The facts indicated by the above Table are, at first sight, some- 

 what contradictory. As Ireland possesses the greatest number of 

 species in common with the mainland of Scotland, it might well be 

 supposed to have been in connection with it up to a later date than 

 even the Inner Islands. On the other hand we have the presence 



J 



of other forms, as of the Field Vole in the Hebrides, and of 

 the same species with the Water Shrew and Water Vole in 

 Orkney, which are conspicuous by their absence from the Irish 

 fauna. It appears to me, however, that this apparent contradic- 

 tion may be explained, if we remember the more northern position 

 of the Scottish Islands and the nature of the country lying between 

 them and the south-western source from which our Mammalian 

 fauna was undoubtedly derived. 



A consideration of the relative depths of the channels which 

 respectively divide Ireland and the Islands from the mainland of 

 Scotland would lead us to the conclusion that the severance 

 of the former took place first, and that the Orkneys remained 

 longest uninsulated. An upheaval of about 240-270 feet would bring 

 the latter again into communication with Caithness, while it would 

 require a rise of about 300-320 feet to reunite the Hebrides with 

 Skj^e, and of from 700 to 900 feet to restore land communication 

 between the various parts of south-western Scotland and north- 

 eastern Ireland. Nor does the distribution of Mammal life seem 

 to me to contradict such a hypothesis. The absence from the 

 known fossil fauna of Scotland and Ireland of most of the 

 characteristic postpliocene English animals shows that the north- 

 ward migration of these forms was slow, gradually advancing as 

 the glacial conditions of the northern parts of our islands decreased 

 in intensity. Thus it is not difficult to suppose that the Hedge- 

 hog, Ermine, Badger, Squirrel, and Mountain Hare, may have 

 found their way through southern Scotland into Ireland long 

 before they were able to penetrate into the still sub-arctic regions 

 of the Highlands. Subsequently, when the continued depression 

 of the land had isolated Ireland, and the improvement of the 

 climate had continued, the Shrews and Voles may well have found 

 their way northwards along the comparatively genial coasts, before 

 the larger beasts of prey could find a sufficient stock of game. 

 When they reached Orkney, however, they appear to have found 

 it a veritable Ultima Thule, for the absence from Shetland of any 

 land animal (except the half-aquatic Otter) seems to indicate that 



