292 THE FORMATION 



sented by a few scattered individuals. This difference is much more striking 

 in separate examples of ihe 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 Irabitat 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 of 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 P^r cent, and the light from .02-.003. One 

 portion may consist chiefly of Pscndotsttga mucronafa, another of Picea 

 engelmannii, and a thirrl of Picca 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, 



