What Mark Twain is basically saying here is that pilots need to be of the ability to make judgement calls, and the ability to make good judgement calls depends on whether or not one is intelligent. He takes his point further by saying that, basically, intelligence is genetic—one is either born with intelligence or one is not. His point is, if one is not born with “brains” (intelligence), one cannot be a pilot because intelligence (according to this statement of Twain) cannot be acquired.