278 ILLINOIS ACADEMY OF SCIENCE 



It will be noted that of the returned species, one is 

 different (P. Integra) while four are \\\\^&mg,Galba 

 caparta, Pliysa gyrina and P. sayii, and Musculkim 

 partiimeium. It frequently happens that when a fauna 

 returns to a habitat from which it has previously been 

 driven, it is made up of a different aggregation of species 

 (See Ortmann, 1909, for additional notes on this sub- 

 ject). 



In the case of the Genesee River we have a striking 

 example of the history of a polluted stream and its ef- 

 fect on one group of the animal life. Previous to the 

 stage of great pollution there is a varied fauna of mol- 

 lusks 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 we find that the mullusks 

 have returned in as great numbers as before the maxi- 

 mum 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. At the present time the sturgeon, 

 which formerly resorted to the river to feed and breed, 

 and had been driven out by the polluted condition of the 

 stream, has returned, which is another indication of im- 

 proved conditions. It is quite probable that the large 

 fall in the river, some 60 feet in height, has had a marked 

 effect in the return of these favorable conditions. 



A study of the Salt Fork of the Big Vermilion River, 

 now^ in progress, indicates that all clean water life, in- 

 cluding mussels and crayfishes, has been excluded from 

 this stream for a distance of fourteen miles, and a normal 

 fauna of these animals is not encountered until a dis- 

 tance of twenty miles has been traversed. The shallow- 

 ness of this stream has evidently provided a sufficient 

 supply of dissolved oxygen and it is apparent that in a 

 deeper stream the ill effects of sewage pollution would 

 be experienced for a much greater distance. 



