40 THE GROWTH OF GROUPS 



to be applied to the house rats of the plains as a whole, 

 all these peculiarities ought to be disregarded, as being 

 " individual variation." But these peculiarities had not 

 that indefinite aspect which is commonly associated with 

 the term " individual variation," on the contrary they 

 were definite in appearance and sporadic in their mode of 

 occurrence; that is to say, abnormal rats showing the 

 same kind of abnormality were obtained along with normal 

 ones from widely separate localities. 



It is necessary to describe the species briefly, in a 

 general manner, as follows : 



Mus rattus (in India). The mean length of the head 

 and body combined is about 175 mm., but it varies 

 within wide limits from 150 to 200 mm. The mean 

 length is of course not quite the same in every district. 

 The length of the tail is greater than that of the head and 

 body, being usually about 120 per cent, of the latter and 

 varying from no per cent, to 130 per cent. 



The appearance of the head is characteristic, because 

 of the large size of the eyes and ears. The ears are so 

 large that when laid forward they wholly or partially 

 cover the eyes. The sole of the hind foot is decorated 

 with six heart-shaped pads, arranged as in the diagram 

 (Fig. i). The form of the skull affords perhaps the most 

 certain means of identification. If the skull of an 

 English rattus be mingled with a number of skulls taken 

 from Indian rats of the same species, it would not be 

 possible to distinguish it from them by any natural mark. 

 The same may be said of many local races of the rattus 

 group which have received specific names, although 

 those species have been defined as possessing types 

 of skull peculiarly their own. It is true that many 



