the origin of species(物种起源)-第23部分

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anner　as　species　and　groups　of　species　have　tried　to　overmaster　other　species　in　the　great　battle　for　life。　The　limbs　divided　into　great　branches；　and　these　into　lesser　and　lesser　branches；　were　themselves　once；　when　the　tree　was　small；　budding　twigs；　and　this　connexion　of　the　former　and　present　buds　by　ramifying　branches　may　well　represent　the　classification　of　all　extinct　and　living　species　in　groups　subordinate　to　groups。　Of　the　many　twigs　which　flourished　when　the　tree　was　a　mere　bush；　only　two　or　three；　now　grown　into　great　branches；　yet　survive　and　bear　all　the　other　branches；　so　with　the　species　which　lived　during　long…past　geological　periods；　very　few　now　have　living　and　modified　descendants。　From　the　first　growth　of　the　tree；　many　a　limb　and　branch　has　decayed　and　dropped　off；　and　these　lost　branches　of　various　sizes　may　represent　those　whole　orders；　families；　and　genera　which　have　now　no　living　representatives；　and　which　are　known　to　us　only　from　having　been　found　in　a　fossil　state。　As　we　here　and　there　see　a　thin　straggling　branch　springing　from　a　fork　low　down　in　a　tree；　and　which　by　some　chance　has　been　favoured　and　is　still　alive　on　its　summit；　so　we　occasionally　see　an　animal　like　the　Ornithorhynchus　or　Lepidosiren；　which　in　some　small　degree　connects　by　its　affinities　two　large　branches　of　life；　and　which　has　apparently　been　saved　from　fatal　competition　by　having　inhabited　a　protected　station。　As　buds　give　rise　by　growth　to　fresh　buds；　and　these；　if　vigorous；　branch　out　and　overtop　on　all　sides　many　a　feebler　branch；　so　by　generation　I　believe　it　has　been　with　the　great　Tree　of　Life；　which　fills　with　its　dead　and　broken　branches　the　crust　of　the　earth；　and　covers　the　surface　with　its　ever　branching　and　beautiful　ramifications。　　

Chapter　5　…　Laws　of　Variation

I　HAVE　hitherto　sometimes　spoken　as　if　the　variations　so　common　and　multiform　in　organic　beings　under　domestication；　and　in　a　lesser　degree　in　those　in　a　state　of　nature　had　been　due　to　chance。　This；　of　course；　is　a　wholly　incorrect　expression；　but　it　serves　to　acknowledge　plainly　our　ignorance　of　the　cause　of　each　particular　variation。　Some　authors　believe　it　to　be　as　much　the　function　of　the　reproductive　system　to　produce　individual　differences；　or　very　slight　deviations　of　structure；　as　to　make　the　child　like　its　parents。　But　the　much　greater　variability；　as　well　as　the　greater　frequency　of　monstrosities；　under　domestication　or　cultivation；　than　under　nature；　leads　me　to　believe　that　deviations　of　structure　are　in　some　way　due　to　the　nature　of　the　conditions　of　life；　to　which　the　parents　and　their　more　remote　ancestors　have　been　exposed　during　several　generations。　I　have　remarked　in　the　first　chapter　but　a　long　catalogue　of　facts　which　cannot　be　here　given　would　be　necessary　to　show　the　truth　of　the　remark　that　the　reproductive　system　is　eminently　susceptible　to　changes　in　the　conditions　of　life；　and　to　this　system　being　functionally　disturbed　in　the　parents；　I　chiefly　attribute　the　varying　or　plastic　condition　of　the　offspring。　The　male　and　female　sexual　elements　seem　to　be　affected　before　that　union　takes　place　which　is　to　form　a　new　being。　In　the　case　of　'sporting'　plants；　the　bud；　which　in　its　earliest　condition　does　not　apparently　differ　essentially　from　an　ovule；　is　alone　affected。　But　why；　because　the　reproductive　system　is　disturbed；　this　or　that　part　should　vary　more　or　less；　we　are　profoundly　ignorant。　Nevertheless；　we　can　here　and　there　dimly　catch　a　faint　ray　of　light；　and　we　may　feel　sure　that　there　must　be　some　cause　for　each　deviation　of　structure；　however　slight。　
How　much　direct　effect　difference　of　climate；　food；　&c。；　produces　on　any　being　is　extremely　doubtful。　My　impression　is；　that　the　effect　is　extremely　small　in　the　case　of　animals；　but　perhaps　rather　more　in　that　of　plants。　We　may；　at　least；　safely　conclude　that　such　influences　cannot　have　produced　the　many　striking　and　complex　co…adaptations　of　structure　between　one　organic　being　and　another；　which　we　see　everywhere　throughout　nature。　Some　little　influence　may　be　attributed　to　climate；　food；　&c。：　thus；　E。　Forbes　speaks　confidently　that　shells　at　their　southern　limit；　and　when　living　in　shallow　water；　are　more　brightly　coloured　than　those　of　the　same　species　further　north　or　from　greater　depths。　Gould　believes　that　birds　of　the　same　species　are　more　brightly　coloured　under　a　clear　atmosphere；　than　when　living　on　islands　or　near　the　coast。　So　with　insects；　Wollaston　is　convinced　that　residence　near　the　sea　affects　their　colours。　Moquin…Tandon　gives　a　list　of　plants　which　when　growing　near　the　sea…shore　have　their　leaves　in　some　degree　fleshy；　though　not　elsewhere　fleshy。　Several　other　such　cases　could　be　given。　
The　fact　of　varieties　of　one　species；　when　they　range　into　the　zone　of　habitation　of　other　species；　often　acquiring　in　a　very　slight　degree　some　of　the　characters　of　such　species；　accords　with　our　view　that　species　of　all　kinds　are　only　well…marked　and　permanent　varieties。　Thus　the　species　of　shells　which　are　confined　to　tropical　and　shallow　seas　are　generally　brighter…coloured　than　those　confined　to　cold　and　deeper　seas。　The　birds　which　are　confined　to　continents　are；　according　to　Mr　Gould；　brighter…coloured　than　those　of　islands。　The　insect…species　confined　to　sea…coasts；　as　every　collector　knows；　are　often　brassy　or　lurid。　Plants　which　live　exclusively　on　the　sea…side　are　very　apt　to　have　fleshy　leaves。　He　who　believes　in　the　creation　of　each　species；　will　have　to　say　that　this　shell；　for　instance；　was　created　with　bright　colours　for　a　warm　sea；　but　that　this　other　shell　became　bright…coloured　by　variation　when　it　ranged　into　warmer　or　shallower　waters。　
When　a　variation　is　of　the　slightest　use　to　a　being；　we　cannot　tell　how　much　of　it　to　attribute　to　the　accumulative　action　of　natural　selection；　and　how　much　to　the　conditions　of　life。　Thus；　it　is　well　known　to　furriers　that　animals　of　the　same　species　have　thicker　and　better　fur　the　more　severe　the　climate　is　under　which　they　have　lived；　but　who　can　tell　how　much　of　this　difference　may　be　due　to　the　warmest…clad　individuals　having　been　favoured　and　preserved　during　many　generations；　and　how　much　to　the　direct　action　of　the　severe　climate？　for　it　would　appear　that　climate　has　some　direct　action　on　the　hair　of　our　domestic　quadrupeds。　
Instances　could　be　given　of　the　same　variety　being　produced　under　conditions　of　life　as　different　as　can　well　be　conceived；　and；　on　the　other　hand；　of　different　varieties　being　produced　from　the　same　species　under　the　same　conditions。　Such　facts　show　how　indirectly　the　conditions　of　life　must　act。　Again；　innumerable　instances　are　known　to　every　naturalist　of　species　keeping　true；　or　not　varying　at　all；　although　living　under　the　most　opposite　climates。　Such　considerations　as　these　incline　me　to　lay　very　little　weight　on　the　direct　action　of　the　conditions　of　life。　Indirectly；　as　already　remarked；　they　seem　to　play　an　important　part　in　affecting　the　reproductive　system；　and　in　thus　inducing　variability；　and　natural　selection　will　then　accumulate　all　profitable　variations；　however　slight；　until　they　become　plainly　developed　and　appreciable　by　us。　
Effects　of　Use　and　Disuse
From　the　facts　alluded　to　in　the　first　chapter；　I　think　there　can　be　little　doubt　that　use　in　our　domestic　animals　strengthens　and　enlarges　certain　parts；　and　disuse　diminishes　them；　and　that　such　modifications　are　inherited。　Under　free　nature；　we　can　have　no　standard　of　comparison；　by　which　to　judge　of　the　effects　of　long…continued　use　or　disuse；　for　we　know　not　the　parent…forms；　but　many　animals　have　structures　which　can　be　explained　by　the　effects　of　disuse。　As　Professor　Owen　has　remarked；　there　is　no　greater　anomaly　in　nature　than　a　bird　that　cannot　fly；　yet　there　are　several　in　this　state。　The　logger…headed　duck　of　South　America　can　only　flap　along　the　surface　of　the　water；　and　has　its　wings　in　nearly　the　same　condition　as　the　domestic　Aylesbury　duck。　As　the　larger　ground…feeding　birds　seldom　take　flight　except　to　escape　danger；　I　believe　that　the　nearly　wingless　condition　of　several　birds；　which　now　inhabit　or　have　lately　inhabited　several　oceanic　islands；　tenanted　by　no　beast　of　prey；　has　been　caused　by　disuse。　The　ostrich　indeed　inhabits　continents　and　is　exposed　to　danger　from　which　it　cannot　escape　by　flight；　but　by　kicking　it　can　defend　itself　from　enemies；　as　well　as　any　of　the　smaller　quadrupeds。　We　may　imagine　that　the　early　progenitor　of　the　ostrich　had　habits　like　those　of　a　bustard；　and　that　as　natural　selection　increased　in　successive　generations　the　size　and　weight　of　its　body；　its　legs　were　used　more；　and　its　wings　less；　until　they　became　incapable　of　flight。　
Kirby　has　remarked　（and　I　have　observed　the　same　fact）　that　the　anterior　tarsi；　or　feet；　of　many　male　dung…feeding　beetles　are　very　often　broken　off；　he　examined　seventeen　specimens　in　his　own　collection；　and　not　one　had　even　a　relic　left。　In　the　Onites　apelles　the　tarsi　are　so　habitually　lost；　that　the　insect　has　been　described　as　not　having　them。　In　some　other　genera　they　are　present；　but　in　a　rudimentary　condition。　In　the　Ateuchus　or　sacred　beetle　of　the　Egyptians；　they　are　totally　deficient。　There　is　not　sufficient　evidence　to　induce　us　to　believe　that　mutilations　are　ever　inherited；　and　I　should　prefer　explaining　the　entire　absence　of　the　anterior　tarsi　in　Ateuchus；　and　their　rudimentary　condition　in　some　other　genera；　by　the　long…continued　effects　of　disuse　in　their　progenitors；　for　as　the　tarsi　are　almost　always　lost　in　many　dung…feeding　beetles；　they　must　be　lost　early　in　life；　and　therefore　cannot　be　much　used　by　these　insects。　
In　some　cases　we　might　easily　put　down　to　disuse　modifications　of　structure　which　are　wholly；　or　mainly；　due　to　natural　selection。　Mr。　Wollaston　has　discovered　the　remarkable　fact　that　200　beetles；　out　of　the　550　species　inhabiting　Madeira；　are　so　far　deficient　in　wings　that　they　cannot　fly；　and　that　of　the　twenty…nine　endemic　genera；　no　less　than　twenty…three　genera　have　all　their　species　in　this　condition！　Several　facts；　namely；　that　beetles　in　many　parts　of　the　world　are　very　frequently　blown　to　sea　and　perish；　that　the　beetles　in　Madeira；　as　observed　by　Mr　Wollaston；　lie　much　concealed；　until　the　wind　lulls　and　th