Mayan gods had affinities and aspects that caused them to merge with one another in ways that seem unbounded. There is a massive array of supernatural characters in the Maya religious tradition, only some of which recur with regularity. Good and evil traits are not permanent characteristics of Maya gods, nor is only "good" admirable. What is inappropriate during one season might come to pass in another since much of the Maya religious tradition is based on cycles and not permanence. Schellhas explains the Mayan gods that represent the natural elements, such as life and death, the sun and moon, giving technical and anthropological evaluation of each god image. He explains the gods worshipped by the Mayans and how they interplay into the daily lives of these spiritual, yet fierce, people. The Maya is a Mesoamerican civilization, noted for the only known fully developed written language of the pre-Columbian Americas, as well as for its art, architecture, and mathematical and astronomical systems. Initially established during the Pre-Classic period (c. 2000 BC to AD 250), according to the Mesoamerican chronology, many Maya cities reached their highest state of development during the Classic period (c. AD 250 to 900), and continued throughout the Post-Classic period until the arrival of the Spanish.