Before entering on this difficult passage, it seems necessary to make some preliminary observations.
(a) The demand was made twice (in itself exceedingly probable), and our Lord's answers were to a great extent identical in substance (in itself not very probable), and when identical in substance were closely identical in language (distinctly less probable). Or perhaps we might suppose that this identity of language was rather due to the narrator than to our Lord himself; familiarity with one answer may in the curly Church have moulded the record of the other.
(b) The demand and tile answer, as recorded, refer to one and the same occasion. But the account existed in more than one of the sources used by St. Matthew, and as each form of it had its own peculiarities, he retained them both. Anyhow, Matthew 16:1-4 seems to have belonged to the Framework, and our passage to the Discourses.
Christ's primary object, then, in his reply was to show to his opponents that heathen Ninevites and a heathen queen accepted the truth without any such sign as that which they were now demanding, and, if possible, to shame them into doing so. Thus verse 40 is to be considered as parenthetical rather than as the main subject.
It has, indeed, been suggested that verse 40 was in fact not spoken at all by the Lord himself, but is only the result of a very early interpretation by the Hebrew Christians of our Lord's phrase, added before the formation of our Gospel. The explanation is tempting, but, in the entire absence of corroboration, cannot be accepted (cf. note there). So far as our present evidence goes, we must attribute verse 40 to Christ, and consider that as he was mentioning the reception of Jonah by the Ninevites, the thought occurred to him that in Jonah's history lay as it were a prefigurement of what he himself would be. Just as on another occasion he illustrated his death and resurrection by the figure of destroying and building the temple (John 2:18, John 2:19), so now he uses the figure of Jonah in the whale's belly.