the origin of species(物种起源)-第21节

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g　thus　been　exposed　to　less　severe　competition。　
To　sum　up　the　circumstances　favourable　and　unfavourable　to　natural　selection；　as　far　as　the　extreme　intricacy　of　the　subject　permits。　I　conclude；　looking　to　the　future；　that　for　terrestrial　productions　a　large　continental　area；　which　will　probably　undergo　many　oscillations　of　level；　and　which　consequently　will　exist　for　long　periods　in　a　broken　condition；　will　be　the　most　favourable　for　the　production　of　many　new　forms　of　life；　likely　to　endure　long　and　to　spread　widely。　For　the　area　will　first　have　existed　as　a　continent；　and　the　inhabitants；　at　this　period　numerous　in　individuals　and　kinds；　will　have　been　subjected　to　very　severe　competition。　When　converted　by　subsidence　into　large　separate　islands；　there　will　still　exist　many　individuals　of　the　same　species　on　each　island：　intercrossing　on　the　confines　of　the　range　of　each　species　will　thus　be　checked：　after　physical　changes　of　any　kind；　immigration　will　be　prevented；　so　that　new　places　in　the　polity　of　each　island　will　have　to　be　filled　up　by　modifications　of　the　old　inhabitants；　and　time　will　be　allowed　for　the　varieties　in　each　to　become　well　modified　and　perfected。　When；　by　renewed　elevation；　the　islands　shall　be　re…converted　into　a　continental　area；　there　will　again　be　severe　competition：　the　most　favoured　or　improved　varieties　will　be　enabled　to　spread：　there　will　be　much　extinction　of　the　less　improved　forms；　and　the　relative　proportional　numbers　of　the　various　inhabitants　of　the　renewed　continent　will　again　be　changed；　and　again　there　will　be　a　fair　field　for　natural　selection　to　improve　still　further　the　inhabitants；　and　thus　produce　new　species。　
That　natural　selection　will　always　act　with　extreme　slowness；　I　fully　admit。　Its　action　depends　on　there　being　places　in　the　polity　of　nature；　which　can　be　better　occupied　by　some　of　the　inhabitants　of　the　country　undergoing　modification　of　some　kind。　The　existence　of　such　places　will　often　depend　on　physical　changes；　which　are　generally　very　slow；　and　on　the　immigration　of　better　adapted　forms　having　been　checked。　But　the　action　of　natural　selection　will　probably　still　oftener　depend　on　some　of　the　inhabitants　becoming　slowly　modified；　the　mutual　relations　of　many　of　the　other　inhabitants　being　thus　disturbed。　Nothing　can　be　effected；　unless　favourable　variations　occur；　and　variation　itself　is　apparently　always　a　very　slow　process。　The　process　will　often　be　greatly　retarded　by　free　intercrossing。　Many　will　exclaim　that　these　several　causes　are　amply　sufficient　wholly　to　stop　the　action　of　natural　selection。　I　do　not　believe　so。　On　the　other　hand；　I　do　believe　that　natural　selection　will　always　act　very　slowly；　often　only　at　long　intervals　of　time；　and　generally　on　only　a　very　few　of　the　inhabitants　of　the　same　region　at　the　same　time。　I　further　believe；　that　this　very　slow；　intermittent　action　of　natural　selection　accords　perfectly　well　with　what　geology　tells　us　of　the　rate　and　manner　at　which　the　inhabitants　of　this　world　have　changed。　
Slow　though　the　process　of　selection　may　be；　if　feeble　man　can　do　much　by　his　powers　of　artificial　selection；　I　can　see　no　limit　to　the　amount　of　change；　to　the　beauty　and　infinite　complexity　of　the　coadaptations　between　all　organic　beings；　one　with　another　and　with　their　physical　conditions　of　life；　which　may　be　effected　in　the　long　course　of　time　by　nature's　power　of　selection。　
Extinction
This　subject　will　be　more　fully　discussed　in　our　chapter　on　Geology；　but　it　must　be　here　alluded　to　from　being　intimately　connected　with　natural　selection。　Natural　selection　acts　solely　through　the　preservation　of　variations　in　some　way　advantageous；　which　consequently　endure。　But　as　from　the　high　geometrical　powers　of　increase　of　all　organic　beings；　each　area　is　already　fully　stocked　with　inhabitants；　it　follows　that　as　each　selected　and　favoured　form　increases　in　number；　so　will　the　less　favoured　forms　decrease　and　become　rare。　Rarity；　as　geology　tells　us；　is　the　precursor　to　extinction。　We　can；　also；　see　that　any　form　represented　by　few　individuals　will；　during　fluctuations　in　the　seasons　or　in　the　number　of　its　enemies；　run　a　good　chance　of　utter　extinction。　But　we　may　go　further　than　this；　for　as　new　forms　are　continually　and　slowly　being　produced；　unless　we　believe　that　the　number　of　specific　forms　goes　on　perpetually　and　almost　indefinitely　increasing；　numbers　inevitably　must　become　extinct。　That　the　number　of　specific　forms　has　not　indefinitely　increased；　geology　shows　us　plainly；　and　indeed　we　can　see　reason　why　they　should　not　have　thus　increased；　for　the　number　of　places　in　the　polity　of　nature　is　not　indefinitely　great；　not　that　we　have　any　means　of　knowing　that　any　one　region　has　as　yet　got　its　maximum　of　species。　probably　no　region　is　as　yet　fully　stocked；　for　at　the　Cape　of　Good　Hope；　where　more　species　of　plants　are　crowded　together　than　in　any　other　quarter　of　the　world；　some　foreign　plants　have　become　naturalised；　without　causing；　as　far　as　we　know；　the　extinction　of　any　natives。　
Furthermore；　the　species　which　are　most　numerous　in　individuals　will　have　the　best　chance　of　producing　within　any　given　period　favourable　variations。　We　have　evidence　of　this；　in　the　facts　given　in　the　second　chapter；　showing　that　it　is　the　common　species　which　afford　the　greatest　number　of　recorded　varieties；　or　incipient　species。　Hence；　rare　species　will　be　less　quickly　modified　or　improved　within　any　given　period；　and　they　will　consequently　be　beaten　in　the　race　for　life　by　the　modified　descendants　of　the　commoner　species。　
From　these　several　considerations　I　think　it　inevitably　follows；　that　as　new　species　in　the　course　of　time　are　formed　through　natural　selection；　others　will　become　rarer　and　rarer；　and　finally　extinct。　The　forms　which　stand　in　closest　competition　with　those　undergoing　modification　and　improvement；　will　naturally　suffer　most。　And　we　have　seen　in　the　chapter　on　the　Struggle　for　Existence　that　it　is　the　most　closely…allied　forms；　varieties　of　the　same　species；　and　species　of　the　same　genus　or　of　related　genera；　which；　from　having　nearly　the　same　structure；　constitution；　and　habits；　generally　come　into　the　severest　competition　with　each　other。　Consequently；　each　new　variety　or　species；　during　the　progress　of　its　formation；　will　generally　press　hardest　on　its　nearest　kindred；　and　tend　to　exterminate　them。　We　see　the　same　process　of　extermination　amongst　our　domesticated　productions；　through　the　selection　of　improved　forms　by　man。　Many　curious　instances　could　be　given　showing　how　quickly　new　breeds　of　cattle；　sheep；　and　other　animals；　and　varieties　of　flowers；　take　the　place　of　older　and　inferior　kinds。　In　Yorkshire；　it　is　historically　known　that　the　ancient　black　cattle　were　displaced　by　the　long…horns；　and　that　these　'were　swept　away　by　the　short…horns'　（I　quote　the　words　of　an　agricultural　writer）　'as　if　by　some　murderous　pestilence。'　
Divergence　of　Character
The　principle；　which　I　have　designated　by　this　term；　is　of　high　importance　on　my　theory；　and　explains；　as　I　believe；　several　important　facts。　In　the　first　place；　varieties；　even　strongly…marked　ones；　though　having　somewhat　of　the　character　of　species　as　is　shown　by　the　hopeless　doubts　in　many　cases　how　to　rank　them　yet　certainly　differ　from　each　other　far　less　than　do　good　and　distinct　species。　Nevertheless；　according　to　my　view；　varieties　are　species　in　the　process　of　formation；　or　are；　as　I　have　called　them；　incipient　species。　How；　then；　does　the　lesser　difference　between　varieties　become　augmented　into　the　greater　difference　between　species？　That　this　does　habitually　happen；　we　must　infer　from　most　of　the　innumerable　species　throughout　nature　presenting　well…marked　differences；　whereas　varieties；　the　supposed　prototypes　and　parents　of　future　well…marked　species；　present　slight　and　ill…defined　differences。　Mere　chance；　as　we　may　call　it；　might　cause　one　variety　to　differ　in　some　character　from　its　parents；　and　the　offspring　of　this　variety　again　to　differ　from　its　parent　in　the　very　same　character　and　in　a　greater　degree；　but　this　alone　would　never　account　for　so　habitual　and　large　an　amount　of　difference　as　that　between　varieties　of　the　same　species　and　species　of　the　same　genus。　
As　has　always　been　my　practice；　let　us　seek　light　on　this　head　from　our　domestic　productions。　We　shall　here　find　something　analogous。　A　fancier　is　struck　by　a　pigeon　having　a　slightly　shorter　beak；　another　fancier　is　struck　by　a　pigeon　having　a　rather　longer　beak；　and　on　the　acknowledged　principle　that　'fanciers　do　not　and　will　not　admire　a　medium　standard；　but　like　extremes；'　they　both　go　on　（as　has　actually　occurred　with　tumbler…pigeons）　choosing　and　breeding　from　birds　with　longer　and　longer　beaks；　or　with　shorter　and　shorter　beaks。　Again；　we　may　suppose　that　at　an　early　period　one　man　preferred　swifter　horses；　another　stronger　and　more　bulky　horses。　The　early　differences　would　be　very　slight；　in　the　course　of　time；　from　the　continued　selection　of　swifter　horses　by　some　breeders；　and　of　stronger　ones　by　others；　the　differences　would　become　greater；　and　would　be　noted　as　forming　two　sub…breeds；　finally；　after　the　lapse　of　centuries；　the　sub…breeds　would　become　converted　into　two　well…established　and　dis