You say in the question,
The natural way I would read this as prose would be:
And ev- | ery spi- | rit up- | on earth
The natural way to read the line is nearly always the best way, and you should have the courage to stick to your guns! The last four syllables of the line form a double iamb, which is a common feature of English verse.
I’ll give six examples of the double iamb, but once you are familiar with it you’ll find it everywhere.
Let me | not to | the mar- | riage of | true minds
William Shakespeare. Sonnet 116, line 1.
At the | round earth’s | ima- | gined cor- | ners, blow
John Donne. Holy Sonnets VIII, line 1.
That Shep- | herd, who | first taught | the cho- | sen Seed
John Milton. Paradise Lost, book I, line 8.
Bright star, | would I | were stead- | fast as | thou art—
John Keats. ‘Bright star’, line 1.
In his | first splen- | dour, val- | ley, rock, | or hill
William Wordsworth. ‘Composed upon Westminster Bridge’, line 10.
I shall | go back | again | to the | bleak shore
Edna St Vincent Millay. ‘I shall go back again’, line 1.
The double iamb does not seem to have been properly recognized until the mid-20th century. At the beginning of the century, the phenomenon was doubtful enough for C. M. Lewis to prefix it with a cautionary disclaimer, “Metrists usually say”:
Metrists usually say that the pyrrhic is sometimes admitted in iambic verse; and some leading authorities point out that it generally is found with a spondee immediately afterwards.
C. M. Lewis (1907). The Principles of English Verse, p. 42. New York: Henry Holt.
Robert Bridges noted it as “a common irregularity” in Milton:
The matter stands thus: in all Milton’s verse there is a frequent occurrence of the following rhythm, that is, a foot of two unstressed short syllables preceding a foot composed of two heavy syllables, as in these lines from Shakespeare’s Midsummer-Night’s Dream,
The ploughman lost his sweat, and the green corn.
Before milkwhite, now purple with love’s wound.
Love takes the meaning in love’s conference.
It is common in Milton’s early verse, which is much influenced by the verse of Shakespeare’s first style; and he always made use of it. Whatever the account may be, it is pleasant to the ear in the smoothest verse, and is so, no doubt, by a kind of compensation in it. In typical cases there is no possibility of stress in the first short foot, and the first heavy syllable of the next foot seems to carry what has been omitted, with an accentuation bearing relation to the sense.
Instances occur everywhere in Milton.
Robert Bridges (1901). Milton's Prosody, p. 58. Oxford University Press.
The double iamb was, I think, first stated in the form of a rule by American poet John Crowe Ransom:
These are substantially the exceptions as codified by Bridges in The Prosody of Milton [sic], the best handbook we have on iambic pentameters. What Bridges codified was Milton’s code, as it had been for several generations the code of Milton’s predecessors, and would be for his successors over a century and a half; since then it has been well known to poet-prosodists, and adhered to systematically when they pleased. But it is not quite complete, in my judgment. I wish Bridges had added:
- Any two successive iambic feet might be replaced by a double or ionic foot.
John Crowe Ransom (1956). ‘The Strange Music of English Verse’. The Kenyon Review 18:3, pp. 470-471.
The name “double iamb” was not introduced until the 1990s:
We should drop the pyrrhic foot and accept in its place the double-iamb, as one of the six foot-terms necessary: iamb, trochee, anapest, dactyl, spondee, double-iamb.
This change was suggested by John Crowe Ransom […] Ransom uses the Greek term “ionic foot”, but plain English seems preferable, carrying with it a reminder that a double-iamb counts as two feet, and having the advantage of suggesting that this pattern is quite normal to the iambic base.
Robert Wallace (1993). ‘Meter in English’. In David Baker, ed. (1996). Meter in English: A Critical Engagement. University of Arkansas Press.
Some metrists objected to “double iamb” on the grounds that the term was also used for a pair of iambs. But it seems to have stuck, perhaps due to the advantages noted by Wallace.
Effect in ‘The Darkling Thrush’
The general effect of a double iamb is to slow the rhythm by placing two stressed syllables adjacent to each other, and to alter the general iambic pattern, avoiding a metronomic regularity and making us pay a bit more attention to the stressed words.
In the case of ‘The Darkling Thrush’, we are led by the rhythm to interpret this line as saying that the spirits upon earth in particular seemed fervorless, with the implication that there are other spirits, not upon earth, that still seem fervored. And indeed, the poem goes on to describe one such spirit in the next two stanzas. Without the change in rhythm, we might miss this implication, interpreting the line as saying that the spirits, which happen to be upon earth, seemed fervorless.
Note on scansion
The rhythm of the poem is not quite as even as suggested in the question. When I read it, I find a stress on “sharp” in line 1, an anapest (“-tury’s corpse”) in line 2, and only one stress in “canopy”, so that the double iamb is not so surprising to me.