on the gait of animals-第3章

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and　behind；　but　that　is　only　because　they　are　narrow　in　breadth。　Even。



in　them　the　right　is　the　sovereign　part；　and　there　is　an　alternate



correspondence　behind；　exactly　as　in　quadrupeds。　The　reason　of　their



flexions　is　their　great　length；　for　just　as　tall　men　walk　with　their



spines　bellied　（undulated）　forward；　and　when　their　right　shoulder　is



leading　in　a　forward　direction　their　left　hip　rather　inclined



backwards；　so　that　their　middle　becomes　hollow　and　bellied



（undulated）；　so　we　ought　to　conceive　snakes　as　moving　in　concave



curves　（undulations）　upon　the　ground。　And　this　is　evidence　that　they



move　themselves　like　the　quadrupeds；　for　they　make　the　concave　in



its　turn　convex　and　the　convex　concave。　When　in　its　turn　the　left　of



the　forward　parts　is　leading；　the　concavity　is　in　its　turn　reversed；



for　the　right　becomes　the　inner。　（Let　the　right　front　point　be　A；



the　left　B；　the　right　hind　C；　the　left　D。）



　　Among　land　animals　this　is　the　character　of　the　movement　of



snakes；　and　among　water　animals　of　eels；　and　conger…eels　and　also



lampreys；　in　fact　of　all　that　have　their　form　snakelike。　However；　some



marine　animals　of　this　shape　have　no　fin；　lampreys　for　example；　but



put　the　sea　to　the　same　use　as　snakes　do　both　land　and　water　（for



snakes　swim　precisely　as　they　move　on　the　ground）。　Others　have　two



fins　only；　for　example　conger…eels　and　eels　and　a　kind　of　cestreus



which　breeds　in　the　lake　of　Siphae。　On　this　account　too　those　that　are



accustomed　to　live　on　land；　for　example　all　the　eels；　move　with



fewer　flexions　in　a　fluid　than　on　land；　while　the　kind　of　cestreus



which　has　two　fins；　by　its　flexion　in　a　fluid　makes　up　the　remaining



points。



　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　8







　　The　reason　why　snakes　are　limbless　is　first　that　nature　makes



nothing　without　purpose；　but　always　regards　what　is　the　best



possible　for　each　individual；　preserving　the　peculiar　essence　of



each　and　its　intended　character；　and　secondly　the　principle　we　laid



down　above　that　no　Sanguineous　creature　can　move　itself　at　more　than



four　points。　Granting　this　it　is　evident　that　Sanguineous　animals　like



snakes；　whose　length　is　out　of　proportion　to　the　rest　of　their



dimensions；　cannot　possibly　have　limbs；　for　they　cannot　have　more　than



four　（or　they　would　be　bloodless）；　and　if　they　had　two　or　four　they



would　be　practically　stationary；　so　slow　and　unprofitable　would



their　movement　necessarily　be。



　　But　every　limbed　animal　has　necessarily　an　even　number　of　such



limbs。　For　those　which　only　jump　and　so　move　from　place　to　place　do



not　need　limbs　for　this　movement　at　least；　but　those　which　not　only



jump　but　also　need　to　walk；　finding　that　movement　not　sufficient　for



their　purposes；　evidently　either　are　better　able　to　progress　with　even



limbs　or　cannot　otherwise　progress　at　all　every　animal　which　has　limbs



must　have　an　even　us　for　as　this　kind　of　movement　is　effected　by



part　of　the　body　at　a　time；　and　not　by　the　whole　at　once　as　in　the



movement　of　leaping；　some　of　the　limbs　must　in　turn　remain　at　rest；



and　others　be　moved；　and　the　animal　must　act　in　each　of　these　cases



with　opposite　limbs；　shifting　the　weight　from　the　limbs　that　are　being



moved　to　those　at　rest。　And　so　nothing　can　walk　on　three　limbs　or　on



one；　in　the　latter　case　it　has　no　support　at　all　on　which　to　rest



the　body's　weight；　in　the　former　only　in　respect　of　one　pair　of



opposites；　and　so　it　must　necessarily　fall　in　endeavouring　so　to　move。



　　Polypods　however；　like　the　Centipede；　can　indeed　make　progress　on　an



odd　number　of　limbs；　as　may　be　seen　by　the　experiment　of　wounding



one　of　their　limbs；　for　then　the　mutilation　of　one　row　of　limbs　is



corrected　by　the　number　of　limbs　which　remain　on　either　side。　Such



mutilated　creatures；　however；　drag　the　wounded　limb　after　them　with



the　remainder；　and　do　not　properly　speaking　walk。　Moreover；　it　is



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　h