第83章

类别:其他 作者:Charles Darwin字数:6085更新时间:18/12/21 17:23:57
Thehistoryofthechanges,whichtheelevatedplainsofLongwoodandDeadwoodhaveundergone,asgiveninGeneralBeatson’saccountoftheisland,isextremelycurious。Bothplains,itissaidinformertimeswerecoveredwithwood,andwerethereforecalledtheGreatWood。Solateastheyear1716thereweremanytrees,butin1724theoldtreeshadmostlyfallen; andasgoatsandhogshadbeensufferedtorangeabout,alltheyoungtreeshadbeenkilled。Itappearsalsofromtheofficialrecords,thatthetreeswereunexpectedly,someyearsafterwards,succeededbyawiregrasswhichspreadoverthewholesurface。[3]GeneralBeatsonaddsthatnowthisplain“iscoveredwithfinesward,andisbecomethefinestpieceofpastureontheisland。”Theextentofsurface,probablycoveredbywoodataformerperiod,isestimatedatnolessthantwothousandacres;atthepresentdayscarcelyasingletreecanbefoundthere。Itisalsosaidthatin1709therewerequantitiesofdeadtreesinSandyBay;thisplaceisnowsoutterlydesert,thatnothingbutsowellattestedanaccountcouldhavemademebelievethattheycouldeverhavegrownthere。 Thefact,thatthegoatsandhogsdestroyedalltheyoungtreesastheysprangup,andthatinthecourseoftimetheoldones,whichweresafefromtheirattacks,perishedfromage,seemsclearlymadeout。Goatswereintroducedintheyear1502;eighty-sixyearsafterwards,inthetimeofCavendish,itisknownthattheywereexceedinglynumerous。Morethanacenturyafterwards,in1731,whentheevilwascompleteandirretrievable,anorderwasissuedthatallstrayanimalsshouldbedestroyed。Itisveryinterestingthustofind,thatthearrivalofanimalsatSt。Helenain1501,didnotchangethewholeaspectoftheisland,untilaperiodoftwohundredandtwentyyearshadelapsed:forthegoatswereintroducedin1502,andin1724itissaid“theoldtreeshadmostlyfallen。”Therecanbelittledoubtthatthisgreatchangeinthevegetationaffectednotonlytheland-shells,causingeightspeciestobecomeextinct,butlikewiseamultitudeofinsects。 St。Helena,situatedsoremotefromanycontinent,inthemidstofagreatocean,andpossessingauniqueFlora,excitesourcuriosity。Theeightland-shells,thoughnowextinct,andonelivingSuccinea,arepeculiarspeciesfoundnowhereelse。Mr。Cuming,however,informsmethatanEnglishHelixiscommonhere,itseggsnodoubthavingbeenimportedinsomeofthemanyintroducedplants。Mr。Cumingcollectedonthecoastsixteenspeciesofsea-shells,ofwhichseven,asfarasheknows,areconfinedtothisisland。Birdsandinsects,[4]asmighthavebeenexpected,areveryfewinnumber;indeedIbelieveallthebirdshavebeenintroducedwithinlateyears。Partridgesandpheasantsaretolerablyabundant; theislandismuchtooEnglishnottobesubjecttostrictgame-laws。I wastoldofamoreunjustsacrificetosuchordinancesthanIeverheardofeveninEngland。Thepoorpeopleformerlyusedtoburnaplant,whichgrowsonthecoast-rocks,andexportthesodafromitsashes;butaperemptoryordercameoutprohibitingthispractice,andgivingasareasonthatthepartridgeswouldhavenowheretobuild。 InmywalksIpassedmorethanonceoverthegrassyplainboundedbydeepvalleys,onwhichLongwoodstands。Viewedfromashortdistance,itappearslikearespectablegentleman’scountry-seat。Infrontthereareafewcultivatedfields,andbeyondthemthesmoothhillofcolouredrockscalledtheFlagstaff,andtheruggedsquareblackmassoftheBarn。Onthewholetheviewwasratherbleakanduninteresting。TheonlyinconvenienceIsufferedduringmywalkswasfromtheimpetuouswinds。OnedayInoticedacuriouscircumstance;standingontheedgeofaplain,terminatedbyagreatcliffofaboutathousandfeetindepth,Isawatthedistanceofafewyardsrighttowindward,sometern,strugglingagainstaverystrongbreeze,whilst,whereIstood,theairwasquitecalm。Approachingclosetothebrink,wherethecurrentseemedtobedeflectedupwardsfromthefaceofthecliff,Istretchedoutmyarm,andimmediatelyfeltthefullforceofthewind:aninvisiblebarrier,twoyardsinwidth,separatedperfectlycalmairfromastrongblast。 IsomuchenjoyedmyramblesamongtherocksandmountainsofSt。Helena,thatIfeltalmostsorryonthemorningofthe14thtodescendtothetown。 BeforenoonIwasonboard,andtheBeaglemadesail。 Onthe19thofJulywereachedAscension。Thosewhohavebeheldavolcanicisland,situatedunderanaridclimate,willatoncebeabletopicturetothemselvestheappearanceofAscension。Theywillimaginesmoothconicalhillsofabrightredcolour,withtheirsummitsgenerallytruncated,risingseparatelyoutofalevelsurfaceofblackruggedlava。Aprincipalmoundinthecentreoftheisland,seemsthefatherofthelessercones。ItiscalledGreenHill:itsnamebeingtakenfromthefaintesttingeofthatcolour,whichatthistimeoftheyearisbarelyperceptiblefromtheanchorage。 Tocompletethedesolatescene,theblackrocksonthecoastarelashedbyawildandturbulentsea。 Thesettlementisnearthebeach;itconsistsofseveralhousesandbarracksplacedirregularly,butwellbuiltofwhitefreestone。Theonlyinhabitantsaremarines,andsomenegroesliberatedfromslave-ships,whoarepaidandvictualledbygovernment。Thereisnotaprivatepersonontheisland。Manyofthemarinesappearedwellcontentedwiththeirsituation; theythinkitbettertoservetheirone-and-twentyyearsonshore,letitbewhatitmay,thaninaship;inthischoice,ifIwereamarine,Ishouldmostheartilyagree。 ThenextmorningIascendedGreenHill,2840feethigh,andthencewalkedacrosstheislandtothewindwardpoint。Agoodcart-roadleadsfromthecoast-settlementtothehouses,gardens,andfields,placednearthesummitofthecentralmountain。Ontheroadsidetherearemilestones,andlikewisecisterns,whereeachthirstypasser-bycandrinksomegoodwater。Similarcareisdisplayedineachpartoftheestablishment,andespeciallyinthemanagementofthesprings,sothatasingledropofwatermaynotbelost:indeedthewholeislandmaybecomparedtoahugeshipkeptinfirst-rateorder。Icouldnothelp,whenadmiringtheactiveindustry,whichhadcreatedsucheffectsoutofsuchmeans,atthesametimeregrettingthatithadbeenwastedonsopoorandtriflinganend。M。Lessonhasremarkedwithjustice,thattheEnglishnationwouldhavethoughtofmakingtheislandofAscensionaproductivespot,anyotherpeoplewouldhavehelditasamerefortressintheocean。 Nearthiscoastnothinggrows;furtherinland,anoccasionalgreencastor-oilplant,andafewgrasshoppers,truefriendsofthedesert,maybemetwith。 Somegrassisscatteredoverthesurfaceofthecentralelevatedregion,andthewholemuchresemblestheworsepartsoftheWelshmountains。Butscantyasthepastureappears,aboutsixhundredsheep,manygoats,afewcowsandhorses,allthrivewellonit。Ofnativeanimals,land-crabsandratsswarminnumbers。Whethertheratisreallyindigenous,maywellbedoubted;therearetwovarietiesasdescribedbyMr。Waterhouse;oneisofablackcolour,withfineglossyfur,andlivesonthegrassysummit,theotherisbrown-colouredandlessglossy,withlongerhairs,andlivesnearthesettlementonthecoast。Boththesevarietiesareone-thirdsmallerthanthecommonblackrat(M。rattus);andtheydifferfromitbothinthecolourandcharacteroftheirfur,butinnootheressentialrespect。 Icanhardlydoubtthattheserats(likethecommonmouse,whichhasalsorunwild)havebeenimported,and,asattheGalapagos,havevariedfromtheeffectofthenewconditionstowhichtheyhavebeenexposed:hencethevarietyonthesummitoftheislanddiffersfromthatonthecoast。 Ofnativebirdstherearenone;buttheguinea-fowl,importedfromtheCapedeVerdIslands,isabundant,andthecommonfowlhaslikewiserunwild。Somecats,whichwereoriginallyturnedouttodestroytheratsandmice,haveincreased,soastobecomeagreatplague。Theislandisentirelywithouttrees,inwhich,andineveryotherrespect,itisveryfarinferiortoSt。Helena。 OneofmyexcursionstookmetowardstheS。W。extremityoftheisland。 Thedaywasclearandhot,andIsawtheisland,notsmilingwithbeauty,butstaringwithnakedhideousness。Thelavastreamsarecoveredwithhummocks,andareruggedtoadegreewhich,geologicallyspeaking,isnotofeasyexplanation。Theinterveningspacesareconcealedwithlayersofpumice,ashesandvolcanictuff。Whilstpassingthisendoftheislandatsea,Icouldnotimaginewhatthewhitepatcheswerewithwhichthewholeplainwasmottled;Inowfoundthattheywereseafowl,sleepinginsuchfullconfidence,thateveninmiddayamancouldwalkupandseizeholdofthem。 ThesebirdsweretheonlylivingcreaturesIsawduringthewholeday。 Onthebeachagreatsurf,althoughthebreezewaslight,cametumblingoverthebrokenlavarocks。 Thegeologyofthisislandisinmanyrespectsinteresting。InseveralplacesInoticedvolcanicbombs,thatis,massesoflavawhichhavebeenshotthroughtheairwhilstfluid,andhaveconsequentlyassumedasphericalorpear-shape。Notonlytheirexternalform,but,inseveralcases,theirinternalstructureshowsinaverycuriousmannerthattheyhaverevolvedintheiraerialcourse。Theinternalstructureofoneofthesebombs,whenbroken,isrepresentedveryaccuratelyinthewoodcut。Thecentralpartiscoarselycellular,thecellsdecreasinginsizetowardstheexterior; wherethereisashell-likecaseaboutthethirdofaninchinthickness,ofcompactstone,whichagainisoverlaidbytheoutsidecrustoffinelycellularlava。Ithinktherecanbelittledoubt,firstthattheexternalcrustcooledrapidlyinthestateinwhichwenowseeit;secondly,thatthestillfluidlavawithin,waspackedbythecentrifugalforce,generatedby[picture] therevolvingofthebomb,againsttheexternalcooledcrust,andsoproducedthesolidshellofstone;andlastly,thatthecentrifugalforce,byrelievingthepressureinthemorecentralpartsofthebomb,allowedtheheatedvapourstoexpandtheircells,thusformingthecoarsecellularmassofthecentre。 Ahill,formedoftheolderseriesofvolcanicrocks,andwhichhasbeenincorrectlyconsideredasthecraterofavolcano,isremarkablefromitsbroad,slightlyhollowed,andcircularsummithavingbeenfilledupwithmanysuccessivelayersofashesandfinescoriae。Thesesaucer-shapedlayerscropoutonthemargin,formingperfectringsofmanydifferentcolours,givingtothesummitamostfantasticappearance;oneoftheseringsiswhiteandbroad,andresemblesacourseroundwhichhorseshavebeenexercised;hencethehillhasbeencalledtheDevil’sRidingSchool。 Ibroughtawayspecimensofoneofthetufaceouslayersofapinkishcolouranditisamostextraordinaryfact,thatProfessorEhrenberg[5]findsitalmostwhollycomposedofmatterwhichhasbeenorganized:hedetectsinitsomesiliceous-shieldedfresh-waterinfusoria,andnolessthantwenty-fivedifferentkindsofthesiliceoustissueofplants,chieflyofgrasses。Fromtheabsenceofallcarbonaceousmatter,ProfessorEhrenbergbelievesthattheseorganicbodieshavepassedthroughthevolcanicfire,andhavebeeneruptedinthestateinwhichwenowseethem。Theappearanceofthelayersinducedmetobelievethattheyhadbeendepositedunderwater,thoughfromtheextremedrynessoftheclimateIwasforcedtoimagine,thattorrentsofrainhadprobablyfallenduringsomegreateruption,andthatthusatemporarylakehadbeenformedintowhichtheashesfell。Butitmaynowbesuspectedthatthelakewasnotatemporaryone。Anyhow,wemayfeelsure,thatatsomeformerepochtheclimateandproductionsofAscensionwereverydifferentfromwhattheynoware。Whereonthefaceoftheearthcanwefindaspot,onwhichcloseinvestigationwillnotdiscoversignsofthatendlesscycleofchange,towhichthisearthhasbeen,is,andwillbesubjected? OnleavingAscension,wesailedforBahia,onthecoastofBrazil,inordertocompletethechronometricalmeasurementoftheworld。WearrivedthereonAugust1st,andstayedfourdays,duringwhichItookseverallongwalks。Iwasgladtofindmyenjoymentintropicalsceneryhadnotdecreasedfromthewantofnovelty,evenintheslightestdegree。Theelementsofthesceneryaresosimple,thattheyareworthmentioning,asaproofonwhattriflingcircumstancesexquisitenaturalbeautydepends。 Thecountrymaybedescribedasalevelplainofaboutthreehundredfeetinelevation,whichinallpartshasbeenwornintoflat-bottomedvalleys。Thisstructureisremarkableinagraniticland,butisnearlyuniversalinallthosesofterformationsofwhichplainsareusuallycomposed。 Thewholesurfaceiscoveredbyvariouskindsofstatelytrees,interspersedwithpatchesofcultivatedground,outofwhichhouses,convents,andchapelsarise。Itmustberememberedthatwithinthetropics,thewildluxurianceofnatureisnotlosteveninthevicinityoflargecities:forthenaturalvegetationofthehedgesandhill-sidesoverpowersinpicturesqueeffecttheartificiallabourofman。Hence,thereareonlyafewspotswherethebrightredsoilaffordsastrongcontrastwiththeuniversalclothingofgreen。Fromtheedgesoftheplaintherearedistantviewseitheroftheocean,orofthegreatBaywithitslow-woodedshores,andonwhichnumerousboatsandcanoesshowtheirwhitesails。Exceptingfromthesepoints,thesceneisextremelylimited;followingthelevelpathways,oneachhand,onlyglimpsesintothewoodedvalleysbelowcanbeobtained。ThehousesImayadd,andespeciallythesacrededifices,arebuiltinapeculiarandratherfantasticstyleofarchitecture。Theyareallwhitewashed;sothatwhenilluminedbythebrilliantsunofmidday,andasseenagainstthepaleblueskyofthehorizon,theystandoutmorelikeshadowsthanrealbuildings。