THE GROWTH OF GROUPS 77 



fact is plain that the white-bellied kind are scattered 

 among the others ; they have, so to speak, no head 

 quarters. 



Let us now inquire into the experience of others who 

 have examined large numbers of Indian rats. Several 

 independent observers have recently described what they 

 saw among these animals, while examining them in the 

 routine of plague investigation. Their opinions are of 

 particular value in this discussion, for they are unbiassed. 

 The question of the origin of races did not concern them. 

 It was sufficient for their purpose to distinguish the true 

 physiological species ; but, fortunately, the most obvious 

 peculiarities of the animals within these groups were also 

 mentioned. The experience of the Plague Commissioners 

 in Bombay, which is published in the Journal of Hygiene 

 for 1907, was based on the examination of several hundreds 

 of animals brought from all parts of the city daily during 

 more than a year. The two principal species are the 

 well-known rattus and decumanus, which were obtained 

 in the proportion of about seven of the former to three of 

 the latter. In regard to the former we read in the above- 

 mentioned journal, " Mus rattus is frequently called the 

 black rat, but the commonest type in Bombay is a brown 

 variety. The black variety is somewhat rarely seen in 

 rats taken from the city, though it occurs more commonly 

 amongst rats trapped from ships in Bombay harbour. 

 The belly is invariably of a lighter colour than the back. 

 Rarely the belly is unusually light coloured ; rats with 

 this peculiarity are considered by some workers as a 

 variety (Mus alexandrinus] but no distinction was made 

 by us on this account." Judging from the words, " rarely 

 the belly is unusually light coloured, etc.," it is evident 



