the origin of species(物种起源)-第3部分

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e　degree　probable　that　man　sufficiently　civilized　to　have　manufactured　pottery　existed　in　the　valley　of　the　Nile　thirteen　or　fourteen　thousand　years　ago；　and　who　will　pretend　to　say　how　long　before　these　ancient　periods；　savages；　like　those　of　Tierra　del　Fuego　or　Australia；　who　possess　a　semi…domestic　dog；　may　not　have　existed　in　Egypt？　
The　whole　subject　must；　I　think；　remain　vague；　nevertheless；　I　may；　without　here　entering　on　any　details；　state　that；　from　geographical　and　other　considerations；　I　think　it　highly　probable　that　our　domestic　dogs　have　descended　from　several　wild　species。　In　regard　to　sheep　and　goats　I　can　form　no　opinion。　I　should　think；　from　facts　communicated　to　me　by　Mr　Blyth；　on　the　habits；　voice；　and　constitution；　&c。；　of　the　humped　Indian　cattle；　that　these　had　descended　from　a　different　aboriginal　stock　from　our　European　cattle；　and　several　competent　judges　believe　that　these　latter　have　had　more　than　one　wild　parent。　With　respect　to　horses；　from　reasons　which　I　cannot　give　here；　I　am　doubtfully　inclined　to　believe；　in　opposition　to　several　authors；　that　all　the　races　have　descended　from　one　wild　stock。　Mr　Blyth；　whose　opinion；　from　his　large　and　varied　stores　of　knowledge；　I　should　value　more　than　that　of　almost　any　one；　thinks　that　all　the　breeds　of　poultry　have　proceeded　from　the　common　wild　Indian　fowl　（Gallus　bankiva）。　In　regard　to　ducks　and　rabbits；　the　breeds　of　which　differ　considerably　from　each　other　in　structure；　I　do　not　doubt　that　they　all　have　descended　from　the　common　wild　duck　and　rabbit。　
The　doctrine　of　the　origin　of　our　several　domestic　races　from　several　aboriginal　stocks；　has　been　carried　to　an　absurd　extreme　by　some　authors。　They　believe　that　every　race　which　breeds　true；　let　the　distinctive　characters　be　ever　so　slight；　has　had　its　wild　prototype。　At　this　rate　there　must　have　existed　at　least　a　score　of　species　of　wild　cattle；　as　many　sheep；　and　several　goats　in　Europe　alone；　and　several　even　within　Great　Britain。　One　author　believes　that　there　formerly　existed　in　Great　Britain　eleven　wild　species　of　sheep　peculiar　to　it！　When　we　bear　in　mind　that　Britain　has　now　hardly　one　peculiar　mammal；　and　France　but　few　distinct　from　those　of　Germany　and　conversely；　and　so　with　Hungary；　Spain；　&c。；　but　that　each　of　these　kingdoms　possesses　several　peculiar　breeds　of　cattle；　sheep；　&c。；　we　must　admit　that　many　domestic　breeds　have　originated　in　Europe；　for　whence　could　they　have　been　derived；　as　these　several　countries　do　not　possess　a　number　of　peculiar　species　as　distinct　parent…stocks？　So　it　is　in　India。　Even　in　the　case　of　the　domestic　dogs　of　the　whole　world；　which　I　fully　admit　have　probably　descended　from　several　wild　species；　I　cannot　doubt　that　there　has　been　an　immense　amount　of　inherited　variation。　Who　can　believe　that　animals　closely　resembling　the　Italian　greyhound；　the　bloodhound；　the　bull…dog；　or　Blenheim　spaniel；　&c。　so　unlike　all　wild　Canidae　ever　existed　freely　in　a　state　of　nature？　It　has　often　been　loosely　said　that　all　our　races　of　dogs　have　been　produced　by　the　crossing　of　a　few　aboriginal　species；　but　by　crossing　we　can　get　only　forms　in　some　degree　intermediate　between　their　parents；　and　if　we　account　for　our　several　domestic　races　by　this　process；　we　must　admit　the　former　existence　of　the　most　extreme　forms；　as　the　Italian　greyhound；　bloodhound；　bull…dog；　&c。；　in　the　wild　state。　Moreover；　the　possibility　of　making　distinct　races　by　crossing　has　been　greatly　exaggerated。　There　can　be　no　doubt　that　a　race　may　be　modified　by　occasional　crosses；　if　aided　by　the　careful　selection　of　those　individual　mongrels；　which　present　any　desired　character；　but　that　a　race　could　be　obtained　nearly　intermediate　between　two　extremely　different　races　or　species；　I　can　hardly　believe。　Sir　J。　Sebright　expressly　experimentised　for　this　object；　and　failed。　The　offspring　from　the　first　cross　between　two　pure　breeds　is　tolerably　and　sometimes　（as　I　have　found　with　pigeons）　extremely　uniform；　and　everything　seems　simple　enough；　but　when　these　mongrels　are　crossed　one　with　another　for　several　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　s