THE GROWTH OF GROUPS 113 



Since in all probability the couple were of the same litter, 

 it was evident that they could not be regarded as belonging 

 to different species, although they exhibited a specific 

 difference. 



Let us now consider some of the established species 

 which are known to systematists as having bicoloured 

 tails and semi-bicoloured tails. 



A completely bicoloured tail is a special characteristic 

 of Mus vicerex. A rat of this species is a short-tailed, 

 thick-furred, white-bellied animal, with a skull of the 

 rattus type. The species was first described from eleven 

 specimens caught in Simla in 1903, it was afterwards 

 shown that the common rats of Kashmir resembled them 

 so closely as to be considered of the same species. It is 

 uncertain whether the kind persists at Simla at the 

 present day, since of fifty-eight rats caught there in 1908, 

 not one was of this type ; all of them had black tails. 

 There is, however, no doubt about the Kashmir rats. I 

 have seen over thirty of these animals, obtained from 

 two different sources, and in all the tail was completely 

 bicoloured. Mus niveiventer is a species which was first 

 recorded from Khatmandu in 1836, it has also been 

 recorded from Simla. It is a thick-furred, white-bellied 

 animal, with a short bicoloured tail. How it differs from 

 vicerex is not clear. Besides these two species, others 

 characterized by a bicoloured tail have been recorded 

 from the Malay States, usually from highland districts. 



Now let us turn to the other type, in which the 

 pigment of the tail is confined to the dorsal surface, but 

 only reaches halfway along it, so that the terminal portion 

 of the tail is pure white in its whole circumference. Rats 

 with tails of this kind have been recorded on more than 



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