South America presents indeed a remarkable similarity to the south-western peninsula of the old continent, in the form of its outlines and the direction of its coast-line. But the internal structure of the soil, and its relative position with respect to the contiguous masses of land, occasion in Africa that remarkable aridity which over a vast area checks the development of organic life. Four-fifths of South America lie beyond the Equator, and therefore in a region which, on account of its abundant waters, as well as from many other causes, is cooler and moister than our northern hemisphere[20]. To this, nevertheless, the most considerable portion of Africa belongs. The extent from east to west of the South American Steppes or Llanos, is only one third that of the African Desert. The former are refreshed by the tropical sea wind, while the latter, situated in the same parallel of latitude as Arabia and Southern Persia, are visited by currents of air which have passed over heat-radiating continents. The venerable father of history, Herodotus, so long insufficiently appreciated, has in the true spirit of a comprehensive observer of nature, described all the deserts of Northern Africa, Yemen, Kerman, and Mekran (the Gedrosia of the Greeks), as far even as Mooltan in Western India, as one sole connected sea of sand[21]. To the action of hot land winds, may be associated in Africa, as far as we know, a deficiency of large rivers, of forests that generate cold by exhaling aqueous vapour, and of lofty mountains. The only spot covered with perpetual snow is the western portion of Mount Atlas[22], whose narrow ridge, seen laterally, appeared to the ancient navigators when coasting the shore, as one solitary and aërial pillar of heaven. This mountain range extends eastward to Dakul, where the famed Carthage, once mistress of the seas, lies in crumbling ruins. This range forms a far extended coast-line or Gætulian rampart, which repels the cool north winds and with them the vapours rising from the Mediterranean. The Mountains of the Moon, Djebel-al-Komr[23], fabulously represented as forming a mountainous parallel between the elevated plain of Habesch—an African Quito—and the sources of the Senegal, were supposed to rise above the lower sea line. Even the Cordilleras of Lupata, which skirt the eastern coast of Mozambique and Monomotapa, in the same manner as the Andes bound the western shores of Peru, are covered with eternal snow in the gold districts of Machinga and Mocanga. But these mountains, abundantly watered, are situated at a considerable distance from the vast desert which extends from the southern declivity of the chain of Atlas to the Niger, whose waters flow in an easterly direction.