510 



emphasis laid upon the subject m connection with my discus- 

 sion of the volumetric data will be found in the statistical data 

 of Part II. of this paper. In this connection it may be noted 

 that the statistical data of other investigators lend some sup- 

 port to the probability that this cyclic movement in the plank- 

 ton will be found to occur in other localities as well as in those 

 examined by us. Instances suggesting this occurrence may be 

 found in the statistical data of Amberg ('00), Steuer ('01), and 

 the more recent work of Cohn ('08). 



This cyclic movement consists in the repetition of rise and 

 decline in production^ — a repetition broken in our records only 

 by the imperfection of our data. Wherever collections are of 

 sufhcient frequency it is possible to trace it — sometimes, how- 

 ever, only with the aid of statistical data^continuously through 

 all the vicissitudes of changing seasons, of summer heat and 

 winter's ice, of the vernal rise and autumnal decline in tem- 

 perature, and of high and low water. Moreover, it appears in 

 all of our localities whenever collections have been made with 

 a weekly, or even fortnightly, interval. 



These pulses vary in duration from 2 to 7 weeks, though 

 the majority occur in limits of 3 to 5. Their amplitudes vary 

 greatly, and are plainly influenced by various environmental 

 factors. Their limits are also frequently modified by these 

 factors, though the evidence is not clear that any of the envi- 

 ronmental factors we have discussed are correlated directly 

 with the cyclic movement itself. 



In the discussion of production in the backwaters the uni- 

 versal approximation in time of these pulses in the various 

 localities, or even their precise coincidence in many instances, 

 was recorded. The cause of this tendency toward uniformity 

 in the direction of movement in production in these various 

 localities at the same time is not apparent from the data at 

 hand. Contributory to it are the facts of a plankton largely 

 composed of the same species of planktonts, of a connection 

 and commingling of waters in all of the localities in flood con- 

 ditions, and of the operation of environmental factors com- 

 mon to all of the localities. 



