62 MANAGEMENT AND BREEDING OF HORSES 



the ball-and-socket kind, which allows free motion of the 

 limb in all directions, becomes keeled and grooved like a 

 pulley wheel, permitting free motion forward and back- 

 ward, but limiting the motion in all other directions and 

 increasing the strength of the joint. The increased 

 length of the lower leg increases the length of the stride 

 without decreasing its quickness, thus giving the animal 

 greater speed ; and the heavy muscling in the upper leg in 

 connection with the increased strength at the joints gives 

 greater strength. Additional strength is obtained by the 

 consolidation of the two bones of the forearm (ulna and 

 radius) and of the leg (tibia and fibula). 



The increase in the length of limb renders it necessary 

 for the grazing animal that the head and neck should 

 increase in length in order to enable the mouth to reach 

 the ground. The increase in length and crown develop- 

 ment of the teeth enables the animal to subsist on the 

 hard, comparatively innutritions grasses of the dry plains, 

 which require much more thorough mastication before 

 they can be used as food than do the softer green foods of 

 the swamps and forests. 



All these changes in the evolution of the horse are 

 adaptations to a life in a region of the level, smooth and 

 open grassy plains which are now his natural habitat. In 

 the beginning, the race was better fitted for a forest life, 

 but it has become more and more completely adapted to 

 live and compete with its enemies or rivals under condi- 

 tions which prevail in the high, dry plains. The increase 

 in size, which has occurred during this evolution, is de- 

 pendent on abundance of food. While a large animal 

 requires more food in proportion to its size than does a 

 small one, in order to keep up a proper amount of activity, 

 yet the large one is better fitted to defend itself against 

 its enemies and rivals. Thus, as long as food is abundant, 

 the large animals have the advantage of the smaller ones 

 and tend to become continually larger until a limit is 

 reached, when sufficient food becomes difficult to obtain, 



