1 92 THE SUCCESSION OF ANIMAL RACES. 



substances unnecessary or injurious to the plants to which 

 they belong. But it has been found that excretion from the 

 roots only takes place when these are wounded. This theory 

 has therefore fallen to the ground. The next theory is founded 

 on the exhaustion of the mineral substances contained in the 

 soil. It has already been shown that inorganic substances 

 are essential to the growth of plants, and that without mineral 

 food plants cannot live. Now it is known that some plants 

 contain a greater amount of certain mineral substances than 

 others: for instance some plants require much potash or soda; 

 others much phosphoric acid; some much lime; others a 

 large quantity of silica. This being the case, it is evident 

 that it would be beneficial to cause one crop, requiring only 

 a small quantity of any particular substance, to succeed 

 another requiring that substance in a larger amount. Such is 

 the explanation of the beneficial results attendant on the 

 rotation of crops." * 



Now although, as Mr. Thompson points out, experi- 

 ments upon this subject are as yet far from conclusive, 

 it is evident that this theory, if it does not contain the 

 whole truth, at least it approximates thereto and that 

 this is one of the main causes, though it may not be 

 the only cause, which explains the natural law of the 

 rotation of crops. 



The laws of Nature are as old as time itself, and 

 like the decrees of the Medes and Persians " unchange- 

 able, " and the law of the rotation of vegetable growths 

 applies alike to the grass of the field and to the mon- 

 arch of the forest. Again we find the same principle 

 running through the whole economy of created nature, 

 where change is the universal rule, everywhere unceas- 

 ingly going on in the animal, as well as in the vege- 



* The Gardener's Assistant, by Robert Thompson, pp. 2623 ( a 

 standard work on Kitchen Gardening, no date of publication given). 



