the origin of species(物种起源)-第17章

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　of　the　appearance　of　favourable　variations。　
If　we　turn　to　nature　to　test　the　truth　of　these　remarks；　and　look　at　any　small　isolated　area；　such　as　an　oceanic　island；　although　the　total　number　of　the　species　inhabiting　it；　will　be　found　to　be　small；　as　we　shall　see　in　our　chapter　on　geographical　distribution；　yet　of　these　species　a　very　large　proportion　are　endemic；　that　is；　have　been　produced　there；　and　nowhere　else。　Hence　an　oceanic　island　at　first　sight　seems　to　have　been　highly　favourable　for　the　production　of　new　species。　But　we　may　thus　greatly　deceive　ourselves；　for　to　ascertain　whether　a　small　isolated　area；　or　a　large　open　area　like　a　continent；　has　been　most　favourable　for　the　production　of　new　organic　forms；　we　ought　to　make　the　comparison　within　equal　times；　and　this　we　are　incapable　of　doing。　
Although　I　do　not　doubt　that　isolation　is　of　considerable　importance　in　the　production　of　new　species；　on　the　whole　I　am　inclined　to　believe　that　largeness　of　area　is　of　more　importance；　more　especially　in　the　production　of　species；　which　will　prove　capable　of　enduring　for　a　long　period；　and　of　spreading　widely。　Throughout　a　great　and　open　area；　not　only　will　there　be　a　better　chance　of　favourable　variations　arising　from　the　large　number　of　individuals　of　the　same　species　there　supported；　but　the　conditions　of　life　are　infinitely　complex　from　the　large　number　of　already　existing　species；　and　if　some　of　these　many　species　become　modified　and　improved；　others　will　have　to　be　improved　in　a　corresponding　degree　or　they　will　be　exterminated。　Each　new　form；　also；　as　soon　as　it　has　been　much　improved；　will　be　able　to　spread　over　the　open　and　continuous　area；　and　will　thus　come　into　competition　with　many　others。　Hence　more　new　places　will　be　formed；　and　the　competition　to　fill　them　will　be　more　severe；　on　a　large　than　on　a　small　and　isolated　area。　Moreover；　great　areas；　though　now　continuous；　owing　to　oscillations　of　level；　will　often　have　recently　existed　in　a　broken　condition；　so　that　the　good　effects　of　isolation　will　generally；　to　a　certain　extent；　have　concurred。　Finally；　I　conclude　that；　although　small　isolated　areas　probably　have　been　in　some　respects　highly　favourable　for　the　production　of　new　species；　yet　that　the　course　of　modification　will　generally　have　been　more　rapid　on　large　areas；　and　what　is　more　important；　that　the　new　forms　produced　on　large　areas；　which　already　have　been　victorious　over　many　competitors；　will　be　those　that　will　spread　most　widely；　will　give　rise　to　most　new　varieties　and　species；　and　will　thus　play　an　important　part　in　the　changing　history　of　the　organic　world。　
We　can；　perhaps；　on　these　views；　understand　some　facts　which　will　be　again　alluded　to　in　our　chapter　on　geographical　distribution；　for　instance；　that　the　productions　of　the　smaller　continent　of　Australia　have　formerly　yielded；　and　apparently　are　now　yielding；　before　those　of　the　larger　Europaeo…Asiatic　area。　Thus；　also；　it　is　that　continental　productions　have　everywhere　become　so　largely　naturalised　on　islands。　On　a　small　island；　the　race　for　life　will　have　been　less　severe；　and　there　will　have　been　less　modification　and　less　extermination。　Hence；　perhaps；　it　comes　that　the　flora　of　Madeira；　according　to　Oswald　Heer；　resembles　the　extinct　tertiary　flora　of　Europe。　All　fresh…water　basins；　taken　together；　make　a　small　area　compared　with　that　of　the　sea　or　of　the　land；　and；　consequently；　the　competition　between　fresh…water　productions　will　have　been　less　severe　than　elsewhere；　new　forms　will　have　been　more　slowly　formed；　and　old　forms　more　slowly　exterminated。　And　it　is　in　fresh　water　that　we　find　seven　genera　of　Ganoid　fishes；　remnants　of　a　once　preponderant　order：　and　in　fresh　water　we　find　some　of　the　most　anomalous　forms　now　known　in　the　world；　as　the　Ornithorhynchus　and　Lepidosiren；　which；　like　fossils；　connect　to　a　certain　extent　orders　now　widely　separated　in　the　natural　scale。　These　anomalous　forms　may　almost　be　called　living　fossils；　they　have　endured　to　the　present　day；　from　having　inhabited　a　confined　area；　and　from　having　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