That a monument of this kind could be transformed into an Olympian god is astounding
- Walter Burkert
Read the article from Wikipedia on Hermai.
Or check out this info from the ancients:
"The ithyphallic images of Hermes [the Hermai]; the production of these came from the Pelasgians, from whom the Athenians were the first Greeks to take it, and then handed it on to others. For the Athenians were then already counted as Greeks when the Pelasgians came to live in the land with them and thereby began to be considered as Greeks. Whoever has been initiated into the rites of the Kabeiroi, which the Samothrakians learned from the Pelasgians and now practice, understands what my meaning is [the Kabeiroi gods were the keepers of a sacred phallus]. Samothrake was formerly inhabited by those Pelasgians who came to live among the Athenians, and it is from them that the Samothrakians take their rites. The Athenians, then, were the first Greeks to make ithyphallic images of Hermes, and they did this because the Pelasgians taught them. The Pelasgians told a certain sacred tale about this, which is set forth in the Samothrakian mysteries."
Pausanias, Guide to Greece 2.38.7:
"At the Arkadian gate [of Ithome, Messenia] leading to Megalopolis is a Herma of Attic style; for the square form of Herma is Athenian, and the rest adopted it thence."
"Mount Parnon, on which the Lakedaemonian border meets the borders of the Argives and Tegeatai [of Arkadia]. On the borders stand stone figures of Hermes, from which the name of the place is derived."