Factors Determining Vertical Distribution. 429 



food, 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 Dear 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 



