MUTUAL RELATION OF PLANTS AND ANIMALS. 183 



efficient for the production of future crops. In course of 

 time, the fertihty of his fields will decrease, no matter 

 what plants he cultivates, or what order of rotation he may 

 adopt. The removal of his crops is nothing else than 

 robbing the ground of the conditions for future harvests. 



A field is not exhausted for corn, clover, tobacco, or 

 turnips, so long as it j^elds remunerative crops, without 

 needing the replacement of those mineral constituents 

 whicli have been carried away. It is exhausted from the 

 time that the hand of man is needed to restore the 

 failing conditions of its fertihty. In this sense, most of 

 om* cidtivated fields are exhausted. 



The fife of men, animals, and plants is most intimately 

 connected with the restoration of all those conditions 

 which cause the vital process to go on. The soil, by its 

 constituents, takes part in the Hfe of the plant ; its per- 

 manent fertihty is inconceivable and impossible, without' 

 the replacement of those conditions which have made it 

 productive. 



The mightiest river which sets in motion thousands of 

 mills and machines must fail, if the streams and brooks 

 supplying its waters run dry; so, too, the streams and 

 brooks wiU run dry if the many little drops of which 

 they consist fail to return in the form of rain to tlie 

 place whence their sources spring. 



A field which, by the successive cultivation of different 

 plants, has lost its fertility, may recover the power of 

 yielding a new series of crops of the same plants, by the 

 appHcation of manure. 



What is manure, and whence comes it ? All manure 

 comes from the farmers' fields : it consists of straw, Avhicli 

 has served as fitter ; of remains of plants, of the hquid 

 and solid excrements of men and animals. The excre- 

 ments are derived from food. 



