LIMNOLOGICAL PKOBLEMS 
401 
Another very peculiar fact connected with the above-mentioned is 
that the ahnost inconceivable richness of forms which characterises all 
communities, both terrestrial and marine, in the tropics, has no parallel 
in the tropical plankton. It seems as if the greatest development 
of the fresh- water plankton is much more to be sought for in the 
temperate zone. 
Explanation of the CosMOPOLrrANis.M 
As is well known, many scientists explain the cosmopolitanism of 
the fresh-water plankton as having resulted from passive migration. 
As distributing agencies birds, wind, and the currents of the ocean are 
principally mentioned. These means of migration may of course be 
of some moment, yet I wish to call attention to a fact which has hardly 
been taken into due consideration, viz., that they can only be of any 
importance when acting over immense periods of time. With regard 
to plankton it must especially be remembered that its resting organs, the 
organs supposed to be transmitted by the above agencies, are wanting 
over large areas, which further diminishes the probability of passive 
migration with regard to lake plankton throughout great parts of the 
world. All in all, the supposition set forth by many authors, that the 
migration by means of wind, waves, and birds is taking place with 
great intensity, must, according to my view, be considered as unten- 
able. The quantity of live germs which are transported by the 
above-mentioned agencies to new localities and there germinate and 
acquire new areas for the species is probably only slight. With regard 
to birds it must be kept in mind that they migrate on an empty 
stomach, and that migratory birds are almost always clean when they 
journey (Andersen in Ostenfeld, 1901, p. 116). Too much stress has 
been laid upon the few instances in which fresh-water organisms have 
been found attached to the feathers and feet of birds. In contradic- 
tion to the signification attributed to water as a distributing factor, 
speaks the well-known fact that lakes lying on a line and fed by the 
same river often exhibit quite different plankton communities. This 
especially holds good with regard to the zooplankton. In my 
opinion, none of the means of distribution at the disposal of this 
plankton which we know of at present can explain its enormous 
distribution. Only one factor, viz. time^ can perhaps give us a sug- 
gestion as to the correct interpretation of the phenomenon. 
Until the facts give some other explanation, I have formed the 
picture that this same fresh-water plankton, which in a horizontal 
direction has at present a more uniform appearance than any other 
community, has also vertically presented the same appearance during 
all times. The fresh-water plmikton is amongst the oldest coinmunities 
of the earth. The researches of recent times have shown us what an 
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