72 MR. SALTER ON THE CRANIUM OF THE SNAKE-RAT. 
There are sorts of rats which will not come within the cate- 
gory of those recognized, or as their intermediate crosses. We 
have in this country a black rat with a white chest: in the 
British Museum are two stuffed rats, chestnut-coloured, with 
white breasts, which were captured in Cambridgeshire. The dis- 
tinguished Irish naturalist, Mr. William Thompson, has described 
a black rat with a white chest as a new species, under the name 
of Mus Hibernicus. 
On the occasion of the reading of my paper on the cranium of 
the Snake-Rat, it was suggested by Mr. Lubbock that it might bea 
“variety”? of one of our other rats. Subsequently, in a discussion 
in the ‘ Field’ newspaper *, by which a great deal of interesting 
information respecting rats was brought out, Mr. Newman put 
forward the idea that these cosmopolitan rodents are, in their 
differences, not so many species, but mere “ geographical races ;” 
and I am much inclined to believe that this is the truth of the 
matter. Certainly if interbreeding and a resultant fertile offspring 
determine the specific identity of varying individuals, there is an 
end of the question. The different rats do interbreed and their 
progeny are fruitful for any length of time and any number of 
generations. 
Rats hold a curious intermediate position between wild and do- 
mestic animals. They are not absolutely either, and they are 
both. They are wild as they are their own masters and roam at 
will: they approach a domestic condition inasmuch as they are 
nearly always associated with man and are indirectly dependent 
on him for their food. Rats are cosmopolitan—they inhabit almost 
if not quite every region where the human race dwells. In viola- 
tion, or at least not in keeping with their dentition and organs of 
primary assimilation, rats are omnivorous: they can live entirely 
ou animal food—they even resort to the predaceous habits of car- 
nivora; or they may have the barest vegetable diet for their sole 
sustenance. Such constitutional capabilities and such adaptability 
of habit afford wonderful conditions for the development of races. 
Mus Alexandrinus appears to be spreading all over the world ; 
its extreme agility and the ready way in which it accommodates 
itself to ship-board naturally tend to such a result. . 
Besides the Eastern localities where it was first found, according 
to Blasius it was observed by Savi in Italy in 1825, and named 
by him Mus tectorum; it was found by Pictet near Geneva in 
1841, and described by him under the title of Mus leucogaster ; 
* For September 8th and 15th, 1860. 
