the origin of species(物种起源)-第1节

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by　Charles　Darwin

　Next　Chapter　

Chapter　1　…　Variation　Under　Domestication

WHEN　we　look　to　the　individuals　of　the　same　variety　or　sub…variety　of　our　older　cultivated　plants　and　animals；　one　of　the　first　points　which　strikes　us；　is；　that　they　generally　differ　much　more　from　each　other；　than　do　the　individuals　of　any　one　species　or　variety　in　a　state　of　nature。　When　we　reflect　on　the　vast　diversity　of　the　plants　and　animals　which　have　been　cultivated；　and　which　have　varied　during　all　ages　under　the　most　different　climates　and　treatment；　I　think　we　are　driven　to　conclude　that　this　greater　variability　is　simply　due　to　our　domestic　productions　having　been　raised　under　conditions　of　life　not　so　uniform　as；　and　somewhat　different　from；　those　to　which　the　parent…species　have　been　exposed　under　nature。　There　is；　also；　I　think；　some　probability　in　the　view　propounded　by　Andrew　Knight；　that　this　variability　may　be　partly　connected　with　excess　of　food。　It　seems　pretty　clear　that　organic　beings　must　be　exposed　during　several　generations　to　the　new　conditions　of　life　to　cause　any　appreciable　amount　of　variation；　and　that　when　the　organisation　has　once　begun　to　vary；　it　generally　continues　to　vary　for　many　generations。　No　case　is　on　record　of　a　variable　being　ceasing　to　be　variable　under　cultivation。　Our　oldest　cultivated　plants；　such　as　wheat；　still　often　yield　new　varieties：　our　oldest　domesticated　animals　are　still　capable　of　rapid　improvement　or　modification。　
It　has　been　disputed　at　what　period　of　time　the　causes　of　variability；　whatever　they　may　be；　generally　act；　whether　during　the　early　or　late　period　of　development　of　the　embryo；　or　at　the　instant　of　conception。　Geoffroy　St　Hilaire's　experiments　show　that　unnatural　treatment　of　the　embryo　causes　monstrosities；　and　monstrosities　cannot　be　separated　by　any　clear　line　of　distinction　from　mere　variations。　But　I　am　strongly　inclined　to　suspect　that　the　most　frequent　cause　of　variability　may　be　attributed　to　the　male　and　female　reproductive　elements　having　been　affected　prior　to　the　act　of　conception。　Several　reasons　make　me　believe　in　this；　but　the　chief　one　is　the　remarkable　effect　which　confinement　or　cultivation　has　on　the　functions　of　the　reproductive　system；　this　system　appearing　to　be　far　more　susceptible　than　any　other　part　of　the　organization；　to　the　action　of　any　change　in　the　conditions　of　life。　Nothing　is　more　easy　than　to　tame　an　animal；　and　few　things　more　difficult　than　to　get　it　to　breed　freely　under　confinement；　even　in　the　many　cases　when　the　male　and　female　unite。　How　many　animals　there　are　which　will　not　breed；　though　living　long　under　not　very　close　confinement　in　their　native　country！　This　is　generally　attributed　to　vitiated　instincts；　but　how　many　cultivated　plants　display　the　utmost　vigour；　and　yet　rarely　or　never　seed！　In　some　few　such　cases　it　has　been　found　out　that　very　trifling　changes；　such　as　a　little　more　or　less　water　at　some　particular　period　of　growth；　will　determine　whether　or　not　the　plant　sets　a　seed。　I　cannot　here　enter　on　the　copious　details　which　I　have　collected　on　this　curious　subject；　but　to　show　how　singular　the　laws　are　which　determine　the　reproduction　of　animals　under　confinement；　I　may　just　mention　that　carnivorous　animals；　even　from　the　tropics；　breed　in　this　country　pretty　freely　under　confinement；　with　the　exception　of　the　plantigrades　or　bear　family；　whereas；　carnivorous　birds；　with　the　rarest　exceptions；　hardly　ever　lay　fertile　eggs。　Many　exotic　plants　have　pollen　utterly　worthless；　in　the　same　exact　condition　as　in　the　most　sterile　hybrids。　When；　on　the　one　hand；　we　see　domesticated　animals　and　plants；　though　often　weak　and　sickly；　yet　breeding　quite　freely　under　confinement；　and　when；　on　the　other　hand；　we　see　individuals；　though　taken　young　from　a　state　of　nature；　perfectly　tamed；　long…lived；　and　healthy　（of　which　I　could　give　numerous　instances）；　yet　having　their　reproductive　system　so　seriously　affected　by　unperceived　causes　as　to　fail　in　acting；　we　need　not　be　surprised　at　this　system；　when　it　does　act　under　confinement；　acting　not　quite　regularly；　and　producing　offspring　not　perfectly　like　their　parents　or　variable。　
Sterility　has　been　said　to　be　the　bane　of　horticulture；　but　on　this　view　we　owe　variability　to　the　same　cause　which　produces　sterility；　and　variability　is　the　source　of　all　the　choicest　productions　of　the　garden。　I　may　add；　that　as　some　organisms　will　breed　most　freely　under　the　most　unnatural　conditions　（for　instance；　the　rabbit　and　ferret　kept　in　hutches）；　showing　that　their　reproductive　system　has　not　been　thus　affected；　so　will　some　animals　and　plants　withstand　domestication　or　cultivation；　and　vary　very　slightly　perhaps　hardly　more　than　in　a　state　of　nature。　
A　long　list　could　easily　be　given　of　'sporting　plants；'　by　this　term　gardeners　mean　a　single　bud　or　offset；　which　suddenly　assumes　a　new　and　sometimes　very　different　character　from　that　of　the　rest　of　the　plant。　Such　buds　can　be　propagated　by　grafting；　&c。；　and　sometimes　by　seed。　These　'sports'　are　extremely　rare　under　nature；　but　far　from　rare　under　cultivation；　and　in　this　case　we　see　that　the　treatment　of　the　parent　has　affected　a　bud　or　offset；　and　not　the　ovules　or　pollen。　But　it　is　the　opinion　of　most　physiologists　that　there　is　no　essential　difference　between　a　bud　and　an　ovule　in　their　earliest　stages　of　formation；　so　that；　in　fact；'sports'　support　my　view；　that　variability　may　be　largely　attributed　to　the　ovules　or　pollen；　or　to　both；　having　been　affected　by　the　treatment　of　the　parent　prior　to　the　act　of　conception。　These　cases　anyhow　show　that　variation　is　not　necessarily　connected；　as　some　authors　have　supposed；　with　the　act　of　generation。　
Seedlings　from　the　same　fruit；　and　the　young　of　the　same　litter；　sometimes　differ　considerably　from　each　other；　though　both　the　young　and　the　parents；　as　Muller　has　remarked；　have　apparently　been　exposed　to　exactly　the　same　conditions　of　life；　and　this　shows　how　unimportant　the　direct　effects　of　the　conditions　of　life　are　in　comparison　with　the　laws　of　reproduction；　and　of　growth；　and　of　inheritance；　for　had　the　action　of　the　conditions　been　direct；　if　any　of　the　young　had　varied；　all　would　probably　have　varied　in　the　same　manner。　To　judge　how　much；　in　the　case　of　any　variation；　we　should　attribute　to　the　direct　action　of　heat；　moisture；　light；　food；　&c。；　is　most　difficult：　my　impression　is；　that　with　animals　such　agencies　have　produced　very　little　direct　effect；　though　apparently　more　in　the　case　of　plants。　Under　this　point　of　view；　Mr　Buckman's　recent　experiments　on　plants　seem　extremely　valuable。　When　all　or　nearly　all　the　individuals　exposed　to　certain　conditions　are　affected　in　the　same　way；　the　change　at　first　appears　to　be　directly　due　to　such　conditions；　but　in　some　cases　it　can　be　shown　that　quite　opposite　conditions　produce　similar　changes　of　structure。　Nevertheless　some　slight　amount　of　change　may；　I　think；　be　attributed　to　the　direct　action　of　the　conditions　of　life　as；　in　some　cases；　increased　size　from　amount　of　food；　colour　from　particular　kinds　of　food　and　from　light；　and　perhaps　the　thickness　of　fur　from　climate。　
Habit　also　has　a　deciding　influence；　as　in　the　period　of　flowering　with　plants　when　transported　from　one　climate　to　another。　In　animals　it　has　a　more　marked　effect；　for　instance；　I　find　in　the　domestic　duck　that　the　bones　of　the　wing　weigh　less　and　the　bones　of　the　leg　more；　in　proportion　to　the　whole　skeleton；　than　do　the　same　bones　in　the　wild…duck；　and　I　presume　that　this　change　may　be　safely　attributed　to　the　domestic　duck　flying　much　less；　and　walking　more；　than　its　wild　parent。　The　great　and　inherited　development　of　the　udders　in　cows　and　goats　in　countries　where　they　are　habitually　milked；　in　comparison　with　the　state　of　these　organs　in　other　countries；　is　another　instance　of　the　effect　of　use。　Not　a　single　domestic　animal　can　be　named　which　has　not　in　some　country　drooping　ears；　and　the　view　suggested　by　some　authors；　that　the　drooping　is　due　to　the　disuse　of　the　muscles　of　the　ear；　from　the　animals　not　being　much　alarmed　by　danger；　seems　probable。　
There　are　many　laws　regulating　variation；　some　few　of　which　can　be　dimly　seen；　and　will　be　hereafter　briefly　mentioned。　I　will　here　only　allude　to　what　may　be　called　correlation　of　growth。　Any　change　in　the　embryo　or　larva　will　almost　certainly　entail　changes　in　the　mature　animal。　In　monstrosities；　the　correlations　between　quite　distinct　parts　are　very　curious；　and　many　instances　are　given　in　Isidore　Geoffroy　St　Hilaire's　great　work　on　this　subject。　Breeders　believe　that　long　limbs　are　almost　always　accompanied　by　an　elongated　head。　Some　instances　of　correlation　are　quite　whimsical；　thus　cats　with　blue　eyes　are　invariably　deaf；　colour　and　constitutional　peculiarities　go　together；　of　which　many　remarkable　cases　could　be　given　amongst　animals　and　plants。　From　the　facts　collected　by　Heusinger；　it　appears　that　white　sheep　and　pigs　are　differently　affected　from　coloured　individuals　by　certain　vegetable　poisons。　Hairless　dogs　have　imperfect　teeth；　long…haired　and　coarse…haired　animals　are　apt　to　have；　as　is　asserted；　long　or　many　horns；　pigeons　with　feathered　feet　have　skin　between　their　outer　toes；　pigeons　with　short　beaks　have　sma