17 6 Proceedings of the Royal Irish Academy. 



reached almost every islet, even the distant Shetlands and St. Hilda, where 

 shrews are unknown. They are represented in Iceland, but are absent from 

 America. 



In the second place, they are much more plastic than the Pygmy Shrew. 

 They occur on the British mainland hi two forms, and then- representatives 

 in the Outer Hebrides. Fair Island, St. Kilda, the Shetlands and Iceland are 

 all distinguishable. Tire last is little known and the least differentiated, so 

 it may owe its position to introduction, as may also the Shetland stock, 

 which cannot be distinguished from that of the mainland. The three former 

 are more primitive in pelage than then - representatives of the mainland, and 

 may be indigenous and are certainly of ancient standing. 



In the third place, Wood Mice, except perhaps in tooth-structure, are less 

 primitive mammals than shrews : in other respects they represent a higher 

 grade of specialization and evolution, particularly in pelage, the ancestral 

 type of coat being relegated to the juvenile stages, and being completely cast 

 off by the adult. 



The Wood Mice, then, are probably older in Britain than the Pygmy 

 Shrew. Like it, they are animals that one might suppose readily capable of 

 introduction. But, even if it could be demonstrated that they owe their 

 presence in St. Kilda and the Outer Hebrides to introduction, the differentia- 

 tion which they show in these localities must place the introductions at a very 

 distant date in the past. 



They are hardy animals, but not so hardy as the Pygmy Shrew ; their 

 diet is more restricted, being preponderatiugly, though not exclusively, 

 vegetable, so that they would not so easily survive glacial conditions. 



The supposition that Wood Mice are of ancient standing in Britain is 

 supported by the fact that their bones occur - numerously in pleistocene 

 deposits, and even in the pliocene Forest Bed (West Bunton). In Ireland then 

 bones were first found in Ballinamintra Cave, Co. Waterford (where they 

 were at first thought to be those of frogs), and, later, numerously in the 

 caves at Kesh, Co. Sligo, and in those of Co. Clare. In the last they occur in 

 all the strata ; at Kesh they were in association with Arctic Lemmings 

 (Dicrostonyx). The Irish remains have never been critically examined, but 

 in England the bones of late pleistocene Wood Mice have been referred to 

 two forms. One of these resembles A. sylvatkm of the present period. The 

 other is a supposed extinct species named A. lewisi ; it bears a close re- 

 semblance, perhaps amounting to identity, to the Yellow-necked Mouse, 

 A. flavicollis, a subspecies of which, A.f. wiidoni, still exists sporadically in 

 England. 



Applying the above facts to the Wood Mice of Clare Island, it is remark- 



