In the cattle zones farther to the north and south, nature is still less bountiful. The labor of maintenance requires a combination of the pastoral art, agriculture and trade. A slave class could not maintain itself and at the same time support a large master class. The labor of a large proportion of the population is, in one way or another, necessary to existence. The nature of the work, so far as it is pastoral or trading, is not especially irksome, but rather fascinating. Tending cattle is full of excitement, and is a kind of substitute for hunting; while trading is an occupation which appeals with wonderful force to all the races of Africa. The impulse to shift labor in the cattle zones is, therefore, very slight, except in the case of a few populations subsisting largely upon agriculture. The ruling classes, therefore, instead of owning many personal slaves, make a practice of subjugating the agricultural groups in such a way as to constitute a kind of feudalism. As land is free the enslaved groups can be made to serve the free class only by coercion. Similar conditions among the natural races all over the world give rise in the same way to the institution of slavery. Ellis thinks that slavery probably originated under the regime of exogamy where the sons born of captured women formed the slave class because they were considered inferior to the sons born of the women of the group.[2] But it is quite evident that slavery originated primarily from economic conditions. For further sociological explanations of slavery in the several zones the reader is referred to the author's first and second volumes on the Negro races. II. THE SLAVE TRADE OF WEST AFRICA AND THE DESERT OF SAHARA