38 ENVIRONMENTAL RELATIONS 
one stratum to another makes the division lines difficult to draw in some 
cases. Still, the recognition of strata is essential but a rigid classification 
undesirable. Consocies boring into the wood of living trees probably 
should be considered as consocies relatively independent of stratification 
phenomena (13). 
Associations are groups of strata uniform over a considerable area. 
The majority of mores, consocies, and strata are different in different 
associations. A minority of strata may be similar. The term is applied 
in particular to stages of formation development of this ranking. The 
unity of associations is dependent upon the migration of the same indi- 
vidual and the same mores from one stratum to another at different times 
of day or at different periods of their life histories. Migration is far 
more frequent from stratum to stratum than from one association to 
another (13). 
Formations are groups of physiologically similar associations. For- 
mations differ from one another in all strata, no two being closely 
similar. The number of species common to two formations is usually 
small (e.g., 5 per cent). Migrations of individuals from one formation 
to another are relatively rare (13). 
Extensive formations are groups of formations clearly influenced by a 
given climate in the case of land formations and by the topographic age 
of a large area and by climate in the case of aquatic formations (13, 58q). 
A sub-formation is an association or a poorly developed phase ot a 
formation. The term is used in comparing communities of the ranking 
associations when viewed from the standpoint of physiological differ- 
ences but without reference to genetic history. Accordingly the same 
community is referred to as an association in the genetic sense and a 
sub-formation when the point of view is that of physiological difference 
or resemblance. 
b) Animal communities of the forest-border region.—The forest-border 
region is the western line of demarkation between forest and steppe (see 
prairie area of Fig. 8, p. 51). The following is a list of the chief animal 
communities of the area about the south end of Lake Michigan. It is 
not intended to be complete, but rather to illustrate the use of the terms 
with particular reference to the communities to be mentioned later on. 
The term community is used in the general sense. Association is 
applied to stages in genetic development, with sub-formation as an 
alternative as defined above. The classification here presented in outline 
is artificial and attempts to combine the historical or genetic with the 
