422 LAWS OF MOTION. 



matter of which the universe is composed, is 

 not more marked, than in the living beings by 

 which it is inhabited ; between those which 

 inhabit the earth only, and those which 

 are amphibious, of earth and of water, be- 

 tween those that exist in water only, and 

 those that exist in water and in air together. 



The gradual difference which exists in the 

 place of their abode, is as evident as in the 

 organisation and form of which they are seve- 

 rally constituted; I shall select, by way of il- 

 lustration, a few of the organs in each. In the 

 organs of respiration, the gradation which 

 exists may easily be traced, from the simplicity 

 in the fabric of the common trachea, or air cell, 

 of the vegetable, to the branchiae, or gills, of 

 fish; from the branchiae, or gills, offish, to the 

 existence of branchiae and lungs, subsisting to- 

 gether, in one and the same subject, as in the 

 syren, until we arrive at lungs only. 



The difference may likewise be traced in the 

 construction of the lungs, from the most simple, 

 to those of the more complicated animals ; from 

 lungs which consist of large cells, with a small 

 absorbing surface, as in the amphibia, to the 

 higher order of animals, that possess lungs with 

 small cells, and a widely extended surface. 



This gradation in the respiratory organs of 

 different beings, may be traced to the different 

 organs by which the blood is conveyed. In 



