172 ALTERNATION 



diversified areas, such as mountains. It is naturally much 

 less characteristic of lands physiographically more uniform. 

 A xerophytic formation -will alternate from ridge to ridge, 

 a mesophytic formation between the intermediate valleys: 

 aquatic vegetation will alternate from pond to pond, or 

 stream to stream. The appearance of new or denuded soils 

 upon which successions establish themselves is the most im- 

 portant cause of the alternation of formations. The 

 weathering of rocks in different areas of the same region 

 produces in each a sequence of similar or identical form- 

 ations. The same statement is true in general of other 

 causes of succession, such as erosion, flooding, burning, 

 cultivation, etc., wherever they operate upon areas 

 physically similar and surrounded by the same type of 

 vegetation. The areas cf more or less heterogeneous for- 

 mations characterized by major physical differences are 

 occupied by associations. In an extensive formation, the 

 same association alternates from one to another of these 

 areas that are similar. When the formation is interrupted 

 and occurs here and there in separate examples, an associ- 

 ation often alternates from one to another of these. An 

 association regularly derives its character from the fact that 

 one or more of the facies of the formation is more intimately 

 connected with certain areas of the latter than with others. 

 This explains why the alternation of associations and facies 

 are usuallv identical. Layers sometimes alternate between 

 different examples of the same forest or thicket formation, 

 when they are suppressed in some by the diffuseness of the 

 light. 



The alternation of species is a typical feature of formations : 

 it is absent only in those rare cases where the latter consist 

 of a single species. The areas of a habitat which show 

 minor physical, or historical (i. e.. competitive) difference! 

 are occupied by groups of individuals belonging to one or 

 more species responsive to these differences. Each of these 

 groups will recur in all areas essentially similar, the in- 

 tervals being occupied of course by slightly different 

 groups. Such groups are constituted by gregarious or 



