Article VIII. — On the Local Distribution of certain Illinois 

 Fishes: an Essay in Statistical Ecology. By S.A.Forbes. 



An animal society is composed of animals habitually oc- 

 curring together in the same locality and the same class of sit- 

 uations. Such an association is, of course, composed of many 

 species, variously related to their special environment, some 

 attracted to it by one set of conditions and some by another. 

 Although their local haunts may be virtually identical, their 

 ecological relations, if determined in detail, may prove to be 

 very different. A pike and a minnow may be members of the 

 same associate group, to whose habitat, however, the pike is 

 especially attracted by the minnow, and the minnow by the 

 facilities which are offered there for concealment or escape 

 from the pike. 



It is usually possible to learn the contents of a local associ- 

 ation of plants by simple inspection and enumeration; but an- 

 imals come and go, elude observation, and refuse to be num- 

 bered, and the details of their associate occurrence can only be 

 learned indirectly, by means of sample collections preserved for 

 subsequent study. If the situations from which such collec- 

 tions are made are carefully chosen and correctly classified, 

 and if the collections themselves are full enough, uniform 

 enough, and numerous enough to be fairly representative of 

 each situation, the essential facts concerning the assemblage 

 of animals corresponding to any unit of environment may be 

 readily made out. The making of such collections for such a 

 purpose is, however, a relatively new thing, and scarcely a be- 

 ginning has been made in the systematic study of animal asso- 

 ciations by this method. 



A knowledge of definitely circumscribed, or merely meas- 

 urably distinct, local associations does not, however, by any 

 means exhaust the subject of associate relations, for the ani- 

 mals of a region cannot be wholly divided up into such definite 



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