THE GROWTH OF GROUPS 109 



concerted efforts of many persons, aided by considerable 

 sums of money. However, the fifty obtained are a fair 

 sample of the community. All of them belong, broadly 

 speaking, to the species Mus rattus ; that is to say, their 

 skulls are indistinguishable from those of the common 

 lowland rats, but they possess certain superficial features 

 of their own. The tail is, on the average, scarcely longer 

 than the combined length of the head and body, the 

 ventral surface is white, though in a few the roots of the 

 ventral hairs are light grey ; the fur generally is longer 

 and more plentiful than in the lowland rats. 



Having enumerated the points of distinction between 

 the Naini rats and their lowland relatives, we must now 

 consider how the former differ amongst themselves. They 

 fall at once into two groups, the members of which are 

 recognizable from one another at a glance. The tails of 

 some are pigmented in their whole length and to an equal 

 depth on both upper and lower surfaces, just as are the 

 tails of lowland rats. Others have lost part of their 

 caudal pigment. In all, the pigment has quite dis- 

 appeared from the lower half of the circumference of the 

 tail, the junction between the black upper surface and 

 the white lower surface being sharply marked in a line 

 along either side. There are no differential features, 

 other than the caudal pigmentation, by which the separa- 

 tion into groups could be effected. If the tails of the fifty 

 animals were to be cut off and the bodies alone presented 

 to an observer he would not be able to sort them into 

 two groups. But the amputated tails could be sorted 

 without difficulty. 



We now come to the most interesting feature of 

 the case. These two kinds, the black-tailed and the 



