132 THE MECHANICAL FUNCTIONS. 



whole period of its existence, is fixed to the spot 

 where it was first produced, and is dependent for 

 the continuance of its life on local circumstances ; 

 such as the nature of the soil in which its roots are 

 embedded, and the qualities of the air and water 

 in its immediate vicinity. It is exposed to the 

 action of the surrounding elements, and affected by 

 their vicissitudes, without the means of retreat, and 

 without the power of reaction. With respect to all 

 external agents, indeed, vegetables may be regarded 

 as passive beings. Very different are the condi- 

 tion and destination of animals. Excepting a few 

 among the lower orders of the creation, such as 

 Zoophytes and Mollusca, all animals are gifted 

 with the power of spontaneously changing their 

 situation, according to their several wants and ne- 

 cessities, and are thus enabled to seek and to choose 

 those objects which are salutary, and to avoid or 

 reject those which are injurious. Nature has, for 

 these purposes, furnished them with a more com- 

 plex 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 animals, 

 according to the particular model on which they 

 are constructed, and the relations which their or- 

 ganization bears to the element assigned as their 

 residence. All the mechanical circumstances in 

 their economy, indeed, are so closely linked toge- 

 ther, as scarcely to admit of being considered sepa- 

 rately. Thus we find, in one animal, a variety of 

 mechanical effects accomplished by one and the 

 same instrument; while, in others, they are each 



