290 
stream is 3.58 per cent; from a floating boat, 11.2 per cent.; at 
intervals of 1—7 days for periods of 2 to 5 days in the more 
stable hydrographic conditions, 14.1 per cent.; and in the 
stream as a whole for 200 miles of its course, 57 (total 
catch) or 89 per cent. (plankton estimated). If, however, we 
break up the 200 miles into four sections representing sub- 
ordinate units of environment, each dominated by some local 
factor, the + departures from the mean are 12, 51, 32, and 76 
per cent. respectively, for estimated plankton (i. e. after silt 
deduction), or 44, 5, 36, and 34 per cent. for the total catches, 
the averages for the two methods being + 48 and + 29.7 per 
cent. 
The average departure from the mean catch in two trans- 
verse series of 10 catches each is + 27.2 or + 22.3 on the basis 
of plankton content perm. If we eliminate the shallow-wa- 
ter shore collections, the departures fall to + 21.9 and +12.1, 
or on the basis of volumes under 1 sq. m.,to + 15.4 and + 20.2. 
The departure from the mean number of planktonts is only + 
7.8 for the whole cross-section. 
These results are in the main within the + error of distri- 
bution of the plankton in lakes arrived at by similar methods 
of computation. The plankton method may therefore be applied 
to the quantitative investigation of the life of a stream as legitimately 
as to that of alake. The laws of the horizontal distribution of the 
plankton are in this respect essentially the same in both types of 
aquatic environment. 
Whether or not a fundamental source of error as large as 
this—probably the greatest of all the errors in the method as 
we have used it—vitiates the utilization of such data for scien- 
tific conclusions must be to some extent a matter of opinion. 
The extent to which it renders conclusions tentative must de- 
pend upon the distribution of the error, the extent of the data, 
and the method of their utilization. Personally I may say that 
close study of the at first sight aberrant data upon which 
this paper is founded, has led me to attach less significance to 
this source of error than I was at first inclined to do. Readers 
