CLASSIFICATION AND RELATIONSHIP 301 
351. Habitat classification. In arranging formations with reference to 
habitats, the direct factors, water and light, 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. 81. Pachylophon (Pachylophus caespitosus), a family o the 
eae 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 Veuetation 24, 27. 1904, 
