FISHES OF ILLINOIS 



and an average of 23. The two highest single ratios of ichthyo- 

 logical affiliation are those of the Illinois and Mississippi rivers (. 72) 

 and of the Big Muddy and Saline (.70). 



Number of Species Common to each Pair of Districts, and Ratios 



of such Common Numbers to the whole Number 



of Species in each Pair 



The data of this table may be generalized by bringing into com- 

 parison the average of the ratios of affiliation for each district with 

 those for all the rest, as shown in the column of figures farthest to 

 the right. If the ten districts are arranged in the order of the size of 

 their average ratios, they readily fall into two groups, the first of six 

 districts, with relatively high ratios, and the second of four, with 

 relatively low ratios. The first group comprises the basins of the 

 larger rivers— the Mississippi, the Rock, the Illinois, the Kaskaskia, 

 the Wabash, and the Ohio, each with its more or less complex system 



