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，orwhentheyhavestripesontheirfaces，orarefaintlystripedasfoalsovernearlytheirwholebodies，withthestripesangularlybentoneundertheotherontheforehead，orirregularlybranchedinotherparts，itwouldberashtoattributesuchdiversifiedcharacterstothereappearanceofthosepropertotheaboriginalwildhorse。AsthreeAfricanspeciesofthegenusaremuchstriped，andaswehaveseenthatthecrossingoftheunstripedspeciesoftenleadstothehybridoffspringbeingconspicuouslystriped——bearingalsoinmindthattheactofcrossingcertainlycausesthereappearanceoflong—lostcharacters——itisamoreprobableviewthattheabove—specifiedstripesareduetoreversion，nottotheimmediatewildparent—horse，buttothestripedprogenitorofthewholegenus。]
Ihavediscussedthissubjectofanalogousvariationatconsiderablelength，becauseitiswellknownthatthevarietiesofonespeciesfrequentlyresembledistinctspecies——afactinperfectharmonywiththeforegoingcases，andexplicableonthetheoryofdescent。Secondly，becausethesefactsareimportantfromshowing，asremarkedinaformerchapter，thateachtriflingvariationisgovernedbylaw，andisdeterminedinamuchhigherdegreebythenatureoftheorganisation，thanbythenatureoftheconditionstowhichthevaryingbeinghasbeenexposed。Thirdly，becausethesefactsaretoacertainextentrelatedtoamoregenerallaw，namely，thatwhichMr。B。D。Walsh（26/30。’Proc。Entomolog。Soc。ofPhiladelphia’October1863page213。）hascalledthe"LawofEQUABLEVARIABILITY，"or，asheexplainsit，"ifanygivencharacterisveryvariableinonespeciesofagroup，itwilltendtobevariableinalliedspecies;andifanygivencharacterisperfectlyconstantinonespeciesofagroup，itwilltendtobeconstantinalliedspecies。"
ThisleadsmetorecalladiscussioninthechapteronSelection，inwhichitwasshownthatwithdomesticraces，whicharenowundergoingrapidimprovement，thosepartsorcharactersvarythemost，whicharethemostvalued。Thisnaturallyfollowsfromrecentlyselectedcharacterscontinuallytendingtoreverttotheirformerlessimprovedstandard，andfromtheirbeingstillactedonbythesameagencies，whateverthesemaybe，whichfirstcausedthecharactersinquestiontovary。Thesameprincipleisapplicabletonaturalspecies，for，asstatedinmy’OriginofSpecies’genericcharactersarelessvariablethanspecificcharacters;andthelatterarethosewhichhavebeenmodifiedbyvariationandnaturalselection，sincetheperiodwhenallthespeciesbelongingtothegenusbranchedofffromacommonprogenitor，whilstgenericcharactersarethosewhichhaveremainedunalteredfromamuchmoreremoteepoch，andaccordinglyarenowlessvariable。ThisstatementmakesanearapproachtoMr。Walsh’slawofEquableVariability。Secondarysexualcharacters，itmaybeadded，rarelyservetocharacterisedistinctgenera，fortheyusuallydiffermuchinthespeciesofthesamegenus，andtheyarehighlyvariableintheindividualsofthesamespecies;wehavealsoseenintheearlierchaptersofthisworkhowvariablesecondarysexualcharactersbecomeunderdomestication。
SUMMARYOFTHETHREEPREVIOUSCHAPTERSONTHELAWSOFVARIATION。
Inthetwenty—thirdchapterwesawthatchangedconditionsoccasionally，orevenoften，actinadefinitemannerontheorganisation，sothatall，ornearlyall，theindividualsthusexposedbecomemodifiedinthesamemanner。
Butafarmorefrequentresultofchangedconditions，whetheractingdirectlyontheorganisationorindirectlythroughthereproductivesystem，isindefiniteandfluctuatingvariability。Inthethreelastchapters，someofthelawsbywhichsuchvariabilityisregulatedhavebeendiscussed。
Increaseduseaddstothesizeofmuscles，togetherwiththeblood—vessels，nerves，ligaments，thecrestsofboneandthewholebones，towhichtheyareattached。Increasedfunctionalactivityincreasesthesizeofvariousglands，andstrengthensthesense—organs。Increasedandintermittentpressurethickenstheepidermis。Achangeinthenatureofthefoodsometimesmodifiesthecoatsofthestomach，andaugmentsordecreasesthelengthoftheintestines。
Continueddisuse，ontheotherhand，weakensanddiminishesallpartsoftheorganisation。Animalswhichduringmanygenerationshavetakenbutlittleexercise，havetheirlungsreducedinsize，andasaconsequencethebonyfabricofthechestandthewholeformofthebodybecomemodified。Withourancientlydomesticatedbirds，thewingshavebeenlittleused，andtheyareslightlyreduced;withtheirdecrease，thecrestofthesternum，thescapulae，coracoids，andfurculum，haveallbeenreduced。
Withdomesticatedanimals，thereductionofapartfromdisuseisnevercarriedsofarthatamererudimentisleft;whereaswehavereasontobelievethatthishasoftenoccurredundernature;theeffectsofdisuseinthislattercasebeingaidedbyeconomyofgrowth，togetherwiththeintercrossingofmanyvaryingindividuals。Thecauseofthisdifferencebetweenorganismsinastateofnature，andunderdomestication，probablyisthatinthelattercasetherehasnotbeentimesufficientforanyverygreatchange，andthattheprincipleofeconomyofgrowthdoesnotcomeintoaction。Onthecontrary，structureswhicharerudimentaryintheparent—species，sometimesbecomepartiallyredevelopedinourdomesticatedproductions。Suchrudimentsasoccasionallymaketheirappearanceunderdomestication，seemalwaystobetheresultofasuddenarrestofdevelopment;neverthelesstheyareofinterest，asshowingthatrudimentsaretherelicsoforgansonceperfectlydeveloped。
Corporeal，periodical，andmentalhabits，thoughthelatterhavebeenalmostpassedoverinthiswork，becomechangedunderdomestication，andthechangesareofteninherited。Suchchangedhabitsinanorganicbeing，especiallywhenlivingafreelife，wouldoftenleadtotheaugmentedordiminisheduseofvariousorgans，andconsequentlytotheirmodification。Fromlong—continuedhabit，andmoreespeciallyfromtheoccasionalbirthofindividualswithaslightlydifferentconstitution，domesticanimalsandcultivatedplantsbecometoacertainextentacclimatisedoradaptedtoaclimatedifferentfromthatpropertotheparent—species。
Throughtheprincipleofcorrelatedvariability，takeninitswidestsense，whenonepartvariesotherpartsvary，eithersimultaneously，oroneaftertheother。Thus，anorganmodifiedduringanearlyembryonicperiodaffectsotherpartssubsequentlydeveloped。Whenanorgan，suchasthebeak，increasesordecreasesinlength，adjoiningorcorrelatedparts，asthetongueandtheorificeofthenostrils，tendtovaryinthesamemanner。Whenthewholebodyincreasesordecreasesinsize，variouspartsbecomemodified;thus，withpigeonstheribsincreaseordecreaseinnumberandbreadth。Homologouspartswhichareidenticalduringtheirearlydevelopmentandareexposedtosimilarconditions，tendtovaryinthesameorinsomeconnectedmanner，——asinthecaseoftherightandleftsidesofthebody，andofthefrontandhindlimbs。
Soitiswiththeorgansofsightandhearing;forinstance，whitecatswithblueeyesarealmostalwaysdeaf。Thereisamanifestrelationthroughoutthebodybetweentheskinandvariousdermalappendages，suchashair，feathers，hoofs，horns，andteeth。InParaguay，horseswithcurlyhairhavehoofslikethoseofamule;thewoolandthehornsofsheepoftenvarytogether;hairlessdogsaredeficientintheirteeth;menwithredundanthairhaveabnormalteeth，eitherbydeficiencyorexcess。Birdswithlongwing—feathersusuallyhavelongtail—feathers。Whenlongfeathersgrowfromtheoutsideofthelegsandtoesofpigeons，thetwooutertoesareconnectedbymembrane;forthewholelegtendstoassumethestructureofthewing。Thereisamanifestrelationbetweenacrestoffeathersontheheadandamarvellousamountofchangeintheskullofvariousfowls;andinalesserdegree，betweenthegreatlyelongated，loppingearsofrabbitsandthestructureoftheirskulls。
Withplants，theleaves，variouspartsoftheflower，andthefruit，oftenvarytogethertoacorrelatedmanner。
Insomecaseswefindcorrelationwithoutbeingableeventoconjecturewhatisthenatureoftheconnection，aswithvariousmonstrositiesanddiseases。
Thisislikewisethecasewiththecolouroftheadultpigeon，inconnectionwiththepresenceofdownontheyoungbird。Numerouscuriousinstanceshavebeengivenofpeculiaritiesofconstitution，incorrelationwithcolour，asshownbytheimmunityofindividualsofonecolourfromcertaindiseases，fromtheattacksofparasitesandfromtheactionofcertainvegetablepoisons。
Correlationisanimportantsubject;forwithspecies，andinalesserdegreewithdomesticraces，wecontinuallyfindthatcertainpartshavebeengreatlymodifiedtoservesomeusefulpurpose;butwealmostinvariablyfindthatotherpartshavelikewisebeenmoreorlessmodified，withoutourbeingabletodiscoveranyadvantageinthechange。Nodoubtgreatcautionisnecessarywithrespecttothislatterpoint，foritisdifficulttooverrateourignoranceontheuseofvariouspartsoftheorganisation;butfromwhatwehaveseen，wemaybelievethatmanymodificationsareofnodirectservice，havingarisenincorrelationwithotherandusefulchanges。
Homologouspartsduringtheirearlydevelopmentoftenbecomefusedtogether。
Multipleandhomologousorgansareespeciallyliabletovaryinnumberandprobablyinform。Asthesupplyoforganisedmatterisnotunlimited，theprincipleofcompensationsometimescomesintoaction;sothat，whenonepartisgreatlydeveloped，adjoiningpartsareapttobereduced;butthisprincipleisprobablyofmuchlessimportancethanthemoregeneraloneoftheeconomyofgrowth。Throughmeremechanicalpressurehardpartsoccasionallyaffectadjoiningparts。Withplantsthepositionoftheflowersontheaxis，andoftheseedsintheovary，sometimesleads，throughamoreorlessfreeflowofsap，tochangesofstructure;butsuchchangesareoftenduetoreversion。Modifications，inwhatevermannercaused，willbetoacertainextentregulatedbythatco—ordinatingpower，orso—callednisusformativus，whichisinfactaremnantofthatsimpleformofreproduction，displayedbymanylowlyorganisedbeingsintheirpoweroffissiparousgenerationandbudding。Finally，theeffectsofthelawswhichdirectlyorindirectlygovernvariability，maybelargelyregulatedbyman’sselection，andwillsofarbedeterminedbynaturalselectionthatchangesadvantageoustoanyracewillbefavoured，anddisadvantageouschangeswillbechecked。
Domesticracesdescendedfromthesamespecies，orfromtwoormorealliedspecies，areliabletoreverttocharactersderivedfromtheircommonprogenitor;and，astheyinheritasomewhatsimilarconstitution，theyareliabletovaryinthesamemanner。Fromthesetwocausesanalogousvarietiesoftenarise。Whenwereflectontheseveralforegoinglaws，imperfectlyasweunderstandthem，andwhenwebearinmindhowmuchremainstobediscovered，weneednotbesurprisedattheintricateandtousunintelligiblemannerinwhichourdomesticproductionshavevaried，andstillgoonvarying。
CHAPTER2。XXVII。
PROVISIONALHYPOTHESISOFPANGENESIS。
PRELIMINARYREMARKS。
FIRSTPART:
THEFACTSTOBECONNECTEDUNDERASINGLEPOINTOFVIEW，NAMELY，THEVARIOUS
KINDSOFREPRODUCTION。
REGROWTHOFAMPUTATEDPARTS。
GRAFT—HYBRIDS。
THEDIRECTACTIONOFTHEMALEELEMENTONTHEFEMALE。
DEVELOPMENT。
THEFUNCTIONALINDEPENDENCEOFTHEUNITSOFTHEBODY。
VARIABILITY。
INHERITANCE。
REVERSION。
SECONDPART:
STATEMENTOFTHEHYPOTHESIS。
HOWFARTHENECESSARYASSUMPTIONSAREIMPROBABLE。
EXPLANATIONBYAIDOFTHEHYPOTHESISOFTHESEVERALCLASSESOFFACTSSPECIFIED
INTHEFIRSTPART。
CONCLUSION。
Inthepreviouschapterslargeclassesoffacts，suchasthosebearingonbud—
variation，thevariousformsofinheritance，thecausesandlawsofvariation，havebeendiscussed;anditisobviousthatthesesubjects，aswellastheseveralmodesofreproduction，standinsomesortofrelationtooneanother。
Ihavebeenled，orratherforced，toformaviewwhichtoacertainextentconnectsthesefactsbyatangiblemethod。Everyonewouldwishtoexplaintohimself，eveninanimperfectmanner，howitispossibleforacharacterpossessedbysomeremoteancestorsuddenlytoreappearintheoffspring;howtheeffectsofincreasedordecreaseduseofalimbcanbetransmittedtothechild;howthemalesexualelementcanactnotsolelyontheovules，butoccasionallyonthemother—form;howahybridcanbeproducedbytheunionofthecellulartissueoftwoplantsindependentlyoftheorgansofgeneration;
howalimbcanbereproducedontheexactlineofamputation，withneithertoomuchnortoolittleadded;howthesameorganismmaybeproducedbysuchwidelydifferentprocesses，asbuddingandtrueseminalgeneration;and，lastly，howoftwoalliedforms，onepassesinthecourseofitsdevelopmentthroughthemostcomplexmetamorphoses，andtheotherdoesnotdoso，thoughwhenmaturebotharealikeineverydetailofstructure。Iamawarethatmyviewismerelyaprovisionalhypothesisorspeculation;butuntilabetteronebeadvanced，itwillservetobringtogetheramultitudeoffactswhichareatpresentleftdisconnectedbyanyefficientcause。AsWhewell，thehistorianoftheinductivesciences，remarks:——"Hypothesesmayoftenbeofservicetoscience，whentheyinvolveacertainportionofincompleteness，andevenoferror。"UnderthispointofviewIventuretoadvancethehypothesisofPangenesis，whichimpliesthateveryseparatepartofthewholeorganisationreproducesitself。Sothatovules，spermatozoa，andpollen—grains，——thefertilisedeggorseed，aswellasbuds，——includeandconsistofamultitudeofgermsthrownofffromeachseparatepartorunit。（27/1。Thishypothesishasbeenseverelycriticisedbymanywriters，anditwillbefairtogivereferencestothemoreimportantarticles。ThebestessaywhichIhaveseenisbyProf。Delpino，entitled’SullaDarwinianaTeoriadellaPangenesi，1869’ofwhichatranslationappearedin’ScientificOpinion’September29，1869andthesucceedingnumbers。Herejectsthehypothesis，butcriticisesitfairly，andIhavefoundhiscriticismsveryuseful。Mr。Mivart（’GenesisofSpecies’
1871chapter10。）followsDelpino，butaddsnonewobjectionsofanyweight。
Dr。Bastian（’TheBeginningsofLife’1872volume2page98）saysthatthehypothesis"lookslikearelicoftheoldratherthanafittingappanageofthenewevolutionphilosophy。"HeshowsthatIoughtnottohaveusedtheterm"pangenesis，"asithadbeenpreviouslyusedbyDr。Grosinanothersense。Dr。
LionelBeale（’Nature’May11，1871page26）sneersatthewholedoctrinewithmuchacerbityandsomejustice。Prof。Wigand（’SchriftenderGesell。dergesammt。Naturwissen。zuMarburg’b。91870）considersthehypothesisasunscientificandworthless。Mr。G。H。Lewes（’FortnightlyReview’November1，1868page503）seemstoconsiderthatitmaybeuseful:hemakesmanygoodcriticismsinaperfectlyfairspirit。Mr。F。Galton，afterdescribinghisvaluableexperiments（’Proc。RoyalSoc。’volume19page393）ontheintertransfusionofthebloodofdistinctvarietiesoftherabbit，concludesbysayingthatinhisopiniontheresultsnegativebeyondalldoubtthedoctrineofPangenesis。Heinformsmethatsubsequentlytothepublicationofhispaperhecontinuedhisexperimentsonastilllargerscalefortwomoregenerations，withoutanysignofmongrelismshowingitselfintheverynumerousoffspring。Icertainlyshouldhaveexpectedthatgemmuleswouldhavebeenpresentintheblood，butthisisnonecessarypartofthehypothesis，whichmanifestlyappliestoplantsandthelowestanimals。Mr。Galton，inaletterto’Nature’（April27，1871page502），alsocriticisesvariousincorrectexpressionsusedbyme。Ontheotherhand，severalwritershavespokenfavourablyofthehypothesis，buttherewouldbenouseingivingreferencestotheirarticles。Imay，however，refertoDr。Ross’work，’TheGraftTheoryofDisease;beinganapplicationofMr。Darwin’shypothesisofPangenesis’1872ashegivesseveraloriginalandingeniousdiscussions。）
IntheFirstPartIwillenumerateasbrieflyasIcanthegroupsoffactswhichseemtodemandconnection;butcertainsubjects，nothithertodiscussed，mustbetreatedatdisproportionatelength。IntheSecondPartthehypothesiswillbegiven;andafterconsideringhowfarthenecessaryassumptionsareinthemselvesimprobable，weshallseewhetheritservestobringunderasinglepointofviewthevariousfacts。
PARTI。
Reproductionmaybedividedintotwomainclasses，namely，sexualandasexual。
Thelatteriseffectedinmanyways——bytheformationofbudsofvariouskinds，andbyfissiparousgeneration，thatisbyspontaneousorartificialdivision。Itisnotoriousthatsomeoftheloweranimals，whencutintomanypieces，reproducesomanyperfectindividuals:LyonnetcutaNaisorfreshwaterwormintonearlyfortypieces，andtheseallreproducedperfectanimals。（27/2。QuotedbyPaget’LecturesonPathology’1853page159。）Itisprobablethatsegmentationcouldbecarriedmuchfurtherinsomeoftheprotozoa;andwithsomeofthelowestplantseachcellwillreproducetheparent—form。JohannesMullerthoughtthattherewasanimportantdistinctionbetweengemmationandfission;forinthelattercasethedividedportion，howeversmall，ismorefullydevelopedthanabud，whichalsoisayoungerformation;butmostphysiologistsarenowconvincedthatthetwoprocessesareessentiallyalike。（27/3。Dr。Lachmannalsoobserves（’AnnalsandMag。ofNat。
History’2ndseriesvolume191857page231）withrespecttoinfusoria，that"fissationandgemmationpassintoeachotheralmostimperceptibly。"Again，Mr。W。C。Minor（’AnnalsandMag。ofNat。Hist。’3rdseriesvolume11page328）
showsthatwithAnnelidsthedistinctionthathasbeenmadebetweenfissionandbuddingisnotafundamentalone。SeealsoProfessorClark’swork’MindinNature’NewYork1865pages62，94。）Prof。Huxleyremarks，"fissionislittlemorethanapeculiarmodeofbudding，"andProf。H。J。Clarkshowsindetailthatthereissometimes"acompromisebetweenself—divisionandbudding。"Whenalimbisamputated，orwhenthewholebodyisbisected，thecutextremitiesaresaidtobudforth（27/4。SeeBonnet’Oeuvresd’Hist。Nat。’tome51781
page339forremarksonthebudding—outoftheamputatedlimbsofSalamanders。）;andasthepapilla，whichisfirstformed，consistsofundevelopedcellulartissuelikethatforminganordinarybud，theexpressionisapparentlycorrect。Weseetheconnectionofthetwoprocessesinanotherway;forTrembleyobservedwiththehydra，thatthereproductionoftheheadafteramputationwascheckedassoonastheanimalputforthreproductivegemmae。（27/5。Paget’LecturesonPathology’1853page158。）
Betweentheproduction，byfissiparousgeneration，oftwoormorecompleteindividuals，andtherepairofevenaveryslightinjury，thereissoperfectagradation，thatitisimpossibletodoubtthatthetwoprocessesareconnected。Asateachstageofgrowthanamputatedpartisreplacedbyoneinthesamestateofdevelopment，wemustalsofollowSirJ。Pagetinadmitting，"thatthepowersofdevelopmentfromtheembryo，areidenticalwiththoseexercisedfortherestorationfrominjuries:inotherwords，thatthepowersarethesamebywhichperfectionisfirstachieved，andbywhich，whenlost，itisrecovered。"（27/6。Ibidpages152，164。）Finally，wemayconcludethattheseveralformsofbudding，fissiparousgeneration，therepairofinjuries，anddevelopment，areallessentiallytheresultsofoneandthesamepower。
SEXUALGENERATION。
Theunionofthetwosexualelementsseemsatfirstsighttomakeabroaddistinctionbetweensexualandasexualgeneration。Buttheconjugationofalgae，bywhichprocessthecontentsoftwocellsuniteintoasinglemasscapableofdevelopment，apparentlygivesusthefirststeptowardssexualunion:andPringsheim，inhismemoironthepairingofZoospores（27/7。
Translatedin’AnnalsandMag。ofNat。Hist。’April1870page272。），showsthatconjugationgraduatesintotruesexualreproduction。Moreover，thenowwell—ascertainedcasesofParthenogenesisprovethatthedistinctionbetweensexualandasexualgenerationisnotnearlysogreataswasformerlythought;
forovaoccasionally，andeveninsomecasesfrequently，becomedevelopedintoperfectbeings，withouttheconcourseofthemale。Withmostoftheloweranimalsandevenwithmammals，theovashowatraceofparthenogeneticpower，forwithoutbeingfertilisedtheypassthroughthefirststagesofsegmentation。（27/8。BischoffasquotedbyvonSiebold"UeberParthenogenesis"
’Sitzungdermath。phys。Classe。’MunichNovember4，1871page240。SeealsoQuatrefages’AnnalesdesSc。Nat。Zoolog。’3rdseries1850page138。）Norcanpseudovawhichdonotneedfertilisation，bedistinguishedfromtrueova，aswasfirstshownbySirJ。Lubbock，andisnowadmittedbySiebold。So，again，thegerm—ballsinthelarvaeofCecidomyiaaresaidbyLeuckart（27/9。’OntheAsexualReproductionofCecidomyideLarvae’translatedin’AnnalsandMag。ofNat。Hist。’March1866pages167，171。）tobeformedwithintheovarium，buttheydonotrequiretobefertilised。Itshouldalsobeobservedthatinsexualgeneration，theovulesandthemaleelementhaveequalpoweroftransmittingeverysinglecharacterpossessedbyeitherparenttotheiroffspring。Weseethisclearlywhenhybridsarepairedinterse，forthecharactersofbothgrandparentsoftenappearintheprogeny，eitherperfectlyorbysegments。Itisanerrortosupposethatthemaletransmitscertaincharactersandthefemaleothercharacters;althoughnodoubt，fromunknowncauses，onesexsometimeshasamuchstrongerpoweroftransmissionthantheother。
Ithas，however，beenmaintainedbysomeauthorsthatabuddiffersessentiallyfromafertilisedgerm，inalwaysreproducingtheperfectcharacteroftheparent—stock;whilstfertilisedgermsgivebirthtovariablebeings。Butthereisnosuchbroaddistinctionasthis。Intheeleventhchapternumerouscaseswereadvancedshowingthatbudsoccasionallygrowintoplantshavingquitenewcharacters;andthevarietiesthusproducedcanbepropagatedforalengthoftimebybuds，andoccasionallybyseed。
Nevertheless，itmustbeadmittedthatbeingsproducedsexuallyaremuchmoreliabletovarythanthoseproducedasexually;andofthisfactapartialexplanationwillhereafterbeattempted。Thevariabilityinbothcasesisdeterminedbythesamegeneralcauses，andisgovernedbythesamelaws。Hencenewvarietiesarisingfrombudscannotbedistinguishedfromthosearisingfromseed。Althoughbud—varietiesusuallyretaintheircharacterduringsuccessivebud—generations，yettheyoccasionallyrevert，evenafteralongseriesofbud—generations，totheirformercharacter。Thistendencytoreversioninbuds，isoneofthemostremarkableoftheseveralpointsofagreementbetweentheoffspringfrombudandseminalreproduction。
Butthereisonedifferencebetweenorganismsproducedsexuallyandasexually，whichisverygeneral。Theformerpassinthecourseoftheirdevelopmentfromaverylowstagetotheirhigheststage，asweseeinthemetamorphosesofinsectsandofmanyotheranimals，andintheconcealedmetamorphosesofthevertebrata。Animalspropagatedasexuallybybudsorfission，ontheotherhand，commencetheirdevelopmentatthatstageatwhichthebuddingorself—
dividinganimalmayhappentobe，andthereforedonotpassthroughsomeofthelowerdevelopmentalstages。（27/10。Prof。Allmanspeaks（’Transact。R。
Soc。ofEdinburgh’volume261870page102）decisivelyonthisheadwithrespecttotheHydroida:hesays，"Itisauniversallawinthesuccessionofzooids，thatnoretrogressionevertakesplaceintheseries。"）Afterwards，theyoftenadvanceinorganisation，asweseeinthemanycasesof"alternategeneration。"Inthusspeakingofalternategeneration，Ifollowthosenaturalistswholookatthisprocessasessentiallyoneofinternalbuddingoroffissiparousgeneration。Someofthelowerplants，however，suchasmossesandcertainalgae，accordingtoDr。L。Radlkofer（27/11。’AnnalsandMag。ofNat。Hist。’2ndseriesvolume201857pages153—455），whenpropagatedasexually，doundergoaretrogressivemetamorphosis。Asfarasthefinalcauseisconcerned，wecantoacertainextentunderstandwhybeingspropagatedbybudsshouldnotpassthroughalltheearlystagesofdevelopment;forwitheachorganismthestructureacquiredateachstagemustbeadaptedtoitspeculiarhabits;andifthereareplacesforthesupportofmanyindividualsatsomeonestage，thesimplestplanwillbethattheyshouldbemultipliedatthisstage，andnotthattheyshouldfirstretrogradeintheirdevelopmenttoanearlierorsimplerstructure，whichmightnotbefittedforthethensurroundingconditions。
Fromtheseveralforegoingconsiderationswemayconcludethatthedifferencebetweensexualandasexualgenerationisnotnearlysogreatasatfirstappears;thechiefdifferencebeingthatanovulecannotcontinuetoliveandtobefullydevelopedunlessituniteswiththemaleelement;buteventhisdifferenceisfarfrominvariable，asshownbythemanycasesofparthenogenesis。Wearethereforenaturallyledtoinquirewhatthefinalcausecanbeofthenecessityinordinarygenerationfortheconcourseofthetwosexualelements。
Seedsandovaareoftenhighlyserviceableasthemeansofdisseminatingplantsandanimals，andofpreservingthemduringoneormoreseasonsinadormantstate;butunimpregnatedseedsorova，anddetachedbuds，wouldbeequallyserviceableforbothpurposes。Wecan，however，indicatetwoimportantadvantagesgainedbytheconcourseofthetwosexes，orratheroftwoindividualsbelongingtooppositesexes;for，asIhaveshowninaformerchapter，thestructureofeveryorganismappearstobeespeciallyadaptedfortheconcurrence，atleastoccasionally，oftwoindividuals。Whenspeciesarerenderedhighlyvariablebychangedconditionsoflife，thefreeintercrossingofthevaryingindividualstendstokeepeachformfittedforitsproperplaceinnature;andcrossingcanbeeffectedonlybysexualgeneration;butwhethertheendthusgainedisofsufficientimportancetoaccountforthefirstoriginofsexualintercourseisextremelydoubtful。Secondly，Ihaveshownfromalargebodyoffacts，that，asaslightchangeintheconditionsoflifeisbeneficialtoeachcreature，so，inananalogousmanner，isthechangeeffectedinthegermbysexualunionwithadistinctindividual;andIhavebeenled，fromobservingthemanywidely—extendedprovisionsthroughoutnatureforthispurpose，andfromthegreatervigourofcrossedorganismsofallkinds，asprovedbydirectexperiments，aswellasfromtheevileffectsofcloseinterbreedingwhenlongcontinued，tobelievethattheadvantagethusgainedisverygreat。
Whythegerm，whichbeforeimpregnationundergoesacertainamountofdevelopment，ceasestoprogressandperishes，unlessitbeactedonbythemaleelement;andwhyconverselythemaleelement，whichinthecaseofsomeinsectsisenabledtokeepaliveforfourorfiveyears，andinthecaseofsomeplantsforseveralyears，likewiseperishes，unlessitactsonoruniteswiththegerm，arequestionswhichcannotbeansweredwithcertainty。Itis，however，probablethatbothsexualelementsperish，unlessbroughtintounion，simplyfromincludingtoolittleformativematterforindependentdevelopment。
QuatrefageshasshowninthecaseoftheTeredo（27/12。’AnnalesdesSc。Nat。’
3rdseries1850tome13。），asdidformerlyPrevostandDumaswithotheranimals，thatmorethanonespermatozoonisrequisitetofertiliseanovum。
ThishaslikewisebeenshownbyNewport（27/13。’Transact。Phil。Soc。’1851
pages196，208，210;1853pages245，247。），whoprovedbynumerousexperiments，that，whenaverysmallnumberofspermatozoaareappliedtotheovaofBatrachians，theyareonlypartiallyimpregnated，andanembryoisneverfullydeveloped。Theratealsoofthesegmentationoftheovumisdeterminedbythenumberofthespermatozoa。Withrespecttoplants，nearlythesameresultswereobtainedbyKolreuterandGartner。Thislastcarefulobserver，aftermakingsuccessivetrialsonaMalvawithmoreandmorepollen—
grains，found（27/14。’BeitragezurKenntniss’etc。1844s。345。），thateventhirtygrainsdidnotfertiliseasingleseed;butwhenfortygrainswereappliedtothestigma，afewseedsofsmallsizewereformed。InthecaseofMirabilisthepollengrainsareextraordinarilylarge，andtheovariumcontainsonlyasingleovule;andthesecircumstancesledNaudin（27/15。
’NouvellesArchivesduMuseum’tome1page27。）tomakethefollowingexperiments:aflowerwasfertilisedbythreegrainsandsucceededperfectly;
twelveflowerswerefertilisedbytwograins，andseventeenflowersbyasinglegrain，andoftheseonefloweraloneineachlotperfecteditsseed:
anditdeservesespecialnoticethattheplantsproducedbythesetwoseedsneverattainedtheirproperdimensions，andboreflowersofremarkablysmallsize。Fromthesefactsweclearlyseethatthequantityofthepeculiarformativematterwhichiscontainedwithinthespermatozoaandpollen—grainsisanall—importantelementintheactoffertilisation，notonlyforthefulldevelopmentoftheseed，butforthevigouroftheplantproducedfromsuchseed。Weseesomethingofthesamekindincertaincasesofparthenogenesis，thatis，whenthemaleelementiswhollyexcluded;forM。Jourdan（27/16。AsquotedbySirJ。Lubbockin’Nat。Hist。Review’1862page345。Weijenberghalsoraised（’Nature’December21，1871page149）twosuccessivegenerationsfromunimpregnatedfemalesofanotherlepidopterousinsect，Liparisdispar。
Thesefemalesdidnotproduceatmostone—twentiethoftheirfullcomplementofeggs，andmanyoftheeggswereworthless。Moreoverthecaterpillarsraisedfromtheseunfertilisedeggs"possessedfarlessvitality"thanthosefromfertilisedeggs。Inthethirdparthenogeneticgenerationnotasingleeggyieldedacaterpillar。）foundthat，outofabout58，000eggslaidbyunimpregnatedsilk—moths，manypassedthroughtheirearlyembryonicstages，showingthattheywerecapableofself—development，butonlytwenty—nineoutofthewholenumberproducedcaterpillars。Thesameprincipleofquantityseemstoholdgoodeveninartificialfissiparousreproduction，forHackel（27/17。’EntwickelungsgeschichtederSiphonophora’1869page73。）foundthatbycuttingthesegmentedandfertilisedovaorlarvaofSiphonophorae（jelly—
fishes）intopieces，thesmallerthepieceswere，theslowerwastherateofdevelopment，andthelarvaethusproducedwerebysomuchthemoreimperfectandinclinedtomonstrosity。Itseems，therefore，probablethatwiththeseparatesexualelementsdeficientquantityofformativematteristhemaincauseoftheirnothavingthecapacityforprolongedexistenceanddevelopment，unlesstheycombineandthusincreaseeachother’sbulk。Thebeliefthatitisthefunctionofthespermatozoatocommunicatelifetotheovuleseemsastrangeone，seeingthattheunimpregnatedovuleisalreadyaliveandgenerallyundergoesacertainamountofindependentdevelopment。