In Act 3, Scene 4, when Macbeth is told by the First Murderer of the successful assassination of Banquo and the escape of Fleance, he says:
Then comes my fit again: I had else been perfect,
Whole as the marble, founded as the rock,
As broad and general as the casing air:
But now I am cabin'd, cribb'd, confined, bound in
To saucy doubts and fears. But Banquo's safe?
First MurdererAy, my good lord: safe in a ditch he bides,
With twenty trenched gashes on his head;
The least a death to nature.
Thanks for that:
There the grown serpent lies; the worm that's fled
Hath nature that in time will venom breed,
No teeth for the present. Get thee gone: to-morrow
We'll hear, ourselves, again.
Shakespeare does not give Fleance's exact age, but he seems to be very young, perhaps not more than ten years old. (The same boy- actor who played Macduff's son in Act 4, Scene 2 probably also played Fleance.) Fleance represents a grave threat to Macbeth, not because, like Duncan's son Malcolm, he can raise an army, but because he is evidently Banquo's only surviving child and hence would have to be the father of the future kings of Scotland, as predicted by the three witches. Macbeth firmly believes in their predictions by this time. But he has other things to worry about and will have to put off thinking about Fleance until later. In the meantime, Malcolm and Macduff raise an army, invade Scotland, and Macduff slays Macbeth.
Fleance's successful escape from the three murderers demonstrates the impossibility of escaping the hand of fate, as Macbeth finally recognizes in the last act of the play.
MACDUFFDespair thy charm;
And let the angel whom thou still hast served
Tell thee, Macduff was from his mother's womb
Accursed be that tongue that tells me so,
For it hath cow'd my better part of man!
And be these juggling fiends no more believed,
That palter with us in a double sense;
That keep the word of promise to our ear,
And break it to our hope.
In Act 2, Scene 1, it seems as if Banquo is talking to Fleance the way a father sometimes talks jokingly to a young child. Banquo says:
There's husbandry in heaven;
Their candles are all out.
In other words, Banquo is pretending that the stars are all candles being used by the occupants of heaven and that the angels are showing frugality by blowing out all their candles on this murky night when not a single star is visible. Banquo does not really believe what he is saying but is just talking to amuse his little boy while really thinking about serious adult matters, as fathers often will. This little jest also adds to the illusion that it is actually nighttime, although the audience would be watching the play in the daylight. The illusion of night and darkness is furthered by the opening lines of the scene and the fact that Fleance is carrying a lighted torch.