 In the version of replication formulated by Sterelny et al. (1996), both copying and biofunction are crucial. Copying is quite obviously a causal phenomenon, but not any old causal connection will do. Similarity of copies is necessary but not sufficient. These copies must be copies of copies. One copy must produce another copy and that copy produce still another copy and so on. For one entity to be a copy of another, it must be the output of a process whose biofunction is to conserve function. The function of copying is to produce from one token another token, which is similar in the relevant respects. Genes fit this definition, but so do lots of examples of non-genetic transmission, such as habitat stability resulting from nest site imprinting, the song that a bird learns, and various microorganism symbionts, not to mention sociocultural transmission.