Sotias—On the Origin of Freshwater Faunas. 103 
Of the marine forms existing in this transformed area, few would probably 
migrate so long as the tidal waters of the ocean ebbed and flowed in it. As 
elevation progressed, many active locomotive forms, to whom the new conditions 
were distasteful, would escape seawards, and others less enterprising, in whom 
fixity of habit was a pronounced feature, would lag behind till escape became 
impossible. The attached forms would be unable to escape, and would therefore 
be entrapped. With complete isolation of the lake would commence a sorting out 
of the remaining fauna: some members would succumb, others would survive, and, 
adapting themselves to their altered circumstances, give rise to a freshwater fauna. 
The Baltic has often been quoted as a marine area in which some such 
change as that described is in progress, and no doubt with truth, but one cannot 
but admit that the change is here taking place under somewhat unfavourable 
circumstances. The climate of the Baltic is severe, and a selection by means of 
clmatal conditions proceeds apace with that due to freshening of the waters. 
The elevation is also proceeding with that slowness which characterises the 
terrestrial movements of the present day. Finally, the Baltic has only just 
emerged, geologically speaking, from a glacial episede which left its fauna poorer 
than it found it. In past times much more favourable transformations of marine 
into lacustrine areas must have occurred. To look no further back than the 
beginning of Tertiary times, we know that then there existed a far more 
uniform, which is the same as saying ‘less severe” climate than characterises the 
temperate regions of the existing period. We have reason to believe that the 
relative level of land and sea was subject to more rapid changes. Glacial epochs 
did not interfere, and the newly-raised rocks need not have always been schists and 
gneisses, but were sometimes probably composed of softer and more porous 
materials, wluch, retaining a certain quantity of salt water i their interstices, 
may have given a brackish character to the first-formed running streams. If the 
origin of our freshwater fauna dates back to a time when climatal conditions 
were more uniform or less severe, then the characters which freshwater animals 
now possess in adaptation to the existing climate would have been subsequently 
produced, and might have been acquired with secular slowness. 
If now we turn to the evidences of geology, we find that the first recognized 
appearance of lakes is to be found in the Old Red Sandstone period. The pre- 
viously existing Silurian marine areas became gradually differentiated into the 
Old Red Lakes and the Devonian Seas, and a freshwater fauna might very well 
have been contemporaneously developed. It is unfortunate that the explored Old 
Red Sandstone strata, like so many deposits of probably freshwater origin, should 
have proved so remarkably unfossiliferous ; still we know of one fossil far from 
rare in the Kiltorcan beds of Kilkenny, which has been pronounced by no less an 
authority than Edward Forbes to be a genuine ancestor of existing pond mussels. 
This shell, known as Anodonta Jukesii, is altogether different in character from the 
TRANS. ROY. DUB. 80C., N.8. VOL. I. Q 
