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``MY DEAR HARRIET,
You will laugh when you know where I am gone, and I cannot help laughing myself at your surprise to-morrow morning, as soon as I am missed. I am going to Gretna Green, and if you cannot guess with who, I shall think you a simpleton, for there is but one man in the world I love, and he is an angel. I should never be happy without him, so think it no harm to be off. You need not send them word at Longbourn of my going, if you do not like it, for it will make the surprise the greater when I write to them and sign my name Lydia Wickham. What a good joke it will be! I can hardly write for laughing. […] Your affectionate friend,
LYDIA BENNET.'' (47.60)
Well, it's nice to know that Lydia really thought Wickham was going to marry her. It seems that Wickham has just been tricking a very naïve and trusting girl, because we find out for certain that Wickham had no intention of marrying Lydia when we read Mrs. Gardiner's letter to Lizzy about the whole scandal and Darcy's involvement. It's almost (almost) enough to make us feel sorry for Lydia.
[Mary:] "Unhappy as the event must be for Lydia, we may draw from it this useful lesson: that loss of virtue in a female is irretrievable; that one false step involves her in endless ruin; that her reputation is no less brittle than it is beautiful; and that she cannot be too much guarded in her behaviour towards the undeserving of the other sex." (47.44)
Here, Mary is clearly echoing the sort of horrible, formal advice given to young ladies by the conduct books (books about how to behave and why) being published at the time. The awful humor here is that, of course, in the actual situation of the Bennets, with actual human beings, with real feelings involved, no one wants to hear this unsympathetic nonsense about the purity of women.