LIMNOLOGICAL PROBLEMS 
407 
factory, one thing is at any rate quite unquestionable : the structures 
they are intended to explain are indisputable facts ; that the plankton 
Daphnids, for example, are longer and narrower in summer than in 
winter is a fact beyond doubt. The plankton investigations have 
further given the following very interesting result. During the 
formation of all those structures on which the increase in the buoyancy 
power depends, all plankton organisms yb/Zozc pa?'«//>Z lines. However 
different their organisation may be, the development of the buoyancy 
apparatus takes place simultaneously (May, June) ; they reach their 
extreme development simultaneously (mid-summer), and they are reduced 
simidianeously (October, November). From this, and because the 
highest development of the buoyancy apparatus unquestionably takes 
place just at the season when the rate of sinking is highest (sunnner), 
we conclude that the variatioiis in the buoyancy or supporting power 
of fresh water folloiving the variations in temperature^ are the older 
inducements which lead to the seasonal vai^iations as the ansiver to these 
% % '% "/6 % -"h 
Fig. 54. — A Rotifer, AspJanchna prioclonta. In May, almost isodiametric ; 
in July, four times longer than broad. 
on the part of the organism. The variations in viscosity and specific 
gravity on the one side and the seasonal variations on the other 
stand in the relation of cause and eflrect. In 1900,^ when this theory 
was set forth, we had not the slightest idea of the manner in which 
the variations took place. It was therefore quite natural that the 
theory, in the eyes of many, seemed only a loose hypothesis. It is 
naturally not to be expected that a scientifically educated naturalist 
should have confidence in a theory based on the observation that 
the very same species in summer is five times longer than in winter, 
or that another species in winter is many times greater than in 
summer, and that without any means of interpreting the manner in « 
which these great variations in body structure take place. 
One of the reasons why the study of the variation in plankton 
organisms has only advanced with the greatest difficulty, is that the 
^ See Wesenberg-Lund, "Von dem Abhangigkeitsverlialtniss zwisclien dem 
Bau der Plankton-organismen und dem specifisclien Gewicht des Siisswassers," 
Biolog. Centralhl., vol. xx. pp. 606, 644, 1900 ; with regard to the viscosity, see 
Ostwald, "Ziir Theorie des Planktons," Biolog. Centralhl, vol. xxii. p. 596, 1902. 
