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ing lizard, use their ribs as organs of progression, 

 not, however, like serpents, in the manner of 

 legs, but in the manner of wings. The flying li- 

 zard is, however, a very harmless little being, 

 and quite unworthy to have given origin to the 

 dreadful stories of fiery dragons so common in 

 nursery books. In the water, most reptiles use 

 their legs almost in the same way as fishes in ge- 

 neral do their fins ; and some of them, as turtles, 

 keep themselves afloat by a collection of air be- 

 low their dorsal shield. The motions of a frog 

 in the water, furnish a very good lesson to man 

 of the manner in which he should use his limbs 

 in a similar situation. 



We have seen that fishes in general move but 

 in the water, but that some are capable of ad- 

 vancing either on land or in the air ; and, that 

 reptiles in general move equally well either in 

 the water or on land, but that some of them also 

 are capable of flying ; and the same is the case 

 with birds. When on land, their progression is 

 effected by either walking or hopping on their 

 posterior extremities only, birds alone being the 

 only proper bipeds among the lower animals ; 

 and they are enabled to keep themselves erect 

 without effort, since their centre of gravity cor- 

 responds to the region where the anterior extre- 

 mities are attached, owing, in most birds, to the 

 legs being directed forwards, and the toes more 

 elongated, but, in some, as the penguin and the 

 puffin, to the trunk of the body being placed al- 

 most vertically. The manner also in which they 



