theyhave，also，longbeenhabituatedtotheirconditionsoflife;butitcannotbesaidthattheyaresubjecttoquiteuniformconditions，andtheyareliabletoacertainamountofvariation。Thecircumstancesunderwhichourdomesticproductionsarerearedarewidelydifferent:theyareprotectedfromcompetition;theyhavenotonlybeenremovedfromtheirnaturalconditionsandoftenfromtheirnativeland，buttheyarefrequentlycarriedfromdistricttodistrict，wheretheyaretreateddifferently，sothattheyrarelyremainduringanyconsiderablelengthoftimeexposedtocloselysimilarconditions。Inconformitywiththis，allourdomesticatedproductions，withtherarestexceptions，varyfarmorethannaturalspecies。Thehive—bee，whichfeedsitselfandfollowsinmostrespectsitsnaturalhabitsoflife，istheleastvariableofalldomesticatedanimals，andprobablythegooseisthenextleastvariable;buteventhegoosevariesmorethanalmostanywildbird，sothatitcannotbeaffiliatedwithperfectcertaintytoanynaturalspecies。Hardlyasingleplantcanbenamed，whichhaslongbeencultivatedandpropagatedbyseed，thatisnothighlyvariable;commonrye（Secalecereale）hasaffordedfewerandlessmarkedvarietiesthanalmostanyothercultivatedplant（22/4。
Metzger’DieGetreidarten’1841s。39。）;butitmaybedoubtedwhetherthevariationsofthis，theleastvaluableofallourcereals，havebeencloselyobserved。
Bud—variation，whichwasfullydiscussedinaformerchapter，showsusthatvariabilitymaybequiteindependentofseminalreproduction，andlikewiseofreversiontolong—lostancestralcharacters。Noonewillmaintainthatthesuddenappearanceofamoss—roseonaProvence—roseisareturntoaformerstate，formossinessofthecalyxhasbeenobservedinnonaturalspecies;thesameargumentisapplicabletovariegatedandlaciniatedleaves;norcantheappearanceofnectarinesonpeach—treesbeaccountedforontheprincipleofreversion。Butbud—variationsmoreimmediatelyconcernus，astheyoccurfarmorefrequentlyonplantswhichhavebeenhighlycultivatedduringalengthoftime，thanonotherandlesshighlycultivatedplants;andveryfewwell—
markedinstanceshavebeenobservedwithplantsgrowingunderstrictlynaturalconditions。Ihavegivenoneinstanceofanash—treegrowinginagentleman’spleasure—grounds;andoccasionallytheremaybeseen，onbeechandothertrees，twigsleafingatadifferentperiodfromtheotherbranches。ButourforesttreesinEnglandcanhardlybeconsideredaslivingunderstrictlynaturalconditions;theseedlingsareraisedandprotectedinnursery—grounds，andmustoftenbetransplantedintoplaceswherewildtreesofthekindwouldnotnaturallygrow。Itwouldbeesteemedaprodigyifadog—rosegrowinginahedgeproducedbybud—variationamoss—rose，orawildbullaceorwildcherry—
treeyieldedabranchbearingfruitofadifferentshapeandcolourfromtheordinaryfruit。Theprodigywouldbeenhancedifthesevaryingbrancheswerefoundcapableofpropagation，notonlybygrafts，butsometimesbyseed;yetanalogouscaseshaveoccurredwithmanyofourhighlycultivatedtreesandherbs。
Theseseveralconsiderationsalonerenderitprobablethatvariabilityofeverykindisdirectlyorindirectlycausedbychangedconditionsoflife。Or，toputthecaseunderanotherpointofview，ifitwerepossibletoexposealltheindividualsofaspeciesduringmanygenerationstoabsolutelyuniformconditionsoflife，therewouldbenovariability。
ONTHENATUREOFTHECHANGESINTHECONDITIONSOFLIFEWHICHINDUCE
VARIABILITY。
Fromaremoteperiodtothepresentday，underclimatesandcircumstancesasdifferentasitispossibletoconceive，organicbeingsofallkinds，whendomesticatedorcultivated，havevaried。Weseethiswiththemanydomesticracesofquadrupedsandbirdsbelongingtodifferentorders，withgoldfishandsilkworms，withplantsofmanykinds，raisedinvariousquartersoftheworld。
InthedesertsofnorthernAfricathedate—palmhasyieldedthirty—eightvarieties;inthefertileplainsofIndiaitisnotorioushowmanyvarietiesofriceandofahostofotherplantsexist;inasinglePolynesianisland，twenty—fourvarietiesofthebread—fruit，thesamenumberofthebanana，andtwenty—twovarietiesofthearum，arecultivatedbythenatives;themulberry—
treeinIndiaandEuropehasyieldedmanyvarietiesservingasfoodforthesilkworm;andinChinasixty—threevarietiesofthebambooareusedforvariousdomesticpurposes。（22/5。Onthedate—palmseeVogel’AnnalsandMag。
ofNat。Hist。’1854page460。OnIndianvarietiesDr。F。Hamilton’Transact。
Linn。Soc。’volume14page296。OnthevarietiescultivatedinTahitiseeDr。
BennettinLoudon’s’Mag。ofN。Hist。’volume51832page484。AlsoEllis’PolynesianResearches’volume1pages370，375。OntwentyvarietiesofthePandanusandothertreesintheMarianneIslandsee’Hooker’sMiscellany’
volume1page308。OnthebambooinChinaseeHuc’ChineseEmpire’volume2
page307。）Thesefacts，andinnumerableotherswhichcouldbeadded，indicatethatachangeofalmostanykindintheconditionsoflifesufficestocausevariability——differentchangesactingondifferentorganisms。
AndrewKnight（22/6。’TreatiseontheCultureoftheApple’etc。page3。）
attributedthevariationofbothanimalsandplantstoamoreabundantsupplyofnourishment，ortoamorefavourableclimate，thanthatnaturaltothespecies。Amoregenialclimate，however，isfarfromnecessary;thekidney—
bean，whichisofteninjuredbyourspringfrosts，andpeaches，whichrequiretheprotectionofawall，havevariedmuchinEngland，ashastheorange—treeinnorthernItaly，whereitisbarelyabletoexist。（22/7。Gallesio’TeoriadellaRiproduzioneVeg。’page125。）Norcanweoverlookthefact，thoughnotimmediatelyconnectedwithourpresentsubject，thattheplantsandshellsoftheArcticregionsareeminentlyvariable。（22/8。SeeDr。Hooker’sMemoironArcticPlantsin’Linn。Transact。’volume23part2。Mr。Woodward，andahigherauthoritycannotbequoted，speaksoftheArcticmolluscainhis’RudimentaryTreatise’1856page355asremarkablysubjecttovariation。）
Moreover，itdoesnotappearthatachangeofclimate，whethermoreorlessgenial，isoneofthemostpotentcausesofvariability;forinregardtoplantsAlph。DeCandolle，inhis’GeographieBotanique’repeatedlyshowsthatthenativecountryofaplant，whereinmostcasesithasbeenlongestcultivated，isthatwhereithasyieldedthegreatestnumberofvarieties。
Itisdoubtfulwhetherachangeinthenatureofthefoodisapotentcauseofvariability。Scarcelyanydomesticatedanimalhasvariedmorethanthepigeonorthefowl，buttheirfood，especiallythatofhighly—bredpigeons，isgenerallythesame。Norcanourcattleandsheephavebeensubjectedtoanygreatchangeinthisrespect。Butinallthesecasesthefoodprobablyismuchlessvariedinkindthanthatwhichwasconsumedbythespeciesinitsnaturalstate。（22/9。Bechsteininhis’NaturgeschichtederStubenvogel’1840s。238，hassomegoodremarksonthissubject。Hestatesthathiscanary—birdsvariedincolour，thoughkeptonuniformfood。）
Ofallthecauseswhichinducevariability，excessoffood，whetherornotchangedinnature，isprobablythemostpowerful。ThisviewwasheldwithregardtoplantsbyAndrewKnight，andisnowheldbySchleiden，moreespeciallyinreferencetotheinorganicelementsofthefood。（22/10。’ThePlant’bySchleidentranslatedbyHenfrey1848page169。SeealsoAlex。Braunin’Bot。Memoirs’RaySoc。1853page313。）Inordertogiveaplantmorefooditsufficesinmostcasestogrowitseparately，andthuspreventotherplantsrobbingitsroots。Itissurprising，asIhaveoftenseen，howvigorouslyourcommonwildspeciesflourishwhenplantedbythemselves，thoughnotinhighlymanuredland;separategrowthis，infact，thefirststepincultivation。Weseetheconverseofthebeliefthatexcessoffoodinducesvariabilityinthefollowingstatementbyagreatraiserofseedsofallkinds（22/11。Messrs。
HardyandSonofMaldonin’Gardener’sChronicle’1856page458。Carriere’ProductionetFixationdesVarietes’1865page31。）:"Itisaruleinvariablywithus，whenwedesiretokeepatruestockofanyonekindofseed，togrowitonpoorlandwithoutdung;butwhenwegrowforquantity，weactcontrary，andsometimeshavedearlytorepentofit。"AccordingalsotoCarriere，whohashadgreatexperiencewithflower—gardenseeds，"Onremarqueengenerallesplantesdevigeurmoyennesontcellesquiconserventlemieuxleurscaracteres。"
Inthecaseofanimalsthewantofaproperamountofexercise，asBechsteinremarked，hasperhapsplayed，independentlyofthedirecteffectsofthedisuseofanyparticularorgan，animportantpartincausingvariability。Wecanseeinavaguemannerthat，whentheorganisedandnutrientfluidsofthebodyarenotusedduringgrowth，orbythewearandtearofthetissues，theywillbeinexcess;andasgrowth，nutrition，andreproductionareintimatelyalliedprocesses，thissuperfluitymightdisturbthedueandproperactionofthereproductiveorgans，andconsequentlyaffectthecharacterofthefutureoffspring。Butitmaybearguedthatneitheranexcessoffoodnorasuperfluityintheorganisedfluidsofthebodynecessarilyinducesvariability。Thegooseandtheturkeyhavebeenwellfedformanygenerations，yethavevariedverylittle。Ourfruit—treesandculinaryplants，whicharesovariable，havebeencultivatedfromanancientperiod，and，thoughtheyprobablystillreceivemorenutrimentthanintheirnaturalstate，yettheymusthavereceivedduringmanygenerationsnearlythesameamount;anditmightbethoughtthattheywouldhavebecomehabituatedtotheexcess。
Nevertheless，onthewhole，Knight’sview，thatexcessoffoodisoneofthemostpotentcausesofvariability，appears，asfarasIcanjudge，probable。
Whetherornotourvariouscultivatedplantshavereceivednutrimentinexcess，allhavebeenexposedtochangesofvariouskinds。Fruit—treesaregraftedondifferentstocks，andgrowninvarioussoils。Theseedsofculinaryandagriculturalplantsarecarriedfromplacetoplace;andduringthelastcenturytherotationofourcropsandthemanuresusedhavebeengreatlychanged。
Slightchangesoftreatmentoftensufficetoinducevariability。Thesimplefactofalmostallourcultivatedplantsanddomesticatedanimalshavingvariedinallplacesandatalltimes，leadstothisconclusion。SeedstakenfromcommonEnglishforest—trees，grownundertheirnativeclimate，nothighlymanuredorotherwiseartificiallytreated，yieldseedlingswhichvarymuch，asmaybeseenineveryextensiveseed—bed。Ihaveshowninaformerchapterwhatanumberofwell—markedandsingularvarietiesthethorn（Crataegusoxycantha）
hasproduced:yetthistreehasbeensubjectedtohardlyanycultivation。InStaffordshireIcarefullyexaminedalargenumberoftwoBritishplants，namelyGeraniumphaeumandpyrenaicum，whichhaveneverbeenhighlycultivated。Theseplantshadspreadspontaneouslybyseedfromacommongardenintoanopenplantation;andtheseedlingsvariedinalmosteverysinglecharacter，bothintheirflowerandfoliage，toadegreewhichIhaveneverseenexceeded;yettheycouldnothavebeenexposedtoanygreatchangeintheirconditions。
Withrespecttoanimals，Azarahasremarkedwithmuchsurprise（22/12。
’QuadrupedesduParaguay’1801tome2page319。）that，whilsttheferalhorsesonthePampasarealwaysofoneofthreecolours，andthecattlealwaysofauniformcolour，yettheseanimals，whenbredontheunenclosedestancias，thoughkeptinastatewhichcanhardlybecalleddomesticated，andapparentlyexposedtoalmostidenticallythesameconditionsaswhentheyareferal，neverthelessdisplayagreatdiversityofcolour。SoagaininIndiaseveralspeciesoffresh—waterfishareonlysofartreatedartificially，thattheyarerearedingreattanks;butthissmallchangeissufficienttoinducemuchvariability。（22/13。M’ClellandonIndianCyprinidae’AsiaticResearches’
volume19part21839pages266，268，313。）
Somefactsontheeffectsofgrafting，inregardtothevariabilityoftrees，deserveattention。Cabanisassertsthatwhencertainpearsaregraftedonthequince，theirseedsyieldagreaternumberofvarietiesthandotheseedsofthesamevarietyofpearwhengraftedonthewildpear。（22/14。QuotedbySageret’Pom。Phys。’1830page43。Thisstatement，however，isnotbelievedbyDecaisne。）Butasthepearandquincearedistinctspecies，thoughsocloselyrelatedthattheonecanbereadilygraftedandsucceedsadmirablyontheother，thefactofvariabilitybeingthuscausedisnotsurprising;aswearehereenabledtoseethecause，namely，theverydifferentnatureofthestockandgraft。SeveralNorthAmericanvarietiesoftheplumandpeacharewellknowntoreproducethemselvestrulybyseed;butDowningasserts（22/15。’TheFruitsofAmerica’1845page5。），"thatwhenagraftistakenfromoneofthesetreesandplaceduponanotherstock，thisgraftedtreeisfoundtoloseitssingularpropertyofproducingthesamevarietybyseed，andbecomeslikeallotherworkedtrees;"——thatis，itsseedlingsbecomehighlyvariable。
Anothercaseisworthgiving:theLalandevarietyofthewalnut—treeleafsbetweenApril20thandMay15th，anditsseedlingsinvariablyinheritthesamehabit;whilstseveralothervarietiesofthewalnutleafinJune。Now，ifseedlingsareraisedfromtheMay—leafingLalandevariety，graftedonanotherMay—leafingvariety，thoughbothstockandgrafthavethesameearlyhabitofleafing，yettheseedlingsleafatvarioustimes，evenaslateasthe5thofJune。（22/16。M。Cardanin’ComptesRendus’December1848quotedin’Gardener’sChronicle’1849page101。）Suchfactsasthesearewellfittedtoshowonwhatobscureandslightcausesvariabilitydepends。
[Imayherejustalludetotheappearanceofnewandvaluablevarietiesoffruit—treesandofwheatinwoodsandwasteplaces，whichatfirstsightseemsamostanomalouscircumstance。InFranceaconsiderablenumberofthebestpearshavebeendiscoveredinwoods;andthishasoccurredsofrequently，thatPoiteauassertsthat"improvedvarietiesofourcultivatedfruitsrarelyoriginatewithnurserymen。"（22/17。M。AlexisJordanmentionsfourexcellentpearsfoundinwoodsinFrance，andalludestoothers（’Mem。Acad。deLyon’
tome21852page159）。Poiteau’sremarkisquotedin’Gardener’sMag。’volume41828page385。See’Gardener’sChronicle’1862page335，foranothercaseofanewvarietyofthepearfoundinahedgeinFrance。Alsoforanothercase，seeLoudon’s’Encyclop。ofGardening’page901。Mr。Rivershasgivenmesimilarinformation。）InEngland，ontheotherhand，noinstanceofagoodpearhavingbeenfoundwildhasbeenrecorded;andMr。Riversinformsmethatheknowsofonlyoneinstancewithapples，namely，theBessPoole，whichwasdiscoveredinawoodinNottinghamshire。Thisdifferencebetweenthetwocountriesmaybeinpartaccou...完整阅读请扫描二维码下载丁香书院APP免费看：