ECOLOGICAL SUCCESSION. 359 



mode of life, reaction to physical factors, time of seasonal appear- 

 ance, are noticeable between the formations and lesser differences 

 of the same sort between the associations. 



We note from Fig. II showing the relation of communities, 

 that intermittent streams become permanent, gravel bottom 

 gains ascendency over riffles and silt bottom over gravel bottom, 

 which is accompanied by a decrease in strength of current. All 

 types of streams converge to the base level stream, all large 

 lakes toward small lakes, which are parallel with the sluggish 

 streams. These small lakes become ponds and finally dry land, 

 in the forest border region, either prairie or forest. Forest 

 margin or thicket is a tension line and may shift position rapidly. 



The relation of the different habitats is a genetic one, the most 

 permanent habitats being the sluggish stream and climax plant 

 communities. Each is characterized by different mores, and 

 as the one habitat is transformed into another the mores change 

 accordingly. 



III. GENERAL DISCUSSION. 



The environmental processes, which we are discussing are those 

 in which organisms have existed since their origin on earth. 

 The stresses and strains to which organisms have been subjected 

 have been in the same direction for long periods. Now that we 

 have learned much concerning organic response to environment, 

 such as physiological response, behavior response, and structural 

 response, we note at once that processes of adjustment and 

 equilibration of living substance may bear important relations, 

 on the one hand to environmental processes and on the other to 

 the physiological aspect of biological phenomena. Ecological 

 classification is then worthy of attention. 



With all of their imperfections and uncertainties, the ideas of 

 phylogeny which are presented in our phylogenetic system of 

 taxonomy are an important asset in zoological thinking from the 

 point of view of structure and development. The classification 

 which ecologists are striving to build up will serve a purpose in 

 behavior, physiology, and ecology, analogous in this respect to 

 that served by the phylogenetic classification in morphological 

 thought. It should however be flexible rather than rigid and 



