290 THE FORMATION 
or stream to stream. The appearance of new or denuded soils upon which 
successions establish themselves is the most important cause of the alterna- 
tion of formations. The weathering of rocks in different areas of the same 
region produces in each a sequence of similar or identical formations. The 
same statement is true in general of other causes of succession, such as ero- 
sion, flooding, burning, cultivation, etc., wherever they operate upon areas 
physically similar and surrounded by the same type of vegetation. The areas 
of more or less heterogeneous formations characterized by major physical 
differences are occupied by consocies. In an extensive formation, the 
same consocies alternates from one to another of these areas that are simi- 
lar. When the formation is interrupted and occurs here and there in sep- 
arate examples, a consocies often alternates from one to another of these. 
A consocies regularly derives its character from the fact that one or more 
of the facies of the formation is more intimately connected with certain 
areas of the latter than with others. This explains why the alternations of 
consocies and facies are usually identical. Layers sometimes alternate be- 
tween different examples of the same forest or thicket formation, when they 
are suppressed in sone by the diffuseness of the light. 
The alternation of species is a typical feature of formations; it is absent 
only in those rare cases where the latter consist of a single species. The 
areas of a habitat which show minor physical or historical (i. e., competi- 
tive) differences are occupied by groups of individuals belonging to one or 
more species responsive to these differences. Each of these groups will recur 
in all areas essentially similar, the intervals being occupied of course by 
slightly different groups. Such groups are constituted by gregarious or copi- 
ous species of restricted adjustability. Sparse plants likewise alternate, but 
they necessarily play a much less conspicuous part. In habitats not too heter- 
ogeneous, a large number of species are sufficiently adjustable to the slight 
differences so that they occur throughout the formation. Often, to be sure, 
they show a characteristic response, expressed in the size or number. This is 
illustrated by the facies and many of the principal species of the prairie for- 
mation. Festuca, Koelera, Panicum, and Andropogon occur throughout, ex- 
cept in the moist ravines which are practically meadows. Astragalus, 
Psoralea, Erigeron, aad Aster grow everywhere on slopes and crests, but 
they are much more abundant in certain situations. Other plants, Lomatium, 
Meriolix, Anemone, Pentsiemon, etc., recur in similar or identical situations 
upon different hills. Lomatium alternates between sandy or sandstone crests, 
Meriolix and Pentstemon occur together upon dry upper slopes, while Ane- 
mone alternates between dry slopes and crests. 
Owing to the accidents of migration and competition, similar areas within 
a habitat are not occupied by the same species, or group of species. A spe- 
