ItisimpossibletoreflectonthechangedstateoftheAmericancontinentwithoutthedeepestastonishment。Formerlyitmusthaveswarmedwithgreatmonsters：nowwefindmerepigmies，comparedwiththeantecedent，alliedraces。IfBuffonhadknownofthegiganticslothandarmadillo-likeanimals，andofthelostPachydermata，hemighthavesaidwithagreatersemblanceoftruththatthecreativeforceinAmericahadlostitspower，ratherthanthatithadneverpossessedgreatvigour。Thegreaternumber，ifnotall，oftheseextinctquadrupedslivedatalateperiod，andwerethecontemporariesofmostoftheexistingsea-shells。Sincetheylived，noverygreatchangeintheformofthelandcanhavetakenplace。What，then，hasexterminatedsomanyspeciesandwholegenera？Themindatfirstisirresistiblyhurriedintothebeliefofsomegreatcatastrophe；butthustodestroyanimals，bothlargeandsmall，inSouthernPatagonia，inBrazil，ontheCordilleraofPeru，inNorthAmericauptoBehring’sStraits，wemustshaketheentireframeworkoftheglobe。Anexamination，moreover，ofthegeologyofLaPlataandPatagonia，leadstothebeliefthatallthefeaturesofthelandresultfromslowandgradualchanges。ItappearsfromthecharacterofthefossilsinEurope，Asia，Australia，andinNorthandSouthAmerica，thatthoseconditionswhichfavourthelifeofthe_larger_quadrupedswerelatelyco-extensivewiththeworld：whatthoseconditionswere，noonehasyetevenconjectured。Itcouldhardlyhavebeenachangeoftemperature，whichataboutthesametimedestroyedtheinhabitantsoftropical，temperate，andarcticlatitudesonbothsidesoftheglobe。InNorthAmericawepositivelyknowfromMr。Lyell，thatthelargequadrupedslivedsubsequentlytothatperiod，whenboulderswerebroughtintolatitudesatwhichicebergsnowneverarrive：fromconclusivebutindirectreasonswemayfeelsure，thatinthesouthernhemispheretheMacrauchenia，also，livedlongsubsequentlytotheice-transportingboulder-period。Didman，afterhisfirstinroadintoSouthAmerica，destroy，ashasbeensuggested，theunwieldyMegatheriumandtheotherEdentata？WemustatleastlooktosomeothercauseforthedestructionofthelittletucutucoatBahiaBlanca，andofthemanyfossilmiceandothersmallquadrupedsinBrazil。Noonewillimaginethatadrought，evenfarsevererthanthosewhichcausesuchlossesintheprovincesofLaPlata，coulddestroyeveryindividualofeveryspeciesfromSouthernPatagoniatoBehring’sStraits。Whatshallwesayoftheextinctionofthehorse？Didthoseplainsfailofpasture，whichhavesincebeenoverrunbythousandsandhundredsofthousandsofthedescendantsofthestockintroducedbytheSpaniards？Havethesubsequentlyintroducedspeciesconsumedthefoodofthegreatantecedentraces？CanwebelievethattheCapybarahastakenthefoodoftheToxodon，theGuanacooftheMacrauchenia，theexistingsmallEdentataoftheirnumerousgiganticprototypes？Certainly，nofactinthelonghistoryoftheworldissostartlingasthewideandrepeatedexterminationsofitsinhabitants。
Nevertheless，ifweconsiderthesubjectunderanotherpointofview，itwillappearlessperplexing。Wedonotsteadilybearinmind，howprofoundlyignorantweareoftheconditionsofexistenceofeveryanimal；nordowealwaysremember，thatsomecheckisconstantlypreventingthetoorapidincreaseofeveryorganizedbeingleftinastateofnature。Thesupplyoffood，onanaverage，remainsconstant，yetthetendencyineveryanimaltoincreasebypropagationisgeometrical；anditssurprisingeffectshavenowherebeenmoreastonishinglyshown，thaninthecaseoftheEuropeananimalsrunwildduringthelastfewcenturiesinAmerica。Everyanimalinastateofnatureregularlybreeds；yetinaspecieslongestablished，any_great_increaseinnumbersisobviouslyimpossible，andmustbecheckedbysomemeans。Weare，nevertheless，seldomablewithcertaintytotellinanygivenspecies，atwhatperiodoflife，oratwhatperiodoftheyear，orwhetheronlyatlongintervals，thecheckfalls；or，again，whatistheprecisenatureofthecheck。Henceprobablyitis，thatwefeelsolittlesurpriseatone，oftwospeciescloselyalliedinhabits，beingrareandtheotherabundantinthesamedistrict；or，again，thatoneshouldbeabundantinonedistrict，andanother，fillingthesameplaceintheeconomyofnature，shouldbeabundantinaneighbouringdistrict，differingverylittleinitsconditions。Ifaskedhowthisis，oneimmediatelyrepliesthatitisdeterminedbysomeslightdifference，inclimate，food，orthenumberofenemies：yethowrarely，ifever，wecanpointouttheprecisecauseandmannerofactionofthecheck！Wearetherefore，driventotheconclusion，thatcausesgenerallyquiteinappreciablebyus，determinewhetheragivenspeciesshallbeabundantorscantyinnumbers。
Inthecaseswherewecantracetheextinctionofaspeciesthroughman，eitherwhollyorinonelimiteddistrict，weknowthatitbecomesrarerandrarer，andisthenlost：itwouldbedifficulttopointoutanyjustdistinction[13]betweenaspeciesdestroyedbymanorbytheincreaseofitsnaturalenemies。Theevidenceofrarityprecedingextinction，ismorestrikinginthesuccessivetertiarystrata，asremarkedbyseveralableobservers；ithasoftenbeenfoundthatashellverycommoninatertiarystratumisnowmostrare，andhasevenlongbeenthoughtextinct。Ifthen，asappearsprobable，speciesfirstbecomerareandthenextinct——ifthetoorapidincreaseofeveryspecies，eventhemostfavoured，issteadilychecked，aswemustadmit，thoughhowandwhenitishardtosay——andifwesee，withoutthesmallestsurprise，thoughunabletoassigntheprecisereason，onespeciesabundantandanothercloselyalliedspeciesrareinthesamedistrict——whyshouldwefeelsuchgreatastonishmentattheraritybeingcarriedonestepfurthertoextinction？
Anactiongoingon，oneverysideofus，andyetbarelyappreciable，mightsurelybecarriedalittlefurther，withoutexcitingourobservation。WhowouldfeelanygreatsurpriseathearingthattheMagalonyxwasformerlyrarecomparedwiththeMegatherium，orthatoneofthefossilmonkeyswasfewinnumbercomparedwithoneofthenowlivingmonkeys？andyetinthiscomparativerarity，weshouldhavetheplainestevidenceoflessfavourableconditionsfortheirexistence。Toadmitthatspeciesgenerallybecomerarebeforetheybecomeextinct——tofeelnosurpriseatthecomparativerarityofonespecieswithanother，andyettocallinsomeextraordinaryagentandtomarvelgreatlywhenaspeciesceasestoexist，appearstomemuchthesameastoadmitthatsicknessintheindividualisthepreludetodeath——tofeelnosurpriseatsickness——butwhenthesickmandiestowonder，andtobelievethathediedthroughviolence。[1]Mr。Waterhousehasdrawnupadetaileddescriptionofthishead，whichIhopehewillpublishinsomeJournal。
Anearlysimilarabnormal，butIdonotknowwhetherhereditary，structurehasbeenobservedinthecarp，andlikewiseinthecrocodileoftheGanges：HistoiredesAnomalies，parM。Isid。GeoffroySt。Hilaire，tom。i。p。244。