Agricult。Soc。’volume4page43。）Althoughfreecrossingisadangerontheonesidewhicheveryonecansee，toocloseinterbreedingisahiddendangerontheotherside。Unfavourableconditionsoflifeoverrulethepowerofselection。Ourimprovedheavybreedsofcattleandsheepcouldnothavebeenformedonmountainouspastures;norcoulddray—horseshavebeenraisedonabarrenandinhospitableland，suchastheFalklandIslands，whereeventhelighthorsesofLaPlatarapidlydecreaseinsize。ItseemsimpossibletopreserveseveralEnglishbreedsofsheepinFrance;forassoonasthelambsareweanedtheirvigourdecaysastheheatofthesummerincreases（21/42。
Malingie—Nouel’JournalR。Agricult。Soc。’volume141853pages215，217。）:itwouldbeimpossibletogivegreatlengthofwooltosheepwithinthetropics;
yetselectionhaskepttheMerinobreednearlytrueunderdiversifiedandunfavourableconditions。Thepowerofselectionissogreat，thatbreedsofthedog，sheep，andpoultry，ofthelargestandsmallestsize，longandshortbeakedpigeons，andotherbreedswithoppositecharacters，havehadtheircharacteristicqualitiesaugmented，thoughtreatedineverywayalike，beingexposedtothesameclimateandfedonthesamefood。Selection，however，iseithercheckedorfavouredbytheeffectsofuseorhabit。Ourwonderfully—
improvedpigscouldneverhavebeenformediftheyhadbeenforcedtosearchfortheirownfood;theEnglishracehorseandgreyhoundcouldnothavebeenimproveduptotheirpresenthighstandardofexcellencewithoutconstanttraining。
Asconspicuousdeviationsofstructureoccurrarely，theimprovementofeachbreedisgenerallytheresultoftheselectionofslightindividualdifferences。Hencetheclosestattention，thesharpestpowersofobservation，andindomitableperseverance，areindispensable。Itis，also，highlyimportantthatmanyindividualsofthebreedwhichistobeimprovedshouldberaised;
forthustherewillbeabetterchanceoftheappearanceofvariationsintherightdirection，andindividualsvaryinginanunfavourablemannermaybefreelyrejectedordestroyed。Butthatalargenumberofindividualsshouldberaised，itisnecessarythattheconditionsoflifeshouldfavourthepropagationofthespecies。Hadthepeacockbeenrearedaseasilyasthefowl，weshouldprobablyerethishavehadmanydistinctraces。Weseetheimportanceofalargenumberofplants，fromthefactofnurserygardenersalmostalwaysbeatingamateursintheexhibitionofnewvarieties。In1845itwasestimated（21/43。Gardener’sChronicle’1845page273。）thatbetween4000
and5000pelargoniumswereannuallyraisedfromseedinEngland，yetadecidedlyimprovedvarietyisrarelyobtained。AtMessrs。Carter’sgrounds，inEssex，wheresuchflowersastheLobelia，Nemophila，Mignonette，etc。，aregrownbytheacreforseed，"scarcelyaseasonpasseswithoutsomenewkindsbeingraised，orsomeimprovementeffectedonoldkinds。"（21/44。’JournalofHorticulture’1862page157。）AtKew，asMr。Beatonremarks，wheremanyseedlingsofcommonplantsareraised，"youseenewformsofLaburnums，Spiraeas，andothershrubs。"（21/45。’CottageGardener’1860page368。）Sowithanimals:Marshall（21/46。’AReviewofReports’1808page406。），inspeakingofthesheepinonepartofYorkshire，remarks，"astheybelongtopoorpeople，andaremostlyinsmalllots，theynevercanbeimproved。"LordRivers，whenaskedhowhesucceededinalwayshavingfirst—rategreyhounds，answered，"Ibreedmany，andhangmany。"This，asanothermanremarks，"wasthesecretofhissuccess;andthesamewillbefoundinexhibitingfowls，——
successfulcompetitorsbreedlargely，andkeepthebest。"（21/47。’Gardener’sChronicle’1853page45。）
Itfollowsfromthisthatthecapacityofbreedingatanearlyageandatshortintervals，aswithpigeons，rabbits，etc。，facilitatesselection;fortheresultisthussoonmadevisible，andperseveranceintheworkencouraged。
Itcanhardlybeanaccidentthatthegreatmajorityoftheculinaryandagriculturalplantswhichhaveyieldednumerousracesareannualsorbiennials，whichthereforearecapableofrapidpropagation，andthusofimprovement。Sea—kale，asparagus，commonandJerusalemartichokes，potatoes，andonions，mustbeexcepted，astheyareperennials:butonionsarepropagatedlikeannuals，andoftheotherplantsjustspecified，none，withtheexceptionofthepotato，haveyieldedinthiscountrymorethanoneortwovarieties。IntheMediterraneanregion，whereartichokesareoftenraisedfromseed，thereareseveralkinds，asIhearfromMr。Bentham。Nodoubtfruit—
trees，whichcannotbepropagatedquicklybyseed，haveyieldedahostofvarieties，thoughnotpermanentraces;butthese，judgingfromprehistoricremains，havebeenproducedatacomparativelylateperiod。
Aspeciesmaybehighlyvariable，butdistinctraceswillnotbeformed，iffromanycauseselectionbenotapplied。Itwouldbedifficulttoselectslightvariationsinfishesfromtheirplaceofhabitation;andthoughthecarpisextremelyvariableandismuchattendedtoinGermany，onlyonewell—
markedracehasbeenformed，asIhearfromLordA。Russell，namelythespiegel—carpe;andthisiscarefullysecludedfromthecommonscalykind。Ontheotherhand，acloselyalliedspecies，thegold—fish，frombeingrearedinsmallvessels，andfromhavingbeencarefullyattendedtobytheChinese，hasyieldedmanyraces。Neitherthebee，whichhasbeensemi—domesticatedfromanextremelyremoteperiod，northecochinealinsect，whichwascultivatedbytheaboriginalMexicans（21/48。IsidoreGeoffroySaint—Hilaire’Hist。Nat。Gen。’
tome3page49。’OntheCochinealInsect’page46。），hasyieldedraces;anditwouldbeimpossibletomatchthequeen—beewithanyparticulardrone，andmostdifficulttomatchcochinealinsects。Silk—moths，ontheotherhand，havebeensubjectedtorigorousselection，andhaveproducedahostofraces。Cats，whichfromtheirnocturnalhabitscannotbeselectedforbreeding，donot，asformerlyremarked，yielddistinctraceswithinthesamecountry。DogsareheldinabominationintheEast，andtheirbreedingisneglected;consequently，asProf。MoritzWagner（21/49。’DieDarwin’scheTheorieunddasMigrationsgesetzderOrganismen’1868page19。）remarks，onekindaloneexiststhere。TheassinEnglandvariesmuchincolourandsize;butasitisananimaloflittlevalueandbredbypoorpeople，therehasbeennoselection，anddistinctraceshavenotbeenformed。Wemustnotattributetheinferiorityofourassestoclimate，forinIndiatheyareofevensmallersizethaninEurope。Butwhenselectionisbroughttobearontheass，allischanged。NearCordova，asIaminformed（February1860）byMr。W。E。Webb，C。E。，theyarecarefullybred，asmuchas200poundshavingbeenpaidforastallionass，andtheyhavebeenimmenselyimproved。InKentucky，asseshavebeenimported（forbreedingmules）
fromSpain，Malta，andFrance;these"seldomaveragedmorethanfourteenhandshigh:buttheKentuckians，bygreatcare，haveraisedthemuptofifteenhands，andsometimeseventosixteen。Thepricespaidforthesesplendidanimals，forsuchtheyreallyare，willprovehowmuchtheyareinrequest。
Onemale，ofgreatcelebrity，wassoldforupwardsofonethousandpoundssterling。"Thesechoiceassesaresenttocattle—shows，adaybeinggivenfortheirexhibition。（21/50。Capt。MarryatquotedbyBlythin’Journ。AsiaticSoc。ofBengal’volume28page229。）
Analogousfactshavebeenobservedwithplants:thenutmeg—treeintheMalayarchipelagoishighlyvariable，buttherehasbeennoselection，andtherearenodistinctraces。（21/51。Mr。Oxley’JournaloftheIndianArchipelago’
volume21848page645。）Thecommonmignonette（Resedaodorata），frombearinginconspicuousflowers，valuedsolelyfortheirfragrance，"remainsinthesameunimprovedconditionaswhenfirstintroduced。"（21/52。Mr。Abbey’JournalofHorticulture’December1，1863page430。）Ourcommonforest—treesareveryvariable，asmaybeseenineveryextensivenursery—ground;butastheyarenotvaluedlikefruit—trees，andastheyseedlateinlife，noselectionhasbeenappliedtothem;consequently，asMr。PatrickMatthewsremarks（21/53。
’OnNavalTimber’1831page107。），theyhavenotyieldeddistinctraces，leafingatdifferentperiods，growingtodifferentsizes，andproducingtimberfitfordifferentpurposes。Wehavegainedonlysomefancifulandsemi—
monstrousvarieties，whichnodoubtappearedsuddenlyaswenowseethem。
Somebotanistshavearguedthatplantscannothavesostrongatendencytovaryasisgenerallysupposed，becausemanyspecieslonggrowninbotanicgardens，orunintentionallycultivatedyearafteryearmingledwithourcorncrops，havenotproduceddistinctraces;butthisisaccountedforbyslightvariationsnothavingbeenselectedandpropagated。Letaplantwhichisnowgrowninabotanicgarden，oranycommonweed，becultivatedonalargescale，andletasharp—sightedgardenerlookoutforeachslightvarietyandsowtheseed，andthen，ifdistinctracesarenotproduced，theargumentwillbevalid。
Theimportanceofselectionislikewiseshownbyconsideringspecialcharacters。Forinstance，withmostbreedsoffowlstheformofthecombandthecolouroftheplumagehavebeenattendedto，andareeminentlycharacteristicofeachrace;butinDorkingsfashionhasneverdemandeduniformityofcomborcolour;andtheutmostdiversityintheserespectsprevails。Rose—combs，double—combs，cup—combs，etc。，andcoloursofallkinds，maybeseeninpurelybredandcloselyrelatedDorkingfowls，whilstotherpoints，suchasthegeneralformofbody，andthepresenceofanadditionaltoe，havebeenattendedto，andareinvariablypresent。Ithasalsobeenascertainedthatcolourcanbefixedinthisbreed，aswellasinanyother。
（21/54。Mr。Bailyin’ThePoultryChronicle’volume21854page150。Alsovolume1page342;volume3page245。）
Duringtheformationorimprovementofabreed，itsmemberswillalwaysbefoundtovarymuchinthosecharacterstowhichespecialattentionisdirected，andofwhicheachslightimprovementiseagerlysoughtandselected。
Thus，withshort—facedtumbler—pigeons，theshortnessofthebeak，shapeofheadandplumage，——withcarriers，thelengthofthebeakandwattle，——withfantails，thetailandcarriage，——withSpanishfowls，thewhitefaceandcomb，——withlong—earedrabbits，thelengthofear，areallpointswhichareeminentlyvariable。Soitisineverycase;andthelargepricepaidforfirst—rateanimalsprovesthedifficultyofbreedingthemuptothehigheststandardofexcellence。Thissubjecthasbeendiscussedbyfanciers（21/55。
’CottageGardener’1855Decemberpage171;1856Januarypages248，323。），andthegreaterprizesgivenforhighlyimprovedbreeds，incomparisonwiththosegivenforoldbreedswhicharenotnowundergoingrapidimprovement，havebeenfullyjustified。Nathusiusmakes（21/56。’UeberShorthornRindvieh’1857s。
51。）asimilarremarkwhendiscussingthelessuniformcharacterofimprovedShorthorncattleandoftheEnglishhorse，incomparison，forexample，withtheunennobledcattleofHungary，orwiththehorsesoftheAsiaticsteppes。
Thiswantofuniformityinthepartswhichatthetimeareundergoingselectionchieflydependsonthestrengthoftheprincipleofreversion;butitlikewisedependstoacertainextentonthecontinuedvariabilityofthepartswhichhaverecentlyvaried。Thatthesamepartsdocontinuevaryinginthesamemannerwemustadmit，forifitwerenotso，therecouldbenoimprovementbeyondanearlystandardofexcellence，andweknowthatsuchimprovementisnotonlypossible，butisofgeneraloccurrence。
Asaconsequenceofcontinuedvariability，andmoreespeciallyofreversion，allhighlyimprovedraces，ifneglectedornotsubjectedtoincessantselection，soondegenerate。YouattgivesacuriousinstanceofthisinsomecattleformerlykeptinGlamorganshire;butinthiscasethecattlewerenotfedwithsufficientcare。Mr。Baker，inhismemoirontheHorse，sumsup:"Itmusthavebeenobservedintheprecedingpagesthat，whenevertherehasbeenneglect，thebreedhasproportionallydeteriorated。"（21/57。’TheVeterinary’
volume13page720。FortheGlamorganshirecattleseeYouatton’Cattle’page51。）Ifaconsiderablenumberofimprovedcattle，sheep，orotheranimalsofthesamerace，wereallowedtobreedfreelytogether，withnoselection，butwithnochangeintheirconditionoflife，therecanbenodoubtthatafterascoreorhundredgenerationstheywouldbeveryfarfromexcellentoftheirkind;but，fromwhatweseeofthemanycommonracesofdogs，cattle，fowls，pigeons，etc。，whichwithoutanyparticularcarehavelongretainednearlythesamecharacter，wehavenogroundsforbelievingthattheywouldaltogetherdepartfromtheirtype。
Itisageneralbeliefamongstbreedersthatcharactersofallkindsbecomefixedbylong—continuedinheritance。ButIhaveattemptedtoshowinthefourteenthchapterthatthisbeliefapparentlyresolvesitselfintothefollowingproposition，namely，thatallcharacterswhatever，whetherrecentlyacquiredorancient，tendtobetransmitted，butthatthosewhichhavealreadylongwithstoodallcounteractinginfluences，will，asageneralrule，continuetowithstandthem，andconsequentlybefaithfullytransmitted。
TENDENCYINMANTOCARRYTHEPRACTICEOFSELECTIONTOANEXTREMEPOINT。
Itisanimportantprinciplethatintheprocessofselectionmanalmostinvariablywishestogotoanextremepoint。Thus，thereisnolimittohisdesiretobreedcertainkindsofhorsesanddogsasfleetaspossible，andothersasstrongaspossible;certainkindsofsheepforextremefineness，andothersforextremelengthofwool;andhewishestoproducefruit，grain，tubers，andotherusefulpartsofplants，aslargeandexcellentaspossible。
Withanimalsbredforamusement，thesameprincipleisevenmorepowerful;forfashion，asweseeinourdress，alwaysrunstoextremes。Thisviewhasbeenexpresslyadmittedbyfanciers。Instancesweregiveninthechaptersonthepigeon，buthereisanother:Mr。Eaton，afterdescribingacomparativelynewvariety，namely，theArchangel，remarks，"WhatfanciersintenddoingwiththisbirdIamatalosstoknow，whethertheyintendtobreeditdowntothetumbler’sheadandbeak，orcarryitouttothecarrier’sheadandbeak;
leavingitastheyfoundit，isnotprogressing。"Ferguson，speakingoffowls，says，"theirpeculiarities，whatevertheymaybe，mustnecessarilybefullydeveloped:alittlepeculiarityformsnoughtbutugliness，seeingitviolatestheexistinglawsofsymmetry。"SoMr。Brent，indiscussingthemeritsofthesub—varietiesoftheBelgiancanary—bird，remarks，"Fanciersalwaysgotoextremes;theydonotadmireindefiniteproperties。"（21/58。J。M。Eaton’A
TreatiseonFancyPigeons’page82;Fergusonon’RareandPrizePoultry’page162;Mr。Brentin’CottageGardener’October1860page13。）
Thisprinciple，whichnecessarilyleadstodivergenceofcharacter，explainsthepresentstateofvariousdomesticraces。Wecanthusseehowitisthatracehorsesanddray—horses，greyhoundsandmastiffs，whichareopposedtoeachotherineverycharacter，——howvarietiessodistinctasCochin—chinafowlsandbantams，orcarrier—pigeonswithverylongbeaks，andtumblerswithexcessivelyshortbeaks，havebeenderivedfromthesamestock。Aseachbreedisslowlyimproved，theinferiorvarietiesarefirstneglectedandfinallylost。Inafewcases，bytheaidofoldrecords，orfromintermediatevarietiesstillexistingincountrieswhere...完整阅读请扫描二维码下载丁香书院APP免费看：