"Suppose a white sailor were
thrown ashore on an island of [Purple People Eaters]
...."
Jenkin supposed that (of course) the white sailor is superior to
the local PPE, and will become King. As such, the 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 sailor 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 sailor in the first generation, the
population cannot become white in the long run.
Darwin told friends
that Jenkin's argument bothered him more than any other.