 He also worked with Bölsche to illustrate 60 dinosaur and other prehistoric animal collecting cards for the Reichardt Cocoa Company, titled "Tiere der Urwelt" ("Animals of the Prehistoric World").One of Harder's contemporaries, Danish paleontologist Gerhard Heilmann, produced a large number of sketches and ink drawings related to "Archaeopteryx" and avian evolution, culminating in his lavishly illustrated and controversial treatise "The Origin of Birds", published in 1926.This classic depiction of dinosaurs remained the status quo until the 1960s, when a minor scientific revolution began changing the perceptions of dinosaurs as tail-dragging, sluggish animals to active, alert creatures.