In addition to Indo-European and Near Eastern origins, some scholars have speculated on the debts of Greek mythology to the still poorly understood pre-Hellenic societies of Greece, such as the Minoans and so-called Pelasgians. This is especially true in the case of chthonic deities and mother goddesses. Historians of religion were fascinated by a number of apparently ancient configurations of myth connencted with Crete: the god as bull — Zeus and Europa; Pasiphaë who yields to the bull and gives birth to the Minotaur; agrarian mysteries with a sacred marriage (Demeter's union with Iasion) etc. Crete, Mycenae, Pylos, Thebes and Orchomenus figure so large in later Greek mythology. For some, the three main generations of gods in Hesiod's "Theogony" (Uranus, Gaia, etc.; the Titans and then the Olympians) suggest a distant echo of a struggle between social groups, mirroring the three major high cultures of Greek civilization: Minoan, Mycenaean and Hellenic. Martin P. Nilsson, Professor of Classical Archaeology, worked on the structure, origins and relationships of the Indo-European languages, and concluded that all great classical Greek myths were tied to Mycenaen centres and were anchored in prehistoric times. Nevertheless, according to Walter Burkert, the iconography of the Cretan Palace Period has provided almost no confirmation of all these theories; nothing points to a bull, sexual symbols are absent and a single seal impression from Knossos showing a boy beneath a sheep is regarded as a scant evidence for the myth of Zeus' childhood.