the origin of species(物种起源)-第7节

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dents；　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　marked　varieties；　hence　the　Thoulouse　and　the　common　breed；　which　differ　only　in　colour；　that　most　fleeting　of　characters；　have　lately　been　exhibited　as　distinct　at　our　poultry…shows。　
I　think　these　views　further　explain　what　has　sometimes　been　noticed　namely　that　we　know　nothing　about　the　origin　or　history　of　any　of　our　domestic　breeds。　But；　in　fact；　a　breed；　like　a　dialect　of　a　language；　can　hardly　be　said　to　have　had　a　definite　origin。　A　man　preserves　and　breeds　from　an　individual　with　some　slight　deviation　of　structure；　or　takes　more　care　than　usual　in　matching　his　best　animals　and　thus　improves　them；　and　the　improved　individuals　slowly　spread　in　the　immediate　neighbourhood。　But　as　yet　they　will　hardly　have　a　distinct　name；　and　from　being　only　slightly　valued；　their　history　will　be　disregarded。　When　further　improved　by　the　same　slow　and　gradual　process；　they　will　spread　more　widely；　and　will　get　recognised　as　something　distinct　and　valuable；　and　will　then　probably　first　receive　a　provincial　name。　In　semi…civilised　countries；　with　little　free　communication；　the　spreading　and　knowledge　of　any　new　sub…breed　will　be　a　slow　process。　As　soon　as　the　points　of　value　of　the　new　sub…breed　are　once　fully　acknowledged；　the　principle；　as　I　have　called　it；　of　unconscious　selection　will　always　tend；　perhaps　more　at　one　period　than　at　another；　as　the　breed　rises　or　falls　in　fashion；　perhaps　more　in　one　district　than　in　another；　according　to　the　state　of　civilisation　of　the　inhabitants　slowly　to　add　to　the　characteristic　features　of　the　breed；　whatever　they　may　be。　But　the　chance　will　be　infinitely　small　of　any　record　having　been　preserved　of　such　slow；　varying；　and　insensible　changes。　
I　must　now　say　a　few　words　on　the　circumstances；　favourable；　or　the　reverse；　to　man's　power　of　selection。　A　high　degree　of　variability　is　obviously　favourable；　as　freely　giving　the　materials　for　selection　to　work　on；　not　that　mere　individual　differences　are　not　amply　sufficient；　with　extreme　care；　to　allow　of　the　accumulation　of　a　large　amount　of　modification　in　almost　any　desired　direction。　But　as　variations　manifestly　useful　or　pleasing　to　man　appear　only　occasionally；　the　chance　of　their　appearance　will　be　much　increased　by　a　large　number　of　individuals　being　kept；　and　hence　this　comes　to　be　of　the　highest　importance　to　success。　On　this　principle　Marshall　has　remarked；　with　respect　to　the　sheep　of　parts　of　Yorkshire；　that　'as　they　generally　belong　to　poor　people；　and　are　mostly　in　small　lots；　they　never　can　be　improved。'　On　the　other　hand；　nurserymen；　from　raising　large　stocks　of　the　same　plants；　are　generally　far　more　successful　than　amateurs　in　getting　new　and　valuable　varieties。　The　keeping　of　a　large　number　of　individuals　of　a　species　in　any　country　requires　that　the　species　should　be　placed　under　favourable　conditions　of　life；　so　as　to　breed　freely　in　that　country。　When　the　individuals　of　any　species　are　scanty；　all　the　individuals；　whatever　their　quality　may　be；　will　generally　be　allowed　to　breed；　and　this　will　effectually　prevent　selectio