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Show 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 m y 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, 3Ius sylvaticus is, on the whole, remarkably constant to a single well-marked type. Throughout the Palaearctic 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 Sciurus vulgaris ; and Mus sylvaticus chevrieri Milne-Edwards, of Tibet, is at first sight surprisingly like Mus s. arianus Blanford, of Persia and Afghanistan, or 3Ius 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 3Ius sylvaticus primarily into two very distinct-a larger long-tailed aiid a smaller short-tailed- subspecies. The larger reaches its maximum both of size and coloration in Eastern Europe (31. 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-band of the same colour as the upper surface of the body (31. s. wintoni). In Denmark and Scandinavia there is a reduction in size (31. s. typicus), but, although the red of the upper surface remains fairly intense, the 'underside is much dulled; and this process has been carried 1o its fullest extent in the representatives of the species on the islands of Lewis |
