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 J\fus Hibernicus. 



On the occasion of the reading of my paper on the cranium of 

 the Snake-Eat, it was suggested by Mr. Lubbock that it might be a 

 " variety " of one of our other rats. Subsequently, in a discussion 

 in the 1 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. 



Iiats 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 

 on animal food — they even resort to the predaceous habits of car- 

 oivora ; 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. 



Mas Alexandrinus appears to be spreading all over the world ; 

 its extreme ability and tin; ready way in which it accommodates 

 itself to ship-board naturally tend to such a result. 



Besidei the Eastern localities where it was first found, according 

 to Klasius it was observed by Savi in Italy in 1825, and named 

 by him Mas tcclorum\ it was found by Pictet near Geneva in 

 1811, and described by him under the title of M us leucogasler ; 



* For September Bth and L6th, L860. 



