462 NATURE AND MAN. 



But the Bird requires a yet more active respiration than the 

 Mammal : being far higher in point of animal activity. It must 

 put forth far more muscular power in proportion to its size, in 

 order to raise itself in the air; and it must be able to sustain 

 that power for a great length of time. Its animal energy can 

 only be kept up by the maintenance of a higher temperature. 

 All this involves a much larger consumption of oxygen, and a 

 greater production of carbonic acid. Hence you would suppose 

 that if " natural selection " had in any way worked out the respi- 

 ratory apparatus of a Bird, it would be a more highly organized 

 instrument than that of a Mammal. So far, however, is this from 

 being the case, that the lung of the Bird is really formed upon the 

 lower plan of the lung of the Reptile. Instead of having the 

 minutely subdivided air-cells of the Mammalian lung, the lung of 

 the Bird is an aggregation of little lunglets, each resembling the 

 entire lung of the Frog ; and instead of the provision made in 

 the general structure of the Mammal for the constant renewal of 

 the air in the cavity of the lungs, we find the diaphragm absent, 

 and the bony framework of the trunk so firmly knit together (thus 

 affording fixed attachments for the powerful muscles of flight) as 

 to be incapable of the movement which our ribs and sternum 

 perform in aid of the action of the diaphragm. How, then, is 

 the more active respiration required by the Bird provided for ? 

 Just as in the Insect, to which Birds have so many analogies, — 

 by the extension of the respiratory surface through the body 

 generally. The long bones, instead of being filled with marrow, 

 are hollow ; and their cavities are connected with each other and 

 with that of the lung on either side : there are also air-sacs dis- 

 posed in various parts, which probably take a share in the same 

 action. Further, by the elasticity of the framework of the trunk, 

 the lungs are kept full of air, the state of emptiness being forced ; 

 so that when they have been compressed by a muscular effort, 

 they fill themselves again spontaneously as soon as the pressure is 

 relaxed. 



Thus, looking at the general plan of the respiratory apparatus, 

 we find it undergoing a uniformly progressive elevation of type, as 

 we pass from the Fish to the Reptile, from the Reptile to the Bird, 

 and from the Bird to the Mammal. But if there was no pre- 



