The introduction of the camel into the Occidental Sahara took place during the time of the Roman Empire, but particularly toward the latter end of it. In one respect the Sahara is the antithesis of the North American and Australian deserts, which were found by European immigration in a virgin state. The animals are provided for by having them carry on their own backs the water and forage they will need in their wanderings. By the time of Ammianus Marcellinus camels were found in Tripolitania by thousands; and in Byzantine Africa at the period of Procopius and Corippus they were already established in their present status and playing an important part as beasts of burden and companions in war. There were similar great Nigritian empires throughout the western Sahara during the Middle Ages, and those of the Ghana, Sonroi and Manding tribes were important enough to attract the attention of the Arabian chroniclers.