Something very important transpires at the level of relationships. For natural history conceives of the relationships between animals in two ways: series and structure. In the case of series I say a resembles b, b resembles c, etc… This is exactly what theologians used to call an analogy of proportion. In the case of structure, I say a is to b as c is to d… This is an analogy of proportionality.
–Deleuze and Guattari, A Thousand Plateaus