According to the seasons animal life fluctuates in the Sahara from north to south. In winter and spring, when heavy rains, falling on its northern borders, provide wide districts, thoroughly parched by the summer heat, with the water and pasturage needed for the herds, the nomadic tribes wander farther into the desert with their camels, horses, sheep, and goats, and retreat again to the coast-lands as the sun gains power. At this time of the year the wild animals-- the lion, the gazelle, and the antelope--also wander farther to the south, which at that time provides them, each according to its taste, with the nourishment which the dry summer is unable to bestow; while the ostrich, who during the summer ranged farther to the north, then retreats to the south; for hot and sandy plains are the paradise in which this singular bird delights to roam. In the southern part of the Sahara the tropical rains, whose limits extend to 19° N. lat., and in some parts still farther to the north, produce similar periodical fluctuations in the animal life of the desert. Under their influence the sandy plains are soon enlivened here and there with grasses, and the parched shrubs clothe themselves with verdure. In the dry season, on the contrary, the green carpet disappears, and the country then changes into a dry waste. Frequently, however, the tropical rains fail to appear on their northern boundaries, and disappoint the hopes of the thirsty desert. Two nomadic nations, the Tuaregs and the Tibbos wander with their camels and sheep over the immense expanse of the Sahara in quest of scanty forage and thorny shrubbery. The abstinence and hardships they frequently endure, the freedom of a roving life, and their predatory habits, give them an evident superiority over the sedentary Berbers, who inhabit the oases, and repay the haughty demeanour of the nomads with hatred and contempt. Yet, in spite of these feelings of ill-will, the bonds of traffic and of a common interest connect the vagrant and the agricultural tribes. Condemned to perpetual migrations, the nomad is forced to confide all the property he is unable to carry about with him to the inhabitants of the oases; he may even possess a small piece of land, the cultivation of which he entrusts to the latter, who, on his part, as soon as he has saved something, buys a sheep or a goat which he gives in charge to the nomad. An unmitigated hatred, on the contrary, exists between the various erratic tribes, as here no mediating self-interest softens the antipathies which are almost universally found to exist between neighbouring barbarians, and their robber expeditions not merely attack the richly-laden caravan, but also the oasis which may be connected by the bonds of intercourse with their hereditary enemies. [Illustration: BEDOUIN WARRIORS.] CHAPTER XI. THE BEDOUINS OF ARABIA.