The study of the Poseidon-cult in Hellas is of more value for the Greek historian than for the student of the higher religions of mankind. It lacks the spiritual and ethical interest of some of the Olympian cults, and from the earliest to the latest period Poseidon remains comparatively a backward god, never intimately associated with the nation's intellectual advance. But the ritual presents us with certain facts of great interest. And early Greek ethnography and the history of the earliest migrations of Hellenic tribes can gather much from a minute inquiry into the diffusion of this worship. Modern historians have become accustomed to use the facts of Greek religion as a clue for their researches into the period that precedes recorded history. But the criterion is often misapplied, and the value of it is still occasionally ignored. Much has still to be done in this branch of inquiry, and much may be effected if the evidence is severely scrutinized according to some fixed principles of criticism, and at the outset of this chapter it may be well to state and consider some of these. The historian of the earliest period, if he believes that he can extract anything from the religion and the mythology, has to reckon with three sources of possible evidence: with cult and ritual, with myth pure and simple, and finally with genealogical tables. Now the value of these sources is by no means equal.