Dakin and Latakche — The Plankton of Lough Keagh. 88 



planktologists to the "vaew that a large number of the European and North 

 American limno-planktonic organisms have migrated south from the northern 

 regions. Other workers have maintained that in addition to the cosmopolitan 

 forms present, there are others, arctic in appearance, in the alpine lakes which 

 are relics of a fauna which existed in Glacial times, and which migrated partly 

 north and partly south with the melting of the ice. Many of the fonns have 

 been considered as relicts. This term has had, however, a very chequered 

 career. 



In 1860 a paper was read by Loven on a marine arctic fauna which was 

 found to occur in certain lakes in Sweden. Amongst the species supposed to 

 represent marine forms were Mysis rdicta Lov., Idothea entomon'Lt., two species 

 of Gammarus.and Pontoporeia affinis Lindstrom. Loven assumed that this faima 

 had migrated from a Polar sea which once covered Finland, into an arm of 

 the sea which eventually was reduced to the fi-esh-water lakes examined. The 

 old marine fauna did not disappear entirely, but left some organisms which 

 had adapted themselves to the new external medium. These lakes were termed 

 relict lakes, and the marine species, now in fresh water, "relicts." This relict 

 hypothesis was very favourably received ; and many lakes containing 

 organisms of supposed marine origin were supposed to be ancient sea-basins. 

 Again, other species of both plants and animals which occur in regions outside 

 their centre of distribution have been regarded as resulting from Glacial 

 influence, and consequently termed Glacial relicts. Both uses have been 

 carried much too far. Where should one end ? Credner's paper in 1898 

 demonstrated that in a great many cases the marine organisms had mounted 

 the rivers. Other causes may have been at work which resulted in the 

 transference of a species arctic in character or marine to another region. It is 

 necessary always to count on the possibilities of a migration active ot passive: 

 and a relict species is not so easily defined as was at one time imagined. 

 The formation of so-called relict forms is, as a matter of fact, a phenomenon of 

 to-day, and of pre-Glacial times as well as GlaciaL Bearing this in mind, we 

 can now consider one of the most characteristic of so-called relicts — one 

 which Loven met with in the Swedish lakes, and to which reference has 

 already been made. 



M)/sis relieta or, better, Mysis oculata Fab. var. relicta (Loven) G. 0. Sars, 

 occurs in Swedish lakes, Lake Ladoga, Finnish lakes, Eussian lakes, Lakes 

 Superior and Michigan, in Great Lakes of Xorth America, Madii, Dratzig See 

 and Tollen See in North Germany, andLough Xeagh and Loch Erne in Ireland. 

 All these lakes would be considered, according to the work of Loven, as 

 having been formerly, in the time when Mysis relicta first occurred, arms of 

 the sea. The distribution is indeed great ; and, according to Tattersall, there 



K.I.A. PROC, VOL. .\XX., SECT, B. [J/] 



