Theguinea—fowlintheWestIndies，whenferal，seemstovarymorethaninthedomesticatedstate。
Withrespecttoplantsrunwild，Dr。Hooker（13/14。’FloraofAustralia’1859
Introductionpage9。）hasstronglyinsistedonwhatslightevidencethecommonbeliefintheirreversiontoaprimitivestaterests。Godron（13/15。’Del’Espece’tome2pages54，58，60。）describeswildturnips，carrots，andcelery;buttheseplantsintheircultivatedstatehardlydifferfromtheirwildprototypes，exceptinthesucculencyandenlargementofcertainparts，——
characterswhichwouldcertainlybelostbyplantsgrowinginpoorsoilandstrugglingwithotherplants。Nocultivatedplanthasrunwildonsoenormousascaleasthecardoon（Cynaracardunculus）inLaPlata。Everybotanistwhohasseenitgrowingthere，invastbeds，ashighasahorse’sback，hasbeenstruckwithitspeculiarappearance;butwhetheritdiffersinanyimportantpointfromthecultivatedSpanishform，whichissaidnottobepricklylikeitsAmericandescendant，orwhetheritdiffersfromthewildMediterraneanspecies，whichissaidnottobesocial（thoughthismaybeduemerelytothenatureoftheconditions），Idonotknow。]
REVERSIONTOCHARACTERSDERIVEDFROMACROSS，INTHECASEOFSUB—VARIETIES，RACES，ANDSPECIES。
Whenanindividualhavingsomerecognisablepeculiarityuniteswithanotherofthesamesub—variety，nothavingthepeculiarityinquestion，itoftenreappearsinthedescendantsafteranintervalofseveralgenerations。Everyonemusthavenoticed，orheardfromoldpeopleofchildrencloselyresemblinginappearanceormentaldisposition，orinsosmallandcomplexacharacterasexpression，oneoftheirgrandparents，orsomemoredistantcollateralrelation。Verymanyanomaliesofstructureanddiseases（13/16。Mr。Sedgwickgivesmanyinstancesinthe’BritishandForeignMed。—Chirurg。Review’AprilandJuly1863pages448，188。）ofwhichinstanceshavebeengiveninthelastchapter，havecomeintoafamilyfromoneparent，andhavereappearedintheprogenyafterpassingovertwoorthreegenerations。Thefollowingcasehasbeencommunicatedtomeongoodauthority，andmay，Ibelieve，befullytrusted:apointer—bitchproducedsevenpuppies;fourweremarkedwithblueandwhite，whichissounusualacolourwithpointersthatshewasthoughttohaveplayedfalsewithoneofthegreyhounds，andthewholelitterwascondemned;butthegamekeeperwaspermittedtosaveoneasacuriosity。Twoyearsafterwardsafriendoftheownersawtheyoungdog，anddeclaredthathewastheimageofhisoldpointer—bitchSappho，theonlyblueandwhitepointerofpuredescentwhichhehadeverseen。Thisledtocloseinquiry，anditwasprovedthathewasthegreat—great—grandsonofSappho;sothat，accordingtothecommonexpression，hehadonly1/16thofherbloodinhisveins。Imaygiveoneotherinstance，ontheauthorityofMr。R。Walker，alargecattle—
breederinKincardineshire。Heboughtablackbull，thesonofablackcowwithwhitelegs，whitebellyandpartofthetailwhite;andin1870acalfthegr。—gr。—gr。—gr。—grandchildofthiscowwasborncolouredinthesameverypeculiarmanner;alltheintermediateoffspringhavingbeenblack。Inthesecasestherecanhardlybeadoubtthatacharacterderivedfromacrosswithanindividualofthesamevarietyreappearedafterpassingoverthreegenerationsintheonecase，andfiveintheother。
Whentwodistinctracesarecrossed，itisnotoriousthatthetendencyintheoffspringtoreverttooneorbothparent—formsisstrong，andenduresformanygenerations。Ihavemyselfseentheclearestevidenceofthisincrossedpigeonsandwithvariousplants。Mr。Sidney（13/17。Inhiseditionof’YouattonthePig’1860page27。）statesthat，inalitterofEssexpigs，twoyoungonesappearedwhichweretheimageoftheBerkshireboarthathadbeenusedtwenty—eightyearsbeforeingivingsizeandconstitutiontothebreed。I
observedinthefarmyardatBetleyHallsomefowlsshowingastronglikenesstotheMalaybreed，andwastoldbyMr。TolletthathehadfortyyearsbeforecrossedhisbirdswithMalays;andthat，thoughhehadatfirstattemptedtogetridofthisstrain，hehadsubsequentlygivenuptheattemptindespair，astheMalaycharacterwouldreappear。
Thisstrongtendencyincrossedbreedstoreverthasgivenrisetoendlessdiscussionsinhowmanygenerationsafterasinglecross，eitherwithadistinctbreedormerelywithaninferioranimal，thebreedmaybeconsideredaspure，andfreefromalldangerofreversion。Noonesupposesthatlessthanthreegenerationssuffices，andmostbreedersthinkthatsix，seven，oreightarenecessary，andsomegotostillgreaterlengths。（13/18。Dr。P。Lucas，’Hered。Nat。’tome2pages314，892:seeagoodpracticalarticleonthesubjectin’Gardener’sChronicle’1856page620。Icouldaddavastnumberofreferences，buttheywouldbesuperfluous。）Butneitherinthecaseofabreedwhichhasbeencontaminatedbyasinglecross，norwhen，intheattempttoformanintermediatebreed，half—bredanimalshavebeenmatchedtogetherduringmanygenerations，cananyrulebelaiddownhowsoonthetendencytoreversionwillbeobliterated。Itdependsonthedifferenceinthestrengthorprepotencyoftransmissioninthetwoparent—forms，ontheiractualamountofdifference，andonthenatureoftheconditionsoflifetowhichthecrossedoffspringareexposed。Butwemustbecarefulnottoconfoundthesecasesofreversiontocharacterswhichweregainedbyacross，withthoseunderthefirstclass，inwhichcharactersoriginallycommontoBOTHparents，butlostatsomeformerperiod，reappear;forsuchcharactersmayrecurafteranalmostindefinitenumberofgenerations。
Thelawofreversionisaspowerfulwithhybrids，whentheyaresufficientlyfertiletobreedtogether，orwhentheyarerepeatedlycrossedwitheitherpureparent—form，asinthecaseofmongrels。Itisnotnecessarytogiveinstances。Withplantsalmosteveryonewhohasworkedonthissubject，fromthetimeofKolreutertothepresentday，hasinsistedonthistendency。
Gartnerhasrecordedsomegoodinstances;butnoonehasgivenmorestrikingonesthanNaudin。（13/19。Kolreutergivescuriouscasesinhis’DritteFortsetzung’1766ss。53，59;andinhiswell—known’MemoirsonLavateraandJalapa。’Gartner’Bastarderzeugung’ss。437，441，etc。Naudininhis"Recherchessurl’Hybridite"’NouvellesArchivesduMuseum’tome1page25。）
Thetendencydiffersindegreeorstrengthindifferentgroups，andpartlydepends，asweshallpresentlysee，onwhethertheparent—plantshavebeenlongcultivated。Althoughthetendencytoreversionisextremelygeneralwithnearlyallmongrelsandhybrids，itcannotbeconsideredasinvariablycharacteristicofthem;itmayalsobemasteredbylong—continuedselection;
butthesesubjectswillmoreproperlybediscussedinafuturechapteronCrossing。Fromwhatweseeofthepowerandscopeofreversion，bothinpureraces，andwhenvarietiesorspeciesarecrossed，wemayinferthatcharactersofalmosteverykindarecapableofreappearingafterhavingbeenlostforagreatlengthoftime。Butitdoesnotfollowfromthisthatineachparticularcasecertaincharacterswillreappear;forinstance，thiswillnotoccurwhenaraceiscrossedwithanotherendowedwithprepotencyoftransmission。
Sometimesthepowerofreversionwhollyfails，withoutourbeingabletoassignanycauseforthefailure:thusithasbeenstatedthatinaFrenchfamilyinwhich85outofabove600members，duringsixgenerations，hadbeensubjecttonight—blindness，"therehasnotbeenasingleexampleofthisaffectioninthechildrenofparentswhowerethemselvesfreefromit。"
（13/20。QuotedbyMr。Sedgwickin’Med。—Chirurg。Review’April1861page485。
Dr。H。Dobellin’Med。—Chirurg。Transactions’volume46givesananalogouscaseinwhich，inalargefamily，fingerswiththickenedjointsweretransmittedtoseveralmembersduringfivegenerations;butwhentheblemishoncedisappeareditneverreappeared。）
REVERSIONTHROUGHBUD—PROPAGATION——PARTIALREVERSION，BYSEGMENTSINTHESAME
FLOWERORFRUIT，ORINDIFFERENTPARTSOFTHEBODYINTHESAMEINDIVIDUAL
ANIMAL。
Intheeleventhchaptermanycasesofreversionbybuds，independentlyofseminalgeneration，weregiven——aswhenaleaf—budonavariegated，acurled，orlaciniatedvarietysuddenlyreassumesitspropercharacter;oraswhenaProvence—roseappearsonamoss—rose，orapeachonanectarine—tree。Insomeofthesecasesonlyhalftheflowerorfruit，orasmallersegment，ormerestripes，reassumetheirformercharacter;andherewehavereversionbysegments。Vilmorin（13/21。Verlot’DesVarietes’1865page63。）hasalsorecordedseveralcaseswithplantsderivedfromseed，offlowersrevertingbystripesorblotchestotheirprimitivecolours:hestatesthatinallsuchcasesawhiteorpale—colouredvarietymustfirstbeformed，and，whenthisispropagatedforalengthoftimebyseed，stripedseedlingsoccasionallymaketheirappearance;andthesecanafterwardsbycarebemultipliedbyseed。
Thestripesandsegmentsjustreferredtoarenotdue，asfarasisknown，toreversiontocharactersderivedfromacross，buttocharacterslostbyvariation。Thesecases，however，asNaudin（13/22。’NouvellesArchivesduMuseum’tome1page25。Alex。Braun（inhis’Rejuvenescence’RaySoc。1853
page315）apparentlyholdsasimilaropinion。）insistsinhisdiscussionondisjunctionofcharacter，arecloselyanalogouswiththosegivenintheeleventhchapter，inwhichcrossedplantshavebeenknowntoproducehalf—and—
halforstripedflowersandfruit，ordistinctkindsofflowersonthesamerootresemblingthetwoparent—forms。Manypiebaldanimalsprobablycomeunderthissamehead。Suchcases，asweshallseeinthechapteronCrossing，apparentlyresultfromcertaincharactersnotreadilyblendingtogether，and，asaconsequenceofthisincapacityforfusion，theoffspringeitherperfectlyresembleoneoftheirtwoparents，orresembleoneparentinonepart，andtheotherparentinanotherpart;orwhilstyoungareintermediateincharacter，butwithadvancingagerevertwhollyorbysegmentstoeitherparent—form，ortoboth。Thus，youngtreesoftheCytisusadamiareintermediateinfoliageandflowersbetweenthetwoparent—forms;butwhenolderthebudscontinuallyreverteitherpartiallyorwhollytobothforms。ThecasesgivenintheeleventhchapteronthechangeswhichoccurredduringgrowthincrossedplantsofTropaeolum，Cereus，Datura，andLathyrusareallanalogous。As，however，theseplantsarehybridsofthefirstgeneration，andastheirbudsafteratimecometoresembletheirparentsandnottheirgrandparents，thesecasesdonotatfirstappeartocomeunderthelawofreversionintheordinarysenseoftheword;nevertheless，asthechangeiseffectedthroughasuccessionofbud—generationsonthesameplant，theymaybethusincluded。
Analogousfactshavebeenobservedintheanimalkingdom，andaremoreremarkable，astheyoccurinthesameindividualinthestrictestsense，andnotaswithplantsthroughasuccessionofbud—generations。Withanimalstheactofreversion，ifitcanbesodesignated，doesnotpassoveratruegeneration，butmerelyovertheearlystagesofgrowthinthesameindividual。
Forinstance，Icrossedseveralwhitehenswithablackcock，andmanyofthechickenswere，duringthefirstyear，perfectlywhite，butacquiredduringthesecondyearblackfeathers;ontheotherhand，someofthechickenswhichwereatfirstblack，becameduringthesecondyearpiebaldwithwhite。Agreatbreeder（13/23。Mr。Teebayin’ThePoultryBook’byMr。Tegetmeier1866page72。）says，thataPencilledBrahmahenwhichhasanyofthebloodoftheLightBrahmainher，will"occasionallyproduceapulletwellpencilledduringthefirstyear，butshewillmostlikelymoultbrownontheshouldersandbecomequiteunlikeheroriginalcoloursinthesecondyear。"ThesamethingoccurswithlightBrahmasifofimpureblood。Ihaveobservedexactlysimilarcaseswiththecrossedoffspringfromdifferentlycolouredpigeons。Buthereisamoreremarkablefact:Icrossedaturbit，whichhasafrillformedbythefeathersbeingreversedonitsbreast，withatrumpeter;andoneoftheyoungpigeonsthusraisedatfirstshowednotatraceofthefrill，but，aftermoultingthrice，asmallyetunmistakablydistinctfrillappearedonitsbreast。AccordingtoGirou（13/24。QuotedbyHofacker’UeberdieEigenschaften’etc。s。98。）calvesproducedfromaredcowbyablackbull，orfromablackcowbyaredbull，arenotrarelybornred，andsubsequentlybecomeblack。Ipossessadog，thedaughterofawhiteterrierbyafox—
colouredbulldog;asapuppyshewasquitewhite，butwhenaboutsixmonthsoldablackspotappearedonhernose，andbrownspotsonherears。Whenalittleoldershewasbadlywoundedontheback，andthehairwhichgrewonthecicatrixwasofabrowncolour，apparentlyderivedfromherfather。Thisisthemoreremarkable，aswithmostanimalshavingcolouredhair，thatwhichgrowsonawoundedsurfaceiswhite。
Intheforegoingcases，thecharacterswhichwithadvancingagereappeared，werepresentintheimmediatelyprecedinggenerations;butcharacterssometimesreappearinthesamemannerafteramuchlongerintervaloftime。
ThusthecalvesofahornlessraceofcattlewhichoriginatedinCorrientes，thoughatfirstquitehornless，astheybecomeadultsometimesacquiresmall，crooked，andloosehorns;andtheseinsucceedingyearsoccasionallybecomeattachedtotheskull。（13/25。Azara’EssaisHist。Nat。deParaguay’tome2
1801page372。）WhiteandblackBantams，bothofwhichgenerallybreedtrue，sometimesassumeastheygrowoldasaffronorredplumage。Forinstance，afirst—rateblackbantamhasbeendescribed，whichduringthreeseasonswasperfectlyblack，butthenannuallybecamemoreandmorered;anditdeservesnoticethatthistendencytochange，wheneveritoccursinabantam，"isalmostcertaintoprovehereditary。"（13/26。ThesefactsaregivenonthehighauthorityofMr。Hewittin’ThePoultryBook’byMr。Tegetmeier1866page248。）Thecuckooorblue—mottledDorkingcock，whenold，isliabletoacquireyellowororangehacklesinplaceofhisproperbluish—greyhackles。（13/27。
’ThePoultryBook’byTegetmeier1866page97。）NowasGallusbankivaiscolouredredandorange，andasDorkingfowlsandbantamsaredescendedfromthisspecies，wecanhardlydoubtthatthechangewhichoccasionallyoccursintheplumageofthesebirdsastheirageadvances，resultsfromatendencyintheindividualtoreverttotheprimitivetype。
CROSSINGASADIRECTCAUSEOFREVERSION。
Ithaslongbeennotoriousthathybridsandmongrelsoftenreverttobothortooneoftheirparent—forms，afteranintervaloffromtwotosevenoreight，or，accordingtosomeauthorities，evenagreaternumberofgenerations。Butthattheactofcrossinginitselfgivesanimpulsetowardsreversion，asshownbythereappearanceoflong—lostcharacters，hasnever，Ibelieve，beenhithertoproved。Theproofliesincertainpeculiarities，whichdonotcharacterisetheimmediateparents，andthereforecannothavebeenderivedfromthem，frequentlyappearingintheoffspringoftwobreedswhencrossed，whichpeculiaritiesneverappear，orappearwithextremerarity，inthesesamebreeds，aslongastheyareprecludedfromcrossing。Asthisconclusionseemstomehighlycuriousandnovel，Iwillgivetheevidenceindetail。
[Myattentionwasfirstcalledtothissubject，andIwasledtomakenumerousexperiments，byMM。BoitardandCorbiehavingstatedthat，whentheycrossedcertainbreedsofpigeons，birdscolouredlikethewildC。livia，orthecommondovecote——namely，slaty—blue，withdoubleblackwing—bars，sometimeschequeredwithblack，whitelo...完整阅读请扫描二维码下载丁香书院APP免费看：