Marsh —The Plankton of Fresh Water Lakes. 165 



brief time at mj disposal of what is now known of the fauna and 

 flora of fresh water lakes. 



Russell, in his work on !N^orth American Lakes, enumerates 

 ten agencies which, acting separately or in combination, may 

 produce lakes. So far as our Wisconsin lakes are concerned, 

 the most important of these agencies is glacial action. Most 

 of our lakes occupy the depressions caused by the unequal 

 distribution of the glacial drift, or by interference with pre- 

 existing drainage lines. Inasmuch as Wisconsin is not a 

 mountainous state, it follows that these depressions are no- 

 where of great depth, and that we have no lakes which com- 

 pare in depth \vith those located in mountainous regions. Most 

 of our lakes are extremely shallow, few being more than forty 

 or fifty feet in depth. Lake Geneva, 142 feet, and Green Lake, 

 237 feet, are our deepest bodies of water, while our largest lake, 

 Lake Winnebago, probably does not exceed twenty-five feet. All 

 lakes are temporary features of the topography. The outflow- 

 ing water is all the time deepening the outlet and increasing the 

 amount of drainage, while the inflowing water is bringing in 

 material which gradually fills up tbe lake bed. This process 

 goes on with comparative rapidity, and even in our new lake 

 areas there are numerous examples of dead lakes, where swamp 

 vegetation entirely covers what was formerly an open sheet of 

 water. The physical processes involved in the lives of lakes and 

 the relation of the lake vegetation to these processes are very in- 

 teresting, but this is not the time or place to discuss them, and 

 they can only be referred to in passing. 



The subject of the fauna of fresh, water lakes has not been 

 especially attractive to zoologists. This is but natural when we 

 consider the great wealth of life in the ocean, and the compara- 

 tive poverty of fresh water. Of the more important divisions 

 of the animal kingdom the echinoderms and tunicates are en- 

 tirelv absent in fresh water, and the coelenterates and melius- 

 coidea are represented by few forms. Even the Crustacea, which, 

 form the greater part of the plankton, and are present in such 

 enormous numbers, have very few forms compared with the Crus- 

 tacea of the sea. It is to be expected that zoologists will be 

 attracted by this wealth of material in the sea, and that most of 

 them will in the future as in the past resort to the sea for their 



