The Selfish Gene

Darwin's theory of evolution focuses on the selection at the organism level. Dawkins realised that actually the selection takes place at the gene level. The difference doesn't matter most of the time but it explains why some organisms are altruistic towards closely related members of it's species.

He extended this idea into culture and used it to describe the way some ideas spread better than others and the way ideas mutate in order to spread better. This is where the word meme comes from.