Vertical Distribution of the Crustacea — Autumn. 387 



of the month in 1895, and in the latter part of August, 1896. 

 The distribution in the first half of October, 1894, is not very 

 different from that two or three weeks earlier in the preceding 

 years. 



The leading general feature of distribution during the late 

 summer and autumn is the progressive occupation by the Crus- 

 tacea of the deeper strata of the lake as the thermocline moves 

 downward through August and September, and the coincident rise 

 in number of the Crustacea toward the fall maximum. It is a fact 

 which was wholly unexpected by me that the 0-3 m. level shows 

 little or no increase in the number of its Crustacea after the 

 early summer maximum in early June or late July. In 1895 its 

 numbers steadily declined, or at best were stationary, after 

 July 15th. (See Figs. 22, 23.) In 1896 there was considerable 

 variation in numbers, but on the whole there was no increase 

 except a sharp temporary rise in late October, due to the occur- 

 rence of great swarms of young Daphnia hyalina at that time. In 

 1894 the numbers in the upper level rose in the autumn, as 

 would be expected, since they were at an abnormally low level 

 in July, owing to the peculiar condition of the vegetation of 

 the lake in that year. 



The Crustacea between 3 and 9 meters show also the same re- 

 lation in their summer and autumn numbers ; while those below 9 

 meters show a great increase, beginning in the 9-12 m. level, 

 as the thermocline moves downward through it in August. 

 The increase steadily proceeds to the the lower levels of the 

 lake. It is very rapid in September and early October, and 

 continues until the storms of late October, when the popula- 

 tion decreases in all levels of the water. This result is the sum 

 from 5 to 7 species of Crustacea, and of course it does not hold 

 accurately for each species. It is also true that since the 

 broods of young appear in the upper level, they may temporarily 

 increase the number of a species there, but this excess of one 

 species is balanced by a deficiency in another, and often for the 

 single species the semi-monthly averages agree pretty well with 

 the general law. 



A good example of the effect of age upon distribution can be 

 seen from the case of Daphnia hyalina in the latter part of Oc- 



