304 THE FORMATION 



habitat system for summarizing the essential structure of a vegetation, in- 

 asmuch as it places the emphasis upon historical rather than structural fea- 

 tures. It is evident that both deal with the same formations, and that the 

 difference is merely one of viewpoint. The habitat classification is simpler 

 in that it considers only those formations actually on the ground, while de- 

 velopment has regularly to take into account stages which have disappeared. 

 Tlie groups of the developmental system, and. the arrangement of formations 

 within them have already been indicated under the nomenclature of succes- 

 sion (sections 326 and 327). 



354. Regional classification. The grouping of formations with respect 

 to the divisions of vegetations is chiefly of geographical value. It indicates a 

 certain general relationship, but its principal use is to summarize the struc- 

 ture of the vegetative covering of a region. The arrangement of formations 

 in the various divisions is made with reference to the outline of North 

 American vegetation (section 341). This is naturally based upon the iden- 

 tity of altitude and latitude zones. In the study of mountain countries, it is 

 often desirable to group formations with reference to altitude alone. In 

 this case, the grouping is based upon the following divisions : ( i ) hathyphy- 

 tia, lowland plant formations; (2) mcsiophytia, midland formations; 

 (3) pcdiophytia, upland formations; (4) pagophytia, foothill formations; 

 (5) orophytia, subalpine formations; (6) acrophytia, alpine formations; 

 (7) chionophytia, niveal formations. 



355. Mixed formations. These are mixtures of two, rarely more, adja- 

 cent formations, or of two consecutive stages of the same succession. Mixed 

 formations are really transitions in space or in time between two distinct 

 formations. Theoretically, they are to be referred to one or the other, ac- 

 cording to the preponderance of species. Actually, however, they often 

 persist in an intermediate condition for many years, and it ljcomes necessary 

 to devote considerable attention to them. In some cases, there is good rea- 

 son to think that the species of two contiguous formations have become per- 

 manently associated, and thus constitute a new formation. This is often 

 apparently true in succession, when the change from one stage to the next 

 requires a long term of years, but it is really true only of the very rare cases 

 in which a succession becomes stabilized m a transition stage. When the 

 mixture is due to development, the formations concerned are often quite dis- 

 fimilar, e. g., grassland and thicket, thicket and forest. If it is the result of 

 position, the formations arc usually similar, i. e., both are grassland, thicket, 

 or forest, since the plants of the lower level are regularly assimilated or de- 

 ftroved, when invasion occurs at two levels. The term mictiiim (fUKTcv, 



