How to Make Mac OS X Mail Show Conversations Chronologically from Top to Bottom

From the sky, cows look like iron filings.
Like these, cattle and deer grazing seem to line up with magnetic fields. More often than not, they stand pointing to the Earth's magnetic poles.
Why they do that appears to be anyone's guess — while you, of course, do know why you want your emails to line up along the threads of time starting up north:
›› Don't like to start reading at the end? For less jerky reading, here's how to set up Mac OS X Mail to show conversations in chronological order: the first message on top and the most recent at the bottom.