PROGRESSION OF ANIMALS, ix. 



body is large, and therefore the leg must be long ; and 

 if this is so, there must necessarily be a bending. For 

 since a standing position is perpendicular, the leg which 

 is moved forΛvard, if it is to be unbent, will either fall 

 as the right angle becomes less, or else it Avill not ad- 

 vance at all ; for if, while one leg is at right angles, the 

 other is advanced, the advanced leg will be greater and 

 at the same time equal ; for it will be equal to the leg 

 which is at rest and also to the side subtending the 

 right angle.'' The advancing leg must therefore be 

 bent, and the animal, as it bends it, must at the same 

 time stretch the other leg and lean forward and make 

 a stride and remain in the perpendicular ; for the legs 

 form an isosceles triangle and the head becomes 

 loAver when it is perpendicular to the base of the 

 triangle.* 



Of animals which are footless, some advance with 

 an undulating motion — this can be of two kinds, for 

 some animals, for example snakes, make their bends 

 on the ground, while others, for instance caterpillars, 

 make them upwards — and undulation is bending. 

 Others move by craΛvling, like the earthAvorms and 

 leeches ; for these advance with one part leading the 

 way, and then draw up all the rest of their body to it, 

 and in this manner make the change from place to 

 place. It is plain that, if the two lines which they 



* When the stride has been completed the result is an 

 isosceles triangle formed by the two legs and the ground ; the 

 head, which is necessarily lower than when the legs were 

 together, is perpendicularly above the base. 



513 



