207 



readily detached in the manipulation of the plankton, so much so 

 that in 1898 less than 6 per cent, remained attached. More or less 

 uncertainty attends the determination of the parentage of detached 

 winter and male eggs, so that decisive proof of sexual reproduction 

 is best obtained from the attached eggs. In Table I. will be found 

 the records of free and attached male and winter eggs recorded in 

 1898. Evidence will be found in this of sexual reproduction at- 

 tending the pulses of March, April, May, September, and December. 

 The presence of winter eggs at intervals throughoutrthe greater part 

 of the year may be due either to their continual production or, as 

 seems more probable, to their continuance in the plankton for some 

 time after their formation. The presence of attached winter eggs, 

 or of larger numbers of free winter eggs, seems to mark the culmina- 

 tion and decline of the pulse. Male eggs, on the other hand, are 

 more generally present during both the rise and decline of the pulses. 

 Somewhat similar evidence of sexual cycles attends many of the 

 larger pulses in years prior to 1898. 



This species affords a striking example of a perennial eulimnetic 

 planktont. It is found in midwinter under the ice in water at the 

 freezing point, and even under these conditions it multiplies, pro- 

 ducing pulses whose amplitude surpasses that of many rotifers of the 

 plankton, and runs a reproductive cycle similar to, though of less 

 amplitude than, those at other seasons of the year. It shares with 

 other organisms the vernal outburst, and repeats the process in 

 summer months under maximum conditions of heat and in waters 

 whose chemical condition is very different from that in which the 

 hiemal and vernal pulses appeared. Successive generations of this 

 species are thus adapted to widely different conditions. Through 

 all the changes incident to ice, stagnation, flood, sewage pollution, 

 changing temperature, the wax and wane and change of food, the 

 constant and unceasing warfare of enemies which prey upon it and 

 of parasites which plague it, and, above all and continuously, the 

 removal of countless individuals from the place of their origin by 

 the ceaseless current of the stream, this species lives on, holds its 

 own in the plankton, and repeats year after year the same sequence 

 of rhythmic pulses of occurrence in the river water. The secret of 

 the process doubtless lies in its capacity to produce repeatedly these 

 crops of winter eggs which serve to seed the environment and start 



