the origin of species(物种起源)-第14章

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But　we　have　better　evidence　on　this　subject　than　mere　theoretical　calculations；　namely；　the　numerous　recorded　cases　of　the　astonishingly　rapid　increase　of　various　animals　in　a　state　of　nature；　when　circumstances　have　been　favourable　to　them　during　two　or　three　following　seasons。　Still　more　striking　is　the　evidence　from　our　domestic　animals　of　many　kinds　which　have　run　wild　in　several　parts　of　the　world：　if　the　statements　of　the　rate　of　increase　of　slow…breeding　cattle　and　horses　in　South　America；　and　latterly　in　Australia；　had　not　been　well　authenticated；　they　would　have　been　quite　incredible。　So　it　is　with　plants：　cases　could　be　given　of　introduced　plants　which　have　become　common　throughout　whole　islands　in　a　period　of　less　than　ten　years；　Several　of　the　plants　now　most　numerous　over　the　wide　plains　of　La　Plata；　clothing　square　leagues　of　surface　almost　to　the　exclusion　of　all　other　plants；　have　been　introduced　from　Europe；　and　there　are　plants　which　now　range　in　India；　as　I　hear　from　Dr　Falconer；　from　Cape　Comorin　to　the　Himalaya；　which　have　been　imported　from　America　since　its　discovery。　In　such　cases；　and　endless　instances　could　be　given；　no　one　supposes　that　the　fertility　of　these　animals　or　plants　has　been　suddenly　and　temporarily　increased　in　any　sensible　degree。　The　obvious　explanation　is　that　the　conditions　of　life　have　been　very　favourable；　and　that　there　has　consequently　been　less　destruction　of　the　old　and　young；　and　that　nearly　all　the　young　have　been　enabled　to　breed。　In　such　cases　the　geometrical　ratio　of　increase；　the　result　of　which　never　fails　to　be　surprising；　simply　explains　the　extraordinarily　rapid　increase　and　wide　diffusion　of　naturalised　productions　in　their　new　homes。　
In　a　state　of　nature　almost　every　plant　produces　seed；　and　amongst　animals　there　are　very　few　which　do　not　annually　pair。　Hence　we　may　confidently　assert；　that　all　plants　and　animals　are　tending　to　increase　at　a　geometrical　ratio；　that　all　would　most　rapidly　stock　every　station　in　which　they　could　any　how　exist；　and　that　the　geometrical　tendency　to　increase　must　be　checked　by　destruction　at　some　period　of　life。　Our　familiarity　with　the　larger　domestic　animals　tends；　I　think；　to　mislead　us：　we　see　no　great　destruction　falling　on　them；　and　we　forget　that　thousands　are　annually　slaughtered　for　food；　and　that　in　a　state　of　nature　an　equal　number　would　have　somehow　to　be　disposed　of。　
The　only　difference　between　organisms　which　annually　produce　eggs　or　seeds　by　the　thousand；　and　those　which　produce　extremely　few；　is；　that　the　slow…breeders　would　require　a　few　more　years　to　people；　under　favourable　conditions；　a　whole　district；　let　it　be　ever　so　large。　The　condor　lays　a　couple　of　eggs　and　the　ostrich　a　score；　and　yet　in　the　same　country　the　condor　may　be　the　more　numerous　of　the　two：　the　Fulmar　petrel　lays　but　one　egg；　yet　it　is　believed　to　be　the　most　numerous　bird　in　the　world。　One　fly　deposits　hundreds　of　eggs；　and　another；　like　the　hippobosca；　a　single　one；　but　this　difference　does　not　determine　how　many　individuals　of　the　two　species　can　be　supported　in　a　district。　A　large　number　of　eggs　is　of　some　importance　to　those　species；　which　depend　on　a　rapidly　fluctuating　amount　of　food；　for　it　allows　them　rapidly　to　increase　in　number。　But　the　real　importance　of　a　large　number　of　eggs　or　seeds　is　to　make　up　for　much　destruction　at　some　period　of　life；　and　this　period　in　the　great　majority　of　cases　is　an　early　one。　If　an　animal　can　in　any　way　protect　its　own　eggs　or　young；　a　small　number　may　be　produced；　and　yet　the　average　stock　be　fully　kept　up；　but　if　many　eggs　or　young　are　destroyed；　many　must　be　produced；　or　the　species　will　become　extinct。　It　would　suffice　to　keep　up　the　full　number　of　a　tree；　which　lived　on　an　average　for　a　thousand　years；　if　a　single　seed　were　produced　once　in　a　thousand　years；　supposing　that　this　seed　were　never　destroyed；　and　could　be　ensured　to　germinate　in　a　fitting　place。　So　that　in　all　cases；　the　average　number　of　any　animal　or　plant　depends　only　indirectly　on　the　number　of　its　eggs　or　seeds。　
In　looking　at　Nature；　it　is　most　necessary　to　keep　the　foregoing　considerations　always　in　mind　never　to　forget　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　vigoro