352 MARINE BIOTIC COMMUNITIES 



of a species as a community indicator from this point of view, it 

 must be: 



1. Abundant. 



2. Uniformly distributed. 



3. Always present in considerable abundance when other spe- 

 cies decline almost to zero. 



4. The limits of its range must coincide with those of various 

 less stable constituents. 



The life forms that signalize the most important constituents serve 

 as important criteria, just as they do for land plants. 



Petersen's use of single species as indicators has led to apparent 

 confusion in the minds of other investigators (Ford, Davis, Stephens, 

 et al.). A closer adherence to the practice of plant ecologists allows 

 a greater latitude. In this, a large series of dominants of a limited 

 number of life forms is recognized, and in some situations nearly all 

 are found mixed together (Clements, 1920; Braun, 1935). They 

 usually segregate into large units or associations, each characterized 

 by a definite group of wide-ranging and restricted species character- 

 istic of the particular association. Toward the outskirts and in local 

 areas some of these drop out and occasionally some species are added. 

 These variations are those called faciations. 



Petersen's reluctance to consider fishes and other motile forms a 

 part of the bottom community left something to be desired. His work, 

 however, was superior, from a bio-ecological viewpoint, to that of 

 most plant ecologists, because he and his associates were primarily 

 concerned with the food of fishes and worked out the interchange of 

 effect between the motile and sessile constituents. They merely found 

 difficulty in connecting fishes with the communities in mapping, and 

 the fishes, etc., were treated in a somewhat detached manner. They 

 made many facts available; however, our attempt to organize them in 

 the preceding pages is doubtless quite imperfect. The same is true 

 of the endeavor to synthesize the animal constituents of the land com- 

 munities considered. A certain amount of reinvestigation will be 

 necessary to establish definite facts in all cases. 



COMPARISON OF MARINE AND TERRESTRIAL COMMUNITIES 



The preceding pages have indicated that the phenomena of dis- 

 tribution of relatively stationary organisms on the sea bottom and on 

 land are quite similar and lend themselves to a similar type of classi- 

 fication for greater ease in description. The failure to find succes- 



