Laudator Temporis Acti

C.M. Bowra, From Virgil to Milton (1945; rpt. London: Macmillan, 1967), pp. 240-241:

[p. 240]

How deep his roots were even Milton did not always know. Describing the number of the fallen angels, he says that they lie

Thick as Autumnal Leaves that strow the Brooks
In Vallombrosa, where th' Etrurian shades
High overarch't imbowr.                          (I, 302-4)

The comparison of spirits in the underworld to fallen leaves is of great antiquity. It may first have appeared in some lost Orphic poem about a descent into Hades. From this Bacchylides, in the fifth century B.C., probably took it when he told how Heracles visited Hades:

    ἔνθα δυστάνων βροτῶν
ψυχὰς ἐδάη παρὰ Κωκυτοῦ ῥεέθροις,
    οἷά τε φύλλ᾽ ἄνεμος
Ἴδας ἀνὰ μηλοβότους
    πρῶνας ἀργηστὰς δονεῖ.¹
                                                   (V, 63-7)

Virgil took up the idea for the ghosts of the unburied dead:

quam multa in silvis autumni frigore primo
lapsa cadunt folia,²                  (VI, 309-10)

and after him Dante told of the ghosts pressing to cross Acheron:

¹ There he saw the ghosts
Of unlucky men by Cocytus' streams,
Like leaves that the wind flutters
On Ida's glittering headlands
Where the flocks graze.

² Thick as in forests at first autumn frost
Leaves drift and fall.

[p. 241]

Come d' autunno si levan le foglie
L'una appresso dell'altra, fin che 'l ramo
Vede alia terra tutte le sue spoglie.¹
                                         (Inf. III, 112-14)

Tasso gave a new turn to the comparison when he made the routed devils go back to Hell:

Nè tante vede mai l' autunno al suolo
Cader co' primi freddi aride foglie.²
                                        (IX, 66, 5-6)

Marlowe picked it up to describes a vast army in Tamerlane:

In number more than are the quivering leaves
Of Ida's forest.

At the end of the succession comes Milton who knew Virgil, Dante, Tasso and Marlowe but not Bacchylides or his unknown predecessor. He picks up the old simile and uses it of the hosts of fallen angels, thus showing some indebtedness to Tasso, who used it of devils, to Marlowe, who used it of an army, and to Virgil and Dante, who used it of spirits in the underworld. Moreover his instinctive genius shows his affinity to classical art when he gives a real place to the fallen leaves. His Vallombrosa is as exact as Bacchylides' Ida and has the immediacy of Greek poetry.

¹ And as the late leaves of November fall
One after one till on the earthen floor
The ruined bough looks on their funeral.
                                                      (L. Binyon)

² Nor leaves in so great numbers fall away
When winter nips them with his new-come frosts,
                                                                       (Fairfax)