英语考试高级阅读文选

第一部分动物、植物、生物学

ClassificationofPlants

Thenewerclassificationsystemlistsallofthemorethan300,000knownplantsinjusttwophyla,the

BryophytesandtheTracheophytes.Bryophytes,themossesandliverworts,areusuallysoftandnonwoodyin

structure,takeinwaterthroughshortroot-likefilamentscalledrhizoids,andmayhavestemsandsimpleleaves

but,unlikethemorecomplexTracheophytes,donothavetruerootsorvasculartissuewhosefunctionitisto

circulatewater,food,andessentialmineralsthroughouttheorganism.

Tracheophytesaredividedintofoursub-phyla:lycopsids,whichnumbersome900livingspecies;sphenopsids,

whosefossilspeciescontributedtocoalformationintheCarboniferousperiod,butwhichhavefewlivingspecies;

psilopsids,anextinctgroupofrelativelysimpleplants,whichfossilstudiesshowtohavebeenmoreadvanced

thananyofthemosses;andpterosids,subdividedintothreedasses,theferns,thegymnosperms,andthe

angiosperms.

Theangiosperms,themosthighlydevelopedandcomplexclassofplants,reproducebymeansofsingleand

doubleseedleavescalledcotyledons.Monocots,suchascorn,wheat,lilies,andorchids,haveleaveswith

parallelveins,whiledicots,whichincludeoaks,maples,rosesandthistles,amongothers,havenet-veined

leavesandstemswithannualgrowthrings.

AdaptationsofDesertPlants

Desertplantpopulationshaveevolvedsophisticatedphysiologicalandbehavioraltraitsthataidsurvivalinarid

conditions.Somesendoutlong,unusuallydeeptaproots;othersutilizeshallowbutwidespreadroots,which

allowthemtoabsorblargeintermittentflowsofwater.Certainplantsprotecttheiraccesstowater.Thecreosote

bushproducesapotentroottoxinwhichinhibitsthegrowthofcompetingrootsystems.Daytimeclosureof

stomataexemplifiesafurthergeneticadaptation;guardcellsworktominimizedaytimewaterloss,laterallowing

thestomatatoopenwhenconditionsaremorefavorabletogasexchangewiththeenvironment.

Certainadaptationsreflecttheprinciplethatalargesurfaceareafacilitateswaterandgasexchange.Mose

plantshavesmallleaves,modifiedleaves(spines),ornoleavesatall.Themainfood-producingoragnisnotthe

leafbutthestem,whichisoftengreenandnon-woody.Thick,waxystemsandcuticles,seeninsucculentssuch

ascactiandagaves,alsohelpconservewater.Spinesandthorns(modifiedbranches)protectagainstpredators

andalsominimizewaterloss.

LeavesMicrobiological

Activityclearlyaffectsthemechanicalstrengthofleaves.Althoughitcannotbedeniedthatwithmostspecies

thelossofmechanicalstrengthistheresultofbothinvertebratefeedingandmicrobiologicalbreakdown,the

exampleofFagussylvaticanillustrateslosswithoutanysignofinvertebrateattackbeingevident.Fagusshows

littlesignofinvertebrateattackevenafterbeingexposedforeightmonthsineitherlakeorstreamenvironment,

butresultsoftherollingfragmentationexperimentshowthatlossofmechanicalstrength,eveninthis

apparentlyresistantspecies,isconsiderable.

Mostspeciesappeartoexhibitahigherrateofdegradationinthestreamenvironmentthaninthelake.Thisis

perhapsmostclearlyshowninthecaseofAlnus.Examinationofthetypeofdestructionsuggeststhatthecause

forthegreaterlossofmaterialinthestreamprocessedleavesisacombinationofbothbiologicalandmechanical

degradation.Theleavesexhibitanangularfragmentation,whichischaracteristicofmechanicaldamage,rather

thantheroundedholestypicaloftheattackbylargeparticlefeeders.Astheleavesbecomelessstrong,thefluid

forcesactingonthestreamnyloncagescausedsuccessivelygreaterfragmentation.

Mechanicalfragmenation,likebiologicalbreakdown,istosomeextentinfluencedbyleafstructureandform.In

someleaveswithastrongmidrib,thelaminabreakup,butthepiecesremainattachedbymeansofthemidrib.

Onetypeofleafmaybreakcleanwhileanothertearsoffandiseasilydestroyedoncethetissuesareweakened

bymicrobialattack.

Inmostspecies,themechanicalbreakdownwilltaketheformofgradualattritionatthemargins.Iftheenergy

oftheenvironmentissufficientlyhigh,brittlespeciesmaybebrokenacrossthemidrib,somethingthatrarely

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happenswithmorepliableleaves.Theresultofattritionisthat,wheretheareasofthewholeleavesfollowa

normaldistribution,abimodaldistributionisproduced,onepeakcomposedmainlyofthefragmentedpieces,

theotherofthelargerremains.

Totestthetheorythatathinleafhasonlyhalfthechanceofathickonforenteringthefossilrecord,allother

thingsbeingequal,Ferguson(1971)cutdiscsoffreshleavesfrom11speciesofdifferentleafthicknessand

rotatedthemwithsandandwaterinarevolvingdrum.Eachrunlasted100hoursandwasrepeatedthreetimes,

butevenafterthistreatment,allspeciesshowedlittlesignofwear.Itthereforeseemsunlikelythatleaf

thicknessalone,withoutsubstantialmicrobialpreconditioning,contributesmuchtotheprobabilitythataleafwill

enteradepositionalenvironmentinarecognizableform.Theresultofexperimentswithwholefreshleaves

showthattheyaremoreresistanttofragmentationthanleavesexposedtomicrobiologicalattack.Unlessthe

leafisexceptionallylargeofsmall,leafsizeandthicknessarenotlikelytobeascriticalindeterminingthe

preservationpotentialofaleaftypeastherateofmicrobiologicaldegradation.

PlantsandGeography

Althoughdifferentplantshavevaryingenvironmentalrequirementsbecauseofphysiologicaldifferences,there

arecertainplantspeciesthatarefoundassociatedwithrelativelyextensivegeographicalareas.Thedistribution

ofplantsdependsuponanumberoffactorsamongwhichare(1)lengthofdaylightanddarkness,(2)

temperaturemeansandextremes,(3)lengthofgrowingseason,and(4)precipitationamounts,types,and

distribution.

Daylightanddarknessarethekeysbywhichaplantregulatesitscycle.Itisnotalwaysobvioushowthe

triggeringfactorworks,butexperimentshaveshowndaylengthtobeakey.Acaseinpointisthatmany

greenhouseplantsbloomonlyinthespringwithoutbeinginfluencedbyoutsideconditionsotherthanlight.

Normally,theplantskeyedtodaylightanddarknessphenomenaarerestrictedtoparticularlatitudes.

Inonewayoranother,everyplantisaffectedbytemperature.SomespeciesareRiledbyfrost;othersrequire

frostandcoldconditionstofruit.Orangeblossomsarekilledbyfrost,butcherryblossomswilldeveloponlyif

thebudshavebeenadequatelychilledforanappropriatetime.Oftentheaccumulationofdegreesorthe

directionoftemperaturesaboveorbelowaspecificfigurecriticallyaffectsplants.Plantdistributionsareoften

comparedwithisothermstosuggestthetemperaturelimitsandrangesfordifferentspecies.Theworld'sgreat

vegetationzonesarecloselyalignedwithtemperaturebelts.

Differentplantspeciesadjusttoseasonalchangesindifferentways.Somemaketheadjustmentbyretarding

growthandarrestingvitalfunctionsduringwinter.Thismayresultintheleaffallofmiddlelatitudedeciduous

trees.Otherplantsdisappearentirelyattheendofthegrowingseasonandonlyreappearthroughtheirseeds.

Thesearetheannuals,andtheyformastrikingcontrasttotheperennials,whichlivefromoneseasonto

another.

Precipitationsuppliesthenecessarysoilwaterforplants,whichtakeitinattheroots.Allplantshavesome

limitingmoisturestresslevelbeyondwhichtheymustbecomeinactiveordie.Droughtresistantplantshavea

varietyofdefensesagainstmoisturedeficiencies,buthydrophytes,whichalsoareadaptedtohumid

environments,havehardlyanydefenseagainstawatershortage.

Whales

Althoughvocalcordsarelackingincetaceans,phonationisundoubtedlycenteredinthelarynx.

Thetoothedwhalesorodontocetes(spermwhaleandporpoises)aremuchmorevociferousthanthewhalebone

whales,ormysticetes.Inthiscountryobservershaverecordedonlyoccasionalsoundsfromtwospeciesof

mysticetes(thehumpbackandrightwhale).ARussiancetologistreportshearingsoundsfromatleastfive

spciesofwhalebonewhalesbutgivesnodetailsofthecircumstancesordescriptionsofthesoundsthemselves.

Althoughcomparisonofthesoundproducingapparatusinthetwowhalegroupscannotyetbemade,itis

interestingtonotethattheauditorycentersofthebrainaremuchmorehighlydevelopedintheodontocetes

thaninthemysticetes,infact,toadegreeunsurpassedbyanyothermammaliangroup.

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BioluminescenceintheSea

Atnight,schoolsofpreyandpredatorsarealmostalwaysspectacularlyilluminatedbythebioluminescence

producedbythemicroscopicandlargerplankton.Thereasonfortheubiquitousproductionoflightbythe

microorganismsofthesearemainsobscure,andsuggestedexplanationsarecontroversial.Ithasbeen

suggestedthatlightisakindofinadvertentby-productoflifeintransparentorganisms.Ithasalsobeen

hypothesizedthattheemissionoflightondisturbanceisadvantageoustotheplanktoninmakingthepredators

oftheplanktonconspicuoustotheirpredators!Unquestionably,itdoesactthisway.Indeed,somefisheries

basethedetectionoftheirpreyonthebioluminescencethatthefishexcite.Itisdifficult,however,todefend

thethesisthatthiseffectwasthedirectfactorintheoriginaldevelopmentofbioluminescence,sincetheeffect

wasofnoadvantagetotheindividualmicroorganismthatfirstdevelopedit.Perhapsthebioluminescenceofa

microorganismalsodiscouragesattackbylight-avoidingpredatorsandisofinitialsurvivalbenefittothe

individual.Asitthenbecomesgeneralinthepopulation,theeffectofrevealingplanktonpredatorstotheir

predatorswouldalsobecomeimportant.

AnimalTorpidity

Afewspeciesdemonstrateconditionswhichareneithercompletehibernationnoraestivation.Insteadofgoing

intoalong"sleep'*duringthemostadverseseason,theybecometorpidforafewhourseachday.Thiskindof

behaviorisknowninotheranimalbatsbecometorpidduringdaytime,andhummingbirdsatnight.Thefirsttime

Iappreciatedthisphenomenonwaswhileworkingwithfatmice(Steatomys)inAfrica.Thesemice,incidentally,

haveamostappropriatename,fortheirbodiesaresofulloffattheyresemblelittlefurryballs.Fatstorageasa

methodofsurvivalhasreboundedtosomeextentasfarasthefatmiceareconcerned.Theyareregardedasa

succulentdelicacybymanyAfricantribeswhohuntthemwithgreattenactiy;whencaptured,themiceare

skeweredandfriedintheirownfat.Acaptivefatmousewasoncekeptwithoutfoodorwaterforthirty-sixdays;

attheendofthattimeithadlostathirdofitsweightbutappearedquitehealthy.Duringthedryseason,some

captivesspentthedayinsuchadeepstateoftorporthattheycouldberoughlyhandledwithoutwaking.The

bodytemperaturewasacoupleofdegreesaboveroomtemperatureandtherespirationwasmostirregular,

severalshortpantsbeingfollowedbyapauseofuptothreeminutes.Justbeforeduskthemicewokeupof

theirownaccordandrespirednormally.Inthiscasethetorpidstatewasnotinducedbyshortageoffoodor

abnormaltemperatures.TheforestdormouseofsouthernAsiaandEuropealsoundergoesperiodsoftorpidity

duringtheday;thisspecieshasbeenrecordedashavingpausesofuptoseventeenminutesbetweenbreaths.

Thereisalsoarecordofaleaf-earedmouseofthePeruviandesertwhichbecametorpidundersevere

conditions.

Mammals

ThehistoryofmammalsdatesbackatleasttoTriassictime.Developmentwasretarded,however,untilthe

suddenaccelerationofevolutionalchangethatoccurredintheoldestPaleocene.Thisledinfocenetimeto

increaseinaveragesize,largermentalcapacity,andspecialadaptationsfordifferentmodesfolife.Inthe

OligoceneEpoch,therewasfurtherimprovement,withappearanceofsomenewlinesandextinctionofothers.

MioceneandPliocenetimewasmarkedbyculminationofseveralgroupsandcontinuedapproachtoward

moderncharacters.Thepeakofthecareerofmammalsinvarietyandaveragelargesizewasattainedinthe

Miocene.

TheadaptationofmammalstoalmostallpossiblemodesoflifeparallelsthatofthereptilesinMesozoictime,

andexceptforgreaterintelligence,themammalsdonotseemtohavedonemuchbetterthancorresponding

reptilianforms.Thebatisdoubtlessabetterflyinganimalthanthepterosaur,butthedolphinandwhaleare

hardlymorefishlikethantheichthyosaur.Manyswift-runningmammalsoftheplains,likethehorseandthe

antelope,mustexcelanyofthedinosaurs.Thetyrannosaurwasamoreponderousandpowerfulcarnivorethan

anyflesh-eatingmammal,butthelionortigerisprobablyamoreefficientanddangerousbeastofpreybecause

ofasuperiorbrain.Thesignificantpointtoobserveisthatdifferentbranchesofthemammalsgraduallyfitted

themselvesforallsortsoflife,grazingontheplainsandabletorunswiftly(horse,deer,bison),livinginrivers

andswamps(hippopotamus,beaver),dwellingintrees(sloth,monkey),diggingunderground(mole,rodent),

feedingonfleshintheforest(tiger)andontheplain(wolf),swimminginthesea(dolphin,whale,seal),and

flyingintheair(bat).Manisablebymechanicalmeanstoconquerthephysicalworldandtoadapthimselfto

almostanysetofconditions.

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Thisadaptationproducesgradualchangesofformandstructure.Itisbiologicallycharacteristicoftheyouthful,

plasticstageofagroup.Earlyinitscareer,ananimalassemblageseemstopossesscapacityforchange,which,

astheunitbecomesoldandfixed,disappears.Thegeneralizedtypesoforganismsretainlongesttheabilityto

makeadjustmentswhenrequired,anditisfromthemthatnew,fecundstockstakeorigincertainlynotfromany

specializedendproducts.So,inthemammals,wewitnessthebirth,plasticspreadinmanydirections,

increasingspecialization,andinsomebranches,theextinction,whichwehavelearnedfromobservationofthe

geologicrecordoflifeisacharacteristicoftheevolutionoflife.

HeredityofHorses

Horseownerswhoplantobreedoneormoremaresshouldhaveaworkingknowledgeofheredityandknow

howtocareforbreedinganimalsandfoals.Thenumberofmaresbredthatactuallyconceivevariesfromabout

40to85percent,withtheaveragerunninglessthan50percent.Somemaresthatdoconceivefailtoproduce

livingfoals.Thismeansthat,ontheaverage,twomaresarekeptawholeyeartoproduceonefoal,andeven

then,somefoalsaredisappointmentsfromthestandpointofquality.

Thegeneistheunitthatdeterminesheredity.Inthebodycellsofhorsestherearemanychromosomes.Inturn,

thechromosomescarrypairsofminuteparticles,calledgenes,whicharethebasichereditarymaterial.The

nucleusofeachbodycellofhorsecontains32pairsofchromosomes,oratotalof64;whereasthereare

thousandsofpairsofgenes.

Whenasexcell(aspermoranegg)isformed,onlyonechromosomeandonegeneofeachpairgoesintoit.

Then,whenmatingandfertilizationoccur,the32singlechromosomesfromthegermcellofeachparentunite

toformnewpaires,andthechromosomeswiththeirgenesareagainpresentinduplicate,inthebodycellsof

theembryo.Thus,withallpossiblecombinationsof32pairsofchromosomesandthegenesthattheybear,itis

notstrangethatfullsisters(exceptidenticaltwinsfromasingleeggsplitafterfertilization)aresodifferent.

Actuallywecanmarvelthattheybearasmuchresemblancetoeachotherastheydo.

Becauseofthissituation,thematingofamarewithafinetrackrecordtoastallionthattransmitsgood

performancecharacteristicswillnotalwaysproduceafoalofameritequaltoitsparents.Thefoalcouldbe

markedlypoorerthantheparentsor,insomecases,itcouldbebetterthaneitherparent.

Smpleandmultiplegeneinheritanceoccursinhorses,asinallanimals.Insimplegeneinheritance,onlyone

pairofgenesisinvolved;thus,apairofgenesmayberesponsibleforsomeonespecifictraitinhorse.However,

mostcharacteristics,suchasspeed,areduetomanygenes;hence,theyarecalledmultiplegenecharacteristics.

Formostcharacteristics,manypairsofgenesareinvolved.forexample,growthrateinfoalsisaffectedby(1)

appetiteandfeedconsumption,(2)theproportionofthefeedeatenthatisabsorbed,and(3)theusetowhich

thenutrientsareputwhethertheyareusedforgrowthorfattening,andeachinturnisprobablyaffectedby

differentgenes.Becausemultiplecharacteristicsshowallmannerofgradationfromhightolowperformance,

theyaresometimesrefeeredtoasquantitativetraits.Thus,quantitativeinheritancereferstothedegreeto

whichacharacteristicisinherited.Forexample,allracehorsescanrunandallinheritsomeabilitytorun,butit

isthedegreetowhichtheyinherittheabilitythatisimportant.

MarineAnimalEvolution

Untilthe1970's,thepatternofearlymarineanimalevolutionseemedtobewellestablished.Mostpresent-day

animalphylahadappearedduringthe"Cambrianexplosion*',anextraordinaryburgeoningofmulticellularlifein

thewarmseasoftheCambrianperiod,between570and500millionyearsago.Itwasassumedthat,despite

theverylargenumberofspeciesthatappearedduringtheCambrianexplosion,nearlyallfitintothesame

rathersmallnumberofphylathatexisttoday.Eachphylumagroupoforganismswiththesamebasicpatternof

organization,suchastheradialsymmetryofjellyfishandothercoelenteratesorthesegmentedstructureof

wormsandotherannelidswasseenasevolutionarilystable.Innumerableindividualspecieshavearisenand

diedoutbutdevelopmentandextinctionwereassumedtotakeplacewithinexistingphyla;theeliminationof

entirephylawasthoughttobeextremelyrare.However,adiversegroupofmarinefossils,knowncollectivelyas

the"Problematica*',presenteddifficultiesforthisinterpretation.TheProblematicashowpatternsoforganization

sobizarrethatitishardtofitanyofthemintopresent-dayphyla.Theyincludethebananashaped

_Tullimonstrum_andthespikedspiny_Hallucigenia_,creatureswhoseverynamesreflecttheclassifier's

discomfort.The"Ediacaranfauna'*,whichrespired,absorbednutrients,andeliminatedwastesdirectlythrough

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theirexternalsurfaces,arealsoincludedamongtheProblematica.Theirswasanapproachtakenbyonlyafew

modernmulticelledcreatures(suchastapeworms)thatareotherwisetotallyunlikethem.Recently,several

theoristshavearguedthattheProblematicaarenotjusthardtoclassifytheyareevidencethattheconventional

viewoftheCambrianexplosioniswrong.TheycontendthattheCambrianexplosionrepresentedthe

simultaneousappearanceofamuchlargernumberofanimalphylathanexiststoday.白chwasaseparate

Hexperimentninbasicbodydesign,andtheCambrianseasteemedwithmanydifferentphyla,orbasicbody

plans,eachrepresentedbyonlyafewspecies.Today,thenumberofphylahasfallendrastically,buteach

survivingphylumcontainsamuchlargernumberofspeciesthereareatleast20,000speciesoffishalone.The

Problematica,then,werenotunsuccessfulvariantswithinpresent-dayphyla;eachrepresentedadistinctphylum

initsownright.

Revisionistsandconventionaltheoristsagreethatmodernmarinespeciesareproductsofnaturalselection.But

therevisionistscontendthattheselectionprocesseliminatednotonlyparticularunfavorabletraits,butentire

bodyplans,entireapproachestosurvival.TheSiacaranfauna,forexample,reprsentedaparticularstructural

solutiontothebasicproblemsofgasandfluidexchangewiththeenviroment.Thisapproachtobody

engineeringwasdiscardedatthesametimeastheEdiacaranfaunathemselveswerewipedout;giventhe

improbabilityofduplicatinganentirebodyplanthroughchancemutation,itwasunlikelythatthisparticular

approachwouldeverbetriedagain.

SavingthePorpoise

JustasthemembersoftheInter-AmericanTropicalTunaCommissionhavesubscribedtoannualquotasonthe

tunaharvest,theyareagreedthatcooperationisessentialinlimitingtheporpoisekill.Thecommoninterestis

preservationofthetunaindustry.Andsincemodernfishingmethodsexploitthecozyrelationshipbetweenthe

yellowfintunaandtheporpoise,tunafishingwouldbecomelessprofitableifthenumberofporpoisesdecreased.

Tunaandporpoiseareoftenfoundtogetheratsea,andthefishermenhavelearnedtocasttheirnetswhere

theyseetheporpoises,usingthemtolocatethetuna.Theproblemisthatmanyporpoisesdieinthenets.

Thecommissiondeliberationsacknowledgedtheenvironmentalpressuresthathaveledtostrictregulationof

U.S.tunacrewsunderfederallaw.Delegatesalsorecognizedthatporpoiseprotectiongoalsarerelatively

meaninglessunlessconservationproceduresareadoptedandfollowedonaninternationalbasis.Commission

supervisionofsurvey,observer,andresearchprogramswongeneralagreementattheeight-nationconference.

Themethodandtimetableforimplementingtheprogram,however,remainuncertain.

ThusthefederalregulationthatleavesU.S.crewsatadisadvantageinthetuna-harvestcompetitionremainsa

threattothesurvivalofthetunafleet.Still,thecommissionmeetingshavefocusedontheworkablesolution.All

vesselsshouldbeequippedwiththebestporpoise-savinggeardevised;crewsshouldbetrainedandmotivated

tosavetheporpoise;asystemmustbeinstitutedtoassurethatrulesareenforced.Aboveall,theresponse

mustbeinternational.Porpoiseconservationcouldwellbeanotherelementinanenvisionedtreatythatremains

unhappilyelusiveatthecontinuingLaw-of-the-SeaConference.

Thetunaindustryinterestinsavingporpoisesisbothersometomanywhoalsowanttosavetheporpoise,but

objecttotheindustrymotivationfordoingso.Forfishermen,savingtheporpoiseisvaluableonlybecausethe

porpoiseleadsthemtotuna.Formorecompassionatesouls,however,theporpoiseisnotjustatunafinder,but,

moreimportant,theseacreaturethatseemsmosthuman.FredsonDelacourte,nationalchairmanofthe

"PeopleforAnimals'*drive,saysthis:*'ltisespeciallysadthattheseseacreatures,inspiteoftheirkeen

intelligence,cannotoutwitthetunafishermenwho,anxioustomeettheirannualquota,ensnareanddestroy

porpoisesaswell.Butitisevensadderthatthetunaindustryissointentuponusingtheporpoisesogreedily".

Mr.Delacourtepraisesthefishingindustryforitsplanstosavetheporpoises,atthesametimethathewishes

theirmotivesweremorealtruistic.Heinsiststhatany"lawofthesea"shouldbeessentiallyamorallawrather

thananeconomicone.

Reefs

Thereareagreatmanypointsaboutcoralreefsthatremainsubjectsofscientificpuzzlement.Onemystery

concernstherelationshipbetweenScleractinia,thecoraltypewhosecolonizationproducesreefs,andtheir

symbioticpartnersthezooxanthellae,theunicellularalgaepresentinthecorals'endodermictissues.Itisknown

thateachsymbiontplaysanintegralpartintheformationofareefsprotectivelimestonefoundation.Thecoral

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polypssecretecalceousexoskeletonswhichcementthemselvesintoanunderlayerofrock,whilethealgae

depositstillmorecalciumcarbonate,whichreactswithseasalttocreateaneventougherlimestonelayer.Itis

alsoknownthat,duetothealgalphotosynthesis,thereefenvironmentishighlyoxygen-saturated,whilethe

similarlyhighamountsofcarbondioxidearecarriedoffrapidly.Allthisaccountsfortheamazingrenewabilityof

coralreefsdespitetheendlesserosioncausedbywaveactivity.However,theprecisemanerinwhichone

symbiontstimulatesthesecretionofcalciumcarbonatebytheotherremainsunclear.

Scientistshavealsoproposedvarioustheoriestoexplainthetransformationof"fringingreefs',,thoseconnected

abovesealeveltolandmasses,into"barrierreefs"thatareseparatedfromshorelinesbywidelagoons,and

thenintofree-floatingatolls.ThoughthetheorypostulatedbyCharlesDarwinisconsideredatleastpartially

correct,somescientiststodayarguethatthecreationofthereefformshasmoretodowiththeriseofsealevel

thataccompaniedtheendoftheIceAgethanwiththegradualsubmergenceofthevolcanicislandstowhich

thefringingreefswereoriginallyattached.However,recentdrillingsatEnewetakatollhaveuncoveredalarge

underlayofvolcanicrock,whichsuggeststhatDarwin'sexplanationmayhavebeenmorevalidafterall.

Eventhenamegivertothereefsissomethingofamisnomer.TheScleractiniathemselvesgenerallycomprise

nomorethan10percentofthebiotaoftheaveragereefcommunity:zooxanthellaecanaccountforupto90

percentofthereefmass,alongwithformainifera,annelidworms,andassortedmolluscs.Moreover,the

conditionsunderwhichreefgrowthoccursaredeterminedbytheneedsofthealgae,notthecorals.Reefscan

flourishonlyinshallow,highlysalinewatersabove70F,becausethealgaerequiresuchcircumstances;yetnon­

reef-buildingcoralscoralswhichlackthealgaepresenceoccurworldwideundervariousenvironmental

conditions,fromtheArctictotheMediterranean,homeoftheredcoralprizedforjewelry.Themostlikely

reasonthattheterm"coralreefs"persistsisthatthebrilliantvarietyofcoralshapesandcolorsmakesaesthetic

considerationsmorevividthanbiologicalones.