326 INTRODUCTION TO EVOLUTION 



from a center of dispersal (pp. 268-272). As animals spread out over the 

 country groups of them became separated from each other by distance and 

 sometimes by geographic barriers. These separate groups, being out of con- 

 tact with each other, gradually came to develop differences, so that each was 

 no longer quite like the original parent stock or, on the other hand, quite 

 Hke the other groups. (Recall our discussion of the processes involved in the 

 evolution of the human races, pp. 250-255.) Each group would then rank as 



FIG. 14.5. The ranges of four geographic races forming a Rassenkreis. Where 

 the ranges come into contact interbreeding occurs in three cases, but it does not 

 occur in the area where Race D comes into contact with Race A. 



a separate subspecies or geographic race. If now the groups became progres- 

 sively more and more different in structure, and if, especially, these differ- 

 ences finally became sufficient to prevent interbreeding whenever members 

 of different groups came in contact, the groups would be considered to have 

 reached the rank of separate species. In brief, according to this view, the 

 subspecies is a step in the development of the species. 



We have referred to "clusters" of subspecies; sometimes these form more 

 or less circular mosaics covering a certain geographic area. Such a mosaic 

 or circle of races has been termed by Rensch (1960) a rassenkreis. Sup- 

 pose, for example, that a rough circle is formed by races A, B, C, and D 



