the origin of species(物种起源)-第12部分

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fly　by　slugs　and　insects。　If　turf　which　has　long　been　mown；　and　the　case　would　be　the　same　with　turf　closely　browsed　by　quadrupeds；　be　let　to　grow；　the　more　vigorous　plants　gradually　kill　the　less　vigorous；　though　fully　grown；　plants：　thus　out　of　twenty　species　growing　on　a　little　plot　of　turf　（three　feet　by　four）　nine　species　perished　from　the　other　species　being　allowed　to　grow　up　freely。　
The　amount　of　food　for　each　species　of　course　gives　the　extreme　limit　to　which　each　can　increase；　but　very　frequently　it　is　not　the　obtaining　food；　but　the　serving　as　prey　to　other　animals；　which　determines　the　average　numbers　of　a　species。　Thus；　there　seems　to　be　little　doubt　that　the　stock　of　partridges；　grouse；　and　hares　on　any　large　estate　depends　chiefly　on　the　destruction　of　vermin。　If　not　one　head　of　game　were　shot　during　the　next　twenty　years　in　England；　and；　at　the　same　time；　if　no　vermin　were　destroyed；　there　would；　in　all　probability；　be　less　game　than　at　present；　although　hundreds　of　thousands　of　game　animals　are　now　annually　killed。　On　the　other　hand；　in　some　cases；　as　with　the　elephant　and　rhinoceros；　none　are　destroyed　by　beasts　of　prey：　even　the　tiger　in　India　most　rarely　dares　to　attack　a　young　elephant　protected　by　its　dam。　
Climate　plays　an　important　part　in　determining　the　average　numbers　of　a　species；　and　periodical　seasons　of　extreme　cold　or　drought；　I　believe　to　be　the　most　effective　of　all　checks。　I　estimated　that　the　winter　of　1854…55　destroyed　four…fifths　of　the　birds　in　my　own　grounds；　and　this　is　a　tremendous　destruction；　when　we　remember　that　ten　per　cent。　is　an　extraordinarily　severe　mortality　from　epidemics　with　man。　The　action　of　climate　seems　at　first　sight　to　be　quite　independent　of　the　struggle　for　existence；　but　in　so　far　as　climate　chiefly　acts　in　reducing　food；　it　brings　on　the　most　severe　struggle　between　the　individuals；　whether　of　the　same　or　of　distinct　species；　which　subsist　on　the　same　kind　of　food。　Even　when　climate；　for　instance　extreme　cold；　acts　directly；　it　will　be　the　least　vigorous；　or　those　which　have　got　least　food　through　the　advancing　winter；　which　will　suffer　most。　When　we　travel　from　south　to　north；　or　from　a　damp　region　to　a　dry；　we　invariably　see　some　species　gradually　getting　rarer　and　rarer；　and　finally　disappearing；　and　the　change　of　climate　being　conspicuous；　we　are　tempted　to　attribute　the　whole　effect　to　its　direct　action。　But　this　is　a　very　false　view：　we　forget　that　each　species；　even　where　it　most　abounds；　is　constantly　suffering　enormous　destruction　at　some　period　of　its　life；　from　enemies　or　from　competitors　for　the　same　place　and　food；　and　if　these　enemies　or　competitors　be　in　the　least　degree　favoured　by　any　slight　change　of　climate；　they　will　increase　in　numbers；　and；　as　each　area　is　already　fully　stocked　with　inhabitants；　the　other　species　will　decrease。　When　we　travel　southward　and　see　a　species　decreasing　in　numbers；　we　may　feel　sure　that　the　cause　lies　quite　as　much　in　other　species　being　favoured；　as　in　this　one　being　hurt。　So　it　is　when　we　travel　northward；　but　in　a　somewhat　lesser　degree；　for　the　number　of　species　of　all　kinds；　and　therefore　of　competitors；　decreases　northwards；　hence　in　going　northward；　or　in　ascending　a　mountain；　we　far　oftener　meet　with　stunted　forms；　due　to　the　directly　injurious　action　of　climate；　than　we　do　in　proceeding　southwards　or　in　descending　a　mountain。　When　we　reach　the　Arctic　regions；　or　snow…capped　summits；　or　absolute　deserts；　the　struggle　for　life　is　almost　exclusively　with　the　elements。　
That　climate　acts　in　main　part　indirectly　by　favouring　other　species；　we　may　clearly　see　in　the　prodigious　number　of　plants　in　our　gardens　which　can　perfectly　well　endure　our　climate；　but　which　never　become　naturalised；　for　they　cannot　compete　with　our　native　plants；　nor　resist　destruction　by　our　native　animals。　
When　a　species；　owing　to　highly　favourable　circumstances；　increases　inordinately　in　numbers　in　a　small　tract；　epidemics　at　least；　this　seems　generally　to　occur　with　our　game　animals　often　ensue：　and　here　we　have　a　limiting　check　independent　of　the　struggle　for　life。　But　even　some　of　these　so…called　epidemics　appear　to　be　due　to　parasitic　worms；　which　have　from　some　cause；　possibly　in　part　through　facility　of　diffusion　amongst　the　crowded　animals；　been　disproportionably　favoured：　and　here　comes　in　a　sort　of　struggle　between　the　parasite　and　its　prey。　
On　the　other　hand；　in　many　cases；　a　large　stock　of　individuals　of　the　same　species；　relatively　to　the　numbers　of　its　enemies；　is　absolutely　necessary　for　its　preservation。　Thus　we　can　easily　raise　plenty　of　corn　and　rape…seed；　&c。；　in　our　fields；　because　the　seeds　are　in　great　excess　compared　with　the　number　of　birds　which　feed　on　them；　nor　can　the　birds；　though　having　a　superabundance　of　food　at　this　one　season；　increase　in　number　proportionally　to　the　supply　of　seed；　as　their　numbers　are　checked　during　winter：　but　any　one　who　has　tried；　knows　how　troublesome　it　is　to　get　seed　from　a　few　wheat　or　other　such　plants　in　a　garden；　I　have　in　this　case　lost　every　single　seed。　This　view　of　the　necessity　of　a　large　stock　of　the　same　species　for　its　preservation；　explains；　I　believe；　some　singular　facts　in　nature；　such　as　that　of　very　rare　plants　being　sometimes　extremely　abundant　in　the　few　spots　where　they　do　occur；　and　that　of　some　social　plants　being　social；　that　is；　abounding　in　individuals；　even　on　the　extreme　confines　of　their　range。　For　in　such　cases；　we　may　believe；　that　a　plant　could　exist　only　where　the　conditions　of　its　life　were　so　favourable　that　many　could　exist　together；　and　thus　save　each　other　from　utter　destruction。　I　should　add　that　the　good　effects　of　frequent　intercrossing；　and　the　ill　effects　of　close　interbreeding；　probably　come　into　play　in　some　of　these　cases；　but　on　this　intricate　subject　I　will　not　here　enlarge。　
Many　cases　are　on　record　showing　how　complex　and　unexpected　are　the　checks　and　relations　between　organic　beings；　which　have　to　struggle　together　in　the　same　country。　I　will　give　only　a　single　instance；　which；　though　a　simple　one；　has　interested　me。　In　Staffordshire；　on　the　estate　of　a　relation　where　I　had　ample　means　of　investigation；　there　was　a　large　and　extremely　barren　heath；　which　had　never　been　touched　by　the　hand　of　man；　but　several　hundred　acres　of　exactly　the　same　nature　had　been　enclosed　twenty…five　years　previously　and　planted　with　Scotch　fir。　The　change　in　the　native　vegetation　of　the　planted　part　of　the　heath　was　most　remarkable；　more　than　is　generally　seen　in　passing　from　one　quite　different　soil　to　another：　not　only　the　proportional　numbers　of　the　heath…plants　were　wholly　changed；　but　twelve　species　of　plants　（not　counting　grasses　and　carices）　flourished　in　the　plantations；　which　could　not　be　found　on　the　heath。　The　effect　on　the　insects　must　have　been　still　greater；　for　six　insectivorous　birds　were　very　common　in　the　plantations；　which　were　not　to　be　seen　on　the　heath；　and　the　heath　was　frequented　by　two　or　three　distinct　insectivorous　birds。　Here　we　see　how　potent　has　been　the　effect　of　the　introduction　of　a　single　tree；　nothing　whatever　else　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　cataclys