126 STRUCTURE AND UNITS OF VEGETATION. 



more or less generally by British ecologists. The latter have also tended to 

 employ community as an inclusive term for any and all umts from the forma- 

 tion to the family. Convenience and accuracy demand such a term, and it is 

 here proposed to restrict community to this sense. For its concrete use to 

 designate the division next below the society, the term clan is proposed. 



The term consocies was first proposed (Clements, 1905 : 296) as a substitute 

 for association, owing to the use of the latter in both an abstract and a concrete 

 sense. The general use of association in the concrete sense has fixed it defi- 

 nitely in ecological terminology. At the same time, its actual application 

 to particular communities has shown the widest divergence of viewpoint. As 

 a consequence of moTe exact knowledge of vegetation, it became evident that 

 a new division was needed between association and society to designate the 

 characteristic dominance of facies (Pound and Clements, 1900:319). The 

 term consocies has been used for this division, since this is precisely the unit 

 for which it was first proposed. Thus, while the relation of formation and 

 association remains the same, consocies would become the term to be applied 

 to by far the larger number of associations as hitherto recognized. This con- 

 cept in particular has been repeatedly tested during the past two years through- 

 out the western half of North America, and has shown itself to be one of the 

 most valid and easily applied of all the units. The term has been used in 

 this sense or essentially so by Shantz (1906 : 36), Jennings (1908 : 292; 1909 : 

 308), Gleason (1910:38), Gates (1912:263), Matthews (1914:139), and 

 Vestal (1914 : 356; 1914^ : 383). 



However, the requirements of a developmental analysis of vegetation make 

 it desirable, if not necessary, to distinguish between climax and developmental 

 consocies. Accordingly, it is proposed to retain consocies for the serai unit, 

 and to employ consociation for the climax unit. Thus, from the standpoint 

 of structure, the following plant communities are recognized, namely, forma- 

 tion, association, consociation, society, clan, and family. Their essential 

 relationship is indicated by the sequence. Since at least the formation, con- 

 sociation, and family permit of objective limitation, the use of the remaining 

 terms may be definitized much more than has been the case hitherto. 



Formation. — The formation is the unit of vegetation. It is the climax com- 

 munity of a natural area in which the essential climatic relations are similar 

 or identical. It is delimited chiefly by development, but this can be traced 

 and analyzed only by means of physiognomy, floristic, and habitat. In a 

 natural formation, development, physiognomy, and floristic are readily seen 

 to be in accord, but this often appears not to be true of habitat. There are 

 several reasons for this. In the first place, complete and exact knowledge of 

 any habitat is still to be obtained. As a consequence, the actual correlation 

 of factors and the critical responses of the plant are as yet untouched. Fin- 

 ally, we think of climate in human terms, and forget that the only trustworthy 

 evidence as to climatic climaxes must be obtained from the responses of the 

 plant and the commumty. Even the exact evidence obtained by recording 

 instruments may be most misleading, unless it is translated into terms of 

 plant life. Thus, while there is every certainty theoretically that the respon- 

 sive unit, the formation, is in harmony with the causal unit, the habitat, our 

 present knowledge is inadequate to prove this. As a consequence, the habitat 

 can only be used m a general way for recognizing formations, until we have a 



