THE GROWTH OF GROUPS 21 



rattus shown in the frontispiece. In jerdoni, the fur of 

 the whole lower surface is white ; the tail is somewhat 

 longer and has no pigment in the lower half of its cir- 

 cumference from root to tip. There are certain small 

 peculiarities in the skull, the most noticeable of which is 

 the smallness and flatness of the tympanic bulla. An- 

 other character, remarkable for its triviality, is an extra 

 ridge on the palate. Most if not all of the muridce have 

 five ridges in the roof of the mouth lying between the 

 molar teeth, each ridge is interrupted in the middle line. 

 In rats of the species jerdoni there is a sixth ridge. The 

 normal series of five is not disarranged, the extra one is 

 interpolated between the two hindmost. In all pro- 

 bability this sixth ridge is constant in the species, at 

 least it has been recognized both in Indian and Javanese 

 specimens. 



These are the most decided peculiarities of jerdoni, 

 but as regards the general contour of the body, the size 

 of the foot and ear in proportion to the size of the body, 

 the peculiar arrangement of the pads on the soles of the 

 feet and in many other ways, the species does not differ 

 from rattus. Mus jerdoni is a good example of a morpho- 

 logical species, i.e. a group of animals each possessing 

 a certain combination of characters, which are used as 

 marks of identification by the naturalist. It is typical 

 of thousands of other groups such as are called 

 species. 



Now let us consider some other group which is less 

 distinct than the last-mentioned but yet is perceptibly 

 a group. As an example, the rats of Poona, a town in the 

 west of India, will be mentioned. They are slightly smaller 

 than the common house rats of many other Indian towns 



