THE GEOLOGY OF ORKNEY. lxxxvii 
hares, rabbits, mice, and voles. It is said that in 
the peat-bogs there are also remains of deer, and in 
all the islands toads are abundant. These animals 
are so generally distributed that it is difficult to 
believe that the islands have not at one time been 
connected together by dry land, and for the same 
reason we may infer that this fauna migrated into 
Orkney from Caithness at some period when a direct 
land connexion was in existence. The distribution 
of the animals over the archipelago can hardly be 
accounted for on the hypothesis that they were all 
accidentally introduced into their present habitats at 
different times by fortuitous circumstances. Many 
naturalists believe that a study of the flora and fauna 
of the Feeroes and Iceland proves that these were 
joined to Europe in post-glacial times by a land 
bridge that has long since gone down beneath the 
sea. Similarly Shetland may have been connected 
with Orkney, or even with some part of Europe. 
The evidence on which these hypotheses rest, how- 
ever, is not geological, but depends on the nature and 
distribution of the living animals and plants. Some 
peculiarities of the fauna of Orkney deserve further 
consideration in this regard. One of these is the 
presence of a species of vole not known elsewhere. 
This would tend to prove that the islands have been 
distinct from the mainland of Scotland for a very 
long time, whether we are to regard this vole as 
having developed from other voles since the islands 
were cut off, or as the survivor of a species that 
once inhabited Seotland also, but has become extinct 
everywhere but in Orkney. Another remarkable fact 
