55 



than on that of the whole year, the latter does 

 not less determine in southern countries the 

 limits, which the species have not been able to 

 pass in their distant migrations. The observa- 

 tion made by Tournefort on the summit of Ara- 



and the peach tree do not ripen. In the equinoctial region, 

 on the contrary, where the difference of the season is as it 

 were nothing, the geographical distribution of plants is regu- 

 lated almost only according to the mean temperature of the 

 whole year, which depends itself on the elevation of the soil 

 above the level of the ocean. In proportion as we advance 

 toward the north, the temperature of the months varies more 

 and more, and the strength and richness of vegetation no 

 longer give the measure of the mean temperature of the whole 

 year. In Lapland, for instance, there are beautiful forests 

 on the continent, at Enontekies, while on the island of Mage- 

 roe we scarce find a few shrubs sprinkled over the rocks ; 

 nevertheless the mean annual temperature of Enontekies is 

 three degrees colder than that of Mageroe. The former is 

 — 2-86% and the latter + 0 07°. (Wahlenberg, in Gilbert's 

 Annals, 181*2, p. 271.) The more vigorous vegetation of 

 Enontekies is the effect of a warmer summer, the mean tem- 

 perature of the months of July being there 15-3° j while at 

 the isle of Mageroe it is only, according to Mr. von Buch, 

 8*2°. These two places offer striking instances of the differ- 

 ence between a continental climate and an insular climate j or, 

 as Mr. Wahlenberg says, between a climate of Siberia, and 

 a climate of Iceland. In general, the problem of the clima- 

 tic distribution of plants is much more complicated in the 

 northern countries than under the tropics. In the former this 

 distribution depends at the same time both on the mean tem- 

 perature of the summer months, and on the temperature of 

 the soil, which differs from the mean heat of the year. 



