Selection at the transmission stage happens through either elimination of certain memes, when the vehicle is destroyed or gets corrupted before it is perceived by another individual, or through differential multiplication, when the vehicle is reproduced into many copies. For example, a manuscript may be put into the shredder or it may be turned into a book which is printed in thousands of copies. A radio communication may get lost because of noise, or it may be broadcasted to millions of listeners. Especially since the emergence of mass media, the transmission stage is the one where the contrast between successful and unsuccessful memes is largest, and where selection may have the largest impact. R represents the proportion of these assimilated memes that are retained in memory.
E is the number of times a retained meme is expressed by the host.
T is the number of copies of an expression that is transmitted to a potential new host. Unlike A and R , E and T do not have an upper bound, although E is likely to be more restricted than T . This expresses the fact that a meme must successfully pass through all four stages in order to replicate. Which memes will most successfully pass all these stages can be modelled by a series of selection criteria. I will here basically situate them with respect to the four replication stages.