Vertical Distribution of Individual Species. 891 
In October the distribution of all of the species of Crustacea is 
approximately equal. In the winter the equality of distribution 
is brought about by the excess of Daphnia and Diaplomus in 
the upper strata, nearly balancing the excess of Cyclops near 
the bottom. (See Fig. 30.) The curve for August shows a 
very large percentage in the upper 3 meters and a very small 
number in the lower water. It is a characteristic distribution 
for middle summer. 
THE VERTICAL DISTRIBUTION OP THE INDIVIDUAL SPECIES. 
After this full discussion of the vertical distribution of the 
total crustacean population I do not intend to describe that of 
the individual species in similar detail, but I shall confine my¬ 
self to pointing out the individual peculiarities of each species, 
devoting more space to those which depart in a marked way 
from the average vertical distribution. One general law holds 
for nearly all the species, as already stated: the broods of young 
appear first in the upper water of the lake and the increase of 
population extends downward, becoming approximately uniform 
at all depths as the species reaches its maximum, and later in 
its life becoming more numerous in the deeper water of the 
lake. To the first part of this rule the only exception is Daph¬ 
nia pulicaria during summer. There are, however, several fac¬ 
tors which prevent the full carrying out of the latter part of the 
rule. The most important of these is the formation of the ther- 
mocline, by which all of the crustacean life is confined to the 
upper waters of the lake during that period when the develop¬ 
ment of several species is going on actively. In the late au¬ 
tumn also the numbers of the Crustacea decline so rapidly after 
the fall broods appear that it is not easy to find any accumula¬ 
tion at any low level of the lake. The downward movement of 
the older forms is shown most clearly by Cyclops and Daphnia 
hyalina during the spring, and by the accumulation of Cyclops 
in the deeper water of the lake during the winter, by the dis¬ 
appearance of D. hyalina and D. retrocurva in autumn. Sim¬ 
ilar, though less striking, illustrations can be found in all of the 
species of limnetic Crustacea. 
