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。