Now，withtheincreasedsizeandchangeddirectionoftheearsnotonlyhasthebonyauditorymeatusbecomechangedinoutline，direction，andgreatlyinsize，butthewholeskullhasbeenslightlymodified。Thiscouldbeclearlyseenin\"half—lops\"——thatis，inrabbitswithonlyoneearloppingforward——
fortheoppositesidesoftheirskullswerenotstrictlysymmetrical。Thisseemstomeacuriousinstanceofcorrelation，betweenhardbonesandorganssosoftandflexible，aswellassounimportantunderaphysiologicalpointofview，astheexternalears。Theresultnodoubtislargelyduetomeremechanicalaction，thatis，totheweightoftheears，onthesameprinciplethattheskullofahumaninfantiseasilymodifiedbypressure。
Theskinandtheappendagesofhair，feathers，hoofs，horns，andteeth，arehomologousoverthewholebody。Everyoneknowsthatthecolouroftheskinandthatofthehairusuallyvarytogether;sothatVirgiladvisestheshepherdtolookwhetherthemouthandtongueoftheramareblack，lestthelambsshouldnotbepurelywhite。Thecolouroftheskinandhair，andtheodouremittedbytheglandsoftheskin，aresaid（25/10。Godron’Surl’Espece’tome2page217。）tobeconnected，eveninthesameraceofmen。
Generallythehairvariesinthesamewayalloverthebodyinlength，fineness，andcurliness。Thesameruleholdsgoodwithfeathers，asweseewiththelacedandfrizzledbreedsbothoffowlsandpigeons。Inthecommoncockthefeathersontheneckandloinsarealwaysofaparticularshape，calledhackles:nowinthePolishbreed，bothsexesarecharacterisedbyatuftoffeathersonthehead，andthroughcorrelationthesefeathersinthemalealwaysassumetheformofhackles。Thewingandtail—feathers，thougharisingfrompartsnothomologous，varyinlengthtogether;sothatlongorshortwingedpigeonsgenerallyhavelongorshorttails。ThecaseoftheJacobin—pigeonismorecurious，forthewingandtailfeathersareremarkablylong;andthisapparentlyhasarisenincorrelationwiththeelongatedandreversedfeathersonthebackoftheneck，whichformthehood。
Thehoofsandhairarehomologousappendages;andacarefulobserver，namelyAzara（25/11。’QuadrupedesduParaguay’tome2page333。），statesthatinParaguayhorsesofvariouscoloursareoftenbornwiththeirhaircurledandtwistedlikethatontheheadofanegro。Thispeculiarityisstronglyinherited。Butwhatisremarkableisthatthehoofsofthesehorses\"areabsolutelylikethoseofamule。\"Thehairalsooftheirmanesandtailsisinvariablymuchshorterthanusual，beingonlyfromfourtotwelveinchesinlength;sothatcurlinessandshortnessofthehairarehere，aswiththenegro，apparentlycorrelated。
Withrespecttothehornsofsheep，Youatt（25/12。’OnSheep’page142。）
remarksthat\"multiplicityofhornsisnotfoundinanybreedofmuchvalue;
itisgenerallyaccompaniedbygreatlengthandcoarsenessofthefleece。\"
Severaltropicalbreedsofsheepwhichareclothedwithhairinsteadofwool，havehornsalmostlikethoseofagoat。Sturm（25/13。’UeberRacen，Kreuzungen’etc。1825s。24。）expresslydeclaresthatindifferentracesthemorethewooliscurledthemorethehornsarespirallytwisted。Wehaveseeninthethirdchapter，whereotheranalogousfactshavebeengiven，thattheparentoftheMauchampbreed，sofamousforitsfleece，hadpeculiarlyshapedhorns。TheinhabitantsofAngoraassert（25/14。QuotedfromConollyin’TheIndianField’February1859volume2page266。）that\"onlythewhitegoatswhichhavehornswearthefleeceinthelongcurlylocksthataresomuchadmired;thosewhicharenothornedhavingacomparativelyclosecoat。\"Fromthesecaseswemayinferthatthehairorwoolandthehornstendtovaryinacorrelatedmanner。（25/15。InthethirdchapterIhavesaidthat\"thehairandhornsaresocloselyrelatedtoeachother，thattheyareapttovarytogether。\"Dr。Wilckens（\"Darwin’sTheorie\"’JahrbuchderDeutschenViehzucht’
18661。Heft）translatesmywordsinto\"lang—undgrobhaarigeThieresollengeneigtersein，langeundvieleHornerzubekommen\"andhethenjustlydisputesthisproposition;butwhatIhavereallysaid，inaccordancewiththeauthoritiesjustquoted，may，Ithink，betrusted。）Thosewhohavetriedhydropathyareawarethatthefrequentapplicationofcoldwaterstimulatestheskin;andwhateverstimulatestheskintendstoincreasethegrowthofthehair，asiswellshownintheabnormalgrowthofhairnearoldinflamedsurfaces。Now，ProfessorLow（25/16。’DomesticatedAnimalsoftheBritishIslands’pages307，368。Dr。Wilckensargues（’Landwirth。Wochenblatt’Nr。10
1869）tothesameeffectwithrespecttodomesticanimalsinGermany。）isconvincedthatwiththedifferentracesofBritishcattlethickskinandlonghairdependonthehumidityoftheclimatewhichtheyinhabit。Wecanthusseehowahumidclimatemightactonthehorns——inthefirstplacedirectlyontheskinandhair，andsecondlybycorrelationonthehorns。Thepresenceorabsenceofhorns，moreover，bothinthecaseofsheepandcattle，acts，aswillpresentlybeshown，bysomesortofcorrelationontheskull。
Withrespecttohairandteeth，Mr。Yarrell（25/17。’ProceedingsZoolog。Soc。’
1833page113。）foundmanyoftheteethdeficientinthreehairless\"Egyptiandogs，\"andinahairlessterrier。Theincisors，canines，andthepremolarssufferedmost，butinonecasealltheteeth，exceptthelargetubercularmolaroneachside，weredeficient。Withmanseveralstrikingcaseshavebeenrecorded（25/18。Sedgwick’Brit。andForeignMedico—Chirurg。Review’April1863page453。）ofinheritedbaldnesswithinheriteddeficiency，eithercompleteorpartial，oftheteeth。Imaygiveananalogouscase，communicatedtomebyMr。W。Wedderburn，ofaHindoofamilyinScinde，inwhichtenmen，inthecourseoffourgenerations，werefurnished，inbothjawstakentogether，withonlyfoursmallandweakincisorteethandwitheightposteriormolars。
Thementhusaffectedhaveverylittlehaironthebody，andbecomebaldearlyinlife。Theyalsosuffermuchduringhotweatherfromexcessivedrynessoftheskin。Itisremarkablethatnoinstancehasoccurredofadaughterbeingthusaffected;andthisfactremindsushowmuchmoreliablemenareinEnglandtobecomebaldthanwomen。Thoughthedaughtersintheabovefamilyareneveraffected，theytransmitthetendencytotheirsons;andnocasehasoccurredofasontransmittingittohissons。Theaffectionthusappearsonlyinalternategenerations，orafterlongerintervals。Thereisasimilarconnectionbetweenhairandteeth，accordingtoMr。Sedgwick，inthoserarecasesinwhichthehairhasbeenrenewedinoldage，forthishas\"usuallybeenaccompaniedbyarenewaloftheteeth。\"Ihaveremarkedinaformerpartofthisvolumethatthegreatreductioninthesizeofthetusksindomesticboarsprobablystandsincloserelationwiththeirdiminishedbristles，duetoacertainamountofprotection;andthatthereappearanceofthetusksinboars，whichhavebecomeferalandarefullyexposedtotheweather，probablydependsonthereappearanceofthebristles。Imayadd，thoughnotstrictlyconnectedwithourpresentpoint，thatanagriculturist（25/19。’Gardener’sChronicle’1849page205。）assertsthat\"pigswithlittlehairontheirbodiesaremostliabletolosetheirtails，showingaweaknessofthetegumentalstructure。Itmaybepreventedbycrossingwithamorehairybreed。\"
Inthepreviouscasesdeficienthair，andteethdeficientinnumberorsize，areapparentlyconnected。Inthefollowingcasesabnormallyredundanthair，andteetheitherdeficientorredundant，arelikewiseconnected。Mr。Crawfurd（25/20。’EmbassytotheCourtofAva’volume1page320。）sawattheBurmeseCourtaman，thirtyyearsold，withhiswholebody，exceptthehandsandfeet，coveredwithstraightsilkyhair，whichontheshouldersandspinewasfiveinchesinlength。Atbirththeearsalonewerecovered。Hedidnotarriveatpuberty，orshedhismilkteeth，untiltwentyyearsold;andatthisperiodheacquiredfiveteethintheupperjaw，namely，fourincisorsandonecanine，andfourincisorteethinthelowerjaw;alltheteethweresmall。Thismanhadadaughterwhowasbornwithhairwithinherears;andthehairsoonextendedoverherbody。WhenCaptainYule（25/21。’NarrativeofaMissiontotheCourtofAvain1855’page94。）visitedtheCourt，hefoundthisgirlgrownup;andshepresentedastrangeappearancewithevenhernosedenselycoveredwithsofthair。Likeherfather，shewasfurnishedwithincisorteethalone。TheKinghadwithdifficultybribedamantomarryher，andofhertwochildren，one，aboyfourteenmonthsold，hadhairgrowingoutofhisears，withabeardandmoustache。Thisstrangepeculiarityhas，therefore，beeninheritedforthreegenerations，withthemolarteethdeficientinthegrandfatherandmother;whethertheseteethwouldlikewisefailintheinfantcouldnotthenbetold。
Aparallelcaseofamanfifty—fiveyearsold，andofhisson，withtheirfacescoveredwithhair，hasrecentlyoccurredinRussia。Dr。Alex。Brandthassentmeanaccountofthiscase，togetherwithspecimensoftheextremelyfinehairfromthecheeks。Themanisdeficientinteeth，possessingonlyfourincisorsinthelowerandtwointheupperjaw。Hisson，aboutthreeyearsold，hasnoteethexceptfourlowerincisors。Thecase，asDr。Brandtremarksinhisletter，nodoubtisduetoanarrestofdevelopmentinthehairandteeth。Wehereseehowindependentoftheordinaryconditionsofexistencesucharrestsmustbe，forthelivesofaRussianpeasantandofanativeofBurmahareasdifferentaspossible。（25/22。IowetothekindnessofM。
Chauman，ofSt。Petersburg，excellentphotographsofthismanandhisson，bothofwhomhavesincebeenexhibitedinParisandLondon。）
HereisanotherandsomewhatdifferentcasecommunicatedtomebyMr。WallaceontheauthorityofDr。Purland，adentist:JuliaPastrana，aSpanishdancer，wasaremarkablyfinewoman，butshehadathickmasculinebeardandahairyforehead;shewasphotographed，andherstuffedskinwasexhibitedasashow;
butwhatconcernsusis，thatshehadinboththeupperandlowerjawanirregulardoublesetofteeth，onerowbeingplacedwithintheother，ofwhichDr。Purlandtookacast。Fromtheredundancyofteethhermouthprojected，andherfacehadagorilla—likeappearance。Thesecasesandthoseofthehairlessdogsforciblycalltomindthefact，thatthetwoordersofmammals——namely，theEdentataandCetacea——whicharethemostabnormalintheirdermalcovering，arelikewisethemostabnormaleitherbydeficiencyorredundancyofteeth。
Theorgansofsightandhearingaregenerallyadmittedtobehomologouswithoneanotherandwithvariousdermalappendages;hencethesepartsareliabletobeabnormallyaffectedinconjunction。Mr。WhiteCowpersays\"thatinallcasesofdoublemicrophthalmiabroughtunderhisnoticehehasatthesametimemetwithdefectivedevelopmentofthedentalsystem。\"Certainformsofblindnessseemtobeassociatedwiththecolourofthehair;amanwithblackhairandawomanwithlight—colouredhair，bothofsoundconstitution，marriedandhadninechildren，allofwhomwerebornblind;ofthesechildren，five\"withdarkhairandbrowniriswereafflictedwithamaurosis;thefourothers，withlight—colouredhairandblueiris，hadamaurosisandcataractconjoined。\"
Severalcasescouldbegiven，showingthatsomerelationexistsbetweenvariousaffectionsoftheeyesandears;thusLiebreichstatesthatoutof241
deaf—mutesinBerlin，nolessthanfourteensufferedfromtherarediseasecalledpigmentaryretinitis。Mr。WhiteCowperandDr。Earlehaveremarkedthatinabilitytodistinguishdifferentcolours，orcolour—blindness，\"isoftenassociatedwithacorrespondinginabilitytodistinguishmusicalsounds。\"
（25/23。ThesestatementsaretakenfromMr。Sedgwickinthe’Medico—Chirurg。
Review’July1861page198;April1863pages455and458。LiebreichisquotedbyProfessorDevayinhis’MariagesConsanguins’1862page116。）
Hereisamorecuriouscase:whitecats，iftheyhaveblueeyes，arealmostalwaysdeaf。Iformerlythoughtthattherulewasinvariable，butIhaveheardofafewauthenticexceptions。Thefirsttwonoticeswerepublishedin1829
andrelatetoEnglishandPersiancats:ofthelatter，theRev。W。T。Breepossessedafemale，andhestates，\"thatoftheoffspringproducedatoneandthesamebirth，suchas，likethemother，wereentirelywhite（withblueeyes）
were，likeher，invariablydeaf;whilethosethathadtheleastspeckofcolourontheirfur，asinvariablypossessedtheusualfacultyofhearing。\"
（25/24。Loudon’s’Mag。ofNat。Hist。’volume11829pages66，178。SeealsoDr。P。Lucas’L’Hered。Nat。’tome1page428ontheinheritanceofdeafnessincats。Mr。LawsonTaitstates（’Nature’1873page323）thatonlymalecatsarethusaffected;butthismustbeahastygeneralisation。ThefirstcaserecordedinEnglandbyMr。Breerelatedtoafemale，andMr。Foxinformsmethathehasbredkittensfromawhitefemalewithblueeyes，whichwascompletelydeaf;hehasalsoobservedotherfemalesinthesamecondition。）
TheRev。W。DarwinFoxinformsmethathehasseenmorethanadozeninstancesofthiscorrelationinEnglish，Persian，andDanishcats;butheadds\"that，ifoneeye，asIhaveseveraltimesobserved，benotblue，thecathears。Ontheotherhand，Ihaveneverseenawhitecatwitheyesofthecommoncolourthatwasdeaf。\"InFranceDr。Sichel（25/25。’AnnalesdesSc。Nat。’Zoolog。
3rdseries1847tome8page239。）hasobservedduringtwentyyearssimilarfacts;headdstheremarkablecaseoftheirisbeginning，attheendoffourmonths，togrowdark—coloured，andthenthecatfirstbegantohear。
Thiscaseofcorrelationincatshasstruckmanypersonsasmarvellous。Thereisnothingunusualintherelationbetweenblueeyesandwhitefur;andwehavealreadyseenthattheorgansofsightandhearingareoftensimultaneouslyaffected。Inthepresentinstancethecauseprobablyliesinaslightarrestofdevelopmentinthenervoussysteminconnectionwiththesense—organs。Kittensduringthefirstninedays，whilsttheireyesareclosed，appeartobecompletelydeaf;Ihavemadeagreatclangingnoisewithapokerandshovelclosetotheirheads，bothwhentheywereasleepandawake，withoutproducinganyeffect。Thetrialmustnotbemadebyshoutingclosetotheirears，fortheyare，evenwhenasleep，extremelysensitivetoabreathofair。Now，aslongastheeyescontinueclosed，theirisisnodoubtblue，forinallthekittenswhichIhaveseenthiscolourremainsforsometimeaftertheeyelidsopen。Hence，ifwesupposethedevelopmentoftheorgansofsightandhearingtobearrestedatthestageoftheclosedeyelids，theeyeswouldremainpermanentlyblueandtheearswouldbeincapableofperceivingsound;
andweshouldthusunderstandthiscuriouscase。As，however，thecolourofthefurisdeterminedlongbeforebirth，andasthebluenessoftheeyesandthewhitenessofthefurareobviouslyconnected，wemustbelievethatsomeprimarycauseactsatamuchearlierperiod。
Theinstancesofcorrelatedvariabilityhithertogivenhavebeenchieflydrawnfromtheanimalkingdom，andwewillnowturntoplants。Leaves，sepals，petals，stamens，andpistilsareallhomologous。Indoubleflowersweseethatthestamensandpistilsvaryinthesamemanner，andassumetheformandcolourofthepetals。Inthedoublecolumbine（Aquilegiavulgaris），thesuccessivewhorlsofstamensareconvertedintocornucopias，whichareenclosedwithinoneanotherandresemblethetruepetals。Inhose—in—hoseflowersthesepalsmockthepetals。Insomecasestheflowersandleavesvarytogetherintint:inallthevarietiesofthecommonpea，whichhavepurpleflowers，apurplemarkmaybeseenonthestipules。
M。FaivrestatesthatwiththevarietiesofPrimulasinensisthecolouroftheflowerisevidentlycorrelatedwiththecolouroftheundersideoftheleaves;andheaddsthatthevarietieswithfimbriatedflowersalmostalwayshavevoluminous，balloon—likecalyces。（25/26。’RevuedesCoursScientifiques’
June5，1869page430。）Withotherplantstheleavesandfruitorseedsvarytogetherincolour，asinacuriouspale—leavedvarietyofthesycamore，whichhasrecentlybeendescribedinFrance（25/27。’Gardener’sChronicle’1864page1202。），andasinthepurple—leavedhazel，inwhichtheleaves，thehuskofthenut，andthepellicleroundthekernelareallcolouredpurple。（25/28。
Verlotgivesseveralotherinstances’DesVarietes’1865page72。）Pomologistscanpredicttoacertainextent，fromthesizeandappearanceoftheleavesoftheirseedlings，theprobablenatureofthefruit;for，asVanMonsremarks（25/29。’ArbresFruitiers’1836tome2pages204，226。）variationsintheleavesaregenerallyaccompaniedbysomemodificationintheflower，andconsequentlyinthefruit。IntheSerpentmelon，whichhasanarrowtortuousfruitaboveayardinlength，thestemoftheplant，thepeduncleofthefemaleflower，andthemiddlelobeoftheleaf，areallelongatedinaremarkablemanner。Ontheotherhand，severalvarietiesofCucurbita，whichhavedwarfedstems，allproduce，asNaudinremarks，leavesofthesamepeculiarshape。Mr。G。MawinformsmethatallthevarietiesofthescarletPelargoniumswhichhavecontractedorimperfectleaveshavecontractedflowers:thedifferencebetween\"Brilliant\"anditsparent\"TomThumb\"isagoodinstanceofthis。ItmaybesuspectedthatthecuriouscasedescribedbyRisso（25/30。’AnnalesduMuseum’tome20page188。），ofavarietyoftheOrangewhichproducesontheyoungshootsroundedleaveswithwingedpetioles，andafterwardselongatedleavesonlongbutwinglesspetioles，isconnectedwiththeremarkablechangeinformandnaturewhichthefruitundergoesduringitsdevelopment。
Inthefollowinginstancewehavethecolourandtheformofthepetalsapparentlycorrelated，andbothdependentonthenatureoftheseason。Anobserver，skilledinthesubject，writes（25/31。’Gardener’sChronicle’1843
page877。），\"Inoticed，duringtheyear1842，thateveryDahliaofwhichthecolourhadanytendencytoscarlet，wasdeeplynotched——indeed，tosogreatanextentastogivethepetalstheappearanceofasaw;theindentureswere，insomeinstances，morethanaquarterofaninchdeep。\"Again，Dahliaswhichhavetheirpetalstippedwithadifferentcolourfromtherestoftheflowerareveryinconstant，andduringcertainyearssome，orevenalltheflowers，becomeuniformlycoloured;andithasbeenobservedwithseveralvarieties（25/32。Ibid1845page102。）thatwhenthishappensthepetalsgrowmuchelongatedandlosetheirpropershape。This，however，maybeduetoreversion，bothincolourandform，totheaboriginalspecies。
Inthisdiscussiononcorrelation，wehavehithertotreatedofcasesinwhichwecanpartlyunderstandthebondofconnection;butIwillnowgivecasesinwhichwecannotevenconjecture，orcanonlyveryobscurelysee，thenatureofthebond。IsidoreGeoffroySaint—Hilaire，inhisworkonMonstrosities，insists（25/33。’Hist。desAnomalies’tome3page402。SeealsoM。CamilleDareste’RecherchessurlesConditions’etc。1863pages16，48。），\"quecertainesanomaliescoexistentraremententr’elles，d’autresfrequemment，d’autresenfinpresqueconstamment，malgreladifferencetres—grandedeleurnature，etquoiqu’ellespuissentparaitreCOMPLETEMENTINDEPENDANTESlesunesdesautres。\"Weseesomethinganalogousincertaindiseases:thusinarareaffectionoftherenalcapsules（ofwhichthefunctionsareunknown），theskinbecomesbronzed;andinhereditarysyphilis，asIhearfromSirJ。Paget，boththemilkandthesecondteethassumeapeculiarandcharacteristicform。
ProfessorRolleston，also，informsmethattheincisorteetharesometimesfurnishedwithavascularrimincorrelationwithintra—pulmonarydepositionoftubercles。Inothercasesofphthisisandofcyanosisthenailsandfinger—
endsbecomeclubbedlikeacorns。Ibelievethatnoexplanationhasbeenofferedoftheseandofmanyothercasesofcorrelateddisease。
Whatcanbemorecuriousandlessintelligiblethanthefactpreviouslygiven，ontheauthorityofMr。Tegetmeier，thatyoungpigeonsofallbreeds，whichwhenmaturehavewhite，yellow，silver—blue，ordun—colouredplumage，comeoutoftheeggalmostnaked;whereaspigeonsofothercolourswhenfirstbornareclothedwithplentyofdown？WhitePea—fowls，ashasbeenobservedbothinEnglandandFrance（25/34。Rev。E。S。Dixon’OrnamentalPoultry’1848page111;
IsidoreGeoffroy’Hist。Anomalies’tome1page211。），andasIhavemyselfseen，areinferiorinsizetothecommoncolouredkind;andthiscannotbeaccountedforbythebeliefthatalbinismisalwaysaccompaniedbyconstitutionalweakness;forwhiteoralbinomolesaregenerallylargerthanthecommonkind。
Toturntomoreimportantcharacters:theniatacattleofthePampasareremarkablefromtheirshortforeheads，upturnedmuzzles，andcurvedlowerjaws。Intheskullthenasalandpremaxillarybonesaremuchshortened，themaxillariesareexcludedfromanyjunctionwiththenasals，andallthebonesareslightlymodified，eventotheplaneoftheocciput。Fromtheanalogouscaseofthedog，hereaftertobegiven，itisprobablethattheshorteningofthenasalandadjoiningbonesistheproximatecauseoftheothermodificationsintheskull，includingtheupwardcurvatureofthelowerjaw，thoughwecannotfollowoutthestepsbywhichthesechangeshavebeeneffected。
Polishfowlshavealargetuftoffeathersontheirheads;andtheirskullsareperforatedbynumerousholes，sothatapincanbedrivenintothebrainwithouttouchinganybone。Thatthisdeficiencyofboneisinsomewayconnectedwiththetuftoffeathersisclearfromtuftedducksandgeeselikewisehavingperforatedskulls。Thecasewouldprobablybeconsideredbysomeauthorsasoneofbalancementorcompensation。InthechapteronFowls，I
haveshownthatwithPolishfowlsthetuftoffeatherswasprobablyatfirstsmall;bycontinuedselectionitbecamelarger，andthenrestedonafibrousmass;andfinally，asitbecamestilllarger，theskullitselfbecamemoreandmoreprotuberantuntilitacquireditspresentextraordinarystructure。
Throughcorrelationwiththeprotuberanceoftheskull，theshapeandeventherelativeconnectionofthepremaxillaryandnasalbones，theshapeoftheorificeofthenostrils，thebreadthofthefrontalbone，theshapeofthepost—lateralprocessesofthefrontalandsquamosalbones，andthedirectionofthebonycavityoftheear，haveallbeenmodified。Theinternalconfigurationoftheskullandthewholeshapeofthebrainhavelikewisebeenalteredinatrulymarvellousmanner。
AfterthiscaseofthePolishfowlitwouldbesuperfluoustodomorethanrefertothedetailspreviouslygivenonthemannerinwhichthechangedformofthecombhasaffectedtheskull，invariousbreedsofthefowl，causingbycorrelationcrests，protuberances，anddepressionsonitssurface。
Withourcattleandsheepthehornsstandincloseconnectionwiththesizeoftheskull，andwiththeshapeofthefrontalbones;thusCline（25/35。’OntheBreedingofDomesticAnimals’1829page6。）foundthattheskullofahornedramweighedfivetimesasmuchasthatofahornlessramofthesameage。Whencattlebecomehornless，thefrontalbonesare\"materiallydiminishedinbreadthtowardsthepoll;\"andthecavitiesbetweenthebonyplates\"arenotsodeep，nordotheyextendbeyondthefrontals。\"（25/36。Youatton’Cattle’
1834page283。）
Itmaybewellheretopauseandobservehowtheeffectsofcorrelatedvariability，oftheincreaseduseofparts，andoftheaccumulationofso—
calledspontaneousvariationsthroughnaturalselection，areinmanycasesinextricablycommingled。WemayborrowanillustrationfromMr。HerbertSpencer，whoremarksthat，whentheIrishelkacquireditsgigantichorns，weighingaboveonehundredpounds，numerousco—ordinatedchangesofstructurewouldhavebeenindispensable，——namely，athickenedskulltocarrythehorns;
strengthenedcervicalvertebrae，withstrengthenedligaments;enlargeddorsalvertebraetosupporttheneck，withpowerfulfore—legsandfeet;allthesepartsbeingsuppliedwithpropermuscles，blood—vessels，andnerves。Howthencouldtheseadmirablyco—ordinatedmodificationsofstructurehavebeenacquired？AccordingtothedoctrinewhichImaintain，thehornsofthemaleelkwereslowlygainedthroughsexualselection，——thatis，bythebest—armedmalesconqueringtheworse—armed，andleavingagreaternumberofdescendants。
Butitisnotatallnecessarythattheseveralpartsofthebodyshouldhavesimultaneouslyvaried。Eachstagpresentsindividualcharacteristics，andinthesamedistrictthosewhichhadslightlyheavierhorns，orstrongernecks，orstrongerbodies，orwerethemostcourageous，wouldsecurethegreaternumberofdoes，andconsequentlyhaveagreaternumberofoffspring。Theoffspringwouldinherit，inagreaterorlessdegree，thesesamequalities，wouldoccasionallyintercrosswithoneanother，orwithotherindividualsvaryinginsomefavourablemanner;andoftheiroffspring，thosewhichwerethebestendowedinanyrespectwouldcontinuemultiplying;andsoonwards，alwaysprogressing，sometimesinonedirection，andsometimesinanother，towardstheexcellentlyco—ordinatedstructureofthemaleelk。Tomakethisclear，letusreflectontheprobablesteps，asshowninthetwentiethchapter，bywhichourraceanddrayhorseshavearrivedattheirpresentstateofexcellence;ifwecouldviewthewholeseriesofintermediateformsbetweenoneoftheseanimalsandanearlyunimprovedprogenitor，weshouldbeholdavastnumberofanimals，notequallyimprovedineachgenerationthroughouttheirentirestructure，butsometimesalittlemoreinonepoint，andsometimesinanother，yetonthewholegraduallyapproachingincharactertoourpresentraceordrayhorses，whicharesoadmirablyfittedintheonecaseforfleetnessandintheotherfordraught。
Althoughnaturalselectionwouldthus（25/37。Mr。HerbertSpencer’PrinciplesofBiology’1864volume1pages452，468takesadifferentview;andinoneplaceremarks:\"Wehaveseenreasontothinkthat，asfastasessentialfacultiesmultiply，andasfastasthenumberoforgansthatco—operateinanygivenfunctionincreases，indirectequilibrationthroughnaturalselectionbecomeslessandlesscapableofproducingspecificadaptations;andremainsfullycapableonlyofmaintainingthegeneralfitnessofconstitutiontoconditions。\"Thisviewthatnaturalselectioncandolittleinmodifyingthehigheranimalssurprisesme，seeingthatman’sselectionhasundoubtedlyeffectedmuchwithourdomesticatedquadrupedsandbirds。）tendtogivetothemaleelkitspresentstructure，yetitisprobablethattheinheritedeffectsofuse，andofthemutualactionofpartonpart，havebeenequallyormoreimportant。Asthehornsgraduallyincreasedinweightthemusclesoftheneck，withthebonestowhichtheyareattached，wouldincreaseinsizeandstrength;andthesepartswouldreactonthebodyandlegs。Normustweoverlookthefactthatcertainpartsoftheskullandtheextremitieswould，judgingbyanalogy，tendfromthefirsttovaryinacorrelatedmanner。Theincreasedweightofthehornswouldalsoactdirectlyontheskull，inthesamemanneraswhenoneboneisremovedinthelegofadog，theotherbone，whichhastocarrythewholeweightofthebody，increasesinthickness。Butfromthefactgivenwithrespecttohornedandhornlesscattle，itisprobablethatthehornsandskullwouldimmediatelyactoneachotherthroughtheprincipleofcorrelation。Lastly，thegrowthandsubsequentwearandtearoftheaugmentedmusclesandboneswouldrequireanincreasedsupplyofblood，andconsequentlyincreasedsupplyoffood;andthisagainwouldrequireincreasedpowersofmastication，digestion，respiration，andexcretion。
COLOURASCORRELATEDWITHCONSTITUTIONALPECULIARITIES。
Itisanoldbeliefthatwithmanthereisaconnectionbetweencomplexionsandconstitution;andIfindthatsomeofthebestauthoritiesbelieveinthistothepresentday。（25/38。Dr。ProsperLucasapparentlydisbelievesinanysuchconnection;’L’Hered。Nat。’tome2pages88—94。）ThusDr。Beddoebyhistablesshows（25/39。’BritishMedicalJournal’1862page433。）thatarelationexistsbetweenliabilitytoconsumptionandthecolourofthehair，eyes，andskin。Ithasbeenaffirmed（25/40。Boudin’Geograph。Medicale’tome1page406。）that，intheFrencharmywhichinvadedRussia，soldiershavingadarkcomplexionfromthesouthernpartsofEurope，withstoodtheintensecoldbetterthanthosewithlightercomplexionsfromthenorth;butnodoubtsuchstatementsareliabletoerror。
InthesecondchapteronSelectionIhavegivenseveralcasesprovingthatwithanimalsandplantsdifferencesincolourarecorrelatedwithconstitutionaldifferences，asshownbygreaterorlessimmunityfromcertaindiseases，fromtheattacksofparasiticplantsandanimals，fromscorchingbythesun，andfromtheactionofcertainpoisons。Whenalltheindividualsofanyonevarietypossessanimmunityofthisnature，wedonotknowthatitstandsinanysortofcorrelationwiththeircolour;butwhenseveralsimilarlycolouredvarietiesofthesamespeciesarethuscharacterised，whilstothercolouredvarietiesarenotthusfavoured，wemustbelieveintheexistenceofacorrelationofthiskind。Thus，intheUnitedStatespurple—
fruitedplumsofmanykindsarefarmoreaffectedbyacertaindiseasethangreenoryellow—fruitedvarieties。Ontheotherhand，yellow—fleshedpeachesofvariouskindssufferfromanotherdiseasemuchmorethanthewhite—fleshedvarieties。IntheMauritiusredsugar—canesaremuchlessaffectedbyaparticulardiseasethanthewhitecanes。Whiteonionsandverbenasarethemostliabletomildew;andinSpainthegreen—fruitedgrapessufferedfromthevine—diseasemorethanothercolouredvarieties。Dark—colouredpelargoniumsandverbenasaremorescorchedbythesunthanvarietiesofothercolours。Redwheatsarebelievedtobehardierthanwhite;andred—floweredhyacinthsweremoreinjuredduringoneparticularwinterinHollandthanothercolouredvarieties。Withanimals，whiteterrierssuffermostfromthedistemper，whitechickensfromaparasiticwormintheirtracheae，whitepigsfromscorchingbythesun，andwhitecattlefromflies;butthecaterpillarsofthesilk—mothwhichyieldwhitecocoonssufferedinFrancelessfromthedeadlyparasiticfungusthanthoseproducingyellowsilk。
Thecasesofimmunityfromtheactionofcertainvegetablepoisons，inconnexionwithcolour，aremoreinteresting，andareatpresentwhollyinexplicable。Ihavealreadygivenaremarkableinstance，ontheauthorityofProfessorWyman，ofallthehogs，exceptingthoseofablackcolour，sufferingseverelyinVirginiafromeatingtherootoftheLachnanthestinctoria。
AccordingtoSpinolaandothers（25/41。Thisfactandthefollowingcases，whennotstatedtothecontrary，aretakenfromaverycuriouspaperbyProf。
Heusingerin’WochenschriftfurHeilkunde’May1846s。277。Settegast’DieThierzucht’1868page39saysthatwhiteorwhite—spottedsheepsufferlikepigs，orevendiefromeatingbuckwheat;whilstblackordark—woolledindividualsarenotintheleastaffected。），buckwheat（Po1ygonumfagopyrum），wheninflower，ishighlyinjurioustowhiteorwhite—spottedpigs，iftheyareexposedtotheheatofthesun，butisquiteinnocuoustoblackpigs。
Accordingtotwoaccounts，theHypericumcrispuminSicilyispoisonoustowhitesheepalone;theirheadsswell，theirwoolfallsoff，andtheyoftendie;butthisplant，accordingtoLecce，ispoisonousonlywhenitgrowsinswamps;noristhisimprobable，asweknowhowreadilythepoisonousprincipleinplantsisinfluencedbytheconditionsunderwhichtheygrow。
ThreeaccountshavebeenpublishedinEasternPrussia，ofwhiteandwhite—
spottedhorsesbeinggreatlyinjuredbyeatingmildewedandhoneydewedvetches;everyspotofskinbearingwhitehairsbecominginflamedandgangrenous。TheRev。J。Rodwellinformsmethathisfatherturnedoutaboutfifteencart—horsesintoafieldoftareswhichinpartsswarmedwithblackaphides，andwhichnodoubtwerehoneydewed，andprobablymildewed;thehorses，withtwoexceptions，werechestnutsandbayswithwhitemarksontheirfacesandpasterns，andthewhitepartsaloneswelledandbecameangryscabs。
Thetwobayhorseswithnowhitemarksentirelyescapedallinjury。InGuernsey，whenhorseseatfool’sparsley（Aethusacynapium）theyaresometimesviolentlypurged;andthisplant\"hasapeculiareffectonthenoseandlips，causingdeepcracksandulcers，particularlyonhorseswithwhitemuzzles。\"
（25/42。Mr。Mogfordinthe’Veterinarian’quotedin’TheField’January22，1861page545。）Withcattle，independentlyoftheactionofanypoison，caseshavebeenpublishedbyYouattandErdtofcutaneousdiseaseswithmuchconstitutionaldisturbance（inoneinstanceafterexposuretoahotsun）
affectingeverysinglepointwhichboreawhitehair，butcompletelypassingoverotherpartsofthebody。Similarcaseshavebeenobservedwithhorses。
（25/43。’EdinburghVeterinaryJournal’October1860page347。）
Wethusseethatnotonlydothosepartsoftheskinwhichbearwhitehairdifferinaremarkablemannerfromthosebearinghairofanyothercolour，butthatsomegreatconstitutionaldifferencemustbecorrelatedwiththecolourofthehair;forintheabove—mentionedcases，vegetablepoisonscausedfever，swellingofthehead，aswellasothersymptoms，andevendeath，toallthewhite，orwhite—spottedanimals。
CHAPTER2。XXVI。
LAWSOFVARIATION，continued。——SUMMARY。
THEFUSIONOFHOMOLOGOUSPARTS。
THEVARIABILITYOFMULTIPLEANDHOMOLOGOUSPARTS。
COMPENSATIONOFGROWTH。
MECHANICALPRESSURE。
RELATIVEPOSITIONOFFLOWERSWITHRESPECTTOTHEAXIS，ANDOFSEEDSINTHE
OVARY，ASINDUCINGVARIATION。
ANALOGOUSORPARALLELVARIETIES。
SUMMARYOFTHETHREELASTCHAPTERS。
THEFUSIONOFHOMOLOGOUSPARTS。
GeoffroySaint—Hilaireformerlypropoundedwhathecalledlaloidel’affinitedesoipoursoi，whichhasbeendiscussedandillustratedbyhisson，Isidore，withrespecttomonstersintheanimalkingdom（26/1。’Hist。desAnomalies’
1832tome1pages22，537—556;tome3page462。），andbyMoquin—Tandon，withrespecttomonstrousplants。
Thislawseemstoimplythathomologouspartsactuallyattractoneanotherandthenunite。Nodoubttherearemanywonderfulcases，inwhichsuchpartsbecomeintimatelyfusedtogether。Thisisperhapsbestseeninmonsterswithtwoheads，whichareunited，summittosummit，orfacetoface，orJanus—like，backtoback，orobliquelysidetoside。Inoneinstanceoftwoheadsunitedalmostfacetoface，butalittleobliquely，fourearsweredeveloped，andononesideaperfectface，whichwasmanifestlyformedbythefusionoftwohalf—faces。Whenevertwobodiesortwoheadsareunited，eachbone，muscle，vessel，andnerveonthelineofjunctionappearsasifithadsoughtoutitsfellow，andhadbecomecompletelyfusedwithit。Lereboullet（26/2。’ComptesRendus’1855pages855，1039。），whocarefullystudiedthedevelopmentofdoublemonstersinfishes，observedinfifteeninstancesthestepsbywhichtwoheadsgraduallybecameunitedintoone。Inallsuchcasesitisnowthoughtbythegreaternumberofcapablejudgesthatthehomologouspartsdonotattracteachother，butthatinthewordsofMr。Lowne（26/3。’CatalogueoftheTeratologicalSeriesintheMuseumoftheR。Coll。ofSurgeons’1872
page16。）:\"Asuniontakesplacebeforethedifferentiationofdistinctorgansoccurs，theseareformedincontinuitywitheachother。\"Headdsthatorgansalreadydifferentiatedprobablyinnocasebecomeunitedtohomologousones。
M。Darestedoesnotspeak（26/4。’ArchivesdeZoolog。Exper。’January1874
page78。）quitedecisivelyagainstthelawofsoipoursoi，butconcludesbysaying，\"Onserendparfaitementcomptedelaformationdesmonstres，sil’onadmetquelesembryonsquisesoudentappartiennentaunmemeoeuf;qu’ilss’unissentenmemetempsqu’ilsseforment，etquelasoudureneseproduitquependantlapremiereperiodedelavieembryonnaire，celleoulesorganesnesontencoreconstituesquepardesblastemeshomogenes。\"
Bywhatevermeanstheabnormalfusionofhomologouspartsiseffected，suchcasesthrowlightonthefrequentpresenceoforganswhicharedoubleduringanembryonicperiod（andthroughoutlifeinotherandlowermembersofthesameclass）butwhichafterwardsunitebyanormalprocessintoasinglemedialorgan。InthevegetablekingdomMoquin—Tandon（26/5。’TeratologieVeg。’
1841livre3。）givesalonglistofcases，showinghowfrequentlyhomologousparts，suchasleaves，petals，stamens，andpistils，flowers，andaggregatesofhomologousparts，suchasbuds，aswellasfruit，becomeblended，bothnormallyandabnormally，withperfectsymmetryintooneanother。
THEVARIABILITYOFMULTIPLEANDHOMOLOGOUSPARTS。
IsidoreGeoffroy（26/6。’Hist。desAnomalies’tome3pages4，5，6。）insiststhat，whenanypartororganisrepeatedmanytimesinthesameanimal，itisparticularlyliabletovarybothinnumberandstructure。Withrespecttonumber，thepropositionmay，Ithink，beconsideredasfullyestablished;buttheevidenceischieflyderivedfromorganicbeingslivingundertheirnaturalconditions，withwhichwearenothereconcerned。Wheneversuchpartsasthevertebraeorteeth，theraysinthefinsoffishes，orthefeathersinthetailsofbirds，orpetals，stamens，pistils，orseeds，areverynumerous，thenumberisgenerallyvariable。Withrespecttothestructureofmultipleparts，theevidenceofvariabilityisnotsodecisive;butthefact，asfarasitmaybetrusted，probablydependsonmultiplepartsbeingoflessphysiologicalimportancethansingleparts;consequentlytheirstructurehasbeenlessrigorouslyguardedbynaturalselection。
COMPENSATIONOFGROWTH，ORBALANCEMENT。
Thislaw，asappliedtonaturalspecies，waspropoundedbyGoetheandGeoffroySaint—Hilaireatnearlythesametime。Itimpliesthat，whenmuchorganisedmatterisusedinbuildingupsomeonepart，otherpartsarestarvedandbecomereduced。Severalauthors，especiallybotanists，believeinthislaw;
othersrejectit。AsfarasIcanjudge，itoccasionallyholdsgood;butitsimportancehasprobablybeenexaggerated。Itisscarcelypossibletodistinguishbetweenthesupposedeffectsofsuchcompensation，andtheeffectsoflong—continuedselectionwhichmayleadtotheaugmentationofonepart，andsimultaneouslytothediminutionofanother。Anyhow，therecanbenodoubtthatanorganmaybegreatlyincreasedwithoutanycorrespondingdiminutionofanadjoiningpart。TorecurtoourformerillustrationoftheIrishelk，itmaybeaskedwhatparthassufferedinconsequenceoftheimmensedevelopmentofthehorns？
Ithasalreadybeenobservedthatthestruggleforexistencedoesnotbearhardonourdomesticatedproductions，andconsequentlytheprincipleofeconomyofgrowthwillseldomcomeintoplay，sothatweoughtnottoexpecttofindwiththemfrequentevidenceofcompensation。Wehave，however，somesuchcases。Moquin—Tandondescribesamonstrousbean（26/7。’TeratologieVeg。’
page156。Seealsomybookon’TheMovementsandHabitsofClimbingPlants’
2ndedition1875page202。），inwhichthestipuleswereenormouslydeveloped，andtheleafletsapparentlyinconsequencecompletelyaborted;thiscaseisinteresting，asitrepresentsthenaturalconditionofLathyrusaphaca，withitsstipulesofgreatsize，anditsleavesreducedtomerethreads，whichactastendrils。DeCandolle（26/8。’MemoiresduMuseum’etc。tome8page178。）
hasremarkedthatthevarietiesofRaphanussativuswhichhavesmallrootsyieldnumerousseedcontainingmuchoil，whilstthosewithlargerootsarenotproductiveinoil;andsoitiswithBrassicaasperifolia。ThevarietiesofCucurbitapepowhichbearlargefruityieldasmallcrop，accordingtoNaudin;
whilstthoseproducingsmallfruityieldavastnumber。Lastly，Ihaveendeavouredtoshowintheeighteenthchapterthatwithmanycultivatedplantsunnaturaltreatmentchecksthefullandproperactionofthereproductiveorgans，andtheyarethusrenderedmoreorlesssterile;consequently，inthewayofcompensation，thefruitbecomesgreatlyenlarged，and，indoubleflowers，thepetalsaregreatlyincreasedinnumber。
Withanimals，ithasbeenfounddifficulttoproducecowswhichyieldmuchmilk，andareafterwardscapableoffatteningwell。Withfowlswhichhavelargetop—knotsandbeardsthecombandwattlesaregenerallymuchreducedinsize;thoughthereareexceptionstothisrule。Perhapstheentireabsenceoftheoil—glandinfantailpigeonsmaybeconnectedwiththegreatdevelopmentoftheirtails。
MECHANICALPRESSUREASACAUSEOFMODIFICATIONS。
Insomefewcasesthereisreasontobelievethatmeremechanicalpressurehasaffectedcertainstructures。VrolikandWeber（26/9。Prichard’Phys。Hist。ofMankind’1851volume1page324。）maintainthattheshapeofthehumanheadisinfluencedbytheshapeofthemother’spelvis。Thekidneysindifferentbirdsdiffermuchinform，andSt。Ange（26/10。’AnnalesdesSc。Nat。’1stseriestome19page327。）believesthatthisisdeterminedbytheformofthepelvis，whichagain，nodoubt，standsincloserelationwiththeirpoweroflocomotion。Insnakes，thevisceraarecuriouslydisplaced，incomparisonwiththeirpositioninothervertebrates;andthishasbeenattributedbysomeauthorstotheelongationoftheirbodies;buthere，asinsomanypreviouscases，itisimpossibletodisentangleadirectresultofthiskindfromthatconsequentonnaturalselection。Godronhasargued（26/11。’ComptesRendus’
December1864page1039。）thattheabortionofthespurontheinnersideoftheflowersinCorydalis，iscausedbythebudsataveryearlyperiodofgrowthwhilstundergroundbeingcloselypressedagainstoneanotherandagainstthestem。Somebotanistsbelievethatthesingulardifferenceintheshapebothoftheseedandcorolla，intheinteriorandexteriorfloretsincertainCompositousandUmbelliferousplants，isduetothepressuretowhichtheinnerfloretsaresubjected;butthisconclusionisdoubtful。
Thefactsjustgivendonotrelatetodomesticatedproductions，andthereforedonotstrictlyconcernus。Buthereisamoreappropriatecase:H。Muller（26/12。\"UeberfotaleRachites\"’WurzburgerMedicin。Zeitschrift’1860b。1s。
265。）hasshownthatinshortfacedracesofthedogsomeofthemolarteethareplacedinaslightlydifferentpositiontothatwhichtheyoccupyinotherdogs，especiallyinthosehavingelongatedmuzzles;andasheremarks，anyinheritedchangeinthearrangementoftheteethdeservesnotice，consideringtheirclassificatoryimportance。Thisdifferenceinpositionisduetotheshorteningofcertainfacialbonesandtheconsequentwantofspace;andtheshorteningresultsfromapeculiarandabnormalstateoftheembryonalcartilagesofthebones。
[RELATIVEPOSITIONOFFLOWERSWITHRESPECTTOTHEAXIS，ANDOFSEEDSINTHE
OVARY，ASINDUCINGVARIATION。
Inthethirteenthchaptervariouspeloricflowersweredescribed，andtheirproductionwasshowntobedueeithertoarresteddevelopment，ortoreversiontoaprimordialcondition。Moquin—Tandonhasremarkedthattheflowerswhichstandonthesummitofthemainstemorofalateralbrancharemoreliabletobecomepeloricthanthoseonthesides（26/13。’TeratologieVeg。’page192。）;
andheadduces，amongstotherinstances，thatofTeucriumcampanulatum。InanotherLabiateplantgrownbyme，viz。，theGaleobdolonluteum，thepeloricflowerswerealwaysproducedonthesummitofthestem，whereflowersarenotusuallyborne。InPelargonium，aSINGLEflowerinthetrussisfrequentlypeloric，andwhenthisoccursIhaveduringseveralyearsinvariablyobservedittobethecentralflower。Thisisofsuchfrequentoccurrencethatoneobserver（26/14。’JournalofHorticulture’July2，1861page253。）givesthenamesoftenvarietiesfloweringatthesametime，ineveryoneofwhichthecentralflowerwaspeloric。Occasionallymorethanoneflowerinthetrussispeloric，andthenofcoursetheadditionalonesmustbelateral。Theseflowersareinterestingasshowinghowthewholestructureiscorrelated。InthecommonPelargoniumtheuppersepalisproducedintoanectarywhichcohereswiththeflower—peduncle;thetwoupperpetalsdifferalittleinshapefromthethreelowerones，andaremarkedwithdarkshadesofcolour;thestamensaregraduatedinlengthandupturned。Inthepeloricflowers，thenectaryaborts;allthepetalsbecomealikebothinshapeandcolour;thestamensaregenerallyreducedinnumberandbecomestraight，sothatthewholeflowerresemblesthatofthealliedgenusErodium。Thecorrelationbetweenthesechangesiswellshownwhenoneofthetwoupperpetalsalonelosesitsdarkmark，forinthiscasethenectarydoesnotentirelyabort，butisusuallymuchreducedinlength。（26/15。Itwouldbeworthtrialtofertilisewiththesamepollenthecentralandlateralflowersofthepelargonium，orofotherhighlycultivatedplants，protectingthemofcoursefrominsects:thentosowtheseedseparately，andobservewhethertheoneortheotherlotofseedlingsvariedthemost。）
Morrenhasdescribed（26/16。Quotedin’JournalofHorticulture’February24，1863page152。）amarvellousflask—shapedfloweroftheCalceolaria，nearlyfourinchesinlength，whichwasalmostcompletelypeloric;itgrewonthesummitoftheplant，withanormalfloweroneachside;Prof。Westwoodalsohasdescribed（26/17。’Gardener’sChronicle’1866page612。ForthePhalaenopsisseeibid1867page211。）threesimilarpeloricflowers，whichalloccupiedacentralpositionontheflower—branches。IntheOrchideousgenus，Phalaenopsis，theterminalflowerhasbeenseentobecomepeloric。