Although unrelated to any intuitive notions of organismal complexity, haploid genome sizes (C values) are correlated with a variety of cellular and organismal parameters in different taxa. In some cases, these relationships are universal--notably, genome size correlates positively with cell size in each of the vertebrate classes. Other relationships are apparently relevant only in particular groups. For example, although genome size is inversely correlated with metabolic rate in both mammals and birds, no such relationship is found in amphibians. More recently, it has been suggested that developmental rate and (or) longevity are related to genome size in birds. In the present study, a large dataset was used to examine possible relationships between genome size and various developmental parameters in both birds and mammals. In neither group does development appear to be of relevance to genome size evolution (except perhaps indirectly in birds through the intermediation of body size and (or) within the rodents), a situation very different from that found in amphibians. These findings make it clear that genome size evolution cannot be understood without reference to the particular biology of the organisms under study.