Some Forms 0/ Mus musculiis, Linn. 161 



process of being, evolved. Indeed, a reference to Trouessart's 

 great catalogue (2), and other sources of information, makes 

 us aware that about a score of these geographical forms have 

 already received names, and to these there might very well 

 be added others which now hold full specific rank. We 

 may be certain, too, that a number of races remain to be 

 described in the near future, as the result of that specialisa- 

 tion which is such a marked feature of modern scientific 

 research. 



The study of these races is, however, a most complicated 

 and difficult one, and has demonstrated in a remarkable 

 manner the high degree of plasticity possessed by this and 

 other small rodents. 



The typical House Mouse {Mils fnuscidus) was until quite 

 recently the only recognised member of its group known to 

 occur in the British Isles, and other parts of northern and 

 western Europe. But in 1895 Mr H. Lyster Jameson (3) 

 discovered a colony of mice inhabiting the sand-hills on the 

 North Bull, an isolated tract in Dublin Bay, which for about 

 a century has been cut off from the mainland by a tidal 

 channel. Here he observed certain mice whose unusually 

 pale tint harmonised strikingly in colour with that of their 

 surroundings of sand. A number — thirty-six — of specimens 

 were procured, and a very large proportion of these were 

 found to be pale examples of Mies musculus. They varied 

 somewhat in shade of fur among themselves, and showed 

 every gradation from the type to light-coloured examples 

 with pale dorsal surface, while in the majority of cases the 

 ventral fur was pale buff or yellowish white. These interest- 

 ing Irish mice show in their whitish under surfaces a 

 tendency to resemble — perhaps it is a tendency to rever- 

 sion — certain strictly wild forms found in Portugal by Mr 

 Old field Thomas (4), by Mr Barrett-Hamilton (5) in Morocco, 

 and elsewhere by others. 



These protectively-coloured mice may fairly be regarded 

 as incipient subspecies; and their discovery must be con- 

 sidered as one of special interest, since it would seem to 

 indicate that less than a century has elapsed since the 

 variation described has been evolved. 



