As is remarked in the Introduction to the third volume of the Cambridge History of Africa, there are obvious pitfalls in marking out periods of African history which are equally valid for all parts of the continent. Africa is a vast land mass, and also the only one to be centred in the tropics. It has presented quite as many and as varied difficulties for its human inhabitants to master as it has resources to be exploited. Thus until the general world-wide acceleration of processes of change brought about by the rise and spread of modern scientific technology, there could be extremely wide variations in the degrees of social and material development to be found among African peoples. During the very long period of history covered by this volume these differences seem to have been accentuated by the fact that the more temperate lands north of the Sahara (and to some extent the eastern littoral also) were in much closer touch with developments in other parts of the world than were the great tropical heartlands of the continent or its southerly temperate zone.

A by-product of this fact that historians cannot escape is an imbalance of historical source materials for the period. Some of the implications of this are more fully discussed later in this Introduction. Here no more need be said than that this imbalance makes it possible to consider the course and the significance of events in northern Africa in greater detail and with less recourse to hypothesis or speculation than is the case for the other three-quarters of the continent.