24 GEOLOGY OF SOIL. 



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. Tem- 

 perature is the great agent which limits the agricultural 

 " habitat " of the grain-bearing plants. 



26. The distribution of plants is governed by the two fol- 

 lowing laws : 



1st. The polar agricultural limits are bounded by lines 

 passing through places of equal summer heat. 



2d. The equatorial limits, by lines of equal winter heat. 



These lines are called respectively, isotheral, and isochi- 

 menal. They by no means coincide. They often cut each 

 other at right angles, and generally, from about the 45th 

 degree north latitude, they are parallel neither to one 

 another, nor to the latitude. They are often highly curved. 



And now for the proof of these general laws. Beginning 

 with barley, the grain which has been cultivated the farthest 

 north, its fields are found in the extremity of Scotland, in 

 the Orkneys and Shetland Isles, 61 degrees N. ; in the 

 Faroe Islands, between 61 and 62|- degrees N. ; in Western 

 Lapland, near North Cape, in latitude of 70 degrees ; on the 

 borders of the White Sea, in Western Russia, between 67 

 and 68 degrees, and near to Archangel, in Eastern Russia, 

 about QQ degrees ; in Central Siberia, the limit of barley is 

 between 58 and 59 degreei N. There are no extended 

 observations of the temperature of the northern portions of 

 our own continent, and therefore the limit of barley in 

 Northern America is left undefined. But its European line 



