64 THE HAKMONIES OF NATURE. 



tained in the minute crevices of the stones or the interstices of 

 the soil thoroughly loosens the cohesion of the ground. On 

 the return of spring, warmth, moisture, and air are thus better 

 able to penetrate below the surface, and to awaken the germs 

 of dormant life. In the Arctic regions, where winter fre- 

 quently holds vegetation for months in iron bondage, this 

 inclement season is at the same time one of its greatest friends, 

 not only through the protecting mantle of snow which it spreads 

 over the earth, but chiefly through the mechanical division of 

 the soil which it causes. Thus death becomes the parent of 

 life ; and thus divine wisdom has made dreary winter the active 

 helpmate of the short summer of the northern regions. 



The roots of one plant do not rob the soil of the same mineral 

 substances as those of another, for the various families of plants 

 are not constructed of identical materials. Thus the grasses and 

 all our cereals chiefly require silica for their nourishment ; the 

 pea and the lupin, chalk ; the potato and the turnip, potash ; 

 the vine, soda; as the chemical analysis of their respective 

 ashes proves. If plants of the same class were cultivated 

 year after year on the same spot, the soil would soon be ex- 

 hausted of the particular mineral substances they require, while 

 by a judicious alternation of silica, potash, or chalk plants, it 

 gains time to replace the mineral particles that have been with- 

 drawn from it by the preceding crops. 



If all plants absorbed the same mineral substances, the fields 

 which now yield an annual return must frequently have lain 

 fallow until the slow progress of mineral dissolution had repaired 

 their losses, and consequently the same extent of territory could 

 only have been able to feed a much smaller population. Thus 

 we see that the wealth and power of all agricultural nations, 

 and, consequently, also the progress of civilisation, depend in 

 a great measure upon the relative importance of the various 

 mineral portions of the soil to the different plants cultivated by 

 man. 



