If，however，theconditionoftheworld，whichhasobtainedthroughoutgeologictime，isbutthesequeltoavastseriesofchangeswhichtookplaceinpre-geologictime，thenitseemsnotunlikelythatthedurationofthislatteristothatoftheformerasthevastextentofgeologictimeistothelengthofthebriefepochwecallthehistoricalperiod；andthateventheoldestrocksarerecordsofanepochalmostinfinitelyremotefromthatwhichcouldhavewitnessedthefirstshapingofourglobe。
Itisprobablethatnomoderngeologistwouldhesitatetoadmitthegeneralvalidityofthesereasoningswhenappliedtothephysicsofhissubject，whenceitisthemoreremarkablethatthemomentthequestionchangesfromoneofphysicsandchemistrytooneofnaturalhistory，scientificopinionsandthepopularprejudices，whichreflecttheminadistortedform，undergoasuddenmetamorphosis。Geologistsandpalaeontologistswriteaboutthe\"beginningoflife\"andthe\"first-createdformsoflivingbeings，\"asiftheywerethemostfamiliarthingsintheworld；andevencautiouswritersseemtobeonquitefriendlytermswiththe\"archetype\"wherebytheCreatorwasguided\"amidstthecrashoffallingworlds。\"Justasitusedtobeimaginedthattheancientworldwasphysicallyopposedtothepresent，soitisstillwidelyassumedthatthelivingpopulationofourglobe，whetheranimalorvegetable，intheolderepochs，exhibitedformssostrikinglycontrastedwiththosewhichweseearoundus，thatthereishardlyanythingincommonbetweenthetwo。Itisconstantlytacitlyassumedthatwehavebeforeusalltheformsoflifewhichhaveeverexisted；andthoughtheprogressofknowledge，yearlyandalmostmonthly，drivesthedefendersofthatpositionfromtheirground，theyentrenchthemselvesinthenewlineofdefencesasifnothinghadhappened，andproclaimthatthe\'new\'beginningisthe\'real\'
beginning。
Withoutforaninstantdenyingorendeavouringtosoftendowntheconsiderablepositivedifferences（thenegativeonesaremetbyanotherlineofargument）whichundoubtedlyobtainbetweentheancientandthemodernworldsoflife，webelievetheyhavebeenvastlyoverstatedandexaggerated，andthisbeliefisbaseduponcertainfactswhosevaluedoesnotseemtohavebeenfullyappreciated，thoughtheyhavelongbeenmoreorlesscompletelyknown。
Themultitudinouskindsofanimalsandplants，bothrecentandfossil，are，asiswellknown，arrangedbyzoologistsandbotanists，inaccordancewiththeirnaturalrelations，intogroupswhichreceivethenamesofsub-kingdoms，classes，orders，families，generaandspecies。
Nowitisamostremarkablecircumstancethat，viewedonthegreatscale，livingbeingshavedifferedsolittlethroughoutallgeologictimethatthereisnosub-kingdomandnoclasswhollyextinctorwithoutlivingrepresentatives。
Ifwedescendtothesmallergroups，wefindthatthenumberofordersofplantsisabouttwohundred；andIhaveitonthebestauthoritythatnotoneoftheseisexclusivelyfossil；sothatthereisabsolutelynotasingleextinctordinaltypeofvegetablelife；anditisnotuntilwedescendtothenextgroup，orthefamilies，thatwefindtypeswhicharewhollyextinct。Thenumberofordersofanimals，ontheotherhand，maybereckonedatahundredandtwenty，orthereabouts，andofthese，eightorninehavenolivingrepresentatives。
Theproportionofextinctordinaltypesofanimalstotheexistingtypes，therefore，doesnotexceedsevenpercent——amarvellouslysmallproportionwhenweconsiderthevastnessofgeologictime。
Anotherclassofconsiderations——ofadifferentkind，itistrue，buttendinginthesamedirection——seemstohavebeenoverlooked。Notonlyisittruethatthegeneralplanofconstructionofanimalsandplantshasbeenthesameinallrecordedtimeasatpresent，butthereareparticularkindsofanimalsandplantswhichhaveexistedthroughoutvastepochs，sometimesthroughthewholerangeofrecordedtime，withverylittlechange。Byreasonofthispersistency，thetypicalformofsuchakindmightbecalleda\"persistenttype，\"incontradistinctiontothosetypeswhichhaveappearedforbutashorttimeinthecourseoftheworld\'shistory。Examplesofthesepersistenttypesareabundantenoughinboththevegetableandtheanimalkingdoms。Theoldestgroupofplantswithwhichwearewellacquaintedisthatofwhoseremainscoalisconstituted；andasfarastheycanbeidentified，thecarboniferousplantsareferns，orclub-mosses，orConiferae，inmanycasesgenericallyidenticalwiththosenowliving！
Amonganimals，instancesofthesamekindmaybefoundineverysub-kingdom。The\'Globigerina\'oftheAtlanticsoundingsisidenticalwiththatwhichoccursinthechalk；andthecastsoflowersilurian\'Foraminifera\'，whichEhrenberghasrecentlydescribed，seemtoindicatetheexistenceatthatremoteperiodofformssingularlylikethosewhichnowexist。Amongthecorals，thepalaeozoic\'Tabulata\'areconstructedonpreciselythesametypeasthemodernmillepores；andifweturntomolluscs，themostcompetentmalacologistsfailtodiscoveranygenericdistinctionbetweenthe\'Craniae\'，\'Lingulae\'and\'Discinae\'ofthesilurianrocksandthosewhichnowlive。Ourexisting\'Nautilus\'hasitsrepresentativespeciesineverygreatformation，fromtheoldesttothenewest；and\'Loligo\'，thesquidofmodernseas，appearsinthelias，oratthebottomofthemesozoicseries，inaform，atmost，specificallydifferentfromitslivingcongeners。Inthegreatassemblageofannuloseanimals，thetwohighestclasses，theinsectsandspidertribe，exhibitawonderfulpersistencyoftype。Thecockroachesofthecarboniferousepochareexceedinglysimilartothosewhichnowrunaboutourcoal-cellars；anditslocusts，termitesanddragon-fliesarecloselyalliedtothemembersofthesamegroupswhichnowchirrupaboutourfields，undermineourhouses，orsailwithswiftgraceaboutthebanksofoursedgypools。And，inlikemanner，thepalaeozoicscorpionscanonlybedistinguishedbytheeyeofanaturalistfromthemodernones。
Finally，withrespecttothe\'Vertebrata\'，thesamelawholdsgood:
certaintypes，suchasthoseoftheganoidandplacoidfishes，havingpersistedfromthepalaeozoicepochtothepresenttimewithoutagreateramountofdeviationfromthenormalstandardthanthatwhichisseenwithinthelimitsofthegroupasitnowexists。Evenamongthe\'Reptilia\'——theclasswhichexhibitsthelargestproportionofentirelyextinctformsofanyonetype，——thatofthe\'Crocodilia\'，haspersistedfromatleastthecommencementoftheMesozoicepochuptothepresenttimewithsomuchconstancy，thattheamountofchangewhichitexhibitsmayfairly，inrelationtothetimewhichhaselapsed，becalledinsignificant。Andtheimperfectknowledgewehaveoftheancientmammalianpopulationofourearthleadstothebeliefthatcertainofitstypes，suchasthatofthe\'Marsupialia\'，havepersistedwithcorrespondinglylittlechangethroughasimilarrangeoftime。