Asnewcharacterssometimesappearbybud—variation，whichdonotoccurintheparent—speciesorinanyalliedspecies，wemayreject，atleastinthesecases，theideathattheyareduetoreversion。Nowitiswellworthwhiletoreflectmaturelyonsomestrikingcaseofbud—variation，forinstancethatofthepeach。Thistreehasbeencultivatedbythemillioninvariouspartsoftheworld，hasbeentreateddifferently，grownonitsownrootsandgraftedonvariousstocks，plantedasastandard，trainedagainstawall，orunderglass;
yeteachbudofeachsub—varietykeepstruetoitskind。Butoccasionally，atlongintervalsoftime，atreeinEngland，orunderthewidelydifferentclimateofVirginia，producesasinglebud，andthisyieldsabranchwhicheverafterwardsbearsnectarines。Nectarinesdiffer，aseveryoneknows，frompeachesintheirsmoothness，size，andflavour;andthedifferenceissogreatthatsomebotanistshavemaintainedthattheyarespecificallydistinct。Sopermanentarethecharactersthussuddenlyacquired，thatanectarineproducedbybud—variationhaspropagateditselfbyseed。Toguardagainstthesuppositionthatthereissomefundamentaldistinctionbetweenbudandseminalvariation，itiswelltobearinmindthatnectarineshavelikewisebeenproducedfromthestoneofthepeach;and，reversely，peachesfromthestoneofthenectarine。Nowisitpossibletoconceiveexternalconditionsmorecloselyalikethanthosetowhichthebudsonthesametreeareexposed？Yetonebudalone，outofthemanythousandsbornebythesametree，hassuddenly，withoutanyapparentcause，producedanectarine。Butthecaseisevenstrongerthanthis，forthesameflower—budhasyieldedafruit，one—halforone—quarteranectarine，andtheotherhalforthree—quartersapeach。Again，sevenoreightvarietiesofthepeachhaveyieldedbybud—variationnectarines:thenectarinesthusproduced，nodoubt，differalittlefromoneanother;butstilltheyarenectarines。Ofcoursetheremustbesomecause，internalorexternal，toexcitethepeach—budtochangeitsnature;butI
cannotimagineaclassoffactsbetteradaptedtoforceonourmindstheconvictionthatwhatwecalltheexternalconditionsoflifeareinmanycasesquiteinsignificantinrelationtoanyparticularvariation，incomparisonwiththeorganisationorconstitutionofthebeingwhichvaries。
ItisknownfromthelaboursofGeoffroySaint—Hilaire，andrecentlyfromthoseofDaresteandothers，thateggsofthefowl，ifshaken，placedupright，perforated，coveredinpartwithvarnish，etc。，producemonstrouschickens。
Nowthesemonstrositiesmaybesaidtobedirectlycausedbysuchunnaturalconditions，butthemodificationsthusinducedarenotofadefinitenature。
Anexcellentobserver，M。CamilleDareste（23/59。’MemoiresurlaProductionArtificielledesMonstruosites’1862pages8—12;’RecherchessurlesConditions，etc。，chezlesMonstres’1863page6。AnabstractisgivenofGeoffroy’sExperimentsbyhisson，inhis’Vie，Travaux’etc。1847page290。），remarks"thatthevariousspeciesofmonstrositiesarenotdeterminedbyspecificcauses;theexternalagencieswhichmodifythedevelopmentoftheembryoactsolelyincausingaperturbation——aperversioninthenormalcourseofdevelopment。"Hecomparestheresulttowhatweseeinillness:asuddenchill，forinstance，affectsoneindividualaloneoutofmany，causingeitheracold，orsore—throat，rheumatism，orinflammationofthelungsorpleura。
Contagiousmatteractsinananalogousmanner。（23/60。Paget’LecturesonSurgicalPathology’1853volume1page483。）Wemaytakeastillmorespecificinstance:sevenpigeonswerestruckbyrattle—snakes（23/61。’ResearchesupontheVenomoftheRattle—snake’January1861byDr。Mitchellpage67。）:somesufferedfromconvulsions;somehadtheirbloodcoagulated，inothersitwasperfectlyfluid;someshowedecchymosedspotsontheheart，othersontheintestines，etc。;othersagainshowednovisiblelesioninanyorgan。Itiswellknownthatexcessindrinkingcausesdifferentdiseasesindifferentmen;
butinthetropicstheeffectsofintemperancedifferfromthosecausedinacoldclimate（23/62。Mr。Sedgwick’BritishandForeignMedico—Chirurg。Review’
July1863page175。）;andinthiscaseweseethedefiniteinfluenceofoppositeconditions。Theforegoingfactsapparentlygiveusasgoodanideaaswearelikelyforalongtimetoobtain，howinmanycasesexternalconditionsactdirectly，thoughnotdefinitely，incausingmodificationsofstructure。
SUMMARY。
Therecanbenodoubt，fromthefactsgiveninthischapter，thatextremelyslightchangesintheconditionsoflifesometimes，probablyoften，actinadefinitemanneronourdomesticatedproductions;and，astheactionofchangedconditionsincausingindefinitevariabilityisaccumulative，soitmaybewiththeirdefiniteaction。Henceconsiderableanddefinitemodificationsofstructureprobablyfollowfromalteredconditionsactingduringalongseriesofgenerations。Insomefewinstancesamarkedeffecthasbeenproducedquicklyonall，ornearlyall，theindividualswhichhavebeenexposedtoamarkedchangeofclimate，food，orothercircumstance。ThishasoccurredwithEuropeanmenintheUnitedStates，withEuropeandogsinIndia，withhorsesintheFalklandIslands，apparentlywithvariousanimalsatAngora，withforeignoystersintheMediterranean，andwithmaizetransportedfromoneclimatetoanother。Wehaveseenthatthechemicalcompoundsofsomeplantsandthestateoftheirtissuesarereadilyaffectedbychangedconditions。Arelationapparentlyexistsbetweencertaincharactersandcertainconditions，sothatifthelatterbechangedthecharacterislost——aswiththecoloursofflowers，thestateofsomeculinaryplants，thefruitofthemelon，thetailoffat—tailedsheep，andthepeculiarfleecesofothersheep。
Theproductionofgalls，andthechangeofplumageinparrotswhenfedonpeculiarfoodorwheninoculatedbythepoisonofatoad，provetouswhatgreatandmysteriouschangesinstructureandcolour，maybethedefiniteresultofchemicalchangesinthenutrientfluidsortissues。
Wenowalmostcertainlyknowthatorganicbeingsinastateofnaturemaybemodifiedinvariousdefinitewaysbytheconditionstowhichtheyhavebeenlongexposed，asinthecaseofthebirdsandotheranimalsinthenorthernandsouthernUnitedStates，andofAmericantreesincomparisonwiththeirrepresentativesinEurope。Butinmanycasesitismostdifficulttodistinguishbetweenthedefiniteresultofchangedconditions，andtheaccumulationthroughnaturalselectionofindefinitevariationswhichhaveprovedserviceable。Ifitprofitedaplanttoinhabitahumidinsteadofanaridstation，afittingchangeinitsconstitutionmightpossiblyresultfromthedirectactionoftheenvironment，thoughwehavenogroundsforbelievingthatvariationsoftherightkindwouldoccurmorefrequentlywithplantsinhabitingastationalittlemorehumidthanusual，thanwithotherplants。
Whetherthestationwasunusuallydryorhumid，variationsadaptingtheplantinaslightdegreefordirectlyoppositehabitsoflifewouldoccasionallyarise，aswehavegoodreasontobelievefromwhatweactuallyseeinothercases。
Theorganisationorconstitutionofthebeingwhichisactedon，isgenerallyamuchmoreimportantelementthanthenatureofthechangedconditions，indeterminingthenatureofthevariation。Wehaveevidenceofthisintheappearanceofnearlysimilarmodificationsunderdifferentconditions，andofdifferentmodificationsunderapparentlynearlythesameconditions。Wehavestillbetterevidenceofthisincloselyparallelvarietiesbeingfrequentlyproducedfromdistinctraces，orevendistinctspecies;andinthefrequentrecurrenceofthesamemonstrosityinthesamespecies。Wehavealsoseenthatthedegreetowhichdomesticatedbirdshavevaried，doesnotstandinanycloserelationwiththeamountofchangetowhichtheyhavebeensubjected。
Torecuronceagaintobud—variations。Whenwereflectonthemillionsofbudswhichmanytreeshaveproduced，beforesomeonebudhasvaried，wearelostinwonderastowhattheprecisecauseofeachvariationcanbe。LetusrecallthecasegivenbyAndrewKnightoftheforty—year—oldtreeoftheyellowmagnumbonumplum，anoldvarietywhichhasbeenpropagatedbygraftsonvariousstocksforaverylongperiodthroughoutEuropeandNorthAmerica，andonwhichasinglebudsuddenlyproducedtheredmagnumbonum。Weshouldalsobearinmindthatdistinctvarieties，andevendistinctspecies，——asinthecaseofpeaches，nectarines，andapricots，——ofcertainrosesandcamellias，——
althoughseparatedbyavastnumberofgenerationsfromanyprogenitorincommon，andalthoughcultivatedunderdiversifiedconditions，haveyieldedbybud—variationcloselyanalogousvarieties。Whenwereflectonthesefactswebecomedeeplyimpressedwiththeconvictionthatinsuchcasesthenatureofthevariationdependsbutlittleontheconditionstowhichtheplanthasbeenexposed，andnotinanyespecialmanneronitsindividualcharacter，butmuchmoreontheinheritednatureorconstitutionofthewholegroupofalliedbeingstowhichtheplantinquestionbelongs。Wearethusdriventoconcludethatinmostcasestheconditionsoflifeplayasubordinatepartincausinganyparticularmodification;likethatwhichasparkplays，whenamassofcombustiblesburstsintoflame——thenatureoftheflamedependingonthecombustiblematter，andnotonthespark。（23/63。ProfessorWeismannarguesstronglyinfavourofthisviewinhis’Saison—DimorphismusderSchmetterlinge’1875pages40—43。）
Nodoubteachslightvariationmusthaveitsefficientcause;butitisashopelessanattempttodiscoverthecauseofeach，astosaywhyachillorapoisonaffectsonemandifferentlyfromanother。Evenwithmodificationsresultingfromthedefiniteactionoftheconditionsoflife，whenallornearlyalltheindividuals，whichhavebeensimilarlyexposed，aresimilarlyaffected，wecanrarelyseethepreciserelationbetweencauseandeffect。Inthenextchapteritwillbeshownthattheincreaseduseordisuseofvariousorgansproducesaninheritedeffect。Itwillfurtherbeseenthatcertainvariationsareboundtogetherbycorrelationaswellasbyotherlaws。Beyondthiswecannotatpresentexplaineitherthecausesornatureofthevariabilityoforganicbeings。
CHAPTER2。XXIV。
LAWSOFVARIATION——USEANDDISUSE，ETC。
NISUSFORMATIVUS，ORTHECO—ORDINATINGPOWEROFTHEORGANISATION。
ONTHEEFFECTSOFTHEINCREASEDUSEANDDISUSEOFORGANS。
CHANGEDHABITSOFLIFE。
ACCLIMATISATIONWITHANIMALSANDPLANTS。
VARIOUSMETHODSBYWHICHTHISCANBEEFFECTED。
ARRESTSOFDEVELOPMENT。
RUDIMENTARYORGANS。
InthisandthetwofollowingchaptersIshalldiscuss，aswellasthedifficultyofthesubjectpermits，theseverallawswhichgovernVariability。
Thesemaybegroupedundertheeffectsofuseanddisuse，includingchangedhabitsandacclimatisation——arrestofdevelopment——correlatedvariation——thecohesionofhomologousparts—thevariabilityofmultipleparts——compensationofgrowth——thepositionofbudswithrespecttotheaxisoftheplant——andlastly，analogousvariation。Theseseveralsubjectssograduateintooneanotherthattheirdistinctionisoftenarbitrary。
Itmaybeconvenientfirstbrieflytodiscussthatcoordinatingandreparativepowerwhichiscommon，inahigherorlowerdegree，toallorganicbeings，andwhichwasformerlydesignatedbyphysiologistsasnisusformativus。
[Blumenbachandothers（24/1。’AnEssayonGeneration’Englishtranslationpage18;Paget’LecturesonSurgicalPathology’1853volume1page209。）haveinsistedthattheprinciplewhichpermitsaHydra，whencutintofragments，todevelopitselfintotwoormoreperfectanimals，isthesamewiththatwhichcausesawoundinthehigheranimalstohealbyacicatrice。SuchcasesasthatoftheHydraareevidentlyanalogoustothespontaneousdivisionorfissiparousgenerationofthelowestanimals，andlikewisetothebuddingofplants。Betweentheseextremecasesandthatofamerecicatricewehaveeverygradation。Spallanzani（24/2。’AnEssayonAnimalReproduction’Englishtranslation1769page79。）bycuttingoffthelegsandtailofaSalamander，gotinthecourseofthreemonthssixcropsofthesemembers;sothat687
perfectboneswerereproducedbyoneanimalduringoneseason。Atwhateverpointthelimbwascutoff，thedeficientpart，andnomore，wasexactlyreproduced。Whenadiseasedbonehasbeenremoved，anewonesometimes"graduallyassumestheregularform，andalltheattachmentsofmuscles，ligaments，etc。，becomeascompleteasbefore。"（24/3。Carpenter’PrinciplesofComp。Physiology’1854page479。）
Thispowerofregrowthdoesnot，however，alwaysactperfectly;thereproducedtailofalizarddiffersintheformofthescalesfromthenormaltail:withcertainOrthopterousinsectsthelargehindlegsarereproducedofsmallersize（24/4。Charlesworth’Mag。ofNat。Hist。’volume11837page145。）:thewhitecicatricewhichinthehigheranimalsunitestheedgesofadeepwoundisnotformedofperfectskin，forelastictissueisnotproducedtilllongafterwards。（24/5。Paget’LecturesonSurgicalPathology’volume1page239。）
"Theactivityofthenisusformativus，"saysBlumenbach，"isinaninverseratiototheageoftheorganisedbody。"Itspowerisalsogreaterwithanimals，thelowertheystandinthescaleoforganisation;andanimalslowinthescalecorrespondwiththeembryosofhigheranimalsbelongingtothesameclass。Newport’sobservations（24/6。QuotedbyCarpenter’Comp。Phys。’page479。）affordagoodillustrationofthisfact，forhefoundthat"myriapods，whosehighestdevelopmentscarcelycarriesthembeyondthelarvaofperfectinsects，canregeneratelimbsandantennaeuptothetimeoftheirlastmoult;"andsocanthelarvaeoftrueinsects，but，exceptinoneorder，notinthematureinsect。SalamanderscorrespondindevelopmentwiththetadpolesorlarvaeofthetaillessBatrachians，andbothpossesstoalargeextentthepowerofregrowth;butnotsothematuretaillessBatrachians。
Absorptionoftenplaysanimportantpartintherepairofinjuries。Whenaboneisbrokenanddoesnotunite，theendsareabsorbedandrounded，sothatafalsejointisformed;oriftheendsunite，butoverlap，theprojectingpartsareremoved。（24/7。Prof。Marey’sdiscussiononthepowerofco—
adaptationinallpartsoftheorganisationisexcellent。’LaMachineAnimale’
1873chapter9。SeealsoPaget’Lectures’etc。page257。）Adislocatedbonewillformforitselfanewsocket。Displacedtendonsandvaricoseveinsexcavatenewchannelsinthebonesagainstwhichtheypress。Butabsorptioncomesintoaction，asVirchowremarks，duringthenormalgrowthofbones;
partswhicharesolidduringyouthbecomehollowedoutforthemedullarytissueastheboneincreasesinsize。Intryingtounderstandthemanywell—
adaptedcasesofregrowthwhenaidedbyabsorption，weshouldrememberthatalmostallpartsoftheorganisation，evenwhilstretainingthesameform，undergoconstantrenewal;sothatapartwhichisnotrenewedwouldbeliabletoabsorption。
Somecases，usuallyclassedundertheso—callednisusformativus，atfirstappeartocomeunderadistincthead;fornotonlyareoldstructuresreproduced，butnewstructuresareformed。Thus，afterinflammation"falsemembranes，"furnish...完整阅读请扫描二维码下载丁香书院APP免费看：