the origin of species(物种起源)-第27节

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　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　himself　and　by　his　domestic　animals；　and　such　facts　as　that　former　species　of　the　elephant　and　rhinoceros　were　capable　of　enduring　a　glacial　climate；　whereas　the　living　species　are　now　all　tropical　or　sub…tropical　in　their　habits；　ought　not　to　be　looked　at　as　anomalies；　but　merely　as　examples　of　a　very　common　flexibility　of　constitution；　brought；　under　peculiar　circumstances；　into　play。　
How　much　of　the　acclimatisation　of　species　to　any　peculiar　climate　is　due　to　mere　habit；　and　how　much　to　the　natural　selection　of　varieties　having　different　innate　constitutions；　and　how　much　to　means　combined；　is　a　very　obscure　question。　That　habit　or　custom　has　some　influence　I　must　believe；　both　from　analogy；　and　from　the　incessant　advice　given　in　agricultural　works；　even　in　the　ancient　Encyclopaedias　of　China；　to　be　very　cautious　in　transposing　animals　from　one　district　to　another；　for　it　is　not　likely　that　man　should　have　succeeded　in　selecting　so　many　breeds　and　sub…breeds　with　constitutions　specially　fitted　for　their　own　districts：　the　result　must；　I　think；　be　due　to　habit。　On　the　other　hand；　I　can　see　no　reason　to　doubt　that　natural　selection　will　continually　tend　to　preserve　those　individuals　which　are　born　with　constitutions　best　adapted　to　their　native　countries。　In　treatises　on　many　kinds　of　cultivated　plants；　certain　varieties　are　said　to　withstand　certain　climates　better　than　others：　this　is　very　strikingly　shown　in　works　on　fruit　trees　published　in　the　United　States；　in　which　certain　varieties　are　habitually　recommended　for　the　northern；　and　others　for　the　southern　States；　and　as　most　of　these　varieties　are　of　recent　origin；　they　cannot　owe　their　constitutional　differences　to　habit。　The　case　of　the　Jerusalem　artichoke；　which　is　never　propagated　by　seed；　and　of　which　consequently　new　varieties　have　not　been　produced；　has　even　been　advanced　for　it　is　now　as　tender　as　ever　it　was　　as　proving　that　acclimatisation　cannot　be　effected！　The　case；　also；　of　the　kidney…bean　has　been　often　cited　for　a　similar　purpose；　and　with　much　greater　weight；　but　until　some　one　will　sow；　during　a　score　of　generations；　his　kidney…beans　so　early　that　a　very　large　proportion　are　destroyed　by　frost；　and　then　collect　seed　from　the　few　survivors；　with　care　to　prevent　accidental　crosses；　and　then　again　get　seed　from　these　seedlings；　with　the　same　precautions；　the　experiment　cannot　be　said　to　have　been　even　tried。　Nor　let　it　be　supposed　that　no　differences　in　the　constitution　of　seedling　kidney…beans　ever　appear；　for　an　account　has　been　published　how　much　more　hardy　some　seedlings　appeared　to　be　than　others。　
On　the　whole；　I　think　we　may　conclude　that　habit；　use；　and　disuse；　have；　in　some　cases；　played　a　considerable　part　in　the　modification　of　the　constitution；　and　of　the　structure　of　various　organs；　but　that　the　effects　of　use　and　disuse　have　often　been　largely　combined　with；　and　sometimes　overmastered　by；　the　natural　selection　of　innate　differences。　
Correlation　of　Growth
I　mean　by　this　expression　that　the　whole　organisation　is　so　tied　togeth