The mesopotamian region and the country directly south of the Plate estuary have, of course, an ample rainfall. South of the latitude of Buenos Aires the rainfall of the Andean region, which has grown steadily less from the northern boundary, begins again to increase. The eastern slopes of the mountains south for an indeterminate distance are well watered, while the Patagonian plains to their east are dry and desolate. The climate varies from tropical, on the northern frontier, to arctic in Tierra del Fuego. The southern pampa and the Andean provinces are temperate or subtropical, and admirably adapted for habitation by men of European descent. Tucuman is the hottest of these provinces. There the average temperature of the coldest month is 53 deg.; at Buenos Aires it is 50 deg.; at Cordoba 47 deg.. The average temperatures in these localities for the whole year are, respectively, 63 deg., 61 deg., and 63 deg.. When Columbus landed in the West Indies, this vast territory was occupied by two separate sets of aborigines. The Andean provinces were a part of the great Inca Empire. South as far as Mendoza, the Andean valleys were filled with a vigorous yet peaceful population who had brought the art of irrigation to a high degree of perfection. Plantations of corn, mandioc, and potatoes flourished on the terraced hillsides and in the fertile valleys. The lower and hotter plains furnished cotton. Constant communication, both commercial and governmental, was kept up with the centre of the Inca power in Cuzco, along roads that followed the easiest routes along the valleys and up over the passes to the Bolivian plateau, and thence to the central provinces of the Empire. Chile, on the other side of the Cordillera, was a sister province, and the passes over the great range were well known and constantly used. The population was greater than it is at the present day. While the political solidity of the Inca Empire is doubtless exaggerated, it is certain that the same civilisation extended from Ecuador to Mendoza and Santiago de Chile, and that the Cordilleran region was the home of twenty millions of people, organised into vigorous, progressive, and expanding communities. The Andean civilisation never showed any tendency to expand over the tropical plains of the great central depressions. The Incas themselves never cared to penetrate far down the wooded and steaming slopes of the Andes lying directly to the east of their own capital. Their dependent states bordering on the Argentine pampa did not cross the desert plains, where irrigating ditches could not reach. So far as we now know, the Andean Indians had never penetrated to the Atlantic.