265 



assigning an empirical coefficient to each catch. This coeffi- 

 cient was decided upon after analysis or inspection of the plank- 

 ton, and in view of its quantity and constituent organisms, the 

 amount and nature of the silt, and the age of the net, the basis 

 of estimation in each case being the coefficient test by the 

 pumping method whose conditions most nearly approached 

 those of the catch in question. These coefficients were decided 

 upon without knowledge of or reference to the effect which 

 they might have upon the theoretical questions arising from 

 the analysis of the quantitative results, and prior to the organ- 

 ization and analysis of the volumetric data. 



Obviously this latter method involves both possible and 

 probable error in estimation of similarities and differences in the 

 catches examined and in maintaining throughout a uniform 

 standard. Nevertheless, for plankton catches as varied as those 

 with which we deal, it is probable that this method involves less 

 distortion of volumetric results than the omission of the coeffi- 

 cient factor or the adoption of a uniform factor for all catches 

 irrespective of the fluctuations in this factor as revealed by 

 our held tests. Accordingly all of the volumes of catches by the 

 drawn net, of plankton, silt, and total catch per cubic meter 

 recorded prior to May 20, 1896, in Tables III.-IX. have been 

 computed with this coefficient as one of the factors, the actual 

 factor employed being given in the tables in each case. 



The results of my efforts (see Kofoid, '97a) to find an ade- 

 quate correction for the loss by leakage through the silk by the 

 use of hard-pressed filter paper and the Berkefeld army filter 

 will be discussed in another connection. 



The volumetric data of the plankton at the seven stations 

 (see PI. II.) at which periodical collections were made, name- 

 ly, the river channel (E), Spoon River (M), Quiver Lake (C), 

 Dogfish Lake (L), Flag Lake (K), Thompson's Lake (G), and 

 Phelps Lake (F) will now be discussed, and the general ques- 

 tions arising from the investigation as a whole will then be 

 treated. 



The chronological series of collections at these seven 



