the origin of species(物种起源)-第17节

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　will　be　able　to　modify　one　sex　in　its　functional　relations　to　the　other　sex；　or　in　relation　to　wholly　different　habits　of　life　in　the　two　sexes；　as　is　sometimes　the　case　with　insects。　And　this　leads　me　to　say　a　few　words　on　what　I　call　Sexual　Selection。　This　depends；　not　on　a　struggle　for　existence；　but　on　a　struggle　between　the　males　for　possession　of　the　females；　the　result　is　not　death　to　the　unsuccessful　competitor；　but　few　or　no　offspring。　Sexual　selection　is；　therefore；　less　rigorous　than　natural　selection。　Generally；　the　most　vigorous　males；　those　which　are　best　fitted　for　their　places　in　nature；　will　leave　most　progeny。　But　in　many　cases；　victory　will　depend　not　on　general　vigour；　but　on　having　special　weapons；　confined　to　the　male　sex。　A　hornless　stag　or　spurless　cock　would　have　a　poor　chance　of　leaving　offspring。　Sexual　selection　by　always　allowing　the　victor　to　breed　might　surely　give　indomitable　courage；　length　to　the　spur；　and　strength　to　the　wing　to　strike　in　the　spurred　leg；　as　well　as　the　brutal　cock…fighter；　who　knows　well　that　he　can　improve　his　breed　by　careful　selection　of　the　best　cocks。　How　low　in　the　scale　of　nature　this　law　of　battle　descends；　I　know　not；　male　alligators　have　been　described　as　fighting；　bellowing；　and　whirling　round；　like　Indians　in　a　war…dance；　for　the　possession　of　the　females；　male　salmons　have　been　seen　fighting　all　day　long；　male　stag…beetles　often　bear　wounds　from　the　huge　mandibles　of　other　males。　The　war　is；　perhaps；　severest　between　the　males　of　polygamous　animals；　and　these　seem　oftenest　provided　with　special　weapons。　The　males　of　carnivorous　animals　are　already　well　armed；　though　to　them　and　to　others；　special　means　of　defence　may　be　given　through　means　of　sexual　selection；　as　the　mane　to　the　lion；　the　shoulder…pad　to　the　boar；　and　the　hooked　jaw　to　the　male　salmon；　for　the　shield　may　be　as　important　for　victory；　as　the　sword　or　spear。　
Amongst　birds；　the　contest　is　often　of　a　more　peaceful　character。　All　those　who　have　attended　to　the　subject；　believe　that　there　is　the　severest　rivalry　between　the　males　of　many　species　to　attract　by　singing　the　females。　The　rock…thrush　of　Guiana；　birds　of　paradise；　and　some　others；　congregate；　and　successive　males　display　their　gorgeous　plumage　and　perform　strange　antics　before　the　females；　which　standing　by　as　spectators；　at　last　choose　the　most　attractive　partner。　Those　who　have　closely　attended　to　birds　in　confinement　well　know　that　they　often　take　individual　preferences　and　dislikes：　thus　Sir　R。　Heron　has　described　how　one　pied　peacock　was　eminently　attractive　to　all　his　hen　birds。　It　may　appear　childish　to　attribute　any　effect　to　such　apparently　weak　means：　I　cannot　here　enter　on　the　details　necessary　to　support　this　view；　but　if　man　can　in　a　short　time　give　elegant　carriage　and　beauty　to　his　bantams；　according　to　his　standard　of　beauty；　I　can　see　no　good　reason　to　doubt　that　female　birds；　by　selecting；　during　thousands　of　generations；　the　most　melodious　or　beautiful　males；　according　to　their　standard　of　beauty；　might　produce　a　marked　effect。　I　strongly　suspect　that　some　well…known　laws　with　respect　to　the　plumage　of　male　and　female　birds；　in　comparison　with　the　plumage　of　the　young；　can　be　explained　on　the　view　of　plumage　having　been　chiefly　modified　by　sexual　selection；　acting　when　the　birds　have　come　to　the　breeding　age　or　during　the　breeding　season；　the　modifications　thus　produced　being　inherited　at　corresponding　ages　or　seasons；　either　by　the　males　alone；　or　by　the　males　and　females；　but　I　have　not　space　here　to　enter　on　this　subject。　
Thus　it　is；　as　I　believe；　that　when　the　males　and　females　of　any　animal　have　the　same　general　habits　of　life；　but　differ　in　structure；　colour；　or　ornament；　such　differences　have　been　mainly　caused　by　sexual　selection；　that　is；　individual　males　have　had；　in　successive　generations；　some　slight　advantage　over　other　males；　in　their　weapons；　means　of　defence；　or　charms；　and　have　transmitted　these　advantages　to　their　male　offspring。　Yet；　I　would　not　wish　to　attribute　all　such　sexual　differences　to　this　agency：　for　we　see　peculiarities　arising　and　becoming　attached　to　the　male　sex　in　our　domestic　animals　（as　the　wattle　in　male　carriers；　horn…like　protuberances　in　the　cocks　of　certain　fowls；　&c。）；　which　we　cannot　believe　to　be　either　useful　to　the　males　in　battle；　or　attractive　to　the　females。　We　see　analogous　cases　under　nature；　for　instance；　the　tuft　of　hair　on　the　breast　of　the　turkey…cock；　which　can　hardly　be　either　useful　or　ornamental　to　this　bird；　indeed；　had　the　tuft　appeared　under　domestication；　it　would　have　been　called　a　monstrosity。　
Illustrations　of　the　action　of　Natural　Selection
In　order　to　make　it　clear　how；　as　I　believe；　natural　selection　acts；　I　must　beg　permission　to　give　one　or　two　imaginary　illustrations。　Let　us　take　the　case　of　a　wolf；　which　preys　on　various　animals；　securing　some　by　craft；　some　by　strength；　and　some　by　fleetness；　and　let　us　suppose　that　the　fleetest　prey；　a　deer　for　instance；　had　from　any　change　in　the　country　increased　in　numbers；　or　that　other　prey　had　decreased　in　numbers；　during　that　season　of　the　year　when　the　wolf　is　hardest　pressed　for　food。　I　can　under　such　circumstances　see　no　reason　to　doubt　that　the　swiftest　and　slimmest　wolves　would　have　the　best　chance　of　surviving；　and　so　be　preserved　or　selected；　provided　always　that　they　retained　strength　to　master　their　prey　at　this　or　at　some　other　period　of　the　year；　when　they　might　be　compelled　to　prey　on　other　animals。　I　can　see　no　more　reason　to　doubt　this；　than　that　man　can　improve　the　fleetness　of　his　greyhounds　by　careful　and　methodical　selection；　or　by　that　unconscious　selection　which　results　from　each　man　trying　to　keep　the　best　dogs　without　any　thought　of　modifying　the　breed。　
Even　without　any　change　in　the　proportional　numbers　of　the　animals　on　which　our　wolf　preyed；　a　cub　might　be　born　with　an　innate　tendency　to　pursue　certain　kinds　of　prey。　Nor　can　this　be　thought　very　improbable；　for　we　often　observe　great　differences　in　the　natural　tendencies　of　our　domestic　animals；　one　cat；　for　instance；　taking　to　catch　rats；　another　mice；　one　cat；　according　to　Mr。　St。　John；　bringing　home　winged　game；　another　hares　or　rabbits；　and　another　hunting　on　marshy　ground　and　almost　nightly　catching　woodcocks　or　snipes。　The　tendency　to　catch　rats　rather　than　mice　is　known　to　be　inherited。　Now；　if　any　slight　innate　change　of　habit　or　of　structure　benefited　an　individual　wolf；　it　would　have　the　best　chance　of　surviving　and　of　leaving　offspring。　Some　of　its　young　would　probably　inherit　the　same　habits　or　structure；　and　by　the　repetition　of　this　process；　a　new　variety　might　be　formed　which　would　either　supplant　or　coexist　with　the　parent…form　of　wolf。　Or；　again；　the　wolves　inhabiting　a　mountainous　district；　and　those　frequenting　the　lowlands；　would　naturally　be　forced　to　hunt　different　prey；　and　from　the　continued　preservation　of　the　individuals　best　fitted　for　the　two　sites；　two　varieties　might　slowly　be　formed。　These　varieties　would　cross　and　blend　where　they　met；　but　to　this　subject　of　intercrossing　we　shall　soon　have　to　return。　I　may　add；　that；　according　to　Mr。　Pierce；　there　are　two　varieties　of　the　wolf　inhabiting　the　Catskill　Mountains　in　the　United　States；　one　with　a　light　greyhound…like　form；　which　pursues　deer；　and　the　other　more　bulky；　with　shorter　legs；　which　more　frequently　attacks　the　shepherd's　flocks。　
Let　us　now　take　a　more　complex　case。　Certain　plants　excrete　a　sweet　juice；　apparently　for　the　sake　of　eliminating　something　injurious　from　their　sap：　this　is　effected　by　glands　at　the　base　of　the　stipules　in　some　Leguminosae；　and　at　the　back　of　the　leaf　of　the　common　laurel。　This　juice；　though　small　in　quantity；　is　greedily　sought　by　insects。　Let　us　now　suppose　a　little　sweet　juice　or　nectar　to　be　excreted　by　the　inner　bases　of　the　petals　of　a　flower。　In　this　case　insects　in　seeking　the　nectar　would　get　dusted　with　pollen；　and　would　certainly　often　transport　the　pollen　from　one　flower　to　the　stigma　of　another　flower。　The　flowers　of　two　distinct　individuals　of　the　same　species　would　thus　get　crossed；　and　the　act　of　crossing；　we　have　good　reason　to　believe　（as　will　hereafter　be　more　fully　alluded　to）；　would　produce　very　vigorous　seedlings；　which　consequently　would　have　the　best　chance　of　flourishing　and　surviving。　Some　of　these　seedlings　would　probably　inherit　the　nectar…excreting　power。　Those　in　individual　flowers　which　had　the　largest　glands　or　nectaries；　and　which　excreted　most　nectar；　would　be　oftenest　visited　by　insects；　and　would　be　oftenest　crossed；　and　so　in　the　long…run　would　gain　the　upper　hand。　Those　flowers；　also；　which　had　their　stamens　and　pistils　placed；　in　relation　to　the　size　and　habits　of　the　particular　insects　which　visited　them；　so　as　to　favour　in　any　degree　the　transportal　of　their　pollen　from　flower　to　flower；　would　likewise　be　favoured　or　selected。　We　might　have　taken　the　case　of　insects　visiting　flowers　for　the　sake　of　collecting　pollen　instead　of　nectar；　and　as　pollen　is　formed　for　the　sole　object　of　fertilisation；　its　destruction　appears　a　simple　loss　to　the　plant；　yet　if　a　little　pollen　were　carried；　at　first　occasionally　and　then　habitually；　by　the　pollen…devouring　insects　from　flower　to　flower；　a