6lO HISTORICAL GEOLOGY 



earlier horses. Besides the reduction in the number of toes, the leg 

 had lengthened as the body thickened, and the animal stood about 

 1 8 inches high. The teeth were still short-crowned and lacked the 

 complicated structure of the later forms. 



A Miocene horse (Protohippus, or Hipparion, Fig. 547) shows the 

 next stage in the evolution of the race. In these animals there was 

 one large toe on each foot, with two smaller slender toes, one on each 

 side of it, which were of no use to the animal, as they did not reach 

 the ground when it walked. The teeth are very like those of the 

 modern horse, in which the plates of enamel form curved, complex, 

 irregular patterns (p. 602), but are shorter and probably wore out 

 at an earlier age. The average height of the animal was about three 

 feet. Associated with this more highly specialized horse (Proto- 

 hippus) were others with short-crowned teeth and with all three toes 

 functional (Parahippus and Hypohippus). 



The stage between the Miocene horse (Protohippus) and the true 

 horse (Equus, Fig. 547) is not* definitely known, but was doubtless 

 represented by an animal with a large central toe and with either 

 very diminutive side toes or large splints, and longer and more per- 

 fect grinding teeth. 



Summary of the Evolution of the Horse. — The changes, there- 

 fore, which took place in the horse family during its geological his- 

 tory are: (1) a reduction in the number of teeth from 44 to 36, 

 accompanied by a lengthening and perfecting of the grinding teeth ; 

 (2) a reduction in the number of the toes from five to one; (3) an 

 improvement of the joints of the legs by means of which motion was 

 permitted in but two directions, forward and backward ; (4) a 

 lengthening of the limbs, especially in the lower portions. This 

 has left the center of gravity high, and the limb, though long, moves 

 quickly like a short pendulum, combining rapidity of movement 

 with a lengthened stride ; (5) an increase in the size of the animal ; 

 (6) a proportionally greater increase in the size of the brain than 

 the body; (7) besides the above, other changes, such as the lengthen- 

 ing of the neck and head to permit the animal of increased height 

 to crop grass from the ground ; (8) the gradual perfection of the 

 body; and others of which space will not permit mention. 



Probable Cause of the Evolution of the Horse. — These radical 

 structural changes seem to be the indirect result of a modification 

 of the climate of the Great Plains region of North America and the 

 accompanying change in the character of the vegetation. Eohippus 



