THE GROWTH OF GROUPS 105 



the Mutation Theory as defined by its author (page 177) 

 is valuable to him. 



The local group found near Atari shows that pecu- 

 liarities of cranial structure may occur in all the members 

 of a family group, as do the peculiarities of coat colour, 

 previously described. Our attention is more often drawn 

 to coat colour than to cranial or other structural pecu- 

 liarities, since the former are more conspicuous than the 

 latter. 



Further, the Atari group affords a good example of 

 what often comes to be known as a rare species, and there- 

 fore proves the unwisdom of commemorating such by 

 names. The group is represented by four adults and a 

 number of young ones taken from a particular field. All 

 have the same cranial type in the narrowest sense, a 

 type that is recognizable from other types known in 

 the district. But there is no evidence that the members 

 of the group are isolated from their neighbours by in- 

 fertility. They may be thinned by disease and weakened 

 by the admission of other types, or the whole group may 

 be drowned in a year when the monsoon is heavier than 

 usual. The rise and fall of such a group may be accom- 

 plished within ten generations, that is within about three 

 years. Why, then, should such a group receive a name ? 

 A single individual showing a peculiar character or two 

 is often considered a sufficient excuse for setting up a new 

 name, and four like individuals taken from the same 

 district would generally be regarded as a sufficient reason 

 for so doing. 



The cranial characters just described though apparently 

 small are important from a taxonomic point of view. 

 For example, the several points of difference between 



