ON TWO RECENTLY DESCRIBED MICE FROM ST. KILDA 135 



long foot, large ears, and pure white belly, separated from the rufous 

 colour of the upper side by a strong and clearly marked line of 

 demarcation. It is true that these peculiarities show a slight 

 tendency to local variation, so that two or three local forms of 

 Mus sylvaticus may be recognised ; but the variation is so slight that 

 it takes a specialist to distinguish Mus chevrieri^ M.-Edw., of Tibet 

 and China from Mus arianus, Blanf, of Persia and Afghanistan, or 

 Mus sylvaticus, Linn., of Europe. 



Within the confines of Europe the animal seems to hold quite 

 firmly to one particular type, so that I am unable to distinguish 

 specimens from Corsica from those of Ireland or France. 



Mus sylvaticus is then obviously a species which, in its long- 

 standing and successful struggle for existence, has attained to a 

 height of specialisation from which it has either very little power of 

 variation, or else which is such as to fulfil all the needs of the species 

 in almost any conditions with which it may be brought into contact. 

 It is a species which further and even minute study may find un- 

 profitable, or even impossible, to split into local subspecies. Not 

 that I wish to imply that local variations are absent or even rare in 

 Mus sylvaticus. They are by no means so, but their presence is 

 infinitely less abundant or conspicuous than is the case with other 

 and perhaps equally widely spread mammals. 



It is then extremely interesting to find that the representatives 

 of Mus sylvaticus in the Hebrides and St. Kilda show as much 

 divergence from the type as examples from any other locality with 

 which we are acquainted, and it is an evident sign of the antiquity 

 of the animal at St. Kilda, and a seemingly irrefutable argument 

 against any theory of its introduction into the island apart from 

 the fact that its presence in the Channel Islands, in Iceland, Norway 

 and Sweden, the Shetlands, Ireland, and the Inner and Outer 

 Hebrides marks it out as the species par excellence of all others in 

 the Paljearctic Region which we should most expect to find in an 

 out-of-the-way island. And, to judge by its large size and robust 

 form, it has had no difficulty in maintaining its existence at St. Kilda. 



I think, then, that we have a good deal of evidence to support 

 us in supposing that Mus hirtcnsis is indigenous to St. Kilda ; and, 

 indeed the very position of this rock, facing as it does the Western 

 Hebrides and with a channel of no very great depth between it and 

 them, throws no difficulty in the way of the hypothesis that the 

 continuous land -area which enabled Mus sylvaticus to reach the 

 Shetlands, Scotland, the Hebrides, and Ireland, should have included 

 also St. Kilda in its surface an event which might be brought about 

 by an elevation of about 60 fathoms only. 



That such a land connection must have been of geologically 

 quite recent existence is a matter of no difficulty for a zoologist, 

 since the whole of the Mammalian fauna in Ireland and Britain is so 



