the origin of species(物种起源)-第28部分

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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：　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：　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：　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　repeat　that　such　cases　certainly　do　occur；　and　seem　to　me　very　remarkable。　
I　will；　however；　give　one　curious　and　complex　case；　not　indeed　as　affecting　any　important　character；　but　from　occurring　in　several　species　of　the　same　genus；　partly　under　domestication　and　partly　under　nature。　It　is　a　case　apparently　of　reversion。　The　ass　not　rarely　has　very　distinct　transverse　bars　on　its　legs；　like　those　of　a　zebra：　it　has　been　asserted　that　these　are　plainest　in　the　foal；　and　from　inquiries　which　I　have　made；　I　believe　this　to　be　true。　It　has　also　been　asserted　that　the　stripe　on　each　shoulder　is　sometimes　double。　The　shoulder…stripe　is　certainly　very　variable　in　length　and　outline。　A　white　ass；　but　not　an　albino；　has　been　described　without　either　spinal　or　shoulder…stripe；　and　these　stripes　are　sometimes　very　obscure；　or　actually　quite　lost；　in　dark…coloured　asses。　The　koulan　of　Pallas　is　said　to　have　been　seen　with　a　double　shoulder…stripe；　but　traces　of　it；　as　stated　by　Mr　Blyth　and　others；　occasionally　appear：　and