Ithasbeensettled，then，inwhatsensetheembryoandthesemenhavesoul，andinwhatsensetheyhavenot；theyhaveitpotentiallybutnotactually。
Nowsemenisasecretionandismovedwiththesamemovementasthatinvirtueofwhichthebodyincreases（thisincreasebeingduetosubdivisionofthenutrimentinitslaststage）。Whenithasenteredtheuterusitputsintoformthecorrespondingsecretionofthefemaleandmovesitwiththesamemovementwherewithitismoveditself。Forthefemale’scontributionalsoisasecretion，andhasalltheartsinitpotentiallythoughnoneofthemactually；ithasinitpotentiallyeventhosepartswhichdifferentiatethefemalefromthemale，forjustastheyoungofmutilatedparentsaresometimesbornmutilatedandsometimesnot，soalsotheyoungbornofafemalearesometimesfemaleandsometimesmaleinstead。Forthefemaleis，asitwere，amutilatedmale，andthecatameniaaresemen，onlynotpure；
forthereisonlyonethingtheyhavenotinthem，theprincipleofsoul。Forthisreason，wheneverawind—eggisproducedbyanyanimal，theeggsoforminghasinitthepartsofbothsexespotentially，buthasnottheprincipleinquestion，sothatitdoesnotdevelopintoalivingcreature，forthisisintroducedbythesemenofthemale。Whensuchaprinciplehasbenimpartedtothesecretionofthefemaleitbecomesanembryo。
Liquidbutcorporealsubstancesbecomesurroundedbysomekindofcoveringonheating，likethesolidscumwhichformsonboiledfoodswhencooling。Allbodiesareheldtogetherbytheglutinous；thisquality，astheembryodevelopsandincreasesinsize，isacquiredbythesinewysubstance，whichholdstogetherthepartsofanimals，beingactualsinewinsomeanditsanalogueinothers。Tothesameclassbelongalsoskin，blood—vessels，membranes，andthelike，forthesedifferinbeingmoreorlessglutinousandgenerallyinexcessanddeficiency。
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Inthoseanimalswhosenatureiscomparativelyimperfect，whenaperfectembryo（which，however，isnotyetaperfectanimal）hasbeenformed，itiscastoutfromthemother，forreasonspreviouslystated。Anembryoisthencompletewhenitiseithermaleorfemale，inthecaseofthoseanimalswhopossessthisdistinction，forsome（i。e。allthosewhicharenotthemselvesproducedfromamaleorfemaleparentnorfromaunionofthetwo）produceanoffspringwhichisneithermalenorfemale。Ofthegenerationoftheseweshallspeaklater。
Theperfectanimals，thoseinternallyviviparous，keepthedevelopingembryowithinthemselvesandincloseconnexionuntiltheygivebirthtoacompleteanimalandbringittolight。
Athirdclassisexternallyviviparousbutfirstinternallyoviparous；theydeveloptheeggintoaperfectcondition，andtheninsomecasestheeggissetfreeaswithcreaturesexternallyoviparous，andtheanimalisproducedfromtheeggwithinthemother’sbody；inothercases，whenthenutrimentfromtheeggisconsumed，developmentiscompletedbyconnectionwiththeuterus，andthereforetheeggisnotsetfreefromtheuterus。Thischaractermarksthecartilaginousfish，ofwhichwemustspeaklaterbythemselves。
Herewemustmakeourfirststartfromthefirstclass；thesearetheperfectorviviparousanimals，andofthesethefirstisman。
Nowthesecretionofthesementakesplaceinallofthemjustasdoesthatofanyotherresidualmatter。Foreachisconveyedtoitsproperplacewithoutanyforcefromthebreathorcompulsionofanyothercause，assomeassert，sayingthatthegenerativepartsattractthesemenlikecupping—glasses，aidedbytheforceofthebreath，asifitwerepossibleforeitherthissecretionortheresidueofthesolidandliquidnutrimenttogoanywhereelsethantheydowithouttheexertionofsuchaforce。Theirreasonisthatthedischargeofbothisattendedbyholdingthebreath，butthisisacommonfeatureofallcaseswhenitisnecessarytomoveanything，becausestrengtharisesthroughholdingthebreath。Why，evenwithoutthisforcethesecretionsorexcretionsaredischargedinsleepifthepartsconcernedarefullofthemandarerelaxed。Onemightaswellsaythatitisbythebreaththattheseedsofplantsarealwayssegregatedtotheplaceswheretheyarewonttobearfruit。
No，therealcause，ashasbeenstatedalready，isthattherearespecialpartsforreceivingallthesecretions，aliketheuseless（astheresiduesoftheliquidandsolidnutriment），andtheblood，whichhastheso—calledblood—vessels。
Toconsidernowtheregionoftheuterusinthefemale—thetwoblood—vessels，thegreatvesselandtheaorta，dividehigherup，andmanyfinevesselsfromthemterminateintheuterus。Thesebecomeover—filledfromthenourishmenttheyconvey，noristhefemalenatureabletoconcoctit，becauseitiscolderthanman’s；sothebloodisexcretedthroughveryfinevesselsintotheuterus，thesebeingunableonaccountoftheirnarrownesstoreceivetheexcessivequantity，andtheresultisasortofhaemorrhage。Theperiodisnotaccuratelydefinedinwomen，buttendstoreturnduringthewaningofthemoon。Thisweshouldexpect，forthebodiesofanimalsarecolderwhentheenvironmenthappenstobecomeso，andthetimeofchangefromonemonthtoanotheriscoldbecauseoftheabsenceofthemoon，whencealsoitresultsthatthistimeisstormierthanthemiddleofthemonth。Whenthentheresidueofthenourishmenthaschangedintoblood，thecatameniatendtooccurattheabove—mentionedperiod，butwhenitisnotconcoctedalittlematteratatimeisalwayscomingaway，andthisiswhy’whites’appearinfemaleswhilestillsmall，infactmerechildren。Ifboththesedischargesofthesecretionsaremoderate，thebodyremainsingoodhealth，fortheyactasapurificationofthesecretionswhicharethecausesofamorbidstateofbody；iftheydonotoccuratalloriftheyareexcessive，theyareinjurious，eithercausingillnessorpullingdownthepatient；hencewhites，ifcontinuousandexcessive，preventgirlsfromgrowing。Thissecretionthenisnecessarilydischargedbyfemalesforthereasonsgiven；for，thefemalenaturebeingunabletoconcoctthenourishmentthoroughly，theremustnotonlybeleftaresidueoftheuselessnutriment，butalsotheremustbearesidueintheblood—vessels，andthisfillingthechannelsofthefinestvesselsmustoverflow。ThenNature，aimingatthebestend，usesitupinthisplaceforthesakeofgeneration，thatanothercreaturemaycomeintobeingofthesamekindastheformerwasgoingtobe，forthemenstrualbloodisalreadypotentiallysuchasthebodyfromwhichitisdischarged。
Inallfemales，then，theremustnecessarilybesuchasecretion，moreindeedinthosethathavebloodandofthesemostofallinman，butintheothersalsosomemattermustbecollectedintheuterineregion。Thereasonwhythereismoreinthosethathavebloodandmostinmanhasbeenalreadygiven，butwhy，ifallfemaleshavesuchasecretion，havenotallmalesonetocorrespond？
Forsomeofthemdonotemitsemenbut，justasthosewhichdoemititfashionbythemovementinthesementhemassformingfromthematerialsuppliedbythefemale，sodotheanimalsinquestionbringthesametopassandexertthesameformativepowerbythemovementwithinthemselvesinthatpartfromwhencethesemenissecreted。Thisistheregionaboutthediaphragminallthoseanimalswhichhaveone，fortheheartoritsanalogueisthefirstprincipleofanaturalbody，whilethelowerpartisamereadditionforthesakeofit。
Nowthereasonwhyitisnotallmalesthathaveagenerativesecretion，whileallfemalesdo，isthattheanimalisabodywithSoulorlife；thefemalealwaysprovidesthematerial，themalethatwhichfashionsit，forthisisthepowerthatwesaytheyeachpossess，andthisiswhatismeantbycallingthemmaleandfemale。
Thuswhileitisnecessaryforthefemaletoprovideabodyandamaterialmass，itisnotnecessaryforthemale，becauseitisnotwithintheworkofartortheembryothatthetoolsorthemakermustexist。Whilethebodyisfromthefemale，itisthesoulthatisfromthemale，forthesoulistherealityofaparticularbody。
Forthisreasonifanimalsofadifferentkindarecrossed（andthisispossiblewhentheperiodsofgestationareequalandconceptiontakesplacenearlyatthesameseasonandthereisnogreatdifferenceintheoftheanimals），thefirstcrosshasacommonresemblancetobothparents，asthehybridbetweenfoxanddog，partridgeanddomesticfowl，butastimegoesonandonegenerationspringsfromanother，thefinalresultresemblesthefemaleinform，justasforeignseedsproduceplantsvaryinginaccordancewiththecountryinwhichtheyaresown。Foritisthesoilthatgivestotheseedsthematerialandthebodyoftheplant。Andhencethepartofthefemalewhichreceivesthesemenisnotamerepassage，buttheuterushasaconsiderablewidth，whereasthemalesthatemitsemenhaveonlypassagesforthispurpose，andthesearebloodless。
Eachofthesecretionsbecomessuchatthemomentwhenitisinitsproperplace；beforethatthereisnothingofthesortunlesswithmuchviolenceandcontrarytonature。
Wehavethusstatedthereasonforwhichthegenerativesecretionsareformedinanimals。Butwhenthesemenfromthemale（inthoseanimalswhichemitsemen）hasentered，itputsintoformthepurestpartofthefemalesecretion（forthegreaterpartofthecatameniaalsoisuselessandfluid，asisthemostfluidpartofthemalesecretion，i。e。inasingleemission，theearlierdischargebeinginmostcasesapttobeinfertileratherthanthelater，havinglessvitalheatthroughwantofconcoction，whereasthatwhichisconcoctedisthickandofamorematerialnature）。
Ifthereisnoexternaldischarge，eitherinwomenorotheranimals，onaccountoftherenotbeingmuchuselessandsuperfluousmatterinthesecretion，thenthequantityformingwithinthefemalealtogetherisasmuchaswhatisretainedwithinthoseanimalswhichhaveanexternaldischarge；thisisputintoformbythepowerofthemaleresidinginthesemensecretedbyhim，or，asisclearlyseentohappeninsomeinsects，bythepartinthefemaleanalogoustotheuterusbeinginsertedintothemale。
Ithasbeenpreviouslystatedthatthedischargeaccompanyingsexualpleasureinthefemalecontributesnothingtotheembryo。Thechiefargumentfortheoppositeviewisthatwhatarecalledbaddreamsoccurbynightwithwomenaswithmen；butthisisnoproof，forthesamethinghappenstoyoungmenalsowhodonotyetemitsemen，andtothosewhodoemitsemenbutwhosesemenisinfertile。
Itisimpossibletoconceivewithouttheemissionofthemaleinunionandwithoutthesecretionofthecorrespondingfemalematerial，whetheritbedischargedexternallyorwhetherthereisonlyenoughwithinthebody。Womenconceive，however，withoutexperiencingthepleasureusualinsuchintercourse，ifthepartchancetobeinheatandtheuterustohavedescended。Butgenerallyspeakingtheoppositeisthecase，becausetheosuteriisnotclosedwhenthedischargetakesplacewhichisusuallyaccompaniedbypleasureinwomenaswellasmen，andwhenthisissothereisareadierwayforthesemenofthemaletobedrawnintotheuterus。
Theactualdischargedoesnottakeplacewithintheuterusassomethink，theosuteribeingtoonarrow，butitisintheregioninfrontofthis，wherethefemaledischargesthemoisturefoundinsomecases，thatthemaleemitsthesemen。Sometimesitremainsinthisplace；
atothertimes，iftheuteruschancetobeconvenientlyplacedandhotonaccountofthepurgationofthecatamenia，itdrawsitwithinitself。Aproofofthisisthatpessaries，thoughwetwhenapplied，areremoveddry。Moreover，inallthoseanimalswhichhavetheuterusnearthehypozoma，asbirdsandviviparousfishes，itisimpossiblethatthesemenshouldbesodischargedastoenterit；itmustbedrawnintoit。Thisregion，onaccountoftheheatwhichisinit，attractsthesemen。Thedischargeandcollectionofthecatameniaalsoexciteheatinthispart。Henceitactslikecone—shapedvesselswhich，whentheyhavebeenwashedoutwithhotwater，theirmouthbeingturneddownwards，drawwaterintothemselves。
Andthisisthewaythingsaredrawnup，butsomesaythatnothingofthekindhappenswiththeorganicpartsconcernedincopulation。
Preciselytheoppositeisthecaseofthosewhosaythewomanemitssemenaswellastheman，forifsheemitsitoutsidetheuterusthismustthendrawitbackagainintoitselfifitistobemixedwiththesemenofthemale。Butthisisasuperfluousproceeding，andNaturedoesnothingsuperfluous。
Whenthematerialsecretedbythefemaleintheuterushasbeenfixedbythesemenofthemale（thisactsinthesamewayasrennetactsuponmilk，forrennetisakindofmilkcontainingvitalheat，whichbringsintoonemassandfixesthesimilarmaterial，andtherelationofthesementothecatameniaisthesame，milkandthecatameniabeingofthesamenature）—when，Isay，themoresolidpartcomestogether，theliquidisseparatedofffromit，andastheearthypartssolidifymembranesformallroundit；thisisbothanecessaryresultandforafinalcause，theformerbecausethesurfaceofamassmustsolidifyonheatingaswellasoncooling，thelatterbecausethefoetusmustnotbeinaliquidbutbeseparatedfromit。
Someofthesearecalledmembranesandotherschoria，thedifferencebeingoneofmoreorless，andtheyexistinoviparaandviviparaalike。
Whentheembryoisonceformed，itactsliketheseedsofplants。
Forseedsalsocontainthefirstprincipleofgrowthinthemselves，andwhenthis（whichpreviouslyexistsinthemonlypotentially）hasbeendifferentiated，theshootandtherootaresentofffromit，anditisbytherootthattheplantgetsnourishment；foritneedsgrowth。Soalsointheembryoallthepartsexistpotentiallyinawayatthesametime，butthefirstprincipleisfurthestontheroadtorealization。Thereforetheheartisfirstdifferentiatedinactuality。
Thisisclearnotonlytothesenses（foritisso）butalsoontheoreticalgrounds。Forwhenevertheyounganimalhasbeenseparatedfrombothparentsitmustbeabletomanageitself，likeasonwhohassetuphouseawayfromhisfather。Henceitmusthaveafirstprinciplefromwhichcomestheorderingofthebodyatalaterstagealso，forifitistocomeinfromoutsideatlaterperiodtodwellinit，notonlymaythequestionbeaskedatwhattimeitistodoso，butalsowemayobjectthat，wheneachofthepartsisseparatingfromtherest，itisnecessarythatthisprincipleshouldexistfirstfromwhichcomesgrowthandmovementtotheotherparts。
（Whereforeallwhosay，asdidDemocritus，thattheexternalpartsofanimalsarefirstdifferentiatedandtheinternallater，aremuchmistaken；itisasiftheyweretalkingofanimalsofstoneorwood。
Forsuchasthesehavenoprincipleofgrowthatall，butallanimalshave，andhaveitwithinthemselves。）Thereforeitisthattheheartappearsfirstdistinctlymarkedoffinallthesanguinea，forthisisthefirstprincipleororiginofbothhomogeneousandheterogeneousparts，sincefromthemomentthattheanimalororganismneedsnourishment，fromthatmomentdoesthisdeservetobecalleditsprincipleororigin。Fortheanimalgrows，andthenutriment，initsfinalstage，ofananimalisthebloodoritsanalogue，andofthistheblood—vesselsarethereceptacle，whereforetheheartistheprincipleororiginofthesealso。（ThisisclearfromtheEnquiriesandtheanatomicaldrawings。）
Sincetheembryoisalreadypotentiallyananimalbutanimperfectone，itmustobtainitsnourishmentfromelsewhere；accordinglyitmakesuseoftheuterusandthemother，asaplantdoesoftheearth，togetnourishment，untilitisperfectedtothepointofbeingnowananimalpotentiallylocomotive。SoNaturehasfirstdesignedthetwoblood—vesselsfromtheheart，andfromthesesmallervesselsbranchofftotheuterus。Thesearewhatiscalledtheumbilicus，forthisisablood—vessel，consistingofoneormorevesselsindifferentanimals。Roundtheseisaskin—likeintegument，becausetheweaknessofthevesselsneedsprotectionandshelter。Thevesselsjoinontotheuterusliketherootsofplants，andthroughthemtheembryoreceivesitsnourishment。Thisiswhytheanimalremainsintheuterus，not，asDemocritussays，thatthepartsoftheembryomaybemouldedinconformitywiththoseofthemother。Thisisplainintheovipara，fortheyhavetheirpartsdifferentiatedintheeggafterseparationfromthematrix。
Hereadifficultymayberaised。Ifthebloodisthenourishment，andiftheheart，whichfirstcomesintobeing，alreadycontainsblood，andthenourishmentcomesfromoutside，whencedidthefirstnourishmententer？Perhapsitisnottruethatallofitcomesfromoutsidejustasintheseedsofplantsthereissomethingofthisnature，thesubstancewhichatfirstappearsmilky，soalsointhematerialoftheanimalembryothesuperfluousmatterofwhichitisformedisitsnourishmentfromthefirst。
Theembryo，then，growsbymeansoftheumbilicusinthesamewayasaplantbyitsroots，orasanimalsthemselveswhenseparatedfromthenutrimentwithinthemother，ofwhichwemustspeaklateratthetimeappropriatefordiscussingthem。Butthepartsarenotdifferentiated，assomesuppose，becauselikeisnaturallycarriedtolike。Besidesmanyotherdifficultiesinvolvedinthistheory，itresultsfromitthatthehomogeneouspartsoughttocomeintobeingeachoneseparatefromtherest，asbonesandsinewsbythemselves，andfleshbyitself，ifoneshouldacceptthiscause。Therealcausewhyeachofthemcomesintobeingisthatthesecretionofthefemaleispotentiallysuchastheanimalisnaturally，andallthepartsarepotentiallypresentinit，butnoneactually。Itisalsobecausewhentheactiveandthepassivecomeincontactwitheachotherinthatwayinwhichtheoneisactiveandtheotherpassive（I
meanintherightmanner，intherightplace，andattherighttime），straightwaytheoneactsandtheotherisactedupon。Thefemale，then，providesmatter，themaletheprincipleofmotion。Andastheproductsofartaremadebymeansofthetoolsoftheartist，ortoputitmoretrulybymeansoftheirmovement，andthisistheactivityoftheart，andtheartistheformofwhatismadeinsomethingelse，soisitwiththepowerofthenutritivesoul。Aslateroninthecaseofmatureanimalsandplantsthissoulcausesgrowthfromthenutriment，usingheatandcoldasitstools（forintheseisthemovementofthesoul），andeachthingcomesintobeinginaccordancewithacertainformula，soalsofromthebeginningdoesitformtheproductofnature。Forthematerialbywhichthislattergrowsisthesameasthatfromwhichitisconstitutedatfirst；
consequentlyalsothepowerwhichactsuponitisidenticalwiththatwhichoriginallygeneratedit；ifthenthisactingpoweristhenutritivesoul，thisisalsothegenerativesoul，andthisisthenatureofeveryorganism，existinginallanimalsandplants。[Buttheotherpartsofthesoulexistinsomeanimals，notinothers。]Inplants，then，thefemaleisnotseparatedfromthemale，butinthoseanimalsinwhichitisseparatedthemaleneedsthefemalebesides。
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Andyetthequestionmayberaisedwhyitisthat，ifindeedthefemalepossessesthesamesoulandifitisthesecretionofthefemalewhichisthematerialoftheembryo，sheneedsthemalebesidesinsteadofgeneratingentirelyfromherself。Thereasonisthattheanimaldiffersfromtheplantbyhavingsense—perception；ifthesensitivesoulisnotpresent，eitheractuallyorpotentially，andeitherwithorwithoutqualification，itisimpossibleforface，hand，flesh，oranyotherparttoexist；itwillbenobetterthanacorpseorpartofacorpse。Ifthen，whenthesexesareseparated，itisthemalethathasthepowerofmakingthesensitivesoul，itisimpossibleforthefemaletogenerateananimalfromitselfalone，fortheprocessinquestionwasseentoinvolvethemalequality。Certainlythatthereisagooddealinthedifficultystatedisplaininthecaseofthebirdsthatlaywind—eggs，showingthatthefemalecangenerateuptoacertainpointunaided。Butthisstillinvolvesadifficulty；inwhatwayarewetosaythattheireggslive？Itneitherpossiblethattheyshouldliveinthesamewayasfertileeggs（forthentheywouldproduceachickactuallyalive），noryetcantheybecalledeggsonlyinthesenseinwhichaneggofwoodorstoneissocalled，forthefactthattheseeggsgobadshowsthattheypreviouslyparticipateinsomewayinlife。Itisplain，then，thattheyhavesomesoulpotentially。Whatsortofsoulwillthisbe？Itmustbethelowestsurely，andthisisthenutritive，forthisexistsinallanimalsandplantsalike。Whythendoesitnotperfectthepartsandtheanimal？Becausetheymusthaveasensitivesoul，forthepartsofanimalsarenotlikethoseofaplant。Andsothefemaleanimalneedsthehelpofthemale，forintheseanimalswearespeakingofthemaleisseparate。Thisisexactlywhatwefind，forthewind—eggsbecomefertileifthemaletreadthefemaleinacertainspaceoftime。Aboutthecauseofthesethings，however，weshallenterintodetaillater。
Ifthereisanykindofanimalwhichisfemaleandhasnomaleseparatefromit，itispossiblethatthismaygenerateayoungonefromitselfwithoutcopulation。Noinstanceofthisworthyofcredithasbeenobserveduptothepresentatanyrate，butonecaseintheclassoffishesmakesushesitate。Nomaleoftheso—callederythrinushaseveryetbeenseen，butfemales，andspecimensfullofroe，havebeenseen。Ofthis，however，wehaveasyetnoproofworthyofcredit。
Again，somemembersoftheclassoffishesareneithermalenorfemale，aseelsandakindofmulletsfoundinstagnantwaters。Butwheneverthesexesareseparatethefemalecannotgenerateperfectlybyherselfalone，forthenthemalewouldexistinvain，andNaturemakesnothinginvain。Henceinsuchanimalsthemalealwaysperfectstheworkofgeneration，forheimpartsthesensitivesoul，eitherbymeansofthesemenorwithoutit。Nowthepartsoftheembryoalreadyexistpotentiallyinthematerial，andsowhenoncetheprincipleofmovementhasbeenimpartedtothemtheydevelopinachainoneafteranother，asthewheelsaremovedonebyanotherintheautomaticmachines。Whensomeofthenaturalphilosopherssaythatlikeisbroughttolike，thismustbeunderstood，notinthesensethatthepartsaremovedaschangingplace，butthattheystaywheretheyareandthemovementisachangeofquality（suchassoftness，hardness，colour，andtheotherdifferencesofthehomogeneousparts）；
thustheybecomeinactualitywhattheypreviouslywereinpotentiality。Andwhatcomesintobeingfirstisthefirstprinciple；thisistheheartinthesanguineaanditsanalogueintherest，ashasbeenoftensaidalready。Thisisplainnotonlytothesenses（thatitisfirsttocomeintobeing），butalsoinviewofitsend；forlifefailsintheheartlastofall，andithappensinallcasesthatwhatcomesintobeinglastfailsfirst，andthefirstlast，Naturerunningadoublecourse，sotosay，andturningbacktothepointfromwhenceshestarted。Fortheprocessofbecomingisfromthenon—existenttotheexistent，andthatofperishingisbackagainfromtheexistenttothenon—existent。
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Afterthis，assaidalready，theinternalpartscomeintobeingbeforetheexternal。Thegreaterbecomevisiblebeforetheless，evenifsomeofthemdonotcomeintobeingbeforethem。Firstthepartsabovethehypozomaaredifferentiatedandaresuperiorinsize；thepartbelowisbothsmallerandlessdifferentiated。Thishappensinallanimalsinwhichexiststhedistinctionofupperandlower，exceptintheinsects；thegrowthofthosethatproduceascolexistowardstheupperpart，forthisissmallerinthebeginning。Thecephalopodaaretheonlylocomotiveanimalsinwhichthedistinctionofupperandlowerdoesnotexist。
Whathasbeensaidappliestoplantsalso，thattheupperportionisearlierindevelopmentthanthelower，fortherootspushoutfromtheseedbeforetheshoots。
Theagencybywhichthepartsofanimalsaredifferentiatedisair，nothoweverthatofthemothernoryetoftheembryoitself，assomeofthephysicistssay。Thisismanifestinbirds，fishes，andinsects。Forsomeoftheseareseparatedfromthemotherandproducedfromanegg，withinwhichthedifferentiationtakesplace；
otheranimalsdonotbreatheatall，butareproducedasascolexoranegg；thosewhichdobreatheandwhosepartsaredifferentiatedwithinthemother’suterusyetdonotbreatheuntilthelungisperfected，andthelungandtheprecedingpartsaredifferentiatedbeforetheybreathe。Moreover，allpolydactylousquadrupeds，asdog，lion，wolf，fox，jackal，producetheiryoungblind，andtheeyelidsdonotseparatetillafterbirth。Manifestlythesameholdsalsoinalltheotherparts；asthequalitative，soalsothequantitativedifferentiacomesintobeing，pre—existingpotentiallybutbeingactualizedlaterbythesamecausesbywhichthequalitativedistinctionisproduced，andsotheeyelidsbecometwoinsteadofone。
Ofcourseairmustbepresent，becauseheatandmoisturearepresent，theformeractingandthelatterbeingactedupon。
Someoftheancientnature—philosolphersmadeanattempttostatewhichpartcomesintobeingafterwhich，butwerenotsufficientlyacquaintedwiththefacts。Itiswiththepartsaswithotherthings；onenaturallyexistspriortoanother。Buttheword’prior’isusedinmoresensesthanone。Forthereisadifferencebetweentheendorfinalcauseandthatwhichexistsforthesakeofit；thelatterispriorinorderofdevelopment，theformerispriorinreality。Again，thatwhichexistsforthesakeoftheendadmitsofdivisionintotwoclasses，（1）theoriginofthemovement，（2）thatwhichisusedbytheend；Imean，forinstance，（1）thatwhichcangenerate，（2）thatwhichservesasaninstrumenttowhatisgenerated，fortheoneofthese，thatwhichmakes，mustexistfirst，astheteacherbeforethelearner，andtheotherlater，asthepipesarelaterthanhewholearnstoplayuponthem，foritissuperfluousthatmenwhodonotknowhowtoplayshouldhavepipes。Thustherearethreethings：first，theend，bywhichwemeanthatforthesakeofwhichsomethingelseexists；secondly，theprincipleofmovementandofgeneration，existingforthesakeoftheend（forthatwhichcanmakeandgenerate，consideredsimplyassuch，existsonlyinrelationtowhatismadeandgenerated）；thirdly，theuseful，thatistosaywhattheenduses。Accordingly，theremustfirstexistsomepartinwhichistheprincipleofmovement（Isayapartbecausethisisfromthefirstonepartoftheendandthemostimportantparttoo）；nextafterthisthewholeandtheend；thirdlyandlastly，theorganicpartsservingtheseforcertainuses。Henceifthereisanythingofthissortwhichmustexistinanimals，containingtheprincipleandendofalltheirnature，thismustbethefirsttocomeintobeing—first，thatis，consideredasthemovingpower，butsimultaneouswiththewholeembryoifconsideredasapartoftheend。
Thereforealltheorganicpartswhosenatureistobringothersintobeingmustalwaysthemselvesexistbeforethem，fortheyareforthesakeofsomethingelse，asthebeginningforthesakeoftheend；
allthosepartswhichareforthesakeofsomethingelsebutarenotofthenatureofbeginningsmustcomeintobeinglater。Soitisnoteasytodistinguishwhichofthepartsareprior，thosewhichareforthesakeofanotherorthatforthesakeofwhicharetheformer。Forthepartswhichcausethemovement，beingpriortotheendinorderofdevelopment，comeintocauseconfusion，anditisnoteasytodistinguishtheseascomparedwiththeorganicparts。Andyetitisinaccordancewiththismethodthatwemustinquirewhatcomesintobeingafterwhat；fortheendislaterthansomepartsandearlierthanothers。Andforthisreasonthatpartwhichcontainsthefirstprinciplecomesintobeingfirst，nexttothistheupperhalfofthebody。Thisiswhythepartsaboutthehead，andparticularlytheeyes，appearlargestintheembryoatanearlystage，whilethepartsbelowtheumbilicus，asthelegs，aresmall；
forthelowerpartsareforthesakeoftheupper，andareneitherpartsoftheendnorabletoformit。
Buttheydonotsaywellnordotheyassignanecessarycausewhosaysimplythat’italwayshappensso’，andimaginethatthisisafirstprincipleinthesecases。ThusDemocritusofAbderasaysthat’thereisnobeginningoftheinfinite；nowthecauseisabeginning，andtheeternalisinfinite；inconsequence，toaskthecauseofanythingofthiskindistoseekforabeginningoftheinfinite’。Yetaccordingtothisargument，whichforbidsustoseekthecause，therewillbenoproofofanyeternaltruthwhatever；butweseethatthereisaproofofmanysuch，whetherby’eternal’wemeanwhatalwayshappensorwhatexistseternally；itisaneternaltruththattheanglesofatrianglearealwaysequaltotworightangles，orthatthediagonalofasquareisincommensurablewiththeside，andneverthelessacauseandaproofcanbegivenforthesetruths。While，then，itiswellsaidthatwemustnottakeonustoseekabeginning（orfirstprinciple）ofallthings，yetthisisnotwellsaidofallthingswhateverthatalwaysareoralwayshappen，butonlyofthosewhichreallyarefirstprinciplesoftheeternalthings；
foritisbyanothermethod，notbyproof，thatweacquireknowledgeofthefirstprinciple。Nowinthatwhichisimmovableandunchangingthefirstprincipleissimplytheessenceofthething，butwhenwecometothosethingswhichcomeintobeingtheprinciplesaremorethanone，varyinginkindandnotallofthesamekind；oneofthisnumberistheprincipleofmovement，andthereforeinallthesanguineatheheartisformedfirst，aswassaidatthebeginning，andintheotheranimalsthatwhichisanalogoustotheheart。
Fromthehearttheblood—vesselsextendthroughoutthebodyasintheanatomicaldiagramswhicharerepresentedonthewall，forthepartslieroundthesebecausetheyareformedoutofthem。Thehomogeneouspartsareformedbyheatandcold，forsomeareputtogetherandsolidifiedbytheoneandsomebytheother。Thedifferencebetweenthesehasalreadybeendiscussedelsewhere，andithasbeenstatedwhatkindsofthingsaresolublebyliquidandfire，andwhatarenotsolublebyliquidandcannotbemeltedbyfire。
Thenutrimentthenoozesthroughtheblood—vesselsandthepassagesineachoftheparts，likewaterinunbakedpottery，andthusisformedthefleshoritsanalogues，beingsolidifiedbycold，whichiswhyitisalsodissolvedbyfire。Butalltheparticlesgivenoffwhicharetooearthy，havingbutlittlemoistureandheat，coolasthemoistureevaporatesalongwiththeheat；sotheybecomehardandearthyincharacter，asnails，horns，hoofs，andbeaks，andthereforetheyaresoftenedbyfirebutnoneofthemismeltedbyit，whilesomeofthem，asegg—shells，aresolubleinliquids。Thesinewsandbonesareformedbytheinternalheatasthemoisturedries，andhencethebonesareinsolublebyfirelikepottery，forlikeittheyhavebeenasitwerebakedinanovenbytheheatintheprocessofdevelopment。Butitisnotanythingwhateverthatismadeintofleshorbonebytheheat，butonlysomethingnaturallyfittedforthepurpose；norisitmadeinanyplaceortimewhatever，butonlyinaplaceandtimenaturallysofitted。Forneitherwillthatwhichexistspotentiallybemadeexceptbythatmovingagentwhichpossessestheactuality，norwillthatwhichpossessestheactualitymakeanythingwhatever；thecarpenterwouldnotmakeaboxexceptoutofwood，norwillaboxbemadeoutofthewoodwithoutthecarpenter。Theheatexistsintheseminalsecretion，andthemovementandactivityinitissufficientinkindandinquantitytocorrespondtoeachoftheparts。Insofarasthereisanydeficiencyorexcess，theresultingproductisinworseconditionorphysicallydefective，inlikemannerasinthecaseofexternalsubstanceswhicharethickenedbyboilingthattheymaybemorepalatableorforanyotherpurpose。Butinthelattercaseitiswewhoapplytheheatinduemeasureforthemotionrequired；intheformeritisthenatureofthemaleparentthatgivesit，orwithanimalsspontaneouslygenerateditisthemovementandheatimpartedbytherightseasonoftheyearthatitisthecause。
Cooling，again，ismeredeprivationofheat。Naturemakesuseofboth；theyhaveofnecessitythepowerofbringingaboutdifferentresults，butinthedevelopmentoftheembryowefindthattheonecoolsandtheotherheatsforsomedefinitepurpose，andsoeachofthepartsisformed；thusitisinonesensebynecessity，inanotherforafinalcause，thattheymakethefleshsoft，thesinewssolidandelastic，thebonessolidandbrittle。Theskin，again，isformedbythedryingoftheflesh，likethescumuponboiledsubstances；itissoformednotonlybecauseitisontheoutside，butalsobecausewhatisglutinous，beingunabletoevaporate，remainsonthesurface。Whileinotheranimalstheglutinousisdry，forwhichreasonthecoveringoftheinvertebratesistestaceousorcrustaceous，inthevertebratesitisratherofthenatureoffat。Inallofthesewhicharenotoftooearthyanaturethefatiscollectedunderthecoveringoftheskin，afactwhichpointstotheskinbeingformedoutofsuchaglutinoussubstance，forfatissomewhatglutinous。Aswesaid，allthesethingsmustbeunderstoodtobeformedinonesenseofnecessity，butinanothersensenotofnecessitybutforafinalcause。
Theupperhalfofthebody，then，isfirstmarkedoutintheorderofdevelopment；astimegoesontheloweralsoreachesitsfullsizeinthesanguinea。Allthepartsarefirstmarkedoutintheiroutlinesandacquirelaterontheircolourandsoftnessorhardness，exactlyasifNaturewereapainterproducingaworkofart，forpainters，too，firstsketchintheanimalwithlinesandonlyafterthatputinthecolours。
Becausethesourceofthesensationsisintheheart，thereforethisisthepartfirstformedinthewholeanimal，andbecauseoftheheatofthisorganthecoldformsthebrain，wheretheblood—vesselsterminateabove，correspondingtotheheatoftheheart。Hencethepartsabouttheheadbegintoformnextinorderaftertheheart，andsurpasstheotherpartsinsize，forthebrainisfromthefirstlargeandfluid。
Thereisadifficultyaboutwhathappenswiththeeyesofanimals。
Thoughfromthebeginningtheyappearverylargeinallcreatures，whethertheywalkorswimorfly，yettheyarethelastofthepartstobeformedcompletely，forintheinterveningtimetheycollapse。
Thereasonisthis。Thesense—organoftheeyesissetuponcertainpassages，asaretheothersense—organs。Whereasthoseoftouchandtastearesimplythebodyitselforsomepartofthebodyofanimals，thoseofsmellandhearingarepassagesconnectingwiththeexternalairandfullthemselvesofinnatespiritus；thesepassagesendatthesmallblood—vesselsaboutthebrainwhichrunthitherfromtheheart。Buttheeyeistheonlysense—organthathasabodilyconstitutionpeculiartoitself。Itisfluidandcold，anddoesnotexistfromthefirstintheplacewhichitoccupieslaterinthesamewayastheotherpartsdo，fortheyexistpotentiallytobeginwithandactuallycomeintobeinglater，buttheeyeisthepurestpartoftheliquidityaboutthebraindrainedoffthroughthepassageswhicharevisiblerunningfromthemtothemembraneroundthebrain。A
proofofthisisthat，apartfromthebrain，thereisnootherpartintheheadthatiscoldandfluidexcepttheeye。Ofnecessitythereforethisregionislargeatfirstbutfallsinlater。Forthesamethinghappenswiththebrain；atfirstitisliquidandlarge，butincourseofevaporationandconcoctionitbecomesmoresolidandfallsin；thisappliesbothtothebrainandtheeyes。Theheadisverylargeatfirst，onaccountofthebrain，andtheeyesappearlargebecauseoftheliquidinthem。Theyarethelastorganstoreachcompletionbecausethebrainisformedwithdifficulty；foritisatalateperiodthatitgetsridofitscoldnessandfluidity；thisappliestoallanimalspossessingabrain，butespeciallytoman。Forthisreasonthe’bregma’isthelastofthebonestobeformed；evenafterbirththisboneisstillsoftinchildren。Thecauseofthisbeingsowithmenmorethanwithotheranimalsisthefactthattheirbrainisthemostfluidandlargest。Thisagainisbecausetheheatinman’sheartispurest。Hisintellectshowshowwellheistempered，formanisthewisestofanimals。Andchildrenforalongtimehavenocontrolovertheirheadsonaccountoftheheavinessofthebrain；andthesameappliestothepartswhichitisnecessarytomove，foritislatethattheprincipleofmotiongetscontrolovertheupperparts，andlastofalloverthosewhosemotionisnotconnecteddirectlywithit，asthatofthelegsisnot。Nowtheeyelidissuchapart。ButsinceNaturemakesnothingsuperfluousnorinvain，itisclearalsothatshemakesnothingtoolateortoosoon，forifshedidtheresultwouldbeeitherinvainorsuperfluous。
Henceitisnecessarythattheeyelidsshouldbeseparatedatthesametimeastheheartisabletomovethem。Sothentheeyesofanimalsareperfectedlatebecauseoftheamountofconcoctionrequiredbythebrain，andlastofallthepartsbecausethemotionmustbeverystrongbeforeitcanaffectpartssofarfromthefirstprincipleofmotionandsocold。Anditisplainthatsuchisthenatureoftheeyelids，foriftheheadisaffectedbyneversolittleheavinessthroughsleepinessordrunkennessoranythingelseofthekind，wecannotraisetheeyelidsthoughtheirownweightissosmall。Somuchforthequestionhowtheeyescomeintobeing，andwhyandforwhatcausetheyarethelasttobefullydeveloped。
Eachoftheotherpartsisformedoutofthenutriment，thosemosthonourableandparticipatinginthesovereignprinciplefromthenutrimentwhichisfirstandpurestandfullyconcocted，thosewhichareonlynecessaryforthesakeoftheformerpartsfromtheinferiornutrimentandtheresiduesleftoverfromtheother。ForNature，likeagoodhouseholder，isnotinthehabitofthrowingawayanythingfromwhichitispossibletomakeanythinguseful。Nowinahouseholdthebestpartofthefoodthatcomesinissetapartforthefreemen，theinferiorandtheresidueofthebestfortheslaves，andtheworstisgiventotheanimalsthatlivewiththem。
Justastheintellectactsthusintheoutsideworldwithaviewtothegrowthofthepersonsconcerned，sointhecaseoftheembryoitselfdoesNatureformfromthepurestmaterialthefleshandthebodyoftheothersense—organs，andfromtheresiduesthereofbones，sinews，hair，andalsonailsandhoofsandthelike；hencethesearelasttoassumetheirform，fortheyhavetowaittillthetimewhenNaturehassomeresiduetospare。
Thebones，then，aremadeinthefirstconformationofthepartsfromtheseminalsecretionorresidue。Astheanimalgrowsthebonesgrowfromthenaturalnourishment，beingthesameasthatofthesovereignparts，butofthistheyonlytakeupthesuperfluousresidues。Foreverywherethenutrimentmaybedividedintotwokinds，thefirstandthesecond；theformeris’nutritious’，beingthatwhichgivesitsessencebothtothewholeandtotheparts；thelatterisconcernedwithgrowth，beingthatwhichcausesquantitativeincrease。Butthesemustbedistinguishedmorefullylateron。Thesinewsareformedinthesamewayasthebonesandoutofthesamematerials，theSeminalandnutritiousresidue。Nails，hair，hoofs，horns，beaks，thespursofcocks，andanyothersimilarparts，areonthecontraryformedfromthenutrimentwhichistakenlaterandonlyconcernedwithgrowth，inotherwordsthatwhichisderivedfromthemother，orfromtheouterworldafterbirth。Forthisreasonthebonesontheonehandonlygrowuptoacertainpoint（forthereisalimitofsizeinallanimals，andthereforealsoofthegrowthofthebones；ifthesehadbeenalwaysabletogrow，allanimalsthathaveboneoritsanaloguewouldgrowaslongastheylived，forthesesetthelimitofsizetoanimals。Whatisthereasonoftheirnotalwaysincreasinginsizemustbestatedlater。）
Hair，onthecontrary，andgrowthsakintohairgoongrowingaslongastheyexistatall，andincreaseyetmoreindiseasesandwhenthebodyisgettingoldandwasting，becausemoreresidualmatterisleftover，asowingtooldageanddiseaselessisexpendedontheimportantparts，thoughwhentheresidualmatteralsofailsthroughagethehairfailswithit。Butthecontraryisthecasewiththebones，fortheywasteawayalongwiththebodyandtheotherparts。
Hairactuallygoesongrowingafterdeath；itdoesnot，however，begingrowingthen。
Abouttheteethadifficultymayberaised。Theyhaveactuallythesamenatureasthebones，andareformedoutofthebones，butnails，hair，horns，andthelikeareformedoutoftheskin，andthatiswhytheychangeincolouralongwithit，fortheybecomewhite，black，andallsortsofcoloursaccordingtothatoftheskin。Buttheteethdonothingofthesort，fortheyaremadeoutofthebonesinallanimalsthathavebothbonesandteeth。Ofallthebonestheyalonegoongrowingthroughlife，asisplainwiththeteethwhichgrowoutofthestraightlinesoasnolongertotoucheachother。Thereasonfortheirgrowth，asafinalcause，istheirfunction，fortheywouldsoonbeworndowniftherewerenotsomemeansofsavingthem；evenasitistheyarealtogetherworndowninoldageinsomeanimalswhicheatmuchandhavenotlargeteeth，theirgrowthnotbeinginproportiontotheirdetrition。AndsoNaturehascontrivedwelltomeetthecaseinthisalso，forshecausesthefailureoftheteethtosynchronizewitholdageanddeath。Iflifelastedforathousandortenthousandyearstheoriginalteethmusthavebeenverylargeindeed，andmanysetsofthemmusthavebeenproduced，foreveniftheyhadgrowncontinuouslytheywouldstillhavebeenwornsmoothandbecomeuselessfortheirwork。Thefinalcauseoftheirgrowthhasbeennowstated，butbesidesthisasamatteroffactthegrowthoftheteethisnotthesameasthatoftheotherbones。Thelatterallcomeintobeinginthefirstformationoftheembryoandnoneofthemlater，buttheteethdosolater。
Thereforeitispossibleforthemtogrowagainafterthefirstsetfallsout，forthoughtheytouchthebonestheyarenotconnatewiththem。Theyareformed，however，outofthenutrimentdistributedtothebones，andsohavethesamenature，evenwhentheboneshavetheirownnumbercomplete。
Otheranimalsareborninpossessionofteethortheiranalogue（unlessincasescontrarytoNature），becausewhentheyaresetfreefromtheparenttheyaremoreperfectthanman；butman（alsounlessincasescontrarytoNature）isbornwithoutthem。
Thereasonwillbestatedlaterwhysometeethareformedandfalloutbutothersdonotfallout。
Itisbecausesuchpartsareformedfromaresiduethatmanisthemostnakedinbodyofallanimalsandhasthesmallestnailsinproportiontohissize；hehastheleastamountofearthyresidue，butthatpartofthebloodwhichisnotconcoctedistheresidue，andtheearthypartinthebodiesofallanimalsistheleastconcocted。
Wehavenowstatedhoweachofthepartsisformedandwhatisthecauseoftheirgeneration。
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Inviviparousanimals，assaidbefore，theembryogetsitsgrowththroughtheumbilicalcord。Forsincethenutritivepowerofthesoul，aswellastheothers，ispresentinanimals，itstraightwaysendsoffthiscordlikearoottotheuterus。Thecordconsistsofblood—vesselsinasheath，morenumerousinthelargeranimalsascattleandthelike，oneinthesmallest，twointhoseofintermediatesize。Throughthiscordtheembryoreceivesitsnourishmentintheformofblood，fortheuterusistheterminationofmanyblood—vessels。Allanimalswithnofrontteethintheupperjaw，andallthosewhichhavetheminbothjawsandwhoseuterushasnotonegreatblood—vesselrunningthroughitbutmanyclosetogetherinstead—
allthesehaveintheuterustheso—calledcotyledons（withwhichtheumbilicalcordconnectsandiscloselyunited；forthevesselswhichpassthroughthecordrunbackwardsandforwardsbetweenembryoanduterusandsplitupintosmallervesselsallovertheuterus；wheretheyterminate，therearefoundthecotyledons）。Theirconvexityisturnedtowardstheuterus，theconcavitytowardstheembryo。Betweenuterusandembryoarethechorionandthemembranes。Astheembryogrowsandapproachesperfectionthecotyledonsbecomesmallerandfinallydisappearwhenitisperfected。ForNaturesendsthesanguineousnutrimentfortheembryointothispartoftheuterusasshesendsmilkintothebreasts，andbecausethecotyledonsaregraduallyaggregatedfrommanyintoafewthebodyofthecotyledonbecomeslikeaneruptionorinflammation。Solongastheembryoiscomparativelysmall，beingunabletoreceivemuchnutriment，theyareplainandlarge，butwhenithasincreasedinsizetheyfallintogether。
Butmostoftheanimalswhichhavefrontteethinbothjawsandnohornshavenocotyledonsintheuterus，buttheumbilicalcordrunstomeetoneblood—vessel，whichislargeandextendsthroughouttheuterus。Ofsuchanimalssomeproduceoneyoungatatime，somemorethanone，butthesamedescriptionappliestoboththeseclasses。
（ThisshouldbestudiedwiththeaidoftheexamplesdrawnintheAnatomyandtheEnquiries。）Fortheyoung，ifnumerous，areattachedeachtoitsumbilicalcord，andthistotheblood—vesselofthemother；theyarearrangednexttooneanotheralongthestreamoftheblood—vesselasalongacanal；andeachembryoisenclosedinitsmembranesandchorion。
Thosewhosaythatchildrenarenourishedintheuterusbysuckingsomelumpoffleshorotheraremistaken。Ifso，thesamewouldhavebeenthecasewithotheranimals，butasitiswedonotfindthis（andthiscaneasilybeobservedbydissection）。Secondly，allembryosalike，whetherofcreaturesthatflyorswimorwalk，aresurroundedbyfinemembranesseparatingthemfromtheuterusandfromthefluidswhichareformedinit；butneitherinthesethemselvesisthereanythingofthekind，norisitpossiblefortheembryototakenourishmentbymeansofanyofthem。Thirdly，itisplainthatallcreaturesdevelopedineggsgrowwhenseparatedfromtheuterus。
Naturalintercoursetakesplacebetweenanimalsofthesamekind。
However，thosealsounitewhosenatureisnearakinandwhoseformisnotverydifferent，iftheirsizeismuchthesameandiftheperiodsofgestationareequal。Inotheranimalssuchcasesarerare，buttheyoccurwithdogsandfoxesandwolves；theIndiandogsalsospringfromtheunionofadogwithsomewilddog—likeanimal。
Asimilarthinghasbeenseentotakeplaceinthosebirdsthatareamative，aspartridgesandhens。Amongbirdsofpreyhawksofdifferentformarethoughttounite，andthesameappliestosomeotherbirds。Nothingworthmentioninghasbeenobservedintheinhabitantsofthesea，buttheso—called’rhinobates’especiallyisthoughttospringfromtheunionofthe’rhini’and’batus’。AndtheproverbaboutLibya，that’Libyaisalwaysproducingsomethingnew’，issaidtohaveoriginatedfromanimalsofdifferentspeciesunitingwithoneanotherinthatcountry，foritissaidthatbecauseofthewantofwaterallmeetatthefewplaceswherespringsaretobefound，andthatevendifferentkindsuniteinconsequence。
Oftheanimalsthatarisefromsuchunionallexceptmulesarefoundtocopulateagainwitheachotherandtobeabletoproduceyoungofbothsexes，butmulesalonearesterile，fortheydonotgeneratebyunionwithoneanotherorwithotheranimals。Theproblemwhyanyindividual，whethermaleorfemale，issterileisageneralone，forsomemenandwomenaresterile，andsoareotheranimalsintheirseveralkinds，ashorsesandsheep。Butthiskind，ofmules，isuniversallyso。Thecausesofsterilityinotheranimalsareseveral。Bothmenandwomenaresterilefrombirthwhenthepartsusefulforunionareimperfect，sothatmennevergrowabeardbutremainlikeeunuchs，andwomendonotattainpuberty；thesamethingmaybefallothersastheiryearsadvance，sometimesonaccountofthebodybeingtoowellnourished（formenwhoareintoogoodconditionandwomenwhoaretoofattheseminalsecretionistakenupintothebody，andtheformerhavenosemen，thelatternocatamenia）；atothertimesbyreasonofsicknessmenemitthesemeninacoldandliquidstate，andthedischargesofwomenarebadandfullofmorbidsecretions。Often，too，inbothsexesthisstateiscausedbyinjuriesinthepartsandregionscontributorytocopulation。Somesuchcasesarecurable，othersincurable，butthesubjectsespeciallyremainsterileifanythingofthesorthashappenedinthefirstformationofthepartsintheembryo，forthenareproducedwomenofamasculineandmenofafeminineappearance，andintheformerthecatameniadonotoccur，inthelatterthesemenisthinandcold。Henceitiswithgoodreasonthatthesemenofmenistestedinwatertofindoutifitisinfertile，forthatwhichisthinandcoldisquicklyspreadoutonthesurface，butthefertilesinkstothebottom，forthatwhichiswellconcoctedishotindeed，butthatwhichisfirmandthickiswellconcocted。Theytestwomenbypessariestoseeifthesmellsthereofpermeatefrombelowupwardstothebreathfromthemouthandbycolourssmearedupontheeyestoseeiftheycolourthesaliva。Iftheseresultsdonotfollowitisasignthatthepassagesofthebody，throughwhichthecatameniaaresecreted，arecloggedandclosed。Fortheregionabouttheeyesis，ofallthehead，thatmostnearlyconnectedwiththegenerativesecretions；aproofofthisisthatitaloneisvisiblychangedinsexualintercourse，andthosewhoindulgetoomuchinthisareseentohavetheireyessunkenin。Thereasonisthatthenatureofthesemenissimilartothatofthebrain，forthematerialofitiswatery（theheatbeingacquiredlater）。Andtheseminalpurgationsarefromtheregionofthediaphragm，forthefirstprincipleofnatureisthere，sothatthemovementsfromthepudendaarecommunicatedtothechest，andthesmellsfromthechestareperceivedthroughtherespiration。
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Inmen，then，andinotherkinds，assaidbefore，suchdeficiencyoccurssporadically，butthewholeofthemulekindissterile。ThereasonhasnotbeenrightlygivenbyEmpedoclesandDemocritus，ofwhomtheformerexpresseshimselfobscurely，thelattermoreintelligibly。Fortheyoffertheirdemonstrationinthecaseofalltheseanimalsalikewhichuniteagainsttheiraffinities。Democritussaysthatthegenitalpassagesofmulesarespoiltinthemother’suterusbecausetheanimalsfromthefirstarenotproducedfromparentsofthesamekind。Butwefindthatthoughthisissowithotheranimalstheyarenonethelessabletogenerate；yet，ifthiswerethereason，allothersthatuniteinthismanneroughttobebarren。Empedoclesassignsashisreasonthatthemixtureofthe’seeds’becomesdense，eachofthetwoseminalfluidsoutofwhichitismadebeingsoft，forthehollowsineachfitintothedensitiesoftheother，andinsuchcasesahardsubstanceisformedoutofsoftones，likebronzemingledwithtin。Nowhedoesnotgivethecorrectreasoninthecaseofbronzeandtin—（wehavespokenofthemintheProblems）—nor，totakegeneralground，doeshetakehisprinciplesfromtheintelligible。Howdothe’hollows’and’solids’
fitintooneanothertomakethemixing，e。g。inthecaseofwineandwater？Thissayingisquitebeyondus；forhowwearetounderstandthe’hollows’ofthewineandwateristoofarbeyondourperception。Again，when，asamatteroffact，horseisbornofhorse，assofass，andmuleofhorseandassintwowaysaccordingastheparentsarestallionandshe—assorjackassandmare，whyinthelastcasedoesthereresultsomethingso’dense’thattheoffspringissterile，whereastheoffspringofmaleandfemalehorse，maleandfemaleass，isnotsterile？Andyetthegenerativefluidofthemaleandfemalehorseissoft。Butbothsexesofthehorsecrosswithbothsexesoftheass，andtheoffspringofbothcrossesarebarren，accordingtoEmpedocles，becausefrombothisproducedsomething’dense’，the’seeds’being’soft’。Ifso，theoffspringofstallionandmareoughtalsotobesterile。Ifoneofthemaloneunitedwiththeass，itmightbesaidthatthecauseofthemule’sbeingunabletogeneratewastheunlikenessofthatonetothegenerativefluidoftheass；but，asitis，whateverbethecharacterofthatgenerativefluidwithwhichitunitesintheass，suchitisalsointheanimalofitsownkind。Then，again，theargumentisintendedtoapplytobothmaleandfemalemulesalike，butthemaledoesgenerateatsevenyearsofage，itissaid；itisthefemalealonethatisentirelysterile，andevensheissoonlybecauseshedoesnotcompletethedevelopmentoftheembryo，forafemalemulehasbeenknowntoconceive。
Perhapsanabstractproofmightappeartobemoreplausiblethanthosealreadygiven；Icallitabstractbecausethemoregeneralitisthefurtherisitremovedfromthespecialprinciplesinvolved。Itrunssomewhatasfollows。Frommaleandfemaleofthesamespeciestherearebornincourseofnaturemaleandfemaleofthesamespeciesastheparents，e。g。maleandfemalepuppiesfrommaleandfemaledog。
Fromparentsofdifferentspeciesisbornayoungonedifferentinspecies；thusifadogisdifferentfromalion，theoffspringofmaledogandlionessoroflionandbitchwillbedifferentfrombothparents。Ifthisisso，thensince（1）mulesareproducedofbothsexesandarenotdifferentinspeciesfromoneanother，and（2）amuleisbornofhorseandassandthesearedifferentinspeciesfrommules，itisimpossiblethatanythingshouldbeproducedfrommules。For（1）anotherkindcannotbe，becausetheproductofmaleandfemaleofthesamespeciesisalsoofthesamespecies，and（2）amulecannotbe，becausethatistheproductofhorseandasswhicharedifferentinform，[anditwaslaiddownthatfromparentsdifferentinformisbornadifferentanimal]。Nowthistheoryistoogeneralandempty。Foralltheoriesnotbasedonthespecialprinciplesinvolvedareempty；theyonlyappeartobeconnectedwiththefactswithoutbeingsoreally。Asgeometricalargumentsmuststartfromgeometricalprinciples，soitiswiththeothers；thatwhichisemptymayseemtobesomething，butisreallynothing。Nowthebasisofthisparticulartheoryisnottrue，formanyanimalsofdifferentspeciesarefertilewithoneanother，aswassaidbefore。Sowemustnotinquireintoquestionsofnaturalscienceinthisfashionanymorethananyotherquestions；weshallbemorelikelytofindthereasonbyconsideringthefactspeculiartothetwokindsconcerned，horseandass。Inthefirstplace，eachofthem，ifmatedwithitsownkind，bearsonlyoneyoungone；secondly，thefemalesarenotalwaysabletoconceivefromthemale（whereforebreedersputthehorsetothemareagainatintervals）。Indeed，boththemareisdeficientincatamenia，discharginglessthananyotherquadruped，andtheshe—assdoesnotadmittheimpregnation，butejectsthesemenwithherurine，whereforemenfollowfloggingherafterintercourse。Againtheassisananimalofcoldnature，andsoisnotwonttobeproducedinwintryregionsbecauseitcannotbearcold，asinScythiaandtheneighbouringcountryandamongtheCeltsbeyondIberia，forthiscountryalsoiscold。Forthiscausetheydonotputthejackassestothefemalesattheequinox，astheydowithhorses，butaboutthesummersolstice，inorderthattheass—foalsmaybeborninawarmseason，forthemothersbearatthesameseasonasthatinwhichtheyareimpregnated，theperiodofgestationinbothhorseandassbeingoneyear。Theanimal，then，being，ashasbeensaidofsuchacoldnature，itssemenalsomustbecold。Aproofofthisisthatifahorsemountafemalealreadyimpregnatedbyanasshedoesnotdestroytheimpregnationoftheass，butiftheassbethesecondtomountherhedoesdestroythatofthehorsebecauseofthecoldnessofhisownsemen。When，therefore，theyunitewitheachother，thegenerativeelementsarepreservedbytheheatoftheoneofthem，thatcontributedbythehorsebeingthehotter；forintheassboththesemenofthemaleandthematerialcontributedbythefemalearecold，andthoseofthehorse，inbothsexes，arehotter。Nowwheneitherhotisaddedtocoldorcoldtohotsoastomix，theresultisthattheembryoitselfarisingfromtheseispreservedandthustheseanimalsarefertilewhencrossedwithoneanother，buttheanimalproducedbythemisnolongerfertilebutunabletoproduceperfectoffspring。