THE GROWTH OF GROUPS 101 



in the strictest sense, i.e. they are descendants of an 

 individual that happened to possess these peculiarities 

 and arose as a sport from the common stock. 



We will now pass to another case which illustrates 

 the occurrence of minor structural characters. 



Hitherto it has been assumed that each of the well- 

 known species has its own type skull, but this is true only 

 in a broad sense. The common rattus, for example, 

 undoubtedly has its own type of skull, which is recogniz- 

 able in animals captured in various parts of the world. 

 But any large collection of these skulls will show minor 

 differences. Strictly speaking these should not be called 

 individual differences, since they only appear as such if 

 the collection is taken haphazard from all parts of a town, 

 moreover the same kind of peculiarity is found again and 

 again in animals taken from separate places. We may 

 recognize certain definite types even among these minor 

 peculiarities. Such peculiarities are seen in the shape 

 of the nasal bones, of the supra temporal ridges, of the 

 plate which bounds the infra orbital foramen, the degree 

 of inflation of the tympanic bone, and other points. It 

 is sometimes possible to observe that minor points such 

 as these occur in every member of a family group. In 

 the case now to be related it happens that these minor 

 peculiarities are so well marked that they may be spoken 

 of with confidence and illustrated graphically. 



In Fig. 6 are seen drawings of the skulls of three 

 mole-rats (referred to as A, B, and C) which were found 

 burrowing in fields near Amritzar within a circle of radius 

 a few miles. The dorsal, the ventral, and the posterior 

 aspects of each are shown in succession from above 

 down wards. 



