90 THE SCOTTISH NATURALIST 



of the ' Scotch Sea.' " ^ Without entering into the question 

 as to whether this " Scotch Sea " may or may not have 

 completely covered the outer islands, it is sufficient to say 

 that the problem of the origin of the fauna of Coll and Tiree 

 is identical with that of Lewis. 



If the former view is correct, and the Outer Hebrides and 

 other western islands have become separated from the main- 

 land in Post-glacial times by subsidence of intervening 

 land-tracts, it is to be expected that some, perhaps most of 

 the water-beetle fauna, has reached the island by a normal 

 process of dispersal by short flights and local floods ; whereas 

 if the islands have been separated from the mainland since 

 Pre-glacial times, we have either got to assume that the 

 fauna is a relict from Pre-glacial times, or that it has arrived 

 there since the Glacial period by crossing the sea. 



As to these island faunas being Pre-glacial relicts, the 

 question seems scarcely worth considering. It is held by 

 some authorities that the so-called " Lusitanian " element in 

 the west of the Britannic area survived the Glacial period 

 either where it now is or on a westward land-extension ; but 

 this element is a very small proportion of the Britannic fauna 

 and flora, and only one of the water-beetles in the Lewis list 

 could possibly be regarded as a member of it. Apart from 

 this, there is ample evidence that the glaciation of— or 

 perhaps iceberg action on — these islands was very complete, 

 so that the probabilities of anything surviving, except 

 perhaps purely arctic species, are very remote. 



We may, therefore, enquire as to whether it is at all 

 reasonable to assume that the present water-beetle fauna 

 could have crossed the intervening sea. I have already 

 mentioned that species are always experimenting with new 

 ground in an endeavour to find something that suits them ; 

 and we find that if, for instance, a new pond is dug in a 

 district it quickly becomes inhabited by water-beetles, the 

 actual kinds which first occupy it varying in different districts 

 and at different periods. One would almost expect that any 

 newly opened habitat would be not only first seized but also 

 permanently retained by the dominant community in the 



' Loc. cit., 1 91 3, p. 51. 



