60 TERRESTRIAL ADAPTATIONS. 



the meantime, observe how this property of the 

 atmospheric changes is made subservient to a 

 further object. 



To this constancy of the climates of each place, 

 the structure of plants is adapted ; almost all 

 vegetables require a particular mean temperature 

 of the year, or of some season of the year, a par- 

 ticular degree of moisture, and similar conditions. 

 This will be seen by observing that the range of 

 most plants as to climate is very limited. A 

 vegetable which flourishes where the mean tem- 

 perature is 55 degrees, would pine and wither 

 when removed to a region where the average is 

 50 degrees. If, therefore, the average at each 

 place were to vary as much as this, our plants 

 with their present constitutions would suffer, 

 languish, and soon die. 



2. It will be readily understood that the same 

 mode of measurement by which we learn the 

 constancy of climate at the same place, serves to 

 show us the variety which belongs to different 

 places. While the variations of the same region 

 vanish when we take the averages even of 

 moderate periods, those of distant countries are 

 fixed and perpetual ; and stand out more clear 

 and distinct, the longer is the interval for which 

 we measure their operation. 



In the way of measuring already described, 

 the mean temperature of Petersburg is 39 de- 

 grees, of Rome 60, of Cairo 72. Such observa- 



