LIMNOLOGICAL PROBLEMS 
419 
the ice had retired. All these fresh waters have probably been 
populated by the same, at all seasons almost invariable, common races 
which even nowadays predominate in those parts of the globe where 
an Ice Age still prevails, or are close to the boundaries of the ice. 
As the temperature rose the lakes became differentiated, and lake- 
types with different biological conditions came into existence; the 
race primarily common to all lakes was split up into various small 
races. Through selection, those mother-animals which produced 
resting-eggs disappeared ; the loss of resting-stages was followed by 
isolation of the colonies and fixation of the races. The result was a 
very distinct local variation. With the improvement in the climatic 
conditions the necessity for increased powers of floating was modified 
in proportion as the specific gravity and viscosity changed. As the 
temperature rose and the bearing power of the fresh water in the 
summer diminished, the plankton organisms had only one of two 
things to do : either to accommodate themselves to the claims for 
increased floating power, or perish. By increasing and developing 
such processes as counteracted the increasing rate of sinking, the 
seasonal variations arose. The deeper basis of the causal connection 
between the variations in the plaiikton organisms and those in the 
bearing power of fresh water is therefore to be sought for in the 
amelioration of the cUinatic conditio?is which began after the Glacial 
Age, the consequent higher temperature of the water, and at the 
same time the continually increasing rate of sinking. As a concomi- 
tant of the amelioration of the conditions may also be emphasised 
better nourishment. My opinion is partly based on the fact that all 
seasonal and local variation is absent, or at any rate is not con- 
spicuous, in the arctic region, partly on the fact that all southern 
local races fall back upon the same winter race, which of all races is 
that which is nearest related to the present arctic race of the species 
concerned. This winter race is therefore to be regarded as a irminis- 
cence preserved from periods remote in the development of our races of 
the present day. 
Just as in the course of a year we see those modifications brought 
about which have been developed in the course of the thousands of 
years which separate us from the remote periods when our waters were 
inhabited only by the poorly equipped arctic races of the present day, 
so we can probably observe quite the same development when we 
study, lake by lake, locality by locality, the conditions in a country 
which reaches the temperate zone in the south and the region of 
eternal snow in the north. 
According to this view, the local variations of the plankton 
organisms may be said to be arranged in series of forms {Formenixihen, 
Sarasin, Plate, Neumayer, etc.). The causes of the origin of these 
