the origin of species(物种起源)-第11部分

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ure　bright　with　gladness；　we　often　see　superabundance　of　food；　we　do　not　see；　or　we　forget；　that　the　birds　which　are　idly　singing　round　us　mostly　live　on　insects　or　seeds；　and　are　thus　constantly　destroying　life；　or　we　forget　how　largely　these　songsters；　or　their　eggs；　or　their　nestlings　are　destroyed　by　birds　and　beasts　of　prey；　we　do　not　always　bear　in　mind；　that　though　food　may　be　now　superabundant；　it　is　not　so　at　all　seasons　of　each　recurring　year。　
I　should　premise　that　I　use　the　term　Struggle　for　Existence　in　a　large　and　metaphorical　sense；　including　dependence　of　one　being　on　another；　and　including　（which　is　more　important）　not　only　the　life　of　the　individual；　but　success　in　leaving　progeny。　Two　canine　animals　in　a　time　of　dearth；　may　be　truly　said　to　struggle　with　each　other　which　shall　get　food　and　live。　But　a　plant　on　the　edge　of　a　desert　is　said　to　struggle　for　life　against　the　drought；　though　more　properly　it　should　be　said　to　be　dependent　on　the　moisture。　A　plant　which　annually　produces　a　thousand　seeds；　of　which　on　an　average　only　one　comes　to　maturity；　may　be　more　truly　said　to　struggle　with　the　plants　of　the　same　and　other　kinds　which　already　clothe　the　ground。　The　missletoe　is　dependent　on　the　apple　and　a　few　other　trees；　but　can　only　in　a　far…fetched　sense　be　said　to　struggle　with　these　trees；　for　if　too　many　of　these　parasites　grow　on　the　same　tree；　it　will　languish　and　die。　But　several　seedling　missletoes；　growing　close　together　on　the　same　branch；　may　more　truly　be　said　to　struggle　with　each　other。　As　the　missletoe　is　disseminated　by　birds；　its　existence　depends　on　birds；　and　it　may　metaphorically　be　said　to　struggle　with　other　fruit…bearing　plants；　in　order　to　tempt　birds　to　devour　and　thus　disseminate　its　seeds　rather　than　those　of　other　plants。　In　these　several　senses；　which　pass　into　each　other；　I　use　for　convenience　sake　the　general　term　of　struggle　for　existence。　
A　struggle　for　existence　inevitably　follows　from　the　high　rate　at　which　all　organic　beings　tend　to　increase。　Every　being；　which　during　its　natural　lifetime　produces　several　eggs　or　seeds；　must　suffer　destruction　during　some　period　of　its　life；　and　during　some　season　or　occasional　year；　otherwise；　on　the　principle　of　geometrical　increase；　its　numbers　would　quickly　become　so　inordinately　great　that　no　country　could　support　the　product。　Hence；　as　more　individuals　are　produced　than　can　possibly　survive；　there　must　in　every　case　be　a　struggle　for　existence；　either　one　individual　with　another　of　the　same　species；　or　with　the　individuals　of　distinct　species；　or　with　the　physical　conditions　of　life。　It　is　the　doctrine　of　Malthus　applied　with　manifold　force　to　the　whole　animal　and　vegetable　kingdoms；　for　in　this　case　there　can　be　no　artificial　increase　of　food；　and　no　prudential　restraint　from　marriage。　Although　some　species　may　be　now　increasing；　more　or　less　rapidly；　in　numbers；　all　cannot　do　so；　for　the　world　would　not　hold　them。　
There　is　no　exception　to　the　rule　that　every　organic　being　naturally　increases　at　so　high　a　rate；　that　if　not　destroyed；　the　earth　would　soon　be　covered　by　the　progeny　of　a　single　pair。　Even　slow…breeding　man　has　doubled　in　twenty…five　years；　and　at　this　rate；　in　a　few　thousand　years；　there　would　literally　not　be　standing　room　for　his　progeny。　Linnaeus　has　calculated　that　if　an　annual　plant　produced　only　two　seeds　and　there　is　no　plant　so　unproductive　as　this　and　their　seedlings　next　year　produced　two；　and　so　on；　then　in　twenty　years　there　would　be　a　million　plants。　The　elephant　is　reckoned　to　be　the　slowest　breeder　of　all　known　animals；　and　I　have　taken　some　pains　to　estimate　its　probable　minimum　rate　of　natural　increase：　it　will　be　under　the　mark　to　assume　that　it　breeds　when　thirty　years　old；　and　goes　on　breeding　till　ninety　years　old；　bringing　forth　three　pairs　of　young　in　this　interval；　if　this　be　so；　at　the　end　of　the　fifth　century　there　would　be　alive　fifteen　million　elephants；　descended　from　the　first　pair。　
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