LIMNOLOGICAL PROBLEMS 
417 
and in which great annual fluctuations of temperature did not occur 
(fig. 61). The investigations in alpine lakes in Switzerland and Austria 
have given quite the same result, whereas the seasonal variations in the 
lowland lakes of the same countries take place quite as in the Baltic 
lakes. Just as the seasonal variations in our lakes do not take place 
at temperatures above 12-16° C, they are not traceable in lakes 
which never reach this temperature. Where the claims for increased 
floating power are not pressed, those structures by which it is 
augmented will not be formed by the organism. 
It is still more peculiar that not only the seasonal variations but 
also the local variations almost totally disappear under arctic con- 
FiG. 61. — Daphnia hyalina in Esromso, Denmark (above), and in Myvatn, Iceland 
(lower). In Esromso we have a conspicuous seasonal variation, in Myvatn this is 
absent. In Esromso, Daphnia hyalina overwinters as a free-swimming organism, 
in Myvatn only in ephippia. 
ditions. On an area so small as Zealand, D. hyalina has in almost 
every lake a special, well-defined race. If, on the other hand, we com- 
pare examples from Greenland, Iceland, Sarek in Sweden, and from the 
northern parts of Finland it will be seen that the race characters 
have almost disappeared. All these forms may without any difficulty 
be referred to one or two types easily distinguishable from the summer 
forms of southern countries^ (fig. 62). Now it has further been 
1 The peculiar phenomenon that the local variation is lost in the high north is 
in full accordance with the fact that the species of Daphnids with which we have 
to do in the arctic region, like the jjlankton organisms in large lakes, have 
preserved the digonic propagation. Everywhere where investigations have 
been made it has been shown that resting-eggs are formed ; sexual propagation, 
which prevents race-formation, is still retained ; and the resting-stages, the means 
of distribution, are carried by means of rivers and wind from lake to lake. 
27 
