1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1159 
little periodicity, the seasonal variations being slight. 
This absence of any marked variation in character is to 
be attributed to the relatively slight changes in the 
temperature of the surface waters at different seasons.” 
6. “ The Desmids were doubtless originally derived from the 
pools and bays of the mountains, and only those species 
have flourished which found the conditions most suitable 
for their existence as pelagic organisms.” 
7. “There is no very obvious maximum development of 
Diatoms, and some of the larger species of the JSTavicu- 
loidese and Surirelloidese have firmly established them- 
selves.” 
8. “The proportion of Myxophycese is relatively small, and 
species of Oscillatoria, Lyngbya, and other genera are 
somewhat scarce.” 
1 am glad to see that in all these points we are of quite the 
same opinion. I take the liberty to point out that I in my 
paper, see especially pp. 423, 426, 427, 430-31, have expressed 
quite the same suppositions. This the authors seem quite 
to have overlooked. Also with regard to the derivation of 
the rich Desmid flora we are mainly in full accordance. Pre- 
viously to the authors’ statement, I had pointed out (p. 430), that 
the plankton Desmids must have been originally derived from tarns 
and moors on the hill-tops , and that the Scottish lakes (p. 431), 
unlike most other larger lakes , present one of those great life- conditions 
which so many of the Desmids seem to require , viz., peaty water rich 
in humic acid, ( cf . Summary, point 4, by Messrs West). 
Besides, the authors add the very important fact that the 
Desmids oive their existence * (p. 515) to the Older Palaeozoan and 
Pre-Cambrian formation. On the other hand, when the authors 
(p. 512) maintain that I was of opinion that many of the Conjugates 
adopt a pelagic life as soon as they arrive in the lakes from the 
peat moors, and, contrary to me, set forth that the plankton Desmids 
have existed under these pelagic conditions for a very long time, 
the authors have totally misunderstood me. In p. 430 I have 
written that the Desmids were originally derived from the tarns and 
pools, and (p. 431) that the home of the Desmids is the sides of the 
* An expression which I do not find very clear. 
