316 Birge — The Crustacea of the Plankton. 



of the most important species in 1896, and had a fair develop- 

 ment in 1895, while in 1894 it was wholly absent. No reason 

 can be given in most cases for these variations in individual 

 species; but where a cause can be assigned, the subject is dis- 

 cussed in the section which deals with the single species in 

 detail. 



Diagrams 8 and 9 show on single charts the numerical rela- 

 tions of the most important limnetic Crustacea during the sea- 

 sons of 1895 and 1896. Several facts become very plain from 

 these diagrams. First, the development of Cyclops precedes 

 that of Daphnia and Diaptomus by nearly a month, and precedes 

 that of D. pulicaria by something more than two weeks. This 

 relation held in both years, although the development of alls 

 the Crustacea was some two weeks earlier in 1896 than in 1895. 

 Second, in both years Daphnia hyalina and Diaptomus began 

 their development together in the spring and rose together to 

 the spring maximum. This coincidence was probably due to the 

 rapid warming of the lake in both seasons. Figs. 1 and 2 

 show that the temperature of the water rose with much the same 

 rapidity in the two years. Diaptomus requires a higher tem- 

 perature for its development than does Daphnia, as is shown 

 by the fact that it declines steadily after the lake falls below a 

 temperature of 20°, while Daphnia has its great autumnal pe- 

 riod of reproduction in the month of October when the tempera- 

 ture is below 15°. In the spring of 1897 the warming of the 

 lake was slower than in either of the two years covered by my 

 study, and the development of Diaptomus lagged decidedly 

 behind that of Daphnia. I am not able, however, to give the 

 exact numerical relations. 



Diagram 9 shows also that Daphnia pulicaria began its course 

 of development about two weeks in advance of Daphnia hyalina. 

 Another fact is disclosed by Figs. 8 and 9, namely, that in 

 each summer some one species of limnetic crustacean appears 

 to take the lead, and decidedly dominates the other forms. In 



1894, as shown by Fig. 7, this species was Diaptomus. In 



1895, as shown by Fig. 8, Daphnia hyalina maintained its- 

 numbers full through July and August, gradually declining 

 through the autumn, and being nearly twice as numerous as- 



