the origin of species(物种起源)-第11节

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　as　geology　plainly　tells　us　that　small　genera　have　in　the　lapse　of　time　often　increased　greatly　in　size；　and　that　large　genera　have　often　come　to　their　maxima；　declined；　and　disappeared。　All　that　we　want　to　show　is；　that　where　many　species　of　a　genus　have　been　formed；　on　an　average　many　are　still　forming；　and　this　holds　good。　
There　are　other　relations　between　the　species　of　large　genera　and　their　recorded　varieties　which　deserve　notice。　We　have　seen　that　there　is　no　infallible　criterion　by　which　to　distinguish　species　and　well…marked　varieties；　and　in　those　cases　in　which　intermediate　links　have　not　been　found　between　doubtful　forms；　naturalists　are　compelled　to　come　to　a　determination　by　the　amount　of　difference　between　them；　judging　by　analogy　whether　or　not　the　amount　suffices　to　raise　one　or　both　to　the　rank　of　species。　Hence　the　amount　of　difference　is　one　very　important　criterion　in　settling　whether　two　forms　should　be　ranked　as　species　or　varieties。　Now　Fries　has　remarked　in　regard　to　plants；　and　Westwood　in　regard　to　insects；　that　in　large　genera　the　amount　of　difference　between　the　species　is　often　exceedingly　small。　I　have　endeavoured　to　test　this　numerically　by　averages；　and；　as　far　as　my　imperfect　results　go；　they　always　confirm　the　view。　I　have　also　consulted　some　sagacious　and　most　experienced　observers；　and；　after　deliberation；　they　concur　in　this　view。　In　this　respect；　therefore；　the　species　of　the　larger　genera　resemble　varieties；　more　than　do　the　species　of　the　smaller　genera。　Or　the　case　may　be　put　in　another　way；　and　it　may　be　said；　that　in　the　larger　genera；　in　which　a　number　of　varieties　or　incipient　species　greater　than　the　average　are　now　manufacturing；　many　of　the　species　already　manufactured　still　to　a　certain　extent　resemble　varieties；　for　they　differ　from　each　other　by　a　less　than　usual　amount　of　difference。　
Moreover；　the　species　of　the　large　genera　are　related　to　each　other；　in　the　same　manner　as　the　varieties　of　any　one　species　are　related　to　each　other。　No　naturalist　pretends　that　all　the　species　of　a　genus　are　equally　distinct　from　each　other；　they　may　generally　be　divided　into　sub…genera；　or　sections；　or　lesser　groups。　As　Fries　has　well　remarked；　little　groups　of　species　are　generally　clustered　like　satellites　around　certain　other　species。　And　what　are　varieties　but　groups　of　forms；　unequally　related　to　each　other；　and　clustered　round　certain　forms　that　is；　round　their　parent…species？　Undoubtedly　there　is　one　most　important　point　of　difference　between　varieties　and　species；　namely；　that　the　amount　of　difference　between　varieties；　when　compared　with　each　other　or　with　their　parent…species；　is　much　less　than　that　between　the　species　of　the　same　genus。　But　when　we　come　to　discuss　the　principle；　as　I　call　it；　of　Divergence　of　Character；　we　shall　see　how　this　may　be　explained；　and　how　the　lesser　differences　between　varieties　will　tend　to　increase　into　the　greater　differences　between　species。　
There　is　one　other　point　which　seems　to　me　worth　notice。　Varieties　generally　have　much　restricted　ranges：　this　statement　is　indeed　scarcely　more　than　a　truism；　for　if　a　variety　were　found　to　have　a　wider　range　than　that　of　its　supposed　parent…species；　their　denominations　ought　to　be　reversed。　But　there　is　also　reason　to　believe；　that　those　species　which　are　very　closely　allied　to　other　species；　and　in　so　far　resemble　varieties；　often　have　much　restricted　ranges。　For　instance；　Mr　H。　C。　Watson　has　marked　for　me　in　the　well…sifted　London　Catalogue　of　plants　（4th　edition）　63　plants　which　are　therein　ranked　as　species；　but　which　he　considers　as　so　closely　allied　to　other　species　as　to　be　of　doubtful　value：　these　63　reputed　species　range　on　an　average　over　6。9　of　the　provinces　into　which　Mr　Watson　has　divided　Great　Britain。　Now；　in　this　same　catalogue；　53　acknowledged　varieties　are　recorded；　and　these　range　over　7。7　provinces；　whereas；　the　species　to　which　these　varieties　belong　range　over　14。3　provinces。　So　that　the　acknowledged　varieties　have　very　nearly　the　same　restricted　average　range；　as　have　those　very　closely　allied　forms；　marked　for　me　by　Mr　Watson　as　doubtful　species；　but　which　are　almost　universally　ranked　by　British　botanists　as　good　and　true　species。　
Finally；　then；　varieties　have　the　same　general　characters　as　species；　for　they　cannot　be　distinguished　from　species；　except；　firstly；　by　the　discovery　of　intermediate　linking　forms；　and　the　occurrence　of　such　links　cannot　affect　the　actual　characters　of　the　forms　which　they　connect；　and　except；　secondly；　by　a　certain　amount　of　difference；　for　two　forms；　if　differing　very　little；　are　generally　ranked　as　varieties；　notwithstanding　that　intermediate　linking　forms　have　not　been　discovered；　but　the　amount　of　difference　considered　necessary　to　give　to　two　forms　the　rank　of　species　is　quite　indefinite。　In　genera　having　more　than　the　average　number　of　species　in　any　country；　the　species　of　these　genera　have　more　than　the　average　number　of　varieties。　In　large　genera　the　species　are　apt　to　be　closely；　but　unequally；　allied　together；　forming　little　clusters　round　certain　species。　Species　very　closely　allied　to　other　species　apparently　have　restricted　ranges。　In　all　these　several　respects　the　species　of　large　genera　present　a　strong　analogy　with　varieties。　And　we　can　clearly　understand　these　analogies；　if　species　have　once　existed　as　varieties；　and　have　thus　originated：　whereas；　these　analogies　are　utterly　inexplicable　if　each　species　has　been　independently　created。　
We　have；　also；　seen　that　it　is　the　most　flourishing　and　dominant　species　of　the　larger　genera　which　on　an　average　vary　most；　and　varieties；　as　we　shall　hereafter　see；　tend　to　become　converted　into　new　and　distinct　species。　The　larger　genera　thus　tend　to　become　larger；　and　throughout　nature　the　forms　of　life　which　are　now　dominant　tend　to　become　still　more　dominant　by　leaving　many　modified　and　dominant　descendants。　But　by　steps　hereafter　to　be　explained；　the　larger　genera　also　tend　to　break　up　into　smaller　genera。　And　thus；　the　forms　of　life　throughout　the　universe　become　divided　into　groups　subordinate　to　groups。　　
The　Origin　of　Species
by　Charles　Darwin

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Chapter　3　…　Struggle　for　Existence

BEF0RE　entering　on　the　subject　of　this　chapter；　I　must　make　a　few　preliminary　remarks；　to　show　how　the　struggle　for　existence　bears　on　Natural　Selection。　It　has　been　seen　in　the　last　chapter　that　amongst　organic　beings　in　a　state　of　nature　there　is　some　individual　variability；　indeed　I　am　not　aware　that　this　has　ever　been　disputed。　It　is　immaterial　for　us　whether　a　multitude　of　doubtful　forms　be　called　species　or　sub…species　or　varieties；　what　rank；　for　instance；　the　two　or　three　hundred　doubtful　forms　of　British　plants　are　entitled　to　hold；　if　the　existence　of　any　well…marked　varieties　be　admitted。　But　the　mere　existence　of　individual　variability　and　of　some　few　well…marked　varieties；　though　necessary　as　the　foundation　for　the　work；　helps　us　but　little　in　understanding　how　species　arise　in　nature。　How　have　all　those　exquisite　adaptations　of　one　part　of　the　organisation　to　another　part；　and　to　the　conditions　of　life；　and　of　one　distinct　organic　being　to　another　being；　been　perfected？　We　see　these　beautiful　co…adaptations　most　plainly　in　the　woodpecker　and　missletoe；　and　only　a　little　less　plainly　in　the　humblest　parasite　which　clings　to　the　hairs　of　a　quadruped　or　feathers　of　a　bird；　in　the　structure　of　the　beetle　which　dives　through　the　water；　in　the　plumed　seed　which　is　wafted　by　the　gentlest　breeze；　in　short；　we　see　beautiful　adaptations　everywhere　and　in　every　part　of　the　organic　world。　
Again；　it　may　be　asked；　how　is　it　that　varieties；　which　I　have　called　incipient　species；　become　ultimately　converted　into　good　and　distinct　species；　which　in　most　cases　obviously　differ　from　each　other　far　more　than　do　the　varieties　of　the　same　species？　How　do　those　groups　of　species；　which　constitute　what　are　called　distinct　genera；　and　which　differ　from　each　other　more　than　do　the　species　of　the　same　genus；　arise？　All　these　results；　as　we　shall　more　fully　see　in　the　next　chapter；　follow　inevitably　from　the　struggle　for　life。　Owing　to　this　struggle　for　life；　any　variation；　however　slight　and　from　whatever　cause　proceeding；　if　it　be　in　any　degree　profitable　to　an　individual　of　any　species；　in　its　infinitely　complex　relations　to　other　organic　beings　and　to　external　nature；　will　tend　to　the　preservation　of　that　individual；　and　will　generally　be　inherited　by　its　offspring。　The　offspring；　also；　will　thus　have　a　better　chance　of　surviving；　for；　of　the　many　individuals　of　any　species　which　are　periodically　born；　but　a　small　number　can　survive。　I　have　called　this　principle；　by　which　each　slight　variation；　if　useful；　is　preserved；　by　the　term　of　Natural　Selection；　in　order　to　mark　its　relation　to　man's　power　of　selection。　We　have　seen　that　man　by　selection　can　certainly　produce　great　results；　and　can　adapt　organic　beings　to　his　own　uses；　through　the　accumulation　of　slight　but　useful　variations；　given　to　him　by　the　hand　of　Nature。　But　Natural　Selection；　as　we　shall　hereafter　see；　is　a　power　incessantly　ready　for　action；　and　is　as　immeasurably　superior　to　man's　feeble　efforts；　as　the　works　of　Nature　ar