Moreover，moresemenmustneedsbeusedingenerationbythelargeranimal，andlittlebythesmaller。Thereforemanysmallonesmaybeproducedtogether，butitishardformanylargeonestobeso，andtothoseintermediateinsizeNaturehasassignedtheintermediatenumber。Wehaveformerlygiventhereasonwhysomeanimalsarelarge，somesmaller，andsomebetweenthetwo，andspeakinggenerally，withregardtothenumberofyoungproduced，thesolid—hoofedproduceone，thecloven—footedfew，themany—toedmany。（Thereasonofthisisthat，generallyspeaking，theirsizescorrespondtothisdifference。）Itisnotso，however，inallcases；foritisthelargenessandsmallnessofthebodythatiscauseoffewormanyyoungbeingborn，notthefactthatthekindofanimalhasone，two，ormanytoes。Aproofofthisisthattheelephantisthelargestofanimalsandyetismany—toed，andthecamel，thenextlargest，iscloven—footed。Andnotonlyinanimalsthatwalkbutalsointhosethatflyorswimthelargeonesproducefew，thesmallmany，forthesamereason。Inlikemanneralsoitisnotthelargestplantsthatbearmostfruit。
Wehaveexplainedthenwhysomeanimalsnaturallyproducemanyyoung，somebutfew，andsomeonlyone；inthedifficultynowstatedwemayratherbesurprisedwithreasonatthosewhichproducemany，sincesuchanimalsareoftenseentoconceivefromasinglecopulation。Whetherthesemenofthemalecontributestothematerialoftheembryobyitselfbecomingapartofitandmixingwiththesemenofthefemale，orwhether，aswesay，itdoesnotactinthiswaybutbringstogetherandfashionsthematerialwithinthefemaleandthegenerativesecretionasthefig—juicedoestheliquidsubstanceofmilk，whatisthereasonwhyitdoesnotformasingleanimalofconsiderablesize？Forcertainlyintheparallelcasethefig—juiceisnotseparatedifithastocurdlealargequantityofmilk，butthemorethemilkandthemorethefig—juiceputintoit，somuchthegreateristhecurdledmass。Nowitisnousetosaythattheseveralregionsoftheuterusattractthesemenandthereforemoreyoungthanoneareformed，becausetheregionsaremanyandthecotyledonsaremorethanone。Fortwoembryosareoftenformedinthesameregionoftheuterus，andtheymaybeseenlyinginarowinanimalsthatproducemany，whentheuterusisfilledwiththeembryos。（Thisisplainfromthedissections。）Ratherthetruthisthis。Asanimalscompletetheirgrowththerearecertainlimitstotheirsize，bothupwardsanddownwards，beyondwhichtheycannotgo，butitisinthespacebetweentheselimitsthattheyexceedorfallshortofoneanotherinsize，anditiswithintheselimitsthatoneman（oranyotheranimal）islargerorsmallerthananother。Soalsothegenerativematerialfromwhicheachanimalisformedisnotwithoutaquantitativelimitinbothdirections，norcanitbeformedfromanyquantityyouplease。Wheneverthenananimal，forthecauseassigned，dischargesmoreofthefemalesecretionthanisneededforbeginningtheexistenceofasingleanimal，itisnotpossiblethatonlyoneshouldbeformedoutofallthis，butanumberlimitedbytheappropriatesizeineachcase；norwillthesemenofthemale，orthepowerresidinginthesemen，formanythingeithermoreorlessthanwhatisaccordingtoNature。Inlikemanner，ifthemaleemitsmoresementhanisnecessary，ormorepowersindifferentpartsofthesemenasitisdivided，howevermuchitisitwillnotmakeanythinggreater；onthecontraryitwilldryupthematerialofthefemaleanddestroyit。Sofirealsodoesnotcontinuetomakewaterhotterinproportionasitisitselfincreased，butthereisafixedlimittotheheatofwhichwateriscapable；ifthatisoncereachedandthefireisthenincreased，thewaternolongergetshotterbutratherevaporatesandatlastdisappearsandisdriedup。Nowsinceitappearsthatthesecretionofthefemaleandthatfromthemaleneedtostandinsomeproportionaterelationtooneanother（Imeaninanimalsofwhichthemaleemitssemen），whathappensinthosethatproducemanyyoungisthis：fromtheveryfirstthesemenemittedbythemalehaspower，beingdivided，toformseveralembryos，andthematerialcontributedbythefemaleissomuchthatseveralcanbeformedoutofit。（Theparallelofcurdlingmilk，whichwespokeofbefore，isnolongerinpointhere，forwhatisformedbytheheatofthesemenisnotonlyofacertainquantitybutalsoofacertainquality，whereaswithfig—juiceandrennetquantityaloneisconcerned。）Thisthenisjustthereasonwhyinsuchanimalstheembryosformedarenumerousanddonotalluniteintoonewhole；itisbecauseanembryoisnotformedoutofanyquantityyouplease，butwhetherthereistoomuchortoolittle，ineithercasetherewillbenoresult，forthereisalimitsetaliketothepoweroftheheatwhichactsonthematerialandtothematerialsoactedupon。
Onthesameprinciplemanyembryosarenotformed，thoughthesecretionismuch，inthelargeanimalswhichproduceonlyoneyoungone，forinthemalsoboththematerialandthatwhichworksuponitareofacertainquantity。Sothentheydonotsecretesuchmaterialintoogreatquantityforthereasonpreviouslystated，andwhattheydosecreteisnaturallyjustenoughforoneembryoalonetobeformedfromit。Ifevertoomuchissecreted，thentwinsareborn。
Hencesuchcasesseemtobemoreportentous，becausetheyarecontrarytothegeneralandcustomaryrule。
Manbelongstoallthreeclasses，forheproducesoneonlyandsometimesmanyorfew，thoughnaturallyhealmostalwaysproducesone。