Cii FISHES OF ILLINOIS 



even approximately clear. So persistent is this turbidity, due to 

 very finely divided matter in suspension, that the chemists of the 

 Water Survey find it almost impossible to free the water wholly from 

 suspended solids even by repeated filtration. Furthermore, this soil 

 has a definitely acid reaction, to which is due a notable physical dif- 

 ference between the soils of this area and those of the later glacia- 

 tions west and north of it. A surplus of lime in a soil coagulates or 

 granulates it, causing its ultimate particles to cohere in larger gran- 

 ules, while in an acid soil this effect is entirely wanting. This lack of 

 granulation in a very finely divided soil increases, of course, the per- 

 manent muddiness of its waters as compared with those of the other 

 areas in which lime in the soil renders it alkaline. 



The acidity of this southern soil seems not to be of a kind or 

 amount to affect the surface waters sensibly and directly, since the 

 water samples from this region analyzed by the State Water Survey 

 show a soft water, slightly alkaline, and chemically unobjectionable 

 as a medium for fishes. 



CLASSIFICATION AND USE OF ECOLOGICAL DATA 



That these conditions are a part, at least, of the cause of the phe- 

 nomenal distribution of southern Illinios fishes may be shown by a 

 comparison of our ecological data for the fishes of the two lists — one 

 composed of those adapted to the conditions of the lower Illinoisan 

 glaciation and the other of those avoiding them. In the organiza- 

 tion of the data of our collections of Illinois fishes, those concerning 

 the character of the water body in which collections were made were 

 classified in a way to show the number of collections of each species 

 taken from each class of situation. By reducing these numbers to 

 ratios of frequency of occurrence, we have a means of exhibiting the 

 preference of species with respect to the situations in which each oc- 

 curs. Pimephales notatus, for example, was found twenty times 

 over a muddy bottom to thirty-four over a bottom of mud and 

 sand, and to forty-six over a bottom of rock and sand. Aphredoderus 

 sayanus, on the other hand, was found sixty-two times on a muddy 

 bottom to nineteen times in each of the other situations. 



By tabulating data of this description separately for each of the 

 two lists of species referred to — thirty-four species in the one list and 

 thirty-five in the other — and averaging the ratios for each group 

 separately, significant evidence was obtained of the factors which 

 affect the distribution of these fishes. 



The species which distribute themselves freely over southern Illi- 

 nois are those which are generally tolerant of turbid waters, as shown 



