288 



overlap, will be less frequently associated, other things being 

 equal, than tliose whose distribution areas are identical; and 

 species which are equally attracted to some local situations 

 and unequally attracted to others, will be less frequently asso- 

 ciated than those whose local i)references are altogether simi- 

 lar. FurtluM'more. if two species which occupy the same situa- 

 tions in the same area have a widely unlike abundance in dif- 

 ferent parts of this area — one being much the most abundant 

 to the north, for example, and the other to the south — these 

 species will occur together in collections less frequently, 

 will have a lower coefficient of association, than if the 

 two were most abundant in the same section and least abund- 

 ant in the same. The number of joint occurrences will be con- 

 ditioned, in part, in each section of the common area, by the 

 abundance there of the less abundant species. It is impossible, 

 consequently, to distinguish, by a simple inspection of a table 

 of coefficients, local from general factors among the determin- 

 ing causes of difference in associative frequency. For this pur- 

 pose maps of species distribution, and tables showing the local- 

 ity preferences of species (like my Table VI.) must be studied 

 in connection with tables of associative coetticients. 



The causes controlling general distribution and local dis- 

 tribution are alike ecological, those affecting general distribu- 

 tion being usually general — climatic, topographic, hydrograph- 

 ic, and the like — and those affecting local distribution being 

 local. In a small area like that of Illinois, one in which there 

 are comparatively few physical barriers to the intermingling 

 of fishes, these two classes of causes are not widely different, 

 but they must nevertheless be distinguished, so far as possible, 

 if we are to have a clear and correct knowledge of ecological 

 relationships. 



Comparative Study of Tables and Maps. 



As an example of the manner in which these factors may 

 be separated by a comparison of my tables and maps, and of 

 the extent to which associate relationships may be accounted 

 for, we may take a few instances of very low, and others of 



