THE GROWTH OF GROUPS 41 



local groups of rats have slight peculiarities of skull 

 formation, such as are seen especially in the nasal 

 bones, the form of the tympanic bones or the length of 

 the series of molar teeth : but even as few as one 

 hundred skulls of rats caught in different parts of a 

 large town will usually comprise individuals showing 

 similar peculiarities* 



The species is most variable in colour. Three of the 

 principal types are shown in the frontispiece, which was 

 executed by an Indian artist from animals captured in 

 Calcutta. That on the left of the plate, which will be 

 referred to as the whole-coloured brown type, is by 

 far the commonest kind in India. It is, however, by no 

 means a pure type. In some individuals red predomi- 

 nates. Others are yellowish. Some are darker than 

 others. 



The white-bellied type, shown in the middle of the 

 plate, is comparatively common ; its distribution in 

 India will be described later on. The black type, seen 

 on the right, occurs but rarely in the interior of India, 

 although it is not rare in seaports and is common on 

 ships. 



According to report the black type was common in 

 England until displaced by the short-tailed Mus norve- 

 gicus, but it is difficult to obtain satisfactory information 

 on the subject. At all events the species which is com- 

 monly referred to as the black rat seems to be uncommon 

 in England at the present day. Of two specimens in the 

 Indian Museum, which were sent from the British Museum 

 for comparison, one was black, the other sombre brown, 

 somewhat darker than the common Indian type, other- 

 wise these English specimens were indistinguishable 



