THE GROWTH OF GROUPS 139 



characters imposed upon it, but not masking it. The 

 facts may be illustrated by reference to certain parts of 

 the body, such as the skull and the hind foot. The 

 drawings shown at Fig. i would serve not only as 

 illustrations of the common Mus rattus, but also of a 

 number of so-called species belonging to the rattus group, 

 but any one acquainted with the subject would recognize 

 that they were not representations of Mus norvegicus. 

 Thus there seems to be reason for believing that the 

 distinction between specific and varietal characters which 

 was recognized by De Vries in plants, is also recognizable 

 among the higher animals. 



But difficulties arise in many cases, for we cannot 

 always say of groups this is a species that is a variety. 

 Let us take a few examples. The black and the white- 

 bellied rats shown in the frontispiece might well be 

 regarded as varieties of Mus rattus, having a single 

 additional character. The miniature M. concolor of 

 Burmah might also be regarded as having the single 

 character of smallness. But M. vicerex has at least three 

 peculiar characters, M. blanfordi has at least five, M. 

 jerdoni has six or seven, and others have so many that 

 they obscure the underlying specific facies more or less. 

 When is the specific character obscured, and when is it 

 not quite obscured ? These are questions which arise 

 in many cases and cannot be answered. 



There is no certainty in the matter at present. It 

 seems, then, that we ought not to use the term mutant, 

 in the strict sense, when speaking of the higher animals. 

 By mutant in the strict sense I mean, as exemplified by 

 those of (Enothem. We cannot believe in mutants, in 

 this sense, among the higher animals until some one has 



