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support themselves while asleep upon a perch, 

 furnishes a beautiful example of adaptation of 

 the most simple means to a necessary end ; the 

 whole mechanism consists in making the mus- 

 cles which close the claws, pass in such a man- 

 ner over the joints of the knee and heel, that 

 upon the mere bending of their joints, they are 

 put upon the stretch without even the conscious- 

 ness of the animal. The muscles of birds are of 

 a redder colour and firmer consistence than those 

 of any other animals, and they are furnished in 

 general with very strong tendons, which, as age 

 advances, are very liable, particularly in the aqua- 

 tic and gallinaceous birds, to become converted 

 into bone. Birds are enabled to float in water 

 owing to their specific gravity being in general 

 less than that of this fluid, and hence they dis- 

 place only as much of it as is equal to their own 

 weight, according to the well known hydrostati- 

 cal law j and they move along its surface by the 

 action of their webbed feet, the swan appearing 

 to use its wings, in addition, almost in the man- 

 ner of sails. But the characteristic organ of lo- 

 co-motion in birds, as in insects, is their wings, 

 corresponding in their more essential parts, as 

 well with the pectoral fins of fishes, as with the 

 fore legs of reptiles and quadrupeds, and the arms 

 of man. The motions of these are effected by a 

 mass of muscles weighing more than all the rest 

 of the muscular system of the animal put toge- 

 ther, and arising from a breast-bone of a larger 

 size than is to be met with in any other class of ani- 



