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CHAPTER X. 



VERTEBRATA CAPABLE OP FLYING. 



§ 1. Verlebrata ivilhout Feathers, for^med for flying. 



Few problems in mechanic art present greater practical 

 difficulties than thft of raising from the ground, and of sus- 

 taining and moving rapidly through the air an animal body^ 

 composed as it must be of many ponderous organs, that are 

 requisite for the performance of the higher functions of life: 

 yet Nature has achieved all this, not only in endless tribes 

 of the more diminutive invertebrate animals, but also in the 

 more solid and massive organizations which are modelled on 

 the vertebrate type. These objects have been accomplished, 

 in all cases, without the employment of any other than the 

 ordinary elements of those organizations; modified, indeed, 

 to suit the particular purpose in view, but yet essentially the 

 same, and regulated by the same laws of development which 

 prevail throughout the whole animal system. The adapta- 

 tion of these elements to the construction of an apparatus of 

 so refined a nature as that which is required for flying, im- 

 plies the deepest foresight, the most extensive plan, and the 

 most artificial combination of means. The foundations for 

 these peculiar forms of mechanism are laid in the primeval 

 constitution of the embryo; and a long and curious series of 

 preparatory changes must take place before the completion 

 of the finished structures. Of this we have already had a 

 remarkable example in the metamorphoses of insects, which 

 exhibit, in their last stage of development, the highest de- 

 gree of perfection compatible with the articulate type. 

 Birds, in like manner, present us with the highest refine- 



