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Show 1899.] THE MICE OF ST. KILDA. 87 typicv.s in houses. It seems to me, therefore, probable that both Mus bactrianus and Mus musculus are developments of some original parent form to suit particular conditions, and we may perhaps look for the latter to some Central Asian species like M. wagneri. Some of the white-bellied forms Avhich are found in a wild state in Western Europe and in other countries Avhere Mus musculus typicus occurs in bouses may be cases of reversion from the latter, which is no doubt almost certainly the origin of such races as are found on islands, such as the Salvage Islands, where the Mice must have been accidentally introduced. But it by no means follows that this is the case with Mus spicilegus, the size and proportions of which are so much finer than in true Mus musculus and the tail shorter. Mus spicilegus, indeed, might even be regarded as a wild parent form of Mus musculus; hence it is not with it, but the forms which are certainly reversions from true Mus musculus, that we must associate Mus muralis of St. Kilda, and it is interesting to note that the similarly derived Mice of the Salvage Islands resemble those of St. Kilda very closely in their robust form. That a Avild race of Mus musculus can be rapidly evolved from Common House-Mice when living in a wild state has been recently shown by m y friend 1 Mr. H. Lyster Jameson, who has clearly made out his case for the formation of an incipient species of Mouse on the North Bull, Dublin Bay, Ireland, a tract of sandhills about three miles in length and almost completely isolated from the mainland. It is known that this sand-bank has not been in existence for more than about 100 years, so that the coloration described by Mr. Jameson must have been evolved in at most a period of that length. Mr. Jameson lays great stress on the value of the change to these mice as a protective feature, and so he has not, I think, given sufficient emphasis to the fact that Ave have here a clear instance of the development of an incipient subspecies of Mouse with an exact period laid down in which the change occurred, and we may fairly, I think, use Mr. Jameson's results in dealing with other species or subspecies of Mice. If we are to judge from the analogy of Mr. Jameson's mice, we must conclude that the Mice of St. Kilda have inhabited that island for a considerable time. Not only are they more distinct in colour than any other local form of Mus musculus Avith which I am acquainted (and I have been through the whole of the specimens in the British Museum Collection), but their line of development seems to have become fixed, and is no longer, as in the case of Mr. Jameson's mice, in a state of uncertain evolution. On the North Bull sand-hills, indeed, Mr. Jameson found not only mice which had progressed for a considerable distance along the path of their new development, but also mice which showed every kind 1 Journ. Linn. Soc, Zool. vol. xxvi. pp. 465-473: " On a probable Case of Protective Coloration in the House-Mouse (Mus musculus, Linn.)." |