 The existence of various evolutionary forces does not rule out the possibility of a trait occurring in all and only the members of a species. But consider the conditions such a trait must satisfy. A species’ essential trait must occur in all the members of a species for the entire life of that species. Moreover, if that trait is to be unique to that species, it cannot occur in any other species for the entire existence of life on this planet. The temporal parameters that species essentialism must satisfy are quite broad. The occurrence of a biological trait in all and only the members of a species is an empirical possibility. But given current biological theory, that possibility is unlikely.