the origin of species(物种起源)-第26节

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ghter…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　the　sun　shines；　that　the　proportion　of　wingless　beetles　is　larger　on　the　exposed　Dezertas　than　in　Madeira　itself；　and　especially　the　extraordinary　fact；　so　strongly　insisted　on　by　Mr。　Wollaston；　of　the　almost　entire　absence　of　certain　large　groups　of　beetles；　elsewhere　excessively　numerous；　and　which　groups　have　habits　of　life　almost　necessitating　frequent　flight；　these　several　considerations　have　made　me　believe　that　the　wingless　condition　of　so　many　Madeira　beetles　is　mainly　due　to　the　action　of　natural　selection；　but　combined　probably　with　disuse。　For　during　thousands　of　successive　generations　each　individual　beetle　which　flew　least；　either　from　its　wings　having　been　ever　so　little　less　perfectly　developed　or　from　indolent　habit；　will　have　had　the　best　chance　of　surviving　from　not　being　blown　out　to　sea；　and；　on　the　other　hand；　those　beetles　which　most　readily　took　to　flight　will　oftenest　have　been　blown　to　sea　and　thus　have　been　destroyed。　
The　insects　in　Madeira　which　are　not　ground…feeders；　and　which；　as　the　flower…feeding　coleoptera　and　lepidoptera；　must　habitually　use　their　wings　to　gain　their　subsistence；　have；　as　Mr。　Wollaston　suspects；　their　wings　not　at　all　reduced；　but　even　enlarged。　This　is　quite　compatible　with　the　action　of　natural　selection。　For　when　a　new　insect　first　arrived　on　the　island；　the　tendency　of　natural　selection　to　enlarge　or　to　reduce　the　wings；　would　depend　on　whether　a　greater　number　of　individuals　were　saved　by　successfully　battling　with　the　winds；　or　by　giving　up　the　attempt　and　rarely　or　never　flying。　As　with　mariners　ship…wrecked　near　a　coast；　it　would　have　been　better　for　the　good　swimmers　if　they　had　been　able　to　swim　still　further；　whereas　it　would　have　been　better　for　the　bad　swimmers　if　they　had　not　been　able　to　swim　at　all　and　had　stuck　to　the　wreck。　
The　eyes　of　moles　and　of　some　burrowing　rodents　are　rudimentary　in　size；　and　in　some　cases　are　quite　covered　up　by　skin　and　fur。　This　state　of　the　eyes　is　probably　due　to　gradual　reduction　from　disuse；　but　aided　perhaps　by　natural　selection。　In　South　America；　a　burrowing　rodent；　the　tuco…tuco；　or　Ctenomys；　is　even　more　subterranean　in　its　habits　than　the　mole；　and　I　was　assured　by　a　Spaniard；　who　had　often　caught　them；　that　they　were　frequently　blind；　one　which　I　kept　alive　was　certainly　in　this　condition；　the　cause；　as　appeared　on　dissection；　having　been　inflammation　of　the　nictitating　membrane。　As　frequent　inflammation　of　the　eyes　must　be　injurious　to　any　animal；　and　as　eyes　are　certainly　not　indispensable　to　animals　with　subterranean　habits；　a　reduction　in　their　size　with　the　adhesion　of　the　eyelids　and　growth　of　fur　over　them；　might　in　such　case　be　an　advantage；　and　if　so；　natural　selection　would　constantly　aid　the　effects　of　disuse。　
It　is　well　known　that　several　animals；　belonging　to　the　most　different　classes；　which　inhabit　the　caves　of　Styria　and　of　Kentucky；　are　blind。　In　some　of　the　crabs　the　foot…stalk　for　the　eye　remains；　though　the　eye　is　gone；　the　stand　for　the　telescope　is　there；　though　the　telescope　with　its　glasses　has　been　lost。　As　it　is　difficult　to　imagine　that　eyes；　though　useless；　could　be　in　any　way　injurious　to　animals　living　in　darkness；　I　attribute　their　loss　wholly　to　disuse。　In　one　of　the　blind　animals；　namely；　the　cave…rat；　the　eyes　are　of　immense　size；　and　Professor　Silliman　thought　that　it　regained；　after　living　some　days　in　the　light；　some　slight　power　of　vision。　In　the　same　manner　as　in　Madeira　the　wings　of　some　of　the　insects　have　been　enlarged；　and　the　wings　of　others　have　been　reduced　by　natural　selection　aided　by　use　and　disuse；　so　in　the　case　of　the　cave…rat　natural　selection　seems　to　have　struggled　with　the　loss　of　light　and　to　have　increased　the　size　of　the　eyes；　whereas　with　all　the　other　inhabitants　of　the　caves；　disuse　by　itself　seems　to　have　done　its　work。　
It　is　difficult　to　imagine　conditions　of　life　more　similar　than　deep　limestone　caverns　under　a　nearly　similar　climate；　so　that　on　the　common　view　of　the　blind　animals　having　been　separately　created　for　the　American　and　European　caver