CLASSIFICATION AND RELATIONSHIP 



301 



351. Habitat classification. In arranging formations with reference to 

 habitats, the direct factors, water and Hght, can alone be used to advantage. 

 Such a system is fundamental, because it is founded upon similarity of hab- 

 itat and of structure. Proposed groupings based upon nutrition-content, or 

 upon the division of factors into climatic and edaphic, have elsewhere^ been 

 shown to be altogether of secondary importance, if not actually erroneous. 

 The basis of the habitat grouping is water-content, which is supplemented 

 by light whenever the factor is decisive. The primary divisions thus ob- 

 tained are water, forest, grassland, and desert, which are characterized re- 



Fig. SI. Pachylophon (Pachylophus caespitosus), a family of the 

 gravel slide formation. 



spectively by associations of hydrophytes, mesophytes, hylophytes, poophytes, 

 and xerophytes respectively. Within these, formations are arranged ac- 

 cording to the type of habitat, i. e., pond, meadow, forest, dune, etc. These 

 divisions comprise all formations which belong to the type by virtue of their 

 physiognomy and structure. Such formations differ from each other very 

 considerably or completely in the matter of floristic, i. e., component species, 

 but they still belong to the same type. A dune formation in the interior and 

 one on the coast may not have a single species in common, and yet they are 

 essentially alike in habitat, development, and structure. 



* Clements, F. E. The Development and Structure of Vegetation, 24, 27. 1904. 



