It is a fact, that, as we count our increasing millions, we are apt to lose sight of—nevertheless it is a fact—that in what we know of the world’s history decadence of population is as common as increase. Whether the aggregate population of the earth is now greater than at any previous epoch is a speculation which can deal only with guesses. Since Montesquieu, in the early part of the last century, asserted, what was then probably the prevailing impression, that the population of the earth had, since the Christian era, greatly declined, opinion has run the other way. But the tendency of recent investigation and exploration has been to give greater credit to what have been deemed the exaggerated accounts of ancient historians and travelers, and to reveal indications of denser populations and more advanced civilizations than had before been suspected, as well as of a higher antiquity in the human race. And in basing our estimates of population upon the development of trade, the advance of the arts, and the size of cities, we are apt to underrate the density of population which the intensive cultivations, characteristic of the earlier civilizations, are capable of maintaining—especially where irrigation is resorted to. As we may see from the closely cultivated districts of China and Europe a very great population of simple habits can readily exist with very little commerce and a much lower stage of those arts in which modern progress has been most marked, and without that tendency to concentrate in cities which modern populations show.[21] Be this as it may, the only continent which we can be sure now contains a larger population than ever before is Europe. But this is not true of all parts of Europe. Certainly Greece, the Mediterranean Islands, and Turkey in Europe, probably Italy, and possibly Spain, have contained larger populations than now, and this may be likewise true of Northwestern and parts of Central and Eastern Europe.