the origin of species(物种起源)-第12章

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　common　red　clover　（Trifolium　pratense）；　as　other　bees　cannot　reach　the　nectar。　Hence　I　have　very　little　doubt；　that　if　the　whole　genus　of　humble…bees　became　extinct　or　very　rare　in　England；　the　heartsease　and　red　clover　would　become　very　rare；　or　wholly　disappear。　The　number　of　humble…bees　in　any　district　depends　in　a　great　degree　on　the　number　of　field…mice；　which　destroy　their　combs　and　nests；　and　Mr　H。　Newman；　who　has　long　attended　to　the　habits　of　humble…bees；　believes　that　'more　than　two　thirds　of　them　are　thus　destroyed　all　over　England。'　Now　the　number　of　mice　is　largely　dependent；　as　every　one　knows；　on　the　number　of　cats；　and　Mr　Newman　says；　'Near　villages　and　small　towns　I　have　found　the　nests　of　humble…bees　more　numerous　than　elsewhere；　which　I　attribute　to　the　number　of　cats　that　destroy　the　mice。'　Hence　it　is　quite　credible　that　the　presence　of　a　feline　animal　in　large　numbers　in　a　district　might　determine；　through　the　intervention　first　of　mice　and　then　of　bees；　the　frequency　of　certain　flowers　in　that　district！　
In　the　case　of　every　species；　many　different　checks；　acting　at　different　periods　of　life；　and　during　different　seasons　or　years；　probably　come　into　play；　some　one　check　or　some　few　being　generally　the　most　potent；　but　all　concurring　in　determining　the　average　number　or　even　the　existence　of　the　species。　In　some　cases　it　can　be　shown　that　widely…different　checks　act　on　the　same　species　in　different　districts。　When　we　look　at　the　plants　and　bushes　clothing　an　entangled　bank；　we　are　tempted　to　attribute　their　proportional　numbers　and　kinds　to　what　we　call　chance。　But　how　false　a　view　is　this！　Every　one　has　heard　that　when　an　American　forest　is　cut　down；　a　very　different　vegetation　springs　up；　but　it　has　been　observed　that　the　trees　now　growing　on　the　ancient　Indian　mounds；　in　the　Southern　United　States；　display　the　same　beautiful　diversity　and　proportion　of　kinds　as　in　the　surrounding　virgin　forests。　What　a　struggle　between　the　several　kinds　of　trees　must　here　have　gone　on　during　long　centuries；　each　annually　scattering　its　seeds　by　the　thousand；　what　war　between　insect　and　insect　between　insects；　snails；　and　other　animals　with　birds　and　beasts　of　prey　all　striving　to　increase；　and　all　feeding　on　each　other　or　on　the　trees　or　their　seeds　and　seedlings；　or　on　the　other　plants　which　first　clothed　the　ground　and　thus　checked　the　growth　of　the　trees！　Throw　up　a　handful　of　feathers；　and　all　must　fall　to　the　ground　according　to　definite　laws；　but　how　simple　is　this　problem　compared　to　the　action　and　reaction　of　the　innumerable　plants　and　animals　which　have　determined；　in　the　course　of　centuries；　the　proportional　numbers　and　kinds　of　trees　now　growing　on　the　old　Indian　ruins！　
The　dependency　of　one　organic　being　on　another；　as　of　a　parasite　on　its　prey；　lies　generally　between　beings　remote　in　the　scale　of　nature。　This　is　often　the　case　with　those　which　may　strictly　be　said　to　struggle　with　each　other　for　existence；　as　in　the　case　of　locusts　and　grass…feeding　quadrupeds。　But　the　struggle　almost　invariably　will　be　most　severe　between　the　individuals　of　the　same　species；　for　they　frequent　the　same　districts；　require　the　same　food；　and　are　exposed　to　the　same　dangers。　In　the　case　of　varieties　of　the　same　species；　the　struggle　will　generally　be　almost　equally　severe；　and　we　sometimes　see　the　contest　soon　decided：　for　instance；　if　several　varieties　of　wheat　be　sown　together；　and　the　mixed　seed　be　resown；　some　of　the　varieties　which　best　suit　the　soil　or　climate；　or　are　naturally　the　most　fertile；　will　beat　the　others　and　so　yield　more　seed；　and　will　consequently　in　a　few　years　quite　supplant　the　other　varieties。　To　keep　up　a　mixed　stock　of　even　such　extremely　close　varieties　as　the　variously　coloured　sweet…peas；　they　must　be　each　year　harvested　separately；　and　the　seed　then　mixed　in　due　proportion；　otherwise　the　weaker　kinds　will　steadily　decrease　in　numbers　and　disappear。　So　again　with　the　varieties　of　sheep：　it　has　been　asserted　that　certain　mountain…varieties　will　starve　out　other　mountain…varieties；　so　that　they　cannot　be　kept　together。　The　same　result　has　followed　from　keeping　together　different　varieties　of　the　medicinal　leech。　It　may　even　be　doubted　whether　the　varieties　of　any　one　of　our　domestic　plants　or　animals　have　so　exactly　the　same　strength；　habits；　and　constitution；　that　the　original　proportions　of　a　mixed　stock　could　be　kept　up　for　half　a　dozen　generations；　if　they　were　allowed　to　struggle　together；　like　beings　in　a　state　of　nature；　and　if　the　seed　or　young　were　not　annually　sorted。　
As　species　of　the　same　genus　have　usually；　though　by　no　means　invariably；　some　similarity　in　habits　and　constitution；　and　always　in　structure；　the　struggle　will　generally　be　more　severe　between　species　of　the　same　genus；　when　they　come　into　competition　with　each　other；　than　between　species　of　distinct　genera。　We　see　this　in　the　recent　extension　over　parts　of　the　United　States　of　one　species　of　swallow　having　caused　the　decrease　of　another　species。　The　recent　increase　of　the　missel…thrush　in　parts　of　Scotland　has　caused　the　decrease　of　the　song…thrush。　How　frequently　we　hear　of　one　species　of　rat　taking　the　place　of　another　species　under　the　most　different　climates！　In　Russia　the　small　Asiatic　cockroach　has　everywhere　driven　before　it　its　great　congener。　One　species　of　charlock　will　supplant　another；　and　so　in　other　cases。　We　can　dimly　see　why　the　competition　should　be　most　severe　between　allied　forms；　which　fill　nearly　the　same　place　in　the　economy　of　nature；　but　probably　in　no　one　case　could　we　precisely　say　why　one　species　has　been　victorious　over　another　in　the　great　battle　of　life。　
A　corollary　of　the　highest　importance　may　be　deduced　from　the　foregoing　remarks；　namely；　that　the　structure　of　every　organic　being　is　related；　in　the　most　essential　yet　often　hidden　manner；　to　that　of　all　other　organic　beings；　with　which　it　comes　into　competition　for　food　or　residence；　or　from　which　it　has　to　escape；　or　on　which　it　preys。　This　is　obvious　in　the　structure　of　the　teeth　and　talons　of　the　tiger；　and　in　that　of　the　legs　and　claws　of　the　parasite　which　clings　to　the　hair　on　the　tiger's　body。　But　in　the　beautifully　plumed　seed　of　the　dandelion；　and　in　the　flattened　and　fringed　legs　of　the　water…beetle；　the　relation　seems　at　first　confined　to　the　elements　of　air　and　water。　Yet　the　advantage　of　plumed　seeds　no　doubt　stands　in　the　closest　relation　to　the　land　being　already　thickly　clothed　by　other　plants；　so　that　the　seeds　may　be　widely　distributed　and　fall　on　unoccupied　ground。　In　the　water…beetle；　the　structure　of　its　legs；　so　well　adapted　for　diving；　allows　it　to　compete　with　other　aquatic　insects；　to　hunt　for　its　own　prey；　and　to　escape　serving　as　prey　to　other　animals。　
The　store　of　nutriment　laid　up　within　the　seeds　of　many　plants　seems　at　first　sight　to　have　no　sort　of　relation　to　other　plants。　But　from　the　strong　growth　of　young　plants　produced　from　such　seeds　（as　peas　and　beans）；　when　sown　in　the　midst　of　long　grass；　I　suspect　that　the　chief　use　of　the　nutriment　in　the　seed　is　to　favour　the　growth　of　the　young　seedling；　whilst　struggling　with　other　plants　growing　vigorously　all　around。　
Look　at　a　plant　in　the　midst　of　its　range；　why　does　it　not　double　or　quadruple　its　numbers？　We　know　that　it　can　perfectly　well　withstand　a　little　more　heat　or　cold；　dampness　or　dryness；　for　elsewhere　it　ranges　into　slightly　hotter　or　colder；　damper　or　drier　districts。　In　this　case　we　can　clearly　see　that　if　we　wished　in　imagination　to　give　the　plant　the　power　of　increasing　in　number；　we　should　have　to　give　it　some　advantage　over　its　competitors；　or　over　the　animals　which　preyed　on　it。　On　the　confines　of　its　geographical　range；　a　change　of　constitution　with　respect　to　climate　would　clearly　be　an　advantage　to　our　plant；　but　we　have　reason　to　believe　that　only　a　few　plants　or　animals　range　so　far；　that　they　are　destroyed　by　the　rigour　of　the　climate　alone。　Not　until　we　reach　the　extreme　confines　of　life；　in　the　arctic　regions　or　on　the　borders　of　an　utter　desert；　will　competition　cease。　The　land　may　be　extremely　cold　or　dry；　yet　there　will　be　competition　between　some　few　species；　or　between　the　individuals　of　the　same　species；　for　the　warmest　or　dampest　spots。　
Hence；　also；　we　can　see　that　when　a　plant　or　animal　is　placed　in　a　new　country　amongst　new　competitors；　though　the　climate　may　be　exactly　the　same　as　in　its　former　home；　yet　the　conditions　of　its　life　will　generally　be　changed　in　an　essential　manner。　If　we　wished　to　increase　its　average　numbers　in　its　new　home；　we　should　have　to　modify　it　in　a　different　way　to　what　we　should　have　done　in　its　native　country；　for　we　should　have　to　give　it　some　advantage　over　a　different　set　of　competitors　or　enemies。　
It　is　good　thus　to　try　in　our　imagination　to　give　any　form　some　advantage　over　another。　Probably　in　no　single　instance　should　we　know　what　to　do；　so　as　to　succeed。　It　will　convince　us　of　our　ignorance　on　the　mutual　relations　of　all　organic　beings；　a　conviction　as　necessary；　as　it　seems　to　be　difficult　to　acquire。　All　that　we　can　do；　is　to　keep　steadily　in　mind　that　each　organic　being　is　striving　to　increase　at　a　geometrical　ratio；　that　each　at　some　period　of　its　life；　during　some　season　of　the　year；　during　each　generation　or　at　intervals；　has　to　struggle　for　life；　and　to　suffer　great　destruction。　When　we　reflect　on　this　struggle；　we　may　console　ourselves　with　the　full　belief；　that　the　war　of　nature　is　not　incessant；　that　no　fear　is　felt；　that　death　is　generally　prompt；　and　that　the　vigorous；　the　healthy；　and　the　happy　survive　and　multiply。　　
Chapter　4　…　Natural　Selection

How　will　the　struggle　for　existence；　discussed　too　briefly　in　the　last　chapter；　act　in　regard　to　variation？　Can　the　principle　of　selection；　which　we　have　seen　is　so　potent　in　the　hands　of　man；　apply　in　nature？　I　think　we　shall　see　that　it　can　act　most　effectually。　Let　it　be　borne　in　mind　in　what　an　endless　number　of　strange　peculiarities　our　domestic　productions；　and；　in　a　lesser　degree；　those　under　nature；　vary；　and　how　strong　the　hereditary　tendency　is。　Under　domestication；　it　may　be　truly　said　that　the；　whole　organisation　becomes　in　some　degree　plastic。　Let　it　be　borne　in　mind　how　infinitely　complex　and　close…fitting　are　the　mutual　relations　of　all　organic　beings　to　each　other　and　to　their　physical　conditions　of　life。　Can　it；　then；　be　thought　improbable；　seeing　that　variations　useful　to　man　have　undoubtedly　occurred；　that　other　variations　useful　in　some　way　to　each　being　in　the　great　and　complex　battle　of　life；　should　sometimes　occur　in　the　course　of　thousands　of　generations？　If　such　do　occur；　can　we　doubt　（remembering　that　many　more　individuals　are　born　than　can　possibly　survive）　that　individuals　having　any　advantage；　however　slight；　over　others；　would　have　the　best　chance　of　surviving　and　of　procreating　their　kind？　On　the　other　hand；　we　may　feel　sure　that　any　variation　in　the　least　degree　injurious　would　be　rigidly　destroyed。　This　preservation　of　favourable　variations　and　the　rejection　of　injurious　variations；　I　call　Natural　Selection。　Variations　neither　useful　nor　injurious　would　not　be　affected　by　natural　selection；　and　would　be　left　a　fluctuating　element；　as　perhaps　we　see　in　the　species　called　polymorphic。　
We　shall　best　und