SOLLAS On the Oriyinof r,rshn;it<-i- Faunas. 103 



Of the marine form- t '.\iMing in tints transformed area, few would probably 

 migrate so long us the tidnl waters of the oc-tan ebbed and flowed in it. As 

 elevation progressed, many active locomotive forms, fo whom the new conditions 

 \\ere 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 

 climatal 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 

 t-iner^ed, geologically speaking, from a glacial episade 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 i> 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, which, retaining a certain quantity of salt water iff their interstices, 

 may lia\. -iven 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 SOT. IH'B. SOC.. V. 8. VOL. III. Q 



