No. 382.] METHODS IN PLANKTOLOGY. 739 
useful plants of his garden, for these also, regarded from the 
physiological point of view, belong to the ‘total production’ 
of the ground.” (Translation in Report of U. S. Commissioner 
of Fish and Fisheries for 1889-91, pp. 565-641, of Plankton 
Studien, Jenaische Zeitschrift, Bd. xxv, 1890.) It would seem 
as if Professor Haeckel overlooked the fact that the farmer 
can readily separate the hay, etc., from stones, dead sticks, 
and other foreign material. He can accurately determine the 
volume and weight of farm products. He does not have to 
contend in this connection with foreign substances, such as 
silt, organic débris, etc., which render inaccurate determina- 
tions by weight and volume of the contents of water from 
ponds, lakes, and oceans. It is these elements which thus 
far have prevented any apparent progress in establishing 
tables of the economic yield of water volumes on the basis 
of weight, volume, and number of individuals, which would 
be of value for comparison in determining the commercial 
importance of any area or any depth of water. 
It is, too, the presence of an undetermined and locally vary- 
ing quantity of organic débris which renders inaccurate the 
estimation of the economic value of water by means of the 
determination of the albuminoid ammonia. 
Numerous methods of Plankton collecting have been devised ; 
the most important of these may be grouped as follows : 
(1) By drawing a fine net through known volumes of water. 
(2) By passing known volumes of water through a filter of 
either (a) fine silk bolting cloth, or (4) fine sand, or (c) a com- 
bination of æ and å. 
(1) The net and the method of using it have been subjected 
to much study by Hensen (87 and ’95), Apstein ('91, '92, and 
'96), Reighard (94), Ward (96 and '96a), Borgert ('96), Kofoid 
(97), and others. From the net method it seems impossible to 
exclude several prolific sources of uncertainty in the results ; 
viz., (a) it is impossible to be certain of the quantity of water 
through which the net is drawn, and consequently of the quan- 
tity which passes through the net even in motionless water ; 
(4) currents in the water almost hopelessly complicate the con- 
ditions ; (c) the progressive clogging of the net cannot be 
