557 
are able to compute the mean annual production on the basis 
of 235 observations extending over a period of five years. At 
present writing no series of observations of like, or even approx- 
imately like, extent has been published concerning any other 
body of water. Comparisons upon this basis are therefore not 
possible. 
The scientific or economic value of comparisons upon other 
bases than the mean annual production or the full seasonal 
course of production cannot be great, unless it be for coinci- 
dent seasons. Furthermore, data of production without com- 
parable environmental data lose much of their significance. 
The volumetric determination of the plankton of streams 
elsewhere has not been carried on to any considerable extent. 
Steuer (01), in his paper upon the entomostracan fauna of the 
backwaters of the Danube at Vienna, gives a brief list of organ- 
isms observed in the plankton of the Danube itself, but no vol- 
umetric determinations. His conclusion regarding the potamo- 
plankton—a term whose very validity he contests—is: “Das 
einzige wichtige Ergebniss der ‘ Potamoplanktonforschung’ 
scheint mir bis jetzt die Feststellung der grossen Armuth un- 
serer fliessenden Gewasser an Mikroérganismen zu sein.” It 
isin a similar vein that Whipple (99) notes the paucity of 
plankton organisms in rivers, resulting presumably from sani- 
tary examinations of streams of New England. The results of 
our investigation upon the Illinois River place limitations on 
these conclusions. It is a question of time and nutrition and 
the absence of deleterious industrial wastes. Given ordinary 
stream water free from poisonous industrial wastes and suffi- 
cient time for breeding, a typical plankton may be expected, it 
seems, in every river, especially the larger ones, and in the 
lower reaches of those of the smaller type. 
Steuer’s volumetric work on the two backwaters of the 
Danube, covering, it seems, nineteen collections in a period of 
fifteen months, from June, 1898, to August, 1899, indicates a 
planktograph somewhat similar to our backwater plankto- 
graphs in that he finds a vernal pulse in May and a midsum- 
