218 Marsh — Limnetic Crustacea of Green Lake. 



marked diurnal migration of most of the forms at the immedi- 

 ate surface, but it would take a series of collections at very short 

 intervals to determine the limits of this genera, movement. 

 These conclusions in regard to the surface phenomena are in 

 harmony with the observations of France ('94, p. 35) and 

 Birge ('95, p. 477). 



THE HORIZONTAL DISTRIBUTION OF THE LIMNETIC CRUSTACEA. 



The results of quantitative plankton determinations are en- 

 tirely dependent on the assumption that the horizontal distri- 

 bution of the plankton material is uniform. The laborious 

 methods formulated by Hensen and his co-workers are founded 

 on the assumption that over wide stretches of the ocean there is 

 a practical uniformity in the distribution of the plankton. 

 They believe that their investigations prove this assumption to 

 be a fact. Their theory, however, has not gained universal as- 

 sent. Haeckel (Haeckel, '90), among others, opposes it strongly. 

 The same question has arisen in regard to lakes, and here it has 

 a great practical importance, for if we can assume the hori- 

 zontal uniformity of the plankton, then collections made in dif- 

 ferent lakes under similar conditions would furnish us accurate 

 means of comparing the lakes in regard to the richness of the 

 fauna and flora. 



If this could be done, it would have a practical value in rela- 

 tion to the cultivation of fish, as we would expect that the lake 

 rich in plankton would be especially adapted to nourish large 

 numbers of fish. The question of horizontal uniformity of dis- 

 tribution in lakes has been actively discussed by many authors,, 

 and thus far with no uniformity of conclusions. Apstein 

 ('92, p. 491) "expressed his conviction .from the measure- 

 ments of plankton hauls and the counting of three comparative 

 collections, that the distribution of the plankton in fresh water 

 was practically uniform. 



Fric and Vavra ('94, p. 118) come to a similar conclusion 

 from their researches on the Unter Pocernitzer Teich. 



France ('94, p. 34 ff.) from his investigations on Bala- 

 ton See comes to directly opposite conclusions, and says that 





