Anothercase，thoughsomewhatdifferent，maybeheregiven，asitishighlyremarkable，andisestablishedonexcellentevidence。Kolreuterminutelydescribesfivevarietiesofthecommontobacco（16/25。’ZweiteForts。’s。53
namelyNicotianamajorvulgaris;（2）perennis;（3）transylvanica;（4）asub—
var。ofthelast;（5）majorlatifol。fl。alb。）whichwerereciprocallycrossed，andtheoffspringwereintermediateincharacterandasfertileastheirparents:fromthisfactKolreuterinferredthattheyarereallyvarieties;andnoone，asfarasIcandiscover，seemstohavedoubtedthatsuchisthecase。HealsocrossedreciprocallythesefivevarietieswithN。
glutinosa，andtheyyieldedverysterilehybrids;butthoseraisedfromthevar。perennis，whetherusedasthefatherormotherplant，werenotsosterileasthehybridsfromthefourothervarieties。（16/26。KolreuterwassomuchstruckwiththisfactthathesuspectedthatalittlepollenofN。glutinosainoneofhisexperimentsmighthaveaccidentallygotmingledwiththatofvar。perennis，andthusaideditsfertilisingpower。ButwenowknowconclusivelyfromGartner（’Bastarderz。’s。34，43）thatthepollenoftwospeciesneveractsCONJOINTLYonathirdspecies;stilllesswillthepollenofadistinctspecies，mingledwithaplant’sownpollen，ifthelatterbepresentinsufficientquantity，haveanyeffect。Thesoleeffectofminglingtwokindsofpollenistoproduceinthesamecapsuleseedswhichyieldplants，sometakingaftertheoneandsomeaftertheotherparent。）Sothatthesexualcapacityofthisonevarietyhascertainlybeeninsomedegreemodified，soastoapproachinnaturethatofN。glutinosa。（16/27。Mr。Scotthasmadesomeobservationsontheabsolutesterilityofapurpleandwhiteprimrose（Primulavulgaris）whenfertilisedbypollenfromthecommonprimrose（’JournalofProc。ofLinn。Soc。’volume81864page98）;buttheseobservationsrequireconfirmation。Iraisedanumberofpurple—floweredlong—
styledseedlingsfromseedkindlysentmebyMr。Scott，and，thoughtheywereallinsomedegreesterile，theyweremuchmorefertilewithpollentakenfromthecommonprimrosethanwiththeirownpollen。Mr。Scotthaslikewisedescribedaredequal—styledcowslip（P。verisibidpage106），whichwasfoundbyhimtobehighlysterilewhencrossedwiththecommoncowslip;butthiswasnotthecasewithseveralequal—styledredseedlingsraisedbymefromhisplant。Thisvarietyofthecowslippresentstheremarkablepeculiarityofcombiningmaleorgansineveryrespectlikethoseoftheshort—styledform，withfemaleorgansresemblinginfunctionandpartlyinstructurethoseofthelong—styledform;sothatwehavethesingularanomalyofthetwoformscombinedinthesameflower。Henceitisnotsurprisingthattheseflowersshouldbespontaneouslyself—fertileinahighdegree。）
Thesefactswithrespecttoplantsshowthatinsomefewcasescertainvarietieshavehadtheirsexualpowerssofarmodified，thattheycrosstogetherlessreadilyandyieldlessseedthanothervarietiesofthesamespecies。Weshallpresentlyseethatthesexualfunctionsofmostanimalsandplantsareeminentlyliabletobeaffectedbytheconditionsoflifetowhichtheyareexposed;andhereafterweshallbrieflydiscusstheconjointbearingofthisfact，andothers，onthedifferenceinfertilitybetweencrossedvarietiesandcrossedspecies。
DOMESTICATIONELIMINATESTHETENDENCYTOSTERILITYWHICHISGENERALWITH
SPECIESWHENCROSSED。
ThishypothesiswasfirstpropoundedbyPallas（16/28。’Act。Acad。St。
Petersburg’1780part2pages84，100。），andhasbeenadoptedbyseveralauthors。Icanfindhardlyanydirectfactsinitssupport;butunfortunatelynoonehascompared，inthecaseofeitheranimalsorplants，thefertilityofancientlydomesticatedvarieties，whencrossedwithadistinctspecies，withthatofthewildparent—specieswhensimilarlycrossed。Noonehascompared，forinstance，thefertilityofGallusbankivaandofthedomesticatedfowl，whencrossedwithadistinctspeciesofGallusorPhasianus;andtheexperimentwouldinallcasesbesurroundedbymanydifficulties。DureaudelaMalle，whohassocloselystudiedclassicalliterature，states（16/29。
’AnnalesdesSc。Nat。’tome211stseriespage61。）thatinthetimeoftheRomansthecommonmulewasproducedwithmoredifficultythanatthepresentday;butwhetherthisstatementmaybetrustedIknownot。Amuchmoreimportant，thoughsomewhatdifferent，caseisgivenbyM。Groenland（16/30。
’Bull。Bot。Soc。deFrance’December27，1861tome8page612。），namely，thatplants，knownfromtheirintermediatecharacterandsterilitytobehybridsbetweenAegilopsandwheat，haveperpetuatedthemselvesunderculturesince1857，WITHARAPIDBUTVARYINGINCREASEOFFERTILITYINEACHGENERATION。Inthefourthgenerationtheplants，stillretainingtheirintermediatecharacter，hadbecomeasfertileascommoncultivatedwheat。
TheindirectevidenceinfavourofthePallasiandoctrineappearstometobeextremelystrong。IntheearlierchaptersIhaveshownthatourvariousbreedsofthedogaredescendedfromseveralwildspecies;andthisprobablyisthecasewithsheep。TherecanbenodoubtthattheZebuorhumpedIndianoxbelongstoadistinctspeciesfromEuropeancattle:thelatter，moreover，aredescendedfromtwoforms，whichmaybecalledeitherspeciesorraces。Wehavegoodevidencethatourdomesticatedpigsbelongtoatleasttwospecifictypes，S。scrofaandindicus。Nowawidelyextendedanalogyleadstothebeliefthatiftheseseveralalliedspecies，whenfirstreclaimed，hadbeencrossed，theywouldhaveexhibited，bothintheirfirstunionsandintheirhybridoffspring，somedegreeofsterility。Nevertheless，theseveraldomesticatedracesdescendedfromthemarenowall，asfarascanbeascertained，perfectlyfertiletogether。Ifthisreasoningbetrustworthy，anditisapparentlysound，wemustadmitthePallasiandoctrinethatlong—
continueddomesticationtendstoeliminatethatsterilitywhichisnaturaltospecieswhencrossedintheiraboriginalstate。
ONINCREASEDFERTILITYFROMDOMESTICATIONANDCULTIVATION。
Increasedfertilityfromdomestication，withoutanyreferencetocrossing，maybeherebrieflyconsidered。Thissubjectbearsindirectlyontwoorthreepointsconnectedwiththemodificationoforganicbeings。AsBuffonlongagoremarked（16/31。QuotedbyIsid。GeoffroySt。Hilaire’Hist。NaturelleGenerale’tome3page476。SincethisMS。hasbeensenttopressafulldiscussiononthepresentsubjecthasappearedinMr。HerbertSpencer’s’PrinciplesofBiology’volume21867page457etseq。），domesticanimalsbreedoftenerintheyearandproducemoreyoungatabirththanwildanimalsofthesamespecies;they，also，sometimesbreedatanearlierage。Thecasewouldhardlyhavedeservedfurthernotice，hadnotsomeauthorslatelyattemptedtoshowthatfertilityincreasesanddecreasesinaninverseratiowiththeamountoffood。Thisstrangedoctrinehasapparentlyarisenfromindividualanimalswhensuppliedwithaninordinatequantityoffood，andfromplantsofmanykindswhengrownonexcessivelyrichsoil，asonadunghill，becomingsterile:buttothislatterpointIshallhaveoccasionpresentlytoreturn。Withhardlyanexception，ourdomesticatedanimals，whichhavebeenlonghabituatedtoaregularandcopioussupplyoffood，withoutthelabourofsearchingforit，aremorefertilethanthecorrespondingwildanimals。Itisnotorioushowfrequentlycatsanddogsbreed，andhowmanyyoungtheyproduceatabirth。Thewildrabbitissaidgenerallytobreedfourtimesyearly，andtoproduceeachtimeatmostsixyoung;thetamerabbitbreedssixorseventimesyearly，producingeachtimefromfourtoelevenyoung;andMr。HarrisonWeirtellsmeofacaseofeighteenyounghavingbeenproducedatabirth，allofwhichsurvived。Theferret，thoughgenerallysocloselyconfined，ismoreprolificthanitssupposedwildprototype。Thewildsowisremarkablyprolific;sheoftenbreedstwiceintheyear，andbearsfromfourtoeightandsometimeseventwelveyoung;butthedomesticsowregularlybreedstwiceayear，andwouldbreedoftenerifpermitted;andasowthatproduceslessthaneightatabirth"isworthlittle，andthesoonersheisfattenedforthebutcherthebetter。"Theamountoffoodaffectsthefertilityofthesameindividual:thussheep，whichonmountainsneverproducemorethanonelambatabirth，whenbroughtdowntolowlandpasturesfrequentlybeartwins。Thisdifferenceapparentlyisnotduetothecoldofthehigherland，forsheepandotherdomesticanimalsaresaidtobeextremelyprolificinLapland。Hardliving，also，retardstheperiodatwhichanimalsconceive;forithasbeenfounddisadvantageousinthenorthernislandsofScotlandtoallowcowstobearcalvesbeforetheyarefouryearsold。（16/32。Forcatsanddogsetc。seeBellingeriin’Annal。desSc。Nat。’2ndseries，Zoolog。tome12page155。ForferretsBechstein’NaturgeschichteDeutschlands’b。11801s。786，795。Forrabbitsdittos。1123，1131;andBronn’GeschichtederNatur。’b。2s。99。Formountainsheepdittos。102。Forthefertilityofthewildsow，seeBechstein’Naturgesch。Deutschlands’b。11801s。534;forthedomesticpigSidney’seditionof’YouattonthePig’1860page62。WithrespecttoLaplandseeAcerbi’TravelstotheNorthCape’Englishtranslationvolume2page222。
AbouttheHighlandcowssee’HoggonSheep’page263。）
[Birdsofferstillbetterevidenceofincreasedfertilityfromdomestication:
thehenofthewildGallusbankivalaysfromsixtoteneggs，anumberwhichwouldbethoughtnothingofwiththedomestichen。Thewildducklaysfromfivetoteneggs;thetameoneinthecourseoftheyearfromeightytoonehundred。Thewildgrey—laggooselaysfromfivetoeighteggs;thetamefromthirteentoeighteen，andshelaysasecondtime;asMr。Dixonhasremarked，"high—feeding，care，andmoderatewarmthinduceahabitofprolificacywhichbecomesinsomemeasurehereditary。"Whetherthesemi—domesticateddovecotepigeonismorefertilethanthewildrock—pigeon，C。livia，Iknownot;butthemorethoroughlydomesticatedbreedsarenearlytwiceasfertileasdovecotes:thelatter，however，whencagedandhighlyfed，becomeequallyfertilewithhousepigeons。IhearfromJudgeCatonthatthewildturkeyintheUnitedStatesdoesnotbreedwhenayearold，asthedomesticatedturkeysthereinvariablydo。Thepeahenaloneofdomesticatedbirdsisrathermorefertile，accordingtosomeaccounts，whenwildinitsnativeIndianhome，thaninEuropewhenexposedtoourmuchcolderclimate。（16/33。FortheeggsofGallusbankivaseeBlythin’AnnalsandMag。ofNat。Hist。’2ndseriesvolume11848page456。ForwildandtameducksMacgillivray’BritishBirds’volume5
page37;and’DieEnten’s。87。ForwildgeeseL。Lloyd’ScandinavianAdventures’volume21854page413;andfortamegeese’OrnamentalPoultry’byRev。E。S。Dixonpage139。OnthebreedingofPigeonsPistor’DasGanzederTaubenzucht’1831s。48;andBoitardandCorbie’LesPigeons’page158。Withrespecttopeacocks，accordingtoTemminck’Hist。Nat。Gen。desPigeons’etc。
1813tome2page41，thehenlaysinIndiaevenasmanyastwentyeggs;butaccordingtoJerdonandanotherwriterquotedinTegetmeier’PoultryBook’
1866pages280，282，shetherelaysonlyfromfourtonineorteneggs:inEnglandsheissaid，inthe’PoultryBook’tolayfiveorsix，butanotherwritersaysfromeighttotwelveeggs。）
Withrespecttoplants，noonewouldexpectwheattotillermore，andeacheartoproducemoregrain，inpoorthaninrichsoil;ortogetinpoorsoilaheavycropofpeasorbeans。Seedsvarysomuchinnumberthatitisdifficulttoestimatethem;butoncomparingbedsofcarrotsinanurserygardenwithwildplants，theformerseemedtoproduceabouttwiceasmuchseed。CultivatedcabbagesyieldedthriceasmanypodsbymeasureaswildcabbagesfromtherocksofSouthWales。Theexcessofberriesproducedbythecultivatedasparagusincomparisonwiththewildplantisenormous。Nodoubtmanyhighlycultivatedplants，suchaspears，pineapples，bananas，sugar—cane，etc。，arenearlyorquitesterile;andIaminclinedtoattributethissterilitytoexcessoffoodandtootherunnaturalconditions;buttothissubjectIshallrecur。]
Insomecases，aswiththepig，rabbit，etc。，andwiththoseplantswhicharevaluedfortheirseed，thedirectselectionofthemorefertileindividualshasprobablymuchincreasedtheirfertility;andinallcasesthismayhaveoccurredindirectly，fromthebetterchanceofsomeofthenumerousoffspringfromthemorefertileindividualshavingbeenpreserved。Butwithcats，ferrets，anddogs，andwithplantslikecarrots，cabbages，andasparagus，whicharenotvaluedfortheirprolificacy，selectioncanhaveplayedonlyasubordinatepart;andtheirincreasedfertilitymustbeattributedtothemorefavourableconditionsoflifeunderwhichtheyhavelongexisted。
CHAPTER2。XVII。
ONTHEGOODEFFECTSOFCROSSING，ANDONTHEEVILEFFECTSOFCLOSE
INTERBREEDING。
DEFINITIONOFCLOSEINTERBREEDING。
AUGMENTATIONOFMORBIDTENDENCIES。
GENERALEVIDENCEOFTHEGOODEFFECTSDERIVEDFROMCROSSING，ANDONTHEEVIL
EFFECTSFROMCLOSEINTERBREEDING。
CATTLE，CLOSELYINTERBRED;HALF—WILDCATTLELONGKEPTINTHESAMEPARKS。
SHEEP。
FALLOW—DEER。
DOGS，RABBITS，PIGS。
MAN，ORIGINOFHISABHORRENCEOFINCESTUOUSMARRIAGES。
FOWLS。
PIGEONS。
HIVE—BEES。
PLANTS，GENERALCONSIDERATIONSONTHEBENEFITSDERIVEDFROMCROSSING。
MELONS，FRUIT—TREES，PEAS，CABBAGES，WHEAT，ANDFOREST—TREES。
ONTHEINCREASEDSIZEOFHYBRIDPLANTS，NOTEXCLUSIVELYDUETOTHEIR
STERILITY。
ONCERTAINPLANTSWHICHEITHERNORMALLYORABNORMALLYARESELF—IMPOTENT，BUT
AREFERTILE，BOTHONTHEMALEANDFEMALESIDE，WHENCROSSEDWITHDISTINCT
INDIVIDUALSEITHEROFTHESAMEORANOTHERSPECIES。
CONCLUSION。
Thegaininconstitutionalvigour，derivedfromanoccasionalcrossbetweenindividualsofthesamevariety，butbelongingtodistinctfamilies，orbetweendistinctvarieties，hasnotbeensolargelyorsofrequentlydiscussed，ashavetheevileffectsoftoocloseinterbreeding。Buttheformerpointisthemoreimportantofthetwo，inasmuchastheevidenceismoredecisive。Theevilresultsfromcloseinterbreedingaredifficulttodetect，fortheyaccumulateslowly，anddiffermuchindegreewithdifferentspecies;
whilstthegoodeffectswhichalmostinvariablyfollowacrossarefromthefirstmanifest。Itshould，however，beclearlyunderstoodthattheadvantageofcloseinterbreeding，asfarastheretentionofcharacterisconcerned，isindisputable，andoftenoutweighstheevilofaslightlossofconstitutionalvigour。Inrelationtothesubjectofdomestication，thewholequestionisofsomeimportance，astoocloseinterbreedinginterfereswiththeimprovementofoldraces。ItisimportantasindirectlybearingonHybridism;andpossiblyontheextinctionofspecies，whenanyformhasbecomesorarethatonlyafewindividualsremainwithinaconfinedarea。Itbearsinanimportantmannerontheinfluenceoffreeintercrossing，inobliteratingindividualdifferences，andthusgivinguniformityofcharactertotheindividualsofthesameraceorspecies;forifadditionalvigourandfertilitybethusgained，thecrossedoffspringwillmultiplyandprevail，andtheultimateresultwillbefargreaterthanotherwisewouldhaveoccurred。Lastly，thequestionisofhighinterest，asbearin...完整阅读请扫描二维码下载丁香书院APP免费看：