THEORIES OF ITS USE. 165 



they are returned to it in the form of excrements. 

 Both views explain how it happens that after corn, 

 corn cannot be raised with advantage, nor after 

 peas, peas ; but they do not explain how a field is 

 improved by lying fallow, and this in proportion to 

 the care with which it is tilled and kept free from 

 weeds ; nor do they show how a soil gains carbon- 

 aceous matter by the cultivation of certain plants 

 such as lucern and esparsette. 



Theoretical considerations on the process of nu- 

 trition, as well as the experience of all agricultur- 

 ists, so beautifully illustrated by the experiments of 

 Macaire-Princep, leave no doubt that substances 

 are excreted from the roots of plants, and that 

 these matters form the means by which the carbon 

 received from humus in the early period of their 

 growth, is restored to the soil. But we may now 

 inquire whether these excrements in the state in 

 which they are expelled, are capable of being em- 

 ployed 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 herbi- 

 vorous 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. 



