RAPIDITY OF THE CIRCULATION. 77 



actions, such as coursing on two feet, as fleetly as 

 antelopes do on four and with the aid of the flexible 

 spine and its muscles, as in the ostrich, plunging into 

 thervvater like the gannet or the cormorant, dashing 

 through that element like the divers, cleaving the air 

 beyond comparison with all terrestrial speed, as in 

 the falcon, the swift, or the pratincole, or breasting 

 the tempest with the majesty of the eagle, require, 

 and are furnished with a supply of blood, proportional 

 to the waste which their great energy must occasion ; 

 yet they are by no means so well suited to an equally 

 rapid breathing by means of lungs. But the applica- 

 tion of renovating air to the blood must, in all animals, 

 be proportional to the circulation , and, among ver- 

 tebrated animals, it is only the reptiles and fishes 

 which have the temperature low and the circulation 

 lagging, and which spend much of their time in a 

 state of comparative inaction, that can carry on their 

 systems in a healthy state with only a partial aeration 

 of the blood. 



If the subject is considered according to our plans 

 in contriving and executing, there is thus a difficulty 

 to be overcome in the case of the birds, similar to 

 which nothing occurs in that of any of the other ver- 

 tebrated animals. They stand more in need of the 

 action of the air than any other animals ; and their 

 habits are such that they are less able to bear even 

 the same action, by means of the ordinary apparatus 

 of lungs. 



Now this is one of those difficulties which human 

 wisdom could never see the means of overcoming, 

 except in the accomplishment of the very object, the 

 means of accomplishing which are the subject of 

 inquiry ; and therefore it is wholly above the reach 

 of the human powers, and in itself a perfect demon- 



