Homologous structures. They're evidence in the sense that organisms with
homologous structures are thought to be descended from a common
ancestor. It's good evidence because it follows the nested pattern of common
descent. All mammals have similarly structured arms, so bats' wings are
made up of fingers. But birds are not descended from mammals, and their
wings are completely different: the underlying limb structures are the
same, but the way they make wings are not.