the origin of species(物种起源)-第25章

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s　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：　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：　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：　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：　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：　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　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；　wh