REPORT OF OFFICERS AND COMMITTEES 33 
7. Ina general way the trend of the associations on the 
black soil and clay prairies follows the changes in the moisture 
content of the soil as the physiography of the regions develop. 
In the sand prairies transpiration, stability of the soil, and 
probably nutrition, are also factors of prime importance. 
8. The data also support the theory that many of the black 
soil prairies of Illinois originated from glacial lakes ‘and 
swamps, and have existed as prairies since glacial times. 
The work of the Natural History Survey on the lakes of the 
Fox River district in Illinois has been under the immediate 
charge of Mr. R. E. Richardson, biologist of the State Lab- 
oratory staff. In these operations, we have returned to an 
area in which the survey did considerable work more than 
thirty years ago, and have consequently the means of compar- 
ing present and former conditions. Twelve collecting stations 
were selected last spring, one on Fox River at Algonquin, 
where the field headquarters were established, three on Pistakee 
Lake, and the others on Fox, Nippersink, Cedar, Deep, Sand, 
Long, Crystal, and Zurich lakes. The stations were each visit- 
ed ten times between June 6 and November 29, for thorough- 
going collections from the open water, the bottom, and the 
shores, a single round of all the stations usually requiring from 
five to eight days. These collecting stations were chosen to 
give as wide a variety as possible of contrasting conditions and 
environments ; and the product of the work is enabling us to 
make some interesting and important comparisons between the 
biota of our lakes and that of our rivers, especially with respect 
to the plankton; between that of the Illinois and of the Fox; 
and that of the several lakes, each fairly typical of its special 
class. The yearly history of the life of a lake, and the trans- 
formations which it undergoes as the season progresses, are 
also proving most interesting, especially as each of the differ- 
ent kinds of lakes seems to have its more or less peculiar sea- 
sonal regimen. It is too soon to generalize from the product 
of our studies, even in a preliminary way, but the field opened 
up by these beginnings is a very promising and inviting one. 
In the course of the year’s work of the State Entomologist’s 
office, two important papers have been published which con- 
tain a considerable amount of ecological data—one by Dr. 
Philip Garman, on the Zygoptera, or damsel-flies, of Illinois, 
