How we cite our quotes:
"I call it noble," I cried. "I'm a Whig, or little better; but I call it noble."
"Ay" said [Alan], "ye're a Whig, but ye're a gentleman; and that's what does it." (12.30-31)
Davie is commenting here on the donations that Ardshiel's former tenants have been making to his upkeep while he's living in exile in France. Davie admits that his politics don't agree with the actions of these Highlanders (since he's a Whig, a supporter of the current king of England), but he can admire them all the same. And Alan says this is because Davie is "a gentleman; and that's what does it." In other words, he and Davie can overcome their political differences because they are gentlemen.
I said nothing, nor so much as lifted my face. I had seen murder done, and a great, ruddy, jovial gentleman struck out of life in a moment; the pity of that sight was still sore within me, and yet that was but a part of my concern. Here was murder done upon the man Alan hated; here was Alan skulking in the trees and running from the troops; and whether his was the hand that fired or only the head that ordered, signified but little. By my way of it, my only friend in that wild country was blood-guilty in the first degree; I held him in horror; I could not look upon his face; I would have rather lain alone in the rain on my cold isle, than in that warm wood beside a murderer. (18. 3)
But…there's a line that can't be crossed. Much of the novel is dedicated to showing how two men of completely different politics, religion, and moral feeling can still be great friends. But Davie cannot allow murder, so it's a good thing Alan didn't do it.
"James Stewart," said Alan, "I will ask ye to speak in Scotch, for here is a young gentleman with me that has nane of the other. This is him," he added, putting his arm through mine, "a young gentleman of the Lowlands, and a laird in his country too, but I am thinking it will be the better for his health if we give his name the go-by." (19.6)
Alan is incredibly careful about making sure that all conversation in front of Davie is conducted in Scots. This is not only polite and generous of him, it's also important for the advancement of the plot. Otherwise, how could Davie realistically claim to understand all of the conversations around him?