the origin of species(物种起源)-第6节

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or　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　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　offsp