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taken¡¡advantage¡¡of¡¡by¡¡natural¡¡and¡¡sexual¡¡selection£»¡¡in¡¡order¡¡to¡¡fit¡¡the¡¡several¡¡species¡¡to¡¡their¡¡several¡¡places¡¡in¡¡the¡¡economy¡¡of¡¡nature£»¡¡and¡¡likewise¡¡to¡¡fit¡¡the¡¡two¡¡sexes¡¡of¡¡the¡¡same¡¡species¡¡to¡¡each¡¡other£»¡¡or¡¡to¡¡fit¡¡the¡¡males¡¡and¡¡females¡¡to¡¡different¡¡habits¡¡of¡¡life£»¡¡or¡¡the¡¡males¡¡to¡¡struggle¡¡with¡¡other¡¡males¡¡for¡¡the¡¡possession¡¡of¡¡the¡¡females¡£¡¡
Finally£»¡¡then£»¡¡I¡¡conclude¡¡that¡¡the¡¡greater¡¡variability¡¡of¡¡specific¡¡characters£»¡¡or¡¡those¡¡which¡¡distinguish¡¡species¡¡from¡¡species£»¡¡than¡¡of¡¡generic¡¡characters£»¡¡or¡¡those¡¡which¡¡the¡¡species¡¡possess¡¡in¡¡common£»¡¡that¡¡the¡¡frequent¡¡extreme¡¡variability¡¡of¡¡any¡¡part¡¡which¡¡is¡¡developed¡¡in¡¡a¡¡species¡¡in¡¡an¡¡extraordinary¡¡manner¡¡in¡¡comparison¡¡with¡¡the¡¡same¡¡part¡¡in¡¡its¡¡congeners£»¡¡and¡¡the¡¡not¡¡great¡¡degree¡¡of¡¡variability¡¡in¡¡a¡¡part£»¡¡however¡¡extraordinarily¡¡it¡¡may¡¡be¡¡developed£»¡¡if¡¡it¡¡be¡¡common¡¡to¡¡a¡¡whole¡¡group¡¡of¡¡species£»¡¡that¡¡the¡¡great¡¡variability¡¡of¡¡secondary¡¡sexual¡¡characters£»¡¡and¡¡the¡¡great¡¡amount¡¡of¡¡difference¡¡in¡¡these¡¡same¡¡characters¡¡between¡¡closely¡¡allied¡¡species£»¡¡that¡¡secondary¡¡sexual¡¡and¡¡ordinary¡¡specific¡¡differences¡¡are¡¡generally¡¡displayed¡¡in¡¡the¡¡same¡¡parts¡¡of¡¡the¡¡organisation£»¡¡are¡¡all¡¡principles¡¡closely¡¡connected¡¡together¡£¡¡All¡¡being¡¡mainly¡¡due¡¡to¡¡the¡¡species¡¡of¡¡the¡¡same¡¡group¡¡having¡¡descended¡¡from¡¡a¡¡common¡¡progenitor£»¡¡from¡¡whom¡¡they¡¡have¡¡inherited¡¡much¡¡in¡¡common£»¡¡to¡¡parts¡¡which¡¡have¡¡recently¡¡and¡¡largely¡¡varied¡¡being¡¡more¡¡likely¡¡still¡¡to¡¡go¡¡on¡¡varying¡¡than¡¡parts¡¡which¡¡have¡¡long¡¡been¡¡inherited¡¡and¡¡have¡¡not¡¡varied£»¡¡to¡¡natural¡¡selection¡¡having¡¡more¡¡or¡¡less¡¡completely£»¡¡according¡¡to¡¡the¡¡lapse¡¡of¡¡time£»¡¡overmastered¡¡the¡¡tendency¡¡to¡¡reversion¡¡and¡¡to¡¡further¡¡variability£»¡¡to¡¡sexual¡¡selection¡¡being¡¡less¡¡rigid¡¡than¡¡ordinary¡¡selection£»¡¡and¡¡to¡¡variations¡¡in¡¡the¡¡same¡¡parts¡¡having¡¡been¡¡accumulated¡¡by¡¡natural¡¡and¡¡sexual¡¡selection£»¡¡and¡¡thus¡¡adapted¡¡for¡¡secondary¡¡sexual£»¡¡and¡¡for¡¡ordinary¡¡specific¡¡purposes¡£¡¡
Distinct¡¡species¡¡present¡¡analogous¡¡variations£»¡¡and¡¡a¡¡variety¡¡of¡¡one¡¡species¡¡often¡¡assumes¡¡some¡¡of¡¡the¡¡characters¡¡of¡¡an¡¡allied¡¡species£»¡¡or¡¡reverts¡¡to¡¡some¡¡of¡¡the¡¡characters¡¡of¡¡an¡¡early¡¡progenitor¡£
These¡¡propositions¡¡will¡¡be¡¡most¡¡readily¡¡understood¡¡by¡¡looking¡¡to¡¡our¡¡domestic¡¡races¡£¡¡The¡¡most¡¡distinct¡¡breeds¡¡of¡¡pigeons£»¡¡in¡¡countries¡¡most¡¡widely¡¡apart£»¡¡present¡¡sub¡­varieties¡¡with¡¡reversed¡¡feathers¡¡on¡¡the¡¡head¡¡and¡¡feathers¡¡on¡¡the¡¡feet£»¡¡characters¡¡not¡¡possessed¡¡by¡¡the¡¡aboriginal¡¡rock¡­pigeon£»¡¡these¡¡then¡¡are¡¡analogous¡¡variations¡¡in¡¡two¡¡or¡¡more¡¡distinct¡¡races¡£¡¡The¡¡frequent¡¡presence¡¡of¡¡fourteen¡¡or¡¡even¡¡sixteen¡¡tail¡­feathers¡¡in¡¡the¡¡pouter£»¡¡may¡¡be¡¡considered¡¡as¡¡a¡¡variation¡¡representing¡¡the¡¡normal¡¡structure¡¡of¡¡another¡¡race£»¡¡the¡¡fantail¡£¡¡I¡¡presume¡¡that¡¡no¡¡one¡¡will¡¡doubt¡¡that¡¡all¡¡such¡¡analogous¡¡variations¡¡are¡¡due¡¡to¡¡the¡¡several¡¡races¡¡of¡¡the¡¡pigeon¡¡having¡¡inherited¡¡from¡¡a¡¡common¡¡parent¡¡the¡¡same¡¡constitution¡¡and¡¡tendency¡¡to¡¡variation£»¡¡when¡¡acted¡¡on¡¡by¡¡similar¡¡unknown¡¡influences¡£¡¡In¡¡the¡¡vegetable¡¡kingdom¡¡we¡¡have¡¡a¡¡case¡¡of¡¡analogous¡¡variation£»¡¡in¡¡the¡¡enlarged¡¡stems£»¡¡or¡¡roots¡¡as¡¡commonly¡¡called£»¡¡of¡¡the¡¡Swedish¡¡turnip¡¡and¡¡Ruta¡¡baga£»¡¡plants¡¡which¡¡several¡¡botanists¡¡rank¡¡as¡¡varieties¡¡produced¡¡by¡¡cultivation¡¡from¡¡a¡¡common¡¡parent£º¡¡if¡¡this¡¡be¡¡not¡¡so£»¡¡the¡¡case¡¡will¡¡then¡¡be¡¡one¡¡of¡¡analogous¡¡variation¡¡in¡¡two¡¡so¡­called¡¡distinct¡¡species£»¡¡and¡¡to¡¡these¡¡a¡¡third¡¡may¡¡be¡¡added£»¡¡namely£»¡¡the¡¡common¡¡turnip¡£¡¡According¡¡to¡¡the¡¡ordinary¡¡view¡¡of¡¡each¡¡species¡¡having¡¡been¡¡independently¡¡created£»¡¡we¡¡should¡¡have¡¡to¡¡attribute¡¡this¡¡similarity¡¡in¡¡the¡¡enlarged¡¡stems¡¡of¡¡these¡¡three¡¡plants£»¡¡not¡¡to¡¡the¡¡vera¡¡causa¡¡of¡¡community¡¡of¡¡descent£»¡¡and¡¡a¡¡consequent¡¡tendency¡¡to¡¡vary¡¡in¡¡a¡¡like¡¡manner£»¡¡but¡¡to¡¡three¡¡separate¡¡yet¡¡closely¡¡related¡¡acts¡¡of¡¡creation¡£¡¡
With¡¡pigeons£»¡¡however£»¡¡we¡¡have¡¡another¡¡case£»¡¡namely£»¡¡the¡¡occasional¡¡appearance¡¡in¡¡all¡¡the¡¡breeds£»¡¡of¡¡slaty¡­blue¡¡birds¡¡with¡¡two¡¡black¡¡bars¡¡on¡¡the¡¡wings£»¡¡a¡¡white¡¡rump£»¡¡a¡¡bar¡¡at¡¡the¡¡end¡¡of¡¡the¡¡tail£»¡¡with¡¡the¡¡outer¡¡feathers¡¡externally¡¡edged¡¡near¡¡their¡¡bases¡¡with¡¡white¡£¡¡As¡¡all¡¡these¡¡marks¡¡are¡¡characteristic¡¡of¡¡the¡¡parent¡¡rock¡­pigeon£»¡¡I¡¡presume¡¡that¡¡no¡¡one¡¡will¡¡doubt¡¡that¡¡this¡¡is¡¡a¡¡case¡¡of¡¡reversion£»¡¡and¡¡not¡¡of¡¡a¡¡new¡¡yet¡¡analogous¡¡variation¡¡appearing¡¡in¡¡the¡¡several¡¡breeds¡£¡¡We¡¡may¡¡I¡¡think¡¡confidently¡¡come¡¡to¡¡this¡¡conclusion£»¡¡because£»¡¡as¡¡we¡¡have¡¡seen£»¡¡these¡¡coloured¡¡marks¡¡are¡¡eminently¡¡liable¡¡to¡¡appear¡¡in¡¡the¡¡crossed¡¡offspring¡¡of¡¡two¡¡distinct¡¡and¡¡differently¡¡coloured¡¡breeds£»¡¡and¡¡in¡¡this¡¡case¡¡there¡¡is¡¡nothing¡¡in¡¡the¡¡external¡¡conditions¡¡of¡¡life¡¡to¡¡cause¡¡the¡¡reappearance¡¡of¡¡the¡¡slaty¡­blue£»¡¡with¡¡the¡¡several¡¡marks£»¡¡beyond¡¡the¡¡influence¡¡of¡¡the¡¡mere¡¡act¡¡of¡¡crossing¡¡on¡¡the¡¡laws¡¡of¡¡inheritance¡£¡¡
No¡¡doubt¡¡it¡¡is¡¡a¡¡very¡¡surprising¡¡fact¡¡that¡¡characters¡¡should¡¡reappear¡¡after¡¡having¡¡been¡¡lost¡¡for¡¡many£»¡¡perhaps¡¡for¡¡hundreds¡¡of¡¡generations¡£¡¡But¡¡when¡¡a¡¡breed¡¡has¡¡been¡¡crossed¡¡only¡¡once¡¡by¡¡some¡¡other¡¡breed£»¡¡the¡¡offspring¡¡occasionally¡¡show¡¡a¡¡tendency¡¡to¡¡revert¡¡in¡¡character¡¡to¡¡the¡¡foreign¡¡breed¡¡for¡¡many¡¡generations¡¡some¡¡say£»¡¡for¡¡a¡¡dozen¡¡or¡¡even¡¡a¡¡score¡¡of¡¡generations¡£¡¡After¡¡twelve¡¡generations£»¡¡the¡¡proportion¡¡of¡¡blood£»¡¡to¡¡use¡¡a¡¡common¡¡expression£»¡¡of¡¡any¡¡one¡¡ancestor£»¡¡is¡¡only¡¡1¡¡in¡¡2048£»¡¡and¡¡yet£»¡¡as¡¡we¡¡see£»¡¡it¡¡is¡¡generally¡¡believed¡¡that¡¡a¡¡tendency¡¡to¡¡reversion¡¡is¡¡retained¡¡by¡¡this¡¡very¡¡small¡¡proportion¡¡of¡¡foreign¡¡blood¡£¡¡In¡¡a¡¡breed¡¡which¡¡has¡¡not¡¡been¡¡crossed£»¡¡but¡¡in¡¡which¡¡both¡¡parents¡¡have¡¡lost¡¡some¡¡character¡¡which¡¡their¡¡progenitor¡¡possessed£»¡¡the¡¡tendency£»¡¡whether¡¡strong¡¡or¡¡weak£»¡¡to¡¡reproduce¡¡the¡¡lost¡¡character¡¡might¡¡be£»¡¡as¡¡was¡¡formerly¡¡remarked£»¡¡for¡¡all¡¡that¡¡we¡¡can¡¡see¡¡to¡¡the¡¡contrary£»¡¡transmitted¡¡for¡¡almost¡¡any¡¡number¡¡of¡¡generations¡£¡¡When¡¡a¡¡character¡¡which¡¡has¡¡been¡¡lost¡¡in¡¡a¡¡breed£»¡¡reappears¡¡after¡¡a¡¡great¡¡number¡¡of¡¡generations£»¡¡the¡¡most¡¡probable¡¡hypothesis¡¡is£»¡¡not¡¡that¡¡the¡¡offspring¡¡suddenly¡¡takes¡¡after¡¡an¡¡ancestor¡¡some¡¡hundred¡¡generations¡¡distant£»¡¡but¡¡that¡¡in¡¡each¡¡successive¡¡generation¡¡there¡¡has¡¡been¡¡a¡¡tendency¡¡to¡¡reproduce¡¡the¡¡character¡¡in¡¡question£»¡¡which¡¡at¡¡last£»¡¡under¡¡unknown¡¡favourable¡¡conditions£»¡¡gains¡¡an¡¡ascendancy¡£¡¡For¡¡instance£»¡¡it¡¡is¡¡probable¡¡that¡¡in¡¡each¡¡generation¡¡of¡¡the¡¡barb¡­pigeon£»¡¡which¡¡produces¡¡most¡¡rarely¡¡a¡¡blue¡¡and¡¡black¡­barred¡¡bird£»¡¡there¡¡has¡¡been¡¡a¡¡tendency¡¡in¡¡each¡¡generation¡¡in¡¡the¡¡plumage¡¡to¡¡assume¡¡this¡¡colour¡£¡¡This¡¡view¡¡is¡¡hypothetical£»¡¡but¡¡could¡¡be¡¡supported¡¡by¡¡some¡¡facts£»¡¡and¡¡I¡¡can¡¡see¡¡no¡¡more¡¡abstract¡¡improbability¡¡in¡¡a¡¡tendency¡¡to¡¡produce¡¡any¡¡character¡¡being¡¡inherited¡¡for¡¡an¡¡endless¡¡number¡¡of¡¡generations£»¡¡than¡¡in¡¡quite¡¡useless¡¡or¡¡rudimentary¡¡organs¡¡being£»¡¡as¡¡we¡¡all¡¡know¡¡them¡¡to¡¡be£»¡¡thus¡¡inherited¡£¡¡Indeed£»¡¡we¡¡may¡¡sometimes¡¡observe¡¡a¡¡mere¡¡tendency¡¡to¡¡produce¡¡a¡¡rudiment¡¡inherited£º¡¡for¡¡instance£»¡¡in¡¡the¡¡common¡¡snapdragon¡¡£¨Antirrhinum£©¡¡a¡¡rudiment¡¡of¡¡a¡¡fifth¡¡stamen¡¡so¡¡often¡¡appears£»¡¡that¡¡this¡¡plant¡¡must¡¡have¡¡an¡¡inherited¡¡tendency¡¡to¡¡produce¡¡it¡£¡¡
As¡¡all¡¡the¡¡species¡¡of¡¡the¡¡same¡¡genus¡¡are¡¡supposed£»¡¡on¡¡my¡¡theory£»¡¡to¡¡have¡¡descended¡¡from¡¡a¡¡common¡¡parent£»¡¡it¡¡might¡¡be¡¡expected¡¡that¡¡they¡¡would¡¡occasionally¡¡vary¡¡in¡¡an¡¡analogous¡¡manner£»¡¡so¡¡that¡¡a¡¡variety¡¡of¡¡one¡¡species¡¡would¡¡resemble¡¡in¡¡some¡¡of¡¡its¡¡characters¡¡another¡¡species£»¡¡this¡¡other¡¡species¡¡being¡¡on¡¡my¡¡view¡¡only¡¡a¡¡well¡­marked¡¡and¡¡permanent¡¡variety¡£¡¡But¡¡characters¡¡thus¡¡gained¡¡would¡¡probably¡¡be¡¡of¡¡an¡¡unimportant¡¡nature£»¡¡for¡¡the¡¡presence¡¡of¡¡all¡¡important¡¡characters¡¡will¡¡be¡¡governed¡¡by¡¡natural¡¡selection£»¡¡in¡¡accordance¡¡with¡¡the¡¡diverse¡¡habits¡¡of¡¡the¡¡species£»¡¡and¡¡will¡¡not¡¡be¡¡left¡¡to¡¡the¡¡mutual¡¡action¡¡of¡¡the¡¡conditions¡¡of¡¡life¡¡and¡¡of¡¡a¡¡similar¡¡inherited¡¡constitution¡£¡¡It¡¡might¡¡further¡¡be¡¡expected¡¡that¡¡the¡¡species¡¡of¡¡the¡¡same¡¡genus¡¡would¡¡occasionally¡¡exhibit¡¡reversions¡¡to¡¡lost¡¡ancestral¡¡characters¡£¡¡As£»¡¡however£»¡¡we¡¡never¡¡know¡¡the¡¡exact¡¡character¡¡of¡¡the¡¡common¡¡ancestor¡¡of¡¡a¡¡group£»¡¡we¡¡could¡¡not¡¡distinguish¡¡these¡¡two¡¡cases£º¡¡if£»¡¡for¡¡instance£»¡¡we¡¡did¡¡not¡¡know¡¡that¡¡the¡¡rock¡­pigeon¡¡was¡¡not¡¡feather¡­footed¡¡or¡¡turn¡­crowned£»¡¡we¡¡could¡¡not¡¡have¡¡told£»¡¡whether¡¡these¡¡characters¡¡in¡¡our¡¡domestic¡¡breeds¡¡were¡¡reversions¡¡or¡¡only¡¡analogous¡¡variations£»¡¡but¡¡we¡¡might¡¡have¡¡inferred¡¡that¡¡the¡¡blueness¡¡was¡¡a¡¡case¡¡of¡¡reversion£»¡¡from¡¡the¡¡number¡¡of¡¡the¡¡markings£»¡¡which¡¡are¡¡correlated¡¡with¡¡the¡¡blue¡¡tint£»¡¡and¡¡which¡¡it¡¡does¡¡not¡¡appear¡¡probable¡¡would¡¡all¡¡appear¡¡together¡¡from¡¡simple¡¡variation¡£¡¡More¡¡especially¡¡we¡¡might¡¡have¡¡inferred¡¡this£»¡¡from¡¡the¡¡blue¡¡colour¡¡and¡¡marks¡¡so¡¡often¡¡appearing¡¡when¡¡distinct¡¡breeds¡¡of¡¡diverse¡¡colours¡¡are¡¡crossed¡£¡¡Hence£»¡¡though¡¡under¡¡nature¡¡it¡¡must¡¡generally¡¡be¡¡left¡¡doubtful£»¡¡what¡¡cases¡¡are¡¡reversions¡¡to¡¡an¡¡anciently¡¡existing¡¡character£»¡¡and¡¡what¡¡are¡¡new¡¡but¡¡analogous¡¡variations£»¡¡yet¡¡we¡¡ought£»¡¡on¡¡my¡¡theory£»¡¡sometimes¡¡to¡¡find¡¡the¡¡varying¡¡offspring¡¡of¡¡a¡¡species¡¡assuming¡¡characters¡¡£¨either¡¡from¡¡reversion¡¡or¡¡from¡¡analogous¡¡variation£©¡¡which¡¡already¡¡occur¡¡in¡¡some¡¡members¡¡of¡¡the¡¡same¡¡group¡£¡¡And¡¡this¡¡undoubtedly¡¡is¡¡the¡¡case¡¡in¡¡nature¡£¡¡
A¡¡considerable¡¡part¡¡of¡¡the¡¡difficulty¡¡in¡¡recognising¡¡a¡¡variable¡¡species¡¡in¡¡our¡¡systematic¡¡works£»¡¡is¡¡due¡¡to¡¡its¡¡varieties¡¡mocking£»¡¡as¡¡it¡¡were£»¡¡come¡¡of¡¡the¡¡other¡¡species¡¡of¡¡the¡¡same¡¡genus¡£¡¡A¡¡considerable¡¡catalogue£»¡¡also£»¡¡could¡¡be¡¡given¡¡of¡¡forms¡¡intermediate¡¡between¡¡two¡¡other¡¡forms£»¡¡which¡¡themselves¡¡must¡¡be¡¡doubtfully¡¡ranked¡¡as¡¡either¡¡varieties¡¡or¡¡species£»¡¡that¡¡the¡¡one¡¡in¡¡varying¡¡has¡¡assumed¡¡some¡¡of¡¡the¡¡characters¡¡of¡¡the¡¡other£»¡¡so¡¡as¡¡to¡¡produce¡¡the¡¡intermediate¡¡form¡£¡¡But¡¡the¡¡best¡¡evidence¡¡is¡¡afforded¡¡by¡¡parts¡¡or¡¡organs¡¡of¡¡an¡¡important¡¡and¡¡uniform¡¡nature¡¡occasionally¡¡varying¡¡so¡¡as¡¡to¡¡acquire£»¡¡in¡¡some¡¡degree£»¡¡the¡¡character¡¡of¡¡the¡¡same¡¡part¡¡or¡¡organ¡¡in¡¡an¡¡allied¡¡species¡£¡¡I¡¡have¡¡collected¡¡a¡¡long¡¡list¡¡of¡¡such¡¡cases£»¡¡but¡¡here£»¡¡as¡¡before£»¡¡I¡¡lie¡¡under¡¡a¡¡great¡¡disadvantage¡¡in¡¡not¡¡being¡¡able¡¡to¡¡give¡¡them¡£¡¡I¡¡can¡¡only¡¡repea

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