第4章

Thesameprincipleisseeninthenaturalisationofplantsthroughman’sagencyinforeignlands。Itmighthavebeenexpectedthattheplantswhichhavesucceededinbecomingnaturalisedinanylandwouldgenerallyhavebeencloselyalliedtotheindigenes。forthesearecommonlylookedatasspeciallycreatedandadaptedfortheirowncountry。Itmight，also，perhapshavebeenexpectedthatnaturalisedplantswouldhavebelongedtoafewgroupsmoreespeciallyadaptedtocertainstationsintheirnewhomes。Butthecaseisverydifferent。andAlph。DeCandollehaswellremarkedinhisgreatandadmirablework，thatflorasgainbynaturalisation，proportionallywiththenumberofthenativegeneraandspecies，farmoreinnewgenerathaninnewspecies。Togiveasingleinstance：inthelasteditionofDrAsaGray’s’ManualoftheFloraoftheNorthernUnitedStates，’260

naturalisedplantsareenumerated，andthesebelongto162genera。Wethusseethatthesenaturalisedplantsareofahighlydiversifiednature。Theydiffer，moreover，toalargeextentfromtheindigenes，foroutofthe162genera，nolessthan100generaarenotthereindigenous，andthusalargeproportionaladditionismadetothegeneraoftheseStates。

Byconsideringthenatureoftheplantsoranimalswhichhavestruggledsuccessfullywiththeindigenesofanycountry，andhavetherebecomenaturalised，wecangainsomecrudeideainwhatmannersomeofthenativeswouldhavehadtobemodified，inordertohavegainedanadvantageovertheothernatives。andwemay，Ithink，atleastsafelyinferthatdiversificationofstructure，amountingtonewgenericdifferences，wouldhavebeenprofitabletothem。

Theadvantageofdiversificationintheinhabitantsofthesameregionis，infact，thesameasthatofthephysiologicaldivisionoflabourintheorgansofthesameindividualbodyasubjectsowellelucidatedbyMilneEdwards。Nophysiologistdoubtsthatastomachbybeingadaptedtodigestvegetablematteralone，orfleshalone，drawsmostnutrimentfromthesesubstances。Sointhegeneraleconomyofanyland，themorewidelyandperfectlytheanimalsandplantsarediversifiedfordifferenthabitsoflife，sowillagreaternumberofindividualsbecapableoftheresupportingthemselves。Asetofanimals，withtheirorganisationbutlittlediversified，couldhardlycompetewithasetmoreperfectlydiversifiedinstructure。

Itmaybedoubted，forinstance，whethertheAustralianmarsupials，whicharedividedintogroupsdifferingbutlittlefromeachother，andfeeblyrepresenting，asMrWaterhouseandothershaveremarked，ourcarnivorous，ruminant，androdentmammals，couldsuccessfullycompetewiththesewell-pronouncedorders。IntheAustralianmammals，weseetheprocessofdiversificationinanearlyandincompletestageofdevelopment。

Aftertheforegoingdiscussion，whichoughttohavebeenmuchamplified，wemay，Ithink，assumethatthemodifieddescendantsofanyonespecieswillsucceedbysomuchthebetterastheybecomemorediversifiedinstructure，andarethusenabledtoencroachonplacesoccupiedbyotherbeings。Nowletusseehowthisprincipleofgreatbenefitbeingderivedfromdivergenceofcharacter，combinedwiththeprinciplesofnaturalselectionandofextinction，willtendtoact。

Theaccompanyingdiagramwillaidusinunderstandingthisratherperplexingsubject。LetAtoLrepresentthespeciesofagenuslargeinitsowncountry。

thesespeciesaresupposedtoresembleeachotherinunequaldegrees，asissogenerallythecaseinnature，andasisrepresentedinthediagrambythelettersstandingatunequaldistances。Ihavesaidalargegenus，becausewehaveseeninthesecondchapter，thatonanaveragemoreofthespeciesoflargegeneravarythanofsmallgenera。andthevaryingspeciesofthelargegenerapresentagreaternumberofvarieties。Wehave，also，seenthatthespecies，whicharethecommonestandthemostwidely-diffused，varymorethanrarespecieswithrestrictedranges。Let(A)beacommon，widely-diffused，andvaryingspecies，belongingtoagenuslargeinitsowncountry。Thelittlefanofdivergingdottedlinesofunequallengthsproceedingfrom(A)，mayrepresentitsvaryingoffspring。Thevariationsaresupposedtobeextremelyslight，butofthemostdiversifiednature。

theyarenotsupposedalltoappearsimultaneously，butoftenafterlongintervalsoftime。noraretheyallsupposedtoendureforequalperiods。

Onlythosevariationswhichareinsomewayprofitablewillbepreservedornaturallyselected。Andheretheimportanceoftheprincipleofbenefitbeingderivedfromdivergenceofcharactercomesin。forthiswillgenerallyleadtothemostdifferentordivergentvariations(representedbytheouterdottedlines)beingpreservedandaccumulatedbynaturalselection。

Whenadottedlinereachesoneofthehorizontallines，andistheremarkedbyasmallnumberedletter，asufficientamountofvariationissupposedtohavebeenaccumulatedtohaveformedafairlywell-markedvariety，suchaswouldbethoughtworthyofrecordinasystematicwork。

Theintervalsbetweenthehorizontallinesinthediagram，mayrepresenteachathousandgenerations。butitwouldhavebeenbetterifeachhadrepresentedtenthousandgenerations。Afterathousandgenerations，species(A)issupposedtohaveproducedtwofairlywell-markedvarieties，namelya1andm1。Thesetwovarietieswillgenerallycontinuetobeexposedtothesameconditionswhichmadetheirparentsvariable，andthetendencytovariabilityisinitselfhereditary，consequentlytheywilltendtovary，andgenerallytovaryinnearlythesamemannerastheirparentsvaried。Moreover，thesetwovarieties，beingonlyslightlymodifiedforms，willtendtoinheritthoseadvantageswhichmadetheircommonparent(A)morenumerousthanmostoftheotherinhabitantsofthesamecountry。

theywilllikewisepartakeofthosemoregeneraladvantageswhichmadethegenustowhichtheparent-speciesbelonged，alargegenusinitsowncountry。Andthesecircumstancesweknowtobefavourabletotheproductionofnewvarieties。

If，then，thesetwovarietiesbevariable，themostdivergentoftheirvariationswillgenerallybepreservedduringthenextthousandgenerations。

Andafterthisinterval，varietya1issupposedinthediagramtohaveproducedvarietya2，whichwill，owingtotheprincipleofdivergence，differmorefrom(A)thandidvarietya1。Varietym1

issupposedtohaveproducedtwovarieties，namelym2ands2，differingfromeachother，andmoreconsiderablyfromtheircommonparent(A)。Wemaycontinuetheprocessbysimilarstepsforanylengthoftime。

someofthevarieties，aftereachthousandgenerations，producingonlyasinglevariety，butinamoreandmoremodifiedcondition，someproducingtwoorthreevarieties，andsomefailingtoproduceany。Thusthevarietiesormodifieddescendants，proceedingfromthecommonparent(A)，willgenerallygoonincreasinginnumberanddivergingincharacter。Inthediagramtheprocessisrepresenteduptotheten-thousandthgeneration，andunderacondensedandsimplifiedformuptothefourteen-thousandthgeneration。

ButImusthereremarkthatIdonotsupposethattheprocessevergoesonsoregularlyasisrepresentedinthediagram，thoughinitselfmadesomewhatirregular。Iamfarfromthinkingthatthemostdivergentvarietieswillinvariablyprevailandmultiply：amediumformmayoftenlongendure，andmayormaynotproducemorethanonemodifieddescendant。fornaturalselectionwillalwaysactaccordingtothenatureoftheplaceswhichareeitherunoccupiedornotperfectlyoccupiedbyotherbeings。andthiswilldependoninfinitelycomplexrelations。Butasageneralrule，themorediversifiedinstructurethedescendantsfromanyonespeciescanberendered，themoreplacestheywillbeenabledtoseizeon，andthemoretheirmodifiedprogenywillbeincreased。Inourdiagramthelineofsuccessionisbrokenatregularintervalsbysmallnumberedlettersmarkingthesuccessiveformswhichhavebecomesufficientlydistincttoberecordedasvarieties。Butthesebreaksareimaginary，andmighthavebeeninsertedanywhere，afterintervalslongenoughtohaveallowedtheaccumulationofaconsiderableamountofdivergentvariation。

Asallthemodifieddescendantsfromacommonandwidely-diffusedspecies，belongingtoalargegenus，willtendtopartakeofthesameadvantageswhichmadetheirparentsuccessfulinlife，theywillgenerallygoonmultiplyinginnumberaswellasdivergingincharacter：thisisrepresentedinthediagrambytheseveraldivergentbranchesproceedingfrom(A)。Themodifiedoffspringfromthelaterandmorehighlyimprovedbranchesinthelinesofdescent，will，itisprobable，oftentaketheplaceof，andsodestroy，theearlierandlessimprovedbranches：thisisrepresentedinthediagrambysomeofthelowerbranchesnotreachingtotheupperhorizontallines。

InsomecasesIdonotdoubtthattheprocessofmodificationwillbeconfinedtoasinglelineofdescent，andthenumberofthedescendantswillnotbeincreased。althoughtheamountofdivergentmodificationmayhavebeenincreasedinthesuccessivegenerations。Thiscasewouldberepresentedinthediagram，ifallthelinesproceedingfrom(A)wereremoved，exceptingthatfroma1toa10Inthesameway，forinstance，theEnglishrace-horseandEnglishpointerhaveapparentlybothgoneonslowlydivergingincharacterfromtheiroriginalstocks，withouteitherhavinggivenoffanyfreshbranchesorraces。

Aftertenthousandgenerations，species(A)issupposedtohaveproducedthreeforms，a10，f10，andm10，which，fromhavingdivergedincharacterduringthesuccessivegenerations，willhavecometodifferlargely，butperhapsunequally，fromeachotherandfromtheircommonparent。Ifwesupposetheamountofchangebetweeneachhorizontallineinourdiagramtobeexcessivelysmall，thesethreeformsmaystillbeonlywell-markedvarieties。ortheymayhavearrivedatthedoubtfulcategoryofsub-species。butwehaveonlytosupposethestepsintheprocessofmodificationtobemorenumerousorgreaterinamount，toconvertthesethreeformsintowell-definedspecies：thusthediagramillustratesthestepsbywhichthesmalldifferencesdistinguishingvarietiesareincreasedintothelargerdifferencesdistinguishingspecies。Bycontinuingthesameprocessforagreaternumberofgenerations(asshowninthediagraminacondensedandsimplifiedmanner)，wegeteightspecies，markedbythelettersbetweena14andm14，alldescendedfrom(A)。Thus，asIbelieve，speciesaremultipliedandgeneraareformed。

Inalargegenusitisprobablethatmorethanonespecieswouldvary。

InthediagramIhaveassumedthatasecondspecies(I)hasproduced，byanalogoussteps，aftertenthousandgenerations，eithertwowell-markedvarieties(w10andz10)ortwospecies，accordingtotheamountofchangesupposedtoberepresentedbetweenthehorizontallines。

Afterfourteenthousandgenerations，sixnewspecies，markedbythelettersn14toz14，aresupposedtohavebeenproduced。Ineachgenus，thespecies，whicharealreadyextremelydifferentincharacter，willgenerallytendtoproducethegreatestnumberofmodifieddescendants。forthesewillhavethebestchanceoffillingnewandwidelydifferentplacesinthepolityofnature：henceinthediagramIhavechosentheextremespecies(A)，andthenearlyextremespecies(I)，asthosewhichhavelargelyvaried，andhavegivenrisetonewvarietiesandspecies。Theotherninespecies(markedbycapitalletters)ofouroriginalgenus，mayforalongperiodcontinuetransmittingunaltereddescendants。andthisisshowninthediagrambythedottedlinesnotprolongedfarupwardsfromwantofspace。

Butduringtheprocessofmodification，representedinthediagram，anotherofourprinciples，namelythatofextinction，willhaveplayedanimportantpart。Asineachfullystockedcountrynaturalselectionnecessarilyactsbytheselectedformhavingsomeadvantageinthestruggleforlifeoverotherforms，therewillbeaconstanttendencyintheimproveddescendantsofanyonespeciestosupplantandexterminateineachstageofdescenttheirpredecessorsandtheiroriginalparent。Foritshouldberememberedthatthecompetitionwillgenerallybemostseverebetweenthoseformswhicharemostnearlyrelatedtoeachotherinhabits，constitution，andstructure。Hencealltheintermediateformsbetweentheearlierandlaterstates，thatisbetweenthelessandmoreimprovedstateofaspecies，aswellastheoriginalparent-speciesitself，willgenerallytendtobecomeextinct。Soitprobablywillbewithmanywholecollaterallinesofdescent，whichwillbeconqueredbylaterandimprovedlinesofdescent。If，however，themodifiedoffspringofaspeciesgetintosomedistinctcountry，orbecomequicklyadaptedtosomequitenewstation，inwhichchildandparentdonotcomeintocompetition，bothmaycontinuetoexist。

Ifthenourdiagrambeassumedtorepresentaconsiderableamountofmodification，species(A)andalltheearliervarietieswillhavebecomeextinct，havingbeenreplacedbyeightnewspecies(a14tom14)。

and(I)willhavebeenreplacedbysix(n14toz14)newspecies。

Butwemaygofurtherthanthis。Theoriginalspeciesofourgenusweresupposedtoresembleeachotherinunequaldegrees，asissogenerallythecaseinnature。species(A)beingmorenearlyrelatedtoB，C，andD，thantotheotherspecies。andspecies(I)moretoG，H，K，L，thantotheothers。Thesetwospecies(A)and(I)，werealsosupposedtobeverycommonandwidelydiffusedspecies，sothattheymustoriginallyhavehadsomeadvantageovermostoftheotherspeciesofthegenus。Theirmodifieddescendants，fourteeninnumberatthefourteen-thousandthgeneration，willprobablyhaveinheritedsomeofthesameadvantages：theyhavealsobeenmodifiedandimprovedinadiversifiedmannerateachstageofdescent，soastohavebecomeadaptedtomanyrelatedplacesinthenaturaleconomyoftheircountry。Itseems，therefore，tomeextremelyprobablethattheywillhavetakentheplacesof，andthusexterminated，notonlytheirparents(A)and(I)，butlikewisesomeoftheoriginalspecieswhichweremostnearlyrelatedtotheirparents。Henceveryfewoftheoriginalspecieswillhavetransmittedoffspringtothefourteen-thousandthgeneration。

Wemaysupposethatonlyone(F)，ofthetwospecieswhichwereleastcloselyrelatedtotheothernineoriginalspecies，hastransmitteddescendantstothislatestageofdescent。

Thenewspeciesinourdiagramdescendedfromtheoriginalelevenspecies，willnowbefifteeninnumber。Owingtothedivergenttendencyofnaturalselection，theextremeamountofdifferenceincharacterbetweenspeciesa14andz14willbemuchgreaterthanthatbetweenthemostdifferentoftheoriginalelevenspecies。Thenewspecies，moreover，willbealliedtoeachotherinawidelydifferentmanner。Oftheeightdescendantsfrom(A)thethreemarkeda14，q14，p14，willbenearlyrelatedfromhavingrecentlybranchedofffroma14。b14andf14，fromhavingdivergedatanearlierperiodfroma5，willbeinsomedegreedistinctfromthethreefirst-namedspecies。andlastly，o14，e14，andm14，willbenearlyrelatedonetotheother，butfromhavingdivergedatthefirstcommencementoftheprocessofmodification，willbewidelydifferentfromtheotherfivespecies，andmayconstituteasub-genusorevenadistinctgenus。Thesixdescendantsfrom(I)willformtwosub-generaorevengenera。Butastheoriginalspecies(I)differedlargelyfrom(A)，standingnearlyattheextremepointsoftheoriginalgenus，thesixdescendantsfrom(I)will，owingtoinheritance，differconsiderablyfromtheeightdescendantsfrom(A)。thetwogroups，moreover，aresupposedtohavegoneondivergingindifferentdirections。

Theintermediatespecies，also(andthisisaveryimportantconsideration)，whichconnectedtheoriginalspecies(A)and(I)，haveallbecome，excepting(F)，extinct，andhaveleftnodescendants。Hencethesixnewspeciesdescendedfrom(I)，andtheeightdescendedfrom(A)，willhavetoberankedasverydistinctgenera，orevenasdistinctsub-families。

Thusitis，asIbelieve，thattwoormoregeneraareproducedbydescent，withmodification，fromtwoormorespeciesofthesamegenus。Andthetwoormoreparent-speciesaresupposedtohavedescendedfromsomeonespeciesofanearliergenus。Inourdiagram，thisisindicatedbythebrokenlines，beneaththecapitalletters，converginginsub-branchesdownwardstowardsasinglepoint。thispointrepresentingasinglespecies，thesupposedsingleparentofourseveralnewsub-generaandgenera。

ItisworthwhiletoreflectforamomentonthecharacterofthenewspeciesF14，whichissupposednottohavedivergedmuchincharacter，buttohaveretainedtheformof(F)，eitherunalteredoralteredonlyinaslightdegree。Inthiscase，itsaffinitiestotheotherfourteennewspecieswillbeofacuriousandcircuitousnature。Havingdescendedfromaformwhichstoodbetweenthetwoparent-species(A)and(I)，nowsupposedtobeextinctandunknown，itwillbeinsomedegreeintermediateincharacterbetweenthetwogroupsdescendedfromthesespecies。Butasthesetwogroupshavegoneondivergingincharacterfromthetypeoftheirparents，thenewspecies(F14)willnotbedirectlyintermediatebetweenthem，butratherbetweentypesofthetwogroups。andeverynaturalistwillbeabletobringsomesuchcasebeforehismind。

Inthediagram，eachhorizontallinehashithertobeensupposedtorepresentathousandgenerations，buteachmayrepresentamillionorhundredmilliongenerations，andlikewiseasectionofthesuccessivestrataoftheearth’scrustincludingextinctremains。Weshall，whenwecometoourchapteronGeology，havetoreferagaintothissubject，andIthinkweshallthenseethatthediagramthrowslightontheaffinitiesofextinctbeings，which，thoughgenerallybelongingtothesameorders，orfamilies，orgenera，withthosenowliving，yetareoften，insomedegree，intermediateincharacterbetweenexistinggroups。andwecanunderstandthisfact，fortheextinctspecieslivedatveryancientepochswhenthebranchinglinesofdescenthaddivergedless。

Iseenoreasontolimittheprocessofmodification，asnowexplained，totheformationofgeneraalone。If，inourdiagram，wesupposetheamountofchangerepresentedbyeachsuccessivegroupofdivergingdottedlinestobeverygreat，theformsmarkeda214top14，thosemarkedb14

andf14，andthosemarkedo14tom14，willformthreeverydistinctgenera。Weshallalsohavetwoverydistinctgeneradescendedfrom(I)andastheselattertwogenera，bothfromcontinueddivergenceofcharacterandfrominheritancefromadifferentparent，willdifferwidelyfromthethreegeneradescendedfrom(A)，thetwolittlegroupsofgenerawillformtwodistinctfamilies，orevenorders，accordingtotheamountofdivergentmodificationsupposedtoberepresentedinthediagram。Andthetwonewfamilies，ororders，willhavedescendedfromtwospeciesoftheoriginalgenus。andthesetwospeciesaresupposedtohavedescendedfromonespeciesofastillmoreancientandunknowngenus。

Wehaveseenthatineachcountryitisthespeciesofthelargergenerawhichoftenestpresentvarietiesorincipientspecies。This，indeed，mighthavebeenexpected。forasnaturalselectionactsthroughoneformhavingsomeadvantageoverotherformsinthestruggleforexistence，itwillchieflyactonthosewhichalreadyhavesomeadvantage。andthelargenessofanygroupshowsthatitsspecieshaveinheritedfromacommonancestorsomeadvantageincommon。Hence，thestrugglefortheproductionofnewandmodifieddescendants，willmainlyliebetweenthelargergroups，whicharealltryingtoincreaseinnumber。Onelargegroupwillslowlyconqueranotherlargegroup，reduceitsnumbers，andthuslessenitschanceoffurthervariationandimprovement。Withinthesamelargegroup，thelaterandmorehighlyperfectedsub-groups，frombranchingoutandseizingonmanynewplacesinthepolityofNature，willconstantlytendtosupplantanddestroytheearlierandlessimprovedsub-groups。Smallandbrokengroupsandsub-groupswillfinallytendtodisappear。Lookingtothefuture，wecanpredictthatthegroupsoforganicbeingswhicharenowlargeandtriumphant，andwhichareleastbrokenup，thatis，whichasyethavesufferedleastextinction，willforalongperiodcontinuetoincrease。Butwhichgroupswillultimatelyprevail，nomancanpredict。forwewellknowthatmanygroups，formerlymostextensivelydeveloped，havenowbecomeextinct。

Lookingstillmoreremotelytothefuture，wemaypredictthat，owingtothecontinuedandsteadyincreaseofthelargergroups，amultitudeofsmallergroupswillbecomeutterlyextinct，andleavenomodifieddescendants。

andconsequentlythatofthespecieslivingatanyoneperiod，extremelyfewwilltransmitdescendantstoaremotefuturity。IshallhavetoreturntothissubjectinthechapteronClassification，butImayaddthatonthisviewofextremelyfewofthemoreancientspecieshavingtransmitteddescendants，andontheviewofallthedescendantsofthesamespeciesmakingaclass，wecanunderstandhowitisthatthereexistbutveryfewclassesineachmaindivisionoftheanimalandvegetablekingdoms。Althoughextremelyfewofthemostancientspeciesmaynowhavelivingandmodifieddescendants，yetatthemostremotegeologicalperiod，theearthmayhavebeenaswellpeopledwithmanyspeciesofmanygenera，families，orders，andclasses，asatthepresentday。SummaryofChapterIfduringthelongcourseofagesandundervaryingconditionsoflife，organicbeingsvaryatallintheseveralpartsoftheirorganisation，andIthinkthiscannotbedisputed。iftherebe，owingtothehighgeometricalpowersofincreaseofeachspecies，atsomeage，season，oryear，aseverestruggleforlife，andthiscertainlycannotbedisputed。then，consideringtheinfinitecomplexityoftherelationsofallorganicbeingstoeachotherandtotheirconditionsofexistence，causinganinfinitediversityinstructure，constitution，andhabits，tobeadvantageoustothem，Ithinkitwouldbeamostextraordinaryfactifnovariationeverhadoccurredusefultoeachbeing’sownwelfare，inthesamewayassomanyvariationshaveoccurredusefultoman。Butifvariationsusefultoanyorganicbeingdooccur，assuredlyindividualsthuscharacterisedwillhavethebestchanceofbeingpreservedinthestruggleforlife。andfromthestrongprincipleofinheritancetheywilltendtoproduceoffspringsimilarlycharacterised。

Thisprincipleofpreservation，Ihavecalled，forthesakeofbrevity，NaturalSelection。Naturalselection，ontheprincipleofqualitiesbeinginheritedatcorrespondingages，canmodifytheegg，seed，oryoung，aseasilyastheadult。Amongstmanyanimals，sexualselectionwillgiveitsaidtoordinaryselection，byassuringtothemostvigorousandbestadaptedmalesthegreatestnumberofoffspring。Sexualselectionwillalsogivecharactersusefultothemalesalone，intheirstruggleswithothermales。

Whethernaturalselectionhasreallythusactedinnature，inmodifyingandadaptingthevariousformsoflifetotheirseveralconditionsandstations，mustbejudgedofbythegeneraltenourandbalanceofevidencegiveninthefollowingchapters。Butwealreadyseehowitentailsextinction。

andhowlargelyextinctionhasactedintheworld’shistory，geologyplainlydeclares。Naturalselection，also，leadstodivergenceofcharacter。formorelivingbeingscanbesupportedonthesameareathemoretheydivergeinstructure，habits，andconstitution，ofwhichweseeproofbylookingattheinhabitantsofanysmallspotoratnaturalisedproductions。Thereforeduringthemodificationofthedescendantsofanyonespecies，andduringtheincessantstruggleofallspeciestoincreaseinnumbers，themorediversifiedthesedescendantsbecome，thebetterwillbetheirchanceofsucceedinginthebattleoflife。Thusthesmalldifferencesdistinguishingvarietiesofthesamespecies，willsteadilytendtoincreasetilltheycometoequalthegreaterdifferencesbetweenspeciesofthesamegenus，orevenofdistinctgenera。

Wehaveseenthatitisthecommon，thewidely-diffused，andwidely-rangingspecies，belongingtothelargergenera，whichvarymost。andthesewilltendtotransmittotheirmodifiedoffspringthatsuperioritywhichnowmakesthemdominantintheirowncountries。Naturalselection，ashasjustbeenremarked，leadstodivergenceofcharacterandtomuchextinctionofthelessimprovedandintermediateformsoflife。Ontheseprinciples，Ibelieve，thenatureoftheaffinitiesofallorganicbeingsmaybeexplained。

Itisatrulywonderfulfactthewonderofwhichweareapttooverlookfromfamiliaritythatallanimalsandallplantsthroughoutalltimeandspaceshouldberelatedtoeachotheringroupsubordinatetogroup，inthemannerwhichweeverywherebeholdnamely，varietiesofthesamespeciesmostcloselyrelatedtogether，speciesofthesamegenuslesscloselyandunequallyrelatedtogether，formingsectionsandsub-genera，speciesofdistinctgeneramuchlesscloselyrelated，andgenerarelatedindifferentdegrees，formingsub-families，families，orders，sub-classes，andclasses。

Theseveralsubordinategroupsinanyclasscannotberankedinasinglefile，butseemrathertobeclusteredroundpoints，andtheseroundotherpoints，andsooninalmostendlesscycles。Ontheviewthateachspecieshasbeenindependentlycreated，Icanseenoexplanationofthisgreatfactintheclassificationofallorganicbeings。but，tothebestofmyjudgment，itisexplainedthroughinheritanceandthecomplexactionofnaturalselection，entailingextinctionanddivergenceofcharacter，aswehaveseenillustratedinthediagram。

Theaffinitiesofallthebeingsofthesameclasshavesometimesbeenrepresentedbyagreattree。Ibelievethissimilelargelyspeaksthetruth。

Thegreenandbuddingtwigsmayrepresentexistingspecies。andthoseproducedduringeachformeryearmayrepresentthelongsuccessionofextinctspecies。

Ateachperiodofgrowthallthegrowingtwigshavetriedtobranchoutonallsides，andtoovertopandkillthesurroundingtwigsandbranches，inthesamemannerasspeciesandgroupsofspecieshavetriedtoovermasterotherspeciesinthegreatbattleforlife。Thelimbsdividedintogreatbranches，andtheseintolesserandlesserbranches，werethemselvesonce，whenthetreewassmall，buddingtwigs。andthisconnexionoftheformerandpresentbudsbyramifyingbranchesmaywellrepresenttheclassificationofallextinctandlivingspeciesingroupssubordinatetogroups。Ofthemanytwigswhichflourishedwhenthetreewasamerebush，onlytwoorthree，nowgrownintogreatbranches，yetsurviveandbearalltheotherbranches。sowiththespecieswhichlivedduringlong-pastgeologicalperiods，veryfewnowhavelivingandmodifieddescendants。Fromthefirstgrowthofthetree，manyalimbandbranchhasdecayedanddroppedoff。andtheselostbranchesofvarioussizesmayrepresentthosewholeorders，families，andgenerawhichhavenownolivingrepresentatives，andwhichareknowntousonlyfromhavingbeenfoundinafossilstate。Aswehereandthereseeathinstragglingbranchspringingfromaforklowdowninatree，andwhichbysomechancehasbeenfavouredandisstillaliveonitssummit，soweoccasionallyseeananimalliketheOrnithorhynchusorLepidosiren，whichinsomesmalldegreeconnectsbyitsaffinitiestwolargebranchesoflife，andwhichhasapparentlybeensavedfromfatalcompetitionbyhavinginhabitedaprotectedstation。Asbudsgiverisebygrowthtofreshbuds，andthese，ifvigorous，branchoutandovertoponallsidesmanyafeeblerbranch，sobygenerationIbelieveithasbeenwiththegreatTreeofLife，whichfillswithitsdeadandbrokenbranchesthecrustoftheearth，andcoversthesurfacewithitseverbranchingandbeautifulramifications。PreviousChapterNextChapterTheOriginofSpecies-Chapter5TheOriginofSpeciesbyCharlesDarwinPreviousChapterNextChapterChapter5-LawsofVariationIHAVEhithertosometimesspokenasifthevariationssocommonandmultiforminorganicbeingsunderdomestication，andinalesserdegreeinthoseinastateofnaturehadbeenduetochance。This，ofcourse，isawhollyincorrectexpression，butitservestoacknowledgeplainlyourignoranceofthecauseofeachparticularvariation。Someauthorsbelieveittobeasmuchthefunctionofthereproductivesystemtoproduceindividualdifferences，orveryslightdeviationsofstructure，astomakethechildlikeitsparents。Butthemuchgreatervariability，aswellasthegreaterfrequencyofmonstrosities，underdomesticationorcultivation，thanundernature，leadsmetobelievethatdeviationsofstructureareinsomewayduetothenatureoftheconditionsoflife，towhichtheparentsandtheirmoreremoteancestorshavebeenexposedduringseveralgenerations。Ihaveremarkedinthefirstchapterbutalongcatalogueoffactswhichcannotbeheregivenwouldbenecessarytoshowthetruthoftheremarkthatthereproductivesystemiseminentlysusceptibletochangesintheconditionsoflife。andtothissystembeingfunctionallydisturbedintheparents，Ichieflyattributethevaryingorplasticconditionoftheoffspring。

Themaleandfemalesexualelementsseemtobeaffectedbeforethatuniontakesplacewhichistoformanewbeing。Inthecaseof’sporting’plants，thebud，whichinitsearliestconditiondoesnotapparentlydifferessentiallyfromanovule，isaloneaffected。Butwhy，becausethereproductivesystemisdisturbed，thisorthatpartshouldvarymoreorless，weareprofoundlyignorant。Nevertheless，wecanhereandtheredimlycatchafaintrayoflight，andwemayfeelsurethattheremustbesomecauseforeachdeviationofstructure，howeverslight。

Howmuchdirecteffectdifferenceofclimate，food，&。c。，producesonanybeingisextremelydoubtful。Myimpressionis，thattheeffectisextremelysmallinthecaseofanimals，butperhapsrathermoreinthatofplants。Wemay，atleast，safelyconcludethatsuchinfluencescannothaveproducedthemanystrikingandcomplexco-adaptationsofstructurebetweenoneorganicbeingandanother，whichweseeeverywherethroughoutnature。Somelittleinfluencemaybeattributedtoclimate，food，&。c。：

thus，E。Forbesspeaksconfidentlythatshellsattheirsouthernlimit，andwhenlivinginshallowwater，aremorebrightlycolouredthanthoseofthesamespeciesfurthernorthorfromgreaterdepths。Gouldbelievesthatbirdsofthesamespeciesaremorebrightlycolouredunderaclearatmosphere，thanwhenlivingonislandsornearthecoast。Sowithinsects，Wollastonisconvincedthatresidenceneartheseaaffectstheircolours。

Moquin-Tandongivesalistofplantswhichwhengrowingnearthesea-shorehavetheirleavesinsomedegreefleshy，thoughnotelsewherefleshy。Severalothersuchcasescouldbegiven。

Thefactofvarietiesofonespecies，whentheyrangeintothezoneofhabitationofotherspecies，oftenacquiringinaveryslightdegreesomeofthecharactersofsuchspecies，accordswithourviewthatspeciesofallkindsareonlywell-markedandpermanentvarieties。Thusthespeciesofshellswhichareconfinedtotropicalandshallowseasaregenerallybrighter-colouredthanthoseconfinedtocoldanddeeperseas。Thebirdswhichareconfinedtocontinentsare，accordingtoMrGould，brighter-colouredthanthoseofislands。Theinsect-speciesconfinedtosea-coasts，aseverycollectorknows，areoftenbrassyorlurid。Plantswhichliveexclusivelyonthesea-sideareveryapttohavefleshyleaves。Hewhobelievesinthecreationofeachspecies，willhavetosaythatthisshell，forinstance，wascreatedwithbrightcoloursforawarmsea。butthatthisothershellbecamebright-colouredbyvariationwhenitrangedintowarmerorshallowerwaters。

Whenavariationisoftheslightestusetoabeing，wecannottellhowmuchofittoattributetotheaccumulativeactionofnaturalselection，andhowmuchtotheconditionsoflife。Thus，itiswellknowntofurriersthatanimalsofthesamespecieshavethickerandbetterfurthemoreseveretheclimateisunderwhichtheyhavelived。butwhocantellhowmuchofthisdifferencemaybeduetothewarmest-cladindividualshavingbeenfavouredandpreservedduringmanygenerations，andhowmuchtothedirectactionofthesevereclimate？foritwouldappearthatclimatehassomedirectactiononthehairofourdomesticquadrupeds。

Instancescouldbegivenofthesamevarietybeingproducedunderconditionsoflifeasdifferentascanwellbeconceived。and，ontheotherhand，ofdifferentvarietiesbeingproducedfromthesamespeciesunderthesameconditions。Suchfactsshowhowindirectlytheconditionsoflifemustact。Again，innumerableinstancesareknowntoeverynaturalistofspecieskeepingtrue，ornotvaryingatall，althoughlivingunderthemostoppositeclimates。Suchconsiderationsastheseinclinemetolayverylittleweightonthedirectactionoftheconditionsoflife。Indirectly，asalreadyremarked，theyseemtoplayanimportantpartinaffectingthereproductivesystem，andinthusinducingvariability。andnaturalselectionwillthenaccumulateallprofitablevariations，howeverslight，untiltheybecomeplainlydevelopedandappreciablebyus。EffectsofUseandDisuseFromthefactsalludedtointhefirstchapter，Ithinktherecanbelittledoubtthatuseinourdomesticanimalsstrengthensandenlargescertainparts，anddisusediminishesthem。andthatsuchmodificationsareinherited。

Underfreenature，wecanhavenostandardofcomparison，bywhichtojudgeoftheeffectsoflong-continueduseordisuse，forweknownottheparent-forms。

butmanyanimalshavestructureswhichcanbeexplainedbytheeffectsofdisuse。AsProfessorOwenhasremarked，thereisnogreateranomalyinnaturethanabirdthatcannotfly。yetthereareseveralinthisstate。

Thelogger-headedduckofSouthAmericacanonlyflapalongthesurfaceofthewater，andhasitswingsinnearlythesameconditionasthedomesticAylesburyduck。Asthelargerground-feedingbirdsseldomtakeflightexcepttoescapedanger，Ibelievethatthenearlywinglessconditionofseveralbirds，whichnowinhabitorhavelatelyinhabitedseveraloceanicislands，tenantedbynobeastofprey，hasbeencausedbydisuse。Theostrichindeedinhabitscontinentsandisexposedtodangerfromwhichitcannotescapebyflight，butbykickingitcandefenditselffromenemies，aswellasanyofthesmallerquadrupeds。Wemayimaginethattheearlyprogenitoroftheostrichhadhabitslikethoseofabustard，andthatasnaturalselectionincreasedinsuccessivegenerationsthesizeandweightofitsbody，itslegswereusedmore，anditswingsless，untiltheybecameincapableofflight。

Kirbyhasremarked(andIhaveobservedthesamefact)thattheanteriortarsi，orfeet，ofmanymaledung-feedingbeetlesareveryoftenbrokenoff。heexaminedseventeenspecimensinhisowncollection，andnotonehadevenarelicleft。IntheOnitesapellesthetarsiaresohabituallylost，thattheinsecthasbeendescribedasnothavingthem。Insomeothergeneratheyarepresent，butinarudimentarycondition。IntheAteuchusorsacredbeetleoftheEgyptians，theyaretotallydeficient。Thereisnotsufficientevidencetoinduceustobelievethatmutilationsareeverinherited。andIshouldpreferexplainingtheentireabsenceoftheanteriortarsiinAteuchus，andtheirrudimentaryconditioninsomeothergenera，bythelong-continuedeffectsofdisuseintheirprogenitors。forasthetarsiarealmostalwayslostinmanydung-feedingbeetles，theymustbelostearlyinlife，andthereforecannotbemuchusedbytheseinsects。

Insomecaseswemighteasilyputdowntodisusemodificationsofstructurewhicharewholly，ormainly，duetonaturalselection。Mr。Wollastonhasdiscoveredtheremarkablefactthat200beetles，outofthe550speciesinhabitingMadeira，aresofardeficientinwingsthattheycannotfly。

andthatofthetwenty-nineendemicgenera，nolessthantwenty-threegenerahavealltheirspeciesinthiscondition！Severalfacts，namely，thatbeetlesinmanypartsoftheworldareveryfrequentlyblowntoseaandperish。

thatthebeetlesinMadeira，asobservedbyMrWollaston，liemuchconcealed，untilthewindlullsandthesunshines。thattheproportionofwinglessbeetlesislargerontheexposedDezertasthaninMadeiraitself。andespeciallytheextraordinaryfact，sostronglyinsistedonbyMr。Wollaston，ofthealmostentireabsenceofcertainlargegroupsofbeetles，elsewhereexcessivelynumerous，andwhichgroupshavehabitsoflifealmostnecessitatingfrequentflight。theseseveralconsiderationshavemademebelievethatthewinglessconditionofsomanyMadeirabeetlesismainlyduetotheactionofnaturalselection，butcombinedprobablywithdisuse。Forduringthousandsofsuccessivegenerationseachindividualbeetlewhichflewleast，eitherfromitswingshavingbeeneversolittlelessperfectlydevelopedorfromindolenthabit，willhavehadthebestchanceofsurvivingfromnotbeingblownouttosea。and，ontheotherhand，thosebeetleswhichmostreadilytooktoflightwilloftenesthavebeenblowntoseaandthushavebeendestroyed。

TheinsectsinMadeirawhicharenotground-feeders，andwhich，astheflower-feedingcoleopteraandlepidoptera，musthabituallyusetheirwingstogaintheirsubsistence，have，asMr。Wollastonsuspects，theirwingsnotatallreduced，butevenenlarged。Thisisquitecompatiblewiththeactionofnaturalselection。Forwhenanewinsectfirstarrivedontheisland，thetendencyofnaturalselectiontoenlargeortoreducethewings，woulddependonwhetheragreaternumberofindividualsweresavedbysuccessfullybattlingwiththewinds，orbygivinguptheattemptandrarelyorneverflying。Aswithmarinersship-wreckednearacoast，itwouldhavebeenbetterforthegoodswimmersiftheyhadbeenabletoswimstillfurther，whereasitwouldhavebeenbetterforthebadswimmersiftheyhadnotbeenabletoswimatallandhadstucktothewreck。

Theeyesofmolesandofsomeburrowingrodentsarerudimentaryinsize，andinsomecasesarequitecoveredupbyskinandfur。Thisstateoftheeyesisprobablyduetogradualreductionfromdisuse，butaidedperhapsbynaturalselection。InSouthAmerica，aburrowingrodent，thetuco-tuco，orCtenomys，isevenmoresubterraneaninitshabitsthanthemole。andIwasassuredbyaSpaniard，whohadoftencaughtthem，thattheywerefrequentlyblind。onewhichIkeptalivewascertainlyinthiscondition，thecause，asappearedondissection，havingbeeninflammationofthenictitatingmembrane。Asfrequentinflammationoftheeyesmustbeinjurioustoanyanimal，andaseyesarecertainlynotindispensabletoanimalswithsubterraneanhabits，areductionintheirsizewiththeadhesionoftheeyelidsandgrowthoffuroverthem，mightinsuchcasebeanadvantage。andifso，naturalselectionwouldconstantlyaidtheeffectsofdisuse。

Itiswellknownthatseveralanimals，belongingtothemostdifferentclasses，whichinhabitthecavesofStyriaandofKentucky，areblind。

Insomeofthecrabsthefoot-stalkfortheeyeremains，thoughtheeyeisgone。thestandforthetelescopeisthere，thoughthetelescopewithitsglasseshasbeenlost。Asitisdifficulttoimaginethateyes，thoughuseless，couldbeinanywayinjurioustoanimalslivingindarkness，I

attributetheirlosswhollytodisuse。Inoneoftheblindanimals，namely，thecave-rat，theeyesareofimmensesize。andProfessorSillimanthoughtthatitregained，afterlivingsomedaysinthelight，someslightpowerofvision。InthesamemannerasinMadeirathewingsofsomeoftheinsectshavebeenenlarged，andthewingsofothershavebeenreducedbynaturalselectionaidedbyuseanddisuse，sointhecaseofthecave-ratnaturalselectionseemstohavestruggledwiththelossoflightandtohaveincreasedthesizeoftheeyes。whereaswithalltheotherinhabitantsofthecaves，disusebyitselfseemstohavedoneitswork。

Itisdifficulttoimagineconditionsoflifemoresimilarthandeeplimestonecavernsunderanearlysimilarclimate。sothatonthecommonviewoftheblindanimalshavingbeenseparatelycreatedfortheAmericanandEuropeancaverns，closesimilarityintheirorganisationandaffinitiesmighthavebeenexpected。but，asSchiö。dteandothershaveremarked，thisisnotthecase，andthecave-insectsofthetwocontinentsarenotmorecloselyalliedthanmighthavebeenanticipatedfromthegeneralresemblanceoftheotherinhabitantsofNorthAmericaandEurope。OnmyviewwemustsupposethatAmericananimals，havingordinarypowersofvision，slowlymigratedbysuccessivegenerationsfromtheouterworldintothedeeperanddeeperrecessesoftheKentuckycaves，asdidEuropeananimalsintothecavesofEurope。Wehavesomeevidenceofthisgradationofhabit。

for，asSchiö。dteremarks，’animalsnotfarremotefromordinaryforms，preparethetransitionfromlighttodarkness。Nextfollowthosethatareconstructedfortwilight。and，lastofall，thosedestinedfortotaldarkness。’

Bythetimethatananimalhadreached，afternumberlessgenerations，thedeepestrecesses，disusewillonthisviewhavemoreorlessperfectlyobliterateditseyes，andnaturalselectionwilloftenhaveeffectedotherchanges，suchasanincreaseinthelengthoftheantennaeorpalpi，asacompensationforblindness。Notwithstandingsuchmodifications，wemightexpectstilltoseeinthecave-animalsofAmerica，affinitiestotheotherinhabitantsofthatcontinent，andinthoseofEurope，totheinhabitantsoftheEuropeancontinent。AndthisisthecasewithsomeoftheAmericancave-animals，asIhearfromProfessorDana。andsomeoftheEuropeancave-insectsareverycloselyalliedtothoseofthesurroundingcountry。Itwouldbemostdifficulttogiveanyrationalexplanationoftheaffinitiesoftheblindcave-animalstotheotherinhabitantsofthetwocontinentsontheordinaryviewoftheirindependentcreation。ThatseveraloftheinhabitantsofthecavesoftheOldandNewWorldsshouldbecloselyrelated，wemightexpectfromthewell-knownrelationshipofmostoftheirotherproductions。

Farfromfeelinganysurprisethatsomeofthecave-animalsshouldbeveryanomalous，asAgassizhasremarkedinregardtotheblindfish，theAmblyopsis，andasisthecasewiththeblindProteuswithreferencetothereptilesofEurope，Iamonlysurprisedthatmorewrecksofancientlifehavenotbeenpreserved，owingtothelessseverecompetitiontowhichtheinhabitantsofthesedarkabodeswillprobablyhavebeenexposed。AcclimatisationHabitishereditarywithplants，asintheperiodofflowering，intheamountofrainrequisiteforseedstogerminate，inthetimeofsleep，&。c。，andthisleadsmetosayafewwordsonacclimatisation。Asitisextremelycommonforspeciesofthesamegenustoinhabitveryhotandverycoldcountries，andasIbelievethatallthespeciesofthesamegenushavedescendedfromasingleparent，ifthisviewbecorrect，acclimatisationmustbereadilyeffectedduringlong-continueddescent。Itisnotoriousthateachspeciesisadaptedtotheclimateofitsownhome：speciesfromanarcticorevenfromatemperateregioncannotendureatropicalclimate，orconversely。Soagain，manysucculentplantscannotendureadampclimate。

Butthedegreeofadaptationofspeciestotheclimatesunderwhichtheyliveisoftenoverrated。Wemayinferthisfromourfrequentinabilitytopredictwhetherornotanimportedplantwillendureourclimate，andfromthenumberofplantsandanimalsbroughtfromwarmercountrieswhichhereenjoygoodhealth。Wehavereasontobelievethatspeciesinastateofnaturearelimitedintheirrangesbythecompetitionofotherorganicbeingsquiteasmuchas，ormorethan，byadaptationtoparticularclimates。

Butwhetherornottheadaptationbegenerallyveryclose，wehaveevidence，inthecaseofsomefewplants，oftheirbecoming，toacertainextent，naturallyhabituatedtodifferenttemperatures，orbecomingacclimatised：

thusthepinesandrhododendrons，raisedfromseedcollectedbyDrHookerfromtreesgrowingatdifferentheightsontheHimalayawerefoundinthiscountrytopossessdifferentconstitutionalpowersofresistingcold。MrThwaitesinformsmethathehasobservedsimilarfactsinCeylon，andanalogousobservationshavebeenmadebyMrH。C。WatsononEuropeanspeciesofplantsbroughtfromtheAzorestoEngland。Inregardtoanimals，severalauthenticcasescouldbegivenofspecieswithinhistoricaltimeshavinglargelyextendedtheirrangefromwarmertocoolerlatitudes，andconversely。butwedonotpositivelyknowthattheseanimalswerestrictlyadaptedtotheirnativeclimate，butinallordinarycasesweassumesuchtobethecase。

nordoweknowthattheyhavesubsequentlybecomeacclimatisedtotheirnewhomes。

AsIbelievethatourdomesticanimalswereoriginallychosenbyuncivilisedmanbecausetheywereusefulandbredreadilyunderconfinement，andnotbecausetheyweresubsequentlyfoundcapableoffar-extendedtransportation，Ithinkthecommonandextraordinarycapacityinourdomesticanimalsofnotonlywithstandingthemostdifferentclimatesbutofbeingperfectlyfertile(afarseverertest)underthem，maybeusedasanargumentthatalargeproportionofotheranimals，nowinastateofnature，couldeasilybebroughttobearwidelydifferentclimates。Wemustnot，however，pushtheforegoingargumenttoofar，onaccountoftheprobableoriginofsomeofourdomesticanimalsfromseveralwildstocks：theblood，forinstance，ofatropicalandarcticwolforwilddogmayperhapsbemingledinourdomesticbreeds。Theratandmousecannotbeconsideredasdomesticanimals，buttheyhavebeentransportedbymantomanypartsoftheworld，andnowhaveafarwiderrangethananyotherrodent，livingfreeunderthecoldclimateofFaroeinthenorthandoftheFalklandsinthesouth，andonmanyislandsinthetorridzones。HenceIaminclinedtolookatadaptationtoanyspecialclimateasaqualityreadilygraftedonaninnatewideflexibilityofconstitution，whichiscommontomostanimals。Onthisview，thecapacityofenduringthemostdifferentclimatesbymanhimselfandbyhisdomesticanimals，andsuchfactsasthatformerspeciesoftheelephantandrhinoceroswerecapableofenduringaglacialclimate，whereasthelivingspeciesarenowalltropicalorsub-tropicalintheirhabits，oughtnottobelookedatasanomalies，butmerelyasexamplesofaverycommonflexibilityofconstitution，brought，underpeculiarcircumstances，intoplay。

Howmuchoftheacclimatisationofspeciestoanypeculiarclimateisduetomerehabit，andhowmuchtothenaturalselectionofvarietieshavingdifferentinnateconstitutions，andhowmuchtomeanscombined，isaveryobscurequestion。ThathabitorcustomhassomeinfluenceImustbelieve，bothfromanalogy，andfromtheincessantadvicegiveninagriculturalworks，evenintheancientEncyclopaediasofChina，tobeverycautiousintransposinganimalsfromonedistricttoanother。foritisnotlikelythatmanshouldhavesucceededinselectingsomanybreedsandsub-breedswithconstitutionsspeciallyfittedfortheirowndistricts：theresultmust，Ithink，beduetohabit。Ontheotherhand，Icanseenoreasontodoubtthatnaturalselectionwillcontinuallytendtopreservethoseindividualswhicharebornwithconstitutionsbestadaptedtotheirnativecountries。Intreatisesonmanykindsofcultivatedplants，certainvarietiesaresaidtowithstandcertainclimatesbetterthanothers：thisisverystrikinglyshowninworksonfruittreespublishedintheUnitedStates，inwhichcertainvarietiesarehabituallyrecommendedforthenorthern，andothersforthesouthernStates。andasmostofthesevarietiesareofrecentorigin，theycannotowetheirconstitutionaldifferencestohabit。

ThecaseoftheJerusalemartichoke，whichisneverpropagatedbyseed，andofwhichconsequentlynewvarietieshavenotbeenproduced，hasevenbeenadvancedforitisnowastenderaseveritwas——asprovingthatacclimatisationcannotbeeffected！Thecase，also，ofthekidney-beanhasbeenoftencitedforasimilarpurpose，andwithmuchgreaterweight。

butuntilsomeonewillsow，duringascoreofgenerations，hiskidney-beanssoearlythataverylargeproportionaredestroyedbyfrost，andthencollectseedfromthefewsurvivors，withcaretopreventaccidentalcrosses，andthenagaingetseedfromtheseseedlings，withthesameprecautions，theexperimentcannotbesaidtohavebeeneventried。Norletitbesupposedthatnodifferencesintheconstitutionofseedlingkidney-beanseverappear，foranaccounthasbeenpublishedhowmuchmorehardysomeseedlingsappearedtobethanothers。

Onthewhole，Ithinkwemayconcludethathabit，use，anddisuse，have，insomecases，playedaconsiderablepartinthemodificationoftheconstitution，andofthestructureofvariousorgans。butthattheeffectsofuseanddisusehaveoftenbeenlargelycombinedwith，andsometimesovermasteredby，thenaturalselectionofinnatedifferences。CorrelationofGrowthImeanbythisexpressionthatthewholeorganisationissotiedtogetherduringitsgrowthanddevelopment，thatwhenslightvariationsinanyonepartoccur，andareaccumulatedthroughnaturalselection，otherpartsbecomemodified。Thisisaveryimportantsubject，mostimperfectlyunderstood。

Themostobviouscaseis，thatmodificationsaccumulatedsolelyforthegoodoftheyoungorlarva，will，itmaysafelybeconcluded，affectthestructureoftheadult。inthesamemannerasanymalconformationaffectingtheearlyembryo，seriouslyaffectsthewholeorganisationoftheadult。

Theseveralpartsofthebodywhicharehomologous，andwhich，atanearlyembryonicperiod，arealike，seemliabletovaryinanalliedmanner：weseethisintherightandleftsidesofthebodyvaryinginthesamemanner。

inthefrontandhindlegs，andeveninthejawsandlimbs，varyingtogether，forthelowerjawisbelievedtobehomologouswiththelimbs。Thesetendencies，Idonotdoubt，maybemasteredmoreorlesscompletelybynaturalselection：

thusafamilyofstagsonceexistedwithanantleronlyononeside。andifthishadbeenofanygreatusetothebreeditmightprobablyhavebeenrenderedpermanentbynaturalselection。

Homologousparts，ashasbeenremarkedbysomeauthors，tendtocohere。

thisisoftenseeninmonstrousplants。andnothingismorecommonthantheunionofhomologouspartsinnormalstructures，astheunionofthepetalsofthecorollaintoatube。Hardpartsseemtoaffecttheformofadjoiningsoftparts。itisbelievedbysomeauthorsthatthediversityintheshapeofthepelvisinbirdscausestheremarkablediversityintheshapeoftheirkidneys。Othersbelievethattheshapeofthepelvisinthehumanmotherinfluencesbypressuretheshapeoftheheadofthechild。Insnakes，accordingtoSchlegel，theshapeofthebodyandthemannerofswallowingdeterminethepositionofseveralofthemostimportantviscera。

Thenatureofthebondofcorrelationisveryfrequentlyquiteobscure。

M。Is。GeoffroyStHilairehasforciblyremarked，thatcertainmalconformationsveryfrequently，andthatothersrarelycoexist，withoutourbeingabletoassignanyreason。Whatcanbemoresingularthantherelationbetweenblueeyesanddeafnessincats，andthetortoise-shellcolourwiththefemalesex。thefeatheredfeetandskinbetweentheoutertoesinpigeons，andthepresenceofmoreorlessdownontheyoungbirdswhenfirsthatched，withthefuturecolouroftheirplumage。or，again，therelationbetweenthehairandteethinthenakedTurkishdog，thoughhereprobablyhomologycomesintoplay？Withrespecttothislattercaseofcorrelation，Ithinkitcanhardlybeaccidental，thatifwepickoutthetwoordersofmammaliawhicharemostabnormalintheirdermalcoverings，viz。Cetacea(whales)

andEdentata(armadilloes，scalyant-eaters，&。c。)，thatthesearelikewisethemostabnormalintheirteeth。

Iknowofnocasebetteradaptedtoshowtheimportanceofthelawsofcorrelationinmodifyingimportantstructures，independentlyofutilityand，therefore，ofnaturalselection，thanthatofthedifferencebetweentheouterandinnerflowersinsomeCompositousandUmbelliferousplants。

Everyoneknowsthedifferenceintherayandcentralfloretsof，forinstance，thedaisy，andthisdifferenceisoftenaccompaniedwiththeabortionofpartsoftheflower。But，insomeCompositousplants，theseedsalsodifferinshapeandsculpture。andeventheovaryitself，withitsaccessoryparts，differs，ashasbeendescribedbyCassini。Thesedifferenceshavebeenattributedbysomeauthorstopressure，andtheshapeoftheseedsintheray-floretsinsomeCompositaecountenancesthisidea。but，inthecaseofthecorollaoftheUmbelliferae，itisbynomeans，asDrHookerinformsme，inspecieswiththedensestheadsthattheinnerandouterflowersmostfrequentlydiffer。Itmighthavebeenthoughtthatthedevelopmentoftheray-petalsbydrawingnourishmentfromcertainotherpartsoftheflowerhadcausedtheirabortion。butinsomeCompositaethereisadifferenceintheseedsoftheouterandinnerfloretswithoutanydifferenceinthecorolla。Possibly，theseseveraldifferencesmaybeconnectedwithsomedifferenceintheflowofnutrimenttowardsthecentralandexternalflowers：

weknow，atleast，thatinirregularflowers，thosenearesttotheaxisareoftenestsubjecttopeloria，andbecomeregular。Imayadd，asaninstanceofthis，andofastrikingcaseofcorrelation，thatIhaverecentlyobservedinsomegardenpelargoniums，thatthecentralflowerofthetrussoftenlosesthepatchesofdarkercolourinthetwoupperpetals。andthatwhenthisoccurs，theadherentnectaryisquiteaborted。whenthecolourisabsentfromonlyoneofthetwoupperpetals，thenectaryisonlymuchshortened。

Withrespecttothedifferenceinthecorollaofthecentralandexteriorflowersofaheadorumbel，IdonotfeelatallsurethatC。C。Sprengel’sideathattheray-floretsservetoattractinsects，whoseagencyishighlyadvantageousinthefertilisationofplantsofthesetwoorders，issofar-fetched，asitmayatfirstappear：andifitbeadvantageous，naturalselectionmayhavecomeintoplay。Butinregardtothedifferencesbothintheinternalandexternalstructureoftheseeds，whicharenotalwayscorrelatedwithanydifferencesintheflowers，itseemsimpossiblethattheycanbeinanywayadvantageoustotheplant：yetintheUmbelliferaethesedifferencesareofsuchapparentimportancetheseedsbeinginsomecases，accordingtoTausch，orthospermousintheexteriorflowersandcoelospermousinthecentralflowers，thattheelderDeCandollefoundedhismaindivisionsoftheorderonanalogousdifferences。Henceweseethatmodificationsofstructure，viewedbysystematistsasofhighvalue，maybewhollyduetounknownlawsofcorrelatedgrowth，andwithoutbeing，asfaraswecansee，oftheslightestservicetothespecies。

Wemayoftenfalselyattributetocorrelationofgrowth，structureswhicharecommontowholegroupsofspecies，andwhichintrutharesimplyduetoinheritance。foranancientprogenitormayhaveacquiredthroughnaturalselectionsomeonemodificationinstructure，and，afterthousandsofgenerations，someotherandindependentmodification。andthesetwomodifications，havingbeentransmittedtoawholegroupofdescendantswithdiversehabits，wouldnaturallybethoughttobecorrelatedinsomenecessarymanner。So，again，Idonotdoubtthatsomeapparentcorrelations，occurringthroughoutwholeorders，areentirelyduetothemanneraloneinwhichnaturalselectioncanact。Forinstance，Alph。DeCandollehasremarkedthatwingedseedsareneverfoundinfruitswhichdonotopen：

Ishouldexplaintherulebythefactthatseedscouldnotgraduallybecomewingedthroughnaturalselection，exceptinfruitswhichopened。sothattheindividualplantsproducingseedswhichwerealittlebetterfittedtobewaftedfurther，mightgetanadvantageoverthoseproducingseedlessfittedfordispersal。andthisprocesscouldnotpossiblygooninfruitwhichdidnotopen。

TheelderGeoffroyandGoethepropounded，ataboutthesameperiod，theirlawofcompensationorbalancementofgrowth。or，asGoetheexpressedit，’inordertospendononeside，natureisforcedtoeconomiseontheotherside。’Ithinkthisholdstruetoacertainextentwithourdomesticproductions：ifnourishmentflowstoonepartororganinexcess，itrarelyflows，atleastinexcess，toanotherpart。thusitisdifficulttogetacowtogivemuchmilkandtofattenreadily。Thesamevarietiesofthecabbagedonotyieldabundantandnutritiousfoliageandacopioussupplyofoil-bearingseeds。Whentheseedsinourfruitsbecomeatrophied，thefruititselfgainslargelyinsizeandquality。Inourpoultry，alargetuftoffeathersontheheadisgenerallyaccompaniedbyadiminishedcomb，andalargebeardbydiminishedwattles。Withspeciesinastateofnatureitcanhardlybemaintainedthatthelawisofuniversalapplication。butmanygoodobservers，moreespeciallybotanists，believeinitstruth。I

willnot，however，heregiveanyinstances，forIseehardlyanywayofdistinguishingbetweentheeffects，ontheonehand，ofapartbeinglargelydevelopedthroughnaturalselectionandanotherandadjoiningpartbeingreducedbythissameprocessorbydisuse，and，ontheotherhand，theactualwithdrawalofnutrimentfromonepartowingtotheexcessofgrowthinanotherandadjoiningpart。

Isuspect，also，thatsomeofthecasesofcompensationwhichhavebeenadvanced，andlikewisesomeotherfacts，maybemergedunderamoregeneralprinciple，namely，thatnaturalselectioniscontinuallytryingtoeconomiseineverypartoftheorganisation。Ifunderchangedconditionsoflifeastructurebeforeusefulbecomeslessuseful，anydiminution，howeverslight，initsdevelopment，willbeseizedonbynaturalselection，foritwillprofittheindividualnottohaveitsnutrimentwastedinbuildingupanuselessstructure。IcanthusonlyunderstandafactwithwhichI

wasmuchstruckwhenexaminingcirripedes，andofwhichmanyotherinstancescouldbegiven：namely，thatwhenacirripedeisparasiticwithinanotherandisthusprotected，itlosesmoreorlesscompletelyitsownshellorcarapace。ThisisthecasewiththemaleIbla，andinatrulyextraordinarymannerwiththeProteolepas：forthecarapaceinallothercirripedesconsistsofthethreehighly-importantanteriorsegmentsoftheheadenormouslydeveloped，andfurnishedwithgreatnervesandmuscles。butintheparasiticandprotectedProteolepas，thewholeanteriorpartoftheheadisreducedtothemerestrudimentattachedtothebasesoftheprehensileantennae。

Nowthesavingofalargeandcomplexstructure，whenrenderedsuperfluousbytheparasitichabitsoftheProteolepas，thougheffectedbyslowsteps，wouldbeadecidedadvantagetoeachsuccessiveindividualofthespecies。

forinthestruggleforlifetowhicheveryanimalisexposed，eachindividualProteolepaswouldhaveabetterchanceofsupportingitself，bylessnutrimentbeingwastedindevelopingastructurenowbecomeuseless。

Thus，asIbelieve，naturalselectionwillalwayssucceedinthelongruninreducingandsavingeverypartoftheorganisation，assoonasitisrenderedsuperfluous，withoutbyanymeanscausingsomeotherparttobelargelydevelopedinacorrespondingdegree。And，conversely，thatnaturalselectionmayperfectlywellsucceedinlargelydevelopinganyorgan，withoutrequiringasanecessarycompensationthereductionofsomeadjoiningpart。

Itseemstobearule，asremarkedbyIs。GeoffroyStHilaire，bothinvarietiesandinspecies，thatwhenanypartororganisrepeatedmanytimesinthestructureofthesameindividual(asthevertebraeinsnakes，andthestamensinpolyandrousflowers)thenumberisvariable。whereasthenumberofthesamepartororgan，whenitoccursinlessernumbers，isconstant。Thesameauthorandsomebotanistshavefurtherremarkedthatmultiplepartsarealsoveryliabletovariationinstructure。Inasmuchasthis’vegetativerepetition，’touseProf。Owen’sexpression，seemstobeasignofloworganisation。theforegoingremarkseemsconnectedwiththeverygeneralopinionofnaturalists，thatbeingslowinthescaleofnaturearemorevariablethanthosewhicharehigher。Ipresumethatlownessinthiscasemeansthattheseveralpartsoftheorganisationhavebeenbutlittlespecialisedforparticularfunctions。andaslongasthesameparthastoperformdiversifiedwork，wecanperhapsseewhyitshouldremainvariable，thatis，whynaturalselectionshouldhavepreservedorrejectedeachlittledeviationofformlesscarefullythanwhentheparthastoserveforonespecialpurposealone。Inthesamewaythataknifewhichhastocutallsortsofthingsmaybeofalmostanyshape。whilstatoolforsomeparticularobjecthadbetterbeofsomeparticularshape。

Naturalselection，itshouldneverbeforgotten，canactoneachpartofeachbeing，solelythroughandforitsadvantage。

Rudimentaryparts，ithasbeenstatedbysomeauthors，andIbelievewithtruth，areapttobehighlyvariable。Weshallhavetorecurtothegeneralsubjectofrudimentaryandabortedorgans。andIwillhereonlyaddthattheirvariabilityseemstobeowingtotheiruselessness，andthereforetonaturalselectionhavingnopowertocheckdeviationsintheirstructure。Thusrudimentarypartsarelefttothefreeplayofthevariouslawsofgrowth，totheeffectsoflong-continueddisuse，andtothetendencytoreversion。Apartdevelopedinanyspeciesinanextraordinarydegreeormanner，incomparisonwiththesamepartinalliedspecies，tendstobehighlyvariable。SeveralyearsagoIwasmuchstruckwitharemark，nearlytotheaboveeffect，publishedbyMrWaterhouse。IinferalsofromanobservationmadebyProfessorOwen，withrespecttothelengthofthearmsoftheourang-outang，thathehascometoanearlysimilarconclusion。ItishopelesstoattempttoconvinceanyoneofthetruthofthispropositionwithoutgivingthelongarrayoffactswhichIhavecollected，andwhichcannotpossiblybehereintroduced。Icanonlystatemyconvictionthatitisaruleofhighgenerality。Iamawareofseveralcausesoferror，butIhopethatIhavemadedueallowanceforthem。Itshouldbeunderstoodthattherulebynomeansappliestoanypart，howeverunusuallydeveloped，unlessitbeunusuallydevelopedincomparisonwiththesamepartincloselyalliedspecies。Thus，thebat’swingisamostabnormalstructureintheclassmammalia。buttherulewouldnothereapply，becausethereisawholegroupofbatshavingwings。itwouldapplyonlyifsomeonespeciesofbathaditswingsdevelopedinsomeremarkablemannerincomparisonwiththeotherspeciesofthesamegenus。Theruleappliesverystronglyinthecaseofsecondarysexualcharacters，whendisplayedinanyunusualmanner。Theterm，secondarysexualcharacters，usedbyHunter，appliestocharacterswhichareattachedtoonesex，butarenotdirectlyconnectedwiththeactofreproduction。Theruleappliestomalesandfemales。butasfemalesmorerarelyofferremarkablesecondarysexualcharacters，itappliesmorerarelytothem。Therulebeingsoplainlyapplicableinthecaseofsecondarysexualcharacters，maybeduetothegreatvariabilityofthesecharacters，whetherornotdisplayedinanyunusualmannerofwhichfactIthinktherecanbelittledoubt。Butthatourruleisnotconfinedtosecondarysexualcharactersisclearlyshowninthecaseofhermaphroditecirripedes。andImayhereadd，thatIparticularlyattendedtoMr。Waterhouse’sremark，whilstinvestigatingthisOrder，andIamfullyconvincedthattherulealmostinvariablyholdsgoodwithcirripedes。

Ishall，inmyfuturework，givealistofthemoreremarkablecases。I

willhereonlybrieflygiveone，asitillustratestheruleinitslargestapplication。Theopercularvalvesofsessilecirripedes(rockbarnacles)

are，ineverysenseoftheword，veryimportantstructures，andtheydifferextremelylittleevenindifferentgenera。butintheseveralspeciesofonegenus，Pyrgoma，thesevalvespresentamarvellousamountofdiversification：

thehomologousvalvesinthedifferentspeciesbeingsometimeswhollyunlikeinshape。andtheamountofvariationintheindividualsofseveralofthespeciesissogreat，thatitisnoexaggerationtostatethatthevarietiesdiffermorefromeachotherinthecharactersoftheseimportantvalvesthandootherspeciesofdistinctgenera。

Asbirdswithinthesamecountryvaryinaremarkablysmalldegree，Ihaveparticularlyattendedtothem，andtheruleseemstomecertainlytoholdgoodinthisclass。Icannotmakeoutthatitappliestoplants，andthiswouldseriouslyhaveshakenmybeliefinitstruth，hadnotthegreatvariabilityinplantsmadeitparticularlydifficulttocomparetheirrelativedegreesofvariability。

Whenweseeanypartororgandevelopedinaremarkabledegreeormannerinanyspecies，thefairpresumptionisthatitisofhighimportancetothatspecies。neverthelessthepartinthiscaseiseminentlyliabletovariation。Whyshouldthisbeso？Ontheviewthateachspecieshasbeenindependentlycreated，withallitspartsaswenowseethem，Icanseenoexplanation。Butontheviewthatgroupsofspecieshavedescendedfromotherspecies，andhavebeenmodifiedthroughnaturalselection，Ithinkwecanobtainsomelight。Inourdomesticanimals，ifanypart，orthewholeanimal，beneglectedandnoselectionbeapplied，thatpart(forinstance，thecombintheDorkingfowl)orthewholebreedwillceasetohaveanearlyuniformcharacter。Thebreedwillthenbesaidtohavedegenerated。

Inrudimentaryorgans，andinthosewhichhavebeenbutlittlespecializedforanyparticularpurpose，andperhapsinpolymorphicgroups，weseeanearlyparallelnaturalcase。forinsuchcasesnaturalselectioneitherhasnotorcannotcomeintofullplay，andthustheorganisationisleftinafluctuatingcondition。Butwhatheremoreespeciallyconcernsusis，thatinourdomesticanimalsthosepoints，whichatthepresenttimeareundergoingrapidchangebycontinuedselection，arealsoeminentlyliabletovariation。Lookatthebreedsofthepigeon。seewhataprodigiousamountofdifferencethereisinthebeakofthedifferenttumblers，inthebeakandwattleofthedifferentcarriers，inthecarriageandtailofourfantails，&。c。，thesebeingthepointsnowmainlyattendedtobyEnglishfanciers。

Eveninthesub-breeds，asintheshort-facedtumbler，itisnotoriouslydifficulttobreedthemnearlytoperfection，andfrequentlyindividualsarebornwhichdepartwidelyfromthestandard。Theremaybetrulysaidtobeaconstantstrugglegoingonbetween，ontheonehand，thetendencytoreversiontoalessmodifiedstate，aswellasaninnatetendencytofurthervariabilityofallkinds，and，ontheotherhand，thepowerofsteadyselectiontokeepthebreedtrue。Inthelongrunselectiongainstheday，andwedonotexpecttofailsofarastobreedabirdascoarseasacommontumblerfromagoodshort-facedstrain。Butaslongasselectionisrapidlygoingon，theremayalwaysbeexpectedtobemuchvariabilityinthestructureundergoingmodification。Itfurtherdeservesnoticethatthesevariablecharacters，producedbyman’sselection，sometimesbecomeattached，fromcausesquiteunknowntous，moretoonesexthantotheother，generallytothemalesex，aswiththewattleofcarriersandtheenlargedcropofpouters。

Nowletusturntonature。Whenaparthasbeendevelopedinanextraordinarymannerinanyonespecies，comparedwiththeotherspeciesofthesamegenus，wemayconcludethatthisparthasundergoneanextraordinaryamountofmodification，sincetheperiodwhenthespeciesbranchedofffromthecommonprogenitorofthegenus。Thisperiodwillseldomberemoteinanyextremedegree，asspeciesveryrarelyendureformorethanonegeologicalperiod。Anextraordinaryamountofmodificationimpliesanunusuallylargeandlong-continuedamountofvariability，whichhascontinuallybeenaccumulatedbynaturalselectionforthebenefitofthespecies。Butasthevariabilityoftheextraordinarily-developedpartororganhasbeensogreatandlong-continuedwithinaperiodnotexcessivelyremote，wemight，asageneralrule，expectstilltofindmorevariabilityinsuchpartsthaninotherpartsoftheorganisation，whichhaveremainedforamuchlongerperiodnearlyconstant。

Andthis，Iamconvinced，isthecase。Thatthestrugglebetweennaturalselectionontheonehand，andthetendencytoreversionandvariabilityontheotherhand，willinthecourseoftimecease。andthatthemostabnormallydevelopedorgansmaybemadeconstant，Icanseenoreasontodoubt。Hencewhenanorgan，howeverabnormalitmaybe，hasbeentransmittedinapproximatelythesameconditiontomanymodifieddescendants，asinthecaseofthewingofthebat，itmusthaveexisted，accordingtomytheory，foranimmenseperiodinnearlythesamestate。andthusitcomestobenomorevariablethananyotherstructure。Itisonlyinthosecasesinwhichthemodificationhasbeencomparativelyrecentandextraordinarilygreatthatweoughttofindthegenerativevariability，asitmaybecalled，stillpresentinahighdegree。Forinthiscasethevariabilitywillseldomasyethavebeenfixedbythecontinuedselectionoftheindividualsvaryingintherequiredmanneranddegree，andbythecontinuedrejectionofthosetendingtoreverttoaformerandlessmodifiedcondition。

Theprincipleincludedintheseremarksmaybeextended。Itisnotoriousthatspecificcharactersaremorevariablethangeneric。Toexplainbyasimpleexamplewhatismeant。Ifsomespeciesinalargegenusofplantshadblueflowersandsomehadred，thecolourwouldbeonlyaspecificcharacter，andnoonewouldbesurprisedatoneofthebluespeciesvaryingintored，orconversely。butifallthespecieshadblueflowers，thecolourwouldbecomeagenericcharacter，anditsvariationwouldbeamoreunusualcircumstance。Ihavechosenthisexamplebecauseanexplanationisnotinthiscaseapplicable，whichmostnaturalistswouldadvance，namely，thatspecificcharactersaremorevariablethangeneric，becausetheyaretakenfrompartsoflessphysiologicalimportancethanthosecommonlyusedforclassinggenera。Ibelievethisexplanationispartly，yetonlyindirectly，true。Ishall，however，havetoreturntothissubjectinourchapteronClassification。Itwouldbealmostsuperfluoustoadduceevidenceinsupportoftheabovestatement，thatspecificcharactersaremorevariablethangeneric。butIhaverepeatedlynoticedinworksonnaturalhistory，thatwhenanauthorhasremarkedwithsurprisethatsomeimportantorganorpart，whichisgenerallyveryconstantthroughoutlargegroupsofspecies，hasdifferedconsiderablyinclosely-alliedspecies，thatithas，also，beenvariableintheindividualsofsomeofthespecies。Andthisfactshowsthatacharacter，whichisgenerallyofgenericvalue，whenitsinksinvalueandbecomesonlyofspecificvalue，oftenbecomesvariable，thoughitsphysiologicalimportancemayremainthesame。Somethingofthesamekindappliestomonstrosities：atleastIs。GeoffroySt。Hilaireseemstoentertainnodoubt，thatthemoreanorgannormallydiffersinthedifferentspeciesofthesamegroup，themoresubjectitistoindividualanomalies。

Ontheordinaryviewofeachspecieshavingbeenindependentlycreated，whyshouldthatpartofthestructure，whichdiffersfromthesamepartinotherindependently-createdspeciesofthesamegenus，bemorevariablethanthosepartswhicharecloselyalikeintheseveralspecies？Idonotseethatanyexplanationcanbegiven。Butontheviewofspeciesbeingonlystronglymarkedandfixedvarieties，wemightsurelyexpecttofindthemstilloftencontinuingtovaryinthosepartsoftheirstructurewhichhavevariedwithinamoderatelyrecentperiod，andwhichhavethuscometodiffer。Ortostatethecaseinanothermanner：thepointsinwhichallthespeciesofagenusresembleeachother，andinwhichtheydifferfromthespeciesofsomeothergenus，arecalledgenericcharacters。andthesecharactersincommonIattributetoinheritancefromacommonprogenitor，foritcanrarelyhavehappenedthatnaturalselectionwillhavemodifiedseveralspecies，fittedtomoreorlesswidely-differenthabits，inexactlythesamemanner：andastheseso-calledgenericcharactershavebeeninheritedfromaremoteperiod，sincethatperiodwhenthespeciesfirstbranchedofffromtheircommonprogenitor，andsubsequentlyhavenotvariedorcometodifferinanydegree，oronlyinaslightdegree，itisnotprobablethattheyshouldvaryatthepresentday。Ontheotherhand，thepointsinwhichspeciesdifferfromotherspeciesofthesamegenus，arecalledspecificcharacters。andasthesespecificcharactershavevariedandcometodifferwithintheperiodofthebranchingoffofthespeciesfromacommonprogenitor，itisprobablethattheyshouldstilloftenbeinsomedegreevariable，atleastmorevariablethanthosepartsoftheorganisationwhichhaveforaverylongperiodremainedconstant。

Inconnexionwiththepresentsubject，Iwillmakeonlytwootherremarks。

Ithinkitwillbeadmitted，withoutmyenteringondetails，thatsecondarysexualcharactersareveryvariable。Ithinkitalsowillbeadmittedthatspeciesofthesamegroupdifferfromeachothermorewidelyintheirsecondarysexualcharacters，thaninotherpartsoftheirorganisation。compare，forinstance，theamountofdifferencebetweenthemalesofgallinaceousbirds，inwhichsecondarysexualcharactersarestronglydisplayed，withtheamountofdifferencebetweentheirfemales。andthetruthofthispropositionwillbegranted。Thecauseoftheoriginalvariabilityofsecondarysexualcharactersisnotmanifest。butwecanseewhythesecharactersshouldnothavebeenrenderedasconstantanduniformasotherpartsoftheorganisation。

forsecondarysexualcharactershavebeenaccumulatedbysexualselection，whichislessrigidinitsactionthanordinaryselection，asitdoesnotentaildeath，butonlygivesfeweroffspringtothelessfavouredmales。

Whateverthecausemaybeofthevariabilityofsecondarysexualcharacters，astheyarehighlyvariable，sexualselectionwillhavehadawidescopeforaction，andmaythusreadilyhavesucceededingivingtothespeciesofthesamegroupagreateramountofdifferenceintheirsexualcharacters，thaninotherpartsoftheirstructure。

Itisaremarkablefact，thatthesecondarysexualdifferencesbetweenthetwosexesofthesamespeciesaregenerallydisplayedintheverysamepartsoftheorganisationinwhichthedifferentspeciesofthesamegenusdifferfromeachother。OfthisfactIwillgiveinillustrationtwoinstances，thefirstwhichhappentostandonmylist。andasthedifferencesinthesecasesareofaveryunusualnature，therelationcanhardlybeaccidental。

Thesamenumberofjointsinthetarsiisacharactergenerallycommontoverylargegroupsofbeetles，butintheEngidae，asWestwoodhasremarked，thenumbervariesgreatly。andthenumberlikewisediffersinthetwosexesofthesamespecies：againinfossorialhymenoptera，themannerofneurationofthewingsisacharacterofthehighestimportance，becausecommontolargegroups。butincertaingeneratheneurationdiffersinthedifferentspecies，andlikewiseinthetwosexesofthesamespecies。Thisrelationhasaclearmeaningonmyviewofthesubject：Ilookatallthespeciesofthesamegenusashavingascertainlydescendedfromthesameprogenitor，ashavethetwosexesofanyoneofthespecies。Consequently，whateverpartofthestructureofthecommonprogenitor，orofitsearlydescendants，becamevariable。variationsofthispartwoulditishighlyprobable，betakenadvantageofbynaturalandsexualselection，inordertofittheseveralspeciestotheirseveralplacesintheeconomyofnature，andlikewisetofitthetwosexesofthesamespeciestoeachother，ortofitthemalesandfemalestodifferenthabitsoflife，orthemalestostrugglewithothermalesforthepossessionofthefemales。