increase in fertility from the edge of the Sahara towards the tropical forests of the Guinea Coast and the Congo. The forests do not spread completely across to the Indian Ocean, but leave a belt of grassy upland which connects the grasslands of the Sudan with those of South Africa, and this immense, open ground, thus continuous from the Sudan to the Cape Veldt, is the home of the antelopes, zebras, and other large, hoofed game, which correspond to the wild horses and wild asses of the Northern Heartland. Though the zebra has not been successfully domesticated, and the South African natives had no usual beast of burden, yet the horse and the one-humped camel of Arabia were early introduced into the Sudan. In both Heartlands, therefore, although to a greater extent in the Northern than in the Southern, mobility by the aid of animals has been available to replace the riverwise and coastwise mobility of the ships of the Atlantic and Pacific coastlands.
The Northern Heartland adjoins Arabia, as we have seen, for many hundred miles where the Iranian Upland drops to the Euphrates Valley; the Southern Heartland, at its northeastern corner in Abyssinia and Somaliland, grasps, though with an interval of sea, the
