256 THE FORMATION 
the earlier stages than in the later development. Constant, gradual move- 
ment toward a stable formation is characteristic of continuous succession. 
Contrasted with this is intermittent succession, in which the succession 
swings for a time in one direction, from xerophytic to mesophytic for ex- 
ample, and then moves in the opposite direction, often passing through the 
same stages. This phenomenon usually is characteristic only of the less 
stable stages, and is generally produced by a climatic swing, in which a series 
of hot or dry vears is followed by one of cold or wet years, or the reverse. 
The same effect upon a vast scale is produced by alternate elevation and 
subsidence, but these operate through such great periods of time that one 
can not trace, but can only conjecture their effects. A normal continuous 
stuiccession frequently changes its direction of movement, or its type, in trans- 
ition regions or in areas where the outposts of a new flora are rapidly ad- 
vancing, as in wide mesophytic valleys that run down into or traverse plains, 
Here the change is often sudden, and grass and desert formations are 
replaced by thickets and forests, resulting in abrupt succession. Species 
guilds are typical examples of this. More rarely, a stage foreign to the 
succession will be interpolated, replacing a normal stage, or slipping in be- 
tween two such, though finally disappearing before the next regular forma- 
tion. This may be distinguished as interpolated succession. 
The apparent terminus of all stabilization is the forest, on account of the 
thoroughness with which it controls the habitat. A close examination of 
vegetation, however, will show that its stable terms are dependent in the first 
degree upon the character of the region in which the formation is indigenous. 
It is obviously impossible that successions in desert lands, in polar barrens, 
or upon alpine stretches should terminate in forest stages. In these, grass- 
land irust be the ultimate condition, except in those extreme habitats, alpine 
and polar, where mosses and lichens represent the highest type of existing 
vegetation. Forests are ultimate for all successions in habitats belonging to 
a region generally wooded, while grassland represents the terminus of prairie 
and plains successions as well as of many arctic-alpine ones. 
CAUSES AND REACTIONS 
315. The initial cause of a succession must be sought in a physical change 
in the habitat; its continuance depends upon the reaction which each stage 
of vegetation exerts upon the physical factors which constitute the habitat. 
A single exception to this is found in anomalous successions, where the 
change of formation often hinges upon the appearance of remote or foreign 
disseminules. The causes which initiate successions have already been con- 
sidered; they may be summarized as follows: (1) weathering, (2) erosion, 
(3) elevation, (4) subsidence, (5) climatic changes, (6) artificial changes. 
