ECOLOGY OF NORTHERN MICHIGAN. 43 



. VI. Hardwood Forest. This region has been reserved to the last, for 

 the conditions are evidently those toward which the other habitats tend to be 

 changed under the present conditions. The conditions are similar to those on 

 the north slope, and the whole region is covered by a similar type of biota. 

 This society thus represents the climax society of the region. It consists 

 of the forms that are adapted to or associated with the conditions which 

 prevail in this region in the last stages of the mutual adjustment of all the 

 environmental processes. As the processes become adjusted to one another, 

 the habitat of the climax society is increased at the expense of the other 

 habitats, and the associated biota tends to become of general geographic 

 extent in the region. 



7. INTERPRETATIONS AND CONCLUSIONS. 



From the conditions of life in this region as summarized above, the follow- 

 ing interpretations and conclusions seem justifiable. 



Owing to the dependence of forms of life on their environment, biotic 

 changes are necessarily closely related to environmental changes. These biotic 

 changes may occur in two ways; the forms must either be able to respond 

 to the new conditions or be supplanted by other forms. That they tend 

 to become adjusted cannot be questioned, but in many cases at least this 

 adjustment lags behind the changing conditions, and the forms are replaced 

 by others from adjacent habitats which are adjusted to the conditions 

 toward which the particular habitat is changing, thus bringing about a succes- 

 sion of societies. 



To understand, therefore, the succession of societies in a region it is neces- 

 sary to know both the environmental conditions and the processes that 

 modify them. The environmental conditions are brought about by the 

 association of certain environmental factors, such as the geographic, physio- 

 graphic, organic, edaphic (the chemical and structural composition of the 

 rock and the depth of the soil), time, and climatic factors. These divisions 

 are arbitrary, for the different factors are so intimately related that they 

 can only rarely be separated, and although certain ones may predominate 

 in different habitats, it is impossible, owing to this interrelation, to explain 

 the distribution of animals or plants on the basis of a single factor, for all 

 are more or less involved in the formation of a habitat. This is one of the 

 primary reasons for emphasizing habitat dynamics. 



It will be seen by reference to Van Hise ('04, p. 40) that the environmental 

 factors given above are the same as the geological factors in the belt of 

 weathering. Each of these factors is the resultant of various processes 

 (composed in turn of physical forces, heat, light, etc.) which when not in 

 equilibrium tend to become so. The adjustment of these processes to each 

 other brings about changes in the conditions which can only approximately 

 cease when they approach an equilibrium, as, for example, when the topo- 

 graphy has been reduced to a base-leveled plain covered by a layer of residual 

 soil. The conditions in habitats where the processes are not in equilibrium 

 are thus being constantly changed in the direction of other habitats in which 

 they have more nearly reached an adjustment, and a succession of societies oc- 

 curs that only ceases when the processes have become approximately adjusted 

 to each other. The forms that are adapted to the adjusted conditions will 

 constitute the climax society. It is necessary to here emphasize the im- 

 portance of the organic factor; this must also become adjusted to the others 

 for the entrance of new forms into a region may greatly change the equiUb- 

 rium of its societies. 



