72 ILLINOIS BIOLOGICAL MONOGRAPHS {170 
It will be noted that of the returned species, one is different (Physa integra), 
while four are missing, Galba caperata, Physa gyrina, Physa sayii, and 
Musculium partumeium. It frequently happens that when a fauna returns 
to a habitat from which it has been driven by unfavorable conditions, it 
is made up of a different aggregation of species (see Ortmann, 1909, for 
additional notes on this subject). 
The Genesee River is a striking example of the history of a polluted 
stream and its effect on one group of animal life. Previous to the stage 
of greatest pollution there is a varied fauna of mollusks very numerous in 
individuals. In the course of eleven years the gill-bearing species are 
forced out and after a lapse of fourteen years all molluscan life ceases to 
live in this part of the river. Seven years later the greater amount of 
sewage is diverted to another outlet. Two years after this change the mol- 
lusks have returned in as great numbers as before the maximum stage of 
pollution. The significance of all this lies in the fact of the early return of 
this life and strikingly indicates that streams may become restocked with 
life in a short period after pollution has ceased to be of an unfavorable 
character, provided, of course, the bottom of the stream has not been 
made permanently untenable by the deposition of poisonous substances 
that cannot be washed away by ordinary river currents. It is quite prob- 
able that the large fall of water, some 60 feet in height, immediately above 
the sewage outlet, has had a marked effect in the return of these favorable 
conditions. 
No additional data are at hand indicating the changes in a polluted 
stream after septic conditions have ceased or become greatly modified. 
It is probable that similar beneficial results would be obtained in other 
streams if the sewage was diverted or treated to remove the large amount 
of organic matter. In the case of a stream like the Salt Fork, the septic 
condition of which will be discussed in the following pages, it would 
probably not require a very long time to reduce the septic conditions if 
the sewage from Urbana and Champaign were properly treated. While 
the putrescible matter at present covers everything, in some places to a 
considerable depth, the high water during the spring would in several 
seasons remove a large part of this material, and if no additional matter 
was permitted to flow into the stream, the lapse of a few years would 
enable nature to bring the stream back to a normal, healthy condition, 
and make it a place to seek for recreation instead of a place to avoid on 
account of its filth, as at present. 
SEWAGE POLLUTION IN THE SALT FORK 
The sewage and other wastes of the Twin Cities of Urbana and Cham- 
paign are discharged into the waters of the Salt Fork by separate systems, 
that of Urbana emptying into the Boneyard near the Big Four shops, while 
