178 ^yisconsin Academy of Sciences^ Arts^ and Letters, 



in detail and very elaborately by Henisen. The collection was- 

 made by a conical net of bolting silk drawn vertically through 

 the water, thus straining out the organisms of a column of water 

 of the size of the opening of the net. Then the material was 

 counted under the compound microscope by a very laborious 

 process. In this way exact numerical values can be obtained 

 not only for the plankton as a whole, but for the individual con- 

 stituents. Most investigators use Hensen's methods with 

 greater or less modifications, and they have been productive of 

 very fruitful results. But, unfortunately — I say unfortunately, 

 because we all like to claim exact results, and are prone to think 

 that nature works according to certain inflexible laws — much of 

 the laborious detail of the work is a waste of time. 



The question of the uniformity of horizontal distribution has 

 been discussed by various authors and with considerable vigor, 

 but I think it must now be acknowledged, that while there is a 

 certain amount of uniformity, so that, by a considerable num- 

 ber of collections, we can express within rather wide limits the 

 amount of plankton at any time on a lake, uniformity in any 

 exact sense does not exist. This lack of uniformity is largely 

 due to a difference in the number of Crustacea, although there 

 are marked differences in the distribution of the Other organ- 

 isms. Surface growing plants, for instance, are moved about 

 under the influence of the winds, and accumulate on the lee- 

 ward side of a lake. If one part of a lake is deeper than 

 another and the lake is not much disturbed by the winds, at the 

 period of maximum growth the number of diatoms will be much 

 greater over the deeper part. Crustacea may be in ill-defined 

 aggregations which may be called swarms, and these swarms are 

 not stationary in all cases, but move slowly, perhaps under the 

 influence of extremely weak currents. Xot only is there a con- 

 siderable variation in collections made at different locations on 

 a lake at the same time, but if a series of collections is made at 

 the same place, the amount of the plankton in some collections 

 may be twice as much as in others. An examination will show 

 that these large variations are generally due to a difference in 

 the numbers of some of the Crustacea, showing conclusively that 

 not only do the numbers of Crustacea vary at different locations, 



