Factors Determining Vertical Distribution . 481 
feebler animals, tend gradually to sink and accumulate in the 
deeper waters of the lake. 
Such aggregations of Cyclops are often found at the bottom 
of the lake in winter. In March, 1895, for example, from fifty 
to seventy per cent, of this species were in the lower three 
meters. Daphnia hyalina shows a similar downward movement 
in late May and early June on the part of those individuals, 
which have lived over winter. In late autumn, also, the adult 
members of this species are far more numerous in the lower 
strata than they are at higher levels. Since, at this time, there 
is a superabundance of food at all depth of the water, and,, 
since the Crustacea are relatively few iu number, this distribu¬ 
tion can hardly be due to any other cause than gravity. (See 
p. 398.) 
Diaptomus and Diaphanosoma with their very powerful swim 
ming organs, rarely show this tendency to sink. Perhaps the 
large amount of fat usually present in Diaptomus also aids in 
preventing sinking. 
Age. 
It is a general rule that the young individuals of a species 
appear near the surface, When the Crustacea begin to multiply 
in the spring, the increase appears first in the 0-3-meter level. 
All very exceptionally large numbers of any species obtained 
during the summer have been caught in the upper three meters, 
and usually consisted of young and half-grown animals. No sim¬ 
ilar aggregations have been found in the deeper water, except as 
noted for Cyclops in the last section. 
When a species is declining in numbers, the distribution is 
more uniform, and as the decline goes on, the lower levels may 
contain a larger number than the upper. If the Crustacea 
obeyed this law with mathematical accuracy, there would be a 
sort of progress of the members of a brood from the top to the 
bottom of the lake, the successive broods of the young contin¬ 
ually displacing the older in the upper strata. 
Good illustrations of the distribution of the young and adult 
•individuals can be obtained from the fall broods of Daphnia 
hyalina , as stated on page 398 
