THE SUCCESSION OF CROPS. 191 



of the old and the success of the new regime: and 

 if we might venture to hazard an opinion upon the 

 subject, we should say that it is because some 

 vital chemical principle of the soil has been exhausted 

 by the ancient forest: the land is therefore in a com- 

 paratively barren condition for producing a fresh crop 

 of the same trees, consequently they either do not 

 germinate at all, or else are of such weakly growth 

 that they are quickly choked by the rank grasses and 

 weeds, and the vigorous undergrowth of bushy shrubs, 

 which are generally the first crops that spring from 

 the fallo\v ground of the clearing. 



On the other hand some other chemical principle, 

 which was not necessary to the growth of the ancient 

 trees, and therefore was not assimilated by them, has 

 accumulated in the soil for ages, and this, being highly 

 favourable to the growth of other kinds of trees, their 

 seed is enabled at once to take root, and oust the former 

 tenants from their inheritance. Such, we venture to 

 suggest, is a simple and probable explanation of this 

 apparent mystery. 



The necessity for a proper rotation of crops is a 

 well-known principle in agriculture, recognised for ages 

 by all competent farmers and gardeners. " It is a 

 well-known fact," says Thompson 



" that the same kind of crop cannot be successfully grown 

 on the same ground for several consecutive years. Farmers 

 know that if land be too frequently cropped with clover, it 

 becomes what is termed " clover-sick, " and refuses to produce 

 that crop, till a considerable space of time has elapsed. Var- 

 ious theories have been formed as to the causes which ren- 

 der the rotation of crops necessary. De Candolle supposes 

 that the roots have the power of excreting, or throwing off, 



