PLANTS AND ANIMALS. 227 



of a more rational mode of feeding stock has become from 

 year to year more conspicuous. 



Liebig, on the other hand, although in full sympathy with 

 the final object of the investigation of Boussingault, differed 

 from him to some extent in regard to the means to gain the 

 same end ; i.e., to establish a rational system of feeding our 

 domesticated animals. His extensive and careful examina- 

 tions into the composition of the liquid and solid constitu- 

 ents of the vegetable and the animal organism, and their 

 relation to the support of life, gave him a closer insight into 

 the mutual relation of vegetable food to animal life than any 

 previous experimenter could claim. He taught already at 

 an early date (1838 to 1842) that " the organic constituents 

 of plants and animals might be properly divided into two 

 great classes, — nitrogenous and non-nitrogenous substances. 

 Judging from the great similarity of the representative com- 

 pounds of each class of these substances, it was but reason- 

 able to assume that the proximate constituents of the animal 

 system are already formed in the plants. 



" In the natural order, the plant precedes the animal. The 

 constituents of the blood, and consequently of the entire 

 body, which is formed and supported by the former, are 

 already, in some form or other, present in the plant. The 

 latter builds up from a single order of combination the most 

 complex organic compounds : the animal system reduces 

 them again to more elementary or less complicated sub- 

 stances. 



"The nitrogenous substances serve for the formation of 

 the blood and the tissues, and may be characterized as the 

 plastic constituents of the food ; whilst the non-nitrogenous 

 substances (carbo-hydrates of the chemists), as starch, sugar, 

 gum, fat, cellular matter, &c, which support the process of 

 respiration, furnishing thereby a source of animal heat, may 

 be named heat-making, or respiratory substances. 



" The particular fitness of any plant, or any part of plants, 

 to support animal life, stands in a direct relation to that 

 amount of their organic and inorganic blood-constituents 

 which are soluble in water, or are rendered soluble within 

 the digestive organs of the animal by means of the intesti- 

 nal liquid secretions. 



" There is no essential difference in the support of life 



