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flexion, the elasticity of the water giving them 

 thus the force of a projectile. Some fishes, lastly, 

 as the flying kinds, are capable of using their long 

 fins in the air, almost in the manner of the wings 

 of birds, while some birds, on the other hand, 

 dive and swim under the water almost as well as 

 fishes ; and a most beautiful sight is thus some- 

 times presented to mariners, of whole flocks of 

 these two classes of animals alternately exchang- 

 ing, as it were, their natural elements, the one 

 with the other. 



Reptiles in general make progress on land 

 either exclusively by crawling, as tortoises and 

 most lizards, or by either crawling or leaping as 

 frogs ; and the manner in which some of them, 

 as lizards, are enabled to move up a perpendicular 

 surface is still by a species of suction, the soles of 

 their feet being provided with a series of soft plaits, 

 which being drawn up at pleasure, produce the 

 requisite vacuum. The muscles of these animals 

 are, in general, of a redder colour than those of 

 fishes ; but they are, for the most part, still with- 

 out tendons. Some reptiles again, as serpents, 

 advance, like eels on land, chiefly by the motions 

 of their spine ; but they assist them by those of 

 their ribs which are, in them, organs rather of 

 loco-motion than of respiration being at inter-, 

 vals raised, or advanced like legs, and the rest of 

 the body afterwards drawn towards them. Some 

 lizards also, as the salamander, seem to advance 

 at least as much by the motions of their spine, 

 as by those of their legs ; while others, as the fly- 



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