Sleep is very ancient. In the electroencephalographicsense we share it with all the primatesand almost all the other mammals and birds: it may extend back as far as the reptiles.

There is some evidence that the two types of sleep, dreaming and dreamless, depend on the life style. of the animal, and that predatorsare 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 immobilizedand 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 animals 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.

復習資料：

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] 本文三段雖然討論的內容各有側重，但都談到了睡眠在各種動物中的差異與進步的歷史，具體用詞如下: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比較準確地總結了本文大意。B只不過概括了第2段的內容;C只提及第3段的論述內容;D 好像涵蓋了三段內容,但過于寬泛,不夠具體。

2.[B] 依據第2段第1句第4個分句可知應選B。A的內容與該段第2句相悖;C中dreamless sleep應為dream sleep。D的內容文中沒涉及。

3.[C] 這個例子是為了證明無夢睡眠比較淺，即第2第3句的前半句，所以選C做合適。所有例子都是為之前或之后的話提供例證，因此,作者用例子的目的可以從該例子之前或之后的一句話找到。本題中，貓和狗的例 子是為了說明其前面的一句話,其他選項與此無關,均可排除。

4.[D] 依據第2段倒數第3句及末句，可以判斷與無夢睡眠相比，深睡不動處于更髙的進化階段。文中雖有提及無夢睡眠對于被捕食動物來講是一種自然選擇，但并不意味著深睡不動不是自然選擇，因此A不正確;該句也 提到“笨拙的動物與敏捷的動物相比，更少有深睡不動的時候”，primates是smart ones,因此更大概深睡不動,B與此相悖;文中暗示無夢睡眠使低級動物更警覺，從而起到肯定的保護用途,C正好與這種理解相反，因此也不正確。

5. [C]最后一段第2句講解了第1句中所說的“海豚等睡眠少”是什么原因，C比較確切地反映了文章內容。A、B、D三項都與最后一段開頭兩句表達的意思無關。