 The obvious answer seems to be this. In the case of identity over time, we can appeal to relations (other than mere similarity) between the states of an individual at different times in its existence. For example, it looks as if we can say that the adult Russell is identical with the infant Russell in virtue of the existence of certain spatiotemporal and causal continuities between his infant state in 1873 and his adult state in (say) 1950 that are characteristic of the continued existence of a human being. But no such relations of continuity are available to ground identities across possible worlds (cf. Quine 1976).