Marsh — The Planhton of Fresh Water LaJces. 179 



but that these swarms are not stationary. It follows, then, that 

 conclusions in regard to the plankton drawn from a few collec- 

 tions may be quite erroneous. If, how^ever, a considerable num- 

 ber of collections is made, especially if they are made from 

 widely-separated localities, the average of all these collections, 

 allowing something of a margin for error, will give a fair idea 

 of the amount of plankton in a lake. Of course, the larger the 

 number of collections the less the amount of error, but anything 

 like very exact results cannot be expected. We are able, in this 

 way, to compare the plankton of one lake with that of another, 

 or to determine the relative amount of plankton at different 

 times of the year on any single lake. But an}^ estimate of the 

 actual amount of plankton produced by a lake at any time or dur- 

 ing the year must be acknowledged to be only an approximation. 

 Care must be taken, too, in comparing one lake with another, 

 that they be compared under similar conditions. The maximum 

 of one lake may not be reached at the same period as that of an- 

 other. The conditions of a deep lake are very different from 

 those of a shallow lake, and a fair comparison can be made only 

 by averaging collections continuing over a considerable period. 



In the abyssal region, because of the lack of light, plant life 

 is impossible, and the fauna is very meager. 



It is true that the list of animals which have been found in the 

 abyssal regions of lakes is a long one, including, as it does, pro- 

 tozoa, coelenterata, worms, molluscs, bryozoa, Crustacea, arach- 

 nida, insect larvae, and some few fish, but an examination of 

 any single lake shows that not only is the numl^er of kinds 

 small, but the numbers of individuals of any kind are very 

 small. In Europe the abyssal fauna of Lake Leman has been 

 Tvorked up with great thoroughness. In this country verj little 

 detailed work has been done on this subject. It is not a fruitful 

 field for research, and it is not strange that it has been neglected. 

 In shallow lakes it is doubtful if there is any distinctive abyssal 

 fauna. The most abundant animals in the mud of the bottom 

 are worms, insect larvae, gasteropod and lamellibranch molluscs, 

 and amphipods. With these may be associated at times great 

 numbers of other animals, as occasionally one finds in the mud 

 of shallow lakes large numbers of fresh- water hydra. Most if 



