556 



made by some finer filter than that of the silk net. The work 

 in this line which I have done since the publication of my tests 

 of the leakage through the silk net (Kofoid, '97b) has only 

 confirmed my opinion as additional data, volumetric and enu- 

 merative, have accumulated. The corroboration of the cor- 

 rectness of my criticisms on this point by the recent work of 

 Lohmann ('03) on marine plankton adds to the testimony 

 against the Hensen plankton-method as a complete quantitative 

 test of the productivity of water. The criticisms which Brandt 

 ('99), Reighard ('98), and Ward ('99) have passed upon my con- 

 clusions in this respect have not stood the test of actual inves- 

 tigation, in so far as the work of Lohmann ('03) and Volk ('01 

 and '03) and my own investigations, as given in the preceding 

 pages, are concerned. To my mind, owing to silt contavunatio)i, 

 no purely volumetric test is sufficient to solve adequately the 

 problem of productivity of water. It may be possible by pure 

 cultures and measurements of many individuals to establish 

 unit values for the various planktonts, so that volumetric de- 

 terminations can be made from enumerative data, and to sup- 

 plement these by chemical analyses, so that chemical values in 

 proteids, silica, etc., can be in like manner approximated with 

 sufficient accuracy for scientific purposes. This may seem chi- 

 merical at a distance, and it raises at once the question as to 

 the utility of so great an undertaking. Something of the sort 

 is, however, necessary if quantitative plankton investigations 

 are to cease being merely desultory and disconnected and be- 

 come joined in a substantial structure which comparative sci- 

 ence alone can rear. The development of a scientific aquicul- 

 ture demands some standard of this kind as a basis for its per- 

 manent success. 



COMPARISON WITH OTHER BODIES OF WATER. 



Comparisons of the quantitative production in the Illinois 

 River with that in other localities are obviously of value only 

 when based on similar or approximately similar data. As a 

 result of our operations upon the Illinois and its backwaters we 



