SoLttas—On the Origin of Freshwater Faunas. 111 
from early Pliocene freshwater ancestors. It is, on the other hand, possible that 
the modification of some marine Littorinid into Limnotrochus, as well perhaps as 
Syrnolis into Syrnolopsis, may have taken place during the conversion of some 
part of the Tertiary sea into the African lakes; but whether these two genera 
are special modifications confined to the African area or not, future research alone 
can decide. 
The lakes of the northern hemisphere must now be briefly referred to. Some 
of them, as those of Norway and the British Isles, were probably submerged 
beneath the sea during the middle of the glacial episode. The same is probably 
true of the North American lakes. This short, temporary submergence, while it 
must have destroyed the freshwater inhabitants of the lakes, probably introduced 
into them the marine and Arctic Crustacea, Mysis relicta and Pontoporeia affnis, 
which became isolated from the sea on the subsequent re-emergence of the land. 
The rest of the present inhabitants of these lakes must have been subsequently 
supplied by the rivers which discharge into them. It is probably owing to the 
severe climate which culminated in glacial conditions that the Melanide are no 
longer to be found in Northern Europe. The cold and the glaciers would, no 
doubt, have operated quite as effectually in North America; but in this case the 
structure of the country has made it possible for the Strepomatide to return from 
their southern exile, and once more to occupy their pre-glacial habitat. The great 
size also of the American lakes might have saved the Melanide from extinction 
during the first and most severe part of the glacial period; and during the great 
submergence, when the climate was warmer, they might have sought refuge from 
the sea-water in the freshly restored river-systems. 
We have, finally, to consider the causes which have led to some of the more 
marked modifications which characterise freshwater genera. In the first place 
they are seldom shared by their nearest marine relations: no marine molluse is 
known to pass through a glochidium stage ; no marine Polyzoon nor sponge produces 
a winter bud or statoblast; no marine Phyllopod an ephippium; and no Tubula- 
rian an egg within a horny shell like that of Hydra. A large number of marine 
molluses, however, lay their eggs in capsules, and some, such as Cymba, are vivi- 
parous. 
The winter eggs of sponges and Polyzoa, since they appear in correspondence 
with a seasonal change, are probably, as Semper has suggested, produced as a pro- 
tection against cold or drought. If they are adaptations to a freshwater climate 
they must have appeared subsequently to the isolation of the organism from the sea ; 
and thus, though now available as a means of distribution, could not have been the 
means by which the organisms producing them exchanged a marine for a freshwater 
habitat. The ephippium of Daphnia and the incapsuled embryo of Hydra may also 
be regarded as modifications induced by the severity of a freshwater climate. The 
fact that the embryo of Hydra hatches out soon after it is laid in a freshwater tank 
TRANS. ROY. DUB. SOC., N.S. VOL. III. R 
