504 MURID/Z—APODEMUS 
Mag. Nat. Hist., January 1907, 121), however, show a similar 
combination of characters, perhaps indicating relationship with 
Apodemus, but for Eurasian mammals the genus is a useful 
and practical institution. 
Although not apparently capable of wide variation from the 
generic type, these mice show, nevertheless, great plasticity 
before the moulding influences of their environment. Consider- 
ing the wide range of the genus, the number of its species is 
relatively few; but most of these species are differentiated 
into numerous sub-species, which are to be regarded as purely 
local developments or adaptations. The characters which 
betray this plasticity are principally those of the pelage, 
coloration, and the proportions of the peripheral organs; the 
size and form of the skull, and, to a lesser degree, of the teeth 
are also subject to local modification. 
In the A. sylvaticus group, to which all the British forms belong, the 
fur is typically soft, the darker hairs of the back are diffusely distributed 
and mingled with lighter ones, and the general colour of the upper 
parts is russet or tawny in more or less sharp contrast with the light 
belly. The ears, tail, and limbs, particularly the feet, are relatively 
long. There are six mamme, the anterior pectoral pair being absent. 
The skull is always without supra-orbital beads. The cheek-teeth have 
the outer and inner rows of tubercles relatively well developed ; cusp 1 
is present in both mz! and 7”; cusp 7 is well developed in m*; the outer 
“ accessories” of mz, are rather large and include vestiges of cusps 6, 7, 
and z. The British representatives of this group are :— 
A. sylvaticus, Britain, Ireland, and many of the smaller islands 
(including Skye and Bute). 
A. hebridensis, Hebrides (exclusive of Skye and Bute). 
A. hirtensis, St Kilda. 
A. fridariensis, Fair Isle and Mid Yell, Shetlands. 
A. flavicollis, South Britain. 
Of these forms A. sylvaticus shows a tendency to split up into 
several local races or sub-species within the British area. A. hebridenszs, 
hirtensis, and fridariensis may be regarded as local developments from 
the primitive sy/vaticus stock of the region, which appear to have 
resulted from the segregation of this stock upon small islands; the 
characters which distinguish them from the parent stock are slight, and 
all three might well be regarded as mere sub-species of sy/vaticus were 
it not for the fact that hedridenszs and fridartensis have in turn under- 
gone a further differentiation into a number of quite recognisable and 
