 Kripke (1971, 1972) and Putnam (1975a) use animal kinds as examples of natural kinds for which a posteriori essences can be found. There is some implication that these essences are microstructural, intrinsic properties, which will be, of necessity, individually necessary and jointly sufficient for an entity to be a member of a kind. However, if species are individuals, then it is not true that species may be individuated on the basis of the intrinsic properties of their members. The various species concepts tend to offer relational criteria of species membership (see above). According to the BSC, for example, membership of a species depends on relational properties, such as membership of a certain population and interbreeding. Alternatively, the PSC refers to shared descent.