leep i very ancient. in the electroencephalographic ene we hare it with all the primate and almot all the other mammal and bird: it may etend back a far a the reptile.
there i ome evidence that the two type of leep, dreaming and dreamle, depend on the life-tyle of the animal, and that predator are tatitically much more likely to dream than prey, which are in turn much more likely to eperience dreamle leep. in dream leep, the animal i powerfully immobilized and remarkably unreponive to eternal timuli. dreamle leep i much hallower, and we have all witneed cat or dog cocking their ear to a ound when apparently fat aleep. the fact that deep dream leep i rare among pray today eem clearly to be a product of natural election, and it make ene that today, when leep i highly evolved, the tupid animal are le frequently immobilized by deep leep than the mart one. but why hould they leep deeply at all? why hould a tate of uch deep immobilization ever have evolved?
perhap one ueful hint about the original function of leep i to be found in the fact that dolphin and whale and aquatic mammal in genera eem to leep very little. there i, by and large, no place to hide in the ocean. could it be that, rather than increaing an animal＇ vulnerability, the univerity of florida and ray meddi of london univerity have uggeted thi to be the cae. it i conceivable that animal who are too tupid to be quite on their own initiative are, during period of high rik, immobilized by the implacable arm of leep. the point eem particularly clear for the young of predatory animal. thi i an intereting notion and probably at leat partly true.由好作网www.hAOzuowEn.com整理