138 THE GROWTH OF GROUPS 



We should be able to recognize our species clearly, each 

 species containing a number of varieties, each variety 

 marked by its particular varietal character or characters, 

 which would not obscure the underlying specific facies. 

 We cannot always draw a clear distinction between 

 species and varieties. But yet we cannot be quite sure 

 that there is no distinction between a specific and a 

 varietal character. The matter is well illustrated by 

 rats. Mus rattus (the common or brown variety) and 

 Mus norvegicus are so alike that an unobservant person 

 might overlook the difference unless it was pointed out 

 to him, that the ears and tail of the latter were relatively 

 shorter. But yet it is true to say that these two species 

 differ from one another as regards their whole constitution. 

 Their skulls and even corresponding portions of them are 

 distinguishable from one another, yet the differences are 

 so small that many people would overlook them ; but when 

 once they are appreciated and remembered the observer 

 will make no mistakes in identification. A number of 

 skulls of the two kinds mixed together can be sorted 

 without hesitation and without mistake. The same is 

 true of other detached and corresponding parts of the 

 body such as the ears, tail, or feet. Hence the relation 

 between these two species appears to be that of mutants 

 in the strict sense of De Vries. Although both are so 

 alike we cannot say that one kind was derived from the 

 other. We can find nothing in their appearance leading 

 us to suppose that one was the original and the other a 

 derived species. But, on the other hand, there are many 

 so-called species of Mus which are obviously derivatives 

 of Mus rattus. They possess what may be called the 

 specific facies of rattus with one or more additional 



