The Coefficient of the Dredge. 285 



of Crustacea had been selected, or if the total of all the Crusta- 

 cea had been chosen. 



There is still a third question relating to the coefficient to 

 the dredge, namely, does the net function similarly on different 

 occasions, or does its coefficient vary irregularly and in such a 

 way as to vitiate conclusions based on the hauls of the net? 

 This question is partially answered by the determination of the 

 dredge coefficient, as shown in Table I. A second answer can 

 also be given. During the winter the numbers of Daphnia and 

 Diaptomus do not increase by reproduction, and the successive 

 catches should therefore show no very great variation. In a sub- 

 sequent section, dealing with the question of swarms, I have 

 given the figures for the catches of these genera during the 

 winter of 1895, from which it appears that the variation in suc- 

 cessive catches made within a short time of each other is no 

 greater than may be found between catches made on the same 

 •day. "Still further, a diagram is given (Fig. 21), showing the 

 numbers of Cyclops caught during the year 1895. This diagram 

 shows plainly that when the average number of Cyclops is ap- 

 proximately constant, the individual catches do not ordinarily 

 vary greatly from the average, no more than would be expected 

 from Cyclops 1 necessarily somewhat irregular distribution in the 

 lake. An examination of the maximum and minimum catches 

 in the tables for the different species shows the same result. 



I do not pretend that I have determined the coefficient of my 

 nets with absolute accuracy, nor that the coefficient of the net 

 is exactly the same on different occasions; but the careful study 

 whose results are summarized above has convinced me that the 

 coefficient of the net is quite as constant as any of the factors 

 entering into the determination of the plankton. The number 

 of the Crustacea certainly varies from point to point in the lake. 

 Where a fraction only of the Crustacea are counted, the deter- 

 mination of the number caught is an approximation and is sub- 

 ject to error. This error, is, of course, multiplied greatly in 

 stating the number of Crustacea in terms of square meter of 

 surface. Among the variables and approximations which en- 

 ter into the statement of the results of plankton work, I think 

 it may fairly be said that the coefficient of the net is one of the 



