the origin of species(物种起源)-第20节

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ng　a　large　amount　of　crossing　with　inferior　animals。　Thus　it　will　be　in　nature；　for　within　a　confined　area；　with　some　place　in　its　polity　not　so　perfectly　occupied　as　might　be；　natural　selection　will　always　tend　to　preserve　all　the　individuals　varying　in　the　right　direction；　though　in　different　degrees；　so　as　better　to　fill　up　the　unoccupied　place。　But　if　the　area　be　large；　its　several　districts　will　almost　certainly　present　different　conditions　of　life；　and　then　if　natural　selection　be　modifying　and　improving　a　species　in　the　several　districts；　there　will　be　intercrossing　with　the　other　individuals　of　the　same　species　on　the　confines　of　each。　And　in　this　case　the　effects　of　intercrossing　can　hardly　be　counterbalanced　by　natural　selection　always　tending　to　modify　all　the　individuals　in　each　district　in　exactly　the　same　manner　to　the　conditions　of　each；　for　in　a　continuous　area；　the　conditions　will　generally　graduate　away　insensibly　from　one　district　to　another。　The　intercrossing　will　most　affect　those　animals　which　unite　for　each　birth；　which　wander　much；　and　which　do　not　breed　at　a　very　quick　rate。　Hence　in　animals　of　this　nature；　for　instance　in　birds；　varieties　will　generally　be　confined　to　separated　countries；　and　this　I　believe　to　be　the　case。　In　hermaphrodite　organisms　which　cross　only　occasionally；　and　likewise　in　animals　which　unite　for　each　birth；　but　which　wander　little　and　which　can　increase　at　a　very　rapid　rate；　a　new　and　improved　variety　might　be　quickly　formed　on　any　one　spot；　and　might　there　maintain　itself　in　a　body；　so　that　whatever　intercrossing　took　place　would　be　chiefly　between　the　individuals　of　the　same　new　variety。　A　local　variety　when　once　thus　formed　might　subsequently　slowly　spread　to　other　districts。　On　the　above　principle；　nurserymen　always　prefer　getting　seed　from　a　large　body　of　plants　of　the　same　variety；　as　the　chance　of　intercrossing　with　other　varieties　is　thus　lessened。　
Even　in　the　case　of　slow…breeding　animals；　which　unite　for　each　birth；　we　must　not　overrate　the　effects　of　intercrosses　in　retarding　natural　selection；　for　I　can　bring　a　considerable　catalogue　of　facts；　showing　that　within　the　same　area；　varieties　of　the　same　animal　can　long　remain　distinct；　from　haunting　different　stations；　from　breeding　at　slightly　different　seasons；　or　from　varieties　of　the　same　kind　preferring　to　pair　together。　
Intercrossing　plays　a　very　important　part　in　nature　in　keeping　the　individuals　of　the　same　species；　or　of　the　same　variety；　true　and　uniform　in　character。　It　will　obviously　thus　act　far　more　efficiently　with　those　animals　which　unite　for　each　birth；　but　I　have　already　attempted　to　show　that　we　have　reason　to　believe　that　occasional　intercrosses　take　place　with　all　animals　and　with　all　plants。　Even　if　these　take　place　only　at　long　intervals；　I　am　convinced　that　the　young　thus　produced　will　gain　so　much　in　vigour　and　fertility　over　the　offspring　from　long…continued　self…fertilisation；　that　they　will　have　a　better　chance　of　surviving　and　propagating　their　kind；　and　thus；　in　the　long　run；　the　influence　of　intercrosses；　even　at　rare　intervals；　will　be　great。　If　there　exist　organic　beings　which　never　intercross；　uniformity　of　character　can　be　retained　amongst　them；　as　long　as　their　conditions　of　life　remain　the　same；　only　through　the　principle　of　inheritance；　and　through　natural　selection　destroying　any　which　depart　from　the　proper　type；　but　if　their　conditions　of　life　change　and　they　undergo　modification；　uniformity　of　character　can　be　given　to　their　modified　offspring；　solely　by　natural　selection　preserving　the　same　favourable　variations。　
Isolation；　also；　is　an　important　element　in　the　process　of　natural　selection。　In　a　confined　or　isolated　area；　if　not　very　large；　the　organic　and　inorganic　conditions　of　life　will　generally　be　in　a　great　degree　uniform；　so　that　natural　selection　will　tend　to　modify　all　the　individuals　of　a　varying　species　throughout　the　area　in　the　same　manner　in　relation　to　the　same　conditions。　Intercrosses；　also；　with　the　individuals　of　the　same　species；　which　otherwise　would　have　inhabited　the　surrounding　and　differently　circumstanced　districts；　will　be　prevented。　But　isolation　probably　acts　more　efficiently　in　checking　the　immigration　of　better　adapted　organisms；　after　any　physical　change；　such　as　of　climate　or　elevation　of　the　land；　&c。；　and　thus　new　places　in　the　natural　economy　of　the　country　are　left　open　for　the　old　inhabitants　to　struggle　for；　and　become　adapted　to；　through　modifications　in　their　structure　and　constitution。　Lastly；　isolation；　by　checking　immigration　and　consequently　competition；　will　give　time　for　any　new　variety　to　be　slowly　improved；　and　this　may　sometimes　be　of　importance　in　the　production　of　new　species。　If；　however；　an　isolated　area　be　very　small；　either　from　being　surrounded　by　barriers；　or　from　having　very　peculiar　physical　conditions；　the　total　number　of　the　individuals　supported　on　it　will　necessarily　be　very　small；　and　fewness　of　individuals　will　greatly　retard　the　production　of　new　species　through　natural　selection；　by　decreasing　the　chance　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　unfavou