the origin of species(物种起源)-第4部分

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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　improbability　of　man　having　formerly　got　seven　or　eight　supposed　species　of　pigeons　to　breed　freely　under　domestication；　these　supposed　species　being　quite　unknown　in　a　wild　state；　and　their　becoming　nowhere　feral；　these　species　having　very　abnormal　characters　in　certain　respects；　as　compared　with　all　other　Columbidae；　though　so　like　in　most　other　respects　to　the　rock…pigeon；　the　blue　colour　and　various　marks　occasionally　appearing　in　all　the　breeds；　both　when　kept　pure　and　when　crossed；　the　mongrel　offspring　being　perfectly　fertile；　from　these　several　reasons；　taken　together；　I　can　feel　no　doubt　that　all　our　domestic　breeds　have　descended　from　the　Columba　livia　with　its　geographical　sub…species。　
In　favour　of　this　view；　I　may　add；　firstly；　that　C。　livia；　or　the　rock…pigeon；　has　been　found　capable　of　domestication　in　Europe　and　in　India；　and　that　it　agrees　in　habits　and　in　a　great　number　of　points　of　structure　with　all　the　domestic　breeds。　Secondly；　although　an　English　carrier　or　short…faced　tumbler　differs　immensely　in　certain　characters　from　the　rock…pigeon；　yet　by　comparing　the　several　sub…breeds　of　these　breeds；　more　especially　those　brought　from　distant　countries；　we　can　make　an　almost　perfect　series　between　the　extremes　of　structure。　Thirdly；　those　characters　which　are　mainly　distinctive　of　each　breed；　for　instance　the　wattle　and　length　of　beak　of　the　carrier；　the　shortness　of　that　of　the　tumbler；　and　the　number　of　tail…feathers　in　the　fantail；　are　in　each　breed　eminently　variable；　and　the　explanation　of　this　fact　will　be　obvious　when　we　come　to　treat　of　selection。　Fourthly；　pigeons　have　been　watched；　and　tended　with　the　utmost　care；　and　loved　by　many　people。　They　have　been　domesticated　for　thousands　of　years　in　several　quarters　of　the　world；　the　earliest　known　record　of　pigeons　is　in　the　fifth　Aegyptian　dynasty；　about　3000　B。C。；　as　was　pointed　out　to　me　by　Professor　Lepsius；　but　Mr　Birch　informs　me　that　pigeons　are　given　in　a　bill　of　fare　in　the　previous　dynasty。　In　the　time　of　the　Romans；　as　we　hear　from　Pliny；　immense　prices　were　given　for　pigeons；　'nay；　they　are　come　to　this　pass；　that　they　can　reckon　up　their　pedigree　and　race。'　Pigeons　were　much　valued　by　Akber　Khan　in　India；　about　the　year　1600；　never　less　than　20；000　pigeons　were　taken　with　the　court。　'The　monarchs　of　Iran　and　Turan　sent　him　some　very　rare　birds；'　and；　continues　the　courtly　historian；　'His　Majesty　by　crossing　the　breeds；　which　method　was　never　practised　before；　has　improved　them　astonishingly。'　About　this　same　period　the　Dutch　were　as　eager　about　pigeons　as　were　the　old　Romans。　The　paramount　importance　of　these　considerations　in　explaining　the　immense　amount　of　variation　which　pigeons　have　undergone；　will　be　obvious　when　we　treat　of　Selection。　We　shall　then；　also；　see　how　it　is　that　the　breeds　so　often　have　a　somewhat　monstrous　character。　It　is　also　a　most　favourable　circumstance　for　the　production　of　distinct　breeds；　that　male　and　female　pigeons　can　be　easily　mated　for　life；　and　thus　different　breeds　can　be　kept　together　in　the　same　aviary。　
I　have　discussed　the　probable　origin　of　domestic　pigeons　at　some；　yet　quite　insufficient；　length；　because　when　I　first　kept　pigeons　and　watched　the　several　kinds；　knowing　well　how　true　they　bred；　I　felt　fully　as　much　difficulty　in　believing　that　they　could　ever　have　descended　from　a　common　parent；　as　any　naturalist　could　in　coming　to　a　similar　conclusion　in　regard　to　the　many　species　of　finches；　or　other　large　groups　of　birds；　in　nature。　One　circumstance　has　struck　me　much；　namely；　that　all　the　breeders　of　the　various　domestic　animals　and　the　cultivators　of　plants；　with　whom　I　have　ever　conversed；　or　whose　treatises　I　have　read；　are　firmly　convinced　that　the　several　breeds　to　which　each　has　attended；　are　descended　from　so　many　aboriginally　distinct　species。　Ask；　as　I　have　asked；　a　celebrated　raiser　of　Hereford　cattle；　whether　his　cattle　might　not　have　descended　from　long　horns；　and　he　will　laugh　you　to　scorn。　I　have　never　met　a　pigeon；　or　poultry；　or　duck；　or　rabbit　fancier；　who　was　not　fully　convinced　that　each　main　breed　was　descended　from　a　distinct　species。　Van　Mons；　in　his　tre