Introductory. 
277 
During the winter observations were made through the ice, 
the net being suspended from a tripod. While it is very easy 
to make a single haul of the net at any temperature in the win¬ 
ter, it is very difficult to make a series if the temperature is ma¬ 
terially below —6° C. At lower temperatures, or even at 
this temperature on a cloudy day and with northerly wind, the 
net freezes so rapidly that work is extremely difficult and slow, 
as time must be taken for the net to thaw in the water before a 
second haul can be made. The line also becomes so heavily 
coated with ice and so slippery and stiff that it is impossible 
to secure accuracy in the time of raising the net. While there¬ 
fore the pleasant warm days of winter offer the best possible oc¬ 
casions for working the dredge, the average work in winter is 
extremely disagreeable. It is, however, more difficult to secure 
continuous observations during the periods immediately pre¬ 
ceding the formation and the breaking up of the ice than it is 
in winter. The lake freezes near the shore so that it is difficult 
to get out with a boat, while the ice is still too thin to bear 
the weight of a man; and as there is no current in the lake, 
the breaking up of the ice in the spring is ordinarily very slow 
and there is always a number of days in which the ice is too 
weak for safety. After the breaking up of the ice a continua¬ 
tion of north winds may keep the sludge ice on the southern 
shore, and thus still further delay observations, as was the case 
in 1896. 
In carrying out this work it has been my endeavor to make 
a contribution to the natural history of an inland lake as “a 
unit of environment,” to employ Eigenmann’s appropriate 
phrase. (Eigenmann ’95, p. 204.) I have, therefore, discussed 
somewhat freely the causes which seem to me to have contrib¬ 
uted to the peculiarities of the annual and vertical distribution 
of the Crustacea. I do not suppose that my conclusions are cor¬ 
rect in all particulars, still less that they are complete. The 
causes determining the biological conditions of a lake are far 
too numerous and various, and their inter-relations far too 
complex to be understood at present with any accuracy. 
It has seemed to me, however, that the aim of plankton investi¬ 
gations should be to reach an understanding of these conditions, 
