EvolutionandWheels 　　Inthepast,evolutionarybiologistscontemplatingtheabsenceofwheelsinnatureagreedthattheexplanationwasnotundesirability:wheelswouldbegoodforanimals,justastheyareforus.Animalswerepreventedfromevolvingwheels,thebiologistsreasoned,bythefollowingdilemma:livingcellsinananimal'sbodyareconnectedtotheheartbybloodvessels,andtothebrainbynerves.Becausearotatingjointisessentialtoawheel,awheelmadeoflivingcellswouldtwistitsarteryveinandnerveconnectionsatthefirstrevolution,makinglivingimpracticable. 　　However,thereisaflawintheargumentthattheevolutionofwheeledanimalswasthwartedbytheinsolublejointproblem.Thetheoryfailstoexplainwhyanimalshavenotevolvedwheelsofdeadtissuewithnoneedforarteriesandnerves.Countlessanimals,includingus,bearexternalstructureswithoutbloodsupplyornerves-forexample,ourhairandfingernails,orthescales,claws,andhornsofotheranimals.Whyhaveratsnotevolvedbonywheels,similartorollerskates?Pawsmightbemoreusefulthanwheelsinsomesituations,butcats'clawsareretractable:whynotretractablewheels?WethusarriveattheseriousbiologicalparadoxflippantlytermedtheRRRdilemma:nature'sfailuretoproduceratswithretractablerollerskates.进化与轮子 　　从前,研究自然界没有轮子的进化论生物学家都同意不能用无此需要来解释这种现象：轮子对于动物会像对于我们人类一样有好处.生物学家们推论,动物没有进化出轮子是由下述困难所致：动物身上的活细胞通过血管与心脏相连,通过神经与大脑相连.因为一个旋转的接头对轮子来说是至关重要的,由活的细胞构成的轮子在第一次转动时便会扭伤其上的动脉和神经的连结,因而不现实. 　　不过,动物未能进化出轮子是受阻于无法解决接头问题的说法有一个缺陷.这种理论无法解释为何动物没有进化出由死组织构成的而无需动脉和神经的轮子.包括人在内的无数动物都有一些没有血液供应和神经的体外构造,例如,我们的头发和指甲,或者鳞片、爪子和其它一些动物的角.为什么老鼠没有进化出类似于滑轮溜冰鞋的骨质的轮子呢?在某些情况下,爪子可能比轮子更有用,但猫的爪子是可以伸缩的：为什么不能有可以伸缩的轮子呢?这样,我们便得出了一个被戏称为RRR的严肃的生物学悖论：大自然未能产生出有可伸缩的滑轮溜冰鞋的老鼠. 　　Johnisthemostdiligentstudentinourclass.Heisoneofmyfriends.Asamatteroffact,hegetsalongwithhisclassmates.Hemightspendjustfivetotenminutestalkingtoyouonceinalongwhile,buthetalkswithenthusiasmandenergy,andhelikestosharehisthinkingandideaswithyou.Heishonest;hedoesnotliketowastehistimeandenergy.Heneverappearssleepyandtiredintheclassroom.Healwayshandsinhomeworkontimeandalsodoeswellinhisexaminations,yethehastimepracticingplayingfootballandhisviolinskill.Iadmirehim.IhopeIcanbeasdiligentasheis;Johnispersistent,attentive,andenergetic. 　　John是我们班上最勤奋的学生.他也是我的一个朋友.实际上,他和班上的同学相处的都很融洽.也许过好长时间他才只是花5分钟或10分钟和你聊天,但是他的谈话充满了活力和热情,而且他喜欢和你分享他的思想和想法.他是诚实的,他不会无谓的浪费他的时间和精力.在教室里,他从没有显得困倦或是疲惫.他常常会按时交作业,考试中也会取得好成绩,然而他仍会有时间踢足球和练小提琴.我羡慕他,我希望自己能象他那样勤奋.John是个执着、专注、充满活力的人. 　　8January202005 　　IhavestayedinShanghaialoneforaweek.It’stimeformetogotomyrealhome—itisinBeijing.Myparentsarewaitingforme.Althoughoutsideisnomatterhowbeautiful,Beijingisstillmyhome.Infact,thereismanyfreshthings.Likeredbayberries.Intheafternoon,ItookabustoBeijing.Iwasveryunluckythatmymp3wasstolen.Iwasverysad.Ididn’tknowwhythieveswenttostealother’sthings.Whydon’ttheyhuntforajob?Idon’twhy!Ihopethereisnothiefinourmotherland. 　　WhenIgothome,itwasverydark.Why?Thereisnoelectric.EveryyearinBeijing,thereislackofelectrictosupply.Becausetherearealotoffactories.Anditisverycoldinwinter.Sothegovernmentdecided:sup