48 PLANKTON OF WINNEBAGO AND UREEN LAKES. 



forms may be very abundant in one year and either entirely 

 lacking or present in small numbers in another. 



So even for qualitative determinations, it is necessary, if one 

 would be strictly accurate, to make collections not only at all 

 seasons of the year, but for a series of years. 



When one makes plankton measurements, and compares one 

 lake with another from such records, the results are valuable, 

 but it must be recognized that they are subject to certain sources 

 of error. It has been already indicated that the horizontal dis- 

 tribution of the plankton is remarkably uniform. Yet this uni- 

 formity is subject to wide variations, so that inferences from 

 single plankton collections might be very erroneous. Safety in 

 drawing conclusions lies only in averages, and the larger the 

 number of collections from which those averages can be drawn 

 the safer are the conclusions. 



It follows, I think, that refinements in plankton measurement 

 are unprofitable. It must be acknowledged that the measure- 

 ment by settling is inaccurate, and that the use of the centri- 

 fuge, while more accurate, nevertheless still leaves a large 

 margin of error. Of course measurement by weighing is exact, 

 but the results hardly justify the labor necessary. 



I would not have it understood, from what has just been said, 

 that I would throw discredit on plankton measurement, for this 

 paper is evidence of the importance I attach to it, but I wish to 

 emphasize the fact that such measurements never can have the 

 merit of exactness, for allowance must always be mjade for 

 error. 



It is very desirable if lakes are to be compared with each 

 other in regard to the amount of plankton, that they should be 

 under continuons observation for a long time, preferably for a 

 term of years, for there may be considerable differences in the 

 plankton of successive years. 



