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 



(The observations on Pettibone Creek were repeated in four succeeding years 

 with the same results. Stars indicate presence; numbers refer to Fig. 2) 



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- 



ffiMir 



