38 THE LIFE OF DEVILS LAKE 



to the habit of swarming among plankton animals, due perhaps 

 to a social instinct, similar to that found in many other groups of 

 the animal kingdom. Plankton swarms are at times visible, even at 

 considerable distances, to the naked eye, 2) With larger samples 

 (19 litres) the variations tend to be reduced, but even here they 

 are at times greater than in the smaller ones (% litre). 3) These 

 variations invalidate the usual assumption that a given sample of 

 water is representative of a large area, at least in respect to its 

 animal inhabitants, and necessitate the collection of large numbers 

 of samples before definite conclusions regarding their distribution 

 or movement can be drawn." 



These difficulties, however, while serious in themselves, are 

 usuallj^ insufficient to obscure the comparative results of an ample 

 series of collections, and it is these comparative results in which 

 the ecologist is chiefly interested. It may, of course, be desirable 

 to know the actual amount of plancton per hectare which a given body 

 of water will produce, in order to know the number of fish which 

 that body will support. But who knows the number of cubic centi- 

 metres of plancton per cubic metre of water which will furnish 

 adequate sustenance for a fish of a given weight and given species? 

 And in general the suitability of any water for the cultivation of fish, 

 or other economic animals, depends more on temperature, chemical 

 character of water, breeding grounds, and oxygen supply, than on 

 the available amount of food; for in any water where the physico- 

 chemical environment is suitable for fish life there is pretty sure 

 to be adequate plancton for the support of as many fish as the 

 water is suited to contain. 



On the other hand the comparative variation in amount of 

 plancton in both time and space, correlated with changes in the 

 environment, answers the ecologist 's questions regarding the reac- 

 tion between organism and environment and adaptation of the former 

 to the latter. 



While no one method is adequate for the purpose, it has not 

 seemed practical for various reasons to employ more than one, and 

 I have accordingly employed that one which seemed best adapted 

 to the general purpose from every standpoint. This method has 

 been checked and supplemented from time to time by others, but 

 the results which follow are based almost entirely upon it. 



In collecting samples I have used a water bottle closed with 

 a two hole rubber stopper, thru one hole of which extended a glass 

 tube with a rubber hose connection reaching to the surface, the 

 other hole being closed with a wooden plug attached to a string. 

 The bottle was contained in a metal box, weighted and open on 

 the sides, so as to insure easy sinking, attached to a marked chain. 

 When the box was lowered to the desired depth, as indicated on 



