IntheexperimentsonthehybridHippeastrums，ifHerberthadfoundthatthepollenoftwoorthreekindsalonehadbeenmoreefficientoncertainkindsthantheirownpollen，itmighthavebeenarguedthatthese，fromtheirmixedparentage，hadaclosermutualaffinitythantheothers;butthisexplanationisinadmissible，forthetrialsweremadereciprocallybackwardsandforwardsonninedifferenthybrids;andacross，whicheverwaytaken，alwaysprovedhighlybeneficial。IcanaddastrikingandanalogouscasefromexperimentsmadebytheRev。A。Rawson，ofBromleyCommon，withsomecomplexhybridsofGladiolus。ThisskilfulhorticulturistpossessedanumberofFrenchvarieties，differingfromeachotheronlyinthecolourandsizeoftheflowers，alldescendedfromGandavensis，awell—knownoldhybrid，saidtobedescendedfromG。natalensisbythepollenofG。oppositiflorus。（17/86。Mr。D。Beatonin’JournalofHort。’1861page453。Lecoqhowever（’DelaFecond。’1862page369）statesthatthishybridisdescendedfromG。psittacinusandcardinalis;
butthisisopposedtoHerbert’sexperience，whofoundthattheformerspeciescouldnotbecrossed。）Mr。Rawson，afterrepeatedtrials，foundthatnoneofthevarietieswouldsetseedwiththeirownpollen，althoughtakenfromdistinctplantsofthesamevariety（whichhad，ofcourse，beenpropagatedbybulbs），butthattheyallseededfreelywithpollenfromanyothervariety。Togivetwoexamples:Ophirdidnotproduceacapsulewithitsownpollen，butwhenfertilisedwiththatofJanire，Brenchleyensis，VulcainandLinne，itproducedtenfinecapsules;butthepollenofOphirwasgood，forwhenLinnewasfertilisedbyitsevencapsuleswereproduced。Thislattervariety，ontheotherhand，wasutterlybarrenwithitsownpollen，whichwehaveseenwasperfectlyefficientonOphir。Altogether，Mr。Rawson，intheyear1861
fertilisedtwenty—sixflowersbornebyfourvarietieswithpollentakenfromothervarieties，andeverysingleflowerproducedafineseed—capsule;whereasfifty—twoflowersonthesameplants，fertilisedatthesametimewiththeirownpollen，didnotyieldasingleseed—capsule。Mr。Rawsonfertilised，insomecases，thealternateflowers，andinothercasesallthosedownonesideofthespike，withpollenofothervarieties，andtheremainingflowerswiththeirownpollen。Isawtheseplantswhenthecapsuleswerenearlymature，andtheircuriousarrangementatoncebroughtfullconvictiontothemindthatanimmenseadvantagehadbeenderivedfromcrossingthesehybrids。
Lastly，IhaveheardfromDr。E。Bornet，ofAntibes，whohasmadenumerousexperimentsincrossingthespeciesofCistus，buthasnotyetpublishedtheresults，that，whenanyofthesehybridsarefertile，theymaybesaidtobe，inregardtofunction，dioecious;"fortheflowersarealwayssterilewhenthepistilisfertilisedbypollentakenfromthesameflowerorfromflowersonthesameplant。Buttheyareoftenfertileifpollenbeemployedfromadistinctindividualofthesamehybridnature，orfromahybridmadebyareciprocalcross。"]
CONCLUSION。
Thatplantsshouldbeself—sterile，althoughbothsexualelementsareinafitstateforreproduction，appearsatfirstsightopposedtoallanalogy。Withrespecttothespecies，alltheindividualsofwhichareinthisstate，althoughlivingundertheirnaturalconditions，wemayconcludethattheirself—sterilityhasbeenacquiredforthesakeofeffectuallypreventingself—
fertilisation。Thecaseiscloselyanalogouswiththatofdimorphicandtrimorphicorheterostyledplants，whichcanbefullyfertilisedonlybyplantsbelongingtoadifferentform，andnot，asintheforegoingcases，indifferentlybyanyotherindividualofthespecies。Someofthesehetero—
styledplantsarecompletelysterilewithpollentakenfromthesameplantorfromthesameform。Withrespecttospecieslivingundertheirnaturalconditions，ofwhichonlycertainindividualsareself—sterile（aswithResedalutea），itisprobablethatthesehavebeenrenderedself—steriletoensureoccasionalcross—fertilisation，whilstotherindividualshaveremainedself—
fertiletoensurethepropagationofthespecies。Thecaseseemstobeparallelwiththatofplantswhichproduce，asHermannMullerhasdiscovered，twoforms——onebearingmoreconspicuousflowerswiththeirstructureadaptedforcross—fertilisationbyinsects，andtheotherformwithlessconspicuousflowersadaptedforself—fertilisation。Theself—sterility，however，ofsomeoftheforegoingplantsisincidentalontheconditionstowhichtheyhavebeensubjected，aswiththeEschscholtzia，theVerbascumphoeniceum（thesterilityofwhichvariedaccordingtotheseason），andwiththePassifloraalata，whichrecovereditsself—fertilitywhengraftedonadifferentstock。
Itisinterestingtoobserveintheaboveseveralcasesthegraduatedseriesfromplantswhich，whenfertilisedbytheirownpollen，yieldthefullnumberofseeds，butwiththeseedlingsalittledwarfedinstature——toplantswhichwhenself—fertilisedyieldfewseeds——tothosewhichyieldnone，buthavetheirovariasomewhatdeveloped——and，lastly，tothoseinwhichtheplant’sownpollenandstigmamutuallyactononeanotherlikepoison。Itisalsointerestingtoobserveonhowslightadifferenceinthenatureofthepollenoroftheovulescompleteself—sterilityorcompleteself—fertilitymustdependinsomeoftheabovecases。Everyindividualoftheself—sterilespeciesappearstobecapableofproducingthefullcomplementofseedwhenfertilisedbythepollenofanyotherindividual（thoughjudgingfromthefactsgivenwithrespecttoAbutilonthenearestkinmustbeexcepted）;butnotoneindividualcanbefertilisedbyitsownpollen。Aseveryorganismdiffersinsomeslightdegreefromeveryotherindividualofthesamespecies，sonodoubtitiswiththeirpollenandovules;andintheabovecaseswemustbelievethatcompleteself—sterilityandcompleteself—fertilitydependonsuchslightdifferencesintheovulesandpollen，andnottheirhavingbeendifferentiatedinsomespecialmannerinrelationtooneanother;foritisimpossiblethatthesexualelementsofmanythousandindividualsshouldhavebeenspecialisedinrelationtoeveryotherindividual。Insome，however，oftheabovecases，aswithcertainPassifloras，anamountofdifferentiationbetweenthepollenandovulessufficientforfertilisationisgainedonlybyemployingpollenfromadistinctspecies;butthisisprobablytheresultofsuchplantshavingbeenrenderedsomewhatsterilefromtheunnaturalconditionstowhichtheyhavebeenexposed。
Exoticanimalsconfinedinmenageriesaresometimesinnearlythesamestateastheabove—describedself—impotentplants;for，asweshallseeinthefollowingchapter，certainmonkeys，thelargercarnivora，severalfinches，geese，andpheasants，crosstogether，quiteasfreelyas，orevenmorefreelythantheindividualsofthesamespeciesbreedtogether。Caseswill，also，begivenofsexualincompatibilitybetweencertain，maleandfemaledomesticatedanimals，which，nevertheless，arefertilewhenmatchedwithanyotherindividualofthesamekind。
Intheearlypartofthischapteritwasshownthatthecrossingofindividualsbelongingtodistinctfamiliesofthesamerace，ortodifferentracesorspecies，givesincreasedsizeandconstitutionalvigourtotheoffspring，and，exceptinthecaseofcrossedspecies，increasedfertility。
Theevidencerestsontheuniversaltestimonyofbreeders（foritshouldbeobservedthatIamnotherespeakingoftheevilresultsofcloseinterbreeding），andispracticallyexemplifiedinthehighervalueofcross—
bredanimalsforimmediateconsumption。Thegoodresultsofcrossinghavealsobeendemonstratedwithsomeanimalsandwithnumerousplants，byactualweightandmeasurement。Althoughanimalsofpurebloodwillobviouslybedeterioratedbycrossing，asfarastheircharacteristicqualitiesareconcerned，thereseemstobenoexceptiontotherulethatadvantagesofthekindjustmentionedarethusgained，evenwhentherehasnotbeenanypreviouscloseinterbreeding;andtheruleappliestosuchanimalsascattleandsheep，whichcanlongresistbreedingin—and—inbetweenthenearestblood—relations。
Inthecaseofcrossedspecies，althoughsize，vigour，precocity，andhardinessare，withrareexceptions，gained，fertility，inagreaterorlessdegree，islost;butthegainintheaboverespectscanhardlybeattributedtotheprincipleofcompensation;forthereisnocloseparallelismbetweentheincreasedsizeandvigourofhybridoffspringandtheirsterility。
Moreover，ithasbeenclearlyprovedthatmongrelswhichareperfectlyfertilegainthesesameadvantagesaswellassterilehybrids。
Withthehigheranimalsnospecialadaptationsforensuringoccasionalcrossesbetweendistinctfamiliesseemtoexist。Theeagernessofthemales，leadingtoseverecompetitionbetweenthem，issufficient;forevenwithgregariousanimals，theoldanddominantmaleswillbedispossessedafteratimeanditwouldbeamerechanceifacloselyrelatedmemberofthesamefamilyweretobethevictorioussuccessor。Thestructureofmanyoftheloweranimals，whentheyarehermaphrodites，issuchastopreventtheovulesbeingfertilisedbythemaleelementofthesameindividual;sothattheconcourseoftwoindividualsisnecessary。Inothercasestheaccessofthemaleelementofadistinctindividualisatleastpossible。Withplants，whichareaffixedtothegroundandcannotwanderfromplacetoplacelikeanimals，thenumerousadaptationsforcross—fertilisationarewonderfullyperfect，ashasbeenadmittedbyeveryonewhohasstudiedthesubject。
Theevilconsequencesoflong—continuedcloseinterbreedingarenotsoeasilyrecognisedasthegoodeffectsfromcrossing，forthedeteriorationisgradual。Nevertheless，itisthegeneralopinionofthosewhohavehadmostexperience，especiallywithanimalswhichpropagatequickly，thatevildoesinevitablyfollowsoonerorlater，butatdifferentrateswithdifferentanimals。Nodoubtafalsebeliefmay，likeasuperstition，prevailwidely;yetitisdifficulttosupposethatsomanyacuteobservershaveallbeendeceivedattheexpenseofmuchcostandtrouble。Amaleanimalmaysometimesbepairedwithhisdaughter，granddaughter，andsoon，evenforsevengenerations，withoutanymanifestbadresult:buttheexperimenthasneverbeentriedofmatchingbrothersandsisters，whichisconsideredtheclosestformofinterbreeding，foranequalnumberofgenerations。Thereisgoodreasontobelievethatbykeepingthemembersofthesamefamilyindistinctbodies，especiallyifexposedtosomewhatdifferentconditionsoflife，andbyoccasionallycrossingthesefamilies，theevilresultsofinterbreedingmaybemuchdiminishedorquiteeliminated。Theseresultsarelossofconstitutionalvigour，size，andfertility;butthereisnonecessarydeteriorationinthegeneralformofthebody，orinothergoodqualities。Wehaveseenthatwithpigsfirst—rateanimalshavebeenproducedafterlong—continuedcloseinterbreeding，thoughtheyhadbecomeextremelyinfertilewhenpairedwiththeirnearrelations。Thelossoffertility，whenitoccurs，seemsnevertobeabsolute，butonlyrelativetoanimalsofthesameblood;sothatthissterilityistoacertainextentanalogouswiththatofself—impotentplantswhichcannotbefertilisedbytheirownpollen，butareperfectlyfertilewithpollenofanyotherindividualofthesamespecies。Thefactofinfertilityofthispeculiarnaturebeingoneoftheresultsoflong—continuedinterbreeding，showsthatinterbreedingdoesnotactmerelybycombiningandaugmentingvariousmorbidtendenciescommontobothparents;foranimalswithsuchtendencies，ifnotatthetimeactuallyill，cangenerallypropagatetheirkind。Althoughoffspringdescendedfromthenearestblood—relationsarenotnecessarilydeterioratedinstructure，yetsomeauthorsbelievethattheyareeminentlyliabletomalformations;andthisisnotimprobable，aseverythingwhichlessensthevitalpowersactsinthismanner。Instancesofthiskindhavebeenrecordedinthecaseofpigs，bloodhounds，andsomeotheranimals。
Finally，whenweconsiderthevariousfactsnowgivenwhichplainlyshowthatgoodfollowsfromcrossing，andlessplainlythatevilfollowsfromcloseinterbreeding，andwhenwebearinmindthatwithverymanyorganismselaborateprovisionshavebeenmadefortheoccasionalunionofdistinctindividuals，theexistenceofagreatlawofnatureisalmostproved;namely，thatthecrossingofanimalsandplantswhicharenotcloselyrelatedtoeachotherishighlybeneficialorevennecessary，andthatinterbreedingprolongedduringmanygenerationsisinjurious。
CHAPTER2。XVIII。
ONTHEADVANTAGESANDDISADVANTAGESOFCHANGEDCONDITIONSOFLIFE:STERILITY
FROMVARIOUSCAUSES。
ONTHEGOODDERIVEDFROMSLIGHTCHANGESINTHECONDITIONSOFLIFE。
STERILITYFROMCHANGEDCONDITIONS，INANIMALS，INTHEIRNATIVECOUNTRYANDIN
MENAGERIES。
MAMMALS，BIRDS，ANDINSECTS。
LOSSOFSECONDARYSEXUALCHARACTERSANDOFINSTINCTS。
CAUSESOFSTERILITY。
STERILITYOFDOMESTICATEDANIMALSFROMCHANGEDCONDITIONS。
SEXUALINCOMPATIBILITYOFINDIVIDUALANIMALS。
STERILITYOFPLANTSFROMCHANGEDCONDITIONSOFLIFE。
CONTABESCENCEOFTHEANTHERS。
MONSTROSITIESASACAUSEOFSTERILITY。
DOUBLEFLOWERS。
SEEDLESSFRUIT。
STERILITYFROMTHEEXCESSIVEDEVELOPMENTOFTHEORGANSOFVEGETATION。
FROMLONG—CONTINUEDPROPAGATIONBYBUDS。
INCIPIENTSTERILITYTHEPRIMARYCAUSEOFDOUBLEFLOWERSANDSEEDLESSFRUIT。
ONTHEGOODDERIVEDFROMSLIGHTCHANGESINTHECONDITIONSOFLIFE。
Inconsideringwhetheranyfactswereknownwhichmightthrowlightontheconclusionarrivedatinthelastchapter，namely，thatbenefitsensuefromcrossing，andthatitisalawofnaturethatallorganicbeingsshouldoccasionallycross，itappearedtomeprobablethatthegoodderivedfromslightchangesintheconditionsoflife，frombeingananalogousphenomenon，mightservethispurpose。Notwoindividuals，andstilllessnotwovarieties，areabsolutelyalikeinconstitutionandstructure;andwhenthegermofoneisfertilisedbythemaleelementofanother，wemaybelievethatitisactedoninasomewhatsimilarmannerasanindividualwhenexposedtoslightlychangedconditions。Now，everyonemusthaveobservedtheremarkableinfluenceonconvalescentsofachangeofresidence，andnomedicalmandoubtsthetruthofthisfact。Smallfarmerswhoholdbutlittlelandareconvincedthattheircattlederivegreatbenefitfromachangeofpasture。Inthecaseofplants，theevidenceisstrongthatagreatadvantageisderivedfromexchangingseeds，tubers，bulbs，andcuttingsfromonesoilorplacetoanotherasdifferentaspossible。
[Thebeliefthatplantsarethusbenefited，whetherornotwellfounded，hasbeenfirmlymaintainedfromthetimeofColumella，whowroteshortlyaftertheChristianera，tothepresentday;anditnowprevailsinEngland，France，andGermany。（18/1。ForEnglandseebelow。ForGermanyseeMetzger’Getreidearten’
1841s。63。ForFranceLoiseleur—Deslongchamps（’Consid。surlesCereales’
1843page200）givesnumerousreferencesonthissubject。ForSouthernFranceseeGodron’FlorulaJuvenalis’1854page28。）Asagaciousobse...完整阅读请扫描二维码下载丁香书院APP免费看：