All living things on the earth need other living things to live. Nothing lives alone. Most animals must live in a group, and even a plant grows close together with others of the same kind. Sometimes one living thing kills another. One eats and the other is eaten. Each kind of life eats another kind of life in order to live, and together they form a food chains( 食物链) Some food chains are simple, while others are not. But all food chains begin with the sun, and all food chains become broken up if one of the links disappears.

    All life needs sunlight to live on. But only plants can use sun-light directly. Plants are “factories”. They make food from sun-light, water and things in the soil and air.

    Plants feed all other living things. Animals can only use the sun's energy (能量)after it has been changed into food by plants. Some animals feed directly on plants. Others eat smaller animals. Meat-eating animals are only eating plants indirectly.

    What about human beings? We are members of many food chains. We eat wheat, rice, vegetable, fruits and so on. We also eat meat and drink milk, but men often break up the food chains. They kill wild animals. They also make rivers, lakes and seas dirty. When these rivers, lakes and seas are polluted, the fish in them can not be eaten. If men eat fish, they will get strange diseases.

Each form of life is liked to all others. Breaking the links puts all life in danger.

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吕思勉先生把古代中国历史划分为“部落”、“封建”、“郡县”三个先后相继的时代。其中与“封建时代”相对应的朝代是（    ）