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Show 1899.] THE MICE OF ST. KILDA. 83 throughout the immense area where it is found it remains remarkably constant to a single well-marked type. Throughout the Palaearctic Region 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 peculiarities 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 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 obtained in Corsica from those of Ireland or France. Mus sylvaticus is, then, obviously a species which in its longstanding 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 with 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 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 wide-spread mammals. It is also 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 Palaearctic 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 evidence 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 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, a state of things which might be produced 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, 1 6* |
