270 APPLICATION OF PRINCIPLES. 



plants render the soil unfit for their support. Every 

 one knows that the soil of a farm will not bear, year 

 after year, the same kind of crop, but that one kind 

 of produce is cultivated on a piece of ground one 

 year, and is succeeded by some other kind ; which 

 practice, in part, constitutes the important system of 

 rotation of crops. Not, however, to refer to matters 

 extra-horticultural, it is notorious that an apple 

 orchard will not immediately succeed upon the site 

 of an old orchard of the same kind of fruit, and that 

 no amount of manuring will enable it to succeed ; a 

 wall border, in which fruit trees have been long 

 grown, becomes at last insensible to manure, and re- 

 quires to be renewed ; and, not to dwell upon an un- 

 disputed fact, Dahlias do not " like" the soil in which 

 Dahlias were grown the previous year. This class 

 of phenomena cannot be explained upon the principle 

 of soil being exhausted, because that exhaustion is 

 made good and yet to no purpose, unless we assume 

 that land contains something mineral which each 

 species prefers to feed on, and which is not contained 

 in manure. But the slender power of selection pos- 

 sessed by the roots of plants (35) would be unfavour- 

 able to this supposition, even if it were open to no 

 other objections. It has of late years been thought 

 that the excretory functions of the root (39) would 

 explain the deterioration of soil, and that the reason 

 why plants cannot grow year after year in the same 

 soil, if it and their roots are disturbed, is, that, under 

 such circumstances, they are perpetually brought into 

 contact with the matter of which nature had previous- 



