I enjoyed doing the gag covers better than the story ones because they were usually simpler. A cover based on an incident in the plot took a great deal of staging to tell a little story that was still part of the book. And it had to make sense on its own. ↗
Although Darwin was able to persuade much of the world that a modern eye could be produced gradually from a much simpler structure, he did not even attempt to explain how the simple light sensitive spot that was his starting point actually worked. ↗