  A number of critics have pointed out that essentialism need not be committed to ‘types’ understood as universalia in re; and on certain accounts of essences any species taxon that meets the standards of BSC does so in virtue of certain essential (though relational and historical) properties. At one extreme, Michael Ghiselin and David Hull have argued that this causal/historical structure of species provides grounds, at least within evolutionary biology, for considering species to be  individuals.[15]  Organisms are not members of a class or set, but ‘parts’ of a phylogenetic unit. Taking a very different tack, Denis Walsh has recently argued that a form of ‘evolutionary essentialism,’ bearing a striking resemblance to the essentialism of Aristotle’s zoological work, is implicit in the work of a number of evolutionary developmental theorists (Walsh, 2006).