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，andassumethefor...完整阅读请扫描二维码下载丁香书院APP免费看：