THE GROWTH OF GROUPS HI 



of one or other class, a careful observer would not say that 

 all possessed the same number and the same kinds of 

 characters. They differ from one another in small but 

 appreciable points, in the length and tint of the fur, in 

 total size or in the proportional size of any part such as 

 the ears or the feet. We may feel sure that these small 

 points are hereditable, since they may be characteristic 

 of a few animals caught together. It seems that a race 

 is never pure in a strict sense, but that the term pure race 

 should only be used in a conventional manner, to indicate 

 a group of animals all possessing some one or more obvious 

 characters which are special to them. When the experi- 

 mental biologist uses the term pure race he means, perhaps, 

 that the race is pure as regards those characters which he 

 is studying, at all events this is the sense in which the 

 term is used here. I purposely ignore the small, less 

 appreciable characters and pay attention for the time 

 only to the pigmentation of the tail for that is the obvious 

 character which is the mark of the classes under discussion. 

 The black-tailed rats of Nainital may, therefore, be con- 

 sidered as a pure race in regard to their caudal pigmen- 

 tation. The white-tailed race, however, cannot be 

 regarded as pure in the same sense, for there are among 

 them two distinct varieties. 



The inquiry at Nainital had a strong influence on my 

 opinions, I will therefore relate the order in which the 

 facts came before me, so that the reader may ask himself 

 how he would have reconciled them with certain of the 

 accepted opinions about species, their origin and distri- 

 bution. At first I examined about twenty rats which 

 had been caught in various houses in the bazaar, i.e. the 

 more central or compact part of the town. I regarded 



