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. 
The 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 
eften desirable to group formations with reference to altitude alone. In 
this case, the grouping is based upon the following divisions: (1) bathyphy- 
tia, lowland plant formations: (2) mesiophytia, midland formations; 
(3) pediophytia, 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 ot the other, ac- 
cording to the preponderance of species. Actually, however, they often 
persist in an intermediate condition for many years, and it becomes 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 
reqnires a long term of years, but it is really true only of the very rare cases 
in which a succession becomes stabilized in a transition stage. When the 
inixture is due to development, the formations concerned are often quite dis- 
similar, e. g., grassland and thicket, thicket and forest. If it is the result of 
position, the formations are usually similar, i. e., both are grassland, thicket, 
or forest, since the plants of the lower level are regularly assimilated or de- 
stroyed, when invasion occurs at two levels. The term mictium (uxrev, 
