1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1095 
of the propagation of the plankton Cladocera in nine of our 
Danish lakes. These explorations will he published in the second 
part of my plankton work. I think they will bring out some new 
facts. Still, all in all, they are in accordance with Weismann’s 
(1876-79) elaborate investigations of the Cladocera in Germany. 
On the other hand, it is shown by my own exploration of the 
Greenland Daphnids (1895) that the propagation in all probability 
is quite different in arctic lakes. I shall particularly point out, 
that the Cladocera of Greenland are always monocyclic, never poly- 
or acyclic, as is the case with many of the species to be found in 
temperate countries. If species which are polycyclic in southern 
countries become monocyclic in Greenland, then it is the sexual 
period occurring in autumn which thus is lost. The cycle is 
abbreviated as far as possible, if the parthenogenetic propagation 
is to be preserved at all. Lastly, I shall draw attention to my 
supposition that the number of eggs produced by the partheno- 
genetic females is smaller than in countries of a temperate climate. 
None of these assertions must be regarded as exact, because the 
material which I have had at my disposal was not collected with 
studies of this kind in view. The possibility of incorrectness is so 
much greater since Zschokke (1892), who has studied the life cycle 
in alpine lakes in Switzerland, arrived at quite different results. 
He maintains that the Cladocera of the alpine lakes are 
ordinarily polycyclic and not monocyclic, and that if Cladocera in 
lakes situated at a great elevation above the level of the sea are 
monocyclic, this is not owing to the loss of the autumnal sexual 
period, but to the merging of the two sexual periods into each 
other, the first of these having set in later and the second having 
commenced somewhat earlier than usual. 
However that may be, one thing may be concluded from 
Weismann’s, Zschokke’s, and my own explorations, — the manner 
in which the life cycle of the Cladocera goes on is not the same 
all over the world, but depends on latitude and the height above 
the sea-level. According to my knowledge, this very interesting 
fact has not been established with so much certainty with regard 
to any other group of animals. 
11. In 1900 I pointed out that in several very different plankton 
organisms the longitudinal axis is simultaneously lengthened 
