Ifweaskourselveswhythisorthatcharacterhasbeenmodifiedunderdomestication，weare，inmostcases，lostinutterdarkness。Manynaturalists，especiallyoftheFrenchschool，attributeeverymodificationtothe\"mondeambiant，\"thatis，tochangedclimate，withallitsdiversitiesofheatandcold，dampnessanddryness，lightandelectricity，tothenatureofthesoil，andtovariedkindsandamountoffood。Bythetermdefiniteaction，asusedinthischapter，Imeananactionofsuchanaturethat，whenmanyindividualsofthesamevarietyareexposedduringseveralgenerationstoanyparticularchangeintheirconditionsoflife，all，ornearlyalltheindividuals，aremodifiedinthesamemanner。Theeffectsofhabit，oroftheincreaseduseanddisuseofvariousorgans，mighthavebeenincludedunderthishead;butitwillbeconvenienttodiscussthissubjectinaseparatechapter。BythetermindefiniteactionImeananactionwhichcausesoneindividualtovaryinonewayandanotherindividualinanotherway，asweoftenseewithplantsandanimalsaftertheyhavebeensubjectedforsomegenerationstochangedconditionsoflife。Butweknowfartoolittleofthecausesandlawsofvariationtomakeasoundclassification。Theactionofchangedconditions，whetherleadingtodefiniteorindefiniteresults，isatotallydistinctconsiderationfromtheeffectsofselection;forselectiondependsonthepreservationbymanofcertainindividuals，orontheirsurvivalundervariousandcomplexnaturalcircumstances，andhasnorelationwhatevertotheprimarycauseofeachparticularvariation。
IwillfirstgiveindetailallthefactswhichIhavebeenabletocollect，renderingitprobablethatclimate，food，etc。，haveactedsodefinitelyandpowerfullyontheorganisationofourdomesticatedproductions，thatnewsub—
varietiesorraceshavebeenthusformedwithouttheaidofselectionbymanornature。Iwillthengivethefactsandconsiderationsopposedtothisconclusion，andfinallywewillweigh，asfairlyaswecan，theevidenceonbothsides。
WhenwereflectthatdistinctracesofalmostallourdomesticatedanimalsexistineachkingdomofEurope，andformerlyevenineachdistrictofEngland，weareatfirststronglyinclinedtoattributetheirorigintothedefiniteactionofthephysicalconditionsofeachcountry;andthishasbeentheconclusionofmanyauthors。Butweshouldbearinmindthatmanannuallyhastochoosewhichanimalsshallbepreservedforbreeding，andwhichshallbeslaughtered。Wehavealsoseenthatbothmethodicalandunconsciousselectionwereformerlypractised，andarenowoccasionallypractisedbythemostbarbarousraces，toamuchgreaterextentthanmighthavebeenanticipated。Henceitisdifficulttojudgehowfardifferencesintheconditionsbetween，forinstance，theseveraldistrictsinEngland，havesufficedtomodifythebreedswhichhavebeenrearedineach。Itmaybearguedthat，asnumerouswildanimalsandplantshaverangedduringmanyagesthroughoutGreatBritain，andstillretainthesamecharacter，thedifferenceinconditionsbetweentheseveraldistrictscouldnothavemodifiedinamarkedmannerthevariousnativeracesofcattle，sheep，pigs，andhorses。Thesamedifficultyofdistinguishingbetweentheeffectsofnaturalselectionandthedefiniteactionofexternalconditionsisencounteredinastillhigherdegreewhenwecomparecloselyalliedspeciesinhabitingtwocountries，suchasNorthAmericaandEurope，whichdonotdiffergreatlyinclimate，natureofsoil，etc。，forinthiscasenaturalselectionwillinevitablyandrigorouslyhaveactedduringalongsuccessionofages。
Prof。Weismannhassuggested（23/1。’UeberdenEinflussderIsolirungaufdieArtbildung’1872。）thatwhenavariablespeciesentersanewandisolatedcountry，althoughthevariationsmaybeofthesamegeneralnatureasbefore，yetitisimprobablethattheyshouldoccurinthesameproportionalnumbers。
Afteralongerorshorterperiod，thespecieswilltendtobecomenearlyuniformincharacterfromtheincessantcrossingofthevaryingindividuals;
butowingtotheproportionoftheindividualsvaryingindifferentwaysnotbeingthesameinthetwocases，thefinalresultwillbetheproductionoftwoformssomewhatdifferentfromoneanother。Incasesofthiskinditwouldfalselyappearasiftheconditionshadinducedcertaindefinitemodifications，whereastheyhadonlyexcitedindefinitevariability，butwiththevariationsinslightlydifferentproportionalnumbers。ThisviewmaythrowsomelightonthefactthatthedomesticanimalswhichformerlyinhabitedtheseveraldistrictsinGreatBritain，andthehalfwildcattlelatelykeptinseveralBritishparks，differedslightlyfromoneanother;fortheseanimalswerepreventedfromwanderingoverthewholecountryandintercrossing，butwouldhavecrossedfreelywithineachdistrictorpark。
[Fromthedifficultyofjudginghowfarchangedconditionshavecauseddefinitemodificationsofstructure，itwillbeadvisabletogiveaslargeabodyoffactsaspossible，showingthatextremelyslightdifferenceswithinthesamecountry，orduringdifferentseasons，certainlyproduceanappreciableeffect，atleastonvarietieswhicharealreadyinanunstablecondition。Ornamentalflowersaregoodforthispurpose，astheyarehighlyvariable，andarecarefullyobserved。Allfloriculturistsareunanimousthatcertainvarietiesareaffectedbyveryslightdifferencesinthenatureoftheartificialcompostinwhichtheyaregrown，andbythenaturalsoilofthedistrict，aswellasbytheseason。Thus，askilfuljudge，inwritingonCarnationsandPicotees（23/2。’Gardener’sChronicle’1853page183。）asks\"wherecanAdmiralCurzonbeseenpossessingthecolour，size，andstrengthwhichithasinDerbyshire？WherecanFlora’sGarlandbefoundequaltothoseatSlough？Wheredohigh—colouredflowersrevelbetterthanatWoolwichandBirmingham？Yetinnotwoofthesedistrictsdothesamevarietiesattainanequaldegreeofexcellence，althougheachmaybereceivingtheattentionofthemostskilfulcultivators。\"Thesamewriterthenrecommendseverycultivatortokeepfivedifferentkindsofsoilandmanure，\"andtoendeavourtosuittherespectiveappetitesoftheplantsyouaredealingwith，forwithoutsuchattentionallhopeofgeneralsuccesswillbevain。\"SoitiswiththeDahlia（23/3。Mr。Wildman’FloriculturalSoc。’February7，1843
reportedin’Gardener’sChronicle’1843page86。）:theLadyCooperrarelysucceedsnearLondon，butdoesadmirablyinotherdistricts;thereverseholdsgoodwithothervarieties;andagain，thereareotherswhichsucceedequallywellinvarioussituations。Askilfulgardener（23/4。Mr。Robsonin’JournalofHorticulture’February13，1866page122。）statesthatheprocuredcuttingsofanoldandwell—knownvariety（pulchella）ofVerbena，whichfromhavingbeenpropagatedinadifferentsituationpresentedaslightlydifferentshadeofcolour;thetwovarietieswereafterwardsmultipliedbycuttings，beingcarefullykeptdistinct;butinthesecondyeartheycouldhardlybedistinguished，andinthethirdyearnoonecoulddistinguishthem。
ThenatureoftheseasonhasanespecialinfluenceoncertainvarietiesoftheDahlia:in1841twovarietieswerepre—eminentlygood，andthenextyearthesesametwowerepre—eminentlybad。Afamousamateur（23/5。’JournalofHorticulture’1861page24。）assertsthatin1861manyvarietiesoftheRosecamesountrueincharacter，\"thatitwashardlypossibletorecognisethem，andthethoughtwasnotseldomentertainedthatthegrowerhadlosthistally。\"Thesameamateur（23/6。Ibid1862page83。）statesthatin1862two—
thirdsofhisAuriculasproducedcentraltrussesofflowers，andsuchtrussesareliablenottokeeptrue;andheaddsthatinsomeseasonscertainvarietiesofthisplantallprovegood，andthenextseasonallprovebad;
whilstexactlythereversehappenswithothervarieties。In1845theeditorofthe’Gardener’sChronicle’（23/7。’Gardener’sChronicle’1845page660。）
remarkedhowsingularitwasthatthisyearmanyCalceolariastendedtoassumeatubularform。WithHeartsease（23/8。Ibid1863page628。）theblotchedsortsdonotacquiretheirpropercharacteruntilhotweathersetsin;whilstothervarietieslosetheirbeautifulmarksassoonasthisoccurs。
Analogousfactshavebeenobservedwithleaves:Mr。Beatonasserts（23/9。
’JournalofHort。’1861pages64，309。）thatheraisedatShrubland，duringsixyears，twentythousandseedlingsfromthePunchPelargonium，andnotonehadvariegatedleaves;butatSurbiton，inSurrey，one—third，orevenagreaterproportion，oftheseedlingsfromthissamevarietyweremoreorlessvariegated。ThesoilofanotherdistrictinSurreyhasastrongtendencytocausevariegation，asappearsfrominformationgivenmebySirF。Pollock。
Verlot（23/10。’DesVarietes’etc。page76。）statesthatthevariegatedstrawberryretainsitscharacteraslongasgrowninadryishsoil，butsoonlosesitwhenplantedinfreshandhumidsoil。Mr。Salter，whoiswellknownforhissuccessincultivatingvariegatedplants，informsmethatrowsofstrawberrieswereplantedinhisgardenin1859，intheusualway;andatvariousdistancesinonerow，severalplantssimultaneouslybecamevariegated;
andwhatmadethecasemoreextraordinary，allwerevariegatedinpreciselythesamemanner。Theseplantswereremoved，butduringthethreesucceedingyearsotherplantsinthesamerowbecamevariegated，andinnoinstanceweretheplantsinanyadjoiningrowaffected。
Thechemicalqualities，odours，andtissuesofplantsareoftenmodifiedbyachangewhichseemstousslight。TheHemlockissaidnottoyieldconicineinScotland。TherootoftheAconitumnapellusbecomesinnocuousinfrigidclimates。ThemedicinalpropertiesoftheDigitalisareeasilyaffectedbyculture。AsthePistacialentiscusgrowsabundantlyintheSouthofFrance，theclimatemustsuitit，butityieldsnomastic。TheLaurussassafrasinEuropelosestheodourpropertoitinNorthAmerica。（23/11。Engel’SurlesProp。MedicalesdesPlantes’1860pages10，25。OnchangesintheodoursofplantsseeDalibert’sExperimentsquotedbyBeckman’Inventions’volume2page344;andNeesinFerussac’Bull。desSc。Nat。’1824tome1page60。Withrespecttotherhubarbetc。seealso’Gardener’sChronicle’1849page355;
1862page1123。）Manysimilarfactscouldbegiven，andtheyareremarkablebecauseitmighthavebeenthoughtthatdefinitechemicalcompoundswouldhavebeenlittleliabletochangeeitherinqualityorquantity。
ThewoodoftheAmericanLocust—tree（Robinia）whengrowninEnglandisnearlyworthless，asisthatoftheOak—treewhengrownattheCapeofGoodHope。
（23/12。Hooker’FloraIndica’page32。）Hempandflax，asIhearfromDr。
Falconer，flourishandyieldplentyofseedontheplainsofIndia，buttheirfibresarebrittleanduseless。Hemp，ontheotherhand，failstoproduceinEnglandthatresinousmatterwhichissolargelyusedinIndiaasanintoxicatingdrug。
ThefruitoftheMelonisgreatlyinfluencedbyslightdifferencesincultureandclimate。Henceitisgenerallyabetterplan，accordingtoNaudin，toimproveanoldkindthantointroduceanewoneintoanylocality。TheseedofthePersianMelonproducesnearParisfruitinferiortothepoorestmarketkinds，butatBordeauxyieldsdeliciousfruit。（23/13。Naudin’AnnalesdesSc。
Nat。’4thseries，Bot。tome111859page81。’Gardener’sChronicle’1859page464。）SeedisannuallybroughtfromThibettoKashmir（23/14。Moorcroft’Travels’etc。volume2page143。）andproducesfruitweighingfromfourtotenpounds，butplantsraisednextyearfromseedsavedinKashmirgivefruitweighingonlyfromtwotothreepounds。ItiswellknownthatAmericanvarietiesoftheAppleproduceintheirnativelandmagnificentandbrightly—
colouredfruit，buttheseinEnglandareofpoorqualityandadullcolour。InHungarytherearemanyvarietiesofthekidney—bean，remarkableforthebeautyoftheirseeds，buttheRev。M。J。Berkeley（23/15。’Gardener’sChronicle’1861
page1113。）foundthattheirbeautycouldhardlyeverbepreservedinEngland，andinsomecasesthecolourwasgreatlychanged。Wehaveseenintheninthchapter，withrespecttowheat，whataremarkableeffecttransportalfromthenorthtothesouthofFrance，andconversely，producedontheweightofthegrain。]
Whenmancanperceivenochangeinplantsoranimalswhichhavebeenexposedtoanewclimateortodifferenttreatment，insectscansometimesperceiveamarkedchange。AcactushasbeenimportedintoIndiafromCanton，ManillaMauritius，andfromthehot—housesofKew，andthereislikewiseaso—callednativekindwhichwasformerlyintroducedfromSouthAmerica;alltheseplantsbelongtothesamespeciesandarealikeinappearance，butthecochinealinsectflourishesonlyonthenativekind，onwhichitthrivesprodigiously。
（23/16。Royle’ProductiveResourcesofIndia’page59。）Humboldtremarks（23/17。’PersonalNarrative’Englishtranslationvolume5page101。ThisstatementhasbeenconfirmedbyKarsten’BeitragzurKenntnissderRhynchoprion’Moscow1864s。39andbyothers。）thatwhitemen\"borninthetorridzonewalkbarefootwithimpunityinthesameapartmentwhereaEuropean，recentlylanded，isexposedtotheattacksofthePulexpenetrans。\"
Thisinsect，thetoowell—knownchigoe，mustthereforebeabletoperceivewhatthemostdelicatechemicalanalysisfailstodiscover，namely，adifferencebetweenthebloodortissuesofaEuropeanandthoseofawhitemanborninthetropics。Butthediscernmentofthechigoeisnotsosurprisingasitatfirstappears;foraccordingtoLiebig（23/18。’OrganicChemistry’
Englishtranslation1steditionpage369。）thebloodofmenwithdifferentcomplexions，thoughinhabitingthesamecountry，emitsadifferentodour。
[Diseasespeculiartocertainlocalities，heights，orclimates，maybeherebrieflynoticed，asshowingtheinfluenceofexternalcircumstancesonthehumanbody。Diseasesconfinedtocertainracesofmandonotconcernus，fortheconstitutionoftheracemayplaythemoreimportantpart，andthismayhavebeendeterminedbyunknowncauses。ThePlicaPolonicastands，inthisrespect，inanearlyintermediateposition;foritrarelyaffectsGermans，whoinhabittheneighbourhoodoftheVistula，wheresomanyPolesaregrievouslyaffected;neitherdoesitaffectRussians，whoaresaidtobelongtothesameoriginalstockasthePoles。（23/19。Prichard’Phys。Hist。ofMankind’1851
volume1page155。）Theelevationofadistrictoftengovernstheappearanceofdiseases;inMexicotheyellowfeverdoesnotextendabove924metres;andinPeru，peopleareaffectedwiththeverugasonlybetween600and1600metresabovethesea;manyothersuchcasescouldbegiven。Apeculiarcutaneouscomplaint，calledtheBoutond’Alep，affectsinAleppoandsomeneighbouringdistrictsalmosteverynativeinfant，andsomefewstrangers;anditseemsfairlywellestablishedthatthissingularcomplaintdependsondrinkingcertainwaters。InthehealthylittleislandofSt。Helenathescarlet—feverisdreadedlikethePlague;analogousfactshavebeenobservedinChiliandMexico。（23/20。Darwin’JournalofResearches’1845page434。）EveninthedifferentdepartmentsofFranceitisfoundthatthevariousinfirmitieswhichrendertheconscriptunfitforservinginthearmy，prevailwithremarkableinequality，revealing，asBoudinobserves，thatmanyofthemareendemic，whichotherwisewouldneverhavebeensuspected。（23/21。ThesestatementsondiseasearetakenfromDr。Boudin’GeographieetStatistiqueMedicale’1857
tome1pages44and52;tome2page315。）Anyonewhowillstudythedistributionofdiseasewillbestruckwithsurpriseatwhatslightdifferencesinthesurroundingcircumstancesgovernthenatureandseverityofthecomplaintsbywhichmanisatleasttemporarilyaffected。
Themodificationsasyetreferredtoareextremelyslight，andinmostcaseshavebeencaused，asfaraswecanjudge，byequallyslightdifferencesintheconditions。Butsuchconditionsactingduringaseriesofgenerationswouldperhapsproduceamarkedeffect。
Withplants，aconsiderablechangeofclimatesometimesproducesaconspicuousresult。Ihavegivenintheninthchapterthemostremarkablecaseknowntome，namely，thatofvarietiesofmaize，whichweregreatlymodifiedinthecourseofonlytwoorthreegenerationswhentakenfromatropicalcountrytoacoolerone，orconversely。Dr。FalconerinformsmethathehasseentheEnglishRibston—pippinapple，aHimalayanoak，PrunusandPyrus，allassumeinthehotterpartsofIndiaafastigiateorpyramidalhabit;andthisfactisthemoreinteresting，asaChinesetropicalspeciesofPyrusnaturallygrowsthus。Althoughinthesecasesthechangedmannerofgrowthseemstohavebeendirectlycausedbythegreatheat，weknowthatmanyfastigiatetreeshaveoriginatedintheirtemperatehomes。IntheBotanicGardensofCeylontheapple—tree（23/22。’Ceylon’bySirJ。E。Tennentvolume11859page89。）\"sendsoutnumerousrunnersunderground，whichcontinuallyriseintosmallstems，andformagrowtharoundtheparent—tree。）ThevarietiesofthecabbagewhichproduceheadsinEuropefailtodosoincertaintropicalcountries（23/23。
Godron’Del’Espece’tome2page52。）TheRhododendronciliatumproducedatKewflowerssomuchlargerandpaler—colouredthanthosewhichitbearsonitsnativeHimalayanmountain，thatDr。Hooker（23/24。’JournalofHorticulturalSoc。’volume71852page117。）wouldhardlyhaverecognisedthespeciesbytheflowersalone。Manysimilarfactswithrespecttothecolourandsizeofflowerscouldbegiven。
TheexperimentsofVilmorinandBuckmanoncarrotsandparsnipsprovethatabundantnutrimentproducesadefiniteandinheritableeffectontheroots，withscarcelyanychangeinotherpartsoftheplant。AlumdirectlyinfluencesthecolouroftheflowersoftheHydrangea。（23/25。’JournalofHort。Soc。’
volume1page160。）Drynessseemsgenerallytofavourthehairinessorvillosityofplants。GartnerfoundthathybridVerbascumsbecameextremelywoollywhengrowninpots。Mr。Masters，ontheotherhand，statesthattheOpuntialeucotricha\"iswellclothedwithbeautifulwhitehairswhengrowninadampheat，butinadryheatexhibitsnoneofthispeculiarity。\"（23/26。SeeLecoqontheVillosityofPlants’Geograph。Bot。’tome3pages287，291;
Gartner’Bastarderz。’s。261;Mr。MastersontheOpuntiain’Gardener’sChronicle’1846page444。）Slightvariationsofmanykinds，notworthspecifyingindetail，areretainedonlyaslongasplantsaregrownincertainsoils，ofwhichSageret（23/27。’Pom。Phys。’page136。）givessomeinstancesfromhisownexperience。Odart，whoinsistsstronglyonthepermanenceofthevarietiesofthegrape，admits（23/28。’Ampelographie’1849page19。）thatsomevarieties，whengrownunderadifferentclimateortreateddifferently，varyinaslightdegree，asinthetintofthefruitandintheperiodofripening。Someauthorshavedeniedthatgraftingcauseseventheslightestdifferenceinthescion;butthereissufficientevidencethatthefruitissometimesslightlyaffectedinsizeandflavour，theleavesinduration，andtheflowersinappearance。（23/29。Gartner’Bastarderz。’s。606，hascollectednearlyallrecordedfacts。AndrewKnightin’Transact。Hort。Soc。’volume2
page160，goessofarastomaintainthatfewvarietiesareabsolutelypermanentincharacterwhenpropagatedbybudsorgrafts。）
Therecanbenodoubt，fromthefactsgiveninthefirstchapter，thatEuropeandogsdeteriorateinIndia，notonlyintheirinstinctsbutinstructure;butthechangeswhichtheyundergoareofsuchanature，thattheymaybepartlyduetoreversiontoaprimitiveform，asinthecaseofferalanimals。InpartsofIndiatheturkeybecomesreducedinsize，\"withthependulousappendageoverthebeakenormouslydeveloped。\"（23/30。Mr。Blyth’AnnalsandMagofNat。Hist。’volume201847page391。）Wehaveseenhowsoonthewildduck，whendomesticated，losesitstruecharacter，fromtheeffectsofabundantorchangedfood，orfromtakinglittleexercise。FromthedirectactionofahumidclimateandpoorpasturethehorserapidlydecreasesinsizeintheFalklandIslands。FrominformationwhichIhavereceived，thisseemslikewisetobethecasetoacertainextentwithsheepinAustralia。
Climatedefinitelyinfluencesthehairycoveringofanimals;intheWestIndiesagreatchangeisproducedinthefleeceofsheep，inaboutthreegenerations。Dr。Falconerstates（23/31。’NaturalHistoryReview’1862page113。）thattheThibetmastiffandgoat，whenbroughtdownfromtheHimalayatoKashmir，losetheirfinewool。AtAngoranotonlygoats，butshepherd—dogsandcats，havefinefleecyhair，andMr。Ainsworth（23/32。’JournalofRoy。
GeographicalSoc。’volume91839page275。）attributesthethicknessofthefleecetotheseverewinters，anditssilkylustretothehotsummers。Burnesstatespositively（23/33。’TravelsinBokhara’volume3page151。）thattheKarakoolsheeplosetheirpeculiarblackcurledfleeceswhenremovedintoanyothercountry。EvenwithinthelimitsofEngland，Ihavebeenassuredthatthewooloftwobreedsofsheepwasslightlychangedbytheflocksbeingpasturedindifferentlocalities。（23/34。SeealsoontheinfluenceofmarshypasturesonthewoolGodron’L’Espece’tome2page22。）Ithasbeenassertedongoodauthority（23/35。IsidoreGeoffroySaint—Hilaire’Hist。Nat。Gen。’tome3page438。）thathorseskeptduringseveralyearsinthedeepcoal—minesofBelgiumbecomecoveredwithvelvetyhair，almostlikethatonthemole。Thesecasesprobablystandincloserelationtothenaturalchangeofcoatinwinterandsummer。Nakedvarietiesofseveraldomesticanimalshaveoccasionallyappeared;butthereisnoreasontobelievethatthisisinanywayrelatedtothenatureoftheclimatetowhichtheyhavebeenexposed。（23/36。Azarahasmadesomegoodremarksonthissubject’QuadrupedesduParaguay’tome2page337。SeeanaccountofafamilyofnakedmiceproducedinEngland’Proc。
Zoolog。Soc。’1856page38。）
Itappearsatfirstsightprobablethattheincreasedsize，thetendencytofatten，theearlymaturityandalteredformsofourimprovedcattle，sheep，andpigs，havedirectlyresultedfromtheirabundantsupplyoffood。Thisistheopinionofmanycompetentjudges，andprobablyistoagreatextenttrue。
Butasfarasformisconcerned，wemustnotoverlookthemorepotentinfluenceoflesseneduseonthelimbsandlungs。Wesee，moreover，asfarassizeisconcerned，thatselectionisapparentlyamorepowerfulagentthanalargesupplyoffood，forwecanthusonlyaccountfortheexistence，asremarkedtomebyMr。Blyth，ofthelargestandsmallestbreedsofsheepinthesamecountry，ofCochin—ChinafowlsandBantams，ofsmallTumblerandlargeRuntpigeons，allkepttogetherandsuppliedwithabundantnourishment。
Neverthelesstherecanbelittledoubtthatourdomesticatedanimalshavebeenmodified，independentlyoftheincreasedorlesseneduseofparts，bytheconditionstowhichtheyhavebeensubjected，withouttheaidofselection。
Forinstance，Prof。Rutimeyer（23/37。’DieFaunaderPfahlbauten’1861s。15。）
showsthatthebonesofdomesticatedquadrupedscanbedistinguishedfromthoseofwildanimalsbythestateoftheirsurfaceandgeneralappearance。ItisscarcelypossibletoreadNathusius’sexcellent’Vorstudien’（23/38。
’Schweineschadel’1864s。99。）anddoubtthat，withthehighlyimprovedracesofthepig，abundantfoodhasproducedaconspicuouseffectonthegeneralformofthebody，onthebreadthoftheheadandface，andevenontheteeth。
NathusiusrestsmuchonthecaseofapurelybredBerkshirepig，whichwhentwomonthsoldbecamediseasedinitsdigestiveorgans，andwaspreservedforobservationuntilnineteenmonthsold;atthisageithadlostseveralcharacteristicfeaturesofthebreed，andhadacquiredalong，narrowhead，oflargesizerelativelytoitssmallbody，andelongatedlegs。Butinthiscaseandinsomeothersweoughtnottoassumethat，becausecertaincharactersarelost，perhapsthroughreversion，underonecourseoftreatment，thereforethattheywereatfirstdirectlyproducedbyanoppositetreatment。
Inthecaseoftherabbit，whichhasbecomeferalontheislandofPortoSanto，weareatfirststronglytemptedtoattributethewholechange——thegreatlyreducedsize，thealteredtintsofthefur，andthelossofcertaincharacteristicmarks——tothedefiniteactionofthenewconditionstowhichithasbeenexposed。Butinallsuchcaseswehavetoconsiderinadditionthetendencytoreversiontoprogenitorsmoreorlessremote，andthenaturalselectionofthefinestshadesofdifference。
Thenatureofthefoodsometimeseitherdefinitelyinducescertainpeculiarities，orstandsinsomecloserelationwiththem。Pallaslongagoassertedthatthefat—tailedsheepofSiberiadegenerateandlosetheirenormoustailswhenremovedfromcertainsalinepastures;andrecentlyErman（23/39。’TravelsinSiberia’Englishtranslationvolume1page228。）statesthatthisoccurswiththeKirgisiansheepwhenbroughttoOrenburgh。
Itiswellknownthathemp—seedcausesbullfinchesandcertainotherbirdstobecomeblack。Mr。Wallacehascommunicatedtomesomemuchmoreremarkablefactsofthesamenature。ThenativesoftheAmazonianregionfeedthecommongreenparrot（Chrysotisfestiva，Linn。）withthefatoflargeSiluroidfishes，andthebirdsthustreatedbecomebeautifullyvariegatedwithredandyellowfeathers。IntheMalayanarchipelago，thenativesofGiloloalterinananalogousmannerthecoloursofanotherparrot，namely，theLoriusgarrulus，Linn。，andthusproducetheLorirajahorKing—Lory。TheseparrotsintheMalayIslandsandSouthAmerica，whenfedbythenativesonnaturalvegetablefood，suchasriceandplaintains，retaintheirpropercolours。Mr。Wallacehas，also，recorded（23/40。A。R。Wallace’TravelsontheAmazonandRioNegro’
page294。）astillmoresingularfact。\"TheIndians（ofS。America）haveacuriousartbywhichtheychangethecoloursofthefeathersofmanybirds。
Theypluckoutthosefromtheparttheywishtopaint，andinoculatethefreshwoundwiththemilkysecretionfromtheskinofasmalltoad。Thefeathersgrowofabrilliantyellowcolour，andonbeingpluckedout，itissaid，growagainofthesamecolourwithoutanyfreshoperation。\"
Bechstein（23/41。’NaturgeschichtederStubenvogel’1840s。262，308。）doesnotentertainanydoubtthatseclusionfromlightaffects，atleasttemporarily，thecoloursofcage—birds。
Itiswellknownthattheshellsofland—molluscaareaffectedbytheabundanceoflimeindifferentdistricts。IsidoreGeoffroySaint—Hilaire（23/42。’Hist。NatGen。’tome3page402。）givesthecaseofHelixlactea，whichhasrecentlybeencarriedfromSpaintotheSouthofFranceandtotheRioPlata，andinbothcountriesnowpresentsadistinctappearance，butwhetherthishasresultedfromfoodorclimateisnotknown。Withrespecttothecommonoyster，Mr。F。Bucklandinformsmethathecangenerallydistinguishtheshellsfromdifferentdistricts;youngoystersbroughtfromWalesandlaiddowninbedswhere\"natives\"areindigenous，intheshortspaceoftwomonthsbegintoassumethe\"native\"character。M。Costa（23/43。’Bull。
deLaSoc。Imp。d’Acclimat。’tome8page351。）hasrecordedamuchmoreremarkablecaseofthesamenature，namely，thatyoungshellstakenfromtheshoresofEnglandandplacedintheMediterranean，atoncealteredtheirmannerofgrowthandformedprominentdivergingrays，likethoseontheshellsoftheproperMediterraneanoyster。Thesameindividualshell，showingbothformsofgrowth，wasexhibitedbeforeasocietyinParis。Lastly，itiswellknownthatcaterpillarsfedondifferentfoodsometimeseitherthemselvesacquireadifferentcolourorproducemothsdifferingincolour。（23/44。SeeanaccountofMr。Gregson’sexperimentsontheAbraxusgrossulariata’Proc。
Entomolog。Soc。’January6，1862:theseexperimentshavebeenconfirmedbyMr。
Greeningin’Proc。oftheNorthernEntomolog。Soc。’July28，1862。FortheeffectsoffoodoncaterpillarsseeacuriousaccountbyM。Michelyin’Bull。
DeLaSoc。Imp。d’Acclimat。’tome8page563。ForanalogousfactsfromDahlbomonHymenopteraseeWestwood’ModernClass。ofInsects’volume2page98。SeealsoDr。L。Moller’DieAbhangigkeitderInsecten’1867s。70。）
Itwouldbetravellingbeyondmyproperlimitsheretodiscusshowfarorganicbeingsinastateofnaturearedefinitelymodifiedbychangedconditions。Inmy’OriginofSpecies’Ihavegivenabriefabstractofthefactsbearingonthispoint，andhaveshowntheinfluenceoflightonthecoloursofbirds，andofresidenceneartheseaontheluridtintsofinsects，andonthesucculencyofplants。Mr。HerbertSpencer（23/45。’ThePrinciplesofBiology’volume2
1866。ThepresentchapterswerewrittenbeforeIhadreadMr。HerbertSpencer’swork，sothatIhavenotbeenabletomakesomuchuseofitasI
shouldotherwiseprobablyhavedone。）hasrecentlydiscussedwithmuchabilitythiswholesubjectongeneralgrounds。Heargues，forinstance，thatwithallanimalstheexternalandinternaltissuesaredifferentlyactedonbythesurroundingconditions，andtheyinvariablydifferinintimatestructure。Soagaintheupperandlowersurfacesoftrueleaves，aswellasofstemsandpetioles，whentheseassumethefunctionandoccupythepositionofleaves，aredifferentlycircumstancedwithrespecttolight，etc。，andapparentlyinconsequencedifferinstructure。But，asMr。HerbertSpenceradmits，itismostdifficultinallsuchcasestodistinguishbetweentheeffectsofthedefiniteactionofphysicalconditionsandtheaccumulationthroughnaturalselectionofinheritedvariationswhichareserviceabletotheorganism，andwhichhavearisenindependentlyofthedefiniteactionoftheseconditions。]
Althoughwearenothereconcernedwiththedefiniteactionoftheconditionsoflifeonorganismsinastateofnature，Imaystatethatmuchevidencehasbeengainedduringthelastfewyearsonthissubject。IntheUnitedStates，forinstance，ithasbeenclearlyproved，moreespeciallybyMr。J。A。Allen，that，withbirds，manyspeciesdifferintint，sizeofbodyandofbeak，andinlengthoftail，inproceedingfromtheNorthtotheSouth;anditappearsthatthesedifferencesmustbeattributedtothedirectactionoftemperature。
（23/46。ProfessorWeismanncomestothesameconclusionwithrespecttocertainEuropeanbutterfliesinhisvaluableessay’UeberdenSaison—
Dimorphismus’1875。Imightalsorefertotherecentworksofseveralotherauthorsonthepresentsubject;forinstancetoKerner’s’GuteundschlechteArten’1866。）WithrespecttoplantsIwillgiveasomewhatanalogouscase:
Mr。Meehan（23/47。’Proc。Acad。Nat。Soc。ofPhiladelphia’January28，1862。），hascomparedtwenty—ninekindsofAmericantreeswiththeirnearestEuropeanallies，allgrownincloseproximityandunderasnearlyaspossiblethesameconditions。IntheAmericanspecieshefinds，withtherarestexceptions，thattheleavesfallearlierintheseason，andassumebeforetheirfallabrightertint;thattheyarelessdeeplytoothedorserrated;thatthebudsaresmaller;thatthetreesaremorediffuseingrowthandhavefewerbranchlets;
and，lastly，thattheseedsaresmaller——allincomparisonwiththecorrespondingEuropeanspecies。Nowconsideringthatthesecorrespondingtreesbelongtoseveraldistinctorders，andthattheyareadaptedtowidelydifferentstations，itcanhardlybesupposedthattheirdifferencesareofanyspecialservicetothemintheNewandOldworlds;andifsosuchdifferencescannothavebeengainedthroughnaturalselection，andmustbeattributedtothelongcontinuedactionofadifferentclimate。
GALLS。
Anotherclassoffacts，notrelatingtocultivatedplants，deservesattention。
Ialludetotheproductionofgalls。Everyoneknowsthecurious，bright—red，hairyproductionsonthewildrose—tree，andthevariousdifferentgallsproducedbytheoak。Someofthelatterresemblefruit，withonefaceasrosyastherosiestapple。Thesebrightcolourscanbeofnoserviceeithertothegall—forminginsectortothetree，andprobablyarethedirectresultoftheactionofthelight，inthesamemannerastheapplesofNovaScotiaorCanadaarebrightercolouredthanEnglishapples。AccordingtoOstenSacken’slatestrevision，nolessthanfifty—eightkindsofgallsareproducedontheseveralspeciesofoak，byCynipswithitssub—genera;andMr。B。D。Walsh（23/48。SeeMr。B。D。Walsh’sexcellentpapersin’Proc。Entomolog。Soc。Philadelphia’
December1866page284。Withrespecttothewillowseeibid1864page546。）
statesthathecanaddmanyotherstothelist。OneAmericanspeciesofwillow，theSalixhumilis，bearstendistinctkindsofgalls。TheleaveswhichspringfromthegallsofvariousEnglishwillowsdiffercompletelyinshapefromthenaturalleaves。Theyoungshootsofjunipersandfirs，whenpuncturedbycertaininsects，yieldmonstrousgrowthsresemblingflowersandfir—cones;
andtheflowersofsomeplantsbecomefromthesamecausewhollychangedinappearance。Gallsareproducedineveryquarteroftheworld;ofseveralsenttomebyMr。ThwaitesfromCeylon，somewereassymmetricalasacompositeflowerwheninbud，otherssmoothandsphericallikeaberry;someprotectedbylongspines，othersclothedwithyellowwoolformedoflongcellularhairs，otherswithregularlytuftedhairs。Insomegallstheinternalstructureissimple，butinothersitishighlycomplex;thusM。Lacaze—Duthiers（23/49。
Seehisadmirable’HistoiredesGalles’in’Annal。desSc。Nat。Bot。’3rdseriestome191853page273。）hasfiguredinthecommonink—gallnolessthansevenconcentriclayers，composedofdistincttissue，namely，theepidermic，sub—epidermic，spongy，intermediate，andthehardprotectivelayerformedofcuriouslythickenedwoodycells，and，lastly，thecentralmass，aboundingwithstarch—granulesonwhichthelarvaefeed。
Gallsareproducedbyinsectsofvariousorders，butthegreaternumberbyspeciesofCynips。ItisimpossibletoreadM。Lacaze—Duthiers’discussionanddoubtthatthepoisonoussecretionoftheinsectcausesthegrowthofthegall;andeveryoneknowshowvirulentisthepoisonsecretedbywaspsandbees，whichbelongtothesamegroupwithCynips。Gallsgrowwithextraordinaryrapidity，anditissaidthattheyattaintheirfullsizeinafewdays（23/50。KirbyandSpence’Entomology’1818volume1page450;Lacaze—
Duthiersibidpage284。）;itiscertainthattheyarealmostcompletelydevelopedbeforethelarvaearehatched。Consideringthatmanygall—insectsareextremelysmall，thedropofsecretedpoisonmustbeexcessivelyminute;
itprobablyactsononeortwocellsalone，which，beingabnormallystimulated，rapidlyincreasebyaprocessofself—division。Galls，asMr。
Walsh（23/51。’Proc。Entomolog。Soc。Philadelphia’1864page558。）remarks，affordgood，constant，anddefinitecharacters，eachkindkeepingastruetoformasdoesanyindependentorganicbeing。Thisfactbecomesstillmoreremarkablewhenwehearthat，forinstance，sevenoutofthetendifferentkindsofgallsproducedonSalixhumilisareformedbygall—gnats（Cecidomyidae）which\"thoughessentiallydistinctspecies，yetresembleoneanothersocloselythatinalmostallcasesitisdifficult，andinmostcasesimpossible，todistinguishthefull—growninsectsonefromtheother。\"（23/52。
Mr。B。D。Walshibidpage633andDecember1866page275。）Forinaccordancewithawide—spreadanalogywemaysafelyinferthatthepoisonsecretedbyinsectssocloselyalliedwouldnotdiffermuchinnature;yetthisslightdifferenceissufficienttoinducewidelydifferentresults。Insomefewcasesthesamespeciesofgall—gnatproducesondistinctspeciesofwillowsgallswhichcannotbedistinguished;theCynipsfecundatrix，also，hasbeenknowntoproduceontheTurkishoak，towhichitisnotproperlyattached，exactlythesamekindofgallasontheEuropeanoak。（23/53。Mr。B。D。Walshibid1864
pages545，411，495;andDecember1866page278。SeealsoLacaze—Duthiers。）
Theselatterfactsapparentlyprovethatthenatureofthepoisonisamorepowerfulagentindeterminingtheformofthegallthanthespecificcharacterofthetreewhichisactedon。
Asthepoisonoussecretionofinsectsbelongingtovariousordershasthespecialpowerofaffectingthegrowthofvariousplants;asaslightdifferenceinthenatureofthepoisonsufficestoproducewidelydifferentresults;andlastly，asweknowthatthechemicalcompoundssecretedbyplantsareeminentlyliabletobemodifiedbychangedconditionsoflife，wemaybelieveitpossiblethatvariouspartsofaplantmightbemodifiedthroughtheagencyofitsownalteredsecretions。Compare，forinstance，themossyandviscidcalyxofamoss—rose，whichsuddenlyappearsthroughbud—variationonaProvence—rose，withthegallofredmossgrowingfromtheinoculatedleafofawildrose，witheachfilamentsymmetricallybranchedlikeamicroscopicalspruce—fir，bearingaglandulartipandsecretingodoriferousgummymatter。
（23/54。Lacaze—Duthiersibidpages325，328。）Orcompare，ontheonehand，thefruitofthepeach，withitshairyskin，fleshycovering，hardshellandkernel，andontheotherhandoneofthemorecomplexgallswithitsepidermic，spongy，andwoodylayers，surroundingtissueloadedwithstarchgranules。Thesenormalandabnormalstructuresmanifestlypresentacertaindegreeofresemblance。Or，again，reflectonthecasesabovegivenofparrotswhichhavehadtheirplumagebrightlydecoratedthroughsomechangeintheirblood，causedbyhavingbeenfedoncertainfishes，orlocallyinoculatedwiththepoisonofatoad。Iamfarfromwishingtomaintainthatthemoss—roseorthehardshellofthepeach—stoneorthebrightcoloursofbirdsareactuallyduetoanychemicalchangeinthesaporblood;butthesecasesofgallsandofparrotsareexcellentlyadaptedtoshowushowpowerfullyandsingularlyexternalagenciesmayaffectstructure。Withsuchfactsbeforeus，weneedfeelnosurpriseattheappearanceofanymodificationinanyorganicbeing。
[Imay，also，herealludetotheremarkableeffectswhichparasiticfungisometimesproduceonplants。Reissek（23/55。’Linnaea’volume171843;quotedbyDr。M。T。Masters，RoyalInstitution，March16，1860。）hasdescribedaThesium，affectedbyanOecidium，whichwasgreatlymodified，andassumedsomeofthecharacteristicfeaturesofcertainalliedspecies，orevengenera。
Suppose，saysReissek，\"theconditionoriginallycausedbythefungustobecomeconstantinthecourseoftime，theplantwould，iffoundgrowingwild，beconsideredasadistinctspeciesorevenasbelongingtoanewgenus。\"I
quotethisremarktoshowhowprofoundly，yetinhownaturalamanner，thisplantmusthavebeenmodifiedbytheparasiticfungus。Mr。Meehan（23/56。
’Proc。Acad。Nat。Sc。，Philadelphia’June16，1874andJuly23，1875。）alsostatesthatthreespeciesofEuphorbiaandPortulacaolereacea，whichnaturallygrowprostrate，becomeerectwhentheyareattackedbytheOecidium。
Euphorbiamaculatainthiscasealsobecomesnodose，withthebranchletscomparativelysmoothandtheleavesmodifiedinshape，approachingintheserespectstoadistinctspecies，namely，theE。hypericifolia。]
FACTSANDCONSIDERATIONSOPPOSEDTOTHEBELIEFTHATTHECONDITIONSOFLIFEACT
INAPOTENTMANNERINCAUSINGDEFINITEMODIFICATIONSOFSTRUCTURE。
Ihavealludedtotheslightdifferencesinspeciesnaturallylivingindistinctcountriesunderdifferentconditions;andsuchdifferenceswefeelatfirstinclinedtoattribute，probablyoftenwithjustice，tothedefiniteactionofthesurroundingconditions。Butitmustbeborneinmindthatthereexistmanyanimalsandplantswhichrangewidelyandhavebeenexposedtogreatdiversitiesofclimate，yetremainuniformincharacter。Someauthors，aspreviouslyremarked，accountforthevarietiesofourculinaryandagriculturalplantsbythedefiniteactionoftheconditionstowhichtheyhavebeenexposedinthedifferentpartsofGreatBritain;butthereareabout200plants（23/57。HewettC。Watson’CybeleBritannica’volume11847page11。）whicharefoundineverysingleEnglishcounty;andtheseplantsmusthavebeenexposedforanimmenseperiodtoconsiderabledifferencesofclimateandsoil，yetdonotdiffer。So，again，，someanimalsandplantsrangeoveralargeportionoftheworld，yetretainthesamecharacter。
[Notwithstandingthefactspreviouslygivenontheoccurrenceofhighlypeculiarlocaldiseasesandonthestrangemodificationsofstructureinplantscausedbytheinoculatedpoisonofinsects，andotheranalogouscases;
stillthereareamultitudeofvariations——suchasthemodifiedskulloftheniataoxandbulldog，thelonghornsofCaffrecattle，theconjoinedtoesofthesolid—hoofedswine，theimmensecrestandprotuberantskullofPolishfowls，thecropofthepouter—pigeon，andahostofothersuchcases——whichwecanhardlyattributetothedefiniteaction，inthesensebeforespecified，oftheexternalconditionsoflife。Nodoubtineverycasetheremusthavebeensomeexcitingcause;butasweseeinnumerableindividualsexposedtonearlythesameconditions，andonealoneisaffected，wemayconcludethattheconstitutionoftheindividualisoffarhigherimportancethantheconditionstowhichithasbeenexposed。Itseems，indeed，tobeageneralrulethatconspicuousvariationsoccurrarely，andinoneindividualaloneoutofmillions，thoughallmayhavebeenexposed，asfaraswecanjudge，tonearlythesameconditions。Asthemoststronglymarkedvariationsgraduateinsensiblyintothemosttrifling，weareledbythesametrainofthoughttoattributeeachslightvariationmuchmoretoinnatedifferencesofconstitution，howevercaused，thantothedefiniteactionofthesurroundingconditions。
Weareledtothesameconclusionbyconsideringthecases，formerlyalludedto，offowlsandpigeons，whichhavevariedandwillnodoubtgoonvaryingindirectlyoppositeways，thoughkeptduringmanygenerationsundernearlythesameconditions。Some，forinstance，arebornwiththeirbeaks，wings，tails，legs，etc。，alittlelonger，andotherswiththesesamepartsalittleshorter。Bythelong—continuedselectionofsuchslightindividualdifferenceswhichoccurinbirdskeptinthesameaviary，widelydifferentracescouldcertainlybeformed;andlong—continuedselection，importantasistheresult，doesnothingbutpreservethevariationswhicharise，asitappearstous，spontaneously。
Inthesecasesweseethatdomesticatedanimalsvaryinanindefinitenumberofparticulars，thoughtreatedasuniformlyasispossible。Ontheotherhand，thereareinstancesofanimalsandplants，which，thoughtheyhavebeenexposedtoverydifferentconditions，bothundernatureanddomestication，havevariedinnearlythesamemanner。Mr。LayardinformsmethathehasobservedamongsttheCaffresofSouthAfricaadogsingularlylikeanarcticEsquimauxdog。PigeonsinIndiapresentnearlythesamewidediversitiesofcolourasinEurope;andIhaveseenchequeredandsimplybarredpigeons，andpigeonswithblueandwhiteloins，fromSierraLeone，Madeira，England，andIndia。NewvarietiesofflowersarecontinuallyraisedindifferentpartsofGreatBritain，butmanyofthesearefoundbythejudgesatourexhibitionstobealmostidenticalwitholdvarieties。Avastnumberofnewfruit—treesandculinaryvegetableshavebeenproducedinNorthAmerica:thesedifferfromEuropeanvarietiesinthesamegeneralmannerastheseveralvarietiesraisedinEuropedifferfromoneanother;andnoonehaseverpretendedthattheclimateofAmericahasgiventothemanyAmericanvarietiesanygeneralcharacterbywhichtheycanberecognised。Nevertheless，fromthefactspreviouslyadvancedontheauthorityofMr。MeehanwithrespecttoAmericanandEuropeanforest—treesitwouldberashtoaffirmthatvarietiesraisedinthetwocountrieswouldnotinthecourseofagesassumeadistinctivecharacter。Dr。M。Mastershasrecordedastrikingfact（23/58。’Gardener’sChronicle’1857page629。）bearingonthissubject:heraisednumerousplantsofHybiscussyriacusfromseedcollectedinSouthCarolinaandtheHolyLand，wheretheparent—plantsmusthavebeenexposedtoconsiderablydifferentconditions;yettheseedlingsfrombothlocalitiesbrokeintotwosimilarstrains，onewithobtuseleavesandpurpleorcrimsonflowers，andtheotherwithelongatedleavesandmoreorlesspinkflowers。
Wemay，also，infertheprepotentinfluenceoftheconstitutionoftheorganismoverthedefiniteactionoftheconditionsoflife，fromtheseveralcasesgivenintheearlierchaptersofparallelseriesofvarieties，——animportantsubject，hereaftertobemorefullydiscussed。Sub—varietiesoftheseveralkindsofwheat，gourds，peaches，andotherplants，andtoalimitedextentsub—varietiesofthefowl，pigeon，anddog，havebeenshowneithertoresembleortodifferfromoneanotherinacloselycorrespondingorparallelmanner。Inothercases，avarietyofonespeciesresemblesadistinctspecies;
orthevarietiesoftwodistinctspeciesresembleoneanother。Althoughtheseparallelresemblancesnodoubtoftenresultfromreversiontotheformercharactersofacommonprogenitor;yetinothercases，whennewcharactersfirstappear，theresemblancemustbeattributedtotheinheritanceofasimilarconstitution，andconsequentlytoatendencytovaryinthesamemanner。Weseesomethingofasimilarkindinthesamemonstrosityappearingandreappearingmanytimesinthesamespeciesofanimal，and，asDr。MaxwellMastershasremarkedtome，inthesamespeciesofplant。]
Wemayatleastconclude，thattheamountofmodificationwhichanimalsandplantshaveundergoneunderdomesticationdoesnotcorrespondwiththedegreetowhichtheyhavebeensubjectedtochangedcircumstances。Asweknowtheparentageofdomesticatedbirdsfarbetterthanofmostquadrupeds，wewillglancethroughthelist。ThepigeonhasvariedinEuropemorethanalmostanyotherbird;yetitisanativespecies，andhasnotbeenexposedtoanyextraordinarychangeofconditions。Thefowlhasvariedequally，oralmostequally，withthepigeon，andisanativeofthehotjunglesofIndia。Neitherthepeacock，anativeofthesamecountry，northeguinea—fowl，aninhabitantofthedrydesertsofAfrica，hasvariedatall，oronlyincolour。Theturkey，fromMexico，hasvariedbutlittle。Theduck，ontheotherhand，anativeofEurope，hasyieldedsomewell—markedraces;andasthisisanaquaticbird，itmusthavebeensubjectedtoafarmoreseriouschangeinitshabitsthanthepigeonoreventhefowl，whichneverthelesshavevariedinamuchhigherdegree。Thegoose，anativeofEuropeandaquaticliketheduck，hasvariedlessthananyotherdomesticatedbird，exceptthepeacock。
Bud—variationis，also，importantunderourpresentpointofview，insomefewcases，aswhenalltheeyesonthesametuberofthepotato，orallthefruitonthesameplum—tree，oralltheflowersonthesameplant，havesuddenlyvariedinthesamemanner，itmightbearguedthatthevariationhadbeendefinitelycausedbysomechangeintheconditionstowhichtheplantshadbeenexposed;yet，inothercases，suchanadmissionisextremelydifficult。