The American continent formed no exception to this law of the invasion of the globe by the emigration of human phalanxes. It is a matter of no great difficulty to pass from Asia to America, across Behring Strait, which is almost always covered with ice, thus permitting of almost a dry passage from one continent to the other. Thus it is that the inhabitants of Northern Asia have found their way into the north of the New World. This communication of one terrestrial hemisphere with the other is less surprising when we consider what modern historical works have shown, namely, that already about the Tenth Century, which would be nearly 400 years before Christopher Columbus, navigators from the coast of Norway had penetrated to the other hemisphere. The inhabitants of Mexico and Chili possess most authentic historical archives, which prove that a most advanced civilization flourished there at an early period. Gigantic monuments which still remain bear witness to the great antiquity of the civilization of the Incas (Peru) and of the Aztecs (Mexico). It is reasonable to suppose that the inhabitants of America, who thus advanced at a rapid pace in the path of civilization, descended from the hordes of Northern Asia which reached the New World by traversing the ice of Behring Strait. To explain, therefore, the presence of man upon all parts of the continent, and in the islands, it is not necessary to insist upon the existence of several centres, where our species was created. If popular traditions went to show that all the regions now inhabited have always been occupied by the same people, and that those who are found there have constantly lived in the same places, there might be reason to admit the hypothesis of multiple creations of the human race; but, on the contrary, traditions for the most part teach us that each country has been peopled progressively by means of conquest or emigration. Tradition shows that the nomadic state of existence has universally preceded fixed settlements. It is, therefore, probable that the first men were constantly on the move. A flood of barbarians, coming from Central Asia, overflowed the Roman Empire, and the Vandals penetrated even into Africa. Modern migrations have been conducted on a still vaster scale, for at the present day we find America almost wholly occupied by Europeans; English, Spanish, and other people of the Latin race fill the vast American hemisphere, and the primitive populations of the New World have almost entirely disappeared, annihilated by the iron yoke of the conqueror. The continent of Asia was peopled little by little by branches of the Aryan race, who came down from the plains of Central Asia, directing their course toward India. As to Africa: that continent received its contingent of population through the Isthmus of Suez, the valley of the Nile, and the coasts of Arabia, by the aid of navigation.