Takingallthesefactstogether，itisobviousthattheview，whichwasentertainedbyMantellandtheprobabilityofwhichwasdemonstratedbyyourowndistinguishedanatomist，Leidy，whilemuchadditionalevidenceinthesamedirectionhasbeenfurnishedbyProfessorCope，thatsomeoftheseanimalsmayhavewalkedupontheirhindlegsasbirdsdo，acquiresgreatweight。
Infact，therecanbenoreasonabledoubtthatoneofthesmallerformsoftheOrnithoscelida，Compsognathus，thealmostentireskeletonofwhichhasbeendiscoveredintheSolenhofenslates，wasabipedalanimal。Thepartsofthisskeletonaresomewhattwistedoutoftheirnaturalrelations，buttheaccompanyingfiguregivesajustviewofthegeneralformofCompsognathusandoftheproportionsofitslimbs；which，insomerespects，aremorecompletelybird-likethanthoseofotherOrnithoscelida。
Fig。7。——RestorationofCompsognathusLongipesWehavehadtostretchthedefinitionoftheclassofbirdssoastoincludebirdswithteethandbirdswithpaw-likeforelimbsandlongtails。ThereisnoevidencethatCompsognathuspossessedfeathers；but，ifitdid，itwouldbehardindeedtosaywhetheritshouldbecalledareptilianbirdoranavianreptile。
AsCompsognathuswalkeduponitshindlegs，itmusthavemadetrackslikethoseofbirds。AndasthestructureofthelimbsofseveralofthegiganticOrnithoscelida，suchasIguanodon，leadstotheconclusionthattheyalsomayhaveconstantly，oroccasionally，assumedthesameattitude，apeculiarinterestattachestothefactthat，intheWealdenstrataofEngland，therearetobefoundgiganticfootsteps，arrangedinorderlikethoseoftheBrontozoum，andwhichtherecanbenoreasonabledoubtweremadebysomeoftheOrnithoscelida，theremainsofwhicharefoundinthesamerocks。And，knowingthatreptilesthatwalkedupontheirhindlegsandsharedmanyoftheanatomicalcharactersofbirdsdidonceexist，itbecomesaveryimportantquestionwhetherthetracksintheTriasofMassachusetts，towhichIreferredsometimeago，andwhichformerlyusedtobeunhesitatinglyascribedtobirds，maynotallhavebeenmadebyornithoscelidanreptiles；andwhether，ifwecouldobtaintheskeletonsoftheanimalswhichmadethesetracks，weshouldnotfindinthemtheactualstepsoftheevolutionalprocessbywhichreptilesgaverisetobirds。
TheevidentialvalueofthefactsIhavebroughtforwardinthisLecturemustbeneitherovernorunderestimated。Itisnothistoricalproofoftheoccurrenceoftheevolutionofbirdsfromreptiles，forwehavenosafegroundforassumingthattruebirdshadnotmadetheirappearanceatthecommencementoftheMesozoicepoch。Itis，infact，quitepossiblethatallthesemoreorlessavi-formreptilesoftheMesozoicepochsarenottermsintheseriesofprogressionfrombirdstoreptilesatall，butsimplythemoreorlessmodifieddescendantsofPalaeozoicformsthroughwhichthattransitionwasactuallyeffected。
WearenotinapositiontosaythattheknownOrnithoscelidaareintermediateintheorderoftheirappearanceontheearthbetweenreptilesandbirds。Allthatcanbesaidisthat，ifindependentevidenceoftheactualoccurrenceofevolutionisproducible，thentheseintercalaryformsremoveeverydifficultyinthewayofunderstandingwhattheactualstepsoftheprocess，inthecaseofbirds，mayhavebeen。
Thatintercalaryformsshouldhaveexistedinancienttimesisanecessaryconsequenceofthetruthofthehypothesisofevolution；and，hence，theevidenceIhavelaidbeforeyouinproofoftheexistenceofsuchforms，is，sofarasitgoes，infavourofthathypothesis。
Thereisanotherseriesofextinctreptileswhichmaybesaidtobeintercalarybetweenreptilesandbirds，insofarastheycombinesomeofthecharactersofboththesegroups；andwhich，astheypossessedthepowerofflight，mayseem，atfirstsight，tobenearerrepresentativesoftheformsbywhichthetransitionfromthereptiletothebirdwaseffected，thantheOrnithoscelida。
ThesearethePterosauria，orPterodactyles，theremainsofwhicharemetwiththroughouttheseriesofMesozoicrocks，fromtheliastothechalk，andsomeofwhichattainedagreatsize，theirwingshavingaspanofeighteenortwentyfeet。
Theseanimals，intheformandproportionsoftheheadandneckrelativelytothebody，andinthefactthattheendsofthejawswereoften，ifnotalways，moreorlessextensivelyensheathedinhornybeaks，remindusofbirds。Moreover，theirbonescontainedaircavities，renderingthemspecificallylighter，asisthecaseinmostbirds。Thebreastbonewaslargeandkeeled，asinmostbirdsandinbats，andtheshouldergirdleisstrikinglysimilartothatofordinarybirds。But，itseemstome，thatthespecialresemblanceofpterodactylestobirdsendshere，unlessImayaddtheentireabsenceofteethwhichcharacterisesthegreatpterodactyles（Pteranodon）
discoveredbyProfessorMarsh。Allotherknownpterodactyleshaveteethlodgedinsockets。Inthevertebralcolumnandthehindlimbstherearenospecialresemblancestobirds，andwhenweturntothewingstheyarefoundtobeconstructedonatotallydifferentprinciplefromthoseofbirds。
Fig。8。——PterodactylusSpectabilis（VonMeyer）。
Therearefourfingers。Thesefourfingersarelarge，andthreeofthem，thosewhichanswertothethumbandtwofollowingfingersinmyhand——areterminatedbyclaws，whilethefourthisenormouslyprolongedandconvertedintoagreatjointedstyle。
Youseeatonce，fromwhatIhavestatedaboutabird’swing，thattherecouldbenothinglesslikeabird’swingthanthisis。Itwasconcludedbygeneralreasoningthatthisfingerhadtheofficeofsupportingawebwhichextendedbetweenitandthebody。Anexistingspecimenprovesthatsuchwasreallythecase，andthatthepterodactylesweredevoidoffeathers，butthatthefingerssupportedavastweblikethatofabat’swing；infact，therecanbenodoubtthatthisancientreptileflewafterthefashionofabat。
Thus，thoughthepterodactyleisareptilewhichhasbecomemodifiedinsuchamannerastoenableittofly，andtherefore，asmightbeexpected，presentssomepointsofresemblancetootheranimalswhichfly；ithas，sotospeak，goneoffthelinewhichleadsdirectlyfromreptilestobirds，andhasbecomedisqualifiedforthechangeswhichleadtothecharacteristicorganisationofthelatterclass。Therefore，viewedinrelationtotheclassesofreptilesandbirds，thepterodactylesappeartometobe，inalimitedsense，intercalaryforms；buttheyarenotevenapproximatelylinear，inthesenseofexemplifyingthosemodificationsofstructurethroughwhichthepassagefromthereptiletothebirdtookplace。
LECTURESONEVOLUTION
III
THEDEMONSTRATIVEEVIDENCEOFEVOLUTION
Theoccurrenceofhistoricalfactsissaidtobedemonstrated，whentheevidencethattheyhappenedisofsuchacharacterastorendertheassumptionthattheydidnothappeninthehighestdegreeimprobable；andthequestionInowhavetodealwithis，whetherevidenceinfavouroftheevolutionofanimalsofthisdegreeofcogencyis，orisnot，obtainablefromtherecordofthesuccessionoflivingformswhichispresentedtousbyfossilremains。
ThosewhohaveattendedtotheprogressofpalaeontologyareawarethatevidenceofthecharacterwhichIhavedefinedhasbeenproducedinconsiderableandcontinually-increasingquantityduringthelastfewyears。Indeed，theamountandthesatisfactorynatureofthatevidencearesomewhatsurprising，whenweconsidertheconditionsunderwhichalonewecanhopetoobtainit。
Itisobviouslyuselesstoseekforsuchevidenceexceptinlocalitiesinwhichthephysicalconditionshavebeensuchastopermitofthedepositofanunbroken，orbutrarelyinterrupted，seriesofstratathroughalongperiodoftime；inwhichthegroupofanimalstobeinvestigatedhasexistedinsuchabundanceastofurnishtherequisitesupplyofremains；andinwhich，finally，thematerialscomposingthestrataaresuchastoensurethepreservationoftheseremainsinatolerablyperfectandundisturbedstate。
Itsohappensthatthecasewhich，atpresent，mostnearlyfulfilsalltheseconditionsisthatoftheseriesofextinctanimalswhichculminatesinthehorses；bywhichtermImeantodenotenotmerelythedomesticanimalswithwhichweareallsowellacquainted，buttheirallies，theass，zebra，quagga，andthelike。Inshort，Iuse"horses"astheequivalentofthetechnicalnameEquidae，whichisappliedtothewholegroupofexistingequineanimals。
Thehorseisinmanywaysaremarkableanimal；notleastsointhefactthatitpresentsuswithanexampleofoneofthemostperfectpiecesofmachineryinthelivingworld。Intruth，amongtheworksofhumaningenuityitcannotbesaidthatthereisanylocomotivesoperfectlyadaptedtoitspurposes，doingsomuchworkwithsosmallaquantityoffuel，asthismachineofnature’smanufacture——thehorse。And，asanecessaryconsequenceofanysortofperfection，ofmechanicalperfectionasofothers，youfindthatthehorseisabeautifulcreature，oneofthemostbeautifulofallland-animals。Lookattheperfectbalanceofitsform，andtherhythmandforceofitsaction。
Thelocomotivemachineryis，asyouareaware，residentinitsslenderforeandhindlimbs；theyareflexibleandelasticlevers，capableofbeingmovedbyverypowerfulmuscles；and，inordertosupplytheengineswhichworktheseleverswiththeforcewhichtheyexpend，thehorseisprovidedwithaveryperfectapparatusforgrindingitsfoodandextractingtherefromtherequisitefuel。
Withoutattemptingtotakeyouveryfarintotheregionofosteologicaldetail，Imustneverthelesstroubleyouwithsomestatementsrespectingtheanatomicalstructureofthehorse；
and，moreespecially，willitbeneedfultoobtainageneralconceptionofthestructureofitsforeandhindlimbs，andofitsteeth。ButIshallonlytouchuponthosepointswhichareabsolutelyessentialtoourinquiry。
Letusturninthefirstplacetothefore-limb。Inmostquadrupeds，asinourselves，thefore-armcontainsdistinctbonescalledtheradiusandtheulna。Thecorrespondingregioninthehorseseemsatfirsttopossessbutonebone。Carefulobservation，however，enablesustodistinguishinthisboneapartwhichclearlyanswerstotheupperendoftheulna。Thisiscloselyunitedwiththechiefmassofthebonewhichrepresentstheradius，andrunsoutintoaslendershaftwhichmaybetracedforsomedistancedownwardsuponthebackoftheradius，andtheninmostcasesthinsoutandvanishes。Ittakesstillmoretroubletomakesureofwhatisneverthelessthefact，thatasmallpartofthelowerendoftheboneofthehorse’sforearm，whichisonlydistinctinaveryyoungfoal，isreallythelowerextremityoftheulna。
Whatiscommonlycalledthekneeofahorseisitswrist。
The"cannonbone"answerstothemiddleboneofthefivemetacarpalbones，whichsupportthepalmofthehandinourselves。The"pastern，""coronary，"and"coffin"bonesofveterinariansanswertothejointsofourmiddlefingers，whilethehoofissimplyagreatlyenlargedandthickenednail。Butifwhatliesbelowthehorse’s"knee"thuscorrespondstothemiddlefingerinourselves，whathasbecomeofthefourotherfingersordigits？Wefindintheplacesofthesecondandfourthdigitsonlytwoslendersplint-likebones，abouttwo-
thirdsaslongasthecannonbone，whichgraduallytapertotheirlowerendsandbearnofingerjoints，or，astheyaretermed，phalanges。Sometimes，smallbonyorgristlynodulesaretobefoundatthebasesofthesetwometacarpalsplints，anditisprobablethattheserepresentrudimentsofthefirstandfifthtoes。Thus，thepartofthehorse’sskeleton，whichcorrespondswiththatofthehumanhand，containsoneovergrownmiddledigit，andatleasttwoimperfectlateraldigits；andtheseanswer，respectively，tothethird，thesecond，andthefourthfingersinman。
Correspondingmodificationsarefoundinthehindlimb。
Inourselves，andinmostquadrupeds，thelegcontainstwodistinctbones，alargebone，thetibia，andasmallerandmoreslenderbone，thefibula。But，inthehorse，thefibulaseems，atfirst，tobereducedtoitsupperend；ashortslenderboneunitedwiththetibia，andendinginapointbelow，occupyingitsplace。Examinationofthelowerendofayoungfoal’sshinbone，however，showsadistinctportionofosseousmatter，whichisthelowerendofthefibula；sothattheapparentlysingle，lowerendoftheshinboneisreallymadeupofthecoalescedendsofthetibiaandfibula，justasthe，apparentlysingle，lowerendofthefore-armboneiscomposedofthecoalescedradiusandulna。
Theheelofthehorseisthepartcommonlyknownasthehock。
Thehindercannonboneanswerstothemiddlemetatarsalboneofthehumanfoot，thepastern，coronary，andcoffinbones，tothemiddletoebones；thehindhooftothenail；asinthefore-
foot。And，asinthefore-foot，therearemerelytwosplintstorepresentthesecondandthefourthtoes。Sometimesarudimentofafifthtoeappearstobetraceable。
Theteethofahorsearenotlesspeculiarthanitslimbs。Thelivingengine，likeallothers，mustbewellstokedifitistodoitswork；andthehorse，ifitistomakegooditswearandtear，andtoexerttheenormousamountofforcerequiredforitspropulsion，mustbewellandrapidlyfed。Tothisend，goodcuttinginstrumentsandpowerfulandlastingcrushersareneedful。Accordingly，thetwelvecuttingteethofahorseareclose-setandconcentratedinthefore-partofitsmouth，likesomanyadzesorchisels。Thegrindersormolarsarelarge，andhaveanextremelycomplicatedstructure，beingcomposedofanumberofdifferentsubstancesofunequalhardness。Theconsequenceofthisisthattheywearawayatdifferentrates；
and，hence，thesurfaceofeachgrinderisalwaysasunevenasthatofagoodmillstone。
Ihavesaidthatthestructureofthegrindingteethisverycomplicated，theharderandthesofterpartsbeing，asitwere，interlacedwithoneanother。Theresultofthisisthat，asthetoothwears，thecrownpresentsapeculiarpattern，thenatureofwhichisnotveryeasilydecipheredatfirst；butwhichitisimportantweshouldunderstandclearly。Eachgrindingtoothoftheupperjawhasanouterwallsoshapedthat，ontheworncrown，itexhibitstheformoftwocrescents，oneinfrontandonebehind，withtheirconcavesidesturnedoutwards。Fromtheinnersideofthefrontcrescent，acrescenticfrontridgepassesinwardsandbackwards，anditsinnerfaceenlargesintoastronglongitudinalfoldorpillar。
Fromthefrontpartofthehindercrescent，abackridge
takesalikedirection，andalsohasitspillar。
Thedeepinterspacesorvalleysbetweentheseridgesandtheouterwallarefilledbybonysubstance，whichiscalledcement，andcoatsthewholetooth。
Thepatternofthewornfaceofeachgrindingtoothofthelowerjawisquitedifferent。Itappearstobeformedoftwocrescent-
shapedridges，theconvexitiesofwhichareturnedoutwards。
Thefreeextremityofeachcrescenthasapillar，andthereisalargedoublepillarwherethetwocrescentsmeet。Thewholestructureis，asitwere，imbeddedincement，whichfillsupthevalleys，asintheuppergrinders。
Ifthegrindingfacesofanupperandofalowermolarofthesamesideareappliedtogether，itwillbeseenthattheopposedridgesarenowhereparallel，butthattheyfrequentlycross；
andthatthus，intheactofmastication，ahardsurfaceintheoneisconstantlyappliedtoasoftsurfaceintheother，andviceversa。Theythusconstituteagrindingapparatusofgreatefficiency，andonewhichisrepairedasfastasitwears，owingtothelong-continuedgrowthoftheteeth。
Someotherpeculiaritiesofthedentitionofthehorsemustbenoticed，astheybearuponwhatIshallhavetosaybyandby。
Thusthecrownsofthecuttingteethhaveapeculiardeeppit，whichgivesrisetothewell-known"mark"ofthehorse。Thereisalargespacebetweentheouterincisorsandthefrontgrinder。
Inthisspacetheadultmalehorsepresents，neartheincisorsoneachside，aboveandbelow，acanineor"tush，"whichiscommonlyabsentinmares。Inayounghorse，moreover，thereisnotunfrequentlytobeseeninfrontofthefirstgrinder，averysmalltooth，whichsoonfallsout。Ifthissmalltoothbecountedasone，itwillbefoundthatthereareseventeethbehindthecanineoneachside；namely，thesmalltoothinquestion，andthesixgreatgrinders，amongwhich，byanunusualpeculiarity，theforemosttoothisratherlargerthanthosewhichfollowit。
Ihavenowenumeratedthosecharacteristicstructuresofthehorsewhichareofmostimportanceforthepurposewehaveinview。
Toanyonewhoisacquaintedwiththemorphologyofvertebratedanimals，theyshowthatthehorsedeviateswidelyfromthegeneralstructureofmammals；andthatthehorsetypeis，inmanyrespects，anextrememodificationofthegeneralmammalianplan。Theleastmodifiedmammals，infact，havetheradiusandulna，thetibiaandfibula，distinctandseparate。Theyhavefivedistinctandcompletedigitsoneachfoot，andnooneofthesedigitsisverymuchlargerthantherest。Moreover，intheleastmodifiedmammals，thetotalnumberoftheteethisverygenerallyforty-four，whileinhorses，theusualnumberisforty，andintheabsenceofthecanines，itmaybereducedtothirty-six；theincisorteetharedevoidofthefoldseeninthoseofthehorse：thegrindersregularlydiminishinsizefromthemiddleoftheseriestoitsfrontend；whiletheircrownsareshort，earlyattaintheirfulllength，andexhibitsimpleridgesortubercles，inplaceofthecomplexfoldingsofthehorse’sgrinders。
Hencethegeneralprinciplesofthehypothesisofevolutionleadtotheconclusionthatthehorsemusthavebeenderivedfromsomequadrupedwhichpossessedfivecompletedigitsoneachfoot；whichhadthebonesofthefore-armandofthelegcompleteandseparate；andwhichpossessedforty-fourteeth，amongwhichthecrownsoftheincisorsandgrindershadasimplestructure；whilethelattergraduallyincreasedinsizefrombeforebackwards，atanyrateintheanteriorpartoftheseries，andhadshortcrowns。
Andifthehorsehasbeenthusevolved，andtheremainsofthedifferentstagesofitsevolutionhavebeenpreserved，theyoughttopresentuswithaseriesofformsinwhichthenumberofthedigitsbecomesreduced；thebonesofthefore-armandleggraduallytakeontheequinecondition；andtheformandarrangementoftheteethsuccessivelyapproximatetothosewhichobtaininexistinghorses。
Letusturntothefacts，andseehowfartheyfulfiltheserequirementsofthedoctrineofevolution。
InEuropeabundantremainsofhorsesarefoundintheQuaternaryandlaterTertiarystrataasfarasthePlioceneformation。
Butthesehorses，whicharesocommoninthecave-depositsandinthegravelsofEurope，areinallessentialrespectslikeexistinghorses。AndthatistrueofallthehorsesofthelatterpartofthePlioceneepoch。But，indepositswhichbelongtotheearlierPlioceneandlaterMioceneepochs，andwhichoccurinBritain，inFrance，inGermany，inGreece，inIndia，wefindanimalswhichareextremelylikehorses——which，infact，aresosimilartohorses，thatyoumayfollowdescriptionsgiveninworksupontheanatomyofthehorseupontheskeletonsoftheseanimals——butwhichdifferinsomeimportantparticulars。
Forexample，thestructureoftheirforeandhindlimbsissomewhatdifferent。Theboneswhich，inthehorse，arerepresentedbytwosplints，imperfectbelow，areaslongasthemiddlemetacarpalandmetatarsalbones；and，attachedtotheextremityofeach，isadigitwiththreejointsofthesamegeneralcharacterasthoseofthemiddledigit，onlyverymuchsmaller。Thesesmalldigitsaresodisposedthattheycouldhavehadbutverylittlefunctionalimportance，andtheymusthavebeenratherofthenatureofthedew-claws，suchasaretobefoundinmanyruminantanimals。TheHipparion，astheextinctEuropeanthree-toedhorseiscalled，infact，presentsafootsimilartothatoftheAmericanProtohippus
（Fig。9），exceptthat，intheHipparion，thesmallerdigitsaresituatedfartherback，andareofsmallerproportionalsize，thanintheProtohippus。
Theulnaisslightlymoredistinctthaninthehorse；andthewholelengthofit，asaveryslendershaft，intimatelyunitedwiththeradius，iscompletelytraceable。Thefibulaappearstobeinthesameconditionasinthehorse。TheteethoftheHipparionareessentiallysimilartothoseofthehorse，butthepatternofthegrindersisinsomerespectsalittlemorecomplex，andthereisadepressiononthefaceoftheskullinfrontoftheorbit，whichisnotseeninexistinghorses。
IntheearlierMiocene，andperhapsthelaterEocenedepositsofsomepartsofEurope，anotherextinctanimalhasbeendiscovered，whichCuvier，whofirstdescribedsomefragmentsofit，consideredtobeaPalaeotherim。Butasfurtherdiscoveriesthrewnewlightuponitsstructure，itwasrecognisedasadistinctgenus，underthenameofAnchitherium。
Initsgeneralcharacters，theskeletonofAnchitherium
isverysimilartothatofthehorse。Infact，LartetandDeBlainvillecalleditPal？otheriumequinumorhippoides；andDeChristol，in1847，saidthatitdifferedfromHipparioninlittlemorethanthecharactersofitsteeth，andgaveitthenameofHipparitherium。Eachfootpossessesthreecompletetoes；
whilethelateraltoesaremuchlargerinproportiontothemiddletoethaninHipparion，anddoubtlessrestedonthegroundinordinarylocomotion。
Theulnaiscompleteandquitedistinctfromtheradius，thoughfirmlyunitedwiththelatter。Thefibulaseemsalsotohavebeencomplete。Itslowerend，thoughintimatelyunitedwiththatofthetibia，isclearlymarkedofffromthelatterbone。
Thereareforty-fourteeth。Theincisorshavenostrongpit。
Thecaninesseemtohavebeenwelldevelopedinbothsexes。
Thefirstofthesevengrinders，which，asIhavesaid，isfrequentlyabsent，and，whenitdoesexist，issmallinthehorse，isagood-sizedandpermanenttooth，whilethegrinderwhichfollowsitisbutlittlelargerthanthehinderones。
Thecrownsofthegrindersareshort，andthoughthefundamentalpatternofthehorse-toothisdiscernible，thefrontandbackridgesarelesscurved，theaccessorypillarsarewanting，andthevalleys，muchshallower，arenotfilledupwithcement。
Sevenyearsago，whenIhappenedtobelookingcriticallyintothebearingofpalaentologicalfactsuponthedoctrineofevolution，itappearedtomethattheAnchitherium，theHipparion，andthemodernhorses，constituteaseriesinwhichthemodificationsofstructurecoincidewiththeorderofchronologicaloccurrence，inthemannerinwhichtheymustcoincide，ifthemodernhorsesreallyaretheresultofthegradualmetamorphosis，inthecourseoftheTertiaryepoch，ofalessspecialisedancestralform。AndIfoundbycorrespondencewiththelateeminentFrenchanatomistandpalaeontologist，M。Lartet，thathehadarrivedatthesameconclusionfromthesamedata。
ThattheAnchitheriumtypehadbecomemetamorphosedintotheHippariontype，andthelatterintotheEquine
type，inthecourseofthatperiodoftimewhichisrepresentedbythelatterhalfoftheTertiarydeposits，seemedtometobetheonlyexplanationofthefactsforwhichtherewasevenashadowofprobability。
And，hence，Ihaveeversinceheldthatthesefactsaffordevidenceoftheoccurrenceofevolution，which，inthesensealreadydefined，maybetermeddemonstrative。
AllwhohaveoccupiedthemselveswiththestructureofAnchitherium，fromCuvieronwards，haveacknowledgeditsmanypointsoflikenesstoawell-knowngenusofextinctEocenemammals，Palaeotherium。Indeed，aswehaveseen，CuvierregardedhisremainsofAnchitheriumasthoseofaspeciesofPalaeotherium。Hence，inattemptingtotracethepedigreeofthehorsebeyondtheMioceneepochandtheAnchitheroidform，InaturallysoughtamongthevariousspeciesofPalaeotheroidanimalsforitsnearestally，andIwasledtoconcludethatthePalaeotheriumminus（Plagiolophus）
representedthenextstepmorenearlythananyformthenknown。
Ithinkthatthisopinionwasfullyjustifiable；buttheprogressofinvestigationhasthrownanunexpectedlightonthequestion，andhasbroughtusmuchnearerthancouldhavebeenanticipatedtoaknowledgeofthetrueseriesoftheprogenitorsofthehorse。
Youareallawarethat，whenyourcountrywasfirstdiscoveredbyEuropeans，therewerenotracesoftheexistenceofthehorseinanypartoftheAmericanContinent。TheaccountsoftheconquestofMexicodwellupontheastonishmentofthenativesofthatcountrywhentheyfirstbecameacquaintedwiththatastoundingphenomenon——amanseateduponahorse。
Nevertheless，theinvestigationsofAmericangeologistshaveprovedthattheremainsofhorsesoccurinthemostsuperficialdepositsofbothNorthandSouthAmerica，justastheydoinEurope。Therefore，forsomereasonorother——nofeasiblesuggestiononthatsubject，sofarasIknow，hasbeenmade——thehorsemusthavediedoutonthiscontinentatsomeperiodprecedingthediscoveryofAmerica。OflateyearstherehasbeendiscoveredinyourWesternTerritoriesthatmarvellousaccumulationofdeposits，admirablyadaptedforthepreservationoforganicremains，towhichIreferredtheotherevening，andwhichfurnishesuswithaconsecutiveseriesofrecordsofthefaunaoftheolderhalfoftheTertiaryepoch，forwhichwehavenoparallelinEurope。Theyhaveyieldedfossilsinanexcellentstateofconservationandinunexamplednumberandvariety。
TheresearchesofLeidyandothershaveshownthatformsalliedtotheHipparionandtheAnchitheriumaretobefoundamongtheseremains。Butitisonlyrecentlythattheadmirablyconceivedandmostthoroughlyandpatientlyworked-outinvestigationsofProfessorMarshhavegivenusajustideaofthevastfossilwealth，andofthescientificimportance，ofthesedeposits。IhavehadtheadvantageofglancingoverthecollectionsinYaleMuseum；andIcantrulysaythat，sofarasmyknowledgeextends，thereisnocollectionfromanyoneregionandseriesofstratacomparable，forextent，orforthecarewithwhichtheremainshavebeengottogether，orfortheirscientificimportance，totheseriesoffossilswhichhehasdepositedthere。Thisvastcollectionhasyieldedevidencebearinguponthequestionofthepedigreeofthehorseofthemoststrikingcharacter。IttendstoshowthatwemustlooktoAmerica，ratherthantoEurope，fortheoriginalseatoftheequineseries；andthatthearchaicformsandsuccessivemodificationsofthehorse’sancestryarefarbetterpreservedherethaninEurope。
ProfessorMarsh’skindnesshasenabledmetoputbeforeyouadiagram，everyfigureinwhichisanactualrepresentationofsomespecimenwhichistobeseenatYaleatthispresenttime（Fig。9）。
Fig。9。
ThesuccessionofformswhichhehasbroughttogethercarriesusfromthetoptothebottomoftheTertiaries。Firstly，thereisthetruehorse。NextwehavetheAmericanPlioceneformofthehorse（Pliohippus）；intheconformationofitslimbsitpresentssomeveryslightdeviationsfromtheordinaryhorse，andthecrownsofthegrindingteethareshorter。ThencomestheProtohippus，whichrepresentstheEuropeanHipparion，havingonelargedigitandtwosmallonesoneachfoot，andthegeneralcharactersofthefore-armandlegtowhichIhavereferred。ButitismorevaluablethantheEuropeanHipparionforthereasonthatitisdevoidofsomeofthepeculiaritiesofthatform——peculiaritieswhichtendtoshowthattheEuropeanHipparionisratheramemberofacollateralbranch，thanaforminthedirectlineofsuccession。
Next，inthebackwardorderintime，istheMiohippus，
whichcorrespondsprettynearlywiththeAnchitheriumofEurope。Itpresentsthreecompletetoes——onelargemedianandtwosmallerlateralones；andthereisarudimentofthatdigit，whichanswerstothelittlefingerofthehumanhand。
TheEuropeanrecordofthepedigreeofthehorsestopshere；
intheAmericanTertiaries，onthecontrary，theseriesofancestralequineformsiscontinuedintotheEoceneformations。
AnolderMioceneform，termedMesohippus，hasthreetoesinfront，withalargesplint-likerudimentrepresentingthelittlefinger；andthreetoesbehind。Theradiusandulna，thetibiaandthefibula，aredistinct，andtheshortcrownedmolarteethareanchitherioidinpattern。
ButthemostimportantdiscoveryofallistheOrohippus，
whichcomesfromtheEoceneformation，andistheoldestmemberoftheequineseries，asyetknown。Herewefindfourcompletetoesonthefrontlimb，threetoesonthehindlimb，awell-
developedulna，awell-developedfibula，andshort-crownedgrindersofsimplepattern。
Thus，thankstotheseimportantresearches，ithasbecomeevidentthat，sofarasourpresentknowledgeextends，thehistoryofthehorse-typeisexactlyandpreciselythatwhichcouldhavebeenpredictedfromaknowledgeoftheprinciplesofevolution。Andtheknowledgewenowpossessjustifiesuscompletelyintheanticipation，thatwhenthestilllowerEocenedeposits，andthosewhichbelongtotheCretaceousepoch，haveyieldeduptheirremainsofancestralequineanimals，weshallfind，first，aformwithfourcompletetoesandarudimentoftheinnermostorfirstdigitinfront，with，probably，arudimentofthefifthdigitinthehindfoot；while，instillolderforms，theseriesofthedigitswillbemoreandmorecomplete，untilwecometothefive-toedanimals，inwhich，ifthedoctrineofevolutioniswellfounded，thewholeseriesmusthavetakenitsorigin。
ThatiswhatImeanbydemonstrativeevidenceofevolution。
Aninductivehypothesisissaidtobedemonstratedwhenthefactsareshowntobeinentireaccordancewithit。Ifthatisnotscientificproof，therearenomerelyinductiveconclusionswhichcanbesaidtobeproved。Andthedoctrineofevolution，atthepresenttime，restsuponexactlyassecureafoundationastheCopernicantheoryofthemotionsoftheheavenlybodiesdidatthetimeofitspromulgation。Itslogicalbasisispreciselyofthesamecharacter——thecoincidenceoftheobservedfactswiththeoreticalrequirements。
Theonlywayofescape，ifitbeawayofescape，fromtheconclusionswhichIhavejustindicated，isthesuppositionthatallthesedifferentequineformshavebeencreatedseparatelyatseparateepochsoftime；and，Irepeat，thatofsuchanhypothesisasthisthereneitheris，norcanbe，anyscientificevidence；and，assuredly，sofarasIknow，thereisnonewhichissupported，orpretendstobesupported，byevidenceorauthorityofanyotherkind。Icanbutthinkthatthetimewillcomewhensuchsuggestionsasthese，suchobviousattemptstoescapetheforceofdemonstration，willbeputuponthesamefootingasthesuppositionmadebysomewriters，whoareI
believenotcompletelyextinctatpresent，thatfossilsaremeresimulacra，arenoindicationsoftheformerexistenceoftheanimalstowhichtheyseemtobelong；butthattheyareeithersportsofnature，orspecialcreations，intended——asIheardsuggestedtheotherday——totestourfaith。
Infact，thewholeevidenceisinfavourofevolution，andthereisnoneagainstit。AndIsaythis，althoughperfectlywellawareoftheseemingdifficultieswhichhavebeenbuiltupuponwhatappearstotheuninformedtobeasolidfoundation。Imeetconstantlywiththeargumentthatthedoctrineofevolutioncannotbewellfounded，becauseitrequiresthelapseofaveryvastperiodoftime；whilethedurationoflifeupontheearththusimpliedisinconsistentwiththeconclusionsarrivedatbytheastronomerandthephysicist。ImayventuretosaythatIamfamiliarwiththoseconclusions，inasmuchassomeyearsago，whenPresidentoftheGeologicalSocietyofLondon，Itookthelibertyofcriticisingthem，andofshowinginwhatrespects，asitappearedtome，theylackedcompleteandthoroughdemonstration。But，puttingthatpointaside，supposethat，astheastronomers，orsomeofthem，andsomephysicalphilosophers，tellus，itisimpossiblethatlifecouldhaveenduredupontheearthforaslongaperiodasisrequiredbythedoctrineofevolution——supposingthattobeproved——Idesiretobeinformed，whatisthefoundationforthestatementthatevolutiondoesrequiresogreatatime？Thebiologistknowsnothingwhateveroftheamountoftimewhichmayberequiredfortheprocessofevolution。ItisamatteroffactthattheequineformswhichIhavedescribedtoyouoccur，intheorderstated，intheTertiaryformations。ButIhavenottheslightestmeansofguessingwhetherittookamillionofyears，ortenmillions，orahundredmillions，orathousandmillionsofyears，togiverisetothatseriesofchanges。Abiologisthasnomeansofarrivingatanyconclusionastotheamountoftimewhichmaybeneededforacertainquantityoforganicchange。Hetakeshistimefromthegeologist。Thegeologist，consideringtherateatwhichdepositsareformedandtherateatwhichdenudationgoesonuponthesurfaceoftheearth，arrivesatmoreorlessjustifiableconclusionsastothetimewhichisrequiredforthedepositofacertainthicknessofrocks；andifhetellsmethattheTertiaryformationsrequired500，000，000yearsfortheirdeposit，Isupposehehasgoodgroundforwhathesays，andI
takethatasameasureofthedurationoftheevolutionofthehorsefromtheOrohippusuptoitspresentcondition。
And，ifheisright，undoubtedlyevolutionisaveryslowprocess，andrequiresagreatdealoftime。Butsuppose，now，thatanastronomeroraphysicist——forinstance，myfriendSirWilliamThomson——tellsmethatmygeologicalauthorityisquitewrong；andthathehasweightyevidencetoshowthatlifecouldnotpossiblyhaveexisteduponthesurfaceoftheearth500，000，000yearsago，becausetheearthwouldhavethenbeentoohottoallowoflife，myreplyis："Thatisnotmyaffair；
settlethatwiththegeologist，andwhenyouhavecometoanagreementamongyourselvesIwilladoptyourconclusion。"
Wetakeourtimefromthegeologistsandphysicists；anditismonstrousthat，havingtakenourtimefromthephysicalphilosopher’sclock，thephysicalphilosophershouldturnrounduponus，andsaywearetoofastortooslow。Whatwedesiretoknowis，isitafactthatevolutiontookplace？Astotheamountoftimewhichevolutionmayhaveoccupied，weareinthehandsofthephysicistandtheastronomer，whosebusinessitistodealwiththosequestions。
Ihavenow，ladiesandgentlemen，arrivedattheconclusionofthetaskwhichIsetbeforemyselfwhenIundertooktodelivertheselectures。Mypurposehasbeen，nottoenablethoseamongyouwhohavepaidnoattentiontothesesubjectsbefore，toleavethisroominaconditiontodecideuponthevalidityortheinvalidityofthehypothesisofevolution；butIhavedesiredtoputbeforeyoutheprinciplesuponwhichallhypothesesrespectingthehistoryofNaturemustbejudged；
andfurthermore，tomakeapparentthenatureoftheevidenceandtheamountofcogencywhichistobeexpectedandmaybeobtainedfromit。Tothisend，Ihavenothesitatedtoregardyouasgenuinestudentsandpersonsdesirousofknowingthetruth。Ihavenotshrunkfromtakingyouthroughlongdiscussions，thatIfearmayhavesometimestriedyourpatience；
andIhaveinflicteduponyoudetailswhichwereindispensable，butwhichmaywellhavebeenwearisome。ButIshallrejoice——I
shallconsiderthatIhavedoneyouthegreatestservicewhichitwasinmypowertodo——ifIhavethusconvincedyouthatthegreatquestionwhichwehavebeendiscussingisnotonetobedealtwithbyrhetoricalflourishes，orbylooseandsuperficialtalk；butthatitrequiresthekeenattentionofthetrainedintellectandthepatienceoftheaccurateobserver。
WhenIcommencedthisseriesoflectures，Ididnotthinkitnecessarytoprefacethemwithaprologue，suchasmightbeexpectedfromastrangerandaforeigner；forduringmybriefstayinyourcountry，Ihavefounditveryhardtobelievethatastrangercouldbepossessedofsomanyfriends，andalmostharderthataforeignercouldexpresshimselfinyourlanguageinsuchawayastobe，toallappearance，soreadilyintelligible。SofarasIcanjudge，thatmostintelligent，andperhaps，Imayadd，mostsingularlyactiveandenterprisingbody，yourpressreporters，donotseemtohavebeendeterredbymyaccentfromgivingthefullestaccountofeverythingthatI
happentohavesaid。
ButthevesselinwhichItakemydepartureto-morrowmorningisevennowreadytosliphermoorings；IawakefrommydelusionthatIamotherthanastrangerandaforeigner。Iamreadytogobacktomyplaceandcountry；but，beforedoingso，letme，bywayofepilogue，tendertoyoumymostheartythanksforthekindandcordialreceptionwhichyouhaveaccordedtome；
andletmethankyoustillmoreforthatwhichisthegreatestcomplimentwhichcanbeaffordedtoanypersoninmyposition——
thecontinuousandundisturbedattentionwhichyouhavebestoweduponthelongargumentwhichIhavehadthehonourtolaybeforeyou。
FOOTNOTES
（1）Theabsenceofanykeelonthebreast-boneandsomeotherosteologicalpeculiarities，observedbyProfessorMarsh，however，suggestthatHesperornismaybeamodificationofalessspecialisedgroupofbirdsthanthattowhichtheseexistingaquaticbirdsbelong。
（2）Asecondspecimen，discoveredin1877，andatpresentintheBerlinmuseum，showsanexcellentlypreservedskullwithteeth；
andthreedigits，allterminatedbyclaws，intheforelimb。
1893。
（3）Iusetheword"type"becauseitishighlyprobablethatmanyformsofAnchitherium-likeandHipparion-likeanimalsexistedintheMioceneandPlioceneepochs，justasmanyspeciesofthehorsetribeexistnow，anditishighlyimprobablethattheparticularspeciesofAnchitheriumorHipparion，whichhappentohavebeendiscovered，shouldbepreciselythosewhichhaveformedpartofthedirectlineofthehorse’spedigree。
（4）Sincethislecturewasdelivered，ProfessorMarshhasdiscoveredanewgenusofequinemammals（Eohippus）fromthelowestEocenedepositsoftheWest，whichcorrespondsverynearlytothisdescription。——AmericanJournalofScience，
November，1876。