Have you ever seen a horse with toes? Millions of years ago, horses had many toes. They had four toes on each front foot. They had three toes on each back foot. The horses were smaller than cats. These small horses lived in the forest. Their many toes helped the horses run over the soft wet ground. It was very hot in the forest. But the weather changed. It became cold. Many trees couldn't live in cold weather. The trees died and fell. Forests became open field. The sun made the ground dry and hard. Horses began to change too. They began to get bigger. This took a long time. On the dry hard land, horses needed only their middle toes for running. At last their middle toes became hard. Long ago, the horse had four toes on each of its   _  .
back feet front feet right feet left feet