Darwin made predictions not found until the mid 1900s. Yep, that's way after the fact.
Ms Cleo also makes predictions which have been proven after the fact - does that make her a great scientist?
The answer is no. Predictions are scientific proof only when all the predictions are proven to be true. Any charlatan can make some predictions that turn out to be true - and Darwin was an excellent charlatan. I have shown some important predictions of Darwin, pretty central to his theory that have been proven false: brachyocephalism, melding of parental traits, proof of evolution from the fossil record, the malthusian basis of evolution, to name just some. So no, his predictions prove him a charlatan, not a scientist.