Article IX. — An Ornithological Cross-section of Illinois in 

 Autumn. By S. A. Forbes. 



The subject of the relations of interaction between organisms 

 and their environment, animate and inanimate, which goes by the 

 name of ecology, may be studied with reference to the welfare of 

 species or to that of the general assemblage of organisms to which 

 the species belong. The ecology of a species is special ecology; that 

 of the assemblage is a phase or division of general ecology — more or 

 less general according to the size and contents of the assemblage 

 considered. In special ecology every ecological factor, every feature 

 of the environment, is valued according to its importance to the 

 species ; in general ecology the various ecological factors are valued 

 according to their significance in the general system of life. In special 

 ecology the species is the all-important, dominating center ; in general 

 ecology each species takes its appropriate place — dominant, important, 

 subordinate, or insignificant — according to its dynamic value as a part 

 of the whole. 



Precise studies in animal ecology have heretofore been made 

 mainly in the special field, necessarily so in the beginning since a 

 knowledge of the ecology of species must precede that of groups or 

 assemblages of species. These special studies are, however, merely 

 preliminary to a general study of the dynamic system of organic life 

 as exhibited in its larger and more complex units. Without the 

 corrective and organizing influence of such a study of the system as 

 a whole, our ideas of that system must be badly proportioned and 

 correspondingly inadequate or misleading — a fact readily illustrated 

 by the state of our knowledge and opinion respecting the ecological 

 significance of birds. 



To learn what we now know of the effects of the activities of 

 birds has required much difficult, expert, time-consuming study, espe- 

 cially of the details of their food, since it is mainly through the food 

 relation that birds affect the welfare of other animals and of plants. 

 These studies, although both qualitative and quantitative as related to 

 the welfare of the various species of birds themselves, have been quali- 

 tative only as concerning the relation of birds to the general welfare; 

 and we have little but vague estimate and doubtful surmise in place 

 of a definite knowledge of the relative ecological values of the various 



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