“I ought to have,” said Perry, “it’s what I want to work at, though I haven’t had much chance in the field yet. Well,” he continued, “after a few hundred thousand years had gone by, Nature improved on the horse. She found she could make a better horse, if he was a little bigger, and that he could run quicker on three toes than on four. So in the next layer of rocks, which we call Oligocene, and which we can liken to the lower layer of the oak shavings, you’re apt to find skeletons of that three-toed horse, which is called Mesohippus, same as you’d find a knife-whittled oak horse in the lower layer of oak shavings.” “And ‘Mesohippus’ means a ‘what-horse’?” queried Round-up Dick, remembering that ‘hippus’ was a ‘horse.’ “A middle-horse,” the boy answered, “halfway between the dawn horse and the modern horse.” “Deal again,” said the cowboy. “Another few hundred thousand years went by, a different series of rocks came and Nature again improved on the horse. She saw that he would be better if he were still larger, and swifter if only one toe reached the ground. So in the next layer, which corresponds, Dick, with the top part of the layer of oak shavings, we find a horse called the Protohippus. He had three toes, but only one of them touched the ground, the other two hung useless on either side.” [Illustration: _Courtesy of American Museum of Natural History._ FINDING THE MESOHIPPUS. The Oreodon formation, in South Dakota, with the Museum expedition in the foreground.] [Illustration: _Courtesy of American Museum of Natural History._ MESOHIPPUS, THE THREE-TOED HORSE. The swifter Middle Horse, the size of a coyote, adapted for hard ground: threateningly watched from a distance by Dinictis, the light-limbed sabre-tooth cat.] The range-rider nodded sagaciously. “An’ what’s the face value o’ ‘Proto-hippus’?” he queried. Perry puzzled for a moment. “The ‘Before-Horse,’ I suppose; it’s really almost a true horse.” “An’ the boxwood layer is the ponies o’ to-day?” “No, Dick,” said the boy. “The next layer, which the geologists would call Pleistocene, has true horses, with a single hoof, and with the other two toes reduced to small splints of bone that don’t appear outside the skin. These are called horses. There were vast herds of them roaming over the plains.” “An’ this little bronc,” said Round-up Dick, slapping his pony’s neck, “has come down from them, eh?” “No,” answered Perry. “All the American horses went out, bang! Why, no one knows.” “Where did we get the broncs, then?” “From the Spaniards and early colonists of America.” The cowboy looked incredulous. “How about the Indian mustang?” “Same thing,” asserted Perry. “Then what did the Indians ride on before the Spaniards came?” “They didn’t ride.” The cowboy turned to Antoine for confirmation, and the young paleontologist nodded in support of Perry’s assertion.