8o THE GROWTH OF GROUPS 



conclusion, for the number of distinct colour types is large, 

 but not indefinitely so. The black type, the reddish-brown 

 the yellowish-brown, the pure white-bellied, the white- 

 bellied with a median brown stripe and others are recog- 

 nizable and may be found again and again in various 

 localities. There is little doubt that by isolating pairs 

 from these several types, we could obtain pure strains 

 from them in many cases, since we often find in nature 

 litters of young rats all of which are of one and the same 

 colour type. In his description of the colour types that 

 may be met with, Blanford is accurate ; but in using the 

 words "sometimes," " generally," "occasionally," he shows 

 uncertainty as to the frequency of occurrence of the various 

 types. Thus he writes, " Below generally white . . . 

 sometimes brown or grey," but we have since learnt from 

 the large number of rats which have been caught in 

 India recently, that the white-bellied type is very much 

 less common than the other. 



Blanford's account, though partly the result of his 

 own experience, is based on the work of Mr. Oldfield 

 Thomas, which was published in the Proceedings of the 

 Zoological Society for 1881, and is to-day the most ade- 

 quate account of Indian rats from the taxonomist's point 

 of view. This account was based on a large number of 

 specimens sent from India. Mr. Thomas refers to the 

 common house rats of India by the name Mus alexandrinus, 

 and in regard to the use of the term writes as follows : 

 " M. alexandrinus would seem to be a more tropical form 

 of M. rattus ; but as it seems always to have much shorter, 

 coarser, and more rufous hair as compared with the black 

 and shining fur of M. rattus, I have provisionally kept 

 them distinct, though I have little doubt that they will 



