the origin of species(物种起源)-第8节

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their　quality　may　be；　will　generally　be　allowed　to　breed；　and　this　will　effectually　prevent　selection。　But　probably　the　most　important　point　of　all；　is；　that　the　animal　or　plant　should　be　so　highly　useful　to　man；　or　so　much　valued　by　him；　that　the　closest　attention　should　be　paid　to　even　the　slightest　deviation　in　the　qualities　or　structure　of　each　individual。　Unless　such　attention　be　paid　nothing　can　be　effected。　I　have　seen　it　gravely　remarked；　that　it　was　most　fortunate　that　the　strawberry　began　to　vary　just　when　gardeners　began　to　attend　closely　to　this　plant。　No　doubt　the　strawberry　had　always　varied　since　it　was　cultivated；　but　the　slight　varieties　had　been　neglected。　As　soon；　however；　as　gardeners　picked　out　individual　plants　with　slightly　larger；　earlier；　or　better　fruit；　and　raised　seedlings　from　them；　and　again　picked　out　the　best　seedlings　and　bred　from　them；　then；　there　appeared　（aided　by　some　crossing　with　distinct　species）　those　many　admirable　varieties　of　the　strawberry　which　have　been　raised　during　the　last　thirty　or　forty　years。　
In　the　case　of　animals　with　separate　sexes；　facility　in　preventing　crosses　is　an　important　element　of　success　in　the　formation　of　new　races；　at　least；　in　a　country　which　is　already　stocked　with　other　races。　In　this　respect　enclosure　of　the　land　plays　a　part。　Wandering　savages　or　the　inhabitants　of　open　plains　rarely　possess　more　than　one　breed　of　the　same　species。　Pigeons　can　be　mated　for　life；　and　this　is　a　great　convenience　to　the　fancier；　for　thus　many　races　may　be　kept　true；　though　mingled　in　the　same　aviary；　and　this　circumstance　must　have　largely　favoured　the　improvement　and　formation　of　new　breeds。　Pigeons；　I　may　add；　can　be　propagated　in　great　numbers　and　at　a　very　quick　rate；　and　inferior　birds　may　be　freely　rejected；　as　when　killed　they　serve　for　food。　On　the　other　hand；　cats；　from　their　nocturnal　rambling　habits；　cannot　be　matched；　and；　although　so　much　valued　by　women　and　children；　we　hardly　ever　see　a　distinct　breed　kept　up；　such　breeds　as　we　do　sometimes　see　are　almost　always　imported　from　some　other　country；　often　from　islands。　Although　I　do　not　doubt　that　some　domestic　animals　vary　less　than　others；　yet　the　rarity　or　absence　of　distinct　breeds　of　the　cat；　the　donkey；　peacock；　goose；　&c。；　may　be　attributed　in　main　part　to　selection　not　having　been　brought　into　play：　in　cats；　from　the　difficulty　in　pairing　them；　in　donkeys；　from　only　a　few　being　kept　by　poor　people；　and　little　attention　paid　to　their　breeding；　in　peacocks；　from　not　being　very　easily　reared　and　a　large　stock　not　kept；　in　geese；　from　being　valuable　only　for　two　purposes；　food　and　feathers；　and　more　especially　from　no　pleasure　having　been　felt　in　the　display　of　distinct　breeds。　
To　sum　up　on　the　origin　of　our　Domestic　Races　of　animals　and　plants。　I　believe　that　the　conditions　of　life；　from　their　action　on　the　reproductive　system；　are　so　far　of　the　highest　importance　as　causing　variability。　I　do　not　believe　that　variability　is　an　inherent　and　necessary　contingency；　under　all　circumstances；　with　all　organic　beings；　as　some　authors　have　thought。　The　effects　of　variability　are　modified　by　various　degrees　of　inheritance　and　of　reversion。　Variability　is　governed　by　many　unknown　laws；　more　especially　by　that　of　correlation　of　growth。　Something　may　be　attributed　to　the　direct　action　of　the　conditions　of　life。　Something　must　be　attributed　to　use　and　disuse。　The　final　result　is　thus　rendered　infinitely　complex。　In　some　cases；　I　do　not　doubt　that　the　intercrossing　of　species；　aboriginally　distinct；　has　played　an　important　part　in　the　origin　of　our　domestic　productions。　When　in　any　country　several　domestic　breeds　have　once　been　established；　their　occasional　intercrossing；　with　the　aid　of　selection；　has；　no　doubt；　largely　aided　in　the　formation　of　new　sub…breeds；　but　the　importance　of　the　crossing　of　varieties　has；　I　believe；　been　greatly　exaggerated；　both　in　regard　to　animals　and　to　those　plants　which　are　propagated　by　seed。　In　plants　which　are　temporarily　propagated　by　cuttings；　buds；　&c。；　the　importance　of　the　crossing　both　of　distinct　species　and　of　varieties　is　immense；　for　the　cultivator　here　quite　disregards　the　extreme　variability　both　of　hybrids　and　mongrels；　and　the　frequent　sterility　of　hybrids；　but　the　cases　of　plants　not　propagated　by　seed　are　of　little　importance　to　us；　for　their　endurance　is　only　temporary。　Over　all　these　causes　of　Change　I　am　convinced　that　the　accumulative　action　of　Selection；　whether　applied　methodically　and　more　quickly；　or　unconsciously　and　more　slowly；　but　more　efficiently；　is　by　far　the　predominant　power。　　

The　Origin　of　Species
by　Charles　Darwin

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Chapter　2　…　Variation　Under　Nature

BEFORE　applying　the　principles　arrived　at　in　the　last　chapter　to　organic　beings　in　a　state　of　nature；　we　must　briefly　discuss　whether　these　latter　are　subject　to　any　variation。　To　treat　this　subject　at　all　properly；　a　long　catalogue　of　dry　facts　should　be　given；　but　these　I　shall　reserve　for　my　future　work。　Nor　shall　I　here　discuss　the　various　definitions　which　have　been　given　of　the　term　species。　No　one　definition　has　as　yet　satisfied　all　naturalists；　yet　every　naturalist　knows　vaguely　what　he　means　when　he　speaks　of　a　species。　Generally　the　term　includes　the　unknown　element　of　a　distinct　act　of　creation。　The　term　'variety'　is　almost　equally　difficult　to　define；　but　here　community　of　descent　is　almost　universally　implied；　though　it　can　rarely　be　proved。　We　have　also　what　are　called　monstrosities；　but　they　graduate　into　varieties。　By　a　monstrosity　I　presume　is　meant　some　considerable　deviation　of　structure　in　one　part；　either　injurious　to　or　not　useful　to　the　species；　and　not　generally　propagated。　Some　authors　use　the　term　'variation'　in　a　technical　sense；　as　implying　a　modification　directly　due　to　the　physical　conditions　of　life；　and　'variations'　in　this　sense　are　supposed　not　to　be　inherited：　but　who　can　say　that　the　dwarfed　condition　of　shells　in　the　brackish　waters　of　the　Baltic；　or　dwarfed　plants　on　Alpine　summits；　or　the　thicker　fur　of　an　animal　from　far　northwards；　would　not　in　some　cases　be　inherited　for　at　least　some　few　generations？　and　in　this　case　I　presume　that　the　form　would　be　called　a　variety。　
Again；　we　have　many　slight　differences　which　may　be　called　individual　differences；　such　as　are　known　frequently　to　appear　in　the　offspring　from　the　same　parents；　or　which　may　be　presumed　to　have　thus　arisen；　from　being　frequently　observed　in　the　individuals　of　the　same　species　inhabiting　the　same　confined　locality。　No　one　supposes　that　all　the　individuals　of　the　same　species　are　cast　in　the　very　same　mould。　These　individual　differences　are　highly　important　for　us；　as　they　afford　materials　for　natural　selection　to　accumulate；　in　the　same　manner　as　man　can　accumulate　in　any　given　direction　individual　differences　in　his　domesticated　productions。　These　individual　differences　generally　affect　what　naturalists　consider　unimportant　parts；　but　I　could　show　by　a　long　catalogue　of　facts；　that　parts　which　must　be　called　important；　whether　viewed　under　a　physiological　or　classificatory　point　of　view；　sometimes　vary　in　the　individuals　of　the　same　species。　I　am　convinced　that　the　most　experienced　naturalist　would　be　surprised　at　the　number　of　the　cases　of　variability；　even　in　important　parts　of　structure；　which　he　could　collect　on　good　authority；　as　I　have　collected；　during　a　course　of　years。　It　should　be　remembered　that　systematists　are　far　from　pleased　at　finding　variability　in　important　characters；　and　that　there　are　not　many　men　who　will　laboriously　examine　internal　and　important　organs；　and　compare　them　in　many　specimens　of　the　same　species。　I　should　never　have　expected　that　the　branching　of　the　main　nerves　close　to　the　great　central　ganglion　of　an　insect　would　have　been　variable　in　the　same　species；　I　should　have　expected　that　changes　of　this　nature　could　have　been　effected　only　by　slow　degrees：　yet　quite　recently　Mr　Lubbock　has　shown　a　degree　of　variability　in　these　main　nerves　in　Coccus；　which　may　almost　be　compared　to　the　irregular　branching　of　the　stem　of　a　tree。　This　philosophical　naturalist；　I　may　add；　has　also　quite　recently　shown　that　the　muscles　in　the　larvae　of　certain　insects　are　very　far　from　uniform。　Authors　sometimes　argue　in　a　circle　when　they　state　that　important　organs　never　vary；　for　these　same　authors　practically　rank　that　character　as　important　（as　some　few　naturalists　have　honestly　confessed）　which　does　not　vary；　and；　under　this　point　of　view；　no　instance　of　any　important　part　varying　will　ever　be　found：　but　under　any　other　point　of　view　many　instances　assuredly　can　be　given。　
There　is　one　point　connected　with　individual　differences；　which　seems　to　me　extremely　perplexing：　I　refer　to　those　genera　which　have　sometimes　been　called　'protean'　or　'polymorphic；'　in　which　the　species　present　an　inordinate　amount　of　variation；　and　hardly　two　naturalists　can　agree　which　forms　to　rank　as　species　and　which　as　varieties。　We　may　instance　Rubus；　Rosa；　and　Hieracium　amongst　plants；　several　genera　of　insects；　and　several　genera　of　Brachiopod　shells。　In　most　polymorphic　genera　some　of　the　species　have　fixed　and　definite　characters。　Genera　which　are　polymorphic　in