550 



culent matter occupies a large part of the field as the catch is 

 seen in the Rafter cell. It is a difficult matter to estimate 

 with any certainty the proportion which such silt forms of the 

 filter-paper catches. Nor is it possible to use the same standard 

 of estimation upon the catches of both the silk and filter-paper, 

 owing largely to the considerable volume of the flocculent debris 

 in the distributed plankton and the uncertainty as to its compres- 

 sibility in the centrifuge. My estimates of the silt, which will 

 be found in the table beginning on page 552, in the light of the 

 tabulated results of the enumeration seem to be too large, 

 probably as the result of the influence of the standard used on 

 the silk catches. It is, therefore, my opinion that the quanti- 

 ties of plankton given in this table are in the main below 

 rather than above the actual amounts present. The facts upon 

 which this opinion is based will appear in the discussion of the 

 results of the enumeration. It should be said that the relative 

 values of the estimated quantities of plankton in this table are 

 probably above the actual ones. They are of more importance 

 in indicating the direction of the seasonal movement in pro- 

 duction than they are in expressing the exact amounts of 

 plankton present. 



Because of this great increase in the proportion and quan- 

 tity of the silt found in these filter-paper catches, even in our 

 clearest waters, the filter-paper method does not satisfactorily 

 solve the vohnnetrie problem. It is not a satisfactory method, 

 and, indeed, it seems probable that the same difficulty will be 

 met in all methods which remove all suspended solids in all 

 but the clearest lake or ocean waters. This same difficulty has 

 balked the efforts of Lohman ('03) to determine satisfactorily 

 the volumetric loss by leakage through the silk in the applica- 

 tion of the Hensen method to a marine plankton, which leaks 

 through the silk. In passing, it may be noted that this plank- 

 tologist of the Hensen school at Kiel, working with the subven- 

 tion of the University of Kiel and the German Commission for 

 the Investigation of the Sea, after testing upon the plankton of 

 the Baltic and Mediterranean seas the correctness of my criti- 



