 Another useful distinction is between synchronic and diachronic unity. Synchronic accounts are ahistorical, assuming no meaningful temporal relations. Diachronic accounts, by contrast, introduce genealogical hypotheses involving asymmetric temporal and causal relations between entities or states of the systems described. Evolutionary models are of this kind; they may be reductive to the extent that the posited original entities are simpler and on a lower level of organization and size. Others simply emphasize connection without overall directionality.