the origin of species(物种起源)-第33部分

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　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　transparent　layers；　and　carefully　selecting　each　alteration　which；　under　varied　circumstances；　may　in　any　way；　or　in　any　degree；　tend　to　produce　a　distincter　image。　We　must　suppose　each　new　state　of　the　instrument　to　be　multiplied　by　the　million；　and　each　to　be　preserved　till　a　better　be　produced；　and　then　the　old　ones　to　be　destroyed。　In　living　bodies；　variation　will　cause　the　slight　alterations；　generation　will　multiply　them　almost　infinitely；　and　natural　selection　will　pick　out　with　unerring　skill　each　improvement。　Let　this　process　go　on　for　millions　on　millions　of　years；　and　during　each　year　on　millions　of　individuals　of　many　kinds；　and　may　we　not　believe　that　a　living　optical　instrument　might　thus　be　formed　as　superior　to　one　of　glass；　as　the　works　of　the　Creator　are　to　those　of　man？　
If　it　could　be　demonstrated　that　any　complex　organ　existed；　which　could　not　possibly　have　been　formed　by　numerous；　successive；　slight　modifications；　my　theory　would　absolutely　break　down。　But　I　can　find　out　no　such　case。　No　doubt　many　organs　exist　of　which　we　do　not　know　the　transitional　grades；　more　especially　if　we　look　to　much…isolated　species；　round　which；　according　to　my　theory；　there　has　been　much　extinction。　Or　again；　if　we　look　to　an　organ　common　to　all　the　members　of　a　large　class；　for　in　this　latter　case　the　organ　must　have　been　first　formed　at　an　extremely　remote　period；　since　which　all　the　many　members　of　the　class　have　been　developed；　and　in　order　to　discover　the　early　transitional　grades　through　which　the　organ　has　passed；　we　should　have　to　look　to　very　ancient　ancestral　forms；　long　since　become　extinct。　
We　should　be　extremely　cautious　in　concluding　that　an　organ　could　not　have　been　formed　by　transitional　gradations　of　some　kind。　Numerous　cases　could　be　given　amongst　the　lower　animals　of　the　same　organ　performing　at　the　same　time　wholly　distinct　functions；　thus　the　alimentary　canal　respires；　digests；　and　excretes　in　the　larva　of　the　dragon…fly　and　in　the　fish　Cobites。　In　the　Hydra；　the　animal　may　be　turned　inside　out；　and　the　exterior　surface　will　then　digest　and　the　stomach　respire。　In　such　cases　natural　selection　might　easily　specialise；　if　any　advantage　were　thus　gained；　a　part　or　organ；　which　had　performed　two　functions；　for　one　function　alone；　and　thus　wholly　change　its　nature　by　insensible　steps。　Two　distinct　organs　sometimes　perform　simultaneously　the　same　function　in　the　same　individual；　to　give　one　instance；　there　are　fish　with　gills　or　branchiae　that　breathe　the　air　dissolved　in　the　water；　at　the　same　time　that　they　breathe　free　air　in　their　swimbladders；　this　latter　organ　having　a　ductus　pneumaticus　for　its　supply；　and　being　divided　by　highly　vascular　partitions。　In　these　cases；　one　of　the　two　organs　might　with　ease　be　modified　and　perfected　so　as　to　perform　all　the　work　by　itself；　being　aided　during　the　process　of　modification　by　the　other　organ；　and　then　this　other　organ　might　be　modified　for　some　other　and　quite　distinct　purpose；　or　be　quite　obliterated。　
The　illustration　of　the　swimbladder　in　fishes　is　a　good　one；　because　it　shows　us　clearly　the　highly　important　fact　that　an　organ　originally　constructed　for　one　purpose；　namely　flotation；　may　be　converted　into　one　for　a　wholly　different　purpose；　namely　respiration。　The　swimbladder　has；　also；　been　worked　in　as　an　accessory　to　the　auditory　organs　of　certain　fish；　or；　for　I　do　not　know　which　view　is　now　generally　held；　a　part　of　the　auditory　apparatus　has　been　worked　in　as　a　complement　to　the　swimbladder。　All　physiologists　admit　that　the　swimbladder　is　homologous；　or　'ideally　similar；'　in　position　and　structure　with　the　lungs　of　the　higher　vertebrate　animals：　hence　there　seems　to　me　to　be　no　great　difficulty　in　believing　that　natural　selection　has　actually　converted　a　swimbladder　into　a　lung；　or　organ　used　exclusively　for　respiration。　
I　can；　indeed；　hardly　doubt　that　all　vertebrate　animals　having　true　lungs　have　descended　by　ordinary　generation　from　an　ancient　prototype；　of　which　we　know　nothing；　furnished　with　a　floating　apparatus　or　swimbladder。　We　can　thus；　as　I　infer　from　Professor　Owen's　interesting　description　of　these　parts；　understand　the　strange　fact　that　every　particle　of　food　and　drink　which　we　swallow　has　to　pass　over　the　orifice　of　the　trachea；　with　some　risk　of　falling　into　the　lungs；　notwithstanding　the　beautiful　contrivance　by　which　the　glottis　is　closed。　In　the　higher　Vertebrata　the　branchiae　have　wholly　disappeared　the　slits　on　the　sides　of　the　neck　and　the　loop…like　course　of　the　arteries　still　marking　in　the　embryo　their　former　position。　But　it　is　conceivable　that　the　now　utterly　lost　branchiae　might　have　been　gradually　worked　in　by　natural　selection　for　some　quite　distinct　purpose：　in　the　same　manner　as；　on　the　view　entertained　by　some　naturalists　that　the　branchiae　and　dorsal　scales　of　Annelids　are　homologous　with　the　wings　and　wing…covers　of　insects；　it　is　probable　that　organs　which　at　a　very　ancient　period　served　for　respiration　have　been　actually　converted　into　organs　of　flight。　
In　considering　transitions　of　organs；　it　is　so　important　to　bear　in　mind　the　probability　of　conversion　from　one　function　to　another；　that　I　will　give　one　more　instance。　Pedunculated　cirripedes　have　two　minute　folds　of　skin；　called　by　me　the　ovigerous　frena；　which　serve；　through　the　means　of　a　sticky　secretion；　to　retain　the　eggs　until　they　are　hatched　within　the　sack。　These　cirripedes　have　no　branchiae；　the　whole　surface　of　the　body　and　sack；　including　the　small　frena；　serving　for　respiration。　The　Balanidae　or　sessile　cirripedes；　on　the　other　hand；　have　no　ovigerous　frena；　the　eggs　lying　loose　at　the　bottom　of　the　sack；　in　the　well…enclosed　shell；　but　they　have　large　folded　branchiae。　Now　I　think　no　one　will　dispute　that　the　ovigerous　frena　in　the　one　family　are　strictly　homologous　with　the　branchiae　of　the　other　family；　indeed；　they　graduate　into　each　other。　Therefore　I　do　not　doubt　that　little　folds　of　skin；　which　originally　served　as　ovigerous　frena；　but　which；　likewise；　very　slightly　aided　the　act　of　respiration；　have　been　gradually　converted　by　natural　sel