the origin of species(物种起源)-第30节

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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：　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　sev