54 



enough to those which other naturalists have 

 adopted before me*, is very important to the 

 geography of plants ; for though in the northern 

 countries the distribution of vegetables on the 

 mountains and in the plains depends, like the 

 height of the perpetual snows, more on the 

 mean temperature of the months of summerf** 



* Every hundred metres of height lower the temperature 

 about half a degree of the common division of our thermome- 

 ters : and if we take for the limit of refrigeration, that which 

 excludes the presence of vegetation, the perpetual ice, with 

 which the summits of mountains are loaded, will represent the 

 perpetual ice with which the pole is covered y and every hun- 

 dred metres of vertical height will correspond to a degree of 

 the distance from the mountain to the pole. Hamond, on 

 the Vegetation of Mountains (Annales du Museum, t, iv, p. 

 396). 



f Decandolle, Flore franpaise, t. i, p. 1, p. 9. Leopold von 

 Buch, Reise nach Lapland, t. ii, p. 276. Wahlenberg, Flora 

 Laponica, 1810, p. 28. In the temperate zone it often hap- 

 pens, that the mean heat of a place, a, is less than that of a 

 place 6, while the mean heat of the summer months is much 

 greater at a than at b. It is for this reason, that a distinction 

 is properly made between a continental climate and an insular 

 climate; in the first, very warm summers succeed very rigor- 

 ous winters j in the second, the contrast of the winters is less ; 

 the summers are less warm, and the winters less cold, on ac- 

 count of the small changes in the temperature of the neigh- 

 bouring ocean, by which the air is cooled in summer and 

 warmed in winter. The perpetual snows descend more in 

 Iceland than on the same parallel in the interior of Norway, 

 and we often see, in the islands and on the coasts of western 

 Europe, the laurel and the arbutus flourish, where the vine 



