So, I'm doing an illustration for somebody-- they want a parody of the famous "march of progress" picture; the one where a walking ape progresses forward into modern man.

I started out by scanning the original (from a time life book) for reference.

But how close to the original is too close? Is it ethical to trace it, if it still bears elements of my own artistic style? Or am I obligated to mimic it only by eye?


http://people.ucsc.edu/~tbabb/shirt_temp/march.png

Above is an example of my style versus the original. (Unfortunately, I can't embed an image or a link because my account is broken and does not allow me to post HTML).

I figured the threadless community seems pretty keen on the issue of copyright, so I ask for your advice.