Factors Determining the Annual Distribution. 357 



am very far from supposing that I can answer the question 

 completely, yet Zacharias's own figures show that at certain 

 times of the year the food supply must be exceedingly small. 

 For example, his figures show that the quantity of plant life is 

 apparently abundant during the spring and early summer, but 

 that in the late summer the amount of vegetation is small in 

 proportion to the number of eaters. 



On August 20, 1895, the number of Crustacea (1. c, p. 45) 

 was nearly 1,360,000 per square meter of surface, the diatoms 

 less than 30,500; Dinobryon, Eudorina, and Ceratium 459,010; 

 and Gloiotrichia 70,650. Thus, including Gloiotrichia, there was 

 less than one colony of algae to 2.5 Crustacea. On Sept. 20, 

 there was hardly more than one plant to 10 Crustacea. Under 

 these conditions a daphnia would have to strain a good many 

 liters of water to satisfy her eternal hunger. 



It never happens in lake Mendota that the ratio of food to 

 Crustacea falls as low as these observations in lake Ploen, and 

 while I am convinced that the occasional scarcity of food is an 

 important factor in limiting the number of Crustacea, I am 

 equally sure that there must be other conditions, still unknown, 

 which at times are even more important. My studies on the 

 vertical distribution of the Crustacea in 1895 and 1896 show that 

 all or nearly all of the increase of the Crustacea which causes 

 the fall maximum is brought about by the increase in the num- 

 bers of the Crustacea in the deeper part of the lake from which 

 they are excluded during the summer. In other words, the nunu 

 ber of Crustacea in the upper three meters of the water remains 

 nearly constant from a date near the close of the spring max- 

 imum to the decline in numbers in late autumn. In 1896 the 

 number of the Crustacea in the upper strata increased somewhat 

 during the autumn, owing to the occasional presence of large 

 numbers of new-hatched individuals, but even in this year more 

 than three-fourths of the increase in the number of the Crustacea 

 was due to the increase of the population of the lake below the 

 nine-meter level. In the upper water, however, the increase of 

 plants is most rapid. It begins in August at latest, and the 

 quantity of vegetation goes on increasing, for two months at 

 least, until in October the amount of food may easily be four 



