5 o THE GROWTH OF GROUPS 



single individual and the small family group it may 

 establish, nor can we draw a line between a large family 

 group and a small race. 



(4) Such groups are common ; they are mostly unknown 

 to us as such, because they can only be appreciated by 

 examining a large proportion of the animals belonging 

 to one species spread over a large area. Whenever, as 

 frequently happens, members of such groups come into 

 the collectors' hands they go to swell the number of the 

 so-called rare species, such as are found once, but never 

 again. 



(5) The peculiarity which marks off a group from 

 the normal multitude is not always special to that group. 

 The same peculiarity may appear in various groups 

 which are found in separate areas : this is true, however, 

 only in the case of groups whose peculiarity appears as a 

 single character unit. Those groups whose peculiarity 

 is made up of several uncorrelated characters arise on one 

 occasion only. This statement is assumptive, the facts 

 relating to it will be found in Chapter V. 



(6) Family groups arise from individuals by in- 

 breeding. 



The representation of Indian house rats is then, a 

 multitude of normal animals spread over the continent, 

 amongst which such family groups are disseminated. 

 Since at one time we may find a group containing perhaps 

 five members, a second group containing ten, a third 

 with fifty or more, we shall understand that groups 

 containing a large number of individuals must have 

 passed through stages exemplified by the smaller groups, 

 in other words they must have grown from unity. 



