 Given the class/individual distinction, Ghiselin and Hull argue that species are individuals, not classes. Their argument assumes that the term ‘species’ is a theoretical term in evolutionary theory, so their argument focuses on the role of ‘species’ in that theory. Here is Hull’s version of the argument, which can be dubbed the ‘evolutionary unit argument.’ Since Darwin, species have been considered units of evolution. When Hull asserts that species are units of evolution, he does not simply mean that the gene frequencies of a species change from one generation to the next. He has a more significant form of evolution in mind, namely a trait going from being rare to being prominent in a species. A number of processes can cause a trait to become prominent in a species. Hull highlights selection. Selection causes a trait to become prominent in a species only if that trait is passed down from one generation to the next. If a trait is not heritable, the frequency of that trait will not increase cumulatively. Hereditary relations, genetic or otherwise, require the generations of a species to be causally and hence spatiotemporally connected. So, if species are to evolve in non trivial ways by natural selection, they must be spatiotemporally continuous entities. Given that species are units of evolution, species are individuals and not classes. (For responses to the Evolutionary Unit Argument see Dupré 2001, Reydon 2003, Crane 2004, and Crawford 2008.)