Have you ever seen a horse with toes (脚趾) 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 no bigger than cats.
These small horses lived in the forest (森林). Their many toes helped the horses run over the soft (柔软的) , wet ground.
It was very hot in the forest. Bat the weather changed. It became cold (寒冷的,伤风). Many trees could not live in cold weather. The trees died and fell (落下;下降). Open fields took the place of forests. 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. Their middle toes became hard. After a long while, horses had only one hard toe on each foot.
At that time, the cat-like horses could climb (攀登,上升) trees.