America also has increased in population during the time we know of it; but this increase is not so great as is popularly supposed, some estimates giving to Peru alone at the time of the discovery a greater population than now exists on the whole continent of South America. And all the indications are that previous to the discovery the population of America had been declining. What great nations have run their course, what empires have arisen and fallen in “that new world which is the old,” we can only imagine. But fragments of massive ruins yet attest a grander pre-Incan civilization; amid the tropical forests of Yucatan and Central America are the remains of great cities forgotten ere the Spanish conquest; Mexico, as Cortez found it, showed the superimposition of barbarism upon a higher social development, while through a great part of what is now the United States are scattered mounds which prove a once relatively dense population, and here and there, as in the Lake Superior copper mines, are traces of higher arts than were known to the Indians with whom the whites came in contact. As to Africa there can be no question. Northern Africa can contain but a fraction of the population that it had in ancient times; the Nile Valley once held an enormously greater population than now, while south of the Sahara there is nothing to show increase within historic times, and widespread depopulation was certainly caused by the slave trade. As for Asia, which even now contains more than half the human race, though it is not much more than half as densely populated as Europe, there are indications that both India and China once contained larger populations than now, while that great breeding ground of men from which issued swarms that overran both countries and sent great waves of people rolling upon Europe, must have been once far more populous. But the most marked change is in Asia Minor, Syria, Babylonia, Persia, and in short that vast district which yielded to the conquering arms of Alexander. Where were once great cities and teeming populations are now squalid villages and barren wastes.