THE CATEGORIES OF VARIANT INDIVIDUALS 71 



geographical race which ' geht gleitend in die Nachbarrassen 

 uber.' He suggests that groups of geographical races which 

 may or may not correspond with taxonomic species should 

 be called ' Rassenkreise.' x Now Rensch's Rassenkreis, as 

 far as we can see, can scarcely be treated as a classificatory 

 unit, but rather as the name of a principle of divergence. It 

 denotes the tendency to form constellations of geographical 

 races. At times the Rassenkreis appears to us to be clearly 

 conterminous with the taxonomists' species. Rensch does 

 not hesitate to give some of his Rassenkreise binominal names 

 (e.g. p. 29, the Rassenkreis of Troglodytes troglodytes). The 

 suggestion is of value in pointing the differences between 

 groups of races connected by transitional forms and more 

 homogeneous and geographically undiversified groups ; but 

 it has a disadvantage in that two terms are applied to what 

 are in practice equivalent degrees of morphological divergence. 

 We are left, in short, with the general result that there is 

 a principle of geographical divergence manifest within the 

 systematic species, and at all early stages in evolutionary 

 divergence, of such a nature that groups very slightly different 

 in structure (often only in a single character, e.g. coat- or 

 plumage-colour) are also distinct in their topographical 

 range. That such divergence is, according to our present 

 knowledge, more clearly seen in some groups than others is 

 quite apparent. But we would point out (a) that it is by no 

 means a universal feature in mammals and birds and (b) that 

 we are a little uncertain as to how far it may not be exaggerated 

 in those groups by the relatively low numbers used in the 

 discrimination of mammalian and bird races. Finally, it is 

 uncertain to what extent many of the subspecies and geo- 

 graphical races described by taxonomists are hereditarily 

 stable. 



IV. Genetigal and Reproductive Categories 



It is convenient to consider here not only the strictly 

 genetical categories, such as the biotype, pure line and the 

 ' petite espece,' but also the clone and the syngameon which 

 depend on the type of reproduction (whether sexual or asexual, 

 interbreeding or not), and the aberration, form, modification and 



1 In all probability the Rassenkreis corresponds to Waagen's ' Collectivart ' 

 and the gens of certain modern palaeontologists (cf. Bather, 1927, p. Ixxxviii). 



