the origin of species(物种起源)-第2章

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and　commoner　deviations　may　be　freely　admitted　to　be　inheritable。　Perhaps　the　correct　way　of　viewing　the　whole　subject；　would　be；　to　look　at　the　inheritance　of　every　character　whatever　as　the　rule；　and　non…inheritance　as　the　anomaly。　
The　laws　governing　inheritance　are　quite　unknown；　no　one　can　say　why　the　same　peculiarity　in　different　individuals　of　the　same　species；　and　in　individuals　of　different　species；　is　sometimes　inherited　and　sometimes　not　so；　why　the　child　often　reverts　in　certain　characters　to　its　grandfather　or　grandmother　or　other　much　more　remote　ancestor；　why　a　peculiarity　is　often　transmitted　from　one　sex　to　both　sexes　or　to　one　sex　alone；　more　commonly　but　not　exclusively　to　the　like　sex。　It　is　a　fact　of　some　little　importance　to　us；　that　peculiarities　appearing　in　the　males　of　our　domestic　breeds　are　often　transmitted　either　exclusively；　or　in　a　much　greater　degree；　to　males　alone。　A　much　more　important　rule；　which　I　think　may　be　trusted；　is　that；　at　whatever　period　of　life　a　peculiarity　first　appears；　it　tends　to　appear　in　the　offspring　at　a　corresponding　age；　though　sometimes　earlier。　In　many　cases　this　could　not　be　otherwise：　thus　the　inherited　peculiarities　in　the　horns　of　cattle　could　appear　only　in　the　offspring　when　nearly　mature；　peculiarities　in　the　silkworm　are　known　to　appear　at　the　corresponding　caterpillar　or　cocoon　stage。　But　hereditary　diseases　and　some　other　facts　make　me　believe　that　the　rule　has　a　wider　extension；　and　that　when　there　is　no　apparent　reason　why　a　peculiarity　should　appear　at　any　particular　age；　yet　that　it　does　tend　to　appear　in　the　offspring　at　the　same　period　at　which　it　first　appeared　in　the　parent。　I　believe　this　rule　to　be　of　the　highest　importance　in　explaining　the　laws　of　embryology。　These　remarks　are　of　course　confined　to　the　first　appearance　of　the　peculiarity；　and　not　to　its　primary　cause；　which　may　have　acted　on　the　ovules　or　male　element；　in　nearly　the　same　manner　as　in　the　crossed　offspring　from　a　short…horned　cow　by　a　long…horned　bull；　the　greater　length　of　horn；　though　appearing　late　in　life；　is　clearly　due　to　the　male　element。　
Having　alluded　to　the　subject　of　reversion；　I　may　here　refer　to　a　statement　often　made　by　naturalists　namely；　that　our　domestic　varieties；　when　run　wild；　gradually　but　certainly　revert　in　character　to　their　aboriginal　stocks。　Hence　it　has　been　argued　that　no　deductions　can　be　drawn　from　domestic　races　to　species　in　a　state　of　nature。　I　have　in　vain　endeavoured　to　discover　on　what　decisive　facts　the　above　statement　has　so　often　and　so　boldly　been　made。　There　would　be　great　difficulty　in　proving　its　truth：　we　may　safely　conclude　that　very　many　of　the　most　strongly…marked　domestic　varieties　could　not　possibly　live　in　a　wild　state。　In　many　cases　we　do　not　know　what　the　aboriginal　stock　was；　and　so　could　not　tell　whether　or　not　nearly　perfect　reversion　had　ensued。　It　would　be　quite　necessary；　in　order　to　prevent　the　effects　of　intercrossing；　that　only　a　single　variety　should　be　turned　loose　in　its　new　home。　Nevertheless；　as　our　varieties　certainly　do　occasionally　revert　in　some　of　their　characters　to　ancestral　forms；　it　seems　to　me　not　improbable；　that　if　we　could　succeed　in　naturalising；　or　were　to　cultivate；　during　many　generations；　the　several　races；　for　instance；　of　the　cabbage；　in　very　poor　soil　（in　which　case；　however；　some　effect　would　have　to　be　attributed　to　the　direct　action　of　the　poor　soil）；　that　they　would　to　a　large　extent；　or　even　wholly；　revert　to　the　wild　aboriginal　stock。　Whether　or　not　the　experiment　would　succeed；　is　not　of　great　importance　for　our　line　of　argument；　for　by　the　experiment　itself　the　conditions　of　life　are　changed。　If　it　could　be　shown　that　our　domestic　varieties　manifested　a　strong　tendency　to　reversion；　that　is；　to　lose　their　acquired　characters；　whilst　kept　under　unchanged　conditions；　and　whilst　kept　in　a　considerable　body；　so　that　free　intercrossing　might　check；　by　blending　together；　any　slight　deviations　of　structure；　in　such　case；　I　grant　that　we　could　deduce　nothing　from　domestic　varieties　in　regard　to　species。　But　there　is　not　a　shadow　of　evidence　in　favour　of　this　view：　to　assert　that　we　could　not　breed　our　cart　and　race…horses；　long　and　short…horned　cattle　and　poultry　of　various　breeds；　and　esculent　vegetables；　for　an　almost　infinite　number　of　generations；　would　be　opposed　to　all　experience。　I　may　add；　that　when　under　nature　the　conditions　of　life　do　change；　variations　and　reversions　of　character　probably　do　occur；　but　natural　selection；　as　will　hereafter　be　explained；　will　determine　how　far　the　new　characters　thus　arising　shall　be　preserved。　
When　we　look　to　the　hereditary　varieties　or　races　of　our　domestic　animals　and　plants；　and　compare　them　with　species　closely　allied　together；　we　generally　perceive　in　each　domestic　race；　as　already　remarked；　less　uniformity　of　character　than　in　true　species。　Domestic　races　of　the　same　species；　also；　often　have　a　somewhat　monstrous　character；　by　which　I　mean；　that；　although　differing　from　each　other；　and　from　the　other　species　of　the　same　genus；　in　several　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