DISTEIBUTION OF PLANTS IN ALTITUDE. 673 



earth's surface into zones. Great difficulty interposes here in any 

 attempt at generalization, since local conditions, arising from aspect, 

 and conformation of surface, either giving more or less of preci- 

 pitous character, accompanied by sudden changes, or producing 

 elevated plains, &c., cause such great differences, even within the 

 limits of single mountain-systems, that no absolute rule can be ap- 

 plied. The rules laid down by Meyen apply pretty well to his 

 zones within the limits of Europe ; but, in the delineation of the 

 regions of altitude of greater extent, great variation presents itself 

 near the equator. 



The Snow-line. According to Meyen's views, the snow-line, beginning 

 at the polar zone, rises between 1900 and 2000 feet above the level of the 

 sea, and in the equatorial zone to 15,500 or 16,600 feet ; and he divides 

 the regions of altitude in accordance with this, raising each region between 

 1900 and 2000 feet in each zone, as he approaches the equator. Now at 

 North Cape, which lies near the polar limit of our subarctic zone, and in 

 Iceland, which is crossed by the same limit, the line of perpetual snow is 

 at about 2000 feet ; we may therefore take this as the snow-line of the 

 arctic zone. The equatorial limit of the subarctic zone falls in Southern 

 Norway, where the snow-line is at about 4000 feet ; while the equatorial 

 limit of our cold temperate zone is not far removed from the Alps and 

 Pyrenees, where the snow-line rises to 8000 feet and more. In the south 

 of Spain, lying within the warm temperate region, snow lies in isolated 

 patches below 11,000 feet. In the district of Sierra Nevada, which is one 

 of the best-known of the mountains of this zone, as regards vegetation, 

 there is a subtropical region up to 600 feet, the true warm temperate 

 vegetation extends up to about 4000 feet, a cold temperate vegetation 

 from about 4000 to 6500 ; the vegetation then passes into a condition 

 allied to the subarctic, but without trees, and characterized by shrubs of 

 a similar nature to those of the arctic zone. This region extends to 8000 

 feet ; and thence to the summits of 11,000 feet there is an alpine summer 

 vegetation (snow lying for eight months out of the twelve), which, again, 

 is intermediate in character between those of the arctic and polar, con- 

 sisting chiefly of perennial herbs like the latter, but presenting a formation 

 of turfy pasture to some extent in the warm season. In the Caucasus 

 the snow-line is much higher. 



In the subtropical zone, on the Peak of Teneriffe, we find the vegeta- 

 tion of the warm temperate zone from about 2000 to 4000 feet, a repre- 

 sentation of the cold temperature from 3000 to over 6000 feet ; at about 

 8000 feet the climate is subarctic. This mountain does not reach the 

 snow-line. 



In Mexico, lying in our tropical zone, the lines are respectively shifted 

 up in about the same ratio. We see throughout, then, a deviation from 

 Meyen's ratio, in the tendency of the colder zones to widen out on the 

 mountains of warmer zones j but this is partly owing to our dividing the 

 zones according to temperature, and not according to latitude. 



If we attempt to lay down the conditions of the mountains of Asia 

 under a similar point of view, we find greater deviations. The mass of 

 elevated land in Central Asia modifies all the climatal conditions very 

 much. The snow-lines of the mountains of the cold-temperate and warm- 



