292 THE FORMATION 
sented by a few scattered individuals. This difference is much more striking 
in separate examples of the same formation, particularly when a normal 
facies is reduced to the numerical value of a secondary species. This is a 
matter of great importance in the study of formations, for it has doubtless 
often resulted in mistaking a consocies for a formation., 
Alternation furnishes the logical basis for what may be called comparative 
phytogeography. The latter is of much broader scope than the old subject 
of geographical distribution, for it treats not only of the distribution of for- 
mations and associations as well as of species, but it also seeks to explain. 
this by means of principles drawn from the relation between isabitat and 
vegetation. When the latter come to be fully based upon physical factor in- 
vestigations, and upon the effects of migration and competition as shown in 
alternation, the comparative study cf formations will represent the highest 
type of phytogeographical activity. 
THE FORMATION IN DETAIL 
346. The rank of the formation. There have been. as many different 
opinions in regard to the application of the term formation as there are con- 
cerning the group which is to be called a species. In taxonomy, however, the 
concept of the species is purely arbitrary, and agreement can not be hoped 
for. In vegetation, on the contrary, the connection between formation and 
habitat is so close that any application of the term to a division greater or 
smaller than the habitat is both illogical and unfortunate. As effect and 
cause, it is inevitable that the unit of the vegetative covering, the formation, 
should correspond to the unit of the earth’s surface, the habitat. This places 
the formation upon a basis which can be accurately determined. It is im- 
perative, however, to have a clear understanding of what constitutes the 
difference between habitats. A society is in entire correspondence with the 
physical factors of its area, and the same is true of the vegetation of a prov- 
ince. Nevertheless, many societies usually occur in a single habitat,’and a 
province contains many habitats. The final test of a habitat is an efficient 
difference in one or more of the direct factors, water-content, humidity, and 
light, by virtue of which the plant covering differs in structure and in spe- 
cies from the areas contiguous to it. A balsam-spruce forest shows within 
itself certain differences of physical factors and of structure. The water- 
content will range from 20-25 per cent, and the light from .o2-.003. One 
portion may consist chiefly of Pseudotsuga mucronata, another of Picea 
engelmannii, and a third of Picea parryana, or these species may be inter- 
mingled. If, however, this forest is compared with the gravel slide, which 
touches it on one side, and the meadow thicket, which meets it on another, 
