ii2 THE GROWTH OF GROUPS 



them as a fair sample of the rodent population which 

 seemed to be as constant in type as that of most lowland 

 towns. They presented the peculiarities already men- 

 tioned, in comparison with lowland rats. They were 

 somewhat grayer in colour with white under-parts, their 

 tails were relatively short and their fur dense. On the 

 first occasion of setting traps in an outlying house, two 

 animals were captured which caused great perplexity. 

 They were captured side by side in a small cupboard, 

 both were adolescent and in the same stage of adolescence. 

 The probability that they were of the same litter was 

 therefore great. The tail of one was bicoloured in its 

 whole length, that is to say the upper surface was black 

 as far as the tip, the lower surface being devoid of pig- 

 ment. In the other, the dorsal pigmentation did not 

 extend so far as the tip of the tail, but came to an end a 

 little beyond the middle of its length. This is the type 

 called semi-bicoloured, Fig. 2 c. 



Except for the colour of their tails they were as alike 

 as two young rats of the same litter usually are. More- 

 over, except by their tails, they could not have been 

 distinguished from many of the adolescent rats which 

 were being caught elsewhere. They were compared with 

 three young black- tailed rats caught in the bazaar. If 

 the tails of all five had been cut off there would have been 

 no obvious means of distinguishing the tailless bodies 

 from one another. 



The interest of the case is this. Both types of caudal 

 pigmentation exhibited by this remarkable pair have been 

 known for a long time as specific characters, which in the 

 first instance were defined by taxonomists and have since 

 been used on several occasions as marks of identification. 



