the origin of species(物种起源)-第13节

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rget　that　every　single　organic　being　around　us　may　be　said　to　be　striving　to　the　utmost　to　increase　in　numbers；　that　each　lives　by　a　struggle　at　some　period　of　its　life；　that　heavy　destruction　inevitably　falls　either　on　the　young　or　old；　during　each　generation　or　at　recurrent　intervals。　Lighten　any　check；　mitigate　the　destruction　ever　so　little；　and　the　number　of　the　species　will　almost　instantaneously　increase　to　any　amount。　The　face　of　Nature　may　be　compared　to　a　yielding　surface；　with　ten　thousand　sharp　wedges　packed　close　together　and　driven　inwards　by　incessant　blows；　sometimes　one　wedge　being　struck；　and　then　another　with　greater　force。　
What　checks　the　natural　tendency　of　each　species　to　increase　in　number　is　most　obscure。　Look　at　the　most　vigorous　species；　by　as　much　as　it　swarms　in　numbers；　by　so　much　will　its　tendency　to　increase　be　still　further　increased。　We　know　not　exactly　what　the　checks　are　in　even　one　single　instance。　Nor　will　this　surprise　any　one　who　reflects　how　ignorant　we　are　on　this　head；　even　in　regard　to　mankind；　so　incomparably　better　known　than　any　other　animal。　This　subject　has　been　ably　treated　by　several　authors；　and　I　shall；　in　my　future　work；　discuss　some　of　the　checks　at　considerable　length；　more　especially　in　regard　to　the　feral　animals　of　South　America。　Here　I　will　make　only　a　few　remarks；　just　to　recall　to　the　reader's　mind　some　of　the　chief　points。　Eggs　or　very　young　animals　seem　generally　to　suffer　most；　but　this　is　not　invariably　the　case。　With　plants　there　is　a　vast　destruction　of　seeds；　but；　from　some　observations　which　I　have　made；　I　believe　that　it　is　the　seedlings　which　suffer　most　from　germinating　in　ground　already　thickly　stocked　with　other　plants。　Seedlings；　also；　are　destroyed　in　vast　numbers　by　various　enemies；　for　instance；　on　a　piece　of　ground　three　feet　long　and　two　wide；　dug　and　cleared；　and　where　there　could　be　no　choking　from　other　plants；　I　marked　all　the　seedlings　of　our　native　weeds　as　they　came　up；　and　out　of　the　357　no　less　than　295　were　destroyed；　chiefly　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　ente