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 believe 
that the still more ancient ancestors of this and all other mam- 
mals had five toes on each foot. In the forefoot of the earliest 
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 (Vzver- 
ride). 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 Hyracother1um, 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. 
1 and 2.t WHyracotherium and Eohippus. Lower EoceENE. 
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 line of descent of the 
modern Horse; see frontispiece. , 
