Sleep is very ancient. In the electroencephalographic (脑电图)sense we share it with all the primates (灵长类)and almost all the other mammals(哺乳动物) and birds. {{U}}(46) {{/U}}
There is some evidence that the two types of sleep, dreaming and dreamless, depend on the life-style of the animal. Predators (捕食者) are much more likely to dream than prey. {{U}}(47) {{/U}} In dream sleep, the animal can't move and remarkably unresponsive to external stimuli(外部刺激).{{U}} (48) {{/U}} The fact that deep dream sleep is rare among prey today seems clearly to be a product of natural selection, and it makes sense that today, when sleep is highly evolved, the stupid animals are less frequently immobilized(不能动) by deep sleep than the smart ones.
But why should they sleep deeply at all? Why should a state of such deep immobilization ever have evolved? {{U}}(49) {{/U}} Wilse Webb of the University of Florida and Ray Meddis of London University have suggested this to be the case. It is conceivable that animals who are too stupid to be quiet on their own initiative are, during periods of high risk, immobilized by the implacable arm of sleep.
{{U}} (50) {{/U}} This is an interesting notion and probably at least partly true.
A. The point seems particularly clear for the young of predatory animals.
B. Dreamless sleep is much shallower, and we have all witnessed cats or dogs cocking their ears to a sound when apparently fast asleep.
C. While prey are in turn much more likely to experience dreamless sleep.
D. Could it be that, rather than increasing an animal's vulnerability(易受伤), the function of sleep is to decrease it?
E. It may extend back as far as the reptiles(爬行类动物).
F. Human being is likely to dream.