the origin of species(物种起源)-第5节

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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　treatise　on　pears　and　apples；　shows　how　utterly　he　disbelieves　that　the　several　sorts；　for　instance　a　Ribston…pippin　or　Codlin…apple；　could　ever　have　proceeded　from　the　seeds　of　the　same　tree。　Innumerable　other　examples　could　be　given。　The　explanation；　I　think；　is　simple：　from　long…continued　study　they　are　strongly　impressed　with　the　differences　between　the　several　races；　and　though　they　well　know　that　each　race　varies　slightly；　for　they　win　their　prizes　by　selecting　such　slight　differences；　yet　they　ignore　all　general　arguments；　and　refuse　to　sum　up　in　their　minds　slight　differences　accumulated　during　many　successive　generations。　May　not　those　naturalists　who；　knowing　far　less　of　the　laws　of　inheritance　than　does　the　breeder；　and　knowing　no　more　than　he　does　of　the　intermediate　links　in　the　long　lines　of　descent；　yet　admit　that　many　of　our　domestic　races　have　descended　from　the　same　parents　may　they　not　learn　a　lesson　of　caution；　when　they　deride　the　idea　of　species　in　a　state　of　nature　being　lineal　descendants　of　other　species？　
Selection
Let　us　now　briefly　consider　the　steps　by　which　domestic　races　have　been　produced；　either　from　one　or　from　several　allied　species。　Some　little　effect　may；　perhaps；　be　attributed　to　the　direct　action　of　the　external　conditions　of　life；　and　some　little　to　habit；　but　he　would　be　a　bold　man　who　would　account　by　such　agencies　for　the　differences　of　a　dray　and　race　horse；　a　greyhound　and　bloodhound；　a　carrier　and　tumbler　pigeon。　One　of　the　most　remarkable　features　in　our　domesticated　races　is　that　we　see　in　them　adaptation；　not　indeed　to　the　animal's　or　plant's　own　good；　but　to　man's　use　or　fancy。　Some　variations　useful　to　him　have　probably　arisen　suddenly；　or　by　one　step；　many　botanists；　for　instance；　believe　that　the　fuller's　teazle；　with　its　hooks；　which　cannot　be　rivalled　by　any　mechanical　contrivance；　is　only　a　variety　of　the　wild　Dipsacus；　and　this　amount　of　change　may　have　suddenly　arisen　in　a　seedling。　So　it　has　probably　been　with　the　turnspit　dog；　and　this　is　known　to　have　been　the　case　with　the　ancon　sheep。　But　when　we　compare　the　dray…horse　and　race…horse；　the　dromedary　and　camel；　the　various　breeds　of　sheep　fitted　either　for　cultivated　land　or　mountain　pasture；　with　the　wool　of　one　breed　good　for　one　purpose；　and　that　of　another　breed　for　another　purpose；　when　we　compare　the　many　breeds　of　dogs；　each　good　for　man　in　very　different　ways；　when　we　compare　the　gamecock；　so　pertinacious　in　battle；　with　other　breeds　so　little　quarrelsome；　with　'everlasting　layers'　which　never　desire　to　sit；　and　with　the　bantam　so　small　and　elegant；　when　we　compare　the　host　of　agricultural；　culinary；　orchard；　and　flower…garden　races　of　plants；　most　useful　to　man　at　different　seasons　and　for　different　purposes；　or　so　beautiful　in　his　eyes；　we　must；　I　think；　look　further　than　to　mere　variability。　We　cannot　suppose　that　all　the　breeds　were　suddenly　produced　as　perfect　and　as　useful　as　we　now　see　them；　indeed；　in　several　cases；　we　know　that　this　has　not　been　their　history。　The　key　is　man's　power　of　accumulative　selection：　nature　gives　successive　variations；　man　adds　them　up　in　certain　directions　useful　to　him。　In　this　sense　he　may　be　said　to　make　for　himself　useful　breeds。　
The　great　power　of　this　principle　of　selection　is　not　hypothetical。　It　is　certain　that　several　of　our　eminent　breeders　have；　even　within　a　single　lifetime；　modified　to　a　large　extent　some　breeds　of　cattle　and　sheep。　In　order　fully　to　realise　what　they　have　done；　it　is　almost　necessary　to　read　several　of　the　many　treatises　devoted　to　this　subject；　and　to　inspect　the　animals。　Breeders　habitually　speak　of　an　animal's　organisation　as　something　quite　plastic；　which　they　can　model　almost　as　they　please。　If　I　had　space　I　could　quote　numerous　passages　to　this　effect　from　highly　competent　authorities。　Youatt；　who　was　probably　better　acquainted　with　the　works　of　agriculturalists　than　almost　any　other　individual；　and　who　was　himself　a　very　good　judge　of　an　animal；　speaks　of　the　principle　of　selection　as　'that　which　enables　the　agriculturist；　not　only　to　modify　the　character　of　his　flock；　but　to　change　it　altogether。　It　is　the　magician's　wand；　by　means　of　which　he　may　summon　into　life　whatever　form　and　mould　he　pleases。'　Lord　Somerville；　speaking　of　what　breeders　have　done　for　sheep；　says：　'It　would　seem　as　if　they　had　chalked　out　upon　a　wall　a　form　perfect　in　itself；　and　then　had　given　it　existence。'　That　most　skilful　breeder；　Sir　John　Sebright；　used　to　say；　with　respect　to　pigeons；　that　'he　would　produce　any　given　feather　in　three　years；　but　it　would　take　him　six　years　to　obtain　head　and　beak。'　In　Saxony　the　importance　of　the　principle　of　selection　in　regard　to　merino　sheep　is　so　fully　recognised；　that　men　follow　it　as　a　trade：　the　sheep　are　placed　on　a　table　and　are　studied；　like　a　picture　by　a　connoisseur；　this　is　done　three　times　at　intervals　of　months；　and　the　sheep　are　each　time　marked　and　classed；　so　that　the　very　best　may　ultimately　be　selected　for　breeding。　
What　English　breeders　have　actually　effected　is　proved　by　the