Prehistoric Religion: Robert Graves & the White Goddess Thesis

Published in 1948, Robert Graves' book "The White Goddess: A Historical Grammar of Poetic Myth" made the argument that the Celtic pantheon evolved from the Greek pantheon, which evolved from the Sumerian. And, if you read the ancient myths and ancient poetry, there is a hidden subtext that reveals the world of the Mother Goddess--a prehistoric faith that goes back o the stone age. But, how true is this?