A legitimate appeal to authority is already based on evidence
Then,
by definition, it's not an appeal to authority - it's an appeal to evidence.
Essentially my argument is that, of three statements:
1) "evolution is correct because Stephen Jay Gould says so."
2) "evolution is correct because Stephen Jay Gould says so based on such-and-such evidence."
3) "evolution is correct because of such-and-such evidence."
1 and 2 are not equivalent, but 2 and 3 are. 1 is a fallacious appeal to authority. 2 and 3 are appeals to evidence. You may feel that 2 is a legitamate appeal to authority; I agree that it's a legitamate argument, but not because of the authority, but because of the evidence.
The evidence is there, supporting their statements, even if not explicitly given.
Why? Simply because they're an authority? What a world of credulity and naivete you must live in. How are we to be sure that an "authority's" conclusions are based on evidence without access to that evidence? And once we have the evidence, why do we need the authority?
I just don't see how a reasonable person could trust that an authority would always be honest enough to support their conclusions with evidence.
In this case, my "argument from authority" is an argument based on evidence, just transitively.
Then it's hardly argument from authority, is it? It's argument from evidence.
Look, if we're just arguing about what words mean, then we're saying the same thing differently. I know - or at least, assume you're reasonable enough to realize - that you don't think "Stephen Jay Gould says so" is a compelling argument. If you believe that "Stephen Gay Gould says so because of such-and-such evidence" is an argument from authority, then I'll accept that as a "legitamate argument from authority." But personally I don't believe that's what is meant by "argument from authority" because to me, that's an argument from evidence.