I haven't read Darwin's work, so obsviously I don't know that a "major sorce" for him was a creationist... nevertheless, as I have no reason to doubt your word, I'll indulge.
Maybe he wanted hard data. Maybe some other naturalists had collected info on morphology that Darwin could use, regardless of the fact they reached very different conclusions from them. Maybe only Darwin is known today because, despite the fact that he had help from info collected by others,
he was the one who have achieved amazing innovative results with it.
That would not be a first. I remember, from when I read a biograph on Keppler, that he relied heavly in observations from another astronomer - who believed that the earth was the center of the solar system and that the orbit of the planets was round (Kepler was a hardcore supporter of Copernicus, and the man who figured out the ellipytical orbit of the planets, to those who don't know). And why? Because the fact that the other guy (sorry, forgot his name) has reached wrong conclusions from his observations didn't make him any less precise in his notes about planet positions in given days, and hence, didn't make the data he collected any less valuable.
Regards

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