Poor as may be the Fauna of the Desert, there is yet cause enough for astonishment that the species which compose it, especially the herbivora, should be able to find subsistence in these seas of sand, where they can find but a few saline plants scattered at rare intervals, and where fresh water is almost wholly wanting. It is, however, well known now-a-days that the wilderness provides its denizens with an aliment, which is sometimes very abundant, suitable for man, the camel, and the beasts, and is considered identical by many authorities with the _manna_ of the Bible.[75] This substance is a cryptogamous vegetable, variously christened _lichen esculentus_ (Acharius), _lecanora esculenta_ (Pallas), _luttarut_ (by the Arabs), and _vasseh-el-ard_, or "earth-dung" (by the Algerines). It sometimes forms on the sand, in the morning, a layer one or two inches in thickness, and appears to have dropped from heaven, or to have sprung spontaneously from the soil, during the night. It is probable that its spores, transported by the wind, are developed by the humidity which is condensed through the nocturnal coldness. A shower of this lichen was observed, in April 1846, in the Russian government of Wilna. It covered the soil for three or four inches in depth, and the inhabitants lived upon it for several days. Its form is that of a small, anfractuous, rounded grain, about the size of a pea, externally of a gray colour, but white and farinaceous within. Its taste is weak, amygdalaceous, with a faint, mushroom-like aroma. Boiled in water, it swells, becomes gelatinous, and may be served up in various ways. In the Sahara, as well as in Arabia, it adheres to any foreign body. Cattle feed upon it eagerly. It certainly facilitates digestion, and contains all the assimilating principles which form the constituents of the wholesomest vegetable food. Such as it is, the _lichen esculentus_ is an inestimable boon to the wandering tribes of the Desert, who would perish of hunger in years of famine but for its heaven-sent nutriment. CHAPTER VII. THE MEN OF THE DESERT.