 Biologists have had a hard time finding biological traits that occur in all and only the members of a species. Even such pre Darwinian essentialists as Linnaeus could not locate the essences of species (Ereshefsky 2001). Evolutionary theory explains why. A number of forces conspire against the universality and uniqueness of a trait in a species (Hull 1965). Suppose a genetically based trait were found in all the members of a species. The forces of mutation, recombination, and random drift can cause the disappearance of that trait in a future member of the species. All it takes is the disappearance of a trait in one member of a species to show that it is not essential. The universality of a biological trait in a species is fragile.