第8章

Fromtheforegoingconsiderationsitcannotbedoubtedthatthegeologicalrecord，viewedasawhole，isextremelyimperfect。butifweconfineourattentiontoanyoneformation，itbecomesmoredifficulttounderstand，whywedonotthereinfindcloselygraduatedvarietiesbetweenthealliedspecieswhichlivedatitscommencementandatitsclose。Somecasesareonrecordofthesamespeciespresentingdistinctvarietiesintheupperandlowerpartsofthesameformation，but，astheyarerare，theymaybeherepassedover。Althougheachformationhasindisputablyrequiredavastnumberofyearsforitsdeposition，Icanseeseveralreasonswhyeachshouldnotincludeagraduatedseriesoflinksbetweenthespecieswhichthenlived。butIcanbynomeanspretendtoassigndueproportionalweighttothefollowingconsiderations。

Althougheachformationmaymarkaverylonglapseofyears，eachperhapsisshortcomparedwiththeperiodrequisitetochangeonespeciesintoanother。Iamawarethattwopalaeontologists，whoseopinionsareworthyofmuchdeference，namelyBronnandWoodward，haveconcludedthattheaveragedurationofeachformationistwiceorthriceaslongastheaveragedurationofspecificforms。Butinsuperabledifficulties，asitseemstome，preventuscomingtoanyjustconclusiononthishead。Whenweseeaspeciesfirstappearinginthemiddleofanyformation，itwouldberashintheextremetoinferthatithadnotelsewherepreviouslyexisted。Soagainwhenwefindaspeciesdisappearingbeforetheuppermostlayershavebeendeposited，itwouldbeequallyrashtosupposethatitthenbecamewhollyextinct。

WeforgethowsmalltheareaofEuropeiscomparedwiththerestoftheworld。norhavetheseveralstagesofthesameformationthroughoutEuropebeencorrelatedwithperfectaccuracy。

Withmarineanimalsofallkinds，wemaysafelyinferalargeamountofmigrationduringclimatalandotherchanges。andwhenweseeaspeciesfirstappearinginanyformation，theprobabilityisthatitonlythenfirstimmigratedintothatarea。Itiswellknown，forinstance，thatseveralspeciesappearedsomewhatearlierinthepalaeozoicbedsofNorthAmericathaninthoseofEurope。timehavingapparentlybeenrequiredfortheirmigrationfromtheAmericantotheEuropeanseas。Inexaminingthelatestdepositsofvariousquartersoftheworld，ithaseverywherebeennoted，thatsomefewstillexistingspeciesarecommoninthedeposit，buthavebecomeextinctintheimmediatelysurroundingsea。or，conversely，thatsomearenowabundantintheneighbouringsea，butarerareorabsentinthisparticulardeposit。ItisanexcellentlessontoreflectontheascertainedamountofmigrationoftheinhabitantsofEuropeduringtheGlacialperiod，whichformsonlyapartofonewholegeologicalperiod。andlikewisetoreflectonthegreatchangesoflevel，ontheinordinatelygreatchangeofclimate，ontheprodigiouslapseoftime，allincludedwithinthissameglacialperiod。Yetitmaybedoubtedwhetherinanyquarteroftheworld，sedimentarydeposits，includingfossilremains，havegoneonaccumulatingwithinthesameareaduringthewholeofthisperiod。Itisnot，forinstance，probablethatsedimentwasdepositedduringthewholeoftheglacialperiodnearthemouthoftheMississippi，withinthatlimitofdepthatwhichmarineanimalscanflourish。forweknowwhatvastgeographicalchangesoccurredinotherpartsofAmericaduringthisspaceoftime。WhensuchbedsasweredepositedinshallowwaternearthemouthoftheMississippiduringsomepartoftheglacialperiodshallhavebeenupraised，organicremainswillprobablyfirstappearanddisappearatdifferentlevels，owingtothemigrationofspeciesandtogeographicalchanges。Andinthedistantfuture，ageologistexaminingthesebeds，mightbetemptedtoconcludethattheaveragedurationoflifeoftheembeddedfossilshadbeenlessthanthatoftheglacialperiod，insteadofhavingbeenreallyfargreater，thatisextendingfrombeforetheglacialepochtothepresentday。

Inordertogetaperfectgradationbetweentwoformsintheupperandlowerpartsofthesameformation，thedepositmusthavegoneonaccumulatingforaverylongperiod，inordertohavegivensufficienttimefortheslowprocessofvariation。hencethedepositwillgenerallyhavetobeaverythickone。andthespeciesundergoingmodificationwillhavehadtoliveonthesameareathroughoutthiswholetime。Butwehaveseenthatathickfossiliferousformationcanonlybeaccumulatedduringaperiodofsubsidence。andtokeepthedepthapproximatelythesame，whichisnecessaryinordertoenablethesamespeciestoliveonthesamespace，thesupplyofsedimentmustnearlyhavecounterbalancedtheamountofsubsidence。

Butthissamemovementofsubsidencewilloftentendtosinktheareawhencethesedimentisderived，andthusdiminishthesupplywhilstthedownwardmovementcontinues。Infact，thisnearlyexactbalancingbetweenthesupplyofsedimentandtheamountofsubsidenceisprobablyararecontingency。

forithasbeenobservedbymorethanonepalaeontologist，thatverythickdepositsareusuallybarrenoforganicremains，exceptneartheirupperorlowerlimits。

Itwouldseemthateachseparateformation，likethewholepileofformationsinanycountry，hasgenerallybeenintermittentinitsaccumulation。Whenwesee，asissooftenthecase，aformationcomposedofbedsofdifferentmineralogicalcomposition，wemayreasonablysuspectthattheprocessofdepositionhasbeenmuchinterrupted，asachangeinthecurrentsoftheseaandasupplyofsedimentofadifferentnaturewillgenerallyhavebeenduetogeographicalchangesrequiringmuchtime。Norwilltheclosestinspectionofaformationgiveanyideaofthetimewhichitsdepositionhasconsumed。Manyinstancescouldbegivenofbedsonlyafewfeetinthickness，representingformations，elsewherethousandsoffeetinthickness，andwhichmusthaverequiredanenormousperiodfortheiraccumulation。

yetnooneignorantofthisfactwouldhavesuspectedthevastlapseoftimerepresentedbythethinnerformation。Manycasescouldbegivenofthelowerbedsofaformationhavingbeenupraised，denuded，submerged，andthenre-coveredbytheupperbedsofthesameformation，facts，showingwhatwide，yeteasilyoverlooked，intervalshaveoccurredinitsaccumulation。

Inothercaseswehavetheplainestevidenceingreatfossilisedtrees，stillstandinguprightastheygrew，ofmanylongintervalsoftimeandchangesoflevelduringtheprocessofdeposition，whichwouldneverevenhavebeensuspected，hadnotthetreeschancedtohavebeenpreserved：

thus，MessrsLyellandDawsonfoundcarboniferousbeds1400feetthickinNovaScotia，withancientroot-bearingstrata，oneabovetheother，atnolessthansixty-eightdifferentlevels。Hence，whenthesamespeciesoccuratthebottom，middle，andtopofaformation，theprobabilityisthattheyhavenotlivedonthesamespotduringthewholeperiodofdeposition，buthavedisappearedandreappeared，perhapsmanytimes，duringthesamegeologicalperiod。Sothatifsuchspeciesweretoundergoaconsiderableamountofmodificationduringanyonegeologicalperiod，asectionwouldnotprobablyincludeallthefineintermediategradationswhichmustonmytheoryhaveexistedbetweenthem，butabrupt，thoughperhapsveryslight，changesofform。

Itisall-importanttorememberthatnaturalistshavenogoldenrulebywhichtodistinguishspeciesandvarieties。theygrantsomelittlevariabilitytoeachspecies，butwhentheymeetwithasomewhatgreateramountofdifferencebetweenanytwoforms，theyrankbothasspecies，unlesstheyareenabledtoconnectthemtogetherbycloseintermediategradations。Andthisfromthereasonsjustassignedwecanseldomhopetoeffectinanyonegeologicalsection。SupposingBandCtobetwospecies，andathird，A，tobefoundinanunderlyingbed。evenifAwerestrictlyintermediatebetweenBandC，itwouldsimplyberankedasathirdanddistinctspecies，unlessatthesametimeitcouldbemostcloselyconnectedwitheitheroneorbothformsbyintermediatevarieties。Norshoulditbeforgotten，asbeforeexplained，thatAmightbetheactualprogenitorofBandC，andyetmightnotatallnecessarilybestrictlyintermediatebetweentheminallpointsofstructure。Sothatwemightobtaintheparent-speciesanditsseveralmodifieddescendantsfromthelowerandupperbedsofaformation，andunlessweobtainednumeroustransitionalgradations，weshouldnotrecognisetheirrelationship，andshouldconsequentlybecompelledtorankthemallasdistinctspecies。

Itisnotoriousonwhatexcessivelyslightdifferencesmanypalaeontologistshavefoundedtheirspecies。andtheydothisthemorereadilyifthespecimenscomefromdifferentsub-stagesofthesameformation。SomeexperiencedconchologistsarenowsinkingmanyoftheveryfinespeciesofD’Orbignyandothersintotherankofvarieties。andonthisviewwedofindthekindofevidenceofchangewhichonmytheoryweoughttofind。Moreover，ifwelooktoratherwiderintervals，namely，todistinctbutconsecutivestagesofthesamegreatformation，wefindthattheembeddedfossils，thoughalmostuniversallyrankedasspecificallydifferent，yetarefarmorecloselyalliedtoeachotherthanarethespeciesfoundinmorewidelyseparatedformations。buttothissubjectIshallhavetoreturninthefollowingchapter。

Oneotherconsiderationisworthnotice：withanimalsandplantsthatcanpropagaterapidlyandarenothighlylocomotive，thereisreasontosuspect，aswehaveformerlyseen，thattheirvarietiesaregenerallyatfirstlocal。andthatsuchlocalvarietiesdonotspreadwidelyandsupplanttheirparent-formsuntiltheyhavebeenmodifiedandperfectedinsomeconsiderabledegree。Accordingtothisview，thechanceofdiscoveringinaformationinanyonecountryalltheearlystagesoftransitionbetweenanytwoforms，issmall，forthesuccessivechangesaresupposedtohavebeenlocalorconfinedtosomeonespot。Mostmarineanimalshaveawiderange。andwehaveseenthatwithplantsitisthosewhichhavethewidestrange，thatoftenestpresentvarieties。sothatwithshellsandothermarineanimals，itisprobablythosewhichhavehadthewidestrange，farexceedingthelimitsoftheknowngeologicalformationsofEurope，whichhaveoftenestgivenrise，firsttolocalvarietiesandultimatelytonewspecies。andthisagainwouldgreatlylessenthechanceofourbeingabletotracethestagesoftransitioninanyonegeologicalformation。

Itshouldnotbeforgotten，thatatthepresentday，withperfectspecimensforexamination，twoformscanseldombeconnectedbyintermediatevarietiesandthusprovedtobethesamespecies，untilmanyspecimenshavebeencollectedfrommanyplaces。andinthecaseoffossilspeciesthiscouldrarelybeeffectedbypalaeontologists。Weshall，perhaps，bestperceivetheimprobabilityofourbeingenabledtoconnectspeciesbynumerous，fine，intermediate，fossillinks，byaskingourselveswhether，forinstance，geologistsatsomefutureperiodwillbeabletoprove，thatourdifferentbreedsofcattle，sheep，horses，anddogshavedescendedfromasinglestockorfromseveralaboriginalstocks。or，again，whethercertainsea-shellsinhabitingtheshoresofNorthAmerica，whicharerankedbysomeconchologistsasdistinctspeciesfromtheirEuropeanrepresentatives，andbyotherconchologistsasonlyvarieties，arereallyvarietiesorare，asitiscalled，specificallydistinct。Thiscouldbeeffectedonlybythefuturegeologistdiscoveringinafossilstatenumerousintermediategradations。andsuchsuccessseemstomeimprobableinthehighestdegree。

Geologicalresearch，thoughithasaddednumerousspeciestoexistingandextinctgenera，andhasmadetheintervalsbetweensomefewgroupslesswidethantheyotherwisewouldhavebeen，yethasdonescarcelyanythinginbreakingdownthedistinctionbetweenspecies，byconnectingthemtogetherbynumerous，fine，intermediatevarieties。andthisnothavingbeeneffected，isprobablythegravestandmostobviousofallthemanyobjectionswhichmaybeurgedagainstmyviews。Henceitwillbeworthwhiletosumuptheforegoingremarks，underanimaginaryillustration。TheMalayArchipelagoisofaboutthesizeofEuropefromtheNorthCapetotheMediterranean，andfromBritaintoRussia。andthereforeequalsallthegeologicalformationswhichhavebeenexaminedwithanyaccuracy，exceptingthoseoftheUnitedStatesofAmerica。IfullyagreewithMrGodwin-Austen，thatthepresentconditionoftheMalayArchipelago，withitsnumerouslargeislandsseparatedbywideandshallowseas，probablyrepresentstheformerstateofEurope，whenmostofourformationswereaccumulating。TheMalayArchipelagoisoneoftherichestregionsofthewholeworldinorganicbeings。yetifallthespeciesweretobecollectedwhichhaveeverlivedthere，howimperfectlywouldtheyrepresentthenaturalhistoryoftheworld！

Butwehaveeveryreasontobelievethattheterrestrialproductionsofthearchipelagowouldbepreservedinanexcessivelyimperfectmannerintheformationswhichwesupposetobethereaccumulating。Isuspectthatnotmanyofthestrictlylittoralanimals，orofthosewhichlivedonnakedsubmarinerocks，wouldbeembedded。andthoseembeddedingravelorsand，wouldnotenduretoadistantepoch。Whereversedimentdidnotaccumulateonthebedofthesea，orwhereitdidnotaccumulateatasufficientratetoprotectorganicbodiesfromdecay，noremainscouldbepreserved。

Inourarchipelago，Ibelievethatfossiliferousformationscouldbeformedofsufficientthicknesstolasttoanage，asdistantinfuturityasthesecondaryformationslieinthepast，onlyduringperiodsofsubsidence。

Theseperiodsofsubsidencewouldbeseparatedfromeachotherbyenormousintervals，duringwhichtheareawouldbeeitherstationaryorrising。

whilstrising，eachfossiliferousformationwouldbedestroyed，almostassoonasaccumulated，bytheincessantcoast-action，aswenowseeontheshoresofSouthAmerica。Duringtheperiodsofsubsidencetherewouldprobablybemuchextinctionoflife。duringtheperiodsofelevation，therewouldbemuchvariation，butthegeologicalrecordwouldthenbeleastperfect。

Itmaybedoubtedwhetherthedurationofanyonegreatperiodofsubsidenceoverthewholeorpartofthearchipelago，togetherwithacontemporaneousaccumulationofsediment，wouldexceedtheaveragedurationofthesamespecificforms。andthesecontingenciesareindispensableforthepreservationofallthetransitionalgradationsbetweenanytwoormorespecies。Ifsuchgradationswerenotfullypreserved，transitionalvarietieswouldmerelyappearassomanydistinctspecies。Itis，also，probablethateachgreatperiodofsubsidencewouldbeinterruptedbyoscillationsoflevel，andthatslightclimatalchangeswouldinterveneduringsuchlengthyperiods。andinthesecasestheinhabitantsofthearchipelagowouldhavetomigrate，andnocloselyconsecutiverecordoftheirmodificationscouldbepreservedinanyoneformation。

Verymanyofthemarineinhabitantsofthearchipelagonowrangethousandsofmilesbeyonditsconfines。andanalogyleadsmetobelievethatitwouldbechieflythesefar-rangingspecieswhichwouldoftenestproducenewvarieties。

andthevarietieswouldatfirstgenerallybelocalorconfinedtooneplace，butifpossessedofanydecidedadvantage，orwhenfurthermodifiedandimproved，theywouldslowlyspreadandsupplanttheirparent-forms。

Whensuchvarietiesreturnedtotheirancienthomes，astheywoulddifferfromtheirformerstate，inanearlyuniform，thoughperhapsextremelyslightdegree，theywould，accordingtotheprinciplesfollowedbymanypalaeontologists，berankedasnewanddistinctspecies。

Ifthen，therebesomedegreeoftruthintheseremarks，wehavenorighttoexpecttofindinourgeologicalformations，aninfinitenumberofthosefinetransitionalforms，whichonmytheoryassuredlyhaveconnectedallthepastandpresentspeciesofthesamegroupintoonelongandbranchingchainoflife。Weoughtonlytolookforafewlinks，somemoreclosely，somemoredistantlyrelatedtoeachother。andtheselinks，letthembeeversoclose，iffoundindifferentstagesofthesameformation，would，bymostpalaeontologists，berankedasdistinctspecies。ButIdonotpretendthatIshouldeverhavesuspectedhowpoorarecordofthemutationsoflife，thebestpreservedgeologicalsectionpresented，hadnotthedifficultyofournotdiscoveringinnumerabletransitionallinksbetweenthespecieswhichappearedatthecommencementandcloseofeachformation，pressedsohardlyonmytheory。

OnthesuddenappearanceofwholegroupsofAlliedSpecies。Theabruptmannerinwhichwholegroupsofspeciessuddenlyappearincertainformations，hasbeenurgedbyseveralpalaeontologists，forinstance，byAgassiz，Pictet，andbynonemoreforciblythanbyProfessorSedgwick，asafatalobjectiontothebeliefinthetransmutationofspecies。Ifnumerousspecies，belongingtothesamegeneraorfamilies，havereallystartedintolifeallatonce，thefactwouldbefataltothetheoryofdescentwithslowmodificationthroughnaturalselection。Forthedevelopmentofagroupofforms，allofwhichhavedescendedfromsomeoneprogenitor，musthavebeenanextremelyslowprocess。andtheprogenitorsmusthavelivedlongagesbeforetheirmodifieddescendants。Butwecontinuallyover-ratetheperfectionofthegeologicalrecord，andfalselyinfer，becausecertaingeneraorfamilieshavenotbeenfoundbeneathacertainstage，thattheydidnotexistbeforethatstage。Wecontinuallyforgethowlargetheworldis，comparedwiththeareaoverwhichourgeologicalformationshavebeencarefullyexamined。weforgetthatgroupsofspeciesmayelsewherehavelongexistedandhaveslowlymultipliedbeforetheyinvadedtheancientarchipelagoesofEuropeandoftheUnitedStates。Wedonotmakedueallowancefortheenormousintervalsoftime，whichhaveprobablyelapsedbetweenourconsecutiveformations，longerperhapsinsomecasesthanthetimerequiredfortheaccumulationofeachformation。Theseintervalswillhavegiventimeforthemultiplicationofspeciesfromsomeoneorsomefewparent-forms。andinthesucceedingformationsuchspecieswillappearasifsuddenlycreated。

Imayhererecallaremarkformerlymade，namelythatitmightrequirealongsuccessionofagestoadaptanorganismtosomenewandpeculiarlineoflife，forinstancetoflythroughtheair。butthatwhenthishadbeeneffected，andafewspecieshadthusacquiredagreatadvantageoverotherorganisms，acomparativelyshorttimewouldbenecessarytoproducemanydivergentforms，whichwouldbeabletospreadrapidlyandwidelythroughouttheworld。

Iwillnowgiveafewexamplestoillustratetheseremarks。andtoshowhowliablewearetoerrorinsupposingthatwholegroupsofspecieshavesuddenlybeenproduced。Imayrecallthewell-knownfactthatingeologicaltreatises，publishednotmanyyearsago，thegreatclassofmammalswasalwaysspokenofashavingabruptlycomeinatthecommencementofthetertiaryseries。Andnowoneoftherichestknownaccumulationsoffossilmammalsbelongstothemiddleofthesecondaryseries。andonetruemammalhasbeendiscoveredinthenewredsandstoneatnearlythecommencementofthisgreatseries。Cuvierusedtourgethatnomonkeyoccurredinanytertiarystratum。butnowextinctspecieshavebeendiscoveredinIndia，SouthAmerica，andinEuropeevenasfarbackastheeocenestage。Themoststrikingcase，however，isthatoftheWhalefamily。astheseanimalshavehugebones，aremarine，andrangeovertheworld，thefactofnotasingleboneofawhalehavingbeendiscoveredinanysecondaryformation，seemedfullytojustifythebeliefthatthisgreatanddistinctorderhadbeensuddenlyproducedintheintervalbetweenthelatestsecondaryandearliesttertiaryformation。ButnowwemayreadintheSupplementtoLyell’s’Manual，’publishedin1858，clearevidenceoftheexistenceofwhalesintheuppergreensand，sometimebeforethecloseofthesecondaryperiod。

Imaygiveanotherinstance，whichfromhavingpassedundermyowneyeshasmuchstruckme。InamemoironFossilSessileCirripedes，Ihavestatedthat，fromthenumberofexistingandextincttertiaryspecies。fromtheextraordinaryabundanceoftheindividualsofmanyspeciesallovertheworld，fromtheArcticregionstotheequator，inhabitingvariouszonesofdepthsfromtheuppertidallimitsto50fathoms。fromtheperfectmannerinwhichspecimensarepreservedintheoldesttertiarybeds。fromtheeasewithwhichevenafragmentofavalvecanberecognised。fromallthesecircumstances，Iinferredthathadsessilecirripedesexistedduringthesecondaryperiods，theywouldcertainlyhavebeenpreservedanddiscovered。

andasnotonespecieshadbeendiscoveredinbedsofthisage，Iconcludedthatthisgreatgrouphadbeensuddenlydevelopedatthecommencementofthetertiaryseries。Thiswasasoretroubletome，addingasIthoughtonemoreinstanceoftheabruptappearanceofagreatgroupofspecies。

Butmyworkhadhardlybeenpublished，whenaskilfulpalaeontologist，M。Bosquet，sentmeadrawingofaperfectspecimenofanunmistakeablesessilecirripede，whichhehadhimselfextractedfromthechalkofBelgium。

And，asiftomakethecaseasstrikingaspossible，thissessilecirripedewasaChthamalus，averycommon，large，andubiquitousgenus，ofwhichnotonespecimenhasasyetbeenfoundeveninanytertiarystratum。Hencewenowpositivelyknowthatsessilecirripedesexistedduringthesecondaryperiod。andthesecirripedesmighthavebeentheprogenitorsofourmanytertiaryandexistingspecies。

Thecasemostfrequentlyinsistedonbypalaeontologistsoftheapparentlysuddenappearanceofawholegroupofspecies，isthatoftheteleosteanfishes，lowdownintheChalkperiod。Thisgroupincludesthelargemajorityofexistingspecies。Lately，ProfessorPictethascarriedtheirexistenceonesub-stagefurtherback。andsomepalaeontologistsbelievethatcertainmucholderfishes，ofwhichtheaffinitiesareasyetimperfectlyknown，arereallyteleostean。Assuming，however，thatthewholeofthemdidappear，asAgassizbelieves，atthecommencementofthechalkformation，thefactwouldcertainlybehighlyremarkable。butIcannotseethatitwouldbeaninsuperabledifficultyonmytheory，unlessitcouldlikewisebeshownthatthespeciesofthisgroupappearedsuddenlyandsimultaneouslythroughouttheworldatthissameperiod。Itisalmostsuperfluoustoremarkthathardlyanyfossil-fishareknownfromsouthoftheequator。andbyrunningthroughPictet’spalaeontologyitwillbeseenthatveryfewspeciesareknownfromseveralformationsinEurope。Somefewfamiliesoffishnowhaveaconfinedrange。theteleosteanfishmightformerlyhavehadasimilarlyconfinedrange，andafterhavingbeenlargelydevelopedinsomeonesea，mighthavespreadwidely。Norhaveweanyrighttosupposethattheseasoftheworldhavealwaysbeensofreelyopenfromsouthtonorthastheyareatpresent。Evenatthisday，iftheMalayArchipelagowereconvertedintoland，thetropicalpartsoftheIndianOceanwouldformalargeandperfectlyenclosedbasin，inwhichanygreatgroupofmarineanimalsmightbemultiplied。andheretheywouldremainconfined，untilsomeofthespeciesbecameadaptedtoacoolerclimate，andwereenabledtodoublethesoutherncapesofAfricaorAustralia，andthusreachotheranddistantseas。

Fromtheseandsimilarconsiderations，butchieflyfromourignoranceofthegeologyofothercountriesbeyondtheconfinesofEuropeandtheUnitedStates。andfromtherevolutioninourpalaeontologicalideasonmanypoints，whichthediscoveriesofeventhelastdozenyearshaveeffected，itseemstometobeaboutasrashinustodogmatizeonthesuccessionoforganicbeingsthroughouttheworld，asitwouldbeforanaturalisttolandforfiveminutesonsomeonebarrenpointinAustralia，andthentodiscussthenumberandrangeofitsproductions。

OnthesuddenappearanceofgroupsofAlliedSpeciesinthelowestknownfossiliferousstrata。Thereisanotherandallieddifficulty，whichismuchgraver。Ialludetothemannerinwhichnumbersofspeciesofthesamegroup，suddenlyappearinthelowestknownfossiliferousrocks。

Mostoftheargumentswhichhaveconvincedmethatalltheexistingspeciesofthesamegrouphavedescendedfromoneprogenitor，applywithnearlyequalforcetotheearliestknownspecies。Forinstance，IcannotdoubtthatalltheSiluriantrilobiteshavedescendedfromsomeonecrustacean，whichmusthavelivedlongbeforetheSilurianage，andwhichprobablydifferedgreatlyfromanyknownanimal。SomeofthemostancientSiluriananimals，astheNautilus，Lingula，&。c。，donotdiffermuchfromlivingspecies。anditcannotonmytheorybesupposed，thattheseoldspeciesweretheprogenitorsofallthespeciesoftheorderstowhichtheybelong，fortheydonotpresentcharactersinanydegreeintermediatebetweenthem。

If，moreover，theyhadbeentheprogenitorsoftheseorders，theywouldalmostcertainlyhavebeenlongagosupplantedandexterminatedbytheirnumerousandimproveddescendants。

Consequently，ifmytheorybetrue，itisindisputablethatbeforethelowestSilurianstratumwasdeposited，longperiodselapsed，aslongas，orprobablyfarlongerthan，thewholeintervalfromtheSilurianagetothepresentday。andthatduringthesevast，yetquiteunknown，periodsoftime，theworldswarmedwithlivingcreatures。

Tothequestionwhywedonotfindrecordsofthesevastprimordialperiods，Icangivenosatisfactoryanswer。Severalofthemosteminentgeologists，withSirR。Murchisonattheirhead，areconvincedthatweseeintheorganicremainsofthelowestSilurianstratumthedawnoflifeonthisplanet。Otherhighlycompetentjudges，asLyellandthelateE。

Forbes，disputethisconclusion。Weshouldnotforgetthatonlyasmallportionoftheworldisknownwithaccuracy。M。BarrandehaslatelyaddedanotherandlowerstagetotheSiluriansystem，aboundingwithnewandpeculiarspecies。TracesoflifehavebeendetectedintheLongmyndbedsbeneathBarrande’sso-calledprimordialzone。Thepresenceofphosphaticnodulesandbituminousmatterinsomeofthelowestazoicrocks，probablyindicatestheformerexistenceoflifeattheseperiods。Butthedifficultyofunderstandingtheabsenceofvastpilesoffossiliferousstrata，whichonmytheorynodoubtweresomewhereaccumulatedbeforetheSilurianepoch，isverygreat。Ifthesemostancientbedshadbeenwhollywornawaybydenudation，orobliteratedbymetamorphicaction，weoughttofindonlysmallremnantsoftheformationsnextsucceedingtheminage，andtheseoughttobeverygenerallyinametamorphosedcondition。ButthedescriptionswhichwenowpossessoftheSiluriandepositsoverimmenseterritoriesinRussiaandinNorthAmerica，donotsupporttheview，thattheolderaformationis，themoreithassufferedtheextremityofdenudationandmetamorphism。

Thecaseatpresentmustremaininexplicable。andmaybetrulyurgedasavalidargumentagainsttheviewshereentertained。Toshowthatitmayhereafterreceivesomeexplanation，Iwillgivethefollowinghypothesis。

Fromthenatureoftheorganicremains，whichdonotappeartohaveinhabitedprofounddepths，intheseveralformationsofEuropeandoftheUnitedStates。andfromtheamountofsediment，milesinthickness，ofwhichtheformationsarecomposed，wemayinferthatfromfirsttolastlargeislandsortractsofland，whencethesedimentwasderived，occurredintheneighbourhoodoftheexistingcontinentsofEuropeandNorthAmerica。Butwedonotknowwhatwasthestateofthingsintheintervalsbetweenthesuccessiveformations。

whetherEuropeandtheUnitedStatesduringtheseintervalsexistedasdryland，orasasubmarinesurfacenearland，onwhichsedimentwasnotdeposited，oragainasthebedofanopenandunfathomablesea。

Lookingtotheexistingoceans，whicharethriceasextensiveastheland，weseethemstuddedwithmanyislands。butnotoneoceanicislandisasyetknowntoaffordevenaremnantofanypalaeozoicorsecondaryformation。Hencewemayperhapsinfer，thatduringthepalaeozoicandsecondaryperiods，neithercontinentsnorcontinentalislandsexistedwhereouroceansnowextend。forhadtheyexistedthere，palaeozoicandsecondaryformationswouldinallprobabilityhavebeenaccumulatedfromsedimentderivedfromtheirwearandtear。andwouldhavebeenatleastpartiallyupheavedbytheoscillationsoflevel，whichwemayfairlyconcludemusthaveintervenedduringtheseenormouslylongperiods。Ifthenwemayinferanythingfromthesefacts，wemayinferthatwhereouroceansnowextend，oceanshaveextendedfromtheremotestperiodofwhichwehaveanyrecord。andontheotherhand，thatwherecontinentsnowexist，largetractsoflandhaveexisted，subjectednodoubttogreatoscillationsoflevel，sincetheearliestsilurianperiod。ThecolouredmapappendedtomyvolumeonCoralReefs，ledmetoconcludethatthegreatoceansarestillmainlyareasofsubsidence，thegreatarchipelagoesstillareasofoscillationsoflevel，andthecontinentsareasofelevation。Buthaveweanyrighttoassumethatthingshavethusremainedfrometernity？Ourcontinentsseemtohavebeenformedbyapreponderance，duringmanyoscillationsoflevel，oftheforceofelevation。butmaynottheareasofpreponderantmovementhavechangedinthelapseofages？Ataperiodimmeasurablyantecedenttothesilurianepoch，continentsmayhaveexistedwhereoceansarenowspreadout。andclearandopenoceansmayhaveexistedwhereourcontinentsnowstand。Norshouldwebejustifiedinassumingthatif，forinstance，thebedofthePacificOceanwerenowconvertedintoacontinent，weshouldtherefindformationsolderthanthesilurianstrata，supposingsuchtohavebeenformerlydeposited。foritmightwellhappenthatstratawhichhadsubsidedsomemilesnearertothecentreoftheearth，andwhichhadbeenpressedonbyanenormousweightofsuperincumbentwater，mighthaveundergonefarmoremetamorphicactionthanstratawhichhavealwaysremainednearertothesurface。Theimmenseareasinsomepartsoftheworld，forinstanceinSouthAmerica，ofbaremetamorphicrocks，whichmusthavebeenheatedundergreatpressure，havealwaysseemedtometorequiresomespecialexplanation。andwemayperhapsbelievethatweseeintheselargeareas，themanyformationslonganteriortothesilurianepochinacompletelymetamorphosedcondition。

Theseveraldifficultiesherediscussed，namelyournotfindinginthesuccessiveformationsinfinitelynumeroustransitionallinksbetweenthemanyspecieswhichnowexistorhaveexisted。thesuddenmannerinwhichwholegroupsofspeciesappearinourEuropeanformations。thealmostentireabsence，asatpresentknown，offossiliferousformationsbeneaththeSilurianstrata，areallundoubtedlyofthegravestnature。Weseethisintheplainestmannerbythefactthatallthemosteminentpalaeontologists，namelyCuvier，Owen，Agassiz，Barrande，Falconer，E。Forbes，&。c。，andallourgreatestgeologists，asLyell，Murchison，Sedgwick，&。c。，haveunanimously，oftenvehemently，maintainedtheimmutabilityofspecies。ButIhavereasontobelievethatonegreatauthority，SirCharlesLyell，fromfurtherreflexionentertainsgravedoubtsonthissubject。Ifeelhowrashitistodifferfromthesegreatauthorities，towhom，withothers，weoweallourknowledge。

Thosewhothinkthenaturalgeologicalrecordinanydegreeperfect，andwhodonotattachmuchweighttothefactsandargumentsofotherkindseveninthisvolume，willundoubtedlyatoncerejectmytheory。Formypart，followingoutLyell’smetaphor，Ilookatthenaturalgeologicalrecord，asahistoryoftheworldimperfectlykept，andwritteninachangingdialect。ofthishistorywepossessthelastvolumealone，relatingonlytotwoorthreecountries。Ofthisvolume，onlyhereandthereashortchapterhasbeenpreserved。andofeachpage，onlyhereandthereafewlines。Eachwordoftheslowly-changinglanguage，inwhichthehistoryissupposedtobewritten，beingmoreorlessdifferentintheinterruptedsuccessionofchapters，mayrepresenttheapparentlyabruptlychangedformsoflife，entombedinourconsecutive，butwidelyseparatedformations。

Onthisview，thedifficultiesabovediscussedaregreatlydiminished，orevendisappear。PreviousChapterNextChapterTheOriginofSpecies-Chapter10TheOriginofSpeciesbyCharlesDarwinPreviousChapterNextChapterChapter10-OnTheGeologicalSuccessionofOrganicBeingsLetusnowseewhethertheseveralfactsandrulesrelatingtothegeologicalsuccessionoforganicbeings，betteraccordwiththecommonviewoftheimmutabilityofspecies，orwiththatoftheirslowandgradualmodification，throughdescentandnaturalselection。

Newspecieshaveappearedveryslowly，oneafteranother，bothonthelandandinthewaters。Lyellhasshownthatitishardlypossibletoresisttheevidenceonthisheadinthecaseoftheseveraltertiarystages。andeveryyeartendstofilluptheblanksbetweenthem，andtomakethepercentagesystemoflostandnewformsmoregradual。Insomeofthemostrecentbeds，thoughundoubtedlyofhighantiquityifmeasuredbyyears，onlyoneortwospeciesarelostforms，andonlyoneortwoarenewforms，havinghereappearedforthefirsttime，eitherlocally，or，asfarasweknow，onthefaceoftheearth。IfwemaytrusttheobservationsofPhilippiinSicily，thesuccessivechangesinthemarineinhabitantsofthatislandhavebeenmanyandmostgradual。Thesecondaryformationsaremorebroken。

but，asBronnhasremarked，neithertheappearancenordisappearanceoftheirmanynowextinctspecieshasbeensimultaneousineachseparateformation。

Speciesofdifferentgeneraandclasseshavenotchangedatthesamerate，orinthesamedegree。Intheoldesttertiarybedsafewlivingshellsmaystillbefoundinthemidstofamultitudeofextinctforms。Falconerhasgivenastrikinginstanceofasimilarfact，inanexistingcrocodileassociatedwithmanystrangeandlostmammalsandreptilesinthesub-Himalayandeposits。TheSilurianLinguladiffersbutlittlefromthelivingspeciesofthisgenus。whereasmostoftheotherSilurianMolluscsandalltheCrustaceanshavechangedgreatly。Theproductionsofthelandseemtochangeataquickerratethanthoseofthesea，ofwhichastrikinginstancehaslatelybeenobservedinSwitzerland。Thereissomereasontobelievethatorganisms，consideredhighinthescaleofnature，changemorequicklythanthosethatarelow：thoughthereareexceptionstothisrule。Theamountoforganicchange，asPictethasremarked，doesnotstrictlycorrespondwiththesuccessionofourgeologicalformations。sothatbetweeneachtwoconsecutiveformations，theformsoflifehaveseldomchangedinexactlythesamedegree。Yetifwecompareanybutthemostcloselyrelatedformations，allthespecieswillbefoundtohaveundergonesomechange。Whenaspecieshasoncedisappearedfromthefaceoftheearth，wehavereasontobelievethatthesameidenticalformneverreappears。Thestrongestapparentexceptiontothislatterrule，isthatoftheso-called`colonies’ofM。Barrande，whichintrudeforaperiodinthemidstofanolderformation，andthenallowthepre-existingfaunatoreappear。butLyell’sexplanation，namely，thatitisacaseoftemporarymigrationfromadistinctgeographicalprovince，seemstomesatisfactory。

Theseseveralfactsaccordwellwithmytheory。Ibelieveinnofixedlawofdevelopment，causingalltheinhabitantsofacountrytochangeabruptly，orsimultaneously，ortoanequaldegree。Theprocessofmodificationmustbeextremelyslow。Thevariabilityofeachspeciesisquiteindependentofthatofallothers。Whethersuchvariabilitybetakenadvantageofbynaturalselection，andwhetherthevariationsbeaccumulatedtoagreaterorlesseramount，thuscausingagreaterorlesseramountofmodificationinthevaryingspecies，dependsonmanycomplexcontingencies，onthevariabilitybeingofabeneficialnature，onthepowerofintercrossing，ontherateofbreeding，ontheslowlychangingphysicalconditionsofthecountry，andmoreespeciallyonthenatureoftheotherinhabitantswithwhichthevaryingspeciescomesintocompetition。Henceitisbynomeanssurprisingthatonespeciesshouldretainthesameidenticalformmuchlongerthanothers。or，ifchanging，thatitshouldchangeless。Weseethesamefactingeographicaldistribution。forinstance，intheland-shellsandcoleopterousinsectsofMadeirahavingcometodifferconsiderablyfromtheirnearestalliesonthecontinentofEurope，whereasthemarineshellsandbirdshaveremainedunaltered。Wecanperhapsunderstandtheapparentlyquickerrateofchangeinterrestrialandinmorehighlyorganisedproductionscomparedwithmarineandlowerproductions，bythemorecomplexrelationsofthehigherbeingstotheirorganicandinorganicconditionsoflife，asexplainedinaformerchapter。Whenmanyoftheinhabitantsofacountryhavebecomemodifiedandimproved，wecanunderstand，ontheprincipleofcompetition，andonthatofthemanyall-importantrelationsoforganismtoorganism，thatanyformwhichdoesnotbecomeinsomedegreemodifiedandimproved，willbeliabletobeexterminated。Hencewecanseewhyallthespeciesinthesameregiondoatlast，ifwelooktowideenoughintervalsoftime，becomemodified。forthosewhichdonotchangewillbecomeextinct。

Inmembersofthesameclasstheaverageamountofchange，duringlongandequalperiodsoftime，may，perhaps，benearlythesame。butastheaccumulationoflong-enduringfossiliferousformationsdependsongreatmassesofsedimenthavingbeendepositedonareaswhilstsubsiding，ourformationshavebeenalmostnecessarilyaccumulatedatwideandirregularlyintermittentintervals。consequentlytheamountoforganicchangeexhibitedbythefossilsembeddedinconsecutiveformationsisnotequal。Eachformation，onthisview，doesnotmarkanewandcompleteactofcreation，butonlyanoccasionalscene，takenalmostathazard，inaslowlychangingdrama。

Wecanclearlyunderstandwhyaspecieswhenoncelostshouldneverreappear，eveniftheverysameconditionsoflife，organicandinorganic，shouldrecur。Forthoughtheoffspringofonespeciesmightbeadapted(andnodoubtthishasoccurredininnumerableinstances)tofilltheexactplaceofanotherspeciesintheeconomyofnature，andthussupplantit。

yetthetwoformstheoldandthenewwouldnotbeidenticallythesame。

forbothwouldalmostcertainlyinheritdifferentcharactersfromtheirdistinctprogenitors。Forinstance，itisjustpossible，ifourfantail-pigeonswerealldestroyed，thatfanciers，bystrivingduringlongagesforthesameobject，mightmakeanewbreedhardlydistinguishablefromourpresentfantail。butiftheparentrock-pigeonwerealsodestroyed，andinnaturewehaveeveryreasontobelievethattheparent-formwillgenerallybesupplantedandexterminatedbyitsimprovedoffspring，itisquiteincrediblethatafantail，identicalwiththeexistingbreed，couldberaisedfromanyotherspeciesofpigeon，orevenfromtheotherwell-establishedracesofthedomesticpigeon，forthenewly-formedfantailwouldbealmostsuretoinheritfromitsnewprogenitorsomeslightcharacteristicdifferences。

Groupsofspecies，thatis，generaandfamilies，followthesamegeneralrulesintheirappearanceanddisappearanceasdosinglespecies，changingmoreorlessquickly，andinagreaterorlesserdegree。Agroupdoesnotreappearafterithasoncedisappeared。oritsexistence，aslongasitlasts，iscontinuous。Iamawarethattherearesomeapparentexceptionstothisrule，buttheexceptionsaresurprisinglyfew，sofew，thatE。

Forbes，Pictet，andWoodward(thoughallstronglyopposedtosuchviewsasImaintain)admititstruth。andtherulestrictlyaccordswithmytheory。

Forasallthespeciesofthesamegrouphavedescendedfromsomeonespecies，itisclearthataslongasanyspeciesofthegrouphaveappearedinthelongsuccessionofages，solongmustitsmembershavecontinuouslyexisted，inordertohavegeneratedeithernewandmodifiedorthesameoldandunmodifiedforms。SpeciesofthegenusLingula，forinstance，musthavecontinuouslyexistedbyanunbrokensuccessionofgenerations，fromthelowestSilurianstratumtothepresentday。

Wehaveseeninthelastchapterthatthespeciesofagroupsometimesfalselyappeartohavecomeinabruptly。andIhaveattemptedtogiveanexplanationofthisfact，whichiftruewouldhavebeenfataltomyviews。

Butsuchcasesarecertainlyexceptional。thegeneralrulebeingagradualincreaseinnumber，tillthegroupreachesitsmaximum，andthen，soonerorlater，itgraduallydecreases。Ifthenumberofthespeciesofagenus，orthenumberofthegeneraofafamily，berepresentedbyaverticallineofvaryingthickness，crossingthesuccessivegeologicalformationsinwhichthespeciesarefound，thelinewillsometimesfalselyappeartobeginatitslowerend，notinasharppoint，butabruptly。itthengraduallythickensupwards，sometimeskeepingforaspaceofequalthickness，andultimatelythinsoutintheupperbeds，markingthedecreaseandfinalextinctionofthespecies。Thisgradualincreaseinnumberofthespeciesofagroupisstrictlyconformablewithmytheory。asthespeciesofthesamegenus，andthegeneraofthesamefamily，canincreaseonlyslowlyandprogressively。fortheprocessofmodificationandtheproductionofanumberofalliedformsmustbeslowandgradual，onespeciesgivingrisefirsttotwoorthreevarieties，thesebeingslowlyconvertedintospecies，whichintheirturnproducebyequallyslowstepsotherspecies，andsoon，likethebranchingofagreattreefromasinglestem，tillthegroupbecomeslarge。OnExtinctionWehaveasyetspokenonlyincidentallyofthedisappearanceofspeciesandofgroupsofspecies。Onthetheoryofnaturalselectiontheextinctionofoldformsandtheproductionofnewandimprovedformsareintimatelyconnectedtogether。Theoldnotionofalltheinhabitantsoftheearthhavingbeensweptawayatsuccessiveperiodsbycatastrophes，isverygenerallygivenup，evenbythosegeologists，asEliedeBeaumont，Murchison，Barrande，&。c。，whosegeneralviewswouldnaturallyleadthemtothisconclusion。

Onthecontrary，wehaveeveryreasontobelieve，fromthestudyofthetertiaryformations，thatspeciesandgroupsofspeciesgraduallydisappear，oneafteranother，firstfromonespot，thenfromanother，andfinallyfromtheworld。Bothsinglespeciesandwholegroupsofspecieslastforveryunequalperiods。somegroups，aswehaveseen，havingenduredfromtheearliestknowndawnoflifetothepresentday。somehavingdisappearedbeforethecloseofthepalaeozoicperiod。Nofixedlawseemstodeterminethelengthoftimeduringwhichanysinglespeciesoranysinglegenusendures。Thereisreasontobelievethatthecompleteextinctionofthespeciesofagroupisgenerallyaslowerprocessthantheirproduction：

iftheappearanceanddisappearanceofagroupofspeciesberepresented，asbefore，byaverticallineofvaryingthickness，thelineisfoundtotapermoregraduallyatitsupperend，whichmarkstheprogressofextermination，thanatitslowerend，whichmarksthefirstappearanceandincreaseinnumbersofthespecies。Insomecases，however，theexterminationofwholegroupsofbeings，asofammonitestowardsthecloseofthesecondaryperiod，hasbeenwonderfullysudden。

Thewholesubjectoftheextinctionofspecieshasbeeninvolvedinthemostgratuitousmystery。Someauthorshaveevensupposedthatastheindividualhasadefinitelengthoflife，sohavespeciesadefiniteduration。

NooneIthinkcanhavemarvelledmoreattheextinctionofspecies，thanIhavedone。WhenIfoundinLaPlatathetoothofahorseembeddedwiththeremainsofMastodon，Megatherium，Toxodon，andotherextinctmonsters，whichallco-existedwithstilllivingshellsataverylategeologicalperiod，Iwasfilledwithastonishment。forseeingthatthehorse，sinceitsintroductionbytheSpaniardsintoSouthAmerica，hasrunwildoverthewholecountryandhasincreasedinnumbersatanunparalleledrate，Iaskedmyselfwhatcouldsorecentlyhaveexterminatedtheformerhorseunderconditionsoflifeapparentlysofavourable。Buthowutterlygroundlesswasmyastonishment！ProfessorOwensoonperceivedthatthetooth，thoughsolikethatoftheexistinghorse，belongedtoanextinctspecies。Hadthishorsebeenstillliving，butinsomedegreerare，nonaturalistwouldhavefelttheleastsurpriseatitsrarity。forrarityistheattributeofavastnumberofspeciesofallclasses，inallcountries。Ifweaskourselveswhythisorthatspeciesisrare，weanswerthatsomethingisunfavourableinitsconditionsoflife。butwhatthatsomethingis，wecanhardlyevertell。Onthesuppositionofthefossilhorsestillexistingasararespecies，wemighthavefeltcertainfromtheanalogyofallothermammals，evenoftheslow-breedingelephant，andfromthehistoryofthenaturalisationofthedomestichorseinSouthAmerica，thatundermorefavourableconditionsitwouldinaveryfewyearshavestockedthewholecontinent。Butwecouldnothavetoldwhattheunfavourableconditionswerewhichcheckeditsincrease，whethersomeoneorseveralcontingencies，andatwhatperiodofthehorse’slife，andinwhatdegree，theyseverallyacted。Iftheconditionshadgoneon，howeverslowly，becominglessandlessfavourable，weassuredlyshouldnothaveperceivedthefact，yetthefossilhorsewouldcertainlyhavebecomerarerandrarer，andfinallyextinct。

itsplacebeingseizedonbysomemoresuccessfulcompetitor。

Itismostdifficultalwaystorememberthattheincreaseofeverylivingbeingisconstantlybeingcheckedbyunperceivedinjuriousagencies。andthatthesesameunperceivedagenciesareamplysufficienttocauserarity，andfinallyextinction。Weseeinmanycasesinthemorerecenttertiaryformations，thatrarityprecedesextinction。andweknowthatthishasbeentheprogressofeventswiththoseanimalswhichhavebeenexterminated，eitherlocallyorwholly，throughman’sagency。ImayrepeatwhatIpublishedin1845，namely，thattoadmitthatspeciesgenerallybecomerarebeforetheybecomeextincttofeelnosurpriseattherarityofaspecies，andyettomarvelgreatlywhenitceasestoexist，ismuchthesameastoadmitthatsicknessintheindividualistheforerunnerofdeathtofeelnosurpriseatsickness，butwhenthesickmandies，towonderandtosuspectthathediedbysomeunknowndeedofviolence。

Thetheoryofnaturalselectionisgroundedonthebeliefthateachnewvariety，andultimatelyeachnewspecies，isproducedandmaintainedbyhavingsomeadvantageoverthosewithwhichitcomesintocompetition。

andtheconsequentextinctionofless-favouredformsalmostinevitablyfollows。Itisthesamewithourdomesticproductions：whenanewandslightlyimprovedvarietyhasbeenraised，itatfirstsupplantsthelessimprovedvarietiesinthesameneighbourhood。whenmuchimproveditistransportedfarandnear，likeourshort-horncattle，andtakestheplaceofotherbreedsinothercountries。Thustheappearanceofnewformsandthedisappearanceofoldforms，bothnaturalandartificial，areboundtogether。Incertainflourishinggroups，thenumberofnewspecificformswhichhavebeenproducedwithinagiventimeisprobablygreaterthanthatoftheoldformswhichhavebeenexterminated。butweknowthatthenumberofspecieshasnotgoneonindefinitelyincreasing，atleastduringthelatergeologicalperiods，sothatlookingtolatertimeswemaybelievethattheproductionofnewformshascausedtheextinctionofaboutthesamenumberofoldforms。

Thecompetitionwillgenerallybemostsevere，asformerlyexplainedandillustratedbyexamples，betweentheformswhicharemostlikeeachotherinallrespects。Hencetheimprovedandmodifieddescendantsofaspecieswillgenerallycausetheexterminationoftheparent-species。andifmanynewformshavebeendevelopedfromanyonespecies，thenearestalliesofthatspecies，i。e。thespeciesofthesamegenus，willbethemostliabletoextermination。Thus，asIbelieve，anumberofnewspeciesdescendedfromonespecies，thatisanewgenus，comestosupplantanoldgenus，belongingtothesamefamily。Butitmustoftenhavehappenedthatanewspeciesbelongingtosomeonegroupwillhaveseizedontheplaceoccupiedbyaspeciesbelongingtoadistinctgroup，andthuscauseditsextermination。andifmanyalliedformsbedevelopedfromthesuccessfulintruder，manywillhavetoyieldtheirplaces。anditwillgenerallybealliedforms，whichwillsufferfromsomeinheritedinferiorityincommon。

Butwhetheritbespeciesbelongingtothesameortoadistinctclass，whichyieldtheirplacestootherspecieswhichhavebeenmodifiedandimproved，afewofthesufferersmayoftenlongbepreserved，frombeingfittedtosomepeculiarlineoflife，orfrominhabitingsomedistantandisolatedstation，wheretheyhaveescapedseverecompetition。Forinstance，asinglespeciesofTrigonia，agreatgenusofshellsinthesecondaryformations，survivesintheAustralianseas。andafewmembersofthegreatandalmostextinctgroupofGanoidfishesstillinhabitourfreshwaters。

Thereforetheutterextinctionofagroupisgenerally，aswehaveseen，aslowerprocessthanitsproduction。

Withrespecttotheapparentlysuddenexterminationofwholefamiliesororders，asofTrilobitesatthecloseofthepalaeozoicperiodandofAmmonitesatthecloseofthesecondaryperiod，wemustrememberwhathasbeenalreadysaidontheprobablewideintervalsoftimebetweenourconsecutiveformations。andintheseintervalstheremayhavebeenmuchslowextermination。

Moreover，whenbysuddenimmigrationorbyunusuallyrapiddevelopment，manyspeciesofanewgrouphavetakenpossessionofanewarea，theywillhaveexterminatedinacorrespondinglyrapidmannermanyoftheoldinhabitants。

andtheformswhichthusyieldtheirplaceswillcommonlybeallied，fortheywillpartakeofsomeinferiorityincommon。

Thus，asitseemstome，themannerinwhichsinglespeciesandwholegroupsofspeciesbecomeextinct，accordswellwiththetheoryofnaturalselection。Weneednotmarvelatextinction。ifwemustmarvel，letitbeatourpresumptioninimaginingforamomentthatweunderstandthemanycomplexcontingencies，onwhichtheexistenceofeachspeciesdepends。

Ifweforgetforaninstant，thateachspeciestendstoincreaseinordinately，andthatsomecheckisalwaysinaction，yetseldomperceivedbyus，thewholeeconomyofnaturewillbeutterlyobscured。Wheneverwecanpreciselysaywhythisspeciesismoreabundantinindividualsthanthat。whythisspeciesandnotanothercanbenaturalisedinagivencountry。then，andnottillthen，wemayjustlyfeelsurprisewhywecannotaccountfortheextinctionofthisparticularspeciesorgroupofspecies。OntheFormsofLifechangingalmostsimultaneouslythroughouttheWorldScarcelyanypalaeontologicaldiscoveryismorestrikingthanthefact，thattheformsoflifechangealmostsimultaneouslythroughouttheworld。

ThusourEuropeanChalkformationcanberecognisedinmanydistantpartsoftheworld，underthemostdifferentclimates，wherenotafragmentofthemineralchalkitselfcanbefound。namely，inNorthAmerica，inequatorialSouthAmerica，inTierradelFuego，attheCapeofGoodHope，andinthepeninsulaofIndia。Foratthesedistantpoints，theorganicremainsincertainbedspresentanunmistakeabledegreeofresemblancetothoseoftheChalk。Itisnotthatthesamespeciesaremetwith。forinsomecasesnotonespeciesisidenticallythesame，buttheybelongtothesamefamilies，genera，andsectionsofgenera，andsometimesaresimilarlycharacterisedinsuchtriflingpointsasmeresuperficialsculpture。Moreoverotherforms，whicharenotfoundintheChalkofEurope，butwhichoccurintheformationseitheraboveorbelow，aresimilarlyabsentatthesedistantpointsoftheworld。IntheseveralsuccessivepalaeozoicformationsofRussia，WesternEuropeandNorthAmerica，asimilarparallelismintheformsoflifehasbeenobservedbyseveralauthors：soitis，accordingtoLyell，withtheseveralEuropeanandNorthAmericantertiarydeposits。EvenifthefewfossilspecieswhicharecommontotheOldandNewWorldsbekeptwhollyoutofview，thegeneralparallelisminthesuccessiveformsoflife，inthestagesofthewidelyseparatedpalaeozoicandtertiaryperiods，wouldstillbemanifest，andtheseveralformationscouldbeeasilycorrelated。

Theseobservations，however，relatetothemarineinhabitantsofdistantpartsoftheworld：wehavenotsufficientdatatojudgewhethertheproductionsofthelandandoffreshwaterchangeatdistantpointsinthesameparallelmanner。Wemaydoubtwhethertheyhavethuschanged：iftheMegatherium，Mylodon，Macrauchenia，andToxodonhadbeenbroughttoEuropefromLaPlata，withoutanyinformationinregardtotheirgeologicalposition，noonewouldhavesuspectedthattheyhadcoexistedwithstilllivingsea-shells。

butastheseanomalousmonsterscoexistedwiththeMastodonandHorse，itmightatleasthavebeeninferredthattheyhadlivedduringoneofthelattertertiarystages。

Whenthemarineformsoflifearespokenofashavingchangedsimultaneouslythroughouttheworld，itmustnotbesupposedthatthisexpressionrelatestothesamethousandthorhundred-thousandthyear，oreventhatithasaverystrictgeologicalsense。forifallthemarineanimalswhichliveatthepresentdayinEurope，andallthosethatlivedinEuropeduringthepleistoceneperiod(anenormouslyremoteperiodasmeasuredbyyears，includingthewholeglacialepoch)，weretobecomparedwiththosenowlivinginSouthAmericaorinAustralia，themostskilfulnaturalistwouldhardlybeabletosaywhethertheexistingorthepleistoceneinhabitantsofEuroperesembledmostcloselythoseofthesouthernhemisphere。So，again，severalhighlycompetentobserversbelievethattheexistingproductionsoftheUnitedStatesaremorecloselyrelatedtothosewhichlivedinEuropeduringcertainlatertertiarystages，thantothosewhichnowlivehere。

andifthisbeso，itisevidentthatfossiliferousbedsdepositedatthepresentdayontheshoresofNorthAmericawouldhereafterbeliabletobeclassedwithsomewhatolderEuropeanbeds。Nevertheless，lookingtoaremotelyfutureepoch，therecan，Ithink，belittledoubtthatallthemoremodernmarineformations，namely，theupperpliocene，thepleistoceneandstrictlymodernbeds，ofEurope，NorthandSouthAmerica，andAustralia，fromcontainingfossilremainsinsomedegreeallied，andfromnotincludingthoseformswhichareonlyfoundintheolderunderlyingdeposits，wouldbecorrectlyrankedassimultaneousinageologicalsense。

Thefactoftheformsoflifechangingsimultaneously，intheabovelargesense，atdistantpartsoftheworld，hasgreatlystruckthoseadmirableobservers，MM。deVerneuilandd’Archiac。AfterreferringtotheparallelismofthepalaeozoicformsoflifeinvariouspartsofEurope，theyadd，`Ifstruckbythisstrangesequence，weturnourattentiontoNorthAmerica，andtherediscoveraseriesofanalogousphenomena，itwillappearcertainthatallthesemodificationsofspecies，theirextinction，andtheintroductionofnewones，cannotbeowingtomerechangesinmarinecurrentsorothercausesmoreorlesslocalandtemporary，butdependongenerallawswhichgovernthewholeanimalkingdom。’M。Barrandehasmadeforcibleremarkstopreciselythesameeffect。Itis，indeed，quitefutiletolooktochangesofcurrents，climate，orotherphysicalconditions，asthecauseofthesegreatmutationsintheformsoflifethroughouttheworld，underthemostdifferentclimates。Wemust，asBarrandehasremarked，looktosomespeciallaw。Weshallseethismoreclearlywhenwetreatofthepresentdistributionoforganicbeings，andfindhowslightistherelationbetweenthephysicalconditionsofvariouscountries，andthenatureoftheirinhabitants。

Thisgreatfactoftheparallelsuccessionoftheformsoflifethroughouttheworld，isexplicableonthetheoryofnaturalselection。Newspeciesareformedbynewvarietiesarising，whichhavesomeadvantageoverolderforms。andthoseforms，whicharealreadydominant，orhavesomeadvantageovertheotherformsintheirowncountry，wouldnaturallyoftenestgiverisetonewvarietiesorincipientspecies。fortheselattermustbevictoriousinastillhigherdegreeinordertobepreservedandtosurvive。Wehavedistinctevidenceonthishead，intheplantswhicharedominant，thatis，whicharecommonestintheirownhomes，andaremostwidelydiffused，havingproducedthegreatestnumberofnewvarieties。Itisalsonaturalthatthedominant，varying，andfar-spreadingspecies，whichalreadyhaveinvadedtoacertainextenttheterritoriesofotherspecies，shouldbethosewhichwouldhavethebestchanceofspreadingstillfurther，andofgivingriseinnewcountriestonewvarietiesandspecies。Theprocessofdiffusionmayoftenbeveryslow，beingdependentonclimatalandgeographicalchanges，oronstrangeaccidents，butinthelongrunthedominantformswillgenerallysucceedinspreading。Thediffusionwould，itisprobable，beslowerwiththeterrestrialinhabitantsofdistinctcontinentsthanwiththemarineinhabitantsofthecontinuoussea。Wemightthereforeexpecttofind，asweapparentlydofind，alessstrictdegreeofparallelsuccessionintheproductionsofthelandthanofthesea。

Dominantspeciesspreadingfromanyregionmightencounterstillmoredominantspecies，andthentheirtriumphantcourse，oreventheirexistence，wouldcease。Weknownotatallpreciselywhatarealltheconditionsmostfavourableforthemultiplicationofnewanddominantspecies。butwecan，Ithink，clearlyseethatanumberofindividuals，fromgivingabetterchanceoftheappearanceoffavourablevariations，andthatseverecompetitionwithmanyalreadyexistingforms，wouldbehighlyfavourable，aswouldbethepowerofspreadingintonewterritories。Acertainamountofisolation，recurringatlongintervalsoftime，wouldprobablybealsofavourable，asbeforeexplained。Onequarteroftheworldmayhavebeenmostfavourablefortheproductionofnewanddominantspeciesontheland，andanotherforthoseinthewatersofthesea。Iftwogreatregionshadbeenforalongperiodfavourablycircumstancedinanequaldegree，whenevertheirinhabitantsmet，thebattlewouldbeprolongedandsevere。andsomefromonebirthplaceandsomefromtheothermightbevictorious。Butinthecourseoftime，theformsdominantinthehighestdegree，whereverproduced，wouldtendeverywheretoprevail。Astheyprevailed，theywouldcausetheextinctionofotherandinferiorforms。andastheseinferiorformswouldbealliedingroupsbyinheritance，wholegroupswouldtendslowlytodisappear。

thoughhereandthereasinglemembermightlongbeenabledtosurvive。

Thus，asitseemstome，theparallel，and，takeninalargesense，simultaneous，successionofthesameformsoflifethroughouttheworld，accordswellwiththeprincipleofnewspecieshavingbeenformedbydominantspeciesspreadingwidelyandvarying。thenewspeciesthusproducedbeingthemselvesdominantowingtoinheritance，andtohavingalreadyhadsomeadvantageovertheirparentsoroverotherspecies。theseagainspreading，varying，andproducingnewspecies。Theformswhicharebeatenandwhichyieldtheirplacestothenewandvictoriousforms，willgenerallybealliedingroups，frominheritingsomeinferiorityincommon。andthereforeasnewandimprovedgroupsspreadthroughouttheworld，oldgroupswilldisappearfromtheworld。andthesuccessionofformsinbothwayswilleverywheretendtocorrespond。

Thereisoneotherremarkconnectedwiththissubjectworthmaking。

Ihavegivenmyreasonsforbelievingthatallourgreaterfossiliferousformationsweredepositedduringperiodsofsubsidence。andthatblankintervalsofvastdurationoccurredduringtheperiodswhenthebedoftheseawaseitherstationaryorrising，andlikewisewhensedimentwasnotthrowndownquicklyenoughtoembedandpreserveorganicremains。DuringtheselongandblankintervalsIsupposethattheinhabitantsofeachregionunderwentaconsiderableamountofmodificationandextinction，andthattherewasmuchmigrationfromotherpartsoftheworld。Aswehavereasontobelievethatlargeareasareaffectedbythesamemovement，itisprobablethatstrictlycontemporaneousformationshaveoftenbeenaccumulatedoververywidespacesinthesamequarteroftheworld。butwearefarfromhavinganyrighttoconcludethatthishasinvariablybeenthecase，andthatlargeareashaveinvariablybeenaffectedbythesamemovements。Whentwoformationshavebeendepositedintworegionsduringnearly，butnotexactlythesameperiod，weshouldfindinboth，fromthecausesexplainedintheforegoingparagraphs，thesamegeneralsuccessionintheformsoflife。butthespecieswouldnotexactlycorrespond。fortherewillhavebeenalittlemoretimeintheoneregionthanintheotherformodification，extinction，andimmigration。

IsuspectthatcasesofthisnaturehaveoccurredinEurope。Mr。Prestwich，inhisadmirableMemoirsontheeocenedepositsofEnglandandFrance，isabletodrawaclosegeneralparallelismbetweenthesuccessivestagesinthetwocountries。butwhenhecomparescertainstagesinEnglandwiththoseinFrance，althoughhefindsinbothacuriousaccordanceinthenumbersofthespeciesbelongingtothesamegenera，yetthespeciesthemselvesdifferinamannerverydifficulttoaccountfor，consideringtheproximityofthetwoareas，unless，indeed，itbeassumedthatanisthmusseparatedtwoseasinhabitedbydistinct，butcontemporaneous，faunas。Lyellhasmadesimilarobservationsonsomeofthelatertertiaryformations。Barrande，also，showsthatthereisastrikinggeneralparallelisminthesuccessiveSiluriandepositsofBohemiaandScandinavia。neverthelesshefindsasurprisingamountofdifferenceinthespecies。Iftheseveralformationsintheseregionshavenotbeendepositedduringthesameexactperiods，aformationinoneregionoftencorrespondingwithablankintervalintheother，andifinbothregionsthespecieshavegoneonslowlychangingduringtheaccumulationoftheseveralformationsandduringthelongintervalsoftimebetweenthem。inthiscase，theseveralformationsinthetworegionscouldbearrangedinthesameorder，inaccordancewiththegeneralsuccessionoftheformoflife，andtheorderwouldfalselyappeartobestrictlyparallel。neverthelessthespecieswouldnotallbethesameintheapparentlycorrespondingstagesinthetworegions。OntheAffinitiesofextinctSpeciestoeachother，andtolivingformsLetusnowlooktothemutualaffinitiesofextinctandlivingspecies。

Theyallfallintoonegrandnaturalsystem。andthisfactisatonceexplainedontheprincipleofdescent。Themoreancientanyformis，themore，asageneralrule，itdiffersfromlivingforms。But，asBucklandlongagoremarked，allfossilscanbeclassedeitherinstillexistinggroups，orbetweenthem。Thattheextinctformsoflifehelptofillupthewideintervalsbetweenexistinggenera，families，andorders，cannotbedisputed。Forifweconfineourattentioneithertothelivingortotheextinctalone，theseriesisfarlessperfectthanifwecombinebothintoonegeneralsystem。WithrespecttotheVertebrata，wholepagescouldbefilledwithstrikingillustrationsfromourgreatpalaeontologist，Owen，showinghowextinctanimalsfallinbetweenexistinggroups。CuvierrankedtheRuminantsandPachyderms，asthetwomostdistinctordersofmammals。butOwenhasdiscoveredsomanyfossillinks，thathehashadtoalterthewholeclassificationofthesetwoorders。andhasplacedcertainpachydermsinthesamesub-orderwithruminants：forexample，hedissolvesbyfinegradationstheapparentlywidedifferencebetweenthepigandthecamel。InregardtotheInvertebrata，Barrande，andahigherauthoritycouldnotbenamed，assertsthatheiseverydaytaughtthatpalaeozoicanimals，thoughbelongingtothesameorders，families，orgenerawiththoselivingatthepresentday，werenotatthisearlyepochlimitedinsuchdistinctgroupsastheynoware。

Somewritershaveobjectedtoanyextinctspeciesorgroupofspeciesbeingconsideredasintermediatebetweenlivingspeciesorgroups。Ifbythistermitismeantthatanextinctformisdirectlyintermediateinallitscharactersbetweentwolivingforms，theobjectionisprobablyvalid。ButIapprehendthatinaperfectlynaturalclassificationmanyfossilspecieswouldhavetostandbetweenlivingspecies，andsomeextinctgenerabetweenlivinggenera，evenbetweengenerabelongingtodistinctfamilies。Themostcommoncase，especiallywithrespecttoverydistinctgroups，suchasfishandreptiles，seemstobe，thatsupposingthemtobedistinguishedatthepresentdayfromeachotherbyadozencharacters，theancientmembersofthesametwogroupswouldbedistinguishedbyasomewhatlessernumberofcharacters，sothatthetwogroups，thoughformerlyquitedistinct，atthatperiodmadesomesmallapproachtoeachother。

Itisacommonbeliefthatthemoreancientaformis，bysomuchthemoreittendstoconnectbysomeofitscharactersgroupsnowwidelyseparatedfromeachother。Thisremarknodoubtmustberestrictedtothosegroupswhichhaveundergonemuchchangeinthecourseofgeologicalages。anditwouldbedifficulttoprovethetruthoftheproposition，foreverynowandthenevenalivinganimal，astheLepidosiren，isdiscoveredhavingaffinitiesdirectedtowardsverydistinctgroups。YetifwecomparetheolderReptilesandBatrachians，theolderFish，theolderCephalopods，andtheeoceneMammals，withthemorerecentmembersofthesameclasses，wemustadmitthatthereissometruthintheremark。

Letusseehowfartheseseveralfactsandinferencesaccordwiththetheoryofdescentwithmodification。Asthesubjectissomewhatcomplex，Imustrequestthereadertoturntothediagraminthefourthchapter。

Wemaysupposethatthenumberedlettersrepresentgenera，andthedottedlinesdivergingfromthemthespeciesineachgenus。Thediagramismuchtoosimple，toofewgeneraandtoofewspeciesbeinggiven，butthisisunimportantforus。Thehorizontallinesmayrepresentsuccessivegeologicalformations，andalltheformsbeneaththeuppermostlinemaybeconsideredasextinct。Thethreeexistinggenera，a14，q14，p14，willformasmallfamily。b14andf14acloselyalliedfamilyorsub-family。ando14，e14，m14，athirdfamily。

Thesethreefamilies，togetherwiththemanyextinctgeneraontheseverallinesofdescentdivergingfromtheparent-formA，willformanorder。

forallwillhaveinheritedsomethingincommonfromtheirancientandcommonprogenitor。Ontheprincipleofthecontinuedtendencytodivergenceofcharacter，whichwasformerlyillustratedbythisdiagram，themorerecentanyformis，themoreitwillgenerallydifferfromitsancientprogenitor。Hencewecanunderstandtherulethatthemostancientfossilsdiffermostfromexistingforms。Wemustnot，however，assumethatdivergenceofcharacterisanecessarycontingency。itdependssolelyonthedescendantsfromaspeciesbeingthusenabledtoseizeonmanyanddifferentplacesintheeconomyofnature。Thereforeitisquitepossible，aswehaveseeninthecaseofsomeSilurianforms，thataspeciesmightgoonbeingslightlymodifiedinrelationtoitsslightlyalteredconditionsoflife，andyetretainthroughoutavastperiodthesamegeneralcharacteristics。ThisisrepresentedinthediagrambytheletterF14。

Allthemanyforms，extinctandrecent，descendedfromA，make，asbeforeremarked，oneorder。andthisorder，fromthecontinuedeffectsofextinctionanddivergenceofcharacter，hasbecomedividedintoseveralsub-familiesandfamilies，someofwhicharesupposedtohaveperishedatdifferentperiods，andsometohaveenduredtothepresentday。

Bylookingatthediagramwecanseethatifmanyoftheextinctforms，supposedtobeembeddedinthesuccessiveformations，werediscoveredatseveralpointslowdownintheseries，thethreeexistingfamiliesontheuppermostlinewouldberenderedlessdistinctfromeachother。If，forinstance，thegeneraa1，a5，a10，m3，m6，m9weredisinterred，thesethreefamilieswouldbesocloselylinkedtogetherthattheyprobablywouldhavetobeunitedintoonegreatfamily，innearlythesamemannerashasoccurredwithruminantsandpachyderms。

Yethewhoobjectedtocalltheextinctgenera，whichthuslinkedthelivinggeneraofthreefamiliestogether，intermediateincharacter，wouldbejustified，astheyareintermediate，notdirectly，butonlybyalongandcircuitouscoursethroughmanywidelydifferentforms。Ifmanyextinctformsweretobediscoveredaboveoneofthemiddlehorizontallinesorgeologicalformationsforinstance，aboveNo。VI。butnonefrombeneaththisline，thenonlythetwofamiliesonthelefthand(namely，a14，&。c。，andb14，&。c。)wouldhavetobeunitedintoonefamily。

andthetwootherfamilies(namely，a14tof14nowincludingfivegenera，ando14tom14)wouldyetremaindistinct。Thesetwofamilies，however，wouldbelessdistinctfromeachotherthantheywerebeforethediscoveryofthefossils。If，forinstance，wesupposetheexistinggeneraofthetwofamiliestodifferfromeachotherbyadozencharacters，inthiscasethegenera，attheearlyperiodmarkedVI。，woulddifferbyalessernumberofcharacters。foratthisearlystageofdescenttheyhavenotdivergedincharacterfromthecommonprogenitoroftheorder，nearlysomuchastheysubsequentlydiverged。Thusitcomesthatancientandextinctgeneraareofteninsomeslightdegreeintermediateincharacterbetweentheirmodifieddescendants，orbetweentheircollateralrelations。

Innaturethecasewillbefarmorecomplicatedthanisrepresentedinthediagram。forthegroupswillhavebeenmorenumerous，theywillhaveenduredforextremelyunequallengthsoftime，andwillhavebeenmodifiedinvariousdegrees。Aswepossessonlythelastvolumeofthegeologicalrecord，andthatinaverybrokencondition，wehavenorighttoexpect，exceptinveryrarecases，tofillupwideintervalsinthenaturalsystem，andthusunitedistinctfamiliesororders。Allthatwehavearighttoexpect，isthatthosegroups，whichhavewithinknowngeologicalperiodsundergonemuchmodification，shouldintheolderformationsmakesomeslightapproachtoeachother。sothattheoldermembersshoulddifferlessfromeachotherinsomeoftheircharactersthandotheexistingmembersofthesamegroups。andthisbytheconcurrentevidenceofourbestpalaeontologistsseemsfrequentlytobethecase。

Thus，onthetheoryofdescentwithmodification，themainfactswithrespecttothemutualaffinitiesoftheextinctformsoflifetoeachotherandtolivingforms，seemtomeexplainedinasatisfactorymanner。Andtheyarewhollyinexplicableonanyotherview。

Onthissametheory，itisevidentthatthefaunaofanygreatperiodintheearth’shistorywillbeintermediateingeneralcharacterbetweenthatwhichprecededandthatwhichsucceededit。Thus，thespecieswhichlivedatthesixthgreatstageofdescentinthediagramarethemodifiedoffspringofthosewhichlivedatthefifthstage，andaretheparentsofthosewhichbecamestillmoremodifiedattheseventhstage。hencetheycouldhardlyfailtobenearlyintermediateincharacterbetweentheformsoflifeaboveandbelow。Wemust，however，allowfortheentireextinctionofsomeprecedingforms，andforthecominginofquitenewformsbyimmigration，andforalargeamountofmodification，duringthelongandblankintervalsbetweenthesuccessiveformations。Subjecttotheseallowances，thefaunaofeachgeologicalperiodundoubtedlyisintermediateincharacter，betweentheprecedingandsucceedingfaunas。Ineedgiveonlyoneinstance，namely，themannerinwhichthefossilsoftheDevoniansystem，whenthissystemwasfirstdiscovered，wereatoncerecognisedbypalaeontologistsasintermediateincharacterbetweenthoseoftheoverlyingcarboniferous，andunderlyingSiluriansystem。Buteachfaunaisnotnecessarilyexactlyintermediate，asunequalintervalsoftimehaveelapsedbetweenconsecutiveformations。

Itisnorealobjectiontothetruthofthestatement，thatthefaunaofeachperiodasawholeisnearlyintermediateincharacterbetweentheprecedingandsucceedingfaunas，thatcertaingeneraofferexceptionstotherule。Forinstance，mastodonsandelephants，whenarrangedbyDrFalconerintwoseries，firstaccordingtotheirmutualaffinitiesandthenaccordingtotheirperiodsofexistence，donotaccordinarrangement。Thespeciesextremeincharacterarenottheoldest，orthemostrecent。norarethosewhichareintermediateincharacter，intermediateinage。Butsupposingforaninstant，inthisandothersuchcases，thattherecordofthefirstappearanceanddisappearanceofthespecieswasperfect，wehavenoreasontobelievethatformssuccessivelyproducednecessarilyendureforcorrespondinglengthsoftime：averyancientformmightoccasionallylastmuchlongerthanaformelsewheresubsequentlyproduced，especiallyinthecaseofterrestrialproductionsinhabitingseparateddistricts。Tocomparesmallthingswithgreat：iftheprincipallivingandextinctracesofthedomesticpigeonwerearrangedaswellastheycouldbeinserialaffinity，thisarrangementwouldnotcloselyaccordwiththeorderintimeoftheirproduction，andstilllesswiththeorderoftheirdisappearance。fortheparentrock-pigeonnowlives。andmanyvarietiesbetweentherock-pigeonandthecarrierhavebecomeextinct。andcarrierswhichareextremeintheimportantcharacteroflengthofbeakoriginatedearlierthanshort-beakedtumblers，whichareattheoppositeendoftheseriesinthissamerespect。

Closelyconnectedwiththestatement，thattheorganicremainsfromanintermediateformationareinsomedegreeintermediateincharacter，isthefact，insistedonbyallpalaeontologists，thatfossilsfromtwoconsecutiveformationsarefarmorecloselyrelatedtoeachother，thanarethefossilsfromtworemoteformations。Pictetgivesasawell-knowninstance，thegeneralresemblanceoftheorganicremainsfromtheseveralstagesofthechalkformation，thoughthespeciesaredistinctineachstage。Thisfactalone，fromitsgenerality，seemstohaveshakenProfessorPictetinhisfirmbeliefintheimmutabilityofspecies。Hewhoisacquaintedwiththedistributionofexistingspeciesovertheglobe，willnotattempttoaccountforthecloseresemblanceofthedistinctspeciesincloselyconsecutiveformations，bythephysicalconditionsoftheancientareashavingremainednearlythesame。Letitberememberedthattheformsoflife，atleastthoseinhabitingthesea，havechangedalmostsimultaneouslythroughouttheworld，andthereforeunderthemostdifferentclimatesandconditions。Considertheprodigiousvicissitudesofclimateduringthepleistoceneperiod，whichincludesthewholeglacialperiod，andnotehowlittlethespecificformsoftheinhabitantsoftheseahavebeenaffected。

Onthetheoryofdescent，thefullmeaningofthefactoffossilremainsfromcloselyconsecutiveformations，thoughrankedasdistinctspecies，beingcloselyrelated，isobvious。Astheaccumulationofeachformationhasoftenbeeninterrupted，andaslongblankintervalshaveintervenedbetweensuccessiveformations，weoughtnottoexpecttofind，asIattemptedtoshowinthelastchapter，inanyoneortwoformationsalltheintermediatevarietiesbetweenthespecieswhichappearedatthecommencementandcloseoftheseperiods。butweoughttofindafterintervals，verylongasmeasuredbyyears，butonlymoderatelylongasmeasuredgeologically，closelyalliedforms，or，astheyhavebeencalledbysomeauthors，representativespecies。

andtheseweassuredlydofind。Wefind，inshort，suchevidenceoftheslowandscarcelysensiblemutationofspecificforms，aswehaveajustrighttoexpecttofind。OnthestateofDevelopmentofAncientFormsTherehasbeenmuchdiscussionwhetherrecentformsaremorehighlydevelopedthanancient。Iwillnothereenteronthissubject，fornaturalistshavenotasyetdefinedtoeachother’ssatisfactionwhatismeantbyhighandlowforms。Butinoneparticularsensethemorerecentformsmust，onmytheory，behigherthanthemoreancient。foreachnewspeciesisformedbyhavinghadsomeadvantageinthestruggleforlifeoverotherandprecedingforms。Ifunderanearlysimilarclimate，theeoceneinhabitantsofonequarteroftheworldwereputintocompetitionwiththeexistinginhabitantsofthesameorsomeotherquarter，theeocenefaunaorflorawouldcertainlybebeatenandexterminated。aswouldasecondaryfaunabyaneocene，andapalaeozoicfaunabyasecondaryfauna。Idonotdoubtthatthisprocessofimprovementhasaffectedinamarkedandsensiblemannertheorganisationofthemorerecentandvictoriousformsoflife，incomparisonwiththeancientandbeatenforms。butIcanseenowayoftestingthissortofprogress。Crustaceans，forinstance，notthehighestintheirownclass，mayhavebeatenthehighestmolluscs。FromtheextraordinarymannerinwhichEuropeanproductionshaverecentlyspreadoverNewZealand，andhaveseizedonplaceswhichmusthavebeenpreviouslyoccupied，wemaybelieve，ifalltheanimalsandplantsofGreatBritainweresetfreeinNewZealand，thatinthecourseoftimeamultitudeofBritishformswouldbecomethoroughlynaturalizedthere，andwouldexterminatemanyofthenatives。Ontheotherhand，fromwhatweseenowoccurringinNewZealand，andfromhardlyasingleinhabitantofthesouthernhemispherehavingbecomewildinanypartofEurope，wemaydoubt，ifalltheproductionsofNewZealandweresetfreeinGreatBritain，whetheranyconsiderablenumberwouldbeenabledtoseizeonplacesnowoccupiedbyournativeplantsandanimals。Underthispointofview，theproductionsofGreatBritain，maybesaidtobehigherthanthoseofNewZealand。Yetthemostskilfulnaturalistfromanexaminationofthespeciesofthetwocountriescouldnothaveforeseenthisresult。