Sleep is very ancient. In the electroencephalographic(脑电图的)sense we share it with all the primates(灵长类动物)and almost all the other mammals and birds: it may extend back as far as the reptiles(爬行动物).

　　There is some evidence that the two types of sleep, dreaming and dreamless, depend on the life style. of the animal, and that predators(食肉动物)are statistically much more likely to dream than prey, which are inturn much more likely to experience dreamless sleep. In dream sleep, the animal is powerfully immobilized(使固定不动)and remarkably unresponsive to external stimuli. Dreamless sleep is much shallower, and we haveall witnessed cats or dogs cocking their ears to a sound when apparently fast asleep. The fact that deep dream sleep is rare among prey today seems clearly to be a product of natural selection, and it makes sensethat today, when sleep is highly evolved, the stupid animals are less frequently immobilized by deep sleepthan the smart ones. But why should they sleep deeply at all? Why should a state of such deep immobilization ever have evolved?

　　Perhaps one useful hint about the original function of sleep is to be found in the fact that dolphins and whales and aquatic mammals in general seem to sleep very little. There is, by and large, no place to hide in the ocean. Could it be that, rather than increasing an animal's vulnerability, Ray Meddis of London University has suggested this to be the case. It is conceivable that animals that are too stupid to be quite on their own initiative are, during periods of high risk, immobilized by the implacable arm of sleep. The point seems particularly clear for the young of predatory animals. This is an interesting notion and probably at least partly true.

　　练习题：

　　Choose correct answers to the question:

　　1.Which of the following might be the best title for this passage?

　　A.Evolution of Sleep.

　　B.Two Types of Sleep.

　　C.The Original Function of Sleep.

　　D.Animals and Sleep.

　　2.Predators are ___________.

　　A.able to prey even when they are in deep dream sleep

　　B.more likely to experience dream sleep

　　C.incapable of preying when immobilized by dreamless sleep

　　D.good at preying on stupid animals

　　3.The example of dogs and cats in the second paragraph is intended to _.

　　A. explain which animals are mammals

　　B. show the differences between mammals

　　C. illustrate how shallow dreamless sleep is

　　D. reveal how smart they are

　　4.Compare with dreamless sleep, deep dream sleep is _______.

　　A. not the result of natural selection

　　B. less likely to appear to primates

　　C. more protective to the animals

　　D. at a higher stage of evolution

　　5.According to some scientists’ research findings, dolphins seldom sleep because __.

　　A.of their stupidity

　　B.of their vulnerability

　　C.there are possible dangers in the ocean

　　D.aquatic mammals do not need sleep

　　1.[A] 本文三段虽然讨论的内容各有侧重，但是都谈到了睡眠在各种动物中的差异以及发展的历史，具体用词如下:第1段第1句中的ancient,最后一句中的extend back;第2段最后一句中的evolved;第3段第1句中的original function。A比较准确地归纳了本文大意。B只是概括了第2段的内容;C只提及第3段的论述内容;D 似乎涵盖了三段内容,但是过于宽泛,不够具体。

　　2.[B] 根据第2段第1句第4个分句可知应选B。A的内容与该段第2句相悖;C中dreamless sleep应为dream sleep。D的内容文中没有涉及。

　　3.[C] 这个例子是为了证明无梦睡眠比较浅，即第2第3句的前半句，所以选C比较合适。所有例子都是为之前或之后的话提供例证，因此,作者使用例子的目的可以从该例子之前或之后的一句话找到。本题中，猫和狗的例 子是为了说明其前面的一句话,其他选项与此无关,均可排除。

　　4.[D] 根据第2段倒数第3句及末句，可以推断与无梦睡眠相比，深睡不动处于更髙的进化阶段。文中虽有提及无梦睡眠对于被捕食动物来说是一种自然选择，但并不意味着深睡不动不是自然选择，因此A不正确;该句也 提到“笨拙的动物与敏捷的动物相比，更少有深睡不动的时候”，primates属于smart ones,因此更有可能深睡不动,B与此相悖;文中暗示无梦睡眠使低级动物更警觉，从而起到一定的保护作用,C正好与这种理解相反，因此也不正确。

　　5. [C]最后一段第2句解释了第1句中所说的“海豚等睡眠少”的原因，C比较确切地反映了文章内容。A、B、D三项都与最后一段开头两句表达的意思无关。