[footnote]*"leplusgrandservicequ’onpuisserendrealascienceestd’yfaireplacenetteavantd’yrienconstruire。"——CUVIER
　　Theevidenceonwhichsuchconclusionsarebasedisoftwokinds，negativeandpositive。Thevalueofnegativeevidence，inconnectionwiththisinquiry，hasbeensofullyandclearlydiscussedinanaddressfromthechairofthisSociety*，whichnoneofushaveforgotten，thatnothingneedatpresentbesaidaboutit；themore，astheconsiderationswhichhavebeenlaidbeforeyouhavecertainlynottendedtoincreaseyourestimationofsuchevidence。Itwillbepreferabletoturntothepositivefactsofpaleontology，andtoinquirewhattheytellus。
　　[footnote]*AnniversaryAddressfor1851，’Quart。Journ。
　　Geol。Soc。’vol。vii。
　　Weareallaccustomedtospeakofthenumberandtheextentofthechangesinthelivingpopulationoftheglobeduringgeologicaltimeassomethingenormous：andindeedtheyareso，ifweregardonlythenegativedifferenceswhichseparatetheolderrocksfromthemoremodern，andifwelookuponspecificandgenericchangesasgreatchanges，whichfromonepointofview，theytrulyare。Butleavingthenegativedifferencesoutofconsideration，andlookingonlyatthepositivedatafurnishedbythefossilworldfromabroaderpointofview——fromthatofthecomparativeanatomistwhohasmadethestudyofthegreatermodificationsofanimalformhischiefbusiness——asurpriseofanotherkinddawnsuponthemind；andunder’this’aspectthesmallnessofthetotalchangebecomesasastonishingaswasitsgreatnessundertheother。
　　Therearetwohundredknownordersofplants；ofthesenotoneiscertainlyknowntoexistexclusivelyinthefossilstate。Thewholelapseofgeologicaltimehasasyetyieldednotasinglenewordinaltypeofvegetablestructure。*
　　[footnote]*SeeHooker’s’IntroductoryEssaytotheFloraofTasmania’，p。xxiii。
　　Thepositivechangeinpassingfromtherecenttotheancientanimalworldisgreater，butstillsingularlysmall。Nofossilanimalissodistinctfromthosenowlivingastorequiretobearrangedeveninaseparateclassfromthosewhichcontainexistingforms。Itisonlywhenwecometotheorders，whichmayberoughlyestimatedataboutahundredandthirty，thatwemeetwithfossilanimalssodistinctfromthosenowlivingastorequireordersforthemselves；andthesedonotamount，onthemostliberalestimate，tomorethanabout10percent。ofthewhole。
　　ThereisnocertainlyknownextinctorderofProtozoa；thereisbutoneamongtheCoelenterata——thatoftherugosecorals；thereisnoneamongtheMollusca；therearethree，theCystidea，Blastoidea，andEdrioasterida，amongtheEchinoderms；andtwo，theTrilobitaandEurypterida，amongtheCrustacea；makingaltogetherfiveforthegreatsub—kingdomofAnnulosa。AmongVertebratesthereisnoordinallydistinctfossilfish：thereisonlyoneextinctorderofAmphibia——theLabyrinthodonts；butthereareatleastfourdistinctordersofReptilia，viz。theIchthyosauria，Plesiosauria，Pterosauria，Dinosauria，andperhapsanotherortwo。ThereisnoknownextinctorderofBirds，andnocertainlyknownextinctorderofMammals，theordinaldistinctnessofthe"Toxodontia"beingdoubtful。
　　Theobjectionthatbroadstatementsofthiskind，afterall，restlargelyonnegativeevidenceisobvious，butithaslessforcethanmayatfirstbesupposed；for，asmightbeexpectedfromthecircumstancesofthecase，wepossessmoreabundantpositiveevidenceregardingFishesandmarineMollusksthanrespectinganyotherformsofanimallife；andyettheseofferus，throughthewholerangeofgeologicaltime，nospeciesordinallydistinctfromthosenowliving；whilethefarlessnumerousclassofEchinodermspresentsthree；andtheCrustaceatwo，suchorders，thoughnoneofthesecomedownlaterthanthePaleozoicage。Lastly，theReptiliapresenttheextraordinaryandexceptionalphenomenonofasmanyextinctasexistingorders，ifnotmore；thefourmentionedmaintainingtheirexistencefromtheLiastotheChalkinclusive。
　　SomeyearsagooneofyourSecretariespointedoutanotherkindofpositivepaleontologicevidencetendingtowardsthesameconclusion——affordedbytheexistenceofwhathetermed"persistenttypes"ofvegetableandofanimallife。*Hestated，ontheauthorityofDr。Hooker，thatthereareCarboniferousplantswhichappeartobegenericallyidenticalwithsomenowliving；thattheconeoftheOolitic’Araucaria’ishardlydistinguishablefromthatofanexistingspecies；thatatrue’Pinus’appearsinthePurbecks，anda’Juglans’intheChalk；while，fromtheBagshotSands，a’Banksia’，thewoodofwhichisnotdistinguishablefromthatofspeciesnowlivinginAustralia，hadbeenobtained。
　　[footnote]*SeetheabstractofaLecture"OnthePersistentTypesofAnimalLife，"inthe’NoticesoftheMeetingsoftheRoyalInstitutionofGreatBritain’。——June3，1859，vol。iii。p。151。
　　Turningtotheanimalkingdom，heaffirmedthetabulatecoralsoftheSilurianrockstobewonderfullylikethosewhichnowexist；whileeventhefamiliesoftheAporosawereallrepresentedintheolderMesozoicrocks。
　　AmongtheMolluskasimilarfactswereadduced。Letitbeborneinmindthat’Avicula’，’Mytails’，’Chiton’，’Natica’，’Patella’，’Trochus’，’Discina’，’Orbicula’，’Lingula’，’Rhynchonella’，and’Nautilus’，allofwhichareexisting’genera’，aregivenwithoutadoubtasSilurianinthelasteditionof’Siluria’；whilethehighestformsofthehighestCephalopodsarerepresentedintheLiasbyagenus，’Belemnoteuthis’，whichpresentstheclosestrelationtotheexisting’Loligo’。
　　ThetwohighestgroupsoftheAnnulosa，theInsectaandtheArachnida，arerepresentedintheCoal，eitherbyexistinggenera，orbyformsdifferingfromexistinggenerainquiteminorpeculiarities。
　　TurningtotheVertebrata，theonlyPaleozoicElasmobranchFishofwhichwehaveanycompleteknowledgeistheDevonianandCarboniferous’Pleuracanthus’，whichdiffersnomorefromexistingSharksthanthesedofromoneanother。
　　Again，vastasisthenumberofundoubtedlyGanoidfossilFishes，andgreatasistheirrangeintime，alargemassofevidencehasrecentlybeenadducedtoshowthatalmostallthoserespectingwhichwepossesssufficientinformation，arereferabletothesamesub—ordinalgroupsastheexisting’Lepidosteus’，’Polypterus’，andSturgeon；andthatasingularrelationobtainsbetweentheolderandtheyoungerFishes；theformer，theDevonianGanoids，beingalmostallmembersofthesamesub—orderas’Polypterus’，whiletheMesozoicGanoidsarealmostallsimilarlyalliedto’Lepidosteus’。*
　　[footnote]*"MemoirsoftheGeologicalSurveyoftheUnitedKingdom。——Decadex。PreliminaryEssayupontheSystematicArrangementoftheFishesoftheDevonianEpoch。"
　　Again，whatcanbemoreremarkablethanthesingularconstancyofstructurepreservedthroughoutavastperiodoftimebythefamilyofthePycnodontsandbythatofthetrueCoelacanths；theformerpersisting，withbutinsignificantmodifications，fromtheCarboniferoustotheTertiaryrocks，inclusive；thelatterexisting，withstilllesschange，fromtheCarboniferousrockstotheChalk，inclusive？
　　AmongReptiles，thehighestlivinggroup，thatoftheCrocodilia，isrepresented，attheearlypartoftheMesozoicepoch，byspeciesidenticalintheessentialcharactersoftheirorganizationwiththosenowliving，anddifferingfromthelatteronlyinsuchmattersastheformofthearticularfacetsofthevertebralcentra，intheextenttowhichthenasalpassagesareseparatedfromthecavityofthemouthbybone，andintheproportionsofthelimbs。
　　AndevenasregardstheMammalia，thescantyremainsofTriassicandOoliticspeciesaffordnofoundationforthesuppositionthattheorganizationoftheoldestformsdifferednearlysomuchfromsomeofthosewhichnowliveasthesedifferfromoneanother。
　　Itisneedlesstomultiplytheseinstances；enoughhasbeensaidtojustifythestatementthat，inviewoftheimmensediversityofknownanimalandvegetableforms，andtheenormouslapseoftimeindicatedbytheaccumulationoffossiliferousstrata，theonlycircumstancetobewonderedatis，notthatthechangesoflife，asexhibitedbypositiveevidence，havebeensogreat，butthattheyhavebeensosmall。
　　Betheygreatorsmall，however，itisdesirabletoattempttoestimatethem。Letus，therefore，takeeachgreatdivisionoftheanimalworldinsuccession，and，wheneveranorderorafamilycanbeshowntohavehadaprolongedexistence，letusendeavourtoascertainhowfarthelatermembersofthegroupdifferfromtheearlierones。Iftheselatermembers，inallorinmanycases，exhibitacertainamountofmodification，thefactis，sofar，evidenceinfavourofagenerallawofchange；and，inaroughway，therapidityofthatchangewillbemeasuredbythedemonstrableamountofmodification。Ontheotherhand，itmustberecollectedthattheabsenceofanymodification，whileitmayleavethedoctrineoftheexistenceofalawofchangewithoutpositivesupport，cannotpossiblydisproveallformsofthatdoctrine，thoughitmayaffordasufficientrefutationofanyofthem。
　　ThePROTOZOA。——TheProtozoaarerepresentedthroughoutthewholerangeofgeologicalseries，fromtheLowerSilurianformationtothepresentday。ThemostancientformsrecentlymadeknownbyEhrenbergareexceedinglylikethosewhichnowexist：noonehaseverpretendedthatthedifferencebetweenanyancientandanymodernForaminiferaisofmorethangenericvalue，noraretheoldestForaminiferaeithersimpler，moreembryonic，orlessdifferentiated，thantheexistingforms。
　　TheCOELENTERATA。——TheTabulateCoralshaveexistedfromtheSilurianepochtothepresentday，butIamnotawarethattheancient’Heliolites’possessesasinglemarkofamoreembryonicorlessdifferentiatedcharacter，orlesshighorganization，thantheexisting’Heliopora’。AsfortheAporoseCorals，inwhatrespectistheSilurian’Paleocyclus’lesshighlyorganizedormoreembryonicthanthemodern’Fungia’，ortheLiassicAporosathantheexistingmembersofthesamefamilies？
　　The’Mollusca’。——Inwhatsenseistheliving’Waldheimia’lessembryonic，ormorespecialized；thanthepaleozoic’Spirifer’；ortheexisting’Rhynchonellae’，’Craniae’，’Discinae’，’Lingulae’，thantheSilurianspeciesofthesamegenera？Inwhatsensecan’Loligo’or’Spirula’besaidtobemorespecialized，orlessembryonic，than’Belemnites’；orthemodernspeciesofLamellibranchandGasteropodgenera，thantheSilurianspeciesofthesamegenera？
　　TheANNULOSA。——TheCarboniferousInsectaandArachnidaareneitherlessspecialized，normoreembryonic，thanthesethatnowlive，noraretheLiassicCirripediaandMacrura；whileseveraloftheBrachyura，whichappearintheChalk，belongtoexistinggenera；andnoneexhibiteitheranintermediate，oranembryonic，character。
　　TheVERTEBRARA。——AmongfishesIhavereferredtotheCoelacanthini（comprisingthegenera’Coelacanthus’，’Holophagus’，’Undina’，and’Macropoma’）asaffordinganexampleofapersistenttype；anditismostremarkabletonotethesmallnessofthedifferencesbetweenanyofthesefishes（affectingatmosttheproportionsofthebodyandfins，andthecharacterandsculptureofthescales），notwithstandingtheirenormousrangeintime。Inalltheessentialsofitsverypeculiarstructure，the’Macropoma’oftheChalkisidenticalwiththe’Coelacanthus’oftheCoal。Lookatthegenus’Lepidotus’，again，persistingwithoutamodificationofimportancefromtheLiassictotheEoceneformationsinclusive。
　　OramongtheTeleostei——inwhatrespectisthe’Beryx’oftheChalkmoreembryonic，orlessdifferentiated，than’Beryxlineatus’ofKingGeorge’sSound？
　　OrtoturntothehigherVertebrata——inwhatsensearetheLiassicCheloniainferiortothosewhichnowexist？HowaretheCretaceousIchthyosauria，Plesiosauria，orPterosaurialessembryonic，ormoredifferentiated，speciesthanthoseoftheLias？
　　Orlastly，inwhatcircumstanceisthe’Phascolotherium’moreembryonic，orofamoregeneralizedtype，thanthemodernOpossum；ora’Lophiodon’，ora’Paleotherium’，thanamodern’Tapirus’or’Hyrax’？
　　Theseexamplesmightbealmostindefinitelymultiplied，butsurelytheyaresufficienttoprovethattheonlysafeandunquestionabletestimonywecanprocure——positiveevidence——failstodemonstrateanysortofprogressivemodificationtowardsalessembryonic，orlessgeneralised，typeinagreatmanygroupsofanimalsoflong—continuedgeologicalexistence。Inthesegroupsthereisabundantevidenceofvariation——noneofwhatisordinarilyunderstoodasprogression；and，iftheknowngeologicalrecordistoberegardedasevenanyconsiderablefragmentofthewhole，itisinconceivablethatanytheoryofanecessarilyprogressivedevelopmentcanstand，forthenumerousordersandfamiliescitedaffordnotraceofsuchaprocess。
　　Butitisamostremarkablefact，that，whilethegroupswhichhavebeenmentioned，andmanybesides，exhibitnosignofprogressivemodification，thereareothers，co—existingwiththem，underthesameconditions，inwhichmoreorlessdistinctindicationsofsuchaprocessseemstobetraceable。AmongsuchindicationsImayremindyouofthepredominanceofHolostomeGasteropodaintheolderrocksascomparedwiththatofSiphonostomeGasteropodainthelater。Acaselessopentotheobjectionofnegativeevidence，however，isthataffordedbytheTetrabranchiateCephalopoda，theformsoftheshellsandoftheseptalsuturesexhibitingacertainincreaseofcomplexityinthenewergenera。Here，however，oneismetatoncewiththeoccurrenceof’Orthoceras’and’Baculites’atthetwoendsoftheseries，andofthefactthatoneofthesimplestGenera，’Nautilus’，isthatwhichnowexists。
　　TheCrinoidea，intheabundanceofstalkedformsintheancientformationsascomparedwiththeirpresentrarity，seemtopresentuswithafaircaseofmodificationfromamoreembryonictowardsalessembryoniccondition。Butthen，oncarefulconsiderationofthefacts，theobjectionarisesthatthestalk，calyx，andarmsofthepaleozoicCrinoidareexceedinglydifferentfromthecorrespondingorgansofalarval’Comatula’；anditmightwithperfectjusticebearguedthat’Actinocrinus’and’Eucalyptocrinus’，forexample，departtothefullaswidely，inonedirection，fromthestalkedembryoof’Comatula’，as’Comatula’itselfdoesintheother。
　　TheEchinidea，again，arefrequentlyquotedasexhibitingagradualpassagefromamoregeneralizedtoamorespecializedtype，seeingthattheelongated，oroval，SpatangoidsappearafterthespheroidalEchinoids。Buthereitmightbeargued，ontheotherhand，thatthespheroidalEchinoids，inreality，departfurtherfromthegeneralplanandfromtheembryonicformthantheelongatedSpatangoidsdo；andthatthepeculiardentalapparatusandthepedicellariaeoftheformeraremarksofatleastasgreatdifferentiationasthepetaloidambulacraandsemitaeofthelatter。
　　Oncemore，theprevalenceofMacrurousbeforeBrachyurousPodophthalmiais，apparently，afairpieceofevidenceinfavourofprogressivemodificationinthesameorderofCrustacea；andyetthecasewillnotstandmuchsifting，seeingthattheMacrurousPodophthalmiadepartasfarinonedirectionfromthecommontypeofPodophthalmia，orfromanyembryonicconditionoftheBrachyura，astheBrachyuradointheother；
　　andthatthemiddletermsbetweenMacruraandBrachyura——theAnomura——arelittlebetterrepresentedintheolderMesozoicrocksthantheBrachyuraare。
　　NoneofthecasesofprogressivemodificationwhicharecitedfromamongtheInvertebrataappeartometohaveafoundationlessopentocriticismthanthese；andifthisbeso，nocarefulreasonerwould，I
　　think，beinclinedtolayverygreatstressuponthem。AmongtheVertebrata，however，thereareafewexampleswhichappeartobefarlessopentoobjection。
　　Itis，infact，trueofseveralgroupsofVertebratawhichhavelivedthroughaconsiderablerangeoftime，thattheendoskeleton（moreparticularlythespinalcolumn）oftheoldergenerapresentsalessossified，and，sofar，lessdifferentiated，conditionthanthatoftheyoungergenera。ThustheDevonianGanoids，thoughalmostallmembersofthesamesub—orderas’Polypterus’，andpresentingnumerousimportantresemblancestotheexistinggenus，whichpossessesbiconcavevertebrae，are，forthemostpart，whollydevoidofossifiedvertebralcentra。TheMesozoicLepidosteidae，again，have，atmost，biconcavevertebrae，whiletheexisting’Lepidosteus’hasSalamandroid，opisthocoelous，vertebrae。So，noneofthePaleozoicSharkshaveshownthemselvestobepossessedofossifiedvertebrae，whilethemajorityofmodernSharkspossesssuchvertebrae。Again，themoreancientCrocodiliaandLacertiliahavevertebraewiththearticularfacetsoftheircentraflattenedorbiconcave，whilethemodernmembersofthesamegrouphavethemprocoelous。Butthemostremarkableexamplesofprogressivemodificationofthevertebralcolumn，incorrespondencewithgeologicalage，arethoseaffordedbythePycnodontsamongfish，andtheLabyrinthodontsamongAmphibia。
　　ThelateableichthyologistHeckelpointedoutthefact，that，whilethePycnodontsneverpossesstruevertebralcentra，theydifferinthedegreeofexpansionandextensionoftheendsofthebonyarchesofthevertebraeuponthesheathofthenotochord；theCarboniferousformsexhibitinghardlyanysuchexpansion，whiletheMesozoicgenerapresentagreaterandgreaterdevelopment，until，intheTertiaryforms，theexpandedendsbecomesuturallyunitedsoastoformasortoffalsevertebra。HermannvonMeyer，again，towhoseluminousresearchesweareindebtedforourpresentlargeknowledgeoftheorganizationoftheolderLabyrinthodonts，hasprovedthattheCarboniferous’Archegosaurus’hadveryimperfectlydevelopedvertebralcentra，whiletheTriassic’Mastodonsaurus’hadthesamepartscompletelyossified。*
　　[footnote]*AstheAddressispassingthroughthepress（March7，1862），evidenceliesbeforemeoftheexistenceofanewLabyrinthodont（’Pholidogaster’），fromtheEdinburghcoal—field，withwell—ossifiedvertebralcentra。
　　Theregularityandevennessofthedentitionofthe’Anoplotherium’，ascontrastedwiththatofexistingArtiodactyles，andtheassumednearerapproachofthedentitionofcertainancientCarnivorestothetypicalarrangement，havealsobeencitedasexemplificationsofalawofprogressivedevelopment，butIknowofnoothercasesbasedonpositiveevidencewhichareworthyofparticularnotice。
　　What，then，doesanimpartialsurveyofthepositivelyascertainedtruthsofpaleontologytestifyinrelationtothecommondoctrinesofprogressivemodification，whichsupposethatmodificationtohavetakenplacebyanecessaryprogressfrommoretolessembryonicforms，orfrommoretolessgeneralizedtypes，withinthelimitsoftheperiodrepresentedbythefossiliferousrocks？
　　Itnegativesthosedoctrines；foriteithershowsusnoevidenceofanysuchmodification，ordemonstratesittohavebeenveryslight；andastothenatureofthatmodification，ityieldsnoevidencewhatsoeverthattheearliermembersofanylong—continuedgroupweremoregeneralizedinstructurethanthelaterones。Toacertainextent，indeed，itmaybesaidthatimperfectossificationofthevertebralcolumnisanembryoniccharacter；but，ontheotherhand，itwouldbeextremelyincorrecttosupposethatthevertebralcolumnsoftheolderVertebrataareinanysenseembryonicintheirwholestructure。
　　Obviously，iftheearliestfossiliferousrocksnowknownarecoevalwiththecommencementoflife，andiftheircontentsgiveusanyjustconceptionofthenatureandtheextentoftheearliestfaunaandflora，theinsignificantamountofmodificationwhichcanbedemonstratedtohavetakenplaceinanyonegroupofanimals，orplants，isquiteincompatiblewiththehypothesisthatalllivingformsaretheresultsofanecessaryprocessofprogressivedevelopment，entirelycomprisedwithinthetimerepresentedbythefossiliferousrocks。
　　Contrariwise，anyadmissiblehypothesisofprogressivemodificationmustbecompatiblewithpersistencewithoutprogression，throughindefiniteperiods。Andshouldsuchanhypothesiseventuallybeprovedtobetrue，intheonlywayinwhichitcanbedemonstrated，viz。byobservationandexperimentupontheexistingformsoflife，theconclusionwillinevitablypresentitself，thatthePaleozoic，Mesozoic，andCainozoicfaunaeandflorae，takentogether，bearsomewhatthesameproportiontothewholeseriesoflivingbeingswhichhaveoccupiedthisglobe，astheexistingfaunaandfloradotothem。
　　Sucharetheresultsofpaleontologyastheyappear，andhaveforsomeyearsappeared，tothemindofaninquirerwhoregardsthatstudysimplyasoneoftheapplicationsofthegreatbiologicalsciences，andwhodesirestoseeitplaceduponthesamesoundbasisasotherbranchesofphysicalinquiry。Iftheargumentswhichhavebeenbroughtforwardarevalid，probablynoone，inviewofthepresentstateofopinion，willbeinclinedtothinkthetimewastedwhichhasbeenspentupontheirelaboration。