276 



A series of ten consecutive hauls made on the afternoon of 

 August 21, 1896, from a floating boat between the bend in the 

 river above the plankton station and the towhead below it (PI. 

 II.) throws some light on the questions of local distribution and 

 of variation in catches from a limited area. Owing to the wind 

 it was not possible to float with the current, and the apparatus 

 also served to impede the boat. The river stood at 7.1 ft. above 

 low water and was falling slowly, so that the current was not 

 so strong as when the ten were made from the anchored boat. 

 The test occupied about eighty minutes, and the boat drifted 

 about a mile, so that the body of water actually passing it, 

 from which the plankton was taken, was less than half a mile 

 in length. Considerable dislodged vegetation and some cattle- 

 yard debris were floating at the time, causing more than 

 the usual inequality in the distribution of the silt which 

 these elements introduce into the plankton. The catches 

 ranged in centrifuged volume from .4 to .575 cm. 3 , averag- 

 ing .48, and showing an average of divergence of ±11.2 per cent, 

 from the mean, with limits of +19.9 and —16.6 — a total of 36.5 

 per cent. The divergence in this test is greater than that 

 from the anchored boat, owing in part to the floating debris, 

 and in part, probably, to the fact that the wind drifted the boat 

 across fully three quarters of the channel. 



These divergences, both in average and limits, fall within 

 the figures of parallel catches in lake waters quoted above from 

 Apstein ('96) and computed from Reighard ('94). The fact 

 that the range of variation on the whole is greater than the 

 average run of Apstein's results is doubtless due in part to the 

 larger number of catches included in my test. 



These two tests thus indicate that the plankton of the 

 main channel waters of the Illinois at the point where our col- 

 lections are made, is distributed quite as evenly as that in lakes 

 thus far examined from this point of view, and in consequence 

 single collections may be utilized for the study of plankton 

 problems with no greater error for the potamoplankton than for 

 the limnoplankton. The divergence from the mean will upon 



