Withextremelyfewexceptions，allanimalsandplantswhichhavebeenlongdomesticatedhavevariedgreatly。Itmattersnotunderwhatclimate，orforwhatpurposetheyarekept，whetherasfoodformanorbeast，fordraughtorhunting，forclothingormerepleasure，——underallthesecircumstancesraceshavebeenproducedwhichdiffermorefromoneanotherthandotheformswhichinastateofnaturearerankedasdifferentspecies。Whycertainanimalsandplantshavevariedmoreunderdomesticationthanotherswedonotknow，anymorethanwhysomearerenderedmoresterilethanothersunderchangedconditionsoflife。Butwehavetojudgeoftheamountofvariationwhichourdomesticproductionshaveundergone，chieflybythenumberandamountofdifferencebetweentheraceswhichhavebeenformed，andwecanoftenclearlyseewhymanyanddistinctraceshavenotbeenformed，namely，becauseslightsuccessivevariationshavenotbeensteadilyaccumulated;andsuchvariationswillneverbeaccumulatedifananimalorplantbenotcloselyobserved，muchvalued，andkeptinlargenumbers。
Thefluctuating，and，asfaraswecanjudge，never—endingvariabilityofourdomesticatedproductions，——theplasticityofalmosttheirwholeorganisation，—
—isoneofthemostimportantlessonswhichwelearnfromthenumerousdetailsgivenintheearlierchaptersofthiswork。Yetdomesticatedanimalsandplantscanhardlyhavebeenexposedtogreaterchangesintheirconditionsoflifethanhavemanynaturalspeciesduringtheincessantgeological，geographical，andclimatalchangestowhichtheworldhasbeensubject;butdomesticatedproductionswilloftenhavebeenexposedtomoresuddenchangesandtolesscontinuouslyuniformconditions。Asmanhasdomesticatedsomanyanimalsandplantsbelongingtowidelydifferentclasses，andashecertainlydidnotchoosewithpropheticinstinctthosespecieswhichwouldvarymost，wemayinferthatallnaturalspecies，ifexposedtoanalogousconditions，would，onanaverage，varytothesamedegree。Fewmenatthepresentdaywillmaintainthatanimalsandplantswerecreatedwithatendencytovary，whichlongremaineddormant，inorderthatfanciersinafteragesmightrear，forinstance，curiousbreedsofthefowl，pigeon，orcanary—bird。
Fromseveralcausesitisdifficulttojudgeoftheamountofmodificationwhichourdomesticproductionshaveundergone。Insomecasestheprimitiveparent—stockhasbecomeextinct;oritcannotberecognisedwithcertainty，owingtoitssupposeddescendantshavingbeensomuchmodified。Inothercasestwoormoreclosely—alliedforms，afterbeingdomesticated，havecrossed;andthenitisdifficulttoestimatehowmuchofthecharacterofthepresentdescendantsoughttobeattributedtovariation，andhowmuchtotheinfluenceoftheseveralparent—stocks。Butthedegreetowhichourdomesticatedbreedshavebeenmodifiedbythecrossingofdistinctspecieshasprobablybeenmuchexaggeratedbysomeauthors。Afewindividualsofoneformwouldseldompermanentlyaffectanotherformexistingingreaternumbers;for，withoutcarefulselection，thestainoftheforeignbloodwouldsoonbeobliterated，andduringearlyandbarbaroustimes，whenouranimalswerefirstdomesticated，suchcarewouldseldomhavebeentaken。
Thereisgoodreasontobelieveinthecaseofthedog，ox，pig，andofsomeotheranimals，thatseveralofourracesaredescendedfromdistinctwildprototypes;neverthelessthebeliefinthemultipleoriginofourdomesticatedanimalshasbeenextendedbysomefewnaturalistsandbymanybreederstoanunauthorisedextent。Breedersrefusetolookatthewholesubjectunderasinglepointofview;Ihavehearditsaidbyaman，whomaintainedthatourfowlsweredescendedfromatleasthalf—a—dozenaboriginalspecies，thattheevidenceofthecommonoriginofpigeons，ducksandrabbits，wasofnoavailwithrespecttofowls。Breedersoverlooktheimprobabilityofmanyspecieshavingbeendomesticatedatanearlyandbarbarousperiod。Theydonotconsidertheimprobabilityofspecieshavingexistedinastateofnaturewhich，iftheyresembledourpresentdomesticbreeds，wouldhavebeenhighlyabnormalincomparisonwithalltheircongeners。Theymaintainthatcertainspecies，whichformerlyexisted，havebecomeextinct，orarenowunknown，althoughformerlyknown。Theassumptionofsomuchrecentextinctionisnodifficultyintheireyes;fortheydonotjudgeofitsprobabilitybythefacilityordifficultyoftheextinctionofotherclosely—alliedwildforms。
Lastly，theyoftenignorethewholesubjectofgeographicaldistributionascompletelyasifitweretheresultofchance。
Althoughfromthereasonsjustassigneditisoftendifficulttojudgeaccuratelyoftheamountofchangewhichourdomesticatedproductionshaveundergone，yetthiscanbeascertainedinthecasesinwhichallthebreedsareknowntobedescendedfromasinglespecies，——aswiththepigeon，duck，rabbit，andalmostcertainlywiththefowl;andbytheaidofanalogythiscanbejudgedoftoacertainextentwithdomesticatedanimalsdescendedfromseveralwildstocks。Itisimpossibletoreadthedetailsgivenintheearlierchaptersandinmanypublishedworks，ortovisitourvariousexhibitions，withoutbeingdeeplyimpressedwiththeextremevariabilityofourdomesticatedanimalsandcultivatedplants。Nopartoftheorganisationescapesthetendencytovary。Thevariationsgenerallyaffectpartsofsmallvitalorphysiologicalimportance，butsoitiswiththedifferenceswhichexistbetweenclosely—alliedspecies。Intheseunimportantcharactersthereisoftenagreaterdifferencebetweenthebreedsofthesamespeciesthanbetweenthenaturalspeciesofthesamegenus，asIsidoreGeoffroyhasshowntobethecasewithsize，andasisoftenthecasewiththecolour，texture，form，etc。，ofthehair，feathers，horns，andotherdermalappendages。
Ithasoftenbeenassertedthatimportantpartsnevervaryunderdomestication，butthisisacompleteerror。Lookattheskullofthepiginanyoneofthehighlyimprovedbreeds，withtheoccipitalcondylesandotherpartsgreatlymodified;orlookatthatoftheniataox。Or，again，intheseveralbreedsoftherabbit，observetheelongatedskull，withthedifferentlyshapedoccipitalforamen，atlas，andothercervicalvertebrae。Thewholeshapeofthebrain，togetherwiththeskull，hasbeenmodifiedinPolishfowls;inotherbreedsofthefowlthenumberofthevertebraeandtheformsofthecervicalvertebraehavebeenchanged。Incertainpigeonstheshapeofthelowerjaw，therelativelengthofthetongue，thesizeofthenostrilsandeyelids，thenumberandshapeoftheribs，theformandsizeoftheoesophagus，haveallvaried。Incertainquadrupedsthelengthoftheintestineshasbeenmuchincreasedordiminished。Withplantsweseewonderfuldifferencesinthestonesofvariousfruits。IntheCucurbitaceaeseveralhighlyimportantcharactershavevaried，suchasthesessilepositionofthestigmasontheovarium，thepositionofthecarpels，andtheprojectionoftheovariumoutofthereceptacle。Butitwouldbeuselesstorunthroughthemanyfactsgivenintheearlierchapters。
Itisnotorioushowgreatlythementaldisposition，tastes，habits，consensualmovements，loquacityorsilence，andtoneofvoicehavevariedandbeeninheritedinourdomesticatedanimals。Thedogoffersthemoststrikinginstanceofchangedmentalattributes，andthesedifferencescannotbeaccountedforbydescentfromdistinctwildtypes。
Newcharactersmayappearandoldonesdisappearatanystageofdevelopment，beinginheritedatacorrespondingstage。Weseethisinthedifferencebetweentheeggs，thedownonthechickensandthefirstplumageofthevariousbreedsofthefowl;andstillmoreplainlyinthedifferencesbetweenthecaterpillarsandcocoonsofthevariousbreedsofthesilk—moth。Thesefacts，simpleastheyappear，throwlightonthedifferencesbetweenthelarvalandadultstatesofalliednaturalspecies，andonthewholegreatsubjectofembryology。Newcharactersfirstappearinglateinlifeareapttobecomeattachedexclusivelytothatsexinwhichtheyfirstarose，ortheymaybedevelopedinamuchhigherdegreeinthisthanintheothersex;oragain，afterhavingbecomeattachedtoonesex，theymaybetransferredtotheoppositesex。Thesefacts，andmoreespeciallythecircumstancethatnewcharactersseemtobeparticularlyliable，fromsomeunknowncause，tobecomeattachedtothemalesex，haveanimportantbearingontheacquirementofsecondarysexualcharactersbyanimalsinastateofnature。
Ithassometimesbeensaidthatourdomesticracesdonotdifferinconstitutionalpeculiarities，butthiscannotbemaintained。Inourimprovedcattle，pigs，etc。，theperiodofmaturity，includingthatoftheseconddentition，hasbeenmuchhastened。Theperiodofgestationvariesmuch，andhasbeenmodifiedinafixedmannerinoneortwocases。Insomebreedsofpoultryandpigeonstheperiodatwhichthedownandthefirstplumageareacquired，differs。Thenumberofmoultsthroughwhichthelarvaeofsilk—mothspass，varies。Thetendencytofatten，toyieldmuchmilk，toproducemanyyoungoreggsatabirthorduringlife，differsindifferentbreeds。Wefinddifferentdegreesofadaptationtoclimate，anddifferenttendenciestocertaindiseases，totheattacksofparasites，andtotheactionofcertainvegetablepoisons。Withplants，adaptationtocertainsoils，thepowerofresistingfrost，theperiodoffloweringandfruiting，thedurationoflife，theperiodofsheddingtheleavesorofretainingthemthroughoutthewinter，theproportionandnatureofcertainchemicalcompoundsinthetissuesorseeds，allvary。
Thereis，however，oneimportantconstitutionaldifferencebetweendomesticracesandspecies;Irefertothesterilitywhichalmostinvariablyfollows，inagreaterorlessdegree，whenspeciesarecrossed，andtotheperfectfertilityofthemostdistinctdomesticraces，withtheexceptionofaveryfewplants，whensimilarlycrossed。Itiscertainlyamostremarkablefactthatmanyclosely—alliedspecies，whichinappearancedifferextremelylittle，shouldyieldwhencrossedonlyafewmoreorlesssterileoffspring，ornoneatall;whilstdomesticraceswhichdifferconspicuouslyfromeachotherare，whenunited，remarkablyfertile，andyieldperfectlyfertileoffspring。Butthisfactisnotinrealitysoinexplicableasitatfirstappears。Inthefirstplace，itwasclearlyshowninthenineteenthchapterthatthesterilityofcrossedspeciesdoesnotdependchieflyondifferencesintheirexternalstructureorgeneralconstitution，butondifferencesinthereproductivesystem，analogoustothosewhichcausethelessenedfertilityoftheillegitimateunionsofdimorphicandtrimorphicplants。Inthesecondplace，thePallasiandoctrine，thatspeciesafterhavingbeenlongdomesticatedlosetheirnaturaltendencytosterilitywhencrossed，hasbeenshowntobehighlyprobableoralmostcertain。Wecannotavoidthisconclusionwhenwereflectontheparentageandpresentfertilityoftheseveralbreedsofthedog，oftheIndianorhumpedandEuropeancattle，andofthetwochiefkindsofpigs。
Henceitwouldbeunreasonabletoexpectthatracesformedunderdomesticationshouldacquiresterilitywhencrossed，whilstatthesametimeweadmitthatdomesticationeliminatesthenormalsterilityofcrossedspecies。Whywithclosely—alliedspeciestheirreproductivesystemsshouldalmostinvariablyhavebeenmodifiedinsopeculiaramannerastobemutuallyincapableofactingoneachother——thoughinunequaldegreesinthetwosexes，asshownbythedifferenceinfertilitybetweenreciprocalcrossesofthesamespecies——wedonotknow，butmaywithmuchprobabilityinferthecausetobeasfollows。
Mostnaturalspecieshavebeenhabituatedtonearlyuniformconditionsoflifeforanincomparablylongertimethanhavedomesticraces;andwepositivelyknowthatchangedconditionsexertanespecialandpowerfulinfluenceonthereproductivesystem。Hencethisdifferencemaywellaccountforthedifferenceinthepowerofreproductionbetweendomesticraceswhencrossedandspecieswhencrossed。Itisprobablyinchiefpartowingtothesamecausethatdomesticracescanbesuddenlytransportedfromoneclimatetoanother，orplacedunderwidelydifferentconditions，andyetretaininmostcasestheirfertilityunimpaired;whilstamultitudeofspeciessubjectedtolesserchangesarerenderedincapableofbreeding。
Theoffspringofcrosseddomesticracesandofcrossedspeciesresembleeachotherinmostrespects，withtheoneimportantexceptionoffertility;theyoftenpartakeinthesameunequaldegreeofthecharactersoftheirparents，oneofwhichisoftenprepotentovertheother;andtheyareliabletoreversionofthesamekind。Bysuccessivecrossesonespeciesmaybemadetoabsorbcompletelyanother，andsoitnotoriouslyiswithraces。Thelatterresemblespeciesinmanyotherways。Theysometimesinherittheirnewly—
acquiredcharactersalmostorevenquiteasfirmlyasspecies。Theconditionsleadingtovariabilityandthelawsgoverningitsnatureappeartobethesameinboth。Varietiescanbeclassedingroupsundergroups，likespeciesundergenera，andtheseunderfamiliesandorders;andtheclassificationmaybeeitherartificial，——thatis，foundedonanyarbitrarycharacter，——ornatural。
Withvarietiesanaturalclassificationiscertainlyfounded，andwithspeciesisapparentlyfounded，oncommunityofdescent，togetherwiththeamountofmodificationwhichtheformshaveundergone。Thecharactersbywhichdomesticvarietiesdifferfromoneanotheraremorevariablethanthosedistinguishingspecies，thoughhardlymoresothanwithcertainpolymorphicspecies;butthisgreaterdegreeofvariabilityisnotsurprising，asvarietieshavegenerallybeenexposedwithinrecenttimestofluctuatingconditionsoflife，andaremuchmoreliabletohavebeencrossed;theyarealsoinmanycasesstillundergoing，orhaverecentlyundergone，modificationbyman’smethodicalorunconsciousselection。
Domesticvarietiesasageneralrulecertainlydifferfromoneanotherinlessimportantpartsthandospecies;andwhenimportantdifferencesoccur，theyareseldomfirmlyfixed;butthisfactisintelligible，ifweconsiderman’smethodofselection。Inthelivinganimalorplanthecannotobserveinternalmodificationsinthemoreimportantorgans;nordoesheregardthemaslongastheyarecompatiblewithhealthandlife。Whatdoesthebreedercareaboutanyslightchangeinthemolarteethofhispigs，orforanadditionalmolartoothinthedog;orforanychangeintheintestinalcanalorotherinternalorgan？
Thebreedercaresforthefleshofhiscattlebeingwellmarbledwithfat，andforanaccumulationoffatwithintheabdomenofhissheep，andthishehaseffected。Whatwouldthefloriculturistcareforanychangeinthestructureoftheovariumoroftheovules？Asimportantinternalorgansarecertainlyliabletonumerousslightvariations，andasthesewouldprobablybetransmitted，formanystrangemonstrositiesareinherited，mancouldundoubtedlyeffectacertainamountofchangeintheseorgans。Whenhehasproducedanymodificationinanimportantpart，hehasgenerallydonesounintentionally，incorrelationwithsomeotherconspicuouspart。Forinstance，hehasgivenridgesandprotuberancestotheskullsoffowls，byattendingtotheformofthecomb，ortotheplumeoffeathersonthehead。Byattendingtotheexternalformofthepouter—pigeon，hehasenormouslyincreasedthesizeoftheoesophagus，andhasaddedtothenumberoftheribs，andgiventhemgreaterbreadth。Withthecarrier—pigeon，byincreasingthroughsteadyselectionthewattlesontheuppermandible，hehasgreatlymodifiedtheformofthelowermandible;andsoinmanyothercases。Naturalspecies，ontheotherhand，havebeenmodifiedexclusivelyfortheirowngood，tofitthemforinfinitelydiversifiedconditionsoflife，toavoidenemiesofallkinds，andtostruggleagainstahostofcompetitors。Hence，undersuchcomplexconditions，itwouldoftenhappenthatmodificationsofthemostvariedkinds，inimportantaswellasinunimportantparts，wouldbeadvantageousorevennecessary;andtheywouldslowlybutsurelybeacquiredthroughthesurvivalofthefittest。Stillmoreimportantisthefactthatvariousindirectmodificationswouldlikewisearisethroughthelawofcorrelatedvariation。
Domesticbreedsoftenhaveanabnormalorsemi—monstrouscharacter，asamongstdogs，theItaliangreyhound，bulldog，Blenheimspaniel，andbloodhound，——somebreedso...完整阅读请扫描二维码下载丁香书院APP免费看：