The Scottish Naturalist. 53 



Alston shows in tabular form the distribution of the twenty-four 

 species alluded to above, and then proceeds to say : " The 

 facts indicated by the above table are at first sight somewhat 

 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 of other forms, as of the Field Vole in 

 the Hebrides, and of the same species with the Water Shrew 

 or Water Vole in Orkney, which are conspicuous by their ab- 

 sence from the Irish fauna. It appears to me, however, that 

 this apparent contradiction 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 un- 

 doubtedly 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 

 longer 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 Skye, 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 distribu- 

 tion 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 ani- 

 mals shows that the northward 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 diffi- 

 cult to suppose that the Hedgehog, 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 



