On the other hand, certain things are known and won't change. For example Abraham Lincoln was not a black man, as one of my students had been taught. He's not going to acquire black DNA over the passage of time.
As a small boy, my youngest son enjoyed watching college football with me. One day he announced, "Daddy, I'm going to get darker and darker, and one day I'll be a football player!" Evidence for him was on the TV screen. He had a mixed-race friend in daycare and might have thought the little boy was going through 'the change' a little early. More evidence!
I don't believe we'll discover that companies of Southern ladies or Chinese soldiers served in the Confederate army, but show me the evidence. I'll reconsider.
David, I know you understand, but for what it's worth, evidence is not someone's unsupported claim, even if it is printed in a book. My student who believed Abraham Lincoln was black read it in a book. It's not someone's assertion who had no actual first-hand knowledge of events or people. For example, I used testimony by Frederick Douglas to help my student understand because Frederick Douglas had first-hand experience with Abraham Lincoln. On the other hand, Douglas had no contact whatsoever with Confederate soldiers.
Finally, lack of evidence does not mean that something may be true. Let's put that one to rest and discuss history as something we actually know and understand.