While I disagree that Lord Julius was "genius," I will agree that what Sim did is not the same thing that Greg Land, J.G. Jones, et al do. One is caricature (which involves original creativity) and the other is merely synthesizing borrowed elements.
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"The trouble with being a ghost writer or artist is that you must remain anonymous without credit.
If one wants the credit, one has to cease being a ghost and become a leader or innovator."
Bob Kane