natural history, there are certain conventional terms that may require explanation before the words used in describing an object are intelligible to the uninitiated.

The Bird is an inhabitant of the air in the fullest sense of the expression. The atmosphere is emphatically the sphere of its activity; it mounts it as it would a ladder; it sails through it in triumph, and rides upon the winds as upon a fleet steed. Moreover, it is the atmosphere itself which endows the feathered Ariel with such capabilities, and it is in the perfection of his respiration that we must search for an explanation of his wonderful achievements.