548 THE MECHANICAL FUNCTIONS. 



has a singularly constructed apparatus, which 

 appears like two wings, affixed to the sides of 

 the back, and quite independent of either the 

 fore or the hind extremities. By the aid of 

 these moveable flaps, the animal is able to de- 

 scend from the tops of trees, or flutter lightly 

 from branch to branch : but this is the utmost 

 that it can accomplish by means of these imper- 

 fect organs. The construction of these anoma- 

 lous members is highly curious in a physiolo- 

 gical point of view ; as showing how Nature, in 

 effecting a new purpose, is inclined to resort to 

 the modification of structures already established 

 as constituent parts of the frame, in preference 

 to creating new organs, or such as have no pro- 

 totype in the model of its formation. Frequent 

 proofs of this law, indeed, are afforded by the 

 comparative examination of the anatomy of the 

 organs of progressive motion. The ribs, in par- 

 ticular, are often the subject of these conversions 

 to uses very different from their ordinary func- 

 tion, which is that of assisting in respiration. 

 Thus we have seen that in the Tortoise they are 

 expanded to form the carapace, uniting with 

 corresponding dilatations of the sternum, and 

 sterno-costal appendages, in composing a ge- 

 neral osseous encasement to the body. In Ser- 

 pents, again, the ribs are employed as organs of 

 progressive motion ; performing the functions of 

 legs, and having affixed to their extremities the 



