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 num¬ 
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 
