All the civilizations before our own have thought, as we do, that they were set firmly on the rock of time by their unforgettable discoveries. They all believed in their immortality. The Incas and their families, who travelled swiftly in their palanquins on the excellent roads, fifteen hundred miles long, that still link Cuzco to Quito, were certainly convinced that their conquests would last for ever. Time, with one blow of his wing, has hurled their empire, like so many others, into the uttermost abyss. These kings of Peru also had their sciences, their machinery, their powerful engines, at the work of which we still stand amazed without being able to guess their construction. They too knew the secret of carrying enormous masses from place to place. They built fortresses by piling, one upon the other, blocks of stone thirty-eight feet long and eighteen wide, such as may be seen in the ruins of Tihuanaco, to which these gigantic building-materials must have been brought from a distance of many miles. Do we know the means used by the engineers of this vanished people to solve such a problem? No more than we know how the vast Cyclopean walls were constructed, the ruins of which, in many parts of Southern Europe, still defy the ravages of time. We must not confuse the causes of a civilization with its results. The causes disappear, and the results are forgotten, when the spirit that gave them birth has departed. If they persist, it is because of a new spirit that takes hold of them, and often succeeds in giving quite a new direction to their activities. The human mind is always in motion. It runs from one point to another, but cannot be in all places at once. It exalts what it embraces, and forgets what it has abandoned. Held prisoner for ever within a circle whose bounds it may not overstep, it never manages to cultivate one part of its domain without leaving the others fallow. It is always at the same time superior and inferior to its forbears. Mankind never goes beyond itself, and so is not capable of infinite progress. CHAPTER XIV PROOF OF THE INTELLECTUAL INEQUALITY OF RACES (_continued_). DIFFERENT CIVILIZATIONS ARE MUTUALLY REPULSIVE. HYBRID RACES HAVE EQUALLY HYBRID CIVILIZATIONS