122 
In gr. 1, where no (or almost no) vegetation is found, Diaptomus 
minutus and Cyclops strenuus are dominating. They are both by all 
authors considered as eulimnetic animals, i. e. animals which entirely 
belong to the free mass of the water. 
A typical lake in gr. 1 is T h i n g v a 11 a v a t n. Diaptomus mi¬ 
nutus is here the all-dominating species, occurring in dense swarms 
in the surface as well as in deeper water. It is besides found in 
almost all ponds in the groups of height II and III, if only the 
vegetation is scarce. In such places it is, however, found in dense 
swarms in shallow water (less than 0,3 m). 
In the Canadian Sea-Territory it occurs, according to Marsch 
and Juday, in quite another way. Here it is found only in the 
greater lakes and never near the beach. In smaller ponds it was 
never found. In Greenland its distribution (acc. to Haberbosch) is 
just the same as in Iceland. Thus Diaptomus minutus has in 
arctic countries quite another mode of living than in 
more Southern countries. 
From Tab. 2 it will be seen that the mean temperature of July 
in the Canadian Sea-Territory is twice as many centigrades as that 
of Iceland and Greenland. It is surely the high temperature in 
the more Southern countries which confines its occurrence to the 
larger lakes, where, during the warm period, it can frequent the 
cold water in the lower layers. 
The faet that Diaptomus minutus in arctic countries is found 
both in small ponds and in the lakes, while in temperate countries 
it is found only in large and deep lakes, makes it probable that 
its primary home is the Arctic, and that thence it has immigrated 
to the temperate America, surely during the glacial period. That 
Diaptomus minutus in Iceland and Greenland lays more eggs than 
in the temperate North-America also indicates the northern descent 
of the species. 
An analogy to this in the old worid was shown by Ekman, 1905, 
in the distribution and habits of Bythotrephes longirnaniis. Thus 
we have here the interesting faet that an exelusively 
nearctic species {Diaptomus minutus) shows the very same 
peculiarities as a palæarctic species {Bythotrephes longi- 
manus). 
