273 



sample of the contents of the water in contiguous parts of the 

 stream, or of the plankton present in the water passing a given 

 point of the stream for any considerable length of time. Will 

 not the conditions pertaining to fluviatile life cause such local 

 variations in the plankton and such changes in it from day to 

 day that chronological series of isolated collections will reveal 

 only erratic and meaningless fluctuations, without significance 

 for the analysis of the factors of the environment and incapa- 

 ble of revealing an orderly regimen of aquatic life ? In other 

 words, is the river a unit of environment sufficiently compact 

 to yield, by the plankton method, data of scientific value com- 

 parable with those derived from other bodies of water, types of 

 which we find in the sea and the lakes ? 



As contrasted with the lake, the river as a unit of environ- 

 ment presents a constant and excessive predominance of the 

 longitudinal over the transverse axis. This feature, combined 

 with the fact that in a river the relative shore development is 

 much greater than it is in the lake, makes it necessary to discuss 

 the longitudinal and transverse distribution of the plankton in 

 the stream separately. 



LONGITUDINAL DISTRIBUTION. 



With a view to testing this question of the local longitudinal 

 distribution of the plankton in the Illinois River, I made a series 

 of ten catches in immediate succession from a boat anchored 

 at our usual station in mid-channel on October 29, 1896. This 

 was at a time of a considerable autumnal development of Sy- 

 nura and Synchcrta, and the quantity of plankton present (see 

 Table III.) was sufficient (PI. X.) to allow room for considera- 

 ble fluctuation and to minimize the error attributable to meas- 

 urement. 



In the following table the volume of the centrifuged plank- 

 ton per m. 3 and the deviations from the mean in volume and in 

 percentages of the mean are given. 



The similarity in the amounts of these successive catches 

 is shown in the fact that the average departure from the mean 

 catch is only ±3.58 per cent, and the total range of the limits of 



