In an attempt to remove this ambiguity between referring to tokens (individual instances) and classes, McMullin introduces two prefixes, `A-' (standing for Actor) and `L-' (standing for Lineage) [McMullin 95]. The former signifies an individual token, and the latter signifies a class (read the `L-' prefix as `a lineage of ...'). For example, an A-replicator is an individual (molecule, strand of DNA, etc.), and an L-replicator is a lineage of A-replicators. McMullin goes on to single out lineages of replicators in which all members are identical to the founder of the lineage with respect to a particular selectively relevant character. These lineages, which he calls similarity lineages, or S-lineages, are, he argues, the fundamental units of selection in a Darwinian evolutionary process.2.27 Phrases such as ``genes are potentially immortal'' refer to L-genes, and, in particular, to S-lineages. Although McMullin regrets the introduction of yet more terminology (as do I), he sees no other way to disambiguate confusing terms such as `replicator'. I shall therefore use these prefixes throughout this thesis in situations where potential ambiguities may otherwise arise.
We are now in a position to be somewhat more precise about the meaning of a number of concepts that were used in Chapter 1.