1899.] THE MICE OF ST. KILDA. 83 



throughout the immense area where it is found it remains remark- 

 ably constant to a single A^ell-raarked type. Throughout the 

 Palaearctic Eegion it is distinguishable at a glance from every 

 other Mouse with which it might possibly be confounded by the 

 pattern of its teeth, its long foot, large ears, and pure white belly, 

 separated from the rufous colour of the upperside by a strong and 

 clearly-marked line of demarcation. It is true that these peculi- 

 arities show a slight tendency to local variation, so that two or 

 three local forms of Mus sylvaticus may be recognized ; 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 Mua 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 obtained in 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 specialization 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 \\'ith which it may be brought 

 into contact. It is a species which further and even minute study 

 may find unprofitable, or even impossible, to split into local sub- 

 species. Not that I wish to iraplj' 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 wide-spread mammals. 



It is also extremely interesting to find that the I'epresentatives 

 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 ai'e 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 Palgearctic Region which we should most expect to 

 find in such 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 

 on St. Kilda. 



I therefore think that we have a good deal of cA'idence to support us 

 in supposing that Mus hirtensis 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 con- 

 tinuous land-area which enabled 3Ius sylvaticus to reach the Shet- 

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

 also St. Kilda in its surface, a state of things which might be pro- 

 duced by an elevation of about 60 fathoms only. 



That such a land-connection must have been of geologically 

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



6* 



