277 
the average, in all probability, fall within +10 per cent. 
Our chronological series of collections affords a few instan- 
ces of catches under somewhat stable conditions of river levels 
and temperature, and at intervals so short that they may be 
utilized as tests of local distribution within certain larger limits 
of error, since the utilization of such data introduces the er- 
rors resulting from changes of chemical conditions due to rot- 
ting of sewage, and from growth, reproduction, and destruction 
of the plankton in the interim between collections. The follow- 
ing tabulated instances (p. 278) from Table III. and Plates X. 
and XI. may be cited as throwing light on this question of local 
distribution along the length of the stream. 
The fourteen groups of collections were selected with refer- 
ence to stability of conditions, therefore in falling or low water 
and in periods of relatively even temperatures. Inspection 
of the tables and plates above referred to will show that the 
selection has not been made so as to eliminate wide varia- 
tions, and it may therefore be regarded as fairly typical. The 
periods included, range from 2 to 15 days in extent, and 
upon estimated rates of current the several tests include plank- 
tons taken at intervals in reaches of channel water from 24 to 
252 miles in length. The average departures from the mean, 
range from +0 to +29.8, and yieldagrand average of +14.1. In 
view of greater number of catches averaged and extended 
time element involved, these results compare very favorably 
with those derived from Reighard’s data and Apstein’s results. 
The probable error resulting from variations in the longitu- 
dinal distribution under stable conditions seems to be less than 
+15 per cent. 
An inspection of Table III. and Plates X.-XIII. will show 
that in the case of invading flood waters the departures from 
the mean of catches at similar intervals would be considerably 
greater than the averages above computed. Also, thatin case 
of plankton pulses in stable conditions—for example in Sep- 
tember and October, 1897—collections at weekly intervals may 
exhibit departures in excess of +50 per cent. Itis evident, how- 
