76 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



to within a few degrees of the Tropic of Cancer ; and even on the higher 

 peaks of that part of the ridge of the Andes, which lies immediately 

 beneath the Equator. The height of the snow-line beneath the Equator 

 is between 15,000 and 16,000 feet above the leJVel of the sea ; on the 

 south side of the Himalayan ridge it is about 15,500 feet,- but on the 

 north side it rises to 18,500 feet ; and in the Swiss Alps it is about 

 8000 feet. Its position is very much affected, however, by local cir- 

 cumstances, such as the neighbourhood of a large expanse of land or of 

 sea; hence the small quantity of land in the Southern Hemisphere, 

 renders its climate generally so much colder than that of the Northern, 

 that in Sandwich Land (which is lat. 59 S., or in the same parallel as 

 the north of Scotland) the whole country, from the summits of the 

 mountains down to the very brink of the sea-cliffs, is covered many 

 fathoms thick with everlasting snow ; and in the Island of Georgia 

 (which is in lat. 54 S., or in the same parallel as Yorkshire), the limit of 

 perpetual snow descends to the level of the ocean, the partial melting 

 in summer only disclosing a few rocks, scantily covered with moss and 

 tufts of grass. Yet the highest mountains of Scotland, which ascend 

 to an elevation of nearly 5000 feet, and are four degrees more distant 

 from the equator, do not attain the limit of perpetual snow ; this is 

 reached, however, by mountains in Norway, at no greater elevation. 



106. If, then, Temperature exert such an influence on Vegetation as 

 has been stated, we ought to find on the sides of lofty mountains in 

 tropical regions, the same progressive alterations in the characters of 

 the Plants that cover them, as we encounter in journeying from the 

 equatorial towards the polar regions. This is actually the case. The 

 proportion of Cryptogamia to Flowering Plants, for example, is no more 

 than one-fifteenth on the plains of the Equatorial region ; whilst it is as 

 much as one-fifth on the mountains. In ascending the Peak of Tene- 

 riffe, Humboldt remarked as many as five distinct zones, which were 

 respectively marked by the products which characterize different 

 climates. Thus at the base, the vegetation is altogether tropical; 

 the Date-Palm, Plantain, Sugar-Cane, Banyan, the succulent Euphor- 

 bia, the Dracaena, and other trees and plants of the torrid zone there 

 flourish. A little higher grow the Olive, the Vine, and other fruit- 

 trees of Southern Europe ; there Wheat flourishes ; and there the 

 ground is covered with grassy herbage. Above this is the woody 

 region, in which are found the Oak, Laurel, Arbutus, and other beau- 

 ful hardy evergreens. Next above is the region of Pines ; characterized 

 by a vast forest of trees resembling the Scottish Fir, intermixed with 

 Juniper. This gives place to a tract remarkable for the abundance of 

 Broom ; and at last the scenery is terminated by Scrofularia, Viola, a 

 few Grasses, and Cryptogamic plants, which extend to the borders of 

 the perpetual snow that caps the summit of the mountain. 



107. The effects of Temperature on Vegetation are not only seen in 

 its influence upon the Geographical distribution of Plants, that is, in 

 the limitation of particular species to particular climates ; for they are 

 shown, perhaps even more remarkably, in the variation in the size of 

 individuals of the same species ; when that species possesses the power 

 of adapting itself to widely different conditions, which is the case with 



