107 



7, 1897, was at 80. A pulse of 48,000 on November 22 at 40 gives 

 evidence of considerable range in adaptation to temperatures. 



In Table I. the seasonal distribution of D. globulosa is given in 

 full. It differs from that of previous years mainly in the fact that 

 the summer pulses do not here have the amplitude reached in other 

 years; for example, in 1896 (252,000) and 1897 (1,240,000). It is 

 characterized by considerable irregularity caused by somewhat 

 abrupt pulses at irregular intervals. A comparison of these occur- 

 rences with the hydrographic conditions (Pt. I., PL XII.) indicates 

 that in the colder months increase in numbers in the plankton at- 

 tends flood waters only, as, for example, in January, February, late 

 October, and November. In the summer, pulses may also come 

 with floods. For example, that of 252,000 on May 25, 1896, ap- 

 peared on the upward slope of the June rise of the year, and that 

 of 80,000 on June 28, 1897, came with the belated June rise of that 

 year. On the other hand, some of the minor fluctuations appear 

 on declining floods, and the maximum one of our records, that of 

 Sept. 7, 1897, came in the midst of the most prolonged period of 

 stable low water (Pt. I., PL XI.) found in the six years of our 

 operations. From these facts it is evident that floods are efficient 

 in increasing the number of D. globulosa in the plankton, and that 

 the amplitude of the pulses to which they contribute is much greater 

 in the warmer months (above 60) than in the colder ones as a 

 result, perhaps, of the greater numbers present in their normal 

 habitat, the shores and bottom, and also as a result of their readier 

 flotation at this season. In so far as their presence is due to floods 

 they are adventitious. On the other hand, it is very probable that 

 they become temporarily eulimnetic in habit during the summer 

 months. The evidence for this lies in their greater numbers in a 

 period which is predominantly one of greater stability. Thus in 

 1898, in the 22 collections between May 1 and October 1, the average 

 number present is 9,731, while in the remaining seven months of 

 colder weather the number is only 5 ,200. Additional evidence arises 

 from the fact that pulses of unusual magnitude have occurred quite 

 independently of any factor such as flood or other disturbance which 

 might cause their adventitious introduction into the plankton. 

 Thus on Sept. 7, 1897, there is a symmetrical pulse whose rise and 

 decline occupy four weeks, as shown in the following table. The 

 total change in river levels in this period of four weeks (Pt. I., PL 



