300 
nant or more typical species. In the case of the total plankton 
some obscurity results at times from the inclusion of unusual 
proportions of an adventitious population with flood waters. The 
selection of particular organisms as representative is also subject to 
some error, since seasonal changes in temperature and other more 
subtile causes often deflect or suppress their development. The totals 
of the Chlorophycee, Bacillariacee, and chlorophyll-bearing Mast:- 
gophora, and of the Rottfera and Entomostraca (Pl. I-IV.) probably 
give as complete and accurate a delineation of the recurrent pulses 
as the statistical data afford, since they include relatively few 
adventitious organisms, cover the entire year, and swamp more or 
less completely individual and temporary divergences of particular 
species. The delineation of the pulses by statistical data is obviously 
more significant than the volumetric method, since it more clearly 
presents the results of the reproductive processes which lie at the 
foundation of the phenomenon of recurrent pulses; and this method 
is also free from the unavoidable error arising from the presence of 
silt in the collections. 
The interval between collections introduces an error of consid- 
erable moment in any effort to determine with accuracy the duration 
of individual pulses, that is, the length of time between their minima 
or maxima. Daily collections would render this feasible, but with 
an interval of a week or more, not only the duration, but in some 
cases the probable separation of the pulses and location or their 
maxima, is to some undetermined degree obscured. 
The duration of the pulses of the five groups of plankton organ- 
isms shown graphically on Plates I.—IV.,in the case of all chlorophyll- 
bearing organisms considered as a Sheil is in 29 out of 36 instances 
between 21 and 35 days, less than 21 in 2 cases, and more than 35 
in 5, reaching extreme limits of 14 and 49 days. They average 
30.25 days between minima and 29.97 between maxima. 
The rotiferan data in the same months may be divided into 36 
periods, in 33 of which pulses are traceable. The duration of pulses 
between minima lies between 21 and 35 days in 23 of the 36 instances, 
falls below 21 in 5, and is above 35 in 8. The extreme limits are 
14 and 49 days. 
In the case of the Entomostraca, where also the pulses are obscure 
in a few of the intervals, we find that 22 of the 36 are between 21 
and 35 days between minima, 5 are below 21, and 9 are above 35. 
