DR. SCHLEIDEN'S THEORY OF AGRlCULTUrvE. 35 



the other extremely constant. Now, certainly, it is not the rich- 

 ness in humus which intluces the multiplied varieties of Alpine 

 plants, or its comparative absence, on the other hand, which 

 causes the poverty of forms in other species. The Alpine plants 

 grow in a soil to which the water contiimally brings down the 

 decomposed elements or the comminuted fragments of the im- 

 pending roclvs. The characteristic of our cultivated fields is 

 equally the richness in soluble mineral constituents, and not the 

 greater proportion of humus. 



We may now then answer the question : What is an Agri- 

 cultural plant ? It is one which is distinguished from wild 

 individuals of the same species by peculiar qualities which con- 

 stitute its fitness for culture, and which depend upon a modifica- 

 tion of chemical action. 



By cultivation, in its confined sense, to the exclusion of all 

 cases of mere promotion of luxui'iant growth, is meant the pro- 

 duction of certain definite qualities in particular individuals. 

 There is however another end in view, namely, that of associa- 

 tion, to the exclusion of all other plants. Even where no pecu' 

 liar qualities distinguish the plant under culture from the wild 

 state of the species, such association is necessary to save the 

 labour of the collection of scattered individuals. Nor is this 

 necessary association without its consequences. Most men con- 

 sider plants so collected as in a far more favourable situation 

 than they occupy by nature ; but the fact is precisely contrary ; 

 for the greater part of cultivation consists in restoring the fer- 

 tility which has been impaired by such association. 



Although it is improbable that different natural laws prevail 

 in the nutrition of cultivated plants from those which are wild, 

 we must not take this for granted, but put it to a strict proof, 

 especially since the most erroneous notions on this subject are 

 prevalent. 



The facts which follow are collected from Loudon's Encyclo- 

 paedia : — ■ 



Up to the 35th degree of latitude, on either side of the 

 equator, the farmer depends not on dung, but on irrigation ; 

 from thence to the 45th degree manure accompanies irrigation, 

 but by no means universally ; from thence to the 67th degree, 

 which is about the limit of cultivation in Norway and Sweden, 

 the ground is drained and manure employed. In tlie southern 

 hemisphere this latter region is of little importance, while the 

 first is of great consequence on both sides of the equator. Three- 

 fourths then of tiie cultivation of the whole globe is quite inde- 

 pendent of manure. 



In the following countries organic manure is not used, and 

 frequently impracticable from the want of beasts to yield it. 



D 2 



