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，asfaraswecansee，earlierorlaterinlife，yetareinheritedatthesameperiodatwhichtheyfirstappeared。
[IntheLambertfamilytheporcupine—likeexcrescencesappearedinthefatherandsonsatthesameage，namely，aboutnineweeksafterbirth。（14/30。
Prichard’Phys。Hist。ofMankind’1851volume1page349。）IntheextraordinaryhairyfamilydescribedbyMr。Crawfurd（14/31。’EmbassytotheCourtofAva’volume1page320。ThethirdgenerationisdescribedbyCapt。
Yuleinhis’NarrativeoftheMissiontotheCourtofAva’1855page94。），childrenwereproducedduringthreegenerationswithhairyears;inthefatherthehairbegantogrowoverhisbodyatsixyearsold;inhisdaughtersomewhatearlier，namely，atoneyear;andinbothgenerationsthemilkteethappearedlateinlife，thepermanentteethbeingafterwardssingularlydeficient。Greynessofhairatanunusuallyearlyagehasbeentransmittedinsomefamilies。Thesecasesborderondiseasesinheritedatcorrespondingperiodsoflife，towhichIshallimmediatelyrefer。
Itisawell—knownpeculiaritywithalmond—tumblerpigeons，thatthefullbeautyandpeculiarcharacteroftheplumagedoesnotappearuntilthebirdhasmoultedtwoorthreetimes。Neumeisterdescribesandfiguresabraceofpigeonsinwhichthewholebodyiswhiteexceptthebreast，neck，andhead;
butintheirfirstplumageallthewhitefeathershavecolourededges。Anotherbreedismoreremarkable:itsfirstplumageisblack，withrusty—redwing—barsandacrescent—shapedmarkonthebreast;thesemarksthenbecomewhite，andremainsoduringthreeorfourmoults;butafterthisperiodthewhitespreadsoverthebody，andthebirdlosesitsbeauty。（14/32。’DasGanzederTaubenzucht’1837s。24tab。4figure2s。21tab。1figure4。）Prizecanary—
birdshavetheirwingsandtailblack:\"thiscolour，however，isonlyretaineduntilthefirstmoult，sothattheymustbeexhibitederethechangetakesplace。Oncemoulted，thepeculiarityhasceased。Ofcourseallthebirdsemanatingfromthisstockhaveblackwingsandtailsthefirstyear。\"（14/33。
Kidd’TreatiseontheCanary’page18。）Acuriousandsomewhatanalogousaccounthasbeengiven（14/34。Charlesworth’Mag。ofNat。Hist。’volume11837
page167。）ofafamilyofwildpiedrookswhichwerefirstobservedin1798，nearChalfont，andwhicheveryyearfromthatdateuptotheperiodofthepublishednotice，viz。，1837\"haveseveraloftheirbroodparticoloured，blackandwhite。Thisvariegationoftheplumage，however，disappearswiththefirstmoult;butamongthenextyoungfamiliestherearealwaysafewpiedones。\"
Thesechangesofplumage，whichareinheritedatvariouscorrespondingperiodsoflifeinthepigeon，canary—bird，androok，areremarkable，becausetheparent—speciespassesthroughnosuchchange。
Inheriteddiseasesaffordevidenceinsomerespectsoflessvaluethantheforegoingcases，becausediseasesarenotnecessarilyconnectedwithanychangeinstructure;butinotherrespectsofmorevalue，becausetheperiodshavebeenmorecarefullyobserved。Certaindiseasesarecommunicatedtothechildapparentlybyaprocesslikeinoculation，andthechildisfromthefirstaffected;suchcasesmaybeherepassedover。Largeclassesofdiseasesusuallyappearatcertainages，suchasSt。Vitus’sdanceinyouth，consumptioninearlymid—life，goutlater，andapoplexystilllater;andthesearenaturallyinheritedatthesameperiod。Butevenindiseasesofthisclass，instanceshavebeenrecorded，aswithSt。Vitus’sdance，showingthatanunusuallyearlyorlatetendencytothediseaseisinheritable。（14/35。Dr。
ProsperLucas’Hered。Nat。’tome2page713。）Inmostcasestheappearanceofanyinheriteddiseaseislargelydeterminedbycertaincriticalperiodsineachperson’slife，aswellasbyunfavourableconditions。Therearemanyotherdiseases，whicharenotattachedtoanyparticularperiod，butwhichcertainlytendtoappearinthechildataboutthesameageatwhichtheparentwasfirstattacked。Anarrayofhighauthorities，ancientandmodern，couldbegiveninsupportofthisproposition。TheillustriousHunterbelievedinit;andPiorry（14/36。’L’Hered。danslesMaladies’1840page135。ForHunterseeHarlan’Med。Researches’page530。）cautionsthephysiciantolookcloselytothechildattheperiodwhenanygraveinheritablediseaseattackedtheparent。Dr。ProsperLucas（14/37。’L’Hered。Nat。’tome2page850。），aftercollectingfactsfromeverysource，assertsthataffectionsofallkinds，thoughnotrelatedtoanyparticularperiodoflife，tendtoreappearintheoffspringatwhateverperiodoflifetheyfirstappearedintheprogenitor。
Asthesubjectisimportant，itmaybewelltogiveafewinstances，simplyasillustrations，notasproof;forproof，recoursemustbehadtotheauthoritiesabovequoted。Someofthefollowingcaseshavebeenselectedforthesakeofshowingthat，whenaslightdeparturefromtheruleoccurs，thechildisaffectedsomewhatearlierinlifethantheparent。InthefamilyofLeCompteblindnesswasinheritedthroughthreegenerations，andnolessthantwenty—sevenchildrenandgrandchildrenwereallaffectedataboutthesameage;theirblindnessingeneralbegantoadvanceaboutthefifteenthorsixteenthyear，andendedintotaldeprivationofsightattheageofabouttwenty—two。（14/38。Sedgwick’Brit。andFor。Med。—Chirurg。Review’April1861
page485。Insomeaccountsthenumberofchildrenandgrandchildrenisgivenas37;butthisseemstobeanerrorjudgingfromthepaperfirstpublishedinthe’BaltimoreMed。andPhys。Reg。’1809ofwhichMr。Sedgwickhasbeensokindastosendmeacopy。）Inanothercaseafatherandhisfourchildrenallbecameblindattwenty—oneyearsold;inanother，agrandmothergrewblindatthirty—five，herdaughteratnineteen，andthreegrandchildrenattheagesofthirteenandeleven。（14/39。ProsperLucas’Hered。Nat。’tome1page400。）Sowithdeafness，twobrothers，theirfatherandpaternalgrandfather，allbecamedeafattheageofforty。（14/40。SedgwickibidJuly1861page202。）
Esquirolgivesseveralstrikinginstancesofinsanitycomingonatthesameage，asthatofagrandfather，father，andson，whoallcommittedsuicideneartheirfiftiethyear。Manyothercasescouldbegiven，asofawholefamilywhobecameinsaneattheageofforty。（14/41。Piorrypage109;ProsperLucastome2page759。）Othercerebralaffectionssometimesfollowthesamerule，——forinstance，epilepsyandapoplexy。Awomandiedofthelatterdiseasewhensixty—threeyearsold;oneofherdaughtersatforty—three，andtheotheratsixty—seven:thelatterhadtwelvechildren，whoalldiedfromtubercularmeningitis。（14/42。ProsperLucastome2page748。）Imentionthislattercasebecauseitillustratesafrequentoccurrence，namely，achangeintheprecisenatureofaninheriteddisease，thoughstillaffectingthesameorgan。
Asthmahasattackedseveralmembersofthesamefamilywhenfortyyearsold，andotherfamiliesduringinfancy。Themostdifferentdiseases，suchasanginapectoris，stoneinthebladder，andvariousaffectionsoftheskin，haveappearedinsuccessivegenerationsatnearlythesameage。Thelittlefingerofamanbeganfromsomeunknowncausetogrowinwards，andthesamefingerinhistwosonsbeganatthesameagetobendinwardsinasimilarmanner。
Strangeandinexplicableneuralgicaffectionshavecausedparentsandchildrentosufferagoniesataboutthesameperiodoflife。（14/43。ProsperLucastome3pages678，700，702;SedgwickibidApril1863page449andJuly1863page162。Dr。J。Steinan’EssayonHereditaryDisease’1843pages27，34。）
Iwillgiveonlytwoothercases，whichareinterestingasillustratingthedisappearanceaswellastheappearanceofdiseaseatthesameage。Twobrothers，theirfather，theirpaternaluncles，sevencousins，andtheirpaternalgrandfather，wereallsimilarlyaffectedbyaskin—disease，calledpityriasisversicolor;\"thedisease，strictlylimitedtothemalesofthefamily（thoughtransmittedthroughthefemales），usuallyappearedatpuberty，anddisappearedatabouttheageoffortyorforty—fiveyears。\"Thesecondcaseisthatoffourbrothers，whowhenabouttwelveyearsoldsufferedalmosteveryweekfromsevereheadaches，whichwererelievedonlybyarecumbentpositioninadarkroom。Theirfather，paternaluncles，paternalgrandfather，andgrandunclesallsufferedinthesamewayfromheadaches，whichceasedattheageoffifty—fourorfifty—fiveinallthosewholivedsolong。Noneofthefemalesofthefamilywereaffected。（14/44。ThesecasesaregivenbyMr。
SedgwickontheauthorityofDr。H。Stewartin’Med。—Chirurg。Review’April1863pages449，477。）]
Itisimpossibletoreadtheforegoingaccounts，andthemanyotherswhichhavebeenrecorded，ofdiseasescomingonduringthreeorevenmoregenerationsinseveralmembersofthesamefamilyatthesameage，especiallyinthecaseofrareaffectionsinwhichthecoincidencecannotbeattributedtochance，andtodoubtthatthereisastrongtendencytoinheritanceindiseaseatcorrespondingperiodsoflife。Whentherulefails，thediseaseisapttocomeonearlierinthechildthanintheparent;theexceptionsintheotherdirectionbeingverymuchrarer。Dr。Lucas（14/45。’Hered。Nat。’tome2
page852。）alludestoseveralcasesofinheriteddiseasescomingonatanearlierperiod。Ihavealreadygivenonestrikinginstancewithblindnessduringthreegenerations;andMr。Bowmanremarksthatthisfrequentlyoccurswithcataract。Withcancerthereseemstobeapeculiarliabilitytoearlierinheritance:SirJ。Paget，whohasparticularlyattendedtothissubject，andtabulatedalargenumberofcases，informsmethathebelievesthatinninecasesoutoftenthelatergenerationsuffersfromthediseaseatanearlierperiodthanthepreviousgeneration。Headds，\"Intheinstancesinwhichtheoppositerelationholds，andthemembersoflatergenerationshavecanceratalateragethantheirpredecessors，Ithinkitwillbefoundthatthenon—
cancerousparentshavelivedtoextremeoldages。\"Sothatthelongevityofanon—affectedparentseemstohavethepowerofinfluencingthefatalperiodintheoffspring;andwethusapparentlygetanotherelementofcomplexityininheritance。
Thefacts，showingthatwithcertaindiseasestheperiodofinheritanceoccasionallyorevenfrequentlyadvances，areimportantwithrespecttothegeneraldescent—theory，fortheyrenderitprobablethatthesamethingwouldoccurwithordinarymodificationsofstructure。Thefinalresultofalongseriesofsuchadvanceswouldbethegradualobliterationofcharacterspropertotheembryoandlarva，whichwouldthuscometoresemblemoreandmorecloselythematureparent—form。Butanystructurewhichwasofservicetotheembryoorlarvawouldbepreservedbythedestructionatthisstageofgrowthofeachindividualwhichmanifestedanytendencytoloseitspropercharacterattooearlyanage。
Finally，fromthenumerousracesofcultivatedplantsanddomesticanimals，inwhichtheseedsoreggs，theyoungorold，differfromoneanotherandfromthoseoftheparent—species;——fromthecasesinwhichnewcharactershaveappearedataparticularperiod，andafterwardsbeeninheritedatthesameperiod;——andfromwhatweknowwithrespecttodisease，wemustbelieveinthetruthofthegreatprincipleofinheritanceatcorrespondingperiodsoflife。
SUMMARYOFTHETHREEPRECEDINGCHAPTERS。
Strongasistheforceofinheritance，itallowstheincessantappearanceofnewcharacters。These，whetherbeneficialorinjurious，——ofthemosttriflingimportance，suchasashadeofcolourinaflower，acolouredlockofhair，orameregesture，——orofthehighestimportance，aswhenaffectingthebrain，oranorgansoperfectandcomplexastheeye，——orofsograveanatureastodeservetobecalledamonstrosity，——orsopeculiarasnottooccurnormallyinanymemberofthesamenaturalclass，——areofteninheritedbyman，bytheloweranimals，andplants。Innumberlesscasesitsufficesfortheinheritanceofapeculiaritythatoneparentaloneshouldbethuscharacterised。
Inequalitiesinthetwosidesofthebody，thoughopposedtothelawofsymmetry，maybetransmitted。Thereisampleevidencethattheeffectsofmutilationsandofaccidents，especiallyorperhapsexclusivelywhenfollowedbydisease，areoccasionallyinherited。Therecanbenodoubtthattheevileffectsofthelong—continuedexposureoftheparenttoinjuriousconditionsaresometimestransmittedtotheoffspring。Soitis，asweshallseeinafuturechapter，withtheeffectsoftheuseanddisuseofparts，andofmentalhabits。Periodicalhabitsarelikewisetransmitted，butgenerally，asitwouldappear，withlittleforce。
Henceweareledtolookatinheritanceastherule，andnon—inheritanceastheanomaly。Butthispoweroftenappearstousinourignorancetoactcapriciously，transmittingacharacterwithinexplicablestrengthorfeebleness。Theverysamepeculiarity，astheweepinghabitoftrees，silkyfeathers，etc。，maybeinheritedeitherfirmlyornotatallbydifferentmembersofthesamegroup，andevenbydifferentindividualsofthesamespecies，thoughtreatedinthesamemanner。Inthislattercaseweseethatthepoweroftransmissionisaqualitywhichismerelyindividualinitsattachment。Aswithsinglecharacters，soitiswiththeseveralconcurrentslightdifferenceswhichdistinguishsub—varietiesorraces;forofthese，somecanbepropagatedalmostastrulyasspecies，whilstotherscannotbereliedon。Thesameruleholdsgoodwithplants，whenpropagatedbybulbs，offsets，etc。，whichinonesensestillformpartsofthesameindividual，forsomevarietiesretainorinheritthroughsuccessivebud—generationstheircharacterfarmoretrulythanothers。
Somecharactersnotpropertotheparent—specieshavecertainlybeeninheritedfromanextremelyremoteepoch，andmaythereforebeconsideredasfirmlyfixed。Butitisdoubtfulwhetherlengthofinheritanceinitselfgivesfixednessofcharacter;thoughthechancesareobviouslyinfavourofanycharacterwhichhaslongbeentransmittedtrueorunalteredstillbeingtransmittedtrueaslongastheconditionsofliferemainthesame。Weknowthatmanyspecies，afterhavingretainedthesamecharacterforcountlessages，whilstlivingundertheirnaturalconditions，whendomesticatedhavevariedinthemostdiversifiedmanner，thatis，havefailedtotransmittheiroriginalform;sothatnocharacterappearstobeabsolutelyfixed。Wecansometimesaccountforthefailureofinheritancebytheconditionsoflifebeingopposedtothedevelopmentofcertaincharacters;andstilloftener，aswithplantscultivatedbygraftsandbuds，bytheconditionscausingnewandslightmodificationsincessantlytoappear。Inthislattercaseitisnotthatinheritancewhollyfails，butthatnewcharactersarecontinuallysuperadded。
Insomefewcases，inwhichbothparentsaresimilarlycharacterised，inheritanceseemstogainsomuchforcebythecombinedactionofthetwoparents，thatitcounteractsitsownpower，andanewmodificationistheresult。
Inmanycasesthefailureoftheparentstotransmittheirlikenessisduetothebreedhavingbeenatsomeformerperiodcrossed;andthechildtakesafterhisgrandparentormoreremoteancestorofforeignblood。Inothercases，inwhichthebreedhasnotbeencrossed，butsomeancientcharacterhasbeenlostthroughvariation，itoccasionallyreappearsthroughreversion，sothattheparentsapparentlyfailtotransmittheirownlikeness。Inallcases，however，wemaysafelyconcludethatthechildinheritsallitscharactersfromitsparents，inwhomcertaincharactersarelatent，likethesecondarysexualcharactersofonesexintheother。When，afteralongsuccessionofbud—
generations，aflowerorfruitbecomesseparatedintodistinctsegments，havingthecoloursorotherattributesofbothparent—forms，wecannotdoubtthatthesecharacterswerelatentintheearlierbuds，thoughtheycouldnotthenbedetected，orcouldbedetectedonlyinanintimatelycommingledstate。
Soitiswithanimalsofcrossedparentage，whichwithadvancingyearsoccasionallyexhibitcharactersderivedfromoneoftheirtwoparents，ofwhichnotatracecouldatfirstbeperceived。Certainmonstrosities，whichresemblewhatnaturalistscallthetypicalformofthegroupinquestion，apparentlycomeunderthesamelawofreversion。Itisassuredlyanastonishingfactthatthemaleandfemalesexualelements，thatbuds，andevenfull—grownanimals，shouldretaincharacters，duringseveralgenerationsinthecaseofcrossedbreeds，andduringthousandsofgenerationsinthecaseofpurebreeds，writtenasitwereininvisibleink，yetreadyatanytimetobeevolvedundercertainconditions。
Whattheseconditionspreciselyare，wedonotknow。Butanycausewhichdisturbstheorganisationorconstitutionseemstobesufficient。Acrosscertainlygivesastrongtendencytothereappearanceoflong—lostcharacters，bothcorporealandmental。Inthecaseofplants，thistendencyismuchstrongerwiththosespecieswhichhavebeencrossedafterlongcultivationandwhichthereforehavehadtheirconstitutionsdisturbedbythiscauseaswellasbycrossing，thanwithspecieswhichhavealwayslivedundertheirnaturalconditionsandhavethenbeencrossed。Areturn，also，ofdomesticatedanimalsandcultivatedplantstoawildstatefavoursreversion;butthetendencyunderthesecircumstanceshasbeenmuchexaggerated。
Whenindividualsofthesamefamilywhichdiffersomewhat，andwhenracesorspeciesarecrossed，theoneisoftenprepotentovertheotherintransmittingitscharacter。Aracemaypossessastrongpowerofinheritance，andyetwhencrossed，aswehaveseenwithtrumpeter—pigeons，yieldtotheprepotencyofeveryotherrace。Prepotencyoftransmissionmaybeequalinthetwosexesofthesamespecies，butoftenrunsmorestronglyinonesex。Itplaysanimportantpartindeterminingtherateatwhichoneracecanbemodifiedorwhollyabsorbedbyrepeatedcrosseswithanother。Wecanseldomtellwhatmakesoneraceorspeciesprepotentoveranother;butitsometimesdependsonthesamecharacterbeingpresentandvisibleinoneparent，andlatentorpotentiallypresentintheother。
Charactersmayfirstappearineithersex，butoftenerinthemalethaninthefemale，andafterwardsbetransmittedtotheoffspringofthesamesex。Inthiscasewemayfeelconfidentthatthepeculiarityinquestionisreallypresentthoughlatentintheoppositesex！hencethefathermaytransmitthroughhisdaughteranycharactertohisgrandson;andthemotherconverselytohergranddaughter。Wethuslearn，andthefactisanimportantone，thattransmissionanddevelopmentaredistinctpowers。Occasionallythesetwopowersseemtobeantagonistic，orincapableofcombinationinthesameindividual;forseveralcaseshavebeenrecordedinwhichthesonhasnotdirectlyinheritedacharacterfromhisfather，ordirectlytransmittedittohisson，buthasreceiveditbytransmissionthroughhisnon—affectedmother，andtransmitteditthroughhisnon—affecteddaughter。Owingtoinheritancebeinglimitedbysex，weseehowsecondarysexualcharactersmayhavearisenundernature;theirpreservationandaccumulationbeingdependentontheirservicetoeithersex。
Atwhateverperiodoflifeanewcharacterfirstappears，itgenerallyremainslatentintheoffspringuntilacorrespondingageisattained，andthenisdeveloped。Whenthisrulefails，thechildgenerallyexhibitsthecharacteratanearlierperiodthantheparent。Onthisprincipleofinheritanceatcorrespondingperiods，wecanunderstandhowitisthatmostanimalsdisplayfromthegermtomaturitysuchamarvelloussuccessionofcharacters。
Finally，thoughmuchremainsobscurewithrespecttoInheritance，wemaylookatthefollowinglawsasfairlywellestablished。
FIRSTLY，atendencyineverycharacter，newandold，tobetransmittedbyseminalandbudgeneration，thoughoftencounteractedbyvariousknownandunknowncauses。
SECONDLY，reversionoratavism，whichdependsontransmissionanddevelopmentbeingdistinctpowers:itactsinvariousdegreesandmannersthroughbothseminalandbudgeneration。
THIRDLY，prepotencyoftransmission，whichmaybeconfinedtoonesex，orbecommontobothsexes。
FOURTHLY，transmission，aslimitedbysex，generallytothesamesexinwhichtheinheritedcharacterfirstappeared;andthisinmany，probablymostcases，dependsonthenewcharacterhavingfirstappearedataratherlateperiodoflife。
FIFTHLY，inheritanceatcorrespondingperiodsoflife，withsometendencytotheearlierdevelopmentoftheinheritedcharacter。
IntheselawsofInheritance，asdisplayedunderdomestication，weseeanampleprovisionfortheproduction，throughvariabilityandnaturalselection，ofnewspecificforms。
CHAPTER2。XV。
ONCROSSING。
FREEINTERCROSSINGOBLITERATESTHEDIFFERENCESBETWEENALLIEDBREEDS。
WHENTHENUMBERSOFTWOCOMMINGLINGBREEDSAREUNEQUAL，ONEABSORBSTHEOTHER。
THERATEOFABSORPTIONDETERMINEDBYPREPOTENCYOFTRANSMISSION，BYTHE
CONDITIONSOFLIFE，ANDBYNATURALSELECTION。
ALLORGANICBEINGSOCCASIONALLYINTERCROSS;APPARENTEXCEPTIONS。
ONCERTAINCHARACTERSINCAPABLEOFFUSION;CHIEFLYOREXCLUSIVELYTHOSEWHICH
HAVESUDDENLYAPPEAREDINTHEINDIVIDUAL。
ONTHEMODIFICATIONOFOLDRACES，ANDTHEFORMATIONOFNEWRACESBYCROSSING。
SOMECROSSEDRACESHAVEBREDTRUEFROMTHEIRFIRSTPRODUCTION。
ONTHECROSSINGOFDISTINCTSPECIESINRELATIONTOTHEFORMATIONOFDOMESTIC
RACES。
Inthetwopreviouschapters，whendiscussingreversionandprepotency，Iwasnecessarilyledtogivemanyfactsoncrossing。InthepresentchapterIshallconsiderthepartwhichcrossingplaysintwoopposeddirections，——firstly，inobliteratingcharacters，andconsequentlyinpreventingtheformationofnewraces;andsecondly，inthemodificationofoldraces，orintheformationofnewandintermediateraces，byacombinationofcharacters。Ishallalsoshowthatcertaincharactersareincapableoffusion。
Theeffectsoffreeoruncontrolledbreedingbetweenthemembersofthesamevarietyorofcloselyalliedvarietiesareimportant;butaresoobviousthattheyneednotbediscussedatmuchlength。Itisfreeintercrossingwhichchieflygivesuniformity，bothundernatureandunderdomestication，totheindividualsofthesamespeciesorvariety，whentheylivemingledtogetherandarenotexposedtoanycauseinducingexcessivevariability。Thepreventionoffreecrossing，andtheintentionalmatchingofindividualanimals，arethecorner—stonesofthebreeder’sart。Nomaninhissenseswouldexpecttoimproveormodifyabreedinanyparticularmanner，orkeepanoldbreedtrueanddistinct，unlessheseparatedhisanimals。Thekillingofinferioranimalsineachgenerationcomestothesamethingastheirseparation。Insavageandsemi—civilisedcountries，wheretheinhabitantshavenotthemeansofseparatingtheiranimals，morethanasinglebreedofthesamespeciesrarelyorneverexists。Informertimes，evenintheUnitedStates，therewerenodistinctracesofsheep，forallhadbeenmingledtogether。（15/1。’CommunicationstotheBoardofAgriculture’volume1page367。）ThecelebratedagriculturistMarshall（15/2。’ReviewofReports，NorthofEngland’1808page200。）remarksthat\"sheepthatarekeptwithinfences，aswellasshepherdedflocksinopencountries，havegenerallyasimilarity，ifnotauniformity，ofcharacterintheindividualsofeachflock;\"fortheybreedfreelytogether，andarepreventedfromcrossingwithotherkinds;
whereasintheunenclosedpartsofEnglandtheunshepherdedsheep，evenofthesameflock，arefarfromtrueoruniform，owingtovariousbreedshavingmingledandcrossed。Wehaveseenthatthehalf—wildcattleineachoftheseveralBritishparksarenearlyuniformincharacter;butinthedifferentparks，fromnothavingmingledandcrossedduringmanygenerations，theydiffertoacertainsmallextent。
Wecannotdoubtthattheextraordinarynumberofvarietiesandsub—varietiesofthepigeon，amountingtoatleastonehundredandfifty，ispartlyduetotheirremaining，differentlyfromotherdomesticatedbirds，pairedforlifeoncematched。Ontheotherhand，breedsofcatsimportedintothiscountrysoondisappear，fortheirnocturnalandramblinghabitsrenderithardlypossibletopreventfreecrossing。Rengger（15/3。’SaugethierevonParaguay’
1830s。212。）givesaninterestingcasewithrespecttothecatinParaguay:
inallthedistantpartsofthekingdomithasassumed，apparentlyfromtheeffectsoftheclimate，apeculiarcharacter，butnearthecapitalthischangehasbeenprevented，owing，asheasserts，tothenativeanimalfrequentlycrossingwithcatsimportedfromEurope。Inallcasesliketheforegoing，theeffectsofanoccasionalcrosswillbeaugmentedbytheincreasedvigourandfertilityofthecrossedoffspring，ofwhichfactevidencewillhereafterbegiven;forthiswillleadtothemongrelsincreasingmorerapidlythanthepureparent—breeds。
Whendistinctbreedsareallowedtocrossfreely，theresultwillbeaheterogeneousbody;forinstance，thedogsinParaguayarefarfromuniform，andcannolongerbeaffiliatedtotheirparent—races。（15/4。Rengger’Saugethiere’etc。s。154。）Thecharacterwhichacrossedbodyofanimalswillultimatelyassumemustdependonseveralcontingencies，——namely，ontherelativemembersoftheindividualsbelongingtothetwoormoreraceswhichareallowedtomingle;ontheprepotencyofoneraceovertheotherinthetransmissionofcharacter;andontheconditionsoflifetowhichtheyareexposed。Whentwocommingledbreedsexistatfirstinnearlyequalnumbers，thewholewillsoonerorlaterbecomeintimatelyblended，butnotsosoon，bothbreedsbeingequallyfavouredinallrespects，asmighthavebeenexpected。Thefollowingcalculation（15/5。White’RegularGradationinMan’
page146。）showsthatthisisthecase:ifacolonywithanequalnumberofblackandwhitemenwerefounded，andweassumethattheymarryindiscriminately，areequallyprolific，andthatoneinthirtyannuallydiesandisborn;then\"in65yearsthenumberofblacks，whites，andmulattoeswouldbeequal。In91yearsthewhiteswouldbe1—10th，theblacks1—10th，andthemulattoes，orpeopleofintermediatedegreesofcolour，8—10thsofthewholenumber。Inthreecenturiesnot1—100thpartofthewhiteswouldexist。\"
Whenoneoftwomingledracesexceedtheothergreatlyinnumber，thelatterwillsoonbewholly，oralmostwholly，absorbedandlost。（15/6。Dr。W。F。
Edwardsinhis’CaracteresPhysiolog。desRacesHumaines’page24firstcalledattentiontothissubject，andablydiscussedit。）ThusEuropeanpigsanddogshavebeenlargelyintroducedintheislandsofthePacificOcean，andthenativeraceshavebeenabsorbedandlostinthecourseofaboutfiftyorsixtyyears（15/7。Rev。D。TyermanandBennett’JournalofVoyages’1821—1829volume1page300。）;buttheimportedracesnodoubtwerefavoured。Ratsmaybeconsideredassemi—domesticatedanimals。Somesnake—rats（Musalexandrinus）
escapedintheZoologicalGardensofLondon\"andforalongtimeafterwardsthekeepersfrequentlycaughtcross—bredrats，atfirsthalf—breds，afterwardswithlessofthecharacterofthesnake—rat，tillatlengthalltracesofitdisappeared。\"（15/8。Mr。S。J。Salter’JournalLinn。Soc。’volume61862page71。）Ontheotherhand，insomepartsofLondon，especiallynearthedocks，wherefreshratsarefrequentlyimported，anendlessvarietyofintermediateformsmaybefoundbetweenthebrown，black，andsnakerat，whichareallthreeusuallyrankedasdistinctspecies。
Howmanygenerationsarenecessaryforonespeciesorracetoabsorbanotherbyrepeatedcrosseshasoftenbeendiscussed（15/9。Sturm’UeberRacen，etc。’
1825s。107。Bronn’GeschichtederNatur’b。2s。170givesatableoftheproportionsofbloodaftersuccessivecrosses。Dr。P。Lucas’L’HerediteNat。’
tome2page308。）;andtherequisitenumberhasprobablybeenmuchexaggerated。Somewritershavemaintainedthatadozenorscore，orevenmoregenerations，arenecessary;butthisinitselfisimprobable，forinthetenthgenerationtherewouldbeonly1—1024thpartofforeignbloodintheoffspring。Gartnerfound（15/10。’Bastarderzeugung’s。463，470。），thatwithplants，onespeciescouldbemadetoabsorbanotherinfromthreetofivegenerations，andhebelievesthatthiscouldalwaysbeeffectedinfromsixtosevengenerations。Inoneinstance，however，Kolreuter（15/11。’NovaActaPetrop。’1794page393:seealsopreviousvolume。）speaksoftheoffspringofMirabilisvulgaris，crossedduringeightsuccessivegenerationsbyM。
longiflora，asresemblingthislatterspeciessoclosely，thatthemostscrupulousobservercoulddetect\"vixaliquamnotabilemdifferentiam\"or，ashesays，hesucceeded，\"adplenariamferetransmutationem。\"Butthisexpressionshowsthattheactofabsorptionwasnoteventhenabsolutelycomplete，thoughthesecrossedplantscontainedonlythe1—256thpartofM。
vulgaris。TheconclusionsofsuchaccurateobserversasGartnerandKolreuterareoffarhigherworththanthosemadewithoutscientificaimbybreeders。
ThemostpreciseaccountwhichIhavemetwithisgivenbyStonehenge（15/12。
’TheDog’1867pages179—184。）andisillustratedbyphotographs。Mr。Hanleycrossedagreyhoundbitchwithabulldog;theoffspringineachsucceedinggenerationbeingrecrossedwithfirst—rategreyhounds。AsStonehengeremarks，itmightnaturallybesupposedthatitwouldtakeseveralcrossestogetridoftheheavyformofthebulldog;butHysterics，thegr—gr—granddaughterofabulldog，showednotracewhateverofthisbreedinexternalform。Sheandallofthesamelitter，however，were\"remarkablydeficientinstoutness，thoughfastaswellasclever。\"Ibelievecleverreferstoskillinturning。
HystericswasputtoasonofBedlamite，\"buttheresultofthefifthcrossisnotasyet，Ibelieve，moresatisfactorythanthatofthefourth。\"Ontheotherhand，withsheep，Fleischmann（15/13。Asquotedinthe’TruePrinciplesofBreeding’byC。H。MacknightandDr。H。Madden1865page11。）showshowpersistenttheeffectsofasinglecrossmaybe:hesays\"thattheoriginalcoarsesheep（ofGermany）have5500fibresofwoolonasquareinch;gradesofthethirdorfourthMerinocrossproducedabout8000，thetwentiethcross27，000，theperfectpureMerinoblood40，000to48，000。\"SothatcommonGermansheepcrossedtwentytimessuccessivelywithMerinodidnotbyanymeansacquirewoolasfineasthatofthepurebreed。Butinallcases，therateofabsorptionwilldependlargelyontheconditionsoflifebeingfavourabletoanyparticularcharacter;andwemaysuspectthattherewouldbeaconstanttendencytodegenerationinthewoolofMerinosundertheclimateofGermany，unlesspreventedbycarefulselection;andthusperhapstheforegoingremarkablecasemaybeexplained。Therateofabsorptionmustalsodependontheamountofdistinguishabledifferencebetweenthetwoformswhicharecrossed，andespecially，asGartnerinsists，onprepotencyoftransmissionintheoneformovertheother。WehaveseeninthelastchapterthatoneoftwoFrenchbreedsofsheepyieldedupitscharacter，whencrossedwithMerinos，verymuchmoreslowlythantheother;andthecommonGermansheepreferredtobyFleischmannmaybeinthisrespectanalogous。Inallcasestherewillbemoreorlessliabilitytoreversionduringmanysubsequentgenerations，anditisthisfactwhichhasprobablyledauthorstomaintainthatascoreormoreofgenerationsarerequisiteforoneracetoabsorbanother。Inconsideringthefinalresultofthecomminglingoftwoormorebreeds，wemustnotforgetthattheactofcrossinginitselftendstobringbacklong—lostcharactersnotpropertotheimmediateparent—forms。
Withrespecttotheinfluenceoftheconditionsoflifeonanytwobreedswhichareallowedtocrossfreely，unlessbothareindigenousandhavelongbeenaccustomedtothecountrywheretheylive，theywill，inallprobability，beunequallyaffectedbytheconditions，andthiswillmodifytheresult。Evenwithindigenousbreeds，itwillrarelyorneveroccurthatbothareequallywelladaptedtothesurroundingcircumstances;moreespeciallywhenpermittedtoroamfreely，andnotcarefullytended，asisgenerallythecasewithbreedsallowedtocross。Asaconsequenceofthis，naturalselectionwilltoacertainextentcomeintoaction，andthebestfittedwillsurvive，andthiswillaidindeterminingtheultimatecharacterofthecommingledbody。