阅读理解

　　Millions of years ago, horses had many toes(脚趾)．They had four toes on each front foot and three toes on each back foot．

　　The horses were no bigger than cats．

　　These small horses lived in the forest．Their toes helped them run over the soft, wet ground．It was very hot in the forest．But the weather gradually(逐渐地)changed．It became colder and colder．Many trees could not live in cold weather．They died and fell, and 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, long time．On the dry and hard land, horses needed only their middle toes for runing．Their middle toes became hard．After a long while, horses had only one hard toe on each foot．