420 
south of Eussia, about the year 1727, having come from the 
neighbouring part of Asia by Astrachan. It appears to have been 
introduced into England about tlie year 1727, and into France a 
short time after. Thence it has sjoread to almost every part of the 
world by means of shipping, and the effect of its introduction on 
the black Eat, which it found already established, has been the 
gradual extermination of the latter; not, perhaps, by direct 
antagonism, but simply from the fact of there not being room 
for two species so similar in habits and food, both of which 
are exceedingly prolific. The result has been that the weaker of 
the two has gone down in the struggle for existence, the black Eat, 
itself not truly indigenous, giving place to its more hardy relative 
Mus decwnanus. The history of the black Eat in this country is 
thus very instructive, as illustrating the faunal changes which we 
know to have taken place. 
bleither the Eabbit, black Eat, brown Eat, common Mouse, 
and probably the Hedgehog and common Field Vole f A. agrestis), 
arc known to occur fossil, they must therefore be regarded as recent 
immigrants. IMr. HcAvton has shown that the remains of the small 
V ole, formerly believed to have belonged to A . agrestis, are really 
those of the continental form A. arvalis. It seems likely, therefore, 
that the latter race has been exterminated in this country by the 
present species f A. agrestis), precisely as the black Eat has been 
displaced by its more robust brown cousin in later times. 
For the sake of comparison I give the following measurements 
of this species, also of M. rattus and M. cJecumanus. 
e 
s 
'3 
S3 
M. vattus. 
.1/. alexandrinus. 
1 
<4 
1 
i: 
< 
No. 1 
(see below). 
No. 3 
(see below). 
In. Lines 
In. Lines 
In. Lines 
In. Lines 
In. Lines 
In. Lines 
Length of head and body 
9 — 
7 — 
6 — 
7 6 
8 7 
8 7 
„ ears 
— 8 
— 10 
— 11 
— 9 
— 8 
— 8 
„ tail 
7 5 
7 6 
8 — 
5 6 
G 1 
7 1 
Note.— Since the above was read I have received, from the same locality 
at different times, through the kindness of Messrs. Squirrcll & Utting, three 
other Rats, exhibiting more or less the characters of kotli M. alexandrinus 
and M. decumanm. The first, received on March 25tli, 1882, had the 
general appearance of M. alexandrimis as to colour, texture of fur, and 
elongated muzzle, but both in size and tail much more nearly resembled 
