6o The Irish Naturalist. March, 



Crustacea, one of which is also found in Scotland ; and 

 together with these certain species of an alpine genus offish, 

 with its headquarters in Scandinavia and high altitudes. 

 Though the science of geology does not yet speak with 

 certain voice upon some more or less local movements of the 

 earth in past ages, yet its testimony is very distinct as to the 

 former continuity of Scandinavia, Scotland, and the North 

 of Ireland, which for a long period subsequent to the Glacial 

 epoch formed a continuous peninsula. As to the purely 

 fresh-water fauna of Ireland, therefore, there can be no 

 difficulty in assigning (at least to a great portion of it) a 

 European origin through Scandinavia. Therefore, if the 

 marine hypothesis of the introduction of Mysis relicta to our 

 two greatest lakes has to be abandoned, its association with 

 Scandinavian Kntomostraca offers a very simple explanation 

 of whence it was derived. If you ask me, however, hozv the 

 introduction was effected of these fresh-water organisms, and 

 how they were conveyed from lake to lake, I can only reply 

 that though some light can be thrown upon it, yet the 

 phenomenon (especially with regard to the introduction of 

 fish, and particularly the delicately-organised SalmonidcE, to 

 which the various species of Charr belong), is one of the 

 most inexplicable of all distributional problems. As, how- 

 ever, it is one of such wide comprehension, w^e need not be 

 staggered by any single example. The difficulty sinks into 

 comparative insignificance if we remember the extraordinary 

 apparition of Arteviia salina in any locality in which brine 

 ponds are established, when the water becomes sufficiently 

 salt to fulfil the requirements of its existence. 



In any case, however, the enormous tracts of time which 

 are involved in the problem of the influx of fresh-water 

 animals hither from North Europe, and also of our newly-found 

 American fresh-w^ater sponges, stagger the mind with their 

 immensity ! How noble a study then is that of Natural 

 History ; opening ever afresh new vistas of an interminable 

 past, full of wondrous energies; and raising our minds from 

 horizon to horizon, shining with the life-giving rays which 

 emanate from the great Source of all Eight and Eife 1 



Drumreaske, Monaghan. 



