52 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



instruments for the agency of light and for the production of organic 

 compounds. The whole nisus of Vegetable life may be considered as 

 manifested in this production ; and, in effecting it, each organism is not 

 only drawing material, but force, from the universe around it. Sup- 

 posing that no Animals existed to consume these organic compounds, 

 they would be all at last restored to the inorganic condition by sponta- 

 neous decay, which would reproduce the carbonic acid, water, and am- 

 monia, from which they were generated. In this decay, however slow, 

 heat and light are given out, in the same amount as when more evidently 

 produced in the ordinary combustive process ; and this sometimes occurs 

 even during the life of the plant, whose vital movements, also, may be 

 considered as restoring to the Inorganic universe a certain measure of 

 the force they have derived from it under other forms. Thus in making 

 use of the stores of Coal which have been prepared for his wants by the 

 luxuriant Flora of past ages, Man is not only restoring to the atmo- 

 sphere the carbonic acid, the water, and the ammonia, of the Carboni- 

 ferous period, but is actually reproducing and applying to his own 

 purposes, the Light and Heat which were operating to produce the 

 growth of vegetation at that remote period in the Earth's history. 



63. But the organic compounds which the agency of Light and Heat 

 upon the Vegetable structures has produced, are designed for a much 

 higher purpose, than that of being merely given back to the Inorganic 

 universe by decay or combustion ; and the forces which hold together 

 their elements have a much more exalted destiny. In serving as the 

 food of Animals, a part of them become the materials of their organized 

 tissues, and the instruments through which the nervous and muscular 

 forces are developed; whilst another part are applied to sustain the 

 combustive process, by which the heat of the higher classes is main- 

 tained quite independently of the external supply of that force. The 

 greater part of the Animal kingdom, however, is dependent, like the 

 Vegetable, upon the Inorganic Universe, for the Heat which serves as 

 its organizing force ; and it is only under the constant influence of this 

 agent, that the operations of growth, development, and maintenance 

 can take place. The Animal is not dependent like the Plant upon Light ; 

 and this is obviously because that agent is chiefly concerned in that 

 preliminary operation, by which the organic compounds are generated 

 as the pabulum of the growing tissues ; in fact, the embryo within the 

 germinating seed, which, like the animal, is nourished upon matter 

 previously prepared for it, is most rapidly developed in the absence of 

 light, up to the time when, its store being exhausted, its further sup- 

 plies must be obtained by its own instrumentality. The Vital activity 

 of Animals, then, may be considered as chiefly sustained by the Che- 

 mical forces subsisting in their food, which are set free when the ele- 

 ments are reconverted to their original state ; and by the Heat which 

 they derive from external sources, or from the combustion of a part of 

 their food. These forces may be considered as in a state of continual 

 restoration to the Inorganic Universe, during the whole life of Animals, 

 in the heat, light, electricity, still more in the motion, which they deve- 

 lope ; and, after their death, in the production of heat and light during 

 the processes of decay. During Animal life, there is a continual resto- 



