the origin of species(物种起源)-第5部分

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not　fully　convinced　that　each　main　breed　was　descended　from　a　distinct　species。　Van　Mons；　in　his　treatise　on　pears　and　apples；　shows　how　utterly　he　disbelieves　that　the　several　sorts；　for　instance　a　Ribston…pippin　or　Codlin…apple；　could　ever　have　proceeded　from　the　seeds　of　the　same　tree。　Innumerable　other　examples　could　be　given。　The　explanation；　I　think；　is　simple：　from　long…continued　study　they　are　strongly　impressed　with　the　differences　between　the　several　races；　and　though　they　well　know　that　each　race　varies　slightly；　for　they　win　their　prizes　by　selecting　such　slight　differences；　yet　they　ignore　all　general　arguments；　and　refuse　to　sum　up　in　their　minds　slight　differences　accumulated　during　many　successive　generations。　May　not　those　naturalists　who；　knowing　far　less　of　the　laws　of　inheritance　than　does　the　breeder；　and　knowing　no　more　than　he　does　of　the　intermediate　links　in　the　long　lines　of　descent；　yet　admit　that　many　of　our　domestic　races　have　descended　from　the　same　parents　may　they　not　learn　a　lesson　of　caution；　when　they　deride　the　idea　of　species　in　a　state　of　nature　being　lineal　descendants　of　other　species？　
Selection
Let　us　now　briefly　consider　the　steps　by　which　domestic　races　have　been　produced；　either　from　one　or　from　several　allied　species。　Some　little　effect　may；　perhaps；　be　attributed　to　the　direct　action　of　the　external　conditions　of　life；　and　some　little　to　habit；　but　he　would　be　a　bold　man　who　would　account　by　such　agencies　for　the　differences　of　a　dray　and　race　horse；　a　greyhound　and　bloodhound；　a　carrier　and　tumbler　pigeon。　One　of　the　most　remarkable　features　in　our　domesticated　races　is　that　we　see　in　them　adaptation；　not　indeed　to　the　animal's　or　plant's　own　good；　but　to　man's　use　or　fancy。　Some　variations　useful　to　him　have　probably　arisen　suddenly；　or　by　one　step；　many　botanists；　for　instance；　believe　that　the　fuller's　teazle；　with　its　hooks；　which　cannot　be　rivalled　by　any　mechanical　contrivance；　is　only　a　variety　of　the　wild　Dipsacus；　and　this　amount　of　change　may　have　suddenly　arisen　in　a　seedling。　So　it　has　probably　been　with　the　turnspit　dog；　and　this　is　known　to　have　been　the　case　with　the　ancon　sheep。　But　when　we　compare　the　dray…horse　and　race…horse；　the　dromedary　and　camel；　the　various　breeds　of　sheep　fitted　either　for　cultivated　land　or　mountain　pasture；　with　the　wool　of　one　breed　good　for　one　purpose；　and　that　of　another　breed　for　another　purpose；　when　we　compare　the　many　breeds　of　dogs；　each　good　for　man　in　very　different　ways；　when　we　compare　the　gamecock；　so　pertinacious　in　battle；　with　other　breeds　so　little　quarrelsome；　with　'everlasting　layers'　which　never　desire　to　sit；　and　with　the　bantam　so　small　and　elegant；　when　we　compare　the　host　of　agricultural；　culinary；　orchard；　and　flower…garden　races　of　plants；　most　useful　to　man　at　different　seasons　and　for　different　purposes；　or　so　beautiful　in　his　eyes；　we　must；　I　think；　look　further　than　to　mere　variability。　We　cannot　suppose　that　all　the　breeds　were　suddenly　produced　as　perfect　and　as　useful　as　we　now　see　them；　indeed；　in　several　cases；　we　know　that　this　has　not　been　their　history。　The　key　is　man's　power　of　accumulative　selection：　nature　gives　successive　variations；　man　adds　them　up　in　certain　directions　useful　to　him。　In　this　sense　he　may　be　said　to　make　for　himself　useful　breeds。　
The　great　power　of　this　principle　of　selection　is　not　hypothetical。　It　is　certain　that　several　of　our　eminent　breeders　have；　even　within　a　single　lifetime；　modified　to　a　large　extent　some　breeds　of　cattle　and　sheep。　In　order　fully　to　realise　what　they　have　done；　it　is　almost　necessary　to　read　several　of　the　many　treatises　devoted　to　this　subject；　and　to　inspect　the　animals。　Breeders　habitually　speak　of　an　animal's　organisation　as　something　quite　plastic；　which　they　can　model　almost　as　they　please。　If　I　had　space　I　could　quote　numerous　passages　to　this　effect　from　highly　competent　authorities。　Youatt；　who　was　probably　better　acquainted　with　the　works　of　agriculturalists　than　almost　any　other　individual；　and　who　was　himself　a　very　good　judge　of　an　animal；　speaks　of　the　principle　of　selection　as　'that　which　enables　the　agriculturist；　not　only　to　modify　the　character　of　his　flock；　but　to　change　it　altogether。　It　is　the　magician's　wand；　by　means　of　which　he　may　summon　into　life　whatever　form　and　mould　he　pleases。'　Lord　Somerville；　speaking　of　what　breeders　have　done　for　sheep；　says：　'It　would　seem　as　if　they　had　chalked　out　upon　a　wall　a　form　perfect　in　itself；　and　then　had　given　it　existence。'　That　most　skilful　breeder；　Sir　John　Sebright；　used　to　say；　with　respect　to　pigeons；　that　'he　would　produce　any　given　feather　in　three　years；　but　it　would　take　him　six　years　to　obtain　head　and　beak。'　In　Saxony　the　importance　of　the　principle　of　selection　in　regard　to　merino　sheep　is　so　fully　recognised；　that　men　follow　it　as　a　trade：　the　sheep　are　placed　on　a　table　and　are　studied；　like　a　picture　by　a　connoisseur；　this　is　done　three　times　at　intervals　of　months；　and　the　sheep　are　each　time　marked　and　classed；　so　that　the　very　best　may　ultimately　be　selected　for　breeding。　
What　English　breeders　have　actually　effected　is　proved　by　the　enormous　prices　given　for　animals　with　a　good　pedigree；　and　these　have　now　been　exported　to　almost　every　quarter　of　the　world。　The　improvement　is　by　no　means　generally　due　to　crossing　different　breeds；　all　the　best　breeders　are　strongly　opposed　to　this　practice；　except　sometimes　amongst　closely　allied　sub…breeds。　And　when　a　cross　has　been　made；　the　closest　selection　is　far　more　indispensable　even　than　in　ordinary　cases。　If　selection　consisted　merely　in　separating　some　very　distinct　variety；　and　breeding　from　it；　the　principle　would　be　so　obvious　as　hardly　to　be　worth　notice；　but　its　importance　consists　in　the　great　effect　produced　by　the　accumulation　in　one　direction；　during　successive　generations；　of　differences　absolutely　inappreciable　by　an　uneducated　eye　differences　which　I　for　one　have　vainly　attempted　to　appreciate。　Not　one　man　in　a　thousand　has　accuracy　of　eye　and　judgement　sufficient　to　become　an　eminent　breeder。　If　gifted　with　these　qualities；　and　he　studies　his　subject　for　years；　and　devotes　his　lifetime　to　it　with　indomitable　perseverance；　he　will　succeed；　and　may　make　great　improvements；　if　he　wants　any　of　these　qualities；　he　will　assuredly　fail。　Few　would　readily　believe　in　the　natural　capacity　and　years　of　practice　requisite　to　become　even　a　skilful　pigeon…fancier。　
The　same　principles　are　followed　by　horticulturists；　but　the　variations　are　here　often　more　abrupt。　No　one　supposes　that　our　choicest　productions　have　been　produced　by　a　single　variation　from　the　aboriginal　stock。　We　have　proofs　that　this　is　not　so　in　some　cases；　in　which　exact　records　have　been　kept；　thus；　to　give　a　very　trifling　instance；　the　steadily…increasing　size　of　the　common　gooseberry　may　be　quoted。　We　see　an　astonishing　improvement　in　many　florists'　flowers；　when　the　flowers　of　the　present　day　are　compared　with　drawings　made　only　twenty　or　thirty　years　ago。　When　a　race　of　plants　is　once　pretty　well　established；　the　seed…raisers　do　not　pick　out　the　best　plants；　but　merely　go　over　their　seed…beds；　and　pull　up　the　'rogues；'　as　they　call　the　plants　that　deviate　from　the　proper　standard。　With　animals　this　kind　of　selection　is；　in　fact；　also　followed；　for　hardly　any　one　is　so　careless　as　to　allow　his　worst　animals　to　breed。　
In　regard　to　plants；　there　is　another　means　of　observing　the　accumulated　effects　of　selection　namely；　by　comparing　the　diversity　of　flowers　in　the　different　varieties　of　the　same　species　in　the　flower…garden；　the　diversity　of　leaves；　pods；　or　tubers；　or　whatever　part　is　valued；　in　the　kitchen…garden；　in　comparison　with　the　flowers　of　the　same　varieties；　and　the　diversity　of　fruit　of　the　same　species　in　the　orchard；　in　comparison　with　the　leaves　and　flowers　of　the　same　set　of　varieties。　See　how　different　the　leaves　of　the　cabbage　are；　and　how　extremely　alike　the　flowers；　how　unlike　the　flowers　of　the　heartsease　are；　and　how　alike　the　leaves；　how　much　the　fruit　of　the　different　kinds　of　gooseberries　differ　in　size；　colour；　shape；　and　hairiness；　and　yet　the　flowers　present　very　slight　differences。　It　is　not　that　the　varieties　which　differ　largely　in　some　one　point　do　not　differ　at　all　in　other　points；　this　is　hardly　ever；　perhaps　never；　the　case。　The　laws　of　correlation　of　growth；　the　importance　of　which　should　never　be　overlooked；　will　ensure　some　differences；　but；　as　a　general　rule；　I　cannot　doubt　that　the　continued　selection　of　slight　variations；　either　in　the　leaves；　the　flowers；　or　the　fruit；　will　produce　races　differing　from　each　other　chiefly　in　these　characters。　
It　may　be　objected　that　the　principle　of　selection　has　been　reduced　to　methodical　practice　for　scarcely　more　than　three…quarters　of　a　century；　it　has　certainly　been　more　attended　to　of　late　years；　and　many　treatises　have　been　published　on　the　subject；　and　the　result；　I　may　add；　has　been；　in　a　corresponding　degree；　rapid　and　important。　But　it　is　very　far　from　true　that　the　principle　is　a　modern　discovery。　I　could　give　several　references　to　the　full　acknowledgement　of　the　importance　of　the　principle　in　works　of　high　antiquity。　In　rude　and　barbarous　periods　of　English　history　choice　animals　were　often　imported；　and　laws　were　passed　to　prevent　their　exportation：　the　destruction　of　horses　under　a　certain　size　was　ordered；　and　this　may　be　compared　to　the　'roguing'　of　plants　by　nurserymen。　The　principle　of　selection　I　find　distinctly　given　in　an　ancient　Chinese　encyclopaedia。　Explicit　rules　are　laid　down　by　some　of　the　Roman　classical　writers。　From　passages　in　Genesis；　it　is　clear　that　the　colour　of　domestic　animals　was　at　that　early　period　attended　to。　Savages　now　sometimes　cross　their　dogs　with　wild　canine　animals；　to　improve　the　breed；　and　they　formerly　did　so；　as　is　attested　by　passages　in　Pliny。　The　savages　in　South　Africa　match　their　draught　cattle　by　colour；　as　do　some　of　the　Esquimaux　their　teams　of　dogs。　Livingstone　shows　how　much　good　domestic　breeds　are　valued　by　the　negroes　of　the　interior　of　Africa　who　have　not　associated　with　Europeans。　Some　of　these　facts　do　not　show　actual　selection；　but　they　show　that　the　breeding　of　domestic　animals　was　carefully　attended　to　in　ancient