the origin of species(物种起源)-第3章

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　trifling　respects；　they　often　differ　in　an　extreme　degree　in　some　one　part；　both　when　compared　one　with　another；　and　more　especially　when　compared　with　all　the　species　in　nature　to　which　they　are　nearest　allied。　With　these　exceptions　（and　with　that　of　the　perfect　fertility　of　varieties　when　crossed；　a　subject　hereafter　to　be　discussed）；　domestic　races　of　the　same　species　differ　from　each　other　in　the　same　manner　as；　only　in　most　cases　in　a　lesser　degree　than；　do　closely…allied　species　of　the　same　genus　in　a　state　of　nature。　I　think　this　must　be　admitted；　when　we　find　that　there　are　hardly　any　domestic　races；　either　amongst　animals　or　plants；　which　have　not　been　ranked　by　some　competent　judges　as　mere　varieties；　and　by　other　competent　judges　as　the　descendants　of　aboriginally　distinct　species。　If　any　marked　distinction　existed　between　domestic　races　and　species；　this　source　of　doubt　could　not　so　perpetually　recur。　It　has　often　been　stated　that　domestic　races　do　not　differ　from　each　other　in　characters　of　generic　value。　I　think　it　could　be　shown　that　this　statement　is　hardly　correct；　but　naturalists　differ　most　widely　in　determining　what　characters　are　of　generic　value；　all　such　valuations　being　at　present　empirical。　Moreover；　on　the　view　of　the　origin　of　genera　which　I　shall　presently　give；　we　have　no　right　to　expect　often　to　meet　with　generic　differences　in　our　domesticated　productions。　
When　we　attempt　to　estimate　the　amount　of　structural　difference　between　the　domestic　races　of　the　same　species；　we　are　soon　involved　in　doubt；　from　not　knowing　whether　they　have　descended　from　one　or　several　parent…species。　This　point；　if　could　be　cleared　up；　would　be　interesting；　if；　for　instance；　it　could　be　shown　that　the　greyhound；　bloodhound；　terrier；　spaniel；　and　bull…dog；　which　we　all　know　propagate　their　kind　so　truly；　were　the　offspring　of　any　single　species；　then　such　facts　would　have　great　weight　in　making　us　doubt　about　the　immutability　of　the　many　very　closely　allied　and　natural　species　for　instance；　of　the　many　foxes　inhabiting　different　quarters　of　the　world。　I　do　not　believe；　as　we　shall　presently　see；　that　all　our　dogs　have　descended　from　any　one　wild　species；　but；　in　the　case　of　some　other　domestic　races；　there　is　presumptive；　or　even　strong；　evidence　in　favour　of　this　view。　
It　has　often　been　assumed　that　man　has　chosen　for　domestication　animals　and　plants　having　an　extraordinary　inherent　tendency　to　vary；　and　likewise　to　withstand　diverse　climates。　I　do　not　dispute　that　these　capacities　have　added　largely　to　the　value　of　most　of　our　domesticated　productions；　but　how　could　a　savage　possibly　know；　when　he　first　tamed　an　animal；　whether　it　would　vary　in　succeeding　generations；　and　whether　it　would　endure　other　climates？　Has　the　little　variability　of　the　ass　or　guinea…fowl；　or　the　small　power　of　endurance　of　warmth　by　the　reindeer；　or　of　cold　by　the　common　camel；　prevented　their　domestication？　I　cannot　doubt　that　if　other　animals　and　plants；　equal　in　number　to　our　domesticated　productions；　and　belonging　to　equally　diverse　classes　and　countries；　were　taken　from　a　state　of　nature；　and　could　be　made　to　breed　for　an　equal　number　of　generations　under　domestication；　they　would　vary　on　an　average　as　largely　as　the　parent　species　of　our　existing　domesticated　productions　have　varied。　
In　the　case　of　most　of　our　anciently　domesticated　animals　and　plants；　I　do　not　think　it　is　possible　to　come　to　any　definite　conclusion；　whether　they　have　descended　from　one　or　several　species。　The　argument　mainly　relied　on　by　those　who　believe　in　the　multiple　origin　of　our　domestic　animals　is；　that　we　find　in　the　most　ancient　records；　more　especially　on　the　monuments　of　Egypt；　much　diversity　in　the　breeds；　and　that　some　of　the　breeds　closely　resemble；　perhaps　are　identical　with；　those　still　existing。　Even　if　this　latter　fact　were　found　more　strictly　and　generally　true　than　seems　to　me　to　be　the　case；　what　does　it　show；　but　that　some　of　our　breeds　originated　there；　four　or　five　thousand　years　ago？　But　Mr　Horner's　researches　have　rendered　it　in　some　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