LECTURESONEVOLUTION
II
THEHYPOTHESISOFEVOLUTION。THENEUTRALAND
THEFAVOURABLEEVIDENCE。
IntheprecedinglectureIpointedoutthattherearethreehypotheseswhichmaybeentertained，andwhichhavebeenentertained，respectingthepasthistoryoflifeupontheglobe。
Accordingtothefirstofthesehypotheses，livingbeings，suchasnowexist，haveexistedfromalleternityuponthisearth。
Wetestedthathypothesisbythecircumstantialevidence，asI
calledit，whichisfurnishedbythefossilremainscontainedintheearth’scrust，andwefoundthatitwasobviouslyuntenable。
Ithenproceededtoconsiderthesecondhypothesis，whichI
termedtheMiltonichypothesis，notbecauseitisofanyparticularconsequencewhetherJohnMiltonseriouslyentertaineditornot，butbecauseitisstatedinaclearandunmistakablemannerinhisgreatpoem。Ipointedouttoyouthattheevidenceatourcommandascompletelyandfullynegativesthathypothesisasitdidtheprecedingone。AndIconfessthatIhadtoomuchrespectforyourintelligencetothinkitnecessarytoaddthatthenegationwasequallyclearandequallyvalid，whateverthesourcefromwhichthathypothesismightbederived，orwhatevertheauthoritybywhichitmightbesupported。Ifurtherstatedthat，accordingtothethirdhypothesis，orthatofevolution，theexistingstateofthingsisthelasttermofalongseriesofstates，which，whentracedback，wouldbefoundtoshownointerruptionandnobreachinthecontinuityofnaturalcausation。Ipropose，inthepresentandthefollowinglecture，totestthishypothesisrigorouslybytheevidenceatcommand，andtoinquirehowfarthatevidencecanbesaidtobeindifferenttoit，howfaritcanbesaidtobefavourabletoit，and，finally，howfaritcanbesaidtobedemonstrative。
Fromalmosttheoriginofthediscussionsabouttheexistingconditionoftheanimalandvegetableworldsandthecauseswhichhavedeterminedthatcondition，anargumenthasbeenputforwardasanobjectiontoevolution，whichweshallhavetoconsiderveryseriously。ItisanargumentwhichwasfirstclearlystatedbyCuvierinhiscriticismofthedoctrinespropoundedbyhisgreatcontemporary，Lamarck。TheFrenchexpeditiontoEgypthadcalledtheattentionoflearnedmentothewonderfulstoreofantiquitiesinthatcountry，andtherehadbeenbroughtbacktoFrancenumerousmummifiedcorpsesoftheanimalswhichtheancientEgyptiansreveredandpreserved，andwhich，atareasonablecomputation，musthavelivednotlessthanthreeorfourthousandyearsbeforethetimeatwhichtheywerethusbroughttolight。Cuvierendeavouredtotestthehypothesisthatanimalshaveundergonegradualandprogressivemodificationsofstructure，bycomparingtheskeletonsandsuchotherpartsofthemummiesaswereinafittingstateofpreservation，withthecorrespondingpartsoftherepresentativesofthesamespeciesnowlivinginEgypt。
Hearrivedattheconvictionthatnoappreciablechangehadtakenplaceintheseanimalsinthecourseofthisconsiderablelapseoftime，andthejusticeofhisconclusionisnotdisputed。
Itisobviousthat，ifitcanbeprovedthatanimalshaveendured，withoutundergoinganydemonstrablechangeofstructure，forsolongaperiodasfourthousandyears，noformofthehypothesisofevolutionwhichassumesthatanimalsundergoaconstantandnecessaryprogressivechangecanbetenable；unless，indeed，itbefurtherassumedthatfourthousandyearsistooshortatimefortheproductionofachangesufficientlygreattobedetected。
Butitisnolessplainthatiftheprocessofevolutionofanimalsisnotindependentofsurroundingconditions；ifitmaybeindefinitelyhastenedorretardedbyvariationsintheseconditions；orifevolutionissimplyaprocessofaccommodationtovaryingconditions；theargumentagainstthehypothesisofevolutionbasedontheunchangedcharacteroftheEgyptianfaunaisworthless。ForthemonumentswhicharecoevalwiththemummiestestifyasstronglytotheabsenceofchangeinthephysicalgeographyandthegeneralconditionsofthelandofEgypt，forthetimeinquestion，asthemummiesdototheunvaryingcharactersofitslivingpopulation。
TheprogressofresearchsinceCuvier’stimehassuppliedfarmorestrikingexamplesofthelongdurationofspecificformsoflifethanthosewhicharefurnishedbythemummifiedIbisesandCrocodilesofEgypt。Aremarkablecaseistobefoundinyourowncountry，intheneighbourhoodofthefallsofNiagara。
Intheimmediatevicinityofthewhirlpool，andagainuponGoatIsland，inthesuperficialdepositswhichcoverthesurfaceoftherockysubsoilinthoseregions，therearefoundremainsofanimalsinperfectpreservation，andamongthem，shellsbelongingtoexactlythesamespeciesasthosewhichatpresentinhabitthestillwatersofLakeErie。Itisevident，fromthestructureofthecountry，thattheseanimalremainsweredepositedinthebedsinwhichtheyoccuratatimewhenthelakeextendedovertheregioninwhichtheyarefound。ThisinvolvestheconclusionthattheylivedanddiedbeforethefallshadcuttheirwaybackthroughthegorgeofNiagara；
and，indeed，ithasbeendeterminedthat，whentheseanimalslived，thefallsofNiagaramusthavebeenatleastsixmilesfurtherdowntheriverthantheyareatpresent。
Manycomputationshavebeenmadeoftherateatwhichthefallsarethuscuttingtheirwayback。Thosecomputationshavevariedgreatly，butIbelieveIamspeakingwithintheboundsofprudence，ifIassumethatthefallsofNiagarahavenotretreatedatagreaterpacethanaboutafootayear。Sixmiles，speakingroughly，are30，000feet；30，000feet，atafootayear，gives30，000years；andthuswearefairlyjustifiedinconcludingthatnolessaperiodthanthishaspassedsincetheshell-fish，whoseremainsareleftinthebedstowhichIhavereferred，werelivingcreatures。
Butthereisstillstrongerevidenceofthelongdurationofcertaintypes。Ihavealreadystatedthat，asweworkourwaythroughthegreatseriesoftheTertiaryformations，wefindmanyspeciesofanimalsidenticalwiththosewhichliveatthepresentday，diminishinginnumbers，itistrue，butstillexisting，inacertainproportion，intheoldestoftheTertiaryrocks。Furthermore，whenweexaminetherocksoftheCretaceousepoch，wefindtheremainsofsomeanimalswhichtheclosestscrutinycannotshowtobe，inanyimportantrespect，differentfromthosewhichliveatthepresenttime。Thatisthecasewithoneofthecretaceouslamp-shells（Terebratula），whichhascontinuedtoexistunchanged，orwithinsignificantvariations，downtothepresentday。SuchisthecasewiththeGlobigerin？，theskeletonsofwhich，aggregatedtogether，formalargeproportionofourEnglishchalk。
ThoseGlobigerinaecanbetraceddowntotheGlobigerinaewhichliveatthesurfaceofthepresentgreatoceans，andtheremainsofwhich，fallingtothebottomofthesea，giverisetoachalkymud。Henceitmustbeadmittedthatcertainexistingspeciesofanimalsshownodistinctsignofmodification，ortransformation，inthecourseofalapseoftimeasgreatasthatwhichcarriesusbacktotheCretaceousperiod；andwhich，whateveritsabsolutemeasure，iscertainlyvastlygreaterthanthirtythousandyears。
Therearegroupsofspeciessocloselyalliedtogether，thatitneedstheeyeofanaturalisttodistinguishthemonefromanother。Ifwedisregardthesmalldifferenceswhichseparatetheseforms，andconsiderallthespeciesofsuchgroupsasmodificationsofonetype，weshallfindthat，evenamongthehigheranimals，sometypeshavehadamarvellousduration。
Inthechalk，forexample，thereisfoundafishbelongingtothehighestandthemostdifferentiatedgroupofosseousfishes，whichgoesbythenameofBeryx。Theremainsofthatfishareamongthemostbeautifulandwell-preservedofthefossilsfoundinourEnglishchalk。Itcanbestudiedanatomically，sofarasthehardpartsareconcerned，almostaswellasifitwerearecentfish。ButthegenusBeryxisrepresented，atthepresentday，byverycloselyalliedspecieswhicharelivinginthePacificandAtlanticOceans。Wemaygostillfartherback。IhavealreadyreferredtothefactthattheCarboniferousformations，inEuropeandinAmerica，containtheremainsofscorpionsinanadmirablestateofpreservation，andthatthosescorpionsarehardlydistinguishablefromsuchasnowlive。Idonotmeantosaythattheyarenotdifferent，butclosescrutinyisneededinordertodistinguishthemfrommodernscorpions。
Morethanthis。AttheverybottomoftheSilurianseries，inbedswhicharebysomeauthoritiesreferredtotheCambrianformation，wherethesignsoflifebegintofailus——eventhere，amongthefewandscantyanimalremainswhicharediscoverable，wefindspeciesofmolluscousanimalswhicharesocloselyalliedtoexistingformsthat，atonetime，theyweregroupedunderthesamegenericname。Irefertothewell-knownLingulaoftheLingulaflags，lately，inconsequenceofsomeslightdifferences，placedinthenewgenusLingulella。Practically，itbelongstothesamegreatgenericgroupastheLingula，whichistobefoundatthepresentdayuponyourownshoresandthoseofmanyotherpartsoftheworld。
Thesametruthisexemplifiedifweturntocertaingreatperiodsoftheearth’shistory——as，forexample，theMesozoicepoch。Therearegroupsofreptiles，suchastheIchthyosauriaandthePlesiosauria，whichappearshortlyafterthecommencementofthisepoch，andtheyoccurinvastnumbers。Theydisappearwiththechalkand，throughoutthewholeofthegreatseriesofMesozoicrocks，theypresentnosuchmodificationsascansafelybeconsideredevidenceofprogressivemodification。
Factsofthiskindareundoubtedlyfataltoanyformofthedoctrineofevolutionwhichpostulatesthesuppositionthatthereisanintrinsicnecessity，onthepartofanimalformswhichhaveoncecomeintoexistence，toundergocontinualmodification；andtheyareasdistinctlyopposedtoanyviewwhichinvolvesthebelief，thatsuchmodificationmayoccur，musttakeplace，atthesamerate，inallthedifferenttypesofanimalorvegetablelife。Thefacts，asIhaveplacedthembeforeyou，obviouslydirectlycontradictanyformofthehypothesisofevolutionwhichstandsinneedofthesetwopostulates。
But，onegreatservicethathasbeenrenderedbyMr。Darwintothedoctrineofevolutioningeneralisthis：hehasshownthattherearetwochieffactorsintheprocessofevolution：oneofthemisthetendencytovary，theexistenceofwhichinalllivingformsmaybeprovedbyobservation；theotheristheinfluenceofsurroundingconditionsuponwhatImaycalltheparentformandthevariationswhicharethusevolvedfromit。
Thecauseoftheproductionofvariationsisamatternotatallproperlyunderstoodatpresent。Whethervariationdependsuponsomeintricatemachinery——ifImayusethephrase——ofthelivingorganismitself，orwhetheritarisesthroughtheinfluenceofconditionsuponthatform，isnotcertain，andthequestionmay，forthepresent，beleftopen。Buttheimportantpointisthat，grantingtheexistenceofthetendencytotheproductionofvariations；then，whetherthevariationswhichareproducedshallsurviveandsupplanttheparent，orwhethertheparentformshallsurviveandsupplantthevariations，isamatterwhichdependsentirelyonthoseconditionswhichgiverisetothestruggleforexistence。Ifthesurroundingconditionsaresuchthattheparentformismorecompetenttodealwiththem，andflourishinthemthanthederivedforms，then，inthestruggleforexistence，theparentformwillmaintainitselfandthederivedformswillbeexterminated。Butif，onthecontrary，theconditionsaresuchastobemorefavourabletoaderivedthantotheparentform，theparentformwillbeextirpatedandthederivedformwilltakeitsplace。Inthefirstcase，therewillbenoprogression，nochangeofstructure，throughanyimaginableseriesofages；inthesecondplacetherewillbemodificationofchangeandform。
Thustheexistenceofthesepersistenttypes，asIhavetermedthem，isnorealobstacleinthewayofthetheoryofevolution。
TakethecaseofthescorpionstowhichIhavejustreferred。
Nodoubt，sincetheCarboniferousepoch，conditionshavealwaysobtained，suchasexistedwhenthescorpionsofthatepochflourished；conditionsinwhichscorpionsfindthemselvesbetteroff，morecompetenttodealwiththedifficultiesintheirway，thananyvariationfromthescorpiontypewhichtheymayhaveproduced；and，forthatreason，thescorpiontypehaspersisted，andhasnotbeensupplantedbyanyotherform。Andthereisnoreason，inthenatureofthings，why，aslongasthisworldexists，iftherebeconditionsmorefavourabletoscorpionsthantoanyvariationwhichmayarisefromthem，theseformsoflifeshouldnotpersist。
Therefore，thestockobjectiontothehypothesisofevolution，basedonthelongdurationofcertainanimalandvegetabletypes，isnoobjectionatall。Thefactsofthischaracter——andtheyarenumerous——belongtothatclassofevidencewhichIhavecalledindifferent。Thatistosay，theymayaffordnodirectsupporttothedoctrineofevolution，buttheyarecapableofbeinginterpretedinperfectconsistencywithit。
Thereisanotherorderoffactsbelongingtotheclassofnegativeorindifferentevidence。ThegreatgroupofLizards，whichaboundinthepresentworld，extendsthroughthewholeseriesofformationsasfarbackasthePermian，orlatestPalaeozoic，epoch。ThesePermianlizardsdifferastonishinglylittlefromthelizardswhichexistatthepresentday。
Comparingtheamountofthedifferencesbetweenthemandmodernlizards，withtheprodigiouslapseoftimebetweenthePermianepochandthepresentday，itmaybesaidthattheamountofchangeisinsignificant。But，whenwecarryourresearchesfartherbackintime，wefindnotraceoflizards，norofanytruereptilewhatever，inthewholemassofformationsbeneaththePermian。
Now，itisperfectlyclearthatifourpalaeontologicalcollectionsaretobetaken，evenapproximately，asanadequaterepresentationofalltheformsofanimalsandplantsthathaveeverlived；andiftherecordfurnishedbytheknownseriesofbedsofstratifiedrockcoversthewholeseriesofeventswhichconstitutethehistoryoflifeontheglobe，suchafactasthisdirectlycontravenesthehypothesisofevolution；becausethishypothesispostulatesthattheexistenceofeveryformmusthavebeenprecededbythatofsomeformlittledifferentfromit。
Here，however，wehavetotakeintoconsiderationthatimportanttruthsowellinsisteduponbyLyellandbyDarwin——theimperfectionofthegeologicalrecord。Itcanbedemonstratedthatthegeologicalrecordmustbeincomplete，thatitcanonlypreserveremainsfoundincertainfavourablelocalitiesandunderparticularconditions；thatitmustbedestroyedbyprocessesofdenudation，andobliteratedbyprocessesofmetamorphosis。Bedsofrockofanythicknesscrammedfulloforganicremains，mayyet，eitherbythepercolationofwaterthroughthem，orbytheinfluenceofsubterraneanheat，losealltraceoftheseremains，andpresenttheappearanceofbedsofrockformedunderconditionsinwhichlivingformswereabsent。
Suchmetamorphicrocksoccurinformationsofallages；and，invariouscases，thereareverygoodgroundsforthebeliefthattheyhavecontainedorganicremains，andthatthoseremainshavebeenabsolutelyobliterated。
Iinsistuponthedefectsofthegeologicalrecordthemorebecausethosewhohavenotattendedtothesemattersareapttosay，\"Itisallverywell，but，whenyougetintoadifficultywithyourtheoryofevolution，youappealtotheincompletenessandtheimperfectionofthegeologicalrecord；\"andIwanttomakeitperfectlycleartoyouthatthisimperfectionisagreatfact，whichmustbetakenintoaccountinallourspeculations，orweshallconstantlybegoingwrong。
Youseethesingularseriesoffootmarks，drawnofitsnaturalsizeinthelargediagramhanginguphere（Fig。2），whichIowetothekindnessofmyfriendProfessorMarsh，withwhomIhadtheopportunityrecentlyofvisitingthepreciselocalityinMassachusettsinwhichthesetracksoccur。Iam，therefore，abletogiveyoumyowntestimony，ifneeded，thatthediagramaccuratelyrepresentswhatwesaw。ThevalleyoftheConnecticutisclassicalgroundforthegeologist。Itcontainsgreatbedsofsandstone，coveringmanysquaremiles，whichhaveevidentlyformedapartofanancientsea-shore，or，itmaybe，lake-
shore。Foracertainperiodoftimeaftertheirdeposition，thesebedshaveremainedsufficientlysofttoreceivetheimpressionsofthefeetofwhateveranimalswalkedoverthem，andtopreservethemafterwards，inexactlythesamewayassuchimpressionsareatthishourpreservedontheshoresoftheBayofFundyandelsewhere。Thediagramrepresentsthetrackofsomegiganticanimal，whichwalkedonitshindlegs。Youseetheseriesofmarksmadealternatelybytherightandbytheleftfoot；sothat，fromoneimpressiontotheotherofthethree-
toedfootonthesameside，isonestride，andthatstride，aswemeasuredit，issixfeetnineinches。Ileaveyou，therefore，toformanimpressionofthemagnitudeofthecreaturewhich，asitwalkedalongtheancientshore，madetheseimpressions。
Fig。2。——TracksofBrontozoum。
Ofsuchimpressionsthereareuntoldthousandsuponthesesandstones。Fiftyorsixtydifferentkindshavebeendiscovered，andtheycovervastareas。But，uptothispresenttime，notabone，notafragment，ofanyoneoftheanimalswhichleftthesegreatfootmarkshasbeenfound；infact，theonlyanimalremainswhichhavebeenmetwithinallthesedeposits，fromthetimeoftheirdiscoverytothepresentday——thoughtheyhavebeencarefullyhuntedover——isafragmentaryskeletonofoneofthesmallerforms。Whathasbecomeofthebonesofalltheseanimals？Youseewearenotdealingwithlittlecreatures，butwithanimalsthatmakeastepofsixfeetnineinches；andtheirremainsmusthavebeenleftsomewhere。Theprobabilityis，thattheyhavebeendissolvedaway，andcompletelylost。
Ihavehadoccasiontoworkoutthenatureoffossilremains，ofwhichtherewasnothingleftexceptcastsofthebones，thesolidmaterialoftheskeletonhavingbeendissolvedoutbypercolatingwater。Itwasachance，inthiscase，thatthesandstonehappenedtobeofsuchaconstitutionastoset，andtoallowthebonestobeafterwarddissolvedout，leavingcavitiesoftheexactshapeofthebones。Hadthatconstitutionbeenotherthanwhatitwas，theboneswouldhavebeendissolved，thelayersofsandstonewouldhavefallentogetherintoonemass，andnottheslightestindicationthattheanimalhadexistedwouldhavebeendiscoverable。
Iknowofnomorestrikingevidencethanthesefactsafford，ofthecautionwhichshouldbeusedindrawingtheconclusion，fromtheabsenceoforganicremainsinadeposit，thatanimalsorplantsdidnotexistatthetimeitwasformed。Ibelievethat，witharightunderstandingofthedoctrineofevolutionontheonehand，andajustestimationoftheimportanceoftheimperfectionofthegeologicalrecordontheother，alldifficultyisremovedfromthekindofevidencetowhichIhaveadverted；andthatwearejustifiedinbelievingthatallsuchcasesareexamplesofwhatIhavedesignatednegativeorindifferentevidence——thatistosay，theyinnowaydirectlyadvancethehypothesisofevolution，buttheyarenottoberegardedasobstaclesinthewayofourbeliefinthatdoctrine。
Inowpassontotheconsiderationofthosecaseswhich，forreasonswhichIwillpointouttoyoubyandby，arenottoberegardedasdemonstrativeofthetruthofevolution，butwhicharesuchasmustexistifevolutionbetrue，andwhichthereforeare，uponthewhole，evidenceinfavourofthedoctrine。Ifthedoctrineofevolutionbetrue，itfollows，that，howeverdiversethedifferentgroupsofanimalsandofplantsmaybe，theymustall，atonetimeorother，havebeenconnectedbygradationalforms；sothat，fromthehighestanimals，whatevertheymaybe，downtothelowestspeckofprotoplasmicmatterinwhichlifecanbemanifested，aseriesofgradations，leadingfromoneendoftheseriestotheother，eitherexistsorhasexisted。
Undoubtedlythatisanecessarypostulateofthedoctrineofevolution。ButwhenwelookuponlivingNatureasitis，wefindatotallydifferentstateofthings。Wefindthatanimalsandplantsfallintogroups，thedifferentmembersofwhichareprettycloselyalliedtogether，butwhichareseparatedbydefinite，largerorsmaller，breaks，fromothergroups。Inotherwords，nointermediateformswhichbridgeoverthesegapsorintervalsare，atpresent，tobemetwith。
ToillustratewhatImean：Letmecallyourattentiontothosevertebrateanimalswhicharemostfamiliartoyou，suchasmammals，birds，andreptiles。Atthepresentday，thesegroupsofanimalsareperfectlywell-definedfromoneanother。Weknowofnoanimalnowlivingwhich，inanysense，isintermediatebetweenthemammalandthebird，orbetweenthebirdandthereptile；but，onthecontrary，therearemanyverydistinctanatomicalpeculiarities，well-definedmarks，bywhichthemammalisseparatedfromthebird，andthebirdfromthereptile。Thedistinctionsareobviousandstrikingifyoucomparethedefinitionsofthesegreatgroupsastheynowexist。
Thesamemaybesaidofmanyofthesubordinategroups，ororders，intowhichthesegreatclassesaredivided。Atthepresenttime，forexample，therearenumerousformsofnon-
ruminantpachyderms，orwhatwemaycallbroadly，thepigtribe，andmanyvarietiesofruminants。Theselatterhavetheirdefinitecharacteristics，andtheformerhavetheirdistinguishingpeculiarities。Butthereisnothingthatfillsupthegapbetweentheruminantsandthepigtribe。Thetwoaredistinct。Suchalsoisthecaseinrespectoftheminorgroupsoftheclassofreptiles。Theexistingfaunashowsuscrocodiles，lizards，snakes，andtortoises；butnoconnectinglinkbetweenthecrocodileandlizard，norbetweenthelizardandsnake，norbetweenthesnakeandthecrocodile，norbetweenanytwoofthesegroups。Theyareseparatedbyabsolutebreaks。
If，then，itcouldbeshownthatthisstateofthingshadalwaysexisted，thefactwouldbefataltothedoctrineofevolution。
Iftheintermediategradations，whichthedoctrineofevolutionrequirestohaveexistedbetweenthesegroups，arenottobefoundanywhereintherecordsofthepasthistoryoftheglobe，theirabsenceisastrongandweightynegativeargumentagainstevolution；while，ontheotherhand，ifsuchintermediateformsaretobefound，thatissomuchtothegoodofevolution；
although，forreasonswhichIwilllaybeforeyoubyandby，wemustbecautiousinourestimateoftheevidentialcogencyoffactsofthiskind。
Itisaveryremarkablecircumstancethat，fromthecommencementoftheseriousstudyoffossilremains，infact，fromthetimewhenCuvierbeganhisbrilliantresearchesuponthosefoundinthequarriesofMontmartre，palaeontologyhasshownwhatshewasgoingtodointhismatter，andwhatkindofevidenceitlayinherpowertoproduce。
Isaidjustnowthat，intheexistingFauna，thegroupofpig-
likeanimalsandthegroupofruminantsareentirelydistinct；
butoneofthefirstofCuvier’sdiscoverieswasananimalwhichhecalledtheAnoplotherium，andwhichprovedtobe，inagreatmanyimportantrespects，intermediateincharacterbetweenthepigs，ontheonehand，andtheruminantsontheother。
Thus，researchintothehistoryofthepastdid，toacertainextent，tendtofillupthebreachbetweenthegroupofruminantsandthegroupofpigs。AnotherremarkableanimalrestoredbythegreatFrenchpalaeontologist，thePalaeotherium，similarlytendedtoconnecttogetheranimalstoallappearancesodifferentastherhinoceros，thehorse，andthetapir。Subsequentresearchhasbroughttolightmultitudesoffactsofthesameorder；andatthepresentday，theinvestigationsofsuchanatomistsasRutimeyerandGaudryhavetendedtofillup，moreandmore，thegapsinourexistingseriesofmammals，andtoconnectgroupsformerlythoughttobedistinct。
ButIthinkitmayhaveanespecialinterestif，insteadofdealingwiththeseexamples，whichwouldrequireagreatdealoftediousosteologicaldetail，Itakethecaseofbirdsandreptiles；groupswhich，atthepresentday，aresoclearlydistinguishedfromoneanotherthatthereareperhapsnoclassesofanimalswhich，inpopularapprehension，aremorecompletelyseparated。Existingbirds，asyouareaware，arecoveredwithfeathers；theiranteriorextremities，speciallyandpeculiarlymodified，areconvertedintowingsbytheaidofwhichmostofthemareabletofly；theywalkuprightupontwolegs；andtheselimbs，whentheyareconsideredanatomically，presentagreatnumberofexceedinglyremarkablepeculiarities，towhichImayhaveoccasiontoadvertincidentallyasIgoon，andwhicharenotmetwith，evenapproximately，inanyexistingformsofreptiles。Ontheotherhand，existingreptileshavenofeathers。
Theymayhavenakedskins，orbecoveredwithhornyscales，orbonyplates，orwithboth。Theypossessnowings；theyneitherflybymeansoftheirfore-limbs，norhabituallywalkuprightupontheirhind-limbs；andthebonesoftheirlegspresentnosuchmodificationsaswefindinbirds。Itisimpossibletoimagineanytwogroupsmoredefinitelyanddistinctlyseparated，notwithstandingcertaincharacterswhichtheypossessincommon。
Aswetracethehistoryofbirdsbackintime，wefindtheirremains，sometimesingreatabundance，throughoutthewholeextentofthetertiaryrocks；but，sofarasourpresentknowledgegoes，thebirdsofthetertiaryrocksretainthesameessentialcharactersasthebirdsofthepresentday。Inotherwords，thetertiarybirdscomewithinthedefinitionoftheclassconstitutedbyexistingbirds，andareasmuchseparatedfromreptilesasexistingbirdsare。Notverylongagonoremainsofbirdshadbeenfoundbelowthetertiaryrocks，andI
amnotsurebutthatsomepersonswerepreparedtodemonstratethattheycouldnothaveexistedatanearlierperiod。But，inthecourseofthelastfewyears，suchremainshavebeendiscoveredinEngland；though，unfortunately，insoimperfectandfragmentaryacondition，thatitisimpossibletosaywhethertheydifferedfromexistingbirdsinanyessentialcharacterornot。Inyourcountrythedevelopmentofthecretaceousseriesofrocksisenormous；theconditionsunderwhichthelatercretaceousstratahavebeendepositedarehighlyfavourabletothepreservationoforganicremains；andtheresearches，fulloflabourandrisk，whichhavebeencarriedonbyProfessorMarshinthesecretaceousrocksofWesternAmerica，haverewardedhimwiththediscoveryofformsofbirdsofwhichwehadhithertonoconception。Byhiskindness，Iamenabledtoplacebeforeyouarestorationofoneoftheseextraordinarybirds，everypartofwhichcanbethoroughlyjustifiedbythemoreorlesscompleteskeletons，inaveryperfectstateofpreservation，whichhehasdiscovered。ThisHesperornis
（Fig。3），whichmeasuredbetweenfiveandsixfeetinlength，isastonishinglylikeourexistingdiversorgrebesinagreatmanyrespects；solikethemindeedthat，hadtheskeletonofHesperornisbeenfoundinamuseumwithoutitsskull，itprobablywouldhavebeenplacedinthesamegroupofbirdsasthediversandgrebesofthepresentday。<1>
ButHesperornisdiffersfromallexistingbirds，andsofarresemblesreptiles，inoneimportantparticular——itisprovidedwithteeth。Thelongjawsarearmedwithteethwhichhavecurvedcrownsandthickroots（Fig。4），andarenotsetindistinctsockets，butarelodgedinagroove。Inpossessingtrueteeth，theHesperornisdiffersfromeveryexistingbird，andfromeverybirdyetdiscoveredinthetertiaryformations，thetooth-likeserrationsofthejawsintheOdontopteryx
oftheLondonclaybeingmereprocessesofthebonysubstanceofthejaws，andnotteethinthepropersenseoftheword。Inviewofthecharacteristicsofthisbirdwearethereforeobligedtomodifythedefinitionsoftheclassesofbirdsandreptiles。
BeforethediscoveryofHesperornis，thedefinitionoftheclassAvesbaseduponourknowledgeofexistingbirdsmighthavebeenextendedtoallbirds；itmighthavebeensaidthattheabsenceofteethwascharacteristicoftheclassofbirds；
butthediscoveryofananimalwhich，ineverypartofitsskeleton，closelyagreeswithexistingbirds，andyetpossessesteeth，showsthattherewereancientbirdswhich，inrespectofpossessingteeth，approachedreptilesmorenearlythananyexistingbirddoes，and，tothatextent，diminishesthehiatusbetweenthetwoclasses。
Fig。3——Hesperornisregalis（Marsh）
Fig。4——Hesperornisregalis（Marsh）
（Sideandupperviewsofhalfthelowerjaw；sideandendviewsofavertebraandaseparatetooth。）
Thesameformationhasyieldedanotherbird，Ichthyornis
（Fig。5），whichalsopossessesteeth；buttheteetharesituatedindistinctsockets，whilethoseofHesperornisarenotsolodged。Thelatteralsohassuchverysmall，almostrudimentarywings，thatitmusthavebeenchieflyaswimmerandadiverlikeaPenguin；whileIchthyornishasstrongwingsandnodoubtpossessedcorrespondingpowersofflight。
Ichthyornisalsodifferedinthefactthatitsvertebraehavenotthepeculiarcharactersofthevertebraeofexistingandofallknowntertiarybirds，butwereconcaveateachend。
Thisdiscoveryleadsustomakeafurthermodificationinthedefinitionofthegroupofbirds，andtopartwithanotherofthecharactersbywhichalmostallexistingbirdsaredistinguishedfromreptiles。
Figure。5——IchthyornisDispar（Marsh）。
Sideandupperviewsofhalfthelowerjaw；andsideandendviewsofavertebra。）
ApartfromthefewfragmentaryremainsfromtheEnglishgreensand，towhichIhavereferred，theMesozoicrocks，olderthanthoseinwhichHesperornisandIchthyornis
havebeendiscovered，haveaffordednocertainevidenceofbirds，withtheremarkableexceptionoftheSolenhofenslates。
Theseso-calledslatesarecomposedofafinegrainedcalcareousmudwhichhashardenedintolithographicstone，andinwhichorganicremainsarealmostaswellpreservedastheywouldbeiftheyhadbeenimbeddedinsomuchplasterofParis。TheyhaveyieldedtheArchaeopteryx，theexistenceofwhichwasfirstmadeknownbythefindingofafossilfeather，orratheroftheimpressionofone。Itiswonderfulenoughthatsuchaperishablethingasafeather，andnothingmore，shouldbediscovered；yet，foralongtime，nothingwasknownofthisbirdexceptitsfeather。ButbyandbyasolitaryskeletonwasdiscoveredwhichisnowintheBritishMuseum。Theskullofthissolitaryspecimenisunfortunatelywanting，anditisthereforeuncertainwhethertheArchaeopteryxpossessedteethornot。<2>Buttheremainderoftheskeletonissowellpreservedastoleavenodoubtrespectingthemainfeaturesoftheanimal，whichareverysingular。Thefeetarenotonlyaltogetherbird-
like，buthavethespecialcharactersofthefeetofperchingbirds，whilethebodyhadaclothingoftruefeathers。
Nevertheless，insomeotherrespects，Archaeopteryxisunlikeabirdandlikeareptile。Thereisalongtailcomposedofmanyvertebrae。Thestructureofthewingdiffersinsomeveryremarkablerespectsfromthatwhichitpresentsinatruebird。Inthelatter，theendofthewinganswerstothethumbandtwofingersofmyhand；butthemetacarpalbones，orthosewhichanswertothebonesofthefingerswhichlieinthepalmofthehand，arefusedtogetherintoonemass；andthewholeapparatus，exceptthelastjointsofthethumb，isboundupinasheathofintegument，whiletheedgeofthehandcarriestheprincipalquill-feathers。IntheArchaeopteryx，theupper-armboneislikethatofabird；andthetwobonesoftheforearmaremoreorlesslikethoseofabird，butthefingersarenotboundtogether——theyarefree。Whattheirnumbermayhavebeenisuncertain；butseveral，ifnotall，ofthemwereterminatedbystrongcurvedclaws，notlikesuchasaresometimesfoundinbirds，butsuchasreptilespossess；sothat，intheArchaeopteryx，wehaveananimalwhich，toacertainextent，occupiesamidwayplacebetweenabirdandareptile。Itisabirdsofarasitsfootandsundryotherpartsofitsskeletonareconcerned；itisessentiallyandthoroughlyabirdbyitsfeathers；butitismuchmoreproperlyareptileinthefactthattheregionwhichrepresentsthehandhasseparatebones，withclawsresemblingthosewhichterminatetheforelimbofareptile。Moreover，ithasalongreptile-liketailwithafringeoffeathersoneachside；while，inalltruebirdshithertoknown，thetailisrelativelyshort，andthevertebraewhichconstituteitsskeletonaregenerallypeculiarlymodified。
LiketheAnoplotheriumandthePalaeotherium，
therefore，Archaeopteryxtendstofilluptheintervalbetweengroupswhich，intheexistingworld，arewidelyseparated，andtodestroythevalueofthedefinitionsofzoologicalgroupsbaseduponourknowledgeofexistingforms。
Andsuchcasesastheseconstituteevidenceinfavourofevolution，insofarastheyprovethat，informerperiodsoftheworld’shistory，therewereanimalswhichoversteppedtheboundsofexistinggroups，andtendedtomergethemintolargerassemblages。Theyshowthatanimalorganisationismoreflexiblethanourknowledgeofrecentformsmighthaveledustobelieve；
andthatmanystructuralpermutationsandcombinations，ofwhichthepresentworldgivesusnoindication，mayneverthelesshaveexisted。
Butitbynomeansfollows，becausethePalaeotheriumhasmuchincommonwiththehorse，ontheonehand，andwiththerhinocerosontheother，thatitistheintermediateformthroughwhichrhinoceroseshavepassedtobecomehorses，orviceversa；onthecontrary，anysuchsuppositionwouldcertainlybeerroneous。NordoIthinkitlikelythatthetransitionfromthereptiletothebirdhasbeeneffectedbysuchaformasArchaeopteryx。Anditisconvenienttodistinguishtheseintermediateformsbetweentwogroups，whichdonotrepresenttheactualpassagefromtheonegrouptotheother，asintercalarytypes，fromthoselinear
typeswhich，moreorlessapproximately，indicatethenatureofthestepsbywhichthetransitionfromonegrouptotheotherwaseffected。
Iconceivethatsuchlinearforms，constitutingaseriesofnaturalgradationsbetweenthereptileandthebird，andenablingustounderstandthemannerinwhichthereptilianhasbeenmetamorphosedintothebirdtype，arereallytobefoundamongagroupofancientandextinctterrestrialreptilesknownastheOrnithoscelida。Theremainsoftheseanimalsoccurthroughouttheseriesofmesozoicformations，fromtheTriastotheChalk，andthereareindicationsoftheirexistenceeveninthelaterPalaeozoicstrata。
Mostofthesereptiles，atpresentknown，areofgreatsize，somehavingattainedalengthoffortyfeetorperhapsmore。
Themajorityresembledlizardsandcrocodilesintheirgeneralform，andmanyofthemwere，likecrocodiles，protectedbyanarmourofheavybonyplates。But，inothers，thehindlimbselongateandtheforelimbsshorten，untiltheirrelativeproportionsapproachthosewhichareobservedintheshort-
winged，flightless，ostrichtribeamongbirds。
Theskullisrelativelylight，andinsomecasesthejaws，thoughbearingteeth，arebeak-likeattheirextremitiesandappeartohavebeenenvelopedinahornysheath。Inthepartofthevertebralcolumnwhichliesbetweenthehaunchbonesandiscalledthesacrum，anumberofvertebraemayunitetogetherintoonewhole，andinthisrespect，asinsomedetailsofitsstructure，thesacrumofthesereptilesapproachesthatofbirds。
Butitisinthestructureofthepelvisandofthehindlimbthatsomeoftheseancientreptilespresentthemostremarkableapproximationtobirds，andclearlyindicatethewaybywhichthemostspecialisedandcharacteristicfeaturesofthebirdmayhavebeenevolvedfromthecorrespondingpartsofthereptile。
InFig。6，thepelvisandhindlimbsofacrocodile，athree-
toedbird，andanornithoscelidanarerepresentedsidebyside；
and，forfacilityofcomparison，incorrespondingpositions；
butitmustberecollectedthat，whilethepositionofthebird’slimbisnatural，thatofthecrocodileisnotso。Inthebird，thethighboneliesclosetothebody，andthemetatarsalbonesofthefoot（ii。，iii。，iv。，Fig。6）are，ordinarily，raisedintoamoreorlessverticalposition；inthecrocodile，thethighbonestandsoutatananglefromthebody，andthemetatarsalbones（i。，ii。，iii。，iv。，Fig。6）lieflatontheground。Hence，inthecrocodile，thebodyusuallyliessquatbetweenthelegs，while，inthebird，itisraiseduponthehindlegs，asuponpillars。
Inthecrocodile，thepelvisisobviouslycomposedofthreebonesoneachside：theilium（Il。），thepubis（Pb。），andtheischium（Is。）。Intheadultbirdthereappearstobebutoneboneoneachside。Theexaminationofthepelvisofachick，however，showsthateachhalfismadeupofthreebones，whichanswertothosewhichremaindistinctthroughoutlifeinthecrocodile。Thereis，therefore，afundamentalidentityofplanintheconstructionofthepelvisofbothbirdandreptile；thoughthedifferenceinform，relativesize，anddirectionofthecorrespondingbonesinthetwocasesareverygreat。
Butthemoststrikingcontrastbetweenthetwoliesinthebonesofthelegandofthatpartofthefoottermedthetarsus，whichfollowsupontheleg。Inthecrocodile，thefibula（F）isrelativelylargeanditslowerendiscomplete。Thetibia（T）hasnomarkedcrestatitsupperend，anditslowerendisnarrowandnotpulley-shaped。Therearetworowsofseparatetarsalbones（As。，Ca。，&c。）andfourdistinctmetatarsalbones，witharudimentofafifth。
Inthebird，thefibulaissmallanditslowerenddiminishestoapoint。Thetibiahasastrongcrestatitsupperendanditslowerextremitypassesintoabroadpulley。Thereseematfirsttobenotarsalbones；andonlyonebone，dividedattheendintothreeheadsforthethreetoeswhichareattachedtoit，appearsintheplaceofthemetatarsus。
Intheyoungbird，however，thepulley-shapedapparentendofthetibiaisadistinctbone，whichrepresentsthebonesmarkedAs。，Ca。，inthecrocodile；whiletheapparentlysinglemetatarsalboneconsistsofthreebones，whichearlyunitewithoneanotherandwithanadditionalbone，whichrepresentsthelowerrowofbonesinthetarsusofthecrocodile。
Inotherwords，itcanbeshownbythestudyofdevelopmentthatthebird’spelvisandhindlimbaresimplyextrememodificationsofthesamefundamentalplanasthatuponwhichthesepartsaremodelledinreptiles。
Oncomparingthepelvisandhindlimboftheornithoscelidanwiththatofthecrocodile，ontheoneside，andthatofthebird，ontheother（Fig。6），itisobviousthatitrepresentsamiddletermbetweenthetwo。Thepelvicbonesapproachtheformofthoseofthebirds，andthedirectionofthepubisandischiumisnearlythatwhichischaracteristicofbirds；
thethighbone，fromthedirectionofitshead，musthavelainclosetothebody；thetibiahasagreatcrest；and，immovablyfittedontoitslowerend，thereisapulley-shapedbone，likethatofthebird，butremainingdistinct。Thelowerendofthefibulaismuchmoreslender，proportionally，thaninthecrocodile。Themetatarsalboneshavesuchaformthattheyfittogetherimmovably，thoughtheydonotenterintobonyunion；
thethirdtoeis，asinthebird，longestandstrongest。
Infact，theornithoscelidanlimbiscomparabletothatofanunhatchedchick。
Fig。6。——Bird。Ornithoscelidan。Crocodile。
Thelettershavethesamesignificationinallthefigures。
Il。，Ilium；a。anteriorend；b。posteriorend；Ia。ischium；Pb。，pubis；T，tibia；
F，fibula；As。，astragalus；Ca。，calcaneum；
I，distalportionofthetarsus；i。，ii。，iii。，iv。，metatarsalbones。