PAPERS OX ZOOLOGY 275 



is badly polluted ( Shelf ord, 1918, p. 27; Wells, 1918, pp. 

 562-567). Young fisli are relatively more sensitive than 

 adult fish. It is noteworthy that the more resistant 

 species of fish are inhabitants of sluggish bodies of 

 water, as ponds and shallow lakes, while the least resist- 

 ant species live in running streams. It seems to be a 

 question of the amount of oxygen necessary for the well 

 being of the fish. 



The ill effect of sewage pollution is most marked on 

 the bottom of bodies of water, where a sludge is foimed, 

 often of great thickness (as nmch as ten feet in several 

 cases), consisting of a mass of soft, black, sediment, with 

 a high content of organic matter, in which only a few 

 organisms, normally inhabitants of polluted streams, can 

 live (e. g. septic Protozoa and Rotifera, foul-water 

 algae, and slime-worms, Tubificidae). This effect on the 

 bottom is perhaps the most serious phase of stream pol- 

 lution because the septic condition of this area continues 

 in operation long after the original source of contamina- 

 tion ceases to operate. This sludge formation renders 

 the bottom unfit for clean-water life upon which many 

 fish depend for food. The time necessary for the recov- 

 ery of the normal biota of such a stream will in most cases 

 be of long duration, and in the case of a stream polluted 

 with wastes from mines and chemical manufacturies, 

 there may never be a return to the original condition. 

 . In Xew York State, the Genesee River, at Rochester, 

 has afforded a striking example of stream pollution, of 

 the eff'ect of this pollution on certain animal life in the 

 river, and of the return of this life when the amount of 

 pollution had been largely reduced. This stream has been 

 under observation by the writer for a period of twenty- 

 seven years (1892 to 1919) and collections of the mollus- 

 ean 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 Rochester, and about a quarter of a mile below 

 the outfall of several large trunk sewers, the sewage be- 

 ing discharged into the stream in a crude condition. 

 Refuse and other waste matter, both liquid and solid, also 



