72 THE GROWTH OF GROUPS 



1. A race of white-bellied rats became established in 

 some place by selection from among rats possessing all 

 degrees of ventral coloration. This selection came 

 about because, under the conditions appertaining to that 

 place, white-bellied rats had a better chance of life than 

 others, better even than those with very light grey or 

 almost white under-parts. Under these peculiar con- 

 ditions, the difference between white and very nearly 

 white under-parts must have been of life-saving value to 

 the rats. From out of a race established in this manner 

 at least one pair wandered, or was conveyed, into the 

 heart of Poona city, where it founded a colony. 



2. A pair of whole-coloured rats living in Poona 

 produced one or more white-bellied offspring, which were 

 the progenitors of the colony. 



One feels convinced that the group arose in the 

 manner described in paragraph 2. An expression of 

 conviction is not a proof, but proof in any case seems to be 

 impossible. Even if the birth of every member of the 

 colony and of their progenitors had been witnessed and 

 every step in their descent traced back to the whole- 

 coloured ancestors, the experiment could not be repeated 

 for a proof. The purpose of exposition is to convince 

 other people, this can only be accomplished fairly by 

 setting forth for the reader's approval the various steps 

 which led to the conclusion. 



It is highly improbable that the progenitors of the 

 colony should have arrived in the city from without, 

 even if white-bellied rats were common in the country 

 around Poona. It is clear from the evidence that the 

 houses of the town are infested with whole-coloured rats. 

 It is difficult to imagine the supposed intruders passing 



