144 THE ART OF CULTURE. 



as well as on their size. It is the duty of the natu- 

 ral philosopher to discover what these conditions 

 are ; for the fundamental principles of agriculture 

 must be based on a knowledge of them. There is 

 no profession which can be compared in importance 

 with that of agriculture, for to it belongs the pro- 

 duction of food for man and animals ; on it depends 

 the welfare and development of the whole human 

 species, the riches of states, and all commerce. 

 There is no other profession in which the applica- 

 tion of correct principles is productive of more bene- 

 ficial effects, or is of greater and more decided in- 

 fluence. Hence it appears quite unaccountable, that 

 we may vainly search for one leading principle in the 

 writings of agriculturists and vegetable physiologists. 



The methods employed in the cultivation of land 

 are different in every country, and in every district ; 

 and when we inquire the causes of these differences, 

 we receive the answer, that they depend upon cir- 

 cumstances. [Les circonstances font les assolements.) 

 No answer could show ignorance more plainly, since 

 no one has ever yet devoted himself to ascertain 

 what these circumstances are. Thus also when we 

 inquire in what manner manure acts, we are answered 

 by the most intelligent men, that its action is covered 

 by the veil of Isis ; and when we demand further 

 what this means, we discover merely that the excre- 

 ments of men and animals are supposed to contain 

 an incomprehensible something which assists in the 

 nutrition of plants, and increases their size. This 

 opinion is embraced without even an attempt being 

 made to discover the component parts of manure, 

 or to become acquainted with its nature."^ 



In addition to the general conditions, such as heat, 

 light, moisture, and the component parts of the atmo- 

 sphere, which are necessary for the growth of all 

 plants, certain substances are found to exercise a 



* This statement is now somewhat too general ; both in this country 

 and in Great Britain agriculture has received important aid from the 

 labors of chemists and physiologists. 



