THE GROWTH OF GROUPS 71 



mentioned before in dealing with the other group, we 

 cannot, from this number, estimate the numerical strength 

 of the colony, although we may feel sure that it contained 

 considerably more than fourteen. 



In the case of the group described in the last chapter 

 it was almost certain that the members of it held the 

 two houses for themselves. If ten rats of a particular 

 kind are captured during a few successive days in certain 

 houses, none of any other kind being caught at the same 

 time, the probability is great that no other kinds were 

 present in those houses. But at Poona the white-bellied 

 rats did not occupy the houses to the exclusion of the 

 common kind, but the fact that there was a group of them 

 in those houses is clear. 



We must now consider how this state of affairs came 

 about. In my opinion there are only two possibilities. 

 The progenitors of the colony were born in the city of 

 normal parents, that is to say as mutants, or they are 

 migrants of another race which arrived there from without. 

 A third view might perhaps be held, although few will 

 consider it tenable, namely, that this small group of white- 

 bellied rats was derived from the common rats in the 

 city of Poona by Natural Selection working upon rats 

 possessing various degrees of ventral coloration, this 

 selective process having occurred in and around the 

 place where the white-bellied rats were discovered in 

 Poona. This view may be disregarded, however, since 

 the more plausible explanation of the facts from the 

 Darwinian standpoint would be that the group was estab- 

 lished by migrants from some other place. 



The alternative explanations are, therefore, as 

 follows : 



