"Suppose a white sailor were thrown ashore on an island of [Purple People Eaters] ...."

The single white sailor in the first generation has four offspring in the second generation, twice as many as a local PPE. All of them are intermediate in color between white and deep purple. These offspring also have an excess number of offspring in the third generation, who are intermediate between lilac and deep purple. In each successive generation, even though the descendants of the original white have more offspring than other individuals in the population, their coloration will always converge on the darker color of the bulk of the population, and eventually the population will again become uniformly dark. No matter the reproductive advantage gained by the introduced white in the first generation, the population cannot become white in the long run.

Darwin told friends that Jenkins' argument bothered him more than any other.