forinstance，somekindsofmossesandlichenshaveneverbeenseentofructifyinFrance。
Someoftheseendemicandnaturalisedplantsareprobablyrenderedsterilefromexcessivemultiplicationbybuds，andtheirconsequentincapacitytoproduceandnourishseed。Butthesterilityofothersmoreprobablydependsonthepeculiarconditionsunderwhichtheylive，asinthecaseoftheivyinthenorthernpartofEurope，andofthetreesintheswampsoftheUnitedStates;yettheseplantsmustbeinsomerespectseminentlywelladaptedforthestationswhichtheyoccupy，fortheyholdtheirplacesagainstahostofcompetitors。]
Finally，thehighdegreeofsterilitywhichoftenaccompaniesthedoublingofflowers，oranexcessivedevelopmentoffruit，seldomsupervenesatonce。Anincipienttendencyisobserved，andcontinuedselectioncompletestheresult。
Theviewwhichseemsthemostprobable，andwhichconnectstogetheralltheforegoingfactsandbringsthemwithinourpresentsubject，is，thatchangedandunnaturalconditionsoflifefirstgiveatendencytosterility;andinconsequenceofthis，theorgansofreproductionbeingnolongerablefullytoperformtheirproperfunctions，asupplyoforganisedmatter，notrequiredforthedevelopmentoftheseed，flowseitherintotheseorgansandrendersthemfoliaceous，orintothefruit，stems，tubers，etc。，increasingtheirsizeandsucculency。Butitisprobablethatthereexists，independentlyofanyincipientsterility，anantagonismbetweenthetwoformsofreproduction，namely，byseedandbuds，wheneitheriscarriedtoanextremedegree。Thatincipientsterilityplaysanimportantpartinthedoublingofflowers，andintheothercasesjustspecified，Iinferchieflyfromthefollowingfacts。Whenfertilityislostfromawhollydifferentcause，namely，fromhybridism，thereisastrongtendency，asGartner（18/118。’Bastarderzeugung’s。565。Kolreuter’DritteFortsetzung’s。73，87，119）alsoshowsthatwhentwospecies，onesingleandtheotherdouble，arecrossed，thehybridsareapttobeextremelydouble。）affirms，forflowerstobecomedouble，andthistendencyisinherited。Moreover，itisnotoriousthatwithhybridsthemaleorgansbecomesterilebeforethefemaleorgans，andwithdoubleflowersthestamensfirstbecomefoliaceous。Thislatterfactiswellshownbythemaleflowersofdioeciousplants，which，accordingtoGallesio（18/119。’TeoriadellaRiproduzioneVeg。’1816page73。）firstbecomedouble。Again，Gartner（18/120。
’Bastarderzeugung’s。573。）ofteninsiststhattheflowersofevenutterlysterilehybrids，whichdonotproduceanyseed，generallyyieldperfectcapsulesorfruit，——afactwhichhaslikewisebeenrepeatedlyobservedbyNaudinwiththeCucurbitaceae;sothattheproductionoffruitbyplantsrenderedsterilethroughanycauseisintelligible。Kolreuterhasalsoexpressedhisunboundedastonishmentatthesizeanddevelopmentofthetubersincertainhybrids;andallexperimentalists（18/121。Ibids。527。）haveremarkedonthestrongtendencyinhybridstoincreasebyroots，runners，andsuckers。Seeingthathybridplants，whichfromtheirnaturearemoreorlesssterile，thustendtoproducedoubleflowers;thattheyhavethepartsincludingtheseed，thatisthefruit，perfectlydeveloped，evenwhencontainingnoseed;thattheysometimesyieldgiganticroots;thattheyalmostinvariablytendtoincreaselargelybysuckersandothersuchmeans;——seeingthis，andknowing，fromthemanyfactsgivenintheearlierpartsofthischapter，thatalmostallorganicbeingswhenexposedtounnaturalconditionstendtobecomemoreorlesssterile，itseemsmuchthemostprobableviewthatwithcultivatedplantssterilityistheexcitingcause，anddoubleflowers，richseedlessfruit，andinsomecaseslargely—developedorgansofvegetation，etc。，aretheindirectresults——theseresultshavingbeeninmostcaseslargelyincreasedthroughcontinuedselectionbyman。