228 AFRICAN POLITICAL SYSTEMS
erosion caused by shifting cultivation and excessive burning of bush for grazing purposes. Expansion, based on these causes, cannot, however, have been continuous. The first two of these causes were checked by epidemics and by adverse luck in war. The third was limited by the checks on the increase in population and live stock and also by the fact that, under traditional conditions at least, the deterioration of the soil was not permanent but temporary, so that periods of expansion were followed by periods of retreat to lands formerly cultivated. The system of land tenure indicates that neither the tribal group nor the clans attached value to the possession of land apart from those stretches actually used or reserved for cultivation and grazing.
The fact that the two ultimate motives in warfare were the raiding of cattle and the conquest of territory has a definite bearing upon the conduct of warfare, as it involves conflicting aims. While it lies in the interest of expansion to carry on aggression in a ruthless manner which drives the enemy away as far as possible, the aim of raiding cattle clearly requires the presence of enemy groups in the neighbourhood. Owing to the necessity of balancing these two aims, warfare tended to be conducted with certain restrictions, above all with provisions for terminating a period of hostilities and with generally observed rules regarding the treatment of slain warriors and of women and children. Such 'rules of warfare' were more pronounced in the conduct of hostilities between the various Bantu groups than between Bantu and non-Bantu. In the latter case, the mutual destruction of the hostile groups was the prominent aim, while in the encounters between groups of Bantu stock the hostile groups conceded their mutual rights of existence and maintained a type of relationship with one another in which warfare functioned chiefly as a regulating and balancing force, making for an approximately even distribution of power and wealth between the tribes
Secondary motives of warfare, the relative importance of which differed in the different areas, were the taking of captives and the raiding of crops. The first was limited mainly to the taking of small boys and girls between the ages of six and ten years, who were adopted and brought up in the family of the warrior who had captured them. As, under traditional conditions, children were of economic value to the family, the adoption of war captives meant a welcome addition to the family and the clan.