is becoming adapted for speed on hard ground. 

 In the horse the axis of the foot lies in the third digit, which 

 grows proportionately longer and stronger as the lateral ones 

 reduce. 



The pendulum-like motion of the limbs being all in one 

 plane, the joints between the bones become pulley-like through 

 the formation of interlocking tongues and grooves, which 

 effectually limit any lateral motion. There is also a reduction 

 of the ulna in the fore-arm and of the fibula in the lower leg, 

 as these bones, especially the former, are associated with more 

 varied movement. 



In this evolution the hinder foot is the more progressive, as 

 the fore limb retains its general utility for a longer time. 

 Finally, however, after vast ages, the fore foot overtakes the 

 hind one, and thenceforth the degree of evolution in each is 



