Takingallthesefactstogether，itisobviousthattheview，whichwasentertainedbyMantellandtheprobabilityofwhichwasdemonstratedbyyourowndistinguishedanatomist，Leidy，whilemuchadditionalevidenceinthesamedirectionhasbeenfurnishedbyProfessorCope，thatsomeoftheseanimalsmayhavewalkedupontheirhindlegsasbirdsdo，acquiresgreatweight。
Infact，therecanbenoreasonabledoubtthatoneofthesmallerformsofthe<i>Ornithoscelida，Compsognathus，</i>thealmostentireskeletonofwhichhasbeendiscoveredintheSolenhofenslates，wasabipedalanimal。Thepartsofthisskeletonaresomewhattwistedoutoftheirnaturalrelations，buttheaccompanyingfiguregivesajustviewofthegeneralformof<i>Compsognathus</i>andoftheproportionsofitslimbs；which，insomerespects，aremorecompletelybird-likethanthoseofother<i>Ornithoscelida。</i>
Fig。7。——RestorationofCompsognathusLongipesWehavehadtostretchthedefinitionoftheclassofbirdssoastoincludebirdswithteethandbirdswithpaw-likeforelimbsandlongtails。Thereisnoevidencethat<i>Compsognathus</i>possessedfeathers；but，ifitdid，itwouldbehardindeedtosaywhetheritshouldbecalledareptilianbirdoranavianreptile。
As<i>Compsognathus</i>walkeduponitshindlegs，itmusthavemadetrackslikethoseofbirds。Andasthestructureofthelimbsofseveralofthegigantic<i>Ornithoscelida，</i>suchas<i>Iguanodon，</i>leadstotheconclusionthattheyalsomayhaveconstantly，oroccasionally，assumedthesameattitude，apeculiarinterestattachestothefactthat，intheWealdenstrataofEngland，therearetobefoundgiganticfootsteps，arrangedinorderlikethoseofthe<i>Brontozoum，</i>andwhichtherecanbenoreasonabledoubtweremadebysomeofthe<i>Ornithoscelida，</i>theremainsofwhicharefoundinthesamerocks。And，knowingthatreptilesthatwalkedupontheirhindlegsandsharedmanyoftheanatomicalcharactersofbirdsdidonceexist，itbecomesaveryimportantquestionwhetherthetracksintheTriasofMassachusetts，towhichIreferredsometimeago，andwhichformerlyusedtobeunhesitatinglyascribedtobirds，maynotallhavebeenmadebyornithoscelidanreptiles；andwhether，ifwecouldobtaintheskeletonsoftheanimalswhichmadethesetracks，weshouldnotfindinthemtheactualstepsoftheevolutionalprocessbywhichreptilesgaverisetobirds。
TheevidentialvalueofthefactsIhavebroughtforwardinthisLecturemustbeneitherovernorunderestimated。Itisnothistoricalproofoftheoccurrenceoftheevolutionofbirdsfromreptiles，forwehavenosafegroundforassumingthattruebirdshadnotmadetheirappearanceatthecommencementoftheMesozoicepoch。Itis，infact，quitepossiblethatallthesemoreorlessavi-formreptilesoftheMesozoicepochsarenottermsintheseriesofprogressionfrombirdstoreptilesatall，butsimplythemoreorlessmodifieddescendantsofPalaeozoicformsthroughwhichthattransitionwasactuallyeffected。
Wearenotinapositiontosaythattheknown<i>Ornithoscelida</i>areintermediateintheorderoftheirappearanceontheearthbetweenreptilesandbirds。Allthatcanbesaidisthat，ifindependentevidenceoftheactualoccurrenceofevolutionisproducible，thentheseintercalaryformsremoveeverydifficultyinthewayofunderstandingwhattheactualstepsoftheprocess，inthecaseofbirds，mayhavebeen。
Thatintercalaryformsshouldhaveexistedinancienttimesisanecessaryconsequenceofthetruthofthehypothesisofevolution；and，hence，theevidenceIhavelaidbeforeyouinproofoftheexistenceofsuchforms，is，sofarasitgoes，infavourofthathypothesis。
Thereisanotherseriesofextinctreptileswhichmaybesaidtobeintercalarybetweenreptilesandbirds，insofarastheycombinesomeofthecharactersofboththesegroups；andwhich，astheypossessedthepowerofflight，mayseem，atfirstsight，tobenearerrepresentativesoftheformsbywhichthetransitionfromthereptiletothebirdwaseffected，thanthe<i>Ornithoscelida。</i>
Thesearethe<i>Pterosauria，</i>orPterodactyles，theremainsofwhicharemetwiththroughouttheseriesofMesozoicrocks，fromtheliastothechalk，andsomeofwhichattainedagreatsize，theirwingshavingaspanofeighteenortwentyfeet。
Theseanimals，intheformandproportionsoftheheadandneckrelativelytothebody，andinthefactthattheendsofthejawswereoften，ifnotalways，moreorlessextensivelyensheathedinhornybeaks，remindusofbirds。Moreover，theirbonescontainedaircavities，renderingthemspecificallylighter，asisthecaseinmostbirds。Thebreastbonewaslargeandkeeled，asinmostbirdsandinbats，andtheshouldergirdleisstrikinglysimilartothatofordinarybirds。But，itseemstome，thatthespecialresemblanceofpterodactylestobirdsendshere，unlessImayaddtheentireabsenceofteethwhichcharacterisesthegreatpterodactyles<i>（Pteranodon）</i>
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"astheequivalentofthetechnicalname<i>Equidae，</i>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。Eachgrindingtoothoftheupperjawhasan<i>outerwall</i>soshapedthat，ontheworncrown，itexhibitstheformoftwocrescents，oneinfrontandonebehind，withtheirconcavesidesturnedoutwards。Fromtheinnersideofthefrontcrescent，acrescentic<i>frontridge</i>passesinwardsandbackwards，anditsinnerfaceenlargesintoastronglongitudinalfoldor<i>pillar。</i>
Fromthefrontpartofthehindercrescent，a<i>backridge</i>
takesalikedirection，andalsohasits<i>pillar。</i>
Thedeepinterspacesor<i>valleys</i>betweentheseridgesandtheouterwallarefilledbybonysubstance，whichiscalled<i>cement，</i>andcoatsthewholetooth。
Thepatternofthewornfaceofeachgrindingtoothofthelowerjawisquitedifferent。Itappearstobeformedoftwocrescent-
shapedridges，theconvexitiesofwhichareturnedoutwards。
Thefreeextremityofeachcrescenthasa<i>pillar，</i>andthereisalargedouble<i>pillar</i>wherethetwocrescentsmeet。Thewholestructureis，asitwere，imbeddedincement，whichfillsupthevalleys，asintheuppergrinders。
Ifthegrindingfacesofanupperandofalowermolarofthesamesideareappliedtogether，itwillbeseenthattheopposedridgesarenowhereparallel，butthattheyfrequentlycross；
andthatthus，intheactofmastication，ahardsurfaceintheoneisconstantlyappliedtoasoftsurfaceintheother，and<i>viceversa。</i>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。The<i>Hipparion，</i>astheextinctEuropeanthree-toedhorseiscalled，infact，presentsafootsimilartothatoftheAmerican<i>Protohippus</i>
（Fig。9），exceptthat，inthe<i>Hipparion，</i>thesmallerdigitsaresituatedfartherback，andareofsmallerproportionalsize，thaninthe<i>Protohippus。</i>
Theulnaisslightlymoredistinctthaninthehorse；andthewholelengthofit，asaveryslendershaft，intimatelyunitedwiththeradius，iscompletelytraceable。Thefibulaappearstobeinthesameconditionasinthehorse。Theteethofthe<i>Hipparion</i>areessentiallysimilartothoseofthehorse，butthepatternofthegrindersisinsomerespectsalittlemorecomplex，andthereisadepressiononthefaceoftheskullinfrontoftheorbit，whichisnotseeninexistinghorses。
IntheearlierMiocene，andperhapsthelaterEocenedepositsofsomepartsofEurope，anotherextinctanimalhasbeendiscovered，whichCuvier，whofirstdescribedsomefragmentsofit，consideredtobea<i>Palaeotherim。</i>Butasfurtherdiscoveriesthrewnewlightuponitsstructure，itwasrecognisedasadistinctgenus，underthenameof<i>Anchitherium。</i>
Initsgeneralcharacters，theskeletonof<i>Anchitherium</i>
isverysimilartothatofthehorse。Infact，LartetandDeBlainvillecalledit<i>Pal？otheriumequinum</i>or<i>hippoides；</i>andDeChristol，in1847，saidthatitdifferedfrom<i>Hipparion</i>inlittlemorethanthecharactersofitsteeth，andgaveitthenameof<i>Hipparitherium。</i>Eachfootpossessesthreecompletetoes；
whilethelateraltoesaremuchlargerinproportiontothemiddletoethanin<i>Hipparion，</i>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，itappearedtomethatthe<i>Anchitherium，</i>the<i>Hipparion，</i>andthemodernhorses，constituteaseriesinwhichthemodificationsofstructurecoincidewiththeorderofchronologicaloccurrence，inthemannerinwhichtheymustcoincide，ifthemodernhorsesreallyaretheresultofthegradualmetamorphosis，inthecourseoftheTertiaryepoch，ofalessspecialisedancestralform。AndIfoundbycorrespondencewiththelateeminentFrenchanatomistandpalaeontologist，M。Lartet，thathehadarrivedatthesameconclusionfromthesamedata。
Thatthe<i>Anchitherium</i>typehadbecomemetamorphosedintothe<i>Hipparion</i>type，andthelatterintothe<i>Equine</i>
type，inthecourseofthatperiodoftimewhichisrepresentedbythelatterhalfoftheTertiarydeposits，seemedtometobetheonlyexplanationofthefactsforwhichtherewasevenashadowofprobability。<3>
And，hence，Ihaveeversinceheldthatthesefactsaffordevidenceoftheoccurrenceofevolution，which，inthesensealreadydefined，maybetermeddemonstrative。
Allwhohaveoccupiedthemselveswiththestructureof<i>Anchitherium，</i>fromCuvieronwards，haveacknowledgeditsmanypointsoflikenesstoawell-knowngenusofextinctEocenemammals，<i>Palaeotherium。</i>Indeed，aswehaveseen，Cuvierregardedhisremainsof<i>Anchitherium</i>asthoseofaspeciesof<i>Palaeotherium。</i>Hence，inattemptingtotracethepedigreeofthehorsebeyondtheMioceneepochandtheAnchitheroidform，InaturallysoughtamongthevariousspeciesofPalaeotheroidanimalsforitsnearestally，andIwasledtoconcludethatthe<i>Palaeotheriumminus（Plagiolophus）</i>
representedthenextstepmorenearlythananyformthenknown。
Ithinkthatthisopinionwasfullyjustifiable；buttheprogressofinvestigationhasthrownanunexpectedlightonthequestion，andhasbroughtusmuchnearerthancouldhavebeenanticipatedtoaknowledgeofthetrueseriesoftheprogenitorsofthehorse。
Youareallawarethat，whenyourcountrywasfirstdiscoveredbyEuropeans，therewerenotracesoftheexistenceofthehorseinanypartoftheAmericanContinent。TheaccountsoftheconquestofMexicodwellupontheastonishmentofthenativesofthatcountrywhentheyfirstbecameacquaintedwiththatastoundingphenomenon——amanseateduponahorse。
Nevertheless，theinvestigationsofAmericangeologistshaveprovedthattheremainsofhorsesoccurinthemostsuperficialdepositsofbothNorthandSouthAmerica，justastheydoinEurope。Therefore，forsomereasonorother——nofeasiblesuggestiononthatsubject，sofarasIknow，hasbeenmade——thehorsemusthavediedoutonthiscontinentatsomeperiodprecedingthediscoveryofAmerica。OflateyearstherehasbeendiscoveredinyourWesternTerritoriesthatmarvellousaccumulationofdeposits，admirablyadaptedforthepreservationoforganicremains，towhichIreferredtheotherevening，andwhichfurnishesuswithaconsecutiveseriesofrecordsofthefaunaoftheolderhalfoftheTertiaryepoch，forwhichwehavenoparallelinEurope。Theyhaveyieldedfossilsinanexcellentstateofconservationandinunexamplednumberandvariety。
TheresearchesofLeidyandothershaveshownthatformsalliedtothe<i>Hipparion</i>andthe<i>Anchitherium</i>aretobefoundamongtheseremains。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（<i>Pliohippus</i>）；intheconformationofitslimbsitpresentssomeveryslightdeviationsfromtheordinaryhorse，andthecrownsofthegrindingteethareshorter。Thencomesthe<i>Protohippus，</i>whichrepresentstheEuropean<i>Hipparion，</i>havingonelargedigitandtwosmallonesoneachfoot，andthegeneralcharactersofthefore-armandlegtowhichIhavereferred。ButitismorevaluablethantheEuropean<i>Hipparion</i>forthereasonthatitisdevoidofsomeofthepeculiaritiesofthatform——peculiaritieswhichtendtoshowthattheEuropean<i>Hipparion</i>isratheramemberofacollateralbranch，thanaforminthedirectlineofsuccession。
Next，inthebackwardorderintime，isthe<i>Miohippus，</i>
whichcorrespondsprettynearlywiththe<i>Anchitherium</i>ofEurope。Itpresentsthreecompletetoes——onelargemedianandtwosmallerlateralones；andthereisarudimentofthatdigit，whichanswerstothelittlefingerofthehumanhand。
TheEuropeanrecordofthepedigreeofthehorsestopshere；
intheAmericanTertiaries，onthecontrary，theseriesofancestralequineformsiscontinuedintotheEoceneformations。
AnolderMioceneform，termed<i>Mesohippus，</i>hasthreetoesinfront，withalargesplint-likerudimentrepresentingthelittlefinger；andthreetoesbehind。Theradiusandulna，thetibiaandthefibula，aredistinct，andtheshortcrownedmolarteethareanchitherioidinpattern。
Butthemostimportantdiscoveryofallisthe<i>Orohippus，</i>
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；<4>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
takethatasameasureofthedurationoftheevolutionofthehorsefromthe<i>Orohippus</i>uptoitspresentcondition。
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，suggestthat<i>Hesperornis</i>maybeamodificationofalessspecialisedgroupofbirdsthanthattowhichtheseexistingaquaticbirdsbelong。
（2）Asecondspecimen，discoveredin1877，andatpresentintheBerlinmuseum，showsanexcellentlypreservedskullwithteeth；
andthreedigits，allterminatedbyclaws，intheforelimb。
1893。
（3）Iusetheword"type"becauseitishighlyprobablethatmanyformsof<i>Anchitherium-</i>likeand<i>Hipparion-</i>likeanimalsexistedintheMioceneandPlioceneepochs，justasmanyspeciesofthehorsetribeexistnow，anditishighlyimprobablethattheparticularspeciesof<i>Anchitherium</i>or<i>Hipparion，</i>whichhappentohavebeendiscovered，shouldbepreciselythosewhichhaveformedpartofthedirectlineofthehorse’spedigree。
（4）Sincethislecturewasdelivered，ProfessorMarshhasdiscoveredanewgenusofequinemammals（<i>Eohippus</i>）fromthelowestEocenedepositsoftheWest，whichcorrespondsverynearlytothisdescription。——<i>AmericanJournalofScience，</i>
November，1876。
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