Muchinerror，then，aretheywhosaythattheconstructionofmanisnotonlyfaulty，butinferiortothatofallotheranimals；
seeingthatheis，astheypointout，bare-footed，naked，andwithoutweaponofwhichtoavailhimself。Forotheranimalshaveeachbutonemodeofdefence，andthistheycanneverchange；sothattheymustperformalltheofficesoflifeandeven，sotospeak，sleepwithsandalson，neverlayingasidewhateverservesasaprotectiontotheirbodies，norchangingsuchsingleweaponastheymaychancetopossess。Buttomannumerousmodesofdefenceareopen，andthese，moreover，hemaychangeatwill；asalsohemayadoptsuchweaponashepleases，andatsuchtimesassuithim。Forthehandistalon，hoof，andhorn，atwill。Sotooitisspear，andsword，andwhatsoeverotherweaponorinstrumentyouplease；forallthesecanitbefromitspowerofgraspingandholdingthemall。Inharmonywiththisvariedofficeistheformwhichnaturehascontrivedforit。Foritissplitintoseveraldivisions，andthesearecapableofdivergence。Suchcapacityofdivergencedoesnotpreventtheiragainconvergingsoastoformasinglecompactbody，whereashadthehandbeenanundividedmass，divergencewouldhavebeenimpossible。Thedivisionsalsomaybeusedsinglyortwotogetherandinvariouscombinations。Thejoints，moreover，ofthefingersarewellconstructedforprehensionandforpressure。Oneofthesealso，andthisnotlongliketherestbutshortandthick，isplacedlaterally。Forwereitnotsoplacedallprehensionwouldbeasimpossible，asweretherenohandatall。Forthepressureofthisdigitisappliedfrombelowupwards，whiletherestactfromabovedownwards；anarrangementwhichisessential，ifthegraspistobefirmandholdlikeatightclamp。Asfortheshortnessofthisdigit，theobjectistoincreaseitsstrength，sothatitmaybeable，thoughbutone，tocounterbalanceitsmorenumerousopponents。
Moreover，wereitlongitwouldbeofnouse。Thisistheexplanationofitsbeingsometimescalledthegreatdigit，inspiteofitssmallsize；forwithoutitalltherestwouldbepracticallyuseless。Thefingerwhichstandsattheotherendoftherowissmall，whilethecentraloneofallislong，likeacentreoarinaship。
Thisisrightlyso；foritismainlybythecentralpartoftheencirclinggraspthatatoolmustbeheldwhenputtouse。
Nolessskilfullycontrivedarethenails。For，whileinmantheseservesimplyascoveringstoprotectthetipsofthefingers，inotheranimalstheyarealsousedforactivepurposes；andtheirformineachcaseissuitedtotheiroffice。
Thearmsinmanandtheforelimbsinquadrupedsbendincontrarydirections，thisdifferencehavingreferencetotheingestionoffoodandtotheotherofficeswhichbelongtotheseparts。Forquadrupedsmustofnecessitybendtheiranteriorlimbsinwardsthattheymayserveinlocomotion，fortheyusethemasfeet。Notbutwhatevenamongquadrupedsthereisatanyrateatendencyforsuchasarepolydactyloustousetheirforefeetnotonlyforlocomotionbutashands。Andtheyareinfactsoused，asanyonemaysee。Fortheseanimalsseizeholdofobjects，andalsorepelassailantswiththeiranteriorlimbs；whereasquadrupedswithsolidhoofsusetheirhindlegsforthislatterpurpose。Fortheirforelimbsarenotanalogoustothearmsandhandsofman。
Itisthishand-likeofficeoftheanteriorlimbswhichexplainswhyinsomeofthepolydactylousquadrupeds，suchaswolves，lions，dogs，andleopards，thereareactuallyfivedigitsoneachforefoot，thoughthereareonlyfouroneachhindone。Forthefifthdigitofthefootcorrespondstothefifthdigitofthehand，andlikeitiscalledthebigone。Itistruethatinthesmallerpolydactylousquadrupedsthehindfeetalsohaveeachfivetoes。Butthisisbecausetheseanimalsarecreepers；andtheincreasednumberofnailsservestogivethematightergrip，andsoenablesthemtocreepupsteepplaceswithgreaterfacility，oreventorunheaddownwards。
Inmanbetweenthearms，andinotheranimalsbetweentheforelegs，lieswhatiscalledthebreast。Thisinmanisbroad，asonemightexpect；forasthearmsaresetlaterallyonthebody，theyoffernoimpedimenttosuchexpansioninthispart。Butinquadrupedsthebreastisnarrow，owingtothelegshavingtobeextendedinaforwarddirectioninprogressionandlocomotion。
Owingtothisnarrownessthemammaeofquadrupedsareneverplacedonthebreast。Butinthehumanbodythereisamplespaceinthispart；moreover，theheartandneighbouringorgansrequireprotection，andforthesereasonsthispartisfleshyandthemammaeareplaceduponitseparately，sidebyside，beingthemselvesofafleshysubstanceinthemaleandthereforeofuseinthewayjuststated；whileinthefemale，nature，inaccordancewithwhatwesayisherfrequentpractice，makesthemministertoanadditionalfunction，employingthemasastore-placeofnutrimentfortheoffspring。Thehumanmammaearetwoinnumber，inaccordancewiththedivisionofthebodyintotwohalves，arightandaleft。Theyaresomewhatfirmerthantheywouldotherwisebe，becausetheribsinthisregionarejoinedtogether；whiletheyformtwoseparatemasses，becausetheirpresenceisinnowiseburdensome。Inotheranimalsthanman，itisimpossibleforthemammaetobeplacedonthebreastbetweentheforelegs，fortheywouldinterferewithlocomotion；theyarethereforedisposedofotherwise，andinavarietyofways。Thusinsuchanimalsasproducebutfewatabirth，whetherhornedquadrupedsorthosewithsolidhoofs，themammaeareplacedintheregionofthethighs，andaretwoinnumber，whileinsuchasproducelitters，orsuchasarepolydactylous，thedugsareeithernumerousandplacedlaterallyonthebelly，asinswineanddogs，orareonlytwoinnumber，beingset，however，inthecentreoftheabdomen，asisthecaseinthelion。Theexplanationofthislatterconditionisnotthatthelionproducesfewatabirth，forsometimesithasmorethantwocubsatatime，butistobefoundinthefactthatthisanimalhasnoplentifulsupplyofmilk。For，beingaflesheater，itgetsfoodatbutrareintervals，andsuchnourishmentasitobtainsisallexpendedonthegrowthofitsbody。
Intheelephantalsotherearebuttwomammae，whichareplacedundertheaxillaeoftheforelimbs。Themammaearenotmorethantwo，becausethisanimalhasonlyasingleyoungoneatabirth；andtheyarenotplacedintheregionofthethighs，becausetheyneveroccupythatpositioninanypolydactylousanimalsuchasthis。Lastly，theyareplacedabove，closetotheaxillae，becausethisisthepositionoftheforemostdugsinallanimalswhosedugsarenumerous，andthedugssoplacedgivethemostmilk。Evidenceofthisisfurnishedbythesow。Forshealwayspresentstheseforemostdugstothefirst-bornofherlitter。Asingleyoungoneisofcourseafirst-born，andsosuchanimalsasonlyproduceasingleyoungonemusthavetheseanteriordugstopresenttoit；thatistheymusthavethedugswhichareundertheaxillae。This，then，isthereasonwhytheelephanthasbuttwomammae，andwhytheyaresoplaced。But，insuchanimalsashavelittersofyoung，thedugsaredisposedaboutthebelly；thereasonbeingthatmoredugsarerequiredbythosethatwillhavemoreyoungtonourish。Nowitisimpossiblethatthesedugsshouldbesettransverselyinrowsofmorethantwo，one，thatis，foreachsideofthebody，therightandtheleft；
theymustthereforebeplacedlengthways，andtheonlyplacewherethereissufficientlengthforthisistheregionbetweenthefrontandhindlegs。Astotheanimalsthatarenotpolydactylousbutproducefewatabirth，orhavehorns，theirdugsareplacedintheregionofthethighs。Thehorse，theass，thecamelareexamples；
allofwhichbearbutasingleyoungoneatatime，andofwhichthetwoformerhavesolidhoofs，whileinthelastthehoofiscloven。
Asstillfurtherexamplesmaybementionedthedeer，theox，thegoat，andallothersimilaranimals。
Theexplanationisthatintheseanimalsgrowthtakesplaceinanupwarddirection；sothattheremustbeanabundantcollectionofresidualmatterandofbloodinthelowerregion，thatistosayintheneighbourhoodoftheorificesforefflux，andherethereforenaturehasplacedthemammae。Fortheplaceinwhichthenutrimentissetinmotionmustalsobetheplacewhencenutrimentcanbederivedbythem。Inmantherearemammaeinthemaleaswellasinthefemale；butsomeofthemalesofotheranimalsarewithoutthem。Such，forinstance，isthecasewithhorses，somestallionsbeingdestituteoftheseparts，whileothersthatresembletheirdamshavethem。Thusmuchthenconcerningthemammae。
Nextafterthebreastcomestheregionofthebelly，whichisleftunenclosedbytheribsforareasonwhichhasalreadybeengiven；
namelythattheremaybenoimpedimenttotheswellingwhichnecessarilyoccursinthefoodasitgetsheated，nortotheexpansionofthewombinpregnancy。
Attheextremeendofwhatiscalledthetrunkarethepartsconcernedintheevacuationofthesolidandalsoofthefluidresidue。Inallsanguineousanimalswithsomefewexceptions，andinallViviparawithoutanyexceptionatall，thesamepartwhichservesfortheevacuationofthefluidresidueisalsomadebynaturetoserveinsexualcongress，andthisalikeinmaleandfemale。
Forthesemenisakindoffluidandresidualmatter。Theproofofthiswillbegivenhereafter，butforthepresentletittakenforgranted。（Thelikeholdsgoodofthemenstrualfluidinwomen，andofthepartwheretheyemitsemen。Thisalso，however，isamatterofwhichamoreaccurateaccountwillbegivenhereafter。Forthepresentletitbesimplystatedasafact，thatthecatameniaofthefemalelikethesemenofthemaleareresidualmatter。Bothofthem，moreover，beingfluid，itisonlynaturalthatthepartswhichserveforvoidanceoftheurineshouldgiveissuetoresidueswhichresembleitincharacter。）Oftheinternalstructureoftheseparts，andofthedifferenceswhichexistbetweenthepartsconcernedwithsemenandthepartsconcernedwithconception，aclearaccountisgiveninthebookofResearchesconcerningAnimalsandinthetreatisesonAnatomy。
Moreover，IshallhavetospeakofthemagainwhenIcometodealwithGeneration。Asregards，however，theexternalshapeoftheseparts，itisplainenoughthattheyareadaptedtotheiroperations，asindeedofnecessitytheymustbe。Thereare，however，differencesinthemaleorgancorrespondingtodifferencesinthebodygenerally。Forallanimalsarenotofanequallysinewynature。Thisorgan，again，istheonlyonethat，independentlyofanymorbidchange，admitsofaugmentationandofdiminutionofbulk。Theformerconditionisofserviceincopulation，whiletheotherisrequiredfortheadvantageofthebodyatlarge。For，weretheorganconstantlyintheformercondition，itwouldbeanincumbrance。Theorganthereforehasbeenformedofsuchconstituentsaswilladmitofeitherstate。Foritispartlysinewy，partlycartilaginous，andthusisenabledeithertocontractortobecomeextended，andiscapableofadmittingair。
Allfemalequadrupedsvoidtheirurinebackwards，becausethepositionofthepartswhichthisimpliesisusefultothemintheactofcopulation。Thisisthecasewithonlysomefewmales，suchasthelynx，thelion，thecamel，andthehare。Noquadrupedwithasolidhoofisretromingent。
Theposteriorportionofthebodyandthepartsaboutthelegsarepeculiarinmanascomparedwithquadrupeds。Nearlyalltheselatterhaveatail，andthiswhethertheyareviviparousoroviparous。For，evenifthetailbeofnogreatsize，yettheyhaveakindofscut，asatanyrateasmallrepresentativeofit。Butmanistail-less。Hehas，however，buttocks，whichexistinnoneofthequadrupeds。Hislegsalsoarefleshy（astooarehisthighsandfeet）；whilethelegsinallotheranimalsthathaveany，whetherviviparousornot，arefleshless，beingmadeofsinewandboneandspinoussubstance。Forallthesedifferencesthereis，sotosay，onecommonexplanation，andthisisthatofallanimalsmanalonestandserect。ItwastofacilitatethemaintenanceofthispositionthatNaturemadehisupperpartslight，takingawaysomeoftheircorporealsubstance，andusingittoincreasetheweightoflithepartsbelow，sothatthebuttocks，thethighs，andthecalvesofthelegswereallmadefleshy。
Thecharacterwhichshethusgavetothebuttocksrendersthematthesametimeusefulinrestingthebody。Forstandingcausesnofatiguetoquadrupeds，andeventhelongcontinuanceofthispostureproducesinthemnoweariness；fortheyaresupportedthewholetimebyfourprops，whichismuchasthoughtheywerelyingdown。Buttomanitisnotasktoremainforanylengthoftimeonhisfeet，hisbodydemandingrestinasittingposition。This，then，isthereasonwhymanhasbuttocksandfleshylegs；andthepresenceofthesefleshypartsexplainswhyhehasnotail。Forthenutrimentwhichwouldotherwisegotothetailisusedupintheproductionoftheseparts，whileatthesametimetheexistenceofbuttocksdoesawaywiththenecessityofatail。Butinquadrupedsandotheranimalsthereverseobtains。Fortheyareofdwarf-likeform，sothatallthepressureoftheirweightandcorporealsubstanceisontheirupperpart，andiswithdrawnfromthepartsbelow。Onthisaccounttheyarewithoutbuttocksandhavehardlegs。Inorder，however，tocoverandprotectthatpartwhichservesfortheevacuationofexcrement，naturehasgiventhematailofsomekindorother，subtractingforthepurposesomeofthenutrimentwhichwouldotherwisegotothelegs。Intermediateinshapebetweenmanandquadrupedsistheape，belongingthereforetoneitherortoboth，andhavingonthisaccountneithertailnorbuttocks；notailinitscharacterofbiped，nobuttocksinitscharacterofquadruped。Thereisgreatdiversityofso-calledtails；andthisorganlikeothersissometimesusedbynatureforby-purposes，beingmadetoservenotonlyasacoveringandprotectiontothefundament，butalsoforotherusesandadvantagesofitspossessor。
Therearedifferencesinthefeetofquadrupeds。Forinsomeoftheseanimalsthereisasolidhoof，andinothersahoofclovenintotwo，andagaininothersafootdividedintomanyparts。
Thehoofissolidwhenthebodyislargeandtheearthymatterpresentingreatabundance；inwhichcasetheearth，insteadofformingteethandhorns，isseparatedinthecharacterofanail，andbeingveryabundantformsonecontinuousnail，thatisahoof，inplaceofseveral。Thisconsumptionoftheearthymatteronthehoofexplainswhytheseanimals，asarule，havenohuckle-bones；asecondreasonbeingthatthepresenceofsuchaboneinthejointofthehindlegsomewhatimpedesitsfreemotion。Forextensionandflexioncanbemademorerapidlyinpartsthathavebutoneanglethaninpartsthathaveseveral。Butthepresenceofahuckle-bone，asaconnectingbolt，istheintroductionasitwereofanewlimb-segmentbetweenthetwoordinaryones。Suchanadditionaddstotheweightofthefoot，butrenderstheactofprogressionmoresecure。Thusitisthatinsuchanimalsashaveahucklebone，itisonlyintheposteriorandnotintheanteriorlimbsthatthisboneisfound。Fortheanteriorlimbs，movingastheydoinadvanceoftheothers，requiretobelightandcapableofreadyflexion，whereasfirmnessandextensibilityarewhatarewantedinthehindlimbs。Moreover，ahuckle-boneaddsweighttotheblowofalimb，andsorendersitasuitableweaponofdefence；andtheseanimalsallusetheirhindlegstoprotectthemselves，kickingoutwiththeirheelsagainstanythingwhichannoysthem。Inthecloven-hoofedquadrupedsthelightercharacterofthehindlegsadmitsoftherebeingahuckle-bone；andthepresenceofthehuckle-bonepreventsthemfromhavingasolidhoof，thebonysubstanceremaininginthejoint，andthereforebeingdeficientinthefoot。Astothepolydactylousquadrupeds，noneofthemhavehuckle-bones。Foriftheyhadtheywouldnotbepolydactylous，butthedivisionsofthefootwouldonlyextendtothatamountofitsbreadthwhichwascoveredbythehuckle-bone。Thusitisthatmostoftheanimalsthathavehuckle-bonesarecloven-hoofed。
Ofallanimalsmanhasthelargestfootinproportiontothesizeofthebody。Thisisonlywhatmightbeexpected。Forseeingthatheistheonlyanimalthatstandserect，thetwofeetwhichareintendedtobearalltheweightofthebodymustbebothlongandbroad。
Equallyintelligibleisitthattheproportionbetweenthesizeofthefingersandthatofthewholehandshouldbeinvertedinthecaseofthetoesandfeet。Forthefunctionofthehandsistotakeholdofobjectsandretainthembypressure；sothatthefingersrequiretobelong。Foritisbyitsflexedportionthatthehandgraspsanobject。Butthefunctionofthefeetistoenableustostandsecurely，andforthistheundividedpartofthefootrequirestobeoflargersizethanthetoes。However，itisbetterfortheextremitytobedividedthantobeundivided。Forinanundividedfootdiseaseofanyonepartwouldextendtothewholeorgan；whereas，ifthefootbedividedintoseparatedigits，thereisnotanequalliabilitytosuchanoccurrence。Thedigits，again，bybeingshortwouldbelessliabletoinjury。Forthesereasonsthefeetinmanaremany-toed，whiletheseparatedigitsareofnogreatlength。Thetoes，finally，arefurnishedwithnailsforthesamereasonasarethefingers，namelybecausesuchprojectingpartsareweakandthereforerequirespecialprotection。
11
Wehavenowdonewithsuchsanguineousanimalsasliveonlandandbringforththeiryoungalive；and，havingdealtwithalltheirmainkinds，wemaypassontosuchsanguineousanimalsasareoviparous。Ofthesesomehavefourfeet，whileothershavenone。Thelatterformasinglegenus，namelytheSerpents；andwhytheseareapodoushasbeenalreadyexplainedinthedissertationonAnimalProgression。
Irrespectiveofthisabsenceoffeet，serpentsresembletheoviparousquadrupedsintheirconformation。
Inalltheseanimalsthereisaheadwithitscomponentparts；itspresencebeingdeterminedbythesamecausesasobtaininthecaseofothersanguineousanimals；andinall，withthesingleexceptionoftherivercrocodile，thereisatongueinsidethemouth。Inthisoneexceptiontherewouldseemtobenoactualtongue，butmerelyaspaceleftvacantforit。Thereasonisthatacrocodileisinawayaland-animalandawater-animalcombined。Initscharacterofland-animalithasaspaceforatongue；butinitscharacterofwater-animalitiswithoutthetongueitself。Forinsomefishes，ashasalreadybeenmentioned，thereisnoappearancewhatsoeverofatongue，unlessthemouthbestretchedopenverywidelyindeed；whileinothersitisindistinctlyseparatedfromtherestofthemouth。Thereasonforthisisthatatonguewouldbeofbutlittleservicetosuchanimals，seeingthattheyareunabletochewtheirfoodortotasteitbeforeswallowing，thepleasurablesensationstheyderivefromitbeinglimitedtotheactofdeglutition。Foritisintheirpassagedownthegulletthatsolidediblescauseenjoyment，whileitisbythetonguethatthesavouroffluidsisperceived。Thusitisduringdeglutitionthattheoiliness，theheat，andothersuchqualitiesoffoodarerecognized；and，infact，thesatisfactionfrommostsolidediblesanddaintiesisderivedalmostentirelyfromthedilatationoftheoesophagusduringdeglutition。Thissensation，then，belongseventoanimalsthathavenotongue，butwhileotheranimalshaveinadditionthesensationsoftaste，tonguelessanimalshave，wemaysay，noothersatisfactionthanit。Whathasnowbeensaidexplainswhyintemperanceasregardsdrinksandsavouryfluidsdoesnotgohandinhandwithintemperanceasregardseatingandsolidrelishes。
Insomeoviparousquadrupeds，namelyinlizards，thetongueisbifid，asalsoitisinserpents，anditsterminaldivisionsareofhair-likefineness，ashasalreadybeendescribed。（Sealsalsohaveaforkedtongue。）Thisitiswhichaccountsforalltheseanimalsbeingsofondofdaintyfood。Theteethinthefour-footedOviparaareofthesharpinterfittingkind，liketheteethoffishes。Theorgansofallthesensesarepresentandresemblethoseofotheranimals。
Thustherearenostrilsforsmell，evesforvision，andearsforhearing。Thelatterorgans，however，donotprojectfromthesidesofthehead，butconsistsimplyoftheduct，asalsoisthecaseinbirds。Thisisdueinbothcasestothehardnessoftheintegument；
birdshavingtheirbodiescoveredwithfeathers，andtheseoviparousquadrupedswithhornyplates。Theseplatesareequivalenttoscales，butofahardercharacter。Thisismanifestintortoisesandrivercrocodiles，andalsointhelargeserpents。Forheretheplatesbecomestrongerthanthebones，beingseeminglyofthesamesubstanceasthese。
Theseanimalshavenouppereyelid，butclosetheeyewiththelowerlidInthistheyresemblebirds，andthereasonisthesameaswasassignedintheircase。Amongbirdstherearesomethatcannotonlythusclosetheeye，butcanalsoblinkbymeansofamembranewhichcomesfromitscorner。Butnoneoftheoviparousquadrupedsblink；fortheireyesareharderthanthoseofbirds。Thereasonforthisisthatkeenvisionandfar-sightednessareofveryconsiderableservicetobirds，flyingastheydointheair，whereastheywouldbeofcomparativelysmallusetotheoviparousquadrupeds，seeingthattheyarealloftroglodytichabits。
Ofthetwoseparateportionswhichconstitutethehead，namelytheupperpartandthelowerjaw，thelatterinmanandintheviviparousquadrupedsmovesnotonlyupwardsanddownwards，butalsofromsidetoside；whileinfishes，andbirdsandoviparousquadrupeds，theonlymovementisupanddown。Thereasonisthatthislattermovementistheonerequiredinbitinganddividingfood，whilethelateralmovementservetoreducesubstancestoapulp。
Tosuchanimals，therefore，ashavegrinder-teeththislateralmotionisofservice；buttothoseanimalsthathavenogrindersitwouldbequiteuseless，andtheyarethereforeinvariablywithoutit。Fornaturenevermakesanythingthatissuperfluous。Whileinallotheranimalsitisthelowerjawthatismovable，intherivercrocodileitisexceptionallytheupper。Thisisbecausethefeetinthiscreaturearesoexcessivelysmallastobeuselessforseizingandholdingprey；onwhichaccountnaturehasgivenitamouththatcanserveforthesepurposesintheirstead。Forthatdirectionofmotionwhichwillgivethegreaterforcetoablowwillbethemoreserviceableoneinholdingorinseizingprey；andablowfromaboveisalwaysmoreforciblethanonefrombelow。Seeing，then，thatboththeprehensionandthemasticationoffoodareofficesofthemouth，andthattheformerofthesetwoisthemoreessentialinananimalthathasneitherhandsnorsuitablyformedfeet，thesecrocodileswillderivegreaterbenefitfromamotionoftheupperjawdownwardsthanfromamotionofthelowerjawupwards。Thesameconsiderationsexplainwhycrabsalsomovetheupperdivisionofeachclawandnotthelower。Fortheirclawsaresubstitutesforhands，andsorequiretobesuitablefortheprehensionoffood，andnotforitscomminution；forsuchcomminutionandbitingistheofficeofteeth。
Incrabs，then，andinsuchotheranimalsasareabletoseizetheirfoodinaleisurelymanner，inasmuchastheirmouthisnotcalledontoperformitsofficewhiletheyarestillinthewater，thetwofunctionsareassignedtodifferentparts，prehensiontothehandsorfeet，bitingandcomminutionoffoodtothemouth。Butincrocodilesthemouthhasbeensoframedbynatureastoservebothpurposes，thejawsbeingmadetomoveinthemannerjustdescribed。
Anotherpartpresentintheseanimalsisaneck，thisbeingthenecessaryconsequenceoftheirhavingalung。Forthewindpipebywhichtheairisadmittedtothelungisofsomelength。If，however，thedefinitionofaneckbecorrect，whichcallsittheportionbetweentheheadandtheshoulders，aserpentcanscarcelybesaidwiththesamerightastherestoftheseanimalstohaveaneck，butonlytohavesomethinganalogoustothatpartofthebody。
Itisapeculiarityofserpents，ascomparedwithotheranimalsalliedtothem，thattheyareabletoturntheirheadbackwardswithoutstirringtherestofthebody。Thereasonofthisisthataserpent，likeaninsect，hasabodythatadmitsofbeingcurledup，itsvertebraebeingcartilaginousandeasilybent。Thefacultyinquestionbelongsthentoserpentssimplyasanecessaryconsequenceofthischaracteroftheirvertebrae；butatthesametimeithasafinalcause，foritenablesthemtoguardagainstattacksfrombehind。Fortheirbody，owingtoitslengthandtheabsenceoffeet，isill-suitedforturningroundandprotectingthehinderparts；andmerelytoliftthehead，withoutthepowerofturningitround，wouldbeofnousewhatsoever。
Theanimalswithwhichwearedealinghave，moreover，apartwhichcorrespondstothebreast；butneitherherenorelsewhereintheirbodyhavetheyanymammae，asneitherhasanybirdorfish。Thisisaconsequenceoftheirhavingnomilk；foramammaisareceptacleformilkand，asitwere，avesseltocontainit。Thisabsenceofmilkisnotpeculiartotheseanimals，butiscommontoallsuchasarenotinternallyviviparous。Forallsuchproduceeggs，andthenutrimentwhichinViviparahasthecharacterofmilkisinthemengenderedintheegg。Ofallthis，however，acleareraccountwillbegiveninthetreatiseonGeneration。Astothemodeinwhichthelegsbend，ageneralaccount，inwhichallanimalsareconsidered，hasalreadybeengiveninthedissertationonProgression。Theseanimalsalsohaveatail，largerinsomeofthem，smallerinothers，andthereasonforthishasbeenstatedingeneraltermsinanearlierpassage。
OfalloviparousanimalsthatliveonlandthereisnonesoleanastheChamaeleon。Forthereisnonethathassolittleblood。Theexplanationofthisistobefoundinthepsychicaltemperamentofthecreature。Foritisofatimidnature，asthefrequentchangesitundergoesinitsoutwardaspecttestify。Butfearisarefrigeration，andresultsfromdeficiencyofnaturalheatandscantinessofblood。Wehavenowdonewithsuchsanguineousanimalsasarequadrupedousandalsosuchasareapodous，andhavestatedwithsufficientcompletenesswhatexternalpartstheypossess，andforwhatreasontheyhavethem。
12
Thedifferencesofbirdscomparedonewithanotheraredifferencesofmagnitude，andofthegreaterorsmallerdevelopmentofparts。Thussomehavelonglegs，othersshortlegs；somehaveabroadtongue，othersanarrowtongue；andsoonwiththeotherparts。Therearefewoftheirpartsthatdiffersaveinsize，takingbirdsbythemselves。Butwhenbirdsarecomparedwithotheranimalsthepartspresentdifferencesofformalso。Forinsomeanimalsthesearehairy，inothersscaly，andinothershavescale-likeplates，whilebirdsarefeathered。
Birds，then，arefeathered，andthisisacharactercommontothemallandpeculiartothem。Theirfeathers，too，aresplitanddistinctinkindfromtheundividedfeathersofinsects；forthebird’sfeatherisbarbed，thesearenot；thebird’sfeatherhasashaft，thesehavenone。Asecondstrangepeculiaritywhichdistinguishesbirdsfromallotheranimalsistheirbeak。Forasinelephantsthenostrilservesinplaceofhands，andasinsomeinsectsthetongueservesinplaceofmouth，soinbirdsthereisabeak，which，beingbony，servesinplaceofteethandlips。Theirorgansofsensehavealreadybeenconsidered。
Allbirdshaveaneckextendingfromthebody；andthepurposeofthisneckisthesameasinsuchotheranimalsashaveone。Thisneckinsomebirdsislong，inothersshort；itslength，asageneralrule，beingprettynearlydeterminedbythatofthelegs。
Forlong-leggedbirdshavealongneck，short-leggedbirdsashortone，towhichrule，however，theweb-footedbirdsformanexception。
Fortoabirdpercheduponlonglegsashortneckwouldbeofnousewhatsoeverincollectingfoodfromtheground；andequallyuselesswouldbealongneck，ifthelegswereshort。Suchbirds，again，asarecarnivorouswouldfindlengthinthispartinterferegreatlywiththeirhabitsoflife。Foralongneckisweak，anditisontheirsuperiorstrengththatcarnivorousbirdsdependfortheirsubsistence。
Nobird，therefore，thathastalonseverhasanelongatedneck。Inweb-footedbirds，however，andinthoseotherbirdsbelongingtothesameclass，whosetoesthoughactuallyseparatehaveflatmarginallobes，theneckiselongated，soastobesuitableforcollectingfoodfromthewater；whilethelegsareshort，soastoserveinswimming。Thebeaksofbirds，astheirfeet，varywiththeirmodesoflife。Forinsomethebeakisstraight，inotherscrooked；
straight，inthosewhouseitmerelyforeating；crooked，inthosethatliveonrawflesh。Foracrookedbeakisanadvantageinfighting；andthesebirdsmust，ofcourse，gettheirfoodfromthebodiesofotheranimals，andinmostcasesbyviolence。Insuchbirds，again，asliveinmarshesandareherbivorousthebeakisbroadandflat，thisformbeingbestsuitedfordiggingandcropping，andforpullingupplants。Insomeofthesemarshbirds，however，thebeakiselongated，astooistheneck，thereasonforthisbeingthatthebirdgetitsfoodfromsomedepthbelowthesurface。Formostbirdsofthiskind，andmostofthosewhosefeetarewebbed，eitherintheirentiretyoreachpartseparately，livebypreyingonsomeofthesmalleranimalsthataretobefoundinwater，andusethesepartsfortheircapture，theneckactingasafishing-rod，andthebeakrepresentingthelineandhook。
Theupperandundersidesofthebody，thatisofwhatinquadrupedsiscalledthetrunk，presentinbirdsoneunbrokensurface，andtheyhavenoarmsorforelegsattachedtoit，butintheirsteadwings，whichareadistinctivepeculiarityoftheseanimals；and，asthesewingsaresubstitutesforarms，theirterminalsegmentslieonthebackintheplaceofashoulder-blade。
Thelegsaretwoinnumber，asinman；nothowever，asinman，bentoutwards，butbentinwardslikethelegsofaquadruped。Thewingsarebentliketheforelegsofaquadruped，havingtheirconvexityturnedoutwards。Thatthefeetshouldbetwoinnumberisamatterofnecessity。Forabirdisessentiallyasanguineousanimal，andatthesametimeessentiallyawingedanimal；andnosanguineousanimalhasmorethanfourpointsformotionInbirds，then，asinthoseothersanguineousanimalsthatliveandmoveupontheground，thelimbsattachedtothetrunkarefourinnumber。But，whileinalltherestthesefourlimbsconsistofapairofarmsandapairoflegs，oroffourlegsasinquadrupeds，inbirdsthearmsorforelegsarereplacedbyapairofwings，andthisistheirdistinctivecharacter。Foritisoftheessenceofabirdthatitshallbeabletofly；anditisbytheextensionofwingsthatthisismadepossible。Ofallarrangements，then，theonlypossible，andsothenecessary，oneisthatbirdsshallhavetwofeet；forthiswiththewingswillgivethemfourpointsformotion。Thebreastinallbirdsissharp-edged，andfleshy。Thesharpedgeistoministertoflight，forbroadsurfacesmovewithconsiderabledifficulty，owingtothelargequantityofairwhichtheyhavetodisplace；whilethefleshycharacteractsasaprotection，forthebreast，owingtoitsform，wouldbeweak，wereitnotamplycovered。
Belowthebreastliesthebelly，extending，asinquadrupedsandinman，totheventandtotheplacewherethelegsarejointedtothetrunk。
Such，then，arethepartswhichliebetweenthewingsandthelegs。Birdslikeallotheranimals，whetherproducedviviparouslyorfromeggs，haveanumbilicusduringtheirdevelopment，but，whenthebirdhasattainedtofullergrowth，nosignsofthisremainvisible。
Thecauseofthisisplainlytobeseenduringtheprocessofdevelopment；forinbirdstheumbilicalcorduniteswiththeintestine，andisnotaportionofthevascularsystem，asisthecaseinviviparousanimals。
Somebirds，again，arewelladaptedforflight，theirwingsbeinglargeandstrong。Such，forinstance，arethosethathavetalonsandliveonflesh。Fortheirmodeofliferendersthepowerofflightanecessity，anditisonthisaccountthattheirfeathersaresoabundantandtheirwingssolarge。Besidesthese，however，therearealsoothergeneraofbirdsthatcanflywell；allthose，namely，thatdependonspeedforsecurity，orthatareofmigratoryhabits。Ontheotherhand，somekindsofbirdshaveheavybodiesandarenotconstructedforflight。Thesearebirdsthatarefrugivorousandliveontheground，orthatareabletoswimandgettheirlivinginwateryplaces。Inthosethathavetalonsthebody，withoutthewings，issmall；forthenutrimentisconsumedintheproductionofthesewings，andoftheweaponsanddefensiveappliances；whereasinbirdsthatarenotmadeforflightthecontraryobtains，andthebodyisbulkyandsoofheavyweight。Insomeoftheseheavy-bodiedbirdsthelegsarefurnishedwithwhatarecalledspurs，whichreplacethewingsasameansofdefence。Spursandtalonsneverco-existinthesamebird。Fornaturenevermakesanythingsuperfluous；andifabirdcanfly，andhastalons，ithasnouseforspurs；fortheseareweaponsforfightingontheground，andonthisaccountareanappanageofcertainheavy-bodiedbirds。Theselatter，again，wouldfindthepossessionoftalonsnotonlyuselessbutactuallyinjurious；
fortheclawswouldstickintothegroundandinterferewithprogression。Thisisthereasonwhyallbirdswithtalonswalksobadly，andwhytheyneversettleuponrocks。Forthecharacteroftheirclawsisill-suitedforeitheraction。
Allthisisthenecessaryconsequenceoftheprocessofdevelopment。
Fortheearthymatterinthebodyissuingfromitisconvertedintopartsthatareusefulasweapons。Thatwhichflowsupwardsgiveshardnessorsizetothebeak；and，shouldanyflowdownwards，iteitherformsspursuponthelegsorgivessizeandstrengthtotheclawsuponthefeet。Butitdoesnotatoneandthesametimeproduceboththeseresults，oneinthelegs，theotherintheclaws；
forsuchadispersionofthisresidualmatterwoulddestroyallitsefficiency。Inotherbirdsthisearthyresiduefurnishesthelegswiththematerialfortheirelongation；orsometimes，inplaceofthis，fillsuptheinterspacesbetweenthetoes。Thusitissimplyamatterofnecessity，thatsuchbirdsasswimshalleitherbeactuallyweb-footed，orshallhaveakindofbroadblade-likemarginrunningalongthewholelengthofeachdistincttoe。Theforms，then，ofthesefeetaresimplythenecessaryresultsofthecausesthathavebeenmentioned。Yetatthesametimetheyareintendedfortheanimal’sadvantage。Fortheyareinharmonywiththemodeoflifeofthesebirds，who，livingonthewater，wheretheirwingsareuseless，requirethattheirfeetshallbesuchastoserveinswimming。Forthesefeetaresodevelopedastoresembletheoarsofaboat，orthefinsofafish；andthedestructionofthefoot-webhasthesameeffectasthedestructionofthefins；thatistosay，itputsanendtoallpowerofswimming。
Insomebirdsthelegsareverylong，thecauseofthisbeingthattheyinhabitmarshes。Isaythecause，becausenaturemakestheorgansforthefunction，andnotthefunctionfortheorgans。Itis，then，becausethesebirdsarenotmeantforswimmingthattheirfeetarewithoutwebs，anditisbecausetheyliveongroundthatgiveswayunderthefootthattheirlegsandtoesareelongated，andthattheselatterinmostofthemhaveanextranumberofjoints。Again，thoughallbirdshavethesamematerialcomposition，theyarenotallmadeforflight；andinthese，therefore，thenutrimentthatshouldgototheirtail-feathersisspentonthelegsandusedtoincreasetheirsize。Thisisthereasonwhythesebirdswhentheyflymakeuseoftheirlegsasatail，stretchingthemoutbehind，andsorenderingthemserviceable，whereasinanyotherpositiontheywouldbesimplyanimpediment。
Inotherbirds，wherethelegsareshort，theseareheldcloseagainstthebellyduringflight。Insomecasesthisismerelytokeepthefeetoutoftheway，butinbirdsthathavetalonsthepositionhasafurtherpurpose，beingtheonebestsuitedforrapine。Birdsthathavealongandathickneckkeepitstretchedoutduringflight；butthosewhoseneckthoughlongisslenderflywithitcoiledup。Forinthispositionitisprotected，andlesslikelytogetbroken，shouldthebirdflyagainstanyobstacle。
Inallbirdsthereisanischium，butsoplacedandofsuchlengththatitwouldscarcelybetakenforanischium，butratherforasecondthigh-bone；foritextendsasfarastothemiddleofthebelly。Thereasonforthisisthatthebirdisabiped，andyetisunabletostanderect。Forifitsischiumextendedbutashortwayfromthefundament，andthenimmediatelycametheleg，asisthecaseinmanandinquadrupeds，thebirdwouldbeunabletostandupatall。Forwhilemanstandserect，andwhilequadrupedshavetheirheavybodiesproppedupinfrontbytheforelegs，birdscanneitherstanderectowingtotheirdwarf-likeshape，norhaveanteriorlegstopropthemup，theselegsbeingreplacedbywings。AsaremedyforthisNaturehasgiventhemalongischium，andbroughtittothecentreofthebody，fixingitfirmly；andshehasplacedthelegsunderthiscentralpoint，thattheweightoneithersidemaybeequallybalanced，andstandingorprogressionrenderedpossible。Suchthenisthereasonwhyabird，thoughitisabiped，doesnotstanderect。Whyitslegsaredestituteoffleshhasalsoalreadybeenstated；forthereasonsarethesameasinthecaseofquadrupeds。
Inallbirdsalike，whetherweb-footedornot，thenumberoftoesineachfootisfour。FortheLibyanostrichmaybedisregardedforthepresent，anditsclovenhoofandotherdiscrepanciesofstructureascomparedwiththetribeofbirdswillbeconsideredfurtheron。Ofthesefourtoesthreeareinfront，whilethefourthpointsbackward，serving，asaheel，togivesteadiness。Inthelong-leggedbirdsthisfourthtoeismuchshorterthantheothers，asisthecasewiththeCrex，butthenumberoftheirtoesisnotincreased。Thearrangementofthetoesissuchashasbeendescribedinallbirdswiththeexceptionofthewryneck。Hereonlytwoofthetoesareinfront，theothertwobehind；andthereasonforthisisthatthebodyofthewryneckisnotinclinedforwardsomuchasthatofotherbirds。Allbirdshavetesticles；buttheyareinsidethebody。ThereasonforthiswillbegiveninthetreatiseOntheGenerationofAnimals。
13
Thusthenarefashionedthepartsofbirds。Butinfishesastillfurtherstuntinghasoccurredintheexternalparts。Forhere，forreasonsalreadygiven，thereareneitherlegsnorhandsnorwings，thewholebodyfromheadtotailpresentingoneunbrokensurface。Thistaildiffersindifferentfishes，insomeapproximatingincharactertothefins，whileinothers，namelyinsomeoftheflatkinds，itisspinousandelongated，becausethematerialwhichshouldhavegonetothetailhasbeendivertedthenceandusedtoincreasethebreadthofthebody。Such，forinstance，isthecasewiththeTorpedos，theTrygons，andwhateverotherSelachiatheremaybeoflikenature。Insuchfishes，then，thetailisspinousandlong；whileinsomeothersitisshortandfleshy，forthesamereasonwhichmakesitspinousandlongintheTorpedo。Fortobeshortandfleshycomestothesamethingastobelongandlessamplyfurnishedwithflesh。
WhathasoccurredintheFishing-frogisthereverseofwhathasoccurredintheotherinstancesjustgiven。Forheretheanteriorandbroadpartofthebodyisnotofafleshycharacter，andsoallthefleshysubstancewhichhasbeenthencedivertedhasbeenplacedbynatureinthetailandhinderportionofthebody。
Infishestherearenolimbsattachedtothebody。Forinaccordancewiththeiressentialconstitutiontheyareswimminganimals；andnaturenevermakesanythingsuperfluousorvoidofuse。Nowinasmuchasfishesaremadeswimmingtheyhavefins，andastheyarenotmadeforwalkingtheyarewithoutfeet；forfeetareattachedtothebodythattheymaybeofuseinprogressiononland。Moreover，fishescannothavefeet，oranyothersimilarlimbs，aswellasfourfins；
fortheyareessentiallysanguineousanimals。TheCordylus，thoughithasgills，hasfeet，forithasnofinsbutmerelyhasitstailflattenedoutandlooseintexture。
Fishes，unless，liketheBatosandtheTrygon，theyarebroadandflat，havefourfins，twoontheupperandtwoontheundersideofthebody；andnofisheverhasmorethanthese。For，ifithad，itwouldbeabloodlessanimal。
Theupperpairoffinsispresentinnearlyallfishes，butnotsotheunderpair；forthesearewantinginsomeofthosefishesthathavelongthickbodies，suchastheeel，theconger，andacertainkindofCestreusthatisfoundinthelakeatSiphae。Whenthebodyisstillmoreelongated，andresemblesthatofaserpentratherthanthatofafish，asisthecaseintheSmuraena，thereareabsolutelynofinsatall；andlocomotioniseffectedbytheflexuresofthebody，thewaterbeingputtothesameusebythesefishesasisthegroundbyserpents。Forserpentsswiminwaterexactlyinthesamewayastheyglideontheground。Thereasonfortheseserpent-likefishesbeingwithoutfinsisthesameasthatwhichcausesserpentstobewithoutfeet；andwhatthisishasbeenalreadystatedinthedissertationsontheProgressionandtheMotionofAnimals。Thereasonwasthis。Ifthepointsofmotionwerefour，motionwouldbeeffectedunderdifficulties；foreitherthetwopairsoffinswouldbeclosetoeachother，inwhichcasemotionwouldscarcelybepossible，ortheywouldbeataveryconsiderabledistanceapart，inwhichcasethelongintervalbetweenthemwouldbejustasgreatanevil。Ontheotherhand，tohavemorethanfoursuchmotorpointswouldconvertthefishesintobloodlessanimals。Asimilarexplanationappliestothecaseofthosefishesthathaveonlytwofins。Forhereagainthebodyisofgreatlengthandlikethatofaserpent，anditsundulationsdotheofficeofthetwomissingfins。Itisowingtothisthatsuchfishescanevencrawlondryground，andcanlivethereforaconsiderabletime；anddonotbegintogaspuntiltheyhavebeenforaconsiderabletimeoutofthewater，whileothers，whosenatureisakintothatofland-animals，donotevendoasmuchasthat。Insuchfishesashavebuttwofinsitistheupperpair（pectorals）thatispresent，exceptingwhentheflatbroadshapeofthebodypreventsthis。Thefinsinsuchcasesareplacedatthehead，becauseinthisregionthereisnoelongation，whichmightserveintheabsenceoffinsasameansoflocomotion；whereasinthedirectionofthetailthereisaconsiderablelengtheningoutinfishesofthisconformation。AsfortheBatiandthelike，theyusethemarginalpartoftheirflattenedbodiesinplaceoffinsforswimming。
IntheTorpedoandtheFishing-frogthebreadthoftheanteriorpartofthebodyisnotsogreatastorenderlocomotionbyfinsimpossible，butinconsequenceofittheupperpair（pectorals）areplacedfurtherbackandtheunderpair（ventrals）areplacedclosetothehead，whiletocompensateforthisadvancementtheyarereducedinsizesoastobesmallerthantheupperones。IntheTorpedothetwoupperfins（pectorals）areplacedonthetail，andthefishusesthebroadexpansionofitsbodytosupplytheirplace，eachlateralhalfofitscircumferenceservingtheofficeofafin。
Thehead，withitsseveralparts，asalsotheorgansofsense，havealreadycomeunderconsideration。
Thereisonepeculiaritywhichdistinguishesfishesfromallothersanguineousanimals，namely，thepossessionofgills。WhytheyhavetheseorganshasbeensetforthinthetreatiseonRespiration。
Thesegillsareinmostfishescoveredbyopercula，butintheSelachia，owingtotheskeletonbeingcartilaginous，therearenosuchcoverings。Foranoperculumrequiresfish-spineforitsformation，andinotherfishestheskeletonismadeofthissubstance，whereasintheSelachiaitisinvariablyformedofcartilage。Again，whilethemotionsofspinousfishesarerapid，thoseoftheSelachiaaresluggish，inasmuchastheyhaveneitherfish-spinenorsinew；butanoperculumrequiresrapidityofmotion，seeingthattheofficeofthegillsistoministerasitweretoexpiration。ForthisreasoninSelachiathebranchialorificesthemselveseffecttheirownclosure，andthusthereisnoneedforanoperculumtoensureitstakingplacewithduerapidity。Insomefishesthegillsarenumerous，inothersfewinnumber；insomeagaintheyaredouble，inotherssingle。
Thelastgillinmostcasesissingle。Foradetailedaccountofallthis，referencemustbemadetothetreatisesonAnatomy，andtothebookofResearchesconcerningAnimals。
Itistheabundanceorthedeficiencyofthecardiacheatwhichdeterminesthenumericalabundanceordeficiencyofthegills。For，thegreaterananimal’sheat，themorerapidandthemoreforcibledoesitrequirethebranchialmovementtobe；andnumerousanddoublegillsactwithmoreforceandrapiditythansuchasarefewandsingle。Thus，too，itisthatsomefishesthathavebutfewgills，andthoseofcomparativelysmallefficacy，canliveoutofwaterforaconsiderabletime；forinthemthereisnogreatdemandforrefrigeration。Such，forexample，aretheeelandallotherfishesofserpent-likeform。
Fishesalsopresentdiversitiesasregardsthemouth。Forinsomethisisplacedinfront，attheveryextremityofthebody，whileinothers，asthedolphinandtheSelachia，itisplacedontheundersurface；sothatthesefishesturnonthebackinordertotaketheirfood。ThepurposeofNatureinthiswasapparentlynotmerelytoprovideameansofsalvationforotheranimals，byallowingthemopportunityofescapeduringthetimelostintheactofturning-forallthefisheswiththiskindofmouthpreyonlivinganimals-butalsotopreventthesefishesfromgivingwaytoomuchtotheirgluttonousraveningafterfood。Forhadtheybeenabletoseizetheirpreymoreeasilythantheydo，theywouldsoonhaveperishedfromover-repletion。Anadditionalreasonisthattheprojectingextremityoftheheadinthesefishesisroundandsmall，andthereforecannotadmitofawideopening。
Again，evenwhenthemouthisnotplacedontheundersurface，therearedifferencesintheextenttowhichitcanopen。Forinsomecasesitcangapewidely，whileinothersitissetatthepointofasmalltaperingsnout；theformerbeingthecaseincarnivorousfishes，suchasthosewithsharpinterfittingteeth，whosestrengthliesintheirmouth，whilethelatterisitsforminallsuchasarenotcarnivorous。
Theskinisinsomefishescoveredwithscales（thescaleofafishisathinandshinyfilm，andthereforeeasilybecomesdetachedfromthesurfaceofthebody）。Inothersitisrough，asforinstanceintheRhine，theBatos，andthelike。Fewestofallarethosewhoseskinissmooth。TheSelachiahavenoscales，butaroughskin。Thisisexplainedbytheircartilaginousskeleton。Fortheearthymaterialwhichhasbeenthencedivertedisexpendedbynatureupontheskin。
Nofishhastesticleseitherexternallyorinternally；asindeedhavenoapodousanimals，amongwhichofcourseareincludedtheserpents。Oneandthesameorificeservesbothfortheexcrementandforthegenerativesecretions，asisthecasealsoinallotheroviparousanimals，whethertwo-footedorfour-footed，inasmuchastheyhavenourinarybladderandformnofluidexcretion。
Suchthenarethecharacterswhichdistinguishfishesfromallotheranimals。ButdolphinsandwhalesandallsuchCetaceaarewithoutgills；and，havingalung，areprovidedwithablow-hole；forthisservesthemtodischargethesea-waterwhichhasbeentakenintothemouth。For，feedingastheydointhewater，theycannotbutletthisfluidenterintotheirmouth，and，havingletitin，theymustofnecessityletitoutagain。Theuseofgills，however，ashasbeenexplainedinthetreatiseonRespiration，islimitedtosuchanimalsasdonotbreathe；fornoanimalcanpossiblypossessgillsandatthesametimebearespiratoryanimal。Inorder，therefore，thattheseCetaceamaydischargethewater，theyareprovidedwithablow-hole。
Thisisplacedinfrontofthebrain；forotherwiseitwouldhavecutoffthebrainfromthespine。Thereasonfortheseanimalshavingalungandbreathing，isthatanimalsoflargesizerequireanexcessofheat，tofacilitatetheirmotion。Alung，therefore，isplacedwithintheirbody，andisfullysuppliedwithblood-heat。Thesecreaturesareafterafashionlandandwateranimalsinone。Forsofarastheyareinhalersofairtheyresembleland-animals，whiletheyresemblewater-animalsinhavingnofeetandinderivingtheirfoodfromthesea。Soalsosealsliehalfwaybetweenlandandwateranimals，andbatshalf-waybetweenanimalsthatliveonthegroundandanimalsthatfly；andsobelongtobothkindsortoneither。Forseals，iflookedonaswater-animals，areyetfoundtohavefeet；and，iflookedonasland-animals，areyetfoundtohavefins。Fortheirhindfeetareexactlylikethefinsoffishes；andtheirteethalsoaresharpandinterfittingasinfishes。Batsagain，ifregardedaswingedanimals，havefeet；and，ifregardedasquadrupeds，arewithoutthem。Soalsotheyhaveneitherthetailofaquadrupednorthetailofabird；noquadruped’stail，becausetheyarewintedanimals；nobird’stail，becausetheyareterrestrial。Thisabsenceoftailistheresultofnecessity。Forbatsflybymeansofamembrane，butnoanimal，unlessithasbarbedfeathers，hasthetailofabird；forabird’stailiscomposedofsuchfeathers。Asforaquadruped’stail，itwouldbeanactualimpediment，ifpresentamongthefeathers。
14
MuchthesamemaybesaidalsooftheLibyanostrich。Forithassomeofthecharactersofabird，someofthecharactersofaquadruped。Itdiffersfromaquadrupedinbeingfeathered；andfromabirdinbeingunabletosoaraloftandinhavingfeathersthatresemblehairandareuselessforflight。Again，itagreeswithquadrupedsinhavinguppereyelashes，whicharethemorerichlysuppliedwithhairsbecausethepartsabouttheheadandtheupperportionoftheneckarebare；anditagreeswithbirdsinbeingfeatheredinallthepartsposteriortothese。Further，itresemblesabirdinbeingabiped，andaquadrupedinhavingaclovenhoof；forithashoofsandnottoes。Theexplanationofthesepeculiaritiesistobefoundinitsbulk，whichisthatofaquadrupedratherthanthatofabird。For，speakinggenerally，abirdmustnecessarilybeofverysmallsize。Forabodyofheavybulkcanwithdifficultyberaisedintotheair。
Thusmuchthenasregardsthepartsofanimals。Wehavediscussedthemall，andsetforththecausewhyeachexists；andinsodoingwehaveseverallyconsideredeachgroupofanimals。Wemustnowpasson，andinduesequencemustnextdealwiththequestionoftheirgeneration。