!6o GEORGE E. NICHOLS Vol. IV, No. 2 



kiaer ('16), and it does not seem necessary to go further into this phase of 

 classification here. 



VI. Classification Based on Geographical Relations 



A. Introductory 



General Correlation between Geography and Vegetation. — The importance 

 of geographic factors in their effect on vegetation is most strikingly seen 

 when areas are compared which differ from one another to a marked degree 

 in either climate or physiography. Thus the dissimilarities in vegetation be- 

 tween arctic and tropical regions and between desert and humid regions are 

 manifestly associated with differences in climate, while the corresponding dis- 

 similarities between lakes and uplands, between ravines and flood plains, and 

 between saline and fresh swamps are quite as manifestly associated with 

 differences in physiography. Fully as significant as these differences, how- 

 ever, are the similarities exhibited by the vegetation of areas which are geo- 

 graphically alike. Thus the similarities in vegetation between arctic and ant- 

 arctic regions, between the desert regions of America and those of Africa, 

 and between tropical regions the world over are manifestly correlated with 

 resemblances in climate, while the similarities between the vegetation of dif- 

 ferent individual ravines, flood plains, lakes, and salt marshes are quite as 

 manifestly correlated with resemblances in physiography. Examples which 

 illustrate the influence of climate and physiography on the character of the 

 vegetation might be multiplied indefinitely. 



With regard to this relationship in general, it may be stated: (i) that 

 uniformity or similarity in geographic conditions is conducive to uniformity 

 or similarity of vegetation; and (2) vice versa, that diversity or dissimilarity 

 of geographic conditions is conducive to diversity or dissimilarity of vegeta- 

 tion. It follows, to be more specific, that an area in which the combined 

 influence of all the locally effective climatic and physiographic factors is 

 essentially homogeneous or uniform throughout — such an area as a sand plain, 

 or a clay bank, or a rock face— will tend to be occupied by a single plant 

 association ; and, on the other hand, that an area in which the combined influ- 

 ence of these geographic factors varies from place to place — such an area as 

 a ravine, or a lake, or a salt marsh, or a continent — will tend to be occupied 

 by more than one association. 36 That these tendencies are not universally 

 realized in nature can be attributed primarily to the influence on the habitat of 

 various biotic agencies which are able to compensate in greater or less degree 

 the influence of geographic agencies, 37 secondarily to the disturbing influence 

 of various human agencies and fire. 



88 Provided of course that these variations are of sufficient magnitude to have a dif- 

 ferentiating effect on the character of the vegetation. 



J ' See further remarks regarding compensating factors on pp. 166, 173. 



