ON TWO RECENTLY DESCRIBED MICE FROM ST. KILDA 139 



there a bactrianus-Yiko. Mouse is found, so that M. bactrianus is 

 perhaps as widely distributed in deserts as is Mus musculus typicus 

 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 which are found in a wild state 

 in Western Europe, and in other countries where Mus musculus 

 typicus occurs in houses, 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 Mus musculus 

 must have been accidentally introduced. But it by no means 

 follows that this is the case with Mus spicilegus, the size and pro- 

 portions 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, and 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 wild race of Mus musculus can be rapidly evolved from 

 Common House Mice when living in a wild state has been recently 

 shown by my friend x Mr. H. Lyster Jameson. This gentleman 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 sand- 

 hills about three miles in length and almost completely isolated from 

 the mainland. 



This sandbank is known not to have been in existence longer 

 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 we have here a clear 

 instance of the rapid 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 been in that island 

 for a considerable time. Not only are they more distinct in colour 

 than any other local form of Mus musculus with 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, as in the case of Mr. Jameson's Mice, no 



1 "Journ. Linn. Soc. Zool.," vol. xxvi., 'On a Probable Case of Protective 

 Coloration in the House Mouse (Mus miisciilics, Linn.),' pp. 465-473. 



