286 



the increase in the initial collection at the east shore is caused 

 in part by the greater abundance of Wolffia drifted there by 

 the prevailing wind. It is obvious that for comparison with 

 lake collections these shore catches should be excluded, for the 

 former are rarely taken so near shore. Furthermore, all our 

 chronological series on which this paper is based were taken in 

 mid-channel, far from the shore belt, and in excluding those 

 marginal collections but one sixth to one third of the total 

 width of the stream is removed from the test. After all al- 

 lowances are made, it is obvious that quantitative differences 

 in the plankton are much greater in a single transverse trav- 

 erse of the stream than they were found to be in a longitudi- 

 nal test extending over approximately thirty times the width 

 of the stream. Indeed, it is to be expected that differences 

 arising from the effect of the shores and of tributary waters 

 would be carried by the current far down the stream. On the 

 basis of volume per m. 3 the probable error of distribution is 

 ±27.23, with a range of —61.7 to +53.1, and a total between 

 limits of 114.89 — all within these limits of variation in Reig- 

 hard's data from Lake St. Clair, but exceeding somewhat the 

 more limited data of Apstein. If we omit the three inshore 

 collections, Nos. 1, 9, and 10, the probable error of distribution 

 falls still lower, — to ±21.9 per cent., with a range of —22.1 to 

 + 39.5, a total of 61.6 per cent, between limits. 



If we take the amount of plankton under one square me- 

 ter as the basis of comparison the results will be much more di- 

 vergent, owing to the greater relative difference in depth in 

 my locations and to the introduction of variation due to verti- 

 cal distribution of the plankton. In Apstein's tests the great- 

 est departure from the mean depth in no case exceeds 10 per 

 cent., and with but four exceptions his 31 tests are in water 

 from 15 to 45 meters in depth, where differences in depth are 

 of less importance than in shoaler water. In Reighard's se- 

 ries the greatest departure from the mean depth is 66.9 per cent., 

 the range being from 1.17 to 5.54 meters. In my test the range 

 is from .56 to 4.88 m., the greatest departure being 81.5 per cent., 



