134 ANNALS OF SCOTTISH NATURAL HISTORY 



to the island. The great amount of variation from the type of a 

 species which varies so little as Mus sylvaticus, as shown in the one 

 case, and the evolution of a perfectly uniform and distinct type of 

 coloration in one so variable as Mus musculus in the other, are both 

 characters which would seem to have taken no inconsiderable time 

 for their development. So that even if, as is possible, the presence 

 of a Mus muscuIus-Mko. species of Mouse on St. Kilda be due in the 

 beginning to a case of introduction, such an introduction could not 

 have taken place at a very recent period in the history of the 

 island, which is known to have been inhabited for at least several 

 centuries. 



The distribution of Mus sylvaticus is almost coterminous with 

 the limits of the Palsearctic Region, the species only just reaching 

 the confines of the Oriental Region " in Gilgit, where it is common 

 from 5000 to 10,000 feet elevation" (Blanford's "Mammals of 

 India," p. 416). In the former region it is probably as widely 

 spread as any other mammal, as it seems to be almost regardless of 

 the influence of temperature, and is found far up the slopes of the 

 mountains. It is equally at home in all the countries, except 

 probably the great sandy deserts, from the Eastern coast-line of 

 China to the Atlantic. It has reached Morocco, Algeria, and 

 Palestine, and has found its way to most of the islands, such as those 

 of the Mediterranean, the Channel Islands, Great Britain, Ireland, the 

 Scotch Islands, the Shetlands, 1 and even Iceland, where the local 

 form (Mus islandicus, Thien.) is said to be the only indigenous species 

 of mammal. 



Its presence in such isolated, yet widely separated islands, as 

 Iceland and Corsica, seem to mark it as a species which has for 

 long maintained a wide area of distribution, and which had already 

 occupied the greater part of its present range before these and the 

 other islands, where it is now found, were finally separated from the 

 Continent as such, but still formed a part of the continuous Palaearctic 

 land area. And of its antiquity we have sufficient proof, for its bones 

 have been found in numerous caves on the Continent and in the 

 English Forest-bed ; and we have no trace of its ancestry, the Pleis- 

 tocene species Mus orthodon, Hensel, and abbotti, E. T. Newton, being 

 at least as specialised as itself. 



Not only is Mus sylvaticus of exceedingly wide distribution, but 

 throughout the immense area where it is found it remains remarkably 

 constant to a single well-marked type. Throughout the Palsearctic 

 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 



1 A set of four from Dunrossness, for which I am indebted to Mr. Henderson, 

 has recently reached me ; I am unable to separate them from RItis sylvaticus 

 of Western Europe and Great Britain, and the same remark applies to some 

 specimens collected for me by Mr. W. Eagle Clarke on Alderney. 



