264 Ohsei-vations on Humboldt's Works 



Xapponica, the contrary proposition is given; the absolute height 

 300 toises, the distance 250 toises. In determining the limit 

 of trees in the equatorial regions, the author employs Escallonia 

 and Alstonia ; but, as he says in his Essay *, when speaking of 

 his Region of Wintera and Escallonia, that it contains shrubs 

 with short stems, and branches trailing on the ground, it 

 appears that we cannot well compare this vegetation with the 

 pine forests of Switzerland, or the birch woods of the north, 

 but rather with Alnus viridis, and the Rhododendra in Switzer- 

 land, and Betula nana in the north. 



It appears, moreover, that from such a comparison between 

 the vegetation of mountains in different climate^ (if we abstract 

 from it all anomalies arising out of particular local relations) we 

 may deduce the law, that a line of vegetation, viz., that of the 

 limit of trees, drawn from the equator towards the north pole, 

 does not run parallel to the snow-line, but converges towards it. 

 It is not difficult to make this apparent. It has been already 

 proved that it is not the annual mean temperature which de- 

 termines the vegetation, but the temperature which prevails 

 within the cycle of vegetation ; hence it follows, that towards 

 the north pole, the vegetation is richer in proportion to the 

 difference between the different periods of the year, as might 

 be inferred from the phaenomena which present themselves in 

 the equatorial mountains, where the annual mean temperature 

 is nearly the same as that of each period of the year. Since, 

 on the other hand, the snow-line, if it be not entirely governed 

 by the mean temperature, stands nevertheless in a more exact 

 relation to it, because the summer temperature has less, and 

 the winter temperature more, effect upon it than upon the plants ; 

 it follows, therefore, that the limit of trees towards the pole 

 must approach nearer the snow-line. In connexion with this 

 stands the circumstance, that during the cycle of vegetation 

 the day towards the pole is always longer, ^ and consequently 

 the plants receive a greater quantity of heat, and the difference 

 between the temperature of day and night is less. The author 



Essai sur 



la GhgrapMe des Plantes, p. 69. 



