In The Blind Watchmaker...?

Darwin was terribly troubled by the comments of a Scottish engineer who provided an example of how natural selection could not possibly lead to evolution. Darwin was unable to refute the apparent implications of a hypothetical example in which a white man lands on an island inhabited by a black tribe. Despite the racism of the example, typical of the times, how did the argument go, and what basic principle did Darwin need in order to understand in order to refute the objections of the engineer?