98 OUR FARM CROPS. 



everywhere forms the limit between the agricultural and 

 the pastoral or nomadic life. From the importance of the 

 cultivation of barley in the North, it is evident that where- 

 ever the human species has attained the first stages of 

 civilization, the attempt will have been made to advance 

 it as far as possible towards the pole. If, then, it is limited 

 by a sinuous curve, as already explained, it is because 

 circumstances of a purely physical nature oppose to it an 

 insurmountable barrier. 



A mean temperature of 46*4 during the summer 

 quarter, seems to be for Europe the only indispensable 

 condition for the cultivation of barley; in the islands of 

 the Atlantic Ocean, and in insular climates generally, a 

 summer temperature of 3 to 4 higher appears to be neces- 

 sary for its success. Iceland, indeed, where this grain 

 cannot be grown at all, presents in its southern districts, 

 at Rejkavik, a mean summer temperature of 49 '4. It 

 appears that there unseasonable rains are the means of 

 preventing all cultivation of cereals. With the exception 

 of districts where such counteracting influences exist, we 

 may consider that the limit of barley cultivation varies 

 between the zones of 46 '4 and 49 of mean summer tem- 

 perature (isotheral), in those countries where it is of im- 

 portance as an article of animal food. On continents 46*4 

 temperature is sufficient, but in islands generally the 

 increased humidity requires to be compensated for by 

 increased temperature in summer. Barley, however, is 

 cultivated as an alimentary plant as far north as either 

 oats or rye; towards the lower latitudes it loses its im- 

 portance ; its cultivation languishes, and ceases altogether 

 on the plains within the tropics, as it suffers more from in- 

 tense heat than either of the other cereals. It will grow, 

 too, at an elevation far above the range of wheat cultiva- 

 tion. Humboldt met with barley growing at an elevation 

 of 13,500 feet on the Andes, and in Switzerland it may 



