LIMNOLOGICAL PROBLEMS 
403 
attribute to the birds, wind, and water a greater importance as dis- 
seminating agencies. 
Means by which the CosMoroLrrANisM has been brought about 
I. Different Modes of Reproduction. — If it should now be 
asked, What means do the species possess to enable them to adapt 
themselves to the varying and different demands imposed upon the 
individual organisms, partly in Greenland, partly in the large African 
lakes, attention must specially be directed to the following points. 
It may be regarded as a well-known fact that a great part of the 
lower fresh- water fauna and flora, and probably more than we know of 
at present, have special resting-stages in which the species remains 
under unfavourable conditions ; e.g. resting-cells in Cyanophyceae, 
resting-spores in Diatoms, cysts in Chlorophyceas, Flagellata, and 
Infusoria ; resting-eggs in Rotifera, Cladocera, and Copepoda. In 
many cases these resting-stages proceed from special modes of repro- 
duction ; a great many plankton forms have several, generally two, 
modes. It further appears that the plankton forms employ the 
different modes of reproduction to a different degree in different 
climates : in certain zones one kind of reproduction prevails, in 
another, the other. In arctic regions reproduction in the Cladocera 
is only to a slight degree parthenogenetic, and is mainly digonic ; the 
farther south we go the more parthenogenetic reproduction prevails, 
and the " sexual becomes restricted to certain short periods. Quite 
the same thing is probably also displayed in the Rotifera. It is also 
certain that if the conditions differ very much in different localities, 
though in the same latitude, then reproduction also differs in these 
localities. In arctic regions reproduction is the same in pools and 
lakes, whereas farther south it differs in lakes and in ponds even for 
the same species. The pelagic races in the south pass from being 
dicyclic in ponds to monocyclv and thence to acycly in lakes. The 
tendency to acycly increases in the pelagic races from north to south. 
D. hyalina and the Bosmince are everywhere monocyclic in arctic 
regions ; in the Baltic territory D. hyalina is dicyclic in ponds, at any 
rate in certain districts ; in the lakes there is a decided tendency to 
acycly ; in the lower-lying alpine Swiss lakes D. hyalina is distinctly 
acyclic, in the high alpine di- or mono-cyclic. Though not so con- 
spicuous, very similar phenomena have been discovered in the Rotifers, 
and the very same can also be pointed out for many other lacustrine 
(bottom and littoral) groups of animals, in which either two sorts of 
eggs are met with (thick-shelled resting-eggs and thin-shelled summer 
eggs), or in which various modes of reproduction occur, the digonic or 
monogonic (parthenogenesis, gemmation, partition). The two modes 
