 Jablonka and Lamb (2005) argue that thinking in terms of information transmission systems also allows us to point out salient differences in the forms of social transmission underlying cultural evolution. They claim that only some forms of social transmission make use of a system of symbols. Consider, for example, that to say that some birds inherit their song by social transmission is not to say that birdsong is a symbolic system. Humans, on the other hand, trade in publicly-accessible symbols. Moreover, repositories of symbols, most obviously in the form of libraries and computer databases, are vital inheritance systems for humans, allowing the preservation and accumulation of knowledge across generations. Note, also, that there are different types of symbol system. In some cases the relationship between a symbol and what is represented is arbitrary. The is the case for a word like ‘man’, which does not look or sound like a human male. In other cases of iconic symbolism, the relationship is one of resemblance: a sign for the gents’ toilet looks like a man.