学习英语的好文章.docx

学习英语的好文章.docx

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学习英语的好文章

itwasfoundthat5percentDDTpowdercausednoincreaseintheirnaturalmortalityrate.Similarresultsamonglicecollectedfromvagrants游民；流浪者inTokyo,fromanasyluminItabashi,andfromrefugeecampsinSyria,Jordan,andeasternEgypt,confirmedtheineffectivenessofDDTforthecontrolofliceandtyphus.Whenby1957thelistofcountriesinwhichlicehadbecomeresistanttoDDTwasextendedtoincludeIran,Turkey,Ethiopia,WestAfrica,SouthAfrica,Peru,Chile,France,Yugoslavia,Afghanistan,Uganda,Mexico,andTanganyika,theinitialtriumphinItalyseemeddimindeed.

ThefirstmalariamosquitotodevelopresistancetoDDTwasAnophelessacharoviinGreece.Extensivesprayingwasbegunin1946withearlysuccess;by1949,however,observersnoticedthatadultmosquitoeswererestinginlargenumbersunderroadbridges,althoughtheywereabsentfromhousesandstablesthathadbeentreated.Soonthishabitofoutsiderestingwasextendedtocaves,outbuildings,andculverts阴沟andtothefoliageandtrunksoforangetrees.ApparentlytheadultmosquitoeshadbecomesufficientlytolerantofDDTtoescapefromsprayedbuildingsandrestandrecoverintheopen.Afewmonthslatertheywereabletoremaininhouses,wheretheywerefoundrestingontreatedwalls.

Thiswasaportent预示；迹象oftheextremelyserioussituationthathasnowdeveloped.Resistancetoinsecticidesbymosquitoesoftheanophelenegrouphassurgedupwardatanastoundingrate,beingcreatedbythethoroughnessoftheveryhouse-sprayingprogramsdesignedtoeliminatemalaria.In1956,only5speciesofthesemosquitoesdisplayedresistance;byearly1960thenumberhadrisenfrom5to28!

ThenumberincludesverydangerousmalariavectorsinWestAfrica,theMiddleEast,CentralAmerica,Indonesia,andtheeasternEuropeanregion.

Amongothermosquitoes,includingcarriersofotherdiseases,thepatternisbeingrepeated.Atropicalmosquitothatcarriesparasitesresponsibleforsuchdiseasesaselephantiasis象皮病hasbecomestronglyresistantinmanypartsoftheworld.InsomeareasoftheUnitedStatesthemosquitovectorofwesternequineencephalitis马脑炎hasdevelopedresistance.Anevenmoreseriousproblemconcernsthevectorofyellowfever,forcenturiesoneofthegreatplaguesoftheworld.Insecticide-resistantstrainsofthismosquitohaveoccurredinSoutheastAsiaandarenowcommonintheCaribbeanregion.

Theconsequencesofresistanceintermsofmalariaandotherdiseasesareindicatedbyreportsfrommanypartsoftheworld.AnoutbreakofyellowfeverinTrinidadin1954followedfailuretocontrolthevectormosquitobecauseofresistance.Therehasbeenaflare-up疾病的发作ofmalariainIndonesiaandIran.InGreece,Nigeria,andLiberiathemosquitoescontinuetoharborandtransmitthemalariaparasite.Areductionofdiarrheal腹泻的diseaseachievedinGeorgiathroughflycontrolwaswipedoutwithinaboutayear.ThereductioninacuteconjunctivitisinEgypt,alsoattainedthroughtemporaryflycontrol,didnotlastbeyond1950.

Lessseriousintermsofhumanhealth,butvexatious令人烦恼的；麻烦的asmanmeasureseconomicvalues,isthefactthatsalt-marshmosquitoesinFloridaalsoareshowingresistance.Althoughthesearenotvectorsofdisease,theirpresenceinbloodthirstyswarmshadrenderedlargeareasofcoastalFloridauninhabitableuntilcontrol—ofanuneasyandtemporarynature—wasestablished.Butthiswasquicklylost.

Theordinaryhousemosquitoishereandtheredevelopingresistance,afactthatshouldgivepausetomanycommunitiesthatnowregularlyarrangeforwholesalespraying.Thisspeciesisnowresistanttoseveralinsecticides,amongwhichisthealmostuniversallyusedDDT,inItaly,Israel,Japan,France,andpartsoftheUnitedStates,includingCalifornia,Ohio,NewJersey,andMassachusetts.

Ticksareanotherproblem.Thewoodtic木虱,vectorofspottedfever斑疹热,hasrecentlydevelopedresistance;inthebrowndogticktheabilitytoescapeachemicaldeathhaslongbeenthoroughlyandwidelyestablished.Thisposesproblemsforhumanbeingsaswellasfordogs.ThebrowndogtickisasemitropicalspeciesandwhenitoccursasfarnorthasNewJerseyitmustliveoverwinterinheatedbuildingsratherthanoutofdoors.JohnC.PallisteroftheAmericanMuseumofNaturalHistoryreportedinthesummerof1959thathisdepartmenthadbeengettinganumberofcallsfromneighboringapartmentsonCentralParkWest.‘Everynowandthen,’Mr.Pallistersaid,‘awholeapartmenthousegetsinfestedwithyoungticks,andthey’rehardtogetridof.AdogwillpickupticksinCentralPark,andthenthetickslayeggsandtheyhatchintheapartment.TheyseemimmunetoDDTorchlordaneormostofourmodernsprays.ItusedtobeveryunusualtohaveticksinNewYorkCity,butnowthey’realloverhereandonLongIsland,inWestchesterandonupintoConnecticut.We’venoticedthisparticularlyinthepastfiveorsixyears.’

TheGermancockroachthroughoutmuchofNorthAmericahasbecomeresistanttochlordane,oncethefavoriteweaponofexterminators根除者；（美）灭鼠药whohavenowturnedtotheorganicphosphates.However,therecentdevelopmentofresistancetotheseinsecticidesconfrontstheexterminatorswiththeproblemofwheretogonext.

Agenciesconcernedwithvector-bornediseaseareatpresentcopingwiththeirproblemsbyswitchingfromoneinsecticidetoanotherasresistancedevelops.Butthiscannotgoonindefinitely,despitetheingenuityofthechemistsinsupplyingnewmaterials.Dr.Brownhaspointedoutthatwearetraveling‘aone-waystreet’.Nooneknowshowlongthestreetis.Ifthedeadendisreachedbeforecontrolofdisease-carryinginsectsisachieved,oursituationwillindeedbecritical.

Withinsectsthatinfestcropsthestoryisthesame.TothelistofaboutadozenagriculturalinsectsshowingresistancetotheinorganicchemicalsofanearliererathereisnowaddedahostofothersresistanttoDDT,BHC,lindane,toxaphene,dieldrin,aldrin,andeventothephosphatesfromwhichsomuchwashoped.Thetotalnumberofresistantspeciesamongcrop-destroyinginsectshadreached65in1960.

ThefirstcasesofDDTresistanceamongagriculturalinsectsappearedintheUnitedStatesin1951,aboutsixyearsafteritsfirstuse.Perhapsthemosttroublesomesituationconcernsthecodlingmoth,whichisnowresistanttoDDTinpracticallyalloftheworld’sapple-growingregions.Resistanceincabbageinsectsiscreatinganotherseriousproblem.PotatoinsectsareescapingchemicalcontrolinmanysectionsoftheUnitedStates.Sixspeciesofcottoninsects,alongwithanassortmentofthrips蓟马,fruitmoths,leafhoppers叶蝉,caterpillars,mites,aphids,wireworms金针虫,andmanyothersnowareabletoignorethefarmer’sassaultwithchemicalsprays.

Thechemicalindustryisperhapsunderstandablyloathtofaceuptotheunpleasantfactofresistance.Evenin1959,withmorethan100majorinsectspeciesshowingdefiniteresistancetochemicals,oneoftheleadingjournalsinthefieldofagriculturalchemistryspokeof‘realorimagined’insectresistance.Yethopefullyastheindustrymayturnitsfacetheotherway,theproblemsimplydoesnotgoaway,anditpresentssomeunpleasanteconomicfacts.Oneisthatthecostofinsectcontrolbychemicalsisincreasingsteadily.Itisnolongerpossibletostockpilematerialswellinadvance;whattodaymaybethemostpromisingofinsecticidalchemicalsmaybethedismalfailureoftomorrow.Theverysubstantialfinancialinvestmentinvolvedinbackingandlaunchinganinsecticidemaybesweptawayastheinsectsproveoncemorethattheeffectiveapproachtonatureisnotthroughbruteforce.Andhoweverrapidlytechnologymayinventnewusesforinsecticidesandnewwaysofapplyingthem,itislikelytofindtheinsectskeepingalap一圈ahead.

Darwinhimselfcouldscarcelyhavefoundabetterexampleoftheoperationofnaturalselectionthanisprovidedbythewaythemechanismofresistanceoperates.Outofanoriginalpopulation,themembersofwhichvarygreatlyinqualitiesofstructure,behavior,orphysiology,itisthe‘tough’insectsthatsurvivechemicalattack.Sprayingkillsofftheweaklings.Theonlysurvivorsareinsectsthathavesomeinherentqualitythatallowsthemtoescapeharm.Thesearetheparentsofthenewgeneration,which,bysimpleinheritance,possessesallthequalitiesof‘toughness’inherentinitsforebears.Inevitablyitfollowsthatintensivesprayingwithpowerfulchemicalsonlymakesworsetheproblemitisdesignedtosolve.Afterafewgenerations,insteadofamixedpopulationofstrongandweakinsects,thereresultsapopulationconsistingentirelyoftough,resistantstrains.

Themeansbywhichinsectsresistchemicalsprobablyvaryandasyetarenotthoroughlyunderstood.Someoftheinsectsthatdefychemicalcontrolarethoughttobeaidedbyastructuraladvantage,hutthereseemstobelittleactualproofofthis.Thatimmunityexistsinsomestrainsisclear,however,fromobservationslikethoseofDr.Briejèr,whoreportswatchingfliesatthePestControlInstituteatSpringforbi,Denmark,‘disporting娱乐；玩耍themselvesinDDTasmuchathomeasprimitivesorcerers魔术师cavorting腾跃；欢跃overred-hotcoals.’

Similarreportscomefromotherpartsoftheworld.InMalaya,atKualaLumpur,mosquitoesatfirstreactedtoDDTbyleavingthetreatedinteriors.Asresistancedeveloped,however,theycouldbefoundatrestonsurfaceswherethedepositofDDTbeneaththemwasclearlyvisiblebytorchlight.AndinanarmycampinsouthernTaiwansamplesofresistantbedbugswerefoundactuallycarryingadepositofDDTpowderontheirbodies.WhenthesebedbugswereexperimentallyplacedinclothimpregnatedwithDDT,theylivedfora