188 QUARTERLY JOURNAL. ^m2 



Animals cannot produce organic from inorpjanic substances. 

 All the organic matter entering into their composition is derived 

 primarily from vegetables, then converted into organized tissue, and 

 finally decomposed and restored to the atmosphere in the form in 

 which it entered the plant. 



The vegetable kingdom is, then, the grand laboratory in which 

 the food of animals is prepared. 



Herbivorous animals live on organic matters as found in vege- 

 tables. Carnivorous animals live on the flesh of the former ; so 

 that ultimately, both classes draw their nourishment from vegeta- 

 bles. 



Just as all the elements of the proximate principles of plants 

 once existed in the atmosphere in the form of water, carbonic acid 

 and ammonia ; so all the organic tissues of animals once existed 

 in vegetables. Thus we trace the matter of which animals are 

 formed, to the vegetable kingdom, and this again to the atmos- 

 phere ; so that ultimately we find that all the matter now making 

 part of vegetables and animals, may be traced to the atmosphere, 

 where it existed in the form of water, carbonic acid and ammonia. 



If we would determine what was the composition of the atmos- 

 phere before plants and animals existed, we must restore to it all 

 the carbon, oxygen, hydrogen and nitrogen now existing in living 

 animals and vegetables, and in their remains, whether fossil or 

 others. The result of such a calculation would not, however, be 

 altogether true, for the reason that carbonic acid and ammonia are 

 constantly passing into the atmosphere from volcanic and other 

 sources, and it is impossible to estimate the activity of these agen- 

 cies in past times. 



The great problem of nutrition may be thus stated : To trace 

 the water, carbonic acid and ammonia through the various pro- 

 cesses in the vegetable kingdom, by which they are converted into 

 organic matters; to trace these organic matters through the pro- 

 cesses in the bodies of animals by which they are converted into 

 organized tissues; and finally, to trace this same vegetable and 

 animal matter in its decompositions until it is restored to the atmos- 

 phere in the form of water, carbonic acid and ammonia. 



These preliminary observations were necessary to prepare for 

 the explanation of the process of nutrition in animals. 



AH the organized tissues of animals contain the four elements. 





