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he¡¡power¡¡of¡¡steady¡¡selection¡¡to¡¡keep¡¡the¡¡breed¡¡true¡£¡¡In¡¡the¡¡long¡¡run¡¡selection¡¡gains¡¡the¡¡day£»¡¡and¡¡we¡¡do¡¡not¡¡expect¡¡to¡¡fail¡¡so¡¡far¡¡as¡¡to¡¡breed¡¡a¡¡bird¡¡as¡¡coarse¡¡as¡¡a¡¡common¡¡tumbler¡¡from¡¡a¡¡good¡¡short¡­faced¡¡strain¡£¡¡But¡¡as¡¡long¡¡as¡¡selection¡¡is¡¡rapidly¡¡going¡¡on£»¡¡there¡¡may¡¡always¡¡be¡¡expected¡¡to¡¡be¡¡much¡¡variability¡¡in¡¡the¡¡structure¡¡undergoing¡¡modification¡£¡¡It¡¡further¡¡deserves¡¡notice¡¡that¡¡these¡¡variable¡¡characters£»¡¡produced¡¡by¡¡man's¡¡selection£»¡¡sometimes¡¡become¡¡attached£»¡¡from¡¡causes¡¡quite¡¡unknown¡¡to¡¡us£»¡¡more¡¡to¡¡one¡¡sex¡¡than¡¡to¡¡the¡¡other£»¡¡generally¡¡to¡¡the¡¡male¡¡sex£»¡¡as¡¡with¡¡the¡¡wattle¡¡of¡¡carriers¡¡and¡¡the¡¡enlarged¡¡crop¡¡of¡¡pouters¡£¡¡
Now¡¡let¡¡us¡¡turn¡¡to¡¡nature¡£¡¡When¡¡a¡¡part¡¡has¡¡been¡¡developed¡¡in¡¡an¡¡extraordinary¡¡manner¡¡in¡¡any¡¡one¡¡species£»¡¡compared¡¡with¡¡the¡¡other¡¡species¡¡of¡¡the¡¡same¡¡genus£»¡¡we¡¡may¡¡conclude¡¡that¡¡this¡¡part¡¡has¡¡undergone¡¡an¡¡extraordinary¡¡amount¡¡of¡¡modification£»¡¡since¡¡the¡¡period¡¡when¡¡the¡¡species¡¡branched¡¡off¡¡from¡¡the¡¡common¡¡progenitor¡¡of¡¡the¡¡genus¡£¡¡This¡¡period¡¡will¡¡seldom¡¡be¡¡remote¡¡in¡¡any¡¡extreme¡¡degree£»¡¡as¡¡species¡¡very¡¡rarely¡¡endure¡¡for¡¡more¡¡than¡¡one¡¡geological¡¡period¡£¡¡An¡¡extraordinary¡¡amount¡¡of¡¡modification¡¡implies¡¡an¡¡unusually¡¡large¡¡and¡¡long¡­continued¡¡amount¡¡of¡¡variability£»¡¡which¡¡has¡¡continually¡¡been¡¡accumulated¡¡by¡¡natural¡¡selection¡¡for¡¡the¡¡benefit¡¡of¡¡the¡¡species¡£¡¡But¡¡as¡¡the¡¡variability¡¡of¡¡the¡¡extraordinarily¡­developed¡¡part¡¡or¡¡organ¡¡has¡¡been¡¡so¡¡great¡¡and¡¡long¡­continued¡¡within¡¡a¡¡period¡¡not¡¡excessively¡¡remote£»¡¡we¡¡might£»¡¡as¡¡a¡¡general¡¡rule£»¡¡expect¡¡still¡¡to¡¡find¡¡more¡¡variability¡¡in¡¡such¡¡parts¡¡than¡¡in¡¡other¡¡parts¡¡of¡¡the¡¡organisation£»¡¡which¡¡have¡¡remained¡¡for¡¡a¡¡much¡¡longer¡¡period¡¡nearly¡¡constant¡£¡¡And¡¡this£»¡¡I¡¡am¡¡convinced£»¡¡is¡¡the¡¡case¡£¡¡That¡¡the¡¡struggle¡¡between¡¡natural¡¡selection¡¡on¡¡the¡¡one¡¡hand£»¡¡and¡¡the¡¡tendency¡¡to¡¡reversion¡¡and¡¡variability¡¡on¡¡the¡¡other¡¡hand£»¡¡will¡¡in¡¡the¡¡course¡¡of¡¡time¡¡cease£»¡¡and¡¡that¡¡the¡¡most¡¡abnormally¡¡developed¡¡organs¡¡may¡¡be¡¡made¡¡constant£»¡¡I¡¡can¡¡see¡¡no¡¡reason¡¡to¡¡doubt¡£¡¡Hence¡¡when¡¡an¡¡organ£»¡¡however¡¡abnormal¡¡it¡¡may¡¡be£»¡¡has¡¡been¡¡transmitted¡¡in¡¡approximately¡¡the¡¡same¡¡condition¡¡to¡¡many¡¡modified¡¡descendants£»¡¡as¡¡in¡¡the¡¡case¡¡of¡¡the¡¡wing¡¡of¡¡the¡¡bat£»¡¡it¡¡must¡¡have¡¡existed£»¡¡according¡¡to¡¡my¡¡theory£»¡¡for¡¡an¡¡immense¡¡period¡¡in¡¡nearly¡¡the¡¡same¡¡state£»¡¡and¡¡thus¡¡it¡¡comes¡¡to¡¡be¡¡no¡¡more¡¡variable¡¡than¡¡any¡¡other¡¡structure¡£¡¡It¡¡is¡¡only¡¡in¡¡those¡¡cases¡¡in¡¡which¡¡the¡¡modification¡¡has¡¡been¡¡comparatively¡¡recent¡¡and¡¡extraordinarily¡¡great¡¡that¡¡we¡¡ought¡¡to¡¡find¡¡the¡¡generative¡¡variability£»¡¡as¡¡it¡¡may¡¡be¡¡called£»¡¡still¡¡present¡¡in¡¡a¡¡high¡¡degree¡£¡¡For¡¡in¡¡this¡¡case¡¡the¡¡variability¡¡will¡¡seldom¡¡as¡¡yet¡¡have¡¡been¡¡fixed¡¡by¡¡the¡¡continued¡¡selection¡¡of¡¡the¡¡individuals¡¡varying¡¡in¡¡the¡¡required¡¡manner¡¡and¡¡degree£»¡¡and¡¡by¡¡the¡¡continued¡¡rejection¡¡of¡¡those¡¡tending¡¡to¡¡revert¡¡to¡¡a¡¡former¡¡and¡¡less¡¡modified¡¡condition¡£¡¡
The¡¡principle¡¡included¡¡in¡¡these¡¡remarks¡¡may¡¡be¡¡extended¡£¡¡It¡¡is¡¡notorious¡¡that¡¡specific¡¡characters¡¡are¡¡more¡¡variable¡¡than¡¡generic¡£¡¡To¡¡explain¡¡by¡¡a¡¡simple¡¡example¡¡what¡¡is¡¡meant¡£¡¡If¡¡some¡¡species¡¡in¡¡a¡¡large¡¡genus¡¡of¡¡plants¡¡had¡¡blue¡¡flowers¡¡and¡¡some¡¡had¡¡red£»¡¡the¡¡colour¡¡would¡¡be¡¡only¡¡a¡¡specific¡¡character£»¡¡and¡¡no¡¡one¡¡would¡¡be¡¡surprised¡¡at¡¡one¡¡of¡¡the¡¡blue¡¡species¡¡varying¡¡into¡¡red£»¡¡or¡¡conversely£»¡¡but¡¡if¡¡all¡¡the¡¡species¡¡had¡¡blue¡¡flowers£»¡¡the¡¡colour¡¡would¡¡become¡¡a¡¡generic¡¡character£»¡¡and¡¡its¡¡variation¡¡would¡¡be¡¡a¡¡more¡¡unusual¡¡circumstance¡£¡¡I¡¡have¡¡chosen¡¡this¡¡example¡¡because¡¡an¡¡explanation¡¡is¡¡not¡¡in¡¡this¡¡case¡¡applicable£»¡¡which¡¡most¡¡naturalists¡¡would¡¡advance£»¡¡namely£»¡¡that¡¡specific¡¡characters¡¡are¡¡more¡¡variable¡¡than¡¡generic£»¡¡because¡¡they¡¡are¡¡taken¡¡from¡¡parts¡¡of¡¡less¡¡physiological¡¡importance¡¡than¡¡those¡¡commonly¡¡used¡¡for¡¡classing¡¡genera¡£¡¡I¡¡believe¡¡this¡¡explanation¡¡is¡¡partly£»¡¡yet¡¡only¡¡indirectly£»¡¡true£»¡¡I¡¡shall£»¡¡however£»¡¡have¡¡to¡¡return¡¡to¡¡this¡¡subject¡¡in¡¡our¡¡chapter¡¡on¡¡Classification¡£¡¡It¡¡would¡¡be¡¡almost¡¡superfluous¡¡to¡¡adduce¡¡evidence¡¡in¡¡support¡¡of¡¡the¡¡above¡¡statement£»¡¡that¡¡specific¡¡characters¡¡are¡¡more¡¡variable¡¡than¡¡generic£»¡¡but¡¡I¡¡have¡¡repeatedly¡¡noticed¡¡in¡¡works¡¡on¡¡natural¡¡history£»¡¡that¡¡when¡¡an¡¡author¡¡has¡¡remarked¡¡with¡¡surprise¡¡that¡¡some¡¡important¡¡organ¡¡or¡¡part£»¡¡which¡¡is¡¡generally¡¡very¡¡constant¡¡throughout¡¡large¡¡groups¡¡of¡¡species£»¡¡has¡¡differed¡¡considerably¡¡in¡¡closely¡­allied¡¡species£»¡¡that¡¡it¡¡has£»¡¡also£»¡¡been¡¡variable¡¡in¡¡the¡¡individuals¡¡of¡¡some¡¡of¡¡the¡¡species¡£¡¡And¡¡this¡¡fact¡¡shows¡¡that¡¡a¡¡character£»¡¡which¡¡is¡¡generally¡¡of¡¡generic¡¡value£»¡¡when¡¡it¡¡sinks¡¡in¡¡value¡¡and¡¡becomes¡¡only¡¡of¡¡specific¡¡value£»¡¡often¡¡becomes¡¡variable£»¡¡though¡¡its¡¡physiological¡¡importance¡¡may¡¡remain¡¡the¡¡same¡£¡¡Something¡¡of¡¡the¡¡same¡¡kind¡¡applies¡¡to¡¡monstrosities£º¡¡at¡¡least¡¡Is¡£¡¡Geoffroy¡¡St¡£¡¡Hilaire¡¡seems¡¡to¡¡entertain¡¡no¡¡doubt£»¡¡that¡¡the¡¡more¡¡an¡¡organ¡¡normally¡¡differs¡¡in¡¡the¡¡different¡¡species¡¡of¡¡the¡¡same¡¡group£»¡¡the¡¡more¡¡subject¡¡it¡¡is¡¡to¡¡individual¡¡anomalies¡£¡¡
On¡¡the¡¡ordinary¡¡view¡¡of¡¡each¡¡species¡¡having¡¡been¡¡independently¡¡created£»¡¡why¡¡should¡¡that¡¡part¡¡of¡¡the¡¡structure£»¡¡which¡¡differs¡¡from¡¡the¡¡same¡¡part¡¡in¡¡other¡¡independently¡­created¡¡species¡¡of¡¡the¡¡same¡¡genus£»¡¡be¡¡more¡¡variable¡¡than¡¡those¡¡parts¡¡which¡¡are¡¡closely¡¡alike¡¡in¡¡the¡¡several¡¡species£¿¡¡I¡¡do¡¡not¡¡see¡¡that¡¡any¡¡explanation¡¡can¡¡be¡¡given¡£¡¡But¡¡on¡¡the¡¡view¡¡of¡¡species¡¡being¡¡only¡¡strongly¡¡marked¡¡and¡¡fixed¡¡varieties£»¡¡we¡¡might¡¡surely¡¡expect¡¡to¡¡find¡¡them¡¡still¡¡often¡¡continuing¡¡to¡¡vary¡¡in¡¡those¡¡parts¡¡of¡¡their¡¡structure¡¡which¡¡have¡¡varied¡¡within¡¡a¡¡moderately¡¡recent¡¡period£»¡¡and¡¡which¡¡have¡¡thus¡¡come¡¡to¡¡differ¡£¡¡Or¡¡to¡¡state¡¡the¡¡case¡¡in¡¡another¡¡manner£º¡¡the¡¡points¡¡in¡¡which¡¡all¡¡the¡¡species¡¡of¡¡a¡¡genus¡¡resemble¡¡each¡¡other£»¡¡and¡¡in¡¡which¡¡they¡¡differ¡¡from¡¡the¡¡species¡¡of¡¡some¡¡other¡¡genus£»¡¡are¡¡called¡¡generic¡¡characters£»¡¡and¡¡these¡¡characters¡¡in¡¡common¡¡I¡¡attribute¡¡to¡¡inheritance¡¡from¡¡a¡¡common¡¡progenitor£»¡¡for¡¡it¡¡can¡¡rarely¡¡have¡¡happened¡¡that¡¡natural¡¡selection¡¡will¡¡have¡¡modified¡¡several¡¡species£»¡¡fitted¡¡to¡¡more¡¡or¡¡less¡¡widely¡­different¡¡habits£»¡¡in¡¡exactly¡¡the¡¡same¡¡manner£º¡¡and¡¡as¡¡these¡¡so¡­called¡¡generic¡¡characters¡¡have¡¡been¡¡inherited¡¡from¡¡a¡¡remote¡¡period£»¡¡since¡¡that¡¡period¡¡when¡¡the¡¡species¡¡first¡¡branched¡¡off¡¡from¡¡their¡¡common¡¡progenitor£»¡¡and¡¡subsequently¡¡have¡¡not¡¡varied¡¡or¡¡come¡¡to¡¡differ¡¡in¡¡any¡¡degree£»¡¡or¡¡only¡¡in¡¡a¡¡slight¡¡degree£»¡¡it¡¡is¡¡not¡¡probable¡¡that¡¡they¡¡should¡¡vary¡¡at¡¡the¡¡present¡¡day¡£¡¡On¡¡the¡¡other¡¡hand£»¡¡the¡¡points¡¡in¡¡which¡¡species¡¡differ¡¡from¡¡other¡¡species¡¡of¡¡the¡¡same¡¡genus£»¡¡are¡¡called¡¡specific¡¡characters£»¡¡and¡¡as¡¡these¡¡specific¡¡characters¡¡have¡¡varied¡¡and¡¡come¡¡to¡¡differ¡¡within¡¡the¡¡period¡¡of¡¡the¡¡branching¡¡off¡¡of¡¡the¡¡species¡¡from¡¡a¡¡common¡¡progenitor£»¡¡it¡¡is¡¡probable¡¡that¡¡they¡¡should¡¡still¡¡often¡¡be¡¡in¡¡some¡¡degree¡¡variable£»¡¡at¡¡least¡¡more¡¡variable¡¡than¡¡those¡¡parts¡¡of¡¡the¡¡organisation¡¡which¡¡have¡¡for¡¡a¡¡very¡¡long¡¡period¡¡remained¡¡constant¡£¡¡
In¡¡connexion¡¡with¡¡the¡¡present¡¡subject£»¡¡I¡¡will¡¡make¡¡only¡¡two¡¡other¡¡remarks¡£¡¡I¡¡think¡¡it¡¡will¡¡be¡¡admitted£»¡¡without¡¡my¡¡entering¡¡on¡¡details£»¡¡that¡¡secondary¡¡sexual¡¡characters¡¡are¡¡very¡¡variable£»¡¡I¡¡think¡¡it¡¡also¡¡will¡¡be¡¡admitted¡¡that¡¡species¡¡of¡¡the¡¡same¡¡group¡¡differ¡¡from¡¡each¡¡other¡¡more¡¡widely¡¡in¡¡their¡¡secondary¡¡sexual¡¡characters£»¡¡than¡¡in¡¡other¡¡parts¡¡of¡¡their¡¡organisation£»¡¡compare£»¡¡for¡¡instance£»¡¡the¡¡amount¡¡of¡¡difference¡¡between¡¡the¡¡males¡¡of¡¡gallinaceous¡¡birds£»¡¡in¡¡which¡¡secondary¡¡sexual¡¡characters¡¡are¡¡strongly¡¡displayed£»¡¡with¡¡the¡¡amount¡¡of¡¡difference¡¡between¡¡their¡¡females£»¡¡and¡¡the¡¡truth¡¡of¡¡this¡¡proposition¡¡will¡¡be¡¡granted¡£¡¡The¡¡cause¡¡of¡¡the¡¡original¡¡variability¡¡of¡¡secondary¡¡sexual¡¡characters¡¡is¡¡not¡¡manifest£»¡¡but¡¡we¡¡can¡¡see¡¡why¡¡these¡¡characters¡¡should¡¡not¡¡have¡¡been¡¡rendered¡¡as¡¡constant¡¡and¡¡uniform¡¡as¡¡other¡¡parts¡¡of¡¡the¡¡organisation£»¡¡for¡¡secondary¡¡sexual¡¡characters¡¡have¡¡been¡¡accumulated¡¡by¡¡sexual¡¡selection£»¡¡which¡¡is¡¡less¡¡rigid¡¡in¡¡its¡¡action¡¡than¡¡ordinary¡¡selection£»¡¡as¡¡it¡¡does¡¡not¡¡entail¡¡death£»¡¡but¡¡only¡¡gives¡¡fewer¡¡offspring¡¡to¡¡the¡¡less¡¡favoured¡¡males¡£¡¡Whatever¡¡the¡¡cause¡¡may¡¡be¡¡of¡¡the¡¡variability¡¡of¡¡secondary¡¡sexual¡¡characters£»¡¡as¡¡they¡¡are¡¡highly¡¡variable£»¡¡sexual¡¡selection¡¡will¡¡have¡¡had¡¡a¡¡wide¡¡scope¡¡for¡¡action£»¡¡and¡¡may¡¡thus¡¡readily¡¡have¡¡succeeded¡¡in¡¡giving¡¡to¡¡the¡¡species¡¡of¡¡the¡¡same¡¡group¡¡a¡¡greater¡¡amount¡¡of¡¡difference¡¡in¡¡their¡¡sexual¡¡characters£»¡¡than¡¡in¡¡other¡¡parts¡¡of¡¡their¡¡structure¡£¡¡
It¡¡is¡¡a¡¡remarkable¡¡fact£»¡¡that¡¡the¡¡secondary¡¡sexual¡¡differences¡¡between¡¡the¡¡two¡¡sexes¡¡of¡¡the¡¡same¡¡species¡¡are¡¡generally¡¡displayed¡¡in¡¡the¡¡very¡¡same¡¡parts¡¡of¡¡the¡¡organisation¡¡in¡¡which¡¡the¡¡different¡¡species¡¡of¡¡the¡¡same¡¡genus¡¡differ¡¡from¡¡each¡¡other¡£¡¡Of¡¡this¡¡fact¡¡I¡¡will¡¡give¡¡in¡¡illustration¡¡two¡¡instances£»¡¡the¡¡first¡¡which¡¡happen¡¡to¡¡stand¡¡on¡¡my¡¡list£»¡¡and¡¡as¡¡the¡¡differences¡¡in¡¡these¡¡cases¡¡are¡¡of¡¡a¡¡very¡¡unusual¡¡nature£»¡¡the¡¡relation¡¡can¡¡hardly¡¡be¡¡accidental¡£¡¡The¡¡same¡¡number¡¡of¡¡joints¡¡in¡¡the¡¡tarsi¡¡is¡¡a¡¡character¡¡generally¡¡common¡¡to¡¡very¡¡large¡¡groups¡¡of¡¡beetles£»¡¡but¡¡in¡¡the¡¡Engidae£»¡¡as¡¡Westwood¡¡has¡¡remarked£»¡¡the¡¡number¡¡varies¡¡greatly£»¡¡and¡¡the¡¡number¡¡likewise¡¡differs¡¡in¡¡the¡¡two¡¡sexes¡¡of¡¡the¡¡same¡¡species£º¡¡again¡¡in¡¡fossorial¡¡hymenoptera£»¡¡the¡¡manner¡¡of¡¡neuration¡¡of¡¡the¡¡wings¡¡is¡¡a¡¡character¡¡of¡¡the¡¡highest¡¡importance£»¡¡because¡¡common¡¡to¡¡large¡¡groups£»¡¡but¡¡in¡¡certain¡¡genera¡¡the¡¡neuration¡¡differs¡¡in¡¡the¡¡different¡¡species£»¡¡and¡¡likewise¡¡in¡¡the¡¡two¡¡sexes¡¡of¡¡the¡¡same¡¡species¡£¡¡This¡¡relation¡¡has¡¡a¡¡clear¡¡meaning¡¡on¡¡my¡¡view¡¡of¡¡the¡¡subject£º¡¡I¡¡look¡¡at¡¡all¡¡the¡¡species¡¡of¡¡the¡¡same¡¡genus¡¡as¡¡having¡¡as¡¡certainly¡¡descended¡¡from¡¡the¡¡same¡¡progenitor£»¡¡as¡¡have¡¡the¡¡two¡¡sexes¡¡of¡¡any¡¡one¡¡of¡¡the¡¡species¡£¡¡Consequently£»¡¡whatever¡¡part¡¡of¡¡the¡¡structure¡¡of¡¡the¡¡common¡¡progenitor£»¡¡or¡¡of¡¡its¡¡early¡¡descendants£»¡¡became¡¡variable£»¡¡variations¡¡of¡¡this¡¡part¡¡would¡¡it¡¡is¡¡highly¡¡probable£»¡¡be¡¡taken¡¡advantage¡¡of¡¡by¡¡natural¡¡and¡¡sexual¡¡selection£»¡¡in¡¡order¡¡to¡¡fit¡¡the¡¡several¡¡species¡¡to¡¡their¡¡sev

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