Chap. 3.] Principles of Agriculture. 33 



tof both thefe kingdoms. It is certain, however, that ve- 

 getables muft in every part of the globe have preceded 

 animals. A feed of mofs lodging in a crevice of a bare 

 rock is noufiftied by the atmofpherei) and the moifture 

 afforded by the rains and dews. It comes to perfection, 

 and fheds its feeds in the mouldering remains of its own 

 fubftance. Its offspring do the fame, till a cruft of 

 vegetable mould is formed fufEciently thick for the flip- 

 port of grafs and other vegetables of the fame growth. 

 The fame procefs going forward, fhrubs, and laftly, the 

 largeft trees, may find a firm fupport on the once barren 

 rock, and brave the efforts of the temped. 



From the advantages derived from a change of 

 -crops in agriculture, it has been fuppofed that different 

 vegetables derive different kinds of nouri foment from 

 the fame foil, feleding what is beft adapted to their 

 own fupport, and leaving a fupply of nourifhment of 

 another kind for vegetables of a different fpecies. Was 

 this, however, the cafe, vegetables would not fo much 

 impede each other's growth when placed near toge- 

 ther. And in the operation of grafting, we have a 

 clear proof, that the juices received by the root of one 

 fpecies of tree may, by the organization of the inferted 

 twig, be fubfervientto the growth of leaves, flowers, 

 and fruit of a different kind. The advantage derived 

 from a change of crops may be better explained on 

 other principles : fome plants extend their roots hori- 

 zontally on the furface of the foil, others ftrike them 

 downwards to a confiderable depth. Some plants are t 

 found to bind or harden the foil, others to loofen it. 

 Thus, for example, wheat and rye -grafs render a foil 

 ftiff, while pulfe, clover and turnips pulverize it. By 

 varying the crops, therefore, the foil is preferved in a 

 middle (late, between too much ftiffhefs and too much 

 friability. Nor is this the only good effect arifmg from 

 VOL. III. D that 



