MR。DARWIN’Slong-standingandwell-earnedscientificeminenceprobablyrendershimindifferenttothatsocialnotorietywhichpassesbythenameofsuccess；butifthecalmspiritofthephilosopherhavenotyetwhollysupersededtheambitionandthevanityofthecarnalmanwithinhim，hemustbewellsatisfiedwiththeresultsofhisventureinpublishingthe’OriginofSpecies’。Overflowingthenarrowboundsofpurelyscientificcircles，the"speciesquestion"divideswithItalyandtheVolunteerstheattentionofgeneralsociety。EverybodyhasreadMr。Darwin’sbook，or，atleast，hasgivenanopinionuponitsmeritsordemerits；pietists，whetherlayorecclesiastic，decryitwiththemildrailingwhichsoundssocharitable；bigotsdenounceitwithignorantinvective；oldladiesofbothsexesconsideritadecidedlydangerousbook，andevensavants，whohavenobettermudtothrow，quoteantiquatedwriterstoshowthatitsauthorisnobetterthananapehimself；whileeveryphilosophicalthinkerhailsitasaveritableWhitworthguninthearmouryofliberalism；andallcompetentnaturalistsandphysiologists，whatevertheiropinionsastotheultimatefateofthedoctrinesputforth，acknowledgethattheworkinwhichtheyareembodiedisasolidcontributiontoknowledgeandinauguratesanewepochinnaturalhistory。
Norhasthediscussionofthesubjectbeenrestrainedwithinthelimitsofconversation。Whenthepubliciseagerandinterested，reviewersmustministertoitswants；andthegenuine’litterateur’istoomuchinthehabitofacquiringhisknowledgefromthebookhejudges——astheAbyssinianissaidtoprovidehimselfwithsteaksfromtheoxwhichcarrieshim——tobewithheldfromcriticismofaprofoundscientificworkbythemerewantoftherequisitepreliminaryscientificacquirement；while，ontheotherhand，themenofsciencewhowishwelltothenewviews，nolessthanthosewhodisputetheirvalidity，havenaturallysoughtopportunitiesofexpressingtheiropinions。HenceitisnotsurprisingthatalmostallthecriticaljournalshavenoticedMr。Darwin’sworkatgreaterorlesslength；andsomanydisquisitions，ofeverydegreeofexcellence，fromthepoorproductofignorance，toooftenstimulatedbyprejudice，tothefairandthoughtfulessayofthecandidstudentofNature，haveappeared，thatitseemsanalmosthopelesstasktoattempttosayanythingnewuponthequestion。
Butitmaybedoubtediftheknowledgeandacumenofprejudgedscientificopponents，orthesubtletyoforthodoxspecialpleaders，haveyetexertedtheirfullforceinmystifyingtherealissuesofthegreatcontroversywhichhasbeensetafoot，andwhoseendishardlylikelytobeseenbythisgeneration；sothat，atthiseleventhhour，andevenfailinganythingnew，itmaybeusefultostateafreshthatwhichistrue，andtoputthefundamentalpositionsadvocatedbyMr。
Darwininsuchaformthattheymaybegraspedbythosewhosespecialstudieslieinotherdirections。Andtheadoptionofthiscoursemaybethemoreadvisable，because，notwithstandingitsgreatdeserts，andindeedpartlyonaccountofthem，the’OriginofSpecies’isbynomeansaneasybooktoread——ifbyreadingisimpliedthefullcomprehensionofanauthor’smeaning。
WedonotspeakjestinglyinsayingthatitisMr。Darwin’smisfortunetoknowmoreaboutthequestionhehastakenupthananymanliving。
Personallyandpracticallyexercisedinzoology，inminuteanatomy，ingeology；astudentofgeographicaldistribution，notonmapsandinmuseumsonly，butbylongvoyagesandlaboriouscollection；havinglargelyadvancedeachofthesebranchesofscience，andhavingspentmanyyearsingatheringandsiftingmaterialsforhispresentwork，thestoreofaccuratelyregisteredfactsuponwhichtheauthorofthe’OriginofSpecies’isabletodrawatwillisprodigious。
Butthisverysuperabundanceofmattermusthavebeenembarrassingtoawriterwho，forthepresent，canonlyputforwardanabstractofhisviews；andthenceitarises，perhaps，thatnotwithstandingtheclearnessofthestyle，thosewhoattemptfairlytodigestthebookfindmuchofitasortofintellectualpemmican——amassoffactscrushedandpoundedintoshape，ratherthanheldtogetherbytheordinarymediumofanobviouslogicalbond；dueattentionwill，withoutdoubt，discoverthisbond，butitisoftenhardtofind。
Again，fromsheerwantofroom，muchhastobetakenforgrantedwhichmightreadilyenoughbeproved；andhence，whiletheadept，whocansupplythemissinglinksintheevidencefromhisownknowledge，discoversfreshproofofthesingularthoroughnesswithwhichalldifficultieshavebeenconsideredandallunjustifiablesuppositionsavoided，ateveryreperusalofMr。Darwin’spregnantparagraphs，thenoviceinbiologyisapttocomplainofthefrequencyofwhathefanciesisgratuitousassumption。
Thuswhileitmaybedoubtedif，forsomeyears，anyoneislikelytobecompetenttopronouncejudgmentonalltheissuesraisedbyMr。Darwin，thereisassuredlyabundantroomforhim，who，assumingthehumbler，thoughperhapsasuseful，officeofaninterpreterbetweenthe’OriginofSpecies’andthepublic，contentshimselfwithendeavouringtopointoutthenatureoftheproblemswhichitdiscusses；todistinguishbetweentheascertainedfactsandthetheoreticalviewswhichitcontains；andfinally，toshowtheextenttowhichtheexplanationitofferssatisfiestherequirementsofscientificlogic。Atanyrate，itisthisofficewhichwepurposetoundertakeinthefollowingpages。
Itmaybesafelyassumedthatourreadershaveageneralconceptionofthenatureoftheobjectstowhichtheword"species"isapplied；butithas，perhaps，occurredtoafew，eventothosewhoarenaturalists’exprofesso’，toreflect，that，ascommonlyemployed，thetermhasadoublesenseanddenotestwoverydifferentordersofrelations。Whenwecallagroupofanimals，orofplants，aspecies，wemayimplythereby，eitherthatalltheseanimalsorplantshavesomecommonpeculiarityofformorstructure；or，wemaymeanthattheypossesssomecommonfunctionalcharacter。ThatpartofbiologicalsciencewhichdealswithformandstructureiscalledMorphology——thatwhichconcernsitselfwithfunction，Physiology——sothatwemayconvenientlyspeakofthesetwosenses，oraspects，of"species"——theoneasmorphological，theotherasphysiological。Regardedfromtheformerpointofview，aspeciesisnothingmorethanakindofanimalorplant，whichisdistinctlydefinablefromallothers，bycertainconstant，andnotmerelysexual，morphologicalpeculiarities。Thushorsesformaspecies，becausethegroupofanimalstowhichthatnameisappliedisdistinguishedfromallothersintheworldbythefollowingconstantlyassociatedcharacters。Theyhave——1，Avertebralcolumn；2，Mammae；3，Aplacentalembryo；4，Fourlegs；5，Asinglewell-developedtoeineachfootprovidedwithahoof；6，Abushytail；
and7，Callositiesontheinnersidesofboththeforeandthehindlegs。Theasses，again，formadistinctspecies，because，withthesamecharacters，asfarasthefifthintheabovelist，allasseshavetuftedtails，andhavecallositiesonlyontheinnersideofthefore-legs。Ifanimalswerediscoveredhavingthegeneralcharactersofthehorse，butsometimeswithcallositiesonlyonthefore-legs，andmoreorlesstuftedtails；oranimalshavingthegeneralcharactersoftheass，butwithmoreorlessbushytails，andsometimeswithcallositiesonbothpairsoflegs，besidesbeingintermediateinotherrespects——thetwospecieswouldhavetobemergedintoone。Theycouldnolongerberegardedasmorphologicallydistinctspecies，fortheywouldnotbedistinctlydefinableonefromtheother。
Howeverbareandsimplethisdefinitionofspeciesmayappeartobe，weconfidentlyappealtoallpracticalnaturalists，whetherzoologists，botanists，orpalaeontologists，tosayif，inthevastmajorityofcases，theyknow，ormeantoaffirmanythingmoreofthegroupofanimalsorplantstheysodenominatethanwhathasjustbeenstated。
Eventhemostdecidedadvocatesofthereceiveddoctrinesrespectingspeciesadmitthis。
"Iapprehend，"saysProfessorOwen*，"thatfewnaturalistsnowadays，indescribingandproposinganameforwhattheycall’anewspecies，’usethattermtosignifywhatwasmeantbyittwentyorthirtyyearsago；
thatis，anoriginallydistinctcreation，maintainingitsprimitivedistinctionbyobstructivegenerativepeculiarities。Theproposerofthenewspeciesnowintendstostatenomorethanheactuallyknows；
as，forexample，thatthedifferencesonwhichhefoundsthespecificcharacterareconstantinindividualsofbothsexes，sofarasobservationhasreached；andthattheyarenotduetodomesticationortoartificiallysuperinducedexternalcircumstances，ortoanyoutwardinfluencewithinhiscognizance；thatthespeciesiswild，orissuchasitappearsbyNature。"
[footnote]*OntheOsteologyoftheChimpanzeesandOrangs：
TransactionsoftheZoologicalSociety，1858。
Ifweconsider，infact，thatbyfarthelargestproportionofrecordedexistingspeciesareknownonlybythestudyoftheirskins，orbones，orotherlifelessexuvia；thatweareacquaintedwithnone，ornexttonone，oftheirphysiologicalpeculiarities，beyondthosewhichcanbededucedfromtheirstructure，orareopentocursoryobservation；andthatwecannothopetolearnmoreofanyofthoseextinctformsoflifewhichnowconstitutenoinconsiderableproportionoftheknownFloraandFaunaoftheworld：itisobviousthatthedefinitionsofthesespeciescanbeonlyofapurelystructural，ormorphological，character。
Itisprobablethatnaturalistswouldhaveavoidedmuchconfusionofideasiftheyhadmorefrequentlybornethenecessarylimitationsofourknowledgeinmind。Butwhileitmaysafelybeadmittedthatweareacquaintedwithonlythemorphologicalcharactersofthevastmajorityofspecies——thefunctionalorphysiological，peculiaritiesofafewhavebeencarefullyinvestigated，andtheresultofthatstudyformsalargeandmostinterestingportionofthephysiologyofreproduction。
ThestudentofNaturewondersthemoreandisastonishedtheless，themoreconversanthebecomeswithheroperations；butofalltheperennialmiraclessheofferstohisinspection，perhapsthemostworthyofadmirationisthedevelopmentofaplantorofananimalfromitsembryo。Examinetherecentlylaideggofsomecommonanimal，suchasasalamanderornewt。Itisaminutespheroidinwhichthebestmicroscopewillrevealnothingbutastructurelesssac，enclosingaglairyfluid，holdinggranulesinsuspension。Butstrangepossibilitiesliedormantinthatsemi-fluidglobule。Letamoderatesupplyofwarmthreachitswaterycradle，andtheplasticmatterundergoeschangessorapid，yetsosteadyandpurposelikeintheirsuccession，thatonecanonlycomparethemtothoseoperatedbyaskilledmodelleruponaformlesslumpofclay。Aswithaninvisibletrowel，themassisdividedandsubdividedintosmallerandsmallerportions，untilitisreducedtoanaggregationofgranulesnottoolargetobuildwithalthefinestfabricsofthenascentorganism。And，then，itisasifadelicatefingertracedoutthelinetobeoccupiedbythespinalcolumn，andmouldedthecontourofthebody；pinchinguptheheadatoneend，thetailattheother，andfashioningflankandlimbintoduesalamandrineproportions，insoartisticaway，that，afterwatchingtheprocesshourbyhour，oneisalmostinvoluntarilypossessedbythenotion，thatsomemoresubtleaidtovisionthananachromatic，wouldshowthehiddenartist，withhisplanbeforehim，strivingwithskilfulmanipulationtoperfecthiswork。
Aslifeadvances，andtheyoungamphibianrangesthewaters，theterrorofhisinsectcontemporaries，notonlyarethenutritiousparticlessuppliedbyitsprey，bytheadditionofwhichtoitsframe，growthtakesplace，laiddown，eachinitsproperspot，andinsuchdueproportiontotherest，astoreproducetheform，thecolour，andthesize，characteristicoftheparentalstock；buteventhewonderfulpowersofreproducinglostpartspossessedbytheseanimalsarecontrolledbythesamegoverningtendency。Cutoffthelegs，thetail，thejaws，separatelyoralltogether，and，asSpallanzanishowedlongago，thesepartsnotonlygrowagain，buttheredintegratedlimbisformedonthesametypeasthosewhichwerelost。Thenewjaw，orleg，isanewt’s，andneverbyanyaccidentmorelikethatofafrog。Whatistrueofthenewtistrueofeveryanimalandofeveryplant；theacorntendstobuilditselfupagainintoawoodlandgiantsuchasthatfromwhosetwigitfell；thesporeofthehumblestlichenreproducesthegreenorbrownincrustationwhichgaveitbirth；andattheotherendofthescaleoflife，thechildthatresembledneitherthepaternalnorthematernalsideofthehousewouldberegardedasakindofmonster。
Sothattheoneendtowhich，inalllivingbeings，theformativeimpulseistending——theoneschemewhichtheArchaeusoftheoldspeculatorsstrivestocarryout，seemstobetomouldtheoffspringintothelikenessoftheparent。Itisthefirstgreatlawofreproduction，thattheoffspringtendstoresembleitsparentorparents，morecloselythananythingelse。
Sciencewillsomedayshowushowthislawisanecessaryconsequenceofthemoregenerallawswhichgovernmatter；but，forthepresent，morecanhardlybesaidthanthatitappearstobeinharmonywiththem。Weknowthatthephenomenaofvitalityarenotsomethingapartfromotherphysicalphenomena，butonewiththem；andmatterandforcearethetwonamesoftheoneartistwhofashionsthelivingaswellasthelifeless。Hencelivingbodiesshouldobeythesamegreatlawsasothermatter——nor，throughoutNature，istherealawofwiderapplicationthanthis，thatabodyimpelledbytwoforcestakesthedirectionoftheirresultant。Butlivingbodiesmayberegardedasnothingbutextremelycomplexbundlesofforcesheldinamassofmatter，asthecomplexforcesofamagnetareheldinthesteelbyitscoerciveforce；
and，sincethedifferencesofsexarecomparativelyslight，or，inotherwords，thesumoftheforcesineachhasaverysimilartendency，theirresultant，theoffspring，mayreasonablybeexpectedtodeviatebutlittlefromacourseparalleltoeither，ortoboth。
Representthereasonofthelawtoourselvesbywhatphysicalmetaphororanalogywewill，however，thegreatmatteristoapprehenditsexistenceandtheimportanceoftheconsequencesdeduciblefromit。Forthingswhichareliketothesameareliketooneanother；andif；inagreatseriesofgenerations，everyoffspringislikeitsparent，itfollowsthatalltheoffspringandalltheparentsmustbelikeoneanother；andthat，givenanoriginalparentalstock，withtheopportunityofundisturbedmultiplication，thelawinquestionnecessitatestheproduction，incourseoftime，ofanindefinitelylargegroup，thewholeofwhosemembersareatonceverysimilarandarebloodrelations，havingdescendedfromthesameparent，orpairofparents。Theproofthatallthemembersofanygivengroupofanimals，orplants，hadthusdescended，wouldbeordinarilyconsideredsufficienttoentitlethemtotherankofphysiologicalspecies，formostphysiologistsconsiderspeciestobedefinableas"theoffspringofasingleprimitivestock。"
Butthoughitisquitetruethatallthosegroupswecallspecies’may’，accordingtotheknownlawsofreproduction，havedescendedfromasinglestock，andthoughitisverylikelytheyreallyhavedoneso，yetthisconclusionrestsondeductionandcanhardlyhopetoestablishitselfuponabasisofobservation。Andtheprimitivenessofthesupposedsinglestock，which，afterall，istheessentialpartofthematter，isnotonlyahypothesis，butonewhichhasnotashadowoffoundation，ifby"primitive"bemeant"independentofanyotherlivingbeing。"Ascientificdefinition，ofwhichanunwarrantablehypothesisformsanessentialpart，carriesitscondemnationwithinitself；but，evensupposingsuchadefinitionwere，inform，tenable，thephysiologistwhoshouldattempttoapplyitinNaturewouldsoonfindhimselfinvolvedingreat，ifnotinextricable，difficulties。Aswehavesaid，itisindubitablethatoffspring’tend’toresembletheparentalorganism，butitisequallytruethatthesimilarityattainedneveramountstoidentity，eitherinformorinstructure。Thereisalwaysacertainamountofdeviation，notonlyfromtheprecisecharactersofasingleparent，butwhen，asinmostanimalsandmanyplants，thesexesarelodgedindistinctindividuals，fromanexactmeanbetweenthetwoparents。Andindeed，ongeneralprinciples，thisslightdeviationseemsasintelligibleasthegeneralsimilarity，ifwereflecthowcomplextheco-operating"bundlesofforces"are，andhowimprobableitisthat，inanycase，theirtrueresultantshallcoincidewithanymeanbetweenthemoreobviouscharactersofthetwoparents。
Whateverbeitscause，however，theco-existenceofthistendencytominorvariationwiththetendencytogeneralsimilarity，isofvastimportanceinitsbearingonthequestionoftheoriginofspecies。
Asageneralrule，theextenttowhichanoffspringdiffersfromitsparentisslightenough；but，occasionally，theamountofdifferenceismuchmorestronglymarked，andthenthedivergentoffspringreceivesthenameofaVariety。Multitudes，ofwhatthereiseveryreasontobelievearesuchvarieties，areknown，buttheoriginofveryfewhasbeenaccuratelyrecorded，andofthesewewillselecttwoasmoreespeciallyillustrativeofthemainfeaturesofvariation。Thefirstofthemisthatofthe"Ancon，"or"Otter"sheep，ofwhichacarefulaccountisgivenbyColonelDavidHumphreys，F。R。S。，inalettertoSirJosephBanks，publishedinthePhilosophicalTransactionsfor1813。ItappearsthatoneSethWright，theproprietorofafarmonthebanksoftheCharlesRiver，inMassachusetts，possessedaflockoffifteenewesandaramoftheordinarykind。Intheyear1791，oneoftheewespresentedherownerwithamalelamb，differing，fornoassignablereason，fromitsparentsbyaproportionallylongbodyandshortbandylegs，whenceitwasunabletoemulateitsrelativesinthosesportiveleapsovertheneighbours’fences，inwhichtheywereinthehabitofindulging，muchtothegoodfarmer’svexation。
ThesecondcaseisthatdetailedbyanolessunexceptionableauthoritythanReaumur，inhis’ArtdefaireeclorelesPoulets’。AMaltesecouple，namedKelleia，whosehandsandfeetwereconstructedupontheordinaryhumanmodel，hadborntothemason，Gratio，whopossessedsixperfectlymovablefingersoneachhand，andsixtoes，notquitesowellformed，oneachfoot。Nocausecouldbeassignedfortheappearanceofthisunusualvarietyofthehumanspecies。
Twocircumstancesarewellworthyofremarkinboththesecases。Ineach，thevarietyappearstohaveariseninfullforce，and，asitwere，’persaltum’；awideanddefinitedifferenceappearing，atonce，betweentheAnconramandtheordinarysheep；betweenthesix-fingeredandsix-toedGratioKelleiaandordinarymen。Inneithercaseisitpossibletopointoutanyobviousreasonfortheappearanceofthevariety。Doubtlessthereweredeterminingcausesfortheseasforallotherphenomena；buttheydonotappear，andwecanbetolerablycertainthatwhatareordinarilyunderstoodaschangesinphysicalconditions，asinclimate，infood，orthelike，didnottakeplaceandhadnothingtodowiththematter。Itwasnocaseofwhatiscommonlycalledadaptationtocircumstances；but，touseaconvenientlyerroneousphrase，thevariationsarosespontaneously。Thefruitlesssearchafterfinalcausesleadstheirpursuersalongway；buteventhosehardyteleologists，whoarereadytobreakthroughallthelawsofphysicsinchaseoftheirfavouritewill-o’-the-wisp，maybepuzzledtodiscoverwhatpurposecouldbeattainedbythestuntedlegsofSethWright’sramorthehexadactylemembersofGratioKelleia。
Varietiesthenariseweknownotwhy；anditismorethanprobablethatthemajorityofvarietieshaveariseninthis"spontaneous"manner，thoughweare，ofcourse，farfromdenyingthattheymaybetraced，insomecases，todistinctexternalinfluences；whichareassuredlycompetenttoalterthecharacterofthetegumentarycovering，tochangecolour，toincreaseordiminishthesizeofmuscles，tomodifyconstitution，and，amongplants，togiverisetothemetamorphosisofstamensintopetals，andsoforth。Buthowevertheymayhavearisen，whatespeciallyinterestsusatpresentis，toremarkthat，onceinexistence，varietiesobeythefundamentallawofreproductionthatliketendstoproducelike；andtheiroffspringexemplifyitbytendingtoexhibitthesamedeviationfromtheparentalstockasthemselves。
Indeed，thereseemstobe，inmanyinstances，apre-potentinfluenceaboutanewly-arisenvarietywhichgivesitwhatonemaycallanunfairadvantageoverthenormaldescendantsfromthesamestock。ThisisstrikinglyexemplifiedbythecaseofGratioKelleia，whomarriedawomanwiththeordinarypentadactyleextremities，andhadbyherfourchildren，Salvator，George，Andre，andMarie。OfthesechildrenSalvator，theeldestboy，hadsixfingersandsixtoes，likehisfather；thesecondandthird，alsoboys，hadfivefingersandfivetoes，liketheirmother，thoughthehandsandfeetofGeorgewereslightlydeformed。Thelast，agirl，hadfivefingersandfivetoes，butthethumbswereslightlydeformed。Thevarietythusreproduceditselfpurelyintheeldest，whilethenormaltypereproduceditselfpurelyinthethird，andalmostpurelyinthesecondandlast：sothatitwouldseem，atfirst，asifthenormaltypeweremorepowerfulthanthevariety。Butallthesechildrengrewupandintermarriedwithnormalwivesandhusband，andthen，notewhattookplace：Salvatorhadfourchildren，threeofwhomexhibitedthehexadactylemembersoftheirgrandfatherandfather，whiletheyoungesthadthepentadactylelimbsofthemotherandgrandmother；sothathere，notwithstandingadoublepentadactyledilutionoftheblood，thehexadactylevarietyhadthebestofit。Thesamepre-potencyofthevarietywasstillmoremarkedlyexemplifiedintheprogenyoftwooftheotherchildren，MarieandGeorge。Marie（whosethumbsonlyweredeformed）gavebirthtoaboywithsixtoes，andthreeothernormallyformedchildren；butGeorge，whowasnotquitesopureapentadactyle，begot，first，twogirls，eachofwhomhadsixfingersandtoes；thenagirlwithsixfingersoneachhandandsixtoesontherightfoot，butonlyfivetoesontheleft；
andlastly，aboywithonlyfivefingersandtoes。Intheseinstances，therefore，thevariety，asitwere，leapedoveronegenerationtoreproduceitselfinfullforceinthenext。Finally，thepurelypentadactyleAndrewasthefatherofmanychildren，notoneofwhomdepartedfromthenormalparentaltype。
Ifavariationwhichapproachesthenatureofamonstrositycanstrivethusforciblytoreproduceitself，itisnotwonderfulthatlessaberrantmodificationsshouldtendtobepreservedevenmorestrongly；
andthehistoryoftheAnconsheepis，inthisrespect，particularlyinstructive。Withthe"’cuteness"characteristicoftheirnation，theneighboursoftheMassachusettsfarmerimagineditwouldbeanexcellentthingifallhissheepwereimbuedwiththestay-at-hometendenciesenforcedbyNatureuponthenewly-arrivedram；andtheyadvisedWrighttokilltheoldpatriarchofhisfold，andinstalltheAnconraminhisplace。Theresultjustifiedtheirsagaciousanticipations，andcoincidedverynearlywithwhatoccurredtotheprogenyofGratioKelleia。TheyounglambswerealmostalwayseitherpureAncons，orpureordinarysheep。*ButwhensufficientAnconsheepwereobtainedtointerbreedwithoneanother，itwasfoundthattheoffspringwasalwayspureAncon。ColonelHumphreys，infact，statesthathewasacquaintedwithonly"onequestionablecaseofacontrarynature。"Here，then，isaremarkableandwell-establishedinstance，notonlyofaverydistinctracebeingestablished’persaltum’，butofthatracebreeding"true"atonce，andshowingnomixedforms，evenwhencrossedwithanotherbreed。
[footnote]*ColonelHumphreys’statementsareexceedinglyexplicitonthispoint：——"WhenanAnconeweisimpregnatedbyacommonram，theincreaseresembleswhollyeithertheeweortheram。TheincreaseofthecommoneweimpregnatedbyanAnconramfollowsentirelytheoneortheother，withoutblendinganyofthedistinguishingandessentialpeculiaritiesofboth。FrequentinstanceshavehappenedwherecommoneweshavehadtwinsbyAnconrams，whenoneexhibitedthecompletemarksandfeaturesoftheewe，theotheroftheram。Thecontrasthasbeenrenderedsingularlystriking，whenoneshort-leggedandonelong-leggedlamb，producedatabirth，havebeenseensuckingthedamatthesametime。"——’PhilosophicalTransactions’，1813，Pt。I。pp。89，90。
BytakingcaretoselectAnconsofbothsexes，forbreedingfrom，itthusbecameeasytoestablishanextremelywell-markedrace；sopeculiarthat，evenwhenherdedwithothersheep，itwasnotedthattheAnconskepttogether。Andthereiseveryreasontobelievethattheexistenceofthisbreedmighthavebeenindefinitelyprotracted；buttheintroductionoftheMerinosheep，whichwerenotonlyverysuperiortotheAnconsinwoolandmeat，butquiteasquietandorderly，ledtothecompleteneglectofthenewbreed，sothat，in1813，ColonelHumphreysfounditdifficulttoobtainthespecimen，whoseskeletonwaspresentedtoSirJosephBanks。Webelievethat，formanyyears，noremnantofithasexistedintheUnitedStates。
GratioKelleiawasnottheprogenitorofaraceofsix-fingeredmen，asSethWright’srambecameanationofAnconsheep，thoughthetendencyofthevarietytoperpetuateitselfappearstohavebeenfullyasstrongintheonecaseasintheother。Andthereasonofthedifferenceisnotfartoseek。SethWrighttookcarenottoweakentheAnconbloodbymatchinghisAnconeweswithanybutmalesofthesamevariety，whileGratioKelleia’ssonsweretoofarremovedfromthepatriarchaltimestointermarrywiththeirsisters；andhisgrandchildrenseemnottohavebeenattractedbytheirsix-fingeredcousins。Inotherwords，intheoneexamplearacewasproduced，because，forseveralgenerations，carewastakento’select’bothparentsofthebreedingstockfromanimalsexhibitingatendencytovaryinthesamecondition；while，intheother，noracewasevolved，becausenosuchselectionwasexercised。Araceisapropagatedvariety；andas，bythelawsofreproduction，offspringtendtoassumetheparentalforms，theywillbemorelikelytopropagateavariationexhibitedbybothparentsthanthatpossessedbyonlyone。
Thereisnoorganofthebodyofananimalwhichmaynot，anddoesnot，occasionally，varymoreorlessfromthenormaltype；andthereisnovariationwhichmaynotbetransmittedandwhich，ifselectivelytransmitted，maynotbecomethefoundationofarace。Thisgreattruth，sometimesforgottenbyphilosophers，haslongbeenfamiliartopracticalagriculturistsandbreeders；anduponitrestallthemethodsofimprovingthebreedsofdomesticanimals，which，forthelastcentury，havebeenfollowedwithsomuchsuccessinEngland。Colour，form，size，textureofhairorwool，proportionsofvariousparts，strengthorweaknessofconstitution，tendencytofattenortoremainlean，togivemuchorlittlemilk，speed，strength，temper，intelligence，specialinstincts；thereisnotoneofthesecharacterswhosetransmissionisnotanevery-dayoccurrencewithintheexperienceofcattle-breeders，stock-farmers，horse-dealers，anddogandpoultryfanciers。Nay，itisonlytheotherdaythataneminentphysiologist，Dr。Brown-Sequard，communicatedtotheRoyalSocietyhisdiscoverythatepilepsy，artificiallyproducedinguinea-pigs，byameanswhichhehasdiscovered，istransmittedtotheiroffspring。
Butarace，onceproduced，isnomoreafixedandimmutableentitythanthestockwhenceitsprang；variationsariseamongitsmembers，andasthesevariationsaretransmittedlikeanyothers，newracesmaybedevelopedoutofthepre-existingone’adinfinitum’，or，atleast，withinanylimitatpresentdetermined。Givensufficienttimeandsufficientlycarefulselection，andthemultitudeofraceswhichmayarisefromacommonstockisasastonishingasaretheextremestructuraldifferenceswhichtheymaypresent。Aremarkableexampleofthisistobefoundintherock-pigeon，whichDr。Darwinhas，inouropinion，satisfactorilydemonstratedtobetheprogenitorofallourdomesticpigeons，ofwhichtherearecertainlymorethanahundredwell-markedraces。Themostnoteworthyoftheseracesare，thefourgreatstocksknowntothe"fancy"astumblers，pouters，carriers，andfantails；birdswhichnotonlydiffermostsingularlyinsize，colour，andhabits，butintheformofthebeakandoftheskull：intheproportionsofthebeaktotheskull；inthenumberoftail-feathers；
intheabsoluteandrelativesizeofthefeet；inthepresenceorabsenceoftheuropygialgland；inthenumberofvertebraeintheback；
inshort，inpreciselythosecharactersinwhichthegeneraandspeciesofbirdsdifferfromoneanother。
Anditismostremarkableandinstructivetoobserve，thatnoneoftheseracescanbeshowntohavebeenoriginatedbytheactionofchangesinwhatarecommonlycalledexternalcircumstances，uponthewildrock-pigeon。Onthecontrary，fromtimeimmemorial，pigeon-fanciershavehadessentiallysimilarmethodsoftreatingtheirpets，whichhavebeenhoused，fed，protectedandcaredforinmuchthesamewayinallpigeonries。Infact，thereisnocasebetteradaptedthanthatofthepigeonstorefutethedoctrinewhichoneseesputforthonhighauthority，that"noothercharactersthanthosefoundedonthedevelopmentofbonefortheattachmentofmuscles"arecapableofvariation。Inprecisecontradictionofthishastyassertion，Mr。
Darwin’sresearchesprovethattheskeletonofthewingsindomesticpigeonshashardlyvariedatallfromthatofthewildtype；while，ontheotherhand，itisinexactlythoserespects，suchastherelativelengthofthebeakandskull，thenumberofthevertebrae，andthenumberofthetail-feathers，inwhichmuscularexertioncanhavenoimportantinfluence，thattheutmostamountofvariationhastakenplace。
Wehavesaidthatthefollowingoutofthepropertiesexhibitedbyphysiologicalspecieswouldleadusintodifficulties，andatthispointtheybegintobeobvious；forif，astheresultofspontaneousvariationandofselectivebreeding，theprogenyofacommonstockmaybecomeseparatedintogroupsdistinguishedfromoneanotherbyconstant，notsexual，morphologicalcharacters，itisclearthatthephysiologicaldefinitionofspeciesislikelytoclashwiththemorphologicaldefinition。Noonewouldhesitatetodescribethepouterandthetumblerasdistinctspecies，iftheywerefoundfossil，oriftheirskinsandskeletonswereimported，asthoseofexoticwildbirdscommonlyare——andwithoutdoubt，ifconsideredalone，theyaregoodanddistinctmorphologicalspecies。Ontheotherhand，theyarenotphysiologicalspecies，fortheyaredescendedfromacommonstock，therock-pigeon。
Underthesecircumstances，asitisadmittedonallsidesthatracesoccurinNature，howarewetoknowwhetheranyapparentlydistinctanimalsarereallyofdifferentphysiologicalspecies，ornot，seeingthattheamountofmorphologicaldifferenceisnosafeguide？Isthereanytestofaphysiologicalspecies？Theusualanswerofphysiologistsisintheaffirmative。Itissaidthatsuchatestistobefoundinthephenomenaofhybridization——intheresultsofcrossingraces，ascomparedwiththeresultsofcrossingspecies。
Sofarastheevidencegoesatpresent，individuals，ofwhatarecertainlyknowntobemereracesproducedbyselection，howeverdistincttheymayappeartobe，notonlybreedfreelytogether，buttheoffspringofsuchcrossedracesareonlyperfectlyfertilewithoneanother。Thus，thespanielandthegreyhound，thedray-horseandtheArab，thepouterandthetumbler，breedtogetherwithperfectfreedom，andtheirmongrels，ifmatchedwithothermongrelsofthesamekind，areequallyfertile。
Ontheotherhand，therecanbenodoubtthattheindividualsofmanynaturalspeciesareeitherabsolutelyinfertileifcrossedwithindividualsofotherspecies，or，iftheygiverisetohybridoffspring，thehybridssoproducedareinfertilewhenpairedtogether。
Thehorseandtheass，forinstance，ifsocrossed，giverisetothemule，andthereisnocertainevidenceofoffspringeverhavingbeenproducedbyamaleandfemalemule。Theunionsoftherock-pigeonandthering-pigeonappeartobeequallybarrenofresult。Here，then，saysthephysiologist，wehaveameansofdistinguishinganytwotruespeciesfromanytwovarieties。Ifamaleandafemale，selectedfromeachgroup，produceoffspring，andthatoffspringisfertilewithothersproducedinthesameway，thegroupsareracesandnotspecies。If，ontheotherhand，noresultensues，oriftheoffspringareinfertilewithothersproducedinthesameway，theyaretruephysiologicalspecies。Thetestwouldbeanadmirableone，if，inthefirstplace，itwerealwayspracticabletoapplyit，andif，inthesecond，italwaysyieldedresultssusceptibleofadefiniteinterpretation。
Unfortunately，inthegreatmajorityofcases，thistouchstoneforspeciesiswhollyinapplicable。
Theconstitutionofmanywildanimalsissoalteredbyconfinementthattheywillnotbreedevenwiththeirownfemales，sothatthenegativeresultsobtainedfromcrossesareofnovalue；andtheantipathyofwildanimalsofthesamespeciesforoneanother，orevenofwildandtamemembersofthesamespecies，isordinarilysogreat，thatitishopelesstolookforsuchunionsinNature。Thehermaphrodismofmostplants，thedifficultyinthewayofinsuringtheabsenceoftheirown，ortheproperworkingofotherpollen，areobstaclesofnolessmagnitudeinapplyingthetesttothem。And，inbothanimalsandplants，issuperaddedthefurtherdifficulty，thatexperimentsmustbecontinuedoveralongtimeforthepurposeofascertainingthefertilityofthemongrelorhybridprogeny，aswellasofthefirstcrossesfromwhichtheyspring。
Notonlydothesegreatpracticaldifficultieslieinthewayofapplyingthehybridizationtest，butevenwhenthisoraclecanbequestioned，itsrepliesaresometimesasdoubtfulasthoseofDelphi。
Forexample，casesarecitedbyMr。Darwin，ofplantswhicharemorefertilewiththepollenofanotherspeciesthanwiththeirown；andthereareothers，suchascertain’fuci’，whosemaleelementwillfertilizetheovuleofaplantofdistinctspecies，whilethemalesofthelatterspeciesareineffectivewiththefemalesofthefirst。Sothat，inthelast-namedinstance，aphysiologist，whoshouldcrossthetwospeciesinoneway，woulddecidethattheyweretruespecies；whileanother，whoshouldcrosstheminthereverseway，would，withequaljustice，accordingtotherule，pronouncethemtobemereraces。
Severalplants，whichthereisgreatreasontobelievearemerevarieties，arealmoststerilewhencrossed；whilebothanimalsandplants，whichhavealwaysbeenregardedbynaturalistsasofdistinctspecies，turnout，whenthetestisapplied，tobeperfectlyfertile。
Again，thesterilityorfertilityofcrossesseemstobearnorelationtothestructuralresemblancesordifferencesofthemembersofanytwogroups。
Mr。Darwinhasdiscussedthisquestionwithsingularabilityandcircumspection，andhisconclusionsaresummedupasfollows，atpage276ofhiswork：——
"Firstcrossesbetweenformssufficientlydistincttoberankedasspecies，andtheirhybrids，areverygenerally，butnotuniversally，sterile。Thesterilityisofalldegrees，andisoftensoslightthatthetwomostcarefulexperimentalistswhohaveeverlivedhavecometodiametricallyoppositeconclusionsinrankingformsbythistest。Thesterilityisinnatelyvariableinindividualsofthesamespecies，andiseminentlysusceptibleoffavourableandunfavourableconditions。Thedegreeofsterilitydoesnotstrictlyfollowsystematicaffinity，butisgovernedbyseveralcuriousandcomplexlaws。Itisgenerallydifferentandsometimeswidelydifferent，inreciprocalcrossesbetweenthesametwospecies。Itisnotalwaysequalindegreeinafirstcross，andinthehybridproducedfromthiscross。
"Inthesamemannerasingraftingtrees，thecapacityofonespeciesorvarietytotakeonanotherisincidentalongenerallyunknowndifferencesintheirvegetativesystems；soincrossing，thegreaterorlessfacilityofonespeciestounitewithanotherisincidentalonunknowndifferencesintheirreproductivesystems。ThereisnomorereasontothinkthatspecieshavebeenspeciallyendowedwithvariousdegreesofsterilitytopreventthemcrossingandbreedinginNature，thantothinkthattreeshavebeenspeciallyendowedwithvariousandsomewhatanalogousdegreesofdifficultyinbeinggraftedtogether，inordertopreventthembecominginarchedinourforests。
"Thesterilityoffirstcrossesbetweenpurespecies，whichhavetheirreproductivesystemsperfect，seemstodependonseveralcircumstances；
insomecaseslargelyontheearlydeathoftheembryo。Thesterilityofhybridswhichhavetheirreproductivesystemsimperfect，andwhichhavehadthissystemandtheirwholeorganizationdisturbedbybeingcompoundedoftwodistinctspecies，seemscloselyalliedtothatsterilitywhichsofrequentlyaffectspurespecieswhentheirnaturalconditionsoflifehavebeendisturbed。Thisviewissupportedbyaparallelismofanotherkind：namely，thatthecrossingofforms，onlyslightlydifferent，isfavourabletothevigourandfertilityoftheoffspring；andthatslightchangesintheconditionsoflifeareapparentlyfavourabletothevigourandfertilityofallorganicbeings。