the origin of species(物种起源)-第18节

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　at　first　occasionally　and　then　habitually；　by　the　pollen…devouring　insects　from　flower　to　flower；　and　a　cross　thus　effected；　although　nine…tenths　of　the　pollen　were　destroyed；　it　might　still　be　a　great　gain　to　the　plant；　and　those　individuals　which　produced　more　and　more　pollen；　and　had　larger　and　larger　anthers；　would　be　selected。　
When　our　plant；　by　this　process　of　the　continued　preservation　or　natural　selection　of　more　and　more　attractive　flowers；　had　been　rendered　highly　attractive　to　insects；　they　would；　unintentionally　on　their　part；　regularly　carry　pollen　from　flower　to　flower；　and　that　they　can　most　effectually　do　this；　I　could　easily　show　by　many　striking　instances。　I　will　give　only　one　not　as　a　very　striking　case；　but　as　likewise　illustrating　one　step　in　the　separation　of　the　sexes　of　plants；　presently　to　be　alluded　to。　Some　holly…trees　bear　only　male　flowers；　which　have　four　stamens　producing　rather　a　small　quantity　of　pollen；　and　a　rudimentary　pistil；　other　holly…trees　bear　only　female　flowers；　these　have　a　full…sized　pistil；　and　four　stamens　with　shrivelled　anthers；　in　which　not　a　grain　of　pollen　can　be　detected。　Having　found　a　female　tree　exactly　sixty　yards　from　a　male　tree；　I　put　the　stigmas　of　twenty　flowers；　taken　from　different　branches；　under　the　microscope；　and　on　all；　without　exception；　there　were　pollen…grains；　and　on　some　a　profusion　of　pollen。　As　the　wind　had　set　for　several　days　from　the　female　to　the　male　tree；　the　pollen　could　not　thus　have　been　carried。　The　weather　had　been　cold　and　boisterous；　and　therefore　not　favourable　to　bees；　nevertheless　every　female　flower　which　I　examined　had　been　effectually　fertilised　by　the　bees；　accidentally　dusted　with　pollen；　having　flown　from　tree　to　tree　in　search　of　nectar。　But　to　return　to　our　imaginary　case：　as　soon　as　the　plant　had　been　rendered　so　highly　attractive　to　insects　that　pollen　was　regularly　carried　from　flower　to　flower；　another　process　might　commence。　No　naturalist　doubts　the　advantage　of　what　has　been　called　the　'physiological　division　of　labour；'　hence　we　may　believe　that　it　would　be　advantageous　to　a　plant　to　produce　stamens　alone　in　one　flower　or　on　one　whole　plant；　and　pistils　alone　in　another　flower　or　on　another　plant。　In　plants　under　culture　and　placed　under　new　conditions　of　life；　sometimes　the　male　organs　and　sometimes　the　female　organs　become　more　or　less　impotent；　now　if　we　suppose　this　to　occur　in　ever　so　slight　a　degree　under　nature；　then　as　pollen　is　already　carried　regularly　from　flower　to　flower；　and　as　a　more　complete　separation　of　the　sexes　of　our　plant　would　be　advantageous　on　the　principle　of　the　division　of　labour；　individuals　with　this　tendency　more　and　more　increased；　would　be　continually　favoured　or　selected；　until　at　last　a　complete　separation　of　the　sexes　would　be　effected。　
Let　us　now　turn　to　the　nectar…feeding　insects　in　our　imaginary　case：　we　may　suppose　the　plant　of　which　we　have　been　slowly　increasing　the　nectar　by　continued　selection；　to　be　a　common　plant；　and　that　certain　insects　depended　in　main　part　on　its　nectar　for　food。　I　could　give　many　facts；　showing　how　anxious　bees　are　to　save　time；　for　instance；　their　habit　of　cutting　holes　and　sucking　the　nectar　at　the　bases　of　certain　flowers；　which　they　can；　with　a　very　little　more　trouble；　enter　by　the　mouth。　Bearing　such　facts　in　mind；　I　can　see　no　reason　to　doubt　that　an　accidental　deviation　in　the　size　and　form　of　the　body；　or　in　the　curvature　and　length　of　the　proboscis；　&c。；　far　too　slight　to　be　appreciated　by　us；　might　profit　a　bee　or　other　insect；　so　that　an　individual　so　characterised　would　be　able　to　obtain　its　food　more　quickly；　and　so　have　a　better　chance　of　living　and　leaving　descendants。　Its　descendants　would　probably　inherit　a　tendency　to　a　similar　slight　deviation　of　structure。　The　tubes　of　the　corollas　of　the　common　red　and　incarnate　clovers　（Trifolium　pratense　and　incarnatum）　do　not　on　a　hasty　glance　appear　to　differ　in　length；　yet　the　hive…bee　can　easily　suck　the　nectar　out　of　the　incarnate　clover；　but　not　out　of　the　common　red　clover；　which　is　visited　by　humble…bees　alone；　so　that　whole　fields　of　the　red　clover　offer　in　vain　an　abundant　supply　of　precious　nectar　to　the　hive…bee。　Thus　it　might　be　a　great　advantage　to　the　hive…bee　to　have　a　slightly　longer　or　differently　constructed　proboscis。　On　the　other　hand；　I　have　found　by　experiment　that　the　fertility　of　clover　greatly　depends　on　bees　visiting　and　moving　parts　of　the　corolla；　so　as　to　push　the　pollen　on　to　the　stigmatic　surface。　Hence；　again；　if　humble…bees　were　to　become　rare　in　any　country；　it　might　be　a　great　advantage　to　the　red　clover　to　have　a　shorter　or　more　deeply　divided　tube　to　its　corolla；　so　that　the　hive…bee　could　visit　its　flowers。　Thus　I　can　understand　how　a　flower　and　a　bee　might　slowly　become；　either　simultaneously　or　one　after　the　other；　modified　and　adapted　in　the　most　perfect　manner　to　each　other；　by　the　continued　preservation　of　individuals　presenting　mutual　and　slightly　favourable　deviations　of　structure。　
I　am　well　aware　that　this　doctrine　of　natural　selection；　exemplified　in　the　above　imaginary　instances；　is　open　to　the　same　objections　which　were　at　first　urged　against　Sir　Charles　Lyell's　noble　views　on　'the　modern　changes　of　the　earth；　as　illustrative　of　geology；'　but　we　now　very　seldom　hear　the　action；　for　instance；　of　the　coast…waves；　called　a　trifling　and　insignificant　cause；　when　applied　to　the　excavation　of　gigantic　valleys　or　to　the　formation　of　the　longest　lines　of　inland　cliffs。　Natural　selection　can　act　only　by　the　preservation　and　accumulation　of　infinitesimally　small　inherited　modifications；　each　profitable　to　the　preserved　being；　and　as　modern　geology　has　almost　banished　such　views　as　the　excavation　of　a　great　valley　by　a　single　diluvial　wave；　so　will　natural　selection；　if　it　be　a　true　principle；　banish　the　belief　of　the　continued　creation　of　new　organic　beings；　or　of　any　great　and　sudden　modification　in　their　structure。　
On　the　Intercrossing　of　Individuals
I　must　here　introduce　a　short　digression。　In　the　case　of　animals　and　plants　with　separated　sexes；　it　is　of　course　obvious　that　two　individuals　must　always　unite　for　each　birth；　but　in　the　case　of　hermaphrodites　this　is　far　from　obvious。　Nevertheless　I　am　strongly　inclined　to　believe　that　with　all　hermaphrodites　two　individuals；　either　occasionally　or　habitually；　concur　for　the　reproduction　of　their　kind。　This　view；　I　may　add；　was　first　suggested　by　Andrew　Knight。　We　shall　presently　see　its　importance；　but　I　must　here　treat　the　subject　with　extreme　brevity；　though　I　have　the　materials　prepared　for　an　ample　discussion。　All　vertebrate　animals；　all　insects；　and　some　other　large　groups　of　animals；　pair　for　each　birth。　Modern　research　has　much　diminished　the　number　of　supposed　hermaphrodites；　and　of　real　hermaphrodites　a　large　number　pair；　that　is；　two　individuals　regularly　unite　for　reproduction；　which　is　all　that　concerns　us。　But　still　there　are　many　hermaphrodite　animals　which　certainly　do　not　habitually　pair；　and　a　vast　majority　of　plants　are　hermaphrodites。　What　reason；　it　may　be　asked；　is　there　for　supposing　in　these　cases　that　two　individuals　ever　concur　in　reproduction？　As　it　is　impossible　here　to　enter　on　details；　I　must　trust　to　some　general　considerations　alone。　
In　the　first　place；　I　have　collected　so　large　a　body　of　facts；　showing；　in　accordance　with　the　almost　universal　belief　of　breeders；　that　with　animals　and　plants　a　cross　between　different　varieties；　or　between　individuals　of　the　same　variety　but　of　another　strain；　gives　vigour　and　fertility　to　the　offspring；　and　on　the　other　hand；　that　close　interbreeding　diminishes　vigour　and　fertility；　that　these　facts　alone　incline　me　to　believe　that　it　is　a　general　law　of　nature　（utterly　ignorant　though　we　be　of　the　meaning　of　the　law）　that　no　organic　being　self…fertilises　itself　for　an　eternity　of　generations；　but　that　a　cross　with　another　individual　is　occasionally　perhaps　at　very　long　intervals　　indispensable。　
On　the　belief　that　this　is　a　law　of　nature；　we　can；　I　think；　understand　several　large　classes　of　facts；　such　as　the　following；　which　on　any　other　view　are　inexplicable。　Every　hybridizer　knows　how　unfavourable　exposure　to　wet　is　to　the　fertilisation　of　a　flower；　yet　what　a　multitude　of　flowers　have　their　anthers　and　stigmas　fully　exposed　to　the　weather！　but　if　an　occasional　cross　be　indispensable；　the　fullest　freedom　for　the　entrance　of　pollen　from　another　individual　will　explain　this　state　of　exposure；　more　especially　as　the　plant's　own　anthers　and　pistil　generally　stand　so　close　together　that　self…fertilisation　seems　almost　inevitable。　Many　flowers；　on　the　other　hand；　have　their　organs　of　fructification　closely　enclosed；　as　in　the　great　papilionaceous　or　pea…family；　but　in　several；　perhaps　in　all；　such　flowers；　there　is　a　very　curious　adaptation　between　the　structure　of　the　flower　and　the　manner　in　which　bees　suck　the　nectar；　for；　in　doing　this；　they　either　push　the　flower's　own　pollen　on　the　stigma；　or　bring　pollen　from　another　flower。　So　necessary　are　the　visits　of　bees　to　papilionaceous　flowers；　that　I　have　found；　by　experiments　published　elsewhere；　that　their　fertility　is　greatly　diminished　if　these　visits　be　prevented。　Now；　it　is　scarcely　possi