378 



In view of this fact, added to the generally rational and therefore 

 explicable nature of the results, it seems not unreasonable to suppose 

 that in the total of one hundred and fifty-two hauls in Peoria Lake 

 between 1913 and 1923 (46 in 1913-1915, as a part of a total of 387 

 for the entire river; and 106 in 1920-1922) we probably obtained in 

 both periods examples of at least most of the commoner species inso- 

 far as these are present during the summer season. 



As all subdivisions of Peoria Lake were well supplied with coarse 

 aquatic vegetation over a large portion of their area up to 1915, in 

 order to obtain fair comparison with our 1920 and 1922 records, w-hich 

 were made after the old weed-beds had been destroyed, it has been 

 necessary to exclude a good many of the less distinctly bottom forms 

 of the pre-1920 lists. The exclusions cover a sizable assortment of 

 free-swimming, weed-dwelling, and air-breathing species, not belong- 

 ing in a proper sense to the bottom, which formerly entered in con- 

 siderable numbers into bottom collections made in the shallower areas 

 where there was more or less vegetation. Most of the excluded forms, 

 it may be noted, have also disappeared since nine or ten years ago, 

 along with the vegetation, over practically the entire area of Peoria 

 Lake. 



Having regard at present, then, only to the more exclusively 

 bottom-species, both in the case of the earlier and the more recent 

 records, we find that we took as nearly as may be calculated a total 

 of 22 or more kinds in the upper lake in 1913-1915, of which at least 

 8 have since disappeared ; 26 or more species in the middle lake, of 

 which 12 have since dropped out ; 43 or more species from the lower 

 lake, of which 19 have disappeared ; and from the bottom muds of 

 all three lakes combined no less than 61 species, of which at least 23 

 are missing in recent collections, although recent hauls have been more 

 than double the number of the earlier ones. 



In evident contrast with the reduction of representation in the 

 lists of more sensitive species, the tabulation (p. 383) indicates good- 

 sized additions in all three lakes to the totals of more tolerant kinds 

 since 1915. While a part of these apparent additions in the ix)llu- 

 tional or tolerant column have undoubtedly taken place, especially in 

 the Tubificidae and Chironomidae, it is probable that the more complete 

 identification of the more recent material accounts for a part of them. 

 It is, however, quite certain that there has been large increase in 

 numerical abundance since 1915 of some of the more pollutional forms. 



