106 THE GROWTH OF GROUPS 



skulls A and B are as a whole more conspicuous than the 

 cranial differences which separate the well-known M. 

 rattus and M. norvegicus. These two common species, 

 although living side by side in many parts of the world, 

 preserve their identity because they are mutually infertile. 

 Similarly, types A or B would no doubt become per- 

 manently established, if they happened to be infertile with 

 all other types. 



