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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£º¡¡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£º¡¡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¡¡selection¡¡into¡¡branchiae£»¡¡simply¡¡through¡¡an¡¡increase¡¡in¡¡their¡¡size¡¡and¡¡the¡¡obliteration¡¡of¡¡their¡¡adhesive¡¡glands¡£¡¡If¡¡all¡¡pedunculated¡¡cirripedes¡¡had¡¡become¡¡extinct£»¡¡and¡¡they¡¡have¡¡already¡¡suffered¡¡far¡¡more¡¡extinction¡¡than¡¡have¡¡sessile¡¡cirripedes£»¡¡who¡¡would¡¡ever¡¡have¡¡imagined¡¡that¡¡the¡¡branchiae¡¡in¡¡this¡¡latter¡¡family¡¡had¡¡originally¡¡existed¡¡as¡¡organs¡¡for¡¡preventing¡¡the¡¡ova¡¡from¡¡being¡¡washed¡¡out¡¡of¡¡the¡¡sack£¿¡¡
Although¡¡we¡¡must¡¡be¡¡extremely¡¡cautious¡¡in¡¡concluding¡¡that¡¡any¡¡organ¡¡could¡¡not¡¡possibly¡¡have¡¡been¡¡produced¡¡by¡¡successive¡¡transitional¡¡gradations£»¡¡yet£»¡¡undoubtedly£»¡¡grave¡¡cases¡¡of¡¡difficulty¡¡occur£»¡¡some¡¡of¡¡which¡¡will¡¡be¡¡discussed¡¡in¡¡my¡¡future¡¡work¡£¡¡
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The¡¡electric¡¡organs¡¡offer¡¡another¡¡and¡¡even¡¡more¡¡serious¡¡difficulty£»¡¡for¡¡they¡¡occur¡¡in¡¡only¡¡about¡¡a¡¡dozen¡¡fishes£»¡¡of¡¡which¡¡several¡¡are¡¡widely¡¡remote¡¡in¡¡their¡¡affinities¡£¡¡Generally¡¡when¡¡the¡¡same¡¡organ¡¡appears¡¡in¡¡several¡¡members¡¡of¡¡the¡¡same¡¡class£»¡¡especially¡¡if¡¡in¡¡members¡¡having¡¡very¡¡different¡¡habits¡¡of¡¡life£»¡¡we¡¡may¡¡attribute¡¡its¡¡presence¡¡to¡¡inheritance¡¡from¡¡a¡¡common¡¡ancestor£»¡¡and¡¡its¡¡absence¡¡in¡¡some¡¡of¡¡the¡¡members¡¡to¡¡its¡¡loss¡¡through¡¡disuse¡¡or¡¡natural¡¡selection¡£¡¡But¡¡if¡¡the¡¡electric¡¡organs¡¡had¡¡been¡¡inherited¡¡from¡¡one¡¡ancient¡¡progenitor¡¡thus¡¡provided£»¡¡we¡¡might¡¡have¡¡expected¡¡that¡¡all¡¡electric¡¡fishes¡¡would¡¡have¡¡been¡¡specially¡¡related¡¡to¡¡each¡¡other¡£¡¡Nor¡¡does¡¡geology¡¡at¡¡all¡¡lead¡¡to¡¡the¡¡belief¡¡that¡¡formerly¡¡most¡¡fishes¡¡had¡¡electric¡¡organs£»¡¡which¡¡most¡¡of¡¡their¡¡modified¡¡descendants¡¡have¡¡lost¡£¡¡The¡¡presence¡¡of¡¡luminous¡¡organs¡¡in¡¡a¡¡few¡¡insects£»¡¡belonging¡¡to¡¡different¡¡families¡¡and¡¡orders£»¡¡offers¡¡a¡¡parallel¡¡case¡¡of¡¡difficulty¡£¡¡Other¡¡cases¡¡could¡¡be¡¡given£»¡¡for¡¡instance¡¡in¡¡plants£»¡¡the¡¡very¡¡curious¡¡contrivance¡¡of¡¡a¡¡mass¡¡of¡¡pollen¡­grains£»¡¡borne¡¡on¡¡a¡¡foot¡­stalk¡¡with¡¡a¡¡sticky¡¡gland¡¡at¡¡the¡¡end£»¡¡is¡¡the¡¡same¡¡in¡¡Orchis¡¡and¡¡Asclepias£»¡¡genera¡¡almost¡¡as¡¡remote¡¡as¡¡possible¡¡amongst¡¡flowering¡¡plants¡£¡¡In¡¡all¡¡these¡¡cases¡¡of¡¡two¡¡very¡¡distinct¡¡species¡¡furnished¡¡with¡¡apparently¡¡the¡¡same¡¡anomalous¡¡organ£»¡¡it¡¡should¡¡be¡¡observed¡¡that£»¡¡although¡¡the¡¡general¡¡appearance¡¡and¡¡function¡¡of¡¡the¡¡organ¡¡may¡¡be¡¡the¡¡same£»¡¡yet¡¡some¡¡fundamental¡¡difference¡¡can¡¡generally¡¡be¡¡detected¡£¡¡I¡¡am¡¡inclined¡¡to¡¡believe¡¡that¡¡in¡¡nearly¡¡the¡¡same¡¡way¡¡as¡¡two¡¡men¡¡have¡¡sometimes¡¡independently¡¡hit¡¡on¡¡the¡¡very¡¡same¡¡invention£»¡¡so¡¡natural¡¡selection£»¡¡working¡¡for¡¡the¡¡good¡¡of¡¡each¡¡being¡¡and¡¡taking¡¡advantage¡¡of¡¡analogous¡¡variations£»¡¡has¡¡sometimes¡¡modified¡¡in¡¡ver

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