70 ILLINOIS BIOLOGICAL MONOGRAPHS [168 
buffalo, and sheepshead, which were formerly very common and taken in 
quantity by the fishermen several years ago, are now either wanting, or 
greatly reduced in numbers. Other fish, not bottom feeders, such as 
sunfishes, crappies, and the basses, are reported to be decreasing in recent 
years as polluted conditions are creeping down the river (Forbes and 
Richardson, 1913:544). It has been observed that fish entering a polluted 
stream from a clean-water tributary soon die if unable to return to clean 
water. The fauna of a polluted stream also becomes gradually of greater 
size as the distance from the source of pollution increases. This has been 
observed by Forbes and Richardson in the Illinois River, by Ortmann in 
the Allegheny River, and by the author in the Big Vermilion River. 
The time necessary for the recovery of the normal biota of such a stream 
will in most cases be of long duration and in the case of a stream polluted 
by wastes from mines and chemical manufacturies, there may never be a 
return to the original condition. 
In New York State, the Genesee River, at Rochester, has afforded a 
striking example of stream pollution, of the effect of this pollution on cer- 
tain animal life in the river, and of the return of this life when the amount 
of pollution has been largely reduced. This stream has been under observa- 
tion by the writer for a period of twenty-seven years (1892 to 1919) and 
collections of the molluscan life have been made from time to time, both 
before the period of maximum pollution and since that time. The portion 
of the river studied lies below the lower falls north of the city, and about a 
quarter of a mile below the outfall of several trunk sewers, the sewage 
being discharged into the river in a crude condition. Refuse and other 
waste matter, both liquid and solid, also enter the stream from gas works, 
tanneries, and manufacturing plants above the lower falls. 
Collections made in 1892, before pollution became notably apparent, 
included nine species of gastropod mollusks, three being water breathers 
and six air breathers. These species included: 
Musculium transversum Physa saytt 
Musculium partumeium Physa oneida 
Bythinia tentaculata Galba catascopium 
Planorbis trivolvis Galba caperata 
Physa gyrina 
Individuals were notably abundant, thickly covering the rocks and 
the shore. In 1897, it was observed that the sewage was increasing in 
volume and pollution was becoming more noticable, the water appearing 
like very heavy, greasy dish water. The river was visited and examined 
at short intervals from 1898 to 1919. Each year it was noted that pollu- 
tion was rapidly increasing. In 1907, the water-breathing mollusks, 
Musculium and Bythinia, had succumbed and none could be found. 
The air-breathers, Galba, Planorbis, and Physa, still held out, though 
