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ave¡¡its¡¡nutriment¡¡wasted¡¡in¡¡building¡¡up¡¡an¡¡useless¡¡structure¡£¡¡I¡¡can¡¡thus¡¡only¡¡understand¡¡a¡¡fact¡¡with¡¡which¡¡I¡¡was¡¡much¡¡struck¡¡when¡¡examining¡¡cirripedes£»¡¡and¡¡of¡¡which¡¡many¡¡other¡¡instances¡¡could¡¡be¡¡given£º¡¡namely£»¡¡that¡¡when¡¡a¡¡cirripede¡¡is¡¡parasitic¡¡within¡¡another¡¡and¡¡is¡¡thus¡¡protected£»¡¡it¡¡loses¡¡more¡¡or¡¡less¡¡completely¡¡its¡¡own¡¡shell¡¡or¡¡carapace¡£¡¡This¡¡is¡¡the¡¡case¡¡with¡¡the¡¡male¡¡Ibla£»¡¡and¡¡in¡¡a¡¡truly¡¡extraordinary¡¡manner¡¡with¡¡the¡¡Proteolepas£º¡¡for¡¡the¡¡carapace¡¡in¡¡all¡¡other¡¡cirripedes¡¡consists¡¡of¡¡the¡¡three¡¡highly¡­important¡¡anterior¡¡segments¡¡of¡¡the¡¡head¡¡enormously¡¡developed£»¡¡and¡¡furnished¡¡with¡¡great¡¡nerves¡¡and¡¡muscles£»¡¡but¡¡in¡¡the¡¡parasitic¡¡and¡¡protected¡¡Proteolepas£»¡¡the¡¡whole¡¡anterior¡¡part¡¡of¡¡the¡¡head¡¡is¡¡reduced¡¡to¡¡the¡¡merest¡¡rudiment¡¡attached¡¡to¡¡the¡¡bases¡¡of¡¡the¡¡prehensile¡¡antennae¡£¡¡Now¡¡the¡¡saving¡¡of¡¡a¡¡large¡¡and¡¡complex¡¡structure£»¡¡when¡¡rendered¡¡superfluous¡¡by¡¡the¡¡parasitic¡¡habits¡¡of¡¡the¡¡Proteolepas£»¡¡though¡¡effected¡¡by¡¡slow¡¡steps£»¡¡would¡¡be¡¡a¡¡decided¡¡advantage¡¡to¡¡each¡¡successive¡¡individual¡¡of¡¡the¡¡species£»¡¡for¡¡in¡¡the¡¡struggle¡¡for¡¡life¡¡to¡¡which¡¡every¡¡animal¡¡is¡¡exposed£»¡¡each¡¡individual¡¡Proteolepas¡¡would¡¡have¡¡a¡¡better¡¡chance¡¡of¡¡supporting¡¡itself£»¡¡by¡¡less¡¡nutriment¡¡being¡¡wasted¡¡in¡¡developing¡¡a¡¡structure¡¡now¡¡become¡¡useless¡£¡¡
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It¡¡seems¡¡to¡¡be¡¡a¡¡rule£»¡¡as¡¡remarked¡¡by¡¡Is¡£¡¡Geoffroy¡¡St¡¡Hilaire£»¡¡both¡¡in¡¡varieties¡¡and¡¡in¡¡species£»¡¡that¡¡when¡¡any¡¡part¡¡or¡¡organ¡¡is¡¡repeated¡¡many¡¡times¡¡in¡¡the¡¡structure¡¡of¡¡the¡¡same¡¡individual¡¡£¨as¡¡the¡¡vertebrae¡¡in¡¡snakes£»¡¡and¡¡the¡¡stamens¡¡in¡¡polyandrous¡¡flowers£©¡¡the¡¡number¡¡is¡¡variable£»¡¡whereas¡¡the¡¡number¡¡of¡¡the¡¡same¡¡part¡¡or¡¡organ£»¡¡when¡¡it¡¡occurs¡¡in¡¡lesser¡¡numbers£»¡¡is¡¡constant¡£¡¡The¡¡same¡¡author¡¡and¡¡some¡¡botanists¡¡have¡¡further¡¡remarked¡¡that¡¡multiple¡¡parts¡¡are¡¡also¡¡very¡¡liable¡¡to¡¡variation¡¡in¡¡structure¡£¡¡Inasmuch¡¡as¡¡this¡¡'vegetative¡¡repetition£»'¡¡to¡¡use¡¡Prof¡£¡¡Owen's¡¡expression£»¡¡seems¡¡to¡¡be¡¡a¡¡sign¡¡of¡¡low¡¡organisation£»¡¡the¡¡foregoing¡¡remark¡¡seems¡¡connected¡¡with¡¡the¡¡very¡¡general¡¡opinion¡¡of¡¡naturalists£»¡¡that¡¡beings¡¡low¡¡in¡¡the¡¡scale¡¡of¡¡nature¡¡are¡¡more¡¡variable¡¡than¡¡those¡¡which¡¡are¡¡higher¡£¡¡I¡¡presume¡¡that¡¡lowness¡¡in¡¡this¡¡case¡¡means¡¡that¡¡the¡¡several¡¡parts¡¡of¡¡the¡¡organisation¡¡have¡¡been¡¡but¡¡little¡¡specialised¡¡for¡¡particular¡¡functions£»¡¡and¡¡as¡¡long¡¡as¡¡the¡¡same¡¡part¡¡has¡¡to¡¡perform¡¡diversified¡¡work£»¡¡we¡¡can¡¡perhaps¡¡see¡¡why¡¡it¡¡should¡¡remain¡¡variable£»¡¡that¡¡is£»¡¡why¡¡natural¡¡selection¡¡should¡¡have¡¡preserved¡¡or¡¡rejected¡¡each¡¡little¡¡deviation¡¡of¡¡form¡¡less¡¡carefully¡¡than¡¡when¡¡the¡¡part¡¡has¡¡to¡¡serve¡¡for¡¡one¡¡special¡¡purpose¡¡alone¡£¡¡In¡¡the¡¡same¡¡way¡¡that¡¡a¡¡knife¡¡which¡¡has¡¡to¡¡cut¡¡all¡¡sorts¡¡of¡¡things¡¡may¡¡be¡¡of¡¡almost¡¡any¡¡shape£»¡¡whilst¡¡a¡¡tool¡¡for¡¡some¡¡particular¡¡object¡¡had¡¡better¡¡be¡¡of¡¡some¡¡particular¡¡shape¡£¡¡Natural¡¡selection£»¡¡it¡¡should¡¡never¡¡be¡¡forgotten£»¡¡can¡¡act¡¡on¡¡each¡¡part¡¡of¡¡each¡¡being£»¡¡solely¡¡through¡¡and¡¡for¡¡its¡¡advantage¡£¡¡
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Several¡¡years¡¡ago¡¡I¡¡was¡¡much¡¡struck¡¡with¡¡a¡¡remark£»¡¡nearly¡¡to¡¡the¡¡above¡¡effect£»¡¡published¡¡by¡¡Mr¡¡Waterhouse¡£¡¡I¡¡infer¡¡also¡¡from¡¡an¡¡observation¡¡made¡¡by¡¡Professor¡¡Owen£»¡¡with¡¡respect¡¡to¡¡the¡¡length¡¡of¡¡the¡¡arms¡¡of¡¡the¡¡ourang¡­outang£»¡¡that¡¡he¡¡has¡¡come¡¡to¡¡a¡¡nearly¡¡similar¡¡conclusion¡£¡¡It¡¡is¡¡hopeless¡¡to¡¡attempt¡¡to¡¡convince¡¡any¡¡one¡¡of¡¡the¡¡truth¡¡of¡¡this¡¡proposition¡¡without¡¡giving¡¡the¡¡long¡¡array¡¡of¡¡facts¡¡which¡¡I¡¡have¡¡collected£»¡¡and¡¡which¡¡cannot¡¡possibly¡¡be¡¡here¡¡introduced¡£¡¡I¡¡can¡¡only¡¡state¡¡my¡¡conviction¡¡that¡¡it¡¡is¡¡a¡¡rule¡¡of¡¡high¡¡generality¡£¡¡I¡¡am¡¡aware¡¡of¡¡several¡¡causes¡¡of¡¡error£»¡¡but¡¡I¡¡hope¡¡that¡¡I¡¡have¡¡made¡¡due¡¡allowance¡¡for¡¡them¡£¡¡It¡¡should¡¡be¡¡understood¡¡that¡¡the¡¡rule¡¡by¡¡no¡¡means¡¡applies¡¡to¡¡any¡¡part£»¡¡however¡¡unusually¡¡developed£»¡¡unless¡¡it¡¡be¡¡unusually¡¡developed¡¡in¡¡comparison¡¡with¡¡the¡¡same¡¡part¡¡in¡¡closely¡¡allied¡¡species¡£¡¡Thus£»¡¡the¡¡bat's¡¡wing¡¡is¡¡a¡¡most¡¡abnormal¡¡structure¡¡in¡¡the¡¡class¡¡mammalia£»¡¡but¡¡the¡¡rule¡¡would¡¡not¡¡here¡¡apply£»¡¡because¡¡there¡¡is¡¡a¡¡whole¡¡group¡¡of¡¡bats¡¡having¡¡wings£»¡¡it¡¡would¡¡apply¡¡only¡¡if¡¡some¡¡one¡¡species¡¡of¡¡bat¡¡had¡¡its¡¡wings¡¡developed¡¡in¡¡some¡¡remarkable¡¡manner¡¡in¡¡comparison¡¡with¡¡the¡¡other¡¡species¡¡of¡¡the¡¡same¡¡genus¡£¡¡The¡¡rule¡¡applies¡¡very¡¡strongly¡¡in¡¡the¡¡case¡¡of¡¡secondary¡¡sexual¡¡characters£»¡¡when¡¡displayed¡¡in¡¡any¡¡unusual¡¡manner¡£¡¡The¡¡term£»¡¡secondary¡¡sexual¡¡characters£»¡¡used¡¡by¡¡Hunter£»¡¡applies¡¡to¡¡characters¡¡which¡¡are¡¡attached¡¡to¡¡one¡¡sex£»¡¡but¡¡are¡¡not¡¡directly¡¡connected¡¡with¡¡the¡¡act¡¡of¡¡reproduction¡£¡¡The¡¡rule¡¡applies¡¡to¡¡males¡¡and¡¡females£»¡¡but¡¡as¡¡females¡¡more¡¡rarely¡¡offer¡¡remarkable¡¡secondary¡¡sexual¡¡characters£»¡¡it¡¡applies¡¡more¡¡rarely¡¡to¡¡them¡£¡¡The¡¡rule¡¡being¡¡so¡¡plainly¡¡applicable¡¡in¡¡the¡¡case¡¡of¡¡secondary¡¡sexual¡¡characters£»¡¡may¡¡be¡¡due¡¡to¡¡the¡¡great¡¡variability¡¡of¡¡these¡¡characters£»¡¡whether¡¡or¡¡not¡¡displayed¡¡in¡¡any¡¡unusual¡¡manner¡¡of¡¡which¡¡fact¡¡I¡¡think¡¡there¡¡can¡¡be¡¡little¡¡doubt¡£¡¡But¡¡that¡¡our¡¡rule¡¡is¡¡not¡¡confined¡¡to¡¡secondary¡¡sexual¡¡characters¡¡is¡¡clearly¡¡shown¡¡in¡¡the¡¡case¡¡of¡¡hermaphrodite¡¡cirripedes£»¡¡and¡¡I¡¡may¡¡here¡¡add£»¡¡that¡¡I¡¡particularly¡¡attended¡¡to¡¡Mr¡£¡¡Waterhouse's¡¡remark£»¡¡whilst¡¡investigating¡¡this¡¡Order£»¡¡and¡¡I¡¡am¡¡fully¡¡convinced¡¡that¡¡the¡¡rule¡¡almost¡¡invariably¡¡holds¡¡good¡¡with¡¡cirripedes¡£¡¡I¡¡shall£»¡¡in¡¡my¡¡future¡¡work£»¡¡give¡¡a¡¡list¡¡of¡¡the¡¡more¡¡remarkable¡¡cases£»¡¡I¡¡will¡¡here¡¡only¡¡briefly¡¡give¡¡one£»¡¡as¡¡it¡¡illustrates¡¡the¡¡rule¡¡in¡¡its¡¡largest¡¡application¡£¡¡The¡¡opercular¡¡valves¡¡of¡¡sessile¡¡cirripedes¡¡£¨rock¡¡barnacles£©¡¡are£»¡¡in¡¡every¡¡sense¡¡of¡¡the¡¡word£»¡¡very¡¡important¡¡structures£»¡¡and¡¡they¡¡differ¡¡extremely¡¡little¡¡even¡¡in¡¡different¡¡genera£»¡¡but¡¡in¡¡the¡¡several¡¡species¡¡of¡¡one¡¡genus£»¡¡Pyrgoma£»¡¡these¡¡valves¡¡present¡¡a¡¡marvellous¡¡amount¡¡of¡¡diversification£º¡¡the¡¡homologous¡¡valves¡¡in¡¡the¡¡different¡¡species¡¡being¡¡sometimes¡¡wholly¡¡unlike¡¡in¡¡shape£»¡¡and¡¡the¡¡amount¡¡of¡¡variation¡¡in¡¡the¡¡individuals¡¡of¡¡several¡¡of¡¡the¡¡species¡¡is¡¡so¡¡great£»¡¡that¡¡it¡¡is¡¡no¡¡exaggeration¡¡to¡¡state¡¡that¡¡the¡¡varieties¡¡differ¡¡more¡¡from¡¡each¡¡other¡¡in¡¡the¡¡characters¡¡of¡¡these¡¡important¡¡valves¡¡than¡¡do¡¡other¡¡species¡¡of¡¡distinct¡¡genera¡£¡¡
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