98 



time, as these internal conditions of animated existence aie 

 severally dependent on certain external conditions, which 

 again are not always fully and equally supplied ; so it 

 follows, that the life of every organized being is determined 

 in its amount, and in the direction of its development, by 

 tlie outward circumstances of its individual situation. For 

 this reason we see, that every animal, and every plant is 

 dependent for its existence, and also for its perfect existence, 

 on conditions both internal and external. 



From this reasoning it may be conceived, how the several 

 parts of the living whole reciprocally act and react. They 

 are, in fact, cause and effect mutually ; and no one can 

 precede another, either in the order of nature, or of time. 

 Thus, in an animal, the digestive and the absorbent, the 

 sanguineous, the respiratory, and the nervous systems are 

 at once relative and correlative. In like manner, in a plant, 

 the same reciprocal pro})ortion is found to hold between the 

 roots and the stem, the branches and the leaves : each 

 modifies and determines the existence of all the others, and 

 is equally affected by all, in its turn. And as their several 

 parts, by means of their union, constitute the organic whole ; 

 and as their functions, by the same means, realize the com- 

 plement of life, which the plant or animal exhibits ; so it is 

 evident, that every living individual is a necessary system, 

 in which no one part can be affected, without affecting the 

 other parts, and throughout which there reigns an intimate 

 sympathy, and a complete harmony of perfection and im- 

 perfection. 



Further ; the external conditions of this internal develop- 

 ment of plants and animals, are food, air, heat, and probably 

 water ; while light, according to most physiologists, seems 

 to be a peculiar condition, indispensably necessary to plants.* 

 Where any one of these conditions is not supplied, the 



* Note HI. 



