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 G-reen 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- 

 itomus 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 



