I will first refer briefly to some of the older arguments in favour of this later date, and then more at length to those based on versification.
(1) In ii. iii. 4-5, 'Here's a farmer that hang'd himself on the expectation of plenty,' Malone found a reference to the exceptionally low price of wheat in 1606.
(2) In the reference in the same speech to the equivocator who could swear in both scales and committed treason enough for God's sake, he found an allusion to the trial of the Jesuit Garnet, in the spring of 1606, for complicity in the Gunpowder Treason and Plot. Garnet protested on his soul and salvation that he had not held a certain conversation, then was obliged to confess that he had, and thereupon 'fell into a large discourse defending equivocation.' This argument, which I have barely sketched, seems to me much weightier than the first; and its weight is increased by the further references to perjury and treason pointed out on p. [397].
(3) Halliwell observed what appears to be an allusion to Macbeth in the comedy of the Puritan, 4to, 1607: 'we'll ha' the ghost i' th' white sheet sit at upper end o' th' table'; and Malone had referred to a less striking parallel in Caesar and Pompey, also pub. 1607:
Why, think you, lords, that 'tis ambition's spur
That pricketh Caesar to these high attempts?
He also found a significance in the references in Macbeth to the genius of Mark Antony being rebuked by Caesar, and to the insane root that takes the reason prisoner, as showing that Shakespeare, while writing Macbeth, was reading Plutarch's Lives, with a view to his next play Antony and Cleopatra (S.R. 1608).
(4) To these last arguments, which by themselves would be of little weight, I may add another, of which the same may be said. Marston's reminiscences of Shakespeare are only too obvious. In his Dutch Courtezan, 1605, I have noticed passages which recall Othello and King Lear, but nothing that even faintly recalls Macbeth. But in reading Sophonisba, 1606, I was several times reminded of Macbeth (as well as, more decidedly, of Othello). I note the parallels for what they are worth.
With Sophonisba, Act i. Sc. ii.:
Upon whose tops the Roman eagles stretch'd
Their large spread wings, which fann'd the evening aire
To us cold breath,
cf. Macbeth i. ii. 49: