Sexualandasexualreproductionarethusseennottodifferessentially;andwehavealreadyshownthatasexualreproduction，thepowerofregrowthanddevelopmentareallpartsofoneandthesamegreatlaw。
REGROWTHOFAMPUTATEDPARTS。
Thissubjectdeservesalittlefurtherdiscussion。Amultitudeoftheloweranimalsandsomevertebratespossessthiswonderfulpower。Forinstance，Spallanzanicutoffthelegsandtailofthesamesalamandersixtimessuccessively，andBonnet（27/18。Spallanzani’AnEssayonAnimalReproduction’
translatedbyDr。Maty1769page79。Bonnet’Oeuvresd’Hist。Nat。’tome5part14to。edition1781pages343，350。）didsoeighttimes;andoneachoccasionthelimbswerereproducedontheexactlineofamputation，withnopartdeficientorinexcess。Analliedanimal，theaxolotl，hadalimbbittenoff，whichwasreproducedinanabnormalcondition，butwhenthiswasamputateditwasreplacedbyaperfectlimb。（27/19。VulpianasquotedbyProf。Faivre’LaVariabilitedesEspeces’1868page112。）Thenewlimbsinthesecasesbudforth，andaredevelopedinthesamemannerasduringtheregulardevelopmentofayounganimal。Forinstance，withtheAmblystomalurida，threetoesarefirstdeveloped，thenthefourth，andonthehind—feetthefifth，andsoitiswithareproducedlimb。（27/20。Dr。P。Hoy’TheAmericanNaturalist’September1871page579。）
Thepowerofregrowthisgenerallymuchgreaterduringtheyouthofananimalorduringtheearlierstagesofitsdevelopmentthanduringmaturity。ThelarvaeortadpolesoftheBatrachiansarecapableofreproducinglostmembers，butnotsotheadults。（27/21。Dr。GuntherinOwen’AnatomyofVertebrates’
volume11866page567。Spallanzanihasmadesimilarobservations。）Matureinsectshavenopowerofregrowth，exceptinginoneorder，whilstthelarvaeofmanykindshavethispower。Animalslowinthescaleareable，asageneralrule，toreproducelostpartsfarmoreeasilythanthosewhicharemorehighlyorganised。Themyriapodsofferagoodillustrationofthisrule;buttherearesomestrangeexceptionstoit——thusNemerteans，thoughlowlyorganised，aresaidtoexhibitlittlepowerofregrowth。Withthehighervertebrata，suchasbirdsandmammals，thepowerisextremelylimited。（27/22。AthrushwasexhibitedbeforetheBritishAssociationatHullin1853whichhadlostitstarsus，andthismember，itwasasserted，hadbeenthricereproduced;havingbeenlost，Ipresume，eachtimebydisease。SirJ。PagetinformsmethathefeelssomedoubtaboutthefactsrecordedbySirJ。Simpson（’MonthlyJournalofMedicalScience’Edinburgh1848newseriesvolume2page890）oftheregrowthoflimbsinthewombinthecaseofman。）
Inthecaseofthoseanimalswhichmaybebisectedorchoppedintopieces，andofwhicheveryfragmentwillreproducethewhole，thepowerofregrowthmustbediffusedthroughoutthewholebody。NeverthelessthereseemstobemuchtruthintheviewmaintainedbyProf。Lessona（27/23。’AttidellaSoc。Ital。
diSc。Nat。’volume111869page493。），thatthiscapacityisgenerallyalocalisedandspecialone，servingtoreplacepartswhichareeminentlyliabletobelostineachparticularanimal。Themoststrikingcaseinfavourofthisview，isthattheterrestrialsalamander，accordingtoLessona，cannotreproducelostparts，whilstanotherspeciesofthesamegenus，theaquaticsalamander，hasextraordinarypowersofregrowth，aswehavejustseen;andthisanimaliseminentlyliabletohaveitslimbs，tail，eyesandjawsbittenoffbyothertritons。（27/24。Lessonastatesthatthisissointhepaperjustreferredto。Seealso’TheAmericanNaturalist’September1871page579。）Evenwiththeaquaticsalamanderthecapacityistoacertainextentlocalised，forwhenM。Philipeaux（27/25。’ComptesRendus’October1，1866andJune1867。）
extirpatedtheentireforelimbtogetherwiththescapula，thepowerofregrowthwascompletelylost。Itisalsoaremarkablefact，standinginoppositiontoaverygeneralrule，thattheyoungoftheaquaticsalamanderdonotpossessthepowerofrepairingtheirlimbsinanequaldegreewiththeadults（27/26。Bonnet’OeuvresHist。Nat。’volume5page294，asquotedbyProf。Rollestoninhisremarkableaddresstothe36thannualmeetingoftheBritishMedicalAssociation。）butIdonotknowthattheyaremoreactive，orcanotherwisebetterescapethelossoftheirlimbs，thantheadults。Thewalking—stickinsect，Diapheromerafemorata，likeotherinsectsofthesameorder，canreproduceitslegsinthematurestate，andthesefromtheirgreatlengthmustbeliabletobelost:butthecapacityislocalised（asinthecaseofthesalamander），forDr。Scudderfound（27/27。’Proc。BostonSoc。ofNat。Hist。’volume121868—69page1。），thatifthelimbwasremovedwithinthetrochanto—femoralarticulation，itwasneverrenewed。Whenacrabisseizedbyoneofitslegs，thisisthrownoffatthebasaljoint，beingafterwardsreplacedbyanewleg;anditisgenerallyadmittedthatthisisaspecialprovisionforthesafetyoftheanimal。Lastly，withgasteropodmolluscs，whicharewellknowntohavethepowerofreproducingtheirheads，Lessonashowsthattheyareveryliabletohavetheirheadsbittenoffbyfishes;therestofthebodybeingprotectedbytheshell。Evenwithplantsweseesomethingofthesamekind，fornon—deciduousleavesandyoungstemshavenopowerofregrowth，thesepartsbeingeasilyreplacedbygrowthfromnewbuds;whilstthebarkandsubjacenttissuesofthetrunksoftreeshavegreatpowerofregrowth，probablyonaccountoftheirincreaseindiameter，andoftheirliabilitytoinjuryfrombeinggnawedbyanimals。
GRAFT—HYBRIDS。
Itiswellknownfrominnumerabletrialsmadeinallpartsoftheworld，thatbudsmaybeinsertedintoastock，andthattheplantsthusraisedarenotaffectedinagreaterdegreethancanbeaccountedforbychangednutrition。
Nordotheseedlingsraisedfromsuchinsertedbudspartakeofthecharacterofthestock，thoughtheyaremoreliabletovarythanareseedlingsfromthesamevarietygrowingonitsownroots。Abud，also，maysportintoanewandstrongly—markedvarietywithoutanyotherbudonthesameplantbeingintheleastdegreeaffected。Wemaythereforeinfer，inaccordancewiththecommonview，thateachbudisadistinctindividual，andthatitsformativeelementsdonotspreadbeyondthepartssubsequentlydevelopedfromit。Nevertheless，wehaveseenintheabstractongraft—hybridisationintheeleventhchapterthatbudscertainlyincludeformativematter，whichcanoccasionallycombinewiththatincludedinthetissuesofadistinctvarietyorspecies;aplantintermediatebetweenthetwoparent—formsbeingthusproduced。Inthecaseofthepotatowehaveseenthatthetubersproducedfromabudofonekindinsertedintoanotherareintermediateincolour，size，shapeandstateofsurface;thatthestems，foliage，andevencertainconstitutionalpeculiarities，suchasprecocity，arelikewiseintermediate。Withthesewell—
establishedcases，theevidencethatgraft—hybridshavealsobeenproducedwiththelaburnum，orange，vine，rose，etc。，seemssufficient。Butwedonotknowunderwhatconditionsthisrareformofreproductionispossible。Fromtheseseveralcaseswelearntheimportantfactthatformativeelementscapableofblendingwiththoseofadistinctindividual（andthisisthechiefcharacteristicofsexualgeneration），arenotconfinedtothereproductiveorgans，butarepresentinthebudsandcellulartissueofplants;andthisisafactofthehighestphysiologicalimportance。
DIRECTACTIONOFTHEMALEELEMENTONTHEFEMALE。
Intheeleventhchapter，abundantproofsweregiventhatforeignpollenoccasionallyaffectsinadirectmannerthemother—plant。Thus，whenGallesiofertilisedanorange—flowerwithpollenfromthelemon，thefruitborestripesofperfectlycharacterisedlemon—peel。Withpeas，severalobservershaveseenthecolouroftheseed—coatsandevenofthepoddirectlyaffectedbythepollenofadistinctvariety。Soithasbeenwiththefruitoftheapple，whichconsistsofthemodifiedcalyxandupperpartoftheflower—stalk。Inordinarycasesthesepartsarewhollyformedbythemother—plant。Wehereseethattheformativeelementsincludedwithinthemaleelementorpollenofonevarietycanaffectandhybridise，notthepartwhichtheyareproperlyadaptedtoaffect，namely，theovules，butthepartially—developedtissuesofadistinctvarietyorspecies。Wearethusbroughthalf—waytowardsagraft—
hybrid，inwhichtheformativeelementsincludedwithinthetissuesofoneindividualcombinewiththoseincludedinthetissuesofadistinctvarietyorspecies，thusgivingrisetoanewandintermediateform，independentlyofthemaleorfemalesexualorgans。
Withanimalswhichdonotbreeduntilnearlymature，andofwhichallthepartsarethenfullydeveloped，itishardlypossiblethatthemaleelementshoulddirectlyaffectthefemale。Butwehavetheanalogousandperfectlywell—ascertainedcaseofthemaleelementaffecting（aswiththequaggaandLordMorton’smare）thefemaleorherova，insuchamannerthatwhensheisimpregnatedbyanothermaleheroffspringareaffectedandhybridisedbythefirstmale。Theexplanationwouldbesimpleifthespermatozoacouldkeepalivewithinthebodyofthefemaleduringthelongintervalwhichhassometimeselapsedbetweenthetwoactsofimpregnation;butnoonewillsupposethatthisispossiblewiththehigheranimals。
DEVELOPMENT。
Thefertilisedgermreachesmaturitybyavastnumberofchanges:theseareeitherslightandslowlyeffected，aswhenthechildgrowsintotheman，oraregreatandsudden，aswiththemetamorphosesofmostinsects。Betweentheseextremeswehaveeverygradation，evenwithinthesameclass;thus，asSirJ。
Lubbockhasshown（27/28。’Transact。Linn。Soc。’volume241863page62。）
thereisanEphemerousinsectwhichmoultsabovetwentytimes，undergoingeachtimeaslightbutdecidedchangeofstructure;andthesechanges，ashefurtherremarks，probablyrevealtousthenormalstagesofdevelopment，whichareconcealedandhurriedthroughorsuppressedinmostotherinsects。Inordinarymetamorphoses，thepartsandorgansappeartobecomechangedintothecorrespondingpartsinthenextstageofdevelopment;butthereisanotherformofdevelopment，whichhasbeencalledbyProfessorOwenmetagenesis。Inthiscase"thenewpartsarenotmouldedupontheinnersurfaceoftheoldones。Theplasticforcehaschangeditscourseofoperation。Theoutercase，andallthatgaveformandcharactertotheprecedentindividual，perishandarecastoff;theyarenotchangedintothecorrespondingpartsofthenewindividual。Theseareduetoanewanddistinctdevelopmentalprocess，"etc。
（27/29。’Parthenogenesis’1849pages25，26。Prof。Huxleyhassomeexcellentremarks（’MedicalTimes’1856page637）onthissubjectinreferencetothedevelopmentofstar—fishes，andshowshowcuriouslymetamorphosisgraduatesintogemmationorzoid—formation，whichisinfactthesameasmetagenesis。）
Metamorphosis，however，graduatessoinsensibly，intometagenesis，thatthetwoprocessescannotbedistinctlyseparated。Forinstance，inthelastchangewhichCirripedesundergo，thealimentarycanalandsomeotherorgansaremouldedonpre—existingparts;buttheeyesoftheoldandtheyounganimalaredevelopedinentirelydifferentpartsofthebody;thetipsofthematurelimbsareformedwithinthelarvallimbs，andmaybesaidtobemetamorphosedfromthem;buttheirbasalportionsandthewholethoraxaredevelopedinaplaneatrightanglestothelarvallimbsandthorax;andthismaybecalledmetagenesis。ThemetageneticprocessiscarriedtoanextremepointinthedevelopmentofsomeEchinoderms，fortheanimalinthesecondstageofdevelopmentisformedalmostlikeabudwithintheanimalofthefirststage，thelatterbeingthencastofflikeanoldvestment，yetsometimesmaintainingforashortperiodanindependentvitality。（27/30。Prof。J。ReayGreeneinGunther’s’RecordofZoolog。Lit。’1865page625。）If，insteadofasingleindividual，severalweretobethusdevelopedmetageneticallywithinapre—
existingform，theprocesswouldbecalledoneofalternategeneration。Theyoungthusdevelopedmayeithercloselyresembletheencasingparent—form，aswiththelarvaeofCecidomyia，ormaydiffertoanastonishingdegree，aswithmanyparasiticwormsandjelly—fishes;butthisdoesnotmakeanyessentialdifferenceintheprocess，anymorethanthegreatnessorabruptnessofthechangeinthemetamorphosesofinsects。
Thewholequestionofdevelopmentisofgreatimportanceforourpresentsubject。Whenanorgan，theeye，forinstance，ismetageneticallyformedinapartofthebodywhereduringthepreviousstageofdevelopmentnoeyeexisted，wemustlookatitasanewandindependentgrowth。Theabsoluteindependenceofnewandoldstructures，althoughcorrespondinginstructureandfunction，isstillmoreobviouswhenseveralindividualsareformedwithinapreviousform，asinthecasesofalternategeneration。Thesameimportantprincipleprobablycomeslargelyintoplayeveninthecaseofapparentlycontinuousgrowth，asweshallseewhenweconsidertheinheritanceofmodificationsatcorrespondingages。
Weareledtothesameconclusion，namely，theindependenceofpartssuccessivelydeveloped，byanotherandquitedistinctgroupoffacts。Itiswellknownthatmanyanimalsbelongingtothesameorder，andthereforenotdifferingwidelyfromeachother，passthroughanextremelydifferentcourseofdevelopment。Thuscertainbeetles，notinanywayremarkablydifferentfromothersofthesameorder，undergowhathasbeencalledahyper—metamorphosis——
thatis，theypassthroughanearlystagewhollydifferentfromtheordinarygrub—likelarva。Inthesamesub—orderofcrabs，namely，theMacroura，asFritzMullerremarks，therivercray—fishishatchedunderthesameformwhichiteverafterwardsretains;theyounglobsterhasdividedlegs，likeaMysis;
thePalaemonappearsundertheformofaZoea，andPeneusundertheNauplius—
form;andhowwonderfullytheselarvalformsdifferfromoneanother，isknowntoeverynaturalist。（27/31。FritzMuller’FurDarwin’1864s。65，71。Thehighestauthorityoncrustaceans，Prof。Milne—Edwards，insists（’Annal。desSci。Nat。’2ndseriesZoolog。tome3page322）onthedifferenceinthemetamorphosisofclosely—alliedgenera。）Someothercrustaceans，asthesameauthorobserves，startfromthesamepointandarriveatnearlythesameend，butinthemiddleoftheirdevelopmentarewidelydifferentfromoneanother。
StillmorestrikingcasescouldbegivenwithrespecttotheEchinodermata。
WiththeMedusaeorjelly—fishesProfessorAllmanobserves，"TheclassificationoftheHydroidawouldbeacomparativelysimpletaskif，ashasbeenerroneouslyasserted，generically—identicalmedusoidsalwaysarosefromgenerically—identicalpolypoids;and，ontheotherhand，thatgenerically—
identicalpolypoidsalwaysgaveorigintogenerically—identicalmedusoids。"Soagain，Dr。StrethillWrightremarks，"Inthelife—historyoftheHydroidaeanyphase，planuloid，polypoid，ormedusoid，maybeabsent。"（27/32。Prof。Allman’AnnalsandMag。ofNat。Hist。’3rdseriesvolume131864page348;Dr。S。
Wrightibidvolume81861page127。Seealsopage358foranalogousstatementsbySars。）
Accordingtothebeliefnowgenerallyacceptedbyourbestnaturalists，allthemembersofthesameorderorclass，forinstance，theMedusaeortheMacrourouscrustaceans，aredescendedfromacommonprogenitor。Duringtheirdescenttheyhavedivergedmuchinstructure，buthaveretainedmuchincommon;andthishasoccurred，thoughtheyhavepassedthroughandstillpassthroughmarvellouslydifferentmetamorphoses。Thisfactwellillustrateshowindependenteachstructure...完整阅读请扫描二维码下载丁香书院APP免费看：