 Dawkins defines replicators as anything in the universe of which copies are made (1982: 82). Hull has a similar definition: a replicator is an entity that passes on its structure largely intact in successive replications (Hull 1988: 408). Germ-line replicators have the potential to have indefinitely many descendants; they contrast with somatic replicators, the genes found in body cells, which produce copies only as part of mitosis and whose lineages of descendants end when the body dies. Active replicators have some causal influence on their probability of being copied in contrast to inactive, “neutral” ones, which do not have any effects on development. Natural selection will occur wherever we find active germ-line replicators (Dawkins 1982: 83). Dawkins distinguished replicators from vehicles, his notion meant to replace and generalize that of organism. Hull proposed the notion of interactor as a similar complement to the notion of replicator. Neither notion, however, is meant to further delineate the circumstances in which selection occurs, or to narrow the scope of application of evolutionary theory (for further discussion of these notions, see entry on  units and levels of selection).  This is evident, for Hull at least, insofar as genes may be both replicators and interactors (1988: 409). 