4 POPULAR GEOGRAPHY OF PLANTS. 



where the hues of the rainbow melt into each other, or to fix 

 the moment when day fades into twilight and twilight into 

 night. One broad rule is nevertheless observed by vege- 

 tation of a gradual development from the lowest state in 

 which it exists near the Poles, to the glorious perfection and 

 luxuriance of form, colour, and organization which it shows 

 at the Equator ; and to trace this will be an interesting task. 

 But a horizontal division of the surface of the globe is 

 not all that has been attempted. It is well known that in 

 ascending high mountains, the temperature becomes gra- 

 dually colder towards the snow -line. Meyen has deeply 

 considered this subject, and gives it as the result of his own 

 personal observations, that a vertical division of mountains 

 may be made with respect to climate and vegetation, cor- 

 responding exactly with the horizontal one of the surface 

 of the globe into zones. Tor example : a mountain at or 

 near the equator, which rises to the limit of perpetual snow, 

 would exhibit every variety of climate, and consequently of 

 vegetation, which is found from the equator to the poles, 

 and might be divided into eight vertical regions, correspond- 

 ing with the eight horizontal zones. In the same way, in 

 whatever zone a mountain stands, the number of regions 

 from the plain to the snow-line (supposing it to reach so 



