489 



Illinois River Plankton, Main Stream at Havana, III., 1895- '98 and 



1909- '10. Given in Cubic Centimeters of Plankton to 



One Cubic Meter of Water 



These are the only years of the earlier collecting period for which 

 our data are sufficiently continuous to serve the purpose of a com- 

 parison. The general average of monthly averages for the year 

 igoQ-'io, it will be seen, was 5.07 cm., those for the earlier years 

 being, respectively, 1.39, 2.10, and 3. 



From a scrutiny of the columns of figures of these three years 

 in comparison with the column for igcx^-'io, it becomes evident that 

 the series of monthly plankton products for i897-'98 has much the 

 closest resemblance to that of i909-'io, and that this is the year 

 that must be chosen for our comparison, so far as the data of aver- 

 age plankton production are concerned. 



If the figures of the following table of gage readings be com- 

 pared in the same manner and for the same purpose, it will be seen 

 that the series for i897-'98 resembles i909-'io more closely than any 

 one of the four years of the earlier period of this table, the principal 

 difference being in the autumnal months, when plankton production 

 is relatively small at best and is of comparatively little significance as 

 an item of fish food. 



