76 CIRCULATION AND RESPIRATION. 



intensity of action generallr, especially in the male 

 birds ; under the latter it displays itself more in 

 change of place. Not a little of the character of 

 birds, both in themselves, and also as they are by 

 analogy (which they are largely and accurately), 

 indices to the rest of nature, might be worked out 

 upon this principle ; but, even though the length of 

 this section did not threaten to exceed all wonted 

 bounds, in treating on the same subject, this is not 

 exactly the place for it. We still want the elements, 

 in the accounts of the modifications of character by 

 the feet and the wings, and also of the bill, as the 

 grand prehensile organ in the feeding system. We 

 shall therefore only further remark that much of the 

 character of the different tribes depends on the varied 

 development of this seasonal energy; and that the 

 character thus produced is much modified, according 

 as the energy is more marked off in the locality which 

 the bird inhabits when it is at first exerted, or 

 prompts it to migration to a distant region. 



CIRCULATION AND RESPIRATION. 



The systems of circulation and respiration, in birds 

 as in other animals, are closely connected, though not 

 so much so as in the mammalia. The circulation in 

 both is performed by means of a double heart, con- 

 eisting of two ventricles for propelling the blood, and 

 two auricles for receiving that fluid on its return ; 

 and connected with these, as in all animals that have 

 the heart double, there are two sets both of arteries 

 and of veins, — a pulmonary set and a systematic one. 

 The circulation is, as has been hinted already, more 

 rapid in birds than in the mammalia, which agrees 

 with the greater violence and longer continuance of 

 some of their actions. But though these more violent 



