Havingnowendeavouredtoshowthattheseveralforegoingassumptionsaretoacertainextentsupportedbyanalogousfacts，andhavingalludedtosomeofthemostdoubtfulpoints，wewillconsiderhowfarthehypothesisbringsunderasinglepointofviewthevariouscasesenumeratedintheFirstPart。Alltheformsofreproductiongraduateintooneanotherandagreeintheirproduct;
foritisimpossibletodistinguishbetweenorganismsproducedfrombuds，fromself—division，orfromfertilisedgerms;suchorganismsareliabletovariationsofthesamenatureandtoreversionsofthesamekind;andas，accordingtoourhypothesis，alltheformsofreproductiondependontheaggregationofgemmulesderivedfromthewholebody，wecanunderstandthisremarkableagreement。Parthenogenesisisnolongerwonderful，andifwedidnotknowthatgreatgoodfollowedfromtheunionofthesexualelementsderivedfromtwodistinctindividuals，thewonderwouldbethatparthenogenesisdidnotoccurmuchoftenerthanitdoes。Onanyordinarytheoryofreproductiontheformationofgraft—hybrids，andtheactionofthemaleelementonthetissuesofthemother—plant，aswellasonthefutureprogenyoffemaleanimals，aregreatanomalies;buttheyareintelligibleonourhypothesis。Thereproductiveorgansdonotactuallycreatethesexualelements;theymerelydeterminetheaggregationandperhapsthemultiplicationofthegemmulesinaspecialmanner。Theseorgans，however，togetherwiththeiraccessoryparts，havehighfunctionstoperform。Theyadaptoneorbothelementsforindependenttemporaryexistence，andformutualunion。Thestigmaticsecretionactsonthepollenofaplantofthesamespeciesinawhollydifferentmannertowhatitdoesonthepollenofonebelongingtoadistinctgenusorfamily。ThespermatophoresoftheCephalopodaarewonderfullycomplexstructures，whichwereformerlymistakenforparasiticworms;andthespermatozoaofsomeanimalspossessattributeswhich，ifobservedinanindependentanimal，wouldbeputdowntoinstinctguidedbysense—organs，——aswhenthespermatozoaofaninsectfindtheirwayintotheminutemicropyleoftheegg。
Theantagonismwhichhaslongbeenobserved（27/55。Mr。HerbertSpencer（’PrinciplesofBiology’volume2page430）hasfullydiscussedthisantagonism。），withcertainexceptions，betweengrowthandthepowerofsexualreproduction（27/56。Themalesalmonisknowntobreedataveryearlyage。
TheTritonandSiredon，whilstretainingtheirlarvalbranchiae，accordingtoFilippiandDumeril（’AnnalsandMag。ofNat。Hist。’3rdseries1866page157）
arecapableofreproduction。ErnstHaeckelhasrecently（’MonatsberichtAkad。
Wiss。Berlin’February2，1865）observedthesurprisingcaseofamedusa，withitsreproductiveorgansactive，whichproducesbybuddingawidelydifferentformofmedusa;andthislatteralsohasthepowerofsexualreproduction。
Krohnhasshown（’AnnalsandMag。ofNat。Hist。’3rdseriesvolume191862
page6）thatcertainothermedusae，whilstsexuallymature，propagatebygemmae。SeealsoKolliker’MorphologieundEntwickelungsgeschichtedesPennatulidenstammes’1872page12。）——betweentherepairofinjuriesandgemmation——andwithplants，betweenrapidincreasebybuds，rhizomes，etc。，andtheproductionofseed，ispartlyexplainedbythegemmulesnotexistinginsufficientnumbersfortheseprocessestobecarriedonsimultaneously。
Hardlyanyfactinphysiologyismorewonderfulthanthepowerofregrowth;
forinstance，thatasnailshouldbeabletoreproduceitshead，orasalamanderitseyes，tail，andlegs，exactlyatthepointswheretheyhavebeencutoff。Suchcasesareexplainedbythepresenceofgemmulesderivedfromeachpart，anddisseminatedthroughoutthebody。Ihaveheardtheprocesscomparedwiththatoftherepairofthebrokenanglesofacrystalbyre—
crystallisation;andthetwoprocesseshavethismuchincommon，thatintheonecasethepolarityofthemoleculesistheefficientcause，andintheothertheaffinityofthegemmulesforparticularnascentcells。Butwehaveheretoencountertwoobjectionswhichapplynotonlytotheregrowthofapart，orofabisectedindividual，buttofissiparousgenerationandbudding。
Thefirstobjectionisthatthepartwhichisreproducedisinthesamestageofdevelopmentasthatofthebeingwhichhasbeenoperatedonorbisected;
andinthecaseofbuds，thatthenewbeingsthusproducedareinthesamestageasthatofthebuddingparent。Thusamaturesalamander，ofwhichthetailhasbeencutoff，doesnotreproducealarvaltail;andacrabdoesnotreproducealarvalleg。Inthecaseofbuddingitwasshowninthefirstpartofthischapterthatthenewbeingthusproduceddoesnotretrogradeindevelopment，——thatis，doesnotpassthroughthoseearlierstages，whichthefertilisedgermhastopassthrough。Nevertheless，theorganismsoperatedonormultiplyingthemselvesbybudsmust，byourhypothesis，includeinnumerablegemmulesderivedfromeverypartorunitoftheearlierstagesofdevelopment;
andwhydonotsuchgemmulesreproducetheamputatedpartorthewholebodyatacorrespondingearlystageofdevelopment？
Thesecondobjection，whichhasbeeninsistedonbyDelpino，isthatthetissues，forinstance，ofamaturesalamanderorcrab，ofwhichalimbhasbeenremoved，arealreadydifferentiatedandhavepassedthroughtheirwholecourseofdevelopment;andhowcansuchtissuesinaccordancewithourhypothesisattractandcombinewiththegemmulesofthepartwhichistobereproduced？Inanswertothesetwoobjectionswemustbearinmindtheevidencewhichhasbeenadvanced，showingthatatleastinalargenumberofcasesthepowerofregrowthisalocalisedfaculty，acquiredforthesakeofrepairingspecialinjuriestowhicheachparticularcreatureisliable;andinthecaseofbudsorfissiparousgeneration，forthesakeofquicklymultiplyingtheorganismataperiodoflifewhenitcanbesupportedinlargenumbers。Theseconsiderationsleadustobelievethatinallsuchcasesastockofnascentcellsorofpartiallydevelopedgemmulesareretainedforthisspecialpurposeeitherlocallyorthroughoutthebody，readytocombinewiththegemmulesderivedfromthecellswhichcomenextinduesuccession。Ifthisbeadmittedwehaveasufficientanswertotheabovetwoobjections。
Anyhow，pangenesisseemstothrowaconsiderableamountoflightonthewonderfulpowerofregrowth。
Itfollows，also，fromtheviewjustgiven，thatthesexualelementsdifferfrombudsinnotincludingnascentcellsorgemmulesinasomewhatadvancedstageofdevelopment，sothatonlythegemmulesbelongingtotheearlieststagesarefirstdeveloped。Asyounganimalsandthosewhichstandlowinthescalegenerallyhaveamuchgreatercapacityforregrowththanolderandhigheranimals，itwouldalsoappearthattheyretaincellsinanascentstate，orpartiallydevelopedgemmules，morereadilythandoanimalswhichhavealreadypassedthroughalongseriesofdevelopmentalchanges。Imayhereaddthatalthoughovulescanbedetectedinmostorallfemaleanimalsatanextremelyearlyage，thereisnoreasontodoubtthatgemmulesderivedfrompartsmodifiedduringmaturitycanpassintotheovules。
Withrespecttohybridism，pangenesisagreeswellwithmostoftheascertainedfacts。Wemustbelieve，aspreviouslyshown，thatseveralgemmulesarerequisiteforthedevelopmentofeachcellorunit。Butfromtheoccurrenceofparthenogenesis，moreespeciallyfromthosecasesinwhichanembryoisonlypartiallyformed，wemayinferthatthefemaleelementgenerallyincludesgemmulesinnearlysufficientnumberforindependentdevelopment，sothatwhenunitedwiththemaleelementthegemmulesaresuperabundant。Now，whentwospeciesorracesarecrossedreciprocally，theoffspringdonotcommonlydiffer，andthisshowsthatthesexualelementsagreeinpower，inaccordancewiththeviewthatbothincludethesamegemmules。Hybridsandmongrelsarealsogenerallyintermediateincharacterbetweenthetwoparent—forms，yetoccasionallytheycloselyresembleoneparentinonepartandtheotherparentinanotherpart，orevenintheirwholestructure:noristhisdifficulttounderstandontheadmissionthatthegemmulesinthefertilisedgermaresuperabundantinnumber，andthatthosederivedfromoneparentmayhavesomeadvantageinnumber，affinity，orvigouroverthosederivedfromtheotherparent。Crossedformssometimesexhibitthecolourorothercharactersofeitherparentinstripesorblotches;andthisoccursinthefirstgeneration，orthroughreversioninsucceedingbudandseminalgenerations，ofwhichfactseveralinstancesweregivenintheeleventhchapter。InthesecaseswemustfollowNaudin（27/57。Seehisexcellentdiscussiononthissubjectin’NouvellesArchivesduMuseum’tome1page151。）andadmitthatthe"essence"
or"element"ofthetwospecies，——termswhichIshouldtranslateintothegemmules，——haveanaffinityfortheirownkind，andthusseparatethemselvesintodistinctstripesorblotches;andreasonsweregiven，whendiscussinginthefifteenthchaptertheincompatibilityofcertaincharacterstounite，forbelievinginsuchmutualaffinity。Whentwoformsarecrossed，oneisnotrarelyfoundtobeprepotentinthetransmissionofitscharactersovertheother;andthiswecanexplainbyagainassumingthattheoneformhassomeadvantageovertheotherinthenumber，vigour，oraffinityofitsgemmules。
Insomecases，however，certaincharactersarepresentintheoneformandlatentintheother;forinstance，thereisalatenttendencyinallpigeonstobecomeblue，and，whenabluepigeoniscrossedwithoneofanyothercolour，thebluetintisgenerallyprepotent。TheexplanationofthisformofprepotencywillbeobviouswhenwecometotheconsiderationofReversion。
Whentwodistinctspeciesarecrossed，itisnotoriousthattheydonotyieldthefullorpropernumberofoffspring;andwecanonlysayonthisheadthat，asthedevelopmentofeachorganismdependsonsuchnicely—balancedaffinitiesbetweenahostofgemmulesandnascentcells，weneednotfeelatallsurprisedthatthecommixtureofgemmulesderivedfromtwodistinctspeciesshouldleadtopartialorcompletefailureofdevelopment。Withrespecttothesterilityofhybridsproducedfromtheunionoftwodistinctspecies，itwasshowninthenineteenthchapterthatthisdependsexclusivelyonthereproductiveorgansbeingspeciallyaffected;butwhytheseorgansshouldbethusaffectedwedonotknow，anymorethanwhyunnaturalconditionsoflife，thoughcompatiblewithhealth，shouldcausesterility;orwhycontinuedcloseinterbreeding，ortheillegitimateunionsofheterostyledplants，inducethesameresult。Theconclusionthatthereproductiveorgansaloneareaffected，andnotthewholeorganisation，agreesperfectlywiththeunimpairedorevenincreasedcapacityinhybridplantsforpropagationbybuds;forthisimplies，accordingtoourhypothesis，thatthecellsofthehybridsthrowoffhybridisedgemmules，whichbecomeaggregatedintobuds，butfailtobecomeaggregatedwithinthereproductiveorgans，soastoformthesexualelements。
Inasimilarmannermanyplants，whenplacedunderunnaturalconditions，failtoproduceseed，butcanreadilybepropagatedbybuds。Weshallpresentlyseethatpangenesisagreeswellwiththestrongtendencytoreversionexhibitedbyallcrossedanimalsandplants。
Eachorganismreachesmaturitythroughalongerorshortercourseofgrowthanddevelopment:theformertermbeingconfinedtomereincreaseofsize，anddevelopmenttochangedstructure。Thechangesmaybesmallandinsensiblyslow，aswhenachildgrowsintoaman，ormany，abrupt，andslight，asinthemetamorphosesofcertainephemerousinsects，or，again，fewandstrongly—
marked，aswithmostotherinsects。Eachnewlyformedpartmaybemouldedwithinapreviouslyexistingandcorrespondingpart，andinthiscaseitwillappear，falselyasIbelieve，tobedevelopedfromtheoldpart;oritmaybeformedwithinadistinctpartofthebody，asintheextremecasesofmetagenesis。Aneye，forinstance，maybedevelopedataspotwherenoeyepreviouslyexisted。Wehavealsoseenthatalliedorganicbeingsinthecourseoftheirmetamorphosessometimesattainnearlythesamestructureafterpassingthroughwidelydifferentforms;orconversely，afterpassingthroughnearlythesameearlyforms，arriveatwidelydifferentmatureforms。Inthesecasesitisverydifficulttoacceptthecommonviewthatthefirst—formedcellsorunitspossesstheinherentpower，independentlyofanyexternalagency，ofproducingnewstructureswhollydifferentinform，position，andfunction。Butallthesecasesbecomeplainonthehypothesisofpangenesis。
Theunits，duringeachstageofdevelopment，throwoffgemmules，which，multiplying，aretransmittedtotheoffspring。Intheoffspring，assoonasanyparticularcellorunitbecomespartiallydeveloped，ituniteswith（or，tospeakmetaphorically，isfertilisedby）thegemmuleofthenextsucceedingcell，andsoonwards。Butorganismshaveoftenbeensubjectedtochangedconditionsoflifeatacertainstageoftheirdevelopment，andinconsequencehavebeenslightlymodified;andthegemmulescastofffromsuchmodifiedpartswilltendtoreproducepartsmodifiedinthesamemanner。Thisprocessmayberepeateduntilthestructureofthepartbecomesgreatlychangedatoneparticularstageofdevelopment，butthiswillnotnecessarilyaffectotherparts，whetherpreviouslyorsubsequentlyformed。Inthismannerwecanunderstandtheremarkableindependenceofstructureinthesuccessivemetamorphoses，andespeciallyinthesuccessivemetagenesesofmanyanimals。
Inthecase，however，ofdiseaseswhichsuperveneduringoldage，subsequentlytotheordinaryperiodofprocreation，andwhich，nevertheless，aresometimesinherited，asoccurswithbrainandheartcomplaints，wemustsupposethattheorganswereaffectedatanearlyageandthrewoffatthisperiodaffectedgemmules;butthattheaffectionbecamevisibleorinjuriousonlyaftertheprolongedgrowth，inthestrictsenseoftheword，ofthepart。Inallthechangesofstructurewhichregularlysuperveneduringoldage，weprobablyseetheeffectsofdeterioratedgrowth，andnotoftruedevelopment。
Theprincipleoftheindependentformationofeachpart，owingtotheunionofthepropergemmuleswithcertainnascentcells，togetherwiththesuperabundanceofthegemmulesderivedfrombothparents，andthesubsequentself—multiplicationofthegemmules，throwslightonawidelydifferentgroupoffacts，whichonanyordinaryviewofdevelopmentappearsverystrange。I
alludetoorganswhichareabnormallytransposedormultiplied。Forinstance，acuriouscasehasbeenrecordedbyDr。ElliottCoues（27/58。’Proc。BostonSoc。ofNat。Hist。’republishedin’ScientificOpinion’November10，1869page488。）ofamonstrouschickenwithaperfectadditionalRIGHTlegarticulatedtotheLEFTsideofthepelvis。Gold—fishoftenhavesupernumeraryfinsplacedonvariouspartsoftheirbodies。Whenthetailofalizardisbrokenoff，adoubletailissometimesreproduced;andwhenthefootofthesalamanderwasdividedlongitudinallybyBonnet，additionaldigitswereoccasionallyformed。
Valentininjuredthecaudalextremityofanembryo，andthreedaysafterwardsitproducedrudimentsofadoublepelvisandofdoublehind—limbs。（27/59。
Todd’Cyclop。ofAnat。andPhys。’volume41849—52page975。）Whenfrogs，toads，etc。，arebornwiththeirlimbsdoubled，assometimeshappens，thedoubling，asGervaisremarks（27/60。’CompteRendus’November14，1865page800。），cannotbeduetothecompletefusionoftwoembryos，withtheexceptionofthelimbs，forthelarvaearelimbless。Thesameargumentisapplicable（27/61。AspreviouslyremarkedbyQuatrefagesinhis’Metamorphosesdel’Homme’etc。1862page129。）tocertaininsectsproducedwithmultiplelegsorantennae，forthesearemetamorphosedfromapodalorantennae—lesslarvae。
AlphonseMilne—Edwards（27/62。Gunther’ZoologicalRecord’1864page279。）hasdescribedthecuriouscaseofacrustaceaninwhichoneeye—pedunclesupp...完整阅读请扫描二维码下载丁香书院APP免费看：