ABSTRACT
Biological species are supposed to have no essences and no necessary characteristics. Biologists today accept that all species they look at come from others, in an evolutionary tree: and they generally delimit species, or characterize them, according to their place in that tree. The primary job of essence, summarized in the foregoing rubric INVARIANCE, is to explain what it is to be this or that. The difficulty with Essential Membership is this: that current characterizations of species, those of the experts, do not seem amenable. Tradition has assigned too many roles to the same notion called 'essence', when tradition has suggested that a species' essence must be essential to individuals. Clusters of theoretically important properties are sometimes supposed to arise by nomological necessity from essence. Reflection on this sort of discrepancy between species concepts might engender skepticism about whether species are one natural category of lineage.
