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THE AMERICAN NATURALIST [Vol. XLY1II 



the sand prairie is the bunch-grass association, well- 

 developed in parts of northwestern, central and north- 

 eastern Illinois, and in northwestern Indiana, in each 

 of which areas, as well as in the sandhills of Nebraska 

 and of eastern Colorado, the writer has studied. Discus- 

 sions of physical, vegetational and animal aspects of the 

 associations of the central Illinois sand prairie, together 

 with an annotated list of the animal species, with data on 

 food, habitat-relations, life-history, etc., are embodied in 

 the writer's paper (1913&), to which constant reference 

 is made. Frequent citations to a more detailed study of 

 local distribution of grasshoppers, in a Michigan area 

 (Vestal, 1913a), and to the many associational studies 

 of Shelford, are to be found. 



The data which have accumulated relate nearly equally 

 to the botanical and zoological aspects of associational 

 study, but since the subject of plant ecology is at present 

 more advanced than that of animal ecology, it has been 

 possible to treat the vegetational side of the problem 

 very briefly, so that more of the discussion relates to 

 animals and animal assemblages. 



The writings most frequently cited are indicated by 

 italic capitals, the full titles appearing in the list of spe- 

 cial references at the end of the paper. 



The writer wishes to thank Dr. Charles C. Adams, Dr. 

 Max M. Ellis and Dr. H. A. Gleason for suggestions and 

 criticism. 



II. INTERNAL ACTIVITIES OF THE ASSOCIATION, AS DETER- 

 MINED BY THE CONSTITUTION OF THE 

 INDIVIDUAL ORGANISM 



The internal activities of the association may be said 

 to be the sum-total of the activities of all the plants and 

 all the animals which make up the association. Such a 

 sum-total of activities may well be thought of as an intri- 

 cate and complicated mass of dependencies. It will 

 simplify the treatment of the entire system of relations 

 if the chief dependencies of the individual organism are 

 first discussed. A knowledge of the ecology of the asso- 



