THE GROWTH OF GROUPS 9 



in the belief that the new-fashioned species which perhaps 

 have only one or two special characters of their own, 

 arose in one place only ; in fact they feel pretty sure 

 that such groups may arise on many occasions in widely 

 separate places, that is to say at various points of the 

 area which is occupied by the species X. They therefore 

 object to x, y, and z being known as species, for if a 

 species may arise in more than one place, all arguments 

 concerning its distribution are futile, hence they object 

 to any group being known as a species unless it has many 

 clear characters of its own. The justice of their conten- 

 tion cannot be disputed. There is good reason to believe 

 that groups such as x, which by some are spoken of as 

 varieties, arise in separate parts of the area inhabited by 

 the parent species, but there is little or no evidence that 

 groups such as X arise in more than one place. 



This discussion about groups X and x, will be clearer 

 if in place of the letters we substitute known groups of 

 animals. Three animals are seen in the frontispiece. 

 All are alike in many ways, in their bodily proportions, 

 in the form of their skulls and in most other respects ; 

 in a broad sense all three belong to the species Mus rattus, 

 a species such as X, for they have many characters in 

 common ; but as regards a single character which deter- 

 mines their coat colour they are different, one has a 

 special character which finds its expression in the whiteness 

 of the underparts, another has the melanic character. 

 There is evidence that groups such as these arise from the 

 parent species in separate places on various occasions, 

 the method of origin being growth from unity. There is 

 no evidence to show how the parent group Mus rattus 

 originated. These so-called varietal groups with their 



