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

    Fleming Jenkin, a Scottish engineer, along with Charles Darwin and other 19th century biologists believed that inheritance was a blending process, that offspring would be a combination of traits from their two parents. Jenkin argued that, under these conditions, it was impossible for a new trait, no matter how advantageous, to become fixed in a population. His argument was presented in terms that were overtly racist and are offensive to modern readers. I have corrected the narrative to involve a white sailor thrown ashore on an island of Purple People Eaters (PPEs).

    Jenkin supposed that, as the White sailor would (of course) be innately superior to the local PPE, he would become King. As such, the sailor in the first generation would have more offspring in the second generation (Line 2) than the local PPE. Because of blending inheritance, they would be an intermediate lilac in color between white and purple. As such, they would succeed their father as rulers. In the third generation (Line 3), these lilac offspring would also have an excess number of offspring, who would be intermediate between lilac and purple. In each successive generation (lines 4 & 5), even if the descendants of the original white sailor continue to constitute a ruling caste with more offspring than the PPE, blending inheritance will make their color converge on the darker color of the bulk of the population. Eventually the population will again become uniformly dark. No matter the reproductive advantage gained by the introduced white sailor or his descendants, the population cannot become white in the long run.

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