on the gait of animals-第3部分

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plain　that　they；　too；　would　make　the　change　of　place　better　if　they



had　an　even　number；　in　fact　if　none　were　missing　and　they　had　the



limbs　which　correspond　to　one　another。　In　this　way　they　could　equalize



their　own　weight；　and　not　oscillate　to　one　side；　if　they　had



corresponding　supports　instead　of　one　section　of　the　opposite　sides



being　unoccupied　by　a　limb。　A　walking　creature　advances　from　each　of



its　members　alternately；　for　in　this　way　it　recovers　the　same　figure



that　it　had　at　first。



　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　9







　　The　fact　that　all　animals　have　an　even　number　of　feet；　and　the



reasons　for　the　fact　have　been　set　forth。　What　follows　will　explain



that　if　there　were　no　point　at　rest　flexion　and　straightening　would　be



impossible。　Flexion　is　a　change　from　a　right　line　to　an　arc　or　an



angle；　straightening　a　change　from　either　of　these　to　a　right　line。



Now　in　all　such　changes　the　flexion　or　the　straightening　must　be



relative　to　one　point。　Moreover；　without　flexion　there　could　not　be



walking　or　swimming　or　flying。　For　since　limbed　creatures　stand　and



take　their　weight　alternately　on　one　or　other　of　the　opposite　legs；　if



one　be　thrust　forward　the　other　of　necessity　must　be　bent。　For　the



opposite　limbs　are　naturally　of　equal　length；　and　the　one　which　is



under　the　weight　must　be　a　kind　of　perpendicular　at　right　angles　to



the　ground。



　　When　then　one　leg　is　advanced　it　becomes　the　hypotenuse　of　a



right…angled　triangle。　Its　square　then　is　equal　to　the　square　on　the



other　side　together　with　the　square　on　the　base。　As　the　legs　then



are　equal；　the　one　at　rest　must　bend　either　at　the　knee　or；　if　there



were　any　kneeless　animal　which　walked；　at　some　other　articulation。　The



following　experiment　exhibits　the　fact。　If　a　man　were　to　walk　parallel



to　a　wall　in　sunshine；　the　line　described　（by　the　shadow　of　his



head》　would　be　not　straight　but　zigzag；　becoming　lower　as　he　bends；



and　higher　when　he　stands　and　lifts　himself　up。



　　It　is；　indeed；　possible　to　move　oneself　even　if　the　leg　be　not　bent；



in　the　way　in　which　children　crawl。　This　was　the　old　though



erroneous　account　of　the　movement　of　elephants。　But　these　kinds　of



movements　involve　a　flexion　in　the　shoulders　or　in　the　hips。　Nothing



at　any　rate　could　walk　upright　continuously　and　securely　without



flexions　at　the　knee；　but　would　have　to　move　like　men　in　the　wrestling



schools　who　crawl　forward　through　the　sand　on　their　knees。　For　the



upper　part　of　the　upright　creature　is　long　so　that　its　leg　has　to　be



correspondingly　long；　in　consequence　there　must　be　flexion。　For



since　a　stationary　position　is　perpendicular；　if　that　which　moves



cannot　bend　it　will　either　fall　forward　as　the　right　angle　becomes



acute　or　will　not　be　able　to　progress。　For　if　one　leg　is　at　right



angles　to　the　ground　and　the　other　is　advanced；　the　latter　will　be



at　once　equal　and　greater。　For　it　will　be　equal　to　the　stationary



leg　and　also　equivalent　to　the　hypotenuse　of　a　right…angled



triangle。　That　which　goes　forward　therefore　must　bend；　and　while



bending　one；　extend　the　other　leg　simultaneously；　so　as　to　incline



forward　and　make　a　stride　and　still　remain　above　the　perpendicular；



for　the　legs　form　an　isosceles　triangle；　and　the　head　sinks　lower　when



it　is　perpendicularly　above　the　base　on　which　it　stands。



　　Of　limbless　animals；　some　progress　by　undulations　（and　this



happens　in　two　ways；　either　they　undulate　on　the　ground；　like



snakes；　or　up　and　down；　like　caterpillars）；　and　undulation　is　a



flexion；　others　by　a　telescopic　action；　like　what　are　called



earthworms　and　leeches。　These　go　forward；　first　one　part　leading　and



then　drawing　the　whole　of　the　rest　of　the　body　up　to　this；　and　so　they



change　from　place　to　place。　It　is　plain　too　that　if　the　two　curves



were　not　greater　than　the　one　line　which　subtends　them　undulating



animals　could　not　move　themselves；　when　the　flexure　is　extended　they



would　not　have　moved　forward　at　all　if　the　flexure　or　arc　were　equal



to　the　chord　subtended；　as　it　is；　it　reaches　further　when　it　is



straightened　out；　and　then　this　part　stays　still　and　it　draws　up



what　is　left　behind。



　　In　all　the　changes　described　that　which　moves　now　extends　itself



in　a　straight　line　to　progress；　and　now　is　hooped；　it　straightens



itself　in　its　leading　part；　and　is　hooped　in　what　follows　behind。　Even



jumping　animals　all　make　a　flexion　in　the　part　of　the　body　which　is



underneath；　and　after　this　fashion　make　their　leaps。　So　too　flying　and



swimming　things　progress；　the　one　straightening　and　bending　their



wings　to　fly；　the　other　their　fins　to　swim。　Of　the　latter　some　have



four　fins；　others　which　are　rather　long；　for　example　eels；　have　only



two。　These　swim　by　substituting　a　flexion　of　the　rest　of　their　body



for　the　（missing）　pair　of　fins　to　complete　the　movement；　as　we　have



said　before。　Flat　fish　use　two　fins；　and　the　flat　of　their　body　as　a



substitute　for　the　absent　pair　of　fins。　Quite　flat　fish；　like　the　Ray；



produce　their　swimming　movement　with　the　actual　fins　and　with　the



two　extremes　or　semicircles　of　their　body；　bending　and　straightening



themselves　alternately。



　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　10







　　A　difficulty　might　perhaps　be　raised　about　birds。　How；　it　may　be



said；　can　they；　either　when　they　fly　or　when　they　walk；　be　said　to



move　at　four　points？　Now　we　did　not　say　that　all　Sanguinea　move　at



four　points；　but　merely　at　not　more　than　four。　Moreover；　they　cannot



as　a　fact　fly　if　their　legs　be　removed；　nor　walk　without　their



wings。　Even　a　man　does　not　walk　without　moving　his　shoulders。



Everything　indeed；　as　we　have　said；　makes　a　change　of　place　by　flexion



and　straightening；　for　all　things　progress　by　pressing　upon　what　being



beneath　them　up　to　a　point　gives　way　as　it　were　gradually；



accordingly；　even　if　there　be　no　flexion　in　another　member；　there　must



be　at　least　in　the　point　whence　motion　begins；　is　in　feathered



（flying）　insects　at　the　base　of　the　'scale…wing'；　in　birds　at　the　base



of　the　wing；　in　others　at　the　base　of　the　corresponding　member；　the



fins；　for　instance；　in　fish。　In　others；　for　example　snakes；　the



flexion　begins　in　the　joints　of　the　body。



　　In　winged　creatures　the　tail　serves；　like　a　ship's　rudder；　to　keep



the　flying　thing　in　its　course。　The　tail　then　must　like　other　limbs　be



able　to　bend　at　the　point　of　attachment。　And　so　flying　insects；　and



birds　（Schizoptera）　whose　tails　are　ill…adapted　for　the　use　in



question；　for　example　peacocks；　and　domestic　cocks；　and　generally



birds　that　hardly　fly；　cannot　steer　a　straight　course。　Flying



insects　have　absolutely　no　tail；　and　so　drift　along　like　a



rudderless　vessel；　and　beat　against　anything　they　happen　upon；　and



this　applies　equally　to　sharded　insects；　like　the　scarab…beetle　and



the　chafer；　and　to　unsharded；　like　bees　and　wasps。　Further；　birds　that



are　not　made　for　flight　have　a　tail　that　is　of　no　use；　for　instance



the　purple　coot　and　the　heron　and　all　water…fowl。　These　fly　stretching



out　their　feet　as　a　substitute　for　a　tail；　and　use　their　legs



instead　of　a　tail　to　direct　their　flight。　The　flight　of　insects　is



slow　and　frail　because　the　character　of　their　feathery　wings　is　not



proportionate　to　the　bulk　of　their　body；　this　is　heavy；　their　wings



small　and　frail；　and　so　the　flight　they　use　is　like　a　cargo　boat



attempting　to　make　its　voyage　with　oars；　now　the　frailty　both　of　the



actual　wings　and　of　the　outgrowths　upon　them　contributes　in　a



measure　to　the　flight　described。　Among　birds；　the　peacock's　tail　is　at



one　time　useless　because　of　its　size；　at　another　because　it　is　shed。



But　birds　are　in　general　at　the　opposite　pole　to　flying　insects　as



regards　their　feathers；　but　especially　the　swiftest　flyers　among　them。



（These　are　the　birds　with　curved　talons；　for　swiftness　of　wing　is



useful　to　their　mode　of　life。）　The　rest　of　their　bodily　structure　is



in　harmony　with　their　peculiar　movement；　the　small　head；　the　slight



neck；　the　strong　and　acute　breastbone　（acute　like　the　prow　of　a



clipper…built　vessel；　so　as　to　be　well…girt；　and　strong　by　dint　of　its



mass　of　flesh）；　in　order　to　be　able　to　push　away　the　air　that　beats



against　it；　and　that　easily　and　without　exhaustion。　The　hind…quarters；



too；　are　light　and　taper　again；　in　order　to　conform　to　the　movement　of



the　front　and　not　by　their　breadth　to　suck　the　air。



　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　11







　　So　much　then　for　these　questions。　But　why　an　animal　that　is　to　stand



erect　must　necessarily　be　not　only　a　biped；　but　must　also　have　the



superior　parts　of　the　body　lighter；　and　those　that　lie　under　these



heavier；　is　plain。　Only　if　situated　like　this　could　it　possibly



carry　itself　easily。　And　so　man；　the　only　erect　animal；　has　legs



longer　and　stouter　relatively　to　the　upper　parts　of　his　body　than



any　other　animal　with　legs。　What　we　observe　in　children　also　is



evidence　of　this。　Children　cannot　walk　erect　because　they　are　always



dwarf…like；　the　upper　parts　of　their　bodies　being　longer　and　stouter



than　the　lower。　With　advancing　years　the　lower　increase



disproportionately；　until　the　children　get　their　appropriate　size；　and



then　and　not　till　then　they　succeed　in　walking　erect。　Birds　are



hunchbacked　yet　stand　on　two　legs　because　their　weight　is　set　back；



after　the　principle　of　horses　fashioned　in　bronze　with　their



forelegs　prancing。　But　their　being　bipeds　and　able　to　stand　is　above



all　due　to　their　having　the　hip…bone　shaped　like　a　thigh；　and　so　large



that　it　looks　as　if　they　had　two　thighs；　one　in　the　leg　before　the



knee…joint；　the　other　joining　his　part　to　the　fundament。　Really　this



is　not　a　thigh　but　a　hip；　and　if　it　were　not　so　large　the　bird　could



not　be　a　biped。　As　in　a　man　or　a　quadruped；　the　thigh　and　the　rest



of