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。）Anyonewhowillstudythedistributionofdiseasewillbestruckwithsurpriseatwhatslightdifferencesinthesurround...完整阅读请扫描二维码下载丁香书院APP免费看：