the origin of species(物种起源)-第6部分

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selection；　but　they　show　that　the　breeding　of　domestic　animals　was　carefully　attended　to　in　ancient　times；　and　is　now　attended　to　by　the　lowest　savages。　It　would；　indeed；　have　been　a　strange　fact；　had　attention　not　been　paid　to　breeding；　for　the　inheritance　of　good　and　bad　qualities　is　so　obvious。　
At　the　present　time；　eminent　breeders　try　by　methodical　selection；　with　a　distinct　object　in　view；　to　make　a　new　strain　or　sub…breed；　superior　to　anything　existing　in　the　country。　But；　for　our　purpose；　a　kind　of　Selection；　which　may　be　called　Unconscious；　and　which　results　from　every　one　trying　to　possess　and　breed　from　the　best　individual　animals；　is　more　important。　Thus；　a　man　who　intends　keeping　pointers　naturally　tries　to　get　as　good　dogs　as　he　can；　and　afterwards　breeds　from　his　own　best　dogs；　but　he　has　no　wish　or　expectation　of　permanently　altering　the　breed。　Nevertheless　I　cannot　doubt　that　this　process；　continued　during　centuries；　would　improve　and　modify　any　breed；　in　the　same　way　as　Bakewell；　Collins；　&c。；　by　this　very　same　process；　only　carried　on　more　methodically；　did　greatly　modify；　even　during　their　own　lifetimes；　the　forms　and　qualities　of　their　cattle。　Slow　and　insensible　changes　of　this　kind　could　never　be　recognised　unless　actual　measurements　or　careful　drawings　of　the　breeds　in　question　had　been　made　long　ago；　which　might　serve　for　comparison。　In　some　cases；　however；　unchanged　or　but　little　changed　individuals　of　the　same　breed　may　be　found　in　less　civilised　districts；　where　the　breed　has　been　less　improved。　There　is　reason　to　believe　that　King　Charles's　spaniel　has　been　unconsciously　modified　to　a　large　extent　since　the　time　of　that　monarch。　Some　highly　competent　authorities　are　convinced　that　the　setter　is　directly　derived　from　the　spaniel；　and　has　probably　been　slowly　altered　from　it。　It　is　known　that　the　English　pointer　has　been　greatly　changed　within　the　last　century；　and　in　this　case　the　change　has；　it　is　believed；　been　chiefly　effected　by　crosses　with　the　fox…hound；　but　what　concerns　us　is；　that　the　change　has　been　effected　unconsciously　and　gradually；　and　yet　so　effectually；　that；　though　the　old　Spanish　pointer　certainly　came　from　Spain；　Mr　Barrow　has　not　seen；　as　I　am　informed　by　him；　any　native　dog　in　Spain　like　our　pointer。　
By　a　similar　process　of　selection；　and　by　careful　training；　the　whole　body　of　English　racehorses　have　come　to　surpass　in　fleetness　and　size　the　parent　Arab　stock；　so　that　the　latter；　by　the　regulations　for　the　Goodwood　Races；　are　favoured　in　the　weights　they　carry。　Lord　Spencer　and　others　have　shown　how　the　cattle　of　England　have　increased　in　weight　and　in　early　maturity；　compared　with　the　stock　formerly　kept　in　this　country。　By　comparing　the　accounts　given　in　old　pigeon　treatises　of　carriers　and　tumblers　with　these　breeds　as　now　existing　in　Britain；　India；　and　Persia；　we　can；　I　think；　clearly　trace　the　stages　through　which　they　have　insensibly　passed；　and　come　to　differ　so　greatly　from　the　rock…pigeon。　
Youatt　gives　an　excellent　illustration　of　the　effects　of　a　course　of　selection；　which　may　be　considered　as　unconsciously　followed；　in　so　far　that　the　breeders　could　never　have　expected　or　even　have　wished　to　have　produced　the　result　which　ensued　namely；　the　production　of　two　distinct　strains。　The　two　flocks　of　Leicester　sheep　kept　by　Mr　Buckley　and　Mr　Burgess；　as　Mr　Youatt　remarks；　'have　been　purely　bred　from　the　original　stock　of　Mr　Bakewell　for　upwards　of　fifty　years。　There　is　not　a　suspicion　existing　in　the　mind　of　any　one　at　all　acquainted　with　the　subject　that　the　owner　of　either　of　them　has　deviated　in　any　one　instance　from　the　pure　blood　of　Mr　Bakewell's　flock；　and　yet　the　difference　between　the　sheep　possessed　by　these　two　gentlemen　is　so　great　that　they　have　the　appearance　of　being　quite　different　varieties。'　
If　there　exist　savages　so　barbarous　as　never　to　think　of　the　inherited　character　of　the　offspring　of　their　domestic　animals；　yet　any　one　animal　particularly　useful　to　them；　for　any　special　purpose；　would　be　carefully　preserved　during　famines　and　other　accidents；　to　which　savages　are　so　liable；　and　such　choice　animals　would　thus　generally　leave　more　offspring　than　the　inferior　ones；　so　that　in　this　case　there　would　be　a　kind　of　unconscious　selection　going　on。　We　see　the　value　set　on　animals　even　by　the　barbarians　of　Tierra　del　Fuego；　by　their　killing　and　devouring　their　old　women；　in　times　of　dearth；　as　of　less　value　than　their　dogs。　
In　plants　the　same　gradual　process　of　improvement；　through　the　occasional　preservation　of　the　best　individuals；　whether　or　not　sufficiently　distinct　to　be　ranked　at　their　first　appearance　as　distinct　varieties；　and　whether　or　not　two　or　more　species　or　races　have　become　blended　together　by　crossing；　may　plainly　be　recognised　in　the　increased　size　and　beauty　which　we　now　see　in　the　varieties　of　the　heartsease；　rose；　pelargonium；　dahlia；　and　other　plants；　when　compared　with　the　older　varieties　or　with　their　parent…stocks。　No　one　would　ever　expect　to　get　a　first…rate　heartsease　or　dahlia　from　the　seed　of　a　wild　plant。　No　one　would　expect　to　raise　a　first…rate　melting　pear　from　the　seed　of　a　wild　pear；　though　he　might　succeed　from　a　poor　seedling　growing　wild；　if　it　had　come　from　a　garden…stock。　The　pear；　though　cultivated　in　classical　times；　appears；　from　Pliny's　description；　to　have　been　a　fruit　of　very　inferior　quality。　I　have　seen　great　surprise　expressed　in　horticultural　works　at　the　wonderful　skill　of　gardeners；　in　having　produced　such　splendid　results　from　such　poor　materials；　but　the　art；　I　cannot　doubt；　has　been　simple；　and；　as　far　as　the　final　result　is　concerned；　has　been　followed　almost　unconsciously。　It　has　consisted　in　always　cultivating　the　best　known　variety；　sowing　its　seeds；　and；　when　a　slightly　better　variety　has　chanced　to　appear；　selecting　it；　and　so　onwards。　But　the　gardeners　of　the　classical　period；　who　cultivated　the　best　pear　they　could　procure；　never　thought　what　splendid　fruit　we　should　eat；　though　we　owe　our　excellent　fruit；　in　some　small　degree；　to　their　having　naturally　chosen　and　preserved　the　best　varieties　they　could　anywhere　find。　
A　large　amount　of　change　in　our　cultivated　plants；　thus　slowly　and　unconsciously　accumulated；　explains；　as　I　believe；　the　well…known　fact；　that　in　a　vast　number　of　cases　we　cannot　recognise；　and　therefore　do　not　know；　the　wild　parent…stocks　of　the　plants　which　have　been　longest　cultivated　in　our　flower　and　kitchen　gardens。　If　it　has　taken　centuries　or　thousands　of　years　to　improve　or　modify　most　of　our　plants　up　to　their　present　standard　of　usefulness　to　man；　we　can　understand　how　it　is　that　neither　Australia；　the　Cape　of　Good　Hope；　nor　any　other　region　inhabited　by　quite　uncivilised　man；　has　afforded　us　a　single　plant　worth　culture。　It　is　not　that　these　countries；　so　rich　in　species；　do　not　by　a　strange　chance　possess　the　aboriginal　stocks　of　any　useful　plants；　but　that　the　native　plants　have　not　been　improved　by　continued　selection　up　to　a　standard　of　perfection　comparable　with　that　given　to　the　plants　in　countries　anciently　civilised。　
In　regard　to　the　domestic　animals　kept　by　uncivilised　man；　it　should　not　be　overlooked　that　they　almost　always　have　to　struggle　for　their　own　food；　at　least　during　certain　seasons。　And　in　two　countries　very　differently　circumstanced；　individuals　of　the　same　species；　having　slightly　different　constitutions　or　structure；　would　often　succeed　better　in　the　one　country　than　in　the　other；　and　thus　by　a　process　of　'natural　selection；'　as　will　hereafter　be　more　fully　explained；　two　sub…breeds　might　be　formed。　This；　perhaps；　partly　explains　what　has　been　remarked　by　some　authors；　namely；　that　the　varieties　kept　by　savages　have　more　of　the　character　of　species　than　the　varieties　kept　in　civilised　countries。　
On　the　view　here　given　of　the　all…important　part　which　selection　by　man　has　played；　it　becomes　at　once　obvious；　how　it　is　that　our　domestic　races　show　adaptation　in　their　structure　or　in　their　habits　to　man's　wants　or　fancies。　We　can；　I　think；　further　understand　the　frequently　abnormal　character　of　our　domestic　races；　and　likewise　their　differences　being　so　great　in　external　characters　and　relatively　so　slight　in　internal　parts　or　organs。　Man　can　hardly　select；　or　only　with　much　difficulty；　any　deviation　of　structure　excepting　such　as　is　externally　visible；　and　indeed　he　rarely　cares　for　what　is　internal。　He　can　never　act　by　selection；　excepting　on　variations　which　are　first　given　to　him　in　some　slight　degree　by　nature。　No　man　would　ever　try　to　make　a　fantail；　till　he　saw　a　pigeon　with　a　tail　developed　in　some　slight　degree　in　an　unusual　manner；　or　a　pouter　till　he　saw　a　pigeon　with　a　crop　of　somewhat　unusual　size；　and　the　more　abnormal　or　unusual　any　character　was　when　it　first　appeared；　the　more　likely　it　would　be　to　catch　his　attention。　But　to　use　such　an　expression　as　trying　to　make　a　fantail；　is；　I　have　no　doubt；　in　most　cases；　utterly　incorrect。　The　man　who　first　selected　a　pigeon　with　a　slightly　larger　tail；　never　dreamed　what　the　descendants　of　that　pigeon　would　become　through　long…continued；　partly　unconscious　and　partly　methodical　selection。　Perhaps　the　parent　bird　of　all　fantails　had　only　fourteen　tail…feathers　somewhat　expanded；　like　the　present　Java　fantail；　or　like　individuals　of　other　and　distinct　breeds；　in　which　as　many　as　seventeen　tail…feathers　have　been　counted。　Perhaps　the　first　pouter…pigeon　did　not　inflate　its　crop　much　more　than　the　turbit　now　does　the　upper　part　of　its　oesophagus；　a　habit　which　is　disregarded　by　all　fanciers；　as　it　is　not　one　of　the　points　of　the　breed。　
Nor　let　it　be　thought　that　some　great　deviation　of　structure　would　be　necessary　to　catch　the　fancier's　eye：　he　perceives　extremely　small　differences；　and　it　is　in　human　nature　to　value　any　novelty；　however　slight；　in　one's　own　possession。　Nor　must　the　value　which　would　formerly　be　set　on　any　slight　differences　in　the　individuals　of　the　same　species；　be　judged　of　by　the　value　which　would　now　be　set　on　them；　after　several　breeds　have　once　fairly　been　established。　Many　slight　differences　might；　and　indeed　do　now；　arise　amongst　pigeons；　which　are　rejected　as　faults　or　deviations　from　the　standard　of　perfection　of　each　breed。　The　common　goose　has　not　given　rise　to　any　mar