THE GROWTH OF GROUPS 39 



them were of the type which has long been known by 

 the name Mus rattus. Rats of this kind form a hetero- 

 geneous group rather than a species in the modern sense. 

 The term Mus rattus is applied to the house rats of India 

 collectively, just as the term Homo sapiens is applied 

 to Man. 



Although obvious differences were to be seen, especially 

 in coat colour, among the members of the collection, 

 yet it was not possible to divide the group into distinct 

 local races, except in the case of those received from hilly 

 districts. For example, the rats received from both 

 Sirinagar and Darjiling, which are among mountains, 

 have each their own special peculiarities by which they 

 could be recognized. The distinguishing features, how- 

 ever, do not extend to fundamental structures such as 

 the skull. If a few skulls of rats from Darjiling or 

 Sirinagar were to be mingled with a number of skulls 

 of rats caught in any of the large towns of India, it would 

 be impossible for any one to sort them out. 



The question as to how the collection should be 

 treated presented some difficulty. In accordance, with 

 the usual procedure the specimens were to be classified 

 according to their resemblances and differences, and the 

 classes were to receive special names, old or new. This 

 method of nomenclature could be applied to the few 

 isolated groups. The rats of Sirinagar had already 

 received the name of Mus vicerex, similarly the Dar- 

 jiling rats might be called Mus nitidus. The difficulty 

 arose in considering how the rats of the plains should be 

 treated, since some of them exhibited peculiarities of 

 colour as visible as those which are the marks of identifi- 

 cation of certain species. If the name Mus rattus was 



