These two animals not only differ in the colour of the fur, one being of
a grizzled brownish colour, and the other black, but there is a
considerable difference in the texture of the fur. In the black
specimen, the fur is very soft and glossy, and the long hairs, which are
abundant, are very slender. In the brown specimen, the fur is of a
harsher nature, the long hairs are not so abundant, but longer, and less
slender. On the other hand, they agree in size, dentition, the length of
the head, tarsus, and ears, and differ but in a trifling degree (about
three lines,) in the length of the tail.
Upon comparing the Ascension Island specimens with _M. Rattus_, I find
that, although in size they are about one-third less, yet the teeth
precisely agree, not only in form, but in size. The relative proportions
of the head, ears, and tarsi, also agree. Besides the general colouring
of the fur, they both differ in having the hairs of the feet uniformly
purple-black, those in _Mus Rattus_ being much paler, and even whitish,
on the toes. In the character of the fur, there is much difference. The
long silky hairs, which are so conspicuous in _Mus Rattus_, are
replaced, in the black specimen, by hairs which are scarcely to be
distinguished from the ordinary fur; and in the other specimen, although
rather longer and more distinct, they are short, compared with those of
the black rat.
“The specimen which has a black, and glossy fur, frequents the short
coarse grass near the summit of the island, where the common mouse
likewise occurs. It is often seen running about by day, and was found in
numbers, when the island was first colonized by the English, a few years
since. The other, and browner coloured variety, lives in the out-houses
near the sea-beach, and feeds chiefly on the offal of the turtles,
slaughtered for the daily food of the inhabitants. If the settlement
were destroyed, I feel no doubt that this latter variety would be
compelled to migrate from the coast. Did it originally descend from the
summit? and, in the case just supposed, would it retreat there? and, if
so, would its black colour return? It must, however, be observed, that
the two localities are separated from each other by a space, some miles
in width, of bare lava and ashes. Does the summit of Ascension, an
island so immensely remote from any continent, and the summit itself
surrounded by a broad fringe of desert volcanic soil, possess a small
quadruped, peculiar to itself? Or, more probably, has this new species
been brought, by some ship, from some unknown quarter of the world? Or,
I am again tempted to ask, as I did in the case of the Galapagos rat,
has the common English species been changed, by its new habitation, into
a strongly marked variety?”—D.
Mr. Darwin seems to have foreseen the difficult problem which these two
rats have furnished, and although I have spent much time in studying the
Muridæ, I must confess I have been exceedingly puzzled by the animals in
question. It appears as if the brown, and black rats, (_M. decumanus_,
and _M. Rattus_,) and likewise the common mouse, (_M. Musculus_,)[16]
all of which follow man in his peregrinations, and which, to a certain
degree, are dependent upon man, and may therefore be termed
semi-domestic animals; like _really_ domestic animals, are subject to a
greater degree of variation than those species which hold themselves
aloof from him.
Upon the whole then I have determined to describe the two Ascension
Island specimens as one species, and as varieties of the _Mus Rattus_,
but with a mark of doubt, since I do not possess sufficient materials
for a rigorous examination, having, in fact, but one skin of each
variety, and neither skull nor skeleton. I have also applied the name of
_insularis_, to designate this variety or species, whichever it may be,
for, supposing it be not a distinct species, it is so marked a variety,
that a name for it is desirable.