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。