396 Birge — The Crustacea of the Plankton. 



quickly forced to move toward the deeper water of the lake r 

 and, since the entire lake is accessible to them in spring, there 

 rarely occurs as great a percentage in the upper stratum as is 

 the case in summer. The highest average per cent, in the O-S 

 m. level, reached in the spring of 1895, was 42.7 in the first 

 part of May; and 35 per cent, was the average in the latter 

 part of April. 1896. In July of each year the percentage in the 

 upper stratum rose to about 50, owing to the coincidence of 

 swarms of young in the upper water while the lower strata con- 

 tained a very scanty population. The fall rise in numbers does 

 not cause any noteworthy increase in the percentage in the- 

 upper strata, since at this time the entire lake is accessible to- 

 the animals and food is abundant at all levels, and the autumnal 

 gales aid to distribute the species through the lake. 



In the winter there is a strong tendency of Cyclops toward 

 the bottom and as many as 50 per cent, may be found in the 

 lower three meters, and as many as 70 per cent, in the lower- 

 six meters of the lake. Illustrations are given on page 379. 

 Since many of the older representatives of the species die during 

 the winter and the new individuals appear towards spring in 

 the upper water, the population of the lower levels decreases in 

 the early spring, both absolutely and relatively. Diagram 30 

 shows the percentile distribution of Cyclops in the first part of 

 March, 1895, and in the latter part of July of the same year, in 

 which the extremes of its distribution were found. 



The spring broods of Cyclops show exceedingly well the progres- 

 sive occupation of the water of the lake by the increasing num- 

 bers of the species; the way in which the numbers of a declin- 

 ing species disappear first from the upper waters of the lake,, 

 where they first appeared; and the equality of distribution 

 during the decline. The following table shows the spring history 

 of Cyclops during 1896. The story for 1895 would be substan- 

 tially the same. 



