24 



FRESH-WATER BIOLOGY 



or muddy bottom. Morainic lakes are usually relatively deep 

 with clay bottoms and sides. Solution lakes and ponds of limestone 

 regions usually have abrupt rocky sides. 



TABLE I 



Showing the Distribution of Fish (Nomenclature after Forbes and 

 Richardson) in Three Illinois Streams at the Times Indicated 



I The lower part of Pettibone Creek has been destroyed by the United States Naval School, 

 otherwise the table would include the records for a point 5 and perhaps a point 6, but probably not 7. 



Physical factors include bottom, currents, light, temperature, 

 density, pressure, viscosity, etc. 



The size of bottom materials is an important condition of exist- 

 ence. In streams the current sorts the materials, leaving the coars' 



