the origin of species(物种起源)-第15节

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d　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　understand　the　probable　course　of　natural　selection　by　taking　the　case　of　a　country　undergoing　some　physical　change；　for　instance；　of　climate。　The　proportional　numbers　of　its　inhabitants　would　almost　immediately　undergo　a　change；　and　some　species　might　become　extinct。　We　may　conclude；　from　what　we　have　seen　of　the　intimate　and　complex　manner　in　which　the　inhabitants　of　each　country　are　bound　together；　that　any　change　in　the　numerical　proportions　of　some　of　the　inhabitants；　independently　of　the　change　of　climate　itself；　would　most　seriously　affect　many　of　the　others。　If　the　country　were　open　on　its　borders；　new　forms　would　certainly　immigrate；　and　this　also　would　seriously　disturb　the　relations　of　some　of　the　former　inhabitants。　Let　it　be　remembered　how　powerful　the　influence　of　a　single　introduced　tree　or　mammal　has　been　shown　to　be。　But　in　the　case　of　an　island；　or　of　a　country　partly　surrounded　by　barriers；　into　which　new　and　better　adapted　forms　could　not　freely　enter；　we　should　then　have　places　in　the　economy　of　nature　which　would　assuredly　be　better　filled　up；　if　some　of　the　original　inhabitants　were　in　some　manner　modified；　for；　had　the　area　been　open　to　immigration；　these　same　places　would　have　been　seized　on　by　intruders。　In　such　case；　every　slight　modification；　which　in　the　course　of　ages　chanced　to　arise；　and　which　in　any　way　favoured　the　individuals　of　any　of　the　species；　by　better　adapting　them　to　their　altered　conditions；　would　tend　to　be　preserved；　and　natural　selection　would　thus　have　free　scope　for　the　work　of　improvement。　
We　have　reason　to　believe；　as　stated　in　the　first　chapter；　that　a　change　in　the　conditions　of　life；　by　specially　acting　on　the　reproductive　system；　causes　or　increases　variability；　and　in　the　foregoing　case　the　conditions　of　life　are　supposed　to　have　undergone　a　change；　and　this　would　manifestly　be　favourable　to　natural　selection；　by　giving　a　better　chance　of　profitable　variations　occurring；　and　unless　profitable　variations　do　occur；　natural　selection　can　do　nothing。　Not　that；　as　I　believe；　any　extreme　amount　of　variability　is　necessary；　as　man　can　certainly　produce　great　results　by　adding　up　in　any　given　direction　mere　individual　differences；　so　could　Nature；　but　far　more　easily；　from　having　incomparably　longer　time　at　her　disposal。　Nor　do　I　believe　that　any　great　physical　change；　as　of　climate；　or　any　unusual　degree　of　isolation　to　check　immigration；　is　actually　necessary　to　produce　new　and　unoccupied　places　for　natural　selection　to　fill　up　by　modifying　and　improving　some　of　the　varying　inhabitants。　For　as　all　the　inhabitants　of　each　country　are　struggling　together　with　nicely　balanced　forces；　extremely　slight　modifications　in　the　structure　or　habits　of　one　inhabitant　would　often　give　it　an　advantage　over　others；　and　still　further　modifications　of　the　same　kind　would　often　still　further　increase　the　advantage。　No　country　can　be　named　in　which　all　the　native　inhabitants　are　now　so　perfectly　adapted　to　each　other　and　to　the　physical　conditions　under　which　they　live；　that　none　of　them