460 Proceedings of the Royal Irish Academy. 



absent from the relic lakes. Had Prof. Credner been acquainted uitb 

 Prof. Sollas' ingenious explanation of the origin of freshwater faunas 

 (79), no doubt be would have greatly modified his views. Dr. Sollas 

 shows that all freshwater organisms in their early stages of develop- 

 ment are provided either with some process enabling them to attach 

 themselves to a foreign object, or that they pass this period within 

 the body of the parent. This is a provision of nature to prevent 

 freshwater organisms from being floated out to the sea, where tbey 

 would perish, until they have reached maturity, and can cope with 

 floods and currents. But the larv« of marine mollusca are all free- 

 swimming. Tliey are a prey to the slightest current, and have no 

 chance of settling down in freshwater lakes permanently, unless a 

 radical change were to take place in their mode of development. 



To judge from the relic fauna in the iXorth European lakes, we 

 may safely assume that the area occupied by the plains in the extreme 

 north of Piussia and in Finland and Sweden was, in recent geological 

 times, occupied by the sea. But we have still to take into conside- 

 ration the views expressed by geologists on this subject from ]iurely 

 geological evidence. 



I have already mentioned, what Murchison, de Temeuil, and von 

 Keyserling's opinions were on this point. In speaking of some Arctic 

 shell-beds, which underlie the boulder-clay on the coast-lands of the 

 Baltic, Prof. J. Geikie {25 a, p. 442) remarks : — <* It would seem, tljen, 

 that before the deposition of the lower boulder-clay of those regions, 

 the Baltic Sea had open communication with the German Ocean. 

 Some geologists have supposed that the Arctic fauna of the East 

 Prussian clay-beds may have immigrated from the north rather than 

 from the west. But [there is no direct evidence that the lands lying 

 between the Baltic and the "White Sea were under water during the 

 formation of the shell-bed in question." Prof. J. Geikie, however, 

 admits that at a later period the Baltic and "White Sea were joined : — 

 "The dissolution," he says (p. 486), "of the great Baltic glacier was 

 accompanied and followed by the depression of a considerable portion 

 of the Scandinavian peninsula. Communications thus obtained between 

 the Isorth Sea and the Baltic by one or more straits across Central 

 Sweden, and there was likewise wide communication between the 

 Baltic and the "White Sea, by way of Lakes Ladoga and Onega." 



The assumption that the Arctic mollusca were admitted from the 

 Atlantic to the German Ocean before the deposition of the lower 

 boulder clay, and then found their way into the Baltic, is altogether 

 unwarranted, as I shall show later on. It is extremely probable that 



