on the gait of animals-第3部分

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　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　7







　　It　is　clear　then　how　locomotion　belongs　to　those　animals　only



which　make　their　changes　of　place　by　means　of　two　or　four　points　in



their　structure；　or　to　such　animals　par　excellence。　Moreover；　since



this　property　belongs　almost　peculiarly　to　Sanguineous　animals；　we　see



that　no　Sanguineous　animal　can　progress　at　more　points　than　four；



and　that　if　it　is　the　nature　of　anything　so　to　progress　at　four　points



it　must　of　necessity　be　Sanguineous。



　　What　we　observe　in　the　animal　world　is　in　agreement　with　the　above



account。　For　no　Sanguineous　animal　if　it　be　divided　into　more　parts



can　live　for　any　appreciable　length　of　time；　nor　can　it　enjoy　the



power　of　locomotion　which　it　possessed　while　it　was　a　continuous　and



undivided　whole。　But　some　bloodless　animals　and　polypods　can　live　a



long　time；　if　divided；　in　each　of　the　severed　parts；　and　can　move　in



the　same　way　as　before　they　were　dismembered。　Examples　are　what　is



termed　the　centipede　and　other　insects　that　are　long　in　shape；　for



even　the　hinder　portion　of　all　these　goes　on　progressing　in　the　same



direction　as　before　when　they　are　cut　in　two。



　　The　explanation　of　their　living　when　thus　divided　is　that　each　of



them　is　constructed　like　a　continuous　body　of　many　separate　living



beings。　It　is　plain；　too；　from　what　was　said　above　why　they　are　like



this。　Animals　constructed　most　naturally　are　made　to　move　at　two　or



four　points；　and　even　limbless　Sanguinea　are　no　exception。　They　too



move　by　dint　of　four　points；　whereby　they　achieve　progression。　They　go



forward　by　means　of　two　flexions。　For　in　each　of　their　flexions



there　is　a　right　and　a　left；　both　before　and　behind　in　their　flat



surface；　in　the　part　towards　the　head　a　right　and　a　left　front



point；　and　in　the　part　towards　the　tail　the　two　hinder　points。　They



look　as　if　they　moved　at　two　points　only；　where　they　touch　before



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　s