Howlongatimeitwouldrequirebeforesuchacrossedbodyofanimalswouldassumeauniformcharacterwithinalimitedarea，noonecansay;thattheywouldultimatelybecomeuniformfromfreeintercrossing，andfromthesurvivalofthefittest，wemayfeelassured;butthecharactersthusacquiredwouldrarelyornever，asmaybeinferredfromthepreviousconsiderations，beexactlyintermediatebetweenthoseofthetwoparent—breeds。Withrespecttotheveryslightdifferencesbywhichtheindividualsofthesamesub—variety，orevenofalliedvarieties，arecharacterised，itisobviousthatfreecrossingwouldsoonobliteratesuchsmalldistinctions。Theformationofnewvarieties，independentlyofselection，wouldalsothusbeprevented;exceptwhenthesamevariationcontinuallyrecurredfromtheactionofsomestronglypredisposingcause。Wemaythereforeconcludethatfreecrossinghasinallcasesplayedanimportantpartingivinguniformityofcharactertoallthemembersofthesamedomesticraceandofthesamenaturalspecies，thoughlargelygovernedbynaturalselectionandbythedirectactionofthesurroundingconditions。
ONTHEPOSSIBILITYOFALLORGANICBEINGSOCCASIONALLYINTERCROSSING。
Butitmaybeasked，canfreecrossingoccurwithhermaphroditeanimalsandplants？Allthehigheranimals，andthefewinsectswhichhavebeendomesticated，haveseparatesexes，andmustinevitablyuniteforeachbirth。
Withrespecttothecrossingofhermaphrodites，thesubjectistoolargeforthepresentvolume，butinthe’OriginofSpecies’Ihavegivenashortabstractofthereasonswhichinducemetobelievethatallorganicbeingsoccasionallycross，thoughperhapsinsomecasesonlyatlongintervalsoftime。（15/14。Withrespecttoplants，anadmirableessayonthissubject（DieGeschlechter—VertheilungbeidenPflanzen:1867）hasbeenpublishedbyDr。
HildebrandwhoarrivesatthesamegeneralconclusionsasIhavedone。Variousothertreatiseshavesinceappearedonthesamesubject，moreespeciallybyHermannMullerandDelpino。）Iwillmerelyrecallthefactthatmanyplants，thoughhermaphroditeinstructure，areunisexualinfunction;——suchasthosecalledbyC。K。SprengelDICHOGAMOUS，inwhichthepollenandstigmaofthesameflowerarematuredatdifferentperiods;orthosecalledbymeRECIPROCALLYDIMORPHIC，inwhichtheflower’sownpollenisnotfittedtofertiliseitsownstigma;oragain，themanykindsinwhichcuriousmechanicalcontrivancesexist，effectuallypreventingself—fertilisation。Thereare，however，manyhermaphroditeplantswhicharenotinanywayspeciallyconstructedtofavourintercrossing，butwhichneverthelesscomminglealmostasfreelyasanimalswithseparatedsexes。Thisisthecasewithcabbages，radishes，andonions，asIknowfromhavingexperimentedonthem:eventhepeasantsofLiguriasaythatcabbagesmustbeprevented"fromfallinginlove"
witheachother。Intheorangetribe，Gallesio（15/15。’TeoriadellaRiproduzioneVegetal’1816page12。）remarksthattheameliorationofthevariouskindsischeckedbytheircontinualandalmostregularcrossing。Soitiswithnumerousotherplants。
Ontheotherhand，somecultivatedplantsrarelyorneverintercross，forinstance，thecommonpeaandsweet—pea（Lathyrusodoratus）;yettheirflowersarecertainlyadaptedforcrossfertilisation。Thevarietiesofthetomatoandaubergine（Solanum）andthepimenta（Pimentavulgaris？）aresaid（15/16。
Verlot’DesVarietes’1865page72。）nevertocross，evenwhengrowingalongsideoneanother。Butitshouldbeobservedthattheseareallexoticplants，andwedonotknowhowtheywouldbehaveintheirnativecountrywhenvisitedbytheproperinsects。Withrespecttothecommonpea，Ihaveascertainedthatitisrarelycrossedinthiscountryowingtoprematurefertilisation。Thereexist，however，someplantswhichundertheirnaturalconditionsappeartobealwaysself—fertilised，suchastheBeeOphrys（Ophrysapifera）andafewotherOrchids;yettheseplantsexhibittheplainestadaptationsforcross—fertilisation。Again，somefewplantsarebelievedtoproduceonlyclosedflowers，calledcleistogene，whichcannotpossiblybecrossed。ThiswaslongthoughttobethecasewiththeLeersiaoryzoides（15/17。DuvalJouve’Bull。Soc。Bot。deFrance’tome101863page194。WithrespecttotheperfectflowerssettingseedseeDr。Aschersonin’Bot。
Zeitung’1864page350。），butthisgrassisnowknownoccasionallytoproduceperfectflowers，whichsetseed。
Althoughsomeplants，bothindigenousandnaturalised，rarelyorneverproduceflowers，oriftheyflowerneverproduceseeds，yetnoonedoubtsthatphanerogamicplantsareadaptedtoproduceflowers，andtheflowerstoproduceseed。Whentheyfail，webelievethatsuchplantsunderdifferentconditionswouldperformtheirproperfunction，orthattheyformerlydidso，andwilldosoagain。Onanalogousgrounds，Ibelievethattheflowersintheabovespecifiedanomalouscaseswhichdonotnowintercross，eitherwoulddosooccasionallyunderdifferentconditions，orthattheyformerlydidso——themeansforaffectingthisbeinggenerallystillretained——andwillagainintercrossatsomefutureperiod，unlessindeedtheybecomeextinct。Onthisviewalone，manypointsinthestructureandactionofthereproductiveorgansinhermaphroditeplantsandanimalsareintelligible，——forinstance，thefactofthemaleandfemaleorgansneverbeingsocompletelyenclosedastorenderaccessfromwithoutimpossible。Hencewemayconcludethatthemostimportantofallthemeansforgivinguniformitytotheindividualsofthesamespecies，namely，thecapacityofoccasionallyintercrossing，ispresent，orhasbeenformerlypresent，withallorganicbeings，except，perhaps，someofthelowest。
[ONCERTAINCHARACTERSNOTBLENDING。
Whentwobreedsarecrossedtheircharactersusuallybecomeintimatelyfusedtogether;butsomecharactersrefusetoblend，andaretransmittedinanunmodifiedstateeitherfrombothparentsorfromone。Whengreyandwhitemicearepaired，theyoungarepiebald，orpurewhiteorgrey，butnotofanintermediatetint;soitiswhenwhiteandcommoncollaredturtle—dovesarepaired。InbreedingGamefowls，agreatauthority，Mr。J。Douglas，remarks，"I
mayherestateastrangefact:ifyoucrossablackwithawhitegame，yougetbirdsofbothbreedsoftheclearestcolour。"SirR。Heroncrossedduringmanyyearswhite，black，brown，andfawn—colouredAngorarabbits，andneveroncegotthesecoloursmingledinthesameanimal，butoftenallfourcoloursinthesamelitter。（15/18。ExtractofaletterfromSirR。Heron1838givenmebyMr。Yarrell。Withrespecttomicesee’Annal。desSc。Nat。’tome1page180;andIhaveheardofothersimilarcases。Forturtle—dovesBoitardandCorbie’LesPigeons’etc。page238。FortheGamefowl’ThePoultryBook’1866
page128。ForcrossesoftaillessfowlsseeBechstein’Naturges。Deutsch。’b。
3s。403。Bronn’GeschichtederNatur’b。2s。170givesanalogousfactswithhorses。OnthehairlessconditionofcrossedSouthAmericandogsseeRengger’SaugethierevonParaguay’s。152;butIsawintheZoologicalGardensmongrels，fromasimilarcross，whichwerehairless，quitehairy，orhairyinpatches，thatis，piebaldwithhair。ForcrossesofDorkingandotherfowlssee’PoultryChronicle’volume2page355。Aboutthecrossedpigs，extractofletterfromSirR。HerontoMr。Yarrell。ForothercasesseeP。Lucas’L’Hered。Nat。’tome1page212。）Fromcaseslikethese，inwhichthecoloursofthetwoparentsaretransmittedquiteseparatelytotheoffspring，wehaveallsortsofgradations，leadingtocompletefusion。Iwillgiveaninstance:
agentlemanwithafaircomplexion，lighthairbutdarkeyes，marriedaladywithdarkhairandcomplexion:theirthreechildrenhaveverylighthair，butoncarefulsearchaboutadozenblackhairswerefoundscatteredinthemidstofthelighthairontheheadsofallthree。
Whenturnspitdogsandanconsheep，bothofwhichhavedwarfedlimbs，arecrossedwithcommonbreeds，theoffspringarenotintermediateinstructure，buttakeaftereitherparent。Whentaillessorhornlessanimalsarecrossedwithperfectanimals，itfrequently，butbynomeansinvariably，happensthattheoffspringareeitherfurnishedwiththeseorgansinaperfectstate，orarequitedestituteofthem。AccordingtoRengger，thehairlessconditionoftheParaguaydogiseitherperfectlyornotatalltransmittedtoitsmongreloffspring;butIhaveseenonepartialexceptioninadogofthisparentagewhichhadpartofitsskinhairy，andpartnaked，thepartsbeingdistinctlyseparatedasinapiebaldanimal。WhenDorkingfowlswithfivetoesarecrossedwithotherbreeds，thechickensoftenhavefivetoesononefootandfourontheother。SomecrossedpigsraisedbySirR。Heronbetweenthesolid—
hoofedandcommonpighadnotallfourfeetinanintermediatecondition，buttwofeetwerefurnishedwithproperlydivided，andtwowithunitedhoofs。
Analogousfactshavebeenobservedwithplants:MajorTrevorClarkecrossedthelittle，glabrous—leaved，annualstock（Matthiola），withpollenofalarge，red—flowered，rough—leaved，biennialstock，calledcocardeaubytheFrench，andtheresultwasthathalftheseedlingshadglabrousandtheotherhalfroughleaves，butnonehadleavesinanintermediatestate。Thattheglabrousseedlingsweretheproductoftherough—leavedvariety，andnotaccidentallyofthemother—plant’sownpollen，wasshownbytheirtallandstronghabitofgrowth。（15/19。’Internat。Hort。andBot。CongressofLondon’1866。）inthesucceedinggenerationsraisedfromtherough—leavedcrossedseedlings，someglabrousplantsappeared，showingthattheglabrouscharacter，thoughincapableofblendingwithandmodifyingtheroughleaves，wasallthetimelatentinthisfamilyofplants。Thenumerousplantsformerlyreferredto，whichIraisedfromreciprocalcrossesbetweenthepeloricandcommonAntirrhinum，offeranearlyparallelcase;forinthefirstgenerationalltheplantsresembledthecommonform，andinthenextgeneration，outofonehundredandthirty—sevenplants，twoalonewereinanintermediatecondition，theothersperfectlyresemblingeitherthepeloricorcommonform。MajorTrevorClarkealsofertilisedtheabove—mentionedred—floweredstockwithpollenfromthepurpleQueenstock，andabouthalftheseedlingsscarcelydifferedinhabit，andnotatallintheredcolouroftheflower，fromthemother—plant，theotherhalfbearingblossomsofarichpurple，closelylikethoseofthepaternalplant。Gartnercrossedmanywhiteandyellow—floweredspeciesandvarietiesofVerbascum;andthesecolourswereneverblended，buttheoffspringboreeitherpurewhiteorpureyellowblossoms;theformerinthelargerproportion。（15/20。’Bastarderzeugung’s。307。Kolreuter’DritteFortsetszung’s。34，39however，obtainedintermediatetintsfromsimilarcrossesinthegenusVerbascum。WithrespecttotheturnipsseeHerbert’Amaryllidaceae’1837page370。）Dr。Herbertraisedmanyseedlings，asheinformedme，fromSwedishturnipscrossedbytwoothervarieties，andtheseneverproducedflowersofanintermediatetint，butalwayslikeoneoftheirparents。Ifertilisedthepurplesweet—pea（Lathyrusodoratus），whichhasadarkreddish—purplestandard—petalandviolet—colouredwingsandkeel，withpollenofthepaintedladysweet—pea，whichhasapalecherry—colouredstandard，andalmostwhitewingsandkeel;andfromthesamepodItwiceraisedplantsperfectlyresemblingbothsorts;thegreaternumberresemblingthefather。Soperfectwastheresemblance，thatIshouldhavethoughttherehadbeensomemistake，iftheplantswhichwereatfirstidenticalwiththepaternalvariety，namely，thepainted—lady，hadnotlaterintheseasonproduced，asmentionedinaformerchapter，flowersblotchedandstreakedwithdarkpurple。Iraisedgrandchildrenandgreat—grandchildrenfromthesecrossedplants，andtheycontinuedtoresemblethepainted—lady，butduringlatergenerationsbecamerathermoreblotchedwithpurple，yetnonerevertedcompletelytotheoriginalmother—plant，thepurplesweet—pea。Thefollowingcaseisslightlydifferent，butstillshowsthesameprinciple:Naudin（15/21。
’NouvellesArchivesduMuseum’tome1page100。）raisednumeroushybridsbetweentheyellowLinariavulgarisandthepurpleL。purpurea，andduringthreesuccessivegenerationsthecolourskeptdistinctindifferentpartsofthesameflower。
Fromcasessuchastheforegoing，inwhichtheoffspringofthefirstgenerationperfectlyresembleeitherparent，wecomebyasmallsteptothosecasesinwhichdifferentlycolouredflowersborneonthesamerootresemblebothparents，andbyanothersteptothoseinwhichthesameflowerorfruitisstripedorblotchedwiththetwoparentalcolours，orbearsasinglestripeofthecolourorothercharacteristicqualityofoneoftheparent—forms。Withhybridsandmongrelsitfrequentlyorevengenerallyhappensthatonepartofthebodyresemblesmoreorlesscloselyoneparentandanotherparttheotherparent;andhereagainsomeresistencetofusion，or，whatcomestothesamething，somemutualaffinitybetweentheorganicatomsofthesamenature，apparentlycomesintoplay，forotherwiseallpartsofthebodywouldbeequallyintermediateincharacter。Soagain，whentheoffspringofhybridsormongrels，whicharethemselvesnearlyintermediateincharacter，reverteitherwhollyorbysegmentstotheirancestors，theprincipleoftheaffinityofsimilar，ortherepulsionofdissimilaratoms，mustcomeintoaction。Tothisprinciple，whichseemstobeextremelygeneral，weshallrecurinthechapteronpangenesis。
Itisremarkable，ashasbeenstronglyinsisteduponbyIsidoreGeoffroySt。
Hilaireinregardtoanimals，thatthetransmissionofcharacterswithoutfusionoccursveryrarelywhenspeciesarecrossed;Iknowofoneexceptionalone，namely，withthehybridsnaturallyproducedbetweenthecommonandhoodedcrow（Corvuscoroneandcornix），which，however，arecloselyalliedspecies，differinginnothingexceptcolour。NorhaveImetwithanywell—
ascertainedcasesoftransmissionofthiskind，evenwhenoneformisstronglyprepotentoveranother，whentworacesarecrossedwhichhavebeenslowlyformedbyman’sselection，andthereforeresembletoacertainextentnaturalspecies。Suchcasesaspuppiesinthesamelittercloselyresemblingtwodistinctbreeds，areprobablyduetosuperfoetation，——thatis，totheinfluenceoftwofathers。Allthecharactersaboveenumerated，whicharetransmittedinaperfectstatetosomeoftheoffspringandnottoothers，——
suchasdistinctcolours，nakednessofskin，smoothnessofleaves，absenceofhornsortail，additionaltoes，pelorism，dwarfedstructure，etc。，——haveallbeenknowntoappearsuddenlyinindividualanimalsandplants。Fromthisfact，andfromtheseveralslight，aggregateddifferenceswhichdistinguishdomesticracesandspeciesfromoneanother，notbeingliabletothispeculiarformoftransmission，wemayconcludethatitisinsomewayconnectedwiththesuddenappearanceofthecharactersinquestion。]
ONTHEMODIFICATIONOFOLDRACESANDTHEFORMATIONOFNEWRACESBYCROSSING。
Wehavehithertochieflyconsideredtheeffectsofcrossingingivinguniformityofcharacter;wemustnowlooktoanoppositeresult。Therecanbenodoubtthatcrossing，withtheaidofrigorousselectionduringseveralgenerations，hasbeenapotentmeansinmodifyingoldraces，andinformingnewones。LordOrfordcrossedhisfamousstudofgreyhoundsoncewiththebulldog，inordertogivethemcourageandperseverance。Certainpointershavebeencrossed，asIhearfromtheRev。W。D。Fox，withthefoxhound，togivethemdashandspeed。CertainstrainsofDorkingfowlshavehadaslightinfusionofGameblood;andIhaveknownagreatfancierwhoonasingleoccasioncrossedhisturbit—pigeonswithbarbs，forthesakeofgaininggreaterbreadthofbeak。
Intheforegoingcasesbreedshavebeencrossedonce，forthesakeofmodifyingsomeparticularcharacter;butwithmostoftheimprovedracesofthepig，whichnowbreedtrue，therehavebeenrepeatedcrosses，——...完整阅读请扫描二维码下载丁香书院APP免费看：