Possibly, however, all the causes of heat and dryness which have been enumerated may have been insufficient to transform such considerable parts of the African plains into a dreadful desert, without the concurrence of some revolution of nature,--such, for instance, as an irruption of the ocean, whereby these flat regions may have been despoiled of their coating of vegetable soil, as well as of the plants which it nourished. Profound obscurity veils the period of such an event, and the force which determined the irruption. Perhaps it may have been caused by the great “rotatory current”[24] which sends the warmer water of the Mexican gulf over the banks of Newfoundland and to the shores of the old continent, and causes West India cocoa-nuts and other tropical fruits to reach the coasts of Ireland and Norway. There is still at least at the present time, an arm of this current directed from the Azores to the south-east, which sometimes produces disasters by carrying ships upon the west coast of Africa, which it strikes at a part lined by sand-hills. Other sea coasts (I particularly recall that of Peru between Amotape and Coquimbo) shew that in these hot regions of the earth, where rain never falls and where neither Lecideas nor other Lichens[25] germinate, centuries and perhaps thousands of years may elapse before the moveable sand can afford to the roots of plants a secure holding place. These considerations are sufficient to explain why, with an external similarity of form, Africa and South America present so marked a difference of character both in respect to climate and to vegetation. But although the South American Steppe is covered with a thin coating of mould or fertile earth, and although it is periodically bathed by rains, and becomes covered at such seasons with luxuriantly sprouting herbage, yet it never could attract the surrounding nations or tribes to forsake the beautiful mountain valleys of Caraccas, the margin of the sea, or the wooded banks of the Orinoco, for the treeless and springless wilderness; and thus, previous to the arrival of European and African settlers, the Steppe was almost entirely devoid of human inhabitants.