1. MUS DECUMANUS.
Mus decumanus, _Auctorum_.
In the extensive collection of Rodent animals brought home by Mr.
Darwin, I find several specimens of the above named species, that is to
say, animals which resemble the European specimens of _Mus Decumanus_ in
all those characters which are the least liable to variation in
individuals of the same species, such as the proportions which the
various parts of the animal bear to each other: they differ, however,
somewhat in colouring.
Buenos Ayres, Maldonado, Valparaiso, East Falkland Island, and Keeling
Island, are each, it appears, infested with the common European rat. I
have now before me two specimens from East Falkland Island, and one
specimen from each of the other localities, and among these I find none
equal in size to the largest European specimens: as regards the
colouring, the Buenos Ayres specimen differs only from the English
specimens of _Mus Decumanus_, in having the upper parts of a richer and
deeper hue, owing to the tips of the shorter hairs being of a deep
yellow instead of pale yellow, and in having a rusty tint over the
haunches.
Mr. Darwin found this variety “common about houses in the country around
Buenos Ayres.”
In the Maldonado variety, the shorter hairs of the upper parts of the
body are of a rusty yellow colour at the apex, in other respects it
resembles the British variety. The rusty yellow colour of the tips of
the hairs produces a general reddish hue, which is the more conspicuous,
when the animal is placed near an English specimen. “Was caught in a
house, at Maldonado. I saw a specimen of the common gray English, or
Norway rat, lying dead in the streets, and it certainly had a very
different appearance from these red rats. The latter, I saw crawling
about the hedges in the interior provinces at Santa Fé, and likewise in
the forest of the island of Chiloe. This latter fact, however, is a
strong argument against its being aboriginal, since I did not find even
one undoubted American species, out of the many which I collected,
inhabiting both sides of the Cordillera.”—D.
The specimen from Valparaiso very closely resembles that from Maldonado;
it is, perhaps, a little less red. “Common about the houses in the town
of Valparaiso.”
The two specimens from East Falkland are of a brighter hue, and have
less gray in their colouring, than in the European variety of the common
rat. “One of them was caught in a Bay, which is sometimes frequented by
shipping, but which is distant thirty or forty miles from any
habitation. These rats have spread, not only over the whole of East and
West Falkland, but even on some of the outlying islets. When the cold,
wet, and gloomy nature of the climate is considered, it is surprising
that these animals should be able to find food to live on.”—D.
The general hue of the Keeling Island specimen, is deep brown, the
longer hairs of the upper parts of the body being, as usual, black; but
the shorter hairs, instead of having the pale yellow tint which we
observe in the European, (or, rather, British) specimens of _Mus
Decumanus_, are of a deep, rusty yellow. The most remarkable difference,
however, consists in the colouring of the under parts being of a
yellowish tint, and, towards the root of the tail, of a very distinct
buff yellow: the feet are brownish.
“This rat is exceedingly numerous on some of the low coral islets
forming the margin of the Lagoon of Keeling Island, in the Indian Ocean.
The climate is dry and hot. The rats are known to have come in a vessel
from the Mauritius, which was wrecked on one of the islets, which is now
called Rat Island. They appeared stunted in their growth, and many of
them were mangy. They are supposed to live chiefly on cocoa-nuts, and
any animal matter the sea may chance to throw up. They have not any
fresh water; but the milk of the cocoa-nut would supply its place.”—D.