PROGRESSION OF ANIMALS, vii.-viii. 



way forward, their left hip indines towards the rear 

 and the middle of the body becomes concave and 

 hollow, so we must suppose that snakes too move 

 upon the ground with their backs hollowed. And 

 that they move in the same manner as quadrupeds 

 is shown by the fact that they change the concave 

 into the convex and the convex into the concave. 

 For when the left forward point is again leading 

 the way, the concavity comes in turn on the other 

 side, for the right again becomes the inner. Let 

 the front point on the right be A, and that on the 



left B, and the rear point on the right C, and that on 

 the left D. 



This is the way that snakes move as land-animals, 

 and eels, conger-eels and lampreys and all the 

 other snake-like creatures as water-animals. Some 

 water-animals, however, of this class, lampreys for 

 example, have no fin and use the sea as snakes 

 use both the sea and the land ; for snakes swim in 

 just the same manner as when they move on land. 

 Others have two fins only, conger-eels for example, 

 and ordinary eels and a species of mullet which occurs 

 in the lake at Siphae." For this reason too those 

 which are accustomed to live on land, the eels for 

 example, move with fewer bends in the water than 

 on dry land. The kind of mullet which has only two 

 fins makes up the number of four points in the water 

 by its bends. ^ VIII. The reason why snakes are 

 footless is, first, that nature creates nothing without 



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