And how does this relate to the jibber-jabber that is Darwin's OntheOriginofSpecies? I can't find the word "gene" in the index.

Undoubtedly because Darwin didn't know what genes were. Genetic change over a period of several generations: sounds like evolution to me.

In addition, I believe you may have misread Eliezer's comment: the gene may take a long time to reach the point where the species dies out. It's only "one more generation" once the gene has evolved to fixation.