Vertical Distribution of Individual Species. 401 
ing the level below which the respective percentages of the 
species were found and these were connected by lines. The dis¬ 
tribution is based on assumption that the individuals of the 
species were uniformly distributed throughout the 3 m. level in 
which they were found. This assumption is peculiarly incorrect 
for D. pulicaria , since the species is limited to the region of the 
thermocline. It is often confined within a space of 1 meter, 
or even less, yet it often passes beyond these narrow lim¬ 
its, as is indicated by the fact that not inconsiderable numbers 
may be found in two or even three levels. While, therefore, the 
diagram spreads out the distribution of the species during the 
summer more than is correct, the general relations are well 
enough indicated by its lines. It will be seen that in the latter 
part of August more than 65 per cent, of the species was found 
between 9 and 12 meters and that the species moved downward 
during September as the thermocline moved down. In 
October, after the breaking up of the thermocline, the 
distribution was much more nearly equal. The center of popula¬ 
tion rose rapidly and regularly from the latter part of September 
to the middle of November, lying near 14 meters in late Septem¬ 
ber and at 4 meters in the first part of November. After a 
small fluctuation in the latter part of November, it rose once 
more, and in the latter part of December lay about two meters 
below the surface, where it remained during the early part of 
the winter, until the decline in numbers came on in March or April. 
If this diagram were reversed it would serve fairly well to indi¬ 
cate the downward migration of the species in the spring. 
In Fig. 30 are given curves for the percentile distribution of 
D. pulicaria for April 16-30, 1896, and August 16-31, 1895, 
showing the extreme variation of its average distribution. 
The diagram is similar to that described on p. 384. 
I have not found any other case recorded of a Daphnia which 
in summer remains at or below the thermocline. At least one 
other species of the genus has the same habit in this region. A 
form which I have identified as D. longiremis Sars, belonging 
to the cristata group, is regularly confined to the region be¬ 
low the thermocline in some of the lakes of the Oconomowoc 
system and in lake Geneva. 
26 
