Factors Determining Vertical Distribution. 429 
iood, therefore, give any reason to the Crustacea for moving to 
any particular level. The effect of light, also, is lessened by 
the declining temperature of the water. Hence the Crustacea 
are far more apt to yield to the action of wind and gravity 
than they do in summer, and become more evenly distributed 
through all levels of the water. 
In the spring a similar distribution occurs immediately after 
the breaking up of the ice, when the lake is homothermous, and 
the Crustacea and the algae have not yet started their spring 
development. Very soon, however, the surface strata contain 
much more food material than those below, and the young Crus¬ 
tacea tend to remain near the surface until crowded down by 
the swarms of newly-hatched forms. The lake, too, rapidly be¬ 
comes heterothermous and the circulation of the water in late 
April and early May is by no means as complete as it is during 
the long homothermous period of the autumn. 
A slight effect is also produced by the wind on the vertical 
distribution of the Crustacea, since it causes the thermocline to 
oscillate through one or more meters. In general, it may be said 
that the on-shore wind tends to depress the thermocline, piling 
up the warm water on top of it; while the off-shore wind tends 
to raise it by stripping off the warm water of the surface. This 
general law, however, is subject to many modifications owing to 
the irregularities in the outline of the lake and in the confor¬ 
mation of its bottom. Whatever effect however, the wind pro¬ 
duces on the thermocline it also exerts, of course, on the lower 
limit to which the Crustacea extend. 
Gravity. 
The action of gravity has more influence on the position of 
Crustacea than I had supposed on beginning this investigation. 
Its effects are most plainly seen in Daphnia , and least in Diap- 
tomus. Gravity does not act as an accelerating force upon the 
movements of the Crustacea, and yet their ordinary movements 
are adjusted with some reference to it. If Daphnias are watched 
in an aquarium, it will be seen that they usually remain at about 
the same level, permitting themselves to sink and then with a few 
