166 THE ALTERNATION OF CROPS. 



er these excrements, in the state in which they are 

 expelled, are capable of being employed as food by 

 other plants. 



The excrements of a carnivorous animal contain 

 no constituents fitted for the nourishment of another 

 of the same species ; but it is possible that an her- 

 bivorous animal, a fish, or a fowl, might find in them 

 undigested matters capable of being digested in 

 their organism, from the very circumstance of their 

 organs of digestion having a different structure. 

 This is the only sense in which we can conceive that 

 the excrements of one animal could yield matter 

 adapted for the nutrition of another. 



A number of substances contained in the food of 

 animals pass through their alimentary organs without 

 change, and are expelled from the system ; these are 

 excrements but not excretions. Now^ a part of such 

 excrementitious matter might be assimilated in pass- 

 ing through the digestive apparatus of another ani- 

 mal. The organs of secretion form combinations of 

 which only the elements were, contained in the food. 

 The production of these new compounds is a conse- 

 quence of the changes which the food undergoes in 

 becoming chyle and chyme, and of the further trans- 

 formations to which these are subjected by entering 

 into the composition of the organism. These mat- 

 ters, likewise, are eliminated in the excrements, 

 which must therefore consist of two different kinds 

 of substances, namely, of the indigestible constitu- 

 ents of the food, and of the new compounds formed 

 by the vital process. The latter substances have 

 been produced in consequence of the formation of 

 fat, muscular fibre, cerebral and nervous substance, 

 and are quite incapable of being converted into the 

 same substances in any other animal organism. 



Exactly similar conditions must subsist in the vi- 

 tal 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 excrements 



