620 PHYSIOGRAPHY 



fauna does not characterize every climate; but the relationship is 

 unmistakable from the quantitative point of view. Almost every- 

 where on the earth's surface we find a dense covering of vegetation 

 corresponding to a certain quantity of light, warmth, and moist- 

 ure, and this covering becomes thinner, in proportion as the warmth 

 or the moisture of the country becomes less. The plant form- 

 ations mirror the most widely varying combination of climatic ele- 

 ments, and since the latter are dependent upon the extent and size 

 of the forms of the earth's surface, so these latter geographical facts 

 may be traced in the varying density of the vegetal covering of the 

 lands. 



Finally, we find that there is the very closest relation between the 

 climate and the minuter modeling of the earth's surface. All the mod- 

 eling forces which work upon the latter are under climatic influence ; 

 the running water and the powerful masses of ice of the glaciers are 

 both products of climate, and the universally present wind can work 

 most effectively where an arid climate causes the absence of the pro- 

 tective covering of plants. Where a single river, such as the Colorado, 

 is eroding, the valley forms are different from those where their 

 slopes are regularly moistened, so that the debris creeps downward. 

 Thus we find that climate, the density of the vegetation, and the finer 

 features of relief of any country bear intimate mutual relationship, 

 and are dependent upon the distribution and mass of the greater land 

 forms, which exert a far-reaching influence. 



This close relation between climate, density of vegetation and 

 the finer land forms finds expression in a definite physiographic 

 correlation, more or less completely corresponding to the position 

 of any district. Very simple consideration of the subject shows us 

 that different grades of development may exist. Imagine a land area 

 emerging from the ocean. As soon as it appears above the surface 

 of the water, it will acquire its appropriate climate, but a certain 

 period of time must elapse before its appropriate covering of vegeta- 

 tion will develop, for it is only the density and not the existence 

 of this cover that is dependent upon the climate. The development 

 of such a cover presumes that germs and seeds shall reach the new 

 land area. If other land areas lie in its neighborhood, this transfer 

 will take place quickly, as, for example, we see the island of Krakatoa 

 already clothing itself again in vegetation, after the fearful explosion 

 of 1883. On the other hand, if other land areas are widely removed 

 from the new one, a very long period of time may elapse before it 

 receives the elements of its appropriate flora. Yet, from the geolog- 

 ical point of view, this reclothing takes place in a rather short time. 

 This is illustrated by the lonely group of the Kerguelen Islands, which 

 during the glacial period was wholly covered with ice, and has devel- 

 oped since that time a flora peculiar to itself. 



