408 
They have greater amplitudes, but are frequently followed by 
more sudden and complete declines. Thus, in this case the 
apex of the pulse is at 18.39 and 6.99 cm.’ respectively in the 
lake and river on June 7 and 14, while the decline has reached 
2.47 and 2.88 on the 21st in the two localities—a fall of 86 and 
59 per cent. respectively. The third instance occurs on Aug. 16 
(lake, .45, river, .61), when a large silt content in Thompson’s 
Lake, due to roiling of the water by heavy wind, obscures the 
actual quantity of the plankton. 
The similarity in the direction of the changes in plankton 
content in Thompson’s Lake and the other localities continues 
in this year even to a greater degree than formerly, owing in 
part at least to the hydrographic conditions above noted and 
to the more complete and uniform records. In the case of 
Thompson’s Lake and the river there are 21 agreements in the 
direction of the changes to 4 exceptions, and in the records of 
Quiver Lake 22 to 3 in the possible 25, This is so far in excess 
of the degree of agreement demanded by chance that we may 
look with confidence for an efficient cause in the common fac- 
tors of the environment, in the similar reproductive cycles of 
the constituents of the plankton found in common in the sey- 
eral localities, and in the uniformity in the reactions of at 
least a predominant portion of the total plankton assemblage 
to the factors of the environment. 
The river levels average 8.01 ft. for the year and stood 
above 6 ft. for 8 months of the 12. The high water increases the 
area of the “open water,” and causes a retreat of the shore-line 
and bottom, and a decrease in the relative occupancy of the 
bodies of water in question by the spheres of influence of the 
immediate environment. Thus the local differentiating char- 
acters of the several environments are in general progressively 
less potent as the open water increases in extent. The loca- 
tions of the 7 exceptions to the similarity in the direction of 
the movement in production are significant. All of them lie 
in the last five months, in the period of low water, and 2in the 
lowest water in August, when local influences are more potent, 
