381 



urn in which the plankton grows. The similarity in the move- 

 ment of production must also be correlated with the fact that 

 these common environmental factors are responded to in the 

 three localities by a plankton composed of identical or closely re- 

 lated species in varying proportions. It is in the main the re- 

 sult of the response of similar organisms to the common fac- 

 tors of an environment, factors, moreover, of fundamental sig- 

 nificance. 



FLAG LAKE. 

 (Table VII. ; PI. XIX., XXXIII., XXXIV.) 



ENVIRONMENTAL CONDITIONS. 



This is the local name for a marsh in the western bottom- 

 lands opposite the location of our plankton station in the river 

 (PL II.). Together with its outlet, Flag Lake Slough, it ex- 

 tends parallel to the river from north to south a distance 

 of about 4| miles, and is generally less than | of a mile in width. 

 It has no precise boundaries, since the fringe of willows which 

 borders it, save for 2 miles along its northwestern margin where 

 it joins Thompson's Lake, merges gradually with the marsh on 

 the one hand and the bottom-land forest on the other. It con- 

 tains about 2^ square miles of permanent marsh, of which but 

 a small area toward the lower end was free from vegetation. 

 The depth depends upon the stage of the river or the extent of 

 the run-off of the impounded water. Its bottom, if we may dig- 

 nify the treacherous ooze from which the vegetation springs by 

 this name, is generally, if not entirely, several feet above low- 

 water mark in the river. In the autumn of 1897, during the 

 prolonged low water of that season, the lake dried up and a 

 road was opened across it to Thompson's Lake. Generally, how- 

 ever, it retains sufficient water to tide over ordinary periods of 

 low levels. 



The hydrographic conditions are such as to make this 

 marsh exempt from all current save at times of most general 

 overflow. Owing to the somewhat elevated banks along Flag 



