404 THE FRESH- WATER LOCHS OF SCOTLAND 
of reproduction are not equally common under all climates and in all 
localities. For the Ostracoda, Bryozoa, Planaria, Dinoflagellata, and 
possibly also Phyllopoda I shall here not enter into more thorough 
details, but refer the reader to the following literature : — Ostracoda 
(S. Jensen, 1904, p. 21) ; Bryozoa (Wesenberg-Lund, 1896b, p. 350, and 
1907, p. 71); Planaria (Zschokke, 1900, p. 86; Thinemann, 1906, 
p. 68 ; V. Hofsten, 1907, p. 4 ; Steinmann, 1906, pp. 199 and 213 ; and 
to valuable papers by Voigt) ; Cerat'mm Mrundinclla (Zederbauer). 
In certain latitudes and under certain conditions monogonic reproduc- 
tion prevails ; under others, digonic reproduction. This phenomenon, 
which, so far as I know, has hitherto not been sufficiently noticed in the 
animal kingdom, is exceedingly well known in plants. It is well 
known to all botanists that numerous plants under certain conditions 
and in certain latitudes only have vegetative reproduction, and that 
different conditions determine vegetative reproduction. It is just in 
the varying modes of reproduction and the ability the organisms 
possess of utilising mainly sometimes the one sometimes the other, 
that we must seek for one of the main causes of the quite pheno- 
menal power of adaptation and the consequently wide geographical dis- 
tribution of the plankton organisms (see Wesenberg-Lund, 1907, p. 70). 
It must be regarded as a well-known fact that the maxima of the 
plankton organisms seem to occur at certain fixed temperatures. If 
the temperature of the lake is too distant from these, the plankton 
organism concerned does not occur there (Cyanophycese rarely in high 
arctic lakes) ; if the right temperatures only prevail for a short time, 
the organism remains in resting-stages throughout the greater part of 
the year ; where they prevail for a long time the resting-stage is but 
short. In arctic regions Ceratium h irundinella is only free and pelagic 
during a few weeks of the year ; in the Baltic lakes, from April till 
October ; in the North Italian lakes it is perennial. Daphnella does 
not occur in arctic lakes ; it is a periodical form in the Baltic lakes, 
perennial in the Italian lakes ; Anahce7ia circinalis is periodical in 
Danish lakes, perennial in the Swiss lakes. The duration of the 
resting-periods decreases towards the south, and in this we find an 
interesting parallel to the life of higher plants in different zones. We 
are thus able to show that the different use of the two modes of 
reproduction, the result of which is the different sort of eggs, germs, 
etc., is one of the chief means by which the plankton organisms 
are able to adapt themselves to the varying conditions in the different 
parts of the world. 
II. Variation. — A second fact on which the cosmopolitanism of 
the plankton organisms depends is the extraordinary capability each 
species has of changing form. It is therefore necessary shortly to 
deal with our knowledge regarding the variation of the plankton 
