394 MR. G. E. H. BARRETT-HAMILTON ON [Apr. 3, 



"Within its own limits, as has been stated above, the Long-tailed 

 Field-mouse is subject to considerable variation, but the variations 

 are usually more subtle than are those of some other mammals. 

 Thus, putting aside the influence of age and season, I find a good 

 deal of individual variation in the amount and intensity of 

 the rufous coloration of the upper surface and of the purity of the 

 underside, in the presence or absence of a spot or band upon the 

 breast, and in the length of the tail. Size too, as in other 

 mammals, contributes its share to individual differences ; and my 

 tables at the end of this paper show that, after allowing for 

 individual differences amongst the various persons who have 

 measured the specimens which I have had under examination, 

 there yet remains a considerable variation above and below the mean 

 for adults. I think it highly probable that such variation would 

 have proved distinctly less had the series from which my averages 

 have been compiled been collected in a locality smaller than that 

 of Great Britain as a whole. It is, in fact, possible that there is 

 an increase in size of Mus sylvaticus even in Great Britain from 

 south to north, or, perhaps, from south-west to north-east. 



As regards geographical or local variations, Mus sylvaticus is, on 

 the whole, remarkably constant to a single well-marked type. 

 Throughout the Palsearctic Eegion it is distinguishable at a glance 

 from every other mouse with which it might possibly be confounded 

 by the possession of a combination of characters, amongst which 

 its size, the pattern of its teeth, its long foot, large ears, and pure 

 white belly, separated from the more or less rufous dorsal regions 

 by a clearly marked line of demarcation, are predominant. Thus 

 its local forms, though distinguishable, are not nearly so readily 

 appreciable as, say, those of iSciurus vulgaris ; and Mus sylvaticus 

 chevrieri Milne-Edwards, of Tibet, is at tirst sight surprisingly like 

 Mus s. arianus Blauford, of Persia and Afghanistan, or Mus s. 

 intermedins Bellamy, of England, considering the enormous tracts 

 of country by which these forms are separated. Yet there is 

 variation and that distinctly geographical, consisting for the most 

 part (as in the case of the individual variations) in differences 

 of general size, tint, and thickness of the coat of the upperside, 

 intensity of the white colour of the belly, presence or absence of a 

 breast-band, length of tail, and size of ear. 



Thus, in Europe, it is easy to divide Mus sylvaticus primarily 

 into two very distinct — a larger long-tailed and a smaller short- 

 tailed — subspecies. The larger reaches its maximum both of size 

 and coloration in Eastern Europe (M. s. princeps), but towards 

 the West becomes both smaller and duller. Colonies, however, 

 persist right into Great Britain, where they are distinguishable by 

 the presence of a remarkable breast-baud of the same colour as 

 the upper surface of the body (M. s. urintmi). In Denmark and 

 Scandinavia thei'e is a reduction in size (M. s. typicus), but, although 

 the red of the upper surface remains fairly intense, the underside 

 is much dulled; and this process has been carried to its fullest 

 extent in the representatives of the species on the islands of Lewis 



