144 THE MECHANICAL FUNCTIONS. 



organization and more varied powers, adapted to 

 a greater diversity of pursuits, and to a higher 

 and more expanded sphere of existence. 



The power of progressive motion is enjoyed in 

 very different degrees by different races of ani- 

 mals, according to the particular model on 

 which they are constructed, and the relations 

 which their organization bears to the element 

 assigned as their residence. All the mechanical 

 circumstances in their economy, indeed, are so 

 closely linked together, as scarcely to admit of 

 being considered separately. Thus we find, in 

 one animal, a variety of mechanical effects ac- 

 complished by one and the same instrument; 

 while, in others, they are each produced by a 

 separate and distinct organ. In some, the lead- 

 ing principle of the construction is simplicity ; 

 in others, the most elaborate mechanism is dis- 

 played. But the means have constant reference 

 to the design, and are ever varied in exact 

 conformity with the change of purpose. The 

 relative advantages of each plan of structure 

 appear to have been carefully estimated, and 

 studiously balanced. Each quality has been 

 bestowed in different degrees of perfection ; so 

 that in following the series of gradation among 

 the successive tribes of animals, we occasion- 

 ally meet with favoured species, endowed with 

 great superiority in some particular faculty. 

 Some animals excel in swiftness ; others in 



