62 THE GROWTH OF GROUPS 



means or other might not have occurred. But it is 

 most improbable that the progenitors of the colony 

 arrived from without. They could not have arrived on 

 a ship from a foreign port. Rats of the species G. benga- 

 lensis have long been known to be common in the fields 

 throughout India. In 1907 it was pointed out that these 

 animals might also be found in houses, and that they were 

 common in some of the larger towns. But the mole-rat 

 is a burrowing animal, and has never been found on a 

 ship. The presence of a group of melanic animals of the 

 species rattus or norvegicus in Rangoon might be accounted 

 for by supposing that they arose from ship-borne emigrants, 

 and in such a case if we wished to inquire as to the where- 

 abouts of the main body from which the emigrants were 

 derived, we should have any part of the world to choose 

 from. But since in this case we are able to exclude the 

 possibility of marine transit the problem of the origin of 

 the group becomes narrower. The progenitors of the 

 group were either born of the common brown mole-rats in 

 Rangoon or they arrived there from some inland part of 

 Burmah. But it seems impossible that there should be, 

 unknown to us, an established race of black mole-rats in 

 Burmah, for since plague broke out the attention of 

 many persons has been directed towards rodents, and the 

 particular animals referred to are of a most noticeable 

 kind. 



Now let us consider the probability of the group 

 having been derived suddenly from brown parents in 

 Rangoon. There is a great difficulty in all such cases. 

 We know that races or groups arise in some way but we 

 cannot know of them until they have arisen, and when they 

 have arisen it is already too late to observe how they 



