418 THE FRESH-WATER LOCHS OF SCOTLAND 
shown that this very same common race, which inhabits the fresh 
waters of the Arctic and which is nearly related to the race from the 
high alpine lakes in Switzerland, is the very savie race which i7i winter 
inhabits the Baltic fresh-water lakes and from which the numerous local 
summer races proceed (fig. 62, upper row, compared with fig. 60, 
lower row). 
We have now obtained the material from which it is possible to 
Fig. 62. — Daphnia hyalina. Upper row : summer forms from lakes which rarely or 
never reach a temperature of about 12-16° C. (Achensee, Brehm ; Sarek, Ekman ; 
Thingvallavatn, Wesenberg-Lund ; Myvatn, Wesenberg-Lund ; Kola, Levander ; 
Mjosen, Huitfeldt-Kaas). Lower row : summer forms from lakes which annually 
reach more than 12-16° C. (Viborgso, Wesenberg-Lund ; Esromso, Wesenberg- 
Lund ; Tjustrupso, Wesenberg-Lund ; Vastergotland, Lilljeborg ; Pomerania, 
Seligo ; Haldsii, Wesenberg-Lund). The local variation is very inconspicuous in 
the cold lakes, but very prominent in the warm lakes. 
understand why all the local summer races are in our lakes condensed 
into one single race in the winter time. 
Influence of the Ice Age on the Fresh-water Plankton 
During the tundra period the northern part of Central Europe 
was covered with innumerable lakes and pools which remained after 
The colonies of the single lake thus escaj^e isolation, and race-formation cannot 
occur. The main cause why a species is able as plankton organism only to form 
its resting-stage in the Arctic is probably that the arctic lakes have not the high 
summer temperatures. Thus, also, the rate of sinking is never so gre^-t in the 
northern lakes, and the mother-animals are therefore able to form and carry 
the resting-stages, which undoubtedly increase the weight of the animals. 
