AQUATIC COLEOPTERA OF THE OUTER HEBRIDES 89 



THE AQUATIC COLEOPTERA OF THE 

 OUTER HEBRIDES. 



By Frank Balfour Browne, M.A. (Oxon. et Cantab.), 

 F.R.S.E., F.Z.S. 



{Continued frotn page 67.) 



We have, therefore, on this island a number of species 

 of water-beetles occupying all the available types of habitat, 

 and the same species dominate the fauna as are dominant in 

 others of these western islands. There are no species not found 

 on the mainland, and the vast majority are found all through 

 the British islands. If Lewis had recently been part of the 

 mainland which had become separated off by subsidence of 

 an intervening land-tract, the water-beetle fauna is just what 

 would be expected, and it has been maintained that this is 

 the correct view of the origin of the faunas and floras of 

 these western Scottish islands. On the other hand, there 

 seems to be no agreement among geologists as to when these 

 islands formed part of the mainland of Scotland, or as to 

 their condition at the time of the Glacial period, when at 

 least most of Scotland was covered with ice and snow. With 

 regard to this we find, according to one view, that during the 

 Glacial period there was a considerable elevation of land in 

 the Britannic area, and that this land stretched far out to the 

 westward ; whereas according to another view, the Glacial 

 period was marked by a subsidence of at least the western 

 part of Scotland, so that these islands were more or less 

 completely submerged. 



In a recent paper on the beetles of Tiree, Mr W. E. Sharp 

 assumes the correctness of the former view, while Mr P. A. 

 Buxton, commenting on this paper, gives the authority of 

 other geologists for saying that " Tiree must almost certainly 

 have been isolated from the mainland since Pre-glacial times. 

 It may be taken as certain (as far as geological facts can be 

 certain) that the island was completely covered by a part 

 40 M 



