482 THE AMERICAN NATURALIST [Vol. LII 



environmental complexity is so great that it is bewilder- 

 ing to many, particularly to those who have not followed 

 the most recent methods of dealing with the vegetation 

 and gross physical environment. For convenience in 

 handling, this complex may be broken up advantageously 

 into smaller systems or units which are the agencies 

 which influence animals. This plan provides for both 

 their qualitative and quantitative relations, because the 

 agents provide for the qualitative units, and their dy- 

 namic relations include their quantitative intensities. 



In dealing with the interaction of systems relating to 

 animals, one of the first points to consider is the classifi- 

 cation of these systems, and the recognition of the sizes 

 of the units. Many groupings are possible, such as the 

 individual animal, its plant and animal associates, and 

 the numerous factors of the physical environment. Fur- 

 ther analytical systems of the vegetational environment 

 can be grouped according to the recognized units current 

 among the students of the genetic aspects of vegetational 

 development (see Cowles, Clements, etc.). For the phys- 

 ical environment the geologists, physiographers and 

 geographers have already made much progress in the 

 analysis of unit systems, which can be used with com- 

 parative ease (see Chamberlin, Salisbury, Van Hise, 

 Davis ( '09), etc.). In the study of all these systems natu- 

 rally more progress has been made in their recognition, 

 than in their complex modes of interaction; and the for- 

 mulation of their laws of interaction is of the greatest 

 importance. There are three models which really come 

 to mind in this connection. These are : 



1. The physical model of the interaction of forces, 

 which leads to resultant motion. 



2. The application of Bancroft's law to the interaction 

 of all systems. 



3. The application of the physical and chemical model 

 of the phase rule of Gibbs to equilibria of all kinds. 



These will now be considered in their respective order : 

 1. The physical model will assist in keeping in mind 



