Donotallowyourselvestobemisledbythecommonnotionthatanhypothesisisuntrustworthysimplybecauseitisanhypothesis。Itisoftenurged，inrespecttosomescientificconclusion，that，afterall，itisonlyanhypothesis。Butwhatmorehavewetoguideusinnine—tenthsofthemostimportantaffairsofdailylifethanhypotheses，andoftenveryill—basedones？Sothatinscience，wheretheevidenceofanhypothesisissubjectedtothemostrigidexamination，wemayrightlypursuethesamecourse。Youmayhavehypothesesandhypotheses。Amanmaysay，ifhelikes，thatthemoonismadeofgreencheese：thatisanhypothesis。Butanotherman，whohasdevotedagreatdealoftimeandattentiontothesubject，andavailedhimselfofthemostpowerfultelescopesandtheresultsoftheobservationsofothers，declaresthatinhisopinionitisprobablycomposedofmaterialsverysimilartothoseofwhichourownearthismadeup：andthatisalsoonlyanhypothesis。ButIneednottellyouthatthereisanenormousdifferenceinthevalueofthetwohypotheses。Thatonewhichisbasedonsoundscientificknowledgeissuretohaveacorrespondingvalue；andthatwhichisamerehastyrandomguessislikelytohavebutlittlevalue。EverygreatstepinourprogressindiscoveringcauseshasbeenmadeinexactlythesamewayasthatwhichI
　　havedetailedtoyou。Apersonobservingtheoccurrenceofcertainfactsandphenomenaasks，naturallyenough，whatprocess，whatkindofoperationknowntooccurinnatureappliedtotheparticularcase，willunravelandexplainthemystery？Henceyouhavethescientifichypothesis；anditsvaluewillbeproportionatetothecareandcompletenesswithwhichitsbasishadbeentestedandverified。Itisinthesemattersasinthecommonestaffairsofpracticallife：theguessofthefoolwillbefolly，whiletheguessofthewisemanwillcontainwisdom。Inallcases，youseethatthevalueoftheresultdependsonthepatienceandfaithfulnesswithwhichtheinvestigatorappliestohishypothesiseverypossiblekindofverification。
　　IdaresayImayhavetoreturntothispointby—and—by；buthavingdealtthusfarwithourlogicalmethods，Imustnowturntosomethingwhich，perhaps，youmayconsidermoreinteresting，or，atanyrate，moretangible。Butinrealitytherearebutfewthingsthatcanbemoreimportantforyoutounderstandthanthementalprocessesandthemeansbywhichweobtainscientificconclusionsandtheories。1Havinggrantedthattheinquiryisaproperone，andhavingdeterminedonthenatureofthemethodswearetopursueandwhichonlycanleadtosuccess，Imustnowturntotheconsiderationofourknowledgeofthenatureoftheprocesseswhichhaveresultedinthepresentconditionoforganicnature。
　　Here，letmesayatonce，lestsomeofyoumisunderstandme，thatIhaveextremelylittletoreport。Thequestionofhowthepresentconditionoforganicnaturecameabout，resolvesitselfintotwoquestions。Thefirstis：Howhasorganicorlivingmattercommenceditsexistence？Andthesecondis：Howhasitbeenperpetuated？OnthesecondquestionI
　　shallhavemoretosayhereafter。Butonthefirstone，whatInowhavetosaywillbeforthemostpartofanegativecharacter。
　　Ifyouconsiderwhatkindofevidencewecanhaveuponthismatter，itwillresolveitselfintotwokinds。Wemayhavehistoricalevidenceandwemayhaveexperimentalevidence。Itis，forexample，conceivable，thatinasmuchasthehardenedmudwhichformsaconsiderableportionofthethicknessoftheearth’scrustcontainsfaithfulrecordsofthepastformsoflife，andinasmuchasthesediffermoreandmoreaswegofurtherdown，——itispossibleandconceivablethatwemightcometosomeparticularbedorstratumwhichshouldcontaintheremainsofthosecreatureswithwhichorganiclifebeganupontheearth。Andifwedidso，andifsuchformsoforganiclifewerepreservable，weshouldhavewhatIwouldcallhistoricalevidenceofthemodeinwhichorganiclifebeganuponthisplanet。Manypersonswilltellyou，andindeedyouwillfinditstatedinmanyworksongeology，thatthishasbeendone，andthatwereallypossesssucharecord；therearesomewhoimaginethattheearliestformsoflifeofwhichwehaveasyetdiscoveredanyrecord，areintruththeformsinwhichanimallifebeganupontheglobe。Thegroundsonwhichtheybasethatsuppositionarethese：——Thatifyougothroughtheenormousthicknessoftheearth’scrustandgetdowntotheolderrocks，thehighervertebrateanimals——thequadrupeds，birds，andfishes——ceasetobefound；beneaththemyoufindonlytheinvertebrateanimals；andinthedeepestandlowestrocksthoseremainsbecomescantierandscantier，notinanyverygradualprogression，however，until，atlength，inwhataresupposedtobetheoldestrocks，theanimalremainswhicharefoundarealmostalwaysconfinedtofourforms——’Oldhamia’，whoseprecisenatureisnotknown，whetherplantoranimal；’Lingula’，akindofmollusc；’Trilobites’，acrustaceananimal，havingthesameessentialplanofconstruction，thoughdifferinginmanydetailsfromalobsterorcrab；andHymenocaris，whichisalsoacrustacean。Sothatyouhaveallthe’Fauna’reduced，atthisperiod，tofourforms：oneakindofanimalorplantthatweknownothingabout，andthreeundoubtedanimals——twocrustaceansandonemollusc。
　　Ithink，consideringtheorganizationofthesemolluscaandcrustacea，andlookingattheirverycomplexnature，thatitdoesindeedrequireaverystrongimaginationtoconceivethatthesewerethefirstcreatedofalllivingthings。Andyoumusttakeintoconsiderationthefactthatwehavenottheslightestproofthatthesewhichwecalltheoldestbedsarereallyso：Irepeat，wehavenottheslightestproofofit。Whenyoufindinsomeplacesthatinanenormousthicknessofrockstherearebutveryscantytracesoflife，orabsolutelynoneatall；andthatinotherpartsoftheworldrocksoftheverysameformationarecrowdedwiththerecordsoflivingforms，Ithinkitisimpossibletoplaceanyrelianceonthesupposition，ortofeeloneselfjustifiedinsupposingthatthesearetheformsinwhichlifefirstcommenced。IhavenottimeheretoenteruponthetechnicalgroundsuponwhichIamledtothisconclusion，——thatcouldhardlybedoneproperlyinhalfadozenlecturesonthatpartalone；——ImustcontentmyselfwithsayingthatIdonotatallbelievethatthesearetheoldestformsoflife。
　　Iturntotheexperimentalsidetoseewhatevidencewehavethere。Toenableustosaythatweknowanythingabouttheexperimentaloriginationoforganizationandlife，theinvestigatoroughttobeabletotakeinorganicmatters，suchascarbonicacid，ammonia，water，andsalines，inanysortofinorganiccombination，andbeabletobuildthemupintoProteinmatter，andthatthatProteinmatteroughttobegintoliveinanorganicform。That，nobodyhasdoneasyet，andI
　　suspectitwillbealongwhilebeforeanybodydoesdoit。Butthethingisbynomeanssoimpossibleasitlooks；fortheresearchesofmodernchemistryhaveshownus——Iwon’tsaytheroadtowardsit，but，ifImaysosay，theyhaveshownthefinger—postpointingtotheroadthatmayleadtoit。
　　Itisnotmanyyearsago——andyoumustrecollectthatOrganicChemistryisayoungscience，notaboveacoupleofgenerationsold，——youmustnotexpecttoomuchofit；itisnotmanyyearsagosinceitwassaidtobeperfectlyimpossibletofabricateanyorganiccompound；thatistosay，anynon—mineralcompoundwhichistobefoundinanorganizedbeing。Itremainedsoforaverylongperiod；butitisnowaconsiderablenumberofyearssinceadistinguishedforeignchemistcontrivedtofabricateUrea，asubstanceofaverycomplexcharacter，whichformsoneofthewasteproductsofanimalstructures。Andoflateyearsanumberofothercompounds，suchasButyricAcid，andothers，havebeenaddedtothelist。IneednottellyouthatchemistryisanenormousdistancefromthegoalIindicate；allIwishtopointouttoyouis，thatitisbynomeanssafetosaythatthatgoalmaynotbereachedoneday。Itmaybethatitisimpossibleforustoproducetheconditionsrequisitetotheoriginationoflife；butwemustspeakmodestlyaboutthematter，andrecollectthatSciencehasputherfootuponthebottomroundoftheladder。Trulyhewouldbeaboldmanwhowouldventuretopredictwhereshewillbefiftyyearshence。
　　Thereisanotherinquirywhichbearsindirectlyuponthisquestion，anduponwhichImustsayafewwords。Youareallofyouawareofthephenomenaofwhatiscalledspontaneousgeneration。Ourforefathers，downtotheseventeenthcentury，orthereabouts，allimagined，inperfectlygoodfaith，thatcertainvegetableandanimalformsgavebirth，intheprocessoftheirdecomposition，toinsectlife。Thus，ifyouputapieceofmeatinthesun，andallowedittoputrefy，theyconceivedthatthegrubswhichsoonbegantoappearweretheresultoftheactionofapowerofspontaneousgenerationwhichthemeatcontained。Andtheycouldgiveyoureceiptsformakingvariousanimalandvegetablepreparationswhichwouldproduceparticularkindsofanimals。AverydistinguishedItaliannaturalist，namedRedi，tookupthequestion，atatimewheneverybodybelievedinit；amongothersourowngreatHarvey，thediscovererofthecirculationoftheblood。Youwillconstantlyfindhisnamequoted，however，asanopponentofthedoctrineofspontaneousgeneration；butthefactis，andyouwillseeitifyouwilltakethetroubletolookintohisworks，Harveybelieveditasprofoundlyasanymanofhistime；buthehappenedtoenunciateaverycuriousproposition——thateverylivingthingcamefroman’egg’；
　　hedidnotmeantousethewordinthesenseinwhichwenowemployit，heonlymeanttosaythateverylivingthingoriginatedinalittleroundedparticleoforganizedsubstance；anditisfromthiscircumstance，probably，thatthenotionofHarveyhavingopposedthedoctrineoriginated。ThencameRedi，andheproceededtoupsetthedoctrineinaverysimplemanner。Hemerelycoveredthepieceofmeatwithsomeveryfinegauze，andthenheexposedittothesameconditions。Theresultofthiswasthatnogrubsorinsectswereproduced；heprovedthatthegrubsoriginatedfromtheinsectswhocameanddepositedtheireggsinthemeat，andthattheywerehatchedbytheheatofthesun。Bythiskindofinquiryhethoroughlyupsetthedoctrineofspontaneousgeneration，forhistimeatleast。
　　Thencamethediscoveryandapplicationofthemicroscopetoscientificinquiries，whichshowedtonaturaliststhatbesidestheorganismswhichtheyalreadyknewaslivingbeingsandplants，therewereanimmensenumberofminutethingswhichcouldbeobtainedapparentlyalmostatwillfromdecayingvegetableandanimalforms。Thus，ifyoutooksomeordinaryblackpepperorsomehay，andsteepeditinwater，youwouldfindinthecourseofafewdaysthatthewaterhadbecomeimpregnatedwithanimmensenumberofanimalculesswimmingaboutinalldirections。Fromfactsofthiskindnaturalistswereledtorevivethetheoryofspontaneousgeneration。TheywereheadedherebyanEnglishnaturalist，——Needham，——andafterwardsinFrancebythelearnedBuffon。
　　Theysaidthatthesethingswereabsolutelybegotteninthewaterofthedecayingsubstancesoutofwhichtheinfusionwasmade。Itdidnotmatterwhetheryoutookanimalorvegetablematter，youhadonlytosteepitinwaterandexposeit，andyouwouldsoonhaveplentyofanimalcules。Theymadeanhypothesisaboutthiswhichwasaveryfairone。Theysaid，thismatteroftheanimalworld，orofthehigherplants，appearstobedead，butinrealityithasasortofdimlifeaboutit，which，ifitisplacedunderfairconditions，willcauseittobreakupintotheformsoftheselittleanimalcules，andtheywillgothroughtheirlivesinthesamewayastheanimalorplantofwhichtheyonceformedapart。
　　Thequestionnowbecameveryhotlydebated。Spallanzani，anItaliannaturalist，tookupoppositeviewstothoseofNeedhamandBuffon，andbymeansofcertainexperimentsheshowedthatitwasquitepossibletostoptheprocessbyboilingthewater，andclosingthevesselinwhichitwascontained。"Oh！"saidhisopponents；"butwhatdoyouknowyoumaybedoingwhenyouheattheairoverthewaterinthisway？Youmaybedestroyingsomepropertyoftheairrequisiteforthespontaneousgenerationoftheanimalcules。"
　　However，Spallanzani’sviewsweresupposedtobeupontherightside，andthoseoftheothersfellintodiscredit；althoughthefactwasthatSpallanzanihadnotmadegoodhisviews。Well，then，thesubjectcontinuedtoberevivedfromtimetotime，andexperimentsweremadebyseveralpersons；buttheseexperimentswerenotaltogethersatisfactory。
　　Itwasfoundthatifyouputaninfusioninwhichanimalculeswouldappearifitwereexposedtotheairintoavesselandboiledit，andthensealedupthemouthofthevessel，sothatnoair，savesuchashadbeenheatedto212degrees，couldreachitscontents，thatthennoanimalculeswouldbefound；butifyoutookthesamevesselandexposedtheinfusiontotheair，thenyouwouldgetanimalcules。Furthermore，itwasfoundthatifyouconnectedthemouthofthevesselwithared—hottubeinsuchawaythattheairwouldhavetopassthroughthetubebeforereachingtheinfusion，thatthenyouwouldgetnoanimalcules。Yetanotherthingwasnoticed：ifyoutooktwoflaskscontainingthesamekindofinfusion，andleftoneentirelyexposedtotheair，andinthemouthoftheotherplacedaballofcottonwool，sothattheairwouldhavetofilteritselfthroughitbeforereachingtheinfusion，thatthen，althoughyoumighthaveplentyofanimalculesinthefirstflask，youwouldcertainlyobtainnonefromthesecond。
　　Theseexperiments，yousee，alltendedtowardsoneconclusion——thattheinfusoriaweredevelopedfromlittleminutesporesoreggswhichwereconstantlyfloatingintheatmosphere，whichlosetheirpowerofgerminationifsubjectedtoheat。Butoneobservernowmadeanotherexperimentwhichseemedtogoentirelytheotherway，andpuzzledhimaltogether。HetooksomeofthisboiledinfusionthatIhavebeenspeakingof，andbytheuseofamercurialbath——akindoftroughusedinlaboratories——hedeftlyinvertedavesselcontainingtheinfusionintothemercury，sothatthelatterreachedalittlebeyondthelevelofthemouthofthe’inverted’vessel。Youseethathethushadaquantityoftheinfusionshutofffromanypossiblecommunicationwiththeouterairbybeinginverteduponabedofmercury。
　　Hethenpreparedsomepureoxygenandnitrogengases，andpassedthembymeansofatubegoingfromtheoutsideofthevessel，upthroughthemercuryintotheinfusion；sothathethushaditexposedtoaperfectlypureatmosphereofthesameconstituentsastheexternalair。
　　Ofcourse，heexpectedhewouldgetnoinfusorialanimalculesatallinthatinfusion；but，tohisgreatdismayanddiscomfiture，hefoundhealmostalwaysdidgetthem。
　　Furthermore，ithasbeenfoundthatexperimentsmadeinthemannerdescribedaboveanswerwellwithmostinfusions；butthatifyoufillthevesselwithboiledmilk，andthenstoptheneckwithcotton—wool，you’will’haveinfusoria。Sothatyouseethereweretwoexperimentsthatbroughtyoutoonekindofconclusion，andthreetoanother；whichwasamostunsatisfactorystateofthingstoarriveatinascientificinquiry。
　　Somefewyearsafterthis，thequestionbegantobeveryhotlydiscussedinFrance。TherewasM。Pouchet，aprofessoratRouen，averylearnedman，butcertainlynotaveryrigidexperimentalist。Hepublishedanumberofexperimentsofhisown，someofwhichwereveryingenious，toshowthatifyouwenttoworkinaproperway，therewasatruthinthedoctrineofspontaneousgeneration。Well，itwasoneofthemostfortunatethingsintheworldthatM。Pouchettookupthisquestion，becauseitinducedadistinguishedFrenchchemist，M。Pasteur，totakeupthequestionontheotherside；andhehascertainlyworkeditoutinthemostperfectmanner。Iamgladtosay，too，thathehaspublishedhisresearchesintimetoenablemetogiveyouanaccountofthem。HeverifiedalltheexperimentswhichIhavejustmentionedtoyou——andthenfindingthoseextraordinaryanomalies，asinthecaseofthemercurybathandthemilk，hesethimselftoworktodiscovertheirnature。Inthecaseofmilkhefoundittobeaquestionoftemperature。Milkinafreshstateisslightlyalkaline；anditisaverycuriouscircumstance，butthisveryslightdegreeofalkalinityseemstohavetheeffectofpreservingtheorganismswhichfallintoitfromtheairfrombeingdestroyedatatemperatureof212degrees，whichistheboilingpoint。Butifyouraisethetemperature10degreeswhenyouboilit，themilkbehaveslikeeverythingelse；andiftheairwithwhichitcomesincontact，afterbeingboiledatthistemperature，ispassedthroughared—hottube，youwillnotgetatraceoforganisms。
　　Hethenturnedhisattentiontothemercurybath，andfoundonexaminationthatthesurfaceofthemercurywasalmostalwayscoveredwithaveryfinedust。Hefoundthateventhemercuryitselfwaspositivelyfulloforganicmatters；thatfrombeingconstantlyexposedtotheair，ithadcollectedanimmensenumberoftheseinfusorialorganismsfromtheair。Well，underthesecircumstanceshefeltthatthecasewasquiteclear，andthatthemercurywasnotwhatithadappearedtoM。Schwanntobe，——abartotheadmissionoftheseorganisms；butthat，inreality，itactedasareservoirfromwhichtheinfusionwasimmediatelysuppliedwiththelargequantitythathadsopuzzledhim。
　　Butnotcontentwithexplainingtheexperimentsofothers，M。Pasteurwenttoworktosatisfyhimselfcompletely。Hesaidtohimself："Ifmyviewisright，andif，inpointoffact，alltheseappearancesofspontaneousgenerationarealtogetherduetothefallingofminutegermssuspendedintheatmosphere，——why，Ioughtnotonlytobeabletoshowthegerms，butIoughttobeabletocatchandsowthem，andproducetheresultingorganisms。"He，accordingly，constructedaveryingeniousapparatustoenablehimtoaccomplishthistrappingofthis"germdust"intheair。Hefixedinthewindowofhisroomaglasstube，inthecentreofwhichhehadplacedaballofgun—cotton，which，asyouallknow，isordinarycotton—wool，which，fromhavingbeensteepedinstrongacid，isconvertedintoasubstanceofgreatexplosivepower。Itisalsosolubleinalcoholandether。Oneendoftheglasstubewas，ofcourse，opentotheexternalair；andattheotherendofitheplacedanaspirator，acontrivanceforcausingacurrentoftheexternalairtopassthroughthetube。Hekeptthisapparatusgoingforfour—and—twentyhours，andthenremovedthe’dusted’gun—cotton，anddissolveditinalcoholandether。Hethenallowedthistostandforafewhours，andtheresultwas，thataveryfinedustwasgraduallydepositedatthebottomofit。Thatdust，onbeingtransferredtothestageofamicroscope，wasfoundtocontainanenormousnumberofstarchgrains。Youknowthatthematerialsofourfoodandthegreaterportionofplantsarecomposedofstarch，andweareconstantlymakinguseofitinavarietyofways，sothatthereisalwaysaquantityofitsuspendedintheair。Itisthesestarchgrainswhichformmanyofthosebrightspecksthatweseedancinginarayoflightsometimes。Butbesidesthese，M。Pasteurfoundalsoanimmensenumberofotherorganicsubstancessuchassporesoffungi，whichhadbeenfloatingaboutintheairandhadgotcagedinthisway。
　　Hewentfarther，andsaidtohimself，"Ifthesereallyarethethingsthatgiverisetotheappearanceofspontaneousgeneration，Ioughttobeabletotakeaballofthis’dusted’gun—cottonandputitintooneofmyvessels，containingthatboiledinfusionwhichhasbeenkeptawayfromtheair，andinwhichnoinfusoriaareatpresentdeveloped，andthen，ifIamright，theintroductionofthisgun—cottonwillgiverisetoorganisms。"
　　Accordingly，hetookoneofthesevesselsofinfusion，whichhadbeenkepteighteenmonths，withouttheleastappearanceoflife，andbyamostingeniouscontrivance，hemanagedtobreakitopenandintroducesuchaballofgun—cotton，withoutallowingtheinfusionorthecottonballtocomeintocontactwithanyairbutthatwhichhadbeensubjectedtoaredheat，andintwenty—fourhourshehadthesatisfactionoffindingalltheindicationsofwhathadbeenhithertocalledspontaneousgeneration。Hehadsucceededincatchingthegermsanddevelopingorganismsinthewayhehadanticipated。
　　Itnowstruckhimthatthetruthofhisconclusionsmightbedemonstratedwithoutalltheapparatushehademployed。Todothis，hetooksomedecayinganimalorvegetablesubstance，suchasurine，whichisanextremelydecomposablesubstance，orthejuiceofyeast，orperhapssomeotherartificialpreparation，andfilledavesselhavingalongtubularneckwithit。HethenboiledtheliquidandbentthatlongneckintoanSshapeorzig—zag，leavingitopenattheend。Theinfusionthengavenotraceofanyappearanceofspontaneousgeneration，howeverlongitmightbeleft，asallthegermsintheairweredepositedinthebeginningofthebentneck。Hethencutthetubeclosetothevessel，andallowedtheordinaryairtohavefreeanddirectaccess；andtheresultofthatwastheappearanceoforganismsinit，assoonastheinfusionhadbeenallowedtostandlongenoughtoallowofthegrowthofthoseitreceivedfromtheair，whichwasaboutforty—eighthours。TheresultofM。Pasteur’sexperimentsproved，therefore，inthemostconclusivemanner，thatalltheappearancesofspontaneousgenerationarosefromnothingmorethanthedepositionofthegermsoforganismswhichwereconstantlyfloatingintheair。
　　Tothisconclusion，however，theobjectionwasmade，thatifthatwerethecause，thentheairwouldcontainsuchanenormousnumberofthesegerms，thatitwouldbeacontinualfog。ButM。Pasteurrepliedthattheyarenotthereinanythinglikethenumberwemightsuppose，andthatanexaggeratedviewhasbeenheldonthatsubject；heshowedthatthechancesofanimalorvegetablelifeappearingininfusions，dependentirelyontheconditionsunderwhichtheyareexposed。Iftheyareexposedtotheordinaryatmospherearoundus，why，ofcourse，youmayhaveorganismsappearingearly。But，ontheotherhand，iftheyareexposedtoairfromagreatheight，orfromsomeveryquietcellar，youwilloftennotfindasingletraceoflife。
　　SothatM。Pasteurarrivedatlastattheclearanddefiniteresult，thatalltheseappearancesarelikethecaseofthewormsinthepieceofmeat，whichwasrefutedbyRedi，simplygermscarriedbytheairanddepositedintheliquidsinwhichtheyafterwardsappear。Formyownpart，Iconceivethat，withtheparticularsofM。Pasteur’sexperimentsbeforeus，wecannotfailtoarriveathisconclusions；andthatthedoctrineofspontaneousgenerationhasreceivedafinal’coupdegrace’。
　　You，ofcourse，understandthatallthisinnowayinterfereswiththe’possibility’ofthefabricationoforganicmattersbythedirectmethodtowhichIhavereferred，remoteasthatpossibilitymaybe。
　　[Footnote]1ThosewhowishtostudyfullythedoctrinesofwhichIhaveendeavouredtogivesomeroughandreadyillustrations，mustreadMr。JohnStuartMill’s’SystemofLogic’。