166 THE INTERCHANGE OF CROPS. 



A number of substances contained in the food of 

 animals pass through their alimentary organs with- 

 out change, and are expelled from the system ; 

 these are excrements but not excretions. Now a 

 part of such excrementitious matter might be as- 

 similated in passing through the digestive apparatus 

 of another animal. The organs of secretion form 

 combinations of which only the elements were con- 

 tained in the food. The production of these new 

 compounds is a consequence of the changes which 

 the food undergoes in becoming chyle and chyme, 

 and of the further transformations to which these 

 are subjected by entering into the composition of 

 the organism. These matters, likewise, are elimi- 

 nated in the excrements, which must therefore con- 

 sist of two different kinds of substances, namely, of 

 the indigestible constituents of the food, and of the 

 new compounds formed by the vital process. 

 The latter substances have been produced in con- 

 sequence of the formation of fat, muscular fibre, 

 cerebral and nervous substance, and are quite inca- 

 pable of being converted into the same substances 

 in any other animal organism. 



Exactly similar conditions must subsist in the 

 vital processes of plants. When substances, which 

 are incapable of being employed in the nutrition of 

 a plant, exist in the matter absorbed by its roots, 

 they must be again returned to the soil. Such ex- 

 crements might be serviceable and even indispens- 

 able to the existence of several other plants. But 



