These lines are founded on an early Christian tradition that when the heavenly host told the shepherds at Bethlehem of the birth of Christ, a deep groan, heard through all the isles of Greece, told that the great Pan was dead, and that all the royalty of Olympus was dethroned and the several deities were sent wandering in cold and darkness. So Milton in his “Hymn on the Nativity”:
“The lonely mountains o’er,
And the resounding shore,
A voice of weeping heard and loud lament;
From haunted spring and dale,
Edged with poplar pale,
The parting Genius is with sighing sent;
With flower-enwoven tresses torn,
The nymphs in twilight shade of tangled thickets mourn.”