the origin of species(物种起源)-第13章

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left　a　fluctuating　element；　as　perhaps　we　see　in　the　species　called　polymorphic。　
We　shall　best　understand　the　probable　course　of　natural　selection　by　taking　the　case　of　a　country　undergoing　some　physical　change；　for　instance；　of　climate。　The　proportional　numbers　of　its　inhabitants　would　almost　immediately　undergo　a　change；　and　some　species　might　become　extinct。　We　may　conclude；　from　what　we　have　seen　of　the　intimate　and　complex　manner　in　which　the　inhabitants　of　each　country　are　bound　together；　that　any　change　in　the　numerical　proportions　of　some　of　the　inhabitants；　independently　of　the　change　of　climate　itself；　would　most　seriously　affect　many　of　the　others。　If　the　country　were　open　on　its　borders；　new　forms　would　certainly　immigrate；　and　this　also　would　seriously　disturb　the　relations　of　some　of　the　former　inhabitants。　Let　it　be　remembered　how　powerful　the　influence　of　a　single　introduced　tree　or　mammal　has　been　shown　to　be。　But　in　the　case　of　an　island；　or　of　a　country　partly　surrounded　by　barriers；　into　which　new　and　better　adapted　forms　could　not　freely　enter；　we　should　then　have　places　in　the　economy　of　nature　which　would　assuredly　be　better　filled　up；　if　some　of　the　original　inhabitants　were　in　some　manner　modified；　for；　had　the　area　been　open　to　immigration；　these　same　places　would　have　been　seized　on　by　intruders。　In　such　case；　every　slight　modification；　which　in　the　course　of　ages　chanced　to　arise；　and　which　in　any　way　favoured　the　individuals　of　any　of　the　species；　by　better　adapting　them　to　their　altered　conditions；　would　tend　to　be　preserved；　and　natural　selection　would　thus　have　free　scope　for　the　work　of　improvement。　
We　have　reason　to　believe；　as　stated　in　the　first　chapter；　that　a　change　in　the　conditions　of　life；　by　specially　acting　on　the　reproductive　system；　causes　or　increases　variability；　and　in　the　foregoing　case　the　conditions　of　life　are　supposed　to　have　undergone　a　change；　and　this　would　manifestly　be　favourable　to　natural　selection；　by　giving　a　better　chance　of　profitable　variations　occurring；　and　unless　profitable　variations　do　occur；　natural　selection　can　do　nothing。　Not　that；　as　I　believe；　any　extreme　amount　of　variability　is　necessary；　as　man　can　certainly　produce　great　results　by　adding　up　in　any　given　direction　mere　individual　differences；　so　could　Nature；　but　far　more　easily；　from　having　incomparably　longer　time　at　her　disposal。　Nor　do　I　believe　that　any　great　physical　change；　as　of　climate；　or　any　unusual　degree　of　isolation　to　check　immigration；　is　actually　necessary　to　produce　new　and　unoccupied　places　for　natural　selection　to　fill　up　by　modifying　and　improving　some　of　the　varying　inhabitants。　For　as　all　the　inhabitants　of　each　country　are　struggling　together　with　nicely　balanced　forces；　extremely　slight　modifications　in　the　structure　or　habits　of　one　inhabitant　would　often　give　it　an　advantage　over　others；　and　still　further　modifications　of　the　same　kind　would　often　still　further　increase　the　advantage。　No　country　can　be　named　in　which　all　the　native　inhabitants　are　now　so　perfectly　adapted　to　each　other　and　to　the　physical　conditions　under　which　they　live；　that　none　of　them　could　anyhow　be　improved；　for　in　all　countries；　the　natives　have　been　so　far　conquered　by　naturalised　productions；　that　they　have　allowed　foreigners　to　take　firm　possession　of　the　land。　And　as　foreigners　have　thus　everywhere　beaten　some　of　the　natives；　we　may　safely　conclude　that　the　natives　might　have　been　modified　with　advantage；　so　as　to　have　better　resisted　such　intruders。　
As　man　can　produce　and　certainly　has　produced　a　great　result　by　his　methodical　and　unconscious　means　of　selection；　what　may　not　nature　effect？　Man　can　act　only　on　external　and　visible　characters：　nature　cares　nothing　for　appearances；　except　in　so　far　as　they　may　be　useful　to　any　being。　She　can　act　on　every　internal　organ；　on　every　shade　of　constitutional　difference；　on　the　whole　machinery　of　life。　Man　selects　only　for　his　own　good；　Nature　only　for　that　of　the　being　which　she　tends。　Every　selected　character　is　fully　exercised　by　her；　and　the　being　is　placed　under　well…suited　conditions　of　life。　Man　keeps　the　natives　of　many　climates　in　the　same　country；　he　seldom　exercises　each　selected　character　in　some　peculiar　and　fitting　manner；　he　feeds　a　long　and　a　short　beaked　pigeon　on　the　same　food；　he　does　not　exercise　a　long…backed　or　long…legged　quadruped　in　any　peculiar　manner；　he　exposes　sheep　with　long　and　short　wool　to　the　same　climate。　He　does　not　allow　the　most　vigorous　males　to　struggle　for　the　females。　He　does　not　rigidly　destroy　all　inferior　animals；　but　protects　during　each　varying　season；　as　far　as　lies　in　his　power；　all　his　productions。　He　often　begins　his　selection　by　some　half…monstrous　form；　or　at　least　by　some　modification　prominent　enough　to　catch　his　eye；　or　to　be　plainly　useful　to　him。　Under　nature；　the　slightest　difference　of　structure　or　constitution　may　well　turn　the　nicely…balanced　scale　in　the　struggle　for　life；　and　so　be　preserved。　How　fleeting　are　the　wishes　and　efforts　of　man！　how　short　his　time！　and　consequently　how　poor　will　his　products　be；　compared　with　those　accumulated　by　nature　during　whole　geological　periods。　Can　we　wonder；　then；　that　nature's　productions　should　be　far　'truer'　in　character　than　man's　productions；　that　they　should　be　infinitely　better　adapted　to　the　most　complex　conditions　of　life；　and　should　plainly　bear　the　stamp　of　far　higher　workmanship？　
It　may　be　said　that　natural　selection　is　daily　and　hourly　scrutinising；　throughout　the　world；　every　variation；　even　the　slightest；　rejecting　that　which　is　bad；　preserving　and　adding　up　all　that　is　good；　silently　and　insensibly　working；　whenever　and　wherever　opportunity　offers；　at　the　improvement　of　each　organic　being　in　relation　to　its　organic　and　inorganic　conditions　of　life。　We　see　nothing　of　these　slow　changes　in　progress；　until　the　hand　of　time　has　marked　the　long　lapses　of　ages；　and　then　so　imperfect　is　our　view　into　long　past　geological　ages；　that　we　only　see　that　the　forms　of　life　are　now　different　from　what　they　formerly　were。　
Although　natural　selection　can　act　only　through　and　for　the　good　of　each　being；　yet　characters　and　structures；　which　we　are　apt　to　consider　as　of　very　trifling　importance；　may　thus　be　acted　on。　When　we　see　leaf…eating　insects　green；　and　bark…feeders　mottled…grey；　the　alpine　ptarmigan　white　in　winter；　the　red…grouse　the　colour　of　heather；　and　the　black…grouse　that　of　peaty　earth；　we　must　believe　that　these　tints　are　of　service　to　these　birds　and　insects　in　preserving　them　from　danger。　Grouse；　if　not　destroyed　at　some　period　of　their　lives；　would　increase　in　countless　numbers；　they　are　known　to　suffer　largely　from　birds　of　prey；　and　hawks　are　guided　by　eyesight　to　their　prey；　so　much　so；　that　on　parts　of　the　Continent　persons　are　warned　not　to　keep　white　pigeons；　as　being　the　most　liable　to　destruction。　Hence　I　can　see　no　reason　to　doubt　that　natural　selection　might　be　most　effective　in　giving　the　proper　colour　to　each　kind　of　grouse；　and　in　keeping　that　colour；　when　once　acquired；　true　and　constant。　Nor　ought　we　to　think　that　the　occasional　destruction　of　an　animal　of　any　particular　colour　would　produce　little　effect：　we　should　remember　how　essential　it　is　in　a　flock　of　white　sheep　to　destroy　every　lamb　with　the　faintest　trace　of　black。　In　plants　the　down　on　the　fruit　and　the　colour　of　the　flesh　are　considered　by　botanists　as　characters　of　the　most　trifling　importance：　yet　we　hear　from　an　excellent　horticulturist；　Downing；　that　in　the　United　States　smooth…skinned　fruits　suffer　far　more　from　a　beetle；　a　curculio；　than　those　with　down；　that　purple　plums　suffer　far　more　from　a　certain　disease　than　yellow　plums；　whereas　another　disease　attacks　yellow…fleshed　peaches　far　more　than　those　with　other　coloured　flesh。　If；　with　all　the　aids　of　art；　these　slight　differences　make　a　great　difference　in　cultivating　the　several　varieties；　assuredly；　in　a　state　of　nature；　where　the　trees　would　have　to　struggle　with　other　trees　and　with　a　host　of　enemies；　such　differences　would　effectually　settle　which　variety；　whether　a　smooth　or　downy；　a　yellow　or　purple　fleshed　fruit；　should　succeed。　
In　looking　at　many　small　points　of　difference　between　species；　which；　as　far　as　our　ignorance　permits　us　to　judge；　seem　to　be　quite　unimportant；　we　must　not　forget　that　climate；　food；　&c。；　probably　produce　some　slight　and　direct　effect。　It　is；　however；　far　more　necessary　to　bear　in　mind　that　there　are　many　unknown　laws　of　correlation　of　growth；　which；　when　one　part　of　the　organisation　is　modified　through　variation；　and　the　modifications　are　accumulated　by　natural　selection　for　the　good　of　the　being；　will　cause　other　modifications；　often　of　the　most　unexpected　nature。　
As　we　see　that　those　variations　which　under　domestication　appear　at　any　particular　period　of　life；　tend　to　reappear　in　the　offspring　at　the　same　period；　for　instance；　in　the　seeds　of　the　many　varieties　of　our　culinary　and　agricultural　plants；　in　the　caterpillar　and　cocoon　stages　of　the　varieties　of　the　silkworm；　in　the　eggs　of　poultry；　and　in　the　colour　of　the　down　of　their　chickens；　in　the　horns　of　our　sheep　and　cattle　when　nearly　adult；　so　in　a　state　of　nature；　natural　selection　will　be　enabled　to　act　on　and　modify　organic　beings　at　any　age；　by　the　accumulation　of　profitable　variations　at　that　age；　and　by　their　inheritance　at　a　corresponding　age。　If　it　profit　a　plant　to　have　its　seeds　more　and　more　widely　disseminated　by　the　wind；　I　can　see　no　greater　difficulty　in　this　being　effected　through　natural　selection；　than　in　the　cotton…planter　increasing　and　improving　by　selection　the　down　in　the　pods　on　his　cotton…trees。　Natural　selection　may　modify　and　adapt　the　larva　of　an　insect　to　a　score　of　contingencies；　wholly　different　from　those　which　concern　the　mature　insect。　These　modifications　will　no　doubt　affect；　through　the　laws　of　correlation；　the　structure　of　the　adult；　and　probably　in　the　case　of　those　insects　which　live　only　for　a　few　hours；　and　which　never　feed；　a　large　part　of　their　structure　is　merely　the　correlated　result　of　successive　changes　in　the　structure　of　their　larvae。　So；　conversely；　modifications　in　the　adult　will　probably　often　affect　the　structure　of　the　larva；　but　in　all　cases　natural　selection　will　ensure　that　modifications　consequent　on　other　modifications　at　a　different　period　of　life；　shall　not　be　in　the　least　degree　injurious：　for　if　they　became　so；　they　would　cause　the　extinction　of　the　species。　
Natural　selection　will　modify　the　structure　of　the　young　in　relation　to　the　parent；　and　of　the　parent　in　relation　to　the　young。　In　social　animals　it　will　adapt　the　structure　of　each　individual　for　the　benefit　of　the　community；　if　each　in　consequence　profits　by　the　selected　change。　What　natural　selection　cannot　do；　is　to　modify　the　structure　of　one　species；　without　giving　it　any　advantage；　for　the　good　of　another　species；　and　though　statements　to　this　effect　may　be　found　in　works　of　natural　history；　I　cannot　find　one　case　which　will　bear　investigation。　A　structure　used　only　once　in　an　animal's　whole　life；　if　of　high　importance　to　it；　might　be　modified　to　any　extent　by　natural　selection；　for　instance；　the　great　jaws　possessed　by　certain　insects；　and　used　exclusively　for　opening　the　cocoon　or　the　hard