263 



ecological unit, as several modern writers have done, is clearly a mis- 

 interpretation of Grisebach's statement. Warming (1909) definitely 

 uses the word association, which he explicitly states is not synony- 

 mous with Grisebach's "formation" but is included under it.* 



Approaching the question from an analytical standpoint, Warm- 

 ing (1909: 140-145) defines a formation as "an expression of certain 

 defined conditions of life" which "is not concerned with floristic dif- 

 ferences," and an association as "a community of definite floristic com- 

 position within a formation" ; to which he adds : "it is, so to speak, 

 a floristic species of a foriuation zvhich is an ecological genus". The 

 ecological unit (association) is equivalent to the taxonomic unit 

 (species). Just as species are grouped to form a genus and genera 

 are grouped to form a family, so are associations grouped to form 

 a formation and formations grouped to form a province. If neces- 

 sary, an association may be divided into consocies, in like manner as 

 species are divided into subspecies. 



Of the apparent properties that ecological associations and taxo- 

 nomic species have in common, HaqDer (1906: 33-34) gives the fol- 

 lowing very pithy statement: "There are many analogies be- 

 tween habitat-groups and taxonomic groups, such as species, though 

 the latter are mutually exclusive categories and the former often are 

 not. For instance, both are able to be discovered, described, named, 

 and associated with certain type-localities. Records of both may be 

 preserved by descriptions, photographs, measurements, and other 

 means. Both have their diagnostic characters, with more or less va- 

 riation and intergradation. Both have passed through processes of 

 evolution, are self-perpetuatmg, and are liable to disappear through 

 geological or climatic changes or the works of man. New ones may 

 also originate, suddenly or gradually. Both have more or less def- 

 inite geographical distributions and regions of best development. 

 Both are capable of being subdivided, combined, or relegated to 

 synonymy, with the increase of our knowledge concerning them. 

 Habitat-groups, like species, can also be aggregated into larger cate- 

 gories analogous to genera and families". 



Just as genera and species present difticulties of delimitation, so 

 do formations and associations. The difficulties of ecological classi- 

 fication show many points of similarity, and require fully as much 



*For a detailed discussion of the questions involved the reader is referred 

 to the following articles : 



Smith, Robert. On the Study of Plant Associations. Nat. Sci., Vol. 14. 1899. 

 Warming, E. Oecolog>' of Plants, p. 139-148. 1909. 



Moss, C. E. The Fundamental Units of Vegetation. New Phytologist 

 9:18-53, 1910. 



