1150 Proceedings of Royal Society of Edinburgh. [sess. 
cycle of the species, especially in the larger and deeper lakes ; they 
only become of consequence in shallow and warm lakes or in ponds. 
I think it very probable that the first of the two generations in 
Thingvallavatn belongs to the form microcephala, the second to the 
obtusifrons. The question arising with regard to the degree in 
which the different generations vary according to tp. is of 
the greatest moment with regard to the understanding of the 
seasonal variations, but it is also extremely difficult to deal with. 
I have studied this problem upon our own plankton organisms 
for quite a while, and a full account will be given of this in the 
second part of my plankton work. With regard to the question 
in the Icelandic lakes I have of course formed some opinions, but 
as these have not been tested by means of more thorough-going 
explorations in the Danish lakes, they are of no value, and will 
therefore not be mentioned here. 
It will then be seen that the life cycle of D. longispina in 
Thingavallvatn is [in accordance with the cycle which Zschokke 
(1892) has found in the Swiss alpine lakes, whereas that of D. longi- 
spina in Myvatn corresponds with what I have found in Greenland 
(1895) and Ekman (1904) in the sub-arctic alpine lakes in Sweden. 
The exploration has further confirmed the supposition first set 
forth by me, and later on confirmed by Ekman, that otherwise 
polycyclical Cladocera nearer the poles will become monocyclic, the 
autumn sexual period being omitted, and that the parthenogenetic 
propagation here is of inferior significance when compared to the 
sexual one. 
Ekman (1904, p. 94) maintains, contrary to my observations with 
regard to the Cladocera of Greenland, that the number of partheno- 
genetic eggs in an alpine or arctic country is by no means less than 
in a lowland country, but even greater. I daresay that Ekman, 
generally speaking, is quite right ; still, I must emphasise the fact 
that the number of eggs which I have found in the Thingvallavatn 
samples has never exceeded 4 and has often numbered only 2 or 3. 
With regard to the number of eggs in Myvatn, I refer to the fact 
stated above : it is very high in spring, but by no means greater 
than may, for instance, occur in Danish lakes at the same time of 
the year. I suppose that, taken all in all, the number of eggs to 
be noted is about the same all the year round as in Danish lakes. 
