506 



in the river than in Phelps Lake. A further inspection of the 

 table indicates that Phelps and Thompson's lakes and Spoon 

 River exhibit the most stable relations in productive rank. 

 It is in these liodies of water that we have found environment- 

 al conditions most uniform. In Flag, Quiver, and Dogfish 

 lakes the divergences in rank are much greater, and it is in 

 these localities that conditions of high and low water and of 

 vegetation afford sharpest contrasts. In the Illinois River 

 itself, where hydrographic fluctuations are most immediately 

 effective, we find apparently the widest divergences in produc- 

 tive rank. Even in these instances of greatest divergence the 

 tendency towards a certain rank in production in each locality 

 is sufficiently evident to warrant the statement that in the 

 main the relative rank in production in the several localities 

 examined by us is well established and generally maintained. 



Still more pertinent to the question of the existence of a 

 normal regimen in production is the question of sequence in 

 the course of production in successive years and in different 

 localities in the same year. Do the planktographs form curves 

 which may be superposed in successive years in the same lo- 

 cality or in different localities in the same year? A single 

 glance at the plates which accompany this paper will suffice 

 to reveal the chaotic complex of lines which such a superposi- 

 tion would produce. There is no such unity or similarity if 

 we base the comparisons on the actual volumetric production. 

 If, on the other hand, we disregard coincidence in amplitude of 

 the curve and consider mainly the direction of the changes, sim- 

 ilarity becomes increasingly apparent. It rarely approaches 

 to the condition of complete parallelism, however, owing to 

 the great variety in the amplitude of production in various 

 years and localities. 



If the data in the table following page 342 be analyzed with 

 reference simply to the upward or dow^nward movement in 

 mean production from month to month and year to year in the 

 various localities, we find certain tendencies appearing which 

 may afford a basis for predicting the probable course of pro- 



