the origin of species(物种起源)-第4节

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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　succeeded　in　thoroughly　domesticating　several　species；　but　that　he　intentionally　or　by　chance　picked　out　extraordinarily　abnormal　species；　and　further；　that　these　very　species　have　since　all　become　extinct　or　unknown。　So　many　strange　contingencies　seem　to　me　improbable　in　the　highest　degree。　
Some　facts　in　regard　to　the　colouring　of　pigeons　well　deserve　consideration。　The　rock…pigeon　is　of　a　slaty…blue；　and　has　a　white　rump　（the　Indian　sub…species；　C。　intermedia　of　Strickland；　having　it　bluish）；　the　tail　has　a　terminal　dark　bar；　with　the　bases　of　the　outer　feathers　externally　edged　with　white；　the　wings　have　two　black　bars：　some　semi…domestic　breeds　and　some　apparently　truly　wild　breeds　have；　besides　the　two　black　bars；　the　wings　chequered　with　black。　These　several　marks　do　not　occur　together　in　any　other　species　of　the　whole　family。　Now；　in　every　one　of　the　domestic　breeds；　taking　thoroughly　well…bred　birds；　all　the　above　marks；　even　to　the　white　edging　of　the　outer　tail…feathers；　sometimes　concur　perfectly　developed。　Moreover；　when　two　birds　belonging　to　two　distinct　breeds　are　crossed；　neither　of　which　is　blue　or　has　any　of　the　above…specified　marks；　the　mongrel　offspring　are　very　apt　suddenly　to　acquire　these　characters；　for　instance；　I　crossed　some　uniformly　white　fantails　with　some　uniformly　black　barbs；　and　they　produced　mottled　brown　and　black　birds；　these　I　again　crossed　together；　and　one　grandchild　of　the　pure　white　fantail　and　pure　black　barb　was　of　as　beautiful　a　blue　colour；　with　the　white　rump；　double　black　wing…bar；　and　barred　and　white…edged　tail…feathers；　as　any　wild　rock…pigeon！　We　can　understand　these　facts；　on　the　well…known　principle　of　reversion　to　ancestral　characters；　if　all　the　domestic　breeds　have　descended　from　the　rock…pigeon。　But　if　we　deny　this；　we　must　make　one　of　the　two　following　highly　improbable　suppositions。　Either；　firstly；　that　all　the　several　imagined　aboriginal　stocks　were　coloured　and　marked　like　the　rock…pigeon；　although　no　other　existing　species　is　thus　coloured　and　marked；　so　that　in　each　separate　breed　there　might　be　a　tendency　to　revert　to　the　very　same　colours　and　markings。　Or；　secondly；　that　each　breed；　even　the　purest；　has　within　a　dozen　or；　at　most；　within　a　score　of　generations；　been　crossed　by　the　rock…pigeon：　I　say　within　a　dozen　or　twenty　generations；　for　we　know　of　no　fact　countenancing　the　belief　that　the　child　ever　reverts　to　some　one　ancestor；　removed　by　a　greater　number　of　generations。　In　a　breed　which　has　been　crossed　only　once　with　some　distinct　breed；　the　tendency　to　reversion　to　any　character　derived　from　such　cross　will　naturally　become　less　and　less；　as　in　each　succeeding　generation　there　will　be　less　of　the　foreign　blood；　but　when　there　has　been　no　cross　with　a　distinct　breed；　and　there　is　a　tendency　in　both　parents　to　revert　to　a　character；　which　has　been　lost　during　some　former　generation；　this　tendency；　for　all　that　we　can　see　to　the　contrary；　may　be　transmitted　undiminished　for　an　indefinite　number　of　generations。　These　two　distinct　cases　are　often　confounded　in　treatises　on　inheritance。　
Lastly；　the　hybrids　or　mongrels　from　between　all　the　domestic　breeds　of　pigeons　are　perfectly　fertile。　I　can　state　this　from　my　own　observations；　purposely　made　on　the　most　distinct　breeds。　Now；　it　is　difficult；　perhaps　impossible；　to　bring　forward　one　case　of　the　hybrid　offspring　of　two　animals　clearly　distinct　being　themselves　perfectly　fertile。　Some　authors　believe　that　long…continued　domestication　eliminates　this　strong　tendency　to　sterility：　from　the　history　of　the　dog　I　think　there　is　some　probability　in　this　hypothesis；　if　applied　to　species　closely　related　together；　though　it　is　unsupported　by　a　single　experiment。　But　to　extend　the　hypothesis　so　far　as　to　suppose　that　species；　aboriginally　as　distinct　as　carriers；　tumblers；　pouters；　and　fantails　now　are；　should　yield　offspring　perfectly　fertile；　inter　se；　seems　to　me　rash　in　the　extreme。　
From　these　several　reasons；　namely；　the