Left:A diagram from Sander Gilman's Difference and Pathology of  racial/ethnic classifications by  physical charaacteristicss, such  as cranial shape and size. (Identity and Difference 308a)
  
This attempts to explain racial differences in terms of the races' biological attributes (i.e. the cranial size and shape), and with the arrangement of the races on the diagram, a kind of hierarchy is implied.