In his book "The Mycenaean World", linguist and classicist John Chadwick argues that many chthonic deities may be remnants of the native Pre-Hellenic religion and that many of the Olympian deities may come from the Proto-Greeks who overran the southern part of the Balkan Peninsula in the late third millennium BC. He does, however, note that this may be somewhat of an overgeneralization and that the origins of chthonic and Olympian deities are probably much more complex. The German classicist Walter Burkert explicitly rejects the notion of chthonic deities as pre-Greek and the Olympian deities as Indo-European in his book "Greek Religion". He comments, "It is the chthonic "choai" which are related to Indo-European, whereas the Olympian sacrifice has connections with Semitic tradition."