FORMATION GROUPS. 143 



(1) vegetation type, subdivided into (2) formation groups, and the latter into 

 formations. Grassland is given as an illustration of the vegetation type, and 

 meadow of the formation group. Clements (1904 : 139; 1905 : 302, 270) 

 arranged formations with reference to habitat, development, and region, but 

 emphasized the developmental classification as primary. Warming and Vahl 

 (1909 : 136) propose 13 classes of formations on the basis of climatic and 

 edaphic distinctions. To the original 4 groups of Warming are added helo- 

 phytes, oxylophytes, psychrophytes, Uthophytes, psammophytes, chersophytes, 

 eremophytes, psUophytes, sclerophyllous, and coniferous plants (cf. Clements, 

 1902:5). Brockmann and Rubel (1913:23) have recognized three major 

 groups: (1) vegetation type, (2) formation class, and (3) formation group. 

 For example, the vegetation type, woodland or "Lignosa," is divided into sev- 

 eral formation classes, e. g., " Pluviilignosa," "Deciduilignosa," etc., and these 

 into groups, such as "Aetatisilvae," "Aetatifruticeta," etc. The primary 

 basis of the classification is physiognomy, with some reference to habitat in 

 many of the classes and groups. 



Bases. — A comparison of the various systems proposed above shows that 

 there are three general bases. These are habitat, physiognomy, and develop- 

 ment. These practically exhaust the list of possibilities, since floristic does 

 not furnish a feasible basis. All systems based primarily upon habitat make 

 use of physiognomy in some degree, and the converse is also true. They do 

 not take development into account, and hence are more or less superficial. 

 The simpUcity and convenience of artificial classifications based upon habitat 

 and physiognomy are so great, and the readiness with which they can be made 

 is so alluring, that they will persist for a long time. They must slowly yield 

 to a natural system based upon development, but such a system in its details 

 demands much more knowledge of vegetation and climate than we possess at 

 present. There can be no serious objection to using a habitat-physiognomy 

 or a physiognomy-habitat system in so far as it is useful and accords with the 

 facts. It should be constantly borne in mind, however, that such classifica- 

 tions are makeshifts against the time when developmental studies have be- 

 come general. 



Developmental groups. — The formation as generally understood is based in 

 no wise upon development. Hence the natural or developmental relation of 

 such formations or associes, as they are called here, is revealed by the physio- 

 graphic classifications of Cowles (1901). Such a system broadened to become 

 purely developmental, and with physiography regarded as but one of several 

 causes is the one which we have already considered in various aspects. The 

 formation as here conceived is a natural unit in which all of its associes, the 

 formations of most authors, fall into their proper developmental relation. 

 It has already been pointed out that such a relation includes all the essentials 

 of habitat and physiognomy. 



The classification of formations, i. e., climax communities, as here under- 

 stood, is a more difficult task. Here again the fundamental basis should be 

 that of development, but we now have to do with the phylogenetic develop- 

 ment of a climax formation, and not with its ontogeny. The ontogenetic devel- 

 opment of a formation, such as the Great Plains grassland, can be studied in 

 hundreds of primary and secondary seres. Its phylogeny is a matter of the 

 past. It not only can not be studied with profit until the present development 



