THIS DIVISION IS GOVERMED BY THE PRESENT COMPARTMENTAL NATURE OF RESEARCH INTO AFRICAN HISTORY. IT MIGHT HAVE SEEMED MORE LOGICAL TO ARRAGE THE VOLUME ACCORDING TO THE CONTINENT'S CHIEF ECOLOGICAL DIVISIONS , WHICH OFFER THE SAME LIVING CONDITIONS TO THE HUMAN GROUPS INHABITING THEM WITHOUT ANY TRUE PHYSICAL BARRIES WHICH SERVE TO BLOCK CULTURAL AND OTHER EXCHANFES WITHIN THE REGION.