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。