Pelagic Fauna of Freshwater Lakes. 325 
animals having been differentiated by the action of the winds 
in other larger lakes. = 
In this way we can easily explain the differentiation of most 
pelagic species, with the exception of two; and these are the 
finest and most interesting of the pelagic Entomostraca— 
namely Leptodora hyalinaand Bythotrephes longimanus. These 
two Cladocera are not related to the freshwater species which 
form the littoral faunas of the lakes, or the palustrine and 
fluviatile faunas*; and therefore we cannot explain their origin 
by differentiation of the littoral forms. For these two species 
we must therefore, like Pavesi, seek a marine origin. Bytho- 
trephes would be derived from an ancestor which was common to 
it and to Podon, its nearest ally, as, indeed, Leydig has already 
indicated. Leptodora, on the contrary, according to Weis- 
mann’s view, would have branched off from a primeval 
Daphnid, of whose direct descendants nothing further is known. 
But how could the passage from salt into fresh water be 
effected ? Pavesi supposes that this may have taken place 
by the closing of a fjord, and its gradual conversion into a 
freshwater lake, so soon as it was separated by a bank from 
the sea. This is possible ; and we have examples of the same 
kind in certain marine forms which occur in the freshwater 
lakes of North Italy and Scandinavia. But how if this 
transition has not taken place by passive migration and trans- 
ference into lagoons which were constantly becoming less 
salt? For the decision of this question we have still no reli- 
able materials. But so soon as the adaptation to fresh water 
had been effected, the distribution of these forms of marine 
origin took place in the same way as with other pelagic fresh- 
water forms, and thus these two forms would be introduced 
into lakes which were never in direct communication with 
the sea. | 
In conclusion, we might draw a parallel between the pelagic 
fauna of freshwater lakes and that of the sea. ‘The analogies 
are numerous and of great interest; but they are so patent 
that it is superfluous to dwell upon them particularly. The 
general facts are the same or very similar; the distinction 
lies chiefly in size and number. In the sea all is large; in 
our lakes every thing is of small and restricted dimensions— 
not only the number and size of the individuals, but also the 
number of species, the extent of their migrations, and their 
range of distribution. 
* G. Joseph has discovered, in two large caves of Carinthia, a second 
species of the genus Leptodora, L. pellucida, which differs essentially from 
the L. hyalina of the pelagic lake-fauna by the absence of eyes. It is the 
only Cladocere that occurs in the cave-fauna (Berl. entom. Zeitschy. 
xXXvi. 3, 1882). 
