1096 Proceedings of Royal Society of Edinburgh, [sess. 
during summer and shortened during winter, and that the forma- 
tion of all the various structures (spines, floating apparatus, etc.), 
considered necessary to enable the organisms to float, are most dis- 
tinctly visible in summer forms and summer individuals. I also 
pointed out that the explanation must he looked for in the varying 
external conditions, which, so to speak, compel the organisms to 
vary regularly in accordance therewith. I ascribed these variations 
mainly to the annual changes in the specific gravity of the water, 
caused by the regular annual fluctuations in the tp. I started 
from the supposition that if the velocity of the falling motion 
of the plankton organisms he not the same at all seasons, the 
organisms must — in order to exist as such during the seasons 
when the velocity of the falling motion is invariably greatest — of 
necessity he capable of developing properties tending to reduce 
the velocity of the falling motion. Knowing that the spherical 
form in all bodies has the quickest falling velocity, and seeing 
that so many organisms with the increasing tp. 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 
factors in determining the seasonal variations in the shape of 
the organisms. It was subsequently pointed out by Ostwald 
(1902) that the lengthening of the longitudinal axis with increase 
of tp., and the shortening of the longitudinal axis with decrease 
of tp., cannot he attributed solely to the variations in the specific 
gravity of the water caused by the rising tp. in spring and 
falling tp. in autumn. He draws attention to the fact that the 
oscillations in the specific gravity of the water, with a tp. 
varying from 0° to 27° C., 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 tp. 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. 
