Darwin was not the only person interested in evolution; others who came before him had similar theories and ideas. Darwin was criticized - by Baden Powell in particular - for not including his influences in The Origin of Species. (Read his response to Baden Powell here; Powell's original letter is missing.) Darwin did add a Historical Sketch to The Origin of Species after it was first published - read more about the sketch's history in "The Preface to Darwin's Origin of Species: The Curious History of the 'Historical Sketch'" by Curtis Johnson.
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