136 ANNALS OF SCOTTISH NATURAL HISTORY 



similar to that of the Continent, that it is inconceivable (unless all 

 the species are introductions) that it can have existed in our islands 

 for any, geologically speaking, long period of time. Even the most 

 plastic of British Mammals, such as the Squirrel, have only advanced 

 a comparatively short distance on the road of differentiation ; and 

 as regards Birds there is a precisely similar story to be told, there 

 being only one really well-differentiated peculiar British species, the 

 Red Grouse (Lagopus scoticus). In fact, one of the strongest 

 arguments against my friend * Dr. R. F. Scharff 's brilliant theories as 

 to the age of the Irish fauna is, that were it so old as he would make 

 it, we should expect to find not only peculiar species but even 

 peculiar genera among the mammals of Ireland, whereas a most 

 careful study has hitherto only sufficed to distinguish one certainly 

 peculiar species, the Irish Stoat (Putorius hibernicus], and that bears 

 in itself very clear evidence of its recent origin. Another species or 

 subspecies, the Irish Hare (Lepus hibernicus\ seems also to be 

 distinguishable, but it is not nearly so distinct as the Stoat. Among 

 Birds, Reptiles, and Amphibians naturalists have hitherto failed to 

 find any peculiar local forms, although it is evident that the Grouse 

 of Western Great Britain and of Ireland is following on the same 

 route as the Irish Stoat and Hare. 



Can there, then, be any great difficulty in supposing that Mus 

 hirtensis is indigenous to St. Kilda, and that it reached the island at 

 a (geologically speaking) comparatively recent period, when there 

 was in existence a land-surface connecting the Shetlands, Orkneys, 

 Scotland, the Hebrides, St. Kilda, and Ireland ; and that that 

 connection must have been so (geologically speaking) recent that 

 few of our native mammals have had time to develop into species or 

 even subspecies distinct from those of the continent of Europe ? 

 That the Mouse of St. Kilda should be the one in which variation 

 has proceeded further than in other localities is quite in accordance 

 with the isolated situation of and confined space on the rock, together 

 with its full exposure to the Atlantic winds. 



To assert that the Mouse of Iceland has reached that island 

 along a formerly continuous land -area would be a very different 

 matter, since not only is there a deep channel between the Faroes 

 and Iceland, and even between the former islands and the Shetlands, 

 but, if we consider that Mus islandicus is native to Iceland, then we 

 should expect to find a similar or representative species in the 

 Faroes, and of that we have as yet no record. 



Yet that there has never been such a land connection will not, I 

 suppose, be contended by anybody, so that the question in reality 

 resolves itself into one dealing with the time at which such a con- 

 nection existed, and whether it has been sufficently recent to allow 

 of a passage along it of such a presumably recent mammal as a 



1 See "Proc. R. I. Acad./' July 1897, p. 427. 



