170 WHAT DOES MAN LIVE UPON? 



most minute characteristics, under the most diverse con- 

 ditions, others run readily into innumerable varieties. 

 While in some, the varieties exhibit very little stability f 

 passing readily again into the wild form, or into new 

 variations, other plants produce manifold aberrant forms, 

 which, after some years' culture, may be propagated with 

 full certainty by their seeds, and thus arise what are called 

 sub-species. It is exactly this character of plants which 

 fits them to become advantageous objects of cultivation ; 

 that they readily produce very different and stable varieties, 

 out of which Man selects those most profitable for his 

 purposes, and receives them into the number of his vege- 

 table subjects. 



We have then three opposite conditions here : the com- 

 mon soil, bog soil and that of gardens. The first nourishes 

 an abundance of different plants, which, however, remain 

 the same, in fixed consequence, through thousands of 

 years. The bog soil is extraordinarily poor in vegetables ; 

 it only brings forth the most formless and useless plants. 

 Lastly, the garden soil not only nourishes in luxuriance 

 every plant that is committed to it, but even continually 

 multiplies the abundance of vegetable forms to infinity, to 

 which, however, opposing climate sets a limit so soon as 

 the favouring influence of culture is withdrawn. Then two 

 other conditions present themselves, in contrast, to our 

 consideration. We have on the one side the common 

 soil, possessing little or no organic remains, and abundance 

 of plants ; on the other, the bog and garden soils, both rich 

 to superabundance in the black constituent called humus, 

 which has been formed by the decomposition of animal 

 and vegetable organisms. And nevertheless, we find such 

 a difference of influence on vegetation between the bog and 

 the garden land. But this is readily explained by the 



