Millions of years passed; great changes took place. Parts of the land slowly sank beneath the seas, and out of the seas rose dry land. Most of the larger reptiles died, but the mammals multiplied. They grew to be as large as the fox or the sheep. At first they were all very much alike and they lived in about the same way. But as they became more and more unlike they had very different ways. Some became like cats, and some like dogs. [Illustration: _An ancestor of our mammals_] Some became like rhinoceroses and some like hogs. Others became like monkeys, and still others became like horses. THINGS TO DO _Look at the picture above and tell how this animal must have changed to become more like a dog, a cat, a pig, a cow, or a horse._ XVII. THINGS TO THINK ABOUT What part of the wild horse's foot touched the ground when he ran? What happens to muscles that are not used? What happens to muscles that are used most? _The Story of the Wild Horse_ Perhaps you have seen the zebra. If you have, you must have noticed its stripes. The first horse-like creatures were probably striped in much the same way. These animals never ate hay and oats; and, at first, they did not eat much grass. There was little, if any, grass at that time. These horse-like creatures lived on marshes and in swamps bordering streams and lakes. They probably ate stems and leaves of plants that grew on the marshy lands. They did not run as horses do to-day, but they plodded along on the soft ground. They spread out their toes as they walked along, so as to keep from sinking. When the dry land began to be covered with grass, little by little they left the marshes. They went to the grassy highlands to live, and became more and more like horses. Some of the cat-like creatures went to the highlands, but they did not go for the grass. They were becoming more and more like tigers, and they went to hunt the wild horses. The timid horses ran when their enemies came, for there was no place in which to hide. When they ran they stepped on the tips of their toes. They used the third toe so much more than the others that it became larger and larger. Its hoof became hard and strong. The first and fifth toes were used so little that after thousands of years they disappeared. Meanwhile the horses had learned to run fast. When a pack of wolves chased them, they galloped away. They found rich grass on the highlands. Colts grew to be larger than their mothers. By the time they were as large as ponies they used only the third toe. Two side toes hung like the dew-claws of a dog, but they did not reach the ground. When the Tree-dwellers lived, little bones beneath the skin were all that was left to show where the side toes had been. The hoof had become round and hard. The wild horses ran very fast. They outran all other creatures.