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 no bigger than the cats. These small horses lived in the forest. The toes helped the horses run on the soft, wet ground.

    At first, it was very hot in the forest. But the weather changed. It became very cold. Many trees could not live in cold weather. The trees died and fell. 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 time, horses had only one hard toe on each foot. Now we call this hard toe a hoof(蹄子).

【青春邀约 款款而来】青春，带着一份特殊的邀约，款款而来。在这个时期，我们的身体会发生一定的变化，我们的内心会面对一些新的问题，我们看待自然和社会的观点也会有所改变。悄悄变化的我们，成长的不仅仅是身体。

我国黄土高原水土流失十分严重，在黄河流域，与其相关的另一种灾害是____。