428 Proceedings of Royal Society of Edinburgh. [sess. 
invariably greatest, of necessity be capable of developing properties 
tending to reduce the velocity of the falling motion. Knowing, as 
we now do, that the spherical form in all bodies has the quickest 
falling velocity, and seeing that so many organisms, with the 
increasing temperature and decreasing specific gravity of the water, 
often obviously became lengthened in form, the thought struck me 
that very probably the seasonal variations in the specific gravity 
of the water were the main factor in determining the seasonal 
variations in the shape of the organisms. Subsequently Ostwald 
(1903) pointed out that the lengthening of the longitudinal 
axis with increase of temperature, and the shortening of the 
longitudinal axis with decrease of temperature, cannot be 
attributed solely to the variations in the specific gravity of the 
water consequent upon the rising temperature in spring and falling 
temperature in autumn ; he draws attention to the fact that the 
oscillations in the specific gravity of the w T ater with a temperature 
varying from 0° to 24° C. (32° to 75° F.) are too slight to account 
for these great seasonal variations in the form of the organisms. 
He agrees with me in taking it for granted that these seasonal 
variations in so many very different plankton-organisms can only 
be due to variations in the external conditions, but he believes 
them to be due to the varying viscosity of the water, which, like 
the specific gravity, is dependent on the oscillations in the 
temperature of the water, while the variations in viscosity are far 
more perceptible than the variations in specific gravity. I think 
that Ostwald’s modification of my views is quite correct. 
The conclusions arrived at by Ostwald and myself have been 
greatly strengthened by recent observations. It is evident that if 
the seasonal variations are occasioned by variations in the external 
conditions, in accordance with the variations in the temperature of 
the water, these seasonal variations must be most conspicuous in 
those lakes having the most pronounced annual variations in 
temperature. It has now been shown that the seasonal variations 
are very conspicuous in a great many lakes in Denmark, South 
Sweden, and North Germany, and many interesting facts regarding 
these seasonal variations, the sinking of short-spined individuals 
during the early summer months, etc. (Max Voigt, 1904, p. 113), 
have been brought to light by the explorers in these countries, 
