forinstance，somekindsofmossesandlichenshaveneverbeenseentofructifyinFrance。
Someoftheseendemicandnaturalisedplantsareprobablyrenderedsterilefromexcessivemultiplicationbybuds，andtheirconsequentincapacitytoproduceandnourishseed。Butthesterilityofothersmoreprobablydependsonthepeculiarconditionsunderwhichtheylive，asinthecaseoftheivyinthenorthernpartofEurope，andofthetreesintheswampsoftheUnitedStates;yettheseplantsmustbeinsomerespectseminentlywelladaptedforthestationswhichtheyoccupy，fortheyholdtheirplacesagainstahostofcompetitors。]
Finally，thehighdegreeofsterilitywhichoftenaccompaniesthedoublingofflowers，oranexcessivedevelopmentoffruit，seldomsupervenesatonce。Anincipienttendencyisobserved，andcontinuedselectioncompletestheresult。
Theviewwhichseemsthemostprobable，andwhichconnectstogetheralltheforegoingfactsandbringsthemwithinourpresentsubject，is，thatchangedandunnaturalconditionsoflifefirstgiveatendencytosterility;andinconsequenceofthis，theorgansofreproductionbeingnolongerablefullytoperformtheirproperfunctions，asupplyoforganisedmatter，notrequiredforthedevelopmentoftheseed，flowseitherintotheseorgansandrendersthemfoliaceous，orintothefruit，stems，tubers，etc。，increasingtheirsizeandsucculency。Butitisprobablethatthereexists，independentlyofanyincipientsterility，anantagonismbetweenthetwoformsofreproduction，namely，byseedandbuds，wheneitheriscarriedtoanextremedegree。Thatincipientsterilityplaysanimportantpartinthedoublingofflowers，andintheothercasesjustspecified，Iinferchieflyfromthefollowingfacts。Whenfertilityislostfromawhollydifferentcause，namely，fromhybridism，thereisastrongtendency，asGartner（18/118。’Bastarderzeugung’s。565。Kolreuter’DritteFortsetzung’s。73，87，119）alsoshowsthatwhentwospecies，onesingleandtheotherdouble，arecrossed，thehybridsareapttobeextremelydouble。）affirms，forflowerstobecomedouble，andthistendencyisinherited。Moreover，itisnotoriousthatwithhybridsthemaleorgansbecomesterilebeforethefemaleorgans，andwithdoubleflowersthestamensfirstbecomefoliaceous。Thislatterfactiswellshownbythemaleflowersofdioeciousplants，which，accordingtoGallesio（18/119。’TeoriadellaRiproduzioneVeg。’1816page73。）firstbecomedouble。Again，Gartner（18/120。
’Bastarderzeugung’s。573。）ofteninsiststhattheflowersofevenutterlysterilehybrids，whichdonotproduceanyseed，generallyyieldperfectcapsulesorfruit，——afactwhichhaslikewisebeenrepeatedlyobservedbyNaudinwiththeCucurbitaceae;sothattheproductionoffruitbyplantsrenderedsterilethroughanycauseisintelligible。Kolreuterhasalsoexpressedhisunboundedastonishmentatthesizeanddevelopmentofthetubersincertainhybrids;andallexperimentalists（18/121。Ibids。527。）haveremarkedonthestrongtendencyinhybridstoincreasebyroots，runners，andsuckers。Seeingthathybridplants，whichfromtheirnaturearemoreorlesssterile，thustendtoproducedoubleflowers;thattheyhavethepartsincludingtheseed，thatisthefruit，perfectlydeveloped，evenwhencontainingnoseed;thattheysometimesyieldgiganticroots;thattheyalmostinvariablytendtoincreaselargelybysuckersandothersuchmeans;——seeingthis，andknowing，fromthemanyfactsgivenintheearlierpartsofthischapter，thatalmostallorganicbeingswhenexposedtounnaturalconditionstendtobecomemoreorlesssterile，itseemsmuchthemostprobableviewthatwithcultivatedplantssterilityistheexcitingcause，anddoubleflowers，richseedlessfruit，andinsomecaseslargely—developedorgansofvegetation，etc。，aretheindirectresults——theseresultshavingbeeninmostcaseslargelyincreasedthroughcontinuedselectionbyman。
CHAPTER2。XIX。
SUMMARYOFTHEFOURLASTCHAPTERS，WITHREMARKSONHYBRIDISM。
ONTHEEFFECTSOFCROSSING。
THEINFLUENCEOFDOMESTICATIONONFERTILITY。
CLOSEINTERBREEDING。
GOODANDEVILRESULTSFROMCHANGEDCONDITIONSOFLIFE。
VARIETIESWHENCROSSEDNOTINVARIABLYFERTILE。
ONTHEDIFFERENCEINFERTILITYBETWEENCROSSEDSPECIESANDVARIETIES。
CONCLUSIONSWITHRESPECTTOHYBRIDISM。
LIGHTTHROWNONHYBRIDISMBYTHEILLEGITIMATEPROGENYOFHETEROSTYLEDPLANTS。
STERILITYOFCROSSEDSPECIESDUETODIFFERENCESCONFINEDTOTHEREPRODUCTIVE
SYSTEM。
NOTACCUMULATEDTHROUGHNATURALSELECTION。
REASONSWHYDOMESTICVARIETIESARENOTMUTUALLYSTERILE。
TOOMUCHSTRESSHASBEENLAIDONTHEDIFFERENCEINFERTILITYBETWEENCROSSED
SPECIESANDCROSSEDVARIETIES。
CONCLUSION。
Itwasshowninthefifteenthchapterthatwhenindividualsofthesamevariety，orevenofadistinctvariety，areallowedfreelytointercross，uniformityofcharacterisultimatelyacquired。Somefewcharacters，however，areincapableoffusion，buttheseareunimportant，astheyareoftenofasemi—monstrousnature，andhavesuddenlyappeared。Hence，topreserveourdomesticatedbreedstrue，ortoimprovethembymethodicalselection，itisobviouslynecessarythattheyshouldbekeptseparate。Nevertheless，awholebodyofindividualsmaybeslowlymodified，throughunconsciousselection，asweshallseeinafuturechapter，withoutseparatingthemintodistinctlots。
Domesticraceshaveoftenbeenintentionallymodifiedbyoneortwocrosses，madewithsomealliedrace，andoccasionallyevenbyrepeatedcrosseswithverydistinctraces;butinalmostallsuchcases，long—continuedandcarefulselectionhasbeenabsolutelynecessary，owingtotheexcessivevariabilityofthecrossedoffspring，duetotheprincipleofreversion。Inafewinstances，however，mongrelshaveretainedauniformcharacterfromtheirfirstproduction。
Whentwovarietiesareallowedtocrossfreely，andoneismuchmorenumerousthantheother，theformerwillultimatelyabsorbthelatter。Shouldbothvarietiesexistinnearlyequalnumbers，itisprobablethataconsiderableperiodwouldelapsebeforetheacquirementofauniformcharacter;andthecharacterultimatelyacquiredwouldlargelydependonprepotencyoftransmissionandontheconditionsoflife;forthenatureoftheseconditionswouldgenerallyfavouronevarietymorethananother，sothatakindofnaturalselectionwouldcomeintoplay。Unlessthecrossedoffspringwereslaughteredbymanwithouttheleastdiscrimination，somedegreeofunmethodicalselectionwouldlikewisecomeintoaction。Fromtheseseveralconsiderationswemayinfer，thatwhentwoormorecloselyalliedspeciesfirstcameintothepossessionofthesametribe，theircrossingwillnothaveinfluenced，insogreatadegreeashasoftenbeensupposed，thecharacteroftheoffspringinfuturetimes;althoughinsomecasesitprobablyhashadaconsiderableeffect。
Domestication，asageneralrule，increasestheprolificnessofanimalsandplants。Iteliminatesthetendencytosterilitywhichiscommontospecieswhenfirsttakenfromastateofnatureandcrossed。Onthislatterheadwehavenodirectevidence;butasourracesofdogs，cattle，pigsetc。，arealmostcertainlydescendedfromaboriginallydistinctstocks，andastheseracesarenowfullyfertiletogether，oratleastincomparablymorefertilethanmostspecieswhencrossed，wemaywithentireconfidenceacceptthisconclusion。
Abundantevidencehasbeengiventhatcrossingaddstothesize，vigour，andfertilityoftheoffspring。Thisholdsgoodwhentherehasbeennopreviouscloseinterbreeding。Itappliestotheindividualsofthesamevarietybutbelongingtodifferentfamilies，todistinctvarieties，sub—species，andeventospecies。Inthelattercase，thoughsizeisgained，fertilityislost;buttheincreasedsize，vigour，andhardinessofmanyhybridscannotbeaccountedforsolelyontheprincipleofcompensationfromtheinactionofthereproductivesystem。Certainplantswhilstgrowingundertheirnaturalconditions，otherswhencultivated，andothersofhybridorigin，arecompletelyself—impotent，thoughperfectlyhealthy;andsuchplantscanbestimulatedtofertilityonlybybeingcrossedwithotherindividualsofthesameorofadistinctspecies。
Ontheotherhand，long—continuedcloseinterbreedingbetweenthenearestrelationsdiminishestheconstitutionalvigour，size，andfertilityoftheoffspring;andoccasionallyleadstomalformations，butnotnecessarilytogeneraldeteriorationofformorstructure。Thisfailureoffertilityshowsthattheevilresultsofinterbreedingareindependentoftheaugmentationofmorbidtendenciescommontobothparents，thoughthisaugmentationnodoubtisoftenhighlyinjurious。Ourbeliefthatevilfollowsfromcloseinterbreedingreststoacertainextentontheexperienceofpracticalbreeders，especiallyofthosewhohaverearedmanyanimalsofquicklypropagatingkinds;butitlikewiserestsonseveralcarefullyrecordedexperiments。Withsomeanimalscloseinterbreedingmaybecarriedonforalongperiodwithimpunitybytheselectionofthemostvigorousandhealthyindividuals;butsoonerorlaterevilfollows。Theevil，however，comesonsoslowlyandgraduallythatiteasilyescapesobservation，butcanberecognisedbythealmostinstantaneousmannerinwhichsize，constitutionalvigour，andfertilityareregainedwhenanimalsthathavelongbeeninterbredarecrossedwithadistinctfamily。
Thesetwogreatclassesoffacts，namely，thegoodderivedfromcrossing，andtheevilfromcloseinterbreeding，withtheconsiderationoftheinnumerableadaptationsthroughoutnatureforcompelling，orfavouring，oratleastpermitting，theoccasionalunionofdistinctindividuals，takentogether，leadtotheconclusionthatitisalawofnaturethatorganicbeingsshallnotfertilisethemselvesforperpetuity。Thislawwasfirstplainlyhintedatin1799，withrespecttoplants，byAndrewKnight（19/1。’TransactionsPhil。
Soc。’1799page202。ForKolreutersee’Mem。del’Acad。deSt。—Petersbourg’
tome31809published1811page197。InreadingC。K。Sprengel’sremarkablework，’DasentdeckteGeheimniss’etc。1793，itiscurioustoobservehowoftenthiswonderfullyacuteobserverfailedtounderstandthefullmeaningofthestructureoftheflowerswhichhehassowelldescribed，fromnotalwayshavingbeforehismindthekeytotheproblem，namely，thegoodderivedfromthecrossingofdistinctindividualplants。）and，notlongafterwards，thatsagaciousobserverKolreuter，aftershowinghowwelltheMalvaceaeareadaptedforcrossing，asks，"anidaliquidinrecessuhabeat，quodhujuscemodifloresnunquampropriosuopulvere，sedsempereoaliarumsuspecieiimpregnentur，meritoquaritur？Certenaturanilfacitfrustra。"AlthoughwemaydemurtoKolreuter’ssayingthatnaturedoesnothinginvain，seeinghowmanyrudimentaryanduselessorgansthereare，yetundoubtedlytheargumentfromtheinnumerablecontrivances，whichfavourcrossing，isofthegreatestweight。Themostimportantresultofthislawisthatitleadstouniformityofcharacterintheindividualsofthesamespecies。Inthecaseofcertainhermaphrodites，whichprobablyintercrossonlyatlongintervalsoftime，andwithunisexualanimalsinhabitingsomewhatseparatedlocalities，whichcanonlyoccasionallycomeintocontactandpair，thegreatervigourandfertilityofthecrossedoffspringwillultimatelytendtogiveuniformityofcharacter。
Butwhenwegobeyondthelimitsofthesamespecies，freeintercrossingisbarredbythelawofsterility。
Insearchingforfactswhichmightthrowlightonthecauseofthegoodeffectsfromcrossing，andoftheevileffectsfromcloseinterbreeding，wehaveseenthat，ontheonehand，itisawidelyprevalentandancientbelief，thatanimalsandplantsprofitfromslightchangesintheirconditionoflife;
anditwouldappearthatthegerm，inasomewhatanalogousmanner，ismoreeffectuallystimulatedbythemaleelement，whentakenfromadistinctindividual，andthereforeslightlymodifiedinnature，thanwhentakenfromamalehavingthesameidenticalconstitution。Ontheotherhand，numerousfactshavebeengiven，showingthatwhenanimalsarefirstsubjectedtocaptivity，evenintheirnativeland，andalthoughallowedmuchliberty，theirreproductivefunctionsareoftengreatlyimpairedorquiteannulled。Somegroupsofanimalsaremoreaffectedthanothers，butwithapparentlycapriciousexceptionsineverygroup。Someanimalsneverorrarelycoupleunderconfinement;somecouplefreely，butneverorrarelyconceive。Thesecondarymalecharacters，thematernalfunctionsandinstincts，areoccasionallyaffected。Withplants，whenfirstsubjectedtocultivation，analogousfactshavebeenobserved。Weprobablyoweourdoubleflowers，richseedlessfruits，andinsomecasesgreatlydevelopedtubers，etc。，toincipientsterilityoftheabovenaturecombinedwithacopioussupplyofnutriment。Animalswhichhavelongbeendomesticated，andplantswhichhavelongbeencultivated，cangenerallywithstand，withunimpairedfertility，greatchangesintheirconditionsoflife;thoughbotharesometimesslightlyaffected。Withanimalsthesomewhatrarecapacityofbreedingfreelyunderconfinement，togetherwiththeirutility，mainlydeterminethekindswhichhavebeendomesticated。
Wecaninnocasepreciselysaywhatisthecauseofthediminishedfertilityofananimalwhenfirstcaptured，orofaplantwhenfirstcultivated;wecanonlyinferthatitiscausedbyachangeofsomekindinthenaturalconditionsoflife。Theremarkablesusceptibilityofthereproductivesystemtosuchchanges，——asusceptibilitynotcommontoanyotherorgan，——apparentlyhasanimportantbearingonVariability，asweshallseeinafuturechapter。
Itisimpossiblenottobestruckwiththedoubleparallelismbetweenthetwoclassesoffactsjustalludedto。Ontheonehand，slightchangesintheconditionsoflife，andcrossesbetweenslightlymodifiedformsorvarieties，arebeneficialasfarasprolificnessandconstitutionalvigourareconcerned。
Ontheotherhand，changesintheconditionsgreaterindegree，orofadifferentnature，andcrossesbetweenformswhichhavebeenslowlyandgreatlymodifiedbynaturalmeans，——inotherwords，betweenspecies，——arehighlyinjurious，asfarasthereproductivesystemisconcerned，andinsomefewinstancesasfarasconstitutionalvigourisconcerned。Canthisparallelismbeaccidental？Doesitnotratherindicatesomerealbondofconnection？Asafiregoesoutunlessitbestirredup，sothevitalforcesarealwaystending，accordingtoMr。HerbertSpencer，toastateofequilibrium，unlessdisturbedandrenovatedthroughtheactionofotherforces。
Insomefewcasesvarietiestendtokeepdistinct，bybreedingatdifferentseasons，bygreatdifferenceinsize，orbysexualpreference。Butthecrossingofvarieties，farfromdiminishing，generallyaddstothefertilityofthefirstunionandofthemongreloffspring。Whetherallthemorewidelydistinctdomesticvarietiesareinvariablyquitefertilewhencrossed，wedonotpositivelyknow;muchtimeandtroublewouldberequisiteforthenecessaryexperiments，andmanydifficultiesoccur，suchasthedescentofthevariousracesfromaboriginallydistinctspecies，andthedoubtswhethercertainformsoughttoberankedasspeciesorvarieties。Nevertheless，thewideexperienceofpracticalbreedersprovesthatthegreatmajorityofvarieties，evenifsomeshouldhereafterprovenottobeindefinitelyfertileinterse，arefarmorefertilewhencrossed，t...完整阅读请扫描二维码下载丁香书院APP免费看：