Vertical Distribution of Individual Species . 395 
was 61.5 percent.; and in June, 1896, where the average for 
the whole month was 69 per cent. Each of these numbers is 
higher than the average for July, 1894, which was less than 53 
per cent., and higher than the highest average per cent, for any 
period of July, 1894, which was 63 per cent, in the second period. 
The variations which are found in the percentile distribution are 
substantially like those which are recorded in my former paper. 
(Birge, ’95, p. 455.) In no case do the older individuals of this 
species show a tendency to accumulate in the deeper water of 
the lake but as the broods which appear in the spring, or later, 
become older and the water becomes more crowded, they migrate 
progressively into the deeper levels, but appear to prefer to 
stay near the surface. 
Marsh (’97, p. 194) finds that the vertical distribution of Di- 
aptomus in Green lake is uniform throughout the year. This 
is entirely different from the facts as I find them, since the up¬ 
per three meters in summer contain more than twice as many 
of the species as they do in winter. Apstein (’96, p. 80) finds that 
Diaptomus was chiefly in the deep water from January to April. 
Here again his observations differ from mine, since there was 
hardly a trace of a descent of the species in lake Mendota. 
Apstein thinks that this descent in winter on the part of Diap¬ 
tomus and Cyclops may be due to their desire to seek the 
warmer water at the bottom of the lake. This motive cannot 
hold in the case of lake Mendota, where the temperature of the 
water is almost the same at all depths during the winter. The 
• aggregations of Cyclops in the deeper water are apparently com¬ 
posed of feeble individuals, which do not rise again to the sur¬ 
face. 
Cyclops. 
Figures 29, 30.—Table E, Appendix. 
Of all the limnetic Crustacea Cyclops seems to be most inde¬ 
pendent of external influences in its vertical distribution. The 
maximum percentage in the upper levels is reached when the 
spring or summer broods appear. While the absolute numbers 
of these broods in the spring are much greater than in summer, 
multiplication goes on so rapidly in May that the animals are 
