12 EVOLUTION OF THE HORSE 



modern quadrupeds have diverged from a single type, each be- 

 coming adapted to the needs of its especial mode of life. 



The earliest known ancestors of the Horse were small animals 

 not larger than the domestic cat, with four complete toes on each 

 forefoot and three on each hindfoot. There is reason to beheve 

 that the still more ancient ancestors of this and ah other mam- 

 mals had five toes on each foot. In the forefoot of the earhest 

 known stage we find a splint-bone or small, slender rudiment rep- 

 resenting the missing first digit or thumb, which no longer ap- 

 pears on the surface of the foot, while in the hindfoot there is a 

 similar rudiment representing the outer or fifth digit, but no 

 trace is left of the innermost or first digit. The proportions of 

 the skull, the short neck and arched back and the limbs of moder- 

 ate length, were very little horse-like ; recalling, on the contrary, 

 some modern carnivorous animals, especially the civets (Viver- 

 ridcc). The teeth were short-crowned and covered with low 

 rounded knobs of enamel, suggesting those of monkeys and of 

 pigs or other omnivorous animals, but not at all like the long- 

 crowned complicated grinders of the Horse. 



Commencing with the Hyracotherium, twelve stages have been 

 recognized from as many successive formations, showing the 

 gradual evolution of the race into its modern form, and each stage 

 is characteristic of its particular geological horizon. Some of the 

 stages have been found in several parts of the world, but by far 

 the most complete and best known series comes from the Tertiary 

 Badlands of the Western States. Besides the main line of de- 

 scent which led into the modern horses, asses and zebras, there 

 were several collateral branches which have left no descendants. 

 Of some stages all parts of the skeleton have been found ; of others 

 only the jaws, or jaws and feet, are known. We can mention 

 only the more important stages. 



I and 2.' Hyracotherium and Eohippus. Lower Eocene. 

 The Hyracotherium is the most primitive stage known, but only 

 the skull has been found, so that it has not been determined 

 exactly what the feet were like. The teeth display six rounded 

 knobs or cusps on the upper molars and four on the lower ones, 



^ These numbers refer to the stages in the direct Hne of descent of the 

 modem Horse; see frontispiece. 



