THE GROWTH OF GROUPS 95 



trap. All, being in the same stage of adolescence, were 

 doubtless of the same litter. Each was marked with a 

 white line on the breast. This shows that all the young 

 produced at a birth may be marked in this manner. Such 

 a litter, if placed upon an island, would probably stock 

 it with their descendants, all possessing the mark. We 

 know, however, that it is scarcely possible for such a 

 group to establish itself in India, since none was found, 

 whereas single sports bearing the mark are quite common. 

 At least one per thousand of Indian rats are marked in 

 this manner, that is to say about a million of them are 

 born yearly in various places, but yet they have not 

 become established in any place so far as we know. 



In the Proceedings of the Zoological Society for 3837 

 Thompson described the species M. hibernicus. The 

 chief characters chosen by him as the marks of this species 

 are as follows : short tail and ears, black coat colour with 

 a white breast mark. There is a drawing of an animal 

 of this species in the Zoologist, September, 1889, which 

 shows plainly that the breast mark is like that noticed 

 among the sports in India. 



Messrs. Clarke and Hamilton have since shown that 

 M. hibernicus is a melanotic variety of the common 

 M. norvegicus. They examined fifty-six specimens from 

 various parts of Ireland and found that the breast mark 

 was present in thirteen only of them, hence it is not a 

 constant specific mark. There is evidently no correla- 

 tion between melanism and the breast mark. The first 

 alone or both may occur in addition to the other 

 characters of M. norvegicus in Ireland. 



We will now pass on to another case. 



During my visit to Rangoon I heard of some peculiar 



