"Had Darwin left it at that, you would have a point. He didn't"
Examples, please.
Above and beyond the fact that Darwin chose to publish Galton's eugenic theory in "Descent Of Man?" Well, here's one:
"Darwin congratulated Galton on the publication of Hereditary Genius, telling his younger cousin in a letter that, “I do not think I ever in all my life read anything more interesting and original.”