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 (’08) 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 (’08) and Volk (01 
and ’03) and my own investigations, as given in the preceding 
pages, are concerned. 'l'o my mind, owing to silt contamination, 
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. Asa 
result of our operations upon the Illinois and its backwaters we 
