IcarefullyexaminedtheflowersofninetyplantsofthecrossedAntirrhinuminthetwobeds，andtheirstructurehadnotbeenintheleastaffectedbythecross，exceptthatinafewinstancestheminuterudimentofthefifthstamen，whichisalwayspresent，wasmorefullyorevencompletelydeveloped。Itmustnotbesupposedthatthisentireobliterationofthepeloricstructureinthecrossedplantscanbeaccountedforbyanyincapacityoftransmission;forI
raisedalargebedofplantsfromthepeloricAntirrhinum，artificiallyfertilisedbyitsownpollen，andsixteenplants，whichalonesurvivedthewinter，wereallasperfectlypeloricastheparent—plant。Herewehaveagoodinstanceofthewidedifferencebetweentheinheritanceofacharacterandthepoweroftransmittingittocrossedoffspring。Thecrossedplants，whichperfectlyresembledthecommonsnapdragon，wereallowedtosowthemselves，andoutofahundredandtwenty—sevenseedlings，eighty—eightprovedtobecommonsnapdragons，twowereinanintermediateconditionbetweenthepeloricandnormalstate，andthirty—sevenwereperfectlypeloric，havingrevertedtothestructureoftheironegrand—parent。Thiscaseseemsatfirstsighttoofferanexceptiontotherulejustgiven，namely，thatacharacterwhichispresentinoneformandlatentintheotherisgenerallytransmittedwithprepotentforcewhenthetwoformsarecrossed。ForinalltheScrophulariaceae，andespeciallyinthegeneraAntirrhinumandLinaria，thereis，aswasshowninthelastchapter，astronglatenttendencytobecomepeloric;butthereisalso，aswehaveseen，astillstrongertendencyinallpeloricplantstoreacquiretheirnormalirregularstructure。Sothatwehavetwoopposedlatenttendenciesinthesameplants。Now，withthecrossedAntirrhinumsthetendencytoproducenormalorirregularflowers，likethoseofthecommonSnapdragon，prevailedinthefirstgeneration;whilstthetendencytopelorism，appearingtogainstrengthbytheintermissionofageneration，prevailedtoalargeextentinthesecondsetofseedlings。Howitispossibleforacharactertogainstrengthbytheintermissionofageneration，willbeconsideredinthechapteronpangenesis。
Onthewhole，thesubjectofprepotencyisextremelyintricate，——fromitsvaryingsomuchinstrength，eveninregardtothesamecharacter，indifferentanimals，——fromitsrunningeitherequallyinbothsexes，or，asfrequentlyisthecasewithanimals，butnotwithplants，muchstrongerinonesexthantheother，——fromtheexistenceofsecondarysexualcharacters，——fromthetransmissionofcertaincharactersbeinglimited，asweshallimmediatelysee，bysex，——fromcertaincharactersnotblendingtogether，——and，perhaps，occasionallyfromtheeffectsofapreviousfertilisationonthemother。Itisthereforenotsurprisingthatnoonehashithertosucceededindrawingupgeneralrulesonthesubjectofprepotency。
INHERITANCEASLIMITEDBYSEX。
Newcharactersoftenappearinonesex，andareafterwardstransmittedtothesamesex，eitherexclusivelyorinamuchgreaterdegreethantotheother。
Thissubjectisimportant，becausewithanimalsofmanykindsinastateofnature，bothhighandlowinthescale，secondarysexualcharacters，notdirectlyconnectedwiththeorgansofreproduction，areconspicuouslypresent。
Withourdomesticatedanimals，charactersofthiskindoftendifferwidelyfromthosedistinguishingthetwosexesoftheparentspecies;andtheprincipleofinheritance，aslimitedbysex，explainshowthisispossible。
[Dr。P。Lucashasshown（14/25。’L’Hered。Nat。’tome2pages137—165。SeealsoMr。Sedgwick’sfourmemoirs，immediatelytobereferredto。）thatwhenapeculiarity，innomannerconnectedwiththereproductiveorgans，appearsineitherparent，itisoftentransmittedexclusivelytotheoffspringofthesamesex，ortoamuchgreaternumberofthemthanoftheoppositesex。Thus，inthefamilyofLambert，thehorn—likeprojectionsontheskinweretransmittedfromthefathertohissonsandgrandsonsalone;soithasbeenwithothercasesofichthyosis，withsupernumerarydigits，withadeficiencyofdigitsandphalanges，andinalesserdegreewithvariousdiseases，especiallywithcolour—blindnessandthehaemorrhagicdiathesis，thatis，anextremeliabilitytoprofuseanduncontrollablebleedingfromtriflingwounds。
Ontheotherhand，mothershavetransmitted，duringseveralgenerations，totheirdaughtersalone，supernumeraryanddeficientdigits，colour—blindnessandotherpeculiarities。Sothattheverysamepeculiaritymaybecomeattachedtoeithersex，andbelonginheritedbythatsexalone;buttheattachmentincertaincasesismuchmorefrequenttoonethantheothersex。Thesamepeculiaritiesalsomaybepromiscuouslytransmittedtoeithersex。Dr。Lucasgivesothercases，showingthatthemaleoccasionallytransmitshispeculiaritiestohisdaughtersalone，andthemothertohersonsalone;buteveninthiscaseweseethatinheritanceistoacertainextent，thoughinversely，regulatedbysex。Dr。Lucas，afterweighingthewholeevidence，comestotheconclusionthateverypeculiaritytendstobetransmittedinagreaterorlesserdegreetothatsexinwhichitfirstappears。Butamoredefiniterule，asIhaveelsewhereshown（14/26。’DescentofMan’2ndeditionpage32。）generallyholdsgood，namely，thatvariationswhichfirstappearineithersexatalateperiodoflife，whenthereproductivefunctionsareactive，tendtobedevelopedinthatsexalone;whilstvariationswhichfirstappearearlyinlifeineithersexarecommonlytransmittedtobothsexes。I
am，however，farfromsupposingthatthisisthesoledeterminingcause。
AfewdetailsfromthemanycasescollectedbyMr。Sedgwick（14/27。OnSexualLimitationinHereditaryDiseases’Brit。andFor。Med。—Chirurg。Review’April1861page477;Julypage198;April1863page445;andJulypage159。Alsoin1867’OntheinfluenceofAgeinHereditaryDisease。’），maybeheregiven。
Colour—blindness，fromsomeunknowncause，showsitselfmuchoftenerinmalesthaninfemales;inupwardsoftwohundredcasescollectedbyMr。Sedgwick，nine—tenthsrelatedtomen;butitiseminentlyliabletobetransmittedthroughwomen。InthecasegivenbyDr。Earle，membersofeightrelatedfamilieswereaffectedduringfivegenerations:thesefamiliesconsistedofsixty—oneindividuals，namely，ofthirty—twomales，ofwhomnine—sixteenthswereincapableofdistinguishingcolour，andoftwenty—ninefemales，ofwhomonlyone—fifteenthwerethusaffected。Althoughcolour—blindnessthusgenerallyclingstothemalesex，nevertheless，inoneinstanceinwhichitfirstappearedinafemale，itwastransmittedduringfivegenerationstothirteenindividuals，allofwhomwerefemales。Thehaemorrhagicdiathesis，oftenaccompaniedbyrheumatism，hasbeenknowntoaffectthemalesaloneduringfivegenerations，beingtransmitted，however，throughthefemales。Itissaidthatdeficientphalangesinthefingershavebeeninheritedbythefemalesaloneduringtengenerations。Inanothercase，amanthusdeficientinbothhandsandfeet，transmittedthepeculiaritytohistwosonsandonedaughter;butinthethirdgeneration，——outofnineteengrandchildren，twelvesonshadthefamilydefect，whilstthesevendaughterswerefree。Inordinarycasesofsexuallimitation，thesonsordaughtersinheritthepeculiarity，whateveritmaybe，fromtheirfatherormother，andtransmitittotheirchildrenofthesamesex;butgenerallywiththehaemorrhagicdiathesis，andoftenwithcolour—blindness，andinsomeothercases，thesonsneverinheritthepeculiaritydirectlyfromtheirfathers，butthedaughtersalonetransmitthelatenttendency，sothatthesonsofthedaughtersaloneexhibitit。Thusthefather，grandson，andgreat—great—grandsonwillexhibitapeculiarity，——
thegrandmother，daughter，andgreat—grand—daughterhavingtransmitteditinalatentstate。Hencewehave，asMr。Sedgwickremarks，adoublekindofatavismorreversion;eachgrandsonapparentlyreceivinganddevelopingthepeculiarityfromhisgrandfather，andeachdaughterapparentlyreceivingthelatenttendencyfromhergrandmother。
FromthevariousfactsrecordedbyDr。ProsperLucas，Mr。Sedgwick，andothers，therecanbenodoubtthatpeculiaritiesfirstappearingineithersex，thoughnotinanywaynecessarilyorinvariablyconnectedwiththatsex，stronglytendtobeinheritedbytheoffspringofthesamesex，butareoftentransmittedinalatentstatethroughtheoppositesex。
Turningnowtodomesticatedanimals，wefindthatcertaincharactersnotpropertotheparentspeciesareoftenconfinedto，andinheritedby，onesexalone;butwedonotknowthehistoryofthefirstappearanceofsuchcharacters。InthechapteronSheep，wehaveseenthatthemalesofcertainracesdiffergreatlyfromthefemalesintheshapeoftheirhorns，thesebeingabsentintheewesofsomebreeds;theydifferalsointhedevelopmentoffatinthetailandintheoutlineoftheforehead。Thesedifferences，judgingfromthecharacterofthealliedwildspecies，cannotbeaccountedforbysupposingthattheyhavebeenderivedfromdistinctparentforms。Thereis，also，agreatdifferencebetweenthehornsofthetwosexesinoneIndianbreedofgoats。Thebullzebuissaidtohavealargerhumpthanthecow。IntheScotchdeer—houndthetwosexesdifferinsizemorethaninanyothervarietyofthedog（14/28。W。Scrope’ArtofDeerStalking’page354。）and，judgingfromanalogy，morethanintheaboriginalparent—species。Thepeculiarcolourcalledtortoise—shellisveryrarelyseeninamalecat;themalesofthisvarietybeingofarustytint。
Invariousbreedsofthefowlthemalesandfemalesoftendiffergreatly;andthesedifferencesarefarfrombeingthesamewiththosewhichdistinguishthetwosexesoftheparent—species，theGallusbankiva;andconsequentlyhaveoriginatedunderdomestication。Incertainsub—varietiesoftheGameracewehavetheunusualcaseofthehensdifferingfromeachothermorethanthecocks。InanIndianbreedofawhitecolourshadedwithblack，thehensinvariablyhaveblackskins，andtheirbonesarecoveredbyablackperiosteum，whilstthecocksareneverormostrarelythuscharacterised。
Pigeonsofferamoreinterestingcase;forthroughoutthewholegreatfamilythetwosexesdonotoftendiffermuch;andthemalesandfemalesoftheparent—form，theC。livia，areundistinguishable:yetwehaveseenthatwithpoutersthemalehasthecharacteristicqualityofpoutingmorestronglydevelopedthanthefemale;andincertainsub—varietiesthemalesalonearespottedorstriatedwithblack，orotherwisedifferincolour。WhenmaleandfemaleEnglishcarrier—pigeonsareexhibitedinseparatepens，thedifferenceinthedevelopmentofthewattleoverthebeakandroundtheeyesisconspicuous。Sothatherewehaveinstancesoftheappearanceofsecondarysexualcharactersinthedomesticatedracesofaspeciesinwhichsuchdifferencesarenaturallyquiteabsent。]
Ontheotherhand，secondarysexualcharacterswhichbelongtothespeciesinastateofnaturearesometimesquitelost，orgreatlydiminished，underdomestication。Weseethisinthesmallsizeofthetusksinourimprovedbreedsofthepig，incomparisonwiththoseofthewildboar。Therearesub—
breedsoffowls，inwhichthemaleshavelostthefine—flowingtail—feathersandhackles;andothersinwhichthereisnodifferenceincolourbetweenthetwosexes。Insomecasesthebarredplumage，whichingallinaceousbirdsiscommonlytheattributeofthehen，hasbeentransferredtothecock，asinthecuckoosub—breeds。Inothercasesmasculinecharactershavebeenpartlytransferredtothefemale，aswiththesplendidplumageofthegolden—spangledHamburghhen，theenlargedcomboftheSpanishhen，thepugnaciousdispositionoftheGamehen，andasinthewell—developedspurswhichoccasionallyappearinthehensofvariousbreeds。InPolishfowlsbothsexesareornamentedwithatopknot，thatofthemalebeingformedofhackle—likefeathers，andthisisanewmalecharacterinthegenusGallus。Onthewhole，asfarasIcanjudge，newcharactersaremoreapttoappearinthemalesofourdomesticatedanimalsthaninthefemales（14/29。Ihavegiveninmy’DescentofMan’2ndeditionpage223sufficientevidencethatmaleanimalsareusuallymorevariablethanthefemales。），andafterwardstobeinheritedexclusivelyormorestronglybythemales。Finally，inaccordancewiththeprincipleofinheritanceaslimitedbysex，thepreservationandaugmentationofsecondarysexualcharactersinnaturalspeciesoffersnoespecialdifficulty，asthiswouldfollowthroughthatformofselectionwhichIhavecalledsexualselection。
INHERITANCEATCORRESPONDINGPERIODSOFLIFE。
Thisisanimportantsubject。Sincethepublicationofmy’OriginofSpecies’
Ihaveseennoreasontodoubtthetruthoftheexplanationtheregivenofoneofthemostremarkablefactsinbiology，namely，thedifferencebetweentheembryoandtheadultanimal。Theexplanationis，thatvariationsdonotnecessarilyorgenerallyoccurataveryearlyperiodofembryonicgrowth，andthatsuchvariationsareinheritedatacorrespondingage。Asaconsequenceofthistheembryo，evenaftertheparent—formhasundergonegreatmodification，isleftonlyslightlymodified;andtheembryosofwidely—differentanimalswhicharedescendedfromacommonprogenitorremaininmanyimportantrespectslikeoneanotherandprobablyliketheircommonprogenitor。Wecanthusunderstandwhyembryologythrowsafloodoflightonthenaturalsystemofclassification，asthisoughttobeasfaraspossiblegenealogical。Whentheembryoleadsanindependentlife，thatis，becomesalarva，ithastobeadaptedtothesurroundingconditionsinitsstructureandinstincts，independentlyofthoseofitsparents;andtheprincipleofinheritanceatcorrespondingperiodsofliferendersthispossible。
Thisprincipleis，indeed，inonewaysoobviousthatitescapesattention。Wepossessanumberofracesofanimalsandplants，which，whencomparedwithoneanotherandwiththeirparent—forms，presentconspicuousdifferences，bothintheirimmatureandmaturestates。Lookattheseedsoftheseveralkindsofpeas，beans，maize，whichcanbepropagatedtruly，andseehowtheydifferinsize，colour，andshape，whilstthefull—grownplantsdifferbutlittle。
Cabbages，ontheotherhand，differgreatlyinfoliageandmannerofgrowth，buthardlyatallintheirseeds;andgenerallyitwillbefoundthatthedifferencesbetweencultivatedplantsatdifferentperiodsofgrowtharenotnecessarilycloselyconnectedtogether，forplantsmaydiffermuchintheirseedsandlittlewhenfull—grown，andconverselymayyieldseedshardlydistinguishable，yetdiffermuchwhenfull—grown。Intheseveralbreedsofpoultry，descendedfromasinglespecies，differencesintheeggsandchickenswhilstcoveredwithdown，intheplumageatthefirstandsubsequentmoults，aswellasinthecombandwattles，areallinherited。Withmanpeculiaritiesinthemilkandsecondteeth（ofwhichIhavereceivedthedetails）areinheritable，andlongevityisoftentransmitted。Soagainwithourimprovedbreedsofcattleandsheep，earlymaturity，includingtheearlydevelopmentoftheteeth，andwithcertainbreedsoffowltheearlyappearanceofsecondarysexualcharacters，allcomeunderthesameheadofinheritanceatcorrespondingperiods。
Numerousanalogousfactscouldbegiven。Thesilk—moth，perhaps，offersthebestinstance;forinthebreedswhichtransmittheircharacterstruly，theeggsdifferinsize，colour，andshape:thecaterpillarsdiffer，inmoultingthreeorfourtimes，incolour，eveninhavingadark—colouredmarklikeaneyebrow，andinthelossofcertaininstincts;——thecocoonsdifferinsize，shape，andinthecolourandqualityofthesilk;theseseveraldifferencesbeingfollowedbyslightorbarelydistinguishabledifferencesinthematuremoth。
Butitmaybesaidthat，ifintheabovecasesanewpeculiarityisinherited，itmustbeatthecorrespondingstageofdevelopment;foraneggorseedcanresembleonlyaneggorseed，andthehorninafull—grownoxcanresembleonlyahorn。Thefollowingcasesshowinheritanceatcorrespondingperiodsmoreplainly，becausetheyrefertopeculiaritieswhichmighthavesupervened，asfaraswe...完整阅读请扫描二维码下载丁香书院APP免费看：