He also believed that the southern centers such as Copán and Quiriguá had been united in the Classical period under what he termed the "Old Empire". This empire mysteriously collapsed, but the remnants later migrated to the northern sites (such as Chichen Itza) to form a "New Empire". It is now generally accepted that at no time was the Maya region united under a single polity, but rather that individual "city-states" maintained a somewhat independent existence, albeit one with its fluctuating conquests and local subservience to more dominant centers. In support of his view, Morley devised a 4-tier classification system of relative importance, which he ascribed to all of the then-known main Maya sites (about 116); many more sites are now known, and his classification system is now seen as an arbitrary one, contradicted in places by the sites' texts which can now be (substantially) read.