InaLaburnum—treeIobservedthataboutafourthpartoftheracemesproducedterminalflowerswhichhadlosttheirpapilionaceousstructure。Thesewereproducedafteralmostalltheotherflowersonthesameracemeshadwithered。
Themostperfectlypelorisedexampleshadsixpetals，eachmarkedwithblackstriaelikethoseonthestandard—petal。Thekeelseemedtoresistthechangemorethantheotherpetals。Dutrochethasdescribed（26/18。’Memoires……desVegetaux’1837tome2page170。）anexactlysimilarcaseinFrance，andI
believethesearetheonlytwoinstancesofpelorisminthelaburnumwhichhavebeenrecorded。Dutrochetremarksthattheracemesonthistreedonotproperlyproduceaterminalflower，sothat（asinthecaseoftheGaleobdolon）theirpositionaswellasstructurearebothanomalies，whichnodoubtareinsomemannerrelated。Dr。Mastershasbrieflydescribedanotherleguminousplant（26/19。’JournalofHorticulture’July23，1861page311。），namely，aspeciesofclover，inwhichtheuppermostandcentralflowerswereregularorhadlosttheirpapilionaceousstructure。Insomeoftheseplantstheflower—headswerealsoproliferous。
Lastly，Linariaproducestwokindsofpeloricflowers，onehavingsimplepetals，andtheotherhavingthemallspurred。Thetwoforms，asNaudinremarks（26/20。’NouvellesArchivesduMuseum’tome1page137。），notrarelyoccuronthesameplant，butinthiscasethespurredformalmostinvariablystandsonthesummitofthespike。
Thetendencyintheterminalorcentralflowertobecomepeloricmorefrequentlythantheotherflowers，probablyresultsfrom"thebudwhichstandsontheendofashootreceivingthemostsap;itgrowsoutintoastrongershootthanthosesituatedlowerdown。"（26/21。HugovonMohl’TheVegetableCell’Englishtranslation1852page76。）Ihavediscussedtheconnectionbetweenpelorismandacentralposition，partlybecausesomefewplantsareknownnormallytoproduceaterminalflowerdifferentinstructurefromthelateralones;butchieflyonaccountofthefollowingcase，inwhichweseeatendencytovariabilityortoreversionconnectedwiththesameposition。A
greatjudgeofAuriculas（26/22。TheRev。H。H。Dombrainin’JournalofHorticulture’1861June4page174;andJune25page234;1862April29page83。）statesthatwhenonethrowsupasidebloomitisprettysuretokeepitscharacter;butthatifitgrowsfromthecentreorheartoftheplant，whateverthecolouroftheedgingoughttobe，"itisjustaslikelytocomeinanyotherclassasintheonetowhichitproperlybelongs。"Thisissonotoriousafact，thatsomefloristsregularlypinchoffthecentraltrussesofflowers。Whetherinthehighlyimprovedvarietiesthedepartureofthecentraltrussesfromtheirpropertypeisduetoreversion，Idonotknow。Mr。
Dombraininsiststhat，whatevermaybethecommonestkindofimperfectionineachvariety，thisisgenerallyexaggeratedinthecentraltruss。Thusonevariety"sometimeshasthefaultofproducingalittlegreenfloretinthecentreoftheflower，"andincentralbloomsthesebecomeexcessiveinsize。
Insomecentralblooms，senttomebyMr。Dombrain，alltheorgansoftheflowerwererudimentaryinstructure，ofminutesize，andofagreencolour，sothatbyalittlefurtherchangeallwouldhavebeenconvertedintosmallleaves。Inthiscaseweclearlyseeatendencytoprolification——atermwhichImayexplain，forthosewhohaveneverattendedtobotany，tomeantheproductionofabranchorflower，orheadofflowers，outofanotherflower。
NowDr。Masters（26/23。’Transact。Linn。Soc。’volume231861page360。）
statesthatthecentraloruppermostfloweronaplantisgenerallythemostliabletoprolification。Thus，inthevarietiesoftheAuricula，thelossoftheirpropercharacterandatendencytoprolification，alsoatendencytoprolificationwithpelorism，areallconnectedtogether，andaredueeithertoarresteddevelopment，ortoreversiontoaformercondition。
Thefollowingisamoreinterestingcase;Metzger（26/24。’DieGetreidearten’
1845s。208，209。）cultivatedinGermanyseveralkindsofmaizebroughtfromthehotterpartsofAmerica，andhefound，aspreviouslydescribed，thatintwoorthreegenerationsthegrainsbecamegreatlychangedinform，size，andcolour;andwithrespecttotworacesheexpresslystatesthatinthefirstgeneration，whilstthelowergrainsoneachheadretainedtheirpropercharacter，theuppermostgrainsalreadybegantoassumethatcharacterwhichinthethirdgenerationallthegrainsacquired。Aswedonotknowtheaboriginalparentofthemaize，wecannottellwhetherthesechangesareinanywayconnectedwithreversion。
Inthetwofollowingcases，reversioncomesintoplayandisdeterminedbythepositionoftheseedinthecapsule。TheBlueImperialpeaistheoffspringoftheBluePrussian，andhaslargerseedandbroaderpodsthanitsparent。NowMr。Masters，ofCanterbury，acarefulobserverandaraiserofnewvarietiesofthepea，states（26/25。’Gardener’sChronicle’1850page198。）thattheBlueImperialalwayshasastrongtendencytoreverttoitsparent—stock，andthereversion"occursinthismanner:thelast（oruppermost）peainthepodisfrequentlymuchsmallerthantherest;andifthesesmallpeasarecarefullycollectedandsownseparately，verymanymore，inproportion，willreverttotheirorigin，thanthosetakenfromtheotherpartsofthepod。"
Again，M。Chate（26/26。Quotedin’Gardener’sChronicle’1866page74。）saysthatinraisingseedlingstockshesucceedsingettingeightypercenttobeardoubleflowers，byleavingonlyafewofthesecondarybranchestoseed;butinadditiontothis，"atthetimeofextractingtheseeds，theupperportionofthepodisseparatedandplacedaside，becauseithasbeenascertainedthattheplantscomingfromtheseedssituatedinthisportionofthepod，giveeightypercentofsingleflowers。"Nowtheproductionofsingle—floweringplantsfromtheseedofdouble—floweringplantsisclearlyacaseofreversion。Theselatterfacts，aswellastheconnectionbetweenacentralpositionandpelorismandprolification，showinaninterestingmannerhowsmalladifference——namely，alittlegreaterorlessfreedomintheflowofsaptowardsonepartoftheplant——determinesimportantchangesofstructure。]
ANALOGOUSORPARALLELVARIATION。
BythistermImeanthatsimilarcharactersoccasionallymaketheirappearanceintheseveralvarietiesorracesdescendedfromthesamespecies，andmorerarelyintheoffspringofwidelydistinctspecies。Wearehereconcerned，notashithertowiththecausesofvariation，butwiththeresults;butthisdiscussioncouldnothavebeenmoreconvenientlyintroducedelsewhere。Thecasesofanalogousvariation，asfarastheiroriginisconcerned，maybegrouped，disregardingminorsubdivisions，undertwomainheads;firstly，thoseduetounknowncausesactingonsimilarlyconstitutedorganisms，andwhichconsequentlyhavevariedinasimilarmanner;andsecondly，thoseduetothereappearanceofcharacterswhichwerepossessedbyamoreorlessremoteprogenitor。Butthesetwomaindivisionscanoftenbeseparatedonlyconjecturally，andgraduate，asweshallpresentlysee，intoeachother。
[Underthefirstheadofanalogousvariations，notduetoreversion，wehavethemanycasesoftreesbelongingtoquitedifferentorderswhichhaveproducedpendulousandfastigiatevarieties。Thebeech，hazel，andbarberryhavegivenrisetopurple—leavedvarieties;and，asBernhardiremarks（26/27。
’UeberdenBegriffderPflanzenart’1834s。14。），amultitudeofplants，asdistinctaspossible，haveyieldedvarietieswithdeeply—cutorlaciniatedleaves。VarietiesdescendedfromthreedistinctspeciesofBrassicahavetheirstems，orso—calledroots，enlargedintoglobularmasses。Thenectarineistheoffspringofthepeach;andthevarietiesofpeachesandnectarinesofferaremarkableparallelisminthefruitbeingwhite，red，oryellowfleshed——inbeingclingstonesorfreestones——intheflowersbeinglargeorsmall——intheleavesbeingserratedorcrenated，furnishedwithgloboseorreniformglands，orquitedestituteofglands。Itshouldberemarkedthateachvarietyofthenectarinehasnotderiveditscharacterfromacorrespondingvarietyofthepeach。Theseveralvarietiesalsoofacloselyalliedgenus，namelytheapricot，differfromoneanotherinnearlythesameparallelmanner。Thereisnoreasontobelievethatanyofthesevarietieshavemerelyreacquiredlong—
lostcharacters;andinmostofthemthiscertainlyisnotthecase。
ThreespeciesofCucurbitahaveyieldedamultitudeofraceswhichcorrespondsocloselyincharacterthat，asNaudininsists，theymaybearrangedinalmoststrictlyparallelseries。Severalvarietiesofthemelonareinterestingfromresembling，inimportantcharacters，otherspecies，eitherofthesamegenusorofalliedgenera;thus，onevarietyhasfruitsolike，bothexternallyandinternally，thefruitofaperfectlydistinctspecies，namely，thecucumber，ashardlytobedistinguishedfromit;anotherhaslongcylindricalfruittwistingaboutlikeaserpent;inanothertheseedsadheretoportionsofthepulp;inanotherthefruit，whenripe，suddenlycracksandfallsintopieces;andallthesehighlyremarkablepeculiaritiesarecharacteristicofspeciesbelongingtoalliedgenera。Wecanhardlyaccountfortheappearanceofsomanyunusualcharactersbyreversiontoasingleancientform;butwemustbelievethatallthemembersofthefamilyhaveinheritedanearlysimilarconstitutionfromanearlyprogenitor。Ourcerealandmanyotherplantsoffersimilarcases。
Withanimalswehavefewercasesofanalogousvariation，independentlyofdirectreversion。Weseesomethingofthekindintheresemblancebetweentheshort—muzzledracesofthedog，suchasthepugandbull—dog;infeather—
footedracesofthefowl，pigeon，andcanary—bird;inhorsesofthemostdifferentracespresentingthesamerangeofcolour;inallblack—and—tandogshavingtan—colouredeye—spotsandfeet，butinthislattercasereversionmaypossiblyhaveplayedapart。Lowhasremarked（26/28。’DomesticatedAnimals’
1845page351。）thatseveralbreedsofcattleare"sheeted，"——thatis，haveabroadbandofwhitepassingroundtheirbodieslikeasheet;thischaracterisstronglyinherited，andsometimesoriginatesfromacross;itmaybethefirststepinreversiontoanearlytype，for，aswasshowninthethirdchapter，whitecattlewithdarkears，darkfeetandtipoftail，formerlyexisted，andnowexistinferalorsemi—feralconditioninseveralquartersoftheworld。
Underoursecondmaindivision，namely，ofanalogousvariationsduetoreversion，thebestcasesareaffordedbypigeons。Inallthemostdistinctbreeds，sub—varietiesoccasionallyappearcolouredexactlyliketheparentrock—pigeon，withblackwing—bars，whiteloins，bandedtail，etc。;andnoonecandoubtthatthesecharactersareduetoreversion。Sowithminordetails;
turbitsproperlyhavewhitetails，butoccasionallyabirdisbornwithadark—colouredandbandedtail;poutersproperlyhavetheirprimarywing—
featherswhite，butnotrarelya"sword—flighted"birdappears，thatis，onewiththefewfirstprimariesdark—coloured;andinthesecaseswehavecharacterspropertotherock—pigeon，butnewtothebreed，evidentlyappearingfromreversion。Insomedomesticvarietiesthewing—bars，insteadofbeingsimplyblack，asintherock—pigeon，arebeautifullyedgedwithdifferentzonesofcolour，andtheythenpresentastrikinganalogywiththewing—barsincertainnaturalspeciesofthesamefamily，suchasPhapschalcoptera;andthismayprobablybeaccountedforbyallthespeciesofthefamilybeingdescendedfromthesameremoteprogenitorandhavingatendencytovaryinthesamemanner。Thus，also，wecanperhapsunderstandthefactofsomeLaugher—pigeonscooingalmostliketurtle—doves，andforseveralraceshavingpeculiaritiesintheirflight，sincecertainnaturalspecies（viz。，C。
torquatrixandpalumbus），displaysingularvagariesinthisrespect。Inothercasesarace，insteadofimitatingadistinctspecies，resemblessomeotherrace;thus，certainruntstrembleandslightlyelevatetheirtails，likefantails;andturbitsinflatetheupperpartoftheiroesophagus，likepouter—
pigeons。
Itisacommoncircumstancetofindcertaincolouredmarkspersistentlycharacterisingallthespeciesofagenus，butdifferingmuchintint;andthesamethingoccurswiththevarietiesofthepigeon:thus，insteadofthegeneralplumagebeingblue，withthewing—barsblack，therearesnow—whitevarietieswithredbars，andblackvarietieswithwhitebars;inothervarietiesthewing—bars，aswehaveseen，areelegantlyzonedwithdifferenttints。TheSpotpigeonischaracterisedbythewholeplumagebeingwhite，exceptingaspotontheforeheadandthetail;butthesepartsmaybered，yellow，orblack。Intherock—pigeonandinmanyvarietiesthetailisblue，withtheouteredgesoftheouterfeatherswhite;butinthesub—varietyofthemonk—pigeonwehaveareversedstyleofcoloration，forthetailiswhite，excepttheouteredgesoftheouterfeathers，whichareblack。（26/29。
Bechstein’NaturgeschichteDeutschlands’b。41795s。31。）
Withsomespeciesofbirds，forinstancewithgulls，certaincolouredpartsappearasifalmostwashedout，andIhaveobservedexactlythesameappearanceintheterminaldarktail—barincertainpigeons，andinthewholeplumageofcertainvarietiesoftheduck。Analogousfactsinthevegetablekingdomcouldbegiven。
Manysub—varietiesofthepigeonhavereversedandsomewhatlengthenedfeathersonthebackpartoftheirheads，andthisiscertainlynotduetoreversiontotheparent—species，whichshowsnotraceofsuchstructure:butwhenwerememberthatsub—varietiesofthefowl，turkey，canary—bird，duck，andgoose，allhaveeithertopknotsorreversedfeathersontheirheads;andwhenwerememberthatscarcelyasinglelargenaturalgroupofbirdscanbenamed，inwhichsomemembershavenotatuftoffeathersontheirheads，wemaysuspectthatreversiontosomeextremelyremoteformhascomeintoaction。
Severalbreedsofthefowlhaveeitherspangledorpencilledfeathers;andthesecannotbederivedfromtheparent—species，theGallusbankiva;thoughofcourseitispossiblethatoneearlyprogenitorofthisspeciesmayhavebeenspangled，andanotherpencilled。But，asmanygallinaceousbirdsareeitherspangledorpencilled，itisamoreprobableviewthattheseveraldomesticbreedsofthefowlhaveacquiredthiskindofplumagefromallthemembersofthefamilyinheritingatendencytovaryinalikemanner。Thesameprinciplemayaccountfortheewesincertainbreedsofsheepbeinghornless，likethefemalesofsomeotherhollow—hornedruminants;itmayaccountforcertaindomesticcatshavingslightly—tuftedears，likethoseofthelynx;andfortheskullsofdomesticrabbitsoftendifferingfromoneanotherinthesamecharactersbywhichtheskullsofthevariousspeciesofthegenusLepusdiffer。
Iwillonlyalludetooneothercase，alreadydiscussed。Nowthatweknowthatthewildparentoftheasscommonlyhasstripedlegs，wemayfeelconfidentthattheoccasionalappearanceofstripesonthelegsofthedomesticassisduetoreversion;butthiswillnotaccountforthelowerendoftheshoulder—
stripebeingsometimesangularlybentorslightlyforked。So，again，whenweseedunandothercolouredhorseswithstripesonthespine，shoulders，andlegs，weareled，fromreasonsformerlygiven，tobelievethattheyreappearthroughreversiontothewildparent—horse。Butwhenhorseshavetwoorthreeshoulder—stripes，withoneofthemoccasionallyforkedatthelowerend，orwhentheyhavestripesontheirface...完整阅读请扫描二维码下载丁香书院APP免费看：