the origin of species(物种起源)-第4章

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eral　generations；　hardly　two　of　them　will　be　alike；　and　then　the　extreme　difficulty；　or　rather　utter　hopelessness；　of　the　task　becomes　apparent。　Certainly；　a　breed　intermediate　between　two　very　distinct　breeds　could　not　be　got　without　extreme　care　and　long…continued　selection；　nor　can　I　find　a　single　case　on　record　of　a　permanent　race　having　been　thus　formed。　
On　the　Breeds　of　the　Domestic　pigeon。
Believing　that　it　is　always　best　to　study　some　special　group；　I　have；　after　deliberation；　taken　up　domestic　pigeons。　I　have　kept　every　breed　which　I　could　purchase　or　obtain；　and　have　been　most　kindly　favoured　with　skins　from　several　quarters　of　the　world；　more　especially　by　the　Hon。　W。　Elliot　from　India；　and　by　the　Hon。　C。　Murray　from　Persia。　Many　treatises　in　different　languages　have　been　published　on　pigeons；　and　some　of　them　are　very　important；　as　being　of　considerably　antiquity。　I　have　associated　with　several　eminent　fanciers；　and　have　been　permitted　to　join　two　of　the　London　Pigeon　Clubs。　The　diversity　of　the　breeds　is　something　astonishing。　Compare　the　English　carrier　and　the　short…faced　tumbler；　and　see　the　wonderful　difference　in　their　beaks；　entailing　corresponding　differences　in　their　skulls。　The　carrier；　more　especially　the　male　bird；　is　also　remarkable　from　the　wonderful　development　of　the　carunculated　skin　about　the　head；　and　this　is　accompanied　by　greatly　elongated　eyelids；　very　large　external　orifices　to　the　nostrils；　and　a　wide　gape　of　mouth。　The　short…faced　tumbler　has　a　beak　in　outline　almost　like　that　of　a　finch；　and　the　common　tumbler　has　the　singular　and　strictly　inherited　habit　of　flying　at　a　great　height　in　a　compact　flock；　and　tumbling　in　the　air　head　over　heels。　The　runt　is　a　bird　of　great　size；　with　long；　massive　beak　and　large　feet；　some　of　the　sub…breeds　of　runts　have　very　long　necks；　others　very　long　wings　and　tails；　others　singularly　short　tails。　The　barb　is　allied　to　the　carrier；　but；　instead　of　a　very　long　beak；　has　a　very　short　and　very　broad　one。　The　pouter　has　a　much　elongated　body；　wings；　and　legs；　and　its　enormously　developed　crop；　which　it　glories　in　inflating；　may　well　excite　astonishment　and　even　laughter。　The　turbit　has　a　very　short　and　conical　beak；　with　a　line　of　reversed　feathers　down　the　breast；　and　it　has　the　habit　of　continually　expanding　slightly　the　upper　part　of　the　oesophagus。　The　Jacobin　has　the　feathers　so　much　reversed　along　the　back　of　the　neck　that　they　form　a　hood；　and　it　has；　proportionally　to　its　size；　much　elongated　wing　and　tail　feathers。　The　trumpeter　and　laugher；　as　their　names　express；　utter　a　very　different　coo　from　the　other　breeds。　The　fantail　has　thirty　or　even　forty　tail…feathers；　instead　of　twelve　or　fourteen；　the　normal　number　in　all　members　of　the　great　pigeon　family；　and　these　feathers　are　kept　expanded；　and　are　carried　so　erect　that　in　good　birds　the　head　and　tail　touch；　the　oil…gland　is　quite　aborted。　Several　other　less　distinct　breeds　might　have　been　specified。　
In　the　skeletons　of　the　several　breeds；　the　development　of　the　bones　of　the　face　in　length　and　breadth　and　curvature　differs　enormously。　The　shape；　as　well　as　the　breadth　and　length　of　the　ramus　of　the　lower　jaw；　varies　in　a　highly　remarkable　manner。　The　number　of　the　caudal　and　sacral　vertebrae　vary；　as　does　the　number　of　the　ribs；　together　with　their　relative　breadth　and　the　presence　of　processes。　The　size　and　shape　of　the　apertures　in　the　sternum　are　highly　variable；　so　is　the　degree　of　divergence　and　relative　size　of　the　two　arms　of　the　furcula。　The　proportional　width　of　the　gape　of　mouth；　the　proportional　length　of　the　eyelids；　of　the　orifice　of　the　nostrils；　of　the　tongue　（not　always　in　strict　correlation　with　the　length　of　beak）；　the　size　of　the　crop　and　of　the　upper　part　of　the　oesophagus；　the　development　and　abortion　of　the　oil…gland；　the　number　of　the　primary　wing　and　caudal　feathers；　the　relative　length　of　wing　and　tail　to　each　other　and　to　the　body；　the　relative　length　of　leg　and　of　the　feet；　the　number　of　scutellae　on　the　toes；　the　development　of　skin　between　the　toes；　are　all　points　of　structure　which　are　variable。　The　period　at　which　the　perfect　plumage　is　acquired　varies；　as　does　the　state　of　the　down　with　which　the　nestling　birds　are　clothed　when　hatched。　The　shape　and　size　of　the　eggs　vary。　The　manner　of　flight　differs　remarkably；　as　does　in　some　breeds　the　voice　and　disposition。　Lastly；　in　certain　breeds；　the　males　and　females　have　come　to　differ　to　a　slight　degree　from　each　other。　
Altogether　at　least　a　score　of　pigeons　might　be　chosen；　which　if　shown　to　an　ornithologist；　and　he　were　told　that　they　were　wild　birds；　would　certainly；　I　think；　be　ranked　by　him　as　well…defined　species。　Moreover；　I　do　not　believe　that　any　ornithologist　would　place　the　English　carrier；　the　short…faced　tumbler；　the　runt；　the　barb；　pouter；　and　fantail　in　the　same　genus；　more　especially　as　in　each　of　these　breeds　several　truly…inherited　sub…breeds；　or　species　as　he　might　have　called　them；　could　be　shown　him。　
Great　as　the　differences　are　between　the　breeds　of　pigeons；　I　am　fully　convinced　that　the　common　opinion　of　naturalists　is　correct；　namely；　that　all　have　descended　from　the　rock…pigeon　（Columba　livia）；　including　under　this　term　several　geographical　races　or　sub…species；　which　differ　from　each　other　in　the　most　trifling　respects。　As　several　of　the　reasons　which　have　led　me　to　this　belief　are　in　some　degree　applicable　in　other　cases；　I　will　here　briefly　give　them。　If　the　several　breeds　are　not　varieties；　and　have　not　proceeded　from　the　rock…pigeon；　they　must　have　descended　from　at　least　seven　or　eight　aboriginal　stocks；　for　it　is　impossible　to　make　the　present　domestic　breeds　by　the　crossing　of　any　lesser　number：　how；　for　instance；　could　a　pouter　be　produced　by　crossing　two　breeds　unless　one　of　the　parent…stocks　possessed　the　characteristic　enormous　crop？　The　supposed　aboriginal　stocks　must　all　have　been　rock…pigeons；　that　is；　not　breeding　or　willingly　perching　on　trees。　But　besides　C。　livia；　with　its　geographical　sub…species；　only　two　or　three　other　species　of　rock…pigeons　are　known；　and　these　have　not　any　of　the　characters　of　the　domestic　breeds。　Hence　the　supposed　aboriginal　stocks　must　either　still　exist　in　the　countries　where　they　were　originally　domesticated；　and　yet　be　unknown　to　ornithologists；　and　this；　considering　their　size；　habits；　and　remarkable　characters；　seems　very　improbable；　or　they　must　have　become　extinct　in　the　wild　state。　But　birds　breeding　on　precipices；　and　good　fliers；　are　unlikely　to　be　exterminated；　and　the　common　rock…pigeon；　which　has　the　same　habits　with　the　domestic　breeds；　has　not　been　exterminated　even　on　several　of　the　smaller　British　islets；　or　on　the　shores　of　the　Mediterranean。　Hence　the　supposed　extermination　of　so　many　species　having　similar　habits　with　the　rock…pigeon　seems　to　me　a　very　rash　assumption。　Moreover；　the　several　above…named　domesticated　breeds　have　been　transported　to　all　parts　of　the　world；　and；　therefore；　some　of　them　must　have　been　carried　back　again　into　their　native　country；　but　not　one　has　ever　become　wild　or　feral；　though　the　dovecot…pigeon；　which　is　the　rock…pigeon　in　a　very　slightly　altered　state；　has　become　feral　in　several　places。　Again；　all　recent　experience　shows　that　it　is　most　difficult　to　get　any　wild　animal　to　breed　freely　under　domestication；　yet　on　the　hypothesis　of　the　multiple　origin　of　our　pigeons；　it　must　be　assumed　that　at　least　seven　or　eight　species　were　so　thoroughly　domesticated　in　ancient　times　by　half…civilized　man；　as　to　be　quite　prolific　under　confinement。　
An　argument；　as　it　seems　to　me；　of　great　weight；　and　applicable　in　several　other　cases；　is；　that　the　above…specified　breeds；　though　agreeing　generally　in　constitution；　habits；　voice；　colouring；　and　in　most　parts　of　their　structure；　with　the　wild　rock…pigeon；　yet　are　certainly　highly　abnormal　in　other　parts　of　their　structure：　we　may　look　in　vain　throughout　the　whole　great　family　of　Columbidae　for　a　beak　like　that　of　the　English　carrier；　or　that　of　the　short…faced　tumbler；　or　barb；　for　reversed　feathers　like　those　of　the　jacobin；　for　a　crop　like　that　of　the　pouter；　for　tail…feathers　like　those　of　the　fantail。　Hence　it　must　be　assumed　not　only　that　half…civilized　man　suc