RESPIRATION IN BIRDS. 331 



on the plan of those of quadrupeds, they must 

 have been twice as large to obtain the same 

 amount of aeration in the blood ; and conse- 

 quently must have been twice as heavy, which 

 would have been a serious inconvenience in an 

 animal formed for flying*. The diffusion of so 

 large a quantity of air throughout the body of 

 animals of this class presents an analogy with 

 a similar purpose apparent in the conformation 

 of insects, w^here the same object is effected by 

 means of tracheae!. 



Thus has the mechanism of respiration been 

 varied in the different classes of animals, and 

 adapted to the particular element, and mode of 

 life designed for each. Combined with the 

 peculiar mode of circulation, it affords a tole- 

 rably accurate criterion of the energy of the 



* I must mention, however, that the correctness of this view 

 of the subject is contested by Dr. Macartney, who thinks it 

 probable that the ab, on its return from the large air-cells, passes 

 directly by the large air-holes into the bronchia, and is not 

 pirought a second time into contact with the blood. 



f The peculiarities of structure in the respiratory system of 

 birds have probably a relation to the capability we see them 

 possess, of bearing with impunity, very quick and violent changes 

 of atmospheric pressure. Thus the Condor of the Andes is 

 often seen to descend rapidly from a height of above 20,000 

 feet, to the edge of the sea, where the air is more than twice the 

 density of that which the bird had been breathing. We are ag 

 yet unable to trace the connexion which probably exists between 

 the structure of the lungs, and this extraordinary power of accom- 

 modation to such great and sudden variations of atmospheric 

 pressure. 



