299 
April 24 and that of the latter about May 3. The temperature 
curves of 1896 pass 60° about sixteen days before they reach 
that point in 1898. This may be the cause of the earlier cul- 
mination of the vernal pulse in 1896. Now in 1895 there is a 
suggestion in the temperature curve of an early spring, and the 
suggestion is borne out by the records. of the U.S. Weather 
Bureau for central Illinois. The normal mean temperature 
for Illinois as a whole in April is 51.8°. In 1896 it was 54.8°. 
In this case we might expect to find an earlier vernal pulse 
culminating, as in 1896, before the end of April, so that our col- 
lection of the 29th would fall upon its decline rather than upon 
its apex. I use the term vernal maxima advisedly, for | am 
inclined to the view that the period from April 29 to June 19 
witnessed a remarkable development of the plankton. The 
reasons for this view are found, first, in the fact that the catch 
of June 19 contains many Moina micrura, numerous males and 
epphippial females being among them, whose presence suggests 
the close of a period of rapid multiplication by parthenogenesis. 
The catch of June 19, though large, may thus represent the 
decline of a still larger population. In the second place, the 
qualitative collections made witha tow-net in the river in the in- 
terim between the quantitative collections of April 29 and June 
19 indicate an exceedingly abundant plankton rich in Moina. 
From the available data in 1895 and the course of the ver- 
nal production in other years it may be inferred with some de- 
eree of possibility, if not indeed of probability, that the vernal 
production in this year was accelerated by the early spring, 
and that a pulse appeared prior to April 29, and that this was 
followed in May-June by another pulse of much larger propor- 
tions and longer duration, a part of which (probably the decline) 
is detected in catches of June 19 and July 6. Of the occurrence 
of this latter and larger pulse there is little doubt, though the 
data are not available for its location and delineation. 
The unusual and prolonged low water of these spring 
months thus seems to result in a marked increase in the plank- 
ton content. The causes which lead to this are not far to seek. 
