Sir James George Frazer (1854–1941).  The Golden Bough.  1922.

§ 3. Recapitulation
WE can now perhaps understand why the ancients identified Hippolytus, the consort of Artemis, with Virbius, who, according to Servius, stood to Diana as Adonis to Venus, or Attis to the Mother of the Gods. For Diana, like Artemis, was a goddess of fertility in general, and of childbirth in particular. As such she, like her Greek counterpart, needed a male partner. That partner, if Servius is right, was Virbius. In his character of the founder of the sacred grove and first king of Nemi, Virbius is clearly the mythical predecessor or archetype of the line of priests who served Diana under the title of Kings of the Wood, and who came, like him, one after the other, to a violent end. It is natural, therefore, to conjecture that they stood to the goddess of the grove in the same relation in which Virbius stood to her; in short, that the mortal King of the Wood had for his queen the woodland Diana herself. If the sacred tree which he guarded with his life was supposed, as seems probable, to be her special embodiment, her priest may not only have worshipped it as his goddess but embraced it as his wife. There is at least nothing absurd in the supposition, since even in the time of Pliny a noble Roman used thus to treat a beautiful beech-tree in another sacred grove of Diana on the Alban hills. He embraced it, he kissed it, he lay under its shadow, he poured wine on its trunk. Apparently he took the tree for the goddess. The custom of physically marrying men and women to trees is still practised in India and other parts of the East. Why should it not have obtained in ancient Latium?   1
  Reviewing the evidence as a whole, we may conclude that the worship of Diana in her sacred grove at Nemi was of great importance and immemorial antiquity; that she was revered as the goddess of woodlands and of wild creatures, probably also of domestic cattle and of the fruits of the earth; that she was believed to bless men and women with offspring and to aid mothers in childbed; that her holy fire, tended by chaste virgins, burned perpetually in a round temple within the precinct; that associated with her was a water-nymph Egeria who discharged one of Diana’s own functions by succouring women in travail, and who was popularly supposed to have mated with an old Roman king in the sacred grove; further, that Diana of the Wood herself had a male companion Virbius by name, who was to her what Adonis was to Venus, or Attis to Cybele; and, lastly, that this mythical Virbius was represented in historical times by a line of priests known as Kings of the Wood, who regularly perished by the swords of their successors, and whose lives were in a manner bound up with a certain tree in the grove, because so long as that tree was uninjured they were safe from attack.   2
  Clearly these conclusions do not of themselves suffice to explain the peculiar rule of succession to the priesthood. But perhaps the survey of a wider field may lead us to think that they contain in germ the solution of the problem. To that wider survey we must now address ourselves. It will be long and laborious, but may possess something of the interest and charm of a voyage of discovery, in which we shall visit many strange foreign lands, with strange foreign peoples, and still stranger customs. The wind is in the shrouds: we shake out our sails to it, and leave the coast of Italy behind us for a time.   3