the origin of species(物种起源)-第7章

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ll　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　one　country　seem　to　be；　with　some　few　exceptions；　polymorphic　in　other　countries；　and　likewise；　judging　from　Brachiopod　shells；　at　former　periods　of　time。　These　facts　seem　to　be　very　perplexing；　for　they　seem　to　show　that　this　kind　of　variability　is　independent　of　the　conditions　of　life。　I　am　inclined　to　suspect　that　we　see　in　these　polymorphic　genera　variations　in　points　of　structure　which　are　of　no　service　or　disservice　to　the　species；　and　which　consequently　have　not　been　seized　on　and　rendered　definite　by　natural　selection；　as　hereafter　will　be　explained。　
Those　forms　which　possess　in　some　considerable　degree　the　character　of　species；　but　which　are　so　closely　similar　to　some　other　forms；　or　are　so　closely　linked　to　them　by　intermediate　gradations；　that　naturalists　do　not　like　to　rank　them　as　distinct　species；　are　in　several　respects　the　most　important　for　us。　We　have　every　reason　to　believe　that　many　of　these　doubtful　and　closely…allied　forms　have　permanently　retained　their　characters　in　their　own　country　for　a　long　time；　for　as　long；　as　far　as　we　know；　as　have　good　and　true　species。　practically；　when　a　naturalist　can　unite　two　forms　together　by　others　having　intermediate　characters；　he　treats　the　one　as　a　variety　of　the　other；　ranking　the　most　common；　but　sometimes　the　one　first　described；　as　the　species；　and　the　other　as　the　variety。　But　cases　of　great　difficulty；　which　I　will　not　here　enumerate；　sometimes　occur　in　deciding　whether　or　not　to　rank　one　form　as　a　variety　of　another；　even　when　they　are　closely　connected　by　intermediate　links；　nor　will　the　commonly…assumed　hybrid　nature　of　the　intermediate　links　always　remove　the　difficulty。　In　very　many　cases；　however；　one　form　is　ranked　as　a　variety　of　another；　not　because　the　intermediate　links　have　actually　been　found；　but　because　analogy　leads　the　observer　to　suppose　either　that　they　do　now　somewhere　exist；　or　may　formerly　have　existed；　and　here　a　wide　door　for　the　entry　of　doubt　and　conjecture　is　opened。　
Hence；　in　determining　whether　a　form　should　be　ranked　as　a　species　or　a　variety；　the　opinion　of　naturalists　having　sound　judgement　and　wide　experience　seems　the　only　guide　to　follow。　We　must；　however；　in　many　cases；　decide　by　a　majority　of　naturalists；　for　few　well…marked　and　well…known　varieties　can　be　named　which　have　not　been　ranked　as　species　by　at　least　some　competent　judges。　
That　varieties　of　this　doubtful　nature　are　far　from　uncommon　cannot　be　disputed。　Compare　the　several　floras　of　Great　Britain；　of　France　or　of　the　United　States；　drawn　up　by　different　botanists；　and　see　what　a　surprising　number　of　forms　have　been　ranked　by　one　botanist　as　good　species；　and　by　another　as　mere　varieties。　Mr　H。　C。　Watson；　to　whom　I　lie　under　deep　obligation　for　assistance　of　all　kinds；　has　marked　for　me　182　British　plants；　which　are　generally　considered　as　varieties；　but　which　have　all　been　ranked　by　botanists　as　species；　and　in　making　this　list　he　has　omitted　many　trifling　varieties；　but　which　nevertheless　have　been　ranked　by　some　botanists　as　species；　and　he　has　entirely　omitted　several　highly　polymorphic　genera。　Under　genera；　including　the　most　polymorphic　forms；　Mr　Babington　gives　251　species；　whereas　Mr　Bentham　gives　only　112；　a　difference　of　139　doubtful　forms！　Amongst　animals　which　unite　for　each　birth；　and　which　are　highly　locomotive；　doubtful　forms；　ranked　by　one　zoologist　as　a　species　and　by　another　as　a　variety；　can　rarely　be　found　within　the　same　country；　but　are　common　in　separated　areas。　How　many　of　those　birds　and　insects　in　North　America　and　Europe；　which　differ　very　slightly　from　each　other；　have　been　ranked　by　one　eminent　naturalist　as　undoubted　species；　and　by　another　as　varieties；　or；　as　they　are　often　called；　as　geographical　races！　Many　years　ago；　when　comparing；　and　seeing　others　compare；　the　birds　from　the　separate　islands　of　the　Galapagos　Archipelago；　both　one　with　another；　and　with　those　from　the　American　mainland；　I　was　much　struck　how　entirely　vague　and　arbitrary　is　the　distinction　between　species　and　varieties。　On　the　islets　of　the　little　Madeira　group　there　are　many　insects　which　are　characterized　as　varieties　in　Mr　Wollaston's　admirable　work；　but　which　it　cannot　be　doubted　would　be　ranked　as　distinct　species　by　many　entomologists。　Even　Ireland　has　a　few　animals；　now　generally　regarded　as　varieties；　but　which　have　been　ranked　as　species　by　some　zoologists。　Several　most　experienced　ornithologists　consider　our　British　red　grouse　as　only　a　strongly…marked　race　of　a　Norwegian　species；　whereas　the　greater　number　rank　it　as　an　undoubted　species　peculiar　to　Great　Britain。　A　wide　distance　between　the　homes　of　two　doubtful　forms　leads　many　naturalists　to　rank　both　as　distinct　species；　but　what　distance；　it　has　been　well　asked；　will　suffice？　if　that　between　America　and　Europe　is　ample；　will　that　between　the　Continent　and　the　Azores；　or　Madeira；　or　the　Canaries；　or　Ireland；　be　sufficient？　It　must　be　admitted　that　many　forms；　considered　by　highly…competent　judges　as　varieties；　have　so　perfectly　the　character　of　species　that　they　are　ranked　by　other　highly…co