the origin of species(物种起源)-第20部分

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We　can　clearly　see　this　in　the　case　of　animals　with　simple　habits。　Take　the　case　of　a　carnivorous　quadruped；　of　which　the　number　that　can　be　supported　in　any　country　has　long　ago　arrived　at　its　full　average。　If　its　natural　powers　of　increase　be　allowed　to　act；　it　can　succeed　in　increasing　（the　country　not　undergoing　any　change　in　its　conditions）　only　by　its　varying　descendants　seizing　on　places　at　present　occupied　by　other　animals：　some　of　them；　for　instance；　being　enabled　to　feed　on　new　kinds　of　prey；　either　dead　or　alive；　some　inhabiting　new　stations；　climbing　trees；　frequenting　water；　and　some　perhaps　becoming　less　carnivorous。　The　more　diversified　in　habits　and　structure　the　descendants　of　our　carnivorous　animal　became；　the　more　places　they　would　be　enabled　to　occupy。　What　applies　to　one　animal　will　apply　throughout　all　time　to　all　animals　that　is；　if　they　vary　for　otherwise　natural　selection　can　do　nothing。　So　it　will　be　with　plants。　It　has　been　experimentally　proved；　that　if　a　plot　of　ground　be　sown　with　several　distinct　genera　of　grasses；　a　greater　number　of　plants　and　a　greater　weight　of　dry　herbage　can　thus　be　raised。　The　same　has　been　found　to　hold　good　when　first　one　variety　and　then　several　mixed　varieties　of　wheat　have　been　sown　on　equal　spaces　of　ground。　Hence；　if　any　one　species　of　grass　were　to　go　on　varying；　and　those　varieties　were　continually　selected　which　differed　from　each　other　in　at　all　the　same　manner　as　distinct　species　and　genera　of　grasses　differ　from　each　other；　a　greater　number　of　individual　plants　of　this　species　of　grass；　including　its　modified　descendants；　would　succeed　in　living　on　the　same　piece　of　ground。　And　we　well　know　that　each　species　and　each　variety　of　grass　is　annually　sowing　almost　countless　seeds；　and　thus；　as　it　may　be　said；　is　striving　its　utmost　to　increase　its　numbers。　Consequently；　I　cannot　doubt　that　in　the　course　of　many　thousands　of　generations；　the　most　distinct　varieties　of　any　one　species　of　grass　would　always　have　the　best　chance　of　succeeding　and　of　increasing　in　numbers；　and　thus　of　supplanting　the　less　distinct　varieties；　and　varieties；　when　rendered　very　distinct　from　each　other；　take　the　rank　of　species。　
The　truth　of　the　principle；　that　the　greatest　amount　of　life　can　be　supported　by　great　diversification　of　structure；　is　seen　under　many　natural　circumstances。　In　an　extremely　small　area；　especially　if　freely　open　to　immigration；　and　where　the　contest　between　individual　and　individual　must　be　severe；　we　always　find　great　diversity　in　its　inhabitants。　For　instance；　I　found　that　a　piece　of　turf；　three　feet　by　four　in　size；　which　had　been　exposed　for　many　years　to　exactly　the　same　conditions；　supported　twenty　species　of　plants；　and　these　belonged　to　eighteen　genera　and　to　eight　orders；　which　shows　how　much　these　plants　differed　from　each　other。　So　it　is　with　the　plants　and　insects　on　small　and　uniform　islets；　and　so　in　small　ponds　of　fresh　water。　Farmers　find　that　they　can　raise　most　food　by　a　rotation　of　plants　belonging　to　the　most　different　orders：　nature　follows　what　may　be　called　a　simultaneous　rotation。　Most　of　the　animals　and　plants　which　live　close　round　any　small　piece　of　ground；　could　live　on　it　（supposing　it　not　to　be　in　any　way　peculiar　in　its　nature）；　and　may　be　said　to　be　striving　to　the　utmost　to　live　there；　but；　it　is　seen；　that　where　they　come　into　the　closest　competition　with　each　other；　the　advantages　of　diversification　of　structure；　with　the　accompanying　differences　of　habit　and　constitution；　determine　that　the　inhabitants；　which　thus　jostle　each　other　most　closely；　shall；　as　a　general　rule；　belong　to　what　we　call　different　genera　and　orders。　
The　same　principle　is　seen　in　the　naturalisation　of　plants　through　man's　agency　in　foreign　lands。　It　might　have　been　expected　that　the　plants　which　have　succeeded　in　becoming　naturalised　in　any　land　would　generally　have　been　closely　allied　to　the　indigenes；　for　these　are　commonly　looked　at　as　specially　created　and　adapted　for　their　own　country。　It　might；　also；　perhaps　have　been　expected　that　naturalised　plants　would　have　belonged　to　a　few　groups　more　especially　adapted　to　certain　stations　in　their　new　homes。　But　the　case　is　very　different；　and　Alph。　De　Candolle　has　well　remarked　in　his　great　and　admirable　work；　that　floras　gain　by　naturalisation；　proportionally　with　the　number　of　the　native　genera　and　species；　far　more　in　new　genera　than　in　new　species。　To　give　a　single　instance：　in　the　last　edition　of　Dr　Asa　Gray's　'Manual　of　the　Flora　of　the　Northern　United　States；'　260　naturalised　plants　are　enumerated；　and　these　belong　to　162　genera。　We　thus　see　that　these　naturalised　plants　are　of　a　highly　diversified　nature。　They　differ；　moreover；　to　a　large　extent　from　the　indigenes；　for　out　of　the　162　genera；　no　less　than　100　genera　are　not　there　indigenous；　and　thus　a　large　proportional　addition　is　made　to　the　genera　of　these　States。　
By　considering　the　nature　of　the　plants　or　animals　which　have　struggled　successfully　with　the　indigenes　of　any　country；　and　have　there　become　naturalised；　we　can　gain　some　crude　idea　in　what　manner　some　of　the　natives　would　have　had　to　be　modified；　in　order　to　have　gained　an　advantage　over　the　other　natives；　and　we　may；　I　think；　at　least　safely　infer　that　diversification　of　structure；　amounting　to　new　generic　differences；　would　have　been　profitable　to　them。　
The　advantage　of　diversification　in　the　inhabitants　of　the　same　region　is；　in　fact；　the　same　as　that　of　the　physiological　division　of　labour　in　the　organs　of　the　same　individual　body　a　subject　so　well　elucidated　by　Milne　Edwards。　No　physiologist　doubts　that　a　stomach　by　being　adapted　to　digest　vegetable　matter　alone；　or　flesh　alone；　draws　most　nutriment　from　these　substances。　So　in　the　general　economy　of　any　land；　the　more　widely　and　perfectly　the　animals　and　plants　are　diversified　for　different　habits　of　life；　so　will　a　greater　number　of　individuals　be　capable　of　there　supporting　themselves。　A　set　of　animals；　with　their　organisation　but　little　diversified；　could　hardly　compete　with　a　set　more　perfectly　diversified　in　structure。　It　may　be　doubted；　for　instance；　whether　the　Australian　marsupials；　which　are　divided　into　groups　differing　but　little　from　each　other；　and　feebly　representing；　as　Mr　Waterhouse　and　others　have　remarked；　our　carnivorous；　ruminant；　and　rodent　mammals；　could　successfully　compete　with　these　well…pronounced　orders。　In　the　Australian　mammals；　we　see　the　process　of　diversification　in　an　early　and　incomplete　stage　of　development。　
After　the　foregoing　discussion；　which　ought　to　have　been　much　amplified；　we　may；　I　think；　assume　that　the　modified　descendants　of　any　one　species　will　succeed　by　so　much　the　better　as　they　become　more　diversified　in　structure；　and　are　thus　enabled　to　encroach　on　places　occupied　by　other　beings。　Now　let　us　see　how　this　principle　of　great　benefit　being　derived　from　divergence　of　character；　combined　with　the　principles　of　natural　selection　and　of　extinction；　will　tend　to　act。　
The　accompanying　diagram　will　aid　us　in　understanding　this　rather　perplexing　subject。　Let　A　to　L　represent　the　species　of　a　genus　large　in　its　own　country；　these　species　are　supposed　to　resemble　each　other　in　unequal　degrees；　as　is　so　generally　the　case　in　nature；　and　as　is　represented　in　the　diagram　by　the　letters　standing　at　unequal　distances。　I　have　said　a　large　genus；　because　we　have　seen　in　the　second　chapter；　that　on　an　average　more　of　the　species　of　large　genera　vary　than　of　small　genera；　and　the　varying　species　of　the　large　genera　present　a　greater　number　of　varieties。　We　have；　also；　seen　that　the　species；　which　are　the　commonest　and　the　most　widely…diffused；　vary　more　than　rare　species　with　restricted　ranges。　Let　（A）　be　a　common；　widely…diffused；　and　varying　species；　belonging　to　a　genus　large　in　its　own　country。　The　little　fan　of　diverging　dotted　lines　of　unequal　lengths　proceeding　from　（A）；　may　represent　its　varying　offspring。　The　variations　are　supposed　to　be　extremely　slight；　but　of　the　most　diversified　nature；　they　are　not　supposed　all　to　appear　simultaneously；　but　often　after　long　intervals　of　time；　nor　are　they　all　supposed　to　endure　for　equal　periods。　Only　those　variations　which　are　in　some　way　profitable　will　be　preserved　or　naturally　selected。　And　here　the　importance　of　the　principle　of　benefit　being　derived　from　divergence　of　character　comes　in；　for　this　will　generally　lead　to　the　most　different　or　divergent　variations　（represented　by　the　outer　dotted　lines）　being　preserved　and　accumulated　by　natural　selection。　When　a　dotted　line　reaches　one　of　the　horizontal　lines；　and　is　there　marked　by　a　small　numbered　letter；　a　sufficient　amount　of　variation　is　supposed　to　have　been　accumulated　to　have　formed　a　fairly　well…marked　variety；　such　as　would　be　thought　worthy　of　record　in　a　systematic　work。　
The　intervals　between　the　horizontal　lines　in　the　diagram；　may　represent　each　a　thousand　generations；　but　it　would　have　been　better　if　each　had　represented　ten　thousand　generations。　After　a　thousand　generations；　species　（A）　is　supposed　to　have　produced　two　fairly　well…marked　varieties；　namely　a1　and　m1。　These　two　varieties　will　generally　continue　to　be　exposed　to　the　same　conditions　which　made　their　parents　variable；　and　the　tendency　to　variability　is　in　itself　hereditary；　consequently　they　will　tend　to　vary；　and　generally　to　vary　in　nearly　the　same　manner　as　their　parents　varied。　Moreover；　these　two　varieties；　being　only　slightly　modified　forms；　will　tend　to　inherit　those　advantages　which　made　their　common　parent　（A）　more　numerous　than　most　of　the　other　inhabitants　of　the　same　country；　they　will　likewise　partake　of　those　more　general　advantages　which　made　the　genus　to　which　the　parent…species　belonged；　a　large　genus　in　its　own　country。　And　these　circumstances　we　know　to　be　favourable　to　the　production　of　new　varieties。　
If；　then；　these　two　varieties　be　variable；　the　most　divergent　of　their　variations　will　generally　be　preserved　during　the　next　thousand　generations。　And　after　this　interval；　variety　a1　is　supposed　in　the　diagram　to　have　produced　variety　a2；　which　will；　owing　to　the　principle　of　divergence；　differ　more　from　（A）　than　did　variety　a1。　Variety　m1　is　supposed　to　have　produced　two　varieties；　namely　m　2　and　s2；　di