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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
