I have while reading Bram Stoker's 'Dracula' stumbled upon a sentence which has ever since been puzzling my mind and I have not been able to penetrate the grammar used. Here goes the sentence:
"I do believe the dear soul thought I might be jealous, lest my poor dear should have fallen in love with any other girl."
What exactly puzzles me are the following grammar structures: "lest" and "should have fallen". What meaning do they convey here? How do I understand them? I feel it's not commonly used grammar, and any help is very much appreciated. I have made bold all the structures I'm not quite sure of. I only know of the "should have + past participle" construction in a sense of regret, like "I should have studied harder." (while in fact I did not, hence the lamentation." But here the construction seems to be following some other frame of application.