Charles Darwin was a prolific letter writer and this passage from one of his letters to botanist Joseph Dalton Hooker made me laugh.

Now, do you agree thus far? if not, it is no use arguing; we must come to swearing, and I am convinced I can swear harder than you, .-. I am right. Q.E.D.

BTW today is Darwin's birthday. In an interesting coincidence he and Abraham Lincoln were both born on February 12, 1809.

Letter excerpt from Popsci.com