273 
sample of the contents of the water in contiguous parts of the 
stream, or of the plankton present in the water passing a given 
point of the stream for any considerable length of time. Will 
not the conditions pertaining to fluviatile life cause such local 
variations in the plankton and such changes in it from day to 
day that chronological series of isolated collections will reveal 
only erratic and meaningless fluctuations, without significance 
for the analysis of the factors of the environment and incapa- 
ble of revealing an orderly regimen of aquatic life? In other 
words, is the river a unit of environment sufficiently compact 
to yield, by the plankton method, data of scientific value com- 
parable with those derived from other bodies of water, types of 
which we find in the sea and the lakes ? 
As contrasted with the lake, the river as a unit of environ- 
ment presents a constant and excessive predominance of the 
longitudinal over the transverse axis. This feature, combined 
with the fact that in a river the relative shore development is 
much greater than it is inthe lake, makes it necessary to discuss 
the longitudinal and transverse distribution of the plankton in 
the stream separately. 
LONGITUDINAL DISTRIBUTION, 
With a view to testing this question ofthe local longitudinal 
distribution of the plankton in the Illinois River, | made a series 
of ten catches in immediate succession from a boat anchored 
at our usual station in mid-channel on October 29, 1596. This 
was at a time of a considerable autumnal development of Sy- 
nura and Syncheta, and the quantity of plankton present (see 
Table III.) was sufficient (Pl. X.) to allow room for considera- 
ble fluctuation and to minimize the error attributable to meas- 
urement. 
In the following table the volume of the centrifuged plank- 
ton per m.3 and the deviations from the mean in volume and in 
percentages of the mean are given. 
The similarity in the amounts of these successive catches 
is shown in the fact that the average departure from the mean 
catch is only +3.58 per cent. and the total range ofthe limits of 
