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。