the origin of species(物种起源)-第36节

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ale；　insects　in　the　water。　Even　in　so　extreme　a　case　as　this；　if　the　supply　of　insects　were　constant；　and　if　better　adapted　competitors　did　not　already　exist　in　the　country；　I　can　see　no　difficulty　in　a　race　of　bears　being　rendered；　by　natural　selection；　more　and　more　aquatic　in　their　structure　and　habits；　with　larger　and　larger　mouths；　till　a　creature　was　produced　as　monstrous　as　a　whale。　
As　we　sometimes　see　individuals　of　a　species　following　habits　widely　different　from　those　both　of　their　own　species　and　of　the　other　species　of　the　same　genus；　we　might　expect；　on　my　theory；　that　such　individuals　would　occasionally　have　given　rise　to　new　species；　having　anomalous　habits；　and　with　their　structure　either　slightly　or　considerably　modified　from　that　of　their　proper　type。　And　such　instances　do　occur　in　nature。　Can　a　more　striking　instance　of　adaptation　be　given　than　that　of　a　woodpecker　for　climbing　trees　and　for　seizing　insects　in　the　chinks　of　the　bark？　Yet　in　North　America　there　are　woodpeckers　which　feed　largely　on　fruit；　and　others　with　elongated　wings　which　chase　insects　on　the　wing；　and　on　the　plains　of　La　Plata；　where　not　a　tree　grows；　there　is　a　woodpecker；　which　in　every　essential　part　of　its　organisation；　even　in　its　colouring；　in　the　harsh　tone　of　its　voice；　and　undulatory　flight；　told　me　plainly　of　its　close　blood…relationship　to　our　common　species；　yet　it　is　a　woodpecker　which　never　climbs　a　tree！　
Petrels　are　the　most　a？rial　and　oceanic　of　birds；　yet　in　the　quiet　Sounds　of　Tierra　del　Fuego；　the　Puffinuria　berardi；　in　its　general　habits；　in　its　astonishing　power　of　diving；　its　manner　of　swimming；　and　of　flying　when　unwillingly　it　takes　flight；　would　be　mistaken　by　any　one　for　an　auk　or　grebe；　nevertheless；　it　is　essentially　a　petrel；　but　with　many　parts　of　its　organisation　profoundly　modified。　On　the　other　hand；　the　acutest　observer　by　examining　the　dead　body　of　the　water…ouzel　would　never　have　suspected　its　sub…aquatic　habits；　yet　this　anomalous　member　of　the　strictly　terrestrial　thrush　family　wholly　subsists　by　diving；　grasping　the　stones　with　its　feet　and　using　its　wings　under　water。　
He　who　believes　that　each　being　has　been　created　as　we　now　see　it；　must　occasionally　have　felt　surprise　when　he　has　met　with　an　animal　having　habits　and　structure　not　at　all　in　agreement。　What　can　be　plainer　than　that　the　webbed　feet　of　ducks　and　geese　are　formed　for　swimming；　yet　there　are　upland　geese　with　webbed　feet　which　rarely　or　never　go　near　the　water；　and　no　one　except　Audubon　has　seen　the　frigate…bird；　which　has　all　its　four　toes　webbed；　alight　on　the　surface　of　the　sea。　On　the　other　hand；　grebes　and　coots　are　eminently　aquatic；　although　their　toes　are　only　bordered　by　membrane。　What　seems　plainer　than　that　the　long　toes　of　grallatores　are　formed　for　walking　over　swamps　and　floating　plants；　yet　the　water…hen　is　nearly　as　aquatic　as　the　coot；　and　the　landrail　nearly　as　terrestrial　as　the　quail　or　partridge。　In　such　cases；　and　many　others　could　be　given；　habits　have　changed　without　a　corresponding　change　of　structure。　The　webbed　feet　of　the　upland　goose　may　be　said　to　have　become　rudimentary　in　function；　though　not　in　structure。　In　the　frigate…bird；　the　deeply…scooped　membrane　between　the　toes　shows　that　structure　has　begun　to　change。　
He　who　believes　in　separate　and　innumerable　acts　of　creation　will　say；　that　in　these　cases　it　has　pleased　the　Creator　to　cause　a　being　of　one　type　to　take　the　place　of　one　of　another　type；　but　this　seems　to　me　only　restating　the　fact　in　dignified　language。　He　who　believes　in　the　struggle　for　existence　and　in　the　principle　of　natural　selection；　will　acknowledge　that　every　organic　being　is　constantly　endeavouring　to　increase　in　numbers；　and　that　if　any　one　being　vary　ever　so　little；　either　in　habits　or　structure；　and　thus　gain　an　advantage　over　some　other　inhabitant　of　the　country；　it　will　seize　on　the　place　of　that　inhabitant；　however　different　it　may　be　from　its　own　place。　Hence　it　will　cause　him　no　surprise　that　there　should　be　geese　and　frigate…birds　with　webbed　feet；　either　living　on　the　dry　land　or　most　rarely　alighting　on　the　water；　that　there　should　be　long…toed　corncrakes　living　in　meadows　instead　of　in　swamps；　that　there　should　be　woodpeckers　where　not　a　tree　grows；　that　there　should　be　diving　thrushes；　and　petrels　with　the　habits　of　auks。　
Organs　of　extreme　perfection　and　complication。　To　suppose　that　the　eye；　with　all　its　inimitable　contrivances　for　adjusting　the　focus　to　different　distances；　for　admitting　different　amounts　of　light；　and　for　the　correction　of　spherical　and　chromatic　aberration；　could　have　been　formed　by　natural　selection；　seems；　I　freely　confess；　absurd　in　the　highest　possible　degree。　Yet　reason　tells　me；　that　if　numerous　gradations　from　a　perfect　and　complex　eye　to　one　very　imperfect　and　simple；　each　grade　being　useful　to　its　possessor；　can　be　shown　to　exist；　if　further；　the　eye　does　vary　ever　so　slightly；　and　the　variations　be　inherited；　which　is　certainly　the　case；　and　if　any　variation　or　modification　in　the　organ　be　ever　useful　to　an　animal　under　changing　conditions　of　life；　then　the　difficulty　of　believing　that　a　perfect　and　complex　eye　could　be　formed　by　natural　selection；　though　insuperable　by　our　imagination；　can　hardly　be　considered　real。　How　a　nerve　comes　to　be　sensitive　to　light；　hardly　concerns　us　more　than　how　life　itself　first　originated；　but　I　may　remark　that　several　facts　make　me　suspect　that　any　sensitive　nerve　may　be　rendered　sensitive　to　light；　and　likewise　to　those　coarser　vibrations　of　the　air　which　produce　sound。　
In　looking　for　the　gradations　by　which　an　organ　in　any　species　has　been　perfected；　we　ought　to　look　exclusively　to　its　lineal　ancestors；　but　this　is　scarcely　ever　possible；　and　we　are　forced　in　each　case　to　look　to　species　of　the　same　group；　that　is　to　the　collateral　descendants　from　the　same　original　parent…form；　in　order　to　see　what　gradations　are　possible；　and　for　the　chance　of　some　gradations　having　been　transmitted　from　the　earlier　stages　of　descent；　in　an　unaltered　or　little　altered　condition。　Amongst　existing　Vertebrata；　we　find　but　a　small　amount　of　gradation　in　the　structure　of　the　eye；　and　from　fossil　species　we　can　learn　nothing　on　this　head。　In　this　great　class　we　should　probably　have　to　descend　far　beneath　the　lowest　known　fossiliferous　stratum　to　discover　the　earlier　stages；　by　which　the　eye　has　been　perfected。　
In　the　Articulata　we　can　commence　a　series　with　an　optic　nerve　merely　coated　with　pigment；　and　without　any　other　mechanism；　and　from　this　low　stage；　numerous　gradations　of　structure；　branching　off　in　two　fundamentally　different　lines；　can　be　shown　to　exist；　until　we　reach　a　moderately　high　stage　of　perfection。　In　certain　crustaceans；　for　instance；　there　is　a　double　cornea；　the　inner　one　divided　into　facets；　within　each　of　which　there　is　a　lens　shaped　swelling。　In　other　crustaceans　the　transparent　cones　which　are　coated　by　pigment；　and　which　properly　act　only　by　excluding　lateral　pencils　of　light；　are　convex　at　their　upper　ends　and　must　act　by　convergence；　and　at　their　lower　ends　there　seems　to　be　an　imperfect　vitreous　substance。　With　these　facts；　here　far　too　briefly　and　imperfectly　given；　which　show　that　there　is　much　graduated　diversity　in　the　eyes　of　living　crustaceans；　and　bearing　in　mind　how　small　the　number　of　living　animals　is　in　proportion　to　those　which　have　become　extinct；　I　can　see　no　very　great　difficulty　（not　more　than　in　the　case　of　many　other　structures）　in　believing　that　natural　selection　has　converted　the　simple　apparatus　of　an　optic　nerve　merely　coated　with　pigment　and　invested　by　transparent　membrane；　into　an　optical　instrument　as　perfect　as　is　possessed　by　any　member　of　the　great　Articulate　class。　
He　who　will　go　thus　far；　if　he　find　on　finishing　this　treatise　that　large　bodies　of　facts；　otherwise　inexplicable；　can　be　explained　by　the　theory　of　descent；　ought　not　to　hesitate　to　go　further；　and　to　admit　that　a　structure　even　as　perfect　as　the　eye　of　an　eagle　might　be　formed　by　natural　selection；　although　in　this　case　he　does　not　know　any　of　the　transitional　grades。　His　reason　ought　to　conquer　his　imagination；　though　I　have　felt　the　difficulty　far　too　keenly　to　be　surprised　at　any　degree　of　hesitation　in　extending　the　principle　of　natural　selection　to　such　startling　lengths。　
It　is　scarcely　possible　to　avoid　comparing　the　eye　to　a　telescope。　We　know　that　this　instrument　has　been　perfected　by　the　long…continued　efforts　of　the　highest　human　intellects；　and　we　naturally　infer　that　the　eye　has　been　formed　by　a　somewhat　analogous　process。　But　may　not　this　inference　be　presumptuous？　Have　we　any　right　to　assume　that　the　Creator　works　by　intellectual　powers　like　those　of　man？　If　we　must　compare　the　eye　to　an　optical　instrument；　we　ought　in　imagination　to　take　a　thick　layer　of　transparent　tissue；　with　a　nerve　sensitive　to　light　beneath；　and　then　suppose　every　part　of　this　layer　to　be　continually　changing　slowly　in　density；　so　as　to　separate　into　layers　of　different　densities　and　thicknesses；　placed　at　different　distances　from　each　other；　and　with　the　surfaces　of　each　layer　slowly　changing　in　form。　Further　we　must　suppose　that　there　is　a　power　always　intently　watching　each　slight　accidental　alteration　in　the