the origin of species(物种起源)-第14节

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　having　been　done；　with　the　exception　that　the　land　had　been　enclosed；　so　that　cattle　could　not　enter。　But　how　important　an　element　enclosure　is；　I　plainly　saw　near　Farnham；　in　Surrey。　Here　there　are　extensive　heaths；　with　a　few　clumps　of　old　Scotch　firs　on　the　distant　hill…tops：　within　the　last　ten　years　large　spaces　have　been　enclosed；　and　self…sown　firs　are　now　springing　up　in　multitudes；　so　close　together　that　all　cannot　live。　When　I　ascertained　that　these　young　trees　had　not　been　sown　or　planted；　I　was　so　much　surprised　at　their　numbers　that　I　went　to　several　points　of　view；　whence　I　could　examine　hundreds　of　acres　of　the　unenclosed　heath；　and　literally　I　could　not　see　a　single　Scotch　fir；　except　the　old　planted　clumps。　But　on　looking　closely　between　the　stems　of　the　heath；　I　found　a　multitude　of　seedlings　and　little　trees；　which　had　been　perpetually　browsed　down　by　the　cattle。　In　one　square　yard；　at　a　point　some　hundreds　yards　distant　from　one　of　the　old　clumps；　I　counted　thirty…two　little　trees；　and　one　of　them；　judging　from　the　rings　of　growth；　had　during　twenty…six　years　tried　to　raise　its　head　above　the　stems　of　the　heath；　and　had　failed。　No　wonder　that；　as　soon　as　the　land　was　enclosed；　it　became　thickly　clothed　with　vigorously　growing　young　firs。　Yet　the　heath　was　so　extremely　barren　and　so　extensive　that　no　one　would　ever　have　imagined　that　cattle　would　have　so　closely　and　effectually　searched　it　for　food。　
Here　we　see　that　cattle　absolutely　determine　the　existence　of　the　Scotch　fir；　but　in　several　parts　of　the　world　insects　determine　the　existence　of　cattle。　Perhaps　Paraguay　offers　the　most　curious　instance　of　this；　for　here　neither　cattle　nor　horses　nor　dogs　have　ever　run　wild；　though　they　swarm　southward　and　northward　in　a　feral　state；　and　Azara　and　Rengger　have　shown　that　this　is　caused　by　the　greater　number　in　Paraguay　of　a　certain　fly；　which　lays　its　eggs　in　the　navels　of　these　animals　when　first　born。　The　increase　of　these　flies；　numerous　as　they　are；　must　be　habitually　checked　by　some　means；　probably　by　birds。　Hence；　if　certain　insectivorous　birds　（whose　numbers　are　probably　regulated　by　hawks　or　beasts　of　prey）　were　to　increase　in　Paraguay；　the　flies　would　decrease　then　cattle　and　horses　would　become　feral；　and　this　would　certainly　greatly　alter　（as　indeed　I　have　observed　in　parts　of　South　America）　the　vegetation：　this　again　would　largely　affect　the　insects；　and　this；　as　we　just　have　seen　in　Staffordshire；　the　insectivorous　birds；　and　so　onwards　in　ever…increasing　circles　of　complexity。　We　began　this　series　by　insectivorous　birds；　and　we　have　ended　with　them。　Not　that　in　nature　the　relations　can　ever　be　as　simple　as　this。　Battle　within　battle　must　ever　be　recurring　with　varying　success；　and　yet　in　the　long…run　the　forces　are　so　nicely　balanced；　that　the　face　of　nature　remains　uniform　for　long　periods　of　time；　though　assuredly　the　merest　trifle　would　often　give　the　victory　to　one　organic　being　over　another。　Nevertheless　so　profound　is　our　ignorance；　and　so　high　our　presumption；　that　we　marvel　when　we　hear　of　the　extinction　of　an　organic　being；　and　as　we　do　not　see　the　cause；　we　invoke　cataclysms　to　desolate　the　world；　or　invent　laws　on　the　duration　of　the　forms　of　life！　
I　am　tempted　to　give　one　more　instance　showing　how　plants　and　animals；　most　remote　in　the　scale　of　nature；　are　bound　together　by　a　web　of　complex　relations。　I　shall　hereafter　have　occasion　to　show　that　the　exotic　Lobelia　fulgens；　in　this　part　of　England；　is　never　visited　by　insects；　and　consequently；　from　its　peculiar　structure；　never　can　set　a　seed。　Many　of　our　orchidaceous　plants　absolutely　require　the　visits　of　moths　to　remove　their　pollen…masses　and　thus　to　fertilise　them。　I　have；　also；　reason　to　believe　that　humble…bees　are　indispensable　to　the　fertilisation　of　the　heartsease　（Viola　tricolor）；　for　other　bees　do　not　visit　this　flower。　From　experiments　which　I　have　tried；　I　have　found　that　the　visits　of　bees；　if　not　indispensable；　are　at　least　highly　beneficial　to　the　fertilisation　of　our　clovers；　but　humble…bees　alone　visit　the　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