Manycasesareonrecordshowinghowcomplexandunexpectedarethechecksandrelationsbetweenorganicbeings，whichhavetostruggletogetherinthesamecountry。Iwillgiveonlyasingleinstance，which，thoughasimpleone，hasinterestedme。InStaffordshire，ontheestateofarelationwhereIhadamplemeansofinvestigation，therewasalargeandextremelybarrenheath，whichhadneverbeentouchedbythehandofman。butseveralhundredacresofexactlythesamenaturehadbeenenclosedtwenty-fiveyearspreviouslyandplantedwithScotchfir。Thechangeinthenativevegetationoftheplantedpartoftheheathwasmostremarkable，morethanisgenerallyseeninpassingfromonequitedifferentsoiltoanother：

notonlytheproportionalnumbersoftheheath-plantswerewhollychanged，buttwelvespeciesofplants(notcountinggrassesandcarices)flourishedintheplantations，whichcouldnotbefoundontheheath。Theeffectontheinsectsmusthavebeenstillgreater，forsixinsectivorousbirdswereverycommonintheplantations，whichwerenottobeseenontheheath。

andtheheathwasfrequentedbytwoorthreedistinctinsectivorousbirds。

Hereweseehowpotenthasbeentheeffectoftheintroductionofasingletree，nothingwhateverelsehavingbeendone，withtheexceptionthatthelandhadbeenenclosed，sothatcattlecouldnotenter。Buthowimportantanelementenclosureis，IplainlysawnearFarnham，inSurrey。Herethereareextensiveheaths，withafewclumpsofoldScotchfirsonthedistanthill-tops：withinthelasttenyearslargespaceshavebeenenclosed，andself-sownfirsarenowspringingupinmultitudes，soclosetogetherthatallcannotlive。WhenIascertainedthattheseyoungtreeshadnotbeensownorplanted，IwassomuchsurprisedattheirnumbersthatIwenttoseveralpointsofview，whenceIcouldexaminehundredsofacresoftheunenclosedheath，andliterallyIcouldnotseeasingleScotchfir，excepttheoldplantedclumps。Butonlookingcloselybetweenthestemsoftheheath，Ifoundamultitudeofseedlingsandlittletrees，whichhadbeenperpetuallybrowseddownbythecattle。Inonesquareyard，atapointsomehundredsyardsdistantfromoneoftheoldclumps，Icountedthirty-twolittletrees。andoneofthem，judgingfromtheringsofgrowth，hadduringtwenty-sixyearstriedtoraiseitsheadabovethestemsoftheheath，andhadfailed。Nowonderthat，assoonasthelandwasenclosed，itbecamethicklyclothedwithvigorouslygrowingyoungfirs。Yettheheathwassoextremelybarrenandsoextensivethatnoonewouldeverhaveimaginedthatcattlewouldhavesocloselyandeffectuallysearcheditforfood。

HereweseethatcattleabsolutelydeterminetheexistenceoftheScotchfir。butinseveralpartsoftheworldinsectsdeterminetheexistenceofcattle。PerhapsParaguayoffersthemostcuriousinstanceofthis。forhereneithercattlenorhorsesnordogshaveeverrunwild，thoughtheyswarmsouthwardandnorthwardinaferalstate。andAzaraandRenggerhaveshownthatthisiscausedbythegreaternumberinParaguayofacertainfly，whichlaysitseggsinthenavelsoftheseanimalswhenfirstborn。

Theincreaseoftheseflies，numerousastheyare，mustbehabituallycheckedbysomemeans，probablybybirds。Hence，ifcertaininsectivorousbirds(whosenumbersareprobablyregulatedbyhawksorbeastsofprey)weretoincreaseinParaguay，theflieswoulddecreasethencattleandhorseswouldbecomeferal，andthiswouldcertainlygreatlyalter(asindeedI

haveobservedinpartsofSouthAmerica)thevegetation：thisagainwouldlargelyaffecttheinsects。andthis，aswejusthaveseeninStaffordshire，theinsectivorousbirds，andsoonwardsinever-increasingcirclesofcomplexity。

Webeganthisseriesbyinsectivorousbirds，andwehaveendedwiththem。

Notthatinnaturetherelationscaneverbeassimpleasthis。Battlewithinbattlemusteverberecurringwithvaryingsuccess。andyetinthelong-runtheforcesaresonicelybalanced，thatthefaceofnatureremainsuniformforlongperiodsoftime，thoughassuredlythemeresttriflewouldoftengivethevictorytooneorganicbeingoveranother。Neverthelesssoprofoundisourignorance，andsohighourpresumption，thatwemarvelwhenwehearoftheextinctionofanorganicbeing。andaswedonotseethecause，weinvokecataclysmstodesolatetheworld，orinventlawsonthedurationoftheformsoflife！

Iamtemptedtogiveonemoreinstanceshowinghowplantsandanimals，mostremoteinthescaleofnature，areboundtogetherbyawebofcomplexrelations。IshallhereafterhaveoccasiontoshowthattheexoticLobeliafulgens，inthispartofEngland，isnevervisitedbyinsects，andconsequently，fromitspeculiarstructure，nevercansetaseed。Manyofourorchidaceousplantsabsolutelyrequirethevisitsofmothstoremovetheirpollen-massesandthustofertilisethem。Ihave，also，reasontobelievethathumble-beesareindispensabletothefertilisationoftheheartsease(Violatricolor)，forotherbeesdonotvisitthisflower。FromexperimentswhichIhavetried，Ihavefoundthatthevisitsofbees，ifnotindispensable，areatleasthighlybeneficialtothefertilisationofourclovers。buthumble-beesalonevisitthecommonredclover(Trifoliumpratense)，asotherbeescannotreachthenectar。HenceIhaveverylittledoubt，thatifthewholegenusofhumble-beesbecameextinctorveryrareinEngland，theheartseaseandredcloverwouldbecomeveryrare，orwhollydisappear。Thenumberofhumble-beesinanydistrictdependsinagreatdegreeonthenumberoffield-mice，whichdestroytheircombsandnests。andMrH。Newman，whohaslongattendedtothehabitsofhumble-bees，believesthat’morethantwothirdsofthemarethusdestroyedalloverEngland。’Nowthenumberofmiceislargelydependent，aseveryoneknows，onthenumberofcats。andMrNewmansays，’NearvillagesandsmalltownsIhavefoundthenestsofhumble-beesmorenumerousthanelsewhere，whichIattributetothenumberofcatsthatdestroythemice。’Henceitisquitecrediblethatthepresenceofafelineanimalinlargenumbersinadistrictmightdetermine，throughtheinterventionfirstofmiceandthenofbees，thefrequencyofcertainflowersinthatdistrict！

Inthecaseofeveryspecies，manydifferentchecks，actingatdifferentperiodsoflife，andduringdifferentseasonsoryears，probablycomeintoplay。someonecheckorsomefewbeinggenerallythemostpotent，butallconcurringindeterminingtheaveragenumberoreventheexistenceofthespecies。Insomecasesitcanbeshownthatwidely-differentchecksactonthesamespeciesindifferentdistricts。Whenwelookattheplantsandbushesclothinganentangledbank，wearetemptedtoattributetheirproportionalnumbersandkindstowhatwecallchance。Buthowfalseaviewisthis！EveryonehasheardthatwhenanAmericanforestiscutdown，averydifferentvegetationspringsup。butithasbeenobservedthatthetreesnowgrowingontheancientIndianmounds，intheSouthernUnitedStates，displaythesamebeautifuldiversityandproportionofkindsasinthesurroundingvirginforests。Whatastrugglebetweentheseveralkindsoftreesmustherehavegoneonduringlongcenturies，eachannuallyscatteringitsseedsbythethousand。whatwarbetweeninsectandinsectbetweeninsects，snails，andotheranimalswithbirdsandbeastsofpreyallstrivingtoincrease，andallfeedingoneachotheroronthetreesortheirseedsandseedlings，orontheotherplantswhichfirstclothedthegroundandthuscheckedthegrowthofthetrees！Throwupahandfuloffeathers，andallmustfalltothegroundaccordingtodefinitelaws。

buthowsimpleisthisproblemcomparedtotheactionandreactionoftheinnumerableplantsandanimalswhichhavedetermined，inthecourseofcenturies，theproportionalnumbersandkindsoftreesnowgrowingontheoldIndianruins！

Thedependencyofoneorganicbeingonanother，asofaparasiteonitsprey，liesgenerallybetweenbeingsremoteinthescaleofnature。

Thisisoftenthecasewiththosewhichmaystrictlybesaidtostrugglewitheachotherforexistence，asinthecaseoflocustsandgrass-feedingquadrupeds。Butthestrugglealmostinvariablywillbemostseverebetweentheindividualsofthesamespecies，fortheyfrequentthesamedistricts，requirethesamefood，andareexposedtothesamedangers。Inthecaseofvarietiesofthesamespecies，thestrugglewillgenerallybealmostequallysevere，andwesometimesseethecontestsoondecided：forinstance，ifseveralvarietiesofwheatbesowntogether，andthemixedseedberesown，someofthevarietieswhichbestsuitthesoilorclimate，orarenaturallythemostfertile，willbeattheothersandsoyieldmoreseed，andwillconsequentlyinafewyearsquitesupplanttheothervarieties。Tokeepupamixedstockofevensuchextremelyclosevarietiesasthevariouslycolouredsweet-peas，theymustbeeachyearharvestedseparately，andtheseedthenmixedindueproportion，otherwisetheweakerkindswillsteadilydecreaseinnumbersanddisappear。Soagainwiththevarietiesofsheep：

ithasbeenassertedthatcertainmountain-varietieswillstarveoutothermountain-varieties，sothattheycannotbekepttogether。Thesameresulthasfollowedfromkeepingtogetherdifferentvarietiesofthemedicinalleech。Itmayevenbedoubtedwhetherthevarietiesofanyoneofourdomesticplantsoranimalshavesoexactlythesamestrength，habits，andconstitution，thattheoriginalproportionsofamixedstockcouldbekeptupforhalfadozengenerations，iftheywereallowedtostruggletogether，likebeingsinastateofnature，andiftheseedoryoungwerenotannuallysorted。

Asspeciesofthesamegenushaveusually，thoughbynomeansinvariably，somesimilarityinhabitsandconstitution，andalwaysinstructure，thestrugglewillgenerallybemoreseverebetweenspeciesofthesamegenus，whentheycomeintocompetitionwitheachother，thanbetweenspeciesofdistinctgenera。WeseethisintherecentextensionoverpartsoftheUnitedStatesofonespeciesofswallowhavingcausedthedecreaseofanotherspecies。Therecentincreaseofthemissel-thrushinpartsofScotlandhascausedthedecreaseofthesong-thrush。Howfrequentlywehearofonespeciesofrattakingtheplaceofanotherspeciesunderthemostdifferentclimates！InRussiathesmallAsiaticcockroachhaseverywheredrivenbeforeititsgreatcongener。Onespeciesofcharlockwillsupplantanother，andsoinothercases。Wecandimlyseewhythecompetitionshouldbemostseverebetweenalliedforms，whichfillnearlythesameplaceintheeconomyofnature。butprobablyinnoonecasecouldwepreciselysaywhyonespecieshasbeenvictoriousoveranotherinthegreatbattleoflife。

Acorollaryofthehighestimportancemaybededucedfromtheforegoingremarks，namely，thatthestructureofeveryorganicbeingisrelated，inthemostessentialyetoftenhiddenmanner，tothatofallotherorganicbeings，withwhichitcomesintocompetitionforfoodorresidence，orfromwhichithastoescape，oronwhichitpreys。Thisisobviousinthestructureoftheteethandtalonsofthetiger。andinthatofthelegsandclawsoftheparasitewhichclingstothehaironthetiger’sbody。

Butinthebeautifullyplumedseedofthedandelion，andintheflattenedandfringedlegsofthewater-beetle，therelationseemsatfirstconfinedtotheelementsofairandwater。Yettheadvantageofplumedseedsnodoubtstandsintheclosestrelationtothelandbeingalreadythicklyclothedbyotherplants。sothattheseedsmaybewidelydistributedandfallonunoccupiedground。Inthewater-beetle，thestructureofitslegs，sowelladaptedfordiving，allowsittocompetewithotheraquaticinsects，tohuntforitsownprey，andtoescapeservingaspreytootheranimals。

Thestoreofnutrimentlaidupwithintheseedsofmanyplantsseemsatfirstsighttohavenosortofrelationtootherplants。Butfromthestronggrowthofyoungplantsproducedfromsuchseeds(aspeasandbeans)，whensowninthemidstoflonggrass，Isuspectthatthechiefuseofthenutrimentintheseedistofavourthegrowthoftheyoungseedling，whilststrugglingwithotherplantsgrowingvigorouslyallaround。

Lookataplantinthemidstofitsrange，whydoesitnotdoubleorquadrupleitsnumbers？Weknowthatitcanperfectlywellwithstandalittlemoreheatorcold，dampnessordryness，forelsewhereitrangesintoslightlyhotterorcolder，damperordrierdistricts。Inthiscasewecanclearlyseethatifwewishedinimaginationtogivetheplantthepowerofincreasinginnumber，weshouldhavetogiveitsomeadvantageoveritscompetitors，orovertheanimalswhichpreyedonit。Ontheconfinesofitsgeographicalrange，achangeofconstitutionwithrespecttoclimatewouldclearlybeanadvantagetoourplant。butwehavereasontobelievethatonlyafewplantsoranimalsrangesofar，thattheyaredestroyedbytherigouroftheclimatealone。Notuntilwereachtheextremeconfinesoflife，inthearcticregionsoronthebordersofanutterdesert，willcompetitioncease。Thelandmaybeextremelycoldordry，yettherewillbecompetitionbetweensomefewspecies，orbetweentheindividualsofthesamespecies，forthewarmestordampestspots。

Hence，also，wecanseethatwhenaplantoranimalisplacedinanewcountryamongstnewcompetitors，thoughtheclimatemaybeexactlythesameasinitsformerhome，yettheconditionsofitslifewillgenerallybechangedinanessentialmanner。Ifwewishedtoincreaseitsaveragenumbersinitsnewhome，weshouldhavetomodifyitinadifferentwaytowhatweshouldhavedoneinitsnativecountry。forweshouldhavetogiveitsomeadvantageoveradifferentsetofcompetitorsorenemies。

Itisgoodthustotryinourimaginationtogiveanyformsomeadvantageoveranother。Probablyinnosingleinstanceshouldweknowwhattodo，soastosucceed。Itwillconvinceusofourignoranceonthemutualrelationsofallorganicbeings。aconvictionasnecessary，asitseemstobedifficulttoacquire。Allthatwecando，istokeepsteadilyinmindthateachorganicbeingisstrivingtoincreaseatageometricalratio。thateachatsomeperiodofitslife，duringsomeseasonoftheyear，duringeachgenerationoratintervals，hastostruggleforlife，andtosuffergreatdestruction。

Whenwereflectonthisstruggle，wemayconsoleourselveswiththefullbelief，thatthewarofnatureisnotincessant，thatnofearisfelt，thatdeathisgenerallyprompt，andthatthevigorous，thehealthy，andthehappysurviveandmultiply。PreviousChapterNextChapterTheOriginofSpecies-Chapter4TheOriginofSpeciesbyCharlesDarwinPreviousChapterNextChapterChapter4-NaturalSelectionHowwillthestruggleforexistence，discussedtoobrieflyinthelastchapter，actinregardtovariation？Cantheprincipleofselection，whichwehaveseenissopotentinthehandsofman，applyinnature？Ithinkweshallseethatitcanactmosteffectually。Letitbeborneinmindinwhatanendlessnumberofstrangepeculiaritiesourdomesticproductions，and，inalesserdegree，thoseundernature，vary。andhowstrongthehereditarytendencyis。Underdomestication，itmaybetrulysaidthatthe，wholeorganisationbecomesinsomedegreeplastic。Letitbeborneinmindhowinfinitelycomplexandclose-fittingarethemutualrelationsofallorganicbeingstoeachotherandtotheirphysicalconditionsoflife。Canit，then，bethoughtimprobable，seeingthatvariationsusefultomanhaveundoubtedlyoccurred，thatothervariationsusefulinsomewaytoeachbeinginthegreatandcomplexbattleoflife，shouldsometimesoccurinthecourseofthousandsofgenerations？Ifsuchdooccur，canwedoubt(rememberingthatmanymoreindividualsarebornthancanpossiblysurvive)

thatindividualshavinganyadvantage，howeverslight，overothers，wouldhavethebestchanceofsurvivingandofprocreatingtheirkind？Ontheotherhand，wemayfeelsurethatanyvariationintheleastdegreeinjuriouswouldberigidlydestroyed。Thispreservationoffavourablevariationsandtherejectionofinjuriousvariations，IcallNaturalSelection。Variationsneitherusefulnorinjuriouswouldnotbeaffectedbynaturalselection，andwouldbeleftafluctuatingelement，asperhapsweseeinthespeciescalledpolymorphic。

Weshallbestunderstandtheprobablecourseofnaturalselectionbytakingthecaseofacountryundergoingsomephysicalchange，forinstance，ofclimate。Theproportionalnumbersofitsinhabitantswouldalmostimmediatelyundergoachange，andsomespeciesmightbecomeextinct。Wemayconclude，fromwhatwehaveseenoftheintimateandcomplexmannerinwhichtheinhabitantsofeachcountryareboundtogether，thatanychangeinthenumericalproportionsofsomeoftheinhabitants，independentlyofthechangeofclimateitself，wouldmostseriouslyaffectmanyoftheothers。

Ifthecountrywereopenonitsborders，newformswouldcertainlyimmigrate，andthisalsowouldseriouslydisturbtherelationsofsomeoftheformerinhabitants。Letitberememberedhowpowerfultheinfluenceofasingleintroducedtreeormammalhasbeenshowntobe。Butinthecaseofanisland，orofacountrypartlysurroundedbybarriers，intowhichnewandbetteradaptedformscouldnotfreelyenter，weshouldthenhaveplacesintheeconomyofnaturewhichwouldassuredlybebetterfilledup，ifsomeoftheoriginalinhabitantswereinsomemannermodified。for，hadtheareabeenopentoimmigration，thesesameplaceswouldhavebeenseizedonbyintruders。Insuchcase，everyslightmodification，whichinthecourseofageschancedtoarise，andwhichinanywayfavouredtheindividualsofanyofthespecies，bybetteradaptingthemtotheiralteredconditions，wouldtendtobepreserved。andnaturalselectionwouldthushavefreescopefortheworkofimprovement。

Wehavereasontobelieve，asstatedinthefirstchapter，thatachangeintheconditionsoflife，byspeciallyactingonthereproductivesystem，causesorincreasesvariability。andintheforegoingcasetheconditionsoflifearesupposedtohaveundergoneachange，andthiswouldmanifestlybefavourabletonaturalselection，bygivingabetterchanceofprofitablevariationsoccurring。andunlessprofitablevariationsdooccur，naturalselectioncandonothing。Notthat，asIbelieve，anyextremeamountofvariabilityisnecessary。asmancancertainlyproducegreatresultsbyaddingupinanygivendirectionmereindividualdifferences，socouldNature，butfarmoreeasily，fromhavingincomparablylongertimeatherdisposal。NordoIbelievethatanygreatphysicalchange，asofclimate，oranyunusualdegreeofisolationtocheckimmigration，isactuallynecessarytoproducenewandunoccupiedplacesfornaturalselectiontofillupbymodifyingandimprovingsomeofthevaryinginhabitants。Forasalltheinhabitantsofeachcountryarestrugglingtogetherwithnicelybalancedforces，extremelyslightmodificationsinthestructureorhabitsofoneinhabitantwouldoftengiveitanadvantageoverothers。andstillfurthermodificationsofthesamekindwouldoftenstillfurtherincreasetheadvantage。

Nocountrycanbenamedinwhichallthenativeinhabitantsarenowsoperfectlyadaptedtoeachotherandtothephysicalconditionsunderwhichtheylive，thatnoneofthemcouldanyhowbeimproved。forinallcountries，thenativeshavebeensofarconqueredbynaturalisedproductions，thattheyhaveallowedforeignerstotakefirmpossessionoftheland。Andasforeignershavethuseverywherebeatensomeofthenatives，wemaysafelyconcludethatthenativesmighthavebeenmodifiedwithadvantage，soastohavebetterresistedsuchintruders。

Asmancanproduceandcertainlyhasproducedagreatresultbyhismethodicalandunconsciousmeansofselection，whatmaynotnatureeffect？

Mancanactonlyonexternalandvisiblecharacters：naturecaresnothingforappearances，exceptinsofarastheymaybeusefultoanybeing。Shecanactoneveryinternalorgan，oneveryshadeofconstitutionaldifference，onthewholemachineryoflife。Manselectsonlyforhisowngood。Natureonlyforthatofthebeingwhichshetends。Everyselectedcharacterisfullyexercisedbyher。andthebeingisplacedunderwell-suitedconditionsoflife。Mankeepsthenativesofmanyclimatesinthesamecountry。heseldomexerciseseachselectedcharacterinsomepeculiarandfittingmanner。

hefeedsalongandashortbeakedpigeononthesamefood。hedoesnotexercisealong-backedorlong-leggedquadrupedinanypeculiarmanner。

heexposessheepwithlongandshortwooltothesameclimate。Hedoesnotallowthemostvigorousmalestostruggleforthefemales。Hedoesnotrigidlydestroyallinferioranimals，butprotectsduringeachvaryingseason，asfarasliesinhispower，allhisproductions。Heoftenbeginshisselectionbysomehalf-monstrousform。oratleastbysomemodificationprominentenoughtocatchhiseye，ortobeplainlyusefultohim。Undernature，theslightestdifferenceofstructureorconstitutionmaywellturnthenicely-balancedscaleinthestruggleforlife，andsobepreserved。

Howfleetingarethewishesandeffortsofman！howshorthistime！andconsequentlyhowpoorwillhisproductsbe，comparedwiththoseaccumulatedbynatureduringwholegeologicalperiods。Canwewonder，then，thatnature’sproductionsshouldbefar’truer’incharacterthanman’sproductions。

thattheyshouldbeinfinitelybetteradaptedtothemostcomplexconditionsoflife，andshouldplainlybearthestampoffarhigherworkmanship？

Itmaybesaidthatnaturalselectionisdailyandhourlyscrutinising，throughouttheworld，everyvariation，eventheslightest。rejectingthatwhichisbad，preservingandaddingupallthatisgood。silentlyandinsensiblyworking，wheneverandwhereveropportunityoffers，attheimprovementofeachorganicbeinginrelationtoitsorganicandinorganicconditionsoflife。Weseenothingoftheseslowchangesinprogress，untilthehandoftimehasmarkedthelonglapsesofages，andthensoimperfectisourviewintolongpastgeologicalages，thatweonlyseethattheformsoflifearenowdifferentfromwhattheyformerlywere。

Althoughnaturalselectioncanactonlythroughandforthegoodofeachbeing，yetcharactersandstructures，whichweareapttoconsiderasofverytriflingimportance，maythusbeactedon。Whenweseeleaf-eatinginsectsgreen，andbark-feedersmottled-grey。thealpineptarmiganwhiteinwinter，thered-grousethecolourofheather，andtheblack-grousethatofpeatyearth，wemustbelievethatthesetintsareofservicetothesebirdsandinsectsinpreservingthemfromdanger。Grouse，ifnotdestroyedatsomeperiodoftheirlives，wouldincreaseincountlessnumbers。theyareknowntosufferlargelyfrombirdsofprey。andhawksareguidedbyeyesighttotheirprey，somuchso，thatonpartsoftheContinentpersonsarewarnednottokeepwhitepigeons，asbeingthemostliabletodestruction。

HenceIcanseenoreasontodoubtthatnaturalselectionmightbemosteffectiveingivingthepropercolourtoeachkindofgrouse，andinkeepingthatcolour，whenonceacquired，trueandconstant。Noroughtwetothinkthattheoccasionaldestructionofananimalofanyparticularcolourwouldproducelittleeffect：weshouldrememberhowessentialitisinaflockofwhitesheeptodestroyeverylambwiththefaintesttraceofblack。

Inplantsthedownonthefruitandthecolourofthefleshareconsideredbybotanistsascharactersofthemosttriflingimportance：yetwehearfromanexcellenthorticulturist，Downing，thatintheUnitedStatessmooth-skinnedfruitssufferfarmorefromabeetle，acurculio，thanthosewithdown。

thatpurpleplumssufferfarmorefromacertaindiseasethanyellowplums。

whereasanotherdiseaseattacksyellow-fleshedpeachesfarmorethanthosewithothercolouredflesh。If，withalltheaidsofart，theseslightdifferencesmakeagreatdifferenceincultivatingtheseveralvarieties，assuredly，inastateofnature，wherethetreeswouldhavetostrugglewithothertreesandwithahostofenemies，suchdifferenceswouldeffectuallysettlewhichvariety，whetherasmoothordowny，ayelloworpurplefleshedfruit，shouldsucceed。

Inlookingatmanysmallpointsofdifferencebetweenspecies，which，asfarasourignorancepermitsustojudge，seemtobequiteunimportant，wemustnotforgetthatclimate，food，&amp。c。，probablyproducesomeslightanddirecteffect。Itis，however，farmorenecessarytobearinmindthattherearemanyunknownlawsofcorrelationofgrowth，which，whenonepartoftheorganisationismodifiedthroughvariation，andthemodificationsareaccumulatedbynaturalselectionforthegoodofthebeing，willcauseothermodifications，oftenofthemostunexpectednature。

Asweseethatthosevariationswhichunderdomesticationappearatanyparticularperiodoflife，tendtoreappearintheoffspringatthesameperiod。forinstance，intheseedsofthemanyvarietiesofourculinaryandagriculturalplants。inthecaterpillarandcocoonstagesofthevarietiesofthesilkworm。intheeggsofpoultry，andinthecolourofthedownoftheirchickens。inthehornsofoursheepandcattlewhennearlyadult。

soinastateofnature，naturalselectionwillbeenabledtoactonandmodifyorganicbeingsatanyage，bytheaccumulationofprofitablevariationsatthatage，andbytheirinheritanceatacorrespondingage。Ifitprofitaplanttohaveitsseedsmoreandmorewidelydisseminatedbythewind，Icanseenogreaterdifficultyinthisbeingeffectedthroughnaturalselection，thaninthecotton-planterincreasingandimprovingbyselectionthedowninthepodsonhiscotton-trees。Naturalselectionmaymodifyandadaptthelarvaofaninsecttoascoreofcontingencies，whollydifferentfromthosewhichconcernthematureinsect。Thesemodificationswillnodoubtaffect，throughthelawsofcorrelation，thestructureoftheadult。

andprobablyinthecaseofthoseinsectswhichliveonlyforafewhours，andwhichneverfeed，alargepartoftheirstructureismerelythecorrelatedresultofsuccessivechangesinthestructureoftheirlarvae。So，conversely，modificationsintheadultwillprobablyoftenaffectthestructureofthelarva。butinallcasesnaturalselectionwillensurethatmodificationsconsequentonothermodificationsatadifferentperiodoflife，shallnotbeintheleastdegreeinjurious：foriftheybecameso，theywouldcausetheextinctionofthespecies。

Naturalselectionwillmodifythestructureoftheyounginrelationtotheparent，andoftheparentinrelationtotheyoung。Insocialanimalsitwilladaptthestructureofeachindividualforthebenefitofthecommunity。

ifeachinconsequenceprofitsbytheselectedchange。Whatnaturalselectioncannotdo，istomodifythestructureofonespecies，withoutgivingitanyadvantage，forthegoodofanotherspecies。andthoughstatementstothiseffectmaybefoundinworksofnaturalhistory，Icannotfindonecasewhichwillbearinvestigation。Astructureusedonlyonceinananimal’swholelife，ifofhighimportancetoit，mightbemodifiedtoanyextentbynaturalselection。forinstance，thegreatjawspossessedbycertaininsects，andusedexclusivelyforopeningthecocoonorthehardtiptothebeakofnestlingbirds，usedforbreakingtheegg。Ithasbeenasserted，thatofthebestshort-beakedtumbler-pigeonsmoreperishintheeggthanareabletogetoutofit。sothatfanciersassistintheactofhatching。

Now，ifnaturehadtomakethebeakofafull-grownpigeonveryshortforthebird’sownadvantage，theprocessofmodificationwouldbeveryslow，andtherewouldbesimultaneouslythemostrigorousselectionoftheyoungbirdswithintheegg，whichhadthemostpowerfulandhardestbeaks，forallwithweakbeakswouldinevitablyperish：or，moredelicateandmoreeasilybrokenshellsmightbeselected，thethicknessoftheshellbeingknowntovarylikeeveryotherstructure。SexualSelectionInasmuchaspeculiaritiesoftenappearunderdomesticationinonesexandbecomehereditarilyattachedtothatsex，thesamefactprobablyoccursundernature，andifso，naturalselectionwillbeabletomodifyonesexinitsfunctionalrelationstotheothersex，orinrelationtowhollydifferenthabitsoflifeinthetwosexes，asissometimesthecasewithinsects。AndthisleadsmetosayafewwordsonwhatIcallSexualSelection。

Thisdepends，notonastruggleforexistence，butonastrugglebetweenthemalesforpossessionofthefemales。theresultisnotdeathtotheunsuccessfulcompetitor，butfewornooffspring。Sexualselectionis，therefore，lessrigorousthannaturalselection。Generally，themostvigorousmales，thosewhicharebestfittedfortheirplacesinnature，willleavemostprogeny。Butinmanycases，victorywilldependnotongeneralvigour，butonhavingspecialweapons，confinedtothemalesex。Ahornlessstagorspurlesscockwouldhaveapoorchanceofleavingoffspring。Sexualselectionbyalwaysallowingthevictortobreedmightsurelygiveindomitablecourage，lengthtothespur，andstrengthtothewingtostrikeinthespurredleg，aswellasthebrutalcock-fighter，whoknowswellthathecanimprovehisbreedbycarefulselectionofthebestcocks。Howlowinthescaleofnaturethislawofbattledescends，Iknownot。malealligatorshavebeendescribedasfighting，bellowing，andwhirlinground，likeIndiansinawar-dance，forthepossessionofthefemales。malesalmonshavebeenseenfightingalldaylong。malestag-beetlesoftenbearwoundsfromthehugemandiblesofothermales。Thewaris，perhaps，severestbetweenthemalesofpolygamousanimals，andtheseseemoftenestprovidedwithspecialweapons。Themalesofcarnivorousanimalsarealreadywellarmed。thoughtothemandtoothers，specialmeansofdefencemaybegiventhroughmeansofsexualselection，asthemanetothelion，theshoulder-padtotheboar，andthehookedjawtothemalesalmon。fortheshieldmaybeasimportantforvictory，astheswordorspear。

Amongstbirds，thecontestisoftenofamorepeacefulcharacter。Allthosewhohaveattendedtothesubject，believethatthereistheseverestrivalrybetweenthemalesofmanyspeciestoattractbysingingthefemales。

Therock-thrushofGuiana，birdsofparadise，andsomeothers，congregate。

andsuccessivemalesdisplaytheirgorgeousplumageandperformstrangeanticsbeforethefemales，whichstandingbyasspectators，atlastchoosethemostattractivepartner。Thosewhohavecloselyattendedtobirdsinconfinementwellknowthattheyoftentakeindividualpreferencesanddislikes：

thusSirR。Heronhasdescribedhowonepiedpeacockwaseminentlyattractivetoallhishenbirds。Itmayappearchildishtoattributeanyeffecttosuchapparentlyweakmeans：Icannothereenteronthedetailsnecessarytosupportthisview。butifmancaninashorttimegiveelegantcarriageandbeautytohisbantams，accordingtohisstandardofbeauty，Icanseenogoodreasontodoubtthatfemalebirds，byselecting，duringthousandsofgenerations，themostmelodiousorbeautifulmales，accordingtotheirstandardofbeauty，mightproduceamarkedeffect。Istronglysuspectthatsomewell-knownlawswithrespecttotheplumageofmaleandfemalebirds，incomparisonwiththeplumageoftheyoung，canbeexplainedontheviewofplumagehavingbeenchieflymodifiedbysexualselection，actingwhenthebirdshavecometothebreedingageorduringthebreedingseason。

themodificationsthusproducedbeinginheritedatcorrespondingagesorseasons，eitherbythemalesalone，orbythemalesandfemales。butI

havenotspaceheretoenteronthissubject。

Thusitis，asIbelieve，thatwhenthemalesandfemalesofanyanimalhavethesamegeneralhabitsoflife，butdifferinstructure，colour，orornament，suchdifferenceshavebeenmainlycausedbysexualselection。

thatis，individualmaleshavehad，insuccessivegenerations，someslightadvantageoverothermales，intheirweapons，meansofdefence，orcharms。

andhavetransmittedtheseadvantagestotheirmaleoffspring。Yet，Iwouldnotwishtoattributeallsuchsexualdifferencestothisagency：forweseepeculiaritiesarisingandbecomingattachedtothemalesexinourdomesticanimals(asthewattleinmalecarriers，horn-likeprotuberancesinthecocksofcertainfowls，&amp。c。)，whichwecannotbelievetobeeitherusefultothemalesinbattle，orattractivetothefemales。Weseeanalogouscasesundernature，forinstance，thetuftofhaironthebreastoftheturkey-cock，whichcanhardlybeeitherusefulorornamentaltothisbird。indeed，hadthetuftappearedunderdomestication，itwouldhavebeencalledamonstrosity。IllustrationsoftheactionofNaturalSelectionInordertomakeitclearhow，asIbelieve，naturalselectionacts，I

mustbegpermissiontogiveoneortwoimaginaryillustrations。Letustakethecaseofawolf，whichpreysonvariousanimals，securingsomebycraft，somebystrength，andsomebyfleetness。andletussupposethatthefleetestprey，adeerforinstance，hadfromanychangeinthecountryincreasedinnumbers，orthatotherpreyhaddecreasedinnumbers，duringthatseasonoftheyearwhenthewolfishardestpressedforfood。Icanundersuchcircumstancesseenoreasontodoubtthattheswiftestandslimmestwolveswouldhavethebestchanceofsurviving，andsobepreservedorselected，providedalwaysthattheyretainedstrengthtomastertheirpreyatthisoratsomeotherperiodoftheyear，whentheymightbecompelledtopreyonotheranimals。Icanseenomorereasontodoubtthis，thanthatmancanimprovethefleetnessofhisgreyhoundsbycarefulandmethodicalselection，orbythatunconsciousselectionwhichresultsfromeachmantryingtokeepthebestdogswithoutanythoughtofmodifyingthebreed。

Evenwithoutanychangeintheproportionalnumbersoftheanimalsonwhichourwolfpreyed，acubmightbebornwithaninnatetendencytopursuecertainkindsofprey。Norcanthisbethoughtveryimprobable。forweoftenobservegreatdifferencesinthenaturaltendenciesofourdomesticanimals。onecat，forinstance，takingtocatchrats，anothermice。onecat，accordingtoMr。St。John，bringinghomewingedgame，anotherharesorrabbits，andanotherhuntingonmarshygroundandalmostnightlycatchingwoodcocksorsnipes。Thetendencytocatchratsratherthanmiceisknowntobeinherited。Now，ifanyslightinnatechangeofhabitorofstructurebenefitedanindividualwolf，itwouldhavethebestchanceofsurvivingandofleavingoffspring。Someofitsyoungwouldprobablyinheritthesamehabitsorstructure，andbytherepetitionofthisprocess，anewvarietymightbeformedwhichwouldeithersupplantorcoexistwiththeparent-formofwolf。Or，again，thewolvesinhabitingamountainousdistrict，andthosefrequentingthelowlands，wouldnaturallybeforcedtohuntdifferentprey。andfromthecontinuedpreservationoftheindividualsbestfittedforthetwosites，twovarietiesmightslowlybeformed。Thesevarietieswouldcrossandblendwheretheymet。buttothissubjectofintercrossingweshallsoonhavetoreturn。Imayadd，that，accordingtoMr。Pierce，therearetwovarietiesofthewolfinhabitingtheCatskillMountainsintheUnitedStates，onewithalightgreyhound-likeform，whichpursuesdeer，andtheothermorebulky，withshorterlegs，whichmorefrequentlyattackstheshepherd’sflocks。

Letusnowtakeamorecomplexcase。Certainplantsexcreteasweetjuice，apparentlyforthesakeofeliminatingsomethinginjuriousfromtheirsap：thisiseffectedbyglandsatthebaseofthestipulesinsomeLeguminosae，andatthebackoftheleafofthecommonlaurel。Thisjuice，thoughsmallinquantity，isgreedilysoughtbyinsects。Letusnowsupposealittlesweetjuiceornectartobeexcretedbytheinnerbasesofthepetalsofaflower。Inthiscaseinsectsinseekingthenectarwouldgetdustedwithpollen，andwouldcertainlyoftentransportthepollenfromoneflowertothestigmaofanotherflower。Theflowersoftwodistinctindividualsofthesamespecieswouldthusgetcrossed。andtheactofcrossing，wehavegoodreasontobelieve(aswillhereafterbemorefullyalludedto)，wouldproduceveryvigorousseedlings，whichconsequentlywouldhavethebestchanceofflourishingandsurviving。Someoftheseseedlingswouldprobablyinheritthenectar-excretingpower。Thoseinindividualflowerswhichhadthelargestglandsornectaries，andwhichexcretedmostnectar，wouldbeoftenestvisitedbyinsects，andwouldbeoftenestcrossed。

andsointhelong-runwouldgaintheupperhand。Thoseflowers，also，whichhadtheirstamensandpistilsplaced，inrelationtothesizeandhabitsoftheparticularinsectswhichvisitedthem，soastofavourinanydegreethetransportaloftheirpollenfromflowertoflower，wouldlikewisebefavouredorselected。Wemighthavetakenthecaseofinsectsvisitingflowersforthesakeofcollectingpolleninsteadofnectar。andaspollenisformedforthesoleobjectoffertilisation，itsdestructionappearsasimplelosstotheplant。yetifalittlepollenwerecarried，atfirstoccasionallyandthenhabitually，bythepollen-devouringinsectsfromflowertoflower，andacrossthuseffected，althoughnine-tenthsofthepollenweredestroyed，itmightstillbeagreatgaintotheplant。

andthoseindividualswhichproducedmoreandmorepollen，andhadlargerandlargeranthers，wouldbeselected。

Whenourplant，bythisprocessofthecontinuedpreservationornaturalselectionofmoreandmoreattractiveflowers，hadbeenrenderedhighlyattractivetoinsects，theywould，unintentionallyontheirpart，regularlycarrypollenfromflowertoflower。andthattheycanmosteffectuallydothis，Icouldeasilyshowbymanystrikinginstances。Iwillgiveonlyonenotasaverystrikingcase，butaslikewiseillustratingonestepintheseparationofthesexesofplants，presentlytobealludedto。Someholly-treesbearonlymaleflowers，whichhavefourstamensproducingratherasmallquantityofpollen，andarudimentarypistil。otherholly-treesbearonlyfemaleflowers。thesehaveafull-sizedpistil，andfourstamenswithshrivelledanthers，inwhichnotagrainofpollencanbedetected。

Havingfoundafemaletreeexactlysixtyyardsfromamaletree，Iputthestigmasoftwentyflowers，takenfromdifferentbranches，underthemicroscope，andonall，withoutexception，therewerepollen-grains，andonsomeaprofusionofpollen。Asthewindhadsetforseveraldaysfromthefemaletothemaletree，thepollencouldnotthushavebeencarried。

Theweatherhadbeencoldandboisterous，andthereforenotfavourabletobees，neverthelesseveryfemaleflowerwhichIexaminedhadbeeneffectuallyfertilisedbythebees，accidentallydustedwithpollen，havingflownfromtreetotreeinsearchofnectar。Buttoreturntoourimaginarycase：

assoonastheplanthadbeenrenderedsohighlyattractivetoinsectsthatpollenwasregularlycarriedfromflowertoflower，anotherprocessmightcommence。Nonaturalistdoubtstheadvantageofwhathasbeencalledthe’physiologicaldivisionoflabour。’hencewemaybelievethatitwouldbeadvantageoustoaplanttoproducestamensaloneinoneflowerorononewholeplant，andpistilsaloneinanotherfloweroronanotherplant。

Inplantsundercultureandplacedundernewconditionsoflife，sometimesthemaleorgansandsometimesthefemaleorgansbecomemoreorlessimpotent。

nowifwesupposethistooccurineversoslightadegreeundernature，thenaspollenisalreadycarriedregularlyfromflowertoflower，andasamorecompleteseparationofthesexesofourplantwouldbeadvantageousontheprincipleofthedivisionoflabour，individualswiththistendencymoreandmoreincreased，wouldbecontinuallyfavouredorselected，untilatlastacompleteseparationofthesexeswouldbeeffected。

Letusnowturntothenectar-feedinginsectsinourimaginarycase：

wemaysupposetheplantofwhichwehavebeenslowlyincreasingthenectarbycontinuedselection，tobeacommonplant。andthatcertaininsectsdependedinmainpartonitsnectarforfood。Icouldgivemanyfacts，showinghowanxiousbeesaretosavetime。forinstance，theirhabitofcuttingholesandsuckingthenectaratthebasesofcertainflowers，whichtheycan，withaverylittlemoretrouble，enterbythemouth。Bearingsuchfactsinmind，Icanseenoreasontodoubtthatanaccidentaldeviationinthesizeandformofthebody，orinthecurvatureandlengthoftheproboscis，&amp。c。，fartooslighttobeappreciatedbyus，mightprofitabeeorotherinsect，sothatanindividualsocharacterisedwouldbeabletoobtainitsfoodmorequickly，andsohaveabetterchanceoflivingandleavingdescendants。Itsdescendantswouldprobablyinheritatendencytoasimilarslightdeviationofstructure。Thetubesofthecorollasofthecommonredandincarnateclovers(Trifoliumpratenseandincarnatum)

donotonahastyglanceappeartodifferinlength。yetthehive-beecaneasilysuckthenectaroutoftheincarnateclover，butnotoutofthecommonredclover，whichisvisitedbyhumble-beesalone。sothatwholefieldsoftheredcloverofferinvainanabundantsupplyofpreciousnectartothehive-bee。Thusitmightbeagreatadvantagetothehive-beetohaveaslightlylongerordifferentlyconstructedproboscis。Ontheotherhand，Ihavefoundbyexperimentthatthefertilityofclovergreatlydependsonbeesvisitingandmovingpartsofthecorolla，soastopushthepollenontothestigmaticsurface。Hence，again，ifhumble-beesweretobecomerareinanycountry，itmightbeagreatadvantagetotheredclovertohaveashorterormoredeeplydividedtubetoitscorolla，sothatthehive-beecouldvisititsflowers。ThusIcanunderstandhowaflowerandabeemightslowlybecome，eithersimultaneouslyoroneaftertheother，modifiedandadaptedinthemostperfectmannertoeachother，bythecontinuedpreservationofindividualspresentingmutualandslightlyfavourabledeviationsofstructure。

Iamwellawarethatthisdoctrineofnaturalselection，exemplifiedintheaboveimaginaryinstances，isopentothesameobjectionswhichwereatfirsturgedagainstSirCharlesLyell’snobleviewson’themodernchangesoftheearth，asillustrativeofgeology。’butwenowveryseldomheartheaction，forinstance，ofthecoast-waves，calledatriflingandinsignificantcause，whenappliedtotheexcavationofgiganticvalleysortotheformationofthelongestlinesofinlandcliffs。Naturalselectioncanactonlybythepreservationandaccumulationofinfinitesimallysmallinheritedmodifications，eachprofitabletothepreservedbeing。andasmoderngeologyhasalmostbanishedsuchviewsastheexcavationofagreatvalleybyasinglediluvialwave，sowillnaturalselection，ifitbeatrueprinciple，banishthebeliefofthecontinuedcreationofneworganicbeings，orofanygreatandsuddenmodificationintheirstructure。OntheIntercrossingofIndividualsImusthereintroduceashortdigression。Inthecaseofanimalsandplantswithseparatedsexes，itisofcourseobviousthattwoindividualsmustalwaysuniteforeachbirth。butinthecaseofhermaphroditesthisisfarfromobvious。NeverthelessIamstronglyinclinedtobelievethatwithallhermaphroditestwoindividuals，eitheroccasionallyorhabitually，concurforthereproductionoftheirkind。Thisview，Imayadd，wasfirstsuggestedbyAndrewKnight。Weshallpresentlyseeitsimportance。butImustheretreatthesubjectwithextremebrevity，thoughIhavethematerialspreparedforanamplediscussion。Allvertebrateanimals，allinsects，andsomeotherlargegroupsofanimals，pairforeachbirth。Modernresearchhasmuchdiminishedthenumberofsupposedhermaphrodites，andofrealhermaphroditesalargenumberpair。thatis，twoindividualsregularlyuniteforreproduction，whichisallthatconcernsus。Butstilltherearemanyhermaphroditeanimalswhichcertainlydonothabituallypair，andavastmajorityofplantsarehermaphrodites。Whatreason，itmaybeasked，isthereforsupposinginthesecasesthattwoindividualseverconcurinreproduction？Asitisimpossibleheretoenterondetails，I

musttrusttosomegeneralconsiderationsalone。

Inthefirstplace，Ihavecollectedsolargeabodyoffacts，showing，inaccordancewiththealmostuniversalbeliefofbreeders，thatwithanimalsandplantsacrossbetweendifferentvarieties，orbetweenindividualsofthesamevarietybutofanotherstrain，givesvigourandfertilitytotheoffspring。andontheotherhand，thatcloseinterbreedingdiminishesvigourandfertility。thatthesefactsaloneinclinemetobelievethatitisagenerallawofnature(utterlyignorantthoughwebeofthemeaningofthelaw)thatnoorganicbeingself-fertilisesitselfforaneternityofgenerations。butthatacrosswithanotherindividualisoccasionallyperhapsatverylongintervals——indispensable。

Onthebeliefthatthisisalawofnature，wecan，Ithink，understandseverallargeclassesoffacts，suchasthefollowing，whichonanyotherviewareinexplicable。Everyhybridizerknowshowunfavourableexposuretowetistothefertilisationofaflower，yetwhatamultitudeofflowershavetheiranthersandstigmasfullyexposedtotheweather！butifanoccasionalcrossbeindispensable，thefullestfreedomfortheentranceofpollenfromanotherindividualwillexplainthisstateofexposure，moreespeciallyastheplant’sownanthersandpistilgenerallystandsoclosetogetherthatself-fertilisationseemsalmostinevitable。Manyflowers，ontheotherhand，havetheirorgansoffructificationcloselyenclosed，asinthegreatpapilionaceousorpea-family。butinseveral，perhapsinall，suchflowers，thereisaverycuriousadaptationbetweenthestructureoftheflowerandthemannerinwhichbeessuckthenectar。for，indoingthis，theyeitherpushtheflower’sownpollenonthestigma，orbringpollenfromanotherflower。Sonecessaryarethevisitsofbeestopapilionaceousflowers，thatIhavefound，byexperimentspublishedelsewhere，thattheirfertilityisgreatlydiminishedifthesevisitsbeprevented。Now，itisscarcelypossiblethatbeesshouldflyfromflowertoflower，andnotcarrypollenfromonetotheother，tothegreatgood，asIbelieve，oftheplant。

Beeswillactlikeacamel-hairpencil，anditisquitesufficientjusttotouchtheanthersofoneflowerandthenthestigmaofanotherwiththesamebrushtoensurefertilisation。butitmustnotbesupposedthatbeeswouldthusproduceamultitudeofhybridsbetweendistinctspecies。

forifyoubringonthesamebrushaplant’sownpollenandpollenfromanotherspecies，theformerwillhavesuchaprepotenteffect，thatitwillinvariablyandcompletelydestroy，ashasbeenshownbyG&auml。rtner，anyinfluencefromtheforeignpollen。

Whenthestamensofaflowersuddenlyspringtowardsthepistil，orslowlymoveoneaftertheothertowardsit，thecontrivanceseemsadaptedsolelytoensureself-fertilisation。andnodoubtitisusefulforthisend：but，theagencyofinsectsisoftenrequiredtocausethestamenstospringforward，asK&ouml。lreuterhasshowntobethecasewiththebarberry。andcuriouslyinthisverygenus，whichseemstohaveaspecialcontrivanceforself-fertilisation，itiswellknownthatifveryclosely-alliedformsorvarietiesareplantedneareachother，itishardlypossibletoraisepureseedlings，solargelydotheynaturallycross。Inmanyothercases，farfromtherebeinganyaidsforself-fertilisation，therearespecialcontrivances，asIcouldshowfromthewritingsofC。C。Sprengelandfrommyownobservations，whicheffectuallypreventthestigmareceivingpollenfromitsownflower：forinstance，inLobeliafulgens，thereisareallybeautifulandelaboratecontrivancebywhicheveryoneoftheinfinitelynumerouspollen-granulesaresweptoutoftheconjoinedanthersofeachflower，beforethestigmaofthatindividualflowerisreadytoreceivethem。andasthisflowerisnevervisited，atleastinmygarden，byinsects，itneversetsaseed，thoughbyplacingpollenfromonefloweronthestigmaofanother，Iraisedplentyofseedlings。andwhilstanotherspeciesofLobeliagrowingcloseby，whichisvisitedbybees，seedsfreely。

Inverymanyothercases，thoughtherebenospecialmechanicalcontrivancetopreventthestigmaofaflowerreceivingitsownpollen，yet，asC。

C。Sprengelhasshown，andasIcanconfirm，eithertheanthersburstbeforethestigmaisreadyforfertilisation，orthestigmaisreadybeforethepollenofthatflowerisready，sothattheseplantshaveinfactseparatedsexes，andmusthabituallybecrossed。Howstrangearethesefacts！Howstrangethatthepollenandstigmaticsurfaceofthesameflower，thoughplacedsoclosetogether，asiffortheverypurposeofself-fertilisation，shouldinsomanycasesbemutuallyuselesstoeachother！Howsimplyarethesefactsexplainedontheviewofanoccasionalcrosswithadistinctindividualbeingadvantageousorindispensable！

Ifseveralvarietiesofthecabbage，radish，onion，andofsomeotherplants，beallowedtoseedneareachother，alargemajority，asIhavefound，oftheseedlingsthusraisedwillturnoutmongrels：forinstance，Iraised233seedlingcabbagesfromsomeplantsofdifferentvarietiesgrowingneareachother，andoftheseonly78weretruetotheirkind，andsomeevenofthesewerenotperfectlytrue。Yetthepistilofeachcabbage-flowerissurroundednotonlybyitsownsixstamens，butbythoseofthemanyotherflowersonthesameplant。How，then，comesitthatsuchavastnumberoftheseedlingsaremongrelised？Isuspectthatitmustarisefromthepollenofadistinctvarietyhavingaprepotenteffectoveraflower’sownpollen。andthatthisispartofthegenerallawofgoodbeingderivedfromtheintercrossingofdistinctindividualsofthesamespecies。Whendistinctspeciesarecrossedthecaseisdirectlythereverse，foraplant’sownpollenisalwaysprepotentoverforeignpollen。buttothissubjectweshallreturninafuturechapter。

Inthecaseofagigantictreecoveredwithinnumerableflowers，itmaybeobjectedthatpollencouldseldombecarriedfromtreetotree，andatmostonlyfromflowertofloweronthesametree，andthatflowersonthesametreecanbeconsideredasdistinctindividualsonlyinalimitedsense。Ibelievethisobjectiontobevalid，butthatnaturehaslargelyprovidedagainstitbygivingtotreesastrongtendencytobearflowerswithseparatedsexes。Whenthesexesareseparated，althoughthemaleandfemaleflowersmaybeproducedonthesametree，wecanseethatpollenmustberegularlycarriedfromflowertoflower。andthiswillgiveabetterchanceofpollenbeingoccasionallycarriedfromtreetotree。ThattreesbelongingtoallOrdershavetheirsexesmoreoftenseparatedthanotherplants，Ifindtobethecaseinthiscountry。andatmyrequestDrHookertabulatedthetreesofNewZealand，andDrAsaGraythoseoftheUnitedStates，andtheresultwasasIanticipated。Ontheotherhand，DrHookerhasrecentlyinformedmethathefindsthattheruledoesnotholdinAustralia。

andIhavemadethesefewremarksonthesexesoftreessimplytocallattentiontothesubject。

Turningforaverybriefspacetoanimals：onthelandtherearesomehermaphrodites，asland-molluscaandearth-worms。buttheseallpair。AsyetIhavenotfoundasinglecaseofaterrestrialanimalwhichfertilisesitself。Wecanunderstandthisremarkablefact，whichofferssostrongacontrastwithterrestrialplants，ontheviewofanoccasionalcrossbeingindispensable，byconsideringthemediuminwhichterrestrialanimalslive，andthenatureofthefertilisingelement。forweknowofnomeans，analogoustotheactionofinsectsandofthewindinthecaseofplants，bywhichanoccasionalcrosscouldbeeffectedwithterrestrialanimalswithouttheconcurrenceoftwoindividuals。Ofaquaticanimals，therearemanyself-fertilisinghermaphrodites。butherecurrentsinthewaterofferanobviousmeansforanoccasionalcross。And，asinthecaseofflowers，Ihaveasyetfailed，afterconsultationwithoneofthehighestauthorities，namely，ProfessorHuxley，todiscoverasinglecaseofanhermaphroditeanimalwiththeorgansofreproductionsoperfectlyenclosedwithinthebody，thataccessfromwithoutandtheoccasionalinfluenceofadistinctindividualcanbeshowntobephysicallyimpossible。Cirripedeslongappearedtometopresentacaseofverygreatdifficultyunderthispointofview。

butIhavebeenenabled，byafortunatechance，elsewheretoprovethattwoindividuals，thoughbothareself-fertilisinghermaphrodites，dosometimescross。

Itmusthavestruckmostnaturalistsasastrangeanomalythat，inthecaseofbothanimalsandplants，speciesofthesamefamilyandevenofthesamegenus，thoughagreeingcloselywitheachotherinalmosttheirwholeorganisation，yetarenotrarely，someofthemhermaphrodites，andsomeofthemunisexual。Butif，infact，allhermaphroditesdooccasionallyintercrosswithotherindividuals，thedifferencebetweenhermaphroditesandunisexualspecies，asfarasfunctionisconcerned，becomesverysmall。

FromtheseseveralconsiderationsandfromthemanyspecialfactswhichIhavecollected，butwhichIamnothereabletogive，Iamstronglyinclinedtosuspectthat，bothinthevegetableandanimalkingdoms，anoccasionalintercrosswithadistinctindividualisalawofnature。Iamwellawarethatthereare，onthisview，manycasesofdifficulty，someofwhichI

amtryingtoinvestigate。Finallythen，wemayconcludethatinmanyorganicbeings，acrossbetweentwoindividualsisanobviousnecessityforeachbirth。inmanyothersitoccursperhapsonlyatlongintervals。butinnone，asIsuspect，canself-fertilisationgoonforperpetuity。CircumstancesfavourabletoNaturalSelectionThisisanextremelyintricatesubject。Alargeamountofinheritableanddiversifiedvariabilityisfavourable，butIbelievemereindividualdifferencessufficeforthework。Alargenumberofindividuals，bygivingabetterchancefortheappearancewithinanygivenperiodofprofitablevariations，willcompensateforalesseramountofvariabilityineachindividual，andis，Ibelieve，anextremelyimportantelementofsuccess。Thoughnaturegrantsvastperiodsoftimefortheworkofnaturalselection，shedoesnotgrantanindefiniteperiod。forasallorganicbeingsarestriving，itmaybesaid，toseizeoneachplaceintheeconomyofnature，ifanyonespeciesdoesnotbecomemodifiedandimprovedinacorrespondingdegreewithitscompetitors，itwillsoonbeexterminated。

Inman’smethodicalselection，abreederselectsforsomedefiniteobject，andfreeintercrossingwillwhollystophiswork。Butwhenmanymen，withoutintendingtoalterthebreed，haveanearlycommonstandardofperfection，andalltrytogetandbreedfromthebestanimals，muchimprovementandmodificationsurelybutslowlyfollowfromthisunconsciousprocessofselection，notwithstandingalargeamountofcrossingwithinferioranimals。

Thusitwillbeinnature。forwithinaconfinedarea，withsomeplaceinitspolitynotsoperfectlyoccupiedasmightbe，naturalselectionwillalwaystendtopreservealltheindividualsvaryingintherightdirection，thoughindifferentdegrees，soasbettertofilluptheunoccupiedplace。

Butiftheareabelarge，itsseveraldistrictswillalmostcertainlypresentdifferentconditionsoflife。andthenifnaturalselectionbemodifyingandimprovingaspeciesintheseveraldistricts，therewillbeintercrossingwiththeotherindividualsofthesamespeciesontheconfinesofeach。

Andinthiscasetheeffectsofintercrossingcanhardlybecounterbalancedbynaturalselectionalwaystendingtomodifyalltheindividualsineachdistrictinexactlythesamemannertotheconditionsofeach。forinacontinuousarea，theconditionswillgenerallygraduateawayinsensiblyfromonedistricttoanother。Theintercrossingwillmostaffectthoseanimalswhichuniteforeachbirth，whichwandermuch，andwhichdonotbreedataveryquickrate。Henceinanimalsofthisnature，forinstanceinbirds，varietieswillgenerallybeconfinedtoseparatedcountries。

andthisIbelievetobethecase。Inhermaphroditeorganismswhichcrossonlyoccasionally，andlikewiseinanimalswhichuniteforeachbirth，butwhichwanderlittleandwhichcanincreaseataveryrapidrate，anewandimprovedvarietymightbequicklyformedonanyonespot，andmighttheremaintainitselfinabody，sothatwhateverintercrossingtookplacewouldbechieflybetweentheindividualsofthesamenewvariety。Alocalvarietywhenoncethusformedmightsubsequentlyslowlyspreadtootherdistricts。Ontheaboveprinciple，nurserymenalwaysprefergettingseedfromalargebodyofplantsofthesamevariety，asthechanceofintercrossingwithothervarietiesisthuslessened。

Eveninthecaseofslow-breedinganimals，whichuniteforeachbirth，wemustnotoverratetheeffectsofintercrossesinretardingnaturalselection。

forIcanbringaconsiderablecatalogueoffacts，showingthatwithinthesamearea，varietiesofthesameanimalcanlongremaindistinct，fromhauntingdifferentstations，frombreedingatslightlydifferentseasons，orfromvarietiesofthesamekindpreferringtopairtogether。

Intercrossingplaysaveryimportantpartinnatureinkeepingtheindividualsofthesamespecies，orofthesamevariety，trueanduniformincharacter。

Itwillobviouslythusactfarmoreefficientlywiththoseanimalswhichuniteforeachbirth。butIhavealreadyattemptedtoshowthatwehavereasontobelievethatoccasionalintercrossestakeplacewithallanimalsandwithallplants。Evenifthesetakeplaceonlyatlongintervals，I

amconvincedthattheyoungthusproducedwillgainsomuchinvigourandfertilityovertheoffspringfromlong-continuedself-fertilisation，thattheywillhaveabetterchanceofsurvivingandpropagatingtheirkind。

andthus，inthelongrun，theinfluenceofintercrosses，evenatrareintervals，willbegreat。Ifthereexistorganicbeingswhichneverintercross，uniformityofcharactercanberetainedamongstthem，aslongastheirconditionsofliferemainthesame，onlythroughtheprincipleofinheritance，andthroughnaturalselectiondestroyinganywhichdepartfromthepropertype。butiftheirconditionsoflifechangeandtheyundergomodification，uniformityofcharactercanbegiventotheirmodifiedoffspring，solelybynaturalselectionpreservingthesamefavourablevariations。

Isolation，also，isanimportantelementintheprocessofnaturalselection。

Inaconfinedorisolatedarea，ifnotverylarge，theorganicandinorganicconditionsoflifewillgenerallybeinagreatdegreeuniform。sothatnaturalselectionwilltendtomodifyalltheindividualsofavaryingspeciesthroughouttheareainthesamemannerinrelationtothesameconditions。Intercrosses，also，withtheindividualsofthesamespecies，whichotherwisewouldhaveinhabitedthesurroundinganddifferentlycircumstanceddistricts，willbeprevented。Butisolationprobablyactsmoreefficientlyincheckingtheimmigrationofbetteradaptedorganisms，afteranyphysicalchange，suchasofclimateorelevationoftheland，&amp。c……andthusnewplacesinthenaturaleconomyofthecountryareleftopenfortheoldinhabitantstostrugglefor，andbecomeadaptedto，throughmodificationsintheirstructureandconstitution。Lastly，isolation，bycheckingimmigrationandconsequentlycompetition，willgivetimeforanynewvarietytobeslowlyimproved。andthismaysometimesbeofimportanceintheproductionofnewspecies。If，however，anisolatedareabeverysmall，eitherfrombeingsurroundedbybarriers，orfromhavingverypeculiarphysicalconditions，thetotalnumberoftheindividualssupportedonitwillnecessarilybeverysmall。andfewnessofindividualswillgreatlyretardtheproductionofnewspeciesthroughnaturalselection，bydecreasingthechanceoftheappearanceoffavourablevariations。

Ifweturntonaturetotestthetruthoftheseremarks，andlookatanysmallisolatedarea，suchasanoceanicisland，althoughthetotalnumberofthespeciesinhabitingit，willbefoundtobesmall，asweshallseeinourchapterongeographicaldistribution。yetofthesespeciesaverylargeproportionareendemic，thatis，havebeenproducedthere，andnowhereelse。Henceanoceanicislandatfirstsightseemstohavebeenhighlyfavourablefortheproductionofnewspecies。Butwemaythusgreatlydeceiveourselves，fortoascertainwhetherasmallisolatedarea，oralargeopenarealikeacontinent，hasbeenmostfavourablefortheproductionofneworganicforms，weoughttomakethecomparisonwithinequaltimes。

andthisweareincapableofdoing。

AlthoughIdonotdoubtthatisolationisofconsiderableimportanceintheproductionofnewspecies，onthewholeIaminclinedtobelievethatlargenessofareaisofmoreimportance，moreespeciallyintheproductionofspecies，whichwillprovecapableofenduringforalongperiod，andofspreadingwidely。Throughoutagreatandopenarea，notonlywilltherebeabetterchanceoffavourablevariationsarisingfromthelargenumberofindividualsofthesamespeciestheresupported，buttheconditionsoflifeareinfinitelycomplexfromthelargenumberofalreadyexistingspecies。andifsomeofthesemanyspeciesbecomemodifiedandimproved，otherswillhavetobeimprovedinacorrespondingdegreeortheywillbeexterminated。Eachnewform，also，assoonasithasbeenmuchimproved，willbeabletospreadovertheopenandcontinuousarea，andwillthuscomeintocompetitionwithmanyothers。Hencemorenewplaceswillbeformed，andthecompetitiontofillthemwillbemoresevere，onalargethanonasmallandisolatedarea。Moreover，greatareas，thoughnowcontinuous，owingtooscillationsoflevel，willoftenhaverecentlyexistedinabrokencondition，sothatthegoodeffectsofisolationwillgenerally，toacertainextent，haveconcurred。Finally，Iconcludethat，althoughsmallisolatedareasprobablyhavebeeninsomerespectshighlyfavourablefortheproductionofnewspecies，yetthatthecourseofmodificationwillgenerallyhavebeenmorerapidonlargeareas。andwhatismoreimportant，thatthenewformsproducedonlargeareas，whichalreadyhavebeenvictoriousovermanycompetitors，willbethosethatwillspreadmostwidely，willgiverisetomostnewvarietiesandspecies，andwillthusplayanimportantpartinthechanginghistoryoftheorganicworld。

Wecan，perhaps，ontheseviews，understandsomefactswhichwillbeagainalludedtoinourchapterongeographicaldistribution。forinstance，thattheproductionsofthesmallercontinentofAustraliahaveformerlyyielded，andapparentlyarenowyielding，beforethoseofthelargerEuropaeo-Asiaticarea。Thus，also，itisthatcontinentalproductionshaveeverywherebecomesolargelynaturalisedonislands。Onasmallisland，theraceforlifewillhavebeenlesssevere，andtherewillhavebeenlessmodificationandlessextermination。Hence，perhaps，itcomesthatthefloraofMadeira，accordingtoOswaldHeer，resemblestheextincttertiaryfloraofEurope。

Allfresh-waterbasins，takentogether，makeasmallareacomparedwiththatoftheseaoroftheland。and，consequently，thecompetitionbetweenfresh-waterproductionswillhavebeenlessseverethanelsewhere。newformswillhavebeenmoreslowlyformed，andoldformsmoreslowlyexterminated。

AnditisinfreshwaterthatwefindsevengeneraofGanoidfishes，remnantsofaoncepreponderantorder：andinfreshwaterwefindsomeofthemostanomalousformsnowknownintheworld，astheOrnithorhynchusandLepidosiren，which，likefossils，connecttoacertainextentordersnowwidelyseparatedinthenaturalscale。Theseanomalousformsmayalmostbecalledlivingfossils。theyhaveenduredtothepresentday，fromhavinginhabitedaconfinedarea，andfromhavingthusbeenexposedtolessseverecompetition。

Tosumupthecircumstancesfavourableandunfavourabletonaturalselection，asfarastheextremeintricacyofthesubjectpermits。Iconclude，lookingtothefuture，thatforterrestrialproductionsalargecontinentalarea，whichwillprobablyundergomanyoscillationsoflevel，andwhichconsequentlywillexistforlongperiodsinabrokencondition，willbethemostfavourablefortheproductionofmanynewformsoflife，likelytoendurelongandtospreadwidely。Fortheareawillfirsthaveexistedasacontinent，andtheinhabitants，atthisperiodnumerousinindividualsandkinds，willhavebeensubjectedtoveryseverecompetition。Whenconvertedbysubsidenceintolargeseparateislands，therewillstillexistmanyindividualsofthesamespeciesoneachisland：intercrossingontheconfinesoftherangeofeachspecieswillthusbechecked：afterphysicalchangesofanykind，immigrationwillbeprevented，sothatnewplacesinthepolityofeachislandwillhavetobefilledupbymodificationsoftheoldinhabitants。

andtimewillbeallowedforthevarietiesineachtobecomewellmodifiedandperfected。When，byrenewedelevation，theislandsshallbere-convertedintoacontinentalarea，therewillagainbeseverecompetition：themostfavouredorimprovedvarietieswillbeenabledtospread：therewillbemuchextinctionofthelessimprovedforms，andtherelativeproportionalnumbersofthevariousinhabitantsoftherenewedcontinentwillagainbechanged。andagaintherewillbeafairfieldfornaturalselectiontoimprovestillfurthertheinhabitants，andthusproducenewspecies。

Thatnaturalselectionwillalwaysactwithextremeslowness，Ifullyadmit。Itsactiondependsontherebeingplacesinthepolityofnature，whichcanbebetteroccupiedbysomeoftheinhabitantsofthecountryundergoingmodificationofsomekind。Theexistenceofsuchplaceswilloftendependonphysicalchanges，whicharegenerallyveryslow，andontheimmigrationofbetteradaptedformshavingbeenchecked。Buttheactionofnaturalselectionwillprobablystilloftenerdependonsomeoftheinhabitantsbecomingslowlymodified。themutualrelationsofmanyoftheotherinhabitantsbeingthusdisturbed。Nothingcanbeeffected，unlessfavourablevariationsoccur，andvariationitselfisapparentlyalwaysaveryslowprocess。Theprocesswilloftenbegreatlyretardedbyfreeintercrossing。Manywillexclaimthattheseseveralcausesareamplysufficientwhollytostoptheactionofnaturalselection。Idonotbelieveso。Ontheotherhand，Idobelievethatnaturalselectionwillalwaysactveryslowly，oftenonlyatlongintervalsoftime，andgenerallyononlyaveryfewoftheinhabitantsofthesameregionatthesametime。Ifurtherbelieve，thatthisveryslow，intermittentactionofnaturalselectionaccordsperfectlywellwithwhatgeologytellsusoftherateandmanneratwhichtheinhabitantsofthisworldhavechanged。

Slowthoughtheprocessofselectionmaybe，iffeeblemancandomuchbyhispowersofartificialselection，Icanseenolimittotheamountofchange，tothebeautyandinfinitecomplexityofthecoadaptationsbetweenallorganicbeings，onewithanotherandwiththeirphysicalconditionsoflife，whichmaybeeffectedinthelongcourseoftimebynature’spowerofselection。ExtinctionThissubjectwillbemorefullydiscussedinourchapteronGeology。butitmustbeherealludedtofrombeingintimatelyconnectedwithnaturalselection。Naturalselectionactssolelythroughthepreservationofvariationsinsomewayadvantageous，whichconsequentlyendure。Butasfromthehighgeometricalpowersofincreaseofallorganicbeings，eachareaisalreadyfullystockedwithinhabitants，itfollowsthataseachselectedandfavouredformincreasesinnumber，sowillthelessfavouredformsdecreaseandbecomerare。Rarity，asgeologytellsus，istheprecursortoextinction。

Wecan，also，seethatanyformrepresentedbyfewindividualswill，duringfluctuationsintheseasonsorinthenumberofitsenemies，runagoodchanceofutterextinction。Butwemaygofurtherthanthis。forasnewformsarecontinuallyandslowlybeingproduced，unlesswebelievethatthenumberofspecificformsgoesonperpetuallyandalmostindefinitelyincreasing，numbersinevitablymustbecomeextinct。Thatthenumberofspecificformshasnotindefinitelyincreased，geologyshowsusplainly。

andindeedwecanseereasonwhytheyshouldnothavethusincreased，forthenumberofplacesinthepolityofnatureisnotindefinitelygreat，notthatwehaveanymeansofknowingthatanyoneregionhasasyetgotitsmaximumofspecies。probablynoregionisasyetfullystocked，forattheCapeofGoodHope，wheremorespeciesofplantsarecrowdedtogetherthaninanyotherquarteroftheworld，someforeignplantshavebecomenaturalised，withoutcausing，asfarasweknow，theextinctionofanynatives。

Furthermore，thespecieswhicharemostnumerousinindividualswillhavethebestchanceofproducingwithinanygivenperiodfavourablevariations。

Wehaveevidenceofthis，inthefactsgiveninthesecondchapter，showingthatitisthecommonspecieswhichaffordthegreatestnumberofrecordedvarieties，orincipientspecies。Hence，rarespecieswillbelessquicklymodifiedorimprovedwithinanygivenperiod，andtheywillconsequentlybebeatenintheraceforlifebythemodifieddescendantsofthecommonerspecies。

FromtheseseveralconsiderationsIthinkitinevitablyfollows，thatasnewspeciesinthecourseoftimeareformedthroughnaturalselection，otherswillbecomerarerandrarer，andfinallyextinct。Theformswhichstandinclosestcompetitionwiththoseundergoingmodificationandimprovement，willnaturallysuffermost。AndwehaveseeninthechapterontheStruggleforExistencethatitisthemostclosely-alliedforms，varietiesofthesamespecies，andspeciesofthesamegenusorofrelatedgenera，which，fromhavingnearlythesamestructure，constitution，andhabits，generallycomeintotheseverestcompetitionwitheachother。Consequently，eachnewvarietyorspecies，duringtheprogressofitsformation，willgenerallypresshardestonitsnearestkindred，andtendtoexterminatethem。Weseethesameprocessofexterminationamongstourdomesticatedproductions，throughtheselectionofimprovedformsbyman。Manycuriousinstancescouldbegivenshowinghowquicklynewbreedsofcattle，sheep，andotheranimals，andvarietiesofflowers，taketheplaceofolderandinferiorkinds。InYorkshire，itishistoricallyknownthattheancientblackcattleweredisplacedbythelong-horns，andthatthese’weresweptawaybytheshort-horns’(Iquotethewordsofanagriculturalwriter)’asifbysomemurderouspestilence。’DivergenceofCharacterTheprinciple，whichIhavedesignatedbythisterm，isofhighimportanceonmytheory，andexplains，asIbelieve，severalimportantfacts。Inthefirstplace，varieties，evenstrongly-markedones，thoughhavingsomewhatofthecharacterofspeciesasisshownbythehopelessdoubtsinmanycaseshowtorankthemyetcertainlydifferfromeachotherfarlessthandogoodanddistinctspecies。Nevertheless，accordingtomyview，varietiesarespeciesintheprocessofformation，orare，asIhavecalledthem，incipientspecies。How，then，doesthelesserdifferencebetweenvarietiesbecomeaugmentedintothegreaterdifferencebetweenspecies？Thatthisdoeshabituallyhappen，wemustinferfrommostoftheinnumerablespeciesthroughoutnaturepresentingwell-markeddifferences。whereasvarieties，thesupposedprototypesandparentsoffuturewell-markedspecies，presentslightandill-defineddifferences。Merechance，aswemaycallit，mightcauseonevarietytodifferinsomecharacterfromitsparents，andtheoffspringofthisvarietyagaintodifferfromitsparentintheverysamecharacterandinagreaterdegree。butthisalonewouldneveraccountforsohabitualandlargeanamountofdifferenceasthatbetweenvarietiesofthesamespeciesandspeciesofthesamegenus。

Ashasalwaysbeenmypractice，letusseeklightonthisheadfromourdomesticproductions。Weshallherefindsomethinganalogous。Afancierisstruckbyapigeonhavingaslightlyshorterbeak。anotherfancierisstruckbyapigeonhavingaratherlongerbeak。andontheacknowledgedprinciplethat’fanciersdonotandwillnotadmireamediumstandard，butlikeextremes，’theybothgoon(ashasactuallyoccurredwithtumbler-pigeons)

choosingandbreedingfrombirdswithlongerandlongerbeaks，orwithshorterandshorterbeaks。Again，wemaysupposethatatanearlyperiodonemanpreferredswifterhorses。anotherstrongerandmorebulkyhorses。

Theearlydifferenceswouldbeveryslight。inthecourseoftime，fromthecontinuedselectionofswifterhorsesbysomebreeders，andofstrongeronesbyothers，thedifferenceswouldbecomegreater，andwouldbenotedasformingtwosub-breeds。finally，afterthelapseofcenturies，thesub-breedswouldbecomeconvertedintotwowell-establishedanddistinctbreeds。Asthedifferencesslowlybecomegreater，theinferioranimalswithintermediatecharacters，beingneitherveryswiftnorverystrong，willhavebeenneglected，andwillhavetendedtodisappear。Here，then，weseeinman’sproductionstheactionofwhatmaybecalledtheprincipleofdivergence，causingdifferences，atfirstbarelyappreciable，steadilytoincrease，andthebreedstodivergeincharacterbothfromeachotherandfromtheircommonparent。

Buthow，itmaybeasked，cananyanalogousprincipleapplyinnature？

Ibelieveitcananddoesapplymostefficiently，fromthesimplecircumstancethatthemorediversifiedthedescendantsfromanyonespeciesbecomeinstructure，constitution，andhabits，bysomuchwilltheybebetterenabledtoseizeonmanyandwidelydiversifiedplacesinthepolityofnature，andsobeenabledtoincreaseinnumbers。

Wecanclearlyseethisinthecaseofanimalswithsimplehabits。Takethecaseofacarnivorousquadruped，ofwhichthenumberthatcanbesupportedinanycountryhaslongagoarrivedatitsfullaverage。Ifitsnaturalpowersofincreasebeallowedtoact，itcansucceedinincreasing(thecountrynotundergoinganychangeinitsconditions)onlybyitsvaryingdescendantsseizingonplacesatpresentoccupiedbyotheranimals：someofthem，forinstance，beingenabledtofeedonnewkindsofprey，eitherdeadoralive。someinhabitingnewstations，climbingtrees，frequentingwater，andsomeperhapsbecominglesscarnivorous。Themorediversifiedinhabitsandstructurethedescendantsofourcarnivorousanimalbecame，themoreplacestheywouldbeenabledtooccupy。Whatappliestooneanimalwillapplythroughoutalltimetoallanimalsthatis，iftheyvaryforotherwisenaturalselectioncandonothing。Soitwillbewithplants。

Ithasbeenexperimentallyproved，thatifaplotofgroundbesownwithseveraldistinctgeneraofgrasses，agreaternumberofplantsandagreaterweightofdryherbagecanthusberaised。Thesamehasbeenfoundtoholdgoodwhenfirstonevarietyandthenseveralmixedvarietiesofwheathavebeensownonequalspacesofground。Hence，ifanyonespeciesofgrassweretogoonvarying，andthosevarietieswerecontinuallyselectedwhichdifferedfromeachotherinatallthesamemannerasdistinctspeciesandgeneraofgrassesdifferfromeachother，agreaternumberofindividualplantsofthisspeciesofgrass，includingitsmodifieddescendants，wouldsucceedinlivingonthesamepieceofground。Andwewellknowthateachspeciesandeachvarietyofgrassisannuallysowingalmostcountlessseeds。

andthus，asitmaybesaid，isstrivingitsutmosttoincreaseitsnumbers。

Consequently，Icannotdoubtthatinthecourseofmanythousandsofgenerations，themostdistinctvarietiesofanyonespeciesofgrasswouldalwayshavethebestchanceofsucceedingandofincreasinginnumbers，andthusofsupplantingthelessdistinctvarieties。andvarieties，whenrenderedverydistinctfromeachother，taketherankofspecies。

Thetruthoftheprinciple，thatthegreatestamountoflifecanbesupportedbygreatdiversificationofstructure，isseenundermanynaturalcircumstances。Inanextremelysmallarea，especiallyiffreelyopentoimmigration，andwherethecontestbetweenindividualandindividualmustbesevere，wealwaysfindgreatdiversityinitsinhabitants。Forinstance，Ifoundthatapieceofturf，threefeetbyfourinsize，whichhadbeenexposedformanyyearstoexactlythesameconditions，supportedtwentyspeciesofplants，andthesebelongedtoeighteengeneraandtoeightorders，whichshowshowmuchtheseplantsdifferedfromeachother。Soitiswiththeplantsandinsectsonsmallanduniformislets。andsoinsmallpondsoffreshwater。Farmersfindthattheycanraisemostfoodbyarotationofplantsbelongingtothemostdifferentorders：naturefollowswhatmaybecalledasimultaneousrotation。Mostoftheanimalsandplantswhichlivecloseroundanysmallpieceofground，couldliveonit(supposingitnottobeinanywaypeculiarinitsnature)，andmaybesaidtobestrivingtotheutmosttolivethere。but，itisseen，thatwheretheycomeintotheclosestcompetitionwitheachother，theadvantagesofdiversificationofstructure，withtheaccompanyingdifferencesofhabitandconstitution，determinethattheinhabitants，whichthusjostleeachothermostclosely，shall，asageneralrule，belongtowhatwecalldifferentgeneraandorders。

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