This period of development, which lasted upward of 400 years, or until about the close of the sixth century, may be called {3} perhaps the "Golden Age of the Maya"; at least it was the first great epoch in their history, and so far as sculpture is concerned, the one best comparable to the classic period of Greek art. While sculpture among the Maya never again reached so high a degree of perfection, architecture steadily developed, almost to the last. Judging from the dates inscribed upon their monuments, all the great cities of the south flourished during this period: Palenque and Yaxchilan in what is now southern Mexico; Piedras Negras, Seibal, Tikal, Naranjo, and Quirigua in the present Guatemala; and Copan in the present Honduras. All these cities rose to greatness and sank again into insignificance, if not indeed into oblivion, before the close of this Golden Age. The causes which led to the decline of civilization in the south are unknown. It has been conjectured that the Maya were driven from their southern homes by stronger peoples pushing in from farther south and from the west, or again, that the Maya civilization, having run its natural course, collapsed through sheer lack of inherent power to advance. Which, if either, of these hypotheses be true, matters little, since in any event one all-important fact remains: Just after the close of Cycle 9 of Maya chronology, toward the end of the sixth century, there is a sudden and final cessation of dates in all the southern cities, apparently indicating that they were abandoned about this time. Still another condition doubtless hastened the general decline if indeed it did no more. There is strong documentary evidence[4] that about the middle or close of the fifth century the southern part of Yucatan was discovered and colonized. In the century following, the southern cities one by one sank into decay; at least none of their monuments bear later dates, and coincidently Chichen Itza, the first great city of the north, was founded and rose to prominence. In the absence of reliable contemporaneous records it is impossible to establish the absolute accuracy of any theory relating to times so {4} remote as those here under consideration; but it seems not improbable that after the discovery of Yucatan and the subsequent opening up of that vast region, the southern cities commenced to decline. As the new country waxed the old waned, so that by the end of the sixth century the rise of the one and the fall of the other had occurred.