Factors Determining Vertical Distribution. 431 



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 in 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 arid adult 

 individuals can be obtained from the fall broods of Daphnia 

 hyalina^ as stated on page 398 



