22 THE GROWTH OF GROUPS 



and their fur is slightly darker in tint. They were carefully 

 compared with the rats of Belgaum, a city lying about 

 two hundred miles to the south. The difference in the 

 mean of the combined length of head and body was about 

 half an inch, being less in the case of the rats at Poona. 

 The difference in colour cannot be expressed in a satis- 

 factory manner, but it is distinctly appreciable. If a 

 large number of dead rats captured in both places were 

 to be mingled together, it would be possible to sort them 

 out with some confidence. Although a few mistakes 

 might be made in sorting, yet it is undeniable that the 

 general appearance of the group is slightly different in 

 the two places. It is the rats of Poona which have 

 departed, although to a very small extent, from that 

 which may be called the Indian type of M. rattus, those 

 of Belgaum may be considered as typical. If we could 

 determine the mean length of the rats in several towns 

 in a very accurate manner, we should find, no doubt, that 

 in each case the results would be different from one 

 another, some being further removed from the theo- 

 retical mean than others. It happens that the rats of 

 Poona depart from the mean to such an extent that many 

 people would notice and be sure of it, without mensural 

 proof, while the differences exhibited by those of other 

 towns, being smaller, would be overlooked. Now let us 

 consider the group jerdoni in comparison with the rats 

 of Poona taken as a group. It seems to me that the 

 peculiarities of the former are of a different order from 

 the slight peculiarities of the latter. Those of jerdoni 

 appear to be characters, the kind of attribute which has 

 been the subject of so much experiment in the last 

 decade. On the other hand, the peculiarities of the rats 



