the origin of species(物种起源)-第24部分

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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　caverns；　close　similarity　in　their　organisation　and　affinities　might　have　been　expected；　but；　as　Schi？dte　and　others　have　remarked；　this　is　not　the　case；　and　the　cave…insects　of　the　two　continents　are　not　more　closely　allied　than　might　have　been　anticipated　from　the　general　resemblance　of　the　other　inhabitants　of　North　America　and　Europe。　On　my　view　we　must　suppose　that　American　animals；　having　ordinary　powers　of　vision；　slowly　migrated　by　successive　generations　from　the　outer　world　into　the　deeper　and　deeper　recesses　of　the　Kentucky　caves；　as　did　European　animals　into　the　caves　of　Europe。　We　have　some　evidence　of　this　gradation　of　habit；　for；　as　Schi？dte　remarks；　'animals　not　far　remote　from　ordinary　forms；　prepare　the　transition　from　light　to　darkness。　Next　follow　those　that　are　constructed　for　twilight；　and；　last　of　all；　those　destined　for　total　darkness。'　By　the　time　that　an　animal　had　reached；　after　numberless　generations；　the　deepest　recesses；　disuse　will　on　this　view　have　more　or　less　perfectly　obliterated　its　eyes；　and　natural　selection　will　often　have　effected　other　changes；　such　as　an　increase　in　the　length　of　the　antennae　or　palpi；　as　a　compensation　for　blindness。　Notwithstanding　such　modifications；　we　might　expect　still　to　see　in　the　cave…animals　of　America；　affinities　to　the　other　inhabitants　of　that　continent；　and　in　those　of　Europe；　to　the　inhabitants　of　the　European　continent。　And　this　is　the　case　with　some　of　the　American　cave…animals；　as　I　hear　from　Professor　Dana；　and　some　of　the　European　cave…insects　are　very　closely　allied　to　those　of　the　surrounding　country。　It　would　be　most　difficult　to　give　any　rational　explanation　of　the　affinities　of　the　blind　cave…animals　to　the　other　inhabitants　of　the　two　continents　on　the　ordinary　view　of　their　independent　creation。　That　several　of　the　inhabitants　of　the　caves　of　the　Old　and　New　Worlds　should　be　closely　related；　we　might　expect　from　the　well…known　relationship　of　most　of　their　other　productions。　Far　from　feeling　any　surprise　that　some　of　the　cave…animals　should　be　very　anomalous；　as　Agassiz　has　remarked　in　regard　to　the　blind　fish；　the　Amblyopsis；　and　as　is　the　case　with　the　blind　Proteus　with　reference　to　the　reptiles　of　Europe；　I　am　only　surprised　that　more　wrecks　of　ancient　life　have　not　been　preserved；　owing　to　the　less　severe　competition　to　which　the　inhabitants　of　these　dark　abodes　will　probably　have　been　exposed。　
Acclimatisation
Habit　is　hereditary　with　plants；　as　in　the　period　of　flowering；　in　the　amount　of　rain　requisite　for　seeds　to　germinate；　in　the　time　of　sleep；　&c。；　and　this　leads　me　to　say　a　few　words　on　acclimatisation。　As　it　is　extremely　common　for　species　of　the　same　genus　to　inhabit　very　hot　and　very　cold　countries；　and　as　I　believe　that　all　the　species　of　the　same　genus　have　descended　from　a　single　parent；　if　this　view　be　correct；　acclimatisation　must　be　readily　effected　during　long…continued　descent。　It　is　notorious　that　each　species　is　adapted　to　the　climate　of　its　own　home：　species　from　an　arctic　or　even　from　a　temperate　region　cannot　endure　a　tropical　climate；　or　conversely。　So　again；　many　succulent　plants　cannot　endure　a　damp　climate。　But　the　degree　of　adaptation　of　species　to　the　climates　under　which　they　live　is　often　overrated。　We　may　infer　this　from　our　frequent　inability　to　predict　whether　or　not　an　imported　plant　will　endure　our　climate；　and　from　the　number　of　plants　and　animals　brought　from　warmer　countries　which　here　enjoy　good　health。　We　have　reason　to　believe　that　species　in　a　state　of　nature　are　limited　in　their　ranges　by　the　competition　of　other　organic　beings　quite　as　much　as；　or　more　than；　by　adaptation　to　particular　climates。　But　whether　or　not　the　adaptation　be　generally　very　close；　we　have　evidence；　in　the　case　of　some　few　plants；　of　their　becoming；　to　a　certain　extent；　naturally　habituated　to　different　temperatures；　or　becoming　acclimatised：　thus　the　pines　and　rhododendrons；　raised　from　seed　collected　by　Dr　Hooker　from　trees　growing　at　different　heights　on　the　Himalaya　were　found　in　this　country　to　possess　different　constitutional　powers　of　resisting　cold。　Mr　Thwaites　informs　me　that　he　has　observed　similar　facts　in　Ceylon；　and　analogous　observations　have　been　made　by　Mr　H。　C。　Watson　on　European　species　of　plants　brought　from　the　Azores　to　England。　In　regard　to　animals；　several　authentic　cases　could　be　given　of　species　within　historical　times　having　largely　extended　their　range　from　warmer　to　cooler　latitudes；　and　conversely；　but　we　do　not　positively　know　that　these　animals　were　strictly　adapted　to　their　native　climate；　but　in　all　ordinary　cases　we　assume　such　to　be　the　case；　nor　do　we　know　that　they　have　subsequently　become　acclimatised　to　their　new　homes。　
As　I　believe　that　our　domestic　animals　were　originally　chosen　by　uncivilised　man　because　they　were　useful　and　bred　readily　under　confinement；　and　not　because　they　were　subsequently　found　capable　of　far…extended　transportation；　I　think　the　common　and　extraordinary　capacity　in　our　domestic　animals　of　not　only　withstanding　the　most　different　climates　but　of　being　perfectly　fertile　（a　far　severer　test）　under　them；　may　be　used　as　an　argument　that　a　large　proportion　of　other　animals；　now　in　a　state　of　nature；　could　easily　be　brought　to　bear　widely　different　climates。　We　must　not；　however；　push　the　foregoing　argument　too　far；　on　account　of　the　probable　origin　of　some　of　our　domestic　animals　from　several　wild　stocks：　the　blood；　for　instance；　of　a　tropical　and　arctic　wolf　or　wild　dog　may　perhaps　be　mingled　in　our　domestic　breeds。　The　rat　and　mouse　cannot　be　considered　as　domestic　animals；　but　they　have　been　transported　by　man　to　many　parts　of　the　world；　and　now　have　a　far　wider　range　than　any　other　rodent；　living　free　under　the　cold　climate　of　Faroe　in　the　north　and　of　the　Falklands　in　the　south；　and　on　many　islands　in　the　torrid　zones。　Hence　I　am　inclined　to　look　at　adaptation　to　any　special　climate　as　a　quality　readily　grafted　on　an　innate　wide　flexibility　of　constitution；　which　is　common　to　most　animals。　On　this　view；　the　capacity　of　enduring　the　most　different　climates　by　man