275 



smaller number in waters of European lakes. For example, 

 Apstein ('96) records 23 instances of hauls on the same date 

 from equal depths and evidently in every case within distances 

 between catches less than that represented in the extremes of 

 our test. The number of hauls did not, however, in any of his 

 tests ex^/Ced four. I have compiled or computed from Apstein's 

 table (pp. 56-57) the average and limits of departure from the 

 mean in these 23 cases. In 12 of the 23 the average de- 

 parture exceeds ±3.58 per cent. — the average departure in 

 our test, in which there were from two and a half to five times 

 the number of hauls. In 10 of the 11 instances in which the 

 departure from the mean in Apstein's records falls below ±3.58, 

 only two hauls were averaged. The total range of the limits of 

 departure also exceeds that found in our test in 8 of the 23 cases. 



In the light of Apstein's results and considering the larger 

 number of catches averaged in our test, and also the considera- 

 ble length of the channel that it covers, it seems beyond rea- 

 sonable doubt that single catches of the plankton in the channel 

 of the Illinois at our station of collection afford as trustworthy 

 a basis for the analysis of plankton problems as do similar 

 catches made in a lake. The margin of error thus introduced 

 is no greater, if indeed so great, as that appearing in investiga- 

 tions in such waters. 



Since these catches were made from an anchored boat, the 

 water from which the plankton was taken was distributed over 

 a considerable length of the stream. The test was made between 

 7:30 and 9:30 a. m. The river stood at 5.1 ft. above low water 

 and was falling rapidly, so that the current was noted at the 

 time as unusually swift, probably approaching two miles an 

 hour in mid-channel at this point. At this rate the collections 

 represent plankton taken at ten intervals from a body of water 

 about three miles in length. This areal distribution is compar- 

 able with, if it does not exceed, the limits of widest distribu- 

 tion of catches in Apstein's tests, but it is much less than that 

 of Reighard's, which lay within an area of about ten by thirty 

 miles. 



