Similaristhecaseofanimalswhoseearsarelongandprojectfarliketheeavesofahouse，asinsomequadrupeds，withtheinternalspiralpassagelong；thesealsocatchthemovementfromafarandpassitontothesense—organ。
　　Inrespectofsense—perceptionatadistance，manis，onemaysay，theworstofallanimalsinproportiontohissize，butinrespectofjudgingthedifferencesofqualityintheobjectsheisthebestofall。Thereasonisthatthesense—organinmanispureandleastearthyandmaterial，andheisbynaturethethinnest—skinnedofallanimalsforhissize。
　　TheworkmanshipofNatureisadmirablealsointheseal，forthoughaviviparousquadrupedithasnoearsbutonlypassagesforhearing。Thisisbecauseitslifeispassedinthewater；nowtheearisapartaddedtothepassagestopreservethemovementoftheairatadistance；thereforeanearisnousetoitbutwouldevenbringaboutthecontraryresultbyreceivingamassofwaterintoitself。
　　Wehavethusspokenofsight，hearing，andsmell。
　　3
　　Asforhair，mendifferinthisthemselvesatdifferentages，andalsofromallotherkindsofanimalsthathavehair。Thesearealmostallwhichareinternallyviviparous，forevenwhenthecoveringofsuchanimalsisspinyitmustbeconsideredasakindofhair，asinthelandhedgehogandanyothersuchanimalamongthevivipara。
　　Hairsdifferinrespectofhardnessandsoftness，lengthandshortness，straightnessandcurliness，quantityandscantiness，andinadditiontothesequalities，intheircolours，whitenessandblacknessandtheintermediateshades。Theydifferalsoinsomeoftheserespectsaccordingtoage，astheyareyoungorgrowingold。Thisisespeciallyplaininman；thehairgetscoarserastimegoeson，andsomegobaldonthefrontofthehead；childrenindeeddonotgobald，nordowomen，butmendosobythetimetheirageisadvancing。
　　Humanbeingsalsogogreyontheheadastheygrowold，butthisisnotvisibleinpracticallyanyotheranimal，thoughmoresointhehorsethanothers。Mengobaldonthefrontofthehead，butturngreyfirstonthetemples；noonegoesbaldfirstontheseoronthebackofthehead。Somesuchaffectionsoccurinacorrespondingmanneralsoinallanimalswhichhavenothairbutsomethinganalogoustoit，asthefeathersofbirdsandscalesintheclassoffish。
　　ForwhatpurposeNaturehasmadehairingeneralforanimalshasbeenpreviouslystatedintheworkdealingwiththecausesofthepartsofanimals；itisthebusinessofthepresentinquirytoshowunderwhatcircumstancesandforwhatnecessarycauseseachparticularkindofhairoccurs。Theprincipalcausethenofthicknessandthinnessistheskin，forthisisthickinsomeanimalsandthininothers，rareinsomeanddenseinothers。Thedifferentqualityoftheincludedmoistureisalsoahelpingcause，forinsomeanimalsthisisgreasyandinotherswatery。Forgenerallyspeakingthesubstratumoftheskinisofanearthynature；beingonthesurfaceofthebodyitbecomessolidandearthyasthemoistureevaporates。Nowthehairsortheiranaloguearenotformedoutofthefleshbutoutoftheskinmoistureevaporatingandexhalinginthem，andthereforethickhairsarisefromathickskinandthinfromthin。Ifthentheskinisrarerandthicker，thehairsarethickbecauseofthequantityofearthymatterandthesizeofthepores，butifitisdensertheyarethinbecauseofthenarrownessofthepores。Further，ifthemoisturebewateryitdriesupquicklyandthehairsdonotgaininsize，butifitbegreasytheoppositehappens，forthegreasyisnoteasilydriedup。Thereforethethicker—skinnedanimalsareasageneralrulethicker—hairedforthecausesmentioned；however，thethickest—skinnedarenotmoresothanotherthick—skinnedones，asisshownbytheclassofswinecomparedtothatofoxenandtotheelephantandmanyothers。Andforthesamereasonalsothehairsoftheheadinmanarethickest，forthispartofhisskinisthickestandliesovermostmoistureandbesidesisveryporous。
　　Thecauseofthehairsbeinglongorshortdependsontheevaporatingmoisturenotbeingeasilydried。Ofthistherearetwocauses，quantityandquality；iftheliquidismuchitdoesnotdryupeasilynorifitisgreasy。Andforthisreasonthehairsoftheheadarelongestinman，forthebrain，beingfluidandcold，suppliesgreatabundanceofmoisture。
　　Thehairsbecomestraightorcurlyonaccountofthevapourarisinginthem。Ifitbesmoke—like，itishotanddryandsomakesthehaircurly，foritistwistedasbeingcarriedwithadoublemotion，theearthyparttendingdownwardsandthehotupwards。Thus，beingeasilybent，itistwistedowingtoitsweakness，andthisiswhatismeantbycurlinessinhair。Itispossiblethenthatthisisthecause，butitisalsopossiblethat，owingtoitshavingbutlittlemoistureandmuchearthymatterinit，itisdriedbythesurroundingairandsocoileduptogether。Forwhatisstraightbecomesbent，ifthemoistureinitisevaporated，andrunstogetherasahairdoeswhenburninguponthefire；curlinesswillthenbeacontractionowingtodeficiencyofmoisturecausedbytheheatoftheenvironment。Asignofthisisthefactthatcurlyhairisharderthanstraight，forthedryishard。Andanimalswithmuchmoisturearestraight—haired；forinthesehairsthemoistureadvancesasastream，notindrops。ForthisreasontheScythiansontheBlackSeaandtheThraciansarestraight—haired，forboththeythemselvesandtheenvironingairaremoist，whereastheAethiopiansandmeninhotcountriesarecurly—haired，fortheirbrainsandthesurroundingairaredry。
　　Some，however，ofthethick—skinnedanimalsarefine—hairedforthecausepreviouslystated，forthefinertheporesarethefinermustthehairsbe。Hencetheclassofsheephavesuchhairs（forwoolisonlyamultitudeofhairs）。
　　Therearesomeanimalswhosehairissoftandyetlessfine，asisthecasewiththeclassofharescomparedwiththatofsheep；insuchanimalsthehairisonthesurfaceoftheskin，notdeeplyrootedinit，andsoisnotlongbutinmuchthesamestateasthescrapingsfromlinen，forthesealsoarenotlongbutaresoftanddonotadmitofweaving。
　　Theconditionofsheepincoldclimatesisoppositetothatofman；thehairoftheScythiansissoftbutthatoftheSauromaticsheepishard。Thereasonofthisisthesameasitisalsoallwildanimals。Thecoldhardensandsolidifiesthembydryingthem，forastheheatispressedoutthemoistureevaporates，andbothhairandskinbecomeearthyandhard。Inwildanimalsthentheexposuretothecoldisthecauseofhardnessinthehair，intheothersthenatureoftheclimateisthecause。Aproofofthisisalsowhathappensinthesea—urchinswhichareusedasaremedyinstranguries。Forthese，too，thoughsmallthemselves，havelargeandhardspinesbecausetheseainwhichtheyliveiscoldonaccountofitsdepth（fortheyarefoundinsixtyfathomsandevenmore）。Thespinesarelargebecausethegrowthofthebodyisdivertedtothem，sincehavinglittleheatinthemtheydonotconcocttheirnutrimentandsohavemuchresidualmatteranditisfromthisthatspines，hairs，andsuchthingsareformed；theyarehardandpetrifiedthroughthecongealingeffectofthecold。Inthesamewayalsoplantsarefoundtobeharder，moreearthy，andstony，iftheregioninwhichtheygrowlookstothenorththanifitlookstothesouth，andthoseinwindyplacesthanthoseinsheltered，fortheyareallmorechilledandtheirmoistureevaporates。
　　Hardening，then，comesofbothheatandcold，forbothcausethemoisturetoevaporate，heatperseandcoldperaccidens（sincethemoisturegoesoutofthingsalongwiththeheat，therebeingnomoisturewithoutheat），butwhereascoldnotonlyhardensbutalsocondenses，heatmakesasubstancerarer。
　　Forthesamereason，asanimalsgrowolder，thehairsbecomeharderinthosewhichhavehairs，andthefeathersandscalesinthefeatheredandscalykinds。Fortheirskinsbecomeharderandthickerastheygetolder，fortheyaredriedup，andoldage，astheworddenotes，isearthybecausetheheatfailsandthemoisturealongwithit。
　　Mengobaldvisiblymorethananyotheranimal，butstillsuchastateissomethinggeneral，foramongplantsalsosomeareevergreenswhileothersaredeciduous，andbirdswhichhibernateshedtheirfeathers。Similartothisistheconditionofbaldnessinthosehumanbeingstowhomitisincident。Forleavesareshedbyallplants，fromonepartoftheplantatatime，andsoarefeathersandhairsbythoseanimalsthathavethem；itiswhentheyareallshedtogetherthattheconditionisdescribedbythetermsmentioned，foritiscalled’goingbald’and’thefalloftheleaf’
　　and’moulting’。Thecauseoftheconditionisdeficiencyofhotmoisture，suchmoisturebeingespeciallytheunctuous，andhenceunctuousplantsaremoreevergreen。（Howeverwemustelsewherestatethecauseofthisphenomenainplants，forothercausesalsocontributetoit。）Itisinwinterthatthishappenstoplants（forthechangefromsummertowinterismoreimportanttothemthanthetimeoflife），andtothoseanimalswhichhibernate（forthese，too，arebynaturelesshotandmoistthanman）；inthelatteritistheseasonsoflifethatcorrespondtosummerandwinter。
　　Hencenoonegoesbaldbeforethetimeofsexualintercourse，andatthattimeitisinthosenaturallyinclinedtosuchintercoursethatbaldnessappears，forthebrainisnaturallythecoldestpartofthebodyandsexualintercoursemakesmencold，beingalossofpurenaturalheat。Thusweshouldexpectthebraintofeeltheeffectofitfirst，foralittlecauseturnsthescalewherethethingconcernedisweakandinpoorcondition。Thusifwereckonupthesepoints，thatthebrainitselfhasbutlittleheat，andfurtherthattheskinrounditmustneedshavestillless，andagainthatthehairmusthavestilllessthantheskininasmuchasitisfurthestremovedfromthebrain，weshouldreasonablyexpectbaldnesstocomeaboutthisageuponthosewhohavemuchsemen。Anditisforthesamereasonthatthefrontpartoftheheadalonegoesbaldinmanandthatheistheonlyanimaltodoso；thefrontpartgoesbaldbecausethebrainisthere，andmanistheonlyanimaltogobaldbecausehisbrainismuchthelargestandthemoistest。Womendonotgobaldbecausetheirnatureislikethatofchildren，bothalikebeingincapableofproducingseminalsecretion。Eunuchsdonotbecomebald，becausetheychangeintothefemalecondition。Andastothehairthatcomeslaterinlife，eunuchseitherdonotgrowitatall，orloseitiftheyhappentohaveit，withtheexceptionofthepubichair；forwomenalsogrowthatthoughtheyhavenottheother，andthismutilationisachangefromthemaletothefemalecondition。
　　Thereasonwhythehairdoesnotgrowagainincasesofbaldness，althoughbothhibernatinganimalsrecovertheirfeathersorhairandtreesthathaveshedtheirleavesgrowleavesagain，isthis。Theseasonsoftheyeararetheturning—pointsoftheirlives，ratherthantheirage，sothatwhentheseseasonschangetheychangewiththembygrowingandlosingfeathers，hairs，orleavesrespectively。Butthewinterandsummer，springandautumnofmanaredefinedbyhisage，sothat，sincehisagesdonotreturn，neitherdotheconditionscausedbythemreturn，althoughthecauseofthechangeofconditionissimilarinmantowhatitisintheanimalsandplantsinquestion。
　　Wehavenowspokenprettymuchofalltheotherconditionsofhair。
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　　Butastotheircolour，itisthenatureoftheskinthatisthecauseofthisinotheranimalsandalsooftheirbeinguni—colouredorvari—coloured）；butinmanitisnotthecause，exceptofthehairgoinggreythroughdisease（notthrougholdage），forinwhatiscalledleprosythehairsbecomewhite；onthecontrary，ifthehairsarewhitethewhitenessdoesnotinvadetheskin。Thereasonisthatthehairsgrowoutofskin；if，then，theskinisdiseasedandwhitethehairbecomesdiseasedwithit，andthediseaseofhairisgreyness。Butthegreynessofhairwhichisduetoageresultsfromweaknessanddeficiencyofheat。Forasthebodydeclinesinvigourwetendtocoldateverytimeoflife，andespeciallyinoldage，thisagebeingcoldanddry。Wemustrememberthatthenutrimentcomingtoeachpartofthebodyisconcoctedbytheheatappropriatetothepart；iftheheatisinadequatethepartlosesitsefficiency，anddestructionordiseaseresults。（Weshallspeakmoreindetailofcausesinthetreatiseongrowthandnutrition。）Whenever，then，thehairinmanhasnaturallylittleheatandtoomuchmoistureentersit，itsownproperheatisunabletoconcoctthemoistureandsoitisdecayedbytheheatintheenvironingair。Alldecayiscausedbyheat，nottheinnateheatbutexternalheat，ashasbeenstatedelsewhere。Andasthereisadecayofwater，ofearth，andallsuchmaterialbodies，sothereisalsooftheearthyvapour，forinstancewhatiscalledmould（formouldisadecayofearthyvapour）。Thusalsotheliquidnutrimentinthehairdecaysbecauseitisnotconcocted，andwhatiscalledgreynessresults。Itiswhitebecausemouldalso，practicallyaloneamongdecayedthings，iswhite。Thereasonofthisisthatithasmuchairinit，allearthyvapourbeingequivalenttothickair。Formouldis，asitwere，theantithesisofhoar—frost；iftheascendingvapourbefrozenitbecomeshoar—frost，ifitbedecayed，mould。Hencebothareonthesurfaceofthings，forvapourissuperficial。Andsothecomicpoetsmakeagoodmetaphorinjestwhentheycallgreyhairs’mouldofoldage’andFortheoneisgenericallythesameasgreyness，theotherspecifically；hoar—frostgenerically（forbothareavapour），mouldspecifically（forbothareaformofdecay）。Aproofthatthisissoisthis：greyhairshaveoftengrownonmeninconsequenceofdisease，andlaterondarkhairsinsteadofthemafterrestorationtohealth。Thereasonisthatinsicknessthewholebodyisdeficientinnaturalheatandsothepartsbesides，eventheverysmallones，participateinthisweakness；andagain，muchresidualmatterisformedinthebodyandallitspartsinillness，whereforetheincapacityinthefleshtoconcoctthenutrimentcausesthegreyhairs。Butwhenmenhaverecoveredhealthandstrengthagaintheychange，becomingasitwereyoungagaininsteadofold；inconsequencethestateschangealso。Indeed，wemayrightlycalldiseaseanacquiredoldage，oldageanaturaldisease；atanyrate，somediseasesproducethesameeffectsasoldage。
　　Mengogreyonthetemplesfirst，becausethebackoftheheadisemptyofmoistureowingtoitscontainingnobrain，andthe’bregma’
　　hasagreatdealofmoisture，alargequantitynotbeingliabletodecay；thehaironthetempleshoweverhasneithersolittlethatitcanconcoctitnorsomuchthatitcannotdecay，forthisregionoftheheadbeingbetweenthetwoextremesisexemptfrombothstates。
　　Thecauseofgreynessinmanhasnowbeenstated。
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　　Thereasonwhythischangedoesnottakeplacevisiblyonaccountofageinotheranimalsisthesameasthatalreadygiveninthecaseofbaldness；theirbrainissmallandlessfluidthaninman，sothattheheatrequiredforconcoctiondoesnotaltogetherfail。
　　Amongthemitismostclearinhorsesofallanimalsthatweknow，becausetheboneaboutthebrainisthinnerinthemthaninothersinproportiontotheirsize。Asignofthisisthatablowtothisspotisfataltothem，whereforeHomeralsohassaid：’wherethefirsthairsgrowontheskullofhorses，andawoundismostfatal。’Asthenthemoistureeasilyflowstothesehairsbecauseofthethinnessofthebone，whilsttheheatfailsonaccountofage，theygogrey。Thereddishhairsgogreysoonerthantheblack，rednessalsobeingasortofweaknessofhairandallweakthingsageingsooner。Itissaid，however，thatcranesbecomedarkerastheygrowold。Thereasonofthiswouldbe，ifitshouldprovetrue，thattheirfeathersarenaturallymoisterthanothersandastheygrowoldthemoistureinthefeathersistoomuchtodecayeasily。
　　Greynesscomesaboutbysomesortofdecay，andisnot，assomethink，awithering。（1）Aproofoftheformerstatementisthefactthathairprotectedbyhatsorothercoveringsgoesgreysooner（forthewindspreventdecayandtheprotectionkeepsoffthewinds），andthefactthatitisaidedbyanointingwithamixtureofoilandwater。For，thoughwatercoolsthings，theoilmingledwithitpreventsthehairfromdryingquickly，waterbeingeasilydriedup。
　　（2）Thattheprocessisnotawithering，thatthehairdoesnotwhitenasgrassdoesbywithering，isshownbythefactthatsomehairsgrowgreyfromthefirst，whereasnothingspringsupinawitheredstate。Manyhairsalsowhitenatthetip，forthereisleastheatintheextremitiesandthinnestparts。
　　Whenthehairsofotheranimalsarewhite，thisiscausedbynature，notbyanyaffection。Thecauseofthecoloursinotheranimalsistheskin；iftheyarewhite，theskiniswhite，iftheyaredarkitisdark，iftheyarepiebaldinconsequenceofamixtureofthehairs，itisfoundtobewhiteintheonepartanddarkintheother。Butinmantheskinisinnowaythecause，forevenwhite—skinnedmenhaveverydarkhair。Thereasonisthatmanhasthethinnestskinofallanimalsinproportiontohissizeandthereforeithasnotstrengthtochangethehairs；onthecontrarytheskinitselfchangesitscolourthroughitsweaknessandisdarkenedbysunandwind，whilethehairsdonotchangealongwithitatall。Butintheotheranimalstheskin，owingtoitsthickness，hastheinfluencebelongingtothesoilinwhichathinggrows，thereforethehairschangeaccordingtotheskinbuttheskindoesnotchangeatallinconsequenceofthewindsandthesun。
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　　Ofanimalssomeareuni—coloured（Imeanbythistermthoseofwhichthekindasawholehasonecolour，asalllionsaretawny；
　　andthisconditionexistsalsoinbirds，fish，andtheotherclassesofanimalsalike）；othersthoughmany—colouredareyetwhole—coloured（Imeanthosewhosebodyasawholehasthesamecolour，asabulliswhiteasawholeordarkasawhole）；othersarevari—coloured。
　　Thislasttermisusedinbothways；sometimesthewholekindisvari—coloured，asleopardsandpeacocks，andsomefish，e。g。theso—called’thrattai’；sometimesthekindasawholeisnotso，butsuchindividualsarefoundinit，aswithcattleandgoatsand，amongbirds，pigeons；thesameappliesalsotootherkindsofbirds。
　　Thewhole—colouredchangemuchmorethantheuniformlycoloured，bothintothesimplecolourofanotherindividualofthesamekind（asdarkchangingintowhiteandviceversa）andintobothcoloursmingled。Thisisbecauseitisanaturalcharacteristicofthekindasawholenottohaveonecolouronly，thekindbeingeasilymovedinbothdirectionssothatthecoloursbothchangemoreintooneanotherandaremorevaried。Theoppositeholdswiththeuniformlycoloured；theydonotchangeexceptbyanaffectionofthecolour，andthatrarely；butstilltheydosochange，forbeforenowwhiteindividualshavebeenobservedamongpartridges，ravens，sparrows，andbears。Thishappenswhenthecourseofdevelopmentisperverted，forwhatissmalliseasilyspoiltandeasilymoved，andwhatisdevelopingissmall，thebeginningofallsuchthingsbeingonasmallscale。
　　Changeisespeciallyfoundinthoseanimalsofwhichbynaturetheindividualiswhole—colouredbutthekindmany—coloured。Thisisowingtothewaterwhichtheydrink，forhotwatersmakethehairwhite，coldmakesitdark，aneffectfoundalsoinplants。Thereasonisthatthehothavemoreairthanwaterinthem，andtheairshiningthroughcauseswhiteness，asalsoinfroth。As，then，skinswhicharewhitebyreasonofsomeaffectiondifferfromthosewhitebynature，soalsointhehairthewhitenessduetodiseaseoragediffersfromthatduetonatureinthatthecauseisdifferent；thelatterarewhitenedbythenaturalheat，theformerbytheexternalheat。Whitenessiscausedinallthingsbythevaporousairimprisonedinthem。Hencealsoinallanimalsnotuniformlycolouredallthepartunderthebellyiswhiter。Forpracticallyallwhiteanimalsarebothhotterandbetterflavouredforthesamereason；theconcoctionoftheirnutrimentmakesthemwell—flavoured，andheatcausestheconcoction。Thesamecauseholdsforthoseanimalswhichareuniformly—coloured，buteitherdarkorwhite；heatandcoldarethecausesofthenatureoftheskinandhair，eachofthepartshavingitsownspecialheat。
　　Thetonguealsovariesincolourinthesimplycolouredascomparedwiththevari—colouredanimals，andagaininthesimplycolouredwhichdifferfromoneanother，aswhiteanddark。Thereasonisthatassignedbefore，thattheskinsofthevari—colouredarevari—coloured，andtheskinsofthewhite—hairedanddark—hairedarewhiteanddarkineachcase。Nowwemustconceiveofthetongueasoneoftheexternalparts，nottakingintoaccountthefactthatitiscoveredbythemouthbutlookingonitaswedoonthehandorfoot；
　　thussincetheskinofthevari—colouredanimalsisnotuniformlycoloured，thisisthecauseoftheskinonthetonguebeingalsovari—coloured。
　　Somebirdsandsomewildquadrupedschangetheircolouraccordingtotheseasonsoftheyear。Thereasonisthat，asmenchangeaccordingtotheirage，sothesamethinghappenstothemaccordingtotheseason；forthismakesagreaterdifferencetothemthanthechangeofage。
　　Themoreomnivorousanimalsaremorevari—colouredtospeakgenerally，andthisiswhatmightbeexpected；thusbeesaremoreuniformlycolouredthanhornetsandwasps。Forifthefoodisresponsibleforthechangeweshouldexpectvariedfoodtoincreasethevarietyinthemovementswhichcausethedevelopmentandsointheresidualmatterofthefood，fromwhichcomeintobeinghairsandfeathersandskins。
　　Somuchforcoloursandhairs。
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　　Astothevoice，itisdeepinsomeanimals，highinothers，inothersagainwell—pitchedandindueproportionbetweenbothextremes。
　　Again，insomeitisloud，inotherssmall，anditdiffersinsmoothnessandroughness，flexibilityandinflexibility。Wemustinquirethenintothecausesofeachofthesedistinctions。
　　Wemustsupposethenthatthesamecauseisresponsibleforhighanddeepvoicesasforthechangewhichtheyundergoinpassingfromyouthtoage。Thevoiceishigherinallotheranimalswhenyounger，butincattlethatofcalvesisdeeper。Wefindthesamethingalsointhemaleandfemalesexes；intheotherkindsofanimalsthevoiceofthefemaleishigherthanthatofthemale（thisbeingespeciallyplaininman，forNaturehasgiventhisfacultytohiminthehighestdegreebecausehealoneofanimalsmakesuseofspeechandthevoiceisthematerialofspeech），butincattletheoppositeobtains，forthevoiceofcowsisdeeperthanthatofbulls。
　　Nowthepurposeforwhichanimalshaveavoice，andwhatismeantby’voice’andby’sound’generally，hasbeenstatedpartlyinthetreatiseonsensation，partlyinthatonthesoul。Butsincelownessofvoicedependsonthemovementoftheairbeingslowanditshighnessonitsbeingquick，thereisadifficultyinknowingwhetheritisthatwhichmovesorthatwhichismovedthatisthecauseoftheslownessorquickness。Forsomesaythatwhatismuchismovedslowly，whatislittlequickly，andthatthequantityoftheairisthecauseofsomeanimalshavingadeepandothersahighvoice。Uptoacertainpointthisiswellsaid（foritseemstoberightlysaidinageneralwaythatthedepthdependsonacertainamountoftheairputinmotion），butnotaltogether，forifthisweretrueitwouldnotbeeasytospeakbothsoftanddeepatonce，noragainbothloudandhigh。Again，thedepthseemstobelongtothenoblernature，andinsongsthedeepnoteisbetterthanthehigh—pitchedones，thebetterlyinginsuperiority，anddepthoftonebeingasortofsuperiority。Butthendepthandheightinthevoicearedifferentfromloudnessandsoftness，andsomehigh—voicedanimalsareloud—voiced，andinlikemannersomesoft—voicedonesaredeep—voiced，andthesameappliestothetoneslyingbetweentheseextremes。Andbywhatelsecanwedefinethese（Imeanloudnessandsoftnessofvoice）exceptbythelargeandsmallamountoftheairputinmotion？Ifthenheightanddeptharetobedecidedinaccordancewiththedistinctionpostulated，theresultwillbethatthesameanimalswillbedeep—andloud—voiced，andthesamewillbehigh—andnotloud—voiced；butthisisfalse。
　　Thereasonofthedifficultyisthatthewords’great’and’small’，’much’and’little’areusedsometimesabsolutely，sometimesrelativelytooneanother。Whetherananimalhasagreat（orloud）voicedependsontheairwhichismovedbeingmuchabsolutely，whetherithasasmallvoicedependsonitsbeinglittleabsolutely；butwhethertheyhaveadeeporhighvoicedependsontheirbeingthusdifferentiatedinrelationtooneanother。Forifthatwhichismovedsurpassthestrengthofthatwhichmovesit，theairthatissentforthmustgoslowly；iftheopposite，quickly。Thestrong，then，onaccountoftheirstrength，sometimesmovemuchairandmakethemovementslow，sometimes，havingcompletecommandoverit，makethemovementswift。Onthesameprincipletheweakeithermovetoomuchairfortheirstrengthandsomakethemovementslow，oriftheymakeitswiftmovebutlittlebecauseoftheirweakness。
　　These，then，arethereasonsofthesecontrarieties，thatneitherareallyounganimalshigh—voicednoralldeep—voiced，norarealltheolder，noryetarethetwosexesthusopposed，andagainthatnotonlythesickspeakinahighvoicebutalsothoseingoodbodilycondition，and，further，thatasmenvergeonoldagetheybecomehigher—voiced，thoughthisageisoppositetothatofyouth。
　　Mostyounganimals，then，andmostfemalessetbutlittleairinmotionbecauseoftheirwantofpower，andareconsequentlyhigh—voiced，foralittleairiscarriedalongquickly，andinthevoicewhatisquickishigh。Butincalvesandcows，intheonecasebecauseoftheirage，intheotherbecauseoftheirfemalenature，thepartbywhichtheysettheairinmotionisnotstrong；atthesametimetheysetagreatquantityinmotionandsoaredeep—voiced；forthatwhichisbornealongslowlyisheavy，andmuchairisbornealongslowly。Andtheseanimalssetmuchinmovementwhereastheotherssetbutlittle，becausethevesselthroughwhichthebreathisfirstbornehasinthemalargeopeningandnecessarilysetsmuchairinmotion，whereasintheresttheairisbetterdispensed。Astheirageadvancesthispartwhichmovestheairgainsmorestrengthineachanimal，sothattheychangeintotheoppositecondition，thehigh—voicedbecomingdeeper—voicedthantheywere，andthedeep—voicedhigher—voiced，whichiswhybullshaveahighervoicethancalvesandcows。Nowthestrengthofallanimalsisintheirsinews，andsothoseintheprimeoflifearestronger，theyoungbeingweakerinthejointsandsinews；moreover，intheyoungtheyarenotyettense，andinthosenowgrowingoldthetensionrelaxes，whereforeboththeseagesareweakandpowerlessformovement。Andbullsareparticularlysinewy，eventheirhearts，andthereforethatpartbywhichtheysettheairinmotionisinatensestate，likeasinewystringstretchedtight。（Thattheheartofbullsisofsuchanatureisshownbythefactthataboneisactuallyfoundinsomeofthem，andbonesarenaturallyconnectedwithsinew。）
　　Allanimalswhencastratedchangetothefemalecharacter，andutteravoicelikethatofthefemalesbecausethesinewystrengthintheprincipleofthevoiceisrelaxed。Thisrelaxationisjustasifoneshouldstretchastringandmakeittautbyhangingsomeweightontoit，aswomendowhoweaveattheloom，fortheystretchthewarpbyattachingtoitwhatarecalled’laiai’。Forinthiswayarethetestesattachedtotheseminalpassages，andtheseagaintotheblood—vesselwhichtakesitsoriginintheheartneartheorganwhichsetsthevoiceinmotion。Henceastheseminalpassageschangetowardstheageatwhichtheyarenowabletosecretethesemen，thispartalsochangesalongwiththem。Asthischanges，thevoiceagainchanges，moreindeedinmales，butthesamethinghappensinfemalestoo，onlynotsoplainly，theresultbeingwhatsomecall’bleating’whenthevoiceisuneven。Afterthisitsettlesintothedeeporhighvoiceofthesucceedingtimeoflife。Ifthetestesareremovedthetensionofthepassagesrelaxes，aswhentheweightistakenoffthestringorthewarp；asthisrelaxes，theorganwhichmovesthevoiceisloosenedinthesameproportion。This，then，isthereasonwhythevoiceandtheformgenerallychangestothefemalecharacterincastratedanimals；itisbecausetheprincipleisrelaxeduponwhichdependsthetensionofthebody；notthat，assomesuppose，thetestesarethemselvesaganglionofmanyprinciples，butsmallchangesarethecausesofgreatones，notpersebutwhenithappensthataprinciplechangeswiththem。Fortheprinciples，thoughsmallinsize，aregreatinpotency；this，indeed，iswhatismeantbyaprinciple，thatitisitselfthecauseofmanythingswithoutanythingelsebeinghigherthanitforittodependupon。
　　Theheatorcoldalsooftheirhabitatcontributestomakesomeanimalsofsuchacharacterastobedeep—voiced，andothershigh—voiced。Forhotbreathbeingthickcausesdepth，coldbreathbeingthintheopposite。Thisisclearalsoinpipe—playing，forifthebreathoftheperformerishotter，thatistosayifitisexpelledasbyagroan，thenoteisdeeper。
　　Thecauseofroughnessandsmoothnessinthevoice，andofallsimilarinequality，isthatthepartororganthroughwhichthevoiceisconveyedisroughorsmoothorgenerallyevenoruneven。Thisisplainwhenthereisanymoistureaboutthetracheaorwhenitisroughenedbyanyaffection，forthenthevoicealsobecomesuneven。
　　Flexibilitydependsonthesoftnessorhardnessoftheorgan，forwhatissoftcanberegulatedandassumeanyform，whilewhatishardcannot；thusthesoftorgancanutteraloudorasmallnote，andaccordinglyahighoradeepone，sinceiteasilyregulatesthebreath，becomingitselfeasilygreatorsmall。Buthardnesscannotberegulated。
　　Letthisbeenoughonallthosepointsconcerningthevoicewhichhavenotbeenpreviouslydiscussedinthetreatiseonsensationandinthatonthesoul。
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　　Withregardtotheteethithasbeenstatedpreviouslythattheydonotexistforasinglepurposenorforthesamepurposeinallanimals，butinsomefornutritiononly，inothersalsoforfightingandforvocalspeech。Wemust，however，consideritnotalientothediscussionofgenerationanddevelopmenttoinquireintothereasonwhythefrontteethareformedfirstandthegrinderslater，andwhythelatterarenotshedbuttheformerareshedandgrowagain。
　　Democritushasspokenofthesequestionsbutnotwell，forheassignsthecausetoogenerallywithoutinvestigatingthefactsinallcases。Hesaysthattheearlyteethareshedbecausetheyareformedinanimalstooearly，foritiswhenanimalsarepracticallyintheirprimethattheygrowaccordingtoNature，andsucklingisthecauseheassignsfortheirbeingfoundtooearly。Yetthepigalsosucklesbutdoesnotsheditsteeth，and，further，alltheanimalswithcarnivorousdentitionsuckle，butsomeofthemdonotshedanyteethexceptthecanines，e。g。lions。Thismistake，then，wasduetohisspeakinggenerallywithoutexaminingwhathappensinallcases；
　　butthisiswhatwetodo，foranyonewhomakesanygeneralstatementmustspeakofalltheparticularcases。
　　Nowweassume，basingourassumptionuponwhatwesee，thatNatureneverfailsnordoesanythinginvainsofarasispossibleineachcase。Anditisnecessary，ifananimalistoobtainfoodafterthetimeoftakingmilkisover，thatitshouldhaveinstrumentsforthetreatmentofthefood。If，then，asDemocritussays，thishappenedaboutthetimeofreachingmaturity，Naturewouldfailinsomethingpossibleforhertodo。And，besides，theoperationofNaturewouldbecontrarytoNature，forwhatisdonebyviolenceiscontrarytoNature，anditisbyviolencethathesaystheformationofthefirstteethisbroughtabout。Thatthisviewthenisnottrueisplainfromtheseandothersimilarconsiderations。
　　Nowtheseteetharedevelopedbeforetheflatteeth，inthefirstplacebecausetheirfunctionisearlier（fordividingcomesbeforecrushing，andtheflatteethareforcrushing，theothersfordividing），inthesecondplacebecausethesmallerisnaturallydevelopedquickerthanthelarger，evenifbothstarttogether，andtheseteetharesmallerinsizethanthegrinders，becausetheboneofthejawisflatinthatpartbutnarrowtowardsthemouth。Fromthegreaterpart，therefore，mustflowmorenutrimenttoformtheteeth，andfromthenarrowerpartless。
　　Theactofsuckinginitselfcontributesnothingtotheformationoftheteeth，buttheheatofthemilkmakesthemappearmorequickly。
　　Aproofofthisisthateveninsucklinganimalsthoseyoungwhichenjoyhottermilkgrowtheirteethquicker，heatbeingconducivetogrowth。
　　Theyareshed，aftertheyhavebeenformed，partlybecauseitisbetterso（forwhatissharpissoonblunted，sothatafreshrelayisneededforthework，whereastheflatteethcannotbebluntedbutareonlysmoothedintimebywearingdown），partlyfromnecessitybecause，whiletherootsofthegrindersarefixedwherethejawisflatandthebonestrong，thoseofthefrontteethareinathinpart，sothattheyareweakandeasilymoved。Theygrowagainbecausetheyareshedwhiletheboneisstillgrowingandtheanimalisstillyoungenoughtogrowteeth。Aproofofthisisthateventheflatteethgrowforalongtime，thelastofthemcuttingthegumatabouttwentyyearsofage；indeedinsomecasesthelastteethhavebeengrowninquiteoldage。Thisisbecausethereismuchnutrimentinthebroadpartofthebones，whereasthefrontpartbeingthinsoonreachesperfectionandnoresidualmatterisfoundinit，thenutrimentbeingconsumedinitsowngrowth。
　　Democritus，however，neglectingthefinalcause，reducestonecessityalltheoperationsofNature。Nowtheyarenecessary，itistrue，butyettheyareforafinalcauseandforthesakeofwhatisbestineachcase。Thusnothingpreventstheteethfrombeingformedandbeingshedinthisway；butitisnotonaccountofthesecausesbutonaccountoftheend（orfinalcause）；thesearecausesonlyinthesenseofbeingthemovingandefficientinstrumentsandthematerial。SoitisreasonablethatNatureshouldperformmostofheroperationsusingbreathasaninstrument，forassomeinstrumentsservemanyusesinthearts，e。g。thehammerandanvilinthesmith’sart，sodoesbreathinthelivingthingsformedbyNature。Buttosaythatnecessityistheonlycauseismuchasifweshouldthinkthatthewaterhasbeendrawnofffromadropsicalpatientonaccountofthelancet，notonaccountofhealth，forthesakeofwhichthelancetmadetheincision。
　　Wehavethusspokenoftheteeth，sayingwhysomeareshedandgrowagain，andothersnot，andgenerallyforwhatcausetheyareformed。Andwehavespokenoftheotheraffectionsofthepartswhicharefoundtooccurnotforanyfinalendbutofnecessityandonaccountofthemotiveorefficientcause。