19年大学英语六级口语练习文章

小编为大家整理了2019年12月大学练习文章，希望可以为备考的同学们带来帮助，一起来学习一下吧。

EvolutionofsleepSleepisveryancient.Intheelectroencephalographicsenseweshareitwithalltheprimatesandalmostalltheothermammalsandbirds:itmayextendbackasfarasthereptiles.Thereissomeevidencethatthetwotypesofsleep,dreaminganddreamless,dependonthelife-styleoftheanimal,andthatpredatorsarestatisticallymuchmorelikelytodreamthanprey,whichareinturnmuchmorelikelytoexperiencedreamlesssleep.Indreamsleep,theanimalispowerfullyimmobilizedandremarkablyunresponsivetoexternalstimuli.Dreamlesssleepismuchshallower,andwehaveallwitnessedcatsordogscockingtheirearstoasoundwhenapparentlyfastasleep.Thefactthatdeepdreamsleepisrareamongpraytodayseemsclearlytobeaproductofnaturalselection,anditmakessensethattoday,

whensleepishighlyevolved,thestupidanimalsarelessfrequentlyimmobilizedbydeepsleepthanthesmartones.Butwhyshouldtheysleepdeeplyatall?Whyshouldastateofsuchdeepimmobilizationeverhaveevolved?Perhapsoneusefulhintabouttheoriginalfunctionofsleepistobefoundinthefactthatdolphinsandwhalesandaquaticmammalsingeneraseemtosleepverylittle.Thereis,byandlarge,noplacetohideintheocean.Coulditbethat,ratherthanincreasingananimal’svulnerability,theUniversityofFloridaandRayMeddisofLondonUniversityhavesuggestedthistobethecase.Itisconceivablethatanimalswhoaretoostupidtobequiteontheirowninitiativeare,duringperiodsofhighrisk,immobilizedbytheimplacablearmofsleep.Thepointseemsparticularlyclearfortheyoungofpredatoryanimals.Thisisaninterestingnotionandprobablyatleastpartlytrue.