12 GEOLOGY OF SOIL. 



all the various formations, which the earth presents, 

 are found. Yet no difference in the quality and 

 quantity of crops of rye, oats, barley, wheat, Indian 

 corn, is found which can be attributed to different 

 geological tracts. 



24. All plants have a natural limit, a peculiar re- 

 gion, in which, unaided by the human race, they 

 flourish and spread spontaneously. The smaller the 

 limit of this natural boundary — the more difficult is 

 the cultivation of the plant. Yet we find that the 

 natural boundary is passed, and so plants come to' 

 live in an artificial region. There is a natural, and 

 there is an artificial " habitat," or region ; and this 

 last is either horticultural or agricultural. The first is 

 unlimited, the second is limited by the great external 

 circumstances of temperature and moisture. 



25. The extreme north and south limits, which 

 bound the cultivation of the food-bearing plants, are 

 determined wholly by physical, physiological and 

 social causes. Temperature is the great agent, 

 which limits the agriculture " habitat" of the grain- 

 bearing plants. 



26. The distribution of plants is governed by the 

 two following laws : 



1st. The polar agricultural limits are bounded by 

 lines passing through places of equal summer heat. 



